Abstract
Integrin signaling modulates trophoblast adhesion to extracellular matrices during blastocyst implantation. Fibronectin (FN)-binding activity on the apical surface of trophoblast cells is strengthened after elevation of intracellular Ca(2+) downstream of integrin ligation by FN. We report here that phosphoinositide-specific phospholipase C (PLC) mediates Ca(2+) signaling in response to FN. Pharmacological agents used to antagonize PLC (U73122) or the inositol phosphate receptor (Xestospongin C) inhibited FN-induced elevation of intracellular Ca(2+) and prevented the upregulation of FN-binding activity. In contrast, inhibitors of Ca(2+) influx through either voltage-gated or non-voltage-gated Ca(2+) channels were without effect. Inhibition of protein tyrosine kinase activity by genistein, but not G-protein inhibition by suramin, blocked FN-induced intracellular Ca(2+) signaling and upregulation of adhesion, consistent with involvement of PLC-gamma. Confocal immunofluorescence imaging of peri-implantation blastocysts demonstrated that PLC-gamma2, but not PLC-gamma1 nor PLC-beta1, accumulated near the outer surface of the embryo. Phosphotyrosine site-directed antibodies revealed phosphorylation of PLC-gamma2, but not PLC-gamma1, upon integrin ligation by FN. These data suggest that integrin-mediated activation of PLC-gamma to initiate phosphoinositide signaling and intracellular Ca(2+) mobilization is required for blastocyst adhesion to FN. Signaling cascades regulating PLC-gamma could, therefore, control a critical feature of trophoblast differentiation during peri-implantation development.