Quantitative autoradiography of Na+-dependent [3H]L-aspartate binding to L-glutamate transporters in rat brain: structure-activity studies using L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC) and 2-(carboxycyclopropyl)-glycine (CCG)

Comparison of [3H]-(2S,4R)-4-methylglutamate and [3H]d-aspartate as ligands for binding and autoradiographic analyses of glutamate transporters

While studies with [(3)H]D-aspartate ([(3)H]d-Asp) illustrate specific interactions with excitatory amino acid transporters (EAATs), new insights into the pharmacological characteristics and localization of specific EAAT subtypes depend upon the availability of novel ligands. One such ligand is [(3)H]-(2S,4R)-4-methylglutamate ([(3)H]4MG) which labels astrocytic EAATs in homogenate binding studies. This study examined the utility of [(3)H]4MG for binding and autoradiography in coronal sections of rat brain. Binding of [(3)H]4MG was optimal in 5mM HEPES buffer containing 96 mM NaCl, pH 7.5. Specific binding of [(3)H]4MG exhibited two components, but was to a single site when glutamate receptor (GluR) sites were masked with kainate (KA; 1 microM): t(1/2) approximately 5 min, K(d) 250 nM and B(max) 5.4 pmol/mg protein. Pharmacological studies revealed that [(3)H]4MG, unlike [(3)H]d-Asp, labeled both EAAT and ionotropic GluR sites. Further studies employed 6-cyano-7-nitroquinoxaline (30 microM) to block GluR sites, but selective EAAT ligands displayed lower potency than expected for binding to transporters relative to drugs possessing mixed transporter/receptor activities. Autoradiography in conjunction with densitometry with [(3)H]4MG and [(3)H]d-Asp revealed wide, but discrete distributions in forebrain; significant differences in binding levels were found in hippocampus, nucleus accumbens and cortical sub-areas. Although EAAT1 and EAAT2 components were detectable using 3-methylglutamate and serine-O-sulphate, respectively, the majority of [(3)H]4MG binding was to KA-related sites. Overall, in tissue sections [(3)H]4MG proved unsuitable for studying the autoradiographic localization of EAATs apparently due to its inability to selectively discriminate Na(+)-dependent binding to Glu transporters.

Characterization of the tritium-labeled analog of L-threo-beta-benzyloxyaspartate binding to glutamate transporters

L-Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. Termination of glutamate receptor activation and maintenance of low extracellular glutamate concentrations are primarily achieved by glutamate transporters (excitatory amino acid transporters 1-5, EAATs1-5) located on both the nerve endings and the surrounding glial cells. To identify the physiological roles of each subtype, subtype-selective EAAT ligands are required. In this study, we developed a binding assay system to characterize EAAT ligands for all EAAT subtypes. We recently synthesized novel analogs of threo-beta-benzyloxyaspartate (TBOA) and reported that they blocked glutamate uptake by EAATs 1-5 much more potently than TBOA. The strong inhibitory activity of the TBOA analogs suggested that they would be suitable to use as radioisotope-labeled ligands, and we therefore synthesized a tritiated derivative of (2S,3S)-3-{3-[4-ethylbenzoylamino]benzyloxy}aspartate ([3H]ETB-TBOA). [3H]ETB-TBOA showed significant high-affinity specific binding to EAAT-transfected COS-1 cell membranes with each EAAT subtype. The Hill coefficient for the Na+-dependence of [3H]ETB-TBOA binding revealed a single class of noncooperative binding sites for Na+, suggesting that Na+ binding in the ligand binding step is different from Na+ binding in the substrate uptake process. The binding was displaced by known substrates and blockers. The rank order of inhibition by these compounds was consistent with glutamate uptake assay results reported previously. Thus, the [3H]ETB-TBOA binding assay will be useful to screen novel EAAT ligands for all EAAT subtypes.

