A splice variant of glutamate transporter GLT1/EAAT2 expressed in neurons: cloning and localization in rat nervous system

A cross talk on the role of contemporary biomarkers in depression

Introduction: Biomarkers can be used to identify determinants of response to various treatments of mental disorders. Evidence to date demonstrates that markers of inflammatory, neurotransmitter, neurotrophic, neuroendocrine, and metabolic function can predict the psychological and physical consequences of depression in individuals, allowing for the development of new therapeutic targets with fewer side effects. Extensive research has included hundreds of potential biomarkers of depression, but their roles in depression, abnormal patients, and how bioinformatics can be used to improve diagnosis, treatment, and prognosis have not been determined or defined. To determine which biomarkers can and cannot be used to predict treatment response, classify patients for specific treatments, and develop targets for new interventions, proprietary strategies, and current research projects need to be tailored.Material and Methods: This review article focuses on - biomarker systems that would help in the further development and expansion of newer targets - which holds great promise for reducing the burden of depression.Results and Discussion: Further, this review point to the inflammatory response, metabolic marker, and microribonucleic acids, long non-coding RNAs, HPA axis which are - related to depression and can serve as future targets.

Astrocytic Glutamate Transporters and Migraine

Glutamate levels and lifetime in the brain extracellular space are dinamically regulated by a family of Na⁺- and K⁺-dependent glutamate transporters, which thereby control numerous brain functions and play a role in numerous neurological and psychiatric diseases. Migraine is a neurological disorder characterized by recurrent attacks of typically throbbing and unilateral headache and by a global dysfunction in multisensory processing. Familial hemiplegic migraine type 2 (FHM2) is a rare monogenic form of migraine with aura caused by loss-of-function mutations in the α2 Na/K ATPase (α2NKA). In the adult brain, this pump is expressed almost exclusively in astrocytes where it is colocalized with glutamate transporters. Knockin mouse models of FHM2 (FHM2 mice) show a reduced density of glutamate transporters in perisynaptic astrocytic processes (mirroring the reduced expression of α2NKA) and a reduced rate of glutamate clearance at cortical synapses during neuronal activity and sensory stimulation. Here we review the migraine-relevant alterations produced by the astrocytic glutamate transport dysfunction in FHM2 mice and their underlying mechanisms, in particular regarding the enhanced brain susceptibility to cortical spreading depression (the phenomenon that underlies migraine aura and can also initiate the headache mechanisms) and the enhanced algesic response to a migraine trigger.

Synapse Dysfunctions in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory disease of the central nervous system (CNS) affecting nearly three million humans worldwide. In MS, cells of an auto-reactive immune system invade the brain and cause neuroinflammation. Neuroinflammation triggers a complex, multi-faceted harmful process not only in the white matter but also in the grey matter of the brain. In the grey matter, neuroinflammation causes synapse dysfunctions. Synapse dysfunctions in MS occur early and independent from white matter demyelination and are likely correlates of cognitive and mental symptoms in MS. Disturbed synapse/glia interactions and elevated neuroinflammatory signals play a central role. Glutamatergic excitotoxic synapse damage emerges as a major mechanism. We review synapse/glia communication under normal conditions and summarize how this communication becomes malfunctional during neuroinflammation in MS. We discuss mechanisms of how disturbed glia/synapse communication can lead to synapse dysfunctions, signaling dysbalance, and neurodegeneration in MS.

Astrocyte Activation Markers

Astrocytes are the most common type of glial cells that provide homeostasis and protection of the central nervous system. Important specific characteristic of astrocytes is manifestation of morphological heterogeneity, which is directly dependent on localization in a particular area of the brain. Astrocytes can integrate into neural networks and keep neurons active in various areas of the brain. Moreover, astrocytes express a variety of receptors, channels, and membrane transporters, which underlie their peculiar metabolic activity, and, hence, determine plasticity of the central nervous system during development and aging. Such complex structural and functional organization of astrocytes requires the use of modern methods for their identification and analysis. Considering the important fact that determining the most appropriate marker for polymorphic and multiple subgroups of astrocytes is of decisive importance for studying their multifunctionality, this review presents markers, modern imaging techniques, and identification of astrocytes, which comprise a valuable resource for studying structural and functional properties of astrocytes, as well as facilitate better understanding of the extent to which astrocytes contribute to neuronal activity.

Distinct gene expression in demyelinated white and grey matter areas of patients with multiple sclerosis

Demyelination of the central nervous system is a prominent pathological hallmark of multiple sclerosis and affects both white and grey matter. However, demyelinated white and grey matter exhibit clear pathological differences, most notably the presence or absence of inflammation and activated glial cells in white and grey matter, respectively. In order to gain more insight into the differential pathology of demyelinated white and grey matter areas, we micro-dissected neighbouring white and grey matter demyelinated areas as well as normal-appearing matter from leucocortical lesions of human post-mortem material and used these samples for RNA sequencing. Our data show that even neighbouring demyelinated white and grey matter of the same leucocortical have a distinct gene expression profile and cellular composition. We propose that, based on their distinct expression profile, pathological processes in neighbouring white and grey matter are likely different which could have implications for the efficacy of treating grey matter lesions with current anti-inflammatory-based multiple sclerosis drugs.

Ceftriaxone Treatment Weakens Long-Term Synaptic Potentiation in the Hippocampus of Young Rats

Disrupted glutamate clearance in the synaptic cleft leads to synaptic dysfunction and neurological diseases. Decreased glutamate removal from the synaptic cleft is known to cause excitotoxicity. Data on the physiological effects of increased glutamate clearance are contradictory. This study investigated the consequences of ceftriaxone (CTX), an enhancer of glutamate transporter 1 expression, treatment on long-term synaptic potentiation (LTP) in the hippocampus of young rats. In this study, 5-day administration of CTX (200 mg/kg) significantly weakened LTP in CA3-CA1 synapses. As shown by electrophysiological recordings, LTP attenuation was associated with weakening of N-Methyl-D-aspartate receptor (NMDAR)-dependent signaling in synapses. However, PCR analysis did not show downregulation of NMDAR subunits or changes in the expression of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits. We assume that extracellular burst stimulation activates fewer synapses in CTX-treated animals because increased glutamate reuptake results in reduced spillover, and neighboring synapses do not participate in neurotransmission. Attenuation of LTP was not accompanied by noticeable behavioral changes in the CTX group, with no behavioral abnormalities observed in the open field test or Morris water maze test. Thus, our experiments show that increased glutamate clearance can impair long-term synaptic plasticity and that this phenomenon can be considered a potential side effect of CTX treatment.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

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.

