Modulation of neuronal glutathione synthesis by EAAC1 and its interacting protein GTRAP3-18

Thyroid hormone deficiency affects anxiety-related behaviors and expression of hippocampal glutamate transporters in male congenital hypothyroid rat offspring

Thyroid hormones are crucial for brain development and their deficiency during fetal and postnatal periods can lead to mood and cognitive disorders. We aimed to examine the consequences of thyroid hormone deficiency on anxiety-related behaviors and protein expression of hippocampal glutamate transporters in congenital hypothyroid male offspring rats. Possible beneficial effects of treadmill exercise have also been examined. Congenital hypothyroidism was induced by adding propylthiouracil (PTU) to drinking water of pregnant Wistar rats from gestational day 6 until the end of the weaning period (postnatal day 28). Next, following 4 weeks of treadmill exercise (5 days per week), anxiety-related behaviors were examined using elevated plus maze (EPM) and light/dark box tests. Thereafter, protein expression of astrocytic (GLAST and GLT-1) and neuronal (EAAC1) glutamate transporters were measured in the hippocampus by immunoblotting. Hypothyroid rats showed decreased anxiety-like behavior, as measured by longer time spent in the open arms of the EPM and in the light area of the light/dark box, compared to control rats. Hypothyroid rats had significantly higher GLAST and GLT-1 and lower EAAC1 protein levels in the hippocampus than did the euthyroid rats. Following exercise, anxiety levels decreased in the euthyroid group while protein expression of EAAC1 increased and returned to normal levels in the hypothyroid group. Our findings indicate that thyroid hormone deficiency was associated with alterations in protein expression of glutamate transporters in the hippocampus. Up-regulation of hippocampal GLAST and GLT-1 could be at least one of the mechanisms associated with the anxiolytic effects of congenital hypothyroidism.

Mouse embryonic stem cell-derived motor neurons are susceptible to ferroptosis

Ferroptosis is a regulated form of cell death driven by the lethal accumulation of lipid peroxides in cell membranes. Several regulators of ferroptosis have been identified using cancer cell lines. However, the cellular pathways of ferroptosis in neurons remain poorly characterized. In this study, we used a mouse embryonic stem cell-derived motor neuron model to investigate how motor neurons respond to ferroptosis inducers. Pharmacological and genetic inhibition of glutathione peroxidase 4 (GPx4) induced ferroptosis in motor neurons, while system x_c ^- inhibition by erastin had no effect. RNA-seq analysis showed that the expression levels of several genes were altered during RSL3-induced ferroptosis. Subsequent bioinformatic analysis revealed alterations in several biological pathways during ferroptosis, including synaptogenesis and calcium signaling. Finally, we found that edaravone, an FDA-approved drug for treating amyotrophic lateral sclerosis (ALS) disease, rescued motor neurons from RSL3-induced ferroptosis. Our data highlight the crucial role of GPx4 in ferroptosis regulation and demonstrate that stem cell-derived motor neuron culture is a valuable model to study ferroptosis at the single-cell level in a neuronal context.

Protein Glutathionylation and Glutaredoxin: Role in Neurodegenerative Diseases

Oxidative stress has been implicated in the pathogenesis and progression of many neurodegenerative disorders including Parkinson's disease and Alzheimer's disease. One of the major enzyme systems involved in the defense against reactive oxygen species are the tripeptide glutathione and oxidoreductase glutaredoxin. Glutathione and glutaredoxin system are very important in the brain because of the oxidative modification of protein thiols to protein glutathione mixed disulfides with the concomitant formation of oxidized glutathione during oxidative stress. Formation of Pr-SSG acts as a sink in the brain and is reduced back to protein thiols during recovery, thus restoring protein functions. This is unlike in the liver, which has a high turnover of glutathione, and formation of Pr-SSG is very minimal as liver is able to quickly quench the prooxidant species. Given the important role glutathione and glutaredoxin play in the brain, both in normal and pathologic states, it is necessary to study ways to augment the system to help maintain the protein thiol status. This review details the importance of glutathione and glutaredoxin systems in several neurodegenerative disorders and emphasizes the potential augmentation of this system as a target to effectively protect the brain during aging.

Targeting JWA for Cancer Therapy: Functions, Mechanisms and Drug Discovery

Simple Summary

JWA has been identified as a potential therapeutic target for several cancers. In this review, we summarize the tumor suppressive functions of the JWA gene and its role in anti-cancer drug development. The focus is on elucidating the key regulatory proteins up and downstream of JWA and their signaling networks. We also discuss current strategies for targeting JWA (JWA peptides, small molecule agonists, and JWA-targeted Pt (IV) prodrugs).

Abstract

Tumor heterogeneity limits the precision treatment of targeted drugs. It is important to find new tumor targets. JWA, also known as ADP ribosylation factor-like GTPase 6 interacting protein 5 (ARL6IP5, GenBank: AF070523, 1998), is a microtubule-associated protein and an environmental response gene. Substantial evidence shows that JWA is low expressed in a variety of malignancies and is correlated with overall survival. As a tumor suppressor, JWA inhibits tumor progression by suppressing multiple oncogenes or activating tumor suppressor genes. Low levels of JWA expression in tumors have been reported to be associated with multiple aspects of cancer progression, including angiogenesis, proliferation, apoptosis, metastasis, and chemotherapy resistance. In this review, we will discuss the structure and biological functions of JWA in tumors, examine the potential therapeutic strategies for targeting JWA and explore the directions for future investigation.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Enhanced Vascular Permeability by Microbubbles and Ultrasound in Drug Delivery

Ultrasound and microbubbles, an ultrasound contrast agent, have recently increased attention to developing novel drug delivery systems. Ultrasound exposure can induce mechanical effects derived from microbubbles behaviors such as an expansion, contraction, and collapse depending on ultrasound conditions. These mechanical effects induce several biological effects, including enhancement of vascular permeability. For drug delivery, one promising approach is enhancing vascular permeability using ultrasound and microbubbles, resulting in improved drug transport to targeted tissues. This approach is applied to several tissues and drugs to cure diseases. This review describes the enhancement of vascular permeability by ultrasound and microbubbles and its therapeutic application, including our recent study. We also discuss the current situation of the field and its potential future perspectives.

