The identification of AMPA, kainate and NMDA glutamate receptor subtypes by Watkins and colleagues underlies much of our understanding of excitatory synaptic transmission in the central nervous ...system of animals. Ongoing large scale genome sequencing projects in species for which physiological analysis of receptor function is challenging are resulting in identification of numerous eukaryotic glutamate receptor ion channels in the animal kingdom of life. On the basis of sequence similarity, these are frequently classified into the three vertebrate subtypes, initially identified using subtype selective ligands. Recent work reveals unexpected ligand binding profiles for these newly identified glutamate receptors, for example, kainate receptors on which NMDA acts as a competitive antagonist, and high affinity homomeric glycine activated glutamate receptors. Structural studies reveal that only subtle changes in the ligand binding domain, often identified only in retrospect, underlie different patterns of ligand binding, and that the biology of glutamate receptors is more complex than first anticipated.
Mossy cells (MCs) of the dentate gyrus are key components of an excitatory associative circuit established by reciprocal connections with dentate granule cells (GCs). MCs are implicated in place ...field encoding, pattern separation, and novelty detection, as well as in brain disorders such as temporal lobe epilepsy and depression. Despite their functional relevance, little is known about the determinants that control MC activity. Here, we examined whether MCs express functional kainate receptors (KARs), a subtype of glutamate receptors involved in neuronal development, synaptic transmission, and epilepsy. Using mouse hippocampal slices, we found that bath application of submicromolar and micromolar concentrations of the KAR agonist kainic acid induced inward currents and robust MC firing. These effects were abolished in GluK2 KO mice, indicating the presence of functional GluK2-containing KARs in MCs. In contrast to CA3 pyramidal cells, which are structurally and functionally similar to MCs and express synaptic KARs at mossy fiber (MF) inputs (i.e., GC axons), we found no evidence for KAR-mediated transmission at MF-MC synapses, indicating that most KARs at MCs are extrasynaptic. Immunofluorescence and immunoelectron microscopy analyses confirmed the extrasynaptic localization of GluK2-containing KARs in MCs. Finally, blocking glutamate transporters, a manipulation that increases extracellular levels of endogenous glutamate, was sufficient to induce KAR-mediated inward currents in MCs, suggesting that MC-KARs can be activated by increases in ambient glutamate. Our findings provide the first direct evidence of functional extrasynaptic KARs at a critical excitatory neuron of the hippocampus.
Hilar mossy cells (MCs) are an understudied population of hippocampal neurons that form an excitatory loop with dentate granule cells. MCs have been implicated in pattern separation, spatial navigation, and epilepsy. Despite their importance in hippocampal function and disease, little is known about how MC activity is recruited. Here, we show for the first time that MCs express extrasynaptic kainate receptors (KARs), a subtype of glutamate receptors critically involved in neuronal function and epilepsy. While we found no evidence for synaptic KARs in MCs, KAR activation induced strong action potential firing of MCs, raising the possibility that extracellular KARs regulate MC excitability
and may also promote dentate gyrus hyperexcitability and epileptogenesis.
Kainate receptors (KARs) are one of the ionotropic glutamate receptors in the central nervous system (CNS) comprised of five subunits, GluK1-GluK5. There is a growing interest in the association ...between KARs and psychiatric disorders, and there have been several studies investigating the behavioral phenotypes of KAR deficient mice, however, the difference in the genetic background has been found to affect phenotype in multiple mouse models of human diseases. Here, we examined GluK1-5 single KO mice in a pure C57BL/6N background and identified that GluK3 KO mice specifically express anxiolytic-like behavior with an alteration in dopamine D2 receptor (D2R)-induced anxiety, and reduced D2R expression in the striatum. Biochemical studies in the mouse cortex confirmed that GluK3 subunits do not assemble with GluK4 and GluK5 subunits, that can be activated by lower concentration of agonists. Overall, we found that GluK3-containing KARs function to express anxiety, which may represent promising anti-anxiety medication targets.
