Degradation of glial glutamate transporter mRNAs is selectively blocked by inhibition of cellular transcription

Cooperative and competitive regulation of the astrocytic transcriptome by neurons and endothelial cells: Impact on astrocyte maturation

Astrocytes have essential roles in central nervous system (CNS) health and disease. During development, immature astrocytes show complex interactions with neurons, endothelial cells, and other glial cell types. Our work and that of others have shown that these interactions are important for astrocytic maturation. However, whether and how these cells work together to control this process remains poorly understood. Here, we test the hypothesis that cooperative interactions of astrocytes with neurons and endothelial cells promote astrocytic maturation. Astrocytes were cultured alone, with neurons, endothelial cells, or a combination of both. This was followed by astrocyte sorting, RNA sequencing, and bioinformatic analysis to detect transcriptional changes. Across culture configurations, 7302 genes were differentially expressed by 4 or more fold and organized into 8 groups that demonstrate cooperative and antagonist effects of neurons and endothelia on astrocytes. We also discovered that neurons and endothelial cells caused splicing of 200 and 781 mRNAs, respectively. Changes in gene expression were validated using quantitative PCR, western blot (WB), and immunofluorescence analysis. We found that the transcriptomic data from the three-culture configurations correlated with protein expression of three representative targets (FAM107A, GAT3, and GLT1) in vivo. Alternative splicing results also correlated with cortical tissue isoform representation of a target (Fibronectin 1) at different developmental stages. By comparing our results to published transcriptomes of immature and mature astrocytes, we found that neurons or endothelia shift the astrocytic transcriptome toward a mature state and that the presence of both cell types has a greater effect on maturation than either cell alone. These results increase our understanding of cellular interactions/pathways that contribute to astrocytic maturation. They also provide insight into how alterations to neurons and/or endothelial cells may alter astrocytes with implications for astrocytic changes in CNS disorders and diseases.

Glutamate Transporters and Mitochondria: Signaling, Co-compartmentalization, Functional Coupling, and Future Directions

In addition to being an amino acid that is incorporated into proteins, glutamate is the most abundant neurotransmitter in the mammalian CNS, the precursor for the inhibitory neurotransmitter γ-aminobutyric acid, and one metabolic step from the tricarboxylic acid cycle intermediate α-ketoglutarate. Extracellular glutamate is cleared by a family of Na⁺-dependent transporters. These transporters are variably expressed by all cell types in the nervous system, but the bulk of clearance is into astrocytes. GLT-1 and GLAST (also called EAAT2 and EAAT1) mediate this activity and are extremely abundant proteins with their expression enriched in fine astrocyte processes. In this review, we will focus on three topics related to these astrocytic glutamate transporters. First, these transporters co-transport three Na⁺ ions and a H⁺ with each molecule of glutamate and counter-transport one K⁺; they are also coupled to a Cl^- conductance. The movement of Na⁺ is sufficient to cause profound astrocytic depolarization, and the movement of H⁺ is linked to astrocytic acidification. In addition, the movement of Na⁺ can trigger the activation of Na⁺ co-transporters (e.g. Na⁺-Ca²⁺ exchangers). We will describe the ways in which these ionic movements have been linked as signals to brain function and/or metabolism. Second, these transporters co-compartmentalize with mitochondria, potentially providing a mechanism to supply glutamate to mitochondria as a source of fuel for the brain. We will provide an overview of the proteins involved, discuss the evidence that glutamate is oxidized, and then highlight some of the un-resolved issues related to glutamate oxidation. Finally, we will review evidence that ischemic insults (stroke or oxygen/glucose deprivation) cause changes in these astrocytic mitochondria and discuss the ways in which these changes have been linked to glutamate transport, glutamate transport-dependent signaling, and altered glutamate metabolism. We conclude with a broader summary of some of the unresolved issues.

