ORAI1/STIM1 Interaction Intervenes in Stroke and in Neuroprotection Induced by Ischemic Preconditioning Through Store-Operated Calcium Entry

Background and Purpose - Disturbance of endoplasmic reticulum (ER) Ca2+ homeostasis causes neuronal cell injury in stroke. By contrast, ischemic preconditioning (IPC) - a brief sublethal ischemic episode affording tolerance to a subsequent ischemic insult - restores ER Ca2+ homeostasis. Under physiological conditions, ER calcium content is continuously refilled by the interaction between the ER-located Ca2+ sensor STIM (stromal interacting molecule) 1 and the plasma membrane channel ORAI1 (a structural component of the CRAC calcium channel) - 2 key mediators of the store-operated calcium entry (SOCE) mechanism. However, the role played by ORAI1 and STIM1 in stroke and in IPC-induced neuroprotection during stroke remains unknown. Therefore, we explored whether ORAI1 and STIM1 might be involved in stroke pathogenesis and in IPC-induced neuroprotection. Methods - Primary cortical neurons were subjected to oxygen and glucose deprivation+reoxygenation to reproduce in vitro brain ischemia. Focal brain ischemia and IPC were induced in rats by transient middle cerebral artery occlusion. Expression of ORAI1 and STIM1 transcripts and proteins and their immunosignals were detected by qRT-PCR, Western blot, and immunocytochemistry, respectively. SOCE and Ca2+ release-activated Ca2+ currents (ICRAC) were measured by Fura-2 AM video imaging and patch-clamp electrophysiology in whole-cell configuration, respectively. Results - STIM1 and ORAI1 protein expression and immunosignals decreased in the ipsilesional temporoparietal cortex of rats subjected to transient middle cerebral artery occlusion followed by reperfusion. Analogously, in primary hypoxic cortical neurons, STIM1 and ORAI1 transcript and protein levels decreased concurrently with SOCE and Ca2+ release-activated Ca2+currents. By contrast, IPC induced SOCE and Ca2+ release-activated Ca2+current upregulation, thereby preventing STIM1 and ORAI1 downregulation induced by oxygen and glucose deprivation+reoxygenation. Silencing of STIM1 or ORAI1 prevented IPC-induced tolerance and caused ER stress, as measured by GRP78 (78-kDa glucose regulated protein) and caspase-3 upregulation. Conclusions - ORAI1 and STIM1, which participate in SOCE, take part in stroke pathophysiology and play an important role in IPC-induced neuroprotection.