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

NMDA receptors mediate synaptic plasticity impairment of hippocampal neurons due to arsenic exposure

Endemic arsenism is a worldwide health problem. Chronic arsenic exposure results in cognitive dysfunction due to arsenic and its metabolites accumulating in hippocampus. As the cellular basis of cognition, synaptic plasticity is pivotal in arsenic-induced cognitive dysfunction. N-methyl-D-aspartate receptors (NMDARs) serve physiological functions in synaptic transmission. However, excessive NMDARs activity contributes to exitotoxicity and synaptic plasticity impairment. Here, we provide an overview of the mechanisms that NMDARs and their downstream signaling pathways mediate synaptic plasticity impairment due to arsenic exposure in hippocampal neurons, ways of arsenic exerting on NMDARs, as well as the potential therapeutic targets except for water improvement.

Glutamate transporters: Critical components of glutamatergic transmission

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

Glial Cells in Glaucoma: Friends, Foes, and Potential Therapeutic Targets

Glaucoma is the second leading cause of blindness worldwide, affecting ~80 million people by 2020 (1, 2). The condition is characterized by a progressive loss of retinal ganglion cells (RGCs) and their axons accompanied by visual field loss. The underlying pathophysiology of glaucoma remains elusive. Glaucoma is recognized as a multifactorial disease, and lowering intraocular pressure (IOP) is the only treatment that has been shown to slow the progression of the condition. However, a significant number of glaucoma patients continue to go blind despite intraocular pressure-lowering treatment (2). Thus, the need for alternative treatment strategies is indisputable. Accumulating evidence suggests that glial cells play a significant role in supporting RGC function and that glial dysfunction may contribute to optic nerve disease. Here, we review recent advances in understanding the role of glial cells in the pathophysiology of glaucoma. A particular focus is on the dynamic and essential interactions between glial cells and RGCs and potential therapeutic approaches to glaucoma by targeting glial cells.

The role of astrocytic glutamate transporters GLT-1 and GLAST in neurological disorders: Potential targets for neurotherapeutics

Glutamate is the primary excitatory neurotransmitter in the central nervous system (CNS) which initiates rapid signal transmission in the synapse before its re-uptake into the surrounding glia, specifically astrocytes. The astrocytic glutamate transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) and their human homologs excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2), respectively, are the major transporters which take up synaptic glutamate to maintain optimal extracellular glutamic levels, thus preventing accumulation in the synaptic cleft and ensuing excitotoxicity. Growing evidence has shown that excitotoxicity is associated with various neurological disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Parkinson's disease (PD), manganism, ischemia, schizophrenia, epilepsy, and autism. While the mechanisms of neurological disorders are not well understood, the dysregulation of GLAST/GLT-1 may play a significant role in excitotoxicity and associated neuropathogenesis. The expression and function of GLAST/GLT-1 may be dysregulated at the genetic, epigenetic, transcriptional or translational levels, leading to high levels of extracellular glutamate and excitotoxicity. Consequently, understanding the regulatory mechanisms of GLAST/GLT-1 has been an area of interest in developing therapeutics for the treatment of neurological disorders. Pharmacological agents including β-lactam antibiotics, estrogen/selective estrogen receptor modulators (SERMs), growth factors, histone deacetylase inhibitors (HDACi), and translational activators have shown significant efficacy in enhancing the expression and function of GLAST/GLT-1 and glutamate uptake both in vitro and in vivo. This comprehensive review will discuss the regulatory mechanisms of GLAST/GLT-1, their association with neurological disorders, and the pharmacological agents which mediate their expression and function. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

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

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

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

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

Organotypic Brain Slice Cultures: An Efficient and Reliable Method for Neurotoxicological Screening and Mechanistic Studies

This paper reviews the current state of the use of organotypic brain slice cultures for neurotoxicological and neuropharmacological screening and mechanistic studies, as exemplified by excitotoxin application. At present, no in vitro systems have been approved by the regulatory authorities for neurotoxicity testing. For the evaluation of the slice culture method, organotypic hippocampal slice cultures were exposed to toxic doses of the excitotoxins, glutamate, N-methyl-D-aspartate (NMDA), kainic acid and 2-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and the glial toxin, DL-alpha-aminoadipic acid (DLAAA). Neuronal cell death was quantified by propidium iodide (PI) uptake, and visualised by Fluoro-Jade (FJ) staining. General cell death was monitored by lactate dehydrogenase (LDH) release into the culture medium. EC50 values for the different compounds, based on PI uptake after exposure for 48 hours in entire cultures, were: glutamate, 3.5 mM; DL-AAA, 2.3 mM; kainic acid, 13 microM; NMDA, 11 microM; and AMPA, 3.7 microM. In the slice cultures, the hippocampal subfields displayed the same differences in vulnerability as those observed in vivo. When subfield analysis was performed on the cultures, the CA1 subfield was most susceptible to glutamate, NMDA and AMPA, while CA3 was most susceptible to kainic acid. The amount of LDH release for DL-AAA was about four times that of L-glutamate, in accordance with the additional toxic effect on glial cells, which was also found by confocal microscopy to stain for FJ. In conclusion, it was found that organotypic brain slice culture, combined with standardised protocols and quantifiable markers, such as PI and FJ staining, is a relevant and feasible in vitro system for neurotoxicity testing. Considering the amount and quality of the available published data, it is recommended that the brain slice culture method could be subjected to pre-validation and formal validation for inclusion in a tiered in vitro neurotoxicity testing scheme to supplement and replace conventional animal tests.

