Myosin II regulates complex cellular arrangement and epithelial architecture in Drosophila

Mitotic cell rounding accelerates epithelial invagination

Mitotic cells assume a spherical shape by increasing their surface tension and osmotic pressure by extensively reorganizing their interphase actin cytoskeleton into a cortical meshwork and their microtubules into the mitotic spindle. Mitotic entry is known to interfere with tissue morphogenetic events that require cell-shape changes controlled by the interphase cytoskeleton, such as apical constriction. However, here we show that mitosis plays an active role in the epithelial invagination of the Drosophila melanogaster tracheal placode. Invagination begins with a slow phase under the control of epidermal growth factor receptor (EGFR) signalling; in this process, the central apically constricted cells, which are surrounded by intercalating cells, form a shallow pit. This slow phase is followed by a fast phase, in which the pit is rapidly depressed, accompanied by mitotic entry, which leads to the internalization of all the cells in the placode. We found that mitotic cell rounding, but not cell division, of the central cells in the placode is required to accelerate invagination, in conjunction with EGFR-induced myosin II contractility in the surrounding cells. We propose that mitotic cell rounding causes the epithelium to buckle under pressure and acts as a switch for morphogenetic transition at the appropriate time.

Dual role of myosin II during Drosophila imaginal disc metamorphosis

The motor protein non-muscle myosin II is a major driver of the movements that sculpt three-dimensional organs from two-dimensional epithelia. The machinery of morphogenesis is well established but the logic of its control remains unclear in complex organs. Here we use live imaging and ex vivo culture to report a dual role of myosin II in regulating the development of the Drosophila wing. First, myosin II drives the contraction of a ring of cells that surround the squamous peripodial epithelium, providing the force to fold the whole disc through about 90°. Second, myosin II is needed to allow the squamous cells to expand and then retract at the end of eversion. The combination of genetics and live imaging allows us to describe and understand the tissue dynamics, and the logic of force generation needed to transform a relatively simple imaginal disc into a more complex and three-dimensional adult wing.

The role of the bHLH protein hairy in morphogenetic furrow progression in the developing Drosophila eye

In Drosophila eye development, a wave of differentiation follows a morphogenetic furrow progressing across the eye imaginal disc. This is subject to negative regulation attributed to the HLH repressor proteins Hairy and Extramacrochaete. Recent studies identify negative feedback on the bHLH gene daughterless as one of the main functions of extramacrochaete. Here the role of hairy was assessed in relation to daughterless and other HLH genes. Hairy was not found to regulate the expression of Daughterless, Extramacrochaete or Atonal, and Hairy expression was largely unregulated by these other genes. Null alleles of hairy did not alter the rate or pattern of differentiation, either alone or in the absence of Extramacrochaete. These findings question whether hairy is an important regulator of the progression of retinal differentiation in Drosophila, alone or redundantly with extramacrochaete.

Fellow travellers: emergent properties of collective cell migration

Cells can migrate individually or collectively. Collective movement is common during normal development and is also a characteristic of some cancers. This review discusses recent insights into features that are unique to collective cell migration, as well as properties that emerge from these features. The first feature is that cells of the collective affect each other through adhesion, force-dependent and signalling interactions. The second feature is that cells of the collective differ from one another: leaders from followers, tip from stalk and front from back. These are dynamic differences that are important for directional movement. Last, an unexpected property is discussed: epithelial cells can rotate persistently in constrained spaces.

AIP1 acts with cofilin to control actin dynamics during epithelial morphogenesis

During epithelial morphogenesis, cells not only maintain tight adhesion for epithelial integrity but also allow dynamic intercellular movement to take place within cell sheets. How these seemingly opposing processes are coordinated is not well understood. Here, we report that the actin disassembly factors AIP1 and cofilin are required for remodeling of adherens junctions (AJs) during ommatidial precluster formation in Drosophila eye epithelium, a highly stereotyped cell rearrangement process which we describe in detail in our live imaging study. AIP1 is enriched together with F-actin in the apical region of preclusters, whereas cofilin displays a diffuse and uniform localization pattern. Cofilin overexpression completely rescues AJ remodeling defects caused by AIP1 loss of function, and cofilin physically interacts with AIP1. Pharmacological reduction of actin turnover results in similar AJ remodeling defects and decreased turnover of E-cadherin, which also results from AIP1 deficiency, whereas an F-actin-destabilizing drug affects AJ maintenance and epithelial integrity. Together with other data on actin polymerization, our results suggest that AIP1 enhances cofilin-mediated actin disassembly in the apical region of precluster cells to promote remodeling of AJs and thus intercellular movement, but also that robust actin polymerization promotes AJ general adhesion and integrity during the remodeling process.

Atonal and EGFR signalling orchestrate rok- and Drak-dependent adherens junction remodelling during ommatidia morphogenesis

Morphogenesis of epithelial tissues relies on the interplay between cell division, differentiation and regulated changes in cell shape, intercalation and sorting. These processes are often studied individually in relatively simple epithelia that lack the complexity found during organogenesis when these processes might all coexist simultaneously. To address this issue, we are making use of the developing fly retinal neuroepithelium. Retinal morphogenesis relies on a coordinated sequence of interdependent morphogenetic events that includes apical cell constriction, localized alignment of groups of cells and ommatidia morphogenesis coupled to neurogenesis. Here, we use live imaging to document the sequence of adherens junction (AJ) remodelling events required to generate the fly ommatidium. In this context, we demonstrate that the kinases Rok and Drak function redundantly during Myosin II-dependent cell constriction, subsequent multicellular alignment and AJ remodelling. In addition, we show that early multicellular patterning characterized by cell alignment is promoted by the conserved transcription factor Atonal (Ato). Further ommatidium patterning requires the epidermal growth factor receptor (EGFR) signalling pathway, which transcriptionally governs rok- and Drak-dependent AJ remodelling while also promoting neurogenesis. In conclusion, our work reveals an important role for Drak in regulating AJ remodelling during retinal morphogenesis. It also sheds new light on the interplay between Ato, EGFR-dependent transcription and AJ remodelling in a system in which neurogenesis is coupled with cell shape changes and regulated steps of cell intercalation.

