Nogo Receptor 1 (RTN4R) as a candidate gene for schizophrenia: analysis using human and mouse genetic approaches

PICK1 links KIBRA and AMPA receptors in coiled-coil-driven supramolecular complexes

The human memory-associated protein KIBRA regulates synaptic plasticity and trafficking of AMPA-type glutamate receptors, and is implicated in multiple neuropsychiatric and cognitive disorders. How KIBRA forms complexes with and regulates AMPA receptors remains unclear. Here, we show that KIBRA does not interact directly with the AMPA receptor subunit GluA2, but that PICK1, a key regulator of AMPA receptor trafficking, can serve as a bridge between KIBRA and GluA2. We identified structural determinants of KIBRA-PICK1-AMPAR complexes by investigating interactions and cellular expression patterns of different combinations of KIBRA and PICK1 domain mutants. We find that the PICK1 BAR domain, a coiled-coil structure, is sufficient for interaction with KIBRA, whereas mutation of the BAR domain disrupts KIBRA-PICK1-GluA2 complex formation. In addition, KIBRA recruits PICK1 into large supramolecular complexes, a process which requires KIBRA coiled-coil domains. These findings reveal molecular mechanisms by which KIBRA can organize key synaptic signaling complexes.

Genomic and Reverse Translational Analysis Discloses a Role for Small GTPase RhoA Signaling in the Pathogenesis of Schizophrenia: Rho-Kinase as a Novel Drug Target

Schizophrenia is one of the most serious psychiatric disorders and is characterized by reductions in both brain volume and spine density in the frontal cortex. RhoA belongs to the RAS homolog (Rho) family and plays critical roles in neuronal development and structural plasticity via Rho-kinase. RhoA activity is regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). Several variants in GAPs and GEFs associated with RhoA have been reported to be significantly associated with schizophrenia. Moreover, several mouse models carrying schizophrenia-associated gene variants involved in RhoA/Rho-kinase signaling have been developed. In this review, we summarize clinical evidence showing that variants in genes regulating RhoA activity are associated with schizophrenia. In the last half of the review, we discuss preclinical evidence indicating that RhoA/Rho-kinase is a potential therapeutic target of schizophrenia. In particular, Rho-kinase inhibitors exhibit anti-psychotic-like effects not only in Arhgap10 S490P/NHEJ mice, but also in pharmacologic models of schizophrenia (methamphetamine- and MK-801-treated mice). Accordingly, we propose that Rho-kinase inhibitors may have antipsychotic effects and reduce cognitive deficits in schizophrenia despite the presence or absence of genetic variants in small GTPase signaling pathways.

NgR1 binding to reovirus reveals an unusual bivalent interaction and a new viral attachment protein

Nogo-66 receptor 1 (NgR1) binds a variety of structurally dissimilar ligands in the adult central nervous system to inhibit axon extension. Disruption of ligand binding to NgR1 and subsequent signaling can improve neuron outgrowth, making NgR1 an important therapeutic target for diverse neurological conditions such as spinal crush injuries and Alzheimer's disease. Human NgR1 serves as a receptor for mammalian orthoreovirus (reovirus), but the mechanism of virus-receptor engagement is unknown. To elucidate how NgR1 mediates cell binding and entry of reovirus, we defined the affinity of interaction between virus and receptor, determined the structure of the virus-receptor complex, and identified residues in the receptor required for virus binding and infection. These studies revealed that central NgR1 surfaces form a bridge between two copies of viral capsid protein σ3, establishing that σ3 serves as a receptor ligand for reovirus. This unusual binding interface produces high-avidity interactions between virus and receptor to prime early entry steps. These studies refine models of reovirus cell-attachment and highlight the evolution of viruses to engage multiple receptors using distinct capsid components.

The 22q11.2 region regulates presynaptic gene-products linked to schizophrenia

It is unclear how the 22q11.2 deletion predisposes to psychiatric disease. To study this, we generated induced pluripotent stem cells from deletion carriers and controls and utilized CRISPR/Cas9 to introduce the heterozygous deletion into a control cell line. Here, we show that upon differentiation into neural progenitor cells, the deletion acted in trans to alter the abundance of transcripts associated with risk for neurodevelopmental disorders including autism. In excitatory neurons, altered transcripts encoded presynaptic factors and were associated with genetic risk for schizophrenia, including common and rare variants. To understand how the deletion contributed to these changes, we defined the minimal protein-protein interaction network that best explains gene expression alterations. We found that many genes in 22q11.2 interact in presynaptic, proteasome, and JUN/FOS transcriptional pathways. Our findings suggest that the 22q11.2 deletion impacts genes that may converge with psychiatric risk loci to influence disease manifestation in each deletion carrier.

The CNS/PNS Extracellular Matrix Provides Instructive Guidance Cues to Neural Cells and Neuroregulatory Proteins in Neural Development and Repair

BACKGROUND

The extracellular matrix of the PNS/CNS is unusual in that it is dominated by glycosaminoglycans, especially hyaluronan, whose space filling and hydrating properties make essential contributions to the functional properties of this tissue. Hyaluronan has a relatively simple structure but its space-filling properties ensure micro-compartments are maintained in the brain ultrastructure, ensuring ionic niches and gradients are maintained for optimal cellular function. Hyaluronan has cell-instructive, anti-inflammatory properties and forms macro-molecular aggregates with the lectican CS-proteoglycans, forming dense protective perineuronal net structures that provide neural and synaptic plasticity and support cognitive learning.

AIMS

To highlight the central nervous system/peripheral nervous system (CNS/PNS) and its diverse extracellular and cell-associated proteoglycans that have cell-instructive properties regulating neural repair processes and functional recovery through interactions with cell adhesive molecules, receptors and neuroregulatory proteins. Despite a general lack of stabilising fibrillar collagenous and elastic structures in the CNS/PNS, a sophisticated dynamic extracellular matrix is nevertheless important in tissue form and function.

CONCLUSIONS

This review provides examples of the sophistication of the CNS/PNS extracellular matrix, showing how it maintains homeostasis and regulates neural repair and regeneration.

Molecular genetics of 22q11.2 deletion syndrome

The 22q11.2 deletion syndrome (22q11.2DS) is a congenital malformation and neuropsychiatric disorder caused by meiotic chromosome rearrangements. One of the goals of this review is to summarize the current state of basic research studies of 22q11.2DS. It highlights efforts to understand the mechanisms responsible for the 22q11.2 deletion that occurs in meiosis. This mechanism involves the four sets of low copy repeats (LCR22) that are dispersed in the 22q11.2 region and the deletion is mediated by nonallelic homologous recombination events. This review also highlights selected genes mapping to the 22q11.2 region that may contribute to the typical clinical findings associated with the disorder and explain that mutations in genes on the remaining allele can uncover rare recessive conditions. Another important aspect of 22q11.2DS is the existence of phenotypic heterogeneity. While some patients are mildly affected, others have severe medical, cognitive, and/or psychiatric challenges. Variability may be due in part to the presence of genetic modifiers. This review discusses current genome-wide efforts to identify such modifiers that could shed light on molecular pathways required for normal human development, cognition or behavior.

