The epithelial-mesenchymal transition of theDrosophilamesoderm requires the Rho GTP exchange factor Pebble

Independent signaling pathways provide a fail-safe mechanism to prevent tumorigenesis

Controlled signaling activity is vital for normal tissue homeostasis and oncogenic signaling activation facilitates tumorigenesis. Here we use single-cell transcriptomics to investigate the effects of pro-proliferative signaling on epithelial homeostasis using the Drosophila follicle cell lineage. Notably, EGFR-Ras overactivation induces cell cycle defects by activating the transcription factors Pointed and E2f1 and impedes differentiation. Hh signaling simultaneously promotes an undifferentiated state and induces differentiation via activation of EMT-associated transcription factors zfh1 and Mef2. As a result, overactivation of Hh signaling generates a transcriptional hybrid state comparable to epithelial-mesenchymal-transition. Co-overactivation of Hh signaling with EGFR-Ras signaling blocks differentiation and induces key characteristics of tumor cells including a loss of tissue architecture caused by reduced expression of cell adhesion molecules, sustained proliferation and an evasion of cell cycle checkpoints. These findings provide new insight into how non-interacting signaling pathways converge at the transcriptional level to prevent malignant cell behavior.

Two RhoGEF isoforms with distinct localisation control furrow position during asymmetric cell division

Cytokinesis partitions cellular content between daughter cells. It relies on the formation of an acto-myosin contractile ring, whose constriction induces the ingression of the cleavage furrow between the segregated chromatids. Rho1 GTPase and its RhoGEF (Pbl) are essential for this process. However, how Rho1 is regulated to sustain furrow ingression while maintaining correct furrow position remains poorly defined. Here, we show that during asymmetric division of Drosophila neuroblasts, Rho1 is controlled by two Pbl isoforms with distinct localisation. Spindle midzone- and furrow-enriched Pbl-A focuses Rho1 at the furrow to sustain efficient ingression, while Pbl-B pan-plasma membrane localization promotes the broadening of Rho1 activity and the subsequent enrichment of myosin on the entire cortex. This enlarged zone of Rho1 activity is critical to adjust furrow position, thereby preserving correct daughter cell size asymmetry. Our work highlights how the use of isoforms with distinct localisation makes an essential process more robust.

Parallels between the extracellular matrix roles in developmental biology and cancer biology

Interaction of a tumor with its microenvironment is an emerging field of investigation, and the crosstalk between cancer cells and the extracellular matrix is of particular interest, since cancer patients with abundant and stiff extracellular matrices display a poorer prognosis. At the post-juvenile stage, the extracellular matrix plays predominantly a structural role by providing support to cells and tissues; however, during development, matrix proteins exert a plethora of diverse signals to guide the movement and determine the fate of pluripotent cells. Taking a closer look at the communication between the extracellular matrix and cells of a developing body may bring new insights into cancer biology and identify cancer weaknesses. This review discusses parallels between the extracellular matrix roles during development and tumor growth.

In vivo Functional Genomics for Undiagnosed Patients: The Impact of Small GTPases Signaling Dysregulation at Pan-Embryo Developmental Scale

While individually rare, disorders affecting development collectively represent a substantial clinical, psychological, and socioeconomic burden to patients, families, and society. Insights into the molecular mechanisms underlying these disorders are required to speed up diagnosis, improve counseling, and optimize management toward targeted therapies. Genome sequencing is now unveiling previously unexplored genetic variations in undiagnosed patients, which require functional validation and mechanistic understanding, particularly when dealing with novel nosologic entities. Functional perturbations of key regulators acting on signals' intersections of evolutionarily conserved pathways in these pathological conditions hinder the fine balance between various developmental inputs governing morphogenesis and homeostasis. However, the distinct mechanisms by which these hubs orchestrate pathways to ensure the developmental coordinates are poorly understood. Integrative functional genomics implementing quantitative in vivo models of embryogenesis with subcellular precision in whole organisms contribute to answering these questions. Here, we review the current knowledge on genes and mechanisms critically involved in developmental syndromes and pediatric cancers, revealed by genomic sequencing and in vivo models such as insects, worms and fish. We focus on the monomeric GTPases of the RAS superfamily and their influence on crucial developmental signals and processes. We next discuss the effectiveness of exponentially growing functional assays employing tractable models to identify regulatory crossroads. Unprecedented sophistications are now possible in zebrafish, i.e., genome editing with single-nucleotide precision, nanoimaging, highly resolved recording of multiple small molecules activity, and simultaneous monitoring of brain circuits and complex behavioral response. These assets permit accurate real-time reporting of dynamic small GTPases-controlled processes in entire organisms, owning the potential to tackle rare disease mechanisms.

Drosophila models of metastasis

An important goal in the fight against cancer is to understand how tumors become invasive and metastatic. A crucial early step in metastasis is thought to be the epithelial mesenchymal transition (EMT), the process in which epithelial cells transition into a more migratory and invasive, mesenchymal state. Since the genetic regulatory networks driving EMT in tumors derive from those used in development, analysis of EMTs in genetic model organisms such as the vinegar fly, Drosophila melanogaster, can provide great insight into cancer. In this review I highlight the many ways in which studies in the fly are shedding light on cancer metastasis. The review covers both normal developmental events in which epithelial cells become migratory, as well as induced events, whereby normal epithelial cells become metastatic due to genetic manipulations. The ability to make such precise genetic perturbations in the context of a normal, in vivo environment, complete with a working innate immune system, is making the fly increasingly important in understanding metastasis.

Defining epithelial-mesenchymal transitions in animal development

Over 50 years after its discovery in early chick embryos, the concept of epithelial-mesenchymal transition (EMT) is now widely applied to morphogenetic studies in both physiological and pathological contexts. Indeed, the EMT field has witnessed exponential growth in recent years, driven primarily by a rapid expansion of cancer-oriented EMT research. This has led to EMT-based therapeutic interventions that bear the prospect of fighting cancer, and has given developmental biologists new impetus to investigate EMT phenomena more closely and to find suitable models to address emerging EMT-related questions. Here, and in the accompanying poster, I provide a brief summary of the current status of EMT research and give an overview of EMT models that have been used in developmental studies. I also highlight dynamic epithelialization and de-epithelialization events that are involved in many developmental processes and that should be considered to provide a broader perspective of EMT. Finally, I put forward a set of criteria to separate morphogenetic phenomena that are EMT-related from those that are not.

