Phosphorylation of the 49-kDa putative subunit of the chick cerebellar kainate receptor and its regulation by kainatergic ligands

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

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

Phosphorylation of GluR4 AMPA-type glutamate receptor subunit by protein kinase C in cultured retina amacrine neurons

We have previously reported that the activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors is potentiated by protein kinase C (PKC) in cultured chick retina amacrine neurons, and that constitutive PKC activity is necessary for basal AMPA receptor activity (Carvalho et al., 1998). In this study, we evaluated the phosphorylation of the GluR4 subunit, which is very abundant in cultured amacrine neurons, to correlate it with the effects of PKC on AMPA receptor activity in these cells. 32P-labelling of GluR4 increased upon AMPA receptor stimulation or cell treatment with phorbol 12-myristate 13-acetate (PMA) before stimulating with kainate. By contrast, phosphorylation of GluR4 was not changed when PKC was inhibited by treating the cells with the selective PKC inhibitor GF 109203X before stimulation with kainate. We conclude that GluR4 is phosphorylated upon PKC activation and/or stimulation of AMPA receptors in cultured amacrine cells. Additionally, AMPA receptor activation with kainate in cultured chick amacrine cells leads to translocation of conventional and novel PKC isoforms to the cell membrane, suggesting that PKC could be activated upon AMPA receptor stimulation in these cells.

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

The effect of L-glutamate (L-Glu) and its structural analogs N-methyl-D-aspartate (NMDA), quisqualate (QA), and kainate (KA) on the DNA binding activity of the Activator Protein 1 (AP-1) and the Ca2+/cAMP Responsive Element Binding Protein (CREB) families of transcription factors was examined in cultured chick retinal Müller glia cells. L-Glu, NMDA, and KA evoked a dose and time dependent increase in AP-1 DNA binding activity and had no effect on CREB binding. The order of potency for stimulating AP-1 DNA binding was NMDA > or = Glu > KA >> QA. L-Glu responses were partially blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by 3-[RS)-2-carboxypiperazin-4-yl)]-propyl-1-phosphonate (CPP) indicating that the increase in DNA binding is mediated both by an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/low affinity KA and a NMDA subtypes of L-Glu receptors. Since Müller glia L-Glu receptors are probably mediators of the efficacy of the excitatory transmission in the retina, the present findings suggest that a stimulus-transcription coupling triggered by L-Glu in the glial cells might have a role in the long-term modulation of these synapses.

High-affinity kainate binding sites in living slices of rat neocortex: characterization and regulation

We have characterized a high-affinity kainate binding site in in vitro living rat neocortical slices using [3H]kainate. [3H]Kainate labelled at least two binding sites, the higher affinity site with a Kd of 7.1 nM and a Bmax of 71.2 fmol/mg protein. This high-affinity binding site showed a pharmacology consistent with a kainate receptor with competition by kainate and domoic acid, as well as the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline. Increases in cellular depolarization induced by 2-h preincubations in veratridine and glutamate led to a significant 55% average decrease in [3H]kainate binding in adult cortex. Similarly, preincubation in kainate led to a significant average 26% decrease in binding. In both instances, Eadie-Hofstee analysis of saturation binding data revealed that the decreased binding reflected changes in receptor number. At different postnatal ages, increases in cellular depolarization significantly decreased binding (< 20 days postnatal age, -86%; > 60 days, -48%). Kainate treatment also significantly decreased binding at all ages (-64% at < 20 days; > 60 days, -18%), with significant differences noted between ages. These age-dependent effects are unlike those previously described for either N-methyl-D-aspartate [Lanius and Shaw (1992) Anat. Rec. 232, 54(A)] or (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate high affinity receptors [Shaw and Lanius (1992) Devl Brain Res. 68, 225-233].(ABSTRACT TRUNCATED AT 250 WORDS)

Glutamate stimulates [3H]phorbol 12,13-dibutyrate binding in cultured Bergmann glia cells

The effect of L-glutamate and its structural analog kainate on the binding of [3H]phorbol 12,13-dibutyrate was examined in cultured chick cerebellar Bergmann glia cells. Both glutamate and kainate evoke a dose-dependent increase in the maximal number of binding sites for [3H]phorbol 12,13-dibutyrate in intact cells reflecting an activation and translocation of the Ca2+/diacylglycerol-dependent protein kinase (protein kinase C, PKC) from cytosol to the plasma membrane. Glutamate and kainate responses were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) indicating that the increase in [3H]phorbol 12,13-dibutyrate binding sites is mediated by an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor. Since Bergmann glia AMPA/kainate receptors are probably mediators of the efficacy of the parallel fiber-Purkinje cell synapse, the present findings suggest that the Ca2+/PKC signalling cascade might play a role in such modulation.

Reversible kinase and phosphatase regulation of brain amino acid receptors in postnatal development

In a previous study [Shaw, C., Pasqualotto, B. and Lanius, R.A., Mol. Neuropharmacol., in press] we have shown that phosphorylation and dephosphorylation actions of protein kinase and alkaline phosphatase lead to decreases or increases in the number of GABAA and AMPA receptors in adult rat neocortex. Using the same in vitro cortical slice preparation, we have now examined the role of these enzymes in regulating GABAA and AMPA receptors at different stages of postnatal development. GABAA receptors were labelled with [3H]SR95531 [Shaw, C. and Scarth, B.A., Mol. Brain Res., 11 (1991) 273-282]; AMPA receptors were labelled with [3H]CNQX [Lanius, R.A. and Shaw, C., Mol. Brain Res., 15 (1992) 256-262]. At postnatal day 14, GABAA receptors showed a decrease in binding in response to alkaline phosphatase treatment as opposed to an increase in binding observed in response to protein kinase treatment. Similar effects were observed for AMPA receptors at 20 days of age. The direction of regulation following the enzyme treatments were opposite to those observed in the adult cortex for both receptor populations. These fundamental changes in the enzymatic nature of regulation for such key inhibitory and excitatory receptor populations in cortex may signal an important role for age-dependent kinases and phosphatases in the events leading to modifications in neuronal function during postnatal development.

