Dynein heavy chain 64C
Dhc64C, encodes a cytoplasmic dynein heavy chain polypeptide. The primary structure of the
Drosophila cytoplasmic dynein heavy chain polypeptide has been determined by the isolation and sequence
analysis of overlapping cDNA clones. Drosophila cytoplasmic dynein is highly similar in sequence and
structure to cytoplasmic dynein isoforms reported for other organisms. The Dhc64C dynein transcript is
differentially expressed during development; the highest levels are detected in the ovaries of adult
females. Within the developing egg chambers of the ovary, the dynein gene is predominantly transcribed in
the nurse cell complex. In contrast, the encoded dynein motor protein displays a striking accumulation in the
single cell that will develop as the oocyte. The temporal and spatial pattern of dynein accumulation in the
oocyte is remarkably similar to that of several maternal effect gene products that are essential for oocyte
differentiation and axis specification. Before bulk transport of nurse cell cytoplasm to the oocyte, an increase is observed in the amount of dynein antigen present in the nurse cell cytoplasmic domain of stage 10 egg chambers. Dynein is also detected at a low and uniform intensity in the follicle cells surrounding early egg chambers. At later stages this follicle cell dynein appears to be concentrated to the apical region of each follicle cell that surrounds the oocyte. This apical pattern of dynein distribution in follicle cells could reflect the role that these cells play in polarizing secretion of components required for the formation of several shell layers that encapsulate the mature egg. In the oocyte, a bright focus of dynein staining is closely apposed to the oocyte nuclei in early chambers; and a perinuclear concentration of the dynein antigen is frequently apparent in stage 6-8 egg chambers. During stage 9, an accumulation of dynein antigen is seen at the posterior end of the oocyte, suggesting that dynein may play a role in the assembly of the pole plasm during stages 8 and 9 of oogenesis. The posterior concentration of dynein appears to diminish at later stages, while an elevated level of dynein appears in the nurse cell cytoplasm. This distribution and its disruption by specific maternal effect
mutations lends support to recent models suggesting that microtubule motors participate in the transport of
these morphogens from the nurse cell cytoplasm to the oocyte (Li, 1994).
Microtubules and the Kinesin heavy chain (the force-generating component of the plus end-directed microtubule motor Kinesin I) are required for the localization of oskar mRNA to the posterior pole of the Drosophila oocyte, an essential step in the determination of the anteroposterior axis. The Kinesin heavy chain is also
required for the posterior localization of Dynein, and for all cytoplasmic movements within the oocyte.
Furthermore, the KHC localizes transiently to the posterior pole in an oskar mRNA-independent manner.
Surprisingly, cytoplasmic streaming still occurs in kinesin light chain null mutants, and both oskar mRNA and Dynein localize to the posterior pole. Thus, the Kinesin heavy chain can function independently of the light chain in the oocyte, indicating that it associates with its cargoes by a novel
mechanism (Palacios, 2002).
It is unclear why dynein localizes to the posterior, but one possibility is
that it is needed to recycle kinesin to the minus ends of the microtubules, so
that kinesin can mediate another round of posterior localization. The only known
phenotype of the Dhc64C mutants that specifically disrupt the
posterior localization of DHC is a reduction in the rate of cytoplasmic
streaming, and this may due to the gradual depletion of the pool of KHC
available for transport. However, this localization may be important for
recycling dynein away from the minus ends of microtubules, so that dynein can
mediate further rounds of minus end-directed transport (Palacios, 2002).
In the Drosophila oocyte, microtubule-dependent processes govern the asymmetric positioning of the nucleus and the localization to distinct cortical domains of mRNAs that function as cytoplasmic determinants. A conserved machinery for mRNA localization and nuclear positioning involving cytoplasmic Dynein has been postulated; however, the precise role of plus- and minus end-directed microtubule-based transport in axis formation is not yet understood. mRNA localization and nuclear positioning at mid-oogenesis is shown to depend on two motor proteins, cytoplasmic Dynein and Kinesin I. Both of these microtubule motors cooperate in the polar transport of bicoid and gurken mRNAs to their respective cortical domains. In contrast, Kinesin I-mediated transport of oskar to the posterior pole appears to be independent of Dynein. Beside their roles in RNA transport, both motors are involved in nuclear positioning and in exocytosis of Gurken protein. Dynein-Dynactin complexes accumulate at two sites within the oocyte: around the nucleus in a microtubule-independent manner and at the posterior pole through Kinesin-mediated transport.
It is concluded that the two microtubule motors, cytoplasmic Dynein and Kinesin I, by driving transport to opposing microtubule ends, function in concert to establish intracellular polarity within the Drosophila oocyte. Furthermore, Kinesin-dependent localization of Dynein suggests that both motors are components of the same complex and therefore might cooperate in recycling each other to the opposite microtubule pole (Januschke, 2002).
The localization of bcd mRNA in the oocyte occurs in multiple steps. Several of these involve active transport along microtubules. bcd mRNA coassembles into particles with Exuperantia (Exu) in the nurse cells and in the oocyte. This complex is essential for the correct localization of bcd to the anterior cortex in a microtubule-dependent manner. During mid-oogenesis, bcd maintenance at the anterior cortex is dependent on Swallow (Swa). This protein harbors a putative double-strand-RNA binding motif and a coiled-coil domain, which interacts with the Dynein light chain (Dlc-1). Swa has been proposed to act as an adaptor between bcd mRNA and the Dynein motor. Swa itself localizes to the anterior cortex of stage-10 oocytes, and this localization requires the coiled-coil domain, suggesting that polar transport of Swa and its cargo, bcd mRNA, occurs in a Dynein-dependent manner. The observation that bcd mRNA is delocalized in oocytes overexpressing
p50 Dynamitin (Dmn), a component of the Dynactin complex provides further support for this suggestion (Januschke, 2002).
Surprisingly, in khc mutant oocytes, bcd mRNA is not tightly concentrated to the anterior cortex but is diffusely spread out in a wide cortical ring that expands toward the posterior. Thus, correct bcd localization depends not only on minus end-directed, but also on plus end-directed, motors. Kinesin I might be directly involved in anchoring of bcd mRNA to the anterior cortex. Alternatively, Kinesin I might be required for efficient Dynein-dependent transport of bcd. The observation that Dynein is mislocalized in khc mutant oocytes supports the latter hypothesis. Dhc fails to accumulate at the posterior pole of khc mutant oocytes and instead is enriched at the anterior cortex. Thus, Kinesin I appears to be necessary to relocate Dynein to the posterior pole after it has moved together with its cargo to the anterior pole. This would allow for renewed rounds of cargo loading and transport to the anterior cortex. Without sustained posterior-to-anterior transport, the bcd mRNA/adaptor complexes might become delocalized by diffusion. This scenario indicates that sustained transport could be an alternative to an independent anchorage step (Januschke, 2002).
In contrast to what was detected with bcd, no dual motor requirement has been detected for osk localization to the posterior, which is Kinesin I dependent. osk mRNA is clearly localized and translated when Dynein function is impaired. However, several features of the phenotypes produced by Dynamitin overexpression suggest that Dynein function is not completely abolished. Thus, it cannot be strictly ruled out that low levels of Dynein are required for efficient osk transport to the posterior pole. Indeed, the mechanism of osk localization is more complex than that of bcd localization. osk localization occurs through a series of distinct steps first to the anterior, then to the middle, and finally to the posterior pole of the oocyte (Januschke, 2002).
The correct positioning of the oocyte nucleus requires two different anchoring processes: one to the lateral cortex and a second to the anterior cortex. The former might be a prerequisite for nuclear movement from the posterior to the anterior pole. The latter occurs after completion of the movement. While Kinesin I appears to be dispensable for nuclear movement, the role of Dynein remains to be clarified. The Dynein-Dynactin complex is essential for nuclear migration in many cell types, from yeast to vertebrates. Overexpression of Dmn, though, seems not to interfere with correct positioning of the nucleus at early stages. Since nuclear migration is targeted toward MT minus ends, like bcd mRNA localization, it is assumed that it requires Dynein and it is suggested that, in the Dmn overexpression experiment, residual Dynein function is present at early stages, which allows nuclear migration (Januschke, 2002).
While the question of migration needs further analysis, both motors are clearly required for nuclear anchorage. Impairment of Dynein leads to nuclear detachment from both the anterior and lateral cortex. Kinesin I, however, is only required for maintaining the nucleus at the anterior cortex. The fact that both anchoring processes fail when Dmn is overexpressed indicates that Dynein fulfils a complex function in nuclear positioning. Two Dynein-Dynactin pools are present in the oocyte: the posterior pool, which is microtubule dependent and maintained by Kinesin I-dependent transport, and the perinuclear pool, whose maintenance is independent of MTs and MT motor activity. The perinuclear Dynein-Dynactin pool appears to be involved in organizing a MT cage around the nucleus, and this cage is likely to be necessary for the attachment of the nucleus to the lateral cortex. The mislocalization of the nucleus in Kinesin mutants might be explained in a similar way as the mislocalization of bcd mRNA. Cortical Dynein activity might be required during some stage after the nucleus has reached the anterior pole. During this period what appears to be anchorage would be the result of sustained minus end-directed movement. To maintain this movement, Dynein is supplied from the posterior pool, which constantly has to be replenished by Kinesin-dependent transport (Januschke, 2002).
The nuclear MT scaffold might not only be important for nuclear migration and nuclear anchoring. It harbors centrosomal components such as Centrin, which probably contribute to the formation of the MT scaffold but might also influence the MT network of the entire oocyte. Due to these properties, the nucleus is likely to have a central role in polarizing the Drosophila oocyte. During migration, it might contribute to the overall anterior-posterior repolarization of the oocyte MT network, which is required to establish the anterior and posterior cortical domains. After migration, MTs emanating from the asymmetrically positioned nucleus are likely to polarize the transport of grk mRNA and Grk protein, which establishes the anterodorsal cortical domain (Januschke, 2002).
grk mRNA is produced by both the nurse cells and the oocyte nucleus. After nuclear migration, grk mRNA accumulates briefly along the anterior margin of the oocyte, before it concentrates in a perinuclear position. The anterior localization of grk is not affected when Dynein function is reduced or if Kinesin I function is completely abolished. However, both motors are required to transport grk to the nucleus. It is suggested that grk mRNA is transported toward the minus ends of MTs, which emanate from the nucleus. This would explain the Dynein requirement for grk transport to the nucleus. The role of Kinesin I in anterodorsal grk transport might again reflect the need to retrieve the Dynein motors for renewed cargo loading, as suggested for bcd and the oocyte nucleus (Januschke, 2002).
This model has to assume, however, that Dynein-Dynactin complexes carrying different cargos can distinguish between distinct populations of MTs: Dynein-Dynactin complexes loaded with bcd mRNA should be transported to and remain at anterior cortex, while those loaded with grk mRNA should be subject to a second transport step toward the nucleus. Deletions within the grk 3'UTR allow anterior localization of grk mRNA but prevent its transport to the nucleus. This suggests that specific factors distinguish anterior and anterodorsal transport of grk. The heterogeneous nuclear RNA binding protein (hnRNP) Squid plays a central role in this process. It regulates both grk localization and translation and binds directly to the grk 3'UTR. Squid protein, like grk, appears to be transiently localized along the anterior cortex during the transition from stage 7 to stage 8 (Januschke, 2002).
grk mRNA, though mislocalized, is frequently translated when Kinesin I or Dynein motor activities are impaired. Since grk mRNA is found around the anterior cortex in those cases, Grk secretion should occur around the entire circumference of the oocyte instead of being restricted to the dorsal side. Secreted Grk induces dorsal follicle cell fates. Thus, ectopic secretion should lead to the formation of dorsalized eggs as in squid and fs(1)K10 mutants in which grk mRNA is also mislocalized. However, impaired MT motor activity leads to ventralized eggs and thus to reduced Grk signaling. An analysis of Grk distribution in oocytes shows that, in contrast to wild-type or squid and fs(1)K10, Grk protein is not closely associated with grk mRNA and fails to reach the plasma membrane. Thus, polar transport of Grk protein and exocytosis requires Dynein and Kinesin I activity. This is not surprising, since both motors have been shown to be involved in Golgi dynamics in higher eukaryotes and it has been shown that vesicular trafficking from the Golgi to the plasma membrane requires Kinesin activity (Januschke, 2002).
Interestingly, no requirement has been detected for the two motors in earlier Grk signaling, which induces posterior follicle cells and prevents the formation of a second micropyle at the posterior pole. In the case of Dynamitin overexpression, this might be due once more to residual levels of Dynein function. In the case of Kinesin I, it is assumed that Grk secretion is only impaired, but not entirely blocked. The phenotypic series of grk mutations suggests that minute amounts of secreted Grk are sufficient to induce posterior follicle cells (Januschke, 2002).
Opposite polarity motors can interact with the same cargo in two fundamentally different ways. They can function in an opposition mode, like Myosin V and Kinesin II in the migration of Xenopus melanophores, or in a coordination mode, like Dynein and Kinesin in the motion of lipid droplets in the Drosophila embryo. In the opposition mode, the two motors produce opposing forces on a single cargo. Inactivation of the minus end-directed motor leads to a delocalization of the cargo to the plus end, whereas inactivation of the plus end-directed motor leads to a delocalization of the cargo to the minus end. In the coordination mode, the motors are not competing with each other. For example, when plus end motors are active, minus end motors are turned off, and vice versa. Inactivation of either of the two motors leads to the delocalization of the cargo to the same side. According to this scheme, Dynein and Kinesin I act in a coordination mode during transport of bcd and grk mRNAs, and the same might be true for their role in the positioning of the oocyte nucleus. The observation that Dynein accumulation at the posterior pole depends on Kinesin I suggests that both motors are associated with the same vesicle or macromolecular complex, as has been proposed for axonal transport in Drosophila. Dynein has to be inactive during Kinesin I-dependent transport to the posterior pole but then has to be activated again for renewed cargo loading and transport to the anterior cortex. If this recycling model holds true for the described polar transport processes in the oocyte, it will be challenging to find those factors that regulate motor activity and cargo loading in successive transport cycles (Januschke, 2002).
Despite their opposing polarity, Dynein and kinesin motors may cooperate in vivo. In Drosophila circumstantial evidence suggests that dynein acts in the localization of determinants and signaling factors during oogenesis. However, the pleiotropic requirement for dynein throughout development has made it difficult to establish its specific role. Dynein function in the oocyte has been examined by disrupting motor activity through temporally restricted expression of the dynactin subunit, dynamitin. The results indicate that dynein is required for several processes that impact patterning; such processes include localization of bicoid (bcd) and gurken (grk) mRNAs and anchoring of the oocyte nucleus to the cell cortex. Surprisingly, dynein function is sensitive to reduction in kinesin levels, and germ line clones lacking kinesin show defects in dorsal follicle cell fate, grk mRNA localization, and nuclear attachment that are similar to those resulting from the loss of dynein. Significantly, dynein and dynactin localization is perturbed in these animals. Conversely, kinesin localization also depends on dynein activity. It is concluded that dynein is required for nuclear anchoring and localization of cellular determinants during oogenesis. Strikingly, mutations in the kinesin motor also disrupt these processes and perturb dynein and dynactin localization. These results indicate that the activity of the two motors is interdependent and suggest a model in which kinesin affects patterning indirectly through its role in the localization and recycling of dynein (Duncan, 2002).
In order to investigate the effects of targeted disruption of dynein activity, heat shock-inducible (hsDmn) and Gal4-responsive (UAS-Dmn) transgenic lines were created. The hsDmn transgene permits tight temporal control of misexpression, whereas UAS-Dmn allows spatially restricted transcription when coupled with the appropriate Gal4 drivers. After mapping the insert position, hsDmn flies were examined for the ability to induce Dmn expression by probing immunoblots with polyclonal antisera against the Drosophila protein. A single band migrating at 45 kDa, close to the predicted size of the endogenous protein, was detected in untreated control flies. This band was present at approximately 5- to 10-fold higher levels in animals that had been heat shocked for 60 min. A time course of induction shows that elevated Dmn levels are present 15 min after heat shock and persist for at least 6 hr. To test whether Dmn overexpression perturbs the stability or localization (or both) of the dynein/dynactin complex in vivo, egg chambers were stained with antisera against Dmn, Gl (the largest subunit of dynactin), and the dynein intermediate chain (Cdic). In the wild-type, Dmn preferentially accumulates in the oocyte during early oogenesis and shows both perinuclear and cortical staining through stage 8. By stage 9 Dmn is enriched in a crescent at the posterior cortex as well as in lateral regions. Overall, this distribution mimics that of Gl and the dynein intermediate and heavy chains. Within 60 min of hsDmn induction, high levels of Dmn were detected throughout the oocyte, nurse cells and follicle cells. In contrast, Gl and Cdic staining was undetectable in stage 9/10 oocytes and was strongly reduced at earlier stages, demonstrating that Dmn overexpression disrupts the localization of the dynein/dynactin complex (Duncan, 2002).
One explanation for the strong genetic interaction observed between kinesin and Dmn overexpression could be that kinesin is required to transport dynein toward microtubule plus ends. This would allow individual dynein complexes to be reused for multiple rounds of minus end-directed motion. A reduction in kinesin levels may compromise this recycling and decrease the pool of available dynein; it would thus affect dynein's ability to translocate cargo toward microtubule minus ends. This model provides a mechanistic basis for why processes that involve dynein, such as nuclear attachment and grk RNA localization, could be severely impacted in oocytes lacking Khc. It is also consistent with the observation that in Khc mutant egg chambers the dynein/dynactin complex is not localized to the lateral cortex of the oocyte after stage 8. Significantly, localization of grk transcript and protein are relatively unaffected prior to stage 8, when defects in dynein/dynactin localization first become apparent (Duncan, 2002).
In this context, it is interesting that the distribution of Khc in the oocyte resembles that of dynein and dynactin components; i.e., it is enriched at the cortex and the perinuclear region, where microtubule minus ends are expected to be most abundant. Such a pattern is consistent with a role for kinesin in recycling dynein from the cortex, similar to its proposed function in transporting osk mRNA, but raises the paradoxical question of how kinesin localization is established. After hsDmn induction cortical staining for Khc is reduced, suggesting that Khc localization is in turn dependent on dynein activity. Transport of kinesin to the cortex could occur as a result of a direct physical interaction between the two motors. Alternatively, kinesin and dynein could bind common cargoes or adaptor proteins; this would be analogous to the situation in the embryo, where dynein and a so-far-unidentified plus-end motor both associate with individual lipid droplets. Transport of the particles and the associated motors could occur in either direction if the activity of the opposite polarity motors is appropriately regulated. Interaction with a common intermediate anchored to the posterior cortex could also explain why kinesin, dynein, and dynactin colocalize in this region. The recent finding that dynein-associated structures move rapidly along microtubules in both directions in Dictyostelium suggests that motor recycling may be a common mechanism for enhancing optimal utilization of a limited pool of these mechanochemical enzymes (Duncan, 2002).
The results indicate a role for dynein in grk transcript localization. The fact that grk mRNA cannot be detected in late-stage oocytes 1-6 hr after Dmn induction argues that dynein could be required for both the transport and anchoring of grk message. When microtubules are depolymerized, grk mRNA forms aggregates on the oocyte nuclear lamina, suggesting that this represents a site where it is anchored. It might therefore be expected that if dynein functions exclusively in transport, inhibition of its activity would cause an increase in the perinuclear concentration of grk mRNA. Furthermore, transcripts that were already at the cortex should not have been disrupted. In oocytes assayed 1 or 6 hr after Dmn induction, grk message was absent from the nuclear periphery and the cortex, irrespective of where the nucleus was positioned. However, 12 hr after Dmn induction, grk mRNA localization to the nucleus had partially recovered. Interestingly, when the oocyte nucleus was incorrectly positioned along the A/P axis but remained cortically attached, grk transcript was also detected at the cortex. This argues that nuclear position and proximity to the cortex are primary determinants of grk localization. It is notable that grk message is insensitive to Dmn overexpresssion and the absence of kinesin in earlier-stage egg chambers, when it may be transported by a diffusion-based mechanism and is known to accumulate even in the absence of microtubules or microfilaments. Localization of bcd message at the anterior of the oocyte is also highly susceptible to Dmn misexpression. Although the results cannot distinguish between inhibition of transport or anchoring of the mRNA, other data argue that dynein is likely to be involved in both of these aspects. Resolution of this issue may require direct observation of RNA localization in live egg chambers after hsDmn induction (Duncan, 2002).
In contrast to the dramatic effect of Dmn overexpression on grk and bcd transcripts, osk mRNA distribution is altered in a more subtle fashion. The increased level of osk mRNA in the cytoplasm after Dmn overexpression is consistent with the proposal that osk transcript binds to cortex throughout the oocyte and that kinesin transports it toward the interior in the anterior and lateral regions. Accordingly, dynein may contribute to osk localization by transporting transcripts toward the cortex or maintaining them there (Duncan, 2002).
The requirement for dynein activity in positioning the oocyte nucleus at the anterior cortex could reflect a role in nuclear anchoring alone or in both nuclear migration and anchoring. Misplaced nuclei are found in stage 10 egg chambers dissected 1 hr after heat shock even though nuclear migration would have occurred 13-25 hr earlier (at stage 7/8). This clearly shows that reduction of dynein activity disrupts nuclear anchoring through a mechanism that is still unclear. One possibility is that sustained activity of perinuclear dynein (acting on microtubules oriented with minus ends toward the cortex) is required to maintain nuclear position. Alternatively, cortically localized dynein may have to be continually active to keep the nucleus 'reeled in' through a subset of microtubules that have the opposite orientation. In either case, the nucleus would be predicted to fall away from the cortex in the absence of dynein activity. With respect to nuclear migration, there is considerable evidence that dynein motors power such a process in fungi. The data do not permit a firm conclusion as to whether dynein also performs this role in the oocyte. Although severely ventralized eggs were obtained 40 hr after hsDmn expression, suggesting a failure of nuclear migration, this could also result from defects in anchoring after migration because of perdurance of excess Dmn (Duncan, 2002).
Compared to oocytes in which dynein activity has been disrupted, those lacking kinesin show a higher frequency of nuclear-positioning defects. One explanation could be that kinesin function is completely abolished in Khc null clones, whereas residual dynein activity remains after hsDmn induction. Alternatively, it is conceivable that kinesin is the primary motor involved in nuclear positioning and that dynein plays an accessory role. In either event, the similarity in nuclear localization defects is consistent with a model in which the function of the two motors is linked. Dynein- and kinesin-related motors also act cooperatively to bring about nuclear migration in S. cerevisiae. Deletion of either of the kinesin-related proteins Kip2p and Kip3p or the dynein heavy chain results in nuclear migration defects. Epistatic analysis suggests that Kip2p acts cooperatively with dynein, whereas Kip3p may affect nuclear migration through an independent pathway involving Kar9p. Similarly, in Aspergillus, where nuclear migration is primarily thought to be dynein mediated, it has recently been shown that kinesin mutations affect nuclear movement and distribution in the hyphae. It is concluded that both dynein and kinesin are required for nuclear anchoring and localization of cellular determinants during oogenesis. The subcellular localization of dynein and dynactin is perturbed in kinesin mutants, and kinesin distribution is affected by Dmn misexpression. The interdependence of the two motors suggests a model in which kinesin affects patterning by localizing and recycling dynein and thus maximizing its utilization (Duncan, 2002 and references therein).
asunder (asun) is a critical regulator of dynein localization during Drosophila spermatogenesis. Because the expression of asun is much higher in Drosophila ovaries and early embryos than in testes, this study sought to determine whether ASUN plays roles in oogenesis and/or embryogenesis. The female germline phenotypes of flies homozygous for a null allele of asun (asund93). asund93 females lay very few eggs and contain smaller ovaries with a highly disorganized arrangement of ovarioles in comparison to wild-type females. asund93 ovaries also contain a significant number of egg chambers with structural defects. A majority of the eggs laid by asund93 females are ventralized to varying degrees, from mild to severe; this ventralization phenotype may be secondary to defective localization of gurken transcripts, a dynein-regulated step, within asund93 oocytes. Dynein localization is aberrant in asund93 oocytes, indicating that ASUN is required for this process in both male and female germ cells. In addition to the loss of gurken mRNA localization, asund93 ovaries exhibit defects in other Dynein-mediated processes such as migration of nurse cell centrosomes into the oocyte during the early mitotic divisions, maintenance of the oocyte nucleus in the anterior-dorsal region of the oocyte in late-stage egg chambers, and coupling between the oocyte nucleus and centrosomes. Taken together, these data indicate that asun is a critical regulator of dynein localization and dynein-mediated processes during Drosophila oogenesis (Sitaram, 2013).
bicoid mRNA localises to the Drosophila oocyte anterior from stage 9 of oogenesis onwards to provide a local source for Bicoid protein for embryonic patterning. Live imaging at stage 9 reveals that bicoid mRNA particles undergo rapid Dynein-dependent movements near the oocyte anterior, but with no directional bias. Furthermore, bicoid mRNA localises normally in shot2A2, which abolishes the polarised microtubule organisation. FRAP and photo-conversion experiments demonstrate that the RNA is stably anchored at the anterior, independently of microtubules. Thus, bicoid mRNA is localised by random active transport and anterior anchoring. Super-resolution imaging reveals that bicoid mRNA forms 110-120nm particles with variable RNA content, but constant size. These particles appear to be well-defined structures that package the RNA for transport and anchoring (Trovisco, 2016).
At the onset of meiosis, each chromosome needs to
find its homologue and pair to ensure proper segregation. In
Drosophila, pairing occurs during the mitotic cycles preceding
meiosis. This study shows that germ cell nuclei undergo marked movements
during this developmental window. It was demonstrated that microtubules and Dynein drive nuclear rotations and
are required for centromere pairing and clustering. It was further found
that Klaroid
(SUN) and Klarsicht (KASH)
co-localize with centromeres at the nuclear envelope and are required
for proper chromosome motions and pairing. Mud (NuMA in vertebrates) was identified
as co-localizing with centromeres, Klarsicht and Klaroid. Mud is also
required to maintain the integrity of the nuclear envelope and for the
correct assembly of the synaptonemal complex. These findings reveal a
mechanism for chromosome pairing in Drosophila, and indicate
that microtubules, centrosomes and associated proteins play a crucial
role in the dynamic organization of chromosomes inside the nucleus (Christophorou, 2015).
Rotations of nuclei have been described previously in somatic cells; their function remains however unclear. In germ cells, meiotic chromosome movements are thought to be required for homologue pairing, removing chromosome entanglements, promoting maturation of recombination intermediates, or for assessing chromosome homology before synapsis, in different model organisms. In Drosophila, a high temporal correlation was found between nuclear rotations and chromosome pairing occurring mainly in 8-cell cysts. This work uncovered a second interesting correlation between the speed of nuclear rotation and the degree of centromere pairing and clustering. Indeed, mutations in klaroid affected the least nuclear rotations and disrupted the least centromere associations and synapsis. Rotations were slowed down more significantly in klarsicht, sas-4 and asl mutant germ cells. Accordingly, strong defects were observed in the initial pairing of centromeres and in synaptonemal complex formation. Finally, nuclear rotations were completely abolished in the absence of Dynein or dynamic microtubules. In dynein mutant germ cells, an average of six centromeres were distinguished during pre-meiotic pairing, which is higher than any mutants tested previously, including null alleles of c(3)G. Similarly, five centromeres on average were coundted during clustering in region 2a, a mutant phenotype that is comparable to the strongest orientation disruptor (ord) or c(3)G mutations (lateral and central elements of synaptonemal complex respectively). Nuclear rotations thus play an important role in homologue chromosome pairing and synaptonemal complex formation (Christophorou, 2015).
It was found that microtubules could be nucleated from the fusome, the nuclear envelope and the centrosome in region 1 germ cells. On the basis of these observations and centrosome mutant analysis, it is speculated that the whip-like movements of microtubules could be the main forces creating cytoplasmic flows, as observed in many biological systems and demonstrated theoretically. In addition, microtubules nucleated by the centrosomes could also push on the nucleus and the cell membrane, which could bias nuclear movement towards one direction of rotation as proposed for the migration of this same oocyte. These two forces depend on microtubules and dynein, and would act redundantly for efficient and unidirectional nuclear rotations. However, even in the absence of dynamic microtubules, centromeres ended up paired, albeit much later in region 2b. Synapsis, on the other hand, was completely disrupted. It is thus believed that, as in yeast and worms, these movements are there to facilitate pairing, synapsis or recombination, but that at least chromosome pairing could occur slowly without motions by redundant mechanisms. In flies, Spag4 is a second SUN-domain protein, but it is only expressed in male testis and is thus not likely to play a role during oogenesis. There is also a second KASH-domain protein called MSP-300/Nesprin, which interacts with the actin cytoskeleton. In the absence of microtubules, nuclei were not ‘rolling’ anymore; however, they still showed some back and forth ‘rocking’ movements. It will be interesting to investigate whether MSP300/Nesprin and the actin cytoskeleton are involved in these rocking movements (Christophorou, 2015).
