Identification of FcγRIIa as the ITAM-bearing receptor mediating αIIbβ3 outside-in integrin signaling in human platelets

Abstract

AbstractImmunoreceptor tyrosine-based activation motif (ITAM)–containing proteins have recently been demonstrated in macrophages and neutrophils to be required for cell surface integrins to transmit activation signals into the cell. To identify ITAM-bearing proteins that mediate signaling via the platelet-specific integrin αIIbβ3, fibrinogen binding was induced by (1) allowing platelets to spread directly on immobilized fibrinogen, or (2) activating the PAR1 thrombin receptor on platelets in suspension. Both initiated strong, ligand binding–dependent tyrosine phosphorylation of the ITAM-bearing platelet Fc receptor, FcγRIIa, as well as downstream phosphorylation of the protein tyrosine kinase Syk and activation of phospholipase Cγ2 (PLCγ2). Addition of Fab fragments of an FcγRIIa-specific monoclonal antibody strongly inhibited platelet spreading on immobilized fibrinogen, as well as downstream tyrosine phosphorylation of FcγRIIa, Syk, and PLCγ2, and platelets from a patient whose platelets express reduced levels of FcγRIIa exhibited markedly reduced spreading on immobilized fibrinogen. Finally, fibrinogen binding–induced FcγRIIa phosphorylation did not occur in human platelets expressing a truncated β3 cytoplasmic domain. Taken together, these data suggest that ligand binding to platelet αIIbβ3 induces integrin cytoplasmic domain–dependent phosphorylation of FcγRIIa, which then enlists selected components of the immunoreceptor signaling cascade to transmit amplification signals into the cell.