Excitatory amino acid-induced AP-1 DNA binding activity in Müller glia

Glutamate transporter-dependent mTOR phosphorylation in Müller glia cells

Glu (glutamate), the excitatory transmitter at the main signalling pathway in the retina, is critically involved in changes in the protein repertoire through the activation of signalling cascades, which regulate protein synthesis at transcriptional and translational levels. Activity-dependent differential gene expression by Glu is related to the activation of ionotropic and metabotropic Glu receptors; however, recent findings suggest the involvement of Na⁺-dependent Glu transporters in this process. Within the retina, Glu uptake is aimed at the replenishment of the releasable pool, and for the prevention of excitotoxicity and is carried mainly by the GLAST/EAAT-1 (Na⁺-dependent glutamate/aspartate transporter/excitatory amino acids transporter-1) located in Müller radial glia. Based on the previous work showing the alteration of GLAST expression induced by Glu, the present work investigates the involvement of GLAST signalling in the regulation of protein synthesis in Müller cells. To this end, we explored the effect of D-Asp (D-aspartate) on Ser-2448 mTOR (mammalian target of rapamycin) phosphorylation in primary cultures of chick Müller glia. The results showed that D-Asp transport induces the time- and dose-dependent phosphorylation of mTOR, mimicked by the transportable GLAST inhibitor THA (threo-β-hydroxyaspartate). Signalling leading to mTOR phosphorylation includes Ca²⁺ influx, the activation of p60^src, phosphatidylinositol 3-kinase, protein kinase B, mTOR and p70^S6K. Interestingly, GLAST activity promoted AP-1 (activator protein-1) binding to DNA, supporting a function for transporter signalling in retinal long-term responses. These results add a novel receptor-independent pathway for Glu signalling in Müller glia, and further strengthen the critical involvement of these cells in the regulation of glutamatergic transmission in the retina.

Role of retinal glial cell glutamate transporters in retinal ganglion cell survival following stimulation of NMDA receptor

PURPOSE

To investigate the role of glutamate transporters (GLTs)in retinal glial cells that were treated with N-methyl-D-aspartate (NMDA), in retinal ganglion cell (RGC) survival.

METHODS

Primary cultures of retinal glial cells or RGCs from 3-day-old Sprague-Dawley rats were employed in the present study. Retinal glial cells were treated with NMDA and changes in GLT mRNA and protein expression were analyzed. The effects of pretreating retinal glial cells with the GLAST-specific inhibitor, rottlerin (ROT), and the GLT-1-specific inhibitor, dihydrokainic acid (DHK), on RGC survival were investigated under exposure to NMDA. The amount of glutamate in the culture medium of retinal glial cells was measured by high-performance liquid chromatography.

RESULTS

NMDA treatment increased GLAST and GLT-1 expression. GLAST and GLT-1 mRNA expression increased by 2.94-fold and 3.36-fold at 12 h after treatment with the highest concentration of NMDA (33 mM), and by 1.41-fold and 1.39-fold at 24 h, respectively. GLT-1 and GLAST protein expression also increased. MK801, an NMDA-receptor antagonist, inhibited the NMDA-induced upregulation of GLT mRNA expression. Co-culture with retinal glial cells increased the survival rate of RGCs. ROT decreased the survival rate of RGCs, whereas DHK significantly increased the survival rate of RGCs treated with 33 mM NMDA. NMDA treatment reduced the total amount of glutamate in the culture medium, particularly when 33 mM NMDA was added to the medium. ROT pretreatment increased the amount of glutamate in the culture medium, whereas DHK pretreatment decreased it.

CONCLUSION

GLAST and GLT-1 may have different roles in the survival of RGCs mediated by retinal glial cells. These results suggest that the NMDA-associated induction of GLTs plays an important role in RGC survival.

ERK1 plays a critical protective role againstN-methyl-D-aspartate-induced retinal injury

Excitotoxicity has been implicated in several ischemic diseases of the retina, including retinal vessel occlusion and diabetic retinopathy. Glutamate signaling mediated through the N-methyl-D-aspartate (NMDA) receptor contributes to ischemic cell death. The NMDA receptor antagonists MK-801 and memantine have substantial neuroprotective effects in experimental retinal disease models, but the mechanisms by which NMDA receptor activity leads to cell death is not clear. Here we describe a previously unknown role for retinal glial cells in NMDA-induced retinal injury that involves the activation of ERK1/2. Within 1 hr after injecting NMDA intravitreally, activation of ERK1/2 and c-Fos induction were observed in retinal Müller cells. The roles of activated ERK1/2 in neuronal damage were examined using ERK1 gene deficient mice (homozygous ERK1(-/-) mice). NMDA-induced ERK1/2 activation in retina was significantly suppressed in ERK1(-/-) mice, and these mice had significantly higher numbers of TUNEL-positive retinal cells than wild-type mice 24 hr after NMDA injection. These data suggest that, during NMDA injury, Müller cells are activated and play a protective role against NMDA-induced retinal cell death. ERK1 appears to play a major role in this process. These new findings on retinal glial cell response during NMDA injury offer an important new therapeutic target for preventing many retinal disorders associated with excitotoxicity.

