Changes in glial glutamate transporters in human epileptogenic hippocampus: Inadequate explanation for high extracellular glutamate during seizures

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Discovery of (R)-N-Benzyl-2-(2,5-dioxopyrrolidin-1-yl)propanamide [], a Novel Orally Bioavailable EAAT2 Modulator with Drug-like Properties and Potent Antiseizure Activity In Vivo

(R)-7 [(R)-AS-1] showed broad-spectrum antiseizure activity across in vivo mouse seizure models: maximal electroshock (MES), 6 Hz (32/44 mA), acute pentylenetetrazol (PTZ), and PTZ-kindling. A remarkable separation between antiseizure activity and CNS-related adverse effects was also observed. In vitro studies with primary glia cultures and COS-7 cells expressing the glutamate transporter EAAT2 showed enhancement of glutamate uptake, revealing a stereoselective positive allosteric modulator (PAM) effect, further supported by molecular docking simulations. (R)-7 [(R)-AS-1] was not active in EAAT1 and EAAT3 assays and did not show significant off-target activity, including interactions with targets reported for marketed antiseizure drugs, indicative of a novel and unprecedented mechanism of action. Both in vivo pharmacokinetic and in vitro absorption, distribution, metabolism, excretion, toxicity (ADME-Tox) profiles confirmed the favorable drug-like potential of the compound. Thus, (R)-7 [(R)-AS-1] may be considered as the first-in-class small-molecule PAM of EAAT2 with potential for further preclinical and clinical development in epilepsy and possibly other CNS disorders.

Role of glutamate excitotoxicity and glutamate transporter EAAT2 in epilepsy: Opportunities for novel therapeutics development

Epilepsy is a complex neurological syndrome characterized by seizures resulting from neuronal hyperexcitability and sudden and synchronized bursts of electrical discharges. Impaired astrocyte function that results in glutamate excitotoxicity has been recognized to play a key role in the pathogenesis of epilepsy. While there are 26 drugs marketed as anti-epileptic drugs no current treatments are disease modifying as they only suppress seizures rather than the development and progression of epilepsy. Excitatory amino acid transporters (EAATs) are critical for maintaining low extracellular glutamate concentrations and preventing excitotoxicity. When extracellular glutamate concentrations rise to abnormal levels, glutamate receptor overactivation and the subsequent excessive influx of calcium into the post-synaptic neuron can trigger cell death pathways. In this review we discuss targeting EAAT2, the predominant glutamate transporter in the CNS, as a promising approach for developing therapies for epilepsy. EAAT2 upregulation via transcriptional and translational regulation has proven successful in vivo in reducing spontaneous recurrent seizures and offering neuroprotective effects. Another approach to regulate EAAT2 activity is through positive allosteric modulation (PAM). Novel PAMs of EAAT2 have recently been identified and are under development, representing a promising approach for the advance of novel therapeutics for epilepsy.

Astrocyte and glutamate involvement in the pathogenesis of epilepsy in Alzheimer's disease

Alzheimer's disease (AD) can increase the risk of epilepsy by up to 10-fold compared to healthy age-matched controls. However, the pathological mechanisms that underlie this increased risk are poorly understood. Because disruption in brain glutamate homeostasis has been implicated in both AD and epilepsy, this might play a mechanistic role in the pathogenesis of epilepsy in AD. Prior to the formation of amyloid beta (Aβ) plaques, the brain can undergo pathological changes as a result of increased production of amyloid precursor protein (APP) and Aβ oligomers. Impairments in the glutamate uptake ability of astrocytes due to astrogliosis are hypothesized to be an early event occurring before Aβ plaque formation. Astrogliosis may increase the susceptibility to epileptogenesis of the brain via accumulation of extracellular glutamate and resulting excitotoxicity. Here we hypothesize that Aβ oligomers and proinflammatory cytokines can cause astrogliosis and accumulation of extracellular glutamate, which then contribute to the pathogenesis of epilepsy in AD. In this review article, we consider the evidence supporting a potential role of dysfunction of the glutamate-glutamine cycle and the astrocyte in the pathogenesis of epilepsy in AD.

Network-Related Changes in Neurotransmitters and Seizure Propagation During Rodent Epileptogenesis

OBJECTIVE

To test the hypothesis that glutamate and GABA are linked to the formation of epilepsy networks and the triggering of spontaneous seizures, we examined seizure initiation/propagation characteristics and neurotransmitter levels during epileptogenesis in a translationally relevant rodent model of mesial temporal lobe epilepsy.

METHODS

The glutamine synthetase (GS) inhibitor methionine sulfoximine was infused into one of the hippocampi in laboratory rats to create a seizure focus. Long-term video-intracranial EEG recordings and brain microdialysis combined with mass spectrometry were used to examine seizure initiation, seizure propagation, and extracellular brain levels of glutamate and GABA.

RESULTS

All seizures (n = 78 seizures, n = 3 rats) appeared first in the GS-inhibited hippocampus of all animals, followed by propagation to the contralateral hippocampus. Propagation time decreased significantly from 11.65 seconds early in epileptogenesis (weeks 1-2) to 6.82 seconds late in epileptogenesis (weeks 3-4, paired t test, p = 0.025). Baseline extracellular glutamate levels were 11.6-fold higher in the hippocampus of seizure propagation (7.3 µM) vs the hippocampus of seizure onset (0.63 µM, analysis of variance/Fisher least significant difference, p = 0.01), even though the concentrations of the major glutamate transporter proteins excitatory amino acid transporter subtypes 1 and 2 and xCT were unchanged between the brain regions. Finally, extracellular GABA in the seizure focus decreased significantly from baseline several hours before a spontaneous seizure (paired t test/false discovery rate).

CONCLUSION

The changes in glutamate and GABA suggest novel and potentially important roles of the amino acids in epilepsy network formation and in the initiation and propagation of spontaneous seizures.

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.

