The interaction of a kainate receptor from goldfish brain with a pertussis toxin-sensitive GTP-binding protein

Kainate receptors with a metabotropic modus operandi

Kainate receptors (KARs), together with AMPA and NMDA, are typically described as ionotropic glutamate receptors. The functions of KARs have begun to be elucidated only in the last decade. Although some the actions of KARs are classically ionotropic, surprisingly others seem to involve the activation of second-messenger cascades and invoke metabotropic roles for this type of glutamate receptor. In this review, we describe these metabotropic actions of KARs in relation to the putative signalling cascades involved. Although it is still a mystery how KARs activate G proteins to stimulate second-messenger cascades, intriguingly, in very recent studies, specific subunits of KARs have been demonstrated to associate with G proteins. Altogether, the body of evidence supports the hypothesis that, together with the canonical ionotropic operation, KARs expedite long-lasting signalling by novel metabotropic modes of action.

Metabotropic actions of kainate receptors in the CNS

Kainate receptors (KARs), together with NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA), are typically described as ionotropic glutamate receptors. Although ionotropic functions for KARs are beginning to be characterized in multiple brain regions, both, in the pre- and post-synaptic compartments of the synapse, there is accumulating evidence that KARs mediate some of their effects without invoking ion-fluxes. Thus, since 1998, when the first metabotropic action of KARs was described in the modulation of GABA release in hippocampal interneurons, there have been increasing reports that some of the functions of KARs involve the participation of intracellular signalling cascades and depend on G protein activation. These surprising observations, attesting metabotropic actions of KARs, akin to those usually attributed to seven transmembrane region G protein-coupled receptors, make the physiological classification and description of glutamate receptors more complex. In the present review, we describe the metabotropic roles of KARs in the CNS and discuss the intriguing properties of this receptor which, structurally shows all the facets of a typical ionotropic receptor, but appears to express a metabotropic remit at some key synapses.

Factors affecting guanine nucleotide binding to rat AMPA receptors

Glutamate receptors are competitively inhibited by guanine nucleotides. Insight into the physiological function of this inhibition would be greatly advanced if nucleotide binding could be eliminated through mutations without altering other aspects of receptor function, or if compounds were discovered that selectively prevent nucleotide binding. It was previously reported that a lysine in the chick kainate binding protein (cKBP) is specifically involved in guanine nucleotide binding. In the present study we mutated the equivalent lysine in the rat AMPA receptor subunit GluR1 flip to alanine (K445A) and assessed changes in nucleotide affinity from the displacement of [(3)H]fluorowillardiine. As in the cKBP, the affinity for nucleotides was greatly reduced while the binding affinity for agonists remained unchanged. The reduction in affinity was largest for GTP (factor of 5.8) and GDP (4.4) and minor for GMP and guanosine. This suggests that K445 is involved in stabilizing the second phosphate of the nucleotide. Given that bulkier analogs like GDP-fucose are also accommodated at this site, it seems likely that nucleotides bind in such a way that their phosphates project out of the cleft. In excised-patch recordings using short pulses of glutamate, the K445A mutation increased the EC(50) for the peak response 1.8-fold and accelerated desensitization and deactivation. This indicates that the effects of this mutation are not as specific as previously suggested. Efforts to selectively eliminate inhibition by nucleotides may therefore depend on mapping out further the docking site. In a first attempt using point mutations we ruled out several amino acids around the cleft as being involved in nucleotide binding. Also, the AMPA receptor modulator PPNDS which competitively inhibits nucleotide binding to purinergic receptors did not affect nucleotide inhibition, suggesting that there are major differences in the topography between purinergic and glutamate receptors. Thus new approaches, including crystallography, may be called for to identify residues uniquely involved in nucleotide binding.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

G protein coupling to M1 and M3 muscarinic receptors in sublingual glands

Rat sublingual gland M2 and M3 muscarinic receptors each directly activate exocrine secretion. To investigate the functional role of coreceptor expression, we determined receptor-G protein coupling. Although membrane proteins of 40 and 41 kDa are ADP-ribosylated by pertussis toxin (PTX), and 44 kDa proteins by cholera toxin (CTX), both carbachol-stimulated high-affinity GTPase activity and the GTP-induced shift in agonist binding are insensitive to CTX or PTX. Carbachol enhances photoaffinity labeling ([alpha-(32)P]GTP-azidoaniline) of only 42-kDa proteins that are subsequently tractable to immunoprecipitation by antibodies specific for Galpha(q) or Galpha(11) but not Galpha(12) or Galpha(13). Carbachol-stimulated photoaffinity labeling as well as phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis is reduced 55% and 60%, respectively, by M1 receptor blockade with m1-toxin. Galpha(q/11)-specific antibody blocks carbachol-stimulated PIP2 hydrolysis. We also provide estimates of the molar ratios of receptors to Galpha(q) and Galpha(11). Although simultaneous activation of M1 and M3 receptors is required for a maximal response, both receptor subtypes are coupled to Galpha(q) and Galpha(11) to stimulate exocrine secretion via redundant mechanisms.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

