Integrinalpha5-dependent fibronectin accumulation for maintenance of somite boundaries in zebrafish embryos

Molecular mechanism for cyclic generation of somites: Lessons from mice and zebrafish

The somite is the most prominent metameric structure observed during vertebrate embryogenesis, and its metamerism preserves the characteristic structures of the vertebrae and muscles in the adult body. During vertebrate somitogenesis, sequential formation of epithelialized cell boundaries generates the somites. According to the "clock and wavefront model," the periodical and sequential generation of somites is achieved by the integration of spatiotemporal information provided by the segmentation clock and wavefront. In the anterior region of the presomitic mesoderm, which is the somite precursor, the orchestration between the segmentation clock and the wavefront achieves morphogenesis of somites through multiple processes such as determination of somite boundary position, generation of morophological boundary, and establishment of the rostrocaudal polarity within a somite. Recently, numerous studies using various model animals including mouse, zebrafish, and chick have gradually revealed the molecular aspect of the "clock and wavefront" model and the molecular mechanism connecting the segmentation clock and the wavefront to the multiple processes of somite morphogenesis. In this review, we first summarize the current knowledge about the molecular mechanisms underlying the clock and the wavefront and then describe those of the three processes of somite morphogenesis. Especially, we will discuss the conservation and diversification in the molecular network of the somitigenesis among vertebrates, focusing on two typical model animals used for genetic analyses, i.e., the mouse and zebrafish. In this review, we described molecular mechanism for the generation of somites based on the spatiotemporal information provided by "segmentation clock" and "wavefront" focusing on the evidences obtained from mouse and zebrafish.

Transmembrane 4 L Six Family Member 5 (TM4SF5)-Mediated Epithelial-Mesenchymal Transition in Liver Diseases

The membrane protein TM4SF5, a member of the transmembrane 4L six family, forms a tetraspanin-enriched microdomain (TEM) on the cell surface, where many different membrane proteins and receptors form a massive protein-protein complex to regulate cellular functions including transdifferentiation, migration, and invasion. We recently reported that TM4SF5 causes epithelial-mesenchymal transition (EMT), eventually contributing to aberrant multilayer cellular growth, drug resistance, enhanced migration, invasion, its circulation in the blood, tumor initiation for successful metastasis, and muscle development in zebrafish. In this review, I summarize the information on the role of TM4SF5 in EMT-related functions at TM4SF5-enriched microdomain (T5EM) on cell surface, where proteins such as TM4SF5, CD151, CD44, integrins, and epidermal growth factor receptor (EGFR) can form numerous protein complexes. TM4SF5-mediated EMT contributes to diverse cellular functions, leading to fibrotic phenotypes and initiating and maintaining tumors in primary and/or metastatic regions, in addition to its role in muscle development in zebrafish. Anti-TM4SF5 strategies for addressing the protein networks can lead to regulation of the fibrotic, tumorigenic, and tumor-maintaining functions of TM4SF5-positive hepatic cells. This review is for us to (re)consider the antifibrotic or antitumorigenic (i.e., anti-EMT-related diseases) strategies of dealing with protein networks that would be involved in cross-talks to regulate various cellular functions during TM4SF5-dependent progression from fibrotic to cancerous hepatic cells.

Integration of cell-cell and cell-ECM adhesion in vertebrate morphogenesis

In this review, we highlight recent re-evaluations of the classical cell sorting models and their application to understanding embryonic morphogenesis. Modern genetic and biophysical techniques reveal that tissue self-assembly is not solely a result of differential adhesion, but rather incorporates dynamic cytoskeletal tension and extracellular matrix assembly. There is growing evidence that these biomechanical modules cooperate to organize developing tissues. We describe the contributions of Cadherins and Integrins to tissue assembly and propose a model in which these very different adhesive regimes affect the same outcome through separate but convergent mechanisms.

Cross-Scale Integrin Regulation Organizes ECM and Tissue Topology

The diverse morphologies of animal tissues are underlain by different configurations of adherent cells and extracellular matrix (ECM). Here, we elucidate a cross-scale mechanism for tissue assembly and ECM remodeling involving Cadherin 2, the ECM protein Fibronectin, and its receptor Integrin α5. Fluorescence cross-correlation spectroscopy within the zebrafish paraxial mesoderm mesenchyme reveals a physical association between Integrin α5 on adjacent cell membranes. This Integrin-Integrin complex correlates with conformationally inactive Integrin. Cadherin 2 stabilizes both the Integrin association and inactive Integrin conformation. Thus, Integrin repression within the adherent mesenchymal interior of the tissue biases Fibronectin fibrillogenesis to the tissue surface lacking cell-cell adhesions. Along nascent somite boundaries, Cadherin 2 levels decrease, becoming anti-correlated with levels of Integrin α5. Simultaneously, Integrin α5 clusters and adopts the active conformation and then commences ECM assembly. This cross-scale regulation of Integrin activation organizes a stereotypic pattern of ECM necessary for vertebrate body elongation and segmentation.

prdm1a functions upstream of itga5 in zebrafish craniofacial development

Cranial neural crest cells are specified and migrate into the pharyngeal arches where they subsequently interact with the surrounding environment. Signaling and transcription factors, such as prdm1a regulate this interaction, but it remains unclear which specific factors are required for posterior pharyngeal arch development. Previous analysis suggests that prdm1a is required for posterior ceratobranchial cartilages in zebrafish and microarray analysis between wildtype and prdm1a mutants at 25 h post fertilization demonstrated that integrin α5 (itga5) is differentially expressed in prdm1a mutants. Here, we further investigate the interaction between prdm1a and itga5 in zebrafish craniofacial development. In situ hybridization for itga5 demonstrates that expression of itga5 is decreased in prdm1a mutants between 18 and 31 h post fertilization and itga5 expression overlaps with prdm1a in the posterior arches, suggesting a temporal window for interaction. Double mutants for prdm1a;itga5 have an additive viscerocranium phenotype more similar to prdm1a mutants, suggesting that prdm1a acts upstream of itga5. Consistent with this, loss of posterior pharyngeal arch expression of dlx2a, ceratobranchial cartilages 2-5, and cell proliferation in prdm1a mutants can be rescued with itga5 mRNA injection. Taken together, these data suggest that prdm1a acts upstream of itga5 and are both necessary for posterior pharyngeal arch development in zebrafish.

