Metabolism, Compartmentation, Transport and Production of Acetate in the Cortical Brain Tissue Slice

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.

Physiological acetic acid concentrations from ethanol metabolism stimulate accumbens shell medium spiny neurons via NMDAR activation in a sex-dependent manner

Recent studies have implicated the ethanol metabolite, acetic acid, as neuroactive, perhaps even more so than ethanol itself. In this study, we investigated sex-specific metabolism of ethanol (1, 2, and 4 g/kg) to acetic acid in vivo to guide electrophysiology experiments in the accumbens shell (NAcSh), a key node in the mammalian reward circuit. There was a sex-dependent difference in serum acetate production, quantified via ion chromatography only at the lowest dose of ethanol (males > females). Ex vivo electrophysiology recordings of NAcSh medium spiny neurons (MSN) in brain slices demonstrated that physiological concentrations of acetic acid (2 mM and 4 mM) increased NAcSh MSN excitability in both sexes. N-methyl-D-aspartate receptor (NMDAR) antagonists, AP5 and memantine, robustly attenuated the acetic acid-induced increase in excitability. Acetic acid-induced NMDAR-dependent inward currents were greater in females compared to males and were not estrous cycle dependent. These findings suggest a novel NMDAR-dependent mechanism by which the ethanol metabolite, acetic acid, may influence neurophysiological effects in a key reward circuit in the brain from ethanol consumption. Furthermore, these findings also highlight a specific sex-dependent sensitivity in females to acetic acid-NMDAR interactions. This may underlie their more rapid advancement to alcohol use disorder and increased risk of alcohol related neurodegeneration compared to males.

S100B Secretion in Astrocytes, Unlike C6 Glioma Cells, Is Downregulated by Lactate

S100B is a calcium-binding protein produced and secreted by astrocytes in response to various extracellular stimuli. C6 glioma cells are a lineage commonly employed for astroglial studies due to the expression of astrocyte specific markers and behavior. However, in high-glucose medium, C6 S100B secretion increases, in contrast to the trend in primary astrocyte cultures. Additionally, S100B secretion decreases due to fluorocitrate (FC), a Krebs cycle inhibitor, highlighting a connection between S100B and metabolism. Herein, we investigate the impact of FC on S100B secretion in primary astrocyte cultures, acute hippocampal slices and C6 glioma cells, as well as lactate mediation. Our results demonstrated that C6 responded similarly to astrocytes in various parameters, despite the decrease in S100B secretion, which was inversely observed in astrocytes and slices. Furthermore, FC inversely altered extracellular lactate in both models, suggesting a role for lactate in S100B secretion. This was reinforced by a decrease in S100B secretion in hippocampal slices treated with lactate and its agonist, but not in C6 cells, despite HCAR1 expression. Our findings indicate that extracellular lactate mediates the decrease in S100B secretion in astrocytes exposed to FC. They also emphasize the differences in C6 glioma cells regarding energetic metabolism. The proposed mechanism via HCAR1 provides further compelling evidence of the relationship between S100B and glucose metabolism.

Physiological acetic acid concentrations from ethanol metabolism stimulate accumbens shell neurons via NMDAR activation in a sex-dependent manner

Recent studies have implicated the ethanol metabolite, acetic acid, as neuroactive, perhaps even more so than ethanol itself. In this study, we investigated sex-specific metabolism of ethanol (1, 2, and 4g/kg) to acetic acid in vivo to guide electrophysiology experiments in the accumbens shell (NAcSh), a key node in the mammalian reward circuit. There was a sex-dependent difference in serum acetate production, quantified via ion chromatography only at the lowest dose of ethanol (males>females). Ex vivo electrophysiology recordings of NAcSh neurons in brain slices demonstrated that physiological concentrations of acetic acid (2 mM and 4 mM) increased NAcSh neuronal excitability in both sexes. N -methyl- D -aspartate receptor (NMDAR) antagonists, AP5, and memantine robustly attenuated the acetic acid-induced increase in excitability. Acetic acid-induced NMDAR-dependent inward currents were greater in females compared to males. These findings suggest a novel NMDAR-dependent mechanism by which the ethanol metabolite, acetic acid, may influence neurophysiological effects in a key reward circuit in the brain.

Differential Effects of Chronic Ethanol Use on Mouse Neuronal and Astroglial Metabolic Activity

Chronic alcohol use disorder, a major risk factor for the development of neuropsychiatric disorders including addiction to other substances, is associated with several neuropathology including perturbed neuronal and glial activities in the brain. It affects carbon metabolism in specific brain regions, and perturbs neuro-metabolite homeostasis in neuronal and glial cells. Alcohol induced changes in the brain neurochemical profile accompany the negative emotional state associated with dysregulated reward and sensitized stress response to withdrawal. However, the underlying alterations in neuro-astroglial activities and neurochemical dysregulations in brain regions after chronic alcohol use are poorly understood. This study evaluates the impact of chronic ethanol use on the regional neuro-astroglial metabolic activity using ¹H-[¹³C]-NMR spectroscopy in conjunction with infusion of [1,6-¹³C₂]glucose and sodium [2-¹³C]acetate, respectively, after 48 h of abstinence. Besides establishing detailed ¹³C labeling of neuro-metabolites in a brain region-specific manner, our results show chronic ethanol induced-cognitive deficits along with a reduction in total glucose oxidation rates in the hippocampus and striatum. Furthermore, using [2-¹³C]acetate infusion, we showed an alcohol-induced increase in astroglial metabolic activity in the hippocampus and prefrontal cortex. Interestingly, increased astroglia activity in the hippocampus and prefrontal cortex was associated with a differential expression of monocarboxylic acid transporters that are regulating acetate uptake and metabolism in the brain.

Ethanol Metabolite, Acetate, Increases Excitability of the Central Nucleus of Amygdala Neurons through Activation of NMDA Receptors

The central nucleus of the amygdala (CeA) is a key brain region involved in emotional and stressor responses due to its many projections to autonomic regulatory centers. It is also a primary site of action from ethanol consumption. However, the influence of active metabolites of ethanol such as acetate on the CeA neural circuitry has yet to be elucidated. Here, we investigated the effect of acetate on CeA neurons with the axon projecting to the rostral ventrolateral medulla (CeA-RVLM), as well as quantified cytosolic calcium responses in primary neuronal cultures. Whole-cell patch-clamp recordings in brain slices containing autonomic CeA-RVLM neurons revealed a dose-dependent increase in neuronal excitability in response to acetate. N-Methyl-d-aspartate receptor (NMDAR) antagonists suppressed the acetate-induced increase in CeA-RVLM neuronal excitability and memantine suppressed the direct activation of NMDAR-dependent inward currents by acetate in brain slices. We observed that acetate increased cytosolic Ca2+ in a time-dependent manner in primary neuronal cell cultures. The acetate enhancement of calcium signaling was abolished by memantine. Computational modeling of acetic acid at NMDAR/NR1 glutamatergic and glycinergic sites suggests potential active site interactions. These findings suggest that within the CeA, acetate is excitatory at least partially through activation of NMDAR, which may underlie the impact of ethanol consumption on autonomic circuitry.

