Regulation of cell polarity, radial intercalation and epiboly inXenopus: novel roles for integrin and fibronectin

Spindle position in symmetric cell divisions during epiboly is controlled by opposing and dynamic apicobasal forces

Orientation of cell division is a vital aspect of tissue morphogenesis and growth. Asymmetric divisions generate cell fate diversity and epithelial stratification, whereas symmetric divisions contribute to tissue growth, spreading, and elongation. Here, we describe a mechanism for positioning the spindle in symmetric cell divisions of an embryonic epithelium. We show that during the early stages of epiboly, spindles in the epithelium display dynamic behavior within the plane of the epithelium but are kept firmly within this plane to give a symmetric division. This dynamic stability relies on balancing counteracting forces: an apically directed force exerted by F-actin/myosin-2 via active cortical flow and a basally directed force mediated by microtubules and myosin-10. When both forces are disrupted, spindle orientation deviates from the epithelial plane, and epithelial surface is reduced. We propose that this dynamic mechanism maintains symmetric divisions while allowing the quick adjustment of division plane to facilitate even tissue spreading.

Fibronectin induces endothelial cell migration through β1 integrin and Src-dependent phosphorylation of fibroblast growth factor receptor-1 at tyrosines 653/654 and 766

The extracellular matrix microenvironment regulates cell phenotype and function. One mechanism by which this is achieved is the transactivation of receptor tyrosine kinases by specific matrix molecules. Here, we demonstrate that the provisional matrix protein, fibronectin (FN), activates fibroblast growth factor (FGF) receptor-1 (FGFR1) independent of FGF ligand in liver endothelial cells. FN activation of FGFR1 requires β1 integrin, as evidenced by neutralizing antibody and siRNA-based studies. Complementary genetic and pharmacologic approaches identify that the non-receptor tyrosine kinase Src is required for FN transactivation of FGFR1. Whereas FGF ligand-induced phosphorylation of FGFR1 preferentially activates ERK, FN-induced phosphorylation of FGFR1 preferentially activates AKT, indicating differential downstream signaling of FGFR1 in response to alternate stimuli. Mutation analysis of known tyrosine residues of FGFR1 reveals that tyrosine 653/654 and 766 residues are required for FN-FGFR1 activation of AKT and chemotaxis. Thus, our study mechanistically dissects a new signaling pathway by which FN achieves endothelial cell chemotaxis, demonstrates how differential phosphorylation profiles of FGFR1 can achieve alternate downstream signals, and, more broadly, highlights the diversity of mechanisms by which the extracellular matrix microenvironment regulates cell behavior through transactivation of receptor tyrosine kinases.

Endothelial-monocyte activating polypeptide II disrupts alveolar epithelial type II to type I cell transdifferentiation

Background

Distal alveolar morphogenesis is marked by differentiation of alveolar type (AT)-II to AT-I cells that give rise to the primary site of gas exchange, the alveolar/vascular interface. Endothelial-Monocyte Activating Polypeptide (EMAP) II, an endogenous protein with anti-angiogenic properties, profoundly disrupts distal lung neovascularization and alveolar formation during lung morphogenesis, and is robustly expressed in the dysplastic alveolar regions of infants with Bronchopulmonary dysplasia. Determination as to whether EMAP II has a direct or indirect affect on ATII→ATI trans-differentiation has not been explored.

Method

In a controlled nonvascular environment, an in vitro model of ATII→ATI cell trans-differentiation was utilized to demonstrate the contribution that one vascular mediator has on distal epithelial cell differentiation.

Results

Here, we show that EMAP II significantly blocked ATII→ATI cell transdifferentiation by increasing cellular apoptosis and inhibiting expression of ATI markers. Moreover, EMAP II-treated ATII cells displayed myofibroblast characteristics, including elevated cellular proliferation, increased actin cytoskeleton stress fibers and Rho-GTPase activity, and increased nuclear:cytoplasmic volume. However, EMAP II-treated cells did not express the myofibroblast markers desmin or αSMA.

Conclusion

Our findings demonstrate that EMAP II interferes with ATII → ATI transdifferentiation resulting in a proliferating non-myofibroblast cell. These data identify the transdifferentiating alveolar cell as a possible target for EMAP II's induction of alveolar dysplasia.

Cadherin function during Xenopus gastrulation

Xenopus gastrulation consists of the orderly deformation of a single, multilayered cell sheet that resembles a multilayered epithelium, and flexible cell-cell adhesion has to provide tissue cohesion while allowing for cell rearrangements that drive gastrulation. A few classic cadherins are expressed in the Xenopus early embryo. The prominent C-cadherin is essential for the cohesion of the animal part of the gastrula including ectoderm and chordamesoderm, and it contributes to the adhesion of endoderm and anterior mesoderm in the vegetal moiety. The cadherin/catenin complex is expressed in a graded pattern which is stable during early development. Regional differences in cell adhesion conform to the graded cadherin/catenin expression pattern. However, although the cadherin/catenin pattern seems to be actively maintained, and cadherin function is modulated to reinforce differential adhesiveness, it is not clear how regional differences in tissue cohesion affect gastrulation. Manipulating cadherin expression or function does not induce cell sorting or boundary formation in the embryo. Moreover, known boundary formation mechanisms in the gastrula are based on active cell repulsion. Cell rearrangement is also compatible with variable tissue cohesion. Thus, identifying roles for differential adhesion in the Xenopus gastrula remains a challenge.

Regulation of integrins by conformation and traffic: it takes two to tango

In multicellular organisms, the execution of complex morphogenetic events, such as gastrulation or vascular morphogenesis, depends on the dynamic modulation of adhesion. Guidance cues, such as chemokines, growth factors, and semaphorins control the attachment of cells to extracellular matrix proteins by regulating the conformational activation of integrin receptors. The endo-exocytic traffic of integrins back and forth from the plasma membrane represents another crucial regulatory aspect in cell adhesion and motility. Recent work added an additional layer of complexity by indicating that distinct molecular machineries are required for trafficking active and inactive integrins.

Essential roles of fibronectin in the development of the left-right embryonic body plan

Studies in Xenopus laevis suggested that cell-extracellular matrix (ECM) interactions regulate the development of the left-right axis of asymmetry; however, the identities of ECM components and their receptors important for this process have remained unknown. We discovered that FN is required for the establishment of the asymmetric gene expression pattern in early mouse embryos by regulating morphogenesis of the node, while cellular fates of the nodal cells, canonical Wnt and Shh signaling within the node were not perturbed by the absence of FN. FN is also required for the expression of Lefty 1/2 and activation of SMADs 2 and 3 at the floor plate, while cell fate specification of the notochord and the floor plate, as well as signaling within and between these two embryonic organizing centers remained intact in FN-null mutants. Furthermore, our experiments indicate that a major cell surface receptor for FN, integrin α5β1, is also required for the development of the left-right asymmetry, and that this requirement is evolutionarily conserved in fish and mice. Taken together, our studies demonstrate the requisite role for a structural ECM protein and its integrin receptor in the development of the left-right axis of asymmetry in vertebrates.

