Genomics and variation of ionotropic glutamate receptors

Enhanced Membrane Incorporation of H289Y Mutant GluK1 Receptors from the Audiogenic Seizure-Prone GASH/Sal Model: Functional and Morphological Impacts on Xenopus Oocytes

Epilepsy is a neurological disorder characterized by abnormal neuronal excitability, with glutamate playing a key role as the predominant excitatory neurotransmitter involved in seizures. Animal models of epilepsy are crucial in advancing epilepsy research by faithfully replicating the diverse symptoms of this disorder. In particular, the GASH/Sal (genetically audiogenic seizure-prone hamster from Salamanca) model exhibits seizures resembling human generalized tonic-clonic convulsions. A single nucleotide polymorphism (SNP; C9586732T, p.His289Tyr) in the Grik1 gene (which encodes the kainate receptor GluK1) has been previously identified in this strain. The H289Y mutation affects the amino-terminal domain of GluK1, which is related to the subunit assembly and trafficking. We used confocal microscopy in Xenopus oocytes to investigate how the H289Y mutation, compared to the wild type (WT), affects the expression and cell-surface trafficking of GluK1 receptors. Additionally, we employed the two-electrode voltage-clamp technique to examine the functional effects of the H289Y mutation. Our results indicate that this mutation increases the expression and incorporation of GluK1 receptors into an oocyte's membrane, enhancing kainate-evoked currents, without affecting their functional properties. Although further research is needed to fully understand the molecular mechanisms responsible for this epilepsy, the H289Y mutation in GluK1 may be part of the molecular basis underlying the seizure-prone circuitry in the GASH/Sal model.

Glutamate receptor endocytosis and signaling in neurological conditions

The non-essential amino acid glutamate acts as a major excitatory neurotransmitter and plays a significant role in the central nervous system (CNS). It binds with two different types of receptors, ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors (mGluRs), responsible for the postsynaptic excitation of neurons. They are important for memory, neural development and communication, and learning. Endocytosis and subcellular trafficking of the receptor are essential for the regulation of receptor expression on the cell membrane and excitation of the cells. The endocytosis and trafficking of the receptor are dependent on its type, ligand, agonist, and antagonist present. This chapter discusses the types of glutamate receptors, their subtypes, and the regulation of their internalization and trafficking. The roles of glutamate receptors in neurological diseases are also briefly discussed.

Analysis of gene variants in the GASH/Sal model of epilepsy

Epilepsy is a complex neurological disorder characterized by sudden and recurrent seizures, which are caused by various factors, including genetic abnormalities. Several animal models of epilepsy mimic the different symptoms of this disorder. In particular, the genetic audiogenic seizure hamster from Salamanca (GASH/Sal) animals exhibit sound-induced seizures similar to the generalized tonic seizures observed in epileptic patients. However, the genetic alterations underlying the audiogenic seizure susceptibility of the GASH/Sal model remain unknown. In addition, gene variations in the GASH/Sal might have a close resemblance with those described in humans with epilepsy, which is a prerequisite for any new preclinical studies that target genetic abnormalities. Here, we performed whole exome sequencing (WES) in GASH/Sal animals and their corresponding controls to identify and characterize the mutational landscape of the GASH/Sal strain. After filtering the results, moderate- and high-impact variants were validated by Sanger sequencing, assessing the possible impact of the mutations by “in silico” reconstruction of the encoded proteins and analyzing their corresponding biological pathways. Lastly, we quantified gene expression levels by RT-qPCR. In the GASH/Sal model, WES showed the presence of 342 variations, in which 21 were classified as high-impact mutations. After a full bioinformatics analysis to highlight the high quality and reliable variants, the presence of 3 high-impact and 15 moderate-impact variants were identified. Gene expression analysis of the high-impact variants of Asb14 (ankyrin repeat and SOCS Box Containing 14), Msh3 (MutS Homolog 3) and Arhgef38 (Rho Guanine Nucleotide Exchange Factor 38) genes showed a higher expression in the GASH/Sal than in control hamsters. In silico analysis of the functional consequences indicated that those mutations in the three encoded proteins would have severe functional alterations. By functional analysis of the variants, we detected 44 significantly enriched pathways, including the glutamatergic synapse pathway. The data show three high-impact mutations with a major impact on the function of the proteins encoded by these genes, although no mutation in these three genes has been associated with some type of epilepsy until now. Furthermore, GASH/Sal animals also showed gene variants associated with different types of epilepsy that has been extensively documented, as well as mutations in other genes that encode proteins with functions related to neuronal excitability, which could be implied in the phenotype of the GASH/Sal. Our findings provide valuable genetic and biological pathway data associated to the genetic burden of the audiogenic seizure susceptibility and reinforce the need to validate the role of each key mutation in the phenotype of the GASH/Sal model.

