Glutamate is an essential excitatory neurotransmitter regulating brain functions. Excitatory amino acid transporter (EAAT)-2 is one of the major glutamate transporters expressed predominantly in astroglial cells and is responsible for 90% of total glutamate uptake. Glutamate transporters tightly regulate glutamate concentration in the synaptic cleft. Dysfunction of EAAT2 and accumulation of excessive extracellular glutamate has been implicated in the development of several neurodegenerative diseases including Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. Analysis of the 2.5-kb human EAAT2 promoter showed that NF-κB is an important regulator of EAAT2 expression in astrocytes. Screening of approximately 1,040 FDA-approved compounds and nutritionals led to the discovery that many β-lactam antibiotics are transcriptional activators of EAAT2 resulting in increased EAAT2 protein levels. Treatment of animals with ceftriaxone (CEF), a β-lactam antibiotic, led to an increase of EAAT2 expression and glutamate transport activity in the brain. CEF has neuroprotective effects in both in vitro and in vivo models based on its ability to inhibit neuronal cell death by preventing glutamate excitotoxicity. CEF increases EAAT2 transcription in primary human fetal astrocytes (PHFA) through the NF-κB signaling pathway. The NF-κB binding site at −272 position was critical in CEF-mediated EAAT2 protein induction. These studies emphasize the importance of transcriptional regulation in controlling glutamate levels in the brain. They also emphasize the potential utility of the EAAT2 promoter for developing both low and high throughput screening assays to identify novel small molecule regulators of glutamate transport with potential to ameliorate pathological changes occurring during and causing neurodegeneration.
Astroglial glutamate transporter EAAT2/GLT1 prevents glutamate-induced excitotoxicity in the central nervous system. Expression of EAAT2/GLT1 is dynamically regulated by neurons. The pathogenesis of amyotrophic lateral sclerosis (ALS) involves astroglial dysfunction, including dramatic loss of EAAT2/GLT1. DNA methylation of gene promoters represents one of the most important epigenetic mechanisms in regulating gene expression. The involvement of DNA methylation in the regulation of astroglial EAAT2/GLT1 expression in different conditions, especially in ALS has not been explored. In this study, we established a procedure to selectively isolate a pure astrocyte population in vitro and in vivo from BAC GLT1 eGFP mice using an eGFP-based fluorescence-activated cell sorting approach. Astrocytes isolated from this procedure are GFAP+ and GLT1+ and respond to neuronal stimulation, enabling direct methylation analysis of GLT1 promoter in these astrocytes. To investigate the role of DNA methylation in physiological and pathological EAAT2/GLT1 expression, methylation status of the EAAT2/GLT1 promoter was analyzed in astrocytes from in vitro and in vivo paradigms or postmortem ALS motor cortex by bisulfite sequencing method. DNA demethylation on selective CpG sites of the GLT1 promoter was highly correlated to increased GLT1 mRNA levels in astrocytes in response to neuronal stimulation; however, low level of methylation was found on CpG sites of EAAT2 promoter from postmortem motor cortex of human amyotrophic lateral sclerosis patients. In summary, hypermethylation on selective CpG sites of the GLT1 promoter is involved in repression of GLT1 promoter activation, but this regulation does not play a role in astroglial dysfunction of EAAT2 expression in patients with ALS.
epigenetic; astrocyte; GLT1
EAAT2 is a predominantly astroglial glutamate transporter responsible for the majority of synaptic glutamate clearance in the mammalian CNS. Its dysfunction has been linked to many neurological disorders, including amyotrophic lateral sclerosis (ALS). Decreases in EAAT2 expression and function have been implicated in causing motor neuron excitotoxic death in ALS. Nevertheless, increasing EAAT2 expression does not significantly improve ALS phenotype in mouse models or in clinical trials. In the SOD1-G93A mouse model of inherited ALS, the cytosolic carboxy-terminal domain is cleaved from EAAT2, conjugated to SUMO1, and accumulated in astrocytes where it triggers astrocyte-mediated neurotoxic effects as disease progresses. However, it is not known whether this fragment is sumoylated after cleavage or if full-length EAAT2 is already sumoylated prior to cleavage as part of physiological regulation. In this study, we show that a fraction of full-length EAAT2 is constitutively sumoylated in primary cultures of astrocytes in vitro and in the CNS in vivo. Furthermore, the extent of sumoylation of EAAT2 does not change during the course of ALS in the SOD1-G93A mouse and is not affected by the expression of ALS-causative mutant SOD1 proteins in astrocytes in vitro, indicating that EAAT2 sumoylation is not driven by pathogenic mechanisms. Most interestingly, sumoylated EAAT2 localizes to intracellular compartments, while non-sumoylated EAAT2 resides on the plasma membrane. In agreement, promoting desumoylation in primary astrocytes causes increased EAAT2–mediated glutamate uptake. These findings could have implications for optimizing therapeutic approaches aimed at increasing EAAT2 activity in the dysfunctional or diseased CNS.
