Effect of glutamate transporter on neuronal damage induced by photochemical thrombotic brain ischemia

The Expression of GLAST and GLT1 in a Transient Cerebral Ischemia Mongolian Gerbil Model

PURPOSE

Excitatory amino acid transporters (EAATs) have an indispensable function in the reuptake of extracellular glutamate. To investigate the relationship and the expression of neuronal and astrocytic markers after brain ischemia, the temporal profile of glial EAATs in both peripheral and core regions of the cortex was examined.

METHODS

Transient common carotid artery occlusion was used to induce unilateral transient forebrain ischemia of Mongolian gerbils, and post-ischemic brains (6 h to 2 w) were collected and prepared for immunohistochemical and Western blotting analysis of glutamine synthetase (GS), GLT-1, GLAST, S100β, and NeuN, and for Alizarin red staining of calcium deposits.

RESULTS

The expression of GLAST and GLT-1 were significantly escalated at 6 h both in the core and periphery regions, while reduced from 12 h to 2 w in the core region post-ischemia. GS-positive cells increased at 6 h both in the core and periphery regions, while the density of Alizarin red-positive cells increased and peaked at 12 h in the ischemic cortex. The density of S100β-positive cells decreased in the ischemic core and increased in the periphery region. Immunofluorescence staining showed that S100β and TUNEL double-positive cells increased at 12 h in the core region.

CONCLUSION

The results of GLT-1 and GLAST expression in the cortex indicate that their up-regulation was time-dependent and occurred in the acute post-ischemia period, whereas their down-regulation was region-dependent and it is involved in the pathological progress of nerve cell and glial cell death, and has a series of cascade reactions.

Hypoxia suppresses glutamate transport in astrocytes

Glutamate uptake by astrocytes is fundamentally important in the regulation of CNS function. Disruption of uptake can lead to excitotoxicity and is implicated in various neurodegenerative processes as well as a consequence of hypoxic/ischemic events. Here, we investigate the effect of hypoxia on activity and expression of the key glutamate transporters excitatory amino acid transporter 1 (EAAT1) [GLAST (glutamate-aspartate transporter)] and EAAT2 [GLT-1 (glutamate transporter 1)]. Electrogenic, Na+-dependent glutamate uptake was monitored via whole-cell patch-clamp recordings from cortical astrocytes. Under hypoxic conditions (2.5 and 1% O2 exposure for 24 h), glutamate uptake was significantly reduced, and pharmacological separation of uptake transporter subtypes suggested that the EAAT2 subtype was preferentially reduced relative to the EAAT1. This suppression was confirmed at the level of EAAT protein expression (via Western blots) and mRNA levels (via real-time PCR). These effects of hypoxia to inhibit glutamate uptake current and EAAT protein levels were not replicated by desferrioxamine, cobalt, FG0041, or FG4496, agents known to mimic effects of hypoxia mediated via the transcriptional regulator, hypoxia-inducible factor (HIF). Furthermore, the effects of hypoxia were not prevented by topotecan, which prevents HIF accumulation. In stark contrast, inhibition of nuclear factor-kappaB (NF-kappaB) with SN50 fully prevented the effects of hypoxia on glutamate uptake and EAAT expression. Our results indicate that prolonged hypoxia can suppress glutamate uptake in astrocytes and that this effect requires activation of NF-kappaB but not of HIF. Suppression of glutamate uptake via this mechanism may be an important contributory factor in hypoxic/ischemic triggered glutamate excitotoxicity.

Down-regulation of the glial glutamate transporter GLT-1 in rat hippocampus and striatum and its modulation by a group III metabotropic glutamate receptor antagonist following transient global forebrain ischemia

