Platelet-derived growth factor rapidly increases activity and cell surface expression of the EAAC1 subtype of glutamate transporter through activation of phosphatidylinositol 3-kinase

Evaluation of the anticonvulsant and neuroprotective effect of intracerebral administration of growth hormone in rats

INTRODUCTION

The growth hormone (GH) has been reported as a crucial neuronal survival factor in the hippocampus against insults of diverse nature. Status epilepticus (SE) is a prolonged seizure that produces extensive neuronal cell death. The goal of this study was to evaluate the effect of intracerebroventricular administration of GH on seizure severity and SE-induced hippocampal neurodegeneration.

METHODOLOGY

Adult male rats were implanted with a guide cannula in the left ventricle and different amounts of GH (70, 120 or 220ng/3μl) were microinjected for 5 days; artificial cerebrospinal fluid was used as the vehicle. Seizures were induced by the lithium-pilocarpine model (3mEq/kg LiCl and 30mg/kg pilocarpine hydrochloride) one day after the last GH administration. Neuronal injury was assessed by Fluoro-Jade B (F-JB) staining.

RESULTS

Rats injected with 120ng of GH did not had SE after 30mg/kg pilocarpine, they required a higher number of pilocarpine injections to develop SE than the rats pretreated with the vehicle, 70ng or 220ng GH. Prefrontal and parietal cortex EEG recordings confirmed that latency to generalized seizures and SE was also significantly higher in the 120ng group when compared with all the experimental groups. FJ-B positive cells were detected in the hippocampus after SE in all rats, and no significant differences in the number of F-JB cells in the CA1 area and the hilus was observed between experimental groups.

CONCLUSION

Our results indicate that, although GH has an anticonvulsive effect in the lithium-pilocarpine model of SE, it does not exert hippocampal neuroprotection after SE.

Dysfunction of EAAT3 Aggravates LPS-Induced Post-Operative Cognitive Dysfunction

Numerous results have revealed an association between inhibited function of excitatory amino acid transporter 3 (EAAT3) and several neurodegenerative diseases. This was also corroborated by our previous studies which showed that the EAAT3 function was intimately linked to learning and memory. With this premise, we examined the role of EAAT3 in post-operative cognitive dysfunction (POCD) and explored the potential benefit of riluzole in countering POCD in the present study. We first established a recombinant adeno-associated-viral (rAAV)-mediated shRNA to knockdown SLC1A1/EAAT3 expression in the hippocampus of adult male mice. The mice then received an intracerebroventricular microinjection of 2 μg lipopolysaccharide (LPS) to construct the POCD model. In addition, for old male mice, 4 mg/kg of riluzole was intraperitoneally injected for three consecutive days, with the last injection administered 2 h before the LPS microinjection. Cognitive function was assessed using the Morris water maze 24 h following the LPS microinjection. Animal behavioral tests, as well as pathological and biochemical assays, were performed to clarify the role of EAAT3 function in POCD and evaluate the effect of activating the EAAT3 function by riluzole. In the present study, we established a mouse model with hippocampal SLC1A1/EAAT3 knockdown and found that hippocampal SLC1A1/EAAT3 knockdown aggravated LPS-induced learning and memory deficits in adult male mice. Meanwhile, LPS significantly inhibited the expression of EAAT3 membrane protein and the phosphorylation level of GluA1 protein in the hippocampus of adult male mice. Moreover, riluzole pretreatment significantly increased the expression of hippocampal EAAT3 membrane protein and also ameliorated LPS-induced cognitive impairment in elderly male mice. Taken together, our results demonstrated that the dysfunction of EAAT3 is an important risk factor for POCD susceptibility and therefore, it may become a promising target for POCD treatment.

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

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

Diagnostic Impact of Radiological Findings and Extracellular Vesicles: Are We Close to Radiovesicolomics?

Simple Summary

Over the years, diagnostic tests such as in radiology and flow cytometry have become more and more powerful in the constant struggle against different pathologies, some of which are life-threatening. The possibility of using these “weapons” in a conjugated manner could result in higher healing and prevention rates, and a decrease in late diagnosis diseases. Different correlations among pathologies, extracellular vesicles (EVs), and radiological findings were recently demonstrated by many authors. Together with the increasing importance of “omics” sciences, and artificial intelligence in this new century, the perspective of a new research field called “radiovesicolomics” could be the missing link, enabling a different approach to disease diagnosis and treatment.

Abstract

Currently, several pathologies have corresponding and specific diagnostic and therapeutic branches of interest focused on early and correct detection, as well as the best therapeutic approach. Radiology never ceases to develop newer technologies in order to give patients a clear, safe, early, and precise diagnosis; furthermore, in the last few years diagnostic imaging panoramas have been extended to the field of artificial intelligence (AI) and machine learning. On the other hand, clinical and laboratory tests, like flow cytometry and the techniques found in the “omics” sciences, aim to detect microscopic elements, like extracellular vesicles, with the highest specificity and sensibility for disease detection. If these scientific branches started to cooperate, playing a conjugated role in pathology diagnosis, what could be the results? Our review seeks to give a quick overview of recent state of the art research which investigates correlations between extracellular vesicles and the known radiological features useful for diagnosis.

Neuroprotective Effects of Guanosine in Ischemic Stroke-Small Steps towards Effective Therapy

Guanosine (Guo) is a nucleotide metabolite that acts as a potent neuromodulator with neurotrophic and regenerative properties in neurological disorders. Under brain ischemia or trauma, Guo is released to the extracellular milieu and its concentration substantially raises. In vitro studies on brain tissue slices or cell lines subjected to ischemic conditions demonstrated that Guo counteracts destructive events that occur during ischemic conditions, e.g., glutaminergic excitotoxicity, reactive oxygen and nitrogen species production. Moreover, Guo mitigates neuroinflammation and regulates post-translational processing. Guo asserts its neuroprotective effects via interplay with adenosine receptors, potassium channels, and excitatory amino acid transporters. Subsequently, guanosine activates several prosurvival molecular pathways including PI3K/Akt (PI3K) and MEK/ERK. Due to systemic degradation, the half-life of exogenous Guo is relatively low, thus creating difficulty regarding adequate exogenous Guo distribution. Nevertheless, in vivo studies performed on ischemic stroke rodent models provide promising results presenting a sustained decrease in infarct volume, improved neurological outcome, decrease in proinflammatory events, and stimulation of neuroregeneration through the release of neurotrophic factors. In this comprehensive review, we discuss molecular signaling related to Guo protection against brain ischemia. We present recent advances, limitations, and prospects in exogenous guanosine therapy in the context of ischemic stroke.

Evaluation of the anticonvulsant and neuroprotective effect of intracerebral administration of growth hormone in rats

INTRODUCTION

The growth hormone (GH) has been reported as a crucial neuronal survival factor in the hippocampus against insults of diverse nature. Status epilepticus (SE) is a prolonged seizure that produces extensive neuronal cell death. The goal of this study was to evaluate the effect of intracerebroventricular administration of GH on seizure severity and SE-induced hippocampal neurodegeneration.

METHODOLOGY

Adult male rats were implanted with a guide cannula in the left ventricle and different amounts of GH (70, 120 or 220ng/3μl) were microinjected for 5 days; artificial cerebrospinal fluid was used as the vehicle. Seizures were induced by the lithium-pilocarpine model (3mEq/kg LiCl and 30mg/kg pilocarpine hydrochloride) one day after the last GH administration. Neuronal injury was assessed by Fluoro-Jade B (F-JB) staining.

