Zebrafish models for studying cognitive enhancers

Cognitive decline is commonly seen both in normal aging and in neurodegenerative and neuropsychiatric diseases. Various experimental animal models represent a valuable tool to study brain cognitive processes and their deficits. Equally important is the search for novel drugs to treat cognitive deficits and improve cognitions. Complementing rodent and clinical findings, studies utilizing zebrafish (Danio rerio) are rapidly gaining popularity in translational cognitive research and neuroactive drug screening. Here, we discuss the value of zebrafish models and assays for screening nootropic (cognitive enhancer) drugs and the discovery of novel nootropics. We also discuss the existing challenges, and outline future directions of research in this field.

Behavioral analysis through the lifespan of disc1 mutant zebrafish identifies defects in sensorimotor transformation

DISC1 is a genetic risk factor for multiple psychiatric disorders. Compared to the dozens of murine Disc1 models, there is a paucity of zebrafish disc1 models-an organism amenable to high-throughput experimentation. We conducted the longitudinal neurobehavioral analysis of disc1 mutant zebrafish across key stages of life. During early developmental stages, disc1 mutants exhibited abrogated behavioral responses to sensory stimuli across multiple testing platforms. Moreover, during exposure to an acoustic sensory stimulus, loss of disc1 resulted in the abnormal activation of neurons in the pallium, cerebellum, and tectum-anatomical sites involved in the integration of sensory perception and motor control. In adulthood, disc1 mutants exhibited sexually dimorphic reduction in anxiogenic behavior in novel paradigms. Together, these findings implicate disc1 in sensorimotor processes and the genesis of anxiogenic behaviors, which could be exploited for the development of novel treatments in addition to investigating the biology of sensorimotor transformation in the context of disc1 deletion.

Zebrafish as a model to investigate the CRH axis and interactions with DISC1

Release of corticotropin-releasing hormone (CRH) from CRH neurons activates the hypothalamo-pituitary-adrenal (HPA) axis, one of the main physiological stress response systems. Complex feedback loops operate in the HPA axis and understanding the neurobiological mechanisms regulating CRH neurons is of great importance in the context of stress disorders. In this article, we review how in vivo studies in zebrafish have advanced knowledge of the neurobiology of CRH neurons. Disrupted-in-schizophrenia 1 (DISC1) mutant zebrafish have blunted stress responses and can be used to model human stress disorders. We propose that DISC1 influences the development and functioning of CRH neurons as a mechanism linking DISC1 to psychiatric disorders.

Developmental Neurotoxicity and Behavioral Screening in Larval Zebrafish with a Comparison to Other Published Results

With the abundance of chemicals in the environment that could potentially cause neurodevelopmental deficits, there is a need for rapid testing and chemical screening assays. This study evaluated the developmental toxicity and behavioral effects of 61 chemicals in zebrafish (Danio rerio) larvae using a behavioral Light/Dark assay. Larvae (n = 16-24 per concentration) were exposed to each chemical (0.0001-120 μM) during development and locomotor activity was assessed. Approximately half of the chemicals (n = 30) did not show any gross developmental toxicity (i.e., mortality, dysmorphology or non-hatching) at the highest concentration tested. Twelve of the 31 chemicals that did elicit developmental toxicity were toxic at the highest concentration only, and thirteen chemicals were developmentally toxic at concentrations of 10 µM or lower. Eleven chemicals caused behavioral effects; four chemicals (6-aminonicotinamide, cyclophosphamide, paraquat, phenobarbital) altered behavior in the absence of developmental toxicity. In addition to screening a library of chemicals for developmental neurotoxicity, we also compared our findings with previously published results for those chemicals. Our comparison revealed a general lack of standardized reporting of experimental details, and it also helped identify some chemicals that appear to be consistent positives and negatives across multiple laboratories.

Behavioral Effects of Developmental Exposure to JWH-018 in Wild-Type and Disrupted in Schizophrenia 1 (disc1) Mutant Zebrafish

Synthetic cannabinoids can cause acute adverse psychological effects, but the potential impact when exposure happens before birth is unknown. Use of synthetic cannabinoids during pregnancy may affect fetal brain development, and such effects could be moderated by the genetic makeup of an individual. Disrupted in schizophrenia 1 (DISC1) is a gene with important roles in neurodevelopment that has been associated with psychiatric disorders in pedigree analyses. Using zebrafish as a model, we investigated (1) the behavioral impact of developmental exposure to 3 μM 1-pentyl-3-(1-naphthoyl)-indole (JWH-018; a common psychoactive synthetic cannabinoid) and (2) whether disc1 moderates the effects of JWH-018. As altered anxiety responses are seen in several psychiatric disorders, we focused on zebrafish anxiety-like behavior. Zebrafish embryos were exposed to JWH-018 from one to six days post-fertilization. Anxiety-like behavior was assessed using forced light/dark and acoustic startle assays in larvae and novel tank diving in adults. Compared to controls, both acutely and developmentally exposed zebrafish larvae had impaired locomotion during the forced light/dark test, but anxiety levels and response to startle stimuli were unaltered. Adult zebrafish developmentally exposed to JWH-018 spent less time on the bottom of the tank, suggesting decreased anxiety. Loss-of-function in disc1 increased anxiety-like behavior in the tank diving assay but did not alter sensitivity to JWH-018. Results suggest developmental exposure to JWH-018 has a long-term behavioral impact in zebrafish, which is not moderated by disc1.

The Role of Zebrafish and Laboratory Rodents in Schizophrenia Research

Schizophrenia is a severe disorder characterized by positive, negative and cognitive symptoms, which are still not fully understood. The development of efficient antipsychotics requires animal models of a strong validity, therefore the aims of the article were to summarize the construct, face and predictive validity of schizophrenia models based on rodents and zebrafish, to compare the advantages and disadvantages of these models, and to propose future directions in schizophrenia modeling and indicate when it is reasonable to combine these models. The advantages of rodent models stem primarily from the high homology between rodent and human physiology, neurochemistry, brain morphology and circuitry. The advantages of zebrafish models stem in the high fecundity, fast development and transparency of the embryo. Disadvantages of both models originate in behavioral repertoires not allowing specific symptoms to be modeled, even when the models are combined. Especially modeling the verbal component of certain positive, negative and cognitive symptoms is currently impossible.

