This paper from Millar and Porteous’s group (Bradshaw et al., 2011) proposes a mechanistic link between PDE4 and the NDE1/NDEL1/LIS1 protein complex via a DISC1 pathway. The data suggested that the PKA phosphorylation of NDE1 modulates the protein binding among NDE1, NDEL1, and LIS1, which is regulated by DISC1-PDE4 interaction. Importantly, the authors specified threonine 131 (T131) of NDE1 as a PKA phosphorylation site and produced a phospho-NDE1-T131 antibody. Thus, the question arising is, What is the effect of the phosphorylation of NDE1 at T131 for brain development? Of interest, the authors observed the accumulation of phosphorylated NDE1 at T131 in the centrosome, spindle pole, and intercellular bridge in mitotic cells, as well as in the post-synaptic density in primary hippocampal neurons. The role for the phosphorylation of NDE1 for neuronal processes, such as cell proliferation, differentiation, and synaptic function, is expected to be studied in vivo.

Another question is whether the phosphorylation of NDE1 at T131...  Read more

This paper from Millar and Porteous’s group (Bradshaw et al., 2011) proposes a mechanistic link between PDE4 and the NDE1/NDEL1/LIS1 protein complex via a DISC1 pathway. The data suggested that the PKA phosphorylation of NDE1 modulates the protein binding among NDE1, NDEL1, and LIS1, which is regulated by DISC1-PDE4 interaction. Importantly, the authors specified threonine 131 (T131) of NDE1 as a PKA phosphorylation site and produced a phospho-NDE1-T131 antibody. Thus, the question arising is, What is the effect of the phosphorylation of NDE1 at T131 for brain development? Of interest, the authors observed the accumulation of phosphorylated NDE1 at T131 in the centrosome, spindle pole, and intercellular bridge in mitotic cells, as well as in the post-synaptic density in primary hippocampal neurons. The role for the phosphorylation of NDE1 for neuronal processes, such as cell proliferation, differentiation, and synaptic function, is expected to be studied in vivo.

Another question is whether the phosphorylation of NDE1 at T131 is involved in the pathophysiologies of major mental disorders, such as schizophrenia. As the authors stated, DISC1, PDE4, NDE1, and NDEL1, are all genes that have been implicated as genetic risk factors for major psychiatric conditions. Our group has recently reported that phosphorylated DISC1 at S710 is the critical switch signaling from cell proliferation to neuronal migration in the developing brain (Ishizuka et al., 2011). The phosphorylation status of both DISC1 and NDE1 could be characterized in patient-derived cells, such as iPS and iN cells, which might in turn identify biological markers for major mental disorders.

References:

Bradshaw NJ, Soares DC, Carlyle BC, Ogawa F, Davidson-Smith H, Christie S, Mackie S, Thomson PA, Porteous DJ, Millar JK. PKA phosphorylation of NDE1 is DISC1/PDE4 dependent and modulates its interaction with LIS1 and NDEL1. J Neurosci . 2011 Jun 15 ; 31(24):9043-54. Abstract

Ishizuka K, Kamiya A, Oh EC, Kanki H, Seshadri S, Robinson JF, Murdoch H, Dunlop AJ, Kubo K, Furukori K, Huang B, Zeledon M, Hayashi-Takagi A, Okano H, Nakajima K, Houslay MD, Katsanis N, Sawa A. DISC1-dependent switch from progenitor proliferation to migration in the developing cortex. Nature . 2011 May 5 ; 473(7345):92-6. Abstract

Comment by Anand Gururjan, Rachel Hill, and Maarten van Den Buuse
Reverse-engineering clinical abnormalities in rodents has been a standard approach to creating models of aspects of psychiatric illness, but with the use of genetic knockouts, we have managed to achieve a level of resolution not previously seen using classical drug-induced, lesion-induced, or neurodevelopmental models. Indeed, the use of knockouts in combination with these techniques would, in theory, provide a more accurate model. However, before such combinations are trialed, we would necessarily need to establish the robustness of the knockout model by itself in terms of its construct and face and predictive validity.

As outlined by Carlisle et al. (2011), a model has been created based on clinical findings that genetic variations in the components of the post-synaptic density fraction (PSD) have been linked to several psychiatric disorders. The PSD machinery plays a very important role in regulating signal strength and also selectivity of signals...  Read more

Comment by Anand Gururjan, Rachel Hill, and Maarten van Den Buuse
Reverse-engineering clinical abnormalities in rodents has been a standard approach to creating models of aspects of psychiatric illness, but with the use of genetic knockouts, we have managed to achieve a level of resolution not previously seen using classical drug-induced, lesion-induced, or neurodevelopmental models. Indeed, the use of knockouts in combination with these techniques would, in theory, provide a more accurate model. However, before such combinations are trialed, we would necessarily need to establish the robustness of the knockout model by itself in terms of its construct and face and predictive validity.

As outlined by Carlisle et al. (2011), a model has been created based on clinical findings that genetic variations in the components of the post-synaptic density fraction (PSD) have been linked to several psychiatric disorders. The PSD machinery plays a very important role in regulating signal strength and also selectivity of signals transmitted to the dendrite. In the context of schizophrenia, the components which are thought to be dysfunctional include DISC1, α-CaMKII, SynGAP, and PSD-95. The authors of this study have investigated the effect of deleting exon 3 of the mouse gene LRRC7, which is the start site for the construction of the PSD scaffolding protein known as densin-180. They then proceeded to characterize the behavioral phenotype of the knockouts, changes in expression of other PSD proteins, the effect on neurotransmission, and the morphology of hippocampal neurons in the knockout mice.

Here, we summarize, highlight, and discuss several findings. In terms of the behavioral phenotype, there are several novel findings which are worth discussing. Densin knockout mice were less active in their home cages, but displayed significant novelty-induced hyperactivity. There were also deficits in two forms of short-term memory, unrelated to changes in locomotor activity. In terms of hippocampus-dependent memory, we wonder whether a more specific test like the Morris water maze would have been more appropriate. Perhaps one of the most important behavioral changes observed in the knockout mice were deficits in prepulse inhibition, but it is unclear as to whether there were any effects on the startle response. If the disruption of PPI in the knockouts was accompanied by changes in the startle response, the interpretation would be entirely different (Csomor et al., 2008). It is also unclear why only male mice were tested for PPI. It is stated that fighting “has been shown to alter the PPI response in mice,” but no references are provided to support this. In experiments in our own laboratory, we often test male mice which had to be isolated because of fighting, and have not found consistent differences with socially housed mice of the same genotype. If this model was to be used to assess antipsychotic potential, it would have probably been important to determine whether drugs (e.g., haloperidol, clozapine) would have any effect on the disruption.

The authors reported a reduction in nesting activity in male knockouts, and even though they relate this to social withdrawal—a key negative symptom of schizophrenia—we wonder whether the use of a social cognition test setup would have been more appropriate. It is unclear why females were not tested. Furthermore, given that nest building is a social activity, for those male mice that were eventually housed individually due to aggressive behaviors, the relevance of this measurement should be questioned. And contrary to the authors discussion point, the use of antipsychotics in fact was reported to disrupt nesting behavior (Li et al., 2004).

There were no differences between knockouts and wild-type mice in terms of their balance and coordination, but knockouts did show increased anxiety in the open field test. Further tests, perhaps using the elevated plus maze, would be required to confirm the anxiogenic effect. Interestingly, even though male and female knockout mice were generated for this study, with the exception of the results for the PPI, nesting, and aggression tests, there was no specific mention as to whether there were any sex-specific differences for the other tests.

In terms of the neuromolecular findings, in the PSD the loss of densin produced a decrease in the level of DISC1 and mGluR5 in the forebrain regions of knockouts, which led the authors to propose the existence of a multiprotein complex of densin, DISC1, and mGluR5. Specifically in the context of schizophrenia, reductions in the expression of DISC1 have been previously reported (Brandon et al., 2009), and there have been rodent models based on mGluR5-hypofunction (Gray et al., 2009). Secondly, there were no effects on CaMKII, SynGAP, or PSD-95 in the PSD. PSD-95 in particular is involved in the trafficking of NMDA receptors (NMDARs). The NMDA receptor hypofunction hypothesis of schizophrenia posits that a reduction in glutamatergic neurotransmission is a key feature and cause of the behavioral phenotype. The deletion of densin has no effect on ionotrophic glutamate receptor expression or localization in the PSD, and, hence, the knockouts would be expected to show normal neurotransmission, and indeed this is what the authors found. So it would be appropriate to conclude that this model does not display the construct validity normally associated with the glutamate hypothesis. Furthermore, from a technical point of view, the analysis of entire forebrain regions of the knockouts may miss out on regionally specific effects of densin deletion which may or may not have been important.

The authors went on to discover that in double knockouts of both densin and NMDARs (GluN1), there was reduced colocalization of CaMKII with PSD-95 in the PSD (~50 percent) compared to NMDAR knockouts alone (~15 percent), even though there was no effect on PSD-95 expression itself. This led the authors to suggest that if either densin or the NMDAR is missing, there is a partial docking of CaMKII together with PSD-95, but if both are missing, there is much less colocalization. The authors then hypothesized that in densin knockouts, CaMKII would bind more to NMDARs as a compensatory response, and, in turn, this would enhance glutamatergic neurotransmission. They found that, while basal CaMKII activity was lower in knockouts, in the presence of the GABAA antagonist bicuculline, phosphorylation of CaMKII was double that observed in wild-types. The authors concluded based on this finding that even though the deletion of densin appears to have no direct impact on the level of expression or localization of CaMKII within the PSD, there are indirect consequences on its localization with respect to other PSD components, and this in turn affects its basal activity and how it responds to neuronal stimuli.

These densin-related changes in PSD architecture and function were thought to result in effects on synaptic plasticity in hippocampal preparations, but this was limited only to LTD. The application of low-frequency stimulation or the application of an mGluR agonist, DHPG, induced no LTD in the knockouts compared to the wild-types. As the authors suggest, the deletion of densin impairs a biochemical step from the point of receptor activation (NMDAR or mGluR5), but to determine whether this was related to the reduced CaMKII microlocalization with PSD-95, the authors could have performed another experiment with the use of the CaMKII inhibitor KN-62, which is known to facilitate DHPG-induced LTD (Schnabel et al., 1999). DHPG is an mGluR1 and mGluR5 receptor agonist, so it could also be suggested that the densin-related decrease in mGluR5 expression in the PSD is not offset by a compensatory increase in mGluR1-receptor activity. It was not reported whether there was any effect of densin deletion on mGluR1 expression in the PSD.

