Regulation of glutamate transporters in health and disease

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.

New progress of isoflurane, sevoflurane and propofol in hypoxic-ischemic brain injury and related molecular mechanisms based on p75 neurotrophic factor receptor

Hypoxic ischemic brain injury (HIBI) is one of the most common clinical disorders, especially in neonates. The complex pathophysiology of HIBI is an important cause of disability and even death of patients, however, being without effective clinical treatments. Common anesthetics (such as isoflurane, propofol and sevoflurane) have an adverse impact on neuronal cells for HIBI via the regulation of p75 neurotrophic factor receptor (P75NTR). In order to protect the injured brains and study the effect of underlying treatments, it is particularly significant to understand and master the developmental mechanism of anesthetics for the occurrence of HIBI related molecular mechanisms. Therefore, this paper will mainly review the corresponding pathogenic and protective mechanisms about HIBI binding to the research progress of the role of P75NTR. In conclusion, the effects of neuroprotection and injured nerves are involved in the expression and activation of P75NTR, mainly increased P75NTR mRNA, protein levels and calpain-dependent for propofol, and inducing neuronal apoptosis for isoflurane and sevoflurane, and we look forward to that connection with P75NTR, common anaesthetic and HIBI may be a new direction of research and gain perfect outcomes in the future.

Early Postnatal Exposure to a Low Dose of Nanoparticulate Silver Induces Alterations in Glutamate Transporters in Brain of Immature Rats

Due to strong antimicrobial properties, silver nanoparticles (AgNPs) are used in a wide range of medical and consumer products, including those dedicated for infants and children. While AgNPs are known to exert neurotoxic effects, current knowledge concerning their impact on the developing brain is scarce. During investigations of mechanisms of neurotoxicity in immature rats, we studied the influence of AgNPs on glutamate transporter systems which are involved in regulation of extracellular concentration of glutamate, an excitotoxic amino acid, and compared it with positive control—Ag citrate. We identified significant deposition of AgNPs in brain tissue of exposed rats over the post-exposure time. Ultrastructural alterations in endoplasmic reticulum (ER) and Golgi complexes were observed in neurons of AgNP-exposed rats, which are characteristics of ER stress. These changes presumably underlie substantial long-lasting downregulation of neuronal glutamate transporter EAAC1, which was noted in AgNP-exposed rats. Conversely, the expression of astroglial glutamate transporters GLT-1 and GLAST was not affected by exposure to AgNPs, but the activity of the transporters was diminished. These results indicate that even low doses of AgNPs administered during an early stage of life create a substantial risk for health of immature organisms. Hence, the safety of AgNP-containing products for infants and children should be carefully considered.

Amitriptyline influences the mechanical withdrawal threshold in bone cancer pain rats by regulating glutamate transporter GLAST

Patients with cancer, especially breast, prostate, and lung cancer, commonly experience bone metastases that are difficult to manage and are associated with bone cancer pain. Amitriptyline is often used to treat chronic pain, such as neuropathic pain. In this study, the effects of amitriptyline on the mechanical withdrawal threshold and its underlying mechanisms were evaluated in rat models of bone cancer pain. Walker 256 rat mammary gland carcinoma cells were injected into the bone marrow cavity of the right tibia of rats to provoke bone cancer pain. Then, amitriptyline was intraperitoneally administered twice daily from fifth day after the operation. Rats with bone cancer showed an apparent decline in the mechanical withdrawal threshold at day 11 after Walker 256 cells inoculation. The levels of the glutamate-aspartate transporter in the spinal cord dorsal horn decreased remarkably, and the concentration of the excitatory amino acid glutamate in the cerebrospinal fluid increased substantially. Amitriptyline injection could prevent the decline of mechanical withdrawal threshold in bone cancer pain rats. In addition, glutamate-aspartate transporter was upregulated on the glial cell surface, and glutamate levels were reduced in the cerebrospinal fluid. However, amitriptyline injection could not prevent the bone cancer pain-induced reduction in glutamate-aspartate transporter in the glial cell cytosol, it further downregulated cytosolic glutamate-aspartate transporter. Amitriptyline had no significant effect on GLAST messenger RNA expression, and bone cancer pain-invoked protein kinase A/protein kinase C upregulation was prevented. Taken together, these results suggest that the intraperitoneal injection of amitriptyline can prevent the decrease of mechanical withdrawal threshold in bone cancer pain rats, the underlying mechanisms may be associated with the inhibition of protein kinase A/protein kinase C expression, thus promoting glutamate-aspartate transporter trafficking onto the glial cell surface and reducing excitatory amino acid concentrations in the cerebrospinal fluid.

The Glycogen Synthase Kinase-3 in the Regulation of Ion Channels and Cellular Carriers

Glycogen synthase kinase-3 (GSK-3) is a highly evolutionarily conserved and ubiquitously expressed serine/threonine kinase, an enzyme protein profoundly specific for glycogen synthase (GS). GSK-3 is involved in various cellular functions and physiological processes, including cell proliferation, differentiation, motility, and survival as well as glycogen metabolism, protein synthesis, and apoptosis. There are two isoforms of human GSK-3 (named GSK-3α and GSK-3β) encoded by two distinct genes. Recently, GSK-3β has been reported to function as a powerful regulator of various transport processes across the cell membrane. This kinase, GSK-3β, either directly or indirectly, may stimulate or inhibit many different types of transporter proteins, including ion channel and cellular carriers. More specifically, GSK-3β-sensitive cellular transport regulation involves various calcium, chloride, sodium, and potassium ion channels, as well as a number of Na+-coupled cellular carriers including excitatory amino acid transporters EAAT2, 3 and 4, high-affinity Na+ coupled glucose carriers SGLT1, creatine transporter 1 CreaT1, and the type II sodium/phosphate cotransporter NaPi-IIa. The GSK-3β-dependent cellular transport regulations are a part of the kinase functions in numerous physiological and pathophysiological processes. Clearly, additional studies are required to examine the role of GSK-3β in many other types of cellular transporters as well as further elucidating the underlying mechanisms of GSK-3β-mediated cellular transport regulation.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as 'glutamosome'. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and β-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Hepatic glutamate transport and glutamine synthesis capacities are decreased in finished vs. growing beef steers, concomitant with increased GTRAP3-18 content

