Dynamic changes in expression of glutamate transporter mRNAs in developing brain

Investigation into the biomolecular bases of blunted cocaine-induced glutamate release within the nucleus accumbens elicited by adolescent exposure to phenylpropanolamine

Globally, phenylpropanolamine (PPA) is a prevalent primary active ingredient in over-the-counter cough and cold, as well as weight-loss medications. Previously, we showed that a sensitization of cocaine-induced glutamate release within the nucleus accumbens (NAC) and the expression of cocaine-conditioned reward is not apparent in adult mice with a prior history of repeated PPA exposure during adolescence. As NAC glutamate is a purported driver of cocaine reward and reinforcement, the present study employed in vivo microdialysis and immunoblotting approaches to inform as to the receptor and transporter anomalies that might underpin the disrupted glutamate response to cocaine in adolescent PPA-exposed mice. For this, male and female C57BL/6J mice were pretreated, once daily, with either 0 or 40mg/kg PPA during post-natal days 35-44. Adolescent PPA pretreatment significantly altered the expression of mGlu2/3 and α2 receptors in the NAC, with less robust changes detected for EAAT2, D2 receptors, DAT and NET. However, we detected no overt change in the capacity of these receptors or transporters to affect extracellular glutamate levels in adolescent PPA-pretreated mice. The present findings contrast with the pronounced changes in the capacity of mGlu2/3 receptors, EAAT, DAT and NET to regulate NAC extracellular glutamate reported previously for juvenile PPA-pretreated mice, indicating further that the long-term biochemical consequences of PPA depend on the critical period of neurodevelopment during which an individual is PPA-exposed, although the specific biomolecular changes underpinning the cocaine phenotype produced by adolescent PPA remain to be elucidated.

Cooperative and competitive regulation of the astrocytic transcriptome by neurons and endothelial cells: Impact on astrocyte maturation

Astrocytes have essential roles in central nervous system (CNS) health and disease. During development, immature astrocytes show complex interactions with neurons, endothelial cells, and other glial cell types. Our work and that of others have shown that these interactions are important for astrocytic maturation. However, whether and how these cells work together to control this process remains poorly understood. Here, we test the hypothesis that cooperative interactions of astrocytes with neurons and endothelial cells promote astrocytic maturation. Astrocytes were cultured alone, with neurons, endothelial cells, or a combination of both. This was followed by astrocyte sorting, RNA sequencing, and bioinformatic analysis to detect transcriptional changes. Across culture configurations, 7302 genes were differentially expressed by 4 or more fold and organized into 8 groups that demonstrate cooperative and antagonist effects of neurons and endothelia on astrocytes. We also discovered that neurons and endothelial cells caused splicing of 200 and 781 mRNAs, respectively. Changes in gene expression were validated using quantitative PCR, western blot (WB), and immunofluorescence analysis. We found that the transcriptomic data from the three-culture configurations correlated with protein expression of three representative targets (FAM107A, GAT3, and GLT1) in vivo. Alternative splicing results also correlated with cortical tissue isoform representation of a target (Fibronectin 1) at different developmental stages. By comparing our results to published transcriptomes of immature and mature astrocytes, we found that neurons or endothelia shift the astrocytic transcriptome toward a mature state and that the presence of both cell types has a greater effect on maturation than either cell alone. These results increase our understanding of cellular interactions/pathways that contribute to astrocytic maturation. They also provide insight into how alterations to neurons and/or endothelial cells may alter astrocytes with implications for astrocytic changes in CNS disorders and diseases.

Regulation of glutamate transport and neuroinflammation in a term newborn rat model of hypoxic–ischaemic brain injury

In the newborn brain, moderate-severe hypoxia–ischaemia induces glutamate excitotoxicity and inflammation, possibly via dysregulation of candidate astrocytic glutamate transporter ( Glt1) and pro-inflammatory cytokines (e.g. Tnfα, Il1β, Il6). Epigenetic mechanisms may mediate dysregulation. Hypotheses: (1) hypoxia–ischaemia dysregulates mRNA expression of these candidate genes; (2) expression changes in Glt1 are mediated by DNA methylation changes; and (3) methylation values in brain and blood are correlated. Seven-day-old rat pups ( n = 42) were assigned to nine groups based on treatment (for each timepoint: naïve ( n = 3), sham ( n = 3), hypoxia–ischaemia ( n = 8) and timepoint for tissue collection (6, 12 and 24 h post-hypoxia). Moderate hypoxic–ischemic brain injury was induced via ligation of the left common carotid artery followed by 100 min hypoxia (8% O2, 36°C). mRNA was quantified in cortex and hippocampus for the candidate genes, myelin ( Mbp), astrocytic ( Gfap) and neuronal ( Map2) markers (qPCR). DNA methylation was measured for Glt1 in cortex and blood (bisulphite pyrosequencing). Hypoxia–ischaemia induced pro-inflammatory cytokine upregulation in both brain regions at 6 h. This was accompanied by gene expression changes potentially indicating onset of astrogliosis and myelin injury. There were no significant changes in expression or promoter DNA methylation of Glt1. This pilot study supports accumulating evidence that hypoxia–ischaemia causes neuroinflammation in the newborn brain and prioritises further expression and DNA methylation analyses focusing on this pathway. Epigenetic blood biomarkers may facilitate identification of high-risk newborns at birth, maximising chances of neuroprotective interventions.

Enduring dysregulation of nucleus accumbens catecholamine and glutamate transmission by developmental exposure to phenylpropanolamine

For over 50 years, the sympathomimetic phenylpropanolamine (PPA; ±-norephedrine) was a primary active ingredient in over-the-counter nasal decongestants for both children and adults and continues to be prevalent in the vast majority of countries today. Previously, we reported that juvenile PPA exposure alters the developmental trajectory of catecholamine and amino acid neurotransmitter systems in the nucleus accumbens (NAC), impacting the motivational valence of cocaine in later life. The present study employed a combination of in vivo microdialysis and immunoblotting approaches to better understand how juvenile PPA exposure impacts catecholamine and glutamate function within the NAC. For this, C57BL/6J mice were pretreated repeatedly with PPA (0 or 40 mg/kg) during postnatal days 21-33. Starting at 70 days of age, the function and expression of receptors and transporters regulating extracellular dopamine and glutamate were determined. Juvenile PPA pretreatment completely abolished the capacity of selective dopamine and epinephrine reuptake inhibitors to increase NAC levels of both catecholamines, without impacting D2 or α2 receptor regulation of catecholamine release. Juvenile PPA pretreatment facilitated the rise in NAC glutamate elicited by dopamine, norepinephrine and glutamate transporter inhibitors and blunted mGlu2/3 inhibition of glutamate release in this region. These data confirm that juvenile exposure to PPA produces protracted perturbations in the regulation of extracellular catecholamine and glutamate levels within the NAC and further the hypothesis that early exposure to sympathomimetic drugs found in cough, cold and allergy medicines, have long-lasting effects upon neurotransmission within brain regions gating motivation.

