The distribution of fibronectin and laminin in the somitogenesis of Xenopus laevis

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.

The Role of Sdf-1α signaling in Xenopus laevis somite morphogenesis

BACKGROUND

Stromal derived factor-1α (sdf-1α), a chemoattractant chemokine, plays a major role in tumor growth, angiogenesis, metastasis, and in embryogenesis. The sdf-1α signaling pathway has also been shown to be important for somite rotation in zebrafish (Hollway et al., 2007). Given the known similarities and differences between zebrafish and Xenopus laevis somitogenesis, we sought to determine whether the role of sdf-1α is conserved in Xenopus laevis.

RESULTS

Using a morpholino approach, we demonstrate that knockdown of sdf-1α or its receptor, cxcr4, leads to a significant disruption in somite rotation and myotome alignment. We further show that depletion of sdf-1α or cxcr4 leads to the near absence of β-dystroglycan and laminin expression at the intersomitic boundaries. Finally, knockdown of sdf-1α decreases the level of activated RhoA, a small GTPase known to regulate cell shape and movement.

CONCLUSION

Our results show that sdf-1α signaling regulates somite cell migration, rotation, and myotome alignment by directly or indirectly regulating dystroglycan expression and RhoA activation. These findings support the conservation of sdf-1α signaling in vertebrate somite morphogenesis; however, the precise mechanism by which this signaling pathway influences somite morphogenesis is different between the fish and the frog.

In vivo analyzes of dystroglycan function during somitogenesis in Xenopus laevis

Dystroglycan (Dg) is a cell adhesion receptor for laminin that has been reported to play a role in skeletal muscle cell stability, cytoskeletal organization, cell polarity, and signaling. Here we show that Dg is expressed at both the notochord/somite and the intersomitic boundaries, where laminin and fibronectin are accumulated during somitogenesis. Inhibition of Dg function with morpholino antisense oligonucleotides or a dominant negative mutant results in the normal segmentation of the presomitic mesoderm but affects the number, the size, and the integrity of somites. Depletion of Dg disrupts proliferation and alignment of myoblasts without affecting XMyoD and XMRF4 expression. It also leads to defects in laminin deposition at the intersomitic junctions, whereas expression of integrin beta1 subunits and fibronectin assembly occur normally. Our results show that Dg is critical for both proliferation and elongation of somitic cells and that the Dg-cytoplasmic domain is required for the laminin assembly at the intersomitic boundaries. Developmental Dynamics 238:1332-1345, 2009. (c) 2008 Wiley-Liss, Inc.

Actomyosin stiffens the vertebrate embryo during crucial stages of elongation and neural tube closure

Physical forces drive the movement of tissues within the early embryo. Classical and modern approaches have been used to infer and, in rare cases, measure mechanical properties and the location and magnitude of forces within embryos. Elongation of the dorsal axis is a crucial event in early vertebrate development, yet the mechanics of dorsal tissues in driving embryonic elongation that later support neural tube closure and formation of the central nervous system is not known. Among vertebrates, amphibian embryos allow complex physical manipulation of embryonic tissues that are required to measure the mechanical properties of tissues. In this paper, we measure the stiffness of dorsal isolate explants of frog (Xenopus laevis) from gastrulation to neurulation and find dorsal tissues stiffen from less than 20 Pascal (Pa) to over 80 Pa. By iteratively removing tissues from these explants, we find paraxial somitic mesoderm is nearly twice as stiff as either the notochord or neural plate, and at least 10-fold stiffer than the endoderm. Stiffness measurements from explants with reduced fibronectin fibril assembly or disrupted actomyosin contractility suggest that it is the state of the actomyosin cell cortex rather than accumulating fibronectin that controls tissue stiffness in early amphibian embryos.

Live imaging of cell protrusive activity, and extracellular matrix assembly and remodeling during morphogenesis in the frog, Xenopus laevis

Cell motility and matrix assembly have traditionally been studied in isolation because of a lack of suitable model systems in which both can be observed simultaneously. With embryonic tissues from the gastrulating frog Xenopus laevis we observe stages of fibronectin fibrillogenesis coincident with protrusive activity in the overlying cells. Using live confocal time-lapse images collected from Cy3-tagged fibronectin and plasma membrane tethered green fluorescent protein, we describe the movement and the elaboration of a complex fibrillar network undergoing topological rearrangements of fibrils on the surface of an embryonic tissue. Discrete processes of annealing, polymerization, stretching, breaking, and recoiling are recorded. Elaboration and maintenance of the complex topology of the extracellular matrix appears to require filamentous actin. These findings support a mechanical-model in which cell tractive forces elaborate the complex topological fibrillar network and are part of a homeostatic mechanism for the regulation of the extracellular matrix.