RESULTS

Rats injected with 120ng of GH did not had SE after 30mg/kg pilocarpine, they required a higher number of pilocarpine injections to develop SE than the rats pretreated with the vehicle, 70ng or 220ng GH. Prefrontal and parietal cortex EEG recordings confirmed that latency to generalized seizures and SE was also significantly higher in the 120ng group when compared with all the experimental groups. FJ-B positive cells were detected in the hippocampus after SE in all rats, and no significant differences in the number of F-JB cells in the CA1 area and the hilus was observed between experimental groups.

CONCLUSION

Our results indicate that, although GH has an anticonvulsive effect in the lithium-pilocarpine model of SE, it does not exert hippocampal neuroprotection after SE.

Constitutive Endocytosis of the Neuronal Glutamate Transporter Excitatory Amino Acid Transporter-3 Requires ARFGAP1

The eukaryotic endocytic pathway regulates protein levels available at the plasma membrane by recycling them into specific endosomal compartments. ARFGAP1 is a component of the coat protein I (COPI) complex but it also plays a role in promoting adapter protein-2 (AP-2) mediated endocytosis. The excitatory amino acid transporter-3 (EAAT3) mediates the reuptake of glutamate from the synaptic cleft to achieve rapid termination of synaptic transmission at glutamatergic synapses. In this study, we identified two interacting proteins of EAAT3 by mass spectrometry (MS) ARFGAP1 and ARF6. We explored the role of ARFGAP1 and ARF6 in the endocytosis of EAAT3. Our data revealed that ARFGAP1 plays a role in the recycling of EAAT3, by utilizing its GTPase activating protein (GAP) activity and ARF6 acting as the substrate. ARFGAP1 promotes cargo sorting of EAAT3 via a single phenylalanine residue (F508) located at the C-terminus of the transporter. ARFGAP1-promoted AP-2 dependent endocytosis is abolished upon neutralizing F508. We utilized a heterologous expression system to identify an additional motif in the C-terminus of EAAT3 that regulates its endocytosis. Impairment in endocytosis did not affect somatodendritic targeting in cultured hippocampal neurons. Our findings support a model where endocytosis of EAAT3 is a multifactorial event regulated by ARFGAP1, occurring via the C-terminus of the transporter, and is the first study to examine the role of ARFGAP1 in the endocytosis of a transport protein.

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.

Excitatory Amino Acid Transporters in Physiology and Disorders of the Central Nervous System

Excitatory amino acid transporters (EAATs) encompass a class of five transporters with distinct expression in neurons and glia of the central nervous system (CNS). EAATs are mainly recognized for their role in uptake of the amino acid glutamate, the major excitatory neurotransmitter. EAATs-mediated clearance of glutamate released by neurons is vital to maintain proper glutamatergic signalling and to prevent toxic accumulation of this amino acid in the extracellular space. In addition, some EAATs also act as chloride channels or mediate the uptake of cysteine, required to produce the reactive oxygen speciesscavenger glutathione. Given their central role in glutamate homeostasis in the brain, as well as their additional activities, it comes as no surprise that EAAT dysfunctions have been implicated in numerous acute or chronic diseases of the CNS, including ischemic stroke and epilepsy, cerebellar ataxias, amyotrophic lateral sclerosis, Alzheimer’s disease and Huntington’s disease. Here we review the studies in cellular and animal models, as well as in humans that highlight the roles of EAATs in the pathogenesis of these devastating disorders. We also discuss the mechanisms regulating EAATs expression and intracellular trafficking and new exciting possibilities to modulate EAATs and to provide neuroprotection in course of pathologies affecting the CNS.

Guanosine prevents oxidative damage and glutamate uptake impairment induced by oxygen/glucose deprivation in cortical astrocyte cultures: involvement of A1 and A2A adenosine receptors and PI3K, MEK, and PKC pathways

Glial cells are involved in multiple cerebral functions that profoundly influence brain tissue viability during ischemia, and astrocytes are the main source of extracellular purines as adenosine and guanosine. The endogenous guanine-based nucleoside guanosine is a neuromodulator implicated in important processes in the brain, such as modulation of glutamatergic transmission and protection against oxidative and inflammatory damage. We evaluated if the neuroprotective effect of guanosine is also observed in cultured cortical astrocytes subjected to oxygen/glucose deprivation (OGD) and reoxygenation. We also assessed the involvement of A₁ and A_2A adenosine receptors and phosphatidylinositol-3 kinase (PI3K), MAPK, and protein kinase C (PKC) signaling pathways on the guanosine effects. OGD/reoxygenation decreased cell viability and glutamate uptake and increased reactive oxygen species (ROS) production in cultured astrocytes. Guanosine treatment prevented these OGD-induced damaging effects. Dipropyl-cyclopentyl-xanthine (an adenosine A₁ receptor antagonist) and 4-[2-[[6-amino-9-(N-ethyl-β-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl] benzenepropanoic acid hydrochloride (an adenosine A_2A receptor agonist) abolished guanosine-induced protective effects on ROS production, glutamate uptake, and cell viability. The PI3K pathway inhibitor 2-morpholin-4-yl-8-phenylchromen-4-one, the extracellular-signal regulated kinase kinase (MEK) inhibitor 2'-amino-3'-methoxyflavone, or the PKC inhibitor chelerythrine abolished the guanosine effect of preventing OGD-induced cells viability reduction. PI3K inhibition partially prevented the guanosine effect of reducing ROS production, whereas MEK and PKC inhibitions prevented the guanosine effect of restoring glutamate uptake. The total immunocontent of the main astrocytic glutamate transporter glutamate transporter-1 (GLT-1) was not altered by OGD and guanosine. However, MEK and PKC inhibitions also abolished the guanosine effect of increasing cell-surface expression of GLT-1 in astrocytes subjected to OGD. Then, guanosine prevents oxidative damage and stimulates astrocytic glutamate uptake during ischemic events via adenosine A₁ and A_2A receptors and modulation of survival signaling pathways, contributing to microenvironment homeostasis that culminates in neuroprotection.

Glutamate transporters: Gene expression regulation and signaling properties

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. During synaptic activity, glutamate is released and binds to specific membrane receptors and transporters activating, in the one hand, a wide variety of signal transduction cascades, while in the other hand, its removal from the synaptic cleft. Extracellular glutamate concentrations are maintained within physiological levels mainly by glia glutamate transporters. Inefficient clearance of this amino acid is neurotoxic due to a prolonged hyperactivation of its postsynaptic receptors, exacerbating a wide array of intracellular events linked to an ionic imbalance, that results in neuronal cell death. This process is known as excitotoxicity and is the underlying mechanisms of an important number of neurodegenerative diseases. Therefore, it is important to understand the regulation of glutamate transporters function. The transporter activity can be regulated at different levels: gene expression, transporter protein targeting and trafficking, and post-translational modifications of the transporter protein. The identification of these mechanisms has paved the way to our current understanding the role of glutamate transporters in brain physiology and will certainly provide the needed biochemical information for the development of therapeutic strategies towards the establishment of novel therapeutic approaches for the treatment and/or prevention of pathologies associated with excitotoxicity insults. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Arundic Acid Increases Expression and Function of Astrocytic Glutamate Transporter EAAT1 Via the ERK, Akt, and NF-κB Pathways

