An EF hand mutation in Stim1 causes premature platelet activation and bleeding in mice

Abstract

Changes in cytoplasmic Ca 2+ levels regulate a variety of fundamental cellular functions in virtually all cells. In nonexcitable cells, a major pathway of Ca 2+ entry involves receptor-mediated depletion of intracellular Ca 2+ stores followed by the activation of store-operated calcium channels in the plasma membrane. We have established a mouse line expressing an activating EF hand motif mutant of stromal interaction molecule 1 (Stim1), an ER receptor recently identified as the Ca 2+ sensor responsible for activation of Ca 2+ releaseactivated (CRAC) channels in T cells, whose function in mammalian physiology is not well understood. Mice expressing mutant Stim1 had macrothrombocytopenia and an associated bleeding disorder. Basal intracellular Ca 2+ levels were increased in platelets, which resulted in a preactivation state, a selective unresponsiveness to immunoreceptor tyrosine activation motif-coupled agonists, and increased platelet consumption. In contrast, basal Ca 2+ levels, but not receptor-mediated responses, were affected in mutant T cells. These findings identify Stim1 as a central regulator of platelet function and suggest a cell type-specific activation or composition of the CRAC complex. IntroductionThe regulation of intracellular Ca 2+ ([Ca 2+ ] i ) is essentially involved in signaling processes in virtually all cells. In nonexcitable cells, including hematopoietic cells, Ca 2+ is released from the ER via inositol 1,4,5-triphosphate-mediated (IP 3 -mediated) receptor activation triggered by ligand-activated plasma membrane receptors. If the limited Ca 2+ reservoir of the ER becomes exhausted, extracellular Ca 2+ enters the cytoplasm by a mechanism known as store-operated Ca 2+ entry (SOCE) (1, 2). Although electrophysiologically well defined for more than a decade, the molecular identity of the pivotal proteins undoubtedly involved in SOCE has been discovered only recently. Stromal interaction molecule 1 (Stim1) is an ER resident protein necessary for the detection of ER Ca 2+ depletion (3-6). The 4-transmembrane domain protein Orai1, or CRACM, was reported recently to confer SOC activity (4,(7)(8)(9)(10)(11)(12). In T cells, Orai1 appears to be the predominant SOC (9), despite the fact that the C-terminal region of Stim1 has been shown to also interact with other SOC candidates such as transient receptor potential channels (TRPCs) 1,

RASA3 is a critical inhibitor of RAP1-dependent platelet activation

“…Last, a recent genome-wide association study ing platelets from CalDAG-GEFI/RAP1-mediated activation and clearance. Consistent with this, Grosse et al recently described a very similar platelet clearance phenotype for mice expressing a gain-of-function mutant of stromal interaction molecule 1 (Stim1 Sax ) (56). STIM1 is an important regulator of store-operated calcium entry (SOCE), the predominant mechanism in the regulation of calcium influx into platelets (57).…”

STIM1-Independent T Cell Development and Effector Function In Vivo

“…The identification of STIM1 as the long-sought molecular link between Ca 2ϩ store depletion and CRAC channel activation has enabled gain-of function (32) and loss-of-function (22,24) studies to examine the significance of SOCE for mammalian physiology and disease processes. Although SOCE has been implicated in negative selection of thymic T cell precursors (33), Stim1 Ϫ/Ϫ mice had a normal composition of thymocyte subsets.…”

Only severe thrombocytopenia results in bleeding and defective thrombus formation in mice

“…31 In brief, injury of mesenteric arterioles was induced by application of a filter paper saturated with 20% FeCl 3. Adhesion and aggregation of fluorescently labeled platelets in arterioles were monitored for 40 minutes or until complete occlusion occurred (blood flow stopped for 1 minute).…”