Hepatic glutamate uptake and conversion to glutamine is critical for whole-body N metabolism, but how this process is regulated during growth is poorly described. The hepatic glutamate uptake activities, protein content of system [Formula: see text] transporters (EAAC1, GLT-1) and regulatory proteins (GTRAP3-18, ARL6IP1), glutamine synthetase (GS) activity and content, and glutathione (GSH) content, were compared in liver tissue of weaned Angus steers randomly assigned (n = 8) to predominantly lean (growing) or predominantly lipid (finished) growth regimens. Steers were fed a cotton seed hull-based diet to achieve final body weights of 301 or 576 kg, respectively, at a constant rate of growth. Liver tissue was collected at slaughter and hepatic membranes fractionated. Total (75%), Na+-dependent (90%), system [Formula: see text]-dependent (abolished) glutamate uptake activity, and EAAC1 content (36%) in canalicular membrane-enriched vesicles decreased as steers developed from growing (n = 6) to finished (n = 4) stages, whereas Na+-independent uptake did not change. In basolateral membrane-enriched vesicles, total (60%), Na+-dependent (60%), and Na+-independent (56%) activities decreased, whereas neither system [Formula: see text]-dependent uptake nor protein content changed. EAAC1 protein content in liver homogenates (n = 8) decreased in finished vs. growing steers, whereas GTRAP3-18 and ARL6IP1 content increased and GLT-1 content did not change. Concomitantly, hepatic GS activity decreased (32%) as steers fattened, whereas GS and GSH contents did not differ. We conclude that hepatic glutamate uptake and GS synthesis capacities are reduced in livers of finished versus growing beef steers, and that hepatic system [Formula: see text] transporter activity/EAAC1 content is inversely proportional to GTRAP3-18 content.

The Selective Antagonism of Adenosine A2B Receptors Reduces the Synaptic Failure and Neuronal Death Induced by Oxygen and Glucose Deprivation in Rat CA1 Hippocampus in Vitro

Ischemia is a multifactorial pathology characterized by different events evolving in time. Immediately after the ischemic insult, primary brain damage is due to the massive increase of extracellular glutamate. Adenosine in the brain increases dramatically during ischemia in concentrations able to stimulate all its receptors, A₁, A_2A, A_2B, and A₃. Although adenosine exerts clear neuroprotective effects through A₁ receptors during ischemia, the use of selective A₁ receptor agonists is hampered by their undesirable peripheral side effects. So far, no evidence is available on the involvement of adenosine A_2B receptors in cerebral ischemia. This study explored the role of adenosine A_2B receptors on synaptic and cellular responses during oxygen and glucose deprivation (OGD) in the CA1 region of rat hippocampus in vitro. We conducted extracellular recordings of CA1 field excitatory post-synaptic potentials (fEPSPs); the extent of damage on neurons and glia was assessed by immunohistochemistry. Seven min OGD induced anoxic depolarization (AD) in all hippocampal slices tested and completely abolished fEPSPs that did not recover after return to normoxic condition. Seven minutes OGD was applied in the presence of the selective adenosine A_2B receptor antagonists MRS1754 (500 nM) or PSB603 (50 nM), separately administered 15 min before, during and 5 min after OGD. Both antagonists were able to prevent or delay the appearance of AD and to modify synaptic responses after OGD, allowing significant recovery of neurotransmission. Adenosine A_2B receptor antagonism also counteracted the reduction of neuronal density in CA1 stratum pyramidale, decreased apoptosis at least up to 3 h after the end of OGD, and maintained activated mTOR levels similar to those of controls, thus sparing neurons from the degenerative effects caused by the simil-ischemic conditions. Astrocytes significantly proliferated in CA1 stratum radiatum already 3 h after the end of OGD, possibly due to increased glutamate release. A_2Breceptor antagonism significantly prevented astrocyte modifications. Both A_2B receptor antagonists did not protect CA1 neurons from the neurodegeneration induced by glutamate application, indicating that the antagonistic effect is upstream of glutamate release. The selective antagonists of the adenosine A_2B receptor subtype may thus represent a new class of neuroprotective drugs in ischemia.

The β-lactam clavulanic acid mediates glutamate transport-sensitive pain relief in a rat model of neuropathic pain

BACKGROUND

Following nerve injury, down-regulation of astroglial glutamate transporters (GluTs) with subsequent extracellular glutamate accumulation is a key factor contributing to hyperexcitability within the spinal dorsal horn. Some β-lactam antibiotics can up-regulate GluTs, one of which, ceftriaxone, displays analgesic effects in rodent chronic pain models.

METHODS

Here, the antinociceptive actions of another β-lactam clavulanic acid, which possesses negligible antibiotic activity, were compared with ceftriaxone in rats with chronic constriction injury (CCI)-induced neuropathic pain. In addition, the protein expression of glutamate transporter-1 (GLT1), its splice variant GLT1b and glutamate-aspartate transporter (GLAST) was measured in the spinal cord of CCI rats. Finally, protein expression of the same GluTs was evaluated in cultured astrocytes obtained from rodents and humans.

RESULTS

Repeated injection of ceftriaxone or clavulanic acid over 10 days alleviated CCI-induced mechanical hypersensitivity, whilst clavulanic acid was additionally able to affect the thermal hypersensitivity. In addition, clavulanic acid up-regulated expression of GLT1b within the spinal cord of CCI rats, whereas ceftriaxone failed to modulate expression of any GluTs in this model. However, both clavulanic acid and ceftriaxone up-regulated GLT1 expression in rat cortical and human spinal astrocyte cultures. Furthermore, clavulanic acid increased expression of GLT1b and GLAST in rat astrocytes in a dose-dependent manner.

CONCLUSIONS

Thus, clavulanic acid up-regulates GluTs in cultured rodent- and human astroglia and alleviates CCI-induced hypersensitivity, most likely through up-regulation of GLT1b in spinal dorsal horn.

SIGNIFICANCE

Chronic dosing of clavulanic acid alleviates neuropathic pain in rats and up-regulates glutamate transporters both in vitro and in vivo. Crucially, a similar up-regulation of glutamate transporters in human spinal astrocytes by clavulanic acid supports the development of novel β-lactam-based analgesics, devoid of antibacterial activity, for the clinical treatment of chronic pain.

Neurons and neuronal activity control gene expression in astrocytes to regulate their development and metabolism

The influence that neurons exert on astrocytic function is poorly understood. To investigate this, we first developed a system combining cortical neurons and astrocytes from closely related species, followed by RNA-seq and in silico species separation. This approach uncovers a wide programme of neuron-induced astrocytic gene expression, involving Notch signalling, which drives and maintains astrocytic maturity and neurotransmitter uptake function, is conserved in human development, and is disrupted by neurodegeneration. Separately, hundreds of astrocytic genes are acutely regulated by synaptic activity via mechanisms involving cAMP/PKA-dependent CREB activation. This includes the coordinated activity-dependent upregulation of major astrocytic components of the astrocyte-neuron lactate shuttle, leading to a CREB-dependent increase in astrocytic glucose metabolism and elevated lactate export. Moreover, the groups of astrocytic genes induced by neurons or neuronal activity both show age-dependent decline in humans. Thus, neurons and neuronal activity regulate the astrocytic transcriptome with the potential to shape astrocyte-neuron metabolic cooperation.

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.