Amyotrophic lateral sclerosis; excitotoxicity; protein trafficking; GLT-1
Excitatory amino acid transporters (EAAT) uptake extracellular glutamate, the major excitatory neurotransmitter in the brain. EAAT type 3 (EAAT3), the main neuronal EAAT, is expressed widely in the central nervous system. We have shown that the volatile anesthetic isoflurane increases EAAT3 activity and trafficking to the plasma membrane. Thus, we hypothesize that EAAT3 mediates isoflurane-induced anesthesia. To test this hypothesis, the potency of isoflurane to induce immobility and hypnosis, two major components of general anesthesia, was compared in the CD-1 wild-type mice and EAAT knockout mice that had a CD-1 strain gene background. Hypnosis was assessed by loss of righting reflex in this study. The expression of EAAT1 and EAAT2, two widely expressed EAATs in the central nervous system, in the cerebral cortex and spinal cord was not different between the EAAT3 knockout mice and wild-type mice. The concentration required for isoflurane to cause immobility to painful stimuli, a response involving primarily reflex loops in the spinal cord, was not changed by EAAT3 knockout. However, the EAAT3 knockout mice were more sensitive to isoflurane-induced hypnotic effects, which may be mediated by hypothalamic sleep neural circuits. Interestingly, the EAAT3 knockout mice did not have an altered sensitivity to the hypnotic effects caused by ketamine, an intravenous anesthetic that is a glutamate receptor antagonist and does not affect EAAT3 activity. These results suggest that EAAT3 modulates the sensitivity of neural circuits to isoflurane. These results, along with our previous findings that isoflurane increases EAAT3 activity, indicate that EAAT3 may regulate isoflurane-induced behavioral changes, including anesthesia.
anesthesia; glutamate transporter; gene expression; hypnosis; isoflurane
The neuron-astrocyte synaptic complex is a fundamental operational unit of the nervous system. Astroglia play a central role in the regulation of synaptic glutamate, via neurotransmitter transport by GLT1/EAAT2. The astroglial mechanisms underlying this essential neuron-glial communication are not known. Here we show that presynaptic terminals are sufficient and necessary for GLT1/EAAT2 transcriptional activation and have identified the molecular pathway that regulates astroglial responses to presynaptic input. Presynaptic terminals regulate astroglial GLT1/EAAT2 via kappa B-motif binding phosphoprotein (KBBP), the mouse homologue of human heterogeneous nuclear ribonucleoprotein K (hnRNP K), which binds to an essential element of GLT1/EAAT2 promoter. This neuron-stimulated factor is required for GLT1/EATT2 transcriptional activation and is responsible for astroglial alterations in neural injury. Denervation of neuron-astrocyte signaling in vivo, by acute corticospinal tract transection, ricin-induced motor neuron death, or chronic neurodegeneration in amyotrophic lateral sclerosis (ALS) all result in reduced astroglial KBBP expression and transcriptional dysfunction of astroglial transporter expression. Our studies indicate that presynaptic elements dynamically coordinate normal astroglial function and also provide a fundamental signaling mechanism by which altered neuronal function and injury leads to dysregulated astroglia in CNS disease.
In the central nervous system (CNS), glutamate rapidly upregulates the activities of different excitatory amino-acid transporter subtypes (EAATs) in order to help protect neurons from excitotoxicity. Since human platelets display a specific sodium-dependent glutamate uptake activity, and express the three major glutamate transporters, which may be affected in neurological disorders, we investigated whether platelets are subject to substrate-induced modulation as described for CNS.A time- and dose-dependent upregulation of [3H]-glutamate uptake (up to two-fold) was observed in platelets preincubated with glutamate. There was an increase in maximal velocity rate without affinity changes. Glutamate receptor agonists and antagonists did not modulate this upregulation and preincubation with glutamate analogues failed to mimic the glutamate effect. Only aspartate preincubation increased the uptake, albeit ∼35% less with respect to glutamate.The effect of glutamate preincubation on the expression of the three major transporters was studied by Western blotting, showing an increase of ∼70% in EAAT1 immunoreactivity that was completely blocked by cycloheximide (CEM). However, L-serine-O-sulphate, at a concentration (200 μM) known to block EAAT1/3 selectively, did not completely inhibit the effect of glutamate stimulation, indicating the possible involvement of EAAT2.In fact, glutamate stimulation was completely abolished only when, following CEM pre-incubation, the experiment was run in the presence of the selective EAAT2 inhibitor dihydrokainic acid. Since surface biotinylation experiments failed to show evidence of EAAT2 translocation, our results suggest the existence of a different way of regulating EAAT2 activity.These findings indicate that human platelets display a substrate-dependent modulation of glutamate uptake mediated by different molecular mechanisms and confirm that ex vivo platelets are a reliable model to investigate the dysfunction of glutamate uptake regulation in patients affected by neurological disorders.