Glutamate transporters: Gene expression regulation and signaling properties

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

Astrocytes Maintain Glutamate Homeostasis in the CNS by Controlling the Balance between Glutamate Uptake and Release

Glutamate is one of the most prevalent neurotransmitters released by excitatory neurons in the central nervous system (CNS); however, residual glutamate in the extracellular space is, potentially, neurotoxic. It is now well-established that one of the fundamental functions of astrocytes is to uptake most of the synaptically-released glutamate, which optimizes neuronal functions and prevents glutamate excitotoxicity. In the CNS, glutamate clearance is mediated by glutamate uptake transporters expressed, principally, by astrocytes. Interestingly, recent studies demonstrate that extracellular glutamate stimulates Ca2+ release from the astrocytes' intracellular stores, which triggers glutamate release from astrocytes to the adjacent neurons, mostly by an exocytotic mechanism. This released glutamate is believed to coordinate neuronal firing and mediate their excitatory or inhibitory activity. Therefore, astrocytes contribute to glutamate homeostasis in the CNS, by maintaining the balance between their opposing functions of glutamate uptake and release. This dual function of astrocytes represents a potential therapeutic target for CNS diseases associated with glutamate excitotoxicity. In this regard, we summarize the molecular mechanisms of glutamate uptake and release, their regulation, and the significance of both processes in the CNS. Also, we review the main features of glutamate metabolism and glutamate excitotoxicity and its implication in CNS diseases.

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

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

Transcriptional Regulation of Glutamate Transporters: From Extracellular Signals to Transcription Factors

Glutamate is the predominant excitatory neurotransmitter in the mammalian CNS. It mediates essentially all rapid excitatory signaling. Dysfunction of glutamatergic signaling contributes to developmental, neurologic, and psychiatric diseases. Extracellular glutamate is cleared by a family of five Na(+)-dependent glutamate transporters. Two of these transporters (GLAST and GLT-1) are relatively selectively expressed in astrocytes. Other of these transporters (EAAC1) is expressed by neurons throughout the nervous system. Expression of the last two members of this family (EAAT4 and EAAT5) is almost exclusively restricted to specific populations of neurons in cerebellum and retina, respectively. In this review, we will discuss our current understanding of the mechanisms that control transcriptional regulation of the different members of this family. Over the last two decades, our understanding of the mechanisms that regulate expression of GLT-1 and GLAST has advanced considerably; several specific transcription factors, cis-elements, and epigenetic mechanisms have been identified. For the other members of the family, little or nothing is known about the mechanisms that control their transcription. It is assumed that by defining the mechanisms involved, we will advance our understanding of the events that result in cell-specific expression of these transporters and perhaps begin to define the mechanisms by which neurologic diseases are changing the biology of the cells that express these transporters. This approach might provide a pathway for developing new therapies for a wide range of essentially untreatable and devastating diseases that kill neurons by an excitotoxic mechanism.

Neonatal hyperoxia induces alterations in neurotrophin gene expression

Each year in the United States, nearly 500,000 infants a year are born prematurely. Babies born before 35 weeks gestation are often placed on ventilators and/or given supplemental oxygen. This increase in oxygen, while critical for survival, can cause long-term damage to lungs, retinas and brains. In particular, hyperoxia causes apoptosis in neurons and alters glial activity. Brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) are members of the neurotrophin family of proteins that function to promote the growth, differentiation and development of the nervous system. We hypothesized that hyperoxia can alter the regulation of these genes and by doing so adversely affect the development of the brain. We predicted that mice exposed to hyperoxic conditions would have differences in BDNF and GDNF mRNA expression and relative level of methylated promoter regions coinciding with differences in the relative levels of DNMT1 and DNMT3a mRNA expression. To test this hypothesis, newborn C57Bl/6 mice and their littermates were placed in hyperoxic or normoxic conditions from postnatal day 7 to 12. There were significant decreases in BDNF mRNA expression in the prefrontal cortex following hyperoxia, but a significant increase in the isocortex. GDNF mRNA expression was significantly increased in both the isocortex and prefrontal cortex following hyperoxia. DNMT1 mRNA expression was significantly decreased in the isocortex but significantly increased in the prefrontal following hyperoxia. Together these data suggest that short-term exposure to hyperoxic conditions can affect the regulation and expression of BDNF and GDNF potentially leading to alterations in neural development.

