Head involution inDrosophila: Genetic and morphogenetic connections to dorsal closure

The Enigmas of Tissue Closure: Inspiration from Drosophila

Hollow structures are essential for development and physiological activity. The construction and maintenance of hollow structures never cease throughout the lives of multicellular animals. Epithelial tissue closure is the main strategy used by living organisms to build hollow structures. The high diversity of hollow structures and the simplicity of their development in Drosophila make it an excellent model for the study of hollow structure morphogenesis. In this review, we summarize the tissue closure processes in Drosophila that give rise to or maintain hollow structures and highlight the molecular mechanisms and distinct cell biology involved in these processes.

Pinching and pushing: fold formation in the Drosophila dorsal epidermis

Epithelial folding is a fundamental morphogenetic process that shapes planar epithelial sheets into complex three-dimensional structures. Multiple mechanisms can generate epithelial folds, including apical constriction, which acts locally at the cellular level, differential growth on the tissue scale, or buckling because of compression from neighboring tissues. Here, we investigate the formation of dorsally located epithelial folds at segment boundaries during the late stages of Drosophila embryogenesis. We found that the fold formation at the segment boundaries occurs through the juxtaposition of two key morphogenetic processes: local apical constriction and tissue-level compressive forces from posterior segments. Further, we found that epidermal spreading and fold formation are accompanied by spatiotemporal pulses of Hedgehog (Hh) signaling. A computational model that incorporates the local forces generated from the differential tensions of the apical, basal, and lateral sides of the cell and active forces generated within the whole tissue recapitulates the overall fold formation process in wild-type and Hh overexpression conditions. In sum, this work demonstrates how epithelial folding depends on multiple, separable physical mechanisms to generate the final morphology of the dorsal epidermis. This work illustrates the modularity of morphogenetic unit operations that occur during epithelial morphogenesis.

Septate junction proteins are required for egg elongation and border cell migration during oogenesis in Drosophila

Protein components of the invertebrate occluding junction—known as the septate junction (SJ)—are required for morphogenetic developmental events during embryogenesis in Drosophila melanogaster. In order to determine whether SJ proteins are similarly required for morphogenesis during other developmental stages, we investigated the localization and requirement of four representative SJ proteins during oogenesis: Contactin, Macroglobulin complement-related, Neurexin IV, and Coracle. A number of morphogenetic processes occur during oogenesis, including egg elongation, formation of dorsal appendages, and border cell (BC) migration. We found that all four SJ proteins are expressed in egg chambers throughout oogenesis, with the highest and the most sustained levels in the follicular epithelium (FE). In the FE, SJ proteins localize along the lateral membrane during early and mid-oogenesis, but become enriched in an apical-lateral domain (the presumptive SJ) by stage 11. SJ protein relocalization requires the expression of other SJ proteins, as well as Rab5 and Rab11 like SJ biogenesis in the embryo. Knocking down the expression of these SJ proteins in follicle cells throughout oogenesis results in egg elongation defects and abnormal dorsal appendages. Similarly, reducing the expression of SJ genes in the BC cluster results in BC migration defects. Together, these results demonstrate an essential requirement for SJ genes in morphogenesis during oogenesis, and suggest that SJ proteins may have conserved functions in epithelial morphogenesis across developmental stages.

Expanding the Junction: New Insights into Non-Occluding Roles for Septate Junction Proteins during Development

The septate junction (SJ) provides an occluding function for epithelial tissues in invertebrate organisms. This ability to seal the paracellular route between cells allows internal tissues to create unique compartments for organ function and endows the epidermis with a barrier function to restrict the passage of pathogens. Over the past twenty-five years, numerous investigators have identified more than 30 proteins that are required for the formation or maintenance of the SJs in Drosophila melanogaster, and have determined many of the steps involved in the biogenesis of the junction. Along the way, it has become clear that SJ proteins are also required for a number of developmental events that occur throughout the life of the organism. Many of these developmental events occur prior to the formation of the occluding junction, suggesting that SJ proteins possess non-occluding functions. In this review, we will describe the composition of SJs, taking note of which proteins are core components of the junction versus resident or accessory proteins, and the steps involved in the biogenesis of the junction. We will then elaborate on the functions that core SJ proteins likely play outside of their role in forming the occluding junction and describe studies that provide some cell biological perspectives that are beginning to provide mechanistic understanding of how these proteins function in developmental contexts.