Ionotropic glutamate receptors play a key role in fast neurotransmission in the CNS and have been linked to several neurological diseases and disorders. One subfamily is the kainate receptors, which ...are grouped into low-affinity (GluK1-3) and high-affinity (GluK4-5) receptors based on their affinity for kainate. Although structures of the ligand-binding domain (LBD) of all low-affinity kainate receptors have been reported, no structures of the high-affinity receptor subunits are available. Here, we present the X-ray structure of GluK4-LBD with kainate at 2.05 Å resolution, together with thermofluor and radiolabel binding affinity data. Whereas binding-site residues in GluK4 are most similar to the AMPA receptor subfamily, the domain closure and D1-D2 interlobe contacts induced by kainate are similar to the low-affinity kainate receptor GluK1. These observations provide a likely explanation for the high binding affinity of kainate at GluK4-LBD.
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•Crystal structure of the high-affinity kainate receptor GluK4 ligand-binding domain•Binding-site residues in GluK4 are most similar to the AMPA receptor subfamily•Kainate-induced domain closure is similar to the low-affinity kainate receptor GluK1•Extensive interaction between lobes D1 and D2 likely contributes to the high affinity
GluK4 is a receptor in the CNS belonging to the family of ionotropic glutamate receptors. Kristensen et al. report the crystal structure of the kainate receptor GluK4 ligand-binding domain. This represents the first structure of a high-affinity kainate receptor.
Kainate-type glutamate receptors play critical roles in excitatory synaptic transmission and synaptic plasticity in the brain. GluK1 and GluK2 possess fundamentally different capabilities in surface ...trafficking as well as synaptic targeting in hippocampal CA1 neurons. Here we find that the excitatory postsynaptic currents (EPSCs) are significantly increased by the chimeric GluK1(SP
) receptor, in which the signal peptide of GluK1 is replaced with that of GluK2. Coexpression of GluK1 signal peptide completely suppresses the gain in trafficking ability of GluK1(SP
), indicating that the signal peptide represses receptor trafficking in a trans manner. Furthermore, we demonstrate that the signal peptide directly interacts with the amino-terminal domain (ATD) to inhibit the synaptic and surface expression of GluK1. Thus, we have uncovered a trafficking mechanism for kainate receptors and propose that the cleaved signal peptide behaves as a ligand of GluK1, through binding with the ATD, to repress forward trafficking of the receptor.
The kainic acid model of temporal lobe epilepsy has greatly contributed to the understanding of the molecular, cellular and pharmacological mechanisms underlying epileptogenesis and ictogenesis. This ...model presents with neuropathological and electroencephalographic features that are seen in patients with temporal lobe epilepsy. It is also characterized by a latent period that follows the initial precipitating injury (i.e., status epilepticus) until the appearance of recurrent seizures, as observed in the human condition. Finally, the kainic acid model can be reproduced in a variety of species using either systemic, intrahippocampal or intra-amygdaloid administrations. In this review, we describe the various methodological procedures and evaluate their differences with respect to the behavioral, electroencephalographic and neuropathological correlates. In addition, we compare the kainic acid model with other animal models of temporal lobe epilepsy such as the pilocarpine and the kindling model. We conclude that the kainic acid model is a reliable tool for understanding temporal lobe epilepsy, provided that the differences existing between methodological procedures are taken into account.
Shellfish poisonings have posed severe risks to human health globally. The Canadian Shellfish Sanitation Program was established in 1948 to monitor the toxin levels at shellfish harvesting sites ...along the coast of six provinces in Canada. Domoic acid has been a causal toxin for amnesic shellfish poisoning, and a macro-scale analysis of the temporal and spatial variation of domoic acid along Canada's coast was conducted in this study. We aggregated the toxin levels by week in blue mussel (Mytilus edulis) and soft-shell clam (Mya arenaria) samples, respectively, over a one-year scale. The subsequent application of Functional Principal Component Analysis unveiled that magnitudes of seasonal variation and peaked DA levels around early summer, spring, or mid-fall formed the largest variation in the toxin levels in blue mussels along the coastlines of British Columbia and Prince Edward Island and in soft-shell calms along those of New Brunswick and Nova Scotia. In Quebec, the DA levels were low and varied mostly in terms of the overall magnitude from spring to fall. Downstream correlation analyses in British Columbia further discovered that, at most sites, the strongest correlations were negative between precipitation as well as inorganic nutrients (including nitrate, nitrite, phosphate, and silicate) on one side and DA a few weeks afterward on the other. These findings indicated associations between amnesic shellfish poisoning and environmental stresses.