Molecular and cellular physiology of sodium-dependent glutamate transporters

Glutamate is the major excitatory transmitter in the vertebrate brain. After its release from presynaptic nerve terminals, it is rapidly taken up by high-affinity sodium-dependent plasma membrane transporters. While both neurons and glial cells express these excitatory amino acid transporters (EAATs), the majority of glutamate uptake is accomplished by astrocytes, which convert synaptically-released glutamate to glutamine or feed it into their own metabolism. Glutamate uptake by astrocytes not only shapes synaptic transmission by regulating the availability of glutamate to postsynaptic neuronal receptors, but also protects neurons from hyper-excitability and subsequent excitotoxic damage. In the present review, we provide an overview of the molecular and cellular characteristics of sodium-dependent glutamate transporters and their associated anion permeation pathways, with a focus on astrocytic glutamate transport. We summarize their functional properties and roles within tripartite synapses under physiological and pathophysiological conditions, exemplifying the intricate interactions and interrelationships between neurons and glial cells in the brain.

Docosahexaenoic acid (DHA) prevents corticosterone-induced changes in astrocyte morphology and function

The many functions of astrocytes, such as glutamate recycling and morphological plasticity, enable them to stabilize synapses environment and protect neurons. Little is known about how they adapt to glucocorticoid-induced stress, and even less about the influence of dietary factors. We previously showed that omega-3 polyunsaturated fatty acids (ω3PUFA), dietary fats which alleviate stress responses, influence the way astroglia regulate glutamatergic synapses. We have explored the role of docosahexaenoic acid (DHA), the main ω3PUFA, in the astroglial responses to corticosterone, the main stress hormone in rodents to determine whether ω3PUFA help astrocytes resist stress. Cultured rat astrocytes were enriched in DHA or arachidonic acid (AA, the main ω6PUFA) and given 100 nM corticosterone for several days. Corticosterone stimulated astrocyte glutamate recycling by increasing glutamate uptake and glutamine synthetase (GS), and altered the astrocyte cytoskeleton. DHA-enriched astrocytes no longer responded to the action of corticosterone on glutamate uptake, had decreased GS, and the cytoskeletal effect of corticosterone was delayed, while AA-enriched cells were unaffected. The DHA-dependent anti-corticosterone effect was related to fewer glucocorticoid receptors, while corticosterone increased DHA incorporation into astrocyte membranes. Thus, DHA helps astrocytes resist the influence of corticosterone, so perhaps promoting a sustainable response by the stressed brain. We show that corticosterone increases the glutamate recycling capacity of rat cortical astrocytes in culture, and alters their morphology, which may be detrimental in the long term. Increasing the membrane incorporation of docosahexaenoic acid (DHA), the main omega-3 in brain, reduces the amount of glucocorticoid receptors (GR) and prevents the effects of corticosterone. This may help the astrocytes maintain a functional phenotype in chronic stress situations.

The Role of Regulatory Transporters in Neuropathic Pain

Neuropathic pain arises from an injury or disease of the somatosensory nervous system rather than stimulation of pain receptors. As a result, the fine balance between excitation and inhibition is perturbed leading to hyperalgesia and allodynia. Various neuropathic pain models provide considerable evidence that changes in the glutamatergic, GABAergic, and monoaminergic systems. Neurotransmitter reuptake transporter proteins have the potential to change the temporal and spatial profile of various neurotransmitters throughout the nervous system. This, in turn, can affect the downstream effects of these neurotransmitters and hence modulate pain. This chapter explores various reuptake transporter systems and implicates their role in pain processing. Understanding the transporter systems will enhance drug discovery targeting different facets of neuropathic pain.

Generation of GFAP::GFP astrocyte reporter lines from human adult fibroblast-derived iPS cells using zinc-finger nuclease technology

Astrocytes are instrumental to major brain functions, including metabolic support, extracellular ion regulation, the shaping of excitatory signaling events and maintenance of synaptic glutamate homeostasis. Astrocyte dysfunction contributes to numerous developmental, psychiatric and neurodegenerative disorders. The generation of adult human fibroblast-derived induced pluripotent stem cells (iPSCs) has provided novel opportunities to study mechanisms of astrocyte dysfunction in human-derived cells. To overcome the difficulties of cell type heterogeneity during the differentiation process from iPSCs to astroglial cells (iPS astrocytes), we generated homogenous populations of iPS astrocytes using zinc-finger nuclease (ZFN) technology. Enhanced green fluorescent protein (eGFP) driven by the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter was inserted into the safe harbor adeno-associated virus integration site 1 (AAVS1) locus in disease and control-derived iPSCs. Astrocyte populations were enriched using Fluorescence Activated Cell Sorting (FACS) and after enrichment more than 99% of iPS astrocytes expressed mature astrocyte markers including GFAP, S100β, NFIA and ALDH1L1. In addition, mature pure GFP-iPS astrocytes exhibited a well-described functional astrocytic activity in vitro characterized by neuron-dependent regulation of glutamate transporters to regulate extracellular glutamate concentrations. Engraftment of GFP-iPS astrocytes into rat spinal cord grey matter confirmed in vivo cell survival and continued astrocytic maturation. In conclusion, the generation of GFAP::GFP-iPS astrocytes provides a powerful in vitro and in vivo tool for studying astrocyte biology and astrocyte-driven disease pathogenesis and therapy.

Buyang Huanwu decoction increases the expression of glutamate transporter-1 and glutamate synthetase in association with PACAP-38 following focal ischemia

The neuroprotective role of Buyang Huanwu decoction (BYHWD) in focal ischemia is associated with decreasing glutamate concentration. However, the mechanisms are not fully understood. The present study aimed to explore whether glutamate transporter-1 (GLT-1) and glutamine synthetase (GS) participated in the decreased level of glutamate and whether pituitary adenylate cyclase-activating polypeptide-38 (PACAP-38) was involved in this process. BYHWD was found to significantly upregulate the expression of GLT-1 and GS in the hippocampal CA1 area compared to the ischemia group, with the difference on day 3 being most significant. BYHWD increased the level of PACAP-38, and PACAP-(6-38) (PACAP receptor antagonist) significantly attenuated the effect of BYHWD on GLT-1 and GS, suggesting that PACAP-38 was involved in the upregulation of GLT-1 and GS induced by BYHWD. In addition, as GLT-1 and GS are mainly located in astrocytes, the changes of astrocytes were detected by glial fibrillary acidic protein (GFAP; an astrocytic marker) immunostaining. The results showed that BYHWD inhibited the expression of GFAP compared with the ischemia group, however, co-administration with PACAP-(6-38), which inhibited the effect of BYHWD on GLT-1 and GS in astrocytes, attenuated this effect, indicating that astrocytes participated in the protective role of BYHWD following focal ischemia. These results provided the evidence for the first time that not only neurons but also astrocytes contribute to the protective role of BYHWD, which opposes previous studies and may be a starting point for traditional medicine.