Glutamate induced neonatal excitotoxicity modifies the expression level of EAAT1 (GLAST) and EAAT2 (GLT-1) proteins in various brain regions of the adult rat

Glutamate-mediated excitatory synaptic signalling is primarily controlled by excitatory amino acid transporters (EAATs), such as EAAT1 and EAAT2, which are located mostly on astrocytes and, together, uptake more than 95 % of extracellular glutamate. Alterations in the functional expression levels of EAATs can lead to excessive extracellular glutamate accumulation, potentially triggering excitotoxicity and seizures, among other neurological disorders. Excitotoxicity induced in early developmental stages can lead to lasting changes in several neurotransmission systems, including the glutamatergic system, which could make the brain more susceptible to a second insult. In this study, the expression levels of EAAT1 (GLAST) and EAAT2 (GLT-1) proteins were assessed in the cerebral motor cortex (CMC), striatum, hippocampus and entorhinal cortex (EC) of male adult rats following the neonatal excitotoxic process triggered by monosodium glutamate (MSG)-treatment (4 g/kg of body weight at postnatal days 1,3,5 and 7, subcutaneously). Western blot analysis showed that neonatal MSG-treatment decreased EAAT1 expression levels in the CMC, striatum and hippocampus, while EAAT2 levels were increased in the striatum and EC and decreased in the CMC. Immunofluorescence staining confirmed the changes in EAAT1 and EAAT2 expression induced by neonatal MSG-treatment, which were accompanied by an increase in the glial fibrillary acidic protein (GFAP) immunofluorescence signalthat was particularly significant in the hippocampus. Our results show that a neonatal excitotoxic processes can induce lasting changes in the expression levels of EAAT1 and EAAT2 proteins and suggest that although astrogliosis occurs, glutamate uptake could be deficient, particularly in the CMC and hippocampus.

Vascular endothelial growth factor receptor-3 regulates astroglial glutamate transporter-1 expression via mTOR activation in reactive astrocytes following pilocarpine-induced status epilepticus

Recent evidence has shown that the vascular endothelial growth factor (VEGF) system plays a crucial role in several neuropathological processes. We previously reported an upregulation of VEGF-C and its receptor, VEGFR-3, in reactive astrocytes after the onset of status epilepticus (SE). However, it remains unknown, which molecules act as downstream signals following VEGFR-3 upregulation, and are involved in reactive astrogliosis after SE. Therefore, we investigated whether VEGFR-3 upregulation within reactive astrocytes is associated with the activation of mammalian target of rapamycin (mTOR) signaling, which we confirmed by assaying for the phosphorylated form of S6 protein (pS6), and whether VEGFR-3-mediated mTOR activation induces astroglial glutamate transporter-1 (GLT-1) expression in the hippocampus after pilocarpine-induced SE. We found that spatiotemporal expression of pS6 was consistent with VEGFR-3 expression in the hippocampus after SE, and that both pS6 and VEGFR-3 were highly expressed in SE-induced reactive astrocytes. Treatment with the mTOR inhibitor rapamycin decreased astroglial VEGFR-3 expression and GLT-1 expression after SE. Treatment with a selective inhibitor for VEGFR-3 attenuated astroglial pS6 expression as well as suppressed GLT-1 expression and astroglial reactivity in the hippocampus after SE. These findings demonstrate that VEGFR-3-mediated mTOR activation could contribute to the regulation of GLT-1 expression in reactive astrocytes during the subacute phase of epilepsy. In conclusion, the present study suggests that VEGFR-3 upregulation in reactive astrocytes may play a role in preventing hyperexcitability induced by continued seizure activity.

Astrocytes and Glutamine Synthetase in Epileptogenesis

The cellular, molecular, and metabolic mechanisms that underlie the development of mesial temporal lobe epilepsy are incompletely understood. Here we review the role of astrocytes in epilepsy development (a.k.a. epileptogenesis), particularly astrocyte pathologies related to: aquaporin 4, the inwardly rectifying potassium channel Kir4.1, monocarboxylate transporters MCT1 and MCT2, excitatory amino acid transporters EAAT1 and EAAT2, and glutamine synthetase. We propose that inhibition, dysfunction or loss of astrocytic glutamine synthetase is an important causative factor for some epilepsies, particularly mesial temporal lobe epilepsy and glioblastoma-associated epilepsy. We postulate that the regulatory mechanisms of glutamine synthetase as well as the downstream effects of glutamine synthetase dysfunction, represent attractive, new targets for antiepileptogenic interventions. Currently, no antiepileptogenic therapies are available for human use. The discovery of such interventions is important as it will fundamentally change the way we approach epilepsy by preventing the disease from ever becoming manifest after an epileptogenic insult to the brain.

Astroglial role in the pathophysiology of status epilepticus: an overview

Status epilepticus is a medical emergency with elevated morbidity and mortality rates, and represents a leading cause of epilepsy-related deaths. Though status epilepticus can occur at any age, it manifests more likely in children and elderly people. Despite the common prevalence of epileptic disorders, a complete explanation for the mechanisms leading to development of self-limited or long lasting seizures (as in status epilepticus) are still lacking. Apart from neurons, research evidence suggests the involvement of immune and glial cells in epileptogenesis. Among glial cells, astrocytes represent an ideal target for the study of the pathophysiology of status epilepticus, due to their key role in homeostatic balance of the central nervous system. During status epilepticus, astroglial cells are activated by the presence of cytokines, damage associated molecular patterns and reactive oxygen species. The persistent activation of astrocytes leads to a decrease in glutamate clearance with a corresponding accumulation in the synaptic extracellular space, increasing the chance of neuronal excitotoxicity. Moreover, major alterations in astrocytic gap junction coupling, inflammation and receptor expression, facilitate the generation of seizures. Astrocytes are also involved in dysregulation of inhibitory transmission in the central nervous system and directly participate in ionic homeostatic alterations during status epilepticus. In the present review, we focus on the functional and structural changes in astrocytic activity that participate in the development and maintenance of status epilepticus, with special attention on concurrent inflammatory alterations. We also include potential astrocytic treatment targets for status epilepticus.

