DISC1 regulates the transport of the NUDEL/LIS1/14-3-3epsilon complex through kinesin-1

Peering into the mind: unraveling schizophrenia's secrets using models

Schizophrenia (SCZ) is a complex mental disorder characterized by a range of symptoms, including positive and negative symptoms, as well as cognitive impairments. Despite the extensive research, the underlying neurobiology of SCZ remain elusive. To overcome this challenge, the use of diverse laboratory modeling techniques, encompassing cellular and animal models, and innovative approaches like induced pluripotent stem cell (iPSC)-derived neuronal cultures or brain organoids and genetically engineered animal models, has been crucial. Immortalized cellular models provide controlled environments for investigating the molecular and neurochemical pathways involved in neuronal function, while iPSCs and brain organoids, derived from patient-specific sources, offer significant advantage in translational research by facilitating direct comparisons of cellular phenotypes between patient-derived neurons and healthy-control neurons. Animal models can recapitulate the different psychopathological aspects that should be modeled, offering valuable insights into the neurobiology of SCZ. In addition, invertebrates' models are genetically tractable and offer a powerful approach to dissect the core genetic underpinnings of SCZ, while vertebrate models, especially mammals, with their more complex nervous systems and behavioral repertoire, provide a closer approximation of the human condition to study SCZ-related traits. This narrative review provides a comprehensive overview of the diverse modeling approaches, critically evaluating their strengths and limitations. By synthesizing knowledge from these models, this review offers a valuable source for researchers, clinicians, and stakeholders alike. Integrating findings across these different models may allow us to build a more holistic picture of SCZ pathophysiology, facilitating the exploration of new research avenues and informed decision-making for interventions.

Discovering functional interactions among schizophrenia-risk genes by combining behavioral genetics with cell biology

Despite much progress in identifying risk genes for polygenic brain disorders, their core pathogenic mechanisms remain poorly understood. In particular, functions of many proteins encoded by schizophrenia risk genes appear diverse and unrelated, complicating the efforts to establish the causal relationship between genes and behavior. Using various mouse lines, recent studies indicate that alterations of parvalbumin-positive (PV+) GABAergic interneurons can lead to schizophrenia-like behavior. PV+ interneurons display fast spiking and contribute to excitation-inhibition balance and network oscillations via feedback and feedforward inhibition. Here, we first summarize different lines of genetically modified mice that display motor, cognitive, emotional, and social impairments used to model schizophrenia and related mental disorders. We highlight ten genes, encoding either a nuclear, cytosolic, or membrane protein. Next, we discuss their functional relationship in regulating fast spiking and other aspects of PV+ interneurons and in the context of other domains of schizophrenia. Future investigations combining behavioral genetics and cell biology should elucidate functional relationships among risk genes to identify the core pathogenic mechanisms underlying polygenic brain disorders.

iPSC-derived models of PACS1 syndrome reveal transcriptional and functional deficits in neuron activity

PACS1 syndrome is a neurodevelopmental disorder characterized by intellectual disability and distinct craniofacial abnormalities resulting from a de novo p.R203W variant in phosphofurin acidic cluster sorting protein 1 (PACS1). PACS1 is known to have functions in the endosomal pathway and nucleus, but how the p.R203W variant affects developing neurons is not fully understood. Here we differentiated stem cells towards neuronal models including cortical organoids to investigate the impact of the PACS1 syndrome-causing variant on neurodevelopment. While few deleterious effects were detected in PACS1(+/R203W) neural precursors, mature PACS1(+/R203W) glutamatergic neurons exhibited impaired expression of genes involved in synaptic signaling processes. Subsequent characterization of neural activity using calcium imaging and multielectrode arrays revealed the p.R203W PACS1 variant leads to a prolonged neuronal network burst duration mediated by an increased interspike interval. These findings demonstrate the impact of the PACS1 p.R203W variant on developing human neural tissue and uncover putative electrophysiological underpinnings of disease.

Differential Expression of Proteins Associated with Bipolar Disorder as Identified Using the PeptideShaker Software

The prevalence of bipolar disorder (BD) in modern society is growing rapidly, but due to the lack of paraclinical criteria, its differential diagnosis with other mental disorders is somewhat challenging. In this regard, the relevance of proteomic studies is increasing due to the development of methods for processing large data arrays; this contributes to the discovery of protein patterns of pathological processes and the creation of new methods of diagnosis and treatment. It seems promising to search for proteins involved in the pathogenesis of BD in an easily accessible material-blood serum. Sera from BD patients and healthy individuals were purified via affinity chromatography to isolate 14 major proteins and separated using 1D SDS-PAGE. After trypsinolysis, the proteins in the samples were identified via HPLC/mass spectrometry. Mass spectrometric data were processed using the OMSSA and X!Tandem search algorithms using the UniProtKB database, and the results were analyzed using PeptideShaker. Differences in proteomes were assessed via an unlabeled NSAF-based analysis using a two-tailed Bonferroni-adjusted t-test. When comparing the blood serum proteomes of BD patients and healthy individuals, 10 proteins showed significant differences in NSAF values. Of these, four proteins were predominantly present in BD patients with the maximum NSAF value: 14-3-3 protein zeta/delta; ectonucleoside triphosphate diphosphohydrolase 7; transforming growth factor-beta-induced protein ig-h3; and B-cell CLL/lymphoma 9 protein. Further exploration of the role of these proteins in BD is warranted; conducting such studies will help develop new paraclinical criteria and discover new targets for BD drug therapy.

Dysregulated Signaling at Postsynaptic Density: A Systematic Review and Translational Appraisal for the Pathophysiology, Clinics, and Antipsychotics' Treatment of Schizophrenia

Emerging evidence from genomics, post-mortem, and preclinical studies point to a potential dysregulation of molecular signaling at postsynaptic density (PSD) in schizophrenia pathophysiology. The PSD that identifies the archetypal asymmetric synapse is a structure of approximately 300 nm in diameter, localized behind the neuronal membrane in the glutamatergic synapse, and constituted by more than 1000 proteins, including receptors, adaptors, kinases, and scaffold proteins. Furthermore, using FASS (fluorescence-activated synaptosome sorting) techniques, glutamatergic synaptosomes were isolated at around 70 nm, where the receptors anchored to the PSD proteins can diffuse laterally along the PSD and were stabilized by scaffold proteins in nanodomains of 50-80 nm at a distance of 20-40 nm creating "nanocolumns" within the synaptic button. In this context, PSD was envisioned as a multimodal hub integrating multiple signaling-related intracellular functions. Dysfunctions of glutamate signaling have been postulated in schizophrenia, starting from the glutamate receptor's interaction with scaffolding proteins involved in the N-methyl-D-aspartate receptor (NMDAR). Despite the emerging role of PSD proteins in behavioral disorders, there is currently no systematic review that integrates preclinical and clinical findings addressing dysregulated PSD signaling and translational implications for antipsychotic treatment in the aberrant postsynaptic function context. Here we reviewed a critical appraisal of the role of dysregulated PSD proteins signaling in the pathophysiology of schizophrenia, discussing how antipsychotics may affect PSD structures and synaptic plasticity in brain regions relevant to psychosis.

