The DISC locus in psychiatric illness

The DISC locus is located at the breakpoint of a balanced t(1;11) chromosomal translocation in a large and unique Scottish family. This translocation segregates in a highly statistically significant manner with a broad diagnosis of psychiatric illness, including schizophrenia, bipolar disorder and major depression, as well as with a narrow diagnosis of schizophrenia alone. Two novel genes were identified at this locus and due to the high prevalence of schizophrenia in this family, they were named Disrupted-in-Schizophrenia-1 (DISC1) and Disrupted-in-Schizophrenia-2 (DISC2). DISC1 encodes a novel multifunctional scaffold protein, whereas DISC2 is a putative noncoding RNA gene antisense to DISC1. A number of independent genetic linkage and association studies in diverse populations support the original linkage findings in the Scottish family and genetic evidence now implicates the DISC locus in susceptibility to schizophrenia, schizoaffective disorder, bipolar disorder and major depression as well as various cognitive traits. Despite this, with the exception of the t(1;11) translocation, robust evidence for a functional variant(s) is still lacking and genetic heterogeneity is likely. Of the two genes identified at this locus, DISC1 has been prioritized as the most probable candidate susceptibility gene for psychiatric illness, as its protein sequence is directly disrupted by the translocation. Much research has been undertaken in recent years to elucidate the biological functions of the DISC1 protein and to further our understanding of how it contributes to the pathogenesis of schizophrenia. These data are the main subject of this review; however, the potential involvement of DISC2 in the pathogenesis of psychiatric illness is also discussed. A detailed picture of DISC1 function is now emerging, which encompasses roles in neurodevelopment, cytoskeletal function and cAMP signalling, and several DISC1 interactors have also been defined as independent genetic susceptibility factors for psychiatric illness. DISC1 is a hub protein in a multidimensional risk pathway for major mental illness, and studies of this pathway are opening up opportunities for a better understanding of causality and possible mechanisms of intervention.

DISC1-binding proteins in neural development, signalling and schizophrenia

In the decade since Disrupted in Schizophrenia 1 (DISC1) was first identified it has become one of the most convincing risk genes for major mental illness. As a multi-functional scaffold protein, DISC1 has multiple identified protein interaction partners that highlight pathologically relevant molecular pathways with potential for pharmaceutical intervention. Amongst these are proteins involved in neuronal migration (e.g. APP, Dixdc1, LIS1, NDE1, NDEL1), neural progenitor proliferation (GSK3β), neurosignalling (Girdin, GSK3β, PDE4) and synaptic function (Kal7, TNIK). Furthermore, emerging evidence of genetic association (NDEL1, PCM1, PDE4B) and copy number variation (NDE1) implicate several DISC1-binding partners as risk factors for schizophrenia in their own right. Thus, a picture begins to emerge of DISC1 as a key hub for multiple critical developmental pathways within the brain, disruption of which can lead to a variety of psychiatric illness phenotypes.

DISC1: Structure, Function, and Therapeutic Potential for Major Mental Illness

Disrupted in schizophrenia 1 (DISC1) is well established as a genetic risk factor across a spectrum of psychiatric disorders, a role supported by a growing body of biological studies, making the DISC1 protein interaction network an attractive therapeutic target. By contrast, there is a relative deficit of structural information to relate to the myriad biological functions of DISC1. Here, we critically appraise the available bioinformatics and biochemical analyses on DISC1 and key interacting proteins, and integrate this with the genetic and biological data. We review, analyze, and make predictions regarding the secondary structure and propensity for disordered regions within DISC1, its protein-interaction domains, subcellular localization motifs, and the structural and functional implications of common and ultrarare DISC1 variants associated with major mental illness. We discuss signaling pathways of high pharmacological potential wherein DISC1 participates, including those involving phosphodiesterase 4 (PDE4) and glycogen synthase kinase 3 (GSK3). These predictions and priority areas can inform future research in the translational and potentially guide the therapeutic processes.

