Nucleotide sequence, genomic organization, and chromosomal localization of genes encoding the human NMDA receptor subunits NR3A and NR3B

Analysis of Surface Expression of NMDAR Subunits in Primary Hippocampal Neurons

The expression and activity of ionotropic glutamate receptors control signal transduction at the excitatory synapses in the CNS. The NMDAR comprises two obligatory GluN1 subunits and two GluN2 or GluN3 subunits in different combinations. Each GluN subunit consists of four domains: the extracellular amino-terminal and agonist-binding domains, the transmembrane domain, and the intracellular C-terminal domain (CTD). The CTD interaction with various classes of intracellular proteins is critical for trafficking and synaptic localization of NMDARs. Amino acid mutations or the inclusion of premature stop codons in the CTD could contribute to the emergence of neurodevelopmental and neuropsychiatric disorders. Here, we describe the method of preparing primary hippocampal neurons and lentiviral particles expressing GluN subunits that can be used as a model to study cell surface expression and synaptic localization of NMDARs. We also show a simple method of fluorescence immunostaining of eGFP-tagged GluN2 subunits and subsequent microscopy technique and image analysis to study the effects of disease-associated mutations in the CTDs of GluN2A and GluN2B subunits.

DNA methylation and the opposing NMDAR dysfunction in schizophrenia and major depression disorders: a converging model for the therapeutic effects of psychedelic compounds in the treatment of psychiatric illness

Psychedelic compounds are being increasingly explored as a potential therapeutic option for treating several psychiatric conditions, despite relatively little being known about their mechanism of action. One such possible mechanism, DNA methylation, is a process of epigenetic regulation that changes gene expression via chemical modification of nitrogenous bases. DNA methylation has been implicated in the pathophysiology of several psychiatric conditions, including schizophrenia (SZ) and major depressive disorder (MDD). In this review, we propose alterations to DNA methylation as a converging model for the therapeutic effects of psychedelic compounds, highlighting the N-methyl D-aspartate receptor (NMDAR), a crucial mediator of synaptic plasticity with known dysfunction in both diseases, as an example and anchoring point. We review the established evidence relating aberrant DNA methylation to NMDAR dysfunction in SZ and MDD and provide a model asserting that psychedelic substances may act through an epigenetic mechanism to provide therapeutic effects in the context of these disorders.

Excitatory GluN1/GluN3A glycine receptors (eGlyRs) in brain signaling

GluN3A is a glycine-binding subunit belonging to the NMDA receptor (NMDAR) family that can assemble with GluN1 subunits to form unconventional NMDARs insensitive to glutamate and activated by glycine only. The existence of such excitatory glycine receptors (eGlyRs) in the central nervous system (CNS) has long remained elusive. Recently, eGlyRs have been identified in specific brain regions, where they represent a novel neuronal signaling modality by which extracellular glycine tunes neuronal excitability, circuit function, and behavior. In this review, we summarize the emerging knowledge regarding these underappreciated receptors. The existence of eGlyRs reshapes current understanding of NMDAR diversity and of glycinergic signaling, previously thought to be primarily inhibitory. Given that GluN3A expression is concentrated in brain regions regulating emotional responses, eGlyRs are potential new targets of therapeutic interest in neuropsychiatry.

Comparative Investigation of Expression of Glutamatergic and GABAergic Genes in the Rat Hippocampus after Focal Brain Ischemia and Central LPS Administration

Among the responses in the early stages of stroke, activation of neurodegenerative and proinflammatory processes in the hippocampus is of key importance for the development of negative post-ischemic functional consequences. However, it remains unclear, what genes are involved in these processes. The aim of this work was a comparative study of the expression of genes encoding glutamate and GABA transporters and receptors, as well as inflammation markers in the hippocampus one day after two types of middle cerebral artery occlusion (according to Koizumi et al. method, MCAO-MK, and Longa et al. method, MCAO-ML), and direct pro-inflammatory activation by central administration of bacterial lipopolysaccharide (LPS). Differences and similarities in the effects of these challenges on gene expression were observed. Expression of a larger number of genes associated with activation of apoptosis and neuroinflammation, glutamate reception, and markers of the GABAergic system changed after the MCAO-ML and LPS administration than after the MCAO-MK. Compared with the MCAO-ML, the MCAO-MK and LPS challenges caused changes in the expression of more genes involved in glutamate transport. The most pronounced difference between the responses to different challenges was the changes in expression of calmodulin and calmodulin-dependent kinases genes observed after MCAO, especially MCAO-ML, but not after LPS. The revealed specific features of the hippocampal gene responses to the two types of ischemia and a pro-inflammatory stimulus could contribute to further understanding of the molecular mechanisms underlying diversity of the post-stroke consequences both in the model studies and in the clinic.

Clonidine ameliorates cerebral ischemia-reperfusion injury by up-regulating the GluN3 subunits of NMDA receptor

This study aimed to investigate the protective effects of the alpha-2 adrenergic receptor (α2-AR) agonist, clonidine, on the cerebral ischemia-reperfusion (I/R) injury and elaborate the underlying mechanisms. Cerebral I/R model was established by middle cerebral artery occlusion (MCAO) for 2 h followed by reperfusion for 4 h in adult male SD rats. Saline, clonidine and yohimbine (an α2-AR antagonist) were intraperitoneally administered each day for one week before surgery. Neurological deficit was evaluated just before decapitation. TTC staining was applied for correlation of cerebral infarction volume. HE staining was performed to observe the neuron morphology. Immunohistochemical staining was performed to detect the localization and expression of GluN3 proteins. Western blot analysis also was used to detect the expression levels of GluN3 proteins. Our data showed that clonidine ameliorated neurological deficit and reduced the cerebral infarction volume of the rats with cerebral I/R. It is worth noting that treatment with clonidine up-regulated the protein expression of GluN3 in the rats with the cerebral I/R, especially in the cell membrane. Moreover, clonidine also up-regulated the transposition from cytoplasm to cell membrane of GluN3 after cerebral I/R. In addition, yohimbine abolished the neuroprotective effects of clonidine. The results indicated that clonidine played a protective role in cerebral I/R injury through regulation of the protein expression of GluN3 subunits of N-methyl-D-aspartate (NMDA) receptor.

Micronutrient supplementation affects DNA methylation in male gonads with potential intergenerational epigenetic inheritance involving the embryonic development through glutamate receptor-associated genes

BACKGROUND

DNA methylation has an important role in intergenerational inheritance. An increasing number of studies have reported evidence of germline inheritance of DNA methylation induced by nutritional signals in mammals. Vitamins and minerals as micronutrients contribute to growth performance in vertebrates, including Atlantic salmon (Salmo salar), and also have a role in epigenetics as environmental factors that alter DNA methylation status. It is important to understand whether micronutrients in the paternal diet can influence the offspring through alterations of DNA methylation signatures in male germ cells.

RESULTS

Here, we show the effect of micronutrient supplementation on DNA methylation profiles in the male gonad through a whole life cycle feeding trial of Atlantic salmon fed three graded levels of micronutrient components. Our results strongly indicate that micronutrient supplementation affects the DNA methylation status of genes associated with cell signalling, synaptic signalling, and embryonic development. In particular, it substantially affects DNA methylation status in the promoter region of a glutamate receptor gene, glutamate receptor ionotropic, NMDA 3A-like (grin3a-like), when the fish are fed both medium and high doses of micronutrients. Furthermore, two transcription factors, histone deacetylase 2 (hdac2) and a zinc finger protein, bind to the hyper-methylated site in the grin3a-like promoter. An estimated function of hdac2 together with a zinc finger indicates that grin3a-like has a potential role in intergenerational epigenetic inheritance and the regulation of embryonic development affected by paternal diet.

