Utilization of alpha-ketoglutarate as a precursor for transmitter glutamate in cultured cerebellar granule cells

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.

Genetic correlation between circulating metabolites and chalazion: a two-sample Mendelian randomization study

Background: Lipid metabolism disorders were observationally associated with chalazion, but the causality of the related circulating metabolites on chalazion remained unknown. Here, we investigated the potential causal relationship between circulating metabolites and chalazion using two-sample Mendelian randomization (MR) analysis. Methods: For the primary analysis, 249 metabolic biomarkers were obtained from the UK Biobank, and 123 circulating metabolites were obtained from the publication by Kuttunen et al. for the secondary analysis. Chalazion summary data were obtained from the FinnGen database. Inverse variance weighted (IVW) is the main MR analysis method, and the MR assumptions were evaluated in sensitivity and colocalization analyses. Results: Two MR analyses results showed that the common metabolite, alanine, exhibited a genetic protective effect against chalazion (primary analysis: odds ratio [OR] = 0.680; 95% confidence interval [CI], 0.507-0.912; p = 0.010; secondary analysis: OR = 0.578; 95% CI, 0.439-0.759; p = 0.00008). The robustness of the findings was supported by heterogeneity and horizontal pleiotropy analysis. Two colocalization analyses showed that alanine did not share a region of genetic variation with chalazion (primary analysis: PPH4 = 1.95%; secondary analysis: PPH4 = 25.3%). Moreover, previous studies have suggested that an increase in the degree of unsaturation is associated with an elevated risk of chalazion (OR = 1.216; 95% CI, 1.055-1.401; p = 0.007), with omega-3 fatty acids (OR = 1.204; 95% CI, 1.054-1.377; p = 0.006) appearing to be the major contributing factor, as opposed to omega-6 fatty acids (OR = 0.850; 95% CI, 0.735-0.982; p = 0.027). Conclusion: This study suggests that alanine and several unsaturated fatty acids are candidate molecules for mechanistic exploration and drug target selection in chalazion.

Effect of electroacupuncture at ST36 on the cerebral metabolic kinetics of rheumatoid arthritis rats

Electroacupuncture (EA) has been shown to enhance the recovery of symptoms in rheumatoid arthritis (RA); however, the underlying mechanism remains unclear. Both the pathogenesis of RA and the therapeutic effects of EA are closely associated with the metabolic activity of the brain. In this study, we investigated the effect of EA at the "Zusanli" acupoint (ST36) on a rat model of collagen-induced rheumatoid arthritis (CIA). The results demonstrated that EA effectively alleviated joint swelling, synovial hyperplasia, cartilage erosion, and bone destruction in CIA rats. Additionally, the metabolic kinetics study revealed a significant increase in the 13C enrichment of GABA2 and Glu4 in the midbrain of CIA rats treated with EA. Correlation network analysis showed that changes in Gln4 levels in the hippocampus were strongly associated with the severity of rheumatoid arthritis. Immunofluorescence staining of c-Fos in the midbrain's periaqueductal gray matter (PAG) and hippocampus demonstrated increased c-Fos expression in these regions following EA treatment. These findings suggest that GABAergic and glutamatergic neurons in the midbrain, along with astrocytes in the hippocampus, may play vital roles in the beneficial effects of EA on RA. Furthermore, the PAG and hippocampus brain regions hold potential as critical targets for future RA treatments. Overall, this study provides valuable insights into the specific mechanism of EA in treating RA by elucidating the perspective of cerebral metabolism.

Silencing of a putative alanine aminotransferase (ALT) gene influences free amino acid composition in hemolymph and fecundity of the predatory bug, Cyrtorhinus lividipennis Reuter

In Asian rice systems, Cyrtorhinus lividipennis Reuter is an important predator that preys on rice planthopper eggs and young nymphs, as a primary food source. Alanine aminotransferase (ALT) acts in many physiological and biochemical processes in insects. We cloned the full-length complementary DNA of C. lividipennis ClALT. Expression analysis showed higher expression in the fat body and midgut compared to other tissues. It is expressed in all C. lividipennis developmental stages and at least four organs. Silencing of ClALT by RNA interference significantly decreased the ClALT enzyme activity and ClALT expression compared to dsGFP-treated controls at 2 days after emergence (DAE). Silencing of ClALT influenced free hemolymph amino acid compositions, resulting in a reduction of Aspartic acid (Asp) and Alanine (Ala) proportions, and increased Cysteine (Cys) and Valine (Val) proportions in females at 2 DAE. dsClALT treatments led to decreased soluble total protein concentrations in ovary and fat body, and to lower reduced vitellogenin (Vg) expression, body weight, and the numbers of laid eggs. The double-stranded RNA viruse treatments also led to prolonged preoviposition periods and hindered ovarian development. Western blot analysis indicated that silencing ClALT also led to reduced fat body Vg protein abundance at 2 DAE. These data support our hypothesis that ClALT influences amino acid metabolism and fecundity in C. lividipennis.

Taurine Increases Zinc Preconditioning-Induced Prevention of Nitrosative Stress, Metabolic Alterations, and Motor Deficits in Young Rats following Intrauterine Ischemia

Oxygen deprivation in newborns leads to hypoxic-ischemic encephalopathy, whose hallmarks are oxidative/nitrosative stress, energetic metabolism alterations, nutrient deficiency, and motor behavior disability. Zinc and taurine are known to protect against hypoxic-ischemic brain damage in adults and neonates. However, the combined effect of prophylactic zinc administration and therapeutic taurine treatment on intrauterine ischemia- (IUI-) induced cerebral damage remains unknown. The present work evaluated this issue in male pups subjected to transient IUI (10 min) at E17 and whose mothers received zinc from E1 to E16 and taurine from E17 to postnatal day 15 (PND15) via drinking water. We assessed motor alterations, nitrosative stress, lipid peroxidation, and the antioxidant system comprised of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Enzymes of neuronal energetic pathways, such as aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH), were also evaluated. The hierarchization score of the protective effect of pharmacological strategies (HSPEPS) was used to select the most effective treatment. Compared with the IUI group, zinc, alone or combined with taurine, improved motor behavior and reduced nitrosative stress by increasing SOD, CAT, and GPx activities and decreasing the GSSG/GSH ratio in the cerebral cortex and hippocampus. Taurine alone increased the AST/ALT, LDH/ALT, and AST/LDH ratios in the cerebral cortex, showing improvement of the neural bioenergetics system. This result suggests that taurine improves pyruvate, lactate, and glutamate metabolism, thus decreasing IUI-caused cerebral damage and relieving motor behavior impairment. Our results showed that taurine alone or in combination with zinc provides neuroprotection in the IUI rat model.

Brain Glucose Metabolism: Integration of Energetics with Function

Glucose is the long-established, obligatory fuel for brain that fulfills many critical functions, including ATP production, oxidative stress management, and synthesis of neurotransmitters, neuromodulators, and structural components. Neuronal glucose oxidation exceeds that in astrocytes, but both rates increase in direct proportion to excitatory neurotransmission; signaling and metabolism are closely coupled at the local level. Exact details of neuron-astrocyte glutamate-glutamine cycling remain to be established, and the specific roles of glucose and lactate in the cellular energetics of these processes are debated. Glycolysis is preferentially upregulated during brain activation even though oxygen availability is sufficient (aerobic glycolysis). Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in excess of oxygen, and adrenergic regulation of aerobic glycolysis draws attention to astrocytic metabolism, particularly glycogen turnover, which has a high impact on the oxygen-carbohydrate mismatch. Aerobic glycolysis is proposed to be predominant in young children and specific brain regions, but re-evaluation of data is necessary. Shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation, neurotransmission, and memory consolidation are controversial topics for which alternative mechanisms are proposed. Nutritional therapy and vagus nerve stimulation are translational bridges from metabolism to clinical treatment of diverse brain disorders.

