Astrocytic energy metabolism and glutamate formation — relevance for 13C-NMR spectroscopy and importance of cytosolic/mitochondrial trafficking

Hertz, Leif

doi:10.1016/j.mri.2011.04.013

Cited by 66 publications

(69 citation statements)

References 101 publications

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“…The fractional PC flux observed in our cells is within the wide 4% to 65% range previously reported a variety of in vivo [31] and cultured cell systems (calculated from [27]). However, it should be noted that whereas astrocytic oxidative metabolism is quite variable in cultured normal astrocytes [32]–[34], PC flux represents about 40% of the pyuruvate flux in normal brain [17], [31]. In contrast, previous studies in glioblastoma cells indicate that the pyruvate flux via PC is much lower.…”

Section: Discussionsupporting

confidence: 85%

Glioma Cells with the IDH1 Mutation Modulate Metabolic Fractional Flux through Pyruvate Carboxylase

et al. 2014

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BackgroundOver 70% of low-grade gliomas carry a heterozygous R132H mutation in the gene coding for isocitrate dehydrogenase 1 (IDH1). This confers the enzyme with the novel ability to convert α-ketoglutarate to 2-hydroxyglutarate, ultimately leading to tumorigenesis. The major source of 2-hydroxyglutarate production is glutamine, which, in cancer, is also a source for tricarboxylic acid cycle (TCA) anaplerosis. An alternate source of anaplerosis is pyruvate flux via pyruvate carboxylase (PC), which is a common pathway in normal astrocytes. The goal of this study was to determine whether PC serves as a source of TCA anaplerosis in IDH1 mutant cells wherein glutamine is used for 2-hydroxyglutarate production.MethodsImmortalized normal human astrocytes engineered to express heterozygous mutant IDH1 or wild-type IDH1 were investigated. Flux of pyruvate via PC and via pyruvate dehydrogenase (PDH) was determined by using magnetic resonance spectroscopy to probe the labeling of [2-13C]glucose-derived 13C-labeled glutamate and glutamine. Activity assays, RT-PCR and western blotting were used to probe the expression and activity of relevant enzymes. The Cancer Genome Atlas (TCGA) data was analyzed to assess the expression of enzymes in human glioma samples.ResultsCompared to wild-type cells, mutant IDH1 cells significantly increased fractional flux through PC. This was associated with a significant increase in PC activity and expression. Concurrently, PDH activity significantly decreased, likely mediated by significantly increased inhibitory PDH phosphorylation by PDH kinase 3. Consistent with the observation in cells, analysis of TCGA data indicated a significant increase in PC expression in mutant IDH-expressing human glioma samples compared to wild-type IDH.ConclusionsOur findings suggest that changes in PC and PDH may be an important part of cellular adaptation to the IDH1 mutation and may serve as potential therapeutic targets.

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Section: Discussionsupporting

confidence: 85%

Glioma Cells with the IDH1 Mutation Modulate Metabolic Fractional Flux through Pyruvate Carboxylase

et al. 2014

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“…In neurons, energy is mainly spent on recovering the transmembrane potential and ion gradients perturbed by the various ion channel-mediated currents, whereas in astrocytes energy is mostly spent on the glutamate and potassium uptake. 2,3,9 To pharmacologically separate the postsynaptic neuronal fraction of the observed response, we used a mixture (cocktail) of postsynaptic receptor blockers consisting of NBQX (antagonist of AMPA receptors, 10 mmol/L), AP-5 (antagonist of NMDA receptors, 40 mmol/L), gabazine (antagonist of GABA A receptors, 10 mmol/L), and E4CPG (nonselective antagonist of metabotropic glutamate receptors, 500 mmol/L). Application of this cocktail completely abolished LFPs (see inset in Figure 2A, blue trace) and resulted in a strong decrease in both NAD(P)H (oxidation phase by 71.6 ± 5.5% (from À 2.51 ± 0.26% to À 0.62 ± 0.06% amplitude), overshoot by 83.7±3% (from 4.38±0.46% to 0.68 ± 0.11% amplitude); n ¼ 10) and pO 2 (by 86.7 ± 1.9%) transients (Figures 2A and 2B).…”

Section: Contribution Of Neurons and Astrocytes To The Energy Productionmentioning

confidence: 99%

Glycolysis and Oxidative Phosphorylation in Neurons and Astrocytes during Network Activity in Hippocampal Slices

