Glycogen Synthase Kinase 3 Beta Controls Presenilin-1-Mediated Endoplasmic Reticulum Ca2+ Leak Directed to Mitochondria in Pancreatic Islets and beta-Cells

Abstract

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.

Calcium Signaling in ß-cell Physiology and Pathology: A Revisit

“…In particular, the responsiveness and proper D-glucose sensing of β-cells to elevated blood D-glucose is determined by the cells' pace of producing ATP when exposed to elevated glucose [123]. Data of our group show strongly enhanced basal respiratory activity and elevated mitochondrial ATP levels in mitochondria of clonal β-cell lines in comparison to non-β-cell lines pointing to such pre-stimulation of mitochondria in β-cells [28]. These data are in line with the new perspective of GSIS that involves a continuous priming of β-cells based on a weak stimulation of β-cell mitochondria to ensure proper responsiveness to elevated blood D-glucose sensing [124].…”

“…Furthermore, the group postulated that the effective activation of these SOCs may be reduced in T2DM, thus confirming the importance of Orai1 to the pancreatic cell's optimal functionality [44]. Notably, the importance of basal mitochondrial Ca 2+ entry, possibly via a TRPC-mediated mechanism has been postulated to be fundamental for the responsiveness to increased glucose [28].…”

“…The presenilin-1-mediated ER Ca 2+ leak is directly sequestered by mitochondria, leading to increased basal matrix Ca 2+ levels that yield enhanced resting activity of mitochondria in the pancreatic β-cells due to pre-stimulation the Ca 2+ -dependent dehydrogenases of the citric acid cycle. Upon elevation of glucose, glucose is metabolized and the pre-activated citric cycle in the mitochondria efficiently converts glucose metabolism to activation of the respiratory chain (OXPHOS) and, subsequently, fast ATP production, thus, ensuring a fast, initial insulin secretion within 10 min of exposure to elevated glucose [28,107].…”

“…As the vigilant reader of the manuscript might have already realized, a new and for some maybe pugnacious finding concerning β-cell responsiveness and regulation of insulin secretion is presented in several sections of the manuscript-namely, the crucial role of a presenilin-1-mediated ER Ca 2+ leak for proper β-cell function [28,107], data which are graphically summarized in Figure 2. As the vigilant reader of the manuscript might have already realized, a new and for some maybe pugnacious finding concerning β-cell responsiveness and regulation of insulin secretion is presented in several sections of the manuscript -namely, the crucial role of a presenilin-1-mediated ER Ca 2+ leak for proper β-cell function [28,107], data which are graphically summarized in Figure 2. Graphical summary of the consequences of the β-cell-specific presenilin-1-mediated ER Ca 2+ leak.…”

“…Insofar as Ca 2+ release from the ER goes, this organelle is known to exhibit considerable Ca 2+ leakage into the cytosol, which must ultimately be corrected for by SERCA pump activity [27]. A likely mechanism for the ER Ca 2+ leak in β-cells was recently outlined to involve presenilin-1 (PS 1) [28].…”