Elevated MeCP2 in Mice Causes Neurodegeneration Involving Tau Dysregulation and Excitotoxicity: Implications for the Understanding and Treatment of MeCP2 Triplication Syndrome

Expression of MeCP2 must be carefully regulated as a reduction or increase results in serious neurological disorders. We are studying transgenic mice in which the MeCP2 gene is expressed at about three times higher than the normal level. Male MeCP2-Tg mice, but not female mice, suffer motor and cognitive deficits and die at 18-20 weeks of age. MeCP2-Tg mice display elevated GFAP and Tau expression within the hippocampus and cortex followed by neuronal loss in these brain regions. Loss of Purkinje neurons, but not of granule neurons in the cerebellar cortex is also seen. Exposure of cultured cortical neurons to either conditioned medium from astrocytes (ACM) derived from male MeCP2-Tg mice or normal astrocytes in which MeCP2 is expressed at elevated levels promotes their death. Interestingly, ACM from male, but not female MeCP2-Tg mice, displays this neurotoxicity reflecting the gender selectivity of neurological symptoms in mice. Male ACM, but not female ACM, contains highly elevated levels of glutamate, and its neurotoxicity can be prevented by MK-801, indicating that it is caused by excitotoxicity. Based on the close phenotypic resemblance of MeCP2-Tg mice to patients with MECP2 triplication syndrome, we suggest for the first time that the human syndrome is a neurodegenerative disorder resulting from astrocyte dysfunction that leads to Tau-mediated excitotoxic neurodegeneration. Loss of cortical and hippocampal neurons may explain the mental retardation and epilepsy in patients, whereas ataxia likely results from the loss of Purkinje neurons.

Faster flux of neurotransmitter glutamate during seizure - Evidence from 13C-enrichment of extracellular glutamate in kainate rat model

The objective is to examine how the flux of neurotransmitter glutamate from neurons to the extracellular fluid, as measured by the rate of ¹³C enrichment of extracellular glutamate (GLU_ECF), changes in response to seizures in the kainate-induced rat model of temporal-lobe epilepsy. Following unilateral intrahippocampal injection of kainate, GLU_ECF was collected by microdialysis from the CA1/CA3 region of awake rats, in combination with EEG recording of chronic-phase recurrent seizures and intravenous infusion of [2,5-¹³C]glucose. The ¹³C enrichment of GLU_ECF C5 at ~ 10 picomol level was measured by gas-chromatography mass-spectrometry. The rate of ¹³C enrichment, expressed as the increase of the fractional enrichment/min, was 0.0029 ± 0.0001/min in frequently seizing rats (n = 4); this was significantly higher (p < 0.01) than in the control (0.00167 ± 0.0001/min; n = 6) or in rats with infrequent seizures (0.00172 ± 0.0001/min; n = 6). This result strongly suggests that the flux of the excitatory neurotransmitter from neurons to the extracellular fluid is significantly increased by frequent seizures. The extracellular [¹²C + ¹³C]glutamate concentration increased progressively in frequently seizing rats. Taken together, these results strongly suggest that the observed seizure-induced high flux of glutamate overstimulated glutamate receptors, which triggered a chain reaction of excitation in the CA3 recurrent glutamatergic networks. The rate of ¹³C enrichment of extracellular glutamine (GLN_ECF) at C5 was 0.00299 ± 0.00027/min in frequently seizing rats, which was higher (p < 0.05) than in controls (0.00227 ± 0.00008/min). For the first time in vivo, this study examined the effects of epileptic seizures on fluxes of the neurotransmitter glutamate and its precursor glutamine in the extracellular fluid of the hippocampus. The advantages, limitations and the potential for improvement of this approach for pre-clinical and clinical studies of temporal-lobe epilepsy are discussed.

Glutamine-Glutamate Cycle Flux Is Similar in Cultured Astrocytes and Brain and Both Glutamate Production and Oxidation Are Mainly Catalyzed by Aspartate Aminotransferase

The glutamine-glutamate cycle provides neurons with astrocyte-generated glutamate/γ-aminobutyric acid (GABA) and oxidizes glutamate in astrocytes, and it returns released transmitter glutamate/GABA to neurons after astrocytic uptake. This review deals primarily with the glutamate/GABA generation/oxidation, although it also shows similarity between metabolic rates in cultured astrocytes and intact brain. A key point is identification of the enzyme(s) converting astrocytic α-ketoglutarate to glutamate and vice versa. Most experiments in cultured astrocytes, including those by one of us, suggest that glutamate formation is catalyzed by aspartate aminotransferase (AAT) and its degradation by glutamate dehydrogenase (GDH). Strongly supported by results shown in Table 1 we now propose that both reactions are primarily catalyzed by AAT. This is possible because the formation occurs in the cytosol and the degradation in mitochondria and they are temporally separate. High glutamate/glutamine concentrations abolish the need for glutamate production from α-ketoglutarate and due to metabolic coupling between glutamate synthesis and oxidation these high concentrations render AAT-mediated glutamate oxidation impossible. This necessitates the use of GDH under these conditions, shown by insensitivity of the oxidation to the transamination inhibitor aminooxyacetic acid (AOAA). Experiments using lower glutamate/glutamine concentration show inhibition of glutamate oxidation by AOAA, consistent with the coupled transamination reactions described here.

Human astrocytes in the diseased brain

Astrocytes are key active elements of the brain that contribute to information processing. They not only provide neurons with metabolic and structural support, but also regulate neurogenesis and brain wiring. Furthermore, astrocytes modulate synaptic activity and plasticity in part by controlling the extracellular space volume, as well as ion and neurotransmitter homeostasis. These findings, together with the discovery that human astrocytes display contrasting characteristics with their rodent counterparts, point to a role for astrocytes in higher cognitive functions. Dysfunction of astrocytes can thereby induce major alterations in neuronal functions, contributing to the pathogenesis of several brain disorders. In this review we summarize the current knowledge on the structural and functional alterations occurring in astrocytes from the human brain in pathological conditions such as epilepsy, primary tumours, Alzheimer's disease, major depressive disorder and Down syndrome. Compelling evidence thus shows that dysregulations of astrocyte functions and interplay with neurons contribute to the development and progression of various neurological diseases. Targeting astrocytes is thus a promising alternative approach that could contribute to the development of novel and effective therapies to treat brain disorders.

Alterations in mRNA and protein expression of glutamate transporters in rat hippocampus after paraoxon exposure

Organophosphates affect brain function through a variety of mechanisms beyond their shared role as cholinesterase inhibitors. The aim of the current study was to investigate the changes in the expression of glial (GLAST and GLT-1) and neuronal (EAAC1) glutamate transporters at mRNA and protein levels in paraoxon-treated rat hippocampus. Adult male Wistar rats were intraperitoneally treated with either vehicle (corn oil) or one of three dosages of paraoxon (0.3, 0.7 or 1mg/kg). After 4 or 18h, both hippocampi of each rat were collected to detect mRNA and protein expression of glutamate transporters using the quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and western blotting, respectively. Animals treated with 0.3mg/kg paraoxon showed no difference in mRNA and protein levels of the glutamate transporters when compared with control group. At 4h after exposure with 0.7 and 1mg/kg paraoxon, the expression of GLAST and GLT-1 increased at mRNA and protein levels and remained elevated after 18h. No difference in the expression of EAAC1 at mRNA and protein levels was observed in any paraoxon-treated groups compared with the control group. This study showed an increased expression of glial (GLAST and GLT-1), but not neuronal (EAAC1) glutamate transporters, in adult rat hippocampus following administration of convulsive dosages of paraoxon. These suggest a protective and compensatory adaptation for effective uptake of glutamate in hippocampus induced by paraoxon and thus attenuating seizure activity.