Quantification of a Neurological Protein in a Single Cell Without Amplification

Proteins are key biomolecules that not only play various roles in the living body but also are used as biomarkers. If these proteins can be quantified at the level of a single cell, understanding the role of proteins will be deepened and diagnosing diseases and abnormality will be further upgraded. In this study, we quantified a neurological protein in a single cell using atomic force microscopy (AFM). After capturing specifically disrupted-in-schizophrenia 1 (DISC1) in a single cell onto a microspot immobilizing the corresponding antibody on the surface, force mapping with AFM was followed to visualize individual DISC1. Although a large variation of the number of DISC1 in a cell was observed, the average number is 4.38 × 10³, and the number agrees with the ensemble-averaged value. The current AFM approach for the quantitative analysis of proteins in a single cell should be useful to study molecular behavior of proteins in depth and to follow physiological change of individual cells in response to external stimuli.

The 14-3-3 Protein Family and Schizophrenia

Schizophrenia is a debilitating mental disorder that affects approximately 1% of the world population, yet the disorder is not very well understood. The genetics of schizophrenia is very heterogenous, making it hard to pinpoint specific alterations that may cause the disorder. However, there is growing evidence from human studies suggesting a link between alterations in the 14-3-3 family and schizophrenia. The 14-3-3 proteins are abundantly expressed in the brain and are involved in many important cellular processes. Knockout of 14-3-3 proteins in mice has been shown to cause molecular, structural, and behavioral alterations associated with schizophrenia. Thus, 14-3-3 animal models allow for further exploration of the relationship between 14-3-3 and schizophrenia as well as the study of schizophrenia pathology. This review considers evidence from both human and animal model studies that implicate the 14-3-3 family in schizophrenia. In addition, possible mechanisms by which alterations in 14-3-3 proteins may contribute to schizophrenia-like phenotypes such as dopaminergic, glutamatergic, and cytoskeletal dysregulations are discussed.

Regulation of sensorimotor gating via Disc1/Huntingtin-mediated Bdnf transport in the cortico-striatal circuit

Sensorimotor information processing underlies normal cognitive and behavioral traits and has classically been evaluated through prepulse inhibition (PPI) of a startle reflex. PPI is a behavioral dimension deregulated in several neurological and psychiatric disorders, yet the mechanisms underlying the cross-diagnostic nature of PPI deficits across these conditions remain to be understood. To identify circuitry mechanisms for PPI, we performed circuitry recording over the prefrontal cortex and striatum, two brain regions previously implicated in PPI, using wild-type (WT) mice compared to Disc1-locus-impairment (LI) mice, a model representing neuropsychiatric conditions. We demonstrated that the corticostriatal projection regulates neurophysiological responses during the PPI testing in WT, whereas these circuitry responses were disrupted in Disc1-LI mice. Because our biochemical analyses revealed attenuated brain-derived neurotrophic factor (Bdnf) transport along the corticostriatal circuit in Disc1-LI mice, we investigated the potential role of Bdnf in this circuitry for regulation of PPI. Virus-mediated delivery of Bdnf into the striatum rescued PPI deficits in Disc1-LI mice. Pharmacologically augmenting Bdnf transport by chronic lithium administration, partly via phosphorylation of Huntingtin (Htt) serine-421 and its integration into the motor machinery, restored striatal Bdnf levels and rescued PPI deficits in Disc1-LI mice. Furthermore, reducing the cortical Bdnf expression negated this rescuing effect of lithium, confirming the key role of Bdnf in lithium-mediated PPI rescuing. Collectively, the data suggest that striatal Bdnf supply, collaboratively regulated by Htt and Disc1 along the corticostriatal circuit, is involved in sensorimotor gating, highlighting the utility of dimensional approach in investigating pathophysiological mechanisms across neuropsychiatric disorders.

Mutations in DISC1 alter IP3R and voltage-gated Ca2+ channel functioning, implications for major mental illness

Disrupted in Schizophrenia 1 (DISC1) participates in a wide variety of developmental processes of central neurons. It also serves critical roles that underlie cognitive functioning in adult central neurons. Here we summarize DISC1’s general properties and discuss its use as a model system for understanding major mental illnesses (MMIs). We then discuss the cellular actions of DISC1 that involve or regulate Ca²⁺ signaling in adult central neurons. In particular, we focus on the tethering role DISC1 plays in transporting RNA particles containing Ca²⁺ channel subunit RNAs, including IP3R1, CACNA1C and CACNA2D1, and in transporting mitochondria into dendritic and axonal processes. We also review DISC1’s role in modulating IP₃R1 activity within mitochondria-associated ER membrane (MAM). Finally, we discuss DISC1-glycogen synthase kinase 3β (GSK3β) signaling that regulates functional expression of voltage-gated Ca²⁺ channels (VGCCs) at central synapses. In each case, DISC1 regulates the movement of molecules that impact Ca²⁺ signaling in neurons.

Beyond Neuronal Microtubule Stabilization: MAP6 and CRMPS, Two Converging Stories

The development and function of the central nervous system rely on the microtubule (MT) and actin cytoskeletons and their respective effectors. Although the structural role of the cytoskeleton has long been acknowledged in neuronal morphology and activity, it was recently recognized to play the role of a signaling platform. Following this recognition, research into Microtubule Associated Proteins (MAPs) diversified. Indeed, historically, structural MAPs—including MAP1B, MAP2, Tau, and MAP6 (also known as STOP);—were identified and described as MT-binding and -stabilizing proteins. Extensive data obtained over the last 20 years indicated that these structural MAPs could also contribute to a variety of other molecular roles. Among multi-role MAPs, MAP6 provides a striking example illustrating the diverse molecular and cellular properties of MAPs and showing how their functional versatility contributes to the central nervous system. In this review, in addition to MAP6’s effect on microtubules, we describe its impact on the actin cytoskeleton, on neuroreceptor homeostasis, and its involvement in signaling pathways governing neuron development and maturation. We also discuss its roles in synaptic plasticity, brain connectivity, and cognitive abilities, as well as the potential relationships between the integrated brain functions of MAP6 and its molecular activities. In parallel, the Collapsin Response Mediator Proteins (CRMPs) are presented as examples of how other proteins, not initially identified as MAPs, fall into the broader MAP family. These proteins bind MTs as well as exhibiting molecular and cellular properties very similar to MAP6. Finally, we briefly summarize the multiple similarities between other classical structural MAPs and MAP6 or CRMPs.In summary, this review revisits the molecular properties and the cellular and neuronal roles of the classical MAPs, broadening our definition of what constitutes a MAP.