In terms of hippocampal dendritic spine morphology, the loss of densin did not result in any significant changes in overall spine density, but there were significant effects on mushroom spine length (longer) and spine neck diameter (thinner). Bigger, voluminous headed spines are believed to have a greater PSD area, and hence are stronger synaptic contact points. In this study, the average spine head volume as calculated using the diameter seems unaffected by densin loss in all three types (mushroom, stubby, and thin). Strictly speaking, however, the association between spine morphology and psychiatric illness is weak and difficult to interpret given the highly plastic nature of their shape, and that it is often unclear whether changes are due to a primary cause or a downstream effect (Nimchinsky et al., 2002).

Overall, densin-180 knockout mice do show some behavioral changes which resemble some of the positive and negative symptoms of schizophrenia, but future studies would be necessary to confirm the findings using more specific testing paradigms and alternative protocols. To their credit, the authors have done a tremendous amount of work to produce the results, but no attempt was made to make a link between the molecular and behavioral changes. Unfortunately, this "missing link" is what many in the field would describe as a significant limitation of this animal model.

References:

Brandon NJ, Millar JK, Korth C, Sive H, Singh KK, Sawa A (2009) Understanding the role of DISC1 in psychiatric disease and during normal development. J Neurosci 29:12768-12775. Abstract

Carlisle HJ, Luong TN, Medina-Marino A, Schenker L, Khorosheva E, Indersmitten T, Gunapala KM, Steele AD, O'Dell TJ, Patterson PH, Kennedy MB (2011) Deletion of densin-180 results in abnormal behaviors associated with mental illness and reduces mGluR5 and DISC1 in the postsynaptic density fraction. J Neurosci 31:16194-16207. Abstract

Csomor PA, Yee BK, Vollenweider FX, Feldon J, Nicolet T, Quednow BB (2008) On the influence of baseline startle reactivity on the indexation of prepulse inhibition. Behav Neurosci 122:885-900. Abstract

Gray L, van den Buuse M, Scarr E, Dean B, Hannan AJ (2009) Clozapine reverses schizophrenia-related behaviours in the metabotropic glutamate receptor 5 knockout mouse: association with N-methyl-D-aspartic acid receptor up-regulation. Int J Neuropsychopharmacol 12:45-60. Abstract

Li M, Davidson P, Budin R, Kapur S, Fleming AS (2004) Effects of typical and atypical antipsychotic drugs on maternal behavior in postpartum female rats. Schizophr Res 70:69-80. Abstract

Nimchinsky EA, Sabatini BL, Svoboda K (2002) Structure and Function of Dendritic Spines. Annual Review of Physiology 64:313-353. Abstract

Schnabel R, Palmer MJ, Kilpatrick IC, Collingridge GL (1999) A CaMKII inhibitor, KN-62, facilitates DHPG-induced LTD in the CA1 region of the hippocampus. Neuropharmacology 38:605-608. Abstract

The two latest additions to the burgeoning DISC1 literature provide additional support for a role of this gene in cognitive function and schizophrenia, and suggest that more comprehensive studies will be useful as we move to a greater understanding of its role in CNS function. Koike et al. (2006) found that a relatively common mouse strain has a naturally occurring mutation in DISC1 resulting in a truncated form of the protein, similar in size (exon 7 vs. exon 8 disruptions) to that observed in the members of the Scottish pedigree in which the translocation was first detected. C57/BL/6J mice, into which mutant alleles were transferred, displayed significant impairments on a spatial working memory task similar to one used in humans (Lencz et al., 2003). These data are similar to those observed by our group (Burdick et al., 2005) and others (  Read more

The two latest additions to the burgeoning DISC1 literature provide additional support for a role of this gene in cognitive function and schizophrenia, and suggest that more comprehensive studies will be useful as we move to a greater understanding of its role in CNS function. Koike et al. (2006) found that a relatively common mouse strain has a naturally occurring mutation in DISC1 resulting in a truncated form of the protein, similar in size (exon 7 vs. exon 8 disruptions) to that observed in the members of the Scottish pedigree in which the translocation was first detected. C57/BL/6J mice, into which mutant alleles were transferred, displayed significant impairments on a spatial working memory task similar to one used in humans (Lencz et al., 2003). These data are similar to those observed by our group (Burdick et al., 2005) and others (Callicott et al., 2005; Hennah et al., 2005; Cannon et al., 2005), although no study to date has utilized the same neurocognitive tasks. Lipska et al. (2006) report that genes and proteins (NUDEL, FEZ1) known to interact with DISC1 are also aberrant in schizophrenia postmortem tissue, with some evidence that DISC1 risk polymorphisms also influence expression across the pathway.

Taken together, these two papers suggest that the assessment of genes involved in the DISC1 pathway may be worthwhile in the evaluation of working memory function. To date, most studies have focused on risk alleles within DISC1, with little attention paid to the critical interacting genes. Studies are now underway assessing the relationship between FEZ1 and NUDEL and risk for schizophrenia in a number of populations, as well as studies examining their role in neurocognitive and neuroimaging parameters. Clearly, as the Lipska paper indicates, studies that attempt to assess multiple genes in this pathway will be critical, although the common concern of power in assessing gene-gene interactions, especially across multiple genes, may be a limitation. Moreover, these studies indicate that interaction studies will need to consider additional phenotypes other than diagnosis, and perhaps “purer” tasks of neurocognitive function may be worthwhile, as suggested by Koike et al. Finally, both of these papers underscore the fact that the next wave of genetic studies of schizophrenia will encompass the use of multiple probes, whether with neurocognitive assessments, postmortem analyses, or animal models of disease, amongst others, to fully validate the relationships between putative risk genes and the pathophysiology of schizophrenia and related disorders.

References:

Burdick KE, Hodgkinson CA, Szeszko PR, Lencz T, Ekholm JM, Kane JM, Goldman D, Malhotra AK. DISC1 and neurocognitive function in schizophrenia. Neuroreport 2005; 16(12): 1399-1402. Abstract

Callicott JH, Straub RE, Pezawas L, Egan MF, Mattay VS, Hariri AR, Verchinski BA, Meyer-Lindenberg A, Balkissoon R, Kolachana B, Goldberg TE, Weinberger DR. Variation in DISC1 affects hippocampal structure and function and increases risk for schizophrenia. Proc Natl Acad Sci U S A. 2005 Jun 14;102(24):8627-32. Epub 2005 Jun 6. Abstract

Cannon TD, Hennah W, van Erp TG, Thompson PM, Lonnqvist J, Huttunen M, Gasperoni T, Tuulio-Henriksson A, Pirkola T, Toga AW, Kaprio J, Mazziotta J, Peltonen L. Association of DISC1/TRAX haplotypes with schizophrenia, reduced prefrontal gray matter, and impaired short- and long-term memory. Arch Gen Psychiatry, 2005; 62(11):1205-1213. Abstract

Hennah W, Tuulio-Henriksson A, Paunio T, Ekelund J, Varilo T, Partonen T, Cannon TD, Lonnquist J, Peltonen L. A haplotype within the DISC1 gene is associated with visual memory functions in families with high density of schizophrenia. Mol Psychiatry 2005; 10(12):1097-1103. Abstract

Lencz T, Bilder RM, Turkel E, Goldman RS, Robinson D, Kane JM, Lieberman JA. Impairments in perceptual competency and maintenance on a visual delayed match-to-sample test in first episode schizophrenia. Arch Gen Psychiatry 2003; 60(3):238-243. Abstract

In their recent paper, Koike et al. provide new evidence in support of a genetic determinant of working memory function in the vicinity of the mouse DISC1 gene. They report their discovery of a naturally occurring DISC1 deletion variant in the 129S6/SvEv mouse strain that leads to reduced protein expression and that provides a potentially very important new tool for analyzing the cellular and behavioral phenotypes associated with DISC1 insufficiency. Given the strong evidence of a relationship between a cytogenetic abnormality that leads to DISC1 truncation in humans and major mental illness (Millar et al., 2000), this murine model stands to greatly serve our understanding of the molecular and cellular determinants of poor cognition in schizophrenia and bipolar disorder.

The authors are parsimonious in reminding us of the substantial limitations of models such as this. Specifically, the current approach does not allow...  Read more

In their recent paper, Koike et al. provide new evidence in support of a genetic determinant of working memory function in the vicinity of the mouse DISC1 gene. They report their discovery of a naturally occurring DISC1 deletion variant in the 129S6/SvEv mouse strain that leads to reduced protein expression and that provides a potentially very important new tool for analyzing the cellular and behavioral phenotypes associated with DISC1 insufficiency. Given the strong evidence of a relationship between a cytogenetic abnormality that leads to DISC1 truncation in humans and major mental illness (Millar et al., 2000), this murine model stands to greatly serve our understanding of the molecular and cellular determinants of poor cognition in schizophrenia and bipolar disorder.

The authors are parsimonious in reminding us of the substantial limitations of models such as this. Specifically, the current approach does not allow for a clear statement that the DISC1 gene itself modulates the traits of interest. The DISC1 deletion variant may simply be in linkage disequilibrium with the actual phenotype-determining gene, and/or variation in DISC1 may influence cognition in a manner that is modified by a nearby genetic region. For example, Cannon et al. recently showed that a 4-SNP haplotype spanning DISC1 and an adjacent gene, translin-associated factor X (TRAX) is more predictive of anatomical and cognitive indices of reduced prefrontal cortical and hippocampal function than are any known haplotypes spanning DISC1 only. Clearly, additional consideration of the genetic environment in which DISC1 lies is necessary, and discovery of flanking regions that contain modifiers of the actions of DISC1, and vice versa, would be extremely interesting.

The greatest impact of the paper by Koike et al. is hinged on the fact that mice carrying one or two copies of the deletion variant exhibit poor choice accuracy in a delayed non-match to position task. Specifically, mutant DISC1 mice made fewer correct choices than did wild-type littermate C57 mice. Because a procedure such as this is necessarily psychologically complex, performance failure is hardly prima facie evidence for impairments of spatial working memory, or for prefrontal cortical dysfunction, in general. Nevertheless, the data are remarkable in establishing a phenotypic bridge between species and in laying the foundation for more sophisticated behavioral studies that will narrow in on the psychological constructs and neural systems affected by variation in this genetic region. Through facilitating a greater understanding of the cognitive phenotypes associated with DISC1 variation, the model should open doors to understanding key phenotypic aspects of schizophrenia and bipolar disorder.

