Genetic variations of the NR3A subunit of the NMDA receptor modulate prefrontal cerebral activity in humans

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.

Lessons Learned From Parsing Genetic Risk for Schizophrenia Into Biological Pathways

The clinically heterogeneous presentation of schizophrenia is compounded by the heterogeneity of risk factors and neurobiological correlates of the disorder. Genome-wide association studies in schizophrenia have uncovered a remarkably high number of genetic variants, but the biological pathways they impact upon remain largely unidentified. Among the diverse methodological approaches employed to provide a more granular understanding of genetic risk for schizophrenia, the use of biological labels, such as gene ontologies, regulome approaches, and gene coexpression have all provided novel perspectives into how genetic risk translates into the neurobiology of schizophrenia. Here, we review the salient aspects of parsing polygenic risk for schizophrenia into biological pathways. We argue that parsed scores, compared to standard polygenic risk scores, may afford a more biologically plausible and accurate physiological modeling of the different dimensions involved in translating genetic risk into brain mechanisms, including multiple brain regions, cell types, and maturation stages. We discuss caveats, opportunities, and pitfalls inherent in the parsed risk approach.

Control of protein synthesis and memory by GluN3A-NMDA receptors through inhibition of GIT1/mTORC1 assembly

De novo protein synthesis is required for synapse modifications underlying stable memory encoding. Yet neurons are highly compartmentalized cells and how protein synthesis can be regulated at the synapse level is unknown. Here, we characterize neuronal signaling complexes formed by the postsynaptic scaffold GIT1, the mechanistic target of rapamycin (mTOR) kinase, and Raptor that couple synaptic stimuli to mTOR-dependent protein synthesis; and identify NMDA receptors containing GluN3A subunits as key negative regulators of GIT1 binding to mTOR. Disruption of GIT1/mTOR complexes by enhancing GluN3A expression or silencing GIT1 inhibits synaptic mTOR activation and restricts the mTOR-dependent translation of specific activity-regulated mRNAs. Conversely, GluN3A removal enables complex formation, potentiates mTOR-dependent protein synthesis, and facilitates the consolidation of associative and spatial memories in mice. The memory enhancement becomes evident with light or spaced training, can be achieved by selectively deleting GluN3A from excitatory neurons during adulthood, and does not compromise other aspects of cognition such as memory flexibility or extinction. Our findings provide mechanistic insight into synaptic translational control and reveal a potentially selective target for cognitive enhancement.

GluN3A NMDA receptor subunits: more enigmatic than ever?

Non-conventional N-methyl-d-aspartate receptors (NMDARs) containing GluN3A subunits have unique biophysical, signalling and localization properties within the NMDAR family, and are typically thought to counterbalance functions of classical NMDARs made up of GluN1/2 subunits. Beyond their recognized roles in synapse refinement during postnatal development, recent evidence is building a wider perspective for GluN3A functions. Here we draw particular attention to the latest developments for this multifaceted and unusual subunit: from finely timed expression patterns that correlate with plasticity windows in developing brains or functional hierarchies in the mature brain to new insight onto presynaptic GluN3A-NMDARs, excitatory glycine receptors and behavioural impacts, alongside further connections to a range of brain disorders.

Temporal Dynamics and Neuronal Specificity of Grin3a Expression in the Mouse Forebrain

GluN3A subunits endow N-Methyl-D-Aspartate receptors (NMDARs) with unique biophysical, trafficking, and signaling properties. GluN3A-NMDARs are typically expressed during postnatal development, when they are thought to gate the refinement of neural circuits by inhibiting synapse maturation, and stabilization. Recent work suggests that GluN3A also operates in adult brains to control a variety of behaviors, yet a full spatiotemporal characterization of GluN3A expression is lacking. Here, we conducted a systematic analysis of Grin3a (gene encoding mouse GluN3A) mRNA expression in the mouse brain by combining high-sensitivity colorimetric and fluorescence in situ hybridization with labeling for neuronal subtypes. We find that, while Grin3a mRNA expression peaks postnatally, significant levels are retained into adulthood in specific brain regions such as the amygdala, medial habenula, association cortices, and high-order thalamic nuclei. The time-course of emergence and down-regulation of Grin3a expression varies across brain region, cortical layer of residence, and sensory modality, in a pattern that correlates with previously reported hierarchical gradients of brain maturation and functional specialization. Grin3a is expressed in both excitatory and inhibitory neurons, with strong mRNA levels being a distinguishing feature of somatostatin interneurons. Our study provides a comprehensive map of Grin3a distribution across the murine lifespan and paves the way for dissecting the diverse functions of GluN3A in health and disease.

