Glia-synapse interaction through Ca2+-permeable AMPA receptors in Bergmann glia

Effects of human immunodeficiency virus type 1 on astrocyte gene expression and function: potential role in neuropathogenesis

Neurodegeneration and dementia caused by human immunodeficiency virus type 1 (HIV-1) infection of the brain are common complications of acquired immunodeficiency syndrome (AIDS). Introduction of highly active antiretroviral therapy (HAART) reduced the incidence of HIV-1-associated dementia, but so far had no effect on the high frequency of milder neurological disorders caused by HIV-1. This indicates that some neuropathogenic processes persist during limited HIV-1 replication in the central nervous system (CNS). The authors are evaluating the hypothesis that interaction of HIV-1 with astrocytes, which bind HIV-1 but support limited productive HIV-1 infection, may contribute to these processes by disrupting astrocyte functions that are important for neuronal activity or survival. Using laser-capture microdissection on brain tissue samples from HIV-1-infected individuals, we found that HIV-1 DNA can be detected in up to 1% of cortical and basal ganglia astrocytes, thus confirming HIV-1 infection in astrocytes from symptomatic patients. Using rapid subtraction hybridization, the authors cloned and identified 25 messenger RNAs in primary human fetal astrocytes either up-regulated or down-regulated by native HIV-1 infection or exposure to gp120 in vitro. Extending this approach to gene microarray analysis using Affymetrix U133A/B gene chips, the authors determined that HIV-1 alters globally and significantly the overall program of gene expression in astrocytes, including changes in transcripts coding for cytokines, G-coupled protein receptors, transcription factors, and others. Focusing on a specific astrocyte function relevant to neuropathogenesis, the authors showed that exposure of astrocytes to HIV-1 or gp120 in vitro impairs the ability of the cells to transport L-glutamate and the authors related this defect to transcriptional inhibition of the EAAT2 glutamate transporter gene. These findings define new pathways through which HIV-1 may contribute to neuropathogenesis under conditions of limited virus replication in the brain.

Diversity of functional astroglial properties in the respiratory network

A population of neurons in the caudal medulla generates the rhythmic activity underlying breathing movements. Although this neuronal network has attracted great attention for studying neuronal aspects of synaptic transmission, functions of glial cells supporting this neuronal activity remain unclear. To investigate the role of astrocytes in the respiratory network, we applied electrophysiological and immunohistochemical techniques to characterize astrocytes in regions involved in the generation and transmission of rhythmic activity. In the ventral respiratory group and the hypoglossal nucleus (XII) of acutely isolated brainstem slices, we analyzed fluorescently labeled astrocytes obtained from TgN(GFAP-EGFP) transgenic mice with the whole-cell voltage-clamp technique. Three subpopulations of astrocytes could be discerned by their distinct membrane current profiles. A first group of astrocytes was characterized by nonrectifying, symmetrical and voltage-independent potassium currents and a robust glutamate transporter response to d-aspartate. A second group of astrocytes showed additional A-type potassium currents, whereas a third group, identified by immunolabeling for the glial progenitor marker NG2, expressed outwardly rectifying potassium currents, smaller potassium inward currents, and only minimal D-aspartate-induced transporter currents. Astrocytes of all groups showed kainate-induced inward currents. We conclude that most of the astrocytes serve as a buffer system of excess extracellular glutamate and potassium; however, a distinct cell population (NG2-positive, A-type potassium currents) may play an important role for network plasticity.

Meteorin: a secreted protein that regulates glial cell differentiation and promotes axonal extension

Glial cells are major components of the nervous system. The roles of these cells are not fully understood, however. We have now identified a secreted protein, designated Meteorin, that is expressed in undifferentiated neural progenitors and in the astrocyte lineage, including radial glia. Meteorin selectively promoted astrocyte formation from mouse cerebrocortical neurospheres in differentiation culture, whereas it induced cerebellar astrocytes to become radial glia. Meteorin also induced axonal extension in small and intermediate neurons of sensory ganglia by activating nearby satellite glia. These observations suggest that Meteorin plays important roles in both glial cell differentiation and axonal network formation during neurogenesis.

Roles of glutamate transporters in shaping excitatory synaptic currents in cerebellar Purkinje cells

Several subtypes of glutamate transporters are abundantly expressed near the excitatory synapses on cerebellar Purkinje cells. We investigated the roles of the glutamate transporters in shaping the excitatory postsynaptic currents (EPSCs) and regulating the levels of extracellular glutamate in the mouse cerebellum using a potent blocker of glutamate transporters, dl-threo-beta-benzyloxyaspartate (dl-TBOA). This drug markedly prolonged AMPA receptor-mediated EPSCs in Purkinje cells evoked by stimulating both parallel fibres and climbing fibres. The decay phase of the prolonged EPSCs was fitted by double exponentials, of which the slower component was preferentially inhibited by a low-affinity competitive antagonist of AMPA receptors, gamma-d-glutamyl-glycine, indicating that the slow component induced by dl-TBOA was the AMPA receptor-mediated current activated by lower concentrations of glutamate than those contributing to the peak of the EPSC. This result suggests that dl-TBOA prolongs the stay of synaptically released glutamate in the synaptic cleft and also induces glutamate spillover to extrasynaptic targets as well as neighbouring synapses. Furthermore, high concentrations of dl-TBOA in the presence of cyclothiazide generated a continuous inward current in Purkinje cells, of which the amplitude reached the peak level of the climbing-fibre EPSC. This continuous inward current was abolished by the blocker of AMPA receptors, indicating that the strong inhibition of glutamate uptake causes the rapid accumulation of glutamate in the extracellular space. These results highlight the importance of glutamate transporters in maintaining the proper glutamatergic transmission in Purkinje cell synapses.

Recent advances in the study of AMPA receptors

As glutamate is a dominant excitatory neurotransmitter in the central nervous system, glutamate receptors, and especially AMPA receptors, are located ubiquitously in all brain areas. In this paper, we reviewed recent advances of studies on AMPA receptor functions. AMPA receptors are cation-conducting complexes composed of various combinations of four subunits (GluR1 to GluR4). The glutamine residue located in the pore-forming segment of GluR2 subunit (Q/R site) is changed to arginine by RNA editing at the pre mRNA stage in normal adult mammalian animal. The edited GluR2 subunit is a major determination of Ca(2+) permeability of the AMPA receptor; only edited GluR2-lacking receptor shows high-Ca(2+) permeability. The assembly of glutamate AMPA receptor subunit is not completely according to the stochastic theory. The heteromeric subunits assembly is more rapid than the homomeric assembly is. The transfer of AMPA receptor subunit to the plasma membrane is conducted in multiple ways. Many molecules that interact with the intracellular domain of AMPA receptor subunits are reported as the modulators of AMPA receptor subunit transfer. In the motoneuron of sporadic amyotrophic lateral sclerosis (ALS) patients, the efficiency of RNA editing at the GluR2 Q/R site is significantly decreased. Relative low level of edited GluR2 subunit expression is likely responsible for motoneuronal death in ALS. Recently, AMPA receptors in glial cells have been studied. Bergmann glial cells in cerebellum express Ca(2+)-permeable AMPA receptors. Conversion of these AMPA receptors to Ca(2+)-impermeable type receptors induces morphological and functional changes. Glioblastoma cells also express Ca(2+)-permeable AMPA receptors, and their conversion to Ca(2+)-impermeable receptors inhibits cell locomotion and induces apoptosis.