Reciprocal communication between astrocytes and endothelial cells is required for astrocytic glutamate transporter 1 (GLT-1) expression

Astrocytes have diverse functions that are supported by their anatomic localization between neurons and blood vessels. One of these functions is the clearance of extracellular glutamate. Astrocytes clear glutamate using two Na⁺-dependent glutamate transporters, GLT-1 (also called EAAT2) and GLAST (also called EAAT1). GLT-1 expression increases during synaptogenesis and is a marker of astrocyte maturation. Over 20 years ago, several groups demonstrated that astrocytes in culture express little or no GLT-1 and that neurons induce expression. We recently demonstrated that co-culturing endothelia with mouse astrocytes also induced expression of GLT-1 and GLAST. These increases were blocked by an inhibitor of γ-secretase. This and other observations are consistent with the hypothesis that Notch signaling is required, but the ligands involved were not identified. In the present study, we used rat astrocyte cultures to further define the mechanisms by which endothelia induce expression of GLT-1 and GLAST. We found that co-cultures of astrocytes and endothelia express higher levels of GLT-1 and GLAST protein and mRNA. That endothelia activate Hes5, a transcription factor target of Notch, in astrocytes. Using recombinant Notch ligands, anti-Notch ligand neutralizing antibodies, and shRNAs, we provide evidence that both Dll1 and Dll4 contribute to endothelia-dependent regulation of GLT-1. We also provide evidence that astrocytes secrete a factor(s) that induces expression of Dll4 in endothelia and that this effect is required for Notch-dependent induction of GLT-1. Together these studies indicate that reciprocal communication between astrocytes and endothelia is required for appropriate astrocyte maturation and that endothelia likely deploy additional non-Notch signals to induce GLT-1.

Region-specific effects of Scrapper on the abundance of glutamate and gamma-aminobutyric acid in the mouse brain

The brain consists of various areas with anatomical features. Neurons communicate with one another via excitatory or inhibitory synaptic transmission. Altered abundance of neurotransmitters, including glutamate and gamma-aminobutyric acid (GABA), in specific brain regions is closely involved in severe neurological diseases, such as schizophrenia and obsessive-compulsive disorder. SCRAPPER, a ubiquitin E3 ligase, regulates synaptic transmission. Scrapper gene deficiency results in defective neurotransmission due to excessive secretion of neurotransmitters. The present study employed matrix-assisted laser desorption/ionization imaging mass spectrometry to analyze the abundance of amino acid neurotransmitters in Scrapper knockout (SCR-KO) mice. SCR-KO mice exhibited significantly increased glutamate levels in the isocortex (CTX), corpus callosum (CC), thalamus (TH), midbrain (MB), cerebellar cortex (CBX), and caudoputamen (CP) and increased GABA levels in the CTX, CC, TH, MB, CBX and hypothalamus (HY) compared with wild-type mice. These findings indicate that Scrapper deficiency leads to upregulated glutamate and GABA levels in multiple regions. Our results show a differential, region-specific effect of Scrapper on the abundance of glutamate and GABA.

Glutamate Transport and Preterm Brain Injury

Preterm birth complications are the leading cause of child death worldwide and a top global health priority. Among the survivors, the risk of life-long disabilities is high, including cerebral palsy and impairment of movement, cognition, and behavior. Understanding the molecular mechanisms of preterm brain injuries is at the core of future healthcare improvements. Glutamate excitotoxicity is a key mechanism in preterm brain injury, whereby the accumulation of extracellular glutamate damages the delicate immature oligodendrocytes and neurons, leading to the typical patterns of injury seen in the periventricular white matter. Glutamate excitotoxicity is thought to be induced by an interaction between environmental triggers of injury in the perinatal period, particularly cerebral hypoxia-ischemia and infection/inflammation, and developmental and genetic vulnerabilities. To avoid extracellular build-up of glutamate, the brain relies on rapid uptake by sodium-dependent glutamate transporters. Astrocytic excitatory amino acid transporter 2 (EAAT2) is responsible for up to 95% of glutamate clearance, and several lines of evidence suggest that it is essential for brain functioning. While in the adult EAAT2 is predominantly expressed by astrocytes, EAAT2 is transiently upregulated in the immature oligodendrocytes and selected neuronal populations during mid-late gestation, at the peak time for preterm brain injury. This developmental upregulation may interact with perinatal hypoxia-ischemia and infection/inflammation and contribute to the selective vulnerability of the immature oligodendrocytes and neurons in the preterm brain. Disruption of EAAT2 may involve not only altered expression but also impaired function with reversal of transport direction. Importantly, elevated EAAT2 levels have been found in the reactive astrocytes and macrophages of human infant post-mortem brains with severe white matter injury (cystic periventricular leukomalacia), potentially suggesting an adaptive mechanism against excitotoxicity. Interestingly, EAAT2 is suppressed in animal models of acute hypoxic-ischemic brain injury at term, pointing to an important and complex role in newborn brain injuries. Enhancement of EAAT2 expression and transport function is gathering attention as a potential therapeutic approach for a variety of adult disorders and awaits exploration in the context of the preterm brain injuries.

Transgenic mouse models for studying adult neurogenesis

The mammalian hippocampus shows a remarkable capacity for continued neurogenesis throughout life. Newborn neurons, generated by the radial neural stem cells (NSCs), are important for learning and memory as well as mood control. During aging, the number and responses of NSCs to neurogenic stimuli diminish, leading to decreased neurogenesis and age-associated cognitive decline and psychiatric disorders. Thus, adult hippocampal neurogenesis has garnered significant interest because targeting it could be a novel potential therapeutic strategy for these disorders. However, if we are to use neurogenesis to halt or reverse hippocampal-related pathology, we need to understand better the core molecular machinery that governs NSC and their progeny. In this review, we summarize a wide variety of mouse models used in adult neurogenesis field, present their advantages and disadvantages based on specificity and efficiency of labeling of different cell types, and review their contribution to our understanding of the biology and the heterogeneity of different cell types found in adult neurogenic niches.

Differential regulation of the glutamate transporters GLT-1 and GLAST by GSK3β

The glutamate transporters GLAST and GLT-1 are mainly expressed in glial cells and regulate glutamate levels in the synapses. GLAST and GLT-1 are the targets of several signaling pathways. In this study we explore the possible functional interaction between these transporters and GSK3β. This kinase is involved in multiple cellular processes including neuronal development and synaptic plasticity. To evaluate whether GLT-1 and GLAST were regulated by GSK3β, we coexpressed these proteins in heterologous expression systems. In both COS-7 cells and Xenopus laevis oocytes, GSK3β stimulated the activity of GLT-1 and reduced that of GLAST. These effects were associated with corresponding changes in the amounts of GLT-1 or GLAST in the plasma membrane. These effects were suppressed by inhibitors of GSK3β or a catalytically inactive form of the kinase. GSK3β also decreases the incorporation of (32)Pi into GLT-1 and increases GLAST phosphorylation. Pharmacological inhibition of endogenous GSK3β in primary cultures of rat brain cortex also leads to a differential modulation of GLT-1 and GLAST. Our results suggest that constitutively active GSK3β is important in controlling the expression of functional glutamate transporters on the plasma membrane. This regulation might be relevant in physiological and pathological conditions in which glutamate transporters and GSK3β signaling are involved.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

Glutamate transporters and presynaptic metabotropic glutamate receptors protect neocortical Cajal-Retzius cells against over-excitation

Cajal-Retzius (CR) cells, early generated neurons in the marginal zone of developing neocortex, are reported to be highly vulnerable to excitotoxic damage. Because extracellular glutamate concentration in the central nervous system is mainly controlled by glutamate transporters (EAATs), we studied the effects of EAAT blockade on CR cells. DL: -TBOA, a specific EAAT antagonist, induced NMDA receptor-dependent bursting discharges in layer 2/3 pyramidal neurons, indicating that EAATs operate in the uptake mode and their blockade leads to elevation of extracellular glutamate concentration. In CR cells, however, DL: -TBOA failed to change either the membrane resistance or holding current, and moreover, it reduced the frequency of spontaneous GABAergic postsynaptic currents. DL: -TBOA decreased the mean amplitude and increased paired-pulse ratio of evoked GABAergic postsynaptic currents, indicating the presynaptic locus of its action. Indeed, LY379268, a specific agonist of group II metabotropic glutamate receptors (mGluR-II), mimicked the DL: -TBOA-mediated effects, and LY341495, an unspecific mGluR antagonist, eliminated the DL: -TBOA-induced effects. As dihydrokainic acid, a specific EAAT2 blocker, failed to affect evoked GABAergic postsynaptic currents, whereas TFB-TBOA, a selective blocker of EAAT1 and EAAT2, produced effects similar to that of DL: -TBOA, extracellular glutamate concentration in the marginal zone is mainly controlled by EAAT1 (GLAST). Thus, even though CR cells are highly vulnerable to excitotoxic damage, a number of mechanisms serve to protect them against excessive extracellular glutamate concentration including a lack of functional glutamatergic synapses, Mg(2+) blockade of NMDA receptors, and presynaptic mGluRs that inhibit transmission at GABAergic synapses.

