Impaired RNA splicing of 5'-regulatory sequences of the astroglial glutamate transporter EAAT2 in human astrocytoma

Glutamine Metabolism in Gliomas

By histological, morphological criteria, and malignancy, brain tumors are classified by WHO into grades I (most benign) to IV (highly malignant), and gliomas are the most frequently occurring class throughout the grades. Similar to peripheral tumors, the growth of glia-derived tumor cells largely depends on glutamine (Gln), which is vividly taken up by the cells, using mostly ASCT2 and SN1 as Gln carriers. Tumor growth-promoting effects of Gln are associated with its phosphate-activated glutaminase (GA) (specifically KGA)-mediated degradation to glutamate (Glu) and/or with its entry to the energy- and intermediate metabolite-generating pathways related to the tricarboxylic acid cycle. However, a subclass of liver-type GA are absent in glioma cells, a circumstance which allows phenotype manipulations upon their transfection to the cells. Gln-derived Glu plays a major role in promoting tumor proliferation and invasion. Glu is relatively inefficiently recycled to Gln and readily leaves the cells by exchange with the extracellular pool of the glutathione (GSH) precursor Cys mediated by xc- transporter. This results in (a) cell invasion-fostering interaction of Glu with ionotropic Glu receptors in the surrounding tissue, (b) intracellular accumulation of GSH which increases tumor resistance to radio- and chemotherapy.

Association of rare variation in the glutamate receptor gene SLC1A2 with susceptibility to bipolar disorder and schizophrenia

The SLC1A2 gene encodes the excitatory amino acid transporter 2 (EAAT2). Glutamate is the major mediator of excitatory neurotransmission and EAAT2 is responsible for clearing the neurotransmitter from the synaptic cleft. Genetic variation in SLC1A2 has been implicated in a range of neurological and neuropsychiatric conditions including schizophrenia (SZ), autism and in core phenotypes of bipolar disorder (BD). The coding and putative regulatory regions of SLC1A2 gene were screened for variants using high resolution melting or sequenced in 1099 or in 32 BD subjects. Thirty-two variants were detected in the SLC1A2 gene. Fifteen potentially etiological variants were selected for genotyping in 1099 BD and 1095 control samples. Five amino acid changing variants were also genotyped in 630 participants suffering from SZ. None of the variants were found to be associated with BD or SZ or with the two diseases combined. However, two recurrent missense variants (rs145827578:G>A, p.(G6S); rs199599866:G>A, p.(R31Q)) and one recurrent 5′-untranslated region (UTR) variant (ss825678885:G>T) were detected in cases only. Combined analysis of the recurrent-case-only missense variants and of the case-only missense and 5′-UTR variants showed nominal evidence for association with the combined diseases (Fisher's P=0.019 and 0.0076). These findings are exploratory in nature and await replication in larger cohorts, however, they provide intriguing evidence that potentially functional rare variants in the SLC1A2 gene may confer susceptibility to psychotic disorders.

Genetic dys-regulation of astrocytic glutamate transporter EAAT2 and its implications in neurological disorders and manganese toxicity

Astrocytic glutamate transporters, the excitatory amino acid transporter (EAAT) 2 and EAAT1 (glutamate transporter 1 and glutamate aspartate transporter in rodents, respectively), are the main transporters for maintaining optimal glutamate levels in the synaptic clefts by taking up more than 90% of glutamate from extracellular space thus preventing excitotoxic neuronal death. Reduced expression and function of these transporters, especially EAAT2, has been reported in numerous neurological disorders, including amyotrophic lateral sclerosis, Alzheimer's disease, Parkinson's disease, schizophrenia and epilepsy. The mechanism of down-regulation of EAAT2 in these diseases has yet to be fully established. Genetic as well as transcriptional dys-regulation of these transporters by various modes, such as single nucleotide polymorphisms and epigenetics, resulting in impairment of their functions, might play an important role in the etiology of neurological diseases. Consequently, there has been an extensive effort to identify molecular targets for enhancement of EAAT2 expression as a potential therapeutic approach. Several pharmacological agents increase expression of EAAT2 via nuclear factor κB and cAMP response element binding protein at the transcriptional level. However, the negative regulatory mechanisms of EAAT2 have yet to be identified. Recent studies, including those from our laboratory, suggest that the transcriptional factor yin yang 1 plays a critical role in the repressive effects of various neurotoxins, such as manganese (Mn), on EAAT2 expression. In this review, we will focus on transcriptional epigenetics and translational regulation of EAAT2.