The extracellular matrix dimension of skeletal muscle development

Cells anchor to substrates by binding to extracellular matrix (ECM). In addition to this anchoring function however, cell-ECM binding is a mechanism for cells to sense their surroundings and to communicate and coordinate behaviour amongst themselves. Several ECM molecules and their receptors play essential roles in muscle development and maintenance. Defects in these proteins are responsible for some of the most severe muscle dystrophies at every stage of life from neonates to adults. However, recent studies have also revealed a role of cell-ECM interactions at much earlier stages of development as skeletal muscle forms. Here we review which ECM molecules are present during the early phases of myogenesis, how myogenic cells interact with the ECM that surrounds them and the potential consequences of those interactions. We conclude that cell-ECM interactions play significant roles during all stages of skeletal muscle development in the embryo and suggest that this "extracellular matrix dimension" should be added to our conceptual network of factors contributing to skeletal myogenesis.

PDGF-A controls mesoderm cell orientation and radial intercalation during Xenopus gastrulation

Radial intercalation is a common, yet poorly understood, morphogenetic process in the developing embryo. By analyzing cell rearrangement in the prechordal mesoderm during Xenopus gastrulation, we have identified a mechanism for radial intercalation. It involves cell orientation in response to a long-range signal mediated by platelet-derived growth factor (PDGF-A) and directional intercellular migration. When PDGF-A signaling is inhibited, prechordal mesoderm cells fail to orient towards the ectoderm, the endogenous source of PDGF-A, and no longer migrate towards it. Consequently, the prechordal mesoderm fails to spread during gastrulation. Orientation and directional migration can be rescued specifically by the expression of a short splicing isoform of PDGF-A, but not by a long matrix-binding isoform, consistent with a requirement for long-range signaling.

Embryo mechanics: balancing force production with elastic resistance during morphogenesis

Morphogenesis requires the spatial and temporal control of embryo mechanics, including force production and mechanical resistance to those forces, to coordinate tissue deformation and large-scale movements. Thus, biomechanical processes play a key role in directly shaping the embryo. Additional roles for embryo mechanics during development may include the patterning of positional information and to provide feedback to ensure the success of morphogenetic movements in shaping the larval body and organs. To understand the multiple roles of mechanics during development requires familiarity with engineering principles of the mechanics of structures, the viscoelastic properties of biomaterials, and the integration of force and stress within embryonic structures as morphogenesis progresses. In this chapter, we review the basic engineering principles of biomechanics as they relate to morphogenesis, introduce methods for quantifying embryo mechanics and the limitations of these methods, and outline a formalism for investigating the role of embryo mechanics in birth defects. We encourage the nascent field of embryo mechanics to adopt standard engineering terms and test methods so that studies of diverse organisms can be compared and universal biomechanical principles can be revealed.

Autocrine fibronectin directs matrix assembly and crosstalk between cell–matrix and cell–cell adhesion in vascular endothelial cells

Cellular fibronectin (cFN) variants harboring extra FN type 3 repeats, namely extra domains B and A, are major constituents of the extracellular matrix around newly forming blood vessels during development and angiogenesis. Their expression is induced by angiogenic stimuli and their assembly into fibrillar arrays is driven by cell-generated tension at α5β1 integrin-based adhesions. Here, we examined the role and functional redundancy of cFN variants in cultured endothelial cells by isoform-selective RNA interference. We show that FN fibrillogenesis is a cell-autonomous process whereby basally directed secretion and assembly of cellular FN are tightly coupled events that play an important role not only in signaling at cell–matrix adhesions but also at cell–cell contacts. Silencing of cFN variants differentially affects integrin usage, cell spreading, motility and capillary morphogenesis in vitro. cFN-deficient cells undergo a switch from α5β1- to αvβ3-based adhesion, accompanied by a Src-regulated disruption of adherens junctions. These studies identify a crucial role for autocrine FN in subendothelial matrix assembly and junctional integrity that provides spatially and temporally restricted control of endothelial plasticity during angiogenic blood vessel remodeling.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.

Non-canonical Wnt signaling induces ubiquitination and degradation of Syndecan4

Dynamic regulation of cell adhesion receptors is required for proper cell migration in embryogenesis, tissue repair, and cancer. Integrins and Syndecan4 (SDC4) are the main cell adhesion receptors involved in focal adhesion formation and are required for cell migration. SDC4 interacts biochemically and functionally with components of the Wnt pathway such as Frizzled7 and Dishevelled. Non-canonical Wnt signaling, particularly components of the planar cell polarity branch, controls cell adhesion and migration in embryogenesis and metastatic events. Here, we evaluate the effect of this pathway on SDC4. We have found that Wnt5a reduces cell surface levels and promotes ubiquitination and degradation of SDC4 in cell lines and dorsal mesodermal cells from Xenopus gastrulae. Gain- and loss-of-function experiments demonstrate that Dsh plays a key role in regulating SDC4 steady-state levels. Moreover, a SDC4 deletion construct that interacts inefficiently with Dsh is resistant to Wnt5a-induced degradation. Non-canonical Wnt signaling promotes monoubiquitination of the variable region of SDC4 cytoplasmic domain. Mutation of these specific residues abrogates ubiquitination and results in increased SDC4 steady-state levels. This is the first example of a cell surface protein ubiquitinated and degraded in a Wnt/Dsh-dependent manner.

Integration of planar cell polarity and ECM signaling in elongation of the vertebrate body plan

The shaping of the vertebrate embryonic body plan depends heavily on the narrowing and lengthening (convergence and extension) of embryonic tissues by cell intercalation, a process by which cells actively crawl between one another along the axis of convergence to produce a narrower, longer array. We discuss recent evidence that the vertebrate non-canonical Wnt/Planar Cell Polarity (PCP) pathway, known to directly function in polarizing the movements of intercalating cells, is also involved in the localized assembly of extracellular matrix (ECM). These cell-ECM interactions, in turn, are necessary for expression of the oriented, polarized cell intercalation. The mechanism of PCP/ECM interactions, their molecular signaling, and their mechanical consequences for morphogenesis are discussed with the goal of identifying important unsolved issues.

