The Rho GTPase and a putative RhoGEF mediate a signaling pathway for the cell shape changes in Drosophila gastrulation

Rho GTPase function in flies: insights from a developmental and organismal perspective

Morphogenesis is a key event in the development of a multicellular organism and is reliant on coordinated transcriptional and signal transduction events. To establish the segmented body plan that underlies much of metazoan development, individual cells and groups of cells must respond to exogenous signals with complex movements and shape changes. One class of proteins that plays a pivotal role in the interpretation of extracellular cues into cellular behavior is the Rho family of small GTPases. These molecular switches are essential components of a growing number of signaling pathways, many of which regulate actin cytoskeletal remodeling. Much of our understanding of Rho biology has come from work done in cell culture. More recently, the fruit fly Drosophila melanogaster has emerged as an excellent genetic system for the study of these proteins in a developmental and organismal context. Studies in flies have greatly enhanced our understanding of pathways involving Rho GTPases and their roles in development.

PDZ-containing proteins: alternative splicing as a source of functional diversity

Scaffold proteins allow specific protein complexes to be assembled in particular regions of the cell at which they organize subcellular structures and signal transduction complexes. This characteristic is especially important for neurons, which are highly polarized cells. Among the domains contained by scaffold proteins, the PSD-95, Discs-large, ZO-1 (PDZ) domains are of particular relevance in signal transduction processes and maintenance of neuronal and epithelial polarity. These domains are specialized in the binding of the carboxyl termini of proteins allowing membrane proteins to be localized by the anchoring to the cytoskeleton mediated by PDZ-containing scaffold proteins. In vivo studies carried out in Drosophila have taught that the role of many scaffold proteins is not limited to a single process; thus, in many cases the same genes are expressed in different tissues and participate in apparently very diverse processes. In addition to the differential expression of interactors of scaffold proteins, the expression of variants of these molecular scaffolds as the result of the alternative processing of the genes that encode them is proving to be a very important source of variability and complexity on a main theme. Alternative splicing in the nervous system is well documented, where specific isoforms play roles in neurotransmission, ion channel function, neuronal cell recognition, and are developmentally regulated making it a major mechanism of functional diversity. Here we review the current state of knowledge about the diversity and the known function of PDZ-containing proteins in Drosophila with emphasis in the role played by alternatively processed forms in the diversity of functions attributed to this family of proteins.

RhoGEF2 and the formin Dia control the formation of the furrow canal by directed actin assembly duringDrosophilacellularisation

The physical interaction of the plasma membrane with the associated cortical cytoskeleton is important in many morphogenetic processes during development. At the end of the syncytial blastoderm of Drosophila the plasma membrane begins to fold in and forms the furrow canals in a regular hexagonal pattern. Every furrow canal leads the invagination of membrane between adjacent nuclei. Concomitantly with furrow canal formation, actin filaments are assembled at the furrow canal. It is not known how the regular pattern of membrane invagination and the morphology of the furrow canal is determined and whether actin filaments are important for furrow canal formation. We show that both the guanyl-nucleotide exchange factor RhoGEF2 and the formin Diaphanous (Dia) are required for furrow canal formation. In embryos from RhoGEF2 or dia germline clones, furrow canals do not form at all or are considerably enlarged and contain cytoplasmic blebs. Both Dia and RhoGEF2 proteins are localised at the invagination site prior to formation of the furrow canal. Whereas they localise independently of F-actin,Dia localisation requires RhoGEF2. The amount of F-actin at the furrow canal is reduced in dia and RhoGEF2 mutants,suggesting that RhoGEF2 and Dia are necessary for the correct assembly of actin filaments at the forming furrow canal. Biochemical analysis shows that Rho1 interacts with both RhoGEF2 and Dia, and that Dia nucleates actin filaments. Our results support a model in which RhoGEF2 and dia control position, shape and stability of the forming furrow canal by spatially restricted assembly of actin filaments required for the proper infolding of the plasma membrane.

Gastrulation Movements: the Logic and the Nuts and Bolts

Gastrulation, the period during the early development of animals when major cell and tissue movements remodel an initially unstructured group of cells, requires coordinated control of different types of cellular activities in different cell populations. A hierarchy of genetic control mechanisms, involving cell signaling and transcriptional regulation, sets up the embryonic axes and specify the territories of the future germ layers. Cells in these territories modulate their cytoskeleton and their adhesive behavior, resulting in shape changes and movement. Similarities among different species in patterning and cell biological mechanisms are beginning to allow us to recognize general, conserved principles and speculate on possible ancestral mechanisms of gastrulation.

Interplay of mechanical deformation and patterned gene expression in developing embryos

The shaping of the early embryo requires pattern formation as well as geometric and topological morphogenesis of the developing tissues. The morphogenetic movements that lead to geometric shape changes are controlled by patterned gene expression. How particular movements are related to patterning genes, and which underlying molecular and cellular mechanisms lead to coordinated macroscopic movements that induce morphogenesis, remain the challenging questions of embryonic development. How morphogenetic movements could modulate the expression of developmental genes is an emerging question, potentially opening new horizons in developmental biology. This question instigates the task of characterizing the molecular and cellular mechanisms underlying these mechano-transcription events.

DRhoGEF2 regulates actin organization and contractility in the Drosophila blastoderm embryo

Morphogenesis of the Drosophila melanogaster embryo is associated with a dynamic reorganization of the actin cytoskeleton that is mediated by small GTPases of the Rho family. Often, Rho1 controls different aspects of cytoskeletal function in parallel, requiring a complex level of regulation. We show that the guanine triphosphate (GTP) exchange factor DRhoGEF2 is apically localized in epithelial cells throughout embryogenesis. We demonstrate that DRhoGEF2, which has previously been shown to regulate cell shape changes during gastrulation, recruits Rho1 to actin rings and regulates actin distribution and actomyosin contractility during nuclear divisions, pole cell formation, and cellularization of syncytial blastoderm embryos. We propose that DRhoGEF2 activity coordinates contractile actomyosin forces throughout morphogenesis in Drosophila by regulating the association of myosin with actin to form contractile cables. Our results support the hypothesis that specific aspects of Rho1 function are regulated by specific GTP exchange factors.

