DISC1 variants 37W and 607F disrupt its nuclear targeting and regulatory role in ATF4-mediated transcription

Structural interaction between DISC1 and ATF4 underlying transcriptional and synaptic dysregulation in an iPSC model of mental disorders

Psychiatric disorders are a collection of heterogeneous mental disorders arising from a contribution of genetic and environmental insults, many of which molecularly converge on transcriptional dysregulation, resulting in altered synaptic functions. The underlying mechanisms linking the genetic lesion and functional phenotypes remain largely unknown. Patient iPSC-derived neurons with a rare frameshift DISC1 (Disrupted-in-schizophrenia 1) mutation have previously been shown to exhibit aberrant gene expression and deficits in synaptic functions. How DISC1 regulates gene expression is largely unknown. Here we show that Activating Transcription Factor 4 (ATF4), a DISC1 binding partner, is more abundant in the nucleus of DISC1 mutant human neurons and exhibits enhanced binding to a collection of dysregulated genes. Functionally, overexpressing ATF4 in control neurons recapitulates deficits seen in DISC1 mutant neurons, whereas transcriptional and synaptic deficits are rescued in DISC1 mutant neurons with CRISPR-mediated heterozygous ATF4 knockout. By solving the high-resolution atomic structure of the DISC1-ATF4 complex, we show that mechanistically, the mutation of DISC1 disrupts normal DISC1-ATF4 interaction, and results in excessive ATF4 binding to DNA targets and deregulated gene expression. Together, our study identifies the molecular and structural basis of an DISC1-ATF4 interaction underlying transcriptional and synaptic dysregulation in an iPSC model of mental disorders.

Host-parasite interaction associated with major mental illness

Clinical studies frequently report that patients with major mental illness such as schizophrenia and bipolar disorder have co-morbid physical conditions, suggesting that systemic alterations affecting both brain and peripheral tissues might underlie the disorders. Numerous studies have reported elevated levels of anti-Toxoplasma gondii (T. gondii) antibodies in patients with major mental illnesses, but the underlying mechanism was unclear. Using multidisciplinary epidemiological, cell biological, and gene expression profiling approaches, we report here multiple lines of evidence suggesting that a major mental illness-related susceptibility factor, Disrupted in schizophrenia (DISC1), is involved in host immune responses against T. gondii infection. Specifically, our cell biology and gene expression studies have revealed that DISC1 Leu607Phe variation, which changes DISC1 interaction with activating transcription factor 4 (ATF4), modifies gene expression patterns upon T. gondii infection. Our epidemiological data have also shown that DISC1 607 Phe/Phe genotype was associated with higher T. gondii antibody levels in sera. Although further studies are required, our study provides mechanistic insight into one of the few well-replicated serological observations in major mental illness.

Mechanisms underlying the role of DISC1 in synaptic plasticity

Disrupted in schizophrenia 1 (DISC1) is an important hub protein, forming multimeric complexes by self-association and interacting with a large number of synaptic and cytoskeletal molecules. The synaptic location of DISC1 in the adult brain suggests a role in synaptic plasticity, and indeed, a number of studies have discovered synaptic plasticity impairments in a variety of different DISC1 mutants. This review explores the possibility that DISC1 is an important molecule for organizing proteins involved in synaptic plasticity and examines why mutations in DISC1 impair plasticity. It concentrates on DISC1's role in interacting with synaptic proteins, controlling dendritic structure and cellular trafficking of mRNA, synaptic vesicles and mitochondria. N-terminal directed mutations appear to impair synaptic plasticity through interactions with phosphodiesterase 4B (PDE4B) and hence protein kinase A (PKA)/GluA1 and PKA/cAMP response element-binding protein (CREB) signalling pathways, and affect spine structure through interactions with kalirin 7 (Kal-7) and Rac1. C-terminal directed mutations also impair plasticity possibly through altered interactions with lissencephaly protein 1 (LIS1) and nuclear distribution protein nudE-like 1 (NDEL1), thereby affecting developmental processes such as dendritic structure and spine maturation. Many of the same molecules involved in DISC1's cytoskeletal interactions are also involved in intracellular trafficking, raising the possibility that impairments in intracellular trafficking affect cytoskeletal development and vice versa. While the multiplicity of DISC1 protein interactions makes it difficult to pinpoint a single causal signalling pathway, we suggest that the immediate-term effects of N-terminal influences on GluA1, Rac1 and CREB, coupled with the developmental effects of C-terminal influences on trafficking and the cytoskeleton make up the two main branches of DISC1's effect on synaptic plasticity and dendritic spine stability.

