BETA(1)-CLASS AND BETA(3)-CLASS INTEGRINS MEDIATE FIBRONECTIN-BINDING ACTIVITY 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 the fibronectin cell binding domain attached to fluorescent microspheres. Fibronectin binding activity on the apical surface of the trophectoderm was confined to the abembryonic pole of the blastocyst, where trophoblast differentiation initiates, and was regulated temporally in accordance with blastocyst outgrowth. Soluble fibronectin (IC50 = 0.2 mu M) or Gly-Arg-Gly-Asp- Ser-Pro, but not laminin, competitively inhibited fibronectin binding activity. Addition of antibodies against the alpha(v), alpha(5), beta(1), or beta(3) integrin subunits also inhibited 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 translocation event that up-regulates fibronectin binding beta(1)- and beta(3)-class integrins on the apical surface.