Structure and assembly mechanism for heteromeric kainate receptors.

Native glutamate receptor ion channels are tetrameric assemblies containing two or more different subunits. NMDA receptors are obligate heteromers formed by coassembly of two or three divergent gene families. While some AMPA and kainate receptors can form functional homomeric ion channels, the KA1 and KA2 subunits are obligate heteromers which function only in combination with GluR5-7. The mechanisms controlling glutamate receptor assembly involve an initial step in which the amino terminal domains (ATD) assemble as dimers. Here, we establish by sedimentation velocity that the ATDs of GluR6 and KA2 coassemble as a heterodimer of K(d) 11 nM, 32,000-fold lower than the K(d) for homodimer formation by KA2; we solve crystal structures for the GluR6/KA2 ATD heterodimer and heterotetramer assemblies. Using these structures as a guide, we perform a mutant cycle analysis to probe the energetics of assembly and show that high-affinity ATD interactions are required for biosynthesis of functional heteromeric receptors.

Distinct functions of kainate receptors in the brain are determined by the auxiliary subunit Neto1.

Ionotropic glutamate receptors principally mediate fast excitatory transmission in the brain. Among the three classes of ionotropic glutamate receptors, kainate receptors (KARs) have a unique brain distribution, which has been historically defined by (3)H-radiolabeled kainate binding. Compared with recombinant KARs expressed in heterologous cells, synaptic KARs exhibit characteristically slow rise-time and decay kinetics. However, the mechanisms responsible for these distinct KAR properties remain unclear. We found that both the high-affinity binding pattern in the mouse brain and the channel properties of native KARs are determined by the KAR auxiliary subunit Neto1. Through modulation of agonist binding affinity and off-kinetics of KARs, but not trafficking of KARs, Neto1 determined both the KAR high-affinity binding pattern and the distinctively slow kinetics of postsynaptic KARs. By regulating KAR excitatory postsynaptic current kinetics, Neto1 can control synaptic temporal summation, spike generation and fidelity.

Oligomerization in the endoplasmic reticulum and intracellular trafficking of kainate receptors are subunit-dependent but not editing-dependent.

Investigating subunit assembly of ionotropic glutamate receptor complexes and their trafficking to the plasma membrane under physiological conditions in live cells has been challenging. By confocal imaging of fluorescently labeled kainate receptor (KAR) subunits combined with digital co-localization and fluorescence resonance energy (FRET) transfer analyses, we investigated the assembly of homomeric and heteromeric receptor complexes and identified the subcellular location of subunit interactions. Our data provide direct evidence for oligomerization of KAR subunits as early as following their biosynthesis in the endoplasmic reticulum (ER). These oligomeric assemblies pass through the Golgi apparatus en route to the plasma membrane. We show that the amino acid at the Q/R editing site of the KAR subunit GluR6 neither determines subunit oligomerization in the ER nor ER exit or plasma membrane expression, and that it does not alter GluR6 interaction with KA2. This finding sets KARs apart from alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, where in the absence of auxiliary proteins Q isoforms exit the ER much more efficiently than R isoforms. Furthermore, although KA2 subunits do not form functional homotetrameric complexes, we visualized their oligomerization (at least dimerization) in the ER. Finally, we demonstrate that plasma membrane expression of GluR6/KA2 heteromeric complexes is modulated not only by GluR6 but also KA2.

High-affinity kainate receptor subunits are necessary for ionotropic but not metabotropic signaling.

Kainate receptors signal through both ionotropic and metabotropic pathways. The high-affinity subunits, GluK4 and GluK5, are unique among the five receptor subunits, as they do not form homomeric receptors but modify the properties of heteromeric assemblies. Disruption of the Grik4 gene locus resulted in a significant reduction in synaptic kainate receptor currents. Moreover, ablation of GluK4 and GluK5 caused complete loss of synaptic ionotropic kainate receptor function. The principal subunits were distributed away from postsynaptic densities and presynaptic active zones. There was also a profound alteration in the activation properties of the remaining kainate receptors. Despite this, kainate receptor-mediated inhibition of the slow afterhyperpolarization current (I(sAHP)), which is dependent on metabotropic pathways, was intact in GluK4/GluK5 knockout mice. These results uncover a previously unknown obligatory role for the high-affinity subunits for ionotropic kainate receptor function and further demonstrate that kainate receptor participation in metabotropic signaling pathways does not require their classic role as ion channels.

Antagonism of recombinant and native GluK3-containing kainate receptors.

