Glutamate dehydrogenase is essential to sustain neuronal oxidative energy metabolism during stimulation

Evolution of Glutamate Metabolism via GLUD2 Enhances Lactate-Dependent Synaptic Plasticity and Complex Cognition

Human evolution is characterized by rapid brain enlargement and the emergence of unique cognitive abilities. Besides its distinctive cytoarchitectural organization and extensive inter-neuronal connectivity, the human brain is also defined by high rates of synaptic, mainly glutamatergic, transmission, and energy utilization. While these adaptations' origins remain elusive, evolutionary changes occurred in synaptic glutamate metabolism in the common ancestor of humans and apes via the emergence of GLUD2, a gene encoding the human glutamate dehydrogenase 2 (hGDH2) isoenzyme. Driven by positive selection, hGDH2 became adapted to function upon intense excitatory firing, a process central to the long-term strengthening of synaptic connections. It also gained expression in brain astrocytes and cortical pyramidal neurons, including the CA1-CA3 hippocampal cells, neurons crucial to cognition. In mice transgenic for GLUD2, theta-burst-evoked long-term potentiation (LTP) is markedly enhanced in hippocampal CA3-CA1 synapses, with patch-clamp recordings from CA1 pyramidal neurons revealing increased sNMDA receptor currents. D-lactate blocked LTP enhancement, implying that glutamate metabolism via hGDH2 potentiates L-lactate-dependent glia-neuron interaction, a process essential to memory consolidation. The transgenic (Tg) mice exhibited increased dendritic spine density/synaptogenesis in the hippocampus and improved complex cognitive functions. Hence, enhancement of neuron-glia communication, via GLUD2 evolution, likely contributed to human cognitive advancement by potentiating synaptic plasticity and inter-neuronal connectivity.

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.

Evolutionary Changes in Primate Glutamate Dehydrogenases 1 and 2 Influence the Protein Regulation by Ligands, Targeting and Posttranslational Modifications

There are two paralogs of glutamate dehydrogenase (GDH) in humans encoded by the GLUD1 and GLUD2 genes as a result of a recent retroposition during the evolution of primates. The two human GDHs possess significantly different regulation by allosteric ligands, which is not fully characterized at the structural level. Recent advances in identification of the GDH ligand binding sites provide a deeper perspective on the significance of the accumulated substitutions within the two GDH paralogs. In this review, we describe the evolution of GLUD1 and GLUD2 after the duplication event in primates using the accumulated sequencing and structural data. A new gibbon GLUD2 sequence questions the indispensability of ancestral R496S and G509A mutations for GLUD2 irresponsiveness to GTP, providing an alternative with potentially similar regulatory features. The data of both GLUD1 and GLUD2 evolution not only confirm substitutions enhancing GLUD2 mitochondrial targeting, but also reveal a conserved mutation in ape GLUD1 mitochondrial targeting sequence that likely reduces its transport to mitochondria. Moreover, the information of GDH interactors, posttranslational modification and subcellular localization are provided for better understanding of the GDH mutations. Medically significant point mutations causing deregulation of GDH are considered from the structural and regulatory point of view.

Glutamate-specific gene linked to human brain evolution enhances synaptic plasticity and cognitive processes

The human brain is characterized by the upregulation of synaptic, mainly glutamatergic, transmission, but its evolutionary origin(s) remain elusive. Here we approached this fundamental question by studying mice transgenic (Tg) for GLUD2, a human gene involved in glutamate metabolism that emerged in the hominoid and evolved concomitantly with brain expansion. We demonstrate that Tg mice express the human enzyme in hippocampal astrocytes and CA1-CA3 pyramidal neurons. LTP, evoked by theta-burst stimulation, is markedly enhanced in the CA3-CA1 synapses of Tg mice, with patch-clamp recordings from CA1 pyramidal neurons revealing increased sNMDA currents. LTP enhancement is blocked by D-lactate, implying that GLUD2 potentiates L-lactate-mediated astrocyte-neuron interaction. Dendritic spine density and synaptogenesis are increased in the hippocampus of Tg mice, which exhibit enhanced responses to sensory stimuli and improved performance on complex memory tasks. Hence, GLUD2 likely contributed to human brain evolution by enhancing synaptic plasticity and metabolic processes central to cognitive functions.

SPARC Controls Migration and Invasion of Hepatocellular Carcinoma Cells Via Regulating GPD2-Mediated Mitochondrial Respiration

Mitochondrial respiration and metabolism play a pivotal role in facilitating the migratory and invasive capacities of cancer cells. In this study, we aimed to explore the potential influence of glycoprotein SPARC on mitochondrial respiration and its subsequent influence on the migration and invasion of hepatocellular carcinoma (HCC) cells. Lentivirus-mediated shRNA delivery was employed to deplete SPARC in HCC cell lines. The mitochondria localization of SPARC was validated using cellular fractionation followed by Western blot analysis, as well as immunofluorescence staining and Proteinase K protection assay. Co-immunoprecipitation was employed to investigate the interaction between SPARC and GPD2. Seahorse XF Cell Mito Stress Test was conducted to assess the mitochondrial respiration and functionality of HCC cells. Our study identifies an active pool of SPARC within the mitochondria of HCC cells, with the mitochondrial subset proving crucial for the regulation of migration and invasion. The mitochondrial SPARC interacts with GPD2, influencing its expression levels and subsequently modulating GPD2-mediated mitochondrial respiration. This regulatory mechanism orchestrates the migratory and invasive phenotypes of HCC cells. Notably, SPARC and GPD2 exhibit upregulated expression in HCC tissues compared to normal liver tissues. High expression levels of both SPARC and GPD2 in HCC patients are associated with a poorer prognosis. Our study unveils a novel role for SPARC in governing HCC cell migration and invasion through regulating GPD2-mediated mitochondrial respiration. These findings underscore the importance of mitochondrial processes in cancer progression and propose the SPARC/GPD2 axis as a promising target for HCC interventions.

