Glutamate oxidation in astrocytes: Roles of glutamate dehydrogenase and aminotransferases

Role of Astrocytic Mitochondria in Limiting Ischemic Brain Injury?

Until recently, astrocyte processes were thought to be too small to contain mitochondria. However, it is now clear that mitochondria are found throughout fine astrocyte processes and are mobile with neuronal activity resulting in positioning near synapses. In this review, we discuss evidence that astrocytic mitochondria confer selective resiliency to astrocytes during ischemic insults and the functional significance of these mitochondria for normal brain function.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

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.

Zika virus-induced hyper excitation precedes death of mouse primary neuron

BACKGROUND

Zika virus infection in new born is linked to congenital syndromes, especially microcephaly. Studies have shown that these neuropathies are the result of significant death of neuronal progenitor cells in the central nervous system of the embryo, targeted by the virus. Although cell death via apoptosis is well acknowledged, little is known about possible pathogenic cellular mechanisms triggering cell death in neurons.

METHODS

We used in vitro embryonic mouse primary neuron cultures to study possible upstream cellular mechanisms of cell death. Neuronal networks were grown on microelectrode array and electrical activity was recorded at different times post Zika virus infection. In addition to this method, we used confocal microscopy and Q-PCR techniques to observe morphological and molecular changes after infection.

RESULTS

Zika virus infection of mouse primary neurons triggers an early spiking excitation of neuron cultures, followed by dramatic loss of this activity. Using NMDA receptor antagonist, we show that this excitotoxicity mechanism, likely via glutamate, could also contribute to the observed nervous system defects in human embryos and could open new perspective regarding the causes of adult neuropathies.

CONCLUSIONS

This model of excitotoxicity, in the context of neurotropic virus infection, highlights the significance of neuronal activity recording with microelectrode array and possibility of more than one lethal mechanism after Zika virus infection in the nervous system.

GSEA of mouse and human mitochondriomes reveals fatty acid oxidation in astrocytes

The prevalent view in neuroenergetics is that glucose is the main brain fuel, with neurons being mostly oxidative and astrocytes glycolytic. Evidence supporting that astrocyte mitochondria are functional has been overlooked. Here we sought to determine what is unique about astrocyte mitochondria by performing unbiased statistical comparisons of the mitochondriome in astrocytes and neurons. Using MitoCarta, a compendium of mitochondrial proteins, together with transcriptomes of mouse neurons and astrocytes, we generated cell-specific databases of nuclear genes encoding for mitochondrion proteins, ranked according to relative expression. Standard and in-house Gene Set Enrichment Analyses (GSEA) of five mouse transcriptomes revealed that genes encoding for enzymes involved in fatty acid oxidation (FAO) and amino acid catabolism are consistently more expressed in astrocytes than in neurons. FAO and oxidative-metabolism-related genes are also up-regulated in human cortical astrocytes versus the whole cortex, and in adult astrocytes versus fetal astrocytes. We thus present the first evidence of FAO in human astrocytes. Further, as shown in vitro, FAO coexists with glycolysis in astrocytes and is inhibited by glutamate. Altogether, these analyses provide arguments against the glucose-centered view of energy metabolism in astrocytes and reveal mitochondria as specialized organelles in these cells.

Metabolic signaling in the brain and the role of astrocytes in control of glutamate and GABA neurotransmission

Neurotransmission mediated by the two amino acids glutamate and GABA is based on recycling of the two signaling molecules between the presynaptic nerve endings and the surrounding astrocytes. During the recycling process, a fraction of the transmitter pool is lost since both transmitters undergo oxidative metabolism. This loss must be replenished by de novo synthesis which involves the action of pyruvate carboxylase, aminotransferases, glutamate dehydrogenase and glutamine synthetase. Among these enzymes, pyruvate carboxylase and glutamine synthetase are selectively expressed in astrocytes and thus these cells are obligatory partners in synaptic replenishment of both glutamate and GABA. The cycling processes also involve transporters for glutamate, GABA and glutamine and the operation of these transporters is discussed. Additionally, astrocytes appear to be essential for production of the neuromodulators, citrate, glycine and d-serine, aspects that will be briefly discussed.

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.

Impairment of astrocytic glutaminolysis in glutaric aciduria type I

Glutaric aciduria type I is a rare, autosomal recessive, inherited defect of glutaryl-CoA dehydrogenase. Deficiency of this protein in L-lysine degradation leads to the characteristic accumulation of nontoxic glutarylcarnitine and neurotoxic glutaric acid (GA), glutaryl-CoA, and 3-hydroxyglutaric acid. Untreated patients develop bilateral lesions of basal ganglia resulting in a complex movement disorder with predominant dystonia in infancy and early childhood. The current pathomechanistic concept strongly focuses on imbalanced neuronal energy metabolism due to accumulating metabolites, whereas little is known about the pathomechanistic role of astrocytes, which are thought to be in constant metabolic crosstalk with neurons. We found that glutaric acid (GA) causes astrocytic cell death under starvation cell culture conditions, i.e. low glucose, without glutamine and fetal calf serum. Glutamine completely abolished GA-induced toxicity, suggesting involvement of glutaminolysis. Increasing dependence on glutaminolysis by chemical induction of hypoxia signaling-potentiated GA-induced toxicity. We further show that GA disturbs glutamine degradation by specifically inhibiting glutamate dehydrogenase. Summarizing our study shows that pathologically relevant concentrations of GA block an important step in the metabolic crosstalk between neurons and astrocytes, ultimately leading to astrocytic cell death.

