Exchange-mediated dilution of brain lactate specific activity: implications for the origin of glutamate dilution and the contributions of glutamine dilution and other pathways

In vivo calibration of genetically encoded metabolite biosensors must account for metabolite metabolism during calibration and cellular volume

Isotopic assays of brain glucose utilization rates have been used for more than four decades to establish relationships between energetics, functional activity, and neurotransmitter cycling. Limitations of these methods include the relatively long time (1-60 min) for the determination of labeled metabolite levels and the lack of cellular resolution. Identification and quantification of fuels for neurons and astrocytes that support activation and higher brain functions are a major, unresolved issues. Glycolysis is preferentially up-regulated during activation even though oxygen level and supply are adequate, causing lactate concentrations to quickly rise during alerting, sensory processing, cognitive tasks, and memory consolidation. However, the fate of lactate (rapid release from brain or cell-cell shuttling coupled with local oxidation) is long disputed. Genetically encoded biosensors can determine intracellular metabolite concentrations and report real-time lactate level responses to sensory, behavioral, and biochemical challenges at the cellular level. Kinetics and time courses of cellular lactate concentration changes are informative, but accurate biosensor calibration is required for quantitative comparisons of lactate levels in astrocytes and neurons. An in vivo calibration procedure for the Laconic lactate biosensor involves intracellular lactate depletion by intravenous pyruvate-mediated trans-acceleration of lactate efflux followed by sensor saturation by intravenous infusion of high doses of lactate plus ammonium chloride. In the present paper, the validity of this procedure is questioned because rapid lactate-pyruvate interconversion in blood, preferential neuronal oxidation of both monocarboxylates, on-going glycolytic metabolism, and cellular volumes were not taken into account. Calibration pitfalls for the Laconic lactate biosensor also apply to other metabolite biosensors that are standardized in vivo by infusion of substrates that can be metabolized in peripheral tissues. We discuss how technical shortcomings negate the conclusion that Laconic sensor calibrations support the existence of an in vivo astrocyte-neuron lactate concentration gradient linked to lactate shuttling from astrocytes to neurons to fuel neuronal activity.

Brain energy metabolism: A roadmap for future research

Although we have learned much about how the brain fuels its functions over the last decades, there remains much still to discover in an organ that is so complex. This article lays out major gaps in our knowledge of interrelationships between brain metabolism and brain function, including biochemical, cellular, and subcellular aspects of functional metabolism and its imaging in adult brain, as well as during development, aging, and disease. The focus is on unknowns in metabolism of major brain substrates and associated transporters, the roles of insulin and of lipid droplets, the emerging role of metabolism in microglia, mysteries about the major brain cofactor and signaling molecule NAD+, as well as unsolved problems underlying brain metabolism in pathologies such as traumatic brain injury, epilepsy, and metabolic downregulation during hibernation. It describes our current level of understanding of these facets of brain energy metabolism as well as a roadmap for future research.

Triheptanoin alters [U-13C6]-glucose incorporation into glycolytic intermediates and increases TCA cycling by normalizing the activities of pyruvate dehydrogenase and oxoglutarate dehydrogenase in a chronic epilepsy mouse model

Triheptanoin is anticonvulsant in several seizure models. Here, we investigated changes in glucose metabolism by triheptanoin interictally in the chronic stage of the pilocarpine mouse epilepsy model. After injection of [U-¹³C₆]-glucose (i.p.), enrichments of ¹³C in intermediates of glycolysis and the tricarboxylic acid (TCA) cycle were quantified in hippocampal extracts and maximal activities of enzymes in each pathway were measured. The enrichment of ¹³C glucose in plasma was similar across all groups. Despite this, we observed reductions in incorporation of ¹³C in several glycolytic intermediates compared to control mice suggesting glucose utilization may be impaired and/or glycogenolysis increased in the untreated interictal hippocampus. Triheptanoin prevented the interictal reductions of ¹³C incorporation in most glycolytic intermediates, suggesting it increased glucose utilization or - as an additional astrocytic fuel - it decreased glycogen breakdown. In the TCA cycle metabolites, the incorporation of ¹³C was reduced in the interictal state. Triheptanoin restored the correlation between ¹³C enrichments of pyruvate relative to most of the TCA cycle intermediates in "epileptic" mice. Triheptanoin also prevented the reductions of hippocampal pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase activities. Decreased glycogen breakdown and increased glucose utilization and metabolism via the TCA cycle in epileptogenic brain areas may contribute to triheptanoin's anticonvulsant effects.

