Abstract
Mitochondrial oxidative metabolism is central to glucose-stimulated insulin secretion (GSIS). Whether Ca2+ uptake into pancreatic β-cell mitochondria potentiates or antagonises this process is still a matter of debate. Although the mitochondrial importer (MCU) complex is thought to represent the main route for Ca2+ transport across the inner mitochondrial membrane, its role in β-cells has not previously been examined in vivo. Here, we inactivated the pore-forming subunit MCUa (MCU) selectively in the β-cell in mice using Ins1Cre-mediated recombination. Glucose-stimulated mitochondrial Ca2+ accumulation, ATP production and insulin secretion were strongly (p<0.05 and p<0.01) inhibited in MCU null animals (βMCU-KO) in vitro. Interestingly, cytosolic Ca2+ concentrations increased (p<0.001) whereas mitochondrial membrane depolarisation improved in βMCU-KO animals. Male βMCU-KO mice displayed impaired in vivo insulin secretion at 5 (p<0.001) but not 15 min. post intraperitoneal (IP) injection of glucose while 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 older animals. We conclude that MCU is crucial for mitochondrial Ca2+ uptake in pancreatic β-cells and is required for normal GSIS. The apparent compensatory mechanisms which maintain glucose tolerance in βMCU-KO mice remain to be established.
Abbreviations
- [Ca2+]cyt
- Cytoplasmic Ca2+ concentration
- [Ca2+]mt
- Mitochondrial Ca2+ concentration
- ADP
- Adenosine diphosphate
- AMPK
- Adenosine monphosphate-activated protein kinase
- ANT
- adenine nucleotide transferase
- ATP
- Adenosine triphosphate
- AUC
- Area under curve
- FCCP
- Carbonyl cyanide-4-phenylhydrazone
- GIP
- Gastric inhibitory polypeptide
- GLP-1
- Glucagon-like peptide-1
- GSIS
- Glucose-stimulated insulin secretion
- HTRF
- homogeneous time-resolved fluorescence-based assay
- IPGTT
- Intraperitoneal glucose tolerance test
- KATP
- ATP-sensitive K+ (KATP) channels
- KCl
- Potassium chloride
- KO
- Knockout
- MCU
- Mitochondrial Ca2+ uniporter
- NCLX
- Na+-Ca2+ exchanger
- OGTT
- Oral gavage tolerance test
- PC
- Pyruvate carboxylase
- PDH
- Pyruvate dehydrogenase
- RAM
- Rapid mode of mitochondrial Ca2+ uptake
- SERCA
- Sarco/endoplasmic reticulum Ca2+-ATPase
- T2D
- Type 2 diabetes
- TCA
- tricarboxylic acid
- TMRE
- Tetramethylrhodamine ethyl ester
- VDCCS
- Voltage-dependent Ca2+ channels
- WT
- Wild type
- βMCU-KO
- Mitochondrial Ca2+ uniporter null animal in the beta cell
- α-KG
- α-ketoglutarate
- Δψm
- Mitochondrial membrane potential