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

Neogenin and RGMa control neural tube closure and neuroepithelial morphology by regulating cell polarity

In humans, neural tube closure defects occur in 1:1000 pregnancies. The design of new strategies for the prevention of such common defects would benefit from an improved understanding of the molecular events underlying neurulation. Neural fold elevation is a key morphological process that acts during neurulation to drive neural tube closure. However, to date, the molecular pathways underpinning neural fold elevation have not been elucidated. Here, we use morpholino knock-down technology to demonstrate that Repulsive Guidance Molecule (RGMa)-Neogenin interactions are essential for effective neural fold elevation during Xenopus neurulation and that loss of these molecules results in disrupted neural tube closure. We demonstrate that Neogenin and RGMa are required for establishing the morphology of deep layer cells in the neural plate throughout neurulation. We also show that loss of Neogenin severely disrupts the microtubule network within the deep layer cells suggesting that Neogenin-dependent microtubule organization within the deep cells is essential for radial intercalation with the overlying superficial cell layer, thereby driving neural fold elevation. In addition, we show that sustained Neogenin activity is also necessary for the establishment of the apicobasally polarized pseudostratified neuroepithelium of the neural tube. Therefore, our study identifies a novel signaling pathway essential for radial intercalation and epithelialization during neural fold elevation and neural tube morphogenesis.

Vav proteins are necessary for correct differentiation of mouse cecal and colonic enterocytes

In the mammalian cecum and colon, a single layer of absorptive, mature enterocytes are a crucial element of the physical barrier to the contents of the lumen. Enterocytic differentiation involves expansion of cytoplasmic cytoskeletal networks, which have been proposed to maintain structural integrity of individual cells and thus the entire epithelial barrier. We sought molecular tools to test this hypothesis in vivo, because in vitro systems displaying full intestinal epithelial differentiation have not yet been developed. Vav proteins are RhoGEFs that modulate cytoskeletal networks in immune cells. We found that Vav proteins were preferentially expressed in terminally differentiating cecal and colonic enterocytes. Loss of Vav protein expression in triple-knockout mice (Vav1(-/-);Vav2(-/-);Vav3(-/-)) resulted in defective expansion of microtubule cytoskeletons, a significant decrease in cell height and diminished expression of differentiation markers. Despite these changes, enterocytes in the triple-mutant mice did not contain measurable alterations in actin cytoskeleton, apical cell-cell junctions, nuclear position or global polarized delivery of proteins involved in terminal differentiation. Aged triple-mutant mice spontaneously developed ulcerative lesions that were, in part, a result of defective wound repair. These studies show that Vav proteins are required for enterocytic differentiation and colonic epithelial barrier integrity.

Remodeling of the adherens junctions during morphogenesis

Morphogenesis of epithelial tissues involves various forms of reshaping of cell layers, such as invagination or bending, convergent extension, and epithelial-mesenchymal transition. At the cellular level, these processes include changes in the shape, position, and assembly pattern of cells. During such morphogenetic processes, epithelial sheets in general maintain their multicellular architecture, implying that they must engage the mechanisms to change the spatial relationship with their neighbors without disrupting the junctions. A major junctional structure in epithelial tissues is the "adherens junction," which is composed of cadherin adhesion receptors and associated proteins including F-actin. The adherens junctions are required for the firm associations between cells, as disruption of them causes disorganization of the epithelial architecture. The adherens junctions, however, appear to be a dynamic entity, allowing the rearrangement of cells within cell sheets. This dynamic nature of the adherens junctions seems to be supported by various mechanisms, such as the interactions of cadherins with actin cytoskeleton, endocytosis and recycling of cadherins, and the cooperation of cadherins with other adhesion receptors. In this chapter, we provide an overview of these mechanisms analyzed in vitro and in vivo.

Cellular and molecular mechanisms underlying the formation of biological tubes

Biological tubes are integral components of many organs. Based on their cellular organization, tubes can be divided into three types: multicellular, unicellular, and intracellular. The mechanisms by which these tubes form during development vary significantly, in many cases even for those sharing a similar final architecture. Here, we present recent advances in studying cellular and molecular aspects of tubulogenesis in different organisms.

Morphogenetic movements driving neural tube closure in Xenopus require myosin IIB

Vertebrate neural tube formation involves two distinct morphogenetic events--convergent extension (CE) driven by mediolateral cell intercalation, and bending of the neural plate driven largely by cellular apical constriction. However, the cellular and molecular biomechanics of these processes are not understood. Here, using tissue-targeting techniques, we show that the myosin IIB motor protein complex is essential for both these processes, as well as for conferring resistance to deformation to the neural plate tissue. We show that myosin IIB is required for actin-cytoskeletal organization in both superficial and deep layers of the Xenopus neural plate. In the superficial layer, myosin IIB is needed for apical actin accumulation, which underlies constriction of the neuroepithelial cells, and that ultimately drive neural plate bending, whereas in the deep neural cells myosin IIB organizes a cortical actin cytoskeleton, which we describe for the first time, and that is necessary for both normal neural cell cortical tension and shape and for autonomous CE of the neural tissue. We also show that myosin IIB is required for resistance to deformation ("stiffness") in the neural plate, indicating that the cytoskeleton-organizing roles of this protein translate in regulation of the biomechanical properties of the neural plate at the tissue-level.

The scaffold protein POSH regulates axon outgrowth

How scaffold proteins integrate signaling pathways with cytoskeletal components to drive axon outgrowth is not well understood. We report here that the multidomain scaffold protein Plenty of SH3s (POSH) regulates axon outgrowth. Reduction of POSH function by RNA interference (RNAi) enhances axon outgrowth in differentiating mouse primary cortical neurons and in neurons derived from mouse P19 cells, suggesting POSH negatively regulates axon outgrowth. Complementation analysis reveals a requirement for the third Src homology (SH) 3 domain of POSH, and we find that the actomyosin regulatory protein Shroom3 interacts with this domain of POSH. Inhibition of Shroom3 expression by RNAi leads to increased process lengths, as observed for POSH RNAi, suggesting that POSH and Shroom function together to inhibit process outgrowth. Complementation analysis and interference of protein function by dominant-negative approaches suggest that Shroom3 recruits Rho kinase to inhibit process outgrowth. Furthermore, inhibition of myosin II function reverses the POSH or Shroom3 RNAi phenotype, indicating a role for myosin II regulation as a target of the POSH-Shroom complex. Collectively, these results suggest that the molecular scaffold protein POSH assembles an inhibitory complex that links to the actin-myosin network to regulate neuronal process outgrowth.