Differential Metabolism of Medium-Chain Fatty Acids in Differentiated Human-Induced Pluripotent Stem Cell-Derived Astrocytes

Medium-chain triglyceride (MCT) ketogenic diets increase ketone bodies, which are believed to act as alternative energy substrates in the injured brain. Octanoic (C8:0) and decanoic (C10:0) acids, which produce ketone bodies through β-oxidation, are used as part of MCT ketogenic diets. Although the ketogenic role of MCT is well-established, it remains unclear how the network metabolism underlying β-oxidation of these medium-chain fatty acids (MCFA) differ. We aim to elucidate basal β-oxidation of these commonly used MCFA at the cellular level. Human-induced pluripotent stem cell-derived (iPSC) astrocytes were incubated with [U-¹³C]-C8:0 or [U-¹³C]-C10:0, and the fractional enrichments (FE) of the derivatives were used for metabolic flux analysis. Data indicate higher extracellular concentrations and faster secretion rates of β-hydroxybutyrate (βHB) and acetoacetate (AcAc) with C8:0 than C10:0, and an important contribution from unlabeled substrates. Flux analysis indicates opposite direction of metabolic flux between the MCFA intermediates C6:0 and C8:0, with an important contribution of unlabeled sources to the elongation in the C10:0 condition, suggesting different β-oxidation pathways. Finally, larger intracellular glutathione concentrations and secretions of 3-OH-C10:0 and C6:0 were measured in C10:0-treated astrocytes. These findings reveal MCFA-specific ketogenic properties. Our results provide insights into designing different MCT-based ketogenic diets to target specific health benefits.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

Does shuttling of glycogen-derived lactate from astrocytes to neurons take place during neurotransmission and memory consolidation?

Glycogen levels in resting brain and its utilization rates during brain activation are high, but the functions fulfilled by glycogenolysis in living brain are poorly understood. Studies in cultured astrocytes have identified glycogen as the preferred fuel to provide ATP for Na⁺ ,K⁺ -ATPase for the uptake of extracellular K⁺ and for Ca²⁺ -ATPase to pump Ca²⁺ into the endoplasmic reticulum. Studies in astrocyte-neuron co-cultures led to the suggestion that glycogen-derived lactate is shuttled to neurons as oxidative fuel to support glutamatergic neurotransmission. Furthermore, both knockout of brain glycogen synthase and inhibition of glycogenolysis prior to a memory-evoking event impair memory consolidation, and shuttling of glycogen-derived lactate as neuronal fuel was postulated to be required for memory. However, lactate shuttling has not been measured in any of these studies, and procedures to inhibit glycogenolysis and neuronal lactate uptake are not specific. Testable alternative mechanisms to explain the observed findings are proposed: (i) disruption of K⁺ and Ca²⁺ homeostasis, (ii) release of gliotransmitters, (iii) imposition of an energy crisis on astrocytes and neurons by inhibition of mitochondrial pyruvate transport by compounds used to block neuronal monocarboxylic acid transporters, and (iv) inhibition of astrocytic filopodial movements that secondarily interfere with glutamate and K⁺ uptake from the synaptic cleft. Evidence that most pyruvate/lactate derived from glycogen is not oxidized and does not accumulate suggests predominant glycolytic metabolism of glycogen to support astrocytic energy demands. Sparing of blood-borne glucose for use by neurons is a reasonable explanation for the requirement for glycogenolysis in neurotransmission and memory processing.

Development of a Model to Test Whether Glycogenolysis Can Support Astrocytic Energy Demands of Na+, K+-ATPase and Glutamate-Glutamine Cycling, Sparing an Equivalent Amount of Glucose for Neurons

Recent studies of glycogen in brain have suggested a much more important role in brain energy metabolism and function than previously recognized, including findings of much higher than previously recognized concentrations, consumption at substantial rates compared with utilization of blood-borne glucose, and involvement in ion pumping and in neurotransmission and memory. However, it remains unclear how glycogenolysis is coupled to neuronal activity and provides support for neuronal as well as astroglial function. At present, quantitative aspects of glycogenolysis in brain functions are very difficult to assess due to its metabolic lability, heterogeneous distributions within and among cells, and extreme sensitivity to physiological stimuli. To begin to address this problem, the present study develops a model based on pathway fluxes, mass balance, and literature relevant to functions and turnover of pathways that intersect with glycogen mobilization. A series of equations is developed to describe the stoichiometric relationships between net glycogen consumption that is predominantly in astrocytes with the rate of the glutamate-glutamine cycle, rates of astrocytic and neuronal glycolytic and oxidative metabolism, and the energetics of sodium/potassium pumping in astrocytes and neurons during brain activation. Literature supporting the assumptions of the model is discussed in detail. The overall conclusion is that astrocyte glycogen metabolism is primarily coupled to neuronal function via fueling glycolytically pumping of Na⁺ and K⁺ and sparing glucose for neuronal oxidation, as opposed to previous proposals of coupling neurotransmission via glutamate transport, lactate shuttling, and neuronal oxidation of lactate.