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.

A change in the pattern of activity affects the developmental regression of the Purkinje cell polyinnervation by climbing fibers in the rat cerebellum

Pattern of activity during development is important for the refinement of the final architecture of the brain. In the cerebellar cortex, the regression from multiple to single climbing fiber innervation of the Purkinje cell occurs during development between postnatal days (P) 5 and 15. However, the regression is hampered by altering in various ways the morpho-functional integrity of the parallel fiber input. In rats we disrupted the normal activity pattern of the climbing fiber, the terminal arbor of the inferior olive neurons, by administering harmaline for 4 days from P9 to P12. At all studied ages (P15-87) after harmaline treatment multiple (double only) climbing fiber EPSC-steps persist in 28% of cells as compared with none in the control. The ratio between the amplitudes of the larger and the smaller climbing fiber-evoked EPSC increases in parallel with the decline of the polyinnervation factor, indicating a gradual enlargement of the synaptic contribution of the winning climbing fiber synapse at the expense of the losing one. Harmaline treatment had no later effects on the climbing fiber EPSC kinetics and I/V relation in Purkinje cells (P15-36). However, there was a rise in the paired-pulse depression indicating a potentiation of the presynaptic mechanisms. In the same period, after harmaline treatment, parallel fiber-Purkinje cell electrophysiology was unaffected. The distribution of parallel fiber synaptic boutons was also not changed. Thus, a change in the pattern of activity during a narrow developmental period may affect climbing fiber-Purkinje cell synapse competition resulting in occurrence of multiple innervation at least up to 3 months of age. Our results extend the current view on the role of the pattern of activity in the refinement of neuronal connections during development. They suggest that many similar results obtained by different gene or receptor manipulations might be simply the consequence of disrupting the pattern of activity.

Adenoviral-mediated expression of functional Na+ channel beta1 subunits tagged with a yellow fluorescent protein

Voltage-gated sodium (Na(+)) channels typically contain a pore-forming alpha subunit and one or two auxiliary beta subunits. Although initial characterization of known alpha and beta subunits has been facilitated by expression in heterologous cells, to understand fully the differences between individual subunits and the functional consequences of selective subunit expression, there is a need to acutely manipulate expression in cells that endogenously express Na(+) channels. To this end, we have constructed a recombinant adenovirus containing a cDNA for a mouse Na(+) channel beta1 subunit with a yellow fluorescent protein fused to its C-terminus (Ad-beta1-EYFP), and with fluorescence microscopy detected beta1-EYFP expression in primary cerebellar neurons and Chinese hamster ovary (CHO) cells upon transduction with this adenovirus, including expression in the plasma membrane. Consistent with this, patch clamp recordings confirmed that Na(+) currents in CHO cells expressing mouse Na(v)1.4 alpha subunits were appropriately modified by the viral-mediated expression of beta1-EYFP subunits. The results demonstrate that adenoviral-mediated gene delivery can be used effectively to express epitope-tagged Na(+) channel subunits with properties similar to wild-type subunits, and suggest that Ad-beta1-EYFP will be a useful reagent for investigating Na(+) channels in a variety of excitable cell types, including neurons.

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.

Glial modulation of synaptic transmission: Insights from the supraoptic nucleus of the hypothalamus

Astrocytes clear synaptically released glutamate from the extracellular space through high-affinity transporters present on their plasma membrane. By controlling the extracellular level of the main excitatory transmitter in the central nervous system, astrocytes thus contribute prominently to the regulation of overall cellular excitability and synaptic information processing. We recently investigated the influence of the glial environment on glutamatergic and GABAergic neurotransmission in the supraoptic nucleus of the rat hypothalamus under physiological conditions such as lactation that significantly reduce astrocytic coverage of its neurons. By performing electrophysiological analyses on this unique model of dynamic neuronal-glial interactions, we have been able to show that the fine astrocytic processes normally enwrapping synapses serve two important functions. First, they govern the level of activation of presynaptic metabotropic glutamate receptors on glutamatergic terminals, thereby regulating synaptic efficacy at excitatory synapses. Second, they act as a physical and functional barrier to diffusion in the extracellular space, limiting spillover of glutamate and other neuroactive substances and therefore contributing to the regulation of heterosynaptic transmission and intercellular communication.

Synaptic and extrasynaptic neurotransmitter receptors in glial precursors' quest for identity

It is widely established that neurotransmitter receptors are expressed in non-neuronal cells, and particularly in neural progenitor cells in the postnatal central nervous system. The functional role of these receptors during development is unclear, but it needs to be revisited now that cells previously considered restricted to glial lineages have been shown to generate neurons. The present review integrates recent advances, to shed new light on how neurotransmitter receptors may, alternatively, serve as excitable mediators of neuron-glia and neuron-neuroblast interactions.

Motor training compensates for cerebellar dysfunctions caused by oligodendrocyte ablation

The role played by oligodendrocytes (OLs), the myelinating cells of the CNS, during brain development has not been fully explored. We have addressed this question by inducing a temporal and reversible ablation of OLs on postnatal CNS development. OL ablation in newborn mice leads to a profound alteration in the structure of the cerebellar cortex, which can be progressively rescued by newly generated cells, leading to a delayed myelination. Nevertheless, the temporal shift of the OL proliferation and myelinating program cannot completely compensate for developmental defects, resulting in impaired motor functions in the adult. Strikingly, we show that, despite these abnormalities, epigenetic factors, such as motor training, are able to fully rescue cerebellar-directed motor skills.

Synaptic signaling between GABAergic interneurons and oligodendrocyte precursor cells in the hippocampus

Oligodendrocyte precursor cells (OPCs) express receptors for many neurotransmitters, but the mechanisms responsible for their activation are poorly understood. We have found that quantal release of GABA from interneurons elicits GABA(A) receptor currents with rapid rise times in hippocampal OPCs. These currents did not exhibit properties of spillover transmission or release by transporters, and immunofluorescence and electron microscopy suggest that interneuronal terminals are in direct contact with OPCs, indicating that these GABA currents are generated at direct interneuron-OPC synapses. The reversal potential of OPC GABA(A) currents was -43 mV, and interneuronal firing was correlated with transient depolarizations induced by GABA(A) receptors; however, GABA application induced a transient inhibition of currents mediated by AMPA receptors in OPCs. These results indicate that OPCs are a direct target of interneuronal collaterals and that the GABA-induced Cl(-) flux generated by these events may influence oligodendrocyte development by regulating the efficacy of glutamatergic signaling in OPCs.

New roles for astrocytes: regulation of CNS synaptogenesis

The notion that astrocytes have a profound influence on the function of synapses between CNS neurons implies that the development of synaptic connections and their glial neighbors are controlled by reciprocally acting signals. Currently, however, synaptogenesis is considered a purely neuronal affair. This article summarizes recent experimental evidence suggesting that this may not be the case. Astrocytes may indeed regulate the formation, maturation and maintenance of synapses. The recent advances caution that synapses cannot develop correctly without astrocytes. Further progress on this issue requires new experimental models to identify signaling pathways and to scrutinize the relevance of glia-synapse interactions in vivo.