Molecular alterations in the medial temporal lobe in schizophrenia

The medial temporal lobe (MTL) and its individual structures have been extensively implicated in schizophrenia pathophysiology, with considerable efforts aimed at identifying structural and functional differences in this brain region. The major structures of the MTL for which prominent differences have been revealed include the hippocampus, the amygdala and the superior temporal gyrus (STG). The different functions of each of these regions have been comprehensively characterized, and likely contribute differently to schizophrenia. While neuroimaging studies provide an essential framework for understanding the role of these MTL structures in various aspects of the disease, ongoing efforts have sought to employ molecular measurements in order to elucidate the biology underlying these macroscopic differences. This review provides a summary of the molecular findings in three major MTL structures, and discusses convergent findings in cellular architecture and inter-and intra-cellular networks. The findings of this effort have uncovered cell-type, network and gene-level specificity largely unique to each brain region, indicating distinct molecular origins of disease etiology. Future studies should test the functional implications of these molecular changes at the circuit level, and leverage new advances in sequencing technology to further refine our understanding of the differential contribution of MTL structures to schizophrenia.

A critical review of zebrafish schizophrenia models: Time for validation?

Schizophrenia is a mental disorder that affects 1% of the population worldwide and is manifested as a broad spectrum of symptoms, from hallucinations to memory impairment. It is believed that genetic and/or environmental factors may contribute to the occurrence of this disease. Recently, the zebrafish has emerged as a valuable and attractive model for various neurological disorders including schizophrenia. In this review, we describe current pharmacological models of schizophrenia with special emphasis on providing insights into the pros and cons of using zebrafish as a behavioural model of this disease. Moreover, we highlight the advantages and utility of using zebrafish for elucidating the genetic mechanisms underlying this psychiatric disorder. We believe that the zebrafish has high potential also in the area of precision medicine and may complement the development of therapeutics, especially for pharmacoresistant patients.

Zebrafish as an Emerging Model for Bioassay-Guided Natural Product Drug Discovery for Neurological Disorders

Most neurodegenerative diseases are currently incurable, with large social and economic impacts. Recently, there has been renewed interest in investigating natural products in the modern drug discovery paradigm as novel, bioactive small molecules. Moreover, the discovery of potential therapies for neurological disorders is challenging and involves developing optimized animal models for drug screening. In contemporary biomedicine, the growing need to develop experimental models to obtain a detailed understanding of malady conditions and to portray pioneering treatments has resulted in the application of zebrafish to close the gap between in vitro and in vivo assays. Zebrafish in pharmacogenetics and neuropharmacology are rapidly becoming a widely used organism. Brain function, dysfunction, genetic, and pharmacological modulation considerations are enhanced by both larval and adult zebrafish. Bioassay-guided identification of natural products using zebrafish presents as an attractive strategy for generating new lead compounds. Here, we see evidence that the zebrafish's central nervous system is suitable for modeling human neurological disease and we review and evaluate natural product research using zebrafish as a vertebrate model platform to systematically identify bioactive natural products. Finally, we review recently developed zebrafish models of neurological disorders that have the potential to be applied in this field of research.

Familial t(1;11) translocation is associated with disruption of white matter structural integrity and oligodendrocyte-myelin dysfunction

Although the underlying neurobiology of major mental illness (MMI) remains unknown, emerging evidence implicates a role for oligodendrocyte-myelin abnormalities. Here, we took advantage of a large family carrying a balanced t(1;11) translocation, which substantially increases risk of MMI, to undertake both diffusion tensor imaging and cellular studies to evaluate the consequences of the t(1;11) translocation on white matter structural integrity and oligodendrocyte-myelin biology. This translocation disrupts among others the DISC1 gene which plays a crucial role in brain development. We show that translocation-carrying patients display significant disruption of  white matter integrity compared with familial controls. At a cellular level, we observe dysregulation of key pathways controlling oligodendrocyte development and morphogenesis in induced pluripotent stem cell (iPSC) derived case oligodendrocytes. This is associated with reduced proliferation and a stunted morphology in vitro. Further, myelin internodes in a humanized mouse model that recapitulates the human translocation as well as after transplantation of t(1;11) oligodendrocyte progenitors were significantly reduced when  compared with controls. Thus we provide evidence that the t(1;11) translocation has biological effects at both the systems and cellular level that together suggest oligodendrocyte-myelin dysfunction.

Overexpression of Truncated Human DISC1 Induces Appearance of Hindbrain Oligodendroglia in the Forebrain During Development

Genetic, neuroimaging, and gene expression studies suggest a role for oligodendrocyte (OLG) dysfunction in schizophrenia (SZ). Disrupted-in-schizophrenia 1 (DISC1) is a risk gene for major psychiatric disorders, including SZ. Overexpression of mutant truncated (hDISC1), but not full-length sequence of human DISC1 in forebrain influenced OLG differentiation and proliferation of glial progenitors in the developing cerebral cortex concurrently with reduction of OLG progenitor markers in the hindbrain. We examined gene and protein expression of the molecular determinants of hindbrain OLG development and their interactions with DISC1 in mutant hDISC1 mice. We found ectopic upregulation of hindbrain glial progenitor markers (early growth response 2 [Egr2] and NK2 homeobox 2 [Nkx2-2]) in the forebrain of hDISC1 (E15) embryos. DISC1 and Nkx2-2 were coexpressed and interacted in progenitor cells. Overexpression of truncated hDISC1 impaired interactions between DISC1 and Nkx2-2, which was associated with increased differentiation of OLG and upregulation of hindbrain mature OLG markers (laminin alpha-1 [LAMA1] and myelin protein zero [MPZ]) suggesting a suppressive function of endogenous DISC1 in OLG specialization of hindbrain glial progenitors during embryogenesis. Consistent with findings in hDISC1 mice, several hindbrain OLG markers (PRX, LAMA1, and MPZ) were significantly upregulated in the superior temporal cortex of persons with SZ. These findings show a significant effect of truncated hDISC1 on glial identity cells along the rostrocaudal axis and their OLG specification. Appearance of hindbrain OLG lineage cells and their premature differentiation may affect cerebrocortical organization and contribute to the pathophysiology of SZ.