The excitatory amino acid transporters: Pharmacological insights on substrate and inhibitor specificity of the EAAT subtypes

L-glutamate serves as the primary excitatory neurotransmitter in the mammalian CNS, where it can contribute to either neuronal communication or neuropathological damage through the activation of a wide variety of excitatory amino acid (EAA) receptors. By regulating the levels of extracellular L-glutamate that have access to these receptors, glutamate uptake systems hold the potential to effect both normal synaptic signaling and the abnormal over-activation of the receptors that can trigger excitotoxic pathology. Among the various membrane transporters that are capable of translocating this dicarboxylic amino acid, the majority of glutamate transport in the CNS, particularly as related to excitatory transmission, is mediated by the high-affinity, sodium-dependent, excitatory amino acid transporters (EAATs). At least 5 subtypes of EAATs have been identified, each of which exhibits a distinct distribution and pharmacology. Our growing appreciation for the functional significance of the EAATs is closely linked to our understanding of their pharmacology and the consequent development of inhibitors and substrates with which to delineate their activity. As was the case with EAA receptors, conformationally constrained glutamate mimics have been especially valuable in this effort. The success of these compounds is based upon the concept that restricting the spatial positions that can be occupied by required functional groups can serve to enhance both the potency and selectivity of the analogues. In the instance of the transporters, useful pharmacological probes have emerged through the introduction of additional functional groups (e.g., methyl, hydroxyl, benzyloxy) onto the acyclic backbone of glutamate and aspartate, as well as through the exploitation of novel ring systems (e.g., pyrrolidine-, cyclopropyl-, azole-, oxazole-, and oxazoline-based analogues) to conformationally lock the position of the amino and carboxyl groups. The focus of the present review is on the pharmacology of the EAATs and, in particular, the potential to identify those chemical properties that differentiate the processes of binding and translocation (i.e., substrates from non-substrate inhibitors), as well as strategies to develop glutamate analogues that act selectively among the various EAAT subtypes.

Group I and II metabotropic glutamate receptors alter brain cortical metabolic and glutamate/glutamine cycle activity: a 13C NMR spectroscopy and metabolomic study

Metabotropic glutamate receptors (mGluR) modulate neuronal function. Here, we tested the effect on metabolism of a range of Group I and II mGluR ligands in Guinea pig brain cortical tissue slices, applying 13C NMR spectroscopy and metabolomic analysis using multivariate statistics. The effects of Group I agonists (S)-3,5-dihydroxyphenylglycine (DHPG) and (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) depended upon concentration and were mostly stimulatory, increasing both net metabolic flux through the Krebs cycle and glutamate/glutamine cycle activity. Only the higher (50 microm) concentrations of CHPG had the opposite effect. The Group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), consistent with its neuroprotective role, caused significant decreases in metabolism. With principal components analysis of the metabolic profiles generated by these ligands, the effects could be separated by two principal components. Agonists at Group II mGluR [(2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC)] generally stimulated metabolism, including glutamate/glutamine cycling, although this varied with concentration. The antagonist (2S)-alpha-ethylglutamic acid (EGLU) stimulated astrocyte metabolism with minimal impact on glutamate/glutamine cycling. (RS)-1-Aminophosphoindan-1-carboxylic acid (APICA) decreased metabolism at 5 microm but had a stimulatory effect at 50 microm. All ligand effects were separated from control and from each other using two principal components. The ramifications of these findings are discussed.

Substrate inhibitors and blockers of excitatory amino acid transporters in the treatment of neurodegeneration: critical considerations

Excessive glutamate release (mediated by reversed uptake) or impaired reuptake contributes to the etiopathology of many neurodegenerative disorders. Thus great effort has been devoted to the discovery of agents that can interfere with high-affinity Na+-dependent glutamate transport, with the aim of finding new therapeutics against neurodegenerative diseases. We developed two different 3D-pharmacophore models for substrate inhibitors and blockers, which led to the rational design of novel and potent glutamate and aspartate analogues that selectively interact with excitatory amino acid transporters (EAAT). Our results indicated that all analysed EAAT ligands share the same orientation of the acidic functions and the protonatable nitrogen, even though the distance between the carboxylic carbons varies from 3.7 to 4.9 A. This distance does not discriminate between substrate inhibitors and blockers, but between glutamate and aspartate derivatives. In contrasts differences in the volume distribution of the rest of the molecule with respect to the axis connecting the two carboxylic groups are responsible for the difference in activity between transportable and nontransportable inhibitors. Thus our 3D receptor interaction model for EAAT substrates and nontransportable inhibitors could lead to the rational design of selective EAAT ligands as possible neuroprotective agents. However, some critical points, such as which glutamate transporter is present on glutamatergic nerve terminals and which glutamate transporter mediates reversed glutamate uptake, still remain to be elucidated.

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.

