BIOLOGICAL OVERVIEW

Par-1 family members share a conserved function in the generation of cell polarity (see Nelson, 1997 for review). Drosophila par-1 mutants show a novel polarity phenotype in which Bicoid mRNA accumulates normally at the anterior, but Oskar mRNA is redirected to the center of the oocyte, resulting in embryonic patterning defects. These phenotypes arise from a disorganization of the oocyte microtubule cytoskeleton (Shulman, 2000).

Homologs of Par-1 participate in cell polarization in organisms as diverse as yeast, C. elegans and mammals. In the C. elegans oocyte, unlike in the Drosophila oocyte, there is no predetermined A/P polarity, and the axis is polarized instead by sperm entry, which defines the posterior pole. This event triggers a rearrangement of the cortical actin cytoskeleton in which actin foci migrate toward the anterior pole, generating cytoplasmic flows in the cell interior that move the P granules to the posterior. The cell then divides asymmetrically to generate a large anterior AB cell and a smaller posterior P1 cell, which inherits the P granules and subsequently gives rise to the germline lineage. Consistent with this actin-based mechanism for cell polarization, actin depolymerizing drugs block the partitioning of the cytoplasmic determinates known as P granules and cause a symmetric division, whereas microtubule depolymerization has no effect on these processes. In addition to actin, a number of genes have been shown to play a role in the establishment of A/P polarity in C. elegans, including the maternal-effect gene par-1. Mutations in par-1 cause a symmetric first division and block the segregation of the P granules and other determinants along the A/P axis. The PAR-1 serine/threonine kinase is itself asymmetrically localized to the posterior cortex of the one-cell zygote and segregated into P1 after the first division (Guo, 1995). The localization of PAR-1 is dependent upon other par gene products, including PAR-2 and PAR-3 (Drosophila homolog: Bazooka), and also requires the actin cytoskeleton and the nonmuscle myosin, NMY-2, which interacts directly with a C-terminal region of PAR-1 (Etemad-Moghadam, 1995; Boyd, 1996; Guo, 1996).

Disruption of an Schizosaccharomyces pombe par-1 homolog, kin1, causes cells to lose their normal rod-like shape and grow as spheres (Levin, 1990). Mammalian PAR-1 homologs localize to the lateral membrane domain of cultured epithelial cells, and dominant-negative versions of these proteins disrupt apical-basal polarity (Bohm, 1997). The first mammalian homologs of PAR-1, the MARKs, were identified as kinases that phosphorylate the microtubule-associated proteins: Tau, MAP2, and MAP4 (Drewes, 1995, Drewes, 1997; Illenberger, 1996). MARK-induced phosphorylation disrupts MAP binding to microtubules in vivo and leads to the destabilization of the microtubule cytoskeleton without affecting the organization of actin (Drewes, 1997; Ebneth, 1999). Drosophila and mammalian homologs of PAR-1 may therefore have a different function from the C. elegans kinase, since the Drosophila and mammalian proteins regulate microtubule dynamics, while the PAR-1-dependent polarization of the A/P axis in C. elegans is apparently microtubule-independent (Shulman, 2000 and references therein).

In Drosophila, the anterior-posterior (A/P) axis becomes polarized very early in oogenesis, when the oocyte moves to the posterior of the germline cyst. Here, the oocyte signals to the adjacent follicle cells, inducing them to adopt a posterior fate, and these cells subsequently send an unidentified signal back to the oocyte to establish A/P polarity. This signal induces the disassembly of a microtubule organizing center (MTOC) positioned at the oocyte posterior, and microtubules are nucleated from a new anterior MTOC to form an A/P gradient in which the minus ends appear to lie at the anterior pole and the plus ends at the posterior. This polarized microtubule network defines the A/P axis by directing the localization of Bicoid and Oskar mRNAs to opposite poles of the oocyte. The posterior localization of OSK mRNA is the key step in pole plasm assembly, since Osk protein nucleates the assembly of the polar granules, at least in part, by directly recruiting components such as Vasa. Mutations in genes that require OSK mRNA localization or the assembly of the polar granules have been shown to disrupt the recruitment of germline and posterior determinants to the posterior pole, resulting in a 'posterior group' phenotype in which embryos lack pole cells and abdominal segments. In Drosophila, mutations in par-1 disrupt the polarized organization of the oocyte microtubule network and block the posterior localization of OSK mRNA, leading to defects in the posterior patterning of the embryo and the formation of the germ cells (Shulman, 2000 and references therein).

Pole plasm formation depends on the stepwise recruitment of a number of posterior group gene products to the posterior pole. The posterior localization of Stauffen and Oskar mRNA leads to the translational activation of the latter to produce Osk protein, which then anchors the complex and recruits Vasa protein. To determine where PAR-1 lies in this hierarchy, its localization was examined in various posterior group mutants. Par-1 localization at the oocyte posterior is unaffected in vas^PD egg chambers, and in homozygotes for osk missense mutations, in which OSK mRNA is localized and anchored at the posterior, but fail to recruit Vasa. In contrast, the strong osk nonsense allele, osk⁵⁴, completely abolishes the posterior localization of Par-1 and null mutations in stau have a similar effect. Thus, the recruitment of PAR-1 to the posterior is upstream and independent of vasa, but requires osk and stau (Shulman, 2000).

Since the posterior localization of OSK mRNA, Stau, and Osk are interdependent, these experiments do not distinguish which of these components is responsible for recruiting PAR-1 to the posterior pole. Therefore use was made of an osk-bcd 3'-UTR transgene, in which the bcd localization signal directs the Stau-independent localization of OSK mRNA and protein to the anterior of the oocyte. In egg chambers expressing this construct, Par-1 localizes to the anterior as well as the posterior pole of the oocyte. Thus, Par-1 must interact directly or indirectly with either Osk protein or a region of OSK mRNA other than the 3'-UTR, which is absent from the transgene. Taken together, these results are most consistent with a model in which Par-1 associates with OSK mRNA, since both show a transient localization at the anterior of wild-type stage 9 oocytes, whereas Osk protein is not translated until the mRNA reaches the posterior (Shulman, 2000).

In C. elegans, the posterior localization of PAR-1 requires the activity of a number of genes, including par-3 and nmy-2 (Etemad-Moghadam, 1995; Guo, 1996). To determine whether Par-1 localization in Drosophila has any features in common with C. elegans, mutant germline clones of spaghetti-squash^AX3, a null allele for the nonmuscle myosin II regulatory light chain, and bazooka⁴, a strong mutation in the par-3 homolog were generated. Neither of these mutations had an effect on Par-1 localization to the posterior, and there appear to be no other homologs of these genes in the Drosophila genome, indicating that the kinase is recruited to the posterior by distinct mechanisms in the two organisms (Shulman, 2000).