Ivanov

Malkov

Waseem

et al. 2013

J Cereb Blood Flow Metab

106

103

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Network activation triggers a significant energy metabolism increase in both neurons and astrocytes. Questions of the primary neuronal energy substrate (e.g., glucose vs. lactate) as well as the relative contributions of glycolysis and oxidative phosphorylation and their cellular origin (neurons vs. astrocytes) are still a matter of debates. Using simultaneous measurements of electrophysiological and metabolic parameters during synaptic stimulation in hippocampal slices from mature mice, we show that neurons and astrocytes use both glycolysis and oxidative phosphorylation to meet their energy demands. Supplementation or replacement of glucose in artificial cerebrospinal fluid (ACSF) with pyruvate or lactate strongly modifies parameters related to network activity-triggered energy metabolism. These effects are not induced by changes in ATP content, pH i , [Ca 2 þ ] i or accumulation of reactive oxygen species. Our results suggest that during network activation, a significant fraction of NAD(P)H response (its overshoot phase) corresponds to glycolysis and the changes in cytosolic NAD(P)H and mitochondrial FAD are coupled. Our data do not support the hypothesis of a preferential utilization of astrocyte-released lactate by neurons during network activation in slices-instead, we show that during such activity glucose is an effective energy substrate for both neurons and astrocytes. Keywords: astrocytes; energy metabolism; glycolysis; lactate; network activity; neurons INTRODUCTION High cellular energy demands during network activation are met by upregulation of cytosolic glycolysis and mitochondrial oxidative phosphorylation. Mitochondrial metabolism provides most of the ATP but glycolysis is also enhanced and may contribute to the energy production. Journal of Cerebral Blood1-4 For elucidation of the cellular basis of neuroenergetics, measurements of metabolic signals including the oxygen utilization and NAD(P)H/FAD autofluorescence provide valuable information for connecting energy metabolism with neuronal activity. NADH (reduced form) is fluorescent when excited with UV light whereas NAD þ is not, leading to a decrease in observed fluorescence as a result of NADH oxidation. In contrast, FAD (oxidized form) is fluorescent, so the oxidation of FADH 2 to FAD causes an increase in fluorescence. The fluorescence of NADH cannot be separated from that of NADPH and their emission is measured in concert (NAD(P)H). NAD(P)H fluorescence represents a 'mixed' signal since this cofactor can be produced by both glycolysis and mitochondria, whereas FAD fluorescence is entirely mitochondrial. 5,6 Measurements of these parameters in combination with electrophysiological recordings have been used in many studies to monitor the energy status during neuronal activity in brain tissues.Typically, NAD(P)H transients induced by synaptic stimulation have a characteristic biphasic waveform: the initial short dip is followed by a long-lasting overshoot. While there exists a common

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“…The relative distribution between PC and whole brain PDH activity can be calculated to be 14.5 % by analysis of labelling patterns in glutamate, glutamine, aspartate and GABA. The equivalent value in adult brain is approximately 10 % [40].…”

Section: Pyruvate Carboxylation (Anaplerosis)mentioning

confidence: 99%

Neuron–Astrocyte Interactions, Pyruvate Carboxylation and the Pentose Phosphate Pathway in the Neonatal Rat Brain

et al. 2013

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Glucose and acetate metabolism and the synthesis of amino acid neurotransmitters, anaplerosis, glutamate-glutamine cycling and the pentose phosphate pathway (PPP) have been extensively investigated in the adult, but not the neonatal rat brain. To do this, 7 day postnatal (P7) rats were injected with [1-C]glucose and [1,2-C]acetate and sacrificed 5, 10, 15, 30 and 45 min later. Adult rats were injected and sacrificed after 15 min. To analyse pyruvate carboxylation and PPP activity during development, P7 rats received [1,2-C]glucose and were sacrificed 30 min later. Brain extracts were analysed using Hand C-NMR spectroscopy. The neonatal brain contained lower levels of glutamate, aspartate and N-acetylaspartate but similar levels of GABA and glutamine compared to adults. Metabolism of [1-C]glucose at the acetyl CoA stage was reduced much more than that of [1,2-C]acetate. The transfer of glutamate from neurons to astrocytes was greatly reduced while transfer of glutamine from astrocytes to glutamatergic neurons was relatively higher compared to adults. However, transport of glutamine from astrocytes to GABAergic neurons was lower. Using [1,2-C]glucose it could be shown that despite much lower pyruvate carboxylation, relatively more pyruvate from glycolysis was directed towards anaplerosis than pyruvate dehydrogenation in astrocytes compared to reports from the adult brain. Moreover, the ratio of PPP/glucose metabolism was higher in P7 compared to adult brain. Our findings indicate that only the part of the glutamateglutamine cycle that transfers glutamine from astrocytes to neurons is operating in the After uptake into the cell, glucose (via pyruvate from glycolysis) and acetate can be converted to the TCA cycle substrate acetyl CoA. It has been reported that glucose oxidation is lower and that the average time the metabolites stay in the TCA cycle before conversion to substances such as neurotransmitters glutamate and thereafter γ-amino butyric acid (GABA) is longer in the neonatal compared to the adult brain [7]. This is in part attributed to the low levels of enzymes for pyruvate metabolism and oxidative glucose metabolism in the postnatal period [8].Pyruvate carboxylase, the brain's exclusive anaplerotic enzyme [9], is present in astrocytes only [10], and is of major importance for glial metabolic support of neurotransmission. Pyruvate carboxylase content is low in the neonatal period and increases 15-fold up to young adult age (postnatal day 30-40) when the level reaches a plateau [8].Most of the glutamate in the brain is found in neurons and is released into the synapse after depolarisation [11]. The ability of astrocytes to take up glutamate from the synapse and convert it into glutamine by the astrocyte specific enzyme glutamine synthetase [12] is vital for normal metabolic homeostasis and as a defence mechanism against excitotoxicity [13]. The subsequent transfer of glutamine from astrocytes to neurons for deamidation to glutamate closes the glutamate-

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Astrocytic energy metabolism and glutamate formation — relevance for 13C-NMR spectroscopy and importance of cytosolic/mitochondrial trafficking

Cited by 66 publications

References 101 publications

Glioma Cells with the IDH1 Mutation Modulate Metabolic Fractional Flux through Pyruvate Carboxylase

Glioma Cells with the IDH1 Mutation Modulate Metabolic Fractional Flux through Pyruvate Carboxylase

Glycolysis and Oxidative Phosphorylation in Neurons and Astrocytes during Network Activity in Hippocampal Slices

Neuron–Astrocyte Interactions, Pyruvate Carboxylation and the Pentose Phosphate Pathway in the Neonatal Rat Brain

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