A directional switch of integrin signaling and a new anti-thrombotic strategy

Integrins are critical in thrombosis and hemostasis(1). Antagonists of the platelet integrin α(IIb)β(3) are potent anti-thrombotic drugs, but also have the life-threatening adverse effect of bleeding(2,3). It is thus desirable to develop new antagonists that do not cause bleeding. Integrins transmit signals bidirectionally(4,5). Inside-out signaling activates integrins via a talin-dependent mechanism(6,7). Integrin ligation mediates thrombus formation and outside-in signaling(8,9), which requires Gα(13) and greatly expands thrombi. Here we show that Gα(13) and talin bind to mutually exclusive, but distinct sites within the integrin β(3) cytoplasmic domain in opposing waves. The first talin binding wave mediates inside-out signaling and also “ligand-induced integrin activation”, but is not required for outside-in signaling. Integrin ligation induces transient talin dissociation and Gα(13) binding to an ExE motif, which selectively mediates outside-in signaling and platelet spreading. The second talin binding wave is associated with clot retraction. An ExE motif-based inhibitor of Gα(13)-integrin interaction selectively abolishes outside-in signaling without affecting integrin ligation, and suppresses occlusive arterial thrombosis without affecting bleeding time. Thus, we have discovered a novel mechanism for the directional switch of integrin signaling and, based on this mechanism, we designed a potent new anti-thrombotic that does not cause bleeding.