A Tribute to Mary C. McKenna: Glutamate as Energy Substrate and Neurotransmitter-Functional Interaction Between Neurons and Astrocytes

Glutamate metabolism in the brain is extremely complex not only involving a large variety of enzymes but also a tight partnership between neurons and astrocytes, the latter cells being in control of de novo synthesis of glutamate. This review provides an account of the processes involved, i.e. pyruvate carboxylation and recycling as well as the glutamate-glutamine cycle, focusing on the many seminal contributions from Dr. Mary McKenna. The ramification of the astrocytic end feet allowing contact and control of hundreds of thousands of synapses at the same time obviously puts these cells in a prominent position to regulate neural activity. Additionally, the astrocytes take active part in the neurotransmission processes by releasing a variety of gliotransmitters including glutamate. Hence, the term "the tripartite synapse", in which there is an active and dynamic interplay between the pre- and post-synaptic neurons and the ensheathing astrocytes, has been coined. The studies of Mary McKenna and her colleagues over several decades have been of paramount importance for the elucidation of compartmentation in astrocytes and synaptic terminals and the intricate metabolic processes underlying the glutamatergic neurotransmission process.

Mitochondrial Citrate Transporter-dependent Metabolic Signature in the 22q11.2 Deletion Syndrome

The congenital disorder 22q11.2 deletion syndrome (22qDS), characterized by a hemizygous deletion of 1.5-3 Mb on chromosome 22 at locus 11.2, is the most common microdeletion disorder (estimated prevalence of 1 in 4000) and the second risk factor for schizophrenia. Nine of ∼30 genes involved in 22qDS have the potential of disrupting mitochondrial metabolism (COMT, UFD1L, DGCR8, MRPL40, PRODH, SLC25A1, TXNRD2, T10, and ZDHHC8). Deficits in bioenergetics during early postnatal brain development could set the basis for a disrupted neuronal metabolism or synaptic signaling, partly explaining the higher incidence in developmental and behavioral deficits in these individuals. Here, we investigated whether mitochondrial outcomes and metabolites from 22qDS children segregated with the altered dosage of one or several of these mitochondrial genes contributing to 22qDS etiology and/or morbidity. Plasma metabolomics, lymphocytic mitochondrial outcomes, and epigenetics (histone H3 Lys-4 trimethylation and 5-methylcytosine) were evaluated in samples from 11 22qDS children and 13 age- and sex-matched neurotypically developing controls. Metabolite differences between 22qDS children and controls reflected a shift from oxidative phosphorylation to glycolysis (higher lactate/pyruvate ratios) accompanied by an increase in reductive carboxylation of α-ketoglutarate (increased concentrations of 2-hydroxyglutaric acid, cholesterol, and fatty acids). Altered metabolism in 22qDS reflected a critical role for the haploinsufficiency of the mitochondrial citrate transporter SLC25A1, further enhanced by HIF-1α, MYC, and metabolite controls. This comprehensive profiling served to clarify the biochemistry of this disease underlying its broad, complex phenotype.

Maintaining the presynaptic glutamate supply for excitatory neurotransmission

Glutamate released from synapses during excitatory neurotransmission must be rapidly recycled to maintain neuronal communication. This review evaluates data from physiological experiments at hippocampal CA3 to CA1 synapses and the calyx of Held synapse in the brainstem to analyze quantitatively the rates of release and resupply of glutamate required to sustain neurotransmission. We calculate that, without efficient recycling, the presynaptic glutamate supply will be exhausted within about a minute of normal synaptic activity. We also discuss replenishment of the presynaptic pool by diffusion from the soma, direct uptake of glutamate back into the presynaptic terminal, and uptake of glutamate precursor molecules. Diffusion of glutamate from the soma is calculated to be fast enough to resupply presynaptic glutamate in the hippocampus but not at the calyx of Held. However, because the somatic cytoplasm will also quickly run out of glutamate and synapses can function continually even if the presynaptic axon is severed, mechanisms other than diffusion must be present to resupply glutamate for release. Direct presynaptic uptake of glutamate is not present at the calyx of Held but may play a role in glutamate recycling in the hippocampus. Alternatively, glutamine or tricarboxylic acid cycle intermediates released from glia can serve as a precursor for glutamate in synaptic terminals, and we calculate that the magnitude of presynaptic glutamine uptake is sufficient to supply enough glutamate to sustain neurotransmission. The nature of these mechanisms, their relative abundance, and the co-ordination between them remain areas of intensive investigation.

Is there in vivo evidence for amino acid shuttles carrying ammonia from neurons to astrocytes?

The high in vivo flux of the glutamate/glutamine cycle puts a strong demand on the return of ammonia released by phosphate activated glutaminase from the neurons to the astrocytes in order to maintain nitrogen balance. In this paper we review several amino acid shuttles that have been proposed for balancing the nitrogen flows between neurons and astrocytes in the glutamate/glutamine cycle. All of these cycles depend on the directionality of glutamate dehydrogenase, catalyzing reductive glutamate synthesis (forward reaction) in the neuron in order to capture the ammonia released by phosphate activated glutaminase, while catalyzing oxidative deamination of glutamate (reverse reaction) in the astrocytes to release ammonia for glutamine synthesis. Reanalysis of results from in vivo experiments using (13)N and (15)N labeled ammonia and (15)N leucine in rats suggests that the maximum flux of the alanine/lactate or branched chain amino acid/branched chain amino acid transaminase shuttles between neurons and astrocytes are approximately 3-5 times lower than would be required to account for the ammonia transfer from neurons to astrocytes needed for glutamine synthesis (amide nitrogen) to sustain the glutamate/glutamine cycle. However, in the rat brain both the total ammonia fixation rate by glutamate dehydrogenase and the total branched chain amino acid transaminase activity are sufficient to support a branched chain amino acid/branched chain keto acid shuttle, as proposed by Hutson and coworkers, which would support the de novo synthesis of glutamine in the astrocyte to replace the ~20 % of neurotransmitter glutamate that is oxidized. A higher fraction of the nitrogen needs of total glutamate neurotransmitter cycling could be supported by hybrid cycles in which glutamate and tricarboxylic acid cycle intermediates act as a nitrogen shuttle. A limitation of all in vivo studies in animals conducted to date is that none have shown transfer of nitrogen for glutamine amide synthesis, either as free ammonia or via an amino acid from the neurons to the astrocytes. Future work will be needed, perhaps using methods for selectively labeling nitrogen in neurons, to conclusively establish the rate of amino acid nitrogen shuttles in vivo and their coupling to the glutamate/glutamine cycle.

Valine but not leucine or isoleucine supports neurotransmitter glutamate synthesis during synaptic activity in cultured cerebellar neurons

Synthesis of neuronal glutamate from α-ketoglutarate for neurotransmission necessitates an amino group nitrogen donor; however, it is not clear which amino acid(s) serves this role. Thus, the ability of the three branched-chain amino acids (BCAAs), leucine, isoleucine, and valine, to act as amino group nitrogen donors for synthesis of vesicular neurotransmitter glutamate was investigated in cultured mouse cerebellar (primarily glutamatergic) neurons. The cultures were superfused in the presence of (15) N-labeled BCAAs, and synaptic activity was induced by pulses of N-methyl-D-aspartate (300 μM), which results in release of vesicular glutamate. At the end of the superfusion experiment, the vesicular pool of glutamate was released by treatment with α-latrotoxin (3 nM, 5 min). This experimental paradigm allows a separate analysis of the cytoplasmic and vesicular pools of glutamate. Amount and extent of (15) N labeling of intracellular amino acids plus vesicular glutamate were analyzed employing HPLC and LC-MS analysis. Only when [(15) N]valine served as precursor did the labeling of both cytoplasmic and vesicular glutamate increase after synaptic activity. In addition, only [(15) N]valine was able to maintain the amount of vesicular glutamate during synaptic activity. This indicates that, among the BCAAs, only valine supports the increased need for synthesis of vesicular glutamate.

