V(+)-fibronectin expression and localization prior to gastrulation in Xenopus laevis embryos

The V-region represents one of three alternatively spliced segments in Xenopus fibronectin. Here, we identify this V-region as binding epitope of the monoclonal antibody (MAb 6D9) that we generated against Xenopus plasma fibronectin. By the use of this antibody we obtained new results that change the present view of the fibronectin expression pattern before gastrulation: (1) the V(+)-fibronectin is the major isoform expressed during early development since only a single fibronectin band is found in Western blots up to tadpole stages. (2) In contrast to previously published data we demonstrate that fibronectin expression is induced by progesterone during oocyte maturation. (3) During cleavage stages the protein is stored in the cytoplasm where it is predominantly associated with plasma membranes. Immunoelectronmicroscopy reveals that V(+)-fibronectin is present at the surface of animal pole blastomeres and secreted into intercellular spaces. This extracellular localization of fibronectin is predominantly observed in the marginal zone, surrounding single cells of the outer cell layer baso-laterally. In the vegetal hemisphere V(+)-fibronectin is restricted to the cytoplasm and accumulated at plasma membranes. With the onset of gastrulation the intracellular and membrane associated fibronectin disappears and fibronectin becomes detectable at the blastocoel roof. Since reaggregation of dissociated blastula cells was not blocked by addition of GRGDS peptide or antibodies against fibronectin, we assume that the early expression and secretion of fibronectin serves as store to allow a rapid matrix assembly with onset of mesodermal cell migration.