Glutamate is the major excitatory neurotransmitter in the brain, but excessive synaptic glutamate must be removed to prevent excitotoxic injury and death. Two astrocytic glutamate transporters, excitatory amino acid transporter (EAAT) 1 and 2, play a major role in eliminating excess glutamate from the synapse. Dysregulation of EAAT1 contributes to the pathogenesis of multiple neurological disorders, such as Alzheimer's disease (AD), ataxia, traumatic brain injuries, and glaucoma. In the present study, we investigated the effect of arundic acid on EAAT1 to determine its efficacy in enhancing the expression and function of EAAT1, and its possible mechanisms of action. The studies were carried out in human astrocyte H4 cells as well as in human primary astrocytes. Our findings show that arundic acid upregulated EAAT1 expression at the transcriptional level by activating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Arundic acid increased astrocytic EAAT1 promoter activity, messenger RNA (mRNA)/protein levels, and glutamate uptake, while pharmacological inhibition of NF-κB or mutation on NF-κB binding sites in the EAAT1 promoter region abrogated these effects. Arundic acid increased NF-κB reporter activity and induced NF-κB nuclear translocation as well as its bindings to the EAAT1 promoter. Furthermore, arundic acid activated the Akt and ERK signaling pathways to enhance EAAT1 mRNA/protein levels. Finally, arundic acid attenuated manganese-induced decrease in EAAT1 expression by inhibiting expression of the transcription factor Ying Yang 1 (YY1). These results demonstrate that arundic acid increases the expression and function of EAAT1 via the Akt, ERK, and NF-κB signaling pathways, and reverses Mn-induced EAAT1 repression by inhibiting the Mn-induced YY1 activation.

From the Cover: Vulnerability of C6 Astrocytoma Cells After Single-Compound and Joint Exposure to Type I and Type II Pyrethroid Insecticides

A primary mode-of-action of all pyrethroid insecticides (PYRs) is the disruption of the voltage-gated sodium channel electrophysiology in neurons of target pests and nontarget species. The neurological actions of PYRs on non-neuronal cells of the nervous system remain poorly investigated. In the present work, we used C6 astrocytoma cells to study PYR actions (0.1-50 μM) under the hypothesis that glial cells may be targeted by and vulnerable to PYRs. To this end, we characterized the effects of bifenthrin (BF), tefluthrin (TF), α-cypermethrin (α-CYP), and deltamethrin (DM) on the integrity of nuclear, mitochondrial, and lysosomal compartments. In general, 24- to 48-h exposures produced concentration-related impairment of cell viability. In single-compound, 24-h exposure experiments, effective concentration (EC)₁₅s 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT assay) were computed as follows (in μM): BF, 16.1; TF, 37.3; α-CYP, 7.8; DM, 5.0. We found concentration-related damage in several C6-cell subcellular compartments (mitochondria, nuclei, and lysosomes) at ≥ 10^-1 μM levels. Last, we examined a mixture of all PYRs (ie, Σ individual EC₁₅) using MTT assays and subcellular analyses. Our findings indicate that C6 cells are responsive to nM levels of PYRs, suggesting that astroglial susceptibility may contribute to the low-dose neurological effects caused by these insecticides. This research further suggests that C6 cells may provide relevant information as a screening platform for pesticide mixtures targeting nervous system cells by expected and unexpected toxicogenic pathways potentially contributing to clinical neurotoxicity.

Numb directs the subcellular localization of EAAT3 through binding the YxNxxF motif

Excitatory amino acid transporter type 3 (EAAT3, also known as SLC1A1) is a high-affinity, Na(+)-dependent glutamate carrier that localizes primarily within the cell and at the apical plasma membrane. Although previous studies have reported proteins and sequence regions involved in EAAT3 trafficking, the detailed molecular mechanism by which EAAT3 is distributed to the correct location still remains elusive. Here, we identify that the YVNGGF sequence in the C-terminus of EAAT3 is responsible for its intracellular localization and apical sorting in rat hepatoma cells CRL1601 and Madin-Darby canine kidney (MDCK) cells, respectively. We further demonstrate that Numb, a clathrin adaptor protein, directly binds the YVNGGF motif and regulates the localization of EAAT3. Mutation of Y503, N505 and F508 within the YVNGGF motif to alanine residues or silencing Numb by use of small interfering RNA (siRNA) results in the aberrant localization of EAAT3. Moreover, both Numb and the YVNGGF motif mediate EAAT3 endocytosis in CRL1601 cells. In summary, our study suggests that Numb is a pivotal adaptor protein that mediates the subcellular localization of EAAT3 through binding the YxNxxF (where x stands for any amino acid) motif.

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.

Characterization of a Novel Mutation in SLC1A1 Associated with Schizophrenia

We have recently described a hemi-deletion on chromosome 9p24.2 at the SLC1A1 gene locus and its co-segregation with schizophrenia in an extended Palauan pedigree. This finding represents a point of convergence for several pathophysiological models of schizophrenia. The present report sought to characterize the biological consequences of this hemi-deletion. Dual luciferase assays demonstrated that the partially deleted allele (lacking exon 1 and the native promoter) can drive expression of a 5'-truncated SLC1A1 using sequence upstream of exon 2 as a surrogate promoter. However, confocal microscopy and electrophysiological recordings demonstrate that the 5'-truncated SLC1A1 lacks normal membrane localization and glutamate transport ability. To identify downstream consequences of the hemi-deletion, we first used a themed qRT-PCR array to compare expression of 84 GABA and glutamate genes in RNA from peripheral blood leukocytes in deletion carriers (n = 11) versus noncarriers (n = 8) as well as deletion carriers with psychosis (n = 5) versus those without (n = 3). Then, targeted RNA-Seq (TREx) was used to quantify expression of 375 genes associated with neuropsychiatric disorders in HEK293 cells subjected to either knockdown of SLC1A1 or overexpression of full-length or 5'-truncated SLC1A1. Expression changes of several genes strongly implicated in schizophrenia pathophysiology were detected (e.g. SLC1A2, SLC1A3, SLC1A6, SLC7A11, GRIN2A, GRIA1 and DLX1).

Ondansetron attenuates the activity of excitatory amino acid transporter type 3 expressed in Xenopus oocytes

The purpose of this study was to evaluate the effect of ondansetron on excitatory amino acid transporter type 3 (EAAT3) and to elucidate the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in the effect. EAAT3 was expressed in Xenopus oocytes following the injection of rat EAAT3 mRNAs. Using the two-electrode voltage clamping method, the inward currents induced by L-glutamate were measured for 1 min in the presence and absence of ondansetron (1-1000 μM). Different concentrations of L-glutamate (3-300 μM) were used to determine the kinetic characteristics of EAAT3. To identify the involvement of PKC and PI3K in the effect, oocytes were exposed to a PKC activator and to PKC inhibitors and PI3K inhibitors, and L-glutamate-induced currents were recorded. Ondansetron decreased EAAT3 activity in a dose-dependent manner. In a kinetic study, ondansetron (10 μM for 3 min) reduced Vmax, but not Km compared with the control group. The PKC activator abolished the ondansetron-induced decrease in EAAT3 activity. The PKC inhibitors (staurosporine and chelerythrine) and ondansetron had not additive or synergistic effects on EAAT3 activity. The PI3K inhibitors (wortmannin and LY294002) decreased the EAAT3 response, although there were no differences among the groups comprising ondansetron, PI3K inhibitors, and PI3K inhibitors plus ondansetron. Our results demonstrate that ondansetron attenuates EAAT3 activity and this effect seems to be mediated by PKC and PI3K.

