Molecular cloning of human brain glutamate/aspartate transporter II

Astrocyte Bioenergetics and Major Psychiatric Disorders

Ongoing research continues to add new elements to the emerging picture of involvement of astrocyte energy metabolism in the pathophysiology of major psychiatric disorders, including schizophrenia, mood disorders, and addictions. This review outlines what is known about the energy metabolism in astrocytes, the most numerous cell type in the brain, and summarizes the recent work on how specific perturbations of astrocyte bioenergetics may contribute to the neuropsychiatric conditions. The role of astrocyte energy metabolism in mental health and disease is reviewed on the organism, organ, and cell level. Data arising from genomic, metabolomic, in vitro, and neurobehavioral studies is critically analyzed to suggest future directions in research and possible metabolism-focused therapeutic interventions.

Change in gene expression levels of GABA, glutamate and neurosteroid pathways due to acoustic trauma in the cochlea

The characteristic feature of noise-induced hearing loss (NIHL) is the loss or malfunction of the outer hair cells (OHC) and the inner hair cells (IHC) of the cochlea. 90-95% of the spiral ganglion neurons, forming the cell bodies of cochlear nerve, synapse with the IHCs. Glutamate is the most potent excitatory neurotransmitter for IHC-auditory nerve synapses. Excessive release of glutamate in response to acoustic trauma (AT), may cause excitotoxicity by causing damage to the spiral ganglion neurons (SGN) or loss of the spiral ganglion dendrites, post-synaptic to the IHCs. Another neurotransmitter, GABA, plays an important role in the processing of acoustic stimuli and central regulation after peripheral injury, so it is potentially related to the regulation of hearing function and sensitivity after noise. The aim of this study is to evaluate the effect of AT on the expressions of glutamate excitotoxicity, GABA inhibition and neurosteroid synthesis genes.We exposed 24 BALB/c mice to AT. Controls were sacrificed without exposure to noise, Post-AT(1) and Post-AT(15) were sacrificed on the 1st and 15th day, respectively, after noise exposure. The expressions of various genes playing roles in glutamate, GABA and neurosteroid pathways were compared between groups by real-time PCR.Expressions of Cyp11a1, Gls, Gabra1, Grin2b, Sult1a1, Gad1, and Slc1a2 genes in Post-AT(15) mice were significantly decreased in comparison to control and Post-AT(1) mice. No significant differences in the expression of Slc6a1 and Slc17a8 genes was detected.These findings support the possible role of balance between glutamate excitotoxicity and GABA inhibition is disturbed during the post AT days and also the synthesis of some neurosteroids such as pregnenolone sulfate may be important in this balance.

Phosphoproteomic analysis reveals Akt isoform-specific regulation of cytoskeleton proteins in human temporal lobe epilepsy with hippocampal sclerosis

Akt is one of the most important downstream effectors of phosphatidylinositol 3-kinase/mTOR pathway. Hyperactivation and expression of this pathway are seen in a variety of neurological disorders including human temporal lobe epilepsy with hippocampal sclerosis (TLE-HS). Nevertheless, the expression and activation profiles of the Akt isoforms, Akt1, Akt2, and Akt3 and their functional roles in human TLE-HS have not been studied. We examined the protein expression and activation (phosphorylation) patterns of Akt and its isoforms in human hippocampal tissue from TLE and non-TLE patients. A phosphoproteomic approach followed by interactome analysis of each Akt isoform was used to understand protein-protein interactions and their role in TLE-HS pathology. Our results demonstrated activation of the Akt/mTOR pathway as well as activation of Akt downstream substrates like GSK3β, mTOR, and S6 in TLE-HS samples. Akt1 isoform levels were significantly increased in the TLE-HS samples as compared to the non-TLE samples. Most importantly, different isoforms were activated in different TLE-HS samples, Akt2 was activated in three samples, Akt2 and Akt1 were simultaneously activated in one sample and Akt3 was activated in two samples. Our phosphoproteomic screen across six TLE-HS samples identified 183 proteins phosphorylated by Akt isoforms, 29 of these proteins belong to cytoskeletal modification. Also, we were able to identify proteins of several other classes involved in glycolysis, neuronal development, protein folding and excitatory amino acid transport functions as Akt substrates. Taken together, our data offer clues to understand the role of Akt and its isoforms in underlying the pathology of TLE-HS and further, modulation of Akt/mTOR pathway using Akt isoforms specific inhibitors may offer a new therapeutic window for treatment of human TLE-HS.

NLRX1 Enhances Glutamate Uptake and Inhibits Glutamate Release by Astrocytes

Uptake of glutamate from the extracellular space and glutamate release to neurons are two major processes conducted by astrocytes in the central nervous system (CNS) that protect against glutamate excitotoxicity and strengthen neuronal firing, respectively. During inflammatory conditions in the CNS, astrocytes may lose one or both of these functions, resulting in accumulation of the extracellular glutamate, which eventually leads to excitotoxic neuronal death, which in turn worsens the CNS inflammation. NLRX1 is an innate immune NOD-like receptor that inhibits the major inflammatory pathways. It is localized in the mitochondria and was shown to inhibit cell death, enhance ATP production, and dampen oxidative stress. In the current work, using primary murine astrocyte cultures from WT and Nlrx1^-/- mice, we demonstrate that NLRX1 potentiates astrocytic glutamate uptake by enhancing mitochondrial functions and the functional activity of glutamate transporters. Also, we report that NLRX1 inhibits glutamate release from astrocytes by repressing Ca²⁺-mediated glutamate exocytosis. Our study, for the first time, identified NLRX1 as a potential regulator of glutamate homeostasis in the CNS.

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca²⁺ release from the astrocytes’ intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

Differential expression of glutamate transporters in cerebral cortex of paraoxon-treated rats

Glutamatergic system is involved in pathological effects of organophosphorus (OP) compounds. We aimed to determine in vivo effects of paraoxon, the bioactive metabolite of parathion, on the expression of glutamate transporters as well as Bax and Bcl2 in rat cerebral cortex. Male Wistar rats received an intraperitoneal (i.p.) injection of one of three doses of paraoxon (0.3, 0.7, or 1mg/kg) or corn oil as vehicle (1ml/kg). After 4 or 18h, cerebral cortices were dissected out and used for quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blot assays to measure mRNA and protein levels, respectively. The cortical glial glutamate transporters (GLAST and GLT-1) were up-regulated in animals treated with 0.7mg/kg of paraoxon, but down-regulated in 1mg/kg group. Neuronal glutamate transporter (EAAC1) was unchanged in 0.7mg/kg treated rats, while reduced in 1mg/kg group. No significant difference was found in the mRNA and protein expression of EAAC1 in animals intoxicated with 0.3mg/kg of paraoxon. Paraoxon (1mg/kg) resulted in an up-regulation of Bax and down-regulation of Bcl2 mRNA levels in the rat cerebral cortex. These results indicate that paraoxon can differentially regulate expression of glutamate transporters at mRNA and protein levels in the cerebral cortex. Changes in the expression of glutamate transporters are closely related to paraoxon-induced seizure activity.