A number of kainate receptor antagonists have shown selectivity for receptors containing the GluK1 subunit. Here, we analyze the effects of these GluK1 antagonists on currents mediated by recombinant homomeric GluK3 and heteromeric GluK2/3 receptors expressed in HEK 293 cells and activated by fast application of glutamate. We show that, amongst these compounds, UBP302, UBP310 and UBP316 effectively block recombinant homomeric GluK3 receptors. However, these antagonists are ineffective in blocking homomeric GluK2 or heteromeric GluK2/3 receptors. In addition, these antagonists do not affect presynaptic kainate receptors at mouse hippocampal mossy fibre synapses, which are thought to be composed of GluK2 and GluK3 subunits. Moreover, the AMPA receptor-selective non-competitive antagonist GYKI 53655 blocks, at high concentrations, GluK3-containing receptors and decreases short-term plasticity at mossy fibre synapses. These results expand the range of targets of kainate receptor antagonists and provide pharmacological tools to study the elusive mechanisms of neurotransmitter control by presynaptic kainate receptors.

Different structural requirements for functional ion pore transplantation suggest different gating mechanisms of NMDA and kainate receptors.

Although considerable progress has been made in characterizing the physiological function of the high-affinity kainate (KA) receptor subunits KA1 and KA2, no homomeric ion channel function has been shown. An ion channel transplantation approach was employed in this study to directly test if homomerically expressed KA1 and KA2 pore domains are capable of conducting currents. Transplantation of the ion pore of KA1 or KA2 into GluR6 generated perfectly functional ion channels that allowed characterization of those electrophysiological and pharmacological properties that are determined exclusively by the ion pore of KA1 or KA2. This demonstrates for the first time that KA1 and KA2 ion pore domains are intrinsically capable of conducting ions even in homomeric pore assemblies. NMDA receptors, similar to KA1- or KA2-containing receptors, function only as heteromeric complexes. They are composed of NR1 and NR2 subunits, which both are non-functional when expressed homomerically. In contrast to NR1, the homomeric NR2B ion pore failed to translate ligand binding into pore opening when transplanted into GluR6. Similarly, heteromeric coexpression of the ion channel domains of both NR1 and NR2 inserted into GluR6 failed to produce functional channels. Therefore, we conclude that the mechanism underlying the ion channel opening in the obligatorily heterotetrameric NMDA receptors differs significantly from that in the facultatively heterotetrameric alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and KA receptors.

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.

Regulation of editing and expression of glutamate alpha-amino-propionic-acid (AMPA)/kainate receptors by antidepressant drugs.

BACKGROUND: Several reports have shown that the glutamatergic system is involved in both the pathogenesis of affective and stress-related disorders and in the action of antidepressant drugs. In particular, antidepressant treatment was shown to modulate expression and function of ionotropic glutamate receptors, to inhibit glutamate release and to restore synaptic plasticity impaired by stress. METHODS: We analyzed the mRNA expression and RNA editing of alpha-amino-propionic-acid (AMPA) and kainate (KA) receptor subunits, in the pre-frontal/frontal cortex (P/FC) and hippocampus (HI) of rats chronically treated with three different drugs: the selective serotonin (5-HT) reuptake inhibitor fluoxetine, the selective noradrenaline (NA) reuptake inhibitor reboxetine and the tricyclic antidepressant desipramine. RESULTS: Our data showed that fluoxetine and desipramine exerted moderate but selective effects on glutamate receptor expression and editing, while reboxetine appeared to be the drug that affects glutamate receptors (GluR) most. The most consistent effect, observed with pronoradrenergic drugs (desipramine and reboxetine), was a decrease of GluR3 expression both in P/FC and HI. Interestingly, in HI, the same drugs also decreased the editing levels of either the flip (desipramine) or flop (reboxetine) form of GluR3. CONCLUSIONS: Overall, these results point to specific and regionally discrete changes in the expression and editing level of glutamate receptors and, in particular, to a selective reduction of conductance for GluR3-containing receptors following treatment with antidepressant drugs. These data support the hypothesis that changes in glutamate neurotransmission are involved in the therapeutic effects induced by these drugs.

Kainate receptor trafficking: physiological roles and molecular mechanisms.