Given the lack of pole plasm in embryos derived from par-1 mothers, the earliest steps of pole plasm assembly during oogenesis were examined. In par-1 egg chambers, OSK mRNA localizes normally through stage 7, but then diverges strikingly from the wild-type pattern. In the strongest viable allelic combination, par-1⁶³²³/par-1^W3, OSK mRNA is never detected at the posterior, and is either mislocalized to an ectopic site in the center of the oocyte (73%) or not localized at all (27%). This unusual OSK mRNA 'dot' forms as early as stage 8, and can persist until stage 11, the latest stage that can be examined. Stau protein shows an identical mislocalization to the middle of the oocyte in these mutants. Weaker allelic combinations show a similar abnormal pattern of Stau and OSK mRNA localization, but with lower penetrance. In addition, these egg chambers often show an intermediate phenotype in which some OSK RNA forms a dot in the middle of the oocyte while the rest localizes normally to the posterior cortex. Occasional dots of mislocalized OSK mRNA are even observed in par-1^W3 heterozygotes, indicating that par-1 has a slight dominant haplo-insufficient phenotype. Overall, the penetrance of the OSK mRNA mislocalization phenotype correlates well with that of abdominal defects for each allelic combination (Shulman, 2000).

The localization of OSK mRNA is microtubule-dependent, and several mutants that disrupt this process do so by altering the organization of the oocyte microtubule network. It was therefore examined whether microtubule organization and polarity are also disrupted in par-1 oocytes. In wild-type oocytes, the microtubules are organized in an A/P gradient at stages 7-9 that can be visualized using a Tau:GFP fusion protein. In addition, the polarity of the microtubules can be assayed by expressing microtubule motor proteins fused to beta-galactosidase (beta-gal). A beta-gal fusion to the plus-end-directed motor, Kinesin (Kin:beta-gal), localizes to the posterior of the oocyte during stages 9-10 like OSK mRNA, whereas a Nod:beta-gal fusion localizes to the anterior. In par-1 mutant oocytes, Tau:GFP labels microtubules uniformly around the cortex, including the posterior pole, where they are never seen in wild type. Moreover, like OSK mRNA and Stau, Kin:beta-gal is mislocalized to the center of the oocyte, although a small amount of residual staining is often seen at the posterior. In contrast, Nod:beta-gal shows a normal localization to the anterior in par-1 mutant oocytes, suggesting that the microtubules are still nucleated from this pole. Like the other aspects of the par-1 phenotype, the disruption of microtubule organization is completely penetrant in par-1⁶³²³/par-1^W3 egg chambers, but less so in the weaker allelic combinations (Shulman, 2000).

The abnormal arrangement of microtubules in par-1 is similar to that seen in cappuccino (capu), spire (spir), and chickadee mutants, which disrupt OSK mRNA localization by causing a premature rearrangement of microtubules into a cortical array. Upon careful comparison, however, the microtubules in par-1 appear more diffuse than the tight, parallel bundles of capu egg chambers. Furthermore, whereas mutations in capu or spir cause the premature initiation of cytoplasmic streaming, the cytoplasmic movements of par-1 oocytes are indistinguishable from wild type. The microtubule organization in par-1 mutants also differs from that seen in grk mutants, which fail to disassemble the posterior MTOC, resulting in a focus of microtubules at the posterior pole that is never seen in par-1 oocytes (Shulman, 2000).

Despite extensive molecular investigation and several large-scale genetic screens, no common components have previously been found to be required for A/P axis polarization in Drosophila and C. elegans. Indeed, the primary axes of these two organisms are specified by different cues, at different stages of development, and by mechanisms with distinct cytoskeletal requirements. Nevertheless, in both systems, the axis is polarized within a single cell by an extrinsic spatial cue that triggers cytoskeletal and cytoplasmic rearrangements; in each case, these events culminate in the posterior localization of germline determinants (Shulman, 2000).

In the nematode, mutations in par-1 disrupt the positioning of the mitotic spindle, leading to a symmetric first division, and block the segregation of the P granules and other determinants to the posterior daughter blastomere, resulting in disorganized embryos that lack germ cells. In Drosophila, mutations in par-1 disrupt the polarized organization of the oocyte microtubule network and block the posterior localization of OSK mRNA, leading to defects in the posterior patterning of the embryo and the formation of the germ cells. The functional analogy revealed by par-1 mutant phenotypes in Drosophila and C. elegans is reinforced by the finding that Par-1 protein localizes to the posterior pole at the time when the A/P axis is being specified in both organisms, even though the mechanisms of localization are distinct. In the nematode, PAR-1 localization to the posterior requires the activity of PAR-2, PAR-3, and NMY-2. In contrast, homologs of PAR-3 and the NMY-2 light chain are not required for the localization of Drosophila Par-1, and its recruitment to the posterior of the oocyte at stage 9 depends instead on OSK mRNA. Nevertheless, Par-1 is one of the first molecules to localize to the posterior in each system, and provides an example of a common mediator and molecular marker of A/P polarity in flies and worms (Shulman, 2000 and references therin).

This conserved requirement for par-1 is somewhat surprising given the apparently distinct mechanisms by which the A/P axes form in Drosophila and C. elegans. The localization of the polar granules in Drosophila depends on the microtubule-dependent transport of OSK mRNA to the posterior of the oocyte, and the results provide support for the direct role for Drosophila Par-1 in remodeling the oocyte microtubule cytoskeleton. In contrast, the completed C. elegans genome reveals no osk homolog, and P-granule segregation during the first cell cycle requires the actin cytoskeleton, but not microtubules. A possible resolution to this paradox is suggested by several recent results that reveal the existence of parallel pathways for localizing the P granules to the germ cell lineage of C. elegans. Mutations in par-2 and pod-1 and RNAi against nmy-2 and mlc-4 severely impair or eliminate the cytoplasmic flows that normally localize the P granules, yet these particles can still segregate to the posterior. Thus, the one-cell zygote must still possess A/P polarity that can direct P-granule segregation in the absence of cytoplasmic flows. Furthermore, par-1 mutants do not disrupt the actin reorganization or the cytoplasmic flows during the first cell cycle. Instead, the P granules disappear and then reappear two divisions later in all cells of the embryo. PAR-1 is therefore required for the stability of the P granules at the one-cell stage, and for an A/P polarity that is independent of cytoplasmic flows. It is interesting to note that subsequent to the first division, P granules continue to segregate within the P lineage in the absence of flows, and localize in a microtubule-dependent manner via an association with one spindle pole during anaphase. A similar mechanism has been described for P-granule segregation during the first cell cycle in some nematode species which, like Drosophila, polarize the A/P axis during oogenesis. Since almost nothing is known about the alternative localization pathway in C. elegans, the role of PAR-1 in this process might therefore be more similar to its function in Drosophila than first appears (Shulman, 2000 and references therin).

