Riluzole increases glutamate uptake by cultured C6 astroglial cells

Blood-Brain Barrier Solute Carrier Transporters and Motor Neuron Disease

Defective solute carrier (SLC) transporters are responsible for neurotransmitter dysregulation, resulting in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). We provided the role and kinetic parameters of transporters such as ASCTs, Taut, LAT1, CAT1, MCTs, OCTNs, CHT, and CTL1, which are mainly responsible for the transport of essential nutrients, acidic, and basic drugs in blood-brain barrier (BBB) and motor neuron disease. The affinity for LAT1 was higher in the BBB than in the ALS model cell line, whereas the capacity was higher in the NSC-34 cell lines than in the BBB. Affinity for MCTs was lower in the BBB than in the NSC-34 cell lines. CHT in BBB showed two affinity sites, whereas no expression was observed in ALS cell lines. CTL1 was the main transporter for choline in ALS cell lines. The half maximal inhibitory concentration (IC50) analysis of [3H]choline uptake indicated that choline is sensitive in TR-BBB cells, whereas amiloride is most sensitive in ALS cell lines. Knowledge of the transport systems in the BBB and motor neurons will help to deliver drugs to the brain and develop the therapeutic strategy for treating CNS and neurological diseases.

Implications of a Neuronal Receptor Family, Metabotropic Glutamate Receptors, in Cancer Development and Progression

Cancer is the second leading cause of death, and incidences are increasing globally. Simply defined, cancer is the uncontrolled proliferation of a cell, and depending on the tissue of origin, the cancer etiology, biology, progression, prognosis, and treatment will differ. Carcinogenesis and its progression are associated with genetic factors that can either be inherited and/or acquired and are classified as an oncogene or tumor suppressor. Many of these genetic factors converge on common signaling pathway(s), such as the MAPK and PI3K/AKT pathways. In this review, we will focus on the metabotropic glutamate receptor (mGluR) family, an upstream protein that transmits extracellular signals into the cell and has been shown to regulate many aspects of tumor development and progression. We explore the involvement of members of this receptor family in various cancers that include breast cancer, colorectal cancer, glioma, kidney cancer, melanoma, oral cancer, osteosarcoma, pancreatic cancer, prostate cancer, and T-cell cancers. Intriguingly, depending on the member, mGluRs can either be classified as oncogenes or tumor suppressors, although in general most act as an oncogene. The extensive work done to elucidate the role of mGluRs in various cancers suggests that it might be a viable strategy to therapeutically target glutamatergic signaling.

Riluzole regulates pancreatic cancer cell metabolism by suppressing the Wnt-β-catenin pathway

Most cancer cells rely on aerobic glycolysis to support uncontrolled proliferation and evade apoptosis. However, pancreatic cancer cells switch to glutamine metabolism to survive under hypoxic conditions. Activation of the Wnt/β-catenin pathway induces aerobic glycolysis by activating enzymes required for glucose metabolism and regulating the expression of glutamate transporter and glutamine synthetase. The results demonstrate that riluzole inhibits pancreatic cancer cell growth and has no effect on human pancreatic normal ductal epithelial cells. RNA-seq experiments identified the involvement of Wnt and metabolic pathways by riluzole. Inhibition of Wnt-β-catenin/TCF-LEF pathway by riluzole suppresses the expression of PDK, MCT1, cMyc, AXIN, and CyclinD1. Riluzole inhibits glucose transporter 2 expression, glucose uptake, lactate dehydrogenase A expression, and NAD + level. Furthermore, riluzole inhibits glutamate release and glutathione levels, and elevates reactive oxygen species. Riluzole disrupts mitochondrial homeostasis by inhibiting Bcl-2 and upregulating Bax expression, resulting in a drop of mitochondrial membrane potential. Finally, riluzole inhibits pancreatic cancer growth in KPC (Pdx1-Cre, LSL-Trp53R172H, and LSL-KrasG12D) mice. In conclusion, riluzole can inhibit pancreatic cancer growth by regulating glucose and glutamine metabolisms and can be used to treat pancreatic cancer.