CONCLUSIONS

The present study demonstrates alterations of gene expression patterns and DNA methylation signatures in the male gonad when Atlantic salmon are fed different levels of micronutrients. Alterations of gene expression patterns are of great interest because the gonads are supposed to have limited metabolic activities compared to other organs, whereas alterations of DNA methylation signatures are of great importance in the field of nutritional epigenetics because the signatures affected by nutrition could be transferred to the next generation. We provide extensive data resources for future work in the context of potential intergenerational inheritance through the male germline.

The roles of GluN3-containing N-methyl-D-aspartate receptor in central nerve system

The N-methyl-D-aspartate receptor (NMDAR) in central nerve system is mostly composed of GluN1 and GluN2 subunits. The classical NMDAR has been intensively studied. However, GluN3‑containing NMDAR is much less expressed and have atypical channel properties. Recently, accumulating evidences have revealed two types of GluN3‑containing NMDAR: glutamate-gated GluN1/GluN2/GluN3 NMDAR and glycine-gated GluN1/GluN3 NMDAR. The former may play important roles in regulating synapse maturation and pruning non-used synapses, and its elevated expression at the adult stage may alter synaptic reorganization in some neuropsychiatric disorders. The latter is expressed in the medial habenula and involves in control of aversion. This article reviews the recent progresses on the expression, functional properties of GluN3‑containing atypical NMDARs and the physiological and pathological relevance.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Transcriptome-wide association study of post-trauma symptom trajectories identified GRIN3B as a potential biomarker for PTSD development

Biomarkers that predict symptom trajectories after trauma can facilitate early detection or intervention for posttraumatic stress disorder (PTSD) and may also advance our understanding of its biology. Here, we aimed to identify trajectory-based biomarkers using blood transcriptomes collected in the immediate aftermath of trauma exposure. Participants were recruited from an Emergency Department in the immediate aftermath of trauma exposure and assessed for PTSD symptoms at baseline, 1, 3, 6, and 12 months. Three empirical symptom trajectories (chronic-PTSD, remitting, and resilient) were identified in 377 individuals based on longitudinal symptoms across four data points (1, 3, 6, and 12 months), using latent growth mixture modeling. Blood transcriptomes were examined for association with longitudinal symptom trajectories, followed by expression quantitative trait locus analysis. GRIN3B and AMOTL1 blood mRNA levels were associated with chronic vs. resilient post-trauma symptom trajectories at a transcriptome-wide significant level (N = 153, FDR-corrected p value = 0.0063 and 0.0253, respectively). We identified four genetic variants that regulate mRNA blood expression levels of GRIN3B. Among these, GRIN3B rs10401454 was associated with PTSD in an independent dataset (N = 3521, p = 0.04). Examination of the BrainCloud and GTEx databases revealed that rs10401454 was associated with brain mRNA expression levels of GRIN3B. While further replication and validation studies are needed, our data suggest that GRIN3B, a glutamate ionotropic receptor NMDA type subunit-3B, may be involved in the manifestation of PTSD. In addition, the blood mRNA level of GRIN3B may be a promising early biomarker for the PTSD manifestation and development.

Splicing and editing of ionotropic glutamate receptors: a comprehensive analysis based on human RNA-Seq data

Ionotropic glutamate receptors (iGluRs) play key roles for signaling in the central nervous system. Alternative splicing and RNA editing are well-known mechanisms to increase iGluR diversity and to provide context-dependent regulation. Earlier work on isoform identification has focused on the analysis of cloned transcripts, mostly from rodents. We here set out to obtain a systematic overview of iGluR splicing and editing in human brain based on RNA-Seq data. Using data from two large-scale transcriptome studies, we established a workflow for the de novo identification and quantification of alternative splice and editing events. We detected all canonical iGluR splice junctions, assessed the abundance of alternative events described in the literature, and identified new splice events in AMPA, kainate, delta, and NMDA receptor subunits. Notable events include an abundant transcript encoding the GluA4 amino-terminal domain, GluA4-ATD, a novel C-terminal GluD1 (delta receptor 1) isoform, GluD1-b, and potentially new GluK4 and GluN2C isoforms. C-terminal GluN1 splicing may be controlled by inclusion of a cassette exon, which shows preference for one of the two acceptor sites in the last exon. Moreover, we identified alternative untranslated regions (UTRs) and species-specific differences in splicing. In contrast, editing in exonic iGluR regions appears to be mostly limited to ten previously described sites, two of which result in silent amino acid changes. Coupling of proximal editing/editing and editing/splice events occurs to variable degree. Overall, this analysis provides the first inventory of alternative splicing and editing in human brain iGluRs and provides the impetus for further transcriptome-based and functional investigations.

Pharmacodynamic elucidation of glutamate & dopamine in ketamine-induced anaesthesia

General anaesthetics are some of the most widely used and essential therapeutic agents. However, despite over a century of research, the molecular mechanisms of general anaesthesia in the central nervous system remain elusive. Ketamine (ketamine hydrochloride) has been approved for use in general anaesthesia either alone or in combination with other medications. It is a superb drug for use in short-term medical procedures that do not require skeletal muscle relaxation, and it has approval for the induction of general anaesthesia as a pre-anaesthetic to other general anaesthetic agents. However, Several questions remain unsolved, including the exact identification of the neural substrate of consciousness and its components, the pharmacodynamic interactions between anaesthetic agents, the mechanisms of cognitive alterations that follow an anaesthetic procedure, the identification of an eventual unitary mechanism of anaesthesia-induced alteration of consciousness, the relationship between network effects and the biochemical targets of anaesthetic agents, leading to difficulties in between-studies comparisons. Thus, the glutamate and dopamine systems play distinct roles in terms of neuronal signalling, yet both have proposed to contribute significantly to the pathophysiology of neuropsychiatric diseases. Imaging of the glutamate system and other aspects of research on the dopamine system have produced less consistent findings, potentially due to methodological limitations and the heterogeneity of the disorder. In this review, we discuss the neural circuits through which the two systems interact and how their disruption may cause psychotic symptoms. We also summarize from a molecular perspective of mechanisms of action of ketamine as general anaesthetics on ligand-gated ion channels mediated modulation of dopamine in the brain region.

The Role of NMDA Receptors in Alzheimer's Disease

In Alzheimer’s disease (AD), early synaptic dysfunction is associated with the increased oligomeric amyloid-beta peptide, which causes NMDAR-dependent synaptic depression and spine elimination. Memantine, low-affinity NMDAR channel blocker, has been used in the treatment of moderate to severe AD. However, clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDARs dysfunction and AD. This review focuses on not only changes in expression of different NMDAR subunits, but also some unconventional modes of NMDAR action.