Although Drosophila par-1 mutations cause typical posterior group phenotypes in the embryo, they have a different effect from all previously identified mutations on the polarization of the oocyte at stage 9. Mutations in genes such as grk, Notch, and PKA disrupt signaling from the posterior follicle cells and cause a similar mislocalization of OSK mRNA and Kin:beta-gal to the center of the oocyte as seen in par-1. However, in contrast to these mutants, the posterior MTOC is disassembled normally in par-1 oocytes, and both BCD mRNA localization and nuclear migration are unaffected. The par-1 phenotype is also distinguishable from that caused by mutations in capu, spir, and chickadee. These mutants show tight microtubule bundles and a complete delocalization of OSK mRNA, whereas par-1 oocytes show comparatively diffuse microtubule arrays and mislocalize osk to an ectopic 'dot.' Furthermore, par-1 has no effect on cytoplasmic streaming, grk mRNA localization, or dorsal/ventral patterning of the egg shell and embryo, processes that are all disrupted by the capu-like mutations. Since the par-1 null allele blocks oocyte development before stage 6, the possibility that the phenotype of the strongest viable par-1 mutant combination reflects an incompletely penetrant grk- or capu-like phenotype cannot be ruled out. This seems highly unlikely, however, because in contrast to weak grk or capu mutants, par-1⁶³²³/par-1^W3 oocytes produce a completely penetrant disruption of microtubule organization and OSK mRNA localization, but do not affect bcd and grk mRNA localization, nuclear migration, or cytoplasmic streaming. Par-1 therefore seems to be required for a novel step in the A/P polarization of the oocyte that is necessary for osk but not BCD mRNA localization (Shulman, 2000 and references therin).

Microtubule stainings and the behavior of the Kin:beta-gal and Nod:beta-gal fusion proteins have led to a simple model for the microtubule organization in the stage 9 oocyte, in which the majority of the minus ends are nucleated from the anterior cortex, with the plus ends extending toward the posterior pole. Between stages 7 and 8, at least two events must take place in the oocyte in order to generate this polarized microtubule array: (1) the signal from the posterior follicle cells induces the disassembly of the MTOC at the posterior of the oocyte, and (2) a new MTOC is activated along the anterior cortex. Both of these events seem to occur normally in par-1 mutants, since BCD mRNA and Nod:beta-gal show a wild-type localization to the anterior of the oocyte, and there is no ectopic focus of microtubules at the posterior pole. Thus, the par-1 phenotype reveals that the polarization of the oocyte microtubule network requires more than just a simple switch from a posterior to an anterior MTOC. If Kin:beta-gal and Nod:beta-gal are reliable markers for the ends of microtubules, the plus ends appear to be abnormally focused on the middle of the oocyte in par-1 mutants, whereas the minus ends are unaffected. Thus, Par-1 activity may be required to regulate the plus ends of the microtubules so that they direct OSK mRNA transport to the posterior pole (Shulman, 2000).

The mammalian homologs of Par-1, the MARKs, were identified for their ability to phosphorylate a repeated motif in the microtubule binding domains of Tau, MAP2, and MAP4 (Drewes, 1997). Phosphorylation at these sites reduces the binding affinity of these MAPs for microtubules and induces microtubule depolymerization. Thus, it is attractive to propose that Drosophila PAR-1 has a similar activity, and functions by phosphorylating MAPs to modulate oocyte microtubule dynamics (Shulman, 2000 and references therin).

Par-1 localizes to the posterior with OSK mRNA during stage 9, but par-1 mutants disrupt the microtubule organization as early as stage 8, and as a consequence, neither OSK mRNA nor Stau protein localize to the posterior pole. Par-1 activity is therefore required for its own osk-dependent localization. Consistent with this, mislocalized Par-1 protein in the center of the oocyte has been occasionally observed in weak par-1 allelic combinations. No asymmetrically localized Par-1 has been detected in the oocyte during the stages when the microtubule reorganization takes place, but it is possible that Par-1 function is localized instead through the regulation of its activation. Indeed, the activity of the MARKs has been shown to depend on the phosphorylation of regulatory sites within the kinase domain, and these residues are conserved in Drosophila Par-1 (Shulman, 2000). Such regulation may also occur in C. elegans, since the posterior localization of PAR-1 is not essential for all of its functions in polarizing the first cell division. In par-2 mutants, for example, the P granules often segregate normally, even though PAR-1 protein is not localized to the posterior, whereas par-1 mutants completely abolish P-granule partitioning (Kemphues, 1988; Boyd, 1996).

The finding that the role of Par-1 in A/P axis formation precedes its osk-dependent localization to the posterior raises the question of whether Par-1 has any additional functions once it has reached the posterior pole. This question is not straightforward to answer, since it is not possible at present to rescue the OSK mRNA localization phenotype of par-1 mutants without also rescuing the posterior localization of Par-1 protein. One possibility is that this constitutes a positive feedback loop that reinforces an earlier function in the polarization of the oocyte by concentrating the protein where it is needed. Alternatively, localized Par-1 could play a direct role in the downstream steps of pole plasm assembly. In C. elegans, PAR-1 has been implicated in the translational activation of pal-1 mRNA in the P1 blastomere, where its activity is required to relieve translational repression mediated by the RNA binding protein MEX-3 (Bowerman, 1997). The association of Drosophila Par-1 with OSK mRNA therefore raises the possibility that it may function in a similar manner to relieve Bruno-dependent translational repression once osk reaches the posterior pole (Shulman, 2000).

Although the polarization of the oocyte microtubule cytoskeleton is most sensitive to reductions in Par-1 activity in Drosophila, this kinase is also required at other stages of development. In the germarium, Par-1 is localized to the fusome, and germline clones of the par-1 null allele block oogenesis during the previtellogenic stages. Par-1 must therefore have an earlier, essential role, perhaps in specifying asymmetry during the divisions that give rise to the germline cyst or during the subsequent determination of the oocyte. Furthermore, consistent with a previous investigation of mammalian Par-1 homologs in cultured epithelial cells, Par-1 is localized to the basolateral membrane of the mature follicular epithelium, suggesting the possibility of a conserved role in epithelial polarization. Finally, par-1 must have additional functions during the development of the zygote, since the null allele is homozygous lethal, and this probably explains why it was not identified in previous screens for maternal-effect mutations that affect embryonic patterning (Shulman, 2000).

In conclusion, the parallels between the localization and function of Par-1 homologs in Drosophila, C. elegans, and mammalian systems indicate that this kinase family shares a conserved function in the generation of cell polarity. Although its exact requirement is not known in any of these contexts, the analysis of the Drosophila par-1 phenotype, coupled with the activity of the mammalian MARKs, strongly suggest that PAR-1 plays a direct role in polarizing the microtubule cytoskeleton, and Drosophila should therefore provide a valuable system for investigating the in vivo activities of these kinases (Shulman, 2000).

PAR-1 kinase phosphorylates Dlg and regulates its postsynaptic targeting at the Drosophila neuromuscular junction

Targeting of synaptic molecules to their proper location is essential for synaptic differentiation and plasticity. PSD-95/Dlg proteins have been established as key components of the postsynapse. However, the molecular mechanisms regulating the synaptic targeting, assembly, and disassembly of PSD-95/Dlg are not well understood. This study shows that PAR-1 kinase, a conserved cell polarity regulator, is critically involved in controlling the postsynaptic localization of Dlg. PAR-1 is prominently localized at the Drosophila neuromuscular junction (NMJ). Loss of PAR-1 function leads to increased synapse formation and synaptic transmission, whereas overexpression of PAR-1 has the opposite effects. PAR-1 directly phosphorylates Dlg at a conserved site and negatively regulates its mobility and targeting to the postsynapse. The ability of a nonphosphorylatable Dlg to largely rescue PAR-1-induced synaptic defects supports the idea that Dlg is a major synaptic substrate of PAR-1. Control of Dlg synaptic targeting by PAR-1-mediated phosphorylation thus constitutes a critical event in synaptogenesis (Zhang, 2007).

