Losing balance: Kainate receptors and psychiatric disorders comorbidities

Functional implications of the exon 9 splice insert in GluK1 kainate receptors

Kainate receptors are key modulators of synaptic transmission and plasticity in the central nervous system. Different kainate receptor isoforms with distinct spatiotemporal expressions have been identified in the brain. The GluK1-1 splice variant receptors, which are abundant in the adult brain, have an extra fifteen amino acids inserted in the amino-terminal domain (ATD) of the receptor resulting from alternative splicing of exon 9. However, the functional implications of this post-transcriptional modification are not yet clear. We employed a multi-pronged approach using cryogenic electron microscopy, electrophysiology, and other biophysical and biochemical tools to understand the structural and functional impact of this splice insert in the extracellular domain of GluK1 receptors. Our study reveals that the splice insert alters the key gating properties of GluK1 receptors and their modulation by the cognate auxiliary Neuropilin and tolloid-like (Neto) proteins 1 and 2. Mutational analysis identified the role of crucial splice residues that influence receptor properties and their modulation. Furthermore, the cryoEM structure of the variant shows that the presence of exon 9 in GluK1 does not affect the receptor architecture or domain arrangement in the desensitized state. Our study thus provides the first detailed structural and functional characterization of GluK1-1a receptors, highlighting the role of the splice insert in modulating receptor properties and their modulation.

Association of the GRIK4 rs1954787 polymorphism with clinical response in antidepressant-treated depressed patients: results from a prospective cohort and meta-analysis

Major Depressive Disorder (MDD) is the leading cause of disability worldwide. Genetic factors influence the effect of its main treatment option, antidepressant drugs (ATD). The GRIK4 rs1954787(T>C) genetic polymorphism was associated with response following 1-3 months of ATD treatment in some studies, but not others. We aimed to analyze its association with clinical outcomes in a cohort of 6-month ATD-treated patients and meta-analysis. Clinical data were obtained at baseline and after 1 (M1), 3 (M3), and 6 (M6) months of ATD treatment in 390 patients of the METADAP cohort. Mixed-effects models were used to assess the association of the GRIK4 rs1954787 polymorphism with the Hamilton Depression Rating Scale (HDRS) score and response and remission rates across time. Meta-analyses of ATD treatment response were performed with previously meta-analyzed data and METADAP. Compared to C allele carriers at M3 (n = 200), TT homozygotes at M3 (n = 66) had higher HDRS scores (coef = 3.37, 95% CI [1.30-5.54], Padj = 0.0046) and lower remission rates (OR = 0.36, 95% CI [0.16-0.76], Padj = 0.029). At M6, greater differences between TT homozygotes (n = 53) and C allele carriers (n = 152) were observed for HDRS scores (coef = 4.68, 95% CI [2.17-7.18], Padj = 0.00091) and remission rates (OR = 0.26, 95% CI [0.12-0.54], Padj = 0.0016). Meta-analyses of response were significant when comparing C vs T alleles (OR = 1.31, 95% CI [1.06-1.62], P = 0.014) and CC vs TT genotypes (OR = 1.63, 95% CI [1.10-2.38], P = 0.019). Altogether, our results support an association of the GRIK4 rs1954787(T>C) polymorphism with clinical improvement following ATD treatment. This association should be further assessed in other longitudinal studies. Its position within the glutamatergic system may help in understanding the mechanism of ATD action.