Impacts of GRIN3A, GRM6 and TPH2 genetic polymorphisms on quality of life in methadone maintenance therapy population

Opioid addiction is a major public health issue worldwide. Methadone maintenance treatment (MMT) is used to detoxify users of illicit opiates, but drug relapse is common and associated with poor quality of life (QoL). This study investigated the associations between the GRIN3A, GRM6, and TPH2 genetic variants and QoL in the MMT population. A total of 319 participants were included in the study, and genotyping of GRIN3A, GRM6, and TPH2 genes was performed using the Sequenom iPLEX. Associations between genotypes and the domains of QoL were examined through posthoc analysis with LSMEANS syntax using SAS 9.1.3. The single nucleotide polymorphisms rs9325202 and rs1487275 in the TPH2 gene were significantly associated with the QoL domain of physical functioning. The least absolute shrinkage and selection operator regression model revealed that the risk allele rs1487275-G was significantly correlated with the domain of physical functioning when clinical characteristics were considered as covariates. The results of the present study illuminate the importance of the genetic basis of QoL in the MMT population, and suggest that genotypes should be considered as a potential QoL indicator.

A Polygenic Risk Score of glutamatergic SNPs associated with schizophrenia predicts attentional behavior and related brain activity in healthy humans

Multiple genetic variations impact on risk for schizophrenia. Recent analyses by the Psychiatric Genomics Consortium (PGC2) identified 128 SNPs genome-wide associated with the disorder. Furthermore, attention and working memory deficits are core features of schizophrenia, are heritable and have been associated with variation in glutamatergic neurotransmission. Based on this evidence, in a sample of healthy volunteers, we used SNPs associated with schizophrenia in PGC2 to construct a Polygenic-Risk-Score (PRS) reflecting the cumulative risk for schizophrenia, along with a Polygenic-Risk-Score including only SNPs related to genes implicated in glutamatergic signaling (Glu-PRS). We performed Factor Analysis for dimension reduction of indices of cognitive performance. Furthermore, both PRS and Glu-PRS were used as predictors of cognitive functioning in the domains of Attention, Speed of Processing and Working Memory. The association of the Glu-PRS on brain activity during the Variable Attention Control (VAC) task was also explored. Finally, in a second independent sample of healthy volunteers we sought to confirm the association between the Glu-PRS and both performance in the domain of Attention and brain activity during the VAC.We found that performance in Speed of Processing and Working Memory was not associated with any of the Polygenic-Risk-Scores. The Glu-PRS, but not the PRS was associated with Attention and brain activity during the VAC. The specific effects of Glu-PRS on Attention and brain activity during the VAC were also confirmed in the replication sample.Our results suggest a pathway specificity in the relationship between genetic risk for schizophrenia, the associated cognitive dysfunction and related brain processing.

Residues in the GluN1 C-terminal domain control kinetics and pharmacology of GluN1/GluN3A N-methyl-d-aspartate receptors

N-methyl-d-aspartate (NMDA) receptors assembled from GluN1 and GluN3 subunits are unique in that they form glycine-gated excitatory channels that are insensitive to glutamate and NMDA. Alternative splicing of the GluN1 subunit mRNA results in eight variants with regulated expression patterns and post-translational modifications. Here we investigate the role of residues in the GluN1 C-terminal alternatively spliced cassettes in receptor gating and modulation. We measured whole-cell currents from recombinant GluN1/GluN3A receptors expressed in HEK293 cells that differed in the sequence of their GluN1 C-terminal tail. We found that these residues controlled the level of steady-state activity and the degree to which activity was facilitated by zinc and protons. Further, we found that the phosphorylation status of sites specific to certain variants can also modulate channel activity. Based on these results we suggest that GluN1 C-terminal domain splicing may confer cell-specific and activity-dependent regulation onto the level and pharmacologic sensitivity of GluN1/GluN3A currents.