Synaptic vesicles are capable of synthesizing the VGLUT substrate glutamate from α-ketoglutarate for vesicular loading

Synaptic vesicle loading of glutamate is a pivotal step in glutamate synaptic transmission. The molecular machinery responsible for this step is comprised of v-type proton-pump ATPase and a vesicular glutamate transporter. Recent evidence indicates that synaptic vesicles are endowed with glycolytic ATP-synthesizing enzymes, providing energy for immediate use by vesicle-bound proton-pump ATPase. In this study, we provide evidence that synaptic vesicles are also capable of synthesizing the vesicular glutamate transporter substrate glutamate, from α-ketoglutarate and l-aspartate (as the amino group donor); glutamate thus produced is taken up into vesicles. We also report a finding that α-ketoglutarate-derived glutamate uptake into synaptic vesicles and aspartate aminotransferase are inhibited by 2,3-pyrazinedicarboxylate. Evidence is given that this is a selective inhibitor for aspartate aminotransferase. These observations provide insight into understanding the nerve endings' mechanism for high efficiency in glutamate transmission. Finding this inhibitor may have implications for further experimentation on the role of α-ketoglutarate-derived glutamate in glutamate transmission.

The sodium-dependent di- and tricarboxylate transporter, NaCT, is not responsible for the uptake of D-, L-2-hydroxyglutarate and 3-hydroxyglutarate into neurons

Concentrations of glutarate (GA) and its derivatives such as 3-hydroxyglutarate (3OHGA), D- (D-2OHGA) and L-2-hydroxyglutarate (L-2OHGA) are increased in plasma, cerebrospinal fluid (CSF) and urine of patients suffering from different forms of organic acidurias. It has been proposed that these derivatives cause neuronal damage in these patients, leading to dystonic and dyskinetic movement disorders. We have recently shown that these compounds are eliminated by the kidneys via the human organic anion transporters, OAT1 and OAT4, and the sodium-dependent dicarboxylate transporter 3, NaDC3. In neurons, where most of the damage occurs, a sodium-dependent citrate transporter, NaCT, has been identified. Therefore, we investigated the impact of GA derivatives on hNaCT by two-electrode voltage clamp and tracer uptake studies. None of these compounds induced substrate-associated currents in hNaCT-expressing Xenopus laevis oocytes nor did GA derivatives inhibit the uptake of citrate, the prototypical substrate of hNaCT. In contrast, D- and L-2OHGA, but not 3OHGA, showed affinities to NaDC3, indicating that D- and L-2OHGA impair the uptake of dicarboxylates into astrocytes thereby possibly interfering with their feeding of tricarboxylic acid cycle intermediates to neurons.

Transgenic expression of Glud1 (glutamate dehydrogenase 1) in neurons: in vivo model of enhanced glutamate release, altered synaptic plasticity, and selective neuronal vulnerability

The effects of lifelong, moderate excess release of glutamate (Glu) in the CNS have not been previously characterized. We created a transgenic (Tg) mouse model of lifelong excess synaptic Glu release in the CNS by introducing the gene for glutamate dehydrogenase 1 (Glud1) under the control of the neuron-specific enolase promoter. Glud1 is, potentially, an important enzyme in the pathway of Glu synthesis in nerve terminals. Increased levels of GLUD protein and activity in CNS neurons of hemizygous Tg mice were associated with increases in the in vivo release of Glu after neuronal depolarization in striatum and in the frequency and amplitude of miniature EPSCs in the CA1 region of the hippocampus. Despite overexpression of Glud1 in all neurons of the CNS, the Tg mice suffered neuronal losses in select brain regions (e.g., the CA1 but not the CA3 region). In vulnerable regions, Tg mice had decreases in MAP2A labeling of dendrites and in synaptophysin labeling of presynaptic terminals; the decreases in neuronal numbers and dendrite and presynaptic terminal labeling increased with advancing age. In addition, the Tg mice exhibited decreases in long-term potentiation of synaptic activity and in spine density in dendrites of CA1 neurons. Behaviorally, the Tg mice were significantly more resistant than wild-type mice to induction and duration of anesthesia produced by anesthetics that suppress Glu neurotransmission. The Glud1 mouse might be a useful model for the effects of lifelong excess synaptic Glu release on CNS neurons and for age-associated neurodegenerative processes.

Design and applications of biodegradable polyester tissue scaffolds based on endogenous monomers found in human metabolism

Synthetic polyesters have deeply impacted various biomedical and engineering fields, such as tissue scaffolding and therapeutic delivery. Currently, many applications involving polyesters are being explored with polymers derived from monomers that are endogenous to the human metabolism. Examples of these monomers include glycerol, xylitol, sorbitol, and lactic, sebacic, citric, succinic, alpha-ketoglutaric, and fumaric acids. In terms of mechanical versatility, crystallinity, hydrophobicity, and biocompatibility, polyesters synthesized partially or completely from these monomers can display a wide range of properties. The flexibility in these macromolecular properties allows for materials to be tailored according to the needs of a particular application. Along with the presence of natural monomers that allows for a high probability of biocompatibility, there is also an added benefit that this class of polyesters is more environmentally friendly than many other materials used in biomedical engineering. While the selection of monomers may be limited by nature, these polymers have produced or have the potential to produce an enormous number of successes in vitro and in vivo.

Determination of enzymatic source of alanine aminotransferase activity in serum from dogs with liver injury

INTRODUCTION

Increase of serum alanine aminotransferase (ALT) activity is widely used as a surrogate marker for tissue damage. Two ALT isoforms, ALT1 and ALT2, have been cloned recently in mammals. The study investigated the source of elevated ALT activity in serum of dogs treated with a hepatotoxic compound.

METHODS

ALT activity was measured by enzyme assay. Immunoblot analysis was performed using generated specific peptide antibodies against dog ALTs. LC-MS/MS-based proteomics analysis was conducted to independently identify dog ALT peptides. Serum samples immunodepleted of major serum components by Seppro IgY-D11 microbead spin column were evaluated by the immunoblot analysis, and compared with those of the ALT activity.

RESULTS

Involvement of ALT enzyme(s) is consistent with the following observations: 1) all the substrates (L-alanine and alpha-ketoglutarate) were required for serum ALT activity as purified porcine ALT1 needed for activity, 2) serum ALT activity was inhibited by L-cycloserine, a known ALT inhibitor, and 3) apparent Km value for the ALT reaction catalyzed by the serum, liver, and skeletal muscle was roughly similar. Immunoblot analysis showed that ALT1 was detected in liver and both ALTs were detected in the skeletal muscle. The relative expression level of ALTs was -: liver ALT1>skeletal muscle ALT1>skeletal muscle ALT2. LC-MS/MS-based proteomics analysis gave similar results. Immunoblot analysis of the depleted serum samples revealed the presence of ALT1 in compound-treated dogs. Intensity of the ALT1 band detected in the sera correlated well with the ALT activity measured by the enzyme assay.

DISCUSSION

Based on these findings, we conclude that the elevation of serum ALT activity in dogs with liver injury is attributed to elevation of ALT1 protein level in serum. The methodology to directly detect ALT proteins in serum could be a tool to facilitate our understanding of biological and toxicological significance of the ALT isoenzymes.

