The Interactive Fly
Zygotically transcribed genes
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 bazYD97/+ 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).
Tissue morphogenesis requires stereotyped cell shape changes, such as apical cell constriction in the mesoderm and cell intercalation in the ventrolateral ectoderm of Drosophila. Both processes require force generation by an actomyosin network. The subcellular localization of Myosin-II (Myo-II) dictates these different morphogenetic processes. In the intercalating ectoderm Myo-II is mostly cortical, but in the mesoderm Myo-II is concentrated in a medial meshwork. Spacial constriction is repressed by JAK/STAT signalling in the lateral ectoderm independently of Twist. Inactivation of the JAK/STAT pathway causes germband extension defects because of apical constriction ventrolaterally. This is associated with ectopic recruitment of Myo-II in a medial web, which causes apical cell constriction as shown by laser nanosurgery. Reducing Myo-II levels rescues the JAK/STAT mutant phenotype, whereas overexpression of the Myo-II heavy chain (also known as Zipper), or constitutive activation of its regulatory light chain, does not cause medial accumulation of Myo-II nor apical constriction. Thus, JAK/STAT controls Myo-II localization by additional mechanisms. Regulation of actin polymerization by Wasp, but not by Dia, is important in this process. Constitutive activation of Wasp, a branched actin regulator, causes apical cell constriction and promotes medial 'web' formation. Wasp is inactivated at the cell cortex in the germband by JAK/STAT signalling. Lastly, wasp mutants rescue the normal cortical enrichment of Myo-II and inhibit apical constriction in JAK/STAT mutants, indicating that Wasp is an effector of JAK/STAT signalling in the germband. Possible models are discussed for the role of Wasp activity in the regulation of Myo-II distribution (Bertet, 2009).
Myo-II subcellular localization controls different cell shape changes such as cell constriction or intercalation. The data shed new light on the mechanisms of the subcellular localization of actomyosin networks in the early Drosophila ectoderm. VL ectodermal cells intercalate via the cortical recruitment of Myo-II at AJs, which drives polarized junction remodelling. This contrasts with the behaviour of immediately adjacent cells in the mesoderm, which undergo apical constriction and recruit Myo-II into a medial apical web. The data indicate that the cortical enrichment of Myo-II in ectodermal intercalating cells is not a 'default pathway', and requires at least activity of the JAK/STAT pathway. Indeed, in JAK/STAT pathway mutants, Myo-II is aberrantly recruited in a medial apical meshwork and cells consequently undergo apical constriction. This is surprising, as apical constriction is normally only observed in mesodermal ventral cells and is considered to be a unique attribute owing to their selective expression of Twist and Snail. Twist and Snail induce expression of the ligand Fog in the ventral cells only, which activates RhoGEF2, Rok and Myo-II. It also regulates expression of the transmembrane protein T48, which participates in the apical recruitment of RhoGEF2 and contributes to apical constriction. However it is not clear whether activation of the RhoGEF2 pathway is sufficient to drive the apical medial recruitment of Myo-II. This study shows that apical constriction is not simply induced in mesodermal cells by Fog, but is also prevented in ectodermal cells by activity of the JAK/STAT pathway and that this is essential for germ-band extension (GBE). In JAK/STAT pathway mutants, ectodermal cells undergo apical constriction despite the absence of ectopic Twist expression. Note, however, that apical constriction is not as rapid in these mutants as in mesodermal cells, so Twist and Snail accelerate or render more efficient the capacity to apically constrict. Moreover, the fact that Wasp mediates JAK/STAT function in the ectoderm but is not required in the mesoderm indicates that the mechanisms promoting medial Myo-II in mesoderm cells are likely to be different (Bertet, 2009).
These findings provide a novel opportunity to investigate the regulation of cortical or medial Myo-II localization in the ectoderm. The data document two novel features of this regulation (Bertet, 2009).
MRLC (Sqh) phosphorylation by the RhoGEF2 and the Rok pathway are both necessary for apical constriction; lowering the dose of RhoGEF2, Rho or Rok suppress the apical constriction observed in upd mutants. However, neither constitutive activation of this pathway by expression of a phosphomimetic form of Sqh, ShqE20E21, which rescues Rok inhibition, nor overexpression of MHC (Zip) is sufficient to promote medial accumulation of Myo-II. The medial accumulation of Myo-II requires additional regulation apart from the activation of Myo-II. Since RhoGEF2 and Rok are key regulators of Myo-II, this suggests that activation of the RhoGEF2/Rok pathway is necessary but not sufficient to explain medial Myo-II accumulation and apical constriction (Bertet, 2009).