Integrin alpha5beta1 function is regulated by XGIPC/kermit2 mediated endocytosis during Xenopus laevis gastrulation

During Xenopus gastrulation α5β1 integrin function is modulated in a temporally and spatially restricted manner, however, the regulatory mechanisms behind this regulation remain uncharacterized. Here we report that XGIPC/kermit2 binds to the cytoplasmic domain of the α5 subunit and regulates the activity of α5β1 integrin. The interaction of kermit2 with α5β1 is essential for fibronectin (FN) matrix assembly during the early stages of gastrulation. We further demonstrate that kermit2 regulates α5β1 integrin endocytosis downstream of activin signaling. Inhibition of kermit2 function impairs cell migration but not adhesion to FN substrates indicating that integrin recycling is essential for mesoderm cell migration. Furthermore, we find that the α5β1 integrin is colocalized with kermit2 and Rab 21 in embryonic and XTC cells. These data support a model where region specific mesoderm induction acts through kermit2 to regulate the temporally and spatially restricted changes in adhesive properties of the α5β1 integrin through receptor endocytosis.

From genes to neural tube defects (NTDs): insights from multiscale computational modeling

The morphogenetic movements, and the embryonic phenotypes they ultimately produce, are the consequence of a series of events that involve signaling pathways, cytoskeletal components, and cell- and tissue-level mechanical interactions. In order to better understand how these events work together in the context of amphibian neurulation, an existing multiscale computational model was augmented. Geometric data for this finite element-based mechanical model were obtained from 3D surface reconstructions of live axolotl embryos and serial sections of fixed specimens. Tissue mechanical properties were modeled using cell-based constitutive equations that include internal force generation and cell rearrangement, and equation parameters were adjusted manually to reflect biochemical changes including alterations in Shroom or the planar-cell-polarity pathway. The model indicates that neural tube defects can arise when convergent extension of the neural plate is reduced by as little as 20%, when it is eliminated on one side of the embryo, when neural ridge elevation is disrupted, when tension in the non-neural ectoderm is increased, or when the ectoderm thickness is increased. Where comparable conditions could be induced in Xenopus embryos, good agreement was found, an important step in model validation. The model reveals the neurulating embryo to be a finely tuned biomechanical system.

Morphogenesis of the primitive gut tube is generated by Rho/ROCK/myosin II-mediated endoderm rearrangements

During digestive organogenesis, the primitive gut tube (PGT) undergoes dramatic elongation and forms a lumen lined by a single-layer of epithelium. In Xenopus, endoderm cells in the core of the PGT rearrange during gut elongation, but the morphogenetic mechanisms controlling their reorganization are undetermined. Here, we define the dynamic changes in endoderm cell shape, polarity, and tissue architecture that underlie Xenopus gut morphogenesis. Gut endoderm cells intercalate radially, between their anterior and posterior neighbors, transforming the nearly solid endoderm core into a single layer of epithelium while concomitantly eliciting "radially convergent" extension within the gut walls. Inhibition of Rho/ROCK/Myosin II activity prevents endoderm rearrangements and consequently perturbs both gut elongation and digestive epithelial morphogenesis. Our results suggest that the cellular and molecular events driving tissue elongation in the PGT are mechanistically analogous to those that function during gastrulation, but occur within a novel cylindrical geometry to generate an epithelial-lined tube.

Binding of APC and dishevelled mediates Wnt5a-regulated focal adhesion dynamics in migrating cells

Wnt5a is a representative ligand that activates the Wnt/beta-catenin-independent pathway, resulting in the regulation of cell adhesion, migration, and polarity, but its molecular mechanism is poorly understood. This report shows that Dishevelled (Dvl) binds to adenomatous polyposis coli (APC) gene product, and this binding is enhanced by Wnt5a. Dvl was involved in the stabilization of the plus end dynamics of microtubules as well as APC. Frizzled2 (Fz2) was present with Wnt5a at the leading edge of migrating cells and formed a complex with APC through Dvl. Fz2 also interacted with integrins at the leading edge, and Dvl and APC associated with and activated focal adhesion kinase and paxillin. The binding of APC to Dvl enhanced the localization of paxillin to the leading edge and was involved in Wnt5a-dependent focal adhesion turnover. Furthermore, this new Wnt5a signalling pathway was important for the epithelial morphogenesis in the three-dimensional culture. These results suggest that the functional and physical interaction of Dvl and APC is involved in Wnt5a/Fz2-dependent focal adhesion dynamics during cell migration and epithelial morphogenesis.

Lipid phosphate phosphatase 3 stabilization of beta-catenin induces endothelial cell migration and formation of branching point structures

Endothelial cell (EC) migration, cell-cell adhesion, and the formation of branching point structures are considered hallmarks of angiogenesis; however, the underlying mechanisms of these processes are not well understood. Lipid phosphate phosphatase 3 (LPP3) is a recently described p120-catenin-associated integrin ligand localized in adherens junctions (AJs) of ECs. Here, we tested the hypothesis that LPP3 stimulates beta-catenin/lymphoid enhancer binding factor 1 (beta-catenin/LEF-1) to induce EC migration and formation of branching point structures. In subconfluent ECs, LPP3 induced expression of fibronectin via beta-catenin/LEF-1 signaling in a phosphatase and tensin homologue (PTEN)-dependent manner. In confluent ECs, depletion of p120-catenin restored LPP3-mediated beta-catenin/LEF-1 signaling. Depletion of LPP3 resulted in destabilization of beta-catenin, which in turn reduced fibronectin synthesis and deposition, which resulted in inhibition of EC migration. Accordingly, reexpression of beta-catenin but not p120-catenin in LPP3-depleted ECs restored de novo synthesis of fibronectin, which mediated EC migration and formation of branching point structures. In confluent ECs, however, a fraction of p120-catenin associated and colocalized with LPP3 at the plasma membrane, via the C-terminal cytoplasmic domain, thereby limiting the ability of LPP3 to stimulate beta-catenin/LEF-1 signaling. Thus, our study identified a key role for LPP3 in orchestrating PTEN-mediated beta-catenin/LEF-1 signaling in EC migration, cell-cell adhesion, and formation of branching point structures.

Identification of a novel Bves function: regulation of vesicular transport

Blood vessel/epicardial substance (Bves) is a transmembrane protein that influences cell adhesion and motility through unknown mechanisms. We have discovered that Bves directly interacts with VAMP3, a SNARE protein that facilitates vesicular transport and specifically recycles transferrin and beta-1-integrin. Two independent assays document that cells expressing a mutated form of Bves are severely impaired in the recycling of these molecules, a phenotype consistent with disruption of VAMP3 function. Using Morpholino knockdown in Xenopus laevis, we demonstrate that elimination of Bves function specifically inhibits transferrin receptor recycling, and results in gastrulation defects previously reported with impaired integrin-dependent cell movements. Kymographic analysis of Bves-depleted primary and cultured cells reveals severe impairment of cell spreading and adhesion on fibronectin, indicative of disruption of integrin-mediated adhesion. Taken together, these data demonstrate that Bves interacts with VAMP3 and facilitates receptor recycling both in vitro and during early development. Thus, this study establishes a newly identified role for Bves in vesicular transport and reveals a novel, broadly applied mechanism governing SNARE protein function.