Alterations in mRNA and protein expression of glutamate transporters in rat hippocampus after paraoxon exposure

Organophosphates affect brain function through a variety of mechanisms beyond their shared role as cholinesterase inhibitors. The aim of the current study was to investigate the changes in the expression of glial (GLAST and GLT-1) and neuronal (EAAC1) glutamate transporters at mRNA and protein levels in paraoxon-treated rat hippocampus. Adult male Wistar rats were intraperitoneally treated with either vehicle (corn oil) or one of three dosages of paraoxon (0.3, 0.7 or 1mg/kg). After 4 or 18h, both hippocampi of each rat were collected to detect mRNA and protein expression of glutamate transporters using the quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blotting, respectively. Animals treated with 0.3mg/kg paraoxon showed no difference in mRNA and protein levels of the glutamate transporters when compared with control group. At 4h after exposure with 0.7 and 1mg/kg paraoxon, the expression of GLAST and GLT-1 increased at mRNA and protein levels and remained elevated after 18h. No difference in the expression of EAAC1 at mRNA and protein levels was observed in any paraoxon-treated groups compared with the control group. This study showed an increased expression of glial (GLAST and GLT-1), but not neuronal (EAAC1) glutamate transporters, in adult rat hippocampus following administration of convulsive dosages of paraoxon. These suggest a protective and compensatory adaptation for effective uptake of glutamate in hippocampus induced by paraoxon and thus attenuating seizure activity.

Clinical aspects of urea cycle dysfunction and altered brain energy metabolism on modulation of glutamate receptors and transporters in acute and chronic hyperammonemia

In living organisms, nitrogen arise primarily as ammonia (NH3) and ammonium (NH4(+)), which is a main component of the nucleic acid pool and proteins. Although nitrogen is essential for growth and maintenance in animals, but when the nitrogenous compounds exceeds the normal range which can quickly lead to toxicity and death. Urea cycle is the common pathway for the disposal of excess nitrogen through urea biosynthesis. Hyperammonemia is a consistent finding in many neurological disorders including congenital urea cycle disorders, reye's syndrome and acute liver failure leads to deleterious effects. Hyperammonemia and liver failure results in glutamatergic neurotransmission which contributes to the alteration in the function of the glutamate-nitric oxide-cGMP pathway, modulates the important cerebral process. Even though ammonia is essential for normal functioning of the central nervous system (CNS), in particular high concentrations of ammonia exposure to the brain leads to the alterations of glutamate transport by the transporters. Several glutamate transporters have been recognized in the central nervous system and each has a unique physiological property and distribution. The loss of glutamate transporter activity in brain during acute liver failure and hyperammonemia is allied with increased extracellular brain glutamate concentrations which may be conscientious for the cerebral edema and ultimately cell death.

GLT-1 transporter: an effective pharmacological target for various neurological disorders

L-Glutamate is the predominant excitatory neurotransmitter in the central nervous system (CNS) and is directly and indirectly involved in a variety of brain functions. Glutamate is released in the synaptic cleft at a particular concentration that further activates the various glutaminergic receptors. This concentration of glutamate in the synapse is maintained by either glutamine synthetase or excitatory amino acid proteins which reuptake the excessive glutamate from the synapse and named as excitatory amino acid transporters (EAATs). Out of all the subtypes GLT-1 (glutamate transporter 1) is abundantly distributed in the CNS. Down-regulation of GLT-1 is reported in various neurological diseases such as, epilepsy, stroke, Alzheimer's disease and movement disorders. Therefore, positive modulators of GLT-1 which up-regulate the GLT-1 expression can serve as a potential target for the treatment of neurological disorders. GLT-1 translational activators such as ceftriaxone are found to have significant protective effects in ALS and epilepsy animal models, suggesting that this translational activation approach works well in rodents and that these compounds are worth further pursuit for various neurological disorders. This drug is currently in human clinical trials for ALS. In addition, a thorough understanding of the mechanisms underlying translational regulation of GLT-1, such as identifying the molecular targets of the compounds, signaling pathways involved in the regulation, and translational activation processes, is very important for this novel drug-development effort. This review mainly emphasizes the role of glutamate and its transporter, GLT-1 subtype in excitotoxicity. Further, recent reports on GLT-1 transporters for the treatment of various neurological diseases, including a summary of the presumed physiologic mechanisms behind the pharmacology of these disorders are also explained.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Non-neuronal cell transplantation as a possible therapeutic approach for epilepsy treatment

This is a letter to the editor about a recently published article. This letter provides evidence that suggest non-neuronal cell transplantation merits investigation as a possible novel therapeutic approach for epilepsy treatment.

The role of excitatory amino acid transporters in cerebral ischemia

Glutamate is an excitatory neurotransmitter that plays a major role in the pathogenesis of ischemia brain injury. The regulation of glutamate neurotransmission is carried out by excitatory amino acid transporters (EAATs) that act through reuptake of glutamate into cells. EAATs may also release glutamate into the extracellular space in a calcium-independent manner during ischemia and dysfunction of EAATs is specifically implicated in the pathology of cerebral ischemia. Recent studies show that up-regulation of EAAT2 provides neuroprotection during ischemic insult. This review summarizes current knowledge regarding the role of EAATs in cerebral ischemia.

Rethinking a drug treatment failure on a traditional ALS target

In a recent issue of Experimental Neurology, Boston-Howes and colleagues used an assay of glutamate transport to screen 1040 FDA approved drugs in an attempt to identify compounds that would increase glutamate transport, a central function of astrocytes, and a potential biological target for neuroprotection for a variety of neurological disorders. They identified the compound nordihydroguaiaretic acid (NDGA) as a particularly good candidate for inducing glutamate transport. Pharmacological increases in glutamate transport could have a number of potential applications to diseases of the nervous system where glutamate excitotoxicity is thought to be a contributing factor to pathogenesis including Amyotrophic Lateral Sclerosis, Alzheimer's disease, Parkinson's disease, stroke, and epilepsy among others. They chose to test this compound in a model of Amyotrophic Lateral Sclerosis (ALS)--the SOD1G93A mouse. In both human ALS and rodent models of the disease, glutamate excitotoxicity and abnormalities in glutamate transporter biology more specifically, have been implicated in ALS disease propagation. Interestingly, while the authors nicely demonstrate that NDGA has a biological effect on glutamate transport in normal (wild type) central nervous system tissues both in vitro and in vivo, it was the somewhat unexpected (and often overlooked) findings in the ALS mouse model that makes this manuscript notable and suggests that rethinking translational approaches to drug therapies in ALS may be on the horizon.

EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and its dysregulation has been associated with multiple psychiatric and neurological disorders. However, investigation of this molecule has been complicated by its complex pattern of alternative splicing, including three coding isoforms and multiple 5'- and 3'-UTRs that may have a regulatory function. It is likely that these sequences permit modulation of EAAT2 expression with spatial, temporal and or activity-dependent specificity; however, few studies have attempted to delineate the function of these sequences. Additionally, there are problems with the use of antibodies to study protein localization, possibly due to posttranslational modification of critical amino acid residues. This review describes what is currently known about the regulation of EAAT2 mRNA and protein isoforms and concludes with a summary of studies showing dysregulation of EAAT2 in psychiatric and neurological disorders. EAAT2 has been either primarily or secondarily implicated in a multitude of neuropsychiatric diseases in addition to the normal physiology of learning and memory. Thus, this molecule represents an intriguing therapeutic target once we improve our understanding of how it is regulated under normal conditions.

Disruption of excitatory amino acid transporters in brains of SIV-infected rhesus macaques is associated with microglia activation

Glutamate-mediated neurodysfunction in human immunodeficiency virus (HIV) infection has been primarily suggested by in vitro studies. The regulation of glutamatergic neurotransmission in inflammation is a complex interaction between activation of immune mediators and adaptive changes in the functional elements of the glutamatergic synapse. We have used simian immunodeficiency virus (SIV)-infected macaques to answer the questions (i) whether perturbation of glutamate neurotransmission is evident during progression of immunodeficiency disease and (ii) what are the mechanisms underlying this impairment. Disease progression in SIV-infected macaques both in the periphery and in the brain was documented by clinical and general pathological examination, plasma and brain viral RNA load, T-cell analysis and brain histopathology. We report for the first time, disruption of excitatory amino acid transporters (EAATs), the cardinal glutamate clearing system, during SIV infection and a dramatic loss of EAATs associated with development of rapid acquired immunodeficiency syndrome (AIDS). EAATs impairment was correlated with activation status of microglia. Our data support the glutamate hypothesis for the development of HIV dementia and suggest that the pathogenetic mechanism for the neurodysfunction is the impairment of glutamate clearing which occurs in the stage of AIDS and which is associated with activated microglia.

Ischemic preconditioning reveals that GLT1/EAAT2 glutamate transporter is a novel PPARgamma target gene involved in neuroprotection

Excessive levels of extracellular glutamate in the nervous system are excitotoxic and lead to neuronal death. Glutamate transport, mainly by glutamate transporter GLT1/EAAT2, is the only mechanism for maintaining extracellular glutamate concentrations below excitotoxic levels in the central nervous system. We recently showed that neuroprotection after experimental ischemic preconditioning (IPC) involves, at least partly, the upregulation of the GLT1/EAAT2 glutamate transporter in astrocytes, but the mechanisms were unknown. Thus, we decided to explore whether activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR) gamma, known for its antidiabetic and antiinflammatory properties, is involved in glutamate transport. First, we found that the PPARgamma antagonist T0070907 inhibits both IPC-induced tolerance and reduction of glutamate release after lethal oxygen-glucose deprivation (OGD) (70.1%+/-3.4% versus 97.7%+/-5.2% of OGD-induced lactate dehydrogenase (LDH) release and 61.8%+/-5.9% versus 85.9%+/-7.9% of OGD-induced glutamate release in IPC and IPC+T0070907 1 mumol/L, respectively, n=6 to 12, P<0.05), as well as IPC-induced astrocytic GLT-1 overexpression. IPC also caused an increase in nuclear PPARgamma transcriptional activity in neurons and astrocytes (122.1%+/-8.1% and 158.6%+/-22.6% of control PPARgamma transcriptional activity, n=6, P<0.05). Second, the PPARgamma agonist rosiglitazone increased both GLT-1/EAAT2 mRNA and protein expression and [(3)H]glutamate uptake, and reduced OGD-induced cell death and glutamate release (76.3%+/-7.9% and 65.5%+/-15.1% of OGD-induced LDH and glutamate release in rosiglitazone 1 mumol/l, respectively, n=6 to 12, P<0.05). Finally, we have identified six putative PPAR response elements (PPREs) in the GLT1/EAAT2 promoter and, consistently, rosiglitazone increased fourfold GLT1/EAAT2 promoter activity. All these data show that the GLT1/EAAT2 glutamate transporter is a target gene of PPARgamma leading to neuroprotection by increasing glutamate uptake.

Early rapid rise in EAAT2 expression follows the period of maximal seizure susceptibility in human brain

Seizures are relatively common in the first weeks of life and can have lasting effects on brain development due to glutamate excitotoxicity. The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and mice with this gene deleted die from seizures. Therefore, we reasoned that developmental changes in the expression of EAAT2 might correlate with the period of increased susceptibility to seizures in humans, reflecting a changing vulnerability to excitotoxic insults. Expression levels of eight splice forms of EAAT2 were measured using quantitative RT-PCR from human prefrontal cortex and hippocampus at 1-2 months, 1-2 years, 8 years, 15-16 years, and 18-22 years of age. There was a significant increase in expression of most isoforms between 1-2 months and 1-2 years with isoform-specific patterns after that period. The increase in EAAT2 expression during the first 2 years of life corresponds to a period of maximal synapse formation and other changes in the glutamatergic system such as increased NMDA receptor expression. Moreover, the low expression of EAAT2 in the first months of life corresponds to the period of maximum susceptibility to seizures.

Translational control of glial glutamate transporter EAAT2 expression

Glutamate is the major excitatory neurotransmitter in the central nervous system. Its activity is carefully modulated in the synaptic cleft by glutamate transporters. The glial glutamate transporter EAAT2 is the main mediator of glutamate clearance. Reduced EAAT2 function could lead to accumulation of extracellular glutamate, resulting in a form of cell death known as excitotoxicity. In amyotrophic lateral sclerosis and Alzheimer disease, EAAT2 protein levels are significantly decreased in affected areas. EAAT2 mRNA levels, however, remain constant, indicating that alterations in EAAT2 expression are due to disturbances at the post-transcriptional level. In the present study, we found that some EAAT2 transcripts contained 5'-untranslated regions (5'-UTRs) greater than 300 nucleotides. The mRNAs that bear long 5'-UTRs are often regulated at the translational level. We tested this possibility initially in a primary astrocyte line that constantly expressed an EAAT2 transcript containing the 565-nt 5'-UTR and found that translation of this transcript was regulated by many extracellular factors, including corticosterone and retinol. Moreover, many disease-associated insults affected the efficiency of translation of this transcript. Importantly, this translational regulation of EAAT2 occurred in vivo (i.e. both in primary cortical neurons-astrocytes mixed cultures and in mice). These results indicate that expression of EAAT2 protein is highly regulated at the translational level and also suggest that translational regulation may play an important role in the differential EAAT2 protein expression under normal and disease conditions.