Associations between liver function and cerebrospinal fluid biomarkers of Alzheimer's disease pathology in non-demented adults: The CABLE study

Liver function has been suggested as a possible factor in the progression of Alzheimer's disease (AD) development. However, the association between liver function and cerebrospinal fluid (CSF) levels of AD biomarkers remains unclear. In this study, we analyzed the data from 1687 adults without dementia from the Chinese Alzheimer's Biomarker and LifestylE study to investigate differences in liver function between pathological and clinical AD groups, as defined by the 2018 National Institute on Aging-Alzheimer's Association Research Framework. We also examined the linear relationship between liver function, CSF AD biomarkers, and cognition using linear regression models. Furthermore, mediation analyses were applied to explore the potential mediation effects of AD pathological biomarkers on cognition. Our findings indicated that, with AD pathological and clinical progression, the concentrations of total protein (TP), globulin (GLO), and aspartate aminotransferase/alanine transaminase (ALT) increased, while albumin/globulin (A/G), adenosine deaminase, alpha-L-fucosidase, albumin, prealbumin, ALT, and glutamate dehydrogenase (GLDH) concentrations decreased. Furthermore, we also identified significant relationships between TP (β = -0.115, pFDR  < 0.001), GLO (β = -0.184, pFDR  < 0.001), and A/G (β = 0.182, pFDR  < 0.001) and CSF β-amyloid1-42 (Aβ1-42 ) (and its related CSF AD biomarkers). Moreover, after 10 000 bootstrapped iterations, we identified a potential mechanism by which TP and GLDH may affect cognition by mediating CSF AD biomarkers, with mediation effect sizes ranging from 3.91% to 16.44%. Overall, our results suggested that abnormal liver function might be involved in the clinical and pathological progression of AD. Amyloid and tau pathologies also might partially mediate the relationship between liver function and cognition. Future research is needed to fully understand the underlying mechanisms and causality to develop an approach to AD prevention and treatment approach.

A preliminary transcriptomic analysis of the orbitofrontal cortex of antisocial individuals

AIMS

Antisocial personality disorder (ASPD) and conduct disorder (CD) are characterized by a persistent pattern of violations of societal norms and others' rights. Ample evidence shows that the pathophysiology of these disorders is contributed by orbitofrontal cortex (OFC) alterations, yet the underlying molecular mechanisms remain elusive. To address this knowledge gap, we performed the first-ever RNA sequencing study of postmortem OFC samples from subjects with a lifetime diagnosis of ASPD and/or CD.

METHODS

The transcriptomic profiles of OFC samples from subjects with ASPD and/or CD were compared to those of unaffected age-matched controls (n = 9/group).

RESULTS

The OFC of ASPD/CD-affected subjects displayed significant differences in the expression of 328 genes. Further gene-ontology analyses revealed an extensive downregulation of excitatory neuron transcripts and upregulation of astrocyte transcripts. These alterations were paralleled by significant modifications in synaptic regulation and glutamatergic neurotransmission pathways.

CONCLUSION

These preliminary findings suggest that ASPD and CD feature a complex array of functional deficits in the pyramidal neurons and astrocytes of the OFC. In turn, these aberrances may contribute to the reduced OFC connectivity observed in antisocial subjects. Future analyses on larger cohorts are needed to validate these results.

Glutamate dehydrogenase: Potential therapeutic targets for neurodegenerative disease

Glutamate dehydrogenase (GDH) is a key enzyme in mammalian glutamate metabolism. It is located at the intersection of multiple metabolic pathways and participates in a variety of cellular activities. GDH activity is strictly regulated by a variety of allosteric compounds. Here, we review the unique distribution and expressions of GDH in the brain nervous system. GDH plays an essential role in the glutamate-glutamine-GABA cycle between astrocytes and neurons. The dysfunction of GDH may induce the occurrence of many neurodegenerative diseases, such as Parkinson's disease, epilepsy, Alzheimer's disease, schizophrenia, and frontotemporal dementia. GDH activators and gene therapy have been found to protect neurons and improve motor disorders in neurodegenerative diseases caused by glutamate metabolism disorders. To date, no medicine has been discovered that specifically targets neurodegenerative diseases, although several potential medicines are used clinically. Targeting GDH to treat neurodegenerative diseases is expected to provide new insights and treatment strategies.

Glial Glutamine Homeostasis in Health and Disease

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

Integration of Metabolomics and Transcriptomics to Reveal the Metabolic Characteristics of Exercise-Improved Bone Mass