The tissue mechanics of vertebrate body elongation and segmentation

England's King Richard III, whose skeleton was recently discovered lying ignobly beneath a parking lot, suffered from a lateral curvature of his spinal column called scoliosis. We now know that his scoliosis was not caused by 'imbalanced bodily humors', rather vertebral defects arise from defects in embryonic elongation and segmentation. This review highlights recent advances in our understanding of post-gastrulation biomechanics of the posteriorly advancing tailbud and somite morphogenesis. These processes are beginning to be deciphered from the level of gene networks to a cross-scale physical model incorporating cellular mechanics, the extracellular matrix, and tissue fluidity.

Balancing cell behavior at boundaries

The restriction of cell intermingling across boundaries is essential for the establishment of discrete tissues. Eph receptor signaling prevents intermingling at many boundaries. In this issue, Luu et al. (2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201409026) report a parallel pathway, mediated by Wnt signaling, Snail1, and paraxial protocadherin (PAPC). This pathway establishes a distinctive organization of cell adhesion and intercellular gaps at the interface between tissues.

Anchors and signals: the diverse roles of integrins in development

Integrins mediate cell adhesion by providing a link between the actin cytoskeleton and the extracellular matrix. As well as acting to anchor cells, integrin adhesions provide sensory input via mechanotransduction and synergism with signaling pathways, and provide the cell with the conditions necessary for differentiation in a permissive manner. In this review, we explore how integrins contribute to development, and what this tells us about how they work. From a signaling perspective, the influence of integrins on cell viability and fate is muted in a developmental context as compared to cell culture. Integrin phenotypes tend to arise from a failure of normally specified cells to create tissues properly, due to defective adhesion. The diversity of integrin functions in development shows how cell adhesion is continuously adjusted, both within and between animals, to fit developmental purpose.

MBTPS1/SKI-1/S1P proprotein convertase is required for ECM signaling and axial elongation during somitogenesis and vertebral development†

Caudal regression syndrome (sacral agenesis), which impairs development of the caudal region of the body, occurs with a frequency of about 2 live births per 100 000 newborns although this incidence rises to 1 in 350 infants born to mothers with gestational diabetes. The lower back and limbs can be affected as well as the genitourinary and gastrointestinal tracts. The axial skeleton is formed during embryogenesis through the process of somitogenesis in which the paraxial mesoderm periodically segments into bilateral tissue blocks, called somites. Somites are the precursors of vertebrae and associated muscle, tendons and dorsal dermis. Vertebral anomalies in caudal regression syndrome may arise through perturbation of somitogenesis or, alternatively, could result from defective bone formation and patterning. We discovered that MBTPS1/SKI-1/S1P, which proteolytically activates a class of transmembrane transcription factors, plays a critical role in somitogenesis and the pathogenesis of lumbar/sacral vertebral anomalies. Conditional deletion of Mbtps1 yields a viable mouse with misshapen, fused and reduced number of lumbar and sacral vertebrae, under-developed hind limb bones and a kinky, shortened tail. We show that Mbtps1 is required to (i) maintain the Fgf8 'wavefront' in the presomitic mesoderm that underpins axial elongation, (ii) sustain the Lfng oscillatory 'clock' activity that governs the periodicity of somite formation and (iii) preserve the composition and character of the somitic extracellular matrix containing fibronectin, fibrillin2 and laminin. Based on this spinal phenotype and known functions of MBTPS1, we reason that loss-of-function mutations in Mbtps1 may cause the etiology of caudal regression syndrome.

The medaka dhc2 mutant reveals conserved and distinct mechanisms of Hedgehog signaling in teleosts

Background

Primary cilia are essential for Hedgehog (Hh) signal transduction in vertebrates. Although the core components of the Hh pathway are highly conserved, the dependency on cilia in Hh signaling is considered to be lower in fish than in mice, suggesting the presence of species-specific mechanisms for Hh signal transduction.

Results

To precisely understand the role of cilia in Hh signaling in fish and explore the evolution of Hh signaling, we have generated a maternal-zygotic medaka (Oryzias latipes) mutant that lacks cytoplasmic dynein heavy chain 2 (dhc2; MZdhc2), a component required for retrograde intraflagellar transport. We found that MZdhc2 exhibited the shortened cilia and partial defects in Hh signaling, although the Hh defects were milder than zebrafish mutants which completely lack cilia. This result suggests that Hh activity in fish depends on the length of cilium. However, the activity of Hh signaling in MZdhc2 appeared to be higher than that in mouse Dnchc2 mutants, suggesting a lower requirement for cilia in Hh signaling in fish. We confirmed that Ptch1 receptor is exclusively localized on the cilium in fish as in mammals. Subsequent analyses revealed that Fused, an essential mediator for Hh signaling in Drosophila and fish but not in mammals, augments the activity of Hh signaling in fish as a transcriptional target of Hh signaling.

Conclusions

Ciliary requirement for Hh signaling in fish is lower than that in mammals, possibly due to fused-mediated positive feedback in Hh signaling. The finding of this fish-specific augmentation provides a novel insight into the evolution of Hh signaling.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0057-x) contains supplementary material, which is available to authorized users.

