Regulation of the system x(C)- cystine/glutamate exchanger by intracellular glutathione levels in rat astrocyte primary cultures

Astrocyte modulation in cerebral ischemia-reperfusion injury: A promising therapeutic strategy

Cerebral ischemia-reperfusion injury (CIRI) poses significant challenges for drug development due to its complex pathogenesis. Astrocyte involvement in CIRI pathogenesis has led to the development of novel astrocyte-targeting drug strategies. To comprehensively review the current literature, we conducted a thorough analysis from January 2012 to December 2023, identifying 82 drugs aimed at preventing and treating CIRI. These drugs target astrocytes to exert potential benefits in CIRI, and their primary actions include modulation of relevant signaling pathways to inhibit neuroinflammation and oxidative stress, reduce cerebral edema, restore blood-brain barrier integrity, suppress excitotoxicity, and regulate autophagy. Notably, active components from traditional Chinese medicines (TCM) such as Salvia miltiorrhiza, Ginkgo, and Ginseng exhibit these important pharmacological properties and show promise in the treatment of CIRI. This review highlights the potential of astrocyte-targeted drugs to ameliorate CIRI and categorizes them based on their mechanisms of action, underscoring their therapeutic potential in targeting astrocytes.

Astrocyte-mediated mechanisms contribute to traumatic brain injury pathology

Astrocytes respond to traumatic brain injury (TBI) with changes to their molecular make-up and cell biology, which results in changes in astrocyte function. These changes can be adaptive, initiating repair processes in the brain, or detrimental, causing secondary damage including neuronal death or abnormal neuronal activity. The response of astrocytes to TBI is often-but not always-accompanied by the upregulation of intermediate filaments, including glial fibrillary acidic protein (GFAP) and vimentin. Because GFAP is often upregulated in the context of nervous system disturbance, reactive astrogliosis is sometimes treated as an "all-or-none" process. However, the extent of astrocytes' cellular, molecular, and physiological adjustments is not equal for each TBI type or even for each astrocyte within the same injured brain. Additionally, new research highlights that different neurological injuries and diseases result in entirely distinctive and sometimes divergent astrocyte changes. Thus, extrapolating findings on astrocyte biology from one pathological context to another is problematic. We summarize the current knowledge about astrocyte responses specific to TBI and point out open questions that the field should tackle to better understand how astrocytes shape TBI outcomes. We address the astrocyte response to focal versus diffuse TBI and heterogeneity of reactive astrocytes within the same brain, the role of intermediate filament upregulation, functional changes to astrocyte function including potassium and glutamate homeostasis, blood-brain barrier maintenance and repair, metabolism, and reactive oxygen species detoxification, sex differences, and factors influencing astrocyte proliferation after TBI. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

Appraising the Role of Astrocytes as Suppliers of Neuronal Glutathione Precursors

The metabolism and intercellular transfer of glutathione or its precursors may play an important role in cellular defense against oxidative stress, a common hallmark of neurodegeneration. In the 1990s, several studies in the Neurobiology field led to the widely accepted notion that astrocytes produce large amounts of glutathione that serve to feed neurons with precursors for glutathione synthesis. This assumption has important implications for health and disease since a reduction in this supply from astrocytes could compromise the capacity of neurons to cope with oxidative stress. However, at first glance, this shuttling would imply a large energy expenditure to get to the same point in a nearby cell. Thus, are there additional underlying reasons for this expensive mechanism? Are neurons unable to import and/or synthesize the three non-essential amino acids that are the glutathione building blocks? The rather oxidizing extracellular environment favors the presence of cysteine (Cys) as cystine (Cis), less favorable for neuronal import. Therefore, it has also been proposed that astrocytic GSH efflux could induce a change in the redox status of the extracellular space nearby the neurons, locally lowering the Cis/Cys ratio. This astrocytic glutathione release would also increase their demand for precursors, stimulating Cis uptake, which these cells can import, further impacting the local decline of the Cis/Cys ratio, in turn, contributing to a more reduced extracellular environment and subsequently favoring neuronal Cys import. Here, we revisit the experimental evidence that led to the accepted hypothesis of astrocytes acting as suppliers of neuronal glutathione precursors, considering recent data from the Human Protein Atlas. In addition, we highlight some potential drawbacks of this hypothesis, mainly supported by heterogeneous cellular models. Finally, we outline additional and more cost-efficient possibilities by which astrocytes could support neuronal glutathione levels, including its shuttling in extracellular vesicles.

Feasibility of [18F]FSPG PET for Early Response Assessment to Combined Blockade of EGFR and Glutamine Metabolism in Wild-Type KRAS Colorectal Cancer

Early response assessment is critical for personalizing cancer therapy. Emerging therapeutic regimens with encouraging results in the wild-type (WT) KRAS colorectal cancer (CRC) setting include inhibitors of epidermal growth factor receptor (EGFR) and glutaminolysis. Towards predicting clinical outcome, this preclinical study evaluated non-invasive positron emission tomography (PET) with (4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG) in treatment-sensitive and treatment-resistant WT KRAS CRC patient-derived xenografts (PDXs). Tumor-bearing mice were imaged with [18F]FSPG PET before and one week following the initiation of treatment with either EGFR-targeted monoclonal antibody (mAb) therapy, glutaminase inhibitor therapy, or the combination. Imaging was correlated with tumor volume and histology. In PDX that responded to therapy, [18F]FSPG PET was significantly decreased from baseline at 1-week post-therapy, prior to changes in tumor volume. In contrast, [18F]FSPG PET was not decreased in non-responding PDX. These data suggest that [18F]FSPG PET may serve as an early metric of response to EGFR and glutaminase inhibition in the WT KRAS CRC setting.

Understanding the mechanistic roles of environmental heavy metal stressors in regulating ferroptosis: adding new paradigms to the links with diseases

Ferroptosis is a new type of iron-dependent cell death induced by a failure of the lipid repair protein GPX4 or the Xc- antiporter, which is essential for glutathione production. Some heavy metals such as arsenic (As), cobalt (Co), cadmium (Cd), iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni), mercury (Hg) as well as zinc (Zn) are shown to induce ferroptotic cell death involving the generation of oxidative stress, mitochondrial dysfunctioning, lipid peroxidation, and several other cellular etiologies. However, selenium (Se) treatment has been shown to enhance adaptive transcription responses to protect cells from ferroptosis. Heavy metals like Cadmium exposure activated ALK4/5 signaling via Smad3 and Akt signaling which leads to cell death mechanism. Continuous exposure to a small dose of mercury can damage tissues, and methylmercury bind to sulfhydryl proteins and GSH, this elevates oxidative stress, free radical accumulation, glutathione depletion, mitochondrial damage, and inhibited the nuclear factor-κB pathway which leads to ferroptotic cell death. Animals exposed to nickel and cobalt may have increased lipid peroxidation which can induce ferroptosis. Glutathione depletion is caused by Zn intoxication and exposure to manganese. These metals are systemic toxins that have been shown adverse effects on humans. Ferroptosis has recently been related to several pathological disorders, including, Alzheimer's disease, Parkinson's disease, Huntington's disease, as well as cardiovascular disease, and any type of cancer. For these disorders and some heavy metal toxicity, ferroptosis suppression needs to be looked upon as a promising therapeutic choice.