Ketogenic Diet and Ketone Bodies against Ischemic Injury: Targets, Mechanisms, and Therapeutic Potential

The ketogenic diet (KD) has been used as a treatment for epilepsy since the 1920s, and its role in the prevention of many other diseases is now being considered. In recent years, there has been an intensive investigation on using the KD as a therapeutic approach to treat acute pathologies, including ischemic ones. However, contradictory data are observed for the effects of the KD on various organs after ischemic injury. In this review, we provide the first systematic analysis of studies conducted from 1980 to 2022 investigating the effects and main mechanisms of the KD and its mimetics on ischemia-reperfusion injury of the brain, heart, kidneys, liver, gut, and eyes. Our analysis demonstrated a high diversity of both the composition of the used KD and the protocols for the treatment of animals, which could be the reason for contradictory effects in different studies. It can be concluded that a true KD or its mimetics, such as β-hydroxybutyrate, can be considered as positive exposure, protecting the organ from ischemia and its negative consequences, whereas the shift to a rather similar high-calorie or high-fat diet leads to the opposite effect.

Consumption and Metabolism of Extracellular Pyruvate by Cultured Rat Brain Astrocytes

Brain astrocytes are considered as glycolytic cell type, but these cells also produce ATP via mitochondrial oxidative phosphorylation. Exposure of cultured primary astrocytes in a glucose-free medium to extracellular substrates that are known to be metabolised by mitochondrial pathways, including pyruvate, lactate, beta-hydroxybutyrate, alanine and acetate, revealed that among the substrates investigated extracellular pyruvate was most efficiently consumed by astrocytes. Extracellular pyruvate was consumed by the cells almost proportional to time over hours in a concentration-dependent manner with apparent Michaelis–Menten kinetics [Km = 0.6 ± 0.1 mM, Vmax = 5.1 ± 0.8 nmol/(min × mg protein)]. The astrocytic consumption of pyruvate was strongly impaired in the presence of the monocarboxylate transporter 1 (MCT1) inhibitor AR-C155858 or by application of a 10-times excess of the MCT1 substrates lactate or beta-hydroxybutyrate. Pyruvate consumption by viable astrocytes was inhibited in the presence of UK5099, an inhibitor of the mitochondrial pyruvate carrier, or after application of the respiratory chain inhibitor antimycin A. In contrast, the mitochondrial uncoupler BAM15 strongly accelerated cellular pyruvate consumption. Lactate and alanine accounted after 3 h of incubation with pyruvate for around 60% and 10%, respectively, of the pyruvate consumed by the cells. These results demonstrate that consumption of extracellular pyruvate by astrocytes involves uptake via MCT1 and that the velocity of pyruvate consumption is strongly modified by substances that affect the entry of pyruvate into mitochondria or the activity of mitochondrial respiration.

Glyceryl triacetate feeding in mice increases plasma acetate levels but has no anticonvulsant effects in acute electrical seizure models

INTRODUCTION

Acetate has been shown to have neuroprotective and anti-inflammatory effects. It is oxidized by astrocytes and can thus provide auxiliary energy to the brain in addition to glucose. Therefore, we hypothesized that it may protect against seizures, which is investigated here by feeding glyceryl triacetate (GTA), to provide high amounts of acetate without raising sodium or acid levels.

METHOD

CD1 male mice were fed controlled diets with or without GTA for up to three weeks. Body weights, blood glucose levels, plasma short-chain fatty acid levels, and other hematological parameters were monitored. Seizure thresholds were determined in 6 Hz and maximal electroshock seizure threshold (MEST) tests. Antioxidant capacities were evaluated in the cerebral cortex and plasma using a ferric reducing antioxidant power (FRAP) assay and Trolox equivalent antioxidant capacity assay.

RESULTS

Body weight gain was similar with both diets with and without GTA in two experiments. Glyceryl triacetate-fed groups showed 2-3- and 1.6-fold increased acetate and propionate levels in plasma, respectively. Glucose levels were unaltered in blood collected from the tail tip but increased in trunk blood. No differences were found in the activity of cerebral cortex acetyl-CoA synthetase. In the 6 Hz threshold test, seizure thresholds were lower by 3 mA and 2.4 mA after 8 and 14 days, respectively, in the GTA compared to the control diet-fed group, but showed no difference on day 16, showing that GTA has small, but inconsistent proconvulsant effects in this model. In MEST tests, a slightly increased seizure threshold (1 mA) was found on day 19 in the GTA-fed group, but not in another experiment on day 21. There were no differences in antioxidant capacity in plasma or cortex between the two groups.

CONCLUSION

Glyceryl triacetate feeding showed no antioxidant effects nor beneficial changes in acute electrical seizure threshold mouse models, despite its ability to increase plasma acetate levels.

Enhanced Cortical Metabolic Activity in Females and Males of a Slow Progressing Mouse Model of Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder with selective degeneration of motor neurons in the central nervous system. The pathophysiology of ALS is not well understood. We have used ¹H-[¹³C]-NMR spectroscopy together with an administration of [1,6-¹³C₂]glucose and [2-¹³C]acetate in female and male SOD1^G37R mice to assess neuronal and astroglial metabolic activity, respectively, in the central nervous system in ALS condition. The female (p = 0.0008) and male (p < 0.0001) SOD1^G37R mice exhibited decreased forelimb strength when compared with wild-type mice. There was a reduction in N-acetylaspartylglutamate level, and elevation in myo-inositol in the spinal cord of female and male SOD1^G37R mice. The transgenic male mice exhibited increased acetate oxidation in the spinal cord (p = 0.05) and cerebral cortex (p = 0.03), while females showed an increase in the spinal cord (p = 0.02) only. As acetate is transported and preferentially metabolized in the astrocytes, the finding of increased rate of acetate oxidation in the transgenic mice is suggestive of astrocytic involvement in the pathogenesis of ALS. The rates of glucose oxidation in glutamatergic (p = 0.0004) and GABAergic neurons (p = 0.0052) were increased in the cerebral cortex of male SOD1^G37R mice when compared with the controls. The female mice showed an increase in glutamatergic (p = 0.039) neurometabolic activity only. The neurometabolic activity was unperturbed in the spinal cord of either sex. These data suggest differential changes in neurometabolic activity across the central nervous system in SOD1^G37R mice.

