Expression cloning of a rat glutamate transporter

Reconstitution of GABA, Glycine and Glutamate Transporters

In contrast to water soluble enzymes which can be purified and studied while in solution, studies of solute carrier (transporter) proteins require both that the protein of interest is situated in a phospholipid membrane and that this membrane forms a closed compartment. An additional challenge to the study of transporter proteins has been that the transport depends on the transmembrane electrochemical gradients. Baruch I. Kanner understood this early on and first developed techniques for studying plasma membrane vesicles. This advanced the field in that the experimenter could control the electrochemical gradients. Kanner, however, did not stop there, but started to solubilize the membranes so that the transporter proteins were taken out of their natural environment. In order to study them, Kanner then had to find a way to reconstitute them (reinsert them into phospholipid membranes). The scope of the present review is both to describe the reconstitution method in full detail as that has never been done, and also to reveal the scientific impact that this method has had. Kanner's later work is not reviewed here although that also deserves a review because it too has had a huge impact.

Role of glutamate excitotoxicity and glutamate transporter EAAT2 in epilepsy: Opportunities for novel therapeutics development

Epilepsy is a complex neurological syndrome characterized by seizures resulting from neuronal hyperexcitability and sudden and synchronized bursts of electrical discharges. Impaired astrocyte function that results in glutamate excitotoxicity has been recognized to play a key role in the pathogenesis of epilepsy. While there are 26 drugs marketed as anti-epileptic drugs no current treatments are disease modifying as they only suppress seizures rather than the development and progression of epilepsy. Excitatory amino acid transporters (EAATs) are critical for maintaining low extracellular glutamate concentrations and preventing excitotoxicity. When extracellular glutamate concentrations rise to abnormal levels, glutamate receptor overactivation and the subsequent excessive influx of calcium into the post-synaptic neuron can trigger cell death pathways. In this review we discuss targeting EAAT2, the predominant glutamate transporter in the CNS, as a promising approach for developing therapies for epilepsy. EAAT2 upregulation via transcriptional and translational regulation has proven successful in vivo in reducing spontaneous recurrent seizures and offering neuroprotective effects. Another approach to regulate EAAT2 activity is through positive allosteric modulation (PAM). Novel PAMs of EAAT2 have recently been identified and are under development, representing a promising approach for the advance of novel therapeutics for epilepsy.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Astrocyte-specific deletion of the transcription factor Yin Yang 1 in murine substantia nigra mitigates manganese-induced dopaminergic neurotoxicity

Manganese (Mn)-induced neurotoxicity resembles Parkinson's disease (PD), but the mechanisms underpinning its effects remain unknown. Mn dysregulates astrocytic glutamate transporters, GLT-1 and GLAST, and dopaminergic function, including tyrosine hydroxylase (TH). Our previous in vitro studies have shown that Mn repressed GLAST and GLT-1 via activation of transcription factor Yin Yang 1 (YY1). Here, we investigated if in vivo astrocytic YY1 deletion mitigates Mn-induced dopaminergic neurotoxicity, attenuating Mn-induced reduction in GLAST/GLT-1 expression in murine substantia nigra (SN). AAV5-GFAP-Cre-GFP particles were infused into the SN of 8-week-old YY1 ^flox/flox mice to generate a region-specific astrocytic YY1 conditional knockout (cKO) mouse model. 3 weeks after adeno-associated viral (AAV) infusion, mice were exposed to 330 μg of Mn (MnCl₂ 30 mg/kg, intranasal instillation, daily) for 3 weeks. After Mn exposure, motor functions were determined in open-field and rotarod tests, followed by Western blotting, quantitative PCR, and immunohistochemistry to assess YY1, TH, GLAST, and GLT-1 levels. Infusion of AAV5-GFAP-Cre-GFP vectors into the SN resulted in region-specific astrocytic YY1 deletion and attenuation of Mn-induced impairment of motor functions, reduction of TH-expressing cells in SN, and TH mRNA/protein levels in midbrain/striatum. Astrocytic YY1 deletion also attenuated the Mn-induced decrease in GLAST/GLT-1 mRNA/protein levels in midbrain. Moreover, YY1 deletion abrogated its interaction with histone deacetylases in astrocytes. These results indicate that astrocytic YY1 plays a critical role in Mn-induced neurotoxicity in vivo, at least in part, by reducing astrocytic GLAST/GLT-1. Thus, YY1 might be a potential target for treatment of Mn toxicity and other neurological disorders associated with dysregulation of GLAST/GLT-1.

Glial glutamate transporter GLT-1 determines susceptibility to spreading depression in the mouse cerebral cortex

Cortical spreading depression (CSD) is a pathological neural excitation that underlies migraine pathophysiology. Since glutamate receptor antagonists impair CSD propagation, susceptibility to CSD might be determined by any of the neuronal (excitatory amino acid carrier 1 [EAAC1]) and glial (GLutamate ASpartate Transporter [GLAST] and glial glutamate transporter 1 [GLT-1]) glutamate transporters, which are responsible for clearing extracellular glutamate. To investigate this hypothesis, we performed electrophysiological, hemodynamic, and electrochemical analyses using EAAC1- (EAAC1 KO), GLAST- (GLAST KO), and conditional GLT1-1-knockout mice (GLT-1 cKO) to assess altered susceptibility to CSD. Despite the incomplete deletion of the gene in the cerebral cortex, GLT-1 cKO mice exhibited significant reduction of GLT-1 protein in the brain without apparent alteration of the cytoarchitecture in the cerebral cortex. Physiological analysis revealed that GLT-1 cKO showed enhanced susceptibility to CSD elicited by chemical stimulation with increased CSD frequency and velocity compared to GLT-1 control. In contrast, the germ-line EAAC1 and GLAST KOs showed no such effect. Intriguingly, both field potential and cerebral blood flow showed faster dynamics with narrower CSD than the controls. An enzyme-based biosensor revealed more rapid accumulation of glutamate in the extracellular space in GLT-1 cKO mice during the early phase of CSD than in GLT-1 control, resulting in an increased susceptibility to CSD. These results provided the first evidence for a novel role of GLT-1 in determining susceptibility to CSD.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

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.

