Signaling pathways directing the movement and fusion of epithelial sheets: lessons from dorsal closure in Drosophila

Dpp signalling orchestrates dorsal closure by regulating cell shape changes both in the amnioserosa and in the epidermis

During the final stages of embryogenesis, the Drosophila embryo exhibits a dorsal hole covered by a simple epithelium of large cells termed the amnioserosa (AS). Dorsal closure is the process whereby this hole is closed through the coordination of cellular activities within both the AS and the epidermis. Genetic analysis has shown that signalling through Jun N-terminal Kinase (JNK) and Decapentaplegic (Dpp), a Drosophila member of the BMP/TGF-beta family of secreted factors, controls these activities. JNK activates the expression of dpp in the dorsal-most epidermal cells, and subsequently Dpp acts as a secreted signal to control the elongation of lateral epidermis. Our analysis shows that Dpp function not only affects the epidermal cells, but also the AS. Embryos defective in Dpp signalling display defects in AS cell shape changes, specifically in the reduction of their apical surface areas, leading to defective AS contraction. Our data also demonstrate that Dpp regulates adhesion between epidermis and AS, and mediates expression of the transcription factor U-shaped in a gradient across both the AS and the epidermis. In summary, we show that Dpp plays a crucial role in coordinating the activity of the AS and its interactions with the LE cells during dorsal closure.

Cell adhesion molecule Echinoid associates with unconventional myosin VI/Jaguar motor to regulate cell morphology during dorsal closure in Drosophila

Echinoid (Ed) is a homophilic immunoglobulin domain-containing cell adhesion molecule (CAM) that localizes to adherens junctions (AJs) and cooperates with Drosophila melanogaster epithelial (DE)-cadherin to mediate cell adhesion. Here we show that Ed takes part in many processes of dorsal closure, a morphogenetic movement driven by coordinated cell shape changes and migration of epidermal cells to cover the underlying amnioserosa. Ed is differentially expressed, appearing in epidermis but not in amnioserosa cells. Ed functions independently from the JNK signaling pathway and is required to regulate cell morphology, and for assembly of actomyosin cable, filopodial protrusion and coordinated cell migration in dorsal-most epidermal cells. The effect of Ed on cell morphology requires the presence of the intracellular domain (Ed(intra)). Interestingly, Ed forms homodimers in vivo and Ed(intra) monomer directly associates with unconventional myosin VI/Jaguar (Jar) motor protein. We further show that ed genetically interacts with jar to control cell morphology. It has previously been shown that myosin VI is monomeric in vitro and that its dimeric form can associate with and travel processively along actin filaments. Thus, we propose that Ed mediates the dimerization of myosin VI/Jar in vivo which in turn regulates the reorganization and/or contraction of actin filaments to control changes in cell shape. Consistent with this, we found that ectopic ed expression in the amnioserosa induces myosin VI/Jar-dependent apical constriction of this tissue.

A mutation in zebrafish hmgcr1b reveals a role for isoprenoids in vertebrate heart-tube formation

In vertebrates, the morphogenetic assembly of the primitive heart tube requires the medial migration and midline fusion of the bilateral myocardial epithelia. Several mutations that result in abnormal heart-tube formation have been studied; however, an understanding of the underlying molecular and cellular mechanisms of the migration and fusion of these epithelial sheets is far from complete. In a forward genetic screen to identify genes regulating early zebrafish heart development, we identified a mutation in the 3-hydroxy-3-methylglutaryl-Coenzyme A reductase 1b (hmgcr1b) gene that affects myocardial migration to the midline and subsequent heart-tube morphogenesis. The mutant phenotype can be rescued with injections of mevalonate, the direct product of HMGCR activity. Furthermore, treatment of embryos with pharmacological inhibitors of isoprenoid synthesis, which occurs downstream of mevalonate production, resulted in defective heart-tube formation. Interestingly, in hmgcr1b mutant embryos and embryos treated with HMGCR inhibitors, both RasCT20-eGFP and RhoaCT32-eGFP fusion proteins were mislocalized away from the plasma membrane in embryonic myocardial cells. We conclude that protein prenylation, acting downstream of Hmgcr1b and possibly through Ras and, or, Rho signaling, is required for the morphogenesis of the myocardial sheets for formation of the primitive heart tube.

Upregulation of forces and morphogenic asymmetries in dorsal closure during Drosophila development

Tissue dynamics during dorsal closure, a stage of Drosophila development, provide a model system for cell sheet morphogenesis and wound healing. Dorsal closure is characterized by complex cell sheet movements, driven by multiple tissue specific forces, which are coordinated in space, synchronized in time, and resilient to UV-laser perturbations. The mechanisms responsible for these attributes are not fully understood. We measured spatial, kinematic, and dynamic antero-posterior asymmetries to biophysically characterize both resiliency to laser perturbations and failure of closure in mutant embryos and compared them to natural asymmetries in unperturbed, wild-type closure. We quantified and mathematically modeled two processes that are upregulated to provide resiliency--contractility of the amnioserosa and formation of a seam between advancing epidermal sheets, i.e., zipping. Both processes are spatially removed from the laser-targeted site, indicating they are not a local response to laser-induced wounding and suggesting mechanosensitive and/or chemosensitive mechanisms for upregulation. In mutant embryos, tissue junctions initially fail at the anterior end indicating inhomogeneous mechanical stresses attributable to head involution, another developmental process that occurs concomitant with the end stages of closure. Asymmetries in these mutants are reversed compared to wild-type, and inhomogeneous stresses may cause asymmetries in wild-type closure.

