Still in Search for an EAAT Activator: GT949 Does Not Activate EAAT2, nor EAAT3 in Impedance and Radioligand Uptake Assays.

Excitatory amino acid transporters (EAATs) are important regulators of amino acid transport and in particular glutamate. Recently, more interest has arisen in these transporters in the context of neurodegenerative diseases. This calls for ways to modulate these targets to drive glutamate transport, EAAT2 and EAAT3 in particular. Several inhibitors (competitive and noncompetitive) exist to block glutamate transport; however, activators remain scarce. Recently, GT949 was proposed as a selective activator of EAAT2, as tested in a radioligand uptake assay. In the presented research, we aimed to validate the use of GT949 to activate EAAT2-driven glutamate transport by applying an innovative, impedance-based, whole-cell assay (xCELLigence). A broad range of GT949 concentrations in a variety of cellular environments were tested in this assay. As expected, no activation of EAAT3 could be detected. Yet, surprisingly, no biological activation of GT949 on EAAT2 could be observed in this assay either. To validate whether the impedance-based assay was not suited to pick up increased glutamate uptake or if the compound might not induce activation in this setup, we performed radioligand uptake assays. Two setups were utilized; a novel method compared to previously published research, and in a reproducible fashion copying the methods used in the existing literature. Nonetheless, activation of neither EAAT2 nor EAAT3 could be observed in these assays. Furthermore, no evidence of GT949 binding or stabilization of purified EAAT2 could be observed in a thermal shift assay. To conclude, based on experimental evidence in the present study GT949 requires specific assay conditions, which are difficult to reproduce, and the compound cannot simply be classified as an activator of EAAT2 based on the presented evidence. Hence, further research is required to develop the tools needed to identify new EAAT modulators and use their potential as a therapeutic target.

Forebrain EAAT3 Overexpression Increases Susceptibility to Amphetamine-Induced Repetitive Behaviors.

Obsessive-compulsive disorder (OCD) is a debilitating psychiatric disorder characterized by intrusive obsessive thoughts and compulsive behaviors. Multiple studies have shown the association of polymorphisms in the SLC1A1 gene with OCD. The most common of these OCD-associated polymorphisms increases the expression of the encoded protein, excitatory amino acid transporter 3 (EAAT3), a neuronal glutamate transporter. Previous work has shown that increased EAAT3 expression results in OCD-relevant behavioral phenotypes in rodent models. In this study, we created a novel mouse model with targeted, reversible overexpression of Slc1a1 in forebrain neurons. The mice do not have a baseline difference in repetitive behavior but show increased hyperlocomotion following a low dose of amphetamine (3 mg/kg) and increased stereotypy following a high dose of amphetamine (8 mg/kg). We next characterized the effect of amphetamine on striatal cFos response and found that amphetamine increased cFos throughout the striatum in both control and Slc1a1-overexpressing (OE) mice, but Slc1a1-OE mice had increased cFos expression in the ventral striatum relative to controls. We used an unbiased machine classifier to robustly characterize the behavioral response to different doses of amphetamine and found a unique response to amphetamine in Slc1a1-OE mice, relative to controls. Lastly, we found that the differences in striatal cFos expression in Slc1a1-OE mice were driven by cFos expression specifically in D1 neurons, as Slc1a1-OE mice had increased cFos in D1 ventral medial striatal neurons, implicating this region in the exaggerated behavioral response to amphetamine in Slc1a1-OE mice.

Changes in excitatory amino acid transporters in response to remote ischaemic preconditioning and glutamate excitotoxicity.

The successful implementation of remote ischaemic conditioning as a clinical neuroprotective strategy requires a thorough understanding of its basic principles, which can be modified for each patient. The mechanisms of glutamate homeostasis appear to be a key component. In the current study, we focused on the brain-to-blood glutamate shift mediated by glutamate transporters (excitatory amino acid transports [EAATs]) and the effect of remote ischaemic preconditioning (RIPC) as a mediator of ischaemic tolerance. We used model mimicking ischaemia-mediated excitotoxicity (intracerebroventricular administration of glutamate) to avoid the indirect effect of ischaemia-triggered mechanisms. We found quantitative changes in EAAT2 and EAAT3 and altered membrane trafficking of EAAT1 on the cells of the choroid plexus. These changes could underlie the beneficial effects of ischaemic tolerance. There was reduced oxidative stress and increased glutathione level after RIPC treatment. Moreover, we determined the stimulus-specific response on EAATs. While glutamate overdose stimulated EAAT2 and EAAT3 overexpression, RIPC induced membrane trafficking of EAAT1 and EAAT2 rather than a change in their expression. Taken together, mechanisms related to glutamate homeostasis, especially EAAT-mediated transport, represents a powerful tool of ischaemic tolerance and allow a certain amount of flexibility based on the stimulus used.

