Astroglial contribution to brain energy metabolism in humans revealed by 13C nuclear magnetic resonance spectroscopy: elucidation of the dominant pathway for neurotransmitter glutamate repletion and measurement of astrocytic oxidative metabolism

Circadian Regulation of the Lactate Metabolic Kinetics in Mice Using the [1H-13C]-NMR Technique

Lactate is not only the energy substrate of neural cells, but also an important signal molecule in brain. In modern societies, disturbed circadian rhythms pose a global challenge. Therefore, exploring the influence of circadian period on lactate and its metabolic kinetics is essential for the advancement of neuroscientific research. In the present study, the different groups of mice (L: 8:00 a.m.; D: 20:00 p.m.; SD: 20:00 p.m. with 12 h acute sleep deprivation) were infused with [3-13C] lactate through the lateral tail vein for a duration of 2 min. After 30-min lactate metabolism, the animals were euthanized and the tissues of brain and liver were obtained and extracted, and then, the [1H-13C] NMR technology was employed to investigate the kinetic information of lactate metabolism in different brain regions and liver to detect the enrichment of various metabolic kinetic information. Results revealed the fluctuating lactate concentrations in the brain throughout the day, with lower levels during light periods and higher levels during dark periods. Most metabolites displayed strong sensitivity to circadian rhythm, exhibiting significant day-night variations. Conversely, only a few metabolites showed changes after acute sleep deprivation, primarily in the temporal brain region. Interestingly, in contrast to brain lactate metabolism, liver lactate metabolism exhibited a significant increase following acute sleep deprivation. This study explored the kinetics of lactate metabolism, hinted at potential clinical implications for disorders involving circadian rhythm disturbances, and providing a new research basis for clinical exploration of brain and liver lactate metabolism.

Soil-dwelling grub larvae of Protaetia brevitarsis biodegrade polystyrene: Responses of gut microbiome and host metabolism

Plastic pollution poses a significant threat to terrestrial ecosystems, yet the potential for soil fauna to contribute to plastic biodegradation remains largely unexplored. In this study, we reveal that soil-dwelling grubs, Protaetia brevitarsis larvae, can effectively biodegrade polystyrene (PS) plastics. Over a period of 4 weeks, these grubs achieved a remarkable 61.5 % reduction in PS foam mass. This biodegradation was confirmed by the depolymerization of ingested PS, formation of oxidative functional groups, noticeable chemical modifications, and an increase of δ13C of residual PS in frass. Additionally, antibiotic treatment to suppress gut microbes led to variations in the biodegradation process. PS ingestion induced a significant shift in the gut microbiome, promoting the growth of degradation-related bacteria such as Promicromonosporaceae, Bacillaceae, and Paenibacillaceae. Furthermore, the digestion of plastic triggered extensive metabolomic reprogramming of grubs' intestines, enhancing redox capabilities and facilitating PS biodegradation. These results indicate that responsive adaptation of both the gut microbiome and the host's intestinal metabolism contributes to PS degradation. Collectively, these findings demonstrate P. brevitarsis larvae's capability to alleviate soil plastic pollution, and highlight the potential of researching soil fauna further for sustainable plastic waste management solutions.

Fatty acid preference for beta-oxidation in mitochondria of murine cultured astrocytes

The brain utilizes glucose as a primary energy substrate but also fatty acids for the β-oxidation in mitochondria. The β-oxidation is reported to occur mainly in astrocytes, but its capacity and efficacy against different fatty acids remain unknown. Here, we show the fatty acid preference for the β-oxidation in mitochondria of murine cultured astrocytes. Fatty acid oxidation assay using an extracellular flux analyzer showed that saturated or monosaturated fatty acids, palmitic acid and oleic acid, are preferred substrates over polyunsaturated fatty acids like arachidonic acid and docosahexaenoic acid. We also report that fatty acid binding proteins expressed in the astrocytes contribute less to fatty acid transport to mitochondria for β-oxidation. Our results could give insight into understanding energy metabolism through fatty acid consumption in the brain.

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

The Role of Glutamine Homeostasis in Emotional and Cognitive Functions

Glutamine (Gln), a non-essential amino acid, is synthesized de novo by glutamine synthetase (GS) in various organs. In the brain, GS is exclusively expressed in astrocytes under normal physiological conditions, producing Gln that takes part in glutamatergic neurotransmission through the glutamate (Glu)-Gln cycle. Because the Glu-Gln cycle and glutamatergic neurotransmission play a pivotal role in normal brain activity, maintaining Gln homeostasis in the brain is crucial. Recent findings indicated that a neuronal Gln deficiency in the medial prefrontal cortex in rodents led to depressive behaviors and mild cognitive impairment along with lower glutamatergic neurotransmission. In addition, exogenous Gln supplementation has been tested for its ability to overcome neuronal Gln deficiency and reverse abnormal behaviors induced by chronic immobilization stress (CIS). Although evidence is accumulating as to how Gln supplementation contributes to normalizing glutamatergic neurotransmission and the Glu-Gln cycle, there are few reviews on this. In this review, we summarize recent evidence demonstrating that Gln supplementation ameliorates CIS-induced deleterious changes, including an imbalance of the Glu-Gln cycle, suggesting that Gln homeostasis is important for emotional and cognitive functions. This is the first review of detailed mechanistic studies on the effects of Gln supplementation on emotional and cognitive functions.

Ketamine and rapid antidepressant action: new treatments and novel synaptic signaling mechanisms

Ketamine is an open channel blocker of ionotropic glutamatergic N-Methyl-D-Aspartate (NMDA) receptors. The discovery of its rapid antidepressant effects in patients with depression and treatment-resistant depression fostered novel effective treatments for mood disorders. This discovery not only provided new insight into the neurobiology of mood disorders but also uncovered fundamental synaptic plasticity mechanisms that underlie its treatment. In this review, we discuss key clinical aspects of ketamine's effect as a rapidly acting antidepressant, synaptic and circuit mechanisms underlying its action, as well as how these novel perspectives in clinical practice and synapse biology form a road map for future studies aimed at more effective treatments for neuropsychiatric disorders.

An Efficient and Cost-Effective Approach to Generate Functional Human Inducible Pluripotent Stem Cell-Derived Astrocytes

Human inducible pluripotent stem cell (hiPSC)-derived astrocytes (iAs) are critical to study astrocytes in health and disease. They provide several advantages over human fetal astrocytes in research, which include consistency, availability, disease modeling, customization, and ethical considerations. The generation of iAs is hampered by the requirement of Matrigel matrix coating for survival and proliferation. We provide a protocol demonstrating that human iAs cultured in the absence of Matrigel are viable and proliferative. Further, through a side-by-side comparison of cultures with and without Matrigel, we show significant similarities in astrocyte-specific profiling, including morphology (shape and structure), phenotype (cell-specific markers), genotype (transcriptional expression), metabolic (respiration), and functional aspects (glutamate uptake and cytokine response). In addition, we report that, unlike other CNS cell types, such as neuronal progenitor cells and neurons, iAs can withstand the absence of Matrigel coating. Our study demonstrates that Matrigel is dispensable for the culture of human iPSC-derived astrocytes, facilitating an easy, streamlined, and cost-effective method of generating these cells.