Glutamate uptake; human platelets; substrate modulation; EAAT1
Dysregulation of glutamate handling ensuing downregulation of expression and activity levels of the astroglial glutamate transporter EAAT2 is implicated in excitotoxic degeneration of motor neurons in amyotrophic lateral sclerosis (ALS). We previously reported that EAAT2 (a.k.a. GLT-1) is cleaved by caspase-3 at its cytosolic carboxy-terminus domain. This cleavage results in impaired glutamate transport activity and generates a proteolytic fragment (CTE) that we found to be post-translationally conjugated by SUMO1. We show here that this sumoylated CTE fragment accumulates in the nucleus of spinal cord astrocytes of the SOD1-G93A mouse model of ALS at symptomatic stages of disease. Astrocytic expression of CTE, artificially tagged with SUMO1 (CTE-SUMO1) to mimic the native sumoylated fragment, recapitulates the nuclear accumulation pattern of the endogenous EAAT2-derived proteolytic fragment. Moreover, in a co-culture binary system, expression of CTE-SUMO1 in spinal cord astrocytes initiates extrinsic toxicity by inducing caspase-3 activation in motor neuron-derived NSC-34 cells or axonal growth impairment in primary motor neurons. Interestingly, prolonged nuclear accumulation of CTE-SUMO1 is intrinsically toxic to spinal cord astrocytes, although this gliotoxic effect of CTE-SUMO1 occurs later than the indirect, non-cell autonomous toxic effect on motor neurons. As more evidence on the implication of SUMO substrates in neurodegenerative diseases emerges, our observations strongly suggest that the nuclear accumulation in spinal cord astrocytes of a sumoylated proteolytic fragment of the astroglial glutamate transporter EAAT2 could participate to the pathogenesis of ALS and suggest a novel, unconventional role for EAAT2 in motor neuron degeneration.
Amyotrophic lateral sclerosis; post-translational modification; SUMO; excitotoxicity
The pharmacological profile of a novel glutamate transport inhibitor, WAY-855 (3-amino-tricyclo[2.2.1.02.6]heptane-1,3-dicarboxylic acid), on the activity of the human forebrain glutamate transporters EAAT1, EAAT2 and EAAT3 expressed in stable mammalian cell lines and in Xenopus laevis oocytes is presented.WAY-855 inhibited glutamate uptake mediated by all three subtypes in a concentration-dependent manner, with preferential inhibition of the CNS-predominant EAAT2 subtype in both cells and oocytes. IC50 values for EAAT2 and EAAT3 inhibition in cells were 2.2 and 24.5 μM, respectively, while EAAT1 activity was inhibited by 50% at 100 μM (IC50 values determined in oocytes were 1.3 μM (EAAT2), 52.5 μM (EAAT3) and 125.9 μM (EAAT1)).Application of WAY-855 to EAAT-expressing oocytes failed to induce a transporter current, and the compound failed to exchange with accumulated [3H]D-aspartate in synaptosomes consistent with a nonsubstrate inhibitor. WAY-855 inhibited D-aspartate uptake into cortical synaptosomes by a competitive mechanism, and with similar potency to that observed for the cloned EAAT2.WAY-855 failed to agonise or antagonise ionotropic glutamate receptors in cultured hippocampal neurones, or the human metabotropic glutamate receptor subtype 4 expressed in a stable cell line.WAY-855 represents a novel structure in glutamate transporter pharmacology, and exploration of this structure might provide insights into the discrimination between EAAT2 and other EAAT subtypes.
Excitatory aminoacid transporter; EAAT; WAY-855; glutamate transporter; inhibitor
Raloxifene (RX), a selective estrogen receptor modulator (SERM), exerts neuroprotection in multiple clinical and experimental settings. Astrocytic glutamate transporters GLT-1 (EAAT2) and GLAST (EAAT1) are the main glutamate transporters in the central nervous system, taking up most of excess glutamate from the synaptic cleft to prevent excitotoxic neuronal death. Since drugs targeting astrocytic glutamate transporters to enhance their expression and function represent potential therapeutics for neurodegenerative disorders associated with excitotoxicity, we tested if RX modulates the expression and function of GLT-1 and GLAST in rat primary astrocytes. The results showed that RX significantly increased glutamate uptake and expression of GLT-1 mRNA and protein levels. RX enhanced GLT-1 expression by the activation of multiple signaling pathways including ERK, EGFR and CREB mediated by estrogen receptors (ERs) ER-α, ER-β and GPR30. At the transcriptional level, NF-κB played a critical role in RX-induced GLT-1 expression as RX increased NF-κB reporter activity and induced binding of NF-κB p65 and p50 to the GLT-1 promoter. RX attenuated the reduction of GLT-1 expression and glutamate uptake induced by manganese (Mn) whose chronic high levels of exposure cause manganism. RX also upregulated GLAST by increasing its promoter activity and protein levels via the NF-κB pathway and ERs. Our findings provide new insight into the mechanism of RX-induced enhancement of GLT-1 and GLAST expression, as well as the attenuation of Mn-reduced expression of these transporters. These findings will be highly valuable for developing therapeutics of neurodegenerative diseases associated with impaired astrocytic glutamate transporters.