From cells to organs: building polarized tissue

How do animal cells assemble into tissues and organs? A diverse array of tissue structures and shapes can be formed by organizing groups of cells into different polarized arrangements and by coordinating their polarity in space and time. Conserved design principles underlying this diversity are emerging from studies of model organisms and tissues. We discuss how conserved polarity complexes, signalling networks, transcription factors, membrane-trafficking pathways, mechanisms for forming lumens in tubes and other hollow structures, and transitions between different types of polarity, such as between epithelial and mesenchymal cells, are used in similar and iterative manners to build all tissues.

Formation of the zebrafish midbrain-hindbrain boundary constriction requires laminin-dependent basal constriction

The midbrain-hindbrain boundary (MHB) is a highly conserved fold in the vertebrate embryonic brain. We have termed the deepest point of this fold the MHB constriction (MHBC) and have begun to define the mechanisms by which it develops. In the zebrafish, the MHBC is formed soon after neural tube closure, concomitant with inflation of the brain ventricles. The MHBC is unusual, as it forms by bending the basal side of the neuroepithelium. At single cell resolution, we show that zebrafish MHBC formation involves two steps. The first is a shortening of MHB cells to approximately 75% of the length of surrounding cells. The second is basal constriction, and apical expansion, of a small group of cells that contribute to the MHBC. In the absence of inflated brain ventricles, basal constriction still occurs, indicating that the MHBC is not formed as a passive consequence of ventricle inflation. In laminin mutants, basal constriction does not occur, indicating an active role for the basement membrane in this process. Apical expansion also fails to occur in laminin mutants, suggesting that apical expansion may be dependent on basal constriction. This study demonstrates laminin-dependent basal constriction as a previously undescribed molecular mechanism for brain morphogenesis.

Identification of the TFII-I family target genes in the vertebrate genome

GTF2I and GTF2IRD1 encode members of the TFII-I transcription factor family and are prime candidates in the Williams syndrome, a complex neurodevelopmental disorder. Our previous expression microarray studies implicated TFII-I proteins in the regulation of a number of genes critical in various aspects of cell physiology. Here, we combined bioinformatics and microarray results to identify TFII-I downstream targets in the vertebrate genome. These results were validated by chromatin immunoprecipitation and siRNA analysis. The collected evidence revealed the complexity of TFII-I-mediated processes that involve distinct regulatory networks. Altogether, these results lead to a better understanding of specific molecular events, some of which may be responsible for the Williams syndrome phenotype.

Dynamic determinations: patterning the cell behaviours that close the amphibian blastopore

We review the dynamic patterns of cell behaviours in the marginal zone of amphibians with a focus on how the progressive nature and the geometry of these behaviours drive blastopore closure. Mediolateral cell intercalation behaviour and epithelial-mesenchymal transition are used in different combinations in several species of amphibian to generate a conserved pattern of circumblastoporal hoop stresses. Although these cell behaviours are quite different and involve different germ layers and tissue organization, they are expressed in similar patterns. They are expressed progressively along presumptive lateral-medial and anterior-posterior axes of the body plan in highly ordered geometries of functional significance in the context of the biomechanics of blastopore closure, thereby accounting for the production of similar patterns of circumblastoporal forces. It is not the nature of the cell behaviour alone, but the context, the biomechanical connectivity and spatial and temporal pattern of its expression that determine specificity of morphogenic output during gastrulation and blastopore closure. Understanding the patterning of these dynamic features of cell behaviour is important and will require analysis of signalling at much greater spatial and temporal resolution than that has been typical in the analysis of patterning tissue differentiation.

Synthetic morphology: prospects for engineered, self-constructing anatomies

This paper outlines prospects for applying the emerging techniques of synthetic biology to the field of anatomy, with the aim of programming cells to organize themselves into specific, novel arrangements, structures and tissues. There are two main reasons why developing this hybrid discipline--synthetic morphology--would be useful. The first is that having a way to engineer self-constructing assemblies of cells would provide a powerful means of tissue engineering for clinical use in surgery and regenerative medicine. The second is that construction of simple novel systems according to theories of morphogenesis gained from study of real embryos will provide a means of testing those theories rigorously, something that is very difficult to do by manipulation of complex embryos. This paper sets out the engineering requirements for synthetic morphology, which include the development of a library of sensor modules, regulatory modules and effector modules that can be connected functionally within cells. A substantial number of sensor and regulatory modules already exist and this paper argues that some potential effector modules have already been identified. The necessary library may therefore be within reach. The paper ends by suggesting a set of challenges, ranging from simple to complex, the achievement of which would provide valuable proofs of concept.

Developmental regulation of central spindle assembly and cytokinesis during vertebrate embryogenesis

Mitosis and cytokinesis not only ensure the proper segregation of genetic information but also contribute importantly to morphogenesis in embryos. Cytokinesis is controlled by the central spindle, a microtubule-based structure containing numerous microtubule motors and microtubule-binding proteins, including PRC1. We show here that central spindle assembly and function differ dramatically between two related populations of epithelial cells in developing vertebrate embryos examined in vivo. Compared to epidermal cells, early neural epithelial cells undergo exaggerated anaphase chromosome separation, rapid furrowing, and a marked reduction of microtubule density in the spindle midzone. Cytokinesis in normal early neural epithelial cells thus resembles that in cultured vertebrate cells experimentally depleted of PRC1. We find that PRC1 mRNA and protein expression is surprisingly dynamic in early vertebrate embryos and that neural-plate cells contain less PRC1 than do epidermal cells. Expression of excess PRC1 ameliorates both the exaggerated anaphase and reduced midzone microtubule density observed in early neural epithelial cells. These PRC1-mediated modifications to the cytokinetic mechanism may be related to the specialization of the midbody in neural cells. These data suggest that PRC1 is a dose-dependent regulator of the central spindle in vertebrate embryos and demonstrate unexpected plasticity to fundamental mechanisms of cell division.

