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

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.

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-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.

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.

Neurotransmitter release from the medial hyperstriatum ventrale of the chick forebrain accompanying filial imprinting behavior, measured by in vivo microdialysis

The imprinting behavior of chicks was quantified as a preference score (correct response ratio) achieved in a running wheel apparatus. A total of 249 chicks were exposed to an imprinting stimulus and tested for stimulus-approaching behavior. The chicks were then classified as good learners (imprinted), poor learners (non-imprinted) and a gray-zone group, those were 46%, 31% and 23% of the total chicks respectively. Using the classified chicks, the acetylcholine (ACh) and glutamate releases from the medial hyperstriatum ventrale (MHV) of the chick forebrains were determined by in vivo microdialysis. The non-imprinted chicks were used as yoked controls. Increases of ACh and glutamate released were observed in the imprinted chicks during exposure to the imprinting stimulus, whereas there were no changes in the release of these neurotransmitters in the non-imprinted chicks during the imprinting exposure. These results might be indicated that cholinergic and glutamatergic synapses which are newly formed as functioning synapses with imprinting stimulus in the MHV are involved in the performance of imprinting behavior.

Pharmacological characterization, anatomical distribution and sex differences of the non-NMDA excitatory amino acid receptors in the quail brain as identified by CNQX binding

The distribution of non-N-methyl-D-aspartate binding sites was studied in coronal and sagittal sections through the brain of adult Japanese quail by quantitative autoradiography, using tritiated 6-cyano-7-nitroquinoxaline-2,3-dione as a radioligand. Saturation binding experiments were, in addition, carried out in areas showing high levels of binding (cerebellar molecular layer, nucleus anterior medialis and nucleus infundibularis) and demonstrated that the binding of tritiated ligand was specific and saturable. Competition studies with alpha-amino-3-hydroxy-methyl-4-isoxazole propionic acid and kainic acid indicated that kainic acid strongly inhibited ligand binding in all brain areas. alpha-Amino-3-hydroxy-methyl-4-isoxazole propionic acid was only a weak inhibitor in the hypothalamic nuclei whereas in the cerebellar molecular layer both high and low affinity inhibitions were detected. The highest binding levels of tritiated ligand were observed in the molecular layer of the cerebellum. Very high levels of binding were detected in various preoptic/hypothalamic sites including the nucleus suprachiasmaticus pars medialis, nucleus anterior medialis hypothalami, nucleus infundibularis, nucleus mammillaris medialis, nucleus posteromediale hypothalami and nucleus hypothalami ventromedialis. High levels of binding were also detected in the bulbus olfactorius, bed nucleus commissuralis anterior, bed nucleus commissuralis pallii, nucleus accumbens, bed nucleus striae terminalis and nucleus interpeduncularis. In the preoptic area/hypothalamus, high levels of binding were clearly present in all areas that contain gonadotropin releasing hormone cells or fibers. In the pons and mesencephalon, moderate levels of binding were associated with catecholaminergic areas such as the area ventralis tegmentalis (area ventralis of Tsai) and the locus coeruleus. Saturation analysis demonstrated the presence of a higher number of binding sites in females than in males in the cerebellar molecular layer, nucleus infundibularis and nucleus anterior medialis. This latter difference was confirmed in the one point assays that also identified higher levels of specific binding in the nucleus suprachiasmaticus pars medialis of males as compared with females. These anatomical data suggest a possible implication of non-N-methyl-D-aspartate receptors in the synthesis and/or release of both gonadotropin releasing hormone and catecholaminergic neurotransmitters that should now be tested by pharmacological experiments.

Structure and pharmacological properties of a molluscan glutamate-gated cation channel and its likely role in feeding behavior

We describe the isolation of a molluscan (Lymnaea stagnalis) full-length complementary DNA that encodes a mature polypeptide (which we have named Lym-eGluR2) with a predicted molecular weight of 105 kDa that exhibits 44-48% identity to the mammalian kainate-selective glutamate receptor GluR5, GluR6, and GluR7 subunits. Injection of in vitro-transcribed RNA from this clone into Xenopus laevis oocytes results in the robust expression of homo-oligomeric cation channels that can be gated by L-glutamate (EC50 = 1.2 +/- 0.3 micron) and several other glutamate receptor agonists; rank order of potency: glutamate >> kainate > ibotenate > AMPA. These currents can be blocked by the mammalian non-NMDA receptor antagonists 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitroquinoxaline-2,3-dione, and 1-(4-chlorobenzoyl)piperazine-2,3-dicarboxylic acid. Ionic-replacement experiments have shown that the agonist-induced current is carried entirely by sodium and potassium ions. In situ hybridization has revealed that the Lym-eGluR2 transcript is present in all 11 ganglia of the Lymnaea CNS, including the 4-cluster motorneurons within the paired buccal ganglia. The pharmacological properties and deduced location of Lym-eGluR2 are entirely consistent with it being (a component of) the receptor, which has been identified previously on buccal motorneurons, that mediates the excitatory effects of glutamate released from neurons within the feeding central pattern generator.