Rearrangement of synaptic protein composition and structure is a fundamental mechanism governing synaptic plasticity. As organizers of the postsynapse, PSD-95/Dlg proteins have been intensively studied as substrates mediating synaptic plasticity. The signaling pathways that couple internal or external cues to the localization and function of PSD-95/Dlg are not well defined. This study has found that PAR-1 kinase plays a critical role in regulating the postsynaptic targeting of Dlg at the Drosophila NMJ. PAR-1 does so by phosphorylating Dlg at a Ser residue in the GUK domain. The conservation of this Ser residue in all members of the MAGUK proteins suggests that this phosphorylation event may represent a general mechanism by which the MAGUK proteins are regulated. This is the first time the PAR-1 family of Ser/Thr kinase has been shown to play an important role in synaptic development and function (Zhang, 2007).

PAR-1 directly phosphorylates Dlg, and overactivation of PAR-1 disrupts Dlg's postsynaptic targeting. The physiological function of PAR-1 in regulating Dlg synaptic targeting is supported by loss-of-function analysis, which indicates that phosphorylation by PAR-1 negatively regulates Dlg synaptic targeting. Consistent with this, in vivo FRAP analysis shows that the nonphosphorylatable DlgSA-GFP recovers much faster than DlgWT-GFP, and that the recovery of DlgWT-GFP is facilitated by PAR-1 loss-of-function, but impeded by PAR-1 overexpression. At first glance, it may seem somewhat counterintuitive that DlgSA-GFP, which accumulates to a greater degree at the synapse than DlgWT-GFP does, is replaced more quickly and to a greater extent that DlgWT-GFP. Since FRAP analysis suggested that the recovered Dlg comes primarily from Dlg protein reserved or newly synthesized in the muscle cytoplasm rather than from diffusion of Dlg protein from the neighboring synapses, the most likely explanation is that PAR-1-mediated phosphorylation regulates the transport of Dlg from the extrasynaptic to the synaptic regions. DlgSA-GFP may be transported more efficiently from the extrasynaptic region to the postsynapse. Upon reaching the postsynapse, DlgSA-GFP may also associate with the synaptic membrane more tightly (Zhang, 2007).

Previous studies have demonstrated that the GUK domain, in which the S797 residue is located, plays an important role in the trafficking and synaptic targeting of Dlg. The importance of the GUK domain in mediating Dlg function is also highlighted by the fact that many of the identified dlg mutations are clustered in this domain. Two types of protein-protein interactions involving the GUK domain have been detected: (1) intramolecular interaction with the SH3 domain, and (2) protein-protein interactions with GUK binding proteins, including an MT binding protein and a kinesin motor. Since MT and MT-based motor proteins provide a major driving force for protein and mRNA trafficking, it is possible that PAR-1-mediated phosphorylation may regulate Dlg interaction with the MT-based transport system (Zhang, 2007).

Morphological and electrophysiological rescue experiments strongly support that Dlg is a critical downstream target through which PAR-1 impacts synapse differentiation and function. However, the rescue of PAR-1 overexpression-induced defects by DlgSA-GFP is not complete, raising the possibility that other synaptic substrates are affected by PAR-1. It is also possible that some of the PAR-1 overexpression phenotypes are neomorphic. A possible neomorphic effect caused by the synaptic upregulation of a kinase has recently been described. None of the known postsynaptic markers, such as CaMKII, FasII, or GluRIIA has the KXGS motif, suggesting that they may not be PAR-1 targets. In other developmental contexts, PAR-1/MARK kinases phosphorylate a number of substrates. Whether any of these PAR-1 substrates function at the synapse awaits further investigation. The existence of other synaptic targets of PAR-1 could also explain why it was not possilbe to effectively rescue par-1 mutant phenotypes with the Dlg-GFP variants, although there are other possible explanations for this result. For example, phosphorylated Dlg may possess certain biological activity that cannot be provided by DlgSD-GFP. Even if some of the phenotypes caused by altered PAR-1 activities are mediated by other substrates, several lines of evidence indicate that the mislocalization of Dlg is a primary effect of PAR-1 phosphorylation of Dlg, rather than a secondary consequence of synaptic damages caused by PAR-1 action on some unknown target(s): (1) the phospho-mimetic DlgSD-GFP is mislocalized in a wild-type background, in the presence of normal synaptic structures; (2) another postsynaptic marker, GluRIIA, retains its predominant postsynaptic localization in a PAR-1 overexpression situation; (3) the subsynaptic reticulum loss and synaptic transmission defects caused by PAR-1 overexpression using DlgSA-GFP could largely be rescued; (4) in a condition where postsynaptic structure was maintained with exogenous DlgSA-GFP, endogenous Dlg was still mislocalized in the presence of overexpressed PAR-1 (Zhang, 2007).

Recent studies suggest that posttranslational modification plays a role in regulating the trafficking of PSD-95/Dlg. In mammalian central synapses, N-terminal palmitoylation is critical for the intracellular sorting, postsynaptic targeting, and surface expression of PSD-95. Cyclin-dependent kinase 5 (Cdk5) phosphorylates the N-terminal region of PSD-95, inhibiting its oligomerization, channel clustering activity, and possibly, synaptic localization. This study establishes PAR-1-mediated phosphorylation at the C-terminal GUK domain as a regulatory mechanism in the synaptic targeting of Dlg. In addition, two independent studies have been conducted to study the function of CaMKII at the Drosophila NMJ. However, divergent results were obtained on the effect of CaMKII on synaptic development and function, and it appears that further studies are needed to clarify the function of CaMKII at the Drosophila NMJ. Future studies of upstream signaling events in the regulation of PAR-1 at the synapse, especially those which potentially regulate the PAR-1-Dlg phosphorylation cascade, will provide new insights on molecular mechanisms that regulate synaptic differentiation and plasticity (Zhang, 2007).

It is interesting to note that in addition to postsynaptic defects, altering PAR-1 activity leads to profound defects in presynaptic development and function. This indicates that PAR-1 regulates the coordinated maturation of pre- and postsynaptic structures. PAR-1 could regulate the adhesion between the pre- and postsynaptic membranes or trans-synaptic signaling. Intriguingly, a previous study has revealed a presynaptic localization and function for Dlg in regulating neurotransmission. Since a fraction of PAR-1 is localized at the presynapse, it raises the possibility that PAR-1 may also play a role there. Further studies are needed to test whether PAR-1 may act through Dlg or other substrates at the presynapse to affect neurotransmission. Previous studies have also implicated BMP as a retrograde signal that modulates presynaptic development and function in response to postsynaptic alterations. It would be interesting to explore the relationship between PAR-1 and Dlg-mediated synaptic effects and BMP-mediated retrograde signaling (Zhang, 2007).

