Endothelin-1 reverses the histone deacetylase inhibitor-induced increase in glial glutamate transporter transcription without affecting histone acetylation levels

Arundic Acid Increases Expression and Function of Astrocytic Glutamate Transporter EAAT1 Via the ERK, Akt, and NF-κB Pathways

Glutamate is the major excitatory neurotransmitter in the brain, but excessive synaptic glutamate must be removed to prevent excitotoxic injury and death. Two astrocytic glutamate transporters, excitatory amino acid transporter (EAAT) 1 and 2, play a major role in eliminating excess glutamate from the synapse. Dysregulation of EAAT1 contributes to the pathogenesis of multiple neurological disorders, such as Alzheimer's disease (AD), ataxia, traumatic brain injuries, and glaucoma. In the present study, we investigated the effect of arundic acid on EAAT1 to determine its efficacy in enhancing the expression and function of EAAT1, and its possible mechanisms of action. The studies were carried out in human astrocyte H4 cells as well as in human primary astrocytes. Our findings show that arundic acid upregulated EAAT1 expression at the transcriptional level by activating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Arundic acid increased astrocytic EAAT1 promoter activity, messenger RNA (mRNA)/protein levels, and glutamate uptake, while pharmacological inhibition of NF-κB or mutation on NF-κB binding sites in the EAAT1 promoter region abrogated these effects. Arundic acid increased NF-κB reporter activity and induced NF-κB nuclear translocation as well as its bindings to the EAAT1 promoter. Furthermore, arundic acid activated the Akt and ERK signaling pathways to enhance EAAT1 mRNA/protein levels. Finally, arundic acid attenuated manganese-induced decrease in EAAT1 expression by inhibiting expression of the transcription factor Ying Yang 1 (YY1). These results demonstrate that arundic acid increases the expression and function of EAAT1 via the Akt, ERK, and NF-κB signaling pathways, and reverses Mn-induced EAAT1 repression by inhibiting the Mn-induced YY1 activation.

Mechanism of 2,3,4',5-Tetrahydroxystilbene 2-O-β-D-Glucoside-Induced Upregulation of Glutamate Transporter 1 Protein Expression in Mouse Primary Astrocytes

Glutamate transporter-1 (GLT-1), a major glutamate transporter expressed in astrocytes, takes up excess glutamate from the micro-environment in order to prevent excitotoxicity. Drugs that increase GLT-1 expression may have therapeutic effects in disorders associated with neuronal excitotoxicity. 2,3,4',5-tetrahydroxystilbene 2-O-β-D-glucoside (TSG), a monomer of stilbene from polygonummultiflorum, exerts neuroprotection in a range of experimental models such as Alzheimer's disease and brain ischemia. In this study, we evaluated the effect of TSG on GLT-1 protein expression in mouse primary-cultured astrocytes. Results showed that TSG markedly increased the GLT-1 protein expression level in mouse primary-cultured astrocytes in a dose- and time-dependent manner, and this increase was mediated by the activation of protein kinase B (Akt) but not by the activation of extracellular signal-regulated protein kinase 1/2. Furthermore, inhibition of cAMP response element-binding protein, but not nuclear factor kappa B, abolished the TSG-mediated increase in GLT-1 protein expression in cultured astrocytes. Collectively, these findings may provide novel insights into the mechanism for TSG in neuroprotection, and would help search new agents targeting neurodegenerative disorders associated with impaired astrocytic glutamate transporters.

Transcriptional Regulation of the Astrocytic Excitatory Amino Acid Transporter 1 (EAAT1) via NF-κB and Yin Yang 1 (YY1)