Neuronal activity regulates glutamate transporter dynamics in developing astrocytes

Glutamate transporters (GluTs) maintain a low ambient level of glutamate in the central nervous system (CNS) and shape the activation of glutamate receptors at synapses. Nevertheless, the mechanisms that regulate the trafficking and localization of transporters near sites of glutamate release are poorly understood. Here, we examined the subcellular distribution and dynamic remodeling of the predominant GluT GLT-1 (excitatory amino acid transporter 2, EAAT2) in developing hippocampal astrocytes. Immunolabeling revealed that endogenous GLT-1 is concentrated into discrete clusters along branches of developing astrocytes that were apposed preferentially to synapsin-1 positive synapses. Green fluorescent protein (GFP)-GLT-1 fusion proteins expressed in astrocytes also formed distinct clusters that lined the edges of astrocyte processes, as well as the tips of filopodia and spine-like structures. Time-lapse three-dimensional confocal imaging in tissue slices revealed that GFP-GLT-1 clusters were dynamically remodeled on a timescale of minutes. Some transporter clusters moved within developing astrocyte branches as filopodia extended and retracted, while others maintained stable positions at the tips of spine-like structures. Blockade of neuronal activity with tetrodotoxin reduced both the density and perisynaptic localization of GLT-1 clusters. Conversely, enhancement of neuronal activity increased the size of GLT-1 clusters and their proximity to synapses. Together, these findings indicate that neuronal activity influences both the organization of GluTs in developing astrocyte membranes and their position relative to synapses.

Disruption of Eaat2b, a glutamate transporter, results in abnormal motor behaviors in developing zebrafish

Analysis of zebrafish mutants that have defects in motor behavior can allow entrée into the hindbrain and spinal cord networks that control locomotion. Here, we report that zebrafish techno trousers (tnt) locomotor mutants harbor a mutation in slc1a2b, which encodes Eaat2b, a plasma membrane glutamate transporter. We used tnt mutants to explore the effects of impaired glutamate transporter activity on locomotor network function. Wild-type larvae perform robust swimming behavior in response to touch stimuli at two and four days after fertilization. In contrast, tnt mutant larvae demonstrate aberrant, exaggerated body bends beginning two days after fertilization and they are almost paralyzed four days after fertilization. We show that slc1a2b is expressed in glial cells in a dynamic fashion across development, which may explain the abnormal sequence of motor behaviors demonstrated by tnt mutants. We also show that tnt larvae demonstrate enhanced excitation of neurons, consistent with the predicted effects of excessive glutamate. These findings illustrate the dynamic regulation and importance of glutamate transporters during development. Since glutamate toxicity caused by EAAT2 dysfunction is thought to promote several different neurological disorders in humans, including epilepsy and neurodegenerative diseases, tnt mutants hold promise as a new tool to better understand these pathologies.

The relative roles of diffusion and uptake in clearing synaptically released glutamate change during early postnatal development

Glutamate uptake by transporters expressed in astrocytes combines with synaptic structure to regulate the time that synaptically released glutamate remains in the extracellular space and, consequently, the duration and location of postsynaptic receptor activation. Both factors change greatly in the rodent hippocampus during the second postnatal week when most synapses become established and begin to mature, processes that are influenced by synaptically released glutamate. Transporter expression increases, potentially speeding removal of synaptically released glutamate, whereas extracellular space decreases, thereby slowing dilution. We investigated whether these competing changes influence the glutamate concentration time course and postsynaptic responses in the CA1 region of the mouse hippocampus during this critical period of synaptic development. Our results suggest that the glutamate concentration time course remains relatively consistent over this period, although the primary mechanisms regulating glutamate clearance change. Before the second postnatal week, clearance of synaptically released glutamate depends primarily on diffusion into large extracellular spaces, whereas later in development it relies more on increased uptake capacity. Thus, increased transporter expression during this period accompanies structural changes in the neuropil, preserving a relatively consistent glutamate concentration time course and ensuring that postsynaptic receptor activation remains brief and primarily localized to receptors close to release sites.

mRNA for the EAAC1 subtype of glutamate transporter is present in neuronal dendrites in vitro and dramatically increases in vivo after a seizure

The neuronal Na(+)-dependent glutamate transporter, excitatory amino acid carrier 1 (EAAC1, also called EAAT3), has been implicated in the control of synaptic spillover of glutamate, synaptic plasticity, and the import of cysteine for neuronal synthesis of glutathione. EAAC1 protein is observed in both perisynaptic regions of the synapse and in neuronal cell bodies. Although amino acid residues in the carboxyl terminal tail have been implicated in the dendritic targeting of EAAC1 protein, it is not known if mRNA for EAAC1 may also be targeted to dendrites. Sorting of mRNA to specific cellular domains provides a mechanism by which signals can rapidly increase translation in a local environment; this form of regulated translation has been linked to diverse biological phenomena ranging from establishment of polarity during embryogenesis to synapse development and synaptic plasticity. In the present study, EAAC1 mRNA sequences were amplified from dendritic samples that were mechanically harvested from low-density hippocampal neuronal cultures. In parallel analyses, mRNA for histone deacetylase 2 (HDAC-2) and glial fibrillary acidic protein (GFAP) was not detected, suggesting that these samples are not contaminated with cell body or glial mRNAs. EAAC1 mRNA also co-localized with Map2a (a marker of dendrites) but not Tau1 (a marker of axons) in hippocampal neuronal cultures by in situ hybridization. In control rats, EAAC1 mRNA was observed in soma and proximal dendrites of hippocampal pyramidal neurons. Following pilocarpine- or kainate-induced seizures, EAAC1 mRNA was present in CA1 pyramidal cell dendrites up to 200μm from the soma. These studies provide the first evidence that EAAC1 mRNA localizes to dendrites and suggest that dendritic targeting of EAAC1 mRNA is increased by seizure activity and may be regulated by neuronal activity/depolarization.

TARPs gamma-2 and gamma-7 are essential for AMPA receptor expression in the cerebellum

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors require auxiliary subunits termed transmembrane AMPA receptor regulatory proteins (TARPs), which promote receptor trafficking to the cell surface and synapses and modulate channel pharmacology and gating. Of six TARPs, gamma-2 and gamma-7 are the two major TARPs expressed in the cerebellum. In the present study, we pursued their roles in synaptic expression of cerebellar AMPA receptors. In the cerebellar cortex, gamma-2 and gamma-7 were preferentially localized at various asymmetrical synapses. Using quantitative Western blot and immunofluorescence, we found severe reductions in GluA2 and GluA3 and mild reduction in GluA4 in gamma-2-knockout (KO) cerebellum, whereas GluA1 and GluA4 were moderately reduced in gamma-7-KO cerebellum. GluA2, GluA3 and GluA4 were further reduced in gamma-2/gamma-7 double-KO (DKO) cerebellum. The large losses of GluA2 and GluA3 in gamma-2-KO mice and further reductions in DKO mice were confirmed at all asymmetrical synapses examined with postembedding immunogold. Most notably, the GluA2 level in the postsynaptic density fraction, GluA2 labeling density at parallel fiber-Purkinje cell synapses, and AMPA receptor-mediated currents at climbing fiber-Purkinje cell synapses were all reduced to approximately 10% of the wild-type levels in DKO mice. On the other hand, the reduction in GluA4 in gamma-7-KO granular layer reflected its loss at mossy fiber-granule cell synapses, whereas that of GluA1 and GluA4 in gamma-7-KO molecular layer was caused, at least partly, by their loss in Bergmann glia. Therefore, gamma-2 and gamma-7 cooperatively promote synaptic expression of cerebellar AMPA receptors, and the latter also promotes glial expression.

