Exogenous glutamate is metabolized to glutamine and exported by rat primary astrocyte cultures

Astrocytes in Parkinson's Disease: From Role to Possible Intervention

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the loss of dopaminergic neurons. While neuronal dysfunction is central to PD, astrocytes also play important roles, both positive and negative, and such roles have not yet been fully explored. This literature review serves to highlight these roles and how the properties of astrocytes can be used to increase neuron survivability. Astrocytes normally have protective functions, such as releasing neurotrophic factors, metabolizing glutamate, transferring healthy mitochondria to neurons, or maintaining the blood-brain barrier. However, in PD, astrocytes can become dysfunctional and contribute to neurotoxicity, e.g., via impaired glutamate metabolism or the release of inflammatory cytokines. Therefore, astrocytes represent a double-edged sword. Restoring healthy astrocyte function and increasing the beneficial effects of astrocytes represents a promising therapeutic approach. Strategies such as promoting neurotrophin release, preventing harmful astrocyte reactivity, or utilizing regional astrocyte diversity may help restore neuroprotection.

Astrocyte metabolism and signaling pathways in the CNS

Astrocytes comprise half of the cells in the central nervous system and play a critical role in maintaining metabolic homeostasis. Metabolic dysfunction in astrocytes has been indicated as the primary cause of neurological diseases, such as depression, Alzheimer's disease, and epilepsy. Although the metabolic functionalities of astrocytes are well known, their relationship to neurological disorders is poorly understood. The ways in which astrocytes regulate the metabolism of glucose, amino acids, and lipids have all been implicated in neurological diseases. Metabolism in astrocytes has also exhibited a significant influence on neuron functionality and the brain's neuro-network. In this review, we focused on metabolic processes present in astrocytes, most notably the glucose metabolic pathway, the fatty acid metabolic pathway, and the amino-acid metabolic pathway. For glucose metabolism, we focused on the glycolysis pathway, pentose-phosphate pathway, and oxidative phosphorylation pathway. In fatty acid metabolism, we followed fatty acid oxidation, ketone body metabolism, and sphingolipid metabolism. For amino acid metabolism, we summarized neurotransmitter metabolism and the serine and kynurenine metabolic pathways. This review will provide an overview of functional changes in astrocyte metabolism and provide an overall perspective of current treatment and therapy for neurological disorders.

The Circadian Molecular Machinery in CNS Cells: A Fine Tuner of Neuronal and Glial Activity With Space/Time Resolution

The circadian molecular machinery is a fine timekeeper with the capacity to harmonize physiological and behavioral processes with the external environment. This tight-knit regulation is coordinated by multiple cellular clocks across the body. In this review, we focus our attention on the molecular mechanisms regulated by the clock in different brain areas and within different cells of the central nervous system. Further, we discuss evidence regarding the role of circadian rhythms in the regulation of neuronal activity and neurotransmitter systems. Not only neurons, but also astrocytes and microglia actively participate in the maintenance of timekeeping within the brain, and the diffusion of circadian information among these cells is fine-tuned by neurotransmitters (e.g., dopamine, serotonin, and γ-aminobutyric acid), thus impacting on the core clock machinery. The bidirectional interplay between neurotransmitters and the circadian clockwork is fundamental in maintaining accuracy and precision in daily timekeeping throughout different brain areas. Deepening the knowledge of these correlations allows us to define the basis of drug interventions to restore circadian rhythms, as well as to predict the onset of drug treatment/side effects that might promote daily desynchronization. Furthermore, it may lead to a deeper understanding of the potential impacts of modulations in rhythmic activities on the pace of aging and provide an insight in to the pathogenesis of psychiatric diseases and neurodegenerative disorders.

Effects on Metabolism in Astrocytes Caused by cGAMP, Which Imitates the Initial Stage of Brain Metastasis

The second messenger 2'3'-cyclic-GMP-AMP (cGAMP) is thought to be transmitted from brain carcinomas to astrocytes via gap junctions, which functions to promote metastasis in the brain parenchyma. In the current study, we established a method to introduce cGAMP into astrocytes, which simulates the state of astrocytes that have been invaded by cGAMP around tumors. Astrocytes incorporating cGAMP were analyzed by metabolomics, which demonstrated that cGAMP increased glutamate production and astrocyte secretion. The same trend was observed for γ-aminobutyric acid (GABA). Conversely, glutamine production and secretion were decreased by cGAMP treatment. Due to the fundamental role of astrocytes in regulation of the glutamine-glutamate cycle, such metabolic changes may represent a potential mechanism and therapeutic target for alteration of the central nervous system (CNS) environment and the malignant transformation of brain carcinomas.

Astrocytic Regulation of Synchronous Bursting in Cortical Cultures: From Local to Global

Synchronous bursting (SB) is ubiquitous in neuronal networks and independent of network structure. Although it is known to be driven by glutamatergic neurotransmissions, its underlying mechanism remains unclear. Recent studies show that local glutamate recycle by astrocytes affects nearby neuronal activities, which indicate that the local dynamics might also be the origin of SBs in networks. We investigated the effects of local glutamate dynamics on SBs in both cultures developed on multielectrode array (MEA) systems and a tripartite synapse simulation. Local glutamate uptake by astrocytes was altered by pharmacological targeting of GLT-1 glutamate transporters, whereas neuronal firing activities and synaptic glutamate level was simultaneously monitored with MEA and astrocyte-specific glutamate sensors (intensity-based glutamate-sensing fluorescent reporter), respectively. Global SB properties were significantly altered on targeting GLT-1. Detailed simulation of a network with astrocytic glutamate uptake and recycle mechanisms, conforming with the experimental observations, shows that astrocytes function as a slow negative feedback to neuronal activities in the network. SB in the network can be realized as an alternation between positive and negative feedback in the neurons and astrocytes, respectively. An understanding of glutamate trafficking dynamics is of general application to explain how astrocyte malfunction can result in pathological seizure-like phenomena in neuronal systems.

