Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

Rottlerin inhibits (Na+, K+)-ATPase activity in brain tissue and alters D-aspartate dependent redistribution of glutamate transporter GLAST in cultured astrocytes

The naturally occurring toxin rottlerin has been used by other laboratories as a specific inhibitor of protein kinase C-delta (PKC-delta) to obtain evidence that the activity-dependent distribution of glutamate transporter GLAST is regulated by PKC-delta mediated phosphorylation. Using immunofluorescence labelling for GLAST and deconvolution microscopy we have observed that D-aspartate-induced redistribution of GLAST towards the plasma membranes of cultured astrocytes was abolished by rottlerin. In brain tissue in vitro, rottlerin reduced apparent activity of (Na+, K+)-dependent ATPase (Na+, K+-ATPase) and increased oxygen consumption in accordance with its known activity as an uncoupler of oxidative phosphorylation ("metabolic poison"). Rottlerin also inhibited Na+, K+-ATPase in cultured astrocytes. As the glutamate transport critically depends on energy metabolism and on the activity of Na+, K+-ATPase in particular, we suggest that the metabolic toxicity of rottlerin and/or the decreased activity of the Na+, K+-ATPase could explain both the glutamate transport inhibition and altered GLAST distribution caused by rottlerin even without any involvement of PKC-delta-catalysed phosphorylation in the process.

Distribution of glutamate transporter GLAST in membranes of cultured astrocytes in the presence of glutamate transport substrates and ATP

Neurotransmitter L-glutamate released at central synapses is taken up and "recycled" by astrocytes using glutamate transporter molecules such as GLAST and GLT. Glutamate transport is essential for prevention of glutamate neurotoxicity, it is a key regulator of neurotransmitter metabolism and may contribute to mechanisms through which neurons and glia communicate with each other. Using immunocytochemistry and image analysis we have found that extracellular D-aspartate (a typical substrate for glutamate transport) can cause redistribution of GLAST from cytoplasm to the cell membrane. The process appears to involve phosphorylation/dephosphorylation and requires intact cytoskeleton. Glutamate transport ligands L-trans-pyrrolidine-2,4-dicarboxylate and DL-threo-3-benzyloxyaspartate but not anti,endo-3,4-methanopyrrolidine dicarboxylate have produced similar redistribution of GLAST. Several representative ligands for glutamate receptors whether of ionotropic or metabotropic type, were found to have no effect. In addition, extracellular ATP induced formation of GLAST clusters in the cell membranes by a process apparently mediated by P2 receptors. The present data suggest that GLAST can rapidly and specifically respond to changes in the cellular environment thus potentially helping to fine-tune the functions of astrocytes.

Glutamatergic hypothesis of schizophrenia: involvement of Na+/K+-dependent glutamate transport

Hypothetical model based on deficient glutamatergic neurotransmission caused by hyperactive glutamate transport in astrocytes surrounding excitatory synapses in the prefrontal cortex is examined in relation to the aetiology of schizophrenia. The model is consistent with actions of neuroleptics, such as clozapine, in animal experiments and it is strongly supported by recent findings of increased expression of glutamate transporter GLT in prefrontal cortex of patients with schizophrenia. It is proposed that mechanisms regulating glutamate transport be investigated as potential targets for novel classes of neuroactive compounds with neuroleptic characteristics. Development of new efficient techniques designed specifically for the purpose of studying rapid activity-dependent translocation of glutamate transporters and associated molecules such as Na+, K+-ATPase is essential and should be encouraged.

Effects of glutamate transport substrates and glutamate receptor ligands on the activity of Na-/K(+)-ATPase in brain tissue in vitro

