The Catalytic Subunit of Protein Phosphatase 1 Gamma Regulates Thrombin-Induced Murine Platelet α(IIb)β(3) Function

BACKGROUND: Hemostasis and thrombosis are regulated by agonist-induced activation of platelet integrin α(IIb)β(3). Integrin activation, in turn is mediated by cellular signaling via protein kinases and protein phosphatases. Although the catalytic subunit of protein phosphatase 1 (PP1c) interacts with α(IIb)β(3), the role of PP1c in platelet reactivity is unclear. METHODOLOGY/PRINCIPAL FINDINGS: Using γ isoform of PP1c deficient mice (PP1cγ(−/−)), we show that the platelets have moderately decreased soluble fibrinogen binding and aggregation to low concentrations of thrombin or protease-activated receptor 4 (PAR4)-activating peptide but not to adenosine diphosphate (ADP), collagen or collagen-related peptide (CRP). Thrombin-stimulated PP1cγ(−/−) platelets showed decreased α(IIb)β(3) activation despite comparable levels of α(IIb)β(3), PAR3, PAR4 expression and normal granule secretion. Functions regulated by outside-in integrin α(IIb)β(3) signaling like adhesion to immobilized fibrinogen and clot retraction were not altered in PP1cγ(−/−) platelets. Thrombus formation induced by a light/dye injury in the cremaster muscle venules was significantly delayed in PP1cγ(−/−) mice. Phosphorylation of glycogen synthase kinase (GSK3)β-serine 9 that promotes platelet function, was reduced in thrombin-stimulated PP1cγ(−/−) platelets by an AKT independent mechanism. Inhibition of GSK3β partially abolished the difference in fibrinogen binding between thrombin-stimulated wild type and PP1cγ(−/−) platelets. CONCLUSIONS/SIGNIFICANCE: These studies illustrate a role for PP1cγ in maintaining GSK3β-serine9 phosphorylation downstream of thrombin signaling and promoting thrombus formation via fibrinogen binding and platelet aggregation.