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

The rs696880 Polymorphism in the Nogo-A Receptor Gene (RTN4R) Is Associated With Susceptibility to Sporadic Amyotrophic Lateral Sclerosis in the Chinese Population

Single-nucleotide polymorphisms (SNPs) in the Nogo-A receptor gene (RTN4R) have been associated with increased risk for sporadic amyotrophic lateral sclerosis (SALS) in the French population. In the present study, we investigated the associations between RTN4R tag SNPs and SALS in a large Chinese population. Four tag SNPs (rs854971, rs887765, rs696880 and rs1567871) in the RTN4R gene with an r² threshold of 0.8 and a minor allele frequency (MAF) greater than 0.2% were selected based on Chinese population data from HapMap. A total of 499 SALS patients and 503 healthy controls were genotyped for the SNPs by SNaPshot technology. Haplotype analysis of the four SNPs was performed using the SHEsis software platform. The results showed a significant association between the rs696880 risk allele (A) and SALS in the Han Chinese population (P = 0.009, odds ratio (OR) = 1.266 [1.06-1.51]). The allele and genotype frequencies of rs854971, rs887765 and rs1567871 were not associated with SALS. The distribution of the GAAT haplotype was different between the case and control groups (P = 0.008, OR = 1.289 [1.066-1.558]). In conclusion, our study showed an association between the RTN4R SNP rs696880 and the risk of SALS in the Han Chinese population, with the A allele increasing risk.

A novel rare variant R292H in RTN4R affects growth cone formation and possibly contributes to schizophrenia susceptibility

Reticulon 4 receptor (RTN4R) plays an essential role in regulating axonal regeneration and plasticity in the central nervous system through the activation of rho kinase, and is located within chromosome 22q11.2, a region that is known to be a hotspot for schizophrenia (SCZ) and autism spectrum disorder (ASD). Recently, rare variants such as copy-number variants and single-nucleotide variants have been a focus of research because of their large effect size associated with increased susceptibility to SCZ and ASD and the possibility of elucidating the pathophysiology of mental disorder through functional analysis of the discovered rare variants. To discover rare variants with large effect size and to evaluate their role in the etiopathophysiology of SCZ and ASD, we sequenced the RTN4R coding exons with a sample comprising 370 SCZ and 192 ASD patients, and association analysis using a large number of unrelated individuals (1716 SCZ, 382 ASD and 4009 controls). Through this mutation screening, we discovered four rare (minor allele frequency <1%) missense mutations (R68H, D259N, R292H and V363M) of RTN4R. Among these discovered rare mutations, R292H was found to be significantly associated with SCZ (P=0.048). Furthermore, in vitro functional assays showed that the R292H mutation affected the formation of growth cones. This study strengthens the evidence for association between rare variants within RTN4R and SCZ, and may shed light on the molecular mechanisms underlying the neurodevelopmental disorder.

Genetic variation is associated with RTN4R expression and working memory processing in healthy humans

The Nogo receptor (NgR) is implicated in neurodevelopmental processes and it participates in inhibiting axonal growth. Consistent with its high levels of expression in the prefrontal cortex, animal studies indicate that NgR is relevant for prefrontal-related cognitive processing. Given that genetic variation may alter mechanisms of gene expression impacting molecular and systems-level phenotypes, we investigated the association of genetic variation with the expression of the NgR coding gene (RTN4R), as well as with prefrontal correlates at progressively greater biological distance from gene effects. First, we studied the association of single nucleotide polymorphisms (SNPs) with RTN4R mRNA expression in postmortem prefrontal cortex of humans without psychiatric illnesses. Then, we probed in peripheral blood mononuclear cells (PBMCs) the association that we found in prefrontal tissue. Thus, we investigated whether functional genetic variation affecting RTN4R expression is also associated with prefrontal activity during working memory. We found that rs696884 (A/G) predicted these phenotypes. Specifically, the AA genotype was associated with lower RTN4R mRNA expression levels in the prefrontal cortex and PBMCs and inefficient prefrontal activity during working memory compared to the GG genotype. These results suggest that genetic variation associated with RTN4R mRNA expression influences prefrontal physiology in healthy individuals. Furthermore, they highlight the need for further investigations of the role of NgR in the pathophysiology of brain disorders associated with prefrontal dysfunction.

A fast and powerful W-test for pairwise epistasis testing

Epistasis plays an essential role in the development of complex diseases. Interaction methods face common challenge of seeking a balance between persistent power, model complexity, computation efficiency, and validity of identified bio-markers. We introduce a novel W-test to identify pairwise epistasis effect, which measures the distributional difference between cases and controls through a combined log odds ratio. The test is model-free, fast, and inherits a Chi-squared distribution with data adaptive degrees of freedom. No permutation is needed to obtain the P-values. Simulation studies demonstrated that the W-test is more powerful in low frequency variants environment than alternative methods, which are the Chi-squared test, logistic regression and multifactor-dimensionality reduction (MDR). In two independent real bipolar disorder genome-wide associations (GWAS) datasets, the W-test identified significant interactions pairs that can be replicated, including SLIT3-CENPN, SLIT3-TMEM132D, CNTNAP2-NDST4 and CNTCAP2-RTN4R The genes in the pairs play central roles in neurotransmission and synapse formation. A majority of the identified loci are undiscoverable by main effect and are low frequency variants. The proposed method offers a powerful alternative tool for mapping the genetic puzzle underlying complex disorders.

Identification of genetic variants of LGI1 and RTN4R (NgR1) linked to schizophrenia that are defective in NgR1-LGI1 signaling

Background

The protein NgR1 is encoded by RTN4R, a gene linked to schizophrenia. We previously reported NgR1 as receptor for the epilepsy‐linked protein LGI1. NgR1 regulates synapse number and synaptic plasticity, whereas LGI1 antagonizes NgR1 signaling and promotes synapse formation. Impairments in synapse formation are common in neurological disease and we hypothesized that an LGI1–NgR1 signaling pathway may contribute to the development of schizophrenia.

Methods

We screened two unrelated schizophrenic populations for variants in RTN4R and LGI1 using whole exome sequencing and Sanger sequencing. We tested the ability of LGI1 to bind rare coding variants of NgR1 using a cell surface binding assays and the signaling ability of NgR1 using COS7 cell‐spreading assays.

Results

We observed a previously reported rare coding variant in RTN4R (c.1195C>T, pR399W). We report the first LGI1 mutations to be identified in individuals with schizophrenia. Three different LGI1 mutations were found, two missense mutations (c.205G>A, p.V69I) and (c.313G>A, V105M), and an intronic variant (g.897T>C) that likely leads to a protein truncation. We found NgR1^R119W and NgR1^277C have a reduced ability to bind LGI1 in a cell surface binding assay. COS7 cell‐spreading assays reveal that NgR1 mutants are impaired in their ability to mediate RhoA activation.