Capulet and Slingshot share overlapping functions during Drosophila eye morphogenesis

Background

CAP/Capulet (Capt), Slingshot (Ssh) and Cofilin/Twinstar (Tsr) are actin-binding proteins that restrict actin polymerization. Previously, it was shown that low resolution analyses of loss-of-function mutations in capt, ssh and tsr all show ectopic F-actin accumulation in various Drosophila tissues. In contrast, RNAi depletion of capt, tsr and ssh in Drosophila S2 cells all affect actin-based lamella formation differently. Whether loss of these three related genes might cause the same effect in the same tissue remains unclear.

Methods

Loss-of-function mutant clones were generated using the MARCM or EGUF system whereas overexpression clones were generated using the Flip-out system. Immunostaining were then performed in eye imaginal discs with clones. FRAP was performed in cultured eye discs.

Results

Here, we compared their loss-of-function phenotype at single-cell resolution, using a sheet of epithelial cells in the Drosophila eye imaginal disc as a model system. Surprisingly, we found that capt and ssh, but not tsr, mutant cells within and posterior to the morphogenetic furrow (MF) shared similar phenotypes. The capt/ssh mutant cells possessed: (1) hexagonal cell packing with discontinuous adherens junctions; and (2) largely complementary accumulation of excessive phosphorylated myosin light chain (p-MLC) and F-actin rings at the apical cortex. We further showed that the capt/ssh mutant phenotypes depended on the inactivation of protein kinase A (PKA) and activation of Rho.

Conclusions

Although Capt, Ssh and Tsr were reported to negatively regulate actin polymerization, we found that Capt and Ssh, but not Tsr, share overlapping functions during eye morphogenesis.

The muscle pattern of the Drosophila abdomen depends on a subdivision of the anterior compartment of each segment

In the past, segments were defined by landmarks such as muscle attachments, notably by Snodgrass, the king of insect anatomists. Here, we show how an objective definition of a segment, based on developmental compartments, can help explain the dorsal abdomen of adult Drosophila. The anterior (A) compartment of each segment is subdivided into two domains of cells, each responding differently to Hedgehog. The anterior of these domains is non-neurogenic and clones lacking Notch develop normally; this domain can express stripe and form muscle attachments. The posterior domain is neurogenic and clones lacking Notch do not form cuticle; this domain is unable to express stripe or form muscle attachments. The posterior (P) compartment does not form muscle attachments. Our in vivo films indicate that early in the pupa the anterior domain of the A compartment expresses stripe in a narrowing zone that attracts the extending myotubes and resolves into the attachment sites for the dorsal abdominal muscles. We map the tendon cells precisely and show that all are confined to the anterior domain of A. It follows that the dorsal abdominal muscles are intersegmental, spanning from one anterior domain to the next. This view is tested and supported by clones that change cell identity or express stripe ectopically. It seems that growing myotubes originate in posterior A and extend forwards and backwards until they encounter and attach to anterior A cells. The dorsal adult muscles are polarised in the anteroposterior axis: we disprove the hypothesis that muscle orientation depends on genes that define planar cell polarity in the epidermis.

Genetic analysis of fibroblast growth factor signaling in the Drosophila eye

The development of eyes in Drosophila involves intricate epithelial reorganization events for accurate positioning of cells and proper formation and organization of ommatidial clusters. We demonstrate that Branchless (Bnl), the fibroblast growth factor ligand, regulates restructuring events in the eye disc primordium from as early as the emergence of clusters from a morphogenetic front to the cellular movements during pupal eye development. Breathless (Btl) functions as the fibroblast growth factor receptor to mediate Bnl signal, and together they regulate expression of DE-cadherin, Crumbs, and Actin. In addition, in the eye Bnl regulates the temporal onset and extent of retinal basal glial cell migration by activating Btl in the glia. We hypothesized that the Bnl functions in the eye are Hedgehog dependent and represent novel aspects of Bnl signaling not explored previously.

Lamellipodia-based migrations of larval epithelial cells are required for normal closure of the adult epidermis of Drosophila

Cell migrations are an important feature of animal development. They are, furthermore, essential to wound healing and tumour progression. Despite recent progress, it is still mysterious how cell migration is spatially and temporally regulated during morphogenesis and how cell migration is coordinated with other cellular behaviours to shape tissues and organs. The formation of the abdominal epithelium of Drosophila during metamorphosis provides an attractive system to study morphogenesis. Here, the diploid adult histoblasts replace the polyploid larval epithelial cells (LECs). Using in vivo 4D microscopy, I show that, besides apical constriction and apoptosis, the LECs undergo extensive coordinated migrations. The migrations follow a transition from a stationary (epithelial) to a migratory mode. The migratory behaviour is stimulated by autocrine Dpp signalling. Directed apical lamellipodia-like protrusions propel the cells. Initially, planar cell polarity determines the orientation of LEC migration. While LECs are migrating they also constrict apically, and changes in activity of the small GTPase Rho1 can favour one behaviour over the other. This study shows that the LECs play a more active role in morphogenesis than previously thought, with their migrations contributing to abdominal closure. It furthermore provides insights into how the migratory behaviour of cells is regulated during morphogenesis.

