Subtypes of sodium-dependent high-affinity L-[3H]glutamate transport activity: pharmacologic specificity and regulation by sodium and potassium

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

Neuronal vs glial glutamate uptake: Resolving the conundrum

Neither normal brain function nor the pathological processes involved in neurological diseases can be adequately understood without knowledge of the release, uptake and metabolism of glutamate. The reason for this is that glutamate (a) is the most abundant amino acid in the brain, (b) is at the cross-roads between several metabolic pathways, and (c) serves as the major excitatory neurotransmitter. In fact most brain cells express glutamate receptors and are thereby influenced by extracellular glutamate. In agreement, brain cells have powerful uptake systems that constantly remove glutamate from the extracellular fluid and thereby limit receptor activation. It has been clear since the 1970s that both astrocytes and neurons express glutamate transporters. However the relative contribution of neuronal and glial transporters to the total glutamate uptake activity, however, as well as their functional importance, has been hotly debated ever since. The present short review provides (a) an overview of what we know about neuronal glutamate uptake as well as an historical description of how we got there, and (b) a hypothesis reconciling apparently contradicting observations thereby possibly resolving the paradox.

GLT-1: The elusive presynaptic glutamate transporter

Historically, glutamate uptake in the CNS was mainly attributed to glial cells for three reasons: 1) none of the glutamate transporters were found to be located in presynaptic terminals of excitatory synapses; 2) the putative glial transporters, GLT-1 and GLAST are expressed at high levels in astrocytes; 3) studies of the constitutive GLT-1 knockout as well as pharmacological studies demonstrated that >90% of glutamate uptake into forebrain synaptosomes is mediated by the operation of GLT-1. Here we summarize the history leading up to the recognition of GLT-1a as a presynaptic glutamate transporter. A major issue now is understanding the physiological and pathophysiological significance of the expression of GLT-1 in presynaptic terminals. To elucidate the cell-type specific functions of GLT-1, a conditional knockout was generated with which to inactivate the GLT-1 gene in different cell types using Cre/lox technology. Astrocytic knockout led to an 80% reduction of GLT-1 expression, resulting in intractable seizures and early mortality as seen also in the constitutive knockout. Neuronal knockout was associated with no obvious phenotype. Surprisingly, synaptosomal uptake capacity (Vmax) was found to be significantly reduced, by 40%, in the neuronal knockout, indicating that the contribution of neuronal GLT-1 to synaptosomal uptake is disproportionate to its protein expression (5-10%). Conversely, the contribution of astrocytic GLT-1 to synaptosomal uptake was much lower than expected. In contrast, the loss of uptake into liposomes prepared from brain protein from astrocyte and neuronal knockouts was proportionate with the loss of GLT-1 protein, suggesting that a large portion of GLT-1 in astrocytic membranes in synaptosomal preparations is not functional, possibly because of a failure to reseal. These results suggest the need to reinterpret many previous studies using synaptosomal uptake to investigate glutamate transport itself as well as changes in glutamate homeostasis associated with normal functions, neurodegeneration, and response to drugs.

Conditional deletion of the glutamate transporter GLT-1 reveals that astrocytic GLT-1 protects against fatal epilepsy while neuronal GLT-1 contributes significantly to glutamate uptake into synaptosomes

GLT-1 (EAAT2; slc1a2) is the major glutamate transporter in the brain, and is predominantly expressed in astrocytes, but at lower levels also in excitatory terminals. We generated a conditional GLT-1 knock-out mouse to uncover cell-type-specific functional roles of GLT-1. Inactivation of the GLT-1 gene was achieved in either neurons or astrocytes by expression of synapsin-Cre or inducible human GFAP-CreERT2. Elimination of GLT-1 from astrocytes resulted in loss of ∼80% of GLT-1 protein and of glutamate uptake activity that could be solubilized and reconstituted in liposomes. This loss was accompanied by excess mortality, lower body weight, and seizures suggesting that astrocytic GLT-1 is of major importance. However, there was only a small (15%) reduction that did not reach significance of glutamate uptake into crude forebrain synaptosomes. In contrast, when GLT-1 was deleted in neurons, both the GLT-1 protein and glutamate uptake activity that could be solubilized and reconstituted in liposomes were virtually unaffected. These mice showed normal survival, weight gain, and no seizures. However, the synaptosomal glutamate uptake capacity (Vmax) was reduced significantly (40%). In conclusion, astrocytic GLT-1 performs critical functions required for normal weight gain, resistance to epilepsy, and survival. However, the contribution of astrocytic GLT-1 to glutamate uptake into synaptosomes is less than expected, and the contribution of neuronal GLT-1 to synaptosomal glutamate uptake is greater than expected based on their relative protein expression. These results have important implications for the interpretation of the many previous studies assessing glutamate uptake capacity by measuring synaptosomal uptake.

Glutamate as a neurotransmitter in the healthy brain

Glutamate is the most abundant free amino acid in the brain and is at the crossroad between multiple metabolic pathways. Considering this, it was a surprise to discover that glutamate has excitatory effects on nerve cells, and that it can excite cells to their death in a process now referred to as "excitotoxicity". This effect is due to glutamate receptors present on the surface of brain cells. Powerful uptake systems (glutamate transporters) prevent excessive activation of these receptors by continuously removing glutamate from the extracellular fluid in the brain. Further, the blood-brain barrier shields the brain from glutamate in the blood. The highest concentrations of glutamate are found in synaptic vesicles in nerve terminals from where it can be released by exocytosis. In fact, glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. It took, however, a long time to realize that. The present review provides a brief historical description, gives a short overview of glutamate as a transmitter in the healthy brain, and comments on the so-called glutamate-glutamine cycle. The glutamate transporters responsible for the glutamate removal are described in some detail.

Magnesium in depression

Magnesium is one of the most essential mineral in the human body, connected with brain biochemistry and the fluidity of neuronal membrane. A variety of neuromuscular and psychiatric symptoms, including different types of depression, was observed in magnesium deficiency. Plasma/serum magnesium levels do not seem to be the appropriate indicators of depressive disorders, since ambiguous outcomes, depending on the study, were obtained. The emergence of a new approach to magnesium compounds in medical practice has been seen. Apart from being administered as components of dietary supplements, they are also perceived as the effective agents in treatment of migraine, alcoholism, asthma, heart diseases, arrhythmias, renal calcium stones, premenstrual tension syndrome etc. Magnesium preparations have an essential place in homeopathy as a remedy for a range of mental health problems. Mechanisms of antidepressant action of magnesium are not fully understood yet. Most probably, magnesium influences several systems associated with development of depression. The first information on the beneficial effect of magnesium sulfate given hypodermically to patients with agitated depression was published almost 100 years ago. Numerous pre-clinical and clinical studies confirmed the initial observations as well as demonstrated the beneficial safety profile of magnesium supplementation. Thus, magnesium preparations seem to be a valuable addition to the pharmacological armamentarium for management of depression.

