Glutamine as an energy substrate in cultured neurons during glucose deprivation

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Disturbed glucose and pyruvate metabolism in glaucoma with neuroprotection by pyruvate or rapamycin

Intraocular pressure-sensitive retinal ganglion cell degeneration is a hallmark of glaucoma, the leading cause of irreversible blindness. Here, we used RNA-sequencing and metabolomics to examine early glaucoma in DBA/2J mice. We demonstrate gene expression changes that significantly impact pathways mediating the metabolism and transport of glucose and pyruvate. Subsequent metabolic studies characterized an intraocular pressure (IOP)-dependent decline in retinal pyruvate levels coupled to dysregulated glucose metabolism prior to detectable optic nerve degeneration. Remarkably, retinal glucose levels were elevated 50-fold, consistent with decreased glycolysis but possibly including glycogen mobilization and other metabolic changes. Oral supplementation of the glycolytic product pyruvate strongly protected from neurodegeneration in both rat and mouse models of glaucoma. Investigating further, we detected mTOR activation at the mechanistic nexus of neurodegeneration and metabolism. Rapamycin-induced inhibition of mTOR robustly prevented glaucomatous neurodegeneration, supporting a damaging role for IOP-induced mTOR activation in perturbing metabolism and promoting glaucoma. Together, these findings support the use of treatments that limit metabolic disturbances and provide bioenergetic support. Such treatments provide a readily translatable strategy that warrants investigation in clinical trials.

Heterogeneous Response in Rabbit Fetal Diaphragmatic Hernia Lungs After Tracheal Occlusion

BACKGROUND

Fetal tracheal occlusion (TO) is an experimental therapeutic approach to stimulate lung growth in the most severe congenital diaphragmatic hernia (CDH) cases. We have previously demonstrated a heterogeneous response of normal fetal rabbit lungs after TO with the appearance of at least two distinct zones. The aim of this study was to examine the fetal lung response after TO in a left CDH fetal rabbit model.

METHODS

Fetal rabbits at 25 d gestation underwent surgical creation of CDH followed by TO at 27 d and harvest on day 30. Morphometric analysis, global metabolomics, and fluorescence lifetime imaging microscopy (FLIM) were performed to evaluate structural and metabolic changes in control, CDH, and CDH + TO lungs.

RESULTS

Right and left lungs were different at the baseline and had a heterogeneous pulmonary growth response in CDH and after TO. The relative percent growth of the right lungs in CDH + TO was higher than the left lungs. Morphometric analyses revealed heterogeneous tissue-to-airspace ratios, in addition to size and number of airspaces within and between the lungs in the different groups. Global metabolomics demonstrated a slower rate of metabolism in the CDH group with the left lungs being less metabolically active. TO stimulated metabolic activity in both lungs to different degrees. FLIM analysis demonstrated local heterogeneity in glycolysis, oxidative phosphorylation (OXPHOS), and FLIM "lipid-surfactant" signal within and between the right and left lungs in all groups.

CONCLUSIONS

We demonstrate that TO leads to a heterogeneous morphologic and metabolic response within and between the right and left lungs in a left CDH rabbit model.

Presymptomatic Alterations in Amino Acid Metabolism and DNA Methylation in the Cerebellum of a Murine Model of Niemann-Pick Type C Disease