Identifying Genetic Players in Cell Sheet Morphogenesis Using a Drosophila Deficiency Screen for Genes on Chromosome 2R Involved in Dorsal Closure

Cell sheet morphogenesis characterizes key developmental transitions and homeostasis, in vertebrates and throughout phylogeny, including gastrulation, neural tube formation and wound healing. Dorsal closure, a process during Drosophila embryogenesis, has emerged as a model for cell sheet morphogenesis. ∼140 genes are currently known to affect dorsal closure and new genes are identified each year. Many of these genes were identified in screens that resulted in arrested development. Dorsal closure is remarkably robust and many questions regarding the molecular mechanisms involved in this complex biological process remain. Thus, it is important to identify all genes that contribute to the kinematics and dynamics of closure. Here, we used a set of large deletions (deficiencies), which collectively remove 98.5% of the genes on the right arm of Drosophila melanogaster’s 2^nd chromosome to identify “dorsal closure deficiencies”. Through two crosses, we unambiguously identified embryos homozygous for each deficiency and time-lapse imaged them for the duration of closure. Images were analyzed for defects in cell shapes and tissue movements. Embryos homozygous for 47 deficiencies have notable, diverse defects in closure, demonstrating that a number of discrete processes comprise closure and are susceptible to mutational disruption. Further analysis of these deficiencies will lead to the identification of at least 30 novel “dorsal closure genes”. We expect that many of these novel genes will identify links to pathways and structures already known to coordinate various aspects of closure. We also expect to identify new processes and pathways that contribute to closure.

Cell Sheet Morphogenesis: Dorsal Closure in Drosophila melanogaster as a Model System

Dorsal closure is a key process during Drosophila morphogenesis that models cell sheet movements in chordates, including neural tube closure, palate formation, and wound healing. Closure occurs midway through embryogenesis and entails circumferential elongation of lateral epidermal cell sheets that close a dorsal hole filled with amnioserosa cells. Signaling pathways regulate the function of cellular structures and processes, including Actomyosin and microtubule cytoskeletons, cell-cell/cell-matrix adhesion complexes, and endocytosis/vesicle trafficking. These orchestrate complex shape changes and movements that entail interactions between five distinct cell types. Genetic and laser perturbation studies establish that closure is robust, resilient, and the consequence of redundancy that contributes to four distinct biophysical processes: contraction of the amnioserosa, contraction of supracellular Actomyosin cables, elongation (stretching?) of the lateral epidermis, and zipping together of two converging cell sheets. What triggers closure and what the emergent properties are that give rise to its extraordinary resilience and fidelity remain key, extant questions.

Two consecutive microtubule-based epithelial seaming events mediate dorsal closure in the scuttle fly Megaselia abdita

Evolution of morphogenesis is generally associated with changes in genetic regulation. Here, we report evidence indicating that dorsal closure, a conserved morphogenetic process in dipterans, evolved as the consequence of rearrangements in epithelial organization rather than signaling regulation. In Drosophila melanogaster, dorsal closure consists of a two-tissue system where the contraction of extraembryonic amnioserosa and a JNK/Dpp-dependent epidermal actomyosin cable result in microtubule-dependent seaming of the epidermis. We find that dorsal closure in Megaselia abdita, a three-tissue system comprising serosa, amnion and epidermis, differs in morphogenetic rearrangements despite conservation of JNK/Dpp signaling. In addition to an actomyosin cable, M. abdita dorsal closure is driven by the rupture and contraction of the serosa and the consecutive microtubule-dependent seaming of amnion and epidermis. Our study indicates that the evolutionary transition to a reduced system of dorsal closure involves simplification of the seaming process without changing the signaling pathways of closure progression.