Foxo and Fos regulate the decision between cell death and survival in response to UV irradiation

Cells damaged by environmental insults have to be repaired or eliminated to ensure tissue homeostasis in metazoans. Recent studies suggest that the balance between cell survival signals and pro-apoptotic stimuli controls the decision between cell repair and death. How these competing signals are integrated and interpreted to achieve accurate control over cell fate in vivo is incompletely understood. Here, we show that the Forkhead Box O transcription factor Foxo and the AP-1 transcription factor DFos are required downstream of Jun-N-terminal kinase signaling for the apoptotic response to UV-induced DNA damage in the developing Drosophila retina. Both transcription factors regulate the pro-apoptotic gene hid. Our results indicate that UV-induced apoptosis is repressed by receptor tyrosine kinase-mediated inactivation of Foxo. These data suggest that integrating stress and survival signals through Foxo drives the decision between cell death and repair of damaged cells in vivo.

Comparative analysis of Hox downstream genes in Drosophila

Functional diversification of body parts is dependent on the formation of specialized structures along the various body axes. In animals, region-specific morphogenesis along the anteroposterior axis is controlled by a group of conserved transcription factors encoded by the Hox genes. Although it has long been assumed that Hox proteins carry out their function by regulating distinct sets of downstream genes, only a small number of such genes have been found, with very few having direct roles in controlling cellular behavior. We have quantitatively identified hundreds of Hox downstream genes in Drosophila by microarray analysis, and validated many of them by in situ hybridizations on loss- and gain-of-function mutants. One important finding is that Hox proteins, despite their similar DNA-binding properties in vitro, have highly specific effects on the transcriptome in vivo, because expression of many downstream genes respond primarily to a single Hox protein. In addition, a large fraction of downstream genes encodes realizator functions, which directly affect morphogenetic processes, such as orientation and rate of cell divisions, cell-cell adhesion and communication, cell shape and migration, or cell death. Focusing on these realizators, we provide a framework for the morphogenesis of the maxillary segment. As the genomic organization of Hox genes and the interaction of Hox proteins with specific co-factors are conserved in vertebrates and invertebrates, and similar classes of downstream genes are regulated by Hox proteins across the metazoan phylogeny, our findings represent a first step toward a mechanistic understanding of morphological diversification within a species as well as between species.

Extrinsic and intrinsic mechanisms directing epithelial cell sheet replacement during Drosophila metamorphosis

The fusion of epithelial sheets is an essential morphogenetic event. Here, we study the development of the abdomen of Drosophila as a model of bounded epithelia expansion and uncover a complex multistep process for the generation of the adult epidermis from histoblasts, founder cells that replace the larval cells during metamorphosis. We find that histoblasts experience a biphasic cell cycle and emit apical projections that direct their invasive planar intercalation in between larval cells. Coordinately, the larval cells extrude from the epithelia by apical constriction of an actomyosin ring and as a consequence die by apoptosis and are removed by circulating haemocytes. We demonstrate that the proliferation of histoblasts and the death of larval cells are triggered by two independent extrinsic Ecdysone hormonal pulses. Finally, we show that histoblast spreading and the death of larval cells depend on a mutual exchange of signals and are non-autonomous processes.

Nonmuscle myosin II generates forces that transmit tension and drive contraction in multiple tissues during dorsal closure

BACKGROUND

The morphogenic movements that characterize embryonic development require the precise temporal and spatial control of cell-shape changes. Drosophila dorsal closure is a well-established model for epithelial sheet morphogenesis, and mutations in more than 60 genes cause defects in closure. Closure requires that four forces, derived from distinct tissues, be precisely balanced. The proteins responsible for generating each of the forces have not been determined.

RESULTS

We document dorsal closure in living embryos to show that mutations in nonmuscle myosin II (encoded by zipper; zip/MyoII) disrupt the integrity of multiple tissues during closure. We demonstrate that MyoII localization is distinct from, but overlaps, F-actin in the supracellular purse string, whereas in the amnioserosa and lateral epidermis each has similar, cortical distributions. In zip/MyoII mutant embryos, we restore MyoII function either ubiquitously or specifically in the leading edge, amnioserosa, or lateral epidermis and find that zip/MyoII function in any one tissue can rescue closure. Using a novel, transgenic mosaic approach, we establish that contractility of the supracellular purse string in leading-edge cells requires zip/MyoII-generated forces; that zip/MyoII function is responsible for the apical contraction of amnioserosa cells; that zip/MyoII is important for zipping; and that defects in zip/MyoII contractility cause the misalignment of the lateral-epidermal sheets during seam formation.

CONCLUSIONS

We establish that zip/MyoII is responsible for generating the forces that drive cell-shape changes in each of the force-generating tissues that contribute to closure. This highly conserved contractile protein likely drives cell-sheet movements throughout phylogeny.

Coordinated cell-shape changes control epithelial movement in zebrafish and Drosophila

Epithelial morphogenesis depends on coordinated changes in cell shape, a process that is still poorly understood. During zebrafish epiboly and Drosophila dorsal closure, cell-shape changes at the epithelial margin are of critical importance. Here evidence is provided for a conserved mechanism of local actin and myosin 2 recruitment during theses events. It was found that during epiboly of the zebrafish embryo, the movement of the outer epithelium (enveloping layer) over the yolk cell surface involves the constriction of marginal cells. This process depends on the recruitment of actin and myosin 2 within the yolk cytoplasm along the margin of the enveloping layer. Actin and myosin 2 recruitment within the yolk cytoplasm requires the Ste20-like kinase Msn1, an orthologue of Drosophila Misshapen. Similarly, in Drosophila, actin and myosin 2 localization and cell constriction at the margin of the epidermis mediate dorsal closure and are controlled by Misshapen. Thus, this study has characterized a conserved mechanism underlying coordinated cell-shape changes during epithelial morphogenesis.

