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

Integrins have a critical role in thrombosis and haemostasis. Antagonists of the platelet integrin α IIb β 3 are potent anti-thrombotic drugs, but also have the life-threatening adverse effect of causing bleeding. It is therefore desirable to develop new antagonists that do not cause bleeding. Integrins transmit signals bidirectionally. Inside-out signalling activates integrins through a talin-dependent mechanism. Integrin ligation mediates thrombus formation and outside-in signalling, which requires Gα ₁₃ and greatly expands thrombi. Here we show that Gα₁₃ and talin bind to mutually exclusive but distinct sites within the integrin β₃ cytoplasmic domain in opposing waves. The first talin-binding wave mediates inside-out signalling and also ligand-induced integrin activation, but is not required for outside-in signalling. Integrin ligation induces transient talin dissociation and Gα₁₃ binding to an EXE motif (in which X denotes any residue), which selectively mediates outside-in signalling and platelet spreading. The second talin-binding wave is associated with clot retraction. An EXE-motif-based inhibitor of Gα₁₃-integrin interaction selectively abolishes outside-in signalling without affecting integrin ligation, and suppresses occlusive arterial thrombosis without affecting bleeding time. Thus, we have discovered a new mechanism for the directional switch of integrin signalling and, on the basis of this mechanism, designed a potent new anti-thrombotic drug that does not cause bleeding.