α-Catenin stabilises Cadherin-Catenin complexes and modulates actomyosin dynamics to allow pulsatile apical contraction

We have investigated how cell contractility and adhesion are functionally integrated during epithelial morphogenesis. To this end, we have analysed the role of α-Catenin, a key molecule linking E-Cadherin-based adhesion and the actomyosin cytoskeleton, during Drosophila embryonic dorsal closure, by studying a newly developed allelic series. We find that α-Catenin regulates pulsatile apical contraction in the amnioserosa, the main force-generating tissue driving closure of the embryonic epidermis. α-Catenin controls actomyosin dynamics by stabilising and promoting the formation of actomyosin foci, and also stabilises DE-Cadherin (Drosophila E-Cadherin, also known as Shotgun) at the cell membrane, suggesting that medioapical actomyosin contractility regulates junction stability. Furthermore, we uncover a genetic interaction between α-Catenin and Vinculin, and a tension-dependent recruitment of Vinculin to amniosersoa apical cell membranes, suggesting the existence of a mechano-sensitive module operating in this tissue.

A novel method for quantifying the rate of embryogenesis uncovers considerable genetic variation for the duration of embryonic development in Drosophila melanogaster

BACKGROUND

Embryogenesis is a highly conserved, canalized process, and variation in the duration of embryogenesis (DOE), i.e., time from egg lay to hatching, has a potentially profound effect on the outcome of within- and between-species competition. There is both intra- and inter-specific variation in this trait, which may provide important fuel for evolutionary processes, particularly adaptation. However, while genetic variation underlying simpler morphological traits, or with large phenotypic effects is well described in the literature, less is known about the underlying genetics of traits, such as DOE, partly due to a lack of tools with which to study them.

RESULTS

Here, we establish a novel microscope-based assay to survey genetic variation for the duration of embryogenesis (DOE). First, to establish the potential importance of DOE in competitive fitness, we performed a set of experiments where we experimentally manipulated the time until hatching, and show that short hatching times result in priority effect in the form of improved larval competitive ability. We then use our assay to measure DOE for 43 strains from the Drosophila Genetic Reference Panel (DGRP). Our assay greatly simplifies the measurement of DOE, making it possible to precisely quantify this trait for 59,295 individual embryos (mean ± S.D. of 1103 ± 293 per DGRP strain, and 1002 ± 203 per control). We find extensive genetic variation in DOE, with a 15 % difference in rate between the slowest and fastest strains measured, and 89 % of phenotypic variation due to DGRP strain. Using sequence information from the DGRP, we perform a genome-wide association study, which suggests that some well-known developmental genes affect the speed of embryonic development.

CONCLUSIONS

We showed that the duration of embryogenesis (DOE) can be efficiently and precisely measured in Drosophila, and that the DGRP strains show remarkable variation in DOE. A genome-wide analysis suggests that some well-known developmental genes are potentially associated with DOE. Further functional assays, or transcriptomic analysis of embryos from the DGRP, can validate the role of our candidates in early developmental processes.

Patterned Contractile Forces Promote Epidermal Spreading and Regulate Segment Positioning during Drosophila Head Involution

Epithelial spreading is a fundamental mode of tissue rearrangement occurring during animal development and wound closure. It has been associated either with the collective migration of cells [1, 2] or with actomyosin-generated forces acting at the leading edge (LE) and pulling the epithelial tissue [3, 4]. During the process of Drosophila head involution (HI), the epidermis spreads anteriorly to envelope the head tissues and fully cover the embryo [5]. This results in epidermal segments of equal width that will give rise to the different organs of the fly [6]. Here we perform a quantitative analysis of tissue spreading during HI. Combining high-resolution live microscopy with laser microsurgery and genetic perturbations, we show that epidermal movement is in part, but not solely, driven by a contractile actomyosin cable at the LE. Additional driving forces are generated within each segment by a gradient of actomyosin-based circumferential tension. Interfering with Hedgehog (Hh) signaling can modulate this gradient, thus suggesting the involvement of polarity genes in the regulation of HI. In particular, we show that disruption of these contractile forces alters segment widths and leads to a mispositioning of segments. Within the framework of a physical description, we confirm that given the geometry of the embryo, a patterned profile of active circumferential tensions can indeed generate propelling forces and control final segment position. Our study thus unravels a mechanism by which patterned tensile forces can regulate spreading and positioning of epithelial tissues.

