Endoplasmic reticulum calcium dynamics and insulin secretion in pancreatic β cells

Abstract

Under normal conditions, ER Ca2+ levels are estimated to be at least three orders of magnitude higher than intracellular Ca2+. This steep Ca2+ concentration gradient is maintained by the balance of Ca2+ uptake into the ER via the sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) pump and ER Ca2+ release through Ryanodine receptors (RyR) and Inositol 1,4,5-triphosphate (IP3) receptors (IP3R). Emerging data suggest that alterations in β cell ER Ca2+ levels lead to diminished insulin secretion and reduced β cell survival in both type 1 and type 2 diabetes. However, the mechanisms leading to β cell ER Ca2+ loss remain incompletely understood, and a specific role for either RyR or IP3R dysfunction in diabetes has been largely untested. To this end, we applied intracellular and ER-Ca2+ imaging techniques in INS-1 β cells and isolated mouse and human islets to define whether RyR or IP3R activity were altered under diabetogenic conditions. Results revealed preferential alterations in RyR function in response to ER stress, while pro-inflammatory cytokine stress primarily impacted IP3R activity. Consistent with this, pharmacological inhibition of RyR and IP3Rs prevented ER Ca2+ loss under ER and pro-inflammatory stress, respectively. However, RyR inhibition was unique in its ability to prevent β cell death, delayed initiation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca2+ oscillations in tunicamycin treated INS-1 β cells and islets from Akita mice. Monitoring at the single cell level revealed that ER stress acutely increased intracellular Ca2+ transients and this was dependent on both ER Ca2+ leak from the RyR and plasma membrane depolarization, suggesting ER Ca2+ dynamics regulate cellular excitability. Collectively, our findings suggest that ER-stress induced RyR dysfunction regulates β cell ER Ca2+ dynamics, propagation of the UPR, insulin secretion, and cell survival. These data indicate that RyR-mediated loss of ER Ca2+ and β cell hyperexcitability may be early pathogenic events in diabetes.