In primary airway epithelial cells, the unjamming transition is distinct from the epithelial-to-mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) and the unjamming transition (UJT) each comprises a gateway to cellular migration, plasticity and remodeling, but the extent to which these core programs are distinct, overlapping, or identical has remained undefined. Here, we triggered partial EMT (pEMT) or UJT in differentiated primary human bronchial epithelial cells. After triggering UJT, cell-cell junctions, apico-basal polarity, and barrier function remain intact, cells elongate and align into cooperative migratory packs, and mesenchymal markers of EMT remain unapparent. After triggering pEMT these and other metrics of UJT versus pEMT diverge. A computational model attributes effects of pEMT mainly to diminished junctional tension but attributes those of UJT mainly to augmented cellular propulsion. Through the actions of UJT and pEMT working independently, sequentially, or interactively, those tissues that are subject to development, injury, or disease become endowed with rich mechanisms for cellular migration, plasticity, self-repair, and regeneration.

Gastrulation in Drosophila melanogaster: Genetic control, cellular basis and biomechanics

Gastrulation is generally understood as the morphogenetic processes that result in the spatial organization of the blastomere into the three germ layers, ectoderm, mesoderm and endoderm. This review summarizes our current knowledge of the morphogenetic mechanisms in Drosophila gastrulation. In addition to the events that drive mesoderm invagination and germband elongation, we pay particular attention to other, less well-known mechanisms including midgut invagination, cephalic furrow formation, dorsal fold formation, and mesoderm layer formation. This review covers topics ranging from the identification and functional characterization of developmental and morphogenetic control genes to the analysis of the physical properties of cells and tissues and the control of cell and tissue mechanics of the morphogenetic movements in the gastrula.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Correlation between FAK and EGF-Induced EMT in Colorectal Cancer Cells

Epithelial-mesenchymal transition (EMT) plays an important role in the invasion and metastasis of colorectal cancer, which is mediated by FAK and EGF. However, whether FAK participates in EMT in colorectal cancer cells through the EGF/EGFR signaling pathway remains unknown. The aim of this study was to investigate the effector mechanisms of FAK in the process of EGF-induced EMT in colorectal cancer cells and to determine whether miR-217 is involved in this process. Caco-2 cancer cells were routinely cultured with and without treatment with 100 ng/mL EGF, and changes in cell morphology were observed using an inverted microscope. In addition, a transwell assay was used to detect cell migration under the condition of EGF treatment. The expression of FAK, pFAK, E-cadherin, vimentin, and β actin was assessed by western blotting, and the expression of miR-217 was assessed using real-time PCR. We found that EGF induced EMT in colorectal cancer cells and enhanced cell migration and invasion ability. Moreover, FAK was involved in the EGF-induced EMT of colorectal cancer cells. EGF upregulated the expression of E-cadherin in colorectal cancer cells by activating FAK, and miR-217 was found to participate in EGF-induced EMT in colorectal cancer cells. Our findings indicate that EGF induces EMT in colorectal cancer cells by activating FAK, and miR-217 is involved in the EGF/FAK/E-cadherin signaling pathway.

SILAC-based quantitative proteomic analysis of Drosophila gastrula stage embryos mutant for fibroblast growth factor signalling

Quantitative proteomic analyses in combination with genetics provide powerful tools in developmental cell signalling research. Drosophila melanogaster is one of the most widely used genetic models for studying development and disease. Here we combined quantitative proteomics with genetic selection to determine changes in the proteome upon depletion of Heartless (Htl) Fibroblast-Growth Factor (FGF) receptor signalling in Drosophila embryos at the gastrula stage. We present a robust, single generation SILAC (stable isotope labelling with amino acids in cell culture) protocol for labelling proteins in early embryos. For the selection of homozygously mutant embryos at the pre-gastrula stage, we developed an independent genetic marker. Our analyses detected quantitative changes in the global proteome of htl mutant embryos during gastrulation. We identified distinct classes of downregulated and upregulated proteins, and network analyses indicate functionally related groups of proteins in each class. In addition, we identified changes in the abundance of phosphopeptides. In summary, our quantitative proteomic analysis reveals global changes in metabolic, nucleoplasmic, cytoskeletal and transport proteins in htl mutant embryos.

Functional Role of Non-Coding RNAs during Epithelial-To-Mesenchymal Transition

Epithelial-to-mesenchymal transition (EMT) is a key biological process involved in a multitude of developmental and pathological events. It is characterized by the progressive loss of cell-to-cell contacts and actin cytoskeletal rearrangements, leading to filopodia formation and the progressive up-regulation of a mesenchymal gene expression pattern enabling cell migration. Epithelial-to-mesenchymal transition is already observed in early embryonic stages such as gastrulation, when the epiblast undergoes an EMT process and therefore leads to the formation of the third embryonic layer, the mesoderm. Epithelial-to-mesenchymal transition is pivotal in multiple embryonic processes, such as for example during cardiovascular system development, as valve primordia are formed and the cardiac jelly is progressively invaded by endocardium-derived mesenchyme or as the external cardiac cell layer is established, i.e., the epicardium and cells detached migrate into the embryonic myocardial to form the cardiac fibrous skeleton and the coronary vasculature. Strikingly, the most important biological event in which EMT is pivotal is cancer development and metastasis. Over the last years, understanding of the transcriptional regulatory networks involved in EMT has greatly advanced. Several transcriptional factors such as Snail, Slug, Twist, Zeb1 and Zeb2 have been reported to play fundamental roles in EMT, leading in most cases to transcriptional repression of cell⁻cell interacting proteins such as ZO-1 and cadherins and activation of cytoskeletal markers such as vimentin. In recent years, a fundamental role for non-coding RNAs, particularly microRNAs and more recently long non-coding RNAs, has been identified in normal tissue development and homeostasis as well as in several oncogenic processes. In this study, we will provide a state-of-the-art review of the functional roles of non-coding RNAs, particularly microRNAs, in epithelial-to-mesenchymal transition in both developmental and pathological EMT.