Mitophagy in Human Diseases

Mitophagy is a selective autophagic process, essential for cellular homeostasis, that eliminates dysfunctional mitochondria. Activated by inner membrane depolarization, it plays an important role during development and is fundamental in highly differentiated post-mitotic cells that are highly dependent on aerobic metabolism, such as neurons, muscle cells, and hepatocytes. Both defective and excessive mitophagy have been proposed to contribute to age-related neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, metabolic diseases, vascular complications of diabetes, myocardial injury, muscle dystrophy, and liver disease, among others. Pharmacological or dietary interventions that restore mitophagy homeostasis and facilitate the elimination of irreversibly damaged mitochondria, thus, could serve as potential therapies in several chronic diseases. However, despite extraordinary advances in this field, mainly derived from in vitro and preclinical animal models, human applications based on the regulation of mitochondrial quality in patients have not yet been approved. In this review, we summarize the key selective mitochondrial autophagy pathways and their role in prevalent chronic human diseases and highlight the potential use of specific interventions.

Polysialylation and disease

Polysialic acid (polySia, PSA) is a unique constituent of the glycocalyx on the surface of bacterial and vertebrate cells. In vertebrates, its biosynthesis is highly regulated, not only in quantity and quality, but also in time and location, which allows polySia to be involved in various important biological phenomena. Therefore, impairments in the expression and structure of polySia sometimes relate to diseases, such as schizophrenia, bipolar disorder, and cancer. Some bacteria express polySia as a tool for protecting themselves from the host immune system during invasion. PolySia is proven to be a biosafe material; polySia, as well as polySia-recognizing molecules, are key therapeutic agents. This review first comprehensive outlines the occurrence, features, biosynthesis, and functions of polySia and subsequently focuses on the related diseases.

Regulation and roles of mitophagy at synapses

Maintenance of synaptic homeostasis is a challenging task, due to the intricate spatial organization and intense activity of synapses. Typically, synapses are located far away from the neuronal cell body, where they orchestrate neuronal signalling and communication, through neurotransmitter release. Stationary mitochondria provide energy required for synaptic vesicle cycling, and preserve ionic balance by buffering intercellular calcium at synapses. Thus, synaptic homeostasis is critically dependent on proper mitochondrial function. Indeed, defective mitochondrial metabolism is a common feature of several neurodegenerative and psychiatric disorders, including Alzheimer's disease (AD), Parkinson's disease (PD), bipolar disorders and schizophrenia among others, which are also accompanied by excessive synaptic abnormalities. Specialized and compartmentalized quality control mechanisms have evolved to restore and maintain synaptic energy metabolism. Here, we survey recent advances towards the elucidation of the pivotal role of mitochondria in neurotransmission and implicating mitophagy in the maintenance of synaptic homeostasis during ageing.

Ser/Thr kinase Trc controls neurite outgrowth in Drosophila by modulating microtubule-microtubule sliding

Correct neuronal development requires tailored neurite outgrowth. Neurite outgrowth is driven in part by microtubule-sliding - the transport of microtubules along each other. We have recently demonstrated that a 'mitotic' kinesin-6 (Pavarotti in Drosophila) effectively inhibits microtubule-sliding and neurite outgrowth. However, mechanisms regulating Pavarotti itself in interphase cells and specifically in neurite outgrowth are unknown. Here, we use a combination of live imaging and biochemical methods to show that the inhibition of microtubule-sliding by Pavarotti is controlled by phosphorylation. We identify the Ser/Thr NDR kinase Tricornered (Trc) as a Pavarotti-dependent regulator of microtubule sliding in neurons. Further, we show that Trc-mediated phosphorylation of Pavarotti promotes its interaction with 14-3-3 proteins. Loss of 14-3-3 prevents Pavarotti from associating with microtubules. Thus, we propose a pathway by which microtubule-sliding can be up- or downregulated in neurons to control neurite outgrowth, and establish parallels between microtubule-sliding in mitosis and post-mitotic neurons.

Evidence of association of the DISC1 interactome gene set with schizophrenia from GWAS

DISC1 was discovered as a gene disrupted by a balanced translocation in a large pedigree that segregated with major mental disorders, including schizophrenia. Further attempts to find genetic association with schizophrenia were inconclusive. Most of the biology of DISC1 was inferred from the functionality of its protein partners. Recently, a gene set constituted by DISC1 and several of its partners has been associated with cognitive performance during development, a well-known schizophrenia endophenotype, by means of burden test of rare disruptive variants. Here, we performed a gene set analysis using common variants from the largest schizophrenia genome-wide association study of the Psychiatric Genomics Consortium to test if this gene set is associated with schizophrenia. The main test was based on the MAGMA software. Several additional tests were performed to analyze the robustness of the main findings. The DISC1 interactome gene set was associated with schizophrenia (P = .0056), confirmed by an additional method (INRICH). This association was robust to removal of the major histocompatibility complex region, different definitions of gene boundaries, or different statistical gene models. Conditional analysis revealed that the association was not solely explained by higher expression in brain. Three genes from the gene set, CLIC1, DST, and PDE4B, were associated with schizophrenia at the gene level. Consideration of other DISC1 interactome gene sets revealed the importance of gene set definition. Therefore, we present the first evidence from genome-wide association studies of the role of DISC1 and interacting partners in schizophrenia susceptibility, reconciling genetic and molecular biology data.

NMDAR Hypofunction Animal Models of Schizophrenia

The N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis has been proposed to help understand the etiology and pathophysiology of schizophrenia. This hypothesis was based on early observations that NMDAR antagonists could induce a full range of symptoms of schizophrenia in normal human subjects. Accumulating evidence in humans and animal studies points to NMDAR hypofunctionality as a convergence point for various symptoms of schizophrenia. Here we review animal models of NMDAR hypofunction generated by pharmacological and genetic approaches, and how they relate to the pathophysiology of schizophrenia. In addition, we discuss the limitations of animal models of NMDAR hypofunction and their potential utility for therapeutic applications.

Acute stress-induced change in polysialic acid levels mediated by sialidase in mouse brain

Stress is an important environmental factor influencing human behaviour and causing several mental disorders. Alterations in the structure of polysialic acid (polySia/PSA) due to genetic alterations in ST8SIA2, which encodes a polySia-synthesizing enzyme, are related to certain mental disorders. However, whether stress as an environmental factor leads to changes in polySia structure is unknown. Here we studied the effects of acute stress on polySia expression and found reductions in both the quantity and quality of polySia in the olfactory bulb and prefrontal cortex, even with short-term exposure to acute stress. The use of inhibitors for sialidase, microglia and astrocytes revealed that these declines were due to a transient action of sialidase from microglia and astrocytes in the olfactory bulb and prefrontal cortex, respectively. These data suggest that sialidase dynamically regulates polySia expression in a brain region-specific manner.

Influence of clozapine on neurodevelopmental protein expression and behavioral patterns in animal model of psychiatric disorder induced by low-level of lead

Exposure to lead during pregnancy is a risk factor for the development of psychiatric disorders in the offspring. In this study, we investigated whether exposure to low levels of lead acetate (0.2%) in drinking water during pregnancy and lactation causes behavioral impairment and affects the expression of proteins associated with neurodevelopment. Lead exposure altered several parameters in rat offspring compared with those unexposed in open-field, social interaction, and pre-pulse inhibition tests. These parameters were restored to normal levels after clozapine treatment. Western blot and immunohistochemical analyses of the hippocampus revealed that several neurodevelopmental proteins were downregulated in lead-exposed rats. The expression was normalized after clozapine treatment (5 mg/kg/day, postnatal day 35–56). These findings demonstrate that downregulation of several proteins in lead-exposed rats affected subsequent behavioral changes. Our results suggest that lead exposure in early life may induce psychiatric disorders and treatment with antipsychotics such as clozapine may reduce their incidence.