This study found that although mud mutant ovaries showed only mild defects in centromere dynamics, significant genetic interactions were uncovered with klaroid and klarsicht in this same process. Striking features of Mud in this study were its co-localization with centromeres in interphasic germline cysts and the formation of polycomplexes in mud mutant cysts. The formation of polycomplexes was associated with a lack of nuclear membrane and diffused DNA in the cytoplasm, suggesting that Mud is required to maintain nuclear envelope integrity. It is proposed that the disappearance of the NE in mud cysts is the primary defect leading first to the de-localization of DNA into the cytoplasm and then the formation of polycomplexes. Polycomplexes could thus be the result of self-assembly of synaptonemal complex components polymerizing in the absence of chromatin. Polycomplexes were also observed in klaroid and klarsicht mutants although at a lower penetrance than in mud mutants. Interestingly, large distortions of the NE were also observed in muscle cell nuclei mutant for unc-84, which encodes a C. elegans SUN protein. These deformations were particularly strong in these cells, because muscle cell nuclei are subjected to mechanical stress. It is likely that rolling nuclei of 8-cell cysts are also exposed to some mechanical forces. Klarsicht, Klaroid and Mud may all participate in maintaining the integrity of the nuclear envelope in these conditions. In their absence, the NE is weakened and cannot resist mechanical forces, which also leads to synaptonemal complex assembly defects. In the most extreme cases the NE completely disappears causing the formation of polycomplexes. Interestingly, Mud initially localizes at the NE of all germline cells in region 1, but then becomes localized only to the cells remaining in meiosis in region 2a, and finally only specifically at the NE of the oocyte. This may hint that the meiotic nucleus is subjected to specific mechanical forces during oogenesis (Christophorou, 2015).
A cytoplasmic dynein motor isoform that is present in extracts of Drosophila embryos has been characterized. A prominent high
molecular weight (HMW) polypeptide (> 400 kDa) is enriched in microtubules prepared from nucleotide-depleted embryonic
extracts. Based on its ATP-sensitive microtubule binding activity, 20 S sedimentation coefficient, sensitivity to UV-vanadate and
nucleotide specificity, the HMW polypeptide resembles cytoplasmic dyneins prepared from other organisms. The Drosophila
cytoplasmic dynein acts as a minus-end motor that promotes microtubule translocation in vitro. A polyclonal antibody raised
against the dynein heavy chain polypeptide was used to localize the dynein antigen in whole-mount preparations of embryos by
immunofluorescence microscopy. These studies show that the dynein motor is associated with microtubules throughout
embryogenesis, including mitotic spindle microtubules and microtubules of the embryonic nervous system (Hays, 1994).
Kinesins and dyneins play important roles during cell division. Using RNA interference (RNAi) to deplete individual (or combinations of) motors followed by immunofluorescence and time-lapse microscopy, the mitotic functions were examined of cytoplasmic dynein and all 25 kinesins in Drosophila S2 cells. Four kinesins are involved in bipolar spindle assembly, four kinesins are involved in metaphase chromosome alignment, Dynein plays a role in the metaphase-to-anaphase transition, and one kinesin is needed for cytokinesis.
Dhc64C (cytoplasmic dynein) may control the timing of anaphase onset, possibly by transporting Rod or other checkpoint proteins away from kinetochores as proposed for the fly embryo. A similar checkpoint inactivation model was proposed for mammalian dynein/dynactin based on inhibition analyses. Functional redundancy and alternative pathways for completing mitosis were observed for many single RNAi knockdowns, and failure to complete mitosis was observed for only three kinesins. As an example, inhibition of two microtubule-depolymerizing kinesins initially produced monopolar spindles with abnormally long microtubules, but cells eventually formed bipolar spindles by an acentrosomal pole-focusing mechanism. From this phenotypic data, a model is constructed for the distinct roles of molecular motors during mitosis in a single metazoan cell type (Goshima, 2003).
Dendrites allow neurons to integrate sensory or snaptic inputs, and the spatial disposition and local density of branches within the dendritic arbor limit the number and type of inputs. Drosophila dendritic arborization (da) neurons provide a model system to study the genetic programs underlying such geometry in vivo. This study reports that mutations of motor-protein genes, including a dynein subunit gene (dlic) and kinesin heavy chain (khc), caused not only downsizing of the overall arbor, but also a marked shift of branching activity to the proximal area within the arbor. This phenotype is suppressed when dominant-negative Rab5 is expressed in the mutant neurons, which deposit early endosomes in the cell body. It was also shown that in dendritic branches of the wild-type neurons, Rab5-containing early endosomes are dynamically transported. When Rab5 function alone is abrogated, terminal branches were almost totally deleted. These results reveal an important link between microtubule motors and endosomes in dendrite morphogenesis (Satoh, 2008).
The microtubule-based molecular motor dynein is essential for proper neuronal morphogenesis. Dynein activity is regulated by cofactors whose role(s) in shaping neuronal structure are still being elucidated. Using Drosophila melanogaster, this study revealed that the loss of the dynein cofactor NudE results in abnormal dendrite arborization. The data show that NudE associates with Golgi outposts, which mediate dendrite branching, suggesting NudE normally influences dendrite patterning by regulating Golgi outpost transport. Neurons lacking NudE also have increased microtubule dynamics, reflecting a change in microtubule stability that likely also contributes to abnormal dendrite growth and branching. These defects in dendritogenesis are rescued by elevating Lis1, another dynein cofactor that interacts with NudE as part of a tripartite complex. These data further show that the NudE C-terminus is dispensable for dendrite morphogenesis and likely modulates NudE activity. It is proposed that a key function of NudE is to enhance an interaction between Lis1 and dynein that is critical for motor activity and dendrite architecture (Arthur, 2015).
Neurons typically assume multipolar, bipolar, or unipolar morphologies. Little is known about the mechanisms underlying the development of these basic morphological types. This study shows that the Kruppel-like transcription factor Dar1 determines the multipolar morphology of postmitotic neurons in Drosophila. Dar1 is specifically expressed in multipolar neurons and loss of dar1 gradually converts multipolar neurons into the bipolar or unipolar morphology without changing neuronal identity. Conversely, misexpression of Dar1 or its mammalian homolog in unipolar and bipolar neurons causes them to assume multipolar morphologies. Dar1 regulates the expression of several dynein genes and nuclear distribution protein C (nudC), which is an essential component of a specialized dynein complex that positions the nucleus in a cell. These genes were shown to be required for Dar1-induced multipolar neuron morphology. Dar1 likely functions as a terminal selector gene for the basic layout of neuron morphology by regulating both dendrite extension and the dendrite-nucleus coupling (Wang, 2015).
Ramon y Cajal placed neurons into three major morphological types based on the number of dendrites connected to the soma (i.e., primary dendrites): unipolar, bipolar, and multipolar and this classification system is universally applicable to different species throughout evolution. Multipolar neurons, like mammalian pyramidal neurons, develop more than one primary dendrite. In contrast, bipolar neurons are defined as having a single primary dendrite that may (e.g., cerebellar Purkinje cells) or may not (e.g., photoreceptors) branch out into an elaborate dendritic arbor. Finally, unipolar neurons such as DRG neurons in vertebrates and the majority of CNS neurons in invertebrates extend a single primary neurite, which usually bifurcates into dendritic and axonal branches (Wang, 2015).
Multipolar morphology separates the dendritic arbor into distinct fields around the soma, which has an impact, not only on the passive current spread and processing of electrical signals in the neuron), but also on the types of synaptic or sensory inputs that the neuron receives . In addition, the three basic morphologies of neurons are relevant to the distinct organizational principles used in both the nervous systems of different animal species and in different parts of a single nervous system. Although all three morphological types are found in different species throughout evolution, the majority of neurons in invertebrates are unipolar, whereas the majority of those in vertebrates are multipolar (Wang, 2015).
In the insect CNS, unipolar neurons extend a single process from the soma to a synapse-enriched neuropil and then bifurcate into dendrites that arborize locally and an axon that typically projects to other neuropil areas or target tissues. Unipolar organization of neuronal processes allows the formation of synaptic connections away from the location of the neuronal cell body, so it is likely an alternative strategy for neuronal migration, which is rare in the insect CNS but common in the vertebrate CNS. Despite the importance of these fundamental organizations of neuronal processes, very little progress has been made toward understanding the molecular and cellular programs that lead postmitotic neurons to develop multipolar, bipolar, or unipolar morphologies since their description a century ago (Wang, 2015).
This study shows that the transcription factor Dar1 determines the multipolar morphology of postmitotic neurons in Drosophila. Dar1 is selectively expressed in postmitotic multipolar neurons and is required for these neurons to assume the multipolar morphology. Ectopic expression in unipolar or bipolar neurons leads to multipolar morphology. Dar1 regulates the expression of several dynein genes and nudC, which is an essential component of a specialized dynein complex that positions the nucleus in a cell. It is further shown that this evolutionarily conserved complex is required for multipolar morphology of neurons. These results suggest that dar1 likely functions as a terminal selector gene for the basic layout of neuron morphology (Wang, 2015).
The universal morphological organization of neuronal dendrites and axons-in the form of the unipolar, bipolar, and multipolar morphologies-is important for information processing in neurons and for the wiring of neural circuits. It has generally been assumed that the formation of the basic morphological types of neurons is determined by the number of dendrites growing out from the cell body (the 'outgrowth model'). This study shows that this model alone is insufficient to explain the formation of multipolar morphology. Nuclear positioning is introduced as a factor in determining the multipolar neuron morphology, and it is proposed that Dar1 determines multipolar morphology by regulating both dendrite extension and primary dendrite-nucleus coupling (Wang, 2015).
A novel, instructive role is reported for Dar1 in determining the multipolar morphology of postmitotic neurons without changing cell fate. First, despite the dendritic defects, axon morphology (Ye, 2011) and targeting are unchanged in dar1 mutant neurons. Second, ectopic expression of Dar1 in postmitotic neurons leads to supernumerary primary dendrites. Third, the remaining dendrites in dar1-/- neurons still follow the branching pattern assumed by wild-type neurons (Ye, 2011). Fourth, dar1 mutations do not affect the expression of neuron type-specific markers. Based on extensive studies in C. elegans, Hobert proposed the concept of terminal selector genes (Hobert, 2008). A terminal selector gene is required for determining specific aspects of a neuron's identity by regulating the expression of genes responsible for these characteristics such as those encoding neurotransmitter receptors, enzymes in a neurotransmitter synthesis pathway, and structural proteins. Loss of a terminal selector gene results in the loss of a specific aspect of the neuron type without affecting the overall neuronal identity. Dar1 plays such a function in the basic layout of neuronal morphology and thus is likely a 'terminal selector gene' for neuronal morphology (Wang, 2015).
Based on the findings in this study, it is proposed that generating neuronal multipolar morphology requires, not only dendritic extension, but also a coupling mechanism between the nucleus and the dendrites. Dar1 promotes both dendritic growth and dendrite-nucleus coupling. Therefore, its misexpression converts unipolar neurons into neurons with multipolar morphology. The results presented in this study raise the interesting possibility that a specialized dynein complex in multipolar neurons with components that are transcriptionally regulated by Dar1 couples the nucleus with the primary dendrites. If the primary dendrite-nucleus coupling is weakened, then the nucleus may move to a different location in relation to the dendrites and axons. It is speculated that there might also be an active or passive force that pulls the nucleus toward the axon, opposing the force that couples the primary dendrites and the nucleus. Consistent with this model, the remaining single primary dendrites of all bipolar-shaped da neurons-caused by loss of dar1, reduced functions of dynein, or nuclear positioning complex-are those that project in the direction opposite the axon. This observation again rules out the possibility that the reduction in number of primary dendrites of dar1-/- da neurons is the result of reduction in dendrite growth. If that were the case, then the remaining single primary dendrites would likely project in random directions and not solely away from the axon. Further studies are needed to determine the cellular and molecular basis of the dendrite-nucleus coupling (Wang, 2015).
Several prior studies have demonstrated that neurons switch between different morphological types during development. These observations suggest that the acquisition of basic morphological types in many neurons includes intermediate morphologies with nucleus-primary dendrite relationships that are different from those seen in the mature neurons. It will be interesting to investigate whether the activity of the nuclear positioning complex and its regulators play a role in these developmental changes in morphological type (Wang, 2015).
In summary, this study offers a novel model for understanding the establishment of the three basic morphological types of neurons. Starting from genetic analysis of the KLF transcription factor Dar1, the study not only uncovers an instructive factor that determines the multipolar morphology of neurons, but also provide a mechanistic model showing that the position of the nucleus is critical for establishing multipolar neuron morphology. This study also demonstrates that the basic morphological types are determined by intrinsic molecular mechanisms in postmitotic neurons rather than in precursor cells. The model presented in this study may also be applied to explaining the changes in basic morphological type during neuron development. This study therefore opens the door for a unifying theory of basic structural organization in neurons (Wang, 2015).
Evidence is presented that synapse retraction occurs during normal synaptic growth at the Drosophila neuromuscular junction (NMJ). An RNAi-based screen to identify the molecular mechanisms that regulate synapse retraction identified Arp-1/centractin, a subunit of the dynactin complex. Arp-1 dsRNA enhances synapse retraction, and this effect is phenocopied by a mutation in P150/Glued, also a dynactin component. The Glued protein is enriched within the presynaptic nerve terminal, and presynaptic expression of a dominant-negative Glued transgene enhances retraction. Retraction is associated with a local disruption of the synaptic microtubule cytoskeleton. Electrophysiological, ultrastructural, and immunohistochemical data support a model in which presynaptic retraction precedes disassembly of the postsynaptic apparatus. These data suggests that dynactin functions locally within the presynaptic arbor to promote synapse stability (Eaton, 2002).
Evidence is presented that regulated synaptic growth at the Drosophila NMJ includes synaptic retraction events in addition to the well-characterized addition of new synaptic boutons. Synaptic retraction events (footprints) are defined as the withdrawal of presynaptic antigens (synapsin, HRP, Futsch) from clearly defined regions of postsynaptic specialization defined by Discs-large immunoreactivity. Multiple lines of evidence, using a variety of analytic tools, including light level and ultrastructural analysis, are presented that a footprint represents a site where the nerve terminal once resided and has since retracted. Previous reports have likely failed to identify synaptic retraction as an important element during synapse development in this system because postsynaptic markers were employed to study synapse development. Synapse retraction events are more frequent at early developmental stages which correlate with the more rapid phase of synapse growth. These data suggest that regulated synaptic growth is achieved by a balance of synaptic growth and retraction at the Drosophila NMJ. Such a balance of growth and retraction may represent a general principle of synaptic growth control in this and other systems (Eaton, 2002).
Employing a functional genomic strategy, the dynactin protein complex has been identified as an essential component of the machinery that achieves synapse stabilization at the Drosophila NMJ. Disruption of the dynactin complex using any of three different perturbations, including Arp-1 RNAi, the Glued1 mutation, or presynaptic overexpression of the DN Glued transgene all result in an increase in the frequency and extent of synaptic retraction events at the NMJ. Presynaptic, but not postsynaptic, overexpression of the DN Glued transgene enhances synapse retraction, phenocopying genetic and RNAi perturbation of the dynactin complex. Consistent with this observation, P150/Glued protein is present at the NMJ and enriched in the presynaptic nerve terminal (Eaton, 2002).
Several lines of evidence suggest that dynactin functions locally within the presynaptic nerve terminal to control synapse stability. Retraction events do not result in the complete elimination of a synapse, but are generally restricted to a specific branch or portion of a branch within the synaptic arbor of a single motoneuron. In addition, MN 6/7 innervates multiple muscle targets, and retraction events are often specific to only one or a few of the muscle targets of this neuron, demonstrating that retraction events can be branch specific on different muscle targets (Eaton, 2002).
An obvious concern is that inhibition of dynactin function sufficiently impairs the health of the motoneuron to cause a secondary retraction of the synapse. Ultimately, an assessment of 'health' can only be achieved by assaying a number of independent variables such as electrophysiology, ultrastructure, morphology, and cell death. Synapse retraction events are not correlated with increased cell death in the motoneurons nor are they correlated with gross changes in the motoneuron microtubule cytoskeleton. Synapse retraction events can be local, affecting only a portion of a motoneuron arbor (demonstrated by analysis at both the light and ultrastructural level). In addition, motoneuron transmitter release properties are normal at a portion of the synapses that overexpress the DN Glued transgene (the most severe manipulation used). This is consistent with the observation that only a portion (40%) of the synapses reveal a retraction event despite the fact that DN Glued is expressed pan neuronally. These data argue against DN Glued simply poisoning the cell. Finally, retraction events are observed in the wild-type animal and occur with higher frequency during early larval development. Taken together, these data suggest that retraction is associated with synapse development rather than impaired health (Eaton, 2002).
The data indicate that increased synapse retraction caused by impaired dynactin activity has a functional consequence for the synapse. Increased synaptic retraction results in fewer synaptic boutons and decreased synaptic efficacy. This is predicted if synapse retraction helps to shape the outcome of synapse development. Thus, the increased frequency of retraction events observed during early synapse development (rapid growth) may be an important aspect of synaptic growth control. It is hypothesized, therefore, that presynaptic dynactin-mediated synapse stabilization may help set the balance between growth and retraction during normal synaptic development (Eaton, 2002).
Synaptic retraction caused by disruption of the dynactin complex has been characterized at the light level, ultrastructurally and electrophysiologically. At each level of analysis, the data support the conclusion that the presynaptic nerve terminal withdraws, followed by the disassembly of the postsynaptic apparatus. At the light level, retraction of presynaptic markers precedes the elimination of postsynaptic Discs-large staining and GluR clusters. In addition, no change was observed in quantal size when DN Glued is driven presynaptically, despite often severely impaired presynaptic release. By comparison, acetylcholine receptors are removed postsynaptically prior to presynaptic elimination at the vertebrate NMJ, and a decrease in quantal size is observed. Finally, it was observed ultrastructurally that the presynaptic membrane appears to separate from the postsynaptic density with high frequency at active zones when dynactin is disrupted presynaptically. This is again consistent with presynaptic retraction preceding disassembly of the postsynaptic apparatus (Eaton, 2002).
The dynactin protein complex binds microtubules and has been shown to bind a number of proteins that localize to the plus end of microtubules including Lis1, Eb1, and Clip170. In addition to their localization, these proteins have also been shown to effect microtubule stability and dynamics. In budding yeast, live imaging studies of microtubules have demonstrated that mutations in dynactin and EB1 lead to altered microtubule structure and defects in cortical association. Based on these observations, one hypothesis is that dynactin and associated proteins participate in the process of microtubule capture at the cell cortex. Alternatively, dynactin could be involved in more complex regulation of microtubule dynamics via the localization or trafficking of microtubule regulators (Eaton, 2002).
Dynactin regulation of the microtubule cytoskeleton is thought to occur at sites of intercellular adhesion and signaling. In epithelial cells, microtubules and cytoplasmic dynein have been observed to contact adherens junctions. In addition, overexpression of the p62 component of the dynactin complex localizes strongly to sites of focal adhesion. Furthermore, dynactin has been shown to bind proteins such as β-catenin and spectrin, which are known to be involved in cell adhesion (Eaton, 2002).
In Drosophila, dynactin has been studied during axon guidance. In dynactin mutations, axons reach their target regions but fail to branch normally. It was not determined whether synapses failed to form in these studies or whether synapse stability was disrupted. These data demonstrate that dynactin is not necessary for growth cone motility. The interpretation of these data has been a failure to transport important trophic molecules in a retrograde manner to the sensory neuron cell body via the dynein retrograde motor. However, these data are equally consistent with a failure of sensory neuron synapse stability within the central nervous system (Eaton, 2002).
The data at the Drosophila NMJ are consistent with an essential role for the dynactin complex in maintaining the appropriate microtubule architecture and, as a consequence, synapse stability. The possibility cannot be ruled out that an essential retrograde signal fails to reach the motoneuron soma. However, since retraction events are observed to affect only portions of a single synapse and since no change is seen in motoneuron cell death, it is suspected that failure to traffic an essential retrograde signal is not the primary cause of synapse retraction at the Drosophila NMJ (Eaton, 2002).
An important aspect of the current data is a possible link between synapse retraction and neural diseases with which dynactin function has been associated. Mutations in the human lis1 gene cause type 1 lissencephaly, a debilitating developmental disease of the nervous system characterized by severe mental retardation and reduced cerebral folds. Interestingly, in addition to defects in neuroblast proliferation and axonal transport, Drosophila Lis1 mutants have abnormal dendritic morphology. In addition, dynactin binds Huntingtin Associated Protein 1 (Hap-1) and is thought to play a role in the trafficking of this protein. The current data are most consistent with a local role for dynactin within the nerve terminal controlling synapse stabilization. With respect to vertebrate disease states, the altered control of synapse stability has not been fully investigated. As such, this aspect of dynactin function characterized is this study may be an important component of disease progression if affected cells lose essential target-derived trophic support as a result of impaired synapse stability (Eaton, 2002).
How a nucleus is positioned within a highly polarized postmitotic animal cell is not well understood. The Dynactin complex (a regulator of the microtubule motor protein Dynein) has been shown to be required to maintain the position of the nucleus within post-mitotic Drosophila photoreceptor neurons. Multiple independent disruptions of Dynactin function cause a relocation of the photoreceptor nucleus toward the brain, and inhibiting Dynactin causes the photoreceptor to acquire a bipolar appearance with long leading and trailing processes. It has been found that while the minus-end directed motor Dynein cooperates with Dynactin in positioning the photoreceptor nucleus, the plus-end directed microtubule motor Kinesin acts antagonistically to Dynactin. These data suggest that the maintenance of photoreceptor nuclear position depends on a balance of plus-end and minus-end directed microtubule motor function (Whited, 2004).
The Dynactin complex is an assembly of 11 different subunits that functions as an activator of Dynein, serving as an adaptor for cargo and enhancing motor processivity. The Dynactin subunit Glued couples Dynactin to Dynein by binding to the Dynein intermediate chain (Dic, encoded by short wing). Overexpression of a truncated form of Glued that binds to Dic but cannot associate with the rest of the Dynactin complex acts as a powerful inhibitor of Dynein and Dynactin function. Overexpression of the Dynactin subunit Dynamitin disrupts Dynactin complex assembly and also inhibits Dynactin function. Biochemical studies have shown that the Dynactin complex also contains Capping Protein, a heterodimer composed of the To further establish that Glued1 defects reflected
disruptions in Dynactin function, two other approaches were used to disrupt the
Dynactin complex. Drosophila Dynamitin, which also inhibits Dynactin function in flies, was overexpressed in
photoreceptor neurons. Loss-of-function mutations in the
Dynactin subunit Cpb were examined by generating animals whose visual systems contained homozygous mutant clones of the cpb strong loss-of-function mutation cpbM143. In these cpbM143 mosaic
animals, the nuclear regions of many photoreceptors were observed in the optic
stalk and brain (Whited, 2004).
To confirm that the cpbM143 mutant photoreceptor defect
was due to a loss of cpb function, an additional strong
loss-of-function cpb allele, cpbF44, was isolated from an EMS mutagenesis and a chromosomal deficiency uncovering the cpb locus, Df(2L)E.2, was obtained. When
animals contained homozygous mutant clones of cpbF44 cells
or homozygous mutant clones of Df(2L)E.2, a similar movement of
photoreceptor nuclear regions toward the brain was observed. cpb/Df(2L)E.2 animals did not survive to third instar, preventing the
classic genetic demonstration that these cpb alleles behaved as
strong loss-of-function mutations. Fortunately, it was found that the
[pYES-ß] genomic transgene, which contains the CPB coding region, was
able to rescue the lethality of cpb/Df(2L)E.2 animals, but did not
rescue the previously described cpb bristle defect. This
suggested that [pYES-ß] was a partially functional rescue construct that
could be used to examine the visual systems of otherwise
cpb/Df(2L)E.2 animals. It was found that
[pYES-ß];cpbM143/Df(2L)E.2 animals display a
photoreceptor defect similar to that of other cpb mutants, consistent
with nuclear mispositioning resulting from the loss of cpb function. It was further confirmed that the defect was due to the loss of cpb function by successfully rescuing the cpbM143/Df(2L)E.2 photoreceptor defects (as well as the cpb bristle defects) by expression of a wild-type Cpb
cDNA under the control of a heterologous promoter. Staining of photoreceptor nuclei directly demonstrated the movement of photoreceptor nuclei out of the eye disc and into the optic stalk in cpb mutants (Whited, 2004).
The bifunctional nature of Cpb, which associates with filaments of actin as
well as filaments of Arp1, means that loss of Cpb also increases filamentous
actin levels (Hopmann, 2003). Nonetheless, previous studies have shown that increases in filamentous actin alone, such as those observed in hypomorphic cpb alleles or in actup mutants, do not cause photoreceptor nuclear
mispositioning. Together with the Glued1 and Dynamitin
data, the cpb observations yield a consistent picture that
alterations in Dynactin subunits cause mispositioning of photoreceptor cell
bodies and nuclei, and indicate that Dynactin, and not just the Glued subunit,
has an important role in photoreceptor development (Whited, 2004).
The mispositioning of photoreceptor nuclei in Dynactin mutants raised the
question of whether these disruptions reflect altered positioning of the
nucleus within the photoreceptor or simply migration of the entire
photoreceptor. To address this question, single photoreceptors were labeled in
wild type and in Glued1 mutants. Wild-type photoreceptors
exhibit a highly polarized morphology in which the region of the photoreceptor
containing the nucleus lies in the apical region of the eye disc and an axon
extends basally into the brain. Glued1 mutant photoreceptors whose
nuclei have entered the optic stalk had highly altered morphologies, with both
leading and trailing processes extending from the regions of the cell where
the misplaced nucleus was located. Leading and trailing processes of misplaced
Glued1 photoreceptors were quantified, considering only those with no other labeled cells or processes nearby. Of these 13 neurons, 12 had clearly
detectable leading and trailing processes. The leading process (axon) extended
into the target region and the trailing process extended back into the eye
disc. These data demonstrate that inhibition of Dynactin function dramatically
alters the position of the nucleus within the photoreceptor (Whited, 2004).
The Dynactin complex also controls the pattern of mitoses within the
Drosophila retina. To determine whether nuclear mispositioning is a secondary
consequence of the earlier mitotic requirement for Dynactin, the
effects of specifically inhibiting the Dynactin complex in postmitotic
photoreceptors was examined. Conditional inhibition of Dynactin function can be achieved through inducible expression of a truncated, dominant-negative form Glued
(GluedDN) that resembles the protein product of
Glued1. GluedDN was expressed under the control of the
postmitotic photoreceptor-specific Glass 38-1 promoter, which initiates
expression in the photoreceptors only after their axons have entered the brain. Expression of GluedDN under the control of Glass 38-1 caused photoreceptor nuclei to move into the optic stalk. Overexpression of Dynamitin under the control of Glass 38-1 caused similar photoreceptor nuclear positioning defects. These data demonstrate that Dynactin is required postmitotically in photoreceptors to maintain nuclear position and that the disruptions in nuclear positioning observed are not simply a secondary consequence of mitotic defects (Whited, 2004).
The displacement of photoreceptor nuclei from apical regions of the eye
disc toward more basal regions could reflect an overall disruption in
apical/basal polarity of the eye disc. The apical/basal polarity of developing
photoreceptors was assessed by examining the distribution of the
Drosophila ß-catenin Armadillo and the PDZ-domain-containing
protein PATJ. Armadillo localizes to the zonula adherens separating the
apical and basolateral membrane domains of developing photoreceptors, while
PATJ localizes to the apical membrane domain. In
wild-type eye discs, Armadillo is concentrated just beneath the apical tips of
the developing photoreceptors. In Glued1 animals Armadillo was still present near apical regions of the eye disc, even in areas completely devoid
of apical photoreceptor nuclei. Thus, this marker of apical/basal polarity was retained even when photoreceptor nuclei moved basally. Similar results were obtained when Glued1 mutants were visualized in cross-section using both
Armadillo and PATJ. Apical localization of PATJ and Armadillo were observed in
Glued1 and the relative apical/basal ordering of these
markers was maintained. These data suggest that the alterations in photoreceptor
morphology are not caused by a loss of apical/basal polarity within the
developing photoreceptors (Whited, 2004).
Dynactin has important functions in the organization of the microtubule
cytoskeleton in many systems. The microtubule cytoskeleton of developing
photoreceptors is highly polarized, with microtubule minus ends concentrated
apical to the nucleus as detected using antisera recognizing gamma-tubulin. A similar apical focus is observed when using the fusion protein Nod:LacZ, which often
co-localizes with microtubule minus ends. The
relatively ubiquitous expression of gamma-tubulin in the retina complicated
the analysis of gamma-tubulin localization when retinal patterning was
disrupted. Therefore, the effect of Glued on factors associated with the microtubule cytoskeleton was examined by expressing Nod:LacZ specifically in postmitotic photoreceptors. In animals expressing GluedDN in postmitotic photoreceptors as well as in Glued1 mutants, Nod:LacZ was no longer exclusively concentrated in apical regions of photoreceptors, but rather spread into the photoreceptor axons. Thus, while the overall apical/basal polarity of the photoreceptors was not disrupted in Glued mutants, the spatial organization of the microtubule cytoskeleton-associated factor Nod:LacZ was affected (Whited, 2004).
Dynactin activates the microtubule motor Dynein, and strong
loss-of-function mutations in dynein intermediate chain
(dic) are dominant enhancers of the rough eye phenotype of
Glued1 mutants. Since Dynein and Dynactin may play multiple roles
together during eye development, the effect of a reduction in
dic gene dosage upon photoreceptor nuclear positioning was examined in
Glued1 animals. A twofold reduction in dic gene
dosage caused a further decrease in the number of photoreceptor nuclei in
apical regions of Glued1 mutant eye discs. This did not
reflect a simple reduction in the number of photoreceptors generated; large
numbers of photoreceptor nuclei were crowded at the base of the eye disc and
entered the optic stalk in both animals. Thus, a larger
fraction of photoreceptor nuclei left apical positions when the level of
dic gene activity was reduced, consistent with Dynein and Dynactin
acting together in this process (Whited, 2004).
