GluK2 Research Articles

Pharmacogenetics Studies in STAR*D: Strengths, Limitations, and Results

Pharmacogenetics Studies in STAR*D: Strengths, Limitations, and Results

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.

Modulation of hippocampal activation by genetic variation in the GRIK4 gene

The GRIK4 gene encodes the kainate glutamate receptor subunit KA1, which is expressed at high levels in the hippocampus. An indel variant in the 30-untranslated region of GRIK4 (rs58575285) has been shown to modify the risk of bipolar disorder. The image on the left shows hippocampal activation in healthy control subjects during a functional magnetic resonance imaging (fMRI) face-processing task. The image on the right shows the effect of genetic variation in the GRIK4 gene at rs58575285 on hippocampal activation using this task. Subjects carrying the deletion associated with a lower risk of bipolar disorder showed greater hippocampal activation than non-carriers. Statistical parametric maps were thresholded at P < 0.001. For more information on this topic, please refer to the article by Whalley et al. on pages 467–468. Molecular Psychiatry (2009) 14, 465 & 2009 Nature Publishing Group All rights reserved 1359-4184/09 $32.00

A preliminary pharmacogenetic investigation of adverse events from topiramate in heavy drinkers.

Topiramate, an anticonvulsant medication, is an efficacious treatment for alcohol dependence. To date, little is known about genetic moderators of side effects from topiramate. The objective of this study was to examine 3 single nucleotide polymorphisms (SNPs) of the glutamate receptor GluR5 gene (GRIK1) as predictors of topiramate-induced side effects in the context of a laboratory study of topiramate. Heavy drinkers (n=51, 19 women and 32 men), 75% of whom met criteria for an alcohol use disorder, completed a 5-week dose escalation schedule to a target dose of either 200 or 300 mg or matched placebo. The combined medication groups were compared with placebo-treated individuals for side effects at target dose. Analyses revealed that an SNP in intron 9 of the GRIK1 gene (rs2832407) was associated with the severity of topiramate-induced side effects and with serum levels of topiramate. Genes underlying glutamatergic neurotransmission, such as the GRIK1 gene, may help predict heterogeneity in topiramate-induced side effects. Future studies in larger samples are needed to more fully establish these preliminary findings.

Detailed phenotype–genotype study in five patients with chromosome 6q16 deletion: narrowing the critical region for Prader–Willi-like phenotype

Most patients with an interstitial deletion of 6q16 have Prader-Willi-like phenotype, featuring obesity, hypotonia, short hands and feet, and developmental delay. In all reported studies, the chromosome rearrangement was detected by karyotype analysis, which provides an overview of the entire genome but has limited resolution. Here we describe a detailed clinical presentation of five patients, two of whom were previously reported, with overlapping interstitial 6q16 deletions and Prader-Willi-like phenotype. Our patients share the following main features with previously reported cases: global developmental delay, hypotonia, obesity, hyperphagia, and eye/vision anomalies. All rearrangement breakpoints have been accurately defined through array-CGH at about 100 Kb resolution. We were able to narrow the shortest region of deletion overlap for the presumed gene(s) involved in the Prader-Willi-like syndrome to 4.1 Mb located at 6q16.1q16.2. Our results support the evidence that haploinsufficiency of the SIM1 gene is responsible for obesity in these patients. A possible involvement of the GRIK2 gene in autistic-like behaviour, of POPDC3 in heart development, and of MCHR2 in the control of feeding behaviour and energy metabolism is also hypothesized.

Genetic Markers Within Glutamate Receptors Associated With Antidepressant Treatment-Emergent Suicidal Ideation

Genetic Markers Within Glutamate Receptors Associated With Antidepressant Treatment-Emergent Suicidal Ideation

No genetic association between polymorphisms in the kainate-type glutamate receptor gene, GRIK4, and schizophrenia in the Chinese population

Background Schizophrenia is a chronic psychiatric disorder with a strong genetic component. Several studies have suggested that dysfunctions in the glutamatergic transmission are linked to the pathogenesis of schizophrenia, and that the kainate ionotropic glutamate receptors are involved in this mechanism. A recent study provides cytogenetic and genetic evidence to support a role for the kainate-type glutamate receptor gene ( GRIK4), in schizophrenia. A systematic case-control association study of GRIK4 involving a Scottish population found that three SNPs, rs4935752, rs6589846 and rs4430518, were associated with schizophrenia. Methods Here, we investigated rs4935752, rs6589846, rs4430518 and other 2 SNPs within the GRIK4 gene in an association study of the Chinese population. Our sample consisted of 288 schizophrenia and 288 control subjects. All recruits were Han Chinese drawn from the city of Shanghai. Results No individual SNP nor any haplotype was associated with schizophrenia in our study. Conclusion These results suggest that the five SNPs within the GRIK4 gene are unlikely to play a major role in the susceptibility to schizophrenia in the Chinese population.