Glioprotective Effects of Resveratrol Against BMAA-Induced Astroglial Dysfunctions

Astroglial cells play important roles in maintaining central nervous system (CNS) homeostasis. The neurotoxin β-N-methylamino-L-alanine (BMAA) has usually been associated with neurodegeneration due to its toxic effects on neurons. However, little is known about the effects of BMAA on astroglial cells. Resveratrol, a natural polyphenol, represents a potential protective strategy against brain injuries. In the present study, we sought to investigate BMAA-induced astroglial dysfunctions and the glioprotective roles of resveratrol. BMAA did not impair astroglial cellular viability, but increased glutamate uptake, glutamate metabolism into glutamine, and reactive oxygen species production, while decreased glutathione (GSH) and superoxide dismutase (SOD)-based antioxidant defenses and triggers an inflammatory response. In contrast, resveratrol was able to prevent most of these BMAA-induced functional changes in astroglial cells. Moreover, both BMAA and resveratrol modulated the gene expression of molecular pathways associated with glutamate metabolism, redox homeostasis, and inflammatory response, which characterize their roles on astroglial functions. In this regard, BMAA downregulated adenosine receptors, peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α), phosphoinositide-3-kinase (PI3K), and Akt, while resveratrol prevented these effects and upregulated nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1). Our study, for the first time, demonstrates that BMAA directly impacts key astroglial functions, contributing to elucidating the cellular and molecular mechanisms of this toxin in the CNS. In addition, we reinforce the glioprotective effects of resveratrol against BMAA-induced astroglial dysfunctions.

Gliotoxicity and Glioprotection: the Dual Role of Glial Cells

Glial cells (astrocytes, oligodendrocytes and microglia) are critical for the central nervous system (CNS) in both physiological and pathological conditions. With this in mind, several studies have indicated that glial cells play key roles in the development and progression of CNS diseases. In this sense, gliotoxicity can be referred as the cellular, molecular, and neurochemical changes that can mediate toxic effects or ultimately lead to impairment of the ability of glial cells to protect neurons and/or other glial cells. On the other hand, glioprotection is associated with specific responses of glial cells, by which they can protect themselves as well as neurons, resulting in an overall improvement of the CNS functioning. In addition, gliotoxic events, including metabolic stresses, inflammation, excitotoxicity, and oxidative stress, as well as their related mechanisms, are strongly associated with the pathogenesis of neurological, psychiatric and infectious diseases. However, glioprotective molecules can prevent or improve these glial dysfunctions, representing glial cells-targeting therapies. Therefore, this review will provide a brief summary of types and functions of glial cells and point out cellular and molecular mechanisms associated with gliotoxicity and glioprotection, potential glioprotective molecules and their mechanisms, as well as gliotherapy. In summary, we expect to address the relevance of gliotoxicity and glioprotection in the CNS homeostasis and diseases.

Reduction of glutamate neurotoxicity: A novel therapeutic approach for Niemann-Pick disease, type C1

Niemann-Pick disease, type C1 is a progressive, lethal, neurodegenerative disorder due to endolysosomal storage of unesterified cholesterol. Cerebellar ataxia, as a result of progressive loss of cerebellar Purkinje neurons, is a major symptom of Nieman-Pick disease, type C1. Comparing single cell RNAseq data from control (Npc1^+/+) and mutant (Npc1^-/-) mice, we observed significantly decreased expression of Slc1a3 in Npc1^-/- astrocytes. Slc1a3 encodes a glutamate transporter (GLAST, EAAT1) which functions to decrease glutamate concentrations in the post synaptic space after neuronal firing. Glutamate is an excitatory neurotransmitter and elevated extracellular levels of glutamate can be neurotoxic. Impaired EAAT1 function underlies type-6 episodic ataxia, a rare disorder with progressive cerebellar dysfunction, thus suggesting that impaired glutamate uptake in Niemann-Pick disease, type C1 could contribute to disease progression. We now show that decreased expression of Slc1a3 in Npc1^-/- mice has functional consequences that include decreased surface protein expression and decreased glutamate uptake by Npc1^-/- astrocytes. To test whether glutamate neurotoxicity plays a role in Niemann-Pick disease, type C1 progression, we treated NPC1 deficient mice with ceftriaxone and riluzole. Ceftriaxone is a β-lactam antibiotic that is known to upregulate the expression of Slc1a2, an alternative glial glutamate transporter. Although ceftriaxone increased Slc1a2 expression, we did not observe a treatment effect in NPC1 mutant mice. Riluzole is a glutamate receptor antagonist that inhibits postsynaptic glutamate receptor signaling and reduces the release of glutamate. We found that treatment with riluzole increased median survival in Npc1^-/- by 12%. Given that riluzole is an approved drug for the treatment of amyotrophic lateral sclerosis, repurposing of this drug may provide a novel therapeutic approach to decrease disease progression in Niemann-Pick disease type, C1 patients.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Amino Acid Transporters on the Guard of Cell Genome and Epigenome