Glutathione-Mediated Neuroprotective Effect of Purine Derivatives.

Numerous basic studies have reported on the neuroprotective properties of several purine derivatives such as caffeine and uric acid (UA). Epidemiological studies have also shown the inverse association of appropriate caffeine intake or serum urate levels with neurodegenerative diseases such as Alzheimer disease (AD) and Parkinson's disease (PD). The well-established neuroprotective mechanisms of caffeine and UA involve adenosine A2A receptor antagonism and antioxidant activity, respectively. Our recent study found that another purine derivative, paraxanthine, has neuroprotective effects similar to those of caffeine and UA. These purine derivatives can promote neuronal cysteine uptake through excitatory amino acid carrier protein 1 (EAAC1) to increase neuronal glutathione (GSH) levels in the brain. This review summarizes the GSH-mediated neuroprotective effects of purine derivatives. Considering the fact that GSH depletion is a manifestation in the brains of AD and PD patients, administration of purine derivatives may be a new therapeutic approach to prevent or delay the onset of these neurodegenerative diseases.

HIF-1α drives resistance to ferroptosis in solid tumors by promoting lactate production and activating SLC1A1.

Solid tumors have developed robust ferroptosis resistance. The mechanism underlying ferroptosis resistance regulation in solid tumors, however, remains elusive. Here, we report that the hypoxic tumor microenvironment potently promotes ferroptosis resistance in solid tumors in a hypoxia-inducible factor 1α (HIF-1α)-dependent manner. In combination with HIF-2α, which promotes tumor ferroptosis under hypoxia, HIF-1α is the main driver of hypoxia-induced ferroptosis resistance. Mechanistically, HIF-1α-induced lactate contributes to ferroptosis resistance in a pH-dependent manner that is parallel to the classical SLC7A11 and FSP1 systems. In addition, HIF-1α also enhances transcription of SLC1A1, an important glutamate transporter, and promotes cystine uptake to promote ferroptosis resistance. In support of the role of hypoxia in ferroptosis resistance, silencing HIF-1α sensitizes mouse solid tumors to ferroptosis inducers. In conclusion, our results reveal a mechanism by which hypoxia drives ferroptosis resistance and identify the combination of hypoxia alleviation and ferroptosis induction as a promising therapeutic strategy for solid tumors.

The Protective Role of Glutathione on Zinc-Induced Neuron Death after Brain Injuries.

Glutathione (GSH) is necessary for maintaining physiological antioxidant function, which is responsible for maintaining free radicals derived from reactive oxygen species at low levels and is associated with improved cognitive performance after brain injury. GSH is produced by the linkage of tripeptides that consist of glutamic acid, cysteine, and glycine. The adequate supplementation of GSH has neuroprotective effects in several brain injuries such as cerebral ischemia, hypoglycemia, and traumatic brain injury. Brain injuries produce an excess of reactive oxygen species through complex biochemical cascades, which exacerbates primary neuronal damage. GSH concentrations are known to be closely correlated with the activities of certain genes such as excitatory amino acid carrier 1 (EAAC1), glutamate transporter-associated protein 3-18 (Gtrap3-18), and zinc transporter 3 (ZnT3). Following brain-injury-induced oxidative stress, EAAC1 function is negatively impacted, which then reduces cysteine absorption and impairs neuronal GSH synthesis. In these circumstances, vesicular zinc is also released into the synaptic cleft and then translocated into postsynaptic neurons. The excessive influx of zinc inhibits glutathione reductase, which inhibits GSH's antioxidant functions in neurons, resulting in neuronal damage and ultimately in the impairment of cognitive function. Therefore, in this review, we explore the overall relationship between zinc and GSH in terms of oxidative stress and neuronal cell death. Furthermore, we seek to understand how the modulation of zinc can rescue brain-insult-induced neuronal death after ischemia, hypoglycemia, and traumatic brain injury.

