The pore-forming subunit MCU of the mitochondrial Ca(2+) uniporter is required for normal glucose-stimulated insulin secretion in vitro and in vivo in mice

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 (βMcu-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 βMcu-KO animals. βMcu-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 βMcu-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 βMcu-KO mice remain to be established. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00125-020-05148-x) contains peer-reviewed but unedited supplementary material, which is available to authorised users.