Identification of C-terminal domain residues involved in protein kinase A-mediated potentiation of kainate receptor subtype 6

Glutamate receptors are the major excitatory receptors in the vertebrate CNS and have been implicated in a number of physiological and pathological processes. Previous work has shown that glutamate receptor function may be modulated by protein kinase A (PKA)-mediated phosphorylation, although the molecular mechanism of this potentiation has remained unclear. We have investigated the phosphorylation of specific amino acid residues in the C-terminal cytoplasmic domain of the rat kainate receptor subtype 6 (GluR6) as a possible mechanism for regulation of receptor function. The C-terminal tail of rat GluR6 can be phosphorylated by PKA on serine residues as demonstrated using [gamma-32P]ATP kinase assays. Whole cell recordings of transiently transfected human embryonic kidney (HEK) 293 cells showed that phosphorylation by PKA potentiates whole cell currents in wildtype GluR6 and that removal of the cytoplasmic C-terminal domain abolishes this potentiation. This suggested that the C-terminal domain may contain residue(s) involved in the PKA-mediated potentiation. Single mutations of each serine residue in the C-terminal domain (S815A, S825A, S828A, and S837A) and a truncation after position 855, which removes all threonines (T856, T864, and T875) from the domain, do not abolish PKA potentiation. However, the S825A/S837A mutation, but no other double mutation, abolishes potentiation. These results demonstrate that phosphorylation of the C-terminal tail of GluR6 by PKA leads to potentiation of whole cell response, and the combination of S825 and S837 in the C-terminal domain is a vital component of the mechanism of GluR6 potentiation by PKA.

Interface interactions modulating desensitization of the kainate-selective ionotropic glutamate receptor subunit GluR6

Ionotropic glutamate receptors from the AMPA and kainate subfamilies share many functional and structural features, but it is unclear whether this similarity extends to the molecular mechanisms underlying receptor desensitization. The current model for desensitization in AMPA receptors involves the rearrangement of dimers formed between subunit agonist binding domains. Key evidence for this has come from a single point mutant (from leucine to tyrosine) that abolished desensitization and that was shown to stabilize the binding domain dimer. However, the desensitization of kainate receptors appears to differ from that of AMPA receptors in several key respects. Although the kinetics of AMPA receptor gating and desensitization are consistent with channels formed from two dimers, similar evidence for the functional involvement of dimers has not been found in kainate receptors. Furthermore, despite the homolog of the nondesensitizing tyrosine in AMPA subunits also being a tyrosine in wild-type kainate subunits, these receptors desensitize rapidly and completely. Using mutagenesis based on the crystal structure of the glutamate receptor subunit GluR6 S1S2 domain in complex with domoate, we identified four residues neighboring this tyrosine that differ between AMPA and kainate subunits and that contribute to the different desensitization kinetics of these receptors. Detailed analysis of the effects of mutations at these sites confirms that there is in fact a common general mechanism for desensitization in non-NMDA receptors, dependent on the stability of the binding domain dimer interface, and reveals the existence of potential agonist-specific desensitization pathways.

Glutamate receptor trafficking: endoplasmic reticulum quality control involves ligand binding and receptor function

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 domains in the N-terminal and M3-M4 loop regions, respectively. In the present study, we sought to confirm that the conserved ligand-binding residues identified in the AMPA receptor S1S2 domains also participate in ligand binding of GluR6 kainate receptors. Amino acid substitutions were made in the GluR6 parent at R523, T690, and E738 to alter their potential interactions with ligand. Mutant receptors were expressed in human embryonic kidney 293 cells, confirmed by Western blot analysis, and tested by [3H]kainate binding and patch-clamp recording. Each of the binding site mutations was sufficient to reduce [3H]kainate binding to undetectable levels and eliminate functional responses to glutamate or kainate. As with our studies of other nonfunctional mutants (Fleck et al., 2003), immunocytochemical staining and cell-surface biotinylation studies showed that the mutant receptors were retained intracellularly and did not traffic to the cell surface. Endoglycosidase-H digests and colocalization with endoplasmic reticulum (ER) markers demonstrated that the mutant receptors are immaturely glycosylated and retained in the ER. Immunoprecipitation, native PAGE, and functional studies confirmed that the GluR6-binding site mutants are capable of multimeric assembly, indicating their retention in the ER does not result from a gross protein folding error. Together, these results confirm the role of R523, T690, and E738 directly in ligand binding to GluR6 and further support our previous report that nonfunctional GluRs are retained intracellularly by a functional checkpoint in ER quality control.

