DRhoGEF2 regulates actin organization and contractility in the Drosophila blastoderm embryo

Structured RhoGEF recruitment drives myosin II organization on large exocytic vesicles

The Rho family of GTPases plays a crucial role in cellular mechanics by regulating actomyosin contractility through the parallel induction of actin and myosin assembly and function. Using exocytosis of large vesicles in the Drosophila larval salivary gland as a model, we followed the spatiotemporal regulation of Rho1, which in turn creates distinct organization patterns of actin and myosin. After vesicle fusion, low levels of activated Rho1 reach the vesicle membrane and drive actin nucleation in an uneven, spread-out pattern. Subsequently, the Rho1 activator RhoGEF2 distributes as an irregular meshwork on the vesicle membrane, activating Rho1 in a corresponding punctate pattern and driving local myosin II recruitment, resulting in vesicle constriction. Vesicle membrane buckling and subsequent crumpling occur at local sites of high myosin II concentrations. These findings indicate that distinct thresholds for activated Rho1 create a biphasic mode of actomyosin assembly, inducing anisotropic membrane crumpling during exocrine secretion.

Origin and development of primary animal epithelia

Epithelia are the first organized tissues that appear during development. In many animal embryos, early divisions give rise to a polarized monolayer, the primary epithelium, rather than a random aggregate of cells. Here, we review the mechanisms by which cells organize into primary epithelia in various developmental contexts. We discuss how cells acquire polarity while undergoing early divisions. We describe cases where oriented divisions constrain cell arrangement to monolayers including organization on top of yolk surfaces. We finally discuss how epithelia emerge in embryos from animals that branched early during evolution and provide examples of epithelia-like arrangements encountered in single-celled eukaryotes. Although divergent and context-dependent mechanisms give rise to primary epithelia, here we trace the unifying principles underlying their formation.

Reshaping the Syncytial Drosophila Embryo with Cortical Actin Networks: Four Main Steps of Early Development

Drosophila development begins as a syncytium. The large size of the one-cell embryo makes it ideal for studying the structure, regulation, and effects of the cortical actin cytoskeleton. We review four main steps of early development that depend on the actin cortex. At each step, dynamic remodelling of the cortex has specific effects on nuclei within the syncytium. During axial expansion, a cortical actomyosin network assembles and disassembles with the cell cycle, generating cytoplasmic flows that evenly distribute nuclei along the ovoid cell. When nuclei move to the cell periphery, they seed Arp2/3-based actin caps which grow into an array of dome-like compartments that house the nuclei as they divide at the cell cortex. To separate germline nuclei from the soma, posterior germ plasm induces full cleavage of mono-nucleated primordial germ cells from the syncytium. Finally, zygotic gene expression triggers formation of the blastoderm epithelium via cellularization and simultaneous division of ~6000 mono-nucleated cells from a single internal yolk cell. During these steps, the cortex is regulated in space and time, gains domain and sub-domain structure, and undergoes mesoscale interactions that lay a structural foundation of animal development.

Early zygotic gene product Dunk interacts with anillin to regulate Myosin II during Drosophila cleavage

Drosophila melanogaster cellularization is a special form of cleavage that converts syncytial embryos into cellular blastoderms by partitioning the peripherally localized nuclei into individual cells. An early event in cellularization is the recruitment of nonmuscle myosin II ("myosin") to the leading edge of cleavage furrows, where myosin forms an interconnected basal array before reorganizing into individual cytokinetic rings. The initial recruitment and organization of basal myosin are regulated by a cellularization-specific gene, dunk, but the underlying mechanism is unclear. Through a genome-wide yeast two-hybrid screen, we identified anillin (Scraps in Drosophila), a conserved scaffolding protein in cytokinesis, as the primary binding partner of Dunk. Dunk colocalizes with anillin and regulates its cortical localization during the formation of cleavage furrows, while the localization of Dunk is independent of anillin. Furthermore, Dunk genetically interacts with anillin to regulate the basal myosin array during cellularization. Similar to Dunk, anillin colocalizes with myosin since the very early stage of cellularization and is required for myosin retention at the basal array, before the well-documented function of anillin in regulating cytokinetic ring assembly. Based on these results, we propose that Dunk regulates myosin recruitment and spatial organization during early cellularization by interacting with and regulating anillin.

Cellularization across eukaryotes: Conserved mechanisms and novel strategies

Many eukaryotes form multinucleated cells during their development. Some cells persist as such during their lifetime, others choose to cleave each nucleus individually using a specialized cytokinetic process known as cellularization. What is cellularization and how is it achieved across the eukaryotic tree of life? Are there common pathways among all species supporting a shared ancestry, or are there key differences, suggesting independent evolutionary paths? In this review, we discuss common strategies and key mechanistic differences in how cellularization is executed across vastly divergent eukaryotic species. We present a number of novel methods and non-model organisms that may provide important insight into the evolutionary origins of cellularization.

Membrane-actin interactions in morphogenesis: Lessons learned from Drosophila cellularization

During morphogenesis, changes in the shapes of individual cells are harnessed to mold an entire tissue. These changes in cell shapes require the coupled remodeling of the plasma membrane and underlying actin cytoskeleton. In this review, we highlight cellularization of the Drosophila embryo as a model system to uncover principles of how membrane and actin dynamics are co-regulated in space and time to drive morphogenesis.

Pseudocleavage furrows restrict plasma membrane-associated PH domain in syncytial Drosophila embryos

Syncytial cells contain multiple nuclei and have local distribution and function of cellular components despite being synthesized in a common cytoplasm. The syncytial Drosophila blastoderm embryo shows reduced spread of organelle and plasma membrane-associated proteins between adjacent nucleo-cytoplasmic domains. Anchoring to the cytoarchitecture within a nucleo-cytoplasmic domain is likely to decrease the spread of molecules; however, its role in restricting this spread has not been assessed. In order to analyze the cellular mechanisms that regulate the rate of spread of plasma membrane-associated molecules in the syncytial Drosophila embryos, we express a pleckstrin homology (PH) domain in a localized manner at the anterior of the embryo by tagging it with the bicoid mRNA localization signal. Anteriorly expressed PH domain forms an exponential gradient in the anteroposterior axis with a longer length scale compared with Bicoid. Using a combination of experiments and theoretical modeling, we find that the characteristic distribution and length scale emerge due to plasma membrane sequestration and restriction within an energid. Loss of plasma membrane remodeling to form pseudocleavage furrows shows an enhanced spread of PH domain but not Bicoid. Modeling analysis suggests that the enhanced spread of the PH domain occurs due to the increased spread of the cytoplasmic population of the PH domain in pseudocleavage furrow mutants. Our analysis of cytoarchitecture interaction in regulating plasma membrane protein distribution and constraining its spread has implications on the mechanisms of spread of various molecules, such as morphogens in syncytial cells.

Dia- and Rok-dependent enrichment of capping proteins in a cortical region

Rho signaling with its major targets the formin Dia, Rho kinase (Rok) and non-muscle myosin II (MyoII, encoded by zip in flies) control turnover, amount and contractility of actomyosin. Much less investigated has been a potential function for the distribution of F-actin plus and minus ends. In syncytial Drosophila embryos, Rho1 signaling is high between actin caps, i.e. the cortical intercap region. Capping protein binds to free plus ends of F-actin to prevent elongation of the filament. Capping protein has served as a marker to visualize the distribution of F-actin plus ends in cells and in vitro. In the present study, we probed the distribution of plus ends with capping protein in syncytial Drosophila embryos. We found that capping proteins are specifically enriched in the intercap region similar to Dia and MyoII but distinct from overall F-actin. The intercap enrichment of Capping protein was impaired in dia mutants and embryos, in which Rok and MyoII activation was inhibited. Our observations reveal that Dia and Rok-MyoII control Capping protein enrichment and support a model that Dia and Rok-MyoII control the organization of cortical actin cytoskeleton downstream of Rho1 signaling.