New insights in ferroptosis: Potential therapeutic targets for the treatment of ischemic stroke

Stroke is a common disease in clinical practice, which seriously endangers people's physical and mental health. The neurovascular unit (NVU) plays a key role in the occurrence and development of ischemic stroke. Different from other classical types of cell death such as apoptosis, necrosis, autophagy, and pyroptosis, ferroptosis is an iron-dependent lipid peroxidation-driven new form of cell death. Interestingly, the function of NVU and stroke development can be regulated by activating or inhibiting ferroptosis. This review systematically describes the NVU in ischemic stroke, provides a comprehensive overview of the regulatory mechanisms and key regulators of ferroptosis, and uncovers the role of ferroptosis in the NVU and the progression of ischemic stroke. We further discuss the latest progress in the intervention of ferroptosis as a therapeutic target for ischemic stroke and summarize the research progress and regulatory mechanism of ferroptosis inhibitors on stroke. In conclusion, ferroptosis, as a new form of cell death, plays a key role in ischemic stroke and is expected to become a new therapeutic target for this disease.

Ceftriaxone as a Novel Therapeutic Agent for Hyperglutamatergic States: Bridging the Gap Between Preclinical Results and Clinical Translation

Dysregulation of glutamate homeostasis is a well-established core feature of neuropsychiatric disorders. Extracellular glutamate concentration is regulated by glutamate transporter 1 (GLT-1). The discovery of a beta-lactam antibiotic, ceftriaxone (CEF), as a safe compound with unique ability to upregulate GLT-1 sparked the interest in testing its efficacy as a novel therapeutic agent in animal models of neuropsychiatric disorders with hyperglutamatergic states. Indeed, more than 100 preclinical studies have shown the efficacy of CEF in attenuating the behavioral manifestations of various hyperglutamatergic brain disorders such as ischemic stroke, amyotrophic lateral sclerosis (ALS), seizure, Huntington’s disease, and various aspects of drug use disorders. However, despite rich and promising preclinical data, only one large-scale clinical trial testing the efficacy of CEF in patients with ALS is reported. Unfortunately, in that study, there was no significant difference in survival between placebo- and CEF-treated patients. In this review, we discussed the translational potential of preclinical efficacy of CEF based on four different parameters: (1) initiation of CEF treatment in relation to induction of the hyperglutamatergic state, (2) onset of response in preclinical models in relation to onset of GLT-1 upregulation, (3) mechanisms of action of CEF on GLT-1 expression and function, and (4) non-GLT-1-mediated mechanisms for CEF. Our detailed review of the literature brings new insights into underlying molecular mechanisms correlating the preclinical efficacy of CEF. We concluded here that CEF may be clinically effective in selected cases in acute and transient hyperglutamatergic states such as early drug withdrawal conditions.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

P2X7 Receptor Augments LPS-Induced Nitrosative Stress by Regulating Nrf2 and GSH Levels in the Mouse Hippocampus

P2X7 receptor (P2X7R) regulates inducible nitric oxide synthase (iNOS) expression/activity in response to various harmful insults. Since P2X7R deletion paradoxically decreases the basal glutathione (GSH) level in the mouse hippocampus, it is likely that P2X7R may increase the demand for GSH for the maintenance of the intracellular redox state or affect other antioxidant defense systems. Therefore, the present study was designed to elucidate whether P2X7R affects nuclear factor-erythroid 2-related factor 2 (Nrf2) activity/expression and GSH synthesis under nitrosative stress in response to lipopolysaccharide (LPS)-induced neuroinflammation. In the present study, P2X7R deletion attenuated iNOS upregulation and Nrf2 degradation induced by LPS. Compatible with iNOS induction, P2X7R deletion decreased S-nitrosylated (SNO)-cysteine production under physiological and post-LPS treated conditions. P2X7R deletion also ameliorated the decreases in GSH, glutathione synthetase, GS and ASCT2 levels concomitant with the reduced S-nitrosylations of GS and ASCT2 following LPS treatment. Furthermore, LPS upregulated cystine:glutamate transporter (xCT) and glutaminase in P2X7R^+/+ mice, which were abrogated by P2X7R deletion. LPS did not affect GCLC level in both P2X7R^+/+ and P2X7R^−/− mice. Therefore, our findings indicate that P2X7R may augment LPS-induced neuroinflammation by leading to Nrf2 degradation, aberrant glutamate-glutamine cycle and impaired cystine/cysteine uptake, which would inhibit GSH biosynthesis. Therefore, we suggest that the targeting of P2X7R, which would exert nitrosative stress with iNOS in a positive feedback manner, may be one of the important therapeutic strategies of nitrosative stress under pathophysiological conditions.

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.

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.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

The Regulation of Astrocytic Glutamate Transporters in Health and Neurodegenerative Diseases

The astrocytic glutamate transporters excitatory amino acid transporters 1 and 2 (EAAT1 and EAAT2) play a key role in nervous system function to maintain extracellular glutamate levels at low levels. In physiology, this is essential for the rapid uptake of synaptically released glutamate, maintaining the temporal fidelity of synaptic transmission. However, EAAT1/2 hypo-expression or hypo-function are implicated in several disorders, including epilepsy and neurodegenerative diseases, as well as being observed naturally with aging. This not only disrupts synaptic information transmission, but in extremis leads to extracellular glutamate accumulation and excitotoxicity. A key facet of EAAT1/2 expression in astrocytes is a requirement for signals from other brain cell types in order to maintain their expression. Recent evidence has shown a prominent role for contact-dependent neuron-to-astrocyte and/or endothelial cell-to-astrocyte Notch signalling for inducing and maintaining the expression of these astrocytic glutamate transporters. The relevance of this non-cell-autonomous dependence to age- and neurodegenerative disease-associated decline in astrocytic EAAT expression is discussed, plus the implications for disease progression and putative therapeutic strategies.