Functions of glutamate transporters in cerebellar Purkinje cell synapses

Glutamate transporters play a critical role in the maintenance of low extracellular concentrations of glutamate, which prevents the overactivation of post-synaptic glutamate receptors. Four distinct glutamate transporters, GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3 and EAAT4, are distributed in the molecular layer of the cerebellum, especially near glutamatergic synapses in Purkinje cells (PCs). This review summarizes the current knowledge about the differential roles of these transporters at excitatory synapses of PCs. Data come predominantly from electrophysiological experiments in mutant mice that are deficient in each of these transporter genes. GLAST expressed in Bergmann glia contributes to the clearing of the majority of glutamate that floods out of the synaptic cleft immediately after transmitter release from the climbing fibre (CF) and parallel fibre (PF) terminals. It is indispensable to maintain a one-to-one relationship in synaptic transmission at the CF synapses by preventing transcellular glutamate spillover. GLT-1 plays a similar but minor role in the uptake of glutamate as GLAST. Although the loss of neither GLAST nor GLT-1 affects cerebellar morphology, the deletion of both GLAST and GLT-1 genes causes the death of the mutant animal and hinders the folium formation of the cerebellum. EAAT4 removes the low concentrations of glutamate that escape from uptake by glial transporters, preventing the transmitter from spilling over into neighbouring synapses. It also regulates the activation of metabotropic glutamate receptor 1 (mGluR1) in perisynaptic regions at PF synapses, which in turn affects mGluR1-mediated events including slow EPSCs and long-term depression. No change in synaptic function is detected in mice that are deficient in EAAC1.

Metabolic biomarkers related to energy metabolism in Saudi autistic children

OBJECTIVES

Energy metabolism is usually manipulated in many neurodegenerative diseases. Autism is considered a definable systemic disorder resulting in a number of diverse factors that may affect the brain development and functions both pre and post natal. The increased prevalence of autism will have enormous future public implications and has stimulated intense research into potential etiologic factors. This study aims to establish a connection between autism and the deterioration accompanied it, especially in the brain cognitive areas through a postulation of energy manipulation.

MATERIALS AND METHODS

The biochemical changes in activities of enzymes and pathways that participate in the production of ATP as the most important high-energy compound needed by the human brain were measured in Saudi autistic children. Na(+)/K(+)ATPase, ectonucleotidases (NTPDases) (ADPase and ATPase) and creatine kinase (CK), were assessed in plasma of 30 Saudi autistic patients and compared to 30 age-matching control samples. In addition, adenosine mono, di and trinucleotides (ATP, ADP, and AMP) were measured calorimetrically in the red blood cells of both groups and the adenylate energy charge (AEC) was calculated. Moreover, lactate concentration in plasma of both groups was monitored.

RESULTS

The obtained data recorded 148.77% and 72.35% higher activities of Na(+)/K(+)ATPase and CK respectively in autistic patients which prove the impairment of energy metabolism in these children compared to age and sex matching healthy controls. While ADPase was significantly higher in autistic patients, ATPase were non-significantly elevated compared to control. In spite of the significant increase of Na(+)/K(+)ATPase activity in autistic patients, there was no significant difference in the levels of ATP, ADP, and AMP in both groups and the calculated AEC values were 0.814+/-0.094 and 0.806+/-0.081 for autistic and control groups respectively. The unchanged AEC value in autistic patients was easily correlated with the induced activity of CK and ADPase as two enzymes playing a critical role in the stabilization of AEC. Lactate as an important energy metabolite for the brain was significantly higher in autistic patients compared to control showing about 40% increase.

CONCLUSION

The present study confirmed the impairment of energy metabolism in Saudi autistic patients which could be correlated to the oxidative stress previously recorded in the same investigated samples. The identification of biochemical markers related to autism would be advantageous for earlier clinical diagnosis and intervention.

Early rapid rise in EAAT2 expression follows the period of maximal seizure susceptibility in human brain

Seizures are relatively common in the first weeks of life and can have lasting effects on brain development due to glutamate excitotoxicity. The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and mice with this gene deleted die from seizures. Therefore, we reasoned that developmental changes in the expression of EAAT2 might correlate with the period of increased susceptibility to seizures in humans, reflecting a changing vulnerability to excitotoxic insults. Expression levels of eight splice forms of EAAT2 were measured using quantitative RT-PCR from human prefrontal cortex and hippocampus at 1-2 months, 1-2 years, 8 years, 15-16 years, and 18-22 years of age. There was a significant increase in expression of most isoforms between 1-2 months and 1-2 years with isoform-specific patterns after that period. The increase in EAAT2 expression during the first 2 years of life corresponds to a period of maximal synapse formation and other changes in the glutamatergic system such as increased NMDA receptor expression. Moreover, the low expression of EAAT2 in the first months of life corresponds to the period of maximum susceptibility to seizures.

From the Cover: Indispensability of the glutamate transporters GLAST and GLT1 to brain development

Previous in vitro studies have shown that the neurotransmitter glutamate is important in brain development. Paradoxically, loss-of-function mouse models of glutamatergic signaling that are generated by genetic deletion of glutamate receptors or glutamate release show normal brain assembly. We examined the direct consequences on brain development of extracellular glutamate buildup due to the depletion of the glutamate transporters GLAST and GLT1. GLAST/GLT1 double knockout mice show multiple brain defects, including cortical, hippocampal, and olfactory bulb disorganization with perinatal mortality. Here, we report abnormal formation of the neocortex in GLAST/GLT1 mutants. Several essential aspects of neuronal development, such as stem cell proliferation, radial migration, neuronal differentiation, and survival of SP neurons, were impaired. These results provide direct in vivo evidence that GLAST and GLT1 are necessary for brain development through regulation of extracellular glutamate concentration and show that an important mechanism is likely to be maintenance of glutamate-mediated synaptic transmission.

Don't get too excited: mechanisms of glutamate-mediated Purkinje cell death

Purkinje cells (PCs) present a unique cellular profile in both the cerebellum and the brain. Because they represent the only output cell of the cerebellar cortex, they play a vital role in the normal function of the cerebellum. Interestingly, PCs are highly susceptible to a variety of pathological conditions that may involve glutamate-mediated 'excitotoxicity', a term coined to describe an excessive release of glutamate, and a subsequent over-activation of excitatory amino acid (NMDA, AMPA, and kainite) receptors. Mature PCs, however, lack functional NMDA receptors, the means by which Ca(2+) enters the cell in classic hippocampal and cortical models of excitotoxicity. In PCs, glutamate predominantly mediates its effects, first via a rapid influx of Ca(2+)through voltage-gated calcium channels, caused by the depolarization of the membrane after AMPA receptor activation (and through Ca(2+)-permeable AMPA receptors themselves), and second, via a delayed release of Ca(2+) from intracellular stores. Although physiological levels of intracellular free Ca(2+) initiate vital second messenger signaling pathways in PCs, excessive Ca(2+) influx can detrimentally alter dendritic spine morphology via interactions with the neuronal cytoskeleton, and thus can perturb normal synaptic function. PCs possess various calcium-binding proteins, such as calbindin-D28K and parvalbumin, and glutamate transporters, in order to prevent glutamate from exerting deleterious effects. Bergmann glia are gaining recognition as key players in the clearance of extracellular glutamate; these cells are also high in S-100beta, a protein with both neurodegenerative and neuroprotective abilities. In this review, we discuss PC-specific mechanisms of glutamate-mediated excitotoxic cell death, the relationship between Ca(2+) and cytoskeleton, and the implications of glutamate, and S-100beta for pathological conditions, such as traumatic brain injury.