References:

Koike H, Arguello PA, Kvajo M, Karayiorgou M, Gogos JA. Disc1 is mutated in the 129S6/SvEv strain and modulates working memory in mice. Proc Natl Acad Sci U S A. 2006 Feb 16; [Epub ahead of print] Abstract

Millar JK, Wilson-Annan JC, Anderson S, Christie S, Taylor MS, Semple CA, Devon RS, Clair DM, Muir WJ, Blackwood DH, Porteous DJ. Disruption of two novel genes by a translocation co-segregating with schizophrenia. Hum Mol Genet. 2000 May 22;9(9):1415-23. Abstract

Cannon TD, Hennah W, van Erp TG, Thompson PM, Lonnqvist J, Huttunen M, Gasperoni T, Tuulio-Henriksson A, Pirkola T, Toga AW, Kaprio J, Mazziotta J, Peltonen L. Association of DISC1/TRAX haplotypes with schizophrenia, reduced prefrontal gray matter, and impaired short- and long-term memory. Arch Gen Psychiatry. 2005 Nov;62(11):1205-13. Abstract

Disrupted In Schizophrenia 1 was first identified as a genetic susceptibility factor in schizophrenia because it is disrupted by a translocation between chromosomes 1 and 11 in a large Scottish family with a high loading of schizophrenia and related mental illness. Since then, numerous genetic studies have implicated DISC1 as a risk factor in psychiatric illness in several populations. Given the limitations on studies using brain tissue from patients, an obvious next step was to engineer knockout mice, but these have been slow in coming. As a first step toward this, Kioke and colleagues now report an unexpected naturally occurring genetic variant in the 129/SvEv mouse strain.

Kioke et al. report that the 129/SvEv mouse strain carries a 25 bp deletion in DISC1 exon 6, and that this results in a shift of open reading frame and introduction of a premature stop codon. Several embryonal stem cell lines have been isolated for the 129 strain, favoring it for gene targeting studies. However, this strain has a number of well-established behavioral characteristics (  Read more

Disrupted In Schizophrenia 1 was first identified as a genetic susceptibility factor in schizophrenia because it is disrupted by a translocation between chromosomes 1 and 11 in a large Scottish family with a high loading of schizophrenia and related mental illness. Since then, numerous genetic studies have implicated DISC1 as a risk factor in psychiatric illness in several populations. Given the limitations on studies using brain tissue from patients, an obvious next step was to engineer knockout mice, but these have been slow in coming. As a first step toward this, Kioke and colleagues now report an unexpected naturally occurring genetic variant in the 129/SvEv mouse strain.

Kioke et al. report that the 129/SvEv mouse strain carries a 25 bp deletion in DISC1 exon 6, and that this results in a shift of open reading frame and introduction of a premature stop codon. Several embryonal stem cell lines have been isolated for the 129 strain, favoring it for gene targeting studies. However, this strain has a number of well-established behavioral characteristics (http://www.informatics.jax.org/external/festing/mouse/docs/129.shtml). Therefore, to assign any phenotype specifically to the DISC1 deletion variant, the 129 DISC1 variant had to be transferred to a C57BL/6J background, with its own, rather different but equally characteristic behavior (http://www.informatics.jax.org/external/festing/mouse/docs/C57BL.shtml). There were no detectable gross morphological alterations in the prefrontal cortex, cortex, and hippocampus on transferring the 129 DISC1 locus onto the C57BL/6J background. However, the mutation did result in working memory deficits, consistent with several reports linking DISC1 to cognition.

It is difficult to know what phenotype to expect from a mouse model for schizophrenia, but in humans it is widely believed that mutations confer only a susceptibility to developing illness. Many susceptible individuals function apparently normally, although subtle neurological endophenotypes are detectable. In individuals who do go on to develop schizophrenia, cognitive deficits are a major characteristic. These mild cognitive deficits in mice with loss of DISC1 function are therefore close to what we might predict.

The molecular mechanism by which DISC1 confers susceptibility to psychiatric illness is the subject of some debate. Sawa and colleagues have suggested that a mutant truncated protein resulting from the t(1;11) is responsible for the psychiatric disorders in the Scottish family. Millar and colleagues, however, report that there is no evidence for such a hypothetical protein in t(1;11) cell lines, but rather that the levels of DISC1 transcript and protein are reduced, consistent with a haploinsufficiency model. Identification of the deletion in mice may shed further light on this debate, since while the mutation does not affect DISC1 transcript levels, no mutant truncated protein is detectable, even though such a protein might theoretically be produced as a result of the premature stop codon. Moreover, both homozygotes and heterozygotes have cognitive impairment, demonstrating that DISC1 haploinsufficiency is sufficient to affect central nervous system function.

In this initial study, Kioke and colleagues have left many questions unanswered. In particular, the behavioral studies are limited to one working memory task and one test of locomotion. Ideally, a whole battery of behavioral and cognitive tests should be performed. Since 129/SvEv mice reportedly have impaired hippocampal function, high levels of anxiety-like behavior and altered NMDA receptor-related activity, it will be interesting to discover which, if any, of these phenotypes also co-segregate with the 129 DISC1 variant. It is also interesting to note that the 129 strain is effectively a null for full-length DISC1, but with no gross alteration in brain morphology. This has to be reconciled with the observed effect of transient RNAi mediated down-regulated expression in utero (Kamiya et al., 2005) and the possible, but still anecdotal observation of embryonic lethality in experimental DISC1 knockouts.

This is outstanding work reporting DISC1 genetically engineered mice. Thus far, one type of DISC1 mutant mouse has been reported, by Gogos and colleagues (Koike et al., 2006).

There are two remarkable points in this work. First, of most importance, John Roder and Steve Clapcote have been very successful in using mice with ENU-induced mutations for their questions. Due to the complexity of the DISC1 gene and isoforms, several groups, including ours, have tried but not succeeded in generating knockout mice. However, Roder and Clapcote found alternative mice that could be used in testing our main hypothesis. I believe that the majority of the success in this work is on this particular point. Indeed, to explore animal models for other susceptibility genes for major mental illnesses, this approach should be considered.

Second, it is very interesting that different mutations in the same gene display different types of phenotypes. I appreciate the excellence in the extensive behavioral assays in this work.

Although we need...  Read more

This is outstanding work reporting DISC1 genetically engineered mice. Thus far, one type of DISC1 mutant mouse has been reported, by Gogos and colleagues (Koike et al., 2006).

There are two remarkable points in this work. First, of most importance, John Roder and Steve Clapcote have been very successful in using mice with ENU-induced mutations for their questions. Due to the complexity of the DISC1 gene and isoforms, several groups, including ours, have tried but not succeeded in generating knockout mice. However, Roder and Clapcote found alternative mice that could be used in testing our main hypothesis. I believe that the majority of the success in this work is on this particular point. Indeed, to explore animal models for other susceptibility genes for major mental illnesses, this approach should be considered.

Second, it is very interesting that different mutations in the same gene display different types of phenotypes. I appreciate the excellence in the extensive behavioral assays in this work.

Although we need to wait for any molecular and mechanistic analyses of these mice in the future, this work provides us outstanding methodologies in studying major mental conditions. I anticipate that four to five papers will come out in this year that report various types of DISC1 genetically engineered mice. Neutral comparison of all the DISC1 mice from different groups will provide important insights for DISC1 and its role in major mental conditions.

This paper demonstrates that mutations in DISC1 can alter mouse behavior, brain structure, and biochemistry, consistent with the idea that DISC1 is related to major psychiatric disorders. This is already an important result. But more strikingly, the authors’ interpretation is that one mutation (L100P) causes a phenotype similar to schizophrenia, while the other mutation (Q31L) results in a phenotype similar to affective disorder.

There are a number of caveats that need to be considered. No patients with equivalent mutations have been identified. The behavioral tests have only a hypothesized or empiric relevance to behavior in the human illnesses. DISC1 itself, while a very strong candidate gene, is still not fully validated, and the best evidence for its role in schizophrenia still arises from the single large pedigree in Scotland.

Despite these caveats, I believe this paper is potentially a major advance. The authors’ interpretations are provocative, and could have far-reaching implications for understanding of the biological bases of psychiatric diseases. The...  Read more

This paper demonstrates that mutations in DISC1 can alter mouse behavior, brain structure, and biochemistry, consistent with the idea that DISC1 is related to major psychiatric disorders. This is already an important result. But more strikingly, the authors’ interpretation is that one mutation (L100P) causes a phenotype similar to schizophrenia, while the other mutation (Q31L) results in a phenotype similar to affective disorder.

There are a number of caveats that need to be considered. No patients with equivalent mutations have been identified. The behavioral tests have only a hypothesized or empiric relevance to behavior in the human illnesses. DISC1 itself, while a very strong candidate gene, is still not fully validated, and the best evidence for its role in schizophrenia still arises from the single large pedigree in Scotland.

Despite these caveats, I believe this paper is potentially a major advance. The authors’ interpretations are provocative, and could have far-reaching implications for understanding of the biological bases of psychiatric diseases. The models provide strong support for further study of DISC1. DISC1 has numerous very interesting interacting proteins and thus may provide an entry into pathogenic pathways for psychiatric diseases. We have suggested that interactors at the centrosome, involved with neuronal development, may be especially relevant to schizophrenia, while interactors at the synapse, or related to signal transduction, may be especially relevant to affective disorder (Ross et al., 2006). The beginnings of an allelic series of DISC1 mutations will presage more detailed genotype-phenotype studies in a variety of mouse models, with potential relevance to both schizophrenia and affective disorder.

Mutant Mice Bring Further Excitement to the DISC1-PDE4 Arena
DISC1 continues to ride a wave of optimism as we look for real breakthroughs in the molecular events underlying major psychiatric disorders including schizophrenia, bipolar, and depression. In 2005, its fortunes became entwined with those of the phosphodiesterase PDE4B as they were shown to functionally and physically interact (Millar et al., 2005). Evidence linking PDE4B to depression has been known for some time, but in the wake of the DISC1 finding, its link to schizophrenia has hardened (Siuciak et al., 2007; Menniti et al., 2006; Pickard et al., 2007).

The Roder and Porteous labs have come together to produce a fantastic paper describing two ENU mutant mice lines with specific mutations in the N-terminus of DISC1. Luck was on their side as the mutations seem to have a direct impact on the interaction with the PDE4B. Furthermore, the two lines look to have...  Read more

Mutant Mice Bring Further Excitement to the DISC1-PDE4 Arena
DISC1 continues to ride a wave of optimism as we look for real breakthroughs in the molecular events underlying major psychiatric disorders including schizophrenia, bipolar, and depression. In 2005, its fortunes became entwined with those of the phosphodiesterase PDE4B as they were shown to functionally and physically interact (Millar et al., 2005). Evidence linking PDE4B to depression has been known for some time, but in the wake of the DISC1 finding, its link to schizophrenia has hardened (Siuciak et al., 2007; Menniti et al., 2006; Pickard et al., 2007).

The Roder and Porteous labs have come together to produce a fantastic paper describing two ENU mutant mice lines with specific mutations in the N-terminus of DISC1. Luck was on their side as the mutations seem to have a direct impact on the interaction with the PDE4B. Furthermore, the two lines look to have distinct phenotypes—one a little schizophrenic, the other depressive. It is known from the clinical and genetic data that DISC1 is associated with schizophrenia, bipolar, and MDD, so this mouse dichotomy is very intriguing.