A screen for X-linked mutations affecting Drosophila photoreceptor differentiation identifies Casein kinase 1α as an essential negative regulator of wingless signaling

The Wnt and Hedgehog signaling pathways are essential for normal development and are misregulated in cancer. The casein kinase family of serine/threonine kinases regulates both pathways at multiple levels. However, it has been difficult to determine whether individual members of this family have distinct functions in vivo, due to their overlapping substrate specificities. In Drosophila melanogaster, photoreceptor differentiation is induced by Hedgehog and inhibited by Wingless, providing a sensitive system in which to identify regulators of each pathway. We used a mosaic genetic screen in the Drosophila eye to identify mutations in genes on the X chromosome required for signal transduction. We recovered mutations affecting the transcriptional regulator CREB binding protein, the small GTPase dynamin, the cytoskeletal regulator Actin-related protein 2, and the protein kinase Casein kinase 1α. Consistent with its reported function in the β-Catenin degradation complex, Casein Kinase 1α mutant cells accumulate β-Catenin and ectopically induce Wingless target genes. In contrast to previous studies based on RNA interference, we could not detect any effect of the same Casein Kinase 1α mutation on Hedgehog signaling. We thus propose that Casein kinase 1α is essential to allow β-Catenin degradation and prevent inappropriate Wingless signaling, but its effects on the Hedgehog pathway are redundant with other Casein kinase 1 family members.

Epithelial organisation revealed by a network of cellular contacts

The emergence of differences in the arrangement of cells is the first step towards the establishment of many organs. Understanding this process is limited by the lack of systematic characterization of epithelial organisation. Here we apply network theory at the scale of individual cells to uncover patterns in cell-to-cell contacts that govern epithelial organisation. We provide an objective characterization of epithelia using network representation where cells are nodes and cell contacts are links. The features of individual cells, together with attributes of the cellular network produce a defining signature that distinguishes epithelia from different organs, species, developmental stages and genetic conditions. The approach permits characterization, quantification and classification of normal and perturbed epithelia and establishes a framework for understanding molecular mechanisms that underpin the architecture of complex tissues.

Dissecting the PCP pathway: one or more pathways?: Does a separate Wnt-Fz-Rho pathway drive morphogenesis?

Planar cell polarity (PCP), the alignment of cells within 2D tissue planes, involves a set of core molecular regulators highly conserved between animals and cell types. These include the transmembrane proteins Frizzled (Fz) and VanGogh and the cytoplasmic regulators Dishevelled (Dsh) and Prickle. It is widely accepted that this core forms part of a 'PCP pathway' for signal transduction, which can affect cell morphology through activation of an evolutionary ancient regulatory module involving Rho family GTPases and Myosin II, and/or the JNK kinase cascade. We have re-examined the evidence for interactions between the proposed PCP pathway components, and question the placing of the cell morphology regulators in the same pathway as the PCP core. While Fz and Dsh are clearly involved in both PCP and Rho-based cell morphology regulation, available evidence cannot currently discriminate whether these processes are linked mechanistically by a shared Fz/Dsh population, or pass by two distinct pathways.

Establishment and maintenance of compartmental boundaries: role of contractile actomyosin barriers

During animal development, tissues and organs are partitioned into compartments that do not intermix. This organizing principle is essential for correct tissue morphogenesis. Given that cell sorting defects during compartmentalization in humans are thought to cause malignant invasion and congenital defects such as cranio-fronto-nasal syndrome, identifying the molecular and cellular mechanisms that keep cells apart at boundaries between compartments is important. In both vertebrates and invertebrates, transcription factors and short-range signalling pathways, such as EPH/Ephrin, Hedgehog, or Notch signalling, govern compartmental cell sorting. However, the mechanisms that mediate cell sorting downstream of these factors have remained elusive for decades. Here, we review recent data gathered in Drosophila that suggest that the generation of cortical tensile forces at compartmental boundaries by the actomyosin cytoskeleton could be a general mechanism that inhibits cell mixing between compartments.

Helicobacter pylori CagA disrupts epithelial patterning by activating myosin light chain

Helicobacter pylori infection is a leading cause of ulcers and gastric cancer. We show that expression of the H. pylori virulence factor CagA in a model Drosophila melanogaster epithelium induces morphological disruptions including ectopic furrowing. We find that CagA alters the distribution and increases the levels of activated myosin regulatory light chain (MLC), a key regulator of epithelial integrity. Reducing MLC activity suppresses CagA-induced disruptions. A CagA mutant lacking EPIYA motifs (CagA^EPISA) induces less epithelial disruption and is not targeted to apical foci like wild-type CagA. In a cell culture model in which CagA^EPISA and CagA have equivalent subcellular localization, CagA^EPISA is equally potent in activating MLC. Therefore, in our transgenic system, CagA is targeted by EPIYA motifs to a specific apical region of the epithelium where it efficiently activates MLC to disrupt epithelial integrity.

Tension and epithelial morphogenesis in Drosophila early embryos

During morphogenesis, tissues are shaped by cell behaviors such as apical cell constriction and cell intercalation, which are the result of cell intrinsic forces, but are also shaped passively by forces acting on the cells. The latter extrinsic forces can be produced either within the deforming tissue by the tissue-scale integration of intrinsic forces, or outside the tissue by other tissue movements or by fluid flows. Here we review the intrinsic and extrinsic forces that sculpt the epithelium of early Drosophila embryos, focusing on three conserved morphogenetic processes: tissue internalization, axis extension, and segment boundary formation. Finally, we look at how the actomyosin cytoskeleton forms force-generating structures that power these three morphogenetic events at the cell and the tissue scales.

