Properties of kainate receptor/channels on cultured Bergmann glia

Aryl Hydrocarbon Receptor Involvement in the Sodium-Dependent Glutamate/Aspartate Transporter Regulation in Cerebellar Bergmann Glia Cells

Glutamate, the major excitatory neurotransmitter in the vertebrate brain, exerts its functions through the activation of specific plasma membrane receptors and transporters. Overstimulation of glutamate receptors results in neuronal cell death through a process known as excitotoxicity. A family of sodium-dependent glutamate plasma membrane transporters is responsible for the removal of glutamate from the synaptic cleft, preventing an excitotoxic insult. Glial glutamate transporters carry out more than 90% of the brain glutamate uptake activity and are responsible for glutamate recycling through the GABA/Glutamate/Glutamine shuttle. The aryl hydrocarbon receptor is a ligand-dependent transcription factor that integrates environmental clues through its ability to heterodimerize with different transcription factors. Taking into consideration the fundamental role of glial glutamate transporters in glutamatergic synapses and that these transporters are regulated at the transcriptional, translational, and localization levels in an activity-dependent fashion, in this contribution, we explored the involvement of the aryl hydrocarbon receptor, as a model of environmental integrator, in the regulation of the glial sodium-dependent glutamate/aspartate transporter. Using the model of chick cerebellar Bergmann glia cells, we report herein that the aryl hydrocarbon receptors exert a time-dependent decrease in the transporter mRNA levels and a diminution of its uptake activity. The nuclear factor kappa light chain enhancer of the activated B cell signaling pathway is involved in this regulation. Our results favor the notion of an environmentally dependent regulation of glutamate removal in glial cells and therefore strengthen the notion of the involvement of glial cells in xenobiotic neurotoxic effects.

Acute Manganese Exposure Modifies the Translation Machinery via PI3K/Akt Signaling in Glial Cells

SUMMARY STATEMENT

We demonstrate herein that short-term exposure of radial glia cells to Manganese, a neurotoxic metal, induces an effect on protein synthesis, altering the protein repertoire of these cells.

DNA Methylation-Dependent Gene Expression Regulation of Glutamate Transporters in Cultured Radial Glial Cells

Exposure to xenobiotics has a significant impact in brain physiology that could be liked to an excitotoxic process induced by a massive release of the main excitatory neurotransmitter, L-glutamate. Overstimulation of extra-synaptic glutamate receptors, mainly of the N-methyl-D-aspartate subtype leads to a disturbance of intracellular calcium homeostasis that is critically involved in neuronal death. Hence, glutamate extracellular levels are tightly regulated through its uptake by glial glutamate transporters. It has been observed that glutamate regulates its own removal, both in the short-time frame via a transporter-mediated decrease in the uptake, and in the long-term through the transcriptional control of its gene expression, a process mediated by glutamate receptors that involves the Ca²⁺/diacylglycerol-dependent protein kinase and the transcription factor Ying Yang 1. Taking into consideration that this transcription factor is a member of the Polycomb complex and thus, part of repressive and activating chromatin remodeling factors, it might direct the interaction of DNA methyltransferases or dioxygenases of methylated cytosines to their target sequences. Here we explored the role of dynamic DNA methylation in the expression and function of glial glutamate transporters. To this end, we used the well-characterized models of primary cultures of chick cerebellar Bergmann glia cells and a human retina-derived Müller glia cell line. A time and dose-dependent increase in global DNA methylation was evident upon glutamate exposure. Under hypomethylation conditions, the glial glutamate transporter protein levels and uptake activity were increased. These results favor the notion that a dynamic DNA methylation program triggered by glutamate in glial cells modulates one of its major functions: glutamate removal.

EAAT1-dependent slc1a3 Transcriptional Control depends on the Substrate Translocation Process

Glutamate, the major excitatory neurotransmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent transporters expressed in neurons and glial cells. The bulk of glutamate uptake activity occurs in glial cells through the sodium-dependent glutamate/aspartate transporter (EAAT1/GLAST) and glutamate transporter 1 (EAAT2/GLT-1). EAAT1/GLAST is the predominant transporter within the cerebellum. It is highly enriched in Bergmann glial cells that span the cerebellar cortex and wrap the most abundant glutamatergic synapses in the central nervous system, the synapse formed by the parallel fibers and the Purkinje cells. In the past years, it has become evident that Bergmann glial cells are involved in glutamatergic transmission. Glutamate transporters are tightly regulated due to their essential role in tripartite synapses. Glutamate regulates EAAT1/GLAST function and gene expression in a receptor-dependent and receptor-independent manner. Through the use of the non-metabolizable EAAT1/GLAST ligand, D-Aspartate, and the well-established chick cerebellar Bergmann glia primary culture, in this contribution, we demonstrate that EAAT1/GLAST down-regulates its expression and function at the transcriptional level through the activation of a signaling pathway that includes the phosphatidyl inositol 3 kinase (PI3K), the Ca2+/diacylglycerol dependent protein kinase PKC and the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB). These results favor the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

Summary statement

EAAT1/GLAST down-regulates its expression and function at the transcriptional level by activating a signaling pathway that includes PI3K, PKC and NF-κB, favoring the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

GLAST Activity is Modified by Acute Manganese Exposure in Bergmann Glial Cells

Glutamate is the major excitatory amino acid neurotransmitter in the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed in neurons and glial cells. Overactivation of glutamate receptors results in neuronal death, known as excitotoxicity. A family of sodium-dependent glutamate transporters enriched in glial cells are responsible of the vast majority of the removal of this amino acid form the synaptic cleft. Therefore, a precise and exquisite regulation of these proteins is required not only for a proper glutamatergic transmission but also for the prevention of an excitotoxic insult. Manganese is a trace element essential as a cofactor for several enzymatic systems, although in high concentrations is involved in the disruption of brain glutamate homeostasis. The molecular mechanisms associated to manganese neurotoxicity have been focused on mitochondrial function, although energy depletion severely compromises the glutamate uptake process. In this context, in this contribution we analyze the effect of manganese exposure in glial glutamate transporters function. To this end, we used the well-established model of chick cerebellar Bergmann glia cultures. A time and dose dependent modulation of [³H]-D-aspartate uptake was found. An increase in the transporter catalytic efficiency, most probably linked to a discrete increase in the affinity of the transporter was detected upon manganese exposure. Interestingly, glucose uptake was reduced by this metal. These results favor the notion of a direct effect of manganese on glial cells, this in turn alters their coupling with neurons and might lead to changes in glutamatergic transmission.