The mutant line Q31L is claimed to have a “depressive-like” phenotype. This comes from behavioral experiments including a range of assays looking at depressive-like behaviors where this strain had severe deficits, treatable with the dual serotonin-noradrenaline reuptake inhibitor (SNRI) bupropion, commonly prescribed for depression. Together these findings could just as easily be linked to the negative symptoms of schizophrenia. Furthermore, Q31L also shows modest deficits in two sensory processing paradigms (latent inhibition and pre-pulse inhibition), for which antipsychotics had no impact, and a working memory deficit, so this strain has characteristics of all the three key domains of schizophrenia. The pharmacology gets more interesting when these animals are dosed with rolipram (PDE4 inhibitor, raises cAMP levels) and look to be resistant to its effects. At the protein level, while it effects no changes in absolute levels of DISC1 and PDE4B, it leads to a 50 percent reduction in PDE4 activity. This information connects together nicely with the rolipram resistance, and thus the authors suggest that elevated cAMP might explain the behaviors observed, but they unfortunately do not show any cAMP levels in these animals. The paper also reports a decreased binding of the mutant form of DISC1 with PDE4B in overexpressed systems; coupled with the decreased PDE activity, this is in slight contradiction to the original Millar paper (Millar et al., 2005), but as the authors explain, the complexity of the DISC1-PDE4 molecular partnership could easily explain this. From my perspective, the lack of data to date on DISC1-PDE4 brain complexes is a major weak point of this story—this needs to be addressed as we move forward. This will also allow us to understand better the role of different DISC1 isoforms.

L100P is the “schizophrenic” brother of Q31P and has severe deficits in two sensory processing paradigms (latent inhibition and pre-pulse inhibition) which is reversed by typical and atypical antipsychotic and rolipram. Rolipram is able to modulate the behavior as PDE4 activity levels are at a wild-type level. Again, it shows decreased levels of DISC1-PDE4 binding.

Together, these two lines, along with the Gogos mice and a further bank of DISC1 mice which we should expect to see in the next year, puts the field in a position where we are now able to start to dissect out the clearly complex biological functions of DISC1. But as I indicated earlier, we need more information on relevant DISC1 isoforms. We know from the DISC1 interactome that there are many exciting partnerships to develop, but we may not have the fortune of an ENU screen to pull out mice with specific effects on an interaction. The differences in the behavior and pharmacology of these two strains is striking. In combination with the impact on PDE4-DISC1 binding and PDE4 activity, it highlights how much still needs to be understood for this interaction alone. More immediately, the mice show clearly that specific DISC1 mutations may give rise to specific clinical end-points and open up DISC1 pharmacogenomics as a real possibility.

Several genetic studies point to involvement of DISC1 in major psychiatric illness, including schizophrenia and bipolar disorder, but to date the only causal variant that has been definitively identified is the translocation between human chromosomes 1 and 11 that co-segregates with major mental illness in a large Scottish family and which directly disrupts the DISC1 gene (Millar at al., 2000). It has been speculated that a truncated form of DISC1 may be expressed from the translocated allele and, if so, that this could exert a dominant-negative effect, but there is no such evidence from studies of the translocation cases. Rather, the evidence from studies of lymphoblastoid cell lines carrying the translocation suggests that haploinsufficiency is the most likely disease mechanism in this family (Millar et al., 2005). The unresolvable caveat to this, of course, is that it has not been possible to determine whether this is true also for the brain. Moreover, it is far from certain that any...  Read more

Several genetic studies point to involvement of DISC1 in major psychiatric illness, including schizophrenia and bipolar disorder, but to date the only causal variant that has been definitively identified is the translocation between human chromosomes 1 and 11 that co-segregates with major mental illness in a large Scottish family and which directly disrupts the DISC1 gene (Millar at al., 2000). It has been speculated that a truncated form of DISC1 may be expressed from the translocated allele and, if so, that this could exert a dominant-negative effect, but there is no such evidence from studies of the translocation cases. Rather, the evidence from studies of lymphoblastoid cell lines carrying the translocation suggests that haploinsufficiency is the most likely disease mechanism in this family (Millar et al., 2005). The unresolvable caveat to this, of course, is that it has not been possible to determine whether this is true also for the brain. Moreover, it is far from certain that any productive product from the translocation chromosome would be a perfectly truncated protein encoded by all the remaining exons, as opposed to an exon-skip isoform, with or without a hybrid protein component borrowing sequence information from chromosome 11. What does seem likely from other human studies is that additional genetic mechanisms, including missense mutations, altered expression, and possibly also copy number variation, play a role in the generality of DISC1 as a risk factor.

The evidence in support of DISC1 as an important biological determinant across a spectrum of major mental illness has now received a further boost from the study in PNAS by Hikida et al. The Sawa lab describes a transgenic approach where a truncated human DISC1 protein is expressed from a CAMKII promoter. The truncation was designed to mimic the hypothetical truncation arising from the Scottish translocation. This forebrain-specific promoter confers preferential expression of the transgene at neonatal stages, as distinct from the expression of the endogenous protein, which is abundant from embryonic development into adulthood. This model therefore permits investigation of the effect of the truncated protein in the forebrain within a specific developmental window, against a background of endogenous mouse DISC1 expression. Since there is no evidence for production of a truncated protein from the translocated allele, the relevance of this model to psychiatric illness remains unclear. However, on the positive side and from a functional perspective, dominant-negative effects as a consequence of expressing the truncated protein have already been demonstrated in cultured cells, altering the subcellular distribution of DISC1 and interaction with DISC1 partner proteins. Moreover, expression of the truncated form of DISC1 mimics downregulation of DISC1 in vivo, inhibiting migration of neurons in the developing mouse cortex (Kamiya et al., 2005). Thus, this model has the genuine potential to enhance our understanding of the biology of DISC1.

This is, in fact, the third study describing mice expressing modified DISC1 alleles. In the first study, Gogos and colleagues (Kioke et al., 2006) studied the effects of a modified DISC1 allele carrying a spontaneous 25 bp deletion in exon 6 that is present in all 129 mouse strains (Koike et al., 2007; see SRF related news story). This allele additionally has an artificial stop codon in exon 8 and a downstream polyadenylation signal. After back-crossing this mutagenised version of the 129 allele onto a C57Bl6 background, they reported a deficit in an assay of working memory in both heterozygous and homozygous mutants, but other standard behavioral tests were unaltered or unreported, and there were no anatomical, electrophysiological, or pharmacological studies included. In the second study, one led by the Roder laboratory, Toronto, we described two mouse strains with missense mutations in exon 2, Q31L and L100P (Clapcote et al., 2007). Reductions in brain volume, deficits in a range of standard behavioral tests, and responses to pharmacological treatments were reported, which can be summarized as consistent with the 100P mutants displaying schizophrenia-like phenotypes and the 31L mutants, mood disorder-like phenotypes. There is a frustrating dearth of consistent biomarkers for schizophrenia, but one of the best replicated findings is by brain imaging of enlarged ventricles in schizophrenia (also, but less markedly, in bipolar disorder). Adding to the observations of Clapcote et al., arguably the most striking claim by Hikida et al. is for altered ventricular brain volume and reduced brain laterality following neonatal transgenic overexpression of truncated DISC1. Additionally, behavioral tests were reported that overlap in part with those reported earlier by Clapcote et al. That three studies all describe behavioral abnormalities consistent with modeling components of schizophrenia is reassuring. That there are clear differences, too, between the phenotypes should come as no surprise either, given the important differences in terms of genetic lesion and developmental expression. Other mouse models are in the pipeline and they, too, will be welcomed. Indeed, this is very much what is needed for the field to move forward. But we should do so with some caution, paying careful attention to the specific nature of the models, what they can and cannot tell us about DISC1 biology, and what they may or may not tell us about the human condition. Although none of these models relates directly to a known causal variant, it appears that the mouse models concur with the human genetic studies in suggesting that there are likely to be several routes by which DISC1 can perturb brain function leading to characteristics of human mental illness. What we need now is for the human genetic studies to catch up with the mouse so that defined pathognomic variants can be modeled.

A new mouse model from the Sawa lab strengthens the evidence for the candidate gene DISC1 playing a role in psychosis and mood disorders. This important paper is the first to address one potential disease mechanism, that of a dominant-negative effect. Expression of the C-terminal deletion of human DISC1—which represented the original rearrangement found by the Porteous group in the Scottish families with schizophrenia and depression—in transgenic mice driven by the α CaMKII promoter, first described by Mark Mayford when a postdoctoral fellow in the Kandel lab, leads to mice showing behaviors consistent with schizophrenia and depression, with enlarged lateral ventricles. Since the Sawa group expressed the human C-terminal truncation in mouse with no change in mouse DISC1 levels, they feel this supports a dominant-negative mechanism. More direct experiments are required. For example, create a null mutant mouse for DISC1 and express the full-length and truncated human DISC1 under the influence of their own promoter in transgenic mice using human BACs. Full-length...  Read more

A new mouse model from the Sawa lab strengthens the evidence for the candidate gene DISC1 playing a role in psychosis and mood disorders. This important paper is the first to address one potential disease mechanism, that of a dominant-negative effect. Expression of the C-terminal deletion of human DISC1—which represented the original rearrangement found by the Porteous group in the Scottish families with schizophrenia and depression—in transgenic mice driven by the α CaMKII promoter, first described by Mark Mayford when a postdoctoral fellow in the Kandel lab, leads to mice showing behaviors consistent with schizophrenia and depression, with enlarged lateral ventricles. Since the Sawa group expressed the human C-terminal truncation in mouse with no change in mouse DISC1 levels, they feel this supports a dominant-negative mechanism. More direct experiments are required. For example, create a null mutant mouse for DISC1 and express the full-length and truncated human DISC1 under the influence of their own promoter in transgenic mice using human BACs. Full-length human DISC1 would, hopefully, rescue the null. If so, then a mixture of full-length and truncated DISC1 proteins could be tried. No rescue by the mixture of full-length and truncated proteins would suggest a dominant-negative mechanism.

The Porteous group has shown no detectable DISC1 protein in lymphoblasts from the patients, and put forward the following implicit model. The C-terminal truncation of DISC1 makes the protein unstable and sensitive to degradation, a plausible alternative notion. In my opinion both are likely in different schizophrenia patients with perturbations in DISC1, most of which are alterations other than the C-terminal truncation. Some have SNPs that lead to as yet uncharacterized disease. It has been shown by the Sawa lab that mice with a partial RNAi knockdown of DISC1 show perturbations in brain development, and if aged to 8-12 weeks these mice might have shown behavioral and neuropathological hallmarks of schizophrenia and depression. There is currently no null mutation in the mouse that would address this issue, although DISC1 is one of the genes being targeted in the NIH knockout mouse project. Fortunately, there are now several mouse models—the more the better. The Gogos lab has a 25bp deletion in exon 6 that removes some, but not all isoforms. The Roder lab used a reverse genetic screen of an ENU archive to generate two missense mutants in non-conserved amino acids. The phenotypes of all these lines are nicely summarized in the Sawa paper. This work represents a step forward in our understanding of the DISC1 gene.