Stimulation of α7 nicotinic acetylcholine receptor regulates glutamate transporter GLAST via basic fibroblast growth factor production in cultured cortical microglia

The α7 nicotinic acetylcholine (nACh) receptor expressed in microglia has a crucial role in neuroprotection. Simulation of α7 nACh receptor leads to increased expression of glutamate/aspartate transporter (GLAST), which in turn decreases synaptic glutamate levels. However, the upregulation of GLAST in cultured rat cortical microglia appears long after (over 18 h) stimulation of the α7 nACh receptor with nicotine. Thus, the current study elucidated the pathway responsible for the induction of GLAST expression in cultured cortical microglia. Nicotine-induced GLAST mRNA expression was significantly inhibited by cycloheximide pretreatment, indicating that a protein intermediary, such as a growth factor, is required for GLAST expression. The expression of fibroblast growth factor-2 (FGF-2) mRNA in cortical microglia was significantly increased 6 and 12h after treatment with nicotine, and this increase was potently inhibited by pretreatment with methyllycaconitine, a selective α7 nACh receptor antagonist. The treatment with nicotine also significantly increased FGF-2 protein expression. Furthermore, treatment with recombinant FGF-2 increased GLAST mRNA, protein expression and (14)C-glutamate uptake, a functional measurement of GLAST activity. Conversely, pretreatment with PD173074, an inhibitor of FGF receptor (FGFR) tyrosine kinase, significantly prevented the nicotine-induced expression of GLAST mRNA, its protein and (14)C-glutamate uptake. Reverse transcription polymerase chain reaction confirmed FGFR1 mRNA expression was confined to cultured cortical microglia. Together, the current findings demonstrate that the neuroprotective effect of activation of microglial α7 nACh receptors could be due to the expression of FGF-2, which in turn increases GLAST expression, thereby clearing glutamate from synapse and decreasing glutamate neurotransmission.

Glutamate transporters in brain ischemia: to modulate or not?

In this review, we briefly describe glutamate (Glu) metabolism and its specific transports and receptors in the central nervous system (CNS). Thereafter, we focus on excitatory amino acid transporters, cystine/glutamate antiporters (system xc-) and vesicular glutamate transporters, specifically addressing their location and roles in CNS and the molecular mechanisms underlying the regulation of Glu transporters. We provide evidence from in vitro or in vivo studies concerning alterations in Glu transporter expression in response to hypoxia or ischemia, including limited human data that supports the role of Glu transporters in stroke patients. Moreover, the potential to induce brain tolerance to ischemia through modulation of the expression and/or activities of Glu transporters is also discussed. Finally we present strategies involving the application of ischemic preconditioning and pharmacological agents, eg β-lactam antibiotics, amitriptyline, riluzole and N-acetylcysteine, which result in the significant protection of nervous tissues against ischemia.

Neuroprotection by GDNF in the ischemic brain

The glial cell line-derived neurotrophic factor (GDNF) was first identified as a survival factor for midbrain dopaminergic neurons, but additional studies provided evidences for a role as a trophic factor for other neurons of the central and peripheral nervous systems. GDNF regulates cellular activity through interaction with glycosyl-phosphatidylinositol-anchored cell surface receptors, GDNF family receptor-α1, which might signal through the transmembrane Ret tyrosine receptors or the neural cell adhesion molecule, to promote cell survival, neurite outgrowth, and synaptogenesis. The neuroprotective effect of exogenous GDNF has been shown in different experimental models of focal and global brain ischemia, by local administration of the trophic factor, using viral vectors carrying the GDNF gene and by transplantation of GDNF-expressing cells. These different strategies and the mechanisms contributing to neuroprotection by GDNF are discussed in this review. Importantly, neuroprotection by GDNF was observed even when administered after the ischemic injury.

Presynaptic malfunction: the neurotoxic effects of cadmium and lead on the proton gradient of synaptic vesicles and glutamate transport

Exposure to Cd(2+) and Pb(2+) has neurotoxic consequences for human health and may cause neurodegeneration. The study focused on the analysis of the presynaptic mechanisms underlying the neurotoxic effects of non-essential heavy metals Cd(2+) and Pb(2+). It was shown that the preincubation of rat brain nerve terminals with Cd(2+) (200 μM) or Pb(2+) (200 μM) resulted in the attenuation of synaptic vesicles acidification, which was assessed by the steady state level of the fluorescence of pH-sensitive dye acridine orange. A decrease in L-[(14)C]glutamate accumulation in digitonin-permeabilized synaptosomes after the addition of the metals, which reflected lowered L-[(14)C]glutamate accumulation by synaptic vesicles inside of synaptosomes, may be considered in the support of the above data. Using isolated rat brain synaptic vesicles, it was found that 50 μM Cd(2+) or Pb(2+) caused dissipation of their proton gradient, whereas the application of essential heavy metal Mn(2+) did not do it within the range of the concentration of 50-500 μM. Thus, synaptic malfunction associated with the influence of Cd(2+) and Pb(2+) may result from partial dissipation of the synaptic vesicle proton gradient that leads to: (1) a decrease in stimulated exocytosis, which is associated not only with the blockage of voltage-gated Ca(2+) channels, but also with incomplete filling of synaptic vesicles; (2) an attenuation of Na(+)-dependent glutamate uptake.