Apical accumulation of Rho in the neural plate is important for neural plate cell shape change and neural tube formation

Although Rho-GTPases are well-known regulators of cytoskeletal reorganization, their in vivo distribution and physiological functions have remained elusive. In this study, we found marked apical accumulation of Rho in developing chick embryos undergoing folding of the neural plate during neural tube formation, with similar accumulation of activated myosin II. The timing of accumulation and biochemical activation of both Rho and myosin II was coincident with the dynamics of neural tube formation. Inhibition of Rho disrupted its apical accumulation and led to defects in neural tube formation, with abnormal morphology of the neural plate. Continuous activation of Rho also altered neural tube formation. These results indicate that correct spatiotemporal regulation of Rho is essential for neural tube morphogenesis. Furthermore, we found that a key morphogenetic signaling pathway, the Wnt/PCP pathway, was implicated in the apical accumulation of Rho and regulation of cell shape in the neural plate, suggesting that this signal may be the spatiotemporal regulator of Rho in neural tube formation.

The forces that shape embryos: physical aspects of convergent extension by cell intercalation

We discuss the physical aspects of the morphogenic process of convergence (narrowing) and extension (lengthening) of tissues by cell intercalation. These movements, often referred to as 'convergent extension', occur in both epithelial and mesenchymal tissues during embryogenesis and organogenesis of invertebrates and vertebrates, and they play large roles in shaping the body plan during development. Our focus is on the presumptive mesodermal and neural tissues of the Xenopus (frog) embryo, tissues for which some physical measurements have been made. We discuss the physical aspects of how polarized cell motility, oriented along future tissue axes, generate the forces that drive oriented cell intercalation and how this intercalation results in convergence and extension or convergence and thickening of the tissue. Our goal is to identify aspects of these morphogenic movements for further biophysical, molecular and cell biological, and modeling studies.

"Developmental mechanics": cellular patterns controlled by adhesion, cortical tension and cell division

How embryos are shaped during development has inspired the work of many, embryologists, geneticists, but also mathematicians such as Turing, and physicists. Despite the inherent complexity of the problems it tackles, developmental biology has produced one of the most spectacular conceptual achievements, demonstrating that embryos are built with conserved molecules that orchestrate pattern and morphogenesis. As the logic of development now emerges, new challenges arise, such as how tissue mechanics is controlled. Quantitative approaches and computational models are essential to predict tissue organization and cell shapes. I review briefly how physical concepts have fueled this research in the past decades.

Epithelial morphogenesis in embryos: asymmetries, motors and brakes

Epithelial cells play a central role in many embryonic morphogenetic processes, during which they undergo highly coordinated cell shape changes. Here, we review some common principles that have recently emerged through genetic and cellular analyses performed mainly with invertebrate genetic models, focusing on morphogenetic processes involving epithelial sheets. All available data argue that myosin II is the main motor that induces cell shape changes during morphogenesis. We discuss the control of myosin II activity during epithelial morphogenesis, as well as the recently described involvement of microtubules in this process. Finally, we examine how forces unleashed by myosin II can be measured, how embryos use specific brakes to control molecular motors and the potential input of mechano-sensation in morphogenesis.

Shroom3-mediated recruitment of Rho kinases to the apical cell junctions regulates epithelial and neuroepithelial planar remodeling

Remodeling of epithelial sheets plays important roles in animal morphogenesis. Shroom3 is known to regulate the apical constriction of epithelial cells. Here, we show that Shroom3 binds ROCKs and recruits them to the epithelial apical junctions. We identified the Shroom3-binding site (RII-C1) on ROCKs, and found that RII-C1 could antagonize the Shroom3-ROCK interaction, interfering with the action of Shroom3 on cell morphology. In the invaginating neural plate/tube, Shroom3 colocalized with ROCKs at the apical junctions; Shroom3 depletion or RII-C1 expression in the tube removed these apically localized ROCKs, and concomitantly blocked neural tube closure. Closing neural plate exhibited peculiar cell assemblies, including rosette formation, as well as a planar-polarized distribution of phosphorylated myosin regulatory light chain, but these were abolished by ROCK inhibition or RII-C1 expression. These results demonstrate that the Shroom3-ROCK interaction is crucial for the regulation of epithelial and neuroepithelial cell arrangement and remodeling.

The FERM protein Epb4.1l5 is required for organization of the neural plate and for the epithelial-mesenchymal transition at the primitive streak of the mouse embryo

During early mouse development, a single-layered epithelium is transformed into the three germ layers that are the basis of the embryonic body plan. Here we describe an ENU-induced mutation, limulus (lulu), which disrupts gastrulation and the organization of all three embryonic germ layers. Positional cloning and analysis of additional alleles show that lulu is a null allele of the FERM-domain gene erythrocyte protein band 4.1-like 5 (Epb4.1l5). During gastrulation, some cells in lulu mutants are trapped in the primitive streak at an intermediate stage of the epithelial-mesenchymal transition; as a result, the embryos have very little paraxial mesoderm. Epithelial layers of the later lulu embryo are also disrupted: definitive endoderm is specified but does not form a gut tube, and the neural plate is broad and forms ectopic folds rather than closing to make the neural tube. In contrast to zebrafish and Drosophila, in which orthologs of Epb4.1l5 control the apical localization and activity of Crumbs proteins, mouse Crumbs proteins are localized normally to the apical surface of the lulu mutant epiblast and neural plate. However, the defects in both the lulu primitive streak and neural plate are associated with disruption of the normal organization of the actin cytoskeleton. We propose that mouse Lulu (Epb4.1l5) helps anchor the actin-myosin contractile machinery to the membrane to allow the dynamic rearrangements of epithelia that mediate embryonic morphogenesis.