GABA and Glutamate Transporters in Brain

The mammalian genome contains four genes encoding GABA transporters (GAT1, slc6a1; GAT2, slc6a13; GAT3, slc6a11; BGT1, slc6a12) and five glutamate transporter genes (EAAT1, slc1a3; EAAT2, slc1a2; EAAT3, slc1a1; EAAT4, slc1a6; EAAT5, slc1a7). These transporters keep the extracellular levels of GABA and excitatory amino acids low and provide amino acids for metabolic purposes. The various transporters have different properties both with respect to their transport functions and with respect to their ability to act as ion channels. Further, they are differentially regulated. To understand the physiological roles of the individual transporter subtypes, it is necessary to obtain information on their distributions and expression levels. Quantitative data are important as the functional capacity is limited by the number of transporter molecules. The most important and most abundant transporters for removal of transmitter glutamate in the brain are EAAT2 (GLT-1) and EAAT1 (GLAST), while GAT1 and GAT3 are the major GABA transporters in the brain. EAAT3 (EAAC1) does not appear to play a role in signal transduction, but plays other roles. Due to their high uncoupled anion conductance, EAAT4 and EAAT5 seem to be acting more like inhibitory glutamate receptors than as glutamate transporters. GAT2 and BGT1 are primarily expressed in the liver and kidney, but are also found in the leptomeninges, while the levels in brain tissue proper are too low to have any impact on GABA removal, at least in normal young adult mice. The present review will provide summary of what is currently known and will also discuss some methodological pitfalls.

Evidence for change in current-flux coupling of GLT1 at high glutamate concentrations in rat primary forebrain neurons and GLT1a-expressing COS-7 cells

Glutamate is the major excitatory neurotransmitter of the central nervous system and is toxic to neurons even at low concentrations. GLT1, the rodent analog of human EAAT2, is primarily responsible for glutamate clearance in the cerebrum. GLT1 was thought to be expressed exclusively in astrocytes in the mature brain. Recently, however, GLT1a was demonstrated in excitatory axon terminals where synaptic glutamate concentration rises above 1 mm during excitatory transmission. GLT1 function in neurons with accurate control of both intracellular and extracellular solutions mimicking synaptic concentration gradients has never been studied. Here we characterized the kinetics of coupled glutamate transporter current in whole-cell configuration and [(3)H]-l-glutamate uptake in cultured rat cerebral neurons across the entire range of synaptic glutamate concentrations. In both neurons and GLT1a-transfected COS-7 cells, the kinetics were similar and revealed two specific components: a high-affinity component with glutamate k(D) value around 15 mum and a low-affinity component with k(D) value around 0.2 mm. The specific low-affinity component was discovered as a result of significant deviation of the transporter current from Michaelis-Menten kinetics in the 100-300 mum concentration range. Activation of the specific low-affinity component led to a two-fold decrease in the current/flux ratio, implying a change in the transport coupling. Our data indicate that GLT1 endogenously expressed in cultured rat forebrain neurons displays high and low glutamate affinity uptake components that are different in current-flux coupling ratios. This property is intrinsic to the protein because it was also observed in GLT1a-transfected COS-7 cells.

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.

Down-regulation of the glial glutamate transporter GLT-1 in rat hippocampus and striatum and its modulation by a group III metabotropic glutamate receptor antagonist following transient global forebrain ischemia

Our goals were to identify biochemical markers for transient global ischemia-induced delayed neuronal death and test possible drug therapies against this neuronal damage. Four-vessel occlusion (4-VO) for 20 min was used as a rat model. The temporal expression profiles of three glutamate transporters (GLT-1, GLAST and EAAC1) were evaluated in the CA1 region of the hippocampus and the striatum. The protein levels of the GLT-1 were significantly down-regulated between 3 and 6 h after ischemia-reperfusion in the CA1 region and striatum, returned to the control (2-VO) levels 24 h after reperfusion and remained unchanged for up to 7 days. The levels of GLAST in the CA1 region and striatum, and EAAC1 in the CA1 region did not change after ischemia from 1 h to 7 days. Pretreatment with group III metabotropic glutamate receptor antagonist s-alpha-MCPA (20 microg/5 microl) 30 min prior to 4-VO significantly restored the GLT-1 levels in the CA1 region caused by global ischemia at both 3 and 6 h after reperfusion. The loss of pyramidal neurons in the CA1 region due to ischemia-reperfusion could also be prevented by intraventricular pretreatment with s-alpha-MCPA. The current findings pinpoint the significance of GLT-1 during ischemia/reperfusion and suggest a potential application of group III metabotropic glutamate receptor antagonist against ischemic/hypoxic neuronal damage.

The substituted aspartate analogue L-beta-threo-benzyl-aspartate preferentially inhibits the neuronal excitatory amino acid transporter EAAT3

The excitatory amino acid transporters (EAATs) play key roles in the regulation of CNS L-glutamate, especially related to synthesis, signal termination, synaptic spillover, and excitotoxic protection. Inhibitors available to delineate EAAT pharmacology and function are essentially limited to those that non-selectively block all EAATs or those that exhibit a substantial preference for EAAT2. Thus, it is difficult to selectively study the other subtypes, particularly EAAT1 and EAAT3. Structure activity studies on a series of beta-substituted aspartate analogues identify L-beta-benzyl-aspartate (L-beta-BA) as among the first blockers that potently and preferentially inhibits the neuronal EAAT3 subtype. Kinetic analysis of D-[(3)H]aspartate uptake into C17.2 cells expressing the hEAATs demonstrate that L-beta-threo-BA is the more potent diastereomer, acts competitively, and exhibits a 10-fold preference for EAAT3 compared to EAAT1 and EAAT2. Electrophysiological recordings of EAAT-mediated currents in Xenopus oocytes identify L-beta-BA as a non-substrate inhibitor. Analyzing L-beta-threo-BA within the context of a novel EAAT2 pharmacophore model suggests: (1) a highly conserved positioning of the electrostatic carboxyl and amino groups; (2) nearby regions that accommodate select structural modifications (cyclopropyl rings, methyl groups, oxygen atoms); and (3) a unique region L-beta-threo-BA occupied by the benzyl moieties of L-TBOA, L-beta-threo-BA and related analogues. It is plausible that the preference of L-beta-threo-BA and L-TBOA for EAAT3 and EAAT2, respectively, could reside in the latter two pharmacophore regions.

