Accelerated Platelet Clearance in Mice with Constitutive Store-Operated Calcium Entry Is Independent of Caldag-GEFI/Rap1 Activation

Calcium (Ca²⁺) is critical for numerous functions of platelets including integrin activation, granule secretion and phosphatidylserine exposure. Circulating platelets maintain low cytosolic Ca²⁺ concentrations which rapidly rise upon cellular activation. Ca²⁺ release from intracellular stores triggers influx of extracellular Ca²⁺, a process known as store-operated Ca²⁺ entry (SOCE). Critical for SOCE is Stromal interaction molecule 1 (Stim1), a protein expressed in the dense tubular system membrane. Loss of Stim1 in platelets partially impairs platelet activation and procoagulant function. On the other hand, gain-of-function mutations in Stim1 are associated with thrombocytopenia and bleeding. In platelets, Ca²⁺ also activates CalDAG-GEFI and in turn Rap1 GTPase, the principal pathway mediating integrin activation. We recently demonstrated that unrestricted activation of Rap1 due to loss of the Rap1 GAP Rasa3 causes severe thrombocytopenia due to CalDAG-GEFI/integrin-dependent platelet clearance. To investigate whether CalDAG-GEFI activation mediates thrombocytopenia seen with constitutive Stim1 activation, we crossed Stim1 gain-of-function mice (Stim1^Sax/+) with CalDAG-GEFI^-/- mice to obtain Stim1^Sax/+ x CalDAG-GEFI^+/- or ^-/- mice. We observed no improvement in platelet count or platelet lifespan in Stim1^Sax/+ mice with 1 or 0 copies of CalDAG-GEFI. While Rasa3 mutant platelets are predominantly cleared in the spleen, Stim1^Sax/+ platelets accumulated to a greater extent in the lung. These results suggest that reduced platelet survival due to constitutive SOCE is not the result of increased CalDAG-GEFI/Rap1 signaling in circulating platelets.