Involvement of brain metabolism in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) affect a significant portion of the global population and have a substantial social and economic impact worldwide. Most NDDs manifest in early childhood and are characterized by deficits in cognition, communication, social interaction and motor control. Due to a limited understanding of the etiology of NDDs, current treatment options primarily focus on symptom management rather than on curative solutions. Moreover, research on NDDs is problematic due to its reliance on a neurocentric approach. However, recent studies are broadening the scope of research on NDDs, to include dysregulations within a diverse network of brain cell types, including vascular and glial cells. This review aims to summarize studies from the past few decades on potential new contributions to the etiology of NDDs, with a special focus on metabolic signatures of various brain cells. In particular, we aim to convey how the metabolic functions are intimately linked to the onset and/or progression of common NDDs such as autism spectrum disorders, fragile X syndrome, Rett syndrome and Down syndrome.

Boundaries and integration between microbiota, the nervous system, and immunity

The enteric nervous system is largely autonomous, and the central nervous system is compartmentalized behind the blood-brain barrier. Yet the intestinal microbiota shapes gut function, local and systemic immune responses, and central nervous system functions including cognition and mood. In this review, we address how the gut microbiota can profoundly influence neural and immune networks. Although many of the interactions between these three systems originate in the intestinal mucosa, intestinal function and immunity are modulated by neural pathways that connect the gut and brain. Furthermore, a subset of microbe-derived penetrant molecules enters the brain and regulates central nervous system function. Understanding how these seemingly isolated entities communicate has the potential to open up new avenues for therapies and interventions.

1H magnetic resonance spectroscopic imaging of deuterated glucose and of neurotransmitter metabolism at 7 T in the human brain

Impaired glucose metabolism in the brain has been linked to several neurological disorders. Positron emission tomography and carbon-13 magnetic resonance spectroscopic imaging (MRSI) can be used to quantify the metabolism of glucose, but these methods involve exposure to radiation, cannot quantify downstream metabolism, or have poor spatial resolution. Deuterium MRSI (2H-MRSI) is a non-invasive and safe alternative for the quantification of the metabolism of 2H-labelled substrates such as glucose and their downstream metabolic products, yet it can only measure a limited number of deuterated compounds and requires specialized hardware. Here we show that proton MRSI (1H-MRSI) at 7 T has higher sensitivity, chemical specificity and spatiotemporal resolution than 2H-MRSI. We used 1H-MRSI in five volunteers to differentiate glutamate, glutamine, γ-aminobutyric acid and glucose deuterated at specific molecular positions, and to simultaneously map deuterated and non-deuterated metabolites. 1H-MRSI, which is amenable to clinically available magnetic-resonance hardware, may facilitate the study of glucose metabolism in the brain and its potential roles in neurological disorders.

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.

Biological embedding of early trauma: the role of higher prefrontal synaptic strength

Background: Early trauma predicts poor psychological and physical health. Glutamatergic synaptic processes offer one avenue for understanding this relationship, given glutamate's abundance and involvement in reward and stress sensitivity, emotion, and learning. Trauma-induced glutamatergic excitotoxicity may alter neuroplasticity and approach/avoidance tendencies, increasing risk for psychiatric disorders. Studies examine upstream or downstream effects instead of glutamatergic synaptic processes in vivo, limiting understanding of how trauma affects the brain.Objective: In a pilot study using a previously published data set, we examine associations between early trauma and a proposed measure of synaptic strength in vivo in one of the largest human samples to undergo Carbon-13 (13C MRS) magnetic resonance spectroscopy. Participants were 18 healthy controls and 16 patients with PTSD (male and female).Method: Energy per cycle (EPC), which represents the ratio of neuronal oxidative energy production to glutamate neurotransmitter cycling, was generated as a putative measure of glutamatergic synaptic strength.Results: Results revealed that early trauma was positively correlated with EPC in individuals with PTSD, but not in healthy controls. Increased synaptic strength was associated with reduced behavioural inhibition, and EPC showed stronger associations between reward responsivity and early trauma for those with higher EPC.Conclusion: In the largest known human sample to undergo 13C MRS, we show that early trauma is positively correlated with EPC, a direct measure of synaptic strength. Our study findings have implications for pharmacological treatments thought to impact synaptic plasticity, such as ketamine and psilocybin.

Chronic lead exposure disrupts neurometabolic activity in mouse brain: An ex vivo1H-[13C]-NMR study

Lead poisoning has been identified as a problem in adults as well as in children. Chronic exposure to lead has been implicated in neurological disorders such as amyotrophic lateral sclerosis, Parkinson's disease, and Alzheimer's disease. In the present study, we evaluated the impact of chronic lead exposure on cerebral glutamatergic and GABAergic metabolic activity in mice. C57BL6 mice were provided lead acetate in drinking water for two months. The regional cerebral metabolic activity was measured using ¹H-[¹³C]-NMR spectroscopy in conjunction with infusion of [1,6-¹³C₂]glucose. The blood Pb²⁺ increased significantly in lead acetate treated mice. Concomitantly, there was a significant reduction in the forelimb strength. The level of myo-inositol was elevated in the cerebral cortex of mice chronically exposed to lead. The glutamatergic neurometabolic activity was found to be reduced following chronic lead exposure in the cerebral cortex, hippocampus, and striatum. In contrast, the GABAergic fluxes were impaired in the hippocampus and thalamus only. The metabolic fluxes in the cerebellum were unperturbed to Pb²⁺ toxicity. In conclusion, we report that chronic lead exposure in mice leads to an impairment in forelimb strength, and a perturbation in neurometabolism in brain regions involving cognition and movement.

Emerging role of substance and energy metabolism associated with neuroendocrine regulation in tumor cells

Cancer is the second most common cause of mortality in the world. One of the unresolved difficult pathological mechanism issues in malignant tumors is the imbalance of substance and energy metabolism of tumor cells. Cells maintain life through energy metabolism, and normal cells provide energy through mitochondrial oxidative phosphorylation to generate ATP, while tumor cells demonstrate different energy metabolism. Neuroendocrine control is crucial for tumor cells' consumption of nutrients and energy. As a result, better combinatorial therapeutic approaches will be made possible by knowing the neuroendocrine regulating mechanism of how the neuroendocrine system can fuel cellular metabolism. Here, the basics of metabolic remodeling in tumor cells for nutrients and metabolites are presented, showing how the neuroendocrine system regulates substance and energy metabolic pathways to satisfy tumor cell proliferation and survival requirements. In this context, targeting neuroendocrine regulatory pathways in tumor cell metabolism can beneficially enhance or temper tumor cell metabolism and serve as promising alternatives to available treatments.

Prenatal exposure to air pollution is associated with altered brain structure, function, and metabolism in childhood

BACKGROUND

Prenatal exposure to air pollution disrupts cognitive, emotional, and behavioral development. The brain disturbances associated with prenatal air pollution are largely unknown.

METHODS

In this prospective cohort study, we estimated prenatal exposures to fine particulate matter (PM_2.5 ) and polycyclic aromatic hydrocarbons (PAH), and then assessed their associations with measures of brain anatomy, tissue microstructure, neurometabolites, and blood flow in 332 youth, 6-14 years old. We then assessed how those brain disturbances were associated with measures of intelligence, ADHD and anxiety symptoms, and socialization.