Hanging on for the ride: adhesion to the extracellular matrix mediates cellular responses in skeletal muscle morphogenesis and disease

Skeletal muscle specification and morphogenesis during early development are critical for normal physiology. In addition to mediating locomotion, skeletal muscle is a secretory organ that contributes to metabolic homeostasis. Muscle is a highly adaptable tissue, as evidenced by the ability to increase muscle cell size and/or number in response to weight bearing exercise. Conversely, muscle wasting can occur during aging (sarcopenia), cancer (cancer cachexia), extended hospital stays (disuse atrophy), and in many genetic diseases collectively known as the muscular dystrophies and myopathies. It is therefore of great interest to understand the cellular and molecular mechanisms that mediate skeletal muscle development and adaptation. Muscle morphogenesis transforms short muscle precursor cells into long, multinucleate myotubes that anchor to tendons via the myotendinous junction. This process requires carefully orchestrated interactions between cells and their extracellular matrix microenvironment. These interactions are dynamic, allowing muscle cells to sense biophysical, structural, organizational, and/or signaling changes within their microenvironment and respond appropriately. In many musculoskeletal diseases, these cell adhesion interactions are disrupted to such a degree that normal cellular adaptive responses are not sufficient to compensate for accumulating damage. Thus, one major focus of current research is to identify the cell adhesion mechanisms that drive muscle morphogenesis, with the hope that understanding how muscle cell adhesion promotes the intrinsic adaptability of muscle tissue during development may provide insight into potential therapeutic approaches for muscle diseases. Our objectives in this review are to highlight recent studies suggesting conserved roles for cell-extracellular matrix adhesion in vertebrate muscle morphogenesis and cellular adaptive responses in animal models of muscle diseases.

Mechanisms of boundary formation by Eph receptor and ephrin signaling

The formation of sharp borders, across which cell intermingling is restricted, has a crucial role in the establishment and maintenance of organized tissues. Signaling of Eph receptors and ephrins underlies formation of a number of boundaries between and within tissues during vertebrate development. Eph-ephrin signaling can regulate several types of cell response-adhesion, repulsion and tension-that can in principle underlie the segregation of cells and formation of sharp borders. Recent studies have implicated each of these cell responses as having important roles at different boundaries: repulsion at the mesoderm-ectoderm border, decreased adhesion at the notochord-presomitic mesoderm border, and tension at boundaries within the hindbrain and forebrain. These distinct responses to Eph receptor and ephrin activation may in part be due to the adhesive properties of the tissue.

The hemodynamically-regulated vascular microenvironment promotes migration of the steroidogenic tissue during its interaction with chromaffin cells in the zebrafish embryo

Background

While the endothelium-organ interaction is critical for regulating cellular behaviors during development and disease, the role of blood flow in these processes is only partially understood. The dorsal aorta performs paracrine functions for the timely migration and differentiation of the sympatho-adrenal system. However, it is unclear how the adrenal cortex and medulla achieve and maintain specific integration and whether hemodynamic forces play a role.

Methodology and Principal Findings

In this study, the possible modulation of steroidogenic and chromaffin cell integration by blood flow was investigated in the teleostean counterpart of the adrenal gland, the interrenal gland, in the zebrafish (Danio rerio). Steroidogenic tissue migration and angiogenesis were suppressed by genetic or pharmacologic inhibition of blood flow, and enhanced by acceleration of blood flow upon norepinephrine treatment. Repressed steroidogenic tissue migration and angiogenesis due to flow deficiency were recoverable following restoration of flow. The regulation of interrenal morphogenesis by blood flow was found to be mediated through the vascular microenvironment and the Fibronectin-phosphorylated Focal Adhesion Kinase (Fn-pFak) signaling. Moreover, the knockdown of krüppel-like factor 2a (klf2a) or matrix metalloproteinase 2 (mmp2), two genes regulated by the hemodynamic force, phenocopied the defects in migration, angiogenesis, the vascular microenvironment, and pFak signaling of the steroidogenic tissue observed in flow-deficient embryos, indicating a direct requirement of mechanotransduction in these processes. Interestingly, epithelial-type steroidogenic cells assumed a mesenchymal-like character and downregulated β-Catenin at cell-cell junctions during interaction with chromaffin cells, which was reversed by inhibiting blood flow or Fn-pFak signaling. Blood flow obstruction also affected the migration of chromaffin cells, but not through mechanosensitive or Fn-pFak dependent mechanisms.

Conclusions and Significance

These results demonstrate that hemodynamically regulated Fn-pFak signaling promotes the migration of steroidogenic cells, ensuring their interaction with chromaffin cells along both sides of the midline during interrenal gland development.

In vivo analysis of hyaloid vasculature morphogenesis in zebrafish: A role for the lens in maturation and maintenance of the hyaloid

Two vascular networks nourish the embryonic eye as it develops - the hyaloid vasculature, located at the anterior of the eye between the retina and lens, and the choroidal vasculature, located at the posterior of the eye, surrounding the optic cup. Little is known about hyaloid development and morphogenesis, however. To begin to identify the morphogenetic underpinnings of hyaloid formation, we utilized in vivo time-lapse confocal imaging to characterize morphogenesis of the zebrafish hyaloid through 5 days post fertilization (dpf). Our data segregate hyaloid formation into three distinct morphogenetic stages: Stage I: arrival of hyaloid cells at the lens and formation of the hyaloid loop; Stage II: formation of a branched hyaloid network; Stage III: refinement of the hyaloid network. Utilizing fixed and dissected tissues, distinct Stage II and Stage III aspects of hyaloid formation were quantified over time. Combining in vivo imaging with microangiography, we demonstrate that the hyaloid system becomes fully enclosed by 5dpf. To begin to identify the molecular and cellular mechanisms underlying hyaloid morphogenesis, we identified a recessive mutation in the mab21l2 gene, and in a subset of mab21l2 mutants the lens does not form. Utilizing these "lens-less" mutants, we determined whether the lens was required for hyaloid morphogenesis. Our data demonstrate that the lens is not required for Stage I of hyaloid formation; however, Stages II and III of hyaloid formation are disrupted in the absence of a lens, supporting a role for the lens in hyaloid maturation and maintenance. Taken together, this study provides a foundation on which the cellular, molecular and embryologic mechanisms underlying hyaloid morphogenesis can be elucidated.

TM4SF5 suppression disturbs integrin α5-related signalling and muscle development in zebrafish

TM4SF5 suppression in zebrafish causes abnormal trunk morphology with aberrant translocation and organization of muscle cells, via altered fibronectin/integrin α5/FAK/vinculin/actin signalling. TM4SF5 controls muscle differentiation via alteration in integrin α5-related signalling.