Acetate metabolism does not reflect astrocytic activity, contributes directly to GABA synthesis, and is increased by silent information regulator 1 activation

[¹³ C]Acetate is known to label metabolites preferentially in astrocytes rather than neurons and it has consequently been used as a marker for astrocytic activity. Recent discoveries suggest that control of acetate metabolism and its contributions to the synthesis of metabolites in brain is not as simple as first thought. Here, using a Guinea pig brain cortical tissue slice model metabolizing [1-¹³ C]D-glucose and [1,2-¹³ C]acetate, we investigated control of acetate metabolism and the degree to which it reflects astrocytic activity. Using a range of [1,2-¹³ C]acetate concentrations, we found that acetate is a poor substrate for metabolism and will inhibit metabolism of itself and of glucose at concentrations in excess of 2 mmol/L. By activating astrocytes using potassium depolarization, we found that use of [1,2-¹³ C]acetate to synthesize glutamine decreases significantly under these conditions showing that acetate metabolism does not necessarily reflect astrocytic activity. By blocking synthesis of glutamine using methionine sulfoximine, we found that significant amount of [1,2-¹³ C]acetate are still incorporated into GABA and its metabolic precursors in neurons, with around 30% of the GABA synthesized from [1,2-¹³ C]acetate likely to be made directly in neurons rather than from glutamine supplied by astrocytes. Finally, to test whether activity of the acetate metabolizing enzyme acetyl-CoA synthetase is under acetylation control in the brain, we incubated slices with the AceCS1 deacetylase silent information regulator 1 (SIRT1) activator SRT 1720 and showed consequential increased incorporation of [1,2-¹³ C]acetate into metabolites. Taken together, these data show that acetate metabolism is not directly nor exclusively related to astrocytic metabolic activity, that use of acetate is related to enzyme acetylation and that acetate is directly metabolized to a significant degree in GABAergic neurons. Changes in acetate metabolism should be interpreted as modulation of metabolism through changes in cellular energetic status via altered enzyme acetylation levels rather than simply as an adjustment of glial-neuronal metabolic activity.

Comparison of Glutamate Turnover in Nerve Terminals and Brain Tissue During [1,6-13C2]Glucose Metabolism in Anesthetized Rats

The ¹³C turnover of neurotransmitter amino acids (glutamate, GABA and aspartate) were determined from extracts of forebrain nerve terminals and brain homogenate, and fronto-parietal cortex from anesthetized rats undergoing timed infusions of [1,6-¹³C₂]glucose or [2-¹³C]acetate. Nerve terminal ¹³C fractional labeling of glutamate and aspartate was lower than those in whole cortical tissue at all times measured (up to 120 min), suggesting either the presence of a constant dilution flux from an unlabeled substrate or an unlabeled (effectively non-communicating on the measurement timescale) glutamate pool in the nerve terminals. Half times of ¹³C labeling from [1,6-¹³C₂]glucose, as estimated by least squares exponential fitting to the time course data, were longer for nerve terminals (Glu_C4, 21.8 min; GABA_C2 21.0 min) compared to cortical tissue (Glu_C4, 12.4 min; GABA_C2, 14.5 min), except for Asp_C3, which was similar (26.5 vs. 27.0 min). The slower turnover of glutamate in the nerve terminals (but not GABA) compared to the cortex may reflect selective effects of anesthesia on activity-dependent glucose use, which might be more pronounced in the terminals. The ¹³C labeling ratio for glutamate-C4 from [2-¹³C]acetate over that of ¹³C-glucose was twice as large in nerve terminals compared to cortex, suggesting that astroglial glutamine under the ¹³C glucose infusion was the likely source of much of the nerve terminal dilution. The net replenishment of most of the nerve terminal amino acid pools occurs directly via trafficking of astroglial glutamine.

Fluctuations in Cytosolic Calcium Regulate the Neuronal Malate-Aspartate NADH Shuttle: Implications for Neuronal Energy Metabolism

The malate-aspartate NADH shuttle (MAS) operates in neurons and other cells to translocate reducing equivalents from the cytosol to the mitochondrial matrix, thus allowing a continued flux through the glycolytic pathway and metabolism of extracellular lactate. Recent discoveries have taught us that MAS is regulated by fluctuations in cytosolic Ca(2+) levels, and that this regulation is required to maintain a tight coupling between neuronal activity and mitochondrial respiration and oxidative phosphorylation. At cytosolic Ca(2+) fluctuations below the threshold of the mitochondrial calcium uniporter, there is a positive correlation between Ca(2+) and MAS activity; however, if cytosolic Ca(2+) increases above the threshold, MAS activity is thought to be reduced by an intricate mechanism. The latter forces the neurons to partly rely on anaerobic glycolysis producing lactate that may be metabolized subsequently, by neurons or other cells. In this review, we will discuss the evidence for Ca(2+)-mediated regulation of MAS that have been uncovered over the last decade or so, together with the need for further verification, and examine the metabolic ramifications for neurons.