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

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

Glutamate-induced inhibition of D-aspartate uptake in Müller glia from the retina

Müller glial cells from the retina "in situ" and in primary culture, mainly express the high-affinity sodium-coupled glutamate/aspartate transporter GLAST-1, which dominates total retinal glutamate (Glu) uptake, suggesting a major role for these cells in the modulation of excitatory transmission. The possible involvement of ionotropic and metabotropic Glu receptors in the regulation of Glu uptake was studied in primary cultures of Müller glia. We demonstrate that exposure to 1 mM L-Glu induces a time-dependent inhibition of D-aspartate (D-Asp) uptake in a Na+-dependent manner, as a result of a reduction in the number of transporters at the plasma membrane. The inhibition of D-Asp uptake by Glu was not mimicked by agonists or modified by antagonists of ionotropic and metabotropic Glu receptors. In contrast, transport was inhibited by GLAST-1 transportable substrates threo-hydroxyaspartate and aspartate-beta-hydroxamate, but not by the nontransportable inhibitors trans-pyrrolidine dicarboxylate or DL-threo-beta-benzyloxyaspartic acid. Under the same experimental conditions, L-Glu did not affect the sodium-dependent transport systems for glycine or GABA. The present results demonstrate that the specific downregulation of glutamate/aspartate transport by L-Glu is unrelated to Glu receptor activation, and results from the internalization of transporter proteins triggered by the transport process itself. Such negative feedback of Glu on Glu transport, could contribute to retinal toxicity under pathological conditions in which high extracellular concentrations of Glu are reached.

Inner retinal neurons display differential responses to N-methyl-D-aspartate receptor activation

The N-methyl-D-aspartate (NMDA) responses of neurons from within the inner rabbit retina were mapped using a channel permeable cation, 1-amino-4-guanidobutane (agmatine, AGB). Serial sections were subsequently probed with immunoglobulins targeting AGB, glutamate, gamma-aminobutyric acid (GABA), and glycine to visualize the NMDA responses of neurochemical subpopulations of neurons. Most inner retinal subpopulations of neurons demonstrated an NMDA concentration-dependent increase in activation. This NMDA-induced activation displayed a distinct pattern, with the most sensitive class to least sensitive class ranking being GC > GABA cAC > GABA/Gly cAC > Gly cAC > GABA dAC (GC, ganglion cells; AC, amacrine cells; c, conventional; d, displaced; Gly, glycine). The variable NMDA response may reflect differences in NMDA receptor subunit disposition or differences in receptor density. In addition to the variable NMDA activation pattern, we found that virtually all ganglion cells (87%) showed NMDA-gated AGB entry, compared with only 58% of amacrine cells. We conclude that a large cohort of amacrine cells do not possess functional NMDA receptors. In addition to most ganglion cells being activated by NMDA, a large subpopulation displayed the highest sensitivity to NMDA application. The functional significance of this finding is that the ganglion cell population will be the first neuronal class to be susceptible to glutamate-induced neurotoxicity mediated through the NMDA receptor. The addition of betaxolol significantly reduced NMDA-mediated AGB entry into most neuronal groups (ganglion cells, GABA, and glycine amacrine cells), with the greatest effect being on ganglion cells. Betaxolol had no significant effect on NMDA-gated entry of AGB on the GABA/Gly amacrine cell population.

Selective and AMPA receptor-dependent astrocyte death following prolonged blockade of glutamate reuptake in rat cerebellar cultures

