runt

DEVELOPMENTAL BIOLOGY

Embryonic

Double-staining immunolocalization experiments were used to determine the overlap of the Runt protein pattern with the patterns of Hairy, Even-skipped, and Fushi tarazu. The patterns of Runt and Hairy are complementary. Their phasing is shifted anteriorly by two cell diameters with respect to the complementary EVE and FTZ patterns (Kania, 1990).

In early germ band extended embryos, the pattern of runt expression changes from a pair-rule pattern to a segmental pattern, consisting of a 13-striped pattern. runt is also expressed in a dorsal patch of cells in the head. Antibodies against the Runt protein reveal that it is expressed in a subset of neuroblasts, ganglion-mother cells and neurons. A subset of these neurons also co-express the segmentation gene even-skipped. Neurons require runt activity independent of its activity during segmentation. Upon completion of germ band retraction, about 50 cells per hemisegment are stained in the CNS (Kanai, 1990). These results have been confirmed using a temperature-sensitive runt allele. The requirement for runt arises during an early stage of neurogenesis (Duffy, 1991).

See Chris Doe's Hyper-Neuroblast map site for information on the expression of runt in specific neuroblasts.

To identify X chromosomal genes required for salivary gland development in the Drosophila embryo, embryos hemizygous for EMS-induced lethal mutations were screened to find mutations causing gross morphological defects in salivary gland development. The parental strain carried a lac Z transgene on the second chromosome, which was specifically expressed in the salivary glands so the mutations could be unambiguously identified. Embryos from 3,383 lines were tested for salivary gland abnormalities following lacZ staining. From 63 lines exhibiting aberrant salivary gland phenotypes, 52 stable lines were established containing mutations affecting salivary gland development. From these, 39 lines could be assigned to nine complementation groups: armadillo, brinker, folded gastrulation, giant, hindsight, Notch, runt, stardust and twisted gastrulation (Lammel, 2000).

Mutations in loci involved in segmentation are expected to affect salivary gland development if they are required for PS 2 formation. runt is required for the refinement of the fushi tarazu (ftz) pattern. In the absence of ftz, Scr expression in most of the anterior cells of PS 2 is missing. Although it may seem attractive to propose that run acts via ftz, a more direct run function also seems plausible (Lammel, 2000 and references therein).

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).

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 Drosophila brain develops from the procephalic neurogenic region of the ectoderm. About 100 neural precursor cells (neuroblasts) delaminate from this region on either side in a reproducible spatiotemporal pattern. Neuroblast maps have been prepared from different stages of the early embryo (stages 9, 10 and 11, when the entire population of neuroblasts has formed), in which about 40 molecular markers representing the expression patterns of 34 different genes are linked to individual neuroblasts. In particular, a detailed description is presented of the spatiotemporal patterns of expression in the procephalic neuroectoderm and in the neuroblast layer of the gap genes empty spiracles, hunchback, huckebein, sloppy paired 1 and tailless; the homeotic gene labial; the early eye genes dachshund, eyeless and twin of eyeless; and several other marker genes (including castor, pdm1, fasciclin 2, klumpfuss, ladybird, runt and unplugged). Based on the combination of genes expressed, each brain neuroblast acquires a unique identity, and it is possible to follow the fate of individual neuroblasts through early neurogenesis. Furthermore, despite the highly derived patterns of expression in the procephalic segments, the co-expression of specific molecular markers discloses the existence of serially homologous neuroblasts in neuromeres of the ventral nerve cord and the brain. Taking into consideration that all brain neuroblasts are now assigned to particular neuromeres and individually identified by their unique gene expression, and that the genes found to be expressed are likely candidates for controlling the development of the respective neuroblasts, these data provide a basic framework for studying the mechanisms leading to pattern and cell diversity in the Drosophila brain, and for addressing those mechanisms that make the brain different from the truncal CNS (Urbach, 2003).

In the trunk, the pair-rule gene runt is expressed in segmental domains of the ventral neuroectoderm and in five NBs of row 2 and 3 and two NBs of row 5. Runt has also been shown to be expressed in an anterodorsal region of the blastoderm, corresponding to the presumptive head region. En/Runt antibody co-labelling reveals that this Runt domain contributes to the ocular segment. In addition to the ocular segment, patches of runt expression are in the intercalary, antennal and clypeolabral ectoderm, and in subsets of protocerebral and deutocerebral NBs. At stage 11, the protein is expressed in a total of 23 brain NBs, some of which initiate Runt expression after delamination from Runt-negative ectoderm, and in a large number of postmitotic cells until the end of embryogenesis (Urbach, 2003).