Contributions of glycogen to astrocytic energetics during brain activation

Glycogen is the major store of glucose in brain and is mainly in astrocytes. Brain glycogen levels in unstimulated, carefully-handled rats are 10-12 μmol/g, and assuming that astrocytes account for half the brain mass, astrocytic glycogen content is twice as high. Glycogen turnover is slow under basal conditions, but it is mobilized during activation. There is no net increase in incorporation of label from glucose during activation, whereas label release from pre-labeled glycogen exceeds net glycogen consumption, which increases during stronger stimuli. Because glycogen level is restored by non-oxidative metabolism, astrocytes can influence the global ratio of oxygen to glucose utilization. Compensatory increases in utilization of blood glucose during inhibition of glycogen phosphorylase are large and approximate glycogenolysis rates during sensory stimulation. In contrast, glycogenolysis rates during hypoglycemia are low due to continued glucose delivery and oxidation of endogenous substrates; rates that preserve neuronal function in the absence of glucose are also low, probably due to metabolite oxidation. Modeling studies predict that glycogenolysis maintains a high level of glucose-6-phosphate in astrocytes to maintain feedback inhibition of hexokinase, thereby diverting glucose for use by neurons. The fate of glycogen carbon in vivo is not known, but lactate efflux from brain best accounts for the major metabolic characteristics during activation of living brain. Substantial shuttling coupled with oxidation of glycogen-derived lactate is inconsistent with available evidence. Glycogen has important roles in astrocytic energetics, including glucose sparing, control of extracellular K(+) level, oxidative stress management, and memory consolidation; it is a multi-functional compound.

Characterization of cerebral glutamine uptake from blood in the mouse brain: implications for metabolic modeling of 13C NMR data

(13)C Nuclear Magnetic Resonance (NMR) studies of rodent and human brain using [1-(13)C]/[1,6-(13)C2]glucose as labeled substrate have consistently found a lower enrichment (∼25% to 30%) of glutamine-C4 compared with glutamate-C4 at isotopic steady state. The source of this isotope dilution has not been established experimentally but may potentially arise either from blood/brain exchange of glutamine or from metabolism of unlabeled substrates in astrocytes, where glutamine synthesis occurs. In this study, the contribution of the former was evaluated ex vivo using (1)H-[(13)C]-NMR spectroscopy together with intravenous infusion of [U-(13)C5]glutamine for 3, 15, 30, and 60 minutes in mice. (13)C labeling of brain glutamine was found to be saturated at plasma glutamine levels >1.0 mmol/L. Fitting a blood-astrocyte-neuron metabolic model to the (13)C enrichment time courses of glutamate and glutamine yielded the value of glutamine influx, VGln(in), 0.036±0.002 μmol/g per minute for plasma glutamine of 1.8 mmol/L. For physiologic plasma glutamine level (∼0.6 mmol/L), VGln(in) would be ∼0.010 μmol/g per minute, which corresponds to ∼6% of the glutamine synthesis rate and rises to ∼11% for saturating blood glutamine concentrations. Thus, glutamine influx from blood contributes at most ∼20% to the dilution of astroglial glutamine-C4 consistently seen in metabolic studies using [1-(13)C]glucose.

Astrocytic energetics during excitatory neurotransmission: What are contributions of glutamate oxidation and glycolysis?

Astrocytic energetics of excitatory neurotransmission is controversial due to discrepant findings in different experimental systems in vitro and in vivo. The energy requirements of glutamate uptake are believed by some researchers to be satisfied by glycolysis coupled with shuttling of lactate to neurons for oxidation. However, astrocytes increase glycogenolysis and oxidative metabolism during sensory stimulation in vivo, indicating that other sources of energy are used by astrocytes during brain activation. Furthermore, glutamate uptake into cultured astrocytes stimulates glutamate oxidation and oxygen consumption, and glutamate maintains respiration as well as glucose. The neurotransmitter pool of glutamate is associated with the faster component of total glutamate turnover in vivo, and use of neurotransmitter glutamate to fuel its own uptake by oxidation-competent perisynaptic processes has two advantages, substrate is supplied concomitant with demand, and glutamate spares glucose for use by neurons and astrocytes. Some, but not all, perisynaptic processes of astrocytes in adult rodent brain contain mitochondria, and oxidation of only a small fraction of the neurotransmitter glutamate taken up into these structures would be sufficient to supply the ATP required for sodium extrusion and conversion of glutamate to glutamine. Glycolysis would, however, be required in perisynaptic processes lacking oxidative capacity. Three lines of evidence indicate that critical cornerstones of the astrocyte-to-neuron lactate shuttle model are not established and normal brain does not need lactate as supplemental fuel: (i) rapid onset of hemodynamic responses to activation delivers oxygen and glucose in excess of demand, (ii) total glucose utilization greatly exceeds glucose oxidation in awake rodents during activation, indicating that the lactate generated is released, not locally oxidized, and (iii) glutamate-induced glycolysis is not a robust phenotype of all astrocyte cultures. Various metabolic pathways, including glutamate oxidation and glycolysis with lactate release, contribute to cellular energy demands of excitatory neurotransmission.