Enabled (Xena) regulates neural plate morphogenesis, apical constriction, and cellular adhesion required for neural tube closure in Xenopus

Regulation of cellular adhesion and cytoskeletal dynamics is essential for neurulation, though it remains unclear how these two processes are coordinated. Members of the Ena/VASP family of proteins are localized to sites of cellular adhesion and actin dynamics and lack of two family members, Mena and VASP, in mice results in failure of neural tube closure. The precise mechanism by which Ena/VASP proteins regulate this process, however, is not understood. In this report, we show that Xenopus Ena (Xena) is localized to apical adhesive junctions of neuroepithelial cells during neurulation and that Xena knockdown disrupts cell behaviors integral to neural tube closure. Changes in the shape of the neural plate as well as apical constriction within the neural plate are perturbed in Xena knockdown embryos. Additionally, we demonstrate that Xena is essential for cell-cell adhesion. These results demonstrate that Xena plays an integral role in coordinating the regulation of cytoskeletal dynamics and cellular adhesion during neurulation in Xenopus.

EPLIN mediates linkage of the cadherin catenin complex to F-actin and stabilizes the circumferential actin belt

The cadherin-catenin complex is the major machinery for cell-cell adhesion in many animal species. This complex in general associates with actin fibers at its cytoplasmic side, organizing the adherens junction (AJ). In epithelial cells, the AJ encircles the cells near their apical surface and forms the "zonula adherens" or "adhesion belt." The mechanism as to how the cadherin-catenin complex and F-actin cooperate to generate these junctional structures, however, remains unknown. Here, we show that EPLIN (epithelial protein lost in neoplasm; also known as Lima-1), an actin-binding protein, couples with alpha-catenin and, in turn, links the cadherin-catenin complex to F-actin. Without EPLIN, this linkage was unable to form. When EPLIN had been depleted in epithelial cells, the adhesion belt was disorganized and converted into zipper-like junctions in which actin fibers were radially arranged. However, nonjunctional actin fibers were not particularly affected by EPLIN depletion. As EPLIN is known to have the ability to suppress actin depolymerization, our results suggest that EPLIN functions to link the cadherin-catenin complex to F-actin and simultaneously stabilizes this population of actin fibers, resulting in the establishment of the adhesion belt.

Myosin II regulates complex cellular arrangement and epithelial architecture in Drosophila

Remodeling epithelia is a primary driver of morphogenesis. Here, we report a central role of myosin II in regulating several aspects of complex epithelial architecture in the Drosophila eye imaginal disc. The epithelial indentation of the morphogenetic furrow is established from a pattern of myosin II activation defined by the developmental signals Hedgehog and Decapentaplegic. More generally, patterned myosin activation can control diverse three-dimensional epithelial sculpting. We have developed a technique to image eye disc development in real time, and we show that myosin II also regulates higher-order organization of cells in the plane of the epithelium. This includes the clustering of cells into ommatidial units and their subsequent coordinated rotation. This later clustering function of myosin II depends on EGF receptor signaling. Our work implies that regulation of the actomyosin cytoskeleton can control morphogenesis by regulating both individual cell shapes and their complex two-dimensional arrangement within epithelia.

Cell surface mechanics and the control of cell shape, tissue patterns and morphogenesis

Embryonic morphogenesis requires the execution of complex mechanisms that regulate the local behaviour of groups of cells. The orchestration of such mechanisms has been mainly deciphered through the identification of conserved families of signalling pathways that spatially and temporally control cell behaviour. However, how this information is processed to control cell shape and cell dynamics is an open area of investigation. The framework that emerges from diverse disciplines such as cell biology, physics and developmental biology points to adhesion and cortical actin networks as regulators of cell surface mechanics. In this context, a range of developmental phenomena can be explained by the regulation of cell surface tension.

The Rap GTPase activator Drosophila PDZ-GEF regulates cell shape in epithelial migration and morphogenesis

Epithelial morphogenesis is characterized by an exquisite control of cell shape and position. Progression through dorsal closure in Drosophila gastrulation depends on the ability of Rap1 GTPase to signal through the adherens junctional multidomain protein Canoe. Here, we provide genetic evidence that epithelial Rap activation and Canoe effector usage are conferred by the Drosophila PDZ-GEF (dPDZ-GEF) exchange factor. We demonstrate that dPDZ-GEF/Rap/Canoe signaling modulates cell shape and apicolateral cell constriction in embryonic and wing disc epithelia. In dPDZ-GEF mutant embryos with strong dorsal closure defects, cells in the lateral ectoderm fail to properly elongate. Postembryonic dPDZ-GEF mutant cells generated in mosaic tissue display a striking extension of lateral cell perimeters in the proximity of junctional complexes, suggesting a loss of normal cell contractility. Furthermore, our data indicate that dPDZ-GEF signaling is linked to myosin II function. Both dPDZ-GEF and cno show strong genetic interactions with the myosin II-encoding gene, and myosin II distribution is severely perturbed in epithelia of both mutants. These findings provide the first insight into the molecular machinery targeted by Rap signaling to modulate epithelial plasticity. We propose that dPDZ-GEF-dependent signaling functions as a rheostat linking Rap activity to the regulation of cell shape in epithelial morphogenesis at different developmental stages.

