Contribution of Extracellular Glutamine as An Anaplerotic Substrate to Neuronal Metabolism: A Re-Evaluation by Multinuclear NMR Spectroscopy in Primary Cultured Neurons

Abstract

Multinuclear NMR spectroscopy is used to investigate the effect of glutamine on neuronal glucose metabolism. Primary neurons were incubated with [1-(13C)]glucose in the absence or presence of glutamine (2 mM) and/or NH4Cl (5 mM). After ammonia-treatment, the concentrations of high-energy phosphates decreased up to 84% of control, which was aggravated in glutamine-containing medium (up to 42% of control). These effects could not be attributed to changes in mitochondrial glucose oxidation. Withdrawal of glutamine decreased amino acid concentrations, e.g. of glutamate to 53%, but also considerably lessened the 13C enrichment in [4-(13C)]glutamate to 8.3% of control, and decreased the 13C-enrichment in acetyl-CoA entering the Krebs cycle (P < 0.001). Thus, although glutamine is potent in replenishing neuronal glutamate stores, glutamate formation is mainly attributed to its de novo synthesis from glucose. Furthermore, mitochondrial glucose metabolism strongly depends on the supply of carbons from glutamine, indicating that exogenous glutamine is a well-suited substrate to replenish neuronal Krebs cycle intermediates.

Metabolic Fingerprints of Altered Brain Growth, Osmoregulation and Neurotransmission in a Rett Syndrome Model

“…This would be consistent with (i) data showing an increase in metabolic activity in glia upon increased glutamate release from neurons [42], and (ii) previously reported reduction of brain ATP levels in Mecp2 -null mice [20] since ATP is also consumed in the synthesis of glutamine from glutamate [43]. Glutamate levels may be kept constant by compensatory glutamate synthesis from glucose and by anaplerosis (Table S2) [44], [45]. In fact, the glutamate-glutamine cycle and neuronal activity are tightly linked to glucose metabolism (Figure S3), and elevated glutamate concentrations found in the CSF of RS patients have also been interpreted in terms of enhanced glucose utilization [22].…”

Metabolic Fingerprints of Altered Brain Growth, Osmoregulation and Neurotransmission in a Rett Syndrome Model

“…This would be consistent with (i) data showing an increase in metabolic activity in glia upon increased glutamate release from neurons [42], and (ii) previously reported reduction of brain ATP levels in Mecp2 -null mice [20] since ATP is also consumed in the synthesis of glutamine from glutamate [43]. Glutamate levels may be kept constant by compensatory glutamate synthesis from glucose and by anaplerosis (Table S2) [44], [45]. In fact, the glutamate-glutamine cycle and neuronal activity are tightly linked to glucose metabolism (Figure S3), and elevated glutamate concentrations found in the CSF of RS patients have also been interpreted in terms of enhanced glucose utilization [22].…”

Glutamine as an energy substrate in cultured neurons during glucose deprivation

“…The modest rate of glutamine utilization by the cerebral cortical neurons is at variance with evidence obtained by Shokati et al (2005) that exogenous glutamine is of major importance to replenish neuronal TCA cycle intermediates. This difference may be developmental because the cells used by these authors were younger than those used in the present study.…”

Acute Increase of the Glutamate–Glutamine Cycling in Discrete Brain Areas after Administration of a Single Dose of Amphetamine

“…In addition to recovering some fraction of released glutamate via the EAAC1 transporter, neurons may synthesize glutamate from tricarboxylic acid cycle intermediates, either imported from astrocytes 39 or generated neuronally 40 . Therefore, in the present study, glutamate levels may be kept constant by compensatory glutamate synthesis by anaplerosis 41 . In opposition to CPu and hippocampus, GLU showed a tendency to increase in the frontal cortex.…”

Metabolic Fingerprints of Altered Brain Growth, Osmoregulation and Neurotransmission in a Rett Syndrome Model

“…This would be consistent with (i) data showing an increase in metabolic activity in glia upon increased glutamate release from neurons [42], and (ii) previously reported reduction of brain ATP levels in Mecp2 -null mice [20] since ATP is also consumed in the synthesis of glutamine from glutamate [43]. Glutamate levels may be kept constant by compensatory glutamate synthesis from glucose and by anaplerosis (Table S2) [44], [45]. In fact, the glutamate-glutamine cycle and neuronal activity are tightly linked to glucose metabolism (Figure S3), and elevated glutamate concentrations found in the CSF of RS patients have also been interpreted in terms of enhanced glucose utilization [22].…”

Glutamine as an energy substrate in cultured neurons during glucose deprivation

“…The modest rate of glutamine utilization by the cerebral cortical neurons is at variance with evidence obtained by Shokati et al (2005) that exogenous glutamine is of major importance to replenish neuronal TCA cycle intermediates. This difference may be developmental because the cells used by these authors were younger than those used in the present study.…”

Acute Increase of the Glutamate–Glutamine Cycling in Discrete Brain Areas after Administration of a Single Dose of Amphetamine

“…In addition to recovering some fraction of released glutamate via the EAAC1 transporter, neurons may synthesize glutamate from tricarboxylic acid cycle intermediates, either imported from astrocytes 39 or generated neuronally 40 . Therefore, in the present study, glutamate levels may be kept constant by compensatory glutamate synthesis by anaplerosis 41 . In opposition to CPu and hippocampus, GLU showed a tendency to increase in the frontal cortex.…”

Metabolic Fingerprints of Altered Brain Growth, Osmoregulation and Neurotransmission in a Rett Syndrome Model

“…This would be consistent with (i) data showing an increase in metabolic activity in glia upon increased glutamate release from neurons [42], and (ii) previously reported reduction of brain ATP levels in Mecp2 -null mice [20] since ATP is also consumed in the synthesis of glutamine from glutamate [43]. Glutamate levels may be kept constant by compensatory glutamate synthesis from glucose and by anaplerosis (Table S2) [44], [45]. In fact, the glutamate-glutamine cycle and neuronal activity are tightly linked to glucose metabolism (Figure S3), and elevated glutamate concentrations found in the CSF of RS patients have also been interpreted in terms of enhanced glucose utilization [22].…”

Glutamine as an energy substrate in cultured neurons during glucose deprivation

“…The modest rate of glutamine utilization by the cerebral cortical neurons is at variance with evidence obtained by Shokati et al (2005) that exogenous glutamine is of major importance to replenish neuronal TCA cycle intermediates. This difference may be developmental because the cells used by these authors were younger than those used in the present study.…”

Acute Increase of the Glutamate–Glutamine Cycling in Discrete Brain Areas after Administration of a Single Dose of Amphetamine

“…In addition to recovering some fraction of released glutamate via the EAAC1 transporter, neurons may synthesize glutamate from tricarboxylic acid cycle intermediates, either imported from astrocytes 39 or generated neuronally 40 . Therefore, in the present study, glutamate levels may be kept constant by compensatory glutamate synthesis by anaplerosis 41 . In opposition to CPu and hippocampus, GLU showed a tendency to increase in the frontal cortex.…”