Immunohistochemical localization of Notch receptors and their ligands in the postnatally developing rat cerebellum

Although mRNAs of Notch receptors and their ligands are known to be expressed in the postnatally developing rodent cerebellum, their protein localization has been poorly investigated. In the present study, we immunohistochemically examined localization of Notch receptors and their ligands during postnatal cerebellar development in rats. During the first two postnatal weeks, intense signals of Notch1-3 were localized in Bergmann fibers (radial fibers of Bergmann glia), as confirmed by double fluorescent immunohistochemistry. After that, the signals gradually declined into adulthood. Among Notch ligands, Jagged1 and 2 were also localized in Bergmann fibers. These results suggest that cell-cell interactions through Jagged-Notch signaling can occur between Bergmann glia during postnatal cerebellar development.

Ectopic Release of Synaptic Vesicles

Exocytosis of synaptic vesicles is generally assumed to occur only at ultrastructurally defined presynaptic active zones. If release is restricted to these sites, receptors not located within the synaptic cleft must be activated by transmitter that diffuses out of the cleft or not be activated at all. Here we report that AMPA receptor-mediated quantal events resulting from climbing fiber release are observed in Bergmann glial cells in the cerebellar cortex. These quantal events are not coincident with quanta recorded in neighboring Purkinje cells which receive input from the same climbing fiber. As Bergmann glial membranes are excluded from the synaptic cleft, we propose that exocytosis can occur from climbing fiber release sites located directly across from Bergmann glial membranes. Such ectopic release may account for the majority of the Bergmann glial AMPA response evoked by climbing fiber stimulation.

Fluorescent detection of Ca2+-permeable AMPA/kainate receptor activation in murine neocortical neurons

Agonist stimulated Co(2+) uptake is used to identify neurons expressing Ca(2+)-permeable AMPA/kaniate receptors (Ca-A/K receptors). Based on the selective permeability of Co(2+) through these receptors we have developed a fluorometric method that utilizes the fluorescence laser imaging plate reader (FLIPR). We used the dye calcein whose fluorescence is stoichiometrically quenched by Co(2+), while being only minimally affected by variations in intracellular Ca(2+). Application of AMPA in the presence of cyclothiazide led to a concentration-dependent increase in Co(2+) uptake in the neocortical neurons. Similar concentration-dependent increments in Co(2+) uptake were observed with kainate treatment. 2,3-Dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX), an AMPA/kainate receptor antagonist, blocked the AMPA-induced Co(2+) influx. The fluorometric method described affords a rapid, high throughput and quantitative procedure for investigation of Ca-A/K receptors in intact neurons.

Glial Cells and Neurotransmission

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.

Muscarine-induced increase in frequency of spontaneous EPSCs in Purkinje cells in the vestibulo-cerebellum of the rat

Cholinergic projections are relatively sparse in the cerebellum compared with other parts of the brain. However, some mossy fibers in the vestibulo-cerebellum are known to be cholinergic. To clarify the functional roles of cholinergic mossy fibers in the vestibulo-cerebellum, we investigated the effects of acetylcholine (ACh) on the membrane electrical properties of both granule cells and Purkinje cells in slices of the cerebellar vermis of the rat using whole-cell patch-clamp techniques. The bath application of ACh induced a marked increase in the frequency of spontaneous EPSCs (sEPSCs) in Purkinje cells specifically in the vestibulo-cerebellum. This effect of ACh was mimicked by muscarine but not by nicotine. It was abolished by application of either tetrodotoxin or the antagonist of AMPA receptors, indicating that the ACh-induced enhancement of sEPSCs occurred indirectly via the activation of neurons sending glutamatergic projections to Purkinje cells. In approximately 15% of granule cells tested in the vestibulo-cerebellum, muscarine elicited membrane depolarization accompanied by a decrease in membrane conductance and increased the neuronal excitability. The muscarine-induced depolarization of granule cells in the vestibulo-cerebellum was attributable to the inhibition of standing-outward K+ currents (IKSO) most likely via the activation of muscarinic M3 receptors. Taken together, these results indicate that ACh increases the firing frequency of granule cells by inhibiting IKSO, which in turn increases the frequency of sEPSCs in Purkinje cells in the rat vestibulo-cerebellum.

Reduced expression of glutamate transporter EAAT2 and impaired glutamate transport in human primary astrocytes exposed to HIV-1 or gp120

L-Glutamate is the major excitatory neurotransmitter in the brain. Astrocytes maintain low levels of synaptic glutamate by high-affinity uptake and defects in this function may lead to neuronal cell death by excitotoxicity. We tested the effects of HIV-1 and its envelope glycoprotein gp120 upon glutamate uptake and expression of glutamate transporters EAAT1 and EAAT2 in fetal human astrocytes in vitro. Astrocytes isolated from fetal tissues between 16 and 19 weeks of gestation expressed EAAT1 and EAAT2 RNA and proteins as detected by Northern blot analysis and immunoblotting, respectively, and the cells were capable of specific glutamate uptake. Exposure of astrocytes to HIV-1 or gp120 significantly impaired glutamate uptake by the cells, with maximum inhibition within 6 h, followed by gradual decline during 3 days of observation. HIV-1-infected cells showed a 59% reduction in V(max) for glutamate transport, indicating a reduction in the number of active transporter sites on the cell surface. Impaired glutamate transport after HIV-1 infection or gp120 exposure correlated with a 40-70% decline in steady-state levels of EAAT2 RNA and protein. EAAT1 RNA and protein levels were less affected. Treatment of astrocytes with tumor necrosis factor-alpha (TNF-alpha) decreased the expression of both EAAT1 and EAAT2, but neither HIV-1 nor gp120 were found to induce TNF-alpha production by astrocytes. These findings demonstrate that HIV-1 and gp120 induce transcriptional downmodulation of the EAAT2 transporter gene in human astrocytes and coordinately attenuate glutamate transport by the cells. Reduction of the ability of HIV-1-infected astrocytes to take up glutamate may contribute to the development of neurological disease.

Induction of rapid, activity-dependent neuronal-glial remodelling in the adult rat hypothalamus in vitro

The hypothalamic oxytocinergic system offers a remarkable model of morphological plasticity in the adult because its neurons and astrocytes undergo mutual remodelling in relation to differing physiological conditions. Among various factors involved in such plasticity, oxytocin (OT) itself appears of primary importance as its central administration resulted in morphological changes similar to those brought on by physiological stimuli. In the present study, we applied OT on acute hypothalamic slices from adult rats that included the supraoptic nucleus. Using ultrastructural morphometric analyses, we found that it induced a significant reduction of astrocytic coverage of OT neurons, leaving their surfaces directly juxtaposed, to an extent similar to that detected in vivo under conditions like lactation. These neuronal-glial changes were rapid and reversible, occurring within a few hours, and specifically mediated via OT receptors. They were potentiated by oestrogen and depended on calcium mobilization and de novo protein synthesis. Moreover, they depended on concurrent neuronal activation brought on by hyperosmotic stimulation or blockade of inhibitory GABAergic neurotransmission; they were inhibited by blockade of glutamatergic receptors. Taken together, our observations show that intrahypothalamic release of OT affects not only neuronal activation of the OT system but its morphological plasticity as well. Moreover, the activity dependence of the OT-induced changes strongly suggests that astrocytes can sense the level of activity of adjacent neurons and/or afferent input and this can subsequently act as a signal to bring on the neuronal and glial conformational changes.