Altered behavioral responses to noxious stimuli and fear in glutamate receptor 5 (GluR5)- or GluR6-deficient mice

Different kainate receptor (KAR) subtypes contribute to the regulation of both excitatory and inhibitory transmission. However, no study has reported a role for KAR subtypes in behavioral responses to persistent pain and fear memory. Here we show that responses to capsaicin or inflammatory pain were significantly reduced in mice lacking glutamate receptor 5 (GluR5) but not GluR6 subunits. In classic fear-memory tests, mice lacking GluR6 but not GluR5 showed a significant reduction in fear memory when measured 3, 7, or 14 d after training. Additionally, synaptic potentiation was significantly reduced in the lateral amygdala of GluR6 but not GluR5 knock-out mice. Our findings provide evidence that distinct KAR subtypes contribute to chemical/inflammatory pain and fear memory. Selectively targeting different KAR subtypes may provide a useful strategy for treating persistent pain and fear-related mental disorders.

A C-terminal determinant of GluR6 kainate receptor trafficking

Intracellular trafficking of ionotropic glutamate receptors is regulated predominantly by determinants in the cytoplasmic C-terminal domain of the subunit proteins. Although AMPA receptors are found at the vast majority of excitatory synapses, synaptic kainate receptors exhibit a much more restricted distribution, suggesting that specific mechanisms exist for selective trafficking of these receptor proteins. In this report, we define a critical forward trafficking motif that is necessary for surface expression of the glutamate receptor 6 (GluR6) kainate receptor as well as chimeric proteins containing only the GluR6 C-terminal domain. The trafficking determinant was identified by tracking surface expression of green fluorescent protein-tagged GluR6 receptors with confocal immunofluorescence in COS-7 cells and cultured neurons and patch-clamp electrophysiology in human embryonic kidney 293 cells. Serial truncation and alanine site mutagenesis of the GluR6 subunit C terminus localized the critical motif to a seven amino acid stretch of predominantly basic residues. Alanine mutation of the trafficking motif reduced kainate receptor current amplitudes by >90% and resulted in retention of the mutated receptors in the endoplasmic reticulum. This forward trafficking domain is the first such identified for kainate receptors.

Caspase-dependent and caspase-independent oligodendrocyte death mediated by AMPA and kainate receptors

Oligodendrocytes are vulnerable to excitotoxic signals mediated by AMPA receptors and by high- and low-affinity kainate receptors. Here we investigated the nature of the cell death triggered by activation of these receptors in primary cultures of oligodendrocytes from the rat optic nerve. Activation of AMPA receptors at both submaximal and maximal concentrations of the agonist induced massive calcium entry, mitochondrial depolarization, and a rise in the level of reactive oxygen species that correlated with a decrease in the levels of reduced glutathione. In addition, excitotoxicity initiated by submaximal, but not maximal, activation of AMPA receptors was prevented by caspase-3 blockade and by the concomitant blockade of caspases 8 and 9. In turn, maximal activation of high- or low-affinity kainate receptors induced mitochondrial events and toxicity levels similar to those observed with submaximal activation of AMPA receptors. In contrast to AMPA receptor-mediated insults, calcineurin inhibition or caspase-9 blockade was sufficient to prevent cell death triggered by both types of kainate receptors. Consistent with these results, prolonged glutamate receptor activation in freshly isolated optic nerves caused selective activation of caspase-3 and chromatin condensation in oligodendrocytes. Overall, the evidence presented here indicates that oligodendrocyte death by excitotoxicity is mediated by caspase-dependent and -independent mechanisms.

Selective antagonism of GluR5 kainate-receptor-mediated synaptic currents by topiramate in rat basolateral amygdala neurons

Topiramate is a widely used antiepileptic agent whose mechanism of action is poorly understood. The drug has been reported to interact with various ion channel types, including AMPA/kainate receptors. In whole-cell voltage-clamp recordings from principal neurons of the rat basolateral amygdala, topiramate at low concentrations (IC50, approximately 0.5 microm) selectively inhibited pharmacologically isolated excitatory synaptic currents mediated by kainate receptors containing the GluR5 subunit. Topiramate also partially depressed predominantly AMPA-receptor-mediated EPSCs, but with lower efficacy. Topiramate did not alter the degree of facilitation in paired-pulse experiments, and it reduced the amplitude of miniature EPSCs without affecting their frequency, demonstrating that the block of synaptic responses occurs postsynaptically. Inhibition of GluR5 kainate receptors could represent a key mechanism underlying the anticonvulsant activity of topiramate. Moreover, these results support the concept that GluR5 kainate receptors represent a novel target for antiepileptic drug development.