Our goals were to identify biochemical markers for transient global ischemia-induced delayed neuronal death and test possible drug therapies against this neuronal damage. Four-vessel occlusion (4-VO) for 20 min was used as a rat model. The temporal expression profiles of three glutamate transporters (GLT-1, GLAST and EAAC1) were evaluated in the CA1 region of the hippocampus and the striatum. The protein levels of the GLT-1 were significantly down-regulated between 3 and 6 h after ischemia-reperfusion in the CA1 region and striatum, returned to the control (2-VO) levels 24 h after reperfusion and remained unchanged for up to 7 days. The levels of GLAST in the CA1 region and striatum, and EAAC1 in the CA1 region did not change after ischemia from 1 h to 7 days. Pretreatment with group III metabotropic glutamate receptor antagonist s-alpha-MCPA (20 microg/5 microl) 30 min prior to 4-VO significantly restored the GLT-1 levels in the CA1 region caused by global ischemia at both 3 and 6 h after reperfusion. The loss of pyramidal neurons in the CA1 region due to ischemia-reperfusion could also be prevented by intraventricular pretreatment with s-alpha-MCPA. The current findings pinpoint the significance of GLT-1 during ischemia/reperfusion and suggest a potential application of group III metabotropic glutamate receptor antagonist against ischemic/hypoxic neuronal damage.

GDNF pre-treatment aggravates neuronal cell loss in oxygen-glucose deprived hippocampal slice cultures: a possible effect of glutamate transporter up-regulation

Besides its neurotrophic and neuroprotective effects on dopaminergic neurons and spinal motoneurons, glial cell line-derived neurotrophic factor (GDNF) has potent neuroprotective effects in cerebral ischemia. The protective effect has so far been related to reduced activation of N-methyl-D-aspartate receptors (NMDAr). This study tested the effects of GDNF on glutamate transporter expression, with the hypothesis that modulation of glutamate transporter activity would affect the outcome of cerebral ischemia. Organotypic hippocampal slice cultures, derived from 1-week-old rats, were treated with 100 ng/ml GDNF for either 2 or 5 days, followed by Western blot analysis of NMDAr subunit 1 (NR1) and two glutamate transporter subtypes, GLAST and GLT-1. After 5-day exposure to GDNF, expression of GLAST and GLT-1 was up-regulated to 169 and 181% of control values, respectively, whereas NR1 was down-regulated to 64% of control. However, despite these changes that potentially would support neuronal resistance to excitotoxicity, the long-term treatment with GDNF was found to aggravate the neuronal damage induced by oxygen-glucose deprivation (OGD). The increased cell death, assessed by propidium iodide (PI) uptake, occurred not only among the most susceptible CA1 pyramidal cells, but also in CA3 and fascia dentata. Given that glutamate transporters are able to release glutamate by reversed action during energy failure, it is suggested that the observed increase in OGD-induced cell death in the GDNF-pretreated cultures was caused by the build-up of excitotoxic concentrations of extracellular glutamate released through the glutamate transporters, which were up-regulated by GDNF. Although the extent and consequences of glutamate release via reversal of GLAST and GLT-1 transporters seem to vary in different energy failure models, the present findings should be taken into account in clinical trials of GDNF.

Free radicals, antioxidants, and neurologic injury: possible relationship to cerebral protection by anesthetics

Oxygen-centered free radicals cause brain injury associated with trauma and stroke. These reactive oxygen species may be detoxified by endogenous antioxidants, but cell death occurs after antioxidants become depleted. General anesthetics penetrate into brain parenchyma, where they may abrogate oxidative injury to neurons by several mechanisms that prevent the initiation of free radical chain reactions or terminate the propagation of highly reactive radicals. First, general anesthetics may inhibit free radical generation because these drugs slow cerebral utilization of oxygen and glucose, inhibit oxidative metabolism in neutrophils, and prevent redox changes in hemoglobin. Second, antioxidant anesthetics, such as thiopental and propofol, directly scavenge reactive oxygen species and inhibit lipid peroxidation. Finally, anesthetics may prevent the elevation of extracellular glutamate concentration and inhibit the activation of excitatory glutamatergic receptors that augment oxidative stress after ischemia.

