Neuronal GSH Synthesis Research Articles

Erythroid Differentiation Regulator 1 as a Regulator of Neuronal GSH Synthesis.

Erythroid differentiation regulator 1 (Erdr1) is a cytokine known to play important roles in cell survival under stressful conditions, maintenance of cellular growth homeostasis, and activation of the immune system. However, the impact of Erdr1 on neurons remains undefined. In this study, we present novel evidence that Erdr1 plays a role in regulating glutathione (GSH) synthesis via glutamate transporter-associated protein 3-18 (GTRAP3-18), an anchor protein in the endoplasmic reticulum that holds excitatory amino acid carrier 1 (EAAC1) in neurons. Both DNA microarray and quantitative real-time PCR analyses revealed an approximately 2-fold increase in Erdr1 levels in the hippocampus of GTRAP3-18-deficient mice compared to those of wild-type mice. Knockdown of Erdr1 in vitro resulted in a decrease in GTRAP3-18 levels, leading to an increase in EAAC1 expression and intracellular GSH levels, and subsequently, cytoprotective effects against oxidative stress. Our findings shed light on the regulatory mechanisms involving Erdr1, GTRAP3-18, EAAC1, and GSH in the context of neuronal defense against oxidative stress. Understanding the intricate interplay among these molecules may pave the way for the development of promising therapeutic strategies for neurodegenerative disorders.

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.

Glutathione and Related Molecules in Parkinsonism

Glutathione (GSH) is the most abundant intrinsic antioxidant in the central nervous system, and its substrate cysteine readily becomes the oxidized dimeric cystine. Since neurons lack a cystine transport system, neuronal GSH synthesis depends on cystine uptake via the cystine/glutamate exchange transporter (xCT), GSH synthesis, and release in/from surrounding astrocytes. Transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), a detoxifying master transcription factor, is expressed mainly in astrocytes and activates the gene expression of various phase II drug-metabolizing enzymes or antioxidants including GSH-related molecules and metallothionein by binding to the antioxidant response element (ARE) of these genes. Accumulating evidence has shown the involvement of dysfunction of antioxidative molecules including GSH and its related molecules in the pathogenesis of Parkinson’s disease (PD) or parkinsonian models. Furthermore, we found several agents targeting GSH synthesis in the astrocytes that protect nigrostriatal dopaminergic neuronal loss in PD models. In this article, the neuroprotective effects of supplementation and enhancement of GSH and its related molecules in PD pathology are reviewed, along with introducing new experimental findings, especially targeting of the xCT-GSH synthetic system and Nrf2–ARE pathway in astrocytes.

Anti-oxidants in astrocytes as target of neuroprotection for Parkinson's disease

Recently, it has been reported that dysfunction of astrocytes is involved vulnerability of neuronal cells in several neurological disorders. Glutathione (GSH) is the most abundant intrinsic antioxidant in the central nervous system, and its substrate cysteine is readily becomes the oxidized dimeric cystine. Since neurons lack a cystine transport system, neuronal GSH synthesis depends on cystine uptake via the cystine/glutamate exchange transporter (xCT), GSH synthesis and release in/from surrounding astrocytes. The expression and release of the zinc-binding protein metallothionein (MT) in astrocytes, which is a strong antioxidant, is induced and exerts neuroprotective in the case of dopaminergic neuronal damage. In addition, the transcription factor Nrf2 induces expression of MT-1 and GSH related molecules. We previously revealed that several antiepileptic drugs, serotonin 5-HT1A receptor agonists, plant-derived chemicals (phytochemicals) increased xCT expression, Nrf2 activation, GSH or MT expression and release in/from astrocytes, and exerted a neuroprotective effect against dopaminergic neurodegeneration in Parkinson's disease model. Our serial studies on neuroprotection via antioxidant defense mechanism of astrocytes have found three target molecular systems of astrocytes for neuroprotection: (1) xCT-GSH synthetic system, (2) Nrf2 system and (3) 5-HT1A receptor-Nrf2-MT system, 5-HT1A-S100β system. In this article, possible neuroprotective strategy for Parkinson's disease has been reviewed targeting antioxidative molecules in astrocytes.