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.

The glutamate transporter GLT1a is expressed in excitatory axon terminals of mature hippocampal neurons

GLT1 is the major glutamate transporter of the brain and has been thought to be expressed exclusively in astrocytes. Although excitatory axon terminals take up glutamate, the transporter responsible has not been identified. GLT1 is expressed in at least two forms varying in the C termini, GLT1a and GLT1b. GLT1 mRNA has been demonstrated in neurons, without associated protein. Recently, evidence has been presented, using specific C terminus-directed antibodies, that GLT1b protein is expressed in neurons in vivo. These data suggested that the GLT1 mRNA detected in neurons encodes GLT1b and also that GLT1b might be the elusive presynaptic transporter. To test these hypotheses, we used variant-specific probes directed to the 3'-untranslated regions for GLT1a and GLT1b to perform in situ hybridization in the hippocampus. Contrary to expectation, GLT1a mRNA was the more abundant form. To investigate further the expression of GLT1 in neurons in the hippocampus, antibodies raised against the C terminus of GLT1a and against the N terminus of GLT1, found to be specific by testing in GLT1 knock-out mice, were used for light microscopic and EM-ICC. GLT1a protein was detected in neurons, in 14-29% of axons in the hippocampus, depending on the region. Many of the labeled axons formed axo-spinous, asymmetric, and, thus, excitatory synapses. Labeling also occurred in some spines and dendrites. The antibody against the N terminus of GLT1 also produced labeling of neuronal processes. Thus, the originally cloned form of GLT1, GLT1a, is expressed as protein in neurons in the mature hippocampus and may contribute significantly to glutamate uptake into excitatory terminals.

Glutamate and aspartate do not exhibit the same changes in their extracellular concentrations in the rat striatum after N-methyl-D-aspartate local administration

To determine whether glutamate (Glu) and aspartate (Asp) undergo a similar regulation of their extracellular levels, Glu and Asp were simultaneously monitored in the striatum of anesthetized rats after local N-methyl-D-aspartate (NMDA) receptor stimulation, using 1-min in vivo microdialysis coupled to capillary electrophoresis with laser-induced fluorescence detection. Application of NMDA (10 min, 10(-3) M) through the dialysis probe induced 1) an increase (+50%) in Asp during the NMDA administration and 2) a surprising biphasic effect on Glu, with a rapid increase (+30%) and a return to baseline before the end of NMDA application, followed by a second increase (+40%) occurring after and linked to the end of NMDA administration. When studied in the presence of 10 microM tetrodotoxin (TTX) or 0.1 mM Ca(2+), the increase in Asp was partially TTX-dependent, and the early increase in Glu appeared to be partially TTX and Ca(2+) dependent, whereas the second increase in Glu was not. The second increase in Glu level was still present when NMDA antagonists (AP5 or MK-801) were administered at the end of NMDA application. Finally, only extracellular Asp was increased through application of lower NMDA concentrations (10(-4) M, 10(-5) M), whereas extracellular Glu was not affected. In conclusion, these results suggest a differential control of Glu and Asp extracellular levels in rat striatum by distinct mechanisms linked to NMDA receptors and involving neuronal or nonneuronal release.

The 'glial' glutamate transporter, EAAT2 (Glt-1) accounts for high affinity glutamate uptake into adult rodent nerve endings

The excitatory amino acid transporters (EAAT) removes neurotransmitters glutamate and aspartate from the synaptic cleft. Most CNS glutamate uptake is mediated by EAAT2 into glia, though nerve terminals show evidence for uptake, through an unknown transporter. Reverse-transcriptase PCR identified the expression of EAAT1, EAAT2, EAAT3 and EAAT4 mRNAs in primary cultures of mouse cortical or striatal neurones. We have used synaptosomes and glial plasmalemmal vesicles (GPV) from adult mouse and rat CNS to identify the nerve terminal transporter. Western blotting showed detectable levels of the transporters EAAT1 (GLAST) and EAAT2 (Glt-1) in both synaptosomes and GPVs. Uptake of [3H]D-aspartate or [3H]L-glutamate into these preparations revealed sodium-dependent uptake in GPV and synaptosomes which was inhibited by a range of EAAT blockers: dihydrokainate, serine-o-sulfate, l-trans-2,4-pyrrolidine dicarboxylate (PDC) (+/-)-threo-3-methylglutamate and (2S,4R )-4-methylglutamate. The IC50 values found for these compounds suggested functional expression of the 'glial, transporter, EAAT2 in nerve terminals. Additionally blockade of the majority EAAT2 uptake sites with 100 micro m dihydrokainate, failed to unmask any functional non-EAAT2 uptake sites. The data presented in this study indicate that EAAT2 is the predominant nerve terminal glutamate transporter in the adult rodent CNS.

Magnesium and affective disorders

There are several findings on the action of magnesium ions supporting their possible therapeutic potential in affective disorders. Examinations of the sleep-electroencephalogram (EEG) and of endocrine systems point to the involvement of the limbic-hypothalamus-pituitary-adrenocortical axis as magnesium affects all elements of this system. Magnesium has the property to suppress hippocampal kindling, to reduce the release of adrenocorticotrophic hormone (ACTH) and to affect adrenocortical sensitivity to ACTH. The role of magnesium in the central nervous system could be mediated via the N-methyl-D-aspartate-antagonistic, gamma-aminobutyric acidA-agonistic or a angiotensin II-antagonistic property of this ion. A direct impact of magnesium on the function of the transport protein p-glycoprotein at the level of the blood-brain barrier has also been demonstrated, possibly influencing the access of corticosteroids to the brain. Furthermore, magnesium dampens the calciumion-proteinkinase C related neurotransmission and stimulates the Na-K-ATPase. All these systems have been reported to be involved in the pathophysiology of depression. Despite the antagonism of lithium to magnesium in some cell-based experimental systems, similarities exist on the functional level, i.e. with respect to kindling, sleep-EEG and endocrine effects. Controlled clinical trials examining the effect of Mg in affective disorder are warranted.

Synthesis and preliminary evaluation of trans-3,4-conformationally-restricted glutamate and pyroglutamate analogues as novel EAAT2 inhibitors

Select trans-4,5-[bi]cyclohexenylglutamic and pyroglutamic acids (3,4-substituted glutamates) were synthesized in three steps and were screened as potential inhibitors of the sodium dependent excitatory amino acid transporters 2 (EAAT2) and 3 (EAAT3), the chloride dependent glial cystine/glutamate exchanger system x(c)(-), and the glutamate vesicular transport system (VGLUT). Two glutamate analogues and one pyroglutamate analogue were found to inhibit EAAT2 with activity comparable to dihydrokainate.