Mitochondrial Metabolism in Astrocytes Regulates Brain Bioenergetics, Neurotransmission and Redox Balance

In the brain, mitochondrial metabolism has been largely associated with energy production, and its dysfunction is linked to neuronal cell loss. However, the functional role of mitochondria in glial cells has been poorly studied. Recent reports have demonstrated unequivocally that astrocytes do not require mitochondria to meet their bioenergetics demands. Then, the question remaining is, what is the functional role of mitochondria in astrocytes? In this work, we review current evidence demonstrating that mitochondrial central carbon metabolism in astrocytes regulates overall brain bioenergetics, neurotransmitter homeostasis and redox balance. Emphasis is placed in detailing carbon source utilization (glucose and fatty acids), anaplerotic inputs and cataplerotic outputs, as well as carbon shuttles to neurons, which highlight the metabolic specialization of astrocytic mitochondria and its relevance to brain function.

Novel insights into ferroptosis: Implications for age-related diseases

Rapid increase in aging populations is an urgent problem because older adults are more likely to suffer from disabilities and age-related diseases (ARDs), burdening healthcare systems and society in general. ARDs are characterized by the progressive deterioration of tissues and organs over time, eventually leading to tissue and organ failure. To date, there are no effective interventions to prevent the progression of ARDs. Hence, there is an urgent need for new treatment strategies. Ferroptosis, an iron-dependent cell death, is linked to normal development and homeostasis. Accumulating evidence, however, has highlighted crucial roles for ferroptosis in ARDs, including neurodegenerative and cardiovascular diseases. In this review, we a) summarize initiation, regulatory mechanisms, and molecular signaling pathways involved in ferroptosis, b) discuss the direct and indirect involvement of the activation and/or inhibition of ferroptosis in the pathogenesis of some important diseases, and c) highlight therapeutic targets relevant for ARDs.

xCT: A Critical Molecule That Links Cancer Metabolism to Redox Signaling

System x_c^- cystine/glutamate antiporter, composed of a light-chain subunit (xCT, SLC7A11) and a heavy-chain subunit (CD98hc, SLC3A2), is mainly responsible for the cellular uptake of cystine in exchange for intracellular glutamate. In recent years, the xCT molecule has been found to play an important role in tumor growth, progression, metastasis, and multidrug resistance in various types of cancer. Interestingly, xCT also exhibits an essential function in regulating tumor-associated ferroptosis. Despite significant progress in targeting the system x_c^- transporter in cancer treatment, the underlying mechanisms still remain elusive. It is also unclear why solid tumors are more sensitive to xCT inhibitors such as sulfasalazine, as compared to hematological malignancies. This review mainly focuses on the role of xCT cystine/glutamate transporter in regard to tumor growth, chemoresistance, tumor-selective ferroptosis, and the mechanisms regulating xCT gene expression. The potential therapeutic implications of targeting the system x_c^- and its combination with chemotherapeutic agents or immunotherapy to suppress tumor growth and overcome drug resistance are also discussed.

The role of astrocytes in oxidative stress of central nervous system: A mixed blessing

Central nervous system (CNS) maintains a high level of metabolism, which leads to the generation of large amounts of free radicals, and it is also one of the most vulnerable organs to oxidative stress. Emerging evidences have shown that, as the key homeostatic cells in CNS, astrocytes are deeply involved in multiple aspects of CNS function including oxidative stress regulation. Besides, the redox level in CNS can in turn affect astrocytes in morphology and function. The complex and multiple roles of astrocytes indicate that their correct performance is crucial for the normal functioning of the CNS, and its dysfunction may result in the occurrence and progression of various neurological disorders. To date, the influence of astrocytes in CNS oxidative stress is rarely reviewed. Therefore, in this review we sum up the roles of astrocytes in redox regulation and the corresponding mechanisms under both normal and different pathological conditions.

Hydrogen peroxide triggers an increase in cell surface expression of system xc- in cultured human glioma cells

System x_c^- exchanges extracellular cystine for intracellular glutamate across the plasma membrane of many cell types. One of the physiological roles of System x_c^- is to provide cystine for synthesis of the antioxidant glutathione. Here we report that hydrogen peroxide (H₂O₂) triggers the translocation of System x_c^- to the plasma membrane within 10 min of the initial exposure. Specifically, we observed a three-fold increase in ³⁵S-l-cystine uptake following a 10 min exposure to 0.3 mM H₂O₂. This effect was dose-dependent with an EC₅₀ for H₂O₂ of 65 μM. We then used cell surface biotinylation analysis to test the hypothesis that the increase in activity is due to an increased number of transporters on the plasma membrane. We demonstrated that the amount of transporter protein, xCT, localized to the plasma membrane doubles within 10 min of H₂O₂ exposure as a result of an increase in its delivery rate and a reduction in its internalization rate. In addition, we demonstrated that H₂O₂ triggered a rapid decrease in total cellular glutathione which recovered within 2 h of the oxidative insult. The kinetics of glutathione recovery matched the time course for the recovery of xCT cell surface expression and System x_c^- activity following removal of the oxidative insult. Collectively, these results suggest that oxidants acutely modulate the activity of System x_c^- by increasing its cell surface expression, and that this process may serve as an important mechanism to increase de novo glutathione synthesis during periods of oxidative stress.

Na+-dependent transporters: The backbone of astroglial homeostatic function

Astrocytes are the principal homeostatic cells of the central nerves system (CNS) that support the CNS function at all levels of organisation, from molecular to organ. Several fundamental homeostatic functions of astrocytes are mediated through plasmalemmal pumps and transporters; most of which are also regulated by the transplasmalemmal gradient of Na⁺ ions. Neuronal activity as well as mechanical or chemical stimulation of astrocytes trigger plasmalemmal Na⁺ fluxes, which in turn generate spatio-temporally organised transient changes in the cytosolic Na⁺ concentration, which represent the substrate of astroglial Na⁺ signalling. Astroglial Na⁺ signals link and coordinate neuronal activity and CNS homeostatic demands with the astroglial homeostatic response.

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.