Proteomic Studies Reveal Disrupted in Schizophrenia 1 as a Player in Both Neurodevelopment and Synaptic Function

A balanced chromosomal translocation disrupting DISC1 (Disrupted in Schizophrenia 1) gene has been linked to psychiatric diseases, such as major depression, bipolar disorder and schizophrenia. Since the discovery of this translocation, many studies have focused on understating the role of the truncated isoform of DISC1, hypothesizing that the gain of function of this protein could be behind the neurobiology of mental conditions, but not so many studies have focused in the mechanisms impaired due to its loss of function. For that reason, we performed an analysis on the cellular proteome of primary neurons in which DISC1 was knocked down with the goal of identifying relevant pathways directly affected by DISC1 loss of function. Using an unbiased proteomic approach, we found that the expression of 31 proteins related to neurodevelopment (e.g., CRMP-2, stathmin) and synaptic function (e.g., MUNC-18, NCS-1) is altered by DISC1 in primary mouse neurons. Hence, this study reinforces the idea that DISC1 is a unifying regulator of both neurodevelopment and synaptic function, thereby providing a link between these two key anatomical and cellular circuitries.

The TRAX, DISC1, and GSK3 complex in mental disorders and therapeutic interventions

Psychiatric disorders (such as bipolar disorder, depression, and schizophrenia) affect the lives of millions of individuals worldwide. Despite the tremendous efforts devoted to various types of psychiatric studies and rapidly accumulating genetic information, the molecular mechanisms underlying psychiatric disorder development remain elusive. Among the genes that have been implicated in schizophrenia and other mental disorders, disrupted in schizophrenia 1 (DISC1) and glycogen synthase kinase 3 (GSK3) have been intensively investigated. DISC1 binds directly to GSK3 and modulates many cellular functions by negatively inhibiting GSK3 activity. The human DISC1 gene is located on chromosome 1 and is highly associated with schizophrenia and other mental disorders. A recent study demonstrated that a neighboring gene of DISC1, translin-associated factor X (TRAX), binds to the DISC1/GSK3β complex and at least partly mediates the actions of the DISC1/GSK3β complex. Previous studies also demonstrate that TRAX and most of its interacting proteins that have been identified so far are risk genes and/or markers of mental disorders. In the present review, we will focus on the emerging roles of TRAX and its interacting proteins (including DISC1 and GSK3β) in psychiatric disorders and the potential implications for developing therapeutic interventions.

TAR DNA-Binding Protein 43 and Disrupted in Schizophrenia 1 Coaggregation Disrupts Dendritic Local Translation and Mental Function in Frontotemporal Lobar Degeneration

BACKGROUND

Neurodegenerative diseases involving protein aggregation often accompany psychiatric symptoms. Frontotemporal lobar degeneration (FTLD) associated with TAR DNA-binding protein 43 (TDP-43) aggregation is characterized by progressive neuronal atrophy in frontal and temporal lobes of cerebral cortex. Furthermore, patients with FTLD display mental dysfunction in multiple behavioral dimensions. Nevertheless, their molecular origin for psychiatric symptoms remains unclear.

METHODS

In FTLD neurons and mouse models with TDP-43 aggregates, we examined coaggregation between TDP-43 and disrupted in schizophrenia 1 (DISC1), a key player in the pathology of mental conditions and its effects on local translation in dendrites and psychiatric behaviors. The protein coaggregation and the expression level of synaptic proteins were also investigated with postmortem brains from patients with FTLD (n = 6).

RESULTS

We found cytosolic TDP-43/DISC1 coaggregates in brains of both FTLD mouse model and patients with FTLD. At the mechanistic levels, the TDP-43/DISC1 coaggregates disrupted the activity-dependent dendritic local translation through impairment of translation initiation and, in turn, reduced synaptic protein expression. Behavioral deficits detected in FTLD model mice were ameliorated by exogenous DISC1 expression.

CONCLUSIONS

Our findings reveal a novel role of the aggregate-prone TDP-43/DISC1 protein complex in regulating local translation, which affects aberrant behaviors relevant to multiple psychiatric dimensions.

Mental disorders and an acidic glycan-from the perspective of polysialic acid (PSA/polySia) and the synthesizing enzyme, ST8SIA2

Mental disorders, such as schizophrenia, bipolar disorder, and autism spectrum disorder, are challenging to manage, worldwide. Understanding the molecular mechanisms underlying these disorders is essential and required. Studies investigating such molecular mechanisms are well performed and important findings are accumulating apace. Based on the fact that these disorders are due in part to the accumulation of genetic and environmental risk factors, consideration of multi-molecular and/or multi-system dependent phenomena might be important. Acidic glycans are an attractive family of molecules for understanding these disorders, because impairment of the fine-tuned glycan system affects a large number of molecules that are deeply involved in normal brain function. One of the candidates of this important family of glycan epitopes in the brain is polysialic acid (PSA/polySia). PSA is a well-known molecule because of its role as an oncodevelopmental antigen and is also widely used as a marker of adult neurogenesis. Recently, several reports have suggested that PSA and PSA-related genes are associated with multiple mental disorders. The relationships among PSA, PSA-related genes, and mental disorders are reviewed here.

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.

Transfer of disrupted-in-schizophrenia 1 aggregates between neuronal-like cells occurs in tunnelling nanotubes and is promoted by dopamine

The disrupted-in-schizophrenia 1 (DISC1) gene was identified as a genetic risk factor for chronic mental illnesses (CMI) such as schizophrenia, bipolar disorder and severe recurrent depression. Insoluble aggregated DISC1 variants were found in the cingular cortex of sporadic, i.e. non-genetic, CMI patients. This suggests protein pathology as a novel, additional pathogenic mechanism, further corroborated in a recent transgenic rat model presenting DISC1 aggregates. Since the potential role of aggregation of DISC1 in sporadic CMI is unknown, we investigated whether DISC1 undergoes aggregation in cell culture and could spread between neuronal cells in a prion-like manner, as shown for amyloid proteins in neurodegenerative diseases. Co-culture experiments between donor cells forming DISC1 aggregates and acceptor cells showed that 4.5% of acceptor cells contained donor-derived DISC1 aggregates, thus indicating an efficient transfer in vitro. DISC1 aggregates were found inside tunnelling nanotubes (TNTs) and transfer was enhanced by increasing TNT formation and notably by dopamine treatment, which also induces DISC1 aggregation. These data indicate that DISC1 aggregates can propagate between cells similarly to prions, thus providing some molecular basis for the role of protein pathology in CMI.

PSD95: A synaptic protein implicated in schizophrenia or autism?