Neurobiology, Functions, and Relevance of Excitatory Amino Acid Transporters (EAATs) to Treatment of Refractory Epilepsy

Epilepsy is one of the most prevalent and devastating neurological disorders characterized by episodes of unusual sensations, loss of awareness, and reoccurring seizures. The frequency and intensity of epileptic fits can vary to a great degree, with almost a third of all cases resistant to available therapies. At present, there is a major unmet need for effective and specific therapeutic intervention. Impairments of the exquisite balance between excitatory and inhibitory synaptic processes in the brain are considered key in the onset and pathophysiology of the disease. As the primary excitatory neurotransmitter in the central nervous system, glutamate has been implicated in the process, with the glutamatergic system holding center stage in the pathobiology as well as in developing disease-modifying therapies. Emerging data pinpoint impairments of glutamate clearance as one of the key causative factors in drug-resistant disease forms. Reinstatement of glutamate homeostasis using pharmacological and genetic modulation of glutamate clearance is therefore considered to be of major translational relevance. In this article, we review the neurobiological and clinical evidence suggesting complex aberrations in the activity and functions of excitatory amino acid transporters (EAATs) in epilepsy, with knock-on effects on glutamate homeostasis as a leading cause for the development of refractory forms. We consider the emerging data on pharmacological and genetic manipulations of EAATs, with reference to seizures and glutamate dyshomeostasis, and review their fundamental and translational relevance. We discuss the most recent advances in the EAATs research in human and animal models, along with numerous questions that remain open for debate and critical appraisal. Contrary to the widely held view on EAATs as a promising therapeutic target for management of refractory epilepsy as well as other neurological and psychiatric conditions related to glutamatergic hyperactivity and glutamate-induced cytotoxicity, we stress that the true relevance of EAAT2 as a target for medical intervention remains to be fully appreciated and verified. Despite decades of research, the emerging properties and functional characteristics of glutamate transporters and their relationship with neurophysiological and behavioral correlates of epilepsy challenge the current perception of this disease and fit unambiguously in neither EAATs functional deficit nor in reversal models. We stress the pressing need for new approaches and models for research and restoration of the physiological activity of glutamate transporters and synaptic transmission to achieve much needed therapeutic effects. The complex mechanism of EAATs regulation by multiple factors, including changes in the electrochemical environment and ionic gradients related to epileptic hyperactivity, impose major therapeutic challenges. As a final note, we consider the evolving views and present a cautious perspective on the key areas of future progress in the field towards better management and treatment of refractory disease forms.

Astrocyte control of glutamatergic activity: Downstream effects on serotonergic function and emotional behavior

Major depressive disorder (MDD) is a leading cause of disability worldwide, with a poorly known pathophysiology and sub-optimal treatment, based on serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors. We review existing theories on MDD, paying special attention to the role played by the ventral anterior cingulate cortex (vACC) or its rodent equivalent, infralimbic cortex (IL), which tightly control the activity of brainstem monoamine neurons (including raphe 5-HT neurons) via descending afferents. Further, astrocytes regulate excitatory synapse activity via glutamate reuptake through astrocytic transporters EAAT1 and EAAT2 (GLAST and GLT-1 in rodents), and alterations of astrocyte number/function have been reported in MDD patients and suicide victims. We recently assessed the impact of reducing GLAST/GLT-1 function in IL on emotional behavior and serotonergic function in rodents. The acute pharmacological blockade of GLT-1 with dihydrokainate (DHK) in rat IL evoked an antidepressant-like effect mediated by local AMPA-R activation and a subsequent enhancement of serotonergic function. No effects were produced by DHK microinfusion in prelimbic cortex (PrL). In the second model, a moderate small interfering RNAs (siRNA)-induced reduction of GLAST and GLT-1 expression in mouse IL markedly increased local glutamatergic neurotransmission and evoked a depressive-like phenotype (reversed by citalopram and ketamine), and reduced serotonergic function and BDNF expression in cortical/hippocampal areas. As for DHK, siRNA microinfusion in PrL did not evoke behavioral/neurochemical effects. Overall, both studies support a critical role of the astrocyte-neuron communication in the control of excitatory neurotransmission in IL, and subsequently, on emotional behavior, via the downstream associated changes on serotonergic function.

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'.

Astrocyte Heterogeneity: Impact to Brain Aging and Disease

Astrocytes, one of the largest glial cell population in the central nervous system (CNS), play a key function in several events of brain development and function, such as synapse formation and function, control of neurotransmitters release and uptake, production of trophic factors and control of neuronal survival. Initially described as a homogenous population, several evidences have pointed that astrocytes are highly heterogeneous, both morphologically and functionally, within the same region, and across different brain regions. Recent findings suggest that the heterogeneity in the expression profile of proteins involved in astrocyte function may predict the selective vulnerability of brain regions to specific diseases, as well as to the age-related cognitive decline. However, the molecular mechanisms underlying these changes, either in aging as well as in brain disease are scarce. Neuroinflammation, a hallmark of several neurodegenerative diseases and aging, is reported to have a dubious impact on glial activation, as these cells release pro- and anti-inflammatory cytokines and chemokines, anti-oxidants, free radicals, and neurotrophic factors. Despite the emerging evidences supporting that reactive astrocytes have a duality in their phenotype, neurotoxic or neuroprotective properties, depending on the age and stimuli, the underlying mechanisms of their activation, cellular interplays and the impact of regional astrocyte heterogeneity are still a matter of discussion. In this review article, we will summarize recent findings on astrocyte heterogeneity and phenotypes, as well as their likely impact for the brain function during aging and neural diseases. We will focus on the molecules and mechanisms triggered by astrocyte to control synapse formation in different brain regions. Finally, we will discuss new evidences on how the modulation of astrocyte phenotype and function could impact the synaptic deficits and glial dysfunction present in aging and pathological states.

Glutamate Transporters: Expression and Function in Oligodendrocytes

Glutamate, the main excitatory neurotransmitter of the vertebrate central nervous system (CNS), is well known as a regulator of neuronal plasticity and neurodevelopment. Such glutamate function is thought to be mediated primarily by signaling through glutamate receptors. Thus, it requires a tight regulation of extracellular glutamate levels and a fine-tuned homeostasis that, when dysregulated, has been associated with a wide range of central pathologies including neuropsychiatric, neurodevelopmental, and neurodegenerative disorders. In the mammalian CNS, extracellular glutamate levels are controlled by a family of sodium-dependent glutamate transporters belonging to the solute carrier family 1 (SLC1) that are also referred to as excitatory amino acid transporters (EAATs). The presumed main function of EAATs has been best described in the context of synaptic transmission where EAATs expressed by astrocytes and neurons effectively regulate extracellular glutamate levels so that synapses can function independently. There is, however, increasing evidence that EAATs are expressed by cells other than astrocytes and neurons, and that they exhibit functions beyond glutamate clearance. In this review, we will focus on the expression and functions of EAATs in the myelinating cells of the CNS, oligodendrocytes. More specifically, we will discuss potential roles of oligodendrocyte-expressed EAATs in contributing to extracellular glutamate homeostasis, and in regulating oligodendrocyte maturation and CNS myelination by exerting signaling functions that have traditionally been associated with glutamate receptors. In addition, we will provide some examples for how dysregulation of oligodendrocyte-expressed EAATs may be involved in the pathophysiology of neurologic diseases.