S-Adenosylmethionine Alleviates Amyloid-β-Induced Neural Injury by Enhancing Trans-Sulfuration Pathway Activity in Astrocytes

BACKGROUND

Glutathione (GSH) is an important endogenous antioxidant protecting cells from oxidative injury. Cysteine (Cys), the substrate limiting the production of GSH, is mainly generated from the trans-sulfuration pathway. S-adenosylmethionine (SAM) is a critical molecule produced in the methionine cycle and can be utilized by the trans-sulfuration pathway. Reductions in GSH and SAM as well as dysfunction in the trans-sulfuration pathway have been documented in the brains of Alzheimer's disease (AD) patients. Our previous in vivo study revealed that SAM administration attenuated oxidative stress induced by amyloid-β (Aβ) through the enhancement of GSH.

OBJECTIVE

To investigate the effect of Aβ-induced oxidative stress on the trans-sulfuration pathway in astrocytes and neurons, respectively, and the protective effect of SAM on neurons.

METHODS

APP/PS1 transgenic mice and the primary cultured astrocytes, neurons, and HT22 cells were used in the current study.

RESULTS

SAM could rescue the low trans-sulfuration pathway activity induced by Aβ only in astrocytes, accompanying with increasing levels of Cys and GSH. The decrease of cellular viability of neurons caused by Aβ was greatly reversed when co-cultured with astrocytes with SAM intervention. Meanwhile, SAM improved cognitive performance in APP/PS1 mice.

CONCLUSION

In terms of astrocyte protection from oxidative stress, SAM might be a potent antioxidant in the therapy of AD patients.

Inhibition of miR-96-5p in the mouse brain increases glutathione levels by altering NOVA1 expression

Glutathione (GSH) is an important antioxidant that plays a critical role in neuroprotection. GSH depletion in neurons induces oxidative stress and thereby promotes neuronal damage, which in turn is regarded as a hallmark of the early stage of neurodegenerative diseases. The neuronal GSH level is mainly regulated by cysteine transporter EAAC1 and its inhibitor, GTRAP3-18. In this study, we found that the GTRAP3-18 level was increased by up-regulation of the microRNA miR-96-5p, which was found to decrease EAAC1 levels in our previous study. Since the 3'-UTR region of GTRAP3-18 lacks the consensus sequence for miR-96-5p, an unidentified protein should be responsible for the intermediate regulation of GTRAP3-18 expression by miR-96-5p. Here, we discovered that RNA-binding protein NOVA1 functions as an intermediate protein for GTRAP3-18 expression via miR-96-5p. Moreover, we show that intra-arterial injection of a miR-96-5p-inhibiting nucleic acid to living mice by a drug delivery system using microbubbles and ultrasound decreased the level of GTRAP3-18 via NOVA1 and increased the levels of EAAC1 and GSH in the dentate gyrus of the hippocampus. These findings suggest that the delivery of a miR-96-5p inhibitor to the brain would efficiently increase the neuroprotective activity by increasing GSH levels via EAAC1, GTRAP3-18 and NOVA1.

Neuregulin-1 inhibits CoCl2-induced upregulation of excitatory amino acid carrier 1 expression and oxidative stress in SH-SY5Y cells and the hippocampus of mice

Excitatory amino acid carrier 1 (EAAC1) is an important subtype of excitatory amino acid transporters (EAATs) and is the route for neuronal cysteine uptake. CoCl₂ is not only a hypoxia-mimetic reagent but also an oxidative stress inducer. Here, we found that CoCl₂ induced significant EAAC1 overexpression in SH-SY5Y cells and the hippocampus of mice. Transient transfection of EAAC1 reduced CoCl₂-induced cytotoxicity in SH-SY5Y cells. Based on this result, upregulation of EAAC1 expression by CoCl₂ is thought to represent a compensatory response against oxidative stress in an acute hypoxic state. We further demonstrated that pretreatment with Neuregulin-1 (NRG1) rescued CoCl₂-induced upregulation of EAAC1 and tau expression. NRG1 plays a protective role in the CoCl₂-induced accumulation of reactive oxygen species (ROS) and reduction in antioxidative enzyme (SOD and GPx) activity. Moreover, NRG1 attenuated CoCl₂-induced apoptosis and cell death. NRG1 inhibited the CoCl₂-induced release of cleaved caspase-3 and reduction in Bcl-X_L levels. Our novel finding suggests that NRG1 may play a protective role in hypoxia through the inhibition of oxidative stress and thereby maintain normal EAAC1 expression levels.

Early Neurotoxic Effects of Inorganic Arsenic Modulate Cortical GSH Levels Associated With the Activation of the Nrf2 and NFκB Pathways, Expression of Amino Acid Transporters and NMDA Receptors and the Production of Hydrogen Sulfide

Exposure to toxic metals and metalloids is an important cause of preventable diseases worldwide. Inorganic arsenic (iAs) affects several organs and tissues, causing neurobehavioral alterations in the central nervous system (CNS) that might lead to neurodegeneration. In this work, we wanted to explore the time- and dose-related changes on glutathione (GSH) levels in several regions of the CNS, such as the cortex, striatum, hippocampus, and cerebellum, to identify the initial cellular changes associated to GSH depletion due to iAs exposure. Mice received a single intraperitoneal injection containing 5 or 14 mg/kg sodium arsenite. Animals were killed at 2, 6, and 24 h. Significant depletion of GSH levels was observed in the cortex at 2 and 6 h, while on the striatum, hippocampus, or cerebellum regions, no significant changes were observed. GSH depletion in the cortex was associated with the activation of the nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor kappa B (NFκB) pathways, which led to the upregulation of xCT, excitatory amino acid carrier 1 (EAAC1), glutamate/aspartate transporter (GLAST), and glial glutamate transporter 1 (GLT-1), and the activation of the transsulfuration pathways, which led to the overproduction of H₂S in the cortex and increased levels of GSH in the cortex and cerebellum at 24 h. In the cortex, the N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B were also altered at 24 h. These early effects were not homogeneous among different brain regions and indicate early neurotoxic alterations in the cortex and cerebellum.