1. It has been suggested that Na+/K(+)-ATPase and Na(+)-dependent glutamate transport (GluT) are tightly linked in brain tissue. In the present study, we have investigated Na+/K(+)-ATPase activity using Rb+ uptake by 'minislices' (prisms) of the cerebral cortex. This preparation preserves the morphology of neurons, synapses and astrocytes and is known to possess potent GluT that has been well characterized. Uptake of Rb+ was determined by estimating Rb+ in aqueous extracts of the minislices, using atomic absorption spectroscopy. 2. We determined the potencies of several known substrates/inhibitors of GluT, such as L-trans-pyrrolidine-2,4-dicarboxylate (LtPDC), DL-threo-3-benzyloxyaspartic acid, (2S,3S,4R)-2-(carboxycyclopropyl)-glycine (L-CCG III) and L-anti,endo-3,4-methanopyrrolidine dicarboxylic acid, as inhibitors of [3H]-L-glutamate uptake by cortical prisms. In addition, we established the susceptibility of GluT, measured as [3H]-L-glutamate uptake in brain cortical prisms, to the inhibition of Na+/K(+)-ATPase by ouabain. Then, we tested the hypothesis that the Na+/K(+)-ATPase (measured as Rb+ uptake) can respond to changes in the activity of GluT produced by using GluT substrates as GluT-specific pharmacological tools. 3. The Na+/K(+)-ATPase inhibitor ouabain completely blocked Rb+ uptake (IC50 = 17 micromol/L), but it also potently inhibited a fraction of GluT (approximately 50% of [3H]-L-glutamate uptake was eliminated; IC50 < 1 micromol/L). 4. None of the most commonly used GluT substrates and inhibitors, such as L-aspartate, D-aspartate, L-CCG III and LtPDC (all at 500 micromol/L), produced any significant changes in Rb+ uptake. 5. The N-methyl-D-aspartate (NMDA) receptor agonists (R,S)-(tetrazol-5-yl)-glycine and NMDA decreased Rb+ uptake in a manner compatible with their known neurotoxic actions. 6. None of the agonists or antagonists for any of the other major classes of glutamate receptors caused significant changes in Rb+ uptake. 7. We conclude that, even if a subpopulation of glutamate transporters in the rat cerebral cortex may be intimately linked to a fraction of Na+/K(+)-ATPase, it is not possible, under the present experimental conditions, to detect regulation of Na+/K(+)-ATPase by GluT.

Brain gene expression, metabolism, and bioenergetics: interrelationships in murine models of cerebral and noncerebral malaria

Malaria infection can cause cerebral symptoms without parasite invasion of brain tissue. We examined the relationships between brain biochemistry, bioenergetics, and gene expression in murine models of cerebral (Plasmodium berghei ANKA) and noncerebral (P. berghei K173) malaria using multinuclear NMR spectroscopy, neuropharmacological approaches, and real-time RT-PCR. In cerebral malaria caused by P. berghei ANKA infection, we found biochemical changes consistent with increased glutamatergic activity and decreased flux through the Krebs cycle, followed by increased production of the hypoxia markers lactate and alanine. This was accompanied by compromised brain bioenergetics. There were few significant changes in expression of mRNA for metabolic enzymes or transporters or in the rate of transport of glutamate or glucose. However, in keeping with a role for endogenous cytokines in malaria cerebral pathology, there was significant up-regulation of mRNAs for TNF-alpha, interferon-gamma, and lymphotoxin. These changes are consistent with a state of cytopathic hypoxia. By contrast, in P. berghei K173 infection the brain showed increased metabolic rate, with no deleterious effect on bioenergetics. This was accompanied by mild up-regulation of expression of metabolic enzymes. These changes are consistent with benign hypermetabolism whose cause remains a subject of speculation.

Morphology and ultrastructure of rat hippocampal formation after i.c.v. administration of N-acetyl-L-aspartyl-L-glutamate

N-Acetyl-L-aspartyl-L-glutamate (NAAG) is one of the most abundant neuroactive compounds in the mammalian CNS. Our recent observations have suggested that NAAG administered into rat cerebral ventricles can cause neuronal death by apparently excitotoxic mechanisms that can be antagonized by the N-methyl-D-aspartate-receptor blockers and by ligands of metabotropic glutamate receptor of Group II. Therefore, the principal aim of the present study has been to use quantitative morphology, electron microscopy and terminal deoxynucleotidyl transferase-mediated biotin dUTP nick-end labeling to study a dose- and time-dependence as well as regional distribution of neurodegeneration in hippocampi of rats after the intraventricular infusion of 0.25 micromol NAAG/ventricle and of equimolar doses of L-glutamate (L-GLU) and N-acetyl-L-aspartate (NAA), breakdown products of NAAG. The degenerative changes were observed after the infusion of 0.25 and 1.25 micromol of NAAG/ventricle, but not when a dose of 0.05 micromol of NAAG/ventricle was injected into each lateral cerebral ventricle. With a dose of 0.25 micromol of NAAG/ventricle the number of degenerated neurons reached a maximum on the fourth day after the infusion. The neuronal damage following bilateral administration of 0.25 micromol of NAAG/lateral cerebral ventricle exhibited features of a delayed neuronal degeneration, expressed mainly in the layer of dentate granule neurons. The degeneration was characterized on the basis of ultrastructural appearance and DNA-fragmentation. The morphological changes caused by L-glutamate and NAA were much smaller than those observed after the administration of NAAG and displayed a different pattern of regional distribution. The present findings suggest that NAAG can cause a loss of hippocampal neurons in vivo, apparently resulting from the neurotoxicity of NAAG itself.

