THE RGD-DEPENDENT AND THE HEP-II BINDING DOMAINS OF FIBRONECTIN GOVERN THE ADHESIVE BEHAVIORS OF AMPHIBIAN EMBRYONIC-CELLS

In early amphibian development, interactions between fibronectin and b oth ectoderm and mesoderm cells are critical in the progression of gas trulation movements. In the Pleurodeles waltl embryo, it has been esta blished that ectoderm cells of the animal hemisphere organize a fibril lar-extracellular matrix containing fibronectin. Mesoderm cells migrat e along the blastocoel roof using these fibronectin fibrils as substra tum. Fibronectin is an adhesive glycoprotein which possesses multiple cell-binding domains. From previous studies, it is clear that amphibia n ectoderm and mesoderm cells interact with fibronectin in an RGD-depe ndent manner, whereas the contributions of RGD-independent domains in the adhesive behaviors of gastrula cells has not been defined. To stud y this question, we have used bacterially expressed Pleurodeles waltl fibronectin-fusion proteins. The approach consisted of in vitro adhesi on assays with either isolated cells or tissue fragments of embryos di ssected at the onset of gastrulation. Tissues were obtained from regio ns of the embryo which represent presumptive ectoderm cells or from th e dorsal-marginal zone which contains cells of the presumptive cephali c, chordal and semitic mesoderm. The results show that both the RGD-de pendent and the Hep II domains of fibronectin mediate attachment and s preading of isolated cells. Both regions cooperate to control the prop er expansion of a sheet of dorsal mesoderm cells. The Hep II domain pr omotes the migration of cells ahead of the mesoderm-cell sheet.