RhoA GTPase inhibition organizes contraction during epithelial morphogenesis

During morphogenesis, contraction of the actomyosin cytoskeleton within individual cells drives cell shape changes that fold tissues. Coordination of cytoskeletal contractility is mediated by regulating RhoA GTPase activity. Guanine nucleotide exchange factors (GEFs) activate and GTPase-activating proteins (GAPs) inhibit RhoA activity. Most studies of tissue folding, including apical constriction, have focused on how RhoA is activated by GEFs to promote cell contractility, with little investigation as to how GAPs may be important. Here, we identify a critical role for a RhoA GAP, Cumberland GAP (C-GAP), which coordinates with a RhoA GEF, RhoGEF2, to organize spatiotemporal contractility during Drosophila melanogaster apical constriction. C-GAP spatially restricts RhoA pathway activity to a central position in the apical cortex. RhoGEF2 pulses precede myosin, and C-GAP is required for pulsation, suggesting that contractile pulses result from RhoA activity cycling. Finally, C-GAP expression level influences the transition from reversible to irreversible cell shape change, which defines the onset of tissue shape change. Our data demonstrate that RhoA activity cycling and modulating the ratio of RhoGEF2 to C-GAP are required for tissue folding.

Mechanical forces drive neuroblast morphogenesis and are required for epidermal closure

Tissue morphogenesis requires myosin-dependent events such as cell shape changes and migration to be coordinated between cells within a tissue, and/or with cells from other tissues. However, few studies have investigated the simultaneous morphogenesis of multiple tissues in vivo. We found that during Caenorhabditis elegans ventral enclosure, when epidermal cells collectively migrate to cover the ventral surface of the embryo, the underlying neuroblasts (neuronal precursor cells) also undergo morphogenesis. We found that myosin accumulates as foci along the junction-free edges of the ventral epidermal cells to form a ring, whose closure is myosin-dependent. We also observed the accumulation of myosin foci and the adhesion junction proteins E-cadherin and α-catenin in the underlying neuroblasts. Myosin may help to reorganize a subset of neuroblasts into a rosette-like pattern, and decrease their surface area as the overlying epidermal cells constrict. Since myosin is required in the neuroblasts for ventral enclosure, we propose that mechanical forces in the neuroblasts influence constriction of the overlying epidermal cells. In support of this model, disrupting neuroblast cell division or altering their fate influences myosin localization in the overlying epidermal cells. The coordination of myosin-dependent events and forces between cells in different tissues could be a common theme for coordinating morphogenetic events during metazoan development.

Ligand-binding and constitutive FGF receptors in single Drosophila tracheal cells: Implications for the role of FGF in collective migration

BACKGROUND

The migration of individual cells relies on their capacity to evaluate differences across their bodies and to move either toward or against a chemoattractant or a chemorepellent signal respectively. However, the direction of collective migration is believed to depend on the internal organization of the cell cluster while the role of the external signal is limited to single out some cells in the cluster, conferring them with motility properties.

RESULTS

Here we analyzed the role of Fibroblast Growth Factor (FGF) signaling in collective migration in the Drosophila trachea. While ligand-binding FGF receptor (FGFR) activity in a single cell can drive migration of a tracheal cluster, we show that activity from a constitutively activated FGFR cannot-an observation that contrasts with previously analyzed cases.

CONCLUSIONS

Our results indicate that individual cells in the tracheal cluster can "read" differences in the distribution of FGFR activity and lead migration of the cluster accordingly. Thus, FGF can act as a chemoattractant rather than as a motogen in collective cell migration. This finding has many implications in both development and pathology.

Discovery of MINC1, a GTPase-activating protein small molecule inhibitor, targeting MgcRacGAP

The Rho family of Ras superfamily small GTPases regulates a broad range of biological processes such as migration, differentiation, cell growth and cell survival. Therefore, the availability of small molecule modulators as tool compounds could greatly enhance research on these proteins and their biological function. To this end, we designed a biochemical, high throughput screening assay with complementary follow-up assays to identify small molecule compounds inhibiting MgcRacGAP, a Rho family GTPase activating protein involved in cytokinesis and transcriptionally upregulated in many cancers. We first performed an in-house screen of 20,480 compounds, and later we tested the assay against 342,046 compounds from the NIH Molecular Libraries Small Molecule Repository. Primary screening hit rates were about 1% with the majority of those affecting the primary readout, an enzyme-coupled GDP detection assay. After orthogonal and counter screens, we identified two hits with high selectivity towards MgcRacGAP, compared with other RhoGAPs, and potencies in the low micromolar range. The most promising hit, termed MINC1, was then examined with cell-based testing where it was observed to induce an increased rate of cytokinetic failure and multinucleation in addition to other cell division defects, suggesting that it may act as an MgcRacGAP inhibitor also in cells.