In this study we examined the effects of prolonged l-trans-pyrrolidine-2,4-dicarboxylate (PDC)-induced glutamate reuptake blockade on the viability of glial cells in cerebellar granule cell cultures. Immunofluorescence staining for the glial-specific intermediate filament protein, GFAP, revealed that the PDC- induced increase of extracellular glutamate concentration was accompanied by increased astrocyte death, while neurons and oligodendrocytes remained intact and viable. Astrocytic cell death was manifested as fragmentation of processes and cell bodies. The selective astrocyte death was completely prevented by the competitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor antagonist, NBQX (10 microM), whereas MK-801 (10 microM), a noncompetitive blocker of N-methyl-D-aspartate receptors, gave only partial protection. Double staining for GFAP and the AMPA receptor subunits GluR2/3 showed that astrocytes had much higher immunoreactivity for GluR2/3 than neurons or oligodendrocytes, suggesting that the number of AMPA receptors is likely to be higher on astrocytes. Furthermore, we employed real-time RT-PCR to measure GluR1-4 subunit mRNA expression in control and PDC-exposed cultures. Following treatment with PDC, GluR1 and GluR4 mRNAs were reduced by 40% and GluR3 was reduced by 70% relative to control levels. In contrast, GluR2 expression was not affected by the PDC treatment, indicating that GluR3 was the dominant type of AMPA receptor subunit expressed on astrocytes. Our results show that astrocytes appear to be more vulnerable than neurons or oligodendrocytes to a gradual increase in the extracellular glutamate concentration, suggesting that astrocytes may be critically involved in the pathophysiology of slowly developing chronic neurodegenerative disorders.

Sodium-dependent glutamate transport in Müller glial cells: regulation by phorbol esters

The regulation of the Na(+)-dependent high affinity glutamate/aspartate transporter system expressed in cultured Müller glia cells from chick retina was studied. Treatment of the cells with the Ca(2+)/diacylglycerol dependent protein kinase C (PKC) activator, phorbol 12-tetradecanoil-13-acetate (TPA) produced a decrease in [(3)H]D-aspartate uptake which was reversed by staurosporine and partially by H7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochoride], two PKC inhibitors. Long-term treatment with TPA resulted in a drastic decrease in the uptake activity, correlated with a substantial fall in the expression of the transporter protein. These findings suggest that PKC is involved in transport modulation at two different levels: phosphorylation and transporter expression in retinal Müller glial cells.

Characterization of glycine transport in cultured Müller glial cells from the retina

Rapid termination of the synaptic action of glutamate (Glu) and glycine (Gly) is achieved by uptake into the presynaptic terminal and glial cells. In the vertebrate CNS, Gly acts both as an inhibitory neurotransmitter and as a Glu modulator or coagonist at postsynaptic N-methyl-D-aspartate (NMDA) receptors. We have previously described NMDA receptors in Müller cells of chick retina coupled to the phosphoinositide cascade, the entry of calcium, and the activation of protein kinase C (PKC; López-Colomé et al. Glia 9:127-135, 1993). A colocalization of Gly transporters and NMDA receptors has been reported in brain tissue (Smith et al. Neuron 8:927-936, 1992); since the concentration of Gly could participate in the modulation of Glu excitatory transmission in the vertical pathways of the retina, transport of Gly in monolayer cultures of Müller cells was studied. Gly transport was found pH-sensitive with an optimum at pH 7.4. Kinetic analysis of the saturation curve for Gly within a concentration range of 0.01-2 mM, revealed two components of transport: a low-affinity system with Km = 1.7 mM, Vmax = 30 nmol/10 min/mg protein, and a high-affinity one with a Km = 27 microM, Vmax = 3 nmol/10 min/mg protein. Both systems were Na+ -dependent; the high-affinity system proved also dependent on external Cl- and was inhibited by sarcosine, characteristic of GLYT1 transporters. The inhibition of low-affinity uptake by 2-(methylamino)isobutyric acid (MeAIB) and 2-aminoisobutyric acid (AIB) suggests the presence of transport system A in Müller cells. The process is energy-requiring, since Gly transport was decreased by metabolic inhibitors. Data obtained are in keeping with a modulatory role for Müller glia on excitatory transmission in the retina.

Mapping glutamatergic drive in the vertebrate retina with a channel-permeant organic cation