Differential regulation of the glutamate transporters GLT-1 and GLAST by GSK3β

The glutamate transporters GLAST and GLT-1 are mainly expressed in glial cells and regulate glutamate levels in the synapses. GLAST and GLT-1 are the targets of several signaling pathways. In this study we explore the possible functional interaction between these transporters and GSK3β. This kinase is involved in multiple cellular processes including neuronal development and synaptic plasticity. To evaluate whether GLT-1 and GLAST were regulated by GSK3β, we coexpressed these proteins in heterologous expression systems. In both COS-7 cells and Xenopus laevis oocytes, GSK3β stimulated the activity of GLT-1 and reduced that of GLAST. These effects were associated with corresponding changes in the amounts of GLT-1 or GLAST in the plasma membrane. These effects were suppressed by inhibitors of GSK3β or a catalytically inactive form of the kinase. GSK3β also decreases the incorporation of (32)Pi into GLT-1 and increases GLAST phosphorylation. Pharmacological inhibition of endogenous GSK3β in primary cultures of rat brain cortex also leads to a differential modulation of GLT-1 and GLAST. Our results suggest that constitutively active GSK3β is important in controlling the expression of functional glutamate transporters on the plasma membrane. This regulation might be relevant in physiological and pathological conditions in which glutamate transporters and GSK3β signaling are involved.

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.

Reduced excitatory amino acid transporter 1 and metabotropic glutamate receptor 5 expression in the cerebellum of fragile X mental retardation gene 1 premutation carriers with fragile X-associated tremor/ataxia syndrome

A premutation (PM) expansion (55-200 CGG) in the fragile X mental retardation gene 1 causes elevated messenger RNA and reduced fragile X mental retardation gene 1 protein. Young PM carriers can develop characteristic physical features and mild cognitive disabilities. In addition, individuals with PM, particularly male carriers, are at high risk to develop fragile X-associated tremor/ataxia syndrome (FXTAS) with aging. Human postmortem FXTAS brains show extensive white matter disease in the cerebellum and the presence of intranuclear inclusions throughout the brain, although their etiologic significance is unknown. In the current work, expression levels of the metabotropic glutamate (Glu) receptor 5 and the Glu transporter excitatory amino acid transporter 1, examined by reverse transcription polymerase chain reaction and western blot analyses, were found to be reduced in the postmortem cerebellum of PM carriers with FXTAS compared with age matched controls, with higher CGG repeat number having greater reductions in both proteins. These data suggests a dysregulation of Glu signaling in PM carriers, which would likely contribute to the development and severity of FXTAS.

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.

Hydrogen sulfide protects PC12 cells against reactive oxygen species and extracellular signal-regulated kinase 1/2-mediated downregulation of glutamate transporter-1 expression induced by chemical hypoxia

Hypoxia and/or ischemia are implicated in neurodegenerative disorders. In these diseases, hypoxia/ischemia may induce oxidative stress, including production of reactive oxygen species (ROS), which result in a decrease in glutamate transporter expression. Hydrogen sulfide (H2S), as the third gasotransmitter, has neuroprotective effects and potent antioxidant properties. In the present study, we investigated the role of glutamate transporter-1 (GLT-1) in the protection of H2S against chemical hypoxia-induced injury in PC12 cells. We found that cobalt chloride (CoCl2), a chemical hypoxia agent, reduced the expression of GLT-1 in a time-dependent manner. Pretreatment with NaHS (a donor of H2S) reversed the CoCl2-induced downregulation of GLT-1 expression. Pretreatment with DHK (a selective inhibitor of GLT-1) for 30 min prior to NaHS preconditioning significantly inhibited the cytoprotection of H2S against CoCl2-induced injuries, leading to an increase in cytotoxicity and apoptosis as well as to a loss of mitochondrial membrane potential (MMP). In addition, we found that similar to the effect of NaHS, pretreatment with NAC (a ROS scavenger) or U0126 (a MEK1/2 inhibitor) blocked the downregulation of GLT-1 expression induced by CoCl2. Collectively, we demonstrated for the first time that ROS and extracellular signal-regulated kinase 1/2 (ERK1/2)-mediated reduction of GLT-1 expression may be involved in chemical hypoxia-induced neural injury and that H2S attenuates this injury partly by upregulating GLT-1 expression in PC12 cells.

The rates of postmortem proteolysis of glutamate transporters differ dramatically between cells and between transporter subtypes

Glutamate transporters (GLT-1, GLAST, EAAC1) limit the actions of excitatory amino acids. Because a disturbed transporter operation can cause or aggravate neurological diseases, transporters are of considerable neuropathological interest. Human samples, however, are seldom obtained fresh. Here, we used mice brains to study how fast glutamate transporters are degraded after death. Immunoblots showed that terminal GLT-1 epitopes (within residues 1-26 and 518-573) had mostly disappeared after 24 hr. GLAST termini (1-25 and 522-543) degraded slightly slower. In contrast, epitopes within central parts of GLT-1 (493-508) and the EAAC1 C-terminus (510-523) were readily detectable after 72 hr. The decline in immunoreactivity of the GLT-1 and GLAST termini was also seen in tissue sections, but proteolysis did not happen synchronously in all cells. At 24 hr, scattered cells remained strongly immunopositive, while the majority of cells were completely immunonegative. GLAST and GLT-1 co-localized in neocortical tissue, but at 12 hr, many GLAST-positive cells had lost the GLT-1 termini. The uneven disappearance of labeling was not observed with the antibodies to GLT-1 residues 493-508. The immunoreactivity to this epitope correlated better with the reported glutamate uptake activity. Thus, postmortem delay may affect epitopes differently, possibly causing erroneous conclusions about relative expression levels.