Conclusion

Variants in NgR1 and LGI1 may be associated with schizophrenia and variants in NgR1 found in schizophrenic patients have impaired LGI1–NgR1 signaling. Impaired LGI1–NgR1 signaling may contribute to disease progression.

Developmental Inhibition of Gsk3 Rescues Behavioral and Neurophysiological Deficits in a Mouse Model of Schizophrenia Predisposition

While the genetic basis of schizophrenia is increasingly well characterized, novel treatments will require establishing mechanistic relationships between specific risk genes and core phenotypes. Rare, highly penetrant risk genes such as the 22q11.2 microdeletion are promising in this regard. Df(16)A(+/-) mice, which carry a homologous microdeletion, have deficits in hippocampal-prefrontal connectivity that correlate with deficits in spatial working memory. These mice also have deficits in axonal development that are accompanied by dysregulated Gsk3β signaling and can be rescued by Gsk3 antagonists. Here we show that developmental inhibition of Gsk3 rescues deficits in hippocampal-prefrontal connectivity, task-related neural activity, and spatial working memory behavior in Df(16)A(+/-) mice. Taken together, these results provide mechanistic insight into how the microdeletion results in cognitive deficits, and they suggest possible targets for novel therapies.

Genetic variants in Nogo receptor signaling pathways may be associated with early life adversity in schizophrenia susceptibility

Background

Schizophrenia is a severe neuropsychiatric disorder thought to result from abnormal brain development. Nogo, an oligodendrocyte bound molecule, signals by binding to the Nogo receptor (NgR) located on axonal membranes. The NgR co-receptors include p75 neurotrophin receptor or TNF receptor orphan Y (TROY). Nogo signaling is responsible for central nervous system myelin regulation and neurite outgrowth during neurodevelopment, and plasticity in the mature brain.

Methods

We examined single nucleotide polymorphisms (SNPs) in NgR, p75, and TROY receptor genes and downstream signaling partner With No Lysine (K) (WNK1) and Myelin transcription factor 1-like (Myt1l) genes in an Australian case–control schizophrenia cohort (n = 268/group). High-throughput SNP genotyping was performed using the MassARRAY® genotyping assay.

Results

Analysis revealed a significant association between the Myt1l SNP rs2304008 and female schizophrenia subjects. The WNK1 SNP rs1468326 and the Myt1l SNP rs3748988 showed significant associations with schizophrenia in subjects with a maternal mental history and in subjects who experienced childhood trauma respectively. Following Bonferroni correction, all significance was lost.

Conclusions

Despite the lack of positive findings in our population after correction for multiple testing, previous gene expression and association studies in schizophrenia suggest the implication of NgR signaling pathway genes in the etiology of schizophrenia remains topical and timely.

General significance

Further investigations will be necessary to fully assess the role of these genes in the pathophysiology of schizophrenia. However these genes may prove useful in further understanding the mechanism by which negative experiences early in life can affect myelin-related processes in the context of schizophrenia.

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NgR signaling polymorphisms were evaluated in schizophrenia patients and healthy controls.

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Prior to Bonferroni correction Myt1l rs2304008 had a significant association in female schizophrenia subjects.

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Prior to Bonferroni correction WNK1 rs1468326 and Myt1l rs3748988 were associated with schizophrenia in subjects who experienced early life adversities.

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Disruption to NgR signaling may be associated with early life adversity in schizophrenia.

Glia-related genes and their contribution to schizophrenia

Schizophrenia, a debilitating disease with 1% prevalence in the general population, is characterized by major neuropsychiatric symptoms, including delusions, hallucinations, and deficits in emotional and social behavior. Previous studies have directed their investigations on the mechanism of schizophrenia towards neuronal dysfunction and have defined schizophrenia as a 'neuron-centric' disorder. However, along with the development of genetics and systematic biology approaches in recent years, the crucial role of glial cells in the brain has also been shown to contribute to the etiopathology of schizophrenia. Here, we summarize comprehensive data that support the involvement of glial cells (including oligodendrocytes, astrocytes, and microglial cells) in schizophrenia and list several acknowledged glia-related genes or molecules associated with schizophrenia. Instead of purely an abnormality of neurons in schizophrenia, an additional 'glial perspective' provides us a novel and promising insight into the causal mechanisms and treatment for this disease.

A common functional allele of the Nogo receptor gene, reticulon 4 receptor (RTN4R), is associated with sporadic amyotrophic lateral sclerosis in a French population

Amyotrophic lateral sclerosis is sporadic (SALS) in 90% of cases and has complex environmental and genetic influences. Nogo protein inhibits neurite outgrowth and is overexpressed in muscle in ALS. Our aims were to study the reticulon 4 receptor gene RTN4R which encodes Nogo 1 receptor (NgR1) in SALS, to test if the variants were associated with variable expression of the gene and whether NgR1 protein expression was modified in a transgenic mouse model of ALS. We genotyped three single nucleotide polymorphisms (SNPs; rs701421, rs701427, and rs1567871) of the RTN4R gene in 364 SALS French patients and 430 controls. We examined expression of RTN4R mRNA by quantitative PCR in control post mortem human brain tissue. We determined the expression of NgR1 protein in spinal motor neurons from a SOD1 G86R ALS mouse model. We observed significant associations between SALS and RTN4R alleles. Messenger RNA expression from RTN4R in human cortical brain tissue correlated significantly with the genotypes of rs701427. NgR1 protein expression was reduced in Nogo A positive motor neurons from diseased transgenic animals. In conclusion, these observations suggest that a functional RTN4R gene variant is associated with SALS. This variant may act in concert with other genetic variants or environmental influences.

NogoR1 and PirB signaling stimulates neural stem cell survival and proliferation

Neural stem cells (NSCs) and neural progenitors (NPs) in the mammalian neocortex give rise to the main cell types of the nervous system. The biological behavior of these NSCs and NPs is regulated by extracellular niche derived autocrine-paracrine signaling factors on a developmental timeline. Our previous reports [Plos One 2010;5:e15341; J Neurochem 2011;117:565-578] have shown that chondroitin sulfate proteoglycan and ApolipoproteinE are autocrine-paracrine survival factors for NSCs. NogoA, a myelin related protein, is expressed in the cortical ventricular zones where NSCs reside. However, the functional role of Nogo signaling proteins in NSC behavior is not completely understood. In this study, we show that NogoA receptors, NogoR1 and PirB, are expressed in the ventricular zone where NSCs reside between E10.5 and 14.5 but not at E15.5. Nogo ligands stimulate NSC survival and proliferation in a dosage-dependent manner in vitro. NogoR1 and PirB are low and high affinity Nogo receptors, respectively and are responsible for the effects of Nogo ligands on NSC behavior. Inhibition of autocrine-paracrine Nogo signaling blocks NSC survival and proliferation. In NSCs, NogoR1 functions through Rho whereas PirB uses Shp1/2 signaling pathways to control NSC behavior. Taken together, this work suggests that Nogo signaling is an important pathway for survival of NSCs.