Association between genetic variations of NMDA receptor NR3 subfamily genes and heroin addiction in male Han Chinese

Growing amounts of evidence suggest that N-Methyl-d-aspartate (NMDA) receptor mediated glutamate neurotransmission may be involved in the pathophysiology of drug dependence. The NMDA receptor consists of three subfamilies (NR1, NR2, and NR3). The ability of subunit NR3 to negatively modulate the NMDA receptor function makes it an attractive candidate gene of heroin addiction. The purpose of this study is to explore the association between four single nucleotide polymorphisms (SNPs) of NR3 gene and heroin addiction. Genotyping of two SNPs (rs3739722 and rs17189632) in GRIN3A and two SNPs (rs4807399 and rs2240158) in GRIN3B was performed using TaqMan SNP genotyping method. The association between heroin addiction and these SNPs was assessed among 332 male heroin dependent patients and 400 male normal control subjects. The results showed the genotype and allele frequencies of rs17189632 and rs2240158 were significantly different between the cases and the controls (nominal P values were 0.0284, 0.0136 for rs17189632; 0.0048, 0.0013 for rs2240158, respectively). After Bonferroni correction, rs2240158 of GRIN3B was still found to be associated with heroin addiction. The frequencies of haplotype C-A at GRIN3A (rs3739722-rs17189632) and of C-C and C-T at GRIN3B (rs4807399-rs2240158) differed significantly between the cases and the controls. The genotype and allele distributions of rs3739722 and rs4807399 were not significantly different between in the cases and in the controls (P>0.05). These results suggest that GRIN3A rs17189632 and GRIN3B rs2240158 may contribute to the susceptibility of heroin addiction.

Heritability and molecular-genetic basis of the P3 event-related brain potential: a genome-wide association study

P3 amplitude is a candidate endophenotype for disinhibitory psychopathology, psychosis, and other disorders. The present study is a comprehensive analysis of the behavioral- and molecular-genetic basis of P3 amplitude and a P3 genetic factor score in a large community sample (N = 4,211) of adolescent twins and their parents, genotyped for 527,829 single nucleotide polymorphisms (SNPs). Biometric models indicated that as much as 65% of the variance in each measure was due to additive genes. All SNPs in aggregate accounted for approximately 40% to 50% of the heritable variance. However, analyses of individual SNPs did not yield any significant associations. Analyses of individual genes did not confirm previous associations between P3 amplitude and candidate genes but did yield a novel association with myelin expression factor 2 (MYEF2). Main effects of individual variants may be too small to be detected by GWAS without larger samples.

Phosphoinositide-Dependent Protein Kinase 1 (PDK1)

Abstract. Phosphatidylinositol-3-kinase (PI3K) signaling influences susceptibility to virus infections, anoxia, obstetric complications, and cancer; which are changed in patients with schizophrenia and their first degree relatives. Therefore PI3K signaling might have impact on the pathophysiology of schizophrenia. PI3K signaling crucially involves phosphoinositide-dependent protein kinase (PDK1). Increased anxiety behavior is observed in PDK1 hypomorphic mice. Here we show enhanced prevalence of schizophrenia in carriers of the PDK1 CC genotype in human beings. Moreover, decreased parietal P300 amplitude, which is a well-studied schizophrenic endophenotype, was observed in PDK1 CC carriers. Glutamate and glutamine concentrations are increased in the frontal lobe of PDK1 dysmorphic mice and human CC individuals. Our results demonstrate that the PDK1 CC genotype is associated with increased risk to develop schizophrenia, a typical endophenotype profile observed in the disease and modified neurotransmitter concentrations in brain regions associated with the disease.

Pretreatment anterior cingulate activity predicts antidepressant treatment response in major depressive episodes

Major depressive disorder leads to substantial individual and socioeconomic costs. Despite the ongoing efforts to improve the treatment for this condition, a trial-and-error approach until an individually effective treatment is established still dominates clinical practice. Searching for clinically useful treatment response predictors is one of the most promising strategies to change this quandary therapeutic situation. This study evaluated the predictive value of a biological and a clinical predictor, as well as a combination of both. Pretreatment EEGs of 31 patients with a major depressive episode were analyzed with neuroelectric brain imaging technique to assess cerebral oscillations related to treatment response. Early improvement of symptoms served as a clinical predictor. Treatment response was assessed after 4 weeks of antidepressant treatment. Responders showed more slow-frequency power in the right anterior cingulate cortex compared to non-responders. Slow-frequency power in this region was found to predict response with good sensitivity (82 %) and specificity (100 %), while early improvement showed lower accuracy (73 % sensitivity and 65 % specificity). Combining both parameters did not further improve predictive accuracy. Pretreatment activity within the anterior cingulate cortex is related to antidepressive treatment response. Our results support the search for biological treatment response predictors using electric brain activity. This technique is advantageous due to its low individual and socioeconomic burden. The benefits of combining both, a clinically and a biologically based predictor, should be further evaluated using larger sample sizes.