Inflammatory and Glutamatergic Homeostasis Are Involved in Successful Aging

Whole body studies using long-lived growth hormone receptor gene disrupted or knock out (GHR-KO) mice report global GH resistance, increased insulin sensitivity, reduced insulin-like growth factor 1 (IGF-1), and cognitive retention in old-age, however, little is known about the neurobiological status of these mice. The aim of this study was to determine if glutamatergic and inflammatory markers that are altered in aging and/or age-related diseases and disorders, are preserved in mice that experience increased healthspan. We examined messenger ribonucleic acid (mRNA) expression levels in the brain of 4- to 6-, 8- to 10-, and 20- to 22-month GHR-KO and normal aging control mice. In the hippocampus, glutamate transporter 1 (GLT-1) and anti-inflammatory nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB)-p50 were elevated in 8- to 10-month GHR-KO mice compared with age-matched controls. In the hypothalamus, NFκB-p50, NFκB-p65, IGF-1 receptor (IGF-1R), glutamate/aspartate transporter (GLAST), and 2-amino-3-(5-methyl-3-oxo 2,3-dihydro-1,2 oxazol-4-yl) propanoic acid receptor subunit 1 (GluA1) were elevated in 8- to 10- and/or 20- to 22-month GHR-KO mice when comparing genotypes. Finally, interleukin 1-beta (IL-1β) mRNA was reduced in 4- to 6- and/or 8- to 10-month GHR-KO mice compared with normal littermates in all brain areas examined. These data support the importance of decreased brain inflammation in early adulthood and maintained homeostasis of the glutamatergic and inflammatory systems in extended longevity.

Glutamate Transporters/Na(+), K(+)-ATPase Involving in the Neuroprotective Effect as a Potential Regulatory Target of Glutamate Uptake

The glutamate (Glu) transporters GLAST and GLT-1, as the two most important transporters in brain tissue, transport Glu from the extracellular space into the cell protecting against Glu toxicity. Furthermore, GLAST and GLT-1 are sodium-dependent Glu transporters (GluTs) that rely on sodium and potassium gradients generated principally by Na(+), K(+)-ATPase to generate ion gradients that drive Glu uptake. There is an interaction between Na(+), K(+)-ATPase and GluTs to modulate Glu uptake, and Na(+), K(+)-ATPase α, β or γ subunit can be directly coupled to GluTs, co-localizing with GLAST or GLT-1 in vivo to form a macromolecular complex and operate as a functional unit to regulate glutamatergic neurotransmission. Therefore, GluTs/Na(+), K(+)-ATPase may be involved in the neuroprotective effect as a potential regulatory target of Glu uptake in neurodegenerative diseases induced by Glu-mediated neurotoxicity as the final common pathway.

Experimentally induced diabetes causes glial activation, glutamate toxicity and cellular damage leading to changes in motor function

Behavioral impairments are the most empirical consequence of diabetes mellitus documented in both humans and animal models, but the underlying causes are still poorly understood. As the cerebellum plays a major role in coordination and execution of the motor functions, we investigated the possible involvement of glial activation, cellular degeneration and glutamate transportation in the cerebellum of rats, rendered diabetic by a single injection of streptozotocin (STZ; 45 mg/kg body weight; intraperitoneally). Motor function alterations were studied using Rotarod test (motor coordination) and grip strength (muscle activity) at 2nd, 4th, 6th, 8th, 10th, and 12th week post-diabetic confirmation. Scenario of glial (astroglia and microglia) activation, cell death and glutamate transportation was gaged using immunohistochemistry, histological study and image analysis. Cellular degeneration was clearly demarcated in the diabetic cerebellum. Glial cells were showing sequential and marked activation following diabetes in terms of both morphology and cell number. Bergmann glial cells were hypertrophied and distorted. Active caspase-3 positive apoptotic cells were profoundly present in all three cerebellar layers. Reduced co-labeling of GLT-1 and GFAP revealed the altered glutamate transportation in cerebellum following diabetes. These results, exclusively derived from histology, immunohistochemistry and cellular quantification, provide first insight over the associative reciprocity between the glial activation, cellular degeneration and reduced glutamate transportation, which presumably lead to the behavioral alterations following STZ-induced diabetes.

A3 adenosine receptor agonist prevents the development of paclitaxel-induced neuropathic pain by modulating spinal glial-restricted redox-dependent signaling pathways

Chemotherapy-induced peripheral neuropathy accompanied by chronic neuropathic pain is the major dose-limiting toxicity of several anticancer agents including the taxane paclitaxel (Taxol). A critical mechanism underlying paclitaxel-induced neuropathic pain is the increased production of peroxynitrite in spinal cord generated in response to activation of the superoxide-generating enzyme, NADPH oxidase. Peroxynitrite in turn contributes to the development of neuropathic pain by modulating several redox-dependent events in spinal cord. We recently reported that activation of the Gi/Gq-coupled A3 adenosine receptor (A3AR) with selective A3AR agonists (ie, IB-MECA) blocked the development of chemotherapy induced-neuropathic pain evoked by distinct agents, including paclitaxel, without interfering with anticancer effects. The mechanism or mechanisms of action underlying these beneficial effects has yet to be explored. We now demonstrate that IB-MECA attenuates the development of paclitaxel-induced neuropathic pain by inhibiting the activation of spinal NADPH oxidase and two downstream redox-dependent systems. The first relies on inhibition of the redox-sensitive transcription factor (NFκB) and mitogen activated protein kinases (ERK and p38) resulting in decreased production of neuroexcitatory/proinflammatory cytokines (TNF-α, IL-1β) and increased formation of the neuroprotective/anti-inflammatory IL-10. The second involves inhibition of redox-mediated posttranslational tyrosine nitration and modification (inactivation) of glia-restricted proteins known to play key roles in regulating synaptic glutamate homeostasis: the glutamate transporter GLT-1 and glutamine synthetase. Our results unravel a mechanistic link into biomolecular signaling pathways employed by A3AR activation in neuropathic pain while providing the foundation to consider use of A3AR agonists as therapeutic agents in patients with chemotherapy-induced peripheral neuropathy.