Altered expressions of glutamate transporter subtypes in rat model of neonatal cerebral hypoxia-ischemia

Glutamate transporters are essential for maintaining the extracellular levels of glutamate at synaptic clefts and are regulated developmentally in a subtype-specific manner. We investigated chronological changes of immunoreactivities for glial glutamate transporters GLAST and GLT-1 and a neuronal glutamate transporter, EAAC1, in postnatal 7-day-old rat neocortices and hippocampi at 12, 24, 48 and 72 h after hypoxia-ischemia. Glutamate transporter subtypes are differentially expressed in the ischemic core and the boundary area of the neonatal rat brain with hypoxia-ischemia. Expressions of these glutamate transporters decreased in the ischemic core at 12 h, then immunoreactivities for GLAST and GLT-1 were recovered at the hippocampus. This was accompanied by a GFAP-positive gliosis at 72 h, whereas these immunoreactivities were reduced at the neocortex in the ischemic core. Glial glutamate transporters, especially GLAST, were noted in some astrocytes appearing as apoptosis as well as shrunken pyramidal neurons mainly in the boundary area of the neocortex. Increased perikaryal expression of EAAC1 was associated with that of MAP2 at the border of the boundary area. These temporal and regional expressions of glutamate transporters may contribute towards understanding the excitotoxic cell death mechanism in hypoxic-ischemic encephalopathy during the perinatal period.

Propofol protection of sodium-hydrogen exchange activity sustains glutamate uptake during oxidative stress

We investigated the role of intracellular pH in protection by propofol of glutamate uptake during oxidative stress. Exposure of primary astrocyte cultures to tert-butylhydroperoxide (t-BOOH, 300 microM) decreased the initial rate of Na-dependent glutamate uptake. Either propofol or alpha-tocopherol, administered 30 min after t-BOOH, attenuated this transport inhibition. These lipophilic antioxidants protected glutamate uptake whether the medium contained 25 mM bicarbonate or was nominally bicarbonate-free. t-BOOH also inhibited Na/H exchanger isoform 1 (NHE1) activation by intracellular protons and propofol prevented this inhibition. Blockade of NHE1 by the potent antagonist, 5-(N-ethyl-N-isopropyl) amiloride (1 microM), abolished the protective effects of small concentrations of propofol (1 microM) and alpha-tocopherol (40 microM) on glutamate uptake during oxidative stress in bicarbonate-free medium. 5-(N-ethyl-N-isopropyl) amiloride had no effect on antioxidant rescue of glutamate transport in medium containing 25 mM bicarbonate. These results indicate that regulation of intracellular pH may contribute to neuroprotection by propofol and other lipophilic antioxidants. Propofol concentrations that are associated with anesthesia and neuroprotection may prevent intracellular acidification during oxidative stress by preserving the NHE1 response to cytosolic protons. However, if intracellular acidification occurs nonetheless, then propofol protection of glutamate uptake activity becomes less effective and the extracellular glutamate concentration may increase to neurotoxic levels.

IMPLICATIONS

Anesthetic concentrations of propofol maintain the capacity of brain cells to extrude protons during oxidative stress. However, if intracellular acidification occurs nonetheless, then propofol's protection of glutamate clearance mechanisms from oxidative damage becomes attenuated, and extracellular glutamate concentration may increase to neurotoxic levels.

Expression of Na(+)-K(+)-Cl(-) cotransporter in rat brain during development and its localization in mature astrocytes

Na(+)-K(+)-Cl(-) cotransporter has been proposed to play an important role in the regulation of intracellular Cl(-) concentration in neurons during development. In this study, the expression pattern of the cotransporter in different regions of rat brain was examined at birth (P0), postnatal days 7 (P7), P14, P21, and adult by Western blotting analysis. In cortex, thalamus, cerebellum and striatum, the cotransporter expression level was low at P0 and significantly increased at P14 (P<0.05). The expression peaked at P21 and was maintained at the same level in adulthood. However, in hippocampus, a peak level of the cotransporter expression was detected in adult brain. The immunocytochemistry study of adult rat brain revealed that an intense staining of the Na(+)-K(+)-Cl(-) cotransporter protein was observed in dendritic processes of CA1-CA3 hippocampal pyramidal neurons. In contrast, abundant immuno-reactive signals of the cotransporter were found in somata of thalamic nucleus. Immunofluorescence double staining demonstrates that the Na(+)-K(+)-Cl(-) cotransporter was expressed in astrocytes within cortex, corpus callosum, hippocampus and cerebellum. In addition, co-localization of the cotransporter and glial fibrillary acidic protein (GFAP), or with aquaporin 4, was found in perivascular astrocytes of cortical cortex and white matter. The results indicate that a time-dependent expression of the Na(+)-K(+)-Cl(-) cotransporter protein occurs not only in cortex but also in hippocampus, striatum, thalamus and cerebellum. In addition, the cotransporter is expressed in astrocytes and perivascular astrocytes of adult rat brain.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Role of excitatory amino acid transporter 1 in neonatal rat neuronal damage induced by hypoxia-ischemia