Hypoxia-reoxygenation of primary astrocytes results in a redistribution of mitochondrial size and mitophagy

Astrocytes serve to maintain proper neuronal function and support neuronal viability, but remain largely understudied in research of cerebral ischemia. Astrocytic mitochondria are core participants in the metabolic activity of astrocytes. The objective of this study is to assess astrocyte mitochondrial competence during hypoxia and post-hypoxia reoxygenation and to determine cellular adaptive and pathological changes in the mitochondrial network. We hypothesize that during metabolic distress in astrocytes; mitochondrial networks undergo a shift in fission-fusion dynamics that results in a change in the morphometric state of the entire mitochondrial network. This mitochondrial network shift may be protective during metabolic distress by priming mitochondrial size and facilitating mitophagy. We demonstrated that hypoxia and post-hypoxia reoxygenation of rat primary astrocytes results in a redistribution of mitochondria to smaller sizes evoked by increased mitochondrial fission. Excessive mitochondrial fission corresponded to Drp-1 dephosphorylation at Ser 637, which preceded mitophagy of relatively small mitochondria. Reoxygenation of astrocytes marked the initiation of elevated mitophagic activity primarily reserved to the perinuclear region where a large number of the smallest mitochondria occurred. Although, during hypoxia astrocytic ATP content was severely reduced, after reoxygenation ATP content returned to near normoxic values and these changes mirrored mitochondrial superoxide production. Concomitant with these changes in astrocytic mitochondria, the number of astrocytic extensions declined only after 10-hours post-hypoxic reoxygenation. Overall, we posit a drastic mitochondrial network change that is triggered by a metabolic crisis during hypoxia; these changes are followed by mitochondrial degradation and retraction of astrocytic extensions during reoxygenation.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca²⁺ release from the astrocytes’ intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Silencing of Transcription Factor Sp1 Promotes SN1 Transporter Regulation by Ammonia in Mouse Cortical Astrocytes

The involvement of the astrocytic SN1 (SNAT3) transporter in ammonia-induced l-glutamine retention was recently documented in mouse-cultured astrocytes. Here we investigated the involvement of specificity protein 1 (Sp1) transcription factor in SN1 regulation in ammonium chloride (“ammonia”)-treated astrocytes. Sp1 expression and its cellular localization were determined using real-time qPCR, Western blot, and confocal microscopy. Sp1 binding to Snat3 promoter was analyzed by chromatin immunoprecipitation. The role of Sp1 in SN1 expression and SN1-mediated [³H]glutamine uptake in ammonia-treated astrocytes was verified using siRNA and mithramycin A. The involvement of protein kinase C (PKC) isoforms in Sp1 level/phosphorylation status was verified using siRNA technology. Sp1 translocation to the nuclei and its enhanced binding to the Snat3 promoter, along with Sp1 dependence of system N-mediated [³H]glutamine uptake, were observed in astrocytes upon ammonia exposure. Ammonia decreased the level of phosphorylated Sp1, and the effect was reinforced by long-term incubation with PKC modulator, phorbol 12-myristate 13-acetate, which is a treatment likely to dephosphorylate Sp1. Furthermore, silencing of the PKCδ isoform appears to enhance the ammonia effect on the Sp1 level. Collectively, the results demonstrate the regulatory role of Sp1 in regulation of SN1 expression and activity in ammonia-treated astrocytes and implicate altered Sp1 phosphorylation status in this capacity.

Glutamate oxidation in astrocytes: Roles of glutamate dehydrogenase and aminotransferases

The cellular distribution of transporters and enzymes related to glutamate metabolism led to the concept of the glutamate-glutamine cycle. Glutamate is released as a neurotransmitter and taken up primarily by astrocytes ensheathing the synapses. The glutamate carbon skeleton is transferred back to the presynaptic neurons as the nonexcitatory amino acid glutamine. The cycle was initially thought to function with a 1:1 ratio between glutamate released and glutamine taken up by neurons. However, studies of glutamate metabolism in astrocytes have shown that a considerable proportion of glutamate undergoes oxidative degradation; thus, quantitative formation of glutamine from the glutamate taken up is not possible. Oxidation of glutamate is initiated by transamination catalyzed by an aminotransferase, or oxidative deamination catalyzed by glutamate dehydrogenase (GDH). We discuss methods available to elucidate the enzymes that mediate this conversion. Methods include pharmacological tools such as the transaminase inhibitor aminooxyacetic acid, studies using GDH knockout mice, and siRNA-mediated knockdown of GDH in astrocytes. Studies in brain slices incubated with [¹⁵ N]glutamate demonstrated activity of GDH in astrocytes in situ. These results, in conjunction with reports in the literature, support the conclusion that GDH is active in astrocytes both in culture and in vivo and that this enzyme plays a significant role in glutamate oxidation. Oxidative metabolism of glutamate, primarily mediated by GDH, but also by transamination by aspartate aminotransferase, provides considerably more energy than is required to maintain the activity of the high-affinity glutamate transporters needed for efficient removal of glutamate from the synaptic cleft. © 2016 Wiley Periodicals, Inc.

Definition, Classification, and Pathophysiology of Canine Glaucoma

Glaucoma is a common ocular condition in humans and dogs leading to optic nerve degeneration and irreversible blindness. Primary glaucoma is a group of spontaneous heterogeneous diseases. Multiple factors are involved in its pathogenesis and these factors vary across human ethnic groups and canine breeds, so the clinical phenotypes are numerous and their classification can be challenging and remain superficial. Aging and oxidative stress are major triggers for the manifestation of disease. Multiple, intertwined inflammatory and biochemical cascades eventually alter cellular and extracellular physiology in the optic nerve and trabecular meshwork and lead to vision loss.

In Vivo NMR Studies of the Brain with Hereditary or Acquired Metabolic Disorders

Metabolic disorders, whether hereditary or acquired, affect the brain, and abnormalities of the brain are related to cellular integrity; particularly in regard to neurons and astrocytes as well as interactions between them. Metabolic disturbances lead to alterations in cellular function as well as microscopic and macroscopic structural changes in the brain with diabetes, the most typical example of metabolic disorders, and a number of hereditary metabolic disorders. Alternatively, cellular dysfunction and degeneration of the brain lead to metabolic disturbances in hereditary neurological disorders with neurodegeneration. Nuclear magnetic resonance (NMR) techniques allow us to assess a range of pathophysiological changes of the brain in vivo. For example, magnetic resonance spectroscopy detects alterations in brain metabolism and energetics. Physiological magnetic resonance imaging (MRI) detects accompanying changes in cerebral blood flow related to neurovascular coupling. Diffusion and T1/T2-weighted MRI detect microscopic and macroscopic changes of the brain structure. This review summarizes current NMR findings of functional, physiological and biochemical alterations within a number of hereditary and acquired metabolic disorders in both animal models and humans. The global view of the impact of these metabolic disorders on the brain may be useful in identifying the unique and/or general patterns of abnormalities in the living brain related to the pathophysiology of the diseases, and identifying future fields of inquiry.