Effects of L-glutamate transport inhibition by a conformationally restricted glutamate analogue (2S,1'S,2'R)-2-(carboxycyclopropyl)glycine (L-CCG III) on metabolism in brain tissue in vitro analysed by NMR spectroscopy

(2S,1'S,2'R)-2-(Carboxycyclopropyl)glycine (L-CCG III) was a substrate of Na(+)-dependent glutamate transporters (GluT) in Xenopus laevis oocytes (IC50 to approximately 13 and to approximately 2 microM for, respec tively, EAAT 1 and EAAT 2) and caused an apparent inhibition of [3H]L-glutamate uptake in "mini-slices" of guinea pig cerebral cortex (IC50 to approximately 12 microM). In slices (350 microM) of guinea pig cerebral cortex, 5 microM L-CCG III increased both the flux of label through pyruvate carboxylase and the fractional enrichment of glutamate, GABA, glutamine and lactate, but had no effect on total metabolite pool sizes. At 50 microM L-CCG III decreased incorporation of 13C from [3-13C]-pyruvate into glutamate C4, glutamine C4, lactate C3 and alanine C3. The total metabolite pool sizes were also decreased with no change in the fractional enrichment. Furthermore, L-CCG III was accumulated in the tissue, probably via GluT. At lower concentration, L-CCG III would compete with L-glutamate for GluT and the changes probably reflect a compensation for the "missing" L-glutamate. At 50 microM, intracellular L-CCG III could reach > 10 mM and metabolism might be affected directly.

Regional mapping of low-affinity kainate receptors in mouse brain using [(3)H](2S,4R)-4-methylglutamate autoradiography

Recent data indicate that (2S,4R)-4-methylglutamate is a selective agonist for low affinity (GluR5 and GluR6) kainate receptor subunits. In the present study, we have employed [(3)H](2S,4R)-4-methylglutamate to examine low affinity kainate receptor distribution in mouse brain. [(3)H](2S,4R)-4-Methylglutamate labelled a single site in murine cerebrocortical membranes (K(d)=9.9+/-2.7 nM, B(max)=296.3+/-27.1 fmol mg protein(-1)). The binding of 8 nM [(3)H](2S,4R)-4-methylglutamate was displaced by several non-NMDA receptor ligands (K(i)+/-S.E.M.): domoate (1.1+/-0.2 nM)>kainate (7.1+/-1.1 nM) >> L-glutamate (187.6+/-31.9 nM) >> (S)-alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) (>50 microM). [(3)H](2S,4R)-4-Methylglutamate autoradiography revealed a widespread regional distribution of low affinity kainate receptors. Highest binding densities occurred within deep layers of the cerebral cortex, olfactory bulb, basolateral amygdala and hippocampal CA3 subregion. Moderate labelling was also evident in the nucleus accumbens, dentate gyrus, caudate putamen, hypothalamus and cerebellar granule cell layer. These data show that [(3)H](2S,4R)-4-methylglutamate is a useful radioligand for selectively labelling low affinity kainate receptors.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

[(3)H](2S,4R)-4-Methylglutamate: a novel ligand for the characterization of glutamate transporters

[(3)H](2S,4R)-4-Methylglutamate ([(3)H]4MG), used previously as a ligand for low-affinity kainate receptors, was employed to establish a binding assay for glutamate transporters (GluTs), as 4MG has also been shown to have affinity for the glial GluTs, GLT1 and GLAST. In rat brain membrane homogenates in the presence of Na(+) ions at 4 degrees C, specific binding of [(3)H]4MG was rapid and saturable (t(1/2) approximately 15 min), representing > 90% of total binding. Dissociation of [(3)H]4MG occurred in a biphasic manner, however, saturation studies and Scatchard analysis indicated a single site of binding (n(H) = 0.85) and a K(d) of 6.2 +/- 0.8 microM with a B(max) of 111.8 +/- 23.8 pmol/mg protein. Specific binding of [(3)H]4MG was Na(+)-dependent and inhibited by K(+) and HCO(3-). Pharmacological inhibition with compounds acting at GluTs revealed that Glu, D- and L-aspartate, L-serine-O-sulfate and Ltrans-pyrrolidine-2,4-dicarboxylate fully displaced specific binding. Drugs having preferential affinity for GLT1, kainate, dihydrokainate and Lthreo-3-methylglutamate, all inhibited approximately 40% of specific binding. The inhibition pattern of L-serine-O-sulfate in the presence of a saturating concentration of dihydrokainate was suggestive of [(3)H]4MG also labelling GLAST. 6-Cyano-7-nitroquinoxaline, a kainate receptor antagonist, and a range of Glu receptor agonists and antagonists failed to significantly inhibit [(3)H]4MG binding. The pharmacological profile of binding of [(3)H]4MG resembled that found for [(3)H]D-aspartate, a ligand specific for GluTs, reinforcing the hypothesis that [(3)H]4MG was labelling GluTs in this assay. Together, these data illustrate the development of an efficient, economic binding assay that is suitable for the characterization of different subtypes of GLuTs.