Proteinopathies and OXPHOS dysfunction in neurodegenerative diseases

Mitochondria participate in essential processes in the nervous system such as energy and intermediate metabolism, calcium homeostasis, and apoptosis. Major neurodegenerative diseases are characterized pathologically by accumulation of misfolded proteins as a result of gene mutations or abnormal protein homeostasis. Misfolded proteins associate with mitochondria, forming oligomeric and fibrillary aggregates. As mitochondrial dysfunction, particularly of the oxidative phosphorylation system (OXPHOS), occurs in neurodegeneration, it is postulated that such defects are caused by the accumulation of misfolded proteins. However, this hypothesis and the pathological role of proteinopathies in mitochondria remain elusive. In this study, we critically review the proposed mechanisms whereby exemplary misfolded proteins associate with mitochondria and their consequences on OXPHOS.

Glutamate Dehydrogenase, a Complex Enzyme at a Crucial Metabolic Branch Point

In-vitro, glutamate dehydrogenase (GDH) catalyzes the reversible oxidative deamination of glutamate to α-ketoglutarate (α-KG). GDH is found in all organisms, but in animals is allosterically regulated by a wide array of metabolites. For many years, it was not at all clear why animals required such complex control. Further, in both standard textbooks and some research publications, there has been some controversy as to the directionality of the reaction. Here we review recent work demonstrating that GDH operates mainly in the catabolic direction in-vivo and that the finely tuned network of allosteric regulators allows GDH to meet the varied needs in a wide range of tissues in animals. Finally, we review the progress in using pharmacological agents to activate or inhibit GDH that could impact a wide range of pathologies from insulin disorders to tumor growth.

Control of brain energy supply by astrocytes

Astrocytes form an anatomical bridge between the vasculature and neuronal synapses. Recent work suggests that they play a key role in regulating brain energy supply by increasing blood flow to regions where neurons are active, and setting the baseline level of blood flow. Controversy persists over whether lactate derived from astrocyte glycolysis is used to power oxidative phosphorylation in neurons, but astrocytes sustain neuronal ATP production by recycling neurotransmitter glutamate that would otherwise need to be resynthesised from glucose, and by providing a short-term energy store in the form of glycogen that can be mobilised when neurons are active.

Glutamate-Dependent Translational Control of Glutamine Synthetase in Bergmann Glia Cells

Glutamate is the major excitatory transmitter of the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed both in neurons and in glial cells. Recent evidence has shown that glutamate uptake systems, particularly enriched in glia cells, trigger biochemical cascades in a similar fashion as receptors. A tight regulation of glutamate extracellular levels prevents neuronal overstimulation and cell death, and it is critically involved in glutamate turnover. Glial glutamate transporters are responsible of the majority of the brain glutamate uptake activity. Once internalized, this excitatory amino acid is rapidly metabolized to glutamine via the astrocyte-enriched enzyme glutamine synthetase. A coupling between glutamate uptake and glutamine synthesis and release has been commonly known as the glutamate/glutamine shuttle. Taking advantage of the established model of cultured Bergmann glia cells, in this contribution, we explored the gene expression regulation of glutamine synthetase. A time- and dose-dependent regulation of glutamine synthetase protein and activity levels was found. Moreover, glutamate exposure resulted in the transient shift of glutamine synthetase mRNA from the monosomal to the polysomal fraction. These results demonstrate a novel mode of glutamate-dependent glutamine synthetase regulation and strengthen the notion of an exquisite glia neuronal interaction in glutamatergic synapses.

Impaired Hippocampal Glutamate and Glutamine Metabolism in the db/db Mouse Model of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a risk factor for the development of Alzheimer's disease, and changes in brain energy metabolism have been suggested as a causative mechanism. The aim of this study was to investigate the cerebral metabolism of the important amino acids glutamate and glutamine in the db/db mouse model of T2DM. Glutamate and glutamine are both substrates for mitochondrial oxidation, and oxygen consumption was assessed in isolated brain mitochondria by Seahorse XFe96 analysis. In addition, acutely isolated cerebral cortical and hippocampal slices were incubated with [U-¹³C]glutamate and [U-¹³C]glutamine, and tissue extracts were analyzed by gas chromatography-mass spectrometry. The oxygen consumption rate using glutamate and glutamine as substrates was not different in isolated cerebral mitochondria of db/db mice compared to controls. Hippocampal slices of db/db mice exhibited significantly reduced ¹³C labeling in glutamate, glutamine, GABA, citrate, and aspartate from metabolism of [U-¹³C]glutamate. Additionally, reduced ¹³C labeling were observed in GABA, citrate, and aspartate from [U-¹³C]glutamine metabolism in hippocampal slices of db/db mice when compared to controls. None of these changes were observed in cerebral cortical slices. The results suggest specific hippocampal impairments in glutamate and glutamine metabolism, without affecting mitochondrial oxidation of these substrates, in the db/db mouse.