Concentric zones, cell migration and neuronal circuits in the Drosophila visual center

The Drosophila optic lobe comprises a wide variety of neurons, which form laminar neuropiles with columnar units and topographic projections from the retina. The Drosophila optic lobe shares many structural characteristics with mammalian visual systems. However, little is known about the developmental mechanisms that produce neuronal diversity and organize the circuits in the primary region of the optic lobe, the medulla. This study describes the key features of the developing medulla and reports novel phenomena that could accelerate understanding of the Drosophila visual system. The identities of medulla neurons are pre-determined in the larval medulla primordium, which is subdivided into concentric zones characterized by the expression of four transcription factors: Drifter, Runt, Homothorax and Brain-specific homeobox (Bsh). The expression pattern of these factors correlates with the order of neuron production. Once the concentric zones are specified, the distribution of medulla neurons changes rapidly. Each type of medulla neuron exhibits an extensive but defined pattern of migration during pupal development. The results of clonal analysis suggest homothorax is required to specify the neuronal type by regulating various targets including Bsh and cell-adhesion molecules such as N-cadherin, while drifter regulates a subset of morphological features of Drifter-positive neurons. Thus, genes that show the concentric zones may form a genetic hierarchy to establish neuronal circuits in the medulla (Hasegawa, 2011).

Concentric genes are expressed in a defined subset of medulla neurons throughout development, suggesting that a part of neuronal identities are pre-determined in the larval medulla primordium. The data suggest that Drf-positive neurons produce nine types of medulla neurons, including lobula projection and medulla intrinsic neurons, while Hth-positive neurons produce at least four types of neurons, including lamina projection and medulla intrinsic neurons. In Hth-positive neurons, Bsh is exclusively expressed in medulla intrinsic Mi1 neurons. A hth mutation caused the neuron to switch type, while a drf mutation affected subsets of morphological features of Drf-positive neurons. Thus, roles of concentric genes may be functionally segregated to form a genetic hierarchy. Apparently, other concentric genes must exist in addition to the four genes reported in this study. Because there are many neurons outside of the Drf domain in the larval medulla, some concentric genes may be expressed in the outer zones. Some transcription factors may have expression patterns that differ from those of concentric genes, and their combined expression may specify restricted subtypes of medulla neurons. For example, apterous (ap) and Cut are widely expressed in medulla neurons. Cut was co-expressed in subsets of Drf-positive neurons, while ap was expressed in all Drf- and Bsh/Hth-positive neurons (Hasegawa, 2011).

Early-born medulla neurons express the inner concentric genes, while late born neurons express the outer ones. Thus, concentric gene expression correlates with neuronal birth order. However, it is still unknown how concentric gene expression is specified. It would be possible to speculate that genes controlling temporal specification of neurons are expressed in NBs to control the concentric gene expression. However, the genes that are known to control neuronal birth order in the embryonic CNS were not expressed in larval medulla NBs. In addition to local temporal mechanisms, such as birth order, global and spatial mechanisms governed by morphogen gradient may also play a role in determining medulla cell type. In addition to birth order or a morphogen gradient, mutual repression among concentric genes may be essential in establishing defined concentric zones. Except for rare occasions, de-repression of other concentric genes was not induced in clones mutant for hth or drf. Additionally, ectopic hth expression did not compromise Drf and Run expression. These results may suggest that unidentified genes act redundantly with these genes to repress expression of other concentric genes and that weak Hth expression in NBs does not play roles in temporal specification of medulla neurons (Hasegawa, 2011).

Various types of cell migration play important roles during vertebrate neurogenesis. Although Drosophila has been a powerful model of neural development, extensive neuronal migrations coupled with layer formation found in this study have not been previously reported. The current findings may establish a model to understand molecular mechanisms that govern brain development via neuronal migrations (Hasegawa, 2011).

It is important to know whether the migration of medulla neurons occurs actively or passively. The distribution of cell bodies in the adult medulla cortex was not random, but organized according to cell type. In particular, the Mi1 neurons identified by Bsh expression migrated outwards and were eventually located in the outermost area of the adult medulla cortex, which was affected in hth mutant clones. The observation that defined localization of cell bodies is under the control of genetic program may not be explained by passive migration. Repression of apoptosis by expressing p35 under the control of elav-Gal4 did not compromise migration of Bsh- and Drf-positive neurons, suggesting that apoptosis is not a driving force of the migration. If neurons migrate actively in an organized manner, what regulates the pattern of migration? In many cases, glial cells play important roles in neuronal migration. Indeed, glial cells and their processes were identified in the medulla cortex. Glial cells or other cell types could provide cues for neuronal migration (Hasegawa, 2011).