The Astrocytic cAMP Pathway in Health and Disease

Astrocytes are major glial cells that play critical roles in brain homeostasis. Abnormalities in astrocytic functions can lead to brain disorders. Astrocytes also respond to injury and disease through gliosis and immune activation, which can be both protective and detrimental. Thus, it is essential to elucidate the function of astrocytes in order to understand the physiology of the brain to develop therapeutic strategies against brain diseases. Cyclic adenosine monophosphate (cAMP) is a major second messenger that triggers various downstream cellular machinery in a wide variety of cells. The functions of astrocytes have also been suggested as being regulated by cAMP. Here, we summarize the possible roles of cAMP signaling in regulating the functions of astrocytes. Specifically, we introduce the ways in which cAMP pathways are involved in astrocyte functions, including (1) energy supply, (2) maintenance of the extracellular environment, (3) immune response, and (4) a potential role as a provider of trophic factors, and we discuss how these cAMP-regulated processes can affect brain functions in health and disease.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Implication of the glutamate-cystine antiporter xCT in schizophrenia cases linked to impaired GSH synthesis

xCT is the specific chain of the cystine/glutamate antiporter, which is widely reported to support anti-oxidant defenses in vivo. xCT is therefore at the crossroads between two processes that are involved in schizophrenia: oxidative stress and glutamatergic neurotransmission. But data from human studies implicating xCT in the illness and clarifying the upstream mechanisms of xCT imbalance are still scarce. Low glutathione (GSH) levels and genetic risk in GCLC (Glutamate–Cysteine Ligase Catalytic subunit), the gene of limiting synthesizing enzyme for GSH, are both associated with schizophrenia. In the present study, we aimed at determining if xCT regulation by the redox system is involved in schizophrenia pathophysiology. We assessed whether modulating GCLC expression impact on xCT expression and activity (i) in fibroblasts from patients and controls with different GCLC genotypes which are known to affect GCLC regulation and GSH levels; (ii) in rat brain glial cells, i.e., astrocytes and oligodendrocytes, with a knock-down of GCLC. Our results highlight that decreased GCLC expression leads to an upregulation of xCT levels in patients’ fibroblasts as well as in astrocytes. These results support the implication of xCT dysregulation in illness pathophysiology and further indicate that it can result from redox changes. Additionally, we showed that these anomalies may already take place at early stages of psychosis and be more prominent in a subgroup of patients with GCLC high-risk genotypes. These data add to the existing evidence identifying the inflammatory/redox systems as important targets to treat schizophrenia already at early stages.

Deficit of antioxidant synthesis in schizophrenia leads to oxidative stress and changes in neurotransmitter transporter. Led by Kim Do, a team of researchers from Lausanne University in Switzerland investigated the role of the cell-surface transport protein xCT in schizophrenia. They found that an enzyme responsible for antioxidant production is disturbed in patients. This leads to decreased antioxidant levels and consequently to oxidative stress—i.e. the accumulation of reactive oxygen molecules, damaging the cells component and impairing cell functioning—which in turn affects the functioning of the antioxidant pathway, including xCT. xCT, which exports the neurotransmitter glutamate, is thus overproduced in schizophrenia. The resulting increase of neurotransmitter activity, alongside the increase in oxidative stress, is thought to play a major role in the pathophysiology of schizophrenia, including at early stages of the disease.

Neuroprotective potential of astroglia

Astroglia are the homoeostatic cells of the central nervous system, which participate in all essential functions of the brain. Astrocytes support neuronal networks by handling water and ion fluxes, transmitter clearance, provision of antioxidants, and metabolic precursors and growth factors. The critical dependence of neurons on constant support from the astrocytes confers astrocytes with intrinsic neuroprotective properties. On the other hand, loss of astrocytic support or their pathological transformation compromises neuronal functionality and viability. Manipulating neuroprotective functions of astrocytes is thus an important strategy to enhance neuronal survival and improve outcomes in disease states. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Cysteine, Glutathione, and Thiol Redox Balance in Astrocytes

This review discusses the current understanding of cysteine and glutathione redox balance in astrocytes. Particular emphasis is placed on the impact of oxidative stress and astrocyte activation on pathways that provide cysteine as a precursor for glutathione. The effect of the disruption of thiol-containing amino acid metabolism on the antioxidant capacity of astrocytes is also discussed.

Resveratrol prevents ammonia-induced mitochondrial dysfunction and cellular redox imbalance in C6 astroglial cells

BACKGROUND

Resveratrol is a polyphenolic compound that presents several protective effects in the central nervous system, including gliotoxicity associated to hyperammonemia, a key element for the development of hepatic encephalopathy. In this condition, mitochondrial dysfunction leads to a reactive oxygen species (ROS) overproduction, which, in turn, exacerbates mitochondrial failure and causes cellular damage.

OBJECTIVE

This study sought to determine whether prevention of mitochondrial dysfunction and the maintenance of cellular redox status by resveratrol contribute to its protective action toward ammonia toxicity.

METHODS

C6 astrocyte cell line was pre-incubated in the presence or absence of resveratrol (100 μM) for 1 hour. After pre-incubation, resveratrol was maintained and 5 mM ammonia was added for 24 hours, followed by the evaluation of ROS production, mitochondrial functionality, antioxidant enzymatic and non-enzymatic defenses, energy metabolic parameters, and genotoxicity.

RESULTS

We showed that resveratrol prevented the increase in ROS production, the decrease of mitochondrial membrane potential (ΔΨ_m), and bioenergetics deficit caused by ammonia in C6 astroglial cells. In addition, resveratrol avoided the ammonia-induced upregulation of NOX activity and impairment in enzymatic and non-enzymatic antioxidant defenses. Ammonia also induced DNA damage that was prevented by resveratrol, indicating its genoprotective effect.

CONCLUSIONS

In summary, our study demonstrates that resveratrol prevents ammonia-induced cytotoxicity, as well as supports the role of resveratrol on mitochondrial/cellular redox functionality.

Serotonin 1A Receptors on Astrocytes as a Potential Target for the Treatment of Parkinson's Disease

Astrocytes are the most abundant neuron-supporting glial cells in the central nervous system. The neuroprotective role of astrocytes has been demonstrated in various neurological disorders such as amyotrophic lateral sclerosis, spinal cord injury, stroke and Parkinson’s disease (PD). Astrocyte dysfunction or loss-of-astrocytes increases the susceptibility of neurons to cell death, while astrocyte transplantation in animal studies has therapeutic advantage. We reported recently that stimulation of serotonin 1A (5-HT_1A) receptors on astrocytes promoted astrocyte proliferation and upregulated antioxidative molecules to act as a neuroprotectant in parkinsonian mice. PD is a progressive neurodegenerative disease with motor symptoms such as tremor, bradykinesia, rigidity and postural instability, that are based on selective loss of nigrostriatal dopaminergic neurons, and with non-motor symptoms such as orthostatic hypotension and constipation based on peripheral neurodegeneration. Although dopaminergic therapy for managing the motor disability associated with PD is being assessed at present, the main challenge remains the development of neuroprotective or disease-modifying treatments. Therefore, it is desirable to find treatments that can reduce the progression of dopaminergic cell death. In this article, we summarize first the neuroprotective properties of astrocytes targeting certain molecules related to PD. Next, we review neuroprotective effects induced by stimulation of 5-HT_1A receptors on astrocytes. The review discusses new promising therapeutic strategies based on neuroprotection against oxidative stress and prevention of dopaminergic neurodegeneration.