Epstein-Barr virus infection controls the concentration of the intracellular antioxidant glutathione by upregulation of the glutamate transporter EAAT3 in tumor cells.

Epstein-Barr virus or human herpesvirus 4 (EBV/HHV-4) is an omnipresent oncovirus etiologically associated with various B-cell lymphomas and epithelial cancers. The malignant transformation associated with the persistent expression of viral proteins often deregulates the host cellular machinery and EBV infection is coupled to elevated levels of reactive oxygen species. Here, we investigated the role that the glutamate transporter EAAT3 plays in regulating the antioxidant system as a protective mechanism of EBV-infected cells against the virus-induced oxidative stress. Our study demonstrated that the expression of EAAT3 was upregulated and localized to the plasma membrane in EBV latently infected and de novo EBV-infected cells. EAAT3 was regulated by the transcription factor NFAT5 in the infected cells. Membrane localized EAAT3 was found to be involved in the transportation of glutamate from the extracellular space into the cell, as EAAT3 and NFAT5 inhibitors markedly reduced the levels of intracellular glutamate levels in EBV latently infected cells. Additionally, our data demonstrated a notable decrease in the intracellular glutathione levels following treatment with an EAAT3 inhibitor. Collectively, our results suggest that upregulation of the glutamate transporter EAAT3 is an adaptation of EBV-infected cells to maintain cellular redox homeostasis against the virus-induced oxidative stress, and that this cellular balance could be therapeutically destroyed by targeting EAAT3 to impede EBV-associated cancers.

Intact amphetamine-induced behavioral sensitization in mice with increased or decreased neuronal glutamate transporter SLC1A1/EAAT3.

Repeated amphetamine treatment results in locomotor sensitization, a phenomenon that may relate to the development of psychosis and addiction. Evidence suggests that interactions between dopaminergic and glutamatergic systems are involved in amphetamine sensitization. We previously demonstrated that the neuronal excitatory amino acid transporter (Slc1a1/EAAT3) produces bidirectional, expression-dependent effects on the response to acute amphetamine. Here, using mice with decreased or increased expression of EAAT3, we found that chronic alterations in EAAT3 expression do not significantly impact amphetamine-induced locomotor sensitization. Compensation by other glutamate transporters cannot be ruled out in this important neuroadaptive phenomenon.

Structural insights into inhibitory mechanism of human excitatory amino acid transporter EAAT2.

Glutamate is a pivotal excitatory neurotransmitter in mammalian brains, but excessive glutamate causes numerous neural disorders. Almost all extracellular glutamate is retrieved by the glial transporter, Excitatory Amino Acid Transporter 2 (EAAT2), belonging to the SLC1A family. However, in some cancers, EAAT2 expression is enhanced and causes resistance to therapies by metabolic disturbance. Despite its crucial roles, the detailed structural information about EAAT2 has not been available. Here, we report cryo-EM structures of human EAAT2 in substrate-free and selective inhibitor WAY213613-bound states at 3.2 Å and 2.8 Å, respectively. EAAT2 forms a trimer, with each protomer consisting of transport and scaffold domains. Along with a glutamate-binding site, the transport domain possesses a cavity that could be disrupted during the transport cycle. WAY213613 occupies both the glutamate-binding site and cavity of EAAT2 to interfere with its alternating access, where the sensitivity is defined by the inner environment of the cavity. We provide the characterization of the molecular features of EAAT2 and its selective inhibition mechanism that may facilitate structure-based drug design for EAAT2.

SLC1A1-mediated cellular and mitochondrial influx of R-2-hydroxyglutarate in vascular endothelial cells promotes tumor angiogenesis in IDH1-mutant solid tumors.