This analysis of the JAK/STAT mutant phenotypes indicates a key role of Wasp in this process. Wasp is shown to be necessary for medial Myo-II accumulation, at least in ectodermal cells, and very strong activation of Wasp at the cortex (myrWasp) also causes medial Myo-II accumulation. Moreover, although Wasp is normally downregulated in VL ectodermal cells, in JAK/STAT pathway mutants Wasp is strongly recruited and hence activated at the plasma membrane, which suggests that JAK/STAT signalling represses the membrane activation of Wasp. Importantly, lowering the dose of Wasp maternally suppresses medial accumulation of Myo-II in upd mutants, and restores prominent accumulation at the cortex, as in wild-type embryos. Consistent with this, ectopic apical constriction is completely rescued in these double mutant embryos (Bertet, 2009).
Dia and Wasp play different roles in the regulation of Myo-II localization. Consistent with previous data, Dia controls the amount of apical Myo-II, but the specific localization of Myo-II at the cortex or in the medial network is not affected by loss of Dia. Dia promotes polymerization of non-branched filaments, and might control the formation of a good substrate for the stable association of Myo-II minifilaments. The fact that in dia heterozygotes the amount of apical Myo-II is reduced indicates that the amount of actin filaments might be limiting and controlled. Indeed, more F-actin is detected at the cortex of intercalating cells, preceding by a few minutes the enrichment of Myo-II. The role of Wasp is more surprising and unique; it is shown to mediate specifically repression of medial Myo-II accumulation and, hence, cell constriction in the germband. Because activation of Wasp leads to activation of medial web formation and reduction of Wasp dosage rescues cortical Myo-II in JAK/STAT mutants, it is concluded that Wasp controls an essential feature of Myo-II subcellular localization that is essential for the regulation of apical constriction. How does Wasp control Myo-II localization? Two non-exclusive models. In the first model, Wasp controls actin branching through activation of the Arp2/3 complex. Because Wasp has been implicated in endocytosis via Arp2/3 in Drosophila, Wasp could promote Myo-II web formation indirectly by regulating endocytosis of a surface protein required to anchor the medial actomyosin network at the membrane, such as E-cadherin. Consistent with this, downregulation of E-cadherin by RNAi disrupts the faint medial Myo-II pool. In mesodermal cells, E-cadherin appears to anchor the strong medial Myo-II pool. In the second model, Wasp might act more directly via the regulation of actin network architecture and its impact on the dynamic interactions between the medial web and the cortex, and thereby might affect the steady-state distribution of Myo-II exchanging between these two pools. Although Wasp uniquely mediates Myo-II regulation via JAK/STAT in the ectoderm and not in the mesoderm, regulation of Arp2/3 might be more generally implicated in the control of Myo-II regulation (Bertet, 2009).
Although wasp is an important mediator of JAK/STAT function in the ectoderm, it is unlikely to be the only one. Indeed wasp mutants rescue the cortical accumulation of Myo-II and apical constriction in upd mutants, but GBE is still strongly affected; it was noticed that cortical Myo-II distribution was not properly polarized in the plane of the epithelium. This suggests that other subcellular processes are also perturbed in the mutant. The fact that a reduction of Myo-II levels suppresses the upd defects indicates that the overall dosage of Myo-II is important as well. Identifying the transcriptional targets of JAK/STAT might shed light on its complex regulatory role during embryonic morphogenesis (Bertet, 2009).
Finally, although this work identifies an important regulator of Myo-II network subcellular distribution in epithelial cells, it is still not clear what regulates the polarized distribution of Myo-II at the cortex (Bertet, 2009).
JAK/STAT signalling controls a number of developmental processes. Importantly, this pathway has been implicated in diverse morphogenetic processes, such as convergent extension movements in the zebrafish embryo, hindgut elongation in Drosophila embryos, which probably involves intercalation movements as well, and posterior spiracle morphogenesis in Drosophila embryos. JAK/STAT signalling also controls border cell migration. The data indicate that JAK/STAT signalling plays an important and hitherto unappreciated morphogenetic function in gastrulating embryos. These data document evidence that JAK/STAT controls, via Wasp, a morphogenetic switch based on the regulation of medial or cortical Myo-II distribution. Interestingly, dorsal cells do not undergo apical constriction in JAK/STAT mutants. Indeed, dorsal cells exhibit neither cortical nor medial web Myo-II and are thus unable to participate in profound tissue remodelling. It appears that DV patterning provides a first general subdivision within the embryonic epithelium whereby Myo-II is globally repressed dorsally, and activated laterally and ventrally. Cortical or medial web distribution then results from the combinatorial input of Fog and JAK/STAT (Bertet, 2009).