Glutamate and GABA synthesis, release, transport and metabolism as targets for seizure control

The synthesis, release, reuptake, and metabolism of the excitatory and inhibitory neurotransmitters glutamate and GABA, respectively, are tightly controlled. Given the role that these two neurotransmitters play in normal and abnormal neurotransmission, it is important to consider the processes whereby they are regulated. This brief review is focused entirely on the metabolic aspects of glutamate and GABA synthesis and neurotransmission. It describes in limited detail the synthesis, release, reuptake, metabolism, cellular compartmentation and pharmacology of the glutamatergic and GABAergic synapse. This review also provides a summary and brief description of the pathologic and phenotypic features of the various genetic animal models that have been developed in an effort to provide a greater understanding of the role that each of the aforementioned metabolic processes plays in controlling excitatory and inhibitory neurotransmission and how their use will hopefully facilitate the development of safer and more efficacious therapies for the treatment of epilepsy and other neurological disorders.

The accessibility in the external part of the TM5 of the glutamate transporter EAAT1 is conformationally sensitive during the transport cycle

Background

Excitatory amino acid transporter 1 (EAAT1) is a glutamate transporter which is a key element in the termination of the synaptic actions of glutamate. It serves to keep the extracellular glutamate concentration below neurotoxic level. However the functional significance and the change of accessibility of residues in transmembrane domain (TM) 5 of the EAAT1 are not clear yet.

Methodology/Principal Findings

We used cysteine mutagenesis with treatments with membrane-impermeable sulfhydryl reagent MTSET [(2-trimethylammonium) methanethiosulfonate] to investigate the change of accessibility of TM5. Cysteine mutants were introduced from position 291 to 300 of the cysteine-less version of EAAT1. We checked the activity and kinetic parameters of the mutants before and after treatments with MTSET, furthermore we analyzed the effect of the substrate and blocker on the inhibition of the cysteine mutants by MTSET. Inhibition of transport by MTSET was observed in the mutants L296C, I297C and G299C, while the activity of K300C got higher after exposure to MTSET. V_max of L296C and G299C got lower while that of K300C got higher after treated by MTSET. The L296C, G299C, K300C single cysteine mutants showed a conformationally sensitive reactivity pattern. The sensitivity of L296C to MTSET was potentiated by glutamate and TBOA,but the sensitivity of G299C to MTSET was potentiated only by TBOA.

Conclusions/Significance

All these facts suggest that the accessibility of some positions of the external part of the TM5 is conformationally sensitive during the transport cycle. Our results indicate that some residues of TM5 take part in the transport pathway during the transport cycle.

EAAT expression by macrophages and microglia: still more questions than answers

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

Time-dependent changes in GLT-1 functioning in striatum of hemi-Parkinson rats

Striatal dopamine loss in Parkinson's disease is accompanied by a dysregulation of corticostriatal glutamatergic neurotransmission. Within this study, we investigated striatal expression and activity of the glial high-affinity Na(+)/K(+)-dependent glutamate transporters, GLT-1 and GLAST, in the 6-hydroxydopamine hemi-Parkinson rat model at different time points after unilateral 6-hydroxydopamine injection into the medial forebrain bundle. Using semi-quantitative Western blotting and an ex vivo D-[(3)H]-aspartate uptake assay, we showed a time-dependent bilateral effect of unilateral 6-hydroxydopamine lesioning on the expression as well as activity of GLT-1. At 3 and 12 weeks post-lesion, striatal GLT-1 function was bilaterally upregulated whereas at 5 weeks there was no change. Even though our data do not allow a straightforward conclusion as for the role of glutamate transporters in the pathogenesis of the disease, they do clearly demonstrate a link between disturbed glutamatergic neurotransmission and glutamate transporter functioning in the striatum of a rat model for Parkinson's disease.

Plasticity in expression of the glutamate transporters GLT-1 and GLAST in spinal dorsal horn glial cells following partial sciatic nerve ligation

BACKGROUND

Clearance of synaptically released glutamate, and hence termination of glutamatergic neurotransmission, is carried out by glutamate transporters, most especially glutamate transporter-1 (GLT-1) and the glutamate-aspartate transporter (GLAST) that are located in astrocytes. It is becoming increasingly well appreciated that changes in the function and expression of GLT-1 and GLAST occur under different physiological and pathological conditions. Here we investigated the plasticity in expression of GLT-1 and GLAST in the spinal dorsal horn using immunohistochemistry following partial sciatic nerve ligation (PSNL) in rats.

RESULTS

Animals were confirmed to develop hypersensitivity to mechanical stimulation by 7 days following PSNL. Baseline expression of GLT-1 and GLAST in naive animals was only observed in astrocytes and not in either microglia or neurons. Microglia and astrocytes showed evidence of reactivity to the nerve injury when assessed at 7 and 14 days following PSNL evidenced by increased expression of OX-42 and GFAP, respectively. In contrast, the total level of GLT-1 and GLAST protein decreased at both 7 and 14 days after PSNL. Importantly, the cellular location of GLT-1 and GLAST was also altered in response to nerve injury. Whereas activated astrocytes showed a marked decrease in expression of GLT-1 and GLAST, activated microglia showed de novo expression of GLT-1 and GLAST at 7 days after PSNL and this was maintained through day 14. Neurons showed no expression of GLT-1 or GLAST at any time point.