Whole blood transcriptomic profiles can differentiate vulnerability to chronic low back pain

The mechanisms underlying the transition from acute to chronic pain remain unclear. Here, we sought to characterize the transcriptome associated with chronic low back pain as well as the transcriptome of the transition from acute to chronic low back pain. For the analysis, we compared the whole blood transcriptome of: (a) patients at the onset of low back pain who no longer had pain within 6 weeks after onset (acute) with patients who developed chronic low back pain at 6 months (chronic T5); and, (b) patients at the onset of low back pain (chronic T1) who developed chronic pain at 6 months with healthy pain-free (normal) controls. The majority of differentially expressed genes were protein coding. We illustrate a unique chronic low back pain transcriptome characterized by significant enrichment for known pain genes, extracellular matrix genes, and genes from the extended major histocompatibility complex (MHC) genomic locus. The transcriptome of the transition from acute to chronic low back pain was characterized by significant upregulation of antigen presentation pathway (MHC class I and II) genes and downregulation of mitochondrial genes associated with oxidative phosphorylation, suggesting a unique genomic signature of vulnerability to low back pain chronicity.

MicroRNAs as Potential Targets for Therapeutic Intervention With Metastasis of Non-small Cell Lung Cancer

The death toll of non-small cell lung cancer (NSCLC) patients is primarily due to metastases, which are poorly amenable to therapeutic intervention. In this review we focus on miRs associated with metastasis of NSCLC as potential new targets for anti-metastatic therapy. We discuss miRs validated as therapeutic targets by in vitro data, identification of target(s) and pathway(s) and in vivo efficacy data in at least one clinically-relevant metastasis-related model. A few of the discussed miRs correlate with the clinical status of NSCLC patients. Using miRs as therapeutic agents has the advantage that targeting a single miR can potentially interfere with several metastatic pathways. Depending on their mode of action, the corresponding miRs can be up- or down-regulated compared to normal matching tissues. Here, we describe therapeutic approaches for reconstitution therapy and miR inhibition, general principles of anti-metastatic therapy as well as current technical pitfalls.

Expression of Glutamate Receptor Subtype 3 Is Epigenetically Regulated in Podocytes under Diabetic Conditions

Background

Recent studies suggest a role of epigenetics in the pathogenesis of diabetic kidney disease. However, epigenetic changes occurring specifically in kidney cells is poorly understood.

Methods

To examine the epigenetic regulation of genes in podocytes under diabetic conditions, we performed DNA methylation and transcriptomic profiling in podocytes exposed to high glucose conditions.

Results

Comparative analysis of genes with DNA methylation changes and correspondingly altered mRNA expression identified 337 hypomethylated genes with increased mRNA expression and only 2 hypermethyated genes (ESX1 and GRIA3) with decreased mRNA expression. Glutamate ionotropic receptor AMPA type subunit 3 (GRIA3) belongs to the ionotropic class of glutamate receptors that mediate fast excitatory synaptic transmission in the central nervous system. As podocytes have glutamate-containing vesicles and various glutamate receptors mediate important biological effects in podocytes, we further examined GRIA3 expression and its function in podocytes. Real-time PCR and western blots confirmed the suppression of GRIA3 expression in podocytes under high glucose conditions, which were abolished in the presence of a DNA methyltransferase inhibitor. Sites of DNA hypermethylation were also confirmed by bisulfite sequencing of the GRIA3 promoter region. GRIA3 mRNA and protein expression was suppressed in diabetic kidneys of human and mouse models, and knockdown of GRIA3 exacerbated high glucose-induced apoptosis in cultured podocytes.