Noradrenaline-induced l-lactate production requires d-glucose entry and transit through the glycogen shunt in single-cultured rat astrocytes

During cognitive efforts mediated by local neuronal networks, approximately 20% of additional energy is required; this is mediated by chemical messengers such as noradrenaline (NA). NA targets astroglial aerobic glycolysis, the hallmark of which is the end product l-lactate, a fuel for neurons. Biochemical studies have revealed that astrocytes exhibit a prominent glycogen shunt, in which a portion of d-glucose molecules entering the cytoplasm is transiently incorporated into glycogen, a buffer and source of d-glucose during increased energy demand. Here, we studied single astrocytes by measuring cytosolic L-lactate ([lac]_i ) with the FRET nanosensor Laconic. We examined whether NA-induced increase in [lac]_i is influenced by: (a) 2-deoxy-d-glucose (2-DG, 3 mM), a molecule that enters the cytosol and inhibits the glycolytic pathway; (b) 1,4-dideoxy-1,4-imino-d-arabinitol (DAB, 300 µM), a potent inhibitor of glycogen phosphorylase and glycogen degradation; and (c) 3-nitropropionic acid (3-NPA, 1 mM), an inhibitor of the Krebs cycle. The results of these pharmacological experiments revealed that d-glucose uptake is essential for the NA-induced increase in [lac]_i , and that this exclusively arises from glycogen degradation, indicating that most, if not all, d-glucose molecules in NA-stimulated cells transit the glycogen shunt during glycolysis. Moreover, under the defined transmembrane d-glucose gradient, the glycolytic intermediates were not only used to produce l-lactate, but also to significantly support oxidative phosphorylation, as demonstrated by an elevation in [lac]_i when Krebs cycle was inhibited. We conclude that l-lactate production via aerobic glycolysis is an essential energy pathway in NA-stimulated astrocytes; however, oxidative metabolism is important at rest.

The ethanol metabolite acetic acid activates mouse nucleus accumbens shell medium spiny neurons

Although ethanol consumption leads to an array of neurophysiological alterations involving the neural circuits for reward, the underlying mechanisms remain unclear. Acetic acid is a major metabolite of ethanol with high bioactivity and potentially significant pharmacological importance in regulating brain function. Yet, the impact of acetic acid on reward circuit function has not been well explored. Given the rewarding properties associated with ethanol consumption, we investigated the acute effects of ethanol and/or acetic acid on the neurophysiological function of medium spiny neurons of the nucleus accumbens shell, a key node in the mammalian reward circuit. We find that acetic acid, but not ethanol, provided a rapid and robust boost in neuronal excitability at physiologically relevant concentrations, whereas both compounds enhanced glutamatergic synaptic activity. These effects were consistent across both sexes in C57BL/6J mice. Overall, our data suggest acetic acid is a promising candidate mediator for ethanol effects on mood and motivation that deserves further investigation.NEW & NOTEWORTHY Ethanol consumption disrupts many neurophysiological processes leading to alterations in behavior and physiological function. The possible involvement of acetic acid, produced via ethanol metabolism, has been insufficiently explored. Here, we demonstrate that acetic acid contributes to rapid neurophysiological alterations in the accumbens shell. These findings raise the interesting possibility that ethanol may serve as a prodrug-generating acetic acid as a metabolite-that may influence ethanol consumption-associated behaviors and physiological responses by altering neurophysiological function.

Acetate Revisited: A Key Biomolecule at the Nexus of Metabolism, Epigenetics and Oncogenesis-Part 1: Acetyl-CoA, Acetogenesis and Acyl-CoA Short-Chain Synthetases

Acetate is a major end product of bacterial fermentation of fiber in the gut. Acetate, whether derived from the diet or from fermentation in the colon, has been implicated in a range of health benefits. Acetate is also generated in and released from various tissues including the intestine and liver, and is generated within all cells by deacetylation reactions. To be utilized, all acetate, regardless of the source, must be converted to acetyl coenzyme A (acetyl-CoA), which is carried out by enzymes known as acyl-CoA short-chain synthetases. Acyl-CoA short-chain synthetase-2 (ACSS2) is present in the cytosol and nuclei of many cell types, whereas ACSS1 is mitochondrial, with greatest expression in heart, skeletal muscle, and brown adipose tissue. In addition to acting to redistribute carbon systemically like a ketone body, acetate is becoming recognized as a cellular regulatory molecule with diverse functions beyond the formation of acetyl-CoA for energy derivation and lipogenesis. Acetate acts, in part, as a metabolic sensor linking nutrient balance and cellular stress responses with gene transcription and the regulation of protein function. ACSS2 is an important task-switching component of this sensory system wherein nutrient deprivation, hypoxia and other stressors shift ACSS2 from a lipogenic role in the cytoplasm to a regulatory role in the cell nucleus. Protein acetylation is a critical post-translational modification involved in regulating cell behavior, and alterations in protein acetylation status have been linked to multiple disease states, including cancer. Improving our fundamental understanding of the "acetylome" and how acetate is generated and utilized at the subcellular level in different cell types will provide much needed insight into normal and neoplastic cellular metabolism and the epigenetic regulation of phenotypic expression under different physiological stressors. This article is Part 1 of 2 - for Part 2 see doi: 10.3389/fphys.2020.580171.

Metabolite Transporters as Regulators of Immunity

In the past decade, the rise of immunometabolism has fundamentally reshaped the face of immunology. As the functions and properties of many (immuno)metabolites have now been well described, their exchange among cells and their environment have only recently sparked the interest of immunologists. While many metabolites bind specific receptors to induce signaling cascades, some are actively exchanged between cells to communicate, or induce metabolic reprograming. In this review, we give an overview about how active metabolite transport impacts immune cell function and shapes immunological responses. We present some examples of how specific transporters feed into metabolic pathways and initiate intracellular signaling events in immune cells. In particular, we focus on the role of metabolite transporters in the activation and effector functions of T cells and macrophages, as prototype adaptive and innate immune cell populations.

