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

Integrins not only bind adhesive ligands(1), they also act as signalling receptors(2), Both functions allow the integrin alpha IIb beta 3 to mediate platelet aggregation(3), Platelet agonists activate alpha IIb beta 3 (inside-out signalling) to allow the binding of soluble fibrinogen. Subsequent platelet aggregation leads to outside-in alpha IIb beta 3 signalling, which results in calcium mobilization(4), tyrosine phosphorylation of numerous proteins(5,6) including beta 3 itself(7) increased cytoskeletal reorganisations and further activation of alpha IIb beta 3 (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 alpha IIb beta 3 in which the tyrosines in the integrin cytoplasmic tyrosine motif have been mutated to phenylalanines. These mice are selectively impaired in outside-in alpha IIb beta 3 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 beta-integrin signals and a potential target for new therapeutic agents.