Glutamate transporter expression and function in human glial progenitors

Glutamate is the major neurotransmitter of the brain, whose extracellular levels are tightly controlled by glutamate transporters. Five glutamate transporters in the human brain (EAAT1-5) are present on both astroglia and neurons. We characterize the profile of three different human astroglial progenitors in vitro: human glial restricted precursors (HGRP), human astrocyte precursors (HAPC), and early-differentiated astrocytes. EAAT 1, EAAT3, and EAAT4 are all expressed in GRPs with a subsequent upregulation of EAAT1 following differentiation of GRPs into GRP-derived astrocytes in the presence of bone morphogenic protein (BMP-4). This corresponds to a significant increase in the glutamate transport capacity of these cells. EAAT2, the transporter responsible for the bulk of glutamate transport in the adult brain, is not expressed as a full-length protein, nor does it appear to have functional significance (as determined by the EAAT2 inhibitor dihydrokainate) in these precursors. A splice variant of EAAT2, termed EAAT2b, does appear to be present in low levels, however. EAAT3 and EAAT4 expression is reduced as glial maturation progresses both in astrocyte precursors and early-differentiated astrocytes and is consistent with their role in adult tissues as primarily neuronal glutamate transporters. These human glial precursors offer several advantages as tools for understanding glial biology because they can be passaged extensively in the presence of mitogens, afford the potential to study the temporal changes in glutamate transporter expression in a tightly controlled fashion, and are cultured in the absence of neuronal coculture, allowing for the independent study of astroglial biology.

Developmental expression of glutamate transporters and glutamate dehydrogenase in astrocytes of the postnatal rat hippocampus

Glutamate is the major excitatory transmitter in the CNS and plays distinct roles in a number of developmental events. Its extracellular concentration, which mediates these activities, is regulated by glutamate transporters in glial cells and neurons. In the present study, we have used nonradioactive in situ hybridization, immunocytochemistry, and immunoblotting to show the cellular and regional expression of the high-affinity glutamate transporters GLAST (EAAT1) and generic GLT1 (EAAT2; glial form of GLT1) in the rat hippocampus during postnatal development (P1-60). The results of transporter expression were compared with the localization and activity pattern of glutamate dehydrogenase (GDH), an important glutamate-metabolizing enzyme. The study showed that both transporters and GDH were demonstrable at P1 (day of birth). The expression of GLAST (detected by in situ hybridization and immunocytochemistry) in the early postnatal development was higher than GLT1. Thereafter, the expression of both transporters increased, showing adult levels at between P20 and P30 (detected by in situ hybridization and immunoblotting). At these time points, the expression of GLT1 appeared to be significantly higher than the GLAST expression. GLT1 and GLAST proteins were demonstrable only in astrocytes. The increase of GDH activities (steepest increase from P5-P8), which were localized preferentially in astrocytes, was in agreement with the increase of transporter expression, preferentially with that of GLT1. These observations suggest that the extent of glutamate transporter expression and of glutamate-metabolizing GDH activity in astrocytes is intimately correlated with the formation of glutamatergic synapses in the developing hippocampus.

HysTag--a novel proteomic quantification tool applied to differential display analysis of membrane proteins from distinct areas of mouse brain

A novel isotopically labeled cysteine-tagging and complexity-reducing reagent, called HysTag, has been synthesized and used for quantitative proteomics of proteins from enriched plasma membrane preparations from mouse fore- and hindbrain. The reagent is a 10-mer derivatized peptide, H(2)N-(His)(6)-Ala-Arg-Ala-Cys(2-thiopyridyl disulfide)-CO(2)H, which consists of four functional elements: i) an affinity ligand (His(6)-tag), ii) a tryptic cleavage site (-Arg-Ala-), iii) Ala-9 residue that contains four (d(4)) or no (d(0)) deuterium atoms, and iv) a thiol-reactive group (2-thiopyridyl disulfide). For differential analysis cysteine residues in the compared samples are modified using either (d(4)) or (d(0)) reagent. The HysTag peptide is preserved in Lys-C digestion of proteins and allows charge-based selection of cysteine-containing peptides, whereas subsequent tryptic digestion reduces the labeling group to a di-peptide, which does not hinder effective fragmentation. Furthermore, we found that tagged peptides containing Ala-d(4) co-elute with their d(0)-labeled counterparts. To demonstrate effectiveness of the reagent, a differential analysis of mouse forebrain versus hindbrain plasma membranes was performed. Enriched plasma membrane fractions were partially denatured, reduced, and reacted with the reagent. Digestion with endoproteinase Lys-C was carried out on nonsolubilized membranes. The membranes were sedimented by ultra centrifugation, and the tagged peptides were isolated by Ni(2+) affinity or cation-exchange chromatography. Finally, the tagged peptides were cleaved with trypsin to release the histidine tag (residues 1-8 of the reagent) followed by liquid chromatography tandem mass spectroscopy for relative protein quantification and identification. A total of 355 unique proteins were identified, among which 281 could be quantified. Among a large majority of proteins with ratios close to one, a few proteins with significant quantitative changes were retrieved. The HysTag offers advantages compared with the isotope-coded affinity tag reagent, because the HysTag reagent is easy to synthesize, economical due to use of deuterium instead of (13)C isotope label, and allows robust purification and flexibility through the affinity tag, which can be extended to different peptide functionalities.

Cytodifferentiation of Bergmann glia and its relationship with Purkinje cells

The Bergmann glia is composed of unipolar protoplasmic astrocytes in the cerebellar cortex. Bergmann glial cells locate their cell bodies around Purkinje cells, and extend radial or Bergmann fibers enwrapping synapses on Purkinje cell dendrites. During development, Bergmann fibers display a tight association with migrating granule cells, from which the concept of glia-guided neuronal migration has been proposed. Thus, it is widely known that the Bergmann glia is associated with granule cells in the developing cerebellum and with Purkinje cells in the adult cerebellum. As the information on how Bergmann glial cells are related structurally and functionally with differentiating Purkinje cells is quite fragmental, this issue has been investigated using cytochemical techniques for Bergmann glial cells. This review classifies the cytodifferentiation of Bergmann glial cells into four stages, that is, radial glia, migration, transformation and protoplasmic astrocytes, and then summarizes their structural relationship with Purkinje cells at each stage. The results conclude that the cytodifferentiation of Bergmann glial cells proceeds in correlation with the migration, dendritogenesis, synaptogenesis and maturation of Purkinje cells. Furthermore, morphological and molecular plasticity of this neuroglia appears to be regulated depending on the cytodifferentiation of nearby Purkinje cells. The functional relevance of this intimate neuron-glial relationship is also discussed with reference to recent studies in cell biology, cell ablation and gene knockout.