Tumorigenesis is driven by metabolic reprogramming. Oncogenic mutations and epigenetic alterations that cause metabolic rewiring may also upregulate the reactive oxygen species (ROS). Precise regulation of the intracellular ROS levels is critical for tumor cell growth and survival. High ROS production leads to the damage of vital macromolecules, such as DNA, proteins, and lipids, causing genomic instability and further tumor evolution. One of the hallmarks of cancer metabolism is deregulated amino acid uptake. In fast-growing tumors, amino acids are not only the source of energy and building intermediates but also critical regulators of redox homeostasis. Amino acid uptake regulates the intracellular glutathione (GSH) levels, endoplasmic reticulum stress, unfolded protein response signaling, mTOR-mediated antioxidant defense, and epigenetic adaptations of tumor cells to oxidative stress. This review summarizes the role of amino acid transporters as the defender of tumor antioxidant system and genome integrity and discusses them as promising therapeutic targets and tumor imaging tools.

Amyloid-ß peptides inhibit the expression of AQP4 and glutamate transporter EAAC1 in insulin-treated C6 glioma cells

Astrocytic aquaporin 4 (AQP4) facilitates glutamate clearance via regulation of the glutamate transporter function, involved in the modulation of brain plasticity and cognitive function to prevent neurodegenerative disorders such as Alzheimer's disease (AD). In in vitro studies, the C6 rat glioma cell line is a widely applied aging model system to investigate changes in glial cells associated with aging or AD. However, the neurotoxicity mechanism whether AQP4 mediate glutamate uptake in Aβ-stimulated C6 cell remain uncertain. In this study, we examined the effects of Aβ on the expression of AQP4, Glu transporters, Glu uptake, and cell viability in insulin-treated C6 cells. Our results showed that the expression of AQP4 mRNA and protein was significantly enhanced by insulin in older cultures (passage 45), and the expression was inhibited by Aβ at 10 μM. In addition, the cell viability and glutamate uptake in Aβ-treated C6 cells were decreased in dose-dependent manners. GFAP showed similar changes in gene and protein expression patterns as AQP4, but no significant alterations were seen in GLAST expression. In C6 cells, the glutamate transport was found to be EAAC1, not GLT-1. EAAC1 expression was decreased by the treatment of Aβ. Taken together, our findings suggest that C6 cells may have astrocytic characteristics, and the astrocytic cytotoxicity induced by Aβ was mediated by reduction of glutamate uptake through AQP4/EAAC1 pathway in C6 cells. This indicates that C6 glioma cells could be used to study the roles of AQP4 on astrocyte function in AD.

The Role of the SLC Transporters Protein in the Neurodegenerative Disorders

The solute carrier (SLC) superfamily is one of the major sub-groups of membrane proteins in mammalian cells. The solute carrier proteins include more than 400 different membrane-spanning solute carriers organized with 65 families in the human. In solute carrier family neurons, neurotransmitter is considered to be a pharmacological target of neuropsychiatric drugs because of their important role in the recovery of neurotransmitters such as GABA, glutamate, serotonin, dopamine and noradrenaline and regulation of their concentration in synaptic regions. Therefore, solute carrier transporters play vital and different roles in neurodegenerative disorders. In this article, the role of solute carrier transporters in neurodegenerative disorders such as Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, Parkinson’s diseases, depression, post-traumatic stress disorder, dementia, schizophrenia, and Epilepsy reviewed and discussed to see how defects or absences in SLC transporter cause neurodegenerative disorders. In this review, we try to summarize what is known about solute carriers with respect to brain distribution and expression. The review summarizes current knowledge on the roles of solute carrier transporters in neurodegenerative disorders.