The significance and scope of evolutionary developmental biology: a vision for the 21st century

Evolutionary developmental biology (evo-devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo-devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines-from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself-and discuss why evo-devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo-devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

Cellular and molecular mechanisms of single and collective cell migrations in Drosophila: themes and variations

The process of cell migration is essential throughout life, driving embryonic morphogenesis and ensuring homeostasis in adults. Defects in cell migration are a major cause of human disease, with excessive migration causing autoimmune diseases and cancer metastasis, whereas reduced capacity for migration leads to birth defects and immunodeficiencies. Myriad studies in vitro have established a consensus view that cell migrations require cell polarization, Rho GTPase-mediated cytoskeletal rearrangements, and myosin-mediated contractility. However, in vivo studies later revealed a more complex picture, including the discovery that cells migrate not only as single units but also as clusters, strands, and sheets. In particular, the role of E-Cadherin in cell motility appears to be more complex than previously appreciated. Here, we discuss recent advances achieved by combining the plethora of genetic tools available to the Drosophila geneticist with live imaging and biophysical techniques. Finally, we discuss the emerging themes such studies have revealed and ponder the puzzles that remain to be solved.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

Functional analysis of the Drosophila embryonic germ cell transcriptome by RNA interference

In Drosophila melanogaster, primordial germ cells are specified at the posterior pole of the very early embryo. This process is regulated by the posterior localized germ plasm that contains a large number of RNAs of maternal origin. Transcription in the primordial germ cells is actively down-regulated until germ cell fate is established. Bulk expression of the zygotic genes commences concomitantly with the degradation of the maternal transcripts. Thus, during embryogenesis, maternally provided and zygotically transcribed mRNAs determine germ cell development collectively. In an effort to identify novel genes involved in the regulation of germ cell behavior, we carried out a large-scale RNAi screen targeting both maternal and zygotic components of the embryonic germ line transcriptome. We identified 48 genes necessary for distinct stages in germ cell development. We found pebble and fascetto to be essential for germ cell migration and germ cell division, respectively. Our data uncover a previously unanticipated role of mei-P26 in maintenance of embryonic germ cell fate. We also performed systematic co-RNAi experiments, through which we found a low rate of functional redundancy among homologous gene pairs. As our data indicate a high degree of evolutionary conservation in genetic regulation of germ cell development, they are likely to provide valuable insights into the biology of the germ line in general.

Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway

Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. We have discovered a new mechanism for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. Our results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.

Functions and Mechanisms of Fibroblast Growth Factor (FGF) Signalling in Drosophila melanogaster

Intercellular signalling via growth factors plays an important role in controlling cell differentiation and cell movements during the development of multicellular animals. Fibroblast Growth Factor (FGF) signalling induces changes in cellular behaviour allowing cells in the embryo to move, to survive, to divide or to differentiate. Several examples argue that FGF signalling is used in multi-step morphogenetic processes to achieve and maintain a transitional state of the cells required for the control of cell fate. In the genetic model Drosophila melanogaster, FGF signalling via the receptor tyrosine kinases Heartless (Htl) and Breathless (Btl) is particularly well studied. These FGF receptors affect gene expression, cell shape and cell–cell interactions during mesoderm layer formation, caudal visceral muscle (CVM) formation, tracheal morphogenesis and glia differentiation. Here, we will address the current knowledge of the biological functions of FGF signalling in the fly on the tissue, at a cellular and molecular level.

Defective FGF signaling causes coloboma formation and disrupts retinal neurogenesis

The optic fissure (OF) is a transient opening on the ventral side of the developing vertebrate eye that closes before nearly all retinal progenitor cell differentiation has occurred. Failure to close the OF results in coloboma, a congenital disease that is a major cause of childhood blindness. Although human genetic studies and animal models have linked a number of genes to coloboma, the cellular and molecular mechanisms driving the closure of the OF are still largely unclear. In this study, we used Cre-LoxP-mediated conditional removal of fibroblast growth factor (FGF) receptors, Fgfr1 and Fgfr2, from the developing optic cup (OC) to show that FGF signaling regulates the closing of the OF. Our molecular, cellular and transcriptome analyses of Fgfr1 and Fgfr2 double conditional knockout OCs suggest that FGF signaling controls the OF closure through modulation of retinal progenitor cell proliferation, fate specification and morphological changes. Furthermore, Fgfr1 and Fgfr2 double conditional mutant retinal progenitor cells fail to initiate retinal ganglion cell (RGC) genesis. Taken together, our mouse genetic studies reveal that FGF signaling is essential for OF morphogenesis and RGC development.

Regulation of Drosophila mesoderm migration by phosphoinositides and the PH domain of the Rho GTP exchange factor Pebble

The Drosophila RhoGEF Pebble (Pbl) is required for cytokinesis and migration of mesodermal cells. In a screen for genes that could suppress migration defects in pbl mutants we identified the phosphatidylinositol phosphate (PtdInsP) regulator pi5k59B. Genetic interaction tests with other PtdInsP regulators suggested that PtdIns(4,5)P2 levels are important for mesoderm migration when Pbl is depleted. Consistent with this, the leading front of migrating mesodermal cells was enriched for PtdIns(4,5)P2. Given that Pbl contains a Pleckstrin Homology (PH) domain, a known PtdInsP-binding motif, we examined PtdInsP-binding of Pbl and the importance of the PH domain for Pbl function. In vitro lipid blot assays showed that Pbl binds promiscuously to PtdInsPs, with binding strength associated with the degree of phosphorylation. Pbl was also able to bind lipid vesicles containing PtdIns(4,5)P2 but binding was strongly reduced upon deletion of the PH domain. Similarly, in vivo, loss of the PH domain prevented localisation of Pbl to the cell cortex and severely affected several aspects of early mesoderm development, including flattening of the invaginated tube onto the ectoderm, extension of protrusions, and dorsal migration to form a monolayer. Pbl lacking the PH domain could still localise to the cytokinetic furrow, however, and cytokinesis failure was reduced in pbl(ΔPH) mutants. Taken together, our results support a model in which interaction of the PH-domain of Pbl with PtdIns(4,5)P2 helps localise it to the plasma membrane which is important for mesoderm migration.