This article has an associated First Person interview with the first authors of the paper.

Summary: Plus ends of actin filaments are enriched at cortical regions rich in Rho signaling in syncytial Drosophila embryos depending on the actin regulator Dia and Rho kinase.

A two-tier junctional mechanism drives simultaneous tissue folding and extension

During embryo development, tissues often undergo multiple concomitant changes in shape. It is unclear which signaling pathways and cellular mechanisms are responsible for multiple simultaneous tissue shape transformations. We focus on the process of concomitant tissue folding and extension that is key during gastrulation and neurulation. We use the Drosophila embryo as model system and focus on the process of mesoderm invagination. Here, we show that the prospective mesoderm simultaneously folds and extends. We report that mesoderm cells, under the control of anterior-posterior and dorsal-ventral gene patterning synergy, establish two sets of adherens junctions at different apical-basal positions with specialized functions: while apical junctions drive apical constriction initiating tissue bending, lateral junctions concomitantly drive polarized cell intercalation, resulting in tissue convergence-extension. Thus, epithelial cells devise multiple specialized junctional sets that drive composite morphogenetic processes under the synergistic control of apparently orthogonal signaling sources.

Spatiotemporal recruitment of RhoGTPase protein GRAF inhibits actomyosin ring constriction in Drosophila cellularization

Actomyosin contractility is regulated by Rho-GTP in cell migration, cytokinesis and morphogenesis in embryo development. Whereas Rho activation by Rho-GTP exchange factor (GEF), RhoGEF2, is well known in actomyosin contractility during cytokinesis at the base of invaginating membranes in Drosophila cellularization, Rho inhibition by RhoGTPase-activating proteins (GAPs) remains to be studied. We have found that the RhoGAP, GRAF, inhibits actomyosin contractility during cellularization. GRAF is enriched at the cleavage furrow tip during actomyosin assembly and initiation of ring constriction. Graf depletion shows increased Rho-GTP, increased Myosin II and ring hyper constriction dependent upon the loss of the RhoGTPase domain. GRAF and RhoGEF2 are present in a balance for appropriate activation of actomyosin ring constriction. RhoGEF2 depletion and abrogation of Myosin II activation in Rho kinase mutants suppress the Graf hyper constriction defect. Therefore, GRAF recruitment restricts Rho-GTP levels in a spatiotemporal manner for inhibiting actomyosin contractility during cellularization.

Rho1 activation recapitulates early gastrulation events in the ventral, but not dorsal, epithelium of Drosophila embryos

Ventral furrow formation, the first step in Drosophila gastrulation, is a well-studied example of tissue morphogenesis. Rho1 is highly active in a subset of ventral cells and is required for this morphogenetic event. However, it is unclear whether spatially patterned Rho1 activity alone is sufficient to recapitulate all aspects of this morphogenetic event, including anisotropic apical constriction and coordinated cell movements. Here, using an optogenetic probe that rapidly and robustly activates Rho1 in Drosophila tissues, we show that Rho1 activity induces ectopic deformations in the dorsal and ventral epithelia of Drosophila embryos. These perturbations reveal substantial differences in how ventral and dorsal cells, both within and outside the zone of Rho1 activation, respond to spatially and temporally identical patterns of Rho1 activation. Our results demonstrate that an asymmetric zone of Rho1 activity is not sufficient to recapitulate ventral furrow formation and reveal that additional, ventral-specific factors contribute to the cell- and tissue-level behaviors that emerge during ventral furrow formation.

A Genetic Analysis of Tumor Progression in Drosophila Identifies the Cohesin Complex as a Suppressor of Individual and Collective Cell Invasion

Metastasis is the leading cause of death for patients with cancer. Consequently it is imperative that we improve our understanding of the molecular mechanisms that underlie progression of tumor growth toward malignancy. Advances in genome characterization technologies have been very successful in identifying commonly mutated or misregulated genes in a variety of human cancers. However, the difficulty in evaluating whether these candidates drive tumor progression remains a major challenge. Using the genetic amenability of Drosophila melanogaster we generated tumors with specific genotypes in the living animal and carried out a detailed systematic loss-of-function analysis to identify conserved genes that enhance or suppress epithelial tumor progression. This enabled the discovery of functional cooperative regulators of invasion and the establishment of a network of conserved invasion suppressors. This includes constituents of the cohesin complex, whose loss of function either promotes individual or collective cell invasion, depending on the severity of effect on cohesin complex function.

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Screen identifies genes that affect tumor behavior in a wide variety of ways

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A functionally validated network of invasion-suppressor genes was generated

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Loss of cohesin complex function can promote individual or collective cell invasion

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The fly pupal notum is an excellent in vivo system to study tumor progression

DE-cadherin and Myosin II balance regulates furrow length for onset of polygon shape in syncytial Drosophila embryos

Cell shape morphogenesis, from spherical to polygonal, occurs in epithelial cell formation in metazoan embryogenesis. In syncytial Drosophila embryos, the plasma membrane incompletely surrounds each nucleus and is organized as a polygonal epithelial-like array. Each cortical syncytial division cycle shows a circular to polygonal plasma membrane transition along with furrow extension between adjacent nuclei from interphase to metaphase. In this study, we assess the relative contribution of DE-cadherin (also known as Shotgun) and Myosin II (comprising Zipper and Spaghetti squash in flies) at the furrow to polygonal shape transition. We show that polygonality initiates during each cortical syncytial division cycle when the furrow extends from 4.75 to 5.75 μm. Polygon plasma membrane organization correlates with increased junctional tension, increased DE-cadherin and decreased Myosin II mobility. DE-cadherin regulates furrow length and polygonality. Decreased Myosin II activity allows for polygonality to occur at a lower length than controls. Increased Myosin II activity leads to loss of lateral furrow formation and complete disruption of the polygonal shape transition. Our studies show that DE-cadherin-Myosin II balance regulates an optimal lateral membrane length during each syncytial cycle for polygonal shape transition.This article has an associated First Person interview with the first author of the paper.

Cofilin-Mediated Actin Stress Response Is Maladaptive in Heat-Stressed Embryos

Environmental stress threatens the fidelity of embryonic morphogenesis. Heat, for example, is a teratogen. Yet how heat affects morphogenesis is poorly understood. Here, we identify a heat-inducible actin stress response (ASR) in Drosophila embryos that is mediated by the activation of the actin regulator Cofilin. Similar to ASR in adult mammalian cells, heat stress in fly embryos triggers the assembly of intra-nuclear actin rods. Rods measure up to a few microns in length, and their assembly depends on elevated free nuclear actin concentration and Cofilin. Outside the nucleus, heat stress causes Cofilin-dependent destabilization of filamentous actin (F-actin) in actomyosin networks required for morphogenesis. F-actin destabilization increases the chance of morphogenesis mistakes. Blocking the ASR by reducing Cofilin dosage improves the viability of heat-stressed embryos. However, improved viability correlates with restoring F-actin stability, not rescuing morphogenesis. Thus, ASR endangers embryos, perhaps by shifting actin from cytoplasmic filaments to an elevated nuclear pool.