A quantitative regional expression profile of EAAT2 known and novel splice variants reopens the question of aberrant EAAT2 splicing in disease

The glutamate transporter 1 (GLT1) in rodents, or EAAT2 in humans, is alternatively spliced in a complex manner including the use of multiple 5' and 3' untranslated regions and several coding variants. We used quantitative RT-PCR to profile these splice variants in human and rat brain. We also used RT-PCR and Northern blotting to demonstrate that a novel isoform of GLT1b has an approximately 11kb 3' UTR extending through intron 9, exon 10 and approximately 5kb into the 3' untranslated region of GLT1. However, our most important finding concerns an aberrant transcript lacking exon 9, which contains a motif permitting translocation from the endoplasmic reticulum. This variant had previously been associated with amyotrophic lateral sclerosis until several groups reported high levels in normal brain tissue. In contrast, our data shows that this aberrant transcript is present at 0.1-0.2% of the major EAAT2 isoforms.

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.

Targets of cytoprotection in acute ischemic stroke: present and future

Although the management of stroke has improved remarkably over the last decade due mainly to the advent of thrombolysis, most neuroprotective agents, although successful in animal studies, have failed in humans. Our increasing knowledge concerning the ischemic cascade is leading to a considerable development of pharmacological tools suggesting that each step of this cascade might be a target for cytoprotection. Glutamate has long been recognized to play key roles in the pathophysiology of ischemia. However, although some trials are still ongoing, the results from several completed trials with drugs interfering with the glutamatergic pathway have been disappointing. Regarding the inhibition of glutamate release as a possible target for cytoprotection, it might be afforded either by decreasing glutamate efflux or by increasing glutamate uptake. In this context, it has been shown that glutamate transport is the primary and only mechanism for maintaining extracellular glutamate concentrations below excitotoxic levels. This transport is executed by the five high-affinity, sodium-dependent plasma membrane glutamate transporters. Among them, the transporter EAAT2 is responsible for up to 90% of all glutamate transport. We will discuss the effect of different neuroprotective tools (membrane stabilizers or endogenous neuroprotection) affecting glutamate efflux and/or expression of EAAT2. We will also describe the finding of a novel polymorphism in the EAAT2 promoter region which could be responsible for differences in both gene function and regulation under pathological conditions such as cerebral ischemia, and which might well account for the failure of glutamate antagonists in the clinical practice. These results may possess important therapeutic implications in the management of patients at risk of ischemic events, since it has been demonstrated that those patients with progressing stroke have higher plasma concentrations of glutamate which remain elevated up to 24 h when compared to the levels in patients without neurological deterioration.

A polymorphism in the EAAT2 promoter is associated with higher glutamate concentrations and higher frequency of progressing stroke

It remains unclear why some individuals are susceptible to excitotoxicity after stroke. A possible explanation is impaired glutamate uptake. We have found a highly prevalent polymorphism in the promoter of the glutamate transporter EAAT2 gene that abolishes a putative regulatory site for activator protein–2 (AP-2) and creates a new consensus binding site for the repressor transcription factor GC-binding factor 2 (GCF2). The mutant genotype is associated with increased plasma glutamate concentrations and with a higher frequency of early neurological worsening in human stroke. After transfection into astrocytes, the mutant promoter was not activated by AP-2 and was effectively repressed by GCF2, and its activity in the presence of GCF2 was reduced when compared with the AP-2–cotransfected wild-type promoter. We also show that GCF2 is expressed in ischemic rat brain, suggesting that decreased glutamate uptake occurs in individuals carrying the mutation after stroke. These findings may explain individual susceptibility to excitotoxic damage after stroke as well as the failure of glutamate antagonists in those patients without this polymorphism.

Regulation and dysregulation of glutamate transporters

Glutamate is the primary excitatory neurotransmitter in the central nervous system. During synaptic activity, glutamate is released into the synaptic cleft and binds to glutamate receptors on the pre- and postsynaptic membrane as well as on neighboring astrocytes in order to start a number of intracellular signaling cascades. To allow for an efficient signaling to occur, glutamate levels in the synaptic cleft have to be maintained at very low levels. This process is regulated by glutamate transporters, which remove excess extracellular glutamate via a sodium-potassium coupled uptake mechanism. When extracellular glutamate levels rise to about normal, glutamate overactivates glutamate receptors, triggering a multitude of intracellular events in the postsynaptic neuron, which ultimately results in neuronal cell death. This phenomenon is known as excitotoxicity and is the underlying mechanisms of a number of neurodegenerative diseases. A dysfunction of the glutamate transporter is thought to contribute to cell death during excitotoxicity. Therefore, efforts have been made to understand the regulation of glutamate transporter function. Transporter activity can be regulated in different ways, including through gene expression, transporter protein targeting and trafficking and through posttranslational modifications of the transporter protein. The identification of these mechanisms has helped to understand the role of glutamate transporters during pathology and will aid in the development of therapeutic strategies with the transporter as a desirable target.

Thrombin-stimulated glutamate uptake in human platelets is predominantly mediated by the glial glutamate transporter EAAT2

Glutamate toxicity has been implicated in the pathogenesis of various neurological diseases. Glial glutamate transporters play a key role in the regulation of extracellular glutamate levels in the brain by removing glutamate from the extracellular fluid. Since human blood platelets possess an active glutamate uptake system, they have been used as a peripheral model of glutamate transport in the central nervous system (CNS). The present study is aimed at identifying the glutamate transporter on blood platelets, and to asses the influence of platelet activation on glutamate uptake. Platelets from healthy donors showed Na+-dependent glutamate uptake (Km, 3.5+/-0.9 microM; Vmax, 2.8+/-0.2 pmol glutamate/75 x 10(6)platelets/30 min), which could be blocked dose-dependently by the EAAT specific inhibitors DL-threo-E-benzyloxyaspartate (TBOA), L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) and high concentrations of the EAAT2 inhibitor dihydrokainate (DHK). Analysis of platelet homogenates on Western blots showed EAAT2 as the predominant glutamate transporter. Platelet activation by thrombin caused an increase in glutamate uptake, which could be inhibited by TBOA and the EAAT2 inhibitor DHK. Kinetic analysis showed recruitment of new transporters to the membrane. Indeed, Western blot analysis of subcellular fractions revealed that alpha-granules, which fuse with the membrane upon thrombin stimulation, contained significant EAAT2 immunoreactivity. Inhibition of the second messengers involved in alpha-granule secretion (protein kinase C, phosphatidylinositol-3-kinase) inhibited thrombin-stimulated uptake, but not basal uptake. These data show that the glial EAAT2 is the predominant glutamate transporter on blood platelets and suggest, that thrombin increases glutamate uptake capacity by recruiting new transporters (EAAT2) from alpha-granules.