Stress-altered synaptic plasticity and DAMP signaling in the hippocampus-PFC axis; elucidating the significance of IGF-1/IGF-1R/CaMKIIα expression in neural changes associated with a prolonged exposure therapy

Traumatic stress patients showed significant improvement in behavior after a prolonged exposure to an unrelated stimulus. This treatment method attempts to promote extinction of the fear memory associated with the initial traumatic experience. However, the subsequent prolonged exposure to such stimulus creates an additional layer of neural stress. Although the mechanism remains unclear, prolonged exposure therapy (PET) likely involves changes in synaptic plasticity, neurotransmitter function and inflammation; especially in parts of the brain concerned with the formation and retrieval of fear memory (Hippocampus and Prefrontal Cortex: PFC). Since certain synaptic proteins are also involved in danger-associated molecular pattern signaling (DAMP), we identified the significance of IGF-1/IGF-1R/CaMKIIα expression as a potential link between the concurrent progression of synaptic and inflammatory changes in stress. Thus, a comparison between IGF-1/IGF-1R/CaMKIIα, synaptic and DAMP proteins in stress and PET may highlight the significance of PET on synaptic morphology and neuronal inflammatory response. In behaviorally characterized Sprague-Dawley rats, there was a significant decline in neural IGF-1 (p<0.001), hippocampal (p<0.001) and cortical (p<0.05) IGF-1R expression. These animals showed a significant loss of presynaptic markers (synaptophysin; p<0.001), and changes in neurotransmitters (VGLUT2, Tyrosine hydroxylase, GABA, ChAT). Furthermore, naïve stressed rats recorded a significant decrease in post-synaptic marker (PSD-95; p<0.01) and synaptic regulator (CaMKIIα; p<0.001). As part of the synaptic response to a decrease in brain CaMKIIα, small ion conductance channel (KCa2.2) was upregulated in the brain of naïve stressed rats (p<0.01). After a PET, an increase in IGF-1 (p<0.05) and IGF-1R was recorded in the Stress-PET group (p<0.001). As such, hippocampal (p<0.001), but not cortical (ns) synaptophysin expression increased in Stress-PET. Although PSD-95 was relatively unchanged in the hippocampus and PFC, CaMKIIα (p<0.001) and KCa2.2 (p<0.01) were upregulated in Stress-PET, and may be involved in extinction of fear memory-related synaptic potentials. These changes were also associated with a normalized neurotransmitter function, and a significant reduction in open space avoidance; when the animals were assessed in elevated plus maze (EPM). In addition to a decrease in IGF-1/IGF-1R, an increase in activated hippocampal and cortical microglia was seen in stress (p<0.05) and after a PET (Stress-PET; p<0.001). Furthermore, this was linked with a significant increase in HMGB1 (Hippocampus: p<0.001, PFC: p<0.05) and TLR4 expression (Hippocampus: p<0.01; PFC: ns) in the neurons. Taken together, this study showed that traumatic stress and subsequent PET involves an event-dependent alteration of IGF1/IGF-1R/CaMKIIα. Firstly, we showed a direct relationship between IGF-1/IGF-1R expression, presynaptic function (synaptophysin) and neurotransmitter activity in stress and PET. Secondly, we identified the possible role of CaMKIIα in post-synaptic function and regulation of small ion conductance channels. Lastly, we highlighted some of the possible links between IGF1/IGF-1R/CaMKIIα, the expression of DAMP proteins, Microglia activation, and its implication on synaptic plasticity during stress and PET.

ρ⁰ Cells Feature De-Ubiquitination of SLC Transporters and Increased Levels and Fluxes of Amino Acids

Solute carrier (SLC) transporters are a diverse group of membrane transporter proteins that regulate the cellular flux and distribution of endogenous and xenobiotic compounds. Post-translational modifications (PTMs), such as ubiquitination, have recently emerged as one of the major regulatory mechanisms in protein function and localization. Previously, we showed that SLC amino acid transporters were on average 6-fold de-ubiquitinated and increased amino acid levels were detected in ρ⁰ cells (lacking mitochondrial DNA, mtDNA) compared to parental cells. Here, we elucidated the altered functionality of SLC transporters and their dynamic ubiquitination status by measuring the uptake of several isotopically labeled amino acids in both human osteosarcoma 143B.TK- and ρ⁰ cells. Our pulse chase analysis indicated that de-ubiquitinated amino acid transporters in ρ⁰ cells were accompanied by an increased transport rate, which leads to higher levels of amino acids in the cell. Finding SLC transport enhancers is an aim of the pharmaceutical industry in order to compensate for loss of function mutations in these genes. Thus, the ubiquitination status of SLC transporters could be an indicator for their functionality, but evidence for a direct connection between de-ubiquitination and transporter activity has to be further elucidated.

