Upregulation of ryanodine receptor 1 is involved in ER stress-induced disruption of beta cell calcium homeostasis.

Abstract

Pancreatic beta cells maintain glucose homeostasis by secreting insulin following a rise in plasma glucose. Insulin secretion is pulsatile, which is brought about because of oscillations in beta-cell cytosolic Ca2+ concentration. The endoplasmic reticulum (ER) normally helps regulate the cytosolic Ca2+ level of beta cells. However, in response to ER stress, which is triggered by the accumulation of unfolded proteins in the ER, the ER Ca2+ is depleted, and that contributes to beta-cell deterioration and increases the risk of type-2 diabetes. To determine whether tunicamycin (TM)-induced ER stress reduced ER Ca2+ by activating ER ryanodine receptors (RyRs), qPCR was used to measure RyR expression in beta cells. RyR1 was found to be upregulated in response to TM, while RyR2 was not. To further probe the respective roles of these receptor channels in TM-induced beta-cell dysfunction, RyR1 and RyR2 were individually knocked down in INS-1(832/13) cells using siRNAs. While control cells treated with 10 mg/ml of TM exhibited altered cytosolic Ca2+ in sub-threshold (5 mM) glucose compared to vehicle controls, knocking down RyR1 but not RyR2 abolished the oscillations. Similarly, the abnormal Ca2+ oscillations seen in TM-treated mouse islets were inhibited by including the RyR1 inhibitor dantrolene without affecting UPR activation. Islets from female diabetic db/db mice but not controls also exhibited sub-threshold Ca2+ oscillations that were abolished by dantrolene. Taken together, these results suggest that RyR1 plays a critical role in beta-cell Ca2+ alterations seen in response to ER stress.