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Abstract

Glucose-stimulated insulin secretion from pancreatic β-cells depends on mitochondrial oxidative metabolism. Mitochondrial dysfunction is believed to be a significant factor in the development of type 2 diabetes (T2D). Mitochondria exist as dynamic networks and the control of mitochondrial biomass and fusion/fission dynamics is essential for cellular health and function. The anti-apoptotic protein Bcl-xL has recently been demonstrated to dampen β-cell mitochondrial metabolism and studies in other cell types suggest Bcl-xL regulates mitochondrial biomass and dynamics. We hypothesize that Bcl-xL is important for β-cell adaptation to metabolic stress by regulating mitochondrial dynamics and mass. To quantitatively study mitochondrial structural changes, we developed an image analysis pipeline for 2D/3D confocal imaging of mitochondria in FIJI. We applied the pipeline to primary islet cells and found that glucose stimulation is correlated with a more fragmented mitochondrial morphology. In vitro Bcl-xL overexpression causes β-cell mitochondria to lose their tubular network structure and aggregate. These changes to network morphology and kinetics are associated with decreased total mitochondrial volume and a marked impairment of β-cell O2 consumption. β-cell specific Bcl-xL knockout islet cells demonstrated increased basal activity and decreased average mitochondrion size, suggesting that they behave more similarly to β-cells undergoing glucose stimulation. Challenging β-cells with prolonged high glucose culture increased the size and overall connectivity of their mitochondrial network. In Bcl-xL knockout β-cells this increase in total mitochondrial mass and networking was significantly amplified, but was associated with reduced morphological and functional glucose-responsiveness of the individual mitochondrion. In conclusion, our in vitro data demonstrate that Bcl-xL affects mitochondrial networking, function, and adaptation to stress in pancreatic β-cells.