Structural characterization and expression of a brain specific gene encoding chick kainate binding protein

Kainate-binding proteins are rendered functional ion channels upon transplantation of two short pore-flanking domains from a kainate receptor

Kainate-binding proteins belong to an elusive class of putative ionotropic glutamate receptors that to date have not been shown to form functional ion channels in heterologous expression systems, despite binding glutamatergic agonists with high affinity. To test the hypothesis that inefficient or interrupted signal transduction from the ligand-binding site via linker domains to the ion pore (gating) might be responsible for this apparent lack of function, we transplanted the short homologous linker sequences from the fully functional rat kainate receptor GluR6 into frog kainate-binding protein. We were able to generate chimeric receptors that are functional in the Xenopus oocyte expression system and in human embryonic kidney 293 cells. The linker domains A and B in particular appear to be crucial for gating, because a functional kainate-binding protein was observed when at least parts of both linkers were derived from GluR6. We speculate that to enable signal transduction from the ligand-binding site to the ion pore of the frog kainate-binding protein, the linker structure of the protein has to undergo an essential conformational alteration, possibly mediated by an as yet unknown subunit or modulatory protein.

Glutamate regulates kainate-binding protein expression in cultured chick Bergmann glia through an activator protein-1 binding site

The expression of the chick kainate-binding protein, a member of the ionotropic glutamate receptor family, is restricted to the cerebellum, specifically to Bergmann glia. Glutamate induces a membrane to nuclei signaling involved in gene expression regulation. Exposure of cultured chick Bergmann glia cells to glutamate leads to an increase in kainate binding protein and mRNA levels, suggesting a transcriptional level of regulation. The 5' proximal region of the chick kainate binding gene was cloned and transfected 4into Bergmann glia cells. Three main regulatory regions could be defined, a minimal promoter region, a negative regulatory region, and interestingly, a glutamate-responsive element. Deletion of this element abolishes the agonist effect. Moreover, electrophoretic mobility shift assays, cotransfection experiments, and site-directed mutagenesis clearly suggest that the glutamate effect is mediated through an AP-1 site by a Fos/Jun heterodimer. The present results favor the notion of a functional role of kainate-binding protein in glutamatergic cerebellar neurotransmission.

Kainate binding proteins possess functional ion channel domains

Kainate binding proteins (KBPs) are highly homologous to ionotropic glutamate receptors; however, no ion channel function has been demonstrated for these proteins. To investigate possible reasons for the apparent lack of ion channel function we transplanted the ion channel domains of five KBPs into glutamate receptors GluR 6 and GluR1. In each case we obtained functional chimeric receptors in which glutamatergic agonists were able to open the KBP-derived ion channel with EC50 values identical to those of the subunit contributing the ligand binding domain. Maximal current amplitudes were significantly smaller than those of the parent clones, however. We also show that the KBP ion channels are highly permeable for calcium and have certain pharmacological properties that are distinct from all other glutamate receptor (GluR) subunits. Thus, all five known KBPs, in addition to their well characterized functional ligand binding sites, have functional ion permeation pathways. Our data suggest that the lack of ion channel function in wild-type KBPs results from a failure to translate ligand binding into channel opening. We interpret our findings to indicate the requirement for a modulatory protein or an additional subunit serving to alter the structure of the KBP subunit complex such that signal transduction is enabled from the ligand binding site to the intrinsically functional ion pore.

Distinct distributions of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits and a related 53,000 M(R) antigen (GR53) in brain tissue

Polyclonal antibodies against specific carboxy-terminal sequences of known alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits (GluR-4) were used to screen regional homogenates and subcellular fractions from rat brain. Affinity purified anti-GluR1 (against amino acids 877-899), anti-GluR2/3 (850-862), and anti-GluR4a and anti-GluR4b (868-881) labeled distinct subunits with the expected molecular weight of approximately 105,000. These antigens were shown to have distinct distributions in the brain. While GluR2/3 epitopes had a distribution profile similar to that of the presynaptic marker synaptophysin, GluR1 was notable for its abundance in the hippocampus and its relatively low density in neocortical areas, and GluR4 was highly enriched in cerebellar tissue. An additional antigen (glutamate receptor-related, GR53) of lower molecular weight (50,000-59,000) was recognized in rat, human, frog, chick and goldfish brain samples by anti-GluR4a as well as by anti-GluR1 at, an antibody that specifically recognizes the extracellular aminoterminal domain of GluR1 (amino acids 163-188). Both antibodies also labeled antigens of approximately 105,000 mol. wt in brain tissue from all species tested. The approximately 53,000 mol. wt antigen was concentrated 10-20-fold in synaptic membranes vs homogenates across rat brain regions. Both the 105,000 and the 53,000 mol. wt proteins were also concentrated in postsynaptic densities, and neither of the two antigens were evident in seven non-brain tissue samples. These data indicate that AMPA receptors have regionally different subunit combinations and that some AMPA receptor composites include proteins other than the conventional 105,000 mol. wt GluR subunits.