Traumatic Brain Injury Induces Alterations in Cortical Glutamate Uptake without a Reduction in Glutamate Transporter-1 Protein Expression

We hypothesize that the primary mechanism for removal of glutamate from the extracellular space is altered after traumatic brain injury (TBI). To evaluate this hypothesis, we initiated TBI in adult male rats using a 2.0 atm lateral fluid percussion injury (LFPI) model. In the ipsilateral cortex and hippocampus, we found no differences in expression of the primary glutamate transporter in the brain (GLT-1) 24 h after TBI. In contrast, we found a decrease in glutamate uptake in the cortex, but not the hippocampus, 24 h after injury. Because glutamate uptake is potently regulated by protein kinases, we assessed global serine-threonine protein kinase activity using a kinome array platform. Twenty-five kinome array peptide substrates were differentially phoshorylated between LFPI and controls in the cortex, whereas 19 peptide substrates were differentially phosphorylated in the hippocampus (fold change ≥ ± 1.15). We identified several kinases as likely to be involved in acute TBI, including protein kinase B (Akt) and protein kinase C (PKC), which are well-characterized modulators of GLT-1. Exploratory studies using an inhibitor of Akt suggest selective activation of kinases in LFPI versus controls. Ingenuity pathway analyses of implicated kinases from our network model found apoptosis and cell death pathways as top functions in acute LFPI. Taken together, our data suggest diminished activity of glutamate transporters in the prefrontal cortex, with no changes in protein expression of the primary glutamate transporter GLT-1, and global alterations in signaling networks that include serine-threonine kinases that are known modulators of glutamate transport activity.

Glutamate Neurotransmission in Rodent Models of Traumatic Brain Injury

Traumatic brain injury (TBI) is a leading cause of death and disability in people younger than 45 and is a significant public health concern. In addition to primary mechanical damage to cells and tissue, TBI involves additional molecular mechanisms of injury, termed secondary injury, that continue to evolve over hours, days, weeks, and beyond. The trajectory of recovery after TBI is highly unpredictable and in many cases results in chronic cognitive and behavioral changes. Acutely after TBI, there is an unregulated release of glutamate that cannot be buffered or cleared effectively, resulting in damaging levels of glutamate in the extracellular space. This initial loss of glutamate homeostasis may initiate additional changes in glutamate regulation. The excitatory amino acid transporters (EAATs) are expressed on both neurons and glia and are the principal mechanism for maintaining extracellular glutamate levels. Diffusion of glutamate outside the synapse due to impaired uptake may lead to increased extrasynaptic glutamate signaling, secondary injury through activation of cell death pathways, and loss of fidelity and specificity of synaptic transmission. Coordination of glutamate release and uptake is critical to regulating synaptic strength, long-term potentiation and depression, and cognitive processes. In this review, we will discuss dysregulation of extracellular glutamate and glutamate uptake in the acute stage of TBI and how failure to resolve acute disruptions in glutamate homeostatic mechanisms may play a causal role in chronic cognitive symptoms after TBI.

Deletion of mTOR in Reactive Astrocytes Suppresses Chronic Seizures in a Mouse Model of Temporal Lobe Epilepsy

Germline and somatic mutations in key genes of the mammalian target of rapamycin (mTOR) pathway have been identified in seizure-associated disorders. mTOR mutations lead to aberrant activation of mTOR signaling, and, although affected neurons are critical for epileptogenesis, the role of mTOR activation in glial cells remains poorly understood. We previously reported a consistent activation of the mTOR pathway in astrocytes in the epileptic foci of temporal lobe epilepsy. In this study, it was demonstrated that mTOR deletion from reactive astrocytes prevents increases in seizure frequency over the disease course. By using a tamoxifen-inducible mTOR conditional knockout system and kainic acid, a model was developed that allowed astrocyte-specific mTOR gene deletion in mice with chronic epilepsy. Animals in which mTOR was deleted from 44 % of the astrocyte population exhibited a lower seizure frequency compared with controls. Down-regulation of mTOR significantly ameliorated astrogliosis in the sclerotic hippocampus but did not rescue mossy fiber sprouting. In cultured astrocytes, the mTOR pathway modulated the stability of the astroglial glutamate transporter 1 (Glt1) and influenced the ability of astrocytes to remove extracellular glutamate. Taken together, these data indicate that astrocytes with activated mTOR signaling may provide conditions that are favorable for spontaneous recurrent seizures.