Mutant isocitrate dehydrogenase 1 (mIDH1) drives tumorigenesis via producing oncometabolite R-2-hydroxyglutarate (R-2-HG) across various tumor types. However, mIDH1 inhibitors appear only effective in hematological tumors. The therapeutic benefit in solid tumors remains elusive, likely due to the complex tumor microenvironment. In this study, we discover that R-2-HG produced by IDH1-mutant tumor cells is preferentially imported into vascular endothelial cells and remodels mitochondrial respiration to promote tumor angiogenesis, conferring a therapeutic vulnerability in IDH1-mutant solid tumors. Mechanistically, SLC1A1, a Na+-dependent glutamate transporter that is preferentially expressed in endothelial cells, facilitates the influx of R-2-HG from the tumor microenvironment into the endothelial cells as well as the intracellular trafficking of R-2-HG from cytoplasm to mitochondria. R-2-HG hijacks SLC1A1 to promote mitochondrial Na+/Ca2+ exchange, which activates the mitochondrial respiratory chain and fuels vascular endothelial cell migration in tumor angiogenesis. SLC1A1 deficiency in mice abolishes mIDH1-promoted tumor angiogenesis as well as the therapeutic benefit of mIDH1 inhibitor in solid tumors. Moreover, we report that HH2301, a newly discovered mIDH1 inhibitor, shows promising efficacy in treating IDH1-mutant cholangiocarcinoma in preclinical models. Together, we identify a new role of SLC1A1 as a gatekeeper of R-2-HG-mediated crosstalk between IDH1-mutant tumor cells and vascular endothelial cells, and demonstrate the therapeutic potential of mIDH1 inhibitors in treating IDH1-mutant solid tumors via disrupting R-2-HG-promoted tumor angiogenesis.

Cysteine Donor-Based Brain-Targeting Prodrug: Opportunities and Challenges.

Overcoming blood-brain barrier (BBB) to improve brain bioavailability of therapeutic drug remains an ongoing concern. Prodrug is one of the most reliable approaches for delivering agents with low-level BBB permeability into the brain. The well-known antioxidant capacities of cysteine (Cys) and its vital role in glutathione (GSH) synthesis indicate that Cys-based prodrug could potentiate therapeutic drugs against oxidative stress-related neurodegenerative disorders. Moreover, prodrug with Cys moiety could be recognized by the excitatory amino acid transporter 3 (EAAT3) that is highly expressed at the BBB and transports drug into the brain. In this review, we summarized the strategies of crossing BBB, properties of EAAT3 and its natural substrates, Cys and its donors, and Cys donor-based brain-targeting prodrugs by referring to recent investigations. Moreover, the challenges that we are faced with and future research orientations were also addressed and proposed. It is hoped that present review will provide evidence for the pursuit of novel Cys donor-based brain-targeting prodrug.

Microscopic Characterization of the Chloride Permeation Pathway in the Human Excitatory Amino Acid Transporter 1 (EAAT1).

Excitatory amino acid transporters (EAATs) are glutamate transporters that belong to the solute carrier 1A (SLC1A) family. They couple glutamate transport to the cotransport of three sodium (Na+) ions and one proton (H+) and the counter-transport of one potassium (K+) ion. In addition to this coupled transport, binding of cotransported species to EAATs activates a thermodynamically uncoupled chloride (Cl-) conductance. Structures of SLC1A family members have revealed that these transporters use a twisting elevator mechanism of transport, where a mobile transport domain carries substrate and coupled ions across the membrane, while a static scaffold domain anchors the transporter in the membrane. We recently demonstrated that the uncoupled Cl- conductance is activated by the formation of an aqueous pore at the domain interface during the transport cycle in archaeal GltPh. However, a pathway for the uncoupled Cl- conductance has not been reported for the EAATs, and it is unclear if such a pathway is conserved. Here, we employ all-atom molecular dynamics (MD) simulations combined with enhanced sampling, free-energy calculations, and experimental mutagenesis to approximate large-scale conformational changes during the transport process and identified a Cl--conducting conformation in human EAAT1 (hEAAT1). Sampling the large-scale structural transitions in hEAAT1 allowed us to capture an intermediate conformation formed during the transport cycle with a continuous aqueous pore at the domain interface. The free-energy calculations performed for the conduction of Cl- and Na+ ions through the captured conformation highlight the presence of two hydrophobic gates that control low-barrier movement of Cl- through the aqueous pathway. Overall, our findings provide insights into the mechanism by which a human neurotransmitter transporter supports functional duality of active transport and passive Cl- permeation and confirm the commonality of this mechanism in different members of the SLC1A family.