To identify additional factors that interact with Dynactin to control nuclear positioning, a genetic screen was performed to identify genes that dominantly enhanced or suppressed the Glued1 external eye phenotype. From a collection of approximately 1800 stocks containing transposon-induced lethal mutations, several stocks were identified that had no dominant effect on eye development in a wild-type background, but were dominant enhancers or suppressors of Glued1. Two dominant suppressors of Glued1, khck13219 and khck13314, were alleles of kinesin heavy chain (khc), which encodes a subunit of the plus-end directed microtubule motor kinesin. The interaction with Glued1 was further confirmed using the null allele khc8. Examination of developing eye discs demonstrated that a twofold reduction of khc gene dosage greatly increased the number of photoreceptor nuclei present in apical regions of Glued1 mutant eye discs. This suggested that khc acts antagonistically to Glued in photoreceptor nuclear positioning (Whited, 2004).
To determine whether khc mutations interacted with
Glued1 in postmitotic photoreceptors, khc gene
dosage was reduced in animals expressing dominant-negative Glued under the
control of the postmitotic Glass38-1 promoter. Wild-type animals
(n >50 hemispheres) or animals containing the dominant-negative
Glued transgene without the Glass 38-1 promoter never contained photoreceptor nuclei within their optic stalks. Glass38-1:GluedDN animals contained an average of 11±1 photoreceptor nuclei within the optic stalk. However, Glass38-1:GluedDN
animals heterozygous for either khck13314 or
khc8 showed a significant reduction in the number of
photoreceptor nuclei in the optic stalk. Thus, a twofold reduction in
khc gene dosage suppressed the effects of postmitotic expression of
dominant-negative Glued, consistent with Glued and khc
acting antagonistically within differentiated photoreceptors to regulate
nuclear positioning (Whited, 2004).
The interaction between Glued and khc in
other photoreceptors was studied by examining the Bolwig organ, a cluster of 12
photosensitive neurons that differentiate during embryonic development and
extend axons into the brain. By second and third instar larval stages, Bolwig
photoreceptor nuclei are located near the anterior tip of the larva and their
axons extend over the eye/antennal disc into the brain, a distance of >200
µm. In wild-type second instar animals, photoreceptor neuron
differentiation has not yet begun in the eye disc and no neuronal nuclei are
present there.
However, when GluedDN was expressed in postmitotic Bolwig
photoreceptors, their nuclei appeared on the surface of the eye/antennal disc. Thus, as in the photoreceptors of the adult eye, expression of GluedDN in Bolwig photoreceptors caused their nuclei to be positioned closer to their axon
termini; in many cases, the Bolwig nuclei were over 150 µm closer than
normal to their axon terminals in the brain (Whited, 2004).
The interaction between Glued and khc in Bolwig
photoreceptors was assessed by counting the number of Bolwig nuclei on the
surface of the eye/antennal disc. While wild-type and UAS:GluedDN animals had no neuronal nuclei in this region, Glass38-1:GluedDN animals contained 7±1. A reduction of khc gene dosage in Glass38-1:GluedDN; khck13314/+ and Glass38-1:GluedDN; khc8/+ animals significantly reduced this to 4±1
and 3±1, respectively. These data further support the functional antagonism of Glued and khc in photoreceptor nuclear positioning (Whited, 2004).
Nuclear translocation, driven by the motility apparatus consisting of the cytoplasmic dynein motor and microtubules, is essential for cell migration during embryonic development. Bicaudal-D (Bic-D), an evolutionarily conserved dynein-interacting protein, is required for developmental control of nuclear migration in Drosophila. Nothing is known about the signaling events that coordinate the function of Bic-D and dynein during development. This study shows that Misshapen (Msn), the fly homolog of the vertebrate Nck-interacting kinase is a component of a novel signaling pathway that regulates photoreceptor (R-cell) nuclear migration in the developing Drosophila compound eye. Msn, like Bic-D, is required for the apical migration of differentiating R-cell precursor nuclei. msn displays strong genetic interaction with Bic-D. Biochemical studies demonstrate that Msn increases the phosphorylation of Bic-D, which appears to be necessary for the apical accumulation of both Bic-D and dynein in developing R-cell precursor cells. It is proposed that Msn functions together with Bic-D to regulate the apical localization of dynein in generating directed nuclear migration within differentiating R-cell precursor cells (Houalla, 2005).
Intracellular mRNA localization is a conserved mechanism for spatially regulating protein production in polarized cells, such as neurons. The mRNA encoding the translational repressor Nanos (Nos) forms ribonucleoprotein (RNP) particles that are dendritically localized in Drosophila larval class IV dendritic arborization (da) neurons. In nos mutants, class IV da neurons exhibit reduced dendritic branching complexity. This study investigated the mechanism of dendritic nos mRNA localization by analyzing requirements for nos RNP particle motility in class IV da neuron dendrites. Dynein motor machinery components were shown to mediate transport of nos mRNA in proximal dendrites. Two factors, the RNA-binding protein Rumpelstiltskin and the germ plasm protein Oskar function in da neurons for formation and transport of nos RNP particles. nos was shown to regulate neuronal function, most likely independently of its dendritic localization and function in morphogenesis. These results reveal adaptability of localization factors for regulation of a target transcript in different cellular contexts (Xu, 2013).
This study has combined a method that allows live imaging of mRNA in intact Drosophila larvae with genetic analysis to investigate the mechanism underlying transport of nos mRNA in class IV da neurons. Live imaging over the short time periods allowed has provided a snapshot into the steady-state behavior of nos*RFP particles in the proximal dendrites of mature da neurons. The results indicate that anterograde transport of nos RNP particles into and within da neuron dendrites is mediated by dynein and is consistent with the minus-end out model for microtubule polarity in the proximal dendrites of da neurons (Zheng, 2008). This model predicts that bidirectional trafficking would be mediated by opposite polarity motors and the predominance of retrograde movement of nos*RFP particles when dynein function is partially compromised is consistent with this. Moreover, Rab-5 endosomes, whose accumulation in class IV da neuron dendrites is dynein-dependent, also exhibit bidirectional movement (Satoh, 2008), suggesting that different cargos may use similar dendritic transport strategies. Unfortunately, the severe defects caused by loss of kinesin have thus far hampered confirmation of a role for kinesin in these events (Xu, 2013).
The observed bidirectional movement of nos RNP particles resembles the constant bidirectional transport observed for dendritic mRNAs near synapses in hippocampal neurons. In contrast to da neurons, proximal dendrites of mammalian neurons have mixed microtubule polarity so that bidirectional trafficking could be mediated by a single motor that switches microtubules or by switching between the activities of plus-end and minus-end motors. The association of kinesin with neuronal RNP granule components and inhibition of CaMKIIα RNA transport by dominant-negative inhibition of kinesin has implicated kinesin as the primary motor for dendritic mRNA transport. However, a recent study showed that dynein mediates unidirectional transport of vesicle cargoes into dendrites of cultured hippocampal neurons as well as bidirectional transport within the dendrites. Whether dynein plays a role in RNP particle transport in mammalian dendrites as it does in Drosophila neurons remains to be determined (Xu, 2013).
Despite its prevalence, the role of bidirectional motility is not yet clear. A recently proposed 'sushi belt' model suggests that neuronal RNP particles traffic back and forth along the dendrite until they are recruited by an active synapse and disassembled for translation (Doyle, 2011). Although da neuron dendrites do not receive synaptic input, this continual motility may provide a reservoir of nos mRNA that can be rapidly mobilized for translation locally in response to external signals that regulate dendrite branching (Xu, 2013).
These studies have shown that nos mRNA can be adapted for different localization mechanisms depending on cellular context: diffusion/entrapment in late oocytes that lack a requisite polarized microtubule cytoskeleton and microtubule-based transport during germ cell formation in the embryo and in class IV da neurons. Surprisingly, Rump and Osk are specifically required for nos localization in both oocytes and da neurons, suggesting that they function in the assembly or recognition of a fundamental nos RNP that can be adapted to both means of localization. However, because it is not possible to distinguish individual particles within the cell body, the possibility cannot be ruled out that Rump and/or Osk mediate coupling of nos RNP particles to dynein motors rather than particle formation. Within the germ plasm, nos associates with Vasa (Vas), a DEAD-box helicase, and is transported together with Vas into germ cells. Although dendritic branching complexity is reduced in vas mutants, no effect on dendritic localization of nos RNP particles was detected, suggesting that only a subset of germ plasm components are shared by neuronal localization machinery. A role for osk in learning and memory was proposed based on the isolation of an enhancer trap insertion upstream of osk in a screen for mutants with defective long-term memory, but osk function in memory formation has not been directly tested. Notably, however, a recent study showed that the osk ortholog in the cricket Gryllus bimaculatus functions in development of the embryonic nervous system rather than in germ cell formation. Thus, the ancestral function of osk appears to be in neural development, whereas its role in germ plasm formation is a later adaptation in higher insects. The results showing that Osk protein function is not limited to Dipteran germ plasm organization but also plays an important role in neuronal development and function supports this idea (Xu, 2013).
The data indicate that the Nos/Pum complex is not only required for da neuron morphogenesis, but also for nociceptive function. However, nociception does not appear to require local function of Nos/Pum in the dendrites and reduced dendritic branching does not necessarily correlate with a deficit in nociception. These results suggest that morphogenesis and function are regulated separately and that Nos/Pum plays a second role in regulating the somatic translation of proteins required for the nociceptive response. Systematic identification of Nos/Pum targets will be essential to further investigate these different roles (Xu, 2013).
In order for eukaryotic cells to function properly, they must establish polarity. The Drosophila oocyte uses mRNA localization to establish polarity and hence provides a genetically tractable model in which to study this process. The spatial restriction of oskar mRNA and its subsequent protein product is necessary for embryonic patterning. The localization of oskar mRNA requires microtubules and microtubule-based motor proteins. Null mutants in Kinesin heavy chain (Khc), the motor subunit of the plus end-directed Kinesin-1, result in oskar mRNA delocalization. Although the majority of oskar particles are non-motile in khc nulls, a small fraction of particles display active motility. Thus, a motor other than Kinesin-1 could conceivably also participate in oskar mRNA localization. This study shows that Dynein heavy chain (Dhc), the motor subunit of the minus end-directed Dynein complex, extensively co-localizes with Khc and oskar mRNA. In addition, immunoprecipitation of the Dynein complex specifically co-precipitated oskar mRNA and Khc. Lastly, germline-specific depletion of Dhc resulted in oskar mRNA and Khc delocalization. These results therefore suggest that efficient posterior localization of oskar mRNA requires the concerted activities of both Dynein and Kinesin-1 (Sanghavi, 2013).
Cell polarity is critical for epithelial structure and function. Adherens
junctions (AJs) often direct this polarity, but it has been found that Bazooka
(Baz) acts upstream of AJs as epithelial polarity is first established in
Drosophila. This prompted an investigation into how Baz is positioned and how downstream polarity is elaborated. Surprisingly, it was found that Baz localizes to an apical domain below (basally to) its typical binding partners atypical protein kinase C (aPKC) and
partitioning defective (PAR)-6 as the Drosophila epithelium first forms. In
fact, Baz positioning is independent of aPKC and PAR-6 relying instead on
cytoskeletal cues, including an apical scaffold and dynein-mediated
basal-to-apical transport. AJ assembly is closely coupled to Baz positioning,
whereas aPKC and PAR-6 are positioned separately. This forms a stratified apical
domain with Baz and AJs localizing basally to aPKC and PAR-6, and
specific mechanisms were identified that keep these proteins apart. These results reveal key
steps in the assembly of the apical domain in Drosophila (Harris, 2005).
These results frame a model of apical domain assembly during epithelial
polarity establishment in Drosophila. During
cellularization, Baz acts as a primary polarity landmark that positions AJs and
aPKC. Baz, itself, is positioned by two cues (an apical scaffold and
dynein-mediated transport). Baz recruits and colocalizes with AJ proteins in a
subapical region while helping direct aPKC to the extreme apical region.
During gastrulation, a third cue becomes important for
Baz and AJ positioning. At this stage, aPKC becomes required for maintaining Baz
and AJs. PAR-6 is also recruited to the extreme apical region and maintains Baz
and AJs. Although Baz can interact
with aPKC and PAR-6 at this stage, Crb blocks these interactions. It is proposed
that this interaction network establishes a
robust, stratified apical domain from the earliest stages of epithelial
development (Harris, 2005).
AJs are often key polarity landmarks.
However, Baz positioning is AJ independent at the time that epithelial polarity is first
established in Drosophila.
Here, Baz appears to act as a primary polarity landmark, but what cues position
Baz?
The data indicate that Baz is initially positioned by cytoskeletal cues that
support an apical Baz-binding scaffold and mediate basal-to-apical Baz
transport. The apical scaffold is saturable. Its function requires actin; Baz
becomes basally mislocalized after actin disruption. However, since Baz
overlaps only the basal reaches of the apical actin network, it is unlikely that Baz
simply binds actin. Interestingly, Baz remains largely membrane associated when
actin is disrupted. One caveat is that there is some residual actin. However,
the same treatment dissociates APC2 from the cortex.
Actin is also required for PAR-3
cortical association in C. elegans one-cell embryos. During Drosophila
cellularization, it is speculated that Baz may have other cortical anchors and that actin
may control their distribution -- of course, actin is critical for many cellular
processes and could play other roles in positioning Baz. It will be important to
identify the apical scaffold for Baz (Harris, 2005).
Baz positioning also requires the minus-end-directed MT motor dynein.
Live imaging of BazGFP revealed basal-to-apical translocation of BazGFP puncta
during cellularization. Baz-GFP that diffuses to ectopic basal positions appears
to engage a preexisting, dynein-based, basal-to-apical transport system. Such a
system transports Golgi vesicles apically during
cellularization. Baz-dynein
associations appear to cease once dynein brings Baz to the apical region, where
Baz presumably docks with its apical scaffold. Although BazGFP puncta move
slower than in vitro dynein velocity measurements,
dynein-mediated lipid droplet movements have
similar speeds during Drosophila cellularization.
In vivo, BazGFP puncta may be slowed because they form large
cortical complexes. Indeed, DE-Cad, aPKC, and PAR-6 associate with these puncta
and Baz oligomerization may promote complex assembly.
Further supporting a role for dynein, endogenous Baz is
positioned near MT minus ends in WT embryos, but mislocalizes basally in
dhc64Cm/z mutants. dhc64C mutations also enhance the
baz mutant embryonic phenotype. This is the first report
of dynein positioning Baz or its homologues (Harris, 2005).
Analysis of dynein mutants also revealed a third mechanism that can
reposition Baz apically during gastrulation. Perhaps the apical Baz-binding
scaffold is strengthened during this stage. Alternatively, a distinct polarizing
mechanism may be activated, or aPKC and PAR-6 may be
involved. Having three Baz positioning mechanisms may ensure proper Baz
localization for regulating downstream polarity (Harris, 2005).
Baz acts upstream of AJs as epithelial polarity is first established in
Drosophila. The following model is proposed
in which AJ assembly may be coupled to Baz positioning. During
cellularization, AJ proteins accumulate in both apical and basal junctions.
Basal junctions form transiently near the base of each invaginating furrow. Baz
is not required for basal junctions, but is required for recruiting AJ proteins
into apical junctions. Apical Baz
may provide a landmark for apical AJ assembly (Harris, 2005).
The data also suggest that Baz may be involved in ferrying DE-Cad to the apical
domain via dynein-mediated transport. Dynein is required for correct apical
positioning of both Baz and DE-Cad, and their colocalization in ectopic basal
complexes in dhc64Cm/z mutants suggests they may normally be
transported to the apical domain together. Indeed, Baz can form complexes with
DE-Cad and Arm. Although most endogenous Baz is apical during WT cellularization, its
basal mislocalization in dhc64Cm/z mutants suggests that some
Baz may normally move basally. In fact, excess BazGFP displaced from the apical
domain preferentially accumulates at basal junctions. It is hypothesized that some
Baz may normally interact transiently with basal junctions. From there, it may
help ferry AJ proteins apically via dynein-mediated transport. MT motors have
been implicated in AJ assembly. For example, dynein interacts with
ß-catenin and may tether MTs to AJs assembling between PtK2 cells.
Kinesin transports AJ proteins to nascent
AJs in cell culture, and the mitotic kinesin-like protein 1 is
required for apical targeting of AJs and other cues in C. elegans
epithelia. It will be important
to see if these targeting mechanisms have commonalities with AJ positioning in
Drosophila, and if Baz homologues are involved (Harris, 2005).
Finally, it is hypothesized that the third Baz-AJ positioning mechanism
revealed in dhc64Cm/z mutants might be related to the normal
maturation/stabilization of AJs at gastrulation. At this stage, precursory spot
AJs fuse into continuous belt junctions around the top of each cell.
In mammalian cell culture, aPKC is
required for such AJ maturation.
Similarly, aPKC is required for proper AJ and Baz positioning
during Drosophila gastrulation, as has been shown for PAR-6.
Considering aPKC and PAR-6 are
positioned apically as dhc64Cm/z mutants gastrulate, they
might recruit Baz and AJs apically in this context as well (Harris, 2005).
Based on their shared roles in polarity in C. elegans, characterized
physical interactions, and colocalization in mammalian cells, Baz, aPKC, and
PAR-6 are thought to function, at least in some cases, as an obligate tripartite
complex. The data suggest that the bulk of cortical Baz and aPKC/PAR-6 do not form
obligate complexes during epithelial development in Drosophila. Instead,
aPKC and PAR-6 localize to an apical region above Baz and AJs, and are
positioned there by distinct mechanisms. Baz/PAR-3 also segregates from aPKC and
PAR-6 in other cell types. In C. elegans one-cell embryos, PAR-3, aPKC,
and PAR-6 each localize in clusters on the anterior cortex, but these different
clusters have limited colocalization (60%-85% fail to colocalize.
aPKC and PAR-6 colocalize without PAR-3 at the leading edge of
migrating mammalian astrocytes.
In Drosophila photoreceptors, Baz colocalizes with AJs below
aPKC, PAR-6, and Crb. Even in
polarized MDCK cells, aPKC and PAR-6 show some segregation above PAR-3, and
although they mainly colocalize at tight junctions,
mammalian PAR-3 can regulate tight junction assembly
independently of aPKC and PAR-6.
Thus, in many contexts interactions between Baz/PAR-3, aPKC, and PAR-6 are
dynamic and/or regulated (Harris, 2005).
Baz (PAR-3), aPKC, and PAR-6 often recruit each other to the cortex, but the
assembly pathways vary. In C. elegans, one-cell embryos, PAR-3, aPKC, and
PAR-6 are mutually dependent for their cortical recruitment.
However, in Drosophila neuroblasts, Baz
can be positioned without aPKC and PAR-6.
Similarly, apical Baz is positioned without aPKC and PAR-6 during
Drosophila cellularization. In contrast, apical aPKC recruitment requires
Baz, whereas PAR-6 is largely nonpolarized at this stage. Given the lack of
extensive colocalization of Baz and aPKC in WT embryos, Baz may control aPKC
positioning indirectly, perhaps regulating binding to a separate apical
scaffold. Alternately, cortical recruitment might involve cytoplasmic
Baz-aPKC complexes. Apical PAR-6 accumulates at gastrulation, and this
appears partially Baz independent. Indeed, cdc42 recruits PAR-6 at this stage,
and at the same time aPKC and PAR-6 become required for maintaining apical Baz.
Thus, although Baz is first positioned
independently of aPKC and PAR-6, these cues soon develop complex
interdependencies (Harris, 2005).
Although Baz can directly bind both aPKC and PAR-6, at least two
mechanisms keep them apart. During cellularization, Baz colocalizes with aPKC
and PAR-6 when overexpressed, but normally it localizes with AJs below aPKC and
PAR-6. This normal segregation may thus involve competition with other binding
partners. After cellularization, Crb also becomes important for segregating Baz
and AJs from aPKC and PAR-6. These segregation mechanisms help form a stratified
apical domain from the earliest stages of epithelial development (Harris, 2005).
A stratified apical domain may strengthen the boundary between the apical and
basolateral domains. This boundary forms via reciprocal antagonism between
polarity cues. For example, aPKC phosphorylates and excludes Lethal giant larvae
(Lgl) from the apical domain in Drosophila epithelia and Lgl appears to
repel PAR-6 from the basolateral domain.
The Crb and Dlg complexes also have mutual antagonism. It is proposed
that the subapical Baz-AJ region may insulate the
apical and basolateral domains. For example, it may inhibit active aPKC from
moving basally. Indeed, PAR-3 binding can block mammalian aPKC kinase activity.
The Baz-AJ subapical region could
also block basolateral cues, since AJs are required to segregate Dlg. In this way, the Baz-AJ
subapical region could help define a distinct apical-basolateral boundary (Harris, 2005).
To conclude, Baz appears to be a primary epithelial polarity landmark in
Drosophila. It is positioned by multiple mechanisms, including an apical
scaffold and dynein-mediated transport, and organizes a stratified apical
domain, in which it colocalizes with AJs below its typical partners aPKC and
PAR-6 (Harris, 2005).
Tissue morphogenesis requires assembling and disassembling individual cell-cell contacts without losing epithelial integrity. This requires dynamic control of adherens junction (AJ) positioning around the apical domain, but the mechanisms involved are unclear. Atypical Protein Kinase C (aPKC) is required for symmetric AJ positioning during Drosophila embryogenesis. aPKC is dispensable for initial apical AJ recruitment, but without aPKC, AJs form atypical planar-polarized puncta at gastrulation. Preceding this, microtubules fail to dissociate from centrosomes, and at gastrulation abnormally persistent centrosomal microtubule asters cluster AJs into the puncta. Dynein enrichment at the puncta suggests it may draw AJs and microtubules together and microtubule disruption disperses the puncta. Through cytoskeletal disruption in wild-type embryos, a balance of microtubule and actin interactions was found to control AJ symmetry versus planar polarity during normal gastrulation. aPKC apparently regulates this balance. Without aPKC, abnormally strong microtubule interactions break AJ symmetry and epithelial structure is lost (Harris, 2007).
This study reveals the roles of aPKC during polarity establishment and elaboration in Drosophila embryos. In contrast to C. elegans, aPKC is not critical during initial polarity establishment; Baz and AJs are initially localized correctly and the embryonic epithelium can undergo initial morphogenesis. However, aPKC plays an early and striking role in maintaining the symmetrical organization of AJs, via effects on MT organization, and also plays an important later role in the elaboration of polarity (Harris, 2007).
aPKC's later role in polarity elaboration may reflect effects on multiple targets. aPKC is critical for the cortical localization of its normal binding partner PAR-6 and the apical determinant Crb. This latter effect is consistent with the fact that aPKC can phosphorylate Crb, and disruption of aPKC phosphorylation sites in Crb destabilizes Crb in the apical domain. Since Crb stabilizes AJs after gastrulation, this likely contributes to the eventual AJ breakdown in apkcm/z mutants. Crb may act in concert with PAR-6 or in parallel. aPKC can also phosphorylate and exclude the basolateral cue Lgl from the apical domain, and consequenct failure to exclude Dlg from the apical domain in apkcm/z mutants. Thus, apical invasion of basolateral cues may also contribute to the eventual loss of epithelial polarity in apkcm/z mutants (Harris, 2007).
However, it is unlikely that these global changes in apical-basal cell polarity are responsible for the early clustering of AJs into planar-polarized puncta in apkcm/z mutants. Indeed, most of these other polarity players affect polarity after gastrulation. crb mutants have normal spot junctions during gastrulation and early germband extension. Lgl and Dlg are not normally excluded from the apical domain until after gastrulation. Similarly, while mammalian aPKC can restrict PAR-1 to the basolateral domain of epithelial cells, Drosophila PAR-1 is not normally excluded from the apical domain at gastrulation. Thus, effects on Crb, Lgl/Dlg, and PAR-1 cannot easily account for the focusing of AJs and Baz into discrete planar-polarized puncta as apkcm/z mutants gastrulate (Harris, 2007).
Instead, the data suggest that aPKC regulates AJ symmetry by regulating MTs. MT regulation may be a common aPKC function. For example, Drosophila aPKC promotes MT stability at synaptic boutons of neuromuscular junctions, where aPKC forms a complex with Futsch (a MAP1B-like protein) and tubulin, recruiting Futsch to boutons to stabilize MTs. aPKC also regulates MT orientation as mammalian astrocytes and fibroblasts undergo directed migration during wound healing, while in MDCK cells, aPKC helps organize the MT cytoskeleton during ciliogenesis (Harris, 2007).
MT organization and reorganization play important roles in epithelial morphogenesis, and the data demonstrate that loss of aPKC disrupts these events. During Drosophila cellularization, strong MT nucleation from apical centrosomes is likely necessary for assembling lateral MTs that support the apical transport of lipids and proteins to form cell membranes. These MTs also help direct the initial apical positioning of AJs and Baz. During later development, the analysis of apkcm/z mutants indicates that centrosomal MTs can affect the symmetric positioning of AJs around the apical domain. Without aPKC activity, the centrosomes become abnormally dominant, bipolar cues, directing AJ clustering and thus disrupting AJ symmetry. Although this abnormal MT organization differs from changes to MT organization observed in AJ mutants, the possibility cannot be ruled out that there is feedback between MTs and AJs during epithelial morphogenesis and that aPKC may regulate these interactions. Indeed, such feedback is very likely and it will be critical to define MT-AJ cross talk mechanisms in future studies (Harris, 2007).
In apkcm/z mutants, MT-associated AJ/Baz puncta assemble at the dorsal and ventral sides of the cells. This suggests that MTs may normally function in AJ assembly at these newly formed cell-cell contacts. However, these polarized AJ assembly events must also be counterbalanced to maintain AJ symmetry and proper epithelial structure. Cytoskeletal inhibitor studies suggest that AJ symmetry may normally be regulated by a balance of MT-AJ and actin-AJ interactions at this stage -- actin appears to counteract MT-based AJ assembly at dorsal and ventral cell contacts. Actin as been shown to be enriched at anterior and posterior cell contacts, suggesting that it may be an early planar polarity cue at this stage. Perhaps this planar-polarized actin stabilizes a pool of AJs at anterior and posterior cell contacts, thereby counterbalancing MT-based AJ assembly at dorsal and ventral contacts. Alternatively, lower levels of actin at dorsal and ventral cell contacts could directly counteract MT-based AJ assembly at these sites. Distinguishing these possibilities requires further study. Nonetheless, the apkcm/z mutant phenotype appears to arise from a gain-of-function effect in which MTs become overactive and the proper balance between MT-AJ and actin-AJ interactions is lost. As a result, there is a break in AJ symmetry in apkcm/z mutants, MT-associated AJ puncta eventually become randomly positioned, and the epithelium dissociates (Harris, 2007).
The data suggest a speculative mechanistic model by which aPKC could normally regulate MT-AJ interactions. This study shows that MT association is responsible for the abnormal AJ asymmetry seen in apkcm/z mutants, and that Dynein accumulates at these abnormal AJ/Baz puncta. Since Dynein plays a role in apical transport of AJ/Baz proteins during cellularization, it is proposed that aPKC may normally regulate release of AJ/Baz complexes from Dynein, allowing a complete transport cycle. In the absence of this release, AJ/Baz complexes could maintain an abnormal association with MTs, and localized Dynein activity may pull the centrosomes and spot junctions together into the abnormal puncta seen in apkcm/z mutants. This abnormal cortical Dynein activity might also stabilize MTs emanating from the centrosomes, resulting in the persistent centrosomal MTs in the mutants. Alternatively, aPKC may function at the centrosomes to decrease MT nucleation or increase MT severing. Future experiments will illuminate these mechanisms and the generality of aPKC's role in controlling MT organization and AJ positioning (Harris, 2007).
Neurotransmission requires anterograde axonal transport of dense core vesicles (DCVs) containing neuropeptides and active zone components from the soma to nerve terminals. However, it is puzzling how one-way traffic could uniformly supply sequential release sites called en passant boutons. In this study Drosophila neuropeptide-containing DCVs are tracked in vivo for minutes with a new method called simultaneous photobleaching and imaging (SPAIM). Surprisingly, anterograde DCVs typically bypass proximal boutons to accumulate initially in the most distal bouton. Then, excess distal DCVs undergo dynactin-dependent retrograde transport back through proximal boutons into the axon. Just before re-entering the soma, DCVs again reverse for another round of anterograde axonal transport. While circulating over long distances, both anterograde and retrograde DCVs are captured sporadically in en passant boutons. Therefore, vesicle circulation, which includes long-range retrograde transport and inefficient bidirectional capture, overcomes the limitations of one-way anterograde transport to uniformly supply release sites with DCVs (Wong, 2012).
To determine how the uniform neuropeptide stores in Drosophila
motoneuron type Ib boutons are supplied,
the Geneswitch (GS) system was used to
induce expression of Emerald GFP-tagged Atrial Natriuretic
Factor (Anf-GFP), a reporter of native neuropeptide packaging
and release in Drosophila larvae. Independent
of Anf-GFP labeling, boutons were detected with a TRITC-conjugated
anti-horseradish peroxidase antibody (TRITC-HRP)
and numbered from distal to proximal. Surprisingly,
neuropeptide accumulated initially in the most distal bouton
(#1) and only later was distributed uniformly among en passant
boutons #1-4. Similar results were obtained with heat shock induction of GFP-tagged Drosophila proinsulin-like peptide 2 (Dilp2-GFP). Therefore, the initial distal accumulation cannot be attributed to a particular neuropeptide or induction mechanism. Finally, when neuropeptide release was inhibited by removing extracellular Ca2+, fluorescence recovery after photobleaching (FRAP) of constitutively
expressed Anf-GFP (i.e., driven by elav-GAL4) also revealed
preferential neuropeptide accumulation in the most distal
bouton. This finding independently verifies the polarized neuropeptide accumulation detected with pulse labeling and furthermore proves that this pattern cannot be explained by differential release by en passant boutons (Wong, 2012).