Membrane translocation of glutaric acid and its derivatives

The neurodegenerative disorder glutaric aciduria type I (GA I) is characterized by increased levels of cytotoxic metabolites such as glutaric acid (GA) and 3-hydroxyglutaric (3OHGA). The present report summarizes recent investigations providing insights into mechanisms of intra- and intercellular translocation of these metabolites. Initiated by microarray analyses in a mouse model of GA I, the sodium-dependent dicarboxylate cotransporter 3 (NaC3) was the first molecule identified to mediate the translocation of GA and 3OHGA with high and low affinity, respectively. More recently, organic anion transporters (OAT) 1 and 4 have been reported to be high-affinity transporters for GA and 3OHGA as well as D-2- and L-2-hydroxyglutaric acid (D2OHGA, L2OHGA). The concerted action of NaC3 and OATs may be important for the directed uptake and excretion of GA, 3OHGA, D2OHGA and L2OHGA in kidney proximal tubule cells. In addition, experimental data on cultured neuronal and glial cells isolated from mouse brain demonstrated that GA rather than 3OHGA may competitively inhibit the anaplerotic supply of tricarboxylic acid cycle intermediates from astrocytes to neurons. The identification of GA and GA derivative transporters may represent targets for new approaches to treat patients with GA I and related disorders.

Energy metabolism in brain cells: effects of elevated ammonia concentrations

Both neurons and astrocytes have high rates of glucose utilization and oxidative metabolism. Fully 20% of glucose consumption is used for astrocytic production of glutamate and glutamine, which during intense glutamatergic activity leads to an increase in glutamate content, but at steady state is compensated for by an equally intense oxidation of glutamate. The amounts of ammonia used for glutamine synthesis and liberated during glutamine hydrolysis are large, compared to the additional demand for glutamine synthesis in hyperammonemic animals and patients with hepatic encephalopathy. Nevertheless, elevated ammonia concentrations lead to an increased astrocytic glutamine production and an elevated content of glutamine combined with a decrease in glutamate content, probably mainly in a cytosolic pool needed for normal activity of the malate-asparate shuttle (MAS); another compartment generated by glutamine hydrolysis is increased. As a result of reduced MAS activity the pyruvate/lactate ratio is decreased in astrocytes but not in neurons and decarboxylation of pyruvate to form acetyl coenzyme A is reduced. Elevated ammonia concentrations also inhibit decarboxylation of alpha-ketoglutarate in the TCA cycle. This effect occurs in both neurons and astrocytes, is unrelated to MAS activity and seen after chronic treatment with ammonia even in the absence of elevated ammonia concentrations.

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.

3-Hydroxyglutaric acid is transported via the sodium-dependent dicarboxylate transporter NaDC3

Patients with glutaryl-CoA dehydrogenase (GCDH) deficiency accumulate glutaric acid (GA) and 3-hydroxyglutaric acid (3OH-GA) in their blood and urine. To identify the transporter mediating the translocation of 3OH-GA through membranes, kidney tissue of Gcdh-/- mice have been investigated because of its central role in urinary excretion of this metabolite. Using microarray analyses of kidney-expressed genes in Gcdh-/- mice, several differentially expressed genes encoding transporter proteins were identified. Real-time polymerase chain reaction analysis confirmed the upregulation of the sodium-dependent dicarboxylate cotransporter 3 (NaDC3) and the organic cation transporter 2 (OCT2). Expression analysis of NaDC3 in Xenopus laevis oocytes by the two-electrode-voltage-clamp technique demonstrated the sodium-dependent translocation of 3OH-GA with a K (M) value of 0.95 mM. Furthermore, tracer flux measurements in Chinese hamster ovary cells overexpressing OCT2 showed that 3OH-GA inhibited significantly the uptake of methyl-4-phenylpyridinium, whereas 3OH-GA is not transported by OCT2. The data demonstrate for the first time the membrane translocation of 3OH-GA mediated by NaDC3 and the cis-inhibitory effect on OCT2-mediated transport of cations.

The Effects of Hypoxia-Ischemia on Neutral Amino Acid Transporters in the Developing Rat Brain

The neutral amino acid transporters SNAT1-3 and ASCT1 play critical roles in the recycling of glutamine, and subsequently glutamate, via the glutamine-glutamate cycle. Hypoxia-ischemia was induced in rat pups using the Rice-Vannucci model. Brains were harvested at 1 h, 24 h and 7 days after ischemia. The expression of NAATs was evaluated using immunoblotting, real-time PCR, and immunohistochemistry. Results were compared with age-matched controls and shams. SNAT1 mRNA decreased at 1 h after injury in both hemispheres when compared with the control animals and correlated with a decrease in protein expression at 24 h in the hippocampus and cortex. SNAT1 protein expression increased globally at 7 days after injury and specifically in the hippocampus. Finally, SNAT2 and 3 demonstrated subtle changes in various brain regions after injury. These data suggest that neutral amino acid transporters remain largely intact after hypoxia-ischemia.

Alanine Aminotransferase in Amphioxus: Presence, Localization and Up-regulation after Acute Lipopolysaccharide Exposure

Alanine aminotransferase (AAT) is mainly synthesized in the liver, and its level in mammalian serum is elevated after acute phase induction. Here we demonstrated that sheep anti-human AAT antibody cross-reacted with amphioxus humoral fluids as well as human serum; and the concentration of AAT in the humoral fluids in amphioxus increased after the acute challenge with lipopolysaccharide, while the level of total proteins remains unchanged. These suggest the presence of the same acute phase response pattern in amphioxus, as observed in some mammalian species. Immunohistochemically, AAT was localized in the hepatic diverticulum, ovary and testis. It appears that the hepatic diverticulum in amphioxus is functionally homologous to the vertebrate liver in respect of AAT synthesis, supporting the hypothesis that the vertebrate liver evolved from the hepatic diverticulum of an amphioxus-like ancestor during early chordate evolution.

The glutamate/GABA-glutamine cycle: aspects of transport, neurotransmitter homeostasis and ammonia transfer

Neurons are metabolically handicapped in the sense that they are not able to perform de novo synthesis of neurotransmitter glutamate and gamma-aminobutyric acid (GABA) from glucose. A metabolite shuttle known as the glutamate/GABA-glutamine cycle describes the release of neurotransmitter glutamate or GABA from neurons and subsequent uptake into astrocytes. In return, astrocytes release glutamine to be taken up into neurons for use as neurotransmitter precursor. In this review, the basic properties of the glutamate/GABA-glutamine cycle will be discussed, including aspects of transport and metabolism. Discussions of stoichiometry, the relative role of glutamate vs. GABA and pathological conditions affecting the glutamate/GABA-glutamine cycling are presented. Furthermore, a section is devoted to the accompanying ammonia homeostasis of the glutamate/GABA-glutamine cycle, examining the possible means of intercellular transfer of ammonia produced in neurons (when glutamine is deamidated to glutamate) and utilized in astrocytes (for amidation of glutamate) when the glutamate/GABA-glutamine cycle is operating. A main objective of this review is to endorse the view that the glutamate/GABA-glutamine cycle must be seen as a bi-directional transfer of not only carbon units but also nitrogen units.

