Investigation of leading edge formation at the interface of amnioserosa and dorsal ectoderm in theDrosophilaembryo

How two extraembryonic epithelia became one: serosa and amnion features and functions of Drosophila's amnioserosa

The conservation of gene networks that specify and differentiate distinct tissues has long been a subject of great interest to evolutionary developmental biologists, but the question of how pre-existing tissue-specific developmental trajectories merge is rarely asked. During the radiation of flies, two extraembryonic epithelia, known as serosa and amnion, evolved into one, called amnioserosa. This unique extraembryonic epithelium is found in fly species of the group Schizophora, including the genetic model organism Drosophila melanogaster, and has been studied in depth. Close relatives of this group develop a serosa and a rudimentary amnion. The scuttle fly Megaselia abdita has emerged as an excellent model organism to study this extraembryonic tissue organization. In this review, development and functions of the extraembryonic tissue complements of Drosophila and Megaselia are compared. It is concluded that the amnioserosa combines cells, genetic pathway components and functions that were previously associated either with serosa development or amnion development. The composite developmental trajectory of the amnioserosa raises the question of whether merging tissue-specific gene networks is a common evolutionary process. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Studying Nonproliferative Roles for Egfr Signaling in Tissue Morphogenesis Using Dorsal Closure of the Drosophila Embryo

For several decades, genetic analysis in Drosophila has made important contributions to the understanding of signaling by Egfr. Egfr has been well characterized with regard to its oncogenic potential but is also being studied for its roles in organismal development. We have recently developed dorsal closure of the Drosophila embryo as a system for characterizing Egfr regulation of events that do not involve proliferation, as no cell divisions occur during this process. Dorsal closure is essentially a developmental wound healing event with parallels to vertebrate developmental epithelial fusions such as neural tube closure and palate fusion. We describe here a set of materials and protocols for studying Egfr signaling during dorsal closure, including assessing effects of altering Egfr signaling on other pathways, gene expression and, using live imaging, morphogenesis and programmed cell death. Although this "tool kit" is designed for looking at Egfr, it can be readily adapted to look at the participation of any signaling molecule in dorsal closure.

Amnioserosa development and function in Drosophila embryogenesis: Critical mechanical roles for an extraembryonic tissue

Despite being a short-lived, extraembryonic tissue, the amnioserosa plays critical roles in the major morphogenetic events of Drosophila embryogenesis. These roles involve both cellular mechanics and biochemical signaling. Its best-known role is in dorsal closure-well studied by both developmental biologists and biophysicists-but the amnioserosa is also important during earlier developmental stages. Here, we provide an overview of amnioserosa specification and its role in several key developmental stages: germ band extension, germ band retraction, and dorsal closure. We also compare embryonic development in Drosophila and its relative Megaselia to highlight how the amnioserosa and its roles have evolved. Placed in context, the amnioserosa provides a fascinating example of how signaling, mechanics, and morphogen patterns govern cell-type specification and subsequent morphogenetic changes in cell shape, orientation, and movement. Developmental Dynamics 245:558-568, 2016. © 2016 Wiley Periodicals, Inc.

Crumbs is an essential regulator of cytoskeletal dynamics and cell-cell adhesion during dorsal closure in Drosophila

The evolutionarily conserved Crumbs protein is required for epithelial polarity and morphogenesis. Here we identify a novel role of Crumbs as a negative regulator of actomyosin dynamics during dorsal closure in the Drosophila embryo. Embryos carrying a mutation in the FERM (protein 4.1/ezrin/radixin/moesin) domain-binding motif of Crumbs die due to an overactive actomyosin network associated with disrupted adherens junctions. This phenotype is restricted to the amnioserosa and does not affect other embryonic epithelia. This function of Crumbs requires DMoesin, the Rho1-GTPase, class-I p21-activated kinases and the Arp2/3 complex. Data presented here point to a critical role of Crumbs in regulating actomyosin dynamics, cell junctions and morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.07398.001

A layer of epithelial cells covers the body surface of animals. Epithelial cells have a property known as polarity; this means that they have two different poles, one of which is in contact with the environment. Midway through embryonic development, the Drosophila embryo is covered by two kinds of epithelial sheets; the epidermis on the front, the belly and the sides of the embryo, and the amnioserosa on the back. In the second half of embryonic development, the amnioserosa is brought into the embryo in a process called dorsal closure, while the epidermis expands around the back of the embryo to encompass it.

One of the major activities driving dorsal closure is the contraction of amnioserosa cells. This contraction depends on the highly dynamic activity of the protein network that helps give cells their shape, known as the actomyosin cytoskeleton. One major question in the field is how changes in the actomyosin cytoskeleton are controlled as tissues take shape (a process known as “morphogenesis”) and how the integrity of epithelial tissues is maintained during these processes.