Drosophila RhoGEF2 associates with microtubule plus ends in an EB1-dependent manner

Members of the Rho/Rac/Cdc42 superfamily of GTPases and their upstream activators, guanine nucleotide exchange factors (GEFs) , have emerged as key regulators of actin and microtubule dynamics. In their GTP bound form, these proteins interact with downstream effector molecules that alter actin and microtubule behavior. During Drosophila embryogenesis, a Galpha subunit (Concertina) and a Rho-type guanine nucleotide exchange factor (DRhoGEF2) have been implicated in the dramatic epithelial-cell shape changes that occur during gastrulation and morphogenesis . Using Drosophila S2 cells as a model system, we show that DRhoGEF2 induces contractile cell shape changes by stimulating myosin II via the Rho1 pathway. Unexpectedly, we found that DRhoGEF2 travels to the cell cortex on the tips of growing microtubules by interaction with the microtubule plus-end tracking protein EB1. The upstream activator Concertina, in its GTP but not GDP bound form, dissociates DRhoGEF2 from microtubule tips and also causes cellular contraction. We propose that DRhoGEF2 uses microtubule dynamics to search for cortical subdomains of receptor-mediated Galpha activation, which in turn causes localized actomyosin contraction associated with morphogenetic movements during development.

Rho GTPases regulate axon growth through convergent and divergent signaling pathways

Rho GTPases are essential regulators of cytoskeletal reorganization, but how they do so during neuronal morphogenesis in vivo is poorly understood. Here we show that the actin depolymerization factor cofilin is essential for axon growth in Drosophila neurons. Cofilin function in axon growth is inhibited by LIM kinase and activated by Slingshot phosphatase. Dephosphorylating cofilin appears to be the major function of Slingshot in regulating axon growth in vivo. Genetic data provide evidence that Rho or Rac/Cdc42, via effector kinases Rok or Pak, respectively, activate LIM kinase to inhibit axon growth. Importantly, Rac also activates a Pak-independent pathway that promotes axon growth, and different RacGEFs regulate these distinct pathways. These genetic analyses reveal convergent and divergent pathways from Rho GTPases to the cytoskeleton during axon growth in vivo and suggest that different developmental outcomes could be achieved by biases in pathway selection.

Compartmentalized morphogenesis in epithelia: From cell to tissue shape

During development, embryonic tissues are shaped in a species-specific manner. Yet, across species, general classes of tissue remodeling events occur, such as tissue infolding and tissue elongation. The spatiotemporal control of these morphogenetic processes is responsible for the organization of different body plans, as well as for organogenesis. Cell morphogenesis in a mesenchyme contributes to the shaping of embryonic tissues. Epithelial cells, despite that they need to maintain an apicobasal organization, play an equally important role during morphogenesis. Moving from apical to basal, we review compartmentalized cellular rearrangements underlying tissue remodeling in Drosophila and compare them with those found in other organisms. Contractile activity at the apical surface triggers tissue folding and invagination. The regulation of adhesion at adherens junctions controls polarized neighbor exchange during intercalation and tissue elongation. Basolateral protrusive activity underlies other cases of intercalation. These localized cell shape changes are spatially regulated by developmental signals. Some signals define a local change in cell behavior (e.g., apical constriction), others orient a dynamic process in the plane of the tissue (e.g., junction remodeling).

A Rho GTPase Signaling Pathway Is Used Reiteratively in Epithelial Folding and Potentially Selects the Outcome of Rho Activation

A single Rho GTPase family member is capable of initiating several different processes, including cell cycle regulation, cytokinesis, cell migration, and transcriptional regulation . It is not clear, however, how the Rho protein selects which of these processes to initiate. Guanine nucleotide exchange factors (GEFs), proteins that activate Rho GTPases, could be important in making this selection. We show here that in vivo, DRhoGEF2, a GEF that is ubiquitously expressed and specific for Rho1, is reiteratively required for epithelial folding and invagination, but not for other processes regulated by Rho. The limitation of DRhoGEF2 function supports the hypothesis that the GEF selects the outcome of Rho activation. DRhoGEF2 exerts its effects in gastrulation through the regulation of Myosin II to orchestrate coordinated apical cell constriction. Apical myosin localization is also regulated by Concertina (Cta), a Galpha(12/13) family member that is thought to activate DRhoGEF2 and is itself activated by a putative ligand, Folded gastrulation (Fog). Fog and Cta also play a role in the morphogenetic events requiring DRhoGEF2, suggesting the existence of a conserved signaling pathway in which Fog, Cta, and DRhoGEF2 locally activate Myosin for epithelial invagination and folding.

Survival of developing motor neurons mediated by Rho GTPase signaling pathway through Rho-kinase

A variety of neurons generated during embryonic development survive or undergo programmed cell death (PCD) at later developmental stages. Survival or death of developing neurons is generally considered to depend on trophic support from various target tissues. The small GTPase Rho regulates diverse cellular processes such as cell morphology, cell adhesion, cell motility, and apoptosis. Rho-dependent serine-threonine protein kinase (Rho-kinase-ROK-ROCK), one of the effector proteins, transmits signals for some Rho-mediated processes. Here, we report the in vivo role of the Rho signaling pathway through Rho-kinase during development of motor neurons (MNs) in the spinal cord. We performed conditional expression of a dominant-negative form for RhoA (RhoA DN) or for Rho-kinase (Rho-K DN) in transgenic mice by using the Cre-loxP system to suppress the activity of these signaling molecules in developing MNs. Expression of RhoA DN reduced the number of MNs in the spinal cord because of increased apoptosis while preserving the gross patterning of motor axons. Expression of Rho-K DN produced developmental defects similar to those observed in RhoA DN expression. In addition, analysis of transgenic mice expressing Rho-K DN showed that the increased apoptosis of MNs was induced at the early embryonic stages before the initiation of PCD, and that MN death at the late embryonic stages corresponding to the period of PCD was moderately enhanced in the transgenic mice. These findings indicate that the Rho signaling pathway, primarily through Rho-kinase, plays a crucial role in survival of spinal MNs during embryogenesis, particularly at the early developmental stages.