Snail-dependent repression of the RhoGEF pebble is required for gastrulation consistency in Drosophila melanogaster

The Rho GTP exchange factor, Pebble (Pbl), long recognised as an essential activator of Rho during cytokinesis, also regulates mesoderm migration at gastrulation. Like other cell cycle components, pbl expression patterns broadly correlate with proliferative tissue. Surprisingly, in spite of its role in the early mesoderm, pbl is downregulated in the presumptive mesoderm before ventral furrow formation. Here, we show that this mesoderm-specific repression of pbl is dependent on the transcriptional repressor Snail (Sna). pbl repression was lost in sna mutants but was unaffected when Sna was ectopically expressed, showing that Sna is necessary, but not sufficient, for pbl repression. Using DamID, the first intron of pbl was identified as a Sna-binding region. Nine sites with the Sna-binding consensus motif CAGGT[GA] were identified in this intron. Mutating these to TAGGC[GA] abolished the ventral repression of pbl. Surprisingly, Sna-dependent repression of pbl was not essential for viability or fertility. Loss of repression did, however, increase the frequency of low-penetrance gastrulation defects. Consistent with this, expression of a pbl-GFP transgene in the presumptive mesoderm generated similar gastrulation defects. Finally, we show that a cluster of Snail-binding sites in the middle of the first intron of pbl orthologues is a conserved feature in the other 11 sequenced Drosophila species. We conclude that pbl levels are precisely regulated to ensure that there is enough protein available for its role in early mesoderm development but not so much as to inhibit the orderly progression of gastrulation.

Ect2, an ortholog of Drosophila Pebble, regulates formation of growth cones in primary cortical neurons

In collaboration with Marshall Nirenberg, we performed in vivo RNA interference (RNAi) genome-wide screening in Drosophila embryos. Pebble has been shown to be involved in Drosophila neuronal development. We have also reported that depletion of Ect2, a mammalian ortholog of Pebble, induces differentiation in NG108-15 neuronal cells. However, the precise role of Ect2 in neuronal development has yet to be studied. Here, we confirmed in PC12 pheochromocytoma cells that inhibition of Ect2 expression by RNAi stimulated neurite outgrowth, and in the mouse embryonic cortex that Ect2 was accumulated throughout the ventricular and subventricular zones with neuronal progenitor cells. Next, the effects of Ect2 depletion were studied in primary cultures of mouse embryonic cortical neurons: Loss of Ect2 did not affect the differentiation stages of neuritogenesis, the number of neurites, or axon length, while the numbers of growth cones and growth cone-like structures were increased. Taken together, our results suggest that Ect2 contributes to neuronal morphological differentiation through regulation of growth cone dynamics.

A Trio-RhoA-Shroom3 pathway is required for apical constriction and epithelial invagination

Epithelial invagination is a common feature of embryogenesis. An example of invagination morphogenesis occurs during development of the early eye when the lens placode forms the lens pit. This morphogenesis is accompanied by a columnar-to-conical cell shape change (apical constriction or AC) and is known to be dependent on the cytoskeletal protein Shroom3. Because Shroom3-induced AC can be Rock1/2 dependent, we hypothesized that during lens invagination, RhoA, Rock and a RhoA guanine nucleotide exchange factor (RhoA-GEF) would also be required. In this study, we show that Rock activity is required for lens pit invagination and that RhoA activity is required for Shroom3-induced AC. We demonstrate that RhoA, when activated and targeted apically, is sufficient to induce AC and that RhoA plays a key role in Shroom3 apical localization. Furthermore, we identify Trio as a RhoA-GEF required for Shroom3-dependent AC in MDCK cells and in the lens pit. Collectively, these data indicate that a Trio-RhoA-Shroom3 pathway is required for AC during lens pit invagination.

A functional genomic screen combined with time-lapse microscopy uncovers a novel set of genes involved in dorsal closure of Drosophila embryos

Morphogenesis, the establishment of the animal body, requires the coordinated rearrangement of cells and tissues regulated by a very strictly-determined genetic program. Dorsal closure of the epithelium in the Drosophila melanogaster embryo is one of the best models for such a complex morphogenetic event. To explore the genetic regulation of dorsal closure, we carried out a large-scale RNA interference-based screen in combination with in vivo time-lapse microscopy and identified several genes essential for the closure or affecting its dynamics. One of the novel dorsal closure genes, the small GTPase activator pebble (pbl), was selected for detailed analysis. We show that pbl regulates actin accumulation and protrusion dynamics in the leading edge of the migrating epithelial cells. In addition, pbl affects dorsal closure dynamics by regulating head involution, a morphogenetic process mechanically coupled with dorsal closure. Finally, we provide evidence that pbl is involved in closure of the adult thorax, suggesting its general requirement in epithelial closure processes.

Ect2, an ortholog of Drosophila's pebble, negatively regulates neurite outgrowth in neuroblastoma × glioma hybrid NG108-15 cells

To identify genes required for brain development, we previously performed in vivo RNA interference (RNAi) screening in Drosophila embryos. We identified pebble as a gene that disrupts development of the Drosophila nervous system. Although pebble has been shown to be involved in neuronal development of Drosophila in several screens, the involvement of Ect2, a mammalian ortholog of pebble, in mammalian neuronal development has not been addressed. To examine the role of Ect2 in neuronal differentiation, we performed Ect2 RNAi in the mouse neuroblastoma × rat glioma NG108-15 cell line. Depletion of Ect2 resulted in an increased proportion of binucleate cells and morphological differentiation of NG108-15 cells characterized by the outgrowth of neurites. These morphological changes were correlated with an increased level of acetylcholine esterase mRNA. In addition, expression of Ect2 was decreased in differentiated NG108-15 cells induced by dibutyryl cyclic AMP. These findings indicate that Ect2 negatively regulates the differentiation of NG108-15 cells and suggest that Ect2 may play a role in neuronal differentiation and brain development in vivo.

Imaging pluripotent cell migration in Drosophila

Drosophila melanogaster offers a powerful system for the analysis of cell migration. In the embryo, -pluripotent cells of the mesodermal and endodermal primordia undergo epithelial-mesenchymal transitions and cell migration, while primordial germ cells migrate through an endodermal barrier to form the gonads. Visualisation of these migrations has traditionally been achieved by staining fixed embryos at different developmental stages or through live imaging of cells using tissue-specific expression of marker fluorescent proteins. More recently, photoactivatable fluorescence proteins have allowed the labelling of small groups of cells or single cells so that their migratory patterns and fate can be followed. By fusing the photoactivatable fluorescent protein to proteins that mark different subcellular components, it is now possible to visualise different aspects of the cells as they migrate. Here, we review previous studies of the migration of pluripotent embryonic cells and describe, in detail, methods for visualising these cells.