Acute Exposure to SiO2 Nanoparticles Affects Protein Synthesis in Bergmann Glia Cells

Attractive due to an alleged high biocompatibility, silica nanoparticles have been widely used in the field of nanomedicine; however, their proven capacity to induce the synthesis and release of pro-inflammatory cytokines in several cellular models has raised concern about their safety. Glutamate, the main excitatory amino acid transmitter triggers a wide variety of signal transduction cascades that regulate protein synthesis at transcriptional and translational levels. A stimulus-dependent dynamic change in the protein repertoire in neurons and glia cells is the molecular framework of higher brain functions. Within the cerebellum, Bergmann glia cells are the most abundant non-neuronal cells and span the entire molecular layer of the cerebellar cortex, wrapping the synapses in this structure. Taking into consideration the functional role of Bergmann glia in terms of the recycling of glutamate, lactate supply to neurons, and prevention of neurotoxic insults, we decided to investigate the possibility that silica nanoparticles affect Bergmann glia and by these means alter the major excitatory neurotransmitter system in the brain. To this end, we exposed cultured chick cerebellar Bergmann glia cells to silica nanoparticles and measured [³⁵S]-methionine incorporation into newly synthesized polypeptides. Our results demonstrate that exposure of the cultured cells to silica nanoparticles exerts a time- and dose-dependent modulation of protein synthesis. Furthermore, altered patterns of eukaryotic initiation factor 2 alpha and eukaryotic elongation factor 2 phosphorylation were present upon nanoparticle exposure. These results demonstrate that glia cells respond to the presence of this nanomaterial modifying their proteome, presumably in an effort to overcome any plausible neurotoxic effect.

Neuronal Nitric Oxide Synthase in Cultured Cerebellar Bergmann Glia: Glutamate-Dependent Regulation

Glutamate exerts its actions through the activation of membrane receptors expressed in neurons and glia cells. The signaling properties of glutamate transporters have been characterized recently, suggesting a complex array of signaling transactions triggered by presynaptic released glutamate. In the cerebellar molecular layer, glutamatergic synapses are surrounded by Bergmann glia cells, compulsory participants of glutamate turnover and supply to neurons. Since a glutamate-dependent increase in cGMP levels has been described in these cells and the nitric oxide-cGMP signaling cascade increases their glutamate uptake activity, we describe here the Bergmann glia expression of neuronal nitric oxide synthetase. An augmentation of neuronal nitric oxide synthase was found upon glutamate exposure. This effect is mediated by glutamate transporters and is related to an increase in the stability of the enzyme. These results strengthen the notion of a complex regulation of glial glutamate uptake that supports neuronal glutamate signaling.

Brown Seaweed Egregia menziesii's Cytotoxic Activity against Brain Cancer Cell Lines

Brown seaweeds contain bioactive compounds that show anti-tumorigenic effects. These characteristics have been repeatedly observed in the Lessoniaceae family. Egregia menziesii, a member of this family, is distributed in the North Pacific and its properties have been barely studied. We evaluated herein the cytotoxic and anti-proliferative activity of extracts of this seaweed, through toxicity assay in Artemia salina and lymphocytes, and MTT proliferation assay, in Bergmann glia cells, 3T3-L1 and brain cancer cell lines. E. menziesii’s extracts inhibited the spread of all the tested cell lines. The hexane extract showed the highest cytotoxic activity, while the methanol extract was moderately cytotoxic. Interestingly, seaweed extracts displayed a selective inhibition pattern. These results suggest that E. menziesii’s extracts might be good candidates for cancer prevention and the development of novel chemotherapies due to its highest cytotoxicity in transformed cells compare to glia primary cultures.

The neurotoxin diethyl dithiophosphate impairs glutamate transport in cultured Bergmann glia cells

Glutamate, the main excitatory neurotransmitter in the vertebrate Central Nervous System, is involved in almost every aspect of brain physiology, and its signaling properties are severely affected in most neurodegenerative diseases. This neurotransmitter has to be efficiently removed from the synaptic cleft in order to prevent an over-stimulation of glutamate receptors that leads to neuronal death. Specific sodium-dependent membrane transporters, highly enriched in glial cells, elicit the clearance of glutamate. Once internalized, it is metabolized to glutamine by the glia-enriched enzyme Glutamine synthetase. Accumulated glutamine is released into the extracellular space for its uptake into pre-synaptic neurons and its conversion to glutamate that is packed into synaptic vesicles completing the glutamate/glutamine cycle. Diverse chemical compounds, like organophosphates, directly affect brain chemistry by altering levels of neurotransmitters in the synaptic cleft. Organophosphate compounds are widely used as pesticides, and all living organisms are continuously exposed to these substances, either in a direct or indirect manner. Its metabolites, like the diethyl dithiophosphate, are capable of causing brain damage through diverse mechanisms including perturbation of neuronal-glial cell interactions and have been associated with attention-deficit disorders and other mental illness. In order to characterize the neurotoxic mechanisms of diethyl dithiophosphate, we took advantage of the well characterized model of chick cerebellar Bergmann glia cultures. A significant impairment of [³H] d-Aspartate transport was found upon exposure to the metabolite. These results indicate that glia cells are targets of neurotoxic substances such as pesticides and that these cells might be critically involved in the associated neuronal death.