DISC1, PDE4B, and NDE1 at the centrosome and synapse

Disrupted-In-Schizophrenia 1 (DISC1) is a risk factor for schizophrenia and other major mental illnesses. Its protein binding partners include the Nuclear Distribution Factor E Homologs (NDE1 and NDEL1), LIS1, and phosphodiesterases 4B and 4D (PDE4B and PDE4D). We demonstrate that NDE1, NDEL1 and LIS1, together with their binding partner dynein, associate with DISC1, PDE4B and PDE4D within the cell, and provide evidence that this complex is present at the centrosome. LIS1 and NDEL1 have been previously suggested to be synaptic, and we now demonstrate localisation of DISC1, NDE1, and PDE4B at synapses in cultured neurons. NDE1 is phosphorylated by cAMP-dependant Protein Kinase A (PKA), whose activity is, in turn, regulated by the cAMP hydrolysis activity of phosphodiesterases, including PDE4. We propose that DISC1 acts as an assembly scaffold for all of these proteins and that the NDE1/NDEL1/LIS1/dynein complex is modulated by cAMP levels via PKA and PDE4.

Proteomic, genomic and translational approaches identify CRMP1 for a role in schizophrenia and its underlying traits

Schizophrenia is a chronic illness of heterogenous biological origin. We hypothesized that, similar to chronic progressive brain conditions, persistent functional disturbances of neurons would result in disturbed proteostasis in the brains of schizophrenia patients, leading to increased abundance of specific misfolded, insoluble proteins. Identification of such proteins would facilitate the elucidation of molecular processes underlying these devastating conditions. We therefore generated antibodies against pooled insoluble proteome of post-mortem brains from schizophrenia patients in order to identify unique, disease-specific epitopes. We successfully identified such an epitope to be present on collapsin-response mediator protein 1 (CRMP1) in biochemically purified, insoluble brain fractions. A genetic association analysis for the CRMP1 gene in a large Finnish population cohort (n = 4651) corroborated the association of physical and social anhedonia with the CRMP1 locus in a DISC1 (Disrupted-in-schizophrenia 1)-dependent manner. Physical and social anhedonia are heritable traits, present as chronic, negative symptoms of schizophrenia and severe major depression, thus constituting serious vulnerability factors for mental disease. Strikingly, lymphoblastoid cell lines derived from schizophrenia patients mirrored aberrant CRMP1 immunoreactivity by showing an increase of CRMP1 expression, suggesting its potential role as a blood-based diagnostic marker. CRMP1 is a novel candidate protein for schizophrenia traits at the intersection of the reelin and DISC1 pathways that directly and functionally interacts with DISC1. We demonstrate the impact of an interdisciplinary approach where the identification of a disease-associated epitope in post-mortem brains, powered by a genetic association study, is rapidly translated into a potential blood-based diagnostic marker.

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).

Recent and recurrent selective sweeps of the antiviral RNAi gene Argonaute-2 in three species of Drosophila

Antagonistic host–parasite interactions can drive rapid adaptive evolution in genes of the immune system, and such arms races may be an important force shaping polymorphism in the genome. The RNA interference pathway gene Argonaute-2 (AGO2) is a key component of antiviral defense in Drosophila, and we have previously shown that genes in this pathway experience unusually high rates of adaptive substitution. Here we study patterns of genetic variation in a 100-kbp region around AGO2 in three different species of Drosophila. Our data suggest that recent independent selective sweeps in AGO2 have reduced genetic variation across a region of more than 50 kbp in Drosophila melanogaster, D. simulans, and D. yakuba, and we estimate that selection has fixed adaptive substitutions in this gene every 30–100 thousand years. The strongest signal of recent selection is evident in D. simulans, where we estimate that the most recent selective sweep involved an allele with a selective advantage of the order of 0.5–1% and occurred roughly 13–60 Kya. To evaluate the potential consequences of the recent substitutions on the structure and function of AGO2, we used fold-recognition and homology-based modeling to derive a structural model for the Drosophila protein, and this suggests that recent substitutions in D. simulans are overrepresented at the protein surface. In summary, our results show that selection by parasites can consistently target the same genes in multiple species, resulting in areas of the genome that have markedly reduced genetic diversity.