The role of excitatory amino acid transporter 1 in neonatal rat neuronal damage was studied following hypoxia-ischemia. To induce hypoxia-ischemia injury, rats on postnatal day 7 were exposed to 8 % oxygen for 2 h following unilateral common carotid artery ligation. According to brain damage scoring based on Cresyl Violet staining, the neuronal damage time-dependently changed in the ischemic regions following hypoxia-ischemia. Immunohistochemical studies showed that excitatory amino acid transporter 1 expression was mainly observed in the cerebral cortex ipsilateral to common carotid artery ligation and markedly increased at 24 h and 48 h following hypoxia-ischemia. Combined with confocal laser scanning microscopic analysis, double staining showed that excitatory amino acid transporter 1 positive staining appeared in neurons as well as astrocytes after hypoxia-ischemia. Most excitatory amino acid transporter 1 positive staining cells exhibited regular morphological characteristics and only a few were double-stained by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick-end labeling. Down-regulation of excitatory amino acid transporter 1 expression by intraventricular administration of specific antisense oligonucleotide exacerbated neuronal damage in hypoxia-ischemia brain. These results suggest that the increase of excitatory amino acid transporter 1 expression may be involved in a pathophysiological process of hypoxia-ischemia brain damage and may reflect a self-compensative mechanism for protecting neurons from further injury.

Glial glutamate transporter GLT-1 down-regulation precedes delayed neuronal death in gerbil hippocampus following transient global cerebral ischemia

Glial (GLT-1 and GLAST) and neuronal (EAAC1) high-affinity transporters mediate the sodium dependent glutamate reuptake in mammalian brain. Their dysfunction leads to neuronal damage by allowing glutamate to remain in the synaptic cleft for a longer duration. The purpose of the present study is to understand their contribution to the ischemic delayed neuronal death seen in gerbil hippocampus following transient global cerebral ischemia. The protein levels of these three transporters were studied by immunoblotting as a function of reperfusion time (6 h to 7 days) following a 10 min occlusion of bilateral common carotid arteries in gerbils. In the vulnerable hippocampus, there was a significant decrease in the protein levels of GLT-1 (by 36-46%, P < 0.05; between 1 and 3 days of reperfusion) and EAAC1 (by 42-68%, P < 0.05; between 1 and 7 days of reperfusion). Histopathological evaluation showed no neuronal loss up to 2 days of reperfusion but an extensive neuronal loss (by approximately 84%, P < 0.01) at 7 days of reperfusion in the hippocampal CA1 region. The time frame of GLT-1 dysfunction (1-3 days of reperfusion) precedes the initiation of delayed neuronal death (2-3 days of reperfusion). This suggests GLT-1 dysfunction as a contributing factor for the hippocampal neuronal death following transient global cerebral ischemia. Furthermore, decreased EAAC1 levels may contribute to GABAergic dysfunction and excitatory/inhibitory imbalance following transient global ischemia.

Glutamate stimulates ascorbate transport by astrocytes

The concentrations of glutamate and ascorbate in brain extracellular fluid increase following seizure activity, trauma and ischemia. Extracellular ascorbate concentration also rises following intracerebral glutamate injection. We hypothesized that glutamate triggers the release of ascorbate from astrocytes. We observed in primary cultures of rat cerebral astrocytes that glutamate increased ascorbate efflux significantly within 30 min. The half-maximal effective concentration of glutamate was 180+/-30 microM. Glutamate-stimulated efflux of ascorbate was attenuated by hypertonic media. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited both Na(+)-dependent glutamate uptake and ascorbate efflux. Two other inhibitors of volume-sensitive organic anion channels (1, 9-dideoxyforskolin and 5-nitro-2-(3-phenylpropylamino) benzoic acid) did not slow glutamate uptake but prevented stimulation of ascorbate efflux. Glutamate also stimulated the uptake of ascorbate by ascorbate-depleted astrocytes. In contrast, glutamate uptake was not affected by intracellular ascorbate, thus ruling out a putative glutamate-ascorbate heteroexchange mechanism. These results are consistent with activation by glutamate of ascorbate-permeant channels in astrocytes.