CDNA cloning of chick brain alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors reveals conservation of structure, function and post-transcriptional processes with mammalian receptors

Several types of functional ionotropic glutamate receptor have been cloned in the recent years from the mammalian central nervous system, but till now, none from other vertebrate species. Here, we report the cloning and functional analysis of four chick brain cDNAs, coding for members of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtype of glutamate receptors. These receptors are highly homologous to the mammalian GluR1-4 (A-D) receptors ( > 90%), and conserve their post-transcriptional modifications. The flip/flop exons are conserved not only at the amino acid level but also at the nucleotide level, and the intron of GluR4 involved in the RNA editing of the R/G site displays a rat-chick sequence conservation of 95%. Significant sequence differences are found only in the region containing the immunogenic epitope of neuroactive anti-GluR3 antibodies. Chick AMPA receptors are expressed in both the cerebrum and cerebellum. The ion channel activities of chick GluR1-4 were analyzed in Xenopus oocytes and found to be similar to those of mammalian AMPA receptors. Though their contribution to kainate binding activity in the cerebellum is minor, the profile of channel activity of the chick GluR1-4 suggests that they account for the kainatergic channel activity expressed by total chick cerebellar mRNAs.

Unraveling the modular design of glutamate-gated ion channels

Glutamate receptors that function as ligand-gated ion channels are essential components of cell-cell communication in the nervous system. Despite a wealth of information concerning these receptors, details of their structure are just beginning to emerge. We propose that glutamate receptors comprise four modules: two modules that are related to bacterial periplasmic-binding proteins, one module that is related to the pore-forming region of K+ channels, and one regulatory module of unknown origin. A K(+)-channel-like domain inserted into a crucial region of a periplasmic-binding protein-like domain suggests a mechanism for transduction of binding energy to channel opening. This modular design also suggests an evolutionary link between a ligand-gated ion-channel family and voltage-gated ion channels.

A topological analysis of goldfish kainate receptors predicts three transmembrane segments

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in vertebrate brain. We have previously cloned cDNAs encoding two homologous kainate receptors (GFKAR alpha, 45 kDa, and GFKAR beta, 41 kDa) from goldfish brain and proposed a topology with three transmembrane domains (Wo, Z. G., and Oswald, R. E. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 7154-7158). These studies have been extended using an in vitro translation/translocation system in conjunction with site-specific antibodies and point and deletion mutations. We report here that the entire region between the previously proposed third and fourth transmembrane segments is translocated and likely to be extracellular in mature receptors. This was based on the following results. 1) The entire segment was protected from Proteinase K and trypsin digestion and could be immunoprecipitated by a site-specific antibody. 2) Functional sites for N-glycosylation are present in the C-terminal half of the segment, and 3) a mutation, constructed with an additional consensus site for N-glycosylation in the N-terminal half of the segment, was found to be glycosylated at that site. Given the fact that the N terminus of the protein is likely to be extracellular, this would place an even number of transmembrane segments between the extracellular N terminus and the glycosylated segment. In addition, results of N-glycosylation and proteolysis protection assays of GFKAR alpha mutations indicated that the previously proposed second transmembrane segment is not a true transmembrane domain. These results provide further evidence in support of a topology with three transmembrane domains that has important implications for the relationship of structure to function in ionotropic glutamate receptors.

Transmembrane topology of two kainate receptor subunits revealed by N-glycosylation

Glutamate receptors are the primary excitatory neurotransmitter receptors in vertebrate brain and are of critical importance to a wide variety of neurological processes. Recent reports suggest that ionotropic glutamate receptors may have a unique transmembrane topology not shared by other ligand-gated ion channels. We report here the cloning of cDNAs from goldfish brain encoding two homologous kainate receptors with protein molecular masses of 41 kDa. Using a cell-free translation/translocation system, we show that (i) a portion of these receptors previously thought to be a large intracellular loop is actually located extracellularly and (ii) the putative second transmembrane region of the receptor thought to line the ion channel may not be a true membrane-spanning domain. An alternative model for the transmembrane topology of kainate receptors is proposed that could potentially serve as a framework for future detailed study of the structure of this important class of neurotransmitter receptors.

Kainate-binding proteins: phylogeny, structures and possible functions

Recent advances have demonstrated that the family of [3H]kainate-binding proteins and kainate receptors comprise a number of related polypeptides. In all the cases so far investigated, the kainate-binding proteins from non-mammalian vertebrates have M(r) values in the range of 40-50 kDa whereas mammalian kainate receptors and kainate-binding proteins have M(r) values in the order of 100 kDa. There have not, as yet, been any reports of 40-50 kDa kainate-binding proteins in mammalian CNS and, despite the cloning of increasing numbers of cDNAs encoding new kainate-binding proteins, the relationships between these two general groups of polypeptides remain unclear. Nonetheless, there is now a wealth of phylogenetic, structural and molecular biological data available about these proteins. In this review, Jeremy Henley outlines the properties and structures of kainate-binding proteins and offers some possibilities as to the roles of these often hugely abundant proteins.