The neuroprotective role of EAAC1 in hippocampal injury following ischemia-reperfusion

Ischemic stroke is one of the most prevalent brain disorders and the major cause of long-term disability. In particularly, hippocampal injury after ischemia-reperfusion is a serious problem as it contributes to vascular dementia. Many researches have revealed that ischemia-reperfusion causes increase in reactive oxygen species production and disruption of neuronal Zn2+ homeostasis in the hippocampus, which induces hippocampal neuron death. Glutathione (GSH) is present in all mammalian cells and plays a crucial role in neuronal cell defense against oxidative stress. On the other hand, thiol group of GSH chemically chelates Zn2+ and functions as a regulator of neuronal Zn2+ homeostasis. These evidences suggest that neuronal GSH levels could be an important factor affecting neuronal surviving. The synthesis of GSH is largely influenced by intracellular cysteine availability. In neurons, excitatory amino acid carrier type 1 (EAAC1) acts as a cysteine transporter and provides cysteine substrate for GSH synthesis. Recently, several animal studies have revealed that promotion of neuronal GSH synthesis through EAAC1 reduces ischemia-induced hippocampal neuron death. This review aims to describe neuroprotective role of GSH against hippocampal injury following ischemia-reperfusion, focusing on EAAC1.

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.

MicroRNA as a new agent for regulating neuronal glutathione synthesis and metabolism

microRNA (miRNA) is a small non-coding RNA molecule that plays a role in the post-transcriptional regulation of gene expression. Recent evidence shows that miRNAs are involved in various diseases,including neurodegenerative diseases (NDs) such as: Parkinson's disease,Alzheimer's disease,Huntington's disease,Amyotrophic lateral sclerosisand multiple system atrophy (MSA). The initiation and progression of NDs is generally considered to be induced by oxidative stress arising from an imbalance of oxidants and antioxidants. One of the most important antioxidants against oxidative stress is glutathione (GSH),which is a tripeptide composed of cysteine,glutamate and glycine. Among these precursor amino acids,cysteine is the determinant of neuronal GSH synthesis. Cysteine uptake in the neurons is mostly mediated by excitatory amino acid carrier 1 (EAAC1),a member of the sodium-dependent excitatory amino acid transporters. Interestingly,it has been reported that one miRNA,miR-96-5p,regulates the neuroprotective effect of GSH by directly regulating EAAC1 expression. Furthermore,the expressions of miR-96-5p and its target EAAC1 are specifically deregulated in the brains of patients with MSA,suggesting that deregulated miR-96-5p induces MSA via EAAC1 down-regulation. Since miR-96-5p regulation of EAAC1 expression and GSH level is indicated to be under circadian control,a greater understanding of rhythmic miRNA regulation could lead to the use of miRNA in chronotherapy for ND. In this review,we focus on the role of miRNA in the mechanism of GSH synthesis and metabolism; particularly with respect to a critical transport system of its rate-limiting substrate via EAAC1,as well as on the implications and chronotherapeutic potential of miRNA for NDs.

Rhythmic oscillations of the microRNA miR-96-5p play a neuroprotective role by indirectly regulating glutathione levels

Glutathione (GSH) is a key antioxidant that plays an important neuroprotective role in the brain. Decreased GSH levels are associated with neurodegenerative diseases such as Parkinson’s disease and Alzheimer’s disease. Here we show that a diurnal fluctuation of GSH levels is correlated with neuroprotective activity against oxidative stress in dopaminergic cells. In addition, we found that the cysteine transporter excitatory amino acid carrier 1 (EAAC1), which is involved in neuronal GSH synthesis, is negatively regulated by the microRNA miR-96-5p, which exhibits a diurnal rhythm. Blocking miR-96-5p by intracerebroventricular administration of an inhibitor increased the level of EAAC1 as well as that of GSH and had a neuroprotective effect against oxidative stress in the mouse substantia nigra. Our results suggest that the diurnal rhythm of miR-96-5p may play a role in neuroprotection by regulating neuronal GSH levels via EAAC1.