CONCLUSION

These results indicate that the expression of glutamate transporters in astrocytes and microglia are differentially regulated following nerve injury.

Expression cloning of neural genes using Xenopus laevis oocytes

Expression cloning requires a representative cDNA or genomic DNA library and a host organism in which the cloned genes can be transcribed and/or translated. It likewise requires a method to detect the expressed protein using, for example, the inherent biological activity of the gene or antibodies specific for the gene product. Most successful expression cloning strategies have employed cDNA libraries constructed in plasmid or bacteriophage lambda vectors and Xenopus oocytes or cultured mammalian cells as hosts. This unit presents several protocols designed for expression cloning paradigms that rely on electrophysiological recordings from Xenopus laevis oocytes.

Behavioral characterization of GLT1 (+/-) mice as a model of mild glutamatergic hyperfunction

GLT1 is one of the major transporters responsible for maintenance of glutamate homeostasis in the brain. In the present study, glutamate transporter 1-deficient GLT1 homozygous (-/-) and heterozygous (+/-) mice were investigated with the intention that they may provide a model of hyperglutamatergic state resulting in various behavioral alterations. The GLT1 (-/-) mice had lower body and brain weight, mild neuronal loss in CA1 hippocampal region as well as focal gliosis and severe focal neuronal paucity in layer II of the neocortex. The short life-span of GLT1 (-/-) precluded us from systematic behavioral studies in these mice. In contrast, GLT1 (+/-) mice exhibiting a 59% decrease in GLT1 immunoreactivity in their brain tissue, showed no apparent morphological brain abnormalities, and their life-span was not markedly different from controls. Behaviorally, GLT1 (+/-) presented moderate behavioral alterations compared to their wildtype littermates, such as: mild sensorimotor impairment, hyperlocomotion (at 3 month of age only), lower anxiety (at 6 months), better learning of cue-based fear conditioning but worse context-based fear conditioning. Our results suggest that GLT1 (+/-) mice may serve as a potentially useful model to study neurodegenerative disease conditions with mild hyperglutamatergic activity.

Subcellular localization of the glutamate transporters GLAST and GLT at the neuromuscular junction in rodents

The vertebrate neuromuscular junction (NMJ) is known to be a cholinergic synapse at which acetylcholine (ACh) is released from the presynaptic terminal to act on postsynaptic nicotinic ACh receptors. There is now growing evidence that glutamate, which is the main excitatory transmitter in the CNS and at invertebrate NMJs, may have a signaling function together with ACh also at the vertebrate NMJ. In the CNS, the extracellular concentration of glutamate is kept at a subtoxic level by Na(+)-driven high-affinity glutamate transporters located in plasma membranes of astrocytes and neurons. The glutamate transporters are also pivotal for shaping glutamate receptor responses at synapses. In order to throw further light on the potential role of glutamate as a cotransmitter at the NMJ we used high-resolution immunocytochemical methods to investigate the localization of the plasma membrane glutamate transporters GLAST (glutamate aspartate transporter) and GLT (glutamate transporter 1) in rat and mice NMJ regions. Confocal laser-scanning immunocytochemistry showed that GLT is restricted to the NMJ in rat and mouse skeletal muscle. Lack of labeling signal in knock-out mice confirmed that the immunoreactivity observed at the NMJ was specific for GLT. GLAST was also localized at the NMJ in rat but not detected in mouse NMJ (while abundant in mouse brain). Post-embedding electron microscopic immunocytochemistry and quantitative analyses in rat showed that GLAST and GLT are enriched in the junctional folds of the postsynaptic membrane at the NMJ. GLT was relatively higher in the slow-twitch muscle soleus than in the fast-twitch muscle extensor digitorum longus, whereas GLAST was relatively higher in extensor digitorum longus than in soleus. The findings show--together with previous demonstration of vesicular glutamate, a vesicular glutamate transporter and glutamate receptors--that mammalian NMJs contain the machinery required for synaptic release and action of glutamate. This indicates a signaling role for glutamate at the normal NMJ and provides a basis for the ability of denervated muscle to be reinnervated by glutamatergic axons from the CNS.

The glutamate-glutamine cycle as an inducible, protective face of macrophage activation

Neuronal damage in HIV infection results mainly from chronic activation of brain tissue and involves inflammation, oxidative stress, and glutamate-related neurotoxicity. Glutamate toxicity acts via two distinct pathways: an excitotoxic one, in which glutamate receptors are hyperactivated, and an oxidative one, in which cystine uptake is inhibited, resulting in glutathione depletion, oxidative stress, and cell degeneration. A number of studies have shown that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. They, in turn, provide the trophic amino acid glutamine via their expression of glutamine synthetase. These protective and trophic actions are inhibited in HIV infection, probably as a result of the effects of inflammatory mediators and viral proteins. In vitro and in vivo studies have demonstrated that activated microglia and brain macrophages (AMM) express the transporters and enzymes of the glutamate cycle. This suggests that in addition to their recognized neurotoxic properties in HIV infection, these cells exhibit some neuroprotective properties, which may partly compensate for the inhibited astrocytic function. This hypothesis might explain the discrepancy between microglial activation, which occurs early in the disease, and neuronal apoptosis and neuronal loss, which are late events. In this review, we discuss the possible neuroprotective and neurotrophic roles of AMM and their relationships with inflammation and oxidative stress.