Cell migration during morphogenesis

During development, functional structures must form with the correct three-dimensional geometry composed of the correct cell types. In many cases cell types are specified at locations distant to where they will ultimately reside for normal biological function. Although cell migration is crucial for normal development and morphogenesis of animal body plans and organ systems, abnormal cell migration during adult life underlies pathological states such as invasion and metastasis of cancer. In both contexts cells migrate either individually, as loosely associated sheets or as clusters of cells. In this review, we summarize, compare and integrate knowledge gained from several in vivo model systems that have yielded insights into the regulation of morphogenic cell migration, such as the zebrafish lateral line primordium and primordial germ cells, Drosophila border cell clusters, vertebrate neural crest migration and angiogenic sprouts in the post-natal mouse retina. Because of its broad multicontextual and multiphylletic distribution, understanding cell migration in its various manifestations in vivo is likely to provide new insights into both the function and malfunction of key embryonic and postembryonic events. In this review, we will provide a succinct phenotypic description of the many model systems utilized to study cell migration in vivo. More importantly, we will highlight, compare and integrate recent advances in our understanding of how cell migration is regulated in these varied model systems with special emphasis on individual and collective cell movements.

Multi-scale mechanics from molecules to morphogenesis

Dynamic mechanical processes shape the embryo and organs during development. Little is understood about the basic physics of these processes, what forces are generated, or how tissues resist or guide those forces during morphogenesis. This review offers an outline of some of the basic principles of biomechanics, provides working examples of biomechanical analyses of developing embryos, and reviews the role of structural proteins in establishing and maintaining the mechanical properties of embryonic tissues. Drawing on examples we highlight the importance of investigating mechanics at multiple scales from milliseconds to hours and from individual molecules to whole embryos. Lastly, we pose a series of questions that will need to be addressed if we are to understand the larger integration of molecular and physical mechanical processes during morphogenesis and organogenesis.

Extracellular matrix assembly and organization during zebrafish gastrulation

Zebrafish gastrulation entails morphogenetic cell movements that shape the body plan and give rise to an embryo with defined anterior-posterior and dorsal-ventral axes. Regulating these cell movements are diverse signaling pathways and proteins including Wnts, Src-family tyrosine kinases, cadherins, and matrix metalloproteinases. While our knowledge of how these proteins impact cell polarity and migration has advanced considerably in the last decade, almost no data exist regarding the organization of extracellular matrix (ECM) during zebrafish gastrulation. Here, we describe for the first time the assembly of a fibronectin (FN) and laminin containing ECM in the early zebrafish embryo. This matrix was first detected at early gastrulation (65% epiboly) in the form of punctae that localize to tissue boundaries separating germ layers from each other and the underlying yolk cell. Fibrillogenesis increased after mid-gastrulation (80% epiboly) coinciding with the period of planar cell polarity pathway-dependent convergence and extension cell movements. We demonstrate that FN fibrils present beneath deep mesodermal cells are aligned in the direction of membrane protrusion formation. Utilizing antisense morpholino oligonucleotides, we further show that knockdown of FN expression causes a convergence and extension defect. Taken together, our data show that similar to amphibian embryos, the formation of ECM in the zebrafish gastrula is a dynamic process that occurs in parallel to at least a portion of the polarized cell behaviors shaping the embryonic body plan. These results provide a framework for uncovering the interrelationship between ECM structure and cellular processes regulating convergence and extension such as directed migration and mediolateral/radial intercalation.

Imaging morphogenesis, in Xenopus with Quantum Dot nanocrystals

Mesoderm migration is a well studied morphogenetic movement that takes place during Xenopus gastrulation. The study of mesoderm migration and other morphogenetic movements has been primarily based on in vitro assays due to the inability to image deep tissue movements in the opaque embryo. We are the first to report the use of Near Infra Red Quantum Dots (NIR QD's) to image mesoderm migration in vivo with single cell resolution and provide quantitative in vivo data regarding migration rates. In addition we use QD's to address the function of the focal adhesion kinase (FAK) in this movement. Inhibition of FAK blocks mesoderm spreading and migration both in vitro and in vivo without affecting convergent extension highlighting the molecular differences between the two movements. These results provide new insights about the role of FAK and of focal adhesions during gastrulation and provide a new tool for the study of morphogenesis in vivo.

Proteins on the catwalk: modelling the structural domains of the CCN family of proteins

The CCN family of proteins (CCN1, CCN2, CCN3, CCN4, CCN5 and CCN6) are multifunctional mosaic proteins that play keys roles in crucial areas of physiology such as angiogenesis, skeletal development tumourigenesis, cell proliferation, adhesion and survival. This expansive repertoire of functions comes through a modular structure of 4 discrete domains that act both independently and in concert. How these interactions with ligands and with neighbouring domains lead to the biological effects is still to be explored but the molecular structure of the domains is likely to play an important role in this. In this review we have highlighted some of the key features of the individual domains of CCN family of proteins based on their biological effects using a homology modelling approach.

Geometric control of tissue morphogenesis

Morphogenesis is the dynamic and regulated change in tissue form that leads to creation of the body plan and development of mature organs. Research over the past several decades has uncovered a multitude of genetic factors required for morphogenesis in animals. The behaviors of individual cells within a developing tissue are determined by combining these genetic signals with information from the surrounding microenvironment. At any point in time, the local microenvironment is influenced by macroscale tissue geometry, which sculpts long range signals by affecting gradients of morphogens and mechanical stresses. The geometry of a tissue thus acts as both a template and instructive cue for further morphogenesis.

The physical state of fibronectin matrix differentially regulates morphogenetic movements in vivo

This study demonstrates that proper spatiotemporal expression and the physical assembly state of fibronectin (FN) matrix play key roles in the regulation of morphogenetic cell movements in vivo. We examine the progressive assembly and 3D fibrillar organization of FN and its role in regulating cell and tissue movements in Xenopus embryos. Expression of the 70 kD N-terminal fragment of FN blocks FN fibril assembly at gastrulation but not initial FN binding to integrins at the cell surface. We find that fibrillar FN is necessary to maintain cell polarity through oriented cell division and to promote epiboly, possibly through maintenance of tissue-surface tension. In contrast, FN fibrils are dispensable for convergence and extension movements required for axis elongation. Closure of the migratory mesendodermal mantle was accelerated in the absence of a fibrillar matrix. Thus, the macromolecular assembly of FN matrices may constitute a general regulatory mechanism for coordination of distinct morphogenetic movements.