Dopamine and glutamate receptor genes interactively influence episodic memory in old age

Both the dopaminergic and glutamatergic systems modulate episodic memory consolidation. Evidence from animal studies suggests that these two neurotransmitters may interact in influencing memory performance. Given that individual differences in episodic memory are heritable, we investigated whether variations of the dopamine D2 receptor gene (rs6277, C957T) and the N-methyl-D-aspartate 3A (NR3A) gene, coding for the N-methyl-D-aspartate 3A subunit of the glutamate N-methyl-D-aspartate receptor (rs10989591, Val362Met), interactively modulate episodic memory in large samples of younger (20-31 years; n = 670) and older (59-71 years; n = 832) adults. We found a reliable gene-gene interaction, which was observed in older adults only: older individuals carrying genotypes associated with greater D2 and N-methyl-D-aspartate receptor efficacy showed better episodic performance. These results are in line with findings showing magnification of genetic effects on memory in old age, presumably as a consequence of reduced brain resources. Our findings underscore the need for investigating interactive effects of multiple genes to understand individual difference in episodic memory.

Genetic association study of the P300 endophenotype in schizophrenia

OBJECTIVE

Although reduced amplitude of the P300 event-related potential is a well-documented intermediate phenotype of schizophrenia, little is known about its genetic underpinnings in patients with schizophrenia. This study aims to examine associations between P300 and a range of candidate genetic variants, selected from either candidate gene studies or genome-wide association studies, in a large sample of patients with schizophrenia.

METHODS

P300 amplitude at the midline parietal electrode and 193 single nucleotide polymorphisms (SNPs) in 67 genes were assessed in 336 patients with schizophrenia. The association between each SNP and P300 amplitude, controlled for illness duration and gender, was evaluated. Associations at p<.01 were considered of potential relevance, while Bonferroni correction was applied to determine formal statistical significance (Bonferroni-corrected threshold of significance p=.0003).

RESULTS

Of the 193 selected SNPs, 4 SNPs showed potentially relevant association with P300 amplitude at a significance level of p<.01. One of these SNPs, rs1045642 in ABCB1, was most convincingly associated with P300 amplitude, reaching formal (Bonferroni-corrected) significance, while there was evidence for possible association with rs1572899 in DISC-1, rs6265 in BDNF and rs1625579 in MIR137.

CONCLUSION

Genetic variation in ABCB1 may be associated with P300 amplitude in patients with schizophrenia. This result may encourage further efforts to elucidate the genetic underpinnings of P300 generation.

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.

Association of GRIN1 and GRIN2A-D with schizophrenia and genetic interaction with maternal herpes simplex virus-2 infection affecting disease risk

N-methyl-D-aspartate (NMDA) receptors are very important for proper brain development and several lines of evidence support that hypofunction of the NMDA receptors are involved in the pathophysiology of schizophrenia. Gene variation and gene-environmental interactions involving the genes encoding the NMDA receptors are therefore likely to influence the risk of schizophrenia. The aim of this study was to determine (1) whether SNP variation in the genes (GRIN1, GRIN2A, GRIN2B, GRIN2C, and GRIN2D) encoding the NMDA receptor were associated with schizophrenia; (2) whether GRIN gene variation in the offspring interacted with maternal herpes simplex virus-2 (HSV-2) seropositivity during pregnancy influencing the risk of schizophrenia later in life. Individuals from three independently collected Danish case control samples were genotyped for 81 tagSNPs (in total 984 individuals diagnosed with schizophrenia and 1,500 control persons) and antibodies against maternal HSV-2 infection were measured in one of the samples (365 cases and 365 controls). Nine SNPs out of 30 in GRIN2B were significantly associated with schizophrenia. One SNP remained significant after Bonferroni correction (rs1806194, P(nominal)  = 0.0008). Significant interaction between maternal HSV-2 seropositivity and GRIN2B genetic variation in the offspring were observed for seven SNPs and two remained significant after Bonferroni correction (rs1805539, P(nominal)  = 0.0001 and rs1806205, P(nominal)  = 0.0008). The significant associations and interactions were located at the 3' region of GRIN2B suggesting that genetic variation in this part of the gene may be involved in the pathophysiology of schizophrenia.

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.

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.