Nogo-A-deficient Transgenic Rats Show Deficits in Higher Cognitive Functions, Decreased Anxiety, and Altered Circadian Activity Patterns

Decreased levels of Nogo-A-dependent signaling have been shown to affect behavior and cognitive functions. In Nogo-A knockout and knockdown laboratory rodents, behavioral alterations were observed, possibly corresponding with human neuropsychiatric diseases of neurodevelopmental origin, particularly schizophrenia. This study offers further insight into behavioral manifestations of Nogo-A knockdown in laboratory rats, focusing on spatial and non-spatial cognition, anxiety levels, circadian rhythmicity, and activity patterns. Demonstrated is an impairment of cognitive functions and behavioral flexibility in a spatial active avoidance task, while non-spatial memory in a step-through avoidance task was spared. No signs of anhedonia, typical for schizophrenic patients, were observed in the animals. Some measures indicated lower anxiety levels in the Nogo-A-deficient group. Circadian rhythmicity in locomotor activity was preserved in the Nogo-A knockout rats and their circadian period (tau) did not differ from controls. However, daily activity patterns were slightly altered in the knockdown animals. We conclude that a reduction of Nogo-A levels induces changes in CNS development, manifested as subtle alterations in cognitive functions, emotionality, and activity patterns.

Orchestrated regulation of Nogo receptors, LOTUS, AMPA receptors and BDNF in an ECT model suggests opening and closure of a window of synaptic plasticity

Electroconvulsive therapy (ECT) is an efficient and relatively fast acting treatment for depression. However, one severe side effect of the treatment is retrograde amnesia, which in certain cases can be long-term. The mechanisms behind the antidepressant effect and the amnesia are not well understood. We hypothesized that ECT causes transient downregulation of key molecules needed to stabilize synaptic structure and to prevent Ca2+ influx, and a simultaneous increase in neurotrophic factors, thus providing a short time window of increased structural synaptic plasticity. Here we followed regulation of NgR1, NgR3, LOTUS, BDNF, and AMPA subunits GluR1 and GluR2 flip and flop mRNA levels in hippocampus at 2, 4, 12, 24, and 72 hours after a single episode of induced electroconvulsive seizures (ECS) in rats. NgR1 and LOTUS mRNA levels were transiently downregulated in the dentate gyrus 2, 4, 12 and 4, 12, 24 h after ECS treatment, respectively. GluR2 flip, flop and GluR1 flop were downregulated at 4 h. GluR2 flip remained downregulated at 12 h. In contrast, BDNF, NgR3 and GluR1 flip mRNA levels were upregulated. Thus, ECS treatment induces a transient regulation of factors important for neuronal plasticity. Our data provide correlations between ECS treatment and molecular events compatible with the hypothesis that both effects and side effects of ECT may be caused by structural synaptic rearrangements.

Copy number variation at 22q11.2: from rare variants to common mechanisms of developmental neuropsychiatric disorders

Recently discovered genome-wide rare copy number variants (CNVs) have unprecedented levels of statistical association with many developmental neuropsychiatric disorders, including schizophrenia, autism spectrum disorders, intellectual disability and attention deficit hyperactivity disorder. However, as CNVs often include multiple genes, causal genes responsible for CNV-associated diagnoses and traits are still poorly understood. Mouse models of CNVs are in use to delve into the precise mechanisms through which CNVs contribute to disorders and associated traits. Based on human and mouse model studies on rare CNVs within human chromosome 22q11.2, we propose that alterations of a distinct set of multiple, noncontiguous genes encoded in this chromosomal region, in concert with modulatory impacts of genetic background and environmental factors, variably shift the probabilities of phenotypes along a predetermined developmental trajectory. This model can be further extended to the study of other CNVs and may serve as a guide to help characterize the impact of genes in developmental neuropsychiatric disorders.

N-Methyl-d-Aspartate Receptor - Nitric Oxide Synthase Pathway in the Cortex of Nogo-A-Deficient Rats in Relation to Brain Laterality and Schizophrenia

It has been suggested that Nogo-A, a myelin-associated protein, could play a role in the pathogenesis of schizophrenia and that Nogo-A-deficient rodents could serve as an animal model for schizophrenic symptoms. Since changes in brain laterality are typical of schizophrenia, we investigated whether Nogo-A-deficient rats showed any signs of disturbed asymmetry in cortical N-methyl-d-aspartate (NMDA) receptor–nitric oxide synthase (NOS) pathway, which is reported as dysfunctional in schizophrenia. In particular, we measured separately in the right and left hemisphere of young and old Nogo-A-deficient male rats the expression of NMDA receptor subunits (NR1, NR2A, and NR2B in the frontal cortex) and activities of NOS isoforms [neuronal (nNOS), endothelial (eNOS), and inducible (iNOS) in the parietal cortex]. In young controls, we observed right/left asymmetry of iNOS activity and three positive correlations (between NR1 in the left and NR2B laterality, between NR2B in the right and left sides, and between NR1 in the right side and nNOS laterality). In old controls, we found bilateral decreases in NR1, an increase in NR2B in the right side, and two changes in correlations in the NR1–nNOS pathway. In young Nogo-A-deficient rats, we observed an increase in iNOS activity in the left hemisphere and two changes in correlations in NR1–nNOS and NR2A–eNOS, compared to young controls. Finally, we revealed in old Nogo-A-deficient animals, bilateral decreases in NR1 and one change in correlation between eNOS–iNOS, compared to old controls. Although some findings from schizophrenic brains did not manifest in Nogo-A-deficient rats (e.g., no alterations in NR2B), others did (e.g., alterations demonstrating accelerated aging in young but not old animals, those occurring exclusively in the right hemisphere in young and old animals and those suggesting abnormal frontoparietal cortical interactions in young animals).

Nogo and Nogo receptor: relevance to schizophrenia?

The membrane protein Nogo-A and its receptor NgR have been extensively characterized for their role in restricting axonal growth, regeneration, and plasticity in the central nervous system. Recent evidence suggests that Nogo and NgR might constitute candidate genes for schizophrenia susceptibility. In this article, we critically review the possibility that dysfunctions related to Nogo-A and NgR might contribute to increased risk for schizophrenia. To this end, we consider the most important insights that have emerged from human genetic and pathological studies and from experimental animal work. Furthermore, we discuss potential mechanisms of Nogo/NgR involvement in neural circuit development and stability, and how mutations or changes in expression levels of these proteins could be developmental risk factors contributing to schizophrenia.

