Some observations on the new report by Li and colleagues: this work is the first to map subregions of DISC1 and to show that a region that binds Nudel and LIS1 is important in generating schizophrenia-like perturbations in vivo. The authors express DISC1 C-terminus in mice, which interacts with Nudel and LIS1. They showed less native mouse DISC1 associations with Nudel mouse following gene induction. This suggests a dominant-negative mechanism.

No data was shown on native DISC1 levels following induction. Work from the Sawa lab shows that if murine DISC1 levels are reduced in non-engineered mice using RNAi, severe perturbations in development of nervous system are seen (Kamiya et al., 2005); however, behavior was not measured in this study. Severe perturbations would be expected based on the neonatal ventral hippocampal lesion model. In this latter model early brain lesions lead to later impairments in PPI and other behaviors consistent with schizophrenic-like behavior.

They use a promoter only expressed in the forebrain,...  Read more

Some observations on the new report by Li and colleagues: this work is the first to map subregions of DISC1 and to show that a region that binds Nudel and LIS1 is important in generating schizophrenia-like perturbations in vivo. The authors express DISC1 C-terminus in mice, which interacts with Nudel and LIS1. They showed less native mouse DISC1 associations with Nudel mouse following gene induction. This suggests a dominant-negative mechanism.

No data was shown on native DISC1 levels following induction. Work from the Sawa lab shows that if murine DISC1 levels are reduced in non-engineered mice using RNAi, severe perturbations in development of nervous system are seen (Kamiya et al., 2005); however, behavior was not measured in this study. Severe perturbations would be expected based on the neonatal ventral hippocampal lesion model. In this latter model early brain lesions lead to later impairments in PPI and other behaviors consistent with schizophrenic-like behavior.

They use a promoter only expressed in the forebrain, so it is puzzling they see expression in the cerebellum. Expressed DISC1 bound to endogenous mouse Nudel and LIS1, presumably exerting a dominant-negative effect. Induction of the C-terminus DISC1 at day 7, but not in the adult, led to deficits in working memory, the forced swim test, and sociability. It would have been reassuring if these tasks were validated using antipsychotics and antidepressants. It is not clear in this study why the female C57 was used as a standard opponent mouse, and what genders of DISC1 mice have been used. Even though young C57 females (6 weeks old) were used as neutral partners, the data might be interpreted also as impaired sexual motivation in DISC1-Tg-Tm7 mice.

The authors made an attempt to translate their mouse data (low sociability) into a human population and found an association between DISC1 haplotypes and social impairments in a Finnish population (n = 232 samples), which supports a DISC1 role in social behavior, one of schizophrenia's symptoms. It would be useful to distinguish deficits in social interactions and impaired sexual behavior.

Deficits in working memory are also an important schizophrenia endophenotype, and it would be interesting to measure how specific the cognitive deficit is in DISC1-Tg-Tm-7 mice, estimating associative memory in classical fear conditioning, for example.

Induction of the transgene early in development to day 7 resulted in small changes in dendritic complexity in granule cells in the dentate gyrus. It is surprising larger changes were not observed. The role of DISC1 in the adult self-renewing progenitor cells in the dentate switches, so that DISC1 acts as a brake for dendritic complexity and migration (Duan et al., 2007). Thus, reductions in DISC1 in the adult dentate gyrus granule cells lead to enhanced dendrite growth/complexity.

In the adult, DISC1 was shown to interact with Nudel in controlling adult neurogenesis and development. It is of interest that in the Li et al. paper the transgene also perturbs native DISC1 binding to Nudel at day 7 but not adult. Synaptic transmission was reduced in CA1. It would have been nice to see a recording from dentate granule cells in which changes in dendritic complexity were found.

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

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

DISC1 may be a promising entry point to explore important disease pathways for schizophrenia and related mental conditions; thus, animal models that can provide us with insights into the pathways involving DISC1 may be helpful. In this sense, the new animal model reported by Li et al. (Silva and Cannon’s group at UCLA) has great significance in this field.

They made mice expressing a short domain of DISC1 that may block interaction of DISC1 with a set of protein interactors, including NUDEL/NDEL1 and LIS1. This approach, if the domain is much shorter, will be an important methodology in exploring the disease pathways based on protein interactions. Although the manuscript is excellent, and appropriate as the first report, the domain expressed in the transgenic mice can interact with more than 30-40 proteins, and the phenotypes that the authors observed might not be attributable to the disturbance of protein interactions of DISC1 and NUDEL or LIS1.