Altered development of glutamatergic synapses in layer V pyramidal neurons in NR3A knockout mice

Expression of the NMDA receptor (NMDAR) subunit NR3A reaches its highest level in layer V of the developing rodent cortex during the second postnatal week, a peak period of synaptogenesis. Incorporation of NR3A leads to the formation of non-canonical, Mg2+-insensitive NMDARs, but it is not known whether they participate in synaptic transmission and maturation. Here we show that in the second postnatal week, layer V pyramidal neurons in the somatosensory cortex of wild type (WT) mice exhibited evoked excitatory postsynaptic currents (eEPSCs) with 3- to 6-fold lower Mg2+ sensitivity than NR3A knockout (KO) mice and their reversal potential was approximately 2 mV more negative compared to KO mice consistent with decreased P(Ca) of NMDARs. Surprisingly, ablation of NR3A also led to a 20-fold reduction of the ratio of AMPAR- to NMDAR-mediated eEPSC amplitudes in KO mice. Insertion of AMPARs at the synapses of layer V pyramidal neurons appears to be facilitated by the expression of Mg2+-insensitive NMDARs. The data indicate that NR3A plays a significant role in the development of excitatory synapses in layer V of the developing neocortex.

Downregulation of NR3A-containing NMDARs is required for synapse maturation and memory consolidation

NR3A is the only NMDA receptor (NMDAR) subunit that downregulates sharply prior to the onset of sensitive periods for plasticity, yet the functional importance of this transient expression remains unknown. To investigate whether removal/replacement of juvenile NR3A-containing NMDARs is involved in experience-driven synapse maturation, we used a reversible transgenic system that prolonged NR3A expression in the forebrain. We found that removal of NR3A is required to develop strong NMDAR currents, full expression of long-term synaptic plasticity, a mature synaptic organization characterized by more synapses and larger postsynaptic densities, and the ability to form long-term memories. Deficits associated with prolonged NR3A were reversible, as late-onset suppression of transgene expression rescued both synaptic and memory impairments. Our results suggest that NR3A behaves as a molecular brake to prevent the premature strengthening and stabilization of excitatory synapses and that NR3A removal might thereby initiate critical stages of synapse maturation during early postnatal neural development.

Using genetic data in cognitive neuroscience: from growing pains to genuine insights

Research that combines genetic and cognitive neuroscience data aims to elucidate the mechanisms that underlie human behaviour and experience by way of 'intermediate phenotypes': variations in brain function. Using neuroimaging and other methods, this approach is poised to make the transition from health-focused investigations to inquiries into cognitive, affective and social functions, including ones that do not readily lend themselves to animal models. The growing pains of this emerging field are evident, yet there are also reasons for a measured optimism.

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.

Molecular mechanisms of schizophrenia

Schizophrenia is a complex disorder, where family, twin and adoption studies have been demonstrating a high heritability of the disease and that this disease is not simply defined by several major genes but rather evolves from addition or potentiation of a specific cluster of genes, which subsequently determines the genetic vulnerability of an individual. Linkage and association studies suggest that a genetic vulnerablility, is not forcefully leading to the disease since triggering factors and environmental influences, i.e. birth complications, drug abuse, urban background or time of birth have been identified. This has lead to the assumption that schizophrenia is not only a genetically defined static disorder but a dynamic process leading to dysregulation of multiple pathways. There are several different hypothesis based on several facets of the disease, some of them due to the relatively well-known mechanisms of therapeutic agents. The most widely considered neurodevelopmental hypothesis of schizophrenia integrates environmental influences and causative genes. The dopamine hypothesis of schizophrenia is based on the fact that all common treatments involve antidopaminergic mechanisms and genes such as DRD2, DRD3, DARPP-32, BDNF or COMT are closely related to dopaminergic system functioning. The glutamatergic hypothesis of schizophrenia lead recently to a first successful mGlu2/3 receptor agonistic drug and is underpinned by significant findings in genes regulating the glutamatergic system (SLC1A6, SLC1A2 GRIN1, GRIN2A, GRIA1, NRG1, ErbB4, DTNBP1, DAAO, G72/30, GRM3). Correspondingly, GABA has been proposed to modulate the pathophysiology of the disease which is represented by the involvement of genes like GABRA1, GABRP, GABRA6 and Reelin. Moreover, several genes implicating immune, signaling and networking deficits have been reported to be involved in the disease, i.e. DISC1, RGS4, PRODH, DGCR6, ZDHHC8, DGCR2, Akt, CREB, IL-1B, IL-1RN, IL-10, IL-1B. However, molecular findings suggest that a complex interplay between receptors, kinases, proteins and hormones is involved in schizophrenia. In a unifying hypothesis, different cascades merge into another that ultimately lead to the development of symptoms adherent to schizophrenic disorders.