Unmasking GluN1/GluN3A excitatory glycine NMDA receptors

GluN3A and GluN3B are glycine-binding subunits belonging to the NMDA receptor (NMDAR) family that can assemble with the GluN1 subunit to form unconventional receptors activated by glycine alone. Functional characterization of GluN1/GluN3 NMDARs has been difficult. Here, we uncover two modalities that have transformative properties on GluN1/GluN3A receptors. First, we identify a compound, CGP-78608, which greatly enhances GluN1/GluN3A responses, converting small and rapidly desensitizing currents into large and stable responses. Second, we show that an endogenous GluN3A disulfide bond endows GluN1/GluN3A receptors with distinct redox modulation, profoundly affecting agonist sensitivity and gating kinetics. Under reducing conditions, ambient glycine is sufficient to generate tonic receptor activation. Finally, using CGP-78608 on P8-P12 mouse hippocampal slices, we demonstrate that excitatory glycine GluN1/GluN3A NMDARs are functionally expressed in native neurons, at least in the juvenile brain. Our work opens new perspectives on the exploration of excitatory glycine receptors in brain function and development.

Excitatory glycine GluN1/GluN3A receptors are atypical NMDARs that have been difficult to study. Here the authors identify new properties of these receptors, including potentiation by the GluN1 antagonist CGP-78608 that allows detection of functional GluN1/GluN3A receptors in the juvenile brain.

Effect of NMDA on proliferation and apoptosis in hippocampal neural stem cells treated with MK-801

The purpose of the present study was to investigate effects of N-methyl-D-aspartate (NMDA) on proliferation and apoptosis of hippocampal neural stem cells (NSCs) treated with dizocilpine (MK-801). Cultures of hippocampal NSCs were randomly divided into four groups consisting of an untreated control, cells treated with MK-801, NMDA and a combination of MK801 and NMDA (M+N). Proliferative and apoptotic responses for each of the experimental groups were determined by MTS and flow cytometry. The results revealed that MK-801 and NMDA exerted significant effects on hippocampal NSCs proliferation. Cell survival rates decreased in MK-801, NMDA and M+N treated groups compared with the control group. Cells survival rates in NMDA and M+N treated groups increased compared with the MK-801 treated group. MK-801 and NMDA were demonstrated to significantly affect apoptosis in hippocampal NSCs. Total and early stages of apoptosis in MK-801 and NMDA groups significantly increased compared with the control group. Total and early apoptosis of NSCs in the M+N group significantly decreased compared with MK-801 and NMDA groups. Late apoptosis of NSCs in MK-801 and NMDA groups significantly decreased compared with the control group. Late apoptosis of NSCs in the M+N group significantly increased compared with MK-801 and NMDA groups. The present study revealed that MK-801 inhibited proliferation and increased apoptosis in hippocampal NSCs. NMDA may reduce the neurotoxicity induced by MK-801, which may be associated with its activity towards NMDA receptors and may describe a novel therapeutic target for the treatment of schizophrenia.

Human miR-26a-5p regulates the glutamate transporter SLC1A1 (EAAT3) expression. Relevance in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system, characterized by chronic inflammation, demyelination and scarring as well as a broad spectrum of signs and symptoms. MicroRNA plays pivotal roles in cellular and developmental processes by regulating gene expression at the post-transcriptional level. Increasing evidence suggests the involvement of microRNAs in the pathogenesis of neurodegenerative diseases, including MS. We have already found that the expression of a specific miRNA, hsa-mir-26a-5p (miR-26a), changed during INF-β treatment in responder Relapsing-Remitting MS patients. Functional annotations of mir-26a targets revealed that a number of genes were implicated in Glutamate Receptor Signaling pathway, which is notoriously altered in neurodegenerative diseases as MS. In this study, the different potential targets were subjected to a validation test based on luciferase reporter constructs transfected in an oligodendroglial cell line. In this functional screening, miR-26a was able to interact with SLC1A1 3' UTR suppressing the reporter activity. Transfection of a miR-26a mimic was then shown to decrease the endogenous SLC1A1 mRNA. Afterward, we have evaluated in blood platelets from interferon-β treated Multiple Sclerosis patients the expression of miR-26a and SLC1A1, finding not only their converse expression, but also a responsiveness to interferon-β therapy. Overall, these data suggest that mir-26a and SLC1A1 may play a role in the MS pathogenesis, and may be potential targets for the development of new biomarkers and/or therapeutic tools.

Actions of Steroids: New Neurotransmitters

Over the past two decades, the classical understanding of steroid action has been updated to include rapid, membrane-initiated, neurotransmitter-like functions. While steroids were known to function on very short time spans to induce physiological and behavioral changes, the mechanisms by which these changes occur are now becoming more clear. In avian systems, rapid estradiol effects can be mediated via local alterations in aromatase activity, which precisely regulates the temporal and spatial availability of estrogens. Acute regulation of brain-derived estrogens has been shown to rapidly affect sensorimotor function and sexual motivation in birds. In rodents, estrogens and progesterone are critical for reproduction, including preovulatory events and female sexual receptivity. Membrane progesterone receptor as well as classical progesterone receptor trafficked to the membrane mediate reproductive-related hypothalamic physiology, via second messenger systems with dopamine-induced cell signals. In addition to these relatively rapid actions, estrogen membrane-initiated signaling elicits changes in morphology. In the arcuate nucleus of the hypothalamus, these changes are needed for lordosis behavior. Recent evidence also demonstrates that membrane glucocorticoid receptor is present in numerous cell types and species, including mammals. Further, membrane glucocorticoid receptor influences glucocorticoid receptor translocation to the nucleus effecting transcriptional activity. The studies presented here underscore the evidence that steroids behave like neurotransmitters to regulate CNS functions. In the future, we hope to fully characterize steroid receptor-specific functions in the brain.

GRIN3B missense mutation as an inherited risk factor for schizophrenia: whole-exome sequencing in a family with a familiar history of psychotic disorders

Glutamate is the most important excitatory neurotransmitter in the brain. The N-methyl-D-aspartate (NMDA) receptor is a glutamate-gated ionotropic cation channel that is composed of several subunits and modulated by a glycine binding site. Many forms of synaptic plasticity depend on the influx of calcium ions through NMDA receptors, and NMDA receptor dysfunction has been linked to a number of neuropsychiatric disorders, including schizophrenia. Whole-exome sequencing was performed in a family with a strong history of psychotic disorders over three generations. We used an iterative strategy to obtain condense and meaningful variants. In this highly affected family, we found a frameshift mutation (rs10666583) in the GRIN3B gene, which codes for the GluN3B subunit of the NMDA receptor in all family members with a psychotic disorder, but not in the healthy relatives. Matsuno et al., also reported this null variant as a risk factor for schizophrenia in 2015. In a broader sample of 22 patients with psychosis, the allele frequency of the rs10666583 mutation variant was increased compared to those of healthy population samples and unaffected relatives. Compared to the 1000 Genomes Project population, we found a significant increase of this variant with a large effect size among patients. The amino acid shift degrades the S1/S2 glycine binding domain of the dominant modulatory GluN3B subunit of the NMDA receptor, which subsequently affects the permeability of the channel pore to calcium ions. A decreased glycine affinity for the GluN3B subunit might cause impaired functional capability of the NMDA receptor and could be an important risk factor for the pathogenesis of psychotic disorders.