Distinct tissue distributions and subcellular localizations of differently phosphorylated forms of the myosin regulatory light chain in Drosophila

Nonmuscle myosin II (myosin hereafter) has well-established roles in generating contractile force on actin filaments during morphogenetic processes in all metazoans. Myosin activation is regulated by phosphorylation of the myosin regulatory light chain (MRLC, encoded by spaghettisquash or sqh in Drosophila) first on Ser21 and subsequently on Thr20. These phosphorylation events are positively controlled by a variety of kinases including myosin light chain kinase, Rho kinase, citron kinase, and AMP kinase and are negatively regulated by myosin phosphatase. The activation of myosin is thus highly regulated and likely developmentally controlled. In order to monitor the activity of myosin during development, we have generated antibodies against the monophosphorylated (Sqh1P) and diphosphorylated (Sqh2P) forms of Sqh. We first show that the antibodies are highly specific. We then used these antibodies to monitor myosin activation in wild type Drosophila tissues. Interestingly, Sqh1P and Sqh2P show distinct patterns of expression in embryos. Sqh1P is expressed nearly ubiquitously and outlines cells consistent with a junctional localization, whereas Sqh2P is strongly expressed on the apical surfaces and in filopodia of tissues undergoing extensive cell shape change or cell movements including the invaginating fore- and hindgut, the invaginating tracheal system, the dorsal pouch and the dorsal most row of epidermal (DME) cells during dorsal closure. In imaginal discs, Sqh1P predominantly localizes in the adherens junction, whereas Sqh2P locates to the apical domain. These antibodies thus have the potential to be very useful in monitoring myosin activation for functional studies of morphogenesis in Drosophila.

Atg1-mediated myosin II activation regulates autophagosome formation during starvation-induced autophagy

Autophagy is a membrane-mediated degradation process of macromolecule recycling. Although the formation of double-membrane degradation vesicles (autophagosomes) is known to have a central role in autophagy, the mechanism underlying this process remains elusive. The serine/threonine kinase Atg1 has a key role in the induction of autophagy. In this study, we show that overexpression of Drosophila Atg1 promotes the phosphorylation-dependent activation of the actin-associated motor protein myosin II. A novel myosin light chain kinase (MLCK)-like protein, Spaghetti-squash activator (Sqa), was identified as a link between Atg1 and actomyosin activation. Sqa interacts with Atg1 through its kinase domain and is a substrate of Atg1. Significantly, myosin II inhibition or depletion of Sqa compromised the formation of autophagosomes under starvation conditions. In mammalian cells, we found that the Sqa mammalian homologue zipper-interacting protein kinase (ZIPK) and myosin II had a critical role in the regulation of starvation-induced autophagy and mammalian Atg9 (mAtg9) trafficking when cells were deprived of nutrients. Our findings provide evidence of a link between Atg1 and the control of Atg9-mediated autophagosome formation through the myosin II motor protein.

Sequential activation of apical and basolateral contractility drives ascidian endoderm invagination

BACKGROUND

Epithelial invagination is a fundamental morphogenetic behavior that transforms a flat cell sheet into a pit or groove. Previous studies of invagination have focused on the role of actomyosin-dependent apical contraction; other mechanisms remain largely unexplored.

RESULTS

We combined experimental and computational approaches to identify a two-step mechanism for endoderm invagination during ascidian gastrulation. During Step 1, which immediately precedes invagination, endoderm cells constrict their apices because of Rho/Rho-kinase-dependent apical enrichment of 1P-myosin. Our data suggest that endoderm invagination itself occurs during Step 2, without further apical shrinkage, via a novel mechanism we call collared rounding: Rho/Rho-kinase-independent basolateral enrichment of 1P-myosin drives apico-basal shortening, whereas Rho/Rho-kinase-dependent enrichment of 1P and 2P myosin in circumapical collars is required to prevent apical expansion and for deep invagination. Simulations show that boundary-specific tension values consistent with these distributions of active myosin can explain the cell shape changes observed during invagination both in normal embryos and in embryos treated with pharmacological inhibitors of either Rho-kinase or Myosin II ATPase. Indeed, we find that the balance of strong circumapical and basolateral tension is the only mechanism based on differential cortical tension that can explain ascidian endoderm invagination. Finally, simulations suggest that mesectoderm cells resist endoderm shape changes during both steps, and we confirm this prediction experimentally.

CONCLUSIONS

Our findings suggest that early ascidian gastrulation is driven by the coordinated apposition of circumapical and lateral endoderm contraction, working against a resisting mesectoderm. We propose that similar mechanisms may operate during other invaginations.

Live imaging of Drosophila imaginal disc development

Live imaging has revolutionized the analysis of developmental biology over the last few years. The ability to track in real time the dynamic processes that occur at tissue and cellular levels gives a much clearer view of development, and allows greater temporal resolution, than is possible with fixed tissue. Drosophila imaginal discs are a particularly important model of many aspects of development, but their small size and location inside the larva and pupa has prevented live imaging techniques from extensively being used in their study. Here, we introduce the use of viscous culture medium to enable high resolution imaging of imaginal disc development. As a proof of principle, we have analyzed the transformation that occurs during metamorphosis of the wing imaginal disc into the mature wing and report several previously unobserved stages of this model of organogenesis. These imaging methods are especially useful to study the complex and dynamic changes that occur during morphogenesis, but we show that they can also be used to analyze other developmental and cellular events. Moreover, our viscous medium creates a platform for future adaptation of other tissue culture conditions to allow imaging of a wide range of developmental events and systems.