Glutamate-Dependent Translational Control of Glutamine Synthetase in Bergmann Glia Cells

Glutamate is the major excitatory transmitter of the vertebrate brain. It exerts its actions through the activation of specific plasma membrane receptors expressed both in neurons and in glial cells. Recent evidence has shown that glutamate uptake systems, particularly enriched in glia cells, trigger biochemical cascades in a similar fashion as receptors. A tight regulation of glutamate extracellular levels prevents neuronal overstimulation and cell death, and it is critically involved in glutamate turnover. Glial glutamate transporters are responsible of the majority of the brain glutamate uptake activity. Once internalized, this excitatory amino acid is rapidly metabolized to glutamine via the astrocyte-enriched enzyme glutamine synthetase. A coupling between glutamate uptake and glutamine synthesis and release has been commonly known as the glutamate/glutamine shuttle. Taking advantage of the established model of cultured Bergmann glia cells, in this contribution, we explored the gene expression regulation of glutamine synthetase. A time- and dose-dependent regulation of glutamine synthetase protein and activity levels was found. Moreover, glutamate exposure resulted in the transient shift of glutamine synthetase mRNA from the monosomal to the polysomal fraction. These results demonstrate a novel mode of glutamate-dependent glutamine synthetase regulation and strengthen the notion of an exquisite glia neuronal interaction in glutamatergic synapses.

Coupling of glutamate and glucose uptake in cultured Bergmann glial cells

Glutamate, the main excitatory neurotransmitter in the vertebrate brain, exerts its actions through specific membrane receptors present in neurons and glial cells. Over-stimulation of glutamate receptors results in neuronal death, phenomena known as excitotoxicity. A family of sodium-dependent, glutamate uptake transporters mainly expressed in glial cells, removes the amino acid from the synaptic cleft preventing neuronal death. The sustained sodium influx associated to glutamate removal in glial cells, activates the sodium/potassium ATPase restoring the ionic balance, additionally, glutamate entrance activates glutamine synthetase, both events are energy demanding, therefore glia cells increase their ATP expenditure favouring glucose uptake, and triggering several signal transduction pathways linked to proper neuronal glutamate availability, via the glutamate/glutamine shuttle. To further characterize these complex transporters interactions, we used the well-established model system of cultured chick cerebellum Bergmann glia cells. A time and dose-dependent increase in the activity, plasma membrane localization and protein levels of glucose transporters was detected upon d-aspartate exposure. Interestingly, this increase is the result of a protein kinase C-dependent signaling cascade. Furthermore, a glutamate-dependent glucose and glutamate transporters co-immunoprecipitation was detected. These results favour the notion that glial cells are involved in glutamatergic neuronal physiology.

Glutamate-Dependent BMAL1 Regulation in Cultured Bergmann Glia Cells

Glutamate, the major excitatory amino acid, activates a wide variety of signal transduction cascades. This neurotransmitter is involved in photic entrainment of circadian rhythms, which regulate physiological and behavioral functions. The circadian clock in vertebrates is based on a transcription-translation feedback loop in which Brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like protein 1 (BMAL1) acts as transcriptional activator of others clock genes. This protein is expressed in nearly all suprachiasmatic nucleus neurons, as well as in the granular layer of the cerebellum. In this context, we decided to investigate the role of glutamate in the molecular mechanisms involved in the processes of transcription/translation of BMAL1 protein. To this end, primary cultures of chick cerebellar Bergmann glial cells were stimulated with glutamatergic ligands and we found that BMAL1 levels increased in a dose- and time dependent manner. Additionally, we studied the phosphorylation of serine residues in BMAL1 under glutamate stimulation and we were able to detect an increase in the phosphorylation of this protein. The increased expression of BMAL1 is most probably the result of a stabilization of the protein after it has been phosphorylated by the cyclic AMP-dependent protein kinase and/or the Ca(2+)/diacylglycerol dependent protein kinase. The present results strongly suggest that glutamate participates in regulating BMAL1 in glial cells and that these cells might prove to be important in the control of circadian rhythms in the cerebellum.

Fluoride exposure regulates the elongation phase of protein synthesis in cultured Bergmann glia cells

Fluoride is an environmental pollutant present in dental products, food, pesticides and water. The latter, is the greatest source of exposure to this contaminant. Structural and functional damages to the central nervous system are present in exposed population. An established consequence of the neuronal is the release of a substantial amount of glutamate to the extracellular space, leading to an excitotoxic insult. Glutamate exerts its actions through the activation of specific plasma membrane receptors and transporters present in neurons and in glia cells and it is the over-activation of glutamate receptors and transporters, the biochemical hallmark of neuronal and oligodendrocyte cell death. In this context, taking into consideration that fluoride leads to degeneration of cerebellar cells, we took the advantage of the well-established model of cerebellar Bergmann glia cultures to gain insight into the molecular mechanisms inherent to fluoride neurotoxicity that might be triggered in glia cells. We could establish that fluoride decreases [(35)S]-methionine incorporation into newly synthesized polypeptides, in a time-dependent manner, and that this halt in protein synthesis is the result of a decrease in the elongation phase of translation, mediated by an augmentation of eukaryotic elongation factor 2 phosphorylation. These results favor the notion of glial cells as targets of fluoride toxicity and strengthen the idea of a critical involvement of glia cells in the function and dysfunction of the brain.