RESULTS

Both exposures were associated with thinning of dorsal parietal cortices and thickening of postero-inferior and mesial wall cortices. They were associated with smaller white matter volumes, reduced organization in white matter of the internal capsule and frontal lobe, higher metabolite concentrations in frontal cortex, reduced cortical blood flow, and greater microstructural organization in subcortical gray matter nuclei. Associations were stronger for PM_2.5 in boys and PAH in girls. Youth with low exposure accounted for most significant associations of ADHD, anxiety, socialization, and intelligence measures with cortical thickness and white matter volumes, whereas it appears that high exposures generally disrupted these neurotypical brain-behavior associations, likely because strong exposure-related effects increased the variances of these brain measures.

CONCLUSIONS

The commonality of effects across exposures suggests PM_2.5 and PAH disrupt brain development through one or more common molecular pathways, such as inflammation or oxidative stress. Progressively higher exposures were associated with greater disruptions in local volumes, tissue organization, metabolite concentrations, and blood flow throughout cortical and subcortical brain regions and the white matter pathways interconnecting them. Together these affected regions comprise cortico-striato-thalamo-cortical circuits, which support the regulation of thought, emotion, and behavior.

Measurement of glucose metabolism in the occipital lobe and frontal cortex after oral administration of [1-13C]glucose at 9.4 T

For the first time, labeling effects after oral intake of [1-13C]glucose are observed in the human brain with pure 1H detection at 9.4 T. Spectral time series were acquired using a short-TE 1H MRS MC-semiLASER (Metabolite Cycling semi Localization by Adiabatic SElective Refocusing) sequence in two voxels of 5.4 mL in the frontal cortex and the occipital lobe. High-quality time-courses of [4-13C]glutamate, [4-13C]glutamine, [3-13C]glutamate + glutamine, [2-13C] glutamate+glutamine and [3-13C]aspartate for individual volunteers and additionally, group-averaged time-courses of labeled and non-labeled brain glucose could be obtained. Using a one-compartment model, mean metabolic rates were calculated for each voxel position: The mean rate of the TCA-cycle (Vtca) value was determined to be 1.36 and 0.93 μmol min^-1 g^-1, the mean rate of glutamine synthesis (Vgln) was calculated to be 0.23 and 0.45 μmol min^-1 g^-1, the mean exchange rate between cytosolic amino acids and mitochondrial Krebs cycle intermediates (Vx) rate was found to be 0.57 and 1.21 μmol min^-1 g^-1 for the occipital lobe and the frontal cortex, respectively. These values were in agreement with previously reported data. Altogether, it can be shown that this most simple technique combining oral administration of [1-13C]Glc with pure 1H MRS acquisition is suitable to measure metabolic rates.

Impact of Inhibition of Glutamine and Alanine Transport on Cerebellar Glial and Neuronal Metabolism

The cerebellum, or “little brain”, is often overlooked in studies of brain metabolism in favour of the cortex. Despite this, anomalies in cerebellar amino acid homeostasis in a range of disorders have been reported. Amino acid homeostasis is central to metabolism, providing recycling of carbon backbones and ammonia between cell types. Here, we examined the role of cerebellar amino acid transporters in the cycling of glutamine and alanine in guinea pig cerebellar slices by inhibiting amino acid transporters and examining the resultant metabolism of [1-¹³C]d-glucose and [1,2-¹³C]acetate by NMR spectroscopy and LCMS. While the lack of specific inhibitors of each transporter makes interpretation difficult, by viewing results from experiments with multiple inhibitors we can draw inferences about the major cell types and transporters involved. In cerebellum, glutamine and alanine transfer is dominated by system A, blockade of which has maximum effect on metabolism, with contributions from System N. Inhibition of neural system A isoform SNAT1 by MeAIB resulted in greatly decreased metabolite pools and reduced net fluxes but showed little effect on fluxes from [1,2-¹³C]acetate unlike inhibition of SNAT3 and other glutamine transporters by histidine where net fluxes from [1,2-¹³C]acetate are reduced by ~50%. We interpret the data as further evidence of not one but several glutamate/glutamine exchange pools. The impact of amino acid transport inhibition demonstrates that the cerebellum has tightly coupled cells and that glutamate/glutamine, as well as alanine cycling, play a major role in that part of the brain.

Rates of pyruvate carboxylase, glutamate and GABA neurotransmitter cycling, and glucose oxidation in multiple brain regions of the awake rat using a combination of [2-13C]/[1-13C]glucose infusion and 1H-[13C]NMR ex vivo

Anaplerosis occurs predominately in astroglia through the action of pyruvate carboxylase (PC). The rate of PC (Vpc) has been reported for cerebral cortex (or whole brain) of awake humans and anesthetized rodents, but regional brain rates remain largely unknown and, hence, were subjected to investigation in the current study. Awake male rats were infused with either [2-¹³C]glucose or [1-¹³C]glucose (n = 27/30) for 8, 15, 30, 60 or 120 min, followed by rapid euthanasia with focused-beam microwave irradiation to the brain. Blood plasma and extracts of cerebellum, hippocampus, striatum, and cerebral cortex were analyzed by ¹H-[¹³C]-NMR to establish ¹³C-enrichment time courses for glutamate-C4,C3,C2, glutamine-C4,C3, GABA-C2,C3,C4 and aspartate-C2,C3. Metabolic rates were determined by fitting a three-compartment metabolic model (glutamatergic and GABAergic neurons and astroglia) to the eighteen time courses. Vpc varied by 44% across brain regions, being lowest in the cerebellum (0.087 ± 0.004 µmol/g/min) and highest in striatum (0.125 ± 0.009) with intermediate values in cerebral cortex (0.106 ± 0.005) and hippocampus (0.114 ± 0.005). Vpc constituted 13-19% of the oxidative glucose consumption rate. Combination of cerebral cortical data with literature values revealed a positive correlation between Vpc and the rates of glutamate/glutamine-cycling and oxidative glucose consumption, respectively, consistent with earlier observations.

Metabolic analysis as a driver for discovery, diagnosis, and therapy

Metabolic anomalies contribute to tissue dysfunction. Current metabolism research spans from organelles to populations, and new technologies can accommodate investigation across these scales. Here, we review recent advancements in metabolic analysis, including small-scale metabolomics techniques amenable to organelles and rare cell types, functional screening to explore how cells respond to metabolic stress, and imaging approaches to non-invasively assess metabolic perturbations in diseases. We discuss how metabolomics provides an informative phenotypic dimension that complements genomic analysis in Mendelian and non-Mendelian disorders. We also outline pressing challenges and how addressing them may further clarify the biochemical basis of human disease.

Prefrontal Glutamate Neurotransmission in PTSD: A Novel Approach to Estimate Synaptic Strength in Vivo in Humans

Background

Trauma and chronic stress are believed to induce and exacerbate psychopathology by disrupting glutamate synaptic strength. However, in vivo in human methods to estimate synaptic strength are limited. In this study, we established a novel putative biomarker of glutamatergic synaptic strength, termed energy-per-cycle (EPC). Then, we used EPC to investigate the role of prefrontal neurotransmission in trauma-related psychopathology.

Methods

Healthy controls (n = 18) and patients with posttraumatic stress (PTSD; n = 16) completed ¹³C-acetate magnetic resonance spectroscopy (MRS) scans to estimate prefrontal EPC, which is the ratio of neuronal energetic needs per glutamate neurotransmission cycle (V_TCA/V_Cycle).