Differential regulation of glutamate transporter subtypes by pro-inflammatory cytokine TNF-α in cortical astrocytes from a rat model of amyotrophic lateral sclerosis

Dysregulation of the astroglial glutamate transporters GLAST and GLT-1 has been implicated in several neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) where a loss of GLT-1 protein expression and activity is reported. Furthermore, the two principal C-terminal splice variants of GLT-1 (namely GLT-1a and GLT-1b) show altered expression ratio in animal models of this disease. Considering the putative link between inflammation and excitotoxicity, we have here characterized the influence of TNF-α on glutamate transporters in cerebral cortical astrocyte cultures from wild-type rats and from a rat model of ALS (hSOD1^G93A). Contrasting with the down-regulation of GLAST, a 72 h treatment with TNF-α substantially increased the expression of GLT-1a and GLT-1b in both astrocyte cultures. However, as the basal level of GLT-1a appeared considerably lower in hSOD1^G93A astrocytes, its up-regulation by TNF-α was insufficient to recapitulate the expression observed in wild-type astrocytes. Also the glutamate uptake activity after TNF-α treatment was lower for hSOD1^G93A astrocytes as compared to wild-type astrocytes. In the presence of the protein synthesis inhibitor cycloheximide, TNF-α did not influence GLT-1 isoform expression, suggesting an active role of dynamically regulated protein partners in the adaptation of astrocytes to the inflammatory environment. Confirming the influence of inflammation on the control of glutamate transmission by astrocytes, these results shed light on the regulation of glutamate transporter isoforms in neurodegenerative disorders.

Interleukin-1β alters glutamate transmission at purkinje cell synapses in a mouse model of multiple sclerosis

Cerebellar deficit contributes significantly to disability in multiple sclerosis (MS). Several clinical and experimental studies have investigated the pathophysiology of cerebellar dysfunction in this neuroinflammatory disorder, but the cellular and molecular mechanisms are still unclear. In experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, proinflammatory cytokines, together with a degeneration of inhibitory neurons, contribute to impair GABAergic transmission at Purkinje cells (PCs). Here, we investigated glutamatergic transmission to gain insight into the pathophysiology of cerebellar dysfunction in EAE. Electrophysiological recordings from PCs showed increased duration of spontaneous excitatory postsynaptic currents (EPSCs) during the symptomatic phase of EAE, suggesting an alteration of glutamate uptake played by Bergmann glia. We indeed observed an impaired functioning of the glutamate-aspartate transporter/excitatory amino acid transporter 1 (GLAST/EAAT1) in EAE cerebellum caused by protein downregulation and in correlation with prominent astroglia activation. We have also demonstrated that the proinflammatory cytokine interleukin-1β (IL-1β), released by a subset of activated microglia/macrophages and infiltrating lymphocytes, was involved directly in such synaptic alteration. In fact, brief incubation of IL-1β in normal cerebellar slices replicated EAE modifications through a rapid GLAST/EAAT1 downregulation, whereas incubation of an IL-1 receptor antagonist (IL-1ra) in EAE slices reduced spontaneous EPSC alterations. Finally, EAE mice treated with intracerebroventricular IL-1ra showed normal glutamatergic and GABAergic transmissions, along with GLAST/EAAT1 normalization, milder inflammation, and reduced motor deficits. These results highlight the crucial role played by the proinflammatory IL-1β in triggering molecular and synaptic events involved in neurodegenerative processes that characterize neuroinflammatory diseases such as MS.

Differential regulation of the glutamate transporter variants GLT-1a and GLT-1b in the cortex and spinal cord of transgenic rats expressing hSOD1(G93A)

Altered expression and activity of GLT-1 have been characterized in amyotrophic lateral sclerosis (ALS) patients and in animal models of the disease. Data suggest that the expression of two C-terminus splice variants of GLT-1 (namely GLT-1a and GLT-1b) can be differentially regulated in this pathological context. We herein characterized the expression of GLT-1a and GLT-1b mRNA and the glutamate uptake activity in the fronto-temporal cortex and the lumbar spinal cord of transgenic rats expressing hSOD1(G93A) at various stages of the disease. We also investigated the expression and activity of the other key glutamate transporters GLAST and EAAC1. While the progression of the disease was associated with a reduction of the overall GLT-1 activity in both cortex and spinal cord, the regulation of GLT-1a and GLT-1b transcripts showed different profiles. In the cortex, GLT-1a mRNA which appears as the most abundant isoform at a pre-symptomatic stage was strongly decreased during the progression of the disease while GLT-1b mRNA increased to reach a similar level as GLT-1a at end-stage. In the lumbar spinal cord of transgenic rats, both GLT-1a and GLT-1b mRNAs, expressed at the same levels before the symptom onset, were strongly decreased in the ventral horns. While no modification of GLAST was detected, EAAC1 mRNA was highly increased at a pre-symptomatic stage in transgenic animals, explaining a higher activity of glutamate transporters at this age. These results demonstrate that glutamate transporters are differentially expressed in nervous structures of wild-type and transgenic animals although the total GLT-1 activity was constantly decreased during the disease progression.