The medulla neurons project axons near their targets forming subsets of dendrites in the larval brain; the cell bodies migrated in the presence of preformed neurites during pupal development. During or following cell body migration, additional dendrites were formed along the axonal shafts. Therefore, cell body migration may somehow contribute to circuit formation in the medulla. Indeed, similar strategies have been reported in sensory neurons of C. elegans and cerebellar granule cells in mammals. Thus, cell body migration in the presence of neurites may be a general conserved mechanism of circuit formation. Cell body migration may also allow developing cells to receive inductive cues provided by cells in the vicinity of the medulla cortex. For example, glial cells placed on the surface of the brain may trigger the expression of specific genes (e.g. ChAT) in Mi1 cells that are located in the outermost area of the adult medulla cortex (Hasegawa, 2011).

In adults, Mi1 neurons have arborization sites at M1 and M5, which coincide with the L1 lamina neuron terminals. In Golgi studies, Mi1 neurons were found in all parts of the retinotopic field. Indeed, the number of Bsh expressing medulla neurons was about 800, a figure similar to the number of ommatidial units. Therefore, the Mi1 neurons identified by Bsh expression are most probably columnar neurons with direct inputs from L1 neurons. Because L1 is known to have inputs from R1-6, which processes motion detection, Mi1 may participate in the motion detection circuit (Hasegawa, 2011).

If the genetic codes that specify each type of neuron are found, it may encourage the functional study of defined neurons. In the medulla, bsh-Gal4 is solely expressed by Mi1 neurons. Although the expression of Bsh is also observed in L4/5 lamina neurons, intersectional strategies such as split Gal4 may enable the activity of Mi1 to be specifically manipulated by inducing expression of neurogenetic tools like shibire^ts. This could provide insight into high-resolution functional neurobiology in the Drosophila visual system (Hasegawa, 2011).

Development of the mammalian central nervous system reiteratively establishes cell identity, directs cell migration and assembles neuronal layers, processes similar to the patterns observed during medulla development. In the cerebral cortex, neurons are generated within the ventricular or subventricular zones and migrate outwards, leaving their birthplace along the radial glial fibers. Later-born neurons migrate radially into the cortical plate, past the deep layer neurons and become the upper layers. The layers of the cortex are thus created inside-out. In the developing spinal cord, neuronal types are specified according to morphogen gradients. Within each domain along the dorsoventral axis, neuronal and glial types are specified according to their birth order. The spinal cord neurons then migrate extensively along the radial, tangential and rostrocaudal axes. Therefore, the initial organization of spinal cord neurons is disrupted in the mature system (Hasegawa, 2011).

The medulla shares intriguing similarities with the mammalian central nervous system. For example, the concentric zones established in the larval medulla resemble the dorsoventral subdivisions of the spinal cord. Extensive migrations of medulla neurons disrupt concentric zones, as observed in the spinal cord. However, this study found that the locations of cell bodies were organized according to neuronal type, a distribution that may be similar to the cortical organization of the cerebral cortex. Thus, the development of the medulla may share characteristics with various forms of neurogenesis found in the mammalian central nervous system. A comprehensive study of important features of neurogenesis will now be possible using the Drosophila visual center and powerful tools of Drosophila genetics. Unveiling all aspects of development in the medulla will not only shed light into the functional neurobiology of the visual system, but also elucidate the developmental neurobiology of vertebrates and invertebrates (Hasegawa, 2011).

A temporal mechanism that produces neuronal diversity in the Drosophila visual center

The brain consists of various types of neurons that are generated from neural stem cells; however, the mechanisms underlying neuronal diversity remain uncertain. A recent study demonstrated that the medulla, the largest component of the Drosophila optic lobe, is a suitable model system for brain development because it shares structural features with the mammalian brain and consists of a moderate number and various types of neurons. The concentric zones in the medulla primordium that are characterized by the expression of four transcription factors, including Homothorax (Hth), Brain-specific homeobox (Bsh), Runt (Run) and Drifter (Drf/Vvl), correspond to types of medulla neurons. This study examined the mechanisms that temporally determine the neuronal types in the medulla primordium. For this purpose, transcription factors were sought that are transiently expressed in a subset of medulla neuroblasts (NBs, neuronal stem cell-like neural precursor cells) and identified five candidates [Hth, Klumpfuss (Klu), Eyeless (Ey), Sloppy paired (Slp) and Dichaete (D)]. The results of genetic experiments at least explain the temporal transition of the transcription factor expression in NBs in the order of Ey, Slp and D. The results also suggest that expression of Hth, Klu and Ey in NBs trigger the production of Hth/Bsh-, Run- and Drf-positive neurons, respectively. These results suggest that medulla neuron types are specified in a birth order-dependent manner by the action of temporal transcription factors that are sequential ly expressed in NBs (Suzuki, 2013).