Disrupted-in-Schizophrenia-1 is essential for normal hypothalamic-pituitary-interrenal (HPI) axis function

Psychiatric disorders arise due to an interplay of genetic and environmental factors, including stress. Studies in rodents have shown that mutants for Disrupted-In-Schizophrenia-1 (DISC1), a well-accepted genetic risk factor for mental illness, display abnormal behaviours in response to stress, but the mechanisms through which DISC1 affects stress responses remain poorly understood. Using two lines of zebrafish homozygous mutant for disc1, we investigated behaviour and functioning of the hypothalamic-pituitary-interrenal (HPI) axis, the fish equivalent of the hypothalamic-pituitary-adrenal (HPA) axis. Here, we show that the role of DISC1 in stress responses is evolutionarily conserved and that DISC1 is essential for normal functioning of the HPI axis. Adult zebrafish homozygous mutant for disc1 show aberrant behavioural responses to stress. Our studies reveal that in the embryo, disc1 is expressed in neural progenitor cells of the hypothalamus, a conserved region of the vertebrate brain that centrally controls responses to environmental stressors. In disc1 mutant embryos, proliferating rx3+ hypothalamic progenitors are not maintained normally and neuronal differentiation is compromised: rx3-derived ff1b+ neurons, implicated in anxiety-related behaviours, and corticotrophin releasing hormone (crh) neurons, key regulators of the stress axis, develop abnormally, and rx3-derived pomc+ neurons are disorganised. Abnormal hypothalamic development is associated with dysfunctional behavioural and neuroendocrine stress responses. In contrast to wild type siblings, disc1 mutant larvae show altered crh levels, fail to upregulate cortisol levels when under stress and do not modulate shoal cohesion, indicative of abnormal social behaviour. These data indicate that disc1 is essential for normal development of the hypothalamus and for the correct functioning of the HPA/HPI axis.

Understanding taurine CNS activity using alternative zebrafish models

Taurine is a highly abundant "amino acid" in the brain. Despite the potential neuroactive role of taurine in vertebrates has long been recognized, the underlying molecular mechanisms related to its pleiotropic effects in the brain remain poorly understood. Due to the genetic tractability, rich behavioral repertoire, neurochemical conservation, and small size, the zebrafish (Danio rerio) has emerged as a powerful candidate for neuropsychopharmacology investigation and in vivo drug screening. Here, we summarize the main physiological roles of taurine in mammals, including neuromodulation, osmoregulation, membrane stabilization, and antioxidant action. In this context, we also highlight how zebrafish models of brain disorders may present interesting approaches to assess molecular mechanisms underlying positive effects of taurine in the brain. Finally, we outline recent advances in zebrafish drug screening that significantly improve neuropsychiatric translational researches and small molecule screens.

Neuregulin-1 Regulates Cortical Inhibitory Neuron Dendrite and Synapse Growth through DISC1

Cortical inhibitory neurons play crucial roles in regulating excitatory synaptic networks and cognitive function and aberrant development of these cells have been linked to neurodevelopmental disorders. The secreted neurotrophic factor Neuregulin-1 (NRG1) and its receptor ErbB4 are established regulators of inhibitory neuron connectivity, but the developmental signalling mechanisms regulating this process remain poorly understood. Here, we provide evidence that NRG1-ErbB4 signalling functions through the multifunctional scaffold protein, Disrupted in Schizophrenia 1 (DISC1), to regulate the development of cortical inhibitory interneuron dendrite and synaptic growth. We found that NRG1 increases inhibitory neuron dendrite complexity and glutamatergic synapse formation onto inhibitory neurons and that this effect is blocked by expression of a dominant negative DISC1 mutant, or DISC1 knockdown. We also discovered that NRG1 treatment increases DISC1 expression and its localization to glutamatergic synapses being made onto cortical inhibitory neurons. Mechanistically, we determined that DISC1 binds ErbB4 within cortical inhibitory neurons. Collectively, these data suggest that a NRG1-ErbB4-DISC1 signalling pathway regulates the development of cortical inhibitory neuron dendrite and synaptic growth. Given that NRG1, ErbB4, and DISC1 are schizophrenia-linked genes, these findings shed light on how independent risk factors may signal in a common developmental pathway that contributes to neural connectivity defects and disease pathogenesis.

Function Over Form: Modeling Groups of Inherited Neurological Conditions in Zebrafish

Zebrafish are a unique cell to behavior model for studying the basic biology of human inherited neurological conditions. Conserved vertebrate genetics and optical transparency provide in vivo access to the developing nervous system as well as high-throughput approaches for drug screens. Here we review zebrafish modeling for two broad groups of inherited conditions that each share genetic and molecular pathways and overlap phenotypically: neurodevelopmental disorders such as Autism Spectrum Disorders (ASD), Intellectual Disability (ID) and Schizophrenia (SCZ), and neurodegenerative diseases, such as Cerebellar Ataxia (CATX), Hereditary Spastic Paraplegia (HSP) and Charcot-Marie Tooth Disease (CMT). We also conduct a small meta-analysis of zebrafish orthologs of high confidence neurodevelopmental disorder and neurodegenerative disease genes by looking at duplication rates and relative protein sizes. In the past zebrafish genetic models of these neurodevelopmental disorders and neurodegenerative diseases have provided insight into cellular, circuit and behavioral level mechanisms contributing to these conditions. Moving forward, advances in genetic manipulation, live imaging of neuronal activity and automated high-throughput molecular screening promise to help delineate the mechanistic relationships between different types of neurological conditions and accelerate discovery of therapeutic strategies.

Zebrafish: a vertebrate tool for studying basal body biogenesis, structure, and function

Understanding the role of basal bodies (BBs) during development and disease has been largely overshadowed by research into the function of the cilium. Although these two organelles are closely associated, they have specific roles to complete for successful cellular development. Appropriate development and function of the BB are fundamental for cilia function. Indeed, there are a growing number of human genetic diseases affecting ciliary development, known collectively as the ciliopathies. Accumulating evidence suggests that BBs establish cell polarity, direct ciliogenesis, and provide docking sites for proteins required within the ciliary axoneme. Major contributions to our knowledge of BB structure and function have been provided by studies in flagellated or ciliated unicellular eukaryotic organisms, specifically Tetrahymena and Chlamydomonas. Reproducing these and other findings in vertebrates has required animal in vivo models. Zebrafish have fast become one of the primary organisms of choice for modeling vertebrate functional genetics. Rapid ex-utero development, proficient egg laying, ease of genetic manipulation, and affordability make zebrafish an attractive vertebrate research tool. Furthermore, zebrafish share over 80 % of disease causing genes with humans. In this article, we discuss the merits of using zebrafish to study BB functional genetics, review current knowledge of zebrafish BB ultrastructure and mechanisms of function, and consider the outlook for future zebrafish-based BB studies.