Increased mitochondrial fission and volume density by blocking glutamate excitotoxicity protect glaucomatous optic nerve head astrocytes

Abnormal structure and function of astrocytes have been observed within the lamina cribrosa region of the optic nerve head (ONH) in glaucomatous neurodegeneration. Glutamate excitotoxicity-mediated mitochondrial alteration has been implicated in experimental glaucoma. However, the relationships among glutamate excitotoxicity, mitochondrial alteration and ONH astrocytes in the pathogenesis of glaucoma remain unknown. We found that functional N-methyl-d-aspartate (NMDA) receptors (NRs) are present in human ONH astrocytes and that glaucomatous human ONH astrocytes have increased expression levels of NRs and the glutamate aspartate transporter. Glaucomatous human ONH astrocytes exhibit mitochondrial fission that is linked to increased expression of dynamin-related protein 1 and its phosphorylation at Serine 616. In BAC ALDH1L1 eGFP or Thy1-CFP transgenic mice, NMDA treatment induced axon loss as well as hypertrophic morphology and mitochondrial fission in astrocytes of the glial lamina. In human ONH astrocytes, NMDA treatment in vitro triggered mitochondrial fission by decreasing mitochondrial length and number, thereby reducing mitochondrial volume density. However, blocking excitotoxicity by memantine (MEM) prevented these alterations by increasing mitochondrial length, number and volume density. In glaucomatous DBA/2J (D2) mice, blocking excitotoxicity by MEM inhibited the morphological alteration as well as increased mitochondrial number and volume density in astrocytes of the glial lamina. However, blocking excitotoxicity decreased autophagosome/autolysosome volume density in both astrocytes and axons in the glial lamina of glaucomatous D2 mice. These findings provide evidence that blocking excitotoxicity prevents ONH astrocyte dysfunction in glaucomatous neurodegeneration by increasing mitochondrial fission, increasing mitochondrial volume density and length, and decreasing autophagosome/autolysosome formation. GLIA 2015;63:736-753.

(13)C NMR spectroscopy applications to brain energy metabolism

(13)C nuclear magnetic resonance (NMR) spectroscopy is the method of choice for studying brain metabolism. Indeed, the most convincing data obtained to decipher metabolic exchanges between neurons and astrocytes have been obtained using this technique, thus illustrating its power. It may be difficult for non-specialists, however, to grasp thefull implication of data presented in articles written by spectroscopists. The aim of the review is, therefore, to provide a fundamental understanding of this topic to facilitate the non-specialists in their reading of this literature. In the first part of this review, we present the metabolic fate of (13)C-labeled substrates in the brain in a detailed way, including an overview of some general neurochemical principles. We also address and compare the various spectroscopic strategies that can be used to study brain metabolism. Then, we provide an overview of the (13)C NMR experiments performed to analyze both intracellular and intercellular metabolic fluxes. More particularly, the role of lactate as a potential energy substrate for neurons is discussed in the light of (13)C NMR data. Finally, new perspectives and applications offered by (13)C hyperpolarization are described.

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

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

The glutamate transporter, GLAST, participates in a macromolecular complex that supports glutamate metabolism

GLAST is the predominant glutamate transporter in the cerebellum and contributes substantially to glutamate transport in forebrain. This astroglial glutamate transporter quickly binds and clears synaptically released glutamate and is principally responsible for ensuring that synaptic glutamate concentrations remain low. This process is associated with a significant energetic cost. Compartmentalization of GLAST with mitochondria and proteins involved in energy metabolism could provide energetic support for glutamate transport. Therefore, we performed immunoprecipitation and co-localization experiments to determine if GLAST might co-compartmentalize with proteins involved in energy metabolism. GLAST was immunoprecipitated from rat cerebellum and subunits of the Na(+)/K(+) ATPase, glycolytic enzymes, and mitochondrial proteins were detected. GLAST co-localized with mitochondria in cerebellar tissue. GLAST also co-localized with mitochondria in fine processes of astrocytes in organotypic hippocampal slice cultures. From these data, we hypothesized that mitochondria participate in a macromolecular complex with GLAST to support oxidative metabolism of transported glutamate. To determine the functional metabolic role of this complex, we measured CO(2) production from radiolabeled glutamate in cultured astrocytes and compared it to overall glutamate uptake. Within 15 min, 9% of transported glutamate was converted to CO(2). This CO(2) production was blocked by inhibitors of glutamate transport and glutamate dehydrogenase, but not by an inhibitor of glutamine synthetase. Our data support a model in which GLAST exists in a macromolecular complex that allows transported glutamate to be metabolized in mitochondria to support energy production.

Roles of glutamine in neurotransmission

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor-product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.

Roles of reactive oxygen and nitrogen species in pain

Peroxynitrite (PN; ONOO⁻) and its reactive oxygen precursor superoxide (SO; O₂•⁻) are critically important in the development of pain of several etiologies including pain associated with chronic use of opiates such as morphine (also known as opiate-induced hyperalgesia and antinociceptive tolerance). This is now an emerging field in which considerable progress has been made in terms of understanding the relative contributions of SO, PN, and nitroxidative stress in pain signaling at the molecular and biochemical levels. Aggressive research in this area is poised to provide the pharmacological basis for development of novel nonnarcotic analgesics that are based upon the unique ability to selectively eliminate SO and/or PN. As we have a better understanding of the roles of SO and PN in pathophysiological settings, targeting PN may be a better therapeutic strategy than targeting SO. This is because, unlike PN, which has no currently known beneficial role, SO may play a significant role in learning and memory. Thus, the best approach may be to spare SO while directly targeting its downstream product, PN. Over the past 15 years, our team has spearheaded research concerning the roles of SO and PN in pain and these results are currently leading to the development of solid therapeutic strategies in this important area.

Counter-regulation of opioid analgesia by glial-derived bioactive sphingolipids

The clinical efficacy of opiates for pain control is severely limited by analgesic tolerance and hyperalgesia. Herein we show that chronic morphine upregulates both the sphingolipid ceramide in spinal astrocytes and microglia, but not neurons, and spinal sphingosine-1-phosphate (S1P), the end-product of ceramide metabolism. Coadministering morphine with intrathecal administration of pharmacological inhibitors of ceramide and S1P blocked formation of spinal S1P and development of hyperalgesia and tolerance in rats. Our results show that spinally formed S1P signals at least in part by (1) modulating glial function because inhibiting S1P formation blocked increased formation of glial-related proinflammatory cytokines, in particular tumor necrosis factor-α, interleukin-1βα, and interleukin-6, which are known modulators of neuronal excitability, and (2) peroxynitrite-mediated posttranslational nitration and inactivation of glial-related enzymes (glutamine synthetase and the glutamate transporter) known to play critical roles in glutamate neurotransmission. Inhibitors of the ceramide metabolic pathway may have therapeutic potential as adjuncts to opiates in relieving suffering from chronic pain.