A key regulator of epidermal and amnioserosa polarity is an evolutionarily conserved protein called Crumbs. The epithelial tissues of mutant embryos that do not produce Crumbs lose polarity and integrity, and the embryos fail to develop properly.

Flores-Benitez and Knust have now studied the role of Crumbs in the morphogenesis of the amnioserosa during dorsal closure. This revealed that fly embryos that produce a mutant Crumbs protein that cannot interact with a protein called Moesin (which links the cell membrane and the actomyosin cytoskeleton) are unable to complete dorsal closure. Detailed analyses showed that this failure of dorsal closure is due to the over-activity of the actomyosin cytoskeleton in the amnioserosa. This results in increased and uncoordinated contractions of the cells, and is accompanied by defects in cell-cell adhesion that ultimately cause the amnioserosa to lose integrity. Flores-Benitez and Knust’s genetic analyses further showed that several different signalling systems participate in this process.

Flores-Benitez and Knust’s results reveal an unexpected role of Crumbs in coordinating polarity, actomyosin activity and cell-cell adhesion. Further work is now needed to understand the molecular mechanisms and interactions that enable Crumbs to coordinate these processes; in particular, to unravel how Crumbs influences the periodic contractions that drive changes in cell shape. It will also be important to investigate whether Crumbs is involved in similar mechanisms that operate in other developmental events in which actomyosin oscillations have been linked to tissue morphogenesis.

DOI:http://dx.doi.org/10.7554/eLife.07398.002

Apical oscillations in amnioserosa cells: basolateral coupling and mechanical autonomy

Holographic laser microsurgery is used to isolate single amnioserosa cells in vivo during early dorsal closure. During this stage of Drosophila embryogenesis, amnioserosa cells undergo oscillations in apical surface area. The postisolation behavior of individual cells depends on their preisolation phase in these contraction/expansion cycles: cells that were contracting tend to collapse quickly after isolation; cells that were expanding do not immediately collapse, but instead pause or even continue to expand for ∼40 s. In either case, the postisolation apical collapse can be prevented by prior anesthetization of the embryos with CO2. These results suggest that although the amnioserosa is under tension, its cells are subjected to only small elastic strains. Furthermore, their postisolation apical collapse is not a passive elastic relaxation, and both the contraction and expansion phases of their oscillations are driven by intracellular forces. All of the above require significant changes to existing computational models.

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.

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.

Loss of PTB or negative regulation of Notch mRNA reveals distinct zones of Notch and actin protein accumulation in Drosophila embryo

Polypyrimidine Tract Binding (PTB) protein is a regulator of mRNA processing and translation. Genetic screens and studies of wing and bristle development during the post-embryonic stages of Drosophila suggest that it is a negative regulator of the Notch pathway. How PTB regulates the Notch pathway is unknown. Our studies of Drosophila embryogenesis indicate that (1) the Notch mRNA is a potential target of PTB, (2) PTB and Notch functions in the dorso-lateral regions of the Drosophila embryo are linked to actin regulation but not their functions in the ventral region, and (3) the actin-related Notch activity in the dorso-lateral regions might require a Notch activity at or near the cell surface that is different from the nuclear Notch activity involved in cell fate specification in the ventral region. These data raise the possibility that the Drosophila embryo is divided into zones of different PTB and Notch activities based on whether or not they are linked to actin regulation. They also provide clues to the almost forgotten role of Notch in cell adhesion and reveal a role for the Notch pathway in cell fusions.

A survey of well conserved families of C2H2 zinc-finger genes in Daphnia

Background

A recent comparative genomic analysis tentatively identified roughly 40 orthologous groups of C2H2 Zinc-finger proteins that are well conserved in "bilaterians" (i.e. worms, flies, and humans). Here we extend that analysis to include a second arthropod genome from the crustacean, Daphnia pulex.

Results

Most of the 40 orthologous groups of C2H2 zinc-finger proteins are represented by just one or two proteins within each of the previously surveyed species. Likewise, Daphnia were found to possess a similar number of orthologs for all of these small orthology groups. In contrast, the number of Sp/KLF homologs tends to be greater and to vary between species. Like the corresponding mammalian Sp/KLF proteins, most of the Drosophila and Daphnia homologs can be placed into one of three sub-groups: Class I-III. Daphnia were found to have three Class I proteins that roughly correspond to their Drosophila counterparts, dSP1, btd, CG5669, and three Class II proteins that roughly correspond to Luna, CG12029, CG9895. However, Daphnia have four additional KLF-Class II proteins that are most similar to the vertebrate KLF1/2/4 proteins, a subset not found in Drosophila. Two of these four proteins are encoded by genes linked in tandem. Daphnia also have three KLF-Class III members, one more than Drosophila. One of these is a likely Bteb2 homolog, while the other two correspond to Cabot and KLF13, a vertebrate homolog of Cabot.

