Platelet ERO1α Is Crucial for Ca2+ Mobilization and Platelet Activation in Atherothrombosis

Endoplasmic reticulum oxidoreductase 1α (ERO1α) is a key oxidase of protein disulfide isomerase (PDI) during protein folding in the ER. Platelet ERO1α was reported to interact with PDI and αIIbβ3 integrin and regulate platelet function. However, the regulatory mechanism and pathophysiological role remain unknown. In the present study, we demonstrate that platelet ERO1α orchestrates intracellular Ca²⁺ signaling during platelet activation and contributes to thrombogenesis. Intravital microscopy using megakaryocyte-specific Ero1α conditional knockout (CKO) and KO mice reveals that platelet ERO1α is crucial for platelet thrombus formation in mouse models of laser-induced cremaster arteriolar thrombosis and FeCl₃-induced carotid arterial thrombosis. Deletion of Ero1α does not affect bleeding times and blood loss at the site of tail amputation, implying that ERO1α is not required for hemostasis. Studies using Ero1α-null platelets, novel function-blocking antibodies, and small-molecule inhibitors reveal that intracellular, but not extracellular, ERO1α promotes P-selectin exposure, αIIbβ3 integrin activation, and platelet aggregation induced by multiple agonists, such as thrombin and collagen-related peptide. Importantly, the regulatory function of ERO1α is independent of PDI activity. Mass spectrometry and immunoprecipitation assays identify stromal interaction molecule 1 (STIM1) and sarco/endoplasmic reticulum Ca²⁺-ATPase 2 (SERCA2) as ERO1α binding partners. We have found that ERO1α-STIM1 binding is enhanced during platelet activation, whereas ERO1α-SERCA2 binding is reduced. Intriguingly, platelet ERO1α interacts with STIM1 through Cys49 and Cys56 residues and promotes its function by oxidizing the Cys49-Cys56 disulfide bond, contributing to Ca²⁺ store content and enhancing Ca²⁺ mobilization during platelet activation. Using a high throughput screen, we identify a small-molecule ERO1α inhibitor, B20, which inhibits ERO1α activity with an IC₅₀ value of 10.0 μM but does not affect the activities of H₂O₂ and PDI. Treatment of platelets with B20 recapitulates the functional defects as seen in Ero1α-null platelets. Intravenous injection of B20 and eptifibatide, an αIIbβ3 integrin antagonist, into WT mice significantly inhibits platelet thrombus formation following laser- or FeCl₃-induced vascular injury. While treatment of mice with eptifibatide markedly prolongs tail bleeding times and causes blood loss, B20 does not impair hemostatic function. Furthermore, treatment with B20 reduces the infarct volume in ischemic stroke. These results indicate that platelet ERO1α is a novel regulator of Ca²⁺ signaling and promotes platelet activation and aggregation in a PDI-independent manner, contributing to the pathogenesis of arterial thrombosis and thromboinflammation. Targeting ERO1α may be an attractive therapeutic strategy for treating patients with thrombotic disease.