GLAST/EAAT1 regulation in cultured Bergmann glia cells: role of the NO/cGMP signaling pathway

Glutamate, the major excitatory amino acid, activates a wide variety of signal transduction cascades. Ionotropic and metabotropic glutamate receptors are critically involved in long-term synaptic changes, although recent findings suggest that the electrogenic Na(+)-dependent glutamate transporters, responsible for its removal from the synaptic cleft participate in the signaling transactions triggered by this amino acid. Glutamate transporters are profusely expressed in glia therefore most of its uptake occurs in this cellular compartment. In the cerebellar cortex, Bergmann glial cells enwrap glutamatergic synapses and participate in the recycling of its neurotransmitter through the glutamate/glutamine shuttle. It has long been acknowledged that glutamatergic transmission in the cerebellar molecular layer results in cGMP accumulation within Bergmann glia cells. In this context, we decided to investigate a plausible role of the nitric oxide/cGMP-signaling pathway in the regulation of Bergmann glia glutamate transporters. To this end, the well-established model of primary cultures of chick cerebellar Bergmann glial cells was used. Confluent monolayers were exposed to the nitric oxide donor, sodium nitroprusside, or to the non-hydrolysable cGMP analog dbcGMP and the [(3)H] D-aspartate uptake activity measured. An increase in uptake activity, related to an augmentation in VMax, was detected with both treatments. The signaling cascade includes NO/cGMP/PKG and Ca(2+) influx through the Na(+)/Ca(2+) exchanger and might be related to the plasma membrane glutamate transporters turnover. Interestingly enough, an inhibitor of the cGMP dependent protein kinase was capable to abolish the sodium nitroprusside induced Ca(2+) influx. These results provide an insight into the physiological role of cGMP in the cerebellum.

Glutamate-dependent translational control in cultured Bergmann glia cells: eIF2α phosphorylation

Glutamate (Glu), the major excitatory amino acid, activates a wide variety of signal transduction cascades. Synaptic plasticity relies on activity-dependent differential protein expression. Glu receptors have been critically involved in long-term synaptic changes, although recent findings suggest that Na(+)-dependent Glu transporters participate in Glu-induced signalling. Within the cerebellum, Bergmann glia cells are in close proximity to glutamatergic synapses and through their receptors and transporters, sense and respond to neuronal glutamatergic activity. Translational control represents the fine-tuning stage of protein expression regulation and Glu modulates this event in glial cells. In this context, we decided to explore the involvement of Glu receptors and transporters in the regulation of the initiation phase of protein synthesis. To this end, Bergmann glia cells were exposed to glutamatergic ligands and the serine 51-phosphorylation pattern of the main regulator of the initiation phase of translation, namely the α subunit of eukaryotic initiation factor 2 (eIF2α), determined. A time and dose-dependent increase in eIF2α phosphorylation was detected. The signalling cascade included Ca(2+) influx, activation of the Ca(2+)/calmodulin-dependent protein kinase II and protein kinase C. These results provide an insight into the molecular targets of the Glu effects at the translational level and strengthen the notion of the critical involvement of glia cells in glutamatergic synaptic function.

GLAST/EAAT1-induced glutamine release via SNAT3 in Bergmann glial cells: evidence of a functional and physical coupling

Glutamate, the major excitatory transmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent glutamate transporters profusely expressed in glial cells. Once internalized, it is metabolized by glutamine synthetase to glutamine and released to the synaptic space through sodium-dependent neutral amino acid carriers of the N System (SNAT3/slc38a3/SN1, SNAT5/slc38a5/SN2). Glutamine is then taken up by neurons completing the so-called glutamate/glutamine shuttle. Despite of the fact that this coupling was described decades ago, it is only recently that the biochemical framework of this shuttle has begun to be elucidated. Using the established model of cultured cerebellar Bergmann glia cells, we sought to characterize the functional and physical coupling of glutamate uptake and glutamine release. A time-dependent Na⁺-dependent glutamate/aspartate transporter/EAAT1-induced System N-mediated glutamine release could be demonstrated. Furthermore, D-aspartate, a specific glutamate transporter ligand, was capable of enhancing the co-immunoprecipitation of Na⁺-dependent glutamate/aspartate transporter and Na⁺-dependent neutral amino acid transporter 3, whereas glutamine tended to reduce this association. Our results suggest that glial cells surrounding glutamatergic synapses may act as sensors of neuron-derived glutamate through their contribution to the neurotransmitter turnover.

Brain-derived neurotrophic factor and its receptors in Bergmann glia cells

Brain-derived neurotrophic factor is an abundant and widely distributed neurotrophin expressed in the Central Nervous System. It is critically involved in neuronal differentiation and survival. The expression of brain-derived neurotrophic factor and that of its catalytic active cognate receptor (TrkB) has been extensively studied in neuronal cells but their expression and function in glial cells is still controversial. Despite of this fact, brain-derived neurotrophic factor is released from astrocytes upon glutamate stimulation. A suitable model to study glia/neuronal interactions, in the context of glutamatergic synapses, is the well-characterized culture of chick cerebellar Bergmann glia cells. Using, this system, we show here that BDNF and its functional receptor are present in Bergmann glia and that BDNF stimulation is linked to the activation of the phosphatidyl-inositol 3 kinase/protein kinase C/mitogen-activated protein kinase/Activator Protein-1 signaling pathway. Accordingly, reverse transcription-polymerase chain reaction (RT-PCR) experiments predicted the expression of full-length and truncated TrkB isoforms. Our results suggest that Bergmann glia cells are able to express and respond to BDNF stimulation favoring the notion of their pivotal role in neuroprotection.

Glutamate-dependent transcriptional control in Bergmann glia: Sox10 as a repressor

Glutamate (Glu) is the major excitatory transmitter in the vertebrate brain. Ligand-gated and G protein-coupled Glu receptors present in glial cells are presumably involved in neuronal function. Activation of Bergmann glial Glu receptors triggers a membrane to nuclei signaling cascade that regulates gene expression at the transcriptional and translational levels. Sry-related high-mobility group box (Sox10), a member of the conserved high-mobility group box transcription factor family is expressed in neural crest stem cells and in a subset of neural crest-derived lineages that include glial, but not neuronal cells. To gain insight into the role of Sox10 in Bergmann glial cells, we explored its expression and regulation. We demonstrate herein that Sox10 is expressed in Bergmann glial cells and that its DNA binding activity, mRNA, and protein levels as well as its transcriptional behavior augments upon the activation of metabotropic Glu receptors. Increase in Sox10-DNA complexes and Sox10 mRNA and protein levels were found upon exposure to Glu. Over-expression of Sox10 leads to transcriptional repression in reporter gene assays and in one of its target genes: the chick kainate binding protein gene. These findings add a new perspective into glial glutamatergic signaling and suggest the participation of Sox10 in cerebellar glutamatergic transactions.