Modeling the glutamate-glutamine neurotransmitter cycle

Glutamate is the principal excitatory neurotransmitter in brain. Although it is rapidly synthesized from glucose in neural tissues the biochemical processes for replenishing the neurotransmitter glutamate after glutamate release involve the glutamate–glutamine cycle. Numerous in vivo¹³C magnetic resonance spectroscopy (MRS) experiments since 1994 by different laboratories have consistently concluded: (1) the glutamate–glutamine cycle is a major metabolic pathway with a flux rate substantially greater than those suggested by early studies of cell cultures and brain slices; (2) the glutamate–glutamine cycle is coupled to a large portion of the total energy demand of brain function. The dual roles of glutamate as the principal neurotransmitter in the CNS and as a key metabolite linking carbon and nitrogen metabolism make it possible to probe glutamate neurotransmitter cycling using MRS by measuring the labeling kinetics of glutamate and glutamine. At the same time, comparing to non-amino acid neurotransmitters, the added complexity makes it more challenging to quantitatively separate neurotransmission events from metabolism. Over the past few years our understanding of the neuronal-astroglial two-compartment metabolic model of the glutamate–glutamine cycle has been greatly advanced. In particular, the importance of isotopic dilution of glutamine in determining the glutamate–glutamine cycling rate using [1−¹³C] or [1,6-¹³C₂] glucose has been demonstrated and reproduced by different laboratories. In this article, recent developments in the two-compartment modeling of the glutamate–glutamine cycle are reviewed. In particular, the effects of isotopic dilution of glutamine on various labeling strategies for determining the glutamate–glutamine cycling rate are analyzed. Experimental strategies for measuring the glutamate–glutamine cycling flux that are insensitive to isotopic dilution of glutamine are also suggested.

Resistance of brain glucose metabolism to thiopental-induced CNS depression in newborn piglets

The transition from mild sedation to deep anaesthesia is marked by the phenomenon of burst suppression (BS). FDG-PET studies show that the cerebral metabolic rate for glucose (CMRglc) declines dramatically with onset of BS in the adult brain. Global CMRglc increases substantially in the post-natal period and achieves its maximum in preadolescence. However, the impact of post-natal brain development on the vulnerability of CMRglc to the onset of BS has not been documented. Therefore, cerebral blood flow and metabolism were measured using a variant of the Kety-Schmidt method, in conjunction with quantitative regional estimation of brain glucose uptake by FDG-PET in groups of neonate and juvenile pigs, under a condition of light sedation or after induction of deep anaesthesia with thiopental. Quantification of simultaneous ECoG recordings was used to establish the correlation between anaesthesia-related changes in brain electrical activity and the observed cerebrometabolic changes. In the condition of light sedation the magnitude of CMRglc was approximately 20% higher in the older pigs, with the greatest developmental increase evident in the cerebral cortex and basal ganglia (P<0.05). Onset of BS was associated with 20-40% declines in CMRglc. Subtraction of the mean parametric maps for CMRglc showed the absolute reductions in CMRglc evoked by thiopental anaesthesia to be two-fold greater in the pre-adolescent pigs than in the neonates (P<0.05). Thus, the lesser suppression of brain energy demand of neonate brain during deep anaesthesia represents a reduced part of thiopental suppressing brain metabolism in neonates.

Substrate competition studies demonstrate oxidative metabolism of glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate in cortical astrocytes from rat brain