One-way anterograde transport of DCVs was believed to fully
explain neuropeptide delivery to nerve terminals. However, SPAIM in different neuronal compartments (i.e., en passant boutons, axonal branches, the proximal axon, and the soma)
showed that neuropeptide accumulation in nerve terminal
release sites is mediated by sporadic anterograde and retrograde
capture of DCVs circulating between the proximal axon
and distal boutons. These findings cannot be attributed to phototoxicity as they are supported by SPAIM-independent induction and mutant experiments, the match between flux measured without photobleaching, the consistency of DCV behavior after multiple bouts of photobleaching, and the detection of release from single DCVs, which
demonstrates viability and function (Wong, 2012).
It was surprising that DCVs travel such far distances and that
their presynaptic capture is inefficient, but this elegantly overcomes
the limitations of relying solely on one-way transport to produce the uniform neuropeptide accumulation that characterizes en passant boutons. Indeed, the
conundrum of how to ensure that all potential release sites can
function effectively regardless of distance from the soma or
presence on different branches, which is critical for neurons, is
resolved by vesicle circulation. Vesicle circulation also accounts
for the long known, but mysterious, abundance of retrograde
DCVs in axons and ensures that DCVs do not readily return to the soma to be degraded.
Furthermore, vesicle circulation provides a novel explanation
for the finding that perturbing retrograde DCV transport in
neurons affects anterograde transport: interrupting
retrograde transport prevents circulating retrograde
DCVs from contributing to anterograde flux and so would reduce
total anterograde transport. Finally, because the same kinesin
motor transports neuropeptides and many other presynaptic
proteins, vesicle circulation could be a general strategy for maintaining the synaptic
function of en passant boutons. In this light, it is interesting that retrograde transport
is compromised in neurodegenerative diseases. A previously
unconsidered contributing factor to the early onset loss of synaptic function could be
that diseases that affect dynactin-dependent retrograde transport perturb vesicle circulation
that normally maintains the terminal. Therefore, vesicle circulation may be significant under
physiological and pathological conditions (Wong, 2012).
SPAIM detects DCVs in a native synapse for many minutes. In contrast, it was found that
photoactivation-based approaches using PAGFP, tdEosFP and mOrange did not work well with Drosophila
DCVs. This may be due to suboptimal targeting, difficulty in
inducing photoactivation in the mildly acidic and oxidizing lumen of Drosophila DCVs, and the lower brightness and poorer folding of many photoactivatable proteins
compared to emerald GFP. Given the availability of bright, well
characterized GFP constructs, SPAIM represents a tenable
method for tracking DCVs in native Drosophila neurons. In
addition to detecting vesicle circulation, SPAIM could be used
to study how presynaptic DCV mobilization is induced by activity
in a native synapse and how DCV traffic changes with synaptic
development and plasticity. Furthermore, SPAIM can detect release from single DCVs induced by nerve
stimulation, which opens the possibility of studying peptidergic
transmission at the level of individual vesicles in a native synapse. Therefore, SPAIM could answer many questions concerning neuronal DCVs (Wong, 2012).
In principle, SPAIM could be applied to any fluorescent
organelle. However, a dual scanner confocal microscope, which
was used in this study, might not be ideal for detecting very dim signals.
When optical sectioning is not required, this limitation could
be overcome by using a single scanner system for initially photobleaching a region of interest and then for continually photobleaching newcomers while simultaneously imaging individual
organelles with a sensitive camera. In fact, performing
SPAIM experiments by coupling a camera with a common single
scanner confocal microscope may be an attractive option to
overcome both the insensitivity and the expense of dual scanner
systems. With the appropriate setup, SPAIM could be used to
study traffic in terminals, dendrites, cilia, and filopodia (Wong, 2012)
Various muscle diseases present with aberrant muscle cell morphologies characterized by smaller myofibers with mispositioned nuclei. The mechanisms that normally control these processes, whether they are linked, and their contribution to muscle weakness in disease, are not known. This study examined the role of Dynein and Dynein-interacting proteins during Drosophila muscle development and found that several factors, including Dynein heavy chain, Dynein light chain and Partner of inscuteable, contribute to the regulation of both muscle length and myonuclear positioning. However, Lis1 contributes only to Dynein-dependent muscle length determination, whereas CLIP-190 and Glued contribute only to Dynein-dependent myonuclear positioning. Mechanistically, microtubule density at muscle poles is decreased in CLIP-190 mutants, suggesting that microtubule-cortex interactions facilitate myonuclear positioning. In Lis1 mutants, Dynein hyperaccumulates at the muscle poles with a sharper localization pattern, suggesting that retrograde trafficking contributes to muscle length. Both Lis1 and CLIP-190 act downstream of Dynein accumulation at the cortex, suggesting that they specify Dynein function within a single location. Finally, defects in muscle length or myonuclear positioning correlate with impaired muscle function in vivo, suggesting that both processes are essential for muscle function (Folker, 2012).
This study has used the Drosophila musculature to investigate the mechanisms that control muscle size and intracellular organization. Muscle length is regulated independently of the number of fusion events and demonstrate that perturbations that affect embryonic muscle length correlate with decreased larval muscle size and poor muscle function. Additionally, it was found that intracellular organization, specifically myonuclear positioning, is essential for muscle function. Moreover, it was shown that the length and intracellular organization of the myofiber are mechanistically independent. Although a number of factors link both processes, this study identified factors that contribute solely to muscle length or myonuclear position and demonstrates that each fature can be independently manipulate (Folker, 2012).
Dynein regulates both muscle length and myonuclear positioning. Some Dynein-interacting proteins, such as Dlc90F and Pins, are necessary for both processes. That these factors regulate both muscle length and myonuclear positioning suggests that specific aspects of muscle growth and the positioning of myonuclei can indeed be coordinated. However, Lis1 affects muscle length specifically, whereas CLIP-190 and Glued specifically affect myonuclear position. Within the contexts of muscle length and myonuclear position CLIP-190 and Lis1 do not genetically interact, illustrating that, although linked, the two processes are mechanistically distinct (Folker, 2012).
Identification of CLIP-190 and Lis1 as regulators of Dynein is not novel. CLIP-190 and Lis1 are known to interact with Dynein both physically and functionally, and they usually cooperate toward a single goal. This study provides the first example of CLIP-190 and Lis1 serving completely independent, Dynein-dependent functions (Folker, 2012).
Mechanistically, the data suggest that Dynein function, with respect to the regulation of muscle length and myonuclear positioning, is specified by Lis1 and CLIP-190 downstream of its localization. That Dynein localization to the myofiber pole is important is illustrated by pins (raps193) mutants in which Dynein does not accumulate at the myofiber pole, and defects were observed in both muscle length and myonuclear positioning. This suggests that Pins recruits and stabilizes Dynein at the cortex as it does during mitosis. Lis1 also affects Dynein localization, but in Lis1 mutants Dynein is localized to the muscle pole and is tightly restricted to the pole compared with controls. This is interpreted to mean that Lis1 is necessary for retrograde trafficking of Dynein away from the myofiber pole. It is hypothesized that, in the absence of Dynein retrograde trafficking, cellular components accumulate at the extending muscle end, thus inhibiting the trafficking of factors necessary for further directed growth. Thus, when Dynein is incapable of moving cargo away from the myofiber pole, muscles are shorter than in control embryos. Indeed, a similar correlation between retrograde trafficking and directed growth was recently reported during mechanosensory bristle growth and axonal transport. Interestingly, it was recently reported that decreased retrograde transport of Dynein results in longer processes in both fibroblasts and neurons in culture (Folker, 2012).
This contradictory finding raises several interesting questions. Is there an opposing pathway in muscle or is another aspect of muscle size increased? For example, does the length of the muscle impact its volume? Complications due to muscle contraction in the live embryo make such analysis difficult, however. Likewise, working in vivo on a dynamically changing tissue located 30-150 microm below the surface of the developing embryo presents challenges for imaging the rapid cellular processes that underlie motor activity, microtubule organization, organelle positioning and cell size. Nevertheless, the link between different aspects of cell size and their relationships to each other and to motor activity are being explored (Folker, 2012).
It is not clear why LT muscle growth/length is more sensitive to Dynein activity than the VL muscles in the embryo. A simple explanation is that the maternally loaded Dynein persists longer in the VL muscles than in the LT muscles. Additionally there might be a physical explanation. Although a cluster of potential tendon cells for the VL muscles exist, they are clustered at the segment border. Therefore, the size of the hemisegments, and thus the distance from segment border to segment border, determines muscle length. Conversely, the cluster of potential tendon cells for the LT muscles might be more broadly dispersed and the location of the muscle pole at the time of tendon specification determines the length of the muscle. Under this hypothesis, inefficient extension of the LT muscles would result in the muscles being shorter during tendon specification/maturation and therefore shorter throughout embryonic development. Alternatively, differences in guidance/signaling systems could explain why the LT muscles, but not the VL muscles, are shorter when Dynein activity is compromised. Different signaling mechanisms are employed by these different muscle types. For example, Derailed (Drl) plays a crucial role in the ability of LT muscles to recognize their target. Perhaps, altered trafficking of Drl or another factor in the signaling pathway causes slight, but significant, changes in LT muscle length (Folker, 2012).
It is interesting that the LT muscles are smaller in Dhc64C, Dlc90F, pins and Lis1 mutant embryos, but that other muscles are unaffected, whereas in larvae all muscles appear to be smaller. During the larval stages, muscles remain stably attached to tendon cells and grow through insulin signaling/Foxo- and dMyc-dependent pathways. The observations are intrepreted to mean that Dynein and its regulatory proteins Dlc90F, Pins and Lis1 contribute to insulin receptor-mediated muscle growth. Indeed, it would not be surprising to find that Dynein-dependent cellular trafficking is essential to that signaling pathway (Folker, 2012).
CLIP-190 does not dramatically affect Dynein localization, but it does affect microtubule organization, which, in turn, affects nuclear positioning. CLIP-190 mutant embryos have fewer microtubules at the myofiber pole, suggesting that, similar to its functions in other systems, the role of CLIP-190 is to stabilize microtubule-cortex interactions, which Dynein then uses to move nuclei towards the myofiber pole. The specification of Dynein function, downstream of its localization, is novel. Although phospho-regulation has been shown to alter Dynein function during mitosis and the possibility for competitive regulation of Dynein has been suggested, this is the first example in which Dynein at a single location has its activity modified through interactions with unique binding partners. The ability to specify Dynein function without dramatically altering its localization is likely to be an important factor during development when temporal constraints are high (Folker, 2012).
With regards to physiology, the small, but significant, changes in myonuclear positioning and muscle size seen in the embryo continue throughout larval development and are associated with impaired muscle function. Additionally, that the same defects and impairments are found in larvae that were depleted of Dynein, Lis1 or CLIP-190 specifically in the muscle shows that the effects are, at least in part, muscle specific (Folker, 2012).
Many muscle myopathies are characterized by smaller myofibers and mispositioned nuclei. However, it is unclear whether these pathologies are linked and which of these defects are paramount in causing the muscle weakness associated with these myopathies. This study has shown that in Drosophila these two processes are linked via the requirement for Dynein activity at the muscle pole. It was further shown that these two processes are mechanistically distinct, yet that both are necessary for muscle function. Together, these data suggest that therapeutics aimed at improving the functional capacity of diseased muscles must counteract effects on both muscle size and myonuclear positioning. This highlights Drosophila as an ideal model system with which to identify the genes and mechanisms required for distinct aspects of muscle morphogenesis and to shed light on key features of muscle disease (Folker, 2012).
In addition to the ubiquitous apical-basal polarity, epithelial cells are often polarized within the plane of the tissue - the phenomenon known as planar cell polarity (PCP). In Drosophila, manifestations of PCP are visible in the eye, wing, and cuticle. Several components of the PCP signaling have been characterized in flies and vertebrates, including the heterotrimeric Go protein. However, Go signaling partners in PCP remain largely unknown. Using a genetic screen Kermit, previously implicated in G protein and PCP signaling, was unvcovered as a novel binding partner of Go. Through pull-down and genetic interaction studies, it was found that Kermit interacts with Go and another PCP component Vang (Strabismis), known to undergo intracellular relocalization during PCP establishment. It was further demonstrated that the activity of Kermit in PCP differentially relies on the motor proteins: the microtubule-based dynein and kinesin motors and the actin-based myosin VI (Jaguar). The results place Kermit as a potential transducer of Go, linking Vang with motor proteins for its delivery to dedicated cellular compartments during PCP establishment (Lin, 2013).
At the top of the signaling hierarchy in PCP lies a G protein-coupled receptor Fz. The heterotrimeric Go protein emerged as an immediate transducer of Fz in Drosophila as well as other organisms. One of the mediators of Go signaling in PCP is the endocytic GTPase Rab5 required for the proper Fz internalization and relocalization. During PCP establishment, Fz concentrates at the distal apical position of wing epithelia. This study describes identification of Kermit as another transducer of Go in PCP. kermit downregulation suppresses the Gαo-overexpression phenotypes, and Gαo and kermit co-overexpression results in a prominent synergism in PCP malformations (Lin, 2013).
Kermit and its mammalian homolog GIPC, through their PDZ domain, are known to interact with a number of proteins in various organisms. Observations in Xenopus and mice indicated that Kermit/GIPC could interact with members of the Fz and RGS protein families -- Fz3, Fz7, and RGS19 (De Vries, 1998; Tan, 2001). Since Go also binds Fz and RGS proteins, it was hypothesized that a quaternary complex consisting of Fz, Go, Kermit, and RGS19 could form in Drosophila PCP, with Kermit as a potential organizer of these interactions. However, Drosophila Kermit was found not interact with Fz. Similarly, no binding between Kermit and the Drosophila RGS19 homolog could be seen. Thus Kermit is unlikely to act as a scaffold in Fz-Go signaling, and another mode of action of Kermit in transducing Go signal exists in PCP (Lin, 2013).
In a recent study using mouse genetics and cellular assays, a role of GIPC1 in regulating Vangl2 (a murine homolog of Drosophila Vang) intracellular trafficking has been revealed (Giese, 2012). In Drosophila PCP, Vang relocalizes to the site opposite to Fz at the proximal apical tip of wing epithelia. This study provides genetic evidence placing Vang downstream from Kermit in Drosophila PCP, suggesting that the Kermit-Vang connection is conserved from insects to mammals (Lin, 2013).
kermit expression is strongly upregulated in the developing wing during PCP establishment, and kermit overexpression induces strong PCP phenotypes (Djiane, 2010). In Xenopus, both up- and down-regulation of kermit lead to defective Fz3-dependent neural crest induction. It is thus surprising that Drosophila kermit loss-of-function alleles were homozygous viable and did not reveal PCP phenotypes. It is proposed that Kermit may regulate Drosophila PCP redundantly with some other PDZ domain-containing proteins, such as Scribble or Patj, which genetically interact with PCP components but on their own also produce only mild phenotypes; of those Scribble has been shown to interact with Vang both in Drosophila and mammals. In general, up to 75% of genes Drosophila are estimated to be phenotypically silent in loss-of-function due to redundancy, and the significance of gain-of-function analysis in discovery of novel important pathway components has been highlighted in a recent large-scale Drosophila-based assay. Kermit, based on the presented overexpression and genetic interaction studies, can thus be considered as an important regulator of Drosophila PCP (Lin, 2013).
A genetic and physical interaction between Kermit and the unconventional actin-based motor MyoVI has been described. This study confirmed that the dominant UAS-kermit PCP phenotypes critically depend on the MyoVI activity. MyoVI has been previously shown to mediate removal of endocytic vesicles away from the cell's periphery. The excessive activity of Kermit or MyoVI may thus result in removal of Vang-containing vesicles from the apical membrane, contributing to mislocalization of Vang and appearance of the PCP defects. In contrast, microtubule-based transport along the apical microtubule cables, polarized below the apical plasma membrane in wing epithelia, mediates the correct relocalizations of Fz and Vang in PCP. It is probable that a competition between the actin-based and microtubule-based motors may exist for the endocytic vesicles containing PCP components, and that excessive Kermit activity unbalances this competition in favor of the actin-based transport. Thus whether reduction in the levels of the microtubule-based transport system would further aggravate the dominant UAS-kermit PCP phenotypes was tested. And indeed, reduction in either the minus end-directed motor dynein or the plus end-directed motor kinesin significantly enhances the UAS-kermit effects (Lin, 2013).
The following model is proposed to collectively explain the results. It is speculated that endocytic vesicles containing PCP components can be transported in a planar manner, along the microtubule meshwork underlying the apical plasma membrane -- the mode of transport required for the proper apical relocalizations of these components. Alternatively, the vesicles can be trapped by the actin cables and transported away from the apical membrane, removing them from the active pool of PCP components. In the case of Vang, the choice between these decisions is regulated by the Kermit protein, which favors the actin-based transport (Lin, 2013).
These findings and model shed new light on the mechanisms of complex inter-regulations ensuring the robust epithelial polarization, likely conserved across the metazoans (Lin, 2013).
Formation of new synapses or maintenance of existing synapses requires the delivery of synaptic components from the soma to the nerve termini via axonal transport. One pathway that is important in synapse formation, maintenance and function of the Drosophila neuromuscular junction (NMJ) is the bone morphogenetic protein (BMP)-signaling pathway. This study shows that perturbations in axonal transport directly disrupt BMP signaling, as measured by its downstream signal, phospho Mad (p-Mad). Components of the BMP pathway genetically interact with both kinesin-1 and dynein motor proteins. Thick vein (TKV) vesicle motility is also perturbed by reductions in kinesin-1 or dynein motors. Interestingly, dynein mutations severely disrupted p-Mad signaling while kinesin-1 mutants showed a mild reduction in p-Mad signal intensity. Similar to mutants in components of the BMP pathway, both kinesin-1 and dynein motor protein mutants also show synaptic morphological defects. Strikingly TKV motility and p-Mad signaling are disrupted in larvae expressing two human disease proteins; expansions of glutamine repeats (polyQ77) and human amyloid precursor protein (APP; see Drosophila Appl) with a familial Alzheimer's disease (AD) mutation (APPswe). Consistent with axonal transport defects, larvae expressing these disease proteins show accumulations of synaptic proteins along axons and synaptic abnormalities. Taken together these results suggest that similar to the NGF-TrkA signaling endosome, a BMP signaling endosome that directly interacts with molecular motors likely exists. Thus problems in axonal transport occurs early, perturbs BMP signaling, and likely contributes to the synaptic abnormalities observed in these two diseases (Kang, 2014 - Open access: 25127478).
In dividing animal cells the endoplasmic reticulum (ER) concentrates around the poles of the spindle apparatus by associating with astral microtubules (MTs), and this association is essential for proper ER partitioning to progeny cells. The mechanisms that associate the ER with astral MTs are unknown. Because astral MT minus-ends are anchored by centrosomes at spindle poles, it is hypothesized that the MT minus-end motor dynein mediates ER concentration around spindle poles. Live in vivo imaging of Drosophila spermatocytes revealed that dynein is required for ER concentration around centrosomes during late interphase. In marked contrast, however, dynein suppression had no effect on ER association with astral MTs and concentration around spindle poles in early M-phase. In fact, there was a sudden onset of ER association with astral MTs in dynein RNAi cells, revealing activation of an M-phase specific mechanism of ER-MT association. ER redistribution to spindle poles also did not require non-claret disjunctional (ncd), the other known Drosophila MT minus-end motor, nor Klp61F, a MT plus-end motor that generates spindle poleward forces. Collectively, these results suggest that a novel, M-phase specific mechanism of ER-MT association that is independent of MT minus-end motors is required for proper ER partitioning in dividing cells (Karabasheva, 2019).
The endoplasmic reticulum (ER) cannot be formed by cells de novo and must be inherited during the process of cell division. While the essential roles of the ER in the biogenesis of proteins, lipids and steroid hormones, as well as calcium signaling, are well recognized, little is known about the molecular mechanisms that ensure proper partitioning of the ER to progeny cells. This knowledge is fundamental to understanding the role of the ER in cell division and stem cell biology, with important implications for proper development, tissue repair, and cancer. Specifically, recent findings indicate that asymmetric partitioning of the ER, and misfolded proteins that accumulate there, has a critical role in maintaining pluripotency in stem cells as they undergo rapid cycles of cell division. Related evidence that ER functions contribute to the proliferative capacity and drug resistance of cancer cells is under active investigation, with an objective of revealing novel therapeutic strategies for treating malignancies. The overall goal of the present study was to establish new understanding of the cellular mechanisms that guide the localization of the ER during cell division, bringing closer a full comprehension of the essential roles of the ER in normal physiology and disease (Karabasheva, 2019).
Recent data suggest that proper partitioning of the ER during cell division, or M-phase, depends on specific association of the organelle with astral microtubules (MTs) of the mitotic spindle in both symmetrically and asymmetrically dividing cells4. However, the specific factors that link the ER to astral MTs remain unknown. Identification of these factors is therefore an important next step in understanding mitotic ER partitioning. Most of the knowledge of ER-MT associations comes from non-dividing interphase cells, in which the ER is distributed throughout the cytoplasm with a distinct clustering or focus around centrosomes, the major MT organizing centers of cells. This distribution depends on MT motor-dependent movements of the ER toward both MT plus-ends and minus-ends, as well as stable attachments of the ER along MT filaments mediated by ER membrane proteins including REEPs, spastin, and CLIMP-6. Transport of the ER toward MT minus-ends is mediated by dynein motors, which are also responsible for focusing the ER around centrosomes where MT minus-ends are anchored. Conversely, MT plus-end transport is likely mediated by kinesins9, and also depends on association with growing MT tips mediated, at least in part, by ER membrane embedded STIM1 and STIM2 proteins10. Collectively these associations point to a carefully orchestrated interplay between MT plus-end and minus-end directed transport mechanisms that determine the cellular distribution of the ER (Karabasheva, 2019).
During the course of M-phase, there is a dramatic reorganization of the MT cytoskeleton, whereby the spindle apparatus forms with MT minus-ends anchored at the spindle poles by centrosomes. As this occurs, the majority of the ER becomes focused around the two centrosomes at the spindle poles and along astral MTs, and virtually none is found in the kinetochore region of the spindle where MT plus-ends reside. Thus, there appears to be a shift from the balanced MT plus- and minus-end directed ER distribution during interphase to predominantly minus-end directed localization around spindle poles during M-phase. Consistent with this conclusion, tracking of the ER with growing MT plus-ends is inhibited during cell division due to mitosis-specific phosphorylation of STIM1. This suppression of MT plus-end directed ER transport, and the totality of ER distribution toward MT minus-ends around spindle poles, suggests a predominant role for the MT-minus end motor dynein in M-phase specific ER distribution. Notably, dynein is highly localized to astral MTs and spindle poles in dividing cells and is required for the spindle pole localization of endosomes. However, despite this compelling case, a definitive role for dynein in M-phase ER distribution has never been directly established. Determining dynein's role in the dramatic redistribution of the ER to spindle poles in M-phase is therefore important to understanding the mechanisms that ensure proper ER partitioning to progeny cells (Karabasheva, 2019).
The primary goal of this study was to test the hypothesis that dynein is required for astral MT association and spindle pole focusing of the ER in dividing cells. This was accomplished by live timelapse imaging of Drosophila spermatocytes undergoing the first meiotic division of spermatogenesis in vivo. This experimental system is particularly well suited for this investigation, as Drosophila spermatocytes allow for the analysis of dividing cells in a physiologic environment and have contributed greatly to understanding of fundamental cell division mechanisms including spindle formation and cytokinesis. Moreover, Drosophila spermatocytes are large cells that exhibit clearly defined redistribution of the ER onto astral MTs early in meiosis. This study presents the surprising finding that although dynein is required for peri-centrosomal focusing of the ER during late interphase, it does not mediate the astral MT-dependent spindle pole focusing of the ER around centrosomes during M-phase. Surprisingly, the results further reveal that redistribution of the ER toward MT minus-ends in dividing cells is mediated by a mechanism of ER-MT association that is entirely specific to M-phase and does not operate during interphase. This report lays the groundwork for identification of this novel mechanism of ER-MT association that is essential for the process of ER inheritance (Karabasheva, 2019).
Proper distribution of the ER in dividing cells is critical, because it ensures that progeny cells receive necessary proportions of this essential organelle. Disruptions in this process may result in cells that are prone to accumulation of misfolded proteins and ER stress, as well as dysregulation of lipid homeostasis and calcium signaling, all of which are involved in a spectrum of diseases including cancer, neurodegeneration, and diabetes. However, the importance of mitotic ER distribution may extend beyond organelle inheritance. For example, it was recently demonstrated that the ER plays an essential role in restricting the distribution of damaged proteins in asymmetrically dividing stem cells. This mechanism, which depends on proper distribution of the ER around the mitotic spindle, may allow vital stem cells to protect themselves by asymmetrically shuttling damaged proteins to non-essential progeny cells. In addition, it has been suggested that the ER delivers highly localized calcium signals that are essential for proper function of the spindle apparatus. Thus, disruption of ER distribution around the mitotic spindle may impair the process of cell division itself with potentially devastating effects on development and tissue homeostasis. Conversely, therapeutic interventions that specifically target ER functions in cancer cells may prove to be effective treatments in neoplastic disease. Identification of the molecular mechanisms that regulate M-phase specific distribution of the ER is critical to understanding these multi-faceted roles for the organelle in dividing cells. The present investigation demonstrates the existence of a novel mechanism that mediates ER-MT association in a manner that is specific to M-phase and independent of dynein and other known MT motors (Karabasheva, 2019).
The first studies to examine ER distribution in dividing cells reported the organelle's distinct organization around the poles of the mitotic spindle, where MT minus-ends are clustered. Subsequent studies have confirmed this observation across multiple species, suggesting the existence of a unifying mechanism of ER partitioning that involves MT-dependent spindle pole association. Accordingly, involvement of a MT minus-end motor to move the ER toward spindle poles has been suggested in multiple studies, with dynein being the most likely candidate. Importantly, dynein associates with ER-rich microsomal fractions and is required for proper ER distribution in interphase cells and extracts, suggesting that the motor can associate with and transport ER membranes. Surprisingly however, the putative role for dynein in regulating ER distribution during cell division has not been directly tested. This question was addressed using meiotic Drosophila spermatocytes, which demonstrate a dramatic, near complete MT-dependent redistribution of the ER to spindle poles during cell division. The results clearly demonstrate a role for dynein in ER transport and localization in these cells, because ER distribution toward MT minus-ends and around centrosomes during late interphase was completely disrupted by dynein suppression. Surprisingly, however, dynein was dispensable for astral MT association and spindle pole focusing of the ER during meiosis. Importantly and consistent with previous reports, dynein suppression had dramatic effects on spindle formation and architecture in Drosophila spermatocytes including failure of centrosome migration to the nuclear envelope and complete dissociation of spindle poles from the focused kinetochore MT fibers. Nevertheless, despite these highly abnormal spindles, this study observed that early in meiosis the ER was still drawn onto the astral MTs and towards the centrosomes, despite the centrosomes' mislocalization at the cell cortex. Moreover, this redistribution of the ER occurred with the same kinetics and to the same extent as in control cells, suggesting that the mechanism of ER-astral MT association is completely independent of dynein. In this regard, data indicating that dynein association with membranous organelles is inhibited during M-phase of cell division suggests the existence of a dynamic mechanism that reciprocally regulates dynein association of the ER with MTs according to the stage of the cell cycle (Karabasheva, 2019).
An important outcome of dynein suppression in spermatocytes is that it allowed a clear separation of interphase versus M-phase mechanisms of ER association with centrosomal MTs, wherein the ER was completely excluded from centrosomal MTs during late interphase but suddenly moved along MTs toward the centrosomes early in M-phase. This reveals that an M-phase specific mechanism of ER-MT association that concentrates the ER around centrosomes is triggered at the onset of cell division, possibly due to specific cyclin/cyclin-dependent kinase activity. This conclusion is further supported by the observations that several of the mechanisms for ER-MT association that operate during interphase are in fact inhibited during M-phase, including those mediated by STIM1, CLIMP-63, and possibly dynein. Collectively, these findings suggest an enticing mechanism whereby most or all interphase mechanisms of ER-MT association are inhibited during M-phase, allowing an M-phase specific mechanism to predominate and ensure ER association with astral MTs and proper partitioning to daughter cells (Karabasheva, 2019).
The challenge moving forward is to identify the M-phase specific mechanism that associates the ER with astral MTs. Importantly, in dynein suppressed spermatocytes it was clear that the ER moved along astral MTs toward the minus-ends, suggesting that a MT minus-end motor distinct from dynein may be involved. However, the present findings indicate that neither ncd, the only other known bona fide MT minus-end motor in Drosophila, nor Klp61F, which generates pole-ward forces in the spindle, are involved in the unique M-phase distribution of ER with astral MTs. Thus, it is possible that an unidentified MT minus-end motor is required, or that a non-motor factor stably attaches ER membranes along MT fibers and tubulin flux then moves these membranes toward centrosomes. In this regard, it was recently demonstrated that human REEP3 and 4, members of the REEP1-4 family of ER membrane proteins, directly associate with MTs and play a role in spindle pole focusing of the ER37. However, whether REEP3/4 associate the ER specifically with astral MTs has yet to be determined. Drosophila have a single orthologue to human REEPs1-4, known as REEPA, and preliminary observations indicate that REEPA also is not required for astral MT association of the ER in meiotic Drosophila spermatocytes. It is also important to note that different mechanisms of ER-MT association may operate during cell division in different cell types. Thus, while the current results cannot rule out a role for dynein or other MT minus-end motors in ER partitioning in cells other than Drosophila spermatocytes, the findings do indicate that these factors are not universally required (Karabasheva, 2019).