A Pak1/Erk signaling module acts through Gata6 to regulate cardiovascular development in zebrafish

Proper neural crest development and migration is critical during embryonic development, but the molecular mechanisms regulating this process remain incompletely understood. Here, we show that the protein kinase Erk, which plays a central role in a number of key developmental processes in vertebrates, is regulated in the developing neural crest by p21-activated protein kinase 1 (Pak1). Furthermore, we show that activated Erk signals by phosphorylating the transcription factor Gata6 on a conserved serine residue to promote neural crest migration and proper formation of craniofacial structures, pigment cells, and the outflow tract of the heart. Our data suggest an essential role for Pak1 as an Erk activator, and Gata6 as an Erk target, during neural crest development.

EphrinB1 interacts with CNK1 and promotes cell migration through c-Jun N-terminal kinase (JNK) activation

The Eph receptors and their membrane-bound ligands, ephrins, play important roles in various biological processes such as cell adhesion and movement. The transmembrane ephrinBs transduce reverse signaling in a tyrosine phosphorylation-dependent or -independent, as well as PDZ-dependent manner. Here, we show that ephrinB1 interacts with Connector Enhancer of KSR1 (CNK1) in an EphB receptor-independent manner. In cultured cells, cotransfection of ephrinB1 with CNK1 increases JNK phosphorylation. EphrinB1/CNK1-mediated JNK activation is reduced by overexpression of dominant-negative RhoA. Overexpression of CNK1 alone is sufficient for activation of RhoA; however, both ephrinB1 and CNK1 are required for JNK phosphorylation. Co-immunoprecipitation data showed that ephrinB1 and CNK1 act as scaffold proteins that connect RhoA and JNK signaling components, such as p115RhoGEF and MKK4. Furthermore, adhesion to fibronectin or active Src overexpression increases ephrinB1/CNK1 binding, whereas blocking Src activity by a pharmacological inhibitor decreases not only ephrinB1/CNK1 binding, but also JNK activation. EphrinB1 overexpression increases cell motility, however, CNK1 depletion by siRNA abrogates ephrinB1-mediated cell migration and JNK activation. Moreover, Rho kinase inhibitor or JNK inhibitor treatment suppresses ephrinB1-mediated cell migration. Taken together, our findings suggest that CNK1 is required for ephrinB1-induced JNK activation and cell migration.

Snail modulates the assembly of fibronectin via α5 integrin for myocardial migration in zebrafish embryos

The Snail family member snail encodes a zinc finger-containing transcriptional factor that is involved in heart formation. Yet, little is known about how Snail regulates heart development. Here, we identified that one of the duplicated snail genes, snai1b, was expressed in the heart region of zebrafish embryos. Depletion of Snai1b function dramatically reduced expression of α5 integrin, disrupted Fibronectin layer in the heart region, especially at the midline, and prevented migration of cardiac precursors, resulting in defects in cardiac morphology and function in zebrafish embryos. Injection of α5β1 protein rescued the Fibronectin layer and then the myocardial precursor migration in snai1b knockdown embryos. The results provide the molecular mechanism how Snail controls the morphogenesis of heart during embryonic development.

Ephrin-B2 reverse signaling increases α5β1 integrin-mediated fibronectin deposition and reduces distal lung compliance

Alveolar growth abnormalities and severe respiratory dysfunction are often fatal. Identifying mechanisms that control epithelial proliferation and enlarged, poorly septated airspaces is essential in developing new therapies for lung disease. The membrane-bound ligand ephrin-B2 is strongly expressed in lung epithelium, and yet in contrast to its known requirement for arteriogenesis, considerably less is known regarding the function of this protein in the epithelium. We hypothesize that the vascular mediator ephrin-B2 governs alveolar growth and mechanics beyond the confines of the endothelium. We used the in vivo manipulation of ephrin-B2 reverse signaling to determine the role of this vascular mediator in the pulmonary epithelium and distal lung mechanics. We determined that the ephrin-B2 gene (EfnB2) is strongly expressed in alveolar Type 2 cells throughout development and into adulthood. The role of ephrin-B2 reverse signaling in the lung was assessed in Efnb2(LacZ/6YFΔV) mutants that coexpress the intracellular truncated ephrin-B2-β-galactosidase fusion and an intracellular point mutant ephrin-B2 protein that is unable to become tyrosine-phosphorylated or to interact with either the SH2 or PDZ domain-containing downstream signaling proteins. In these viable mice, we observed pulmonary hypoplasia and altered pulmonary mechanics, as evidenced by a marked reduction in lung compliance. Associated with the reduction in lung compliance was a significant increase in insoluble fibronectin (FN) basement membrane matrix assembly with FN deposition, and a corresponding increase in the α5 integrin receptor required for FN fibrillogenesis. These experiments indicate that ephrin-B2 reverse signaling mediates distal alveolar formation, fibrillogenesis, and pulmonary compliance.

Early fish myoseptal cells: insights from the trout and relationships with amniote axial tenocytes

The trunk muscle in fish is organized as longitudinal series of myomeres which are separated by sheets of connective tissue called myoseptum to which myofibers attach. In this study we show in the trout that the myoseptum separating two somites is initially acellular and composed of matricial components such as fibronectin, laminin and collagen I. However, myoseptal cells forming a continuum with skeletogenic cells surrounding axial structures are observed between adjacent myotomes after the completion of somitogenesis. The myoseptal cells do not express myogenic markers such as Pax3, Pax7 and myogenin but express several tendon-associated collagens including col1a1, col5a2 and col12a1 and angiopoietin-like 7, which is a secreted molecule involved in matrix remodelling. Using col1a1 as a marker gene, we observed in developing trout embryo an initial labelling in disseminating cells ventral to the myotome. Later, labelled cells were found more dorsally encircling the notochord or invading the intermyotomal space. This opens the possibility that the sclerotome gives rise not only to skeletogenic mesenchymal cells, as previously reported, but also to myoseptal cells. We furthermore show that myoseptal cells differ from skeletogenic cells found around the notochord by the specific expression of Scleraxis, a distinctive marker of tendon cells in amniotes. In conclusion, the location, the molecular signature and the possible sclerotomal origin of the myoseptal cells suggest that the fish myoseptal cells are homologous to the axial tenocytes in amniotes.