(1) Background: Exercise is effective in promoting and maintaining bone mass. The aim of this study was to detect the exercise-induced metabolic changes in bone tissue of zebrafish. (2) Methods: Thirty-eight zebrafish (Danio rerio, six months old) were analyzed. The exercise group (n = 19) received 8 weeks of counter-current swimming training. The control group (n = 19) was not subjected to exercise. Mineralization was quantified, and alkaline phosphatase (Alp) and anti-tartrate acid phosphatase (Trap) activities were estimated (n = 12). The metabolomics (n = 12) and transcriptomics (n = 14) data of bone tissue were used for the integration analyses. (3) Results: The results showed that the exercise training improved the bone mineralization of zebrafish, e.g., the exercise group (5.74 × 104 ± 7.63 × 103) had a higher mean optical density than the control group (5.26 × 104 ± 8.56 × 103, p = 0.046) for the caudal vertebrae. The amount of mineralized matrix in scales of the exercised zebrafish was also higher (0.156 ± 0.012 vs. 0.102 ± 0.003, p = 0.005). Both histological staining and biochemical analysis revealed increased Alp activity (0.81 ± 0.26 vs. 0.76 ± 0.01, p = 0.002) and decreased Trap activity (1.34 ± 0.01 vs. 1.36 ± 0.01, p = 0.005) in the exercise group. A total of 103 different metabolites (DMs, VIP ≥ 1, fold change (FC) ≥ 1.20 or ≤0.83, p < 0.050) were identified. Alanine, aspartate and glutamate metabolism, β-alanine metabolism, pyrimidine metabolism, and pantothenate and CoA biosynthesis were the significantly enriched metabolic pathways (p < 0.050). A total of 35 genes (q ≤ 0.050 (BH), |Log2FC| ≥ 0.5) were coenriched with the 103 DMs in the four identified pathways. Protein–protein interaction network analysis of the 35 genes showed that entpd3, entpd1, and cmpk2 were the core genes. (4) Conclusions: The results of this study suggest that alanine, aspartate and glutamate metabolism, β-alanine metabolism, pyrimidine metabolism, and pantothenate and CoA biosynthesis contributed to exercise-induced improvements in bone mass.

Single-nucleus transcriptional profiling uncovers the reprogrammed metabolism of astrocytes in Alzheimer's disease

Astrocytes play an important role in the pathogenesis of Alzheimer's disease (AD). It is widely involved in energy metabolism in the brain by providing nutritional and metabolic support to neurons; however, the alteration in the metabolism of astrocytes in AD remains unknown. Through integrative analysis of single-nucleus sequencing datasets, we revealed metabolic changes in various cell types in the prefrontal cortex of patients with AD. We found the depletion of some important metabolites (acetyl-coenzyme A, aspartate, pyruvate, 2-oxoglutarate, glutamine, and others), as well as the inhibition of some metabolic fluxes (glycolysis and tricarbocylic acid cycle, glutamate metabolism) in astrocytes of AD. The abnormality of glutamate metabolism in astrocytes is unique and important. Downregulation of GLUL (GS) and GLUD1 (GDH) may be the cause of glutamate alterations in astrocytes in AD. These results provide a basis for understanding the characteristic changes in astrocytes in AD and provide ideas for the study of AD pathogenesis.

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.

Astrocyte energy and neurotransmitter metabolism in Alzheimer's disease: integration of the glutamate/GABA-glutamine cycle

Astrocytes contribute to the complex cellular pathology of Alzheimer's disease (AD). Neurons and astrocytes function in close collaboration through neurotransmitter recycling, collectively known as the glutamate/GABA-glutamine cycle, which is essential to sustain neurotransmission. Neurotransmitter recycling is intimately linked to astrocyte energy metabolism. In the course of AD, astrocytes undergo extensive metabolic remodeling, which may profoundly affect the glutamate/GABA-glutamine cycle. The consequences of altered astrocyte function and metabolism in relation to neurotransmitter recycling are yet to be comprehended. Metabolic alterations of astrocytes in AD deprive neurons of metabolic support, thereby contributing to synaptic dysfunction and neurodegeneration. In addition, several astrocyte-specific components of the glutamate/GABA-glutamine cycle, including glutamine synthesis and synaptic neurotransmitter uptake, are perturbed in AD. Integration of the complex astrocyte biology within the context of AD is essential for understanding the fundamental mechanisms of the disease, while restoring astrocyte metabolism may serve as an approach to arrest or even revert clinical progression of AD.

AGC1 Deficiency: Pathology and Molecular and Cellular Mechanisms of the Disease

AGC1/Aralar/Slc25a12 is the mitochondrial carrier of aspartate-glutamate, the regulatory component of the NADH malate-aspartate shuttle (MAS) that transfers cytosolic redox power to neuronal mitochondria. The deficiency in AGC1/Aralar leads to the human rare disease named "early infantile epileptic encephalopathy 39" (EIEE 39, OMIM # 612949) characterized by epilepsy, hypotonia, arrested psychomotor neurodevelopment, hypo myelination and a drastic drop in brain aspartate (Asp) and N-acetylaspartate (NAA). Current evidence suggest that neurons are the main brain cell type expressing Aralar. However, paradoxically, glial functions such as myelin and Glutamine (Gln) synthesis are markedly impaired in AGC1 deficiency. Herein, we discuss the role of the AGC1/Aralar-MAS pathway in neuronal functions such as Asp and NAA synthesis, lactate use, respiration on glucose, glutamate (Glu) oxidation and other neurometabolic aspects. The possible mechanism triggering the pathophysiological findings in AGC1 deficiency, such as epilepsy and postnatal hypomyelination observed in humans and mice, are also included. Many of these mechanisms arise from findings in the aralar-KO mice model that extensively recapitulate the human disease including the astroglial failure to synthesize Gln and the dopamine (DA) mishandling in the nigrostriatal system. Epilepsy and DA mishandling are a direct consequence of the metabolic defect in neurons due to AGC1/Aralar deficiency. However, the deficits in myelin and Gln synthesis may be a consequence of neuronal affectation or a direct effect of AGC1/Aralar deficiency in glial cells. Further research is needed to clarify this question and delineate the transcellular metabolic fluxes that control brain functions. Finally, we discuss therapeutic approaches successfully used in AGC1-deficient patients and mice.

