Pancreatic beta cell selective deletion of mitofusins 1 and 2 ( Mfn1 and Mfn2 ) disrupts mitochondrial architecture and abrogates glucose-stimulated insulin secretion in vivo

Background and aimsMitochondria are highly dynamic organelles, fundamental to cellular energy homeostasis. Mitochondrial metabolism of glucose is essential for the initiation of insulin release from pancreatic beta cells. Whether mitochondrial ultra-structure, and the proteins controlling fission and fusion, are important for glucose recognition are unclear. Mitochondrial fusion is supported by proteins including mitofusin 1 (MFN1), mitofusin 2 (MFN2) and optic atrophy (OPA1), and fission by dynamin-related protein 1 (DRP1). Here, we generated mice with beta cell-selective, adult-restricted deletion of Mfn1 and Mfn2 ({beta}Mfn1/2-KO), and explored the impact on insulin secretion and glucose homeostasis in vivo and in vitro. Materials and methodsC57BL/6J mice bearing Mfn1 and Mfn2 alleles with loxP sites, were crossed to animals carrying an inducible Cre recombinase at the Pdx1 locus (PdxCreERT). Isolated islets were used for live beta cell fluorescence imaging of cytosolic (Cal-520) or mitochondrial (Pericam) free Ca2+ concentration and membrane potential (TMRE). Mitochondrial network characteristics were quantified using super resolution fluorescence and transmission electron microscopy. Beta cell-beta cell connectivity was assessed using the Pearson (R) analysis and Monte Carlo simulation in intact mouse islets. Intravital imaging was performed in mice injected with an adeno-associated virus to express the cytosolic Ca2+ sensor GCaMP6s selectively in beta cells and TMRM to visualise mitochondria using multiphoton microscopy. Results{beta}Mfn1/2-KO mice displayed higher fasting glycaemia than control littermates at 14 weeks (8.6 vs 6.4 mmol/L, p>0.05) and a >five-fold decrease in plasma insulin post-intraperitoneal glucose injection (5-15 min, p<0.0001). Mitochondrial length, and glucose-induced Ca2+ accumulation, mitochondrial hyperpolarisation and beta cell connectivity were all significantly reduced in {beta}Mfn1/2-KO mouse islets. Examined by intravital imaging of the exteriorised pancreas, antiparallel changes in cytosolic Ca2+ and mitochondrial membrane potential, observed in control animals in vivo, were suppressed after Mfn1/2 deletion. ConclusionMitochondrial fusion and fission cycles are essential in the beta cell to maintain normal mitochondrial bioenergetics and glucose sensing both in vitro and in the living mouse. Such cycles may be disrupted in some forms of diabetes to impair mitochondrial function and, consequently, insulin secretion.