In platelets, STIM1 has been recognized as the key regulatory
protein in store-operated Ca(2+) entry (SOCE) with Orai1 as principal Ca(2+) entry channel. Both
proteins contribute to
collagen-dependent arterial
thrombosis in mice in vivo. It is unclear whether STIM2 is involved. A key platelet response relying on Ca(2+) entry is the surface exposure of
phosphatidylserine (PS), which accomplishes platelet procoagulant activity. We studied this response in mouse platelets deficient in STIM1, STIM2, or Orai1. Upon high shear flow of blood over
collagen, Stim1(-/-) and Orai1(-/-) platelets had greatly impaired
glycoprotein (GP) VI-dependent Ca(2+) signals, and they were deficient in PS exposure and
thrombus formation. In contrast, Stim2(-/-) platelets reacted normally. Upon blood flow in the presence of
thrombin generation and coagulation, Ca(2+) signals of Stim1(-/-) and Orai1(-/-) platelets were partly reduced, whereas the PS exposure and formation of
fibrin-rich thrombi were normalized. Washed Stim1(-/-) and Orai1(-/-) platelets were deficient in GPVI-induced PS exposure and
prothrombinase activity, but not when
thrombin was present as co-agonist. Markedly,
SKF96365, a blocker of (receptor-operated) Ca(2+) entry, inhibited Ca(2+) and procoagulant responses even in Stim1(-/-) and Orai1(-/-) platelets. These data show for the first time that: (i) STIM1 and Orai1 jointly contribute to GPVI-induced SOCE, procoagulant activity, and
thrombus formation; (ii) a compensating Ca(2+) entry pathway is effective in the additional presence of
thrombin; (iii) platelets contain two mechanisms of Ca(2+) entry and PS exposure, only one relying on STIM1-Orai1 interaction.