Specific Roles of NMDA Receptor Subunits in Mental Disorders

N-methyl-D-aspartate (NMDA) receptor plays important roles in learning and memory. NMDA receptors are a tetramer that consists of two glycine-binding subunits GluN1, two glutamate-binding subunits (i.e., GluN2A, GluN2B, GluN2C, and GluN2D), a combination of a GluN2 subunit and glycine-binding GluN3 subunit (i.e., GluN3A or GluN3B), or two GluN3 subunits. Recent studies revealed that the specific expression and distribution of each subunit are deeply involved in neural excitability, plasticity, and synaptic deficits. The present article summarizes reports on the dysfunction of NMDA receptors and responsible subunits in various neurological and psychiatric disorders, including schizophrenia, autoimmune-induced glutamatergic receptor dysfunction, mood disorders, and autism. A key role for the GluN2D subunit in NMDA receptor antagonist-induced psychosis has been recently revealed.

The contribution of rare and common variants in 30 genes to risk nicotine dependence

Genetic and functional studies have revealed that both common and rare variants of several nicotinic acetylcholine receptor subunits are associated with nicotine dependence (ND). In this study, we identified variants in 30 candidate genes including nicotinic receptors in 200 sib pairs selected from the Mid-South Tobacco Family population with equal numbers of African Americans (AAs) and European Americans (EAs). We selected 135 of the rare and common variants and genotyped them in the Mid-South Tobacco Case-Control (MSTCC) population, which consists of 3088 AAs and 1430 EAs. None of the genotyped common variants showed significant association with smoking status (smokers vs non-smokers), Fagerström Test for ND scores or indexed cigarettes per day after Bonferroni correction. Rare variants in NRXN1, CHRNA9, CHRNA2, NTRK2, GABBR2, GRIN3A, DNM1, NRXN2, NRXN3 and ARRB2 were significantly associated with smoking status in the MSTCC AA sample, with weighted sum statistic (WSS) P-values ranging from 2.42 × 10(-3) to 1.31 × 10(-4) after 10(6) phenotype rearrangements. We also observed a significant excess of rare nonsynonymous variants exclusive to EA smokers in NRXN1, CHRNA9, TAS2R38, GRIN3A, DBH, ANKK1/DRD2, NRXN3 and CDH13 with WSS P-values between 3.5 × 10(-5) and 1 × 10(-6). Variants rs142807401 (A432T) and rs139982841 (A452V) in CHRNA9 and variants V132L, V389L, rs34755188 (R480H) and rs75981117 (N549S) in GRIN3A are of particular interest because they are found in both the AA and EA samples. A significant aggregate contribution of rare and common coding variants in CHRNA9 to the risk for ND (SKAT-C: P=0.0012) was detected by applying the combined sum test in MSTCC EAs. Together, our results indicate that rare variants alone or combined with common variants in a subset of 30 biological candidate genes contribute substantially to the risk of ND.

A naturally occurring null variant of the NMDA type glutamate receptor NR3B subunit is a risk factor of schizophrenia

Hypofunction of the N-methyl-D-aspartate type glutamate receptor (NMDAR) has been implicated in the pathogenesis of schizophrenia. Here, we investigated the significance of a common human genetic variation of the NMDAR NR3B subunit that inserts 4 bases within the coding region (insCGTT) in the pathogenesis of schizophrenia. The cDNA carrying this polymorphism generates a truncated protein, which is electrophysiologically non-functional in heterologous expression systems. Among 586 schizophrenia patients and 754 healthy controls, insCGTT was significantly overrepresented in patients compared to controls (odds ratio = 1.37, p = 0.035). Among 121 schizophrenia patients and 372 healthy controls, genetic analyses of normal individuals revealed that those carrying insCGTT have a predisposition to schizotypal personality traits (F_1,356 = 4.69, p = 0.031). Furthermore, pre-pulse inhibition, a neurobiological trait disturbed in patients with schizophrenia, was significantly impaired in patients carrying insCGTT compared with those with the major allele (F_1,116 = 5.72, p = 0.018, F_1,238 = 4.46, p = 0.036, respectively). These results indicate that a naturally occurring null variant in NR3B could be a risk factor of schizophrenia.

Structure-based discovery of antagonists for GluN3-containing N-methyl-D-aspartate receptors

NMDA receptors are ligand-gated ion channels that assemble into tetrameric receptor complexes composed of glycine-binding GluN1 and GluN3 subunits (GluN3A-B) and glutamate-binding GluN2 subunits (GluN2A-D). NMDA receptors can assemble as GluN1/N2 receptors and as GluN3-containing NMDA receptors, which are either glutamate/glycine-activated triheteromeric GluN1/N2/N3 receptors or glycine-activated diheteromeric GluN1/N3 receptors. The glycine-binding GluN1 and GluN3 subunits display strikingly different pharmacological selectivity profiles. However, the pharmacological characterization of GluN3-containing receptors has been hampered by the lack of methods and pharmacological tools to study GluN3 subunit pharmacology in isolation. Here, we have developed a method to study the pharmacology of GluN3 subunits in recombinant diheteromeric GluN1/N3 receptors by mutating the orthosteric ligand-binding pocket in GluN1. This method is suitable for performing compound screening and characterization of structure-activity relationship studies on GluN3 ligands. We have performed a virtual screen of the orthosteric binding site of GluN3A in the search for antagonists with selectivity for GluN3 subunits. In the subsequent pharmacological evaluation of 99 selected compounds, we identified 6-hydroxy-[1,2,5]oxadiazolo[3,4-b]pyrazin-5(4H)-one (TK80) a novel competitive antagonist with preference for the GluN3B subunit. Serendipitously, we also identified [2-hydroxy-5-((4-(pyridin-3-yl)thiazol-2-yl)amino]benzoic acid (TK13) and 4-(2,4-dichlorobenzoyl)-1H-pyrrole-2-carboxylic acid (TK30), two novel non-competitive GluN3 antagonists. These findings demonstrate that structural differences between the orthosteric binding site of GluN3 and GluN1 can be exploited to generate selective ligands.

Glutamatergic receptors at developing synapses: the role of GluN3A-containing NMDA receptors and GluA2-lacking AMPA receptors

During brain development excitatory synapses exhibit significant changes in their postsynaptic receptors and activated signaling pathways. Calcium represents the most crucial signaling factor in synaptic transmission and plasticity. Therefore developmental changes in calcium-permeable channels on the membrane contribute significantly to the modulation of neurotransmission at excitatory synapses. The present review focuses on two types of "non-canonical" glutamate receptors in terms of calcium permeability: GluN3A-containing NMDA receptors (calcium-impermeable) and GluA2-lacking AMPA receptors (calcium permeable). The involvement of these receptors during development and their potential function in synaptic plasticity are discussed. The synaptic incorporation of these receptors would alter calcium permeability, and therefore the threshold/direction of further plastic changes. We believe that characterizing the dynamics of non-canonical glutamate receptors during development could provide insight into how these receptors are recruited or removed in pathological conditions.