In addition to identifying the molecular factors that link the ER to astral MTs during cell division, another important question is how does this association discriminate astral MTs from other MTs of the spindle apparatus? Certainly, suppression of interphase mechanisms plays a role, as expression of nonphosphorylatable STIM1, which remains associated with MTs during mitosis, results in mislocalization of the ER to kinetochore MTs. It is also possible that differences in tubulin post-translational modifications facilitate discrimination between different MT populations within the spindle, as demonstrated for kinesin-7 motors that carry chromosomes toward the spindle equator along detyrosinated MTs of the inner spindle (Karabasheva, 2019).
In conclusion, this study has demonstrated that an M-phase specific mechanism associates the ER with astral MTs and partitions the organelle to spindle poles in dividing cells. Surprisingly, and in contrast to many previous suggestions, this ER localization is not mediated by dynein or other known MT minus-end motors. Identification of the mechanisms involved is an important next step in better understanding the role of the ER in cell division, stem cell longevity and pluripotency, and tissue architecture. This knowledge will facilitate development of novel therapeutics for pathological processes underlying cancer and age-related tissue degeneration (Karabasheva, 2019).
The microtubule motor cytoplasmic dynein has been implicated in a variety of intracellular transport processes. The previously
identified and characterized Drosophila gene Dhc64C encodes a cytoplasmic dynein heavy chain. To investigate the
function of the cytoplasmic dynein motor, a mutational analysis of the Dhc64C dynein gene was initiated. A small deletion that
removes the chromosomal region containing the heavy chain gene was used to isolate EMS-induced lethal mutations that define at
least eight essential genes in the region. Germline transformation with a Dhc64C transgene rescues 16 mutant alleles in the single
complementation group that identifies the dynein heavy chain gene. All 16 alleles are hemizygous lethal, which demonstrates that
the cytoplasmic dynein heavy chain gene Dhc64C is essential for Drosophila development. Furthermore, failure to recover
somatic clones of cells homozygous for a Dhc64C mutation indicates that cytoplasmic dynein function is required for cell viability
in several Drosophila tissues. The intragenic complementation of dynein alleles reveals multiple mutant phenotypes including male
and/or female sterility, bristle defects, and defects in eye development (Gepner, 1996).
The Drosophila Glued gene product shares sequence homology with the p150 component of vertebrate dynactin. Dynactin is a
multiprotein complex that stimulates cytoplasmic dynein-mediated vesicle motility in vitro. Biochemical,
cytological, and genetic evidence is presented that demonstrates a functional similarity between the Drosophila Glued complex and vertebrate
dynactin. Similar to the vertebrate homologs in dynactin, the Glued polypeptides are components of a 20S
complex. Biochemical studies further reveal differential expression of the Glued polypeptides, all of which copurify as
microtubule-associated proteins. In an analysis of the Glued polypeptides encoded by the dominant mutation, Glued, a truncated polypeptide was identified that fails to assemble into the wild-type 20S complex, but retains the ability to copurify with microtubules.
The spatial and temporal distribution of the Glued complex during oogenesis is shown by immunocytochemistry methods to be
identical to the pattern previously described for cytoplasmic dynein. Significantly, the pattern of Glued distribution in oogenesis is
dependent on dynein function, as well as several other gene products known to be required for proper dynein localization. In
genetic complementation studies, certain mutations in the cytoplasmic dynein heavy chain gene Dhc64C act are shown to act as
dominant suppressors or enhancers of the rough eye phenotype of the dominant Glued mutation. Furthermore, a
mutation that was previously isolated as a suppressor of the Glued mutation is an allele of Dhc64C. Together with the observed
dependency of Glued localization on dynein function, these genetic interactions demonstrate a functional association between the
Drosophila dynein motor and Glued complexes (McGrail, 1995).
During animal development, cellular differentiation is often preceded by an asymmetric cell division whose polarity is determined
by the orientation of the mitotic spindle. In Drosophila the oocyte differentiates in a 16-cell syncytium
that arises from a cystoblast that undergoes 4 synchronous divisions with incomplete cytokinesis. During these divisions,
spindle orientation is highly ordered and is thought to impart a polarity to the cyst that is necessary for the subsequent
differentiation of the oocyte. Using mutations in the Drosophila cytoplasmic dynein heavy chain gene Dhc64C, it has been shown that
cytoplasmic dynein is required at two stages of oogenesis. Early in oogenesis, dynein mutations disrupt spindle orientation in
dividing cysts and block oocyte determination. The localization of dynein in mitotic cysts suggests spindle orientation is mediated
by the microtubule motor cytoplasmic dynein. Later in oogenesis, dynein function is necessary for proper differentiation, but does
not appear to participate in morphogen localization within the oocyte. These results provide evidence for a novel developmental
role for the cytoplasmic dynein motor in cellular determination and differentiation (McGrail, 1997).
Cytoplasmic dynein is a multisubunit minus-end-directed microtubule motor that serves multiple cellular functions. Genetic studies in
Drosophila and mouse have demonstrated that dynein function is essential in metazoan organisms. However, whether the essential
function of dynein reflects a mitotic requirement, and what specific mitotic tasks require dynein remains controversial. Drosophila is an
excellent genetic system in which to analyze dynein function in mitosis, providing excellent cytology in embryonic and somatic cells. Recessive lethal mutations in the dynein heavy chain gene, Dhc64C, were used to reveal the contributions of the
dynein motor to mitotic centrosome behavior in the syncytial embryo. Embryos lacking wild-type cytoplasmic dynein heavy chain were
analyzed by in vivo analysis of rhodamine-labeled microtubules, as well as by immunofluorescence in situ methods. Comparisons between wild-type and Dhc64C
mutant embryos reveal that dynein function is required for the attachment and migration of centrosomes along the nuclear envelope during interphase/prophase, and to
maintain the attachment of centrosomes to mitotic spindle poles. The disruption of these centrosome attachments in mutant embryos reveals a critical role for dynein
function and centrosome positioning in the spatial organization of the syncytial cytoplasm of the developing embryo (Robinson, 1999).
Recessive lethal alleles of the dynein heavy chain gene, Dhc64C, were identified that exhibit intragenic complementation and reveal mitotic phenotypes during the rapid divisions of the syncytial embryo. The nonlethal Dhc64C6-8/Dhc64C6-6 combination of mutations results in fully viable transheterozygous adult females that produce eggs that are endowed with strictly mutant dynein heavy chain. The rapid rounds of nuclear divisions that follow fertilization of these mutant embryos are compromised by the defective dynein and result in maternal effect lethality with greater than 94% of the embryos dying. The nature of the lesions within the dynein heavy chain mutations, Dhc64C6-8 and Dhc64C6-6, are not known. However, the presence of a wild-type dynein heavy chain transgene rescues the mitotic defects, as well as maternal effect lethality, demonstrating that the phenotype is specific to a loss in dynein function. Importantly, the mitotic defects that were uncovered are not unique to the syncytial nature of early nuclear divisions. Similar defects occur in the larval neuroblasts of late-lethal alleles of the dynein heavy chain (Robinson, 1999).
Within the mutant syncytium, nuclear cycles proceed and repeatedly show defects in specific centrosome behaviors and spindle morphogenesis at each nuclear cycle. This progression of the nuclear cycles and the repetitive occurrence of centrosome misbehavior and aberrant multipolar spindle formation is consistent with the defects being a primary consequence of dynein dysfunction. In this regard, it is suggested that the combination of hypomorphic heavy chain alleles provides a means to specifically attenuate dynein function in order to investigate its mitotic function in early syncytial embryos. Strong loss of function alleles or null mutations in the dynein heavy chain are cell lethal and prohibit such analysis. This result contrasts with findings in other genetically tractable systems, such as yeast, in which it has been shown that dynein is not an essential gene. The results demonstrate that the mitotic function(s) of cytoplasmic dynein are essential in Drosophila (Robinson, 1999).
Analysis of centrosome behavior in vivo within the dynein mutant embryos occasionally reveals centrosomes detaching from the nuclear envelope. In time-lapse movies some centrosomes leave the envelope never to return, while other centrosomes detach briefly and then moved back to the nucleus and reattach. These events are never seen in wild-type embryos and provide evidence for a novel function for dynein in maintaining the association of centrosomes with the nuclear envelope. The reattachment of centrosomes, as well as the low penetrance of the detached centrosome phenotype, is consistent with the prediction that the hypomorphic dynein gene products are only partially compromised for nuclear attachment. One interpretation of these phenotypes is that dynein is associated with the nuclear envelope, where it acts as a minus-end motor to draw in centrosomal microtubules and secure the centrosomes to the nuclear membrane. Alternatively, or in addition, dynein may reside in the centrosome and act to stabilize the attachment of nucleated microtubules that are themselves required for nuclear attachment. In this case, loss of dynein function may increase the frequency of microtubule release from centrosomes and thus weaken nuclear attachment. Evidence for active dynein complex associated with the nuclear envelope has been reported in vitro in Xenopus extracts. In Drosophila, cytoplasmic dynein is localized to the oocyte nucleus, where it might power nuclear migration. However, in embryos, dynein is present on the mitotic spindle and appears concentrated at spindle poles, but no accumulation on nuclear envelopes has yet been detected. In mammalian cell lines, dynein is localized to kinetochores, centrosomes, and at the nuclear periphery (Robinson, 1999 and references).
Most centrosomes observed in mutant embryos retained their nuclear attachments, but exhibit defects in migration along the nuclear membrane during prophase. Time-lapse analysis in living embryos demonstrates that dynein is required for the initial separation of centrosomes along the nuclear envelope and is distinct from the function of antagonistic motors that maintain the separation of centrosomes once initial separation is complete. There is an observed failure of the duplicated centrosomes to fully migrate along the nuclear envelope to a position 180° apart from one another before nuclear envelope breakdown occurs. The centrosome migration defect is consistent with results from antibody knockout experiments performed in a vertebrate cell culture system. The predominant defect in centrosome migration can be viewed as an intermediate phenotype, the consequence of only partial loss of dynein function. The 'detached-centrosome' phenotype might occur when the same dynein-based mechanism is further compromised. However, whether dynein function in centrosome attachment and centrosome migration are mechanistically related remains to be determined. Indeed, recent studies show that centrosome migration in Xenopus extracts depends upon the activity of the plus-end directed kinesin-like protein Xklp2. Xklp2 is a member of the BimC class of conserved kinesin-like proteins which likely play similar roles in several different eukaryotes. Furthermore, the localization of a COOH-terminal fragment of Xklp2 to the minus-ends of spindle and astral microtubules requires the activity of cytoplasmic dynein. Mutations in dynein may affect centrosome separation by reducing or preventing the normal accumulation of BimC class motor proteins to astral and spindle microtubules. An opposing category of models for dynein involvement in centrosome separation predicts that force production occurs within the cortical cytoplasm and acts to pull on centrosomal microtubules. While such a model readily explains the centrosome migration defect, this mechanism does not account for the observed detachment of centrosomes (Robinson, 1999 and references).
In dynein mutant embryos, the release of centrosomes from spindle poles was observed, as well as 'loosely attached' centrosomes, where the distance between a centrosome and the associated metaphase spindle pole is significantly greater than in wild-type. Furthermore, the morphology of the spindle poles which lose a centrosome is affected in a manner consistent with current models of dynein function in spindle morphogenesis. In mutant embryos the detachment of a centrosome from bipolar spindles results in the partial collapse of the affected pole. In some cases, the blunt-ended pole becomes refocused, suggesting that either a residual function of the mutant dynein or an additional minus-end motor can rescue the spindle pole. Loss of a centrosome from multipolar spindles also results in collapse of the affected pole. In living embryos the maintenance of a focused spindle pole requires dynein and the stabilizing influence of a centrosome. This result is not contingent upon the syncytial environment of the embryo since a similar requirement for centrosomes in the organization of spindle poles is apparent in Drosophila larval neuroblasts (Robinson, 1999).
In vivo time-lapsed analysis provides a temporal understanding of the relationship between centrosome behavior and spindle morphogenesis, and reveals another novel aspect to the dynein mutant phenotype. Nuclei that undergo incomplete centrosome migration are predisposed to suffer further defects in spindle assembly, which frequently lead to multipolar spindle configurations. As a further consequence, the size of interphase nuclei is variable and suggests a significant defect in karyokinesis. Although dynein is likely to be present on Drosophila kinetochores, as it is in other organisms, evidence for a direct role for dynein in chromatid congression or segregation is lacking. The alignment of chromosomes at metaphase is apparently normal in mutant embryos and the interpretation that aberrant nuclear size results predominantly from abnormal chromatid segregation on multipolar spindles is favored, rather than a direct effect on kinetochore-mediated chromatid movement (Robinson, 1999).
How does a loss in dynein function and defective centrosome behavior lead to multipolar spindle assembly? The regular spacing between metaphase spindles in the Drosophila syncytium is dependent upon centrosome positions on the nuclear envelope before M phase. One interesting possibility is that organization of the cortical cytoskeleton and the pseudocleavage furrow acts to help isolate nuclei from one another during late nuclear division cycles and is disrupted by mispositioned centrosomes. In this case, loss of spindle autonomy and the formation of multipolar spindle configurations is an indirect effect of the role of dynein in centrosome positioning. However, mutations known to disrupt the cortical cytoskeleton and pseudocleavage furrows are reported to promote spindle fusions during late nuclear cycles when nuclear density is high. For dynein mutant embryos, spindle fusions are detected during the earliest rounds of division before cortical migration and when nuclear density is quite low (Robinson, 1999 and references).
Alternatively, a reduction in dynein function in mitotic spindles and/or the syncytial cytoplasm may allow spindle or centrosomal microtubule bundles to interact inappropriately with neighboring arrays. Measurements of an increase in spindle girth in the mutant embryos is consistent with a reduced organization of microtubules within mutant spindles. In spite of the reduced affinity of centrosomes for both nuclear envelopes and spindle poles in the dynein mutants, centrosomes retain a strong capacity to organize spindle poles. Ectopic spindle pole formation has been observed on neighboring mitotic arrays by both free centrosomes and spindle-associated centrosomes. The ectopic poles form by splitting off bundles of microtubules from the adjacent spindle, rather than by nucleation of microtubule bundles from the errant centrosome toward the adjacent spindle. Subsequently, the formation of interspindle microtubule bundles and the fusion of neighboring spindles may result from the action of other motor activities (Robinson, 1999). For example, the separation of spindles during late nuclear cycles in Drosophila embryos has been shown to require the function of KLP61F (Sharp, 1999).
In summary, evidence has been provided that cytoplasmic dynein is required for the attachment of centrosomes to prophase nuclei. Time-lapsed analysis has further demonstrated in living embryos the role of dynein in the initial migration of centrosomes along the nuclear envelope before spindle assembly, as well as in the attachment of centrosomes to spindle poles. The inappropriate behaviors of centrosomes that result from the reduction in dynein function disrupt spindle morphogenesis. These results show that dynein function in centrosome attachments is essential for autonomous spindle function, and the global spatial organization of early development in the Drosophila embryo (Sharp, 1999).
During early Drosophila oogenesis, one cell from a cyst of
16 germ cells is selected to become the oocyte, and
accumulates oocyte-specific proteins and the centrosomes
from the other 15 cells. The microtubule
cytoskeleton and the centrosomes follow the same
stepwise restriction to one cell as other oocyte markers.
Surprisingly, the centrosomes still localize to one cell after
colcemid treatment, and in BicD and egl mutants, which
abolish the localization of all other oocyte markers and the
polarization of the microtubule cytoskeleton. In contrast,
the centrosomes fail to migrate in cysts mutant for Dynein heavy chain 64C, which disrupts the fusome. Thus, centrosome migration is independent of the organization of the microtubule cytoskeleton, and seems to depend instead
on the polarity of the fusome (Bolivar, 2001).
Since BicD and egl abolish the polarization of the MT
cytoskeleton in the cyst, the only candidate for a polarized
structure that could direct centrosome migration in these
mutants is the fusome. In fact, the cells that accumulate the centrosomes of egl and BicD mutant cysts possess the largest portion of the degenerating fusome. This observation demonstrates
that, as in wild type, the asymmetry established during
fusome morphogenesis persists until the stages when the
centrosomes migrate in BicD and egl cysts.
In order to test a direct role for the fusome in centrosome
movement, the behavior of the centrosomes was analyzed in a
mutant that affects the integrity of the fusome. Germline clones
of null alleles of Dynein heavy chain 64C divide correctly and
produce cysts that show a very similar phenotype to BicD and
egl. These mutant cysts contain 16 nurse cells in which oocyte
cytoplasmic markers do not accumulate in a single cell. In addition to this phenotype, a null allele of Dhc64C affects the integrity of the
fusome. Dhc64C mutant cysts possess a normal-looking
fusome in region 1 and early in region 2a. However, the fusome
of older region-2a cysts shows a fragmented appearance.
Interestingly, the centrosomes of these cysts fail to migrate to
a single cell, strongly suggesting that centrosome migration
requires an intact fusome (Bolivar, 2001).
Dhc64C is necessary for the localization of centrosomes and cytoplasmic markers to the oocyte. It was then
investigated if Dhc64C is also required for the restriction of
meiosis to the oocyte and the distribution of the
synaptonemal complex was analysed in Dhc64C germline clones. Like BicD mutants, Dhc64C is required for the formation of
the SC. Thus, lack of function of dynein blocks the three
asymmetries present in region-3 oocytes, suggesting that the
restriction of meiosis to the oocyte, the organization of a polarized
MT centered in this cell, and the migration of centrosomes to the
oocyte, depend upon the polarization of the fusome (Bolivar, 2001).
Since the MT cytoskeleton seems to be polarized toward the
oocyte prior to the migration of the centrosomes, this suggests
that most of the centrosomes of region-2 cysts might have
lost their MT nucleating properties. These post-mitotic
centrosomes thus would act differently from their region-1
counterparts, which retain the ability to grow microtubules, at
least during the mitotic divisions of the cyst. A test was performed to see whether the molecular composition of post-mitotic
centrosomes is different to mitotic ones. The distribution of Centrosomin (Cnn), a marker for the active centrosomes of mitotic cells, was examined. Cnn, like
gamma-tubulin, is present in region-1 centrosomes. In
contrast, Cnn is absent or barely detectable in region-2 and -3
cysts. This change in composition of centrosomes
depends upon the activity of egl, since in egl mutant cysts Cnn
reappears in post-mitotic region-2b centrosomes and by region 3 they possess a noticeable staining with the alpha-cnn
antibody. Although no explanation for this difference is available, it suggests that the correct determination of the oocyte among the cells
of the cyst affects the composition of the germline
centrosomes (Bolivar, 2001).
To understand the role of Bicaudal-D in the establisment of oocyte polarity, a screen was carried out for genetic enhancers of Bic-D mutation. This screen identified the mutation E415 as a dominant genetic enhancers of Bic-D.
The mutation E415 maps to a genetic locus that codes Drosophila Lissencephaly-1. Mutations in the human Lissencephaly-1 (Lis-1) gene cause Miller-Dieker syndrome. This syndrome is characterized by a
smoothened brain surface and disorganized cortical layers, resulting from
failed neuronal migration during brain development. Homologs of Lis-1
have also been identified in the filamentous fungus Aspergillus nidulans and in Saccharomyces cerevisiae, and both function with the microtubule minus-end-directed motor dynein/dynactin in nuclear migration (Swan, 1999).
Patterning of the Drosophila embryo depends on the correct localization
of patterning determinants within the oocyte, beginning in mid-oogenesis.
Starting in stage 8, specific mRNAs begin to accumulate either at the posterior
end of the oocyte or along the anterior cortex; this pattern is dependent
on Bic-D and microtubules. The distribution of anteriorly localized factors (orb, egl, nos and bcd mRNAs and Orb protein) and posteriorly localized factors (osk mRNA and Osk protein) were studied in E415 ovaries. All of these factors show disruptions in their normal subcellular localization within the oocyte. These effects on localization could be due to effects
on transport, anchoring or stability of transcripts in the mutant (Swan, 1999).
In stage 7 of oogenesis, the oocyte nucleus migrates from the posterior to the future
dorsal-anterior corner of the oocyte. In about half of stage-9 and stage-10 E415/E415 mutant egg chambers, the oocyte nucleus is positioned incorrectly. This could be due to a failure in nuclear migration or anchoring. Since the oocyte nucleus takes up almost the entire oocyte in stage-8 and earlier mutant egg chambers, it is not possible to determine whether nuclear positioning is affected before stage 9. Bic-D is also required for positioning of the oocyte nucleus2, and therefore E415 could function in nuclear positioning through the Bic-D-Egl complex. Consistent with this view, Bic-D and Egl proteins accumulate at high levels in a region between the oocyte cortex and nucleus: this localization is abolished in E415 mutants. Microtubules are also required for oocyte nuclear positioning; whereas microtubule organization in E415 homozygous mutants appears to be unaffected in early oogenesis, some alterations in microtubules are observed after stage 7. In wild-type oocytes, microtubules appear to be concentrated at the anterior cortex, apparently reflecting their nucleation from anterior sites. Microtubules still appear to be organized in this way in E415 mutants, but are often less focused (Swan, 1999).
DLis-1 shares with the dynein heavy chain gene, Dhc, a requirement
in oocyte determination, indicating that the dynein heavy chain may function like its fungal homologs in a pathway with dynein/dynactin. A specific allele of Dhc (Dhc6-6) dominantly suppresses the rough-eye phenotype produced by a mutation in the dynactin component Glued (Gl1 mutation) whereas a deficiency removing Dhc has no effect. This allele-specific interaction
is evidence that Gl and Dhc may act in the same pathway. Similarly,
Dhc6-6 behaves as a strong dominant suppressor of the DLis-1E415 homozygous phenotype, resulting in fertility, proper nuclear positioning and near normal oocyte growth. A deficiency of Dhc and the point mutation Dhc 6-12 have no effect on the DLis-1E415 phenotype. Therefore Dhc shows the same allele-specific interaction with DLis-1 as it does with Gl, indicating that DLis-1 may function in a genetic pathway with Dhc, and implicating dynein in nurse-cell-to-oocyte transport and nuclear positioning in the oocyte. Tests were also made for genetic interactions between Gl and DLis-1 and between Gl and Bic-D. The antimorphic Gl1 mutation confers lethality in a DLis-1E415 homozygote and in a Bic-DPA66/Df(2L)TW119 background, indicating
that both DLis-1 and Bic-D may function in the same essential
process as dynactin (Swan, 1999).
To determine how DLis-1 could interact with dynein/dynactin, the localizations of DLis-1 and Dhc proteins were studied in wild-type oocytes and
in oocytes with mutations in either gene. DLis-1 signal is concentrated along
the cortex of wild-type oocytes from as early as stage 5 of oogenesis. This localization is not detectable in DLis-1 E415/DLis-1E415 ovaries, indicating that the antiserum specifically recognizes DLis-1. To determine whether DLis-1 localization is dependent on dynein function,
DLis-1 distribution was studied in Dhc6-6/
Dhc6-12 mutants. This hypomorphic allelic combination
has no effect on the cortical accumulation of DLis-1 protein.
DLis-1 localization was mapped in egg chambers treated with the microtubule-destabilizing
drug colchicine. Under conditions that disrupt microtubules and dynein localization, cortical DLis-1 signal is still present, indicating that its cortical localization or maintenance does not depend on localized dynein or microtubules. In contrast, Dynein localization is dependent on DLis-1. In wild-type egg chambers, Dhc localizes to the presumptive oocyte in region 2 of the germarium and remains enriched in the oocyte throughout oogenesis. In DLis-1E415 mutants, this
specific accumulation is completely abolished. This strong effect of DLis-1E415 on Dhc localization contrasts with its subtle effect on the accumulation of other oocyte-specific factors, indicating that
DLis-1 may specifically regulate Dhc localization and that the localization
of most oocyte-specific factors to the oocyte is independent of DLis-1
and Dhc function. Thus there appear to be two distinct microtubule-dependent nurse-cell-to-oocyte transport mechanisms at work during oogenesis. One mechanism involving Bic-D, DLis-1 and Dhc is involved in bringing oocyte determinants into the presumptive oocyte, and, subsequently, is responsible for oocyte growth in stages 1-7 of oogenesis. A second microtubule-based transport
mechanism would function during these stages in the transport of oocyte-specific
mRNAs and proteins into the oocyte, possibly using other dyneins. In wild-type egg chambers at stages 7-9, Dhc accumulates along the
oocyte cortex and around the oocyte nucleus. Later in stage 9, Dhc accumulates mainly at the posterior and
anterior oocyte margins. These aspects of Dhc localization are disrupted
in DLis-1 mutants and, therefore, DLis-1 is necessary for most or
all aspects of dynein localization within the female germ line (Swan, 1999).
Lis-1 and Bic-D function in nuclear migration in neurons. Given the role of Lis-1 homologs in fungi, it has been suggested that the requirement for human Lis-1 in neuronal migration could also reflect a role in nuclear migration. In the developing cerebral cortex and cerebellum, migrating neurons project out a cytoplasmic extension towards their target, and then their nucleus translocates along this extension. An analogous process occurs during neural development in Drosophila. In the
third-instar eye imaginal disc, undifferentiated cells lie at the basal surface
and extend processes apically. As photoreceptor cells differentiate
posterior to the morphogenetic furrow, their nuclei translocate to the apical
surface of the eye disc. In serial confocal sections nuclei start
to appear ~4 µm below the apical surface and are no longer visible beyond
8 µm. In flies homozygous for a
pupal-lethal allele of DLis-1, nuclei also start to appear 4
µm below the apical surface, but many nuclei are also found more basally. This phenotype is identical to that of mutants for Glued,
suggesting that DLis-1 functions with dynein/dynactin in nuclear migration
in these neural cells. Given that DLis-1 is 70% identical to the human
Lis-1, these findings strongly support the possibility that the failure in
neuronal migration in Miller-Dieker syndrome also results from a failure in
dynein/dynactin-dependent nuclear migration (Swan, 1999).
As in the ovary, DLis-1 appears to function with Bic-D in nuclear
migration during eye development. Bic-D mutant eye discs also exhibit a severe
defect in nuclear positioning, with photoreceptor nuclei being found at all
levels basal to 4 µm and, frequently, in the axons that project basally from
these cells. The requirement for DLis-1 and Bic-D in nuclear positioning in the
developing eye imaginal disc could reflect a role for these genes in
microtubule organization. Microtubules in the eye disc extend longitudinally
along the apical-basal axis, but the polarity of these microtubules is not
known. To establish the orientation of these microtubules, third-instar eye imaginal discs were immunostained with antibodies to gamma-tubulin. In wild-type imaginal discs, gamma-tubulin
is found at high levels along the apical cortex and within this region in a
single strong focus about 2 µm below the apical cortex and 2 µm apical to
the photoreceptor nuclei. In
DLis-1K13209 mutants, gamma-tubulin still accumulates at the
apical surface, indicating that microtubules are orientated normally within
the mutant photoreceptors. However, the subapical focus of
gamma-tubulin is more diffuse and is undetectable in many
photoreceptors, indicating a requirement for DLis-1 in focusing
microtubule minus ends. A similar effect has been noted in Glued mutants, suggesting that DLis-1 and dynein/dynactin also function in the
same pathway in focusing microtubule minus ends in the eye. Interestingly,
whereas Bic-D mutants consistently exhibit a more marked defect in
nuclear positioning, localization of gamma-tubulin is normal in these
mutants (Swan, 1999).
Several models for nuclear localization have been advanced. This
analysis of DLis-1 allows for the proposal of a model for nuclear migration in
the Drosophila oocyte in which a cortical protein (DLis-1)
anchors microtubules via the minus-end-directed microtubule motor
dynein: DLis-1 appears to function at the cortex, and its localization to the
cortex is independent of microtubules and dynein. The dynein
heavy chain, Dhc, also associates with the oocyte cortex and this
localization requires DLis-1. Association of the nucleus with
these microtubules would then allow it to be anchored to the cell cortex.
The cortical localization of Dhc is also microtubule dependent,
and this could indicate that dynein uses microtubules to reach the cortex.
Bic-D-Egl could mediate the interaction between DLis-1 and dynein or the
interaction between microtubules and the oocyte nucleus. As well as
localizing to the oocyte cortex, Dhc also associates with the oocyte nucleus
throughout oogenesis, indicating that it may function as well in linking the
nucleus to microtubules (Swan, 1999).
The minus-end-directed motor cytoplasmic dynein has
been implicated in nuclear migration in several fungal
systems. In addition, genetic epistasis data suggest that
nudF, the Aspergillus homolog of Drosophila Lis1, acts upstream of
dynein in mediating nuclear migration. Lis1 egg chambers were examined to determine
whether the level or distribution of Dhc64C protein was
affected. In wild-type ovaries high levels of Dhc64C can be
detected in the oocyte from region 2 in the germarium
onward. Later, in stage 9 egg chambers
Dhc64C is enriched at the posterior of the oocyte and also
outlines the oocyte nucleus. In
Lis1 mutants Dhc64C localization appears unperturbed
through early oogenesis. However, in stage 9 mutant egg
chambers localization to the posterior of the oocyte is
strongly reduced, suggesting that Lis1 activity is required
for dynein localization or activity (Lei, 2000).
The dependence of dynein localization on Lis1 activity
suggests a functional interaction between these two genes.
A test was performed to see whether the Lis1 phenotype is
affected by mutations in the Gl and Dhc64C genes that
encode structural components of the dynein-dynactin microtubule
motor. The Gl1
allele causes a synthetic lethality with strong heteroallelic
combinations of Lis1 mutations, while in combinations
with weaker Lis1 alleles Gl1 causes a reduction in
viability. Adult escapers from the latter crosses
generally do not survive for more than a few days. More
interestingly, they display defects in eye morphology,
bristles, and abdominal tergites.