A novel role for MAPKAPK2 in morphogenesis during zebrafish development

One of the earliest morphogenetic processes in the development of many animals is epiboly. In the zebrafish, epiboly ensues when the animally localized blastoderm cells spread, thin over, and enclose the vegetally localized yolk. Only a few factors are known to function in this fundamental process. We identified a maternal-effect mutant, betty boop (bbp), which displays a novel defect in epiboly, wherein the blastoderm margin constricts dramatically, precisely when half of the yolk cell is covered by the blastoderm, causing the yolk cell to burst. Whole-blastoderm transplants and mRNA microinjection rescue demonstrate that Bbp functions in the yolk cell to regulate epiboly. We positionally cloned the maternal-effect bbp mutant gene and identified it as the zebrafish homolog of the serine-threonine kinase Mitogen Activated Protein Kinase Activated Protein Kinase 2, or MAPKAPK2, which was not previously known to function in embryonic development. We show that the regulation of MAPKAPK2 is conserved and p38 MAP kinase functions upstream of MAPKAPK2 in regulating epiboly in the zebrafish embryo. Dramatic alterations in calcium dynamics, together with the massive marginal constrictive force observed in bbp mutants, indicate precocious constriction of an F-actin network within the yolk cell, which first forms at 50% epiboly and regulates epiboly progression. We show that MAPKAPK2 activity and its regulator p38 MAPK function in the yolk cell to regulate the process of epiboly, identifying a new pathway regulating this cell movement process. We postulate that a p38 MAPKAPK2 kinase cascade modulates the activity of F-actin at the yolk cell margin circumference allowing the gradual closure of the blastopore as epiboly progresses.

One of the earliest cell movement processes in the development of many animals is epiboly. In the zebrafish, epiboly ensues when the blastoderm cells spread over and enclose the yolk cell. Only a few factors are known to function in this fundamental process. We identified a maternal-effect mutant, betty boop (bbp), which displays a novel defect in epiboly, wherein the blastoderm margin constricts dramatically, precisely when half of the yolk cell is covered by the blastoderm, causing the yolk cell to burst. We demonstrate that Bbp functions in the yolk cell to regulate epiboly. We positionally cloned the bbp mutant gene and identified it as the serine-threonine kinase Mitogen Activated Protein Kinase Activated Protein Kinase 2, or MAPKAPK2, which was not previously known to function in embryonic development. We show that the regulation of MAPKAPK2 is conserved within a p38 MAP kinase pathway, thus identifying a new pathway in the regulation of this fundamental cell movement process. We postulate that a p38 MAPKAPK2 kinase cascade modulates F-actin contraction at the yolk cell margin circumference, allowing the gradual closure of the cells over the yolk cell as epiboly progresses.

Noncanonical Wnt/PCP signaling during vertebrate gastrulation

The branch of the Wnt pathway, related to planar cell polarity signaling in Drosophila, is fundamental not only to the establishment of tissue polarity but also to a variety of morphogenetic processes in vertebrates. The genetic pathway has been noted for its similarity as well as divergence of between vertebrates and Drosophila. This review focuses on issues related to the complexity of the output of the planar cell polarity pathway during gastrulation in zebrafish and Xenopus and, to a lesser extent, during gastrulation/neurulation in mice.

Cadherin adhesion, tissue tension, and noncanonical Wnt signaling regulate fibronectin matrix organization

In this study we demonstrate that planar cell polarity signaling regulates morphogenesis in Xenopus embryos in part through the assembly of the fibronectin (FN) matrix. We outline a regulatory pathway that includes cadherin adhesion and signaling through Rac and Pak, culminating in actin reorganization, myosin contractility, and tissue tension, which, in turn, directs the correct spatiotemporal localization of FN into a fibrillar matrix. Increased mechanical tension promotes FN fibril assembly in the blastocoel roof (BCR), while reduced BCR tension inhibits matrix assembly. These data support a model for matrix assembly in tissues where cell-cell adhesions play an analogous role to the focal adhesions of cultured cells by transferring to integrins the tension required to direct FN fibril formation at cell surfaces.

Fibronectins in vascular morphogenesis

Fibronectin is an extracellular matrix protein found only in vertebrate organisms containing endothelium-lined vasculature and is required for cardiovascular development in fish and mice. Fibronectin and its splice variants containing EIIIA and EIIIB domains are highly upregulated around newly developing vasculature during embryogenesis and in pathological conditions including atherosclerosis, cardiac hypertrophy, and tumorigenesis. However, their molecular roles in these processes are not entirely understood. We review genetic studies examining functions of fibronectin and its splice variants during embryonic cardiovascular development, and discuss potential roles of fibronectin in vascular disease and tumor angiogenesis.

Cell adhesion in amphibian gastrulation

The amphibian gastrula can be regarded as a single coherent tissue which folds and distorts itself in a reproducible pattern to establish the embryonic germ layers. It is held together by cadherins which provide the flexible adhesion required for the massive cell rearrangements that accompany gastrulation. Cadherin expression and adhesiveness increase as one goes from the vegetal cell mass through the anterior mesendoderm to the chordamesoderm, and then decrease again slightly in the ectoderm. Together with a basic random component of cell motility, this flexible, differentially expressed adhesiveness generates surface and interfacial tension effects which, in principle, can exert strong forces. However, conclusive evidence for an in vivo role of differential adhesion-related effects in gastrula morphogenesis is still lacking. The most important morphogenetic process in the amphibian gastrula seems to be intercellular migration, where cells crawl actively across each other's surface. The crucial aspect of this process is that cell motility is globally oriented, leading for example to mediolateral intercalation of bipolar cells during convergent extension of the chordamesoderm or to the directional migration of unipolar cells during translocation of the anterior mesendoderm on the ectodermal blastocoel roof. During these movements, the boundary between ectoderm and mesoderm is maintained by a tissue separation process.

PACSIN2 regulates cell adhesion during gastrulation in Xenopus laevis

We previously identified the adaptor protein PACSIN2 as a negative regulator of ADAM13 proteolytic function. In Xenopus embryos, PACSIN2 is ubiquitously expressed, suggesting that PACSIN2 may control other proteins during development. To investigate this possibility, we studied PACSIN2 function during Xenopus gastrulation and in XTC cells. Our results show that PACSIN2 is localized to the plasma membrane via its coiled-coil domain. We also show that increased levels of PACSIN2 in embryos inhibit gastrulation, fibronectin (FN) fibrillogenesis and the ability of ectodermal cells to spread on a FN substrate. These effects require PACSIN2 coiled-coil domain and are not due to a reduction of FN or integrin expression and/or trafficking. The expression of a Mitochondria Anchored PACSIN2 (PACSIN2-MA) sequesters wild type PACSIN2 to mitochondria, and blocks gastrulation without interfering with cell spreading or FN fibrillogenesis but perturbs both epiboly and convergence/extension. In XTC cells, the over-expression of PACSIN2 but not PACSIN2-MA prevents the localization of integrin beta1 to focal adhesions (FA) and filamin to stress fiber. PACSIN2-MA prevents filamin localization to membrane ruffles but not to stress fiber. We propose that PACSIN2 may regulate gastrulation by controlling the population of activated alpha5beta1 integrin and cytoskeleton strength during cell movement.