Glutamate Transport System as a Novel Therapeutic Target in Chronic Pain: Molecular Mechanisms and Pharmacology

The vast majority of peripheral neurons sensing noxious stimuli and conducting pain signals to the dorsal horn of the spinal cord utilize glutamate as a chemical transmitter of excitation. High-affinity glutamate transporter subtypes GLAST/EAAT1, GLT1/EAAT2, EAAC1/EAAT3, and EAAT4, differentially expressed on sensory neurons, postsynaptic spinal interneurons, and neighboring glia, ensure fine modulation of glutamate neurotransmission in the spinal cord. The glutamate transport system seems to play important roles in molecular mechanisms underlying chronic pain and analgesia. Downregulation of glutamate transporters (GluTs) often precedes or occurs simultaneously with development of hypersensitivity to thermal or tactile stimuli in various models of chronic pain. Moreover, antisense knockdown or pharmacological inhibition of these membrane proteins can induce or aggravate pain. In contrast, upregulation of GluTs by positive pharmacological modulators or by viral gene transfer to the spinal cord can reverse the development of such pathological hypersensitivity. Furthermore, some multi-target drugs displaying analgesic properties (e.g., tricyclic antidepressant amitriptyline, riluzole, anticonvulsant valproate, tetracycline antibiotic minocycline, β-lactam antibiotic ceftriaxone and its structural analog devoid of antibacterial activity, clavulanic acid) can significantly increase the spinal glutamate uptake. Thus, mounting evidence points at GluTs as prospective therapeutic target for chronic pain treatment. However, design and development of new analgesics based on the modulation of glutamate uptake will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of this transport system in the spinal cord.

Excess cerebral TNF causing glutamate excitotoxicity rationalizes treatment of neurodegenerative diseases and neurogenic pain by anti-TNF agents

The basic mechanism of the major neurodegenerative diseases, including neurogenic pain, needs to be agreed upon before rational treatments can be determined, but this knowledge is still in a state of flux. Most have agreed for decades that these disease states, both infectious and non-infectious, share arguments incriminating excitotoxicity induced by excessive extracellular cerebral glutamate. Excess cerebral levels of tumor necrosis factor (TNF) are also documented in the same group of disease states. However, no agreement exists on overarching mechanism for the harmful effects of excess TNF, nor, indeed how extracellular cerebral glutamate reaches toxic levels in these conditions. Here, we link the two, collecting and arguing the evidence that, across the range of neurodegenerative diseases, excessive TNF harms the central nervous system largely through causing extracellular glutamate to accumulate to levels high enough to inhibit synaptic activity or kill neurons and therefore their associated synapses as well. TNF can be predicted from the broader literature to cause this glutamate accumulation not only by increasing glutamate production by enhancing glutaminase, but in addition simultaneously reducing glutamate clearance by inhibiting re-uptake proteins. We also discuss the effects of a TNF receptor biological fusion protein (etanercept) and the indirect anti-TNF agents dithio-thalidomides, nilotinab, and cannabinoids on these neurological conditions. The therapeutic effects of 6-diazo-5-oxo-norleucine, ceptriaxone, and riluzole, agents unrelated to TNF but which either inhibit glutaminase or enhance re-uptake proteins, but do not do both, as would anti-TNF agents, are also discussed in this context. By pointing to excess extracellular glutamate as the target, these arguments greatly strengthen the case, put now for many years, to test appropriately delivered ant-TNF agents to treat neurodegenerative diseases in randomly controlled trials.

The importance of the excitatory amino acid transporter 3 (EAAT3)

The neuronal excitatory amino acid transporter 3 (EAAT3) is fairly ubiquitously expressed in the brain, though it does not necessarily maintain the same function everywhere. It is important in maintaining low local concentrations of glutamate, where its predominant post-synaptic localization can buffer nearby glutamate receptors and modulate excitatory neurotransmission and synaptic plasticity. It is also the main neuronal cysteine uptake system acting as the rate-limiting factor for the synthesis of glutathione, a potent antioxidant, in EAAT3 expressing neurons, while on GABAergic neurons, it is important in supplying glutamate as a precursor for GABA synthesis. Several diseases implicate EAAT3, and modulation of this transporter could prove a useful therapeutic approach. Regulation of EAAT3 could be targeted at several points for functional modulation, including the level of transcription, trafficking and direct pharmacological modulation, and indeed, compounds and experimental treatments have been identified that regulate EAAT3 function at different stages, which together with observations of EAAT3 regulation in patients is giving us insight into the endogenous function of this transporter, as well as the consequences of altered function. This review summarizes work done on elucidating the role and regulation of EAAT3.