Glutamate transporters, EAAT1 and EAAT2, are potentially important in the pathophysiology and treatment of schizophrenia and affective disorders

Glutamate is the predominant excitatory neurotransmitter in the human brain and it has been shown that prolonged activation of the glutamatergic system leads to nerve damage and cell death. Following release from the pre-synaptic neuron and synaptic transmission, glutamate is either taken up into the pre-synaptic neuron or neighbouring glia by transmembrane glutamate transporters. Excitatory amino acid transporter (EAAT) 1 and EAAT2 are Na⁺-dependant glutamate transporters expressed predominantly in glia cells of the central nervous system. As the most abundant glutamate transporters, their primary role is to modulate levels of glutamatergic excitability and prevent spill over of glutamate beyond the synapse. This role is facilitated through the binding and transportation of glutamate into astrocytes and microglia. The function of EAAT1 and EAAT2 is heavily regulated at the levels of gene expression, post-transcriptional splicing, glycosylation states and cell-surface trafficking of the protein. Both glutamatergic dysfunction and glial dysfunction have been proposed to be involved in psychiatric disorder. This review will present an overview of the roles that EAAT1 and EAAT2 play in modulating glutamatergic activity in the human brain, and mount an argument that these two transporters could be involved in the aetiologies of schizophrenia and affective disorders as well as represent potential drug targets for novel therapies for those disorders.

Physiology of Astroglia

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

Physiology of Astroglia

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

Astrocytic Regulation of Glutamate Transmission in Schizophrenia

According to the glutamate hypothesis of schizophrenia, the abnormality of glutamate transmission induced by hypofunction of NMDA receptors (NMDARs) is causally associated with the positive and negative symptoms of schizophrenia. However, the underlying mechanisms responsible for the changes in glutamate transmission in schizophrenia are not fully understood. Astrocytes, the major regulatory glia in the brain, modulate not only glutamate metabolism but also glutamate transmission. Here we review the recent progress in understanding the role of astrocytes in schizophrenia. We focus on the astrocytic mechanisms of (i) glutamate synthesis via the glutamate-glutamine cycle, (ii) glutamate clearance by excitatory amino acid transporters (EAATs), (iii) D-serine release to activate NMDARs, and (iv) glutamatergic target engagement biomarkers. Abnormality in these processes is highly correlated with schizophrenia phenotypes. These findings will shed light upon further investigation of pathogenesis as well as improvement of biomarkers and therapies for schizophrenia.

Single Cell Immuno-Laser Microdissection Coupled to Label-Free Proteomics to Reveal the Proteotypes of Human Brain Cells After Ischemia

Cerebral ischemia entails rapid tissue damage in the affected brain area causing devastating neurological dysfunction. How each component of the neurovascular unit contributes or responds to the ischemic insult in the context of the human brain has not been solved yet. Thus, the analysis of the proteome is a straightforward approach to unraveling these cell proteotypes. In this study, post-mortem brain slices from ischemic stroke patients were obtained corresponding to infarcted (IC) and contralateral (CL) areas. By means of laser microdissection, neurons and blood brain barrier structures (BBB) were isolated and analyzed using label-free quantification. MS data are available via ProteomeXchange with identifier PXD003519. Ninety proteins were identified only in neurons, 260 proteins only in the BBB and 261 proteins in both cell types. Bioinformatics analyses revealed that repair processes, mainly related to synaptic plasticity, are outlined in microdissected neurons, with nonexclusive important functions found in the BBB. A total of 30 proteins showing p < 0.05 and fold-change> 2 between IC and CL areas were considered meaningful in this study: 13 in neurons, 14 in the BBB and 3 in both cell types. Twelve of these proteins were selected as candidates and analyzed by immunohistofluorescence in independent brains. The MS findings were completely verified for neuronal SAHH2 and SRSF1 whereas the presence in both cell types of GABT and EAA2 was only validated in neurons. In addition, SAHH2 showed its potential as a prognostic biomarker of neurological improvement when analyzed early in the plasma of ischemic stroke patients. Therefore, the quantitative proteomes of neurons and the BBB (or proteotypes) after human brain ischemia presented here contribute to increasing the knowledge regarding the molecular mechanisms of ischemic stroke pathology and highlight new proteins that might represent putative biomarkers of brain ischemia or therapeutic targets.

The role of glutamate transporters in the pathophysiology of neuropsychiatric disorders

Altered glutamate transporter expression is a common feature of many neuropsychiatric conditions, including schizophrenia. Excitatory amino acid transporters (EAATs) are responsible for the reuptake of glutamate, preventing non-physiological spillover from the synapse. Postmortem studies have revealed significant dysregulation of EAAT expression in various brain regions at the cellular and subcellular level. Recent animal studies have also demonstrated a role for glutamate spillover as a mechanism of disease. In this review, we describe current evidence for the role of glutamate transporters in regulating synaptic plasticity and transmission. In neuropsychiatric conditions, EAAT splice variant expression is altered. There are changes in the localization of the transporters and disruption of the metabolic and structural protein network that supports EAAT activity. This results in aberrant neuroplasticity and excitatory signaling, contributing to the symptoms associated with neuropsychiatric disease. Understanding the complex functions of glutamate transporters will clarify the relevance of their role in the pathophysiology of neuropsychiatric disorders.

Stratification of astrocytes in healthy and diseased brain

Astrocytes, a subtype of glial cells, come in variety of forms and functions. However, overarching role of these cell is in the homeostasis of the brain, be that regulation of ions, neurotransmitters, metabolism or neuronal synaptic networks. Loss of homeostasis represents the underlying cause of all brain disorders. Thus, astrocytes are likely involved in most if not all of the brain pathologies. We tabulate astroglial homeostatic functions along with pathological condition that arise from dysfunction of these glial cells. Classification of astrocytes is presented with the emphasis on evolutionary trails, morphological appearance and numerical preponderance. We note that, even though astrocytes from a variety of mammalian species share some common features, human astrocytes appear to be the largest and most complex of all astrocytes studied thus far. It is then an imperative to develop humanized models to study the role of astrocytes in brain pathologies, which is perhaps most abundantly clear in the case of glioblastoma multiforme.

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.