Animal models of gene-environment interaction in schizophrenia: A dimensional perspective

Schizophrenia has long been considered as a disorder with multifactorial origins. Recent discoveries have advanced our understanding of the genetic architecture of the disease. However, even with the increase of identified risk variants, heritability estimates suggest an important contribution of non-genetic factors. Various environmental risk factors have been proposed to play a role in the etiopathogenesis of schizophrenia. These include season of birth, maternal infections, obstetric complications, adverse events at early childhood, and drug abuse. Despite the progress in identification of genetic and environmental risk factors, we still have a limited understanding of the mechanisms whereby gene-environment interactions (G × E) operate in schizophrenia and psychoses at large. In this review we provide a critical analysis of current animal models of G × E relevant to psychotic disorders and propose that dimensional perspective will advance our understanding of the complex mechanisms of these disorders.

Phenoscape: Identifying Candidate Genes for Evolutionary Phenotypes

Phenotypes resulting from mutations in genetic model organisms can help reveal candidate genes for evolutionarily important phenotypic changes in related taxa. Although testing candidate gene hypotheses experimentally in nonmodel organisms is typically difficult, ontology-driven information systems can help generate testable hypotheses about developmental processes in experimentally tractable organisms. Here, we tested candidate gene hypotheses suggested by expert use of the Phenoscape Knowledgebase, specifically looking for genes that are candidates responsible for evolutionarily interesting phenotypes in the ostariophysan fishes that bear resemblance to mutant phenotypes in zebrafish. For this, we searched ZFIN for genetic perturbations that result in either loss of basihyal element or loss of scales phenotypes, because these are the ancestral phenotypes observed in catfishes (Siluriformes). We tested the identified candidate genes by examining their endogenous expression patterns in the channel catfish, Ictalurus punctatus. The experimental results were consistent with the hypotheses that these features evolved through disruption in developmental pathways at, or upstream of, brpf1 and eda/edar for the ancestral losses of basihyal element and scales, respectively. These results demonstrate that ontological annotations of the phenotypic effects of genetic alterations in model organisms, when aggregated within a knowledgebase, can be used effectively to generate testable, and useful, hypotheses about evolutionary changes in morphology.

ASSOCIATION STUDY BETWEEN OLIGODENDROCYTE TRANSCRIPTION FACTOR 2 GENE AND OBSESSIVE-COMPULSIVE DISORDER IN A CHINESE HAN POPULATION

BACKGROUND

Oligodendrocyte transcription factor 2 (OLIG2) is primarily concentrated in the brain and spinal cord ventricular zone, where this protein stimulates oligodendrocytes and specific neurons, determines motor neuron and oligodendrocyte differentiation, and sustains replication in early development. Recent studies have demonstrated that OLIG2 gene is associated with mental disorders, such as schizophrenia, mood disorder, and obsessive-compulsive disorder (OCD).

METHODS

The aim of the present study was to explore whether OLIG2 gene is associated with OCD in a Chinese Han population through the assessment and analysis of three single nucleotide polymorphisms (SNPs), namely, rs762178, rs1059004, and rs9653711, selected from OLIG2 gene sequences from 400 OCD samples and 459 healthy controls in a case-controlled association study.

RESULTS

We demonstrated three principal results. First, SNP rs762178 was associated with OCD, female OCD, and early-onset OCD; rs1059004 was associated with OCD and early-onset OCD; and rs9653711 was also associated with OCD and early-onset OCD. Second, the pairs of loci rs762178 and rs1059004, rs1059004 and rs9653711, and rs762178 and rs9653711 exhibited linkage disequilibrium. Third, the three-locus A-C-G haplotype was associated with early-onset OCD.

CONCLUSIONS

The present study is the first to verify the associations of SNPs rs762178, rs1059004, and rs9653711 of the OLIG2 gene with OCD in a Chinese Han population. Thus, OLIG2 might serve as a potential target for OCD treatment in future studies. Further studies should verify the current findings.

Sonic hedgehog functions upstream of disrupted-in-schizophrenia 1 (disc1): implications for mental illness

DISRUPTED-IN-SCHIZOPHRENIA (DISC1) has been one of the most intensively studied genetic risk factors for mental illness since it was discovered through positional mapping of a translocation breakpoint in a large Scottish family where a balanced chromosomal translocation was found to segregate with schizophrenia and affective disorders. While the evidence for it being central to disease pathogenesis in the original Scottish family is compelling, recent genome-wide association studies have not found evidence for common variants at the DISC1 locus being associated with schizophrenia in the wider population. It may therefore be the case that DISC1 provides an indication of biological pathways that are central to mental health issues and functional studies have shown that it functions in multiple signalling pathways. However, there is little information regarding factors that function upstream of DISC1 to regulate its expression and function. We herein demonstrate that Sonic hedgehog (Shh) signalling promotes expression of disc1 in the zebrafish brain. Expression of disc1 is lost in smoothened mutants that have a complete loss of Shh signal transduction, and elevated in patched mutants which have constitutive activation of Shh signalling. We previously demonstrated that disc1 knockdown has a dramatic effect on the specification of oligodendrocyte precursor cells (OPC) in the hindbrain and Shh signalling is known to be essential for the specification of these cells. We show that disc1 is prominently expressed in olig2-positive midline progenitor cells that are absent in smo mutants, while cyclopamine treatment blocks disc1 expression in these cells and mimics the effect of disc1 knock down on OPC specification. Various features of a number of psychiatric conditions could potentially arise through aberrant Hedgehog signalling. We therefore suggest that altered Shh signalling may be an important neurodevelopmental factor in the pathobiology of mental illness.

Polymorphism within a Neuronal Activity-Dependent Enhancer of NgR1 Is Associated with Corpus Callosum Morphology in Humans

The human Nogo-66 receptor 1 (NgR1) gene, also termed Nogo receptor 1 or reticulon 4 receptor (RTN4R) and located within 22q11.2, inhibits axonal growth and synaptic plasticity. Patients with the 22q11.2 deletion syndrome show multiple changes in brain morphology, with corpus callosum (CC) abnormalities being among the most prominent and frequently reported. Thus, we hypothesized that, in humans, NgR1 may be involved in CC formation. We focused on rs701428, a single nucleotide polymorphism of NgR1, which is associated with schizophrenia. We investigated the effects of the rs701428 genotype on CC structure in 50 healthy participants using magnetic resonance imaging. Polymorphism of rs701428 was associated with CC structural variation in healthy participants; specifically, minor A allele carriers had larger whole CC volumes and lower radial diffusivity in the central CC region compared with major G allele homozygous participants. Furthermore, we showed that the NgR1 3' region, which contains rs701428, is a neuronal activity-dependent enhancer, and that the minor A allele of rs701428 is susceptible to regulation of enhancer activity by MYBL2. Our results suggest that NgR1 can influence the macro- and microstructure of the white matter of the human brain.