Regulation of astrocyte glutamate transporter-1 (GLT1) and aquaporin-4 (AQP4) expression in a model of epilepsy

Astrocytes regulate extracellular glutamate and water homeostasis through the astrocyte-specific membrane proteins glutamate transporter-1 (GLT1) and aquaporin-4 (AQP4), respectively. The role of astrocytes and the regulation of GLT1 and AQP4 in epilepsy are not fully understood. In this study, we investigated the expression of GLT1 and AQP4 in the intrahippocampal kainic acid (IHKA) model of temporal lobe epilepsy (TLE). We used real-time polymerase chain reaction (RT-PCR), Western blot, and immunohistochemical analysis at 1, 4, 7, and 30days after kainic acid-induced status epilepticus (SE) to determine hippocampal glial fibrillary acidic protein (GFAP, a marker for reactive astrocytes), GLT1, and AQP4 expression changes during the development of epilepsy (epileptogenesis). Following IHKA, all mice had SE and progressive increases in GFAP immunoreactivity and GFAP protein expression out to 30days post-SE. A significant initial increase in dorsal hippocampal GLT1 immunoreactivity and protein levels were observed 1day post SE and followed by a marked downregulation at 4 and 7days post SE with a return to near control levels by 30days post SE. AQP4 dorsal hippocampal protein expression was significantly downregulated at 1day post SE and was followed by a gradual return to baseline levels with a significant increase in ipsilateral protein levels by 30days post SE. Transient increases in GFAP and AQP4 mRNA were also observed. Our findings suggest that specific molecular changes in astrocyte glutamate transporters and water channels occur during epileptogenesis in this model, and suggest the novel therapeutic strategy of restoring glutamate and water homeostasis.

Plasticity of Hippocampal Excitatory-Inhibitory Balance: Missing the Synaptic Control in the Epileptic Brain

Synaptic plasticity is the capacity generated by experience to modify the neural function and, thereby, adapt our behaviour. Long-term plasticity of glutamatergic and GABAergic transmission occurs in a concerted manner, finely adjusting the excitatory-inhibitory (E/I) balance. Imbalances of E/I function are related to several neurological diseases including epilepsy. Several evidences have demonstrated that astrocytes are able to control the synaptic plasticity, with astrocytes being active partners in synaptic physiology and E/I balance. Here, we revise molecular evidences showing the epileptic stage as an abnormal form of long-term brain plasticity and propose the possible participation of astrocytes to the abnormal increase of glutamatergic and decrease of GABAergic neurotransmission in epileptic networks.

50 Hz hippocampal stimulation in refractory epilepsy: Higher level of basal glutamate predicts greater release of glutamate

OBJECTIVE

The effect of electrical stimulation on brain glutamate release in humans is unknown. Glutamate is elevated at baseline in the epileptogenic hippocampus of patients with refractory epilepsy, and increases during spontaneous seizures. We examined the effect of 50 Hz stimulation on glutamate release and its relationship to interictal levels in the hippocampus of patients with epilepsy. In addition, we measured basal and stimulated glutamate levels in a subset of these patients where stimulation elicited a seizure.

METHODS

Subjects (n = 10) were patients with medically refractory epilepsy who were undergoing intracranial electroencephalography (EEG) evaluation in an epilepsy monitoring unit. Electrical stimulation (50 Hz) was delivered through implanted hippocampal electrodes (n = 11), and microdialysate samples were collected every 2 min. Basal glutamate, changes in glutamate efflux with stimulation, and the relationships between peak stimulation-associated glutamate concentrations, basal zero-flow levels, and stimulated seizures were examined.

RESULTS

Stimulation of epileptic hippocampi in patients with refractory epilepsy caused increases in glutamate efflux (p = 0.005, n = 10), and 4 of ten patients experienced brief stimulated seizures. Stimulation-induced increases in glutamate were not observed during the evoked seizures, but rather were related to the elevation in interictal basal glutamate (R(2) = 0.81, p = 0.001). The evoked-seizure group had lower basal glutamate levels than the no-seizure group (p = 0.04), with no stimulation-induced change in glutamate efflux (p = 0.47, n = 4). Conversely, increased glutamate was observed following stimulation in the no-seizure group (p = 0.005, n = 7). Subjects with an atrophic hippocampus had higher basal glutamate levels (p = 0.03, n = 7) and higher stimulation-induced glutamate efflux.

SIGNIFICANCE

Electrical stimulation of the epileptic hippocampus either increased extracellular glutamate efflux or induced seizures. The magnitude of stimulated glutamate increase was related to elevation in basal interictal glutamate, suggesting a common mechanism, possibly impaired glutamate metabolism. Divergent mechanisms may exist for seizure induction and increased glutamate in patients with epilepsy. These data highlight the potential risk of 50 Hz stimulation in patients with epilepsy.

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

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.

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.

Aberrant expression of miR-218 and miR-204 in human mesial temporal lobe epilepsy and hippocampal sclerosis-convergence on axonal guidance

OBJECTIVE

Mesial temporal lobe epilepsy (MTLE) is one of the most common types of the intractable epilepsies and is most often associated with hippocampal sclerosis (HS), which is characterized by pronounced loss of hippocampal pyramidal neurons. microRNAs (miRNAs) have been shown to be dysregulated in epilepsy and neurodegenerative diseases, and we hypothesized that miRNAs could be involved in the pathogenesis of MTLE and HS.

METHODS

miRNA expression was quantified in hippocampal specimens from human patients using miRNA microarray and quantitative real-time polymerase chain reaction RT-PCR, and by RNA-seq on fetal brain specimens from domestic pigs. In situ hybridization was used to show the spatial distribution of miRNAs in the human hippocampus. The potential effect of miRNAs on targets genes was investigated using the dual luciferase reporter gene assay.