The molecular components of the postsynaptic density (PSD) in excitatory synapses of the brain are currently being investigated as one of the major etiologies of neurodevelopmental disorders such as schizophrenia (SCZ) and autism. Postsynaptic density protein-95 (PSD-95) is a major regulator of synaptic maturation by interacting, stabilizing and trafficking N-methyl-d-aspartic acid receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isox-azoleproprionic acid receptors (AMPARs) to the postsynaptic membrane. Recently, there has been overwhelming evidence that associates PSD-95 disruption with cognitive and learning deficits observed in SCZ and autism. For instance, recent genomic and sequencing studies of psychiatric patients highlight the aberrations at the PSD of glutamatergic synapses that include PSD-95 dysfunction. In animal studies, PSD-95 deficiency shows alterations in NMDA and AMPA-receptor composition and function in specific brain regions that may contribute to phenotypes observed in neuropsychiatric pathologies. In this review, we describe the role of PSD-95 as an essential scaffolding protein during synaptogenesis and neurodevelopment. More specifically, we discuss its interactions with NMDA receptor subunits that potentially affect glutamate transmission, and the formation of silent synapses during critical time points of neurodevelopment. Furthermore, we describe how PSD-95 may alter dendritic spine morphologies, thus regulating synaptic function that influences behavioral phenotypes in SCZ versus autism. Understanding the role of PSD-95 in the neuropathologies of SCZ and autism will give an insight of the cellular and molecular attributes in the disorders, thus providing treatment options in patients affected.

Kinesin 1 regulates cilia length through an interaction with the Bardet-Biedl syndrome related protein CCDC28B

Bardet-Biedl syndrome (BBS) is a ciliopathy characterized by retinal degeneration, obesity, polydactyly, renal disease and mental retardation. CCDC28B is a BBS-associated protein that we have previously shown plays a role in cilia length regulation whereby its depletion results in shortened cilia both in cells and Danio rerio (zebrafish). At least part of that role is achieved by its interaction with the mTORC2 component SIN1, but the mechanistic details of this interaction and/or additional functions that CCDC28B might play in the context of cilia remain poorly understood. Here we uncover a novel interaction between CCDC28B and the kinesin 1 molecular motor that is relevant to cilia. CCDC28B interacts with kinesin light chain 1 (KLC1) and the heavy chain KIF5B. Notably, depletion of these kinesin 1 components results in abnormally elongated cilia. Furthermore, through genetic interaction studies we demonstrate that kinesin 1 regulates ciliogenesis through CCDC28B. We show that kinesin 1 regulates the subcellular distribution of CCDC28B, unexpectedly, inhibiting its nuclear accumulation, and a ccdc28b mutant missing a nuclear localization motif fails to rescue the phenotype in zebrafish morphant embryos. Therefore, we uncover a previously unknown role of kinesin 1 in cilia length regulation that relies on the BBS related protein CCDC28B.

Mitochondrial roles of the psychiatric disease risk factor DISC1

Ion transport during neuronal signalling utilizes the majority of the brain's energy supply. Mitochondria are key sites for energy provision through ATP synthesis and play other important roles including calcium buffering. Thus, tightly regulated distribution and function of these organelles throughout the intricate architecture of the neuron is essential for normal synaptic communication. Therefore, delineating mechanisms coordinating mitochondrial transport and function is essential for understanding nervous system physiology and pathology. While aberrant mitochondrial transport and dynamics have long been associated with neurodegenerative disease, they have also more recently been linked to major mental illness including schizophrenia, autism and depression. However, the underlying mechanisms have yet to be elucidated, due to an incomplete understanding of the combinations of genetic and environmental factors contributing to these conditions. Consequently, the DISC1 gene has undergone intense study since its discovery at the site of a balanced chromosomal translocation, segregating with mental illness in a Scottish pedigree. The precise molecular functions of DISC1 remain elusive. Reported functions of DISC1 include regulation of intracellular signalling pathways, neuronal migration and dendritic development. Intriguingly, a role for DISC1 in mitochondrial homeostasis and transport is fast emerging. Therefore, a major function of DISC1 in regulating mitochondrial distribution, ATP synthesis and calcium buffering may be disrupted in psychiatric disease. In this review, we discuss the links between DISC1 and mitochondria, considering both trafficking of these organelles and their function, and how, via these processes, DISC1 may contribute to the regulation of neuronal behavior in normal and psychiatric disease states.

Dynein binds and stimulates axonal motility of the endosome adaptor and NEEP21 family member, calcyon

The neuron-enriched, endosomal protein Calcyon (Caly) regulates endocytosis and vesicle sorting, and is important for synaptic plasticity and brain development. In the current investigation of Caly interacting proteins in brain, the microtubule retrograde motor subunit, cytoplasmic dynein 1 heavy chain (DYNC1H), and microtubule structural proteins, α and β tubulin, were identified as Caly associated proteins by MALDI-ToF/ToF. Direct interaction of the Caly-C terminus with dynein and tubulin was further confirmed in in vitro studies. In Cos-7 cells, mCherry-Caly moved along the microtubule network in organelles largely labeled by the late endosome marker Rab7. Expression of the dynein inhibitor CC1, produced striking alterations in Caly distribution, consistent with retrograde motors playing a prominent role in Caly localization and movement. In axons of cultured adult rat sensory neurons, Caly-positive organelles co-localized with dynein intermediate chain (DYNC1I1-isoform IC-1B) and the dynein regulator, lissencephaly 1 (LIS1), both of which co-precipitated from brain with the Caly C-terminus. Manipulation of dynein function in axons altered the motile properties of Caly indicating that Caly vesicles utilize the retrograde motor. Altogether, the current evidence for association with dynein motors raises the possibility that the endocytic and cargo sorting functions of Caly in neurons could be regulated by interaction with the microtubule transport system.

Role of DISC1 in Neuronal Trafficking and its Implication in Neuropsychiatric Manifestation and Neurotherapeutics

Disrupted-in-schizophrenia 1 (DISC1) was initially identified as a gene disrupted by a translocation mutation co-segregating with a variety of psychotic and mood disorders in a Scottish pedigree. In agreement with this original finding, mouse models that perturb Disc1 display deficits of behaviors in specific dimensions, such as cognition and emotion, but not a motor dimension. Although DISC1 is not a risk gene for sporadic cases of specific psychiatric disorders defined by categorical diagnostic criteria (e.g., schizophrenia and major depressive disorder), DISC1 is now regarded as an important molecular lead to decipher molecular pathology for specific dimensions relevant to major mental illnesses. Emerging evidence points to the role of DISC1 in the regulation of intracellular trafficking of a wide range of neuronal cargoes. We will review recent progress in this aspect of DISC1 biology and discuss how we could utilize this body of knowledge to better understand the pathophysiology of mental illnesses.

