Opposing Roles for the α Isoform of the Catalytic Subunit of Protein Phosphatase 1 in Inside-Out and Outside-In Integrin Signaling in Murine Platelets

Platelet activation during hemostasis and thrombosis is facilitated by agonist-induced inside-out and integrin alpha IIb beta 3-initiated outside-in signaling via protein kinases and phosphatases. Pharmacological inhibitor studies suggest that the serine/threonine protein phosphatase 1 (PP1) promotes platelet activation. However, since phosphatase inhibitors block all the isoforms of the catalytic subunit of PP1 (PP1c), the role of specific PP1c isoform in platelet signaling remains unclear. Here, we employed a platelet-specific PP1c alpha-/- mice to explore the contribution of a major PP1 isoform in platelet functions. Loss of PP1c alpha moderately decreased activation of integrin alpha IIb beta 3, binding of soluble fibrinogen, and aggregation to low-dose thrombin, ADP, and collagen. In contrast, PP1c alpha-/- platelets displayed increased adhesion to immobilized fibrinogen, fibrin clot retraction, and thrombus formation on immobilized collagen. Mechanistically, post-fibrinogen engagement potentiated p38 mitogen-activated protein kinase (MAPK) activation in PP1c alpha-/- platelets and the p38 inhibitor blocked the increased integrin-mediated outside-in signaling function. Tail bleeding time and light-dye injury-induced microvascular thrombosis in the cremaster venules and arterioles were not altered in PP1c alpha-/- mice. Thus, PP1c alpha displays pleiotropic signaling in platelets as it amplifies agonist-induced signaling and attenuates integrin-mediated signaling with no impact on hemostasis and thrombosis.