Nrf2-dysregulation correlates with reduced synthesis and low glutathione levels in experimental autoimmune encephalomyelitis

This study investigates the possible mechanism(s) underlying glutathione (GSH) deficiency in the mouse spinal cord during the course of myelin oligodendrocyte glycoprotein35-55 peptide-induced experimental autoimmune encephalomyelitis (EAE), a commonly used animal model of multiple sclerosis. Using the classical enzymatic recycling method and a newly developed immunodot assay, we first demonstrated that total GSH levels (i.e. free GSH plus all its adducts) are reduced in EAE, suggesting an impaired synthesis. The decline in the levels of this essential antioxidant tripeptide in EAE coincides temporally and in magnitude with a reduction in the amount of γ-glutamylcysteine ligase, the rate-limiting enzyme in GSH synthesis. Other enzymes involved in GSH biosynthesis, whose genes also contain antioxidant-response elements, including glutathione synthetase, cystine/glutamate antiporter, and γ-glutamyl transpeptidase (γ-GT) are diminished in EAE as well. Low levels of γ-glutamylcysteine ligase, glutathione synthetase, and γ-GT are the consequence of reduced mRNA expression, which correlates with diminished expression of the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) in both the cytosol and nucleus. Interestingly, the low Nrf2 expression does not seem to be caused by increased degradation via Kelch-like ECH-associated protein 1-dependent or Kelch-like ECH-associated protein 1-independent mechanisms (such as glycogen synthetase kinase-3β activation), or by reduced levels of Nrf2 mRNA. This suggests that translation of this important transcription factor and/or other still unidentified post-translational processes are altered in EAE. These novel findings are central toward understanding how critical antioxidant and protective responses are lost in inflammatory demyelinating disorders.

S-Glutathionylation and Redox Protein Signaling in Drug Addiction

Drug addiction is a chronic relapsing disorder that comes at a high cost to individuals and society. Therefore understanding the mechanisms by which drugs exert their effects is of prime importance. Drugs of abuse increase the production of reactive oxygen and nitrogen species resulting in oxidative stress. This change in redox homeostasis increases the conjugation of glutathione to protein cysteine residues; a process called S-glutathionylation. Although traditionally regarded as a protective mechanism against irreversible protein oxidation, accumulated evidence suggests a more nuanced role for S-glutathionylation, namely as a mediator in redox-sensitive protein signaling. The reversible modification of protein thiols leading to alteration in function under different physiologic/pathologic conditions provides a mechanism whereby change in redox status can be translated into a functional response. As such, S-glutathionylation represents an understudied means of post-translational protein modification that may be important in the mechanisms underlying drug addiction. This review will discuss the evidence for S-glutathionylation as a redox-sensing mechanism and how this may be involved in the response to drug-induced oxidative stress. The function of S-glutathionylated proteins involved in neurotransmission, dendritic spine structure, and drug-induced behavioral outputs will be reviewed with specific reference to alcohol, cocaine, and heroin.

Mutation of ataxia-telangiectasia mutated is associated with dysfunctional glutathione homeostasis in cerebellar astroglia

Astroglial dysfunction plays an important role in neurodegenerative diseases otherwise attributed to neuronal loss of function. Here we focus on the role of astroglia in ataxia-telangiectasia (A-T), a disease caused by mutations in the ataxia-telangiectasia mutated (ATM) gene. A hallmark of A-T pathology is progressive loss of cerebellar neurons, but the mechanisms that impact neuronal survival are unclear. We now provide a possible mechanism by which A-T astroglia affect the survival of cerebellar neurons. As astroglial functions are difficult to study in an in vivo setting, particularly in the cerebellum where these cells are intertwined with the far more numerous neurons, we conducted in vitro coculture experiments that allow for the generation and pharmacological manipulation of purified cell populations. Our analyses revealed that cerebellar astroglia isolated from Atm mutant mice show decreased expression of the cystine/glutamate exchanger subunit xCT, glutathione (GSH) reductase, and glutathione-S-transferase. We also found decreased levels of intercellular and secreted GSH in A-T astroglia. Metabolic labeling of l-cystine, the major precursor for GSH, revealed that a key component of the defect in A-T astroglia is an impaired ability to import this rate-limiting precursor for the production of GSH. This impairment resulted in suboptimal extracellular GSH supply, which in turn impaired survival of cerebellar neurons. We show that by circumventing the xCT-dependent import of L-cystine through addition of N-acetyl-L-cysteine (NAC) as an alternative cysteine source, we were able to restore GSH levels in A-T mutant astroglia providing a possible future avenue for targeted therapeutic intervention.

SLC7A11 expression is associated with seizures and predicts poor survival in patients with malignant glioma

Glioma is the most common malignant primary brain tumor. Its rapid growth is aided by tumor-mediated glutamate release, creating peritumoral excitotoxic cell death and vacating space for tumor expansion. Glioma glutamate release may also be responsible for seizures, which complicate the clinical course for many patients and are often the presenting symptom. A hypothesized glutamate release pathway is the cystine/glutamate transporter System xc (-) (SXC), responsible for the cellular synthesis of glutathione (GSH). However, the relationship of SXC-mediated glutamate release, seizures, and tumor growth remains unclear. Probing expression of SLC7A11/xCT, the catalytic subunit of SXC, in patient and mouse-propagated tissues, we found that ~50% of patient tumors have elevated SLC7A11 expression. Compared with tumors lacking this transporter, in vivo propagated and intracranially implanted SLC7A11-expressing tumors grew faster, produced pronounced peritumoral glutamate excitotoxicity, induced seizures, and shortened overall survival. In agreement with animal data, increased SLC7A11 expression predicted shorter patient survival according to genomic data in the REMBRANDT (National Institutes of Health Repository for Molecular Brain Neoplasia Data) database. In a clinical pilot study, we used magnetic resonance spectroscopy to determine SXC-mediated glutamate release by measuring acute changes in glutamate after administration of the U.S. Food and Drug Administration-approved SXC inhibitor, sulfasalazine (SAS). In nine glioma patients with biopsy-confirmed SXC expression, we found that expression positively correlates with glutamate release, which is acutely inhibited with oral SAS. These data suggest that SXC is the major pathway for glutamate release from gliomas and that SLC7A11 expression predicts accelerated growth and tumor-associated seizures.

