Anti-Thrombotic Effects of Targeting Outside-in αIIbβ3 Signaling in Platelets

Platelet activation by agonists stimulates high affinity binding of adhesive ligands, such as fibrinogen, to integrin αIIbβ3. In turn, ligand binding initiates Src kinase-dependent outside-in signaling, platelet spreading on sub-endothelial matrices, and platelet aggregation. Short-term integrin blockade by parenteral αIIbβ3 antagonists is effectively anti-thrombotic in certain clinical situations, but long-term blockade by oral αIIbβ3 antagonists has been problematic. As an alternative approach, we evaluated the feasibility of disrupting outside-in αIIbβ3 signaling in vivo. A “β3(Δ760-762)” knock-in mouse was generated that lacked the three C-terminal β3 amino acid residues (RGT) necessary for αIIbβ3-dependent c-Src signaling, but retained β3 residues necessary for agonist- and talin-dependent fibrinogen binding. β3(Δ760-762) mice were compared with wild-type β3+/+ littermates, β3+/− heterozygotes, and “β3/β1(EGK)” knock-in mice where β3 RGT was replaced by three C-terminal residues in β1 (EGK) to potentially enable outside-in integrin signaling by certain Src kinases other than c-Src. Whereas β3+/+, β3+/− and β3/β1(EGK) platelets spread and underwent tyrosine phosphorylation normally on fibrinogen, β3(Δ760-762) platelets spread poorly and exhibited reduced tyrosine phosphorylation of c-Src substrates, including the β3 cytoplasmic domain itself (Tyr-747). Furthermore, unlike the three groups of control mice, all of the β3(Δ760-762) mice tested were protected from carotid artery occlusive thrombosis following vessel injury with FeCl3. The median tail bleeding time for the β3(Δ760-762) mice was approximately two-fold longer than that of the control mice. However, β3(Δ760-762) mice did not exhibit spontaneous bleeding, excess bleeding after surgical exposure of the carotid artery, fecal blood loss or anemia. Fibrinogen binding to β3(Δ760-762) platelets was normal in response to saturating concentrations of agonists to the PAR4 thrombin receptor or the GP VI collagen receptor; however, responses to ADP were impaired. These studies establish that deletion of β3 RGT disrupts outside-in αIIbβ3 signaling and confers protection from arterial thrombosis in vivo. Currently available anti-platelet drugs either inhibit agonist-induced activation of αIIbβ3 or block fibrinogen binding to the integrin. The studies reported here suggest that targeting outside-in αIIbβ3 signaling may represent an alternative anti-thrombotic strategy.