The predictive role of preoperative serum glutamate dehydrogenase levels in microvascular invasion and hepatocellular carcinoma prognosis following liver transplantation-a single center retrospective study

Background

As a critical metabolic substrate, glutamine is not only involved in the progression of many cancers but is also related to angiogenesis. Glutamate dehydrogenase (GLDH), a key enzyme in glutamine metabolism, has been reported to regulate tumor proliferation; however, its relationship with microvascular invasion (MVI) is unclear. This study evaluated the ability of preoperative serum GLDH levels to predict MVI and the long-term survival of hepatocellular carcinoma (HCC) patients after liver transplantation (LT).

Methods

HCC patients that underwent LT from January 2015 to May 2020 at the First Affiliated Hospital of Sun Yat-Sen University were enrolled in our retrospective analysis. Clinicopathological variables were extracted from medical records. A receiver operating characteristic curve was created to determine the optimal cut-off value of GLDH for MVI.

Results

Preoperative GLDH was significantly elevated in the MVI-positive group (U = 454.00, p = 0.000). The optimal cut-off value of GLDH for MVI was 7.45 U/L, with an area under the curve of 0.747 (95% CI [0.639–0.856], p = 0.000). The sensitivity was 79.3%, while the specificity was 64.5%. GLDH > 7.45 U/L (p = 0.023) and maximum diameter >5 cm (p = 0.001) were independent risk factors for the presence of MVI. Patients with GLDH > 7.45 U/L had significantly poorer overall survival (p = 0.001) and recurrence-free survival (p = 0.001) after LT than patients with GLDH ≤ 7.45 U/L. Similarly, patients with MVI were associated with poor survival (p = 0.000).

Conclusions

Preoperative elevated serum GLDH levels predict MVI and poorer long-term survival for HCC after LT.

Irisin injection mimics exercise effects on the brain proteome

Physical inactivity can endanger human health and increase the incidence of neurodegenerative disease. Exercise has tremendous beneficial effects on brain health and cognitive function, especially in older adults. It also improves brain-related outcomes in depression, epilepsy and neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease. Irisin is a mediator of the beneficial effects of exercise. This study aimed to assess the proteome alterations in adult male National Maritime Research Institute (NMRI) mice brain tissue upon three different conditions including endurance exercise, resistance exercise and irisin injection. Quantification of irisin levels in blood was performed using irisin-ELISA Kit. Quantification and identification of proteins via two-dimensional gel electrophoresis and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry (MS)/MS showed the alteration of at least 21 proteins due to different treatments. Cellular pathway analysis revealed common beneficial effects of sole irisin treatment and different exercise procedures suggesting the capability of irisin injection to substitute the exercise when physical activity is not possible.

Hippocampal disruptions of synaptic and astrocyte metabolism are primary events of early amyloid pathology in the 5xFAD mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is an unremitting neurodegenerative disorder characterized by cerebral amyloid-β (Aβ) accumulation and gradual decline in cognitive function. Changes in brain energy metabolism arise in the preclinical phase of AD, suggesting an important metabolic component of early AD pathology. Neurons and astrocytes function in close metabolic collaboration, which is essential for the recycling of neurotransmitters in the synapse. However, this crucial metabolic interplay during the early stages of AD development has not been sufficiently investigated. Here, we provide an integrative analysis of cellular metabolism during the early stages of Aβ accumulation in the cerebral cortex and hippocampus of the 5xFAD mouse model of AD. Our electrophysiological examination revealed an increase in spontaneous excitatory signaling in the 5xFAD hippocampus. This hyperactive neuronal phenotype coincided with decreased hippocampal tricarboxylic acid (TCA) cycle metabolism mapped by stable ¹³C isotope tracing. Particularly, reduced astrocyte TCA cycle activity and decreased glutamine synthesis led to hampered neuronal GABA synthesis in the 5xFAD hippocampus. In contrast, the cerebral cortex of 5xFAD mice displayed an elevated capacity for oxidative glucose metabolism, which may suggest a metabolic compensation in this brain region. We found limited changes when we explored the brain proteome and metabolome of the 5xFAD mice, supporting that the functional metabolic disturbances between neurons and astrocytes are early primary events in AD pathology. In addition, synaptic mitochondrial and glycolytic function was selectively impaired in the 5xFAD hippocampus, whereas non-synaptic mitochondrial function was maintained. These findings were supported by ultrastructural analyses demonstrating disruptions in mitochondrial morphology, particularly in the 5xFAD hippocampus. Collectively, our study reveals complex regional and cell-specific metabolic adaptations in the early stages of amyloid pathology, which may be fundamental for the progressing synaptic dysfunctions in AD.

Mitochondrial enzyme GPT2 regulates metabolic mechanisms required for neuron growth and motor function in vivo

The metabolic needs for postnatal growth of the human nervous system are vast. Recessive loss-of-function mutations in the mitochondrial enzyme glutamate pyruvate transaminase 2 (GPT2) in humans cause postnatal undergrowth of brain, and cognitive and motor disability. We demonstrate that GPT2 governs critical metabolic mechanisms in neurons required for neuronal growth and survival. These metabolic processes include neuronal alanine synthesis and anaplerosis, the replenishment of tricarboxylic acid (TCA) cycle intermediates. We performed metabolomics across postnatal development in Gpt2-null mouse brain to identify the trajectory of dysregulated metabolic pathways: alterations in alanine occur earliest; followed by reduced TCA cycle intermediates and reduced pyruvate; followed by elevations in glycolytic intermediates and amino acids. Neuron-specific deletion of GPT2 in mice is sufficient to cause motor abnormalities and death pre-weaning, a phenotype identical to the germline Gpt2-null mouse. Alanine biosynthesis is profoundly impeded in Gpt2-null neurons. Exogenous alanine is necessary for Gpt2-null neuronal survival in vitro, but is not needed for Gpt2-null astrocytes. Dietary alanine supplementation in Gpt2-null mice enhances animal survival, and improves the metabolic profile of Gpt2-null brain, but does not alone appear to correct motor function. In surviving Gpt2-null animals, we observe smaller upper and lower motor neurons in vivo. We also observe selective death of lower motor neurons in vivo with worsening motor behavior with age. In conclusion, these studies of the pathophysiology of GPT2 Deficiency have identified metabolic mechanisms required for neuronal growth and that potentially underlie selective neuronal vulnerabilities in motor neurons.