Developmental impact of glutamate transporter overexpression on dopaminergic neuron activity and stereotypic behavior.

Obsessive-compulsive disorder (OCD) is a disabling condition that often begins in childhood. Genetic studies in OCD have pointed to SLC1A1, which encodes the neuronal glutamate transporter EAAT3, with evidence suggesting that increased expression contributes to risk. In mice, midbrain Slc1a1 expression supports repetitive behavior in response to dopaminergic agonists, aligning with neuroimaging and pharmacologic challenge studies that have implicated the dopaminergic system in OCD. These findings suggest that Slc1a1 may contribute to compulsive behavior through altered dopaminergic transmission; however, this theory has not been mechanistically tested. To examine the developmental impact of Slc1a1 overexpression on compulsive-like behaviors, we, therefore, generated a novel mouse model to perform targeted, reversible overexpression of Slc1a1 in dopaminergic neurons. Mice with life-long overexpression of Slc1a1 showed a significant increase in amphetamine (AMPH)-induced stereotypy and hyperlocomotion. Single-unit recordings demonstrated that Slc1a1 overexpression was associated with increased firing of dopaminergic neurons. Furthermore, dLight1.1 fiber photometry showed that these behavioral abnormalities were associated with increased dorsal striatum dopamine release. In contrast, no impact of overexpression was observed on anxiety-like behaviors or SKF-38393-induced grooming. Importantly, overexpression solely in adulthood failed to recapitulate these behavioral phenotypes, suggesting that overexpression during development is necessary to generate AMPH-induced phenotypes. However, doxycycline-induced reversal of Slc1a1/EAAT3 overexpression in adulthood normalized both the increased dopaminergic firing and AMPH-induced responses. These data indicate that the pathologic effects of Slc1a1/EAAT3 overexpression on dopaminergic neurotransmission and AMPH-induced stereotyped behavior are developmentally mediated, and support normalization of EAAT3 activity as a potential treatment target for basal ganglia-mediated repetitive behaviors.

Pre-Steady-State Kinetics and Reverse Transport in Rat Glutamate Transporter EAAC1 with an Immobilized Transport Domain.

Plasma membrane glutamate transporters move glutamate across the cell membrane in a process that is thought to involve elevator-like movement of the transport domain relative to the static trimerization domain. Conformational changes associated with this elevator-like movement have been blocked by covalent crosslinking of cysteine pairs inserted strategically in several positions in the transporter structure, resulting in inhibition of steady-state transport activity. However, it is not known how these crosslinking restraints affect other partial reactions of the transporter that were identified based on pre-steady-state kinetic analysis. Here, we re-examine two different introduced cysteine pairs in the rat glutamate transporter EAAC1 recombinantely expressed in HEK293 cells, W440C/K268C and K64C/V419C, with respect to the molecular mechanism of their impairment of transporter function. Pre-steady-state kinetic studies of glutamate-induced partial reactions were performed using laser photolysis of caged glutamate to achieve sub-millisecond time resolution. Crosslinking of both cysteine pairs abolished steady-state transport current, as well as the majority of pre-steady-state glutamate-induced charge movements, in both forward and reverse transport mode, suggesting that it is not only the elevator-like movement associated with translocation, but also other transporter partial reactions that are inhibited. In contrast, sodium binding to the empty transporter, and glutamate-induced anion conductance were still intact after the W440C/K268C crosslink. Our results add to the previous mechanistic view of how covalent restraints of the transporter affect function and structural changes linked to individual steps in the transport cycle.

Enmein Decreases Synaptic Glutamate Release and Protects against Kainic Acid-Induced Brain Injury in Rats.

