The pore-forming subunit MCU of the mitochondrial Ca2+ uniporter is required for normal glucose-stimulated insulin secretion in vitro and in vivo in mice

Abstract

Abstract Aims/hypothesis Mitochondrial oxidative metabolism is central to glucose-stimulated insulin secretion (GSIS). Whether Ca 2+ uptake into pancreatic beta cell mitochondria potentiates or antagonises this process is still a matter of debate. Although the mitochondrial Ca 2+ importer (MCU) complex is thought to represent the main route for Ca 2+ transport across the inner mitochondrial membrane, its role in beta cells has not previously been examined in vivo. Methods Here, we inactivated the pore-forming subunit of the MCU, encoded by Mcu , selectively in mouse beta cells using Ins1 Cre -mediated recombination. Whole or dissociated pancreatic islets were isolated and used for live beta cell fluorescence imaging of cytosolic or mitochondrial Ca 2+ concentration and ATP production in response to increasing glucose concentrations. Electrophysiological recordings were also performed on whole islets. Serum and blood samples were collected to examine oral and i.p. glucose tolerance. Results Glucose-stimulated mitochondrial Ca 2+ accumulation ( p < 0.05), ATP production ( p < 0.05) and insulin secretion ( p < 0.01) were strongly inhibited in beta cell-specific Mcu -null (β Mcu -KO) animals, in vitro, as compared with wild-type (WT) mice. Interestingly, cytosolic Ca 2+ concentrations increased ( p < 0.001), whereas mitochondrial membrane depolarisation improved in β Mcu -KO animals. β Mcu -KO mice displayed impaired in vivo insulin secretion at 5 min ( p < 0.001) but not 15 min post-i.p. injection of glucose, whilst the opposite phenomenon was observed following an oral gavage at 5 min. Unexpectedly, glucose tolerance was improved ( p < 0.05) in young β Mcu -KO (<12 weeks), but not in older animals vs WT mice. Conclusions/interpretation MCU is crucial for mitochondrial Ca 2+ uptake in pancreatic beta cells and is required for normal GSIS. The apparent compensatory mechanisms that maintain glucose tolerance in β Mcu -KO mice remain to be established.