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•Multi-decade domoic acid (DA) in shellfish along Canada's coast was studied.•Seasonal increases of DA differed the most in BC, NB, NS, and PE.•The relatively low DA levels in QC mainly differed in overall amplitude.•Strong seasonal increases appeared at locations with a high amount of water mixing.•Precipitation and inorganic nutrients negatively correlated with DA in BC.
Kainate receptors play an important role in the brain. They contribute to postsynaptic depolarization, modulate the release of neurotransmitters such as GABA and glutamate, affect the development of ...the neuronal network. At the same time, their functions depend not only on the type of neuron expressing them but also on their localization (pre- or postsynaptic). It has been shown in present work that activation of kainate receptors by domoic acid stimulates the secretion of both glutamate and GABA. This effect is observed at a concentration of 100 nM. At higher levels (200–500 nM), domoic acid selectively activates a specific population of GABAergic neurons. The peculiarity of these neurons is increased excitability in the network. This phenomenon can be explained by the weak GABA(A)R-mediated inhibition, as well as by the lower activation threshold of voltage-gated channels. Moreover, activation of these GABAergic neurons by domoic acid leads to the suppression of activity in the network under ammonium-induced hyperexcitation. As shown by inhibitory analysis, this effect is mediated by GABA(A) receptors. The obtained data may be of interest since the suppression of hyperexcitation via the selective activation of GABAergic neurons can be considered as a new potential approach to the treatment of diseases accompanied by increased neuronal activity such as epilepsy, ischemia and hepatic encephalopathy.
•Domoic acid selectively activates a specific population of GABAergic neurons.•The peculiarity of these neurons is increased excitability in the network.•Activation of these neurons leads to the suppression of network hyperexcitation.
The development of rapid, cost-effective, and single-step methods for the detection of small molecules is crucial for improving the quality and efficiency of many applications ranging from life ...science to environmental analysis. Unfortunately, current methodologies still require multiple complex, time-consuming washing and incubation steps, which limit their applicability. In this work we present a competitive DNA-based platform that makes use of both programmable DNA-switches and antibodies to detect small target molecules. The strategy exploits both the advantages of proximity-based methods and structure-switching DNA-probes. The platform is modular and versatile and it can potentially be applied for the detection of any small target molecule that can be conjugated to a nucleic acid sequence. Here the rational design of programmable DNA-switches is discussed, and the sensitive, rapid, and single-step detection of different environmentally relevant small target molecules is demonstrated.
Kainate receptors (KARs) form a family of ionotropic glutamate receptors that regulate the activity of neuronal networks by both presynaptic and postsynaptic mechanisms. Their implication in ...pathologies is well documented for epilepsy. The higher prevalence of epileptic symptoms in Alzheimer's disease (AD) patients questions the role of KARs in AD. Here we investigated whether the synaptic expression and function of KARs was impaired in mouse models of AD. We addressed this question by immunostaining and electrophysiology at synapses between mossy fibers and CA3 pyramidal cells, in which KARs are abundant and play a prominent physiological role. We observed a decrease of the immunostaining for GluK2 in the stratum lucidum in CA3, and of the amplitude and decay time of synaptic currents mediated by GluK2-containing KARs in an amyloid mouse model (APP/PS1) of AD. Interestingly, a similar phenotype was observed in CA3 pyramidal cells in male and female mice with a genetic deletion of either presenilin or APP/APLP2 as well as in organotypic cultures treated with γ-secretase inhibitors. Finally, the GluK2 protein interacts with full-length and C-terminal fragments of APP. Overall, our data suggest that APP stabilizes KARs at synapses, possibly through a transsynaptic mechanism, and this interaction is under the control the γ-secretase proteolytic activity of presenilin.
Synaptic impairment correlates strongly with cognitive deficits in Alzheimer's disease (AD). In this context, many studies have addressed the dysregulation of AMPA and NMDA ionotropic glutamate receptors. Kainate receptors (KARs), which form the third family of iGluRs, represent an underestimated actor in the regulation of neuronal circuits and have not yet been examined in the context of AD. Here we provide evidence that synaptic KARs are markedly impaired in a mouse model of AD. Additional experiments indicate that the γ-secretase activity of presenilin acting on the amyloid precursor protein controls synaptic expression of KAR. This study clearly indicates that KARs should be taken into consideration whenever addressing synaptic dysfunction and related cognitive deficits in the context of AD.
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