Conclusion

Consistent with their likely roles as fundamental determinants of bilaterian form and function, most of the 40 groups of C2H2 zinc-finger proteins are conserved in kind and number in Daphnia. However, the KLF family includes several additional genes that are most similar to genes present in vertebrates but missing in Drosophila.

Epithelial reorganization events during late extraembryonic development in a hemimetabolous insect

As extra-embryonic tissues, the amnion and serosa are not considered to contribute materially to the insect embryo, yet they must execute an array of morphogenetic movements before they are dispensable. In hemimetabolous insects, these movements have been known for over a century, but they have remained virtually unexamined. This study addresses late extraembryonic morphogenesis in the milkweed bug, Oncopeltus fasciatus. Cell shape changes and apoptosis profiles are used to characterize the membranes as they undergo a large repertoire of final reorganizational events that reposition the embryo (katatrepsis), and eliminate the membranes themselves in an ordered fashion (dorsal closure). A number of key features were identified. First, amnion-serosa "fusion" involves localized apoptosis in the amnion and the formation of a supracellular actin purse string at the amnion-serosa border. During katatrepsis, a 'focus' of serosal cells undergoes precocious columnarization and may serve as an anchor for contraction. Lastly, dorsal closure involves novel modifications of the amnion and embryonic flank that are without counterpart during the well-known process of dorsal closure in the fruit fly Drosophila melanogaster. These data also address the long-standing question of the final fate of the amnion: it undergoes apoptosis during dorsal closure and thus is likely to be solely extraembryonic.

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.

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.

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.

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.

Gradients and thresholds: BMP response gradients unveiled in Drosophila embryos

Bone morphogenetic proteins (BMP) direct dorsal-ventral patterning in both invertebrate and vertebrate embryos, with strong evolutionary conservation of molecular components of the pathway. Dorsal-ventral patterning of the early Drosophila embryo is a powerful experimental system to probe mechanisms of BMP gradient formation and interpretation. Recent studies have found that spatial patterns of activated BMP signal transducers in Drosophila go through an unexpected transition: a shallow gradient of weak responses at mid-cellularization changes to a step gradient of stronger responses in cellularized embryos. The transition between two gradients of different shape yields new insights into the progression of Drosophila dorsal-ventral patterning and raises new issues about the mechanisms of gradient formation.

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.

Epithelial fusions in the embryo

Morphogenesis in the embryo involves the bending, folding and fusing of epithelial tissues to create the final complex shapes of the various organs and structures in the body. One essential process that occurs frequently during development is the drawing together and fusion of epithelial edges. Drosophila dorsal closure is perhaps the most genetically tractable of this type of movement, and several recent advances have revealed much about the signals regulating the dynamic actin cytoskeletal machineries that underlie the zippering-closed of this hole in the embryonic fly. It is now clear that there are intriguing parallels with more complex morphogenetic tissue movements in vertebrates.

Dynamic analysis of dorsal closure in Drosophila: from genetics to cell biology

Throughout development a series of epithelial bendings, sweepings, and fusions occur that collectively give shape to the embryo. These morphogenetic movements are driven by coordinated assembly and contraction of the actomyosin cytoskeleton in restricted populations of epithelial cells. One well-studied example of such a morphogenetic episode is dorsal closure in Drosophila embryogenesis. This process is tractable at a genetic level and has recently become the focus of live cell biology analysis because of the availability of flies expressing GFP-fusion proteins. This marriage of genetics and cell biology is very powerful and is allowing the dissection of fundamental signaling mechanisms that regulate the cytoskeletal reorganizations and contractions underlying coordinated tissue movements in the embryo.

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

Wound healing in embryos and various developmental events in metazoans require the spreading and fusion of epithelial sheets. The complex signaling pathways regulating these processes are being pieced together through genetic, cell biological, and biochemical approaches. At present, dorsal closure of the Drosophila embryo is the best-characterized example of epithelial sheet movement. Dorsal closure involves migration of the lateral epidermal flanks to close a hole in the dorsal epidermis occupied by an epithelium called the amnioserosa. Detailed genetic studies have revealed a network of interacting signaling molecules regulating this process. At the center of this network is a Jun N-terminal kinase cascade acting at the leading edge of the migrating epidermis that triggers signaling by the TGF-beta superfamily member Decapentaplegic and which interacts with the Wingless pathway. These signaling modules regulate the cytoskeletal reorganization and cell shape change necessary to drive dorsal closure. Activation of this network requires signals from the amnioserosa and input from a variety of proteins at cell-cell junctions. The Rho family of small GTPases is also instrumental, both in activation of signaling and regulation of the cytoskeleton. Many of the proteins regulating dorsal closure have been implicated in epithelial movement in other organisms, and dorsal closure has emerged as an ideal model system for the study of the migration and fusion of epithelial sheets.