Genetic analysis of slipper/mixed lineage kinase reveals requirements in multiple Jun-N-terminal kinase-dependent morphogenetic events during Drosophila development

Mixed lineage kinases (MLKs) function as Jun-N-terminal kinase (JNK) kinase kinases to transduce extracellular signals during development and homeostasis in adults. slipper (slpr), which encodes the Drosophila homolog of mammalian MLKs, has previously been implicated in activation of the JNK pathway during embryonic dorsal epidermal closure. To further define the specific functions of SLPR, we analyzed the phenotypic consequences of slpr loss and gain of function throughout development, using a semiviable maternal-effect allele and wild-type or dominant-negative transgenes. From these analyses we confirm that failure of dorsal closure is the null phenotype in slpr germline clones. In addition, there is a functional maternal contribution, which can suffice for embryogenesis in the zygotic null mutant, but rarely suffices for pupal metamorphosis, revealing later functions for slpr as the maternal contribution is depleted. Zygotic null mutants that eclose as adults display an array of morphological defects, many of which are shared by hep mutant animals, deficient in the JNK kinase (JNKK/MKK7) substrate for SLPR, suggesting that the defects observed in slpr mutants primarily reflect loss of hep-dependent JNK activation. Consistent with this, the maternal slpr contribution is sensitive to the dosage of positive and negative JNK pathway regulators, which attenuate or potentiate SLPR-dependent signaling in development. Although SLPR and TAK1, another JNKKK family member, are differentially used in dorsal closure and TNF/Eiger-stimulated apoptosis, respectively, a Tak1 mutant shows dominant genetic interactions with slpr, suggesting potential redundant or combinatorial functions. Finally, we demonstrate that SLPR overexpression can induce ectopic JNK signaling and that the SLPR protein is enriched at the epithelial cell cortex.

JNK signaling coordinates integrin and actin functions during Drosophila embryogenesis

Epithelial movements are key morphogenetic events in animal development. They are driven by multiple mechanisms, including signal-dependent changes in cytoskeletal organization and in cell adhesion. Such processes must be controlled precisely and coordinated to accurately sculpt the three-dimensional form of the developing organism. By observing the Drosophila epidermis during embryonic development using confocal time-lapse microscopy, we have investigated how signaling through the Jun-N-terminal kinase (JNK) pathway governs the tissue sheet movements that result in dorsal closure (DC). We find that JNK controls the polymerization of actin into a cable at the epidermal leading edge as previously suggested, as well as the joining (zipping) of the contralateral epithelial cell sheets. Here, we show that zipping is mediated by regulation of the integrins myospheroid and scab. Our data demonstrate that JNK signaling regulates a set of target genes that cooperate to facilitate epithelial movement and closure.

The striatin family: a new signaling platform in dendritic spines

Proteins of the striatin family have been identified in all multicellular animals. They are multidomain molecules containing several protein-interacting motifs. In mammals, these proteins are principally expressed in neurons with a somato-dendritic localization and high concentration in dendritic spines. Recent reports suggest that the proteins of the striatin family are molecular scaffolds that act as links between signal transduction and vesicular trafficking.

Drosophila Dok is required for embryonic dorsal closure

Embryonic dorsal closure (DC) in Drosophila is a series of morphogenetic movements involving the bilateral dorsal movement of the epidermis (cell stretching) and dorsal suturing of the leading edge (LE) cells to enclose the viscera. The Syk family tyrosine kinase Shark plays a crucial role in this Jun amino-terminal kinase (JNK)-dependent process, where it acts upstream of JNK in LE cells. Using a yeast two-hybrid screen, the unique Drosophila homolog of the downstream of kinase (Dok) family, Ddok,was identified by its ability to bind Shark SH2 domains in a tyrosine phosphorylation-dependent fashion. In cultured S2 embryonic cells, Ddok tyrosine phosphorylation is Src dependent; Shark associates with Ddok and Ddok localizes at the cell cortex, together with a portion of the Shark protein. The embryonic expression pattern of Ddok resembles the expression pattern of Shark. Ddok loss-of-function mutant (DdokPG155)germ-line clones possess DC defects, including the loss of JNK-dependent expression of dpp mRNA in LE cells, and decreased epidermal F-actin staining and LE actin cable formation. Epistatic analysis indicates that Ddok functions upstream of shark to activate JNK signaling during DC. Consistent with these observations, Ddok mutant embryos exhibit decreased levels of tyrosine phosphorylated Shark at the cell periphery of LE and epidermal cells. As there are six mammalian Dok family members that exhibit some functional redundancy, analysis of the regulation of DC by Ddok is expected to provide novel insights into the function of the Dok adapter proteins.

Cabut, a C2H2 zinc finger transcription factor, is required during Drosophila dorsal closure downstream of JNK signaling

During dorsal closure, the lateral epithelia on each side of the embryo migrate dorsally over the amnioserosa and fuse at the dorsal midline. Detailed genetic studies have revealed that many molecules are involved in this epithelial sheet movement, either with a signaling function or as structural or motor components of the process. Here, we report the characterization of cabut (cbt), a new Drosophila gene involved in dorsal closure. cbt is expressed in the yolk sac nuclei and in the lateral epidermis. The Cbt protein contains three C2H2-type zinc fingers and a serine-rich domain, suggesting that it functions as a transcription factor. cbt mutants die as embryos with dorsal closure defects. Such embryos show defects in the elongation of the dorsal-most epidermal cells as well as in the actomyosin cable assembly at the leading edge. A combination of molecular and genetic analyses demonstrates that cbt expression is dependent on the JNK cascade during dorsal closure, and it functions downstream of Jun regulating dpp expression in the leading edge cells.