It is well established that astrocytes can utilize many substrates to support oxidative energy metabolism; however, use of energy substrates in the presence of other substrates, as would occur in vivo, has not been systematically evaluated. Substrate competition studies were used to determine changes in the rates of (14)CO(2) production since little is known about the interaction of energy substrates in astrocytes. The rates of (14)CO(2) production from 1 mM D-[6-(14)C]glucose, L-[U-(14)C]glutamate, L-[U-(14)C]glutamine, D-3-hydroxy[3-(14)C]butyrate, L-[U-(14)C]lactate and L-[U-(14)C]malate by primary cultures of astrocytes from rat brain were determined to be 1.17 ± 0.19, 85.30 ± 12.25, 28.04 ± 2.84, 13.55 ± 4.56, 14.84 ± 2.40 and 5.20 ± 1.20 nmol/h/mg protein (mean ± SEM), respectively. The rate of (14)CO(2) production from glutamate oxidation was higher than that of the other substrates Addition of unlabeled glutamate significantly decreased the rates of (14)CO(2) production from all other substrates studied; however, glutamate oxidation was not altered by the addition of any of the other substrates. The rate of (14)CO(2) production of glutamine was decreased by glutamate, but not altered by other substrates. The rate of (14)CO(2) production from glucose was significantly decreased by the addition of unlabeled glutamate, glutamine or lactate, but not by 3-hydroxybutyrate or malate. Addition of unlabeled glucose did not significantly alter the (14)CO(2) production from any other substrate. (14)CO(2) production from lactate was decreased by the addition of unlabeled glutamine or glutamate and increased by addition of malate. The (14)CO(2) production from malate was decreased by the addition of unlabeled glutamate or lactate, but was not altered by the other substrates. The substrate utilization for oxidative energy metabolism in astrocytes is very different than the profile previously reported for synaptic terminals. These studies demonstrate the potential use of multiple substrates including glucose, glutamate, glutamine, lactate and 3-hydroxybutyrate as energy substrates for astrocytes. The data also provide evidence of interactions of substrates and multiple compartments of TCA cycle activity in cultured astrocytes.

Fueling and imaging brain activation

Metabolic signals are used for imaging and spectroscopic studies of brain function and disease and to elucidate the cellular basis of neuroenergetics. The major fuel for activated neurons and the models for neuron–astrocyte interactions have been controversial because discordant results are obtained in different experimental systems, some of which do not correspond to adult brain. In rats, the infrastructure to support the high energetic demands of adult brain is acquired during postnatal development and matures after weaning. The brain's capacity to supply and metabolize glucose and oxygen exceeds demand over a wide range of rates, and the hyperaemic response to functional activation is rapid. Oxidative metabolism provides most ATP, but glycolysis is frequently preferentially up-regulated during activation. Underestimation of glucose utilization rates with labelled glucose arises from increased lactate production, lactate diffusion via transporters and astrocytic gap junctions, and lactate release to blood and perivascular drainage. Increased pentose shunt pathway flux also causes label loss from C1 of glucose. Glucose analogues are used to assay cellular activities, but interpretation of results is uncertain due to insufficient characterization of transport and phosphorylation kinetics. Brain activation in subjects with low blood-lactate levels causes a brain-to-blood lactate gradient, with rapid lactate release. In contrast, lactate flooding of brain during physical activity or infusion provides an opportunistic, supplemental fuel. Available evidence indicates that lactate shuttling coupled to its local oxidation during activation is a small fraction of glucose oxidation. Developmental, experimental, and physiological context is critical for interpretation of metabolic studies in terms of theoretical models.

Brain lactate metabolism: the discoveries and the controversies

Potential roles for lactate in the energetics of brain activation have changed radically during the past three decades, shifting from waste product to supplemental fuel and signaling molecule. Current models for lactate transport and metabolism involving cellular responses to excitatory neurotransmission are highly debated, owing, in part, to discordant results obtained in different experimental systems and conditions. Major conclusions drawn from tabular data summarizing results obtained in many laboratories are as follows: Glutamate-stimulated glycolysis is not an inherent property of all astrocyte cultures. Synaptosomes from the adult brain and many preparations of cultured neurons have high capacities to increase glucose transport, glycolysis, and glucose-supported respiration, and pathway rates are stimulated by glutamate and compounds that enhance metabolic demand. Lactate accumulation in activated tissue is a minor fraction of glucose metabolized and does not reflect pathway fluxes. Brain activation in subjects with low plasma lactate causes outward, brain-to-blood lactate gradients, and lactate is quickly released in substantial amounts. Lactate utilization by the adult brain increases during lactate infusions and strenuous exercise that markedly increase blood lactate levels. Lactate can be an 'opportunistic', glucose-sparing substrate when present in high amounts, but most evidence supports glucose as the major fuel for normal, activated brain.

A comprehensive metabolic profile of cultured astrocytes using isotopic transient metabolic flux analysis and C-labeled glucose