Figard et al. show that heat stress induces an actin stress response (ASR) in early Drosophila embryos. This ASR is mediated by a heat-induced increase in Cofilin activity. Increased Cofilin activity destabilizes F-actin structures required for morphogenesis. In addition, the Cofilin-mediated ASR reduces embryo viability.

Drosophila pericentrin-like protein promotes the formation of primordial germ cells

Primordial germ cells (PGCs) are the precursors to the adult germline stem cells that are set aside early during embryogenesis and specified through the inheritance of the germ plasm, which contains the mRNAs and proteins that function as the germline fate determinants. In Drosophila melanogaster, formation of the PGCs requires the microtubule and actin cytoskeletal networks to actively segregate the germ plasm from the soma and physically construct the pole buds (PBs) that protrude from the posterior cortex. Of emerging importance is the central role of centrosomes in the coordination of microtubule dynamics and actin organization to promote PGC development. We previously identified a requirement for the centrosome protein Centrosomin (Cnn) in PGC formation. Cnn interacts directly with Pericentrin-like protein (PLP) to form a centrosome scaffold structure required for pericentriolar material recruitment and organization. In this study, we identify a role for PLP at several discrete steps during PGC development. We find PLP functions in segregating the germ plasm from the soma by regulating microtubule organization and centrosome separation. These activities further contribute to promoting PB protrusion and facilitating the distribution of germ plasm in proliferating PGCs.

Apical polarity proteins recruit the RhoGEF Cysts to promote junctional myosin assembly

Silver et al. show that the RhoGEF Cysts links apical polarity proteins to Rho1 and myosin activation at adherens junctions to support junctional and epithelial integrity in the Drosophila ectoderm.

The spatio-temporal regulation of small Rho GTPases is crucial for the dynamic stability of epithelial tissues. However, how RhoGTPase activity is controlled during development remains largely unknown. To explore the regulation of Rho GTPases in vivo, we analyzed the Rho GTPase guanine nucleotide exchange factor (RhoGEF) Cysts, the Drosophila orthologue of mammalian p114RhoGEF, GEF-H1, p190RhoGEF, and AKAP-13. Loss of Cysts causes a phenotype that closely resembles the mutant phenotype of the apical polarity regulator Crumbs. This phenotype can be suppressed by the loss of basolateral polarity proteins, suggesting that Cysts is an integral component of the apical polarity protein network. We demonstrate that Cysts is recruited to the apico-lateral membrane through interactions with the Crumbs complex and Bazooka/Par3. Cysts activates Rho1 at adherens junctions and stabilizes junctional myosin. Junctional myosin depletion is similar in Cysts- and Crumbs-compromised embryos. Together, our findings indicate that Cysts is a downstream effector of the Crumbs complex and links apical polarity proteins to Rho1 and myosin activation at adherens junctions, supporting junctional integrity and epithelial polarity.

The Crk adapter protein is essential for Drosophila embryogenesis, where it regulates multiple actin-dependent morphogenic events

Small Src homology domain 2 (SH2) and 3 (SH3) adapter proteins regulate cell fate and behavior by mediating interactions between cell surface receptors and downstream signaling effectors in many signal transduction pathways. The CT10 regulator of kinase (Crk) family has tissue-specific roles in phagocytosis, cell migration, and neuronal development and mediates oncogenic signaling in pathways like that of Abelson kinase. However, redundancy among the two mammalian family members and the position of the Drosophila gene on the fourth chromosome precluded assessment of Crk's full role in embryogenesis. We circumvented these limitations with short hairpin RNA and CRISPR technology to assess Crk's function in Drosophila morphogenesis. We found that Crk is essential beginning in the first few hours of development, where it ensures accurate mitosis by regulating orchestrated dynamics of the actin cytoskeleton to keep mitotic spindles in syncytial embryos from colliding. In this role, it positively regulates cortical localization of the actin-related protein 2/3 complex (Arp2/3), its regulator suppressor of cAMP receptor (SCAR), and filamentous actin to actin caps and pseudocleavage furrows. Crk loss leads to the loss of nuclei and formation of multinucleate cells. We also found roles for Crk in embryonic wound healing and in axon patterning in the nervous system, where it localizes to the axons and midline glia. Thus, Crk regulates diverse events in embryogenesis that require orchestrated cytoskeletal dynamics.

Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels

Src64 is required for actomyosin contraction during cellularization of the Drosophila embryonic blastoderm. The mechanism of actomyosin ring constriction is poorly understood even though a number of cytoskeletal regulators have been implicated in the assembly, organization, and contraction of these microfilament rings. How these cytoskeletal processes are regulated during development is even less well understood. To investigate the role of Src64 as an upstream regulator of actomyosin contraction, we conducted a proteomics screen to identify proteins whose expression levels are controlled by src64. Global levels of actin are reduced in src64 mutant embryos. Furthermore, we show that reduction of the actin isoform Actin 5C causes defects in actomyosin contraction during cellularization similar to those caused by src64 mutation, indicating that a relatively high level of Actin 5C is required for normal actomyosin contraction and furrow canal structure. However, reduction of Actin 5C levels only slows down actomyosin ring constriction rather than preventing it, suggesting that src64 acts not only to modulate actin levels, but also to regulate the actomyosin cytoskeleton by other means.

Expressional Profiling of Carpet Glia in the Developing Drosophila Eye Reveals Its Molecular Signature of Morphology Regulators

Homeostasis in the nervous system requires intricate regulation and is largely accomplished by the blood-brain barrier (BBB). The major gate keeper of the vertebrate BBB is vascular endothelial cells, which form tight junctions (TJs). To gain insight into the development of the BBB, we studied the carpet glia, a subperineurial glial cell type with vertebrate TJ-equivalent septate junctions, in the developing Drosophila eye. The large and flat, sheet-like carpet glia, which extends along the developing eye following neuronal differentiation, serves as an easily accessible experimental system to understand the cell types that exhibit barrier function. We profiled transcribed genes in the carpet glia using targeted DNA adenine methyl-transferase identification, followed by next-generation sequencing (targeted DamID-seq) and found that the majority of genes expressed in the carpet glia function in cellular activities were related to its dynamic morphological changes in the developing eye. To unravel the morphology regulators, we silenced genes selected from the carpet glia transcriptome using RNA interference. The Rho1 gene encoding a GTPase was previously reported as a key regulator of the actin cytoskeleton. The expression of the pathetic (path) gene, encoding a solute carrier transporter in the developing eye, is specific to the carpet glia. The reduced expression of Rho1 severely disrupted the formation of intact carpet glia, and the silencing path impaired the connection between the two carpet glial cells, indicating the pan-cellular and local effects of Rho1 and Path on carpet glial cell morphology, respectively. Our study molecularly characterized a particular subperineurial cell type providing a resource for a further understanding of the cell types comprising the BBB.

Coordination of Embryogenesis by the Centrosome in Drosophila melanogaster

The first 3 h of Drosophila melanogaster embryo development are exemplified by rapid nuclear divisions within a large syncytium, transforming the zygote to the cellular blastoderm after 13 successive cleavage divisions. As the syncytial embryo develops, it relies on centrosomes and cytoskeletal dynamics to transport nuclei, maintain uniform nuclear distribution throughout cleavage cycles, ensure generation of germ cells, and coordinate cellularization. For the sake of this review, we classify six early embryo stages that rely on processes coordinated by the centrosome and its regulation of the cytoskeleton. The first stage features migration of one of the female pronuclei toward the male pronucleus following maturation of the first embryonic centrosomes. Two subsequent stages distribute the nuclei first axially and then radially in the embryo. The remaining three stages involve centrosome-actin dynamics that control cortical plasma membrane morphogenesis. In this review, we highlight the dynamics of the centrosome and its role in controlling the six stages that culminate in the cellularization of the blastoderm embryo.