The roles of CUX1 homeodomain proteins in the establishment of a transcriptional program required for cell migration and invasion

The Cut homeobox gene 1 (CUX1) codes for several homeodomain proteins that display distinct DNA binding and transcriptional properties. Some CUX1 isoforms were previously shown to stimulate entry into S phase. More recently, siRNA-mediated knockdown of CUX1 was shown to cause a decrease in cell migration and invasion. In contrast, ectopic expression of p110 or p75 CUX1 stimulated cell migration and invasion in tissue culture. Moreover, metastasis to the lung was observed in a few cases following development of mammary tumors in p75 CUX1 transgenic mice. Chromatin immunoprecipitation (ChIP) assays followed by hybridization on promoter arrays (ChIP-chip) led to the identification of more than 20 genes that are directly regulated by CUX1 and code for proteins involved in cytoskeleton remodeling, cell-cell and cell-matrix adhesion, epithelial to mesenchymal transition and transcriptional regulation. Many targets of CUX1 are regulators of Rho GTPases that play a role both in cell cycle progression and cell motility. Interestingly, some genes that promote cell motility are activated by CUX1, while some genes that inhibit cell motility are repressed by CUX1. The dual function of CUX1 as an activator and repressor is best exemplified by the regulatory cascade whereby CUX1 activates expression of the Snail and Slug transcription factors and then cooperates with them to repress the E-cadherin and occludin genes, thereby causing a severe disorganization of cell-cell junctions. Together, these studies indicate that CUX1 stimulates cell motility by regulating a large number of genes involved in various molecular functions.

Signalling through the RhoGEF Pebble in Drosophila

Small GTPase pathways of the Ras superfamily are implicated in a wide range of signalling processes in animal cells. Small GTPases control pathways by acting as molecular switches. They are converted from an inactive GDP-bound form to an active GTP-bound form by GTP exchange factors (GEFs). The spatial and temporal regulation of GEFs is a major component of the regulation of small GTPases. Here we review the role of the Drosophila RhoGEF, Pebble (the Drosophila ortholog of mammalian ECT2). We discuss its roles in cytokinesis and cell migration, highlighting the diversity with which Rho family signalling pathways operate in biological systems.

Epithelial to mesenchymal transition in human skin wound healing is induced by tumor necrosis factor-alpha through bone morphogenic protein-2

Epithelial-mesenchymal transition (EMT), characterized by loss of epithelial adhesion and gain of mesenchymal features, is an important mechanism to empower epithelial cells into the motility that occurs during embryonic development and recurs in cancer and fibrosis. Whether and how EMT occurs in wound healing and fibrosis in human skin remains unknown. In this study we found that migrating epithelial cells in wound margins and deep epithelial ridges had gained mesenchymal features such as vimentin and FSP1 expression. In hypertrophic scars, EMT-related genes were elevated along with inflammatory cytokines, indicating a causal relationship. To reconstitute EMT in vitro, normal human skin and primary keratinocytes were exposed to cytokines such as tumor necrosis factor-alpha (TNF-alpha), resulting in expression of vimentin, FSP1, and matrix metalloproteinases. Moreover, TNF-alpha-induced EMT was impaired by antagonists against bone morphogen proteins (BMP) 2/4, suggesting that BMP mediates the TNF-alpha-induced EMT in human skin. Indeed, TNF-alpha could induce BMP-2 and its receptor (BMPR1A) in human skin and primary keratinocytes, and BMP2 could induce EMT features in skin explants and primary keratinocytes. In summary, we uncovered EMT features in both acute and fibrotic cutaneous wound healing of human skin. Moreover, we propose that the mesenchymal induction in wound healing is motivated by TNF-alpha, in part, through induction of BMP.

Quantitative imaging of collective cell migration during Drosophila gastrulation: multiphoton microscopy and computational analysis

This protocol describes imaging and computational tools to collect and analyze live imaging data of embryonic cell migration. Our five-step protocol requires a few weeks to move through embryo preparation and four-dimensional (4D) live imaging using multi-photon microscopy, to 3D cell tracking using image processing, registration of tracking data and their quantitative analysis using computational tools. It uses commercially available equipment and requires expertise in microscopy and programming that is appropriate for a biology laboratory. Custom-made scripts are provided, as well as sample datasets to permit readers without experimental data to carry out the analysis. The protocol has offered new insights into the genetic control of cell migration during Drosophila gastrulation. With simple modifications, this systematic analysis could be applied to any developing system to define cell positions in accordance with the body plan, to decompose complex 3D movements and to quantify the collective nature of cell migration.

Regulation of the Rac GTPase pathway by the multifunctional Rho GEF Pebble is essential for mesoderm migration in the Drosophila gastrula

The Drosophila guanine nucleotide exchange factor Pebble (Pbl) is essential for cytokinesis and cell migration during gastrulation. In dividing cells, Pbl promotes Rho1 activation at the cell cortex, leading to formation of the contractile actin-myosin ring. The role of Pbl in fibroblast growth factor-triggered mesoderm spreading during gastrulation is less well understood and its targets and subcellular localization are unknown. To address these issues we performed a domain-function study in the embryo. We show that Pbl is localized to the nucleus and the cell cortex in migrating mesoderm cells and found that, in addition to the PH domain, the conserved C-terminal tail of the protein is crucial for cortical localization. Moreover, we show that the Rac pathway plays an essential role during mesoderm migration. Genetic and biochemical interactions indicate that during mesoderm migration, Pbl functions by activating a Rac-dependent pathway. Furthermore, gain-of-function and rescue experiments suggest an important regulatory role of the C-terminal tail of Pbl for the selective activation of Rho1-versus Rac-dependent pathways.