Precursors of neurons, neuroglia, and ependymal cells in the CNS: what are they? Where are they from? How do they get where they are going?

Neurons, neuroglia (astrocytes and oligodendrocytes), and ependymal cells are three distinct categories of neural cells in the central nervous system. In the mature brain and spinal cord, the classical histological criteria define these cells by their microscopic structure very well. During development, the precursors for all of these cells reside within the epithelium of the neural plate and its successor, the neural tube. These precursor cells are the undifferentiated, primitive neuroepithelium of the classical literature. As the cerebral vesicles enlarge and their walls thicken, the primitive neuroepithelial cells elongate, maintaining a radial orientation until they migrate. Although many, but not all, of these cells span the extent of the ventricular wall, they are the precursors of neurons, neuroglia, and ependymal cells. Thus, it is useful to retain their classical designation as primitive neuroepithelial cells and to treat them as neural precursor cells. Neural precursor cells are neither neuroglia nor neurons. It is not appropriate to call them radial glial cells anymore than it is to call them radial neuronal cells. The term "radial glia" has long been used to describe the mature, elongated astrocytes, represented by Bergmann cells in the cerebellum and Müller cells in the retina. Inevitably, during development, transitional forms between neural precursor cells and the neurons, neuroglia, and ependymal cells will occur. Such transitional cells are known as neuroblasts, glioblasts, or ependymoblasts, even though they may be postmitotic. Alternative terms are "immature neurons," "immature neuroglia," and "immature ependymal cells." The migration of many neural precursor cells is accomplished by translocation rather than free cellular locomotion. There is both direct and indirect evidence to document the translocation of the nuclear/perikaryal/somal complex through the leading process of primitive neuroepithelial cells. This is conspicuous in the neocortex, where the discrete radial arrangement of pyramidal cells may result from translocation of neuroblasts, while their leading processes still contact the pial surface. Migration by translocation occurs throughout the CNS. GLIA 43:6-18, 2003.

Identification of gene products suppressed by human immunodeficiency virus type 1 infection or gp120 exposure of primary human astrocytes by rapid subtraction hybridization

Neurodegeneration and human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) are the major disease manifestations of HIV-1 colonization of the central nervous system (CNS). In the brain, HIV-1 replicates in microglial cells and infiltrating macrophages and it persists in a low-productive, noncytolytic state in astrocytes. Astrocytes play critical roles in the maintenance of the brain microenvironment, responses to injury, and in neuronal signal transmission, and disruption of these functions by HIV-1 could contribute to HAD. To better understand the potential effects of HIV-1 on astrocyte biology, the authors investigated changes in gene expression using an efficient and sensitive rapid subtraction hybridization approach, RaSH. Primary human astrocytes were isolated from abortus brain tissue, low-passage cells were infected with HIV-1 or mock infected, and total cellular RNAs were isolated at multiple time points over a period of 1 week. This approach is designed to identify gene products modulated early and late after HIV-1 infection and limits the cloning of genes displaying normal cell-cycle fluctuations in astrocytes. By subtracting temporal cDNAs derived from HIV-1-infected astrocytes from temporal cDNAs made from uninfected cells, 10 genes displaying reduced expression in infected cells, termed astrocyte suppressed genes (ASGs), were identified and their suppression was confirmed by Northern blot hybridization. Both known and novel ASGs, not reported in current DNA databases, that are down-regulated by HIV-1 infection are described. Northern blotting confirms suppression of the same panel of ASGs by treatment of astrocytes with recombinant HIV-1 envelope glycoprotein, gp120. These results extend our previous analysis of astrocyte genes induced or enhanced by HIV-1 infection and together they suggest that HIV-1 and viral proteins have profound effects on astrocyte physiology, which may influence their function in the CNS.

Extracellular vestibule determinants of Ca2+ influx in Ca2+-permeable AMPA receptor channels

At certain synapses in the brain, Ca2+-permeable AMPA receptor (AMPAR) channels represent an important pathway for synaptically controlled Ca2+ entry. However, the molecular determinants of this Ca2+ influx are poorly defined. In NMDA receptor (NMDAR) channels, where the influx is much greater, the extracellular vestibule, specifically the M3 segment and regions C-terminal to it in the NR1 subunit, contains elements critical to their high Ca2+ influx under physiological conditions. We therefore investigated the contribution of homologous positions in AMPAR as well as kainate receptor (KAR) subunits to the process of Ca2+ influx. Substitutions of a conserved asparagine (N) in M3 of AMPAR GluR-B(Q) channels strongly attenuated Ca2+ permeability measured using reversal potentials under biionic conditions and fractional Ca2+ currents recorded under physiological conditions. Hence, as in NMDAR channels, the conserved N makes a significant contribution to Ca2+ influx in AMPAR channels. In addition, C-terminal to M3, substitutions of negatively (glutamate, E) or positively (arginine, R) charged residues also altered Ca2+ influx. However, in contrast to charged residues occupying homologous positions in NMDAR channels, these effects were about equal and opposite suggesting that this ER in AMPARs does not contribute significantly to the mechanism of Ca2+ influx. Opposite charge substitutions of two negative residues C-terminal to M3 in KAR GluR-6(Q) subunits had no effect on Ca2+ permeability. We conclude that the different contribution of residues C-terminal to M3 to Ca2+ permeation in NMDAR and non-NMDAR channels reflects a different positioning of these residues relative to the tip of the M2 loop.

Overexpression of Ca2+-permeable AMPA receptor promotes delayed cell death of hippocampal CA1 neurons following transient forebrain ischemia

To examine the role of Ca(2+) entry through AMPA receptors in the pathogenesis of the ischemia-induced cell death of hippocampal neurons, we delivered cDNA of Q/R site-unedited form (GluR2Q) of AMPA receptor subunit GluR2 in the hippocampus by using an HVJ-liposome-mediated gene transfer technique. Two days prior to transient forebrain ischemia, we injected an HVJ-liposome containing cDNA of the GluR2Q-myc fusion gene into a rat unilateral hippocampus. In the absence of ischemic insult, overexpression of Ca(2+)-permeable GluR2Q did not cause any neurodegeneration in the cDNA-injected hippocampus. In ischemic rats, overexpression of Ca(2+)-permeable GluR2Q markedly promoted ischemic cell death of CA1 pyramidal neurons, while complete rescue of CA1 pyramidal neurons from ischemic damage occurred in the hippocampal hemisphere opposite the GluR2Q expression. Overexpression of the Q/R-site edited form (GluR2R) of subunit GluR2 did not affect the ischemia-induced damage of CA1 pyramidal neurons. From these results, we suggest that the Ca(2+)-permeability of AMPA receptors does not have a direct contribution to glutamate receptor-mediated neurotoxicity but has a promotive action in the evolution of ischemia-induced neurodegeneration of vulnerable neurons.