The RhoGEF protein Plekhg5 regulates apical constriction of bottle cells during gastrulation

Apical constriction regulates epithelial morphogenesis during embryonic development, but how this process is controlled is not understood completely. Here, we identify a Rho guanine nucleotide exchange factor (GEF) gene plekhg5 as an essential regulator of apical constriction of bottle cells during Xenopus gastrulation. plekhg5 is expressed in the blastopore lip and its expression is sufficient to induce ectopic bottle cells in epithelia of different germ layers in a Rho-dependent manner. This activity is not shared by arhgef3, which encodes another organizer-specific RhoGEF. Plekhg5 protein is localized in the apical cell cortex via its pleckstrin homology domain, and the GEF activity enhances its apical recruitment. Plekhg5 induces apical actomyosin accumulation and cell elongation. Knockdown of plekhg5 inhibits activin-induced bottle cell formation and endogenous blastopore lip formation in gastrulating frog embryos. Apical accumulation of actomyosin, apical constriction and bottle cell formation fail to occur in these embryos. Taken together, our data indicate that transcriptional regulation of plekhg5 expression at the blastopore lip determines bottle cell morphology via local polarized activation of Rho by Plekhg5, which stimulates apical actomyosin activity to induce apical constriction.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

The flipflop orphan genes are required for limb bud eversion in the Tribolium embryo

Background

Unlike Drosophila but similar to other arthropod and vertebrate embryos, the flour beetle Tribolium castaneum develops everted limb buds during embryogenesis. However, the molecular processes directing the evagination of epithelia are only poorly understood.

Results

Here we show that the newly discovered genes Tc-flipflop1 and Tc-flipflop2 are involved in regulating the directional budding of appendages. RNAi-knockdown of Tc-flipflop results in a variety of phenotypic traits. Most prominently, embryonic limb buds frequently grow inwards rather than out, leading to the development of inverted appendages inside the larval body. Moreover, affected embryos display dorsal closure defects. The Tc-flipflop genes are evolutionarily non-conserved, and their molecular function is not evident. We further found that Tc-RhoGEF2, a highly-conserved gene known to be involved in actomyosin-dependent cell movement and cell shape changes, shows a Tc-flipflop-like RNAi-phenotype.

Conclusions

The similarity of the inverted appendage phenotype in both the flipflop- and the RhoGEF2 RNAi gene knockdown led us to conclude that the Tc-flipflop orphan genes act in a Rho-dependent pathway that is essential for the early morphogenesis of polarised epithelial movements. Our work describes one of the few examples of an orphan gene playing a crucial role in an important developmental process.

Electronic supplementary material

The online version of this article (10.1186/s12983-017-0234-9) contains supplementary material, which is available to authorized users.

Analysis of mitochondrial organization and function in the Drosophila blastoderm embryo

Mitochondria are inherited maternally as globular and immature organelles in metazoan embryos. We have used the Drosophila blastoderm embryo to characterize their morphology, distribution and functions in embryogenesis. We find that mitochondria are relatively small, dispersed and distinctly distributed along the apico-basal axis in proximity to microtubules by motor protein transport. Live imaging, photobleaching and photoactivation analyses of mitochondrially targeted GFP show that they are mobile in the apico-basal axis along microtubules and are immobile in the lateral plane thereby associating with one syncytial cell. Photoactivated mitochondria distribute equally to daughter cells across the division cycles. ATP depletion by pharmacological and genetic inhibition of the mitochondrial electron transport chain (ETC) activates AMPK and decreases syncytial metaphase furrow extension. In summary, we show that small and dispersed mitochondria of the Drosophila blastoderm embryo localize by microtubule transport and provide ATP locally for the fast syncytial division cycles. Our study opens the possibility of use of Drosophila embryogenesis as a model system to study the impact of maternal mutations in mitochondrial morphology and metabolism on embryo patterning and differentiation.

Epithelial Tumors Originate in Tumor Hotspots, a Tissue-Intrinsic Microenvironment

Malignant tumors are caused by uncontrolled proliferation of transformed mutant cells that have lost the ability to maintain tissue integrity. Although a number of causative genetic backgrounds for tumor development have been discovered, the initial steps mutant cells take to escape tissue integrity and trigger tumorigenesis remain elusive. Here, we show through analysis of conserved neoplastic tumor-suppressor genes (nTSGs) in Drosophila wing imaginal disc epithelia that tumor initiation depends on tissue-intrinsic local cytoarchitectures, causing tumors to consistently originate in a specific region of the tissue. In this “tumor hotspot” where cells constitute a network of robust structures on their basal side, nTSG-deficient cells delaminate from the apical side of the epithelium and begin tumorigenic overgrowth by exploiting endogenous Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling activity. Conversely, in other regions, the “tumor coldspot” nTSG-deficient cells are extruded toward the basal side and undergo apoptosis. When the direction of delamination is reversed through suppression of RhoGEF2, an activator of the Rho family small GTPases, and JAK/STAT is activated ectopically in these coldspot nTSG-deficient cells, tumorigenesis is induced. These data indicate that two independent processes, apical delamination and JAK/STAT activation, are concurrently required for the initiation of nTSG-deficient-induced tumorigenesis. Given the conservation of the epithelial cytoarchitecture, tumorigenesis may be generally initiated from tumor hotspots by a similar mechanism.

A genetic study in Drosophila reveals a mechanism of tumorigenesis by which pro-tumor cells initiate dysplastic tumor growth at specific "hotspot" locations in epithelial tissues.

Transformed mutant cells (pro-tumor cells) can evolve through a multistep process in which they become tumorigenic and invasive. Many genes that are involved in the different steps towards cancer development have been identified; however, how certain mutant cells destroy normal tissue organization and undergo uncontrolled proliferation during the initial stages of this process remains largely unclear. Using the epithelial tissue of the wing imaginal discs of the fruit fly (Drosophila melanogaster) larvae as a model system, we have analyzed these initial stages of inducing tumors by depletion of a neoplastic tumor suppressor gene (nTSG). We discovered that these tumors always originate from specific regions of the epithelial tissue of the wing disc. We show that in other regions that we dubbed “tumor coldspots” and that lack specific cellular structures, pro-tumor cells are eliminated from the epithelial tissue by the surrounding cells. However, in “tumor hotspots,” cells constitute specific structures in their basal side, and we found that pro-tumor cells successfully avoid potential elimination and deviate from the apical side of the tissue, initiating tumorous overgrowth. Our findings reveal the molecular and cellular mechanisms underlying the initial steps of tumorigenesis at tumor hotspots in the Drosophila imaginal wing discs.

Flow-dependent myosin recruitment during Drosophila cellularization requires zygotic dunk activity

Actomyosin contractility underlies force generation in morphogenesis ranging from cytokinesis to epithelial extension or invagination. In Drosophila, the cleavage of the syncytial blastoderm is initiated by an actomyosin network at the base of membrane furrows that invaginate from the surface of the embryo. It remains unclear how this network forms and how it affects tissue mechanics. Here, we show that during Drosophila cleavage, myosin recruitment to the cleavage furrows proceeds in temporally distinct phases of tension-driven cortical flow and direct recruitment, regulated by different zygotic genes. We identify the gene dunk, which we show is transiently transcribed when cellularization starts and functions to maintain cortical myosin during the flow phase. The subsequent direct myosin recruitment, however, is Dunk-independent but requires Slam. The Slam-dependent direct recruitment of myosin is sufficient to drive cleavage in the dunk mutant, and the subsequent development of the mutant is normal. In the dunk mutant, cortical myosin loss triggers misdirected flow and disrupts the hexagonal packing of the ingressing furrows. Computer simulation coupled with laser ablation suggests that Dunk-dependent maintenance of cortical myosin enables mechanical tension build-up, thereby providing a mechanism to guide myosin flow and define the hexagonal symmetry of the furrows.