Glutathione Deficiency and Alterations in the Sulfur Amino Acid Homeostasis during Early Postnatal Development as Potential Triggering Factors for Schizophrenia-Like Behavior in Adult Rats

Impaired glutathione (GSH) synthesis and dopaminergic transmission are important factors in the pathophysiology of schizophrenia. Our research aimed to assess the effects of l-buthionine-(S,R)-sulfoximine (BSO), a GSH synthesis inhibitor, and GBR 12909, a dopamine reuptake inhibitor, administered alone or in combination, to Sprague–Dawley rats during early postnatal development (p5–p16), on the levels of GSH, sulfur amino acids, global DNA methylation, and schizophrenia-like behavior. GSH, methionine (Met), homocysteine (Hcy), and cysteine (Cys) contents were determined in the liver, kidney, and in the prefrontal cortex (PFC) and hippocampus (HIP) of 16-day-old rats. DNA methylation in the PFC and HIP and schizophrenia-like behavior were assessed in adulthood (p90–p93). BSO caused the tissue-dependent decreases in GSH content and alterations in Met, Hcy, and Cys levels in the peripheral tissues and in the PFC and HIP. The changes in these parameters were accompanied by alterations in the global DNA methylation in the studied brain structures. Parallel to changes in the global DNA methylation, deficits in the social behaviors and cognitive functions were observed in adulthood. Only BSO + GBR 12909-treated rats exhibited behavioral alterations resembling positive symptoms in schizophrenia patients. Our results suggest the usefulness of this neurodevelopmental model for research on the pathomechanism of schizophrenia.

Prophylactic Riluzole Attenuates Oxidative Stress Damage in Spinal Cord Distraction

Spinal cord injury (SCI) without radiographical abnormalities (SCIWORA) presents a significant challenge because of the loss of function despite an apparent normal anatomy. The cause of dysfunction is not understood, and specific treatment options are lacking. Some scoliosis corrective surgeries result in SCIWORA, where stretching of the spinal cord can lead to vascular compromise and hypoxia. The iatrogenic nature of this injury allows for the implantation of neuroprotective strategies that are designed to prevent damage. We utilized a model of atraumatic SCI to evaluate the efficacy of the sodium-channel blocker, riluzole, as a prophylactic neuroprotectant. As expected, the stretch injury caused a significant reduction in intraparenchymal oxygen in distraction (-53.09 ± 22.23%) and riluzole pre-treated distraction animals (-43.04 ± 22.86%). However, in contrast to the oxidative stress and metabolic impairments observed in vehicle-treated distraction animals, in which protein carbonylation increased significantly (5.88 ± 1.3 nmol/mL), riluzole kept these levels within the normal range (1.8 ± 1.0 nmol/mL). This neurprotection also prevented ventral motor neuron hypoplasia and pyknosis, characteristic features of this atraumatic SCI model, and maintained normal gait function (e.g., stride length and stance time). This study provides evidence for the use of prophylactic neuroprotective strategies in which thoracic or spine surgeries present the risk of causing atraumatic SCI.

Ethanol (E) Impairs Fetal Brain GSH Homeostasis by Inhibiting Excitatory Amino-Acid Carrier 1 (EAAC1)-Mediated Cysteine Transport

Central among the fetotoxic responses to in utero ethanol (E) exposure is redox-shift related glutathione (GSH) loss and apoptosis. Previously, we reported that despite an E-generated Nrf2 upregulation, fetal neurons still succumb. In this study, we investigate if the compromised GSH results from an impaired inward transport of cysteine (Cys), a precursor of GSH in association with dysregulated excitatory amino acid carrier1 (EAAC1), a cysteine transporter. In utero binge model involves administration of isocaloric dextrose or 20% E (3.5 g/kg)/ by gavage at 12 h intervals to pregnant Sprague Dawley (SD) rats, starting gestation day (gd) 17 with a final dose on gd19, 2 h prior to sacrifice. Primary cerebral cortical neurons (PCNs) from embryonic day 16–17 fetal SD rats were the in vitro model. E reduced both PCN and cerebral cortical GSH and Cys up to 50% and the abridged GSH could be blocked by administration of N-acetylcysteine. E reduced EAAC1 protein expression in utero and in PCNs (p < 0.05). This was accompanied by a 60–70% decrease in neuron surface expression of EAAC1 along with significant reductions of EAAC1/Slc1a1 mRNA (p < 0.05). In PCNs, EAAC1 knockdown significantly decreased GSH but not oxidized glutathione (GSSG) illustrating that while not the sole provider of Cys, EAAC1 plays an important role in neuron GSH homeostasis. These studies strongly support the concept that in both E exposed intact fetal brain and cultured PCNs a mechanism underlying E impairment of GSH homeostasis is reduction of import of external Cys which is mediated by perturbations of EAAC1 expression/function.

Effect of early weaning on the expression of excitatory amino acid transporter 1 in the jejunum and ileum of piglets

The present study aimed to compare the expression levels of excitatory amino acid transporters (EAATs) and growth status of piglets weaned at 10–20 days after birth with suckling piglets. A total of 40 hybrid piglets (Landrace × Large White × Duroc) born to 40 different sows, with similar body weight were selected for the present study. They were randomly divided into two groups (n=20 per group): Control group (suckling piglets) and experimental group (weaned piglets, reared in isolation). The experiment lasted for 10 days. At the end of the experiment, 12 piglets were randomly selected from each group and the jejunum and the ileum were collected in order to determine excitatory amino acid carrier 1 (EAAC1) expression levels and free amino acid content. The present study determined that early weaning significantly reduced EAAC1 gene and protein (57 and 73 kDa) expression levels and glutamate transporter associate protein 3–18 (GTRAP3-18; 50 kDa) in the jejunum and the ileum compared with the suckling group (P<0.05). Weaning led to an increased content of free glutamic acid (Glu) and total amino acids in the jejunum; however, content of free Glu and total amino acids in the ileum was significantly reduced (P<0.05). Early weaning reduced the expression of EAAC1 and GTRAP3-18, which was possibly due to the amino acid absorption and transport disorder in the small intestine due to the Glu deficiency.