The fatal neurodegenerative disorder Niemann-Pick type C (NPC) is caused in most cases by mutations in NPC1, which encodes the late endosomal NPC1 protein. Loss of NPC1 disrupts cholesterol trafficking from late endosomes to the endoplasmic reticulum and plasma membrane, causing cholesterol accumulation in late endosomes/lysosomes. Neurons are particularly vulnerable to this cholesterol trafficking defect, but the pathogenic mechanisms through which NPC1 deficiency causes neuronal dysfunction remain largely unknown. Herein, we have investigated amino acid metabolism in cerebella of NPC1-deficient mice at different stages of NPC disease. Imbalances in amino acid metabolism were evident from increased branched chain amino acid and asparagine levels and altered expression of key enzymes of glutamine/glutamate metabolism in presymptomatic and early symptomatic NPC1-deficient cerebellum. Increased levels of several amino acid intermediates of one-carbon metabolism indicated disturbances in folate and methylation pathways. Alterations in DNA methylation were apparent in decreased expression of DNA methyltransferase 3a and methyl-5'-cytosine-phosphodiester-guanine-domain binding proteins, reduced 5-methylcytosine immunoreactivity in the molecular and Purkinje cell layers, demethylation of genome-wide repetitive LINE-1 elements, and hypermethylation in specific promoter regions of single-copy genes in NPC1-deficient cerebellum at early stages of the disease. Alterations in amino acid metabolism and epigenetic changes in the cerebellum at presymptomatic stages of NPC disease represent previously unrecognized mechanisms of NPC pathogenesis.

Inhibition of mitochondrial pyruvate transport by zaprinast causes massive accumulation of aspartate at the expense of glutamate in the retina

Transport of pyruvate into mitochondria by the mitochondrial pyruvate carrier is crucial for complete oxidation of glucose and for biosynthesis of amino acids and lipids. Zaprinast is a well known phosphodiesterase inhibitor and lead compound for sildenafil. We found Zaprinast alters the metabolomic profile of mitochondrial intermediates and amino acids in retina and brain. This metabolic effect of Zaprinast does not depend on inhibition of phosphodiesterase activity. By providing (13)C-labeled glucose and glutamine as fuels, we found that the metabolic profile of the Zaprinast effect is nearly identical to that of inhibitors of the mitochondrial pyruvate carrier. Both stimulate oxidation of glutamate and massive accumulation of aspartate. Moreover, Zaprinast inhibits pyruvate-driven O2 consumption in brain mitochondria and blocks mitochondrial pyruvate carrier in liver mitochondria. Inactivation of the aspartate glutamate carrier in retina does not attenuate the metabolic effect of Zaprinast. Our results show that Zaprinast is a potent inhibitor of mitochondrial pyruvate carrier activity, and this action causes aspartate to accumulate at the expense of glutamate. Our findings show that Zaprinast is a specific mitochondrial pyruvate carrier (MPC) inhibitor and may help to elucidate the roles of MPC in amino acid metabolism and hypoglycemia.

Insights into the metabolic response to traumatic brain injury as revealed by (13)C NMR spectroscopy

The present review highlights critical issues related to cerebral metabolism following traumatic brain injury (TBI) and the use of (13)C labeled substrates and nuclear magnetic resonance (NMR) spectroscopy to study these changes. First we address some pathophysiologic factors contributing to metabolic dysfunction following TBI. We then examine how (13)C NMR spectroscopy strategies have been used to investigate energy metabolism, neurotransmission, the intracellular redox state, and neuroglial compartmentation following injury. (13)C NMR spectroscopy studies of brain extracts from animal models of TBI have revealed enhanced glycolytic production of lactate, evidence of pentose phosphate pathway (PPP) activation, and alterations in neuronal and astrocyte oxidative metabolism that are dependent on injury severity. Differential incorporation of label into glutamate and glutamine from (13)C labeled glucose or acetate also suggest TBI-induced adaptations to the glutamate-glutamine cycle.

Effect of glucose on glutamine metabolism in rat brain slices: a cellular metabolomic study with Effect of glucose ¹³C NMR