The current model predicts that PAR-1 overactivation causes Dlg hyperphosphorylation and delocalization from the synapse, producing certain Dlg loss-of-function effects. In contrast, loss of PAR-1 function has the opposite effect, causing Dlg overactivation phenotypes. Most of the phenotypes observed are consistent with this model. For example, at the morphological level, PAR-1 overexpression and Dlg inactivation both lead to SSR loss, whereas loss of PAR-1 and overexpression of Dlg promote SSR growth. At the electrophysiological level, PAR-1 overexpression or Dlg loss of function leads to reduced EJC amplitude, whereas loss of PAR-1 has the opposite effect. The genetic interaction between PAR-1 and Dlg is also consistent with an antagonistic effect of PAR-1 on Dlg. Some inconsistencies of certain PAR-1 overexpression phenotypes and previously published dlg mutant phenotypes are noted. For example, overexpression of PAR-1 in the postsynapse causes reductions in both active zone number and synaptic vesicle density, whereas quite variable phenotypes, ranging from no obvious structural alteration in the presynapse to reduction in synaptic vesicle density or increase in active zone number, have been described for different dlg mutant alleles. Similarly, in electrophysiological studies, a decrease was found in both mEJC and EJC amplitudes, but no significant change in quantal content, in both Mhc>PAR-1 animals and dlgX1-2 mutants. These neurotransmission phenotypes are different from those previously reported for dlg mutant alleles dlgm52 and dlgv59, in which EJC was increased, whereas mEJC was not changed. However, a recent study also reported features of reduced neurotransmission in dlgX1-2 mutant. It is therefore possible that different dlg mutant alleles may differentially affect synaptic function (Zhang, 2007).

Recent studies have revealed a tight correlation between synaptic dysfunction and the pathogenesis of neurodegenerative diseases and other neurological disorders. In AD in particular, synaptic dysfunction occurs decades before the onset of amyloid plaque and neurofibrillary tangle formation and discernable neuronal loss. Intriguingly, loss of PSD-95 protein has been observed in AD patients. It is conceivable that under disease conditions, an increase of PAR-1/MARK activity might occur in response to certain neurotoxic insults, leading to abnormal phosphorylation and delocalization of PSD-95 from the postsynapse, eventually leading to neuronal dysfunction and death. Further studies in human AD postmortem tissues and mouse AD models will test the potential role of PAR-1/MARK kinases in regulating PSD-95 function and disease pathogenesis (Zhang, 2007).

PAR-1 promotes microtubule breakdown during dendrite pruning in Drosophila

Pruning of unspecific neurites is an important mechanism during neuronal morphogenesis. Drosophila sensory neurons prune their dendrites during metamorphosis. Pruning dendrites are severed in their proximal regions. Prior to severing, dendritic microtubules undergo local disassembly, and dendrites thin extensively through local endocytosis. Microtubule disassembly requires a katanin homologue, but the signals initiating microtubule breakdown are not known. This study shows that the kinase PAR-1 is required for pruning and dendritic microtubule breakdown. The data show that neurons lacking PAR-1 fail to break down dendritic microtubules, and PAR-1 is required for an increase in neuronal microtubule dynamics at the onset of metamorphosis. Mammalian PAR-1 is a known Tau kinase, and genetic interactions suggest that PAR-1 promotes microtubule breakdown largely via inhibition of Tau also in Drosophila. Finally, PAR-1 is also required for dendritic thinning, suggesting that microtubule breakdown might precede ensuing plasma membrane alterations. The results shed light on the signaling cascades and epistatic relationships involved in neurite destabilization during dendrite pruning (Herzmann, 2017).

The kinase PAR-1 is part of a pathway for microtubule disassembly during dendrite pruning. The data show that PAR-1 acts to enhance microtubule dynamics specifically during the early pupal phase. In the absence of PAR-1, c4da neurons accumulate stable microtubules at a time when control neurons have already degraded most of their dendritic microtubules. Genetic data suggest that Tau is a major target for PAR-1 in this process and that PAR-1 is required during pruning to remove, or inactivate Tau. It is known that Tau itself stabilizes microtubules; therefore, Tau inhibition likely serves to destabilize microtubules. Interestingly, Tau removal might also serve to activate the katanin homologue Kat-60L1 during dendrite pruning. This is an attractive possibility because Tau, but not the Futsch homolog MAP1B, has been shown to be a potent katanin inhibitor in mammalian cells, exactly matching the observed genetic interactions with PAR-1 during dendrite pruning. Tau also becomes depleted from mammalian sensory neuron axons after trophic support withdrawal in an in vitro pruning model system. Tau depletion was not sufficient to induce pruning in mammalian sensory neurons, matching observations in c4da neurons. However, while not sufficient, it is interesting to speculate that Tau inactivation might also be required for pruning in mammalian neurons (Herzmann, 2017).

The data suggest that PAR-1 acts specifically during the pupal phase, but PAR-1 protein levels do not seem to increase at this stage. PAR-1 can be activated through phosphorylation by upstream kinases such as LKB1. lkb1 mutants showed only mild pruning defects that likely cannot fully explain the stronger defects caused by PAR-1 downregulation. Interestingly, PAR-1 interacts genetically with ik2, another kinase required for dendrite pruning. Thus, PAR-1 activation during dendrite pruning might depend on the interplay of several kinases. Given the temporal specificity of the PAR-1 effect, it is interesting to speculate that PAR-1 might be directly activated by a ecdysone-responsive factor (Herzmann, 2017).

It was also found that loss of PAR-1 prevents several processes at the dendritic plasma membrane during the pruning process: It prevented the local loss of membrane-associated Ank2XL from proximal dendrites, abrogated Ca2+ transients, and displayed strong enhancing genetic interactions with the thinning factor Shibire. As the genetic data indicate that Tau is the primary PAR-1 target during dendrite pruning, this suggests that microtubule breakdown is required for these plasma membrane alterations. In this scenario, the data actually suggest that microtubule disruption is closely linked to plasma membrane alterations, such that it is interesting to speculate that microtubule loss might trigger local endocytosis and thinning formation during dendrite pruning. Thus, a model is proposed where PAR-1, via Tau and possibly Kat-60L1, promotes microtubule disruption. In this model, these processes are placed epistatically over plasma membrane alterations during dendrite pruning (Herzmann, 2017).