Down-regulation of astrocytic GLAST by microglia-related inflammation is abrogated in dibutyryl cAMP-differentiated cultures

The influence of neuroinflammation on glutamate uptake by glial cells was examined after exposing primary cultures of rat astrocytes to conditioned culture medium from lipopolysaccharide-activated microglia. While such treatment triggered an inflammatory response in astrocytes, as revealed by the induction of cytokine expression, a significant decrease in GLAST expression and activity was observed after 72 h. This regulation of glutamate transporter was not observed with medium from naive microglia, but was mimicked by direct addition of tumor necrosis factor-alpha (TNF-alpha), a major cytokine released from activated microglia. Hence, on its own, TNF-alpha also triggered inflammation in astrocyte cultures, highlighting complex cross-talk between astrocytes and microglia in inflammatory conditions. This putatively detrimental regulation of GLAST in response to inflammation was also studied in cells exposed to dibutyryl cAMP, recognized as a model of astrocytes exhibiting a typical differentiated or activated phenotype. In this model, the conditioned culture medium from activated microglia, as well as TNF-alpha, were found to increase glutamate uptake capacity. Consistently, both of these treatments caused only modest induction of an inflammatory response in dibutyryl cAMP-matured astrocytes as compared to undifferentiated astrocytes. Together, these results suggest that differentiated/activated astrocytes are endowed with the capacity to confront inflammatory insults and that drugs influencing the astrocytes phenotype would deserve further consideration in the treatment of neurological disorders.

Suppression of host gene expression by nsp1 proteins of group 2 bat coronaviruses

nsp1 protein of severe acute respiratory syndrome coronavirus (SARS-CoV), a group 2b CoV, suppresses host gene expression by promoting host mRNA degradation and translation inhibition. The present study analyzed the activities of nsp1 proteins from the group 2 bat CoV strains Rm1, 133, and HKU9-1, belonging to groups 2b, 2c, and 2d, respectively. The host mRNA degradation and translational suppression activities of nsp1 of SARS-CoV and Rm1 nsp1 were similar and stronger than the activities of the nsp1 proteins of 133 and HKU9-1. Rm1 nsp1 expression in trans strongly inhibited the induction of type I interferon (IFN-I) and IFN-stimulated genes in cells infected with an IFN-inducing SARS-CoV mutant, while 133 and HKU9-1 nsp1 proteins had relatively moderate IFN-inhibitory activities. The results of our studies suggested a conserved function among nsp1 proteins of SARS-CoV and group 2 bat CoVs.

A glutamatergic component of lead toxicity in adult brain: the role of astrocytic glutamate transporters

Astroglial cells have a variety of roles in the central nervous system (CNS), providing a homeostasis for the proper functioning of neuronal cells. The classical view concerning the supportive role of astroglia towards associated neurons has to be extended. A great number of new evidences suggest that astrocytes interact closely with neurons being involved in the active control of neuronal activity and metabolism, forming with pre- and postsynaptic nerve terminals a tripartite synapse. Astrocytes control many aspects of brain function. Regulation of extracellular glutamate concentration, potentially neurotoxic neurotransmitter, is fundamental. Glial glutamate transporters system is of importance in protection against glutamate excitotoxicity and antioxidant defence system which is glutathione. When astrocytes fail to function properly, they influence the degree of neuronal damage. Thus, astrocytes are involved to a very great extent into numerous brain pathologies, including toxicity of heavy metals, like lead (Pb). Under pathological conditions they appear to express two opposite features: they are neuroprotective (until they can) or deleterious for neurons and may participate in neuronal damage. The very well known affinity of Pb to astroglia and the changes in glutamatergic transmission upon Pb toxicity, led us to discuss the role of astroglia and astrocytic glutamate transporters in the neurotoxicity of this metal. Our observations are viewed against a background of other results.