Distribution of the glutamate transporters GLT-1 (SLC1A2) and GLAST (SLC1A3) in peripheral organs

The glutamate transporters GLT-1 and GLAST are widely expressed in astrocytes in the brain where they fulfill important functions during glutamatergic neurotransmission. The present study examines their distribution in peripheral organs using in situ hybridization (ISH) and immunocytochemistry. GLAST was found to be more widely distributed than GLT-1. GLAST was expressed primarily in epithelial cells, cells of the macrophage-lineage, lymphocytes, fat cells, interstitial cells, and salivary gland acini. GLT-1 was primarily expressed in glandular tissue, including mammary gland, lacrimal gland, and ducts and acini in salivary glands, but also by perivenous hepatocytes and follicular dendritic cells in spleen and lymph nodes. The findings demonstrate that, although expressed by the same cells in the brain, these two glutamate transporters have different distribution patterns in peripheral tissues and that they fulfill glutamate transport functions apart from glutamatergic neurotransmission in these areas.

Regulated expression of sodium-dependent glutamate transporters and synthetase: a neuroprotective role for activated microglia and macrophages in HIV infection?

It is now widely accepted that neuronal damage in HIV infection results mainly from microglial activation and involves apoptosis, oxidative stress and glutamate-mediated neurotoxicity. Glutamate toxicity acts via 2 distinct pathways: an excitotoxic one in which glutamate receptors are hyperactivated, and an oxidative one in which cystine uptake is inhibited, resulting in glutathione depletion and oxidative stress. A number of studies show that astrocytes normally take up glutamate, keeping extracellular glutamate concentration low in the brain and preventing excitotoxicity. This action is inhibited in HIV infection, probably due to the effects of inflammatory mediators and viral proteins. Other in vitro studies as well as in vivo experiments in rodents following mechanical stimulation, show that activated microglia and brain macrophages express high affinity glutamate transporters. These data have been confirmed in chronic inflammation of the brain, particularly in SIV infection, where activated microglia and brain macrophages also express glutamine synthetase. Recent studies in humans with HIV infection show that activated microglia and brain macrophages express the glutamate transporter EAAT-1 and that expression varies according to the disease stage. This suggests that, besides their recognized neurotoxic properties in HIV infection, these cells also have a neuroprotective function, and may partly make up for the inhibited astrocytic function, at least temporarily. This hypothesis might explain the discrepancy between microglial activation which occurs early in the disease, and neuronal apoptosis and neuronal loss which is a late event. In this review article, we discuss the possible neuroprotective and neurotrophic roles of activated microglia and macrophages that may be generated by the expression of high affinity glutamate transporters and glutamine synthetase, 2 major effectors of glial glutamate metabolism, and the implications for HIV-induced neuronal dysfunction, the underlying cause of HIV dementia.

Roles of glial glutamate transporters in shaping EPSCs at the climbing fiber-Purkinje cell synapses

Glial glutamate transporters, GLAST and GLT-1, are co-localized in processes of Bergmann glia (BG) wrapping excitatory synapses on Purkinje cells (PCs). Although GLAST is expressed six-fold more abundantly than GLT-1, no change is detected in the kinetics of climbing fiber (CF)-mediated excitatory postsynaptic currents (CF-EPSCs) in PCs in GLAST(-/-) mice compared to the wild-type mice (WT). Here we aimed to clarify the mechanism(s) underlying this unexpected finding using a selective GLT-1 blocker, dihydrokainate (DHK), and a novel antagonist of glial glutamate transporter, (2S,3S)-3-[3-(4-methoxybenzoylamino)benzyloxy]aspartate (PMB-TBOA). In the presence of cyclothiazide (CTZ), which attenuates the desensitization of AMPA receptors, DHK prolonged the decay time constant (tau(w)) of CF-EPSCs in WT, indicating that GLT-1 plays a partial role in the removal of glutamate. The application of 100 nM PMB-TBOA, which inhibited CF-mediated transporter currents in BG by approximately 80%, caused no change in tau(w) in WT in the absence of CTZ, whereas it prolonged tau(w) in the presence of CTZ. This prolonged value of tau(w) was similar to that in GLAST(-/-) mice in the presence of CTZ. These results indicate that glial glutamate transporters can apparently retain the fast decay kinetics of CF-EPSCs if a small proportion ( approximately 20%) of functional transporters is preserved.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

Neurotoxicité et neuroprotection, les deux facettes de l’activation microgliale au cours de l’infection par le virus de l’immunodéficience humaine (VIH)

Microglial cells and macrophages are the only cells within the central nervous system, in which productive HIV infection has been unquestionably demonstrated. Those cells play a key role in the origin of the neuronal dysfunction underlying HIV-related cognitive disorders. The neurotoxicity of the cells is both direct, related to HIV proteins, and indirect, through the release by activated macrophages and microglial cells (AMM) of multiple neurotoxic factors. The mechanisms of neuronal damage, the final irreversible stage of which is neuronal apoptosis, are only partly understood but appear to involve oxidative stress and glutamate-receptor mediated toxicity. On the other hand, recent experimental in vitro and in vivo studies, and neuropathological studies in HIV infected patients at different stages of the disease, tend to show that AMM express excitatory amino acid transporters (EAAT) suggesting that in addition to their neurotoxic properties, they also have a neuroprotective role by clearing extra-cellular glutamate and producing antioxidant glutathione. This neuroprotective role could counteract, at least in the early stages of the disease, the neurotoxicity of AMM explaining the discrepancy between the conspicuous microglial activation at that stage and the absence of cognitive disorder, neuronal loss and neuronal apoptosis. It could also explain the regression of the cognitive disorders in some patients who received highly active antiretroviral treatment.