Localization of the Na(+)-coupled neutral amino acid transporter 2 in the cerebral cortex

We studied the distribution and cellular localization of Na(+)-coupled neutral amino acid transporter 2, a member of the system A family of amino acid transporters, in the rat and human cerebral cortex using immunocytochemical methods. Na(+)-coupled neutral amino acid transporter 2-positive neurons were pyramidal and non-pyramidal, and Na(+)-coupled neutral amino acid transporter 2/GABA double-labeling studies revealed that Na(+)-coupled neutral amino acid transporter 2 was highly expressed by GABAergic neurons. Double-labeling studies with the synaptophysin indicated that rare axon terminals express Na(+)-coupled neutral amino acid transporter 2. Na(+)-coupled neutral amino acid transporter 2-immunoreactivity was also found in astrocytes, leptomeninges, ependymal cells and choroid plexus. Electron microscopy showed robust Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in the somato-dendritic compartment of neurons and in glial processes, but, as in the case of double-labeling studies, failed to reveal Na(+)-coupled neutral amino acid transporter 2-immunoreactivity in terminals. To rule out the possibility that the absence of Na(+)-coupled neutral amino acid transporter 1- and Na(+)-coupled neutral amino acid transporter 2-positive terminals was due to insufficient antigen detection, we evaluated Na(+)-coupled neutral amino acid transporter 1/synaptophysin and Na(+)-coupled neutral amino acid transporter 2/synaptophysin coexpression using non-standard immunocytochemical procedures and found that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2+ terminals were rare in all conditions. These findings indicate that Na(+)-coupled neutral amino acid transporter 1 and Na(+)-coupled neutral amino acid transporter 2 are virtually absent in cortical terminals, and suggest that they do not contribute significantly to replenishing the Glu and GABA transmitter pools through the glutamate-glutamine cycle. The strong expression of Na(+)-coupled neutral amino acid transporter 2 in the somato-dendritic compartment and in non-neuronal elements that are integral parts of the blood-brain and brain-cerebrospinal fluid barrier suggests that Na(+)-coupled neutral amino acid transporter 2 plays a role in regulating the levels of Gln and other amino acids in the metabolic compartment of cortical neurons.

Functional and molecular identification of sodium-coupled dicarboxylate transporters in rat primary cultured cerebrocortical astrocytes and neurons

Na+-coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na+-coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na+-coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na+-dependent succinate and citrate transport in primary cultures of astrocytes and neurons from rat cerebral cortex. Uptake of succinate was Na+ dependent, Li+ sensitive and saturable with a Michaelis constant (Kt) value of 28.4 microM in rat astrocytes. Na+ activation kinetics revealed that the Na+ to succinate stoichiometry was 3:1 and the concentration of Na+ necessary for half-maximal transport was 53 mM. Although uptake of citrate in astrocytes was also Na+ dependent and saturable, its Kt value was significantly higher (approximately 1.2 mM) than that of succinate. Unlabeled succinate (2 mM) inhibited Na+-dependent [14C]succinate (18 microM) and [14C]citrate (4.5 microM) transport completely, whereas unlabeled citrate inhibited Na+-dependent [14C]succinate uptake more weakly. Interestingly, N-acetyl-L-aspartate, which is the second most abundant amino acid in the nervous system, also completely inhibited Na+-dependent succinate transport in rat astrocytes. The inhibition constant (Ki) for the inhibition of [14C]succinate uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 microM respectively. In primary cultures of neurons, uptake of citrate was also Na+ dependent and saturable with a Kt value of 16.2 microM, which was different from that observed in astrocytes, suggesting that different Na+-dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.

Functional characterization of Na+-coupled citrate transporter NaC2/NaCT expressed in primary cultures of neurons from mouse cerebral cortex

Neurons are known to express a high-affinity Na+ -coupled dicarboxylate transporter(s) for uptake of tricarboxylic acid cycle intermediates, such as alpha-ketoglutarate and malate, which are precursors for neurotransmitters including glutamate and gamma-aminobutyric acid. There is, however, little information available on the molecular identity of the transporters responsible for this uptake process in neurons. In the present study, we investigated the characteristics of Na+ -dependent citrate transport in primary cultures of neurons from mouse cerebral cortex and established the molecular identity of this transport system as the Na+ -coupled citrate transporter (NaC2/NaCT). Reverse transcriptase (RT)-PCR and immunocytochemical analyses revealed that only NaC2/NaCT was expressed in mouse cerebrocortical neurons but not in astrocytes. Uptake of citrate in neurons was Na+ -dependent, Li+ -sensitive, and saturable with the Kt value of 12.3 microM. This Kt value was comparable with that in the case of Na+ -dependent succinate transport (Kt = 9.2 microM). Na+ -activation kinetics revealed that the Na+ -to-citrate stoichiometry was 3.4:1 and concentration of Na+ necessary for half-maximal activation (K0.5(Na)) was 45.7 mM. Na+ -dependent uptake of [14C]citrate (18 microM) was significantly inhibited by unlabeled citrate as well as dicarboxylates such as succinate, malate, fumarate, and alpha-ketoglutarate. This is the first report demonstrating the molecular identity of the Na+ -coupled di/tricarboxylate transport system expressed in neurons as NaC2/NaCT, which can transport the tricarboxylate citrate as well as dicarboxylates such as succinate, alpha-ketoglutarate, and malate.

The glutamine commute: lost in the tube?

The "glutamate-glutamine" cycle appears to have an important, albeit not exclusive role, in the recycling of glutamate (Glu) between neurons and astrocytes. Recent studies show that the efflux of glutamine (Gln) from astrocytes is mediated by SNAT3 (formerly SN1), a system N amino acid transporter localized to perisynaptic astrocytes, whereas its influx into neurons is thought to be mediated by transporters of the system A family, specifically SNAT1 and SNAT2. However, the results of our confocal and electron microscopy immunocytochemical studies of the localization of these transporters in the cerebral cortex show that SNAT1 and SNAT2 are robustly expressed in the somatodendritic domain of cortical neurons, but rarely to axon terminals. To rule out a possible influence of fixation and procedural variables on detection of SNAT1 and SNAT2 immunoreactivity in axon terminals, we used non-conventional immunocytochemical methods, which, in certain cases, improve antigen detection. Though evidencing a slightly increased percentage of axon terminals expressing the two transporters, these techniques demonstrated that SNAT1 and SNAT2 are indeed rarely localized to axon terminals. Our data thus suggest that neither SNAT1 nor SNAT2 meet the criteria for their postulated role in the "glutamate-glutamine" cycle, and indicate that other Gln transporters (either orphan or yet to be identified) must be expressed at axon terminals and sustain the Glu (and gamma-aminobutyric acid) neurotransmitter pool (s).

cDNA cloning, expression, purification, distribution, and characterization of biologically active canine alanine aminotransferase-1

Alanine aminotransferase (ALT) is a pyridoxal enzyme found mainly in the liver and kidney, but also in small amounts in the heart, muscle, fat, and brain. Serum aminotransferase activities have been used broadly as surrogate markers for tissue injury and disease in human and veterinary clinical settings and in safety assessment of chemicals and pharmaceuticals. Because of its relative abundance in liver, increased serum ALT activity is generally considered indicative of liver damage. Two ALT isoenzymes, ALT1 and ALT2, are known and have been cloned and sequenced from human, rat, and mouse. In this study, we have cloned the complementary DNA encoding the canine orthologue of ALT1 (cALT1). The complete cDNA sequence comprised 1852 bases and contained a 1485-base open reading frame, which encodes a polypeptide of 494 amino acid residues. Canine ALT1 shares 87.7, 87.2, and 87.0% amino acid identity to its human, mouse, and rat orthologues, respectively. The cDNA was expressed in Escherichia coli, with a N-terminal His (6x) tag, and the recombinant enzyme was purified using immobilized metal-affinity chromatography. The final yield of the purified recombinant cALT1 was greater than 5mg/L culture. The alanine transaminase activity of purified cALT1 was 229.81U/mg protein, which is approximately 38-fold higher than that of total soluble recombinant E. coli cell lysate, confirming that the enzyme is a functional ALT. Evaluation of various canine tissues by RT-PCR revealed that the level of ALT1 expression is in the order of: heart>liver>fat approximately brain approximately gastrocnemius>kidney. The purified cALT1 will be helpful to develop isoenzyme-specific anti-bodies, which could further improve the diagnostic resolution of current ALT assays in drug safety studies.