Nonmuscle myosin II is required for cell proliferation, cell sheet adhesion and wing hair morphology during wing morphogenesis

Metazoan development involves a myriad of dynamic cellular processes that require cytoskeletal function. Nonmuscle myosin II plays essential roles in embryonic development; however, knowledge of its role in post-embryonic development, even in model organisms such as Drosophila melanogaster, is only recently being revealed. In this study, truncation alleles were generated and enable the conditional perturbation, in a graded fashion, of nonmuscle myosin II function. During wing development they demonstrate novel roles for nonmuscle myosin II, including in adhesion between the dorsal and ventral wing epithelial sheets; in the formation of a single actin-based wing hair from the distal vertex of each cell; in forming unbranched wing hairs; and in the correct positioning of veins and crossveins. Many of these phenotypes overlap with those observed when clonal mosaic analysis was performed in the wing using loss of function alleles. Additional requirements for nonmuscle myosin II are in the correct formation of other actin-based cellular protrusions (microchaetae and macrochaetae). We confirm and extend genetic interaction studies to show that nonmuscle myosin II and an unconventional myosin, encoded by crinkled (ck/MyoVIIA), act antagonistically in multiple processes necessary for wing development. Lastly, we demonstrate that truncation alleles can perturb nonmuscle myosin II function via two distinct mechanisms--by titrating light chains away from endogenous heavy chains or by recruiting endogenous heavy chains into intracellular aggregates. By allowing myosin II function to be perturbed in a controlled manner, these novel tools enable the elucidation of post-embryonic roles for nonmuscle myosin II during targeted stages of fly development.

Overlapping roles of Drosophila Drak and Rok kinases in epithelial tissue morphogenesis

Dynamic regulation of cytoskeletal contractility through phosphorylation of the nonmuscle Myosin-II regulatory light chain (MRLC) provides an essential source of tension for shaping epithelial tissues. Rho GTPase and its effector kinase ROCK have been implicated in regulating MRLC phosphorylation in vivo, but evidence suggests that other mechanisms must be involved. Here, we report the identification of a single Drosophila homologue of the Death-associated protein kinase (DAPK) family, called Drak, as a regulator of MRLC phosphorylation. Based on analysis of null mutants, we find that Drak broadly promotes proper morphogenesis of epithelial tissues during development. Drak activity is largely redundant with that of the Drosophila ROCK orthologue, Rok, such that it is essential only when Rok levels are reduced. We demonstrate that these two kinases synergistically promote phosphorylation of Spaghetti squash (Sqh), the Drosophila MRLC orthologue, in vivo. The lethality of drak/rok mutants can be rescued by restoring Sqh activity, indicating that Sqh is the critical common effector of these two kinases. These results provide the first evidence that DAPK family kinases regulate actin dynamics in vivo and identify Drak as a novel component of the signaling networks that shape epithelial tissues.

Reiterative use of signalling pathways controls multiple cellular events during Drosophila posterior spiracle organogenesis

Organogenesis proceeds in multiple steps and events that need to be coordinated in time and space. Yet the genetic and molecular control of such coordination remains poorly understood. In this study we have investigated the contribution of three signalling pathways, Wnt/Wingless (Wg), Hedgehog (Hh), and epidermal growth factor receptor (EGFR), to posterior spiracle morphogenesis, an organ that forms under Abdominal-B (AbdB) control in the eighth abdominal segment. Using targeted signalling inactivation, we show that these pathways are reiteratively used to control multiple cellular events during posterior spiracle organogenesis, including cell survival and maintenance of cell polarity and adhesion required for tissue integrity. We propose that the reiterative use of the Wg, Hh, and EGFR signalling pathways serves to coordinate in time and space the sequential deployment of events that collectively allow proper organogenesis.

Epithelial relaxation mediated by the myosin phosphatase regulator Mypt1 is required for brain ventricle lumen expansion and hindbrain morphogenesis

We demonstrate that in the zebrafish hindbrain, cell shape, rhombomere morphogenesis and, unexpectedly, brain ventricle lumen expansion depend on the contractile state of the neuroepithelium. The hindbrain neural tube opens in a specific sequence, with initial separation along the midline at rhombomere boundaries, subsequent openings within rhombomeres and eventual coalescence of openings into the hindbrain ventricle lumen. A mutation in the myosin phosphatase regulator mypt1 results in a small ventricle due to impaired stretching of the surrounding neuroepithelium. Although initial hindbrain opening remains normal, mypt1 mutant rhombomeres do not undergo normal morphological progression. Three-dimensional reconstruction demonstrates cell shapes within rhombomeres and at rhombomere boundaries are abnormal in mypt1 mutants. Wild-type cell shape requires that surrounding cells are also wild type, whereas mutant cell shape is autonomously regulated. Supporting the requirement for regulation of myosin function during hindbrain morphogenesis, wild-type embryos show dynamic levels of phosphorylated myosin regulatory light chain (pMRLC). By contrast, mutants show continuously high pMRLC levels, with concentration of pMRLC and myosin II at the apical side of the epithelium, and myosin II and actin concentration at rhombomere boundaries. Brain ventricle lumen expansion, rhombomere morphology and cell shape are rescued by inhibition of myosin II function, indicating that each defect is a consequence of overactive myosin. We suggest that the epithelium must ;relax', via activity of myosin phosphatase, to allow for normal hindbrain morphogenesis and expansion of the brain ventricular lumen. Epithelial relaxation might be a widespread strategy to facilitate tube inflation in many organs.