RESULTS

miRNA expression profiling showed that 25 miRNAs were up-regulated and 5 were down-regulated in hippocampus biopsies of MTLE/HS patients compared to controls. We showed that miR-204 and miR-218 were significantly down-regulated in MTLE and HS, and both were expressed in neurons in all subfields of normal hippocampus. Moreover, miR-204 and miR-218 showed strong changes in expression during fetal development of the hippocampus in pigs, and we identified four target genes, involved in axonal guidance and synaptic plasticity, ROBO1, GRM1, SLC1A2, and GNAI2, as bona fide targets of miR-218. GRM1 was also shown to be a direct target of miR-204.

SIGNIFICANCE

miR-204 and miR-218 are developmentally regulated in the hippocampus and may contribute to the molecular mechanisms underlying the pathogenesis of MTLE and HS.

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.

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.

Abnormal partitioning of hexokinase 1 suggests disruption of a glutamate transport protein complex in schizophrenia

Excitatory amino acid transporter 2 (EAAT2) belongs to a family of Na(+) dependent glutamate transporters that maintain a low synaptic concentration of glutamate by removing glutamate from the synaptic cleft into astroglia and neurons. EAAT2 activity depends on Na(+) and K(+) gradients generated by Na(+)/K(+) ATPase and ATP. Hexokinase 1 (HK1), an initial enzyme of glycolysis, binds to mitochondrial outer membrane where it couples cytosolic glycolysis to mitochondrial oxidative phosphorylation, producing ATP utilized by the EAAT2/Na(+)/K(+) ATPase protein complex to facilitate glutamate reuptake. In this study, we hypothesized that the protein complex formed by EAAT2, Na(+)/K(+) ATPase and mitochondrial proteins in human postmortem prefrontal cortex may be disrupted, leading to abnormal glutamate transmission in schizophrenia. We first determined that EAAT2, Na(+)/K(+) ATPase, HK1 and aconitase were found in both EAAT2 and Na(+)/K(+) ATPase interactomes by immunoisolation and mass spectrometry in human postmortem prefrontal cortex. Next, we measured levels of glutamate transport complex proteins in subcellular fractions in the dorsolateral prefrontal cortex and found increases in the EAAT2B isoform of EAAT2 in a fraction containing extrasynaptic membranes and increased aconitase 1 in a mitochondrial fraction. Finally, an increased ratio of HK1 protein in the extrasynaptic membrane/mitochondrial fraction was found in subjects with schizophrenia, suggesting that HK1 protein is abnormally partitioned in this illness. Our findings indicate that the integrity of the glutamate transport protein complex may be disrupted, leading to decreased perisynaptic buffering and reuptake of glutamate, as well as impaired energy metabolism in schizophrenia.

Mechanisms of ictogenesis

Epilepsy is a paroxysmal condition characterized by repeated transient seizures separated by longer interictal periods. Ictogenesis describes the processes of transition from the interictal state to a seizure. The processes include a preictal state, with specific clinical signs and a distinct electrophysiology which may provide opportunities to anticipate, or even prevent, seizures. Biological mechanisms of ictogenesis remain poorly understood and may vary between conditions/syndromes. We review here ictogenic processes including the involvement of pyramidal cells, interneurons and astrocytes, GABAergic and glutamatergic signaling, and ionic perturbations. Our review suggests that specific excitatory influences at the transition to an ictal event include (1) GABA receptor activation with a neuronal Cl(-) load and (2) a transient increase in external K(+).

Contributions of astrocytes to epileptogenesis following status epilepticus: opportunities for preventive therapy?

Status epilepticus (SE) is a life threatening condition that often precedes the development of epilepsy. Traditional treatments for epilepsy have been focused on targeting neuronal mechanisms contributing to hyperexcitability, however, approximately 30% of patients with epilepsy do not respond to existing neurocentric pharmacotherapies. A growing body of evidence has demonstrated that profound changes in the morphology and function of astrocytes accompany SE and persist in epilepsy. Astrocytes are increasingly recognized for their diverse roles in modulating neuronal activity, and understanding the changes in astrocytes following SE could provide important clues about the mechanisms underlying seizure generation and termination. By understanding the contributions of astrocytes to the network changes underlying epileptogenesis and the development of epilepsy, we will gain a greater appreciation of the contributions of astrocytes to dynamic circuit changes, which will enable us to develop more successful therapies to prevent and treat epilepsy. This review summarizes changes in astrocytes following SE in animal models and human temporal lobe epilepsy and addresses the functional consequences of those changes that may provide clues to the process of epileptogenesis.

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.

The paradox of the paroxysm: can seizure precipitants help explain human ictogenesis?

An epileptic brain is permanently in a diseased state, but seizures occur rarely and without warning. Here we examine this paradox, common to paroxysmal diseases. We review the problem in the context of the prototypic acquired epilepsies of the medial temporal lobe. We ask how an epileptic temporal lobe differs from a healthy one and examine biological mechanisms that may explain the transition to seizure. Attempts to predict seizure timing from analyses of brain electrical activity suggest that the neurological processes involved may be initiated significantly before a seizure. Furthermore, whereas seizures are said to occur without warning, some patients say they know when a seizure is imminent. Several factors, including sleep deprivation, oscillations in hormonal levels, or withdrawal from drugs, increase the probability of a seizure. We ask whether these seizure precipitants might act through common neuronal mechanisms. Several precipitating factors seem to involve relief from a neurosteroid modulation of gamma-amino butyric acid receptor type A (GABAA) receptors. We propose tests of this hypothesis.

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Glutamate transporter EAAT2: a new target for the treatment of neurodegenerative diseases

Glutamate is the primary excitatory amino acid neurotransmitter in the CNS. The concentration of glutamate in the synaptic cleft is tightly controlled by interplay between glutamate release and glutamate clearance. Abnormal glutamate release and/or dysfunction of glutamate clearance can cause overstimulation of glutamate receptors and result in neuronal injury known as excitotoxicity. The glial glutamate transporter EAAT2 plays a major role in glutamate clearance. Dysfunction or reduced expression of EAAT2 has been documented in many neurodegenerative diseases. In addition, many studies in animal models of disease indicate that increased EAAT2 expression provides neuronal protection. Here, we summarize these studies and suggest that EAAT2 is a potential target for the prevention of excitotoxicity. EAAT2 can be upregulated by transcriptional or translational activation. We discuss current progress in the search for EAAT2 activators, which is a promising direction for the treatment of neurodegenerative diseases.