Summary: During Drosophila cellularisation, myosin recruitment to the cleavage furrows proceeds in temporally and mechanistically distinct phases separately regulated by dunk and slam.

Syndapin promotes pseudocleavage furrow formation by actin organization in the syncytial Drosophila embryo

F-BAR domain–containing proteins link the actin cytoskeleton to the membrane during membrane remodeling. Syndapin associates with the pseudocleavage furrow membrane and is essential for furrow morphology, actin organization, and extension downstream of initiation factor RhoGEF2.

Coordinated membrane and cytoskeletal remodeling activities are required for membrane extension in processes such as cytokinesis and syncytial nuclear division cycles in Drosophila. Pseudocleavage furrow membranes in the syncytial Drosophila blastoderm embryo show rapid extension and retraction regulated by actin-remodeling proteins. The F-BAR domain protein Syndapin (Synd) is involved in membrane tubulation, endocytosis, and, uniquely, in F-actin stability. Here we report a role for Synd in actin-regulated pseudocleavage furrow formation. Synd localized to these furrows, and its loss resulted in short, disorganized furrows. Synd presence was important for the recruitment of the septin Peanut and distribution of Diaphanous and F-actin at furrows. Synd and Peanut were both absent in furrow-initiation mutants of RhoGEF2 and Diaphanous and in furrow-progression mutants of Anillin. Synd overexpression in rhogef2 mutants reversed its furrow-extension phenotypes, Peanut and Diaphanous recruitment, and F-actin organization. We conclude that Synd plays an important role in pseudocleavage furrow extension, and this role is also likely to be crucial in cleavage furrow formation during cell division.

Drak Is Required for Actomyosin Organization During Drosophila Cellularization

The generation of force by actomyosin contraction is critical for a variety of cellular and developmental processes. Nonmuscle myosin II is the motor that drives actomyosin contraction, and its activity is largely regulated by phosphorylation of the myosin regulatory light chain. During the formation of the Drosophila cellular blastoderm, actomyosin contraction drives constriction of microfilament rings, modified cytokinesis rings. Here, we find that Drak is necessary for most of the phosphorylation of the myosin regulatory light chain during cellularization. We show that Drak is required for organization of myosin II within the microfilament rings. Proper actomyosin contraction of the microfilament rings during cellularization also requires Drak activity. Constitutive activation of myosin regulatory light chain bypasses the requirement for Drak, suggesting that actomyosin organization and contraction are mediated through Drak's regulation of myosin activity. Drak is also involved in the maintenance of furrow canal structure and lateral plasma membrane integrity during cellularization. Together, our observations suggest that Drak is the primary regulator of actomyosin dynamics during cellularization.

The Rho Guanine Nucleotide Exchange Factor DRhoGEF2 Is a Genetic Modifier of the PI3K Pathway in Drosophila

The insulin/IGF-1 signaling pathway mediates various physiological processes associated with human health. Components of this pathway are highly conserved throughout eukaryotic evolution. In Drosophila, the PTEN ortholog and its mammalian counterpart downregulate insulin/IGF signaling by antagonizing the PI3-kinase function. From a dominant loss-of-function genetic screen, we discovered that mutations of a Dbl-family member, the guanine nucleotide exchange factor DRhoGEF2 (DRhoGEF2^2(l)04291), suppressed the PTEN-overexpression eye phenotype. dAkt/dPKB phosphorylation, a measure of PI3K signaling pathway activation, increased in the eye discs from the heterozygous DRhoGEF2 wandering third instar larvae. Overexpression of DRhoGEF2, and it’s functional mammalian ortholog PDZ-RhoGEF (ArhGEF11), at various stages of eye development, resulted in both dPKB/Akt-dependent and -independent phenotypes, reflecting the complexity in the crosstalk between PI3K and Rho signaling in Drosophila.

Spindle Assembly and Chromosome Segregation Requires Central Spindle Proteins in Drosophila Oocytes

Oocytes segregate chromosomes in the absence of centrosomes. In this situation, the chromosomes direct spindle assembly. It is still unclear in this system which factors are required for homologous chromosome bi-orientation and spindle assembly. The Drosophila kinesin-6 protein Subito, although nonessential for mitotic spindle assembly, is required to organize a bipolar meiotic spindle and chromosome bi-orientation in oocytes. Along with the chromosomal passenger complex (CPC), Subito is an important part of the metaphase I central spindle. In this study we have conducted genetic screens to identify genes that interact with subito or the CPC component Incenp. In addition, the meiotic mutant phenotype for some of the genes identified in these screens were characterized. We show, in part through the use of a heat-shock-inducible system, that the Centralspindlin component RacGAP50C and downstream regulators of cytokinesis Rho1, Sticky, and RhoGEF2 are required for homologous chromosome bi-orientation in metaphase I oocytes. This suggests a novel function for proteins normally involved in mitotic cell division in the regulation of microtubule-chromosome interactions. We also show that the kinetochore protein, Polo kinase, is required for maintaining chromosome alignment and spindle organization in metaphase I oocytes. In combination our results support a model where the meiotic central spindle and associated proteins are essential for acentrosomal chromosome segregation.

Cooperation of the BTB-Zinc finger protein, Abrupt, with cytoskeletal regulators in Drosophila epithelial tumorigenesis

The deregulation of cell polarity or cytoskeletal regulators is a common occurrence in human epithelial cancers. Moreover, there is accumulating evidence in human epithelial cancer that BTB-ZF genes, such as Bcl6 and ZBTB7A, are oncogenic. From our previous studies in the vinegar fly, Drosophila melanogaster, we have identified a cooperative interaction between a mutation in the apico-basal cell polarity regulator Scribble (Scrib) and overexpression of the BTB-ZF protein Abrupt (Ab). Herein, we show that co-expression of ab with actin cytoskeletal regulators, RhoGEF2 or Src64B, in the developing eye-antennal epithelial tissue results in the formation of overgrown amorphous tumours, whereas ab and DRac1 co-expression leads to non-cell autonomous overgrowth. Together with ab, these genes affect the expression of differentiation genes, resulting in tumours locked in a progenitor cell fate. Finally, we show that the expression of two mammalian genes related to ab, Bcl6 and ZBTB7A, which are oncogenes in mammalian epithelial cancers, significantly correlate with the upregulation of cytoskeletal genes or downregulation of apico-basal cell polarity neoplastic tumour suppressor genes in colorectal, lung and other human epithelial cancers. Altogether, this analysis has revealed that upregulation of cytoskeletal regulators cooperate with Abrupt in Drosophila epithelial tumorigenesis, and that high expression of human BTB-ZF genes, Bcl6 and ZBTB7A, shows significant correlations with cytoskeletal and cell polarity gene expression in specific epithelial tumour types. This highlights the need for further investigation of the cooperation between these genes in mammalian systems.