Mechanisms, mechanics and function of epithelial-mesenchymal transitions in early development

Epithelial-mesenchymal transitions (EMTs) are an important mechanism for reorganizing germ layers and tissues during embryonic development. They have both a morphogenic function in shaping the embryo and a patterning function in bringing about new juxtapositions of tissues, which allow further inductive patterning events to occur [Genesis 28 (2000) 23]. Whereas the mechanics of EMT in cultured cells is relatively well understood [reviewed in Biochem. Pharmacol. 60 (2000) 1091; Cell 105 (2001) 425; Bioessays 23 (2001) 912], surprisingly little is known about EMTs during embryonic development [reviewed in Acta Anat. 154 (1995) 8], and nowhere is the entire process well characterized within a single species. Embryonic (developmental) EMTs have properties that are not seen or are not obvious in culture systems or cancer cells. Developmental EMTs are part of a specific differentiative path and occur at a particular time and place. In some types of embryos, a relatively intact epithelium must be maintained while some of its cells de-epithelialize during EMT. In most cases de-epithelialization (loss of apical junctions) must occur in an orderly, patterned fashion in order that the proper morphogenesis results. Interestingly, we find that de-epithelialization is not always necessarily tightly coupled to the expression of mesenchymal phenotypes.Developmental EMTs are multi-step processes, though the interdependence and obligate order of the steps is not clear. The particulars of the process vary between tissues, species, and specific embryonic context. We will focus on 'primary' developmental EMTs, which are those occurring in the initial epiblast or embryonic epithelium. 'Secondary' developmental EMT events are those occurring in epithelial tissues that have reassembled within the embryo from mesenchymal cells. We will review and compare a number of primary EMT events from across the metazoans, and point out some of the many open questions that remain in this field.

The repressor function of snail is required for Drosophila gastrulation and is not replaceable by Escargot or Worniu

Mesoderm formation in the Drosophila embryo depends on the maternal Toll signaling pathway. The Toll pathway establishes the Dorsal nuclear gradient, which regulates many zygotic genes to establish the mesodermal fate and promote the invagination of ventral cells. An important target gene of Dorsal is snail, which is required for proper mesoderm invagination. The Snail protein contains five zinc fingers and is a transcriptional repressor. However, it is not clear whether repressing target genes is a requirement for Snail to control ventral invagination. To examine such requirement, we conducted a series of genetic rescue experiments in snail mutant embryos. Snail, Worniu, and Escargot are closely related zinc-finger proteins and have equal functions during neuroblast development. However, among these three proteins, only Snail can rescue the mesoderm invagination phenotype. Moreover, the ability of various Snail mutant constructs to repress gene expression correlates with their ability to control invagination. This unique property of Snail in mesoderm formation can be attributed mostly to the CtBP co-repressor interaction motifs in the N-terminus, not to the C-terminal DNA-binding zinc fingers. Ectopic expression of Snail outside the ventral domain is not sufficient to induce cell movement even though repression of target genes still occurs. Together, the results show that the repressor function of Snail is essential for gastrulation. The repression of target genes by Snail may permit other factors in the ventral cells to positively promote mesoderm invagination.

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

Drosophila pebble (pbl) encodes a Rho-family GTP exchange factor (GEF) required for cytokinesis. The accumulation of high levels of PBL protein during interphase and the developmentally regulated expression of pbl in mesodermal tissues suggested that the primary cytokinetic mutant phenotype might be masking other roles. Using various muscle differentiation markers, we found that Even skipped (EVE) expression in the dorsal mesoderm is greatly reduced in pbl mutant embryos. EVE expression in the dorsalmost mesodermal cells is induced in response to DPP secreted by the dorsal epidermal cells. Further analysis revealed that this phenotype is likely to be a consequence of an earlier defect. pblmutant mesodermal cells fail to undergo the normal epithelial-mesenchymal transition (EMT) and dorsal migration that follows ventral furrow formation. This phenotype is not a secondary consequence of failed cytokinesis, as it is rescued by a mutant form of pbl that does not rescue the cytokinetic defect. In wild-type embryos, newly invaginated cells at the lateral edges of the mesoderm extend numerous protrusions. In pbl mutant embryos,however, cells appear more tightly adhered to their neighbours and extend very few protrusions. Consistent with the dependence of the mesoderm EMT and cytokinesis on actin organisation, the GTP exchange function of the PBL RhoGEF is required for both processes. By contrast, the N-terminal BRCT domains of PBL are required only for the cytokinetic function of PBL. These studies reveal that a novel PBL-mediated intracellular signalling pathway operates in mesodermal cells during the transition from an epithelial to migratory mesenchymal morphology during gastrulation.

Shroom induces apical constriction and is required for hingepoint formation during neural tube closure

BACKGROUND

The morphogenetic events of early vertebrate development generally involve the combined actions of several populations of cells, each engaged in a distinct behavior. Neural tube closure, for instance, involves apicobasal cell heightening, apical constriction at hingepoints, convergent extension of the midline, and pushing by the epidermis. Although a large number of genes are known to be required for neural tube closure, in only a very few cases has the affected cell behavior been identified. For example, neural tube closure requires the actin binding protein Shroom, but the cellular basis of Shroom function and how it influences neural tube closure remain to be elucidated.

RESULTS

We show here that expression of Shroom is sufficient to organize apical constriction in transcriptionally quiescent, naive epithelial cells but not in non-polarized cells. Shroom-induced apical constriction was associated with enrichment of apically localized actin filaments and required the small GTPase Rap1 but not Rho. Endogenous Xenopus shroom was found to be expressed in cells engaged in apical constriction. Consistent with a role for Shroom in organizing apical constriction, disrupting Shroom function resulted in a specific failure of hingepoint formation, defective neuroepithelial sheet-bending, and failure of neural tube closure.

CONCLUSIONS

These data demonstrate that Shroom is an essential regulator of apical constriction during neurulation. The finding that a single protein can initiate this process in epithelial cells establishes that bending of epithelial sheets may be patterned during development by the regulation of expression of single genes.

RhoA/ROCK Signaling Suppresses Hypertrophic Chondrocyte Differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes is required for normal growth and development of the endochondral skeleton, but little is known about the intracellular signal transduction pathways regulating these processes. We have investigated the roles of the GTPase RhoA and its effector kinases ROCK1/2 in hypertrophic chondrocyte differentiation. RhoA, ROCK1, and ROCK2 are expressed throughout chondrogenic differentiation. RhoA overexpression in chondrogenic ATDC5 cells results in increased proliferation and a marked delay of hypertrophic differentiation, as shown by decreased induction of alkaline phosphatase activity, mineralization, and expression of the hypertrophic markers collagen X, bone sialoprotein, and matrix metalloproteinase 13. These effects are accompanied by activation of cyclin D1 transcription and repression of the collagen X promoter by RhoA. In contrast, inhibition of Rho/ROCK signaling by the pharmacological inhibitor Y27632 inhibits chondrocyte proliferation and accelerates hypertrophic differentiation. Dominant-negative RhoA also inhibits induction of the cyclin D1 promoter by parathyroid hormone-related peptide. Finally, Y27632 treatment partially rescues the effects of RhoA overexpression. In summary, we identify the RhoA/ROCK signaling pathway as a novel and important regulator of chondrocyte proliferation and differentiation.