A cell surface biotinylation assay to reveal membrane-associated neuronal cues: Negr1 regulates dendritic arborization

A complex and still not comprehensively resolved panel of transmembrane proteins regulates the outgrowth and the subsequent morphological and functional development of neuronal processes. In order to gain a more detailed description of these events at the molecular level, we have developed a cell surface biotinylation assay to isolate, detect, and quantify neuronal membrane proteins. When we applied our assay to investigate neuron maturation in vitro, we identified 439 differentially expressed proteins, including 20 members of the immunoglobulin superfamily. Among these candidates, we focused on Negr1, a poorly described cell adhesion molecule. We demonstrated that Negr1 controls the development of neurite arborization in vitro and in vivo. Given the tight correlation existing among synaptic cell adhesion molecules, neuron maturation, and a number of neurological disorders, our assay results are a useful tool that can be used to support the understanding of the molecular bases of physiological and pathological brain function.

Nicotine decreases the activity of glutamate transporter type 3

Nicotine, the main ingredient of tobacco, elicits seizures in animal models and cigarette smoking is regarded as a behavioral risk factor associated with epilepsy or seizures. In the hippocampus, the origin of nicotine-induced seizures, most glutamate uptake could be performed primarily by excitatory amino acid transporter type 3 (EAAT3). An association between temporal lobe epilepsy and EAAT3 downregulation has been reported. Therefore, we hypothesized that nicotine may elicit seizures through the attenuation of EAAT3 activity. We investigated chronic nicotine exposure (72 h) cause reduction of the activity of EAAT3 in a Xenopus oocyte expression system using a two-electrode voltage clamp. The roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) were also determined. Nicotine (0.001-1 μM) resulted in a time- and dose-dependent decrease in EAAT3 activity with maximal inhibition at nicotine concentrations of 0.03 μM or higher and at an exposure time of 72 h. Vmax on the glutamate response was significantly reduced in the nicotine group (0.03 μM for 72 h), but the Km value of EAAT3 for glutamate was not altered. When nicotine-exposed oocytes (0.03 μM for 72 h) were pretreated with phorbol-12-myristate-13-acetate (PMA, a PKC activator), the nicotine-induced reduction in EAAT3 activity was abolished. PKC inhibitors (staurosporine, chelerythrine, and calphostin C) significantly reduced basal EAAT3 activity, but there were no significant differences among the PKC inhibitors, nicotine, and PKC inhibitors+nicotine groups. Similar response patterns were observed among PI3K inhibitors (wortmannin and LY294002), nicotine, and PI3K inhibitors+nicotine. In conclusion, this study suggests that nicotine decreases EAAT3 activity, and that this inhibition seems to be dependent on PKC and PI3K. Our results may provide an additional mechanism for nicotine-induced seizure.

Interleukin-1β alters glutamate transmission at purkinje cell synapses in a mouse model of multiple sclerosis

Cerebellar deficit contributes significantly to disability in multiple sclerosis (MS). Several clinical and experimental studies have investigated the pathophysiology of cerebellar dysfunction in this neuroinflammatory disorder, but the cellular and molecular mechanisms are still unclear. In experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, proinflammatory cytokines, together with a degeneration of inhibitory neurons, contribute to impair GABAergic transmission at Purkinje cells (PCs). Here, we investigated glutamatergic transmission to gain insight into the pathophysiology of cerebellar dysfunction in EAE. Electrophysiological recordings from PCs showed increased duration of spontaneous excitatory postsynaptic currents (EPSCs) during the symptomatic phase of EAE, suggesting an alteration of glutamate uptake played by Bergmann glia. We indeed observed an impaired functioning of the glutamate-aspartate transporter/excitatory amino acid transporter 1 (GLAST/EAAT1) in EAE cerebellum caused by protein downregulation and in correlation with prominent astroglia activation. We have also demonstrated that the proinflammatory cytokine interleukin-1β (IL-1β), released by a subset of activated microglia/macrophages and infiltrating lymphocytes, was involved directly in such synaptic alteration. In fact, brief incubation of IL-1β in normal cerebellar slices replicated EAE modifications through a rapid GLAST/EAAT1 downregulation, whereas incubation of an IL-1 receptor antagonist (IL-1ra) in EAE slices reduced spontaneous EPSC alterations. Finally, EAE mice treated with intracerebroventricular IL-1ra showed normal glutamatergic and GABAergic transmissions, along with GLAST/EAAT1 normalization, milder inflammation, and reduced motor deficits. These results highlight the crucial role played by the proinflammatory IL-1β in triggering molecular and synaptic events involved in neurodegenerative processes that characterize neuroinflammatory diseases such as MS.

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

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

Estrogen attenuates manganese-induced glutamate transporter impairment in rat primary astrocytes

The astrocytic glutamate transporters (GLT-1, GLAST) are critical for removing excess glutamate from synaptic sites, thereby maintaining glutamate homeostasis within the brain. 17β-Estradiol (E2) is one of the most active estrogen hormones possessing neuroprotective effects both in in vivo and in vitro models, and it has been shown to enhance astrocytic glutamate transporter function (Liang et al. in J Neurochem 80:807-814, 2002; Pawlak et al. in Brain Res Mol Brain Res 138:1-7, 2005). However, E2 is not clinically optimal for neuroprotection given its peripheral feminizing and proliferative effects; therefore, brain selective estrogen receptor modulators (neuro SERMs) (Zhao et al. in Neuroscience 132:299-311, 2005) that specifically target estrogenic mechanisms, but lack the systemic estrogen side effects offer more promising therapeutic modality for the treatment of conditions associated with excessive synaptic glutamate levels. This review highlights recent studies from our laboratory showing that E2 and SERMs effectively reverse glutamate transport inhibition in a manganese (Mn)-induced model of glutamatergic deregulation. Specifically, we discuss mechanisms by which E2 restores the expression and activity of glutamate uptake. We advance the hypothesis that E2 and related compounds, such as tamoxifen may offer a potential therapeutic modality in neurodegenerative disorders, which are characterized by altered glutamate homeostasis.

Neuroprotective effect of guanosine against glutamate-induced cell death in rat hippocampal slices is mediated by the phosphatidylinositol-3 kinase/Akt/ glycogen synthase kinase 3β pathway activation and inducible nitric oxide synthase inhibition

Excitotoxicity and cell death induced by glutamate are involved in many neurodegenerative disorders. We have previously demonstrated that excitotoxicity induced by millimolar concentrations of glutamate in hippocampal slices involves apoptotic features and glutamate-induced glutamate release. Guanosine, an endogenous guanine nucleoside, prevents excitotoxicity by its ability to modulate glutamate transport. In this study, we have evaluated the neuroprotective effect of guanosine against glutamate-induced toxicity in hippocampal slices and the mechanism involved in such an effect. We have found that guanosine (100 μM) was neuroprotective against 1 mM glutamate-induced cell death through the inhibition of glutamate release induced by glutamate. Guanosine also induced the phosphorylation and, thus, activation of protein kinase B (PKB/Akt), a downstream target of phosphatidylinositol-3 kinase (PI3K), as well as phosphorylation of glycogen synthase kinase 3β, which has been reported to be inactivated by Akt after phosphorylation at Ser9. Glutamate treated hippocampal slices showed increased inducible nitric oxide synthase (iNOS) expression that was prevented by guanosine. Slices preincubated with SNAP (an NO donor), inhibited the protective effect of guanosine. LY294002 (30 μM), a PI3K inhibitor, attenuated guanosine-induced neuroprotection, guanosine prevention of glutamate release, and guanosine-induced GSK3β(Ser9) phosphorylation but not guanosine reduction of glutamate-induced iNOS expression. Taken together, the results of this study show that guanosine protects hippocampal slices by a mechanism that involves the PI3K/Akt/GSK3β(Ser9) pathway and prevention of glutamate-induced glutamate release. Furthermore, guanosine also reduces glutamate-induced iNOS by a PI3K/Akt-independent mechanism.