Cell rearrangements shape the Drosophila embryo via spatially regulated changes in cell shape and adhesion. This study of axis elongation (germband extension) shows that Bazooka/Par-3 (Baz) is required for the planar polarized distribution of myosin II and adherens junction proteins and polarized intercalary behavior is disrupted in baz mutants. The myosin II activator Rho-kinase is asymmetrically enriched at the anterior and posterior borders of intercalating cells in a pattern complementary to Baz. Loss of Rho-kinase results in expansion of the Baz domain, and activated Rho-kinase is sufficient to exclude Baz from the cortex. The planar polarized distribution of Baz requires its C-terminal domain. Rho-kinase can phosphorylate this domain and inhibit its interaction with phosphoinositide membrane lipids, suggesting a mechanism by which Rho-kinase could regulate Baz association with the cell cortex. These results demonstrate that Rho-kinase plays an instructive role in planar polarity by targeting Baz/Par-3 and myosin II to complementary cortical domains (de Matos Simões, 2010).
The spatially regulated activity of protein kinases with multiple substrates provides an efficient strategy for the control of cell polarity in different contexts. This study shows that Rho-kinase is an asymmetrically localized protein that plays an instructive role in planar polarity in the Drosophila embryo by excluding its substrate Baz/Par-3 from the cell cortex. Rho-kinase prevents expansion of the Baz domain and Baz in turn directs the localization of contractile and adherens junction proteins that are required for axis elongation, converting a localized source of kinase activity into a robust bias in polarized cell behavior. The effect of Rho-kinase on Baz planar polarity appears to be independent of its role in regulating myosin II, as Baz localization is not affected in myosin mutants and activated myosin does not reproduce the effects of Rho-kinase in culture. Instead, Rho-kinase can directly phosphorylate the Baz C-terminal coiled-coil domain that is required for Baz association with the cortex. Deletions within the Baz C-terminal domain or replacement of the Baz C-terminus with a heterologous phospholipid binding motif abolish Baz planar polarity in vivo. These results are consistent with a model in which Rho-kinase directly inhibits the association of the Baz C-terminal domain with specific regions of the cell cortex (de Matos Simões, 2010).
Rho-kinase has been shown to phosphorylate mammalian Par-3 in cultured cells, disrupting its interaction with the Par complex proteins Par-6 and aPKC (Nakayama, 2008). The Par complex is necessary for some aspects of epithelial organization but dispensable for others. Par-6 and aPKC are not required for Baz planar polarity in Drosophila, suggesting that the role of Rho-kinase in this process is unlikely to occur through a similar mechanism. This study provides evidence for a different mechanism of regulation by Rho-kinase involving the Baz C-terminal domain, which is phosphorylated by Rho-kinase in vitro and is necessary for Baz planar polarity in vivo. The Baz C-terminus has been shown to bind directly to phosphoinositide membrane lipids including PI(3,4,5)P3, PI(3,4)P2 and PIP (Krahn, 2010). This study shows that Rho-kinase inhibits the association of Baz with phosphoinositide membrane lipids in vitro, consistent with a model in which Rho-kinase directly regulates Baz association with the cortex. Alternatively, Rho-kinase could regulate Baz localization indirectly through other proteins that interact with the Baz C-terminal domain. Despite potential differences in the mechanism, these results demonstrate that the regulation of Par-3 localization or activity by Rho-kinase is a conserved feature of cell polarity in Drosophila and mammals (de Matos Simões, 2010).
The results demonstrate that Rho-kinase is an asymmetrically localized protein that initiates a cascade of events required for the planar polarized distribution of contractile and adherens junction proteins in intercalating cells. The upstream signals that generate localized Rho-kinase activity are not known. Differences between cells conferred by striped or graded patterns of gene expression orient cell movement during axis elongation, and AP patterning genes expressed in stripes are necessary for the asymmetric localization of Rho-kinase. These findings raise the possibility that planar cell polarity may be generated by the local activation of a Rho GTPase signaling pathway. The Drosophila genome contains 21 RhoGEFs and 19 RhoGAPs that are candidate upstream regulators in this process. Rho GTPase pathways are activated by a number of upstream signals including G protein-coupled receptors, receptor tyrosine kinases, cytokine receptors, and cell-cell and cell-substrate adhesion. Identification of the signals upstream of Rho-kinase will help to elucidate the spatial cues that initiate planar polarity in the Drosophila embryo (de Matos Simões, 2010).
The role of Rho-kinase in planar cell polarity is reinforced by the effect of Baz on the localization of contractile and adherens junction proteins. The relationship between Baz and myosin II is complex. In the C. elegans zygote, a contractile myosin network carries PAR-3 to the anterior cell cortex, suggesting a positive relationship between these proteins. In other cell types myosin appears to be dispensable for Baz localization. PAR-3 is required to sustain myosin contractility in C. elegans and Drosophila, and Baz promotes myosin apical localization during C. elegans gastrulation and in the Drosophila follicular epithelium. The ectopic association of myosin with DV cell boundaries in baz mutants, and the complementary distributions of Baz and myosin in several contexts, raise the possibility of inhibitory effects of Baz on myosin. This regulation could also occur indirectly through effects of Baz on apical-basal polarity (de Matos Simões, 2010).
