Hypoxia-induced Acidosis Uncouples the STIM-Orai Calcium Signaling Complex*

Background: STIM proteins are calcium sensors controlling Orai calcium entry channels. Results: Hypoxia causes decreased intracellular pH and prevents Orai channel activation in response to calcium store depletion. Conclusion: Hypoxia and intracellular acidification prevent coupling of STIM to Orai channels. Significance: pH-mediated uncoupling of STIM-Orai may protect cells from hypoxia-mediated calcium overload. The endoplasmic reticulum Ca2+-sensing STIM proteins mediate Ca2+ entry signals by coupling to activate plasma membrane Orai channels. We reveal that STIM-Orai coupling is rapidly blocked by hypoxia and the ensuing decrease in cytosolic pH. In smooth muscle cells or HEK293 cells coexpressing STIM1 and Orai1, acute hypoxic conditions rapidly blocked store-operated Ca2+ entry and the Orai1-mediated Ca2+ release-activated Ca2+ current (ICRAC). Hypoxia-induced blockade of Ca2+ entry and ICRAC was reversed by NH4+-induced cytosolic alkalinization. Hypoxia and acidification both blocked ICRAC induced by the short STIM1 Orai-activating region. Although hypoxia induced STIM1 translocation into junctions, it did not dissociate the STIM1-Orai1 complex. However, both hypoxia and cytosolic acidosis rapidly decreased Förster resonance energy transfer (FRET) between STIM1-YFP and Orai1-CFP. Thus, although hypoxia promotes STIM1 junctional accumulation, the ensuing acidification functionally uncouples the STIM1-Orai1 complex providing an important mechanism protecting cells from Ca2+ overload under hypoxic stress conditions.

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