Tumour associated epilepsy and glutamate excitotoxicity in patients with gliomas

Tumour associated epilepsy (TAE) is common, debilitating and often not successfully controlled by surgical resection of the tumour and administration of multiple anti-epileptic drugs. It represents a cause of significant lost quality of life in an incurable disease and is therefore an important subject for ongoing research. The pathogenesis of TAE is likely to be multifactorial and involve, on the microscopic level, the interaction of genetic factors, changes in the peritumoural microenvironment, alterations in synaptic neurotransmitter release and re-uptake, and the excitotoxic effects of glutamate. On a macroscopic level, the occurrence of TAE is likely to be influenced by tumour size, location and interaction with environmental factors. The optimal treatment of TAE requires a multi-disciplinary approach with input from neurosurgeons, neurologists, radiologists, pathologists and basic scientists. This article reviews the current literature regarding the incidence, treatment, and aetiology of TAE.

Angiotensin receptor type 1 antagonists protect against neuronal injury induced by oxygen-glucose depletion

BACKGROUND AND PURPOSE

Several clinical trials and in vivo animal experiments have suggested that blockade of angiotensin receptor type 1 (AT(1)) improves ischaemic outcomes. However, the mechanism(s) underlying these effects has not been elucidated. Here, we have investigated the protective effects of pretreatment with AT(1) receptor antagonists, losartan or telmisartan, against ischaemic insult to neurons in vitro.

EXPERIMENTAL APPROACH

Primary rat neuron-astrocyte co-cultures and astrocyte-defined medium (ADM)-cultured pure astrocyte cultures were prepared. Ischaemic injury was modelled by oxygen-glucose depletion (OGD) and lactate dehydrogenase release after OGD was measured with or without AT(1) receptor antagonists or agonists (L162313), AT(2) receptor antagonist (PD123319) or agonist (CGP-42112A) pretreatment, for 48 h. Activity of glutamate transporter 1 (GLT-1) was evaluated by [(3)H]-glutamate uptake assays, after AT(1) receptor agonists or antagonists. Immunoblot and real-time PCR were used for analysis of protein and mRNA levels of GLT-1.

KEY RESULTS

AT(1) receptor agonists augmented OGD-induced cellular damage, which was attenuated by AT(1) receptor antagonists. AT(1) receptor antagonists also suppressed OGD-induced extracellular glutamate release, reactive oxygen species production and nitric oxide generation. GLT-1 expression and glutamate uptake activity were significantly enhanced by AT(1) receptor antagonists and impaired by AT(1) receptor agonists. AT(1) receptor stimulation suppressed both ADM-induced GLT-1 protein expression and mRNA levels. AT(1)b receptor knock-down with siRNA enhanced GLT-1 expression. In postnatal (P1-P21) rat brains, protein levels of GLT-1 and AT(1) receptors were inversely correlated.

CONCLUSIONS AND IMPLICATIONS

Suppression of AT(1) receptor stimulation induced GLT-1 up-regulation, which ameliorated effects of ischaemic injury.

Molecular mechanisms of necrosis in glioblastoma: the role of glutamate excitotoxicity

Glioblastomas continue to rank among the most lethal primary human tumors. Despite treatment with the most rigorous surgical interventions along with the most optimal chemotherapeutic and radiation regimens, the median survival is just 12-15 mo for patients with glioblastoma. Among the histological hallmarks of glioblastoma, necrosis has been demonstrated to be a powerful predictor of poor patient prognosis. Over the years, there have been many advances in our understanding of the molecular mechanisms underlying glioblastoma formation, yet the mechanisms that lead to tumor necrosis remain unclear. One pathway that may lead to necrosis in glioblastoma involves the neurotransmitter, glutamate, which has been shown to accumulate in the peritumoral fluid as a result of decreased cellular uptake by glioblastoma cells. This accumulation leads to subsequent glutamate excitotoxicity and probable necrosis through a massive elevation of intracellular Ca(2+) and reduction in cellular ATP levels. We propose that a pathway involving tumor necrosis factor-alpha (TNFalpha), astrocyte-elevated gene-1 (AEG-1) and nuclear factor-kappaB (NFkappaB) leads to decreased glutamate uptake through coordinated downregulation of the excitatory amino acid transporter 2 (EAAT2), the glutamate transporter responsible for the majority of glutamate uptake in the human brain. In addition, we suggest that AEG-1 signaling, loss of phosphatase and tensin homolog (PTEN), and ionotropic glutamate receptor activity lead to AKT pathway activation, which results in nutrient overconsumption and necrosis. Together, these pathways provide a new perspective on glioblastoma necrosis involving the process of glutamate excitotoxicity. Future research should address the components of these molecular pathways in order to better understand the mechanism of necrosis in glioblastoma and to begin to develop targeted therapies that may improve patient prognosis in the future.