Excessive activation of AhR signaling disrupts neuronal migration in the hippocampal CA1 region in the developing mouse

The aryl hydrocarbon receptor (AhR) avidly binds dioxin, a ubiquitous environmental contaminant. Disruption of downstream AhR signaling has been reported to alter neuronal development, and rodent offspring exposed to dioxin during gestation and lactation showed abnormalities in learning and memory, emotion, and social behavior. However, the mechanism behind the disrupted AhR signaling and developmental neurotoxicity induced by xenobiotic ligands remains elusive. Therefore, we studied how excessive AhR activation affects neuronal migration in the hippocampal CA1 region of the developing mouse brain. We transfected constitutively active (CA)-AhR, AhR, or control vector plasmids into neurons via in utero electroporation on gestational day 14 and analyzed neuronal positioning in the hippocampal CA1 region of offspring on postnatal day 14. CA-AhR transfection affected neuronal positioning, whereas no change was observed in AhR-transfected or control hippocampus. These results suggest that constitutively activated AhR signaling disrupts neuronal migration during hippocampal development. Further studies are needed to investigate whether such developmental disruption in the hippocampus leads to the abnormal cognition and behavior of rodent offspring upon maternal exposure to AhR xenobiotic ligands.

IP3 receptor mutations and brain diseases in human and rodents

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃ R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃ , which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃ R genes, which have distinct properties of IP₃ -binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃ R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃ Rs, the type 1 receptor (IP₃ R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃ R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃ R1 and its functional implication in human brain diseases, as revealed by IP₃ R mutation studies and analysis of human disease-associated genes. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Primary Cilia as a Possible Link between Left-Right Asymmetry and Neurodevelopmental Diseases

Cilia have multiple functions in the development of the entire organism, and participate in the development and functioning of the central nervous system. In the last decade, studies have shown that they are implicated in the development of the visceral left-right asymmetry in different vertebrates. At the same time, some neuropsychiatric disorders, such as schizophrenia, autism, bipolar disorder, and dyslexia, are known to be associated with lateralization failure. In this review, we consider possible links in the mechanisms of determination of visceral asymmetry and brain lateralization, through cilia. We review the functions of seven genes associated with both cilia, and with neurodevelopmental diseases, keeping in mind their possible role in the establishment of the left-right brain asymmetry.

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.

PLEKHM1/DEF8/RAB7 complex regulates lysosome positioning and bone homeostasis

Mutations of the Plekhm1 gene in humans and rats cause osteopetrosis, an inherited bone disease characterized by diminished bone resorption by osteoclasts. PLEKHM1 binds to RAB7 and is critical for lysosome trafficking. However, the molecular mechanisms by which PLEKHM1 regulates lysosomal pathways remain unknown. Here, we generated germline and conditional Plekhm1-deficient mice. These mice displayed no overt abnormalities in major organs, except for an increase in trabecular bone mass. Furthermore, loss of PLEKHM1 abrogated the peripheral distribution of lysosomes and bone resorption in osteoclasts. Mechanistically, we indicated that DEF8 interacts with PLEKHM1 and promotes its binding to RAB7, whereas the binding of FAM98A and NDEL1 with PLEKHM1 connects lysosomes to microtubules. Importantly, suppression of these proteins results in lysosome positioning and bone resorption defects similar to those of Plekhm1-null osteoclasts. Thus, PLHKEM1, DEF8, FAM98A, and NDEL1 constitute a molecular complex that regulates lysosome positioning and secretion through RAB7.

Early postnatal GABAA receptor modulation reverses deficits in neuronal maturation in a conditional neurodevelopmental mouse model of DISC1

Exploring drug targets based on disease-associated molecular mechanisms during development is crucial for the generation of novel prevention and treatment strategies for neurodevelopmental psychiatric conditions. We report that prefrontal cortex (PFC)-specific postnatal knockdown of DISC1 via in utero electroporation combined with an inducible knockdown expression system drives deficits in synaptic GABAA function and dendritic development in pyramidal neurons, as well as abnormalities in sensorimotor gating, albeit without profound memory deficits. We show for the first time that DISC1 is specifically involved in regulating cell surface expression of α2 subunit-containing GABAA receptors in immature developing neurons, but not after full maturation. Notably, pharmacological intervention with α2/3 subtype-selective GABAA receptor positive allosteric modulators during the early postnatal period ameliorates dendritic deficits and behavioral abnormalities induced by knockdown of DISC1. These findings highlight a critical role of DISC1-mediated disruption of postnatal GABA signaling in aberrant PFC maturation and function.

Dynein Separately Partners with NDE1 and Dynactin To Orchestrate T Cell Focused Secretion

Helper and cytotoxic T cells accomplish focused secretion through the movement of vesicles toward the microtubule organizing center (MTOC) and translocation of the MTOC to the target contact site. In this study, using Jurkat cells and OT-I TCR transgenic primary murine CTLs, we show that the dynein-binding proteins nuclear distribution E homolog 1 (NDE1) and dynactin (as represented by p150(Glued)) form mutually exclusive complexes with dynein, exhibit nonoverlapping distributions in target-stimulated cells, and mediate different transport events. When Jurkat cells expressing a dominant negative form of NDE1 (NDE1-enhanced GFP fusion) were activated by Staphylococcus enterotoxin E-coated Raji cells, NDE1 and dynein failed to accumulate at the immunological synapse (IS) and MTOC translocation was inhibited. Knockdown of NDE1 in Jurkat cells or primary mouse CTLs also inhibited MTOC translocation and CTL-mediated killing. In contrast to NDE1, knockdown of p150(Glued), which depleted the alternative dynein/dynactin complex, resulted in impaired accumulation of CTLA4 and granzyme B-containing intracellular vesicles at the IS, whereas MTOC translocation was not affected. Depletion of p150(Glued) in CTLs also inhibited CTL-mediated lysis. We conclude that the NDE1/Lissencephaly 1 and dynactin complexes separately mediate two key components of T cell-focused secretion, namely translocation of the MTOC and lytic granules to the IS, respectively.

Upstream Pathways Controlling Mitochondrial Function in Major Psychosis: A Focus on Bipolar Disorder

Mitochondrial dysfunction is commonly observed in bipolar disorder (BD) and schizophrenia (SCZ) and may be a central feature of psychosis. These illnesses are complex and heterogeneous, which is reflected by the complexity of the processes regulating mitochondrial function. Mitochondria are typically associated with energy production; however, dysfunction of mitochondria affects not only energy production but also vital cellular processes, including the formation of reactive oxygen species, cell cycle and survival, intracellular Ca(2+) homeostasis, and neurotransmission. In this review, we characterize the upstream components controlling mitochondrial function, including 1) mutations in nuclear and mitochondrial DNA, 2) mitochondrial dynamics, and 3) intracellular Ca(2+) homeostasis. Characterizing and understanding the upstream factors that regulate mitochondrial function is essential to understand progression of these illnesses and develop biomarkers and therapeutics.