Pathway to a phenocopy: Heat stress effects in early embryogenesis

BACKGROUND

Heat shocks applied at the onset of gastrulation in early Drosophila embryos frequently lead to phenocopies of U-shaped mutants-having characteristic failures in the late morphogenetic processes of germband retraction and dorsal closure. The pathway from nonspecific heat stress to phenocopied abnormalities is unknown.

RESULTS

Drosophila embryos subjected to 30-min, 38 °C heat shocks at gastrulation appear to recover and restart morphogenesis. Post-heat-shock development appears normal, albeit slower, until a large fraction of embryos develop amnioserosa holes (diameters > 100 µm). These holes are positively correlated with terminal U-shaped phenocopies. They initiate between amnioserosa cells and open over tens of minutes by evading normal wound healing responses. They are not caused by tissue-wide increases in mechanical stress or decreases in cell-cell adhesion, but instead appear to initiate from isolated apoptosis of amnioserosa cells.

CONCLUSIONS

The pathway from heat shock to U-shaped phenocopies involves the opening of one or more large holes in the amnioserosa that compromise its structural integrity and lead to failures in morphogenetic processes that rely on amnioserosa-generated tensile forces. The proposed mechanism by which heat shock leads to hole initiation and expansion is heterochonicity, i.e., disruption of morphogenetic coordination between embryonic and extra-embryonic cell types.

Mummy, A UDP-N-acetylglucosamine pyrophosphorylase, modulates DPP signaling in the embryonic epidermis of Drosophila

The evolutionarily conserved JNK/AP-1 (Jun N-terminal kinase/activator protein 1) and BMP (Bone Morphogenetic Protein) signaling cascades are deployed hierarchically to regulate dorsal closure in the fruit fly Drosophila melanogaster. In this developmental context, the JNK/AP-1 signaling cascade transcriptionally activates BMP signaling in leading edge epidermal cells. Here we show that the mummy (mmy) gene product, which is required for dorsal closure, functions as a BMP signaling antagonist. Genetic and biochemical tests of Mmy's role as a BMP-antagonist indicate that its function is independent of AP-1, the transcriptional trigger of BMP signal transduction in leading edge cells. pMAD (phosphorylated Mothers Against Dpp) activity data show the mmy gene product to be a new type of epidermal BMP regulator - one which transforms a BMP ligand from a long- to a short-range signal. mmy codes for the single UDP-N-acetylglucosamine pyrophosphorylase in Drosophila, and its requirement for attenuating epidermal BMP signaling during dorsal closure points to a new role for glycosylation in defining a highly restricted BMP activity field in the fly. These findings add a new dimension to our understanding of mechanisms modulating the BMP signaling gradient.

Modulation of morphogenesis by Egfr during dorsal closure in Drosophila

During Drosophila embryogenesis the process of dorsal closure (DC) results in continuity of the embryonic epidermis, and DC is well recognized as a model system for the analysis of epithelial morphogenesis as well as wound healing. During DC the flanking lateral epidermal sheets stretch, align, and fuse along the dorsal midline, thereby sealing a hole in the epidermis occupied by an extra-embryonic tissue known as the amnioserosa (AS). Successful DC requires the regulation of cell shape change via actomyosin contractility in both the epidermis and the AS, and this involves bidirectional communication between these two tissues. We previously demonstrated that transcriptional regulation of myosin from the zipper (zip) locus in both the epidermis and the AS involves the expression of Ack family tyrosine kinases in the AS in conjunction with Dpp secreted from the epidermis. A major function of Ack in other species, however, involves the negative regulation of Egfr. We have, therefore, asked what role Egfr might play in the regulation of DC. Our studies demonstrate that Egfr is required to negatively regulate epidermal expression of dpp during DC. Interestingly, we also find that Egfr signaling in the AS is required to repress zip expression in both the AS and the epidermis, and this may be generally restrictive to the progression of morphogenesis in these tissues. Consistent with this theme of restricting morphogenesis, it has previously been shown that programmed cell death of the AS is essential for proper DC, and we show that Egfr signaling also functions to inhibit or delay AS programmed cell death. Finally, we present evidence that Ack regulates zip expression by promoting the endocytosis of Egfr in the AS. We propose that the general role of Egfr signaling during DC is that of a braking mechanism on the overall progression of DC.