Rho activity critically and selectively regulates endothelial cell organization during angiogenesis

The mechanisms that control organization of endothelial cells (ECs) into new blood vessels are poorly understood. We hypothesized that the GTPase Rho, which regulates cytoskeletal architecture, is important for EC organization during neovascularization. To test this hypothesis, we designed a highly versatile mouse skin model that used vascular endothelial growth factor-expressing cells together with packaging cells producing retroviruses encoding RhoA GTPase mutants. In this animal model, dominant negative N19RhoA selectively impaired assembly of ECs into new blood vessels; and, in contrast, active V14RhoA stimulated ECs to form blood vessels with functional lumens. In vitro, dominant negative N19RhoA reduced EC actin stress fibers and prevented ECs from contracting and reorganizing into precapillary cords within collagen gels. In contrast, active V14RhoA promoted EC stress fiber formation, contractility, and organization into cords. Neither N19RhoA nor V14RhoA significantly affected EC proliferation or migration in vitro; and, similarly, neither mutant significantly affected EC density during angiogenesis in vivo. Thus, these studies identify a critical and selective role for Rho activity in regulating EC assembly into new blood vessels, and they identify both negative and positive manipulation of Rho activity, respectively, as strategies for suppressing or promoting the organizational stages of neovascularization.

Cell migration: Rho GTPases lead the way

Rho GTPases control signal transduction pathways that link cell surface receptors to a variety of intracellular responses. They are best known as regulators of the actin cytoskeleton, but in addition they control cell polarity, gene expression, microtubule dynamics and vesicular trafficking. Through these diverse functions, Rho GTPases influence many aspects of cell behavior. This review will focus specifically on their role in cell migration.

Tre1, a G protein-coupled receptor, directs transepithelial migration of Drosophila germ cells

In most organisms, germ cells are formed distant from the somatic part of the gonad and thus have to migrate along and through a variety of tissues to reach the gonad. Transepithelial migration through the posterior midgut (PMG) is the first active step during Drosophila germ cell migration. Here we report the identification of a novel G protein-coupled receptor (GPCR), Tre1, that is essential for this migration step. Maternal tre1 RNA is localized to germ cells, and tre1 is required cell autonomously in germ cells. In tre1 mutant embryos, most germ cells do not exit the PMG. The few germ cells that do leave the midgut early migrate normally to the gonad, suggesting that this gene is specifically required for transepithelial migration and that mutant germ cells are still able to recognize other guidance cues. Additionally, inhibiting small Rho GTPases in germ cells affects transepithelial migration, suggesting that Tre1 signals through Rho1. We propose that Tre1 acts in a manner similar to chemokine receptors required during transepithelial migration of leukocytes, implying an evolutionarily conserved mechanism of transepithelial migration. Recently, the chemokine receptor CXCR4 was shown to direct migration in vertebrate germ cells. Thus, germ cells may more generally use GPCR signaling to navigate the embryo toward their target.

A novel G protein-coupled receptor (GPCR) is shown to be essential for transepithelial migration of Drosophila germ cells. Leukocyte transepithelial migration also requires GPCR signaling, suggesting a conserved mechanism.

Identification and molecular characterization of the G alpha12-Rho guanine nucleotide exchange factor pathway in Caenorhabditis elegans

G alpha 12/13-mediated pathways have been shown to be involved in various fundamental cellular functions in mammalian cells such as axonal guidance, apoptosis, and chemotaxis. Here, we identified a homologue of Rho-guanine nucleotide exchange factor (GEF) in Caenorhabditis elegans (CeRhoGEF), which functions downstream of gpa-12, the C. elegans homologue of G alpha 12/13. CeRhoGEF contains a PSD-95/Dlg/ZO-1 domain and a regulator of G protein signaling (RGS) domain upstream of the Dbl homology-pleckstrin homology region similar to mammalian RhoGEFs with RGS domains, PSD-95/Dlg/ZO-1-RhoGEF and leukemia-associated RhoGEF. It has been shown in mammalian cells that these RhoGEFs interact with activated forms of G alpha 12 or G alpha 13 through their RGS domains. We demonstrated by coimmunoprecipitation that the RGS domain of CeRhoGEF interacts with GPA-12 in an AIF4- activation-dependent manner and confirmed that the Dbl homology-pleckstrin homology domain of CeRhoGEF was capable of Rho-dependent signaling. These results proved conservation of the G alpha 12-RhoGEF pathway in C. elegans. Expression of DsRed or GFP under the control of the promoter of CeRhoGEF or gpa-12 revealed an overlap of their expression patterns in ventral cord motor neurons and several neurons in the head. RNA-mediated gene interference for CeRhoGEF and gpa-12 resulted in similar phenotypes such as embryonic lethality and sensory and locomotive defects in adults. Thus, the G alpha 12/13-RhoGEF pathway is likely to be involved in embryonic development and neuronal function in C. elegans.

Stepwise formation of a SMAD activity gradient during dorsal-ventral patterning of theDrosophilaembryo

Genetic evidence suggests that the Drosophila ectoderm is patterned by a spatial gradient of bone morphogenetic protein (BMP). Here we compare patterns of two related cellular responses, both signal-dependent phosphorylation of the BMP-regulated R-SMAD, MAD, and signal-dependent changes in levels and sub-cellular distribution of the co-SMAD Medea. Our data demonstrate that nuclear accumulation of the co-SMAD Medea requires a BMP signal during blastoderm and gastrula stages. During this period, nuclear co-SMAD responses occur in three distinct patterns. At the end of blastoderm,a broad dorsal domain of weak SMAD response is detected. During early gastrulation, this domain narrows to a thin stripe of strong SMAD response at the dorsal midline. SMAD response levels continue to rise in the dorsal midline region during gastrulation, and flanking plateaus of weak responses are detected in dorsolateral cells. Thus, the thresholds for gene expression responses are implicit in the levels of SMAD responses during gastrulation. Both BMP ligands, DPP and Screw, are required for nuclear co-SMAD responses during these stages. The BMP antagonist Short gastrulation (SOG) is required to elevate peak responses at the dorsal midline as well as to depress responses in dorsolateral cells. The midline SMAD response gradient can form in embryos with reduced dpp gene dosage, but the peak level is reduced. These data support a model in which weak BMP activity during blastoderm defines the boundary between ventral neurogenic ectoderm and dorsal ectoderm. Subsequently, BMP activity creates a step gradient of SMAD responses that patterns the amnioserosa and dorsomedial ectoderm.

