Circulating platelets maintain low cytosolic Ca2+ concentrations. At sites of vascular injury, agonist-induced Ca2+ release from platelet intracellular stores triggers influx of extracellular Ca2+, a process known as store-operated Ca2+ entry (SOCE). Stromal interaction molecule 1 (Stim1) senses reduced Ca2+ stores and triggers SOCE. Gain-of-function (GOF) mutations in Stim1, such as described for Stormorken syndrome patients or mutant mice (Stim1Sax), are associated with marked thrombocytopenia and increased platelet turnover. We hypothesized that reduced platelet survival in Stim1Sax/+ 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 (Rasa3hlb/hlb). Stim1Sax/+ mice were crossed with mice deficient in CalDAG-GEFI, a critical calcium-regulated Rap1-GEF in platelets. In contrast to Rasa3hlb/hlb x Caldaggef1-/- mice, only a small increase in the peripheral platelet count, but not platelet lifespan, was observed in Stim1Sax/+ x Caldaggef1-/- mice. Similarly, inhibition of αIIbβ3 integrin in vivo only minimally raised the peripheral platelet count in Stim1Sax/+ mice. Compared to controls, Stim1Sax/+ 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.
Keywords: Calcium; Rap1; Stim1, thrombocytopenia; platelet.
What do we know? Platelets are small blood cells which act to prevent blood loss, which circulate in a resting state but are rapidly activated upon exposure to ligands at the site of vascular injuryCalcium (Ca2+) is critical for platelet activation, especially for activation of integrins which support platelet–platelet interactionsIf platelet activation occurs in circulation, platelets can be prematurely cleared from blood and unable to function in hemostasisDisorders of Ca2+ dysregulation such as Stormorken syndrome are associated with reduced platelet counts (thrombocytopenia) and bleedingWhat did we discover? We used a mouse model expressing a mutation causing higher Ca2+ levels in cells including platelets (Stim1Sax), and investigated whether thrombocytopenia is due to stimulation of a specific pathway for integrin activation, mediated by a protein called Rap1 GTPaseWe crossed Stim1Sax mice with mice lacking an important activator of Rap1, the Ca2+-regulated protein CalDAG-GEFI, and saw no major improvement in thrombocytopeniaWe also observed more Stim1Sax platelets in the lung but not the liver or spleen, in contrast to mice with activation of platelet integrins in circulationWhat is the impact? Our results rule out activation of the CalDAG-GEFI/Rap1/integrin pathway as a major cause of thrombocytopenia in Stim1Sax miceOur findings help to narrow down potential causes of thrombocytopenia in disorders such as Stormorken syndrome.