SLC transporters as therapeutic targets: emerging opportunities

Solute carrier (SLC) transporters - a family of more than 300 membrane-bound proteins that facilitate the transport of a wide array of substrates across biological membranes - have important roles in physiological processes ranging from the cellular uptake of nutrients to the absorption of drugs and other xenobiotics. Several classes of marketed drugs target well-known SLC transporters, such as neurotransmitter transporters, and human genetic studies have provided powerful insight into the roles of more-recently characterized SLC transporters in both rare and common diseases, indicating a wealth of new therapeutic opportunities. This Review summarizes knowledge on the roles of SLC transporters in human disease, describes strategies to target such transporters, and highlights current and investigational drugs that modulate SLC transporters, as well as promising drug targets.

Downregulation of solute carriers of glutamate in gliosomes and synaptosomes may explain local brain metastasis in anaplastic glioblastoma

Advanced grades of glioblastoma are highly aggressive, especially in terms of multisite spread within the brain or even to distant sites at the spinal cord. In advanced grades of glioblastoma, glutamate and glutamine are reported to be increased in concentration in the extracellular fluid. It has been reported that glutamate acts as an extracellular signaling molecule for facilitating local spread of advanced grades of glioblastoma. In the present study, we aimed to examine whether glutamate uptake mechanisms is impaired in advanced glioblastoma. The possible downregulated mechanisms of glutamate uptake would facilitate persistence of glutamate in the extracellular environment, rather than intracellular uptake. We obtained biobanked human specimens of glioblastoma and tested expression of proteins belonging to the solute carrier families of proteins that are known to function as membrane-located excitatory amino acid like glutamate transporters. The present study provides preliminary evidence of the downregulation of membrane expression of excitatory amino acid transporters solute carrier family 1 member 3 (SLC1A3) and its palmitoylated form in gliosomes, as well as SLC1A2 in the glio-synaptosomes. Compounds like riluzole used in the treatment of amyotrophic lateral sclerosis and the antibiotic ceftriaxone have the potential to facilitate glutamate uptake. These medications may be examined as adjunct chemotherapy in the massively aggressive tumor glioblastoma multiforme.

NMDA antagonists under investigation for the treatment of major depressive disorder

INTRODUCTION

Mood disorders, including depression, are becoming increasingly prevalent in the developed world. Furthermore, treatment of depression therapeutics, mainly influencing the serotonergic and adrenergic systems, is considered insufficient. The original NMDA-glutamate hypothesis mechanism of antidepressant action was first proposed ∼ 20 years ago. Since then, a number of preclinical and clinical studies have examined its rationale.

AREAS COVERED

This review highlights the recent clinical evidence for the use of functional NMDA receptor antagonists as antidepressants. Furthermore, the authors present the mechanism(s) of antidepressant action derived mostly from preclinical paradigms.

EXPERT OPINION

Currently, clinical studies mostly use ketamine (a noncompetitive high-potency NMDA antagonist) as an agent for rapid relief of depressive symptoms. However, due to the ketamine-induced psychotomimetic effects, new NMDA receptor antagonists (modulators) are continuously being introduced for rapid antidepressant action, especially for use in treatment-resistant patients. Recent clinical reports for the use of CP-101,606, MK-0657 (selective GluN2B subunit NMDA receptor antagonists), GLYX-13 and d-cycloserine (glycine site partial agonists) are optimistic but await further support.

Sumoylation of critical proteins in amyotrophic lateral sclerosis: emerging pathways of pathogenesis

Emerging lines of evidence suggest a relationship between amyotrophic lateral sclerosis (ALS) and protein sumoylation. Multiple studies have demonstrated that several of the proteins involved in the pathogenesis of ALS, including superoxide dismutase 1, fused in liposarcoma, and TAR DNA-binding protein 43 (TDP-43), are substrates for sumoylation. Additionally, recent studies in cellular and animal models of ALS revealed that sumoylation of these proteins impact their localization, longevity, and how they functionally perform in disease, providing novel areas for mechanistic investigations and therapeutics. In this article, we summarize the current literature examining the impact of sumoylation of critical proteins involved in ALS and discuss the potential impact for the pathogenesis of the disease. In addition, we report and discuss the implications of new evidence demonstrating that sumoylation of a fragment derived from the proteolytic cleavage of the astroglial glutamate transporter, EAAT2, plays a direct role in downregulating the expression levels of full-length EAAT2 by binding to a regulatory region of its promoter.