Genetic Interactions Between the RhoA and Stubble-stubbloid Loci Suggest a Role for a Type II Transmembrane Serine Protease in Intracellular Signaling During Drosophila Imaginal Disc Morphogenesis

The Drosophila RhoA (Rho1) GTPase is essential for postembryonic morphogenesis of leg and wing imaginal discs. Mutations in RhoA enhance leg and wing defects associated with mutations in zipper, the gene encoding the heavy chain of nonmuscle myosin II. We demonstrate here that mutations affecting the RhoA signaling pathway also interact genetically with mutations in the Stubble-stubbloid (Sb-sbd) locus that encodes an unusual type II transmembrane serine protease required for normal leg and wing morphogenesis. In addition, a leg malformation phenotype associated with overexpression of Sb-sbd in prepupal leg discs is suppressed when RhoA gene dose is reduced, suggesting that RhoA and Sb-sbd act in a common pathway during leg morphogenesis. We also characterized six mutations identified as enhancers of zipper mutant leg defects. Three of these genes encode known members of the RhoA signaling pathway (RhoA, DRhoGEF2, and zipper). The remaining three enhancer of zipper mutations interact genetically with both RhoA and Sb-sbd mutations, suggesting that they encode additional components of the RhoA signaling pathway in imaginal discs. Our results provide evidence that the type II transmembrane serine proteases, a class of proteins linked to human developmental abnormalities and pathology, may be associated with intracellular signaling required for normal development.

Distinct functions of Rho and Rac are required for convergent extension during Xenopus gastrulation

We have undertaken the first detailed analysis of Rho GTPase function during vertebrate development by analyzing how RhoA and Rac1 control convergent extension of axial mesoderm during Xenopus gastrulation. Monitoring of a number of parameters in time-lapse recordings of mesoderm explants revealed that Rac and Rho have both distinct and overlapping roles in regulating the motility of axial mesoderm cells. The cell behaviors revealed by activated or inhibitory versions of these GTPases in native tissue were clearly distinct from those previously documented in cultured fibroblasts. The dynamic properties and polarity of protrusive activity, along with lamellipodia formation, were controlled by the two GTPases operating in a partially redundant manner, while Rho and Rac contributed separately to cell shape and filopodia formation. We propose that Rho and Rac operate in distinct signaling pathways that are integrated to control cell motility during convergent extension.

Molecular mechanisms of epithelial morphogenesis

Epithelial morphogenesis comprises the various processes by which epithelia contribute to organ formation and body shape. These complex and diverse events play a central role in animal development and regeneration. Recently, the characterization of some of the molecular mechanisms involved in epithelial morphogenesis has provided an abundance of new information on the role and regulation of the cytoskeleton, cell-cell adhesion, and cell-matrix adhesion in these processes. In this review, we discuss our current understanding of the molecular mechanisms driving cell shape changes, cell intercalation, fusion of epithelia, ingression, egression, and cell migration. Our discussion is mostly focused on results from Drosophila and mammalian tissue culture but also draws on the insights gained from other organisms.

Local activation of protein kinase A inhibits morphogenetic movements during Xenopus gastrulation

cAMP-dependent protein kinase (PKA) has various biological roles in many organisms. However, little is known about its role in the developmental processes of vertebrates. In this study, we describe the functional analysis of PKA during gastrulation movements in Xenopus laevis. Overexpression of constitutively active PKA (cPKA) in the dorsal equatorial region of the embryo affects morphogenetic movement during gastrulation. We also show that intrinsic differences of PKA activities along the dorsoventral axis are set up and the level of PKA activity on the dorsal region is lower than that on the ventral region from late blastula to gastrula stages. In addition, PKA activation in animal explants inhibits activin-induced elongation. In cPKA-injected embryos, there were no changes in the expressions of markers involved in mesoderm specification, although the correct expression domains of these genes were altered. The effects of PKA activation can be restored by coexpression of PKI, a pseudosubstrate of PKA. We further analyzed the effects of PKA activation on the behavior of migratory gastrulating cells in vitro. Expression of cPKA in head mesoderm cells causes less polarized and/or randomized migration as demonstrated by a directional cell migration assay. Finally, we show that RhoA GTPase lies downstream of PKA, affecting activin-induced convergent extension movements. Taken together, these results suggest that overexpressed PKA can modulate a pathway responsible for morphogenetic movements during Xenopus gastrulation.

How we are shaped: the biomechanics of gastrulation

Although it is rarely considered so in modern developmental biology, morphogenesis is fundamentally a biomechanical process, and this is especially true of one of the first major morphogenic transformations in development, gastrulation. Cells bring about changes in embryonic form by generating patterned forces and by differentiating the tissue mechanical properties that harness these forces in specific ways. Therefore, biomechanics lies at the core of connecting the genetic and molecular basis of cell activities to the macroscopic tissue deformations that shape the embryo. Here we discuss what is known of the biomechanics of gastrulation, primarily in amphibians but also comparing similar morphogenic processes in teleost fish and amniotes, and selected events in several species invertebrates. Our goal is to review what is known and identify problems for further research.

Mechanisms of cell positioning during C. elegans gastrulation

Cell rearrangements are crucial during development. In this study, we use C. elegans gastrulation as a simple model to investigate the mechanisms of cell positioning. During C. elegans gastrulation, two endodermal precursor cells move from the ventral surface to the center of the embryo, leaving a gap between these ingressing cells and the eggshell. Six neighboring cells converge under the endodermal precursors, filling this gap. Using an in vitro system, we observed that these movements occurred consistently in the absence of the eggshell and the vitelline envelope. We found that movement of the neighbors towards each other is not dependent on chemotactic signaling between these cells. We further found that C. elegans gastrulation requires intact microfilaments, but not microtubules. The primary mechanism of microfilament-based motility does not appear to be through protrusive structures, such as lamellipodia or filopodia. Instead, our results suggest an alternative mechanism. We found that myosin activity is required for gastrulation, that the apical sides of the ingressing cells contract, and that the ingressing cells determine the direction of movement of their neighboring cells. Based on these results, we propose that ingression is driven by an actomyosin-based contraction of the apical side of the ingressing cells, which pulls neighboring cells underneath. We conclude that apical constriction can function to position blastomeres in early embryos, even before anchoring junctions form between cells.

