Abstract
Aims/hypothesis Mitochondrial glucose metabolism is essential for stimulated insulin release from pancreatic beta cells. Whether mitochondrial networks may be important for glucose or incretin sensing has yet to be determined.
Methods Here, we generated mice with beta cell-selective, adult-restricted deletion of the mitofusin genes Mfn1 and Mfn2 (βMfn1/2 dKO). Whole or dissociated pancreatic islets were used for live beta cell fluorescence imaging of cytosolic or mitochondrial Ca2+ concentration and ATP production or GSIS in response to increasing glucose concentrations or GLP-1 receptor agonists. Serum and blood samples were collected to examine oral and i.p. glucose tolerance.
Results βMfn1/2 dKO mice displayed elevated fed and fasted glycaemia (p<0.01, p<0.001) and a >five-fold decrease (p<0.0001) in plasma insulin. Mitochondrial length, glucose-induced polarisation, ATP synthesis and cytosolic Ca2+ increases were all reduced (p<0.05,p<0.01,p<0.0001) in dKO islets, and beta cell Ca2+ dynamics were suppressed in vivo (p<0.001). In contrast, oral glucose tolerance was near normal in βMfn1/2 dKO mice (p<0.05, p<0.01) and GLP-1 or GIP receptor agonists largely corrected defective GSIS from isolated islets through an EPAC-dependent signalling activation.
Conclusions/interpretation Mitochondrial fusion and fission cycles are thus essential in the beta cell to maintain normal glucose, but not incretin, sensing. Defects in these cycles in some forms of diabetes might therefore provide opportunities for novel incretin-based or other therapies.
Research in context What is already known about this subject?
Mitochondrial ultrastructural variations and number are altered in beta cells of human T2D patients [1].
Mice lacking Opa1, which controls mitochondrial fusion and inner membrane cristae structure, in beta cells, develop hyperglycaemia and defects in GSIS [2].
What is the key question?
Is an interconnected mitochondrial network essential in primary mouse beta cells for normal insulin secretion and glucose homeostasis?
What are the new findings?
We generated mice with beta cell-selective, adult-restricted deletion of the mitofusin genes Mfn1 and Mfn2 and show that insulin secretion and glucose homeostasis are strongly reduced in vivo.
Cytosolic and mitochondrial Ca2+ increases, Δψm, ATP production and beta cell connectivity are impaired in βMfn1/2 dKO animals.
Incretins bypass the above defects through an exchange protein directly activated by cAMP (EPAC)-dependent signalling mechanism.
How might this impact on clinical practice in the foreseeable future?
The ability of incretins to bypass defects in mitochondrial function might be exploited by the design of new agonists which target this pathway.
Competing Interest Statement
GAR is a consultant for Sun Pharmaceuticals and has received grant support from Les Laboratories Servier
Footnotes
Now included are new data on the signalling pathways through which incretins prompt insulin secretion after the disruption of Mfn1 and Mfn2, notably roles for Epac2. New controls are also included showing that the Pdx1CreERT deleter strain per se has no impact on glycemia, insulin secretion or the cellular composition of islets. N-values for IPGTTs have been increased
List of abbreviations
- [Ca2+]cyt
- Cytoplasmic Ca2+ concentration
- [Ca2+]mito
- Mitochondrial free Ca2+ concentration
- AA
- Antimycin A
- Ach
- Acetylcholine
- Clec16aΔpanc
- Pancreatic islet specific Clec16a knock-out
- Diaz
- Diazoxide
- dKO
- double knock-out
- Ex4
- Exendin-4
- FCCP
- Carbonyl cyanide-4-phenylhydrazone
- GIP
- Glucose-dependent insulinotropic peptide
- GLP-1
- Glucagon-like peptide-1
- GSIS
- Glucose-stimulated insulin secretion
- IMM
- Inner mitochondria membrane
- IPGTT
- Intraperitoneal glucose tolerance test
- OGTT
- Oral gavage and glucose tolerance test
- Oligo
- Oligomycin
- OMM
- Outer mitochondrial membrane
- r
- Pearson correlation coefficient
- Rot
- Rotenone
- TMRE
- Tetramethylrhodamine ethyl ester
- βMfn1/2 dKO
- beta cell specific Mitofusin 1 and 2 double knock-out
- Δψm
- Mitochondrial membrane potential