Enhanced expression of the high affinity glutamate transporter GLT-1 in C6 glioma cells delays tumour progression in rat

High grade gliomas are known to release excitotoxic concentrations of glutamate, a process thought to contribute to their malignant phenotype through enhanced autocrine stimulation of their proliferation and destruction of the surrounding nervous tissue. A model of C6 glioma cells in which expression of the high affinity glutamate transporter GLT-1 can be manipulated both in vivo and in vitro was used in order to investigate the consequences of increasing glutamate clearance on tumour progression. These cells were grafted in the striatum of Wistar rats and doxycycline was administered after validation of tumour development by magnetic resonance imaging. Both GLT-1 expression examined by immunohistochemistry and glutamate transport activity measured on synaptosomes appeared robustly increased in samples from doxycycline-treated animals. Moreover, these rats showed extended survival times as compared to vehicle-treated animals, an effect that was consistent with volumetric data revealing delayed tumour growth. As constitutive deficiency in glutamate clearance at the vicinity of brain tumours is well established, these data illustrate the potential benefit that could be obtained by enhancing glutamate transport by glioma cells in order to reduce their invasive behaviour.

The concentrations and distributions of three C-terminal variants of the GLT1 (EAAT2; slc1a2) glutamate transporter protein in rat brain tissue suggest differential regulation

The neurotransmitter glutamate is inactivated by cellular uptake; mostly catalyzed by the glutamate transporter GLT1 (slc1a2, excitatory amino acid transporter [EAAT2]) subtype which is expressed at high levels in brain astrocytes and at lower levels in neurons. Three coulombs-terminal variants of GLT1 exist (GLT1a, GLT1b and GLT1c). Their cellular distributions are currently being debated (that of GLT1b in particular). Here we have made antibodies to the variants and produced pure preparations of the individual variant proteins. The immunoreactivities of each variant per amount of protein were compared to that of total GLT1 immunoisolated from Wistar rat brains. At eight weeks of age GLT1a, GLT1b and GLT1c represented, respectively 90%+/-1%, 6+/-1% and 1%+/-0.5% (mean+/-SEM) of total hippocampal GLT1. The levels of all three variants were low at birth and increased towards adulthood, but GLT1a increased relatively more than the other two. At postnatal day 14 the levels of GLT1b and GLT1c relative to total GLT1 were, respectively, 1.7+/-0.1 and 2.5+/-0.1 times higher than at eight weeks. In tissue sections, antibodies to GLT1a gave stronger labeling than antibodies to GLT1b, but the distributions of GLT1a and GLT1b were similar in that both were predominantly expressed in astroglia, cell bodies as well as their finest ramifications. GLT1b was not detected in nerve terminals in normal brain tissue. The findings illustrate the need for quantitative measurements and support the notion that the importance of the variants may not be due to the transporter molecules themselves, but rather that their expression represents the activities of different regulatory pathways.

Glutamate-induced glioma cell proliferation is prevented by functional expression of the glutamate transporter GLT-1

A tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of l-cystine. The addition of glutamate (100 microM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. This suggests that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation.

Chemical hypoxia facilitates alternative splicing of EAAT2 in presymptomatic APP23 transgenic mice

Hypoxia is one of the major common components of vascular risk factors for pathogenesis of Alzheimer's disease. This study investigated the possible relationship between hypoxia and alternative splicing of the excitatory amino acid transporter 2 (EAAT2) in a transgenic model for Alzheimer's disease. We used an APP23 mouse model prior to amyloid deposition and subjected it to chemical hypoxia treatment as induced by 3-nitropropionic acid. One hour after administration of 3-nitropropionic acid changes in the expression of the 5'-splice forms mEAAT2/5UT3, mEAAT2/5UT4, and mEAAT2/5UT5 were found in the frontal cortex, hippocampus and cerebellum of the APP23 model. In untreated APP23 animals the expression of EAAT2 splice variants was unchanged. Our results demonstrate that hypoxia facilitates alternative splicing of EAAT2 in the APP23 model. This may be a molecular mechanism linking vascular factors to early pathophysiology of Alzheimer's disease.

Increase in glutamate as a sensitive indicator of extracellular matrix integrity in peritumoral edema: a 3.0-tesla proton magnetic resonance spectroscopy study

OBJECT

The authors of previous studies based on diffusion tensor imaging have indicated that there are two types of peritumoral edema-namely, edema with preserved structural integrity of the glial matrix and edema with compromised glial matrix. The authors of this study hypothesized that functionality of the glutamate (Glu)-glutamine shuttle, a vital neuron-glia interaction, may be differentially affected by peritumoral edema. They tested this hypothesis using proton magnetic resonance (MR) spectroscopy on a 3.0-tesla system that is capable of quantifying Glu without need of editing.