The Jak/STAT pathway in model organisms: emerging roles in cell movement

The JAK/STAT pathway was originally identified in mammals. Studies of this pathway in the mouse have revealed that JAK/STAT signaling plays a central role during hematopoeisis and other developmental processes. The role of JAK/STAT signaling in blood appears to be conserved throughout evolution, as it is also required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and zebrafish have shown that the JAK/STAT pathway is also required in an unusually broad set of developmental decisions, including cell proliferation, cell fate determination, cell migration, planar polarity, convergent extension, and immunity. There is increasing evidence that the versatility of this pathway relies on its cooperation with other signal transduction pathways. In this review, we discuss the components of the JAK/STAT pathway in model organisms and what is known about its requirement in cellular and developmental processes. In particular, we emphasize recent insights into the role that this pathway plays in the control of cell movement.

B plexins activate Rho through PDZ-RhoGEF

Plexins are receptors for the repulsive axon guidance molecules semaphorins. Previously, we have shown that plexin-B1 binds activated Rac, but that clustering of plexin-B1 causes Rho activation, resulting in stress fiber formation. Using the yeast two-hybrid system, we found that the C-terminus of B plexins interacted directly with Rho-specific exchange factors, via their PDZ domain. Mutation of the carboxy-terminal amino acids of plexin-B1 or coexpression of a dominant negative PDZ-RhoGEF abrogated the ability of plexin-B1 to cause stress fiber formation. Our results demonstrate a role for PDZ-RhoGEF in B plexin-mediated activation of Rho/Rho kinase signaling, implicated in the regulation of axon guidance and cell migration.

Cellular processes associated with germ band retraction in Drosophila

Large-scale movements of epithelial sheets are necessary for most embryonic and regenerative morphogenetic events. We have characterized the cellular processes associated with germ band retraction (GBR) in the Drosophila embryo. During GBR, the caudal end of the embryo retracts to its final posterior position. We show using time-lapse recordings that, in contrast to germ band extension, cells within the lateral germ band do not intercalate. In addition, the germ band and amnioserosa move as one coherent sheet, and the amnioserosa strongly shortens along its dorsal-ventral axis. Furthermore, during GBR, the amnioserosa adheres to and migrates over the caudal end of the germ band via lamellipodia. Expression of both dominant-negative and constitutively active RhoA in the amnioserosa disrupts GBR. As RhoA acts on both actomyosin contractility and cell-matrix adhesion, it suggests a role for such processes in the amnioserosa during GBR. The results establish the cellular movements and shape changes occurring during GBR and provide the basis for an analysis of the forces acting during GBR.

Cell movements during gastrulation: snail dependent and independent pathways

The morphogenetic process of gastrulation requires multiple inputs and intricate coordination. Genetic analyses demonstrate critical roles of vertebrate and invertebrate Snail proteins in this process. Together with other regulatory molecules including Wnt and BMP, the Snail pathways specify cell fate and reorganize cellular machineries to coordinate morphological changes and cell movements during gastrulation.

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

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

ACK family tyrosine kinase activity is a component of Dcdc42 signaling during dorsal closure in Drosophila melanogaster

We have characterized Drosophila melanogaster ACK (DACK), one of two members of the ACK family of nonreceptor tyrosine kinases in Drosophila. The ACKs are likely effectors for the small GTPase Cdc42, but signaling by these proteins remains poorly defined. ACK family tyrosine kinase activity functions downstream of Drosophila Cdc42 during dorsal closure of the embryo, as overexpression of DACK can rescue the dorsal closure defects caused by dominant-negative Dcdc42. Similar to known participants in dorsal closure, DACK is enriched in the leading edge cells of the advancing epidermis, but it does not signal through activation of the Jun amino-terminal kinase cascade operating in these cells. Transcription of DACK is responsive to changes in Dcdc42 signaling specifically at the leading edge and in the amnioserosa, two tissues involved in dorsal closure. Unlike other members of the ACK family, DACK does not contain a conserved Cdc42-binding motif, and transcriptional regulation may be one route by which Dcdc42 can affect DACK function. Expression of wild-type and kinase-dead DACK transgenes in embryos, and in the developing wing and eye, reveals that ACK family tyrosine kinase activity is involved in a range of developmental events similar to that of Dcdc42.

Low-molecular-weight protein tyrosine phosphatase is a positive component of the fibroblast growth factor receptor signaling pathway

Low-molecular-weight protein tyrosine phosphatase (LMW-PTP) has been implicated in the regulation of cell growth and actin rearrangement mediated by several receptor tyrosine kinases, including platelet-derived growth factor and epidermal growth factor. Here we identify the Xenopus laevis homolog of LMW-PTP1 (XLPTP1) as an additional positive regulator in the fibroblast growth factor (FGF) signaling pathway during Xenopus development. XLPTP1 has an expression pattern that displays substantial overlap with FGF receptor 1 (FGFR1) during Xenopus development. Using morpholino antisense technology, we show that inhibition of endogenous XLPTP1 expression dramatically restricts anterior and posterior structure development and inhibits mesoderm formation. In ectodermal explants, loss of XLPTP1 expression dramatically blocks the induction of the early mesoderm gene, Xbrachyury (Xbra), by FGF and partially blocks Xbra induction by Activin. Moreover, FGF-induced activation of mitogen-activated protein (MAP) kinase is also inhibited by XLPTP1 morpholino antisense oligonucleotides; however, introduction of RNA encoding XLPTP1 is able to rescue morphological and biochemical effects of antisense inhibition. Inhibition of FGF-induced MAP kinase activity due to loss of XLPTP1 is also rescued by an active Ras, implying that XLPTP1 may act upstream of or parallel to Ras. Finally, XLPTP1 physically associates only with an activated FGFR1, and this interaction requires the presence of SNT1/FRS-2 (FGFR substrate 2). Although LMW-PTP1 has been shown to participate in other receptor systems, the data presented here also reveal XLPTP1 as a new and important component of the FGF signaling pathway.