Filtering of Data-Driven Gene Regulatory Networks Using Drosophila melanogaster as a Case Study

Gene Regulatory Networks (GRNs) allow the study of regulation of gene expression of whole genomes. Among the most relevant advantages of using networks to depict this key process, there is the visual representation of large amounts of information and the application of graph theory to generate new knowledge. Nonetheless, despite the many uses of GRNs, it is still difficult and expensive to assign Transcription Factors (TFs) to the regulation of specific genes. ChIP-Seq allows the determination of TF Binding Sites (TFBSs) over whole genomes, but it is still an expensive technique that can only be applied one TF at a time and requires replicates to reduce its noise. Once TFBSs are determined, the assignment of each TF and its binding sites to the regulation of specific genes is not trivial, and it is often performed by carrying out site-specific experiments that are unfeasible to perform in all possible binding sites. Here, we addressed these relevant issues with a two-step methodology using Drosophila melanogaster as a case study. First, our protocol starts by gathering all transcription factor binding sites (TFBSs) determined with ChIP-Seq experiments available at ENCODE and FlyBase. Then each TFBS is used to assign TFs to the regulation of likely target genes based on the TFBS proximity to the transcription start site of all genes. In the final step, to try to select the most likely regulatory TF from those previously assigned to each gene, we employ GENIE3, a random forest-based method, and more than 9,000 RNA-seq experiments from D. melanogaster. Following, we employed known TF protein-protein interactions to estimate the feasibility of regulatory events in our filtered networks. Finally, we show how known interactions between co-regulatory TFs of each gene increase after the second step of our approach, and thus, the consistency of the TF-gene assignment. Also, we employed our methodology to create a network centered on the Drosophila melanogaster gene Hr96 to demonstrate the role of this transcription factor on mitochondrial gene regulation.

Glutamate metabolism and recycling at the excitatory synapse in health and neurodegeneration

Glutamate is the primary excitatory neurotransmitter of the brain. Cellular homeostasis of glutamate is of paramount importance for normal brain function and relies on an intricate metabolic collaboration between neurons and astrocytes. Glutamate is extensively recycled between neurons and astrocytes in a process known as the glutamate-glutamine cycle. The recycling of glutamate is closely linked to brain energy metabolism and is essential to sustain glutamatergic neurotransmission. However, a considerable amount of glutamate is also metabolized and serves as a metabolic hub connecting glucose and amino acid metabolism in both neurons and astrocytes. Disruptions in glutamate clearance, leading to neuronal overstimulation and excitotoxicity, have been implicated in several neurodegenerative diseases. Furthermore, the link between brain energy homeostasis and glutamate metabolism is gaining attention in several neurological conditions. In this review, we provide an overview of the dynamics of synaptic glutamate homeostasis and the underlying metabolic processes with a cellular focus on neurons and astrocytes. In particular, we review the recently discovered role of neuronal glutamate uptake in synaptic glutamate homeostasis and discuss current advances in cellular glutamate metabolism in the context of Alzheimer's disease and Huntington's disease. Understanding the intricate regulation of glutamate-dependent metabolic processes at the synapse will not only increase our insight into the metabolic mechanisms of glutamate homeostasis, but may reveal new metabolic targets to ameliorate neurodegeneration.

Redirected nuclear glutamate dehydrogenase supplies Tet3 with α-ketoglutarate in neurons

Tet3 is the main α-ketoglutarate (αKG)-dependent dioxygenase in neurons that converts 5-methyl-dC into 5-hydroxymethyl-dC and further on to 5-formyl- and 5-carboxy-dC. Neurons possess high levels of 5-hydroxymethyl-dC that further increase during neural activity to establish transcriptional plasticity required for learning and memory functions. How αKG, which is mainly generated in mitochondria as an intermediate of the tricarboxylic acid cycle, is made available in the nucleus has remained an unresolved question in the connection between metabolism and epigenetics. We show that in neurons the mitochondrial enzyme glutamate dehydrogenase, which converts glutamate into αKG in an NAD⁺-dependent manner, is redirected to the nucleus by the αKG-consumer protein Tet3, suggesting on-site production of αKG. Further, glutamate dehydrogenase has a stimulatory effect on Tet3 demethylation activity in neurons, and neuronal activation increases the levels of αKG. Overall, the glutamate dehydrogenase-Tet3 interaction might have a role in epigenetic changes during neural plasticity.

α-ketoglutarate (αKG) is an intermediate in the tricarboxylic acid cycle that is required in the nucleus for genomic DNA demethylation by Tet3. Here, the authors show that the enzyme glutamate dehydrogenase, which converts glutamate to αKG, is redirected from the mitochondria to the nucleus.

Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-¹³C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM ¹⁵N-labeled NH₄Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

Optogenetic and chemogenetic modulation of astroglial secretory phenotype

Astrocytes play a major role in brain function and alterations in astrocyte function that contribute to the pathogenesis of many brain disorders. The astrocytes are attractive cellular targets for neuroprotection and brain tissue regeneration. Development of novel approaches to monitor and to control astroglial function is of great importance for further progress in basic neurobiology and in clinical neurology, as well as psychiatry. Recently developed advanced optogenetic and chemogenetic techniques enable precise stimulation of astrocytes in vitro and in vivo, which can be achieved by the expression of light-sensitive channels and receptors, or by expression of receptors exclusively activated by designer drugs. Optogenetic stimulation of astrocytes leads to dramatic changes in intracellular calcium concentrations and causes the release of gliotransmitters. Optogenetic and chemogenetic protocols for astrocyte activation aid in extracting novel information regarding the function of brain's neurovascular unit. This review summarizes current data obtained by this approach and discusses a potential mechanistic connection between astrocyte stimulation and changes in brain physiology.

Ammonia inhibits energy metabolism in astrocytes in a rapid and glutamate dehydrogenase 2-dependent manner

Astrocyte dysfunction is a primary factor in hepatic encephalopathy (HE) impairing neuronal activity under hyperammonemia. In particular, the early events causing ammonia-induced toxicity to astrocytes are not well understood. Using established cellular HE models, we show that mitochondria rapidly undergo fragmentation in a reversible manner upon hyperammonemia. Further, in our analyses, within a timescale of minutes, mitochondrial respiration and glycolysis were hampered, which occurred in a pH-independent manner. Using metabolomics, an accumulation of glucose and numerous amino acids, including branched chain amino acids, was observed. Metabolomic tracking of 15N-labeled ammonia showed rapid incorporation of 15N into glutamate and glutamate-derived amino acids. Downregulating human GLUD2 [encoding mitochondrial glutamate dehydrogenase 2 (GDH2)], inhibiting GDH2 activity by SIRT4 overexpression, and supplementing cells with glutamate or glutamine alleviated ammonia-induced inhibition of mitochondrial respiration. Metabolomic tracking of 13C-glutamine showed that hyperammonemia can inhibit anaplerosis of tricarboxylic acid (TCA) cycle intermediates. Contrary to its classical anaplerotic role, we show that, under hyperammonemia, GDH2 catalyzes the removal of ammonia by reductive amination of α-ketoglutarate, which efficiently and rapidly inhibits the TCA cycle. Overall, we propose a critical GDH2-dependent mechanism in HE models that helps to remove ammonia, but also impairs energy metabolism in mitochondria rapidly.

Mitochondria metabonomics of Huangqi Jianzhong Tang against chronic atrophic gastritis

Huangqi Jianzhong Tang (HQJZ) is a representative prescription used for clinical treatment of chronic atrophic gastritis (CAG) in Chinese medicine. Our previous study had revealed that energy regulation was one of the important mechanisms of HQJZ action against CAG. In this study, ultra-high-performance liquid chromatography coupled with quadrupole-Exactive mass spectrometry (UHPLC-Q-Exactive MS) based metabonomics was used to find the potential mitochondrial biomarkers and metabolic pathways of HQJZ in CAG rats, which focused on a specific organelle (mitochondria) isolated from gastric tissue samples. A total of 16 biomarkers from CAG tissues were identified with 11 of these significantly regulated by HQJZ treatment. These biomarkers was mainly involved in glycine, serine, and threonine metabolism; aminoacyl-tRNA biosynthesis metabolism; and taurine and hypotaurine metabolism. Our results show that HQJZ could protect from CAG by altering the mitochondrial function. These findings deepen our understanding of the mitochondrial metabolic changes that occur with CAG and shine a light on the mechanism of HQJZ.

Conditional Knockout of GLT-1 in Neurons Leads to Alterations in Aspartate Homeostasis and Synaptic Mitochondrial Metabolism in Striatum and Hippocampus

Expression of the glutamate transporter GLT-1 in neurons has been shown to be important for synaptic mitochondrial function in the cerebral cortex. Here we determined whether neuronal GLT-1 plays a similar role in the hippocampus and striatum, using conditional GLT-1 knockout mice in which GLT-1 was inactivated in neurons by expression of synapsin-Cre (synGLT-1 KO). Ex vivo ¹³C-labelling using [1,2-¹³C]acetate, representing astrocytic metabolism, yielded increased [4,5-¹³C]glutamate levels, suggesting increased astrocyte-neuron glutamine transfer, in the striatum but not in the hippocampus of the synGLT-1 KO. Moreover, aspartate concentrations were reduced - 38% compared to controls in the hippocampus and the striatum of the synGLT-1 KO. Mitochondria isolated from the hippocampus of synGLT-1 KO mice exhibited a lower oxygen consumption rate in the presence of oligomycin A, indicative of a decreased proton leak across the mitochondrial membrane, whereas the ATP production rate was unchanged. Electron microscopy revealed reduced mitochondrial inter-cristae distance within excitatory synaptic terminals in the hippocampus and striatum of the synGLT-1 KO. Finally, dilution of ¹³C-labelling originating from [U-¹³C]glucose, caused by metabolism of unlabelled glutamate, was reduced in hippocampal synGLT-1 KO synaptosomes, suggesting that neuronal GLT-1 provides glutamate for synaptic tricarboxylic acid cycle metabolism. Collectively, these data demonstrate an important role of neuronal expression of GLT-1 in synaptic mitochondrial metabolism in the forebrain.