Nashchekin, D., Fernandes, A. R. and St Johnston, D. (2016). Patronin/Shot cortical foci assemble the noncentrosomal microtubule array that specifies the Drosophila anterior-posterior axis. Dev Cell 38: 61-72. PubMed ID: 27404359

Patronin/Shot cortical foci assemble the noncentrosomal microtubule array that specifies the Drosophila anterior-posterior axis

Noncentrosomal microtubules play an important role in polarizing differentiated cells, but little is known about how these microtubules are organized. This study identified the spectraplakin, Short stop (Shot), as the cortical anchor for noncentrosomal microtubule organizing centers (ncMTOCs) in the Drosophila oocyte. Shot interacts with the cortex through its actin-binding domain and recruits the microtubule minus-end-binding protein, Patronin, to form cortical ncMTOCs. Shot/Patronin foci do not co-localize with gamma-tubulin, suggesting that they do not nucleate new microtubules. Instead, they capture and stabilize existing microtubule minus ends, which then template new microtubule growth. Shot/Patronin foci are excluded from the oocyte posterior by the Par-1 polarity kinase to generate the polarized microtubule network that localizes axis determinants. Both proteins also accumulate apically in epithelial cells, where they are required for the formation of apical-basal microtubule arrays. Thus, Shot/Patronin ncMTOCs may provide a general mechanism for organizing noncentrosomal microtubules in differentiated cells (Nashchekin, 2016).

The recent discovery of the Patronin family of MT minus-end-binding proteins, consisting of Patronin in Drosophila, CAMSAP1, 2, and 3 in mammals, and PTRN-1 in worms, has begun to reveal how the minus ends of noncentrosomal MTs are organized and maintained. The Patronins recognize and stabilize free MT minus ends by protecting them from depolymerization. Patronins appear to play a particularly important role in organizing MTs in differentiated cells. CAMSAP3 localizes to the apical domain in epithelial cells, where it is required for the formation of the apical-basal array of MTs. CAMSAP2 stabilizes neuronal MTs in axon and dendrites, and its knockdown leads to defects in axon specification and dendritic branch formation. Similarly, Caenorhabditis elegans PTRN-1 is required for normal neurite morphology and axon regeneration. The function of Drosophila Patronin has only been examined in cultured S2 cells, where its depletion leads to a decrease in MT number and an increase in free moving MTs (Nashchekin, 2016 and references therein).

The polarized arrangement of the MTs in the Drosophila oocyte depends on the posterior crescent of the Par-1 kinase, which excludes MT minus ends from the posterior cortex. This study shows that Par-1 acts by preventing the association of Shot with the posterior actin cortex, thereby restricting the formation of noncentrosomal MTOCs to the anterior and lateral cortex. Computer modeling has shown that this asymmetric localization of MT minus ends is sufficient to explain the formation of the weakly polarized MT network that directs the transport of oskar mRNA to the posterior pole. Thus, the regulation of the interaction of Shot with the cortex by Par-1 transmits cortical PAR polarity into the polarization of the MT cytoskeleton that localizes the axis determinants (Nashchekin, 2016).

The mechanism by which Par-1 excludes Shot is unknown. The interaction of Shot with the cortex depends on its N-terminal calponin homology domains, which bind to F-actin. Thus, Par-1 could phosphorylate Shot to inhibit its binding to the cortex. If this is the case, Par-1 would have to modify the activity or accessibility of the N-terminal ABD of Shot, as this domain recapitulates the posterior exclusion and cortical recruitment of the full-length protein. Phosphorylation of the ABD by Par-1 was not detected in vitro, however, and it seems more likely that Par-1 acts by modifying the cortex to block the binding of Shot (Nashchekin, 2016).

Shot and its vertebrate ortholog, MACF1, have previously been shown to interact with the MT plus-end tracking protein EB1 through their C-terminal SxIP motifs and with the MT lattice through their Gas2 and C-terminal domains (Nashchekin, 2016).

The current results indicate that in addition to binding to MT plus ends and to the MT lattice, Shot also interacts with MT minus ends through its association with the Patronin/Katanin complex. The exact nature of the interaction between Shot and the Patronin complex is unclear, but Shot was found to interact with Katanin 60 in a high-throughput yeast two-hybrid screen. Thus, one possibility is that Katanin acts as a link between Shot and Patronin. Since Shot is exclusively cortical in the oocyte, the protein does not appear to bind to MT plus ends or along the body of MTs in this system. It will therefore be interesting to investigate whether the different modes of MT binding by Shot are mutually exclusive and how this is regulated (Nashchekin, 2016).

Several models have been proposed to explain the formation of noncentrosomal MTs. Upon centrosome inactivation in postmitotic Drosophila tracheal cells and C. elegans intestinal cells, γ-TuRC complexes and other pericentriolar material (PCM) components are released from the centrosome and transported toward the apical membrane, where they nucleate MT. Whole MTs released from the centrosome can also be delivered and anchored to the apical domain or cell junctions by Ninein. Alternatively, new MTs can grow from MT ends generated by severing enzymes, a mechanism that is thought to be important in plant cells and neurons (Nashchekin, 2016).

This study presents evidence that this latter mechanism is responsible for the formation of the MT array that directs Drosophila axis formation. Firstly, Shot/Patronin ncMTOCs contain stable minus ends even after treatment with the MT-depolymerizing drug, colcemid, as shown by the persistent recruitment of Tau-GFP and EB1-GFP to these foci. This is consistent with the ability of Patronin and CAMPSAPs to capture and stabilize minus ends of single MTs in vitro and in cells. Secondly, MTs start to grow out in all directions from the Shot/Patronin foci immediately after colcemid inactivation. Indeed all visible growing MTs emanate from Patronin foci, indicating that they are the principal source of MTs in the oocyte. Thirdly, the foci contain no detectable γ-tubulin and do not co-localize with PCM proteins. This is consistent with observations in Caco-2 cells, which showed that CAMSAP2 and CAMSAP3 do not co-localize with γ-tubulin and in the C. elegans epidermis, where PTRN-1 and γ-tubulin function in parallel pathways to assemble circumferential MTs (Nashchekin, 2016).

Taken together, these results suggest a model in which the Shot/Patronin foci act as ncMTOCs by capturing and stabilizing MT minus-end stumps that then act as templates for new MT growth. One attractive feature of this model is that it uncouples MT organization from MT nucleation in both space and time. The Shot/Patronin complex bypasses the need to continually nucleate new MTs by preventing existing microtubules from completely depolymerizing. Thus, once a cell has nucleated sufficient MTs, it can maintain and reorganize its MT cytoskeleton by stabilizing MT minus-end stumps in appropriate locations and using these, rather than the γ-tubulin ring complex, to provide the seeds from which new MTs grow. The number of MTs can even increase in the absence of new MT nucleation if MT-severing proteins chop up existing MTs to produce new minus ends that can then be captured and stabilized. The presence of the severing protein, Katanin, in the Shot/Patronin foci is intriguing in this context, as it raises the possibility that it severs existing MTs to provide a local source of minus ends for Patronin to capture (Nashchekin, 2016).

Shot and Patronin also co-localize at the apical cortex of the epithelial follicle cells, where they are required for apical-basal MT organization. This consistent with the recent observation that CAMSAP3 is required for the recruitment of MT minus ends to the apical cortex of mammalian intestinal epithelial cells (Toya, 2016). Thus, this function of Patronin has been evolutionarily conserved. Furthermore, the similarities between roles of Shot and Patronin in the oocyte and the follicle cells suggest that the complex may provide a general mechanism for organizing noncentrosomal MTs. The relationship between Shot and Patronin is different in the follicle cells compared with the oocyte, however, as Shot is not required for the apical recruitment of Patronin. Nevertheless, loss of either protein produces a very similar loss of apical MT and a reduction in overall MT density. Although it cannot be ruled out that they act in parallel pathways, this observation suggests that they collaborate to anchor MTs to the apical cortex. The combination of Patronin binding to the MT minus ends and Shot binding to the MT lattice may therefore provide a robust anchor to retain MTs at the apical cortex (Nashchekin, 2016).