Metabolic models have been used to elucidate important aspects of brain metabolism in recent years. This work applies for the first time the concept of isotopic transient 13C metabolic flux analysis (MFA) to estimate intracellular fluxes in primary cultures of astrocytes. This methodology comprehensively explores the information provided by 13C labeling time-courses of intracellular metabolites after administration of a 13C-labeled substrate. Cells were incubated with medium containing [1-13C]glucose for 24 h and samples of cell supernatant and extracts collected at different time points were then analyzed by mass spectrometry and/or high performance liquid chromatography. Metabolic fluxes were estimated by fitting a carbon labeling network model to isotopomer profiles experimentally determined. Both the fast isotopic equilibrium of glycolytic metabolite pools and the slow labeling dynamics of TCA cycle intermediates are described well by the model. The large pools of glutamate and aspartate which are linked to the TCA cycle via reversible aminotransferase reactions are likely to be responsible for the observed delay in equilibration of TCA cycle intermediates. Furthermore, it was estimated that 11% of the glucose taken up by astrocytes was diverted to the pentose phosphate pathway. In addition, considerable fluxes through pyruvate carboxylase [PC; PC/pyruvate dehydrogenase (PDH) ratio = 0.5], malic enzyme (5% of the total pyruvate production), and catabolism of branched-chained amino acids (contributing with ∼40% to total acetyl-CoA produced) confirmed the significance of these pathways to astrocytic metabolism. Consistent with the need of maintaining cytosolic redox potential, the fluxes through the malate–aspartate shuttle and the PDH pathway were comparable. Finally, the estimated glutamate/α-ketoglutarate exchange rate (∼0.7 μmol mg prot−1 h−1) was similar to the TCA cycle flux. In conclusion, this work demonstrates the potential of isotopic transient MFA for a comprehensive analysis of energy metabolism.

Noninvasive measurement of brain glycogen by nuclear magnetic resonance spectroscopy and its application to the study of brain metabolism

Glycogen is the reservoir for glucose in the brain. Beyond the general agreement that glycogen serves as an energy source in the central nervous system, its exact role in brain energy metabolism has yet to be elucidated. Experiments performed in cell and tissue culture and animals have shown that glycogen content is affected by several factors, including glucose, insulin, neurotransmitters, and neuronal activation. The study of in vivo glycogen metabolism has been hindered by the inability to measure glycogen noninvasively, but, in the past several years, the development of a noninvasive localized (13) C nuclear magnetic resonance (NMR) spectroscopy method has allowed the study of glycogen metabolism in the conscious human. With this technique, (13) C-glucose is administered intravenously, and its incorporation into and washout from brain glycogen is tracked. One application of this method has been to the study of brain glycogen metabolism in humans during hypoglycemia: data have shown that mobilization of brain glycogen is augmented during hypoglycemia, and, after a single episode of hypoglycemia, glycogen synthesis rate is increased, suggesting that glycogen stores rebound to levels greater than baseline. Such studies suggest that glycogen may serve as a potential energy reservoir in hypoglycemia and may participate in the brain's adaptation to recurrent hypoglycemia and eventual development of hypoglycemia unawareness. Beyond this focused area of study, (13) C NMR spectroscopy has a broad potential for application in the study of brain glycogen metabolism and carries the promise of a better understanding of the role of brain glycogen in diabetes and other conditions.

Compartmentalized Cerebral Metabolism of [1,6-(13)C]Glucose Determined by in vivo (13)C NMR Spectroscopy at 14.1 T

Cerebral metabolism is compartmentalized between neurons and glia. Although glial glycolysis is thought to largely sustain the energetic requirements of neurotransmission while oxidative metabolism takes place mainly in neurons, this hypothesis is matter of debate. The compartmentalization of cerebral metabolic fluxes can be determined by (13)C nuclear magnetic resonance (NMR) spectroscopy upon infusion of (13)C-enriched compounds, especially glucose. Rats under light α-chloralose anesthesia were infused with [1,6-(13)C]glucose and (13)C enrichment in the brain metabolites was measured by (13)C NMR spectroscopy with high sensitivity and spectral resolution at 14.1 T. This allowed determining (13)C enrichment curves of amino acid carbons with high reproducibility and to reliably estimate cerebral metabolic fluxes (mean error of 8%). We further found that TCA cycle intermediates are not required for flux determination in mathematical models of brain metabolism. Neuronal tricarboxylic acid cycle rate (V(TCA)) and neurotransmission rate (V(NT)) were 0.45 ± 0.01 and 0.11 ± 0.01 μmol/g/min, respectively. Glial V(TCA) was found to be 38 ± 3% of total cerebral oxidative metabolism, accounting for more than half of neuronal oxidative metabolism. Furthermore, glial anaplerotic pyruvate carboxylation rate (V(PC)) was 0.069 ± 0.004 μmol/g/min, i.e., 25 ± 1% of the glial TCA cycle rate. These results support a role of glial cells as active partners of neurons during synaptic transmission beyond glycolytic metabolism.

