Background and Aims: Mitochondrial metabolism of glucose is vital for the initiation of insulin secretion in pancreatic beta cells. Whether mitochondrial ultra-structure, and the proteins controlling their fission and fusion, are essential for glucose recognition is less clear. In the present study, beta cell-selective adult mouse restricted deletion of Mfn1 and Mfn2 was used to investigate the role of mitochondrial fusion in insulin secretion and glucose homeostasis in vivo and in vitro.
Methods: C57BL6 mice bearing Mfn1 and Mfn2 alleles with FloxP sites, were crossed to animals carrying an inducible Cre recombinase at the Pdx1 locus (PdxCreERT). After induction of recombination with tamoxifen, live beta cell fluorescence imaging of mitochondrial [Ca2+] and membrane potential were performed on isolated islets. Mitochondrial ultra-structure was measured using super resolution fluorescence and transmission electron microscopy. Intraperitoneal glucose tolerance, and insulin secretion in vivo and in vitro, were measured using standard techniques.
Results: Mitochondrial length was strongly (to 77±0.9% of controls, p<0.0001) and selectively diminished in beta cells from Mfn1/2 KO mice. KO animals displayed higher fasting glycaemia than control animals at 11-12 weeks (8.6 vs. 6.4 mmol/L, p>0.05). An increase in circulating glucose levels was also observed (p<0.05 at 30 min and p<0.01 at 60 min) and was linked to a considerable (>five-fold) decrease in plasma insulin (5-15 min, p<0.0001) post-intraperitoneal glucose injection. Mitochondrial Ca2+ accumulation and membrane potential were significantly reduced (p<0.01) in response to high glucose in KO animals.
Conclusions: Mitochondrial fusion and fission cycles are indispensable in the beta cell in order to preserve normal mitochondrial bioenergetics and glucose sensing, and may be disrupted in some forms of diabetes.
E. Georgiadou: None. P.L. Chabosseau: None. A. Tomas: None. I. Leclerc: Consultant; Spouse/Partner; Sun Pharmaceutical Industries Ltd. Research Support; Spouse/Partner; Servier. G.A. Rutter: Consultant; Self; Sun Pharmaceutical Industries Ltd. Research Support; Self; Servier, Sun Pharmaceutical Industries Ltd.
UK Wellcome Trust (WT098424AIA, 212625/Z/18/Z); UK Medical Researh Council (MR/R022259/1, MR/J0003042/1, MR/L020149/1, MR/L02036X/1, MR/N00275X/1); Diabetes UK (BDA/11/0004210, BDA/15/0005275, BDA16/0005485)