Integrin cytoplasmic tyrosine motif is required for outside-in αIIbβ3 signalling and platelet function

Integrins not only bind adhesive ligands1, they also act as signalling receptors2. Both functions allow the integrin αIIbβ3 to mediate platelet aggregation3. Platelet agonists activate αIIbβ3 (inside-out signalling) to allow the binding of soluble fibrinogen. Subsequent platelet aggregation leads to outside-in αIIbβ3 signalling, which results in calcium mobilization4, tyrosine phosphorylation of numerous proteins5,6 including β3 itself7, increased cytoskeletal reorganisation8 and further activation of αIIbβ3 (ref. 2). Thus, outside-in signals enhance aggregation, although the mechanisms and functional consequences of specific signalling events remain unclear. Here we describe a mouse that expresses an αIIbβ3 in which the tyrosines in the integrin cytoplasmic tyrosine motif have been mutated to phenylalanines. These mice are selectively impaired in outside-in αIIbβ3 signalling, with defective aggregation and clot-retraction responses in vitro, and an in vivo bleeding defect which is characterized by a pronounced tendency to rebleed. These data provide evidence for an important role of outside-in signalling in platelet physiology. Furthermore, they identify the integrin cytoplasmic tyrosine motif as a key mediator of β-integrin signals and a potential target for new therapeutic agents.