Importance of glutamate-generating metabolic pathways for memory consolidation in chicks

Glutamatergic and noradrenergic stimulation is essential for formation of memory of single-trial discriminative avoidance of colored beads in the 1-day-old chick. Transmitter glutamate is released soon after training and again before memory consolidation 30 min after training. Memory consolidation is abolished by posttraining injection of iodoacetate, which inhibits glycolysis and thus not only energy metabolism but also pyruvate carboxylase-dependent glucose conversion to glutamate, needed for consolidation; a similar effect is evoked by the antagonists propranolol acting at beta(2)-adrenoceptors or SR59230A acting at beta(3)-adrenoceptors. This paper shows that the effect of these inhibitors can be overcome by central injection of glutamine, providing an alternate source of transmitter glutamate and compensating for the inhibition of glycolysis by iodoacetate or the blockade of adrenergic stimulation of glycogenolysis by propranolol or of glucose uptake by SR59230A. Conversely, inhibition of memory consolidation by methionine sulfoximine (MSO), an inhibitor of glutamine synthetase and thus of the glutamate-glutamine cycle, essential for neuronal reaccumulation of previously released transmitter glutamate, could be challenged by noradrenaline, stimulating glucose uptake and glycogenolysis and providing glutamate synthesis from glucose to compensate for the lack of return of previously released glutamate. Also, administration of either glutamine or noradrenaline could prevent the spontaneous decay of labile memory 30 min after training on a weakened stimulus, suggesting that direct supply of glutamate from glucose may secure sufficient supplies of transmitter glutamate for release prior to memory consolidation at 30 min.

Differential roles of alanine in GABAergic and glutamatergic neurons

Studies in different preparations of neurons and astrocytes of alanine transport and activities of its metabolizing enzyme alanine aminotransferase have led to the proposal that this amino acid is preferentially synthesized in astrocytes and transferred from the astrocytic to the neuronal compartment. From a functional point of view this may well be the case in a GABAergic synapse since theoretically alanine can be utilized as a metabolic fuel in GABAergic neurons where the GABA shunt is operating. Thus, a metabolic scheme is proposed, according to which alanine catabolism is coupled to the TCA cycle where the GABA shunt replaces the alpha-ketoglutarate dehydrogenase/succinyl CoA synthetase reactions. In a glutamatergic synapse in which the large demand for synthesis of neurotransmitter glutamate leads to a large production of ammonia, it is possible that alanine could play a completely different role. Hence, experimental evidence is reviewed suggesting that alanine may serve as a carrier of ammonia nitrogen from the neuronal compartment to the astrocytic compartment using a flux of lactate in the opposite direction to account for transfer of the C-3 carbon skeleton.

Functional properties and cellular distribution of the system A glutamine transporter SNAT1 support specialized roles in central neurons

Glutamine, the preferred precursor for neurotransmitter glutamate and GABA, is likely to be the principal substrate for the neuronal System A transporter SNAT1 in vivo. We explored the functional properties of SNAT1 (the product of the rat Slc38a1 gene) by measuring radiotracer uptake and currents associated with SNAT1 expression in Xenopus oocytes and determined the neuronal-phenotypic and cellular distribution of SNAT1 by confocal laser-scanning microscopy alongside other markers. We found that SNAT1 mediates transport of small, neutral, aliphatic amino acids including glutamine (K0.5 approximately 0.3 mm), alanine, and the System A-specific analogue 2-(methylamino)isobutyrate. Amino acid transport is driven by the Na+ electrochemical gradient. The voltage-dependent binding of Na+ precedes that of the amino acid in a simultaneous transport mechanism. Li+ (but not H+) can substitute for Na+ but results in reduced Vmax. In the absence of amino acid, SNAT1 mediates Na+-dependent presteady-state currents (Qmax approximately 9 nC) and a nonsaturable cation leak with selectivity Na+, Li+ >> H+, K+. Simultaneous flux and current measurements indicate coupling stoichiometry of 1 Na+ per 1 amino acid. SNAT1 protein was detected in somata and proximal dendrites but not nerve terminals of glutamatergic and GABAergic neurons throughout the adult CNS. We did not detect SNAT1 expression in astrocytes but detected its expression on the luminal membranes of the ependyma. The functional properties and cellular distribution of SNAT1 support a primary role for SNAT1 in glutamine transport serving the glutamate/GABA-glutamine cycle in central neurons. Localization of SNAT1 to certain dopaminergic neurons of the substantia nigra and cholinergic motoneurons suggests that SNAT1 may play additional specialized roles, providing metabolic fuel (via alpha-ketoglutarate) or precursors (cysteine, glycine) for glutathione synthesis.

Effect of vigabatrin on glutamate dehydrogenase in the hippocampus of seizure prone gerbils

Vigabatrin (VGB, gamma-vinyl-gamma-aminobutyric acid (GABA)), an irreversible inhibitor of GABA transaminase, increases regional inhibitory effects by elevating GABA concentration and reducing glutamate synthesis. In the present study, changes in glutamate dehydrogenase (GDH) activity and its immunoreactivity in the seizure prone gerbil hippocampus after treating VGB were investigated to identify the effect of VGB on energy and/or glutamate metabolism via GDH. In the VGB treated group, GDH immunoreactivity and its activity in the hippocampus were significantly decreased, as compared with those of controls. These findings suggest that VGB administration may suppress the development and spread of seizures not only by elevating the level of GABA, but also by affecting the glutamate signaling and energy metabolism in neurons.

Compartmentation of metabolism probed by [2-13C]alanine: improved 13C NMR sensitivity using a CryoProbe detects evidence of a glial metabolon

The labelling of metabolites with the NMR active nucleus 13C allows not only metabolite enrichments to be monitored, but also the relative fluxes through competing pathways to be delineated. [2-13C, 15N]alanine was used as a metabolic probe to investigate compartmentation in superfused cerebral slices. Perchloric acid extracts of the tissue were investigated using 13C NMR spectroscopy. The spectra were obtained using a CryoProbe optimised for 13C detection (dual CryoProbe [13C, 1H]) in which the receiver and transmitter coils are cooled to approximately 20K to reduce contributions to noise in the signal obtained. Compared with conventional inverse geometry probe, the signal-to-noise ratio (S/N) was increased by approximately 17-fold using this device. A large proportion of alanine was initially metabolised over the first 20 min by glial cells, as indicated by the relative importance of the glial, only enzyme pyruvate carboxylase to the labelling pattern of glutamate, with the ratio of pyruvate carboxylase to pyruvate dehydrogenase derived glutamate being 0.25, and exported [2-13C, 15N]aspartate. Using the increased sensitivity of the CryoProbe, [2-13C, 15N]aspartate was also detected in the extracts of cerebral tissue. This metabolite could only have been derived via the pyruvate carboxylase pathway, and given the large export of the metabolite into the superfusion buffer suggests the occurrence of a "metabolon" arrangement of enzymes within glial cells.