Xenopus Cdc42 regulates convergent extension movements during gastrulation through Wnt/Ca2+ signaling pathway

Rho GTPases are molecular switches that regulate many essential cellular processes, including actin dynamics, cell adhesion, cell-cycle progression, and transcription. We have isolated the Xenopus homolog of Rho GTPase Cdc42 and examined its potential role during gastrulation movements in early Xenopus embryos. XCdc42 is expressed in tissues undergoing extensive morphogenetic changes, such as the deep layers of involuting mesoderm and posterior neuroectoderm during gastrulation, and somitic mesoderm at neurula stages. Overexpression of either wild-type (WT) or dominant-negative (DN) XCdc42 interferes with convergent extension movements in intact embryos, activin-stimulated animal caps, and dorsal marginal zone explants. These effects occur without affecting mesodermal specification. Overexpression of WT or DN XCdc42 leads to the decrease and increase of cell adhesiveness of blastomeres, respectively, as demonstrated by the cell adhesion assay. In addition, when overexpressed, PKC-alpha, XWnt-5a, and Mfz-3 inhibit activin-induced convergent extension in animal cap explants. This inhibition can be rescued by coexpression of DN XCdc42, implying that XCdc42 acts downstream of the Wnt/Ca2+ signaling pathway involving PKC activation. XCdc42 also lies downstream of XWnt-5a in the regulation of Ca2+-dependent cell adhesion. Taken together, our results suggest that XCdc42 plays a role in the regulation of convergent extension movements during gastrulation through the protein kinase C-mediated Wnt/Ca2+ pathway.

Inhibition of Rho family GTPases by Rho GDP dissociation inhibitor disrupts cardiac morphogenesis and inhibits cardiomyocyte proliferation

Studies of Rho GTPases in Drosophila and Xenopus suggest that Rho family proteins may play an important role in embryogenesis. A reverse genetic approach was employed to explore the role of Rho GTPases in murine cardiac development. Cardiac-specific inhibition of Rho family protein activities was achieved by expressing Rho GDIα, a specific GDP dissociation inhibitor for Rho family proteins, using the α-myosin heavy chain promoter, active at embryonic day (E)8.0 during morphogenesis of the linear heart tube. RhoA, Rac1 and Cdc42 activities were significantly inhibited, as shown by decreased membrane translocation of these proteins in the transgenic hearts. Transgenic F1 mice for each of two independent lines expressing the highest levels of the transgene, died around E10.5. Homozygotes of the middle copy-number lines, in which Rho GDIα expression was increased four-fold over normal levels, were also embryonic lethal. Cardiac morphogenesis in these embryos was disrupted, with incomplete looping, lack of chamber demarcation, hypocellularity and lack of trabeculation. Cell proliferation was inhibited in the transgenic hearts, as shown by immunostaining with anti-phosphohistone H3, a marker of mitosis. In addition, ventricular hypoplasia was associated with up-regulation of p21, an inhibitor of cyclin-dependent kinases, and with down-regulation of cyclin A, while cell survival was not affected. These results reveal new biological functions for Rho family proteins as essential determinants of cell proliferation signals at looping and chamber maturation stages in mammalian cardiac development.

The snail superfamily of zinc-finger transcription factors

The Snail superfamily of zinc-finger transcription factors is involved in processes that imply pronounced cell movements, both during embryonic development and in the acquisition of invasive and migratory properties during tumour progression. Different family members have also been implicated in the signalling cascade that confers left right identity, as well as in the formation of appendages, neural differentiation, cell division and cell survival.

It takes guts: the Drosophila hindgut as a model system for organogenesis

The Drosophila hindgut is fruitful territory for investigation of events common to many types of organogenesis. The development of the Drosophila hindgut provides, in microcosm, a genetic model system for studying processes such as establishment (patterning) of an epithelial primordium, its internalization by gastrulation, development of left--right asymmetric looping, patterning in both the anteroposterior and dorsoventral axes, innervation, investment of an epithelium with mesoderm, reciprocal epitheliomesenchymal interactions, cell shape change, and cell rearrangement. We review the genetic control of these processes during development of the Drosophila hindgut, and compare these to related processes in other bilaterians, particularly vertebrates. We propose that caudal/Cdx, brachyenteron/Brachyury, fork head/HNF-3, and wingless/Wnt constitute a conserved "cassette" of genes expressed in the blastopore and later in the gut, involved in posterior patterning, cell rearrangement, and gut maintenance. Elongation of the internalized Drosophila hindgut primordium is similar to elongation of the archenteron and also of the entire embryonic axis (both during and after gastrulation), as well as of various tubules (e.g., nephric ducts, Malpighian tubules), as it is driven by cell rearrangement. The genes drumstick, bowl, and lines (which encode putative transcriptional regulators) are required for this cell rearrangement, as well as for spatially localized gene expression required to establish the three morphologically distinct subregions of the hindgut. Expression of signaling molecules regulated by drumstick, bowl, and lines, in particular of the JAK/STAT activator Unpaired at the hindgut anterior, may play a role in controlling hindgut cell rearrangement. Other cell signaling molecules expressed in the hindgut epithelium are required to establish its normal size (Dpp and Hh), and to establish and maintain the hindgut visceral mesoderm (Wg and Hh). Both maternal gene activity and zygotic gene activity are required for asymmetric left--right looping of the hindgut. Some of the same genes (caudal and brachyenteron) required for embryonic hindgut development also act during pupation to construct a new hindgut from imaginal cells. Application of the plethora of genetic techniques available in Drosophila, including forward genetic screens, should identify additional genes controlling hindgut development and thus shed light on a variety of common morphogenetic processes.

Stat3 Controls Cell Movements during Zebrafish Gastrulation

Vertebrate axis formation requires both the correct specification of cell fates and the coordination of gastrulation movements. We report that the zebrafish signal transducer and activator of transcription 3 (Stat3) is activated on the dorsal side by the maternal Wnt/beta-catenin pathway. Zebrafish embryos lacking Stat3 activity display abnormal cell movements during gastrulation, resulting in a mispositioned head and a shortened anterior-posterior axis, but show no defects in early cell fate specification. Time course analysis, cell tracing, and transplantation experiments revealed that Stat3 activity is required cell autonomously for the anterior migration of dorsal mesendodermal cells and non-cell autonomously for the convergence of neighboring paraxial cells. These results reveal a role for Stat3 in controlling cell movements during gastrulation.

The art and design of genetic screens: Drosophila melanogaster

The success of Drosophila melanogaster as a model organism is largely due to the power of forward genetic screens to identify the genes that are involved in a biological process. Traditional screens, such as the Nobel-prize-winning screen for embryonic-patterning mutants, can only identify the earliest phenotype of a mutation. This review describes the ingenious approaches that have been devised to circumvent this problem: modifier screens, for example, have been invaluable for elucidating signal-transduction pathways, whereas clonal screens now make it possible to screen for almost any phenotype in any cell at any stage of development.