Novel neuroglial and glioglial relationships mediated by L-serine metabolism

L-Serine is a non-essential amino acid that can be synthesized in the body. It derives from an intermediate of the glycolytic pathway, 3-phosphoglycerate, and utilized for the syntheses of proteins, other amino acids, membrane lipids, heme, and nucleotides. Emerging evidence indicates that L-serine functions as a glia-derived trophic factor, which strongly promotes the survival and differentiation of cultured neurons. L-Serine biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) and small neutral amino acid transporter ASCT1 have been revealed to be expressed preferentially in the radial glia-astrocyte lineage and olfactory ensheathing glia of both adult and developing rodent brains. In contrast, these biosynthetic and transporter molecules for L-serine are faint or undetectable in neurons and phagocytic cells. In this review, we summarize recent progress to propose that L-serine synthesis in these glial cells and its supply to nearby neurons and other glia constitute a novel metabolic unit in the brain. Based on these neuroglial and glioglial relationships, glucose in neurons and phogocytes can be strategically used for energy production, while a variety of L-serine-derived biomolecules required for their proliferaton, survival, differentiation, and function are synthesized in and supplied from the radial glia-astrocyte lineage and olfactory ensheathing glia. A transient capillary expression of ASCT1 in fetal and neonatal brains further suggests that, in addition to the glia-borne L-serine, an active transport of blood-borne L-serine would play an essential role in neural development.

Metabotropic glutamate receptor activation enhances the activities of two types of Ca2+-activated k+ channels in rat hippocampal astrocytes

The influence of activation of glutamate receptor (GluR) on outward K(+) current in cultured neonate rat hippocampal astrocytes was investigated. Patch-clamp analysis of K(+) channel currents in cultured astrocytes identified the existence of 71 +/- 6 and 161 +/- 11 pS single-channel K(+) currents that were sensitive to changes in voltage and [Ca(2+)](i) and blocked by external TEA but not by charybdotoxin, iberiotoxin, apamin, or 4-aminopyridine. Reverse transcriptase (RT)-PCR and Northern blot analysis revealed transcripts of the Ca(2+)-activated K(+) channel (K(Ca)) beta(4)-subunit (beta4) (KCNMB4) in cultured astrocytes. Expression of the metabotropic glutamate receptor (mGluR) subtypes mGluR1 and mGluR5 and the ionotropic glutamate receptor (iGluR) subtypes iGluR1 and iGluR4 were detected by RT-PCR and immunofluorescence analysis in cultured astrocytes. The mGluR agonists L-glutamate and quisqualate increased the open state probability (NP(o)) of the 71 and 161 pS K(+) channel currents that were prevented by the mGluR receptor antagonists 1-aminoindan-1,5-dicarboxylic acid or L-(+)-2-amino-3-phosphonopropionic acid and not by the iGluR antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate or CNQX. Activation of the two types of K(+) channel currents by mGluR agonists was attenuated by pertussis toxin and by inhibition of phospholipase C (PLC) or cytochrome P450 arachidonate epoxygenase. These results indicate that brain astrocytes contain the KCNMB4 transcript and express two novel types of K(Ca) channels that are gated by activation of a G-protein coupled metabotropic glutamate receptor functionally linked to PLC and cytochrome P450 arachidonate epoxygenase activity.

Transfer of NMDAR2 cDNAs increases endogenous NMDAR1 protein and induces expression of functional NMDA receptors in PC12 cells

The pheochromocytoma cell line (PC12) has been used as a model system for the study of regulation of expression of NMDA receptors. PC12 cells express a substantial amount of NMDAR1 subunit (NR1) mRNA, whereas they express only a small amount of NR1 protein. The level of functional NMDA receptor expression is almost negligible. To test the possibility that NMDAR2 subunits (NR2) control expression of functional NMDA receptors as well as NR1 protein, we transferred NR2A-D cDNAs into PC12 cells using adenovirus vectors. Prominent NMDA receptor-mediated currents were recorded in PC12 cells to which NR2A or NR2B cDNA was delivered without NR1 cDNA. The amplitudes of these responses were similar to those in PC12 cells to which NR1 cDNA was delivered together with NR2A or NR2B cDNA. In cells to which either NR2C or NR2D cDNA alone was delivered, NMDA receptor-mediated currents were also detected, although to a much lesser extent. These results showed that NR2 proteins produced by gene transfer are co-assembled with the endogenous NR1 protein to form functional heteromeric receptors. The delivery of NR2A-D cDNAs also increased the amount of NR1 protein but not that of NR1 mRNA, suggesting that this protein increase is due to post-transcriptional mechanisms. The effects of NR2A-B gene transfer on expression of NR1 protein were much more efficient than those of NR2C-D gene transfer.

Insights into glutamate transport regulation in human astrocytes: cloning of the promoter for excitatory amino acid transporter 2 (EAAT2)

Glutamate transport is central to neurotransmitter functions in the brain. Impaired glutamate transport induces neurotoxicity associated with numerous pathological processes, including stroke/ischemia, temporal lobe epilepsy, Alzheimer's disease, amyotrophic lateral sclerosis, Huntington's disease, HIV-1-associated dementia, and growth of malignant gliomas. Excitatory amino acid transporter-2 (EAAT2) is a major glutamate transporter in the brain expressed primarily in astrocytes. We presently describe the cloning and characterization of the human EAAT2 promoter, demonstrating elevated expression in astrocytes. Regulators of EAAT2 transport, both positive and negative, alter EAAT2 transcription, promoter activity, mRNA, and protein. These findings imply that transcriptional processes can regulate EAAT2 expression. Moreover, they raise the intriguing possibility that the EAAT2 promoter may be useful for targeting gene expression in the brain and for identifying molecules capable of modulating glutamate transport that could potentially inhibit, ameliorate, or prevent various neurodegenerative diseases.

Excitotoxic lesioning of the rat basal forebrain with S-AMPA: consequent mineralization and associated glial response