Propofol inhibits inflammatory cytokine-mediated glutamate uptake dysfunction to alleviate learning/memory impairment in depressed rats undergoing electroconvulsive shock

Electroconvulsive therapy (ECT) is an effective treatment for major depression, but can result in memory impairment. Several studies have shown that anesthetic propofol can alleviate the impairment of memory induced by ECT. However, the underlying molecular mechanisms remain unclear. We aimed to investigate the effects of propofol and electroconvulsive shock (ECS, analog of ECT in animals) on hippocampal inflammatory cytokines and glutamate uptake in depressed rats. The chronic unpredictable mild stress (CUMS) procedure was adopted to establish a model of depression. Sixty adult Sprague-Dawley rats were randomly divided into 5 groups with the following assignments (n=12 for each group): group C: control group without treatment; group D: CUMS+sham ECS; group DE: CUMS+ECS; group DP: CUMS+propofol (80 mg/kg, i.p.); group DPE: CUMS+propofol (80 mg/kg, i.p.)+ECS. Sucrose preference test and Morris water maze were used to assess behavioral changes. Hippocampal glutamate levels were measured with high performance liquid chromatography and the expression levels of IL-1β, TNF-α, GLAST and GLT-1 was quantificational analyzed by real time PCR or Western Blotting. The results demonstrated that ECS increased the levels of IL-1β and TNF-α, down-regulated the expression of GLT-1, GLAST expression remains stable, heightened the concentration of glutamate in the hippocampus and aggravated learning and memory impairment of depressed rats. Propofol suppressed IL-1β and TNF-α production, up-regulated the expression of GLT-1, decreased the concentration of glutamate in the hippocampus and attenuated the impairment of learning and memory induced by ECS. Propofol alleviate the learning and memory impairment induced by ECS could be partly attributed to its anti-inflammatory effects. This article is part of a Special Issue entitled Brain and Memory.

Glutamate as a neurotransmitter in the healthy brain

Glutamate is the most abundant free amino acid in the brain and is at the crossroad between multiple metabolic pathways. Considering this, it was a surprise to discover that glutamate has excitatory effects on nerve cells, and that it can excite cells to their death in a process now referred to as "excitotoxicity". This effect is due to glutamate receptors present on the surface of brain cells. Powerful uptake systems (glutamate transporters) prevent excessive activation of these receptors by continuously removing glutamate from the extracellular fluid in the brain. Further, the blood-brain barrier shields the brain from glutamate in the blood. The highest concentrations of glutamate are found in synaptic vesicles in nerve terminals from where it can be released by exocytosis. In fact, glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. It took, however, a long time to realize that. The present review provides a brief historical description, gives a short overview of glutamate as a transmitter in the healthy brain, and comments on the so-called glutamate-glutamine cycle. The glutamate transporters responsible for the glutamate removal are described in some detail.

Characterization of Gcf2/Lrrfip1 in experimental cerebral ischemia and its role as a modulator of Akt, mTOR and β-catenin signaling pathways

Leucine-rich repeat in Flightless-1 interaction protein 1 (Lrrfip1) is an up-regulated protein after cerebral ischemia whose precise role in the brain both in healthy and ischemic conditions is unclear. Different Lrrfip1 isoforms with distinct roles have been reported in human and mouse species. The present study aimed to analyze the Lrrfip1 transcriptional variants expressed in rat cortex, to characterize their expression patterns and subcellular location after ischemia, and to define their putative role in the brain. Five transcripts were identified and three of them (Lrrfip1, CRA_g and CRA_a' (Fli-I leucine-rich repeat associated protein 1 - Flap-1)) were analyzed by quantitative real-time polymerase chain reaction (qPCR). All the transcripts were up-regulated and showed differential expression patterns after in vivo and in vitro ischemia models. The main isoform, Lrrfip1, was found to be up-regulated from the acute to the late phases of ischemia in the cytoplasm of neurons and astrocytes of the peri-infarct area. This study demonstrates that Lrrfip1 activates β-catenin, Akt, and mammalian target of rapamycin (mTOR) proteins in astrocytes and positively regulates the expression of the excitatory amino acid transporter subtype 2 (GLT-1). Our findings point to Lrrfip1 as a key brain protein that regulates pro-survival pathways and proteins and encourages further studies to elucidate its role in cerebral ischemia as a potential target to prevent brain damage and promote functional recovery after stroke.

SLC1 glutamate transporters

The plasma membrane transporters for the neurotransmitter glutamate belong to the solute carrier 1 family. They are secondary active transporters, taking up glutamate into the cell against a substantial concentration gradient. The driving force for concentrative uptake is provided by the cotransport of Na(+) ions and the countertransport of one K(+) in a step independent of the glutamate translocation step. Due to eletrogenicity of transport, the transmembrane potential can also act as a driving force. Glutamate transporters are expressed in many tissues, but are of particular importance in the brain, where they contribute to the termination of excitatory neurotransmission. Glutamate transporters can also run in reverse, resulting in glutamate release from cells. Due to these important physiological functions, glutamate transporter expression and, therefore, the transport rate, are tightly regulated. This review summarizes recent literature on the functional and biophysical properties, structure-function relationships, regulation, physiological significance, and pharmacology of glutamate transporters. Particular emphasis is on the insight from rapid kinetic and electrophysiological studies, transcriptional regulation of transporter expression, and reverse transport and its importance for pathophysiological glutamate release under ischemic conditions.

Lesion-induced alterations in astrocyte glutamate transporter expression and function in the hippocampus

Astrocytes express the sodium-dependent glutamate transporters GLAST and GLT-1, which are critical to maintain low extracellular glutamate concentrations. Here, we analyzed changes in their expression and function following a mechanical lesion in the CA1 area of organotypic hippocampal slices. 6-7 days after lesion, a glial scar had formed along the injury site, containing strongly activated astrocytes with increased GFAP and S100 β immunoreactivity, enlarged somata, and reduced capability for uptake of SR101. Astrocytes in the scar's periphery were swollen as well, but showed only moderate upregulation of GFAP and S100 β and efficiently took up SR101. In the scar, clusters of GLT-1 and GLAST immunoreactivity colocalized with GFAP-positive fibers. Apart from these, GLT-1 immunoreactivity declined with increasing distance from the scar, whereas GLAST expression appeared largely uniform. Sodium imaging in reactive astrocytes indicated that glutamate uptake was strongly reduced in the scar but maintained in the periphery. Our results thus show that moderately reactive astrocytes in the lesion periphery maintain overall glutamate transporter expression and function. Strongly reactive astrocytes in the scar, however, display clusters of GLAST and GLT-1 immunoreactivity together with reduced glutamate transport activity. This reduction might contribute to increased extracellular glutamate concentrations and promote excitotoxic cell damage at the lesion site.