A neuronal glutamate transporter contributes to neurotransmitter GABA synthesis and epilepsy

The predominant neuronal glutamate transporter, EAAC1 (for excitatory amino acid carrier-1), is localized to the dendrites and somata of many neurons. Rare presynaptic localization is restricted to GABA terminals. Because glutamate is a precursor for GABA synthesis, we hypothesized that EAAC1 may play a role in regulating GABA synthesis and, thus, could cause epilepsy in rats when inactivated. Reduced expression of EAAC1 by antisense treatment led to behavioral abnormalities, including staring-freezing episodes and electrographic (EEG) seizures. Extracellular hippocampal and thalamocortical slice recordings showed excessive excitability in antisense-treated rats. Patch-clamp recordings of miniature IPSCs (mIPSCs) conducted in CA1 pyramidal neurons in slices from EAAC1 antisense-treated animals demonstrated a significant decrease in mIPSC amplitude, indicating decreased tonic inhibition. There was a 50% loss of hippocampal GABA levels associated with knockdown of EAAC1, and newly synthesized GABA from extracellular glutamate was significantly impaired by reduction of EAAC1 expression. EAAC1 may participate in normal GABA neurosynthesis and limbic hyperexcitability, whereas epilepsy can result from a disruption of the interaction between EAAC1 and GABA metabolism.

Expression of a variant form of the glutamate transporter GLT1 in neuronal cultures and in neurons and astrocytes in the rat brain

To identify glutamate transporters expressed in forebrain neurons, we prepared a cDNA library from rat forebrain neuronal cultures, previously shown to transport glutamate with high affinity and capacity. Using this library, we cloned two forms, varying in the C terminus, of the glutamate transporter GLT1. This transporter was previously found to be localized exclusively in astrocytes in the normal mature brain. Specific antibodies against the C-terminal peptides were used to show that forebrain neurons in culture express both GLT1a and GLT1b proteins. The pharmacological properties of glutamate transport mediated by GLT1a and GLT1b expressed in COS-7 cells and in neuronal cultures were indistinguishable. Both GLT1a and GLT1b were upregulated in astrocyte cultures by exposure to dibutyryl cAMP. We next investigated the expression of GLT1b in vivo. Northern blot analysis of forebrain RNA revealed two transcripts of approximately 3 and 11 kb that became more plentiful with developmental age. Immunoblot analysis showed high levels of expression in the cortex, hippocampus, striatum, thalamus, and midbrain. Pre-embedding electron microscopic immunocytochemistry with silver-enhanced immunogold detection was used to localize GLT1b in vivo. In the rat somatosensory cortex, GLT1b was clearly expressed in neurons in presynaptic terminals and dendritic shafts, as well as in astrocytes. The presence of GLT1b in neurons may offer a partial explanation for the observed uptake of glutamate by presynaptic terminals, for the preservation of input specificity at excitatory synapses, and may play a role in the pathophysiology of excitotoxicity.

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.

Evaluation of drugs acting at glutamate transporters in organotypic hippocampal cultures: new evidence on substrates and blockers in excitotoxicity

Removal of L-glutamate (Glu) from the synapse is critical to maintain normal transmission and to prevent excitotoxicity, and is performed exclusively by excitatory amino acid transporters (EAATs). We investigated the effects of substrates and blockers of EAATs on extracellular Glu and cellular viability in organotypic cultures of rat hippocampus. Seven-day treatment with a range of drugs (L-trans-pyrrolidine-2,4-dicarboxylate, (2S,4R)-4-methyl-glutamate, (+/-)-threo-3-methylglutamate and DL-threo-beta-benzyloxyaspartate), in the presence of 300 microM added Glu, resulted in increased extracellular Glu and a significant correlation between Glu concentration and cellular injury (as indicated by lactate dehydrogenase release). In contrast, (2S,3s,4R)-2-(carboxycyclopropyl)glycine (L-CCG-III) exerted a novel neuroprotection against this toxicity, and elevations in extracellular Glu were not toxic in the presence of this compound. Similar results were obtained following two-week treatment of cultures without added Glu. Whilst blockade of GLT-1 alone was relatively ineffective in producing excitotoxic injury, heteroexchange of Glu by EAAT substrates may exacerbate excitotoxicity.

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.

Substrate exchange properties of the high-affinity glutamate transporter EAAT2

A stable cell line expressing the predominant brain glutamate transporter EAAT2 was used for the characterization of substrate exchange as a biochemical index for discriminating between substrate and non-substrate inhibitors of the cloned EAAT2 transporter. Addition of 1 mM unlabeled D-aspartate to cells equilibrated with [3H]D-aspartate produced a time-dependent depletion of the [3H] label retained by the cells. L-Aspartate, L-glutamate and L-cysteate produced an equivalent degree of [3H] exchange to that observed with D-aspartate, although the non-substrate EAAT2 inhibitor dihydrokainate and D-glutamate, which does not interact with the substrate binding site, failed to stimulate [3H]D-aspartate exchange. Estimation of EC50 values for the stimulation of [3H] exchange by D-aspartate, L-glutamate and L-trans-2,4-pyrollidine carboxylate (trans-PDC) produced values that were in excellent agreement with the corresponding IC50 values for the same compounds to inhibit EAAT2 uptake. Moreover, trans-PDC was found to produce a lower maximal exchange than that observed with D-aspartate, consistent with the known partial EAAT2 substrate activity of trans-PDC. The estimate of drug induced [3H] efflux with the cloned EAAT2 transporter represents a convenient biochemical assay for the discrimination of substrate and non-substrate inhibitors of the EAAT2 subtype.

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.

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.

Comparative studies on the distribution of glutamate transporters in the retinae of the Mongolian gerbil and the rat

Glutamate is the major excitatory amino acid transmitter in vertebrate retinae. Glutamate transporters therefore play an important role in the precise control of glutamate concentration in the synaptic cleft by regulating extracellular glutamate concentration. In the present study, we performed an analysis of the expressions of three glutamate transporters in gerbil retina using immunohistochemistry. In the gerbil retina, excitatory amino acid carrier 1 and glutamate transporter 1 immunoreactivity was predominant in the ganglion cells but not amacrine or bipolar cells. Glutamate/aspartate transporter (GLAST) immunoreactivity was observed in the radial gliocytes of which the dense network of fine processes was localized in the inner and outer plexiform layers. GLAST immunoreactivity was also detected in astrocytes in the nerve fibre layer. These results demonstrate that three glutamate transporters show specific distributions in the gerbil retina and suggest that the glutamate re-uptake system in the gerbil retina may be different from that of the rat.