Differential adhesion is sufficient to drive cell sorting in culture and has been proposed to influence tissue morphogenesis in vivo. This study shows that Rho-kinase and Baz regulate the planar polarized localization of the adherens junction protein β-catenin. Rho-kinase has been shown to downregulate adhesion in culture, an activity that is thought to occur through myosin II, which can play positive and negative roles in junctional stabilization. The ability of Rho-kinase to exclude the Baz/Par-3 junctional regulator from the cortex suggests an alternative mechanism for the regulation of adherens junctions by Rho GTPases. These results suggest that Rho-kinase can both promote contractility and inhibit adhesion, providing a single molecular mechanism linking cortical contraction with adherens junction disassembly during tissue morphogenesis (de Matos Simões, 2010).
Integrating individual cell movements to create tissue-level shape change is essential to building an animal. This study explored mechanisms of adherens junction (AJ):cytoskeleton linkage and roles of the linkage regulator Canoe/afadin during Drosophila germband extension (GBE), a convergent-extension process elongating the body axis. Surprising parallels were found between GBE and a quite different morphogenetic movement, mesoderm apical constriction. Germband cells have an apical actomyosin network undergoing cyclical contractions. These coincide with a novel cell shape change--cell extension along the anterior-posterior (AP) axis. In Canoe's absence, GBE is disrupted. The apical actomyosin network detaches from AJs at AP cell borders, reducing coordination of actomyosin contractility and cell shape change. Normal GBE requires planar polarization of AJs and the cytoskeleton. Canoe loss subtly enhances AJ planar polarity and dramatically increases planar polarity of the apical polarity proteins Bazooka/Par3 and atypical protein kinase C. Changes in Bazooka localization parallel retraction of the actomyosin network. Globally reducing AJ function does not mimic Canoe loss, but many effects are replicated by global actin disruption. Strong dose-sensitive genetic interactions between canoe and bazooka are consistent with them affecting a common process. A model is proposed in which an actomyosin network linked at AP AJs by Canoe and coupled to apical polarity proteins regulates convergent extension (Sawyer, 2011).
The data suggest that coupling AJs to a contractile apical actomyosin cytoskeleton plays an important role in a very different cell movement: convergent extension during Drosophila GBE. A novel cell shape change, AP cell elongation, was identified that contributes to WT GBE. Furthermore, it was found that Cno is required for maintaining attachment of the apical actomyosin network AJs in a planar-polarized way. Disrupting this connection results in failure of GBE and prevents coordination of apical myosin contractility and cell shape change. These data are consistent with a model in which Cno tightly couples apical actomyosin to AP AJs and coordinates apical polarity proteins with the network, helping to integrate individual cell shape changes across the tissue (Sawyer, 2011).
Previous studies illustrated how an apical contractile actomyosin network powers apical constriction. In contrast, convergent extension during Drosophila GBE was thought to involve planar-polarized enhancement of contractile actomyosin cables, driving cell intercalation and body elongation. It was surprising to find that, in addition to junctional cables, germband cells also have an apical actomyosin network that undergoes cyclical constriction and relaxation. This coincides with and may help to drive cell shape change. The asymmetric cue of planar-polarized myosin is likely to impose asymmetry. Together, asymmetric cortical myosin and cyclical contractions may help to extend cells in one dimension instead of shrinking them in all dimensions, thus contributing to tissue elongation. While this manuscript was being revised, two other papers independently discovered and described the apical network: the Lecuit lab data further suggest that myosin condensations preferentially move toward AP borders, helping to drive cell rearrangement (Rauzi, 2010; Fernandez-Gonzalez, 2011). Both the current data on Cno and the Lecuit lab's data on β-catenin further suggest that different proteins linking this apical network to AJs are critical for the fidelity and coupling of apical myosin contraction to cell shape change (Sawyer, 2011).
Also, a novel cell shape change was identified that may help to drive AP body axis extension - AP cell elongation. Cno and presumably linkage of the apical actomyosin network to AJs are important for this cell shape change. One speculative possibility is that an asymmetric ratchet acts in germband cells, selectively preventing elongation along the DV body axis while allowing cell elongation along the AP body axis. It is also possible that outside forces, such as shape changes of the first cells to divide, help reshape ectodermal cells, but it is thought that this is less likely, as cell shapes were examined during early GBE before germband mitotic domains divide. Ratchets have also been proposed during mesoderm invagination and during dorsal closure, where amnioserosal cells apically constrict. Before dorsal closure onset, amnioserosal cells have periodic apical actomyosin contractions, but cells only retain changes in shape after a junctional actomyosin purse string appears. Disrupting the purse string disrupts dorsal closure, suggesting that a junctional actomyosin cable can act as a ratchet (Sawyer, 2011).