Actomyosin contractility and microtubules drive apical constriction in Xenopus bottle cells

Cell shape changes are critical for morphogenetic events such as gastrulation, neurulation, and organogenesis. However, the cell biology driving cell shape changes is poorly understood, especially in vertebrates. The beginning of Xenopus laevis gastrulation is marked by the apical constriction of bottle cells in the dorsal marginal zone, which bends the tissue and creates a crevice at the blastopore lip. We found that bottle cells contribute significantly to gastrulation, as their shape change can generate the force required for initial blastopore formation. As actin and myosin are often implicated in contraction, we examined their localization and function in bottle cells. F-actin and activated myosin accumulate apically in bottle cells, and actin and myosin inhibitors either prevent or severely perturb bottle cell formation, showing that actomyosin contractility is required for apical constriction. Microtubules were localized in apicobasally directed arrays in bottle cells, emanating from the apical surface. Surprisingly, apical constriction was inhibited in the presence of nocodazole but not taxol, suggesting that intact, but not dynamic, microtubules are required for apical constriction. Our results indicate that actomyosin contractility is required for bottle cell morphogenesis and further suggest a novel and unpredicted role for microtubules during apical constriction.

Control of morphogenetic cell movements in the early zebrafish myotome

As the vertebrate myotome is generated, myogenic precursor cells undergo extensive and coordinated movements as they differentiate into properly positioned embryonic muscle fibers. In the zebrafish, the "adaxial" cells adjacent to the notochord are the first muscle precursors to be specified. After initially differentiating into slow-twitch myosin-expressing muscle fibers, these cells have been shown to undergo a remarkable radial migration through the lateral somite, to populate the superficial layer of slow-twitch muscle of the mature myotome. Here we characterize an earlier set of adaxial cell behaviors; the transition from a roughly 4x5 array of cuboidal cells to a 1x20 stack of elongated cells, prior to the migration event. We find that adaxial cells display a highly stereotypical series of behaviors as they undergo this rearrangement. Furthermore, we show that the actin regulatory molecule, Cap1, is specifically expressed in adaxial cells and is required for the progression of these behaviors. The requirement of Cap1 for a cellular apical constriction step is reminiscent of similar requirements of Cap during apical constriction in Drosophila development, suggesting a conservation of gene function for a cell biological event critical to many developmental processes.

Tumorhead distribution to cytoplasmic membrane of neural plate cells is positively regulated by Xenopus p21-activated kinase 1 (X-PAK1)

Tumorhead (TH) regulates neural plate cell proliferation during Xenopus early development, and gain or loss of function prevents neural differentiation. TH shuttles between the nuclear and cytoplasmic/cortical cell compartments in embryonic cells. In this study, we show that subcellular distribution of TH is important for its functions. Targeting TH to the cell cortex/membrane potentiates a TH gain of function phenotype and results in neural plate expansion and inhibition of neuronal differentiation. We have found that TH subcellular localization is regulated, and that its shuttling between the nucleus and the cell cortex/cytoplasm is controlled by the catalytic activity of p21-activated kinase, X-PAK1. The phenotypes of embryos that lack, or have excess, X-PAK1 activity mimic the phenotypes induced by loss or gain of TH functions, respectively. We provide evidence that X-PAK1 is an upstream regulator of TH and discuss potential functions of TH at the cell cortex/cytoplasmic membrane and in the nucleus.

Stem cell growth becomes predominant while neural plate progenitor pool decreases during spinal cord elongation

The antero-posterior dispersion of clonally related cells is a prominent feature of axis elongation in vertebrate embryos. Two major models have been proposed: (i) the intercalation of cells by convergent-extension and (ii) the sequential production of the forming axis by stem cells. The relative importance of both of these cell behaviors during the long period of elongation is poorly understood. Here, we use a combination of single cell lineage tracing in the mouse embryo, computer modeling and confocal video-microscopy of GFP labeled cells in the chick embryo to address the mechanisms involved in the antero-posterior dispersion of clones. In the mouse embryo, clones appear as clusters of labeled cells separated by intervals of non-labeled cells. The distribution of intervals between clonally related clusters correlates with a statistical model of a stem cell mode of growth only in the posterior spinal cord. A direct comparison with published data in zebrafish suggests that elongation of the anterior spinal cord involves similar intercalation processes in different vertebrate species. Time-lapse analyses of GFP labeled cells in cultured chick embryos suggest a decrease in the size of the neural progenitor pool and indicate that the dispersion of clones involves ordered changes of neighborhood relationships. We propose that a pre-existing stem zone of growth becomes predominant to form the posterior half of the axis. This temporal change in tissue-level motion is discussed in terms of the clonal and genetic continuities during axis elongation.

Shroom family proteins regulate gamma-tubulin distribution and microtubule architecture during epithelial cell shape change

Cell shape changes require the coordination of actin and microtubule cytoskeletons. The molecular mechanisms by which such coordination is achieved remain obscure, particularly in the context of epithelial cells within developing vertebrate embryos. We have identified a novel role for the actin-binding protein Shroom3 as a regulator of the microtubule cytoskeleton during epithelial morphogenesis. We show that Shroom3 is sufficient and also necessary to induce a redistribution of the microtubule regulator gamma-tubulin. Moreover, this change in gamma-tubulin distribution underlies the assembly of aligned arrays of microtubules that drive apicobasal cell elongation. Finally, experiments with the related protein, Shroom1, demonstrate that gamma-tubulin regulation is a conserved feature of this protein family. Together, the data demonstrate that Shroom family proteins govern epithelial cell behaviors by coordinating the assembly of both microtubule and actin cytoskeletons.

Synapses: sites of cell recognition, adhesion, and functional specification

Synapses are specialized adhesive contacts characteristic of many types of cell-cell interactions involving neurons, immune cells, epithelial cells, and even pathogens and host cells. Cell-cell adhesion is mediated by structurally diverse classes of cell-surface glycoproteins, which form homophilic or heterophilic interactions across the intercellular space. Adhesion proteins bind to a cytoplasmic network of scaffolding proteins, regulators of the actin cytoskeleton, and signal transduction pathways that control the structural and functional organization of synapses. The themes of this review are to compare the organization of synapses in different cell types and to understand how different classes of cell adhesion proteins and cytoplasmic protein networks specify the assembly of functionally distinct synapses in different cell contexts.

