Activation of SK2 channels preserves ER Ca²⁺ homeostasis and protects against ER stress-induced cell death.

Alteration of endoplasmic reticulum (ER) Ca(2+) homeostasis leads to excessive cytosolic Ca(2+) accumulation and delayed neuronal cell death in acute and chronic neurodegenerative disorders. While our recent studies established a protective role for SK channels against excessive intracellular Ca(2+) accumulation, their functional role in the ER has not been elucidated yet. We show here that SK2 channels are present in ER membranes of neuronal HT-22 cells, and that positive pharmacological modulation of SK2 channels with CyPPA protects against cell death induced by the ER stressors brefeldin A and tunicamycin. Calcium imaging of HT-22 neurons revealed that elevated cytosolic Ca(2+) levels and decreased ER Ca(2+) load during sustained ER stress could be largely prevented by SK2 channel activation. Interestingly, SK2 channel activation reduced the amount of the unfolded protein response transcription factor ATF4, but further enhanced the induction of CHOP. Using siRNA approaches we confirmed a detrimental role for ATF4 in ER stress, whereas CHOP regulation was dispensable for both, brefeldin A toxicity and CyPPA-mediated protection. Cell death induced by blocking Ca(2+) influx into the ER with the SERCA inhibitor thapsigargin was not prevented by CyPPA. Blocking the K(+) efflux via K(+)/H(+) exchangers with quinine inhibited CyPPA-mediated neuroprotection, suggesting an essential role of proton uptake and K(+) release in the SK channel-mediated neuroprotection. Our data demonstrate that ER SK2 channel activation preserves ER Ca(2+) uptake and retention which determines cell survival in conditions where sustained ER stress contributes to progressive neuronal death.