Myelination-related genes are associated with decreased white matter integrity in schizophrenia

Disruptions in white matter (WM) tract structures have been implicated consistently in the pathophysiology of schizophrenia. Global WM integrity--as measured by fractional anisotropy (FA)--is highly heritable and may provide a good endophenotype for genetic studies of schizophrenia. WM abnormalities in schizophrenia are not localized to one specific brain region but instead reflect global low-level decreases in FA coupled with focal abnormalities. In this study, we sought to investigate whether functional gene sets associated with schizophrenia are also associated with WM integrity. We analyzed FA and genetic data from the Mind Research Network Clinical Imaging Consortium to study the effect of multiple oligodendrocyte gene sets on schizophrenia and WM integrity using a functional gene set analysis in 77 subjects with schizophrenia and 104 healthy controls. We found that a gene set involved in myelination was significantly associated with schizophrenia and FA. This gene set includes 17 genes that are expressed in oligodendrocytes and one neuronal gene (NRG1) that is known to regulate myelination. None of the genes within the gene set were associated with schizophrenia or FA individually, suggesting that no single gene was driving the association of the gene set. Our findings support the hypothesis that multiple genetic variants in myelination-related genes contribute to the observed correlation between schizophrenia and decreased WM integrity as measured by FA.

Neuregulin 1 Type II-ErbB Signaling Promotes Cell Divisions Generating Neurons from Neural Progenitor Cells in the Developing Zebrafish Brain

Post-mitotic neurons are generated from neural progenitor cells (NPCs) at the expense of their proliferation. Molecular and cellular mechanisms that regulate neuron production temporally and spatially should impact on the size and shape of the brain. While transcription factors such as neurogenin1 (neurog1) and neurod govern progression of neurogenesis as cell-intrinsic mechanisms, recent studies show regulatory roles of several cell-extrinsic or intercellular signaling molecules including Notch, FGF and Wnt in production of neurons/neural progenitor cells from neural stem cells/radial glial cells (NSCs/RGCs) in the ventricular zone (VZ). However, it remains elusive how production of post-mitotic neurons from neural progenitor cells is regulated in the sub-ventricular zone (SVZ). Here we show that newborn neurons accumulate in the basal-to-apical direction in the optic tectum (OT) of zebrafish embryos. While neural progenitor cells are amplified by mitoses in the apical ventricular zone, neurons are exclusively produced through mitoses of neural progenitor cells in the sub-basal zone, later in the sub-ventricular zone, and accumulate apically onto older neurons. This neurogenesis depends on Neuregulin 1 type II (NRG1-II)-ErbB signaling. Treatment with an ErbB inhibitor, AG1478 impairs mitoses in the sub-ventricular zone of the optic tectum. Removal of AG1478 resumes sub-ventricular mitoses without precedent mitoses in the apical ventricular zone prior to basal-to-apical accumulation of neurons, suggesting critical roles of ErbB signaling in mitoses for post-mitotic neuron production. Knockdown of NRG1-II impairs both mitoses in the sub-basal/sub-ventricular zone and the ventricular zone. Injection of soluble human NRG1 into the developing brain ameliorates neurogenesis of NRG1-II-knockdown embryos, suggesting a conserved role of NRG1 as a cell-extrinsic signal. From these results, we propose that NRG1-ErbB signaling stimulates cell divisions generating neurons from neural progenitor cells in the developing vertebrate brain.

Disturbance of oligodendrocyte function plays a key role in the pathogenesis of schizophrenia and major depressive disorder

The major psychiatric disorders such as schizophrenia (SZ) and major depressive disorder (MDD) are thought to be multifactorial diseases related to both genetic and environmental factors. However, the genes responsible and the molecular mechanisms underlying the pathogenesis of SZ and MDD remain unclear. We previously reported that abnormalities of disrupted-in-Schizophrenia-1 (DISC1) and DISC1 binding zinc finger (DBZ) might cause major psychiatric disorders such as SZ. Interestingly, both DISC and DBZ have been further detected in oligodendrocytes and implicated in regulating oligodendrocyte differentiation. DISC1 negatively regulates the differentiation of oligodendrocytes, whereas DBZ plays a positive regulatory role in oligodendrocyte differentiation. We have reported that repeated stressful events, one of the major risk factors of MDD, can induce sustained upregulation of plasma corticosterone levels and serum/glucocorticoid regulated kinase 1 (Sgk1) mRNA expression in oligodendrocytes. Repeated stressful events can also activate the SGK1 cascade and cause excess arborization of oligodendrocyte processes, which is thought to be related to depressive-like symptoms. In this review, we discuss the expression of DISC1, DBZ, and SGK1 in oligodendrocytes, their roles in the regulation of oligodendrocyte function, possible interactions of DISC1 and DBZ in relation to SZ, and the activation of the SGK1 signaling cascade in relation to MDD.

Neurodevelopment, GABA system dysfunction, and schizophrenia

The origins of schizophrenia have eluded clinicians and researchers since Kraepelin and Bleuler began documenting their findings. However, large clinical research efforts in recent decades have identified numerous genetic and environmental risk factors for schizophrenia. The combined data strongly support the neurodevelopmental hypothesis of schizophrenia and underscore the importance of the common converging effects of diverse insults. In this review, we discuss the evidence that genetic and environmental risk factors that predispose to schizophrenia disrupt the development and normal functioning of the GABAergic system.

Genetic underpinnings of white matter 'connectivity': heritability, risk, and heterogeneity in schizophrenia

Schizophrenia is a highly heritable disorder. Thus, the combination of genetics and brain imaging may be a useful strategy to investigate the effects of risk genes on anatomical connectivity, and for gene discovery, i.e. discovering the genetic correlates of white matter phenotypes. Following a database search, I review evidence for heritability of white matter phenotypes. I also review candidate gene investigations, examining association of putative risk variants with white matter phenotypes, as well as the recent flurry of research exploring relationships of genome-wide significant risk loci with white matter phenotypes. Finally, I review multivariate and polygene approaches, which constitute a new wave of imaging-genetics research, including large collaborative initiatives aiming to discover new genes that may predict aspects of white matter microstructure. The literature supports the heritability of white matter phenotypes. Loci in genes intimately implicated in oligodendrocyte and myelin development, growth and maintenance, and neurotrophic systems are associated with white matter microstructure. GWAS variants have not yet sufficiently been explored using DTI-based evaluation of white matter to draw conclusions, although micro-RNA 137 is promising due to its potential regulation of other GWAS schizophrenia genes. Many imaging-genetic studies only include healthy participants, which, while helping control for certain confounds, cannot address questions related to disease heterogeneity or symptom expression, and thus more studies should include participants with schizophrenia. With sufficiently large sample sizes, the future of this field lies in polygene strategies aimed at risk prediction and heterogeneity dissection of schizophrenia that can translate to personalized interventions.