Mitochondrial dysfunction in bipolar disorder: Evidence, pathophysiology and translational implications

Bipolar disorder (BD) is a chronic psychiatric illness characterized by severe and biphasic changes in mood. Several pathophysiological mechanisms have been hypothesized to underpin the neurobiology of BD, including the presence of mitochondrial dysfunction. A confluence of evidence points to an underlying dysfunction of mitochondria, including decreases in mitochondrial respiration, high-energy phosphates and pH; changes in mitochondrial morphology; increases in mitochondrial DNA polymorphisms; and downregulation of nuclear mRNA molecules and proteins involved in mitochondrial respiration. Mitochondria play a pivotal role in neuronal cell survival or death as regulators of both energy metabolism and cell survival and death pathways. Thus, in this review, we discuss the genetic and physiological components of mitochondria and the evidence for mitochondrial abnormalities in BD. The final part of this review discusses mitochondria as a potential target of therapeutic interventions in BD.

DISC1 is a coordinator of intracellular trafficking to shape neuronal development and connectivity

The long, asymmetric and specialised architecture of neuronal processes necessitates a properly regulated transport network of molecular motors and cytoskeletal tracks. This allows appropriate distribution of cargo for correct formation and activity of the synapse, and thus normal neuronal communication. This communication is impaired in psychiatric disease, and ongoing studies have proposed that Disrupted in schizophrenia 1 (DISC1) is an important genetic risk factor for these disorders. The mechanisms by which DISC1 dysfunction might increase propensity to psychiatric disease are not completely understood; however, an emerging theme is that DISC1 can function as a key regulator of neuronal intracellular trafficking. Transport of a wide range of potential cargoes - including mRNAs, neurotransmitter receptors, vesicles and mitochondria - can be modulated by DISC1, and therefore is susceptible to DISC1 dysfunction. This theme highlights the importance of understanding precisely how DISC1 can regulate intracellular trafficking, and suggests that a novel approach to the treatment of psychiatric disorders could be provided by targeting this protein and the trafficking machinery with which it interacts.

14-3-3 eta isoform colocalizes TDP-43 on the coarse granules in the anterior horn cells of patients with sporadic amyotrophic lateral sclerosis

The immunolocalization of the 14-3-3 eta isoform in the anterior horn cells (AHCs) of patients with sporadic amyotrophic lateral sclerosis (ALS) and controls was examined. Compared with the immunolocalization of other 14-3-3 isoforms, the immunolocalization of the 14-3-3 eta isoform was either synaptic at the periphery of AHCs, spindle-shaped in neurites, or granular in the cytoplasm. By double labeling with phosphorylated (p-)TDP-43, the transactivation response DNA binding protein of 43kDa (TDP-43) demonstrated frequent colocalization of the 14-3-3 eta isoform in granular structures (90%) and spindle-shaped structures (85.4%), but not in p-TDP-43-positive round inclusions. It is speculated that the 14-3-3 eta isoform is associated with not only a synaptic pathology of ALS but also TDP-positive small lesions in the cytoplasm and neurites. The absence of eta-like immunoreactivity in p-TDP-43-positive large inclusions suggests the restricted relevance of the 14-3-3 eta isoform during ALS pathogenesis to some phases of the p-TDP pathology.

Identification of the BRD1 interaction network and its impact on mental disorder risk

Background

The bromodomain containing 1 (BRD1) gene has been implicated with transcriptional regulation, brain development, and susceptibility to schizophrenia and bipolar disorder. To advance the understanding of BRD1 and its role in mental disorders, we characterized the protein and chromatin interactions of the BRD1 isoforms, BRD1-S and BRD1-L.

Methods

Stable human cell lines expressing epitope tagged BRD1-S and BRD1-L were generated and used as discovery systems for identifying protein and chromatin interactions. Protein-protein interactions were identified using co-immunoprecipitation followed by mass spectrometry and chromatin interactions were identified using chromatin immunoprecipitation followed by next generation sequencing. Gene expression profiles and differentially expressed genes were identified after upregulating and downregulating BRD1 expression using microarrays. The presented functional molecular data were integrated with human genomic and transcriptomic data using available GWAS, exome-sequencing datasets as well as spatiotemporal transcriptomic datasets from the human brain.

Results

We present several novel protein interactions of BRD1, including isoform-specific interactions as well as proteins previously implicated with mental disorders. By BRD1-S and BRD1-L chromatin immunoprecipitation followed by next generation sequencing we identified binding to promoter regions of 1540 and 823 genes, respectively, and showed correlation between BRD1-S and BRD1-L binding and regulation of gene expression. The identified BRD1 interaction network was found to be predominantly co-expressed with BRD1 mRNA in the human brain and enriched for pathways involved in gene expression and brain function. By interrogation of large datasets from genome-wide association studies, we further demonstrate that the BRD1 interaction network is enriched for schizophrenia risk.

Conclusion

Our results show that BRD1 interacts with chromatin remodeling proteins, e.g. PBRM1, as well as histone modifiers, e.g. MYST2 and SUV420H1. We find that BRD1 primarily binds in close proximity to transcription start sites and regulates expression of numerous genes, many of which are involved with brain development and susceptibility to mental disorders. Our findings indicate that BRD1 acts as a regulatory hub in a comprehensive schizophrenia risk network which plays a role in many brain regions throughout life, implicating e.g. striatum, hippocampus, and amygdala at mid-fetal stages.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-016-0308-x) contains supplementary material, which is available to authorized users.

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.

DISC1 Protein Regulates γ-Aminobutyric Acid, Type A (GABAA) Receptor Trafficking and Inhibitory Synaptic Transmission in Cortical Neurons

Association studies have suggested that Disrupted-in-Schizophrenia 1 (DISC1) confers a genetic risk at the level of endophenotypes that underlies many major mental disorders. Despite the progress in understanding the significance of DISC1 at neural development, the mechanisms underlying DISC1 regulation of synaptic functions remain elusive. Because alterations in the cortical GABA system have been strongly linked to the pathophysiology of schizophrenia, one potential target of DISC1 that is critically involved in the regulation of cognition and emotion is the GABAA receptor (GABAAR). We found that cellular knockdown of DISC1 significantly reduced GABAAR-mediated synaptic and whole-cell current, whereas overexpression of wild-type DISC1, but not the C-terminal-truncated DISC1 (a schizophrenia-related mutant), significantly increased GABAAR currents in pyramidal neurons of the prefrontal cortex. These effects were accompanied by DISC1-induced changes in surface GABAAR expression. Moreover, the regulation of GABAARs by DISC1 knockdown or overexpression depends on the microtubule motor protein kinesin 1 (KIF5). Our results suggest that DISC1 exerts an important effect on GABAergic inhibitory transmission by regulating KIF5/microtubule-based GABAAR trafficking in the cortex. The knowledge gained from this study would shed light on how DISC1 and the GABA system are linked mechanistically and how their interactions are critical for maintaining a normal mental state.

Inhibition of 14-3-3 Proteins Leads to Schizophrenia-Related Behavioral Phenotypes and Synaptic Defects in Mice

BACKGROUND

The 14-3-3 family of proteins is implicated in the regulation of several key neuronal processes. Previous human and animal studies suggested an association between 14-3-3 dysregulation and schizophrenia.

METHODS

We characterized behavioral and functional changes in transgenic mice that express an isoform-independent 14-3-3 inhibitor peptide in the brain.