Reduction of glial glutamate transporters in the parietal cortex and hippocampus of the EL mouse

There is extensive experimental evidence indicating a crucial role for glutamate in epileptogenesis and epileptic activity. The glial glutamate transporters GLT1 and GLAST are proposed to account for the majority of extracellular glutamate re-uptake. In the present study, polyclonal antibodies specific to GLT1 and GLAST were generated and characterized, revealing distribution patterns for the two transporters confirming those previously reported. In situ hybridization and immunoblotting were then used to compare levels of these two transporters in the parietal cortex and hippocampus of unstimulated and stimulated EL mice with DDY control mice. Additionally, HPLC determined tissue glutamate concentrations in the same regions of these animals. These experiments revealed reductions in GLT1 mRNA and protein in the parietal cortex of unstimulated and stimulated EL mice compared with DDY controls, accompanied by an increase in tissue glutamate concentration in the stimulated EL mice group. GLT1 mRNA was also reduced in the CA3 hippocampal subfield of both unstimulated and stimulated EL mice. GLAST protein was reduced in the hippocampus of the stimulated EL mice group, while no changes in GLAST mRNA or protein were detected in the parietal cortex of EL mice when compared with DDY controls. The glial glutamate transporter down-regulation reported here may play a role in seizure initiation, spread and maintenance in the EL mouse.

Transient upregulation of the glial glutamate transporter GLAST in response to fibroblast growth factor, insulin-like growth factor and epidermal growth factor in cultured astrocytes

Although expression of the glial glutamate transporter GLAST is tightly regulated during development and under pathophysiological conditions, little is known about endogenous modulators of GLAST expression. Because growth factors are generally believed to regulate glial functions, we addressed their possible contribution to GLAST regulation in cultured rat astrocytes. Of the six growth factors tested (basic fibroblast growth factor (bFGF), insulin-like growth factor-1 (IGF-1), epidermal growth factor (EGF), insulin, platelet-derived growth factor, and hepatocyte growth factor), bFGF, IGF-1 and EGF enhanced [3H]glutamate transport activity in a concentration-dependent manner. These effects were accompanied by an increase in the Vmax value for transport activity and in GLAST protein and mRNA levels, which suggests that GLAST expression is transcriptionally regulated by the growth factors. Interestingly, the effects reached a peak after 36 hours of exposure to growth factors, and rapidly returned to baseline by 48 hours. A combination of IGF-1 with either bFGF or EGF showed an additive effect on the glutamate uptake activity, but a combination of bFGF and EGF did not. Pharmacological blockade of protein kinase C inhibited the effects of IGF-1 and EGF, but not bFGF. By contrast, genistein, an inhibitor of tyrosine kinases, blocked the effects of bFGF and EGF without affecting the effect of IGF-1. These results suggest that the growth factors activate different signaling pathways for GLAST upregulation. The present study may indicate a novel regulatory system of glial glutamate transporters.

Glutamate uptake

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

Functional striatal hypodopaminergic activity in mice lacking adenosine A(2A) receptors

Adenosine and caffeine modulate locomotor activity and striatal gene expression, partially through the activation and blockade of striatal A(2A) receptors, respectively. The elucidation of the roles of these receptors benefits from the construction of A(2A) receptor-deficient mice (A(2A)-R(-/-)). These mice presented alterations in locomotor behaviour and striatal expression of genes studied so far, which are unexpected regarding the specific expression of A(2A) receptor by striatopallidal neurones. To clarify the functions of A(2A) receptors in the striatum and to identify the mechanisms leading to these unexpected modifications, we studied the basal expression of immediate early and constitutive genes as well as dopamine and glutamate neurotransmission in the striatum. Basal zif268 and arc mRNAs expression was reduced in mutant mice by 60-80%, not only in the striatum but also widespread in the cerebral cortex and hippocampus. Striatal expression of substance P and enkephalin mRNAs was reduced by about 50% and 30%, respectively, whereas the expression of GAD67 and GAD65 mRNAs was slightly increased and unaltered, respectively. In vivo microdialysis in the striatum revealed a 45% decrease in the extracellular dopamine concentration and three-fold increase in extracellular glutamate concentration. This was associated with an up-regulation of D(1) and D(2) dopamine receptors expression but not with changes in ionotropic glutamate receptors. The levels of tyrosine hydroxylase and of striatal and cortical glial glutamate transporters as well as adenosine A(1) receptors expression were indistinguishable between A(2A)-R(-/-) and wild-type mice. Altogether these results pointed out that the lack of A(2A) receptors leads to a functional hypodopaminergic state and demonstrated that A(2A) receptors are necessary to maintain a basal level in immediate early and constitutive genes expression in the striatum and cerebral cortex, possibly via their control of dopamine pathways.

Differential distribution of the glutamate transporters GLT-1 and GLAST in tanycytes of the third ventricle

The ventral one-third of the ventricular lining in the hypothalamus is formed by specialized ependymal cells called the tanycytes. These cells may serve a neuroendocrine transport function because of their structural specializations, which include apical microvili on the ventricular surface and long basal processes that terminate on blood vessels or on the glia limitans. Here, we describe the expression of mRNA and protein for the glutamate transporters GLT-1 and GLAST in unique tanycyte populations of the third ventricle in rat brain. Using nonisotopic in situ hybridization, we demonstrate GLAST mRNA labeling in tanycytes of the ventral floor and lateral walls in the tuberal and mammillary recess portions of the third ventricle. This GLAST mRNA labeling had a higher intensity than the labeling intensity observed in regular ependymal cells throughout the ventricular system. Furthermore, we have identified strong GLT-1 mRNA labeling in a population of tanycytes situated in the dorsolateral walls of caudal tuberal and mammillary recess portions. Immunocytochemical staining indicates that both GLT-1 and GLAST protein are expressed in the tanycyte populations as well. These data corroborate previous findings that third ventricle tanycytes are functionally heterogeneous. Furthermore, the GLT-1-expressing tanycytes represent a population of tanycytes that, to date, has not been recognized as functionally distinct. The strong GLAST expression by the ventral tanycytes in the hypophysiotropic area suggests a role of tanycyte-mediated glutamate transport in neuroendocrine activity. The functional role of GLT-1 in dorsal wall tanycytes remains to be explored.

Altered expression of glutamate transporters under hypoxic conditions in vitro

Regulation of extracellular excitotoxins by glial and neuronal glutamate transporters is critical to maintain synaptic terminal integrity. Factors interfering with the normal functioning of these transporters might be involved in neurodegeneration. Among them, recent studies have shown that hypoxia alters glutamate transporter function; however, it is unclear if hypoxia has an effect on the expression of glutamate transporters and which intracellular signaling pathways are involved. The C6 rat glial and GT1--7 mouse neuronal cell lines were exposed to hypoxic conditions (5% CO(2), 95% N(2)) and levels of glutamate transporter mRNA were determined by ribonuclease protection assay. After 21 hr, there was a 100% increase in levels of rat excitatory amino acid transporter 3 (EAAT3) mRNA in C6 cells and a 600% increase in levels of murine EAAT2 mRNA in GT1--7 cells. There was a similar increase in mRNA levels after hypoxia in C6 cells transfected with human EAAT2, whereas reoxygenation normalized the expression levels of glutamate transporters. Although the expression of EAATs was associated with increased immunoreactivity by Western blot, functioning of the transporters was decreased as evidenced by D-aspartate uptake. Finally, although the protein kinase C stimulator phorbol-12-myristate-13-acetate enhanced EAAT2 mRNA levels after hypoxia, protein kinase C inhibitor bisindolylmaleimide I had the opposite effect. Taken together, this study suggests that the hypoxia is capable of upregulating levels of EAATs via a protein kinase C-dependent compensatory mechanism. This increased expression is not sufficient to overcome the decreased functioning of the EAATs associated with decreased ATP production and mitochondrial dysfunction.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Fast removal of synaptic glutamate by postsynaptic transporters

Glutamate transporters are believed to remove glutamate from the synaptic cleft only slowly because they cycle slowly. However, we show that when glutamate binds to postsynaptic transporters at the cerebellar climbing fiber synapse, it evokes a conformation change and inward current that reflect glutamate removal from the synaptic cleft within a few milliseconds, a time scale much faster than the overall cycle time. Contrary to present models, glutamate removal does not require binding of an extracellular proton, and the time course of transporter anion conductance activation differs from that of glutamate removal. The charge movement associated with glutamate removal is consistent with the majority of synaptically released glutamate being removed from the synaptic cleft by postsynaptic transporters.