Astrocytic glutamate transporter excitatory amino acid transporter (EAAT) 1, also known as glutamate aspartate transporter (GLAST) in rodents, is one of two glial glutamate transporters that are responsible for removing excess glutamate from synaptic clefts to prevent excitotoxic neuronal death. Despite its important role in neurophysiological functions, the molecular mechanisms of EAAT1 regulation at the transcriptional level remain to be established. Here, we report that NF-κB is a main positive transcription factor for EAAT1, supported by the following: 1) EAAT1 contains two consensus sites for NF-κB, 2) mutation of NF-κB binding sites decreased EAAT1 promoter activity, and 3) activation of NF-κB increased, whereas inhibition of NF-κB decreased EAAT1 promoter activity and mRNA/protein levels. EGF increased EAAT1 mRNA/protein levels and glutamate uptake via NF-κB. The transcription factor yin yang 1 (YY1) plays a role as a critical negative regulator of EAAT1, supported by the following: 1) the EAAT1 promoter contains multiple consensus sites for YY1, 2) overexpression of YY1 decreased EAAT1 promoter activity and mRNA/protein levels, and 3) knockdown of YY1 increased EAAT1 promoter activity and mRNA/protein levels. Manganese decreased EAAT1 expression via YY1. Epigenetic modifiers histone deacetylases (HDACs) served as co-repressors of YY1 to further decrease EAAT1 promoter activity, whereas inhibition of HDACs reversed manganese-induced decrease of EAAT1 expression. Taken together, our findings suggest that NF-κB is a critical positive regulator of EAAT1, mediating the stimulatory effects of EGF, whereas YY1 is a negative regulator of EAAT1 with HDACs as co-repressors, mediating the inhibitory effects of manganese on EAAT1 regulation.

Excitotoxicity and mitochondrial dysfunction underlie age-dependent ischemic white matter injury

The central nervous system white matter is damaged during an ischemic stroke and therapeutic strategies derived from experimental studies focused exclusively on young adults and gray matter have been unsuccessful in the more clinically relevant aging population. The risk for stroke increases with age and the white matter inherently becomes more susceptible to injury as a function of age. Age-related changes in the molecular architecture of white matter determine the principal injury mechanisms and the functional outcome. A prominent increase in the main plasma membrane Na(+)-dependent glutamate transporter, GLT-1/EAAT2, together with increased extracellular glutamate levels may reflect an increased need for glutamate signaling in the aging white matter to maintain its function. Mitochondria exhibit intricate dynamics to efficiently buffer Ca(2+), to produce sufficient ATP, and to effectively scavenge reactive oxygen species (ROS) in response to excitotoxicity to sustain axon function. Aging exacerbates mitochondrial fusion, leading to progressive alterations in mitochondrial dynamics and function, presumably to effectively buffer increased Ca(2+) load and ROS production. Interestingly, these adaptive adjustments become detrimental under ischemic conditions, leading to increased and early glutamate release and a rapid exhaustion of mitochondrial capacity to sustain energy status of axons. Consequently, protective interventions in young white matter become injurious or ineffective to promote recovery in aging white matter after an ischemic episode. An age-specific understanding of the mechanisms of injury processes in white matter is vital in order to design dynamic therapeutic approaches for stroke victims.

SLC1 glutamate transporters

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.

Histone deacetylase inhibitors preserve function in aging axons

Aging increases the vulnerability of aging white matter to ischemic injury. Histone deacetylase (HDAC) inhibitors preserve young adult white matter structure and function during ischemia by conserving ATP and reducing excitotoxicity. In isolated optic nerve from 12-month-old mice, deprived of oxygen and glucose, we show that pan- and Class I-specific HDAC inhibitors promote functional recovery of axons. This protection correlates with preservation of axonal mitochondria. The cellular expression of HDAC 3 in the central nervous system (CNS), and HDAC 2 in optic nerve considerably changed with age, expanding to more cytoplasmic domains from nuclear compartments, suggesting that changes in glial cell protein acetylation may confer protection to aging axons. Our results indicate that manipulation of HDAC activities in glial cells may have a universal potential for stroke therapy across age groups.

Trichostatin A enhances glutamate transporter GLT-1 mRNA levels in C6 glioma cells via neurosteroid-mediated cell differentiation

The neurotoxic effects of excitatory amino acids (EAAs) are suggested to be connected with the chronic loss of neuronal cells, thereby being responsible for the age-related neurodegenerative diseases. Therefore, it seems conceivable that the excitatory amino acid transporters may contribute to the protection of neuronal cells against the excitotoxic damage by facilitating the removal of EAAs from the brain tissue. On the other hand, previous studies have suggested that glial cell differentiation may be involved in the protection and recovery of neural function probably through the elevation of BDNF gene expression in the brain. Based on these findings, histone deacetylase (HDAC) inhibitors are assumed to induce glutamate transporter-1 (GLT-1) gene expression probably through the promotion of glial cell differentiation. Then, we examined the effects of HDAC inhibitors on GLT-1 mRNA levels in rat C6 glioma cells and found that trichostatin A can induce GLT-1 gene transcription following steroid 5α-reductase and GFAP gene expression. Therefore, it seems conceivable that glial cell differentiation may play a potential role in the removal of EAAs probably through the expression of GLT-1, thereby being involved in the protection of neuronal cells against the chronic excitotoxic insults in the brain.