Role of glutamatergic neurotransmission in the enteric nervous system and brain-gut axis in health and disease

Several studies have been carried out in the last 30 years in the attempt to clarify the possible role of glutamate as a neurotransmitter/neuromodulator in the gastrointestinal tract. Such effort has provided immunohistochemical, biomolecular and functional data suggesting that the entire glutamatergic neurotransmitter machinery is present in the complex circuitries of the enteric nervous system (ENS), which participates to the local coordination of gastrointestinal functions. Glutamate is also involved in the regulation of the brain-gut axis, a bi-directional connection pathway between the central nervous system (CNS) and the gut. The neurotransmitter contributes to convey information, via afferent fibers, from the gut to the brain, and to send appropriate signals, via efferent fibers, from the brain to control gut secretion and motility. In analogy with the CNS, an increasing number of studies suggest that dysregulation of the enteric glutamatergic neurotransmitter machinery may lead to gastrointestinal dysfunctions. On the whole, this research field has opened the possibility to find new potential targets for development of drugs for the treatment of gastrointestinal diseases. The present review analyzes the more recent literature on enteric glutamatergic neurotransmission both in physiological and pathological conditions, such as gastroesophageal reflux, gastric acid hypersecretory diseases, inflammatory bowel disease, irritable bowel syndrome and intestinal ischemia/reperfusion injury.

GLT-1: The elusive presynaptic glutamate transporter

Historically, glutamate uptake in the CNS was mainly attributed to glial cells for three reasons: 1) none of the glutamate transporters were found to be located in presynaptic terminals of excitatory synapses; 2) the putative glial transporters, GLT-1 and GLAST are expressed at high levels in astrocytes; 3) studies of the constitutive GLT-1 knockout as well as pharmacological studies demonstrated that >90% of glutamate uptake into forebrain synaptosomes is mediated by the operation of GLT-1. Here we summarize the history leading up to the recognition of GLT-1a as a presynaptic glutamate transporter. A major issue now is understanding the physiological and pathophysiological significance of the expression of GLT-1 in presynaptic terminals. To elucidate the cell-type specific functions of GLT-1, a conditional knockout was generated with which to inactivate the GLT-1 gene in different cell types using Cre/lox technology. Astrocytic knockout led to an 80% reduction of GLT-1 expression, resulting in intractable seizures and early mortality as seen also in the constitutive knockout. Neuronal knockout was associated with no obvious phenotype. Surprisingly, synaptosomal uptake capacity (Vmax) was found to be significantly reduced, by 40%, in the neuronal knockout, indicating that the contribution of neuronal GLT-1 to synaptosomal uptake is disproportionate to its protein expression (5-10%). Conversely, the contribution of astrocytic GLT-1 to synaptosomal uptake was much lower than expected. In contrast, the loss of uptake into liposomes prepared from brain protein from astrocyte and neuronal knockouts was proportionate with the loss of GLT-1 protein, suggesting that a large portion of GLT-1 in astrocytic membranes in synaptosomal preparations is not functional, possibly because of a failure to reseal. These results suggest the need to reinterpret many previous studies using synaptosomal uptake to investigate glutamate transport itself as well as changes in glutamate homeostasis associated with normal functions, neurodegeneration, and response to drugs.

Clinical aspects of urea cycle dysfunction and altered brain energy metabolism on modulation of glutamate receptors and transporters in acute and chronic hyperammonemia

In living organisms, nitrogen arise primarily as ammonia (NH3) and ammonium (NH4(+)), which is a main component of the nucleic acid pool and proteins. Although nitrogen is essential for growth and maintenance in animals, but when the nitrogenous compounds exceeds the normal range which can quickly lead to toxicity and death. Urea cycle is the common pathway for the disposal of excess nitrogen through urea biosynthesis. Hyperammonemia is a consistent finding in many neurological disorders including congenital urea cycle disorders, reye's syndrome and acute liver failure leads to deleterious effects. Hyperammonemia and liver failure results in glutamatergic neurotransmission which contributes to the alteration in the function of the glutamate-nitric oxide-cGMP pathway, modulates the important cerebral process. Even though ammonia is essential for normal functioning of the central nervous system (CNS), in particular high concentrations of ammonia exposure to the brain leads to the alterations of glutamate transport by the transporters. Several glutamate transporters have been recognized in the central nervous system and each has a unique physiological property and distribution. The loss of glutamate transporter activity in brain during acute liver failure and hyperammonemia is allied with increased extracellular brain glutamate concentrations which may be conscientious for the cerebral edema and ultimately cell death.

Shifts in excitatory/inhibitory balance by juvenile stress: A role for neuron-astrocyte interaction in the dentate gyrus

Childhood trauma is a well-described risk factor for the development of stress-related psychopathology such as posttraumatic stress disorder or depression later in life. Childhood adversity can be modeled in rodents by juvenile stress (JS) protocols, resulting in impaired coping with stressful challenges in adulthood. In the current study, we investigated the long-lasting impact of JS on the expression of molecular factors for glutamate and γ-aminobutyric acid (GABA) uptake and turnover in sublayers of the dentate gyrus (DG) using laser microdissection and quantitative real-time polymerase chain reaction. We observed reduced mRNA expression levels after JS for factors mediating astrocytic glutamate and GABA uptake and degradation. These alterations were prominently observed in the dorsal but not ventral DG granule cell layer, indicating a lasting change in astrocytic GABA and glutamate metabolism that may affect dorsal DG network activity. Indeed, we observed increased inhibition and a lack of facilitation in response to paired-pulse stimulation at short interstimulus intervals in the dorsal DG after JS, while no alterations were evident in basal synaptic transmission or forms of long-term plasticity. The shift in paired-pulse response was mimicked by pharmacologically blocking the astrocytic GABA transporter GAT-3 in naïve animals. Accordingly, reduced expression levels of GAT-3 were confirmed at the protein level in the dorsal granule cell layer of rats stressed in juvenility. Together, these data demonstrate a lasting shift in the excitatory/inhibitory balance of dorsal DG network activity by JS that appears to be mediated by decreased GABA uptake into astrocytes.

Glutamate transporter EAAT2: regulation, function, and potential as a therapeutic target for neurological and psychiatric disease

Glutamate is the predominant excitatory neurotransmitter in the central nervous system. Excitatory amino acid transporter 2 (EAAT2) is primarily responsible for clearance of extracellular glutamate to prevent neuronal excitotoxicity and hyperexcitability. EAAT2 plays a critical role in regulation of synaptic activity and plasticity. In addition, EAAT2 has been implicated in the pathogenesis of many central nervous system disorders. In this review, we summarize current understanding of EAAT2, including structure, pharmacology, physiology, and functions, as well as disease relevancy, such as in stroke, Parkinson's disease, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, major depressive disorder, and addiction. A large number of studies have demonstrated that up-regulation of EAAT2 protein provides significant beneficial effects in many disease models suggesting EAAT2 activation is a promising therapeutic approach. Several EAAT2 activators have been identified. Further understanding of EAAT2 regulatory mechanisms could improve development of drug-like compounds that spatiotemporally regulate EAAT2.