KEYWORDS

Bergmann glial cells, glutamate, metabotropic glutamate receptors, signaling, Sox10, transcriptional control.

Valproate-dependent transcriptional regulation of GLAST/EAAT1 expression: involvement of Ying-Yang 1

Valproate, a widely used anti-epileptic drug also employed in the treatment of neurological diseases such as bipolar disorder and migraine, regulates the glutamatergic and GABAergic systems, although its effects in cell physiology have not been thoroughly characterized. High concentrations of glutamate reached during abnormal neurotransmission if not removed properly, become neurotoxic. Glutamate clearance is carried out by high affinity Na(+)-dependent glutamate transporter systems. The glutamate/aspartate transporter GLAST/EAAT1 plays the major role in glutamate removal and is regulated at different levels: transcription, post-translational modifications and cytoplasmic trafficking. The aim of this work was to gain insight into a plausible effect of valproate in GLAST function. Using cultured Bergmann glia cells from chick cerebellum we demonstrate here that valproate exposure elicits a dual regulatory effect on GLAST. In the short-term, valproate increases its Na(+)-dependent [(3)H]-d-aspartate uptake activity in a cytochalasin B-sensitive manner. Interestingly, a synergism between valproate and a histone deacetylase inhibitor was observed. Long-term valproate treatment up-regulates chglast promoter activity, GLAST mRNA levels, GLAST molecules at the plasma membrane and its uptake activity. Furthermore, valproate induces histone 3 lysine 14 acetylation and regulates Ying-Yang 1 (YY1) transcriptional repression on the chglast promoter. These results suggest that valproate elicits its effect through its histone deacetylase inhibitor properties.

On the hypes and falls in neuroprotection: targeting the NMDA receptor

Activation of the NMDA (N-methyl-D-aspartate) responsive subclass of glutamate receptors is an important mechanism of excitatory synaptic transmission. Moreover, NMDA receptors are widely involved in many forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which are thought to underlie complex tasks, including learning and memory. Dysfunction of these ligand-gated cation channels has been identified as an underlying molecular mechanism in neurological disorders ranging from acute stroke to chronic neurodegeneration in amyotrophic lateral sclerosis. Excessive glutamate levels have been detected following brain trauma and cerebral ischemia, resulting in an unregulated stimulation of NMDA receptors. These conditions are thought to elicit a cascade of excitation-mediated neuronal damage where massive increases in intracellular calcium concentrations finally trigger neuronal damage and apoptosis. Consistent with the hypothesis of NMDA receptors as essential mediators of excitotoxicity, the different functional domains of these ion channels have been identified as potential targets for neuroprotective agents. Following an initial hype on potential NMDA receptor therapeutics, the authors currently see a period of skepticism that, in reverse, appears to neglect the therapeutic potential of this receptor class. This review attempts a reappraisal of this important class of neurotransmitter receptors, with a focus on NMDA receptor heterogeneity, ligand binding domains, and candidate diseases for a potential neuroprotective therapy.

Glutamate‐dependent transcriptional regulation of GLAST/EAAT1: a role for YY1

Glutamate is the major excitatory transmitter in the vertebrate brain and its extracellular levels are tightly regulated to prevent excitotoxic effects. The Na(+)-dependent glutamate/aspartate transporter GLAST/EAAT1 is regulated in the short and in the long term by glutamate. A receptors-independent change in its membrane translocation rate, accounts for an acute modulation in GLAST/EAAT1 transport. In contrast, activation of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate subtype of glutamate receptors represses the transcription of the chick glast gene. A glutamate responsive element has been mapped to the promoter region of this gene containing a bonafide binding site for the transcription factor Ying-Yang 1. Using cultured chick cerebellar Bergmann glia cells, glutamate elicited a time and dose-dependent increase in Ying-Yang 1 DNA binding consistent with the negative response generated in a reporter gene construct controlled for Ying-Yang 1. Over-expression of this transcription factor leads to a substantial reduction in GLAST/EAAT1 transporter uptake and an important decrease in mRNA levels, all associated with the transcriptional repression of the chick glast promoter activity. These results provide evidence for an involvement of Ying-Yang 1 in the transcriptional response to glutamate in glial cells and favor the notion of a relevant role of this factor in GLAST/EAAT1 transcriptional control.

Bergmann glia expression of polyglutamine-expanded ataxin-7 produces neurodegeneration by impairing glutamate transport

Non-neuronal cells may be pivotal in neurodegenerative disease, but the mechanistic basis of this effect remains ill-defined. In the polyglutamine disease spinocerebellar ataxia type 7 (SCA7), Purkinje cells undergo non-cell-autonomous degeneration in transgenic mice. We considered the possibility that glial dysfunction leads to Purkinje cell degeneration, and generated mice that express ataxin-7 in Bergmann glia of the cerebellum with the Gfa2 promoter. Bergmann glia-specific expression of mutant ataxin-7 was sufficient to produce ataxia and neurodegeneration. Expression of the Bergmann glia-specific glutamate transporter GLAST was reduced in Gfa2-SCA7 mice and was associated with impaired glutamate transport in cultured Bergmann glia, cerebellar slices and cerebellar synaptosomes. Ultrastructural analysis of Purkinje cells revealed findings of dark cell degeneration consistent with excitotoxic injury. Our studies indicate that impairment of glutamate transport secondary to glial dysfunction contributes to SCA7 neurodegeneration, and suggest a similar role for glial dysfunction in other polyglutamine diseases and SCAs.

