Rap1b is critical for glycoprotein VI-mediated but not ADP receptor-mediated α2β1 activation

Background: The platelet alpha(2)beta(1) integrin functions as both an adhesion and signaling receptor upon exposure to collagen. Recent studies have indicated that alpha(2)beta(1) function can be activated via inside-out signaling, similar to the prototypical platelet integrin alpha(IIb)beta(3). However, signaling molecules that regulate alpha(2)beta(1) activation in platelets are not well defined. A strong candidate molecule is the small GTPase Rap1b, the dominant platelet isoform of Rap1, which regulates alpha(IIb)beta(3) activation. Objectives: We hypothesized that Rap1b positively regulates alpha(2)beta(1) during agonist-induced platelet activation. Methods: To test whether Rap1b activates alpha(2)beta(1) downstream of glycoprotein (GP)VI or other platelet receptors, we stimulated platelets purified from Rap1b(-/-) or wild-type mice with diverse agonists and measured alpha(2)beta(1) activation using fluorescein isothiocyanate-labeled monomeric collagen. We also examined the role of Rap1b in outside-in signaling pathways by analyzing adhesion and spreading of Rap1b(-/-) or wild-type platelets on monomeric, immobilized collagen. Finally, we monitored the activation status of related Rap GTPases to detect changes in signaling pathways potentially associated with Rap1b-mediated events. Results: Rap1b(-/-) platelets displayed comparable ADP-induced or thrombin-induced alpha(2)beta(1) activation as wild-type platelets, but reduced convulxin-dependent alpha(2)beta(1) activation. Rap1b(-/-) platelets exhibited increased spreading on immobilized collagen but similar adhesion to immobilized collagen compared to wild-type platelets. Rap1b(-/-) platelets also showed Rap1a and Rap2 activation upon agonist stimulation, possibly revealing functional compensation among Rap family members. Conclusions: Rap1b is required for maximal GPVI-induced but not ADP-induced activation of alpha(2)beta(1) in murine platelets.