CUX1 and E2F1 regulate coordinated expression of the mitotic complex genes Ect2, MgcRacGAP, and MKLP1 in S phase

Rho GTPases are critical for mitosis progression and completion of cytokinesis. During mitosis, the GDP/GTP cycle of Rho GTPases is regulated by the exchange factor Ect2 and the GTPase activating protein MgcRacGAP which associates with the kinesin MKLP1 in the centralspindlin complex. We report here that expression of Ect2, MgcRacGAP, and MKLP1 is tightly regulated during cell cycle progression. These three genes share similar cell cycle-related signatures within their promoter regions: (i) cell cycle gene homology region (CHR) sites located at -20 to +40 nucleotides of their transcription start sites that are required for repression in G(1), (ii) E2F binding elements, and (iii) tandem repeats of target sequences for the CUX1 transcription factor. CUX1 and E2F1 bind these three promoters upon S-phase entry, as demonstrated by chromatin immunoprecipitation, and regulate transcription of these genes, as established using promoter-luciferase reporter constructs and expression of activated or dominant negative transcription factors. Overexpression of either E2F1 or CUX1 increased the levels of the endogenous proteins whereas small interfering RNA knockdown of E2F1 or use of a dominant negative E2F1 reduced their expression levels. Thus, CUX1, E2F, and CHR elements provide the transcriptional controls that coordinate induction of Ect2, MgcRacGAP, and MKLP1 in S phase, leading to peak expression of these interacting proteins in G(2)/M, at the time they are required to regulate cytokinesis.

Transitions between epithelial and mesenchymal states in development and disease

The ancestors of modern Metazoa were constructed in large part by the foldings and distortions of two-dimensional sheets of epithelial cells. This changed approximately 600 million years ago with the evolution of mesenchymal cells. These cells arise as the result of epithelial cell delamination through a reprogramming process called an epithelial to mesenchymal transition (EMT) [Shook D, Keller R. Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development. Mech Dev 2003;120:1351-83; Thiery JP, Sleeman JP. Complex networks orchestrate epithelial-mesenchymal transitions. Nat Rev Mol Cell Biol 2006;7:131-42]. Because mesenchymal cells are free to migrate through the body cavity, the evolution of the mesenchyme opened up new avenues for morphological plasticity, as cells evolved the ability to take up new positions within the embryo and to participate in novel cell-cell interactions; forming new types of internal tissues and organs such as muscle and bone [Thiery JP, Sleeman, JP. Complex networks orchestrate epithelial-mesenchymal transitions. Nat Rev Mol Cell Biol 2006;7:131-42; Hay ED, Zuk A. Transformations between epithelium and mesenchyme: normal, pathological, and experimentally induced. Am J Kidney Dis 1995;26:678-90]. After migrating to a suitable site, mesenchymal cells coalesce and re-polarize to form secondary epithelia, in a so-called mesenchymal-epithelial transition (MET). Such switches between mesenchymal and epithelial states are a frequent feature of Metazoan gastrulation [Hay ED, Zuk A. Transformations between epithelium and mesenchyme: normal, pathological, and experimentally induced. Am J Kidney Dis 1995;26:678-90] and the neural crest lineage [Duband JL, Monier F, Delannet M, Newgreen D. Epitheliu-mmesenchyme transition during neural crest development. Acta Anat 1995;154:63-78]. Significantly, however, when hijacked during the development of cancer, the ability of cells to undergo EMT, to leave the primary tumor and to undergo MET at secondary sites can have devastating consequences on the organism, allowing tumor cells derived from epithelia to invade surrounding tissues and spread through the host [Thiery JP, Sleeman JP. Complex networks orchestrate epithelial-mesenchymal transitions. Nat Rev Mol Cell Biol 2006;7:131-42; Hay ED, Zuk A. Transformations between epithelium and mesenchyme: normal, pathological, and experimentally induced. Am J Kidney Dis 1995;26:678-90]. Thus, the molecular and cellular mechanisms underpinning EMT are both an essential feature of Metazoan development and an important area of biomedical research. In this review, we discuss the common molecular and cellular mechanisms involved in EMT in both cases.

Photoactivatable GFP resolves Drosophila mesoderm migration behaviour

Mesoderm migration is a pivotal event in the early embryonic development of animals. One of the best-studied examples occurs during Drosophila gastrulation. Here, mesodermal cells invaginate, undergo an epithelial-to-mesenchymal transition (EMT), and spread out dorsally over the inner surface of the ectoderm. Although several genes required for spreading have been identified, our inability to visualise mesodermal cells in living embryos has left us to speculate about the cell rearrangements involved. Several mechanisms, such as chemotaxis towards a dorsally expressed attractant, differential affinity between mesodermal cells and the ectoderm, and convergent extension, have been proposed. Here we resolve the behaviour of Drosophila mesodermal cells in live embryos using photoactivatable-GFP fused to alpha-Tubulin (PAGFP-Tub). By photoactivating presumptive mesodermal cells before gastrulation, we could observe their migration over non-fluorescent ectodermal cells. We show that the outermost (outer) cells, which are in contact with the ectoderm, migrate dorsolaterally as a group but can be overtaken by more internal (inner) cells. Using laser-photoactivation of individual cells, we then show that inner cells adjacent to the centre of the furrow migrate dorsolaterally away from the midline to reach dorsal positions, while cells at the centre of the furrow disperse randomly across the mesoderm, before intercalating with outer cells. These movements are dependent on the FGF receptor Heartless. The results indicate that chemotactic movement and differential affinity are the primary drivers of mesodermal cell spreading. These characterisations pave the way for a more detailed analysis of gene function during early mesoderm development.

