Identification of Fc{gamma}Rlla as the ITAM-bearing receptor mediating {alpha}llb{beta}3 outside-in integrin signaling in human platelets

Abstract

Immunoreceptor 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 αllbβ3. fibrinogen binding was induced by (1) allowing platelets to spread directly on immobilized fibrinogen, or (2) activating the PARI 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 (PLC-γ2). Addition of Fab fragments of an FcγRIIa-specific monoclonal antibody strongly inhibited platelet spreading on immobilized fibrinogen, as well as downstream tyrosine phosphorylation of FcγRlla, Syk, and PLCγ2, and platelets from a patient whose platelets express reduced levels of Fc-γRlla exhibited markedly reduced spreading on immobilized fibrinogen. Finally, fibrinogen binding-induced Fc-γRlla phosphorylation did not occur in human platelets expressing a truncated β3 cytoplasmic domain. Taken together, these data suggest that ligand binding to platelet αllbβ3 induces integrin cytoplasmic domaindependent phosphorylation of FcγRIIa, which then enlists selected components of the immunoreceptor signaling cascade to transmit amplification signals into the cell. © 2008 by The American Society of Hematology.