Recently, there has been intense interest in the mechanisms regulating the trafficking and synaptic targeting of kainate receptors in neurons. This topic is still in its infancy when compared with studies of trafficking of other ionotropic glutamate receptors; however, it is already clear that mechanisms exist for subunit- and splice variant-specific trafficking of kainate receptors. There is also enormous diversity of kainate receptor targeting, with the best-studied neurons in this regard being hippocampal CA3 pyramidal neurons and CA1 GABAergic interneurons. This review summarizes the current state of knowledge on this topic, focusing on the molecular mechanisms of kainate receptor trafficking and the potential for these mechanisms to regulate neuronal kainate receptor function.

Involvement of the transcription factor E2F1/Rb in kainic acid-induced death of murine cerebellar granule cells.

The full mechanisms underlying neuronal death following excitotoxic insult remain unclear, despite many in vivo and in vitro studies. Recent work has focused on various signaling molecules and pathways, normally strictly regulated, that can trigger death if perturbed. The transcription factor, E2F1 is pivotal in controlling cell death under stress situations. The current study aimed to investigate the role of this transcription factor in modulating neuronal death following kainic acid (KA) treatment of cultured mouse cerebellar granule cells (CGCs). KA-induced death of CGCs was attenuated by the selective KA/AMPA receptor antagonist CNQX, but not MK-801. Such neuronal death was caspase-3-independent and did not activate many known death genes, such as Fas receptor, caspase-8 and p38. However, hyperphosphorylation of Rb showed a transient increase which may lead to activation of E2F1. Indeed E2F1 +/+ and -/- CGCs showed a differential response to KA-mediated toxicity, in that E2F1 -/- neurons were significantly less susceptible to KA compared to E2F1 +/+ neurons, albeit both E2F1 +/+ and -/- neurons expressed similar levels of KA receptors and responded similarly to kainate antagonist, CNQX. Using selective inhibitors to CDKs, such as olomoucine, roscovitine and flavopiridol, and the inhibitor SB203580 to p38 MAPK, we ruled out the possibility that Rb inactivation through hyperphosphorylation was due to either upstream kinases. Therefore activation of Rb/E2F1 pathway appears to involve novel interactions yet to be elucidated.

AMPA/kainate receptors in mouse spinal cord cell-specific display of receptor subunits by oligodendrocytes and astrocytes and at the nodes of Ranvier.

Spinal cord white matter is susceptible to AMPA/kainate (KA)-type glutamate receptor-mediated excitotoxicity. To understand this vulnerability, it is important to characterize the distribution of AMPA/KA receptor subunits in this tissue. Using immunohistochemistry and laser confocal microscopy, we studied the expression sites of AMPA/KA receptor subunits in mouse spinal cord. The white matter showed consistent immunoreactivity for AMPA receptor subunit GluR2/3 and KA receptor subunits GluR6/7 and KA2. In contrast, antibodies against GluR1, GluR2, GluR4 (AMPA), and GluR5 (KA) subunits showed only weak and occasional labeling of white matter. However, gray matter neurons did express GluR1 and GluR2, as well as GluR2/3. The white matter astrocytes were GluR2/3 and GluR6/7 immunopositive, while the gray matter astrocytes displayed primarily GluR6/7. Both exclusively and abundantly, KA2 labeled oligodendrocytes and myelin, identified by CNPase expression. Interestingly, myelin basic protein, another myelin marker, showed less correlation with KA2 expression, placing KA2 at specific CNPase-containing subdomains. Focal points of dense KA2 labeling showed colocalization with limited, but distinct, axonal regions. These regions were identified as nodes of Ranvier by coexpressing the nodal marker, ankyrin G. Overall, axonal tracts showed little, if any, AMPA/KA receptor expression. The proximity of oligodendrocytic KA2 to the axonal node and the paucity of axonal AMPA/kainate receptor expression suggest that excitotoxic axonal damage may be secondary and, possibly, mediated by oligodendrocytes. Our data demonstrate differential expression of glutamate AMPA and KA receptor subunits in mouse spinal cord white matter and point to astrocytes and oligodendrocytes as potential targets for pharmacological intervention in white matter glutamate excitotoxicity.

Stereostructure-activity studies on agonists at the AMPA and kainate subtypes of ionotropic glutamate receptors.