The Fog signaling pathway: insights into signaling in morphogenesis

Epithelia form the building blocks of many tissue and organ types. Epithelial cells often form a contiguous 2-dimensional sheet that is held together by strong adhesions. The mechanical properties conferred by these adhesions allow the cells to undergo dramatic three-dimensional morphogenetic movements while maintaining cell–cell contacts during embryogenesis and post-embryonic development. The Drosophila Folded gastrulation pathway triggers epithelial cell shape changes that drive gastrulation and tissue folding and is one of the most extensively studied examples of epithelial morphogenesis. This pathway has yielded key insights into the signaling mechanisms and cellular machinery involved in epithelial remodeling. In this review, we discuss principles of morphogenesis and signaling that have been discovered through genetic and cell biological examination of this pathway. We also consider various regulatory mechanisms and the system's relevance to mammalian development. We propose future directions that will continue to broaden our knowledge of morphogenesis across taxa.

Coordinating the cytoskeleton and endocytosis for regulated plasma membrane growth in the early Drosophila embryo

Plasma membrane organization is under the control of cytoskeletal networks and endocytic mechanisms, and a growing literature is showing how closely these influences are interconnected. Here, we review how plasma membranes are formed around individual nuclei of the syncytial Drosophila embryo. Specifically, we outline the pathways that promote and maintain the growth of pseudocleavage and cellularization furrows, as well as specific pathways that keep furrow growth in check. This system has become important for studies of actin regulators, such as Rho1, Diaphanous, non-muscle myosin II and Arp2/3, and endocytic regulators, such as a cytohesin Arf-GEF (Steppke), clathrin, Amphiphysin and dynamin. More generally, it provides a model for understanding how cytoskeletal-endocytic cross-talk regulates the assembly of a cell.

Septins promote F-actin ring formation by crosslinking actin filaments into curved bundles

Animal cell cytokinesis requires a contractile ring of crosslinked actin filaments and myosin motors. How contractile rings form and are stabilized in dividing cells remains unclear. We address this problem by focusing on septins, highly conserved proteins in eukaryotes whose precise contribution to cytokinesis remains elusive. We use the cleavage of the Drosophila melanogaster embryo as a model system, where contractile actin rings drive constriction of invaginating membranes to produce an epithelium in a manner akin to cell division. In vivo functional studies show that septins are required for generating curved and tightly packed actin filament networks. In vitro reconstitution assays show that septins alone bundle actin filaments into rings, accounting for the defects in actin ring formation in septin mutants. The bundling and bending activities are conserved for human septins, and highlight unique functions of septins in the organization of contractile actomyosin rings.

Function and dynamics of slam in furrow formation in early Drosophila embryo

The Drosophila embryo undergoes a developmental transition in the blastoderm stage switching from syncytial to cellular development. The cleavage furrow, which encloses nuclei into cells, is a prominent morphological feature of this transition. It is not clear how the pattern of the furrow array is defined and how zygotic genes trigger the formation and invagination of interphase furrows. A key to these questions is provided by the gene slam, which has been previously implicated in controlling furrow invagination. Here we investigate the null phenotype of slam, the dynamics of Slam protein, and its control by the recycling endosome. We find that slam is essential for furrow invagination during cellularisation and together with nullo, for specification of the furrow. During cellularisation, Slam marks first the furrow, which is derived from the metaphase furrow of the previous mitosis. Slightly later, Slam accumulates at new furrows between daughter cells early in interphase. Slam is stably associated with the furrow canal except for the onset of cellularisation as revealed by FRAP experiments. Restriction of Slam to the furrow canal and Slam mobility during cellularisation is controlled by the recycling endosome and centrosomes. We propose a three step model. The retracting metaphase furrow leaves an initial mark. This mark and the border between corresponding daughter nuclei are refined by vesicular transport away from pericentrosomal recycling endosome towards the margins of the somatic buds. Following the onset of zygotic gene expression, Slam and Nullo together stabilise this mark and Slam triggers invagination of the cleavage furrow.

The plasma membrane flattens out to fuel cell-surface growth during Drosophila cellularization

Cell-shape change demands cell-surface growth, but how growth is fueled and choreographed is still debated. Here we use cellularization, the first complete cytokinetic event in Drosophila embryos, to show that cleavage furrow ingression is kinetically coupled to the loss of surface microvilli. We modulate furrow kinetics with RNAi against the Rho1-GTPase regulator slam and show that furrow ingression controls the rate of microvillar depletion. Finally, we directly track the microvillar membrane and see it move along the cell surface and into ingressing furrows, independent of endocytosis. Together, our results demonstrate that the kinetics of the ingressing furrow regulate the utilization of a microvillar membrane reservoir. Because membranes of the furrow and microvilli are contiguous, we suggest that ingression drives unfolding of the microvilli and incorporation of microvillar membrane into the furrow. We conclude that plasma membrane folding/unfolding can contribute to the cell-shape changes that promote embryonic morphogenesis.

The maternal-to-zygotic transition targets actin to promote robustness during morphogenesis

Robustness is a property built into biological systems to ensure stereotypical outcomes despite fluctuating inputs from gene dosage, biochemical noise, and the environment. During development, robustness safeguards embryos against structural and functional defects. Yet, our understanding of how robustness is achieved in embryos is limited. While much attention has been paid to the role of gene and signaling networks in promoting robust cell fate determination, little has been done to rigorously assay how mechanical processes like morphogenesis are designed to buffer against variable conditions. Here we show that the cell shape changes that drive morphogenesis can be made robust by mechanisms targeting the actin cytoskeleton. We identified two novel members of the Vinculin/α-Catenin Superfamily that work together to promote robustness during Drosophila cellularization, the dramatic tissue-building event that generates the primary epithelium of the embryo. We find that zygotically-expressed Serendipity-α (Sry-α) and maternally-loaded Spitting Image (Spt) share a redundant, actin-regulating activity during cellularization. Spt alone is sufficient for cellularization at an optimal temperature, but both Spt plus Sry-α are required at high temperature and when actin assembly is compromised by genetic perturbation. Our results offer a clear example of how the maternal and zygotic genomes interact to promote the robustness of early developmental events. Specifically, the Spt and Sry-α collaboration is informative when it comes to genes that show both a maternal and zygotic requirement during a given morphogenetic process. For the cellularization of Drosophilids, Sry-α and its expression profile may represent a genetic adaptive trait with the sole purpose of making this extreme event more reliable. Since all morphogenesis depends on cytoskeletal remodeling, both in embryos and adults, we suggest that robustness-promoting mechanisms aimed at actin could be effective at all life stages.

Every embryo develops under its own unique set of circumstances, with variable inputs coming from mother, father, and the environment. To then ensure a reliable outcome, mechanisms are built into development to buffer against challenges like genetic deficiency, maternal fever, alcohol exposure, etc. This buffering, called “robustness”, can be overwhelmed, ending in miscarriage, pre-mature birth, and structural and functional birth defects. Thus, we need to understand how robustness arises in order to define an embryo's susceptibilities to genetic background and environment; and to ultimately promote healthy reproduction. In this work we provide new insight into how morphogenesis, the process of tissue building in embryos, is made more robust. First, we show that early gene expression by the embryo, or zygote, supplements the stockpile of proteins already supplied by the mother to ensure the robustness of early morphogenesis. Specifically, our data suggests that a specific gene, sry-α, and its expression by the embryo at the maternal-to-zygotic transition, is a genetic adaptation with the sole function of making the first tissue building event in the fruit fly more robust. In addition, we show that the robustness of this morphogenetic event is promoted by mechanisms regulating the actin cytoskeleton.