Nicergoline Enhances Glutamate Uptake via Glutamate Transporters in Rat Cortical Synaptosomes

To elucidate the mechanisms of neuroprotective action of nicergoline, we examined its effect on glutamate transport in rat cortical synaptosomes and cloned glutamate transporters. In synaptosomes, nicergoline enhanced the glutamate uptake at 1-10 microM in standard medium and suppressed the increase of extracellular glutamate by reversed transport in low Na(+) medium. Apparent increase of extracellular glutamate concentration by dihydrokinate, an inhibitor of glial glutamate transporter GLT-1, was antagonized by nicergoline. In Xenopus oocytes expressing mouse neuronal glutamate transporter (mEAAC1), the glutamate-induced inward current was enhanced by nicergoline. These results suggest that nicergoline reduces the extracellular glutamate concentration through its effect on glutamate transporters.

Measurement of Glutamate Uptake and Reversed Transport by Rat Synaptosome Transporters

To establish an assay system for evaluation of the uptake and reversed transport of glutamate, we examined the effects of Na(+)-concentration and pharmacological agents on the extracellular glutamate concentration ([Glu](o)) in rat cortical synaptosomes in vitro. There was a decrease and increase of the [Glu](o) at high and low Na(+) concentrations, respectively, in a Ca(2+)-free medium. The changes in [Glu](o) in both directions were temperature-sensitive, and reversed at around 30 mM of Na(+). Dihydrokainate (DHK), a non-transportable inhibitor selective for glial glutamate transporter GLT-1, suppressed the decrease in [Glu](o), and the reversal of [Glu](o) change was shifted to about 60 mM Na(+). There was no change in the maximum [Glu](o) at total Na(+) substitution. Further pharmacological analysis revealed that D-aspartate and DL-threo-beta-hydroxy-aspartate (THA), transportable substrates of glutamate transporters, increased the [Glu](o) in standard media. In contrast, beta-phenylglutamic acid, a structural analogue of glutamate, suppressed both the decrease in [Glu](o) in standard medium and the increase in [Glu](o) in low Na(+) medium. It is, thus, concluded that both the direction and the amount of [Glu](o) changes are determined by a balance of the uptake and reversed transport of glutamate, and that this assay system is suitable for evaluation of the effect of this on glutamate transporters.

Functional changes of glial glutamate transporter GLT-1 during ischemia: an in vivo study in the hippocampal CA1 of normal mice and mutant mice lacking GLT-1

Glutamate transporters remove glutamate from the extracellular space and maintain it below neurotoxic levels under normal conditions. However, the dynamics under ischemic conditions remain to be determined. In the present study, we evaluated the function of the glial glutamate transporter (GLT-1) during brain ischemia by using an in vivo brain microdialysis technique in GLT-1 mutant mice. A microdialysis probe was placed in the hippocampal CA1 of GLT-1 mutant and wild-type mice, and glutamate levels were measured during 5 and 20 min ischemia. The glutamate levels in mice lacking GLT-1 were significantly higher than the corresponding glutamate levels in wild-type mice during 5 min ischemia. Delayed neuronal death was induced in the CA1 of the mice lacking GLT-1 but not in the CA1 of the wild-type mice. When ischemia was elongated to the duration of 20 min, the glutamate levels in wild-type mice were significantly higher than the corresponding glutamate levels in mice lacking GLT-1 during the last 12.5 min of 20 min ischemia. Acute neuronal death was also observed in the CA1 of wild-type mice. These results suggest that GLT-1 takes up extracellular glutamate to protect neurons in the early stage of ischemia and then releases glutamate, triggering acute neuronal death, when ischemic conditions are elongated. The function of GLT-1 may change from neuroprotective to neurodegenerative during ischemia.

Altered expression and uptake activity of spinal glutamate transporters after nerve injury contribute to the pathogenesis of neuropathic pain in rats

The central glutamatergic system has been implicated in the pathogenesis of neuropathic pain, and a highly active central glutamate transporter (GT) system regulates the uptake of endogenous glutamate. Here we demonstrate that both the expression and uptake activity of spinal GTs changed after chronic constriction nerve injury (CCI) and contributed to neuropathic pain behaviors in rats. CCI induced an initial GT upregulation up to at least postoperative day 5 primarily within the ipsilateral spinal cord dorsal horn, which was followed by a GT downregulation when examined on postoperative days 7 and 14 by Western blot and immunohistochemistry. Intrathecal administration of the tyrosine kinase receptor inhibitor K252a and the mitogen-activated protein kinase inhibitor PD98059 for postoperative days 1-4 reduced and nearly abolished the initial GT upregulation in CCI rats, respectively. Prevention of the CCI-induced GT upregulation by PD98059 resulted in exacerbated thermal hyperalgesia and mechanical allodynia reversible by the noncompetitive NMDA receptor antagonist MK-801, indicating that the initial GT upregulation hampered the development of neuropathic pain behaviors. Moreover, CCI significantly reduced glutamate uptake activity of spinal GTs when examined on postoperative day 5, which was prevented by riluzole (a positive GT activity regulator) given intrathecally twice a day for postoperative days 1-4. Consistently, riluzole attenuated and gradually reversed neuropathic pain behaviors when the 4 d riluzole treatment was given for postoperative days 1-4 and 5-8, respectively. These results indicate that changes in the expression and glutamate uptake activity of spinal GTs may play a critical role in both the induction and maintenance of neuropathic pain after nerve injury via the regulation of regional glutamate homeostasis, a new mechanism relevant to the pathogenesis of neuropathic pain.