Studies in Xenopus suggest that the role of a dynamic, planar-polarized apical actomyosin network in convergent extension is conserved. Myosin organizes actin into dynamic foci that move within intercalating cells along their mediolateral axis. In myosin's absence, actin foci are lost and convergent extension is disrupted. Thus dynamic actomyosin foci may play a conserved role in convergent extension (Sawyer, 2011).
It will be interesting to identify regulators shaping contractile activity in different tissues. Jak/Stat signaling restricts apical constriction to the mesoderm; in its absence apical myosin accumulates in the ectoderm, and those cells inappropriately apically constrict. Thus, although both mesoderm and ectoderm share an apical contractile network, its regulation is tuned differently. Furthermore, different actin regulators regulate apical and junctional myosin, with Wasp regulating the apical pool (Sawyer, 2011).
Linking AJs to actin is key in diverse processes from adhesion itself to morphogenetic movements as different as apical constriction and collective cell migration. Cno regulates linkage during mesoderm apical constriction, but isn'n required for cell adhesion (Sawyer, 2009). Other AJ-actin linkers act in other contexts, suggesting that cells use distinct linkers in circumstances with different force regimes. The current data suggest that during GBE, Cno regulates AJ:actomyosin network connections, acting specifically along AP borders (Sawyer, 2011).
Core AJ proteins are more reduced on AP borders in cnoMZ mutants than in WT. In WT, slightly reducing AJ proteins on AP borders may facilitate shrinkage of these borders during GBE. It is tempting to speculate that Cno enhancement along AP borders provides extra support when DEcad/Arm is reduced, strengthening AJ:actomyosin linkages along AP borders yet still allowing cell shape change. In this model, when Cno is absent, AJ:actomyosin linkage is weakened at AP borders, leading to inefficient cell shape change, impairing GBE, and accentuating reduction of AJ proteins (Sawyer, 2011).
The data further suggest that Cno is not the only AJ:actomyosin linker during GBE. Although the actomyosin network detaches from AJs in cnoMZ, it does not collapse into a ball; instead, cables remain 0.2-0.5 microm distant from AJs. A second connection is also supported by the appearance of apical strands of DEcadherin stretching from the cortex to detached myosin in cnoMZ. It will be interesting to determine what proteins compose these other AJ:actomyosin links. β-Catenin regulates actin:AJ linkage just prior to this stage and also plays a role in GBE, although how β-catenin mediates linkage remains mysterious (Sawyer, 2011).
Both myosin and Baz/Par3 are important GBE regulators. One of the most surprising consequences of Cno loss was dramatic change in Baz and aPKC localization. Their strong reduction along AP borders and restricted localization along DV borders correlates well with altered localization of apical actomyosin, which detached from AP AJs and retracted along DV borders from vertices. These data suggest that coordination of the actomyosin network and Baz/aPKC facilitates efficient cell shape change. Consistent with this, an interesting recent paper demonstrated that Baz is required for reciprocal planar-polarized distribution of myosin and AJs. Baz localization, in turn, is restricted by the cytoskeletal regulator Rho-kinase (Rok), leaving Baz enriched at DV borders (Simoes Sde, 2010). This suggests a complex network of interactions (Sawyer, 2011).
In C. elegans a contractile actomyosin cytoskeleton positions apical-polarity proteins (PAR3/PAR6/aPKC) anteriorly in one-cell embryos, and this complex then alters the actomyosin network, promoting asymmetric cortical flow to maintain anterior and posterior domains. It is tempting to speculate that the germband contractile actomyosin network plays a similar role. In this model, planar polarization of the network would create a symmetry break, helping to trigger Baz/aPKC planar polarization. They, in turn, may feed back to modulate actomyosin contractility, driving GBE. Strengthening AJ:actomyosin linkages via Cno could help to ensure efficient cell shape changes that are integrated across the tissue (Sawyer, 2011).
