Nicotinamide nucleotide transhydrogenase and the glucose stimulation of insulin secretion

Nicotinamide nucleotide transhydrogenase (NNT) couples proton influx to NADPH production in the mitochondrial matrix. Its spontaneous inactivating truncation in C57Bl/6J mice reduces glucose-stimulated insulin secretion (GSIS), but the mechanisms involved are not clear. Here, we studied the role of NNT in β cell stimulus-secretion coupling by comparing islets from female C57Bl/6J mice (J) with truncated NNT to islets from genetically close C57Bl/6N mice (N) with wildtype NNT. Islets were cultured up to 4 days in RPMI and incubated for 30-60 min or perifused at 0.5, 15 and 30 mM glucose in Krebs containing 4.8 or 30 mM K+ (K4.8/K30). At K4.8, glucose acutely increased NADH/NAD+, mitochondrial membrane potential, ATP/ADP and cytosolic Ca2+ to the same extent in J and N islets. In contrast, an acute increase in NADPH/NADP+ and reduction in mitochondrial glutathione redox potential (mt-EGSH) were observed in N but not J islets, unless the latter were infected 2-4 days earlier with an adenovirus coding NNT. GSIS (expressed per ng of islet DNA) was ~66% lower in J vs. N islets, despite their similar insulin content. Insulin secretion was also ~66% lower in J vs. N islets incubated at K30 despite similar rises in cytosolic Ca2+, but the amplifying action of glucose on Ca2+-induced secretion was similar in both types of islets. Again, adenovirus-mediated expression of NNT in J islets restored GSIS and K30-induced secretion in proportion to the percentage of infected β cells (~25%). Finally, addition of 10 μM acetylcholine (ACh), but not 20 nM exenatide, much better amplified GSIS in J than N islets, and thereby reduced their relative defect in insulin secretion. In conclusion, the spontaneous inactivating truncation of NNT in C57Bl/6J mice is responsible for a lack of glucose-induced changes in NADPH/NADP+ and mt-EGSH in their pancreatic islets. It does not alter the triggering pathway of GSIS but markedly reduces the efficacy of Ca2+ on exocytosis by a mechanism that is largely corrected by ACh