Myelin, myelin-related disorders, and psychosis

The neuropathological basis of schizophrenia and related psychoses remains elusive despite intensive scientific investigation. Symptoms of psychosis have been reported in a number of conditions where normal myelin development is interrupted. The nature, location, and timing of white matter pathology seem to be key factors in the development of psychosis, especially during the critical adolescent period of association area myelination. Numerous lines of evidence implicate myelin and oligodendrocyte function as critical processes that could affect neuronal connectivity, which has been implicated as a central abnormality in schizophrenia. Phenocopies of schizophrenia with a known pathological basis involving demyelination or dysmyelination may offer insights into the biology of schizophrenia itself. This article reviews the pathological changes in white matter of patients with schizophrenia, as well as demyelinating diseases associated with psychosis. In an attempt to understand the potential role of dysmyelination in schizophrenia, we outline the evidence from a number of both clinically-based and post-mortem studies that provide evidence that OMR genes are genetically associated with increased risk for schizophrenia. To further understand the implication of white matter dysfunction and dysmyelination in schizophrenia, we examine diffusion tensor imaging (DTI), which has shown volumetric and microstructural white matter differences in patients with schizophrenia. While classical clinical-neuropathological correlations have established that disruption in myelination can produce a high fidelity phenocopy of psychosis similar to schizophrenia, the role of dysmyelination in schizophrenia remains controversial.

Neuroprotective effects of neuregulin-1 ß on oligodendrocyte type 2 astrocyte progenitors following oxygen and glucose deprivation

BACKGROUND

Hypoxic-ischemic brain injury in neonates, especially in premature infants, is one of the main contributors to the mortality of newborns and can cause nervous system dysfunction in children. The major pathogenesis seems to be cerebral ischemia/reperfusion in the immature white matter that preferentially targets vulnerable premyelinating oligodendrocytes.

OBJECTIVES

The goal of this study was to culture oligodendrocyte type 2 astrocyte cells in an oxygen and glucose deprivation environment to simulate ischemia injury and examine the cellular and molecular mechanisms involved in the neuroprotective effects of neuregulin-1ß on ischemia-induced immature oligodendrocytes.

METHODS

Oligodendrocyte type 2 astrocyte cells were cultured from neonatal Sprague-Dawley rat cerebra. The cells were divided into two groups: one was subjected to oxygen and glucose deprivation for 9 hours and the other was treated with 50 ng/mL or 100 ng/mL neuregulin-1β during oxygen and glucose deprivation. Cell survival was determined by Trypan Blue staining and cell apoptosis were observed by fluorescein isothiocyanate-Annexin V and propidium iodide double staining. To study if the PI3K-Akt signaling pathway was involved in the mechanism of protective effect of neuregulin-1ß, Western blot analysis was used to quantitative the changes of protein.

RESULTS

Treatment with neuregulin-1ß within the period of oxygen and glucose deprivation significantly increased cell survival and also resulted in a significant decrease in cell apoptosis. The neuroprotective effects of neuregulin-1ß were prevented by treatment with Ly294002, an inhibitor of the phosphatidylinositol-3-kinase/Akt pathway.

CONCLUSIONS

These results suggest that neuregulin-1ß could protect the oligodendrocyte type 2 astrocyte progenitors against hypoxic injury, and the mechanism may be associated with the PI3K-Akt signaling pathway.

DISC1 (disrupted-in-schizophrenia-1) regulates differentiation of oligodendrocytes

Disrupted-in-schizophrenia 1 (DISC1) is a gene disrupted by a translocation, t(1;11) (q42.1;q14.3), that segregates with major psychiatric disorders, including schizophrenia, recurrent major depression and bipolar affective disorder, in a Scottish family. Here we report that mammalian DISC1 endogenously expressed in oligodendroglial lineage cells negatively regulates differentiation of oligodendrocyte precursor cells into oligodendrocytes. DISC1 expression was detected in oligodendrocytes of the mouse corpus callosum at P14 and P70. DISC1 mRNA was expressed in primary cultured rat cortical oligodendrocyte precursor cells and decreased when oligodendrocyte precursor cells were induced to differentiate by PDGF deprivation. Immunocytochemical analysis showed that overexpressed DISC1 was localized in the cell bodies and processes of oligodendrocyte precursor cells and oligodendrocytes. We show that expression of the myelin related markers, CNPase and MBP, as well as the number of cells with a matured oligodendrocyte morphology, were decreased following full length DISC1 overexpression. Conversely, both expression of CNPase and the number of oligodendrocytes with a mature morphology were increased following knockdown of endogenous DISC1 by RNA interference. Overexpression of a truncated form of DISC1 also resulted in an increase in expression of myelin related proteins and the number of mature oligodendrocytes, potentially acting via a dominant negative mechanism. We also identified involvement of Sox10 and Nkx2.2 in the DISC1 regulatory pathway of oligodendrocyte differentiation, both well-known transcription factors involved in the regulation of myelin genes.

DBZ, a CNS-specific DISC1 binding protein, positively regulates oligodendrocyte differentiation

Recent studies have shown changes in myelin genes and alterations in white matter structure in a wide range of psychiatric disorders. Here we report that DBZ, a central nervous system (CNS)-specific member of the DISC1 interactome, positively regulates the oligodendrocyte (OL) differentiation in vivo and in vitro. In mouse corpus callosum (CC), DBZ mRNA is expressed in OL lineage cells and expression of DBZ protein peaked before MBP expression. In the CC of DBZ-KO mice, we observed delayed myelination during the early postnatal period. Although the myelination delay was mostly recovered by adulthood, OLs with immature structural features were more abundant in adult DBZ-KO mice than in control mice. DBZ was also transiently upregulated during rat OL differentiation in vitro before myelin marker expression. DBZ knockdown by RNA interference resulted in a decreased expression of myelin-related markers and a low number of cells with mature characteristics, but with no effect on the proliferation of oligodendrocyte precursor cells. We also show that the expression levels of transcription factors having a negative-regulatory role in OL differentiation were upregulated when endogenous DBZ was knocked down. These results strongly indicate that OL differentiation in rodents is regulated by DBZ.