Aggregation of scaffolding protein DISC1 dysregulates phosphodiesterase 4 in Huntington's disease

Huntington's disease (HD) is a polyglutamine (polyQ) disease caused by aberrant expansion of the polyQ tract in Huntingtin (HTT). While motor impairment mediated by polyQ-expanded HTT has been intensively studied, molecular mechanisms for nonmotor symptoms in HD, such as psychiatric manifestations, remain elusive. Here we have demonstrated that HTT forms a ternary protein complex with the scaffolding protein DISC1 and cAMP-degrading phosphodiesterase 4 (PDE4) to regulate PDE4 activity. We observed pathological cross-seeding between DISC1 and mutant HTT aggregates in the brains of HD patients as well as in a murine model that recapitulates the polyQ pathology of HD (R6/2 mice). In R6/2 mice, consequent reductions in soluble DISC1 led to dysregulation of DISC1-PDE4 complexes, aberrantly increasing the activity of PDE4. Importantly, exogenous expression of a modified DISC1, which binds to PDE4 but not mutant HTT, normalized PDE4 activity and ameliorated anhedonia in the R6/2 mice. We propose that cross-seeding of mutant HTT and DISC1 and the resultant changes in PDE4 activity may underlie the pathology of a specific subset of mental manifestations of HD, which may provide an insight into molecular signaling in mental illness in general.

TLR3 downregulates expression of schizophrenia gene Disc1 via MYD88 to control neuronal morphology

Viral infection during fetal or neonatal stages increases the risk of developing neuropsychiatric disorders such as schizophrenia and autism spectrum disorders. Although neurons express several key regulators of innate immunity, the role of neuronal innate immunity in psychiatric disorders is still unclear. Using cultured neurons and in vivo mouse brain studies, we show here that Toll‐like receptor 3 (TLR3) acts through myeloid differentiation primary response gene 88 (MYD88) to negatively control Disrupted in schizophrenia 1 (Disc1) expression, resulting in impairment of neuronal development. Cytokines are not involved in TLR3‐mediated inhibition of dendrite outgrowth. Instead, TLR3 signaling suppresses expression of several psychiatric disorder‐related genes, including Disc1. The impaired dendritic arborization caused by TLR3 activation is rescued by MYD88 deficiency or DISC1 overexpression. In addition, TLR3 activation at the neonatal stage increases dendritic spine density, but narrows spine heads at postnatal day 21 (P21), suggesting a long‐lasting effect of TLR3 activation on spinogenesis. Our study reveals a novel mechanism of TLR3 in regulation of dendritic morphology and provides an explanation for how environmental factors influence mental health.

Additive sex-specific influence of common non-synonymous DISC1 variants on amygdala, basal ganglia, and white cortical surface area in healthy young adults

The disrupted-in-schizophrenia-1 (DISC1) gene is known for its role in the development of mental disorders. It is also involved in neurodevelopment, cognition, and memory. To investigate the association between DISC1 variants and brain morphology, we analyzed the influence of the three common non-synonymous polymorphisms in DISC1 on specific brain structures in healthy young adults. The volumes of brain regions were determined in 145 subjects by magnetic resonance imaging and automated analysis using FreeSurfer. Genotyping was performed by high resolution melting of amplified products. In an additive genetic model, rs6675281 (Leu607Phe), rs3738401 (Arg264Gln), and rs821616 (Ser704Cys) significantly explained the volume variance of the amygdala (p = 0.007) and the pallidum (p = 0.004). A higher cumulative portion of minor alleles was associated with larger volumes of the amygdala (p = 0.005), the pallidum (p = 0.001), the caudate (p = 0.024), and the putamen (p = 0.007). Sex-stratified analysis revealed a strong genetic effect of rs6675281 on putamen and pallidum in females but not in males and an opposite influence of rs3738401 on the white cortical surface in females compared to males. The strongest single association was found for rs821616 and the amygdala volume in male subjects (p < 0.001). No effect was detected for the nucleus accumbens. We report-to our knowledge-for the first time a significant and sex-specific influence of common DISC1 variants on volumes of the basal ganglia, the amygdala and on the cortical surface area. Our results demonstrate that the additive model of all three polymorphisms outperforms their single analysis.