METHODS

Twenty-three patients, each with a single brain tumor mass and peritumoral edema (nine high-grade gliomas, eight metastatic brain tumors, and six meningiomas), and nine healthy individuals participated in this study. Single-voxel proton MR imaging targeting the region of peritumoral edema was performed using a 3.0-tesla system. Glutamate levels in the peritumoral edema of nonglial tumors was significantly elevated (p < 0.01) compared with edema associated with glial tumors or normal white matter. The finding confirmed that peritumoral edema in nonglial tumors is distinct from that of glial tumors, as previously indicated in diffusion tensor imaging studies. The authors hypothesized that the former condition represents a compensatory increase in activities of the Glu-glutamine shuttle brought about by simple expansion of the extracellular space due to edema.

CONCLUSIONS

The assessment of Glu concentrations in peritumoral edema using 3.0-tesla proton MR spectroscopy may be developed into an objective index of the structural integrity of the glial matrix.

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.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

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.

The excitatory amino acid transporter-2 induces apoptosis and decreases glioma growth in vitro and in vivo

Accumulating evidence suggests that glutamate plays a key role in the proliferation and invasion of glioblastoma tumors. Astrocytic tumors have been shown to release glutamate at high levels, which may stimulate tumor cell proliferation and motility via activation of glutamate receptors. Excess glutamate has also been found to facilitate tumor invasion by causing excitotoxic damage to normal brain thereby paving a pathway for tumor migration. Results from tissue microarray analyses showed decreased excitatory amino acid transporter-2 (EAAT-2) expression in high-grade glial tumors compared with low-grade astrocytomas and normal brain. EAAT-2 expression was inversely correlated with tumor grade, implicating its potential role in glial tumor progression, which was reflected by an undetectable level of EAAT-2 protein in glioma cell lines. In this study, we sought to investigate the effect of reconstituted EAAT-2 on glioma cell growth in vitro and in vivo by adenoviral-mediated gene transfer. Infection of glioma cells with Ad-EAAT-2 resulted in a physiologic level of functional EAAT-2, and a subsequent dose-dependent reduction in cell proliferation in all glioma cell lines tested compared with controls. Interestingly, results from analyses of Annexin V staining, detection of poly(ADP-ribose)polymerase cleavage and caspase-3 activation all indicated that Ad-EAAT-2 infection elicited apoptosis in glioma cells. Ex vivo experiments in nude mice showed a total suppression of tumor growth at sites that received Ad-EAAT-2-infected cells. Collectively, our results uncovered a new function of EAAT-2 in controlling glioma proliferation. Further studies will improve our knowledge of the role of glutamate in glioma growth and may provide useful prognostic information and alternative therapeutic targets for the treatment of glioma.

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.

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.

Differential regulation of 5' splice variants of the glutamate transporter EAAT2 in an in vivo model of chemical hypoxia induced by 3-nitropropionic acid

Defective glutamate uptake has been implicated as a pathogenic event of neuronal damage related to cerebral ischemia and hypoxia. In several models of ischemia-hypoxia, a reduced immunoreactivity and altered RNA expression of excitatory amino acid transporter 2 (EAAT2), the major excitatory amino acid transporter, have been reported. However, the gene regulation of EAAT2 under these conditions is incompletely understood. In this study, we investigated alternative splicing of EAAT2 in an in vivo mouse model of chemical hypoxia as induced by 3-nitropropionic acid (3-NP). The neurotoxin 3-NP is an inhibitor of mitochondrial energy production. Furthermore, it is known to inhibit glutamate reuptake directly, representing at least one of the mechanisms responsible for 3-NP-induced neurodegeneration. Here we report an expression analysis of five known (mEAAT2/5UT1-5) and two novel (mEAAT2/5UT6, -7) 5' splice variants of EAAT2 using semiquantitative PCR. The RNA expression was studied at 2, 12, 24, 48, and 72 hr and 7 days after 3-NP administration. mEAAT2/5UT4 and mEAAT2/5UT5 were up-regulated in the frontal cortex and down-regulated in the hippocampus 12-72 hr after chemical hypoxia. In the cerebellum, there was an increased expression of mEAAT2/5UT4 and a down-regulation of mEAAT2/5UT5. mEAAT2/5UT3 show a different regional expression pattern, being regulated in the cerebellum only. mEAAT2/5UT1-7 encoded distinct 5' regulatory sequences, including conserved elements of translational control. It is easily conceivable that expression alterations of 5' splice variants of EAAT2 are related to glutamate transporter malfunction after chemical hypoxia. Our findings contribute to the hypothesis that RNA splicing events can serve as a molecular mechanism of posthypoxic gene regulation.