Regulation of glutamate transporter GLT-1 by MAGI-1

The sodium-dependent glutamate transporter, glutamate transporter subtype 1 (GLT-1) is one of the main glutamate transporters in the brain. GLT-1 contains a COOH-terminal sequence similar to one in an isoform of Slo1 K(+) channel protein previously shown to bind MAGI-1 (membrane-associated guanylate kinase with inverted orientation protein-1). MAGI-1 is a scaffold protein which allows the formation of complexes between certain transmembrane proteins, actin-binding proteins, and other regulatory proteins. The glutathione S-transferase pull-down assay demonstrated that MAGI-1 was a binding partner of GLT-1. The interaction between MAGI-1 and GLT-1 was confirmed by co-immunoprecipitation. Immunofluorescence of MAGI-1 and GLT-1 demonstrated that the distribution of MAGI-1 and GLT-1 overlapped in astrocytes. Co-expression of MAGI-1 with GLT-1 in C6 Glioma cells resulted in a significant reduction in the surface expression of GLT-1, as assessed by cell-surface biotinylation. On the other hand, partial knockdown of endogenous MAGI-1 expression by small interfering RNA in differentiated cultured astrocytes increased glutamate uptake and the surface expression of endogenous GLT-1. Knockdown of MAGI-1 increased dihydrokainate-sensitive, Na(+) -dependent glutamate uptake, indicating that MAGI-1 regulates GLT-1 mediated glutamate uptake. These data suggest that MAGI-1 regulates surface expression of GLT-1 and the level of glutamate in the hippocampus.

Guanosine is neuroprotective against oxygen/glucose deprivation in hippocampal slices via large conductance Ca²+-activated K+ channels, phosphatidilinositol-3 kinase/protein kinase B pathway activation and glutamate uptake

Guanine derivatives (GD) have been implicated in many relevant brain extracellular roles, such as modulation of glutamate transmission and neuronal protection against excitotoxic damage. GD are spontaneously released to the extracellular space from cultured astrocytes and during oxygen/glucose deprivation (OGD). The aim of this study has been to evaluate the potassium channels and phosphatidilinositol-3 kinase (PI3K) pathway involvement in the mechanisms related to the neuroprotective role of guanosine in rat hippocampal slices subjected to OGD. The addition of guanosine (100 μM) to hippocampal slices subjected to 15 min of OGD and followed by 2 h of re-oxygenation is neuroprotective. The presence of K+ channel blockers, glibenclamide (20 μM) or apamin (300 nM), revealed that neuroprotective effect of guanosine was not dependent on ATP-sensitive K+ channels or small conductance Ca²+-activated K+ channels. The presence of charybdotoxin (100 nM), a large conductance Ca²+-activated K+ channel (BK) blocker, inhibited the neuroprotective effect of guanosine. Hippocampal slices subjected to OGD and re-oxygenation showed a significant reduction of glutamate uptake. Addition of guanosine in the re-oxygenation period has blocked the reduction of glutamate uptake. This guanosine effect was inhibited when hippocampal slices were pre-incubated with charybdotoxin or wortmanin (a PI3K inhibitor, 1 μM) in the re-oxygenation period. Guanosine promoted an increase in Akt protein phosphorylation. However, the presence of charybdotoxin blocked such effect. In conclusion, the neuroprotective effect of guanosine involves augmentation of glutamate uptake, which is modulated by BK channels and the activation of PI3K pathway. Moreover, neuroprotection caused by guanosine depends on the increased expression of phospho-Akt protein.

Corticosterone decreases the activity of rat glutamate transporter type 3 expressed in Xenopus oocytes

Glucocorticoids can increase the extracellular concentrations of glutamate, the major excitatory neurotransmitter. We investigated the effects of corticosterone on the activity of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1; also called excitatory amino acid transporter type 3 [EAAT3]), and the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in regulating these effects. Rat EAAC1 was expressed in Xenopus oocytes by injecting mRNA. L-Glutamate (30 μM)-induced membrane currents were measured using the two-electrode voltage clamp technique. Exposure of these oocytes to corticosterone (0.01-1 μM) for 72 h decreased EAAC1 activity in a dose-dependent fashion, and this inhibition was incubation time-dependent. Corticosterone (0.01 μM for 72 h) significantly decreased the V(max), but not the K(m), of EAAC1 for glutamate. Furthermore, pretreatment of oocytes with staurosporine, a PKC inhibitor, significantly decreased EAAC1 activity (1.00±0.06 to 0.70±0.05 μC; P<0.05). However, no statistical differences were observed between oocytes treated with staurosporine, corticosterone, or corticosterone plus staurosporine. Similar patterns of responses were achieved by chelerythrine or calphostin C, other PKC inhibitors. Phorbol-12-myristate-13-acetate (PMA), a PKC activator, inhibited corticosterone-induced reduction in EAAC1 activity. Pretreating oocytes with wortmannin or LY294002, PI3K inhibitors, also significantly reduced EAAC1 activity, but no difference was observed between oocytes treated with wortmannin, corticosterone, or wortmannin plus corticosterone. The above results suggest that corticosterone exposure reduces EAAC1 activity and this effect is PKC- and PI3K-dependent.

Muscarinic receptor stimulation of D-aspartate uptake into human SH-SY5Y neuroblastoma cells is attenuated by hypoosmolarity

In addition to its function as an excitatory neurotransmitter, glutamate plays a major role as an osmolyte within the central nervous system (CNS). Accordingly, mechanisms that regulate glutamate release and uptake are of physiological importance not only during conditions in which cell volume remains constant but also when cells are subjected to hypoosmotic stress. In the present study, the ability of muscarinic cholinergic receptors (mAChRs) to regulate the uptake of glutamate (monitored as D-aspartate) into human SH-SY5Y neuroblastoma cells under isotonic or hypotonic conditions has been examined. In isotonic media, agonist activation of mAChRs resulted in a significant increase (250-300% of control) in the uptake of D-aspartate and, concurrently, a cellular redistribution of the excitatory amino acid transporter 3 (EAAT3) to the plasma membrane. mAChR-mediated increases in d-aspartate uptake were potently blocked by the EAAT3 inhibitor l-beta-threo-benzyl-aspartate. In hypotonic media, the ability of mAChR activation to facilitate D-aspartate uptake was significantly attenuated (40-50%), and the cellular distribution of EAAT3 was disrupted. Reduction of mAChR-stimulated D-aspartate uptake under hypoosmotic conditions could be fully reversed upon re-exposure of the cells to isotonic media. Under both isotonic and hypotonic conditions, mAChR-mediated increases in D-aspartate uptake depended on cytoskeletal integrity, protein kinase C and phosphatidylinositol 3-kinase activities, and the availability of intracellular Ca2+. In contrast, dependence on extracellular Ca2+ was observed only under isotonic conditions. The results suggest that, although the uptake of D-aspartate into SH-SY5Y cells is enhanced after mAChR activation, this process is markedly attenuated by hypoosmolarity.