Bromocriptine, an ergot alkaloid, inhibits excitatory amino acid release mediated by glutamate transporter reversal

Bromocriptine, a dopamine D(2) receptor agonist, has widely been used for patients with Parkinson's disease. The aim of the present study was to investigate the effect of bromocriptine on glutamate transporter. Since the astroglial glutamate transporter GLT-1 (EAAT2) is the predominant isoform in the forebrain, we generated EAAT2-expressing human embryonic kidney cells and immortalized mouse astrocytes. In the present studies, we observed a GLT-1-immunoreactive band and significant Na(+)-dependent d-[(3)H] aspartate uptake. Furthermore, the glutamate transporter inhibitors, dl-threo-beta-benzyloxyaspartic acid (TBOA) and dihydrokainate (DHK), displayed a dose-dependent reduction of d-[(3)H] aspartate uptake in both types of cells. In contrast, cells exposed to either chemical anoxia or high KCl elicited a marked release of d-[(3)H] aspartate, and the release was inhibited by TBOA and DHK, implying the contribution of glutamate transporter reversal. Interestingly, we found that bromocriptine dose-dependently inhibits d-[(3)H] aspartate release elicited by chemical anoxia or high KCl, while no changes occurred in the uptake. The inhibitory action of bromocriptine was not affected by sulpiride, a dopamine D(2) receptor antagonist. On the other hand, bromocriptine had no effect on swelling-induced d-[(3)H] aspartate release, which is mediated by volume-regulated anion channels. In vivo studies revealed that bromocriptine suppresses the excessive elevation of glutamate levels in gerbils subjected to transient forebrain ischemia in a manner similar to DHK. Taken together, these results provide evidence that bromocriptine inhibits excitatory amino acid release via reversed operation of GLT-1 without altering forward transport.

Optimization of chemically defined cell culture media--replacing fetal bovine serum in mammalian in vitro methods

Quality assurance is becoming increasingly important. Good laboratory practice (GLP) and good manufacturing practice (GMP) are now established standards. The biomedical field aims at an increasing reliance on the use of in vitro methods. Cell and tissue culture methods are generally fast, cheap, reproducible and reduce the use of experimental animals. Good cell culture practice (GCCP) is an attempt to develop a common standard for in vitro methods. The implementation of the use of chemically defined media is part of the GCCP. This will decrease the dependence on animal serum, a supplement with an undefined and variable composition. Defined media supplements are commercially available for some cell types. However, information on the formulation by the companies is often limited and such supplements can therefore not be regarded as completely defined. The development of defined media is difficult and often takes place in isolation. A workshop was organised in 2009 in Copenhagen to discuss strategies to improve the development and use of serum-free defined media. In this report, the results from the meeting are discussed and the formulation of a basic serum-free medium is suggested. Furthermore, recommendations are provided to improve information exchange on newly developed serum-free media.

Tolerance to oxygen nutrient deprivation in the hippocampal slices of the naked mole rats

Hypoxia tolerance in the naked mole rats (Heterocephalus glaber) represents a unique physiological phenomenon characterized by the capability to regulate oxygen demand to attenuate energetically costly response to low oxygen condition. Several aspects of tolerance to hypoxia in the naked mole rat are consistent with a state of neuroprotection; however, it remains to be established if such protective capability is expressed in the brain cells of mole rats subjected to hypoxia insults. The objective of this study was to determine whether evidence of tolerance to oxygen nutrient deprivation exists in the chronic cultures of the naked mole rats hippocampal slices. We used oxygen nutrient deprivation (OND), an in vitro model of hypoxia tolerance, to determine neuronal survival in the hippocampal slices of mole rats and rats (Rattus sp.). Our results indicate that hippocampal slices of mole rats kept in hypoxic condition consistently tolerate OND right from the onset time of 5 hrs and the tolerance to OND is maintained for 24 hrs, suggesting that there is evidence of tolerance to OND in hippocampal slices of mole rats.

Driving GDNF expression: the green and the red traffic lights

Glial cell line-derived neurotrophic factor (GDNF) is widely recognized as a potent survival factor for dopaminergic neurons of the nigrostriatal pathway that degenerate in Parkinson's disease (PD). In animal models of PD, GDNF delivery to the striatum or the substantia nigra protects dopaminergic neurons against subsequent toxin-induced injury and rescues previously damaged neurons, promoting recovery of the motor function. Thus, GDNF was proposed as a potential therapy to PD aimed at slowing down, halting or reversing neurodegeneration, an issue addressed in previous reviews. However, the use of GDNF as a therapeutic agent for PD is hampered by the difficulty in delivering it to the brain. Another potential strategy is to stimulate the endogenous expression of GDNF, but in order to do that we need to understand how GDNF expression is regulated. The aim of this review is to do a comprehensive analysis of the state of the art on the control of endogenous GDNF expression in the nervous system, focusing mainly on the nigrostriatal pathway. We address the control of GDNF expression during development, in the adult brain and after injury, and how damaged neurons signal glial cells to up-regulate GDNF. Pharmacological agents or natural molecules that increase GDNF expression and show neuroprotective activity in animal models of PD are reviewed. We also provide an integrated overview of the signalling pathways linking receptors for these molecules to the induction of GDNF gene, which might also become targets for neuroprotective therapies in PD.

