Glycogen Synthase Kinase 3 Beta Controls Presenilin-1-Mediated Endoplasmic Reticulum Ca(2+) Leak Directed to Mitochondria in Pancreatic Islets and β-Cells

BACKGROUND/AIMS: In pancreatic β-cells, the intracellular Ca(2+) homeostasis is an essential regulator of the cells’ major functions. The endoplasmic reticulum (ER) as interactive intracellular Ca(2+) store balances cellular Ca(2+). In this study basal ER Ca(2+) homeostasis was evaluated in order to reveal potential β-cell-specificity of ER Ca(2+) handling and its consequences for mitochondrial Ca(2+), 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 Ca(2+) 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 Ca(2+) leak was observed exclusively in pancreatic islets and β-cells. This continuous ER Ca(2+) 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 Ca(2+) leak in HeLa and EA.hy926 cells. The ER Ca(2+) loss in β-cells was compensated at steady state by Ca(2+) entry that is linked to the activity of TRPC3. CONCLUSION: Pancreatic β-cells establish a cell-specific ER Ca(2+) leak that is under the control of GSK3β and directed to mitochondria, thus, reflecting a cell-specific intracellular Ca(2+) handling for basal mitochondrial activity.