Na+-dependent high-affinity glutamate transport in macrophages

Excessive accumulation of glutamate in the CNS leads to excitotoxic neuronal damage. However, glutamate clearance is essentially mediated by astrocytes through Na+-dependent high-affinity glutamate transporters (excitatory amino acid transporters (EAATs)). Nevertheless, EAAT function was recently shown to be developmentally restricted in astrocytes and undetectable in mature astrocytes. This suggests a need for other cell types for clearing glutamate in the brain. As blood monocytes infiltrate the CNS in traumatic or inflammatory conditions, we addressed the question of whether macrophages expressed EAATs and were involved in glutamate clearance. We found that macrophages derived from human blood monocytes express both the cystine/glutamate antiporter and EAATs. Kinetic parameters were similar to those determined for neonatal astrocytes and embryonic neurons. Freshly sorted tissue macrophages did not possess EAATs, whereas cultured human spleen macrophages and cultured neonatal murine microglia did. Moreover, blood monocytes did not transport glutamate, but their stimulation with TNF-alpha led to functional transport. This suggests that the acquisition of these transporters by macrophages could be under the control of inflammatory molecules. Also, monocyte-derived macrophages overcame glutamate toxicity in neuron cultures by clearing this molecule. This suggests that brain-infiltrated macrophages and resident microglia may acquire EAATs and, along with astrocytes, regulate extracellular glutamate concentration. Moreover, we showed that EAATs are involved in the regulation of glutathione synthesis by providing intracellular glutamate. These observations thus offer new insight into the role of macrophages in excitotoxicity and in their response to oxidative stress.

Kinetic and pharmacological analysis of L-[35S]cystine transport into rat brain synaptosomes

The synaptosomal transport of L-[35S]cystine occurs by three mechanisms that are distinguishable on the basis of their ionic dependence, kinetics of transport and the specificity of inhibitors. They are (a) low affinity sodium-dependent transport (Km 463 +/- 86 microM, Vmax 185 +/- 20 nmol mg protein-1 min-1), (b) high affinity sodium-independent transport (Km 6.90 +/- 2.1 microM, Vmax 0.485 +/- 0.060 nmol mg protein(-1) min(-1)) and (c) low affinity sodium-independent transport (Km 327 +/- 29 microM, Vmax 4.18 +/- 0.25 nmol mg protein(-1) min(-1)). The sodium-dependent transport of L-cystine was mediated by the X(AG)- family of glutamate transporters, and accounted for almost 90% of the total quantity of L-[35S]cystine accumulated into synaptosomes. L-glutamate (Ki 11.2 +/- 1.3 microM) was a non-competitive inhibitor of this transporter, and at 100 microM L-glutamate, the Vmax for L-[35S]cystine transport was reduced to 10% of control. L-cystine did not inhibit the high-affinity sodium-dependent transport of D-[3H]aspartate into synaptosomes. L-histidine and glutathione were the most potent inhibitors of the low affinity sodium-independent transport of L-[35S]cystine. L-homocysteate, L-cysteine sulphinate and L-homocysteine sulphinate were also effective inhibitors. 1 mM L-glutamate reduced the sodium-independent transport of L-cystine to 63% of control. These results suggest that the vast majority of the L-cystine transported into synaptosomes occurs by the high-affinity glutamate transporters, but that L-cystine may bind to a site that is distinct from that to which L-glutamate binds. The uptake of L-cystine by this mechanism is sensitive to inhibition by increased extracellular concentrations of L-glutamate. The importance of these results for understanding the mechanism of glutamate-mediated neurotoxicity is discussed.

Substrate-stereoselectivity of a high-affinity glutamate transporter cloned from the CNS of the cockroach Diploptera punctata

A cDNA encoding a Na(+)-dependent glutamate transporter has been cloned from the brain of the cockroach Diploptera punctata. The cDNA encodes a transporter protein of 481 amino acids, designated DipEAAT1, which when expressed in baculovirus infected insect cells, resulted in a 40-50 fold increase in [(3)H]L-glutamate uptake. DipEAAT1 mRNA is expressed in the brain, as is the RNA encoding TrnEAAT1, a related transporter recently isolated from the caterpillar Trichoplusia ni. The affinity of these transporters for L-glutamate and several structural analogues was compared. Both have a high affinity for L-glutamate, their presumed primary substrate, but quite different affinities for D-aspartate. TrnEAAT1 was found to be similar to other glutamate transporters in that its ability to transport [(3)H]L-glutamate into cells was inhibited strongly by D- and L- isomers of aspartate and its analogues. DipEAAT1, by contrast, was inhibited weakly by all D- isomers tested. The affinity of DipEAAT1 for [(3)H]D-aspartate was found to be an order of magnitude lower than that of TrnEAAT1, revealing an unusual stereoselectivity for aspartate substrates by the cockroach transporter. The activity of DipEAAT1 was also unaffected by the presence of Zn(++) in the bathing solution, despite the presence of a putative Zn(++)-binding motif conferring Zn(++)-sensitivity on some mammalian glutamate transporters.

Inducible expression and pharmacology of the human excitatory amino acid transporter 2 subtype of L-glutamate transporter

1. In this study we have examined the use of the ecdysone-inducible mammalian expression system (Invitrogen) for the regulation of expression of the predominant L-glutamate transporter EAAT2 (Excitatory Amino Acid Transporter) in HEK 293 cells. 2. HEK 293 cells which were stably transformed with the regulatory vector pVgRXR (EcR 293 cells) were used for transfection of the human EAAT2 cDNA using the inducible vector pIND and a clone designated HEK/EAAT2 was selected for further characterization. 3. Na+-dependent L-glutamate uptake activity (3.2 pmol min-1 mg-1) was observed in EcR 293 cells and this was increased approximately 2 fold in the uninduced HEK/EAAT2 cells, indicating a low level of basal EAAT2 activity in the absence of exogenous inducing agent. Exposure of HEK/EAAT2 cells to the ecdysone analogue Ponasterone A (10 microM for 24 h) resulted in a > or = 10 fold increase in the Na+-dependent activity. 4. L-glutamate uptake into induced HEK/EAAT2 cells followed first-order Michaelis-Menten kinetics and Eadie-Hofstee transformation of the saturable uptake data produced estimates of kinetic parameters as follows; Km 52.7+/-7.5 microM, Vmax 3.8+/-0.9 nmol min-1 mg-1 protein. 5. The pharmacological profile of the EAAT2 subtype was characterized using a series of L-glutamate transport inhibitors and the rank order of inhibitory potency was similar to that described previously for the rat homologue GLT-1 and in synaptosomal preparations from rat cortex. 6. Addition of the EAAT2 modulator arachidonic acid resulted in an enhancement (155+/-5% control in the presence of 30 microM) of the L-glutamate transport capacity in the induced HEK/EAAT2 cells. 7. This study demonstrates that the expression of EAAT2 can be regulated in a mammalian cell line using the ecdysone-inducible mammalian expression system.