JWA regulates TRAIL-induced apoptosis via MARCH8-mediated DR4 ubiquitination in cisplatin-resistant gastric cancer cells

Platinum chemotherapeutics are widely used to treat solid malignant tumors, including gastric cancer (GC). Drug resistance to platinum compounds may result in cancer relapse and decreased survival. The identification and development of novel agents to reactivate apoptosis pathways in platinum-resistant cancer cells is therefore necessary. Here we report that cisplatin-resistant human GC cells (BGC823/DDP and SGC7901/DDP) but not their parental cells (BGC823 and SGC7901) exhibit high sensitivity to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) as a result of overexpression of death receptor 4 (DR4). Furthermore, we found that JWA, a molecule that promotes cisplatin-induced apoptosis in GC cells, suppressed TRAIL-induced apoptosis via negative regulation of DR4. Mechanistically, JWA promoted the ubiquitination of DR4 at K273 via upregulation of the ubiquitin ligase membrane-associated RING-CH-8 (MARCH8). In human GC tissues, JWA and DR4 protein levels were negatively correlated. Thus TRAIL may serve as an auxiliary treatment for cisplatin-resistant GC, and JWA may be a potential predictive marker of TRAIL sensitivity and may improve personalized therapeutics for treating human GC.

Lens glutathione homeostasis: Discrepancies and gaps in knowledge standing in the way of novel therapeutic approaches

Cataract is the major cause of blindness worldwide. The WHO has estimated around 20 million people have bilateral blindness from cataract, and that number is expected to reach 50 million in 2050. The cataract surgery is currently the main treatment approach, though often associated with complications, such as Posterior Capsule Opacification (PCO)-also known as secondary cataract. The lens is an avascular ocular structure equipped with an unusually high level of glutathione (GSH), which plays a vital role in maintaining lens transparency by regulating lenticular redox state. The lens epithelium and outer cortex are thought to be responsible for providing the majority of lens GSH via GSH de novo synthesis, assisted by a continuous supply of constituent amino acids from the aqueous humor, as well as extracellular GSH recycling from the gamma-glutamyl cycle. However, when de novo synthesis is impaired, in the presence of low GSH levels, as in the aging human lens, compensatory mechanisms exist, suggesting that the lens is able to uptake GSH from the surrounding ocular tissues. However, these uptake mechanisms, and the GSH source and its origin, are largely unknown. The lens nucleus does not have the ability to synthesize its own GSH and fully relies on transport from the outer cortex by yet unknown mechanisms. Understanding how aging reduces GSH levels, particularly in the lens nucleus, how it is associated with age-related nuclear cataract (ARNC), and how the lens compensates for GSH loss via external uptake should be a major research priority. The intent of this review, which is dedicated to the memory of David C. Beebe, is to summarize our current understanding of lens GSH homeostasis and highlight discrepancies and gaps in knowledge that stand in the way of pharmacologically minimizing the impact of declining GSH content in the prevention of age-related cataract.

A novel dual-emission fluorescent probe for the simultaneous detection of H2S and GSH

A novel chlorinated coumarin–malononitrile fluorescent probe was synthesized for the simultaneous detection of H₂S and GSH from different emission channels.

Role of glutathione, glutathione transferase, and glutaredoxin in regulation of redox-dependent processes

Over the last decade fundamentally new features have been revealed for the participation of glutathione and glutathione-dependent enzymes (glutathione transferase and glutaredoxin) in cell proliferation, apoptosis, protein folding, and cell signaling. Reduced glutathione (GSH) plays an important role in maintaining cellular redox status by participating in thiol-disulfide exchange, which regulates a number of cell functions including gene expression and the activity of individual enzymes and enzyme systems. Maintaining optimum GSH/GSSG ratio is essential to cell viability. Decrease in the ratio can serve as an indicator of damage to the cell redox status and of changes in redox-dependent gene regulation. Disturbance of intracellular GSH balance is observed in a number of pathologies including cancer. Consequences of inappropriate GSH/GSSG ratio include significant changes in the mechanism of cellular redox-dependent signaling controlled both nonenzymatically and enzymatically with the participation of isoforms of glutathione transferase and glutaredoxin. This review summarizes recent data on the role of glutathione, glutathione transferase, and glutaredoxin in the regulation of cellular redox-dependent processes.

L-Cysteine metabolism and its nutritional implications

L-Cysteine is a nutritionally semiessential amino acid and is present mainly in the form of L-cystine in the extracellular space. With the help of a transport system, extracellular L-cystine crosses the plasma membrane and is reduced to L-cysteine within cells by thioredoxin and reduced glutathione (GSH). Intracellular L-cysteine plays an important role in cellular homeostasis as a precursor for protein synthesis, and for production of GSH, hydrogen sulfide (H(2)S), and taurine. L-Cysteine-dependent synthesis of GSH has been investigated in many pathological conditions, while the pathway for L-cysteine metabolism to form H(2)S has received little attention with regard to prevention and treatment of disease in humans. The main objective of this review is to highlight the metabolic pathways of L-cysteine catabolism to GSH, H(2)S, and taurine, with special emphasis on therapeutic and nutritional use of L-cysteine to improve the health and well-being of animals and humans.