Vascular mediators in chronic lung disease of infancy: role of endothelial monocyte activating polypeptide II (EMAP II)

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of prematurity. Over the years, the BPD phenotype has evolved, but despite various advances in neonatal management approaches, the reduction in the BPD burden is minimal. With the advent of surfactant, glucocorticoids, and new ventilation strategies, BPD has evolved from a disease of structural injury into a new BPD, marked by an arrest in alveolar growth in the lungs of extremely premature infants. This deficient alveolar growth has been associated with a diminution of pulmonary vasculature. Several investigators have described the epithelial / vascular co-dependency and the significant role of crosstalk between vessel formation, alveologenesis, and lung dysplasia's; hence identification and study of factors that regulate pulmonary vascular emergence and inflammation has become crucial in devising effective therapeutic approaches for this debilitating condition. The potent antiangiogenic and proinflammatory protein Endothelial Monocyte Activating Polypeptide II (EMAP II) has been described as a mediator of pulmonary vascular and alveolar formation and its expression is inversely related to the periods of vascularization and alveolarization in the developing lung. Hence the study of EMAP II could play a vital role in studying and devising appropriate therapeutics for diseases of aberrant lung development, such as BPD. Herein, we review the vascular contribution to lung development and the implications that vascular mediators such as EMAP II have in distal lung formation during the vulnerable stage of alveolar genesis.

Membrane-myofibril cross-talk in myofibrillogenesis and in muscular dystrophy pathogenesis: lessons from the zebrafish

Striated muscle has a highly ordered structure in which specialized domains of the cell membrane involved in force transmission (costameres) and excitation-contraction coupling (T tubules) as well as the internal membranes of the sarcoplasmic reticulum are organized over specific regions of the sarcomere. Optimal muscle function is dependent on this high level of organization but how it established and maintained is not well understood. Due to its ex utero development and transparency, the zebrafish embryo is an excellent vertebrate model for the study of dynamic relationships both within and between cells during development. Transgenic models have allowed the delineation of cellular migration and complex morphogenic rearrangements during the differentiation of skeletal myocytes and the assembly and organization of new myofibrils. Molecular targeting of genes and transcripts has allowed the identification of key requirements for myofibril assembly and organization. With the recent advances in gene editing approaches, the zebrafish will become an increasingly important model for the study of human myopathies and muscular dystrophies. Its high fecundity and small size make it well suited to high-throughput screenings to identify novel pharmacologic and molecular therapies for the treatment of a broad range of neuromuscular conditions. In this review, we examine the lessons learned from the zebrafish model regarding the complex interactions between the sarcomere and the sarcolemma that pattern the developing myocyte and discuss the potential for zebrafish as a model system to examine the pathophysiology of, and identify new treatments for, human myopathies and muscular dystrophies.

Low temperature mitigates cardia bifida in zebrafish embryos

The coordinated migration of bilateral cardiomyocytes and the formation of the cardiac cone are essential for heart tube formation. We investigated gene regulatory mechanisms involved in myocardial migration, and regulation of the timing of cardiac cone formation in zebrafish embryos. Through screening of zebrafish treated with ethylnitrosourea, we isolated a mutant with a hypomorphic allele of mil (s1pr2)/edg5, called s1pr2^as10 (as10). Mutant embryos with this allele expressed less mil/edg5 mRNA and exhibited cardia bifida prior to 28 hours post-fertilization. Although the bilateral hearts of the mutants gradually fused together, the resulting formation of two atria and one tightly-packed ventricle failed to support normal blood circulation. Interestingly, cardia bifida of s1pr2^as10 embryos could be rescued and normal circulation could be restored by incubating the embryos at low temperature (22.5°C). Rescue was also observed in gata5 and bon cardia bifida morphants raised at 22.5°C. The use of DNA microarrays, digital gene expression analyses, loss-of-function, as well as mRNA and protein rescue experiments, revealed that low temperature mitigates cardia bifida by regulating the expression of genes encoding components of the extracellular matrix (fibronectin 1, tenascin-c, tenascin-w). Furthermore, the addition of N-acetyl cysteine (NAC), a reactive oxygen species (ROS) scavenger, significantly decreased the effect of low temperature on mitigating cardia bifida in s1pr2^as10 embryos. Our study reveals that temperature coordinates the development of the heart tube and somitogenesis, and that extracellular matrix genes (fibronectin 1, tenascin-c and tenascin-w) are involved.

Cell-fibronectin interactions propel vertebrate trunk elongation via tissue mechanics

During embryonic development and tissue homeostasis, cells produce and remodel the extracellular matrix (ECM). The ECM maintains tissue integrity and can serve as a substrate for cell migration. Integrin α5 (Itgα5) and αV (ItgαV) are the α subunits of the integrins most responsible for both cell adhesion to the ECM protein fibronectin (FN) and FN matrix fibrillogenesis. We perform a systems-level analysis of cell motion in the zebrafish tail bud during trunk elongation in the presence and absence of normal cell-FN interactions. Itgα5 and ItgαV have well-described roles in cell migration in vitro. However, we find that concomitant loss of itgα5 and itgαV leads to a trunk elongation defect without substantive alteration of cell migration. Tissue-specific transgenic rescue experiments suggest that the FN matrix on the surface of the paraxial mesoderm is required for body elongation via its role in defining tissue mechanics and intertissue adhesion.

Functional cooperation of spns2 and fibronectin in cardiac and lower jaw development

The lipid mediator sphingosine-1-phosphate (S1P) is a regulator of cardiac development in zebrafish, as disruption of its receptor s1pr2 or transporter spns2 causes migration defects in cardiac progenitors. To examine the genetic interaction of S1P signaling and the cell adhesion molecule fibronectin, we have established a fn;spns2 double mutant. Cardiac migration defects in fn;spns2 mutants were more severe than those in fn or spns2 mutants. We further found that the lower jaw morphology was disorganized in the fn;spns2 mutant, while it had a slightly shortened anterior–posterior distance in the ventral pharyngeal arch in fn and spns2 mutants relative to wild type. Knockdown of fn in the s1pr2 mutant, but not in the s1pr1 mutant, resulted in severe defects in cardiac migration and ventral pharyngeal arch arrangement. Further, in the background of the fn mutant, knockdown of endothelin receptor A (ednra), which was downregulated in the spns2 mutant, caused pharyngeal defects resembling those in the fn;spns2 mutant. These results strongly suggest that Spns2-S1PR2 signaling and fibronectin cooperatively regulate both cardiac and lower jaw development in zebrafish.

