The Ig-ITIM superfamily member PECAM-1 regulates the outside-in signaling properties of integrin αIIbβ3 in platelets

Previous studies have implicated the immunoglobulin (Ig)-immunoreceptor tyrosine-based inhibitory motif (ITIM) superfamily member platelet endothelial cell adhesion molecule-1 (PECAM-1) in the regulation of integrin function. While PECAM-1 has been demonstrated to play a role as an inhibitory coreceptor of immunoreceptor tyrosine-based activation motif (ITAM)-associated Fc gamma receptor IIa (Fc gamma RIIa) and glycoprotein VI (GPVI)/FcR gamma-chain signaling pathways in platelets, its physiologic role in integrin alpha(IIb)beta(3)-mediated platelet function is unclear. In this study, we investigate the functional importance of PECAM-1 in murine platelets. Using PECAM-1-deficient mice, we show that the platelets have impaired outside-in integrin 01103 signaling with impaired platelet spreading on fibrinogen, failure to retract fibrin clots in vitro, and reduced tyrosine phosphorylation of focal adhesion kinase p125 ((125)FAK) following integrin alpha(IIb)beta(3)-mediated platelet aggregation. This functional integrin alpha(IIb)beta(3) defect could not be attributed to altered expression of integrin alpha(IIb)beta(3)-mediated platelets displayed normal platelet alpha granule secretion, normal platelet aggregation to protease-activated receptor-4 (PAR-4), adenosine diphosphate (ADP), and calcium ionophore, and static platelet adhesion. In addition, PECAM-1(-/-) platelets displayed normal inside-out integrin 0003 signaling properties as demonstrated by normal agonist-induced binding of soluble fluoroscein isothiocyanate (FITC)-fibrinogen, JON/A antibody binding, and increases in cytosolic-free calcium and inositol (1,4,5)P-3 triphosphate (IP3) levels. This study provides direct evidence that PECAM-1 is essential for normal integrin alpha(IIb)beta(3)-mediated platelet function and that disruption of PECAM-1 induced a moderate outsidein integrin alpha(IIb)beta(3) signaling defect.