In the embryonic central nervous system, the heterochronic transcription factors suchas Hb, Kr, Pdm, Cas and Grh are expressed in NBs to regulate the temporal specification of neuronal identity. They regulate each other to achieve sequential changes in their expression in NBs without cell-extrinsic factors. However, expression of the embryonic heterochronic genes was not detected in the medulla NBs.Instead this study found that Hth, Klu, Ey, Slp and D are transiently and sequentially expressed in medulla NBs. The expression of Hth and Klu was observed in lateral NBs, while that of Ey/Slp and D was observed in intermediate and medial NBs, respectively. These observations suggest that the expression of heterochronic transcription factors changes sequentially as each NB ages, as observed in the development of the embryonic central nervous system (Suzuki, 2013).

This study demonstrates that at least three of the temporal factors Ey, Slp and D regulate each other to form a genetic cascade that ensures the transition from Ey expression to D expression in the medulla NBs. Ey expression in NBs activates Slp, while Slp inactivates Ey expression. Similarly, Slp expression in NBs activates D expression, while D inactivates Slp expression. In fact, the expression of Slp is not strong in newer NBs in which Ey is strongly expressed, but is up regulated in older NBs in which Ey is weakly expressed in the wildtype medulla. A similar relationship is found between Slp and D, supporting the idea that Ey, Slp and D regulate each other's expression to control the transition from Ey-expression to D-expression. In the embryonic central nervous system, similar interaction is mainly observed between adjacent genes of the cascade hb-Kr-pdm-cas-grh, and this concept may also be applied to the medulla primordium. The expression pattern and function of Ey, Slp and D suggest that they are adjacent to each other in the cascade of transcription factor expression in medulla NBs (Suzuki, 2013).

However, no such relationship was found between Hth, Klu and the other temporal factors.The sequential expression of Hth and Klu could be regulated by an unidentified mechanism that is totally different from the genetic cascade that controls the transition through Ey-Slp-D. Or, there might be unidentified temporal factors that are expressed in lateral NBs which act upstream of Hth and Klu to regulate their expression. It is necessary to identify additional transcription factors that are transiently expressed in medulla NBs (Suzuki, 2013).

The expression of concentric transcription factors in the medulla neurons correlates with the temporal sequence of neuron production from the medulla NBs (Hasegawa, 2011). In the larval medulla primordium, the neurons are located in the order of Hth/Bsh-, Run- and Drf-positive cells from inside to outside, and these domains are adjacent to each other (Hasegawa, 2011). Given that NBs generate neurons toward the center of the developing medulla, Hth/Bsh-positive neurons are produced at first, and then Run-positive and Drf-positive neurons. Thus Hth/Bsh, Run and Drf were used as markers to examine roles of Hth, Klu, Ey, Slp and D expressed in NBs in specifying types of medulla neurons. The continuous expression of Hth and Ey from NBs to neurons and the results of clonal analyses that visualize the progeny of NBs expressing each one of the temporal transcription factors suggest that the temporal windows of NBs expressing Hth, Klu and Ey approximately correspond to the production of Hth/Bsh-, Run- and Drf- positive neurons, respectively. Indeed, the results of the genetic study suggest that Hth and Ey are necessary and sufficient to induce the production of Hth/Bsh- and Drf-positive neurons,respectively (Hasegawa, 2011, 2013). Ectopic Klu expression at least induces the produc tion of Run-positive neurons (Suzuki, 2013).

Slp and D expression in NBs may correspond to the temporal windows that produce medulla neurons in the outer domains of the concentric zones, which are most likely produced after the production of Drf-positive neurons. The results at least suggest that Slp is necessary and sufficient and D is sufficient to repress the production of Drf-positive neurons. Identification of additional markers that are expressed in the outer concentric zones compared to the Drf-positive domain would be needed to elucidate the roles of Slp and D in specification of medulla neuron types (Suzuki, 2013).

D mutant clones did not produce any significant phenotype except for derepression of Slp expression in NBs. Drf expression in neurons was not affected either. Since D is a Sox family transcription factor, SoxN, another Sox family transcription factor, is a potential candidate molecule that acts together with D in the medulla NBs. However, its expression was found in neuroepithelia cells and lateral NBs that overlap with Hth-positive cells but not with D-positive cells. All the potential heterochronic transcription factors examined in this study are expressed in three to five cell rows of NBs. Nevertheless, one NB has been observed to produce one Bsh- positive and one Run-positive neuron (Hasegawa, 2011). Therefore, the expression pattern of the heterochronic transcription factors is not sufficient to explain the stable production of one Bsh-positive and one Run-positive neuron from a single NB.The combinatorial action of multiple temporal factors expressed in NBs may play important roles in the specification of Bsh- and Run- positive neurons (Suzuki, 2013).