The role of the microbiome in the neurobiology of social behaviour

Microbes colonise all multicellular life, and the gut microbiome has been shown to influence a range of host physiological and behavioural phenotypes. One of the most intriguing and least understood of these influences lies in the domain of the microbiome's interactions with host social behaviour, with new evidence revealing that the gut microbiome makes important contributions to animal sociality. However, little is known about the biological processes through which the microbiome might influence host social behaviour. Here, we synthesise evidence of the gut microbiome's interactions with various aspects of host sociality, including sociability, social cognition, social stress, and autism. We discuss evidence of microbial associations with the most likely physiological mediators of animal social interaction. These include the structure and function of regions of the 'social' brain (the amygdala, the prefrontal cortex, and the hippocampus) and the regulation of 'social' signalling molecules (glucocorticoids including corticosterone and cortisol, sex hormones including testosterone, oestrogens, and progestogens, neuropeptide hormones such as oxytocin and arginine vasopressin, and monoamine neurotransmitters such as serotonin and dopamine). We also discuss microbiome-associated host genetic and epigenetic processes relevant to social behaviour. We then review research on microbial interactions with olfaction in insects and mammals, which contribute to social signalling and communication. Following these discussions, we examine evidence of microbial associations with emotion and social behaviour in humans, focussing on psychobiotic studies, microbe-depression correlations, early human development, autism, and issues of statistical power, replication, and causality. We analyse how the putative physiological mediators of the microbiome-sociality connection may be investigated, and discuss issues relating to the interpretation of results. We also suggest that other candidate molecules should be studied, insofar as they exert effects on social behaviour and are known to interact with the microbiome. Finally, we consider different models of the sequence of microbial effects on host physiological development, and how these may contribute to host social behaviour.

Metabolomic Profiling in Neuromyelitis Optica Spectrum Disorder Biomarker Discovery

There is no specific test for diagnosing neuromyelitis optica spectrum disorder (NMOSD), a disabling autoimmune disease of the central nervous system. Instead, diagnosis relies on ruling out other related disorders with overlapping clinical symptoms. An urgency for NMOSD biomarker discovery is underscored by adverse responses to treatment following misdiagnosis and poor prognosis following the delayed onset of treatment. Pathogenic autoantibiotics that target the water channel aquaporin-4 (AQP4) and myelin oligodendrocyte glycoprotein (MOG) contribute to NMOSD pathology. The importance of early diagnosis between AQP4-Ab⁺ NMOSD, MOG-Ab⁺ NMOSD, AQP4-Ab⁻ MOG-Ab⁻ NMOSD, and related disorders cannot be overemphasized. Here, we provide a comprehensive data collection and analysis of the currently known metabolomic perturbations and related proteomic outcomes of NMOSD. We highlight short chain fatty acids, lipoproteins, amino acids, and lactate as candidate diagnostic biomarkers. Although the application of metabolomic profiling to individual NMOSD patient care shows promise, more research is needed.

β-Hydroxybutyrate Oxidation Promotes the Accumulation of Immunometabolites in Activated Microglia Cells

Metabolic regulation of immune cells has arisen as a critical set of processes required for appropriate response to immunological signals. While our knowledge in this area has rapidly expanded in leukocytes, much less is known about the metabolic regulation of brain-resident microglia. In particular, the role of alternative nutrients to glucose remains poorly understood. Here, we use stable-isotope (¹³C) tracing strategies and metabolomics to characterize the oxidative metabolism of β-hydroxybutyrate (BHB) in human (HMC3) and murine (BV2) microglia cells and the interplay with glucose in resting and LPS-activated BV2 cells. We found that BHB is imported and oxidised in the TCA cycle in both cell lines with a subsequent increase in the cytosolic NADH:NAD⁺ ratio. In BV2 cells, stimulation with LPS upregulated the glycolytic flux, increased the cytosolic NADH:NAD⁺ ratio and promoted the accumulation of the glycolytic intermediate dihydroxyacetone phosphate (DHAP). The addition of BHB enhanced LPS-induced accumulation of DHAP and promoted glucose-derived lactate export. BHB also synergistically increased LPS-induced accumulation of succinate and other key immunometabolites, such as α-ketoglutarate and fumarate generated by the TCA cycle. Finally, BHB upregulated the expression of a key pro-inflammatory (M1 polarisation) marker gene, NOS2, in BV2 cells activated with LPS. In conclusion, we identify BHB as a potentially immunomodulatory metabolic substrate for microglia that promotes metabolic reprogramming during pro-inflammatory response.

Acetate Promotes T Cell Effector Function during Glucose Restriction

Competition for nutrients like glucose can metabolically restrict T cells and contribute to their hyporesponsiveness during cancer. Metabolic adaptation to the surrounding microenvironment is therefore key for maintaining appropriate cell function. For instance, cancer cells use acetate as a substrate alternative to glucose to fuel metabolism and growth. Here, we show that acetate rescues effector function in glucose-restricted CD8⁺ T cells. Mechanistically, acetate promotes histone acetylation and chromatin accessibility and enhances IFN-γ gene transcription and cytokine production in an acetyl-CoA synthetase (ACSS)-dependent manner. Ex vivo acetate treatment increases IFN-γ production by exhausted T cells, whereas reducing ACSS expression in T cells impairs IFN-γ production by tumor-infiltrating lymphocytes and tumor clearance. Thus, hyporesponsive T cells can be epigenetically remodeled and reactivated by acetate, suggesting that pathways regulating the use of substrates alternative to glucose could be therapeutically targeted to promote T cell function during cancer.

Metabolic signature in nucleus accumbens for anti-depressant-like effects of acetyl-L-carnitine

Emerging evidence suggests that hierarchical status provides vulnerability to develop stress-induced depression. Energy metabolic changes in the nucleus accumbens (NAc) were recently related to hierarchical status and vulnerability to develop depression-like behavior. Acetyl-L-carnitine (LAC), a mitochondria-boosting supplement, has shown promising antidepressant-like effects opening therapeutic opportunities for restoring energy balance in depressed patients. We investigated the metabolic impact in the NAc of antidepressant LAC treatment in chronically-stressed mice using ¹H-magnetic resonance spectroscopy (¹H-MRS). High rank, but not low rank, mice, as assessed with the tube test, showed behavioral vulnerability to stress, supporting a higher susceptibility of high social rank mice to develop depressive-like behaviors. High rank mice also showed reduced levels of several energy-related metabolites in the NAc that were counteracted by LAC treatment. Therefore, we reveal a metabolic signature in the NAc for antidepressant-like effects of LAC in vulnerable mice characterized by restoration of stress-induced neuroenergetics alterations and lipid function.