NMDA-receptor activation and nitroxidative regulation of the glutamatergic pathway during nociceptive processing

The role of peroxynitrite (PN) as a mediator of nociceptive signaling is emerging. We recently reported that the development of central sensitization that follows the intraplantar injection of carrageenan in rats is associated with spinal PN synthesis. We now demonstrate that a significant pathway through which spinal PN modulates central sensitization is post-translational tyrosine nitration of key proteins involved in the glutamatergic pathway, namely glutamate transporter GLT-1 and glutamine synthetase (GS). We also reveal that spinal activation of the N-methyl-d-aspartate (NMDA) receptor provides a source of PN in this setting. Intraplantar injection of carrageenan led to the development of thermal hyperalgesia as well as nitration of GLT-1 and GS in dorsal horn tissues. Pretreatment with the PN decomposition catalyst FeTM-4-PyP(5+) [Fe(III)5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin] or the NMDA receptor antagonist MK-801 blocked the development of hyperalgesia. Carrageenan-induced hyperalgesia was also associated with nitration and inactivation of spinal mitochondrial superoxide dismutase (MnSOD) known to provide a critical source of PN during central sensitization. Nitration of GLT-1 and GS contributes to central sensitization by enhancing glutamatergic neurotransmission. Our results support the critical role of nitroxidative stress in the development of hyperalgesia and suggest that post-translational nitration of enzymes and transporters linked to glutamatergic neurotransmission represent a novel mechanism of central sensitization.

The human GLUD2 glutamate dehydrogenase: localization and functional aspects

In all mammals, glutamate dehydrogenase (GDH), an enzyme central to the metabolism of glutamate, is encoded by a single gene (GLUD1 in humans) which is expressed widely (housekeeping). Humans and other primates also possess a second gene, GLUD2, which encodes a highly homologous GDH isoenzyme (hGDH2) expressed predominantly in retina, brain and testis. There is evidence that GLUD1 was retro-posed <23 million years ago to the X chromosome, where it gave rise to GLUD2 through random mutations and natural selection. These mutations provided the novel enzyme with unique properties thought to facilitate its function in the particular milieu of the nervous system. hGDH2, having been dissociated from GTP control (through the Gly456Ala change), is mainly regulated by rising levels of ADP/l-leucine. To achieve full-range regulation by these activators, hGDH2 needs to set its basal activity at low levels (<10% of full capacity), a property largely conferred by the evolutionary Arg443Ser change. Studies of structure/function relationships have identified residues in the regulatory domain of hGDH2 that modify basal catalytic activity and regulation. In addition, enzyme concentration and buffer ionic strength can influence basal enzyme activity. While mature hGDH1 and hGDH2 isoproteins are highly homologous, their predicted leader peptide sequences show a greater degree of divergence. Study of the subcellular sites targeted by hGDH2 in three different cultured cell lines using a GLUD2/EGFP construct revealed that hGDH2 localizes mainly to mitochondria and to a lesser extent to the endoplasmic reticulum of these cells. The implications of these findings for the potential role of this enzyme in the biology of the nervous system in health and disease are discussed.

Proposed cycles for functional glutamate trafficking in synaptic neurotransmission

To date, the glutamate-glutamine cycle has been the dominant paradigm for understanding the coordinated, compartmentalized activities of phosphate-activated glutaminase (PAG) and glutamine synthetase (GS) in support of functional glutamate trafficking in vivo. However, studies in cell cultures have repeatedly challenged the notion that functional glutamate trafficking is accomplished via the glutamate-glutamine cycle alone. The present study introduces and elaborates alternative cycles for functional glutamate trafficking that integrate glucose metabolism, glutamate anabolism, transport, and catabolism, and trafficking of TCA cycle intermediates from astrocytes to presynaptic neurons. Detailed stoichiometry for each of these alternative cycles is established by strict application of the principle of conservation of atomic species to cytosolic and mitochondrial compartments in both presynaptic neurons and astrocytes. In contrast to the glutamate-glutamine cycle, which requires ATP, but not necessarily oxidative metabolism, to function, cycles for functional glutamate trafficking based on intercellular transport of TCA cycle intermediates require oxidative processes to function. These proposed alternative cycles are energetically more efficient than, and incorporate an inherent mechanism for transporting nitrogen from presynaptic neurons to astrocytes in support of the coordinated activities of PAG and GS that is absent in, the glutamate-glutamine cycle. In light of these newly elaborated alternative cycles, it is premature to presuppose that functional glutamate trafficking in synaptic neurotransmission in vivo is sustained by the glutamate-glutamine cycle alone.

Glutamine: a Trojan horse in ammonia neurotoxicity

Mechanisms involved in hepatic encephalopathy still remain to be defined. Nonetheless, it is well recognized that ammonia is a major factor in its pathogenesis, and that the astrocyte represents a major target of its CNS toxicity. In vivo and in vitro studies have shown that ammonia evokes oxidative/nitrosative stress, mitochondrial abnormalities (the mitochondrial permeability transition, MPT) and astrocyte swelling, a major component of the brain edema associated with fulminant hepatic failure. How ammonia brings about these changes in astrocytes is not well understood. It has long been accepted that the conversion of glutamate to glutamine, catalyzed by glutamine synthetase, a cytoplasmic enzyme largely localized to astrocytes in brain, represented the principal means of cerebral ammonia detoxification. Yet, the "benign" aspect of glutamine synthesis has been questioned. This article highlights evidence that, at elevated levels, glutamine is indeed a noxious agent. We also propose a mechanism by which glutamine executes its toxic effects in astrocytes, the "Trojan horse" hypothesis. Much of the newly synthesized glutamine is subsequently metabolized in mitochondria by phosphate-activated glutaminase, yielding glutamate and ammonia. In this manner, glutamine (the Trojan horse) is transported in excess from the cytoplasm to mitochondria serving as a carrier of ammonia. We propose that it is the glutamine-derived ammonia within mitochondria that interferes with mitochondrial function giving rise to excessive production of free radicals and induction of the MPT, two phenomena known to bring about astrocyte dysfunction, including cell swelling. Future therapeutic approaches might include controlling excessive transport of newly synthesized glutamine to mitochondria and its subsequent hydrolysis.