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.

Membrane-type 1 matrix metalloproteinase regulates cell migration during zebrafish gastrulation: evidence for an interaction with non-canonical Wnt signaling

Key to invasiveness is the ability of tumor cells to modify the extracellular matrix, become motile, and engage in directed migration towards the vasculature. One significant protein associated with metastatic progression is membrane-type 1 matrix metalloproteinase (MT1-MMP/MMP14). How MMP14 activity is coordinated with other signaling pathways to regulate cell migration in vivo is largely unknown. Here we have used zebrafish embryogenesis as a model to understand the potential relationship between MMP14-dependent pericellular proteolysis, cell polarity, and motility. Knockdown of zebrafish Mmp14 function disrupted gastrulation convergence and extension cell movements and craniofacial morphogenesis. Using time-lapse imaging and morphometric analyses, we show that Mmp14 is required for proper cell polarity underlying the directed migration of mesodermal cells during gastrulation. We have identified a genetic interaction between mmp14 and non-canonical Wnt signaling, a pathway that also regulates cell polarity in embryonic tissues and is increasingly being linked with tumor cell migration. Finally, we demonstrate that Van Gogh-like 2, a key regulator of the non-canonical Wnt pathway, co-localizes with MMP14 and becomes redistributed towards the leading edge of polarized human cancer cells. Together, our results support the notion that pathways regulating pericellular proteolysis and cell polarity converge to promote efficient cell migration.

Morphogenic machines evolve more rapidly than the signals that pattern them: lessons from amphibians

The induction of mesoderm and the patterning of its dorsal-ventral and anterior-posterior axes seems to be relatively conserved throughout the chordates, as do the morphogenic movements that produce a phylotypic stage embryo. What is not conserved is the initial embryonic architecture of the fertilized egg, and the specific cell behaviors used to drive mesoderm morphogenesis. How then do conserved patterning pathways adapt to diverse architectures and where do they diverge to direct the different cell behaviors used to shape the phylotypic body plan? Amphibians in particular, probably because of their broad range of reproductive strategies, show diverse embryonic architectures across their class and use diverse cell behaviors during their early morphogenesis, making them an interesting comparative group. We examine three examples from our work on amphibians that show variations in the use of cell behaviors to drive the morphogenesis of the same tissues. We also consider possible points where the conserved patterning pathways might diverge to produce different cell behaviors.

Integrating morphogenesis with underlying mechanics and cell biology

Morphogenesis integrates a wide range of cellular processes into a self-organizing, self-deforming tissue. No single molecular "magic bullet" controls morphogenesis. Wide ranging cellular processes, often without parallels in conventional cell culture systems, work together to generate the architecture and modulate forces that produce and guide shape changes in the embryo. In this review we summarize the early development of the frog Xenopus laevis from a biomechanical perspective. We describe processes operating in the embryo from whole embryo scale, the tissue-scale, to the cellular and extracellular matrix scale. We focus on describing cells, their behaviors and the unique microenvironments they traverse during gastrulation and discuss the role of tissue mechanics in these processes.

Integrin alpha5 is required for somite rotation and boundary formation in Xenopus

The morphogenesis of somites in Xenopus laevis is characterized by a complex process of cell turning that requires coordinated regulation of cell shape, adhesion, and motility. The integrin alpha5 subunit has been implicated in the formation of somite boundaries in organisms utilizing epithelialization to create morphologically distinct somites, but its function has not been examined in Xenopus. We used a splice-blocking morpholino to knock down expression of integrin alpha5 during somite formation. Loss of integrin alpha5 delayed somite turning and accumulation of integrin beta1 at somite boundaries, and disrupted the fibronectin matrix surrounding developing somites. Irregular somite boundaries with a sparse and discontinuous fibronectin matrix formed upon eventual completion of somite turning. Recovery of somite morphology was improved, but still incomplete in far posterior somites. These data demonstrate that the role of integrin alpha5 in somite boundary formation is conserved in a species using a unique mechanism of somitogenesis.

Multiple cardiovascular defects caused by the absence of alternatively spliced segments of fibronectin

Alternatively spliced variants of fibronectin (FN) containing exons EIIIA and EIIIB are expressed around newly forming vessels in development and disease but are downregulated in mature vasculature. The sequences and patterns of expression of these splice variants are highly conserved among vertebrates, suggestive of their biological importance; however the functions of EIIIA and EIIIB-containing FNs are unknown. To understand the role(s) of these splice variants, we deleted both EIIIA and EIIIB exons from the FN gene and observed embryonic lethality with incomplete penetrance by embryonic day 10.5. Deletion of both EIIIA and EIIIB exons did not affect synthesis or cell surface deposition of FN, indicating that embryonic lethality was due specifically to the absence of EIIIA and EIIIB exons from FN. EIIIA/EIIIB double-null embryos displayed multiple embryonic cardiovascular defects, including vascular hemorrhage, failure of remodeling embryonic and yolk sac vasculature, defective placental angiogenesis and heart defects. In addition, we observed defects in coverage and association with dorsal aortae of alpha-smooth-muscle-actin-positive cells. Our studies indicate that the presence or absence of EIIIA and EIIIB exons alters the function of FN and demonstrate the requirement for these alternatively spliced exons in cardiovascular development.

Multiscale computational analysis of Xenopus laevis morphogenesis reveals key insights of systems-level behavior

Background

Tissue morphogenesis is a complex process whereby tissue structures self-assemble by the aggregate behaviors of independently acting cells responding to both intracellular and extracellular cues in their environment. During embryonic development, morphogenesis is particularly important for organizing cells into tissues, and although key regulatory events of this process are well studied in isolation, a number of important systems-level questions remain unanswered. This is due, in part, to a lack of integrative tools that enable the coupling of biological phenomena across spatial and temporal scales. Here, we present a new computational framework that integrates intracellular signaling information with multi-cell behaviors in the context of a spatially heterogeneous tissue environment.

