Interactions of human truncated DISC1 proteins: implications for schizophrenia

DISC1 and reelin interact to alter cognition, inhibition, and neurogenesis in a novel mouse model of schizophrenia

The etiology of schizophrenia (SCZ) is multifactorial, and depending on a host of genetic and environmental factors. Two putative SCZ susceptibility genes, Disrupted-in-Schizophrenia-1 (DISC1) and reelin (RELN), interact at a molecular level, suggesting that combined disruption of both may lead to an intensified SCZ phenotype. To examine this gene-gene interaction, we produced a double mutant mouse line. Mice with heterozygous RELN haploinsufficiency were crossed with mice expressing dominant-negative c-terminal truncated human DISC1 to produce offspring with both mutations (HRM/DISC1 mice). We used an array of behavioral tests to generate a behavioral phenotype for these mice, then examined the prefrontal cortex and hippocampus using western blotting and immunohistochemistry to probe for SCZ-relevant molecular and cellular alterations. Compared to wild-type controls, HRM/DISC1 mice demonstrated impaired pre-pulse inhibition, altered cognition, and decreased activity. Diazepam failed to rescue anxiety-like behaviors, paradoxically increasing activity in HRM/DISC1 mice. At a cellular level, we found increased α1-subunit containing GABA receptors in the prefrontal cortex, and a reduction in fast-spiking parvalbumin positive neurons. Maturation of adult-born neurons in the hippocampus was also altered in HRM/DISC1 mice. While there was no difference in the total number proliferating cells, more of these cells were in immature stages of development. Homozygous DISC1 mutation combined with RELN haploinsufficiency produces a complex phenotype with neuropsychiatric characteristics relevant to SCZ and related disorders, expanding our understanding of how multiple genetic susceptibility factors might interact to influence the variable presentation of these disorders.

An alternative splicing hypothesis for neuropathology of schizophrenia: evidence from studies on historical candidate genes and multi-omics data

Alternative splicing of schizophrenia risk genes, such as DRD2, GRM3, and DISC1, has been extensively described. Nevertheless, the alternative splicing characteristics of the growing number of schizophrenia risk genes identified through genetic analyses remain relatively opaque. Recently, transcriptomic analyses in human brains based on short-read RNA-sequencing have discovered many "local splicing" events (e.g., exon skipping junctions) associated with genetic risk of schizophrenia, and further molecular characterizations have identified novel spliced isoforms, such as AS3MT^d2d3 and ZNF804A^E3E4. In addition, long-read sequencing analyses of schizophrenia risk genes (e.g., CACNA1C and NRXN1) have revealed multiple previously unannotated brain-abundant isoforms with therapeutic potentials, and functional analyses of KCNH2-3.1 and Ube3a1 have provided examples for investigating such spliced isoforms in vitro and in vivo. These findings suggest that alternative splicing may be an essential molecular mechanism underlying genetic risk of schizophrenia, however, the incomplete annotations of human brain transcriptomes might have limited our understanding of schizophrenia pathogenesis, and further efforts to elucidate these transcriptional characteristics are urgently needed to gain insights into the illness-correlated brain physiology and pathology as well as to translate genetic discoveries into novel therapeutic targets.

Pathological oligodendrocyte precursor cells revealed in human schizophrenic brains and trigger schizophrenia-like behaviors and synaptic defects in genetic animal model

Although the link of white matter to pathophysiology of schizophrenia is documented, loss of myelin is not detected in patients at the early stages of the disease, suggesting that pathological evolution of schizophrenia may occur before significant myelin loss. Disrupted-in-schizophrenia-1 (DISC1) protein is highly expressed in oligodendrocyte precursor cells (OPCs) and regulates their maturation. Recently, DISC1-Δ3, a major DISC1 variant that lacks exon 3, has been identified in schizophrenia patients, although its pathological significance remains unknown. In this study, we detected in schizophrenia patients a previously unidentified pathological phenotype of OPCs exhibiting excessive branching. We replicated this phenotype by generating a mouse strain expressing DISC1-Δ3 gene in OPCs. We further demonstrated that pathological OPCs, rather than myelin defects, drive the onset of schizophrenic phenotype by hyperactivating OPCs' Wnt/β-catenin pathway, which consequently upregulates Wnt Inhibitory Factor 1 (Wif1), leading to the aberrant synaptic formation and neuronal activity. Suppressing Wif1 in OPCs rescues synaptic loss and behavioral disorders in DISC1-Δ3 mice. Our findings reveal the pathogenetic role of OPC-specific DISC1-Δ3 variant in the onset of schizophrenia and highlight the therapeutic potential of Wif1 as an alternative target for the treatment of this disease.