Glutathione in Cellular Redox Homeostasis: Association with the Excitatory Amino Acid Carrier 1 (EAAC1)

Reactive oxygen species (ROS) are by-products of the cellular metabolism of oxygen consumption, produced mainly in the mitochondria. ROS are known to be highly reactive ions or free radicals containing oxygen that impair redox homeostasis and cellular functions, leading to cell death. Under physiological conditions, a variety of antioxidant systems scavenge ROS to maintain the intracellular redox homeostasis and normal cellular functions. This review focuses on the antioxidant system’s roles in maintaining redox homeostasis. Especially, glutathione (GSH) is the most important thiol-containing molecule, as it functions as a redox buffer, antioxidant, and enzyme cofactor against oxidative stress. In the brain, dysfunction of GSH synthesis leading to GSH depletion exacerbates oxidative stress, which is linked to a pathogenesis of aging-related neurodegenerative diseases. Excitatory amino acid carrier 1 (EAAC1) plays a pivotal role in neuronal GSH synthesis. The regulatory mechanism of EAAC1 is also discussed.

Functional contributions of glutamate transporters at the parallel fibre to Purkinje neuron synapse-relevance for the progression of cerebellar ataxia

Background

Rapid uptake of glutamate by neuronal and glial glutamate transporters (EAATs, a family of excitatory amino acid transporters) is critical for shaping synaptic responses and for preventing excitotoxicity. Two of these transporters, EAAT4 in Purkinje neurons (PN) and EAAT1 in Bergmann glia are both enriched within the cerebellum and altered in a variety of human ataxias.

Results

PN excitatory synaptic responses and firing behaviour following high frequency parallel fibre (PF) activity commonly encountered during sensory stimulation in vivo were adversely influenced by acute inhibition of glutamate transporters. In the presence of a non-transportable blocker of glutamate transporters we observed very large amplitude and duration excitatory postsynaptic currents accompanied by excessive firing of the PNs. A combination of AMPA and mGluR1, but not NMDA, type glutamate receptor activation powered the hyper-excitable PN state. The enhanced PN excitability also recruited a presynaptic mGluR4 dependent mechanism that modified short term plasticity at the PF synapse.

Conclusions

Our findings indicate that reduced glutamate transporter activity, as occurs in the early stages of some forms of human cerebellar ataxias, excessively excites PNs and disrupts the timing of their output. Our findings raise the possibility that sustaining cerebellar glutamate uptake may provide a therapeutic approach to prevent this disruption and the glutamate excitotoxicity-induced PN death that signals the end point of the disease.

Changes in the expression of the glutamate transporter EAAT3/EAAC1 in health and disease

Excitatory amino acid transporters (EAATs) are high-affinity Na(+)-dependent carriers of major importance in maintaining glutamate homeostasis in the central nervous system. EAAT3, the human counterpart of the rodent excitatory amino acid carrier 1 (EAAC1), is encoded by the SLC1A1 gene. EAAT3/EAAC1 is ubiquitously expressed in the brain, mostly in neurons but also in other cell types, such as oligodendrocyte precursors. While most of the glutamate released in the synapses is taken up by the "glial-type" EAATs, EAAT2 (GLT-1 in rodents) and EAAT1 (GLAST), the functional role of EAAT3/EAAC1 is related to the subtle regulation of glutamatergic transmission. Moreover, because it can also transport cysteine, EAAT3/EAAC1 is believed to be important for the synthesis of intracellular glutathione and subsequent protection from oxidative stress. In contrast to other EAATs, EAAT3/EAAC1 is mostly intracellular, and several mechanisms have been described for the rapid regulation of the membrane trafficking of the transporter. Moreover, the carrier interacts with several proteins, and this interaction modulates transport activity. Much less is known about the slow regulatory mechanisms acting on the expression of the transporter, although several recent reports have identified changes in EAAT3/EAAC1 protein level and activity related to modulation of its expression at the gene level. Moreover, EAAT3/EAAC1 expression is altered in pathological conditions, such as hypoxia/ischemia, multiple sclerosis, schizophrenia, and epilepsy. This review summarizes these results and provides an overall picture of changes in EAAT3/EAAC1 expression in health and disease.

Widespread microRNA dysregulation in multiple system atrophy - disease-related alteration in miR-96

MicroRNA (miRNA) are short sequences of RNA that function as post-transcriptional regulators by binding to target mRNA transcripts resulting in translational repression. A number of recent studies have identified miRNA as being involved in neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the role of miRNA in multiple system atrophy (MSA), a progressive neurodegenerative disorder characterized by oligodendroglial accumulation of alpha-synuclein remains unexamined. In this context, this study examined miRNA profiles in MSA cases compared with controls and in transgenic (tg) models of MSA compared with non-tg mice. The results demonstrate a widespread dysregulation of miRNA in MSA cases, which is recapitulated in the murine models. The study employed a cross-disease, cross-species approach to identify miRNA that were either specifically dysregulated in MSA or were commonly dysregulated in neurodegenerative conditions such as Alzheimer's disease, dementia with Lewy bodies, progressive supranuclear palsy and corticobasal degeneration or the tg mouse model equivalents of these disorders. Using this approach we identified a number of miRNA that were commonly dysregulated between disorders and those that were disease-specific. Moreover, we identified miR-96 as being up-regulated in MSA. Consistent with the up-regulation of miR-96, mRNA and protein levels of members of the solute carrier protein family SLC1A1 and SLC6A6, miR-96 target genes, were down-regulated in MSA cases and a tg model of MSA. These results suggest that miR-96 dysregulation may play a role in MSA and its target genes may be involved in the pathogenesis of MSA.