Acetate Mediates Alcohol Excitotoxicity in Dopaminergic-like PC12 Cells

Neuronal excitotoxicity is the major cause of alcohol-related brain damage, yet the underlying mechanism remains poorly understood. Using dopaminergic-like PC12 cells, we evaluated the effect of N-methyl-d-aspartate receptors (NMDAR) on acetate-induced changes in PC12 cells: cell death, cytosolic calcium, and expression levels of the pro-inflammatory cytokine tumor necrosis factor alpha (TNFα). Treatment of PC12 cells with increasing concentrations of acetate for 4 h caused a dose-dependent increase in the percentage of cells staining positive for cell death using propidium iodide (PI) exclusion and cytosolic reactive oxygen species (ROS) using cell ROX detection analyzed via flow cytometry. The EC₅₀ value for acetate was calculated and found to be 4.40 mM for PI and 1.81 mM for ROS. Ethanol up to 100 mM had no apparent changes in the percent of cells staining positive for PI or ROS. Acetate (6 mM) treatment caused an increase in cytosolic calcium measured in real-time with Fluo-4AM, which was abolished by coapplication with the NMDAR blocker memantine (10 μM). Furthermore, cells treated with acetate (6 mM) for 4 h had increased expression levels of TNFα relative to control, which was abolished by coapplication of memantine (10 μM). Co-application of acetate (6 mM) and memantine had no apparent reduction in acetate-induced cell death. These findings suggest that acetate is capable of increasing cytosolic calcium concentrations and expression levels of the pro-inflammatory cytokine TNFα through an NMDAR-dependent mechanism. Cell death from acetate was not reduced through NMDAR blockade, suggesting alternative pathways independent of NMDAR activation for excitotoxicity.

Interplay between NAD+ and acetyl‑CoA metabolism in ischemia-induced mitochondrial pathophysiology

Brain injury caused by ischemic insult due to significant reduction or interruption in cerebral blood flow leads to disruption of practically all cellular metabolic pathways. This triggers a complex stress response followed by overstimulation of downstream enzymatic pathways due to massive activation of post-translational modifications (PTM). Mitochondria are one of the most sensitive organelle to ischemic conditions. They become dysfunctional due to extensive fragmentation, inhibition of acetyl‑CoA production, and increased activity of NAD⁺ consuming enzymes. These pathologic conditions ultimately lead to inhibition of oxidative phosphorylation and mitochondrial ATP production. Both acetyl‑CoA and NAD⁺ are essential intermediates in cellular bioenergetics metabolism and also serve as substrates for post-translational modifications such as acetylation and ADP‑ribosylation. In this review we discuss ischemia/reperfusion-induced changes in NAD⁺ and acetyl‑CoA metabolism, how these affect relevant PTMs, and therapeutic approaches that restore the physiological levels of these metabolites leading to promising neuroprotection.

Glia-neuron energy metabolism in health and diseases: New insights into the role of nervous system metabolic transporters

The brain is, by weight, only 2% the volume of the body and yet it consumes about 20% of the total glucose, suggesting that the energy requirements of the brain are high and that glucose is the primary energy source for the nervous system. Due to this dependence on glucose, brain physiology critically depends on the tight regulation of glucose transport and its metabolism. Glucose transporters ensure efficient glucose uptake by neural cells and contribute to the physiology and pathology of the nervous system. Despite this, a growing body of evidence demonstrates that for the maintenance of several neuronal functions, lactate, rather than glucose, is the preferred energy metabolite in the nervous system. Monocarboxylate transporters play a crucial role in providing metabolic support to axons by functioning as the principal transporters for lactate in the nervous system. Monocarboxylate transporters are also critical for axonal myelination and regeneration. Most importantly, recent studies have demonstrated the central role of glial cells in brain energy metabolism. A close and regulated metabolic conversation between neurons and both astrocytes and oligodendroglia in the central nervous system, or Schwann cells in the peripheral nervous system, has recently been shown to be an important determinant of the metabolism and function of the nervous system. This article reviews the current understanding of the long existing controversies regarding energy substrate and utilization in the nervous system and discusses the role of metabolic transporters in health and diseases of the nervous system.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

Detection of Bacteria-Specific Metabolism Using Hyperpolarized [2-13C]Pyruvate

The differentiation of bacterial infection from other causes of inflammation is difficult in clinical practice and is critical where patient outcomes rely heavily on early interventions. In addition to physical exam and laboratory markers, several imaging modalities are frequently employed, but these techniques generally target the host immune response, rather than the living microorganisms themselves. Here, we describe a method to detect bacteria-specific metabolism using hyperpolarized (HP) ¹³C magnetic resonance spectroscopy. This technology allows visualization of the real-time conversion of enriched ¹³C substrates to their metabolic products, identified by their distinct chemical shifts. We have identified the rapid metabolism of HP [2-¹³C]pyruvate to [1-¹³C]acetate as a metabolic signature of common bacterial pathogens. We demonstrate this conversion in representative Gram-negative and Gram-positive bacteria, namely, Escherichia coli and Staphylococcus aureus, and its absence in key mammalian cell types. Furthermore, this conversion was successfully modulated in three mutant strains, corresponding to deletions of relevant enzymes.

How Can a Ketogenic Diet Improve Motor Function?

A ketogenic diet (KD) is a normocaloric diet composed by high fat (80-90%), low carbohydrate, and low protein consumption that induces fasting-like effects. KD increases ketone body (KBs) production and its concentration in the blood, providing the brain an alternative energy supply that enhances oxidative mitochondrial metabolism. In addition to its profound impact on neuro-metabolism and bioenergetics, the neuroprotective effect of specific polyunsaturated fatty acids and KBs involves pleiotropic mechanisms, such as the modulation of neuronal membrane excitability, inflammation, or reactive oxygen species production. KD is a therapy that has been used for almost a century to treat medically intractable epilepsy and has been increasingly explored in a number of neurological diseases. Motor function has also been shown to be improved by KD and/or medium-chain triglyceride diets in rodent models of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and spinal cord injury. These studies have proposed that KD may induce a modification in synaptic morphology and function, involving ionic channels, glutamatergic transmission, or synaptic vesicular cycling machinery. However, little is understood about the molecular mechanisms underlying the impact of KD on motor function and the perspectives of its use to acquire the neuromuscular effects. The aim of this review is to explore the conditions through which KD might improve motor function. First, we will describe the main consequences of KD exposure in tissues involved in motor function. Second, we will report and discuss the relevance of KD in pre-clinical and clinical trials in the major diseases presenting motor dysfunction.