Glutamate levels and transport in cat (Felis catus) area 17 during cortical reorganization following binocular retinal lesions

Glutamate is known to play a crucial role in the topographic reorganization of visual cortex after the induction of binocular central retinal lesions. In this study we investigated the possible involvement of the glial high-affinity Na+/K+-dependent glutamate transporters in cortical plasticity using western blotting and intracortical microdialysis. Basal extracellular glutamate levels and the re-uptake activity for glutamate have been determined by comparing the extracellular glutamate concentration before and during the blockage of glutamate removal from the synaptic cleft with the potent transporter inhibitor l-trans-pyrrolidine-3,4-dicarboxylic acid. In cats with central retinal lesions we observed increased basal extracellular glutamate concentrations together with a decreased re-uptake activity in non-deprived, peripheral area 17, compared with the sensory-deprived, central cortex of the same animal as well as the topographically matching regions of area 17 in normal subjects. Western blotting experiments revealed a parallel decrease in the expression level of the glial glutamate transporter proteins GLT-1 and GLAST in non-deprived cortex compared with sensory-deprived cortex of lesion cats and the corresponding regions of area 17 of normal subjects. This study shows that partial sensory deprivation of the visual cortex affects the removal of glutamate from the synaptic cleft and implicates a role for glial-neuronal interactions in adult brain plasticity.

Amyloid beta protein enhances the clearance of extracellular L-glutamate by cultured rat cortical astrocytes

To explore the impact of Alzheimer's disease amyloid beta protein (Abeta) on astrocyte functions, we investigated the effect of Abeta on glutamate clearance capacity of cultured rat cortical astrocytes. When L-glutamate (50-200 microM) was added to astrocyte cultures and incubated, the extracellular L-glutamate concentration declined with time. The time-dependent decline of extracellular L-glutamate was significantly faster in cultures treated with 10-20 microM Abeta for 24 h than in intact cultures, suggesting that Abeta enhances the L-glutamate clearance capacity of astrocytes. The effect of Abeta was not affected by antioxidants including catalase, propyl gallate or Trolox. Relatively long treatment time (8-48 h) was required for Abeta to exert this effect. Western blot analysis revealed that expression level of the glutamate transporter GLAST was increased by treatment with 10-20 microM Abeta for 8-48 h. These results suggest that Abeta upregulates a glutamate uptake system of astrocytes and enhances the clearance of extracellular L-glutamate.

Amygdaloid kindling in glutamate transporter (GLAST) knockout mice

PURPOSE

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system. We previously reported abnormal glutamate release during seizures after kindling. GLAST and GLT-1 are astrocytic glutamate transporters, highly concentrated in the cerebellum and the telencephalon, respectively. We have investigated whether stages of amygdala kindling in knockout (KO) mice deficient in GLAST are the same as those of wild mice.

METHODS

Electrodes were implanted into the basolateral amygdala, bilaterally, in C57BL/6J mice and GLAST KO mice under anesthesia. Once-daily stimulation was applied on one side through these electrodes, and bilateral EEG recordings were obtained.

RESULTS

The behavioral manifestations of kindling in mice were (a) arrest of behavior, (b) head nodding, (c) forelimb clonus, (d) bilateral forelimb clonus with rearing, (e) tonic generalized convulsion with elevation of tail and falling with generalized tonic convulsion (GTC). Results of kindling: (a) Mutant mice kindled more slowly than wild-type mice (more stimulations were required to reach each stage in mutants); (b) after kindling was completed, additional stimulations induced shorter afterdischarges (ADs) in mutants than in wild-type mice; (c) Interictal epileptic spikes were more frequent in the mutants than in the wild-type mice, both in the stimulated amygdala and in the contralateral amygdala.

CONCLUSIONS

GLAST KO mice, which are missing one type of astrocytic glutamate transporter, demonstrate kindling of an epileptic focus. However, the kindling is significantly slower in the GLAST mutant than in wild-type mice. These findings suggest that glutamate transport by the astrocytic GLAST transporter may have a role in normal epileptogenesis.

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.

Differential expression patterns of three glutamate transporters (GLAST, GLT1 and EAAC1) in the rat main olfactory bulb

Glutamate is the main neurotransmitter in the olfactory bulb. Therefore, glutamate transporters, which regulate the concentration of extracellular glutamate, might play pivotal roles in odor processing. In this study, we examined expressions of three glutamate transporters (GLAST, GLT1 and EAAC1) in the olfactory bulb using in situ hybridization and immunohistochemistry. EAAC1 mRNA was expressed in neurons, such as periglomerular cells, tufted cells, mitral cells and granule cells as shown before in other brain areas. In contrast, GLAST and GLT1 were found in glial cells throughout the olfactory bulb, with intenser expressions in the glomerular layer, external plexiform layer and internal plexiform layer where glutamatergic synapses are concentrated. In addition, using double staining immunohistochemistry we clearly showed that GLAST and GLT1 were expressed in astrocytes. Furthermore, we found that GLAST was also intensely expressed in the subependymal layer where precursor cells exist. These results suggest each glutamate transporter plays its unique role not only in glutamatergic neurotransmission but also in cell differentiation and migration in the olfactory bulb.

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.

Requirement of appropriate glutamate concentrations in the synaptic cleft for hippocampal LTP induction

Although glutamate transporters maintain low extracellular levels of the excitatory neurotransmitter glutamate in the nervous system, little is known about their roles in synaptic plasticity. Here, using knockout mice lacking GLT-1, that is the most abundant glial subtype of glutamate transporters, we showed that long-term potentiation (LTP) induced by tetanic stimulation in mutant mice was impaired in the hippocampal CA1 region. When tetanic stimulation was applied in the presence of low concentrations of an N-methyl-D-aspartate (NMDA) receptor antagonist, the impairment was overcome. Consistent with these results, the increased glutamate in the synaptic cleft of mutant mice preferentially activated NMDA receptors. Furthermore, analyses of mutant mice revealed that the magnitude of NMDA receptor-dependent transient synaptic potentiation during low-frequency stimulation depended on the concentration of glutamate in the synaptic cleft. These findings suggest that GLT-1 plays critical roles in LTP induction, as well as in short-term potentiation, through regulation of extracellular levels of glutamate, which enables appropriate NMDA receptor activation.