Glutamate transporters: a key piece in the glutamate puzzle of major depressive disorder

Glutamatergic therapies are emerging as the new path for the treatment of Major Depression Disorder. Recent reports reviewing the use of glutamate activity modulators in the treatment of resistant depression advocate the importance of understanding the alterations of the diverse components of this complex system in mood disorders. In this postmortem study we used in situ hybridization and microarray analysis to evaluate the gene expression of the membrane transporters SLC1A2 and SLCA3 and the vesicular transporter SLCA17A7 in the hippocampus of Major Depressive Disorder (MDD) and Bipolar Disorder (BPD) subjects. Samples from 8 controls, 11 MDD and 6 BPD subjects were processed for in situ hybridization using cRNA probes for SLC1A2, SLC1A3 and SLC17A7. Laser capture microdissection was used to collect tissue from adjacent sections for microarray analysis. The results showed that the expression of the membrane transporters SLC1A2 and SLC1A3 was diminished in the MDD group compared to controls. The expression of the vesicular glutamate transporter SLC17A7 on the other hand was increased in MDD subjects. As for the BPD group, all three transporters showed trends similar to those observed in MDD, but the changes observed did not reach significance. We hypothesize that the decreased expression of the membrane glutamate transporters and the increased expression of the vesicular transporter in the hippocampus would affect the balance of the glutamatergic circuitry of the hippocampus, and that this effect may be a major contributor to depressive symptoms.

GABA and Glutamate Transporters in Brain

The mammalian genome contains four genes encoding GABA transporters (GAT1, slc6a1; GAT2, slc6a13; GAT3, slc6a11; BGT1, slc6a12) and five glutamate transporter genes (EAAT1, slc1a3; EAAT2, slc1a2; EAAT3, slc1a1; EAAT4, slc1a6; EAAT5, slc1a7). These transporters keep the extracellular levels of GABA and excitatory amino acids low and provide amino acids for metabolic purposes. The various transporters have different properties both with respect to their transport functions and with respect to their ability to act as ion channels. Further, they are differentially regulated. To understand the physiological roles of the individual transporter subtypes, it is necessary to obtain information on their distributions and expression levels. Quantitative data are important as the functional capacity is limited by the number of transporter molecules. The most important and most abundant transporters for removal of transmitter glutamate in the brain are EAAT2 (GLT-1) and EAAT1 (GLAST), while GAT1 and GAT3 are the major GABA transporters in the brain. EAAT3 (EAAC1) does not appear to play a role in signal transduction, but plays other roles. Due to their high uncoupled anion conductance, EAAT4 and EAAT5 seem to be acting more like inhibitory glutamate receptors than as glutamate transporters. GAT2 and BGT1 are primarily expressed in the liver and kidney, but are also found in the leptomeninges, while the levels in brain tissue proper are too low to have any impact on GABA removal, at least in normal young adult mice. The present review will provide summary of what is currently known and will also discuss some methodological pitfalls.

HIV-1, methamphetamine and astrocyte glutamate regulation: combined excitotoxic implications for neuro-AIDS

Glutamate, the most abundant excitatory transmitter in the brain can lead to neurotoxicity when not properly regulated. Excitotoxicity is a direct result of abnormal regulation of glutamate concentrations in the synapse, and is a common neurotoxic mediator associated with neurodegenerative disorders. It is well accepted that methamphetamine (METH), a potent central nervous stimulant with high abuse potential, and human immunodeficiency virus (HIV)-1 are implicated in the progression of neurocognitive malfunction. Both have been shown to induce common neurodegenerative effects such as astrogliosis, compromised blood brain barrier integrity, and excitotoxicity in the brain. Reduced glutamate uptake from neuronal synapses likely leads to the accumulation of glutamate in the extracellular spaces. Astrocytes express the glutamate transporters responsible for majority of the glutamate uptake from the synapse, as well as for vesicular glutamate release. However, the cellular and molecular mechanisms of astrocyte-mediated excitotoxicity in the context of METH and HIV-1 are undefined. Topics reviewed include dysregulation of the glutamate transporters, specifically excitatory amino acid transporter-2, metabotropic glutamate receptor(s) expression and the release of glutamate by vesicular exocytosis. We also discuss glutamate concentration dysregulation through astrocytic expression of enzymes for glutamate synthesis and metabolism. Lastly, we discuss recent evidence of various astrocyte and neuron crosstalk mechanisms implicated in glutamate regulation. Astrocytes play an essential role in the neuropathologies associated with METH/HIV-1-induced excitotoxicity. We hope to shed light on common cellular and molecular pathways astrocytes share in glutamate regulation during drug abuse and HIV-1 infection.

Harmine, a natural beta-carboline alkaloid, upregulates astroglial glutamate transporter expression

Glutamate is the predominant excitatory amino acid neurotransmitter in the mammalian central nervous system (CNS). Glutamate transporter EAAT2/GLT-1 is the physiologically dominant astroglial protein that inactivates synaptic glutamate. Previous studies have shown that EAAT2 dysfunction leads to excessive extracellular glutamate and may contribute to various neurological disorders including amyotrophic lateral sclerosis (ALS). The recent discovery of the neuroprotective properties of ceftriaxone, a beta lactam antibiotic, suggested that increasing EAAT2/GLT-1 gene expression might be beneficial in ALS and other neurological/psychiatric disorders by augmenting astrocytic glutamate uptake. Here we report our efforts to develop a new screening assay for identifying compounds that activate EAAT2 gene expression. We generated fetal derived-human immortalized astroglial cells that are stably expressing a firefly luciferase reporter under the control of the human EAAT2 promoter. When screening a library of 1040 FDA approved compounds and natural products, we identified harmine, a naturally occurring beta-carboline alkaloid, as one of the top hits for activating the EAAT2 promoter. We further tested harmine in our in vitro cell culture systems and confirmed its ability to increase EAAT2/GLT1 gene expression and functional glutamate uptake activity. We next tested its efficacy in both wild type animals and in an ALS animal model of disease and demonstrated that harmine effectively increased GLT-1 protein and glutamate transporter activity in vivo. Our studies provide potential novel neurotherapeutics by modulating the activity of glutamate transporters via gene activation. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Down-regulation of astrocytic GLAST by microglia-related inflammation is abrogated in dibutyryl cAMP-differentiated cultures

The influence of neuroinflammation on glutamate uptake by glial cells was examined after exposing primary cultures of rat astrocytes to conditioned culture medium from lipopolysaccharide-activated microglia. While such treatment triggered an inflammatory response in astrocytes, as revealed by the induction of cytokine expression, a significant decrease in GLAST expression and activity was observed after 72 h. This regulation of glutamate transporter was not observed with medium from naive microglia, but was mimicked by direct addition of tumor necrosis factor-alpha (TNF-alpha), a major cytokine released from activated microglia. Hence, on its own, TNF-alpha also triggered inflammation in astrocyte cultures, highlighting complex cross-talk between astrocytes and microglia in inflammatory conditions. This putatively detrimental regulation of GLAST in response to inflammation was also studied in cells exposed to dibutyryl cAMP, recognized as a model of astrocytes exhibiting a typical differentiated or activated phenotype. In this model, the conditioned culture medium from activated microglia, as well as TNF-alpha, were found to increase glutamate uptake capacity. Consistently, both of these treatments caused only modest induction of an inflammatory response in dibutyryl cAMP-matured astrocytes as compared to undifferentiated astrocytes. Together, these results suggest that differentiated/activated astrocytes are endowed with the capacity to confront inflammatory insults and that drugs influencing the astrocytes phenotype would deserve further consideration in the treatment of neurological disorders.