Several recent studies on disruptions of the DISC1 gene in mice illustrate the great potential of genetic approaches to studying functions of putative schizophrenia susceptibility genes but also signal the complexity of the problem. An initial rationale for studying the effects of mutations in DISC1 came from the discovery of the chromosomal translocation, resulting in a breakpoint in the DISC1 gene that co-segregated with major mental illness in a Scottish family (reviewed by Porteous et al., 2006). These clinical findings were followed by a number of association studies, which reported that numerous SNPs across the gene were associated with schizophrenia and mood disorders and a variety of intermediate phenotypes, suggesting that other problems in the DISC1 gene may exist in other subjects/populations.

Recent animal models designed to mimic partial loss of DISC1 function suggested that DISC1 is necessary to support development of the cerebral cortex as its loss resulted in impaired neurite...  Read more

Several recent studies on disruptions of the DISC1 gene in mice illustrate the great potential of genetic approaches to studying functions of putative schizophrenia susceptibility genes but also signal the complexity of the problem. An initial rationale for studying the effects of mutations in DISC1 came from the discovery of the chromosomal translocation, resulting in a breakpoint in the DISC1 gene that co-segregated with major mental illness in a Scottish family (reviewed by Porteous et al., 2006). These clinical findings were followed by a number of association studies, which reported that numerous SNPs across the gene were associated with schizophrenia and mood disorders and a variety of intermediate phenotypes, suggesting that other problems in the DISC1 gene may exist in other subjects/populations.

Recent animal models designed to mimic partial loss of DISC1 function suggested that DISC1 is necessary to support development of the cerebral cortex as its loss resulted in impaired neurite outgrowth and the spectrum of behavioral abnormalities characteristic of major mental disorders ( Kamiya et al., 2005; Koike et al., 2006; Clapcote et al., 2007; Hikida et al. 2007). Unexpectedly, however, the paper by Duan et al., 2007, is showing that DISC1 may also function as a brake and master regulator of neuronal development, and that its partial loss could lead to the opposite effects than previously described, i.e., dendritic overgrowth and accelerated synapse formation and faster maturation of newly generated neurons. In contrast to previous studies, they have used the DISC1 knockdown model achieved by RNA interference in a subpopulation of single cells of the dentate gyrus. Other emerging studies continue to reveal the highly complex nature of the DISC1 gene with multiple isoforms exhibiting different functions, perhaps depending on localization, timing, and interactions with a multitude of other genes’ products, some of which confer susceptibility to mental illness independent of DISC1. Similar molecular complexity has also emerged in other susceptibility genes for schizophrenia: GRM3 (Sartorius et al., 2006), NRG1 (Tan et al., 2007), and COMT (Tunbridge et al., 2007). With the growing knowledge about transcript complexity, it becomes increasingly clear that subtle disturbances of isoform(s) of susceptibility gene products and disruptions of intricate interactions between the susceptibility genes may account for the etiology of neuropsychiatric disorders. Research in animals will have a critical role in disentangling this web of interwoven genetic pathways.

I am very glad that our colleagues at Johns Hopkins University have published a very intriguing paper in Cell, showing a novel role for DISC1 in adult hippocampus. This is very consistent with previous publications (Miyoshi et al., 2003; Kamiya et al., 2005; and others; reviewed by Ishizuka et al., 2006), and adds a new insight into a key role for DISC1 during neurodevelopment. In short, DISC1 is a very important regulator in various phases of neurodevelopment, which is reinforced in this study. Specifically, DISC1 is crucial for regulating neuronal migration and dendritic development—for acceleration in the developing cerebral cortex, and for braking in the adult hippocampus.

There is precedence for signaling molecules playing the same role in different contexts, with the resulting molecular activity going in different directions. For example, FOXO3 (a member of the Forkhead transcription factor family) plays a role in...  Read more

I am very glad that our colleagues at Johns Hopkins University have published a very intriguing paper in Cell, showing a novel role for DISC1 in adult hippocampus. This is very consistent with previous publications (Miyoshi et al., 2003; Kamiya et al., 2005; and others; reviewed by Ishizuka et al., 2006), and adds a new insight into a key role for DISC1 during neurodevelopment. In short, DISC1 is a very important regulator in various phases of neurodevelopment, which is reinforced in this study. Specifically, DISC1 is crucial for regulating neuronal migration and dendritic development—for acceleration in the developing cerebral cortex, and for braking in the adult hippocampus.

There is precedence for signaling molecules playing the same role in different contexts, with the resulting molecular activity going in different directions. For example, FOXO3 (a member of the Forkhead transcription factor family) plays a role in cell survival/death in a bidirectional manner (Brunet et al., 2004). FOXO3 endows cells with resistance to oxidative stress in some contexts, and induces apoptosis in other contexts. SIRT1 (known as a key modulator of organismal lifespan) deacetylates FOXO3 and tips FOXO3-dependent responses away from apoptosis and toward stress resistance. In analogy to FOXO3, context-dependent post-translational modifications, such as phosphorylation, may be an underlying mechanism for DISC1 to function in a bidirectional manner. Indeed, a collaborative team at Johns Hopkins, including Pletnikov's lab, Song's lab, and ours, has started exploring, in both cell and animal models, the molecular switch that makes DISC1's effects bidirectional.

References:

Brunet A, Sweeney LB, Sturgill JF, Chua KF, Greer PL, Lin Y, Tran H, Ross SE, Mostoslavsky R, Cohen HY, Hu LS, Cheng HL, Jedrychowski MP, Gygi SP, Sinclair DA, Alt FW, Greenberg ME. Stress-dependent regulation of FOXO transcription factors by the SIRT1 deacetylase. Science. 2004 Mar 26;303(5666):2011-5. Abstract

Recent findings, including the interactome study by Camargo et al., 2007, and this beautiful study by Duan and colleagues, implicate DISC1 (a leading candidate schizophrenia susceptibility gene) in synaptic function, consistent with prevailing ideas of the disorder as one of the synapse and connectivity (see Stephan et al., 2006). As we learn more about DISC1 and its protein partners, evidence demonstrating the importance of microtubules in the regulation of several neuronal processes (see Eastwood et al., 2006, for review) suggests that DISC1’s interactions with microtubule associated proteins (MAPs) may underpin its pathogenic influence.

DISC1 has been shown to bind to several MAPs (e.g., MAP1A, MIPT3) and other proteins important in regulating microtubule function (see Kamiya et al., 2005; Porteous et al., 2006). As a key component of the cell...  Read more

Recent findings, including the interactome study by Camargo et al., 2007, and this beautiful study by Duan and colleagues, implicate DISC1 (a leading candidate schizophrenia susceptibility gene) in synaptic function, consistent with prevailing ideas of the disorder as one of the synapse and connectivity (see Stephan et al., 2006). As we learn more about DISC1 and its protein partners, evidence demonstrating the importance of microtubules in the regulation of several neuronal processes (see Eastwood et al., 2006, for review) suggests that DISC1’s interactions with microtubule associated proteins (MAPs) may underpin its pathogenic influence.

DISC1 has been shown to bind to several MAPs (e.g., MAP1A, MIPT3) and other proteins important in regulating microtubule function (see Kamiya et al., 2005; Porteous et al., 2006). As a key component of the cell cytoskeleton, microtubules are involved in many cellular processes including mitosis, motility, vesicle transport, and morphology, and their dynamics are regulated by MAPs, which modulate microtubule polymerization, stability, and arrangement. Decreased microtubule stability in mutant mice for one MAP, stable tubule only polypeptide (STOP; MAP6), results in behavioral changes relevant to schizophrenia and altered synaptic protein expression (Andrieux et al., 2002; Eastwood et al., 2006), indicating the importance of microtubules in synaptic function and suggesting that they may be a molecular mechanism contributing to the pathogenesis of schizophrenia. Likewise, DISC1 mutant mice exhibit behavioral alterations characteristic of psychiatric disorders (e.g., Clapcote et al., 2007), and altered microtubule dynamics are thought to underlie perturbations in cerebral cortex development and neurite outgrowth caused by decreased DISC1 expression or that of a schizophrenia-associated DISC1 mutation (Kamiya et al., 2005).

Our interpretation of the possible functions of DISC1 has been complicated by the unexpected findings of Duan and colleagues that DISC1 downregulation during adult hippocampal neurogenesis leads to overextended neuronal migration and accelerated dendritic outgrowth and synaptic formation. In terms of neuronal positioning, they suggest that their results indicate that DISC1 may relay positional signals to the intracellular machinery, rather than directly mediate migration. In this way, decreased DISC1 expression may result in the mispositioning of newly formed neurons rather than a simple decrease or increase in their migratory distance. Of note, MAP1B, a neuron-specific MAP important in regulating microtubule stability and the crosstalk between microtubules and actin, is required for neurons to correctly respond to netrin 1 signaling during neuronal migration and axonal guidance (Del Rio et al., 2004), and DISC1 may function similarly during migration. Reconciling differences between the effect of decreased DISC1 expression upon neurite outgrowth during neurodevelopment and adult neurogenesis is more difficult, but could be due to differences in the complement of MAPs expressed by different neuronal populations at different times. Regardless, the results of Duan and colleagues have provided additional evidence implicating DISC1 in neuronal functions thought to go awry in schizophrenia. Further characterization of DISC1’s interactions with microtubules and MAPs may lead to a better understanding of the role of DISC1 in the pathogenesis of psychiatric disorders.