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

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

Extracellular adenosine triphosphate induces glutamate transporter-1 expression in hippocampus

ATP can be significantly released following various brain insults and activates the extracellular signal-regulated protein kinase (ERK) pathway in astrocytes. Glutamate transporter-1 (GLT1) is the major forebrain astroglial glutamate transporter and its expression is stimulated also via ERK1/2 phosphorylation. We thus hypothesized that extracellular ATP could be a signal to GLT1 modulation in hippocampal slices obtained from rat. We indeed observed by western blot analysis that, after 1 mM ATP exposure, GLT1 expression, but not the glutamate-aspartate transporter, was enhanced. At the same time, high ATP induced significant rates of cell death in piramidal and granule cell layers, as shown by propidium iodide uptake, and increased glutamate uptake through GLT1 transporter. Also using confocal laser-scanning microscopy, we observed that ATP induced a vigorous and extensive GLT1-labeling on glial fibrillary acidic protein-positive cells. This stimulation was abolished by purine/pyrimidine nucleotide receptor antagonists and by MEK1/2 inhibitor. The present study demonstrates a novel mechanism of GLT1 regulation by extracellular ATP, reinforcing the evidence of cross talk between glutamatergic and purinergic systems.

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 microg/10 microl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

Seizure suppression by GDNF gene therapy in animal models of epilepsy

Temporal lobe epilepsy patients remain refractory to available anti-epileptic drugs in 30% of cases, indicating a need for novel therapeutic strategies. In this context, glial cell line-derived neurotrophic factor (GDNF) emerges as a possible new agent for epilepsy treatment. However, a limited number of studies, use of different epilepsy models, and different methods of GDNF delivery preclude understanding of the mechanisms for the seizure-suppressant action of GDNF. Here we show that recombinant adeno-associated viral (rAAV) vector-based GDNF overexpression in the rat hippocampus suppresses seizures in two models of temporal lobe epilepsy. First, when rAAV-GDNF was injected before hippocampal kindling, the number of generalized seizures decreased, and the prolongation of behavioral convulsions in fully kindled animals was prevented. Second, injection of rAAV-GDNF after kindling increased the seizure induction threshold. Third, rAAV-GDNF decreased the frequency of generalized seizures during the self-sustained phase of status epilepticus. Our data demonstrate the complexity of mechanisms and the beneficial action of GDNF in epilepsy. Furthermore, we show that ectopic rAAV-mediated GDNF gene expression in the seizure focus is a feasible way to mitigate seizures and provides proof of principle that the neurotrophic factor-based gene therapy approach has the potential to be developed as alternative strategy for epilepsy treatment.

Ischemia-Induced Neurogenesis: Role of Growth Factors

The neurogenic response in ischemic brain to growth factors is the net result of cell division and cell survival in specific regions of the brain. To increase the cell number, these physiologic processes should be active. Hence, when growth factors are infused into the brain, they might stimulate survival, cell division, or both to enhance neurogenesis. The end result is the interplay of all the endogenous factors with the infused exogenous factors. It is essential to understand the growth factors and their regulators that are expressed after ischemia if one is to pharmacologically enhance neurogenesis. It seems that a combinational therapy of factors or their inhibitors may provide powerful therapeutic potential for enhancing stroke-induced neurogenesis and restoring the damaged tissue to function.

Estrogen and the development and protection of nigrostriatal dopaminergic neurons: concerted action of a multitude of signals, protective molecules, and growth factors

The nigrostriatal dopamine system comprises the dopaminergic neurons located in the ventral midbrain, their axonal connections to the forebrain, and their direct cellular target cells in the striatal complex, i.e. GABAergic neurons. The major function of the nigrostriatal dopaminergic unit is the coordination and fine tuning of motor functions at the extrapyramidal level. Numerous biologically active factors including different types of growth factors (neurotrophins, members of the TGFbeta family, IGFs) and peptide/steroid hormones have been identified in the past to be implicated in the regulation of developmental aspects of this neural system. Some of these developmentally active determinants have in addition been found to play a crucial role in the mediation of neuroprotection concerning dopaminergic neurons. Estrogen was identified as such a compound interfering with embryonic neuronal differentiation and cell survival. The physiological mechanisms underlying these effects are very complex and include interactions with other developmental signals (growth factors), inflammatory processes as well as apoptotic events, but also require the activation of nonneuronal cells such as astrocytes. It appears that estrogen is assuming control over or at least influences a multitude of developmental and protective cellular mechanisms rather than taking over the part of a singular protagonist.

Persistent tolerance to oxygen and nutrient deprivation and N-methyl-D-aspartate in cultured hippocampal slices from hibernating Arctic ground squirrel

Hibernating Arctic ground squirrel (hAGS), Spermophilus parryii, survive profound decreases in cerebral perfusion during torpor and return to normal blood flow during intermittent rewarming periods without neurologic damage. Hibernating AGS tolerate traumatic brain injury in vivo, and acute hippocampal slices from hibernating animals tolerate oxygen and glucose deprivation. It remains unclear, however, if neuroprotection results from intrinsic tissue properties or from differences in response to acute trauma associated with slice preparation. The goal of this work was therefore to determine whether an intrinsic tissue tolerance persists in chronic culture of AGS hippocampal slices at 37 degrees C. A second goal was to address N-methyl-D-aspartate (NMDA) receptor involvement and channel arrest as potential mechanisms of intrinsic tissue tolerance. Baseline neuronal survival and tolerance to oxygen and nutrient deprivation (OND), an in vitro model of ischemia-reperfusion, were assessed in the CA1 region of hippocampal slices from juvenile, hAGS and interbout euthermic AGS (ibeAGS). Early in culture (insult onset at 3 h), slices from both hAGS and ibeAGS tolerate OND (4 h deprivation followed by 20 h recovery) and 500 micromol/L NMDA plus 20 mmol/L KCl. Later in culture (insult onset at 24 h), tolerance persists in slices from hAGS but not in slices from ibeAGS. Ouabain (Na(+)K(+)ATPase inhibitor) administered 24 h in culture enhances survival of slices from hAGS (assessed 24 h later). Thus, tolerance to OND in slices from hAGS is due to intrinsic tissue properties likely involving NMDA receptors and ion channel arrest.