Conclusion

These results indicate that decreased GRIA3 expression in podocytes in diabetic condition heightens podocyte apoptosis and loss.

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.

Phylogenetic analysis of ionotropic L-glutamate receptor genes in the Bilateria, with special notes on Aplysia californica

Background

The neurotransmitter L-Glutamate (L-Glu) acting at ionotropic L-Glu receptors (iGluR) conveys fast excitatory signal transmission in the nervous systems of all animals. iGluR-dependent neurotransmission is a key component of the synaptic plasticity that underlies learning and memory. During learning, two subtypes of iGluR, α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) and N-methyl-D-aspartate receptors (NMDAR), are dynamically regulated postsynaptically in vertebrates. Invertebrate organisms such as Aplysia californica (Aplysia) are well-studied models for iGluR-mediated function, yet no studies to date have analyzed the evolutionary relationships between iGluR genes in these species and those in vertebrates, to identify genes that may mediate plasticity. We conducted a thorough phylogenetic analysis spanning Bilateria to elucidate these relationships. The expression status of iGluR genes in the Aplysia nervous system was also examined.

Results

Our analysis shows that ancestral genes for both NMDAR and AMPAR subtypes were present in the common bilaterian ancestor. NMDAR genes show very high conservation in motifs responsible for forming the conductance pore of the ion channel. The number of NMDAR subunits is greater in vertebrates due to an increased number of splice variants and an increased number of genes, likely due to gene duplication events. AMPAR subunits form an orthologous group, and there is high variability in the number of AMPAR genes in each species due to extensive taxon specific gene gain and loss. qPCR results show that all 12 Aplysia iGluR subunits are expressed in all nervous system ganglia.

Conclusions

Orthologous NMDAR subunits in all species studied suggests conserved function across Bilateria, and potentially a conserved mechanism of neuroplasticity and learning. Vertebrates display an increased number of NMDAR genes and splice variants, which may play a role in their greater diversity of physiological responses. Extensive gene gain and loss of AMPAR genes may result in different physiological properties that are taxon specific. Our results suggest a significant role for L-Glu mediated responses throughout the Aplysia nervous system, consistent with L-Glu’s role as the primary excitatory neurotransmitter.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0871-1) contains supplementary material, which is available to authorized users.

Modulation of behavior by scaffolding proteins of the post-synaptic density

Scaffolding proteins of the neuronal post-synaptic density (PSD) are principal organizers of glutamatergic neurotransmission that bring together glutamate receptors and signaling molecules at discrete synaptic locations. Genetic alterations of individual PSD scaffolds therefore disrupt the function of entire multiprotein modules rather than a single glutamatergic mechanism, and thus induce a range of molecular and structural abnormalities in affected neurons. Despite such broad molecular consequences, knockout, knockdown, or knockin of glutamate receptor scaffolds typically affect a subset of specific behaviors and thereby mold and specialize the actions of the ubiquitous glutamatergic neurotransmitter system. Approaches designed to control the function of neuronal scaffolds may therefore have high potential to restore behavioral morbidities and comorbidities in patients with psychiatric disorders. Here we summarize a series of experiments with genetically modified mice revealing the roles of main N-methyl-d-aspartate (NMDA) and group I metabotropic glutamate (mGluR1/5) receptor scaffolds in behavior, discuss the clinical implications of the findings, and propose future research directions.