Evaluation of Factors Associated With Elevated Levels of Platelet-Derived Microparticles in the Acute Phase of Cerebral Infarction

Background: Platelet-derived microparticles (PDMPs) have attracted attention as blood coagulation-promoting, endothelial cell-activating factors. The objective of this study was to determine the parameters associated with elevated PDMP levels and examine their relationship with atherosclerotic lesions of main intracranial and extracranial arteries. Participants and Methods: Participants included a control group (C) of 61 patients with no apparent cerebral vascular lesions and 110 patients with acute-phase cerebral infarction, consisting of a small-vessel occlusion group (S) of 34 patients, a large-artery atherosclerosis group (L) of 41 patients, a cardioembolism group (CE) of 20 patients, and a stroke of undetermined etiology group (U) of 15 patients. Platelet-derived microparticle levels were measured using enzyme-linked immunosorbent assay (ELISA) at the time of admission, and the patients were reclassified into group CP (control level PDMPs), consisting of 70 patients with control PDMP levels, and group HP (high PDMPs), consisting of 40 patients with elevated PDMP levels. All patients underwent cranial magnetic resonance (MR) and carotid ultrasound examinations. Results: Platelet-derived microparticle levels were significantly higher in groups S and L than in group C (P < .01). Concomitant intima-media thickness (IMT; odds ratio [OR] = 1.29, P < .05) and concomitant intracranial stenosis (OR = 3.95, P < .01) were significantly correlated with elevated PDMP levels. Fibrinogen and high-sensitivity CRP levels were significantly higher in group HP than in group CP. Conclusion: Alterations in PDMP levels correlated with the presence of atherothrombotic lesions, and PDMP levels are expected to be useful as a clinical indicator, reflecting the presence of intracranial atherosclerotic lesions in the acute phase of cerebral infarction.

Atorvastatin prevents hippocampal cell death due to quinolinic acid-induced seizures in mice by increasing Akt phosphorylation and glutamate uptake

Statins are cholesterol-lowering agents due to the inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Recent studies have shown statins possess pleiotropic effects, which appear to be independent from its cholesterol-lowering action. In this study, we investigated whether atorvastatin would have protective effects against hippocampal cell death promoted by quinolinic acid (QA)-induced seizures in mice. Mice were pretreated with Atorvastatin (1 or 10 mg/kg) or vehicle (saline, 0.9%), orally, once a day for 7 days before the intracerebroventricular (i.c.v.) QA infusion (36.8 nmol/site). Atorvastatin treatment with 1 mg/kg/day did not significantly prevent QA-induced seizures (13.34%). However, administration of atorvastatin 10 mg/kg/day prevented the clonic and/or tonic seizures induced by QA in 29.41% of the mice. Additionally, administration of atorvastatin 10 mg/kg/day significantly prevented QA-induced cell death in the hippocampus. Atorvastatin treatment promoted an increased Akt phosphorylation, which was sustained after QA infusion in both convulsed and non-convulsed mice. Moreover, atorvastatin pretreatment prevented the reduction in glutamate uptake into hippocampal slices induced by QA i.c.v. infusion. These results show that atorvastatin attenuated QA-induced hippocampal cellular death involving the Akt pathway and glutamate transport modulation. Therefore, atorvastatin treatment might be a useful strategy in the prevention of brain injury caused by the exacerbation of glutamatergic toxicity in neurological diseases such as epilepsy.

Ubiquitination-mediated internalization and degradation of the astroglial glutamate transporter, GLT-1

Sodium-dependent glutamate uptake is essential for limiting excitotoxicity, and dysregulation of this process has been implicated in a wide array of neurological disorders. The majority of forebrain glutamate uptake is mediated by the astroglial glutamate transporter, GLT-1. We and others have shown that this transporter undergoes endocytosis and degradation in response to activation of protein kinase C (PKC), however, the mechanisms involved remain unclear. In the current study, transfected C6 glioma cells or primary cortical cultures were used to show that PKC activation results in incorporation of ubiquitin into GLT-1 immunoprecipitates. Mutation of all 11 lysine residues in the amino and carboxyl-terminal domains to arginine (11R) abolished this signal. Selective mutation of the seven lysine residues in the carboxyl terminus (C7K-R) did not eliminate ubiquitination, but it completely blocked PKC-dependent internalization and degradation. Two families of variants of GLT-1 were prepared with various lysine residues mutated to arginine. Analyses of these constructs indicated that redundant lysine residues in the carboxyl terminus were sufficient for the appearance of ubiquitinated product and degradation of GLT-1. Together these data define a novel mechanism by which the predominant forebrain glutamate transporter can be rapidly targeted for degradation.

Morphine induces ubiquitin-proteasome activity and glutamate transporter degradation

Glutamate transporters play a crucial role in physiological glutamate homeostasis, neurotoxicity, and glutamatergic regulation of opioid tolerance. However, how the glutamate transporter turnover is regulated remains poorly understood. Here we show that chronic morphine exposure induced posttranscriptional down-regulation of the glutamate transporter EAAC1 in C6 glioma cells with a concurrent decrease in glutamate uptake and increase in proteasome activity, which were blocked by the selective proteasome inhibitor MG-132 or lactacystin but not the lysosomal inhibitor chloroquin. At the cellular level, chronic morphine induced the PTEN (phosphatase and tensin homolog deleted on chromosome Ten)-mediated up-regulation of the ubiquitin E3 ligase Nedd4 via cAMP/protein kinase A signaling, leading to EAAC1 ubiquitination and proteasomal degradation. Either Nedd4 or PTEN knockdown with small interfering RNA prevented the morphine-induced EAAC1 degradation and decreased glutamate uptake. These data indicate that cAMP/protein kinase A signaling serves as an intracellular regulator upstream to the activation of the PTEN/Nedd4-mediated ubiquitin-proteasome system activity that is critical for glutamate transporter turnover. Under an in vivo condition, chronic morphine exposure also induced posttranscriptional down-regulation of the glutamate transporter EAAC1, which was prevented by MG-132, and transcriptional up-regulation of PTEN and Nedd4 within the spinal cord dorsal horn. Thus, inhibition of the ubiquitin-proteasome-mediated glutamate transporter degradation may be an important mechanism for preventing glutamate overexcitation and may offer a new strategy for treating certain neurological disorders and improving opioid therapy in chronic pain management.

Glutamate-induced glioma cell proliferation is prevented by functional expression of the glutamate transporter GLT-1

A tetracycline-dependent inducible system was used to achieve controlled expression of the glutamate transporter 1 (GLT-1) in C6 glioma cells. Non-induced cells show modest glutamate uptake and, in the presence of L-cystine, these cells tend to release substantial amounts of glutamate. Overnight exposure to doxycycline increased D-[3H]-aspartate uptake, reaching similar capacity as observed in cultured astrocytes. Efficient clearance of exogenously applied glutamate was evidenced in these cells, even in the presence of l-cystine. The addition of glutamate (100 microM) to the medium of non-induced cells significantly increased their proliferation rate, an effect that was blocked when the expression of GLT-1 was induced. This suggests that impaired glutamate uptake capacity in glioma cells indirectly contributes to their proliferation.

