| Popis: |
BACKGROUND: Many RAS family small GTPases are expressed in platelets, including RAC, RHOA, RAP, and H-/N-/RRAS1, but most of their signaling and cellular functions remain poorly understood. Like RRAS1, TC21/RRAS2 reverses HRAS-induced suppression of integrin activation in CHO cells. However, a role for TC21 in platelets has not been explored. OBJECTIVES: To determine TC21 expression in platelets, TC21 activation in response to platelet agonists, and roles of TC21 in platelet function in vitro and in vivo thrombosis. RESULTS: We demonstrate that TC21 is expressed in human and murine platelets and is activated in response to agonists for the glycoprotein VI (GPVI)/FcRγ immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor, in a Src-dependent manner. GPVI-induced platelet aggregation, integrin α(IIb)β(3) activation, and α- and dense-granule secretion, as well as phosphorylation of Syk, PLCγ2, AKT, and ERK, were inhibited in TC21-deficient platelets ex vivo. In contrast, these responses were normal in TC21-deficient platelets following stimulation with P2Y, PAR4 and CLEC-2 receptor agonists, indicating that the function of TC21 in platelets is GPVI/FcRγ-ITAM-specific. TC21 was required for GPVI-induced activation of Rap1b. TC21-deficient mice did not show a significant delay in injury-induced thrombosis compared to WT controls; however, thrombi were unstable. Hemostatic responses showed similar effects. CONCLUSIONS: TC21 is essential for GPVI/FcRγ-mediated platelet activation and for thrombus stability in vivo via control of Rap1b and integrins. |