Gastrulation: Wnts signal constriction

Recent work shows that Wnt signaling directly regulates the apical constriction that drives gastrulation movements in Caenorhabditis elegans, and also promotes invagination in sea urchins, providing a novel and possibly conserved mode of developmental regulation.

Wnt/Frizzled signaling controls C. elegans gastrulation by activating actomyosin contractility

BACKGROUND

Embryonic patterning mechanisms regulate the cytoskeletal machinery that drives morphogenesis, but there are few cases where links between patterning mechanisms and morphogenesis are well understood. We have used a combination of genetics, in vivo imaging, and cell manipulations to identify such links in C. elegans gastrulation. Gastrulation in C. elegans begins with the internalization of endodermal precursor cells in a process that depends on apical constriction of ingressing cells.

RESULTS

We show that ingression of the endodermal precursor cells is regulated by pathways, including a Wnt-Frizzled signaling pathway, that specify endodermal cell fate. We find that Wnt signaling has a role in gastrulation in addition to its earlier roles in regulating endodermal cell fate and cell-cycle timing. In the absence of Wnt signaling, endodermal precursor cells polarize and enrich myosin II apically but fail to contract their apical surfaces. We show that a regulatory myosin light chain normally becomes phosphorylated on the apical side of ingressing cells at a conserved site that can lead to myosin-filament formation and contraction of actomyosin networks and that this phosphorylation depends on Wnt signaling.

CONCLUSIONS

We conclude that Wnt signaling regulates C. elegans gastrulation through regulatory myosin light-chain phosphorylation, which results in the contraction of the apical surface of ingressing cells. These findings forge new links between cell-fate specification and morphogenesis, and they represent a novel mechanism by which Wnt signaling can regulate morphogenesis.

Shroom2 (APXL) regulates melanosome biogenesis and localization in the retinal pigment epithelium

Shroom family proteins have been implicated in the control of the actin cytoskeleton, but so far only a single family member has been studied in the context of developing embryos. Here, we show that the Shroom-family protein,Shroom2 (previously known as APXL) is both necessary and sufficient to govern the localization of pigment granules at the apical surface of epithelial cells. In Xenopus embryos that lack Shroom2 function, we observed defects in pigmentation of the eye that stem from failure of melanosomes to mature and to associate with the apical cell surface. Ectopic expression of Shroom2 in naïve epithelial cells facilitates apical pigment accumulation, and this activity specifically requires the Rab27a GTPase. Most interestingly, we find that Shroom2, like Shroom3 (previously called Shroom),is sufficient to induce a dramatic apical accumulation of the microtubule-nucleating protein γ-tubulin at the apical surfaces of naïve epithelial cells. Together, our data identify Shroom2 as a central regulator of RPE pigmentation, and suggest that, despite their diverse biological roles, Shroom family proteins share a common activity. Finally,because the locus encoding human SHROOM2 lies within the critical region for two distinct forms of ocular albinism, it is possible that SHROOM2mutations may be a contributing factor in these human visual system disorders.

N-cadherin is required for the polarized cell behaviors that drive neurulation in the zebrafish

Through the direct analysis of cell behaviors, we address the mechanisms underlying anterior neural tube morphogenesis in the zebrafish and the role of the cell adhesion molecule N-cadherin (N-cad) in this process. We demonstrate that although the mode of neurulation differs at the morphological level between amphibians and teleosts, the underlying cellular mechanisms are conserved. Contrary to previous reports, the zebrafish neural plate is a multi-layered structure, composed of deep and superficial cells that converge medially while undergoing radial intercalation, to form a single cell-layered neural tube. Time-lapse recording of individual cell behaviors reveals that cells are polarized along the mediolateral axis and exhibit protrusive activity. In N-cad mutants, both convergence and intercalation are blocked. Moreover, although N-cad-depleted cells are not defective in their ability to form protrusions, they are unable to maintain them stably. Taken together, these studies uncover key cellular mechanisms underlying neural tube morphogenesis in teleosts, and reveal a role for cadherins in promoting the polarized cell behaviors that underlie cellular rearrangements and shape the vertebrate embryo.

Smurf1 regulates neural patterning and folding in Xenopus embryos by antagonizing the BMP/Smad1 pathway

The ubiquitin ligase Smurf1 can target a handful of signaling proteins for ubiquitin-mediated proteasomal destruction or functional modification, including TGF-beta receptors, Smads, transcription factors, RhoA and MEKK2. Smurf1 was initially implicated in BMP pathway regulation in embryonic development, but its potential role in vertebrate embryogenesis has yet to be clarified. Here we demonstrate that inhibition of Smurf1 in Xenopus laevis embryos with an antisense morpholino oligonucleotide or a dominant-negative protein disrupts early development, with the nervous system being the principal target. Smurf1 is enriched on the dorsal side of gastrula stage embryos, and blocking Smurf1 disturbs neural folding and neural, but not mesoderm differentiation, enhances BMP/Smad1 signaling, and elevates phospho-Smad1 levels in the dorsal ectoderm. We conclude that in Xenopus embryos, the BMP pathway is a major physiological target of Smurf1, and we propose that in normal development Smurf1 cooperates with secreted BMP antagonists to limit BMP signaling in dorsal ectoderm. Our data also reveal a novel role for Smurf1 and Smad1 in neural plate morphogenesis.

Differential actin-dependent localization modulates the evolutionarily conserved activity of Shroom family proteins

Shroom is an actin-associated determinant of cell morphology that is required for neural tube closure in both mice and frogs. Shroom regulates this process by causing apical constriction of epithelial cells via a pathway involving myosin II. Here we report on characterization of the Shroom-related proteins Apxl and KIAA1202 and their role in cell architecture. Shroom, Apxl, and KIAA1202 exhibit differing abilities to interact with the actin cytoskeleton. In fibroblasts, Shroom readily associates with actin stress fibers and induces bundling, Apxl is found on cortical actin, and KIAA1202 is localized to a cytoplasmic population of F-actin. In epithelial cells, Apxl and KIAA1202 do not induce apical constriction as Shroom does, but have the capacity to do so if targeted to the apical junctional complex. To determine whether the activity of Shroom-like proteins is conserved in invertebrates, we have tested the ability of the lone Shroomrelated protein in Drosophila, CG8603, to activate the constriction pathway. A chimeric protein consisting of the Shroom targeting domain and the Drosophila protein elicits constriction. Finally, we show that Apxl is involved in regulating the cytoskeletal organization and architecture of endothelial cells. We predict that the ability of Shroom-like proteins to regulate cellular morphology is conserved in evolution and is regulated in part by subcellular localization.