A global transcriptome analysis reveals molecular hallmarks of neural stem cell death, survival, and differentiation in response to partial FGF-2 and EGF deprivation

Neurosphere cell culture is a commonly used model to study the properties and potential applications of neural stem cells (NSCs). However, standard protocols to culture NSCs have yet to be established, and the mechanisms underlying NSC survival and maintenance of their undifferentiated state, in response to the growth factors FGF-2 and EGF are not fully understood. Using cultures of embryonic and adult olfactory bulb stem cells (eOBSCs and aOBSCs), we analyzed the consequences of FGF-2 and EGF addition at different intervals on proliferation, cell cycle progression, cell death and differentiation, as well as on global gene expression. As opposed to cultures supplemented daily, addition of FGF-2 and EGF every 4 days significantly reduced the neurosphere volume and the total number of cells in the spheres, mainly due to increased cell death. Moreover, partial FGF-2 and EGF deprivation produced an increase in OBSC differentiation during the proliferative phase. These changes were more evident in aOBSC than eOBSC cultures. Remarkably, these effects were accompanied by a significant upregulation in the expression of endogenous Fgf-2 and genes involved in cell death and survival (Cryab), lipid catabolic processes (Pla2g7), cell adhesion (Dscaml1), cell differentiation (Dscaml1, Gpr17, S100b, Ndrg2) and signal transduction (Gpr17, Ndrg2). These findings support that a daily supply of FGF-2 and EGF is critical to maintain the viability and the undifferentiated state of NSCs in culture, and they reveal novel molecular hallmarks of NSC death, survival and the initiation of differentiation.

GluN3 subunit-containing NMDA receptors: not just one-trick ponies

The two GluN3 subunits were the last NMDA receptor subunits to be cloned some 15 years ago. Strikingly, despite the steadily growing interest in their function, their physiological role remains elusive. The original billing as dominant-negative modulators of classical NMDA receptors composed of GluN1 and GluN2 subunits has given way to proposals of much more complex functions, including roles in synaptogenesis and synaptic plasticity. In addition, GluN3 subunits in the absence of GluN2 surprisingly assemble with GluN1 into excitatory glycine receptors. This review provides an overview of the unique spatial and temporal expression patterns of the GluN3 subunits, discusses proposed functions and physiological roles for receptors comprising these subunits, and briefly summarizes their putative involvement in several neural diseases.

Overview of glutamatergic neurotransmission in the nervous system

This introductory article to the special edition on glutamate neurotransmission in neuropsychiatric disorders provides an overview of glutamate neurotransmitter system physiology and pharmacology. Glutamate was only relatively recently recognized as the major excitatory neurotransmitter in the mammalian brain, in part due to its ubiquitous nature and diverse metabolic roles within the CNS. The extremely high concentration of glutamate in brain tissue paired with its excitotoxic potential requires tight physiological regulation of extracellular glutamate levels and receptor signaling in order to assure optimal excitatory neurotransmission but limits excitotoxic damage. In order to achieve this high level of control, the system has developed a complex physiology with multiple regulatory processes modulating glutamate metabolism, release, receptor signaling, and uptake. The basic physiology of the various regulatory components of the system including the rich receptor pharmacology is briefly reviewed. Potential contributions from each of the system's components to the pathophysiology of neuropsychiatric illnesses are briefly discussed, as are the many new pharmacological targets for drug development provided by the system, especially as they pertain to the proceeding preclinical and clinical articles in this issue.

Separation of domain contacts is required for heterotetrameric assembly of functional NMDA receptors

The precise knowledge of the subunit assembly process of NMDA receptors (NMDA-Rs) is essential to understand the receptor architecture and underlying mechanism of channel function. Because NMDA-Rs are obligatory heterotetramers requiring the GluN1 subunit, it is critical to investigate how GluN1 and GluN2 type subunits coassemble into tetramers. By combining approaches in cell biology, biochemistry, single particle electron microscopy, and x-ray crystallography, we report the mechanisms and phenotypes of mutant GluN1 subunits that are defective in receptor maturation. The T110A mutation in the N-terminal domain (NTD) of the GluN1 promotes heterodimerization between the NTDs of GluN1 and GluN2, whereas the Y109C mutation in the adjacent residue stabilizes the homodimer of the NTD of GluN1. The crystal structure of the NTD of GluN1 revealed the mechanism underlying the biochemical properties of these mutants. Effects of these mutations on the maturation of heteromeric NMDA-Rs were investigated using a receptor trafficking assay. Our results suggest that the NTDs of the GluN1 subunit initially form homodimers and the subsequent dimer dissociation is critical for forming heterotetrameric NMDA-Rs containing GluN2 subunits, defining a molecular determinant for receptor assembly. The domain arrangement of the dimeric NTD of GluN1 is unique among the ionotropic glutamate receptors and predicts that the structure and mechanism around the NTDs of NMDA-Rs are different from those of the homologous AMPA and kainate receptors.

Residues at the tip of the pore loop of NR3B-containing NMDA receptors determine Ca2+ permeability and Mg2+ block

Background

Members of the complex N-methyl-D-aspartate receptor (NMDAR) subfamily of ionotropic glutamate receptors (iGluRs) conventionally assemble from NR1 and NR2 subunits, the composition of which determines receptor properties. Hallmark features of conventional NMDARs include the requirement for a coagonist, voltage-dependent block by Mg²⁺, and high permeability for Ca²⁺. Both Mg²⁺ sensitivity and Ca²⁺ permeability are critically dependent on the amino acids at the N and N+1 positions of NR1 and NR2. The recently discovered NR3 subunits feature an unprecedented glycine-arginine combination at those critical sites within the pore. Diheteromers assembled from NR1 and NR3 are not blocked by Mg²⁺ and are not permeable for Ca²⁺.

Results

Employing site-directed mutagenesis of receptor subunits, electrophysiological characterization of mutants in a heterologous expression system, and molecular modeling of the NMDAR pore region, we have investigated the contribution of the unusual NR3 N and N+1 site residues to the unique functional characteristics of receptors containing these subunits. Contrary to previous studies, we provide evidence that both the NR3 N and N+1 site amino acids are critically involved in mediating the unique pore properties. Ca²⁺ permeability could be rescued by mutating the NR3 N site glycine to the NR1-like asparagine. Voltage-dependent Mg²⁺ block could be established by providing an Mg²⁺ coordination site at either the NR3 N or N+1 positions. Conversely, "conventional" receptors assembled from NR1 and NR2 could be made Mg²⁺ insensitive and Ca²⁺ impermeable by equipping either subunit with the NR3-like glycine at their N positions, with a stronger contribution of the NR1 subunit.

Conclusions

This study sheds light on the structure-function relationship of the least characterized member of the NMDAR subfamily. Contrary to previous reports, we provide evidence for a critical functional involvement of the NR3 N and N+1 site amino acids, and propose them to be the essential determinants for the unique pore properties mediated by this subunit.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Oligomerized Abeta25-35 induces increased tyrosine phosphorylation of NMDA receptor subunit 2A in rat hippocampal CA1 subfield

Amyloid-beta peptide (Abeta) plays a causal role in the pathogenesis of Alzheimer's disease (AD). To elucidate the mechanisms underlying the over-activation of NMDA receptors in AD, we investigated the alteration of NR2A tyrosine phosphorylation after intracerebroventricular infusion of Abeta25-35 oligomers. Abeta25-35 treatment resulted in the elevated tyrosine phosphorylation of NR2A in rat hippocampal CA1 subfield and facilitated the interactions of NR2A or PSD-95 with Src kinases. PP2, a specific inhibitor of Src family protein tyrosine kinases (SrcPTKs), not only attenuated the Abeta25-35-induced increases in the tyrosine phosphorylation of NR2A and in the associations among Src, NR2A, and PSD-95, but also protected against neuronal loss in the CA1 region. Preapplication of a noncompetitive NMDA receptor antagonist amantadine, an NR2A-selective NMDA receptor antagonist NVP-AAM077, or an NR2B-selective NMDA receptor antagonist Ro25-6981 inhibited the increased tyrosine phosphorylation of NR2A and prevented the associations among Src, NR2A, and PSD-95, but Ro25-6981 had less contribution. These results suggest that the activation of NMDA receptors after Abeta treatment promotes the formation of NR2A-PSD-95-Src complex and thus increases the tyrosine phosphorylation of NR2A by Src kinases, which up-regulates the function of NMDA receptors. Such positive feedback mediates the Abeta-induced over-activation of NMDA receptors and is involved in neuronal impairment.