GLT-1 Transport Stoichiometry Is Constant at Low and High Glutamate Concentrations when Chloride Is Substituted by Gluconate

Glutamate is the major excitatory neurotransmitter, but prolonged exposure even at micromolar concentrations causes neuronal death. Extracellular glutamate is maintained at nanomolar level by glutamate transporters, which, however, may reverse transport and release glutamate. If and when the reverse occurs depends on glutamate transport stoichiometry (GTS). Previously we found that in the presence of chloride, the coupled GLT-1 glutamate transporter current and its relationship to radiolabeled glutamate flux significantly decreased when extracellular glutamate concentration increased above 0.2 mM, which implies a change in GTS. Such high concentrations are feasible near GLT-1 expressed close to synaptic release site during excitatory neurotransmission. The aim of this study was to determine GLT-1 GTS at both low (19–75 μM) and high (300–1200 μM) glutamate concentration ranges. GTS experiments were conducted in the absence of chloride to avoid contributions by the GLT-1 uncoupled chloride conductance. Mathematical analysis of the transporter thermodynamic equilibrium allowed us to derive equations revealing the number of a particular type of ion transported per elementary charge based on the measurements of the transporter reversal potential. We found that GLT-1a expressed in COS-7 cells co-transports 1.5 Na⁺, 0.5 Glu^-, 0.5 H⁺ and counter-transports 0.6 K⁺ per elementary charge in both glutamate concentration ranges, and at both 37°C and 26°C temperatures. The thermodynamic parameter Q₁₀ = 2.4 for GLT-1 turnover rate of 19 s^-1 (37°C, -50 mV) remained constant in the 10 μM–10 mM glutamate concentration range. Importantly, the previously reported decrease in the current/flux ratio at high glutamate concentration was not seen in the absence of chloride in both COS-7 cells and cultured rat neurons. Therefore, only in the absence of chloride, GLT-1 GTS remains constant at all glutamate concentrations. Possible explanations for why apparent GTS might vary in the presence of chloride are discussed.

Effects of ceftriaxone on GLT1 isoforms, xCT and associated signaling pathways in P rats exposed to ethanol

RATIONALE

Several studies have demonstrated a correlation between extracellular glutamate concentration in the mesolimbic reward pathway and alcohol craving. Extracellular glutamate concentration is regulated by several glutamate transporters. Glial glutamate transporter 1 (GLT1) is one of them that regulates the majority of extracellular glutamate concentration. In addition, cystine/glutamate antiporter (xCT) is another transporter that regulates extracellular glutamate.

OBJECTIVES

We focus in this study to determine the effects of ceftriaxone, β-lactam antibiotic, on glial proteins such as GLT1 isoforms, xCT, glutamate aspartate transporter (GLAST), and several associated signaling pathways as well as ethanol intake in P rats. Additionally, to examine the onset of signaling pathways associated with GLT1 upregulation following ceftriaxone treatment, we tested 2- versus 5-day daily dosing of ceftriaxone.

RESULTS

Ceftriaxone treatment (100 mg/kg), 2 and 5 days, resulted in about five fold reduction in ethanol intake by P rats. The reduction in ethanol intake was associated with significantly enhanced expression of GLT1, GLT1a, GLT1b, and xCT in the nucleus accumbens (NAc) and prefrontal cortex (PFC) of 5-day ceftriaxone-treated P rats. Two-day-treated P rats showed marked changes in expression of these glutamate transporters in the PFC but not in the NAc. Importantly, ceftriaxone-treated P rats (2 and 5 days) demonstrated enhanced phosphorylation of Akt and nuclear translocation of nuclear factor kappaB (NFκB) in the NAc and PFC compared to control animals.

CONCLUSIONS

These findings demonstrate that ceftriaxone treatment induced upregulation of GLT1, GLT1 isoforms, and xCT in association with activation of the Akt-NFκB signaling pathway.

Differential regulation of two isoforms of the glial glutamate transporter EAAT2 by DLG1 and CaMKII

The gene for EAAT2, the major astrocytic glutamate transporter, generates two carrier isoforms (EAAT2a and EAAT2b) that vary at their C termini as a consequence of alternative RNA splicing. The EAAT2b cytoplasmic C terminus contains a postsynaptic density-95/Discs large/zona occludens-1 (PDZ) ligand, which is absent in EAAT2a. To understand how the distinct C termini might affect transporter trafficking and surface localization, we generated Madin-Darby canine kidney (MDCK) cells that stably express EGFP-EAAT2a or EGFP-EAAT2b and found robust basolateral membrane expression of the EAAT2b isoform. In contrast, EAAT2a displayed a predominant distribution within intracellular vesicle compartments, constitutively cycling to and from the membrane. Addition of the PDZ ligand to EAAT2a as well as its deletion from EAAT2b confirmed the importance of the motif for cell-surface localization. Using EAAT2 constructs with an extracellular biotin acceptor tag to directly assess surface proteins, we observed significant PDZ ligand-dependent EAAT2b surface expression in cultured astrocytes, consistent with observations in cell lines. Discs large homolog 1 (DLG1; SAP97), a PDZ protein prominent in both astrocytes and MDCK cells, colocalized and coimmunoprecipitated with EAAT2b. shRNA knockdown of DLG1 expression decreased surface EAAT2b in both MDCK cells and cultured astrocytes, suggesting that the DLG scaffolding protein stabilizes EAAT2b at the surface. DLG1 can be phosphorylated by Ca(2+)/calmodulin-dependent protein kinase (CaMKII), resulting in disruption of its PDZ-mediated interaction. In murine astrocytes and acute brain slices, activation of CaMKII decreases EAAT2b surface expression but does not alter the distribution of EAAT2a. These data indicate that the surface expression and function of EAAT2b can be rapidly modulated through the disruption of its interaction with DLG1 by CaMKII activation.

Appearance of fast astrocytic component in voltage-sensitive dye imaging of neural activity

Background

Voltage-sensitive dye (VSD) imaging and intrinsic optical signals (IOS) are widely used methods for monitoring spatiotemporal neural activity in extensive networks. In spite of that, identification of their major cellular and molecular components has not been concluded so far.

Results

We addressed these issues by imaging spatiotemporal spreading of IOS and VSD transients initiated by Schaffer collateral stimulation in rat hippocampal slices with temporal resolution comparable to standard field potential recordings using a 464-element photodiode array. By exploring the potential neuronal and astroglial molecular players in VSD and IOS generation, we identified multiple astrocytic mechanisms that significantly contribute to the VSD signal, in addition to the expected neuronal targets. Glutamate clearance through the astroglial glutamate transporter EAAT2 has been shown to be a significant player in VSD generation within a very short (<5 ms) time-scale, indicating that astrocytes do contribute to the development of spatiotemporal VSD transients previously thought to be essentially neuronal. In addition, non-specific anion channels, astroglial K⁺ clearance through K_ir4.1 channel and astroglial Na⁺/K⁺ ATPase also contribute to IOS and VSD transients.

Conclusion

VSD imaging cannot be considered as a spatially extended field potential measurement with predominantly neuronal origin, instead it also reflects a fast communication between neurons and astrocytes.