Mechanisms of neurodevelopmental toxicity of topiramate

Prescriptions for antiseizure medications (ASMs) have been rapidly growing over the last several decades due, in part, to an expanding list of clinical indications for which they are now prescribed. This trend has raised concern for potential adverse neurodevelopmental outcomes in ASM-exposed pregnancies. Recent large scale population studies have suggested that the use of topiramate (TOPAMAX, Janssen-Cilag), when prescribed for seizure control, migraines, and/or weight management, is associated with an increased risk for autism spectrum disorder (ASD), intellectual disability, and attention-deficit/hyperactivity disorder (ADHD) in exposed offspring. Here, we critically review epidemiologic evidence demonstrating the neurobehavioral teratogenicity of topiramate and speculate on the neuromolecular mechanisms by which prenatal exposure may perturb neurocognitive development. Specifically, we explore the potential role of topiramate's pharmacological interactions with ligand- and voltage-gated ion channels, especially GABAergic signaling, its effects on DNA methylation and histone acetylation, whether topiramate induces oxidative stress, and its association with fetal growth restriction as possible mechanisms contributing to neurodevelopmental toxicity. Resolving this biology will be necessary to reduce the risk of adverse pregnancy outcomes caused by topiramate or other ASMs.

Kainate receptors regulate synaptic integrity and plasticity by forming a complex with synaptic organizers in the cerebellum

Kainate (KA)-type glutamate receptors (KARs) are implicated in various neuropsychiatric and neurological disorders through their ionotropic and metabotropic actions. However, compared to AMPA- and NMDA-type receptor functions, many aspects of KAR biology remain incompletely understood. Our study demonstrates an important role of KARs in organizing climbing fiber (CF)-Purkinje cell (PC) synapses and synaptic plasticity in the cerebellum, independently of their ion channel or metabotropic functions. The amino-terminal domain (ATD) of the GluK4 KAR subunit binds to C1ql1, provided by CFs, and associates with Bai3, an adhesion-type G protein-coupled receptor expressed in PC dendrites. Mice lacking GluK4 exhibit no KAR-mediated responses, reduced C1ql1 and Bai3 levels, and fewer CF-PC synapses, along with impaired long-term depression and oculomotor learning. Remarkably, introduction of the ATD of GluK4 significantly improves all these phenotypes. These findings demonstrate that KARs act as synaptic scaffolds, orchestrating synapses by forming a KAR-C1ql1-Bai3 complex in the cerebellum.

GluK1 kainate receptors are necessary for functional maturation of parvalbumin interneurons regulating amygdala circuit function

Parvalbumin expressing interneurons (PV INs) are key players in the local inhibitory circuits and their developmental maturation coincides with the onset of adult-type network dynamics in the brain. Glutamatergic signaling regulates emergence of the unique PV IN phenotype, yet the receptor mechanisms involved are not fully understood. Here we show that GluK1 subunit containing kainate receptors (KARs) are necessary for development and maintenance of the neurochemical and functional properties of PV INs in the lateral and basal amygdala (BLA). Ablation of GluK1 expression specifically from PV INs resulted in low parvalbumin expression and loss of characteristic high firing rate throughout development. In addition, we observed reduced spontaneous excitatory synaptic activity at adult GluK1 lacking PV INs. Intriguingly, inactivation of GluK1 expression in adult PV INs was sufficient to abolish their high firing rate and to reduce PV expression levels, suggesting a role for GluK1 in dynamic regulation of PV IN maturation state. The PV IN dysfunction in the absence of GluK1 perturbed the balance between evoked excitatory vs. inhibitory synaptic inputs and long-term potentiation (LTP) in LA principal neurons, and resulted in aberrant development of the resting-state functional connectivity between mPFC and BLA. Behaviorally, the absence of GluK1 from PV INs associated with hyperactivity and increased fear of novelty. These results indicate a critical role for GluK1 KARs in regulation of PV IN function across development and suggest GluK1 as a potential therapeutic target for pathologies involving PV IN malfunction.

