Glutaminolysis and insulin secretion: from bedside to bench and back

Identification of regulatory mutations of glutamate dehydrogenase (GDH) in a form of congenital hyperinsulinism (GDH-HI) is providing a model for basal insulin secretion (IS) and amino acid (AA)-stimulated insulin secretion (AASIS) in which glutaminolysis plays a key role. Leucine and ADP are activators and GTP is an inhibitor of GDH. GDH-HI mutations impair GDH sensitivity to GTP inhibition, leading to fasting hypoglycemia, leucine hypersensitivity, and protein-induced hypoglycemia, indicating the importance of GDH in basal secretion and AASIS. The proposed model for glutaminolysis in IS is based on GDH providing NADH and alpha-ketoglutarate (alpha-KG) to the Krebs cycle, hence increasing the beta-cell ATP-to-ADP ratio to effect insulin release. The process operates with 1) sufficient lowering of beta-cell phosphate potential (i.e., fasting) and when 2) AAs provide leucine for allosteric activation and glutamate from transaminations. To test this hypothesis, IS studies were performed in rat and GDH-HI mouse models. In the rat study, rat islets were isolated, cultured, and then perifused in Krebs-Ringer bicarbonate buffer with 2 mmol/l glutamine using 10 mmol/l 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH) or a BCH ramp after 50 or 120 min of glucose deprivation. In the GDH-HI mouse study, the H454Y GDH-HI mutation driven by the rat insulin promoter was created for H454Y beta-cell-specific expression. Cultured, isolated islets were perifused in leucine 0-10 mmol/l with 2 mmol/l glutamine 0-25 mmol/l, AA 0-10 mmol/l, or glucose 0-25 mmol/l. Rat islets displayed enhanced BCH-stimulated IS after 120 min of glucose deprivation, but not when energized by fuel. H454Y and control islets had similar glucose-stimulated IS, but H454Y mice had lower random blood glucose. Leucine-stimulated IS and AASIS occurred at lower thresholds and were greater in H454Y versus control islets. Glutamine stimulated IS in H454Y but not control islets. The clinical manifestations of GDH-HI and related animal studies suggest that GDH regulates basal IS and AASIS. Energy deprivation enhanced GDH-mediated IS, and H454Y mice were hypoglycemic, substantiating roles for GDH and its regulation by the phosphate potential in basal IS. Excessive IS from H454Y islets upon exposure to GDH substrates or stimuli indicate that regulation of GDH by the beta-cell phosphate potential plays a critical role in AASIS. These findings provide a foundation for defining pathways of basal secretion and AASIS, augmenting our understanding of beta-cell function.