The CpG island shore of the GLT-1 gene acts as a methylation-sensitive enhancer

Astrocytic lineage commitment and brain region-dependent specialization of glia are partly ascribed to epigenetic processes. Clearance of glutamate is an essential task, which astrocytes assume in a temporal-spatial fashion by distinct glutamate transporter expression. Glutamate transporter subtype 1 (GLT-1) is predominant in cortex (CTX), while it plays an inferior role in cerebellum (CER). Here, we set out to identify regulatory elements that could account for the differences in brain region-specific activity as well as response to dexamethasone (DEX) or epigenetic factors. We found a distal promoter element at the shore of the CpG island exhibiting enhancer function in response to DEX in reporter gene assays. This shore region showed slight enrichment in repressive trimethyl-histone H3 (Lys27) and under-representation of acetyl-histone H4 (H4ac) marks in DEX nonresponsive CER astrocytes as determined by chromatin immunoprecipitation. In addition, CpG sites of the shore region displayed higher methylation in CER than in CTX cells. Targeted in vitro methylation of CpG sites within the shore abrogated the stimulatory effects of DEX. Interestingly, the shore was characterized by a pronounced epigenetic plasticity in CTX cells since DEX exposure elicited an increase of H4ac in CTX in comparison to DEX nonresponsive CER. The transcriptional activity of this region was also affected by histone deacetylase inhibitors in a methylation- and brain region-dependent manner. Together, our study highlights the impact of an epigenetically adaptive DNA element of the GLT-1 promoter being decisive for brain region-specific activity and reactivity.

Histone deacetylase inhibitors preserve white matter structure and function during ischemia by conserving ATP and reducing excitotoxicity

The importance of white matter (WM) injury to stroke pathology has been underestimated in experimental animal models and this may have contributed to the failure to translate potential therapeutics into the stroke clinic. Histone deacetylase (HDAC) inhibitors are neuroprotective and also promote neurogenesis. These properties make them ideal candidates for stroke therapy. In a pure WM tract (isolated mouse optic nerve), we show that pan- and class I-specific HDAC inhibitors, administered before or after a period of oxygen and glucose deprivation (OGD), promote functional recovery of axons and preserve WM cellular architecture. This protection correlates with the upregulation of an astrocyte glutamate transporter, delayed and reduced glutamate accumulation during OGD, preservation of axonal mitochondria and oligodendrocytes, and maintenance of ATP levels. Interestingly, the expression of HDACs 1, 2, and 3 is localized to astrocytes, suggesting that changes in glial cell gene transcription and/or protein acetylation may confer protection to axons. Our findings suggest that a therapeutic opportunity exists for the use of HDAC inhibitors, targeting mitochondrial energy regulation and excitotoxicity in ischemic WM injury.

Comparative structural and functional analysis of the GLT-1/EAAT-2 promoter from man and rat

In the vertebrate CNS, glutamate transport predominantly occurs through the glutamate transporter subtype, GLT-1/EAAT-2, which prevails in astrocytes. GLT-1/EAAT-2 expression is impaired in many acute and chronic brain diseases, leading to increases in extracellular glutamate and subsequent excitotoxic neuronal cell death. An obvious therapeutical approach to prevent glutamate-induced brain damage would be targeting GLT-1/EAAT-2 expression. Since so far, insights into the mechanisms modulating GLT-1/EAAT-2 expression mostly originated from work with rat astrocytes, we now sought to determine whether this modulatory network would also apply to humans. To this end, we have cloned the previously unknown rat GLT-1/EAAT-2 promoter and compared it to the human promoter sequence. In reporter assays, the cloned 2.7-kb region immediately flanking the 5'-end of the rat GLT-1/EAAT-2 gene allowed for similar increases in constitutive gene expression as the human promoter sequence. Sequence analysis demonstrated the presence of highly conserved regions on the rat and human GLT-1/EAAT-2 promoters, which turned out to be likewise essential for constitutive GLT-1/EAAT-2 expression, stimulation of gene transcription by EGF, TGFalpha, and PACAP as well as inhibition of gene transcription by TNFalpha. Intriguingly, endothelin-1 which inhibits endogenous GLT-1/EAAT-2 expression, promoted activity of both rat and human reporter constructs, indicating the existence of (an) inhibitory mechanism(s) not operational in the reporter gene assay. Our findings establish close similarities in the regulation of GLT-1/EAAT-2 expression in rat and man and, hence, validate rat astrocytes as an assay system for studying the molecular mechanisms affecting glutamate homeostasis in the healthy and diseased human brain.