Several mechanistic hypotheses are consistent with these data that are not mutually exclusive. First, Cno may directly affect Baz/aPKC localization during assembly or maintenance, working in parallel or in series with Rok (Simoes Sde, 2010), with actomyosin positioning and contractility then modulated by Baz/aPKC. Consistent with this, previous work revealed that Baz remains apical in the absence of AJs; residual epithelial cells retain polarized actin but have hyperconstricted apical ends. Furthermore, PAR proteins regulate actomyosin contractility during DC. Second, Cno could alter the actomyosin network, which in turn may affect proper Baz/aPKC localization. Baz apical positioning requires the actin cytoskeleton. Actin disruption and Cno loss alter Baz localization similarly, consistent with this hypothesis. Finally, Baz/aPKC may mediate Cno apical positioning, as Baz does for AJs. Of course, more complex interplay with feedback between actomyosin and Baz/aPKC seems likely, creating a network of interactions rather than a linear pathway. Teasing out the complex coordination of AJs, apical polarity protein, and the actomyosin network during morphogenesis is an exciting challenge (Sawyer, 2011).
Elongation of the head-to-tail body axis by convergent extension is a conserved developmental process throughout metazoans. In Drosophila, patterns of transcription factor expression provide spatial cues that induce systematically oriented cell movements and promote tissue elongation. However, the mechanisms by which patterned transcriptional inputs control cell polarity and behaviour have long been elusive. This study demonstrates that three Toll family receptors, Toll-2 (18 wheeler), Toll-6 and Toll-8, are expressed in overlapping transverse stripes along the anterior-posterior axis and act in combination to direct planar polarity and polarized cell rearrangements during convergent extension. Simultaneous disruption of all three receptors strongly reduces actomyosin-driven junctional remodelling and axis elongation, and an ectopic stripe of Toll receptor expression is sufficient to induce planar polarized actomyosin contractility. These results demonstrate that tissue-level patterns of Toll receptor expression provide spatial signals that link positional information from the anterior-posterior patterning system to the essential cell behaviours that drive convergent extension (Pare, 2014).
Together, these results demonstrate that the spatial signals that establish planar polarity and direct polarized cell behaviour during convergent extension in Drosophila are encoded at the cell surface by three Toll family receptors expressed in overlapping stripes along the AP axis of the embryo. Simultaneous disruption of Toll-2, Toll-6 and Toll-8 significantly impairs planar polarity, cell intercalation, and convergent extension, and removing one or two receptors disrupts planar polarity in distinct subsets of cells, indicating that these proteins serve non-redundant and highly localized functions. These findings support a model in which planar polarity is induced by interactions between neighbouring cells with different levels of Toll receptor activity. Therefore, Drosophila Toll receptors provide the basis of a spatial code that translates patterned Eve and Runt transcriptional activity into planar polarized actomyosin contractility, linking positional information provided by the embryonic AP patterning system to the essential cell behaviours that drive convergent extension. The Toll receptor code is incomplete in certain regions, such as the parasegmental boundaries, suggesting the existence of additional polarity cues at these interfaces. Toll-2,6,8 mutants are similar to runt mutants with respect to all measures of cell rearrangement and planar polarity, but are not as severe as eve mutants. Thus, although Toll-2,6,8 mutants recapitulate much of the eve mutant phenotype, Eve likely has additional targets important for planar polarity (Pare, 2014).
Toll family receptors have a highly conserved structure in vertebrates and invertebrates, including extracellular LRR motifs that are often present in proteins involved in cell adhesion and cell-cell recognition. Although individual receptors are not orthologous between flies and humans, mammalian Toll-like receptors are required for epithelial regeneration and wound healing, processes that involve dynamic and spatially regulated changes in cell adhesion. In the innate immune system, pathogen detection by Toll family receptors activates transcriptional pathways mediated by NF-κB and MAP kinase signalling. However, the spatial information provided by patterned Toll receptor expression in Drosophila, as well as the rapid timescale of cell rearrangements during convergent extension, suggest a more direct connection between Toll receptor signalling and the cellular contractile machinery. Consistent with this possibility, activation of mammalian Toll-like receptors in dendritic cells induces a rapid remodelling of the actin cytoskeleton and mammalian Toll-like receptors can inhibit neurite outgrowth and trigger rapid growth cone collapse in neurons, reminiscent of Toll receptor functions in the Drosophila nervous system. Elucidating the mechanisms that link Toll family receptors to dynamic changes in cell polarity and behaviour may provide insight into conserved and relatively unexplored aspects of Toll receptor signalling (Pare, 2014).
Axis elongation is a conserved process in which the head-to-tail or anterior-posterior (AP) axis of an embryo extends. In Drosophila, cellular rearrangements drive axis elongation. Cells exchange neighbours by converging into transient multicellular vertices which resolve through the assembly of new cell interfaces parallel to the AP axis. This study found that new interfaces elongate in pulses correlating with periodic contractions of the surrounding cells. Inhibiting actomyosin contractility globally, or specifically in the cells around multicellular vertices, disrupts the rate and directionality of new interface assembly. Laser ablation indicates that new interfaces sustain greater tension than non-elongating ones. A method to apply ectopic tension was developed, and increasing AP tension locally was found to increase the elongation rate of new edges by 2.1-fold. Increasing dorsal-ventral tension results in vertex resolution perpendicular to the AP direction. The study proposes that local, periodic contractile forces polarize vertex resolution to drive Drosophila axis elongation (Yu, 2016).