Studies on the potential link between antidepressant effect of Xiaoyao San and its pharmacological activity of hepatoprotection based on multi-platform metabolomics

ETHNOPHARMACOLOGICAL RELEVANCE

In traditional Chinese medicine (TCM) theory, depression is considered to be "liver qi stagnation", and relieving "liver qi stagnation" is regarded as an effective method for treating depression. Xiaoyao San (XYS) is a well-known TCM formula for the treatment of depression by relieving "liver qi stagnation". This formula consists of Radix Paeoniae Alba (Paeonia lactiflora Pall.), Radix Bupleuri (Bupleurum chinense DC.), Poria (Poria cocos (Schw.) Wolf), Rhizoma Atractylodis Macrocephalae (Atractylodes macrocephala Koidz.), Radix Angelicae Sinensis (Angelica sinensis (Oliv.) Diels), Radix Glycyrrhizae (Glycyrrhiza uralensis Fisch.), Rhizoma Zingiberis Recens (Zingiber officinale Roscoe) and Herba Menthae Haplocalycis (Mentha haplocalyx Briq.).

AIM OF THE STUDY

Several studies have suggested that depression is associated with liver injury. XYS was a well-known TCM formula for the treatment of depression and liver stagnancy. However, it was still unknown whether the antidepressant effect of XYS is related to the pharmacological activity of hepatoprotection. The aim of this study was to elucidate the potential link between the antidepressant and hepatoprotective effect of XYS.

MATERIALS AND METHODS

A depression rat model was established by the CUMS (chronic unpredictable mild stress) procedure. The antidepressant effect of XYS was assessed by the behavioral indicators, and the hepatoprotective effect of XYS was evaluated through biochemical assays. ¹H-NMR and LC/MS-based liver metabolomics were performed to discover key metabolic pathways involved in the antidepressant and hepatoprotective effects of XYS. Further, the key pathway was validated using commercial kits.

RESULTS

The results demonstrated that XYS pretreatment could significantly improve the depressive symptom induced by CUMS. More importantly, the results demonstrated that liver injury was observed in the CUMS model rats, and XYS had a hepatoprotective effect by reducing the activities of AST and ALT in serum, increasing the levels of SOD and GSH-Px and reducing the contents of MDA, IL-6, and IL-1β in the liver. In addition, the NMR and LC/MS-based metabolomics results indicated that XYS improved 23 of the 35 perturbed potential liver biomarkers that were induced by CUMS. Among them, 9 biomarkers were significantly correlated with both depression and liver pathology, according to Pearson correlation analysis. Metabolic pathway analyses of these 9 biomarkers showed that glutamine and glutamate metabolism were the most important metabolic pathways. Furthermore, to verify glutamine and glutamate metabolism, the levels of glutamine and glutamate, and the activity of glutamine synthetase (GS) and glutaminase (GLS) were quantitatively determined in the liver by commercial kits, and these results were consistent with the metabolomics results.

CONCLUSIONS

XYS could significantly improve the depressive and liver injury symptoms induced by CUMS. The metabolomics results indicate that the regulation of glutamine and glutamate metabolism to maintain the balance of ammonia and promote energy metabolism is a potential junction between the antidepressant and hepatoprotective effects of XYS.

AMP-activated protein kinase (AMPK) regulates astrocyte oxidative metabolism by balancing TCA cycle dynamics

AMP-activated protein kinase (AMPK) is an important energy sensor located in cells throughout the human body. From the periphery, AMPK is known to be a metabolic master switch controlling the use of energy fuels. The energy sensor is activated when the energy status of the cell is low, initiating energy-producing pathways and deactivating energy-consuming pathways. All brain cells are crucially dependent on energy production for survival, and the availability of energy substrates must be closely regulated. Intriguingly, the role of AMPK in the regulation of brain cell metabolism has been sparsely investigated, particularly in astrocytes. By investigating metabolism of ¹³ C-labeled energy substrates in acutely isolated hippocampal slices and cultured astrocytes, with subsequent mass spectrometry analysis, we here show that activation of AMPK increases glycolysis as well as the capacity of the TCA cycle, that is, anaplerosis, through the activity of pyruvate carboxylase (PC) in astrocytes. In addition, we demonstrate that AMPK activation leads to augmented astrocytic glutamate oxidation via pyruvate recycling (i.e., cataplerosis). This regulatory mechanism induced by AMPK activation is mediated via glutamate dehydrogenase (GDH) shown in a CNS-specific GDH knockout mouse. Collectively, these findings demonstrate that AMPK regulates TCA cycle dynamics in astrocytes via PC and GDH activity. AMPK functionality has been shown to be hampered in Alzheimer's and Parkinson's disease and our findings may therefore add to the toolbox for discovery of new metabolic drug targets.

Deletion of Neuronal GLT-1 in Mice Reveals Its Role in Synaptic Glutamate Homeostasis and Mitochondrial Function