An environmental analysis of genes associated with schizophrenia: hypoxia and vascular factors as interacting elements in the neurodevelopmental model

Investigating and understanding gene-environment interaction (G × E) in a neurodevelopmentally and biologically plausible manner is a major challenge for schizophrenia research. Hypoxia during neurodevelopment is one of several environmental factors related to the risk of schizophrenia, and links between schizophrenia candidate genes and hypoxia regulation or vascular expression have been proposed. Given the availability of a wealth of complex genetic information on schizophrenia in the literature without knowledge on the connections to environmental factors, we now systematically collected genes from candidate studies (using SzGene), genome-wide association studies (GWAS) and copy number variation (CNV) analyses, and then applied four criteria to test for a (theoretical) link to ischemia-hypoxia and/or vascular factors. In all, 55% of the schizophrenia candidate genes (n=42 genes) met the criteria for a link to ischemia-hypoxia and/or vascular factors. Genes associated with schizophrenia showed a significant, threefold enrichment among genes that were derived from microarray studies of the ischemia-hypoxia response (IHR) in the brain. Thus, the finding of a considerable match between genes associated with the risk of schizophrenia and IHR and/or vascular factors is reproducible. An additional survey of genes identified by GWAS and CNV analyses suggested novel genes that match the criteria. Findings for interactions between specific variants of genes proposed to be IHR and/or vascular factors with obstetric complications in patients with schizophrenia have been reported in the literature. Therefore, the extended gene set defined here may form a reasonable and evidence-based starting point for hypothesis-based testing of G × E interactions in clinical genetic and translational neuroscience studies.

The reticulons: guardians of the structure and function of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by a specific class of curvature stabilizing proteins, the reticulons and DP1; however it is still unclear how the sheets are assembled. The ER is the cellular compartment responsible for secretory and membrane protein synthesis. The reducing conditions of ER lead to the intra/inter-chain formation of new disulphide bonds into polypeptides during protein folding assessed by enzymatic or spontaneous reactions. Moreover, ER represents the main intracellular calcium storage site and it plays an important role in calcium signaling that impacts many cellular processes. Accordingly, the maintenance of ER function represents an essential condition for the cell, and ER morphology constitutes an important prerogative of it. Furthermore, it is well known that ER undergoes prominent shape transitions during events such as cell division and differentiation. Thus, maintaining the correct ER structure is an essential feature for cellular physiology. Now, it is known that proper ER-associated proteins play a fundamental role in ER tubules formation. Among these ER-shaping proteins are the reticulons (RTN), which are acquiring a relevant position. In fact, beyond the structural role of reticulons, in very recent years new and deeper functional implications of these proteins are emerging in relation to their involvement in several cellular processes.

The Nogo-66 receptor family in the intact and diseased CNS

The Nogo-66 receptor family (NgR) consists in three glycophosphatidylinositol (GPI)-anchored receptors (NgR1, NgR2 and NgR3), which are primarily expressed by neurons in the central and peripheral mammalian nervous system. NgR1 was identified as serving as a high affinity binding protein for the three classical myelin-associated inhibitors (MAIs) Nogo-A, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMgp), which limit axon regeneration and sprouting in the injured brain. Recent studies suggest that NgR signaling may also play an essential role in the intact adult CNS in restricting axonal and synaptic plasticity and are involved in neurodegenerative diseases, particularly in Alzheimer's disease pathology through modulation of β-secretase cleavage. Here, we outline the biochemical properties of NgRs and their functional roles in the intact and diseased CNS.

NOGO-66 receptor deficient mice show slow acquisition of spatial memory task performance

The Nogo-66 receptor (NgR1) is part of a co-receptor complex on neurons that transmits a signal for inhibition of neurite outgrowth. In addition, NgR1 function has also been related to other disorders such as schizophrenia and Alzheimer's disease. Here, we studied the effect of life-long deletion of NgR1 (ngr(-/-)) in tests for cognition and positive symptoms of schizophrenia. In the water maze, ngr(-/-) mice learned to locate the hidden platform as well as wild type mice, although with slower acquisition. Deletion of NgR1 did not affect amphetamine- or phencyclidine (PCP)-induced hyperactivity, two models of positive symptoms of schizophrenia. Taken together, ngr(-/-) animals show slower acquisition of a spatial learning and memory task.

Understanding aberrant white matter development in schizophrenia: an avenue for therapy?

Although historically gray matter changes have been the focus of neuropathological and neuroradiological studies in schizophrenia, in recent years an increasing body of research has implicated white matter structures and its constituent components (axons, their myelin sheaths and supporting oligodendrocytes). This article summarizes this body of literature, examining neuropathological, neurogenetic and neuroradiological evidence for white matter pathology in schizophrenia. We then look at the possible role that antipsychotic medication may play in these studies, examining both its role as a potential confounder in studies examining neuronal density and brain volume, but also the possible role that these medications may play in promoting myelination through their effects on oligodendrocytes. Finally, the role of potential novel therapies is discussed.

Lack of association of the RTN4R genetic variations with risk of schizophrenia and SPEM abnormality in a Korean population

This study examined the association of the reticulon 4 receptor (RTN4R) gene with schizophrenia and smooth pursuit eye movement (SPEM) abnormality in a Korean population. Although we failed to provide convincing evidence that RTN4R is associated with schizophrenia development and SPEM impairment, our findings may be useful for further genetic studies.

Molecular mechanisms in 22q11 deletion syndrome

It is now well recognized that as well as having a characteristic facial dysmorphology and a range of congenital abnormalities, individuals with chromosome 22q11 deletion syndrome (22q11DS) have a greatly increased risk of developing psychosis, in particular schizophrenia. The majority of deletions span a large 3Mb region at 22q11. However, the presence of affected individuals carrying smaller deletions have not been sufficient to satisfactorily reduce the critical region for the behavioral phenotype beyond a ~1.5Mb region that contains at least 28 genes. By having a shared genetic variant that greatly increases risk to psychosis, individuals with 22q11DS are a relatively homogeneous population to study psychiatric disease. Despite this, the large volume of research performed over the last 15 years suggest that the mechanism by which haploinsufficiency at 22q11 increases risk to psychiatric illness is likely to be complex and it remains uncertain why individuals carrying identical 22q11 deletions can present with such a wide range of neuropsychiatric phenotypes. This review will therefore consider the ways in which deletions at 22q11 are expected to increase risk to develop psychiatric disease by summarizing the work that has been done to investigate three of the most likely disease causing mechanisms: (a) gene dosage sensitivity; (b) unmasking of recessive alleles or functional polymorphism; and (c) position effect.