Impaired glial glutamate uptake induces extrasynaptic glutamate spillover in the spinal sensory synapses of neuropathic rats

Glial cell dysfunction and excessive glutamate receptor activation in spinal dorsal horn neurons are hallmark mechanisms of pathological pain. The way in which glial cell dysfunction leads to excessive glutamate receptor activation in the spinal sensory synapses remains unknown. We and others recently reported the downregulation of glial glutamate transporter (GT) protein expression in the spinal dorsal horn of neuropathic rats. In this study, we showed that excitatory postsynaptic currents originating from N-methyl-d-aspartate receptor activation (NMDA EPSCs) elicited by peripheral synaptic input in the spinal sensory synapses were enhanced in neuropathic rats with mechanical allodynia induced by partial sciatic nerve ligation. The enhanced NMDA EPSCs were accompanied by an increased proportion of NR2B receptor activation. Physically blocking the extrasynaptic glutamate with dextran or chemically scavenging the glutamate with glutamic-pyruvic transaminase ameliorated the abnormal NMDA EPSCs in neuropathic rats. Pharmacological blockade of glial GTs with dihydrokainic acid enhanced NMDA receptor activation elicited by synaptic input or puffed glutamate in normal control rats, but this effect was precluded in neuropathic rats. Thus extrasynaptic glutamate spillover and extrasynaptic NMDA receptor activation induced by deficient glial glutamate uptake in the synapses resulted in the excessive activation of NMDA receptors in neuropathic rats. It is suggested that extrasynaptic glutamate spillover may be a key synaptic mechanism related to phenotypic alterations induced by nerve injury in the spinal dorsal horn and that glial GTs are potential new targets in the development of analgesics.

Role of spinal cord glia in the central processing of peripheral pain perception

BACKGROUND

The discovery that glial activation plays a critical role in the modulation of neuronal functions and affects the spinal processing of nociceptive signalling has brought new understanding on the mechanisms underlying central sensitization involved in chronic pain facilitation. Spinal glial activation is now considered an important component in the development and maintenance of allodynia and hyperalgesia in various models of chronic pain, including neuropathic pain and pain associated with peripheral inflammation. In addition, spinal glial activation is also involved in some forms of visceral hyperalgesia.

PURPOSE

We discuss the signalling pathways engaged in central glial activation, including stress pathways, and the neuron-glia bidirectional relationships involved in the modulation of synaptic activity and pain facilitation. In this expanding field of research, the characterization of the mechanisms by which glia affect spinal neuro-transmission will increase our understanding of central pain facilitation, and has the potential for the development of new therapeutic agents for common chronic pain conditions.

Expression of glutamate transporter subtypes in cultured retinal pigment epithelial and retinoblastoma cells

PURPOSE

Glutamate is the major excitatory neurotransmitter in the retina and glutamate uptake is essential for normal glutamate signalling. Retinal diseases may induce neurochemical changes which affect retinal cells including retinal pigment epithelium (RPE). The aim of the study was to investigate the expression of glutamate transporter subtypes in RPE and retinoblastoma cells and to clarify the effect of proliferation modulators on the levels of the expressed transporter in the RPE cell line.

METHODS

Cultured pig RPE cells and two human RPE cell lines, D407 and ARPE-19, as well as the human retinoblastoma cell line Y79 were used. Glutamate transporter expression was evaluated with Western blot analysis and immunocytochemistry.

RESULTS

The study revealed unexpected expression of neuronal glutamate transporter/chloride channel EAAT4 in these three cell lines, but not in cultured pig RPE cells, whereas another glutamate carrier, EAAC1, was present in all cell types utilized. Other transporter subtypes, GLT1, GLAST and EAAT5 were not found. Neither tamoxifen, known to inhibit both proliferation and glutamate uptake in RPE cells, nor retinoic acid nor insulin, also known to affect cell proliferation rates, were capable of changing the total levels of EAAT4 in APRE-19 cells.

CONCLUSIONS

Neuronal glutamate transporter EAAC1 is expressed in RPE cells. The robust expression of EAAT4 in cell lines may reflect a role of EAAT4 in cell proliferation and migration. Unaltered steady-state expression of this carrier and chloride-channel protein hints at posttranslational mechanisms of regulation of EAAT4.

Glutamate transporters prevent excessive activation of NMDA receptors and extrasynaptic glutamate spillover in the spinal dorsal horn

Activation of N-methyl-d-aspartate (NMDA) receptors in the spinal dorsal horn neurons is a key process related to sensory transmission, neural plasticity, and pathogenesis of pain. In this study, we investigated how activation of NMDA receptors in spinal substantia gelatinosa neurons is regulated by glutamate re-uptake through glutamate transporters located in the astrocytic and neuronal plasma membranes. Using visualized whole cell patch recording techniques, NMDA excitatory postsynaptic currents evoked by graded peripheral inputs in spinal substantia gelatinosa neurons of spinal slices from young adult rats were analyzed before and after combined inhibition of glial and neuronal glutamate transporters by d-threo-beta-benzyloxyaspartate (TBOA). Blockade of glutamate transporters increased the number and duration of NMDA receptors activated by weak and by strong primary afferent inputs as well as by exogenous glutamate. The enhancement in activation of NMDA receptors induced by TBOA was greater in neurons that have weaker synaptic input at baseline. Impaired glutamate uptake increased the open probability of NMDA channels and caused glutamate spillover outside the active synapses, leading to activation of extrasynaptic NMDA receptors and/or receptors located in neighboring synapses. Finally, blockade of glutamate transporters resulted in an increased proportion of NR2B subunit activation induced by peripheral input, and this increase was further augmented by stronger afferent input. These data indicate that glutamate transporters regulate spatiotemporal and intensity coding for sensory input and prevent excessive activation of glutamate receptors in the spinal dorsal horn. It is suggested that remedying dysfunctional glutamate transporters may be a potential new avenue to prevent the pathogenesis of pain.