The rates of postmortem proteolysis of glutamate transporters differ dramatically between cells and between transporter subtypes

Glutamate transporters (GLT-1, GLAST, EAAC1) limit the actions of excitatory amino acids. Because a disturbed transporter operation can cause or aggravate neurological diseases, transporters are of considerable neuropathological interest. Human samples, however, are seldom obtained fresh. Here, we used mice brains to study how fast glutamate transporters are degraded after death. Immunoblots showed that terminal GLT-1 epitopes (within residues 1-26 and 518-573) had mostly disappeared after 24 hr. GLAST termini (1-25 and 522-543) degraded slightly slower. In contrast, epitopes within central parts of GLT-1 (493-508) and the EAAC1 C-terminus (510-523) were readily detectable after 72 hr. The decline in immunoreactivity of the GLT-1 and GLAST termini was also seen in tissue sections, but proteolysis did not happen synchronously in all cells. At 24 hr, scattered cells remained strongly immunopositive, while the majority of cells were completely immunonegative. GLAST and GLT-1 co-localized in neocortical tissue, but at 12 hr, many GLAST-positive cells had lost the GLT-1 termini. The uneven disappearance of labeling was not observed with the antibodies to GLT-1 residues 493-508. The immunoreactivity to this epitope correlated better with the reported glutamate uptake activity. Thus, postmortem delay may affect epitopes differently, possibly causing erroneous conclusions about relative expression levels.

Increased glial glutamate transporter EAAT2 expression reduces epileptogenic processes following pilocarpine-induced status epilepticus

Several lines of evidence indicate that glutamate plays a crucial role in the initiation of seizures and their propagation; abnormal glutamate release causes synchronous firing of large populations of neurons, leading to seizures. In the present study, we investigated whether enhanced glutamate uptake by increased glial glutamate transporter EAAT2, the major glutamate transporter, could prevent seizure activity and reduce epileptogenic processes. EAAT2 transgenic mice, which have a 1.5-2 fold increase in EAAT2 protein levels as compared to their non-transgenic counterparts, were tested in a pilocarpine-induced status epilepticus (SE) model. Several striking phenomena were observed in EAAT2 transgenic mice compared with their non-transgenic littermates. First, the post-SE mortality rate and chronic seizure frequency were significantly decreased. Second, neuronal degeneration in hippocampal subfields after SE were significantly reduced. Third, the SE-induced neurogenesis and mossy fiber sprouting were significantly decreased. The severity of cell loss in epileptic mice was positively correlated with that of mossy fiber sprouting and chronic seizure frequency. Our results suggest that increased EAAT2 expression can protect mice against SE-induced death, neuropathological changes, and chronic seizure development. This study suggests that enhancing EAAT2 protein expression is a potential therapeutic approach.

Bmi-1 absence causes premature brain degeneration

Bmi-1, a polycomb transcriptional repressor, is implicated in cell cycle regulation and cell senescence. Its absence results in generalized astrogliosis and epilepsy during the postnatal development, but the underlying mechanisms are poorly understood. Here, we demonstrate the occurrence of oxidative stress in the brain of four-week-old Bmi-1 null mice. The mice showed various hallmarks of neurodegeneration including synaptic loss, axonal demyelination, reactive gliosis and brain mitochondrial damage. Moreover, astroglial glutamate transporters and glutamine synthetase decreased in the Bmi-1 null hippocampus, which might contribute to the sporadic epileptic-like seizures in these mice. These results indicate that Bmi-1 is required for maintaining endogenous antioxidant defenses in the brain, and its absence subsequently causes premature brain degeneration.

Short-term alterations in hippocampal glutamate transport system caused by one-single neonatal seizure episode: implications on behavioral performance in adulthood

Impairment in the activity and expression of glutamate transporters has been found in experimental models of epilepsy in adult animals. However, there are few studies investigating alterations on glutamate transporters caused by epilepsy in newborn animals, especially in the early periods after seizures. In this study, alterations in the hippocampal glutamate transporters activity and immunocontent were investigated in neonatal rats (7 days old) submitted to kainate-induced seizures model. Glutamate uptake, glutamate transporters (GLT-1, GLAST, EAAC1) and glutamine synthetase (GS) were assessed in hippocampal slices obtained 12 h, 24 h, 48 h, 72 h and 60 days after seizures. Immunoreactivity for hippocampal GFAP, NeuN and DAPI were assessed 24 h after seizure. Behavioral analysis (elevated-plus maze and inhibitory avoidance task) was also investigated in the adult animals (60 days old). The decrease on glutamate uptake was observed in hippocampal slices obtained 24 h after seizures. The immunocontent of GLT-1 increased at 12 h and decreased at 24 h (+62% and -20%, respectively), while GLAST increased up to 48 h after seizures. No alterations were observed for EAAC1 and GS. It should be mentioned that there were no long-term changes in tested glutamate transporters at 60 days after kainate treatment. GFAP immunoreactivity increased in all hippocampal subfields (CA1, CA3 and dentate gyrus) with no alterations in NeuN and DAPI staining. In the adulthood, kainate-treated rats showed anxiety-related behavior and lower performance in the inhibitory avoidance task. Our findings indicate that acute modifications on hippocampal glutamate transporters triggered by a single convulsive event in early life may play a role in the behavioral alterations observed in adulthood.

Hypo-osmotic swelling modifies glutamate-glutamine cycle in the cerebral cortex and in astrocyte cultures

In our previous work, we found that perfusion of the rat cerebral cortex with hypo-osmotic medium triggers massive release of the excitatory amino acid L-glutamate but decreases extracellular levels of L-glutamine (R. E. Haskew-Layton et al., PLoS ONE, 3: e3543). The release of glutamate was linked to activation of volume-regulated anion channels, whereas mechanism(s) responsible for alterations in extracellular glutamine remained unclear. When mannitol was added to the hypo-osmotic medium to reverse reductions in osmolarity, changes in microdialysate levels of glutamine were prevented, indicating an involvement of cellular swelling. As the main source of brain glutamine is astrocytic synthesis and export, we explored the impact of hypo-osmotic medium on glutamine synthesis and transport in rat primary astrocyte cultures. In astrocytes, a 40% reduction in medium osmolarity moderately stimulated the release of L-[(3) H]glutamine by ∼twofold and produced no changes in L-[(3) H]glutamine uptake. In comparison, hypo-osmotic medium stimulated the release of glutamate (traced with D-[(3) H]aspartate) by more than 20-fold. In whole-cell enzymatic assays, we discovered that hypo-osmotic medium caused a 20% inhibition of astrocytic conversion of L-[(3) H]glutamate into L-[(3) H]glutamine by glutamine synthetase. Using an HPLC assay, we further found a 35% reduction in intracellular levels of endogenous glutamine. Overall, our findings suggest that cellular swelling (i) inhibits astrocytic glutamine synthetase activity, and (ii) reduces substrate availability for this enzyme because of the activation of volume-regulated anion channels. These combined effects likely lead to reductions in astrocytic glutamine export in vivo and may partially explain occurrence of hyperexcitability and seizures in human hyponatremia.