Multiple 5′-splice variants of the rat glutamate transporter-1

In most brain areas, uptake of extracellular glutamate predominantly occurs through the glutamate transporter subtype, glutamate transporter-1 (GLT-1), which is enriched in astroglia. Here, we report the identification of five splice variants of the 5'-leader sequence of rat GLT-1 which contain varying numbers of upstream open reading frames and encode putative GLT-1 proteins with two distinct N-terminal modifications. We further demonstrate that the identified rat 5'-GLT-1 splice variants are expressed in a brain region-specific manner. Our findings point to potential influences of RNA splicing on glial glutamate transport in the intact and injured rat brain.

Distribution of Glutamate Transporters in the Human Placenta

Glutamate metabolism is known to be important for growth and development of the human fetus. The glutamate transporters EAAT1, EAAT2 and EAAT3 are key components of the glutamate-glutamine cycle and responsible for active transport of glutamate over the cell membrane. The placenta is thought to regulate glutamate transport during fetal development. Glutamate transporters have been found in placentae of rats, but their distribution in the human placenta is unknown. Therefore, the distribution of glutamate transporters EAAT1, EAAT2 and EAAT3 were analysed in the human placenta during normal pregnancies ending between 8 and 40 weeks of gestation and in placentae of intrauterine growth restricted infants with gestational ages between 28 and 35 weeks of pregnancy. Using immunohistochemistry, EAAT1 expression was found in the syncytiotrophoblast layer, while EAAT2 was detected in the syncytiotrophoblast layer and in endothelial cells of about 5 per cent of all fetal blood vessels. EAAT3 was observed in the endothelium of the fetal blood vessels in all placentae examined. However, expression was also found in the syncytio- and the cytotrophoblast layer of the fetal villi at 8 weeks of gestational age. The expression patterns of EAAT1, EAAT2 and EAAT3 suggest involvement in active transport of glutamate between the fetal and maternal blood circulation. No differences were found in the distribution of the glutamate transporters between control and IUGR placentae. Our data show specific localization of EAAT1, EAAT2 and EAAT3 in the human placenta during development.

Glutamate transporters: animal models to neurologic disease

Glutamate is the primary excitatory amino acid neurotransmitter in the central nervous system and its activity is carefully modulated in the synaptic cleft by glutamate transporters. A number of glutamate transporters have been identified in the central nervous system and each has a unique physiologic property and distribution. Glutamate transporter dysfunction may either be an initiating event or part of a cascade leading to cellular dysfunction and ultimately cell death. Animal models of glutamate transporter dysfunction have revealed a significant role for these proteins in pathologic conditions such as neurodegenerative diseases, epilepsy, stroke, and central nervous system tumors. Recent work has focused on glutamate transporter biology in human diseases with an emphasis on how manipulation of these transporter proteins may lead to therapeutic interventions in neurologic disease.

Glutamate transporter expression and function in human glial progenitors

Glutamate is the major neurotransmitter of the brain, whose extracellular levels are tightly controlled by glutamate transporters. Five glutamate transporters in the human brain (EAAT1-5) are present on both astroglia and neurons. We characterize the profile of three different human astroglial progenitors in vitro: human glial restricted precursors (HGRP), human astrocyte precursors (HAPC), and early-differentiated astrocytes. EAAT 1, EAAT3, and EAAT4 are all expressed in GRPs with a subsequent upregulation of EAAT1 following differentiation of GRPs into GRP-derived astrocytes in the presence of bone morphogenic protein (BMP-4). This corresponds to a significant increase in the glutamate transport capacity of these cells. EAAT2, the transporter responsible for the bulk of glutamate transport in the adult brain, is not expressed as a full-length protein, nor does it appear to have functional significance (as determined by the EAAT2 inhibitor dihydrokainate) in these precursors. A splice variant of EAAT2, termed EAAT2b, does appear to be present in low levels, however. EAAT3 and EAAT4 expression is reduced as glial maturation progresses both in astrocyte precursors and early-differentiated astrocytes and is consistent with their role in adult tissues as primarily neuronal glutamate transporters. These human glial precursors offer several advantages as tools for understanding glial biology because they can be passaged extensively in the presence of mitogens, afford the potential to study the temporal changes in glutamate transporter expression in a tightly controlled fashion, and are cultured in the absence of neuronal coculture, allowing for the independent study of astroglial biology.

Altered expression of the glutamate transporter EAAT2b in neurological disease

Functional studies suggest that up to 95% of all glutamate transport is handled by the glutamate transporter EAAT2. Amino and C-terminal antibodies demonstrate that under normal conditions EAAT2 is specific to astrocytes. A truncated splice variant of EAAT2, known as EAAT2b, also has been identified in astrocytes and some neurons. In vitro studies suggest EAAT2b transports glutamate similar to EAAT2, although the contribution of EAAT2b to normal clearance of extracellular glutamate is unknown. To investigate EAAT2b biology in pathological conditions, we examined the cellular and regional distribution of EAAT2b in amyotrophic lateral sclerosis. Using epitope-specific, affinity purified antibodies, we found that EAAT2b tissue levels were increased by more than twofold in amyotrophic lateral sclerosis motor cortex, whereas EAAT2 levels were decreased by up to 95%. EAAT2b distribution in normal human cortex was largely confined to the neuropil-like EAAT2, with occasional faint neuronal expression. In contrast, amyotrophic lateral sclerosis motor cortex had an obvious qualitative increase in neuropil EAAT2b staining and a drastic increase in neuronal soma and dendritic EAAT2b immunostaining. Despite these increases in EAAT2b immunostaining, functional transporter studies demonstrated a large loss of EAAT2 function. These studies clearly document altered regulation and splicing of the dominant glutamate transporter EAAT2 under conditions of neurological stress.