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.

Metabolic Changes in Synaptosomes in an Animal Model of Schizophrenia Revealed by 1H and 1H,13C NMR Spectroscopy

Synaptosomes are isolated nerve terminals that contain synaptic components, including neurotransmitters, metabolites, adhesion/fusion proteins, and nerve terminal receptors. The essential role of synaptosomes in neurotransmission has stimulated keen interest in understanding both their proteomic and metabolic composition. Mass spectrometric (MS) quantification of synaptosomes has illuminated their proteomic composition, but the determination of the metabolic composition by MS has been met with limited success. In this study, we report a proof-of-concept application of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy for analyzing the metabolic composition of synaptosomes. We utilize this approach to compare the metabolic composition synaptosomes from a wild-type rat with that from a newly generated genetic rat model (Disc1 svΔ2), which qualitatively recapitulates clinically observed early DISC1 truncations associated with schizophrenia. This study demonstrates the feasibility of using NMR spectroscopy to identify and quantify metabolites within synaptosomal fractions.

Fasciculation and Elongation Protein Zeta-1 Expression in Reactive Astrocytes in a Rat Model of Frontal Lobe Injury

There are reports that depression induced by frontal lobe injury (FLI) has a devastating effect on human mental health. We previously reported that fasciculation and elongation protein zeta-1 (FEZ1) was essential for astrocytic protection of dopamine neurons. Studies of glutamate-glutamine cycle in mental illness have been reported, whereas not from the perspective of astrocytes. This study was designed to investigate the roles of astrocytic FEZ1 and glutamate-glutamine cycle after FLI. A model of FLI was established by inserting a blade into the right frontal lobe of rats. Behavioral tests were used to observe the behavioral changes of FLI rats. Neuropathologic examinations, including immunohistochemistry, were conducted. Behavioral tests showed that FLI decreased exploratory activity. Western blot analysis revealed that the expression of astroglial proteins overall decreased in the initial injury stage, as well as FEZ1. Immunohistochemistry showed a shift of FEZ1 localization from neurons in sham-lesioned rats to astrocytes in FLI rats, and showed the expression profile of glutamate transporter 1 and glutamine synthetase (GS) was consistent with Western blot observation. Our results indicate that astrocytic FEZ1 and glutamate-glutamine cycle dysfunction may be involved in the pathogenesis of depression after FLI.

Exercise ameliorates deficits in neural microstructure in a Disc1 model of psychiatric illness

Recent studies have investigated the effectiveness of aerobic exercise to improve physical and mental health outcomes in schizophrenia; however, few have explicitly explored the impact of aerobic exercise on neural microstructure, which is hypothesized to mediate the behavioral changes observed. Neural microstructure is influenced by numerous genetic factors including DISC1, which is a major molecular scaffold protein that interacts with partners like GSK3β, NDEL1, and PDE4. DISC1 has been shown to play a role in neurogenesis, neuronal migration, neuronal maturation, and synaptic signaling. As with other genetic variants that present an increased risk for disease, mutations of the DISC1 gene have been implicated in the molecular intersection of schizophrenia and numerous other major psychiatric illnesses. This study investigated whether short-term exercise recovers deficits in neural microstructure in a novel genetic Disc1 svΔ2 rat model. Disc1 svΔ2 animals and age- and sex-matched controls were subjected to a treadmill exercise protocol. Subsequent ex-vivo diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI) compared neural microstructure in regions of interest (ROI) between sedentary and exercise wild-type animals and between sedentary and exercise Disc1 svΔ2 animals. Short-term exercise uncovered no significant differences in neural microstructure between sedentary and exercise control animals but did lead to significant differences between sedentary and exercise Disc1 svΔ2 animals in neocortex, basal ganglia, corpus callosum, and external capsule, suggesting a positive benefit derived from a short-term exercise regimen. Our findings suggest that Disc1 svΔ2 animals are more sensitive to the effects of short-term exercise and highlight the ameliorating potential of positive treatment interventions such as exercise on neural microstructure in genetic backgrounds of psychiatric disease susceptibility.

