Submitted October 10, 2006
Accepted December 3, 2006
Structure-function analysis reveals discrete 
3 integrin inside-out and outside-in signaling pathways in platelets
Zhiying Zou, Hong Chen, Alec A Schmaier, Richard O Hynes, and Mark L Kahn*
Division of Cardiology and Department of Medicine, University of Pennsylvania, Philadelphia, PA
Center for Cancer Research, MIT, Cambridge, MA
* Corresponding author; email: markkahn{at}mail.med.upenn.edu.
A unique aspect of integrin receptor function is the transmission of bidirectional signals. In platelets 
IIbb
3 integrins require "inside-out" signals to bind fibrinogen and form thrombi. Following ligand binding, IIbb3 integrins generate "outside-in" signals that contribute to thrombus stability. Since 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 IIbb3 integrins with mutant 3 tails in mouse platelets and test mechanisms of bidirectional signaling. Using this approach we identify mutations, e.g. 3Y747A, that confer loss of signaling in both directions and others, e.g. 3T762A, that confer a selective loss of outside-in signals. These results reveal the presence of discrete bidirectional signaling pathways controlled by integrin subunits in platelets and describe a high-throughput means of further investigating these pathways in vivo.