Metabolic alterations induced by ischemia in primary cultures of astrocytes: merging 13C NMR spectroscopy and metabolic flux analysis

Disruption of brain energy metabolism is the hallmark of cerebral ischemia, a major cause of death worldwide. Astrocytes play a key role in the regulation of brain metabolism and their vulnerability to ischemia has been described. Aiming to quantify the effects of an ischemic insult in astrocytic metabolism, primary cultures of astrocytes were subjected to 5 h of oxygen and glucose deprivation in a bioreactor. Flux distributions, before and after ischemia, were estimated by metabolic flux analysis using isotopic information and the consumption/secretion rates of relevant extracellular metabolites as constraints. During ischemia and early recovery, 30% of cell death was observed; several metabolic alterations were also identified reflecting a metabolic response by the surviving cells. In the early recovery ( approximately 10 h), astrocytes up-regulated glucose utilization by 30% and increased the pentose phosphate pathway and tricarboxylic acid cycle fluxes by three and twofold, respectively. Additionally, a two to fivefold enhancement in branched-chain amino acids catabolism suggested the importance of anaplerotic molecules to the fast recovery of the energetic state, which was corroborated by measured cellular ATP levels. Glycolytic metabolism was predominant in the late recovery. In summary, this work demonstrates that changes in fluxes of key metabolic pathways are implicated in the recovery from ischemia in astrocytes.

Trafficking of glucose, lactate, and amyloid-beta from the inferior colliculus through perivascular routes

Metabolic brain imaging is widely used to evaluate brain function and disease, and quantitative assays require local retention of compounds used to register changes in cellular activity. As labeled metabolites of [1- and 6-(14)C]glucose are rapidly released in large quantities during brain activation, this study evaluated release of metabolites and proteins through perivascular fluid flow, a pathway that carries solutes from brain to peripheral lymphatic drainage sites. Assays with [3,4-(14)C]glucose ruled out local oxidation of glucose-derived lactate as a major contributor of label loss. Brief infusion of [1-(14)C]glucose and D-[(14)C]lactate into the inferior colliculus of conscious rats during acoustic stimulation labeled the meninges, consistent with perivascular clearance of [(14)C]metabolites from interstitial fluid. Microinfusion of Evans blue albumin and amyloid-beta(1-40) (Abeta) caused perivascular labeling in the inferior colliculus, labeled the surrounding meninges, and Abeta-labeled-specific blood vessels in the caudate and olfactory bulb and was deposited in cervical lymph nodes. Efflux of extracellular glucose, lactate, and Abeta into perivascular fluid pathways is a normal route for clearance of material from the inferior colliculus that contributes to underestimates of brain energetics. Convergence of 'watershed' drainage to common pathways may facilitate perivascular amyloid plaque formation and pathway obstruction in Alzheimer's disease.

Astrocytes are poised for lactate trafficking and release from activated brain and for supply of glucose to neurons

Brain is a highly-oxidative organ, but during activation, glycolytic flux is preferentially up-regulated even though oxygen supply is adequate. The biochemical and cellular basis of metabolic changes during brain activation and the fate of lactate produced within brain are important, unresolved issues central to understanding brain function, brain images, and spectroscopic data. Because in vivo brain imaging studies reveal rapid efflux of labeled glucose metabolites during activation, lactate trafficking among astrocytes and between astrocytes and neurons was examined after devising specific, real-time, sensitive enzymatic fluorescent assays to measure lactate and glucose levels in single cells in adult rat brain slices. Astrocytes have a 2- to 4-fold faster and higher capacity for lactate uptake from extracellular fluid and for lactate dispersal via the astrocytic syncytium compared to neuronal lactate uptake from extracellular fluid or shuttling of lactate to neurons from neighboring astrocytes. Astrocytes can also supply glucose to neurons as well as glucose can be taken up by neurons from extracellular fluid. Astrocytic networks can provide neuronal fuel and quickly remove lactate from activated glycolytic domains, and the lactate can be dispersed widely throughout the syncytium to endfeet along the vasculature for release to blood or other brain regions via perivascular fluid flow.

Metabolic effects of blocking lactate transport in brain cortical tissue slices using an inhibitor specific to MCT1 and MCT2

A novel inhibitor of lactate transport, AR-C122982, was used to study the effect of inhibiting the monocarboxylate transporters MCT1 and MCT2 on cortical brain slice metabolism. We studied metabolism of L-[3-13C]lactate, and D-[1-13C]glucose under a range of conditions. Experiments using L-[3-13C]lactate showed that the inhibitor AR-C122982 altered exchange of lactate. Under depolarizing conditions, net flux of label from D-[1-13C]glucose was barely altered by 10 or 100 nM AR-C122982. In the presence of AMPA or glutamate there were increases in net flux of label and metabolic pool sizes. These data suggest lactate may supply compartments in the brain not usually directly accessed by glucose. In general, it would appear that movement of lactate between cell types is not essential for metabolic activity, with the heavy metabolic workloads imposed being unaffected by inhibition of MCT1 and MCT2. Further experiments investigating the mechanism of operation of AR-C122982 are necessary to corroborate this finding.