Glutamate transporter splice variant expression in an enriched pyramidal cell population in schizophrenia

Dysregulation of the glutamate transporters EAAT1 and EAAT2 and their isoforms have been implicated in schizophrenia. EAAT1 and EAAT2 expression has been studied in different brain regions but the prevalence of astrocytic glutamate transporter expression masks the more subtle changes in excitatory amino acid transporters (EAATs) isoforms in neurons in the cortex. Using laser capture microdissection, pyramidal neurons were cut from the anterior cingulate cortex of postmortem schizophrenia (n = 20) and control (n = 20) subjects. The messenger RNA (mRNA) levels of EAAT1, EAAT2 and the splice variants EAAT1 exon9skipping, EAAT2 exon9skipping and EAAT2b were analyzed by real time PCR (RT-PCR) in an enriched population of neurons. Region-level expression of these transcripts was measured in postmortem schizophrenia (n = 25) and controls (n = 25). The relationship between selected EAAT polymorphisms and EAAT splice variant expression was also explored. Anterior cingulate cortex pyramidal cell expression of EAAT2b mRNA was increased (P < 0.001; 67%) in schizophrenia subjects compared with controls. There was no significant change in other EAAT variants. EAAT2 exon9skipping mRNA was increased (P < 0.05; 38%) at region level in the anterior cingulate cortex with no significant change in other EAAT variants at region level. EAAT2 single-nucleotide polymorphisms were significantly associated with changes in EAAT2 isoform expression. Haloperidol decanoate-treated animals, acting as controls for possible antipsychotic effects, did not have significantly altered neuronal EAAT2b mRNA levels. The novel finding that EAAT2b levels are increased in populations of anterior cingulate cortex pyramidal cells further demonstrates a role for neuronal glutamate transporter splice variant expression in schizophrenia.

GLT-1 transporter: an effective pharmacological target for various neurological disorders

L-Glutamate is the predominant excitatory neurotransmitter in the central nervous system (CNS) and is directly and indirectly involved in a variety of brain functions. Glutamate is released in the synaptic cleft at a particular concentration that further activates the various glutaminergic receptors. This concentration of glutamate in the synapse is maintained by either glutamine synthetase or excitatory amino acid proteins which reuptake the excessive glutamate from the synapse and named as excitatory amino acid transporters (EAATs). Out of all the subtypes GLT-1 (glutamate transporter 1) is abundantly distributed in the CNS. Down-regulation of GLT-1 is reported in various neurological diseases such as, epilepsy, stroke, Alzheimer's disease and movement disorders. Therefore, positive modulators of GLT-1 which up-regulate the GLT-1 expression can serve as a potential target for the treatment of neurological disorders. GLT-1 translational activators such as ceftriaxone are found to have significant protective effects in ALS and epilepsy animal models, suggesting that this translational activation approach works well in rodents and that these compounds are worth further pursuit for various neurological disorders. This drug is currently in human clinical trials for ALS. In addition, a thorough understanding of the mechanisms underlying translational regulation of GLT-1, such as identifying the molecular targets of the compounds, signaling pathways involved in the regulation, and translational activation processes, is very important for this novel drug-development effort. This review mainly emphasizes the role of glutamate and its transporter, GLT-1 subtype in excitotoxicity. Further, recent reports on GLT-1 transporters for the treatment of various neurological diseases, including a summary of the presumed physiologic mechanisms behind the pharmacology of these disorders are also explained.

Localization of excitatory amino acid transporters EAAT1 and EAAT2 in human postmortem cortex: a light and electron microscopic study

The process of glutamate release, activity, and reuptake involves the astrocyte, the presynaptic and postsynaptic neurons. Glutamate is released into the synapse and may occupy and activate receptors on both neurons and astrocytes. Glutamate is rapidly removed from the synapse by a family of plasma membrane excitatory amino acid transporters (EAATs), also localized to neurons and astrocytes. The purpose of the present study was to examine EAAT labeling in the postmortem human cortex at the light and electron microscopic (EM) levels. The postmortem prefrontal cortex was processed for EAAT1 and EAAT2 immunohistochemistry. At the light microscopic level, EAAT1 and EAAT2 labeling was found in both gray and white matter. Most cellular labeling was in small cells which were morphologically similar to glia. In addition, EAAT1-labeled neurons were scattered throughout, some of which were pyramidal in shape. At the EM level, EAAT1 and EAAT2 labeling was found in astrocytic soma and processes surrounding capillaries. EAAT labeling was also found in small astrocytic processes adjacent to axon terminals forming asymmetric (glutamatergic) synapses. While EAAT2 labeling was most prevalent in astrocytic processes, EAAT1 labeling was also present in neuronal processes including the soma, axons, and dendritic spines. Expression of EAAT1 protein on neurons may be due to the hypoxia associated with the postmortem interval, and requires further confirmation. The localization of EAATs on the astrocytic plasma membrane and adjacent to excitatory synapses is consistent with the function of facilitating glutamate reuptake and limiting glutamate spillover. Establishment that EAAT1 and EAAT2 can be measured at the EM level in human postmortem tissues will permit testing of hypotheses related to these molecules in diseases lacking analogous animal models.

Laser-scanning astrocyte mapping reveals increased glutamate-responsive domain size and disrupted maturation of glutamate uptake following neonatal cortical freeze-lesion

Astrocytic uptake of glutamate shapes extracellular neurotransmitter dynamics, receptor activation, and synaptogenesis. During development, glutamate transport becomes more robust. How neonatal brain insult affects the functional maturation of glutamate transport remains unanswered. Neonatal brain insult can lead to developmental delays, cognitive losses, and epilepsy; the disruption of glutamate transport is known to cause changes in synaptogenesis, receptor activation, and seizure. Using the neonatal freeze-lesion (FL) model, we have investigated how insult affects the maturation of astrocytic glutamate transport. As lesioning occurs on the day of birth, a time when astrocytes are still functionally immature, this model is ideal for identifying changes in astrocyte maturation following insult. Reactive astrocytosis, astrocyte proliferation, and in vitro hyperexcitability are known to occur in this model. To probe astrocyte glutamate transport with better spatial precision we have developed a novel technique, Laser Scanning Astrocyte Mapping (LSAM), which combines glutamate transport current (TC) recording from astrocytes with laser scanning glutamate photolysis. LSAM allows us to identify the area from which a single astrocyte can transport glutamate and to quantify spatial heterogeneity in the rate of glutamate clearance kinetics within that domain. Using LSAM, we report that cortical astrocytes have an increased glutamate-responsive area following FL and that TCs have faster decay times in distal, as compared to proximal processes. Furthermore, the developmental shift from GLAST- to GLT-1-dominated clearance is disrupted following FL. These findings introduce a novel method to probe astrocyte glutamate uptake and show that neonatal cortical FL disrupts the functional maturation of cortical astrocytes.