One of the key morphogenetic processes used during development is the controlled intercalation of cells between their neighbors. This process has been co-opted into a range of developmental events, and it also underlies an event that occurs in each major group of bilaterians: elongation of the embryo along the anterior-posterior axis. In Drosophila, a novel component of this process was recently discovered by Pare (2014), who showed that three Toll genes, Toll-2 (18 Wheeler), Toll-6 and Toll-8 (Tollo), function together to drive cell intercalation during germband extension. This finding raises the question of whether this role of Toll genes is an evolutionary novelty of flies or a general mechanism of embryonic morphogenesis. This study shows that the Toll gene function in axis elongation is, in fact, widely conserved among arthropods. First, two Toll genes were functionally demonstrated to be required for cell intercalation in the beetle Tribolium castaneum. These genes belong to a previously undescribed Toll subfamily, and members of this subfamily exhibit striped expression (as seen in Tribolium and previously reported in Drosophila) in embryos of six other arthropod species spanning the entire phylum. Last, it was shown that two of these Toll genes are required for normal morphogenesis during anterior-posterior embryo elongation in the spider Parasteatoda tepidariorum, a member of the most basally branching arthropod lineage. From these findings, it is hypothesized that Toll genes had a morphogenetic function in embryo elongation in the last common ancestor of all arthropods, which existed over 550 million years ago (Benton, 2016).
Tissue morphogenesis relies on repeated use of dynamic behaviors at the levels of intracellular structures, individual cells, and cell groups. Rapidly accumulating live imaging datasets make it increasingly important to formalize and automate the task of mapping recurrent dynamic behaviors (motifs), as it is done in speech recognition and other data mining applications. This study presents a "template-based search" approach for accurate mapping of sub- to multi-cellular morphogenetic motifs using a time series data mining framework. The task of motif mapping was formulated as a subsequence matching problem, and it was solved using dynamic time warping, while relying on high throughput graph-theoretic algorithms for efficient exploration of the search space. This formulation allows the algorithm to accurately identify the complete duration of each instance and automatically label different stages throughout its progress, such as cell cycle phases during cell division. To illustrate this approach, cell intercalations were mapped during germband extension in the early Drosophila embryo. This framework enabled statistical analysis of intercalary cell behaviors in wild-type and mutant embryos, comparison of temporal dynamics in contracting and growing junctions in different genotypes, and the identification of a novel mode of iterative cell intercalation. This formulation of tissue morphogenesis using time series opens new avenues for systematic decomposition of tissue morphogenesis (Stern, 2020).
Distinct patterns of actomyosin contractility are often associated with particular epithelial tissue shape changes during development. For example, a planar polarized pattern of myosin II localization regulated by Rho1 signaling during Drosophila body axis elongation is thought to drive cell behaviors that contribute to convergent extension. This study developed optogenetic tools to activate (optoGEF) or deactivate (optoGAP) Rho1 signaling. These tools were used to manipulate myosin patterns at the apical side of the germband epithelium during Drosophila axis elongation, and the effects on contractile cell behaviors were analyzed. Uniform activation or inactivation of Rho1 signaling across the apical surface of the germband is sufficient to disrupt the planar polarized pattern of myosin at cell junctions on the timescale of 3-5 min, leading to distinct changes in junctional and medial myosin patterns in optoGEF and optoGAP embryos. These two perturbations to Rho1 activity both disrupt axis elongation and cell intercalation These studies demonstrate that acute optogenetic perturbations to Rho1 activity are sufficient to rapidly override the endogenous planar polarized myosin pattern in the germband during axis elongation. Moreover, these results reveal that the levels of Rho1 activity and the balance between medial and junctional myosin play key roles, not only in organizing the cell rearrangements that are known to directly contribute to axis elongation, but also in regulating cell area fluctuations and cell packings, which have been proposed to be important factors influencing the mechanics of tissue deformation and flow (Herrera-Perez, 2021).
In the early Drosophila embryo, the elongation of the anterior-posterior body axis is driven by cell intercalation in the germband epithelium. Neighboring cells intercalate through the contraction of AP interfaces (between anterior-posterior neighbors) into higher-order vertices, which then resolve through the extension of new DV interfaces (between dorsal-ventral neighbors). Although interface contraction has been extensively studied, less is known about how new interfaces are established. This study shows that DV interface elongation behaviors initiate at the same time as AP contractions, and that DV interfaces which are newly created from resolution of higher-order vertices do not appear to possess a unique 'identity'; instead, all horizontal interfaces undergo lengthening, elongating through ratchet-like sliding behaviors analogous to those found in AP interfaces. Cortical F-actin networks are essential for high area oscillation amplitudes required for effective ratcheting. These results suggest that, contrary to canonical models, the elongation of new DV interfaces is not produced by a mechanistically separate process. Instead, medial Myosin populations drive oscillating radial forces in the cells to generate transient force asymmetries at all tricellular vertices, which - combined with planar polarized stabilization - produce directional ratcheted sliding to generate both AP interface contraction and DV interface elongation (Vanderleest, 2022).