Early and Late Pathomechanisms in Alzheimer's Disease: From Zinc to Amyloid-β Neurotoxicity

There are several systemic and intracerebral pathologic conditions, which limit provision and utilization of energy precursor metabolites in neuronal cells. Energy deficits cause excessive depolarization of neuronal cells triggering glutamate-zinc evoked excitotoxic cascade. The intracellular zinc excess hits several intraneuronal targets yielding collapse of energy balance and impairment functional and structural impairments cholinergic neurons. Disturbances in metabolism of acetyl-CoA, which is a direct precursor for energy, acetylcholine, N-acetyl-L-aspartate and acetylated proteins synthesis, play an important role in these pathomechanisms. Disruption of brain homeostasis activates slow accumulation of amyloid-β 1-42 , which extra and intracellular oligomeric deposits disrupt diverse transporting and signaling processes in all membrane structures of the cell. Both neurotoxic signals may combine aggravating detrimental effects on neuronal cell. Different neuroglial and neuronal cell types may display differential susceptibility to similar pathogenic insults depending on specific features of their energy and functional parameters. This review, basing on findings gained from cellular and animal models of Alzheimer's disease, discusses putative energy/acetyl-CoA dependent mechanism in early and late stages of neurodegeneration.

Improved cerebral energetics and ketone body metabolism in db/db mice

It is becoming evident that type 2 diabetes mellitus is affecting brain energy metabolism. The importance of alternative substrates for the brain in type 2 diabetes mellitus is poorly understood. The aim of this study was to investigate whether ketone bodies are relevant candidates to compensate for cerebral glucose hypometabolism and unravel the functionality of cerebral mitochondria in type 2 diabetes mellitus. Acutely isolated cerebral cortical and hippocampal slices of db/db mice were incubated in media containing [U-¹³C]glucose, [1,2-¹³C]acetate or [U-¹³C]β-hydroxybutyrate and tissue extracts were analysed by mass spectrometry. Oxygen consumption and ATP synthesis of brain mitochondria of db/db mice were assessed by Seahorse XFe96 and luciferin-luciferase assay, respectively. Glucose hypometabolism was observed for both cerebral cortical and hippocampal slices of db/db mice. Significant increased metabolism of [1,2-¹³C]acetate and [U-¹³C]β-hydroxybutyrate was observed for hippocampal slices of db/db mice. Furthermore, brain mitochondria of db/db mice exhibited elevated oxygen consumption and ATP synthesis rate. This study provides evidence of several changes in brain energy metabolism in type 2 diabetes mellitus. The increased hippocampal ketone body utilization and improved mitochondrial function in db/db mice, may act as adaptive mechanisms in order to maintain cerebral energetics during hampered glucose metabolism.

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.

Acetate as a Metabolic and Epigenetic Modifier of Cancer Therapy

Metabolic networks are significantly altered in neoplastic cells. This altered metabolic program leads to increased glycolysis and lipogenesis and decreased dependence on oxidative phosphorylation and oxygen consumption. Despite their limited mitochondrial respiration, cancer cells, nonetheless, derive sufficient energy from alternative carbon sources and metabolic pathways to maintain cell proliferation. They do so, in part, by utilizing fatty acids, amino acids, ketone bodies, and acetate, in addition to glucose. The alternative pathways used in the metabolism of these carbon sources provide opportunities for therapeutic manipulation. Acetate, in particular, has garnered increased attention in the context of cancer as both an epigenetic regulator of posttranslational protein modification, and as a carbon source for cancer cell biomass accumulation. However, to date, the data have not provided a clear understanding of the precise roles that protein acetylation and acetate oxidation play in carcinogenesis, cancer progression or treatment. This review highlights some of the major issues, discrepancies, and opportunities associated with the manipulation of acetate metabolism and acetylation-based signaling in cancer development and treatment.

Severity of experimental traumatic brain injury modulates changes in concentrations of cerebral free amino acids

In this study, concentrations of free amino acids (FAA) and amino group containing compounds (AGCC) following graded diffuse traumatic brain injury (mild TBI, mTBI; severe TBI, sTBI) were evaluated. After 6, 12, 24, 48 and 120 hr aspartate (Asp), glutamate (Glu), asparagine (Asn), serine (Ser), glutamine (Gln), histidine (His), glycine (Gly), threonine (Thr), citrulline (Cit), arginine (Arg), alanine (Ala), taurine (Tau), γ‐aminobutyrate (GABA), tyrosine (Tyr), S‐adenosylhomocysteine (SAH), l‐cystathionine (l‐Cystat), valine (Val), methionine (Met), tryptophane (Trp), phenylalanine (Phe), isoleucine (Ile), leucine (Leu), ornithine (Orn), lysine (Lys), plus N‐acetylaspartate (NAA) were determined in whole brain extracts (n = 6 rats at each time for both TBI levels). Sham‐operated animals (n = 6) were used as controls. Results demonstrated that mTBI caused modest, transient changes in NAA, Asp, GABA, Gly, Arg. Following sTBI, animals showed profound, long‐lasting modifications of Glu, Gln, NAA, Asp, GABA, Ser, Gly, Ala, Arg, Citr, Tau, Met, SAH, l‐Cystat, Tyr and Phe. Increase in Glu and Gln, depletion of NAA and Asp increase, suggested a link between NAA hydrolysis and excitotoxicity after sTBI. Additionally, sTBI rats showed net imbalances of the Glu‐Gln/GABA cycle between neurons and astrocytes, and of the methyl‐cycle (demonstrated by decrease in Met, and increase in SAH and l‐Cystat), throughout the post‐injury period. Besides evidencing new potential targets for novel pharmacological treatments, these results suggest that the force acting on the brain tissue at the time of the impact is the main determinant of the reactions ignited and involving amino acid metabolism.

Functional metabolic interactions of human neuron-astrocyte 3D in vitro networks

The generation of human neural tissue-like 3D structures holds great promise for disease modeling, drug discovery and regenerative medicine strategies. Promoting the establishment of complex cell-cell interactions, 3D culture systems enable the development of human cell-based models with increased physiological relevance, over monolayer cultures. Here, we demonstrate the establishment of neuronal and astrocytic metabolic signatures and shuttles in a human 3D neural cell model, namely the glutamine-glutamate-GABA shuttle. This was indicated by labeling of neuronal GABA following incubation with the glia-specific substrate [2-(13)C]acetate, which decreased by methionine sulfoximine-induced inhibition of the glial enzyme glutamine synthetase. Cell metabolic specialization was further demonstrated by higher pyruvate carboxylase-derived labeling in glutamine than in glutamate, indicating its activity in astrocytes and not in neurons. Exposure to the neurotoxin acrylamide resulted in intracellular accumulation of glutamate and decreased GABA synthesis. These results suggest an acrylamide-induced impairment of neuronal synaptic vesicle trafficking and imbalanced glutamine-glutamate-GABA cycle, due to loss of cell-cell contacts at synaptic sites. This work demonstrates, for the first time to our knowledge, that neural differentiation of human cells in a 3D setting recapitulates neuronal-astrocytic metabolic interactions, highlighting the relevance of these models for toxicology and better understanding the crosstalk between human neural cells.