Speciation of manganese in cells and mitochondria: a search for the proximal cause of manganese neurotoxicity

Recent studies of speciation of manganese (Mn) in brain mitochondria, neuron-like cells, and astrocytes are reviewed. No evidence is found for oxidation of Mn(2+) complexes to a Mn(3+) complex. The only evidence for any Mn(3+) complex is found in a spectrum essentially identical to that of mitochondrial manganese superoxide dismutase (MnSOD). While this does not prove that no Mn(3+) is produced in these tissues by oxidation of Mn(2+), it does suggest that formation of an active Mn(3+) complex by oxidation of Mn(2+) probably does not play as important a role in Mn toxicity as has been suggested earlier. Since these results suggest that we should look elsewhere for the proximal causes of Mn neurotoxicity, we consider the possibilities that Mn(3+) may be transported into the cell via transferrin and that Mn(2+) may inhibit Ca(2+)-activation and control of the rate of ATP production by oxidative phosphorylation.

Determining the oxidation states of manganese in NT2 cells and cultured astrocytes

Excessive brain manganese (Mn) can produce a syndrome called "manganism", which correlates with loss of striatal dopamine and cell death in the striatum and globus pallidus. The prevalent hypothesis for the cause of this syndrome has been oxidation of cell components by the strong oxidizing agent, Mn(3+), either formed by oxidation of intracellular Mn(2+) or transported into the cell as Mn(3+). We have recently used X-ray absorption near edge structure spectroscopy (XANES) to determine the oxidation states of manganese complexes in brain and liver mitochondria and in nerve growth factor (NGF)-induced and non-induced PC12 cells. No evidence was found for stabilization or accumulation of Mn(3+) complexes because of oxidation of Mn(2+) by reactive oxygen species in these tissues. Here we extend these studies of manganese oxidation state to cells of brain origin, human neuroteratocarcinoma (NT2) cells and primary cultures of rat astrocytes. Again we find no evidence for stabilization or accumulation of any Mn(3+) complex derived from oxidation of Mn(2+) under a range of conditions.

Molecular basis of human glutamate dehydrogenase regulation under changing energy demands

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is located in the mitochondria although there is evidence for extramitochondrial localization of GDH. In the human, housekeeping and nerve tissue-specific isoforms, encoded by the GLUD1 and GLUD2 genes, have been identified. The two isoenzymes differ markedly in their baseline activities, allosteric regulation, and thermal stability. GTP potently inhibits GLUD1-derived GDH (IC(50) = 0.2 muM), whereas the GLUD2-derived isoenzyme is resistant to this compound. The GLUD2-derived GDH shows low basal activity and has the capacity to be activated fully by ADP or L-leucine. We used molecular biological tools to study the subcellular localization of GLUD1-derived GDH in cultured cells and the molecular basis of its regulation. COS7 cells, transfected with a GLUD1-pEGFP-N3 vector, revealed a GFP fluorescence pattern nearly identical to that of the mitochondrial marker pDsRed2-Mito. Site-directed mutagenesis of GLUD1 gene showed that replacement of Gly456 by Ala made the enzyme resistant to GTP (IC(50) = 2.8 +/- 0.15 microM) without altering its regulation by ADP. Substitution of Ser for Arg443 rendered the enzyme virtually inactive at its basal state, but fully responsive to ADP activation. The Arg443Ser mutant was more active at pH 7.0 than at pH 8.0. The Gly456Ala change therefore dissociated GLUD2-derived GDH function from GTP, whereas the Arg443Ser change made enzyme regulation possible without this inhibitor. These properties may allow the brain isoenzyme to function well under conditions of intracellular acidification and increased turnover of ATP to ADP, as occurs in synaptic astrocytes during excitatory transmission.

Manganese exposure and induced oxidative stress in the rat brain

Neurotoxicity linked to excessive brain manganese levels can occur as a result of high level Mn exposures and/or metabolic aberrations (liver disease and decreased biliary excretion). Increased brain manganese levels have been reported to induce oxidative stress, as well as alterations in neurotransmitter metabolism with concurrent neurobehavioral and motor deficits. Two putative mechanisms in which manganese can produce oxidative stress in the brain are: (1) via its oxidation of dopamine, and (2) interference with normal mitochondrial respiration. Measurements of antioxidant species (e.g., glutathione and metallothionein), and the abundance of proteins (enzymes) exquisitely sensitive to oxidation (e.g., glutamine synthetase) have been commonly used as biomarkers of oxidative stress, particularly in rat brain tissue. This paper examines the link between manganese neurotoxicity in the rat brain and common pathways to oxidative stress.

Intracellular metabolic compartmentation assessed by 13C magnetic resonance spectroscopy

Our understanding of the brain has developed from the theory that it is one continuous cell to the knowledge that there are many brain cells originally termed neurons and, furthermore to the discovery of glial cells and their multiple functions. Thus, an increasing complexity was unraveled and we have not reached a complete understanding of the phenomenon which comprises the compartmentation of metabolic pathways and metabolites. This is an important principle needed to fully understand the metabolic processes of the brain. At the cellular level this concept is well established whereas intracellular compartmentation has yet to be explored. Using magnetic resonance spectroscopy (MRS) for analysis of isotopomer composition combined with quantification of amino acid contents it is possible to construct models that describe intracellular compartmentation. Results of studies of cultures of astrocytes and neurons incubated in media containing [U- 13C]glutamate in the presence or absence of thiopental may be used to propose an intracellular three compartment model of mitochondrial function. Due to the experimental paradigm only certain aspects of metabolism can be described. The present model consists of compartments assigned to CO(2) production, glutamate synthesis from ketoglutarate and finally synthesis of a four-carbon metabolite which is shuttled between compartments. It is likely that metabolism may be far more complex than this and we are only beginning to glimpse some aspects of compartmentation at the cellular level.

Hypoxia augments TNF-alpha-mediated endothelin-1 release and cell proliferation in human optic nerve head astrocytes

The effect of hypoxia (24 h) on TNF-alpha-mediated release of endothelin-1 (ET-1) from human optic nerve head astrocytes (hONAs) and TNF-alpha- and ET-1-induced hONA proliferation was determined. ET-1 synthesis and release was quantitated using ELISA while TNF-alpha (10 nM)- and ET-1 (100 nM)-mediated hONA proliferation was assessed by CellTiter 96 aqueous one-solution cell proliferation assay, respectively. hONAs appeared to be more rounded with fewer processes following 24 h hypoxia compared to thodr seen in normoxia. Hypoxia enhanced TNF-alpha-mediated ET-1 synthesis and release (by 5-fold) and also significantly increased TNF-alpha- and ET-1-mediated hONA proliferation. PD142893 (1 microM), an ET(A/B) receptor antagonist, blocked ET-1-mediated hONA proliferation both under normoxia and hypoxia, while doing so only under normoxia following TNF-alpha treatment. Also, U0126 (10 microM; an upstream ERK1/2 inhibitor) completely blocked agonist-induced hONA proliferation in normoxia and partially blocked the same in hypoxia. These results demonstrate for the first time that hONAs secrete ET-1 and that TNF-alpha and hypoxia can regulate its levels. Moreover, hypoxia augments the proliferative responses of hONAs to TNF-alpha and ET-1. These agonist-mediated effects following hypoxia could contribute to astroglial activation as seen in glaucomatous optic nerve heads.