Patterns of neuronal excitation in complex populations can be mapped anatomically by activating ionotropic glutamate receptors in the presence of 1-amino-4-guanidobutane (AGB), a channel-permeant guanidinium analogue. Intracellular AGB signals were trapped with conventional glutaraldehyde fixation and were detected by probing registered serial thin sections with anti-AGB and anti-amino acid immunoglobulins, revealing both the accumulated AGB and the characteristic neurochemical signatures of individual cells. In isolated rabbit retina, both glutamate and the ionotropic glutamate receptor agonists alpha-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA), kainic acid (KA), and N-methyl-D-aspartic acid (NMDA) activated permeation of AGB into retinal neurons in dose-dependent and pharmacologically specific modes. Horizontal cells and bipolar cells were dominated by AMPA/KA receptor activation with little or no evidence of NMDA receptor involvement. Strong NMDA activation of AGB permeation was restricted to subsets of the amacrine and ganglion cell populations. Threshold agonist doses for the most responsive cell groups (AMPA, 300 nm; KA, 2 microM; NMDA, 63 microm; glutamate, 1 mM) were similar to values obtained from electrophysiological and neurotransmitter release measures. The threshold for activation of AGB permeation by exogenous glutamate was shifted to <200 microM in the presence of the glutamate transporter antagonist dihydrokainate, indicating substantial spatial buffering of extracellular glutamate levels in vitro. Agonist-activated permeation of AGB into neurons persisted under blockades of Na+ -dependent transporters, voltage-activated Ca2+ and Na+ channels, and ionotropic gamma-aminobutyric acid and glycine receptors. Cholinergic agonists evoked no permeation.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Changes in GluR4 expression induced by metabotropic receptor activation in radial glia cultures

The expression of neurotransmitter receptors in glial cells has suggested a regulatory role of these cells in synaptic function. In radial glia, glutamate receptors elicit a cascade from the membrane to the nucleus and a consequent change in gene expression. In order to gain insight into this process, we address the question of whether receptor activation leads to changes in the repertoire of AMPA/KA glutamate receptor subunits in Bergmann and Müller glial cells. Of the subunits investigated, only GluR4 was up-regulated in Bergmann glial cells both at mRNA and protein levels. In contrast, in Müller glial cells Glu treatment leads to a reduction in GluR4 mRNA and protein expression. Both effects are receptor-mediated and must probably involve group I of metabotropic glutamate receptors. Accordingly, using Northern blot analysis and RT-PCR we detected the expression of both mGluR1 and mGluR5 transcripts in the cultured cells. Our results confirm that glutamate receptors in Bergmann and Müller cells modulate gene expression and further strengthen a plausible role of glial cells in long-lasting changes in the central nervous system.

Activation of p42 mitogen-activated protein kinase by glutamate in cultured radial glia

The effect of L-glutamate (Glu) and its structural analogs N-methyl-D-aspartate (NMDA), kainate (KA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), on the activation of p42 mitogen activated protein kinase (MAPK) was examined in cultured chick radial glia cells, namely retinal Muller cells and cerebellar Bergmann cells. Glu, NMDA, AMPA and KA evoked a dose and time dependent increase in MAPK activity. AMPA and KA responses were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) whereas NMDA responses were sensitive to 3-[(RS)-2-carboxypiperazin-4-yl)]-propyl-1-phosphonate (CPP) indicating that the increase in MAPK activity is mediated by AMPA/low affinity KA and NMDA subtypes of Glu receptors. The present findings open the possibility of a MAPK cascade involvement in the regulation of Glu-induced gene expression in radial glia.

Pharmacological characterization of inositol-1,4,5,-trisphosphate binding to membranes from retina and retinal cultures

Light and excitatory amino acids (EAA) stimulate the phosphoinositide cycle in the vertebrate retina. The regulation of Ca2+ release from intracellular stores by inositol-1,4, 5-trisphosphate (IP3) involves an interaction of this compound with specific receptors. By means of [3H]IP3-specific binding, we studied the kinetic and pharmacological properties of IP3 receptors in the chick retina as well as in primary cultures of neurons and glia from this tissue. The equilibrium time for the binding reaction was 15 min and was optimal at alkaline pH (8.3). IP3 receptor displayed high affinity (K(B) approximately 40 nM) and selectivity for D-IP3, compared to D-IP4 > L-IP3 > D-IP2 > D-IP1. These characteristics were the same in subcellular fractions from outer (P1) and thinner (P2) plexiform layers, binding sites being more abundant in P2 (2.65 pmol/mg protein). IP3 receptors were present in both neuronal and glial cultures, but were concentrated in neuronal cultures. Binding was not affected by ryanodine, or caffeine, related to calcium-induced calcium release (CICR) channels, nor by the endoplasmic reticulum Ca2+ ATPase inhibitor thapsigargin, while heparin affectively inhibited IP3 binding. GSSG and thimerosal increased the affinity of [3H]IP3 binding from IC50 approximately 80 nM to IC50 approximately 40 nM; this effect was reversed by DTT. Binding in zero Ca2+ was decreased by low concentrations of Ca2+ (350 nM). These results suggest that actions of IP3 in the retina are regulated by physiological changes in intracellular pH and Ca2+ concentrations, as well as by the oxidation state of the receptor. Additionally, the presence of IP3 receptors in Müller glia opens the possibility of IP3 participation in nonsynaptic signalling through Ca2+ waves in glial cells.