Results

Patients with PTSD were found to have 28% reduction in prefrontal EPC (t = 3.0; df = 32, P = .005). There was no effect of sex on EPC, but age was negatively associated with prefrontal EPC across groups (r = -0.46, n = 34, P = .006). Controlling for age did not affect the study results.

Conclusion

The feasibility and utility of estimating prefrontal EPC using ¹³C-acetate MRS were established. Patients with PTSD were found to have reduced prefrontal glutamatergic synaptic strength. These findings suggest that reduced glutamatergic synaptic strength may contribute to the pathophysiology of PTSD and could be targeted by new treatments.

Brain Metabolic Alterations in Alzheimer's Disease

The brain is one of the most energy-consuming organs in the body. Satisfying such energy demand requires compartmentalized, cell-specific metabolic processes, known to be complementary and intimately coupled. Thus, the brain relies on thoroughly orchestrated energy-obtaining agents, processes and molecular features, such as the neurovascular unit, the astrocyte-neuron metabolic coupling, and the cellular distribution of energy substrate transporters. Importantly, early features of the aging process are determined by the progressive perturbation of certain processes responsible for adequate brain energy supply, resulting in brain hypometabolism. These age-related brain energy alterations are further worsened during the prodromal stages of neurodegenerative diseases, namely Alzheimer's disease (AD), preceding the onset of clinical symptoms, and are anatomically and functionally associated with the loss of cognitive abilities. Here, we focus on concrete neuroenergetic features such as the brain's fueling by glucose and lactate, the transporters and vascular system guaranteeing its supply, and the metabolic interactions between astrocytes and neurons, and on its neurodegenerative-related disruption. We sought to review the principles underlying the metabolic dimension of healthy and AD brains, and suggest that the integration of these concepts in the preventive, diagnostic and treatment strategies for AD is key to improving the precision of these interventions.

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.

Integrated Metabolomics and Lipidomics Reveal High Accumulation of Glycerophospholipids in Human Astrocytes under the Lipotoxic Effect of Palmitic Acid and Tibolone Protection

Lipotoxicity is a metabolic condition resulting from the accumulation of free fatty acids in non-adipose tissues which involves a series of pathological responses triggered after chronic exposure to high levels of fatty acids, severely detrimental to cellular homeostasis and viability. In brain, lipotoxicity affects both neurons and other cell types, notably astrocytes, leading to neurodegenerative processes, such as Alzheimer (AD) and Parkinson diseases (PD). In this study, we performed for the first time, a whole lipidomic characterization of Normal Human Astrocytes cultures exposed to toxic concentrations of palmitic acid and the protective compound tibolone, to establish and identify the set of potential metabolites that are modulated under these experimental treatments. The study covered 3843 features involved in the exo- and endo-metabolome extracts obtained from astrocytes with the mentioned treatments. Through multivariate statistical analysis such as PCA (principal component analysis), partial least squares (PLS-DA), clustering analysis, and machine learning enrichment analysis, it was possible to determine the specific metabolites that were affected by palmitic acid insult, such as phosphoethanolamines, phosphoserines phosphocholines and glycerophosphocholines, with their respective metabolic pathways impact. Moreover, our results suggest the importance of tibolone in the generation of neuroprotective metabolites by astrocytes and may be relevant to the development of neurodegenerative processes.

Identification of astroglia-like cardiac nexus glia that are critical regulators of cardiac development and function

Glial cells are essential for functionality of the nervous system. Growing evidence underscores the importance of astrocytes; however, analogous astroglia in peripheral organs are poorly understood. Using confocal time-lapse imaging, fate mapping, and mutant genesis in a zebrafish model, we identify a neural crest-derived glial cell, termed nexus glia, which utilizes Meteorin signaling via Jak/Stat3 to drive differentiation and regulate heart rate and rhythm. Nexus glia are labeled with gfap, glast, and glutamine synthetase, markers that typically denote astroglia cells. Further, analysis of single-cell sequencing datasets of human and murine hearts across ages reveals astrocyte-like cells, which we confirm through a multispecies approach. We show that cardiac nexus glia at the outflow tract are critical regulators of both the sympathetic and parasympathetic system. These data establish the crucial role of glia on cardiac homeostasis and provide a description of nexus glia in the PNS.

A large portion of the astrocyte proteome is dedicated to perivascular endfeet, including critical components of the electron transport chain

The perivascular astrocyte endfoot is a specialized and diffusion-limited subcellular compartment that fully ensheathes the cerebral vasculature. Despite their ubiquitous presence, a detailed understanding of endfoot physiology remains elusive, in part due to a limited understanding of the proteins that distinguish the endfoot from the greater astrocyte body. Here, we developed a technique to isolate astrocyte endfeet from brain tissue, which was used to study the endfoot proteome in comparison to the astrocyte somata. In our approach, brain microvessels, which retain their endfoot processes, were isolated from mouse brain and dissociated, whereupon endfeet were recovered using an antibody-based column astrocyte isolation kit. Our findings expand the known set of proteins enriched at the endfoot from 10 to 516, which comprised more than 1/5th of the entire detected astrocyte proteome. Numerous critical electron transport chain proteins were expressed only at the endfeet, while enzymes involved in glycogen storage were distributed to the somata, indicating subcellular metabolic compartmentalization. The endfoot proteome also included numerous proteins that, while known to have important contributions to blood-brain barrier function, were not previously known to localize to the endfoot. Our findings highlight the importance of the endfoot and suggest new routes of investigation into endfoot function.

NMR-based metabolomics of human cerebrospinal fluid identifies signature of brain death

INTRODUCTION

Brain death (BD) is the irreversible cessation of all functions of the entire brain, including the brainstem. Cerebrospinal fluid (CSF) is a biological liquid that circulates in brain and spine. Metabolomics is able to reveal the response of biological systems to diverse factors in a specific moment or condition. Therefore, the study of this neurological condition through metabolic profiling using high resolution Nuclear Magnetic Resonance (NMR) spectroscopy is important for understanding biochemical events.

OBJECTIVES

The aim of the current study is to identify the metabolomics signature of BD using ¹H-NMR spectroscopy in human CSF.

METHODS

¹H-NMR spectroscopy has been employed for metabolomic untargeted analysis in 46 CSF samples: 22 control and 24 with BD. Spectral data were further subjected to multivariate analysis.

RESULTS

Statistically significant multivariate models separated subject's samples with BD from controls and revealed twenty one discriminatory metabolites. The statistical analysis of control and BD subjects using Orthogonal Projections to Latent Structures Discriminant Analysis (OPLS-DA) model resulted in R²X of 0.733 and Q² of 0.635. An elevation in the concentration of statistically discriminant metabolites in BD was observed.

CONCLUSION

This study identifies a metabolic signature associated with BD and the most relevant enriched selected metabolic pathways.