Maternal transmission effect of a PDGF-C SNP on nonsyndromic cleft lip with or without palate from a Chinese population

Cleft lip with or without palate (CL/P) is a common congenital anomaly with a high birth prevalence in China. Based on a previous linkage signal of nonsyndromic CL/P (NSCL/P) on the chromosomal region 4q31–q32 from the Chinese populations, we screened the 4q31–q32 region for susceptibility genes in 214 trios of Han Chinese. PDGF-C, an important developmental factor, resides in the region and has been implicated in NSCL/P. However, in our family-based association test (transmission disequilibrium test; TDT), we could not conclude an association between PDGF-C and NSCL/P as previously suggested. Instead, we found strong evidence for parent-of-origin effect at a PDGF-C SNP, rs17035464, by a likelihood ratio test (unadjusted p-value = 0.0018; I_m = 2.46). The location of rs17035464 is 13 kb downstream of a previously reported, NSCL/P-associated SNP, rs28999109. Furthermore, a patient from our sample trios was observed with a maternal segmental uniparental isodisomy (UPD) in a region containing rs17035464. Our findings support the involvement of PDGF-C in the development of oral clefts; moreover, the UPD case report contributes to the collective knowledge of rare variants in the human genome.

Trisomy 21 and early brain development

Trisomy for human chromosome 21 (Hsa21) results in Down syndrome (DS). The finished human genome sequence provides a thorough catalog of the genetic elements whose altered dosage perturbs development and function in DS. However, understanding how small alterations in the steady state transcript levels for <2% of human genes can disrupt development and function of essentially every cell presents a more complicated problem. Mouse models that recapitulate specific aspects of DS have been used to identify changes in brain morphogenesis and function. Here we provide a few examples of how trisomy for specific genes affects the development of the cortex and cerebellum to illustrate how gene dosage effects might contribute to divergence between the trisomic and euploid brains.

A de novo 8.8-Mb deletion of 21q21.1-q21.3 in an autistic male with a complex rearrangement involving chromosomes 6, 10, and 21

We report here on a normal-appearing male with pervasive developmental disorder who was found to have a de novo, apparently balanced complex rearrangement involving chromosomes 6, 10, and 21: 46,XY,ins(21;10)(q11.2;p11.2p13)t(6;21)(p23;q11.2). Further analysis by high-density oligonucleotide microarray was performed, showing an 8.8-Mb heterozygous deletion at 21q21.1-q21.3. Interestingly, the deletion is distal to the translocation breakpoint on chromosome 21. The deletion involves 19 genes, including NCAM2 and GRIK1, both of which are associated with normal brain development and function, and have been considered as possible candidate genes in autism and other neurobehavioral disorders. This case underscores the utility of genomewide microarray analysis for the detection of copy number alterations in patients with apparently balanced complex rearrangements and abnormal phenotypes.

Furthering pharmacological and physiological assessment of the glutamatergic receptors at the Drosophila neuromuscular junction

Drosophila melanogaster larval neuromuscular junctions (NMJs) serve as a model for synaptic physiology. The molecular sequences of the postsynaptic glutamate receptors have been described; however, the pharmacological profile has not been fully elucidated. The postsynaptic molecular sequence suggests a novel glutamate receptor subtype. Kainate does not depolarize the muscle, but dampens evoked EPSP amplitudes. Quantal responses show a decreased amplitude and area under the voltage curve indicative of reduced postsynaptic receptor sensitivity to glutamate transmission. ATPA, a kainate receptor agonist, did not mimic kainate's action. The metabotropic glutamate receptor agonist t-ACPD had no effect. Domoic acid, a kainate/AMPA receptor agonist, blocks the postsynaptic receptors without depolarizing the muscle. However, SYM 2081, a kainate receptor agonist, did depolarize the muscle and reduce the EPSP amplitude at 1 mM but not at 0.1 mM. This supports the notion that these are generally a quisqualate subtype receptors with some oddities in the pharmacological profile. The results suggest a direct postsynaptic action of kainate due to partial antagonist action on the quisqualate receptors. There does not appear to be presynaptic auto-regulation via a kainate receptor subtype or a metabotropic auto-receptor. This study aids in furthering the pharmokinetic profiling and specificity of the receptor subtypes.

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.