To examine the effect of glucose on the cerebral metabolism of glutamine, rat brain slices were incubated with 5mM [3-(13)C]glutamine without and with 5mM unlabeled glucose. Tissue plus medium extracts were analyzed by using enzymatic and (13)C NMR techniques and fluxes through the enzymatic steps involved were calculated with a mathematical model. We demonstrate that glucose increased alanine, pyruvate and glutamate accumulations and decreased ammonium ions accumulation, aspartate accumulation and labeling, and GABA labeling. In order to determine the participation of glutamine synthetase when glucose was added to the incubation medium, we incubated rat brain slices with 5mM [3-(13)C]glutamine plus 5mM unlabeled glucose without and with 2mM methionine sulfoximine (MSO). The results indicate that 77% of the newly appeared glutamine was formed via glutamine synthetase and 23% from endogenous sources; the stimulation of [3-(13)C]glutamine removal by MSO also strongly suggests the existence of a cycle between [3-(13)C]glutamine and [3-(13)C]glutamate. This work also demonstrates that glucose increased fluxes through hexokinase, pyruvate kinase, lactate dehydrogenase, alanine aminotransferase, pyruvate carboxylase, pyruvate dehydrogenase, citrate synthase, flux from α-ketoglutarate to glutamate and flux through glutamine synthetase whereas it inhibited fluxes through aspartate aminotransferase, glutamic acid decarboxylase and GABA aminotransferase.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Glutamate and glutathione interplay in a motor neuronal model of amyotrophic lateral sclerosis reveals altered energy metabolism

Impairment of mitochondrial function might contribute to oxidative stress associated with neurodegeneration in amyotrophic lateral sclerosis (ALS). Glutamate levels in tissues of ALS patients are sometimes altered. In neurons, mitochondrial metabolism of exogenous glutamine is mainly responsible for the net synthesis of glutamate, which is a neurotransmitter, but it is also necessary for the synthesis of glutathione, the main endogenous antioxidant. We investigated glutathione synthesis and glutamine/glutamate metabolism in a motor neuronal model of familial ALS. In standard culture conditions (with glutamine) or restricting glutamine or cystine, the level of glutathione was always lower in the cell line expressing the mutant (G93A) human Cu, Zn superoxide dismutase (G93ASOD1) than in the line expressing wild-type SOD1. With glutamine the difference in glutathione was associated with a lower glutamate and impairment of the glutamine/glutamate metabolism as evidenced by lower glutaminase and cytosolic malate dehydrogenase activity. d-β-hydroxybutyrate, as an alternative to glutamine as energy substrate in addition to glucose, reversed the decreases of cytosolic malate dehydrogenase activity and glutamate and glutathione. However, in the G93ASOD1 cell line, in all culture conditions the expression of pyruvate dehydrogenase kinase l protein, which down-regulates pyruvate dehydrogenase activity, was induced, together with an increase in lactate release in the medium. These findings suggest that the glutathione decrease associated with mutant SOD1 expression is due to mitochondrial dysfunction caused by the reduction of the flow of glucose-derived pyruvate through the TCA cycle; it implies altered glutamate metabolism and depends on the different mitochondrial energy substrates.

Glutamine effect on cultured granule neuron death induced by glucose deprivation and chemical hypoxia

Using a specific fluorescent probe of mitochondrial membrane potential (tetramethylrhodamine ethyl ester), we have shown that glucose deprivation (GD) of cultured cerebellar granule neurons (CGN) for 3 h lowers mitochondrial membrane potential in these cells. Longer glucose starvation (24 h) causes CGN death that is not prevented by blockers of ionotropic glutamate receptors (MK-801 (10 µM) and NBQX (10 µM)). Glutamine or pyruvate (2 mM) maintain membrane potential of mitochondria and decrease CGN death under GD conditions. In the presence of glucose the mitochondrial respiratory chain blocker rotenone induces neuron death potentiated by glutamine. The potentiation effect is completely prevented by blockers of ionotropic glutamate receptors. These results show that glutamine under conditions of GD can be utilized by mitochondria as substrate, but at the same time, in the case of mitochondrial function deterioration, metabolism of this amino acid results in glutamate accumulation to toxic level.