Modular development of the teleost trunk along the dorsoventral axis and zic1/zic4 as selector genes in the dorsal module

Teleost fish exhibit remarkable diversity in morphology, such as fins and coloration, particularly on the dorsal side. These structures are evolutionary adaptive because their back is highly visible to other individuals. However, owing to the late phenotypic appearance (from larva to adult) and lack of appropriate mutants, the genetic mechanisms that regulate these dorsoventrally asymmetric external patterns are largely unknown. To address this, we have analyzed the spontaneous medaka mutant Double anal fin (Da), which exhibits a mirror-image duplication of the ventral half across the lateral midline from larva to adult. Da is an enhancer mutant for zic1 and zic4 in which their expression in dorsal somites is lost. We show that the dorsoventral polarity in Da somites is lost and then demonstrate using transplantation techniques that somites and their derived tissues globally determine the multiple dorsal-specific characteristics of the body (fin morphology and pigmentation) from embryo to adult. Intriguingly, the zic1/zic4 expression in the wild type persists throughout life in the dorsal parts of somite derivatives, i.e. the myotome, dermis and vertebrae, forming a broad dorsal domain in the trunk. Comparative analysis further implies a central role for zic1/zic4 in morphological diversification of the teleost body. Taken together, we propose that the teleost trunk consists of dorsal/ventral developmental modules and that zic1/zic4 in somites function as selector genes in the dorsal module to regulate multiple dorsal morphologies.

Segmental assembly of fibronectin matrix requires rap1b and integrin α5

BACKGROUND

During segmentation of the zebrafish embryo, inside-out signaling activates Integrin α5, which is necessary for somite border morphogenesis. The direct activator of Integrin α5 during this process is unknown. One candidate is Rap1b, a small monomeric GTPase implicated in Integrin activation in the immune system.

RESULTS

Knockdown of rap1b, or overexpression of a dominant negative rap1b, causes a mild axis elongation defect in zebrafish. However, disruption of rap1b function in integrin α5(-/-) mutants results in a strong reduction in Fibronectin (FN) matrix assembly in the paraxial mesoderm and a failure in somite border morphogenesis along the entire anterior-posterior axis. Somite patterning appears unaffected, as her1 oscillations are maintained in single and double morphants/mutants, but somite polarity is gradually lost in itgα5(-/-) ; rap1b MO embryos.

CONCLUSIONS

In itgα5(-) (/) (-) mutants, rap1b is required for proper somite border morphogenesis in zebrafish. The loss of somite borders is not a result of aberrant segmental patterning. Rather, somite boundary formation initiates but is not completed, due to the failure to assemble FN matrix along the nascent boundary. We propose a model in which Rap1b activates Integrin/Fibronectin receptors as part of an "inside-out" signaling pathway that promotes Integrin binding to FN, FN matrix assembly, and subsequent stabilization of morphological somite boundaries.

The myotomal basement membrane: insight into laminin-111 function and its control by Sonic hedgehog signaling

The importance of laminin-containing basement membranes (BM) for adult muscle function is well established, in particular due to the severe phenotype of congenital muscular dystrophies in patients with mutations disrupting the BM-muscle cell interaction. Developing muscles in the embryo are also dependent on an intact BM. However, the processes controlled by BM-muscle cell interactions in the embryo are only beginning to be elucidated. In this review, we focus on the myotomal BM to illustrate the critical role of laminin-111 in BM assembly and function at the surface of embryonic muscle cells. The myotomal BM provides also an interesting paradigm to study the complex interplay between laminins-containing BM and growth factor-mediated signaling and activity.

Integrin α5/fibronectin1 and focal adhesion kinase are required for lens fiber morphogenesis in zebrafish

Lens fiber formation and morphogenesis requires a precise orchestration of cell- extracellular matrix (ECM) and cell-cell adhesive changes in order for a lens epithelial cell to adopt a lens fiber fate, morphology, and migratory ability. The cell-ECM interactions that mediate these processes are largely unknown, and here we demonstrate that fibronectin1 (Fn1), an ECM component, and integrin α5, its cellular binding partner, are required in the zebrafish lens for fiber morphogenesis. Mutations compromising either of these proteins lead to cataracts, characterized by defects in fiber adhesion, elongation, and packing. Loss of integrin α5/Fn1 does not affect the fate or viability of lens epithelial cells, nor does it affect the expression of differentiation markers expressed in lens fibers, although nucleus degradation is compromised. Analysis of the intracellular mediators of integrin α5/Fn1 activity focal adhesion kinase (FAK) and integrin-linked kinase (ILK) reveals that FAK, but not ILK, is also required for lens fiber morphogenesis. These results support a model in which lens fiber cells use integrin α5 to migrate along a Fn-containing substrate on the apical side of the lens epithelium and on the posterior lens capsule, likely activating an intracellular signaling cascade mediated by FAK in order to orchestrate the cytoskeletal changes in lens fibers that facilitate elongation, migration, and compaction.

Development and fibronectin signaling requirements of the zebrafish interrenal vessel

Background

The early morphogenetic steps of zebrafish interrenal tissue, the teleostean counterpart of the mammalian adrenal gland, are modulated by the peri-interrenal angioblasts and blood vessels. While an organized distribution of intra-adrenal vessels and extracellular matrix is essential for the fetal adrenal cortex remodeling, whether and how an intra-interrenal buildup of vasculature and extracellular matrix forms and functions during interrenal organogenesis in teleosts remains unclear.