The RING-finger scaffold protein Plenty of SH3s targets TAK1 to control immunity signalling in Drosophila

Imd-mediated innate immunity is activated in response to infection by Gram-negative bacteria and leads to the activation of Jun amino-terminal kinase (JNK) and Relish, a nuclear factor-kappaB transcription factor responsible for the expression of antimicrobial peptides. Plenty of SH3s (POSH) has been shown to function as a scaffold protein for JNK activation, leading to apoptosis in mammals. Here, we report that POSH controls Imd-mediated immunity signalling in Drosophila. In POSH-deficient flies, JNK activation and Relish induction were delayed and sustained, which indicated that POSH is required for properly timed activation and termination of the cascade. The RING finger of POSH, possessing ubiquitin-ligase activity, was essential for termination of JNK activation. We show that POSH binds to and degrades TAK1, a crucial activator of both the JNK and the Relish signalling pathways. These results establish a novel role for POSH in the Drosophila immune system.

Amnioserosa is required for dorsal closure in Drosophila

Dorsal closure in the fruit fly Drosophila melanogaster is a complex morphogenetic process, driven by sequential signaling cascades and involving multiple forces, which contribute to cell movements and rearrangements as well as to changes in cell shape. During closure, lateral epidermal cells elongate along the dorsoventral axis and subsequently spread dorsally to cover the embryonic dorsal surface. Amnioserosal cells, which are the original occupants of the most dorsal position in the developing embryo, constrict during closure; thus, the increase in epidermal surface area is accommodated by a reduction in the amnioserosal surface area. Several of the epidermal requirements for closure have been established in functional assays. In contrast, amnioserosal requirements for closure have remained elusive, in part because laser ablation and clonal approaches are limited to only subsets of amnioserosal cells. Here, we report our use of the UAS-GAL4 system to target expression of the cell autonomous toxin Ricin-A to all cells of the amnioserosa. We show that ablation of the amnioserosa leads to clear defects in dorsal closure and, thus, directly demonstrate a role for the amnioserosa in dorsal closure. We also show that DJNK (Drosophila Jun N-terminal kinase) signaling, an epidermal trigger of closure, is unaffected by amnioserosal ablation. These data, together with our demonstration that amnioserosal ablated and Dpp signaling mutant embryos exhibit shared loss-of-function phenotypes, point to a requirement for the amnioserosa in dorsal closure that is downstream of Dpp, perhaps as part of a paracrine response to this signaling cascade.

The small GTPase Rac plays multiple roles in epithelial sheet fusion—dynamic studies of Drosophila dorsal closure

The coordinated migration and fusion of epithelial sheets is a crucial morphogenetic tool used on numerous occasions during the normal development of an embryo and re-activated as part of the wound healing response. Drosophila dorsal closure, whereby a hole in the embryonic epithelium is zipped closed late in embryogenesis, serves as an excellent, genetically tractable model for epithelial migration. Using live confocal imaging, we have dissected multiple roles for the small GTPase Rac in this process. We show that constitutive activation of Rac1 leads to excessive assembly of lamellipodia and precocious halting of epithelial sweeping, possibly through premature activation of contact-inhibition machinery. Conversely, blocking Rac activity, either by loss-of-function mutations or expression of dominant negative Rac1, disables the assembly of both actin cable and protrusions by epithelial cells. Movies of mutant embryos show that continued contraction of the amnioserosa is sufficient to draw the epithelial edges towards one another, allowing the zipper machinery to bypass non-functioning regions of leading edge. In addition to illustrating the key role of Rac in organization of leading edge actin, loss-of-function mutants also provide substantive proof that Rac acts upstream in the Jun N-terminal kinase (JNK) cascade to direct epithelial cell shape changes during dorsal closure.

Concentric zones of active RhoA and Cdc42 around single cell wounds

Rho GTPases control many cytoskeleton-dependent processes, but how they regulate spatially distinct features of cytoskeletal function within a single cell is poorly understood. Here, we studied active RhoA and Cdc42 in wounded Xenopus oocytes, which assemble and close a dynamic ring of actin filaments (F-actin) and myosin-2 around wound sites. RhoA and Cdc42 are rapidly activated around wound sites in a calcium-dependent manner and segregate into distinct, concentric zones around the wound, with active Cdc42 in the approximate middle of the F-actin array and active RhoA on the interior of the array. These zones form before F-actin accumulation, and then move in concert with the closing array. Microtubules and F-actin are required for normal zone organization and dynamics, as is crosstalk between RhoA and Cdc42. Each of the zones makes distinct contributions to the organization and function of the actomyosin wound array. We propose that similar rho activity zones control related processes such as cytokinesis.