Levels of Par-1 kinase determine the localization of Bruchpilot at the Drosophila neuromuscular junction synapses

Functional synaptic networks are compromised in many neurodevelopmental and neurodegenerative diseases. While the mechanisms of axonal transport and localization of synaptic vesicles and mitochondria are relatively well studied, little is known about the mechanisms that regulate the localization of proteins that localize to active zones. Recent finding suggests that mechanisms involved in transporting proteins destined to active zones are distinct from those that transport synaptic vesicles or mitochondria. This study reports that localization of BRP-an essential active zone scaffolding protein in Drosophila, depends on the precise balance of neuronal Par-1 kinase. Disruption of Par-1 levels leads to excess accumulation of BRP in axons at the expense of BRP at active zones. Temporal analyses demonstrate that accumulation of BRP within axons precedes the loss of synaptic function and its depletion from the active zones. Mechanistically, it was found that Par-1 co-localizes with BRP and is present in the same molecular complex, raising the possibility of a novel mechanism for selective localization of BRP-like active zone scaffolding proteins. Taken together, these data suggest an intriguing possibility that mislocalization of active zone proteins like BRP might be one of the earliest signs of synapse perturbation and perhaps, synaptic networks that precede many neurological disorders (Barber, 2018).

Par-1 is an evolutionarily conserved serine threonine kinase that has many diverse roles, including important roles in regulating cell polarity and regulating microtubule stability. Genome-wide association studies have implicated Par-1 (MARK) in Alzheimer's disease (AD). While accumulations of Aβ and tau are implicated in the widespread neuronal death found in late stages of AD, synapse instability is often associated with early stages during the progression of AD. Indeed, animal models of tauopathy show an increase in synapse instability. Therefore, it is proposed that synapse instability might be one of the early events in neurodegenerative diseases like AD and that increase in Par-1/ MARK4 could facilitate the instability and hasten the demise of synapses (Barber, 2018).

Synaptic plasticity is determined by its ability to modulate its response to stimulation. Generally, activity leads to strengthening of synapses, which is bigger response to stimulation. Therefore, maintenance of synapses is important in maintaining the synaptic networks, which are disrupted in both neurodevelopmental and neurodegenerative diseases. Indeed, mutations in cysteine string protein (CSP), which plays an important role in synaptic maintenance, causes a progressive motor neuron disorder characterized by neurodegeneration. Thus, maintaining stable synapses might be important to avoid the failure of synaptic networks (Barber, 2018).

At the Drosophila NMJ synapses, active zones can be rapidly modified to induce synaptic homeostatic changes, which are partly dependent on BRP. Interestingly, in a Drosophila model of ALS, disruption of shape and size of T-bars, which consists primarily of BRP, precedes synapse degeneration. These data suggest that disruption of T-bars might be an early marker for synapse breakdown. The current data support this hypothesis because it was found that the doughnut shape of T-bars is dramatically altered in flies overexpressing Par-1 and this happens before the decrease in the number of AZs marked by BRP. Finally, it is posited that loss of BRP from synapses could lead to a failure of synaptic homeostasis because BRP plays an important role in synaptic vesicle release. Interestingly, loss of synaptic homeostasis has been implicated in early phases of neurodegeneration and, restoring synaptic homeostasis can restore synaptic strength in a Drosophila model of ALS. Thus, gradual loss of BRP from synapse may impair the ability of a synapse to efficaciously respond to changes that perturb synaptic homeostasis leading to catastrophic failure of neural networks (Barber, 2018).

One of the vital functions performed by axonal transport is to maintain steady state levels of synaptic proteins required for the efficacious release of neurotransmitter release. Disruption of axonal transport has been implicated in neurodegenerative diseases. Indeed, mutations that affect axonal transport lead to neurodegenerative diseases. A recent study suggests that active zone density is maintained during the developmental stages but is significantly decreased with aging. Interestingly, axonal transport also declines with aging suggesting that a combination of decreased axonal transport of active zone proteins along with aging may lead to a gradual decrease in the maintenance of active zones. This may eventually lead to a failure to maintain synaptic function and ultimately lead to synapse degeneration. While this hypothesis is generally accepted, it has proven difficult to determine whether axonal transport is a cause or consequence of synapse loss. Temporal analysis suggests that following sequence of events: Par-1 localizes to the axons followed by BRP accumulation in axons likely leading to the decreased synaptic function and finally the reduction of BRP from synaptic active zones likely leading to synapse instability. Together, these findings support the hypothesis that defects in axonal transport cause synapse degeneration (Barber, 2018).

While so far it is not precisely understandood how active zone scaffold proteins like BRP are localized, based on the present study, it is speculated that phosphorylation of Par-1 substrate may be important in determining the localization of BRP. This is because while the expression of WT Par-1 causes accumulation of BRP within axons, expression of inactive Par-1 does not lead to show any aberrant localization of BRP. The data suggest that defects in BRP localization are not mediated either by tau or Futsch but BRP may be a possible substrate of Par-1. This is because the data indicate that BRP and Par-1 may be in the same molecular complex. However, it remains to be determined whether Par-1 can phosphorylate BRP and whether phosphorylation of BRP is required for its localization. Previous studies have shown that BRP can be acetylated, and that this posttranslational modification is important in regulating the structure of T-bars but whether BRP can be phosphorylated remains to be studied. Finally, the data indicate that presynaptic Par-1 levels are important in determining BRP localization because Par-1 knockdown also results in the accumulation of BRP within the axons. Thus, Par-1 not only has an important role in postsynaptic compartment but also has an important function on the presynaptic side. Finally, it should be noted that this study is a limited but an important extension of a previous study of how Par-1 regulates the localization of important active zone proteins such as BRP. This study also opens up a lot of questions. For example, what is the half life of BRP at the AZs? Does BRP get replaced? If so, at what rate? These are some important questions that should be addressed by future studies but this study opens up the possibility to study these processes in much more detail (Barber, 2018).

A PAR-1-dependent orientation gradient of dynamic microtubules directs posterior cargo transport in the Drosophila oocyte

Cytoskeletal organization is central to establishing cell polarity in various cellular contexts, including during messenger ribonucleic acid sorting in Drosophila melanogaster oocytes by microtubule (MT)-dependent molecular motors. However, MT organization and dynamics remain controversial in the oocyte. This study used rapid multichannel live-cell imaging with novel image analysis, tracking, and visualization tools to characterize MT polarity and dynamics while imaging posterior cargo transport. All MTs in the oocyte were highly dynamic and were organized with a biased random polarity that increased toward the posterior. This organization originated through MT nucleation at the oocyte nucleus and cortex, except at the posterior end of the oocyte, where PAR-1 suppressed nucleation. These findings explain the biased random posterior cargo movements in the oocyte that establish the germline and posterior (Parton, 2011).