(13)C glucose labelling studies using 2D NMR are a useful tool for determining ex vivo whole organ metabolism during hypothermic machine perfusion of kidneys

Background

The aim of this study is to determine the feasibility of using nuclear magnetic resonance (NMR) tracer studies (¹³C-enriched glucose) to detect ex vivo de novo metabolism in the perfusion fluid and cortical tissue of porcine kidneys during hypothermic machine perfusion (HMP).

Methods

Porcine kidneys (n = 6) were subjected to 24 h of HMP using the Organ Recovery Systems LifePort Kidney perfusion device. Glucose, uniformly enriched with the stable isotope ¹³C ([U-¹³C] glucose), was incorporated into KPS-1-like perfusion fluid at a concentration of 10 mM. Analysis of perfusate was performed using both 1D ¹H and 2D ¹H,¹³C heteronuclear single quantum coherence (HSQC) NMR spectroscopy. The metabolic activity was then studied by quantifying the proportion of key metabolites containing ¹³C in both perfusate and tissue samples.

Results

There was significant enrichment of ¹³C in a number of central metabolites present in both the perfusate and tissue extracts and was most pronounced for lactate and alanine. The total amount of enriched lactate (per sample) in perfusion fluid increased during HMP (31.1 ± 12.2 nmol at 6 h vs 93.4 ± 25.6 nmol at 24 h p < 0.01). The total amount of enriched alanine increased in a similar fashion (1.73 ± 0.89 nmol at 6 h vs 6.80 ± 2.56 nmol at 24 h p < 0.05). In addition, small amounts of enriched acetate and glutamic acid were evident in some samples.

Conclusions

This study conclusively demonstrates that de novo metabolism occurs during HMP and highlights active metabolic pathways in this hypothermic, hypoxic environment. Whilst the majority of the ¹³C-enriched glucose is metabolised into glycolytic endpoint metabolites such as lactate, the presence of non-glycolytic pathway derivatives suggests that metabolism during HMP is more complex than previously thought. Isotopic labelled ex vivo organ perfusion studies using 2D NMR are feasible and informative.

Evaluating Acetate Metabolism for Imaging and Targeting in Multiple Myeloma

PURPOSE

We hypothesized that in multiple myeloma cells (MMC), high membrane biosynthesis will induce acetate uptake in vitro and in vivo Here, we studied acetate metabolism and targeting in MMC in vitro and tested the efficacy of ¹¹C-acetate-positron emission tomography (PET) to detect and quantitatively image myeloma treatment response in vivo EXPERIMENTAL DESIGN: Acetate fate tracking using ¹³C-edited-¹H NMR (nuclear magnetic resonance) was performed to study in vitro acetate uptake and metabolism in MMC. Effects of pharmacological modulation of acetate transport or acetate incorporation into lipids on MMC cell survival and viability were assessed. Preclinical mouse MM models of subcutaneous and bone tumors were evaluated using ¹¹C-acetate-PET/CT imaging and tissue biodistribution.

RESULTS

In vitro, NMR showed significant uptake of acetate by MMC and acetate incorporation into intracellular metabolites and membrane lipids. Inhibition of lipid synthesis and acetate transport was toxic to MMC, while sparing resident bone cells or normal B cells. In vivo, ¹¹C-acetate uptake by PET imaging was significantly enhanced in subcutaneous and bone MMC tumors compared with unaffected bone or muscle tissue. Likewise, ¹¹C-acetate uptake was significantly reduced in MM tumors after treatment.

CONCLUSIONS

Uptake of acetate from the extracellular environment was enhanced in MMC and was critical to cellular viability. ¹¹C-Acetate-PET detected the presence of myeloma cells in vivo, including uptake in intramedullary bone disease. ¹¹C-Acetate-PET also detected response to therapy in vivo Our data suggested that acetate metabolism and incorporation into lipids was crucial to MM cell biology and that ¹¹C-acetate-PET is a promising imaging modality for MM. Clin Cancer Res; 23(2); 416-29. ©2016 AACR.

Mutant IDH1 expression is associated with down-regulation of monocarboxylate transporters

Mutations in isocitrate dehydrogenase 1 (IDH1) are characteristic of low-grade gliomas. We recently showed that mutant IDH1 cells reprogram cellular metabolism by down-regulating pyruvate dehydrogenase (PDH) activity. Reduced pyruvate metabolism via PDH could lead to increased pyruvate conversion to lactate. The goal of this study was therefore to investigate the impact of the IDH1 mutation on the pyruvate-to-lactate flux. We used 13C magnetic resonance spectroscopy and compared the conversion of hyperpolarized [1-13C]-pyruvate to [1-13C]-lactate in immortalized normal human astrocytes expressing mutant or wild-type IDH1 (NHAIDHmut and NHAIDHwt). Our results indicate that hyperpolarized lactate production is reduced in NHAIDHmut cells compared to NHAIDHwt. This reduction was associated with lower expression of the monocarboxylate transporters MCT1 and MCT4 in NHAIDHmut cells. Furthermore, hyperpolarized lactate production was comparable in lysates of NHAIDHmut and NHAIDHwt cells, wherein MCTs do not impact hyperpolarized pyruvate delivery and lactate production. Collectively, our findings indicated that lower MCT expression was a key contributor to lower hyperpolarized lactate production in NHAIDHmut cells. The SLC16A3 (MCT4) promoter but not SLC16A1 (MCT1) promoter was hypermethylated in NHAIDHmut cells, pointing to possibly different mechanisms mediating reduced MCT expression. Finally analysis of low-grade glioma patient biopsy data from The Cancer Genome Atlas revealed that MCT1 and MCT4 expression was significantly reduced in mutant IDH1 tumors compared to wild-type. Taken together, our study shows that reduced MCT expression is part of the metabolic reprogramming of mutant IDH1 gliomas. This finding could impact treatment and has important implications for metabolic imaging of mutant IDH1 gliomas.

Overview of PET Tracers for Brain Tumor Imaging

This article provides an overview of the key considerations for the development and application of molecular imaging agents for brain tumors and the major classes of PET tracers that have been used for imaging brain tumors in humans. The mechanisms of uptake, biological implications, primary applications, and limitations of PET tracers in neuro-oncology are reviewed. The available data indicate that several of these classes of tracers, including radiolabeled amino acids, have imaging properties superior to those of (18)F-fluorodeoxyglucose, and can complement contrast-enhanced magnetic resonance imaging in the evaluation of brain tumors.