Mechanisms of substrate transport-induced clustering of a glial glutamate transporter GLT-1 in astroglial-neuronal cultures

Glutamate uptake by the Na(+)-dependent glutamate transporter GLT-1, which is predominantly expressed in astrocytes, is crucial for regulating glutamate concentration at the synaptic cleft and achieving proper excitatory neurotransmission. A body of evidence suggests that GLT-1 constitutively traffics between the plasma membrane and endosomes via an endocytosis/recycling pathway, and forms a cluster. Here, we report substrate transport via GLT-1-induced formation of GLT-1 cluster accompanied by intracellular trafficking in rat astroglial-neuronal cultures. We constructed a recombinant adenovirus expressing enhanced green fluorescence protein (EGFP)-tagged GLT-1. Adenoviral infection resulted in the expression of functional GLT-1-EGFP preferentially in astrocytes, partly as clusters. Treatment with glutamate, but not N-methyl-D-aspartate, dramatically increased the number of GLT-1 clusters within 1 h. The estimated EC(50) value of glutamate was 240 microm. In addition, glutamate decreased the cell surface expression and increased the intracellular expression of GLT-1. The GLT-1 clusters were found in early and recycling endosomes and partly in lysosomes, and were inhibited by blockade of endocytotic pathways. Ionotropic and metabotropic glutamate receptor antagonists had no effect on glutamate-induced GLT-1 clustering. The non-transportable glutamate uptake inhibitors (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate and dihydrokainate, as well as Na(+)-free conditions, prevented the glutamate-induced GLT-1 clustering, whereas the competitive substrates, aspartate and L-trans-pyrrolidine-2,4-dicarboxylate, induced GLT-1 clustering. Furthermore, the Na(+)/K(+)-ATPase inhibitor, ouabain, and the Na(+) ionophores, gramicidin and monensin, produced GLT-1 clustering. Modulators of intracellular Ca(2+)signaling or membrane depolarization had no effect on GLT-1 clustering. Taken together, these results suggest that Na(+) influx associated with GLT-1 substrate transport triggers the formation of GLT-1 clusters accompanied by intracellular trafficking via endocytotic pathways in astrocytes.

The astrocyte odyssey

Neurons have long held the spotlight as the central players of the nervous system, but we must remember that we have equal numbers of astrocytes and neurons in the brain. Are these cells only filling up the space and passively nurturing the neurons, or do they also contribute to information transfer and processing? After several years of intense research since the pioneer discovery of astrocytic calcium waves and glutamate release onto neurons in vitro, the neuronal-glial studies have answered many questions thanks to technological advances. However, the definitive in vivo role of astrocytes remains to be addressed. In addition, it is becoming clear that diverse populations of astrocytes coexist with different molecular identities and specialized functions adjusted to their microenvironment, but do they all belong to the umbrella family of astrocytes? One population of astrocytes takes on a new function by displaying both support cell and stem cell characteristics in the neurogenic niches. Here, we define characteristics that classify a cell as an astrocyte under physiological conditions. We will also discuss the well-established and emerging functions of astrocytes with an emphasis on their roles on neuronal activity and as neural stem cells in adult neurogenic zones.

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.

Chemical hypoxia facilitates alternative splicing of EAAT2 in presymptomatic APP23 transgenic mice

Hypoxia is one of the major common components of vascular risk factors for pathogenesis of Alzheimer's disease. This study investigated the possible relationship between hypoxia and alternative splicing of the excitatory amino acid transporter 2 (EAAT2) in a transgenic model for Alzheimer's disease. We used an APP23 mouse model prior to amyloid deposition and subjected it to chemical hypoxia treatment as induced by 3-nitropropionic acid. One hour after administration of 3-nitropropionic acid changes in the expression of the 5'-splice forms mEAAT2/5UT3, mEAAT2/5UT4, and mEAAT2/5UT5 were found in the frontal cortex, hippocampus and cerebellum of the APP23 model. In untreated APP23 animals the expression of EAAT2 splice variants was unchanged. Our results demonstrate that hypoxia facilitates alternative splicing of EAAT2 in the APP23 model. This may be a molecular mechanism linking vascular factors to early pathophysiology of Alzheimer's disease.

The immune-mediated alteration of serotonin and glutamate: towards an integrated view of depression

Beside the well-known deficiency in serotonergic neurotransmission as pathophysiological correlate of major depression (MD), recent evidence points to a pivotal role of increased glutamate receptor activation as well. However, cause and interaction of these neurotransmitter alterations are not understood. In this review, we present a hypothesis integrating current concepts of neurotransmission and hypothalamus-pituitary-adrenal (HPA) axis dysregulation with findings on immunological alterations and alterations in brain morphology in MD. An immune activation including increased production of proinflammatory cytokines has repeatedly been described in MD. Proinflammatory cytokines such as interleukin-2, interferon-gamma, or tumor necrosis factor-alpha activate the tryptophan- and serotonin-degrading enzyme indoleamine 2,3-dioxygenase (IDO). Depressive states during inflammatory somatic disorders are also associated with increased proinflammatory cytokines and increased consumption of tryptophan via activation of IDO. An enhanced consumption of serotonin and its precursor tryptophan through IDO activation could well explain the reduced availability of serotonergic neurotransmission in MD. An increased activation of IDO and its subsequent enzyme kynurenine monooxygenase by proinflammatory cytokines, moreover, leads to an enhanced production of quinolinic acid, a strong agonist of the glutamatergic N-methyl-D-aspartate receptor. In inflammatory states of the central nervous system, IDO is mainly activated in microglial cells, which preferentially metabolize tryptophan to the NMDA receptor agonist quinolinic acid, whereas astrocytes - counteracting this metabolism due to the lack of an enzyme of this metabolism - have been observed to be reduced in MD. Therefore the type 1/type 2 immune response imbalance, associated with an astrocyte/microglia imbalance, leads to serotonergic deficiency and glutamatergic overproduction. Astrocytes are further strongly involved in re-uptake and metabolic conversion of glutamate. The reduced number of astrocytes could contribute to both, a diminished counterregulation of IDO activity in microglia and an altered glutamatergic neurotransmission. Further search for antidepressant agents should take into account anti-inflammatory drugs, for example, cyclooxygenase-2 inhibitors, might exert antidepressant effects by acting on serotonergic deficiency, glutamatergic hyperfunction and antagonizing neurotoxic effects of quinolinic acid.

Neuroinflammation and regulation of glial glutamate uptake in neurological disorders

Oxidative stress, neuroinflammation, and excitotoxicity are frequently considered distinct but common hallmarks of several neurological disorders, including Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and Alzheimer's disease. Although neuron degeneration and death are the ultimate consequences of these pathological processes, it is now widely accepted that alterations in the function of surrounding glial cells are key features in the progression of these diseases. In response to alteration in their local environment, microglia, commonly considered the resident immune cells of the nervous parenchyma, become activated and release a variety of soluble factors. Among these, proinflammatory cytokines and free radicals actively participate in the degenerative insults. In addition, excitotoxic neuronal damage resulting from excessive glutamate is frequently associated with impaired handling of extracellular glutamate by gliotic astrocytes. Although several research projects have focused on the biochemical mechanisms of the regulation of glial glutamate transporters, a relationship between activation of microglia and modulation of astrocytic glutamate uptake is now suggested. The aim of this review is to summarize and discuss the data showing an influence of inflammatory mediators and related free radicals on the expression and activity of glial glutamate transporters.