raloxifene; GLT-1; GLAST; manganese; estrogen receptors; NF-κB
In this study we have examined the use of the ecdysone-inducible mammalian expression system (Invitrogen) for the regulation of expression of the predominant L-glutamate transporter EAAT2 (Excitatory Amino Acid Transporter) in HEK 293 cells.HEK 293 cells which were stably transformed with the regulatory vector pVgRXR (EcR 293 cells) were used for transfection of the human EAAT2 cDNA using the inducible vector pIND and a clone designated HEK/EAAT2 was selected for further characterization.Na+-dependent L-glutamate uptake activity (3.2 pmol min−1 mg−1) was observed in EcR 293 cells and this was increased approximately 2 fold in the uninduced HEK/EAAT2 cells, indicating a low level of basal EAAT2 activity in the absence of exogenous inducing agent. Exposure of HEK/EAAT2 cells to the ecdysone analogue Ponasterone A (10 μM for 24 h) resulted in a ⩾10 fold increase in the Na+-dependent activity.L-glutamate uptake into induced HEK/EAAT2 cells followed first-order Michaelis-Menten kinetics and Eadie-Hofstee transformation of the saturable uptake data produced estimates of kinetic parameters as follows; Km 52.7±7.5 μM, Vmax 3.8±0.9 nmol min−1 mg−1 protein.The pharmacological profile of the EAAT2 subtype was characterized using a series of L-glutamate transport inhibitors and the rank order of inhibitory potency was similar to that described previously for the rat homologue GLT-1 and in synaptosomal preparations from rat cortex.Addition of the EAAT2 modulator arachidonic acid resulted in an enhancement (155±5% control in the presence of 30 μM) of the L-glutamate transport capacity in the induced HEK/EAAT2 cells.This study demonstrates that the expression of EAAT2 can be regulated in a mammalian cell line using the ecdysone-inducible mammalian expression system.
Excitatory amino acid transporter; EAAT2 pharmacology; L-glutamate; ecdysone; kainate; dihydrokainate; arachidonic acid
Background and Purpose
Excitatory amino acid transporters (EAATs) in the CNS contribute to the clearance of glutamate released during neurotransmission. The aim of this study was to explore the role of EAATs in the regulation of locus coeruleus (LC) neurons by glutamate.
We measured the effect of different EAAT subtype inhibitors/enhancers on glutamate- and KCl-induced activation of LC neurons in rat slices. EAAT2–3 expression in the LC was also characterized by immunohistochemistry.
The EAAT2–5 inhibitor DL-threo-β-benzyloxaspartic acid (100 μM), but not the EAAT2, 4, 5 inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (100 μM) or the EAAT2 inhibitor dihydrokainic acid (DHK; 100 μM), enhanced the glutamate- and KCl-induced activation of the firing rate of LC neurons. These effects were blocked by ionotropic, but not metabotrobic, glutamate receptor antagonists. DHK (100 μM) was the only EAAT inhibitor that increased the spontaneous firing rate of LC cells, an effect that was due to inhibition of EAAT2 and subsequent AMPA receptor activation. Chronic treatment with ceftriaxone (200 mg·kg−1 i.p., once daily, 7 days), an EAAT2 expression enhancer, increased the actions of glutamate and DHK, suggesting a functional impact of EAAT2 up-regulation on the glutamatergic system. Immuhistochemical data revealed the presence of EAAT2 and EAAT3 surrounding noradrenergic neurons and EAAT2 on glial cells in the LC.
Conclusions and Implications
These results remark the importance of EAAT2 and EAAT3 in the regulation of rat LC by glutamate. Neuronal EAAT3 would be responsible for terminating the action of synaptically released glutamate, whereas glial EAAT2 would regulate tonic glutamate concentrations in this nucleus.
locus coeruleus; EAAT2; EAAT3; KCl; glutamate; firing
Drugs which upregulate astrocyte glutamate transport may be useful neuroprotective compounds by preventing excitotoxicity. We set up a new system to identify potential neuroprotective drugs which act through GLT-1. Primary mouse striatal astrocytes grown in the presence of the growth-factor supplement G5 express high levels of the functional glutamate transporter, GLT-1 (also known as EAAT2) as assessed by Western blotting and 3H-glutamate uptake assay, and levels decline following growth factor withdrawal. The GLT-1 transcriptional enhancer dexamethasone (0.1 or 1 μM) was able to prevent loss of GLT-1 levels and activity following growth factor withdrawal. In contrast, ceftriaxone, a compound previously reported to enhance GLT-1 expression, failed to regulate GLT-1 in this system. The neuroprotective compound riluzole (100 μM) upregulated GLT-1 levels and activity, through a mechanism that was not dependent on blockade of voltage-sensitive ion channels, since zonasimide (1 mM) did not regulate GLT-1. Finally, CDP-choline (10 μM – 1 mM), a compound which promotes association of GLT-1/EAAT2 with lipid rafts was unable to prevent GLT-1 loss under these conditions. This observation extends the known pharmacological actions of riluzole, and suggests that this compound may exert its neuroprotective effects through an astrocyte-dependent mechanism.