This study investigated the effects of enmein, an active constituent of Isodon japonicus Hara, on glutamate release in rat cerebrocortical nerve terminals (synaptosomes) and evaluated its neuroprotective potential in a rat model of kainic acid (KA)-induced glutamate excitotoxicity. Enmein inhibited depolarization-induced glutamate release, FM1-43 release, and Ca2+ elevation in cortical nerve terminals but had no effect on the membrane potential. Removing extracellular Ca2+ and blocking vesicular glutamate transporters, N- and P/Q-type Ca2+ channels, or protein kinase C (PKC) prevented the inhibition of glutamate release by enmein. Enmein also decreased the phosphorylation of PKC, PKC-α, and myristoylated alanine-rich C kinase substrates in synaptosomes. In the KA rat model, intraperitoneal administration of enmein 30 min before intraperitoneal injection of KA reduced neuronal cell death, glial cell activation, and glutamate elevation in the hippocampus. Furthermore, in the hippocampi of KA rats, enmein increased the expression of synaptic markers (synaptophysin and postsynaptic density protein 95) and excitatory amino acid transporters 2 and 3, which are responsible for glutamate clearance, whereas enmein decreased the expression of glial fibrillary acidic protein (GFAP) and CD11b. These results indicate that enmein not only inhibited glutamate release from cortical synaptosomes by suppressing Ca2+ influx and PKC but also increased KA-induced hippocampal neuronal death by suppressing gliosis and decreasing glutamate levels by increasing glutamate uptake.

Regulation of NRG-1-ErbB4 signaling and neuroprotection by exogenous neuregulin-1 in a mouse model of epilepsy.

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy. Dysregulation of glutamate transporters has been a common finding across animal models of epilepsy and in patients with TLE. In this study, we investigate NRG-1/ErbB4 signaling in epileptogenesis and the neuroprotective effects of NRG-1 treatment in a mouse model of temporal lobe epilepsy. Using immunohistochemistry, we report the first evidence for NRG-1/ErbB4-dependent selective upregulation of glutamate transporter EAAC1 and bihemispheric neuroprotection by exogeneous NRG-1 in the intrahippocampal kainic acid (IHKA) model of TLE. Our findings provide evidence that dysregulation of glutamate transporter EAAC1 contributes to the development of epilepsy and can be therapeutically targeted to reduce neuronal death following IHKA-induced status epilepticus (SE).

SLC1A1 mediated glutamine addiction and contributed to natural killer T-cell lymphoma progression with immunotherapeutic potential.

BACKGROUND: Metabolic reprogramming plays an essential role on lymphoma progression. Dysregulation of glutamine metabolism is implicated in natural-killer T-cell lymphoma (NKTCL) and tumor cell response to asparaginase-based anti-metabolic treatment. METHODS: To understand the metabolomic alterations and determine the potential therapeutic target of asparaginase, we assessed metabolomic profile using liquid chromatography-mass spectrometry in serum samples of 36 NKTCL patients, and integrated targeted metabolic analysis and RNA sequencing in tumor samples of 102 NKTCL patients. The biological function of solute carrier family 1 member 1 (SLC1A1) on metabolic flux, lymphoma cell growth, and drug sensitivity was further examined in vitro in NK-lymphoma cell line NK-92 and SNK-6, and in vivo in zebrafish xenograft models. FINDINGS: In NKTCL patients, serum metabolomic profile was characterized by aberrant glutamine metabolism and SLC1A1 was identified as a central regulator of altered glutaminolysis. Both in vitro and in vivo, ectopic expression of SLC1A1 increased cellular glutamine uptake, enhanced glutathione metabolic flux, and induced glutamine addiction, leading to acceleration of cell proliferation and tumor growth. Of note, SLC1A1 overexpression was significantly associated with PD-L1 downregulation and reduced cytotoxic CD3+/CD8+ T cell activity when co-cultured with peripheral blood mononuclear cells. Asparaginase treatment counteracted SLC1A1-mediated glutamine addiction, restored SLC1A1-induced impaired T-cell immunity. Clinically, high EAAT3 (SLC1A1-encoded protein) expression independently predicted superior progression-free and overall survival in 90 NKTCL patients treated with asparaginase-based regimens. INTERPRETATION: SLC1A1 functioned as an extracellular glutamine transporter, promoted tumor growth through reprogramming glutamine metabolism of NKTCL, while rendered tumor cells sensitive to asparaginase treatment. Moreover, SLC1A1-mediated modulation of PD-L1 expression might provide clinical rationale of co-targeting metabolic vulnerability and immunosuppressive microenvironment in NKTCL. FUNDING: This study was supported, in part, by research funding from the National Natural Science Foundation of China (82130004, 81830007 and 81900192), Chang Jiang Scholars Program, Shanghai Municipal Education Commission Gaofeng Clinical Medicine Grant Support (20152206 and 20152208), Clinical Research Plan of SHDC (2020CR1032B), Multicenter Clinical Research Project by Shanghai Jiao Tong University School of Medicine (DLY201601), Shanghai Chenguang Program (19CG15), Shanghai Sailing Program (19YF1430800), Medical-Engineering Cross Foundation of Shanghai Jiao Tong University (ZH2018QNA46), and Shanghai Yi Yuan Xin Xing Program.