Results

We have developed a computational simulation of mesendoderm migration in the Xenopus laevis explant model, which is a well studied biological model of tissue morphogenesis that recapitulates many features of this process during development in humans. The simulation couples, via a JAVA interface, an ordinary differential equation-based mass action kinetics model to compute intracellular Wnt/β-catenin signaling with an agent-based model of mesendoderm migration across a fibronectin extracellular matrix substrate. The emergent cell behaviors in the simulation suggest the following properties of the system: maintaining the integrity of cell-to-cell contact signals is necessary for preventing fractionation of cells as they move, contact with the Fn substrate and the existence of a Fn gradient provides an extracellular feedback loop that governs migration speed, the incorporation of polarity signals is required for cells to migrate in the same direction, and a delicate balance of integrin and cadherin interactions is needed to reproduce experimentally observed migratory behaviors.

Conclusion

Our computational framework couples two different spatial scales in biology: intracellular with multicellular. In our simulation, events at one scale have quantitative and dynamic impact on events at the other scale. This integration enables the testing and identification of key systems-level hypotheses regarding how signaling proteins affect overall tissue-level behavior during morphogenesis in an experimentally verifiable system. Applications of this approach extend to the study of tissue patterning processes that occur during adulthood and disease, such as tumorgenesis and atherogenesis.

Convergent extension and the hexahedral cell

During development, embryonic cells sculpt three-dimensional tissues. Although cell polarity is commonly analysed along one, and sometimes two, dimensions, this perspective illustrates how higher-order cell polarity regulates convergent extension - the coordinated cell rearrangement that produces solid tissue elongation.

Filamin B mutations cause chondrocyte defects in skeletal development

Filamin B (FLNB) is a cytoplasmic protein that regulates the cytoskeletal network by cross-linking actin, linking cell membrane to the cytoskeleton and regulating intracellular signaling pathways responsible for skeletal development (Stossel, T.P., Condeelis, J., Cooley, L., Hartwig, J.H., Noegel, A., Schleicher, M. and Shapiro, S.S. (2001) Filamins as integrators of cell mechanics and signalling. Nat. Rev. Mol. Cell Biol., 2, 138-145). Mutations in FLNB cause human skeletal disorders [boomerang dysplasia, spondylocarpotarsal (SCT), Larsen, and atelosteogenesis I/III syndromes], which are characterized by disrupted vertebral segmentation, joint formation and endochondral ossification [Krakow, D., Robertson, S.P., King, L.M., Morgan, T., Sebald, E.T., Bertolotto, C., Wachsmann-Hogiu, S., Acuna, D., Shapiro, S.S., Takafuta, T. et al. (2004) Mutations in the gene encoding filamin B disrupt vertebral segmentation, joint formation and skeletogenesis. Nat. Genet., 36, 405-410; Bicknell, L.S., Morgan, T., Bonafe, L., Wessels, M.W., Bialer, M.G., Willems, P.J., Cohn, D.H., Krakow, D. and Robertson, S.P. (2005) Mutations in FLNB cause boomerang dysplasia. J. Med. Genet., 42, e43]. Here we show that Flnb deficient mice have shortened distal limbs with small body size, and develop fusion of the ribs and vertebrae, abnormal spinal curvatures, and dysmorphic facial/calvarial bones, similar to the human phenotype. Characterization of the mutant mice demonstrated increased apoptosis along the bone periphery of the distal appendages, consistent with reduced bone width. No changes in the initial proliferative rate of chondrocytes were observed, but the progressive differentiation of chondrocyte precursors was impaired, consistent with reduced bone length. The extracellular matrix appeared disrupted and phosphorylated beta1-integrin (a collagen receptor and Flnb binding partner) expression was diminished in the mutant growth plate. Like integrin-deficient chondrocytes, adhesion to the ECM was decreased in Flnb(-/-) chondrocytes, and inhibition of beta1-integrin in these cells led to further impairments in cell spreading. These data suggest that disruption of the ECM-beta1-integrin-Flnb pathway contributes to defects in vertebral and distal limb development, similar to those seen in the human autosomal recessive SCT due to Flnb mutations.

Xenopus Tetraspanin-1 regulates gastrulation movements and neural differentiation in the early Xenopus embryo

The tetraspanin family of four-pass transmembrane proteins has been implicated in fundamental biological processes, including cell adhesion, migration, and proliferation. Tetraspanins interact with various transmembrane proteins, establishing a network of large multimolecular complexes that allows specific lateral secondary interactions. Here we report the identification and functional characterization of Xenopus Tetraspanin-1 (xTspan-1). At gastrula and neurula, xTspan-1 is expressed in the dorsal ectoderm and neural plate, respectively, and in the hatching gland, cement gland, and posterior neural tube at tailbud stages. The expression of xTspan-1 in the early embryo is negatively regulated by bone morphogenetic protein (BMP) and stimulated by Notch signals. Microinjection of xTspan-1 mRNA interfered with gastrulation movements and reduced ectodermal cell adhesion in a cadherin-dependent manner. Morpholino knock-down of endogenous xTspan-1 protein revealed a requirement of xTspan-1 for gastrulation movements and primary neurogenesis. Our data suggest that xTspan-1 could act as a molecular link between BMP signalling and the regulation of cellular interactions that are required for gastrulation movements and neural differentiation in the early Xenopus embryo.

Using Xenopus embryos to investigate integrin function

Xenopus embryos are a useful and important system for cell biological studies of integrin adhesion and signaling. Explants prepared from gastrulating embryos undergo normal morphogenetic movements when cultured in simple salt solutions. These preparations are accessible to a variety of experimental perturbations and time-lapse imaging at high resolution, making it possible to elucidate mechanisms of integrin function in intact tissues and whole embryos. Methods used for the visualization of integrins, cadherins, extracellular matrix, and cytoskeletal linkages in both fixed and live tissues are described. We also discuss the use of a novel explant preparation suitable for following the normal deposition and assembly of fibronectin fibrils by ectoderm and mesoderm at gastrulation.

Wnt'er in liver: expression of Wnt and frizzled genes in mouse

The Wnt signaling pathway is essential for a wide array of developmental and physiological processes. Wnts are extracellular ligands that bind to frizzled (Fz) receptors at the membrane, canonically inducing beta-catenin nuclear translocation and activation. Although beta-catenin has been shown to be critical in liver biology, the expression of the 19 Wnt and 10 Fz genes in liver remains undetermined. We report comprehensive analysis of Wnt and Fz expression in whole liver as well as individual cell types: freshly isolated and plated hepatocytes, biliary epithelial cells, normal and activated stellate and Kupffer cells, and sinusoidal endothelial cells (SECs). Oligonucleotides for the 19 Wnt, 10 frizzled receptors genes, and secreted Frizzled-related protein-1 (sFRP or Fzb) were synthesized based on the available sequences. A total of 11 Wnts and 8 Fz genes and Fzb were expressed in normal liver. Although only 6 Wnt and 5 Fz genes were expressed in freshly isolated hepatocytes, 8 Wnt genes, 7 Fz genes, and Fzb were expressed in plated hepatocytes. Although 12 Wnt and 7 Fz genes were expressed in biliary tree, additional Fz9 and Fzb were only expressed in cultured biliary epithelial cells. The same 14 Wnt and 7 Fz genes were expressed in both activated and normal stellate and Kupffer cells; only Fzb was expressed in their activated state. Also, 11 Wnt, seven Fz, and Fzb genes were expressed in SECs.