Short-term frequency-dependent plasticity at recurrent mossy fiber synapses of the epileptic brain

The recurrent mossy fiber pathway of the dentate gyrus expands dramatically in human temporal lobe epilepsy and in animal models of this disorder, creating monosynaptic connections among granule cells. This novel granule cell network can support reverberating excitation but is difficult to activate with low-frequency stimulation. This study used hippocampal slices from pilocarpine-treated rats to explore the dependence of synaptic transmission in this pathway on stimulus frequency. Minimal electrically evoked EPSCs exhibited a high failure rate ( approximately 60%). Stimulus trains delivered at a frequency of <1 Hz depressed synaptic transmission, as evidenced by an increase in response failures. Conversely, stimulus trains delivered at higher frequencies reduced the percentage of response failures and increased the amplitude of compound EPSCs, including pharmacologically isolated NMDA receptor-mediated EPSCs. Short-term frequency-dependent facilitation was of modest size compared with mossy fiber synapses on other neuronal types. Facilitation depended on the activation of kainate receptors by released glutamate and was inhibited by feedback activation of type II metabotropic glutamate receptors. These results suggest that the recurrent mossy fiber pathway may be functionally silent during baseline asynchronous granule cell activity in vivo attributable, in part, to progressive transmission failure. The pathway may synchronize granule cell firing and may promote seizure propagation most effectively during the brief periods of high-frequency granule cell firing that occur during normal behavior, during the periods of hypersynchronous fast activity characteristic of epileptic brain and, most importantly, during the period of increasing granule cell activity that precedes a spontaneous seizure.

Kainate receptor-dependent short-term plasticity of presynaptic Ca2+ influx at the hippocampal mossy fiber synapses

Transmitter release at the hippocampal mossy fiber (MF)-CA3 synapse exhibits robust use-dependent short-term plasticity with an extremely wide dynamic range. Recent studies revealed that presynaptic kainate receptors (KARs), which specifically localized on the MF axons, mediate unusually large facilitation at this particular synapse in concert with the action of residual Ca2+. However, it is currently unclear how activation of kainate autoreceptors enhances transmitter release in an activity-dependent manner. Using fluorescence recordings of presynaptic Ca2+ and voltage in hippocampal slices, here we demonstrate that paired-pulse stimulation (with 20-200 msec intervals) resulted in facilitation of Ca2+ influx into the MF terminals, as opposed to other synapses, such as the Schaffer collateral-CA1 synapse. These observations deviate from typical residual Ca2+ hypothesis of facilitation, assuming an equal amount of Ca2+ influx per action potential. Pharmacological experiments reveal that the facilitation of presynaptic Ca2+ influx is mediated by activation of KARs. We also found that action potentials of MF axons are followed by prominent afterdepolarization, which is partly mediated by activation of KARs. Notably, the time course of the afterdepolarization approximates to that of the paired-pulse facilitation of Ca2+ influx, suggesting that these two processes are closely related to each other. These results suggest that the novel mechanism amplifying presynaptic Ca2+ influx may underlie the robust short-term synaptic plasticity at the MF-CA3 synapse in the hippocampus, and this process is mediated by KARs whose activation evokes prominent afterdepolarization of MF axons and thereby enhances action potential-driven Ca2+ influx into the presynaptic terminals.

Developmental and activity dependent regulation of ionotropic glutamate receptors at synapses

Glutamate receptors mediate the majority of excitatory responses in the central nervous system. The establishment and refinement of glutamatergic synaptic connections depend on the concerted actions of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), N-methyl-D-aspartate (NMDA), and kainate (KA) type ionotropic glutamate receptors (iGluRs) and G-protein coupled metabotropic receptors. While a lot remains to be clarified, the most is known about the mechanisms by which the iGluR subtypes are targeted and how this is influenced by synaptic activity on both short and long time scales. Changes in their subunit compositions are also input specific and developmentally regulated. The identification of key molecular components of the postsynaptic density (PSD) and novel proteins that influence receptor targeting and clustering have started to reveal the underlying molecular mechanisms of the trafficking and targeting of iGluRs. Here we discuss the evidence that these basic mechanisms are used during developmental synaptic plasticity.