Metabolic fate of a high concentration of glutamine and glutamate in rat brain slices: a ¹³C NMR study

This study was performed to analyze the metabolic fate of a high concentration (5 mM) of glutamine and glutamate in rat brain slices and the participation of these amino acids in the glutamine-glutamate cycle. For this, brain slices were incubated for 60 min with [3-¹³C]glutamine or [3-¹³C]glutamate. Tissue plus medium extracts were analyzed by enzymatic and ¹³C NMR measurements and fluxes through pathways of glutamine and glutamate metabolism were calculated. We demonstrate that both substrates were utilized and oxidized at high rates by rat brain slices and served as precursors of neurotransmitters, tricarboxylic acid (TCA) cycle intermediates and alanine. In order to determine the participation of glutamine synthetase in the appearance of new glutamine molecules with glutamine as substrate, brain slices were incubated with [3-¹³C]glutamine in the presence of methionine sulfoximine, a specific inhibitor of glutamine synthetase. Our results indicate that 36.5% of the new glutamine appeared was glutamine synthetase-dependent and 63.5% was formed from endogenous substrates. Flux through glutamic acid decarboxylase was higher with glutamine than with glutamate as substrate whereas fluxes from α-ketoglutarate to glutamate and through glutamine synthetase, malic enzyme, pyruvate dehydrogenase, pyruvate carboxylase and citrate synthase were in the same range with both substrates.

Astrocyte oxidative metabolism and metabolite trafficking after fluid percussion brain injury in adult rats

Despite various lines of evidence pointing to the compartmentation of metabolism within the brain, few studies have reported the effect of a traumatic brain injury (TBI) on neuronal and astrocyte compartments and/or metabolic trafficking between these cells. In this study we used ex vivo ¹³C NMR spectroscopy following an infusion of [1-¹³C] glucose and [1,2-¹³C₂] acetate to study oxidative metabolism in neurons and astrocytes of sham-operated and fluid percussion brain injured (FPI) rats at 1, 5, and 14 days post-surgery. FPI resulted in a decrease in the ¹³C glucose enrichment of glutamate in neurons in the injured hemisphere at day 1. In contrast, enrichment of glutamine in astrocytes from acetate was not significantly decreased at day 1. At day 5 the ¹³C enrichment of glutamate and glutamine from glucose in the injured hemisphere of FPI rats did not differ from sham levels, but glutamine derived from acetate metabolism in astrocytes was significantly increased. The ¹³C glucose enrichment of the C3 position of glutamate (C3) in neurons was significantly decreased ipsilateral to FPI at day 14, whereas the enrichment of glutamine in astrocytes had returned to sham levels at this time point. These findings indicate that the oxidative metabolism of glucose is reduced to a greater extent in neurons compared to astrocytes following a FPI. The increased utilization of acetate to synthesize glutamine, and the acetate enrichment of glutamate via the glutamate-glutamine cycle, suggests an integral protective role for astrocytes in maintaining metabolic function following TBI-induced impairments in glucose metabolism.

Direct measurement of oxidative metabolism in the living brain by microdialysis: a review

This review summarizes microdialysis studies that address the question of which compounds serve as energy sources in the brain. Microdialysis was used to introduce 14C-labeled glucose, lactate, pyruvate, glutamate, glutamine, and acetate into the interstitial fluid of the brain to observe their metabolism to 14CO2. Although glucose uptake from the systemic system supplies the carbon source for these compounds, compounds synthesized from glucose by the brain are subject to recycling including complete metabolism to CO2. Therefore, the brain utilizes multiple compounds in its domain to provide the energy needed to fulfill its function. The physiological conditions controlling metabolism and the contribution of compartmentation into different brain regions, cell types, and subcellular spaces are still unresolved. The aconitase inhibitor fluorocitrate, with a lower inhibition threshold in glial cells, was used to identify the proportion of lactate and glucose that was oxidized in glial cells versus neurons. The fluorocitrate data suggest that glial and neuronal cells are capable of utilizing both lactate and glucose for energy metabolism.

Availability of neurotransmitter glutamate is diminished when beta-hydroxybutyrate replaces glucose in cultured neurons