Properties of excitatory amino acid transport in the human U373 astrocytoma cell line

In this study, we describe the presence of Na(+)-dependent high-affinity L-glutamate transport activity in the human U373 astrocytoma cell line. U373 cells exhibited a robust accumulation of L-glutamate which was predominantly (85%) extracellular Na(+)-dependent. Kinetic analysis of this transport activity revealed that the uptake followed first-order Michaelis-Menten kinetics and was high-affinity in nature. The kinetic parameters estimated by Eadie-Hofstee transformation of the saturable uptake were 37.3 +/- 5.1 microM for K(m) and 0.13 +/- 0.02 nmol min-1 mg-1 protein for Vmax. A total of 14 known inhibitors of high-affinity L-glutamate transport were examined for their abilities to inhibit L-glutamate uptake by U373 cells. Three compounds, kainate (KA), dihydrokainate (DHK) and alpha-aminoadipic acid produced less than 30% inhibition at 1 mM. The lack of effect of both KA and DHK indicates that the predominant astroglial L-glutamate transporter EAAT2 (excitatory amino acid transporter 2) does not contribute to the uptake activity present in these cells. The rank order of inhibitory potency for the remaining 11 compounds tested was L-cysteine sulphinate = L-CCG-III = L-cysteate = L-aspartate = threo-beta-hydroxyaspartate > trans-PDC > D-aspartate = MPDC > beta-glutamate > L-CCG-IV = L-aspartate-beta-hydroxamate. Pre-treatment of U373 cells with phorbol ester for 30 min resulted in a 56% decrease in L-glutamate uptake and this effect was blocked in a concentration-dependent manner by the PKC inhibitor bisindolylmaleimide I. Expression of L-glutamate transporters by U373 cells was examined by reverse transcriptase polymerase chain reaction (RT-PCR) and Western analysis. Transcripts for both the EAAT1 and EAAT3 transporter subtypes were detected but not for EAATs 2, 4, and 5. Immunoblot analysis confirmed the presence of EAAT3 protein, however, we were unable to detect EAAT1 protein. In conclusion, the Na(+)-dependent high-affinity L-glutamate transport into human U373 astrocytoma cells appears to be mediated predominantly by the EAAT3 subtype.

PhTx4, a new class of toxins from Phoneutria nigriventer spider venom, inhibits the glutamate uptake in rat brain synaptosomes

We report the characterization of a new class of glutamate uptake inhibitors isolated from Phoneutria nigriventer venom. Glutamate transport activity was assayed in rat cerebrocortical synaptosomes by using [(3)H]-L-glutamate. PhTx4 inhibited glutamate uptake in a dose dependent manner. The IC(50) value obtained was 2.35+/-0.9 microg/ml which is in the observed range reported for glutamate uptake blockers. Tx4-7, one of PhTx4 toxins, showed the strongest inhibitory activity (50.3+/-0.69%, n=3).

Excitatory amino acid transporters: a family in flux

As the most predominant excitatory neurotransmitter, glutamate has the potential to influence the function of most neuronal circuits in the central nervous system. To limit receptor activation during signaling and prevent the overstimulation of glutamate receptors that can trigger excitotoxic mechanisms and cell death, extracellular concentrations of excitatory amino acids are tightly controlled by transport systems on both neurons and glial cells. L-Glutamate is a potent neurotoxin, and the inadequate clearance of excitatory amino acids may contribute to the neurodegeneration seen in a variety of conditions, including epilepsy, ischemia, and amyotrophic lateral sclerosis. To establish the contributions of carrier systems to the etiology of neurological disorders, and to consider their potential utility as therapeutic targets, a detailed understanding of transporter function and pharmacology is required. This review summarizes current knowledge of the structural and functional diversity of excitatory amino acid transporters and explores how they might serve as targets for drug design.

Cyclobutane quisqualic acid analogues as selective mGluR5a metabotropic glutamic acid receptor ligands

The conformationally constrained cyclobutane analogues of quisqualic acid (Z)- and (E)-1-amino-3-[2'-(3',5'-dioxo-1',2', 4'-oxadiazolidinyl)]cyclobutane-1-carboxylic acid, compounds 2 and 3, respectively, were synthesized. Both 2 and 3 stimulated phosphoinositide (PI) hydrolysis in the hippocampus with EC50 values of 18 +/- 6 and 53 +/- 19 microM, respectively. Neither analogue stimulated PI hydrolysis in the cerebellum. The effects of 2 and 3 were also examined in BHK cells which expressed either mGluR1a or mGluR5a receptors. Compounds 2 and 3 stimulated PI hydrolysis in cells expressing mGluR5a but not in those cells expressing mGluR1a. The EC50 value for 2 was 11 +/- 4 microM, while that for 3 was 49 +/- 25 microM. Both 2 and 3 did not show any significant effect on cells expressing the mGluR2 and mGluR4a receptors. In addition, neither compound blocked [3H]glutamic acid uptake into synaptosomal membranes, and neither compound was able to produce the QUIS effect as does quisqualic acid. This pharmacological profile indicates that 2 and 3 are selective ligands for the mGluR5a metabotropic glutamic acid receptor.

The pharmacological profile of L-glutamate transport in human NT2 neurones is consistent with excitatory amino acid transporter 2

The human teratocarcinoma cell line NTera2/D1 can be differentiated to produce post-mitotic neurones (NT2-N cells) by prolonged (> 3 week) exposure to retinoic acid. In this study, we describe the characterisation of high-affinity Na+-dependent L-glutamate transport activity in post-mitotic differentiated NT2-N cells. NT2-N cells, but not the undifferentiated precursor cells, transported L-glutamate in a Na+-dependent manner, as determined by equimolar replacement of Na+ with choline. L-glutamate uptake was saturable and Eadie-Hofstee transformation of the saturation data revealed a Km of 10.6+/-0.8 microM, and a maximum transport capacity (Vmax) of 100.3+/-12.3 pmol min(-1) mg(-1) protein. Pharmacological characterisation of the transport activity in NT2-N cells produced a rank order of inhibitory activity which was identical to that determined for the human excitatory amino acid transporter 2 which we have analysed in a stable mammalian cell line (Madin Darby Canine Kidney (MDCK) cells). Of particular note, L-glutamate transport by NT2-N cells was sensitive to both dihydrokainate and kainate. The expression of human excitatory amino acid transporter mRNAs was studied using reverse transcriptase polymerase chain reaction. NT2-N cells expressed transcripts for excitatory amino acid transporters 2 and 3, but not for the subtypes 1, 4 and 5. We conclude that although the mRNA expression studies suggest the presence of transcripts for both excitatory amino acid transporter 2 and 3 in NT2-N cells, the sensitivity to dihydrokainate and kainate determined in the pharmacological analysis indicates that, of the known transporter subtypes, excitatory amino acid transporter 2 contributes to the bulk of the L-glutamate transport activity present in these cells.