Flies carrying the dominant Gl1
allele have small, rough
eyes with irregularly positioned bristles. A reduction in Lis1
activity results in an enhancement of the Gl1
eye phenotype. In addition, the scutellar bristles of
escaper flies are reduced in size and frequently lost. Most adult escapers lack bristles on the ventral side of the abdomen, and in many animals abdominal
tergites are completely absent in one or more segments. Lis1 mutants show similar but
weaker interactions with Dhc64C alleles. A deficiency for
Dhc64C was viable in combination with Lis1
mutants. However, the adults display defects in scutellar
and abdominal bristle morphology resembling those seen in
DLis18.25.3/DLis111702
;Gl1/+ adults. Thus
the genetic interactions between Dhc64C, Gl and Lis1 are
consistent with a functional relationship between these
genes (Lei, 2000).
Asymmetric mRNA localization targets proteins to their cytoplasmic site of function. The
mechanism of apical localization of wingless and pair-rule transcripts in the Drosophila blastoderm embryo has been elucidated by
directly visualizing intermediates along the entire path of transcript movement. After release from their site of
transcription, mRNAs diffuse within the nucleus and are exported to all parts of the cytoplasm, regardless of
their cytoplasmic destinations. Endogenous and injected apical RNAs assemble selectively into cytoplasmic
particles that are transported apically along microtubules. Cytoplasmic dynein is required for correct localization
of endogenous transcripts and apical movement of injected RNA particles. It is proposed that dynein-dependent movement of RNA particles is a widely deployed mechanism for mRNA localization (Wilkie, 2001).
To study the mechanism of apical localization, whether actin and/or MTs are necessary for localization of injected mRNA was tested by preinjecting cytoskeletal inhibitors 10 min before injecting the RNA. It was found that preinjection of Cytochalasin B, at concentrations that disrupt the organization of actin filaments, has no affect on Runt mRNA localization. However, a similar disruption of nuclear position has been observed in the cortical cytoplasm. In contrast, preinjection of colcemid, which destabilizes blastoderm MTs, disrupts runt, wingless, and fushi tarazu RNA localization almost entirely. It is concluded that an intact MT cytoskeleton is required for apical localization of injected RNA and that actin does not play a major role in the process. However, some minor role for actin in apical localization of RNA cannot be excluded (Wilkie, 2001).
Whether the localization of injected RNA occurs by minus end directed MT-dependent motor movement was tested by preinjecting embryos with antibodies against Drosophila cytoplasmic dynein heavy chain (dhc). Two independently raised monoclonal antibodies against dhc are each sufficient to inhibit RUN, FTZ, and WG mRNA apical localization in most, or all, embryos. Either one, the anti-dynein antibody or the colcemid injections, is sufficient to cause apical RNA to partly diffuse away from the site of injection in a similar manner to embryos injected with HB RNA alone. Injected apical RNA does not diffuse in the absence of anti-dynein antibodies or Colcemid preinjections. These results suggest that apical RNA is tethered to MTs by dynein and that dynein is required for the transport of RNA particles (Wilkie, 2001).
To further test the involvement of cytoplasmic dynein in apical transcript localization, RNA was injected into mutant cytoplasmic dynein heavy chain (Dhc64C) embryos. A marked reduction was found in the speed of movement of injected apical targeted RNAs in dynein mutants. Cytoplasmic dynein is essential for many cellular processes, so strong mutations in Dhc64C are homozygous lethal in Drosophila and cannot be studied at the blastoderm stage. Instead hypomorphic allelic combinations of Dhc64C, which are viable in trans due to intragenic complementation, were used. In two different allelic combinations of Dhc64C, injected RNA particles move at speeds 60% to 70% slower than they do in wild-type. Staining Dhc64C mutant embryos with anti-tubulin antibodies showsthat MT distribution is indistinguishable from wild-type, indicating that the reduced speed of localization is not due indirectly to a disruption of the MTs. Instead, the reduction in speed is likely to show a direct requirement for dynein in particle transport (Wilkie, 2001).
To test whether cytoplasmic dynein is also required for apical localization of endogenous transcripts, the effects of Dhc64C hypomorphic mutants and anti-dhc antibodies on the apical localization of endogenous FTZ transcripts was tested by in situ hybridization. As expected, hypomorphic Dhc64C mutants show no detectable effects on FTZ apical mRNA localization since injected RNA localizes correctly, but more slowly. In contrast, injection of anti-dhc antibody disrupts endogenous FTZ localization, leading to unlocalized stripes of ftz mRNA 20–30 min after injection. Given that FTZ mRNA has a half-life of 6 min in the blastoderm, the FTZ transcripts observed are likely to have been synthesized after the injection. It is concluded that endogenous apical mRNA localization is also dynein dependent (Wilkie, 2001).
Dynactin is a protein complex that is involved in coordinating the activities of cytoplasmic dynein, and is thought to be required for most forms of dynein-based transport. To test whether dynactin is also required for apical RNA localization, a large excess of p50/dynamitin is preinjected into embryos 10 min before injecting apically targeted RNA. p50/dynamitin causes a significant reduction in the speed of RNA particle movement. p50/dynamitin is a subunit of dynactin whose overexpression is a widely used method of disrupting the dynactin complex and demonstrating conclusively dynein-dependent motility. Dynactin is required for some cargo binding and for dynein processivity. It is concluded that apical transcript localization in the blastoderm embryo occurs by cytoplasmic dynein- and dynactin-mediated transport along MTs toward their minus ends (Wilkie, 2001).
It is thought that export and localization of apical mRNA in the blastoderm embryo can be divided into six distinct steps. (1) During or after completion of transcription and processing, transcripts are assembled into particles, which contain various hnRNPs and export factors, some of which may form part of the cytoplasmic localization machinery. (2) mRNA particles diffuse freely after release from the site of transcription and processing until they reach nuclear pore complexes (NPCs) on the nuclear periphery. (3) mRNA particles are exported through NPCs in all parts of the nuclear envelope. (4) The composition of the particles probably changes during export from the nucleus and in the cytoplasm to recruit dynein, dynactin, and associated proteins. (5) Particles attach to MTs and are actively transported to the apical cytoplasm. (6) Particle movement arrests in the apical cytoplasm, where they may associate with other particles and become anchored (Wilkie, 2001).
The first three steps of apical localization are thought to be common to most mRNAs, because they are essential universal processes in eukaryotic cells. However, the last three steps of the localization pathway are likely to vary among different kinds of transcripts, since the key determinant in sorting different mRNAs to their correct cytoplasmic destinations is presumably RNP particle composition in the cytoplasm. It is possible that some components required for cytoplasmic sorting are preassembled in the nucleus, as suggested by studies showing that the localization of injected FTZ mRNA depends on preincubation with the hnRNPA1 protein Squid. Indeed, a requirement for hnRNPs has also been shown for GRK mRNA localization in the oocyte, for myelin basic protein mRNA in rat oligodendrocytes, and for Vg1 transcripts in Xenopus oocytes. However, the data in this study show that injected protein-free apical RNA assembles in the cytoplasm into particles that localize correctly, arguing that all the factors needed to assemble competent localization particles can also be recruited in the cytoplasm (Wilkie, 2001).
The hypothesis was tested that amyloid precursor protein (APP) and its relatives function as vesicular receptor proteins for kinesin-I. Deletion of the Drosophila APP-like gene (Appl) or overexpression of human APP695 (an alternatively spliced version of APP) or APPL constructs causes axonal transport phenotypes similar to kinesin and dynein mutants. Genetic reduction of kinesin-I expression enhances while genetic reduction of dynein expression suppresses these phenotypes. Deletion of the C terminus of APP695 or APPL, including the kinesin binding region, disrupts axonal transport of APP695 and APPL and abolishes the organelle accumulation phenotype. Neuronal apoptosis was induced only by overexpression of constructs containing both the C-terminal and Ab regions of APP695. The possibility is discussed that axonal transport disruption may play a role in the neurodegenerative pathology of Alzheimer's disease (Gunawardena, 2001).
Although reducing the amount of kinesin-I to 50% of normal by deleting one of two copies of either the klc or khc gene ordinarily has no significant phenotype, such a reduction in an animal overexpressing APP proteins that contain the cytoplasmic C terminus is predicted to significantly enhance the axonal blockage phenotype. This behavior is expected, because if kinesin-I becomes limiting by virtue of binding excess APP C termini, then further reduction of kinesin-I by deleting one gene copy should dramatically enhance the axonal transport phenotype. To test this prediction, larvae were generated that overexpressed APPL or APP695 and that were also heterozygous for a null mutation, khc8, so that kinesin-I was reduced to 50% of normal. Although larvae overexpressing APPL or APP by themselves or reduced in KHC dosage alone have no striking organismal phenotype, larvae combining these two features exhibit a dramatic new neuromuscular phenotype. These larvae flip their tail and head upwards during crawling, rocking back and forth as they struggled to crawl. Their neurons also contained an enhanced number of organelle accumulations. The extent of accumulations in larvae expressing wild-type APP695 and APPL in the context of reduced KHC dosage was comparable to homozygotes for kinesin-I or dynein mutants and was similarly lethal. Quantitative analysis has revealed a statistically significant difference between siblings with a normal and reduced dose of KHC (Gunawardena, 2001).
To confirm the specificity of the genetic interactions observed between reduction in KHC and overexpression of APP695 or APPL, larvae heterozygous for a null mutation of klc [Df(3L)8ex94, which removes the entire kinesin light chain gene] were generated in combination with constructs expressing APPL and APP695. Neurons from these larvae contain an increased number of organelle jams relative to larvae with a normal dose of KLC. Quantitative analysis reveals a statistically significant difference between siblings containing a normal and reduced dose of KLC, although the extent of enhancement is not as dramatic as when the dosage of KHC is reduced. Although these larvae do not show a larval neuromuscular phenotype as dramatic as was observed when the dosage of KHC was reduced, these larvae show a clear posterior paralysis phenotype. This result is again consistent with a direct functional interaction of the C-terminal region of APP695 and APPL with kinesin-I (Gunawardena, 2001).
Although reducing the amount of cytoplasmic dynein to 50% of normal by deleting one of two copies of either dynein heavy chain (dhc) or dynein light chain (dlc) genes ordinarily has no significant phenotype, such a reduction in an animal overexpressing APP family members that contain the cytoplasmic C terminus is predicted to suppress significantly the severity of the axonal accumulation phenotype. The basis for this prediction is that dynein drives retrograde axonal transport, which is antagonistic to kinesin-I-mediated anterograde axonal transport. In addition, many vesicles or organelles that exhibit net anterograde movement experience periods of retrograde movement owing to the simultaneous presence of kinesins and dyneins on the same vesicle or organelle. Thus, vesicle stalling and axonal accumulations induced by APP are predicted to be ameliorated by dynein reduction by (1) reducing the rate at which vesicles and organelles moved by dynein are transported into regions that have stalled or accumulated vesicles caused by APP expression; or (2) reducing the contribution of dynein-driven movement to a vesicle experiencing stalling because of reduced kinesin-driven activity; this reduction should attenuate vesicle stalling by restoring the balance of movements toward normal (Gunawardena, 2001).
To test this prediction, larvae overexpressing APP695 or APPL were generated that were also heterozygous for either a deficiency of either dhc [Df(3L)GN24] or dlc (roblk). Reduction of dynein suppresses the extent of organelle accumulations in APP695 or APPL transgenic lines. In addition, no significant larval crawling phenotype was observed in these animals. Surprisingly, reduction in dynein dosage also rescues the inviability of males overexpressing APP695. No effect is seen in the lines expressing a C-terminal deletion of APP695 or APPL. Thus, reduction in dosage of a retrograde motor protein appears to be sufficient to decrease organelle accumulations induced by APP or APPL expression (Gunawardena, 2001).
Whether larvae bearing heterozygous deletions of dynein components and the Appl gene in combination would exhibit abnormal axonal transport was investigated. In contrast to the situation with kinesin and APPL, female larvae with one copy (50% dose) of Appl and one copy of dhc or dlc do not have typical axonal accumulations. Intriguingly, male larvae bearing a deletion of the Appl gene, and hence lacking all APPL function, in combination with one copy of dhc or dlc also lack axonal accumulations. Thus, reduction in dynein levels appears to suppress axonal accumulations induced by loss of APPL (Gunawardena, 2001).
Genetic analysis in Drosophila strongly supports the hypothesis that mammalian APP and its homolog, APPL, have kinesin-I receptor functions in vivo. The genetic data and tests complement the in vitro biochemical evidence for a kinesin-I receptor function for APP. In these experiments, APP has been shown to form a complex with conventional kinesin by directly binding to KLC. Transport of APP depends upon kinesin-I and KLC in particular. In addition, the finding that, in Drosophila, APP695 can enter and be transported down axons to neuromuscular junctions and that this transport depends upon the cytoplasmic C terminus containing the proposed KLC binding region supports this view. In toto, these data strongly support the hypothesis that APP functions as a kinesin-I receptor. Perhaps APP bound to kinesin-I may be required for the axonal transport of a subset of cargoes, such as vesicles containing signaling or other molecules used at the synapse. Identifying these vesicles and their cargoes is an important next step (Gunawardena, 2001).
A surprising finding is the suppression of APP and APPL-induced organelle accumulations by genetic reduction of cytoplasmic dynein. Although further work is needed to define the mechanism of this suppression, one simple explanation comes from previous observations about the functionally antagonistic relationship of dynein and kinesin. A general observation is that kinesin and dynein are both present on many of the same axonal vesicles and organelles. Such vesicles and organelles often exhibit alternating anterograde (kinesin) and retrograde (dynein) movements, with net anterograde or retrograde movement resulting from a regulated bias in the balance of opposing movements along the microtubule. Thus, reduction of kinesin-I on non-APP vesicles caused by binding of kinesin-I to excess APP might cause vesicle stalling and organelle accumulations. Stalling of these vesicles and subsequent phenotypes might be rescued by reducing the antagonistic component of movement produced by dynein (Gunawardena, 2001).
After its specification, the Drosophila oocyte undergoes a critical polarization event that involves a reorganization of the microtubules (MT) and relocalization of the determinant Orb within the oocyte. This polarization requires Par-1 kinase and the PDZ-containing Par-3 homolog, Bazooka (Baz). Par-1 has been observed on the fusome, which degenerates before the onset of oocyte polarization. How Par-1 acts to polarize the oocyte has been unclear. Par-1 is shown to become restricted to the oocyte in a MT-dependent fashion after disappearance of the fusome. At the time of polarization, the kinase itself and the determinant BicaudalD (BicD) are relocalized from the anterior to the posterior of the oocyte. Par-1 and BicD are interdependent and require MT and the minus end-directed motor Dynein for their relocalization. baz is required for Par-1 relocalization within the oocyte and the distributions of Baz and Par-1 in the Drosophila oocyte are complementary and strikingly reminiscent of the two PAR proteins in the C. elegans embryo. It is proposed that, through the combined actions of the fusome, MT, and Baz, Par-1 is selectively enriched and localized within the oocyte, where, in conjunction with BicD, Egalitarian (Egl), and Dynein, it acts on the MT cytoskeleton to effect polarization (Vaccari, 2002).
During oocyte specification, localization of the determinants BicD, Egl, and Orb to the early oocyte relies on the asymmetric distribution of microtubules in the cyst, evident as a dense focus of MT in the oocyte. Depolymerization of the MT by colchicine abolishes the localization of BicD, Egl, and Orb and results in egg chambers with 16 nurse cells and no oocyte. It was therefore asked if the restriction of Par-1 to the oocyte during the transition from region 2a to region 2b is also MT dependent. Ovaries of flies fed with colchicine for 12 hr fail to localize Par-1 and Orb to the oocyte, indicating that Par-1 restriction to the oocyte is indeed MT dependent. This is in contrast to the localization of Par-1 to the fusome, which occurs independently of MT (Vaccari, 2002).
The distribution of Par-1 within the oocyte was further examined by focusing on the transition between regions 2b and 3, when par-1-dependent polarization of the oocyte occurs. In germarial region 2b, Par-1 is enriched anterior to the oocyte nucleus. In region 3, the protein is mainly detected at the posterior of the oocyte, where it remains. During this relocalization, Par-1 colocalizes completely with BicD in the germline. Because par-1 is required for BicD relocalization within the oocyte, the distribution of Par-1 was examined in BicD hypomorphs that allow differentiation of an oocyte. Par-1 is detected but mislocalized in an anterior dot within the BicD mutant oocytes. Hence, Par-1 and BicD are interdependent for their relocalization to the posterior of the oocyte region 3b (Vaccari, 2002).
To assess whether the MT cytoskeleton mediates relocalization of Par-1 and BicD within the oocyte, wild-type ovaries were dissected a short time after treatment with colchicine. A screen was carried out for region 3 egg chambers in which the focus of oocyte MT was destroyed. In these, BicD and Par-1 remain anterior to the oocyte nucleus, indicating that MTs are required for oocyte polarization (Vaccari, 2002).
The MT motor Dynein has been reported to influence development of the germline cyst. Loss-of-function mutants in dhc64C, encoding the heavy chain of the minus end-directed molecular motor Dynein, fail to develop an egg chamber because of mitotic failure in the germarium. However, hypomorphic dhc64C mutants develop an oocyte and 15 nurse cells. In a high percentage of such egg chambers, both Par-1 and BicD remain at the anterior of the oocyte in region 3. Hence, after its initial requirement in cyst formation, the minus end-directed motor Dynein is involved in the relocalization of Par-1 and BicD to the posterior of the oocyte (Vaccari, 2002).
Thus, impairment of the MT cytoskeleton and mutations in BicD and dhc64C affect Par-1 relocalization within the oocyte. Conversely, in par-1 mutants, the MT cytoskeleton is not focused in the oocyte, BicD fails to relocalize, and Dynein is not enriched in the oocyte. The mutual interdependence of these genes and the MT suggests that all these components cooperate to form a polarization complex in the oocyte. Interestingly, the N1 antibody begins to detect Par-1 only when its function is genetically required, suggesting that, in region 2, the kinase may undergo a change in conformation or in its association with other factors (Vaccari, 2002).
The presence and localization of Par-1 in the oocyte at the time of its determination and polarization complements the previously reported localization of Par-1 on the fusome prior to oocyte determination and establishes Par-1 as a unique oocyte marker, for at least two reasons. (1) Absence of any one of the oocyte determinants, BicD, Egl, or Orb, prevents the concentration of the two other determinants in this cell. In contrast, in the absence of Par-1, it is the relocalization of the determinants within the oocyte that is specifically affected. (2) BicD, Egl, and Orb are not present on the fusome, and the observed enrichment of these determining factors in the oocyte is the result of the enrichment of their RNAs in this cell during its specification. In contrast, no par-1 RNA is detected in the germline at such early stages. The idea that Par-1 is initially loaded on the fusome, where it perdures during the cyst divisions, and that it is later preferentially inherited by the oocyte, is favored. Taken together, the facts that par-1 mutants show no fusomal defects and that accumulation of Par-1 itself in the oocyte requires MT suggest that Par-1 does not affect the oocyte MT cytoskeleton from its fusomal location. It is proposed that, through the combined actions of the fusome, MT, and Baz, Par-1 is selectively enriched and localized within the oocyte, where it acts in conjunction with BicD, Egl, and Dynein to effect polarization (Vaccari, 2002).
In many organisms, germ cells are segregated from the soma through the inheritance of the specialized germ plasm, which contains mRNAs and proteins that specify germ cell fate and promote germline development. Whereas germ plasm assembly has been well characterized, mechanisms mediating germ plasm inheritance are poorly understood. In the Drosophila embryo, germ plasm is anchored to the posterior cortex, and nuclei that migrate into this region give rise to the germ cell progenitors, or pole cells. How the germ plasm interacts with these nuclei for pole cell induction and is selectively incorporated into the forming pole cells is not known. Live imaging of two conserved germ plasm components, nanos mRNA and Vasa protein, revealed that germ plasm segregation is a dynamic process involving active transport of germ plasm RNA-protein complexes coordinated with nuclear migration (see graphical abstract). Centrosomes accompanying posterior nuclei induce release of germ plasm from the cortex and recruit these components by dynein-dependent transport on centrosome-nucleated microtubules. As nuclei divide, continued transport on astral microtubules partitions germ plasm to daughter nuclei, leading to its segregation into pole cells. Disruption of these transport events prevents incorporation of germ plasm into pole cells and impairs germ cell development. These results indicate that active transport of germ plasm is essential for its inheritance and ensures the production of a discrete population of germ cell progenitors endowed with requisite factors for germline development. Transport on astral microtubules may provide a general mechanism for the segregation of cell fate determinants (Lerit, 2011).
This study has uncovered a dynamic mechanism for germ plasm inheritance involving release of germ plasm RNPs from the posterior cortical actin anchor coordinated with their dynein-dependent transport to centrosomes that are associated with posterior nuclei. Transport of these RNPs occurs primarily, if not exclusively, on astral microtubules throughout the mitotic cycle. The results suggest that directed transport of germ plasm components during pole bud formation ensures the production of a discrete population of germ cell progenitors and partitions factors required for germline development during subsequent divisions. Through this process, germline fate determinants are segregated away from somatic nuclei (Lerit, 2011).
Pole cell formation is highly sensitive to the dosage of germ plasm components, because mutations that reduce the accumulation of germ plasm at the posterior pole result in fewer pole cells. This study found that pole cell formation is similarly reduced when germ plasm transport is disrupted, as it is in Dhc mutants or in mutants that affect centrosome function. Although the molecular mechanism by which germ plasm promotes pole cell formation is unknown, the results suggest that directed transport of germ plasm components toward the small subset of nuclei that are the first to arrive at the posterior pole provides the requisite concentration of one or more factors necessary to impart germline fate and induce pole cell formation. In addition, because the first divisions of the nascent pole cells occur before budding is complete, the persistence of germ plasm transport toward centrosomes during these divisions would ensure that factors required for germline development, such as nos, are maintained within pole buds, segregated to daughter nuclei, and ultimately incorporated into the forming germ cells (Lerit, 2011).
Germ plasm produced ectopically in osk-bcd3′UTR (transgene used to generate embryos with germ plasm containing nos*GFP localized ectopically at the anterior of the embryo in addition to its normal localization at the posterior) and Khc mutant embryos is transported to nearby nuclei, indicating that nuclei are not predetermined to recruit germ plasm. Thus, the release of germ plasm from its actin-based anchor and the onset of germ plasm motility must be tightly coordinated with the arrival of nuclei at the posterior cortex to target germ plasm specifically to these nuclei and prevent the misspecification of cell fate. Egg activation triggers the release of bcd mRNA from the anterior cortex, probably through a generalized activation-dependent restructuring of the cortical actin cytoskeleton. This event does not release nos and Vas, however. Nor is germ plasm release scheduled by an intrinsic timing mechanism, as was shown in this study. Consistent with the observation that nos release is delayed at the anterior in osk-bcd3′UTR embryos, formation of ectopic germ cells at the anterior lags behind pole cell formation at the posterior in these animals. Moreover, centrosomes isolated from nuclei, either pharmacologically or genetically, are sufficient to trigger germ plasm release from the posterior. These data thus support a model whereby centrosomes and/or centrosome-nucleated microtubules associated with migrating nuclei trigger germ plasm release from the cortical anchor (Lerit, 2011).
Astral microtubules provide the tracks along which germ plasm RNPs travel upon their initial release from the cortex. During mitosis in the syncytial embryo, astral microtubules appear to secure the partitioning of germ plasm RNPs to daughter nuclei. The preferential association with astral microtubules may also prevent the dilution of inductive signals during asymmetric division events, when only one aster is proximal to the germ plasm. The apparent specificity for astral microtubules suggests that the RNP-motor complexes may include factors that recognize particular microtubule-associated proteins or modifications that distinguish these microtubules as preferred tracks (Lerit, 2011).
The observed dynein-dependent transport of nos during pole cell formation contrasts with its diffusion-based mode of localization during oogenesis. Given that dynein-dependent transport of bcd mRNA to the oocyte anterior is ongoing during late oogenesis, it is essential that nos be excluded from interaction with the dynein transport machinery. nos may reside in a dynein-associated transport complex that is inactive or incompatible with the various oocyte microtubule subpopulations. Alternatively, the composition of the nos RNP in the oocyte may simply preclude its association with the dynein motor complex. The observed cotransport of nos and Vas in the embryo suggests that nos becomes linked to dynein through its packaging into a complex with Vas and other germ plasm components. Whether germ plasm RNPs are coupled to dynein motors while they are anchored at the posterior or only after their release remains a subject for future investigation. A similar switch between motor-independent and motor-dependent modes of germ plasm mRNA translocation may occur in Xenopus, although the role of motors in Xenopus germ plasm inheritance is not yet clear (Lerit, 2011).
Recent in situ hybridization studies have now identified over 50 mRNAs that are localized at the posterior of the Drosophila embryo and incorporated into pole cells. Further characterization of a subset of these mRNAs showed that they accumulate near posterior nuclei, suggesting that they may be transported similarly to nos. Determining whether the different transcripts are cotransported will require the development of methods to simultaneously visualize multiple RNAs and germ plasm proteins. However, packaging of even subsets of RNAs together into germ plasm RNPs competent for dynein-mediated transport would greatly simplify the problem of partitioning a complex pool of transcripts to pole cells (Lerit, 2011).
During the formation of the metaphase spindle in animal somatic cells, kinetochore microtubule bundles (K fibers) are often disconnected from centrosomes, because they are released from centrosomes or directly generated from chromosomes. To create the tightly focused, diamond-shaped appearance of the bipolar spindle, K fibers need to be interconnected with centrosomal microtubules (C-MTs) by minus end-directed motor proteins. This study characterized the roles of two minus end-directed motors, dynein and Ncd, in such processes in Drosophila S2 cells using RNA interference and high resolution microscopy. Even though these two motors have overlapping functions, Ncd is primarily responsible for focusing K fibers, whereas dynein has a dominant function in transporting K fibers to the centrosomes. A novel localization of Ncd to the growing tips of C-MTs is reported, that is shown is mediated by the plus end-tracking protein, EB1. Computer modeling of the K fiber focusing process suggests that the plus end localization of Ncd could facilitate the capture and transport of K fibers along C-MTs. From these results and simulations, a model is proposed on how two minus end-directed motors cooperate to ensure spindle pole coalescence during mitosis (Goshima, 2005; full text of article).
EB1 is a highly conserved microtubule plus end-tracking protein that binds various cargo proteins (e.g., APC [adenomatous polyposis coli protein]). Ncd also was recently observed to bind to an EB1 affinity column. It was therefore of interest investigate whether the plus end accumulation of Ncd-GFP is mediated by EB1. After EB1 depletion by RNAi in Ncd-GFP-NES cell line, no plus end accumulation of Ncd-GFP-NES was seen; instead the microtubules were evenly labeled with this protein. After EB1 RNAi, microtubules become less dynamic and frequently enter a pause state where they exhibit minimal growth or shrinkage. However, even the subset of growing microtubule never accumulated Ncd-GFP-NES at their tips (Goshima, 2005).
Next, an in vitro interaction between purified Ncd and EB1 was tested using a GST pull-down assay. It was found that the nonmotor 'tail' domain (aa 1-290) of Ncd can bind directly to the COOH terminus domain of EB1 (EB1-C; aa 208-278), albeit weakly. This binding was competed by addition of a fragment of human APC protein (2744-2843 aa) that binds to EB1's COOH-terminal domain, suggesting that the Ncd tail and APC bind to the same site on EB1. However, expression of the Ncd tail domain (1-290 aa) fused to GFP did not track along microtubule plus ends in vivo, suggesting that the motor domain may augment affinity for the microtubules and thereby aid plus end localization (Goshima, 2005).
No specific mutagenesis strategy was availabe for eliminating plus end tracking of Ncd while retaining its other critical mitotic activities such as microtubule cross-bridging. However, the consequences were tested of pole focusing and Ncd-GFP localization in mitosis after RNAi of EB1. Time-lapse imaging of mitotic EB1 RNAi cells showed no plus end microtubule enrichment, as expected from the observed interphase results. However, Ncd-GFP still strongly and dynamically (revealed by FRAP) localized to spindle MTs, indicating that K fiber binding does not require EB1. In this setting of EB1 RNAi where Ncd was mislocalized from microtubule plus ends but not the spindle, centrosome detachment and K fiber focusing was examined. Qualitative study of EB1 RNAi found both centrosome detachment and pole defocusing. When these phenotypes were examined quantitatively, it was found that the EB1 phenotype consists of pronounced K fiber defocusing and has less pronounced centrosome detachment, which is more similar to the Ncd RNAi than the dynein RNAi phenotype. Although not definitive proof because EB1 RNAi causes microtubule dynamics defects in addition to displacing Ncd from the microtubule plus end, this result is consistent with a functional link between EB1-dependent localization of Ncd to microtubule tips and K fiber coalescence (Goshima, 2005).
Based on live cell imaging and computer simulation analyses, it is proposed that the coalescence of spindle poles involves the following steps: (1) inter-K fiber cross-linking, (2) 'search and capture' of K fibers by the tip of growing C-MTs, and (3) K fiber transport on C-MTs. RNAi analysis of dynein and Ncd as well as live cell imaging of Ncd-GFP has provided insight into the roles of these minus end-directed motor proteins in these processes. By quantitatively comparing Ncd and dynein knockdown phenotypes in the same cell type, it was found that these Ncd and dynein have distinct but overlapping functions in the three steps of pole focusing (Goshima, 2005).