(S)-Glutamic acid (Glu), the major excitatory neurotransmitter in the central nervous system, operates through ionotropic as well as metabotropic receptors and is considered to be involved in certain neurological disorders and degenerative brain diseases that are currently without any satisfactory therapeutic treatment. Until recently, development of selective Glu receptor agonists had mainly been based on lead compounds, which were frequently naturally occurring excitants structurally related to Glu. These Glu receptor agonists generally contain heterocyclic acidic moieties, which has stimulated the use of bioisosteric replacement approaches for the design of subtype-selective agonists. Furthermore, most of these leads are conformationally restricted and stereochemically well-defined Glu analogs. Crystallization of the agonist binding domain of the GluR2 subunit of the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor subtype of ionotropic Glu receptors in the presence or absence of an agonist has provided important information about ligand-receptor interaction mechanisms. The availability of these binding domain crystal structures has formed the basis for rational design of ligands, especially for the AMPA and kainate subtypes of ionotropic Glu receptors. This mini-review will focus on structure-activity relationships on AMPA and kainate receptor agonists with special emphasis on stereochemical and three-dimensional aspects.

Two populations of kainate receptors with separate signaling mechanisms in hippocampal interneurons.

Consistent with the epileptogenic and deleterious effects of the potent neurotoxin kainate, the activation of kainate receptors reduces the synaptic inhibition induced by the amino acid gamma-aminobutyric acid (GABA). Extrapolating from these data led to the conclusion that kainate receptors are located presynaptically. However, kainate directly depolarizes the inhibitory interneurons, causing them to fire repeatedly. This effect might indirectly decrease the size of inhibitory postsynaptic currents recorded from pyramidal cells and places in doubt the presynaptic location for kainate receptors. Here we show that both effects, membrane depolarization and the reduction of inhibitory potentials, can be dissociated by several means, particularly by the natural agonist of kainate receptors, glutamate. Indeed, when applied at low concentrations, glutamate inhibited GABA release without affecting the firing rate of GABA interneurons. These results indicate that CA1 interneurons contain two populations of kainate receptors, each with different agonist sensitivity and coupled to distinct signaling pathways.

New developments in the molecular pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate and kainate receptors.

Separation of non-N-methyl-D-aspartate subtypes of glutamate receptors, known as alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate receptors, is traced through conventional pharmacology to molecular biology. The physiology and pharmacology of recombinant receptor subtypes (GluR1-7 and KA1-2) are described. Competitive antagonists, e.g., the quinoxalinedione, 2,3-dihyroxy-6-nitro-7-sulphamoyl-benz(F)quinoxaline, and the decahydroisoquinoline, 3S,4aR,6R, 8aR-6-[2-(1(2)H-tetrazol-5-yl)ethyl]-decahydroisoquinolin e-3-carboxylate, have a broad antagonist spectrum, except that the latter is inactive on GluR6. The 2,3-benzodiazepines noncompetitively antagonise the AMPA receptor GluR1-4. Desensitisation of AMPA (GluR1-4) and kainate (GluR5-7 and KA1-2) receptors is blocked by cyclothiazide and concanavalin A, respectively. Polyamine toxins block AMPA receptors not containing GluR2 and unedited kainate receptors (GluR5-6). These data correlate well with results on native neurons characterised by techniques such as in situ hybridisation.

Expression of AMPA, kainate, and NMDA receptor subunits in cochlear and vestibular ganglia.

Glutamate is believed to be the principal afferent neurotransmitter in the peripheral auditory and vestibular systems. In this report, we present a comprehensive molecular analysis of ionotropic glutamate receptor gene expression in the cochlear and vestibular ganglia of the rat. Fourteen glutamate receptor subunits were studied: GluR1-4 (including flip and flop variants), GluR5-7, KA1&2, NR1, and NR2A-D. Reverse transcription of RNA followed by DNA amplification with the polymerase chain reaction was used for the initial analysis. Immunocytochemistry and in situ hybridization with subunit-specific oligonucleotides were subsequently used for cellular localization of receptor expression. AMPA (GluR2-4), kainate (GluR5&6 and KA1&2), and NMDA receptor (NR1 and NR2A-D) subunit expression was detected. Based on the relative amounts of mRNA detected by in situ hybridization, the predominant receptors expressed by cochlear and vestibular ganglion cells appear to be GluR2, GluR3, GluR4, GluR5, and NR1. At a moderate level were GluR6, NR2B, and NR2D. KA1, KA2, NR2A, and NR2C mRNAs were also expressed in ganglion cells, but at lower levels. Only the AMPA receptor subunit GluR1 and the kainate receptor subunit GluR7 were not found to be expressed in vestibulocochlear neurons. These studies suggest that functional AMPA, kainate, and NMDA receptors are present at the hair cell/vestibulocochlear nerve synapse.

Neurotransmitters. Elusive glutamate receptors.

Kainate-preferring glutamate receptors appear to be abundant in the central nervous system. We have recently begun to understand their properties, but their functions remain to be described.