A RHO GTPase-mediated pathway is required during P cell migration in Caenorhabditis elegans

The Rho family of guanine triphosphate hydrolases controls various cellular processes, including cell migration. We describe here the demonstration of a role for a RhoA GTPase homologue during cell migration in Caenorhabditis elegans. We show that eliminating or reducing rho-1 gene function by using a dominant-negative transgene or dsRNA interference results in a severe defect in migration of hypodermal P cells to a ventral position. Biochemical and genetic data also suggest that unc-73, which encodes a Trio-like guanine nucleotide exchange factor, may act as an activator of rho-1 in the migration process. Mutations in let-502 ROCK, a homologue of a RhoA effector in mammals, also cause defects in P cell migration, suggesting that it may be one of several effectors acting downstream of rho-1 during P cell migration. Finally, we provide evidence to support the idea that other small Rac subfamily small GTPases act redundantly and in parallel to RHO-1 in this specific cell migration event.

A gene encoding a putative GTPase regulator is mutated in familial amyotrophic lateral sclerosis 2

Amyotrophic lateral sclerosis 2 (ALS2) is an autosomal recessive form of juvenile ALS and has been mapped to human chromosome 2q33. Here we report the identification of two independent deletion mutations linked to ALS2 in the coding exons of the new gene ALS2. These deletion mutations result in frameshifts that generate premature stop codons. ALS2 is expressed in various tissues and cells, including neurons throughout the brain and spinal cord, and encodes a protein containing multiple domains that have homology to RanGEF as well as RhoGEF. Deletion mutations are predicted to cause a loss of protein function, providing strong evidence that ALS2 is the causative gene underlying this form of ALS.

The Rac GTPase-activating protein RotundRacGAP interferes with Drac1 and Dcdc42 signalling in Drosophila melanogaster

RhoGTPases are negatively regulated by GTPase-activating proteins (GAPs). Here we demonstrate that Drosophila RotundRacGAP is active in vitro on Drac1 and Dcdc42 but not Drho1. Similarly, in yeast, RotundRacGAP interacts specifically with Drac1 and Dcdc42, as well as with their activated V12 forms, showing a particularly strong interaction with Dcdc42V12. In the fly, lowering RotundRacGAP dosage specifically modifies eye defects induced by expressing Drac1 or Dcdc42 but not Drho1, confirming that Drac1 and Dcdc42 are indeed in vivo targets of RotundRacGAP. Furthermore, embryonic-directed expression of either RotundRacGAP, or dominant negative Drac1N17, transgenes induces similar defects in dorsal closure and inhibits Drac1-dependent cytoskeleton assembly at the leading edge. Expression of truncated forms of RotundRacGAP shows that the GAP domain of RotundRacGAP is essential for its function. Unexpectedly, transgenes encoding Drac1N17, Dcdc42N17, or RotundRacGAP do not affect the c-Jun N-terminal kinase-dependent gene expression of decapentaplegic and puckered, indicating that another Drac1-independent signal redundantly activates this pathway. Finally, in a situation where Drac1 is constitutively activated, RotundRacGAP greatly reduces the ectopic expression of decapentaplegic, possibly by negatively regulating Dcdc42.

RotundRacGAP functions with Ras during spermatogenesis and retinal differentiation in Drosophila melanogaster

Our analysis of rotund (rn) null mutations in Drosophila melanogaster revealed that deletion of the rn locus affects both spermatid and retinal differentiation. In the male reproductive system, the absence of RnRacGAP induced small testes, empty seminal vesicles, short testicular cysts, reduced amounts of interspermatid membrane, the absence of individualization complexes, and incomplete mitochondrial condensation. Flagellar growth continued within the short rn null cysts to produce large bulbous terminations of intertwined mature flagella. Organization of the retina was also severely perturbed as evidenced by grossly misshapen ommatidia containing reduced numbers of photoreceptor and pigment cells. These morphological phenotypes were rescued by genomic rnRacGAP transgenes, demonstrating that RnRacGAP function is critical to spermatid and retinal differentiation. The testicular phenotypes were suppressed by heterozygous hypomorphic mutations in the Dras1 and drk genes, indicating cross talk between RacGAP-regulated signaling and that of the Ras pathway. The observed genetic interactions are consistent with a model in which Rac signaling is activated by Ras and negatively regulated by RnRacGAP during spermatid differentiation. RnRacGAP and Ras cross talk also operated during retinal differentiation; however, while the heterozygous hypomorphic drk mutation continued to act as a suppressor of the rn null mutation, the heterozygous hypomorphic Dras1 mutation induced novel retinal phenotypes.

Rac 'n Rho: the music that shapes a developing embryo

The small GTPases of the Rho subfamily constitute a group of evolutionarily conserved proteins that mediate signaling pathways that regulate a variety of cellular processes, many of which are associated with dynamic cytoskeletal reorganization. These processes determine the shape, adhesive properties, and movement of cells, and the Rho GTPases have therefore been implicated in the complex morphogenesis of tissues in developing multicellular organisms. The Drosophila genetic system has proved particularly useful in establishing the in vivo functions of several of the Rho GTPases and their associated signaling pathway components during development.

Rho kinases play an obligatory role in vertebrate embryonic organogenesis

Rho-associated kinases (Rho kinases), which are downstream effectors of RhoA GTPase, regulate diverse cellular functions including actin cytoskeletal organization. We have demonstrated that Rho kinases also direct the early stages of chick and mouse embryonic morphogenesis. We observed that Rho kinase transcripts were enriched in cardiac mesoderm, lateral plate mesoderm and the neural plate. Treatment of neurulating embryos with Y27632, a specific inhibitor of Rho kinases, blocked migration and fusion of the bilateral heart primordia, formation of the brain and neural tube, caudalward movement of Hensen’s node, and establishment of normal left-right asymmetry. Moreover, Y27632 induced precocious expression of cardiac α-actin, an early marker of cardiomyocyte differentiation, coincident with the upregulated expression of serum response factor and GATA4. In addition, specific antisense oligonucleotides significantly diminished Rho kinase mRNA levels and replicated many of the teratologies induced by Y27632. Thus, our study reveals new biological functions for Rho kinases in regulating major morphogenetic events during early chick and mouse development.