Integrin and ligand-independent PDGFr signaling synergistically contribute to directional migration of Xenopus mesendoderm

Abstract

Both PDGF signaling and adhesion to fibronectin (FN) matrix have been implicated in the directional collective migration of Xenopus mesendoderm cells at gastrulation. However, mesendoderm explants cultured on FN-coated substrates migrate directionally even in the absence of a source of PDGF. Integrin adhesion has been reported to up-regulate PDGF ligand-independent signaling through the PDGF receptor (PDGFr) in cultured mammalian cells. In order to address whether a similar mechanism stimulates PDGFr signaling in the absence of PDGF-A ligand in amphibian mesendoderm, isolated cells were cultured on bacterial fusion proteins containing the Type-III repeats 9-11 of FN (GST-9.11). Type III9-11 contains the RGD and \"synergy\" (PPSRN) sites required for integrin 5{beta}1 adhesion and activation but lacks the PDGF-A ligand-binding site present in the full-length FN protein. In order to ensure mesendoderm was not exposed to PDGF in vivo prior to removal and culture in vitro, antisense morpholinos were used to inhibit normal expression of PDGF-A ligand in embryos. P-Akt levels were reduced two-fold when either the PDGFr- was knocked down or when cells were plated on GST-9.11a, which contains a point mutation (PPSRN>PPSAN) that prevents both full activation of integrin 5{beta}1 and cell spreading. Reduced expression of PDGFr- was accompanied by perturbations in tissue migration, cytoskeletal organization, polarity of cell protrusions, and focal adhesion area. Mesendoderm cells became rounded, and the actin and cytokeratin filaments appeared collapsed and often colocalized near the cell center. Taken together, these findings suggest that integrin adhesion to FN, acting in synergy with PDGFr-, is sufficient to elevate PI3K-Akt signaling in the mesendoderm even in the absence of the PDGF-A ligand, and to promote forward-directed protrusions and directional tissue migration.