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Structure-function analysis reveals discrete beta3 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 alphaIIbbeta3 integrins require "inside-out" signals to bind fibrinogen and form thrombi. Following ligand binding, alphaIIbbeta3 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 intracellular proteins, but the molecular basis for integrin signal transduction remains poorly understood. In the present study we have used retroviral vectors to express alphaIIbbeta3 integrins with mutant beta3 tails in mouse platelets and test mechanisms of bidirectional signaling. Using this approach we identify mutations (eg, beta3Y747A) that confer loss of signaling in both directions and others (eg, beta3T762A) 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.