Glycogen synthase kinase 3 beta controls presenilin-1-mediated endoplasmic reticulum Ca2+ leak directed to mitochondria in pancreatic islets and β-cells

Abstract

Background/Aims: In pancreatic β-cells, the intracellular Ca2+ homeostasis is an essential regulator of the cells’ major functions. The endoplasmic reticulum (ER) as interactive intracellular Ca2+ store balances cellular Ca2+. In this study basal ER Ca2+ homeostasis was evaluated in order to reveal potential β-cell-specificity of ER Ca2+ handling and its consequences for mitochondrial Ca2+, ATP and respiration. Methods: The two pancreatic cell lines INS-1 and MIN-6, freshly isolated pancreatic islets, and the two non-pancreatic cell lines HeLA and EA.hy926 were used. Cytosolic, ER and mitochondrial Ca2+ and ATP measurements were performed using single cell fluorescence microscopy and respective (genetically-encoded) sensors/dyes. Mitochondrial respiration was monitored by respirometry. GSK3β activity was measured with ELISA. Results: An atypical ER Ca2+ leak was observed exclusively in pancreatic islets and β-cells. This continuous ER Ca2+ efflux is directed to mitochondria and increases basal respiration and organellar ATP levels, is established by GSK3β-mediated phosphorylation of presenilin-1, and is prevented by either knockdown of presenilin-1 or an inhibition/knockdown of GSK3β. Expression of a presenlin-1 mutant that mimics GSK3β-mediated phosphorylation established a β-cell-like ER Ca2+ leak in HeLa and EA.hy926 cells. The ER Ca2+ loss in β-cells was compensated at steady state by Ca2+ entry that is linked to the activity of TRPC3. Conclusion: Pancreatic β-cells establish a cell-specific ER Ca2+ leak that is under the control of GSK3β and directed to mitochondria, thus, reflecting a cell-specific intracellular Ca2+ handling for basal mitochondrial activity.