MAP1B binds to the NMDA receptor subunit NR3A and affects NR3A protein concentrations

Incorporation of the N-methyl-d-aspartate receptor (NMDAR) subunit NR3A into functional NMDARs results in reduced channel conductance and Ca(2+) permeability. To further investigate the function of NR3A, we have set out to characterize its intracellular binding partners. Here, we report a novel protein interaction between NR3A and microtubule associated-protein (MAP) 1B, which both are localized to dendritic shafts and filopodia. NR3A protein levels were increased in MAP1B deficient (-/-) mice, with a corresponding decrease in NR1 levels, but the fraction of filopodia immunoreactive for NR3A was equal in cells from -/- and wild type (WT) mice. NR3A has previously been shown to interact with another member of the MAP1 family, MAP1S. We showed that MAP1S binds to microtubules in a similar manner as MAP1B, and suggest that MAP1S and MAP1B both are involved in regulating trafficking of NR3A-containing NMDAR.

Significant association of glutamate receptor, ionotropic N-methyl-D-aspartate 3A (GRIN3A), with nicotine dependence in European- and African-American smokers

The glutamate receptor gene, ionotropic N-methyl-D-aspartate 3A (GRIN3A), is one of the seven that code for subunits of N-methyl-D-aspartate receptors, which play an essential role at many synapses in the brain, regulating ion flow across membranes in response to glutamate signaling. In this study, we analyzed 25 single nucleotide polymorphisms (SNPs) within GRIN3A for association with nicotine dependence (ND), which was assessed by smoking quantity, heaviness of smoking index, and the Fagerström test for ND. Both individual SNP and haplotype association tests were performed in African-American (AA) and European-American (EA) samples as well as in the pooled sample consisting of 2,037 individuals from 602 nuclear families. Individual SNP analysis revealed significant associations of 5, 5, and 4 SNPs with at least one ND measure in the pooled, EA, and AA samples, respectively. Of them, SNPs rs17189632 and rs10121600 in the pooled sample and rs11788456 in the EA sample remained significant after correction for multiple testing. On the basis of the blocks determined with Haploview, we performed haplotype-based association analysis and found 2, 4, and 1 haplotype(s) that are significantly associated with at least one ND measure in the pooled, EA, and AA samples, respectively. Some of them remained significant after correction for multiple testing. We concluded that GRIN3A represents a strong candidate for involvement in the etiology of ND and warrants further investigation in independent samples.

Influence of the NR3A subunit on NMDA receptor functions

Various combinations of subunits assemble to form the NMDA-type glutamate receptor (NMDAR), generating diversity in its functions. Here we review roles of the unique NMDAR subunit, NR3A, which acts in a dominant-negative manner to suppress receptor activity. NR3A-containing NMDARs display striking regional and temporal expression specificity, and, unlike most other NMDAR subtypes, they have a low conductance, are only modestly permeable to Ca(2+), and pass current at hyperpolarized potentials in the presence of magnesium. While glutamate activates triheteromeric NMDARs composed of NR1/NR2/NR3A subunits, glycine is sufficient to activate diheteromeric NR1/NR3A-containing receptors. NR3A dysfunction may contribute to neurological disorders involving NMDARs, and the subunit offers an attractive therapeutic target given its distinct pharmacological and structural properties.

Overexpression of the PDZ1 domain of PSD-95 diminishes ischemic brain injury via inhibition of the GluR6.PSD-95.MLK3 pathway

Recent studies have shown that kainate (KA) receptors are involved in neuronal cell death induced by seizure, which is mediated by the GluR6.PSD-95.MLK3 signaling module and subsequent JNK activation. In our previous studies, we demonstrated the neuroprotective role of a GluR6 c-terminus containing peptide against KA or cerebral ischemia-induced excitotoxicity in vitro and in vivo. Here, we first report that overexpression of the PDZ1 domain of PSD-95 protein exerts a protective role against neuronal death induced by cerebral ischemia-reperfusion in vivo and can prevent neuronal cell death induced by oxygen-glucose deprivation. Further studies show that overexpression of PDZ1 can perturb the interaction of GluR6 with PSD-95 and suppress the assembly of the GluR6.PSD-95.MLK3 signaling module and therefore inhibit JNK activation. Thus, it not only inhibits phosphorylation of c-Jun and down-regulates Fas ligand expression but also inhibits phosphorylation of 14-3-3 and decreases Bax translocation to mitochondria, decreases the release of cytochrome c, and decreases caspase-3 activation. Overall, the essential role of the PDZ1 domain of PSD-95 in apoptotic cell death in neurons provides an experimental foundation for gene therapy of neurodegenerative diseases with overexpression of the PDZ1 domain.

Exomic sequencing of the ionotropic glutamate receptor N-methyl-D-aspartate 3A gene (GRIN3A) reveals no association with schizophrenia

BACKGROUND

Growing evidence suggests that dysregulation of N-methyl-D-aspartate receptor (NMDAR)-mediated glutamate neurotransmission may be involved in the pathophysiology of schizophrenia. The NMDAR is a heteromeric protein complex consisting of subunits from three subfamilies (NR1, NR2A, 2B, 2C, 2D and NR3A, 3B). The unique ability of NR3A to modulate the NMDAR function makes it an attractive candidate gene of schizophrenia. The purpose of this study was to investigate the involvement of the gene encoding the human NR3A subunit (GRIN3A) in the liability to schizophrenia.

METHODS

We searched for genetic variants in the putative core promoter region and all the exons (including UTR ends) of the GRIN3A gene in 333 Han Chinese patients with schizophrenia and 369 control subjects from Taiwan using direct polymerase chain reaction (PCR) autosequencing, and assessed their association with schizophrenia.

RESULTS

We identified 22 single nucleotide polymorphisms (SNPs) in the GRIN3A gene in this sample. SNP- and haplotype-based analyses showed no association of these 22 SNPs with schizophrenia. Nevertheless, we identified two missense mutations (D133N and Q1091H), one nonsense mutation (R1024X), and two synonymous mutations (Y873Y and E889E) of the GRIN3A gene in 6 out of 333 (1.8%) patients, while no rare mutations were found in 369 control subjects (p=0.011, Fisher's exact test, one-tailed). In silico analysis showed that the R1024X and Q1091H mutations are possibly damaging.

CONCLUSIONS

Although the functional significance of these mutations remains to be characterized, our study indicates that rare mutations in the GRIN3A gene may contribute to the pathogenesis of schizophrenia in certain patients.