Metabolic interactions between glutamatergic and dopaminergic neurotransmitter systems are mediated through D(1) dopamine receptors

Interactions between the dopaminergic and glutamatergic neurotransmission systems were investigated in the adult brain of wild-type (WT) and transgenic mice lacking the dopamine D(1) or D(2) receptor subtypes. Activity of the glutamine cycle was evaluated by using (13)C NMR spectroscopy, and striatal activity was assessed by c-Fos expression and motor coordination. Brain extracts from (1,2-(13)C(2)) acetate-infused mice were prepared and analyzed by (13)C NMR to determine the incorporation of the label into the C4 and C5 carbons of glutamate and glutamine. D(1)R(-/-) mice showed a significantly higher concentration of cerebral (4,5-(13)C(2)) glutamine, consistent with an increased activity of the glutamate-glutamine cycle and of glutamatergic neurotransmission. Conversely, D(2)R(-/-) mice did not show any significant changes in (4,5-(13)C(2)) glutamate or (4,5-(13)C(2)) glutamine, suggesting that alterations in glutamine metabolism are mediated through D(1) receptors. This was confirmed with D(1)R(-/-) and WT mice treated with reserpine, a dopamine-depleting drug, or with reserpine followed by L-DOPA, a dopamine precursor. Exposure to reserpine increased (4,5-(13)C(2)) glutamine in WT to levels similar to those found in untreated D(1)R(-/-) mice. These values were the same as those reached in the reserpine-treated D(1)R(-/-) mice. Treatment of WT animals with L-DOPA returned (4,5-(13)C(2)) glutamine levels to normal, but this was not verified in D(1)R(-/-) animals. Reserpine impaired motor coordination and decreased c-Fos expression, whereas L-DOPA restored both variables to normal values in WT but not in D(1)R(-/-). Together, our results reveal novel neurometabolic interactions between glutamatergic and dopaminergic systems that are mediated through the D(1), but not the D(2), dopamine receptor subtype.

A first glimpse at the transcriptome of Physarum polycephalum

Background

Physarum polycephalum, an acellular plasmodial species belongs to the amoebozoa, a major branch in eukaryote evolution. Its complex life cycle and rich cell biology is reflected in more than 2500 publications on various aspects of its biochemistry, developmental biology, cytoskeleton, and cell motility. It now can be genetically manipulated, opening up the possibility of targeted functional analysis in this organism.

Methods

Here we describe a large fraction of the transcribed genes by sequencing a cDNA library from the plasmodial stage of the developmental cycle.

Results

In addition to the genes for the basic metabolism we found an unexpected large number of genes involved in sophisticated signaling networks and identified potential receptors for environmental signals such as light. In accordance with the various developmental options of the plasmodial cell we found that many P. polycephalum genes are alternatively spliced. Using 30 donor and 30 acceptor sites we determined the splicing signatures of this species.

Comparisons to various other organisms including Dictyostelium, the closest relative, revealed that roughly half of the transcribed genes have no detectable counterpart, thus potentially defining species specific adaptations. On the other hand, we found highly conserved proteins, which are maintained in the metazoan lineage, but absent in D. discoideum or plants. These genes arose possibly in the last common ancestor of Amoebozoa and Metazoa but were lost in D. discoideum.

Conclusion

This work provides an analysis of up to half of the protein coding genes of Physarum polycephalum. The definition of splice motifs together with the description of alternatively spliced genes will provide a valuable resource for the ongoing genome project.