References:

Andrieux A, Salin PA, Vernet M, Kujala P, Baratier J, Gory Faure S, Bosc C, Pointu H, Proietto D, Schweitzer A, Denarier E, Klumperman J, Job D (2002). The suppression of brain cold-stable microtubules in mice induces synaptic deficits associated with neuroleptic-sensitive behavioural disorders. Genes Dev. 16: 2350-2364. Abstract

Camargo LM, Collura V, Rain JC, Mizuguchi K, Hermjakob H, Kerrien S, Bonnert TP, Whiting PJ, Brandon NJ (2007). Disrupted in Schizophrenia 1 Interactome: evidence for the close connectivity of risk genes and a potential synaptic basis for schizophrenia. Mol. Psychiatry 12: 74-86. Abstract

Clapcote SJ, Lipina TV, Millar JK, Mackie S, Christie S, Ogawa F, Lerch JP, Trimble K, Uchiyama M, Sakuraba Y, Kaneda H, Shiroishi T, Houslay MD, Henkelman RM, Sled JG, Gondo Y, Porteous DJ, Roder JC (2007). Behavioral phenotypes of Disc1 missense mutations in mice. Neuron 54: 387-402. Abstract

Del Rio, J.A., Gonzalez-Billault, C., Urena, J.M., Jimenez, E.M., Barallobre, M.J., Pascual, M., Pujadas, L., Simo, S., La Torre, A., Wandosell, F., Avila, J. and Soriano, E. (2004). MAP1B is required for netrin 1 signaling in neuronal migration and axonal guidance. Cur. Biol. 14: 840-850. Abstract

Eastwood SL, Lyon L, George L, Andrieux A, Job D, Harrison PJ (2006). Altered expression of synaptic protein mRNAs in STOP (MAP6) mutant mice. J. Psychopharm. 21: 635-644. Abstract

Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME, Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nat Cell Biol. 2005 Dec;7(12):1167-78. Epub 2005 Nov 20. Erratum in: Nat Cell Biol. 2006 Jan;8(1):100. Abstract

Porteous DJ, Thomson P, Brandon NJ, Millar JK (2006). The genetics and biology of DISC1-an emerging role in psychosis and cognition. Biol. Psychiatry 60: 123-131. Abstract

Stephan KE, Baldeweg T, Friston KJ (2006). Synaptic plasticity and disconnection in schizophrenia. Biol. Psychiatry 59: 929-939. Abstract

This is a useful model from Pletnikov, Ross, and colleagues, but like all models, it has some limitations. Since DISC1 is known to have a strong role in development and physiology, the development of inducible mutants is necessary to separate the two.

In the TeT-off system used in the paper, mice must be treated with doxycycline for their entire lives to keep the expression of this gene off. Doxycycline must be used at high levels and may have side effects when used this long. The TeT-on system is better because doxycycline is only used transiently for 1 week for maximum induction then washed away. The TeT-on system is also available for the same promoter used in the paper, that of the CaMKII gene.

The phenotype of reduced neurite length was obtained from in vitro neuron cultures, which are prone to artifacts. There are ways of labeling these neurons in vivo for measuring neurite length and spines. The brain phenotype was obtained by MRI. There are ways, such as adding manganese, of enhancing active pathways. This has been done in the bird brain to map song...  Read more

This is a useful model from Pletnikov, Ross, and colleagues, but like all models, it has some limitations. Since DISC1 is known to have a strong role in development and physiology, the development of inducible mutants is necessary to separate the two.

In the TeT-off system used in the paper, mice must be treated with doxycycline for their entire lives to keep the expression of this gene off. Doxycycline must be used at high levels and may have side effects when used this long. The TeT-on system is better because doxycycline is only used transiently for 1 week for maximum induction then washed away. The TeT-on system is also available for the same promoter used in the paper, that of the CaMKII gene.

The phenotype of reduced neurite length was obtained from in vitro neuron cultures, which are prone to artifacts. There are ways of labeling these neurons in vivo for measuring neurite length and spines. The brain phenotype was obtained by MRI. There are ways, such as adding manganese, of enhancing active pathways. This has been done in the bird brain to map song pathways.

The behavioral phenotype was similar to the recent paper from the Sawa group (Hikida et al., 2007) in that it also analyzed a transgenic mouse expressing the same C-terminal truncation of the human DISC1 gene, using the same CaMKII promoter. An important difference in the findings was a reduction of murine DISC1 (50 percent at protein level) in the Pletnikov et al. mice but none in the Sawa group mice. This issue is important because of a recent paper in Cell by the Song group (Duan et al., 2007). In that paper, RNAi was used to reduce wild-type native murine DISC1. Individual neurons with targeted DISC1 knockdown showed accelerated neurite development, greater synapse formation and enhanced excitability. Hippocampal granule cells showed accelerated morphological integration resulting in mispositioning. Unfortunately, in the Song paper they analyzed only cells with complete or no knockdown of DISC1. Partial knockdown vectors were made that achieved 75 percent reduction at the protein level but were not analyzed. Only then would it be possible to compare these morphological data with those from Pletnikov et al., which was a 50 percent reduction. Another difference was that the Song group found that DISC1 needed to interact with Nudel. Pletnikov et al. found normal levels of Nudel in the mice but lower LIS1, which could explain the brain development phenotype.

Mao and colleagues present an impressive body of work implicating GSK3β/β-catenin signaling in the function of Disc1. However, several key experimental controls are missing that detract from the impact of their study, and it is unclear whether this function of Disc1 among its many others is the critical link between the t(1;11) translocation and psychopathology in the Scottish family.

The results of Mao et al. suggest that acute knockdown of Disc1 in embryonic brain causes premature exit from the proliferative cell cycle and premature differentiation into neurons. In fact, they observe fewer GFP+ cells in the VZ/SVZ and greater GFP+ cells within the cortical plate. This is in contrast to the study by Kamiya et al. (2005), in which they find that knocking down Disc1 caused greater retention of cells in the VZ/SVZ and fewer in the cortical plate, suggesting retarded migration. Although the timing of electroporation (E13 vs. E14.5) and examination (E15 vs. P2) differed between the two studies, these results are not...  Read more

Mao and colleagues present an impressive body of work implicating GSK3β/β-catenin signaling in the function of Disc1. However, several key experimental controls are missing that detract from the impact of their study, and it is unclear whether this function of Disc1 among its many others is the critical link between the t(1;11) translocation and psychopathology in the Scottish family.

The results of Mao et al. suggest that acute knockdown of Disc1 in embryonic brain causes premature exit from the proliferative cell cycle and premature differentiation into neurons. In fact, they observe fewer GFP+ cells in the VZ/SVZ and greater GFP+ cells within the cortical plate. This is in contrast to the study by Kamiya et al. (2005), in which they find that knocking down Disc1 caused greater retention of cells in the VZ/SVZ and fewer in the cortical plate, suggesting retarded migration. Although the timing of electroporation (E13 vs. E14.5) and examination (E15 vs. P2) differed between the two studies, these results are not easily reconciled.

The authors also suggest that they can rescue the deficits in proliferation by overexpressing human wild-type DISC1, stabilizing β-catenin expression, or inhibiting GSK3β activity, and thus conclude that Disc1 is acting through this pathway. This conclusion, however, rests on an error in logic. If increasing X causes an increase in Y, and decreasing Z causes a decrease in Y, this does not mean that X and Z are operating via the same mechanism. In fact, overexpressing WT-DISC1, stabilizing β-catenin, or inhibiting GSK3β activity all increase proliferation in control cells. Thus, the fact that these manipulations also work in progenitors with Disc1 silenced only tells us that these effects are independent or downstream of Disc1. What are needed are studies that show a differential sensitivity of Disc1-silenced cells to manipulations of β-catenin or GSK3β. In other words, is there a shift in the dose response curves? This is what is to be expected given that Mao et al. show changes in β-catenin levels and changes in the phosphorylation of GSK3β substrates in Disc1 silenced cells.

Furthermore, it is surprising that a restricted silencing of Disc1 in the adult dentate gyrus produces changes in affective behaviors, when total ablation of dentate neurogenesis in the adult produces little effects on depression-related behaviors (Santarelli et al., 2003; Airen et al., 2007). The fact that inhibiting GSK3β increases proliferation in both control and Disc1 knockdown animals to a similar degree suggests that the “rescue” of any behavioral deficits is independent of the drug’s effects on proliferation. Correlating measures of proliferation with behavioral performance would help address this issue.

How this study will lead to new or improved therapeutic interventions is also an open question. Lithium is well known for its mood-stabilizing properties, and this study may point to better, more efficient ways to address these symptoms. However, it is also known that lithium does little for, if not worsens, cognitive symptoms in patients (Pachet and Wisniewski, 2003), and it is this symptom domain that is in dire need of drug development.

It is also important to keep in mind that acute silencing of Disc1 in a restricted set of cells will not necessarily recapitulate the pathogenetic process of a disease-associated mutation. It remains to be seen if similar results are obtained in animal models of the Disc1 mutation (Clapcote et al., 2007; Hikida et al., 2007; Li et al., 2007).

References:

Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME, Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nat Cell Biol. 2005 Dec 1;7(12):1167-78. Abstract

Santarelli, L. et al. Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants. Science 301, 805–809 (2003). Abstract

Airan, R.D. et al. High-speed imaging reveals neurophysiological links to behavior in an animal model of depression. Science 317, 819-23 (2007). Abstract

Pachet AK, Wisniewski AM. The effects of lithium on cognition: an updated review. Psychopharmacology (Berl). 2003 Nov;170(3):225-34. Review. Abstract

Clapcote SJ, Lipina TV, Millar JK, Mackie S, Christie S, et al. (2007) Behavioral phenotypes of Disc1 missense mutations in mice. Neuron 54: 387–402. Abstract

Hikida T, Jaaro-Peled H, Seshadri S, Oishi K, Hookway C, et al. (2007) Dominant-negative DISC1 transgenic mice display schizophrenia-associated phenotypes detected by measures translatable to humans. Proc Natl Acad Sci U S A 104: 14501–14506. Abstract

Li W, Zhou Y, Jentsch JD, Brown RA, Tian X, et al. (2007) Specific developmental disruption of disrupted-in-schizophrenia-1 function results in schizophrenia-related phenotypes in mice. Proc Natl Acad Sci U S A 104: 18280–18285. Abstract

This is an intriguing paper that builds on a growing body of evidence implicating wnt regulation of GSK3 signaling in psychotic illness (Lovestone et al., 2007).

It is interesting that the authors report that binding of DISC1 to GSK3 results in no change in the inhibitory Ser9 phosphorylation site of GSK3 but a change in Y216 activation site and that this resulted in effects on some but not all GSK3 substrates. This poses a challenge both in terms of understanding the role of GSK3 signaling in schizophrenia and other psychotic disorders and in drug discovery.

The authors cite some of the other evidence for regulation of GSK3 signaling in psychosis, including, for example, the evidence for a role of AKT signaling alteration in schizophrenia and lithium, an inhibitor of GSK3, as a treatment for bipolar disorder. But in both cases, AKT (Cross et al., 1995) and lithium (Jope, 2003), the effect on GSK3 is predominantly via Ser9...  Read more

This is an intriguing paper that builds on a growing body of evidence implicating wnt regulation of GSK3 signaling in psychotic illness (Lovestone et al., 2007).

It is interesting that the authors report that binding of DISC1 to GSK3 results in no change in the inhibitory Ser9 phosphorylation site of GSK3 but a change in Y216 activation site and that this resulted in effects on some but not all GSK3 substrates. This poses a challenge both in terms of understanding the role of GSK3 signaling in schizophrenia and other psychotic disorders and in drug discovery.