A homolog of Drosophila grainy head is essential for epidermal integrity in mice

The Drosophila cuticle is essential for maintaining the surface barrier defenses of the fly. Integral to cuticle resilience is the transcription factor grainy head, which regulates production of the enzyme required for covalent cross-linking of the cuticular structural components. We report that formation and maintenance of the epidermal barrier in mice are dependent on a mammalian homolog of grainy head, Grainy head-like 3. Mice lacking this factor display defective skin barrier function and deficient wound repair, accompanied by reduced expression of transglutaminase 1, the key enzyme involved in cross-linking the structural components of the superficial epidermis. These findings suggest that the functional mechanisms involving protein cross-linking that maintain the epidermal barrier and induce tissue repair are conserved across 700 million years of evolution.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

Restoration of Wnt-7a expression reverses non-small cell lung cancer cellular transformation through frizzled-9-mediated growth inhibition and promotion of cell differentiation

The Wnt signaling pathway is critical in normal development, and mutation of specific components is frequently observed in carcinomas of diverse origins. However, the potential involvement of this pathway in lung tumorigenesis has not been established. In this study, analysis of multiple Wnt mRNAs in non-small cell lung cancer (NSCLC) cell lines and primary lung tumors revealed markedly decreased Wnt-7a expression compared with normal short-term bronchial epithelial cell lines and normal uninvolved lung tissue. Wnt-7a transfection in NSCLC cell lines reversed cellular transformation, decreased anchorage-independent growth, and induced epithelial differentiation as demonstrated by soft agar and three-dimensional cell culture assays in a subset of the NSCLC cell lines. The action of Wnt-7a correlated with expression of the specific Wnt receptor Frizzled-9 (Fzd-9), and transfection of Fzd-9 into a Wnt-7a-insensitive NSCLC cell line established Wnt-7a sensitivity. Moreover, Wnt-7a was present in Fzd-9 immunoprecipitates, indicating a direct interaction of Wnt-7a and Fzd-9. In NSCLC cells, Wnt-7a and Fzd-9 induced both cadherin and Sprouty-4 expression and stimulated the JNK pathway, but not beta-catenin/T cell factor activity. In addition, transfection of gain-of-function JNK strongly inhibited anchorage-independent growth. Thus, this study demonstrates that Wnt-7a and Fzd-9 signaling through activation of the JNK pathway induces cadherin proteins and the receptor tyrosine kinase inhibitor Sprouty-4 and represents a novel tumor suppressor pathway in lung cancer that is required for maintenance of epithelial differentiation and inhibition of transformed cell growth in a subset of human NSCLCs.

Dissecting JNK signaling, one KKKinase at a time

Specificity in signal transduction is essential to ensure distinct and appropriate cellular responses to extracellular cues. Determining the mechanisms that mediate specificity is key to understanding complex cell behaviors in development, when multiple pathways fire simultaneously and individual pathways are used recurrently. Jun kinase (JNK) signal transduction exemplifies a pathway that is used multiple times in animal development and homeostasis. Indeed, molecular genetic analysis of JNK signaling in Drosophila has shown that a core signaling module consisting of Hep (JNKK), Bsk (JNK), and Jun regulates various processes, including tissue morphogenesis, wound repair, stress response, innate immune response, and others. Six putative JNKK kinase (JNKKK) family members are present in the fly genome, which could activate the core module in response to distinct stimuli. The diversity of kinases at this level of the signaling hierarchy could substantially increase the number of possible signals that feed into activation of the core module. Recent studies have described the distinct phenotypic consequences of mutations in three of the genes, Slpr (dMLK), Tak1, and Mekk1. These data, together with Drosophila cell culture and genomic array analyses support the contention that the choice of JNKKK may contribute to signaling specificity in vivo. Whether this is achieved by individual JNKKKs or by means of a combinatorial mechanism will require a systematic characterization of compound mutants and a toolbox of transcriptional reporters specific for distinct JNK-dependent processes.

The integrin effector PINCH regulates JNK activity and epithelial migration in concert with Ras suppressor 1

Cell adhesion and migration are dynamic processes requiring the coordinated action of multiple signaling pathways, but the mechanisms underlying signal integration have remained elusive. Drosophila embryonic dorsal closure (DC) requires both integrin function and c-Jun amino-terminal kinase (JNK) signaling for opposed epithelial sheets to migrate, meet, and suture. Here, we show that PINCH, a protein required for integrin-dependent cell adhesion and actin-membrane anchorage, is present at the leading edge of these migrating epithelia and is required for DC. By analysis of native protein complexes, we identify RSU-1, a regulator of Ras signaling in mammalian cells, as a novel PINCH binding partner that contributes to PINCH stability. Mutation of the gene encoding RSU-1 results in wing blistering in Drosophila, demonstrating its role in integrin-dependent cell adhesion. Genetic interaction analyses reveal that both PINCH and RSU-1 antagonize JNK signaling during DC. Our results suggest that PINCH and RSU-1 contribute to the integration of JNK and integrin functions during Drosophila development.

Multiple rows of cells behind an epithelial wound edge extend cryptic lamellipodia to collectively drive cell-sheet movement

The mechanism by which epithelial, endothelial and other strongly cell-cell adhesive cells migrate collectively as continuous sheets is not clear, even though this process is crucial for embryonic development and tissue repair in virtually all multicellular animals. Wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers involves Rac GTPase-dependent migration of cells both at and behind the wound edge. We report here for the first time that cells behind the margin of wounded MDCK cell monolayers, even hundreds of microns from the edge, extend 'cryptic' lamellipodia against the substratum beneath cells in front of them, toward the wound, as determined by confocal, two-photon and transmission electron microscopy. These so-called submarginal cells nevertheless strictly maintain their more apical cell-cell contacts when they migrate as part of a coherent cell sheet, hiding their basal protrusions from conventional microscopy. The submarginal protrusions display the hallmarks of traditional lamellipodia based on morphology and dynamics. Cells behind the margin therefore actively crawl, instead of just moving passively when cells at the margin pull on them. The rate of migration is inversely proportional to the distance from the margin, and cells move co-ordinately, yet still in part autonomously, toward the wound area. We also clarify the ancillary role played by nonprotrusive contractile actin bundles that assemble in a Rho GTPase-dependent manner at the margin after wounding. In addition, some cell proliferation occurs at a delay after wounding but does not contribute to closure. Instead, it apparently serves to replace damaged cells so that intact spread cells can revert to their normal cuboidal morphology and the original cell density of the unbroken sheet can be restored.