Regulation of somitogenesis by Ena/VASP proteins and FAK during Xenopus development

The metameric organization of the vertebrate body plan is established during somitogenesis as somite pairs sequentially form along the anteroposterior axis. Coordinated regulation of cell shape, motility and adhesion are crucial for directing the morphological segmentation of somites. We show that members of the Ena/VASP family of actin regulatory proteins are required for somitogenesis in Xenopus. Xenopus Ena (Xena) localizes to the cell periphery in the presomitic mesoderm (PSM), and is enriched at intersomitic junctions and at myotendinous junctions in somites and the myotome, where it co-localizes with β1-integrin, vinculin and FAK. Inhibition of Ena/VASP function with dominant-negative mutants results in abnormal somite formation that correlates with later defects in intermyotomal junctions. Neutralization of Ena/VASP activity disrupts cell rearrangements during somite rotation and leads to defects in the fibronectin (FN) matrix surrounding somites. Furthermore, inhibition of Ena/VASP function impairs FN matrix assembly, spreading of somitic cells on FN and autophosphorylation of FAK, suggesting a role for Ena/VASP proteins in the modulation of integrin-mediated processes. We also show that inhibition of FAK results in defects in somite formation, blocks FN matrix deposition and alters Xena localization. Together, these results provide evidence that Ena/VASP proteins and FAK are required for somite formation in Xenopus and support the idea that Ena/VASP and FAK function in a common pathway to regulate integrin-dependent migration and adhesion during somitogenesis.

Assembly and remodeling of the fibrillar fibronectin extracellular matrix during gastrulation and neurulation inXenopus laevis

Fibronectin, a major component of the extracellular matrix is critical for processes of cell traction and cell motility. Whole-mount confocal imaging of the three-dimensional architecture of the extracellular matrix is used to describe dynamic assembly and remodeling of fibronectin fibrils during gastrulation and neurulation in the early frog embryo. As previously reported, fibrils first appear under the prospective ectoderm. We describe here the first evidence for regulated assembly of fibrils along the somitic mesoderm/endoderm boundary as well as at the notochord/somitic mesoderm boundary and clearing of fibrils from the dorsal and ventral surfaces of the notochord that occurs over the course of a few hours. As gastrulation proceeds, fibrils are restored to the dorsal surface of the notochord, where the notochord contacts the prospective floor plate. As the neural folds form, fibrils are again remodeled as deep neural plate cells move medially. The process of neural tube closure leaves a region of the ectoderm overlying the neural crest transiently bare of fibrils. Fibrils are assembled surrounding the dorsal surface of the neural tube as the neural tube lumen is restored.

The origin and morphogenesis of amphibian somites

The origin and development of the amphibian somitic mesoderm is summarized and reviewed with the goal of identifying issues most profitably pursued in these organisms. The location of the prospective somitic mesoderm as well as the cell movements bringing this tissue into its definitive position varies among amphibians. These variations have implications for the tissue interactions patterning the embryo, the design of the gastrulation movements, the role of the somitic mesoderm in early patterning and morphogenic processes, and the nature of the developmental pathway leading to somites. The presegmentation morphogenesis, the process of segmentation, and the subsequent, postsegmentation morphogenesis of the somitic mesoderm also varies considerably among amphibians. Although segmentation in amphibians shares what may be highly conserved and general patterning mechanisms with other vertebrates, the somitic developmental pathway as a whole is not conservative and has been capable of accommodating the use of a number of quite different morphogenic processes, all leading to very similar ends. The major challenges in studying amphibian somitogenesis are to develop molecular markers for major components of the somite, to determine the derivatives of the somite with better cell tracing experiments, and learning to work with the small dermatomal and sclerotomal cell populations found in most species. A potential advantage is that the diversity of somitogenesis among the amphibians makes this group ideal for studying the evolution of developmental processes. In addition, many amphibians allow direct observation of somitogenesis with great resolution and permit biomechanical analysis of tissues participating in morphogenesis, thus making it possible to analyze cellular mechanisms of morphogenesis in ways not possible in most other systems.