Neuronal glutamate transporter EAAT4 is expressed in astrocytes

High-affinity excitatory amino acid transporters (EAATs) are essential to terminate glutamatergic neurotransmission and to prevent excitotoxicity. To date, five distinct EAATs have been cloned from animal and human tissues: GLAST (EAAT1), GLT-1 (EAAT2), EAAC1 (EAAT3), EAAT4, and EAAT5. EAAT1 and EAAT2 are commonly known as glial glutamate transporters, whereas EAAT3, EAAT4, and EAAT5 are neuronal. EAAT4 is largely expressed in cerebellar Purkinje cells. In this study, using immunohistochemistry and Western blotting, we found that EAAT4-like immunoreactivity (ir) is enriched in the spinal cord and forebrain. Double-labeled fluorescent immunostaining and confocal image analysis indicated that EAAT4-like ir colocalizes with an astrocytic marker, glial fibrillary acidic protein (GFAP). The astrocytic localization of EAAT4 was further confirmed in astrocyte cultures by double-labeled fluorescent immunocytochemistry and Western blotting. Reverse transcriptase-polymerase chain reaction analysis demonstrated mRNA expression of EAAT4 in astrocyte cultures. Sequencing confirmed the specificity of the amplified fragment. These results demonstrate that EAAT4 is expressed in astrocytes. This astrocytic localization of neuronal EAAT4 may reveal a new function of EAAT4 in the central nervous system.

The expression of the glutamate re-uptake transporter excitatory amino acid transporter 1 (EAAT1) in the normal human CNS and in motor neurone disease: an immunohistochemical study

A monoclonal antibody to excitatory amino acid transporter 1 (EAAT1) has been generated which robustly stains paraffin-embedded, formaldehyde-fixed as well as snap-frozen human post-mortem brain tissue. We have used this antibody to map the distribution of EAAT1 throughout normal human CNS tissue. In addition this antibody has been used to perform a semi-quantitative immunohistochemical analysis of the expression of EAAT1 in motor cortex and cervical cord tissue taken from motor neurone disease cases (n=17) and neurologically normal controls (n=12). By comparing the relative optical density measurements of identical regions of motor cortex and cervical spinal cord an increase in the expression levels of EAAT1 was observed in motor neurone disease tissue compared to the control tissue and in both motor cortex and cervical spinal cord (9-17% and 13-33% increases respectively). EAAT1 was observed to be the most abundant transporter in more "caudal" brain regions such as the diencephalon and brainstem and its expression in other regions was frequently more uniform than that of EAAT2. In the motor cortex, EAAT1 immunoreactivity was present in all grey matter laminae, with some staining of individual astrocytes in the white matter. In spinal cord, EAAT1 immunoreactivity was strongest in the substantia gelatinosa. In the ventral horn, motor neurones were surrounded with a dense rim of perisomatic EAAT1 immunoreactivity, and the neuropil showed diffuse staining. Additional studies using double-labelling immunocytochemistry demonstrated that astrocytic co-localisation of EAAT1 and EAAT2 may occasionally be seen, but was not widespread in the human CNS and that in general astrocytes were positive for either EAAT1 or EAAT2. These results demonstrate that the EAAT1 has a widespread abundance throughout all regions of the human CNS examined and that there exist discrete populations of astrocytes that are positive solely for either EAAT1 or EAAT2. Furthermore, there is evidence to suggest that altered EAAT1 expression in motor neurone disease follows a different pattern to the reported changes of EAAT2 expression in this condition, indicating that the role of glutamate transporters in the pathogenesis of motor neurone disease appears more complex than previously appreciated.

Distribution of glutamate transporters in the hippocampus of patients with pharmaco-resistant temporal lobe epilepsy

In patients suffering from temporal lobe epilepsy (TLE), increased extracellular glutamate levels in the epileptogenic hippocampus both during and after clinical seizures have been reported. These increased glutamate levels could be the result of malfunctioning and/or downregulation of glutamate transporters (also known as EAATs; excitatory amino acid transporters). In this study, the distribution of protein and mRNA of EAAT subtypes was examined in the hippocampus of TLE patients with hippocampal sclerosis (HS group) and without hippocampal sclerosis (non-HS group), and in autopsy controls without neurological disorders. EAAT protein localization was studied by immunohistochemistry on paraffin sections using specific poly- and monoclonal antibodies against the glial glutamate transporters EAAT1 and EAAT2 and the neuronal glutamate transporter EAAT3. Antibody specificity was shown by immunoblotting. In the HS group, a small decrease in EAAT1-immunoreactivity (IR) was observed in CA4 and in the polymorphic and supragranular layer of the dentate gyrus, compared with the control group. The strongest changes were found for EAAT2 levels. In the non-HS group, increased EAAT2-IR was detected in the CA1 and CA2 field, compared with non-epileptic controls. EAAT2-IR was decreased in the HS compared with the non-HS group. Fewer EAAT3-positive cells were found in the HS group than in the non-HS and control group. In both TLE groups, increased EAAT3 levels were observed in individual neurones. In the HS group, the percentage of EAAT3-IR neurones was increased in CA2 and in the granule cell layer of the dentate gyrus. Radioactive in situ hybridization for EAAT1-3 confirmed our immunohistochemical results. Non-radioactive in situ hybridization showed that not only astrocytes, but also neurones express EAAT2 mRNA. Taken together, differences in both mRNA and protein levels of glutamate transporter subtypes were found in specific regions in the TLE hippocampus, with most severe changes found for EAAT2 and EAAT3 levels. The results indicate an upregulation of EAAT2 protein expression in CA1 and CA2 in neurones in the non-HS group. This is in line with decreased EAAT2 protein levels in the HS group, since these hippocampi are characterized by severe neuronal cell loss. The functional consequences (glutamate transport capacity) of the reported changes in EAAT2 and EAAT3 remain to be determined.

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.

The expression of excitatory amino acid transporter 2 (EAAT2) in forensic autopsy cases

Glutamate is the major excitatory neurotransmitter and the greater part of this amino acid is removed from the synaptic cleft by excitatory amino acid transporter 2 (EAAT2) located on perisynaptic astrocytes. Recently, it was reported that the EAAT2 protein content changed in rats following forebrain ischemia and administration of methamphetamine. We planned to demonstrate the immunohistochemical distribution of EAAT2 in the human brain and discuss the significance of its pathophysiological roles. Thirty-two cases were used from forensic autopsies. The tissues were sampled from the cerebral cortex, striatum and hippocampus. The distribution of EAAT2 was difficult to identify in cases of electrical fatalities. However, continuous and extensive staining of EAAT2 was observed in cases of death from hypothermia. In almost all asphyxia death, we were able to observe a weak stain of EAAT2. In case of solvent abuse, EAAT2 staining was continuous and extensive as in the cases of hypothermia, and patchy negative zones were mixed. This study clearly showed the differences in EAAT2 localization according to the cause of death. These findings suggested that the differences in EAAT2 staining depended on the cause and course (pathophysiological conditions) of death.