Ionotropic glutamate receptors and glutamate transporters are involved in necrotic neuronal cell death induced by oxygen-glucose deprivation of hippocampal slice cultures

Organotypic hippocampal slice cultures represent a feasible model for studies of cerebral ischemia and the role of ionotropic glutamate receptors in oxygen-glucose deprivation-induced neurodegeneration. New results and a review of existing data are presented in the first part of this paper. The role of glutamate transporters, with special reference to recent results on inhibition of glutamate transporters under normal and energy-failure (ischemia-like) conditions is reviewed in the last part of the paper. The experimental work is based on hippocampal slice cultures derived from 7 day old rats and grown for about 3 weeks. In such cultures we investigated the subfield neuronal susceptibility to oxygen-glucose deprivation, the type of induced cell death and the involvement of ionotropic glutamate receptors. Hippocampal slice cultures were also used in our studies on glutamate transporters reviewed in the last part of this paper. Neurodegeneration was monitored and/or shown by cellular uptake of propidium iodide, loss of immunocytochemical staining for microtubule-associated protein 2 and staining with Fluoro-Jade B. To distinguish between necrotic vs. apoptotic neuronal cell death we used immunocytochemical staining for active caspase-3 (apoptosis indicator) and Hoechst 33342 staining of nuclear chromatin. Our experimental studies on oxygen-glucose deprivation confirmed that CA1 pyramidal cells were the most susceptible to this ischemia-like condition. Judged by propidium iodide uptake, a selective CA1 lesion, with only minor affection on CA3, occurred in cultures exposed to oxygen-glucose deprivation for 30 min. Nuclear chromatin staining by Hoechst 33342 and staining for active caspase-3 showed that oxygen-glucose deprivation induced necrotic cell death only. Addition of 10 microM of the N-methyl-D-aspartate glutamate receptor antagonist MK-801, and 20 microM of the non-N-methyl-D-aspartate glutamate receptor antagonist 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline to the culture medium confirmed that both N-methyl-D-aspartate and non-N-methyl-D-aspartate ionotropic glutamate receptors were involved in the oxygen-glucose deprivation-induced cell death. Glutamate is normally quickly removed, from the extracellular space by sodium-dependent glutamate transporters. Effects of blocking the transporters by addition of the DL-threo-beta-benzyloxyaspartate are reviewed in the last part of the paper. Under normal conditions addition of DL-threo-beta-benzyloxyaspartate in concentrations of 25 microM or more to otherwise untreated hippocampal slice cultures induced neuronal cell death, which was prevented by addition of 2,3-dihyroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline and MK-801. In energy failure situations, like cerebral ischemia and oxygen-glucose deprivation, the transporters are believed to reverse and release glutamate to the extracellular space. Blockade of the transporters by a subtoxic (10 microM) dose of DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation (but not during the next 48 h after oxygen-glucose deprivation) significantly reduced the oxygen-glucose deprivation-induced propidium iodide uptake, suggesting a neuroprotective inhibition of reverse transporter activity by DL-threo-beta-benzyloxyaspartate during oxygen-glucose deprivation under these conditions. Adding to this, other results from our laboratory have demonstrated that pre-treatment of the slice cultures with glial cell-line derived neurotrophic factor upregulates glutamate transporters. As a logical, but in some glial cell-line derived neurotrophic factor therapy-related conditions clearly unwanted consequence the susceptibility for oxygen-glucose deprivation-induced glutamate receptor-mediated cell death is increased after glial cell-line derived neurotrophic factor treatment. In summary, we conclude that both ionotropic glutamate receptors and glutamate transporters are involved in oxygen-glucose deprivation-induced necrotic cell death in hippocampal slice cultures, which have proven to be a feasible tool in experimental studies on this topic.

High-affinity glutamate transporter GLAST/EAAT1 regulates cell surface expression of glutamine/neutral amino acid transporter ASCT2 in human fetal astrocytes