Cloning and characterization of cDNAs encoding the integrin alpha2 and alpha3 subunits from Xenopus laevis

Integrins containing the alpha2 and alpha3 subunits associate with the beta1 subunit to form distinct receptors with partially overlapping adhesive specificities. We report the cloning and sequence of cDNAs that encode the Xenopus orthologues of integrins alpha2 and alpha3 and the expression of these subunits during embryogenesis. Integrin alpha2 and alpha3 mRNAs are first expressed in the dorsal mesoderm and developing notochord at gastrulation. We also show that alpha3 mRNAs are expressed in the entire marginal zone of gastrulae dorsalized with LiCl but that this localization is lost in embryos ventralized by ultraviolet light. Immunoblots reveal that the alpha3 protein is expressed throughout early development, however, the alpha2 protein is not detected until late tailbud stages. Injection of full-length alpha3 transcripts into the animal poles of fertilized eggs results in embryonic defects in paraxial mesoderm attributed to the failure of somites to form segments. Injection of the alpha3 transcripts into the vegetal pole and overexpression of a 5'-truncated alpha3 control construct have no apparent affect on development or somite formation. These data suggest that normal position-specific expression of integrins is important in maintaining the proper organization of tissues during early amphibian morphogenesis.

ADAM 13: a novel ADAM expressed in somitic mesoderm and neural crest cells during Xenopus laevis development

Embryonic development involves a series of cell adhesive interactions that provide mechanical and instructive information required for morphogenesis. The ADAMs family of membrane-anchored proteins, containing a disintegrin and metalloprotease domain, is well suited for participating in such developmental events. They encode not only a potential adhesive function, through an integrin-binding disintegrin domain, but also a potential antiadhesive function, through a zinc-dependent metalloprotease domain. In order to investigate the role of ADAMs in early development we cloned a cDNA encoding a novel member of the ADAM family from a Xenopus laevis neurula stage library. We call this cDNA, and the 915-amino-acid protein it encodes, ADAM 13, X-ADAM 13 RNA is expressed during embryogenesis from the midblastula stage through tadpole stage 45. X-ADAM 13 is localized to somitic mesoderm and cranial neural crest cells during gastrulation, neurulation, and in tail bud stages. Sequence analyses of the X-ADAM 13 metalloprotease and disintegrin domains indicate that the protein is likely to be involved in both proteolytic and cell-adhesive functions. The X-ADAM 13 sequence is most closely related to that of mouse meltrin alpha, which is implicated in myoblast fusion. Our data suggest that X-ADAM 13 may be involved in neural crest cell adhesion and migration as well as myoblast differentiation.

Beta 1 integrins regulate axon outgrowth and glial cell spreading on a glial-derived extracellular matrix during development and regeneration

In the present study we have investigated functional roles for beta 1 integrin receptors in regulating axon outgrowth, and glial cell adhesion and spreading in the Xenopus retina. The XR1 glial cell line, isolated from Xenopus retinal neuroepithelium, deposits a proteinaceous extracellular matrix (ECM) with potent neurite outgrowth promoting activity. To investigate a potential role of the integrins as cellular receptors for these glial cell-derived ECM components, embryonic and regenerating retinal explants were cultured in the presence of polyclonal antibodies directed against the beta 1 integrin receptor complex. The IgGs and Fabs of the anti-beta 1 integrin antibody strongly inhibited ganglion cell axon outgrowth on the glial cell-derived ECM, although axons grew freely across the surfaces of glial cells surrounding the explants. The antibodies also inhibited outgrowth on purified laminin containing substrates in a dose-dependent fashion. In addition, the anti-beta 1 antibodies were effective at inhibiting the spreading of glial cells that migrated out from the embryonic explants, and also inhibited attachment and spreading of Xenopus XR1 glial cells on ECM substrates. These results show that the beta 1 integrins play important functional roles in axon outgrowth during development and regeneration, and also serve in regulating retinal glial cell attachment and spreading in vitro, and thus are likely to play similar roles in vivo.