Emerging Roles for the Unfolded Protein Response in the Developing Nervous System

The unfolded protein response (UPR) is a homeostatic signaling pathway triggered by protein misfolding in the endoplasmic reticulum (ER). Beyond its protective role, it plays important functions during normal development in response to elevated demand for protein folding. Several UPR effectors show dynamic temporal and spatial expression patterns that correlate with milestones of the central nervous system (CNS) development. Here, we discuss recent studies suggesting that a dynamic regulation of UPR supports generation, maturation, and maintenance of differentiated neurons in the CNS. We further highlight studies supporting a developmental vulnerability of CNS to UPR dysregulation, which underlies neurodevelopmental disorders. We believe that a better understanding of UPR functions may provide novel opportunities for therapeutic strategies to fight ER/UPR-associated human neurological disorders.

DISC1-dependent Regulation of Mitochondrial Dynamics Controls the Morphogenesis of Complex Neuronal Dendrites

The DISC1 protein is implicated in major mental illnesses including schizophrenia, depression, bipolar disorder, and autism. Aberrant mitochondrial dynamics are also associated with major mental illness. DISC1 plays a role in mitochondrial transport in neuronal axons, but its effects in dendrites have yet to be studied. Further, the mechanisms of this regulation and its role in neuronal development and brain function are poorly understood. Here we have demonstrated that DISC1 couples to the mitochondrial transport and fusion machinery via interaction with the outer mitochondrial membrane GTPase proteins Miro1 and Miro2, the TRAK1 and TRAK2 mitochondrial trafficking adaptors, and the mitochondrial fusion proteins (mitofusins). Using live cell imaging, we show that disruption of the DISC1-Miro-TRAK complex inhibits mitochondrial transport in neurons. We also show that the fusion protein generated from the originally described DISC1 translocation (DISC1-Boymaw) localizes to the mitochondria, where it similarly disrupts mitochondrial dynamics. We also show by super resolution microscopy that DISC1 is localized to endoplasmic reticulum contact sites and that the DISC1-Boymaw fusion protein decreases the endoplasmic reticulum-mitochondria contact area. Moreover, disruption of mitochondrial dynamics by targeting the DISC1-Miro-TRAK complex or upon expression of the DISC1-Boymaw fusion protein impairs the correct development of neuronal dendrites. Thus, DISC1 acts as an important regulator of mitochondrial dynamics in both axons and dendrites to mediate the transport, fusion, and cross-talk of these organelles, and pathological DISC1 isoforms disrupt this critical function leading to abnormal neuronal development.

708 Common and 2010 rare DISC1 locus variants identified in 1542 subjects: analysis for association with psychiatric disorder and cognitive traits

A balanced t(1;11) translocation that transects the Disrupted in schizophrenia 1 (DISC1) gene shows genome-wide significant linkage for schizophrenia and recurrent major depressive disorder (rMDD) in a single large Scottish family, but genome-wide and exome sequencing-based association studies have not supported a role for DISC1 in psychiatric illness. To explore DISC1 in more detail, we sequenced 528 kb of the DISC1 locus in 653 cases and 889 controls. We report 2718 validated single-nucleotide polymorphisms (SNPs) of which 2010 have a minor allele frequency of <1%. Only 38% of these variants are reported in the 1000 Genomes Project European subset. This suggests that many DISC1 SNPs remain undiscovered and are essentially private. Rare coding variants identified exclusively in patients were found in likely functional protein domains. Significant region-wide association was observed between rs16856199 and rMDD (P=0.026, unadjusted P=6.3 × 10(-5), OR=3.48). This was not replicated in additional recurrent major depression samples (replication P=0.11). Combined analysis of both the original and replication set supported the original association (P=0.0058, OR=1.46). Evidence for segregation of this variant with disease in families was limited to those of rMDD individuals referred from primary care. Burden analysis for coding and non-coding variants gave nominal associations with diagnosis and measures of mood and cognition. Together, these observations are likely to generalise to other candidate genes for major mental illness and may thus provide guidelines for the design of future studies.