Ultra-low-dose naloxone restores the antinociceptive effect of morphine and suppresses spinal neuroinflammation in PTX-treated rats

The aim of the present study was to examine the effect of ultra-low-dose naloxone on pertussis toxin (PTX)-induced thermal hyperalgesia in rats and its underlying mechanisms. Male Wistar rats, implanted with an intrathecal catheter with or without a microdialysis probe, received a single intrathecal injection of PTX (1 microg in 5 microl saline). Four days after PTX injection, they were randomly given a different dose of naloxone (either 15 microg or 15 ng in 5 microl saline), followed by a morphine injection (10 microg in 5 microl saline) after 30 min. The results found that PTX injection induced thermal hyperalgesia and increasing excitatory amino acid (EAA; L-glutamate and L-aspartate) concentration in the spinal CSF dialysates. Ultra-low-dose naloxone not only preserved the antinociceptive effect of morphine but also suppressed the PTX-evoked EAA release as well. Moreover, ultra-low-dose naloxone plus morphine administration inhibited the downregulation of L-glutamate transporters (GTs) and the L-glutamate-metabolizing enzyme glutamine synthetase (GS), and, moreover, inhibited microglial activation and suppressed cytokine expression in PTX-treated rat spinal cords. These results show that ultra-low-dose naloxone preserves the antinociceptive effect of morphine in PTX-treated rats. The mechanisms include (a) inhibition of pro-inflammatory cytokine expression, (b) attenuation of PTX-evoked EAA release, and (c) reversion of the downregulation of GT expression.

Ubiquitination-mediated internalization and degradation of the astroglial glutamate transporter, GLT-1

Sodium-dependent glutamate uptake is essential for limiting excitotoxicity, and dysregulation of this process has been implicated in a wide array of neurological disorders. The majority of forebrain glutamate uptake is mediated by the astroglial glutamate transporter, GLT-1. We and others have shown that this transporter undergoes endocytosis and degradation in response to activation of protein kinase C (PKC), however, the mechanisms involved remain unclear. In the current study, transfected C6 glioma cells or primary cortical cultures were used to show that PKC activation results in incorporation of ubiquitin into GLT-1 immunoprecipitates. Mutation of all 11 lysine residues in the amino and carboxyl-terminal domains to arginine (11R) abolished this signal. Selective mutation of the seven lysine residues in the carboxyl terminus (C7K-R) did not eliminate ubiquitination, but it completely blocked PKC-dependent internalization and degradation. Two families of variants of GLT-1 were prepared with various lysine residues mutated to arginine. Analyses of these constructs indicated that redundant lysine residues in the carboxyl terminus were sufficient for the appearance of ubiquitinated product and degradation of GLT-1. Together these data define a novel mechanism by which the predominant forebrain glutamate transporter can be rapidly targeted for degradation.

Internalization and degradation of the glutamate transporter GLT-1 in response to phorbol ester

Activation of protein kinase C (PKC) decreases the activity and cell surface expression of the predominant forebrain glutamate transporter, GLT-1. In the present study, C6 glioma were used as a model system to define the mechanisms that contribute to this decrease in cell surface expression and to determine the fate of internalized transporter. As was previously observed, phorbol 12-myristate 13-acetate (PMA) caused a decrease in biotinylated GLT-1. This effect was blocked by sucrose or by co-expression with a dominant-negative variant of dynamin 1, and it was attenuated by co-expression with a dominant-negative variant of the clathrin heavy chain. Depletion of cholesterol with methyl-beta-cyclodextrin, co-expression with a dominant-negative caveolin-1 mutant (Cav1/S80E), co-expression with dominant-negative variants of Eps15 (epidermal-growth-factor receptor pathway substrate clone 15), or co-expression with dominant-negative Arf6 (T27N) had no effect on the PMA-induced loss of biotinylated GLT-1. Long-term treatment with PMA caused a time-dependent loss of biotinylated GLT-1 and decreased the levels of GLT-1 protein. Inhibitors of lysosomal degradation (chloroquine or ammonium chloride) or co-expression with a dominant-negative variant of a small GTPase implicated in trafficking to lysosomes (Rab7) prevented the PMA-induced decrease in protein and caused an intracellular accumulation of GLT-1. These results suggest that the PKC-induced redistribution of GLT-1 is dependent upon clathrin-mediated endocytosis. These studies identify a novel mechanism by which the levels of GLT-1 could be rapidly down-regulated via lysosomal degradation. The possibility that this mechanism may contribute to the loss of GLT-1 observed after acute insults to the CNS is discussed.