Neutral amino acid transporter ASCT2, together with high-affinity glutamate transporters, belongs to the SLC1 gene family of Na(+)-dependent solute carriers and is one of the major transporters of glutamine in cultured astrocytes. Besides glutamine and other high-affinity substrates--alanine, serine, cysteine or threonine, ASCT2 can also translocate protonated glutamate. The present study elucidated substrate-dependent trafficking of ASCT2 in differentiated primary cultures of human fetal astrocytes. The differentiation induced by 8-bromo-cAMP caused dramatic up-regulation of two co-localized and functionally linked astroglial proteins--glutamate transporter GLAST, that is the only high-affinity router of glutamate into cultured astrocytes, and glutamine synthetase (GS), a cytosolic enzyme that converts at least a part of the arriving glutamate into glutamine. In order to distinguish individual intracellular effects of these two substrates on ASCT2, in some cultures glutamine synthetase was effectively knocked down using siRNA silencing technique. In control conditions, regardless of GS levels, almost the entire ASCT2 immunoreactivity was restricted to the cytosol. Both glutamine and alanine, though to different extents, induced partial redistribution of ASCT2 from the cytosolic compartment to the plasma membrane. However, in cultures with high GS expression, micromolar concentrations of glutamate exhibited more pronounced effect on ASCT2 trafficking than the preferred substrates of this carrier. In contrast, glutamate had no effect on ASCT2 distribution in cultures devoid of GS. D-Aspartate, a metabolically inert substrate effectively transported by GLAST, had no effect in any cell culture utilized. It seems that intracellular glutamine produced by GS from glutamate that, in turn, is supplied by GLAST, is a more potent inducer of ASCT2 trafficking to the cell surface than the ASCT2-mediated translocation of extracellular substrates. At lower pH values (6.2-6.7), the cell surface pool of ASCT2 was significantly larger than at physiological pH. In addition, high concentrations of glutamate, independently from GLAST or glutamate receptor activation, induced further arrival of ASCT2 to the plasma membrane. The pH-dependent functional activation of ASCT2 and the ASCT2-mediated glutamate uptake may play important roles during ischemic acidosis or synaptic activity-induced local acidification.

Role of astrocytes in glutamate homeostasis: implications for excitotoxicity

Glutamate homeostasis in the brain is maintained by its well balanced release, uptake and metabolism. It appears that astrocytes play a prominent role in this context since they possess a very powerful battery of glutamate transporters. Thus, malfunction of astrocytic glutamate transporters will lead to an excessively high extracellular glutamate concentration which may result in neurodegeneration caused by the excitotoxic action of glutamate.

Insonation facilitates plasmid DNA transfection into the central nervous system and microbubbles enhance the effect

Many of the diseases which affect the central nervous system are intractable to conventional therapies and therefore require alternative treatments such as gene therapy. Therapy requires safety, since the central nervous system is a critical organ. Choice of nonviral vectors such as naked plasmid DNA may have merit. However, transfection efficiencies of these vectors are low. We have investigated the use of 210.4 kHz ultrasound and found that 5.0 W/cm(2) of insonation for 5 s most effectively transfected a plasmid DNA into culture slices of mouse brain (147.68-fold increase compared with 0 W/cm(2) of insonation for 5 s). The effect was reinforced by combination with echo contrast agent, Levovist. One hundred fifty mg/mL of Levovist significantly increased gene transfection by ultrasound (5.23-fold when insonated at 5.0 W/cm(2) for 5 s). When DNA was intracranially injected, Levovist also enhanced gene transfection in newborn mice (4.49-fold increase when insonated at 5.0 W/cm(2) for 5 s). Since ultrasound successfully transfected naked plasmid DNA into the neural tissue and Levovist enhanced the effect, this approach may have a significant role in gene transfer to the central nervous system.

Over-expression of the human EAAT2 glutamate transporter within neurons of mouse organotypic hippocampal slice cultures leads to increased vulnerability of CA1 pyramidal cells

Excitatory amino acid transporters (EAATs) maintain the balance between pathological and physiological conditions by limiting the extracellular concentration of glutamate within the CNS and thus preventing excitotoxic injury. The loss of EAAT2 has been associated with the development of neurological diseases such as amyotrophic lateral sclerosis. It has therefore been suggested that the over-expression of specific EAATs may provide some degree of neuroprotection. However, the inability to isolate and study the function of the different EAAT isoforms in a cell type-specific manner has made it difficult to determine the exact contribution of individual EAATs toward neuroprotection or neurodegeneration in the context of excitotoxic injury. To address this question, we transduced hippocampal slice cultures from 1-week-old C57B/6 mice with recombinant adeno-associated virus carrying an EAAT2 gene expression cassette. EAAT2 gene expression was driven in neurons with the neuron-specific enolase promoter. Using this model system, we were able to induce a significant increase in the expression of functional EAAT2. Consequently, a significant increase in CA1 neuronal damage was observed in slices over-expressing EAAT2 in neurons following an acute exposure to exogenous glutamate. These data suggest that the increased expression of EAAT2 within neurons may contribute to neurodegeneration.

Molecular and functional characterisation of glutamate transporters in rat cortical astrocytes exposed to a defined combination of growth factors during in vitro differentiation

In vitro culture of astroglial progenitors can be obtained from early post-natal brain tissues and several methods have been reported for promoting their maturation into differentiated astrocytes. Hence, a combination of several nutriments/growth factors -- the G5 supplement (insulin, transferrin, selenite, biotin, hydrocortisone, fibroblast growth factor and epidermal growth factor) -- is widely used as a culture additive favouring the growth, differentiation and maturation of primary cultured astrocytes. Considering the key role played by glial cells in the clearance of glutamate in the synapses, cultured astrocytes are frequently used as a model for the study of glutamate transporters. Indeed, it has been shown that when tested separately, growth factors influence the expression and activity of the GLAST and GLT-1. The present study aimed at characterising the functional expression of these transporters during the time course of differentiation of cultured cortical astrocytes exposed to the supplement G5. After a few days, the vast majority of cells exposed to this supplement adopted a typical stellate morphology (fibrous or type II astrocytes) and showed intense expression of the glial fibrillary acidic protein. Both RT-PCR and immunoblotting studies revealed that the expression of both GLAST and GLT-1 rapidly increased in these cells. While this was correlated with a significant increase in specific uptake of radiolabelled aspartate, fluorescence monitoring of the Na+ influx associated with glutamate transporters activity revealed that the exposure to the G5 supplement considerably increased the percentage of cells participating in the uptake. Biochemical and pharmacological studies revealed that this activity did not involve GLT-1 but most likely reflected an increase in GLAST-mediated uptake. Together, these data indicate that the addition of this classical combination of growth factors and nutriments drives the rapid differentiation toward a homogenous culture of fibrous astrocytes expressing functional glutamate transporters.