Common variants in psychiatric risk genes predict brain structure at birth

Studies in adolescents and adults have demonstrated that polymorphisms in putative psychiatric risk genes are associated with differences in brain structure, but cannot address when in development these relationships arise. To determine if common genetic variants in disrupted-in-schizophrenia-1 (DISC1; rs821616 and rs6675281), catechol-O-methyltransferase (COMT; rs4680), neuregulin 1 (NRG1; rs35753505 and rs6994992), apolipoprotein E (APOE; ε3ε4 vs. ε3ε3), estrogen receptor alpha (ESR1; rs9340799 and rs2234693), brain-derived neurotrophic factor (BDNF; rs6265), and glutamate decarboxylase 1 (GAD1; rs2270335) are associated with individual differences in brain tissue volumes in neonates, we applied both automated region-of-interest volumetry and tensor-based morphometry to a sample of 272 neonates who had received high-resolution magnetic resonance imaging scans. ESR1 (rs9340799) predicted intracranial volume. Local variation in gray matter (GM) volume was significantly associated with polymorphisms in DISC1 (rs821616), COMT, NRG1, APOE, ESR1 (rs9340799), and BDNF. No associations were identified for DISC1 (rs6675281), ESR1 (rs2234693), or GAD1. Of note, neonates homozygous for the DISC1 (rs821616) serine allele exhibited numerous large clusters of reduced GM in the frontal lobes, and neonates homozygous for the COMT valine allele exhibited reduced GM in the temporal cortex and hippocampus, mirroring findings in adults. The results highlight the importance of prenatal brain development in mediating psychiatric risk.

DISC1 complexes with TRAK1 and Miro1 to modulate anterograde axonal mitochondrial trafficking

Disrupted-In-Schizophrenia 1 (DISC1) is a candidate risk factor for schizophrenia, bipolar disorder and severe recurrent depression. Here, we demonstrate that DISC1 associates robustly with trafficking-protein-Kinesin-binding-1 which is, in turn, known to interact with the outer mitochondrial membrane proteins Miro1/2, linking mitochondria to the kinesin motor for microtubule-based subcellular trafficking. DISC1 also associates with Miro1 and is thus a component of functional mitochondrial transport complexes. Consistent with these observations, in neuronal axons DISC1 promotes specifically anterograde mitochondrial transport. DISC1 thus participates directly in mitochondrial trafficking, which is essential for neural development and neurotransmission. Any factor affecting mitochondrial DISC1 function is hence likely to have deleterious consequences for the brain, potentially contributing to increased risk of psychiatric illness. Intriguingly, therefore, a rare putatively causal human DISC1 sequence variant, 37W, impairs the ability of DISC1 to promote anterograde mitochondrial transport. This is likely related to a number of mitochondrial abnormalities induced by expression of DISC1-37W, which redistributes mitochondrial DISC1 and enhances kinesin mitochondrial association, while also altering protein interactions within the mitochondrial transport complex.

DISC1-ATF4 transcriptional repression complex: dual regulation of the cAMP-PDE4 cascade by DISC1

Disrupted-In-Schizophrenia 1 (DISC1), a risk factor for major mental illnesses, has been studied extensively in the context of neurodevelopment. However, the role of DISC1 in neuronal signaling, particularly in conjunction with intracellular cascades that occur in response to dopamine, a neurotransmitter implicated in numerous psychiatric disorders, remains elusive. Previous data suggest that DISC1 interacts with numerous proteins that impact neuronal function, including activating transcription factor 4 (ATF4). In this study, we identify a novel DISC1 and ATF4 binding region in the genomic locus of phosphodiesterase 4D (PDE4D), a gene implicated in psychiatric disorders. We found that the loss of function of either DISC1 or ATF4 increases PDE4D9 transcription, and that the association of DISC1 with the PDE4D9 locus requires ATF4. We also show that PDE4D9 is increased by D1-type dopamine receptor dopaminergic stimulation. We demonstrate that the mechanism for this increase is due to DISC1 dissociation from the PDE4D locus in mouse brain. We further characterize the interaction of DISC1 with ATF4 to show that it is regulated via protein kinase A-mediated phosphorylation of DISC1 serine-58. Our results suggest that the release of DISC1-mediated transcriptional repression of PDE4D9 acts as feedback inhibition to regulate dopaminergic signaling. Furthermore, as DISC1 loss-of-function leads to a specific increase in PDE4D9, PDE4D9 itself may represent an attractive target for therapeutic approaches in psychiatric disorders.