Exocytotic release of alanine from cultured cerebellar neurons

Many neurons release a variety of amino acids in response to depolarizing stimuli. Although some of these amino acids, namely, glutamate, aspartate, and gamma-aminobutyric acid (GABA), have been qualified as neurotransmitters, functional roles of the other amino acids including alanine remain obscure. We investigated the mechanism and the origin of alanine release from cultured rat cerebellar cells. High-K(+)-induced depolarization produced a considerable amount (139+/-8 pmol/2 min/dish) of alanine release, comparable to that of glutamate (103+/-7 pmol/2 min/dish). Other depolarizing agents including veratridine or 4-aminopyridine also induced alanine release, suggesting that the major source is excitable neurons, rather than non-excitable glial cells. Depolarization-evoked alanine release was suppressed in the absence of extracellular Ca(2+), and was almost abolished by treating the cells with botulinum type B neurotoxin (BoNT/B), indicating that alanine is released by Ca(2+)-dependent exocytosis of vesicle-associated membrane protein-2 (VAMP-2)-containing vesicles. The properties of alanine release were different from those of glutamate and GABA in several aspects: (a) Depolarization-dependent alanine release appeared as early as 7 days in vitro, much earlier than that of GABA. (b) Fifty microM kainate, which causes selective cell death of GABAergic neurons in the culture, only partially reduced alanine release, whereas it had no effect on glutamate release. (c) Alanine release was not affected by phorbol ester, which enhanced glutamate and GABA release in a kinase-dependent manner. We therefore conclude that alanine release occurs via exocytosis of a pool of synaptic vesicles distinct from those containing glutamate or GABA.

cDNA cloning, genomic structure, chromosomal mapping, and functional expression of a novel human alanine aminotransferase

Alanine aminotransferase (ALT) catalyzes the reversible transamination between alanine and 2-oxoglutarate to form pyruvate and glutamate, and thereby has a key role in the intermediary metabolism of glucose and amino acids. Two ALT isoenzymes are known to exist, but only one ALT gene has been cloned, GPT. In this study, we cloned a homolog of GPT and named it GPT2, and the corresponding protein ALT2. GPT2 shares 69% identity and 78% similarity at the protein level to the previously cloned GPT. The human gene GPT2 encodes a 3.9-kb mRNA, consists of 12 exons, spanning approximately 50 kb of the genome, and maps to chromosome 16q12.1. GPT2 and GPT differ in mRNA expression in that GPT2 is highly expressed in muscle, fat, and kidney, whereas GPT is mainly expressed in kidney, liver, and heart. In addition, GPT2 seems to be the predominant form of GPT at the mRNA level in these tissues. Expression of ALT2 protein in Escherichia coli produced a functional recombinant enzyme that catalyzes alanine transamination, confirming that the enzyme is an ALT. The more abundant expression of GPT2 than GPT, especially in muscle and fat, suggests a unique and previously unrecognized role of this gene product in glucose, amino acid, and fatty acid metabolism and homeostasis.

Citrate, beneficial or deleterious in the CNS?

Cerebellar granule neurons were incubated with or without glucose (3 mM) in the presence or absence of citrate (20 mM) using normoxic and/or hypoxic incubation conditions. During 4 h of hypoglycemia and also during hypoxia plus hypoglycemia, citrate increased lactate dehydrogenase (LDH) leakage from the cells and decreased mitochondrial activity, the latter was also the case in the presence of glucose. After 24 h of hypoglycemia, however, citrate decreased LDH leakage slightly, possibly due to its metabolism in the tricarboxylic acid cycle under these conditions. It should be noted that during mild hypoxia plus hypoglycemia a reduced LDH leakage was observed when compared to hypoglycemia alone. The 4 h low oxygen period did protect the neurons also during the 20 h re-oxygenation period. The present study might indicate that incubation of brain cell cultures in an atmosphere of air (30% oxygen) and 5% CO2, which is used in most laboratories, can be toxic and that oxygen concentration should be lowered considerably to mimic conditions in the brain.

Influence of convulsants on rat brain activities of alanine aminotransferase and aspartate aminotransferase

There exist differences between 12-day-old and adult rats in the onset of seizures induced by some inhibitors of glutamate decarboxylase (GAD). The aim of study was to investigate if there are differences between both groups in activities of rat brain alanine aminotransferase (ALT) and aspartate aminotransferase (AST), the enzymes involved in glutamate metabolism, after the administration of 3-mercaptopropionic acid as specific GAD inhibitor or isoniazid as less specific general inhibitor of pyridoxal enzymes. Activities of both aminotransferases in a supernatant 20,000 g of the whole brain (containing predominantly cytosolic isoforms of enzymes) were increased at the beginning of 3-mercaptopropionic acid-induced generalized tonic-clonic seizures. At isoniazid-induced generalized tonic-clonic seizures, a significant increase in both enzyme activities was observed in adult rat brain. In the 12-day-old rat brain, ALT and AST activities reached about 40% and about 50-60% of adult control levels, respectively. In in vitro experiments, no influence of 3-mercaptopropionic acid on transaminase activities was found and an inhibitory effect of isoniazid on the enzymes was confirmed. Increased aminotransferase activities might participate in the enhanced synthesis of excitatory amino acid neurotransmitters in the nervous system, which may take a part in the initiation of epileptic seizures. Alternatively, the increased AST activity may be connected with an increased transport of NADH from the cytosol to mitochondria, while the increased ALT activity would represent the transformation of pyruvate to alanine as a consequence of increased glycolysis.

Cloning and functional characterization of a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain

Neurons contain a high-affinity Na(+)/dicarboxylate cotransporter for absorption of neurotransmitter precursor substrates, such as alpha-ketoglutarate and malate, which are subsequently metabolized to replenish pools of neurotransmitters, including glutamate. We have isolated the cDNA coding for a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain, called mNaDC-3. The mRNA coding for mNaDC-3 is found in brain and choroid plexus as well as in kidney and liver. The mNaDC-3 transporter has a broad substrate specificity for dicarboxylates, including succinate, alpha-ketoglutarate, fumarate, malate, and dimethylsuccinate. The transport of citrate is relatively insensitive to pH, but the transport of succinate is inhibited by acidic pH. The Michaelis-Menten constant for succinate in mNaDC-3 is 140 microM in transport assays and 16 microM at -50 mV in two-electrode voltage clamp assays. Transport is dependent on sodium, although lithium can partially substitute for sodium. In conclusion, mNaDC-3 likely codes for the high-affinity Na(+)/dicarboxylate cotransporter in brain, and it has some unusual electrical properties compared with the other members of the family.

Role of astrocytes in pathogenesis of ischemic brain injury

Astrocytes play an important role in the homeostasis of the CNS both in normal conditions and after ischemic injury. The swelling of astrocytes is observed during and several seconds after brain ischemia. Then ischemia stimulates sequential morphological and biochemical changes in glia and induces its proliferation. Reactive astrocytes demonstrate stellate morphology, increased glial fibrillary acidic protein (GFAP) immunoreactivity, increased number of mitochondria as well as elevated enzymatic and non-enzymatic antioxidant activities. Astrocytes can re-uptake and metabolize glutamate and in this way they control its extracellular concentration. The ability of astrocytes to protect neurons against the toxic action of free radicals depends on their specific energy metabolism, high glutathione level, increased antioxidant enzyme activity (catalase, superoxide dismutase, glutathione peroxidase) and overexpression of antiapoptotic bcl-2 gene. Astrocytes produce cytokines (TNF-alpha, IL-1, IL-6) involved in the initiation and maintaining of immunological response in the CNS. In astrocytes, like in neurones, ischemia induces the expression of immediate early genes: c-fos, c-jun, fos B, jun B, jun D, Krox-24, NGFI-B and others. The protein products of these genes modulate the expression of different proteins, both destructive ones and those involved in the neuroprotective processes.