Another possible mechanism that guarantees the production of a limited number of the same neuronal type from multiple rows of NBs expressing a temporal transcription factor could be a mutual repression between concentric transcription factors expressed in medulla neurons. For example, Hth/Bsh, Run and Drf may repress each other to restrict the number of neurons that express either of these transcription factors. However, expression of Run and Drf was not essentially affected in hth mutant clones and in clones expressing Hth (Hasegawa, 2011). Similarly, expression of Hth and Drf was not essentially affected in clones expressing run RNAi under the control of AyGal4, in which Run expression is eliminated. Hth and Run expression was not affected in drf mutant clones (Hasegawa, 2011). These results suggest that Hth/Bsh, Run and Drf do not essentially regulate each other during the formation of concentric zones in the medulla (Suzuki, 2013).

During embryonic development, the heterochronic genes that are expressed in NBs (hb-Kr-pdm-cas-grh) are maintained and act in GMCs to specify neuronal type. Similarly, Hth and Ey are continuously expressed from NBs to neurons, suggesting that their expression may also be inherited through GMCs (Hasegawa, 2011). However, this type of regulatory mechanism may be somewhat modified in the case of Klu, Slp and D (Suzuki, 2013).

Klu is expressed in NBs and GMCs, but not in neurons. Slp and D are predominantly detected in NBs and neurons visualized by Dpn and Elav, respectively. Occasionally, however, expression of D was found in putative GMCs, which are situated between NBs and neurons. Additionally, both D-positive and D-negative cells were found among Miranda-positive GMCs. Slp expression was not found in Miranda-positive GMCs. Finally, D is expressed in medulla neurons forming a concentric zone in addition to its expression in medial NBs. However, D expression was abolished in slp mutant NBs but remained in the mutant neurons, suggesting that D expression in medulla neurons is not inherited from the NBs. These results suggest that Slp and D expression are not maintained from NBs to neurons and that not all the temporal transcription factors expressed in NBs are inherited through GMCs. However, it is possible to speculate that Klu, Slp and D regulate expression of unidentified transcription factors in NBs that are inherited from NBs to neurons through GMCs (Suzuki, 2013).

Effects of Mutation

Slam servers as a molecular marker for polarized cell behavior revealing functions of Eve, Runt, Myosin II and Bazooka in germband extension

During convergent extension in Drosophila, polarized cell movements cause the germband to narrow along the dorsal-ventral (D-V) axis and more than double in length along the anterior-posterior (A-P) axis. This tissue remodeling requires the correct patterning of gene expression along the A-P axis, perpendicular to the direction of cell movement. A-P patterning information results in the polarized localization of cortical proteins in intercalating cells. In particular, cell fate differences conferred by striped expression of the even-skipped and runt pair-rule genes are both necessary and sufficient to orient planar polarity. This polarity consists of an enrichment of nonmuscle myosin II at A-P cell borders and Bazooka/PAR-3 protein at the reciprocal D-V cell borders. Moreover, bazooka mutants are defective for germband extension. These results indicate that spatial patterns of gene expression coordinate planar polarity across a multicellular population through the localized distribution of proteins required for cell movement (Zallen, 2004).

Polarized cell movement during convergent extension ultimately derives from the asymmetric localization of proteins that direct cell motility. Interestingly, intercalating cells in the Drosophila germband display a polarized localization of the ectopically expressed Slam protein (Lecuit, 2002). Slam is present in a bipolar distribution that correlates spatially and temporally with intercalary behavior. These observations indicate that Slam can serve as a molecular marker for polarized cell behavior. Pair-rule patterning genes expressed in stripes along the A-P axis are necessary for Slam localization and, conversely, altering the geometry of their expression is sufficient to reorient Slam polarity. An endogenous planar polarity in intercalating cells has been shown to be manifested by the accumulation of nonmuscle myosin II at A-P cell borders and Bazooka/PAR-3 at D-V cell borders. Moreover, germband extension is defective in bazooka mutant embryos, supporting a model where molecular polarization of the cell surface is a prerequisite for polarized cell movement. Therefore, differences in gene expression along the A-P axis may direct planar polarity in intercalating cells through the creation of molecularly distinct cell-cell interfaces that differ in migratory potential (Zallen, 2004).

Cell movement during germband extension is oriented along the D-V axis, suggesting a mechanism that restricts the productive generation of motility to dorsal and ventral cell surfaces. Molecules that are asymmetrically localized during convergent extension may therefore contribute to the spatial regulation of cell motility. Interestingly, intercalating cells in the Drosophila germband display a polarized localization of the ectopically expressed Slam protein, a novel cytoplasmic factor required for cellularization in the early embryo (Lecuit, 2002). While proteins such as Armadillo/β-catenin are uniformly distributed at the cell surface, ectopic Slam is enriched in borders between neighboring cells along the A-P axis. This polarized Slam population is present in a punctate apical distribution, coincident with the adherens junction component Armadillo/β-catenin. Therefore, intercalating cells have distinct apical junctional domains that differ in their capacity for Slam association (Zallen, 2004).