Interindividual differences in the levels of the glutamate transporters GLAST and GLT, but no clear correlation with Alzheimer's disease

Alzheimer's disease is a common progressive neurodegenerative disease of unknown etiology. Several different pathological processes have been identified in the brains of Alzheimer patients. To determine if reduced glutamate uptake is a contributing factor, we have measured the levels of the glutamate transporter proteins GLAST (EAAT1) and GLT (EAAT2) in human autopsy samples. The postmortem proteolysis of these proteins turned out to be fairly rapid. Brains from 10 Alzheimer and 10 control patients were therefore obtained with a relatively short postmortem delay (5 hr on average). GLT (N-terminal and central parts), GLAST (C-terminal), glial fibrillary acidic protein (GFAP) and inositol (1,4,5)-triphosphate (IP3)-receptor immunoreactivities were determined in the cingulate and inferior temporal gyri by immunoblotting. The Na+-dependent "binding" of D-[3H]aspartate and the glutamate uptake after solubilization and reconstitution in liposomes were determined for comparison. An individual variation in GLAST and GLT levels was found, but no significant correlation with Alzheimer's disease, except for a 14% lower ratio of N-terminal to central GLT immunoreactivity (P < 0.04). The levels of GLAST and GLT showed negative correlation in agreement with the idea that these proteins are differentially regulated. In conclusion, Alzheimer's disease brains can have both normal and reduced levels of GLAST and GLT.

The expression of the glial glutamate transporter protein EAAT2 in motor neuron disease: an immunohistochemical study

Emerging evidence suggests that a disturbance of the glutamate neurotransmitter system may be a contributory factor to motor neuron injury in motor neuron disease. Previous autoradiographic and immunoblotting studies have suggested that there may be reduced expression of glutamate transporter proteins in pathologically affected areas of the CNS in motor neuron disease. This study further explores the possible alteration in expression of the excitatory amino acid transporter protein EAAT2 in MND, by examining the protein expression in situ, in frozen sections, using immunohistochemistry. The aim of the study was to compare the distribution and density of EAAT2 in the motor cortex and spinal cord of MND cases (n = 16) compared with neurologically normal controls (n = 12), matched for relevant parameters. A novel, previously characterized, monoclonal antibody to EAAT2 was employed. EAAT2 immunoreactivity in motor neuron disease and control cases was compared using relative optical density measurements generated by computerized image analysis. In the motor cortex, EAAT2 immunoreactivity was laminated comprising a superficial intense band (corresponding to layers 1 and 2); a paler middle band (layer 3 and part of 5) and a more intense deep layer (layers 5 and 6). In the spinal cord, the ventral horn showed strong immunoreactivity with dense perisomatic staining around motor neuron cell bodies, the substantia gelatinosa showed moderate diffuse staining and the intermediate spinal laminae showed weak staining. This general pattern of immunoreactivity was preserved in the motor neuron disease cases. However, in the motor neuron disease cases compared with controls, the optical density values for EAAT2 immunoreactivity were significantly reduced in all grey matter regions of the lumbar spinal cord (P < 0.001) and were increased in the middle laminae of the motor cortex (P < 0.05). This study indicates that glutamate transporter pathology in motor neuron disease may be a more complex phenomenon than previously recognized.

5'-heterogeneity of the human excitatory amino acid transporter cDNA EAAT2 (GLT-1)

TWO novel transcripts of the human excitatory amino acid transporter EAAT2 (GLT-1) were cloned using PCR cloning techniques. Comparative sequence analysis of the 5'-untranslated region (UTR) of five EAAT2 transcripts led to the definition of five putative 5'-untranslated exons that are combined in a variable manner. Alternative splicing is a likely mechanism for the 5' complexity of the EAAT2 cDNA. The potential functional meaning of this finding such as differential expression of the EAAT2 transcripts in the central nervous system remains to be demonstrated.

Mutations in the glutamate transporter EAAT2 gene do not cause abnormal EAAT2 transcripts in amyotrophic lateral sclerosis

Recently, variant mRNA transcripts for the astroglial glutamate transporter EAAT2 have been detected in brain tissues of 60% of patients with sporadic amyotrophic lateral sclerosis (SALS). We have tested the hypothesis that the gene for EAAT2 may be defective in some ALS cases. In 16 familial ALS (FALS) pedigrees without mutations in SOD1, we failed to detect genetic linkage to the EAAT2 locus. We next characterized the genomic organization of the EAAT2 gene and used single-strand conformation polymorphism analysis of genomic DNA to identify one novel mutation in a single SALS patient and two novel mutations in 2 affected FALS siblings. In the SALS patient, the mutation substitutes serine for an asparagine that might be involved in N-linked glycosylation of the EAAT2 protein. In the 2 affected individuals in the FALS family, we detected both a mutation in the 5' end of intron 7 and a silent G --> A transition at codon 234 in exon 5. It remains unclear whether this intron 7 mutation is related to the defective mRNA splicing. These studies indicate that germline mutations in the EAAT2 gene are infrequent and do not explain the presence of variant mRNA transcripts of EAAT2 in more than one-half of ALS cases.

Identification of alternative splicing forms of GLT-1 mRNA in the spinal cord of amyotrophic lateral sclerosis patients

The glutamate transporter plays an essential role in regulating glutamate levels in the synaptic cleft. It has been postulated that the dysfunction of GLT-1, one subtype of glutamate transporter, may be etiologically related to amyotrophic lateral sclerosis (ALS). Two alternative splicing forms of GLT-1 messenger RNA (mRNA) were found in the cervical spinal cord of five ALS patients and three controls. Analysis with reverse transcription-polymerase chain reaction (RT-PCR) showed that the shorter mRNA was a result of exon 8 skipping. A truncated transcript containing an intronic sequence at the 3' end of exon 7 was also demonstrated. However, the incidence of both alternative mRNAs was not different between the five ALS patients and three controls. Interestingly, the mRNA were also found in the cerebral cortex of a control subject. These results suggest that alternative splicing forms of GLT-1 mRNAs do not play a pathogenetic role in ALS but rather a physiological one in the normal spinal cord and brain.