Characterisation of the interaction between guanyl nucleotides and AMPA receptors in rat brain

Guanyl nucleotides inhibit the binding of the AMPA receptor agonists [3H]fluorowillardiine and [3H]AMPA and the competitive antagonist [3H]CNQX to rat brain cerebrocortical membranes. The rank order of inhibition for each of the radioligands tested was GTP > GDP > GMP. The nucleotides CTP and ATP showed no effect. GTP inhibition was unaffected by the presence or absence of NaCl and MgCl2. Pre-treatment of the membranes with GTP, and its removal before addition of radioligand, did not inhibit binding. Quantitative autoradiography demonstrated that GTP inhibition occurred throughout the brain. These results are consistent with guanyl nucleotides acting at an extracellular site present on all AMPA receptor subunits, occupation of which inhibits agonist and antagonist binding.

Asn-265 of frog kainate binding protein is a functional glycosylation site: implications for the transmembrane topology of glutamate receptors

Kainate binding proteins (KBPs) from frog and goldfish brain are glycosylated, integral membrane proteins. These KBPs are homologous (35-40%) to the C-terminal half of AMPA and kainate receptors which have been shown to form glutamate-gated ion channels. We report here that the frog KBP has three functional N-glycosylation sites. Of particular interest, Asn-265, a residue located between two putative membrane spanning regions of the frog KBP, is a functional N-glycosylation site. A mutation of Ser-267 to Gly renders this site non-functional as shown using an in vitro translation system and by transient expression in human embryonic kidney (HEK 293) cells. The mutant receptor protein (S267G), when expressed in HEK cells, binds kainate with high affinity (Kd = 16 nM). These results further support a topology with three transmembrane segments for KBPs and, by sequence homology, for glutamate-gated ion channels.

AMPA/kainate receptor activation in murine oligodendrocyte precursor cells leads to activation of a cation conductance, calcium influx and blockade of delayed rectifying K+ channels

Studies during the last few years have shown that glial cells can express a large repertoire of neurotransmitter receptors. In this study, we have characterized the properties of a glutamate receptor in oligodendrocytes and their precursor cells from cultures of mouse brain, using the patch-clamp technique to measure ligand-activated currents and a fura-2 imaging system to determine changes in free cytosolic Ca2+ concentration ([Ca2+]i). The precursor cells were identified by their characteristic morphology and their voltage-gated currents as described previously [Sontheimer H. et al. (1989) Neuron 2, 1135-1145]. The ligands kainate, domoate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), as well as L-glutamate but not trans-1-amino-1,3-cyclopentanedicarboxylate elicited inward currents at a holding potential of -70 mV and the antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the glutamate- and kainate-induced response reversibly, indicating the expression of an AMPA/kainate-type glutamate receptor. The response is due to the activation of a cationic conductance as revealed by analysing the reversal potential of the kainate-activated current. Receptor activation is accompanied by two additional responses: (i) an increase in [Ca2+]i mediated by depolarization and a subsequent activation of voltage-gated Ca2+ channels and (ii) a transient blockade of a delayed rectifying K+ current, but not of the A-type K+ current. The blockade of the K+ current was not due to the increase in [Ca2+]i since it was also observed in Ca(2+)-free bathing solution when no increase in [Ca2+]i was detectable after exposure to kainate. In contrast to precursor cells, oligodendrocytes responded weakly or not at all to glutamate or related ligands. We conclude that glutamate activates a complex pattern of physiological events in the glial precursor cells, which may play a role during the differentiation process of these cells.

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.

Quantitative analysis of the distributions of glutamatergic ligand binding sites in goldfish brain

Goldfish brain is a widely used model system for the study of the mechanisms involved in neuronal regeneration and synaptic plasticity. Because of the proposed role of glutamate receptors in these processes we have investigated the anatomical localisations of [3H]AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate), [3H]kainate, [3H]CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) and [3H]L-glutamate binding sites in horizontal and sagittal sections. Binding sites for [3H]L-glutamate were the most widespread and both NMDA (N-methyl-D-aspartate) and non-NMDA sensitive components were detected. The density of [3H]kainate binding was very high in the cerebellum compared to other regions and in comparison with the other radioligands used. Conversely, relatively low amounts of [3H]AMPA binding were present with the telencephalon being the most densely labelled structure. [3H]CNQX binding was most densely localised in the tectum with the cerebellum also possessing high binding. In addition, there was a small population of [3H]CNQX binding sites located in the telencephalon and lobus vagi that appeared insensitive to AMPA and kainate.

Characterization of adenylyl cyclase in goldfish brain

The rate of production of cAMP by the adenylyl cyclase enzyme from goldfish brain was linear with time and with protein concentration. In agreement with mammalian adenylyl cyclase systems the enzyme is divalent cation dependent, being activated in the presence of either Mg2+ or Mn2+. Forskolin also stimulated the rate of reaction in a dose-dependent manner with a half-maximal effect of 1 microM. The activated enzyme was inhibited by high concentrations of Ca2+ but was independent of Na+ concentration. The presence of guanine nucleotide binding proteins (G-proteins) was demonstrated by the fact that both NaF and guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) stimulated the basal rate. In addition, the p[NH]ppG dose-response curve of the forskolin-stimulated enzyme was biphasic, similar to that observed for other systems. At low concentrations of p[NH]ppG a small inhibition was observed while higher concentrations produced a stimulation. These data suggest that the goldfish brain adenylyl cyclase enzyme complex includes both stimulatory and inhibitory G-proteins in addition to the catalytic unit. A series of known and putative goldfish neurotransmitter substances failed to either stimulate or inhibit the adenylyl cyclase activity. The endogenous neurotransmitters which interact with this second messenger system remain to be determined.