Improving myelin/oligodendrocyte-related dysfunction: a new mechanism of antipsychotics in the treatment of schizophrenia?

Schizophrenia is a severe psychiatric disorder with complex clinical manifestations and its aetiological factors remain unclear. During the past decade, the oligodendrocyte-related myelin dysfunction was proposed as a hypothesis for schizophrenia, supported initially by a series of neuroimaging studies and genetic evidence. Recently, the effects of antipsychotics on myelination and oligodendroglial lineage development and their underlying molecular mechanisms were evaluated. Data from those studies suggest that the antipsychotics-resulting improvement in myelin/oligodendrocyte-related dysfunction may contribute, at least in part, to their therapeutic effect on schizophrenia. Importantly, these findings may provide the basis for a new insight into the therapeutic strategy by targeting the oligodendroglia lineage cells against schizophrenia.

The developing utility of zebrafish models for cognitive enhancers research

Whereas cognitive impairment is a common symptom in multiple brain disorders, predictive and high-throughput animal models of cognition and behavior are becoming increasingly important in the field of translational neuroscience research. In particular, reliable models of the cognitive deficits characteristic of numerous neurobehavioral disorders such as Alzheimer’s disease and schizophrenia have become a significant focus of investigation. While rodents have traditionally been used to study cognitive phenotypes, zebrafish (Danio rerio) are gaining popularity as an excellent model to complement current translational neuroscience research. Here we discuss recent advances in pharmacological and genetic approaches using zebrafish models to study cognitive impairments and to discover novel cognitive enhancers and neuroprotective mechanisms.

DISC1 genetics, biology and psychiatric illness

Psychiatric disorders are highly heritable, and in many individuals likely arise from the combined effects of genes and the environment. A substantial body of evidence points towards DISC1 being one of the genes that influence risk of schizophrenia, bipolar disorder and depression, and functional studies of DISC1 consequently have the potential to reveal much about the pathways that lead to major mental illness. Here, we review the evidence that DISC1 influences disease risk through effects upon multiple critical pathways in the developing and adult brain.

Less efficient information transfer in Cys-allele carriers of DISC1: a brain network study based on diffusion MRI

Previous neuroimaging studies of brain networks have revealed less efficient information transfer in patients with schizophrenia. However, the underlying genetic basis remains largely unexplored. In this study, we investigated the brain anatomical networks of 278 healthy volunteers with different genotypes in the common missense variant (Ser704Cys) of the Disrupted-in-Schizophrenia-1 (DISC1) gene, which is one of the main susceptibility genes of schizophrenia and other psychiatric disorders. The anatomical brain network for each individual was constructed using fiber tractography technique based on diffusion magnetic resonance imaging (dMRI). The properties of this network were then calculated using graph theory. This revealed that Cys-allele carriers showed significantly lower global efficiency of their brain networks than Ser homozygotes, thereby supporting our hypothesis that genetic variation in DISC1 may relate to the risk of schizophrenia by affecting the efficiency of brain network. Additional dMRI analyses were also performed at different levels, with a convergent trend towards decreased white matter integrity being consistently observed in Cys-allele carriers. Together these findings not only provide new clues for understanding DISC1 function, but also suggest that network analyses based on graph theory combined with neuroimaging techniques may reveal structural disruptions related to genetic risk in the brains of healthy individuals.

500,000 fish phenotypes: The new informatics landscape for evolutionary and developmental biology of the vertebrate skeleton

The rich phenotypic diversity that characterizes the vertebrate skeleton results from evolutionary changes in regulation of genes that drive development. Although relatively little is known about the genes that underlie the skeletal variation among fish species, significant knowledge of genetics and development is available for zebrafish. Because developmental processes are highly conserved, this knowledge can be leveraged for understanding the evolution of skeletal diversity. We developed the Phenoscape Knowledgebase (KB; http://kb.phenoscape.org) to yield testable hypotheses of candidate genes involved in skeletal evolution. We developed a community anatomy ontology for fishes and ontology-based methods to represent complex free-text character descriptions of species in a computable format. With these tools, we populated the KB with comparative morphological data from the literature on over 2,500 teleost fishes (mainly Ostariophysi) resulting in over 500,000 taxon phenotype annotations. The KB integrates these data with similarly structured phenotype data from zebrafish genes (http://zfin.org). Using ontology-based reasoning, candidate genes can be inferred for the phenotypes that vary across taxa, thereby uniting genetic and phenotypic data to formulate evo-devo hypotheses. The morphological data in the KB can be browsed, sorted, and aggregated in ways that provide unprecedented possibilities for data mining and discovery.

Linking neurodevelopmental and synaptic theories of mental illness through DISC1

Recent advances in our understanding of the underlying genetic architecture of psychiatric disorders has blown away the diagnostic boundaries that are defined by currently used diagnostic manuals. The disrupted in schizophrenia 1 (DISC1) gene was originally discovered at the breakpoint of an inherited chromosomal translocation, which segregates with major mental illnesses. In addition, many biological studies have indicated a role for DISC1 in early neurodevelopment and synaptic regulation. Given that DISC1 is thought to drive a range of endophenotypes that underlie major mental conditions, elucidating the biology of DISC1 may enable the construction of new diagnostic categories for mental illnesses with a more meaningful biological foundation.