Regional depositions of calcium within the basal ganglia, cortex, cerebellum, and white matter and at perivascular sites have been observed in several pathological conditions. These generally indicate signs of ongoing apoptosis or necrotic processes, whereby the activation of glutamate receptors causes a rise in intracellular calcium levels leading to mineralization of neurons, and ultimately to cell death. The selective degeneration of cholinergic neurons in the basal forebrain is a major neuropathological component of Alzheimer's disease, and may result in abnormal deposition of calcium. In experimental models, selective lesions of the basal forebrain can be induced by intraparenchymal infusions of excito- or immunotoxins targeting cholinergic neurons. Excitotoxic lesions are often accompanied by calcium deposition within affected areas. In a previous study we also noted the presence of unusual deposition in areas close to the site of injections following unilateral S-AMPA-induced lesions of the basal forebrain (T. Perry, H. Hodges, and J. A. Gray, 2001, Brain Res. Bull. 54, 29-48). In this paper, we have characterized these deposits histologically and evaluated the microglial (CD11b) and astrocytic (GFAP) responses at 8 and 16 weeks following lesioning of the nucleus basalis magnocellularis with S-AMPA. The resulting deposits were heterogeneous in morphology and composed primarily of calcium. Small granular deposits were detected around blood vessels, whereas larger calcospherites were situated within the parenchyma. These deposits were more widely dispersed at 16 weeks postlesioning, affected neighboring nuclei, and displayed a progressive increase in size and frequency of occurrence. However, calcification within these regions was differentially associated with microglial and astrocytic reactivity at the two time points. Both microglial and astrocytic responses were pronounced at 8 weeks, whereas at 16 weeks, astrocytic reactivity prevailed and the microglial response was markedly attenuated. Importantly, the pattern of reactivity for microglia detected at 8 weeks was specifically localized to vulnerable nucleated areas prior to their substantial accumulation of calcium deposits, which was clearly evident by 16 weeks. We suggest that the initial microglial response could be used as a selective predictor of tissue necrosis and subsequent calcification, and that astrocytes, which form a glial scar in the affected tissues, may contribute toward the buildup of calcium deposits. The functional relevance of these findings is discussed.

Modulation of glutamatergic transmission by bergmann glial cells in rat cerebellum in situ

We obtained patch-clamp recordings from neuron-glial cell pairs in cerebellar brain slices to examine the contribution of glutamate (Glu) uptake by Bergmann glial cells to shaping excitatory postsynaptic currents (EPSCs) at the parallel fiber to Purkinje cell synapse. We show that electrical stimulation of parallel fibers not only activates EPSCs in Purkinje cells but also activates inward currents in antigenically identified Bergmann glial cells that invest Purkinje cell synapse with their processes. The inward current is partially due to 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX)- and 2-amino-5-phosphonopentanoic acid (AP5)-sensitive ionotropic Glu receptors, but >/=70% of the current was mediated by D,L-threo-beta-hydroxyaspartate (THA)-sensitive Glu transporters. Glu inward currents were completely and reversibly inhibited by depolarization of Bergmann glial cells to positive membrane potentials allowing biophysical inhibition of Glu uptake into a single glial cell. Inhibition of Glu transport into Bergmann glial cells by voltage-clamping the cell to depolarized potentials caused a reversible increase in spontaneous EPSC frequency in the Purkinje cell. This increase could also be achieved by pharmacological inhibition of Glu transport with the Glu transport inhibitor THA, suggesting that inhibition of Glu uptake into Bergmann glial cells is responsible for the modulation of postsynaptic EPSCs. THA modulation of spontaneous EPSCs could only be observed in the absence of TTX, suggesting primarily a presynaptic effect. Taken together these data suggest that glial Glu uptake can profoundly affect excitatory transmission in the cerebellum, most likely by regulating presynaptic glutamate release.

Changes in splice variant expression and subunit assembly of AMPA receptors during maturation of hippocampal astrocytes

Astrocytes in the hippocampus express glutamate receptors of the AMPA subtype. An increasing body of evidence suggests a contribution of astroglial AMPA receptors to a direct signaling between neurons and glial cells in vivo. Here, we have combined functional analysis with singlecell RT-PCR to investigate whether hippocampal astrocytes express Ca(2+)-permeable AMPA receptors. We show that by postnatal day 5, a mosaic of Ca(2+)-permeable and less Ca(2+)-permeable AMPA receptors coexists in individual astrocytes, while receptors with a more uniform, low divalent permeability dominate in older cells. Moreover, we report an upregulation of the flip form of the GluR2 subunit during maturation, while the splicing status of GluR1 and GluR4 remains unchanged. Due to its specific properties, Ca(2+)-permeable AMPA receptors in astrocytes might strengthen neuron-to-glia signaling and enable proper formation of structural and functional connections between glial cells and glutamatergic synapses in the developing hippocampus.

Functional NMDA receptor channels generated by NMDAR2B gene transfer in rat cerebellar Purkinje cells

The adult cerebellar Purkinje cell is an exceptional neuron in the central nervous system in that it expresses high levels of NMDAR1 (NR1) mRNA without expressing any NMDAR2 (NR2) mRNAs. It has no functional NMDA receptor (NMDAR) channels, although it receives enormous numbers of excitatory inputs. Despite the high level of NR1 mRNA expression, the presence and localization of NR1 protein in mature Purkinje cells are controversial. To examine the presence of NR1 protein and its ability to form functional NMDARs, we expressed the NR2B subunit in rat mature Purkinje neurons by Sindbis viral-mediated gene transfer. The recombinant virus encoding both the NR2B and enhanced green fluorescent protein (GFP) genes (designated as SIN-EG-NR2B) infected Purkinje cells without infecting glial cells. GFP fluorescence was detected in the soma and throughout dendrites of Purkinje cells 18-24 h postinfection. In most of GFP-positive cells, the expression of NR2B protein was detected by immunostaining with NR2B-specific antibodies. In Purkinje cells infected with SIN-EG-NR2B, the iontophoretic application of NMDA induced prominent NMDAR-mediated current responses, indicating that the exogenous NR2B was assembled with endogenous NR1 to form functional NMDARs. Furthermore, NMDAR-mediated synaptic currents were detected at both the climbing fibre and parallel fibre synapses in infected Purkinje cells. Thus, the mature Purkinje cell produces NR1 protein that is ready to combine with NR2 to form functional NMDARs in excitatory synapses.

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.

Induction of increased intracellular calcium in astrocytes by glutamate through activating NMDA and AMPA receptors

To study the effect of glutamate on the intracellular calcium signal of pure cultured rat astrocytes and the role of NMDA and AMPA receptors in the procedure, the change of calcium signal was investigated by monitoring the fluctuation of intracellular Ca2+ concentration ([Ca2+]i) on the basis of Fura-2 single cell fluorescent ratio (F345/F380). The changes in the effect of glutamate on the intracellular calcium signal were observed after blockage of NMDA and (or) AMPA receptors. It was found that L-glutamate could induce an increased [Ca2+]i in most of the cells in concentration- and time-dependent manner. D-(-)-2-amino-5-phosphonopentanoic acid (D-AP-5, a selective antagonist of the NMDA receptor) and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, a selective antagonist of the AMPA receptor) could abolish the effects of NMDA and AMPA respectively. The treatment of D-AP-5 and CNQX simultaneously or respectively could attenuate the effect of L-glutamate at varying degrees. All these indicated that glutamate could modulate intracellular Ca2+ of pure cultured rat astrocytes through different pathways. The activation of NMDA and AMPA receptors took part in the complex mechanisms.