Relationships between glutamine, glutamate, and GABA in nerve endings under Pb-toxicity conditions

Glutamine (Gln), glutamate (Glu) and gamma-amino butyric acid (GABA) are essential amino acids for brain metabolism and function. Astrocytic-derived glutamine is the precursor of the two most important neurotransmitters: glutamate, an excitatory neurotransmitter, and GABA, an inhibitory neurotransmitter. In addition to their roles in neurotransmission these neurotransmitters act as alternative metabolic substrates that enable metabolic coupling between astrocytes and neurons. The relationships between Gln, Glu and GABA were studied under lead (Pb) toxicity conditions using synaptosomal fractions obtained from adult rat brains to investigate the cause of Pb neurotoxicity-induced seizures. We have found that diminished transport of [(14)C]GABA occurs after Pb treatment. Both uptake and depolarization-evoked release decrease by 40% and 30%, respectively, relative to controls. Lower expression of glutamate decarboxylase (GAD), the GABA synthesizing enzyme, is also observed. In contrast to impaired synaptosomal GABA function, the GABA transporter GAT-1 protein is overexpressed (possibly as a compensative mechanism). Furthermore, similar decreases in synaptosomal uptake of radioactive glutamine and glutamate are observed. However, the K(+)-evoked release of Glu increases by 20% over control values and the quantity of neuronal EAAC1 transporter for glutamate reaches remarkably higher levels after Pb treatment. In addition, Pb induces decreased activity of phosphate-activated glutaminase (PAG), which plays a role in glutamate metabolism. Most noteworthy is that the overexpression and reversed action of the EAAC1 transporter may be the cause of the elevated extracellular glutamate levels. In addition to the impairment of synaptosomal processes of glutamatergic and GABAergic transport, the results indicate perturbed relationships between Gln, Glu and GABA that may be the cause of altered neuronal-astrocytic interactions under conditions of Pb neurotoxicity.

Manganese neurotoxicity and glutamate-GABA interaction

Brain extracellular concentrations of amino acids (e.g. aspartate, glutamate, taurine) and divalent metals (e.g. zinc, copper, manganese) are primarily regulated by astrocytes. Adequate glutamate homeostasis is essential for the normal functioning of the central nervous system (CNS). Glutamate is of central importance for nitrogen metabolism and, along with aspartate, is the primary mediator of the excitatory pathways in the brain. Similarly, the maintenance of proper manganese levels is important for normal brain functioning. Several in vivo and in vitro studies have linked increased manganese concentrations with alterations in the content and metabolism of neurotransmitters, namely dopamine, gamma-aminobutyric acid, and glutamate. It has been reported by our laboratory and others, that cultured rat primary astrocytes exposed to manganese displayed decreased glutamate uptake, thereby increasing the excitotoxic potential of glutamate. Furthermore, decreased uptake of glutamate has been associated with decreased gene expression of glutamate:aspartate transporter (GLAST) in manganese-exposed astroctyes. Additional studies have suggested that attenuation of astrocytic glutamate uptake by manganese may be a consequence of reactive oxygen species (ROS) generation. Collectively, these data suggest that excitotoxicity may occur due to manganese-induced altered glutamate metabolism, representing a proximate mechanism for manganese-induced neurotoxicity.

Study of structure-function relationships in human glutamate dehydrogenases reveals novel molecular mechanisms for the regulation of the nerve tissue-specific (GLUD2) isoenzyme

In mammalian brain, glutamate dehydrogenase (GDH) is located predominantly in astrocytes, where is thought to play a role in transmitter glutamate's metabolism. Human GDH exists in GLUD1 (housekeeping) and GLUD2 (nerve tissue-specific) isoforms, which share all but 15 out of their 505 amino acids. The GLUD1 GDH is potently inhibited by GTP, whereas the GLUD2 enzyme is resistant to this compound. On the other hand, the GLUD2 isoform assumes in the absence of GTP a conformational state associated with little catalytic activity, but it remains amenable to full activation by ADP and/or L-leucine. Site-directed mutagenesis of the GLUD1 gene at sites that differ from the corresponding residues of the GLUD2 gene showed that replacement of Gly456 by Ala made the enzyme resistant to GTP (IC(50)=2.8+/-0.15 microM) compared to the wild-type GDH (IC(50)=0.19+/-0.01 microM). In addition, substitution of Ser for Arg443 virtually abolished basal activity and rendered the enzyme dependent on ADP for its function. These properties may permit the neural enzyme to be recruited under conditions of low energy charge (high ADP:ATP ratio), similar to those that prevail in synaptic astrocytes during intense glutamatergic transmission. Hence, substitution of Ser for Arg443 and Ala for Gly456 are the main evolutionary changes that led to the adaptation of the GLUD2 GDH to the unique metabolic needs of the nerve tissue.

Regional differences in glutamine synthetase inhibition by L-methionine sulfoximine: a microdialysis study in the rabbit brain

Elevated levels of glutamate, an endogenous excitatory amino acid, contribute to the development of neuronal injury in various cerebral diseases. Using a microdialysis approach, the response of extracellular levels of amino acids and metabolic parameters to glutamine synthetase inhibition by l-methionine sulfoximine was monitored simultaneously in the hippocampal formation and in the frontal cortex of the rabbit brain. In the hippocampal formation the decrease of glutamine levels during l-methionine sulfoximine treatment was more pronounced than in the frontal cortex, and was accompanied by a delayed decline of extracellular glutamate concentrations. Furthermore, l-methionine sulfoximine diminished the increase of lactate and pyruvate concentrations in the hippocampal formation, but not in the frontal cortex. Neither l-methionine sulfoximine treatment nor microdialysis probe insertion caused neuronal apoptosis, as measured by in situ tailing. An impaired function of hippocampal astrocyte glutamate uptake mechanisms or a higher functional capacity of the cortical glutamine synthetase may be possible explanations for the differences demonstrated. The present data are in accordance with regional differences in glutamine synthetase activation during bacterial meningitis and may explain, in part, the higher susceptibility of certain areas of the hippocampal formation (i.e., the dentate gyrus) to neuronal injury.