Sex-specific deficits in neurite density and white matter integrity are associated with targeted disruption of exon 2 of the Disc1 gene in the rat

Diffusion tensor imaging (DTI) has provided remarkable insight into our understanding of white matter microstructure and brain connectivity across a broad spectrum of psychiatric disease. While DTI and other diffusion weighted magnetic resonance imaging (MRI) methods have clarified the axonal contribution to the disconnectivity seen in numerous psychiatric diseases, absent from these studies are quantitative indices of neurite density and orientation that are especially important features in regions of high synaptic density that would capture the synaptic contribution to the psychiatric disease state. Here we report the application of neurite orientation dispersion and density imaging (NODDI), an emerging microstructure imaging technique, to a novel Disc1 svΔ2 rat model of psychiatric illness and demonstrate the complementary and more specific indices of tissue microstructure found in NODDI than those reported by DTI. Our results demonstrate global and sex-specific changes in white matter microstructural integrity and deficits in neurite density as a consequence of the Disc1 svΔ2 genetic variation and highlight the application of NODDI and quantitative measures of neurite density and neurite dispersion in psychiatric disease.

Disrupted in schizophrenia 1 (DISC1) is a constituent of the mammalian mitochondrial contact site and cristae organizing system (MICOS) complex, and is essential for oxidative phosphorylation

Disrupted in Schizophrenia-1 (DISC1) has been associated with a broad spectrum of mental disorders. DISC1 is a multi-compartmentalized protein found in the cytoplasm, centrosome, nuclei and mostly enriched in mitochondria. In order to shed light on DISC1 mitochondrial function, we have studied its topology within the organelle. We show in here that in mammals DISC1 resides in the 'Mitochondrial contact site and Cristae Organizing system' (MICOS) complex, involved in cristae organization. DISC1 knockdown in SH-SY5Y cells causes MICOS disassembly and fragmentation of the mitochondrial morphology network. Moreover, DISC1 depleted cells have decreased mitochondrial DNA (mtDNA) content and steady state levels of oxidative phosphorylation (OXPHOS) subunits. As a consequence, OXPHOS complexes and supercomplexes are partially disassembled in DISC1 knockdown cells, which suffer severe bioenergetic defects, evidenced by impaired oxygen consumption, adenosine triphosphate synthesis and mitochondrial membrane potential. Transfection of recombinant full-length human DISC1 restores MICOS complex assembly and rescues OXPHOS function, meanwhile overexpression of the DISC1 truncated form Δ597-854, known to be pathogenic, fails to rescue the bioenergetic impairment caused by DISC1 knockdown. These results should contribute to reveal DISC1 physiological function and potential pathogenic role in severe mental illnesses.

Beyond the brain: disrupted in schizophrenia 1 regulates pancreatic β-cell function via glycogen synthase kinase-3β

Individuals with schizophrenia and their first-degree relatives have higher rates of type 2 diabetes (T2D) than the general population (18-30 vs. 1.2-6.3%), independent of body mass index and antipsychotic medication, suggesting shared genetic components may contribute to both diseases. The cause of this association remains unknown. Mutations in disrupted in schizophrenia 1 (DISC1) increase the risk of developing psychiatric disorders [logarithm (base 10) of odds = 7.1]. Here, we identified DISC1 as a major player controlling pancreatic β-cell proliferation and insulin secretion via regulation of glycogen synthase kinase-3β (GSK3β). DISC1 expression was enriched in developing mouse and human pancreas and adult β- and ductal cells. Loss of DISC1 function, through siRNA-mediated depletion or expression of a dominant-negative truncation that models the chromosomal translocation of human DISC1 in schizophrenia, resulted in decreased β-cell proliferation (3 vs. 1%; P < 0.01), increased apoptosis (0.1 vs. 0.6%; P < 0.01), and glucose intolerance in transgenic mice. Insulin secretion was reduced (0.5 vs. 0.1 ng/ml; P < 0.05), and critical β-cell transcription factors Pdx1 and Nkx6.1 were significantly decreased. Impaired DISC1 allowed inappropriate activation of GSK3β in β cells, and antagonizing GSK3β (SB216763; IC50 = 34.3 nM) rescued the β-cell defects. These results uncover an unexpected role for DISC1 in normal β-cell physiology and suggest that DISC1 dysregulation contributes to T2D independently of its importance for cognition.