Astrocytes protect neurons from nitric oxide toxicity by a glutathione-dependent mechanism

Nitric oxide (NO) contributes to neuronal death in cerebral ischemia and other conditions. Astrocytes are anatomically well positioned to shield neurons from NO because astrocyte processes surround most neurons. In this study, the capacity of astrocytes to limit NO neurotoxicity was examined using a cortical co-culture system. Astrocyte-coated dialysis membranes were placed directly on top of neuronal cultures to provide a removable astrocyte layer between the neurons and the culture medium. The utility of this system was tested by comparing neuronal death produced by glutamate, which is rapidly cleared by astrocytes, and N-methyl-D-aspartate (NMDA), which is not. The presence of an astrocyte layer increased the LD(50) for glutamate by approximately four-fold, but had no effect on NMDA toxicity. Astrocyte effects on neuronal death produced by the NO donors S-nitroso-N-acetyl penicillamine and spermine NONOate were examined by placing these compounds into the medium of co-cultures containing either a control astrocyte layer or an astrocyte layer depleted of glutathione by prior exposure to buthionine sulfoximine. Neurons in culture with the glutathione-depleted astrocytes exhibited a two-fold increase in cell death over a range of NO donor concentrations. These findings suggest that astrocytes protect neurons from NO toxicity by a glutathione-dependent mechanism.

L-glutamate suppresses astrocyte stellation induced by actin breakdown in culture

We have recently found that L-glutamate suppresses morphological changes of astrocytes induced by amyloid beta protein, adenosine 3',5'-cyclic monophosphate or phorbol ester in culture. To test the possibility that L-glutamate affects organization of the cytoskeleton, we investigated its effect on morphological changes induced by disruption of actin filaments with cytochalasin B. Cultured rat cortical astrocytes exhibited flat, polygonal morphology in the absence of stimulation, and changed into process-bearing stellate cells following treatment with cytochalasin B (50 microM). L-Glutamate strongly suppressed the stellation induced by cytochalasin B. The effect of L-glutamate was mimicked by D- and L-aspartate and transportable glutamate uptake inhibitors. These results suggest that glutamate transporter activity leads to cytoskeletal actin organization in astrocytes.

Glial transporters for glutamate, glycine and GABA I. Glutamate transporters

The termination of chemical neurotransmission in the CNS involves the rapid removal of neurotransmitter from synapses by specific transport systems. Such mechanism operates for the three major amino acid neurotransmitters glutamate, gamma-aminobutyric acid (GABA) and glycine. To date, five different high-affinity Na(+)-dependent glutamate (Glu) transporters have been cloned: GLT1, GLAST, EAAC1, EAAT4 and EAAT5. The first two are expressed mainly by glial cells, and seem to be the predominant Glu transporters in the brain. A major function of Glu uptake in the nervous system is to prevent extracellular Glu concentrations from raising to neurotoxic levels in which glial transporters seem to play a critical role in protecting neurons from glutamate-induced excitotoxicity. Under particular conditions, glial GluTs have been shown to release Glu by reversal of activity, in a Ca(2+)--and energy-independent fashion. Furthermore, an activity of these transporters as ion channels or transducing units coupled to G-proteins has recently been reported. The localization, stoichiometry, and regulation of glial GluTs are outlined, as well as their possible contributions to nervous system diseases as ALS, AD and ischemic damage.

Expression of the high-affinity glutamate transporter EAAT4 in mammalian cerebral cortex

RT-PCR, immunocytochemistry and Western blotting were used to study the expression of the glutamate transporter EAAT4 in the cerebral cortex of cat and mouse. By means of RT-PCR we were able to detect EAAT4 mRNA in the cerebral cortex of both species. Sequencing ensured the specificity of the amplified fragment. Immunocytochemistry and Western blotting enabled us to localize EAAT4 protein in cat and mouse cerebral cortex. Intense EAAT4 immunoreactivity was found in the soma and dendrites of neurons mainly of layers II, III and V. For both species, the signal in the cerebellum was very intense and confined to the molecular and Purkinje cell layer.

Possible linkage between glutamate transporter and mitogen-activated protein kinase cascade in cultured rat cortical astrocytes

The mitogen-activated protein kinases (MAPKs) play a pivotal role in the mediation of cellular responses to a variety of signalling molecules. In the present study, we investigated possible linkage between glutamate signalling and the MAPK cascade in cultured rat cortical astrocytes. Exposure of the cells to L-glutamate (100-1000 microM) resulted in an increase in phosphorylated p44/42 MAPK (ERK1/2) in a concentration- and time-dependent manner. The glutamate-induced ERK1/2 phosphorylation was blocked by U0126 and PD98059, specific inhibitors of the MAPK-activating enzyme MEK. Furthermore, L-glutamate-induced ERK1/2 phosphorylation was not mimicked by glutamate receptor agonists and was not blocked by glutamate receptor antagonists. In contrast, the effect of L-glutamate was mimicked by D- and L-aspartate and transportable glutamate uptake inhibitors. These results suggest that the MEK/ERK cascade is activated by a mechanism related to glutamate transporters. We propose that the glutamate transporter functions as a receptor transmitting extracellular glutamate signal to intracellular messengers.

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.