An Arf-GEF regulates antagonism between endocytosis and the cytoskeleton for Drosophila blastoderm development

BACKGROUND

Actin cytoskeletal networks push and pull the plasma membrane (PM) to control cell structure and behavior. Endocytosis also regulates the PM and can be promoted or inhibited by cytoskeletal networks. However, endocytic regulation of the general membrane cytoskeleton is undocumented.

RESULTS

Here, we provide evidence for endocytic inhibition of actomyosin networks. Specifically, we find that Steppke, a cytohesin Arf-guanine nucleotide exchange factor (GEF), controls initial PM furrow ingression during the syncytial nuclear divisions and cellularization of the Drosophila embryo. Acting at the tips of ingressing furrows, Steppke promotes local endocytic events through its Arf-GEF activity and in cooperation with the AP-2 clathrin adaptor complex. These Steppke activities appear to reduce local Rho1 protein levels and ultimately restrain actomyosin networks. Without Steppke, Rho1 pathways linked to actin polymerization and myosin activation abnormally expand the membrane cytoskeleton into taut sheets emanating perpendicularly from the furrow tips. These expansions lead to premature cellularization and abnormal expulsions of nuclei from the forming blastoderm. Finally, consistent with earlier reports, we also find that actomyosin activity can act reciprocally to inhibit the endocytosis at furrow tips.

CONCLUSIONS

We propose that Steppke-dependent endocytosis keeps the cytoskeleton in check as early PM furrows form. Specifically, a cytohesin Arf-GEF-Arf G protein-AP-2 endocytic axis appears to antagonize Rho1 cytoskeletal pathways to restrain the membrane cytoskeleton. However, as furrows lengthen during cellularization, the cytoskeleton gains strength, blocks the endocytic inhibition, and finally closes off the base of each cell to form the blastoderm.

The F-BAR protein Cip4/Toca-1 antagonizes the formin Diaphanous in membrane stabilization and compartmentalization

During Drosophila embryogenesis, the first epithelium with defined cortical compartments is established during cellularization. Actin polymerization is required for the separation of lateral and basal domains as well as suppression of tubular extensions in the basal domain. The actin nucleator mediating this function is unknown. We found that the formin Diaphanous (Dia) is required for establishing and maintaining distinct lateral and basal domains during cellularization. In dia mutant embryos lateral marker proteins, such as Discs-large and Armadillo/β-Catenin spread into the basal compartment. Furthermore, high-resolution and live-imaging analysis of dia mutant embryos revealed an increased number of membrane extensions and endocytic activity at the basal domain, indicating a suppressing function of dia on membrane invaginations. Dia function might be based on an antagonistic interaction with the F-BAR protein Cip4/Toca-1, a known activator of the WASP/WAVE-Arp2/3 pathway. Dia and Cip4 physically and functionally interact and overexpression of Cip4 phenocopies dia loss-of-function. In vitro, Cip4 inhibits mainly actin nucleation by Dia. Thus, our data support a model in which linear actin filaments induced by Dia stabilize cortical compartmentalization by antagonizing membrane turnover induced by WASP/WAVE-Arp2/3.

Cytokinesis: RhoGEFs control a developmental cleavage switch

New findings provide evidence that developmentally staged RhoGEFs control assembly of two alternative forms of cleavage furrows: the 'Rappaport' furrows, which govern division of Drosophila syncytial cortical division cycles, and conventional spindle-directed furrows.

RhoGEF and positioning of rappaport-like furrows in the early Drosophila embryo

Early Drosophila embryogenesis is characterized by shifting from astral microtubule-based to central spindle-based positioning of cleavage furrows. Before cellularization, astral microtubules determine metaphase furrow position by producing Rappaport-like furrows, which encompass rather than bisect the spindle. Their positioning is explained by our finding that the conserved central spindle components centralspindlin (mKLP1 and RacGAP50C), Polo, and Fascetto (Prc1) localize to the astral microtubule overlap region. These components and the chromosomal passenger complex localize to the central spindle, though no furrow forms there. We identify the maternally supplied RhoGEF2 as a key factor in metaphase furrow positioning. Unlike the zygotic, central spindle-localized RhoGEF (Pebble), RhoGEF2 localizes to metaphase furrows, a function distinct from RhoGEF/Pebble and likely due to the absence of a RacGAP50C binding domain. Accordingly, we find that ectopic activation of Rho GTPase generates furrows perpendicular to the central spindle during syncytial divisions. Whereas metaphase furrow formation is myosin independent, these ectopic furrows, like conventional furrows, require myosin as well as microtubules. These studies demonstrate that early Drosophila embryogenesis is primed to form furrows at either overlapping astral microtubules or the central spindle. We propose that the shift to the latter is driven by a corresponding shift from RhoGEF2 to Pebble in controlling furrow formation.

DRhoGEF2 regulates cellular tension and cell pulsations in the Amnioserosa during Drosophila dorsal closure

Coordination of apical constriction in epithelial sheets is a fundamental process during embryogenesis. Here, we show that DRhoGEF2 is a key regulator of apical pulsation and constriction of amnioserosal cells during Drosophila dorsal closure. Amnioserosal cells mutant for DRhoGEF2 exhibit a consistent decrease in amnioserosa pulsations whereas overexpression of DRhoGEF2 in this tissue leads to an increase in the contraction time of pulsations. We probed the physical properties of the amnioserosa to show that the average tension in DRhoGEF2 mutant cells is lower than wild-type and that overexpression of DRhoGEF2 results in a tissue that is more solid-like than wild-type. We also observe that in the DRhoGEF2 overexpressing cells there is a dramatic increase of apical actomyosin coalescence that can contribute to the generation of more contractile forces, leading to amnioserosal cells with smaller apical surface than wild-type. Conversely, in DRhoGEF2 mutants, the apical actomyosin coalescence is impaired. These results identify DRhoGEF2 as an upstream regulator of the actomyosin contractile machinery that drives amnioserosa cells pulsations and apical constriction.

Spatial regulation of Dia and Myosin-II by RhoGEF2 controls initiation of E-cadherin endocytosis during epithelial morphogenesis

E-cadherin plays a pivotal role in epithelial morphogenesis. It controls the intercellular adhesion required for tissue cohesion and anchors the actomyosin-driven tension needed to change cell shape. In the early Drosophila embryo, Myosin-II (Myo-II) controls the planar polarized remodelling of cell junctions and tissue extension. The E-cadherin distribution is also planar polarized and complementary to the Myosin-II distribution. Here we show that E-cadherin polarity is controlled by the polarized regulation of clathrin- and dynamin-mediated endocytosis. Blocking E-cadherin endocytosis resulted in cell intercalation defects. We delineate a pathway that controls the initiation of E-cadherin endocytosis through the regulation of AP2 and clathrin coat recruitment by E-cadherin. This requires the concerted action of the formin Diaphanous (Dia) and Myosin-II. Their activity is controlled by the guanine exchange factor RhoGEF2, which is planar polarized and absent in non-intercalating regions. Finally, we provide evidence that Dia and Myo-II control the initiation of E-cadherin endocytosis by regulating the lateral clustering of E-cadherin.