Ethanol increases the activity of rat excitatory amino acid transporter type 4 expressed in Xenopus oocytes: role of protein kinase C and phosphatidylinositol 3-kinase

BACKGROUND

Glutamate is the major excitatory neurotransmitter in the central nervous system and is critical for essentially all physiological processes, such as learning, memory, central pain transduction, and control of motor function. Excitatory amino acid transporters (EAATs) play a key role in regulating glutamate neurotransmission by uptake of glutamate into cells. EAAT4 is the major EAAT in the cerebellar Purkinje cells. The authors investigated the effects of ethanol on EAAT4 and the mediatory effects of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in this context.

METHODS

Excitatory amino acid transporter 4 was expressed in Xenopus oocytes by injecting EAAT4 mRNA. l-aspartate-induced membrane currents were measured using a two-electrode voltage clamp. Responses were quantified by integrating current traces and are represented in microCoulombs (microC).

RESULTS

Ethanol increased EAAT4 activity in a dose-dependent manner. At ethanol concentrations of 25, 50, 100, and 200 mM, the responses were significantly higher than untreated control values. Ethanol (25 mM) significantly increased the V(max) (1.5 +/- 0.1 microC for control vs. 2.0 +/- 0.1 microC for ethanol, p < 0.05), but did not affect K(m) (2.3 +/- 0.6 microM for control vs. 1.7 +/- 0.7 microM for ethanol, p > 0.05) of EAAT4 for l-aspartate. Preincubation of oocytes with phorbol-12-myristate-13-acetate (PMA, a PKC activator) significantly increased EAAT4 activity. However, combinations of PMA and ethanol versus PMA or ethanol alone did not increase responses further. Two PKC inhibitors, chelerythrine and staurosporine did not reduce basal EAAT4 activity but abolished ethanol-enhanced EAAT4 activity. Pretreatment with wortmannin (a PI3K inhibitor) also abolished ethanol-enhanced EAAT4 activity.

CONCLUSIONS

These results demonstrate that acute ethanol exposure increases EAAT4 activity at clinically relevant concentrations and that PKC and PI3K may mediate this. The effects of ethanol on EAAT4 may play a role in the cerebellar dysfunction caused by ethanol intoxication.

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.

Caveolin-1 regulates the delivery and endocytosis of the glutamate transporter, excitatory amino acid carrier 1

The sodium-dependent glutamate transporter, excitatory amino acid carrier 1 (EAAC1), has been implicated in the regulation of excitatory signaling and prevention of cell death in the nervous system. There is evidence that EAAC1 constitutively cycles on and off the plasma membrane and that under steady state conditions up to 80% of the transporter is intracellular. As is observed with other neurotransmitter transporters, the activity of EAAC1 is regulated by a variety of molecules, and some of these effects are associated with redistribution of EAAC1 on and off the plasma membrane. In the present study we tested the hypothesis that a structural component of lipid rafts, caveolin-1 (Cav-1), may participate in EAAC1 trafficking. Using C6 glioma cells as a model system, co-expression of Cav-1 S80E (a dominant-negative variant) or small interfering RNA-mediated knock-down of caveolin-1 reduced cell surface expression of myc epitope-tagged EAAC1 or endogenous EAAC1, respectively. Cav-1 S80E slowed the constitutive delivery and endocytosis of myc-EAAC1. In primary cultures derived from caveolin-1 knock-out mice, a similar reduction in delivery and internalization of endogenous EAAC1 was observed. We also found that caveolin-1, caveolin-2, or Cav-1 S80E formed immunoprecipitable complexes with EAAC1 in C6 glioma and/or transfected HEK cells. Together, these data provide strong evidence that caveolin-1 contributes to the trafficking of EAAC1 on and off the plasma membrane and that these effects are associated with formation of EAAC1-caveolin complexes.

Proteomic analyses of retina of excitatory amino acid carrier 1 deficient mice

BACKGROUND

Excitatory amino acid carrier 1 (EAAC1) is a glutamate transporter found in neuronal tissues and is extensively expressed in the retina. EAAC1 plays a role in a variety of neural functions, but its biological functions in the retina has not been fully determined. The purpose of this study was to identify proteins regulated by EAAC1 in the retina of mice. To accomplish this, we used a proteomics-based approach to identify proteins that are up- or down-regulated in EAAC1-deficient (EAAC1-/-) mice.

RESULTS

Proteomic analyses and two-dimensional gel electorphoresis were performed on the retina of EAAC1-/- mice, and the results were compared to that of wild type mice. The protein spots showing significant differences were selected for identification by mass spectrometric analyses. Thirteen proteins were differentially expressed; nine proteins were up-regulated and five proteins were down-regulated in EAAC1-/- retina. Functional clustering showed that identified proteins are involved in various cellular process, e.g. cell cycle, cell death, transport and metabolism.

CONCLUSION

We identified thirteen proteins whose expression is changed in EAAC-/- mice retinas. These proteins are known to regulate cell proliferation, death, transport, metabolism, cell organization and extracellular matrix.

Fyn-mediated phosphorylation of NR2B Tyr-1336 controls calpain-mediated NR2B cleavage in neurons and heterologous systems

Cleavage of the intracellular carboxyl terminus of the N-methyl-d-aspartate (NMDA) receptor 2 subunit (NR2) by calpain regulates NMDA receptor function and localization. Here, we show that Fyn-mediated phosphorylation of NR2B controls calpain-mediated NR2B cleavage. In cultured neurons, calpain-mediated NR2B cleavage is significantly attenuated by blocking NR2B phosphorylation of Tyr-1336, but not Tyr-1472, via inhibition of Src family kinase activity or decreasing Fyn levels by small interfering RNA. In HEK cells, mutation of Tyr-1336 eliminates the potentiating effect of Fyn on calpain-mediated NR2B cleavage. The potentiation of NR2B cleavage by Fyn is limited to cell surface receptors and is associated with calpain translocation to plasma membranes during NMDA receptor activation. Finally, reducing full-length NR2B by calpain does not decrease extrasynaptic NMDA receptor function, and truncated NR1/2B receptors similar to those generated by calpain have electrophysiological properties matching those of wild-type receptors. Thus, the Fyn-controlled regulation of NMDA receptor cleavage by calpain may play critical roles in controlling NMDA receptor properties during synaptic plasticity and excitotoxicity.

N-methyl-D-aspartate receptor-dependent regulation of the glutamate transporter excitatory amino acid carrier 1

The neuronal transporter excitatory amino acid carrier 1 (EAAC1) is enriched in perisynaptic regions, where it may regulate synaptic spillover of glutamate. In this study we examined potential interactions between EAAC1 and ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA) receptor subunits NR1, NR2A, and NR2B, but not the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit GluR2, were co-immunoprecipitated with EAAC1 from neuron-enriched hippocampal cultures. A similar interaction was observed in C6 glioma and human embryonic kidney cells after co-transfection with Myc epitope-tagged EAAC1 and NMDA receptor subunits. Co-transfection of C6 glioma with the combination of NR1 and NR2 subunits dramatically increased (approximately 3-fold) the amount of Myc-EAAC1 that can be labeled with a membrane-impermeable biotinylating reagent. In hippocampal cultures, brief (5 min), robust (100 microM NMDA, 10 microM glycine) activation of the NMDA receptor decreased biotinylated EAAC1 to approximately 50% of control levels. This effect was inhibited by an NMDA receptor antagonist, intracellular or extracellular calcium chelators, or hypertonic sucrose. Glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid with cyclothiazide, and thapsigargin mimicked the effects of NMDA. These studies suggest that NMDA receptors interact with EAAC1, facilitate cell surface expression of EAAC1 under basal conditions, and control internalization of EAAC1 upon activation. This NMDA receptor-dependent regulation of EAAC1 provides a novel mechanism that may shape excitatory signaling during synaptic plasticity and/or excitotoxicity.