Estrogen regulates GFAP-expression in specific subnuclei of the female rat interpeduncular nucleus: a potential role for estrogen receptor beta

We previously demonstrated that in rat, astrocytic glial fibrillary acidic protein- (GFAP) expression in the interpeduncular nucleus (IPN) was responsive to testosterone and in females the intensity of GFAP-immunoreactivity (IR) followed the periodic hormonal changes of the estrous cycle. The aim of this study was to test whether 17beta-estradiol (E(2)), in the absence of other ovarian hormones, can influence GFAP-expression within individual subnuclei of the IPN and to determine the cellular distribution of estrogen receptor beta (ERbeta) in the IPN. Quantitative surface-density analysis was used to compare the intensity of GFAP-IR at different anterio-posterior (AP) levels of the IPN in ovariectomized female rats 24 h after treatment with E(2) or vehicle. Estrogen-treatment resulted in a significant increase in GFAP-IR in the rostrolateral subnucleus of the IPN at AP: -5.60, in the lateral-, dorsolateral-, dorsomedial- and central subnuclei at -6.04 and in the lateral subnucleus at -6.72. No significant differences were observed at -5.80 and -6.30. These results indicate that E(2), in the absence of other ovarian hormones, modulates GFAP-expression within select IPN subnuclei and that these affects are dependent on position along the AP axis. To determine whether ERbeta was a possible mediator of the observed estrogenic effects, adjacent section pairs of the IPN were immunostained for ERbeta or GFAP. Using the 'mirror' method, ERbeta-IR was detected in the cytoplasm of GFAP-immunopositive astroglia and in the nuclei of GFAP-immunonegative neurons. These findings suggest that in the IPN, E(2) may directly modulate GFAP-expression through ERbeta-mediated mechanisms.

Solid-phase synthesis of polyamine toxin analogues: potent and selective antagonists of Ca2+-permeable AMPA receptors

The wasp toxin philanthotoxin-433 (PhTX-433) is a nonselective and noncompetitive antagonist of ionotropic receptors, such as ionotropic glutamate receptors and nicotinic acetylcholine receptors. Polyamine toxins are extensively used for the characterization of subtypes of ionotropic glutamate receptors, in particular Ca(2+)-permeable AMPA and kainate receptors. We have previously shown that an analogue of PhTX-433 with one of the amino groups replaced by a methylene group, philanthotoxin-83 (PhTX-83) is a selective and potent antagonist of AMPA receptors. We now describe the solid-phase synthesis of analogues of PhTX-83 and the electrophysiological characterization of these analogues on cloned AMPA and kainate receptors. The polyamine portion of PhTX-83 was modified systematically by changing the position of the secondary amino group along the polyamine chain. In another series of analogues, the acyl moiety of PhTX-83 was replaced by acids of different size and lipophilicity. Using electrophysiological techniques, PhTX-56 was shown to be a highly potent (K(i) = 3.3 +/- 0.78 nM) and voltage-dependent antagonist of homomeric GluR1 receptors and was more than 1000-fold less potent when tested on heteromeric GluR1+GluR2, as well as homomeric GluR5(Q) receptors, thus being selective for Ca(2+)-permeable AMPA receptors. Variation of the acyl group of PhTX-83 had only minor effect on antagonist potency at homomeric GluR1 receptors but led to a significant decrease in the voltage-dependence. In conclusion, PhTX-56 is a novel, very potent, and selective antagonist of Ca(2+)-permeable AMPA receptors and is a promising tool for structure/function studies of the ion channel of the AMPA receptor.

Long-lasting modulation of synaptic input to Purkinje neurons by Bergmann glia stimulation in rat brain slices

Information processing in the nervous system is achieved primarily at chemical synapses between neurons. Recent evidence suggests that glia-neuron interactions contribute in multiple ways to the synaptic process. In the present study we used the frequency of spontaneous postsynaptic currents (sPSC) in Purkinje neurons in acute cerebellar brain slices from juvenile rats (13-19 days old) as a measure of synaptic activity. Following 50 depolarizing pulses to an adjacent Bergmann glial cell (50 mV; duration 0.5 s; 1 Hz) the sPSC frequency of the Purkinje neuron was reduced to 65 +/- 7 % of control values within 10 min after glial stimulation and remained depressed for at least 40 min. Depolarizing pulses to 0 mV had a comparable effect (70 +/- 5 % of control). The frequency of miniature PSCs, as recorded in 300 nM TTX, was not modulated after glial stimulation. Blockade of ionotropic glutamate receptors (iGluRs) with kynurenic acid (1 mM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 microM) suppressed the reduction of neuronal activity induced by glial depolarization, whereas the glial modulation of synaptic activity was not inhibited by a block of N-methyl-D-aspartate iGluRs, metabotropic glutamate receptors, cannabinoid receptors or GABA(B) receptors. Fluorometric measurements of the intraglial Ca(2+) concentration revealed no glial Ca(2+) transients during the depolarization series, and glial cell stimulation reduced the neuronal sPSC frequency even after loading the glial cell with 20 mM of the Ca(2+) chelator BAPTA. Our results indicate a glia-induced long-lasting depression of neuronal communication mediated by iGluRs.

Neuroplasticity in Alzheimer's disease

Ramon y Cajal proclaimed in 1928 that "once development was ended, the founts of growth and regeneration of the axons and dendrites dried up irrevocably. In the adult centers the nerve paths are something fixed, ended and immutable. Everything must die, nothing may be regenerated. It is for the science of the future to change, if possible, this harsh decree." (Ramon y Cajal, 1928). In large part, despite the extensive knowledge gained since then, the latter directive has not yet been achieved by 'modern' science. Although we know now that Ramon y Cajal's observation on CNS plasticity is largely true (for lower brain and primary cortical structures), there are mechanisms for recovery from CNS injury. These mechanisms, however, may contribute to the vulnerability to neurodegenerative disease. They may also be exploited therapeutically to help alleviate the suffering from neurodegenerative conditions.

Retrograde signaling in the regulation of synaptic transmission: focus on endocannabinoids

This review covers recent developments in the cellular neurophysiology of retrograde signaling in the mammalian central nervous system. Normally at a chemical synapse a neurotransmitter is released from the presynaptic element and diffuses to the postsynaptic element, where it binds to and activates receptors. In retrograde signaling a diffusible messenger is liberated from the postsynaptic element, and travels "backwards" across the synaptic cleft, where it activates receptors on the presynaptic cell. Receptors for retrograde messengers are usually located on or near the presynaptic nerve terminals, and their activation causes an alteration in synaptic transmitter release. Although often considered in the context of long-term synaptic plasticity, retrograde messengers have numerous roles on the short-term regulation of synaptic transmission. The focus of this review will be on a group of molecules from different chemical classes that appear to act as retrograde messengers. The evidence supporting their candidacy as retrograde messengers is considered and evaluated. Endocannabinoids have recently emerged as one of the most thoroughly investigated, and widely accepted, classes of retrograde messenger in the brain. The study of the endocannabinoids can therefore serve as a model for the investigation of other putative messengers, and most attention is devoted to a discussion of systems that use these new messenger molecules.