Progressive development of synchronous activity in the hippocampal neuronal network is modulated by GluK1 kainate receptors

Kainate receptors are potent modulators of circuit excitability and have been repeatedly implicated in pathophysiological synchronization of limbic networks. While the role of aberrant GluK2 subunit containing KARs in generation of epileptiform hypersynchronous activity is well described, the contribution of other KAR subtypes, including GluK1 subunit containing KARs remain less well understood. To investigate the contribution of GluK1 KARs in developmental and pathological synchronization of the hippocampal neural network, we used multielectrode array recordings on organotypic hippocampal slices that display first multi-unit activity and later spontaneous population discharges resembling ictal-like epileptiform activity (IEA). Chronic blockage of GluK1 activity using selective antagonist ACET or lentivirally delivered shRNA significantly delayed developmental synchronization of the hippocampal CA3 network and generation of IEA. GluK1 overexpression, on the other hand, had no significant effect on occurrence of IEA, but enhanced the size of the neuron population participating in the population discharges. Correlation analysis indicated that local knockdown of GluK1 locally in the CA3 neurons reduced their functional connectivity, while GluK1 overexpression increased the connectivity to both CA1 and DG. These data suggest that GluK1 KARs regulate functional connectivity between the excitatory neurons, possibly via morphological changes in glutamatergic circuit, affecting synchronization of neuronal populations. The significant effects of GluK1 manipulations on network activity call for further research on GluK1 KAR as potential targets for antiepileptic treatments, particularly during the early postnatal development when GluK1 KARs are strongly expressed in the limbic neural networks.

Positive and negative allosteric modulation of GluK2 kainate receptors by BPAM344 and antiepileptic perampanel

Kainate receptors (KARs) are a subtype of ionotropic glutamate receptors that control synaptic transmission in the central nervous system and are implicated in neurological, psychiatric, and neurodevelopmental disorders. Understanding the regulation of KAR function by small molecules is essential for exploring these receptors as drug targets. Here, we present cryoelectron microscopy (cryo-EM) structures of KAR GluK2 in complex with the positive allosteric modulator BPAM344, competitive antagonist DNQX, and negative allosteric modulator, antiepileptic drug perampanel. Our structures show that two BPAM344 molecules bind per ligand-binding domain dimer interface. In the absence of an agonist or in the presence of DNQX, BPAM344 stabilizes GluK2 in the closed state. The closed state is also stabilized by perampanel, which binds to the ion channel extracellular collar sites located in two out of four GluK2 subunits. The molecular mechanisms of positive and negative allosteric modulation of KAR provide a guide for developing new therapeutic strategies.

Kainate Receptor Antagonists: Recent Advances and Therapeutic Perspective

Since the 1990s, ionotropic glutamate receptors have served as an outstanding target for drug discovery research aimed at the discovery of new neurotherapeutic agents. With the recent approval of perampanel, the first marketed non-competitive antagonist of AMPA receptors, particular interest has been directed toward 'non-NMDA' (AMPA and kainate) receptor inhibitors. Although the role of AMPA receptors in the development of neurological or psychiatric disorders has been well recognized and characterized, progress in understanding the function of kainate receptors (KARs) has been hampered, mainly due to the lack of specific and selective pharmacological tools. The latest findings in the biology of KA receptors indicate that they are involved in neurophysiological activity and play an important role in both health and disease, including conditions such as anxiety, schizophrenia, epilepsy, neuropathic pain, and migraine. Therefore, we reviewed recent advances in the field of competitive and non-competitive kainate receptor antagonists and their potential therapeutic applications. Due to the high level of structural divergence among the compounds described here, we decided to divide them into seven groups according to their overall structure, presenting a total of 72 active compounds.

Kainate receptor subunit 1 (GRIK1) risk variants and GRIK1 deficiency were detected in the Indian ADHD probands