Glutamate-induced octamer DNA binding and transcriptional control in cultured radial glia cells

Glutamate, the main excitatory neurotransmitter in the vertebrate brain, is critically involved in gene expression regulation in neurons and in glia cells. Neuron-glia interactions provide the framework for synaptic plasticity. Retinal and cerebellar radial glia cells surround glutamatergic excitatory synapses and sense synaptic activity through glutamate receptors expressed in their membranes. Several glutamate-dependent membrane to nuclei signaling cascades have been described in these cells. Octamer DNA binding factors, namely Oct-1 and Oct-2 recognize similar DNA sequences on regulatory regions, but their final transcriptional effect depends on several factors. By these means, different responses can be achieved in different cell types. Here, we describe a comparison between the glutamate-induced DNA binding of octamer factors and their functional activities in two important types of radial glia, retinal Müller and cerebellar Bergmann glial cells. While Oct-1 is expressed in both cell types and in both glutamate treatments results in an increase in Oct-1 DNA binding, this complex is capable of transactivating a reporter gene only in Müller glia cells. In contrast, Oct-2 expression is restricted to Bergmann glia cells in which glutamate treatment results in an augmentation of Oct-2 DNA binding complexes and the repression of kainate binding protein gene transcription. Our present findings demonstrate a differential role for Oct-1 and Oct-2 transcription factors in glial glutamate signaling, and further strengthen the notion of an important role for glial cells in glutamatergic transactions in the central nervous system.

Glutamate-dependent translational regulation in cultured Bergmann glia cells: involvement of p70S6K

Glutamate, the main excitatory amino acid transmitter in the vertebrate brain is involved in the dynamic changes in protein repertoire that underlie synaptic plasticity. Activity-dependent differential expression patterns occur not only in neurons but also in glial cells. In fact, a membrane to nuclei signaling has been described after ionotropic glutamate receptor stimulation in cultured chick cerebellar Bergmann glia cells. In order to characterize other levels of protein expression regulation, we explored the effect of glutamate treatment in [35S]-methionine incorporation into newly synthesized polypeptides. A time-dependent modification in protein synthesis was found. An important component of translational control is the ribosomal S6 protein kinase. Threonine phosphorylation renders the kinase active increasing translation initiation. Glutamate exposure results in ribosomal S6 protein kinase Thr389 phosphorylation in a dose and time-dependent manner that matches perfectly with the overall protein synthesis profile detected upon the excitatory amino acid. Pharmacological characterization of the receptors involved suggests the participation of both ionotropic as well as metabotropic glutamate receptors. The non-receptor tyrosine kinase Src, phosphatidylinositol 3-kinase, protein kinase B and the mammalian target of rapamycin are mediators of the glutamate effect. These results not only demonstrate that glutamate receptors activation is critically involved in translational control in glial cells adjacent to synaptic processes like cerebellar Bergmann glia cells, but also further strengthen the notion of an active participation of glial cells in synaptic transmission.

Glutamate Activates Protein Kinase B (PKB/Akt) through AMPA Receptors in Cultured Bergmann Glia Cells

Glutamate is involved in gene expression regulation in neurons and glial cells through the activation of a diverse array of signaling cascades. In Bergmann glia, Ca2+ -permeable alpha-hydroxy-5-methyl-4-isoazole-propionic acid (AMPA) receptors become tyrosine phosphorylated after ligand binding and by these means form multiprotein signaling complexes. Of the various proteins that associate to these receptors, the phosphatidylinositol 3-kinase (PI-3K) deserves special attention since D3-phosphorylated phosphoinositides are docking molecules for signaling proteins with a pleckstrin homology domain. In order to characterize the role of PI-3K in AMPA receptors signaling, in the present report we analyze the involvement of the serine/threonine protein kinase B in this process. Our results demonstrate an augmentation in protein kinase B phosphorylation and activity after glutamate exposure. Interestingly, the effect is independent of Ca2+ influx, but sensitive to Src blockers. Our present findings broaden our current knowledge of glial glutamate receptors signaling and their involvement glutamatergic neurotransmission.

Characteristics and Regulation of Glycine Transport in Bergmann Glia

In the vertebrate CNS, glycine acts as an inhibitory neurotransmitter and as the obligatory coagonist of glutamate at N-methyl-D-aspartate receptors. These roles depend on extracellular glycine levels, regulated by Na+/Cl--dependent transporters GLYT1, present mainly in glial cells, and GLYT2, predominantly neuronal. In Bergmann glia, GLYT1 mediates both, glycine uptake and efflux, which, in turn, influences excitatory neurotransmission at Purkinje cell synapses. The biochemical properties of GLYTs and their regulation by signaling pathways in these cells are largely unknown. We characterized Gly uptake in confluent primary cultures of Bergmann glia from chick cerebellum. Transport was found to be energy- and Na+-dependent, and was resolved into a high (Km=25 microM) and a low affinity (Km=1.1 mM) components identified as GLYT1 and transport System A, respectively. Results show that high affinity transport by GLYT1 is regulated by calcium from intracellular stores, calmodulin, and myosin light chain kinase through an actin cytoskeleton-mediated action.

Glutamate regulates dystrophin-71 levels in glia cells

Glial glutamate receptors are likely to be involved in neuronal differentiation, migration, and plasticity. Dystrophin, the protein defective in Duchenne muscular dystrophy (DMD) is widely expressed in the Central Nervous System. Activation of internal promoters of the DMD gene leads to the production of several proteins, the Dystrophin-71 (Dp-71) being the most abundant in the encephalon. This protein is known to stabilize neurotransmitter receptors in clusters and its absence has been correlated with cognitive deficits in a mouse model. Using cultured chick Bergmann glia cells and mouse cerebellar fusiform astrocytes, we demonstrate here that glutamate receptor activation results in a time and dose dependent decrease of Dp-71 levels. This effect is mediated through alphaamino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. The present results suggest an involvement of Dp-71 in glutamate receptor signaling and possibly clustering and further support the notion of an active role of glia in the physiology of glutamatergic transmission.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors signaling complexes in Bergmann glia