Interactions between the amnioserosa and the epidermis revealed by the function of the u-shaped gene

Dorsal closure (DC) is an essential step during Drosophila development whereby a hole is sealed in the dorsal epidermis and serves as a model for cell sheet morphogenesis and wound healing. It involves the orchestrated interplay of transcriptional networks and dynamic regulation of cell machinery to bring about shape changes, mechanical forces, and emergent properties. Here we provide insight into the regulation of dorsal closure by describing novel autonomous and non-autonomous roles for U-shaped (Ush) in the amnioserosa, the epidermis, and in mediation of communication between the tissues. We identified Ush by gene expression microarray analysis of Dpp signaling targets and show that Ush mediates some DC functions of Dpp. By selectively restoring Ush function in either the AS or the epidermis in ush mutants, we show that the AS makes a greater (Ush-dependent) contribution to closure than the epidermis. A signal from the AS induces epidermal cell elongation and JNK activation in the DME, while cable formation requires Ush on both sides of the leading edge, i.e. in both the AS and epidermis. Our study demonstrates that the amnioserosa and epidermis communicate at several steps during the process: sometimes the epidermis instructs the amnioserosa, other times the AS instructs the epidermis, and still other times they appear to collaborate.

Developmental expression of Drosophila Wiskott-Aldrich Syndrome family proteins

BACKGROUND

Wiskott-Aldrich Syndrome (WASP) family proteins participate in many cellular processes involving rearrangements of the actin cytoskeleton. To the date, four WASP subfamily members have been described in Drosophila: Wash, WASp, SCAR, and Whamy. Wash, WASp, and SCAR are essential during early Drosophila development where they function in orchestrating cytoplasmic events including membrane-cytoskeleton interactions. A mutant for Whamy has not yet been reported.

RESULTS

We generated monoclonal antibodies that are specific to Drosophila Wash, WASp, SCAR, and Whamy, and use these to describe their spatial and temporal localization patterns. Consistent with the importance of WASP family proteins in flies, we find that Wash, WASp, SCAR, and Whamy are dynamically expressed throughout oogenesis and embryogenesis. For example, we find that Wash accumulates at the oocyte cortex. WASp is highly expressed in the PNS, while SCAR is the most abundantly expressed in the CNS. Whamy exhibits an asymmetric subcellular localization that overlaps with mitochondria and is highly expressed in muscle.

CONCLUSIONS

All four WASP family members show specific expression patterns, some of which reflect their previously known roles and others revealing new potential functions. The monoclonal antibodies developed offer valuable new tools to investigate how WASP family proteins regulate actin cytoskeleton dynamics.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

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.

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.

Formation of the insect head involves lateral contribution of the intercalary segment, which depends on Tc-labial function

The insect head is composed of several segments. During embryonic development, the segments fuse to form a rigid head capsule where obvious segmental boundaries are lacking. Hence, the assignment of regions of the insect head to specific segments is hampered, especially with respect to dorsal (vertex) and lateral (gena) parts. We show that upon Tribolium labial (Tc-lab) knock down, the intercalary segment is deleted but not transformed. Furthermore, we find that the intercalary segment contributes to lateral parts of the head cuticle in Tribolium. Based on several additional mutant and RNAi phenotypes that interfere with gnathal segment development, we show that these segments do not contribute to the dorsal head capsule apart from the dorsal ridge. Opposing the classical view but in line with findings in the vinegar fly Drosophila melanogaster and the milkweed bug Oncopeltus fasciatus, we propose a "bend and zipper" model for insect head capsule formation.