Now we have at least five different types of animal models for DISC1, all of which have unique advantages and...  Read more

DISC1 may be a promising entry point to explore important disease pathways for schizophrenia and related mental conditions; thus, animal models that can provide us with insights into the pathways involving DISC1 may be helpful. In this sense, the new animal model reported by Li et al. (Silva and Cannon’s group at UCLA) has great significance in this field.

They made mice expressing a short domain of DISC1 that may block interaction of DISC1 with a set of protein interactors, including NUDEL/NDEL1 and LIS1. This approach, if the domain is much shorter, will be an important methodology in exploring the disease pathways based on protein interactions. Although the manuscript is excellent, and appropriate as the first report, the domain expressed in the transgenic mice can interact with more than 30-40 proteins, and the phenotypes that the authors observed might not be attributable to the disturbance of protein interactions of DISC1 and NUDEL or LIS1.

Now we have at least five different types of animal models for DISC1, all of which have unique advantages and disadvantages: 1) mice with a spontaneous mutation in an exon, which seem to lack some, but not all, DISC1 isoforms, from Gogos’s lab (see Koike et al., 2006; Ishizuka et al., 2007); 2) mice with mutations induced by a mutagen from Roder’s lab (Clapcote et al., 2007); 3) transgenic mice that express a dominant-negative mutant DISC1 from Sawa’s lab (Hikida et al., 2007); 4) transgenic mice that express a dominant-negative mutant DISC1 in an inducible manner from Pletkinov’s lab (Pletnikov et al., 2007); and 5) the mice from Silva’s and Cannon’s labs.

It is impossible to reach a firm conclusion on how the Scottish mutation of the DISC1 gene leads to molecular dysfunction until the data from autopsied brains of patients in the Scottish family become available. Millar and colleagues have published data of DISC1 in lymphoblastoid cells from the family members and propose an intriguing suggestion of how DISC1 is potentially disturbed in the pedigree (Millar et al., 2005); however, this remains in the realm of hypothesis/suggestion from peripheral cells. Thus, it is very important to compare the various types of DISC1 animal models in approaching how disturbance of DISC1 in brain leads to the pathophysiology of schizophrenia and related disorders.

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 Mar 7;103(10):3693-7. Abstract

Ishizuka K, Chen J, Taya S, Li W, Millar JK, Xu Y, Clapcote SJ, Hookway C, Morita M, Kamiya A, Tomoda T, Lipska BK, Roder JC, Pletnikov M, Porteous D, Silva AJ, Cannon TD, Kaibuchi K, Brandon NJ, Weinberger DR, Sawa A. Evidence that many of the DISC1 isoforms in C57BL/6J mice are also expressed in 129S6/SvEv mice. Mol Psychiatry. 2007 Oct ;12(10):897-9. 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. Behavioral phenotypes of Disc1 missense mutations in mice. Neuron. 2007 May 3;54(3):387-402. Abstract

Hikida T, Jaaro-Peled H, Seshadri S, Oishi K, Hookway C, Kong S, Wu D, Xue R, Andradé M, Tankou S, Mori S, Gallagher M, Ishizuka K, Pletnikov M, Kida S, Sawa A. Dominant-negative DISC1 transgenic mice display schizophrenia-associated phenotypes detected by measures translatable to humans. Proc Natl Acad Sci U S A. 2007 Sep 4;104(36):14501-6. Abstract

Pletnikov MV, Ayhan Y, Nikolskaia O, Xu Y, Ovanesov MV, Huang H, Mori S, Moran TH, Ross CA. Inducible expression of mutant human DISC1 in mice is associated with brain and behavioral abnormalities reminiscent of schizophrenia. Mol Psychiatry. 2007 Sep 11; Abstract

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

On the DISC1 bus
You wait ages for a bus, then a string of them come one behind the other. First, Koike et al. (2006) reported that the 129 strain of mouse had a small detection of the DISC1 gene and this was associated with a deficit on a learning task. The interpretation of this observation was somewhat complicated by the subsequent recognition that the majority, if not all, major DISC1 isoforms are unaffected by the deletion, but this needs further work (Ishizuka et al., 2007). Then, Clapcote et al. (2007) provided a very detailed characterization of two independent ENU-induced mouse missense mutations of DISC1, showing selective brain shrinkage and marked behavioral abnormalities that in one mutant were schizophrenia-like, the other more akin to mood disorder. Importantly, these phenotypes could be differentially rescued by antipsychotics or antidepressants. The main finger pointed to disruption of the interaction with PDE4...  Read more