The Glutamate Dehydrogenase Pathway and Its Roles in Cell and Tissue Biology in Health and Disease

Glutamate dehydrogenase (GDH) is a hexameric enzyme that catalyzes the reversible conversion of glutamate to α-ketoglutarate and ammonia while reducing NAD(P)⁺ to NAD(P)H. It is found in all living organisms serving both catabolic and anabolic reactions. In mammalian tissues, oxidative deamination of glutamate via GDH generates α-ketoglutarate, which is metabolized by the Krebs cycle, leading to the synthesis of ATP. In addition, the GDH pathway is linked to diverse cellular processes, including ammonia metabolism, acid-base equilibrium, redox homeostasis (via formation of fumarate), lipid biosynthesis (via oxidative generation of citrate), and lactate production. While most mammals possess a single GDH1 protein (hGDH1 in the human) that is highly expressed in the liver, humans and other primates have acquired, via duplication, an hGDH2 isoenzyme with distinct functional properties and tissue expression profile. The novel hGDH2 underwent rapid evolutionary adaptation, acquiring unique properties that enable enhanced enzyme function under conditions inhibitory to its ancestor hGDH1. These are thought to provide a biological advantage to humans with hGDH2 evolution occurring concomitantly with human brain development. hGDH2 is co-expressed with hGDH1 in human brain, kidney, testis and steroidogenic organs, but not in the liver. In human cerebral cortex, hGDH1 and hGDH2 are expressed in astrocytes, the cells responsible for removing and metabolizing transmitter glutamate, and for supplying neurons with glutamine and lactate. In human testis, hGDH2 (but not hGDH1) is densely expressed in the Sertoli cells, known to provide the spermatids with lactate and other nutrients. In steroid producing cells, hGDH1/2 is thought to generate reducing equivalents (NADPH) in the mitochondria for the biosynthesis of steroidal hormones. Lastly, up-regulation of hGDH1/2 expression occurs in cancer, permitting neoplastic cells to utilize glutamine/glutamate for their growth. In addition, deregulation of hGDH1/2 is implicated in the pathogenesis of several human disorders.

Amino Acid Catabolism in Alzheimer's Disease Brain: Friend or Foe?

There is a dire need to discover new targets for Alzheimer's disease (AD) drug development. Decreased neuronal glucose metabolism that occurs in AD brain could play a central role in disease progression. Little is known about the compensatory neuronal changes that occur to attempt to maintain energy homeostasis. In this review using the PubMed literature database, we summarize evidence that amino acid oxidation can temporarily compensate for the decreased glucose metabolism, but eventually altered amino acid and amino acid catabolite levels likely lead to toxicities contributing to AD progression. Because amino acids are involved in so many cellular metabolic and signaling pathways, the effects of altered amino acid metabolism in AD brain are far-reaching. Possible pathological results from changes in the levels of several important amino acids are discussed. Urea cycle function may be induced in endothelial cells of AD patient brains, possibly to remove excess ammonia produced from increased amino acid catabolism. Studying AD from a metabolic perspective provides new insights into AD pathogenesis and may lead to the discovery of dietary metabolite supplements that can partially compensate for alterations of enzymatic function to delay AD or alleviate some of the suffering caused by the disease.

Neural Signaling Metabolites May Modulate Energy Use in Hibernation

Despite an epidemic in obesity and metabolic syndrome limited means exist to effect adiposity or metabolic rate other than life style changes. Here we review evidence that neural signaling metabolites may modulate thermoregulatory pathways and offer novel means to fine tune energy use. We extend prior reviews on mechanisms that regulate thermogenesis and energy use in hibernation by focusing primarily on the neural signaling metabolites adenosine, AMP and glutamate.

Enzyme Complexes Important for the Glutamate-Glutamine Cycle

Transient multienzyme and/or multiprotein complexes (metabolons) direct substrates toward specific pathways and can significantly influence the metabolism of glutamate and glutamine in the brain. Glutamate is the primary excitatory neurotransmitter in brain. This neurotransmitter has essential roles in normal brain function including learning and memory. Metabolism of glutamate involves the coordinated activity of astrocytes and neurons and high affinity transporter proteins that are selectively distributed on these cells. This chapter describes known and possible metabolons that affect the metabolism of glutamate and related compounds in the brain, as well as some factors that can modulate the association and dissociation of such complexes, including protein modifications by acylation reactions (e.g., acetylation, palmitoylation, succinylation, SUMOylation, etc.) of specific residues. Development of strategies to modulate transient multienzyme and/or enzyme-protein interactions may represent a novel and promising therapeutic approach for treatment of diseases involving dysregulation of glutamate metabolism.