1-Aminoindan-1,5-dicarboxylic acid and (S)-(+)-2-(3'-carboxybicyclo[1.1.1] pentyl)-glycine, two mGlu1 receptor-preferring antagonists, reduce neuronal death in in vitro and in vivo models of cerebral ischaemia

Metabotropic glutamate (mGlu) receptors have been implicated in a number of physiological and pathological responses to glutamate, but the exact role of group I mGlu receptors in causing postischaemic injury is not yet clear. In this study, we examined whether the recently-characterized and relatively selective mGlu1 receptor antagonists 1-aminoindan-1,5-dicarboxylic acid (AIDA) and (S)-(+)-2-(3'-carboxybicyclo[1.1.1]pentyl)-glycine (CBPG) could reduce neuronal death in vitro, following oxygen-glucose deprivation (OGD) in murine cortical cell and rat organotypic hippocampal cultures, and in vivo, after global ischaemia in gerbils. When present in the incubation medium during the OGD insult and the subsequent 24 h recovery period, AIDA and CBPG significantly reduced neuronal death in vitro. The extent of protection was similar to that observed with the nonselective mGlu receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine [(+)MCPG] and with typical ionotropic glutamate (iGlu) receptor antagonists. Neuroprotection was also observed when AIDA or CBPG were added only after the OGD insult was terminated. Neuronal injury was not attenuated by the inactive isomer (-)MCPG, but was significantly enhanced by the nonselective mGlu receptor agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid [(1S,3R)-ACPD] and the group I mGlu receptor agonist 3,5-dihydroxyphenylglycine (3,5-DHPG). The antagonists (+)MCPG, AIDA and CBPG were also neuroprotective in vivo, because i. c.v. administration reduced CA1 pyramidal cell degeneration examined 7 days following transient carotid occlusion in gerbils. Our results point to a role of mGlu1 receptors in the pathological mechanisms responsible for postischaemic neuronal death and propose a new target for neuroprotection.

Diffusion enhancement of drugs by loaded nanoparticles in vitro

1. Dalargin, a Leu-enkephaline analogue, does normally not pass the blood-brain barrier (BBB). When it was adsorbed onto the surface of polybutylcyanoacrylate, nanoparticles dalargin can cross the BBB and induce central analgesic effects after intravenously as well as after oral application. 2. The mechanisms of this effect are unknown. Therefore, the authors evaluated whether neuronal transport was involved in this effect. In hippocampal synaptosomes and in tissue slices in vitro the active neuronal uptake and diffusion processes were determined by use of labelled D-aspartate as a marker of the aspartate/glutamate transporter and orotic acid as marker of diffusion. 3. Transporter-mediated uptake into hippocampal tissue preparations was not altered in comparison to control whereas diffusion processes were enhanced. These data indicate that the nanoparticles can modify neuronal uptake mechanisms.

Excitatory amino acid transporters: a family in flux

As the most predominant excitatory neurotransmitter, glutamate has the potential to influence the function of most neuronal circuits in the central nervous system. To limit receptor activation during signaling and prevent the overstimulation of glutamate receptors that can trigger excitotoxic mechanisms and cell death, extracellular concentrations of excitatory amino acids are tightly controlled by transport systems on both neurons and glial cells. L-Glutamate is a potent neurotoxin, and the inadequate clearance of excitatory amino acids may contribute to the neurodegeneration seen in a variety of conditions, including epilepsy, ischemia, and amyotrophic lateral sclerosis. To establish the contributions of carrier systems to the etiology of neurological disorders, and to consider their potential utility as therapeutic targets, a detailed understanding of transporter function and pharmacology is required. This review summarizes current knowledge of the structural and functional diversity of excitatory amino acid transporters and explores how they might serve as targets for drug design.