GSK3β Is a Negative Regulator of Platelet Function In Vitro and In Vivo.

GSK3β is a ser-thr kinase that is itself phosphorylated on ser9 by the kinase Akt. Because Akt has recently been shown to regulate platelet aggregation and arterial thrombosis in mice, we sought to identify Akt substrates in platelets that may play important roles in platelet function. We show here that the Akt effector, GSK3β, is present in platelets and becomes phosphorylated after treatment of mouse or human platelets with ADP or thrombin receptor-activating peptides (TRAP). Agonist-dependent phosphorylation of GSK3β is reduced by pre-treatment of mouse or human platelets with the PI3K inhibitor LY294002 and is also reduced in platelets from Akt2−/−Akt1+/− mice relative to non-littermate controls, suggesting that agonist-induced GSK3β phosphorylation is partially PI3K- and Akt-dependent. To determine whether GSK3β plays a role in platelet function, aggregation and secretion of dense granule contents were evaluated in human platelets treated with the GSK3 inhibitors, LiCl or SB216763. The dose-response curves for agonist-induced platelet aggregation and secretion were left-shifted in the presence of either inhibitor compared to untreated control platelets, suggesting that GSK3 activity suppresses platelet aggregation. Comparative immunoblots suggest that GSK3β is more highly expressed in platelets than GSK3α. Therefore, to confirm that GSKβ plays a suppressive role in platelet function, the aggregation of platelet-rich plasma (PRP) from GSK3β+/− mice was compared to that of non-littermate controls (GSK3β −/− mice die in utero). PRP from GSK3β+/− mice showed enhanced aggregation and secretion in response to U46619 or TRAP compared to control PRP. TRAP-induced binding of AlexaFluor-fibrinogen to platelet surfaces was also enhanced in washed platelets from GSK3β+/− mice compared to control platelets. Finally, the effect of GSK3β on platelet function in vivo was evaluated using two thrombosis models: a ferric chloride injury model of arterial thrombosis and a collagen-induced model of disseminated thrombosis. In the arterial thrombosis model, all GSK3β+/− mice (n=5) formed stable occlusive thrombi after ferric chloride injury to the carotid artery, whereas the majority of wildtype mice (67%) formed no thrombi, 27% formed stable occlusive thrombi, and 7% formed unstable thrombi under the same conditions (n=15). In a model of disseminated thrombosis, injection of a combination of collagen (170 μg/kg) and epinephrine (350 μM/kg) resulted in reduced survival of GSK3β+/− mice 10 minutes post-injection relative to wildtype mice (20%, n=5 versus 83%, n=6, respectively). Histological examination of lung sections suggested that all mice that died did so due to pulmonary embolism. These data suggest that removal of a single allele of GSK3β in mice confers enhanced sensitivity to thrombotic insult. Taken together, these results suggest that GSK3β is a substrate of Akt-dependent phosphorylation in platelets and, in contrast to the function of Akt, acts as a negative regulator of platelet function in vitro and in vivo.