Nucleotide exchange factor ECT2 regulates epithelial cell polarity

Cell polarity regulates diverse biological events such as localization of embryonic determinants and establishment of tissue and organ architecture. Epithelial cell polarity is regulated by the polarity complex Par6/Par3/atypical protein kinase C (aPKC). We previously found that the nucleotide exchange factor ECT2 associates with this polarity complex and regulates aPKC activity, but the role of ECT2 in cell polarity is still unclear. Here we show that expression of a dominant negative (ECT2-N2) or constitutively active (ECT2-DeltaN5) form of ECT2 inhibits normal cyst formation of MDCK cells in 3-dimensional collagen gels. Central lumens were not observed in cysts formed by cells expressing either ECT2-DeltaN5 or ECT2-N2. Apical localization of ZO-1 and basolateral localization of beta-catenin were no longer observed in these cells. Interestingly, cells expressing ECT2-N2 did form normal cysts when cultured in the basement membrane matrix Matrigel instead of collagen gels. Addition of a major Matrigel component, laminin, partially rescued the normal cyst formation inhibited by ECT2-N2 in 3-dimensional collagen gels. Thus, signaling through laminin might override the defects of signaling through collagen and ECT2. Whereas ECT2-N2 inhibited the lumen formation of MDCK cysts, caspase-3, which is reportedly involved in lumen formation through apoptosis, was activated at various locations of cells in the cysts. It is likely that perturbation of ECT2 signaling inhibits the establishment of epithelial cell polarity leading to the inhibition of selected elimination of cells at the center of cysts. Thus, ECT2 appears to play a critical role in epithelial cell polarity.

Complex networks orchestrate epithelial-mesenchymal transitions

Epithelial-mesenchymal transition is an indispensable mechanism during morphogenesis, as without mesenchymal cells, tissues and organs will never be formed. However, epithelial-cell plasticity, coupled to the transient or permanent formation of mesenchyme, goes far beyond the problem of cell-lineage segregation. Understanding how mesenchymal cells arise from an epithelial default status will also have a strong impact in unravelling the mechanisms that control fibrosis and cancer progression.

Tum/RacGAP50C provides a critical link between anaphase microtubules and the assembly of the contractile ring in Drosophila melanogaster

A central question in understanding cytokinesis is how the cleavage plane is positioned. Although the positioning signal is likely to be transmitted via the anaphase microtubule array to the cell cortex, exactly how the microtubule array determines the site of contractile ring formation remains unresolved. By analysing tum/RacGAP50C mutant Drosophila embryos we show that cells lacking Tum do not form furrows and fail to localise the key cytokinetic components Pebble (a RhoGEF), Aurora B kinase, Diaphanous, Pav-KLP and Anillin. The GAP activity of Tum is required for cytokinesis: in its absence cytokinesis fails early even though Tum is present on microtubules at the cell equator where the furrow should form. Disruption of the Pebble-interacting domain leaves Tum localised to the cell equator on cortically associated microtubules, again with no evidence of furrowing. These data support a model in which Tum/RacGAP, via its interaction with Pbl, provides a critical link between the anaphase microtubule spindle and cytokinetic furrow formation in Drosophila cells.

The Caenorhabditis elegans ect-2 RhoGEF gene regulates cytokinesis and migration of epidermal P cells

A reduction-of-function mutation in ect-2 was isolated as a suppressor of the Multivulva phenotype of a lin-31 mutation. Analysis using markers indicates that this mutation causes defects in both the cytokinesis and migration of epidermal P cells, phenotypes similar to those caused by expressing a rho-1 dominant-negative construct. ect-2 encodes the Caenorhabditis elegans orthologue of the mouse Ect2 and Drosophila Pebble that function as guanine nucleotide exchange factors (GEFs) for Rho GTPases. The ect-2Colon, two colonsGFP reporter is expressed in embryonic cells and P cells. RNA interference of ect-2 causes sterility and embryonic lethality, in addition to the P-cell defects. We have determined the lesions of two ect-2 alleles, and described their differences in phenotypes in specific tissues. We propose a model in which ECT-2GEF not only activates RHO-1 for P-cell cytokinesis, but also collaborates with UNC-73GEF and at least two Rac proteins to regulate P-cell migration.

The Caenorhabditis elegans homologue of the proto-oncogene ect-2 positively regulates RAS signalling during vulval development

Guanine nucleotide exchange factors (GEFs) regulate the activity of small GTP-binding proteins in a variety of biological processes. We have identified a gain-of-function mutation in the Caenorhabditis elegans GEF ect-2, the homologue of the mammalian ect2 proto-oncogene that has an essential role during cytokinesis. Here, we report that, in addition to its known function during mitosis, ECT-2 promotes the specification of the primary vulval cell fate by activating RAS/mitogen-activated protein kinase (MAPK) signalling before the end of the S-phase. Epistasis analysis indicates that ECT-2 crosstalks to the canonical RAS/MAPK cascade upstream of the RAS GEF SOS-1 by means of a RHO-1 signalling pathway. Our results raise the possibility that the transforming activity of the mammalian ect-2 oncogene could be due to hyperactivation of the RAS/MAPK pathway.

beta-Catenin-dependent Wnt signalling controls the epithelial organisation of somites through the activation of paraxis

The regulation of cell adhesion in epithelia is a fundamental process governing morphogenesis in embryos and a key step in the progression of invasive cancers. Here, we have analysed the molecular pathways controlling the epithelial organisation of somites. Somites are mesodermal epithelial structures of vertebrate embryos that undergo several changes in cell adhesion during early embryonic life. We show that Wnt6 in the ectoderm overlaying the somites, but not Wnt1 in the neighbouring neural tube, is the most likely candidate molecule responsible for the maintenance of the epithelial structure of the dorsal compartment of the somite: the dermomyotome. We characterised the signalling pathway that mediates Wnt6 activity. Our experiments suggest that the Wnt receptor molecule Frizzled7 probably transduces the Wnt6 signal. Intracellularly, this leads to the activation of the beta-catenin/LEF1-dependent pathway. Finally, we demonstrate that the bHLH transcription factor paraxis, which was previously shown to be a major player in the epithelial organisation of somites, is a target of the beta-catenin signal. We conclude that beta-catenin activity, initiated by Wnt6 and mediated by paraxis, is required for the maintenance of the epithelial structure of somites.