Methodology and Principal Findings

We characterized the process of interrenal gland vascularization by identifying the interrenal vessel (IRV); which develops from the axial artery through angiogenesis and is associated with highly enriched Fibronectin (Fn) accumulation at its microenvironment. The loss of Fn1 by either antisense morpholino (MO) knockdown or genetic mutation inhibited endothelial invasion and migration of the steroidogenic tissue. The accumulation of peri-IRV Fn requires Integrin α5 (Itga5), with its knockdown leading to interrenal and IRV morphologies phenocopying those in the fn1 morphant and mutant. fn1b, another known fn gene in zebrafish, is however not involved in the IRV formation. The distribution pattern of peri-IRV Fn could be modulated by the blood flow, while a lack of which altered angiogenic direction of the IRV as well as its ability to integrate with the steroidogenic tissue. The administration of Fn antagonist through microangiography exerted reducing effects on both interrenal vessel angiogenesis and steroidogenic cell migration.

Conclusions and Significance

This work is the first to identify the zebrafish IRV and to characterize how its integration into the developing interrenal gland requires the Fn-enriched microenvironment, which leads to the possibility of using the IRV formation as a platform for exploring organ-specific angiogenesis. In the context of other developmental endocrinology studies, our results indicate a highly dynamic interrenal-vessel interaction immediately before the onset of stress response in the zebrafish embryo.

Mesogenin causes embryonic mesoderm progenitors to differentiate during development of zebrafish tail somites

The molecular mechanism underlying somite development differs along the embryonic antero-posterior axis. In zebrafish, cell lineage tracing and genetic analysis have revealed a difference in somite development between the trunk and tail. For instance, spadetail/tbx16 (spt) mutant embryos lack trunk somites but not tail ones. Trunk and tail somites are developed from mesodermal progenitor cells (MPCs) located in the tailbud. While the undifferentiated state of MPCs is maintained by mutual activation between Wnt and Brachyury/Ntl, the mechanism by which the MPCs differentiate into presomitic mesoderm (PSM) cells remains largely unclear. Especially, the molecules that promote PSM differentiation during tail development should be clarified. Here, we show that zebrafish embryos defective in mesogenin1 (msgn1) and spt failed to differentiate into PSM cells in tail development and show increased expression of wnt8 and ntl. Msgn1 acted in a cell-autonomous manner and as a transcriptional activator in PSM differentiation. The expression of msgn1 initially overlapped with that of ntl in the ventral tailbud, as previously reported; and its mis-expression caused ectopic expression of tbx24, a PSM marker gene, only in the tailbud and posterior notochord, both of which expressed ntl in zebrafish embryos. Furthermore, the PSM-inducing activity of misexpressed msgn1 was enhanced by co-expression with ntl. Thus, Msgn1 exercised its PSM-inducing activity in cells expressing ntl. Based on these results, we speculate that msgn1 expression in association with that of ntl may allow the differentiation of progenitor cells to proceed during development of somites in the tail.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

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.

Brain regionalization: of signaling centers and boundaries

Our knowledge of the general mechanisms controlling the formation of the vertebrate central nervous system has advanced tremendously in the last decade. Here, we discuss the impact of the combined use of cell manipulation, in vivo imaging and genetics in the zebrafish on recent progress in understanding how signaling processes progressively control regionalization of the central nervous system. We highlight the unresolved issues and speculate upon the fundamental role the zebrafish will continue having in answering them.

Non-SH2/PDZ reverse signaling by ephrins

Great strides have been made regarding our understanding of the processes and signaling events influenced by Eph/ephrin signaling that play a role in cell adhesion and cell movement. However, the precise mechanisms by which these signaling events regulate cell and tissue architecture still need further resolution. The Eph/ephrin signaling pathways and the ability to regulate cell-cell adhesion and motility constitutes an impressive system for regulating tissue separation and morphogenesis (Pasquale, 2005, 2008 [1,2]). Moreover, the de-regulation of this signaling system is linked to the promotion of aggressive and metastatic tumors in humans [2]. In the following section, we discuss some of the interesting mechanisms by which ephrins can signal through their own intracellular domains (reverse signaling) either independent of forward signaling or in addition to forward signaling through a cognate receptor. In this review we discuss how ephrins (Eph ligands) "reverse signal" through their intracellular domains to affect cell adhesion and movement, but the focus is on modes of action that are independent of SH2 and PDZ interactions.

Molecular mechanisms of cell segregation and boundary formation in development and tumorigenesis

The establishment and maintenance of precisely organized tissues requires the formation of sharp borders between distinct cell populations. The maintenance of segregated cell populations is also required for tissue homeostasis in the adult, and deficiencies in segregation underlie the metastatic spreading of tumor cells. Three classes of mechanisms that underlie cell segregation and border formation have been uncovered. The first involves differences in cadherin-mediated cell-cell adhesion that establishes interfacial tension at the border between distinct cell populations. A second mechanism involves the induction of actomyosin-mediated contraction by intercellular signaling, such that cortical tension is generated at the border. Third, activation of Eph receptors and ephrins can lead to both decreased adhesion by triggering cleavage of E-cadherin, and to repulsion of cells by regulation of the actin cytoskeleton, thus preventing intermingling between cell populations. These mechanisms play crucial roles at distinct boundaries during development, and alterations in cadherin or Eph/ephrin expression have been implicated in tumor metastasis.

Eph/ephrin signaling in cell-cell and cell-substrate adhesion

Cell-cell and cell-matrix adhesion are critical processes for the formation and maintenance of tissue patterns during development, as well as control of invasion and metastasis of cancer cells. Although great strides have been made regarding our understanding of the processes that play a role in cell adhesion and cell movement, the precise mechanisms by which diverse signaling events regulate cell and tissue architecture are poorly understood. One group of cell surface molecules, Eph receptor tyrosine kinases, and their membrane-bound ligands, ephrins, are key regulators in these processes. It is the ability of Eph/ephrin signaling pathways to regulate cell-cell adhesion and motility that establishes this family as a formidable system for regulating tissue separation and morphogenesis. Moreover, the de-regulation of this signaling system is linked to the promotion of more aggressive and metastatic tumors in humans.