NDE1 and NDEL1: multimerisation, alternate splicing and DISC1 interaction

Nuclear Distribution Factor E Homolog 1 (NDE1) and NDE-Like 1 (NDEL1) are highly homologous mammalian proteins. However, whereas NDEL1 is well studied, there is remarkably little known about NDE1. We demonstrate the presence of multiple isoforms of both NDE1 and NDEL1 in the brain, showing that NDE1 binds directly to multiple isoforms of Disrupted in Schizophrenia 1 (DISC1), and to itself. We also show that NDE1 can complex with NDEL1. Together these results predict a high degree of complexity of DISC1-mediated regulation of neuronal activity.

NDE1 and NDEL1 from genes to (mal)functions: parallel but distinct roles impacting on neurodevelopmental disorders and psychiatric illness

NDE1 (Nuclear Distribution Element 1, also known as NudE) and NDEL1 (NDE-Like 1, also known as NudEL) are the mammalian homologues of the fungus nudE gene, with important and at least partially overlapping roles for brain development. While a large number of studies describe the various properties and functions of these proteins, many do not directly compare the similarities and differences between NDE1 and NDEL1. Although sharing a high degree structural similarity and multiple common cellular roles, each protein presents several distinct features that justify their parallel but also unique functions. Notably both proteins have key binding partners in dynein, LIS1 and DISC1, which impact on neurodevelopmental and psychiatric illnesses. Both are implicated in schizophrenia through genetic and functional evidence, with NDE1 also strongly implicated in microcephaly, as well as other neurodevelopmental and psychiatric conditions through copy number variation, while NDEL1 possesses an oligopeptidase activity with a unique potential as a biomarker in schizophrenia. In this review, we aim to give a comprehensive overview of the various cellular roles of these proteins in a "bottom-up" manner, from their biochemistry and protein-protein interactions on the molecular level, up to the consequences for neuronal differentiation, and ultimately to their importance for correct cortical development, with direct consequences for the pathophysiology of neurodevelopmental and mental illness.

The mitosis and neurodevelopment proteins NDE1 and NDEL1 form dimers, tetramers, and polymers with a folded back structure in solution

Paralogs NDE1 (nuclear distribution element 1) and NDEL1 (NDE-like 1) are essential for mitosis and neurodevelopment. Both proteins are predicted to have similar structures, based upon high sequence similarity, and they co-complex in mammalian cells. X-ray diffraction studies and homology modeling suggest that their N-terminal regions (residues 8-167) adopt continuous, extended α-helical coiled-coil structures, but no experimentally derived information on the structure of their C-terminal regions or the architecture of the full-length proteins is available. In the case of NDE1, no biophysical data exists. Here we characterize the structural architecture of both full-length proteins utilizing negative stain electron microscopy along with our established paradigm of chemical cross-linking followed by tryptic digestion, mass spectrometry, and database searching, which we enhance using isotope labeling for mixed NDE1-NDEL1. We determined that full-length NDE1 forms needle-like dimers and tetramers in solution, similar to crystal structures of NDEL1, as well as chain-like end-to-end polymers. The C-terminal domain of each protein, required for interaction with key protein partners dynein and DISC1 (disrupted-in-schizophrenia 1), includes a predicted disordered region that allows a bent back structure. This facilitates interaction of the C-terminal region with the N-terminal coiled-coil domain and is in agreement with previous results showing N- and C-terminal regions of NDEL1 and NDE1 cooperating in dynein interaction. It sheds light on recently identified mutations in the NDE1 gene that cause truncation of the encoded protein. Additionally, analysis of mixed NDE1-NDEL1 complexes demonstrates that NDE1 and NDEL1 can interact directly.

NDE1 and NDEL1: twin neurodevelopmental proteins with similar 'nature' but different 'nurture'

Nuclear distribution element 1 (NDE1, also known as NudE) and NDE-like 1 (NDEL1, also known as Nudel) are paralogous proteins essential for mitosis and neurodevelopment that have been implicated in psychiatric and neurodevelopmental disorders. The two proteins possess high sequence similarity and have been shown to physically interact with one another. Numerous lines of experimental evidence in vivo and in cell culture have demonstrated that these proteins share common functions, although instances of differing functions between the two have recently emerged. We review the key aspects of NDE1 and NDEL1 in terms of recent advances in structure elucidation and cellular function, with an emphasis on their differing mechanisms of post-translational modification. Based on a review of the literature and bioinformatics assessment, we advance the concept that the twin proteins NDE1 and NDEL1, while sharing a similar 'nature' in terms of their structure and basic functions, appear to be different in their 'nurture', the manner in which they are regulated both in terms of expression and of post-translational modification within the cell. These differences are likely to be of significant importance in understanding the specific roles of NDE1 and NDEL1 in neurodevelopment and disease.

