Molecular Physiology of Kainate Receptors

28 páginas, 15 figuras, 3 tablas.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, and4) 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.Work in the laboratory of J. Lerma has been supported by grants from the Spanish Ministry of Education and Culture (DGICYT Grants PB93/0150 and UE96/0007 as well as DGESIC Grants PM-0008/96 and PM99–0106), the Ministry of Health (FISSS Grant 95/0869), the Comunidad de Madrid (Grant 08.5/ 0042/1998), and the European Union (Grant BIO2-CT930243). J. C. López-García is the recipient of a long-term fellowship awarded by EMBO.Peer reviewe

Impact of the first wave of the SARS-CoV-2 pandemic on the outcome of neurosurgical patients: A nationwide study in Spain

Objective To assess the effect of the first wave of the SARS-CoV-2 pandemic on the outcome of neurosurgical patients in Spain. Settings The initial flood of COVID-19 patients overwhelmed an unprepared healthcare system. Different measures were taken to deal with this overburden. The effect of these measures on neurosurgical patients, as well as the effect of COVID-19 itself, has not been thoroughly studied. Participants This was a multicentre, nationwide, observational retrospective study of patients who underwent any neurosurgical operation from March to July 2020. Interventions An exploratory factorial analysis was performed to select the most relevant variables of the sample. Primary and secondary outcome measures Univariate and multivariate analyses were performed to identify independent predictors of mortality and postoperative SARS-CoV-2 infection. Results Sixteen hospitals registered 1677 operated patients. The overall mortality was 6.4%, and 2.9% (44 patients) suffered a perioperative SARS-CoV-2 infection. Of those infections, 24 were diagnosed postoperatively. Age (OR 1.05), perioperative SARS-CoV-2 infection (OR 4.7), community COVID-19 incidence (cases/10 5 people/week) (OR 1.006), postoperative neurological worsening (OR 5.9), postoperative need for airway support (OR 5.38), ASA grade =3 (OR 2.5) and preoperative GCS 3-8 (OR 2.82) were independently associated with mortality. For SARS-CoV-2 postoperative infection, screening swab test <72 hours preoperatively (OR 0.76), community COVID-19 incidence (cases/10 5 people/week) (OR 1.011), preoperative cognitive impairment (OR 2.784), postoperative sepsis (OR 3.807) and an absence of postoperative complications (OR 0.188) were independently associated. Conclusions Perioperative SARS-CoV-2 infection in neurosurgical patients was associated with an increase in mortality by almost fivefold. Community COVID-19 incidence (cases/10 5 people/week) was a statistically independent predictor of mortality. Trial registration number CEIM 20/217

A Role for Extracellular Na+ in the Channel Gating of Native and Recombinant Kainate Receptors

8 páginas, 7 figuras.Ionotropic glutamate receptors of the kainate and AMPA subtypes share a number of structural features, both topographical and in terms of stoichiometry. In addition, AMPA and kainate receptors share similar pharmacological and biophysical properties in that they are activated by common agonists and display rapid activation and desensitization characteristics. However, we show here that in contrast to AMPA receptor-mediated responses (native or recombinant GluR3 receptor), the response of native and recombinant (GluR6) kainate receptors to glutamate was drastically reduced in the absence of extracellular Na+ (i.e., when replaced by Cs+). Removal of Na+ increases the rate of desensitization, indicating that external Na+ modulates channel gating. Whereas the size of the substituting cation is important in mimicking the action of Na+ (Li+>K+>Cs+), modulation was voltage independent. These results indicate the existence of different gating mechanisms for AMPA and kainate receptors. By using chimeric AMPA-kainate receptors derived from GluR3 and GluR6, we have identified a key residue in the S2 segment of GluR6 (M770) that is largely responsible for the sensitivity of the receptor to external Na+. Thus, these results show the existence of a specific kainate receptor gating mechanism that requires external Na+ to be operative.This work was supported by grants to J.L. from the Dirección General de Ensen anza Superior e Investigación Cientifica (PM99-0106) and the European Union (QLG3-CT2001-00929) and to Y.S.-B. from The Israel Science Foundation (496/00-2), the Israeli Ministry of Science, and the Spanish Ministry of Foreign Affairs in the form of an Israeli–Spanish Scientific Cooperation grant. A.V.P. is a postdoctoral fellow of the Community of Madrid.Peer reviewe