dPak is required for integrity of the leading edge cytoskeleton during Drosophila dorsal closure but does not signal through the JNK cascade

The Pak kinases are effectors for the small GTPases Rac and Cdc42 and are divided into two subfamilies. Group I Paks possess an autoinhibitory domain that can suppress their kinase activity in trans. In Drosophila, two Group I kinases have been identified, dPak and Pak3. Rac and Cdc42 participate in dorsal closure of the embryo, a process in which a hole in the dorsal epidermis is sealed through migration of the epidermal flanks over a tissue called the amnioserosa. Dorsal closure is driven in part by an actomyosin contractile apparatus at the leading edge of the epidermis, and is regulated by a Jun amino terminal kinase (JNK) cascade. Impairment of dPak function using either loss-of-function mutations or expression of a transgene encoding the autoinhibitory domain of dPak led to disruption of the leading edge cytoskeleton and defects in dorsal closure but did not affect the JNK cascade. Group I Pak kinase activity in the amnioserosa is required for correct morphogenesis of the epidermis, and may be a component of the signaling known to occur between these two tissues. We conclude that dorsal closure requires Group I Pak function in both the amnioserosa and the epidermis.

Invasive cell behavior during Drosophila imaginal disc eversion is mediated by the JNK signaling cascade

Drosophila imaginal discs are monolayered epithelial invaginations that grow during larval stages and evert at metamorphosis to assemble the adult exoskeleton. They consist of columnar cells, forming the imaginal epithelium, as well as squamous cells, which constitute the peripodial epithelium and stalk (PS). Here, we uncover a new morphogenetic/cellular mechanism for disc eversion. We show that imaginal discs evert by apposing their peripodial side to the larval epidermis and through the invasion of the larval epidermis by PS cells, which undergo a pseudo-epithelial-mesenchymal transition (PEMT). As a consequence, the PS/larval bilayer is perforated and the imaginal epithelia protrude, a process reminiscent of other developmental events, such as epithelial perforation in chordates. When eversion is completed, PS cells localize to the leading front, heading disc expansion. We found that the JNK pathway is necessary for PS/larval cells apposition, the PEMT, and the motile activity of leading front cells.

Alpha-catenin: at the junction of intercellular adhesion and actin dynamics

Alpha-catenin has often been considered to be a non-regulatory intercellular adhesion protein, in contrast to beta-catenin, which has well-documented dual roles in cell-cell adhesion and signal transduction. Recently, however, alpha-catenin has been found to be important not only in connecting the E-cadherin-beta-catenin complex to the actin cytoskeleton, but also in coordinating actin dynamics and inversely correlating cell adhesion with proliferation. As the number of alpha-catenin-interacting partners increases, intriguing new connections imply even more complex regulatory functions for this protein.

PVR plays a critical role via JNK activation in thorax closure during Drosophila metamorphosis

PVR, the Drosophila homolog of the PDGF/VEGF receptor, has been implicated in border cell migration during oogenesis and hemocyte migration during embryogenesis. It was earlier shown that Mbc, a CDM family protein, and its effector, Rac, transduced the guidance signal from PVR during border cell migration. Here we demonstrate that PVR is also required for the morphogenetic process, thorax closure, during metamorphosis. The results of genetic and biochemical experiments indicate that PVR activates the JNK pathway. We present evidence showing Crk (an adaptor molecule), Mbc, ELMO (a homolog of Caenorhabditis elegans CED-12 and mammalian ELMO), and Rac to be mediators of JNK activation by PVR. In addition, we suppose that not only Rac but also Cdc42 is activated and involved in JNK activation downstream of PVR.

Wound healing and inflammation: embryos reveal the way to perfect repair

Tissue repair in embryos is rapid, efficient and perfect and does not leave a scar, an ability that is lost as development proceeds. Whereas adult wound keratinocytes crawl forwards over the exposed substratum to close the gap, a wound in the embryonic epidermis is closed by contraction of a rapidly assembled actin purse string. Blocking assembly of this cable in chick and mouse embryos, by drugs or by inactivation of the small GTPase Rho, severely hinders the re-epithelialization process. Live studies of epithelial repair in GFP-actin-expressing Drosophila embryos reveal actin-rich filopodia associated with the cable, and although these protrusions from leading edge cells appear to play little role in epithelial migration, they are essential for final zippering of the wound edges together-inactivation of Cdc42 prevents their assembly and blocks the final adhesion step. This wound re-epithelialization machinery appears to recapitulate that used during naturally occurring morphogenetic episodes as typified by Drosophila dorsal closure. One key difference between embryonic and adult repair, which may explain why one heals perfectly and the other scars, is the presence of an inflammatory response at sites of adult repair where there is none in the embryo. Our studies of repair in the PU. 1 null mouse, which is genetically incapable of raising an inflammatory response, show that inflammation may indeed be partly responsible for scarring, and our genetic studies of inflammation in zebrafish (Danio rerio) larvae suggest routes to identifying gene targets for therapeutically modulating the recruitment of inflammatory cells and thus improving adult healing.