Interestingly, the polarized distribution of ectopic Slam protein is spatially and temporally correlated with intercalary behavior. Slam polarity is not observed in Stage 6 embryos prior to the onset of intercalation. Slam accumulation at A-P cell borders first appears in late Stage 7, when cells of the germband initiate intercalation, and reaches its full extent during the period of sustained intercalation in Stage 8. In contrast, Slam is uniformly distributed in cells of the head region and the dorsal ectoderm, tissues which do not undergo intercalary movements. These results indicate that the polarized distribution of ectopic Slam protein is specific to intercalating cells and that Slam can therefore serve as a molecular marker for the visualization of polarized cell behavior (Zallen, 2004).

The enrichment of Slam at borders between neighboring cells along the A-P axis is consistent with two modes of localization: Slam could mark one side of each cell in a unipolar distribution, or Slam could localize to both anterior and posterior surfaces in a bipolar pattern. To distinguish between these possibilities, mosaic embryos were generated where Slam-expressing cells were juxtaposed with unlabeled cells, using the Horka mutation to induce sporadic chromosome loss in early embryos. Slam protein accumulates at anterior and posterior boundaries of mosaic clone, indicating that ectopic Slam protein is targeted to both anterior and posterior surfaces of intercalating cells in a symmetric, bipolar distribution. The bipolar localization of ectopic Slam corresponds well with the bidirectionality of cell movement during germband extension, where cells are equally likely to migrate dorsally or ventrally during intercalation. Bipolar motility is also observed during convergent extension in the presumptive Xenopus and Ciona notochords and in Xenopus neural plate cells in the absence of midline structures (Zallen, 2004).

To extend the spatial and temporal correlation between Slam polarity and cell movement, it was asked if this polarized Slam localization is achieved in mutants that are defective for intercalation. Cell intercalation is dependent on the transcriptional cascade that generates cell fates along the A-P axis, in the direction of tissue elongation and perpendicular to the migrations of individual cells. A-P patterning reflects the hierarchical action of maternal, gap, and pair-rule genes. Cell fate differences along the A-P axis are abolished in embryos maternally deficient for the bicoid, nanos, and torso-like genes (referred to as bicoid nanos torso-like mutants), and these mutant embryos do not exhibit intercalary behavior. Ectopic Slam is correctly targeted to the apical cell surface in bicoid nanos torso-like mutants, but fails to adopt a polarized distribution in the plane of the epithelium (Zallen, 2004).

Downstream of the maternal patterning genes, gap genes establish overlapping subdomains along the A-P axis. A quadruple mutant for the gap genes knirps, hunchback, forkhead, and tailless lacks A-P pattern within the germband while retaining terminal structures. This quadruple mutant exhibits severely reduced cell intercalation, and mutant embryos also display a loss of Slam polarity. The absence of planar polarity in A-P patterning mutants correlates with a more hexagonal appearance of germband cells, in contrast to the irregular morphology of wild-type intercalating cells (Zallen, 2004).

In response to maternal and gap genes, pair-rule patterning genes expressed in narrow stripes act in combination to assign each cell a distinct fate along the A-P axis. In particular, the even-skipped (eve) and runt pair-rule genes are essential for germband extension. This strong requirement for eve and runt during germband extension contrasts with the more subtle effects in mutants for other pair-rule genes such as hairy and ftz. Consistent with these defects in intercalation, eve and runt mutants also display aberrant Slam localization. These results establish a correlation between intercalary behavior and the polarized localization of the ectopic Slam marker (Zallen, 2004).

While eve and runt mutants fail to complete germband extension, they extend further than embryos lacking maternal and gap genes, suggesting that some intercalary behavior is retained. Consistent with this possibility, Slam polarity is only partially disrupted in eve and runt mutants. While some cells display an aberrant uniform Slam distribution, in other cells Slam is correctly enriched at A-P cell interfaces. Therefore, the residual intercalation in eve and runt mutants correlates with the establishment of planar polarity in a subset of germband cells (Zallen, 2004).

The disruption of Slam polarity in A-P patterning mutants demonstrates that proper gene expression along the A-P axis is required for planar polarity in intercalating cells. In particular, the Eve and Runt transcription factors are expressed in 7 stripes at the onset of germband extension and 14 stripes as intercalation proceeds. Each cell in the germband is assigned a fate distinct from its anterior and posterior neighbors through the graded and partially overlapping expression of these and other pair-rule genes. Slam preferentially accumulates at contacts between cells with different levels of pair-rule gene activity, suggesting a model where cells concentrate specific proteins at interfaces with neighbors that differ in A-P identity. To directly address this model, mosaic embryos were generated with altered patterns of pair-rule gene expression in order to artificially introduce differences between dorsal and ventral neighbors. It was then asked if Slam protein is aberrantly recruited to these ectopic juxtapositions between different cell types, even at interfaces that are perpendicular to the normal axis of polarity (Zallen, 2004).

