DISTINCT ROLES for Rap1b In PLATELET SECRETION and INTEGRIN a_IIBb₃ OUTSIDE-In SIGNALING

Abstract 2200

Rap1b is activated by platelet agonists, and plays a critical role in integrin a_IIbb₃ inside-out signaling and platelet aggregation. In this study, we identify two novel functions of Rap1b in platelets. We show that agonist-induced Rap1b activation plays an important role in stimulating secretion of platelet granules. We also show that a_IIbb₃ outside-in signaling can activate Rap1b, and integrin outside-in signaling-mediated Rap1b activation is important in facilitating platelet spreading on fibrinogen and clot retraction. Rap1b deficient platelets had diminished ATP secretion and P-selectin expression induced by thrombin or collagen. Defect in secretion of Rap1b deficient platelets was not due to reduced granule contents, because the amount of serotonin (5HT) and platelet factor 4 (PF4) in Rap1b deficient platelets is similar to that in wild type platelets. Data from transmission electron microscopy indicate that under resting conditions, wild type and Rap1b deficient platelets had normal discoid shapes with similar numbers of granules. When stimulated with thrombin, wild type platelets showed an irregular appearance with protruding filopodia and lack of granules. In contrast, thrombin-stimulated Rap1b^−/– platelets showed that more Rap1b^−/– platelets contained visible a granules than wild type platelets. Dense granules were also more obvious in thrombin-stimulated Rap1b^−/– platelets than wild type platelets. Importantly, addition of low doses of ADP and/or fibrinogen restored aggregation of Rap1b deficient platelets. Furthermore, we found that Rap1b was activated by platelet spreading on immobilized fibrinogen, a process that was not affected by P2Y₁₂ or TP deficiency, but was inhibited by the selective Src inhibitor PP2, the PKC inhibitor Ro-31-8220, or the calcium chelator demethyl-BAPTA. Clot retraction was abolished, and platelet spreading on fibrinogen was diminished in Rap1b deficient platelets compared with wild type controls. The defects in clot retraction and spreading on fibrinogen of Rap1b deficient platelets were not rescued by addition of MnCl₂, which elicits a_IIbb₃ outside-in signaling in the absence of inside-out signaling. Thus, our results reveal two different activation mechanisms of Rap1b as well as novel functions of Rap1b in platelet secretion and in integrin a_IIbb₃ outside-in signaling.