Rapid Upregulation of Orai1 Abundance in the Plasma Membrane of Platelets Following Activation with Thrombin and Collagen Related Peptide

Background: Blood platelets accomplish primary hemostasis following vascular injury and contribute to the orchestration of occlusive vascular disease. Platelets are activated by an increase of cytosolic Ca 2+ -activity ([Ca 2+ ] i ), which is accomplished by Ca 2+ -release from intracellular stores and subsequent store operated Ca 2+ entry (SOCE) through Ca 2+ release activated Ca 2+ channel moiety Orai1. Powerful activators of platelets include thrombin and collagen related peptide (CRP), which are in part effective by activation of small G- protein Rac1. The present study explored the influence of thrombin and CRP on Orai1 protein abundance and cytosolic Ca 2+ -activity ([Ca 2+ ] i ) in platelets drawn from wild type mice. Methods: Orai1 protein surface abundance was quantified utilizing CF™488A conjugated antibodies, and [Ca 2+ ] i was determined with Fluo3-fluorescence. Results: In resting platelets, Orai1 protein abundance and [Ca 2+ ] i were low. Thrombin (0.02 U/ml) and CRP (5ug/ml) within 2 min increased [Ca 2+ ] i and Orai1 protein abundance at the platelet surface. [Ca 2+ ] i was further increased by Ca 2+ ionophore ionomycin (1 µM) and by store depletion with the sarcoendoplasmatic Ca 2+ ATPase inhibitor thapsigargin (1 µM). However, Orai1 protein abundance at the platelet surface was not significantly affected by ionomycin and only slightly increased by thapsigargin. The effect of thrombin and CRP on Orai1 abundance and [Ca 2+ ] i was significantly blunted by Rac1 inhibitor NSC23766 (50 µM). Conclusion: The increase of [Ca 2+ ] i following stimulation of platelets with thrombin and collagen related peptide is potentiated by ultrarapid Rac1 sensitive translocation of Orai1 into the cell membrane.