Overexpression of PDZ1 domain prevents apoptosis of rat hippocampal neurons induced by kainic acid

In our previous studies, Tat-GluR6-9c (a glutamate receptor 6 C-terminus peptide fused the TAT protein transduction sequence) not only inhibited the activation of MLK3 (mixed lineage kinase 3) and JNK (c-Jun N-terminal kinase) via the GluR6.PSD-95 (postsynaptic density protein 95).MLK3 signaling module but also diminished neuronal death induced by kainic acid or transient cerebral ischemia in rat hippocampus. Here, we investigate whether overexpression of the PDZ1 domain of PSD-95 protein could suppress the binding of GluR6 with PSD-95 and the activation of MLK3, MKK7 (mitogen-activated kinase kinase 7) and JNK1/2, and rescused neuronal cell death induced by kainic acid. Our results showed that overexpression of the PDZ1 domain of PSD-95 protein could prevent nuclear accumulation and abrogate neuronal cell death in SD (Sprague-Dawley) rat hippocampal neuronal cells. Further studies indicated that overexpression of PDZ1 could inhibit the enhancement of binding of GluR6 to PSD-95 and prevent the activation of MLK3, MKK7 and JNK1/2 induced by kainic acid. Taken together, the essential role of the PDZ1 domain of PSD-95 in apoptotic cell death in neurons provides an experimental foundation for gene therapy of neurodegenerative diseases with overexpression of the PDZ1 domain.

Circadian and developmental regulation of N-methyl-d-aspartate-receptor 1 mRNA splice variants and N-methyl-d-aspartate-receptor 3 subunit expression within the rat suprachiasmatic nucleus

The circadian rhythms of mammals are generated by the circadian clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Its intrinsic period is entrained to a 24 h cycle by external cues, mainly by light. Light impinging on the SCN at night causes either advancing or delaying phase shifts of the circadian clock. N-methyl-d-aspartate receptors (NMDAR) are the main glutamate receptors mediating the effect of light on the molecular clockwork in the SCN. They are composed of multiple subunits, each with specific characteristics whose mutual interactions strongly determine properties of the receptor. In the brain, the distribution of NMDAR subunits depends on the region and developmental stage. Here, we report the circadian expression of the NMDAR1 subunit in the adult rat SCN and depict its splice variants that may constitute the functional receptor channel in the SCN. During ontogenesis, expression of two of the NMDAR1 subunit splice variants, as well as the NMDAR3A and 3B subunits, exhibits developmental loss around the time of eye opening. Moreover, we demonstrate the spatial and developmental characteristics of the expression of the truncated splice form of NMDAR1 subunit NR1-E in the brain. Our data suggest that specific properties of the NMDAR subunits we describe within the SCN likely influence the photic transduction pathways mediating the clock entrainment. Furthermore, the developmental changes in NMDAR composition may contribute to the gradual postnatal maturation of the entrainment pathways.

Calpain and the glutamatergic synapse

Calpain is a ubiquitous protease found in different tissue types and in many organisms including mammals. It generally does not destroy its large variety of substrates, but more commonly disrupts their function. In neurons, many of its substrates become dysregulated as a result of cleavage of their regulatory domain by this protease, leading to altered signaling between cells. In glutamatergic synaptic transmission, direct targets of calpain include all of the major glutamate receptors: NMDA receptors, AMPA receptors and mGluR. By cleaving these receptors and associated intracellular proteins, calpain may regulate the physiology at glutamatergic synapses. As a result, calpain-mediated cleavage in neurons might not only be involved in pathological events like excitotoxicity, but may also have neuroprotective effects and roles in physiological synaptic transmission.

Immunolocalization of NMDA receptor subunit NR3B in selected structures in the rat forebrain, cerebellum, and lumbar spinal cord

N-methyl-D-aspartate (NMDA) receptors have been implicated in many neurological disorders. Although NMDA receptors are best known for their high calcium permeability, the recently discovered NR3 subunits, NR3A and NR3B, have been shown to reduce the calcium permeability of the NMDA receptor. Thus, NR3 subunits may be important players in modulating synaptic plasticity in neurons. Although NR3B expression in the rodent and human brain has been studied, little is known about its distribution in different cell types. Here we used immunolabeling with a specific NR3B antibody together with antibodies against established neurochemical markers to determine the cellular and subcellular localization of NR3B. The nucleus was concurrently stained with NR3B immunolabeling to show that NR3B is widely expressed by many cells in each brain region. Our findings indicate that NR3B is widely expressed in the structures examined in the rat forebrain (hippocampus, cerebral cortex, caudoputamen, and nucleus accumbens), cerebellum, and lumbar sections of the spinal cord. Within these regions NR3B was found to be expressed in all the substructures of the hippocampus (CA1, CA3, dentate gyrus), the various layers of the cerebral cortex, projection neurons and interneurons of the striatum, different cell types of the cerebellum, and motor neurons of the spinal cord. Furthermore, when stained with NR1-the obligatory subunit responsible for forming functional NMDA receptors-the distribution of NR3B appears to be as ubiquitous as NR1. Taken together, our data suggest that there may be a population of NR3B-containing NMDA receptors conferring new functional roles in the mammalian central nervous system.

Neuroprotection of GluR5-containing kainate receptor activation against ischemic brain injury through decreasing tyrosine phosphorylation of N-methyl-D-aspartate receptors mediated by Src kinase

Previous studies indicate that cerebral ischemia breaks the dynamic balance between excitatory and inhibitory inputs. The neural excitotoxicity induced by ionotropic glutamate receptors gain the upper hand during ischemia-reperfusion. In this paper, we investigate whether GluR5 (glutamate receptor 5)-containing kainate receptor activation could lead to a neuroprotective effect against ischemic brain injury and the related mechanism. The results showed that (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), a selective GluR5 agonist, could suppress Src tyrosine phosphorylation and interactions among N-methyl-D-aspartate (NMDA) receptor subunit 2A (NR2A), postsynaptic density protein 95 (PSD-95), and Src and then decrease NMDA receptor activation through attenuating tyrosine phosphorylation of NR2A and NR2B. More importantly, ATPA had a neuroprotective effect against ischemia-reperfusion-induced neuronal cell death in vivo. However, four separate drugs were found to abolish the effects of ATPA. These were selective GluR5 antagonist NS3763; GluR5 antisense oligodeoxynucleotides; CdCl(2), a broad spectrum blocker of voltage-gated calcium channels; and bicuculline, an antagonist of gamma-aminobutyric acid A (GABA(A)) receptor. GABA(A) receptor agonist muscimol could attenuate Src activation and interactions among NR2A, PSD-95 and Src, resulting the suppression of NMDA receptor tyrosine phosphorylation. Moreover, patch clamp recording proved that the activated GABA(A) receptor could inhibit NMDA receptor-mediated whole-cell currents. Taken together, the results suggest that during ischemia-reperfusion, activated GluR5 may facilitate Ca(2+)-dependent GABA release from interneurons. The released GABA can activate postsynaptic GABA(A) receptors, which then attenuates NMDA receptor tyrosine phosphorylation through inhibiting Src activation and disassembling the signaling module NR2A-PSD-95-Src. The final result of this process is that the pyramidal neurons are rescued from hyperexcitability.