Revisiting Disrupted-in-Schizophrenia 1 as a scaffold protein

Disrupted-in-Schizophrenia 1 (DISC1) is a widely-accepted genetic risk factor for schizophrenia and many other major mental illnesses. Traditionally DISC1 has been referred to as a ‘scaffold protein’ because of its ability to bind to a wide array of other proteins, including those of importance for neurodevelopment. Here, we review the characteristic properties shared between established scaffold proteins and DISC1. We find DISC1 to have many, but not all, of the characteristics of a scaffold protein, as it affects a considerable number of different, but related, signaling pathways, in most cases through inhibition of key enzymes. Using threading algorithms, the C-terminal portion of DISC1 could be mapped to extended helical structures, yet it may not closely resemble any of the known tertiary folds. While not completely fitting the classification of a classical scaffold protein, DISC1 does appear to be a tightly regulated and multi-faceted inhibitor of a wide range of enzymes from interrelated signaling cascades (Diverse Inhibitor of Signaling Cascades), which together contribute to neurodevelopment and synaptic homeostasis. Consequently, disruption of this complex regulation would be expected to lead to the range of major mental illnesses in which the DISC1 gene has been implicated.

Aggregation of the protein TRIOBP-1 and its potential relevance to schizophrenia

We have previously proposed that specific proteins may form insoluble aggregates as a response to an illness-specific proteostatic dysbalance in a subset of brains from individuals with mental illness, as is the case for other chronic brain conditions. So far, established risk factors DISC1 and dysbindin were seen to specifically aggregate in a subset of such patients, as was a novel schizophrenia-related protein, CRMP1, identified through a condition-specific epitope discovery approach. In this process, antibodies are raised against the pooled insoluble protein fractions (aggregomes) of post mortem brain samples from schizophrenia patients, followed by epitope identification and confirmation using additional techniques. Pursuing this epitope discovery paradigm further, we reveal TRIO binding protein (TRIOBP) to be a major substrate of a monoclonal antibody with a high specificity to brain aggregomes from patients with chronic mental illness. TRIOBP is a gene previously associated with deafness which encodes for several distinct protein species, each involved in actin cytoskeletal dynamics. The 3′ splice variant TRIOBP-1 is found to be the antibody substrate and has a high aggregation propensity when over-expressed in neuroblastoma cells, while the major 5′ splice variant, TRIOBP-4, does not. Endogenous TRIOBP-1 can also spontaneously aggregate, doing so to a greater extent in cell cultures which are post-mitotic, consistent with aggregated TRIOBP-1 being able to accumulate in the differentiated neurons of the brain. Finally, upon expression in Neuroscreen-1 cells, aggregated TRIOBP-1 affects cell morphology, indicating that TRIOBP-1 aggregates may directly affect cell development, as opposed to simply being a by-product of other processes involved in major mental illness. While further experiments in clinical samples are required to clarify their relevance to chronic mental illness in the general population, TRIOBP-1 aggregates are thus implicated for the first time as a biological element of the neuropathology of a subset of chronic mental illness.

An unpredicted aggregation-critical region of the actin-polymerizing protein TRIOBP-1/Tara, determined by elucidation of its domain structure