Activity and protein localization of multiple glutamate transporters in gestation day 14 vs. day 20 rat placenta

Concentrative absorption of glutamate by the developing placenta is critical for proper fetal development. The expression of GLAST1, GLT1, EAAC1, and EAAT4, known to be capable of D-aspartate-inhibitable and Na(+)-coupled glutamate transport (system X-AG), was evaluated in day 14 vs. day 20 rat chorioallantoic placenta. Steady-state mRNA levels were greater at day 20 for all transporters. Immunohistochemistry determined that the expression of GLAST1, GLT1, and EAAC1 was greater throughout the day 20 placenta and was asymmetric with respect to cellular localization. EAAT4 protein was not detected. System X-AG activity was responsible for most of the Na(+)-dependent glutamate uptake and was greater in day 20 than in day 14 apical and basal membrane subdomains of the labyrinth syncytiotrophoblast. Greater quantities of EAAC1 and GLAST1 protein were identified on day 20, and quantities were greater in basal than in apical membranes. GLT1 expression, unchanged in apical membranes, was decreased in basal membranes. These data correlate transporter mRNA and protein content with transport activity and demonstrate an increasing capacity for glutamate absorption by the developing placenta.

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.

Alternative splicing of the glutamate transporter EAAT2 (GLT-1)

The human glutamate transporter EAAT2 (GLT-1) is of major importance for synaptic glutamate reuptake, and reportedly, a candidate gene for neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease and epilepsy. Here we report the polymerase chain reaction (PCR) cloning of two novel EAAT2 transcripts, named EAAT2-C1 and EAAT2-C2, which originate from alternative splicing of the human EAAT2 gene. EAAT2-C1 results from skipping of the protein coding exon eight. In contrast, EAAT2-C2 is characterized by usage of internal splice sites in the exons five and six. The splicing events lead to a deletion of 45 and 107 amino acids, respectively, located in the C-terminal and central part of the putative protein.

Expression of the glial glutamate transporter EAAT2 in the human CNS: an immunohistochemical study

Glutamate transporters play an essential role in terminating the excitatory glutamatergic signal at post-synaptic receptors and in protecting neurones from excitotoxic effects, as well as replenishing the neurotransmitter supply at glutamatergic synapses. The distribution and density of glutamate transporters may be important determinants of vulnerability to glutamate-mediated injury. There is emerging evidence that glutamate transporter dysfunction may be present in motor neurone disease (MND). In this study, a monoclonal antibody, suitable for immunohistochemistry (IHC) in human post-mortem tissue, was produced to the human astrocytic glutamate transporter EAAT2 (excitatory amino acid transporter 2). Western blotting of homogenates of human cortical tissue with the EAAT2 antibody produced a discrete band at 66 kDa. Detailed IHC analysis of the expression of the EAAT2 protein in the human CNS was undertaken. EAAT2 was exclusively localised to astrocytes, with preferential expression in the caudate nucleus, nucleus basalis of Meynert, spinal ventral horn, cerebral cortex and hippocampus, but with lower levels of expression throughout many other CNS regions. Motor neurone groups vulnerable to neurodegeneration in MND appeared distinctive in being surrounded by extensive, coarse, strongly immunoreactive perisomatic glial profiles. Motor neurone groups which tend to be spared in MND, such as those present in the oculomotor nucleus, showed a lower expression of EAAT2, with fewer perisomatic profiles. The EAAT2 antibody will provide a useful tool for increasing our understanding of the role of EAAT2 in excitatory neurotransmission in health and disease states.

Immunohistochemical localization of the neuron-specific glutamate transporter EAAC1 (EAAT3) in rat brain and spinal cord revealed by a novel monoclonal antibody

Neuronal regulation of glutamate homeostasis is mediated by high-affinity sodium-dependent and highly hydrophobic plasma membrane glycoproteins which maintain low levels of glutamate at central synapses. To further elucidate the molecular mechanisms that regulate glutamate metabolism and glutamate flux at central synapses, a monoclonal antibody was produced to a synthetic peptide corresponding to amino acid residues 161-177 of the deduced sequence of the human neuron-specific glutamate transporter III (EAAC1). Immunoblot analysis of human and rat brain total homogenates and isolated synaptosomes from frontal cortex revealed that the antibody immunoreacted with a protein band of apparent Mr approximately 70 kDa. Deglycosylation of immunoprecipitates obtained using the monoclonal antibody yielded a protein with a lower apparent Mr (approximately 65 kDa). These results are consistent with the molecular size of the human EAAC1 predicted from the cloned cDNA. Analysis of the transfected COS-1 cells by immunocytochemistry confirmed that the monoclonal antibody is specific for the neuron-specific glutamate transporter. Immunocytochemical studies of rat cerebral cortex, hippocampus, cerebellum, substantia nigra and spinal cord revealed intense labeling of neuronal somata, dendrites, fine-caliber fibers and puncta. Double-label immunofluorescence using antibody to glial fibrillary acidic protein as a marker for astrocytes demonstrated that astrocytes were not co-labeled for EAAC1. The localization of EAAC1 immunoreactivity in dendrites and particularly in cell somata suggests that this transporter may function in the regulation of other aspects of glutamate metabolism in addition to terminating the action of synaptically released glutamate at central synapses.

New beta-hydroxyaspartate derivatives are competitive blockers for the bovine glutamate/aspartate transporter

Four subtypes of excitatory amino acid transporters (EAAT1-4) have been identified in the mammalian brain. A number of pharmacological agents have been developed to study their intrinsic properties and function. Up to now, blockers were available only for EAAT2, whereas all the inhibitors of glutamate uptake active on the other subtypes were proved to be substrates of the transporters. We synthesized five new derivatives of DL-threo-beta-hydroxyaspartic acid, a well known general substrate of EAATs, and investigated their potential blocking activity on the cloned bovine EAAT1 expressed in the Xenopus oocyte system, by using radiotracer and voltage-clamp techniques. Two of our derivatives proved to be substrates for bovine EAAT1, with reduced electrogenicity compared with their parent compound, and an affinity of 40 and 64 microM. The last three derivatives displayed a blocking activity on bovine EAAT1. The affinity of DL-threo-beta-benzoyloxyaspartate and DL-threo-beta-(1-naphthoyl)oxyaspartate was determined by Schild analysis as 17.2 and 52.1 microM, respectively. These blockers should help in the better understanding of the key intrinsic properties of EAAT1. Moreover, they appear as good candidates for a general blocking activity on EAATs.

Genomic organization of the human excitatory amino acid transporter gene GLT-1

We present the genomic structure of the human glutamate transporter GLT-1 coding region, the intronic sequences adjacent to the exons, and oligonucleotide primer sequences for single strand conformational analysis. The exon-intron boundaries were determined using long-distance PCR and direct sequencing. The human GLT-1 coding region is composed of 10 exons spanning > 50 kb of genomic DNA. The exons range from 127 to 251 bp in length. The intron lengths vary considerably from 2.2 kb to > 15 kb. These data provide the basis for implementing a comprehensive screen for genetic alterations in the human GLT-1 gene using genomic DNA as a template.