Valproate and amitriptyline exert common and divergent influences on global and gene promoter-specific chromatin modifications in rat primary astrocytes

Aberrant biochemical processes in the brain frequently go along with subtle shifts of the cellular epigenetic profile that might support the pathogenic progression of psychiatric disorders. Although recent reports have implied the ability of certain antidepressants and mood stabilizers to modulate epigenetic parameters, studies comparing the actions of these compounds under the same conditions are lacking. In this study, we screened amitriptyline (AMI), venlafaxine, citalopram, as well as valproic acid (VPA), carbamazepine, and lamotrigine for their potential actions on global and local epigenetic modifications in rat primary astrocytes. Among all drugs, VPA exposure evoked the strongest global chromatin modifications, including histone H3/H4 hyperacetylation, 2MeH3K9 hypomethylation, and DNA demethylation, as determined by western blot and luminometric methylation analysis, respectively. CpG demethylation occurred independently of DNA methyltransferase (DNMT) suppression. Strikingly, AMI also induced slight cytosine demethylation, paralleled by the reduction in DNMT enzymatic activity, without affecting the global histone acetylation status. Locally, VPA-induced chromatin modifications were reflected at the glutamate transporter (GLT-1) promoter as shown by bisulfite sequencing and acetylated histone H4 chromatin immunoprecipitation analysis. Distinct CpG sites in the distal part of the GLT-1 promoter were demethylated and enriched in acetylated histone H4 in response to VPA. For the first time, we could show that these changes were associated with an enhanced transcription of this astrocyte-specific gene. In contrast, AMI failed to stimulate GLT-1 transcription and to alter promoter methylation levels. In conclusion, VPA and AMI globally exerted chromatin-modulating activities using different mechanisms that divergently precipitated at an astroglial gene locus.

High extracellular glutamate modulates expression of glutamate transporters and glutamine synthetase in cultured astrocytes

Astroglial cells clear extracellular glutamate through the glutamate transporters, GLT-1 and GLAST, and subsequently convert the incorporated glutamate into glutamine by the enzyme, glutamine synthetase (GS). Several forms of acute brain injury are associated with the increased expression of GS and the decreased expression of GLT-1 and/or GLAST, eventually leading to the accumulation of excitotoxic extracellular glutamate concentrations. Although of clinical interest, the actual trigger of these injury-related changes of glial glutamate turnover remains unknown. Our present studies provide evidence that increases in extracellular glutamate, as present in many brain injuries, are sufficient to modulate the expression of glutamate transporters and GS. Subjecting cultured cortical astrocytes to glutamate concentrations of 0.5-20 mM resulted in a 25% loss of GLT-1 and GLAST protein levels after 24 h; GLT-1 and GLAST levels maximally decreased by 40% and 75%, respectively, after 72 h. This decline was not due to astroglial cell death, since glutamate up to 50 mM did not affect the survival of cultured astrocytes within 72 h. Major astrocytic cell death, however, occurred in cultures maintained under severe (4% O(2)), but not mild (9% O(2)), hypoxia, as well as in the presence of aspartate (>or=20 mM). Glutamate at >or=1 mM induced a prolonged increase of GS expression in contrast to glutamate transporters. Neither the decline of glutamate transporter expression nor the increase in GS expression induced by high extracellular glutamate was further modulated by mild hypoxia. Whereas the stimulatory influences of glutamate on GS expression were prevented by the non-competitive NMDA receptor antagonist, MK801, the inhibitory influences on glutamate transporter expression were neither sensitive to MK801, the non-competitive mGluR5 antagonist, MTEP, nor the non-competitive AMPA receptor antagonist, GYKI52466, implying that glutamate controls glial glutamate transport by a glutamate receptor-independent mechanism.