Identification of novel Ras-cooperating oncogenes in Drosophila melanogaster: a RhoGEF/Rho-family/JNK pathway is a central driver of tumorigenesis

We have shown previously that mutations in the apico-basal cell polarity regulators cooperate with oncogenic Ras (Ras(ACT)) to promote tumorigenesis in Drosophila melanogaster and mammalian cells. To identify novel genes that cooperate with Ras(ACT) in tumorigenesis, we carried out a genome-wide screen for genes that when overexpressed throughout the developing Drosophila eye enhance Ras(ACT)-driven hyperplasia. Ras(ACT)-cooperating genes identified were Rac1 Rho1, RhoGEF2, pbl, rib, and east, which encode cell morphology regulators. In a clonal setting, which reveals genes conferring a competitive advantage over wild-type cells, only Rac1, an activated allele of Rho1 (Rho1(ACT)), RhoGEF2, and pbl cooperated with Ras(ACT), resulting in reduced differentiation and large invasive tumors. Expression of RhoGEF2 or Rac1 with Ras(ACT) upregulated Jun kinase (JNK) activity, and JNK upregulation was essential for cooperation. However, in the whole-tissue system, upregulation of JNK alone was not sufficient for cooperation with Ras(ACT), while in the clonal setting, JNK upregulation was sufficient for Ras(ACT)-mediated tumorigenesis. JNK upregulation was also sufficient to confer invasive growth of Ras(V12)-expressing mammalian MCF10A breast epithelial cells. Consistent with this, HER2(+) human breast cancers (where human epidermal growth factor 2 is overexpressed and Ras signaling upregulated) show a significant correlation with a signature representing JNK pathway activation. Moreover, our genetic analysis in Drosophila revealed that Rho1 and Rac are important for the cooperation of RhoGEF2 or Pbl overexpression and of mutants in polarity regulators, Dlg and aPKC, with Ras(ACT) in the whole-tissue context. Collectively our analysis reveals the importance of the RhoGEF/Rho-family/JNK pathway in cooperative tumorigenesis with Ras(ACT).

Genetic evidence for antagonism between Pak protein kinase and Rho1 small GTPase signaling in regulation of the actin cytoskeleton during Drosophila oogenesis

During Drosophila oogenesis, basally localized F-actin bundles in the follicle cells covering the egg chamber drive its elongation along the anterior-posterior axis. The basal F-actin of the follicle cell is an attractive system for the genetic analysis of the regulation of the actin cytoskeleton, and results obtained in this system are likely to be broadly applicable in understanding tissue remodeling. Mutations in a number of genes, including that encoding the p21-activated kinase Pak, have been shown to disrupt organization of the basal F-actin and in turn affect egg chamber elongation. pak mutant egg chambers have disorganized F-actin distribution and remain spherical due to a failure to elongate. In a genetic screen to identify modifiers of the pak rounded egg chamber phenotype several second chromosome deficiencies were identified as suppressors. One suppressing deficiency removes the rho1 locus, and we determined using several rho1 alleles that removal of a single copy of rho1 can suppress the pak phenotype. Reduction of any component of the Rho1-activated actomyosin contractility pathway suppresses pak oogenesis defects, suggesting that Pak counteracts Rho1 signaling. There is ectopic myosin light chain phosphorylation in pak mutant follicle cell clones in elongating egg chambers, probably due at least in part to mislocalization of RhoGEF2, an activator of the Rho1 pathway. In early egg chambers, pak mutant follicle cells have reduced levels of myosin phosphorylation and we conclude that Pak both promotes and restricts myosin light chain phosphorylation in a temporally distinct manner during oogenesis.

Specific isoforms of drosophila shroom define spatial requirements for the induction of apical constriction

Vertebrate Shroom proteins define cytoskeletal organization and cellular architecture by binding directly to F-actin and Rho-kinase and spatially regulating the activity of nonmuscle myosin II (myosin II). Here, we report characterization and gain-of-function analysis of Drosophila Shroom. The dShrm locus expresses at least two protein isoforms, dShrmA and dShrmB, which localize to adherens junctions and the apical membrane, respectively. dShrmA and dShrmB exhibit differing abilities to induce apical constriction that are based on their subcellular distribution and the subsequent assembly of spatially and organizationally distinct actomyosin networks that are dependent on the ability to engage Rho-kinase and the activity of myosin II. These data show that the differential subcellular distribution of naturally occurring isoforms of Shroom proteins can define both the position and organization of actomyosin networks in vivo. We further hypothesize that differentially positioned contractile arrays have distinct effects on cellular morphologies and behaviors.

A mitotic kinesin-6, Pav-KLP, mediates interdependent cortical reorganization and spindle dynamics in Drosophila embryos

We investigated the role of Pav-KLP, a kinesin-6, in the coordination of spindle and cortical dynamics during mitosis in Drosophila embryos. In vitro, Pav-KLP behaves as a dimer. In vivo, it localizes to mitotic spindles and furrows. Inhibition of Pav-KLP causes defects in both spindle dynamics and furrow ingression, as well as causing changes in the distribution of actin and vesicles. Thus, Pav-KLP stabilizes the spindle by crosslinking interpolar microtubule bundles and contributes to actin furrow formation possibly by transporting membrane vesicles, actin and/or actin regulatory molecules along astral microtubules. Modeling suggests that furrow ingression during cellularization depends on: (1) a Pav-KLP-dependent force driving an initial slow stage of ingression; and (2) the subsequent Pav-KLP-driven transport of actin- and membrane-containing vesicles to the furrow during a fast stage of ingression. We hypothesize that Pav-KLP is a multifunctional mitotic motor that contributes both to bundling of interpolar microtubules, thus stabilizing the spindle, and to a biphasic mechanism of furrow ingression by pulling down the furrow and transporting vesicles that deliver new material to the descending furrow.

Cortical actin dynamics facilitate early-stage centrosome separation

Proper centrosome separation is a prerequisite for positioning the bipolar spindle. Although studies demonstrate that microtubules (MTs) and their associated motors drive centrosome separation [1], the role of actin in centrosome separation remains less clear. Studies in tissue culture cells indicate that actin- and myosin-based cortical flow is primarily responsible for driving late centrosome separation [2], whereas other studies suggest that actin plays a more passive role by serving as an attachment site for astral MTs to pull centrosomes apart [3-6]. Here we demonstrate that prior to nuclear envelope breakdown (NEB) in Drosophila embryos, proper centrosome separation does not require myosin II but requires dynamic actin rearrangements at the growing edge of the interphase cap. Both Arp2/3- and Formin-mediated actin remodeling are required for separating the centrosome pairs before NEB. The Apc2-Armadillo complex appears to link cap expansion to centrosome separation. In contrast, the mechanisms driving centrosome separation after NEB are independent of the actin cytoskeleton and compensate for earlier separation defects. Our studies show that the dynamics of actin polymerization drive centrosome separation, and this has important implications for centrosome positioning during processes such as cell migration [7, 8], cell polarity maintenance [9, 10], and asymmetric cell division [11, 12].

Using total internal reflection fluorescence (TIRF) microscopy to visualize cortical actin and microtubules in the Drosophila syncytial embryo

The Drosophila syncytial embryo is a powerful developmental model system for studying dynamic coordinated cytoskeletal rearrangements. Confocal microscopy has begun to reveal more about the cytoskeletal changes that occur during embryogenesis. Total internal reflection fluorescence (TIRF) microscopy provides a promising new approach for the visualization of cortical events with heightened axial resolution. We have applied TIRF microscopy to the Drosophila embryo to visualize cortical microtubule and actin dynamics in the syncytial blastoderm. Here, we describe the details of this technique, and report qualitative assessments of cortical microtubules and actin in the Drosophila syncytial embryo. In addition, we identified a peak of cortical microtubules during anaphase of each nuclear cycle in the syncytial blastoderm, and using images generated by TIRF microscopy, we quantitatively analyzed microtubule dynamics during this time.