Regulation of N-methyl-D-aspartate receptors by disrupted-in-schizophrenia-1

BACKGROUND

Genetic studies have implicated disrupted-in-schizophrenia-1 (DISC1) as a risk factor for a wide range of mental conditions, including schizophrenia. Because N-methyl-D-aspartate receptor (NMDAR) dysfunction has been strongly linked to the pathophysiology of these conditions, we examined whether the NMDAR is a potential target of DISC1.

METHODS

DISC1 was knocked down with a small inference RNA. NMDAR-mediated currents were recorded and NMDAR expression was measured.

RESULTS

We found that cellular knockdown of DISC1 significantly increased NMDAR currents in cortical cultures, which were accompanied by an increase in the expression of NMDAR subunit, GluN2A. NMDAR-mediated synaptic response in prefrontal cortical pyramidal neurons was also increased by DISC1 knockdown in vivo. The effect of DISC1 knockdown on NMDAR currents in cortical cultures was blocked by protein kinase A (PKA) inhibitor, occluded by PKA activator, and prevented by phosphodiesterase 4 inhibitor. Knockdown of DISC1 caused a significant increase of cyclic adenosine monophosphate response element-binding protein (CREB) activity. Inhibiting CREB prevented the DISC1 deficiency-induced increase of NMDAR currents and GluN2A clusters.

CONCLUSIONS

Our results suggest that DISC1 exerts an important impact on NMDAR expression and function through a phosphodiesterase 4/PKA/CREB-dependent mechanism, which provides a potential molecular basis for the role of DISC1 in influencing NMDAR-dependent cognitive and emotional processes.

A t(1;11) translocation linked to schizophrenia and affective disorders gives rise to aberrant chimeric DISC1 transcripts that encode structurally altered, deleterious mitochondrial proteins

Disrupted-In-Schizophrenia 1 (DISC1) was identified as a risk factor for psychiatric illness through its disruption by a balanced chromosomal translocation, t(1;11)(q42.1;q14.3), that co-segregates with schizophrenia, bipolar disorder and depression. We previously reported that the translocation reduces DISC1 expression, consistent with a haploinsufficiency disease model. Here we report that, in lymphoblastoid cell lines, the translocation additionally results in the production of abnormal transcripts due to the fusion of DISC1 with a disrupted gene on chromosome 11 (DISC1FP1/Boymaw). These chimeric transcripts encode abnormal proteins, designated CP1, CP60 and CP69, consisting of DISC1 amino acids 1-597 plus 1, 60 or 69 amino acids, respectively. The novel 69 amino acids in CP69 induce increased α-helical content and formation of large stable protein assemblies. The same is predicted for CP60. Both CP60 and CP69 exhibit profoundly altered functional properties within cell lines and neurons. Both are predominantly targeted to mitochondria, where they induce clustering and loss of membrane potential, indicative of severe mitochondrial dysfunction. There is currently no access to neural material from translocation carriers to confirm these findings, but there is no reason to suppose that these chimeric transcripts will not also be expressed in the brain. There is thus potential for the production of abnormal chimeric proteins in the brains of translocation carriers, although at substantially lower levels than for native DISC1. The mechanism by which inheritance of the translocation increases risk of psychiatric illness may therefore involve both DISC1 haploinsufficiency and mitochondrial deficiency due to the effects of abnormal chimeric protein expression. GenBank accession numbers: DISC1FP1 (EU302123), Boymaw (GU134617), der 11 chimeric transcript DISC1FP1 exon 2 to DISC1 exon 9 (JQ650115), der 1 chimeric transcript DISC1 exon 4 to DISC1FP1 exon 4 (JQ650116), der 1 chimeric transcript DISC1 exon 6 to DISC1FP1 exon 3a (JQ650117).

DISC1 at 10: connecting psychiatric genetics and neuroscience

Psychiatric genetics research, as exemplified by the DISC1 gene, aspires to inform on mental health etiology and to suggest improved strategies for intervention. DISC1 was discovered in 2000 through the molecular cloning of a chromosomal translocation that segregated with a spectrum of major mental illnesses in a single large Scottish family. Through in vitro experiments and mouse models, DISC1 has been firmly established as a genetic risk factor for a spectrum of psychiatric illness. As a consequence of its protein scaffold function, the DISC1 protein impacts on many aspects of brain function, including neurosignaling and neurodevelopment. DISC1 is a pathfinder for understanding psychopathology, brain development, signaling and circuitry. Although much remains to be learnt and understood, potential targets for drug development are starting to emerge, and in this review, we will discuss the 10 years of research that has helped us understand key roles of DISC1 in psychiatric disease.