Systematic investigation of mouse models of Parkinson's disease by transcriptome mapping on a brain-specific genome-scale metabolic network

Genome-scale metabolic networks enable systemic investigation of metabolic alterations caused by diseases by providing interpretation of omics data. Although Mus musculus (mouse) is one of the most commonly used model organisms for neurodegenerative diseases, a brain-specific metabolic network model of mice has not yet been reconstructed. Here we reconstructed the first brain-specific metabolic network model of mice, iBrain674-Mm, by a homology-based approach, which consisted of 992 reactions controlled by 674 genes and distributed over 48 pathways. We validated the newly reconstructed network model by showing that it predicts healthy resting-state metabolic phenotypes of mouse brain compatible with the literature. We later used iBrain674-Mm to interpret various experimental mouse models of Parkinson's Disease (PD) at the transcriptome level. To this end, we applied a constraint-based modelling based biomarker prediction method called TIMBR (Transcriptionally Inferred Metabolic Biomarker Response) to predict altered metabolite production from transcriptomic data. Systemic analysis of seven different PD mouse models by TIMBR showed that the neuronal levels of glutamate, lactate, creatine phosphate, neuronal acetylcholine, bilirubin and formate increased in most of the PD mouse models, whereas the levels of melatonin, epinephrine, astrocytic formate and astrocytic bilirubin decreased. Although most of the predictions were consistent with the literature, there were some inconsistencies among different PD mouse models, signifying that there is no perfect experimental model to reflect PD metabolism. The newly reconstructed brain-specific genome-scale metabolic network model of mice can make important contributions to the interpretation and development of experimental mouse models of PD and other neurodegenerative diseases.

Effects of ketogenic diet and ketone monoester supplement on acute alcohol withdrawal symptoms in male mice

RATIONALE

After alcohol ingestion, the brain partly switches from consumption of glucose to consumption of the alcohol metabolite acetate. In heavy drinkers, the switch persists after abrupt abstinence, leading to the hypothesis that the resting brain may be "starved" when acetate levels suddenly drop during abstinence, despite normal blood glucose, contributing to withdrawal symptoms. We hypothesized that ketone bodies, like acetate, could act as alternative fuels in the brain and alleviate withdrawal symptoms.

OBJECTIVES

We previously reported that a ketogenic diet during alcohol exposure reduced acute withdrawal symptoms in rats. Here, our goals were to test whether (1) we could reproduce our findings, in mice and with longer alcohol exposure; (2) ketone bodies alone are sufficient to reduce withdrawal symptoms (clarifying mechanism); (3) introduction of ketogenic diets at abstinence (a clinically more practical implementation) would also be effective.

METHODS

Male C57BL/6NTac mice had intermittent alcohol exposure for 3 weeks using liquid diet. Somatic alcohol withdrawal symptoms were measured as handling-induced convulsions; anxiety-like behavior was measured using the light-dark transition test. We tested a ketogenic diet, and a ketone monoester supplement with a regular carbohydrate-containing diet.

RESULTS

The regular diet with ketone monoester was sufficient to reduce handling-induced convulsions and anxiety-like behaviors in early withdrawal. Only the ketone monoester reduced handling-induced convulsions when given during abstinence, consistent with faster elevation of blood ketones, relative to ketogenic diet.

CONCLUSIONS

These findings support the potential utility of therapeutic ketosis as an adjunctive treatment in early detoxification in alcohol-dependent patients seeking to become abstinent.

TRIAL REGISTRATION

clinicaltrials.gov NCT03878225, NCT03255031.

Impaired GABAergic and glutamatergic neurometabolic activity in aged mice brain as measured by 1 H-[13 C]-NMR spectroscopy

Healthy aging is associated with a decline in cognitive function, and is a major risk factor for many neurodegenerative diseases. Although, there are several evidence that brain mitochondrial function is altered with aging its significance at the cellular level is elusive. In this study, we have investigated mitochondrial TCA cycle and neurotransmitter cycle fluxes associated with glutamatergic, GABAergic neurons and astroglia in the cerebral cortex and hippocampus of young (6 months) and aged (24 months) C57BL6 mice by using ¹ H-[¹³ C]-NMR spectroscopy together with timed infusion of ¹³ C-labeled glucose and acetate. The ratio V_Cyc /V_TCA was determined from a steady-state [2-¹³ C]acetate experiment. Metabolic fluxes were obtained by fitting a three-compartment metabolic model to ¹³ C turnover of amino acids from glucose. Levels of glutamate, aspartate and taurine were reduced in the cerebral cortex, while glutamine and choline were elevated in the hippocampus of aged mice. Interestingly, the rate of acetate oxidation increased in the cerebral cortex, while the flux of mitochondrial TCA cycle of glutamatergic neurons decreased in the cerebral cortex (P < .0001) and hippocampus (P = .025) of aged mice. The glutamate-glutamine neurotransmitter cycle flux was reduced in the cerebral cortex (P < .0001). The GABAergic TCA cycle flux was reduced in the cerebral cortex (P = .0008), while GABA-glutamine neurotransmitter cycling flux was also reduced in the cerebral cortex (P = .011) and hippocampus (P = .042) of aged brain. In conclusion, the reduction in excitatory and inhibitory neurotransmitter activity of glutamatergic and GABAergic neurons in the cerebral cortex and hippocampus correlates qualitatively with declined cognitive function in aged mice.

Glial Metabolic Reprogramming in Amyotrophic Lateral Sclerosis

ALS is a human neurodegenerative disorder that induces a progressive paralysis of voluntary muscles due to motor neuron loss. The causes are unknown, and there is no curative treatment available. Mitochondrial dysfunction is a hallmark of ALS pathology; however, it is currently unknown whether it is a cause or a consequence of disease progression. Recent evidence indicates that glial mitochondrial function changes to cope with energy demands and critically influences neuronal death and disease progression. Aberrant glial cells detected in the spinal cord of diseased animals are characterized by increased proliferation rate and reduced mitochondrial bioenergetics. These features can be compared with cancer cell behavior of adapting to nutrient microenvironment by altering energy metabolism, a concept known as metabolic reprogramming. We focus on data that suggest that aberrant glial cells in ALS undergo metabolic reprogramming and profound changes in glial mitochondrial activity, which are associated with motor neuron death in ALS. This review article emphasizes on the association between metabolic reprogramming and glial reactivity, bringing new paradigms from the area of cancer research into neurodegenerative diseases. Targeting glial mitochondrial function and metabolic reprogramming may result in promising therapeutic strategies for ALS.

Non-invasive biomarkers for monitoring the immunotherapeutic response to cancer

Immunotherapy is an efficient way to cure cancer by modulating the patient’s immune response. However, the immunotherapy response is heterogeneous and varies between individual patients and cancer subtypes, reinforcing the need for early benefit predictors. Evaluating the infiltration of immune cells in the tumor and changes in cell-intrinsic tumor characteristics provide potential response markers to treatment. However, this approach requires invasive sampling and may not be suitable for real-time monitoring of treatment response. The recent emergence of quantitative imaging biomarkers provides promising opportunities. In vivo imaging technologies that interrogate T cell responses, metabolic activities, and immune microenvironment could offer a powerful tool to monitor the cancer response to immunotherapy. Advances in imaging techniques to identify tumors' immunological characteristics can help stratify patients who are more likely to respond to immunotherapy. This review discusses the metabolic events that occur during T cell activation and differentiation, anti-cancer immunotherapy-induced T cell responses, focusing on non-invasive imaging techniques to monitor T cell metabolism in the search for novel biomarkers of response to cancer immunotherapy.

Bioenergetic adaptations to HIV infection. Could modulation of energy substrate utilization improve brain health in people living with HIV-1?