Aggregation of specific proteins in the brains of patients with chronic mental illness as a result of disruptions in proteostasis is an emerging theme in the study of schizophrenia in particular. Proteins including DISC1 (disrupted in schizophrenia 1) and dysbindin-1B are found in insoluble forms within brain homogenates from such patients. We recently identified TRIOBP-1 (Trio-binding protein 1, also known as Tara) to be another such protein through an epitope discovery and proteomics approach by comparing post-mortem brain material from schizophrenia patients and control individuals. We hypothesized that this was likely to occur as a result of a specific subcellular process and that it, therefore, should be possible to identify a region of the TRIOBP-1 protein that is essential for its aggregation to occur. Here, we probe the domain organization of TRIOBP-1, finding it to possess two distinct coiled-coil domains: the central and C-terminal domains. The central domain inhibits the depolymerization of F-actin and is also responsible for oligomerization of TRIOBP-1. Along with an N-terminal pleckstrin homology domain, the central domain affects neurite outgrowth. In neuroblastoma cells it was found that the aggregation propensity of TRIOBP-1 arises from its central domain, with a short "linker" region narrowed to within amino acids 324-348, between its first two coiled coils, as essential for the formation of TRIOBP-1 aggregates. TRIOBP-1 aggregation, therefore, appears to occur through one or more specific cellular mechanisms, which therefore have the potential to be of physiological relevance for the biological process underlying the development of chronic mental illness.

A structural organization for the Disrupted in Schizophrenia 1 protein, identified by high-throughput screening, reveals distinctly folded regions, which are bisected by mental illness-related mutations

Disrupted in Schizophrenia 1 (DISC1) is a scaffolding protein of significant importance for neurodevelopment and a prominent candidate protein in the pathology of major mental illness. DISC1 modulates a number of critical neuronal signaling pathways through protein-protein interactions; however, the mechanism by which this occurs and how DISC1 causes mental illness is unclear, partly because knowledge of the structure of DISC1 is lacking. A lack of homology with known proteins has hindered attempts to define its domain composition. Here, we employed the high-throughput Expression of Soluble Proteins by Random Incremental Truncation (ESPRIT) technique to identify discretely folded regions of human DISC1 via solubility assessment of tens of thousands of fragments of recombinant DISC1. We identified four novel structured regions, named D, I, S, and C, at amino acids 257-383, 539-655, 635-738, and 691-836, respectively. One region (D) is located in a DISC1 section previously predicted to be unstructured. All regions encompass coiled-coil or α-helical structures, and three are involved in DISC1 oligomerization. Crucially, three of these domains would be lost or disrupted by a chromosomal translocation event after amino acid 597, which has been strongly linked to major mental illness. Furthermore, we observed that a known illness-related frameshift mutation after amino acid 807 causes the C region to form aberrantly multimeric and aggregated complexes with an unstable secondary structure. This newly revealed domain architecture of DISC1, therefore, provides a powerful framework for understanding the critical role of this protein in a variety of devastating mental illnesses.

Loss of Reelin protects mice against arterial thrombosis by impairing integrin activation and thrombus formation under high shear conditions

Reelin is a secreted glycoprotein and essential for brain development and plasticity. Recent studies provide evidence that Reelin modifies platelet actin cytoskeletal dynamics. In this study we sought to dissect the contribution of Reelin in arterial thrombus formation. Here we analyzed the impact of Reelin in arterial thrombosis ex vivo and in vivo using Reelin deficient (reeler) and wildtype mice. We found that Reelin is secreted upon platelet activation and mediates signaling via glycoprotein (GP)Ib, the amyloid precursor protein (APP) and apolipoprotein E receptor 2 (ApoER2) to induce activation of Akt, extracellular signal-regulated kinase (Erk), SYK and Phospholipase Cγ2. Moreover, our data identifies Reelin as first physiological ligand for platelet APP. Platelets from reeler mice displayed attenuated platelet adhesion and significantly reduced thrombus formation under high shear conditions indicating an important role for Reelin in GPIb-dependent integrin α_IIbβ₃ activation. Accordingly, adhesion to immobilized vWF as well as integrin activation and the phosphorylation of Erk and Akt after GPIb engagement was reduced in Reelin deficient platelets. Defective Reelin signaling translated into protection from arterial thrombosis and cerebral ischemia/reperfusion injury beside normal hemostasis. Furthermore, treatment with an antagonistic antibody specific for Reelin protects wildtype mice from occlusive thrombus formation. Mechanistically, GPIb co-localizes to the major Reelin receptor APP in platelets suggesting that Reelin-induced effects on GPIb signaling are mediated by APP-GPIb interaction. These results indicate that Reelin is an important regulator of GPIb-mediated platelet activation and may represent a new therapeutic target for the prevention and treatment of cardio- and cerebrovascular diseases.