Avoiding mouse traps in schizophrenia genetics: lessons and promises from current and emerging mouse models

Schizophrenia is one of the most common psychiatric disorders, but despite progress in identifying the genetic factors implicated in its development, the mechanisms underlying its etiology and pathogenesis remain poorly understood. Development of mouse models is critical for expanding our understanding of the causes of schizophrenia. However, translation of disease pathology into mouse models has proven to be challenging, primarily due to the complex genetic architecture of schizophrenia and the difficulties in the re-creation of susceptibility alleles in the mouse genome. In this review we highlight current research on models of major susceptibility loci and the information accrued from their analysis. We describe and compare the different approaches that are necessitated by diverse susceptibility alleles, and discuss their advantages and drawbacks. Finally, we discuss emerging mouse models, such as second-generation pathophysiology models based on innovative approaches that are facilitated by the information gathered from the current genetic mouse models.

Role of leucine-rich repeat proteins in the development and function of neural circuits

The nervous system consists of an ensemble of billions of neurons interconnected in a highly specific pattern that allows proper propagation and integration of neural activities. The organization of these specific connections emerges from sequential developmental events including axon guidance, target selection, and synapse formation. These events critically rely on cell-cell recognition and communication mediated by cell-surface ligands and receptors. Recent studies have uncovered central roles for leucine-rich repeat (LRR) domain-containing proteins, not only in organizing neural connectivity from axon guidance to target selection to synapse formation, but also in various nervous system disorders. Their versatile LRR domains, in particular, serve as key sites for interactions with a wide diversity of binding partners. Here, we focus on a few exquisite examples of secreted or membrane-associated LRR proteins in Drosophila and mammals and review the mechanisms by which they regulate diverse aspects of nervous system development and function.

Association study of Nogo-related genes with schizophrenia in a Japanese case-control sample

Many studies have suggested that myelin dysfunction may be causally involved in the pathogenesis of schizophrenia. Nogo (RTN4), myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (OMG) all bind to the common receptor, Nogo-66 receptor 1 (RTN4R). We examined 68 single nucleotide polymorphisms (SNPs) (51 with genotyping and 17 with imputation analysis) from these four genes for genetic association with schizophrenia, using a 2,120 case-control sample from the Japanese population. Allelic tests showed nominally significant association of two RTN4 SNPs (P = 0.047 and 0.037 for rs11894868 and rs2968804, respectively) and two MAG SNPs (P = 0.034 and 0.029 for rs7249617 and rs16970218, respectively) with schizophrenia. The MAG SNP rs7249617 also showed nominal significance in a genotypic test (P = 0.017). In haplotype analysis, the MAG haplotype block including rs7249617 and rs16970218 showed nominal significance (P = 0.008). These associations did not remain significant after correction for multiple testing, possibly due to their small genetic effect. In the imputation analysis of RTN4, the untyped SNP rs2972090 showed nominally significant association (P = 0.032) and several imputed SNPs showed marginal associations. Moreover, in silico analysis (PolyPhen) of a missense variant (rs11677099: Asp357Val), which is in strong linkage disequilibrium with rs11894868, predicted a deleterious effect on Nogo protein function. Despite a failure to detect robust associations in this Japanese cohort, our nominally positive signals, taken together with previously reported biological and genetic findings, add further support to the "disturbed myelin system theory of schizophrenia" across different populations.

Contribution of nonprimate animal models in understanding the etiology of schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by positive and negative symptoms and cognitive impairments. The etiology of the disorder is complex, and it is thought to follow a multifactorial threshold model of inheritance with genetic and neurodevelop mental contributions to risk. Human studies are particularly useful in capturing the richness of the phenotype, but they are often limited to the use of correlational approaches. By assessing behavioural abnormalities in both humans and rodents, nonprimate animal models of schizophrenia provide unique insight into the etiology and mechanisms of the disorder. This review discusses the phenomenology and etiology of schizophrenia and the contribution of current nonprimate animal models with an emphasis on how research with models of neuro transmitter dysregulation, environmental risk factors, neurodevelopmental disruption and genetic risk factors can complement the literature on schizophrenia in humans.

Deletion of CB2 cannabinoid receptor induces schizophrenia-related behaviors in mice

The possible role of the CB(2) receptor (CB(2)r) in psychiatric disorders has been considered. Several animal models use knockout (KO) mice that display schizophrenia-like behaviors and this study evaluated the role of CB(2)r in the regulation of such behaviors. Mice lacking the CB(2)r (CB(2)KO) were challenged in open field, light-dark box, elevated plus-maze, tail suspension, step down inhibitory avoidance, and pre-pulse inhibition tests (PPI). Furthermore, the effects of treatment with cocaine and risperidone were evaluated using the OF and the PPI test. Gene expression of dopamine D(2) (D(2)r), adrenergic-α(2C) (α(2C)r), serotonergic 5-HT(2A) and 5-HT(2C) receptors (5-HT(2A)r and 5-HT(2C)r) were studied by RT-PCR in brain regions related to schizophrenia. Deletion of CB(2)r decreased motor activity in the OF test, but enhanced response to acute cocaine and produced mood-related alterations, PPI deficit, and cognitive impairment. Chronic treatment with risperidone tended to impair PPI in WT mice, whereas it 'normalized' the PPI deficit in CB(2)KO mice. CB(2)KO mice presented increased D(2)r and α(2C)r gene expressions in the prefrontal cortex (PFC) and locus coeruleus (LC), decreased 5-HT(2C)r gene expression in the dorsal raphe (DR), and 5-HT(2A)r gene expression in the PFC. Chronic risperidone treatment in WT mice left α(2C)r gene expression unchanged, decreased D(2)r gene expression (15 μg/kg), and decreased 5-HT(2C)r and 5-HT(2A)r in PFC and DR. In CB(2)KO, the gene expression of D(2)r in the PFC, of α(2C)r in the LC, and of 5-HT(2C)r and 5-HT(2A)r in PFC was reduced; 5-HT(2C)r and 5-HT(2A)r gene expressions in DR were increased after treatment with risperidone. These results suggest that deletion of CB(2)r has a relation with schizophrenia-like behaviors. Pharmacological manipulation of CB(2)r may merit further study as a potential therapeutic target for the treatment of schizophrenia-related disorders.

Missense mutation of the reticulon-4 receptor alters spatial memory and social interaction in mice

The reticulon-4 receptor, encoded by RTN4R, limits axonal sprouting and neural plasticity by inhibiting the outgrowth of neurites. Human association studies have implicated mutations in RTN4R in the development of schizophrenia, including the identification of several rare nonconservative missense mutations of RTN4R in schizophrenia patients. To investigate the effects of missense mutation of the reticulon-4 receptor on phenotypes relevant to schizophrenia, we behaviourally characterized a novel Rtn4r mutant mouse line with an amino acid substitution (R189H) in the Nogo-66 binding site. Behavioural assays included prepulse inhibition of acoustic startle, locomotor activity, social interaction and spatial cognition. When compared with wildtype littermates, Rtn4r mutant mice exhibited greater social preference, which may reflect a social-anxyolitic effect, and a mild impairment in spatial cognition. Given the mild effect of the R189H mutation of Rtn4r on behavioural phenotypes relevant to schizophrenia, our results do not support missense mutation of RTN4R as a strong risk factor in the pathogenesis of schizophrenia.