The human brain consumes more energy than any other organ in the body and it relies on an uninterrupted supply of energy in the form of adenosine triphosphate (ATP) to maintain normal cognitive function. This constant supply of energy is made available through an interdependent system of metabolic pathways in neurons, glia and endothelial cells that each have specialized roles in the delivery and metabolism of multiple energetic substrates. Perturbations in brain energy metabolism is associated with a number of different neurodegenerative conditions including impairments in cognition associated with infection by the Human Immunodeficiency Type 1 Virus (HIV-1). Adaptive changes in brain energy metabolism are apparent early following infection, do not fully normalize with the initiation of antiretroviral therapy (ART), and often worsen with length of infection and duration of anti-retroviral therapeutic use. There is now a considerable amount of cumulative evidence that suggests mild forms of cognitive impairments in people living with HIV-1 (PLWH) may be reversible and are associated with specific modifications in brain energy metabolism. In this review we discuss brain energy metabolism with an emphasis on adaptations that occur in response to HIV-1 infection. The potential for interventions that target brain energy metabolism to preserve or restore cognition in PLWH are also discussed.

The far-reaching HAND of cART: cART effects on astrocytes

Following the introduction of combination antiretroviral therapy (cART), the morbidity and mortality from human immunodeficiency virus (HIV) infection has been drastically curtailed and HIV has now become a chronic manageable disease. Persons living with HIV (PLWH) are living longer and experiencing significant co-morbidities and conditions of aging. NeuroHIV, clinically defined as HIV-Associated Neurocognitive Disorders (HAND) and pathologically manifested by persistent inflammation in the CNS despite cART, is a significant co-morbid condition for PLWH. In the pre-cART era, HIV mediated much of the pathogenesis in the Central Nervous System (CNS); in the cART era, with low to undetectable viremia, other mechanisms may be contributing to persistent neuroinflammation. Emerging data point to the adverse effects at the cellular level of cART, independent of HIV. Astrocytes are the most abundant cells in the CNS, playing vital roles in maintaining CNS homeostasis (e.g. metabolic support to neurons, clearance of neurotransmitters, ion balance, modulation of synaptic functions and maintaining the structural integrity of the blood brain barrier (BBB). Therefore, any disruption of their function will have wide repercussions in the CNS. In this review, we will address current knowledge and gaps on the impact of antiretrovirals (ARVs) on astrocytes and physiologic consequences in the CNS. Understanding the status of this field, will provide a practical framework to elucidate the potential role of cART-mediated dysregulation of astrocytes in neuroHIV pathogenesis and inform therapeutic strategies that are "neuro-friendly". Graphical abstract CNS-penetrating cART have the potential to cause resting astrocytes to become activated into an A1 or neurotoxic phenotype. These cells can in turn secrete inflammatory cytokines that affect surrounding microglia macrophages, as well as neurotoxic factors that impact nearby neurons. In addition, impairment in the physiologic functions of astrocytes will result in altered BBB permeability and disrupted metabolic homeostasis. CNS=Central Nervous System; cART=combined antiretroviral therapy; BBB=blood brain barrier.

Emerging technologies to study glial cells

Development, physiological functions, and pathologies of the brain depend on tight interactions between neurons and different types of glial cells, such as astrocytes, microglia, oligodendrocytes, and oligodendrocyte precursor cells. Assessing the relative contribution of different glial cell types is required for the full understanding of brain function and dysfunction. Over the recent years, several technological breakthroughs were achieved, allowing "glio-scientists" to address new challenging biological questions. These technical developments make it possible to study the roles of specific cell types with medium or high-content workflows and perform fine analysis of their mutual interactions in a preserved environment. This review illustrates the potency of several cutting-edge experimental approaches (advanced cell cultures, induced pluripotent stem cell (iPSC)-derived human glial cells, viral vectors, in situ glia imaging, opto- and chemogenetic approaches, and high-content molecular analysis) to unravel the role of glial cells in specific brain functions or diseases. It also illustrates the translation of some techniques to the clinics, to monitor glial cells in patients, through specific brain imaging methods. The advantages, pitfalls, and future developments are discussed for each technique, and selected examples are provided to illustrate how specific "gliobiological" questions can now be tackled.

Local and Interregional Neurochemical Associations Measured by Magnetic Resonance Spectroscopy for Studying Brain Functions and Psychiatric Disorders

Magnetic resonance spectroscopy (MRS) studies have found significant correlations among neurometabolites (e.g., between glutamate and GABA) across individual subjects and altered correlations in neuropsychiatric disorders. In this article, we discuss neurochemical associations among several major neurometabolites which underpin these observations by MRS. We also illustrate the role of spectral editing in eliminating unwanted correlations caused by spectral overlapping. Finally, we describe the prospects of mapping macroscopic neurochemical associations across the brain and characterizing excitation-inhibition balance of neural networks using glutamate- and GABA-editing MRS imaging.

Computational singular perturbation analysis of brain lactate metabolism

Lactate in the brain is considered an important fuel and signalling molecule for neuronal activity, especially during neuronal activation. Whether lactate is shuttled from astrocytes to neurons or from neurons to astrocytes leads to the contradictory Astrocyte to Neuron Lactate Shuttle (ANLS) or Neuron to Astrocyte Lactate Shuttle (NALS) hypotheses, both of which are supported by extensive, but indirect, experimental evidence. This work explores the conditions favouring development of ANLS or NALS phenomenon on the basis of a model that can simulate both by employing the two parameter sets proposed by Simpson et al. (J Cereb. Blood Flow Metab., 27:1766, 2007) and Mangia et al. (J of Neurochemistry, 109:55, 2009). As most mathematical models governing brain metabolism processes, this model is multi-scale in character due to the wide range of time scales characterizing its dynamics. Therefore, we utilize the Computational Singular Perturbation (CSP) algorithm, which has been used extensively in multi-scale systems of reactive flows and biological systems, to identify components of the system that (i) generate the characteristic time scale and the fast/slow dynamics, (ii) participate to the expressions that approximate the surfaces of equilibria that develop in phase space and (iii) control the evolution of the process within the established surfaces of equilibria. It is shown that a decisive factor on whether the ANLS or NALS configuration will develop during neuronal activation is whether the lactate transport between astrocytes and interstitium contributes to the fast dynamics or not. When it does, lactate is mainly generated in astrocytes and the ANLS hypothesis is realised, while when it doesn't, lactate is mainly generated in neurons and the NALS hypothesis is realised. This scenario was tested in exercise conditions.

Oxidation and alkylation stresses activate ribosome-quality control

Oxidation and alkylation of nucleobases are known to disrupt their base-pairing properties within RNA. It is, however, unclear whether organisms have evolved general mechanism(s) to deal with this damage. Here we show that the mRNA-surveillance pathway of no-go decay and the associated ribosome-quality control are activated in response to nucleobase alkylation and oxidation. Our findings reveal that these processes are important for clearing chemically modified mRNA and the resulting aberrant-protein products. In the absence of Xrn1, the level of damaged mRNA significantly increases. Furthermore, deletion of LTN1 results in the accumulation of protein aggregates in the presence of oxidizing and alkylating agents. This accumulation is accompanied by Hel2-dependent regulatory ubiquitylation of ribosomal proteins. Collectively, our data highlight the burden of chemically damaged mRNA on cellular homeostasis and suggest that organisms evolved mechanisms to counter their accumulation.