Localization of integrin subunits alpha 6 and beta 1 during somitogenesis in the long-tailed macaque (M. fascicularis)

The distribution of integrin subunits alpha 6 and beta 1, and the alpha 6 beta 1 integrin ligand, laminin, was examined during somitogenesis in developmental stages 11, 13, and 16 in the long-tailed macaque, using peroxidase immunocytochemistry. Within differentiating somites in stage 11, alpha 6 expression was observed in the sclerotome, basal surface of dermamyotomal cells adjacent to the basal lamina and on scattered cells throughout the dermamyotome. In further advanced somites in stages 13 and 16, alpha 6 immunoreactivity became restricted to the myotome. alpha 6 was expressed on mesenchymal core cells within the myocele of undifferentiated epithelioid somites and the ventromedial wall of somites commencing differentiation at each stage. beta 1 distribution resembled that of alpha 6 in stage 11 somitic tissue, however, it remained present on myotome and sclerotome cells in the later stages, and was also expressed on dermatomal cells in stage 16. Laminin immunoreactivity, while more intense and prevalent than alpha 6 and beta 1 in each stage examined, occurred on the same somite cell populations as the 2 integrin subunits. These results show a defined distribution of alpha 6 on somitic tissue, and suggest this integrin is involved in somite differentiation. They also support a possible role for alpha 6 in myoblast formation and migration. Overlapping of beta 1 and laminin immunoreactivity with that of alpha 6 further suggests that alpha 6 pairs with beta 1 as a functional heterodimer for laminin in defined somitic regions.

Defects in mesoderm, neural tube and vascular development in mouse embryos lacking fibronectin

To examine the role of fibronectin in vivo, we have generated mice in which the fibronectin gene is inactivated. Heterozygotes have one half normal levels of plasma fibronectin, yet appear normal. When homozygous, the mutant allele causes early embryonic lethality, proving that fibronectin is required for embryogenesis. However, homozygous mutant embryos implant and initiate gastrulation normally including extensive mesodermal movement. Neural folds also form but the mutant embryos subsequently display shortened anterior-posterior axes, deformed neural tubes and severe defects in mesodermally derived tissues. Notochord and somites are absent; the heart and embryonic vessels are variable and deformed, and the yolk sac, extraembryonic vasculature and amnion are also defective. These abnormalities can be interpreted as arising from fundamental deficits in mesodermal migration, adhesion, proliferation or differentiation as a result of the absence of fibronectin. The nature of these embryonic defects leads to reevaluation of suggested roles for fibronectin during early development based on results obtained in vitro and in embryos of other species.

Mesodermal cell adhesion to fibronectin-rich fibrillar extracellular matrix is required for normal Rana pipiens gastrulation

New observations on thin strips of cells from the leading edge of the involuting presumptive mesoderm explanted onto FN-coated substrate show a striking preferential cellular emigration from the leading edge of explants. Microinjected probes (Fab' anti-FN, Fab' anti-integrin and RGD-peptides) that disrupt cell adhesion to the FN-matrix on basal surface of the blastocoel roof also disrupt normal anuran gastrulation, producing blocked embryos with no adhesion of leading edge mesodermal cells to the blastocoel roof, abnormal epiboly, and defects of mesodermal cell spreading across the basal surface of the blastocoel roof toward the animal pole. These results show that the FN-rich fibrillar extracellular matrix on the basal surface of the blastocoel roof is required for normal gastrulation in Rana pipiens embryos.

Ambystoma maculatum gastrulae have an oriented, fibronectin-containing extracellular matrix

During early development of the urodele Ambystoma maculatum, the appearance and distribution of fibronectin-containing fibrillar extracellular materials were studied by immunocytochemistry. Fibronectin (FN) first appears in the early blastula (stage 7) as thin punctate fibrils on the cell surface concentrated in the marginal zone. In late blastula (stage 9), thin fibrils are found throughout the blastocoel roof. Early gastrulae (stage 10) have numerous fibrils and multifibrillar strands concentrated in the dorsal lip region and oriented preferentially along a line parallel to the dorsal lip-animal pole axis. There is a striking increase in the amount of FN fibrils during the rest of gastrulation. This FN-containing network can be transferred to plastic substrata with preservation of the preferential orientation observed in vivo. Dorsal marginal zone explants placed on such conditioned substrata show polarized outgrowth toward the animal pole region of conditioned areas when placed on the dorsal lip side or the ventral marginal zone side of conditioned substrata. This outgrowth occurs symmetrically on bovine plasma FN-coated substrata, is prevented by Fab' fragments of antibodies to FN but fails to occur on laminin coated substrata. When migrating mesodermal cells from early gastrulae are cultured on substrata conditioned by deposition of the fibrillar matrix, these cells exhibit striking contact inhibition of locomotion, a phenomenon that may explain dispersal of migrating mesodermal cells across the blastocoel roof. When leading edges of mesodermal cells collide, cells abruptly change direction. When leading edges collide with trailing edges, the trailing edges detach from the substratum and cells move apart in the direction of the leading edge.