Interaction of guanine nucleotides with [3H]kainate and 6-[3H]cyano-7-nitroquinoxaline-2,3-dione binding in goldfish brain

Recent reports have suggested that a major proportion of [3H]kainate binding in goldfish brain is to a novel form of G-protein-linked glutamate receptor. Here we confirm that guanine nucleotides decrease [3H]kainate binding in goldfish brain membranes, but that binding is also reduced to a similar extent under conditions where G-protein modulation should be minimised. Inclusion of GTP gamma S resulted in an approximately twofold decrease in the affinity of [3H]kainate binding and a 50% reduction in the apparent Bmax values in both Mg2+/Na+ and Mg2+/Na(+)-free buffer when assayed at 0 degrees C. The pharmacology of [3H]kainate binding is similar to that of well-characterised ionotropic kainate receptors but unlike that of known metabotropic glutamate receptors, with neither 1S,3R-amino-1,3-cyclopentanedicarboxylic acid (1S,3R-ACPD) nor ibotenic acid being effective competitors. The molecular mass of the [3H]kainate binding protein, as determined by radiation inactivation, was 40 kDa, similar to the subunit sizes of other lower vertebrate kainate binding proteins that are believed to comprise ligand-gated ion channels. Furthermore, GTP gamma S also inhibited the binding of the non-NMDA receptor-selective antagonist 6-[3H]cyano-7-nitroquinoxaline-2,3-dione. These data strongly suggest that the regulatory interaction between guanine nucleotides and [3H]kainate and 6-[3H]cyano-7-nitroquinoxaline-2,3-dione binding is complex and involves competition at the agonist/antagonist binding site in addition to any G-protein-mediated modulation.

A ligand binding study of the interactions of guanine nucleotides with non-NMDA receptors

The interactions of guanine nucleotides, and particularly GTP, with the [3H]-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) and [3H]-kainate (KA) binding sites present on brain membranes was studied, using the ligand binding methodology and Scatchard analysis, in order to establish the competitive/non competitive nature of the interaction and determine whether guanine nucleotides, KA and AMPA share common binding sites. GTP was found to block [3H]-AMPA and [3H]-KA binding to rat cortical membranes with IC50 values of 0.4 mM and 1 mM respectively and the [3H] KA-binding to chick cerebellar membranes with a IC50 value of 20 microM. Scatchard analysis of [3H]-KA binding performed in the absence or presence of 1 mM GTP or 0.25 mM AMPA reveals that the high affinity [3H]-KA binding component is not affected by GTP but blocked in a non competitive fashion by AMPA while the low affinity [3H]-KA binding component is not affected by AMPA but blocked by GTP. Scatchard analysis of [3H]-KA binding to chick cerebellar membranes performed in the absence or presence of 33 microM GTP reveals a single binding site blocked in a competitive fashion by GTP. Scatchard analysis of [3H]-AMPA binding performed in the absence or presence of 0.5 mM GTP or 30 microM KA reveals that the high affinity [3H]-AMPA binding component is affected in a non competitive fashion by both GTP and KA while the low affinity [3H] AMPA binding component is affected in a competitive fashion by both GTP and KA.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular size of the kainate binding protein in goldfish brain determined by radiation inactivation

Radiation inactivation analysis was used to estimate the target size of a putative glutamate receptor subtype in goldfish brain. A simple, linear inactivation curve was obtained. The calculated molecular size of the [3H]kainate binding site was 33.8 kDa. The results presented here are comparable to the molecular masses determined for putative glutamate receptors in other lower vertebrates but are markedly different from the sizes of the corresponding glutamate receptor subtypes in mammalian central nervous system.

Regulation of gene expression in astrocytes by excitatory amino acids

We have studied the effect of excitatory amino acids on the expression of mRNA for the immediate early genes c-fos, c-jun, jun-B, and NGF-1A in isolated cortical astrocytes. The expression of the different genes was induced by 100 microM kainate, quisqualate, AMPA and high concentrations of K+ (140 mM). NMDA did not induce the expression of any of the genes studied. The effect of quisqualate stimulation was not inhibited by the antagonist CNQX or by withdrawal of external Ca2+. In contrast the kainate effect was abolished by CNQX but not by the removal of external Ca2+. However, elevated K+ induced c-fos only when calcium was present in the external medium. These findings suggest that type-1 astrocytes lack NMDA receptors and that the induction of genes by quisqualate and kainate is in part independent of the presence of calcium in the external medium and may be mediated through second messenger pathways.

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis)

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.