Can the emerging field of immunometabolism provide insights into neuroinflammation?

In the past few years it has become increasingly clear that an understanding of the interaction between metabolism and immune function can provide an insight into cellular responses to challenges. Significant progress has been made in terms of how macrophages are metabolically re-programmed in response to inflammatory stimuli but, to date, little emphasis has been placed on evaluating equivalent changes in microglia. The need to make progress is driven by the fact that, while microglial activation and the cell's ability to adopt an inflammatory phenotype is necessary to fulfil the neuroprotective function of the cell, persistent activation of microglia and the associated neuroinflammation is at the heart of several neurodegenerative diseases. Understanding the metabolic changes that accompany microglial responses may broaden our perspective on how dysfunction might arise and be tempered. This review will evaluate the current literature that addresses the interplay between inflammation and metabolic reprogramming in microglia, reflecting on the parallels that exist with macrophages. It will consider the changes that take place with age including those that have been reported in neurons and astrocytes with the development of non-invasive imaging techniques, and reflect on the literature that is currently available relating to metabolic reprogramming of microglia with age and in neurodegeneration. Finally it will consider the possibility that manipulating microglial metabolism may provide a valuable approach to modulating neuroinflammation.

Glial mitochondrial function and dysfunction in health and neurodegeneration

Mitochondria play essential metabolic roles in neural cells. Mitochondrial dysfunction has profound effects on the brain. In primary mitochondrial diseases, mutations that impair specific oxidative phosphorylation (OXPHOS) proteins or OXPHOS assembly factors lead to isolated biochemical defects and a heterogeneous group of clinical phenotypes, including mitochondrial encephalopathies. A broader defect of OXPHOS function, due to mutations in proteins involved in mitochondrial DNA maintenance, mitochondrial biogenesis, or mitochondrial tRNAs can also underlie severe mitochondrial encephalopathies. While primary mitochondrial dysfunction causes rare genetic forms of neurological disorders, secondary mitochondrial dysfunction is involved in the pathophysiology of some of the most common neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. Many studies have investigated mitochondrial function and dysfunction in bulk central nervous system (CNS) tissue. However, the interpretation of these studies has been often complicated by the extreme cellular heterogeneity of the CNS, which includes many different types of neurons and glial cells. Because neurons are especially dependent on OXPHOS for ATP generation, mitochondrial dysfunction is thought to be directly involved in cell autonomous neuronal demise. Despite being metabolically more flexible than neurons, glial mitochondria also play an essential role in the function of the CNS, and have adapted specific metabolic and mitochondrial features to support their diversity of functions. This review analyzes our current understanding and the gaps in knowledge of mitochondrial properties of glia and how they affect neuronal functions, in health and disease.

In vivo 13 C and 1 H-[13 C] MRS studies of neuroenergetics and neurotransmitter cycling, applications to neurological and psychiatric disease and brain cancer

In the last 25 years ¹³ C MRS has been established as the only noninvasive method for measuring glutamate neurotransmission and cell specific neuroenergetics. Although technically and experimentally challenging ¹³ C MRS has already provided important new information on the relationship between neuroenergetics and neuronal function, the high energy cost of brain function in the resting state and the role of altered neuroenergetics and neurotransmitter cycling in disease. In this paper we review the metabolic and neurotransmitter pathways that can be measured by ¹³ C MRS and key findings on the linkage between neuroenergetics, neurotransmitter cycling, and brain function. Applications of ¹³ C MRS to neurological and psychiatric disease as well as brain cancer are reviewed. Recent technological developments that may help to overcome spatial resolution and brain coverage limitations of ¹³ C MRS are discussed.

Analysis of a Mathematical Model for the Glutamate/Glutamine Cycle in the Brain

Our aim in this article is to study the well-posedness and properties of a system with delay which is related with brain glutamate and glutamine kinetics. In particular, we prove the existence and uniqueness of nonnegative solutions. We also give numerical simulations and compare their order of magnitude with experimental data.

Effects of the antidepressant medication duloxetine on brain metabolites in persistent depressive disorder: A randomized, controlled trial

Background

To assess whether patients with Persistent Depressive Disorder (PDD) have abnormal levels of N-acetyl-aspartate (NAA) and whether those levels normalize following treatment with the antidepressant medication duloxetine. Furthermore, we conducted post hoc analyses of other important brain metabolites to understand better the cellular and physiological determinants for changes in NAA levels.

Methods

We acquired proton (1H) magnetic resonance spectroscopic imaging (MRSI) data on a 3 Tesla (3T), GE Magnetic Resonance Imaging (MRI) scanner in 41 patients (39.9±10.4 years, 22 males) with PDD at two time points: before the start and at the end of a 10-week, placebo-controlled, double-blind, randomized controlled trial (RCT) of the antidepressant medication duloxetine. Patients were randomized such that 21 patients received the active medication and 20 patients received placebo during the 10 week period of the trial. In addition, we acquire 1H MRSI data once in 29 healthy controls (37.7±11.2 years, 17 males).

Findings

Patients had significantly higher baseline concentrations of NAA across white matter (WM) pathways and subcortical gray matter, and in direct proportion to the severity of depressive symptoms. NAA concentrations declined in duloxetine-treated patients over the duration of the trial in the direction toward healthy values, whereas concentrations increased in placebo-treated patients, deviating even further away from healthy values. Changes in NAA concentration did not mediate medication effects on reducing symptom severity, however; instead, changes in symptom severity partially mediated the effects of medication on NAA concentration, especially in the caudate and putamen.

Interpretation

These findings, taken together, suggest that PDD is not a direct consequence of elevated NAA concentrations, but that a more fundamental pathophysiological process likely causes PDD and determines the severity of its symptoms. The findings also suggest that although duloxetine normalized NAA concentrations in patients, it did so by modulating the severity of depressive symptoms. Medication presumably reduced depressive symptoms through other, as yet unidentified, brain processes.

Trial registration

ClinicalTrials.gov NCT00360724.

Physiological and pathophysiological roles of hypothalamic astrocytes in metabolism

The role of glial cells, including astrocytes, in metabolic control has received increasing attention in recent years. Although the original interest in these macroglial cells was a result of astrogliosis being observed in the hypothalamus of diet-induced obese subjects, studies have also focused on how they participate in the physiological control of appetite and energy expenditure. Astrocytes express receptors for numerous hormones, growth factors and neuropeptides. Some functions of astrocytes include transport of nutrients and hormones from the circulation to the brain, storage of glycogen, participation in glucose sensing, synaptic plasticity, uptake and metabolism of neurotransmitters, release of substances to modify neurotransmission, and cytokine production, amongst others. In the hypothalamus, these physiological glial functions impact on neuronal circuits that control systemic metabolism to modify their outputs. The initial response of astrocytes to poor dietary habits and obesity involves activation of neuroprotective mechanisms but, with chronic exposure to these situations, hypothalamic astrocytes participate in the development of some of the damaging secondary effects. The present review discusses not only some of the physiological functions of hypothalamic astrocytes in metabolism, but also their role in the secondary complications of obesity, such as insulin resistance and cardiovascular affectations.

