230-LB: Differential Roles of Beta-Cell IP3R and RyR ER Ca2+ Channels in Tunicamycin-Induced Disruption of Beta-Cell Ca2+ Homeostasis

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 the concentrations of beta-cell cytosolic Ca2+. The endoplasmic reticulum (ER) helps to regulate the cytosolic Ca2+ level, thereby playing a role in Ca2+-induced insulin release. ER stress, triggered by the accumulation of unfolded proteins in the ER, can lead to the ER Ca2+ depletion, which in turn can contribute to beta-cell deterioration and an increased risk of type-2 diabetes. We sought to determine the effects of tunicamycin (TM)-induced ER stress on ER Ca2+ channels, inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs) and ryanodine receptors (RyRs), and subsequent alterations in beta-cell Ca2+ homeostasis that result from these alterations. To determine the roles of these receptors in TM-induced beta-cell dysfunction, we treated mouse pancreatic islets with the RyR1 inhibitor dantrolene (Dan) or the IP3R inhibitor xestospongin C (XeC) along with TM. Beta cells treated with TM exhibited altered cytosolic and mitochondrial Ca2+ when in sub-threshold glucose compared to vehicle controls. As TM treatment also reduced ER Ca2+, this raised the possibility that the mitochondrial and cytosolic Ca2+ oscillations seen in stressed cells resulted from increased ER Ca2+ efflux mediated by RyRs and/or IP3Rs. We found that TM-induced ER Ca2+ depletion, as well as cytosolic and mitochondrial Ca2+ oscillations were inhibited by co-treatment with Dan, whereas the inclusion of XeC had little or no effect. Taken together, these results suggest that RyRs, and more specifically RyR1 plays a critical role in mediating the disturbed cellular Ca2+ homeostasis seen in response to the induction of ER stress. Disclosure I. Zhang: None. V. S. Parekh: None. J. J. Leon: None. L. S. Satin: None. Funding National Institutes of Health (R01DK46409); University of Michigan; JDRF (2-SRA-2018-539-A-B)