Micromolar L-glutamate induces extensive apoptosis in an apoptotic-necrotic continuum of insult-dependent, excitotoxic injury in cultured cortical neurones

Excitotoxicity induced by L-glutamate (Glu), when examined in a pure neuronal cortical culture, involved widespread apoptosis at concentrations of 1-10 microM as part of a continuum of injury, which at its most servere was purely necrotic. Cells, maintained in chemically defined neurobasal/B27 medium, were exposed at d7 for 2 h to Glu (1-500 microM), and cellular injury was analysed 2 and 24 h after insult using morphology (phase-contrast microscopy), a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) viability assay, nuclear staining with 4,6-diamidino-2-phenylindole (DAPI), terminal transferase-mediated dUTP nick end-labelling (TUNEL) and DNA fragmentation by gel electrophoresis. Glu-mediated neurotoxicity was prevented by MK-801 (5 microM), whilst CNQX (20 microM) attenuated injury by 20%. Exposure to intensive insults (100 and 500 microM Glu) induced necrosis characterized by rapid cell swelling (< 2 h) and lack of chromatin condensation, confirmed by DAPI nuclear staining. In contrast, mild insults (< 20 microM Glu) failed to produce acute neuronal swelling at < 2 h, but 24 h after injury resulted in a large number of apoptotic nuclei as confirmed by TUNEL and electrophoretic evidence of DNA fragmentation, which was attenuated by cycloheximide (0.1 microg/ml). Our findings indicate for the first time that physiological concentrations of Glu produce neuronal injury across a continuum involving apoptosis (< 20 microM) and increasingly necrosis(> 20 microM), dependent on the severity of the initial insult.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Preparation of difluoro analogs of CCGs and their pharmacological evaluations

All the stereoisomers of 2-(2-carboxy-3,3-difluorocyclopropyl)glycines (F2CCGs) were synthesized in enantiomerically pure forms using (R)-2,3-O-isopropyl-ideneglyceraldehyde as a chiral precursor. L-F2CCG-I, one of the stereoisomers corresponding to an extended form of L-glutamate was found to be a potent agonist for metabotropic glutamate receptors (mGluRs).

Dihydrokainate-sensitive neuronal glutamate transport is required for protection of rat cortical neurons in culture against synaptically released glutamate

Glutamate transport in nearly pure rat cortical neurons in culture (less than 0.2% astrocytes) is potently inhibited by dihydrokainate, l-serine-O-sulphate, but not by l-alpha-amino-adipate. This system allows for a test of the hypothesis that glutamate transport is important for protecting neurons against the toxicity of endogenous synaptically released glutamate. In support of this hypothesis, a 20-24 h exposure to 1 mm dihydrokainate reduced cell survival to only 14.8 +/- 9.8% in neuronal cultures (P < 0.001; n = 3), although it had no effect on neuronal survival in astrocyte-rich cultures (P > 0.05; n = 3). Dihydrokainate also significantly caused accumulation of glutamate in the extracellular medium of cortical neuronal cultures (6.6 +/- 4.9 micrometer, compared to 1.2 +/- 0.3 micrometer in control, n = 14, P < 0.01). The neurotoxicity of dihydrokainate was blocked by 10 micrometer MK-801, 10 micrometer tetrodotoxin, and an enzyme system that degrades extracellular glutamate. The latter two also abolished the accumulation of glutamate in the extracellular medium. Dihydrokainate (1 mm) inhibited the 45calcium uptake stimulated by 30 micrometer N-methyl-d-aspartate (NMDA), but not by higher concentrations consistent with a weak antagonist action of dihydrokainate at the NMDA receptor. Whole cell recordings showed that 1 mm dihydrokainate produced approximately 25% inhibition of 30 micrometer NMDA-induced current in cortical neurons. Dihydrokainate (1 mm) alone generated a small current (17% of the current produced by 30 micrometer NMDA) that was blocked by 30 micrometer 5,7-dichlorokynurenate and only weakly by 10 micrometer cyano-7-nitroquinoxaline-2,3-dione (CNQX). These results suggest that the toxicity of dihydrokainate in neuronal cultures is due to its ability to block glutamate transport in these cultures, and that dihydrokainate-sensitive neuronal glutamate transport may be important in protecting neurons against the toxicity of synaptically released glutamate.

Molecular pharmacology and physiology of glutamate transporters in the central nervous system

1. Glutamate is the predominant excitatory neurotransmitter in the brain, but it is also a potent neurotoxin. Following release of glutamate from presynaptic vesicles into the synapse and activation of a variety of ionotropic and metabotropic glutamate receptors, glutamate is removed from the synapse. This is achieved through active uptake of glutamate by transporters located pre- and also post-synaptically or, alternatively, glutamate can diffuse out of the synapse and be taken up by transporters located on the cell surface of glial cells. 2. Complementary DNA encoding a number of glutamate transporters have recently been cloned and form a family of structurally related membrane proteins with a high degree of amino acid sequence conservation. Expression of the cloned glutamate transporters in various cell types has aided in the characterization of the functional properties of the different transporter subtypes. 3. Glutamate transport is coupled to sodium, potassium and pH gradients across the cell membrane creating an electrogenic process. This allows transport to be measured using electrophysiological techniques, which has greatly aided in understanding some of the basic mechanisms of the transport process and has also allowed a detailed understanding of the molecular pharmacology of the different transporter subtypes. 4. In the present review I shall discuss some of the recent advances in understanding the molecular basis for glutamate transporter function and then highlight some of the unanswered questions concerning the physiological roles of these proteins and suggest possible strategies for pharmacological manipulation of transporters for the treatment of neurological disorders.