The authors cite some of the other evidence for regulation of GSK3 signaling in psychosis, including, for example, the evidence for a role of AKT signaling alteration in schizophrenia and lithium, an inhibitor of GSK3, as a treatment for bipolar disorder. But in both cases, AKT (Cross et al., 1995) and lithium (Jope, 2003), the effect on GSK3 is predominantly via Ser9 phosphorylation and not via Y216. The unstated implication is at least two, possibly three, mechanisms for regulation of GSK3 are all involved in psychotic illness—the auto-phosphorylation at Y216, the exogenous signal transduction regulated Ser9 site inhibition and, if the association of schizophrenia with the wnt inhibitor DKK4 we reported is true (Proitsi et al., 2008), also via the wnt signaling effects on disruption of the macromolecular complex that brings GSK3 together with β-catenin. On the one hand, this might be taken as positive evidence of a role for GSK3 in psychosis—all of its regulatory mechanisms have been implicated; therefore, the case is stronger. On the other hand, GSK3 lies at the intersection point of very many signaling pathways and so is likely to be implicated in many disorders (as it is), and the fact that in cellular and animal models related to psychosis there is no consistent effect on the enzyme is troublesome.

From a drug discovery perspective, those with GSK3 inhibitors in the pipeline will be watching this space carefully. However, it is worth noting that Mao et al. find very selective effects of DISC1 on GSK3 substrates. Despite convincing evidence of an increase in Y216 phosphorylation, which one would expect to increase activity of GSK3 against all substrates, the authors find no evidence of effects on phosphorylation of the GSK3 substrates Ngn2 or C/EBPα. This is somewhat puzzling and merits further attention, especially as in vitro direct binding of a DISC1 fragment to GSK3 inhibited the action of GSK3 on a range of substrates. Might there be more to the direct interaction of DISC1 with GSK3 than a regulation of Y216 autophosphorylation and activation? If, however, GSK3 regulation turns out to be part of the mechanism of schizophrenia or bipolar disorder, then identifying which of the substrates and which of the many activities of GSK3, including on plasticity and hence cognition (Peineau et al., 2007; Hooper et al., 2007), are important in disease will become the critical task.

References:

Lovestone S, Killick R, Di Forti M, Murray R. Schizophrenia as a GSK-3 dysregulation disorder. Trends Neurosci. 2007 Apr 1 ; 30(4):142-9. Abstract

Cross DA, Alessi DR, Cohen P, Andjelkovich M, Hemmings BA. Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B. Nature . 1995 Dec 21-28 ; 378(6559):785-9. Abstract

Jope RS. Lithium and GSK-3: one inhibitor, two inhibitory actions, multiple outcomes. Trends Pharmacol Sci . 2003 Sep 1 ; 24(9):441-3. Abstract

Proitsi P, Li T, Hamilton G, Di Forti M, Collier D, Killick R, Chen R, Sham P, Murray R, Powell J, Lovestone S. Positional pathway screen of wnt signaling genes in schizophrenia: association with DKK4. Biol Psychiatry . 2008 Jan 1 ; 63(1):13-6. Abstract

Peineau S, Taghibiglou C, Bradley C, Wong TP, Liu L, Lu J, Lo E, Wu D, Saule E, Bouschet T, Matthews P, Isaac JT, Bortolotto ZA, Wang YT, Collingridge GL. LTP inhibits LTD in the hippocampus via regulation of GSK3beta. Neuron . 2007 Mar 1 ; 53(5):703-17. Abstract

Hooper C, Markevich V, Plattner F, Killick R, Schofield E, Engel T, Hernandez F, Anderton B, Rosenblum K, Bliss T, Cooke SF, Avila J, Lucas JJ, Giese KP, Stephenson J, Lovestone S. Glycogen synthase kinase-3 inhibition is integral to long-term potentiation. Eur J Neurosci . 2007 Jan 1 ; 25(1):81-6. Abstract

Li-huei Tsai and colleagues have identified another pathway in which the candidate gene DISC1 looks to have a critical regulatory role, namely the wnt signaling pathway, in progenitor cell proliferation. In recent years we have seen that DISC1 has a vital role at the centrosome (Kamiya et al., 2005), in cAMP signaling (Millar et al., 2005), and in multiple steps of adult hippocampal neurogenesis (Duan et al., 2007). They have shown a pivotal role for DISC1 in neural progenitor cell proliferation through regulation of GSK3 signaling using a spectacular combination of cellular and in utero manipulations with shRNAs and GSK3 inhibitor compounds. These findings clearly implicate DISC1 in another “druggable” pathway but at this stage do not really identify new approach/targets, except perhaps to confirm that manipulating adult neurogenesis and the wnt pathway holds much potential hope for therapeutics. Perhaps understanding the mechanism of...  Read more

Li-huei Tsai and colleagues have identified another pathway in which the candidate gene DISC1 looks to have a critical regulatory role, namely the wnt signaling pathway, in progenitor cell proliferation. In recent years we have seen that DISC1 has a vital role at the centrosome (Kamiya et al., 2005), in cAMP signaling (Millar et al., 2005), and in multiple steps of adult hippocampal neurogenesis (Duan et al., 2007). They have shown a pivotal role for DISC1 in neural progenitor cell proliferation through regulation of GSK3 signaling using a spectacular combination of cellular and in utero manipulations with shRNAs and GSK3 inhibitor compounds. These findings clearly implicate DISC1 in another “druggable” pathway but at this stage do not really identify new approach/targets, except perhaps to confirm that manipulating adult neurogenesis and the wnt pathway holds much potential hope for therapeutics. Perhaps understanding the mechanism of inhibition of GSK3 by DISC1 in more detail might reveal more novel approaches or encourage more innovative work around this pathway. In addition, I have read the other comment (by Rahman), and though I agree that this work still leaves many questions to be answered, the paper is much more significant and likely reconcilable with previous papers than appreciated. The commentary from Lovestone was very insightful and brings up additional gaps and issues with the present work. Additional experimentation I am sure will tease out more key facets of the DISC1-wnt interaction in the near future.

There are many avenues now to proceed with this work. In particular, from the DISC1-centric view, a GSK3 binding site on DISC1 overlaps with one of the critical core PDE4 binding site. Mao et al. show that residues 211 to 225 are a core part of a GSK3 binding site. Previously, Miles Houslay had shown very elegantly that residues 191-230 form a common binding site (known as common site 1) for both PDE4B and 4D families (Murdoch et al., 2007). It will be important to understand the relationship between GSK3 and PDE4 related signaling in reference to the activity of DISC1 starting at whether a trimolecular complex among DISC1-PDE4-GSK3 can form. Then it will be critical to understand the regulatory interplay among these molecules. For example, it is known that PKA can regulate GSK3 activity (Torii et al., 2008) and the interaction between DISC1 and PDE4, while both GSK3 and PKA can phosphorylate β-catenin (Taurin et al., 2006). The output of these relationships on progenitor proliferation will further deepen insights into the role of DISC1 complexes in neuronal processes. This type of situation is not really surprising for a molecule (DISC1) which has been shown to interact with >100 proteins (Camargo et al., 2007). The context of these interactions in both normal development and disease is likely to be critical to allow understanding of its complete functional repertoire.

Another area where these new findings need to be exploited is in the study of additional animal models. Though the two behavioral endpoint models used in the paper (amphetamine hyperactivity and forced swim test) provide a tantalizing glimpse of the behavioral importance of the complex, it would be critical to look in additional models relevant for schizophrenia and mood disorders. Furthermore, it will be very interesting to look at the effects of GSK3β inhibitors in some of the DISC1 animal models already available and to see if they can reverse all or a subset of reported behaviors. In reviewing a summary of the phenotypes available to date (Shen et al., 2008) there is clearly a number of lines which share the properties with mice injected with DISC1 shRNA into the dentate gyrus and would be of value to look at.

A very exciting paper which I am sure will drive additional research into understanding the role of DISC1 in psychiatry and hopefully encourage drug discovery efforts around this molecular pathway (Wang et al., 2008).

References:

1. Kamiya A, Kubo K, Tomoda T, Takaki M, Youn R, Ozeki Y, Sawamura N, Park U, Kudo C, Okawa M, Ross CA, Hatten ME, Nakajima K, Sawa A. A schizophrenia-associated mutation of DISC1 perturbs cerebral cortex development. Nat Cell Biol . 2005 Dec 1 ; 7(12):1167-78. Abstract

2. Millar JK, Pickard BS, Mackie S, James R, Christie S, Buchanan SR, Malloy MP, Chubb JE, Huston E, Baillie GS, Thomson PA, Hill EV, Brandon NJ, Rain JC, Camargo LM, Whiting PJ, Houslay MD, Blackwood DH, Muir WJ, Porteous DJ. DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling. Science . 2005 Nov 18 ; 310(5751):1187-91. Abstract

3. Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell . 2007 Sep 21 ; 130(6):1146-58. Abstract

4. Murdoch H, Mackie S, Collins DM, Hill EV, Bolger GB, Klussmann E, Porteous DJ, Millar JK, Houslay MD. Isoform-selective susceptibility of DISC1/phosphodiesterase-4 complexes to dissociation by elevated intracellular cAMP levels. J Neurosci . 2007 Aug 29 ; 27(35):9513-24. Abstract

5. Torii K, Nishizawa K, Kawasaki A, Yamashita Y, Katada M, Ito M, Nishimoto I, Terashita K, Aiso S, Matsuoka M. Anti-apoptotic action of Wnt5a in dermal fibroblasts is mediated by the PKA signaling pathways. Cell Signal . 2008 Jul 1 ; 20(7):1256-66. Abstract

6. Taurin S, Sandbo N, Qin Y, Browning D, Dulin NO. Phosphorylation of beta-catenin by cyclic AMP-dependent protein kinase. J Biol Chem . 2006 Apr 14 ; 281(15):9971-6. Abstract

7. Camargo LM, Collura V, Rain JC, Mizuguchi K, Hermjakob H, Kerrien S, Bonnert TP, Whiting PJ, Brandon NJ. Disrupted in Schizophrenia 1 Interactome: evidence for the close connectivity of risk genes and a potential synaptic basis for schizophrenia. Mol Psychiatry . 2007 Jan 1 ; 12(1):74-86. Abstract

8. Shen S, Lang B, Nakamoto C, Zhang F, Pu J, Kuan SL, Chatzi C, He S, Mackie I, Brandon NJ, Marquis KL, Day M, Hurko O, McCaig CD, Riedel G, St Clair D. Schizophrenia-related neural and behavioral phenotypes in transgenic mice expressing truncated Disc1. J Neurosci . 2008 Oct 22 ; 28(43):10893-904. Abstract

9. Wang Q, Jaaro-Peled H, Sawa A, Brandon NJ. How has DISC1 enabled drug discovery? Mol Cell Neurosci . 2008 Feb 1 ; 37(2):187-95. Abstract

Mao and colleagues’ present outstanding work sheds light on a novel function of DISC1. Because DISC1 is a multifunctional protein, the addition of new functions is not surprising. Thus, for the past several years, the field has focused on how DISC1 can have distinct functions in different cell contexts (for example, progenitor cells vs. postmitotic neurons, or developing cortex vs. adult dentate gyrus). In addition to Mao and colleagues, I understand that several groups, including ours, have obtained preliminary, unpublished evidence that DISC1 regulates progenitor cell proliferation, at least in part via GSK3β. Thus, I am very supportive of this new observation.