Association of GRIK4 With Outcome of Antidepressant Treatment in the STAR*D Cohort

An initial pharmacogenetic study of the Sequenced Treatment Alternatives to Relieve Depression (STAR*D) clinical trial reported an association between genetic variation in the HTR2A gene and outcome of citalopram treatment. By design, the study analyzed only those markers that showed reproducible association in the first wave of genotypes (comprising 1,297 patients) in the complete cohort of patients. The purpose of the present study was to utilize a second wave of genotype results, for a more powerful analysis, in the complete cohort of patients with available deoxyribonucleic acid (DNA) samples. The authors tested the association between treatment response and 768 markers that were genotyped in the full set of 1,816 eligible patients from the STAR*D cohort. In order to control for multiple testing, the subjects were divided into two study groups: discovery and replication. In addition to the previously identified marker in the HTR2A gene, a new marker (rs1954787) in the GRIK4 gene, which codes for the kainic acid-type glutamate receptor KA1, was observed. The effect size of the GRIK4 marker alone was modest, but homozygote carriers of the treatment-response-associated marker alleles of both the GRIK4 and HTR2A genes were 23% less likely to experience nonresponse to treatment relative to participants who did not carry any of these marker alleles. The findings demonstrate that genetic variation in a kainic acid-type glutamate receptor is reproducibly associated with response to the antidepressant citalopram. This finding suggests that the glutamate system plays an important role in modulating response to selective serotonin reuptake inhibitors (SSRIs).

Genetic analysis of candidate genes modifying the age-at-onset in Huntington’s disease

The expansion of a polymorphic CAG repeat in the HD gene encoding huntingtin has been identified as the major cause of Huntington's disease (HD) and determines 42-73% of the variance in the age-at-onset of the disease. Polymorphisms in huntingtin interacting or associated genes are thought to modify the course of the disease. To identify genetic modifiers influencing the age at disease onset, we searched for polymorphic markers in the GRIK2, TBP, BDNF, HIP1 and ZDHHC17 genes and analysed seven of them by association studies in 980 independent European HD patients. Screening for unknown sequence variations we found besides several silent variations three polymorphisms in the ZDHHC17 gene. These and polymorphisms in the GRIK2, TBP and BDNF genes were analysed with respect to their association with the HD age-at-onset. Although some of the factors have been defined as genetic modifier factors in previous studies, none of the genes encoding GRIK2, TBP, BDNF and ZDHHC17 could be identified as a genetic modifier for HD.

Characterisation of UBP296: a novel, potent and selective kainate receptor antagonist

Willardiine derivatives with an N 3-benzyl substituent bearing an acidic group have been synthesized with the aim of producing selective antagonists for GLU K5-containing kainate receptors. UBP296 was found to be a potent and selective antagonist of native GLU K5-containing kainate receptors in the spinal cord, with activity residing in the S enantiomer (UBP302). In cells expressing human kainate receptor subunits, UBP296 selectively depressed glutamate-induced calcium influx in cells containing GLU K5 in homomeric or heteromeric forms. In radioligand displacement binding studies, the willardiine analogues displaced [ 3H]kainate binding with IC 50 values >100 μM at rat GLU K6, GLU K2 or GLU K6/GLU K2. An explanation of the GLU K5 selectivity of UBP296 was obtained using homology models of the antagonist bound forms of GLU K5 and GLU K6. In rat hippocampal slices, UBP296 reversibly blocked ATPA-induced depressions of synaptic transmission at concentrations subthreshold for affecting AMPA receptor-mediated synaptic transmission directly. UBP296 also completely blocked the induction of mossy fibre LTP, in medium containing 2 mM (but not 4 mM) Ca 2+. These data provide further evidence for a role for GLU K5-containing kainate receptors in mossy fibre LTP. In conclusion, UBP296 is the most potent and selective antagonist of GLU K5-containing kainate receptors so far described.

Sequencing of the GRIK1 gene in patients with juvenile absence epilepsy does not reveal mutations affecting receptor structure.