Cognitive, behavioural and psychiatric phenotype in 22q11.2 deletion syndrome

22q11.2 Deletion syndrome has become an important model for understanding the pathophysiology of neurodevelopmental conditions, particularly schizophrenia which develops in about 20-25% of individuals with a chromosome 22q11.2 microdeletion. From the initial discovery of the syndrome, associated developmental delays made it clear that changes in brain development were a key part of the expression. Once patients were followed through childhood into adult years, further neurobehavioural phenotypes became apparent, including a changing cognitive profile, anxiety disorders and seizure diathesis. The variability of expression is as wide as for the myriad physical features associated with the syndrome, with the addition of evolving phenotype over the developmental trajectory. Notably, variability appears unrelated to length of the associated deletion. Several mouse models of the deletion have been engineered and are beginning to reveal potential molecular mechanisms for the cognitive and behavioural phenotypes observable in animals. Both animal and human studies hold great promise for further discoveries relevant to neurodevelopment and associated cognitive, behavioural and psychiatric disorders.

The 22q11.2 microdeletion: fifteen years of insights into the genetic and neural complexity of psychiatric disorders

Over the last fifteen years it has become established that 22q11.2 deletion syndrome (22q11DS) is a true genetic risk factor for schizophrenia. Carriers of deletions in chromosome 22q11.2 develop schizophrenia at rate of 25-30% and such deletions account for as many as 1-2% of cases of sporadic schizophrenia in the general population. Access to a relatively homogeneous population of individuals that suffer from schizophrenia as the result of a shared etiological factor and the potential to generate etiologically valid mouse models provides an immense opportunity to better understand the pathobiology of this disease. In this review we survey the clinical literature associated with the 22q11.2 microdeletions with a focus on neuroanatomical changes. Then, we highlight results from work modeling this structural mutation in animals. The key biological pathways disrupted by the mutation are discussed and how these changes impact the structure and function of neural circuits is described.

Analysis of RTN4 3'UTR insertion/deletion polymorphisms in ventricular septal defect in a Chinese Han population

Congenital heart disease is the most common type of birth defect and the leading cause of infant mortality in the first year of life. Ventricular septal defect (VSD) is one of the most general congenital heart defects and is a defect in the wall between the right and left ventricles of the heart. The pathogenesis of VSD has been extensively investigated for many years, but it remains uncertain. To determine whether reticulon 4 gene (RTN4) 3'UTR insertion/deletion polymorphisms are associated with VSD, we genotyped the TATC and CAA insertion/deletion polymorphisms of RTN4 by polymerase chain reaction-polyacrylamide gel electrophoresis in 151 VSD patients and 308 unrelated healthy subjects in a Chinese Han population. No significant differences in 3'UTR TATC and CAA insertion/deletion polymorphisms genotype and allele frequencies were observed between the VSD and controls. These data indicate that, for the first time, RTN4 3'UTR insertion/deletion polymorphisms may not appear to play a role in the susceptibility of VSD in Chinese Han population.

Emerging functions of myelin-associated proteins during development, neuronal plasticity, and neurodegeneration

Adult mammalian central nervous system (CNS) axons have a limited regrowth capacity following injury. Myelin-associated inhibitors (MAIs) limit axonal outgrowth, and their blockage improves the regeneration of damaged fiber tracts. Three of these proteins, Nogo-A, MAG, and OMgp, share two common neuronal receptors: NgR1, together with its coreceptors [p75(NTR), TROY, and Lingo-1]; and the recently described paired immunoglobulin-like receptor B (PirB). These proteins impair neuronal regeneration by limiting axonal sprouting. Some of the elements involved in the myelin inhibitory pathways may still be unknown, but the discovery that blocking both PirB and NgR1 activities leads to near-complete release from myelin inhibition, sheds light on one of the most competitive and intense fields of neuroregeneration study in recent decades. In parallel with the identification and characterization of the roles and functions of these inhibitory molecules in axonal regeneration, data gathered in the field strongly suggest that most of these proteins have roles other than axonal growth inhibition. The discovery of a new group of interacting partners for myelin-associated receptors and ligands, as well as functional studies within or outside the CNS environment, highlights the potential new physiological roles for these proteins in processes, such as development, neuronal homeostasis, plasticity, and neurodegeneration.

Functions of Nogo proteins and their receptors in the nervous system

The membrane protein Nogo-A was initially characterized as a CNS-specific inhibitor of axonal regeneration. Recent studies have uncovered regulatory roles of Nogo proteins and their receptors--in precursor migration, neurite growth and branching in the developing nervous system--as well as a growth-restricting function during CNS maturation. The function of Nogo in the adult CNS is now understood to be that of a negative regulator of neuronal growth, leading to stabilization of the CNS wiring at the expense of extensive plastic rearrangements and regeneration after injury. In addition, Nogo proteins interact with various intracellular components and may have roles in the regulation of endoplasmic reticulum (ER) structure, processing of amyloid precursor protein and cell survival.

Linking oligodendrocyte and myelin dysfunction to neurocircuitry abnormalities in schizophrenia

Multiple lines of evidence in schizophrenia, from brain imaging, studies in postmortem brains, and genetic association studies, have implicated oligodendrocyte and myelin dysfunction in this disease. Recent studies suggest that oligodendrocyte and myelin dysfunction leads to changes in synaptic formation and function, which could lead to cognitive dysfunction, a core symptom of schizophrenia. Furthermore, there is accumulating data linking oligodendrocyte and myelin dysfunction with dopamine and glutamate abnormalities, both of which are found in schizophrenia. These findings implicate oligodendrocyte and myelin dysfunction as a primary change in schizophrenia, not only as secondary consequences of the illness or treatment. Strategies targeting oligodendrocyte and myelin abnormalities could therefore provide therapeutic opportunities for patients suffering from schizophrenia.

22q11.2 microdeletions: linking DNA structural variation to brain dysfunction and schizophrenia

Recent studies are beginning to paint a clear and consistent picture of the impairments in psychological and cognitive competencies that are associated with microdeletions in chromosome 22q11.2. These studies have highlighted a strong link between this genetic lesion and schizophrenia. Parallel studies in humans and animal models are starting to uncover the complex genetic and neural substrates altered by the microdeletion. In addition to offering a deeper understanding of the effects of this genetic lesion, these findings may guide analysis of other copy-number variants associated with cognitive dysfunction and psychiatric disorders.

Cognition in mouse models of schizophrenia susceptibility genes

Cognitive deficits are core features of psychiatric disorders and contribute substantially to functional outcome. It is still unclear, however, how cognitive deficits are related to underlying genetic liability and overt clinical symptoms. Fortunately, animal models of susceptibility genes can illuminate how the products of disease-associated genetic variants affect brain function and ultimately alter behavior. Using as a reference findings from the Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia program and the SchizophreniaGene database, we review cognitive data from mutant models of rare and common genetic variants associated with schizophrenia.