Executive dysfunctions caused by structural and functional abnormalities of the prefrontal cortex were reported in patients with Attention deficit hyperactivity disorder (ADHD). Owing to a higher expression of the glutamate ionotropic receptor kainate type subunit 1 (GluK1), encoded by the GRIK1 gene, in brain regions responsible for learning and memory, we hypothesized that GRIK1 might have a role in ADHD. GRIK1 variants rs363504 and rs363538, affecting the receptor function, were analyzed by case-control and family-based methods to identify the association with ADHD. The impact of these variants on ADHD-associated traits and pharmacological intervention were also analyzed. GRIK1 expression was quantified in the peripheral blood. The probands and their fathers had a higher frequency of rs363504 'CC' and rs363538 'CA' genotypes. Family-based investigation revealed maternal over transmission of rs363504 'C' and rs363538 'A' alleles to the probands. Quantitative trait analysis exhibited an association of rs363504 'TT' and rs363538 'AA' genotypes with higher hyperactivity scores of the probands. In the presence of rs363504 'TT' and rs363538 'CC' genotypes, MPH treatment improved hyperactivity and inattention, respectively. GRIK1 expression was significantly downregulated in the probands. We infer that GRIK1 affects ADHD etiology, warranting further in-depth investigation involving a larger cohort and more functional variants.

Discovery of the First Highly Selective Antagonist of the GluK3 Kainate Receptor Subtype

Kainate receptors belong to the family of glutamate receptors ion channels, which are responsible for the majority of rapid excitatory synaptic transmission in the central nervous system. The therapeutic potential of kainate receptors is still poorly understood, which is also due to the lack of potent and subunit-selective pharmacological tools. In search of selective ligands for the GluK3 kainate receptor subtype, a series of quinoxaline-2,3-dione analogues was synthesized and pharmacologically characterized at selected recombinant ionotropic glutamate receptors. Among them, compound 28 was found to be a competitive GluK3 antagonist with submicromolar affinity and unprecedented high binding selectivity, showing a 400-fold preference for GluK3 over other homomeric receptors GluK1, GluK2, GluK5 and GluA2. Furthermore, in functional assays performed for selected metabotropic glutamate receptor subtypes, 28 did not show agonist or antagonist activity. The molecular determinants underlying the observed affinity profile of 28 were analyzed using molecular docking and molecular dynamics simulations performed for individual GluK1 and GluK3 ligand-binding domains.

Structure, Function, and Regulation of the Kainate Receptor

Neural communication and modulation are complex processes. Ionotropic glutamate receptors (iGluRs) significantly contribute to mediating the fast-excitatory branch of neurotransmission in the mammalian brain. Kainate receptors (KARs), a subfamily of the iGluRs, act as modulators of the neuronal circuitry by playing important roles at both the post- and presynaptic sites of specific neurons. The functional tetrameric receptors are formed by two different gene families, low agonist affinity (GluK1-GluK3) and high agonist affinity (GluK4-GluK5) subunits. These receptors garnered attention in the past three decades, and since then, much work has been done to understand their localization, interactome, physiological functions, and regulation. Cloning of the receptor subunits (GluK1-GluK5) in the early 1990s led to recombinant expression of kainate receptors in heterologous systems. This facilitated understanding of the functional differences between subunit combinations, splice variants, trafficking, and drug discovery. Structural studies of individual domains and recent full-length homomeric and heteromeric kainate receptors have revealed unique functional mechanisms, which have answered several long-standing questions in the field of kainate receptor biology. In this chapter, we review the current understanding of kainate receptors and associated disorders.

Postsynaptic autism spectrum disorder genes and synaptic dysfunction

This review provides an overview of the synaptic dysfunction of neuronal circuits and the ensuing behavioral alterations caused by mutations in autism spectrum disorder (ASD)-linked genes directly or indirectly affecting the postsynaptic neuronal compartment. There are plenty of ASD risk genes, that may be broadly grouped into those involved in gene expression regulation (epigenetic regulation and transcription) and genes regulating synaptic activity (neural communication and neurotransmission). Notably, the effects mediated by ASD-associated genes can vary extensively depending on the developmental time and/or subcellular site of expression. Therefore, in order to gain a better understanding of the mechanisms of disruptions in postsynaptic function, an effort to better model ASD in experimental animals is required to improve standardization and increase reproducibility within and among studies. Such an effort holds promise to provide deeper insight into the development of these disorders and to improve the translational value of preclinical studies.