CONCLUSION

These data indicate that most Wnt and frizzled genes are expressed in the liver and might be playing important roles in liver pathobiology via canonical and noncanonical pathways.

Zebrafish Gastrulation: Cell Movements, Signals, and Mechanisms

Gastrulation is a morphogenetic process that results in the formation of the embryonic germ layers. Here we detail the major cell movements that occur during zebrafish gastrulation: epiboly, internalization, and convergent extension. Although gastrulation is known to be regulated by signaling pathways such as the Wnt/planar cell polarity pathway, many questions remain about the underlying molecular and cellular mechanisms. Key factors that may play a role in gastrulation cell movements are cell adhesion and cytoskeletal rearrangement. In addition, some of the driving force for gastrulation may derive from tissue interactions such as those described between the enveloping layer and the yolk syncytial layer. Future exploration of gastrulation mechanisms relies on the development of sensitive and quantitative techniques to characterize embryonic germ-layer properties.

Wnt11 controls cell contact persistence by local accumulation of Frizzled 7 at the plasma membrane

Wnt11 is a key signal, determining cell polarization and migration during vertebrate gastrulation. It is known that Wnt11 functionally interacts with several signaling components, the homologues of which control planar cell polarity in Drosophila melanogaster. Although in D. melanogaster these components are thought to polarize cells by asymmetrically localizing at the plasma membrane, it is not yet clear whether their subcellular localization plays a similarly important role in vertebrates. We show that in zebrafish embryonic cells, Wnt11 locally functions at the plasma membrane by accumulating its receptor, Frizzled 7, on adjacent sites of cell contacts. Wnt11-induced Frizzled 7 accumulations recruit the intracellular Wnt signaling mediator Dishevelled, as well as Wnt11 itself, and locally increase cell contact persistence. This increase in cell contact persistence is mediated by the local interaction of Wnt11, Frizzled 7, and the atypical cadherin Flamingo at the plasma membrane, and it does not require the activity of further downstream effectors of Wnt11 signaling, such as RhoA and Rok2. We propose that Wnt11, by interacting with Frizzled 7 and Flamingo, modulates local cell contact persistence to coordinate cell movements during gastrulation.

Regulation of Xenopus gastrulation by ErbB signaling

During Xenopus gastrulation, mesendodermal cells are internalized and display different movements. Head mesoderm migrates along the blastocoel roof, while trunk mesoderm undergoes convergent extension (C&E). Different signals are implicated in these processes. Our previous studies reveal that signals through ErbB receptor tyrosine kinases modulate Xenopus gastrulation, but the mechanisms employed are not understood. Here we report that ErbB signals control both C&E and head mesoderm migration. Inhibition of ErbB pathway blocks elongation of dorsal marginal zone explants and activin-treated animal caps without removing mesodermal gene expression. Bipolar cell shape and cell mixing in the dorsal region are impaired. Inhibition of ErbB signaling also interferes with migration of prechordal mesoderm on fibronectin. Cell-cell and cell-matrix interaction and cell spreading are reduced when ErbB signaling is blocked. Using antisense morpholino oligonucleotides, we show that ErbB4 is involved in Xenopus gastrulation morphogenesis, and it partially regulates cell movements through modulation of cell adhesion and membrane protrusions. Our results reveal for the first time that vertebrate ErbB signaling modulates gastrulation movements, thus providing a novel pathway, in addition to non-canonical Wnt and FGF signals, that controls gastrulation. We further demonstrate that regulation of cell adhesive properties and cell morphology may underlie the functions of ErbBs in gastrulation.

Integrin alpha5beta1 and fibronectin regulate polarized cell protrusions required for Xenopus convergence and extension

BACKGROUND

Integrin recognition of fibronectin is required for normal gastrulation including the mediolateral cell intercalation behaviors that drive convergent extension and the elongation of the frog dorsal axis; however, the cellular and molecular mechanisms involved are unclear.

RESULTS

We report that depletion of fibronectin with antisense morpholinos blocks both convergent extension and mediolateral protrusive behaviors in explant preparations. Both chronic depletion of fibronectin and acute disruptions of integrin alpha5beta1 binding to fibronectin increases the frequency and randomizes the orientation of polarized cellular protrusions, suggesting that integrin-fibronectin interactions normally repress frequent random protrusions in favor of fewer mediolaterally oriented ones. In the absence of integrin alpha5beta1 binding to fibronectin, convergence movements still occur but result in convergent thickening instead of convergent extension.

CONCLUSIONS

These findings support a role for integrin signaling in regulating the protrusive activity that drives axial extension. We hypothesize that the planar spatial arrangement of the fibrillar fibronectin matrix, which delineates tissue compartments within the embryo, is critical for promoting productive oriented protrusions in intercalating cells.

Syndecan-4 regulates non-canonical Wnt signalling and is essential for convergent and extension movements in Xenopus embryos

Early shaping of Xenopus laevis embryos occurs through convergent and extension movements, a process that is driven by intercalation of polarized dorsal mesodermal cells and regulated by non-canonical Wnt signalling. Here, we have identified Xenopus syndecan-4 (xSyn4), a cell-surface transmembrane heparan sulphate proteoglycan. At the gastrula stage, xSyn4 is expressed in the involuting dorsal mesoderm and the anterior neuroectoderm. Later, it is found in the pronephros, branchial arches, brain and tailbud. Both gain- and loss-of-function of xSyn4 impaired convergent extension movements in Xenopus embryos and in activin-treated ectodermal explants. xSyn4 interacts functionally and biochemically with the Wnt receptor Frizzled7 (xFz7) and its signal transducer Dishevelled (xDsh). Furthermore, xSyn4 is necessary and sufficient for translocation of xDsh to the plasma membrane - a landmark in the activation of non-canonical Wnt signalling. Our results suggest that the ability of xSyn4 to translocate xDsh is regulated by fibronectin, a component of the extracellular matrix required for proper convergent extension movements. We propose a model where xSyn4 and fibronectin cooperate with xFz7 and Wnt in the specific activation of the non-canonical Wnt pathway.