Deletion of Orai2 augments endogenous CRAC currents and degranulation in mast cells leading to enhanced anaphylaxis

Abstract All three members of the Orai family of cation channels–Orai1, Orai2 and Orai3–are integral membrane proteins that can form store-operated Ca 2+ channels resembling endogenous calcium release-activated channels (CRAC) in many aspects. Loss of function studies in human and murine models revealed many functions of Orai1 proteins not only for Ca 2+ homeostasis, but also for cellular and systemic functions in many cell types. By contrast, the knowledge regarding the contribution of Orai2 and Orai3 proteins in these processes is sparse. In this study, we report the generation of mouse models with targeted inactivation of the Orai2 gene to study Orai2 function in peritoneal mast cells (PMC), a classical cell model for CRAC channels and Ca 2+ -dependent exocytosis of inflammatory mediators. We show that the Ca 2+ rise triggered by agonists acting on high-affinity Fc receptors for IgE or on MAS-related G protein-coupled receptors is significantly increased in Orai2-deficient mast cells. Ca 2+ entry triggered by depletion of intracellular stores (SOCE) is also increased in Orai2 −/− PMCs at high (2 mM) extracellular Ca 2+ concentration, whereas SOCE is largely reduced upon re-addtion of lower (0.1 mM) Ca 2+ concentration. Likewise, the density of CRAC currents, Ca 2+ -dependent mast cell degranulation, and mast cell-mediated anaphylaxis are intensified in Orai2-deficient mice. These results show that the presence of Orai2 proteins limits receptor-evoked Ca 2+ transients, store-operated Ca 2+ entry (SOCE) as well as degranulation of murine peritoneal mast cells but also raise the idea that Orai2 proteins contribute to Ca 2+ entry in connective tissue type mast cells in discrete operation modes depending on the availability of calcium ions in the extracellular space.