Regulation of System xc(-) by Pharmacological Manipulation of Cellular Thiols

The cystine/glutamate exchanger (system xc (-)) mediates the transport of cystine into the cell in exchange for glutamate. By releasing glutamate, system xc (-) can potentially cause excitotoxicity. However, through providing cystine to the cell, it regulates the levels of cellular glutathione (GSH), the main endogenous intracellular antioxidant, and may protect cells against oxidative stress. We tested two different compounds that deplete primary cortical cultures containing both neurons and astrocytes of intracellular GSH, L-buthionine-sulfoximine (L-BSO), and diethyl maleate (DEM). Both compounds caused significant concentration and time dependent decreases in intracellular GSH levels. However; DEM caused an increase in radiolabeled cystine uptake through system xc (-), while unexpectedly BSO caused a decrease in uptake. The compounds caused similar low levels of neurotoxicity, while only BSO caused an increase in oxidative stress. The mechanism of GSH depletion by these two compounds is different, DEM directly conjugates to GSH, while BSO inhibits γ-glutamylcysteine synthetase, a key enzyme in GSH synthesis. As would be expected from these mechanisms of action, DEM caused a decrease in intracellular cysteine, while BSO increased cysteine levels. The results suggest that negative feedback by intracellular cysteine is an important regulator of system xc (-) in this culture system.

Expression of xCT and activity of system xc(-) are regulated by NRF2 in human breast cancer cells in response to oxidative stress

Cancer cells adapt to high levels of oxidative stress in order to survive and proliferate by activating key transcription factors. One such master regulator, the redox sensitive transcription factor NF E2 Related Factor 2 (NRF2), controls the expression of cellular defense genes including those encoding intracellular redox-balancing proteins involved in glutathione (GSH) synthesis. Under basal conditions, Kelch-like ECH-associated protein 1 (KEAP1) targets NRF2 for ubiquitination. In response to oxidative stress, NRF2 dissociates from KEAP1, entering the nucleus and binding to the antioxidant response element (ARE) in the promoter of its target genes. Elevated reactive oxygen species (ROS) production may deplete GSH levels within cancer cells. System xc(-), an antiporter that exports glutamate while importing cystine to be converted into cysteine for GSH synthesis, is upregulated in cancer cells in response to oxidative stress. Here, we provided evidence that the expression of xCT, the light chain subunit of system xc(-), is regulated by NRF2 in representative human breast cancer cells. Hydrogen peroxide (H2O2) treatment increased nuclear translocation of NRF2, also increasing levels of xCT mRNA and protein and extracellular glutamate release. Overexpression of NRF2 up-regulated the activity of the xCT promoter, which contains a proximal ARE. In contrast, overexpression of KEAP1 repressed promoter activity and decreased xCT protein levels, while siRNA knockdown of KEAP1 up-regulated xCT protein levels and transporter activity. These results demonstrate the importance of the KEAP1/NRF2 pathway in balancing oxidative stress in breast cancer cells through system xc(-). We have previously shown that xCT is upregulated in various cancer cell lines under oxidative stress. In the current investigation, we focused on MCF-7 cells as a model for mechanistic studies.

Researching glutamate - induced cytotoxicity in different cell lines: a comparative/collective analysis/study

Although glutamate is one of the most important excitatory neurotransmitters of the central nervous system, its excessive extracellular concentration leads to uncontrolled continuous depolarization of neurons, a toxic process called, excitotoxicity. In excitotoxicity glutamate triggers the rise of intracellular Ca²⁺ levels, followed by up regulation of nNOS, dysfunction of mitochondria, ROS production, ER stress, and release of lysosomal enzymes. Excessive calcium concentration is the key mediator of glutamate toxicity through over activation of ionotropic and metabotropic receptors. In addition, glutamate accumulation can also inhibit cystine (CySS) uptake by reversing the action of the CySS/glutamate antiporter. Reversal of the antiporter action reinforces the aforementioned events by depleting neurons of cysteine and eventually glutathione’s reducing potential. Various cell lines have been employed in the pursuit to understand the mechanism(s) by which excitotoxicity affects the cells leading them ultimately to their demise. In some cell lines glutamate toxicity is exerted mainly through over activation of NMDA, AMPA, or kainate receptors whereas in other cell lines lacking such receptors, the toxicity is due to glutamate induced oxidative stress. However, in the greatest majority of the cell lines ionotropic glutamate receptors are present, co-existing to CySS/glutamate antiporters and metabotropic glutamate receptors, supporting the assumption that excitotoxicity effect in these cells is accumulative. Different cell lines differ in their responses when exposed to glutamate. In this review article the responses of PC12, SH-SY5Y, HT-22, NT-2, OLCs, C6, primary rat cortical neurons, RGC-5, and SCN2.2 cell systems are systematically collected and analyzed.

A Cystine-Rich Whey Supplement (Immunocal(®)) Delays Disease Onset and Prevents Spinal Cord Glutathione Depletion in the hSOD1(G93A) Mouse Model of Amyotrophic Lateral Sclerosis

Depletion of the endogenous antioxidant, glutathione (GSH), underlies progression of the devastating neurodegenerative disease, amyotrophic lateral sclerosis (ALS). Thus, strategies aimed at elevating GSH may yield new therapeutics for ALS. Here, we investigated the effects of a unique non-denatured whey protein supplement, Immunocal(®), in the transgenic Gly position 93 to Ala (G93A) mutant hSOD1 (hSOD1(G93A)) mouse model of ALS. Immunocal(®) is rich in the GSH precursor, cystine, and is therefore capable of bolstering GSH content. Transgenic hSOD1(G93A) mice receiving Immunocal(®) displayed a significant delay in disease onset compared to untreated hSOD1(G93A) controls. Additionally, Immunocal(®) treatment significantly decreased the rate of decline in grip strength and prevented disease-associated reductions in whole blood and spinal cord tissue GSH levels in end-stage hSOD1(G93A) mice. However, Immunocal(®) did not extend survival, likely due to its inability to preserve the mitochondrial GSH pool in spinal cord. Combination treatment with Immunocal(®) and the anti-glutamatergic compound, riluzole, delayed disease onset and extended survival in hSOD1(G93A) mice. These findings demonstrate that sustaining tissue GSH with Immunocal(®) only modestly delays disease onset and slows the loss of skeletal muscle strength in hSOD1(G93A) mice. Moreover, the inability of Immunocal(®) to rescue mitochondrial GSH in spinal cord provides a possible mechanism for its lack of effect on survival and is a limiting factor in the potential utility of this supplement as a therapeutic for ALS.