A new model of ischemic preconditioning using young adult hippocampal slice cultures

In ischemic preconditioning (IPC), brief sublethal ischemia protects neurons from a subsequent lethal ischemia. In vivo models faithfully display preconditioning, yet, these models are technically challenging, time-consuming and expensive. In vitro models of preconditioning have also been developed that are technically easier and less expensive. A drawback of pre-existing in vitro models is that since susceptibility to ischemic injury is age-dependent; neuroprotection is being studied in neurons that have intrinsic resistance to oxygen-glucose deprivation (OGD). This study introduces a new in vitro model of ischemic preconditioning in hippocampal slice cultures isolated from 20-30-day-old rats. Slice cultures show a high susceptibility and sharp thresholds toward ischemia that is comparable to that found in vivo. A 5-min OGD treatment was not neurotoxic to young adult slice cultures, while a 10-min OGD treatment was neurotoxic. In addition, the sublethal 5-min OGD treatment protected against a 10-min OGD treatment that was delivered 24 h later. Neuroprotection was seen in preconditioned slice cultures stained with propidium iodide (PI) or with antisera against the neuron-specific antigen NeuN. Energy failure is hypothesized to trigger ischemic preconditioning and a 5-min OGD treatment induced transient energy failure in young adult slice cultures. This model may assist in the search for new therapeutics for the prevention and/or treatment of stroke.

Role of astrocytic transport processes in glutamatergic and GABAergic neurotransmission

The fine tuning of both glutamatergic and GABAergic neurotransmission is to a large extent dependent upon optimal function of astrocytic transport processes. Thus, glutamate transport in astrocytes is mandatory to maintain extrasynaptic glutamate levels sufficiently low to prevent excitotoxic neuronal damage. In GABA synapses hyperactivity of astroglial GABA uptake may lead to diminished GABAergic inhibitory activity resulting in seizures. As a consequence of this the expression and functional activity of astrocytic glutamate and GABA transport is regulated in a number of ways at transcriptional, translational and post-translational levels. This opens for a number of therapeutic strategies by which the efficacy of excitatory and inhibitory neurotransmission may be manipulated.

Beta-amyloid and brain-derived neurotrophic factor, BDNF, up-regulate the expression of glutamate transporter GLT-1/EAAT2 via different signaling pathways utilizing transcription factor NF-kappaB

Malfunctioning of high-affinity glutamate transporters is believed to contribute to the accumulation of toxic concentrations of glutamate and, thus, trigger the cellular mechanisms of neurodegeneration. Emerging data point to the presence of excitotoxic component in Alzheimer's disease (AD) and aberrant expression of glutamate transporters in this neurodegenerative malady. Neuronal soluble factors are essential for differential expression and fine tuning of the astroglial glutamate transporters, GLT-1/EAAT2 and GLAST/EAAT1. However, the nature of factors specifically affecting glutamate uptake in AD is largely unknown. The overproduction of neurotoxic beta-amyloid peptide (Abeta), a major constituent of amyloid plaques, and marked down-regulation of BDNF, a neuroprotective factor, are hallmarks of AD pathophysiology. None of these typically neuronal factors was capable of changing the pattern of glutamate transporter expression in undifferentiated rat astrocytes that predominantly expressed GLAST. In differentiated astrocytes, BDNF and, to a lesser extent, subtoxic concentrations of Abeta 1-42 (1-5 microM) induced the expression of GLT-1 and increased glutamate uptake, whereas the GLAST levels were unaltered by these factors. The BDNF-dependent up-regulation of GLT-1 in differentiated astrocytes was partially antagonized by the activation of metabotropic glutamate receptor 4 (mGluR4), but not by group I or II mGluRs. Activation of transcription factor NF-kappaB appeared to be a shared essential, but not a sufficient molecular event in the BDNF- or Abeta-dependent induction of GLT-1. The BDNF-dependent activation of NF-kappaB and up-regulation of GLT-1 was critically dependent on the upstream activation of p42/p44 MAP kinase signaling, whereas the inhibition of these MAP kinases dramatically increased the Abeta-dependent activation of NF-kappaB and production of GLT-1. The capacity to up-regulate astroglial glutamate uptake system, that apparently represents a novel element in the neuroprotective repertoire of BDNF, can, however, provide adverse effect under certain insults when glutamate transporters start operating in reverse direction. The Abeta-dependent up-regulation of GLT-1/EAAT2, more pronounced under the deficit of MAP kinase signaling, may attenuate synaptic efficacy and, thus contribute to the impairment of neuroplasticity in AD.