Chronic olanzapine treatment causes differential expression of genes in frontal cortex of rats as revealed by DNA microarray technique

Recent emerging biochemical data indicate that several important neuroregulatory genes and proteins may be involved in the etiology of schizophrenia and bipolar disorder. Additionally, the same genes appear to be targets of several psychotropic medications that are used to treat these disorders. Recent DNA microarray studies show that genes involved in synaptic neurotransmission, signal transduction, and glutamate/GABA regulation may be differentially regulated in brains of subjects with schizophrenia. We hypothesized that chronic administration of olanzapine to rats would alter expression of various genes that may be involved in the etiology of schizophrenia and mood disorders. Rats were administered olanzapine (N=20, 2 mg/kg/day) or sterile saline intraperitoneally (N=20) daily for 21 days. Control and olanzapine-treated frontal cortices were analyzed using cDNA microarray technology. The results showed significant downregulation of 31 genes and upregulation of 38 genes by greater than two-fold in the drug-treated brains vs controls. Our results provide evidence for altered regulation of genes involved with signal transduction and cell communication, metabolism and energy pathways, transport, immune response, nucleic acid metabolism, and neuronal growth factors. Real-time quantitative RT-PCR analysis verified the direction and magnitude of change in six genes of interest: calbindin 3, homer 1, regulator of G-protein signaling (RGS) 2, pyruvate kinase, Reelin and insulin 2. Western blotting showed significant upregulation in protein products for Reelin 410 and Reelin 180 kDa and downregulation for NMDA3B and RGS2. Our results show for the first time that olanzapine causes changes in levels of several important genes that may be involved in the etiology and treatment of schizophrenia and other psychiatric disorders.

AUNX1, a novel locus responsible for X linked recessive auditory and peripheral neuropathy, maps to Xq23-27.3

BACKGROUND

We report here the genetic characterisation of a large five generation Chinese family with the phenotypic features of auditory neuropathy and progressive peripheral sensory neuropathy, and the genetic feature of X linked recessive inheritance. Disease onset was at adolescence (at an average age of 13 years for six affected subjects). The degree of hearing impairment varied from mild to severe, with decreased otoacoustic emissions; auditory brainstem responses were lacking from onset.

METHODS

Two-point and multipoint model based linkage analysis using the MILNK and LINKMAP programs of the FASTLINK software package produced maximum two-point and multipoint LOD scores of 2.41 and 2.41, respectively.

RESULTS

These findings define a novel X linked auditory neuropathy locus/region (AUNX1, Xq23-q27.3). This region is 42.09 cM long and contains a 28.07 Mb region with flanking markers DXS1220 and DXS8084, according to the Rutgers Combined Linkage-Physical Map, build 35. However, mutation screen of the candidate gene SLC6A14 within the region did not identify the causative genetic determinant for this large Chinese family.

The excitoprotective effect of N-methyl-D-aspartate receptors is mediated by a brain-derived neurotrophic factor autocrine loop in cultured hippocampal neurons

The neuroprotective effect and molecular mechanisms underlying preconditioning with N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons have not been described. Pre-incubation with subtoxic concentrations of the endogenous neurotransmitter glutamate protects vulnerable neurons against NMDA receptor-mediated excitotoxicity. As a result of physiological preconditioning, NMDA significantly antagonizes the neurotoxicity resulting from subsequent exposure to an excitotoxic concentration of glutamate. The protective effect of glutamate or NMDA is time- and concentration-dependent, suggesting that sufficient agonist and time are required to establish an intracellular neuroprotective state. In these cells, the TrkB ligand, brain-derived neurotrophic factor (BDNF) attenuates glutamate toxicity. Therefore, we tested the hypothesis that NMDA protects neurons via a BDNF-dependent mechanism. Exposure of hippocampal cultures to a neuroprotective concentration of NMDA (50 microM) evoked the release of BDNF within 2 min without attendant changes in BDNF protein or gene expression. The accumulated increase of BDNF in the medium is followed by an increase in the phosphorylation (activation) of TrkB receptors and a later increase in exon 4-specific BDNF mRNA. The neuroprotective effect of NMDA was attenuated by pre-incubation with a BDNF-blocking antibody and TrkB-IgG, a fusion protein known to inhibit the activity of extracellular BDNF, suggesting that BDNF plays a major role in NMDA-mediated survival. These results demonstrate that low level stimulation of NMDA receptors protect neurons against glutamate excitotoxicity via a BDNF autocrine loop in hippocampal neurons and suggest that activation of neurotrophin signaling pathways plays a key role in the neuroprotection of NMDA.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.