GluR5 and GluR6 Kainate Receptor Subunits Coexist in Hippocampal Neurons and Coassemble to Form Functional Receptors

10 páginas, 8 figuras.We have performed nonradioactive double in situ hybridization to study the expression of glutamic acid decarboxylase and GluR6 or GluR5 subunits in hippocampal slices. Our results indicate that although GluR6 is primarily expressed by pyramidal cells and dentate granule neurons and GluR5 is prominently expressed in nonpyramidal cells, there is a significant population of GABAergic interneurons that coexpress the two glutamate receptor subunits. To assess whether the two subunits could coassemble to form heteromeric receptors, we studied the electrophysiological responses when both subunits were coexpressed in HEK293 cells. Responses evoked by rapid application of either glutamate, (RS)--amino-3-hydroxy-5-tert-butyl-4-isoxazolepropionic acid (ATPA) the selective agonist of GluR5 receptors), and AMPA in cells cotransfected with GluR6(R) and GluR5(Q) presented a similar degree of outward rectification. This can only be attributed to the fact that all receptors have at least one GluR6(R) subunit in their structure, conferring outward rectification, and at least one GluR5(Q) subunit to confer sensitivity to ATPA and AMPA. More than 80% of the receptors expressed by a single cell were found to be GluR5/R6 heteromers, presenting different desensitization and gating properties to homomeric R6 receptors. These results lead us to believe that a population of interneurons in the hippocampus express receptors made up of both GluR5 and GluR6 subunits and provide evidence for a greater diversity of kainate receptors in the brain than previously thought, that may account for a higher functional complexity.This work was supported in part by grants to J.L. from the Dirección General de Enseñanza Superior e Investigación Científica (P.M.96/0008) and to J.L. and M.A.N. from the Community of Madrid (08.5/0042/1998). M.T.H. held a fellowship from the Instituto de Cooperación Iberoamericana.Peer reviewe

Unbalanced dendritic inhibition of CA1 neurons drives spatial-memory deficits in the Ts2Cje Down syndrome model

Overinhibition is assumed one of the main causes of cognitive deficits (e.g. memory impairment) in mouse models of Down syndrome (DS). Yet the mechanisms that drive such exaggerated synaptic inhibition and their behavioral effects remain unclear. Here we report the existence of bidirectional alterations to the synaptic inhibition on CA1 pyramidal cells in the Ts2Cje mouse model of DS which are associated to impaired spatial memory. Furthermore, we identify triplication of the kainate receptor (KAR) encoding gene Grik1 as the cause of these phenotypes. Normalization of Grik1 dosage in Ts2Cje mice specifically restored spatial memory and reversed the bidirectional alterations to CA1 inhibition, but not the changes in synaptic plasticity or the other behavioral modifications observed. We propose that modified information gating caused by disturbed inhibitory tone rather than generalized overinhibition underlies some of the characteristic cognitive deficits in DS.The authors gratefully acknowledge the financial support received from the Spanish Agency of Research (AEI) under the grant BFU2015-64656-R (to J.L.), co-financed by the European Regional Development Fund (ERDF), and the “Severo Ochoa” Program for Centres of Excellence in R&D (SEV-2013-0317 and SEV-2017-0723). This work was also supported by the Generalitat Valenciana through the program PrometeoII/2015/012 (to J.L.), the FPI fellowship program (to S.V. and A.G.), and the SyMBaD – Marie Curie ITN (agreement # 238608) (to W.M.)Peer reviewe