Glucose metabolism and astrocyte-neuron interactions in the neonatal brain

Glucose is essentially the sole fuel for the adult brain and the mapping of its metabolism has been extensive in the adult but not in the neonatal brain, which is believed to rely mainly on ketone bodies for energy supply. However, glucose is absolutely indispensable for normal development and recent studies have shed light on glycolysis, the pentose phosphate pathway and metabolic interactions between astrocytes and neurons in the 7-day-old rat brain. Appropriately (13)C labeled glucose was used to distinguish between glycolysis and the pentose phosphate pathway during development. Experiments using (13)C labeled acetate provided insight into the GABA-glutamate-glutamine cycle between astrocytes and neurons. It could be shown that in the neonatal brain the part of this cycle that transfers glutamine from astrocytes to neurons is operating efficiently while, in contrast, little glutamate is shuttled from neurons to astrocytes. This lack of glutamate for glutamine synthesis is compensated for by anaplerosis via increased pyruvate carboxylation relative to that in the adult brain. Furthermore, compared to adults, relatively more glucose is prioritized to the pentose phosphate pathway than glycolysis and pyruvate dehydrogenase activity. The reported developmental differences in glucose metabolism and neurotransmitter synthesis may determine the ability of the brain at various ages to resist excitotoxic insults such as hypoxia-ischemia.

Silent information regulator 1 modulator resveratrol increases brain lactate production and inhibits mitochondrial metabolism, whereas SRT1720 increases oxidative metabolism

Silent information regulators (SIRTs) have been shown to deacetylate a range of metabolic enzymes, including those in glycolysis and the Krebs cycle, and thus alter their activity. SIRTs require NAD(+) for their activity, linking cellular energy status to enzyme activity. To examine the impact of SIRT1 modulation on oxidative metabolism, this study tests the effect of ligands that are either SIRT-activating compounds (resveratrol and SRT1720) or SIRT inhibitors (EX527) on the metabolism of (13)C-enriched substrates by guinea pig brain cortical tissue slices with (13)C and (1)H nuclear magnetic resonance spectroscopy. Resveratrol increased lactate labeling but decreased incorporation of (13)C into Krebs cycle intermediates, consistent with effects on AMPK and inhibition of the F0/F1-ATPase. By testing with resveratrol that was directly applied to astrocytes with a Seahorse analyzer, increased glycolytic shift and increased mitochondrial proton leak resulting from interactions of resveratrol with the mitochondrial electron transport chain were revealed. SRT1720, by contrast, stimulated incorporation of (13)C into Krebs cycle intermediates and reduced incorporation into lactate, although the inhibitor EX527 paradoxically also increased Krebs cycle (13)C incorporation. In summary, the various SIRT1 modulators show distinct acute effects on oxidative metabolism. The strong effects of resveratrol on the mitochondrial respiratory chain and on glycolysis suggest that caution should be used in attempts to increase bioavailability of this compound in the CNS.

The anticonvulsant actions of carisbamate associate with alterations in astrocyte glutamine metabolism in the lithium-pilocarpine epilepsy model

As reported previously, in the lithium-pilocarpine model of temporal lobe epilepsy (TLE), carisbamate (CRS) produces strong neuroprotection, leads to milder absence-like seizures, and prevents behavioral impairments in a subpopulation of rats. To understand the metabolic basis of these effects, here we injected 90 mg/kg CRS or vehicle twice daily for 7 days starting 1 h after status epilepticus (SE) induction in rats. Two months later, we injected [1-¹³ C]glucose and [1,2-¹³ C]acetate followed by head microwave fixation after 15 min. ¹³ C incorporation into metabolites was analyzed using ¹³ C magnetic resonance spectroscopy. We found that SE reduced neuronal mitochondrial metabolism in the absence but not in the presence of CRS. Reduction in glutamate level was prevented by CRS and aspartate levels were similar to controls only in rats displaying absence-like seizures after treatment [CRS-absence-like epilepsy (ALE)]. Glutamine levels in CRS-ALE rats were higher compared to controls in hippocampal formation and limbic structures while unchanged in rats displaying motor spontaneous recurrent seizures after treatment (CRS-TLE). Astrocytic mitochondrial metabolism was reduced in CRS-TLE, and either enhanced or unaffected in CRS-ALE rats, which did not affect the transfer of glutamine from astrocytes to neurons. In conclusion, CRS prevents reduction in neuronal mitochondrial metabolism but its effect on astrocytes is likely key in determining outcome of treatment in this model. To understand the metabolic basis of the strong neuroprotection and reduction in seizure severity caused by carisbamate (CRS) in the lithium-pilocarpine (Li-Pilo) model of temporal lobe epilepsy (TLE), we injected CRS for 7 days starting 1 h after status epilepticus and 2 months later [1-¹³ C]glucose and [1,2-¹³ C]acetate. ¹³ C Magnetic resonance spectroscopy analysis was performed on brain extracts and we found that CRS prevented reduction in neuronal mitochondrial metabolism but its effect on astrocytes was likely key in determining outcome of treatment in this model. ALE = absence like epilepsy; acetyl CoA = acetyl coenzyme A; GS = glutamine synthetase; PAG = phosphate activated glutaminase; PC = pyruvate carboxylase; OAA = oxaloacetate; TCA cycle = tricarboxylic acid cycle.

The molecular mechanisms affecting N-acetylaspartate homeostasis following experimental graded traumatic brain injury

To characterize the molecular mechanisms of N-acetylaspartate (NAA) metabolism following traumatic brain injury (TBI), we measured the NAA, adenosine triphosphate (ATP) and adenosine diphosphate (ADP) concentrations and calculated the ATP/ADP ratio at different times from impact, concomitantly evaluating the gene and protein expressions controlling NAA homeostasis (the NAA synthesizing and degrading enzymes N-acetyltransferase 8-like and aspartoacylase, respectively) in rats receiving either mild or severe TBI. The reversible changes in NAA induced by mild TBI were due to a combination of transient mitochondrial malfunctioning with energy crisis (decrease in ATP and in the ATP/ADP ratio) and modulation in the gene and protein levels of N-acetyltransferase 8-like and increase of aspartoacylase levels. The irreversible decrease in NAA following severe TBI, was instead characterized by profound mitochondrial malfunctioning (constant 65% decrease of the ATP/ADP indicating permanent impairment of the mitochondrial phosphorylating capacity), dramatic repression of the N-acetyltransferase 8-like gene and concomitant remarkable increase in the aspartoacylase gene and protein levels. The mechanisms underlying changes in NAA homeostasis following graded TBI might be of note for possible new therapeutic approaches and will help in understanding the effects of repeat concussions occurring during particular periods of the complex NAA recovery process, coincident with the so called window of brain vulnerability.