Hereditary factors play a major role in the etiology of juvenile absence epilepsy (JAE) that is a common subtype of idiopathic generalized epilepsy (IGE). Sander et al. [1997: Am J Med Genet 74:416-421] reported an allelic association of JAE with the nine-copy allele of a tetranucleotide repeat polymorphism in the third intron of the kainate-selective GluR5 receptor gene (GRIK1) and supportive evidence for linkage of IGE to GRIK1 in families of JAE probands. These findings suggest that a major genetic determinant of GRIK1 confers susceptibility to JAE. Assuming that the GRIK1 tetranucleotide repeat polymorphism is unlikely to have functional relevance itself, we have sequenced the coding regions and regulatory sequences of the GRIK1 gene in eight JAE patients who carry the nine-repeat allele of the GRIK1 tetranucleotide repeat polymorphism to detect a putative functional GRIK1 mutation that is in linkage disequilibrium with the nine-repeat allele. Seven of them were derived from families showing positive evidence for linkage to GRIK1. Our mutation analysis of coding regions and splice junctions revealed only two silent polymorphisms (A522C and C1173T) out of the five SNPs present in public databases and no mutations affecting protein structure. No significant differences were found in the allele frequencies of the detected polymorphisms between the JAE patients and controls. High levels of sequence conservation were also found in the promoter, in the 5' and both the 3' untranslated regions and in the RNA secondary structure involved in the editing reaction. The results presented indicate that mutations in the coding sequences, in the intron-exon boundaries and in the main regulatory regions of the GRIK1 are not commonly involved in the etiology of JAE.

Genomic organization of the human GRIK2 gene and evidence for multiple splicing variants

Fast excitatory transmission in the vertebrate central nervous system is mediated mainly by l-glutamate. Here we present the genomic organization of the human GRIK2 gene, which codes for the kainate GluR6 receptor subunit, deduced from sequence data present in the public databases and analyzed by bioinformatic tools. By similarity search using the human GluR6 cDNA sequence against non-redundant databases, we found three positive entries (AP002528, AP002529, and AP002530 deposited by Hirakawa et al., 2000) which are part of a BAC contig of about 1 Mb spanning region 6q21. The GRIK2 gene was found to be split into 17 exons, covering about 670 kb of the region. The availability of the data on the genomic organization allowed the study of GRIK2 gene expression by RT-PCR analysis which was performed on human teratocarcinoma cell cultures (NT2) and on mRNA obtained from human hippocampus (Clontech). The study gives evidence for several different splicing variants in addition to the previously cloned human GluR6 cDNA (ID: U16126). The splicing mechanism leading to the different isoforms involves exons 11, 12 and 16. The mRNA containing exon 16 at the 3′ end is the homolog to the mouse GluR6-2. The translation of this mRNA would code for a different intracellular C-terminus, as compared to that coded by the known human isoform. The newly identify isoform is the predominant form expressed in human teratocarcinoma NT2 cells. All the newly identified mRNAs isoforms are expressed in NT2 cells and in human hippocampus mRNA at variable levels and would be responsible for the production of five different putative GluR6 receptor subunits, some differing in the C-terminal domains (mouse homolog) and some lacking specific transmembrane domains.

Allelic association of juvenile absence epilepsy with a GluR5 kainate receptor gene (GRIK1) polymorphism

Allelic association of juvenile absence epilepsy with a GluR5 kainate receptor gene (GRIK1) polymorphism

Differential assembly of coexpressed glutamate receptor subunits in neurons of rat cerebral cortex

In the rat, subunits of the glutamate receptor family fall into three pharmacologically distinct groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid preferring receptors (Glu R1-4), kainate preferring receptors (Glu R5-7, KA 1, KA 2), and N-methyl-D-aspartate preferring receptors (NMDA R1, NMDA R2A-2D). In the present study, we demonstrate immunocytochemically that the majority of neurons in rat cerebral cortex coexpress members of all three groups of glutamate receptor subunits, Glu R2/3, Glu R5/6/7, and NMDA R1. Using immunoaffinity purified or immunoprecipitated alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, we show that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors containing Glu R1-4, kainate receptors containing Glu R6, Glu R7, and KA 2 and N-methyl-D-aspartate receptors containing NMDA R1 each form distinct protein complexes that do not share subunits. Our data indicate that a mechanism exists which allows for the specific assembly of selected glutamate receptor subunits into functionally and structurally distinct heteromeric receptors.