c-Jun N-terminal kinase regulates lamellipodial protrusion and cell sheet migration during epithelial wound closure by a gene expression-independent mechanism

c-Jun N-terminal kinase (JNK) is emerging as an important regulator of cell migration. Perturbing the JNK signaling pathway with three structurally and mechanistically distinct inhibitors that selectively target either JNKs themselves or the upstream mixed-lineage kinases, we found dramatic inhibition of membrane protrusion and cell sheet migration during wound closure in Madin-Darby canine kidney (MDCK) epithelial cell monolayers. Extension of lamellipodia is blocked from the earliest times after wounding in the presence of JNK pathway inhibitors, whereas assembly of non-protrusive actin bundles at the wound margin is unaffected. Inhibitors of the other mitogen-activated protein kinase (MAPK) pathways, the extracellular signal-regulated kinase and p38 MAPK pathways, only have comparatively weak or marginal inhibitory effects on wound closure. Multiple splice variants of both JNK1 and JNK2 are expressed in MDCK cells, and JNK1 and JNK2 are rapidly and transiently activated upon wounding. Phosphorylation of c-Jun does not appear relevant to MDCK wound closure, and membrane protrusion directly after wounding is not affected by inhibitors of RNA or protein synthesis. While most known substrates of JNK are transcription factors or proteins regulating apoptosis, our data indicate that JNK regulates protrusion and migration in a gene expression-independent manner and suggest an important cytoplasmic role for JNK in the control of cell motility.

Parallels between tissue repair and embryo morphogenesis

Wound healing involves a coordinated series of tissue movements that bears a striking resemblance to various embryonic morphogenetic episodes. There are several ways in which repair recapitulates morphogenesis. We describe how almost identical cytoskeletal machinery is used to repair an embryonic epithelial wound as is involved during the morphogenetic episodes of dorsal closure in Drosophila and eyelid fusion in the mouse foetus. For both naturally occurring and wound-activated tissue movements, JNK signalling appears to be crucial, as does the tight regulation of associated cell divisions and adhesions. In the embryo, both morphogenesis and repair are achieved with a perfect end result, whereas repair of adult tissues leads to scarring. We discuss whether this may be due to the adult inflammatory response, which is absent in the embryo.

Armadillo/beta-catenin-dependent Wnt signalling is required for the polarisation of epidermal cells during dorsal closure in Drosophila

At the end of germband retraction, the dorsal epidermis of the Drosophila embryo exhibits a discontinuity that is covered by the amnioserosa. The process of dorsal closure (DC) involves a coordinated set of cell-shape changes within the epidermis and the amnioserosa that result in epidermal continuity. Polarisation of the dorsal-most epidermal (DME) cells in the plane of the epithelium is an important aspect of DC. The DME cells of embryos mutant for wingless or dishevelled exhibit polarisation defects and fail to close properly. We have investigated the role of the Wingless signalling pathway in the polarisation of the DME cells and DC. We find that the beta-catenin-dependent Wingless signalling pathway is required for polarisation of the DME cells. We further show that although the DME cells are polarised in the plane of the epithelium and present polarised localisation of proteins associated with the process of planar cell polarity (PCP) in the wing, e.g. Flamingo, PCP Wingless signalling is not involved in DC.

Wounding Induces Motility in Sheets of Corneal Epithelial Cells through Loss of Spatial Constraints

Cellular responses to wounding have often been studied at a molecular level after disrupting cell layers by mechanical means. This invariably results in damage to cells at the edges of the wounds, which has been suggested to be instrumental for initiating wound healing. To test this, we devised an alternative procedure to introduce gaps in layers of corneal epithelial cells by casting agarose strips on tissue culture plates. In contrast to mechanical wounding, removal of the strips did not lead to detectable membrane leakage or to activation of the stress-activated kinase JNK. Nonetheless, cells at the edge underwent the typical morphological transition to a highly motile phenotype, and the gaps closed at rates similar to those of mechanically induced wounds. To allow biochemical analysis of cell extracts, a procedure was devised that makes cell-free surface area acutely available to a large proportion of cells in culture. Rapid activation of the epidermal growth factor receptor (EGFR) was detected by immunoblotting, and the addition of an EGFR-blocking antibody completely abolished wound healing. In addition, wound healing was inhibited by agents that block signaling by the heparin-binding epidermal growth factor-like growth factor (HB-EGF). Cells stimulated with cell-free tissue culture surface released a soluble factor that induced activation of the EGFR, which was distinct from HB-EGF. These studies suggest that the triggering event for the induction of motility in corneal epithelial cells is related to the sudden availability of permissive surface area rather than to mechanical damage, and they demonstrate a central role of signaling through HB-EGF.

bullwinkle is required for epithelial morphogenesis during Drosophila oogenesis

Many organs, such as the liver, neural tube, and lung, form by the precise remodeling of flat epithelial sheets into tubes. Here we investigate epithelial tubulogenesis in Drosophila melanogaster by examining the development of the dorsal respiratory appendages of the eggshell. We employ a culture system that permits confocal analysis of stage 10-14 egg chambers. Time-lapse imaging of GFP-Moesin-expressing egg chambers reveals three phases of morphogenesis: tube formation, anterior extension, and paddle maturation. The dorsal-appendage-forming cells, previously thought to represent a single cell fate, consist of two subpopulations, those forming the tube roof and those forming the tube floor. These two cell types exhibit distinct morphological and molecular features. Roof-forming cells constrict apically and express high levels of Broad protein. Floor cells lack Broad, express the rhomboid-lacZ marker, and form the floor by directed cell elongation. We examine the morphogenetic phenotype of the bullwinkle (bwk) mutant and identify defects in both roof and floor formation. Dorsal appendage formation is an excellent system in which cell biological, molecular, and genetic tools facilitate the study of epithelial morphogenesis.