Kainate receptors expressed by a subpopulation of developing nociceptors rapidly switch from high to low Ca2+ permeability

Dorsal root ganglion (DRG) neurons first express kainate receptor subunits, predominantly GluR5, during embryonic development. In the DRG and throughout the nervous system, substantial editing of GluR5 mRNA occurs with developmental maturation (Bernard et al., 1999). The accompanying change in Ca(2+) permeability of functional kainate receptors that is the predicted outcome of this developmental regulation of mRNA editing has not been investigated. Here we report that kainate receptors on DRG neurons from late embryonic and newborn rats are predominantly Ca(2+) permeable but then become fully Ca(2+) impermeable later in the first postnatal week. Using multiple markers for nociceptor subpopulations, we show that this switch in Ca(2+) permeability is not caused by the appearance of a new subpopulation of nociceptors with different receptor properties. Instead, the change in Ca(2+) permeability matches the time course of post-transcriptional RNA editing of GluR5 at the Q/R site within the pore of the channel, indicating that the change is probably caused by developmentally regulated RNA editing. We also report that, on the basis of the strong correlation of receptor expression with expression of the surface markers LA4, isolectin B4, and LD2, kainate receptors are present on C-fiber-type neurons projecting to lamina II of spinal cord dorsal horn. These results raise the possibility that kainate receptors in their Ca(2+)-permeable form serve a developmental role in synapse formation between this population of C-fibers and their targets in the spinal cord dorsal horn. Thereafter, the receptors may serve a new function that does not require Ca(2+) permeability.

Structural similarities between glutamate receptor channels and K(+) channels examined by scanning mutagenesis

The pores of glutamate receptors and K(+) channels share sequence homology, suggesting a conserved secondary structure. Scanning mutagenesis with substitution of alanine and tryptophan in GluR6 channels was performed based on the structure of KcsA. Our assay used disruption of voltage-dependent polyamine block to test for changes in the packing of pore-forming regions. Alanine scanning from D567 to R603 revealed reduced rectification resulting from channel block in two regions. A periodic pattern from F575 to M589 aligned with the pore helix in KcsA, whereas a cluster of sensitive positions around Q590, a site regulated by RNA editing, mapped to the selectivity filter in KcsA. Tryptophan scanning from D567 to R603 revealed similar patterns, but with a complete disruption of spermine block for 7 out of the 37 positions and a pM dissociation constant for Q590W. Molecular modeling with KcsA coordinates showed that GluR6 pore helix mutants disrupting polyamine block pack against M1 and M2, and are not exposed in the ion channel pore. In the selectivity filter, tryptophan creates an aromatic cage consistent with the pM dissociation constant for Q590W. A scan with glutamate substitution was used to map the cytoplasmic entrance to the pore based on charge neutralization experiments, which established that E594 was uniquely required for high affinity polyamine block. In E594Q mutants, introduction of glutamate at positions S593-L600 restored polyamine block at positions corresponding to surface-exposed residues in KcsA. Our results reinforce proposals that the pore region of glutamate receptors contains a helix and pore loop analogous to that found in K(+) channels. At the cytoplasmic entrance of the channel, a negatively charged amino acid, located in an extended loop with solvent-exposed side chains, is required for high affinity polyamine block and probably attracts cations via a through space electrostatic mechanism.

Unequal expression of allelic kainate receptor GluR7 mRNAs in human brains

We describe here the first example of an exonic polymorphism that affects the primary structure of a human ionotropic glutamate receptor. The human kainate receptor GluR7 gene contains a thymine (T)/guanine (G) nucleotide variation that determines a serine or alanine at position 310 in the extracellular region of GluR7 receptor subunits. Our finding contrasts with a previous report that suggested that GluR7 transcripts were RNA-edited at this site. Whole-cell patch-clamp recordings did not detect differences in receptor activation and desensitization between the human GluR7 receptor isoforms expressed in HEK-293 cells. Analysis of 41 tissue samples obtained from 30 human brains revealed expression level differences between GluR7 alleles expressed in the same brain. The expression level of the allelic GluR7 mRNAs differed in 27 samples from 1.2- to 12.7-fold. Unequal expression level of allelic mRNAs is characteristic for genes that are affected by genomic imprinting or that contain mutations. Genomic imprinting in most cases is conserved between human and mice. However, we did not detect unequal expression of allelic GluR7 mRNAs in mice. Our results are important for future studies that explore a potential role or roles for GluR7 receptors in the brain and for neurological disorders.