Organization and expression of the gene encoding chick kainate binding protein, a member of the glutamate receptor family

The gene encoding chick cerebellar Bergmann glia-specific kainate binding protein (chKBP), has been isolated, characterized and expressed in heterologous systems. The structural gene spans 11.2 kb and contains 11 exons and 10 introns. Several of the exons encode specific receptor domains, including each of the predicted transmembrane regions. Exon/intron boundaries flanking the second, putative channel-forming transmembrane domain are conserved between chKBP and other glutamate/kainate receptor subunits. The putative promoter region 5' to the first exon displays high GC content and TATA, CAAT and AP1 consensus sequences. Transcription of the chKBP gene is evident prior to full cerebellar cortical maturation. Transcripts are abundant in cells consistent with Bergmann glia, as revealed by in situ hybridization. Transfection of 293 kidney cell cultures with chKBP cDNA or chKBP gene expression constructs confers CNQX-sensitive kainate binding with the pharmacological specificity displayed by both chKBP and kainate receptors. However, expression of the same constructs in Xenopus oocytes fails to yield detectable agonist-activated currents.

Glycinergic ligands modulate the rate of phosphorylation of the glycine receptor by protein kinase C

The alpha subunit of the glycine receptor purified from rat spinal cord is rapidly and specifically phosphorylated by protein kinase C (Ruiz-Gómez et al., (1991) J. Biol. Chem. 266, 559-566). We report here that the rate of phosphorylation of the glycine receptor by this kinase is higher in the presence of agonists (glycine, beta-alanine) than in the presence of antagonists (strychnine, RU-5135). These results suggest that activated glycine receptors would be a preferential target for functional regulation through phosphorylation mechanisms.

Cortical AMPA receptors: age-dependent regulation by cellular depolarization and agonist stimulation

We have recently shown that a high-affinity AMPA receptor labelled with the antagonist [3H]CNQX can be regulated in a 'living' cortical slice preparation by agonist stimulation or changes in electrical activity (Lanius, R.A. and Shaw, C. (1992) Anat. Rec., in press). Based on a study of GABAA receptors (Shaw, C. and Scarth, B.A. (1992) Mol. Brain Res., in press), which showed age-dependent changes in regulation, we have now investigated the regulation of high-affinity AMPA receptors in neocortex at different stages in postnatal development. The results show that regulation by agonist stimulation and increases in bioelectric activity are age-dependent in amount and, in the latter case, in direction. Agonist stimulation using quisqualate resulted in a significant receptor down-regulation of approximately 7% at ages less than 20 days postnatal; in adult rats quisqualate led to a significant 23% decrease. Changes in bioelectric activity induced by a combination of veratridine and glutamate showed a significant increase in AMPA receptor number of 16% at ages less than 20 days, whereas such treatment resulted in a significant 18% decrease in adult rats. The present data reveal a near mirror-image to the effects of veratridine and glutamate and agonist on GABAA receptors in the same preparation, but with a temporal mismatch in the amount and direction of regulation. We speculate that the age-dependent differences in direction of regulation for the receptor populations which serve key excitatory and inhibitory functions in cortex may provide a molecular basis for the gradual decline of neuronal plasticity during the critical period.

Properties of kainate receptor/channels on cultured Bergmann glia

Following the localization, at the electron microscope level, of the immunoreactivity towards a putative kainate receptor on Bergmann glial cells in the chick cerebellar cortex, cultures of Bergmann glia were used to establish the presence of functional kainate receptor/channels and study their properties. Bergmann glia were identified by their fusiform morphology and their ability to bind an anti-kainate binding protein monoclonal antibody, a kainate receptor high affinity ligand--kainyl-bovine serum albumin--and a glial marker--anti-vimentin monoclonal antibody. Membranes prepared from the culture cells displayed, using 25 nM [3H]kainate, the binding of 4.1 pmol of [3H]kainate/mg protein and showed the presence in Western blots of the two polypeptides of 49 and 93 kDa attributed to the kainate binding protein. Kainate, at concentrations above 0.1 mM, was found to increase the influx into cultured Bergmann glia of 22Na+, 86Rb+, 45Ca2+ and 36Cl- ions. The traffic of 22Na+, induced by kainate and glutamate, observed only in the presence of 1 mM ouabain, was blocked by kainate receptor antagonists and by 0.01 mM quisqualate. Analysis of the kinetics of incorporation of 22Na+ and 45Ca2+ ions showed an initial accumulation of 22Na+ and 45Ca2+ ions followed by their total dissipation. The results indicate that the kainate-induced influx of Na+ ions through the kainate receptor/channel causes the reverse transport of Na+ ions, by activation of the Na+/Ca2+ and Na+/H+ exchangers which remove intracellular Na+ ions. Pre-exposure of the cells to 0.5 mM dibutyryl cAMP was found to greatly enhance the kainate-induced 22Na+ ion influx. We propose that the Bergmann glia kainate receptors modulate the efficacy of the glutamatergic synapses between the parallel fibers and Purkinje cell spines and form part of a glial machinery responsible for plastic changes in synaptic transmission.