Glutamate transporters in neonatal cerebellar subarachnoid hemorrhage

We have previously described the immunoreactivities of glutamate transporters, EAAT4 and GLAST, in the developing human cerebellum. In the present report, we demonstrate the different expression of EAAT4 and GLAST in the pathologic condition, neonatal subarachnoid hemorrhage. EAAT4 and GLAST were characteristically disturbed in the cerebellar cortices beneath the subarachnoid hemorrhage. In preterm infants with subarachnoid hemorrhage the decrease in EAAT4 immunoreactivity was more prominent than in term infants, and GLAST immunoreactivity in the inner granular cell layer decreased and reappeared later than in term infants with subarachnoid hemorrhage. Although Bergmann's glia removes glutamate from the extracellular space surrounding Purkinje cells in the early stage of hypoxic-ischemic brain damage, the reaction of EAAT4 and GLAST in the cerebellar cortex under the subarachnoid hemorrhage was decreased, and immature glia had a delayed reaction. These characteristics of glutamate transporters in immature cells may lead to cell death and olivocerebellar degeneration.

Glutamate transport blockade has a differential effect on AMPA and NMDA receptor-mediated synaptic transmission in the developing barrel cortex

High affinity glutamate transport plays an important role in maintaining a low extracellular glutamate concentration in the CNS. Excitotoxicity due to a loss of glutamate transporter function has been implicated in disease processes such as stroke and amyotrophic lateral sclerosis (ALS). We studied the effects of glutamate transport inhibitors on thalamocortical synapses at developing (postnatal day 3-8) layer IV neurons in the barrel cortex using the thalamocortical slice preparation and whole-cell recordings. Inhibition of glutamate transport by D,L-threo-beta-hydroxyaspartate (THA), a combination of THA and dihydrokainate (DHK), or by L-trans-pyrrolidine-2,4-dicarboxylate (tPDC), caused a reversible blockade of AMPA and kainate receptor-mediated dual component excitatory postsynaptic currents (AMPA/KA EPSCs). This effect was not blocked by cyclothiazide (CTZ) indicating that is was not due to desensitisation of AMPARs. Under conditions in which NMDA receptors were unblocked the transport inhibitors caused the massive activation of NMDA receptors leading to the rapid loss of recordings. Previous studies using these transport inhibitors on brain slices from older animals reported no or only modest effects on synaptic transmission. Therefore the data in the present study suggest that neurons in the developing neocortex are particularly sensitive to glutamate transporter function. Furthermore the effects of transport inhibition are dependent upon whether neurons are sufficiently depolarised to relieve the voltage-dependent block of NMDA receptors.

Down-regulated expression of glutamate transporter GLAST in Purkinje cell-associated astrocytes of reeler and weaver mutant cerebella

The glutamate transporter plays an important role in rapid removal of glutamate from the synaptic cleft. Glutamate transporter GLAST is highly expressed in the Bergmann glia (BG), a unipolar cerebellar astrocyte associated structurally and functionally with Purkinje cells (PCs). Here we investigated the expression and localization in the reeler and weaver mutant cerebella with disorganized cytoarchitecture and disrupted synaptic circuitry. In the cortex of both cerebella, GLAST-expressing cells were astrocytes associating PCs; they were located around PC somata and primary dendrites, and extended glial fibrillary acidic protein (GFAP)-immunopositive processes surrounding PC somata and dendrites. Additional signals were detected in astrocytes of the reeler subcortex; they were dispersed among ectopic PCs and had GFAP-positive processes apposing to PC somata and stunted dendrites. Therefore, GLAST expression in PC-associated astrocytes was conserved in these mutants. Compared to the wild-type BG, however, the transcription level in individual mutant astrocytes was significantly reduced to about one-third level in the reeler and weaver cortex or one-sixth level in the reeler subcortex. Taking previous results on remarkable up-regulation during dendritogenic/synaptogenic stages and down-regulation following experimental glutamatergic denervation, it is suggested that GLAST expression in cerebellar astrocytes is regulated correlatively with cytological and/or synaptic differentiation of neighboring PCs.

Increase in glutamate-aspartate transporter (GLAST) mRNA during kanamycin-induced cochlear insult in rats

Kanamycin (KM)-induced changes in expression of the gene for glutamate-aspartate transporter (GLAST) in the rat cochlea were analyzed by Northern blotting. With the administration of KM (600 mg/kg/day) once daily for 20 days, the expression of GLAST mRNA gradually increased and reached a peak on day 20. Although the expression of GLAST mRNA remained at a high level until 12 days after the completion of the KM treatment, it then fell to the normal level within 2 months. Such KM treatment resulted in loss of both inner and outer hair cells and a concomitant profound permanent threshold shift. The present findings suggest that during KM administration, high concentrations of extracellular glutamate released by collapsing hair cells induced GLAST mRNA expression. Increased GLAST mRNA might play an important role in the prevention of the secondary death of spiral ganglion neurons from glutamate neurotoxicity.

Developmental regulation of glutamate transporters and glutamine synthetase activity in astrocyte cultures differentiated in vitro

Glutamate plays an important role in brain development, physiological function, and neurodegeneration. Astrocytes control synaptic concentration of glutamate via the high affinity glutamate transporters, GLT-1 and GLAST, and the glutamate catabolizing enzyme, glutamine synthetase. In this study we show that astrocytes cultured from rat brain in various stages of development including embryonic (E18), postnatal (P1-P21) and mature (P50), show distinct patterns of GLT-1 and GLAST expression, glutamine synthetase activity, and phenotypic changes induced by dibutyryl-cyclic adenosine monophosphate. The transcripts for GLT-1 message were detectable in embryonic astrocytes only, whereas the GLAST message was highly expressed in E18 and P1-P4 astrocyte cultures, declined in P10-P21, and was undetectable in P50 astrocytes. Uptake of 3H-glutamate correlated well with GLAST expression in astrocyte cultures of all developmental stages. Glutamine synthetase activity significantly declined from high embryonic levels in P4 astrocytes and remained low throughout postnatal maturation. Exposure of astrocyte cultures to the differentiating agent, db-cAMP (250-500 microM; 6 days), resulted in a pronounced stellation, up-regulation of GLT-1 and GLAST in E18, and GLAST in P4 cultures, while it was ineffective in P10 astrocytes. By contrast, db-cAMP induced a more pronounced stimulation of glutamine synthetase activity (up to 10-fold above basal) in P10 than in E18 cultures (up to 2 times above basal). The differences in expression/inducibility of glutamate transporters and glutamine synthetase observed in astrocyte cultures derived from various stages of fetal and postnatal development suggest that astrocytes in vivo might also respond differently to environmental or injurious stimuli during development and maturation.

Changes in expression of neuronal and glial glutamate transporters in rat hippocampus following kainate-induced seizure activity

The expression of excitatory amino acid transporters (EAATs) in rat hippocampus was studied following kainic acid-induced seizure activity in vivo and in hippocampal slice cultures. Protein and mRNA levels of the glial (EAAT2) and neuronal (EAAT3) transporters were determined with affinity-purified antibodies and oligonucleotide probes, respectively. Kainate treatment decreased EAAT3 immunoreactivity in stratum lacunosum moleculare within 4 h of seizure onset. Upon pyramidal cell death (5 days after kainate treatment), EAAT3 immunoreactivity in stratum pyramidale of CA1 and in stratum lacunosum moleculare was almost completely eliminated. The rapid effect of kainate on EAAT3 expression was confirmed by in situ hybridization; EAAT3 mRNA levels were decreased in CA1 and CA3 regions within 4-8 h of seizure onset. Kainate treatment had an opposite effect on levels and expression of EAAT2. Developmental studies indicated that the rapid regulation of transporter expression was not observed in rats younger than 21 days, an observation congruent with previous reports regarding the resistance of young rats to kainate. In hippocampal organotypic cultures, which lack a major excitatory input from the entorhinal cortex, kainate produced a slow decrease in [3H]d-aspartate uptake. This study indicates that an early effect of kainate treatment consists of down-regulation of the neuronal transporter EAAT3 in restricted hippocampal regions, together with a modest increase in the expression of the glial transporter EAAT2. Differential regulation of neuronal and glial glutamate transporters may thus play a role in kainate-induced seizure, neurotoxicity and neuronal plasticity.

