Akt2 Supports aIIbb3-Dependent Reorganization of the Actin Cytoskeleton in Platelets.

Akt is a serine-threonine kinase with well-described roles in growth and metabolism. Previous studies from our laboratory have shown that Akt also plays important roles in platelet function in vitro and in thrombus formation in vivo. Two isoforms are present in mouse platelets, Akt1 and Akt2, with Akt2 being the dominant isoform. Our previous studies have shown that platelets from Akt2−/− mice have marked defects in aggregation, a- and dense granule secretion, and fibrinogen binding. Each of these platelet functions depends in part on the function of the major platelet integrin, aIIbb3. However, whether Akt might regulate the function of aIIbb3 is still unknown. To determine whether Akt regulates aIIbb3-dependent signaling, first the rate of thrombin-initiated clot retraction was compared in platelet-rich plasma (PRP) from wild-type versus Akt2−/− mice. Akt2−/− platelets have a delay in the aIIbb3-dependent retraction of the fibrin clot compared to their wildtype counterparts. Akt2−/− platelets also have a delay in aIIbb3-dependent spreading on immobilized fibrinogen. However, adhesion to immobilized fibrinogen was normal in these platelets, suggesting that the spreading defect is due to a defect in outside-in signaling by the integrin, rather than in fibrinogen binding. Furthermore, unstimulated platelets expressing a constitutively active form of Akt spread more rapidly on fibrinogen-coated slides and generate more filopodial extensions than wildtype platelets, suggesting that Akt activation may be sufficient to induce outside-in signaling and /or cytoskeletal remodeling. Interestingly, integrin activation was not sufficient to induce Akt phoshorylation, suggesting that integrin activity is downstream rather than upstream of Akt activation. Taken together, these results suggest that integrin aIIbb3 is not necessary for Akt activation; however, Akt promotes outside-in signaling and cytoskeletal remodeling by aIIbb3.