Avian Purkinje neuronal cultures: extrinsic control of morphology by cell type and glutamate

An in vitro coculture system is described to study the avian Purkinje neuron and the interactions occurring with astrocytes and granule cells during development in the cerebellum. Astrocytes initially and granule cells later regulate Purkinje neuron morphology. The coculture system presented here provides an excellent system for investigating the morphological, immunocytochemical, and electrophysiological differentiation of Purkinje neurons under controlled conditions and for studying cell-cell interactions and extrinsic factors, e.g., glutamate in normal and neuropathological conditions.

Uptake of [3H]L-serine in rat brain synaptosomal fractions

Accumulation of [3H]L-serine in crude synaptosomal fractions freshly prepared from rat brain has been found to be temperature-sensitive and to consist of both Na(+)-dependent and Na(+)-independent components. The accumulation of [3H]L-serine measured at submicromolar concentrations had a distinct substrate selectivity, different from the uptake of [3H]L-proline, [3H]L-glutamate and [3H]GABA. It was fully inhibited by L-glutamine, L-asparagine, L-cysteine, L-alanine, L-leucine, L-isoleucine, L-tyrosine, L-phenylalanine, L-threonine and by the synthetic marker for the large neutral amino acid transport systems 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid, but not influenced by beta-alanine, taurine, glycine nor was it inhibited by the marker for the A system, L-2-methylamino isobutyric acid. D-Serine at 1 mM concentration produced no significant inhibition of the accumulation of 10 nM [3H]L-serine. We conclude that L-serine uptake observed in the present study is mediated by at least two distinct transport systems: a Na(+)-dependent one of lower affinity (K(m) in mM range) and a Na(+)-independent system of higher affinity (K(m) approximately 20-100 micro M). Characteristics of [3H]L-serine accumulation displayed at low substrate concentrations suggest that it was mediated neither by the typical 'A', nor by the 'large neutral', amino acid transport systems but predominantly by transporters belonging to the recently identified LAT (L-amino acid transporter) family.

Molecular pharmacology of the Na+-dependent transport of acidic amino acids in the mammalian central nervous system

The Na+-dependent transport of L-glutamate (GluT) has been identified in brain tissue more than thirty years ago. Neurochemical studies, performed in various experimental models during 1970's, defined the basic rules for the selection or synthesis of GluT-specific substrates and inhibitors. The protein molecules (transporters) that mediate the translocation of the substrates across the plasma membrane have been cloned and studied during the last ten years. The sites on the transporters that bind the substrates favour glutamate-like or aspartate-like molecules with one positively charged and two negatively charged ionised groups. Substituents at C3 and C4 are often tolerated but substitutions at C2 or alterations of the ionisable groups usually impede the binding. The substrate binding sites display an "anomalous" selectivity towards stereoisomers. These structural requirements are shared by all Na+-dependent glutamate transporters thus making the design of transporter-selective ligands a challenging task. Moreover, the molecular mechanisms of the transport have not yet been adequately elucidated. Data from a wide variety of experimental studies strongly indicate that Na+-dependent GluT regulates the functioning of the glutamatergic excitatory synapses-the most important rapid inter-neuronal signalling system in the mammalian brain. Altered structural and/or functional properties of the Na+-dependent glutamate transporters have been implicated in the damage to the brain tissue following cerebral ischaemia and in the progressive loss of neurons in conditions such as Alzheimer dementia and amyotrophic lateral sclerosis. Furthermore, it seems that fine-tuning of glutamatergic neurotransmission by regulating the Na+-dependent GluT could be useful in the therapy of schizophrenia.