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Rac1-dependent collective cell migration is required for specification of the anterior-posterior body axis of the mouse.

ABSTRACT: Cell migration and cell rearrangements are critical for establishment of the body plan of vertebrate embryos. The first step in organization of the body plan of the mouse embryo, specification of the anterior-posterior body axis, depends on migration of the anterior visceral endoderm from the distal tip of the embryo to a more proximal region overlying the future head. The anterior visceral endoderm (AVE) is a cluster of extra-embryonic cells that secretes inhibitors of the Wnt and Nodal pathways to inhibit posterior development. Because Rac proteins are crucial regulators of cell migration and mouse Rac1 mutants die early in development, we tested whether Rac1 plays a role in AVE migration. Here we show that Rac1 mutant embryos fail to specify an anterior-posterior axis and, instead, express posterior markers in a ring around the embryonic circumference. Cells that express the molecular markers of the AVE are properly specified in Rac1 mutants but remain at the distal tip of the embryo at the time when migration should take place. Using tissue specific deletions, we show that Rac1 acts autonomously within the visceral endoderm to promote cell migration. High-resolution imaging shows that the leading wild-type AVE cells extend long lamellar protrusions that span several cell diameters and are polarized in the direction of cell movement. These projections are tipped by filopodia-like structures that appear to sample the environment. Wild-type AVE cells display hallmarks of collective cell migration: they retain tight and adherens junctions as they migrate and exchange neighbors within the plane of the visceral endoderm epithelium. Analysis of mutant embryos shows that Rac1 is not required for intercellular signaling, survival, proliferation, or adhesion in the visceral endoderm but is necessary for the ability of visceral endoderm cells to extend projections, change shape, and exchange neighbors. The data show that Rac1-mediated epithelial migration of the AVE is a crucial step in the establishment of the mammalian body plan and suggest that Rac1 is essential for collective migration in mammalian tissues.

SUBMITTER: Migeotte I

PROVIDER: S-EPMC2914637 | biostudies-literature | 2010 Aug

REPOSITORIES: biostudies-literature

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Rac1-dependent collective cell migration is required for specification of the anterior-posterior body axis of the mouse.

Migeotte Isabelle I Omelchenko Tatiana T Hall Alan A Anderson Kathryn V KV

PLoS biology 20100803 8

Cell migration and cell rearrangements are critical for establishment of the body plan of vertebrate embryos. The first step in organization of the body plan of the mouse embryo, specification of the anterior-posterior body axis, depends on migration of the anterior visceral endoderm from the distal tip of the embryo to a more proximal region overlying the future head. The anterior visceral endoderm (AVE) is a cluster of extra-embryonic cells that secretes inhibitors of the Wnt and Nodal pathway ...[more]

PMID: 20689803

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Project description:During vertebrate axis extension, adjacent tissue layers undergo profound morphological changes: within the neuroepithelium, neural tube closure and neural crest formation are occurring, while within the paraxial mesoderm somites are segmenting from the presomitic mesoderm (PSM). Little is known about the signals between these tissues that regulate their coordinated morphogenesis. Here, we analyze the posterior axis truncation of mouse Fgf3 null homozygotes and demonstrate that the earliest role of PSM-derived FGF3 is to regulate BMP signals in the adjacent neuroepithelium. FGF3 loss causes elevated BMP signals leading to increased neuroepithelium proliferation, delay in neural tube closure and premature neural crest specification. We demonstrate that elevated BMP4 depletes PSM progenitors in vitro, phenocopying the Fgf3 mutant, suggesting that excessive BMP signals cause the Fgf3 axis defect. To test this in vivo we increased BMP signaling in Fgf3 mutants by removing one copy of Noggin, which encodes a BMP antagonist. In such mutants, all parameters of the Fgf3 phenotype were exacerbated: neural tube closure delay, premature neural crest specification, and premature axis termination. Conversely, genetically decreasing BMP signaling in Fgf3 mutants, via loss of BMP receptor activity, alleviates morphological defects. Aberrant apoptosis is observed in the Fgf3 mutant tailbud. However, we demonstrate that cell death does not cause the Fgf3 phenotype: blocking apoptosis via deletion of pro-apoptotic genes surprisingly increases all Fgf3 defects including causing spina bifida. We demonstrate that this counterintuitive consequence of blocking apoptosis is caused by the increased survival of BMP-producing cells in the neuroepithelium. Thus, we show that FGF3 in the caudal vertebrate embryo regulates BMP signaling in the neuroepithelium, which in turn regulates neural tube closure, neural crest specification and axis termination. Uncovering this FGF3-BMP signaling axis is a major advance toward understanding how these tissue layers interact during axis extension with important implications in human disease.

Project description:The Dishevelled proteins transduce both canonical Wnt/β-catenin and non-canonical Wnt/planar cell polarity (PCP) signaling pathways to regulate many key developmental processes during embryogenesis. Here, we disrupt both canonical and non-canonical Wnt pathways by targeting the entire Dishevelled family of genes (Dvl1, Dvl2, and Dvl3) to investigate their functional roles in the early embryo. We identified several defects in anterior-posterior axis specification and mesoderm patterning in Dvl1+/-; Dvl2-/-; Dvl3-/- embryos. Homozygous deletions in all three Dvl genes (Dvl TKO) resulted in defects in distal visceral endoderm migration and a complete failure to induce mesoderm formation. To identify potential mechanisms that lead to the defects in the developmental processes preceding gastrulation, we generated Dvl TKO mouse embryonic stem cells (mESCs) and compared the transcriptional profile of these cells with wild-type (WT) mESCs during germ lineage differentiation into 3D embryoid bodies (EBs). While the Dvl TKO mESCs displayed similar morphology, self-renewal properties, and minor transcriptional variation from WT mESCs, we identified major transcriptional dysregulation in the Dvl TKO EBs during differentiation in a number of genes involved in anterior-posterior pattern specification, gastrulation induction, mesenchyme morphogenesis, and mesoderm-derived tissue development. The absence of the Dvls leads to specific down-regulation of BMP signaling genes. Furthermore, exogenous activation of canonical Wnt, BMP, and Nodal signaling all fail to rescue the mesodermal defects in the Dvl TKO EBs. Moreover, endoderm differentiation was promoted in the absence of mesoderm in the Dvl TKO EBs, while the suppression of ectoderm differentiation was delayed. Overall, we demonstrate that the Dvls are dispensable for maintaining self-renewal in mESCs but are critical during differentiation to regulate key developmental signaling pathways to promote proper axis specification and mesoderm formation.

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Rac1-dependent collective cell migration is required for specification of the anterior-posterior body axis of the mouse.

Publications

Rac1-dependent collective cell migration is required for specification of the anterior-posterior body axis of the mouse.

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