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

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Regional distribution and pharmacological characteristics of [3H]N-acetyl-aspartyl-glutamate (NAAG) binding sites in rat brain

Autoradiographical studies revealed that 10 nM [3H]N-acetyl-aspartyl-glutamate (NAAG) labelled grey matter structures, particularly in the hippocamus, cerebral neocortex, striatum, septal nuclei and the cerebellar cortex. The binding was inhibited by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG IV), an agonist at group II metabotropic glutamate receptors (mGluR II). (RS)-alpha-Methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-cyclopropyl-4-phosphonoglycine (CPPG) and (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE), all antagonists at mGluR II and mGluR III, also inhibited [3H]NAAG binding. Other inhibitors were (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a broad-spectrum mGluR agonist with preference for groups I and II and the mGluR I agonists/mGluR II antagonists (S)-3-carboxy-4-hydroxyphenylglycine (3,4-CHPG) and (S)-4-carboxy-3-hydroxyphenylglycine (4,3-CHPG). Neither the mGluR I specific agonist (S)-dihydroxyphenylglycine nor any of the ionotropic glutamate receptor ligands such as kainate, AMPA and MK-801 had strong effects (except for the competitive NMDA antagonist CGS 19755, which produced 20-40% inhibition at 100 microM) suggesting that, at low nM concentrations, [3H]NAAG binds predominantly to metabotropic glutamate receptors, particularly those of the mGluR II type. Several studies have indicated that NAAG can interact with mGluR II and the present study supports this notion by demonstrating that sites capable of binding NAAG at low concentrations and displaying pharmacological characteristics of mGluR II exist in the central nervous tissue. Furthermore, the results show that autoradiography of [3H]NAAG binding can be used to quantify the distribution of such sites in distinct brain regions and study their pharmacology at the same time.

Syntheses of optically pure beta-hydroxyaspartate derivatives as glutamate transporter blockers

DL-threo-beta-benzyloxyaspartate (DL-TBOA) is a non-transportable blocker of the glutamate transporters that serves as an indispensable tool for the investigation of the physiological roles of the transporters. To examine the precise interaction between a blocker and the transporters, we synthesized the optically pure isomers (L- and D-TBOA) and its erythro-isomers. L-TBOA is the most potent blocker for the human excitatory amino acid transporters (EAAT1-3), while D-TBOA revealed a difference in the pharmacophores between EAAT1 and EAAT3. We also synthesized the substituent variants (methyl or naphthylmethyl derivatives) of L-TBOA. The results obtained here suggest that bulky substituents are crucial for non-transportable blockers.

Strategies for studies of neurotoxic mechanisms involving deficient transport of L-glutamate: antisense knockout in rat brain in vivo and changes in the neurotransmitter metabolism following inhibition of glutamate transport in guinea pig brain slices

This communication briefly reviews characteristics of glutamate transport in the central nervous system and is involved in the aetiology of slow neurodegenerative diseases. Data in the literature suggest that antisense oligonucleotides targeted against glutamate transporters and administered in vivo over a period of days could be used to test the hypothesis. Data from our laboratory have indicated that single intraventricular doses of antisense oligonucleotides can also results in significant reductions in the numbers of substrate binding sites associated with glutamate transporters and may even cause subtle changes in their characteristics. In order to study metabolism in brain tissue, we have used 13C-nuclear magnetic resonance spectroscopy to analyse extracts of slices of guinea pig cerebral cortex exposed to glutamate transport inhibitor L-anti,endo-methanopyrrolidine dicarboxylate (L-a,e-MPDC). The results have shown-for the first time in an experimental model that preserves the relationship between glia and neurones within the context of brain tissue-that inhibition of L-glutamate transport can exert a significant influence on neurotransmitter-related metabolism. These findings suggest that metabolic disturbances caused by deficient glutamate transport could play a significant role in the death of neurones under pathological conditions in vivo.