Compartmental boundaries physically separate developing tissues into distinct regions, which is fundamental for the organisation of the body plan in both insects and vertebrates. In many examples, this physical segregation is caused by a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. This study performed an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, from the start of germband extension at gastrulation and throughout the germband extended stages (stages 6 to 11). First, it was found that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared with wild-type and ftz mutant embryos, it was shown that Tartan is required during germband extension but not beyond. Candidate genes were identified that could take over from Tartan at PSBs, and it was confirmed that at germband extended stages, actomyosin enrichment at PSBs requires Wingless signalling (Sharrock, 2022).
Embryos extend their anterior-posterior (AP) axis in the conserved process of axis elongation. Drosophila axis elongation occurs in an epithelial monolayer, the germband, and is driven by cell intercalation, cell shape changes, and oriented cell divisions at the posterior germband. Anterior germband cells also divide during axis elongation. Image analysis and pattern recognition methods were developed to track dividing cells from confocal microscopy movies in a generally-applicable approach. Mesectoderm cells, forming the ventral midline, divided parallel to the AP axis, while lateral cells displayed a uniform distribution of division orientations. Mesectoderm cells did not intercalate and sustained increased AP strain before cell division. After division, mesectoderm cell density increased along the AP axis, thus relieving strain. Laser ablation was used to isolate mesectoderm cells from other tissues. Uncoupling the mesectoderm from intercalating cells did not affect cell division orientation. Conversely, separating the mesectoderm from the anterior and posterior poles of the embryo resulted in uniformly-oriented divisions. These data suggest that mesectoderm cells align their division angle to reduce strain caused by mechanical forces along the AP axis of the embryo (Wang, 2017).
Epithelial remodeling determines the structure of many organs in the body through changes in cell shape, polarity and behavior and is a major area of study in developmental biology. Accurate and high-throughput methods are necessary to systematically analyze epithelial organization and dynamics at single-cell resolution. This study developed SEGGA, an easy-to-use software for automated image segmentation, cell tracking and quantitative analysis of cell shape, polarity and behavior in epithelial tissues. SEGGA is free, open source, and provides a full suite of tools that allow users with no prior computational expertise to independently perform all steps of automated image segmentation, semi-automated user-guided error correction, and data analysis. This study uses SEGGA to analyze changes in cell shape, cell interactions and planar polarity during convergent extension in the Drosophila embryo. These studies demonstrate that planar polarity is rapidly established in a spatiotemporally regulated pattern that is dynamically remodeled in response to changes in cell orientation. These findings reveal an unexpected plasticity that maintains coordinated planar polarity in actively moving populations through the continual realignment of cell polarity with the tissue axes (Farrell, 2017).
In Drosophila melanogaster, the germband forms directly on the egg surface and solely consists of embryonic tissue. In contrast, most insect embryos undergo a complicated set of tissue rearrangements to generate a condensed, multilayered germband. The ventral side of the germband is embryonic, while the dorsal side is thought to be an extraembryonic tissue called the amnion. While this tissue organisation has been accepted for decades and has been widely reported in insects, its accuracy has not been directly tested in any species. Using live cell tracking and differential cell labelling in the short germ beetle Tribolium castaneum, this study shows that most of the cells previously thought to be amnion actually give rise to large parts of the embryo. This process occurs via the dorsal-to-ventral flow of cells and contributes to germband extension (GBE). In addition, true 'amnion' cells in Tribolium were shown to originate from a small region of the blastoderm. Together, these findings show that development in the short germ embryos of Tribolium and the long germ embryos of Drosophila is more similar than previously proposed. Dorsal-to-ventral cell flow also occurs in Drosophila during GBE, and this study argues that the flow is driven by a conserved set of underlying morphogenetic events in both species. Furthermore, the revised Tribolium fate map that this study presents is far more similar to that of Drosophila than the classic Tribolium fate map. Lastly, these findings show that there is no qualitative difference between the tissue structure of the cellularised blastoderm and the short/intermediate germ germband. As such, the same tissue patterning mechanisms could function continuously throughout the cellularised blastoderm and germband stages, and easily shift between them over evolutionary time (Benton, 2018).
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date revised: 15 May 2017
Genes involved in organ development
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