The Horka mutation was used to generate embryos that ectopically express Eve or Runt in a mosaic pattern. When these genes are ubiquitously expressed, planar polarity is generally disrupted and Slam displays a more uniform localization. This disruption of Slam polarity correlates with defective germband extension in Eve and Runt overexpressing embryos. The effects of Eve and Runt overexpression are not mimicked by overexpression of other pair-rule proteins such as Paired, Odd-paired, or Sloppy-paired. Moreover, localized sources of Eve or Runt expression direct aberrant patterns of polarity in mosaic embryos. For example, mosaic embryos display circles of Slam polarity that are bordered by ectopic Eve clones. Similarly, Slam polarity in germband cells is diverted from its normal orientation to follow boundaries of Runt misexpression. These results demonstrate that ectopic sites of Eve and Runt expression can reorient Slam polarity at clone boundaries, even when these interfaces are perpendicular to the normal axis of polarity (Zallen, 2004).

In contrast to the reorientation of planar polarity at boundaries of Eve and Runt misexpression, cells distant from the clone often exhibited complex patterns of Slam localization. These patterns may arise from nonautonomous effects of pair-rule gene activity, as well as aberrant cell movements and ectopic folds that form at clone boundaries, suggestive of a disruption in cell adhesion. Therefore mosaic embryos were examined at Stage 7, prior to the onset of cell movement and ectopic fold formation. While early Stage 7 embryos do not normally exhibit Slam polarity, ectopic Eve induces a precocious accumulation of Slam at clone boundaries. In contrast, ectopic Runt only occasionally induces a subtle polarity at Stage 7. The more potent effect of the eve transgene may reflect higher levels of ectopic expression compared to the endogenous eve stripes. These mosaic experiments indicate that differences in gene expression play an instructive role in the generation of planar polarity in intercalating cells. While Eve and Runt are both sufficient for planar polarity, the absence of either gene alone disrupts polarity. However, the defects in eve or runt single mutants may result from a combined disruption of multiple pair-rule genes; loss of eve leads to altered runt expression and vice versa (Zallen, 2004).

Generation of planar polarity by ectopic Eve expression is subject to the same spatial requirements as in wild-type polarity: Eve clones in the head region failed to induce polarity, suggesting that these cells are resistant to Eve-dependent polarization. In contrast, ectopic Runt expression in the head led to a concentration of Slam at clone boundaries, despite the fact that these cells do not normally display Slam polarity or intercalary behavior. These results indicate that in contrast to Eve, Runt can induce planar polarity in head cells, raising the possibility of functional distinctions between Eve and Runt target genes (Zallen, 2004).

The Eve and Runt transcription factors ultimately direct Slam polarity and cell intercalation through the transcriptional regulation of target genes. To identify downstream effectors involved in this process, components of the noncanonical planar cell polarity (PCP) pathway, which is required for convergent extension in vertebrates, were examined. Germband extension occurs normally in the majority of embryos lacking the Frizzled and Frizzled2 receptors. Similarly, germband extension is unaffected in the absence of Dishevelled. Moreover, dishevelled mutants exhibit a normal polarization of the Slam marker. These results demonstrate that molecular and behavioral properties of planar polarity in the Drosophila germband do not require Frizzled or Dishevelled function (Zallen, 2004).

The polarized distribution of ectopic Slam in intercalating cells provides the first clue to a molecular distinction between D-V cell interfaces that generate productive cell motility and A-P interfaces that do not. However, endogenous Slam mRNA and protein are not detected during germband extension, indicating that Slam may not play a functional role in cell intercalation. Slam colocalizes with the Zipper nonmuscle myosin II heavy chain subunit during cellularization and when Slam is ectopically expressed at germband extension (Lecuit, 2002). Therefore, the endogenous distribution of myosin II was examined during germband extension in wild-type embryos. During cell intercalation, myosin II is present in a punctate distribution at the apical cell surface, colocalizing with the adherens junction component Armadillo/β-catenin. In Stage 8 embryos, apical myosin II protein accumulates at interfaces between cells along the A-P axis. Slam can enhance this polarized localization when ectopically expressed (Lecuit, 2002), suggesting that Slam and myosin II may associate with a common localization machinery. Myosin II polarity is not apparent in Stage 6 or early Stage 7 embryos that have not begun intercalation, indicating that the enrichment of myosin II at A-P interfaces is specific to intercalating cells (Zallen, 2004).