Repression of Wasp by JAK/STAT signalling inhibits medial actomyosin network assembly and apical cell constriction in intercalating epithelial cells

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. We report that apical 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. We show that 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. We discuss possible models for the role of Wasp activity in the regulation of Myo-II distribution.

Epithelial cell shape is regulated by Lulu proteins via myosin-II

Cell-shape change in epithelial structures is fundamental to animal morphogenesis. Recent studies identified myosin-II as the major generator of driving forces for cell-shape changes during morphogenesis. Lulu (Epb41l5) is a major regulator of morphogenesis, although the downstream molecular and cellular mechanisms remain obscure in mammals. In Drosophila and zebrafish, Lulu proteins were reported to negatively regulate Crumbs, an apical domain regulator, thus regulating morphogenesis. In this study, we show that mammalian Lulu activates myosin-II, thus regulating epithelial cell shape. In our experiments, Lulu expression in epithelial cells resulted in apical constriction and lateral elongation in the cells, accompanied by upregulation of myosin-II. The inhibition of myosin-II activity almost completely blocked this Lulu-driven cell-shape change. We further found that Rock participates in the myosin-II activation. Additionally, RNAi-mediated depletion of Lulu in epithelial cells resulted in disorganization of myosin-II and a concomitant loss of proper lateral domain organization in the cells. From these results, we propose that Lulu regulates epithelial cell shape by controlling myosin-II activity.

Apical constriction and invagination downstream of the canonical Wnt signaling pathway require Rho1 and Myosin II

The tumor suppressor Adenomatous polyposis coli (APC) is a negative regulator of Wnt signaling and functions in cytoskeletal organization. Disruption of human APC in colonic epithelia initiates benign polyps that progress to carcinoma following additional mutations. The early events of polyposis are poorly understood, as is the role of canonical Wnt signaling in normal epithelial architecture and morphogenesis. To determine the consequences of complete loss of APC in a model epithelium, we generated APC2 APC1 double null clones in the Drosophila wing imaginal disc. APC loss leads to segregation, apical constriction, and invagination that result from transcriptional activation of canonical Wnt signaling. Further, we show that Wnt-dependent changes in cell fate can be decoupled from Wnt-dependent changes in cell shape. Wnt activation is reported to upregulate DE-cadherin in wing discs, and elevated DE-cadherin is thought to promote apical constriction. We find that apical constriction and invagination of APC null tissue are independent of DE-cadherin elevation, but are dependent on Myosin II activity. Further, we show that disruption of Rho1 suppresses apical constriction and invagination in APC null cells. Our data suggest a novel link between canonical Wnt signaling and epithelial structure that requires activation of the Rho1 pathway and Myosin II.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

Cell mechanics and feedback regulation of actomyosin networks

Actomyosin contractility is the major force-generating machinery that shapes cells and tissues during morphogenesis. New evidence from Drosophila demonstrates that these forces are spatially organized by a combination of biochemical and mechanical signals that provide dynamic feedback in a complex cellular environment.

Hedgehog, but not Odd skipped, induces segmental grooves in the Drosophila epidermis

The formation of segmental grooves during mid embryogenesis in the Drosophila epidermis depends on the specification of a single row of groove cells posteriorly adjacent to cells that express the Hedgehog signal. However, the mechanism of groove formation and the role of the parasegmental organizer, which consists of adjacent rows of hedgehog- and wingless-expressing cells, are not well understood. We report that although groove cells originate from a population of Odd skipped-expressing cells, this pair-rule transcription factor is not required for their specification. We further find that Hedgehog is sufficient to specify groove fate in cells of different origin as late as stage 10, suggesting that Hedgehog induces groove cell fate rather than maintaining a pre-established state. Wingless activity is continuously required in the posterior part of parasegments to antagonize segmental groove formation. Our data support an instructive role for the Wingless/Hedgehog organizer in cellular patterning.

Myosin II dynamics are regulated by tension in intercalating cells

Axis elongation in Drosophila occurs through polarized cell rearrangements driven by actomyosin contractility. Myosin II promotes neighbor exchange through the contraction of single cell boundaries, while the contraction of myosin II structures spanning multiple pairs of cells leads to rosette formation. Here we show that multicellular actomyosin cables form at a higher frequency than expected by chance, indicating that cable assembly is an active process. Multicellular cables are sites of increased mechanical tension as measured by laser ablation. Fluorescence recovery after photobleaching experiments show that myosin II is stabilized at the cortex in regions of increased tension. Myosin II is recruited in response to an ectopic force and relieving tension leads to a rapid loss of myosin, indicating that tension is necessary and sufficient for cortical myosin localization. These results demonstrate that myosin II dynamics are regulated by tension in a positive feedback loop that leads to multicellular actomyosin cable formation and efficient tissue elongation.

Cell rearrangements, cell divisions and cell death in a migrating epithelial sheet in the abdomen of Drosophila

During morphogenesis, cell movements, cell divisions and cell death work together to form complex patterns and to shape organs. These events are the outcome of decisions made by many individual cells, but how these decisions are controlled and coordinated is elusive. The adult abdominal epidermis of Drosophila is formed during metamorphosis by divisions and extensive cell migrations of the diploid histoblasts, which replace the polyploid larval cells. Using in vivo 4D microscopy, we have studied the behaviour of the histoblasts and analysed in detail how they reach their final position and to what extent they rearrange during their spreading. Tracking individual cells, we show that the cells migrate in two phases that differ in speed, direction and amount of cellular rearrangement. Cells of the anterior (A) and posterior (P) compartments differ in their behaviour. Cells near the A/P border are more likely to change their neighbours during migration. The mitoses do not show any preferential orientation. After mitosis, the sisters become preferentially aligned with the direction of movement. Thus, in the abdomen, it is the extensive cell migrations that appear to contribute most to morphogenesis. This contrasts with other developing epithelia, such as the wing imaginal disc and the embryonic germband in Drosophila, where oriented mitoses and local cell rearrangements appear to direct morphogenesis. Furthermore, our results suggest that an active force created by the histoblasts contributes to the formation of the adult epidermis. Finally, we show that histoblasts occasionally undergo apoptosis.