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

Glutathione (GSH) plays essential roles in different processes such as antioxidant defenses, cell signaling, cell proliferation, and apoptosis in the central nervous system. GSH is a tripeptide composed of glutamate, cysteine, and glycine. The concentration of cysteine in neurons is much lower than that of glutamate or glycine, so that cysteine is the rate-limiting substrate for neuronal GSH synthesis. Most neuronal cysteine uptake is mediated through the neuronal sodium-dependent glutamate transporter, known as excitatory amino acid carrier 1 (EAAC1). Glutamate transporters are vulnerable to oxidative stress and EAAC1 dysfunction impairs neuronal GSH synthesis by reducing cysteine uptake. This may start a vicious circle leading to neurodegeneration. Intracellular signaling molecules functionally regulate EAAC1. Glutamate transporter-associated protein 3-18 (GTRAP3-18) activation down-regulates EAAC1 function. Here, we focused on the interaction between EAAC1 and GTRAP3-18 at the plasma membrane to investigate their effects on neuronal GSH synthesis. Increased level of GTRAP3-18 protein induced a decrease in GSH level and, thereby, increased the vulnerability to oxidative stress, while decreased level of GTRAP3-18 protein induced an increase in GSH level in vitro. We also confirmed these results in vivo. Our studies demonstrate that GTRAP3-18 regulates neuronal GSH level by controlling the EAAC1-mediated uptake of cysteine.

Caffeine and uric acid mediate glutathione synthesis for neuroprotection

Several lines of epidemiological studies have indicated that caffeine consumption and plasma uric acid (UA) level were negatively correlated with the incidence of some neurodegenerative diseases. We report here a novel mechanism by which these purine derivatives increase neuronal glutathione (GSH) synthesis. Intraperitoneal injection of caffeine or UA into male C57BL/6 mice significantly increased total GSH levels in the hippocampus. Neither SCH58261, an adenosine A2A receptor antagonist, nor rolipram, a phosphodiesterase-4 inhibitor, increased GSH levels. Pretreatment with allopurinol, a drug to inhibit UA production, did not change the GSH level in the caffeine-treated mice. Hippocampal CA1 pyramidal neurons treated with caffeine or UA were resistant to oxidant exposure in the slice culture experiments. In experiments with the SH-SY5Y cell line, cysteine uptake was sodium-dependent and pretreatment with caffeine or UA increased cysteine uptake significantly as compared with the control conditions. Slice culture experiments using the hippocampus also showed increased cysteine and GSH contents after the treatment with caffeine or UA. Immunohistochemical analysis showed increased GSH levels in the hippocampal excitatory amino acid carrier-1 (EAAC1)-positive neurons of mice treated with caffeine or UA. These findings suggest that purine derivatives caffeine and UA induce neuronal GSH synthesis by promoting cysteine uptake, leading to neuroprotection.

Astroglial glutathione export and the supply of glutathione precursors to neurons

Neurons rely on the availability of extracellular cysteine for their glutathione (GSH) synthesis. This amino acid is generated by processing of extracellular GSH. Astrocytes play an important role in the GSH metabolism of the brain, since of the different brain cell type investigated only astrocytes release substantial amounts of GSH. The multidrug resistance protein 1 (Mrp1) which is expressed in astrocytes in vitro and in vivo mediates about 60% of the GSH release from cultured astrocytes. Extracellular GSH serves as substrate for the astroglial ectoenzyme gamma‐glutamyl transpeptidase (gGT). Inhibition of this enzyme prevents the astroglia‐mediated increase in the GSH level of cocultured neurons. The dipeptide CysGly, which is produced by gGT, is hydrolysed by the neuronal aminopeptidase N to glycine and cysteine. By providing the glutamate‐precursor glutamine and the dipeptide CysGly astrocytes supply to neurons precursors for all three amino acids which are required for neuronal GSH synthesis.

The potential role of astrocyte glutathione and tetrahydrobiopterin in the neurodegenerative process

Astrocytic generation of nitric oxide (NO) has been implicated in the neurodegenerative process. Cell culture studies reveal that astrocyte NO can cause neuronal mitochondrial damage. However, it appears that this damage may be limited by up‐regulation of neuronal glutathione (GSH) status. Thus, astrocytes have the capacity to release GSH/GSH precursors that can be utilized for neuronal GSH synthesis. Furthermore, the released GSH is prevented form oxidative degradation by the possible concomitant release of superoxide dismutase. Astrocytic NO generation requires the NO synthase cofactor, tetrahydrobiopterin (BH4). However, BH4 availability is compromised as a result of exposure to NO. Under these conditions, peroxynitrite synthesis may occur which could have a more deleterious effect upon adjacent neurones. Therefore, loss of ability to release GSH, by astrocytes, coupled with BH4 deficiency may be a factor in the neurodegenerative process.