Glutamate clearance mechanisms in resected cortical dysplasia

Object

Changes in the expression of glutamate transporters (GLTs) may play a role in the expression of epileptogenicity. Previous studies have shown an increased number of neuronal GLTs in human dysplastic neurons. The expression of glial and neuronal GLTs and glutamine synthetase (GS) in balloon cells (BCs) and BC-containing cortical dysplasia has not been studied.

Methods

The authors analyzed neocortical samples that were resected in 5 patients who had cortical dysplasia–induced medically intractable focal epilepsy and who underwent extraoperative prolonged electrocorticographic (ECoG) recordings. The expressions of glial (GLT1/EAAT2) and neuronal (EAAT3, EAAC1) GLTs and GS proteins were immunohistochemically studied in all 5 resected samples. The authors also assessed in situ colocalization of GLTs and GS with neuronal and glial markers.

Results

Balloon cell–containing cortical dysplasia lesions did not exhibit ictal patterns on prolonged extraoperative ECoG recordings. There was a differential expression of glial and neuronal GLTs in BCs and dysplastic neurons: the majority of BCs highly expressed glial but not neuronal GLTs. Dysplastic neurons showed increased immunohistochemical staining with neuronal EAAT3 but not with EAAT2/GLT1. Moreover, only glial fibrillary acidic protein–positive BCs also expressed GS.

Conclusions

There is a differential GLT expression in dysplastic and balloon cells. The presence of glial GLTs and GS in balloon cell cortical dysplasia suggests a possible antiepileptic role for BCs and is consistent with the reported increased epileptogenicity in GLT1-deficient animals.

Cellular and Synaptic Localization of EAAT2a in Human Cerebral Cortex

We used light and electron microscopic immunocytochemical techniques to analyze the distribution, cellular and synaptic localization of EAAT2, the main glutamate transporter, in normal human neocortex. EAAT2a-immunoreactivity (ir) was in all layers and consisted of small neuropilar puncta and rare cells. In white matter EAAT2a+ cells were numerous. Electron microscopic studies showed that in gray matter ∼77% of immunoreactive elements were astrocytic processes, ∼14% axon terminals, ∼2.8% dendrites, whereas ∼5% were unidentifiable. In white matter, ∼81% were astrocytic processes, ∼17% were myelinated axons, and ∼2.0% were unidentified. EAAT2a-ir was never in microglial cells and oligodendrocytes. Pre-embedding electron microscopy showed that ∼67% of EAAT2a expressed at (or in the vicinity of) asymmetric synapses was in astrocytes, ∼17% in axon terminals, while ∼13% was both in astrocytes and in axons. Post-embedding electron microscopy studies showed that in astrocytic processes contacting asymmetric synapses and in axon terminals, gold particle density was ∼25.1 and ∼2.8 particles/μm², respectively, and was concentrated in a membrane region extending for ∼300 nm from the active zone edge. Besides representing the first detailed description of EAAT2a in human cerebral cortex, these findings may contribute to understanding its role in the pathophysiology of neuropsychiatric diseases.

Involvement of Glutamate transporter-1 in neuroprotection against global brain ischemia-reperfusion injury induced by postconditioning in rats

Ischemic postconditioning refers to several transient reperfusion and ischemia cycles after an ischemic event and before a long duration of reperfusion. The procedure produces neuroprotective effects. The mechanisms underlying these neuroprotective effects are poorly understood. In this study, we found that most neurons in the CA1 region died after 10 minutes of ischemia and is followed by 72 hours of reperfusion. However, brain ischemic postconditioning (six cycles of 10 s/10 s reperfusion/re-occlusion) significantly reduced neuronal death. Significant up-regulation of Glutamate transporter-1 was found after 3, 6, 24, 72 hours of reperfusion. The present study showed that ischemic postconditioning decreases cell death and that upregulation of GLT-1 expression may play an important role on this effect.

Astrocytes and epilepsy

Astrocytes form a significant constituent of seizure foci in the human brain. For a long time it was believed that astrocytes play a significant role in the causation of seizures. With the increase in our understanding of the unique biology of these cells, their precise role in seizure foci is receiving renewed attention. This article reviews the information now available on the role of astrocytes in the hippocampal seizure focus in patients with temporal lobe epilepsy with hippocampal sclerosis. Our intent is to try to integrate the available data. Astrocytes at seizure foci seem to not be a homogeneous population of cells, and in addition to typical glial fibrillary acidic protein, positive reactive astrocytes also include a population of neuron glia-2-like cells The astrocytes in sclerotic hippocampi differ from those in nonsclerotic hippocampi in their membrane physiology, having elevated Na+ channels and reduced inwardly rectifying potassium ion channels, and some having the capacity to generate action potentials. They also have reduced glutamine synthetase and increased glutamate dehydrogenase activity. The molecular interface between the astrocyte and microvasculature is also changed. The astrocytes are also associated with increased expression of many molecules normally concerned with immune and inflammatory functions. A speculative mechanism postulates that neuron glia-2-like cells may be involved in creating a high glutamate environment, whereas the function of more typical reactive astrocytes contribute to maintain high extracellular K+ levels; both factors contributing to the hyperexcitability of subicular neurons to generate epileptiform activity. The functions of the astrocyte vascular interface may be more critical to the processes involved in epileptogenesis.

The expression of astroglial glutamate transporters in patients with focal cortical dysplasia: an immunohistochemical study

PURPOSE

An abnormal increase in the extracellular glutamate is thought to play a crucial role in the initiation, spread, and maintenance of seizure activity.In normal conditions, the majority of this excess glutamate is cleared via glial glutamate transporters (EAAT-1 and EAAT-2). We aimed to examine the immunohistochemical expression of these transporters in the dysplastic tissues of patients with focal cortical dysplasia (FCD).