A new standard nomenclature for proteins related to Apx and Shroom

Shroom is a recently-described regulator of cell shape changes in the developing nervous system. This protein is a member of a small family of related proteins that are defined by sequence similarity and in most cases by some link to the actin cytoskeleton. At present these proteins are named Shroom, APX, APXL, and KIAA1202. In light of the growing interest in this family of proteins, we propose here a new standard nomenclature.

Regulatory network for cell shape changes during Drosophila ventral furrow formation

Rapid and sequential cell shape changes take place during the formation of the ventral furrow (VF) at the beginning of Drosophila gastrulation. At the cellular level, this morphogenetic event demands close coordination of the proteins involved in actin cytoskeletal reorganization. In order to construct a regulatory network that describes these cell shape changes, we have used published genetic and molecular data for 18 genes encoding transcriptional regulators and signaling pathway components. Based on the dynamic behavior of this network we explored the hypothesis that the combination of three recognizable phenotypes describing wild type or mutant cell types, during VF invagination, correspond to different activation states of a specific set of these gene products, which are point attractors of the regulatory network. From our results, we recognize missing components in the regulatory network and suggest alternative pathways in the regulation of cell shape changes during VF formation.

A requirement for NF-protocadherin and TAF1/Set in cell adhesion and neural tube formation

Neurulation in vertebrates is an intricate process requiring extensive alterations in cell contacts and cellular morphologies as the cells in the neural ectoderm shape and form the neural folds and neural tube. Despite these complex interactions, little is known concerning the molecules that mediate cell adhesion within the embryonic neural plate and neural folds. Here, we demonstrate the requirement for NF-protocadherin (NFPC) and its cytosolic partner TAF1/Set for proper neurulation in Xenopus. Both NFPC and TAF1 function in cell-cell adhesion in the neural ectoderm, and disruptions in either NFPC or TAF1 result in a failure of the neural tube to close. This neural tube defect can be attributed to a lack of proper organization of the cells in the dorsal neural folds, manifested by a loss in the columnar epithelial morphology and apical localization of F-actin. However, the epidermal ectoderm is still able to migrate and cover the open neural tube, indicating that the fusions of the neural tube and epidermis are separate events. These studies demonstrate that NFPC and TAF1 function to maintain proper cell-cell interactions within the neural folds and suggest that NFPC and TAF1 participate in novel adhesive mechanisms that contribute to the final events of vertebrate neurulation.

Using genomewide mutagenesis screens to identify the genes required for neural tube closure in the mouse

BACKGROUND

Neural tube closure is a critical embryological process that requires the coordination of many molecular and cellular events. Only recently has the molecular basis of the cell movements that drive neural tube closure begun to be elucidated. This has been accomplished in part due to the analysis of a growing number of genetically targeted and naturally occurring mouse mutant strains that have neural tube defects (NTDs). Currently there are more than 100 genes that when mutated result in NTDs in the mouse. Yet only approximately 10% of genes in the mouse genome have been mutated and their gross phenotype analyzed, suggesting that only a small percentage of the genes that can cause NTDs have been identified.

METHODS

In order to more systematically and fully understand the genetic basis of neural tube closure and to begin to define the molecular pathways that direct this key embryonic event, our laboratories have undertaken a forward genetic screen in mice. From this we hope to gain a better understanding of the regulation of this complex morphogenic processes.

CONCLUSIONS

The mouse provides a good model for human neural tube closure, and therefore the information gained from generating novel mouse models of NTDs will help to predict the genes responsible for human NTDs and provide experimental evidence for how they function.

Shroom4 (Kiaa1202) is an actin-associated protein implicated in cytoskeletal organization

All animal cells utilize a specialized set of cytoskeletal proteins to determine their overall shape and the organization of their intracellular compartments and organelles. During embryonic development, the dynamic nature of the actin cytoskeleton is critical for virtually all morphogenic events requiring changes in cell shape, migration, adhesion, and division. The behavior of the actin cytoskeleton is modulated by a myriad of accessory proteins. Shroom3 is an actin binding protein that regulates neural tube morphogenesis by eliciting changes in cell shape through a myosin II-dependent pathway. The Shroom-related gene SHROOM4 (formerly called KIAA1202) has also been implicated in neural development, as mutations in this gene are associated with human X-linked mental retardation. To better understand the function of Shrm4 in embryonic development, we have cloned mouse Shroom4 and characterized its protein product in vivo and in vitro. Shroom4 is expressed in a wide range of cell types during mouse development, including vascular endothelium and the polarized epithelium of the neural tube and kidney. In endothelial cells and embryo fibroblasts, endogenous Shroom4 co-distributes with myosin II to a distinct cytoplasmic population of F-actin and ectopic expression of Shroom4 in multiple cell types enhances or induces the formation of this actin-based structure. This localization is mediated, at least in part, by the direct interaction of Shroom4 and F-actin. Our results suggest that Shroom4 is a regulator of cytoskeletal architecture that may play an important role in vertebrate development.