Ketone bodies serve as alternative energy substrates for the brain in cases of low glucose availability such as during starvation or in patients treated with a ketogenic diet. The ketone bodies are metabolized via a distinct pathway confined to the mitochondria. We have compared metabolism of [2,4-(13)C]beta-hydroxybutyrate to that of [1,6-(13)C]glucose in cultured glutamatergic neurons and investigated the effect of neuronal activity focusing on the aspartate-glutamate homeostasis, an essential component of the excitatory activity in the brain. The amount of (13)C incorporation and cellular content was lower for glutamate and higher for aspartate in the presence of [2,4-(13)C]beta-hydroxybutyrate as opposed to [1,6-(13)C]glucose. Our results suggest that the change in aspartate-glutamate homeostasis is due to a decreased availability of NADH for cytosolic malate dehydrogenase and thus reduced malate-aspartate shuttle activity in neurons using beta-hydroxybutyrate. In the presence of glucose, the glutamate content decreased significantly upon activation of neurotransmitter release, whereas in the presence of only beta-hydroxybutyrate, no decrease in the glutamate content was observed. Thus, the fraction of the glutamate pool available for transmitter release was diminished when metabolizing beta-hydroxybutyrate, which is in line with the hypothesis of formation of transmitter glutamate via an obligatory involvement of the malate-aspartate shuttle.

A metabolic switch in brain: glucose and lactate metabolism modulation by ascorbic acid

In this review, we discuss a novel function of ascorbic acid in brain energetics. It has been proposed that during glutamatergic synaptic activity neurons preferably consume lactate released from glia. The key to this energetic coupling is the metabolic activation that occurs in astrocytes by glutamate and an increase in extracellular [K(+)]. Neurons are cells well equipped to consume glucose because they express glucose transporters and glycolytic and tricarboxylic acid cycle enzymes. Moreover, neuronal cells express monocarboxylate transporters and lactate dehydrogenase isoenzyme 1, which is inhibited by pyruvate. As glycolysis produces an increase in pyruvate concentration and a decrease in NAD(+)/NADH, lactate and glucose consumption are not viable at the same time. In this context, we discuss ascorbic acid participation as a metabolic switch modulating neuronal metabolism between rest and activation periods. Ascorbic acid is highly concentrated in CNS. Glutamate stimulates ascorbic acid release from astrocytes. Ascorbic acid entry into neurons and within the cell can inhibit glucose consumption and stimulate lactate transport. For this switch to occur, an ascorbic acid flow is necessary between astrocytes and neurons, which is driven by neural activity and is part of vitamin C recycling. Here, we review the role of glucose and lactate as metabolic substrates and the modulation of neuronal metabolism by ascorbic acid.

Bioenergetics of cerebral ischemia: a cellular perspective

In cerebral ischemia survival of neurons, astrocytes, oligodendrocytes and endothelial cells is threatened during energy deprivation and/or following re-supply of oxygen and glucose. After a brief summary of characteristics of different cells types, emphasizing the dependence of all on oxidative metabolism, the bioenergetics of focal and global ischemia is discussed, distinguishing between events during energy deprivation and subsequent recovery attempt after re-circulation. Gray and white matter ischemia are described separately, and distinctions are made between mature and immature brains. Next comes a description of bioenergetics in individual cell types in culture during oxygen/glucose deprivation or exposure to metabolic inhibitors and following re-establishment of normal aerated conditions. Due to their expression of NMDA and non-NMDA receptors neurons and oligodendrocytes are exquisitely sensitive to excitotoxicity by glutamate, which reaches high extracellular concentrations in ischemic brain for several reasons, including failing astrocytic uptake. Excitotoxicity kills brain cells by energetic exhaustion (due to Na(+) extrusion after channel-mediated entry) combined with mitochondrial Ca(2+)-mediated injury and formation of reactive oxygen species. Many (but not all) astrocytes survive energy deprivation for extended periods, but after return to aerated conditions they are vulnerable to mitochondrial damage by cytoplasmic/mitochondrial Ca(2+) overload and to NAD(+) deficiency. Ca(2+) overload is established by reversal of Na(+)/Ca(2+) exchangers following Na(+) accumulation during Na(+)-K(+)-Cl(-) cotransporter stimulation or pH regulation, compensating for excessive acid production. NAD(+) deficiency inhibits glycolysis and eventually oxidative metabolism, secondary to poly(ADP-ribose)polymerase (PARP) activity following DNA damage. Hyperglycemia can be beneficial for neurons but increases astrocytic death due to enhanced acidosis.