EAAT2; neuroprotection; citicholine; Parkinson’s Disease; glutamate uptake; glutamate transporters
Excitotoxicity has been implicated as the mechanism of neuronal damage resulting from acute insults such as stroke, epilepsy, and trauma, as well as during the progression of adult-onset neurodegenerative disorders such as Alzheimer’s disease and amyotrophic lateral sclerosis (ALS). Excitotoxicity is defined as excessive exposure to the neurotransmitter glutamate or overstimulation of its membrane receptors, leading to neuronal injury or death. One potential approach to protect against excitotoxic neuronal damage is enhanced glutamate reuptake. The glial glutamate transporter EAAT2 is the quantitatively dominant glutamate transporter and plays a major role in clearance of glutamate. Expression of EAAT2 protein is highly regulated at the translational level. In an effort to identify compounds that can induce translation of EAAT2 transcripts, a cell-based enzyme-linked immunosorbent assay was developed using a primary astrocyte line stably transfected with a vector designed to identify modulators of EAAT2 translation. This assay was optimized for high-throughput screening, and a library of approximately 140,000 compounds was tested. In the initial screen, 293 compounds were identified as hits. These 293 hits were retested at 3 concentrations, and a total of 61 compounds showed a dose-dependent increase in EAAT2 protein levels. Selected compounds were tested in full 12-point dose-response experiments in the screening assay to assess potency as well as confirmed by Western blot, immunohistochemistry, and glutamate uptake assays to evaluate the localization and function of the elevated EAAT2 protein. These hits provide excellent starting points for developing therapeutic agents to prevent excitotoxicity.
excitotoxicity; glutamate transporter; EAAT2; high-throughput screen; neurodegeneration
Klotho, a transmembrane protein, which can be cleaved off as β-glucuronidase and hormone, is released in both, kidney and choroid plexus and encountered in blood and cerebrospinal fluid. Klotho deficiency leads to early appearance of age-related disorders and premature death. Klotho may modify transport by inhibiting 1,25(OH)2D3 formation or by directly affecting channel and carrier proteins. The present study explored whether Klotho influences the activity of the Na+-coupled excitatory amino acid transporters EAAT3 and EAAT4, which are expressed in kidney (EAAT3), intestine (EAAT3) and brain (EAAT3 and EAAT4). To this end, cRNA encoding EAAT3 or EAAT4 was injected into Xenopus oocytes with and without additional injection of cRNA encoding Klotho. EAAT expressing Xenopus oocytes were further treated with recombinant human β-Klotho protein with or without β-glucuronidase inhibitor D-saccharic acid 1,4-lactone monohydrate (DSAL). Electrogenic excitatory amino acid transport was determined as L-glutamate-induced current (Iglu) in two electrode voltage clamp experiments. EAAT3 and EAAT4 protein abundance in the Xenopus oocyte cell membrane was visualized by confocal microscopy and quantified utilizing chemiluminescence. As a result, coexpression of Klotho cRNA significantly increased Iglu in both, EAAT3 or EAAT4-expressing Xenopus oocytes. Klotho cRNA coexpression significantly increased the maximal current and cell membrane protein abundance of both EAAT3 and EAAT4. The effect of Klotho coexpression on EAAT3 and EAAT4 activity was mimicked by treating EAAT3 or EAAT4-expressing Xenopus oocytes with recombinant human β-Klotho protein. The effects of Klotho coexpression and of treatment with recombinant human β-Klotho protein were both abrogated in the presence of DSAL (10 µM). In conclusion, Klotho is a novel, powerful regulator of the excitatory amino acid transporters EAAT3 and EAAT4.
Equilibrative nucleoside transporter 1 (ENT1) and excitatory amino acid transporter 2 (EAAT2) are predominantly expressed in astrocytes where they are thought to regulate synaptic adenosine and glutamate levels. Because mice lacking ENT1 display increased glutamate levels in the ventral striatum, we investigated whether ENT1 regulates the expression and function of EAAT2 in astrocytes, which could contribute to altered glutamate levels in the striatum.
We examined the effect of ENT1 inhibition and overexpression on the expression of EAAT2 using quantitative real-time PCR and measured glutamate uptake activity in cultured astrocytes. We also examined the effect of 0 to 200 mM ethanol doses for 0 to 24 hours of ethanol exposure on EAAT2 expression and glutamate uptake activity. We further examined the effect of ENT1 knockdown by a specific siRNA on ethanol-induced EAAT2 expression.
An ENT1-specific antagonist and siRNA treatments significantly reduced both EAAT2 expression and glutamate uptake activity while ENT1 overexpression up-regulated EAAT2 mRNA expression. Interestingly, 100 or 200 mM ethanol exposure increased EAAT2 mRNA expression as well as glutamate uptake activity. Moreover, we found that ENT1 knockdown inhibited the ethanol-induced EAAT2 up-regulation.
Our results suggest that ENT1 regulates glutamate uptake activity by altering EAAT2 expression and function, which might be implicated in ethanol intoxication and preference.