NMR spectroscopic study on the metabolic fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures: implications for glia-neuron interactions in neurotransmitter metabolism

Nuclear magnetic resonance (NMR) spectroscopy and biochemical assays were used to study the fate of [3-(13)C]alanine in astrocytes, neurons, and cocultures. (1)H- and (13)C-NMR analysis of the media demonstrated a high and comparable uptake of [3-(13)C]alanine by the cells. Thereafter, alanine is transaminated predominantly to [3-(13)C]pyruvate, from which the (13)C-label undergoes different metabolic pathways in astrocytes and neurons: Lactate is almost exclusively synthesized in astrocytes, while in neurons and cocultures labeled neurotransmitter amino acids are formed, i.e., glutamate and gamma-aminobutyric acid (GABA). A considerable contribution of the anaplerotic pathway is observed in cocultures, as concluded from the ratio (C-2-C-3)/C-4 of labeled glutamine. Analysis of the multiplet pattern of glutamate isotopomers indicates carbon scrambling through the TCA cycle and the use of alanine also as energy substrate in neurons. In cocultures, astrocyte-deduced lactate and unlabeled exogenous carbon substrates contribute to glutamate synthesis and dilute the [2-(13)C]acetyl-CoA pool by 30%. The coupling of neuronal activity with shuttling of tricarboxylic acid (TCA) cycle-derived metabolites between astrocytes and neurons is concluded from the use of [4-(13)C]-monolabeled glutamate leaving the first TCA cycle turn already for glutamine and GABA synthesis, as well as from the labeling pattern of extracellular glutamine. Further evidence of a metabolic interaction between astrocytes and neurons is obtained, as alanine serves as a carbon and nitrogen carrier through the synthesis and regulated release of lactate from astrocytes for use by neurons. Complementary to the glutamine-glutamate cycle in the brain, a lactate-alanine shuttle between astrocytes and neurons would account for the nitrogen exchange of the glutamatergic neurotransmitter cycle in mammalian brain.

Amino acid concentrations in rat cochlear nucleus and superior olive

Distributions of 10 amino acids were mapped in the cochlear nucleus and superior olive of rats by microdissection of freeze-dried sections combined with high performance liquid chromatography. Glutamate concentrations were relatively high in regions containing granule cell bodies, axons and terminals, whereas aspartate concentrations were higher in the rest of the cochlear nucleus. The distribution of glutamine, a metabolic precursor of glutamate, correlated highly with that of glutamate. In the superior olive, glutamate concentrations were similar among the nuclei, whereas aspartate concentrations were higher in the more dorsal nuclei. Glycine concentrations were relatively high in dorsal portions of the cochlear nucleus and superior olive and were much higher in all regions than those of gamma-aminobutyrate (GABA). Both GABA and taurine showed decreasing gradients from superficial to deep layers of the dorsal cochlear nucleus. Concentrations of serine, threonine, arginine and alanine were generally lower than those of the other six amino acids. The results support other evidence for prominent roles of glutamate and glycine as neurotransmitters in the cochlear nucleus and superior olive. They support a neurotransmitter role also for GABA, especially in the superficial layers of the dorsal cochlear nucleus, but less in the superior olive. The literature related to our results is reviewed.

A possible role of alanine for ammonia transfer between astrocytes and glutamatergic neurons

The metabolism of [U-(13)C]lactate (1 mM) in the presence of unlabeled glucose (2.5 mM) was investigated in glutamatergic cerebellar granule cells, cerebellar astrocytes, and corresponding co-cultures. It was evident that lactate is primarily a neuronal substrate and that lactate produced glycolytically from glucose in astrocytes serves as a substrate in neurons. Alanine was highly enriched with (13)C in the neurons, whereas this was not the case in the astrocytes. Moreover, the cellular content and the amount of alanine released into the medium were higher in neurons than astrocytes. On incubation of the different cell types in medium containing alanine (1 mM), the astrocytes exhibited the highest level of accumulation. Altogether, these results indicate a preferential synthesis and release of alanine in glutamatergic neurons and uptake in cerebellar astrocytes. A new functional role of alanine may be suggested as a carrier of nitrogen from glutamatergic neurons to astrocytes, a transport that may operate to provide ammonia for glutamine synthesis in astrocytes and dispose of ammonia generated by the glutaminase reaction in glutamatergic neurons. Hence, a model of a glutamate-glutamine/lactate-alanine shuttle is presented. To elucidate if this hypothesis is compatible with the pattern of alanine metabolism observed in the astrocytes and neurons from cerebellum, the cells were incubated in a medium containing [(15)N]alanine (1 mM) and [5-(15)N]glutamine (0.5 mM), respectively. Additionally, neurons were incubated with [U-(13)C]glutamine to estimate the magnitude of glutamine conversion to glutamate. Alanine was labeled from [5-(15)N]glutamine to 3.3% and [U-(13)C]glutamate generated from [U-(13)C]glutamine was labeled to 16%. In spite of the modest labeling in alanine, it is clear that nitrogen from ammonia is transferred to alanine via transamination with glutamate formed by reductive amination of alpha-ketoglutarate. With regard to the astrocytic part of the shuttle, glutamine was labeled to 22% in one nitrogen atom whereas 3.2% was labeled in two when astrocytes were incubated in [(15)N]alanine. Moreover, in co-cultures, [U-(13)C]alanine labeled glutamate and glutamine equally, whereas [U-(13)C]lactate preferentially labeled glutamate. Altogether, these results support the role proposed above of alanine as a possible ammonia nitrogen carrier between glutamatergic neurons and surrounding astrocytes and they show that lactate is preferentially metabolized in neurons and alanine in astrocytes.

Differential distribution of the enzymes glutamate dehydrogenase and aspartate aminotransferase in cortical synaptic mitochondria contributes to metabolic compartmentation in cortical synaptic terminals

There have been numerous studies on the activity and localization of aspartate aminotransferase (AAT) and glutamate dehydrogenase (GDH) in brain tissue. However, there is still a controversy as to the specific roles and relative importance of these enzymes in glutamate and glutamine metabolism in astrocytes and neurons or synaptic terminals. There are many reports documenting GDH activity in synaptic terminals, yet the misconception that it is a glial enzyme persists. Furthermore, there is evidence that this tightly regulated enzyme may have an increased role in synaptic metabolism in adverse conditions such as low glucose and hyperammonemia that could compromise synaptic function. In the present study, we report high activity of both AAT and GDH in mitochondrial subfractions from cortical synaptic terminals. The relative amount of GDH/AAT activity was higher in SM2 mitochondria, compared to SM1 mitochondria. Such a differential distribution of enzymes can contribute significantly to the compartmentation of metabolism. There is evidence that the metabolic capabilities of the SM1 and SM2 subfractions of synaptic mitochondria are compatible with the compartments A and B of neuronal metabolism proposed by Waagepetersen et al. (1998b. Dev. Neurosci. 20, 310-320).

A novel system A isoform mediating Na+/neutral amino acid cotransport

A cDNA clone encoding a plasma membrane alanine-preferring transporter (SAT2) has been isolated from glutamatergic neurons in culture and represents the second member of the system A family of neutral amino acid transporters. SAT2 displays a widespread distribution and is expressed in most tissues, including heart, adrenal gland, skeletal muscle, stomach, fat, brain, spinal cord, colon, and lung, with lower levels detected in spleen. No signal is detected in liver or testis. In the central nervous system, SAT2 is expressed in neurons. SAT2 is significantly up-regulated during differentiation of cerebellar granule cells and is absent from astrocytes in primary culture. The functional properties of SAT2, examined using transfected fibroblasts and in cRNA-injected voltage-clamped Xenopus oocytes, show that small aliphatic neutral amino acids are preferred substrates and that transport is voltage- and Na(+)-dependent (1:1 stoichiometry), pH-sensitive, and inhibited by alpha-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of system A. Kinetic analyses of alanine and MeAIB uptake by SAT2 are saturable, with Michaelis constants (K(m)) of 200-500 microm. In addition to its ubiquitous role as a substrate for oxidative metabolism and a major vehicle of nitrogen transport, SAT2 may provide alanine to function as the amino group donor to alpha-ketoglutarate to provide an alternative source for neurotransmitter synthesis in glutamatergic neurons.