Structural and quantitative analysis of astrocytes in the mouse hippocampus

We revealed the structural features of astrocytes by means of light microscopy, confocal laser scanning microscopy and high voltage electron microscopy, and estimated their numerical densities in the mouse hippocampus. The high voltage electron microscope examinations of Golgi-impregnated astrocytes clearly disclosed their fine leaflet-like processes in the masses occupied by individual astrocytes. The intracellular injection of two different fluorescent tracers into two neighboring astrocytes revealed that each astrocyte occupied a discrete area with a limited overlap only at its peripheral portion. In a quantitative analysis using an optical dissector, the numerical densities of astrocytes identified as S100-immunoreactive cells were only slightly different in their areal and laminar distributions. The numerical densities were higher in the stratum lacunosum-moleculare and dentate hilus, while they were slightly lower in the principal cell layers than the average (24.2 x 10(3) mm(-3)) in whole hippocampal regions. As for the dorsoventral difference, the numerical densities were significantly larger at the ventral level in the dentate gyrus, whereas such tendency was not apparent in the hippocampus proper. The projection area of the astrocytes estimated from Golgi-impregnated samples was roughly in inverse relation to the numerical densities; the areas in the stratum lacunosum-moleculare were somewhat smaller than the other layers, where the numerical densities were high. The present study indicates that astrocytes are distributed rather evenly without any prominent areal or laminar differences and that the individual astrocytes have their own domains; the periphery of the domain of a given astrocyte is interdigitated intricately with the processes of adjacent astrocytes whereas its inner core portion is not penetrated by them.

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.

Role of glia in synapse development

Recent studies suggest that glial cells regulate certain aspects of synapse development. Neurons can form synapses without glia, but may require glia-derived cholesterol to form numerous and efficient synapses. During synapse maturation, soluble and contact-dependent factors from glia may influence the composition of the postsynaptic density. Finally, synaptic connections appear to require glia to support their structural stability. Given the new evidence, it may be time now to acknowledge glia as a source for synaptogenesis-promoting signals. Scrutinizing the molecular mechanisms underlying this new function of glia and testing its relevance in vivo may help to understand how synapses develop and why they degenerate under pathological conditions.

Blockage of Ca(2+)-permeable AMPA receptors suppresses migration and induces apoptosis in human glioblastoma cells

Glioblastoma multiforme is the most undifferentiated type of brain tumor, and its prognosis is extremely poor. Glioblastoma cells exhibit highly migratory and invasive behavior, which makes surgical intervention unsuccessful. Here, we showed that glioblastoma cells express Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors assembled from the GluR1 and/or GluR4 subunits, and that their conversion to Ca(2+)-impermeable receptors by adenovirus-mediated transfer of the GluR2 cDNA inhibited cell locomotion and induced apoptosis. In contrast, overexpression of Ca(2+)-permeable AMPA receptors facilitated migration and proliferation of the tumor cells. These findings indicate that Ca(2+)-permeable AMPA receptors have crucial roles in growth of glioblastoma. Blockage of these Ca(2+)-permeable receptors may be a useful therapeutic strategy for the prevention of glioblastoma invasion.

Beyond the role of glutamate as a neurotransmitter

Glutamate is the principal excitatory neurotransmitter of the central nervous system, but many studies have expanded its functional repertoire by showing that glutamate receptors are present in a variety of non-excitable cells. How does glutamate receptor activation modulate their activity? Do non-excitable cells release glutamate, and, if so, how? These questions remain enigmatic. Here, we review the current knowledge on glutamatergic signalling in non-neuronal cells, with a special emphasis on astrocytes.

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.

Identification and cloning of human astrocyte genes displaying elevated expression after infection with HIV-1 or exposure to HIV-1 envelope glycoprotein by rapid subtraction hybridization, RaSH

Neurodegeneration and dementia are common complications of AIDS caused by human immunodeficiency virus type 1 (HIV-1) infection of the central nervous system. HIV-1 target cells in the brain include microglia, infiltrating macrophages and astrocytes, but rarely neurons. Astrocytes play an important role in the maintenance of the synaptic micro-environment and in neuronal signal transmission. To investigate potential changes in cellular gene expression associated with HIV-1 infection of astrocytes, we employed an efficient and sensitive rapid subtraction hybridization approach, RaSH. Primary human astrocytes were isolated from abortus brain tissue and low-passage cells were infected with HIV-1. To identify genes that display both early and late expression modifications after HIV-1 infection and to avoid cloning genes displaying normal cell cycle fluctuations in astrocytes, RNAs were isolated and pooled from 6, 12, 24 h and 3 and 7 day uninfected and infected cells and used for RaSH. Temporal cDNA libraries were prepared from double-stranded cDNAs that were enzymatically digested into small fragments, ligated to adapters, PCR amplified, and hybridized by incubation of tester and driver PCR fragments. By subtracting temporal cDNAs derived from uninfected astrocytes from temporal cDNAs made from HIV-1 infected cells, genes displaying elevated expression in virus infected cells, termed astrocyte elevated genes (AEGs), were identified. Both known and novel AEGs, not reported in current DNA databases, are described that display early or late expression kinetics following HIV-1 infection or treatment with recombinant HIV-1 envelope glycoprotein (gp120). For selected AEGs, expression of their protein products was also tested by Western blotting and found to display elevated expression following HIV-1 infection. The comparable pattern of regulation of the AEGs following HIV-1 infection or gp120 treatment suggest that HIV-1 exposure of astrocytes, even in the absence of productive infection, can induce changes in cellular gene expression.

Astroglia induce neurogenesis from adult neural stem cells

During an investigation of the mechanisms through which the local environment controls the fate specification of adult neural stem cells, we discovered that adult astrocytes from hippocampus are capable of regulating neurogenesis by instructing the stem cells to adopt a neuronal fate. This role in fate specification was unexpected because, during development, neurons are generated before most of the astrocytes. Our findings, together with recent reports that astrocytes regulate synapse formation and synaptic transmission, reinforce the emerging view that astrocytes have an active regulatory role--rather than merely supportive roles traditionally assigned to them--in the mature central nervous system.

Anatomical aspects of glia-synapse interaction: the perisynaptic glial sheath consists of a specialized astrocyte compartment

Amongst several forms of glia-neuronal communication, glia-synaptic interaction appears particularly interesting in the light of the well-known examples of two-way signaling between neurons and astrocytes. We review recent structural and physiological evidence showing that the structural correlate of glia-synaptic interaction is the peripheral astrocyte process (PAP) positioned next to the synaptic cleft. The structural and functional properties of these processes suggest that the PAP represents a separate astroglial compartment, in particular since it is characterized by the restricted localization of the actin-binding ERM protein ezrin. The structural properties of PAPs and this protein form the basis of rapid morphological changes of PAPs. The physiological relevance of PAP plasticity is illustrated by the example of the suprachiasmatic nucleus, where astrocytes display a high degree of activity-dependent plasticity reflecting circadian time.

Unraveling oligodendrocyte origin and function by cell-specific transgenesis

Besides the role of mature oligodendrocytes in myelin synthesis during the development of the central nervous system (CNS), the oligodendrocyte lineage also encompasses the largest pool of postnatal proliferating progenitors whose behavior in vivo remains broadly elusive in health and disease. We describe here transgenic models that allow us to track the functions and origins of such cells by using proteolipid protein and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) gene promoters to direct oligodendroglial expression of different reporters, in particular the green fluorescent protein (GFP). We emphasize that the CNP-GFP mouse, which targets the entire oligodendroglial lineage from embryonic life to adulthood, provides an outstanding tool to study the in vivo properties of oligodendrocyte progenitor cells in normal and damaged CNS.