Despite the importance of MTs in the oocyte, how they are organized and to what extent they are dynamic have remained highly controversial. Moreover, the prevailing models for MT organization have mostly relied on a static view of MT distribution and on indirect measures of polarity, such as the steady-state distribution of motor fusions and cargoes. By using live-cell imaging and developing novel image analysis and global visualization tools, the dynamics and polarity of MTs were characterized directly in living oocytes. MTs were found to form a dynamic cortical network extending into the posterior with a bias in net orientation that increases toward the posterior. It was established that posterior-directed cargo is actively transported on these dynamic MTs, with no evidence for preferential transport by a subpopulation of more stable, posttranslationally modified MTs. Significantly, the magnitude and distribution of the observed bias in cargo movements parallels closely the polarity of the MT network. These findings explain the previously reported subtle biased random transport of posterior cargoes and lead to the proposal of the following model for posterior cargo localization: posterior cargo is transported on the entire dynamic MT network and the overall net bias in MT orientation directs the net movement of cargo to the posterior cap, where it becomes anchored (Parton, 2011).

The results reveal that the establishment of the biased MT network is dependent on a specific distribution of MT nucleation sites around the oocyte cortex, with a critical, PAR-1-dependent exclusion of MT nucleation from the posterior cortex. This extends upon previous observations that PAR-1 affects MT organization, leading to an increased density of MTs at the posterior. By using highly sensitive imaging techniques in live oocytes, it was demonstrated that, in contrast to previous work, in stage 9 oocytes, MT nucleation is also distributed along the anterior and lateral cortexes. Initiation of MTs is predominantly from the anterior of the oocyte with a sharp decrease in nucleation along the posterior two thirds of the oocyte cortex. Those MTs nucleating along the anterior are constrained to grow in a more posterior orientation, whereas nucleation along the lateral cortex is more random in orientation. The combination of these two contributions to the network of MTs present in the oocyte results in a slight excess of plus ends extending in a posterior direction, which increases in magnitude closer to the posterior. Despite the fact that, at the extreme posterior, there are no MT nucleation sites, it was found that, even at the extreme posterior, a percentage of MTs appear to orient toward the anterior. This is caused by MTs bending around as they extend into the posterior. Importantly, this detailed analysis of cargo movements reveals a bias in cargo movement directionality at the posterior that matches precisely the bias in MT orientation (Parton, 2011).

It is interesting to consider how the PAR-1 kinase might prevent the nucleation of MTs at the posterior cortex. The PAR genes are conserved polarity determinants with common functions in a variety of organisms. PAR-1 is known to function in association with other PAR proteins, so it is possible that the other PAR proteins also function together with PAR-1 to inhibit MT nucleation in the oocyte. However, several other factors may also be involved. PAR-1 could affect MTs through its association with Tau, which has been shown in mammalian cells and proposed in the Drosophila oocyte, but this remains contentious, as the presence of Tau is not absolutely required for PAR-1 function. Another possibility is that PAR-1 could act through the components of the γ-TuRC complex or some other MT nucleation components, rather than through a direct affect on MTs. Whatever the molecular mechanism of PAR-1 inhibition of MT nucleation, it is most likely to involve the phosphorylation of a downstream target of the PAR-1 kinase (Parton, 2011).

The live-imaging results highlight the role of a dynamic MT network in establishing cell polarity in the oocyte, in which no stable, posttranslationally modified MTs were detected. This raises the question of why this should be the case when, in some other cells, subsets of either stably bundled or completely stable, posttranslationally modified MTs have been observed and proposed to have functional roles in directing cell polarity. Moreover, in many polarized cell types, including the blastoderm embryo and secretory columnar epithelial cells of egg chambers, MTs are organized with a very strong apical-basal polarity and include stable MTs. This makes functional sense in both cases, as cargoes have to be transported very rapidly either apically or basally. In contrast, in the oocyte, MTs perform three key functions that are not necessarily all compatible with having a very strict apical-basal polarity. First, they provide a means of randomly distributing generic components, such as mitochondria and lipid droplets, throughout the cytoplasm. Second, they provide a network to gather cargoes and redistribute them to distinct intracellular destinations, initiating and maintaining cell polarity. Third, they provide a scaffold that maintains structural integrity. It is proposed that the dynamic, subtly biased network of MTs in the oocyte provides an efficient compromise for dealing with these multiple conflicting biological requirements. During mid-oogenesis, the oocyte undergoes a huge expansion in size, when many cellular components are transported from the nurse cells or secreted from the overlying follicle cells. Although generic cellular components, such as Golgi, mitochondria, and lipid droplets, must be kept distributed throughout the ooplasm, the nucleus and specific mRNAs and proteins must be transported to different poles to establish the embryonic axes. A biased random network of MTs enables the mixing of generic components by continuous transport using molecular motors with opposing polarities, while, at the same time, allowing specific components to be transported by motors with single polarities to the anterior or posterior poles for anchoring. Furthermore, a network of highly dynamic MTs would allow efficient capturing of cargo by the motors throughout the entire ooplasm in a rapidly growing and developing oocyte. The fact that the MT network is highly dynamic also makes considerable functional sense for such a rapidly developing system. The MT cytoskeleton is reorganized extensively during Drosophila oogenesis but most dramatically during stage 7. This fits well with observations in other cell types showing that MTs are highly dynamic in nature and are often reorganized to direct cellular polarization (Parton, 2011).

MTs certainly play critical roles in driving cell polarization and extension in many kinds of eukaryotic cells, for example, during guidance of extending neuronal growth cones, in migratory cells, in dorsal closure, and in fields of bristles with planar polarity in fly wings. In all these cases, the polarity and dynamics of MTs have tended to be studied quite crudely because of an inability to follow the subtleties of global MT polarity and dynamics. Therefore, it is highly likely that MTs in such cells are more complex and subtle than previously thought. Interestingly, at least in Xenopus laevis oocytes in which hook decoration and EM were used as the previous gold standard for determining the polarity of individual MTs, a network of MTs is nucleated at the cortex, leading to a bias of polarity rather than an absolute polarity. The methods used in this study are significantly easier to apply technically than hook decoration methods and are, therefore, more generally applicable to study the orientation and dynamics of MTs in most cells. For example, it has been possible to apply these analysis tools to examine subtleties of MT organization in migratory border cells (Parton, 2011).

It is proposed that during cellular reorganization and repolarization, as in the oocyte, the establishment of a dynamic, subtly biased MT network is a widespread phenomenon and provides a general mechanism by which strong cell polarity can be initiated and maintained while efficiently handling the transport requirements of cargoes distributed throughout the cytoplasm. The tools developed in this study to quantitate global or local bias in a complex field of MTs can now be applied widely to other oocytes and other cell types to test the generality of the proposed biased random model for MT organization (Parton, 2011).

date revised: 22 October 2023

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