Apical constriction: a cell shape change that can drive morphogenesis

Biologists have long recognized that dramatic bending of a cell sheet may be driven by even modest shrinking of the apical sides of cells. Cell shape changes and tissue movements like these are at the core of many of the morphogenetic movements that shape animal form during development, driving processes such as gastrulation, tube formation, and neurulation. The mechanisms of such cell shape changes must integrate developmental patterning information in order to spatially and temporally control force production-issues that touch on fundamental aspects of both cell and developmental biology and on birth defects research. How does developmental patterning regulate force-producing mechanisms, and what roles do such mechanisms play in development? Work on apical constriction from multiple systems including Drosophila, Caenorhabditis elegans, sea urchin, Xenopus, chick, and mouse has begun to illuminate these issues. Here, we review this effort to explore the diversity of mechanisms of apical constriction, the diversity of roles that apical constriction plays in development, and the common themes that emerge from comparing systems.

Wingless signaling and the control of cell shape in Drosophila wing imaginal discs

The control of cell morphology is important for shaping animals during development. Here we address the role of the Wnt/Wingless signal transduction pathway and two of its target genes, vestigial and shotgun (encoding E-cadherin), in controlling the columnar shape of Drosophila wing disc cells. We show that clones of cells mutant for arrow (encoding an essential component of the Wingless signal transduction pathway), vestigial or shotgun undergo profound cell shape changes and are extruded towards the basal side of the epithelium. Compartment-wide expression of a dominant-negative form of the Wingless transducer T-cell factor (TCF/Pangolin), or double-stranded RNA targeting vestigial or shotgun, leads to abnormally short cells throughout this region, indicating that these genes act cell autonomously to maintain normal columnar cell shape. Conversely, overexpression of Wingless, a constitutively-active form of the Wingless transducer beta-catenin/Armadillo, or Vestigial, results in precocious cell elongation. Co-expression of Vestigial partially suppresses the abnormal cell shape induced by dominant-negative TCF. We conclude that Wingless signal transduction plays a cell-autonomous role in promoting and maintaining the columnar shape of wing disc cells. Furthermore, our data suggest that Wingless controls cell shape, in part, through maintaining vestigial expression.

Dpp signaling promotes the cuboidal-to-columnar shape transition of Drosophila wing disc epithelia by regulating Rho1

Morphogenesis is largely driven by changes in the shape of individual cells. However, how cell shape is regulated in developing animals is not well understood. Here, we show that the onset of TGFβ/Dpp signaling activity correlates with the transition from cuboidal to columnar cell shape in developing Drosophila melanogaster wing disc epithelia. Dpp signaling is necessary for maintaining this elongated columnar cell shape and overactivation of the Dpp signaling pathway results in precocious cell elongation. Moreover, we provide evidence that Dpp signaling controls the subcellular distribution of the activities of the small GTPase Rho1 and the regulatory light chain of non-muscle myosin II (MRLC). Alteration of Rho1 or MRLC activity has a profound effect on apical-basal cell length. Finally, we demonstrate that a decrease in Rho1 or MRLC activity rescues the shortening of cells with compromised Dpp signaling. Our results identify a cell-autonomous role for Dpp signaling in promoting and maintaining the elongated columnar shape of wing disc cells and suggest that Dpp signaling acts by regulating Rho1 and MRLC.

Sds22, a PP1 phosphatase regulatory subunit, regulates epithelial cell polarity and shape [Sds22 in epithelial morphology]

Background

How epithelial cells adopt their particular polarised forms is poorly understood. In a screen for genes regulating epithelial morphology in Drosophila, we identified sds22, a conserved gene previously characterised in yeast.

Results

In the columnar epithelia of imaginal discs or follicle cells, mutation of sds22 causes contraction of cells along their apical-basal axis, resulting in a more cuboidal morphology. In addition, the mutant cells can also display altered cell polarity, forming multiple layers in follicle cells and leaving the epithelium in imaginal discs. In yeast, sds22 encodes a PP1 phosphatase regulatory subunit. Consistent with this, we show that Drosophila Sds22 binds to all four Drosophila PP1s and shares an overlapping phenotype with PP1beta9c. We also show that two previously postulated PP1 targets, Spaghetti Squash and Moesin are hyper-phosphorylated in sds22 mutants. This function is shared by the human homologue of Sds22, PPP1R7.

Conclusion

Sds22 is a conserved PP1 phosphatase regulatory subunit that controls cell shape and polarity.

Principles of Drosophila eye differentiation

The Drosophila eye is one of nature's most beautiful structures and one of its most useful. It has emerged as a favored model for understanding the processes that direct cell fate specification, patterning, and morphogenesis. Though composed of thousands of cells, each fly eye is a simple repeating pattern of perhaps a dozen cell types arranged in a hexagonal array that optimizes coverage of the visual field. This simple structure combined with powerful genetic tools make the fly eye an ideal model to explore the relationships between local cell fate specification and global tissue patterning. In this chapter, I discuss the basic principles that have emerged from three decades of close study. We now understand at a useful level some of the basic principles of cell fate selection and the importance of local cell-cell communication. We understand less of the processes by which signaling combines with morphogenesis and basic cell biology to create a correctly patterned neuroepithelium. Progress is being made on these fundamental issues, and in this chapter I discuss some of the principles that are beginning to emerge.