Role of glutamate transporters in redox homeostasis of the brain

Redox homeostasis is especially important in the brain where high oxygen consumption produces an abundance of harmful oxidative by-products. Glutathione (GSH) is a tripeptide non-protein thiol. It is the central nervous system's most abundant antioxidant and the master controller of brain redox homeostasis. The glutamate transporters, System xc(-) (SXC) and the Excitatory Amino Acid Transporters (EAAT), play important, synergistic roles in the synthesis of GSH. In glial cells, SXC mediates the uptake of cystine, which after intracellular reduction to cysteine, reacts with glutamate during the rate-limiting step of GSH synthesis. EAAT3 mediates direct cysteine uptake for neuronal GSH synthesis. SXC and EAAT work in concert in glial cells to provide two intracellular substrates for GSH synthesis, cystine and glutamate. Their cyclical basal function also prevents a buildup of extracellular glutamate, which SXC releases extracellularly in exchange for cystine uptake. Maintaining extracellular glutamate homeostasis is critical to prevent neuronal toxicity, as well as glutamate-mediated SXC inhibition, which could lead to a depletion of intracellular GSH and loss of cellular redox control. Many neurological diseases show evidence of GSH dysfunction, and increased GSH has been widely associated with chemotherapy and radiotherapy resistance of gliomas. We present evidence suggesting that gliomas expressing elevated levels of SXC are more reliant on GSH for growth and survival. They have an increased inherent radiation resistance, however, inhibition of SXC can increase tumor sensitivity at low radiation doses. GSH depletion through SXC inhibition may be a viable mechanism to enhance current glioma treatment strategies and make tumors more sensitive to radiation and chemotherapy protocols.

Genetic deletion of the neuronal glutamate transporter, EAAC1, results in decreased neuronal death after pilocarpine-induced status epilepticus

Excitatory amino acid carrier 1 (EAAC1 also called EAAT3) is a Na(+)-dependent glutamate transporter expressed by both glutamatergic and GABAergic neurons. It provides precursors for the syntheses of glutathione and GABA and contributes to the clearance of synaptically released glutamate. Mice deleted of EAAC1 are more susceptible to neurodegeneration in models of ischemia, Parkinson's disease, and aging. Antisense knock-down of EAAC1 causes an absence seizure-like phenotype. Additionally, EAAC1 expression increases after chemonvulsant-induced seizures in rodent models and in tissue specimens from patients with refractory epilepsy. The goal of the present study was to determine if the absence of EAAC1 affects the sensitivity of mice to seizure-induced cell death. A chemoconvulsant dose of pilocarpine was administered to EAAC1(-/-) mice and to wild-type controls. Although EAAC1(-/-) mice experienced increased latency to seizure onset, no significant differences in behavioral seizure severity or mortality were observed. We examined EAAC1 immunofluorescence 24h after pilocarpine administration and confirmed that pilocarpine causes an increase in EAAC1 protein. Forty-eight hours after induction of seizures, cell death was measured in hippocampus and in cortex using Fluoro-Jade C. Surprisingly, there was ∼2-fold more cell death in area CA1 of wild-type mice than in the corresponding regions of the EAAC1(-/-) mice. Together, these studies indicate that absence of EAAC1 results in either a decrease in pilocarpine-induced seizures that is not detectable by behavioral criteria (surprising, since EAAC1 provides glutamate for GABA synthesis), or that the absence of EAAC1 results in less pilocarpine/seizure-induced cell death, possible explanations as discussed.

Glutamate drugs and pharmacogenetics of OCD: a pathway-based exploratory approach

INTRODUCTION

Neuropharmacology research in glutamate-modulating drugs supports their development and use in the management of neuropsychiatric disorders, including major depression, Alzheimer's disorder and schizophrenia. Concomitantly, there is a growing use of these agents used in the treatment of obsessive-compulsive disorder (OCD).

AREAS COVERED

This article provides a review of glutamate-modulating drugs used in the treatment of OCD. Specifically, the authors examine riluzole, N-acetylcysteine, d-cycloserine, glycine, ketamine, memantine and acamprosate as treatments. Furthermore, recent genetic epidemiology research findings are presented with a focus on the positional candidate genes SLC1A1 (a glutamate transporter), ADAR3 (an RNA-editing enzyme), RYR3 (a Ca(2+) channel), PBX1 (a homeobox transcription factor) and a GWAS candidate gene, DLGAP1 (a protein interacting with post-synaptic density). These genetic findings are submitted to a curated bioinformatics database to conform a biological network for discerning potential pharmacological targets.

EXPERT OPINION

In the genetically informed network, known genes and identified key connecting components, including DLG4 (a developmental gene), PSD-95 (a synaptic scaffolding protein) and PSEN1 (presenilin, a regulator of secretase), conform a group of potential pharmacological targets. These potential targets can be explored, in the future, to deliver new therapeutic approaches to OCD. There is also the need to develop a better understanding of neuroprotective mechanisms as a foundation for future OCD drug discovery.

Oral administration of MSG increases expression of glutamate receptors and transporters in the gastrointestinal tract of young piglets

Glutamate receptors and transporters, including T1R1 and T1R3 (taste receptor 1, subtypes 1 and 3), mGluRs (metabotropic glutamate receptors), EAAC-1 (excitatory amino acid carrier-1), GLAST-1 (glutamate-aspartate transporter-1), and GLT-1 (glutamate transporter-1), are expressed in the gastrointestinal tract. This study determined effects of oral administration of monosodium glutamate [MSG; 0, 0.06, 0.5, or 1 g/kg body weight (BW)/day] for 21 days on expression of glutamate receptors and transporters in the stomach and jejunum of sow-reared piglets. Both mRNA and protein levels for gastric T1R1, T1R3, mGluR1, mGluR4, EAAT1, EAAT2, EAAT3, and EAAT4 and mRNA levels for jejunal T1R1, T1R3, EAAT1, EAAT2, EAAT3 and EAAT4 were increased (P < 0.05) by MSG supplementation. Among all groups, mRNA levels for gastric EAAT1, EAAT2, EAAT3, and EAAT4 were highest (P < 0.05) in piglets receiving 1 g MSG/kg BW/day. EAAT1 and EAAT2 mRNA levels in the stomach and jejunum of piglets receiving 0.5 g MSG/kg BW/day, as well as jejunal EAAT3 and EAAT4 mRNA levels in piglets receiving 1 g MSG/kg BW/day, were higher (P < 0.05) than those in the control and in piglets receiving 0.06 g MSG/kg BW/day. Furthermore, protein levels for jejunal T1R1 and EAAT3 were higher (P < 0.05) in piglets receiving 1 g MSG/kg BW/day than those in the control and in piglets receiving 0.06 g MSG/kg BW/day. Collectively, these findings indicate that dietary MSG may beneficially stimulate glutamate signaling and sensing in the stomach and jejunum of young pigs, as well as their gastrointestinal function.