Integrin-α5 coordinates assembly of posterior cranial placodes in zebrafish and enhances Fgf-dependent regulation of otic/epibranchial cells

Vertebrate sensory organs develop in part from cranial placodes, a series of ectodermal thickenings that coalesce from a common domain of preplacodal ectoderm. Mechanisms coordinating morphogenesis and differentiation of discrete placodes are still poorly understood. We have investigated whether placodal assembly in zebrafish requires Integrin- α5 (itga5), an extracellular matrix receptor initially expressed throughout the preplacodal ectoderm. Morpholino knockdown of itga5 had no detectable effect on anterior placodes (pituitary, nasal and lens), but posterior placodes developed abnormally, resulting in disorganization of trigeminal and epibranchial ganglia and reduction of the otic vesicle. Cell motion analysis in GFP-transgenic embryos showed that cell migration in itga5 morphants was highly erratic and unfocused, impairing convergence and blocking successive recruitment of new cells into these placodes. Further studies revealed genetic interactions between itga5 and Fgf signaling. First, itga5 morphants showed changes in gene expression mimicking modest reduction in Fgf signaling. Second, itga5 morphants showed elevated apoptosis in the otic/epibranchial domain, which was rescued by misexpression of Fgf8. Third, knockdown of the Fgf effector erm had no effect by itself but strongly enhanced defects in itga5 morphants. Finally, proper regulation of itga5 requires dlx3b/4b and pax8, which are themselves regulated by Fgf. These findings support a model in which itga5 coordinates cell migration into posterior placodes and augments Fgf signaling required for patterning of these tissues and cell survival in otic/epibranchial placodes.

Obscurin depletion impairs organization of skeletal muscle in developing zebrafish embryos

During development, skeletal myoblasts differentiate into myocytes and skeletal myotubes with mature contractile structures that are precisely oriented with respect to surrounding cells and tissues. Establishment of this highly ordered structure requires reciprocal interactions between the differentiating myocytes and the surrounding extracellular matrix to form correctly positioned and well-organized attachments from the skeletal muscle to the bony skeleton. Using the developing zebrafish embryo as a model, we examined the relationship between new myofibril assembly and the organization of the membrane domains involved in cell-extracellular matrix interactions. We determined that depletion of obscurin, a giant muscle protein, resulted in irregular cell morphology and disturbed extracellular matrix organization during skeletal muscle development. The resulting impairment of myocyte organization was associated with disturbance of the internal architecture of the myocyte suggesting that obscurin participates in organizing the internal structure of the myocyte and translating those structural cues to surrounding cells and tissues.

α5β1 integrin-mediated adhesion to fibronectin is required for axis elongation and somitogenesis in mice

The arginine-glycine-aspartate (RGD) motif in fibronectin (FN) represents the major binding site for α5β1 and αvβ3 integrins. Mice lacking a functional RGD motif in FN (FN^RGE/RGE) or α5 integrin develop identical phenotypes characterized by embryonic lethality and a severely shortened posterior trunk with kinked neural tubes. Here we show that the FN^RGE/RGE embryos arrest both segmentation and axis elongation. The arrest is evident at about E9.0, corresponding to a stage when gastrulation ceases and the tail bud-derived presomitic mesoderm (PSM) induces α5 integrin expression and assumes axis elongation. At this stage cells of the posterior part of the PSM in wild type embryos are tightly coordinated, express somitic oscillator and cyclic genes required for segmentation, and form a tapered tail bud that extends caudally. In contrast, the posterior PSM cells in FN^RGE/RGE embryos lost their tight associations, formed a blunt tail bud unable to extend the body axis, failed to induce the synchronised expression of Notch1 and cyclic genes and cease the formation of new somites. Mechanistically, the interaction of PSM cells with the RGD motif of FN is required for dynamic formation of lamellipodia allowing motility and cell-cell contact formation, as these processes fail when wild type PSM cells are seeded into a FN matrix derived from FN^RGE/RGE fibroblasts. Thus, α5β1-mediated adhesion to FN in the PSM regulates the dynamics of membrane protrusions and cell-to-cell communication essential for elongation and segmentation of the body axis.

Skeletal muscle differentiation and fusion are regulated by the BAR-containing Rho-GTPase-activating protein (Rho-GAP), GRAF1

Although RhoA activity is necessary for promoting myogenic mesenchymal stem cell fates, recent studies in cultured cells suggest that down-regulation of RhoA activity in specified myoblasts is required for subsequent differentiation and myotube formation. However, whether this phenomenon occurs in vivo and which Rho modifiers control these later events remain unclear. We found that expression of the Rho-GTPase-activating protein, GRAF1, was transiently up-regulated during myogenesis, and studies in C2C12 cells revealed that GRAF1 is necessary and sufficient for mediating RhoA down-regulation and inducing muscle differentiation. Moreover, forced expression of GRAF1 in pre-differentiated myoblasts drives robust muscle fusion by a process that requires GTPase-activating protein-dependent actin remodeling and BAR-dependent membrane binding or sculpting. Moreover, morpholino-based knockdown studies in Xenopus laevis determined that GRAF1 expression is critical for muscle development. GRAF1-depleted embryos exhibited elevated RhoA activity and defective myofibrillogenesis that resulted in progressive muscle degeneration, defective motility, and embryonic lethality. Our results are the first to identify a GTPase-activating protein that regulates muscle maturation and to highlight the functional importance of BAR domains in myotube formation.

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.

Tissue morphogenesis coupled with cell shape changes

When different tissues successively emerge during development, they need to be morphologically segregated from each other. Morphological segregation of tissues is often accompanied by robust changes in cell shape, and these two events are precisely coordinated. We overview recent progress in understanding how such coordination is regulated at the cellular and molecular levels using vertebrate somitogenesis asa unique model. In the formation of the somitic gap and its concomitant cell epithelialization, Ephrin-Eph intercellular signals play a central role, with Ephrin transducing intracellular signals via suppression of Cdc42. These signals lead to the activation of integrin where the segment border of somites forms, which in turn induces the assembly of fibronectin, the final player for the coordination. Intimate coupling of tissue-shape and cell-shape changes is also relevant to tumor suppression.