Disruptions of the novel KIAA1202 gene are associated with X-linked mental retardation

The extensive heterogeneity underlying the genetic component of mental retardation (MR) is the main cause for our limited understanding of the aetiology of this highly prevalent condition. Hence we set out to identify genes involved in MR. We investigated the breakpoints of two balanced X;autosome translocations in two unrelated female patients with mild/moderate MR and found that the Xp11.2 breakpoints disrupt the novel human KIAA1202 (hKIAA1202) gene in both cases. We also identified a missense exchange in this gene, segregating with the Stocco dos Santos XLMR syndrome in a large four-generation pedigree but absent in >1,000 control X-chromosomes. Among other phenotypic characteristics, the affected males in this family present with severe MR, delayed or no speech, seizures and hyperactivity. Molecular studies of hKIAA1202 determined its genomic organisation, its expression throughout the brain and the regulation of expression of its mouse homologue during development. Transient expression of the wild-type KIAA1202 protein in HeLa cells showed partial colocalisation with the F-actin based cytoskeleton. On the basis of its domain structure, we argue that hKIAA1202 is a new member of the APX/Shroom protein family. Members of this family contain a PDZ and two ASD domains of unknown function and have been shown to localise at the cytoskeleton, and play a role in neurulation, cellular architecture, actin remodelling and ion channel function. Our results suggest that hKIAA1202 may be important in cognitive function and/or development.

Shroom regulates epithelial cell shape via the apical positioning of an actomyosin network

The actin-binding protein Shroom is essential for neural tube morphogenesis in multiple vertebrate organisms, indicating its function is evolutionarily conserved. Shroom facilitates neurulation by regulating the morphology of neurepithelial cells. Shroom localizes to the apical tip of adherens junctions of neural ectoderm cells in vivo and to the apical junctional complex (AJC) in MDCK cells. Induced expression of Shroom in polarized epithelia elicits apical constriction and dramatic reorganization of the apical arrangement and packing of cells without altering apical-basal polarity. These events likely mimic the cell shape changes and cellular movements required for neurulation in vivo. The observed phenotypes depend on the ability of Shroom to alter F-actin distribution and regulate the formation of a previously uncharacterized contractile actomyosin network associated with the AJC. Targeting the C-terminal domain of Shroom to the apical plasma membrane elicits constriction and reorganization of the actomyosin network, indicting that this domain mediates Shroom's activity. In vivo, Shroom-mutant neural epithelia show a marked reduction in apically positioned myosin. Thus, Shroom likely facilitates neural tube closure by regulating cell shape changes via the apical positioning of an actomyosin network in the neurepithelium.

xBtg-x regulates Wnt/beta-Catenin signaling during early Xenopus development

In Xenopus, two signaling systems, maternal beta-Catenin and Nodal-related, are required for induction of the Spemann organizer and establishment of the body plan. By screening cDNA macroarrays for genes activated by these two signaling pathways, we identified Xenopus xBtg-x, a novel member of the Btg/Tob gene family of antiproliferative proteins. We show that xBtg-x is expressed in the dorsal mesendoderm (Spemann organizer tissue) of gastrula stage embryos and that its expression is regulated by both beta-Catenin and Nodal-related signals. Microinjection of synthetic xBtg-x mRNA into Xenopus embryos induced axis duplication and completely rescued the ventralizing effects of UV irradiation through the activation of the canonical Wnt/beta-Catenin signaling pathway. Interestingly, xBtg-x stimulated beta-Catenin-dependent transcription without affecting the stability of beta-Catenin protein. These data suggest that xBtg-x is a novel component of the Wnt/beta-Catenin signaling pathway regulating early embryonic patterning.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Neural tube closure and neural tube defects: studies in animal models reveal known knowns and known unknowns

The vertebrate central nervous system is a hollow structure that develops first as a flat sheet of cells and subsequently rolls into a tube during embryogenesis. Failure of this rolling process, called neural tube closure, results in a class of common human birth defects called neural tube defects. The cellular and molecular mechanisms governing neural tube closure have been studied extensively in animal models, but much remains to be elucidated. In this review, I will highlight recent progress in understanding neural tube closure mechanisms and how these studies can inform our search for the genes that underlie human neural tube defects. Supplementary material for this article can be found on the American Journal of Medical Genetics (Part C) website (http://www.mrw.interscience.wiley.com/suppmat/1552-4868/suppmat/2005/135/v135.1.wallingford.html)

aPKC, Crumbs3 and Lgl2 control apicobasal polarity in early vertebrate development

In early vertebrate development, apicobasally polarised blastomeres divide to produce inner non-polarised cells and outer polarised cells that follow different fates. How the polarity of these early blastomeres is established is not known. We have examined the role of Crumbs3, Lgl2 and the apical aPKC in the polarisation of frog blastomeres. Lgl2 localises to the basolateral membrane of blastomeres, while Crumbs3 localises to the apical and basolateral membranes. Overexpression aPKC and Crumbs3 expands the apical domain at the expense of the basolateral and repositions tight junctions in the new apical-basolateral interface. Loss of aPKC function causes loss of apical markers and redirects basolateral markers ectopically to the apical membrane. Cell polarity and tight junctions, but not cell adhesion,are lost and outer polarised cells become inner-like apolar cells. Overexpression of Xenopus Lgl2 phenocopies the aPKCknockout, suggesting that Lgl2 and aPKC act antagonistically. This was confirmed by showing that aPKC and Lgl2 can inhibit the localisation of each other and that Lgl2 rescues the apicalisation caused by aPKC. We conclude that an instrumental antagonistic interaction between aPKC and Lgl2 defines apicobasal polarity in early vertebrate development.

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.

The interface between cell and developmental biology

Cell differentiation, morphology, migration, polarity, intercellular communication and adhesion are all cellular processes that control embryo morphogenesis and lie at the interface of cell and developmental biology. The interface between these two fields is best illustrated, however, in studies of axiation and cytoskeletal remodeling during development. Recent advances reveal novel mechanisms for axiation, including the role of RNA and protein degradation in regulating the timely expression of morphogenetic signals. Significant progress has also been made in identifying components of the cytoskeleton and the extracellular matrix that mediate embryonic cell migration and polarity. Cellular processes at the interface of cell and developmental biology are overseen by the Wnt signaling cascade that coordinates both axiation and cytoskeletal remodeling during development.