Abstract
Alterations in endoplasmic reticulum (ER) calcium (Ca2+) levels diminish insulin secretion and reduce β-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca2+ loss in β cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP3R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca2+ imaging techniques in INS-1 β cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP3R function. Our results revealed that the RyR is sensitive mainly to ER stress-induced dysfunction, whereas cytokine stress specifically alters IP3R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP3R with xestospongin C prevented ER Ca2+ 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 Ca2+ 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 Ca2+ transients that depend on both ER Ca2+ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca2+ dynamics in β cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca2+ may be an early pathogenic event in diabetes.
Highlights
Alterations in endoplasmic reticulum (ER) calcium (Ca2؉) levels diminish insulin secretion and reduce -cell survival in both major forms of diabetes
The pathophysiology of type 1 and type 2 diabetes involves both  cell ER stress and cytokine-induced -cell dysfunction [8, 15, 16]. To define how these stress paradigms influenced ER Ca2ϩ storage, INS-1  cells were treated with 300 nM tunicamycin (TM) or 5 ng/ml interleukin 1 combined with 25 mM high glucose (ILHG) in time-course experiments
The goal of our study was to test whether ryanodine receptor (RyR) or IP3R dysfunction contributed to altered  cell ER Ca2؉ storage under diabetic conditions
Summary
Alterations in endoplasmic reticulum (ER) calcium (Ca2؉) levels diminish insulin secretion and reduce -cell survival in both major forms of diabetes. A specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP3R) dysfunction in the pathophysiology of diabetes remains largely untested To this end, here we applied intracellular and ER Ca2؉ imaging techniques in INS-1  cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP3R function. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca2؉ transients that depend on both ER Ca2؉ leakage from the RyR and plasma membrane depolarization These findings indicate that RyR dysfunction shapes ER Ca2؉ dynamics in  cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca2؉ may be an early pathogenic event in diabetes. Genetic mouse models expressing mutated forms of the RyR2, leading to increased ER Ca2ϩ leak, exhibited reduced insulin secretion, whereas pharmacological antagonists of the RyR and IP3R were found to reduce -cell death in response to thapsigargin treatment [4, 12,13,14]
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