Effect of gamma-glutamyl transpeptidase inhibitors on the transport of glutamate into neuronal and glial primary cultures

Inhibitors of gamma-glutamyl transpeptidase (mixture of serine and borate--13 mM, kainic acid--5 mM and 6-diazo-5-oxo-L-norleucine--2 mM) significantly suppressed glutamate uptake into cultured neurones and glial cells. The simultaneous application of any of these inhibitors with ouabain resulted in a further decline in glutamate uptake. It can be speculated that gamma-glutamyl transpeptidase significantly contributes to glutamate transport into nerve cells in the early period of brain development until the Na(+)-K(+)-gradient is fully constituted.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.

Astrocytes and brain injury

Astrocytes are the most numerous cell type in the central nervous system. They provide structural, trophic, and metabolic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions during brain ischemia and other insults can critically influence neuron survival. Astrocyte functions that are known to influence neuronal survival include glutamate uptake, glutamate release, free radical scavenging, water transport, and the production of cytokines and nitric oxide. Long-term recovery after brain injury, through neurite outgrowth, synaptic plasticity, or neuron regeneration, is influenced by astrocyte surface molecule expression and trophic factor release. In addition, the death or survival of astrocytes themselves may affect the ultimate clinical outcome and rehabilitation through effects on neurogenesis and synaptic reorganization.

Endothelin: is it a contributor to glaucoma pathophysiology?

Endothelin is a vasoactive peptide that has been shown to play an important role in vascular homeostasis. Recently, endothelin and its receptors have been found in ocular tissues where it appears to have a regulatory function. Endothelin is found in both the aqueous and vitreous humors and its concentration is elevated in glaucoma patients and in animal models of glaucoma. In the current review, the authors present information about the distribution of endothelin and endothelin receptors in the eye and the ocular actions of endothelins. Specifically, endothelin/aqueous humor dynamics, endothelin/nitric oxide interactions, endothelin and ischemia, and endothelin/optic nerve head effects. Observations concerning the potential role of endothelin in glaucoma pathophysiology is presented and discussed relative to its effects on the optic nerve head and in relation to glaucoma theories.

Modulation of glial glutamate transport through cell interactions with the extracellular matrix

Glial glutamate transport plays a pivotal role in maintaining glutamate homeostasis in the central nervous system. Expression of glutamate transporters is highly regulated during brain development, and a number of pathological conditions are associated with deficits in expression and/or function of glutamate transports. While several soluble factors have been shown to regulate the expression of glutamate transporter, the contribution of cell-cell interaction and cell-environmental interaction in the regulation of glutamate transport is unknown. Extracellular matrix (ECM) molecules are essential components in cell-cell and cell-environmental interactions, and the ECM has been shown to play critical role in normal development and during brain pathogenesis. We, therefore, investigated the possibility that ECM molecules may regulate astrocytic glutamate transport. Therefore, we cultured rat cortical astrocytes with different ECMs and determined expression levels of the two astrocytic glutamate transporters GLT-1 and GLAST by Western Blot and determined transporter activity through measurements of 3H-D-aspartate uptake. Astrocytes grown on poly-ornithine or poly-D/L-lysine showed approximately two-fold higher GLT-1 expression than sister cells grown on plastic dishes without ECM. Naturally occurring ECM's, including laminin and collagen, showed a dose-dependent regulation of GLT-1 protein expression. These effects were specific for GLT-1 as GLAST expression was unaffected by different ECMs. Surprisingly, however, none of the examined ECMs altered the apparent glutamate uptake activity. In probing blots side-by-side for expression of Na(+)/K(+)-ATPase, we found that ECMs affected expression of Na(+)/K(+)-ATPase and GLT-1 in a reciprocal fashion. Poly-ornithine, for example, enhanced GLT-1 expression, but reduced expression of Na(+)/K(+)-ATPase. Na(+) transport may, thus, be a limiting factor for glutamate uptake.

Aspartate aminotransferase isotope exchange reactions: implications for glutamate/glutamine shuttle hypothesis

Aspartate aminotransferase (AAT) catalyzes amino group transfer from glutamate (Glu) or aspartate (Asp) to a keto acid acceptor-oxaloacetate (OA) or alpha-ketoglutarate (KG), respectively. Data presented here show that AAT catalyzes two partial reactions resulting in isotope exchange between 3H-labeled Glu or 3H-labeled Asp and the cognate keto acid in the absence of the keto acid acceptor required for the net reaction. Tritiated keto acid product was detected by release of 3H2O from C-3 during base-induced enolization. Tritium released directly from C-2 (or C-3) by the enzyme was also evaluated and is a small fraction of that released because of exchange to the keto acid pool. Exchange is dependent on AAT concentration, time-dependent, proportional to the amino-to-keto acid ratio, and blocked by aminooxyacetate (AOA), an AAT inhibitor. Enzymatic conversion of [3H]KG to Glu by glutamic dehydrogenase (GDH) or of [3H]OA to malate by malic dehydrogenase (MDH) "protects" the label from release by base, showing that base-induced isotope release is from keto acid rather than a result of release during the exchange process. AAT isotope exchange is discussed in the context of the glutamate/glutamine shuttle hypothesis for astrocyte/neuron carbon cycling.

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

Increasing evidence supports a crucial role for glial metabolism in maintaining proper synaptic function and in the etiology of neurological disease. However, the study of glial metabolism in humans has been hampered by the lack of noninvasive methods. To specifically measure the contribution of astroglia to brain energy metabolism in humans, we used a novel noninvasive nuclear magnetic resonance spectroscopic approach. We measured carbon 13 incorporation into brain glutamate and glutamine in eight volunteers during an intravenous infusion of [2-13C] acetate, which has been shown in animal models to be metabolized specifically in astroglia. Mathematical modeling of the three established pathways for neurotransmitter glutamate repletion indicates that the glutamate/glutamine neurotransmitter cycle between astroglia and neurons (0.32 +/- 0.07 micromol x gm(-1) x min(-1)) is the major pathway for neuronal glutamate repletion and that the astroglial TCA cycle flux (0.14 +/- 0.06 micromol x gm(-1) x min(-1)) accounts for approximately 14% of brain oxygen consumption. Up to 30% of the glutamine transferred to the neurons by the cycle may derive from replacement of oxidized glutamate by anaplerosis. The further application of this approach could potentially enlighten the role of astroglia in supporting brain glutamatergic activity and in neurological and psychiatric disease.