Expression of NMDA 2B receptor subunit mRNA in Bergmann glia

N-methyl-D-aspartate (NMDA)-activated glutamate receptor subunits are invariably expressed in neurons, although NMDA-activated currents have been recently described in Bergmann glia. To date, the NMDA receptor subunit 2B (NMDAR2B) was thought not to be expressed in adult cerebellum. In the present study we provide evidence, from in situ hybridization histochemistry, that Bergmann glial cells in rat brain express mRNA encoding the NMDAR2B subunit, most probably co-expressed with the ubiquitous NMDAR1 subunit, while transcripts for other NMDAR2 subunits (A,C,D) were either not resolved or detected. Our findings suggest that Bergmann glial cells contain the molecular machinary to synthesize the NMDA receptor 2B subunit. The role of physiological NMDA receptors in the interaction between Bergmann glia and Purkinje neurons is not yet known.

Autoradiographic characterization of [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding sites in adult chick brain

The non-N-methyl-D-aspartate binding sites have been characterized in chick brain using quantitative autoradiography, and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) as a radioligand. [3H]CNQX binding sites were densely localized in the molecular layer of cerebellum. Other areas of prominent binding were the superficial layers of optic tectum, one of the isthmic nuclei, the hippocampus, the hyperstriatum accessorium and the archistriatum ventrale. Analysis of equilibrium binding data in the cerebellar molecular layer indicated that [3H]CNQX bound to a single class of sites (Kd = 78.9 +/- 11.8 nM and Bmax = 41.2 +/- 3.0 pmol/mg protein). Competition experiments in six different regions of chick brain gave the Ki and Bmax values for the inhibition of [3H]CNQX binding by various standard compounds and indicated that: (i) [3H]CNQX labelled non-N-methyl-D-aspartate binding sites with high affinity of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate, (ii) the displacement curves for AMPA and kainate were biphasic in all regions studied, and (iii) the potencies of AMPA and kainate in displacing [3H]CNQX binding were different in the regions studied. Our results indicate that [3H]CNQX labelled non-N-methyl-D-aspartate binding sites in chick brain. In the cerebellar molecular layer, these sites were more sensitive to kainate than AMPA, as were the binding sites in the superficial layers of optic tectum and nucleus isthmi magnocellularis. However, non-N-methyl-D-aspartate binding sites in forebrain regions such as hippocampus and hyperstriatum ventrale appeared to be different in being equally sensitive to AMPA and kainate.

The segregation and expression of glutamate receptor subunits in cultured hippocampal neurons

The distribution and expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-selective glutamate receptor subunits (GluR1-4) were studied in cultured hippocampal neurons using antibodies generated against peptides corresponding to the C-termini of GluR1, GluR2/3 and GluR4, and with a set of oligonucleotide probes designed complementary to specific pan, flip and flop GluR1-4 messenger RNA sequences. GluR1-4 subunit proteins were localized in fixed hippocampal neurons (2 h to three weeks after plating) by immunocytochemistry with light and electron microscopy. At early stages in culture, moderate staining with antibodies to GluR1 and GluR2/3 and very light staining with antibody to GluR4 was observed in cell bodies and proximal portions of all neurites of some neurons. Upon establishment of identified axons and dendrites by seven days in culture, staining was intense with specific antibodies to GluR1 and GluR2/3 and light with anti-GluR4 antibody in cell bodies and dendrites. Little or no staining was observed in axons. Cells at seven days in culture exhibited a variety of morphologies. However, we could not assign a pattern of staining to a particular type. As the cultures matured over two and three weeks, staining was limited to the somatodendritic compartment. The intensity of glutamate receptor subunit staining increased and the extent of staining proceeded to the distal extreme of many dendrites. Moreover, antibodies to GluR1-4 subunits were co-localized in neurons. Immunocytochemistry on living neurons did not result in any significant labeling, suggesting that the epitope is either not expressed on the surface of the neurons, or is present, but inaccessible to the antibody. Electron microscopy demonstrated receptor localization similar to that found in brain, with staining of postsynaptic membrane and density, dendritic cytoplasm and cell body, but not within the synaptic cleft. We examined the possible role of "cellular compartmentation" in the pattern of glutamate receptor expression in hippocampal neurons. Compartmentalization studies of the subcellular distribution of messenger RNAs encoding GluR1-4 subunits was determined in mature cultures by in situ hybridization. Significant silver grain appearance was restricted to the cell body, indicating that the synthesis of glutamate receptor subunits is limited largely to the neuronal cell body. The expression of microtubule-associated protein 2 was studied in parallel. Microtubule-associated protein 2 expression appeared 6 h after plating, while glutamate receptor subunit expression was present at 2 h. This indicates that microtubule-associated protein 2 does not regulate the initial distribution of glutamate receptor subunits into neurites.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cloning of the kainate-binding protein and calmodulin genes which are induced by an imprinting stimulus in ducklings

For the formation of imprinting in birds, protein synthesis is known to be essential in the medial hyperstriatum ventrale (MHV) of the forebrain after presentation of an imprinting stimulus. We have searched for the genes whose expressions are increased in duckling's MHV during formation of imprinting, and identified kainate-binding protein and calmodulin genes. This may reflect the formation of glutamatergic pathways in MHV.

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.