Expression of the neuregulin receptor ErbB4 in the brain of the rhesus monkey (Macaca mulatta)

We demonstrated recently that frontal cortical expression of the Neuregulin (NRG) receptor ErbB4 is restricted to interneurons in rodents, macaques, and humans. However, little is known about protein expression patterns in other areas of the brain. In situ hybridization studies have shown high ErbB4 mRNA levels in various subcortical areas, suggesting that ErbB4 is also expressed in cell types other than cortical interneurons. Here, using highly-specific monoclonal antibodies, we provide the first extensive report of ErbB4 protein expression throughout the cerebrum of primates. We show that ErbB4 immunoreactivity is high in association cortices, intermediate in sensory cortices, and relatively low in motor cortices. The overall immunoreactivity in the hippocampal formation is intermediate, but is high in a subset of interneurons. We detected the highest overall immunoreactivity in distinct locations of the ventral hypothalamus, medial habenula, intercalated nuclei of the amygdala and structures of the ventral forebrain, such as the islands of Calleja, olfactory tubercle and ventral pallidum, and medium expression in the reticular thalamic nucleus. While this pattern is generally consistent with ErbB4 mRNA expression data, further investigations are needed to identify the exact cellular and subcellular sources of mRNA and protein expression in these areas. In contrast to in situ hybridization in rodents, we detected only low levels of ErbB4-immunoreactivity in mesencephalic dopaminergic nuclei but a diffuse pattern of immunofluorescence that was medium in the dorsal striatum and high in the ventral forebrain, suggesting that most ErbB4 protein in dopaminergic neurons could be transported to axons. We conclude that the NRG-ErbB4 signaling pathway can potentially influence many functional systems throughout the brain of primates, and suggest that major sites of action are areas of the “corticolimbic” network. This interpretation is functionally consistent with the genetic association of NRG1 and ERBB4 with schizophrenia.

Disc1 regulates both β-catenin-mediated and noncanonical Wnt signaling during vertebrate embryogenesis

Disc1 is a schizophrenia risk gene that engages multiple signaling pathways during neurogenesis and brain development. Using the zebrafish as a tool, we analyze the function of zebrafish Disc1 (zDisc1) at the earliest stages of brain and body development. We define a "tool" as a biological system that gives insight into mechanisms underlying a human disorder, although the system does not phenocopy the disorder. A zDisc1 peptide binds to GSK3β, and zDisc1 directs early brain development and neurogenesis, by promoting β-catenin-mediated Wnt signaling and inhibiting GSK3β activity. zDisc1 loss-of-function embryos additionally display a convergence and extension phenotype, demonstrated by abnormal movement of dorsolateral cells during gastrulation, through changes in gene expression, and later through formation of abnormal, U-shaped muscle segments, and a truncated tail. These phenotypes are caused by alterations in the noncanonical Wnt pathway, via Daam and Rho signaling. The convergence and extension phenotype can be rescued by a dominant negative GSK3β construct, suggesting that zDisc1 inhibits GSK3β activity during noncanonical Wnt signaling. This is the first demonstration that Disc1 modulates the noncanonical Wnt pathway and suggests a previously unconsidered mechanism by which Disc1 may contribute to the etiology of neuropsychiatric disorders.

Expression of mutant human DISC1 in mice supports abnormalities in differentiation of oligodendrocytes

Abnormalities in oligodendrocyte (OLG) differentiation and OLG gene expression deficit have been described in schizophrenia (SZ). Recent studies revealed a critical requirement for Disrupted-in-Schizophrenia 1 (DISC1) in neural development. Transgenic mice with forebrain restricted expression of mutant human DISC1 (ΔhDISC1) are characterized by neuroanatomical and behavioral abnormalities reminiscent of some features of SZ. We sought to determine whether the expression of ΔhDISC1 may influence the development of OLGs in this mouse model. OLG- and cell cycle-associated gene and protein expression were characterized in the forebrain of ΔhDISC1 mice during different stages of neurodevelopment (E15 and P1 days) and in adulthood. The results suggest that the expression of ΔhDISC1 exerts a significant influence on oligodendrocyte differentiation and function, evidenced by premature OLG differentiation and increased proliferation of their progenitors. Additional findings showed that neuregulin 1 and its receptors may be contributing factors to the observed upregulation of OLG genes. Thus, OLG function may be perturbed by mutant hDISC1 in a model system that provides new avenues for studying aspects of the pathogenesis of SZ.

Characterization of the deleted in autism 1 protein family: implications for studying cognitive disorders

Autism spectrum disorders (ASDs) are a group of commonly occurring, highly-heritable developmental disabilities. Human genes c3orf58 or Deleted In Autism-1 (DIA1) and cXorf36 or Deleted in Autism-1 Related (DIA1R) are implicated in ASD and mental retardation. Both gene products encode signal peptides for targeting to the secretory pathway. As evolutionary medicine has emerged as a key tool for understanding increasing numbers of human diseases, we have used an evolutionary approach to study DIA1 and DIA1R. We found DIA1 conserved from cnidarians to humans, indicating DIA1 evolution coincided with the development of the first primitive synapses. Nematodes lack a DIA1 homologue, indicating Caenorhabditis elegans is not suitable for studying all aspects of ASD etiology, while zebrafish encode two DIA1 paralogues. By contrast to DIA1, DIA1R was found exclusively in vertebrates, with an origin coinciding with the whole-genome duplication events occurring early in the vertebrate lineage, and the evolution of the more complex vertebrate nervous system. Strikingly, DIA1R was present in schooling fish but absent in fish that have adopted a more solitary lifestyle. An additional DIA1-related gene we named DIA1-Like (DIA1L), lacks a signal peptide and is restricted to the genomes of the echinoderm Strongylocentrotus purpuratus and cephalochordate Branchiostoma floridae. Evidence for remarkable DIA1L gene expansion was found in B. floridae. Amino acid alignments of DIA1 family gene products revealed a potential Golgi-retention motif and a number of conserved motifs with unknown function. Furthermore, a glycine and three cysteine residues were absolutely conserved in all DIA1-family proteins, indicating a critical role in protein structure and/or function. We have therefore identified a new metazoan protein family, the DIA1-family, and understanding the biological roles of DIA1-family members will have implications for our understanding of autism and mental retardation.

Neuroplasticity signaling pathways linked to the pathophysiology of schizophrenia

Schizophrenia is a severe mental illness that afflicts nearly 1% of the world's population. One of the cardinal pathological features of schizophrenia is perturbation in synaptic connectivity. Although the etiology of schizophrenia is unknown, it appears to be a developmental disorder involving the interaction of a potentially large number of risk genes, with no one gene producing a strong effect except rare, highly penetrant copy number variants. The purpose of this review is to detail how putative schizophrenia risk genes (DISC-1, neuregulin/ErbB4, dysbindin, Akt1, BDNF, and the NMDA receptor) are involved in regulating neuroplasticity and how alterations in their expression may contribute to the disconnectivity observed in schizophrenia. Moreover, this review highlights how many of these risk genes converge to regulate common neurotransmitter systems and signaling pathways. Future studies aimed at elucidating the functions of these risk genes will provide new insights into the pathophysiology of schizophrenia and will likely lead to the nomination of novel therapeutic targets for restoring proper synaptic connectivity in the brain in schizophrenia and related disorders.