Genes encoding novel secreted and transmembrane proteins are temporally and spatially regulated during Drosophila melanogaster embryogenesis

BACKGROUND

Morphogenetic events that shape the Drosophila melanogaster embryo are tightly controlled by a genetic program in which specific sets of genes are up-regulated. We used a suppressive subtractive hybridization procedure to identify a group of developmentally regulated genes during early stages of D. melanogaster embryogenesis. We studied the spatiotemporal activity of these genes in five different intervals covering 12 stages of embryogenesis.

RESULTS

Microarrays were constructed to confirm induction of expression and to determine the temporal profile of isolated subtracted cDNAs during embryo development. We identified a set of 118 genes whose expression levels increased significantly in at least one developmental interval compared with a reference interval. Of these genes, 53% had a phenotype and/or molecular function reported in the literature, whereas 47% were essentially uncharacterized. Clustering analysis revealed demarcated transcript groups with maximum gene activity at distinct developmental intervals. In situ hybridization assays were carried out on 23 uncharacterized genes, 15 of which proved to have spatiotemporally restricted expression patterns. Among these 15 uncharacterized genes, 13 were found to encode putative secreted and transmembrane proteins. For three of them we validated our protein sequence predictions by expressing their cDNAs in Drosophila S2R+ cells and analyzed the subcellular distribution of recombinant proteins. We then focused on the functional characterization of the gene CG6234. Inhibition of CG6234 by RNA interference resulted in morphological defects in embryos, suggesting the involvement of this gene in germ band retraction.

CONCLUSION

Our data have yielded a list of developmentally regulated D. melanogaster genes and their expression profiles during embryogenesis and provide new information on the spatiotemporal expression patterns of several uncharacterized genes. In particular, we recovered a substantial number of unknown genes encoding putative secreted and transmembrane proteins, suggesting new components of signaling pathways that might be incorporated within the existing regulatory networks controlling D. melanogaster embryogenesis. These genes are also good candidates for additional targeted functional analyses similar to those we conducted for CG6234.See related minireview by Vichas and Zallen: http://www.jbiol.com/content/8/8/76.

JNK signalling influences intracellular trafficking during Drosophila morphogenesis through regulation of the novel target gene Rab30

JNK-mediated closure of the Drosophila dorsal epidermis during embryogenesis is a well-characterised model for morphogenesis. However, little is known about how JNK signalling modifies particular cellular behaviours such as intracellular transport. Here we demonstrate that the gene encoding the small GTPase Rab30 is a new JNK transcriptional target whose function is required during embryonic and adult morphogenesis including JNK-dependent dorsal closure, embryonic head involution and thorax closure. Using immuno-fluorescence and live imaging, we show that EGFP-Rab30 localises to trans-Golgi in addition to small unidentified vesicles, and moves in a microtubule-dependent, polarised dorso-ventral manner in the leading edge during dorsal closure. We propose that JNK activity upregulates genes involved in intracellular transport in order to provide an increased level of trafficking activity in cells undergoing complex morphogenetic arrangements such as dorsal closure.

Leading edge-secreted Dpp cooperates with ACK-dependent signaling from the amnioserosa to regulate myosin levels during dorsal closure

Dorsal closure of the Drosophila embryo is an epithelial fusion in which the epidermal flanks migrate to close a hole in the epidermis occupied by the amnioserosa, a process driven in part by myosin-dependent cell shape change. Dpp signaling is required for the morphogenesis of both tissues, where it promotes transcription of myosin from the zipper (zip) gene. Drosophila has two members of the activated Cdc42-associated kinase (ACK) family: DACK and PR2. Overexpression of DACK in embryos deficient in Dpp signaling can restore zip expression and suppress dorsal closure defects, while reducing the levels of DACK and PR2 simultaneously using mutations or amnioserosa-specific knock down by RNAi results in loss of zip expression. ACK function in the amnioserosa may generate a signal cooperating with Dpp secreted from the epidermis in driving zip expression in these two tissues, ensuring that cell shape changes in dorsal closure occur in a coordinated manner.