Ammonia-induced extracellular accumulation of taurine in the rat striatum in vivo: role of ionotropic glutamate receptors

Accumulation of taurine (Tau), glutamate (Glu) and glutamine (Gln) was measured in vivo in microdialysates of the rat striatum following a direct application to the microdialysis tube of 60 mM ammonium chloride which renders the final ammonia concentration in the extracellular space to approximately 5 mM. The following compounds were coadministered with ammonia to distinguish between the different mechanisms that may underlie the accumulation of amino acids: ion transport inhibitors, diisothiocyanostilbene-2,2'-disulfonate (DIDS) and furosemide, a Glu transport inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (PDC), an NMDA receptor antagonist dizocilpine (MK-801) and an 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (KA) receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). Ammonia stimulated Tau accumulation in the microdialysates to approximately 250% of the basal value. Furosemide did not significantly affect the stimulation by ammonia and DIDS only moderately depressed the effect. The ammonia-dependent Tau accumulation was increased by approximately 50% in the presence of PDC and reduced by approximately 35% in the presence dizocilpine and DNQX. In the microdialysates ammonia stimulated Glu and Gln accumulation somewhat less than Tau accumulation. Except for stimulation of Gln accumulation by DNQX, the effects were not modified by any of the cotreatments. The results are consistent with the assumption that ammonia stimulates Tau efflux mainly via activation of ionotropic Glu receptors.

Effect of glutamine and GABA on [U-(13)C]glutamate metabolism in cerebellar astrocytes and granule neurons

To probe the effect of glutamine and GABA on metabolism of [U-(13)C]glutamate, cerebellar astrocytes were incubated with [U-(13)C]glutamate (0.5 mM) in the presence and absence of glutamine (2.5 mM) or GABA (0.2 mM). It could be shown that consumption of [U-(13)C]glutamate was decreased in the presence of glutamine and release of labeled aspartate and [1,2,3-(13)C]glutamate decreased as well, whereas the concentrations of these metabolites increased inside the cells. Glutamine decreased energy production from [U-(13)C]glutamate presumably by substituting for glutamate as an energy substrate. No additional effect was seen in the presence of both glutamine and GABA. When cerebellar granule neurons were incubated with [U-(13)C]glutamate (0.25 mM) and GABA (0.05 mM), less [U-(13)C]glutamate was used for energy production than in controls. Because the barbiturate thiopental did not elicit such response (Qu et al., 2000, Neurochem Int 37:207-215) it appears that GABA also has a metabolic function in the glutamatergic cerebellar granule neurons in contrast to the astrocytes.

Regulation of human glutamate dehydrogenases: implications for glutamate, ammonia and energy metabolism in brain

Glutamate dehydrogenase (GDH) catalyzes the oxidative deamination of glutamate to alpha-ketoglutarate using NAD or NADP as cofactors. In mammalian brain, GDH is located predominantly in astrocytes, where it is probably involved in the metabolism of transmitter glutamate. The exact mechanisms that regulate glutamate fluxes through this pathway, however, have not been fully understood. In the human, GDH exists in heat-resistant and heat-labile isoforms, encoded by the GLUD1 (housekeeping) and GLUD2 (nerve tissue-specific) genes, respectively. These forms differ in their catalytic and allosteric properties. Kinetic studies showed that the K(m) value for glutamate for the nerve tissue GDH is within the range of glutamate levels in astrocytes (2.43 mM), whereas for the housekeeping enzyme, this value is significantly higher (7.64 mM; P < 0.01). The allosteric activators ADP (0.1-1.0 mM) and L-leucine (1.0-10.0 mM) induce a concentration-dependent enzyme stimulation that is proportionally greater for the nerve tissue-specific GDH (up to 1,600%) than for the housekeeping enzyme (up to 150%). When used together at lower concentrations, ADP (10-50 mM) and L-leucine (75-200 microM) act synergistically in stimulating GDH activity. GTP exerts a powerful inhibitory effect (IC(50) = 0.20 mM) on the housekeeping GDH; in contrast, the nerve tissue isoenzyme is resistant to GTP inhibition. Thus, although the housekeeping GDH is regulated primarily by GTP, the nerve tissue GDH activity depends largely on available ADP or L-leucine levels. Conditions associated with enhanced hydrolysis of ATP to ADP (e.g., intense glutamatergic transmission) are likely to activate nerve tissue-specific GDH leading to an increased glutamate flux through this pathway.

(1R,3S)-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD) reduces intracellular glutamate levels in astrocytes

(+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) is an equimolar mixture of two enantiomers: (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) and 1R,3S-1-Aminocyclopentane-1,3-dicarboxylic acid (RS-ACPD). t-ACPD and SR-ACPD have been commonly used as agonists for metabotropic glutamate receptors (mGluR). Here we demonstrated that RS-ACPD, but not SR-ACPD, is transported into astrocytes with a K(m) of 6.51 +/- 2.38 mM and V(max) of 22.8 +/- 3.4 nmol/mg/min. This low-affinity transport is Na(+)-dependent and is competitively blocked by glutamate or other substrates for the glutamate transporter. RS-ACPD therefore is probably taken up by the glutamate transporter. Prolonged incubation with high levels of RS-ACPD (> 500 microM) induced significant swelling of astrocytes. At lower concentrations (100 microM), RS-ACPD reduced intracellular glutamate content ([Glu](i)) by > 50% without obvious morphological changes. The reduction in [Glu](i) was accompanied by an increase in [glutamine](i). The RS-ACPD's effect on [Glu](i) required glutamine and high levels of phosphate, suggesting that RS-ACPD inhibited phosphate-activated glutaminase (PAG). These data suggest that astrocytic PAG is actively involved in determining the equilibrium between intracellular glutamate and glutamine. By reducing [Glu](i), RS-ACPD reduces the amount of glutamate available for release.

Glutamate-glutamine cycle and aging in striatum of the awake rat: effects of a glutamate transporter blocker

This study investigated the effects of aging on the actions of a specific glutamate reuptake blocker, L-trans-pyrrolidine-2, 4-dicarboxylic acid (PDC), in extracellular glutamate and glutamine in striatum of the awake rat. Microdialysis experiments were performed on young (2-3 months), middle-aged (12-14 months), aged (27-32 months) and very aged (37 months) male Wistar rats. Local infusion of PDC (1-4 mM) in striatum increased the dialysate concentration of glutamate and decreased dialysate concentration of glutamine in all the age-groups. In young rats, decreases of dialysate glutamine were correlated with increases of dialysate glutamate. The same profile glutamine/glutamate as in young rats was found in middle-aged, aged and very aged rats, which suggests that the action of glutamate on the glutamate-glutamine cycle in striatum of the awake rat is not modified as a consequence of aging. We also found a significant correlation between the increases of glutamate produced by PDC and the basal dialysate concentration of glutamine, a relationship that did show a significant change with age. Although the significance of this latter finding remains to be elucidated, it may be important to understand the changes in glutamate-glutamine cycle during aging.