The glutamate transporter GLT-1 is highly expressed in astrocytes but also in neurons, primarily in axon terminals. We generated a conditional neuronal GLT-1 KO using synapsin 1-Cre (synGLT-1 KO) to elucidate the metabolic functions of GLT-1 expressed in neurons, here focusing on the cerebral cortex. Both synaptosomal uptake studies and electron microscopic immunocytochemistry demonstrated knockdown of GLT-1 in the cerebral cortex in the synGLT-1 KO mice. Aspartate content was significantly reduced in cerebral cortical extracts as well as synaptosomes from cerebral cortex of synGLT-1 KO compared with control littermates. ¹³C-Labeling of tricarboxylic acid cycle intermediates originating from metabolism of [U-¹³C]-glutamate was significantly reduced in synGLT-1 KO synaptosomes. The decreased aspartate content was due to diminished entry of glutamate into the tricarboxylic acid cycle. Pyruvate recycling, a pathway necessary for full glutamate oxidation, was also decreased. ATP production was significantly increased, despite unaltered oxygen consumption, in isolated mitochondria from the synGLT-1 KO. The density of mitochondria in axon terminals and perisynaptic astrocytes was increased in the synGLT-1 KO. Intramitochondrial cristae density of synGLT-1 KO mice was increased, suggesting increased mitochondrial efficiency, perhaps in compensation for reduced access to glutamate. SynGLT-1 KO synaptosomes exhibited an elevated oxygen consumption rate when stimulated with veratridine, despite a lower baseline oxygen consumption rate in the presence of glucose. GLT-1 expressed in neurons appears to be required to provide glutamate to synaptic mitochondria and is linked to neuronal energy metabolism and mitochondrial function.SIGNIFICANCE STATEMENT All synaptic transmitters need to be cleared from the extracellular space after release, and transporters are used to clear glutamate released from excitatory synapses. GLT-1 is the major glutamate transporter, and most GLT-1 is expressed in astrocytes. Only 5%-10% is expressed in neurons, primarily in axon terminals. The function of GLT-1 in axon terminals remains unknown. Here, we used a conditional KO approach to investigate the significance of the expression of GLT-1 in neurons. We found multiple abnormalities of mitochondrial function, suggesting impairment of glutamate utilization by synaptic mitochondria in the neuronal GLT-1 KO. These data suggest that GLT-1 expressed in axon terminals may be important in maintaining energy metabolism and biosynthetic activities mediated by presynaptic mitochondria.

Localization of Human Glutamate Dehydrogenases Provides Insights into Their Metabolic Role and Their Involvement in Disease Processes

Glutamate dehydrogenase (GDH) catalyzes the reversible deamination of L-glutamate to α-ketoglutarate and ammonia. In mammals, GDH contributes to important processes such as amino acid and carbohydrate metabolism, energy production, ammonia management, neurotransmitter recycling and insulin secretion. In humans, two isoforms of GDH are found, namely hGDH1 and hGDH2, with the former being ubiquitously expressed and the latter found mainly in brain, testis and kidney. These two iso-enzymes display highly divergent allosteric properties, especially concerning their basal activity, ADP activation and GTP inhibition. On the other hand, both enzymes are thought to predominantly localize in the mitochondrial matrix, even though alternative localizations have been proposed. To further study the subcellular localization of the two human iso-enzymes, we created HEK293 cell lines stably over-expressing hGDH1 and hGDH2. In these cell lines, immunofluorescence and enzymatic analyses verified the overexpression of both hGDH1 and hGDH2 iso-enzymes, whereas subcellular fractionation followed by immunoblotting showed their predominantly mitochondrial localization. Given that previous studies have only indirectly compared the subcellular localization of the two iso-enzymes, we co-expressed them tagged with different fluorescent dyes (green and red fluorescent protein for hGDH1 and hGDH2, respectively) and found them to co-localize. Despite the wealth of information related to the functional properties of hGDH1 and hGDH2 and the availability of the hGDH1 structure, there is still an ongoing debate concerning their metabolic role and their involvement in disease processes. Data on the localization of hGDHs, as the ones presented here, could contribute to better understanding of the function of these important human enzymes.

Dynamic metabolic patterns tracking neurodegeneration and gliosis following 26S proteasome dysfunction in mouse forebrain neurons

Metabolite profiling is an important tool that may better capture the multiple features of neurodegeneration. With the considerable parallels between mouse and human metabolism, the use of metabolomics in mouse models with neurodegenerative pathology provides mechanistic insight and ready translation into aspects of human disease. Using 400 MHz nuclear magnetic resonance spectroscopy we have carried out a temporal region-specific investigation of the metabolome of neuron-specific 26S proteasome knockout mice characterised by progressive neurodegeneration and Lewy-like inclusion formation in the forebrain. An early significant decrease in N-acetyl aspartate revealed evidence of neuronal dysfunction before cell death that may be associated with changes in brain neuroenergetics, underpinning the use of this metabolite to track neuronal health. Importantly, we show early and extensive activation of astrocytes and microglia in response to targeted neuronal dysfunction in this context, but only late changes in myo-inositol; the best established glial cell marker in magnetic resonance spectroscopy studies, supporting recent evidence that additional early neuroinflammatory markers are needed. Our results extend the limited understanding of metabolite changes associated with gliosis and provide evidence that changes in glutamate homeostasis and lactate may correlate with astrocyte activation and have biomarker potential for tracking neuroinflammation.

Glutamate Dehydrogenase as a Neuroprotective Target Against Neurodegeneration

Regulation of glutamate metabolism via glutamate dehydrogenase (GDH) might be the promising therapeutic approach for treating neurodegenerative disorders. In the central nervous system, glutamate functions both as a major excitatory neurotransmitter and as a key intermediate metabolite for neurons. GDH converts glutamate to α-ketoglutarate, which serves as a TCA cycle intermediate. Dysregulated GDH activity in the central nervous system is highly correlated with neurological disorders. Indeed, studies conducted with mutant mice and allosteric drugs have shown that deficient or overexpressed GDH activity in the brain can regulate whole body energy metabolism and affect early onset of Parkinson's disease, Alzheimer's disease, temporal lobe epilepsy, and spinocerebellar atrophy. Moreover, in strokes with excitotoxicity as the main pathophysiology, mice that overexpressed GDH exhibited smaller ischemic lesion than mice with normal GDH expression. In additions, GDH activators improve lesions in vivo by increasing α-ketoglutarate levels. In neurons exposed to an insult in vitro, enhanced GDH activity increases ATP levels. Thus, in an energy crisis, neuronal mitochondrial activity is improved and excitotoxic risk is reduced. Consequently, modulating GDH activity in energy-depleted conditions could be a sound strategy for maintaining the mitochondrial factory in neurons, and thus, protect against metabolic failure.