RNAi results suggest that Ncd plays a role in K fiber focusing through three mechanisms described above. Ncd's major role is likely to be in inter-K fiber cross-linking, as evidenced by the splaying of K fibers after Ncd RNAi and the prominent localization of Ncd-GFP to K fibers. This process most likely involves cross-linking of microtubules by Ncd's force generating motor domain and its positively charged 'tail' domain that also binds to microtubules independently of the motor. This process occurs in the absence of centrosomes and C-MTs, because acentrosomal spindles created by centrosomin RNAi also show severe K fiber unfocusing when Ncd is also knocked down by RNAi. It is also showm that the process of lateral K fiber interactions is highly dynamic, because K fibers are continually splaying and coalescing. Such observations are also consistent with FRAP measurements of Ncd-GFP, which show that these motors are associating and dissociating with K fibers on a rapid time scale and thus are not behaving as static cross-linkers. Albeit less efficient than dynein, Ncd also likely contributes to minus end-directed transport of K fibers along C-MTs, because the centrosome to K fiber distance is somewhat greater in the Ncd/Dhc64C double RNAi compared with Dhc64C alone. Finally, it is believed that Ncd at the tips of C-MTs may act to capture K fibers and facilitate subsequent minus end transport of the K fiber (Goshima, 2005).
Cytoplasmic dynein in S2 cells plays a dominant role in transporting K fibers along microtubules, as evidenced by finding that Dhc64C RNAi causes detachment of centrosomes from the minus ends of K fibers. Although secondary to Ncd, dynein also contributes to the focusing of the minus ends of K fibers. This role of dynein becomes particularly clear after Ncd depletion, since Ncd/Dhc64c double RNAi causes very severe splaying of K fibers. It is believed that this K fiber focusing effect also primarily involves dynein's role as a transporter of K fiber bundles along C-MTs, which causes the coalescence of most peripheral K fibers toward the centrosome as shown in computer simulations. However, dynein may have other roles in K fiber coalescence, such as potentially transporting and concentrating cross-linking proteins at minus ends of K fibers. Indeed, synthetic effects of kinesin-14 and dynein motors on pole focusing have been reported in centrosome-free spindles reconstituted in Xenopus egg extract (Goshima, 2005).
The molecular properties of kinesin-14/Ncd and cytoplasmic dynein are well designed to support the above proposed functions of these two motors in pole focusing. Cytoplasmic dynein is a fast, processive motor. Thus, small numbers of dyneins could rapidly transport K fibers along C-MTs. In contrast, Ncd is nonprocessive, slow motor that is not designed for cargo transport. Instead, its ability to bind two microtubules and its slow motor activity makes it an effective cross-bridger between microtubules in the spindle. These properties are likely to be advantageous for inter-K fiber cross-linking, as well as for crossbridging of C-MTs plus ends to K fiber. However, the rapid on-off rates of these cross-bridges, as shown by FRAP data, would still enable dynein to effectively transport the K fibers along the C-MTs (Goshima, 2005).
Phenotypic RNAi analyses may account for differences in the pole unfocusing phenotypes of Ncd or dynein depletions that have been described in the literature. Specifically, spindle architecture in the given system (e.g., the presence or absence of centrosome and differences in microtubule dynamics) may determine whether Ncd or dynein acts as the essential contributor to pole coalescence. For example, Ncd/kinesin-14 function, is particularly important for K fiber focusing, may become more crucial when centrosomes are detached or absent from the spindle. Consistent with this idea, the most dramatic pole unfocusing phenotypes for kinesin-14 mutations/depletions have been described in plant mitosis and animal meiosis, systems in which spindle assembly occurs through a centrosome-independent mechanism and in which interactions between C-MTs and K fibers are simply absent. However, dynein also is likely to play crucial roles in pole coalescence in some acentrosomal spindles, as shown convincingly in Xenopus extract system. In contrast, somatic animal cell mitosis utilizes centrosomes, and kinesin-14 is less important for pole focusing in such cells (e.g., Ncd is nonessential in fly development). Microtubule dynamics, specifically the relative number of K- to C-MTs in the bipolar spindle, also may alter the relative contribution of the two motors. For example, if C-MTs are very abundant, the high probability of close approximation of K- and C-MTs may enable dynein to easily link these two networks without any assistance from kinesin-14 motors at microtubule plus ends (Goshima, 2005).
An unexpected finding of this study is the microtubule plus end tracking of Ncd. Localization of a kinesin-14 motor protein to the plus end of interphase microtubules has been recently reported in plants. The accumulation of kinesin-14 at the plus end overlap zone in mitotic spindle has been shown but whether this localization reflects localization of plus ends of individual microtubules is not known. Nevertheless, this work does suggest that the plus end tracking in mitotic microtubules might be a broadly conserved feature of kinesin-14 motors. Yeast Kar3p also was shown to accumulate at the plus ends of microtubules at the shmoo, but it is more enriched on the depolymerizing microtubules, which is not observed for Ncd. Even though it was found that C-MTs are still dynamic after Ncd RNAi, it is possible that Ncd at the plus end also modulates microtubule dynamics, as does EB1 or other tip-localized proteins (Goshima, 2005).
The enhanced K fiber unfocusing in EB1 RNAi-treated cells, which displaces Ncd from plus ends but not K fibers, suggests that plus end tracking of Ncd may serve a function in pole focusing. It is proposed that plus end tracking of Ncd on newly nucleated C-MTs, as a 'capture factor,' facilitates their connection to K fibers, possibly using its second microtubule binding site located in its NH2-terminal tail domain. This idea is analogous to a 'search and capture' model for how C-MTs find chromosomes. In this case, the tip-localized motor Ncd enables C-MTs to 'search' for and then 'capture' a second major microtubule network in the spindle, the K fibers. Ncd may generate a connection between K fiber and C-MTs temporary, and thereby facilitate the recruitment of minus end-directed transporter (primarily dynein but Ncd contributing as well) for the transport of K fibers. Additionally, Ncd at the plus end may act as a K fiber transporter once it binds, although this transport would be less efficient than that produced by fast and processive dynein motors. It is also noted that the simulations are two-dimensional and encounters between C-MTs and K fibers would become less likely in three dimensions, and one might expect the effect of a C-MT-mediated capture/transport mechanism to become more important under such circumstances (Goshima, 2005).
The microtubule plus end search-and-capture mechanism might apply to other aspects of metazoan cell division. For example, cross-linking interactions between antiparallel microtubules occurs at overlap zone of microtubules, and genetic study demonstrates that Ncd produces an inward force on antiparallel microtubules during early mitosis. Ncd at the tips of growing microtubules may act to capture microtubules that arise from the opposite pole. Another possible target of tip-localized Ncd may be free microtubules, which are either released from centrosomes or generated de novo in cytoplasm and are eventually incorporated into the spindle by a dynein-dependent transport process (Goshima, 2005).
Many membrane proteins, including Drosophila Dscam, are enriched in dendrites or axons within neurons. However, little is known about how the differential distribution is established and maintained. Dscam isoforms carrying exon 17.1 (Dscam[TM1]) are largely restricted to dendrites, while Dscam isoforms with exon 17.2 (Dscam[TM2]) are enriched in axons. This study investigated the mechanisms underlying the dendritic targeting of Dscam[TM1]. Through forward genetic mosaic screens and by silencing specific genes via targeted RNAi, it was found that several genes, encoding various components of the dynein-dynactin complex, are required for restricting Dscam[TM1] to the mushroom body dendrites. In contrast, compromising dynein/dynactin function did not affect dendritic targeting of two other dendritic markers, Nod and Rdl. Tracing newly synthesized Dscam[TM1] further revealed that compromising dynein/dynactin function did not affect the initial dendritic targeting of Dscam[TM1], but disrupted the maintenance of its restriction to dendrites. The results of this study suggest multiple mechanisms of dendritic protein targeting. Notably, dynein-dynactin plays a role in excluding dendritic Dscam, but not Rdl, from axons by retrograde transport (Yang, 2008).
Multiple lines of evidence indicate that the dynein/dynactin complex has an important function in maintaining proper distribution of dendritic Dscam in MB neurons. First, mutations in three components (Lis1, Dmn and p24) of the dynein/dynactin complex were recovered based on mislocalization of dendritic Dscam through a MARCM-based genetic mosaic screen. Second, silencing other components of the complex with RNAi also resulted in mistargeting of dendritic Dscam to axons. Third, disrupting dynein/dynactin function with dominant-negative Glued reproduced the mislocalization phenotype. Further, newly synthesized Dscam[TM1] was preferentially targeted to dendrites. Interestingly, compromising dynein/dynactin function did not affect the targeting from cell bodies to dendrites but disrupted the continuous exclusion of dendritic Dscam from axons. Altogether, these findings show that dynein/dynactin normally acts to prevent Dscam[TM1] from entering axons by retrograde axonal transport (Yang, 2008).
Acute induction by TARGET, in which GAL4-dependent expression of UAS-transgene is acutely controlled by a temperature-sensitive GAL4 repressor, GAL80ts, revealed two mechanisms underlying the dendritic distribution of Dscam[TM1]. Newly synthesized Dscam[TM1] was largely excluded from axons, suggesting directed dendritic targeting and the involvement of selective transport in the dendritic distribution of Dscam[TM1]. Though dynein/dynactin is essential for restricting Dscam[TM1] to dendrites, knocking down dynein/dynactin function did not disrupt the directed dendritic targeting. This leads to the belief that dynein/dynactin is required for preventing dendritic Dscam from misdistributing into axons. When dynein/dynaction function was compromised, newly synthesized Dscam[TM1] remained consistently targeted to dendrites but later leaked into axons. Dendritic Dscam gradually filled the axons; and it took about six hours for Dscam[TM1] to reach the axon termini. This protracted process of mislocalization suggests that dendritic Dscam passively leaks into the axons, and that dynein/dynactin-mediated retrograde axonal transport normally acts to rapidly move leaked Dscam[TM1]-containing vesicles out of the axons. In summary, these phenomena not only demonstrate a dynein-dynactin-independent mechanism of selective transport that preferentially targets Dscam[TM1]-containing vesicles to dendrites, but also implicate the involvement of retrograde axonal transport in preventing accumulation of Dscam[TM1] in axons. These two independent mechanisms act together to ensure restriction of dendritic Dscam to the dendrites (Yang, 2008).
Although the dynein/dynactin complex is essential for maintaining dendritic distribution of Dscam[TM1], the results do not reveal whether mislocalized Dscam[TM1] is on the plasma membrane or in vesicles inside the cytoplasm. It is possible that dendritic Dscam passively leaks into axons either through membrane diffusion or mistargeting of vesicles. Since blocking endocytosis with temperature-sensitive shibire mutant showed no obvious effect on Dscam dendritic distribution, the model is favored that dynein/dynactin acts to prevent axonal accumulation of Dscam[TM1] by actively moving mistargeted Dscam[TM1]-containing vesicles out of axons by retrograde axonal transport (Yang, 2008).
Dscam[TM1]-containing cargos are primarily targeted to dendrites via a dynein/dynactin-independent process. In addition, they are effectively excluded from the axons by dynein/dynactin-mediated retrograde axonal transport. However, dynein/dynactin is not routinely needed for excluding dendritic proteins from the axons. Since no biological process can be carried out with absolute fidelity, it is conceivable that dendritic molecules of most kinds may accidentally leak into the axons. Some salvage mechanism(s) should exist for actively clearing mislocalized molecules to prevent any significant accumulation in the wrong places. One of the possibilities is that dynein/dynactin mediates retrograde axonal transport and can serve as a general mechanism for removing dendritic molecules out of axons. This hypothesis remains to be tested thoroughly. Nonetheless, blocking dynein/dynactin function did not affect the distribution of two other dendritic markers checked. Nod-β-gal is a reliable minus-end reporter of microtubules, and misdistribution of Nod-β-gal in MB axons has been shown in short stop mutant clones, in which microtubule polarity is perturbed. Absence of Nod-β-gal from the axons of dynein/dynactin mutant neurons demonstrates that the microtubules in axons remained uniformly polarized with minus ends pointing toward cell bodies, and rules out the possibility that dendritic Dscam became mislocalized due to abnormal microtubule organization. As to Rdl-HA, which, like Dscam[TM1], is a membrane protein, a lack of effect on its somatodendritic distribution indicates that dynein/dynactin is selectively involved in preventing dendritic Dscam from leaking into the axons. Diverse mechanisms may be utilized to efficiently clear different dendritic proteins in axons (Yang, 2008).
Regarding the mechanism(s) of selective transport, directed dendritic targeting apparently requires motor proteins that selectively move cargos toward the dendrites. Since dendrites, but not axons, carry microtubules with minus ends pointing away from cell bodies, potential candidates that underlie directed dendritic targeting include all minus-end-directed microtubule motors. Notably, dynein/dynactin is dispensable to the initial dendritic targeting of Dscam[TM1] or the continuous dendritic restriction of Rdl, arguing against any critical role for minus-end-directed dynein/dynactin in transporting cargos into the dendrites. Other microtubule motors that might support such directional movement include dendrite-specific plus-end-directed motors (e.g. KIF17 and KIF21B), though it remains mysterious how a plus-end-directed motor can be well restricted to dendrites. In theory, forward genetic mosaic screens will ultimately allow uncovering of the diverse mechanisms of dendritic protein targeting. Encouragingly, mutants have been obtained that exhibit different mislocalization phenotypes, further characterization of which should shed additional light on neuron polarity and its underlying cellular/molecular mechanisms. Notably, in DC-B9 mutant clones, mistargeted Dscam[TM1]::GFP existed abundantly in the MB peduncle, preferentially accumulated at the end of the peduncle, but never extended into the axon lobes. This intriguing phenotype suggests presence of distribution barriers not only in the beginning of axons but also at the junction between the proximal axon domain (peduncle) and the distal axon segment (lobe), and implies another possible mechanism for restricting Dscam[TM1] to the dendritic membrane (Yang, 2008).
Furthermore, the functional roles of each subunit of the dynein/dynactin complex have not been fully determined. Although several studies of the dynein light chains in mammalian cells indicate that dynein subunits can be functionally specialized, studies in Drosophila show that strong loss-of-function mutations in different dynein/dynactin subunits show extensive overlap in the resulting mutant phenotypes. The current data indicate that Lis1, Dmn, Glued, p24, p25, Dhc64C, Dhc62B, and Dlc90F all participate in the complete function of dynein/dynactin complex in maintaining dendritic distribution of Dscam. This result supports the idea that all the dynein/dynactin subunits work together to fulfill its diverse functions, and loss of any subunits may result in different degrees of similar dynein/dynactin-dysfunctional phenotypes (Yang, 2008).
With respect to Dscam targeting motifs, the cytoplasmic juxtamembrane domain of Dscam may dictate its TM-dependent subcellular localization. However, further structure-distribution analysis only allowed location of an axonal targeting motif to the cytoplasmic juxtamembrane region of TM2, leaving its dendritic targeting motif(s) still undetermined. In addition, using the same system it could not be determined whether any of the mutants recovered here also affects the axonal targeting of Dscam[TM2], since transgenic Dscam[TM2] becomes uniformly distributed upon overexpression following an analogous induction. The involvement of multiple mechanisms in targeting specific Dscams to specific neuronal domains further supports the notion that Dscam isoform compositions in the dendrites versus axons of the same neurons need to be independently regulated, elucidation of the physiological significance of which promises to shed new light on how the brain develops and operates (Yang, 2008).
In summary, this study has uncovered a scavenger mechanism for maintaining dendritic distribution of Dscam[TM1] and provide an in vivo model to study neuron polarity and differential protein targeting. On top of the many known functions of dynein/dynactin (including mitosis, vesicular transport, retrograde signaling, neuronal migration), dynein/dynactin helps restrict certain dendritic proteins to the somatodendritic domain of neurons by preventing them from spreading into the axons. Notably, multiple independent mechanisms act together to locate Dscam[TM1] to dendrites; and diverse mechanisms are utilized to target different dendritic proteins to the dendrites (Yang, 2008).
Axons and dendrites differ in both microtubule organization and in the organelles and proteins they contain. This study shows that the microtubule motor dynein has a crucial role in polarized transport and in controlling the orientation of axonal microtubules in Drosophila melanogaster dendritic arborization (da) neurons. Changes in organelle distribution within the dendritic arbors of dynein mutant neurons correlate with a proximal shift in dendritic branch position. Dynein is also necessary for the dendrite-specific localization of Golgi outposts and the ion channel Pickpocket. Axonal microtubules are normally oriented uniformly plus-end-distal; however, without dynein, axons contain both plus- and minus-end distal microtubules. These data suggest that dynein is required for the distinguishing properties of the axon and dendrites: without dynein, dendritic organelles and proteins enter the axon and the axonal microtubules are no longer uniform in polarity (Zheng, 2008).
Proper cellular morphology and function depends on the polarized localization of organelles and proteins to specific subcellular compartments. This study shows that dynein plays a crucial role in dendrite arbour patterning and in organizing distinct functional compartments (the axon and dendrites) of a neuron. The position of branches within a dendritic arbour has a key role in determining the inputs a neuron receives from pre-synaptic axons or, in the case of sensory neurons, the local environment. This study shows that dynein is necessary for proper positioning of dendritic branches relative to the soma. As a motor, dynein likely influences branch formation by mediating the distribution of cargos that affect branch growth and dynamics. Notwithstanding an overall decrease in dendrite extension and branching in dynein mutants, time-lapse analysis of a few dendrites revealed that they extend normally but have fewer and less stable terminal branches, suggesting that decreased terminal branching is not simply caused by a decrease in dendrite growth. One likely explanation is that 'branching machinery' (including Golgi outposts, endosomes and potentially other proteins and/or organelles) that are normally transported distally for dendrite extension and maintenance become trapped in the proximal arbour in the dynein mutant neurons, resulting in decreased distal branching and the formation of ectopic branches close to the cell body (Zheng, 2008).
Without dynein, Golgi outposts and Pickpocket (Ppk) are present ectopically in axons, revealing a previously unappreciated role for dynein in mediating the dendrite-specific localization of organelles and proteins. One possible explanation for axonal mislocalization is that Golgi outposts and Ppk interact with a MT plus end-directed motor (e.g., kinesin) that transports them into axons in the absence of dynein. Dynein might normally transport such cargo directly to dendrites; alternatively, it is also possible that cargo first enters axons but that dynein counteracts kinesin and carries this cargo out of axons. Since Golgi outposts moving from the soma into the axon are never seen in wild type neurons, the data favour the former possibility. In contrast to the mislocalization of dendritic protein and organelles, Kin-βgal and proteins destined for the axonal terminal retain their polarized distribution, perhaps to be expected given that kinesin mediates the majority of anterograde axonal transport (Zheng, 2008).
In mammalian and fly neurons, axonal MTs are arrayed plus end-distal whereas dendritic MT orientation is mixed. A long-standing question concerns the mechanism(s) that establish and maintain different MT orientations in axons and dendrites. Loss of dynein function causes the axonal localization of Nod-βgal and retrograde movement of EB1-GFP, indicating that minus end-distal MTs are present in these mutant axons. How might dynein regulate the orientation of axonal MTs? The sliding filament model of axonal MT transport proposes that a subset of dynein in the axon is stationary (via an interaction with stable MTs and/or actin) and that dyneins motor domain interacts with short MT polymers, propelling plus end-distal MTs down the axon as the motor moves to the MT minus end. In vivo data support the idea that in addition to transporting MTs, dynein functions as a 'gatekeeper' to move minus end-distal MTs towards the soma, excluding them from the axon. Neurons lacking functional dynein would still transport MTs, likely via kinesin, but now minus end-distal MTs would infiltrate the axon. Proximal axons likely have unique properties, providing a possible explanation for how minus end distal MTs would be excluded from axons but not dendrites (Zheng, 2008).
Recent studies indicate that the trans Golgi network (TGN), which comprises part of the Golgi outposts, can also function as a MT organizing center (MTOC) and influence MT organization. Although it is conceivable that Golgi outposts mislocalized to dynein mutant axons could alter MT polarity, expressing lava lamp dominant-negative, which prevents Golgi from associating with dynein without affecting dynein function, causes Golgi outposts to mislocalize to axons without altering MT orientation (Ye, 2007). Moreover, the change in axonal MT orientation is not likely to be simply a consequence of altered axon morphology because the loss of dynein function also alters the MT orientation of class I neuron axons, which appear relatively normal. With the current level of understanding, the model in which dynein acts as a 'gatekeeper' is most consistent with the results and the findings of others (Zheng, 2008).
Intense investigation has identified an elaborate protein network controlling epithelial polarity. Although precise subcellular targeting of apical and basolateral determinants is required for epithelial architecture, little is known about how the individual determinant proteins become localized within the cell. Through a genetic screen for epithelial defects in the Drosophila follicle cells, it was found that the cytoplasmic Dynein motor is an essential regulator of apico-basal polarity. The data suggest that Dynein acts through the cytoplasmic scaffolding protein Stardust (Sdt) to localize the transmembrane protein Crumbs, in part through the apical targeting of specific sdt mRNA isoforms. The sdt mRNA localization signal maps to an alternatively spliced coding exon. Intriguingly, the presence or absence of this exon corresponds to a developmental switch in sdt mRNA localization in which apical transcripts are found only during early stages of epithelial development, while unlocalized transcripts predominate in mature epithelia. This work represents the first demonstration that Dynein is required for epithelial polarity and suggests that mRNA localization may have a functional role in the regulation of apico-basal organization. A unique mechanism is introduced in this study in which alternative splicing of a coding exon is used to control mRNA localization during development (Horne-Badovinac, 2008).
Dynein's role in MT-based apical transport has been studied in the epithelia of both mammals and flies, but an explicit link between Dynein and apico-basal polarity has not been found. One cause for this deficiency may be that Dynein is required for a number of essential cellular processes, making it difficult to study this motor under strong loss-of-function conditions. For instance, previous studies of Dynein function in Drosophila embryos have relied on combinations of hypomorphic Dhc alleles or injection of anti-Dhc antibodies which only partially block Dynein function. Notably, these manipulations have failed to produce epithelial polarity phenotypes. This study shows that strong loss of Dynein function in the FCs disrupts both molecular and morphological aspects of apico-basal polarity, providing the first direct evidence for the role of this motor in this key cell biological process. It is currently unclear why the FCs tolerate strong loss of Dynein function better than other tissues, but this property provides a unique opportunity to begin dissecting the many roles that Dynein is likely to play in epithelial organization (Horne-Badovinac, 2008).
How does Dynein regulate apico-basal polarity? Since Dynein is a minus-end directed MT motor and minus ends are apically oriented in epithelia, whether Dynein might ferry components of the Baz and/or the Crb complexes to their sites of action at the apical surface was investigated. Two pieces of data indicate that the Baz complex is not the primary Dynein cargo contributing to FC polarity. Although Baz is occasionally relocalized to the lateral FC surface in Dhc clones, most mutant cells display significant amounts of apical Baz. Furthermore, ECs in which the entire epithelium is mutant for baz or aPKC display multilayering at both EC poles, a phenotype that is distinct from the posterior multilayering in Dhc mutant epithelia. These observations are consistent with recent studies in the embryonic blastoderm, where it was shown that Dynein does play a role in Baz localization, but that it is not the only means by which this protein is targeted to the apical domain. Together, these data indicate that, while Dynein likely does play a role in the apical targeting of Baz in both the embryo and the FCs, Dynein's major contribution to FC apico-basal polarity corresponds to a different cargo (Horne-Badovinac, 2008).
The results favor a model in which Dynein works primarily through the Crb complex to influence epithelial polarity. When Dynein function is reduced in the FCs, Crb and Sdt disappear from the apical surface, even in Dhc cells that remain cuboidal. Moreover, the morphology of an egg chamber in which the entire epithelium is mutant for Dhc is quite similar to that seen for crb and sdt mutant egg chambers, as all three display multilayering predominantly in the posterior. A major challenge comes in deciphering which Crb complex components are specifically transported by Dynein. This study focused on the relationship between Dynein and Sdt because genetic interaction and molecular epistasis experiments indicate that loss of apical Sdt can account for many aspects of the Dhc polarity phenotype. The finding that the apical localization of sdt transcripts is Dynein-dependent suggests a mechanism by which Dynein could localize Sdt, in part, through the apical targeting and localized translation of its mRNA. sdt transcripts cannot be the only Crb complex component transported by Dynein, however, as the phenotype of sdtEH681 mutant FCs does not recapitulate all aspects of the polarity phenotype of Dhc mutant clones. Interestingly, crb transcripts are also targeted to the apical FC cytoplasm in a Dynein-dependent manner, although at a later stage than sdt. This finding raises the possibility that, in addition to Baz, crb mRNA and/or Crb complex proteins represent other Dynein cargoes required for full epithelial polarization. Future work will be required to more finely dissect Dynein's complex contributions to the apical targeting of the Crb complex (Horne-Badovinac, 2008).
While investigating whether sdt mRNA was likely to be a primary Dynein cargo contributing to apico-basal polarity, the surprising discovery was made that the apical targeting of sdt transcripts is regulated through the alternative splicing of a coding exon, exon 3. Only two other genes are known in which transcript localization is regulated in this way. In both instances, however, the signal lies within the 3'UTR, so the splicing event does not affect protein structure. It is curious that alternative splicing of the sdt mRNA localization signal also deletes 433 amino acids from the protein. The role of the amino acids encoded by exon 3 in Sdt function is not yet known. These amino acids are not conserved in vertebrate Sdt homologs. Moreover, Sdt A and Sdt B bind the intracellular domain of Crb with equal efficiency in vitro and this work with the sdtEH681 allele, as well as over-expression studies with the sdt A and sdt B transgenes indicate that both protein isoforms stabilize Crb in vivo. Although the possibility that exon 3 regulates both mRNA localization and protein function cannot be ruled out, together these observations suggest that the splicing of exon 3 may primarily regulate mRNA localization (Horne-Badovinac, 2008).
A potential role for Dynein-dependent apical targeting of sdt mRNA in epithelial polarity is supported by analysis of the relative contributions of the sdt A and sdt B isoforms. Although the lack of Sdt A in sdtEH681 FC clones reduces Crb at the apical membrane, this deficit leads to relatively mild effects on other aspects of polarity. This apparent discrepancy can be explained by the extrinsic cue for apical identity that is provided to the FCs through their direct contact with the germline, which may compensate for reduced Crb complex function in this tissue. However, in the embryo, where no such cue is available, sdtEH681 has a nearly null phenotype, which is rescued much more efficiently by sdt A than sdt B. Overall, these data suggest that the apical targeting of sdt transcripts may represent an important mechanism contributing to apico-basal polarity (Horne-Badovinac, 2008).
Interestingly, the apical targeting of sdt mRNA is developmentally regulated in both embryonic and adult epithelia. Specifically, it was shown that apically localized sdt transcripts are found only during early stages, while unlocalized transcripts predominate at later stages. Why is sdt transcript localization regulated in this way? It is tempting to speculate that apical transcripts are required primarily for the establishment of apico-basal polarity but not its maintenance. In reality, however, the functional distinction between the two phases of sdt mRNA localization is almost certainly more subtle. When apical sdt A transcripts are present, the epithelia are relatively immature; they tend to be proliferative, display incomplete junctional structures and have yet to adopt their final cell morphology. By contrast, when unlocalized sdt B transcripts predominate, the epithelia are more likely to be post-mitotic and highly differentiated. These observations raise the possibility that a concentrated pool of Sdt protein, generated by localized translation of apical transcripts, could function to stabilize Crb primarily during periods when cell polarity is labile, but that this dynamic regulatory mechanism would be dispensable in fully differentiated cells (Horne-Badovinac, 2008).
Seamless tubes form intracellularly without cell-cell or autocellular junctions (see Labarsky, 2003). Such tubes have been described across phyla, but remain mysterious despite their simple architecture. In Drosophila, seamless tubes are found within tracheal terminal cells, which have dozens of branched protrusions extending hundreds of micrometres. This study has found that mutations in multiple components of the dynein motor complex block seamless tube growth, raising the possibility that the lumenal membrane forms through minus-end-directed transport of apical membrane components along microtubules. Growth of seamless tubes is polarized along the proximodistal axis by Rab35 and its apical membrane-localized GAP, Whacked. Strikingly, loss of whacked (or constitutive activation of Rab35) leads to tube overgrowth at terminal cell branch tips, whereas overexpression of Whacked (or dominant-negative Rab35) causes formation of ectopic tubes surrounding the terminal cell nucleus. Thus, vesicle trafficking has key roles in making and shaping seamless tubes (Schottenfeld-Roames, 2013).
Three tube types -- multicellular, autocellular and seamless -- are found in the Drosophila trachea. Most tracheal cells contribute to multicellular tubes or make themselves into unicellular tubes by wrapping around a lumenal space and forming autocellular adherens junctions, but two specialized tracheal cell types, fusion cells and terminal cells, make 'seamless' tubes. How seamless tubes are made and how they are shaped are largely unknown. One hypothesis holds that seamless tubes are built by 'cell hollowing', in which vesicles traffic to the centre of the cell and fuse to form an internal tube of apical membrane, whereas an alternative model proposes that apical membrane is extended internally from the site of intercellular adhesion. In both models, transport of apical membrane would probably play a key role. As terminal cells make seamless tubes continuously during larval life, they serve as an especially sensitive model system in which to dissect the genetic program (Schottenfeld-Roames, 2013).
Tracheal cells are initially organized into epithelial sacs with their apical surface facing the sac lumen. During tubulogenesis, γ-tubulin becomes localized to the lumenal membrane of each tracheal cell, generating microtubule networks oriented with minus ends towards the apical membrane. Terminal and fusion cells are first selected as tip cells that undergo a partial epithelial-to-mesenchymal transition and initiate branching morphogenesis: they lose all but one or two cell-cell contacts and become migratory. Branchless-FGF signalling induces a subpopulation of tip cells to differentiate as terminal cells. During larval life, terminal cells ramify on tissues spread across several hundred micrometres, with branching patterns that reflect local hypoxia. A single seamless tube forms within each branched extension of the terminal cell (Schottenfeld-Roames, 2013).