Effects of heat shock on the pattern of fibronectin and laminin during somitogenesis in Xenopus laevis

Heat shock causes partial disruption of the segmentation pattern during somitogenesis in Xenopus as well as in other vertebrates. However, Xenopus undergoes a different type of somite formation than that of most vertebrates: Somites are formed by rotation of cell blocks out of the paraxial mesoderm. We attempted to determine whether or not the segmentation disorder following heat shock is caused by an altered pattern of fibronectin and/or laminin, that could then effect the rotation of cell blocks. Therefore, we carried out heat shock experiments and analyzed the distribution of both ECM proteins in correlation to the position of somitic cells. Our results reveal that heat shock causes an incorrect deposition of fibronectin as well as laminin during somite formation. This leads to an intermingling of cells from different segments and to an anchorage of cells at the lateral matrix. Immunoblots show that the defects in the pattern of these ECM proteins do not correlate with a decrease of both proteins. However, immunohistological staining patterns demonstrate that oversized blocks of 20-cell width, instead of the normal ones of about 9-cell width, are separated out of the paraxial mesoderm following heat shock treatment. This indicates that the altered pattern of fibronectin and laminin might be a secondary effect caused by incorrect segregation and detachment of cell blocks during somitogenesis. Since anchorage of somitic cells is mostly affected by the altered distribution of fibronectin and laminin, it is more likely that both ECM proteins function in anchorage of migrating presomitic cells and in maintaining of segment borders rather than in stimulating cell rotation movements.

Mesodermal cell migration during Xenopus gastrulation

The adhesive glycoprotein fibronectin (FN), which is a component of the network of extracellular matrix fibrils on the inner surface of the blastocoel roof (BCR), has been proposed to play a major role in directing mesodermal cell migration during amphibian gastrulation. In the first part of this paper, the adhesion of Xenopus mesodermal cells to FN in vitro is examined. Cells from several mesoderm regions, which differ in developmental fate and morphogenetic activity, are able to bind specifically to the RGD cell-binding site of FN. Dorsal mesodermal cell adhesion to FN varies along the anterior-posterior (a-p) axis: adhesion is strongest in the anterior head mesoderm, and gradually decreases posteriorly. This a-p gradient of mesodermal adhesiveness to FN does not change during mesodermal involution, and is reflected in the morphology of mesodermal explants on FN. An a-p strip of mesoderm develops a spreading, leading anterior margin and a compact, retracting posterior end, thus moving slowly and directionally over the FN substrate at about 0.8 micron/min. Although dissociated cells from all levels of the dorsal mesodermal axis adhere to FN, only the anterior, leading prospective head mesoderm cells migrate as single cells on a FN substrate in vitro. Locomotion by means of lamelliform protrusions occurs at an average rate of about 1.5 micron/min. Cells of the more posterior axial mesoderm merely shift position at random without substantial net translocation, and preinvolution mesoderm cells are completely stationary. On the BCR, the in vivo substrate for mesodermal cell migration, dissociated prospective head mesoderm cells spread and migrate as on FN in vitro, at 2.2 microns/min. In the presence of an RGD peptide which inhibits cell-FN interaction, cells remain globular and do not spread. They are still motile, but change direction frequently, which leads to less efficient net translocation. Apparently, interaction with the RGD cell-binding site of FN and concomitant spreading of head mesoderm cells is required for the stabilization of cell locomotion. In contrast to the directional migration of the mesoderm cell population toward the animal pole in the embryo, the pathways of dissociated cells on the BCR are randomly oriented. Coherent explants of migratory mesoderm do not move at all on the BCR, although they translocate on FN in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

Appearance and distribution of laminin during development of Xenopus laevis

A cloned cDNA fragment, homologous to domain II of the mouse laminin B1 chain, was obtained from a Xenopus neurula cDNA library. Using this probe Northern-blot analysis over the course of embryogenesis revealed a first signal of a laminin transcript at midgastrula (stage 11). Affinity-purified polyclonal antibodies directed against Xenopus laminin were obtained via a lacZ fusion protein. Immunohistology demonstrated the appearance of the antigen at about one developmental stage after the detection of the transcript. At stage 12 1/2 a faint immunofluorescence staining surrounded the developing notochord and somitogenic mesodermal cells. Thereafter laminin appeared in distinct locations outlining notochord, somites and neural tube. The epidermal basement membrane seemed to be endowed with laminin only at the relatively late postneurula stage. The observation that laminin could not be detected before stage 12 1/2 is discussed with respect to a proposed role of laminin in the gastrulation process.