Structural selectivity and molecular nature of L-glutamate transport in cultured human fibroblasts

Uptake of L-[3H]glutamate by monolayers of fibroblasts cultured from human embryonic skin has been studied in the presence of several nonradioactive structural analogs of glutamate and aspartate. Results have suggested that the structural specificites of glutamate transporters in cultured human fibroblasts are similar to those of glutamate transporters in the mammalian brain. Only subtle differences have been detected: in the mammalian cerebral cortex, enantiomers of threo-3-hydroxyaspartate are almost equipotent as inhibitors of L-[3H]glutamate uptake while, in human fibroblasts, the D-isomer has been found to be an order of magnitude less potent than the corresponding L-isomer. Kinetic analysis of a model in which substrates are recognized by the glutamate transporter binding site(s) as both alpha- and beta-amino acids indicated that such a mechanism cannot explain the apparent negative cooperativity characterizing the effects of D- and L-aspartate. Molecular modeling has been used to estimate the optimum conformation of L-glutamate as it interacts with the transporter(s). Flow cytometry has indicated that all fibroblasts in culture express at least moderate levels of four glutamate transporters cloned from human brain. Small subpopulations (< 3%) of cells, however, were strongly labeled with antibodies against EAAT1 (GLAST) and EAAT2 (GLT-1) transporters. We conclude that these two transporters--known to be strongly expressed in brain tissue--can be principally responsible for the "high affinity" transport of glutamate also in nonneural cells.

Cellular distribution and kinetic properties of high-affinity glutamate transporters

L-glutamic acid is a key chemical transmitter of excitatory signals in the nervous system. The termination of glutamatergic transmission occurs via uptake of glutamate by a family of high-affinity glutamate transporters that utilize the Na+/K+ electrochemical gradient as a driving force. The stoichiometry of a single translocation cycle is still debatable, although all proposed models stipulate an inward movement of a net positive charge. This electrogenic mechanism is capable of translocating the neurotransmitter against its several thousand-fold concentration gradient, therefore, keeping the resting glutamate concentration below the treshold levels. The five cloned transporters (GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, EAAT4, and EAAT5) exhibit distinct distribution patterns and kinetic properties in different brain regions, cell types, and reconstitution systems. Moreover, distinct pharmacological profiles were revealed among the species homologues. GLAST and GLT1, the predominant glutamate transporters in the brain, are coexpressed in astroglial processes, whereas neuronal carriers are mainly located in the dendrosomatic compartment. Some of these carrier proteins may possess signal transducing properties, distinct from their transporter activity. Some experimental conditions and several naturally occurring and synthetic compounds are capable of regulating the expression of glutamate transporters. However, selective pharmacological tools interfering with the individual glutamate carriers have yet to be developed.

DL-threo-beta-benzyloxyaspartate, a potent blocker of excitatory amino acid transporters

DL-threo-beta-Benzyloxyaspartate (DL-TBOA), a novel derivative of DL-threo-beta-hydroxyaspartate, was synthesized and examined as an inhibitor of sodium-dependent glutamate/aspartate (excitatory amino acid) transporters. DL-TBOA inhibited the uptake of [14C]glutamate in COS-1 cells expressing the human excitatory amino acid transporter-1 (EAAT1) (Ki = 42 microM) with almost the same potency as DL-threo-beta-hydroxyaspartate (Ki = 58 microM). With regard to the human excitatory amino acid transporter-2 (EAAT2), the inhibitory effect of DL-TBOA (Ki = 5.7 microM) was much more potent than that of dihydrokainate (Ki = 79 microM), which is well known as a selective blocker of this subtype. Electrophysiologically, DL-TBOA induced no detectable inward currents in Xenopus laevis oocytes expressing human EAAT1 or EAAT2. However, it significantly reduced the glutamate-induced currents, indicating the prevention of transport. The dose-response curve of glutamate was shifted by adding DL-TBOA without a significant change in the maximum current. The Kb values for human EAAT1 and EAAT2 expressed in X. laevis oocytes were 9.0 microM and 116 nM, respectively. These results demonstrated that DL-TBOA is, so far, the most potent competitive blocker of glutamate transporters. DL-TBOA did not show any significant effects on either the ionotropic or metabotropic glutamate receptors. Moreover, DL-TBOA is chemically much more stable than its benzoyl analog, a previously reported blocker of excitatory amino acid transporters; therefore, DL-TBOA should be a useful tool for investigating the physiological roles of transporters.

Potentiation by DL-alpha-aminopimelate of the inhibitory action of a novel mGluR agonist (L-F2CCG-I) on monosynaptic excitation in the rat spinal cord

1. Neuropharmacological actions of all the possible stereoisomers of 3',3'-difluoro-2-(carboxycyclopropyl)glycine (3',3'-difluoro-CCG) were compared with those of the corresponding 2-(carboxycyclopropyl)glycine (CCG) isomers in the isolated spinal cord of newborn rats. (2S,1'S,2'S)- and (2S,1'R,2'S)-2-(2-carboxy-3,3-difluorocyclopropyl)glycine (L-F2CCG-I and L-F2CCG-IV) were the most potent in causing depolarization, their threshold concentrations being approximately 1 microM. 2. The depolarization evoked by L-F2CCG-I (30 microM) was depressed by (+)-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mM (n=4)) to 17+/-3% of the control: this depolarizing action was not decreased by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 100 microM), and only slightly decreased by high concentrations of D-2-amino-5-phosphonopentanoic acid (D-AP5, 100 microM), suggesting that L-F2CCG-I activates mainly metabotropic glutamate receptors. 3. L-F2CCG-I preferentially depressed the monosynaptic component of the spinal reflex approximately 3 times more effectively than (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). The depressant action of L-F2CCG-I (0.2 microM-0.7 microM) on monosynaptic excitation was antagonized by (2S,1'S,2'S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG, 0.3 mM-1 mM) and (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4, 0.3 mM). 4. DL-alpha-aminopimelate (10 and 100 microM) selectively potentiated the depression of monosynaptic excitation caused by L-CCG-I (0.2 microM) and L-F2CCG-I (0.1 microM). The actions of (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) (50 nM-0.2 microM), L-2-amino-4-phosphonobutanoic acid (L-AP4) (0.3-1 microM), (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD) (1-7 microM) and baclofen (0.1-0.7 microM) were unaffected by DL-alpha-aminopimelate. The threshold concentration for the potentiating actions of DL-alpha-aminopimelate was 3 microM. 5. The depolarization induced by quisqualate (3 microM, 10 s application) was increased to 115+/-2% and 137+/-5% of the control values during combined application of quisqualate with either 30 microM or 100 microM DL-alpha-aminopimelate, respectively. 6. Following the application and subsequent washout of L-F2CCG-I, DL-alpha-aminopimelate (3-100 microM) decreased the amplitude of the monosynaptic component of spinal reflexes in a concentration-dependent manner, indicating a 'priming' effect of L-F2CCG-I.