Angiotensin Converting Enzymes as druggable features of psychiatric and neurodegenerative disorders

The Renin-Angiotensin System (RAS) plays essential roles in maintaining peripheral cardiovascular homeostasis, with its potential roles in the brain only being recognized more recently. Angiotensin I-Converting Enzyme (ACE) is the main component of the RAS, and it has been implicated in various disorders of the brain. ACE and other RAS components, including the related enzyme ACE2, angiotensin peptides and their respective receptors, can participate in the pathological state, as well as with potential to contribute for neuroprotection and/or to complement existing treatments for psychiatric illness. In this narrative review, we aimed to identify the main studies describing the functions of the RAS and ACEs in the brain and their association with brain disorders. These include the neurodegenerative disorders such as Parkinson's and Alzheimer's diseases, psychiatric illnesses such as schizophrenia, bipolar disorder, and depression. We also discuss the possible association of a functional polymorphism of the ACE gene with these brain diseases and the relevance of the neuroprotective and anti-inflammatory properties of ACE inhibitors (ACEis) and angiotensin receptor blockers (ARBs). Based on this, we conclude that there is a significant potential value to the inclusion of ACEis and/or ARBs as a novel integrated approach for the treatment for various disorders of the brain, and particularly for psychiatric illness.

The NDE1 genomic locus can affect treatment of psychiatric illness through gene expression changes related to microRNA-484

Genetic studies of familial schizophrenia in Finland have observed significant associations with a group of biologically related genes, DISC1, NDE1, NDEL1, PDE4B and PDE4D, the ‘DISC1 network’. Here, we use gene expression and psychoactive medication use data to study their biological consequences and potential treatment implications. Gene expression levels were determined in 64 individuals from 18 families, while prescription medication information has been collected over a 10-year period for 931 affected individuals. We demonstrate that the NDE1 SNP rs2242549 associates with significant changes in gene expression for 2908 probes (2542 genes), of which 794 probes (719 genes) were replicable. A significant number of the genes altered were predicted targets of microRNA-484 (p = 3.0 × 10⁻⁸), located on a non-coding exon of NDE1. Variants within the NDE1 locus also displayed significant genotype by gender interaction to early cessation of psychoactive medications metabolized by CYP2C19. Furthermore, we demonstrate that miR-484 can affect the expression of CYP2C19 in a cell culture system. Thus, variation at the NDE1 locus may alter risk of mental illness, in part through modification of miR-484, and such modification alters treatment response to specific psychoactive medications, leading to the potential for use of this locus in targeting treatment.

The TRIOBP Isoforms and Their Distinct Roles in Actin Stabilization, Deafness, Mental Illness, and Cancer

The TRIOBP (TRIO and F-actin Binding Protein) gene encodes multiple proteins, which together play crucial roles in modulating the assembly of the actin cytoskeleton. Splicing of the TRIOBP gene is complex, with the two most studied TRIOBP protein isoforms sharing no overlapping amino acid sequence with each other. TRIOBP-1 (also known as TARA or TAP68) is a mainly structured protein that is ubiquitously expressed and binds to F-actin, preventing its depolymerization. It has been shown to be important for many processes including in the cell cycle, adhesion junctions, and neuronal differentiation. TRIOBP-1 has been implicated in schizophrenia through the formation of protein aggregates in the brain. In contrast, TRIOBP-4 is an entirely disordered protein with a highly specialized expression pattern. It is known to be crucial for the bundling of actin in the stereocilia of the inner ear, with mutations in it causing severe or profound hearing loss. Both of these isoforms are implicated in cancer. Additional longer isoforms of TRIOBP exist, which overlap with both TRIOBP-1 and 4. These appear to participate in the functions of both shorter isoforms, while also possessing unique functions in the inner ear. In this review, the structures and functions of all of these isoforms are discussed, with a view to understanding how they operate, both alone and in combination, to modulate actin and their consequences for human illness.

