Structure-function analysis reveals discrete β3 integrin inside-out and outside-in signaling pathways in platelets

A unique aspect of integrin receptor function is the transmission of bidirectional signals. In platelets alpha llb beta 3 integrins require inside-out signals to bind fibrinogen and form thrombi. Following ligand binding, alpha llb beta 3 integrins generate outside-in signals that contribute to thrombus stability. Because integrin cytoplasmic tails are short and lack enzymatic activity, bidirectional signals are believed to be mediated by interactions with intra-cellular proteins, but the molecular basis for integrin signal transduction remains poorly understood. In the present study we have used retroviral vectors to express alpha llb beta 3 integrins with mutant beta 3 tails in mouse platelets and test mechanisms of bidirectional signaling. Using this approach we identify mutations (eg, beta 3Y747A) that confer loss of signaling in both directions and others (eg, beta 3T762A) that confer a selective loss of outside-in signals. These results reveal the presence of discrete bidirectional signaling pathways controlled by integrin beta subunits in platelets and describe a high-throughput means of further investigating these pathways in vivo.