Excitatory Amino Acid Transporter 2 (EAAT2); Equilibrative Nucleoside Transporter 1 (ENT1); Glutamate Uptake; Adenosine Uptake
Glial glutamate transporter EAAT2 plays a major role in glutamate clearance in synaptic clefts. Several lines of evidence indicate that strategies designed to increase EAAT2 expression have potential for preventing excitotoxicity, which contributes to neuronal injury and death in neurodegenerative diseases. We previously discovered several classes of compounds that can increase EAAT2 expression through translational activation. Here, we present efficacy studies of the compound LDN/OSU-0212320, which is a pyridazine derivative from one of our lead series. In a murine model, LDN/OSU-0212320 had good potency, adequate pharmacokinetic properties, no observed toxicity at the doses examined, and low side effect/toxicity potential. Additionally, LDN/OSU-0212320 protected cultured neurons from glutamate-mediated excitotoxic injury and death via EAAT2 activation. Importantly, LDN/OSU-0212320 markedly delayed motor function decline and extended lifespan in an animal model of amyotrophic lateral sclerosis (ALS). We also found that LDN/OSU-0212320 substantially reduced mortality, neuronal death, and spontaneous recurrent seizures in a pilocarpine-induced temporal lobe epilepsy model. Moreover, our study demonstrated that LDN/OSU-0212320 treatment results in activation of PKC and subsequent Y-box–binding protein 1 (YB-1) activation, which regulates activation of EAAT2 translation. Our data indicate that the use of small molecules to enhance EAAT2 translation may be a therapeutic strategy for the treatment of neurodegenerative diseases.
Volatile anesthetics enhance the activity of glutamate transporter type 3 (also called excitatory amino acid transporter type 3, EAAT3), the major neuronal EAAT. In addition to glutamate, EAAT3 can also uptake L-cysteine, the rate-limiting substrate for the synthesis of glutathione. Our previous study showed that oxidative stress inhibited glutamate-induced EAAT3 activity. We determined whether oxidative stress would reduce L-cysteine-induced EAAT3 activity and whether this reduction would be attenuated by volatile anesthetics.
Rat EAAT3 was expressed in Xenopus oocytes. L-glutamate- and L-cysteine-induced membrane currents were recorded using the two-electrode voltage clamp technique. The peak current was quantified to reflect the amount of transported substrates because transport of substrates via EAATs is electrogenic.
Exposure of oocytes to 5 mM tert-butyl hydroperoxide, an organic oxidant, for 10 min reduced the Vmax, but did not affect the Km, of EAAT3 for L-cysteine. The volatile anesthetics isoflurane, sevoflurane and desflurane at concentrations from 1 to 3% attenuated the tert-butyl hydroperoxide-reduced EAAT3 activity for L-glutamate and L-cysteine.
Our results suggest that volatile anesthetics preserve EAAT3 function to transport L-glutamate and L-cysteine under oxidative stress, which may be a mechanism for the neuroprotective effects of volatile anesthetics.
The excitatory amino acid transporters (EAATs) are transmembrane proteins responsible for the uptake of (S)-glutamate from the synaptic cleft. To date, five subtypes EAAT1-5 have been identified for which selective inhibitors have been discovered for EAAT1 and EAAT2. By screening of a commercially available compound library consisting of 4,000 compounds, N-acyl-N-phenylpiperazine analog (±)-exo-1 was identified to be a non-selective inhibitor at EAAT1-3 displaying IC50 values in the mid-micromolar range (10 μM, 40 μM and 30 μM at EAAT1, 2 and 3, respectively). Subsequently, we designed and synthesized a series of analogs to explore the structure-activity-relationship of this scaffold in the search for analogs characterized by increased inhibitory potency and/or EAAT subtype selectivity. Despite extensive efforts, all analogs of (±)-exo-1 proved to be either inactive or to have least 3-fold lower inhibitory potency than the lead, and furthermore none of the active analogs displayed selectivity for a particular subtype amongst the EAAT1-3. On the basis of our findings, we speculate that (±)-exo-1 binds to a recess (deepening) on the EAAT proteins than a well-defined pocket.
Excitatory amino acid transporters; EAATs; Rational ligand design; Medicinal chemistry
The EAAT2 glutamate transporter, accounts for >90% of hippocampal glutamate uptake. Although EAAT2 is predominantly expressed in astrocytes, ∼10% of EAAT2 molecules are found in axon terminals. Despite the lower level of EAAT2 expression in glutamatergic terminals, when hippocampal slices are incubated with low concentration of d-aspartate (an EAAT2 substrate), axon terminals accumulate d-aspartate as quickly as astroglia. This implies an unexplained mismatch between the distribution of EAAT2 protein and of EAAT2-mediated transport activity. One hypothesis is that (1) heteroexchange of internal substrate with external substrate is considerably faster than net uptake and (2) terminals favor heteroexchange because of high levels of internal glutamate. However, it is currently unknown whether heteroexchange and uptake have similar or different rates. To address this issue, we used a reconstituted system to compare the relative rates of the two processes in rat and mice. Net uptake was sensitive to changes in the membrane potential and was stimulated by external permeable anions in agreement with the existence of an uncoupled anion conductance. By using the latter, we also demonstrate that the rate of heteroexchange also depends on the membrane potential. Additionally, our data further suggest the presence of a sodium leak in EAAT2. By incorporating the new findings in our previous model of glutamate uptake by EAAT2, we predict that the voltage sensitivity of exchange is caused by the voltage-dependent third Na+ binding. Further, both our experiments and simulations suggest that the relative rates of net uptake and heteroexchange are comparable in EAAT2.