Disrupted in Schizophrenia 1 regulates the processing of reelin in the perinatal cortex

Disrupted in Schizophrenia 1 (DISC1) is a prominent gene in mental illness research, encoding a scaffold protein known to be of importance in the developing cerebral cortex. Reelin is a critical extracellular protein for development and lamination of the prenatal cortex and which has also been independently implicated in mental illness. Regulation of reelin activity occurs through processing by the metalloproteinases ADAMTS-4 and ADAMTS-5. Through cross-breeding of heterozygous transgenic DISC1 mice with heterozygous reeler mice, which have reduced reelin, pups heterozygous for both phenotypes were generated. From these, we determine that transgenic DISC1 leads to a reduction in the processing of reelin, with implications for its downstream signalling element Dab1. An effect of DISC1 on reelin processing was confirmed in vitro, and revealed that intracellular DISC1 affects ADAMTS-4 protein, which in turn is exported and affects processing of extracellular reelin. In transgenic rat cortical cultures, an effect of DISC1 on reelin processing could also be seen specifically in early, immature neurons, but was lost in calretinin and reelin-positive mature neurons, suggesting cell-type specificity. DISC1 therefore acts upstream of reelin in the perinatal cerebral cortex in a cell type/time specific manner, leading to regulation of its activity through altered proteolytic cleavage. Thus a functional link is demonstrated between two proteins, each of independent importance for both cortical development and associated cognitive functions leading to behavioural maladaptation and mental illness.

TRIOBP-1 Protein Aggregation Exists in Both Major Depressive Disorder and Schizophrenia, and Can Occur through Two Distinct Regions of the Protein

The presence of proteinopathy, the accumulation of specific proteins as aggregates in neurons, is an emerging aspect of the pathology of schizophrenia and other major mental illnesses. Among the initial proteins implicated in forming such aggregates in these conditions is Trio and F-actin Binding Protein isoform 1 (TRIOBP-1), a ubiquitously expressed protein involved in the stabilization of the actin cytoskeleton. Here we investigate the insolubility of TRIOBP-1, as an indicator of aggregation, in brain samples from 25 schizophrenia patients, 25 major depressive disorder patients and 50 control individuals (anterior cingulate cortex, BA23). Strikingly, insoluble TRIOBP-1 is considerably more prevalent in both of these conditions than in controls, further implicating TRIOBP-1 aggregation in schizophrenia and indicating a role in major depressive disorder. These results were only seen using a high stringency insolubility assay (previously used to study DISC1 and other proteins), but not a lower stringency assay that would be expected to also detect functional, actin-bound TRIOBP-1. Previously, we have also determined that a region of 25 amino acids in the center of this protein is critical for its ability to form aggregates. Here we attempt to refine this further, through the expression of various truncated mutant TRIOBP-1 vectors in neuroblastoma cells and examining their aggregation. In this way, it was possible to narrow down the aggregation-critical region of TRIOBP-1 to just 8 amino acids (333–340 of the 652 amino acid-long TRIOBP-1). Surprisingly our results suggested that a second section of TRIOBP-1 is also capable of independently inducing aggregation: the optionally expressed 59 amino acids at the extreme N-terminus of the protein. As a result, the 597 amino acid long version of TRIOBP-1 (also referred to as “Tara” or “TAP68”) has reduced potential to form aggregates. The presence of insoluble TRIOBP-1 in brain samples from patients, combined with insight into the mechanism of aggregation of TRIOBP-1 and generation of an aggregation-resistant mutant TRIOBP-1 that lacks both these regions, will be of significant use in further investigating the mechanism and consequences of TRIOBP-1 aggregation in major mental illness.

Biophysical insights from a single chain camelid antibody directed against the Disrupted-in-Schizophrenia 1 protein

Accumulating evidence suggests an important role for the Disrupted-in-Schizophrenia 1 (DISC1) protein in neurodevelopment and chronic mental illness. In particular, the C-terminal 300 amino acids of DISC1 have been found to mediate important protein-protein interactions and to harbor functionally important phosphorylation sites and disease-associated polymorphisms. However, long disordered regions and oligomer-forming subdomains have so far impeded structural analysis. VHH domains derived from camelid heavy chain only antibodies are minimal antigen binding modules with appreciable solubility and stability, which makes them well suited for the stabilizing proteins prior to structural investigation. Here, we report on the generation of a VHH domain derived from an immunized Lama glama, displaying high affinity for the human DISC1 C region (aa 691-836), and its characterization by surface plasmon resonance, size exclusion chromatography and immunological techniques. The VHH-DISC1 (C region) complex was also used for structural investigation by small angle X-ray scattering analysis. In combination with molecular modeling, these data support predictions regarding the three-dimensional fold of this DISC1 segment as well as its steric arrangement in complex with our VHH antibody.