Change of cystine/glutamate antiporter expression in ethanol-dependent rats

Background: Some drugs of abuse down regulate the expression of cystine/glutamate (xCT) antiporter in the nucleus accumbens (Acb) after extinction or withdrawal. The altered level of xCT exchanger in Acb, a structure involved in ethanol reinforcement, may contribute to the pathological glutamatergic signaling, linked to addiction. We hypothesized that the expression of xCT may be changed in Acb and whole brain also in non-dependent (occasional drinkers), ethanol-dependent rats, as well as, during ethanol withdrawal.

Methods: Wistar rats were made ethanol-dependent by chronic exposure to an alcoholic milk beverage (from 2.4 to 7.2% v/v ethanol). Ethanol non-dependent rats were exposed to a similar, but non-alcoholic liquid diet and self-administered ethanol (10%) twice a week. Withdrawal in ethanol-dependent rats was studied at 12 h after the last ethanol-enriched diet exposure. Immediately after the measurement of somatic signs of withdrawal, Western blot analysis with a polyclonal antibody against xCT was carried out in a naïve control group, non-dependent and ethanol-dependent rats as well as withdrawal rats, in order to study the level of xCT expression in Acb and whole brain.

Results: Non-dependent rats self-administered an average dose of 1.21 ± 0.02 g/kg per session (30 min). Daily ethanol consumption during chronic exposure to the alcoholic beverage ranged from 6.30 ± 0.16 to 13.99 ± 0.66 g/kg. Ethanol dependent rats after suspension of the ethanol-enriched diet have shown significant somatic signs of withdrawal. Western blotting analysis of Acb lysates revealed that xCT was over expressed in ethanol-dependent rats whereas in whole brain preparations xCT was over expressed in both non-dependent and ethanol-dependent rats compared to control group. On the contrary, xCT expression during withdrawal was down regulated in Acb and restored to control level in whole brain preparations.

Conclusions: The changes of xCT expression in both Acb and whole brain following ethanol dependence and withdrawal indicate that xCT might represent a novel therapeutic target for the treatment of ethanol addiction.

Dysregulation of system xc(-) expression induced by mutant huntingtin in a striatal neuronal cell line and in R6/2 mice

Oxidative stress has been implicated in the pathogenesis of Huntington's disease (HD), however, the origin of the oxidative stress is unknown. System xc(-) plays a role in the import of cystine to synthesize the antioxidant glutathione. We found in the STHdh(Q7/Q7) and STHdh(Q111/Q111) striatal cell lines, derived from neuronal precursor cells isolated from knock-in mice containing 7 or 111 CAG repeats in the huntingtin gene, that there is a decrease in system xc(-) function. System xc(-) is composed of two proteins, the substrate specific transporter, xCT, and an anchoring protein, CD98. The decrease in function in system xc(-) that we observed is caused by a decrease in xCT mRNA and protein expression in the STHdh(Q111/Q111) cells. In addition, we found a decrease in protein and mRNA expression in the transgenic R6/2 HD mouse model at 6weeks of age. STHdh(Q111/Q111) cells have lower basal levels of GSH and higher basal levels of ROS. Acute inhibition of system xc(-) causes greater increase in oxidative stress in the STHdh(Q111/Q111) cells than in the STHdh(Q7/Q7) cells. These results suggest that a defect in the regulation of xCT may be involved in the pathogenesis of HD by compromising xCT expression and increasing susceptibility to oxidative stress.

Glutamate transporters in brain ischemia: to modulate or not?

In this review, we briefly describe glutamate (Glu) metabolism and its specific transports and receptors in the central nervous system (CNS). Thereafter, we focus on excitatory amino acid transporters, cystine/glutamate antiporters (system xc-) and vesicular glutamate transporters, specifically addressing their location and roles in CNS and the molecular mechanisms underlying the regulation of Glu transporters. We provide evidence from in vitro or in vivo studies concerning alterations in Glu transporter expression in response to hypoxia or ischemia, including limited human data that supports the role of Glu transporters in stroke patients. Moreover, the potential to induce brain tolerance to ischemia through modulation of the expression and/or activities of Glu transporters is also discussed. Finally we present strategies involving the application of ischemic preconditioning and pharmacological agents, eg β-lactam antibiotics, amitriptyline, riluzole and N-acetylcysteine, which result in the significant protection of nervous tissues against ischemia.

Cysteine catabolism: a novel metabolic pathway contributing to glioblastoma growth

The relevance of cysteine metabolism in cancer has gained considerable interest in recent years, largely focusing on its role in generating the antioxidant glutathione. Through metabolomic profiling using a combination of high-throughput liquid and gas chromatography-based mass spectrometry on a total of 69 patient-derived glioma specimens, this report documents the discovery of a parallel pathway involving cysteine catabolism that results in the accumulation of cysteine sulfinic acid (CSA) in glioblastoma. These studies identified CSA to rank as one of the top metabolites differentiating glioblastoma from low-grade glioma. There was strong intratumoral concordance of CSA levels with expression of its biosynthetic enzyme cysteine dioxygenase 1 (CDO1). Studies designed to determine the biologic consequence of this metabolic pathway identified its capacity to inhibit oxidative phosphorylation in glioblastoma cells, which was determined by decreased cellular respiration, decreased ATP production, and increased mitochondrial membrane potential following pathway activation. CSA-induced attenuation of oxidative phosphorylation was attributed to inhibition of the regulatory enzyme pyruvate dehydrogenase. Studies performed in vivo abrogating the CDO1/CSA axis using a lentiviral-mediated short hairpin RNA approach resulted in significant tumor growth inhibition in a glioblastoma mouse model, supporting the potential for this metabolic pathway to serve as a therapeutic target. Collectively, we identified a novel, targetable metabolic pathway involving cysteine catabolism contributing to the growth of aggressive high-grade gliomas. These findings serve as a framework for future investigations designed to more comprehensively determine the clinical application of this metabolic pathway and its contributory role in tumorigenesis.