A Mosaic of Functional Kainate Receptors in Hippocampal Interneurons

8 páginas, 7 figuras.Although some physiological functions of kainate receptors (KARs) still remain unclear, recent advances have highlighted a role in synaptic physiology. In hippocampal slices, kainate depresses GABA-mediated synaptic inhibition and increases the firing rate of interneurons. However, the sensitivity to agonists of these responses differs, suggesting that the presynaptic and somatic KARs have a distinct molecular composition. Hippocampal interneurons express several distinct KAR subunits that can assemble into heteromeric receptors with a variety of pharmacological properties and that, in principle, could fulfill different roles. To address which receptor types mediate each of the effects of kainate in interneurons, we used new compounds and mice deficient for specific KAR subunits. In a recombinant assay, 5-carboxyl-2,4-di-benzamido-benzoic acid (NS3763) acted exclusively on homomeric glutamate receptor subunit 5 (GluR5), whereas 3S,4aR,6S,8aR-6-((4-carboxyphenyl)methyl) 1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884) antagonized homomeric GluR5 and any heteromeric combination containing GluR5 subunits. In hippocampal slices, LY382884, but not NS3763, was able to prevent kainate-induced depression of evoked IPSC. In contrast, neither prevented the concomitant increase in spontaneous IPSC frequency. The selectivity of these compounds was seen additionally in knock-out mice, such that they were inactive in GluR5-/- mice but completely effective in GluR6-/- mice. Our data indicate that in wild-type mice, CA1 interneurons express heteromeric GluR6 -KA2 receptors in their somatic compartments and GluR5-GluR6 or GluR5-KA2 at presynaptic terminals. However, functional compensation appears to take place in the null mutants, a new pharmacological profile emerging more compatible with the activity of homomeric receptors in both compartments: GluR5 in GluR6-/- mice and GluR6 in GluR5-/- mice.This work was supported by Ministerio de Ciencia y Tecnología Grant BFI2003-00161 (J.L.) and European Union Grant QLG3-CT2001-00929. A.V.P. is a postdoctoral fellow of the Autonomous Community of Madrid.Peer reviewe

Canales activados por glutamato

10 páginas, 3 figuras.Estos trabajos han sido financiados por la Dirección General de Investigación Científica y Técnica (PB89-0061) y el Fondo de Investigaciones Sanitarias de la Seguridad Social (92/0266). F. Somohano disfrutó de una beca posdoctoral de la CEE y A.V. Paternain disfruta de una beca Glaxo-CSIC.Peer reviewe

Dual signaling by mGluR5a results in bi-directional modulation of N-type Ca2+ channels

5 páginas, 3 figuras.We have studied how N-type Ca2+ channels are modulated by the metabotropic glutamate receptor 5a (mGluR5a) in Xenopus oocytes. Stimulation of the receptor with glutamate initiated two parallel responses, a rapid inhibition followed by an upregulation of the Ca2+ current. Although a subsequent stimulation did not upregulate the Ca2+ current, it did still produce a reduction in the amplitude of the current. The upregulation of Ca2+ channels was prevented by the protein kinases inhibitor staurosporine and it was mimicked by the activation of PKC with phorbol esters. In contrast, the inhibition of the Ca2+ current was insensitive to staurosporine. These results show that mGluR5a exerts a bi-directional influence on Ca2+ channels, which may explain how group I mGluRs facilitate and inhibit glutamate release at central synapses.This work was supported by grants from the Community of Madrid to J. S.-P. (08.5/ 0016/1998 and 08.5/0075.1/2000) and from the MCYT to JL (PM99-0106). AVP was a postdoctoral fellowship from the Community of Madrid.Peer reviewe

Noncanonical Signaling by Ionotropic Kainate Receptors

11 páginas, 8 figuras.The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.This work was supported by grants (to J.L.) from the European Union (QLG3-CT2001-00929). J.L.R. is the recipient of a predoctoral fellowship from the Ministry of Education, and A.V.P. is a postdoctoral fellow of the Community of Madrid.Peer reviewe

Noncanonical Signaling by Ionotropic Kainate Receptors

11 páginas, 8 figuras.The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.This work was supported by grants (to J.L.) from the European Union (QLG3-CT2001-00929). J.L.R. is the recipient of a predoctoral fellowship from the Ministry of Education, and A.V.P. is a postdoctoral fellow of the Community of Madrid.Peer reviewe