Synaptic energy metabolism and neuronal excitability, in sickness and health

Most of the energy produced in the brain is dedicated to supporting synaptic transmission. Glucose is the main fuel, providing energy and carbon skeletons to the cells that execute and support synaptic function: neurons and astrocytes, respectively. It is unclear, however, how glucose is provided to and used by these cells under different levels of synaptic activity. It is even more unclear how diseases that impair glucose uptake and oxidation in the brain alter metabolism in neurons and astrocytes, disrupt synaptic activity, and cause neurological dysfunction, of which seizures are one of the most common clinical manifestations. Poor mechanistic understanding of diseases involving synaptic energy metabolism has prevented the expansion of therapeutic options, which, in most cases, are limited to symptomatic treatments. To shed light on the intersections between metabolism, synaptic transmission, and neuronal excitability, we briefly review current knowledge of compartmentalized metabolism in neurons and astrocytes, the biochemical pathways that fuel synaptic transmission at resting and active states, and the mechanisms by which disorders of brain glucose metabolism disrupt neuronal excitability and synaptic function and cause neurological disease in the form of epilepsy.

Mitochondrial Function in Astrocytes Is Essential for Normal Emergence from Anesthesia in Mice

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics.Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission.

WHAT THIS ARTICLE TELLS US THAT IS NEW

In a tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse, the induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection. However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were half that of the controls.Similarly, the induction EC50s for loss of righting reflex were similar between the control and astrocyte-specific Ndufs4(KO) mice; concentrations for regain of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were much less than the control.Thus, mitochondrial complex I function within astrocytes is essential for normal emergence from anesthesia.

BACKGROUND

In mice, restriction of loss of the mitochondrial complex I gene Ndufs4 to glutamatergic neurons confers a profound hypersensitivity to volatile anesthetics similar to that seen with global genetic knockout of Ndufs4. Astrocytes are crucial to glutamatergic synapse functioning during excitatory transmission. Therefore, the authors examined the role of astrocytes in the anesthetic hypersensitivity of Ndufs4(KO).

METHODS

A tamoxifen-activated astrocyte-specific Ndufs4(KO) mouse was constructed. The specificity of the astrocyte-specific inducible model was confirmed by using the green fluorescent protein reporter line Ai6. Approximately 120 astrocyte-specific knockout and control mice were used for the experiments. Mice were anesthetized with varying concentrations of isoflurane or halothane; loss of righting reflex and response to a tail clamp were determined and quantified as the induction and emergence EC50s. Because norepinephrine has been implicated in emergence from anesthesia and astrocytes respond to norepinephrine to release gliotransmitters, the authors measured norepinephrine levels in the brains of control and knockout Ndufs4 animals.

RESULTS

The induction EC50s for tail clamp in both isoflurane and halothane were similar between the control and astrocyte-specific Ndufs4(KO) mice at 3 weeks after 4-hydroxy tamoxifen injection (induction concentration, EC50(ind)-isoflurane: control = 1.27 ± 0.12, astrocyte-specific knockout = 1.21 ± 0.18, P = 0.495; halothane: control = 1.28 ± 0.05, astrocyte-specific knockout = 1.20 ± 0.05, P = 0.017). However, the emergent concentrations in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the controls for tail clamp; (emergence concentration, EC50(em)-isoflurane: control = 1.18 ± 0.10, astrocyte-specific knockout = 0.67 ± 0.11, P < 0.0001; halothane: control = 1.08 ± 0.09, astrocyte-specific knockout = 0.59 ± 0.12, P < 0.0001). The induction EC50s for loss of righting reflex were also similar between the control and astrocyte-specific Ndufs4(KO) mice (EC50(ind)-isoflurane: control = 1.02 ± 0.10, astrocyte-specific knockout = 0.97 ± 0.06, P = 0.264; halothane: control = 1.03 ± 0.05, astrocyte-specific knockout = 0.99 ± 0.08, P = 0.207). The emergent concentrations for loss of righting reflex in both anesthetics for the astrocyte-specific Ndufs4(KO) mice were less than the control (EC50(em)-isoflurane: control = 1.0 ± 0.07, astrocyte-specific knockout = 0.62 ± 0.12, P < 0.0001; halothane: control = 1.0 ± 0.04, astrocyte-specific KO = 0.64 ± 0.09, P < 0.0001); N ≥ 6 for control and astrocyte-specific Ndufs4(KO) mice. For all tests, similar results were seen at 7 weeks after 4-hydroxy tamoxifen injection. The total norepinephrine content of the brain in global or astrocyte-specific Ndufs4(KO) mice was unchanged compared to control mice.

CONCLUSIONS

The only phenotype of the astrocyte-specific Ndufs4(KO) mouse was a specific impairment in emergence from volatile anesthetic-induced general anesthesia. The authors conclude that normal mitochondrial function within astrocytes is essential for emergence from anesthesia.

Brain Tumors of Glial Origin

Gliomas are a heterogeneous group of tumors with evolving classification based on genotype. Isocitrate dehydrogenase (IDH) mutation is an early event in the formation of some diffuse gliomas, and is the best understood mechanism of their epigenetic dysregulation. Glioblastoma may evolve from lower-grade lesions with IDH mutations, or arise independently from copy number changes in platelet-derived growth factor receptor alpha (PDGFRA) and phosphatase and tensin homolog (PTEN). Several molecular subtypes of glioblastoma arise from a common proneural precursor with a tendency toward transition to a mesenchymal subtype. Following oncogenic transformation, gliomas escape growth arrest through a distinct step of aberrant telomere reverse transcriptase (TERT) expression, or mutations in either alpha thalassemia/mental retardation syndrome (ATRX) or death-domain associated protein (DAXX) genes. Metabolic reprogramming allows gliomas to thrive in harsh microenvironments such as hypoxia, acidity, and nutrient depletion, which contribute to tumor initiation, maintenance, and treatment resistance.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Deuterium metabolic imaging (DMI) for MRI-based 3D mapping of metabolism in vivo

Currently, the only widely available metabolic imaging technique in the clinic is positron emission tomography (PET) detection of the radioactive glucose analog 2-¹⁸F-fluoro-2-deoxy-d-glucose (¹⁸FDG). However, ¹⁸FDG-PET does not inform on metabolism downstream of glucose uptake and often provides ambiguous results in organs with intrinsic high glucose uptake, such as the brain. Deuterium metabolic imaging (DMI) is a novel, noninvasive approach that combines deuterium magnetic resonance spectroscopic imaging with oral intake or intravenous infusion of nonradioactive ²H-labeled substrates to generate three-dimensional metabolic maps. DMI can reveal glucose metabolism beyond mere uptake and can be used with other ²H-labeled substrates as well. We demonstrate DMI by mapping metabolism in the brain and liver of animal models and human subjects using [6,6'-²H₂]glucose or [²H₃]acetate. In a rat glioma model, DMI revealed pronounced metabolic differences between normal brain and tumor tissue, with high-contrast metabolic maps depicting the Warburg effect. We observed similar metabolic patterns and image contrast in two patients with a high-grade brain tumor after oral intake of ²H-labeled glucose. Further, DMI used in rat and human livers showed [6,6'-²H₂]glucose stored as labeled glycogen. DMI is a versatile, robust, and easy-to-implement technique that requires minimal modifications to existing clinical magnetic resonance imaging scanners. DMI has great potential to become a widespread method for metabolic imaging in both (pre)clinical research and the clinic.