The CalDAG-GEFI/Rap1/αIIbβ3 axis minimally contributes to accelerated platelet clearance in mice with constitutive store-operated calcium entry.

Circulating platelets maintain low cytosolic Ca 2+ concentrations. At sites of vascular injury, agonist-induced Ca 2+ release from platelet intracellular stores triggers influx of extracellular Ca 2+ , a process known as store-operated Ca 2+ entry (SOCE). Stromal interaction molecule 1 (Stim1) senses reduced Ca 2+ stores and triggers SOCE. Gain-of-function (GOF) mutations in Stim1, such as described for Stormorken syndrome patients or mutant mice ( Stim1 Sax ), are associated with marked thrombocytopenia and increased platelet turnover. We hypothesized that reduced platelet survival in Stim1 Sax/+ mice is due to increased Rap1/integrin signaling and platelet clearance in the spleen, similar to what we recently described for mice expressing a mutant version of the Rap1-GAP, Rasa3 ( Rasa3 hlb/hlb ). Stim1 Sax/+ mice were crossed with mice deficient in CalDAG-GEFI, a critical calcium-regulated Rap1-GEF in platelets. In contrast to Rasa3 hlb/hlb x Caldaggef1 - /- mice, only a small increase in the peripheral platelet count, but not platelet lifespan, was observed in Stim1 Sax/+ x Caldaggef1 - /- mice. Similarly, inhibition of αIIbβ3 integrin in vivo only minimally raised the peripheral platelet count in Stim1 Sax/+ mice. Compared to controls, Stim1 Sax/+ mice exhibited increased platelet accumulation in the lung, but not the spleen or liver. These results suggest that CalDAG-GEFI/Rap1/integrin signaling contributes only minimally to accelerated platelet turnover caused by constitutive SOCE.