Neuroprotective properties of the excitatory amino acid carrier 1 (EAAC1)

Extracellular glutamate should be maintained at low levels to conserve optimal neurotransmission and prevent glutamate neurotoxicity in the brain. Excitatory amino acid transporters (EAATs) play a pivotal role in removing extracellular glutamate in the central nervous system (CNS). Excitatory amino acid carrier 1 (EAAC1) is a high-affinity Na⁺-dependent neuronal EAAT that is ubiquitously expressed in the brain. However, most glutamate released in the synapses is cleared by glial EAATs, but not by EAAC1 in vivo. In the CNS, EAAC1 is widely distributed in somata and dendrites but not in synaptic terminals. The contribution of EAAC1 to the control of extracellular glutamate levels seems to be negligible in the brain. However, EAAC1 can transport not only extracellular glutamate but also cysteine into the neurons. Cysteine is an important substrate for glutathione (GSH) synthesis in the brain. GSH has a variety of neuroprotective functions, while its depletion induces neurodegeneration. Therefore, EAAC1 might exert a critical role for neuroprotection in neuronal GSH metabolism rather than glutamatergic neurotransmission, while EAAC1 dysfunction would cause neurodegeneration. Despite the potential importance of EAAC1 in the brain, previous studies have mainly focused on the glutamate neurotoxicity induced by glial EAAT dysfunction. In recent years, however, several studies have revealed regulatory mechanisms of EAAC1 functions in the brain. This review will summarize the latest information on the EAAC1-regulated neuroprotective functions in the CNS.

Dysregulation of glutathione homeostasis in neurodegenerative diseases

Dysregulation of glutathione homeostasis and alterations in glutathione-dependent enzyme activities are increasingly implicated in the induction and progression of neurodegenerative diseases, including Alzheimer’s, Parkinson’s and Huntington’s diseases, amyotrophic lateral sclerosis, and Friedreich’s ataxia. In this review background is provided on the steady-state synthesis, regulation, and transport of glutathione, with primary focus on the brain. A brief overview is presented on the distinct but vital roles of glutathione in cellular maintenance and survival, and on the functions of key glutathione-dependent enzymes. Major contributors to initiation and progression of neurodegenerative diseases are considered, including oxidative stress, protein misfolding, and protein aggregation. In each case examples of key regulatory mechanisms are identified that are sensitive to changes in glutathione redox status and/or in the activities of glutathione-dependent enzymes. Mechanisms of dysregulation of glutathione and/or glutathione-dependent enzymes are discussed that are implicated in pathogenesis of each neurodegenerative disease. Limitations in information or interpretation are identified, and possible avenues for further research are described with an aim to elucidating novel targets for therapeutic interventions. The pros and cons of administration of N-acetylcysteine or glutathione as therapeutic agents for neurodegenerative diseases, as well as the potential utility of serum glutathione as a biomarker, are critically evaluated.

Riluzole-triggered GSH synthesis via activation of glutamate transporters to antagonize methylmercury-induced oxidative stress in rat cerebral cortex

OBJECTIVE

This study was to evaluate the effect of riluzole on methylmercury- (MeHg-) induced oxidative stress, through promotion of glutathione (GSH) synthesis by activating of glutamate transporters (GluTs) in rat cerebral cortex.

METHODS

Eighty rats were randomly assigned to four groups, control group, riluzole alone group, MeHg alone group, and riluzole + MeHg group. The neurotoxicity of MeHg was observed by measuring mercury (Hg) absorption, pathological changes, and cell apoptosis of cortex. Oxidative stress was evaluated via determining reactive oxygen species (ROS), 8-hydroxy-2-deoxyguanosine (8-OHdG), malondialdehyde (MDAs), carbonyl, sulfydryl, and GSH in cortex. Glutamate (Glu) transport was studied by measuring Glu, glutamine (Gln), mRNA, and protein of glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1).

RESULT

(1) MeHg induced Hg accumulation, pathological injury, and apoptosis of cortex; (2) MeHg increased ROS, 8-OHdG, MDA, and carbonyl, and inhibited sulfydryl and GSH; (3) MeHg elevated Glu, decreased Gln, and downregulated GLAST and GLT-1 mRNA expression and protein levels; (4) riluzole antagonized MeHg-induced downregulation of GLAST and GLT-1 function and expression, GSH depletion, oxidative stress, pathological injury, and apoptosis obviously.

CONCLUSION

Data indicate that MeHg administration induced oxidative stress in cortex and that riluzole could antagonize this situation through elevation of GSH synthesis by activating of GluTs.

Glutathione and modulation of cell apoptosis

Apoptosis is a highly organized form of cell death that is important for tissue homeostasis, organ development and senescence. To date, the extrinsic (death receptor mediated) and intrinsic (mitochondria derived) apoptotic pathways have been characterized in mammalian cells. Reduced glutathione, is the most prevalent cellular thiol that plays an essential role in preserving a reduced intracellular environment. glutathione protection of cellular macromolecules like deoxyribose nucleic acid proteins and lipids against oxidizing, environmental and cytotoxic agents, underscores its central anti-apoptotic function. Reactive oxygen and nitrogen species can oxidize cellular glutathione or induce its extracellular export leading to the loss of intracellular redox homeostasis and activation of the apoptotic signaling cascade. Recent evidence uncovered a novel role for glutathione involvement in apoptotic signaling pathways wherein post-translational S-glutathiolation of protein redox active cysteines is implicated in the potentiation of apoptosis. In the present review we focus on the key aspects of glutathione redox mechanisms associated with apoptotic signaling that includes: (a) changes in cellular glutathione redox homeostasis through glutathione oxidation or GSH transport in relation to the initiation or propagation of the apoptotic cascade, and (b) evidence for S-glutathiolation in protein modulation and apoptotic initiation.