The localized distribution of myosin II is not as pronounced as that of ectopic Slam, suggesting that additional asymmetries contribute to the polarization of intercalating cells. To identify such proteins, the localization was examined of components implicated in cell polarity in other cell types. In particular, the PDZ domain protein Bazooka/PAR-3 participates in both apical-basal and planar polarity. Bazooka/PAR-3 also exhibits a polarized distribution in intercalating cells. Bazooka, like myosin II, is present in a punctate apical distribution, coincident with the adherens junction component Armadillo/β-catenin. However, in contrast to the accumulation of myosin II at A-P cell interfaces, Bazooka is enriched in the reciprocal D-V interfaces. Bazooka polarity is specific to intercalating cells, where it first appears at the onset of intercalary movements in late Stage 7. Bazooka polarity is not observed in cells of the head region, which do not undergo intercalation, nor is it observed in germband cells following the completion of germband extension at Stage 9 (Zallen, 2004).

To characterize the relationship between cell shape and the polarized localization of cortical proteins, the orientation of cell borders was measured as an angle relative to the A-P axis (with A-P interfaces closer to 90° and D-V interfaces closer to 0° and 180°). Interfaces from embryos stained for Bazooka and myosin II were ranked according to mean fluorescence intensity as a relative measure of protein distribution. These results illustrate that Bazooka and myosin II are enriched in distinct sets of cell-cell interfaces that adopt largely nonoverlapping orientations relative to the A-P axis. This quantitation confirms the visual impression from confocal images and demonstrates that the molecular composition of a cell surface domain is a reliable predictor of its orientation within the epithelial cell sheet (Zallen, 2004).

The polarized localization of Bazooka is abolished in the absence of A-P patterning information in bicoid nanos torso-like mutant embryos. A similar disruption of myosin II polarity is observed in A-P patterning mutants. The A-P patterning system may therefore mediate cell intercalation through the polarized accumulation of cell surface-associated proteins. Bazooka participates in a conserved protein complex containing the atypical PKC (DaPKC), and DaPKC is also enriched in D-V cell interfaces during germband extension (Zallen, 2004).

To determine whether the polarized Bazooka/PAR-3 protein is functionally required for germband extension, homozygous bazooka (baz) mutant embryos were examined. In zygotic baz mutants, residual Bazooka protein persists from maternal stores and is often, but not always, correctly distributed along the apical-basal and planar axes. Despite this maternal Bazooka contribution, loss of zygotic Bazooka disrupts germband extension. In wild-type embryos, the posterior end of the extended germband is located at 70% egg length from the posterior pole. Of the progeny of baz^YD97/+ females and wild-type males, 72% were wild-type-like, 25% were partially defective, and 3% were strongly defective. These results demonstrate that Bazooka is required for normal germband extension (Zallen, 2004).

Bazooka/PAR-3 and the associated DmPAR-6 and DaPKC components also influence epithelial cell polarity along the apical-basal axis. To address the possibility that germband extension defects may occur indirectly as a result of disrupted apical-basal polarity, properties of apical-basal polarity were examined in zygotic baz mutants, where some functions are carried out by maternal gene products. Zygotic baz mutant embryos exhibit several signs of normal apical-basal polarity at gastrulation, including a monolayer epithelial morphology in the germband and the correct distribution of proteins to apical and lateral membrane domains. This is consistent with findings that zygotic baz mutants exhibit proper localization of the Armadillo/β-catenin adherens junction component prior to Stage 10 of embryogenesis. These results demonstrate that properties of apical-basal polarity are established correctly in baz mutant embryos during germband extension, consistent with a direct role for Bazooka in cell movements along the planar axis, independent of its later effects on apical-basal polarity (Zallen, 2004).

The local reorientation of planar polarity in response to Eve and Runt expression argues that planar polarity is generated by cell-cell interactions, rather than a distant polarizing cue. In addition to these local effects of Eve and Runt on planar polarity, Slam polarity frequently adopted a circular pattern in mosaic embryos, even when Eve and Runt were not present along the entire circumference of the circle. This unexpected configuration indicates that polarizing information can propagate from cell to cell downstream of an Eve-dependent signal. A similar relay mechanism is suggested by the swirling patterns of wing hair polarity that persist in Drosophila mutants defective for the PCP signaling pathway. Therefore, mechanisms of cell-cell communication may reinforce local polarizing events in the organization of a two-dimensional cell population (Zallen, 2004).

Planar polarity in Drosophila germband extension is locally established through the concentration of specific proteins at sites of contact between cells with different levels of Eve and Runt expression. Cells can monitor the identity of their neighbors through qualitative or quantitative differences in the activity of cell surface proteins, perhaps through ligand-receptor mediated signaling events or adhesion-based cell sorting. Transcriptional targets of Eve and Runt are therefore likely to include components that mediate intercellular signaling events involved in the transmission of polarizing information during multicellular reorganization (Zallen, 2004).

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date revised:  5 October 2021





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