Potential mechanisms for astrocyte-TIMP-1 downregulation in chronic inflammatory diseases

The pathogenesis of many neurodegenerative disorders, including human immunodeficiency virus (HIV)-1 associated dementia, is exacerbated by an imbalance between matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of metalloproteinases (TIMPs). In the context of disease, TIMP-1 has emerged as an important multifunctional protein capable of regulating inflammation. We previously reported differential TIMP-1 expression in acute versus chronic activation of astrocytes. This study investigates possible mechanisms underlying TIMP-1 downregulation in chronic neuroinflammation. We used interleukin (IL)-1beta as a model pro-inflammatory stimulus and measured TIMP-1 binding to extracellular matrix, cell death, receptor downregulation, TIMP-1 mRNA stability and transcriptional regulation in activated astrocytes. TIMP-1 remained localized to the cell body or was secreted into the cell supernatant. DNA fragmentation ELISA and MTT assay showed that prolonged IL-1beta activation of astrocytes induced significant astrocyte death. In acute and chronic IL-1beta-activated astrocytes, IL-1 receptor levels were not significantly different. TIMP-1 mRNA stability was measured in astrocytes and U87 astroglioma cells by real-time PCR, and TIMP-1 promoter activation was studied using TIMP-1-luciferase reporter constructs in transfected astrocytes. Our results indicated that TIMP-1 expression is regulated through multiple mechanisms. Transcriptional control and loss of mRNA stabilization are, however, the most likely primary contributors to chronic downregulation of TIMP-1. These data are important for unraveling the mechanisms underlying astrocyte responses during chronic neuroinflammation and have broader implications in other inflammatory diseases that involve MMP/TIMP imbalance.

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.

Autoantigen specific T cells inhibit glutamate uptake in astrocytes by decreasing expression of astrocytic glutamate transporter GLAST: a mechanism mediated by tumor necrosis factor‐α

Glutamate excitotoxicity is increasingly being recognized as a pathogenic mechanism in autoimmune inflammatory disorders of the central nervous system (CNS). Astrocytes are the predominant players in clearing the extracellular space from glutamate and normally have extensive spare capacities in terms of glutamate uptake. We asked what might be the basis of glutamate accumulation in T cell triggered autoimmune inflammation. In vitro, coculture of primary rat astrocytes with activated myelin basic protein (MBP)-specific T cells resulted in a decrease of astrocytic glutamate uptake rates (Vmax). In parallel, the amount of the Na+-dependent glutamate transporter GLAST was reduced within 48-60 h. Significant decreases of GLAST protein were observed in astrocytes harvested after incubation with T cells activated by MBP during coculture or after incubation with T cell blasts preactivated in the presence of splenocytes beforehand. Since exposure of astrocytes to cell-free supernatant of MBP-activated T cells also resulted in reduced expression of GLAST, a humoral factor appeared to be the driving agent. In blocking experiments using neutralizing antibodies and by incubation of astrocytes with recombinant cytokines, tumor necrosis factor-alpha (TNF-alpha) was identified as being responsible for the down-modulation of GLAST. GLAST was also down-regulated in the CNS of autoimmune encephalomyelitic rats but not in animals suffering from systemic inflammation. Since the loss of GLAST was not confined to inflammatory infiltrates, here too, a humoral factor seemed to be causative. In conclusion, T cell derived TNF-alpha impairs glutamate clearance capacity of astrocytes in vitro and probably also in vivo providing a pathogenic link to glutamate excitotoxicity that may contribute to early axonal dysfunction remote from active autoimmune inflammatory demyelination.