Glutamate, the major excitatory neurotransmitter, induces a wide array of signals from the membrane to the nucleus regulating gene expression. In Bergmann glia, Ca2+ -permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole- propionic acid (AMPA) receptors are involved in the short- and long-term interactions between these cells and the neurons that they surround. After activation, AMPA receptors become tyrosine phosphorylated and by these means form multiprotein signaling complexes. To characterize these events, cultured chick Bergmann glia cells as well as chick cerebellar slices were exposed to glutamate, and, by using a combination of immunoprecipitation assays coupled to Western blot analysis, we identified several signaling proteins that become associated with these receptors. A dose- and time-dependent association among AMPA receptors, the focal adhesion kinase pp125FAK, the phosphatidylinositol-3 kinase and paxillin was found. These results extend the concept of the transducisome to AMPA receptors and provide a framework in which a plausible control of the cytoskeletal network by glutamate is taking place, most possibly through AMPA receptors.

Glutamate activation of Oct-2 in cultured chick Bergmann glia cells: Involvement of NFκB

Glutamate, the major excitatory neurotransmitter in the central nervous system, is critically involved in gene expression regulation at the transcriptional and translational levels. Its activity through ionotropic as well as metabotropic receptors modifies the protein repertoire in neurons and glial cells. In avian cerebellar Bergmann glia cells, glutamate receptors trigger a diverse array of signaling cascades that include activity-dependent transcription factors such as the activator protein-1, the cAMP response-element binding protein, and Oct-2. We analyze the upstream regulatory elements involved in Oct-2 activation. Our results demonstrate that Ca2+ influx, protein kinase C, phosphatidylinositol-3 kinase, Src, and nuclear factor (NF)kappaB are involved in this signaling pathway. Our findings link alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor activation to a negative phase of chkbp gene regulation, controlled by NFkappaB.

Glutamate-dependent transcriptional regulation of GLAST: role of PKC

The Na+-dependent glutamate/aspartate transporter GLAST plays a major role in the removal of glutamate from the synaptic cleft. Short-term, as well as long-term changes in transporter activity are triggered by glutamate. An important locus of regulation is the density of transporter molecules present at the plasma membrane. A substrate-dependent change in the translocation rate of the transporter molecules accounts for the short-term effect, whereas the long-term modulation apparently involves transcriptional regulation. Using cultured chick cerebellar Bergmann glial cells, we report here that glutamate receptors activation mediate a substantial reduction in the transcriptional activity of the chglast promoter through the Ca2+/diacylglicerol-dependent protein kinase (PKC) signaling cascade. Overexpression of constitutive active PKC isoforms of mimic the glutamate effect. Accordingly, increased levels of c-Jun or c-Fos, but not Jun-B, Jun-D or Fos-B, lower the chglast promoter activity. Serial deletions and electrophorectic mobility shift assays were used to define a specific region within the 5' proximal region of the chglast promoter, associated with transcriptional repression. A putative glutamate response element could be defined in the proximal promoter stretch more likely between nts -40 and -78. These results demonstrate that GLAST is under glutamate-dependent transcriptional control through PKC, and support the notion of a pivotal role of this neurotransmitter in the regulation of its own removal from the synaptic cleft, thereby modulating, mainly in the long term, glutamatergic transmission.

Mammalian septin Sept2 modulates the activity of GLAST, a glutamate transporter in astrocytes

Sept2 is a member of the septin family of GTPases. Septins form filaments in a GTP-form dependent manner, and are involved in cytokinesis from yeast to mammals; however, some mammalian septins, including Sept2, are expressed in the brain, a tissue in which almost all the cells are postmitotic. Recently, some functions of mammalian septin other than cytokinesis such as vesicle transport have been reported. However, mammalian septin's physiological functions are still unclear. The present study revealed that Sept2 co-localizes with the astrocyte glutamate transporter GLAST in the Bergmann glial processes facing axons and synapses. Biochemical analyses demonstrated that Sept2 bound directly to the carboxy-terminal region of GLAST in a GDP-form dependent manner. Expression of constitutive GDP-form Sept2 mutant reduced the glutamate uptake activity of GLAST via internalization of GLAST from cell surface. Thus Sept2 may regulate GLAST-mediated glutamate uptake by astrocytes, which is important for appropriate transmitter signalling in the cerebellum.

Glutamate regulates Oct-2 DNA-binding activity through α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors in cultured chick Bergmann glia cells

Ionotropic glutamate receptors in cerebellar Bergmann glial cells are linked to transcriptional regulation and, by these means, are thought to play an important role in plasticity, learning and memory and in several neuropathologies. Within the CNS, the transcription factors of the POU family bind their target DNA sequences after a growth factor-dependent phosphorylation-dephosphorylation cascade. Exposure of cultured Bergmann glial cells to glutamate leads to a time- and dose-dependent increase in Oct-2 DNA-binding activity. The use of specific pharmacological tools established the involvement of Ca2+-permeable alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. Furthermore, the signaling cascade includes phosphatidyl inositol 3-kinase as well as protein kinase C activation. Interestingly, transcriptional as well as translational inhibitors abolish the glutamate effect, suggesting a transcriptional up-regulation of the oct-2 gene. These data demonstrate that Oct-2 expression is not restricted to neurons and further strengthen the notion that the glial glutamate receptors participate in the modulation of glutamatergic cerebellar neurotransmission.

Transcriptional regulation through glutamate receptors: Involvement of tyrosine kinases

Glutamate receptors play a key role in neuronal plasticity, learning and memory, and in several neuropathologies. Short-term and long-term changes in synaptic efficacy are triggered by glutamate. Although an enhanced glutamate-dependent tyrosine phosphorylation has been described in several systems, its role in membrane-to-nuclei signaling is unclear. Taking advantage of the fact that the gene encoding the chick kainate-binding protein undergoes a glutamate-dependent transcriptional regulation via an activator protein-1 (AP-1) site, we evaluated the involvement of tyrosine kinases in this process. We describe here the participation of receptor and non-receptor tyrosine kinases in the signaling cascade triggered by glutamate. Our results suggest that in Bergmann glia cells, glutamate receptors transactivate receptor tyrosine kinases, favoring the idea of a complex network of signals activated by this excitatory neurotransmitter that results in regulation of gene expression.

