Cooperative integrin/ITAM signaling in platelets enhances thrombus formation in vitro and in vivo

The integrin family is composed of a series of 24 alpha beta heterodimer transmembrane adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. Adaptor molecules bearing immunoreceptor tyrosine-based activation motifs (ITAMs) have recently been shown to cooperate with specific integrins to increase the efficiency of transmitting ligand-binding-induced signals into cells. In human platelets, Fc receptor gamma-chain IIa (Fc gamma RIIa) has been identified as an ITAM-bearing transmembrane receptor responsible for mediating "outside-in" signaling through alpha IIb beta 3, the major adhesion receptor on the platelet surface. To explore the importance of Fc gamma RIIa in thrombosis and hemostasis, we subjected Fc gamma RIIa-negative and Fc gamma RIIa-positive murine platelets to a number of well-accepted models of platelet function. Compared with their Fc gamma RIIa-negative counterparts, Fc gamma RIIa-positive platelets exhibited increased tyrosine phosphorylation of Syk and phospholipase C gamma 2 and increased spreading upon interaction with immobilized fibrinogen, retracted a fibrin clot faster, and showed markedly enhanced thrombus formation when perfused over a collagen-coated flow chamber under conditions of arterial and venous shear. They also displayed increased thrombus formation and fibrin deposition in in vivo models of vascular injury. Taken together, these data establish Fc gamma RIIa as a physiologically important functional conduit for alpha(IIb)beta(3)-mediated outside-in signaling, and suggest that modulating the activity of this novel integrin/ITAM pair might be effective in controlling thrombosis.