glutamate uptake; liposomes; membrane protein purification; neuronal glutamate uptake; neurotransmitter transport; transport kinetics
Excitatory amino acid transporters (EAATs) limit glutamatergic signaling and maintain extracellular glutamate concentrations below neurotoxic levels. Of the five known EAAT isoforms (EAATs 1–5), only the neuronal isoform, EAAT3 (EAAC1), can efficiently transport the uncharged amino acid L-cysteine. EAAT3-mediated cysteine transport has been proposed to be a primary mechanism used by neurons to obtain cysteine for the synthesis of glutathione, a key molecule in preventing oxidative stress and neuronal toxicity. The molecular mechanisms underlying the selective transport of cysteine by EAAT3 have not been elucidated. Here we propose that the transport of cysteine through EAAT3 requires formation of the thiolate form of cysteine in the binding site. Using Xenopus oocytes and HEK293 cells expressing EAAT2 and EAAT3, we assessed the transport kinetics of different substrates and measured transporter-associated currents electrophysiologically. Our results show that L-selenocysteine, a cysteine analog that forms a negatively-charged selenolate ion at physiological pH, is efficiently transported by EAATs 1–3 and has a much higher apparent affinity for transport when compared to cysteine. Using a membrane tethered GFP variant to monitor intracellular pH changes associated with transport activity, we observed that transport of either L-glutamate or L-selenocysteine by EAAT3 decreased intracellular pH, whereas transport of cysteine resulted in cytoplasmic alkalinization. No change in pH was observed when cysteine was applied to cells expressing EAAT2, which displays negligible transport of cysteine. Under conditions that favor release of intracellular substrates through EAAT3 we observed release of labeled intracellular glutamate but did not detect cysteine release. Our results support a model whereby cysteine transport through EAAT3 is facilitated through cysteine de-protonation and that once inside, the thiolate is rapidly re-protonated. Moreover, these findings suggest that cysteine transport is predominantly unidirectional and that reverse transport does not contribute to depletion of intracellular cysteine pools.
The etiology of multiple sclerosis (MS) is currently unknown. However, one potential mechanism involved in the disease may be excitotoxicity. The elevation of glutamate in cerebrospinal fluid, as well as changes in the expression of glutamate receptors (iGluRs and mGluRs) and excitatory amino acid transporters (EAATs), have been observed in the brains of MS patients and animals subjected to experimental autoimmune encephalomyelitis (EAE), which is the predominant animal model used to investigate the pathophysiology of MS. In the present paper, the effects of glutamatergic receptor antagonists, including amantadine, memantine, LY 367583, and MPEP, on glutamate transport, the expression of mRNA of glutamate transporters (EAATs), the kinetic parameters of ligand binding to N-methyl-D-aspartate (NMDA) receptors, and the morphology of nerve endings in EAE rat brains were investigated. The extracellular level of glutamate in the brain is primarily regulated by astrocytic glutamate transporter 1 (GLT-1) and glutamate-aspartate transporter (GLAST). Excess glutamate is taken up from the synaptic space and metabolized by astrocytes. Thus, the extracellular level of glutamate decreases, which protects neurons from excitotoxicity. Our investigations showed changes in the expression of EAAT mRNA, glutamate transport (uptake and release) by synaptosomal and glial plasmalemmal vesicle fractions, and ligand binding to NMDA receptors; these effects were partially reversed after the treatment of EAE rats with the NMDA antagonists amantadine and memantine. The antagonists of group I metabotropic glutamate receptors (mGluRs), including LY 367385 and MPEP, did not exert any effect on the examined parameters. These results suggest that disturbances in these mechanisms may play a role in the processes associated with glutamate excitotoxicity and the progressive brain damage in EAE.
Adenosine signaling is implicated in several neuropsychiatric disorders, including alcoholism. Among its diverse functions in the brain, adenosine regulates glutamate release and has an essential role in ethanol sensitivity and preference. However, the molecular mechanisms underlying adenosine-mediated glutamate signaling in neuroglial interaction remain elusive. We have previously shown that mice lacking the ethanol-sensitive adenosine transporter, type 1 equilibrative nucleoside transporter (ENT1), drink more ethanol compared with wild-type mice and have elevated striatal glutamate levels. In addition, ENT1 inhibition or knockdown reduces glutamate transporter expression in cultured astrocytes. Here, we examined how adenosine signaling in astrocytes contributes to ethanol drinking. Inhibition or deletion of ENT1 reduced the expression of type 2 excitatory amino-acid transporter (EAAT2) and the astrocyte-specific water channel, aquaporin 4 (AQP4). EAAT2 and AQP4 colocalization was also reduced in the striatum of ENT1 null mice. Ceftriaxone, an antibiotic compound known to increase EAAT2 expression and function, elevated not only EAAT2 but also AQP4 expression in the striatum. Furthermore, ceftriaxone reduced ethanol drinking, suggesting that ENT1-mediated downregulation of EAAT2 and AQP4 expression contributes to excessive ethanol consumption in our mouse model. Overall, our findings indicate that adenosine signaling regulates EAAT2 and astrocytic AQP4 expressions, which control ethanol drinking in mice.