The SLC3 and SLC7 families of amino acid transporters

Amino acids are necessary for all living cells and organisms. Specialized transporters mediate the transfer of amino acids across plasma membranes. Malfunction of these proteins can affect whole-body homoeostasis giving raise to diverse human diseases. Here, we review the main features of the SLC3 and SLC7 families of amino acid transporters. The SLC7 family is divided into two subfamilies, the cationic amino acid transporters (CATs), and the L-type amino acid transporters (LATs). The latter are the light or catalytic subunits of the heteromeric amino acid transporters (HATs), which are associated by a disulfide bridge with the heavy subunits 4F2hc or rBAT. These two subunits are glycoproteins and form the SLC3 family. Most CAT subfamily members were functionally characterized and shown to function as facilitated diffusers mediating the entry and efflux of cationic amino acids. In certain cells, CATs play an important role in the delivery of L-arginine for the synthesis of nitric oxide. HATs are mostly exchangers with a broad spectrum of substrates and are crucial in renal and intestinal re-absorption and cell redox balance. Furthermore, the role of the HAT 4F2hc/LAT1 in tumor growth and the application of LAT1 inhibitors and PET tracers for reduction of tumor progression and imaging of tumors are discussed. Finally, we describe the link between specific mutations in HATs and the primary inherited aminoacidurias, cystinuria and lysinuric protein intolerance.

Zinc promotes the death of hypoxic astrocytes by upregulating hypoxia-induced hypoxia-inducible factor-1alpha expression via poly(ADP-ribose) polymerase-1

AIM

Pathological release of excess zinc ions has been implicated in ischemic brain cell death. However, the underlying mechanisms remain to be elucidated. In stroke, ischemia-induced zinc release and hypoxia-inducible factor-1 (HIF-1) accumulation concurrently occur in the ischemic tissue. The present study tests the hypothesis that the presence of high intracellular zinc concentration is a major cause of modifications to PARP-1 and HIF-1α during hypoxia, which significantly contributes to cell death during ischemia.

METHODS

Primary cortical astrocytes and C8-D1A cells were exposed to different concentrations of zinc chloride. Cell death rate and protein expression of HIF-1 and Poly(ADP-ribose) polymerase (PARP)-1 were examined after 3-h hypoxic treatment.

RESULTS

Although 3-h hypoxia or 100 μM of zinc alone did not induce noticeable cytotoxicity, their combination led to a dramatic increase in astrocytic cell death in a zinc-concentration-dependent manner. Exposure of astrocytes to hypoxia for 3 h remarkably increased the levels of intracellular zinc and HIF-1α protein, which was further augmented by added exogenous zinc. Notably, HIF-1α knockdown blocked zinc-induced astrocyte death. Moreover, knockdown of PARP-1, another important protein in the response of hypoxia, attenuated the overexpression of HIF-1α and reduced the cell death rate.

CONCLUSIONS

Our studies show that zinc promotes hypoxic cell death through overexpression of the hypoxia response factor HIF-1α via the cell fate determine factor PARP-1 modification, which provides a novel mechanism for zinc-mediated ischemic brain injury.

Reduction in phencyclidine induced sensorimotor gating deficits in the rat following increased system xc⁻ activity in the medial prefrontal cortex

RATIONALE

Aspects of schizophrenia, including deficits in sensorimotor gating, have been linked to glutamate dysfunction and/or oxidative stress in the prefrontal cortex. System xc(-), a cystine-glutamate antiporter, is a poorly understood mechanism that contributes to both cellular antioxidant capacity and glutamate homeostasis.

OBJECTIVES

Our goal was to determine whether increased system xc(-) activity within the prefrontal cortex would normalize a rodent measure of sensorimotor gating.

METHODS

In situ hybridization was used to map messenger RNA (mRNA) expression of xCT, the active subunit of system xc(-), in the prefrontal cortex. Prepulse inhibition was used to measure sensorimotor gating; deficits in prepulse inhibition were produced using phencyclidine (0.3-3 mg/kg, sc). N-Acetylcysteine (10-100 μM) and the system xc(-) inhibitor (S)-4-carboxyphenylglycine (CPG, 0.5 μM) were used to increase and decrease system xc(-) activity, respectively. The uptake of (14)C-cystine into tissue punches obtained from the prefrontal cortex was used to assay system xc(-) activity.

RESULTS

The expression of xCT mRNA in the prefrontal cortex was most prominent in a lateral band spanning primarily the prelimbic cortex. Although phencyclidine did not alter the uptake of (14)C-cystine in prefrontal cortical tissue punches, intraprefrontal cortical infusion of N-acetylcysteine (10-100 μM) significantly reduced phencyclidine- (1.5 mg/kg, sc) induced deficits in prepulse inhibition. N-Acetylcysteine was without effect when coinfused with CPG (0.5 μM), indicating an involvement of system xc(-).

CONCLUSIONS

These results indicate that phencyclidine disrupts sensorimotor gating through system xc(-) independent mechanisms, but that increasing cystine-glutamate exchange in the prefrontal cortex is sufficient to reduce behavioral deficits produced by phencyclidine.

Characterization of adult rat astrocyte cultures

Astrocytes, a major class of glial cells, regulate neurotransmitter systems, synaptic processing, ion homeostasis, antioxidant defenses and energy metabolism. Astrocyte cultures derived from rodent brains have been extensively used to characterize astrocytes' biochemical, pharmacological and morphological properties. The aims of this study were to develop a protocol for routine preparation and to characterize a primary astrocyte culture from the brains of adult (90 days old) Wistar rats. For this we used enzymatic digestion (trypsin and papain) and mechanical dissociation. Medium exchange occurred from 24 h after obtaining a culture and after, twice a week up to reach the confluence (around the 4^th to 5^th week). Under basal conditions, adult astrocytes presented a polygonal to fusiform and flat morphology. Furthermore, approximately 95% the cells were positive for the main glial markers, including GFAP, glutamate transporters, glutamine synthetase and S100B. Moreover, the astrocytes were able to take up glucose and glutamate. Adult astrocytes were also able to respond to acute H₂O₂ exposure, which led to an increase in reactive oxygen species (ROS) levels and a decrease in glutamate uptake. The antioxidant compound resveratrol was able to protect adult astrocytes from oxidative damage. A response of adult astrocytes to an inflammatory stimulus with LPS was also observed. Changes in the actin cytoskeleton were induced in stimulated astrocytes, most likely by a mechanism dependent on MAPK and Rho A signaling pathways. Taken together, these findings indicate that the culture model described in this study exhibits the biochemical and physiological properties of astrocytes and may be useful for elucidating the mechanisms related to the adult brain, exploring changes between neonatal and adult astrocytes, as well as investigating compounds involved in cytotoxicity and cytoprotection.

Thinking outside the cleft to understand synaptic activity: contribution of the cystine-glutamate antiporter (System xc-) to normal and pathological glutamatergic signaling

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.