EMBRYONIC MESODERM CELLS SPREAD IN RESPONSE TO PLATELET-DERIVED GROWTH-FACTOR AND SIGNALING BY PHOSPHATIDYLINOSITOL 3-KINASE

Abnormal mesoderm movement, leading to defects in axial organization, is observed in mouse and Xenopus laevis embryos deprived of platelet d erived growth factor (PDGF) AA signaling, However, neither the cellula r response to PDGF nor the signaling pathways involved are understood. Herein we describe an in vitro assay to examine the direct effect of PDGF AA on aggregates of Xenopus embryonic mesoderm cells. We find tha t PDGF AA stimulates aggregates to spread on fibronectin. This behavio r is similar to that of migrating mesoderm cells in vivo that spread a nd form lamellipodia and filipodia on contact with fibronectin-rich ex tracellular matrix. We go on to show two lines of evidence that implic ate phosphatidylinositol 3-kinase (PI3K) as an important component of PDGF-induced mesoderm cell spreading. (i) The fungal metabolite wortma nnin, which inhibits signaling by PI3K, blocks mesoderm spreading in r esponse to PDGF AA. (ii) Activation of a series of receptors with spec ific tyrosine-to-phenylalanine mutations revealed PDGF-induced spreadi ng of mesoderm cells depends on PI3K but not on other signaling molecu les that interact with PDGF receptors including phospholipase C gamma, Ras GTPase-activating protein, and phosphotyrosine phosphatase SHPTP2 , These results indicate that a PDGF signal, mediated by PI3K, can fac ilitate embryonic mesoderm cell spreading on fibronectin, We propose t hat PDGF, produced by the ectoderm, influences the adhesive properties of the adjacent mesoderm cells during gastrulation.