Pattern formation in the Drosophila eye disc

Differentiation of the Drosophila compound eye from the eye imaginal disc is a progressive process: columns of cells successively differentiate in a posterior to anterior sequence, clusters of cells form at regularly spaced intervals within each column, and individual photoreceptors differentiate in a defined order within each cluster. The progression of differentiation across the eye disc is driven by a positive autoregulatory loop of expression of the secreted molecule Hedgehog, which is temporally delayed by the intercalation of a second signal, Spitz. Hedgehog refines the spatial position at which each column initiates its differentiation by inducing secondary signals that act over different ranges to control the expression of positive and negative regulators. The position of clusters within each column is controlled by secreted inhibitory signals from clusters in the preceding column, and a single founder neuron, R8, is singled out within each cluster by Notch-mediated lateral inhibition. R8 then sequentially recruits surrounding cells to differentiate by producing a short-range signal, Spitz, which induces a secondary short-range signal, Delta. Intrinsic transcription factors act in combination with these two signals to produce cell-type diversity within the ommatidium. The Hedgehog and Spitz signals are transported along the photoreceptor axons and reused within the brain as long-range and local cues to trigger the differentiation and assembly of target neurons.

Multicellular dynamics during epithelial elongation

The reorganization of multicellular populations to produce an elongated tissue structure is a conserved mechanism for shaping the body axis and several organ systems. In the Drosophila germband epithelium, this process is accompanied by the formation of a planar polarized network of junctional and cytoskeletal proteins in response to striped patterns of gene expression. Actomyosin cables and adherens junctions are dynamically remodeled during intercalation, providing the basis for polarized cell behavior. Quantitative analysis of cell behavior in living embryos reveals unexpected cell population dynamics that include the formation of multicellular rosette structures as well as local neighbor exchange.

The abdomen of Drosophila: does planar cell polarity orient the neurons of mechanosensory bristles?

BACKGROUND

In the adult abdomen of Drosophila, the shafts of mechanosensory bristles point consistently from anterior to posterior. This is an example of planar cell polarity (PCP); some genes responsible for PCP have been identified. Each adult bristle is made by a clone of four cells, including the neuron that innervates it, but little is known as to how far the formation or positions of these cells depends on PCP. The neurons include a single dendrite and an axon; it is not known whether the orientation of these processes is influenced by PCP.

RESULTS

We describe the development of the abdominal mechanosensory bristles in detail. The division of the precursor cell gives two daughters, one (pIIa) divides to give rise to the bristle shaft and socket cell and the other (pIIb) generates the neuron, the sheath and the fifth cell. Although the bristles and their associated shaft and socket cells are consistently oriented, the positioning and behaviour of the neuron, the sheath and the fifth cell, as well as the orientation of the axons and the dendritic paths, depend on location. For example, in the anterior zone of the segment, the axons grow posteriorly, while in the posterior zone, they grow anteriorly. Manipulating the PCP genes can reverse bristle orientation, change the path taken by the dendrite and the position of the cell body of the neuron. However, the paths taken by the axon are not affected.

CONCLUSION

PCP genes, such as starry night and dachsous orient the bristles and position the neuronal cell body and affect the shape of the dendrites. However, these PCP genes do not appear to change the paths followed by the sensory axons, which must, therefore, be polarised by other factors.

Hedgehog and Dpp signaling induce cadherin Cad86C expression in the morphogenetic furrow during Drosophila eye development

During Drosophila eye development, cell differentiation is preceded by the formation of a morphogenetic furrow, which progresses across the epithelium from posterior to anterior. Cells within the morphogenetic furrow are apically constricted and shortened along their apical-basal axis. However, how these cell shape changes and, thus, the progression of the morphogenetic furrow are controlled is not well understood. Here we show that cells simultaneously lacking Hedgehog and Dpp signal transduction fail to shorten and do not enter the morphogenetic furrow. Moreover, we have identified a gene, cadherin Cad86C, which is highly expressed in cells of the leading flank of the morphogenetic furrow. Ectopic activation of either the Hedgehog or Dpp signal transduction pathway results in elevated Cad86C expression. Conversely, simultaneous loss of both Hedgehog and Dpp signal transduction leads to decreased Cad86C expression. Finally, ectopic expression of Cad86C in either eye-antennal imaginal discs or wing imaginal discs results in apical constriction and shortening of cells. We conclude that Hedgehog and Dpp signaling promote the shortening of cells within the morphogenetic furrow. Induction of Cad86C expression might be one mechanism through which Hedgehog and Dpp promote these cell shape changes.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

Modulation of Drosophila retinal epithelial integrity by the adhesion proteins capricious and tartan

BACKGROUND

The development of the Drosophila eye imaginal disc requires complex epithelial rearrangements. Cells of the morphogenetic furrow are apically constricted and this leads to a physical indentation in the epithelium. Posterior to the furrow, cells start to rearrange into distinct clusters and eventually form a precisely patterned array of ommatidia. These morphogenetic processes include regulated changes of adhesion between cells.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we show that two transmembrane adhesion proteins, Capricious and Tartan, have dynamic and complementary expression patterns in the eye imaginal disc. We also describe novel null mutations in capricious and double null mutations in capricious and tartan. We report that they have redundant functions in regulating the architecture of the morphogenetic furrow and ommatidial spacing.

CONCLUSIONS/SIGNIFICANCE

We conclude that Capricious and Tartan contribute to the adhesive properties of the cells in the morphogenetic furrow and that this regulated adhesion participates in the control of spacing ommatidial clusters.