METHODS

The parafin-embedded dysplastic tissues of 33 patients who were operated on due to medically intractable epilepsy and histopathologically diagnosed with FCD between 2001 and 2006 were stained immunohistochemically with appropriate antibodies, and the distribution and intensity of immunoreactivity (IR) of EAAT-1 and EAAT-2 were examined.The findings were compared with the histologically normal tissues of five patients who underwent temporal lobectomy for epilepsy surgery and 10 fresh postmortem cases.

RESULTS

In the majority of the patients, the EAAT-1 and EAAT-2 IR were decreased, their astrocytic expression were lower, and the pattern of distribution were more diffused when compared to the control groups.Analyzing these findings according to the types of FCD revealed that as the severity of the dysplasia increased, the IR and astrocytic expression of both transporters are decreased and their distribution tend to be more "diffused."

CONCLUSION

The results of this study suggest a relationship between the decreased glutamate transporter expressions in dysplastic tissues which,in turn, may cause increased extracellular concentrations of glutamate and FCD pathophysiology.Further studies with larger patient populations,investigating the expression of glutamate transporters at mRNA and protein levels, are required to clarify their roles in the pathophysiology of FCD.

Three-dimensional relationships between perisynaptic astroglia and human hippocampal synapses

Perisynaptic astroglia are critical for normal synaptic development and function. Little is known, however, about perisynaptic astroglia in the human hippocampus. When mesial temporal lobe epilepsy (MTLE) is refractory to medication, surgical removal is required for seizure quiescence. To investigate perisynaptic astroglia in human hippocampus, we recovered slices for several hours in vitro from three surgical specimens and then quickly fixed them to achieve high-quality ultrastructure. Histological samples from each case were found to have mesial temporal sclerosis with Blumcke Type 1a (mild, moderate) or 1b (severe) pathology. Quantitative analysis through serial section transmission electron microscopy in CA1 stratum radiatum revealed more synapses in the mild (10/10 microm(3)) than the moderate (5/10 microm(3)) or severe (1/10 microm(3)) cases. Normal spines occurred in mild and moderate cases, but a few multisynaptic spines were all that remained in the severe case. Like adult rat hippocampus, perisynaptic astroglial processes were preferentially associated with larger synapses in the mild and moderate cases, but rarely penetrated the cluster of axonal boutons surrounding multisynaptic spines. Synapse perimeters were only partially surrounded by astroglial processes such that all synapses had some access to substances in the extracellular space, similar to adult rat hippocampus. Junctions between astroglial processes were observed more frequently in moderate than mild case, but were obscured by densely packed intermediate filaments in astroglial processes of the severe case. These findings suggest that perisynaptic astroglial processes associate with synapses in human hippocampus in a manner similar to model systems and are disrupted by severe MTLE pathology.

Mitochondria, oxidative stress, and temporal lobe epilepsy

Mitochondrial oxidative stress and dysfunction are contributing factors to various neurological disorders. Recently, there has been increasing evidence supporting the association between mitochondrial oxidative stress and epilepsy. Although certain inherited epilepsies are associated with mitochondrial dysfunction, little is known about its role in acquired epilepsies such as temporal lobe epilepsy (TLE). Mitochondrial oxidative stress and dysfunction are emerging as key factors that not only result from seizures, but may also contribute to epileptogenesis. The occurrence of epilepsy increases with age, and mitochondrial oxidative stress is a leading mechanism of aging and age-related degenerative disease, suggesting a further involvement of mitochondrial dysfunction in seizure generation. Mitochondria have critical cellular functions that influence neuronal excitability including production of adenosine triphosphate (ATP), fatty acid oxidation, control of apoptosis and necrosis, regulation of amino acid cycling, neurotransmitter biosynthesis, and regulation of cytosolic Ca(2+) homeostasis. Mitochondria are the primary site of reactive oxygen species (ROS) production making them uniquely vulnerable to oxidative stress and damage which can further affect cellular macromolecule function, the ability of the electron transport chain to produce ATP, antioxidant defenses, mitochondrial DNA stability, and synaptic glutamate homeostasis. Oxidative damage to one or more of these cellular targets may affect neuronal excitability and increase seizure susceptibility. The specific targeting of mitochondrial oxidative stress, dysfunction, and bioenergetics with pharmacological and non-pharmacological treatments may be a novel avenue for attenuating epileptogenesis.

Expression of EAAT-1 distinguishes choroid plexus tumors from normal and reactive choroid plexus epithelium

Microscopic distinction of normal choroid plexus (CP) from choroid plexus tumors (CPT) may be difficult, especially in small samples of well-differentiated CP papillomas. So far, there are no established markers that reliably distinguish normal and neoplastic CP epithelium. Recently, a correlation between expression/function of glial glutamate transporters EAAT-1 (GLAST) and EAAT-2 (Glt-1) and tumor proliferation has been reported. Furthermore, we previously found that CPTs frequently express EAAT-1, but not EAAT-2. We now compared expression of EAAT-1, EAAT-2 and GFAP in non-neoplastic CP (n = 68) and CPT (n = 79) by immunohistochemistry. Tissue of normal CP was obtained from 50 autopsy cases (20 normal and 30 pathologic brains) and 18 neurosurgical specimens that included 17 fetal, 21 pediatric and 30 adult cases. In non-neoplastic postnatal CP (n = 51), focal expression of EAAT-1 was found in only two pediatric cases (4%). In CPT, expression of EAAT-1 was found in 64 of 79 (81%) tumor samples and was significantly age-dependent (P < 0.0001). Hence, EAAT-1 expression distinguishes neoplastic from normal CP, both in children (P = 0.0032) and in adults (P < 0.0001). Immunostaining for EAAT-2 in selected samples from cases of different ages showed that normal CP (n = 15) or CPT (n = 16) lacked EAAT-2 expression. GFAP expression was found in 3 of 32 (10%) normal CP and in 28 of 73 (38%) tumor samples. In conclusion, in contrast to neoplastic CP samples, expression of EAAT-1 is exceptionally rare in non-neoplastic CP. Thus, EAAT-1 is superior to GFAP as a helpful diagnostic tool in CP samples.