If there might be a missing point in this paper, it is unclear whether suppression of GSK3β occurs in several different biological contexts in brain in vivo. In other words, it is uncertain whether DISC1’s actions on GSK3β are constitutive or context-dependent. How can we reconcile differential roles for DISC1 in progenitor cells in contrast to postmitotic neurons? We have already obtained a...  Read more

Mao and colleagues’ present outstanding work sheds light on a novel function of DISC1. Because DISC1 is a multifunctional protein, the addition of new functions is not surprising. Thus, for the past several years, the field has focused on how DISC1 can have distinct functions in different cell contexts (for example, progenitor cells vs. postmitotic neurons, or developing cortex vs. adult dentate gyrus). In addition to Mao and colleagues, I understand that several groups, including ours, have obtained preliminary, unpublished evidence that DISC1 regulates progenitor cell proliferation, at least in part via GSK3β. Thus, I am very supportive of this new observation.

If there might be a missing point in this paper, it is unclear whether suppression of GSK3β occurs in several different biological contexts in brain in vivo. In other words, it is uncertain whether DISC1’s actions on GSK3β are constitutive or context-dependent. How can we reconcile differential roles for DISC1 in progenitor cells in contrast to postmitotic neurons? We have already obtained a preliminary promising answer to this question, which is currently being validated very intensively. These two phenotypes (progenitor cell control and postmitotic migration) may compensate for each other in cortical development; thus, overall cortical pathology looks milder in adults, at least in our preliminary unpublished data using DISC1 knockout mice. We are not sure how this novel function of DISC1 may account for the pathology of Scottish cases. Although I have great respect for the Scottish pioneers of DISC1 study, such as St. Clair, Blackwood, and Muir (I believe that the St. Clair et al., 1990 Lancet paper is one of the best publications in psychiatry), now is the time to pay more and more attention to the question of the molecular pathway(s) involving DISC1 in general schizophrenia (see 2009 SRF roundtable discussion). Unlike the role of APP in Alzheimer’s disease, DISC1 is not a key biological target in general schizophrenia, instead being an entry point to explore much more important targets for schizophrenia. There may be no more need to stick to DISC1 itself in the unique Scottish cases in schizophrenia research. In sum, although there may still be key missing points in this study, I wish to congratulate the authors on their outstanding work.

References:

St Clair D, Blackwood D, Muir W, Carothers A, Walker M, Spowart G, Gosden C, Evans HJ. Association within a family of a balanced autosomal translocation with major mental illness. Lancet . 1990 Jul 7 ; 336(8706):13-6. Abstract

This recent and important paper by Sawa's group adds another layer to the complex story of DISC1 function in neurodevelopment. Their findings clarify and integrate two streams of research implicating DISC1 in both neuron proliferation and migration. The identification of the S170 phosphorylation site also raises the exciting possibility that pharmacological strategies targeted at this phosphorylation-dependent switch might be useful in correcting or preventing mental illness-related problems with brain development. It would be interesting in this context to explore whether disease-associated DISC1 gene variants in humans affect DISC1 phosphorylation, and the subsequent balance between neuron proliferation and migration.

I agree with Atsushi Kamiya that further work is needed to understand which of the many effects of DISC1 perturbation are specific to human psychiatric disease phenotypes. Again, from a treatment perspective, it is vital to know which cellular abnormality underlies the most debilitating symptoms so that new treatments can be screened for effects on these...  Read more

This recent and important paper by Sawa's group adds another layer to the complex story of DISC1 function in neurodevelopment. Their findings clarify and integrate two streams of research implicating DISC1 in both neuron proliferation and migration. The identification of the S170 phosphorylation site also raises the exciting possibility that pharmacological strategies targeted at this phosphorylation-dependent switch might be useful in correcting or preventing mental illness-related problems with brain development. It would be interesting in this context to explore whether disease-associated DISC1 gene variants in humans affect DISC1 phosphorylation, and the subsequent balance between neuron proliferation and migration.

I agree with Atsushi Kamiya that further work is needed to understand which of the many effects of DISC1 perturbation are specific to human psychiatric disease phenotypes. Again, from a treatment perspective, it is vital to know which cellular abnormality underlies the most debilitating symptoms so that new treatments can be screened for effects on these specific abnormalities. Another recent paper from our group reinforces this point (Lee et al., 2011). We found that genetic inactivation of GSK3α restored dendritic spine deficits in DISC1 L100P mutant mice, in parallel with amelioration of behavioral abnormalities as previously reported (Lipina et al., 2011). However, other abnormalities in dendrite morphology caused by the DISC1 L100P mutation were not corrected by GSK3α inactivation.

References:

Lee FH, Kaidanovich-Beilin O, Roder JC, Woodgett JR, Wong AH. Genetic inactivation of GSK3α rescues spine deficits in Disc1-L100P mutant mice, Schizophrenia Research. 2011;Apr 16. Abstract

Lipina TV, Kaidanovich-Beilin O, Patel S, Wang M, Clapcote SJ, Liu F, Woodgett JR, Roder JC. Genetic and pharmacological evidence for schizophrenia-related Disc1 interaction with GSK-3. Synapse. 2011;65:234-248. Abstract

A couple of recently published papers have provided insights into the cell physiology of DISC1. Although DISC1 is one of the most extensively studied susceptibility genes for psychiatric illness, the promoter of DISC1 has not been characterized so far. In a systematic approach based on luciferase reporter genes, Walker et al. (Walker et al., 2012) describe a repressive and an enhancing promoter region upstream of the transcription start. The DISC1 promoter is negatively regulated by FOXP2; hence, affected FOXP2 mutation carriers might show a higher DISC1 expression. Therefore, it would be interesting to know if these FOXP2 mutation carriers also display a higher level of insoluble DISC1, since increased expression leads to an increase of insoluble DISC1 (Leliveld et al., 2008). As a result, and possibly through aggregation, DISC1 loses its ability to bind to specific interaction partners, thereby disrupting some cellular pathways (Atkin et...  Read more

A couple of recently published papers have provided insights into the cell physiology of DISC1. Although DISC1 is one of the most extensively studied susceptibility genes for psychiatric illness, the promoter of DISC1 has not been characterized so far. In a systematic approach based on luciferase reporter genes, Walker et al. (Walker et al., 2012) describe a repressive and an enhancing promoter region upstream of the transcription start. The DISC1 promoter is negatively regulated by FOXP2; hence, affected FOXP2 mutation carriers might show a higher DISC1 expression. Therefore, it would be interesting to know if these FOXP2 mutation carriers also display a higher level of insoluble DISC1, since increased expression leads to an increase of insoluble DISC1 (Leliveld et al., 2008). As a result, and possibly through aggregation, DISC1 loses its ability to bind to specific interaction partners, thereby disrupting some cellular pathways (Atkin et al., 2012) and potentially leading to other gain-of-function effects. In this context, Malavasi et al. (Malavasi et al., 2012) report in detail on the control of DISC1 over the transcriptional activity of ATF4. ATF4 itself acts as a key protein in emotional learning and memory via its ability to repress CREB activity. The authors provide intriguing results on how full-length DISC1 protein and its non-synonymous polymorphisms 37W and 607F differentially inhibit ATF4 activity by distinct mechanisms. Both genetic variants—the rare, putatively causal substitution 37W and the common variant 607F—exclude the protein from the nucleus, thereby reducing ATF4 inhibition.

Eykelenboom et al. (Eykelenboom et al., 2012) also report on an abnormal subcellular distribution of mutated DISC1 by elegantly expanding the concept of DISC1 translocation-derived fusion proteins proposed previously (Zhou et al., 2008; Zhou et al., 2010). This is the first paper to confirm the existence of three different transcripts from the translocated DISC1 gene, potentially giving rise to DISC1 proteins adding 1, 60 or 69 amino acids to the N-terminus (1-597). Upon biophysical characterization, the two larger proteins termed CP60 and CP69 exhibit a higher helical amount and larger protein assemblies. When the recombinant fusion proteins were expressed in cells, they mediated abnormal mitochondrial localization and altered mitochondrial membrane potential.

The last two publications show that altered DISC1 protein structure, ranging from single amino acid changes to large, chimeric fusion proteins, can culminate in changes of the protein cellular distribution, oligomerization status, and abnormal cellular function. Increasing evidence suggests that defined DISC1 protein species have particular local functions within the neuron or glia cells, and that at least a part of the DISC1-mediated pathology is dependent on abnormal cellular distribution of the protein.

References:

Atkin TA, Brandon NJ, Kittler JT. Disrupted in Schizophrenia 1 forms pathological aggresomes that disrupt its function in intracellular transport. Hum Mol Genet . 2012 May 1 ; 21(9):2017-28. Abstract

Eykelenboom JE, Briggs GJ, Bradshaw NJ, Soares DC, Ogawa F, Christie S, Malavasi EL, Makedonopoulou P, Mackie S, Malloy MP, Wear MA, Blackburn EA, Bramham J, McIntosh AM, Blackwood DH, Muir WJ, Porteous DJ, Millar JK. 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. Hum Mol Genet . 2012 May 16. Abstract

Leliveld SR, Bader V, Hendriks P, Prikulis I, Sajnani G, Requena JR, Korth C. Insolubility of disrupted-in-schizophrenia 1 disrupts oligomer-dependent interactions with nuclear distribution element 1 and is associated with sporadic mental disease. J Neurosci . 2008 Apr 9 ; 28(15):3839-45. Abstract

Malavasi EL, Ogawa F, Porteous DJ, Millar JK. DISC1 variants 37W and 607F disrupt its nuclear targeting and regulatory role in ATF4-mediated transcription. Hum Mol Genet . 2012 Jun 15 ; 21(12):2779-92. Abstract

Walker RM, Hill AE, Newman AC, Hamilton G, Torrance HS, Anderson SM, Ogawa F, Derizioti P, Nicod J, Vernes SC, Fisher SE, Thomson PA, Porteous DJ, Evans KL. The DISC1 promoter: characterization and regulation by FOXP2. Hum Mol Genet . 2012 Apr 4. Abstract

Zhou X, Geyer MA, Kelsoe JR. Does disrupted-in-schizophrenia (DISC1) generate fusion transcripts? Mol Psychiatry . 2008 Apr ; 13(4):361-3. Abstract

Zhou X, Chen Q, Schaukowitch K, Kelsoe JR, Geyer MA. Insoluble DISC1-Boymaw fusion proteins generated by DISC1 translocation. Mol Psychiatry . 2010 Jul 1 ; 15(7):669-72. Abstract