Developmental regulation of the NMDA receptor subunits, NR3A and NR1, in human prefrontal cortex

Subunit composition of N-methyl-D-aspartate-type glutamate receptors (NMDARs) dictates their function, yet the ontogenic profiles of human NMDAR subunits from gestation to adulthood have not been determined. We examined NMDAR mRNA and protein development in human dorsolateral prefrontal cortex (DLPFC), an area in which NMDARs are critical for higher cognitive processing and NMDAR hypofunction is hypothesized in schizophrenia. Using quantitative reverse transcriptase-polymerase chain reaction and western blotting, we found NR1 expression begins low prenatally, peaks in adolescence, yet remains high throughout life, suggesting lifelong importance of NMDAR function. In contrast, NR3A levels are low during gestation, surge soon after birth, and decline progressively through adolescence and into adulthood. Because NR3A subunits uniquely attenuate NMDAR-mediated currents, limit calcium influx, and suppress dendritic spine formation, high levels during early childhood may be important for regulating neuroprotection and activity-dependent sculpting of synapses. We also examined whether subunit changes underlie reduced NMDAR activity in schizophrenia. Our results reveal normal NR1 and NR3A protein levels in DLPFC from schizophrenic patients, indicating that NMDAR hypofunction is unlikely to be maintained by gross changes in NR3A-containing NMDARs or overall NMDAR numbers. These data provide insights into NMDAR functions in the developing CNS and will contribute to designing pharmacotherapies for neurological disorders.

Exercise-induced activation of NMDA receptor promotes motor unit development and survival in a type 2 spinal muscular atrophy model mouse

Spinal muscular atrophy (SMA) is an inborn neuromuscular disorder caused by low levels of survival motor neuron protein, and for which no efficient therapy exists. Here, we show that the slower rate of postnatal motor-unit maturation observed in type 2 SMA-like mice is correlated with the motor neuron death. Physical exercise delays motor neuron death and leads to an increase in the postnatal maturation rate of the motor-units. Furthermore, exercise is capable of specifically enhancing the expression of the gene encoding the major activating subunit of the NMDA receptor in motor neurons, namely the NR2A subunit, which is dramatically downregulated in the spinal cord of type 2 SMA-like mice. Accordingly, inhibiting NMDA-receptor activity abolishes the exercise-induced effects on muscle development, motor neuron protection and life span gain. Thus, restoring NMDA-receptor function could be a promising therapeutic approach to SMA treatment.

Modulation of NMDA receptor properties and synaptic transmission by the NR3A subunit in mouse hippocampal and cerebrocortical neurons

Expression of the NR3A subunit with NR1/NR2 in Xenopus oocytes or mammalian cell lines leads to a reduction in N-methyl-d-aspartate (NMDA)-induced currents and decreased Mg(2+) sensitivity and Ca(2+) permeability compared with NR1/NR2 receptors. Consistent with these findings, neurons from NR3A knockout (KO) mice exhibit enhanced NMDA-induced currents. Recombinant NR3A can also form excitatory glycine receptors with NR1 in the absence of NR2. However, the effects of NR3A on channel properties in neurons and synaptic transmission have not been fully elucidated. To study physiological roles of NR3A subunits, we generated NR3A transgenic (Tg) mice. Cultured NR3A Tg neurons exhibited two populations of NMDA receptor (NMDAR) channels, reduced Mg(2+) sensitivity, and decreased Ca(2+) permeability in response to NMDA/glycine, but glycine alone did not elicit excitatory currents. In addition, NMDAR-mediated excitatory postsynaptic currents (EPSCs) in NR3A Tg hippocampal slices showed reduced Mg(2+) sensitivity, consistent with the notion that NR3A subunits incorporated into synaptic NMDARs. To study the function of endogenous NR3A subunits, we compared NMDAR-mediated EPSCs in NR3A KO and WT control mice. In NR3A KO mice, the ratio of the amplitudes of the NMDAR-mediated component to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated component of the EPSC was significantly larger than that seen in WT littermates. This result suggests that NR3A subunits contributed to the NMDAR-mediated component of the EPSC in WT mice. Taken together, these results show that NR3A subunits contribute to NMDAR responses from both synaptic and extrasynaptic receptors, likely composed of NR1, NR2, and NR3 subunits.

Genetic ablation of NMDA receptor subunit NR3B in mouse reveals motoneuronal and nonmotoneuronal phenotypes

NR3B is a modulatory subunit of the NMDA receptor, abundantly expressed in both cranial and spinal somatic motoneurons and at lower levels in other regions of the brain as well. Recently, we found the human NR3B gene (GRIN3B) to be highly genetically heterogeneous, and that approximately 10% of the normal European-American population lacks NR3B due to homozygous occurrence of a null allele in the gene. Therefore, it is especially important to understand the phenotypic consequences of the genetic loss of NR3B in both humans and animal models. We here provide results of behavioral analysis of mice genetically lacking NR3B, which is an ideal animal model due to homogeneity in genetic and environmental background. The NR3B(-/-) mice are viable and fertile. Consistent with the expression of NR3B in somatic motoneurons, the NR3B(-/-) mice showed a moderate but significant impairment in motor learning or coordination, and decreased activity in their home cages. Remarkably, the NR3B(-/-) mice showed a highly increased social interaction with their familiar cage mates in their home cage but moderately increased anxiety-like behaviour and decreased social interaction in a novel environment, consistent with the inhibitory role of NR3B on the functions of NMDA receptors. This work is the first reporting of the functional significance of NR3B in vivo and may give insight into the contribution of genetic variability of NR3B in the phenotypic heterogeneity among human population.

Analysis of NR3A receptor subunits in human native NMDA receptors

NR3A, representing the third class of NMDA receptor subunits, was first studied in rats, demonstrating ubiquitous expression in the developing central nervous system (CNS), but in the adult mainly expressed in spinal cord and some forebrain nuclei. Subsequent studies showed that rodent and non-human primate NR3A expression differs. We have studied the distribution of NR3A in the human CNS and show a widespread distribution of NR3A protein in adult human brain. NR3A mRNA and protein were found in all regions of the cerebral cortex, and also in the subcortical forebrain, midbrain and hindbrain. Only very low levels of NR3A mRNA and protein could be detected in homogenized adult human spinal cord, and in situ hybridization showed that expression was limited to ventral motoneurons. We found that NR3A is associated with NR1, NR2A and NR2B in adult human CNS, suggesting the existence of native NR1-NR2A/B-NR3A assemblies in adult human CNS. While NR1 and NR2A could only be efficiently solubilized by deoxycholate, NR3A was extracted by all detergents, suggesting that a large fraction is weakly anchored to cell membranes and other proteins. Using size exclusion chromatography we found that just as for NR1, a large fraction of NR3A exists as monomers and dimers, suggesting that these two glycine binding subunits behave similarly with regard to receptor assembly and trafficking.

The NMDAR subunit NR3A interacts with microtubule-associated protein 1S in the brain

When screening a brain cDNA library, we found that the N-methyl-D-aspartate receptor subunit NR3A binds to microtubule-associated protein (MAP) 1S/chromosome 19 open reading frame 5 (C19ORF5). The interaction was confirmed in vitro and in vivo, and binding of MAP1S was localized to the membrane-proximal part of the NR3A C-terminus. MAP1S belongs to the same family as MAP1A and MAP1B, and was found to be abundant in both postnatal and adult rat brain. In hippocampal neurons the distribution-pattern of MAP1S resembled that of beta-tubulin III, but a fraction of the protein colocalized with synaptic markers synapsin and postsynaptic density protein 95 (PSD95), in beta-tubulin III-negative filopodia-like protrusions. There was coexistance between MAP1S and NR3A immunoreactivity in neurite shafts and occasionally in filopodia-like processes. MAP1S potentially links NR3A to the cytoskeleton, and may stabilize NR3A-containing receptors at the synapse and regulate their movement between synaptic and extrasynaptic sites.