BETA(1)-CLASS AND BETA(3)-CLASS INTEGRINS MEDIATE FIBRONECTIN-BINDINGACTIVITY AT THE SURFACE OF DEVELOPING MOUSE PERIIMPLANTATION BLASTOCYSTS - REGULATION BY LIGAND-INDUCED MOBILIZATION OF STORED RECEPTOR

Implanting mouse blastocysts adhere through their abembryonic surfaces to the endometrial extracellular matrix. Because blastocysts cultured on fibronectin in vitro dissociate to form trophoblast outgrowths, it is unclear whether this adhesion is initially mediated by fibronectin receptors on the apical or basolateral surface of the trophectoderm. Intact blastocysts were examined in a ligand binding assay utilizing t he fibronectin cell binding domain attached to fluorescent microsphere s. Fibronectin binding activity on the apical surface of the trophecto derm was confined to the abembryonic pole of the blastocyst, where tro phoblast differentiation initiates, and was regulated temporally in ac cordance with blastocyst outgrowth. Soluble fibronectin (IC50 = 0.2 mu M) or Gly-Arg-Gly-Asp- Ser-Pro, but not laminin, competitively inhibi ted fibronectin binding activity. Addition of antibodies against the a lpha(v), alpha(5), beta(1), or beta(3) integrin subunits also inhibite d binding activity. Blastocysts cultured in the absence of an adhesive substratum exhibited fibronectin binding activity only after exposure to immobilized or soluble ligand. Potentiation of binding activity by Ligand was unaffected by cycloheximide but was sensitive to brefeldin A inhibition of protein trafficking. These findings suggest that the interaction of fibronectin with the trophectoderm induces a translocat ion event that up-regulates fibronectin binding beta(1)- and beta(3)-c lass integrins on the apical surface.