How trafficking contributes to seamless tube morphogenesis is unknown. Despite clues that vesicle transport plays a role in the genesis of seamless tubes, the tube morphogenesis genes remain elusive. This study characterized the cytoskeletal polarity of larval terminal cells, shows that a minus-end-directed microtubule motor complex is required for seamless tube growth, and characterizes mutations in whacked (wkd) that uncouple seamless tube growth from the normal spatial cues. Sequence analysis indicates that wkd encodes a RabGAP, and it was shown that Rab35 is the essential target of Wkd, and that together, Wkd and Rab35 can polarize the growth of seamless tubes (Schottenfeld-Roames, 2013).
Apical-basal polarity and cytoskeletal organization was examined in mature larval terminal cells. The lumenal membrane was decorated by puncta of Crumbs, a definitive apical membrane marker. Actin filaments were found enriched in three distinct subcellular domains: surrounding seamless tubes, decorating filopodia and outlining short stretches of basolateral membrane. The microtubule cytoskeleton also seemed polarized, with γ-tubulin lining the seamless tubes and enriched at tube tips. These data are consistent with tracheal studies in the embryo. EB1::GFP analyses of growing (plus-end) microtubules demonstrated that some are oriented towards the soma and others towards branch tips. Stable acetylated microtubules ran parallel to the tubes and extended beyond the lumen at branch tips where they may template tube growth. Consistent with such a role, microtubule-tract-associated fragments of apical membrane were observed distal to the blind ends of the seamless tubes. Filopodia extended past the stable microtubules as expected. These data indicate that mature terminal cells maintain the polarity and organization described for embryonic terminal cells. On the basis of γ-tubulin localization, it is inferred that a subset of microtubules is nucleated at the apical membrane, and that apically targeted transport along such microtubules would require minus-end motor proteins. Indeed, homozygous mutant Lissencephaly-1. As γ-tubulin lines the entire apical membrane, growth through minus-end-directed transport might be expected to occur all along the length of seamless tubes, and indeed, a pulse of CD8::GFP (transmembrane protein tagged with GFP) synthesis uniformly labelled the apical membrane as it first became detectable (Schottenfeld-Roames, 2013).
The cytoplasmic dynein motor complex drives minus-end-directed transport of intracellular vesicles in many cell types; to test for its requirement in seamless tube formation, terminal cells were examined mutant for any of four dynein motor complex genes: Dynein heavy chain 64C (Dhc64C), Dynein light intermediate chain (dlic), Dynactin p150 (Glued) and Lis-1. Mutant terminal cells showed a cell autonomous requirement for these genes. Mutant terminal cells had thin cytoplasmic branches that lacked air-filling, and antibody staining revealed that seamless tubes did not extend into these branches although acetylated microtubules often did. It was also noted that formation of filopodia at branch tips is disrupted in dynein motor complex mutants, which may account for the decreased number of branches in mutant terminal cells. Ectopic seamless tubes that were not air-filled were detected near the nucleus, as described below. Interestingly, discontinuous apical membrane fragments (similar to those in Lis-1 embryos) were found in terminal branches lacking seamless tubes, and were associated with microtubule tracts. Whereas γ-tubulin was enriched on truncated tubes and on these presumptive seamless tube intermediates, diffuse γ-tubulin staining was detected throughout the mutant cells, indicating that assembly of apical membrane is required to establish or maintain γ-tubulin localization. Likewise, Crumbs seemed reduced and aberrantly localized. Reduced levels of acetylated microtubule staining in these cells may reflect loss of apical γ-tubulin. Importantly, these data show that stable microtubules extend through cellular projections that lack seamless tubes. Thus, without minus-end-directed transport, stable microtubules are insufficient to promote seamless tube formation, but stable cellular projections are formed and maintained in the absence of seamless tubes (Schottenfeld-Roames, 2013).
In contrast to these defects in seamless tube generation, mutations in wkd confer overly exuberant tube growth. Examination of wkd terminal cell tips revealed a 'U-turn'; phenotype in which seamless tubes executed a series of 180 degree turns - the possibility is entertained that branch retraction, similar to that observed in talin mutants, could contribute to the U-turn defect (Schottenfeld-Roames, 2013).
Homozygous wkd animals survived until pharate adult stages, and, other than the seamless tube defects, had normal tracheal tubes at the third larval instar. Mosaic analysis revealed a terminal cell autonomous requirement for wkd. Mutant clones in multicellular tubes, and in unicellular tubes that lumenize by making autocellular adherens junctions, were of normal morphology. Strikingly, fusion cells, which also form seamless tubes, were unaffected by loss of wkd (Schottenfeld-Roames, 2013).
To determine the molecular nature of wkd, a positional cloning approach was taken. Mapping techniques defined a candidate gene interval of ~ 75 kilobases (kb). Focused was placed on CG5344 as it encodes a protein containing a TBC (Tre2/Bub2/Cdc16) domain characteristic of Rab GTPase-activating proteins, and hence was likely to participate in vesicular trafficking, a process that could lie at the heart of seamless tube formation. A single nucleotide change was identified that resulted in mis-sense (PC24) and non-sense (220) mutations in the CG5344 coding sequence. Pan-tracheal knockdown of wkd by RNA-mediated interference (RNAi) caused terminal cell-specific U-turn defects (other defects characteristic of the ethyl methanesulfonate (EMS)-induced alleles of wkd were detected at a low frequency). A genomic rescue construct for CG5344 rescued wkd mutants, confirming gene identity. On the basis of these results, it is concluded that wkd is CG5344 and that it probably regulates vesicular trafficking during seamless tube morphogenesis (Schottenfeld-Roames, 2013).
To determine the Rab target(s) of Wkd regulation, whether tracheal expression of constitutively active 'GTP-locked'; Rab isoforms (henceforth, RabCA) might phenocopy wkd was investigated. RabCA for 31 of the 33 Drosophila Rabs were tested individually in the tracheal system. Rab35CA alone conferred terminal-cell-specific U-turns defects (Schottenfeld-Roames, 2013).
To evaluate Wkd overexpression, UAS-wkd was expressed in wild-type animals in a pan-tracheal pattern. Excess Wkd caused formation of ectopic seamless tubes surrounding the terminal cell nucleus. At higher levels of expression, small spheres of apical membrane were found adjacent to the nucleus and less abundantly at more distal sites. Consistent with Wkd regulation of vesicle trafficking by modulation of Rab35, expression of a dominant-negative Rab35 (henceforth, Rab35DN) caused formation of ectopic proximal tubules (Schottenfeld-Roames, 2013).
Attempts were made to determine whether Rab35 was the essential target of Wkd GAP activity. Wkd primary structure is equally conserved in three human RabGAPs. All three act as Rab35GAPs, although each has been proposed to have additional targets. To further determine if Wkd acts as a Rab35GAP, whether Rab35DN could suppress wkd mutants was examined; tracheal-specific expression of Rab35DN strongly suppressed the 'U-turn'; defects of wkd-null animals and, surprisingly, also rescued the lethality of wkd. Since mutant Rab35 isoforms phenocopy wkd gain and loss of function, Rab35DN bypasses the requirement for wkd and human Wkd orthologues are Rab35GAPs, it is concluded that the critical function of Wkd is as a GAP for Drosophila Rab35 (Schottenfeld-Roames, 2013).
In other systems Rab35 is implicated in polarized membrane addition to plasma membrane compartments - for example, immune synapse, cytokinetic furrow and so on - or, in actin regulation. A role for actin in fusion cell seamless tube formation has been proposed, so whether Wkd and Rab35 act by modulation of the terminal cell actin cytoskeleton was examined. As the actin-bundling protein Fascin (Drosophila singed) was recently identified biochemically as a Rab35 effector, a role of singed in terminal cell tubes was examined, but found no evidence was found for one. Furthermore, overexpression of Wkd, or of Rab35DN, did not significantly alter the terminal cell actin cytoskeleton, leading to the conclusion that actin regulation is not a primary function of Wkd/Rab35 during seamless tube morphogenesis (Schottenfeld-Roames, 2013).
The alternative model -- that Rab35 acts in polarized membrane addition -- was found to be attractive, because extra Rab35-GTP activity promoted seamless tube growth at branch tips whereas depletion of Rab35-GTP promoted tube growth at the cell soma. To test this model, advantage was taken of the information that expression of an activated Breathless-FGFR (lambdaBtl in terminal cells induces robust growth of ectopic seamless tubes surrounding the nucleus; whether growth of the ectopic tubes could be redirected from the soma to the branch tips was investigated by eliminating wkd. The activated FGFR phenotype was not altered in wkd heterozygotes, but in wkd mutant animals (or wkd-RNAi animals) the site of ectopic seamless tube growth was strikingly different. In some cells, extra tubes were found throughout the cell - in the soma and at branch tip - whereas in others extra tubes were present only at the branch tip. Thus, the position of seamless tube growth is dependent on Wkd activity, although Wkd itself is not essential for tube formation. These data provide evidence against branch retraction (as occurs in talin mutants) as the mechanism for generating a U-turn phenotype, because branch retraction would not redirect ectopic tube growth (Schottenfeld-Roames, 2013).
To better understand how Wkd and Rab35 determined the site of seamless tube growth, their subcellular distribution was examined. Pan-tracheal expression of mKate2-tagged Wkd (Wkd::mKate2) rescued wkd-null animals. The steady-state subcellular localization of Wkd::mKate2 was restricted to the lumenal membrane with higher accumulation at the growing tips of seamless tubes. At lower levels, cytoplasmic puncta of Wkd::mKate2 were noted that could reflect vesicular localization, as well as labeling of filopodia. It was found that YFP::Rab35 was distributed in a diffuse pattern throughout the terminal cell cytoplasm with some apical enrichment, and notable localization to filopodia. Substantial co-localization of Wkd::mKate2with YFP::Rab35 was found at the apical membrane, in cytoplasmic puncta, and in filopodia. Among endosomal Rabs, Rab35 seemed uniquely abundant within filopodia, and showed the greatest overlap with Wkd at the apical membrane. Substantial overlap was noted between Wkd/Rab35 and acetylated microtubules, including at positions distal to the blind end of seamless tubes. The enrichment of Wkd along seamless tubes indicates that Rab35 functions in an apical membrane trafficking event, leading to the speculation that recycling endosomes at filopodia might be targeted to the growing seamless tube by minus-end motor transport (Schottenfeld-Roames, 2013).
In a similar vein, it is speculated that vesicles might be transported from the soma towards branch tips in a process regulated by Wkd and Rab35. Disruption of such transport might explain why overexpression of Wkd leads to ectopic seamless tube growth in the soma. Whether Wkd::mKate2 localization was compromised in dynein motor complex mutants was examined. As these cells have branches that lack apical membrane/seamless tubes, disruption in the localization pattern of Wkd was anticipated, but it was wondered whether co-localization with acetylated tubulin would be intact, indicative of a microtubule association independent of dynein motor transport. It was found that Wkd::mKate2 is broadly distributed throughout the cytoplasm of dynein motor complex mutants, and does not show enrichment on acetylated microtubule tracts; indeed, substantial basal enrichment was detected of Wkd::mKate2. If Wkd/Rab35-dependent trafficking of apical vesicles was dynein motor complex dependent, ectopic seamless tubes would be expected in the soma of dynein motor complex mutants, similar to those seen with Wkd overexpression or expression of Rab35DN. In fact, such ectopic tubes were consistently found in the dynein motor complex mutants, consistent with dynein-dependent trafficking of Rab35 vesicles. It cannot be ruled out that these defects are due to dynein-dependent processes unrelated to Wkd and Rab35; however, whether the ectopic tubes could be redirected distally by expression of Rab35CA, or elimination of Wkd, was examined. The motor complex ectopic tube phenotype could not be altered, indicating that the phenotype does not arise as an indirect consequence of altered Wkd localization or Rab35 activity (Schottenfeld-Roames, 2013).
The roles of RabGAP proteins have started to become clear only in recent years. Historically, it has been difficult to determine which Rab proteins are substrates of specific RabGAPs. Tests of in vitro GAP activity produced conflicting results, and in some cases did not seem indicative of in vivo function. Indeed, the specificity of Carabin (also known as Wkd orthologue TBC1D10C) has been controversial: it was first shown to act as a RasGAP, whereas later studies indicate a Rab35-specific GAP activity. The in vivo genetic data for wkd, together with recent studies characterizing the function of all three human Wkd-like TBC protein, make a compelling case that this family of proteins acts as GAPs for Rab35. Furthermore, this study establishes a role for classical vesicle trafficking proteins in seamless tube growth. As seamless tubes, but not multicellular or autocellular tracheal tubes, are affected by mutations in wkd and Rab35, this study also establishes an in vivo cell-type-specific requirement for trafficking genes in tube morphogenesis (Schottenfeld-Roames, 2013).
It is concluded that Wkd and Rab35 regulate polarized growth of seamless tubes, and it is speculated that Wkd and Rab35 direct transport of apical membrane vesicles to the distal tip of terminal cell branches (when equilibrium is shifted towards active Rab35-GTP), or to a central location adjacent to the terminal cell nucleus (when equilibrium is shifted towards inactive Rab35-GDP). Analogous to its previously described roles in targeting vesicles to the immune synapse in T cells, the cytokinetic furrow in Drosophila S2 cells and the neuromuscular junction in motor neurons, Rab35 would promote transport of vesicles from a recycling endosome compartment to the apical membrane. It is further speculated that Breathless-FGFR activation at branch tips may couple terminal cell branching with seamless tube growth within that new branch (Schottenfeld-Roames, 2013).
Epithelial remodelling is an essential mechanism for organogenesis, during which cells change shape and position while maintaining contact with each other. Adherens junctions (AJs) mediate stable intercellular cohesion but must be actively reorganised to allow morphogenesis. Vesicle trafficking and the microtubule (MT) cytoskeleton contribute to regulating AJs but their interrelationship remains elusive. This study reports a detailed analysis of the role of MTs in cell remodelling during formation of the tracheal system in the Drosophila embryo. Induction of MT depolymerisation specifically in tracheal cells showed that MTs were essential during a specific time frame of tracheal cell elongation while the branch extended. In the absence of MTs, one tracheal cell
per branch overelongated, ultimately leading to branch break.
Three-dimensional quantifications revealed that MTs were crucial to
sustain E-Cadherin (Shotgun) and
Par-3 (Bazooka) levels at AJs.
Maintaining E-Cadherin/Par-3 levels at the apical domain required de
novo synthesis rather than internalisation and recycling from and to
the apical plasma membrane. However, apical targeting of E-Cadherin
and Par-3 required functional recycling endosomes, suggesting an
intermediate role for this compartment in targeting de novo
synthesized E-Cadherin to the plasma membrane. The apical enrichment
of recycling endosomes was dependent on the MT motor Dynein and essential for the
function of this vesicular compartment. In addition, E-Cadherin
dynamics and MT requirement differed in remodelling tracheal cells versus planar epithelial cells. Altogether, these results uncover an MT-Dynein-dependent apical restriction of recycling endosomes that controls adhesion by sustaining Par-3 and E-Cadherin levels at AJs during morphogenesis (Le Droguen, 2015).
This study has reveal the importance of the functional interplay between MTs, vesicular trafficking and the control of AJ dynamics in cells undergoing extensive remodelling through collective migration. MT depletion in tracheal cells induces the formation of intracellular dots containing E-Cad and Par-3. Interestingly, these intracellular accumulations are only seen in remodelling tracheal cells and not in planar epithelia. These dots are still detected when endocytosis is affected, showing that they are de novo synthesis route intermediates. Altogether, this suggests that maintaining the correct level of E-Cad/Par-3 at the apical domain requires a continuous supply of newly synthesized proteins, which could be essential for the intensive AJ reorganisation that occurs during cell intercalation and elongation of the tracheal branch (Le Droguen, 2015).
Using the photo-convertible E-Cad-EosFP in flat epithelium, a previous study showed that E-Cad that is engaged in homophilic interactions at the AJs forms very stable domains. This study has demonstratde that MT depletion does not affect the integrity of this 2D epithelium. In addition, it was shown that the pool of photo-converted E-Cad-EosFP is less stable in tracheal cells than in epidermal cells. Together with a FRAP assay suggesting that AJs are more dynamic in tracheal cells than in epithelial cells, the results highlight a specific fine-tuning of AJ components in tracheal cells undergoing cell movement and cell shape changes in 3D through cell intercalation, cell elongation and thereby organ formation. This fine-tuning is likely to cycle between internalisation, recycling, degradation and de novo synthesis, the latter being MT dependent. When the balance is altered in the absence of MTs, and thus when E-Cad or Par-3 are reduced at AJs, tracheal cells overelongate after completing intercalation. During branch elongation without MTs, the two migrating leading cells generate a pulling force on the following stalk cells, which display a critical reduction in AJ components. Either the tip cell or the base cell of the stalk becomes unable to maintain its integrity in response to this force. Consequently, this tracheal cell overelongates by a factor 1.8, preventing the remaining stalk cells from reaching their average size. As a result, DBs present a single overelongated cell and several underelongated cells (Le Droguen, 2015).
An overlap was detected between the E-Cad intracellular dots generated in the absence of MTs and recycling endosome vesicles. Interfering with Rab11 function in tracheal cells induces cell overelongation and affects E-Cad and Par-3 distribution at the AJs, as does MT depletion. These results illustrate that the E-Cad de novo synthesis pathway passes through the Rab11-positive recycling endosome compartment. The overlap of E-Cad and recycling endosome markers represents only a small proportion of the total intracellular E-Cad, suggesting a transient residence in this vesicular compartment for this newly synthesized protein. Recent studies conducted in different model systems have revealed that some newly synthesized apical plasma membrane proteins, such as E-Cad and Rhodopsin, leave the trans-Golgi network to cross Rab11-positive recycling endosome compartments before reaching the apical surface). This apical trafficking route is used specifically in tracheal cells and requires the MT network. Quantification of Rab11DN-associated defects upon tracheal branch formation reveals that impairing recycling endosome function has a similar effect to altering the distribution of E-Cad at AJs by depleting the MT network. Moreover, interfering with Rab11 function reduces Par-3 levels at the AJs of tracheal cells. Interestingly, Par-3 does not colocalise with the E-Cad cytoplasmic pool, indicating that functional recycling endosomes are required by Par-3 and E-Cad to assemble as a complex and to be targeted to the apical domain of tracheal cells. However, as E-Cad and Par-3 can be apically targeted in the absence of the other, this suggests that apical targeting of E-Cad and Par-3 can be independent in tracheal cells or that redundant pathways could sustain the localisation of each protein. For example, the Nectin protein Echinoid is required for Par-3 localisation at AJs in shg mutant cells. Moreover, the apical distribution of PI(4,5)P2 in the Drosophila follicular epithelium sustains Par-3 apical anchorage at the plasma membrane. Furthermore, Par-3-independent localisation of E-Cad has been observed (Le Droguen, 2015).
Dynamic MTs in the ectoderm locally upregulate AJ turnover through RhoA activity. RhoA stabilises cellular contacts through acto-myosin regulation. In tracheal cells, MT depletion does not alter actin distribution. Moreover, tracheal cells mutant for the Myosin light chain zipper (zip) and also expressing a dominant-negative form of Zip do not present obvious defects in E-Cad distribution at stage 14. By contrast, MT depletion induces the cytoplasmic accumulation of E-Cad and Par-3 in tracheal cells only and not in ectodermal cells at the same developmental stage. Thus far, cytoplasmic accumulation of E-Cad and Par-3 has only been observed after colchicine-induced MT depolymerisation during polarity establishment at embryo cellularisation, when AJs are extremely dynamic and vesicular trafficking is strongly active. This study demonstrated that the E-Cad distribution in tracheal cells is more sensitive to MT depolymerisation and to Rab11DN overexpression than that in the overlying ectodermal cells. The comparison of the maximum recovery of the E-Cad signal in a FRAP assay in tracheal cells and in ectodermal cells, together with the differences in stability of the photo-converted E-Cad-EosFP between these two tissues, suggest that AJs are more dynamic in tracheae. It is thus conceivable that tracheal cells, which undergo extensive cell shape changes through collective cell migration, require more dynamic AJs as sustained by an efficient targeting of E-Cad. These discrepancies between ectodermal and tracheal epithelia underline the importance of investigating MT function in different morphogenetic contexts under different constraints (Le Droguen, 2015).
This study has demonstrated that the MT minus-end motor Dynein is essential for the restricted localisation of recycling endosomes in a developing organism. The Dynein requirement for the apical enrichment of recycling endosomes is in agreement with the MT minus ends being anchored at the apical plasma membrane. The asymmetric distribution of recycling endosome vesicles has been observed in various differentiated cell types, especially during cell division. Vesicles are found enriched either in the apical domain or at the microtubule-organising centre (MTOC; i.e. the centrosome) during mitosis. Indeed, in vivo, Dynein physically interacts with Nuf. During metaphase, Dynein is required for the maintenance of Nuf at the centrosome. This study demonstrates that Dynein is also required for the apical distribution of recycling endosomes in non-dividing tracheal cells. In a context in which Dynein function is altered, Nuf-positive recycling endosome vesicles are dispersed but colocalise with E-Cad and Par-3 intracellular dots, indicating that the recycling endosome compartment remains functional for the assembly of such a complex (Le Droguen, 2015).
A previous study has characterised the relocalisation of the MTOC in tracheal cells; MTs are nucleated and anchored at the apical domain just above the AJs. It has also been demonstrated that such MT organisation is crucial for tracheal morphogenesis. Non-centrosomal MT organisation occurs in many differentiated cell types but the functional relevance of such an organisation is still poorly understood. It will be informative to investigate whether such non-centrosomal MT organisation provides a means to regulate epithelial remodelling by controlling the apical enrichment of recycling endosomes and thus AJ dynamics (Le Droguen, 2015).
Radial glial progenitors (RGPs) are elongated epithelial cells that give rise to neurons, glia, and adult stem cells during brain development. RGP nuclei migrate basally during G1, apically using cytoplasmic dynein during G2, and undergo mitosis at the ventricular surface. By live imaging of in utero electroporated rat brain, this study found that two distinct G2-specific mechanisms for dynein nuclear pore recruitment are essential for apical nuclear migration. The 'RanBP2-BicD2' and 'Nup133-CENP-F' pathways act sequentially, with Nup133 or CENP-F RNAi arresting nuclei close to the ventricular surface in a premitotic state. Forced targeting of dynein to the nuclear envelope rescues nuclear migration and cell-cycle progression, demonstrating that apical nuclear migration is not simply correlated with cell-cycle progression from G2 to mitosis, but rather, is a required event. These results reveal that cell-cycle control of apical nuclear migration occurs by motor protein recruitment and identify a role for nucleus- and centrosome-associated forces in mitotic entry (Hu, 2013).
The results provide evidence on the mechanism for cell-cycle
control of interkinetic nuclear migration. Blocking apical migration of the RGP nucleus,
even within 1-5 μm of the ventricular surface, inhibited mitotic
entry, suggesting that a mitotic entry signal is tightly restricted
to the ventricular terminus of the cell. Why a mechanism has
evolved to ensure this behavior is unknown. It is speculated that
ectopic cell divisions must interfere in some substantial way
with proper neurogenesis. Nucleokinesis at the ventricular surface
provides access of the condensing chromosomes to the
centrosomes, which assemble the mitotic spindle, and to apical
cues associated with the cell cortex, which are likely important in
controlling cell fate through spindle orientation. The specific
consequences of ectopic mitosis remain an important question
for further study. What can be concluded from the current study is that altered interkinetic nuclear migration reduces RGP proliferation. The consequences on brain size seem evident and may have important implications for understanding and controlling brain developmental disease (Hu, 2013).
Nuclei are precisely positioned within all cells, and mispositioned nuclei are a hallmark of many muscle diseases. Myonuclear positioning is dependent on Kinesin and Dynein, but interactions between these motor proteins and their mechanisms of action are unclear. This study found that in developing Drosophila muscles, Dynein and Kinesin work together to move nuclei in a single direction by two separate mechanisms that are spatially segregated. First, the two motors work together in a sequential pathway that acts from the cell cortex at the muscle poles. This mechanism requires Kinesin-dependent localization of Dynein to cell cortex near the muscle pole. From this location Dynein can pull microtubule minus-ends and the attached myonuclei toward the muscle pole. Second, the motors exert forces directly on individual nuclei independently of the cortical pathway. However, the activities of the two motors on the nucleus are polarized relative to the direction of myonuclear translocation: Kinesin acts at the leading edge of the nucleus, whereas Dynein acts at the lagging edge of the nucleus. Consistent with the activities of Kinesin and Dynein being polarized on the nucleus, nuclei rarely change direction, and those that do, reorient to maintain the same leading edge. Conversely, nuclei in both Kinesin and Dynein mutant embryos change direction more often and do not maintain the same leading edge when changing directions. These data implicate Kinesin and Dynein in two distinct and independently regulated mechanisms of moving myonuclei, which together maximize the ability of myonuclei to achieve their proper localizations within the constraints imposed by embryonic development (Folker, 2013).
Myonuclei in the developing Drosophila embryo undergo several discrete nuclear movements governed by distinct mechanisms that have been best characterized in the lateral transverse (LT) muscles. At the completion of fusion, all of the myonuclei within the muscles reside in a single cluster near the ventral pole of each LT muscle. Shortly after fusion [stage 14, 10:20-11:20 hours after egg laying (AEL)] this single cluster separates into two clusters -- one ventral and one dorsal. During stage 15 (11:20-13 hours AEL) and stage 16 (13-16 hours AEL) these clusters move toward their respective poles. Finally, during stage 17 (16-20 hours AEL), the myonuclei become evenly spaced throughout the muscle. These different myonuclear movements appear to be mechanistically distinct. For example, embryos that are mutant for the MT-associated protein Ensconsin fail to separate the single post-fusion nuclear cluster into distinct dorsal and ventral clusters. However, Dynein and Kinesin appear to be dispensable for the initial separation of the single post-fusion cluster into two, but are required to move the individual clusters toward the muscle poles. More specifically, Dynein localized to the muscle pole pulls MT minus-ends and attached myonuclei toward the muscle pole. The role of Kinesin is less clear, and whether Dynein and Kinesin drive distinct aspects of myonuclear movement or work within a common pathway is not known (Folker, 2013).
This study examined the translocation of myonuclei toward the muscle poles during stage 15 (11:20-13 hours AEL); in developing Drosophila muscles, Dynein and Kinesin were found to work together to move nuclei in a single direction. This contrasts with other systems, such as interkinetic nuclear migration in mammalian brains and the C. elegans hypodermis, where Kinesin and Dynein move nuclei in opposing directions. As part of this analysis, it was found that MTs are bidirectionally oriented throughout most of the developing muscle, unlike in these other in vivo contexts, perhaps promoting the cooperation of the two motors. Furthermore, Dynein and Kinesin cooperate to move nuclei by two separate mechanisms that are spatially segregated. First, the two motors work together in a pathway that acts from the cell cortex at the muscle poles. This mechanism requires Kinesin-dependent localization of Dynein to the cell cortex near the muscle pole. At this location, Dynein can pull MT minus-ends and the attached myonuclei toward the muscle pole. Second, the motors exert forces directly on individual nuclei independently of Dynein activity from the cortex, consistent with recent reports in cell culture. However, this study demonstrated that the activities of the two motors on the nucleus are polarized relative to the direction of myonuclear translocation: Kinesin acts at the leading edge of the nucleus, whereas Dynein acts at the lagging edge of the nucleus. Consistent with the activities of Kinesin and Dynein being polarized on the nucleus, nuclei rarely change directions, and those that do, reorient to maintain the same leading edge. Conversely, nuclei in both Kinesin and Dynein mutant embryos change direction more often and do not maintain the same leading edge when changing directions. It is hypothesized that these functions increase myonuclear translocation efficiency in dense in vivo environments and enable myonuclei to reach their proper position within the time constraints of development (Folker, 2013).
This study investigates how microtubule motors organize microtubules in Drosophila neurons. It was shown that, during the initial stages of axon outgrowth, microtubules display mixed polarity and minus-end-out microtubules push the tip of the axon, consistent with kinesin-1 driving outgrowth by sliding antiparallel microtubules. At later stages, the microtubule orientation in the axon switches from mixed to uniform polarity with plus-end-out. Dynein knockdown prevents this rearrangement and results in microtubules of mixed orientation in axons and accumulation of microtubule minus-ends at axon tips. Microtubule reorganization requires recruitment of dynein to the actin cortex, as actin depolymerization phenocopies dynein depletion, and direct recruitment of dynein to the membrane bypasses the actin requirement. These results show that cortical dynein slides 'minus-end-out' microtubules from the axon, generating uniform microtubule arrays. The study speculates that differences in microtubule orientation between axons and dendrites could be dictated by differential activity of cortical dynein (Del Castillo, 2015). Long-distance intracellular transport of organelles, mRNA, and proteins ("cargo") occurs along the microtubule cytoskeleton by the action of kinesin and dynein motor proteins, but the vast network of factors involved in regulating intracellular cargo transport are still unknown. This study capitalized on the Drosophila melanogaster S2 model cell system to monitor lysosome transport along microtubule bundles, which require enzymatically active kinesin-1 motor protein for their formation. This study used an automated tracking program and a naive Bayesian classifier for the multivariate motility data to analyze 15,683 gene phenotypes and find 98 proteins involved in regulating lysosome motility along microtubules and 48 involved in the formation of microtubule filled processes in S2 cells. Innate immunity genes, ion channels, and signaling proteins were identified having a role in lysosome motility regulation; an unexpected relationship was found between the dynein motor, Rab7a, and lysosome motility regulation (Jolly, 2016).
Dynein heavy chain 64C:
Biological Overview
| Evolutionary Homologs
| Regulation
| Developmental Biology
| References
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