Tumour associated epilepsy and glutamate excitotoxicity in patients with gliomas

Tumour associated epilepsy (TAE) is common, debilitating and often not successfully controlled by surgical resection of the tumour and administration of multiple anti-epileptic drugs. It represents a cause of significant lost quality of life in an incurable disease and is therefore an important subject for ongoing research. The pathogenesis of TAE is likely to be multifactorial and involve, on the microscopic level, the interaction of genetic factors, changes in the peritumoural microenvironment, alterations in synaptic neurotransmitter release and re-uptake, and the excitotoxic effects of glutamate. On a macroscopic level, the occurrence of TAE is likely to be influenced by tumour size, location and interaction with environmental factors. The optimal treatment of TAE requires a multi-disciplinary approach with input from neurosurgeons, neurologists, radiologists, pathologists and basic scientists. This article reviews the current literature regarding the incidence, treatment, and aetiology of TAE.

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.

Impairment of neuronal glutamate uptake and modulation of the glutamate transporter GLT-1 induced by retinal ischemia

Excitotoxicity has been implicated in the retinal neuronal loss in several ocular pathologies including glaucoma. Dysfunction of Excitatory Amino Acid Transporters is often a key component of the cascade leading to excitotoxic cell death. In the retina, glutamate transport is mainly operated by the glial glutamate transporter GLAST and the neuronal transporter GLT-1. In this study we evaluated the expression of GLAST and GLT-1 in a rat model of acute glaucoma based on the transient increase of intraocular pressure (IOP) and characterized by high glutamate levels during the reperfusion that follows the ischemic event associated with raised IOP. No changes were reported in GLAST expression while, at neuronal level, a reduction of glutamate uptake and of transporter reversal-mediated glutamate release was observed in isolated retinal synaptosomes. This was accompanied by modulation of GLT-1 expression leading to the reduction of the canonical 65 kDa form and upregulation of a GLT-1-related 38 kDa protein. These results support a role for neuronal transporters in glutamate accumulation observed in the retina following an ischemic event and suggest the presence of a GLT-1 neuronal new alternative splice variant, induced in response to the detrimental stimulus.

Functional modulation of the glutamate transporter variant GLT1b by the PDZ domain protein PICK1

The dominant glutamate transporter isoform in the mammalian brain, GLT1, exists as at least three splice variants, GLT1a, GLT1b, and GLT1c. GLT1b interacts with the scaffold protein PICK1 (protein interacting with kinase C1), which is implicated in glutamatergic neurotransmission via its regulatory effect on trafficking of AMPA-type glutamate receptors. The 11 extreme C-terminal residues specific for the GLT1b variant are essential for its specific interaction with the PICK1 PDZ domain, but a functional consequence of this interaction has remained unresolved. To identify a functional effect of PICK1 on GLT1a or GLT1b separately, we employed the Xenopus laevis expression system. GLT1a and GLT1b displayed similar electrophysiological properties and EC50 for glutamate. Co-expressed PICK1 localized efficiently to the plasma membrane and resulted in a 5-fold enhancement of the leak current in GLT1b-expressing oocytes with only a minor effect on [(3)H]glutamate uptake. Three different GLT1 substrates all caused a slow TBOA-sensitive decay in the membrane current upon prolonged application, which provides support for the leak current being mediated by GLT1b itself. Leak and glutamate-evoked currents in GLT1a-expressing oocytes were unaffected by PICK1 co-expression. PKC activation down-regulated GLT1a and GLT1b activity to a similar extent, which was not affected by co-expression of PICK1. In conclusion, PICK1 may not only affect glutamatergic neurotransmission by its regulatory effect on glutamate receptors but may also affect neuronal excitability via an increased GLT1b-mediated leak current. This may be particularly relevant in pathological conditions such as amyotrophic lateral sclerosis and cerebral hypoxia, which are associated with neuronal GLT1b up-regulation.

Expression of EAAT2 in neurons and protoplasmic astrocytes during human cortical development

The major regulators of synaptic glutamate in the cerebral cortex are the excitatory amino acid transporters 1-3 (EAAT1-3). In this study, we determined the cellular and temporal expression of EAAT1-3 in the developing human cerebral cortex. We applied single- and double-label immunocytochemistry to normative frontal or parietal (associative) cortex samples from 14 cases ranging in age from 23 gestational weeks to 2.5 postnatal years. The most striking finding was the transient expression of EAAT2 in layer V pyramidal neuronal cell bodies up until 8 postnatal months prior to its expression in protoplasmic astrocytes at 41 postconceptional weeks onward. EAAT2 was also expressed in neurons in layer I (presumed Cajal-Retzius cells), and white matter (interstitial) neurons. This expression in neurons in the developing human cortex contrasts with findings by others of transient expression exclusively in axon tracts in the developing sheep and rodent brain. With western blotting, we found that EAAT2 was expressed as a single band until 2 postnatal months, after which it was expressed as two bands. The expression of EAAT2 in pyramidal neurons during human brain development may contribute to cortical vulnerability to excitotoxicity during the critical period for perinatal hypoxic-ischemic encephalopathy. In addition, by studying the expression of EAAT1 and EAAT2 glutamate transporters, it was possible to document the development of protoplasmic astrocytes.

Enteric glia and neuroprotection: basic and clinical aspects

The enteric nervous system (ENS), a major regulatory system for gastrointestinal function, is composed of neurons and enteric glial cells (EGCs). Enteric glia have long been thought to provide only structural support to neurons. However, recent evidence indicates enteric glia-neuron cross talk significantly contributes to neuronal maintenance, survival, and function. Thus damage to EGCs may trigger neurodegenerative processes thought to play a role in gastrointestinal dysfunctions and symptoms. The purpose of this review is to provide an update on EGCs, particularly focusing on their possible neuroprotective features and the resultant enteric neuron abnormalities subsequent to EGC damage. These neuroprotective mechanisms may have pathogenetic relevance in a variety of functional and inflammatory gut diseases. Basic and clinical (translational) studies support a neuroprotective role mediated by EGCs. Different models have been developed to test whether selective EGC damage/ablation has an impact on gut functions and the ENS. Preclinical data indicated that selective EGC alterations were associated with changes in gut physiology related to enteric neuron abnormalities. In humans, a substantial loss of EGCs was described in patients with various functional and/or inflammatory gastrointestinal diseases. However, whether EGC changes precede or follow neuronal degeneration and loss and how this damage occurs is not defined. Additional studies on EGC neuroprotective capacity are expected to improve knowledge of gut diseases and pave the way for targeted therapeutic strategies of underlying neuropathies.