Ontogeny of ethanol-induced locomotor activity and hypothermia differences in selectively bred FAST and SLOW mice

The replicate lines of selectively bred FAST and SLOW mice differ in locomotor response to 2 g/kg ethanol (EtOH). FAST mice show enhanced locomotion; SLOW mice exhibit no change or locomotor depression. Little is known about the responses of FAST and SLOW mice to EtOH during development. We assessed the locomotor responses of FAST and SLOW mice at postnatal days (P) 10, 15, 30, and 60. A genetically correlated response, EtOH-induced hypothermia, was also investigated. Although all animals demonstrated their respective selection phenotypes in adulthood, developing FAST mice exhibited ethanol stimulation by P15 (replicate 1) or P30 (replicate 2). At these ages, responses of FAST mice differed from those of SLOW. The stimulant response in FAST mice was adult-like at P30. EtOH-induced hypothermia was seen in SLOW mice by P15. These data suggest that sensitivity to the locomotor stimulant effects of EtOH changes during postnatal development, and may mirror developmental profiles for certain neurotransmitter systems.

Glutamate transport in cultures from developing avian cerebellum: presence of GLT-1 immunoreactivity in Purkinje neurons

Immunocytochemical studies indicated that Purkinje cells cultured from chick embryonic cerebellum (embryonic day 8) strongly express a glutamate transporter EAAT2 cloned from human brain (GLT-1 in rat brain). At both 7 days and 14 days in culture, Purkinje neurons accumulated 1 microM [3H]L-glutamate via a potent "high-affinity" transport system that could be inhibited by D- and L-threo-3-hydroxyaspartate (D- and L-t-3OHA) and by L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC). The order of potency of the three inhibitors was L-t-PDC approximately L-t-3OHA > D-t-30HA. Only the value of IC50 (concentration causing 50% inhibition) for D-t-3OHA significantly changed between 7 days (116 microM) and 14 days in culture (40 microM). All nH approximately 1, except in the case of the inhibition by D-t-3OHA at 14 days in culture (nH = 0.57), indicating the possible appearance of heterogeneity of the transport sites at later stages of culturing. Chronic inhibition of L-glutamate transport by L-t-PDC resulted in major changes in the morphology of Purkinje cells; particularly, the neurites almost completely regressed.

Glial contribution to glutamate uptake at Schaffer collateral-commissural synapses in the hippocampus

Astrocytes in the hippocampus express high-affinity glutamate transporters that are important for lowering the concentration of extracellular glutamate after release at excitatory synapses. These transporters exhibit a permeability to chaotropic anions that is associated with transport, allowing their activity to be monitored in cell-fee patches when highly permeant anions are present. Astrocyte glutamate transporters are highly temperature sensitive, because L-glutamate-activated, anion-potentiated transporter currents in outside-out patches from these cells exhibited larger amplitudes and faster kinetics at 36 degreesC than at 24 degreesC. The cycling rate of these transporters was estimated by using paired applications of either L-glutamate or D-aspartate to measure the time necessary for the peak of the transporter current to recover from the steady-state level. Transporter currents in patches recovered with a time constant of 11.6 msec at 36 degreesC, suggesting that either the turnover rate of native transporters is much faster than previously reported for expressed EAAT2 transporters or the efficiency of these transporters is very low. Synaptically activated transporter currents persisted in astrocytes at physiological temperatures, although no evidence of these currents was found in CA1 pyramidal neurons in response to afferent stimulation. L-glutamate-gated transporter currents were also not detected in outside-out patches from pyramidal neurons. These results are consistent with the hypothesis that astrocyte transporters are responsible for taking up the majority of glutamate released at Schaffer collateral-commissural synapses in the hippocampus.

Glutamate transporter GLT-1 is transiently localized on growing axons of the mouse spinal cord before establishing astrocytic expression

The glutamate transporter GLT-1 is expressed in astrocytes of the mature brain and spinal cord. In the present study, we examined its expression in the developing mouse spinal cord. By in situ hybridization, 35S-labeled antisense oligonucleotide probes for GLT-1 mRNA consistently labeled the mantle zone/gray matter from embryonic day 11 through the adult stage. However, immunohistochemistry with a specific antibody visualized distinct regional and cellular localizations during the time between the fetal and postnatal stages. At fetal stages, GLT-1 immunoreactivity predominated in the marginal zone/white matter, observed as tiny puncta in cross-sections and as thin fibers in longitudinal sections. The GLT-1-immunopositive structures were also labeled for neuron-specific enolase, a glycolytic enzyme specific to postmitotic neurons and endocrine cells. By electron microscopy, GLT-1 immunoreactivity was detected in axons forming frequent enlargements and was focally localized on a small portion of the axolemma, particularly that facing adjacent axons. At early postnatal stages, GLT-1 disappeared from axons in white matter tracts and, instead, appeared in astrocytic processes surrounding various neuronal elements in the gray matter. Therefore, before switching to astrocytic expression, GLT-1 is transiently expressed in neurons and localized in differentiating axons. Together with our previous finding on the localization of glutamate transporter GLAST in radial glial fibers, GLT-1 and GLAST are thus localized during development on distinct directional cellular elements along which young neurons elongate their axons or move their cell bodies, respectively.

Neuronal expression of the glutamate transporter GLT-1 in hippocampal microcultures

To address the question of the relative contributions of glial and neuronal glutamate transport in the vertebrate CNS, we studied the distribution of forebrain glutamate transporters in rat hippocampal microcultures, a preparation in which physiological functions of glutamate transporters have been well characterized. Two of the three transporters, GLAST (EAAT1) and EAAC1 (EAAT3), are localized to microculture glia and neurons, respectively, as expected. However, we find strong immunoreactivity for the third glutamate transporter GLT-1 (EAAT2), a putatively glial transporter, in microculture neurons and in a small subset of microculture glia. Indistinguishable immunohistochemical staining patterns for GLT-1 were obtained with antibodies directed against both the N terminal and C terminal of the GLT-1 protein. Double-labeling experiments suggest that neuronal GLT-1 protein is primarily localized to the dendrites of excitatory neurons. Neuronal electrogenic transport currents in response to D-aspartate applications were occluded by the selective GLT-1 inhibitor dihydrokainate. In contrast, glia exhibited a larger transporter current density than did neurons, and the glial transport current was less sensitive to dihydrokainate. Neuronal transport currents were potentiated less than were glial currents when the chaotropic anion thiocyanate was substituted for gluconate in the whole-cell recording pipette, consistent with the previously reported lower anion permeability of EAAC1 and GLT-1 compared with that of GLAST. After microculture glia were rendered nonviable, excitatory autaptic currents (EACs) were prolonged in the presence of dihydrokainate, suggesting that neuronal GLT-1 is capable of participating in the clearance of synaptically released glutamate. Our results suggest that the initially proposed characterization of GLT-1 as a purely glial transporter is too simplistic and that under certain conditions functional GLT-1 protein can be expressed in brain neurons. The study suggests that changes in GLT-1 levels that occur with pathology or experimental manipulations cannot be assumed to be glial.