NMDA-Dependent modulation of hippocampal kainate receptors by calcineurin and Ca(2+)/calmodulin-dependent protein kinase

Neurotransmitter receptor function can be influenced by the phosphorylation state of the receptor or of associated proteins. Here we show that kainate receptors expressed in cultured hippocampal neurons can be modulated by Ca(2+)/calmodulin-dependent phosphatase (calcineurin) and Ca(2+)/calmodulin-dependent kinase (CaMK). Ca(2+) influx through NMDA receptor or voltage-sensitive calcium channels resulted in a transient depression of the kainate receptor current. This calcium-induced depression of the kainate receptor current depended on the activation of the phosphatase calcineurin. The amplitude of the kainate receptor currents returned to the baseline level in approximately 9 sec (tau = 3.6 sec), and the recovery of the current amplitude depended on CaMK activity. The effect on kainate receptor currents was dependent on the frequency of NMDA receptor activation. Although low-frequency (0.1 Hz) NMDA application induced depression followed by recovery of the kainate receptor currents, higher frequency (1 Hz) NMDA applications induced a more prolonged depression. Kainate receptors have been shown to modulate synaptic transmission by both presynaptic and postsynaptic mechanisms. Our results suggest that synaptic activity mediated by NMDA receptors, or other routes of Ca(2+) influx, may, in turn, modulate the function of kainate receptors.

Lectin-induced inhibition of desensitization of the kainate receptor GluR6 depends on the activation state and can be mediated by a single native or ectopic N-linked carbohydrate side chain

The ionotropic glutamate receptor GluR6 exhibits strongly and rapidly desensitizing current responses. Treatment of heterologically expressed GluR6 with the lectin concanavalin A (ConA) in Xenopus oocytes as well as in human embryonic kidney-293 cells results in a considerable increase of the steady-state current, presumably by inhibiting receptor desensitization. In the present study, we investigated the molecular basis of this effect using a systematic mutagenesis approach. We found that although N-glycosylation is an absolute prerequisite for the lectin-mediated inhibition of desensitization, no single one of the nine extracellular consensus sites for N-glycosylation of GluR6 is required. Rather, each of the nine N-linked carbohydrate side chains is independently capable of modulatory interaction with the lectin. Moreover, even artificially introduced N-glycosylation sites can substitute for native sites. Thus, the specific site of the lectin binding does not appear to be important for its desensitization-inhibiting action. Furthermore, we show that the extent of the receptor's ConA sensitivity depends on its state of activation, because the desensitized GluR6 exhibits significantly lower lectin sensitivity than the nondesensitized receptor. We conclude that binding of ConA "locks" the receptor in the activatable state, thereby inhibiting conformational changes required to shift the receptor to the desensitized state.

Agonist-induced changes in substituted cysteine accessibility reveal dynamic extracellular structure of M3-M4 loop of glutamate receptor GluR6

Recent evidence suggests that the transmembrane topology of ionotropic glutamate receptors differs from other members of the ligand-gated ion channel superfamily. However, the structure of the segment linking membrane domains M3 and M4 (the M3-M4 loop) remains controversial. Although various data indicate that this loop is extracellular, other results suggest that serine residues in this segment are sites of phosphorylation and channel modulation by intracellular protein kinases. To reconcile these data, we hypothesized that the M3-M4 loop structure is dynamic and, more specifically, that the portion containing putative phosphorylation sites may be translocated across the membrane to the cytoplasmic side during agonist binding. To test this hypothesis, we mutated Ser 684, a putative cAMP-dependent protein kinase site in the kainate-type glutamate receptor GluR6, to Cys. Results of biochemical and electrophysiological experiments are consistent with Cys 684 being accessible, in the unliganded state, from the extracellular side to modification by a Cys-specific biotinylating reagent followed by streptavidin (SA). Interestingly, our data suggest that this residue becomes inaccessible to the extracellular biotinylating reagent during agonist binding. However, we find it unlikely that Cys 684 undergoes membrane translocation, because the addition of SA to Cys-biotinylated GluR6(S684C) has no effect on peak glutamate-evoked current and only a small effect on macroscopic desensitization. We conclude that residue 684 in GluR6 is extracellular in the receptor-channel's closed, unliganded state and does not cross the membrane after agonist binding. However, an agonist-induced conformational change in the receptor substantially alters accessibility of position 684 to the extracellular environment.

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.