Alternate splicing and expression of the glutamate transporter EAAT5 in the rat retina

Excitatory amino acid transporter 5 (EAAT5) is an unusual glutamate transporter that is expressed in the retina, where it is localised to two populations of glutamatergic neurons, namely the bipolar neurons and photoreceptors. EAAT5 exhibits two distinct properties, acting both as a slow glutamate transporter and as a glutamate-gated inhibitory receptor. The latter property is attributable to a co-associated chloride conductance. EAAT5 has previously been thought to exist only as a full-length form. We now demonstrate by PCR cloning and sequencing, the presence of five novel splice variant forms of EAAT5 which skip either partial or complete exons in the rat retina. Furthermore, we demonstrate that each of these variants is expressed at the protein level as assessed by Western blotting using splice-specific antibodies that we have generated. We conclude that EAAT5 exists in multiple spliced forms, and propose, based upon retention or absence of key structural features, that these variant forms may potentially exhibit distinct properties relative to the originally described form of EAAT5.

Role of excitatory amino acid transporter-2 (EAAT2) and glutamate in neurodegeneration: opportunities for developing novel therapeutics

Glutamate is an essential excitatory neurotransmitter regulating brain functions. Excitatory amino acid transporter (EAAT)-2 is one of the major glutamate transporters expressed predominantly in astroglial cells and is responsible for 90% of total glutamate uptake. Glutamate transporters tightly regulate glutamate concentration in the synaptic cleft. Dysfunction of EAAT2 and accumulation of excessive extracellular glutamate has been implicated in the development of several neurodegenerative diseases including Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis. Analysis of the 2.5  kb human EAAT2 promoter showed that NF-κB is an important regulator of EAAT2 expression in astrocytes. Screening of approximately 1,040 FDA-approved compounds and nutritionals led to the discovery that many β-lactam antibiotics are transcriptional activators of EAAT2 resulting in increased EAAT2 protein levels. Treatment of animals with ceftriaxone (CEF), a β-lactam antibiotic, led to an increase of EAAT2 expression and glutamate transport activity in the brain. CEF has neuroprotective effects in both in vitro and in vivo models based on its ability to inhibit neuronal cell death by preventing glutamate excitotoxicity. CEF increases EAAT2 transcription in primary human fetal astrocytes through the NF-κB signaling pathway. The NF-κB binding site at -272 position was critical in CEF-mediated EAAT2 protein induction. These studies emphasize the importance of transcriptional regulation in controlling glutamate levels in the brain. They also emphasize the potential utility of the EAAT2 promoter for developing both low and high throughput screening assays to identify novel small molecule regulators of glutamate transport with potential to ameliorate pathological changes occurring during and causing neurodegeneration.

Localisation of novel forms of glutamate transporters and the cystine-glutamate antiporter in the choroid plexus: Implications for CSF glutamate homeostasis

The choroid plexus is a structure within each ventricle of the brain that is composed of fenestrated vessels surrounded by secretory epithelial cells. The epithelial cells are linked by tight junctions to create a permeability barrier. The epithelial cells are derived from neuroectoderm, and are thus defined by some authors as a subtype of macroglia. Glutamate is a tightly regulated substance in the CSF, as it is in the rest of the brain. In the brain macroglia express multiple sodium dependent and independent glutamate transporters and are the main regulators of extracellular glutamate. However, the identities of the transporters in the choroid plexus and their localisations have remained poorly defined. In this study we examined the expression and distribution of multiple splice variants of classical sodium-dependent glutamate transporters, as well as the cystine-glutamate antiporter, and the PDZ protein NHERF1, (which acts as a molecular anchor for proteins such as the glutamate transporter GLAST). We identified three forms of sodium-dependent transporters (GLAST1a, GLAST1c and GLT1b) that are expressed at the apical surface of the epithelial cells, a location that matches the distribution of NHERF1 and the cystine-glutamate antiporter. We propose that this coincident localisation of GLAST1a/GLAST1c/GLT1b and the cystine-glutamate antiporter would permit the cyclical trafficking of glutamate and thus optimise the accumulation of cystine for the formation of glutathione in the choroid plexus.

The glial regenerative response to central nervous system injury is enabled by pros-notch and pros-NFκB feedback

Organisms are structurally robust, as cells accommodate changes preserving structural integrity and function. The molecular mechanisms underlying structural robustness and plasticity are poorly understood, but can be investigated by probing how cells respond to injury. Injury to the CNS induces proliferation of enwrapping glia, leading to axonal re-enwrapment and partial functional recovery. This glial regenerative response is found across species, and may reflect a common underlying genetic mechanism. Here, we show that injury to the Drosophila larval CNS induces glial proliferation, and we uncover a gene network controlling this response. It consists of the mutual maintenance between the cell cycle inhibitor Prospero (Pros) and the cell cycle activators Notch and NFκB. Together they maintain glia in the brink of dividing, they enable glial proliferation following injury, and subsequently they exert negative feedback on cell division restoring cell cycle arrest. Pros also promotes glial differentiation, resolving vacuolization, enabling debris clearance and axonal enwrapment. Disruption of this gene network prevents repair and induces tumourigenesis. Using wound area measurements across genotypes and time-lapse recordings we show that when glial proliferation and glial differentiation are abolished, both the size of the glial wound and neuropile vacuolization increase. When glial proliferation and differentiation are enabled, glial wound size decreases and injury-induced apoptosis and vacuolization are prevented. The uncovered gene network promotes regeneration of the glial lesion and neuropile repair. In the unharmed animal, it is most likely a homeostatic mechanism for structural robustness. This gene network may be of relevance to mammalian glia to promote repair upon CNS injury or disease.

Sequences of the non-coding RNA, NTAB, are contained within the 3'-UTR of human and rat EAAT2/GLT-1 transcripts and act as transcriptional enhancers

In the CNS, extracellular glutamate is predominantly cleared by astroglial cells through the high-affinity glutamate transporter subtype, EAAT2/GLT-1. Expression of EAAT2/GLT-1 is perturbed in various acute and chronic brain diseases eventually allowing for the onset of neurotoxic extracellular glutamate concentrations and subsequent excitotoxic neuronal cell death. The idea that glutamate-induced brain damage could be prevented by restoring glutamate homeostasis in the injured brain, spurred considerable interest in identifying the mechanisms controlling EAAT2/GLT-1 expression. Since to date most of this study was done with rat astrocytes, an emerging issue is to whether these findings would also apply to humans. While so far it is known that the promoter region of the EAAT2/GLT-1 gene is strikingly similar in rat and man, little information is available on the function of the EAAT2/GLT-1 3'-UTR in the control of EAAT2/GLT-1 expression in general as well as across both species. We now report on the presence of a homologous sequence within the 3'-UTR of the human and rat EAAT2/GLT-1 gene which we identified as a partial sequence of the putative non-coding RNA, Ntab. We further demonstrate that fragments of Ntab act as enhancers of EAAT2/GLT-1 transcription. Finally, we unravel that partial Ntab sequences are selectively present in the vicinity of the EAAT2/GLT-1 gene in several other mammalians, implying a conserved function of this sequence in the vertebrate CNS.