The mitochondrial Ca²⁺uniporter MCU is required for normal glucose-stimulated insulin secretionin vitroandin vivo

Abstract

22Mitochondrial oxidative metabolism is central to glucose-stimulated insulin secretion (GSIS). 23 Whether Ca 2+ uptake into pancreatic β-cell mitochondria potentiates or antagonises this process 24 is still a matter of debate. Although the mitochondrial importer (MCU) complex is thought to 25 represent the main route for Ca 2+ transport across the inner mitochondrial membrane, its role 26 in β-cells has not previously been examined in vivo. Here, we inactivated the pore-forming 27 subunit MCUa (MCU) selectively in the β-cell in mice using Ins1Cre-mediated recombination. 28 Glucose-stimulated mitochondrial Ca 2+ accumulation, ATP production and insulin secretion 29 were strongly (p<0.05 and p<0.01) inhibited in MCU null animals (βMCU-KO) in vitro. 30 Interestingly, cytosolic Ca 2+ concentrations increased (p<0.001) whereas mitochondrial 31 membrane depolarisation improved in βMCU-KO animals. Male βMCU-KO mice displayed 32 impaired in vivo insulin secretion at 5 (p<0.001) but not 15 min. post intraperitoneal (IP) 33 injection of glucose while the opposite phenomenon was observed following an oral gavage at 34 5 min. Unexpectedly, glucose tolerance was improved (p<0.05) in young βMCU-KO (<12 35 weeks), but not older animals. We conclude that MCU is crucial for mitochondrial Ca 2+ uptake 36 in pancreatic β-cells and is required for normal GSIS. The apparent compensatory mechanisms 37 which maintain glucose tolerance in βMCU-KO mice remain to be established. 38 39 40Defective insulin secretion underlies diabetes mellitus, a disease affecting almost 1 in 8 of the 41 adult population worldwide (https://www.idf.org/). The most prevalent form of this condition 42 is Type 2 diabetes (T2D) where pancreatic β-cell failure usually, though not always, occurs in 43 the face of insulin resistance in other tissues (1). Current therapeutic strategies have limited 44 efficacy and there remains a desperate need to develop more effective treatments to tackle this 45 growing epidemic. 46Pancreatic β-cells ensure blood glucose homeostasis by responding to a rise in circulating 47 nutrient levels with insulin secretion. Glucose-induced increases in mitochondrial oxidative 48 metabolism are central to the stimulation of insulin secretion, and drive an increase in cytosolic 49 ATP:ADP ratio, closure of ATP-sensitive K + (KATP) channels, Ca 2+ influx and exocytosis (2). 50Ca 2+ ions are also taken up by mitochondria (3; 4) and this has been suggested to activate 51 tricarboxylate (TCA) cycle and other intra-mitochondrial enzymes (5) in order to enhance the 52 production of reducing equivalents for the electron transport chain and ATP generation (2). 53Although a number of approaches have been used previously to explore the role of intra-54 mitochondrial Ca 2+ in controlling insulin secretion, the role of these ions in modifying ATP 55 synthesis, and hence exocytosis, in these cells is still debated (6; 7). 56Importantly, there is accumulating evidence to suggest that mitochondrial dysfunction in the 57 pancreatic β-cell ...