Endoplasmic reticulum stress alters ryanodine receptor function in the murine pancreatic β cell

Abstract

Alterations in endoplasmic reticulum (ER) calcium (Ca 2+ ) levels diminish insulin secretion and reduce β-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca 2+ loss in β cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP 3 R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca 2+ imaging techniques in INS-1 β cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP 3 R function. Our results revealed that the RyR is sensitive mainly to ER stress–induced dysfunction, whereas cytokine stress specifically alters IP 3 R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP 3 R with xestospongin C prevented ER Ca 2+ loss under ER and cytokine stress conditions, respectively. However, RyR blockade distinctly prevented β-cell death, propagation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca 2+ oscillations in tunicamycin-treated INS-1 β cells and mouse islets and Akita islets. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca 2+ transients that depend on both ER Ca 2+ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca 2+ dynamics in β cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca 2+ may be an early pathogenic event in diabetes.