Glutamate down-regulates GLAST expression through AMPA receptors in Bergmann glial cells

The Na(+)-dependent glutamate/aspartate transporter GLAST plays a major role in the removal of glutamate from the synaptic cleft. Short-, as well as long-term changes in transporter activity are triggered by glutamate. An important locus of regulation is the density of transporter molecules at the plasma membrane. A substrate-dependent change in the translocation rate accounts for the short-term effect, whereas the mechanisms of long-term modulation are less understood. Using cultured chick cerebellar Bergmann glial cells, we report here that glutamate receptors mediate a substantial reduction in GLAST mRNA levels, suggesting a transcriptional level of regulation. Moreover, when the 5' proximal region of the GLAST gene was cloned and transfected into Bergmann glia cells, a decrease in promoter activity was induced by glutamate exposure. The use of specific pharmacological tools established the involvement of Ca(2+)-permeable alpha-amino 3-hydroxy-5-methyl-4-isoaxazolepropionate (AMPA) receptors via protein kinase C and c-Jun. These results demonstrate that GLAST is under transcriptional control through glutamate receptors activation, and further supports the participation of Bergmann glia cells in the modulation of glutamatergic transmission.

Bergmann glia GABA(A) receptors concentrate on the glial processes that wrap inhibitory synapses

We studied the cellular and subcellular distribution of GABA(A) receptors in the Bergmann glia and Purkinje cells in the molecular layer of the cerebellum by using electron microscopy postembedding immunogold techniques. Gold particles corresponding to alpha2 and gamma1 immunoreactivity were localized in Bergmann glia processes that wrapped Purkinje cell somata, dendritic shafts, and some dendritic spines. The gold particles were mainly located on the glial plasma membrane or intracellularly but near the plasma membrane. The density of gold particles corresponding to alpha2 and gamma1 GABA(A) receptor subunits was 4.3-fold higher in the glial processes wrapping Purkinje cell somata than in the glial processes wrapping Purkinje cell dendritic spines. Moreover, the Bergmann glia GABA(A) receptors were often located in close proximity to the type II GABAergic synapses made by the basket cell axons on Purkinje cell somata. These GABAergic synapses were enriched in neuronal GABA(A) receptors containing alpha1 and beta2/3 subunits. Unexpectedly, 2.8% of the Purkinje cell dendritic spines also showed immunoreactivity for the neuronal alpha1 or beta2/3 subunits, which were located on the spine in type I synapses or extrasynaptically. Double-labeling immunogold experiments showed that approximately 50% of the dendritic spines that were immunolabeled with the neuronal GABA(A) receptors were wrapped by Bergmann glia processes containing glial GABA(A) receptors. These results are consistent with a role of the Bergmann glial GABA(A) receptors in sensing GABAergic synaptic function.

Glutamate-dependent transcriptional regulation of the chkbp gene: signaling mechanisms

Glutamate, the major excitatory neurotransmitter, induces a signal from the membrane to the nucleus that regulates gene expression. The gene encoding the chick kainate binding protein undergoes a glutamate-dependent transcriptional regulation via an activator protein-1 site within its promoter region. To characterize this event, cultured chick Bergmann glia cells were exposed to glutamate, and a dose-dependent increase in promoter activity was established. The glutamate effect is mediated through Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. The signaling cascade includes phosphatidyl inositol 3-kinase, Ca(2+)/calmodulin-dependent protein kinase II, mitogen-activated protein kinase, and p90 ribosomal S6 kinase activation. The cAMP response-element binding protein becomes phosphorylated and activates fos transcription. Finally, the activator protein-1 complex binds to the glutamate response element in the chick kainate binding protein promoter region inducing its activity. We propose that the mitogen-activated protein kinase/p90 ribosomal S6 kinase pathway plays a critical role in glutamate-induced gene transcription.

Identification and characterization of a novel mitochondrial tricarboxylate carrier

Here we report the identification and functional characterization of a novel mitochondrial tricarboxylate carrier protein, designated BBG-TCC, in rat brain. The cDNA encodes the predicted protein of 342-amino acid residues with five putative membrane-spanning domains. The protein has apparent similarity with a mitochondrial tricarboxylate carrier TCC, but is distinct from the other mitochondria anion transporters. BBG-TCC shows a citrate transport activity. It is specifically expressed in the brain and localizes in the mitochondria of Bergmann glial cells. In contrast, the expression of TCC is rather ubiquitous and strong in neuronal cells in the brain. This new family of proteins may contribute to biosynthesis and bioenergetics in the brain.

Insulin-like growth factor-1 increases activity and surface levels of the GLAST subtype of glutamate transporter

Glutamate uptake systems are the primary mechanisms involved in excitatory amino acids clearance, their regulation is extremely important for proper neuronal function. Using cultured chick cerebellar Bergmann glia cells, the involvement of receptor tyrosine kinases in glutamate uptake was studied. Treatment of the cells with insulin-like growth factor-1 but not epidermal growth factor or neuronal growth factor, induces a dose and time dependent increase in [(3)H]-D-aspartate uptake that is sensitive to wortmannin, an inhibitor of phosphatidylinositol 3-kinase. Saturation experiments show a significant increase in V(max), suggesting that the amount of transporter molecules at the cell membrane under insulin-like growth factor-1 treatment is augmented. This interpretation was strengthen by equilibrium-binding experiments and by the fact that the increase in [(3)H]-D-aspartate uptake was not dependent on protein synthesis. The present studies suggest that insulin-like growth factor-1 signaling is involved in modulation of glutamate transporter cell surface expression.