Evidence for the interaction of COX-2 with mGluR5 in the regulation of EAAT1 and EAAT3 protein levels in the mouse hippocampus. The influence of oxidative stress mechanisms.

Since we found that inhibition of cyclooxygenase-2 (COX-2) with concomitant application of a metabotropic glutamate receptor subtype 5 (mGluR5) antagonist (MTEP) down-regulates mGluR7 in the hippocampus (HC) and changes behavior of mice, our team decided to investigate the mechanism responsible for the observed changes. The amino acid glutamate (Glu) is a major excitatory neurotransmitter in the brain. Glu uptake is regulated by excitatory amino acid transporters (EAAT). There are five transporters with documented expression in neurons and glia in the central nervous system (CNS). EAATs, maintain the correct transmission of the Glu signal and prevent its toxic accumulation by removing Glu from the synapse. It has been documented that the toxic level of Glu is one of the main causes of mental and cognitive abnormalities. Given the above mechanisms involved in the functioning of the Glu synapse, we hypothesized modification of Glu uptake, involving EAATs as the cause of the observed changes. This study investigated the level of selected EAATs in the HC after chronic treatment with mGluR5 antagonist MTEP, NS398, and their combination using Western blot. Concomitant MTEP treatment with NS398 or a single administration of the above causes changes in LTP and modulation of EAAT levels in mouse HC. As EAATs are cellular markers of oxidative stress mechanisms, the E. coli lipopolysaccharide (LPS) challenge was performed. The modified Barnes maze test (MBM) revealed alterations in the mouse spatial learning abilities. This study reports an interaction between the mGluR5 and COX-2 in the HC, with EAAT1 and EAAT3 involvement.

Glutathione in the Brain.

Glutathione (GSH) is the most abundant non-protein thiol, and plays crucial roles in the antioxidant defense system and the maintenance of redox homeostasis in neurons. GSH depletion in the brain is a common finding in patients with neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease, and can cause neurodegeneration prior to disease onset. Excitatory amino acid carrier 1 (EAAC1), a sodium-dependent glutamate/cysteine transporter that is selectively present in neurons, plays a central role in the regulation of neuronal GSH production. The expression of EAAC1 is posttranslationally controlled by the glutamate transporter-associated protein 3-18 (GTRAP3-18) or miR-96-5p in neurons. The regulatory mechanism of neuronal GSH production mediated by EAAC1 may be a new target in therapeutic strategies for these neurodegenerative diseases. This review describes the regulatory mechanism of neuronal GSH production and its potential therapeutic application in the treatment of neurodegenerative diseases.

Kinetic mechanism of Na+-coupled aspartate transport catalyzed by GltTk.

It is well-established that the secondary active transporters GltTk and GltPh catalyze coupled uptake of aspartate and three sodium ions, but insight in the kinetic mechanism of transport is fragmentary. Here, we systematically measured aspartate uptake rates in proteoliposomes containing purified GltTk, and derived the rate equation for a mechanism in which two sodium ions bind before and another after aspartate. Re-analysis of existing data on GltPh using this equation allowed for determination of the turnover number (0.14 s-1), without the need for error-prone protein quantification. To overcome the complication that purified transporters may adopt right-side-out or inside-out membrane orientations upon reconstitution, thereby confounding the kinetic analysis, we employed a rapid method using synthetic nanobodies to inactivate one population. Oppositely oriented GltTk proteins showed the same transport kinetics, consistent with the use of an identical gating element on both sides of the membrane. Our work underlines the value of bona fide transport experiments to reveal mechanistic features of Na+-aspartate symport that cannot be observed in detergent solution. Combined with previous pre-equilibrium binding studies, a full kinetic mechanism of structurally characterized aspartate transporters of the SLC1A family is now emerging.