Cloning of the promoter of NDE1, a gene implicated in psychiatric and neurodevelopmental disorders through copy number variation

Copy number variation at 16p13.11 has been associated with a range of neurodevelopmental and psychiatric conditions, with duplication of this region being more common in individuals with schizophrenia. A prominent candidate gene within this locus is NDE1 (Nuclear Distribution Element 1) given its known importance for neurodevelopment, previous associations with mental illness and its well characterized interaction with the Disrupted in Schizophrenia 1 (DISC1) protein. In order to accurately model the effect of NDE1 duplication, it is important to first gain an understanding of how the gene is expressed. The complex promoter system of NDE1, which produces three distinct transcripts, each encoding for the same full-length NDE1 protein (also known as NudE), was therefore cloned and tested in human cell lines. The promoter for the longest of these three NDE1 transcripts was found to be responsible for the majority of expression in these systems, with its extended 5' untranslated region (UTR) having a limiting effect on its expression. These results thus highlight and clone the promoter elements required to generate systems in which the NDE1 protein is exogenously expressed under its native promoter, providing a biologically relevant model of 16p13.11 duplication in major mental illness.

Co-Aggregation and Parallel Aggregation of Specific Proteins in Major Mental Illness

BACKGROUND

Disrupted proteostasis is an emerging area of research into major depressive disorder. Several proteins have been implicated as forming aggregates specifically in the brains of subsets of patients with psychiatric illnesses. These proteins include CRMP1, DISC1, NPAS3 and TRIOBP-1. It is unclear, however, whether these proteins normally aggregate together in the same individuals and, if so, whether each protein aggregates independently of each other ("parallel aggregation") or if the proteins physically interact and aggregate together ("co-aggregation").

MATERIALS AND METHODS

Post mortem insular cortex samples from major depressive disorder and Alzheimer's disease patients, suicide victims and control individuals had their insoluble fractions isolated and tested by Western blotting to determine which of these proteins are insoluble and, therefore, likely to be aggregating. The ability of the proteins to co-aggregate (directly interact and form common aggregate structures) was tested by systematic pairwise expression of the proteins in SH-SY5Y neuroblastoma cells, which were then examined by immunofluorescent microscopy.

RESULTS

Many individuals displayed multiple insoluble proteins in the brain, although not enough to imply interaction between the proteins. Cell culture analysis revealed that only a few of the proteins analyzed can consistently co-aggregate with each other: DISC1 with each of CRMP1 and TRIOBP-1. DISC1 was able to induce aggregation of full length TRIOBP-1, but not individual domains of TRIOBP-1 when they were expressed individually.

CONCLUSIONS

While specific proteins are capable of co-aggregating, and appear to do so in the brains of individuals with mental illness and potentially also with suicidal tendency, it is more common for such proteins to aggregate in a parallel manner, through independent mechanisms. This information aids in understanding the distribution of protein aggregates among mental illness patients and is therefore important for any future diagnostic or therapeutic approaches based on this aspect of mental illness pathology.

The interaction of schizophrenia-related proteins DISC1 and NDEL1, in light of the newly identified domain structure of DISC1

DISC1 and NDEL1 are both key proteins in cortical neurodevelopment, which are each also implicated in the pathogenesis of mental illness. That the two proteins interact with each other in a functionally important manner is well established, but two distinct binding domains for NDEL1 on DISC1 have been proposed. A partial domain structure for DISC1 has recently been described, consisting of 4 structured regions referred to as “D,” “I,” “S” and “C” respectively, with one of the NDEL1 binding sites lying in the “C” region of DISC1. In light of this domain structure, it can be deduced that this site is the likely location at which NDEL1 binds, although the other proposed site (which lies in the DISC1 “I” and “S” regions) may indirectly impact on DISC1-NDEL1 interactions through determination of the oligomeric state of DISC1.