Disrupted-in-schizophrenia-1 Gln31Leu polymorphism results in social anhedonia associated with monoaminergic imbalance and reduction of CREB and β-arrestin-1,2 in the nucleus accumbens in a mouse model of depression

Disrupted-in-schizophrenia-1 (DISC1) is associated with mental disorders, including major depression. We previously showed that DISC1-Q31L mutant mice have depression-like behaviors and can therefore be used to study neurobiological mechanisms of depression and antidepressant (AD) medication action. First, we found reduced levels of dopamine, serotonin and norepinephrine in the nucleus accumbens (NAC) of DISC1-Q31L mutants. Next, we assessed social-conditioned place preference as a reward-dependent task and the capacity of distinct ADs to correct impaired social behavior in DISC1-Q31L mice. Bupropion, but not fluoxetine or desipramine, was able to correct deficient social facilitation, social reward, and social novelty in DISC1-Q31L mutants, whereas all three ADs were able to improve social motivation and behavioral despair in DISC1-Q31L mutants. Furthermore, we sought to correlate social anhedonia with molecular and cellular features including dendritic spine density, β-arrestin-1,2, and cAMP-response-element-binding protein (CREB) in the NAC as biomarkers related to depression and the DISC1 pathway. DISC1-Q31L mutants showed reduced levels of β-arrestin-1,2, CREB, and spine density in the NAC, further supporting the construct validity of the genetic model. Bupropion induced the greatest effect on CREB in DISC1-Q31L mutants, whereas all studied ADs corrected the reduced levels of β-arrestin-1,2 and modestly ameliorated deficient spine density in this brain region. Overall, we find neurobiological changes accompanying social anhedonia in the NAC of DISC1-Q31L mutant mice, consistent with a role for DISC1 in regulating social reward as an endophenotype of depression.

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.

DISC1: a key lead in studying cortical development and associated brain disorders

For the past decade, DISC1 has been studied as a promising lead to understand the biology underlying major mental illnesses, such as schizophrenia. Consequently, many review articles on DISC1 have been published. In this article, rather than repeating comprehensive overviews of research articles, we will introduce the utility of DISC1 in the study of cortical development in association with a wide range of developmental brain disorders. Cortical development involves cell autonomous and cell nonautonomous mechanisms as well as host responses to environmental factors, all of which involve DISC1 function. Thus, we will discuss the significance of DISC1 in forming an overall understanding of multiple mechanisms that orchestrate corticogenesis and can serve as therapeutic targets in diseases caused by abnormal cortical development.

An epigenetic framework for neurodevelopmental disorders: from pathogenesis to potential therapy

Neurodevelopmental disorders (NDDs) are characterized by aberrant and delayed early-life development of the brain, leading to deficits in language, cognition, motor behaviour and other functional domains, often accompanied by somatic symptoms. Environmental factors like perinatal infection, malnutrition and trauma can increase the risk of the heterogeneous, multifactorial and polygenic disorders, autism and schizophrenia. Conversely, discrete genetic anomalies are involved in Down, Rett and Fragile X syndromes, tuberous sclerosis and neurofibromatosis, the less familiar Phelan-McDermid, Sotos, Kleefstra, Coffin-Lowry and "ATRX" syndromes, and the disorders of imprinting, Angelman and Prader-Willi syndromes. NDDs have been termed "synaptopathies" in reference to structural and functional disturbance of synaptic plasticity, several involve abnormal Ras-Kinase signalling ("rasopathies"), and many are characterized by disrupted cerebral connectivity and an imbalance between excitatory and inhibitory transmission. However, at a different level of integration, NDDs are accompanied by aberrant "epigenetic" regulation of processes critical for normal and orderly development of the brain. Epigenetics refers to potentially-heritable (by mitosis and/or meiosis) mechanisms controlling gene expression without changes in DNA sequence. In certain NDDs, prototypical epigenetic processes of DNA methylation and covalent histone marking are impacted. Conversely, others involve anomalies in chromatin-modelling, mRNA splicing/editing, mRNA translation, ribosome biogenesis and/or the regulatory actions of small nucleolar RNAs and micro-RNAs. Since epigenetic mechanisms are modifiable, this raises the hope of novel therapy, though questions remain concerning efficacy and safety. The above issues are critically surveyed in this review, which advocates a broad-based epigenetic framework for understanding and ultimately treating a diverse assemblage of NDDs ("epigenopathies") lying at the interface of genetic, developmental and environmental processes. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.