EAAT4 phosphorylation at the SGK1 consensus site is required for transport modulation by the kinase

EAAT4 (SLC1A6) is a Purkinje-Cell-specific post-synaptic excitatory amino acid transporter that plays a major role in clearing synaptic glutamate. EAAT4 abundance and function is known to be modulated by the serum and glucocorticoid inducible kinase (SGK) 1 but the precise mechanism of kinase action has not been defined yet. The present work aims to identify the molecular mechanism of EAAT4 modulation by the kinase. The EAAT4 sequence bears two putative SGK1 consensus sites (at Thr40 and Thr504) at the amino and carboxy terminus that are conserved among species. Expression studies in Xenopus oocytes demonstrated that EAAT4-mediated [(3)H] glutamate uptake and cell surface abundance are enhanced by co-expression of SGK1. Disruption of the SGK1 phosphorylation site at threonine 40 ((T40A)EAAT4) or of both phosphorylation sites ((T40AT504A)EAAT4) abrogated the effect of SGK1 on transporter function and expression. SGK1 modulates several transport proteins via inhibition of the ubiquitin ligase Nedd4-2. Co-expression of Nedd4-2 inhibited wild-type EAAT4 but not the (T40AT504A)EAAT4 mutant. Besides, RNA interference-mediated reduction of endogenous Nedd4-2 (xNedd4-2) expression increased the activity of the transporter. In conclusion, maximal glutamate transport modulation by SGK1 is accomplished by direct EAAT4 stimulation and to a lesser extent by inhibition of intrinsic Nedd4-2.

Acute regulation of sodium-dependent glutamate transporters: a focus on constitutive and regulated trafficking

The acidic amino acid glutamate activates a family of ligand-gated ion channels to mediate depolarization that can be as short-lived as a few milliseconds and activates a family of G protein-coupled receptors that couple to both ion channels and other second messenger pathways. Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and is required for essentially all motor, sensory, and cognitive functions. In addition, glutamate-mediated signaling is required for development and the synaptic plasticity thought to underlie memory formation and retrieval. The levels of glutamate in brain approach 10 mmol/kg and most cells in the CNS express at least one of the receptor subtypes. Unlike acetylcholine that mediates "rapid" excitatory neurotransmission at the neuromuscular junction, there is no evidence for extracellular inactivation of glutamate. Instead, glutamate is cleared by a family of Na(+)-dependent transport systems that are found on glial processes that sheath the synapse and found on the pre- and postsynaptic elements of neurons. These transporters ensure crisp excitatory transmission by maintaining synaptic concentrations below those required for tonic activation of glutamate receptors under baseline conditions (approximately 1 microM) and serve to limit activation of glutamate receptors after release. During the past few years, it has become clear that like many of the other neurotransmitter transporters discussed in this volume of Handbook of Experimental Pharmacology, the activity of these transporters can be rapidly regulated by a variety of effectors. In this chapter, a broad overview of excitatory signaling will be followed by a brief introduction to the family of Na(+)-dependent glutamate transporters and a detailed discussion of our current understanding of the mechanisms that control transporter activity. The focus will be on our current understanding of the mechanisms that could regulate transporter activity within minutes, implying that this regulation is independent of transcriptional or translational control mechanisms. The glutamate transporters found in forebrain are regulated by redistributing the proteins to or from the plasma membrane; the signals involved and the net effects on transporter activity are being defined. In addition, there is evidence to suggest that the intrinsic activity of these transporters is also regulated by mechanisms that are independent of transporter redistribution; less is known about these events. As this field progresses, it should be possible to determine how this regulation affects physiologic and pathologic events in the CNS.

The neuronal excitatory amino acid transporter EAAC1/EAAT3: does it represent a major actor at the brain excitatory synapse?

EAAC1/EAAT3 is a transporter of glutamate (Glu) present at the post-synaptic neuronal element, in opposition to the two other main transporters, GLAST/EAAT1 and GLT1/EAAT2, expressed at the excitatory amino acid (EAA) synapse by surrounding astrocytes. Although, in the adult, EAAC1/EAAT3 exhibits a rather low expression level and is considered to make a minor contribution to Glu removal from the synapse, its early expression during brain development, before the astrocytes are functional, suggests that such a neuronal transporter is involved in the developmental effects of EAA and, possibly, in the biosynthesis and trophic role of GABA, which is excitatory in nature in different brain regions during the earlier stages of brain development. This neuronal Glu transporter is considered to have a dual action as it is apparently involved in the neuronal uptake of cysteine, which acts as a key substrate for the synthesis of glutathione, a major anti-oxidant, because the neurones do not express the Xc(-) transport system in the mature brain. Interestingly, EAAC1/EAAT3 activity/expression was shown to be highly regulated by neuronal activity as well as by intracellular signalling pathways involving primarily alpha protein kinase C (alphaPKC) and phosphatidylinositol-3-kinase (PI3K). Such regulatory processes could act either at the post-traductional level or at the transcriptional level. It is worth noting that EAAC1/EAAT3 exhibits specificity, compared with other EAA transporters, because it is present mainly in the intracellular compartment and only for about 20% at the plasma membrane. Variations in neuronal Glu uptake were shown to be associated with rapid changes in the trafficking of the transporter protein altering the membranar location of the transporter. More recent data show that astrocyte-secreted factors such as cholesterol could also influence rapid changes in the location of EAAC1/EAAT3 between the plasma membrane and the cytoplasmic compartment. Such a highly regulated process of EAAC1/EAAT3 activity/expression may have implications in the physiopathology of major diseases affecting EAA brain signalling, which is further supported by data obtained in animal models of hypoxia-anoxia, for example.

The transcription factor regulatory factor X1 increases the expression of neuronal glutamate transporter type 3

Glutamate transporters (excitatory amino acid transporters, EAAT) play an important role in maintaining extracellular glutamate homeostasis and regulating glutamate neurotransmission. However, very few studies have investigated the regulation of EAAT expression. A binding sequence for the regulatory factor X1 (RFX1) exists in the promoter region of the gene encoding for EAAT3, a neuronal EAAT, but not in the promoter regions of the genes encoding for EAAT1 and EAAT2, two glial EAATs. RFX proteins are transcription factors binding to X-boxes of DNA sequences. Although RFX proteins are necessary for the normal function of sensory neurons in Caenorhabditis elegans, their roles in the mammalian brain are not known. We showed that RFX1 increased EAAT3 expression and activity in C6 glioma cells. RFX1 binding complexes were found in the nuclear extracts of C6 cells. The activity of EAAT3 promoter as measured by luciferase reporter activity was increased by RFX1 in C6 cells and the neuron-like SH-SY5Y cells. However, RFX1 did not change the expression of EAAT2 proteins in the NRK52E cells. RFX1 proteins were expressed in the neurons of rat brain. A high expression level of RFX1 proteins was found in the neurons of cerebral cortex and Purkinje cells. Knockdown of the RFX1 expression by RFX1 antisense oligonucleotides decreased EAAT3 expression in rat cortical neurons in culture. These results suggest that RFX1 enhances the activity of EAAT3 promoter to increase the expression of EAAT3 proteins. This study provides initial evidence for the regulation of gene expression in the nervous cells by RFX1.