RESULTS

We recently showed that 14-3-3 functional knockout mice (FKO) exhibit impairments in associative learning and memory. We report here that these 14-3-3 FKO mice display other behavioral deficits that correspond to the core symptoms of schizophrenia. These behavioral deficits may be attributed to alterations in multiple neurotransmission systems in the 14-3-3 FKO mice. In particular, inhibition of 14-3-3 proteins results in a reduction of dendritic complexity and spine density in forebrain excitatory neurons, which may underlie the altered synaptic connectivity in the prefrontal cortical synapse of the 14-3-3 FKO mice. At the molecular level, this dendritic spine defect may stem from dysregulated actin dynamics secondary to a disruption of the 14-3-3-dependent regulation of phosphorylated cofilin.

CONCLUSIONS

Collectively, our data provide a link between 14-3-3 dysfunction, synaptic alterations, and schizophrenia-associated behavioral deficits.

Extracellular and Intracellular Signaling for Neuronal Polarity

Neurons are one of the highly polarized cells in the body. One of the fundamental issues in neuroscience is how neurons establish their polarity; therefore, this issue fascinates many scientists. Cultured neurons are useful tools for analyzing the mechanisms of neuronal polarization, and indeed, most of the molecules important in their polarization were identified using culture systems. However, we now know that the process of neuronal polarization in vivo differs in some respects from that in cultured neurons. One of the major differences is their surrounding microenvironment; neurons in vivo can be influenced by extrinsic factors from the microenvironment. Therefore, a major question remains: How are neurons polarized in vivo? Here, we begin by reviewing the process of neuronal polarization in culture conditions and in vivo. We also survey the molecular mechanisms underlying neuronal polarization. Finally, we introduce the theoretical basis of neuronal polarization and the possible involvement of neuronal polarity in disease and traumatic brain injury.

NEURODEVELOPMENT. Adult cortical plasticity depends on an early postnatal critical period

Development of the cerebral cortex is influenced by sensory experience during distinct phases of postnatal development known as critical periods. Disruption of experience during a critical period produces neurons that lack specificity for particular stimulus features, such as location in the somatosensory system. Synaptic plasticity is the agent by which sensory experience affects cortical development. Here, we describe, in mice, a developmental critical period that affects plasticity itself. Transient neonatal disruption of signaling via the C-terminal domain of "disrupted in schizophrenia 1" (DISC1)—a molecule implicated in psychiatric disorders—resulted in a lack of long-term potentiation (LTP) (persistent strengthening of synapses) and experience-dependent potentiation in adulthood. Long-term depression (LTD) (selective weakening of specific sets of synapses) and reversal of LTD were present, although impaired, in adolescence and absent in adulthood. These changes may form the basis for the cognitive deficits associated with mutations in DISC1 and the delayed onset of a range of psychiatric symptoms in late adolescence.

Network Cluster Analysis of Protein-Protein Interaction Network-Identified Biomarker for Type 2 Diabetes

Type 2 diabetes mellitus (T2DM) is a complex disease that is caused by an impairment in the secretion of β-cell insulin and by a peripheral resistance to insulin. Most patients suffering from T2DM and from obesity exhibit insulin resistance in the muscles, liver, and fat, resulting in a reduced response of these tissues to insulin. In healthy individuals, pancreatic islet β-cells secrete insulin to regulate the increase in blood glucose levels. Once these β-cells fail to function, T2DM develops. Despite the progress achieved in this field in recent years, the genetic causes for insulin resistance and for T2DM have not yet been fully discovered. The present study aims to characterize T2DM by comparing its gene expression with that of normal controls, as well as to identify biomarkers for early T2DM. Gene expression profiles were downloaded from the Gene Expression Omnibus, and differentially expressed genes (DEGs) were identified for type 2 diabetes. Furthermore, functional analyses were conducted for the gene ontology and for the pathway enrichment. In total, 781 DEGs were identified in the T2DM samples relative to healthy controls. These genes were found to be involved in several biological processes, including cell communication, cell proliferation, cell shape, and apoptosis. We constructed a protein-protein interaction (PPI) network, and the clusters in the PPI were analyzed by using ClusterONE. Six functional genes that may play important roles in the initiation of T2DM were identified within the network.

Potential involvement of kinesin-1 in the regulation of subcellular localization of Girdin

Girdin is an actin-binding protein that has multiple functions in postnatal neural development and cancer progression. We previously showed that Girdin is a regulator of migration for neuroblasts born from neural stem cells in the subventricular zone (SVZ) and the dentate gyrus of the hippocampus in the postnatal brain. Despite a growing list of Girdin-interacting proteins, the mechanism of Girdin-mediated migration has not been fully elucidated. Girdin interacts with Disrupted-In-Schizophrenia 1 and partitioning-defective 3, both of which have been shown to interact with the kinesin microtubule motor proteins. Based on this, we have identified that Girdin also interacts with kinesin-1, a member of neuronal kinesin proteins. Although a direct interaction of Girdin and kinesin-1 has not been determined, it is of interest to find that Girdin loss-of-function mutant mice with the mutation of a basic amino acid residue-rich region (Basic mut mice) exhibit limited interaction with kinesin-1. Furthermore, expression of a kinesin-1 mutant with motor defects, leads to Girdin mislocalization. Finally, consistent with previous studies on the role of kinesin proteins in trafficking a cell-cell adhesion molecule N-cadherin, Basic mut mice showed an aberrant expression pattern of N-cadherin in migrating SVZ neuroblasts. These findings suggest a potential role of Girdin/kinesin-1 interaction in the regulation of neuroblast migration in the postnatal brain.

Identification of Rare, Single-Nucleotide Mutations in NDE1 and Their Contributions to Schizophrenia Susceptibility

BACKGROUND

Nuclear distribution E homolog 1 (NDE1), located within chromosome 16p13.11, plays an essential role in microtubule organization, mitosis, and neuronal migration and has been suggested by several studies of rare copy number variants to be a promising schizophrenia (SCZ) candidate gene. Recently, increasing attention has been paid to rare single-nucleotide variants (SNVs) discovered by deep sequencing of candidate genes, because such SNVs may have large effect sizes and their functional analysis may clarify etiopathology.

METHODS AND RESULTS

We conducted mutation screening of NDE1 coding exons using 433 SCZ and 145 pervasive developmental disorders samples in order to identify rare single nucleotide variants with a minor allele frequency ≤5%. We then performed genetic association analysis using a large number of unrelated individuals (3554 SCZ, 1041 bipolar disorder [BD], and 4746 controls). Among the discovered novel rare variants, we detected significant associations between SCZ and S214F (P = .039), and between BD and R234C (P = .032). Furthermore, functional assays showed that S214F affected axonal outgrowth and the interaction between NDE1 and YWHAE (14-3-3 epsilon; a neurodevelopmental regulator).

CONCLUSIONS

This study strengthens the evidence for association between rare variants within NDE1 and SCZ, and may shed light into the molecular mechanisms underlying this severe psychiatric disorder.