Transcriptional regulation of human excitatory amino acid transporter 1 (EAAT1): cloning of the EAAT1 promoter and characterization of its basal and inducible activity in human astrocytes

Excitatory amino acid transporter 1 (EAAT1) is one of the two glial glutamate transporters that clear the extracellular glutamate generated during neuronal signal transmission. Here, we cloned and characterized a 2.1-kb promoter region of human EAAT1 and investigated its function in the transcriptional regulation of the EAAT1 gene in human primary astrocytes. The full-length promoter region lacked TATA and CCAAT boxes and an initiator element, it contained several potential transcription factor-binding sites and it exhibited promoter activity in primary astrocytes and in several types of transformed cells. Consecutive 5'-deletion analysis of the EAAT1 promoter indicated the presence of negative and positive regulatory regions and a putative core promoter between -57 bp and +20 bp relative to the transcription start site (TSS). The core promoter contained a single GC-box in position -52/-39 and one E-box near the TSS and the GC-box site that was responsible for 90% of the basal promoter activity as determined by mutational analysis. Electrophoretic mobility shift, supershift and competition assays demonstrated binding of stimulating proteins (Sp) 1 and 3 to the GC-box and upstream stimulating factor (USF) 1 to the E-box. Treatment of primary human astrocytes with cellular modulators 8-bromo cyclic AMP and epidermal growth factor increased EAAT1 promoter activity in transient transfection assays and increased cellular EAAT1 mRNA expression and glutamate uptake by astrocytes. Conversely, tumor necrosis factor-alpha reduced both EAAT promoter activity and cellular EAAT1 mRNA expression. These results enable studies of transcriptional regulation of EAAT1 gene at the promoter level.

Cytoplasmic vacuolation in cultured rat astrocytes induced by an organophosphorus agent requires extracellular signal-regulated kinase activation

There are various toxic chemicals that cause cell death. However, in certain cases deleterious agents elicit various cellular responses prior to cell death. To determine the cellular mechanisms by which such cellular responses are induced is important, but sufficient attention has not been paid to this issue to date. In this study, we showed the characteristic effects of an organophosphorus (OP) agent, bis(pinacolyl methyl)phosphonate (BPMP), which we synthesized for the study of OP nerve agents, on cultured rat astrocytes. Morphologically, BPMP induced cytoplasmic vacuolation and stellation in the rat astrocytes. Cytoplasmic vacuolation is a cell pathological change observed, for example, in vacuolar degeneration, and stellation has been reported in astrocytic reactions against various stimuli. By pretreatment with cycloheximide, a protein synthesis inhibitor, stellation was inhibited, although vacuolation was not. Cell staining with a mitochondrion-selective dye indicated that the vacuolation probably occurs in the mitochondria that are swollen and vacuolatred in the center. Interestingly, the extracellular signal-regulated kinase (ERK) cascade inhibitor inhibited vacuolation and, to some extent, stellation. These results suggest that the ERK signaling cascade is important for the induction of mitochondrial vacuolation. We expect that a detailed study of these astrocytic reactions will provide us new perspectives regarding the variation and pathological significance of cell morphological changes, such as vacuolar degeneration, and also the mechanisms underlying various neurological disorders.

Veratridine, but not elevated K+, inhibits excitatory amino acid transporter activity in rat hippocampal slices

PURPOSE

Excitatory amino acid transporter (EAAT) activity prevents Glu from reaching toxic levels, but their contribution to epileptogenesis remains controversial. We examined how the convulsant veratridine causes inhibition of EAAT activity and how it differs from the effects of another convulsant, high (50 mM) K+, that also increases Na+ conductance.

METHODS

Transverse rat hippocampal slices were incubated for 1 h with 100 microM veratridine in oxygenated artificial cerebrospinal fluid (aCSF) with or without extracellular Ca2+. The medium was replaced by 50 microM[(3)H]glutamate in aCSF, and the slices incubated for 10 min at 37 degrees C. The slices were washed 3 times with cold aCSF after removal of the extracellular medium, and the radioactivity was quantified after solubilization of the slices.

RESULTS

Veratridine caused a time- and dose-dependent decrease, whereas high K+ had no effect on EAAT activity. The effects of veratridine on EAAT activity were not prevented by tetrodotoxin (TTX; 10 microM). Coincubation of ouabain with veratridine resulted in further decreases of EAAT activity. Removal of extracellular Ca2+ potentiated the inhibitory effects of veratridine (and other convulsants) on EAAT activity. Chelation of intracellular Ca2+ with BAPTA also increased the inhibitory effects of veratridine on EAAT activity.

CONCLUSIONS

Veratridine caused changes Ca2+ dynamics that led to inhibition of EAAT activity. Such changes in EAAT activity can contribute to the sustained epileptiform activity caused by veratridine.

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.