Regulation of glutamate metabolism and insulin secretion by glutamate dehydrogenase in hypoglycemic children

Abstract

In addition to its extracellular roles as a neurotransmitter/sensory molecule, glutamate serves important intracellular signaling functions via its metabolism through glutamate dehydrogenase (GDH). GDH is a mitochondrial matrix enzyme that catalyzes the oxidative deamination of glutamate to alpha-ketoglutarate in a limited number of tissues in humans, including the liver, the kidney, the brain, and the pancreatic islets. GDH activity is subject to complex regulation by negative (GTP, palmitoyl-coenzyme A) and positive (ADP, leucine) allosteric effectors. This complex regulation allows GDH activity to be modulated by changes in energy state and amino acid availability. The importance of GDH regulation has been highlighted by the discovery of a novel hypoglycemic disorder in children, the hyperinsulinism-hyperammonemia syndrome, which is caused by dominantly expressed, activating mutations of the enzyme that impair its inhibition by GTP. Affected children present in infancy with hypoglycemic seizures after brief periods of fasting or the ingestion of a high-protein meal. Patients have characteristic persistent 3- to 5-fold elevations of blood ammonia concentrations but do not display the usual neurologic symptoms of hyperammonemia. The mutant GDH enzyme shows impaired responses to GTP inhibition. Isolated islets from mice that express the mutant GDH in pancreatic beta cells show an increased rate of glutaminolysis, increased insulin release in response to glutamine, and increased sensitivity to leucine-stimulated insulin secretion. The novel hyperinsulinism-hyperammonemia syndrome indicates that GDH-catalyzed glutamate metabolism plays important roles in 3 tissues: in beta cells, the regulation of amino acid-stimulated insulin secretion; in hepatocytes, the modulation of amino acid catabolism and ammoniagenesis; and in brain neurons, the maintenance of glutamate neurotransmitter concentrations.

FIGURE 1

Glutamate metabolism by glutamate dehydrogenase (GDH). GDH oxidative deamination of glutamate is controlled by multiple allosteric inhibitors, particularly GTP, and by activators, particularly ADP and leucine. These allow control of glutamate oxidation by the intracellular energy potential and by the supply of amino acids (through leucine) and, possibly, also fatty acids. α-KG, α-ketoglutarate.

FIGURE 2

Pedigree with hyperinsulinism-hyperammonemia syndrome. Note the elevations of plasma ammonia in the 3 affected persons. Mutation analysis revealed heterozygosity for an Arg269Cys missense mutation in all 3 affected persons. Gel heteroduplex analysis of genomic DNA–polymerase chain reaction amplicons shows that mutation carriers are heterozygous for a mutation in exon 7. Circles, females; boxes, males; solid symbols, affected individuals.

FIGURE 3

Mechanism of hyperinsulinism and hyperammonemia in hyperinsulinism-hyperammonemia syndrome. In the β cell, oxidation of fuels, such as glucose, increases the ATP:ADP ratio to inhibit the ATP-dependent potassium channel, which triggers the influx of calcium to release insulin from stored granules. Amino acids feed into this triggering pathway via glutamate dehydrogenase (GDH) oxidation of glutamate under the control of GTP and ADP, as well as leucine. In the liver, ammonia is generated from glutamate via GDH; glutamate also generates N-acetylglutamate to regulate ammonia detoxification into urea. GK, glucokinase; SUR, sulfonylurea receptor; K_ATP channel, ATP-dependent potassium channel; Kir, potassium pore; CPS, carbamyl-phosphate synthetase.

FIGURE 4

Role of glutamine in amplifying glucose-stimulated insulin secretion. High glucose raises the β cell ATP:ADP ratio, which not only triggers membrane depolarization and elevation of cytosolic calcium but also inhibits glutamate oxidation and redirects glutamate toward synthesis of glutamine. Glutamine then amplifies insulin release at some as-yet-unknown site. GLUT2, glucose transporter 2; G6P, glucose-6-phosphate; GS, glutamine synthetase; GDH, glutamate dehydrogenase; K_ATP channel, ATP-dependent potassium channel; Ψ, plasma membrane depolarization.