Mammalian formin-1 participates in adherens junctions and polymerization of linear actin cables

During epithelial sheet formation, linear actin cables assemble at nascent adherens junctions. This process requires alpha-catenin and actin polymerization, although the underlying mechanism is poorly understood. Here, we show that formin-1 interacts with alpha-catenin, localizes to adherens junctions and nucleates unbranched actin filaments. Furthermore, disruption of the alpha-catenin-formin-1 interaction blocks assembly of radial actin cables and perturbs intercellular adhesion. A fusion protein of the beta-catenin-binding domain of alpha-catenin with the actin polymerization domains of formin-1 rescues formation of adherens junctions and associated actin cables in alpha-catenin-null keratinocytes. These findings provide new insight into how alpha-catenin orchestrates actin dynamics during intercellular junction formation.

Menin, a tumor suppressor, associates with nonmuscle myosin II-A heavy chain

MEN1 is a likely tumor suppressor gene that encodes a novel protein, menin. Menin is a 610 amino-acid residue protein with as yet unknown function(s). We have used tandem affinity purification and mass spectroscopy to isolate and identify proteins associating with menin from cultured HeLa cell extracts. This strategy has resulted in the isolation and identification of nonmuscle myosin type II-A heavy chain (NMHC II-A) as a menin interacting protein. This interaction was confirmed by glutathione-S-transferase pulldown assays, by coimmunoprecipitation, and by actin selection of myosin. We have further identified the amino-terminal region of menin and the head domain of NMHC II-A to be regions required for this interaction. Moreover menin was seen to colocalize with this myosin isoform in the cleavage furrow of dividing cells by indirect immunofluoresence. These data indicate that menin through binding to NMHC II-A could participate in cell division and in other processes that involve NMHC II-A.

The Sac1 Lipid Phosphatase Regulates Cell Shape Change and the JNK Cascade during Dorsal Closure in Drosophila

The Sac1 lipid phosphatase dephosphorylates several phosphatidylinositol (PtdIns) phosphates and, in yeast, regulates a diverse range of cellular processes including organization of the actin cytoskeleton and secretion. We have identified mutations in the gene encoding Drosophila Sac1. sac1 mutants die as embryos with defects in dorsal closure (DC). DC involves the migration of the epidermis to close a hole in the dorsal surface of the embryo occupied by the amnioserosa. It requires cell shape change in both the epidermis and amnioserosa and activation of a Jun N-terminal kinase (JNK) MAPK cascade in the leading edge cells of the epidermis [2]. Loss of Sac1 leads to the improper activation of two key events in DC: cell shape change in the amnioserosa and JNK signaling. sac1 interacts genetically with other participants in these two events, and our data suggest that loss of Sac1 leads to upregulation of one or more signals controlling DC. This study is the first report of a role for Sac1 in the development of a multicellular organism.

Drosophila morphogenesis: the Newtonian revolution

Recent quantitative modeling of dorsal closure in the fruitfly Drosophila has revealed how multiple forces drive sealing of the two symmetrical epithelial sheets. A predictive model based on the new data allows gene function to be linked to the forces that drive tissue movement.

Forces for morphogenesis investigated with laser microsurgery and quantitative modeling

We investigated the forces that connect the genetic program of development to morphogenesis in Drosophila. We focused on dorsal closure, a powerful model system for development and wound healing. We found that the bulk of progress toward closure is driven by contractility in supracellular "purse strings" and in the amnioserosa, whereas adhesion-mediated zipping coordinates the forces produced by the purse strings and is essential only for the end stages. We applied quantitative modeling to show that these forces, generated in distinct cells, are coordinated in space and synchronized in time. Modeling of wild-type and mutant phenotypes is predictive; although closure in myospheroid mutants ultimately fails when the cell sheets rip themselves apart, our analysis indicates that beta(PS) integrin has an earlier, important role in zipping.

Motility cues in the tumor microenvironment

It is now increasingly recognized that the microenvironment plays a critical role in the progression of tumors. Perhaps less obvious is the concept that the microenvironment may share responsibility in determining the "malignant" traits of tumor cells, i.e. invasiveness and metastasis. If tumors are tissues, however unbalanced, rather than a collection of "malignant" cells recruiting local resources for the purpose of growth, then it is inevitable that tumor cells will respond to local stimuli. These stimuli include cues for motility and migration, which normally appear in tissues undergoing formation, remodeling or healing. Carcinoma cells are likely to be sensitive to the motility cues that normally regulate epithelial morphogenetic movements such as ingression, delamination, invagination, and tube or sheet migration. "Malignant" tumors, then, can be redefined as those in which these cues arise more frequently or act more effectively. Here, we expand on this view and propose that invasion and metastasis may be the outcome of tumor cell responses to microenvironmental motility cues. Understanding how such motility cues arise and act, both in normal and tumor tissue, should be a high priority in cancer research.

Wound healing recapitulates morphogenesis in Drosophila embryos

The capacity to repair a wound is a fundamental survival mechanism that is activated at any site of damage throughout embryonic and adult life. To study the cell biology and genetics of this process, we have developed a wounding model in Drosophila melanogaster embryos that allows live imaging of rearrangements and changes in cell shape, and of the cytoskeletal machinery that draws closed an in vivo wound. Using embryos expressing green fluorescent protein (GFP) fusion proteins, we show that two cytoskeletal-dependent elements -- an actin cable and dynamic filopodial/lamellipodial protrusions -- are expressed by epithelial cells at the wound edge and are pivotal for repair. Modulating the activities of the small GTPases Rho and Cdc42 demonstrates that these actin-dependent elements have differing cellular functions, but that either alone can drive wound closure. The actin cable operates as a 'purse-string' to draw the hole closed, whereas filopodia are essential for the final 'knitting' together of epithelial cells at the end of repair. Our data suggest a more complex model for epithelial repair than previously envisaged and highlight remarkable similarities with the well-characterized morphogenetic movement of dorsal closure in Drosophila.