The role of amino acid transporters in GSH synthesis in the blood-brain barrier and central nervous system

Glutathione (GSH) plays a critical role in protecting cells from oxidative stress and xenobiotics, as well as maintaining the thiol redox state, most notably in the central nervous system (CNS). GSH concentration and synthesis are highly regulated within the CNS and are limited by availability of the sulfhydryl amino acid (AA) l-cys, which is mainly transported from the blood, through the blood-brain barrier (BBB), and into neurons. Several antiporter transport systems (e.g., x(c)(-), x(-)(AG), and L) with clearly different luminal and abluminal distribution, Na(+), and pH dependency have been described in brain endothelial cells (BEC) of the BBB, as well as in neurons, astrocytes, microglia and oligodendrocytes from different brain structures. The purpose of this review is to summarize information regarding the different AA transport systems for l-cys and its oxidized form l-cys(2) in the CNS, such as expression and activity in blood-brain barrier endothelial cells, astrocytes and neurons and environmental factors that modulate transport kinetics.

Transient striatal GLT-1 blockade increases EAAC1 expression, glutamate reuptake, and decreases tyrosine hydroxylase phosphorylation at ser(19)

Three glutamate transporters, GLT-1, GLAST, and EAAC1, are expressed in striatum. GLT-1 and, to a lesser extent, GLAST are thought to play a primary role in glutamate reuptake and mitigate excitoxicity. Progressive tyrosine hydroxylase (TH) loss seen in Parkinson's disease (PD) is associated with increased extracellular glutamate. Glutamate receptor antagonists reduce nigrostriatal loss in PD models. These observations suggest that excess synaptic glutamate contributes to nigrostriatal neuron loss seen in PD. Decreased GLT-1 expression occurs in neurodegenerative disease and PD models, suggesting decreased GLT-1-mediated glutamate reuptake contributes to excitotoxicity. To determine how transient GLT-1 blockade affects glutamate reuptake dynamics and a Ca(2+)-dependent process in nigrostriatal terminals, ser(19) phosphorylation of TH, the GLT-1 inhibitor dihydrokainic acid (DHK) was delivered unilaterally to striatum in vivo and glutamate reuptake was quantified ex vivo in crude synaptosomes 3h later. Ca(2+)-influx is associated with excitotoxic conditions. The phosphorylation of TH at ser(19) is Ca(2+)-dependent, and a change resulting from GLT-1 blockade may signify the potential for excitotoxicity to nigrostriatal neurons. Synaptosomes from DHK infused striatum had a 43% increase in glutamate reuptake in conjunction with decreased ser(19) TH phosphorylation. Using a novel GLAST inhibitor and DHK, we determined that the GLAST-mediated component of increased glutamate reuptake increased 3-fold with no change in GLAST or GLT-1 protein expression. However, GLT-1 blockade increased EAAC1 protein expression ~20%. Taken together, these results suggest that GLT-1 blockade produces a transient increase in GLAST-mediated reuptake and EAAC1 expression coupled with reduced ser(19) TH phosphorylation. These responses could represent an endogenous defense against excitotoxicity to the nigrostriatal pathway.

The oxidative stress-inducible cystine/glutamate antiporter, system x (c) (-) : cystine supplier and beyond

The oxidative stress-inducible cystine/glutamate exchange system, system x (c) (-) , transports one molecule of cystine, the oxidized form of cysteine, into cells and thereby releases one molecule of glutamate into the extracellular space. It consists of two protein components, the 4F2 heavy chain, necessary for membrane location of the heterodimer, and the xCT protein, responsible for transport activity. Previously, system x (c) (-) has been regarded to be a mere supplier of cysteine to cells for the synthesis of proteins and the antioxidant glutathione (GSH). In that sense, oxygen, electrophilic agents, and bacterial lipopolysaccharide trigger xCT expression to accommodate with increased oxidative stress by stimulating GSH biosynthesis. However, emerging evidence established that system x (c) (-) may act on its own as a GSH-independent redox system by sustaining a redox cycle over the plasma membrane. Hallmarks of this cycle are cystine uptake, intracellular reduction to cysteine and secretion of the surplus of cysteine into the extracellular space. Consequently, increased levels of extracellular cysteine provide a reducing microenvironment required for proper cell signaling and communication, e.g. as already shown for the mechanism of T cell activation. By contrast, the enhanced release of glutamate in exchange with cystine may trigger neurodegeneration due to glutamate-induced cytotoxic processes. This review aims to provide a comprehensive picture from the early days of system x (c) (-) research up to now.

Regulation of xCT expression and system x (c) (-) function in neuronal cells

The glutamate/cystine antiporter system x(c)(-) transports cystine into cells in exchange for glutamate at a ratio of 1:1. It is composed of a specific light chain, xCT, and a heavy chain, 4F2, linked by a disulfide bridge. Intracellularly, cystine is reduced into cysteine, the rate-limiting precursor of glutathione (GSH), an important small molecule antioxidant. Several lines of evidence suggest that the expression of xCT and thereby the presence system x(c)(-) activity plays an important role in the brain. First, it regulates extracellular glutamate concentrations. Second, as brain is prone to oxidative stress due to its high oxygen consumption and lipid content, system x(c)(-) by favoring GSH synthesis, may prevent oxidative damage. Thus, to understand how xCT expression and system x(c)(-) activity are regulated in the central nervous system is of utmost importance. In this review, we will summarize the current knowledge about the molecular basis by which xCT expression and system x(c)(-) activity are regulated in neuronal cell lines, especially the hippocampal cell line, HT22. In addition, we will relate these pathways to findings in other cell types, especially those found in the central nervous system. We will focus on the signaling pathways that modulate the transcription of the xCT gene. Furthermore, we describe possible pathways that modify system x(c)(-) activity beyond the level of xCT transcription, including regulation on the level of membrane trafficking and substrate availability, especially the regulation by glutamate transport through excitatory amino acid transporters.