Modeling the positive symptoms of schizophrenia in genetically modified mice: pharmacology and methodology aspects

In recent years, there have been huge advances in the use of genetically modified mice to study pathophysiological mechanisms involved in schizophrenia. This has allowed rapid progress in our understanding of the role of several proposed gene mechanisms in schizophrenia, and yet this research has also revealed how much still remains unresolved. Behavioral studies in genetically modified mice are reviewed with special emphasis on modeling psychotic-like behavior. I will particularly focus on observations on locomotor hyperactivity and disruptions of prepulse inhibition (PPI). Recommendations are included to address pharmacological and methodological aspects in future studies. Mouse models of dopaminergic and glutamatergic dysfunction are then discussed, reflecting the most important and widely studied neurotransmitter systems in schizophrenia. Subsequently, psychosis-like behavior in mice with modifications in the most widely studied schizophrenia susceptibility genes is reviewed. Taken together, the available studies reveal a wealth of available data which have already provided crucial new insight and mechanistic clues which could lead to new treatments or even prevention strategies for schizophrenia.

Constitutive genetic deletion of the growth regulator Nogo-A induces schizophrenia-related endophenotypes

The membrane protein Nogo-A, which is predominantly expressed by oligodendrocytes in the adult CNS and by neurons mainly during development, is well known for limiting neurite outgrowth and regeneration in the injured mammalian CNS. In addition, it has recently been proposed that abnormal Nogo-A expression or Nogo receptor (NgR) mutations may confer genetic risks for neuropsychiatric disorders of presumed neurodevelopmental origin, such as schizophrenia. We therefore evaluated whether Nogo-A deletion may lead to schizophrenia-like abnormalities in a mouse model of genetic Nogo-A deficiency. Here, we show that systemic, lifelong knock-out of the Nogo-A gene can lead to specific behavioral abnormalities resembling schizophrenia-related endophenotypes: deficient sensorimotor gating, disrupted latent inhibition, perseverative behavior, and increased sensitivity to the locomotor stimulating effects of amphetamine. These behavioral phenotypes were accompanied by altered monoaminergic transmitter levels in specific striatal and limbic structures, as well as changes in dopamine D2 receptor expression in the same brain regions. Nogo-A deletion was further associated with elevated expression of growth-related markers. In contrast, acute antibody-mediated Nogo-A neutralization in adult wild-type mice failed to produce such phenotypes, suggesting that the phenotypes observed in the knock-out mice might be of developmental origin, and that Nogo-A normally subserves critical functions in neurodevelopment. This study provides the first experimental demonstration that Nogo-A bears neuropsychiatric relevance, and alterations in its expression may be one etiological factor in schizophrenia and related disorders.

Mice mutant for genes associated with schizophrenia: common phenotype or distinct endophenotypes?

Schizophrenia is a complex neuropsychiatric disorder whose etiology involves a mixture of genetic and environmental factors. By virtue of this complexity, schizophrenia is a field of research in which a number of key technologies converge: in particular, identification of putative susceptibility genes through association studies in clinical populations leads to investigation of the behavioural roles of these genes by targeted manipulation in mice and their phenotypic characterisation ('gene-driven' approach); in a complementary manner, identification of putative pathophysiological processes and therapeutic pathways leads to investigation of behavioural phenotype in mice mutant for genes regulating such processes and pathways ('phenotype-driven' approach). As several susceptibility genes for schizophrenia and numerous genes implicated in the pathophysiology of schizophrenia have now been genetically manipulated in mice, it is timely to consider the roles of these genes in abnormal brain development and the ontogeny of putative schizophrenia-like phenotypes. The aim of this review is to outline existing knowledge from mutant studies concerning the contribution of these genes to the development of a common schizophrenia phenotype vis-à-vis discrete schizophrenia endophenotypes. Emphasis is also placed on the importance of studying gene x environment and gene x gene interactions, as well as addressing methodological issues related to genetic modelling and phenotyping strategies.

Identification of new putative susceptibility genes for several psychiatric disorders by association analysis of regulatory and non-synonymous SNPs of 306 genes involved in neurotransmission and neurodevelopment

A fundamental difficulty in human genetics research is the identification of the spectrum of genetic variants that contribute to the susceptibility to common/complex disorders. We tested here the hypothesis that functional genetic variants may confer susceptibility to several related common disorders. We analyzed five main psychiatric diagnostic categories (substance-abuse, anxiety, eating, psychotic, and mood disorders) and two different control groups, representing a total of 3,214 samples, for 748 promoter and non-synonymous single nucleotide polymorphisms (SNPs) at 306 genes involved in neurotransmission and/or neurodevelopment. We identified strong associations to individual disorders, such as growth hormone releasing hormone (GHRH) with anxiety disorders, prolactin regulatory element (PREB) with eating disorders, ionotropic kainate glutamate receptor 5 (GRIK5) with bipolar disorder and several SNPs associated to several disorders, that may represent individual and related disease susceptibility factors. Remarkably, a functional SNP, rs945032, located in the promoter region of the bradykinin receptor B2 gene (BDKRB2) was associated to three disorders (panic disorder, substance abuse, and bipolar disorder), and two additional BDKRB2 SNPs to obsessive-compulsive disorder and major depression, providing evidence for common variants of susceptibility to several related psychiatric disorders. The association of BDKRB2 (odd ratios between 1.65 and 3.06) to several psychiatric disorders supports the view that a common genetic variant could confer susceptibility to clinically related phenotypes, and defines a new functional hint in the pathophysiology of psychiatric diseases.

Linking white and grey matter in schizophrenia: oligodendrocyte and neuron pathology in the prefrontal cortex

Neuronal circuitry relies to a large extent on the presence of functional myelin produced in the brain by oligodendrocytes. Schizophrenia has been proposed to arise partly from altered brain connectivity. Brain imaging and neuropathologic studies have revealed changes in white matter and reduction in myelin content in patients with schizophrenia. In particular, alterations in the directionality and alignment of axons have been documented in schizophrenia. Moreover, the expression levels of several myelin-related genes are decreased in postmortem brains obtained from patients with schizophrenia. These findings have led to the formulation of the oligodendrocyte/myelin dysfunction hypothesis of schizophrenia. In this review, we present a brief overview of the neuropathologic findings obtained on white matter and oligodendrocyte status observed in schizophrenia patients, and relate these changes to the processes of brain maturation and myelination. We also review recent data on oligodendrocyte/myelin genes, and present some recent mouse models of myelin deficiencies. The use of transgenic and mutant animal models offers a unique opportunity to analyze oligodendrocyte and neuronal changes that may have a clinical impact. Lastly, we present some recent morphological findings supporting possible causal involvement of white and grey matter abnormalities, in the aim of determining the morphologic characteristics of the circuits whose alteration leads to the cortical dysfunction that possibly underlies the pathogenesis of schizophrenia.