ENT1; astrocytes; aquaporin 4; EAAT2; Alcoholism; ceftriaxone; ENT1; Astrocytes; Aquaporin 4; EAAT2; Alcoholism; Ceftriaxone
In multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE), impairment of glial “Excitatory Amino Acid Transporters” (EAATs) together with an excess glutamate-release by invading immune cells causes excitotoxic damage of the central nervous system (CNS). In order to identify pathways to dampen excitotoxic inflammatory CNS damage, we assessed the effects of a β-lactam antibiotic, ceftriaxone, reported to enhance expression of glial EAAT2, in “Myelin Oligodendrocyte Glycoprotein” (MOG)-induced EAE. Ceftriaxone profoundly ameliorated the clinical course of murine MOG-induced EAE both under preventive and therapeutic regimens. However, ceftriaxone had impact neither on EAAT2 protein expression levels in several brain areas, nor on the radioactive glutamate uptake capacity in a mixed primary glial cell-culture and the glutamate-induced uptake currents in a mammalian cell line mediated by EAAT2. Moreover, the clinical effect of ceftriaxone was preserved in the presence of the EAAT2-specific transport inhibitor, dihydrokainate, while dihydrokainate alone caused an aggravated EAE course. This demonstrates the need for sufficient glial glutamate uptake upon an excitotoxic autoimmune inflammatory challenge of the CNS and a molecular target of ceftriaxone other than the glutamate transporter. Ceftriaxone treatment indirectly hampered T cell proliferation and proinflammatory INFγ and IL17 secretion through modulation of myelin-antigen presentation by antigen-presenting cells (APCs) e.g. dendritic cells (DCs) and reduced T cell migration into the CNS in vivo. Taken together, we demonstrate, that a β-lactam antibiotic attenuates disease course and severity in a model of autoimmune CNS inflammation. The mechanisms are reduction of T cell activation by modulation of cellular antigen-presentation and impairment of antigen-specific T cell migration into the CNS rather than or modulation of central glutamate homeostasis.
We recently found evidence for anatomic and physical linkages between the astroglial Na+-dependent glutamate transporters (GLT-1/EAAT2 and GLAST/EAAT1) and mitochondria. In these same studies, we found that the glutamate dehydrogenase (GDH) inhibitor, epigallocatechin-monogallate (EGCG), inhibits both glutamate oxidation and Na+-dependent glutamate uptake in astrocytes. In the present study, we extend this finding by exploring the effects of EGCG on Na+-dependent l-[3H]-glutamate (Glu) uptake in crude membranes (P2) prepared from rat brain cortex. In this preparation, uptake is almost exclusively mediated by GLT-1. EGCG inhibited l-[3H]-Glu uptake in cortical membranes with an IC50 value of 230 μM. We also studied the effects of two additional inhibitors of GDH, hexachlorophene (HCP) and bithionol (BTH). Both of these compounds also caused concentration-dependent inhibition of glutamate uptake in cortical membranes. Pre-incubating with HCP for up to 15 min had no greater effect than that observed with no pre-incubation, showing that the effects occur rapidly. HCP decreased the Vmax for glutamate uptake without changing the Km, consistent with a non-competitive mechanism of action. EGCG, HCP, and BTH also inhibited Na+-dependent transport of d-[3H]-aspartate (Asp), a non-metabolizable transporter substrate, and [3H]-γ-aminobutyric acid (GABA). In contrast to the forebrain, glutamate uptake in crude cerebellar membranes (P2) is likely mediated by GLAST (EAAT1). Therefore, the effects of these compounds were examined in cerebellar membranes. In this region, none of these compounds had any effect on uptake of either l-[3H]-Glu or d-[3H]-Asp, but they all inhibited [3H]-GABA uptake. Together these studies suggest that GDH is preferentially required for glutamate uptake in forebrain as compared to cerebellum, and GDH may be required for GABA uptake as well. They also provide further evidence for a functional linkage between glutamate transport and mitochondria.
glutamate; GLT-1; EAAT2; GLAST; GABA; glutamate dehydrogenase; sodium-dependent uptake; epigallocatechin-monogallate
Impairment of astrocytic glutamate transporter (GLT-1; EAAT2) function is associated with multiple neurodegenerative diseases, including Parkinson's disease (PD) and manganism, the latter being induced by chronic exposure to high levels of manganese (Mn). Mn decreases EAAT2 promoter activity and mRNA and protein levels, but the molecular mechanism of Mn-induced EAAT2 repression at the transcriptional level has yet to be elucidated. We reveal that transcription factor Yin Yang 1 (YY1) is critical in repressing EAAT2 and mediates the effects of negative regulators, such as Mn and tumor necrosis factor alpha (TNF-α), on EAAT2. YY1 overexpression in astrocytes reduced EAAT2 promoter activity, while YY1 knockdown or mutation of the YY1 consensus site of the EAAT2 promoter increased its promoter activity and attenuated the Mn-induced repression of EAAT2. Mn increased YY1 promoter activity and mRNA and protein levels via NF-κB activation. This led to increased YY1 binding to the EAAT2 promoter region. Epigenetically, histone deacetylase (HDAC) classes I and II served as corepressors of YY1, and, accordingly, HDAC inhibitors increased EAAT2 promoter activity and reversed the Mn-induced repression of EAAT2 promoter activity. Taken together, our findings suggest that YY1, with HDACs as corepressors, is a critical negative transcriptional regulator of EAAT2 and mediates Mn-induced EAAT2 repression.