Mitochondrial activity serves as a key factor for the initiation of insulin release in β-cells. However, it is still unclear how -cells precisely sense exclusively (blood) glucose. In fact, in non -cells ATP production is under the control of mitochondrial Ca2+ that stimulates matrix dehydrogenases, while in β-cells mitochondrial ATP production is solely controlled by (blood) glucose. Our aim was to elucidate the molecular mechanism that makes mitochondrial ATP production in β-cells uniquely controlled by (blood) glucose. We hypothesize that in resting β-cells the Ca2+-dependent mitochondrial dehydrogenases are already fully stimulated by Ca2+, thus, making the redox-equivalents (i.e. NADH + H+, FADH2) derived from glucose metabolism the exclusive trigger for respiratory chain and mitochondrial ATP production. This would ensure a fast and precise insulin response upon glucose stimulation in β-cells that is independent from any mitochondrial Ca2+ signal and all-triggered by (blood) glucose. In the two insulin-producing cell lines, INS-1 and MIN-6 we found an increased ER Ca2+ leak which was not detectable in the non-insulin producing cell lines HeLa and the human umbilical vein endothelial cell line, EA.hy926. ER Ca2+ leak in β-cells was not a result from differences in SERCA activity and was compensated by continuous ER Ca2+ refilling by TRPC3. ER Ca2+ leak in β-cells was established by presenilin-1 that is phosphorylated at serine 353/357 by glycogen synthase kinase 3 (GSK3). Subsequently to the ER Ca2+ leak, resting mitochondrial Ca2+ and basal mitochondrial dehydrogenase activity, measured as enhanced basal respiration and organelle ATP content was enhanced in β-cells. By preventing ER Ca2+ leak, responsiveness of β-cells to increased extracellular glucose, measured as lag time to cytosolic Ca2+ spiking, and, ultimately, insulin release were strongly delayed. The increased ER Ca2+ leak in β-beta cells appears to be of great physiological importance for the proper responsiveness of β-cells to elevated glucose, thus, possibly representing the molecular link between diabetes and Alzheimer’s disease (AD). As presenilin-1, which was found herein to be essential for β-cells sensitivity, is also of utmost importance in the development of Alzheimer’s disease further work will focus on verifying this potential link between diabetes and Alzheimer’s disease.
Presenilin-1-mediated ER Ca2+ leak is an essential trigger for β-cell responsiveness to glucose
Klec, C. (Autor/-in). 2019
Studienabschlussarbeit: Dissertation