On the DISC1 bus
You wait ages for a bus, then a string of them come one behind the other. First, Koike et al. (2006) reported that the 129 strain of mouse had a small detection of the DISC1 gene and this was associated with a deficit on a learning task. The interpretation of this observation was somewhat complicated by the subsequent recognition that the majority, if not all, major DISC1 isoforms are unaffected by the deletion, but this needs further work (Ishizuka et al., 2007). Then, Clapcote et al. (2007) provided a very detailed characterization of two independent ENU-induced mouse missense mutations of DISC1, showing selective brain shrinkage and marked behavioral abnormalities that in one mutant were schizophrenia-like, the other more akin to mood disorder. Importantly, these phenotypes could be differentially rescued by antipsychotics or antidepressants. The main finger pointed to disruption of the interaction with PDE4 to misregulate cAMP signaling (Millar et al., 2005; Murdoch et al., 2007).

Then, back-to-back came two variants of DISC1 transgenic models from Johns Hopkins University (Pletnikov et al., 2007; Hikida et al., 2007) (see also SCZ Forum). Both Pletnikov and Hikida overexpressed a truncated form of DISC1 under the control of the CaMKII promoter (in Pletnikov’s case with an inducible CaMKU promoter). Both groups reported brain structural and behavioral abnormalities that aligned rather nicely with the earlier work of Clapcote et al. (2007). Pletnikov et al. showed that neurite outgrowth was attenuated in primary cortical neurons. They also showed that endogenous DISC1, LIS1, and SNAP25, but not NDEL1 or PSD-95, was reduced in mouse forebrain.

Now, Li et al. (2007) introduce yet another transgenic DISC1 model mouse, this time overexpressing a carboxy tail fragment of DISC1, so the opposite end of the DISC1 molecule from that overexpressed by Pletnikov and by Hikida. Intriguingly, Li et al. (as with all the preceding models) report significant behavioral differences for wild-type littermates. The point of added interest and significance here is that by using an inducible transgenic construct, they could elicit behavioral abnormalities if carboxy terminal DISC1 was expressed on postnatal day 7 only, but not in adult life. What are we to make of this and how do the models align? Li et al. interpret their results to suggest that DISC1 plays a crucial role, through NDEL1 and LIS1, in postnatal (but not adult) brain development. This study obviously raises some key questions. What is the developmental window of DISC1 effect? How can the lack of effect in the adult be reconciled with the rather striking effect on neurogenesis consequent upon downregulation of DISC1 in the adult mouse brain reported by Duan et al. (2007). And if overexpressing 5’ (Hikida, Pletnikov) or 3’ constructs (Li) can elicit similar phenotypes as seen in ENU-induced missense variants within exon 2 (Clapcote), can we come up with a unifying explanation? Perhaps not yet, but these various mouse models certainly emphasize the value of a multi-pronged mouse modeling approach. Combinations of “null,” transgenic, inducible, and missense mutants will help dissect the underlying processes. These studies also suggest that a variety of DISC1 variants in humans might elicit rather similar and also subtly different phenotypes. Indeed, Li et al. try to link their findings on the mouse to human studies, but here I feel there is cause for caution. The genetic association referred to maps to a haplotype in a quite distinct region of DISC1 and the direct or indirect functional effect of the haplotype is far from clear. It is, however, conceptually unlikely that this risk haplotype has a specific or restricted effect on Nudel and/or Lis1 binding. The corollary between a genetic association for a selected, but poorly defined sub-phenotype of schizophrenia with a poorly defined behavioral phenotype in the mouse may be a corollary too far too soon. Finally, whereas the focus of attention by Li, Pletnikov, and Hikida has been on the well-established/neurodevelopmental role of NDEL1 (and LIS1), the potential role of PDE4B both in neurosignaling (related to behavior, learning, and memory) and possibly also neurodevelopment should not be overlooked. In this regard it is noteworthy that PDE4 interacts both with the head and the carboxy tail domain of DISC1 (Hannah et al., 2007) and this most likely contributes to the phenotype in all the models described to date.

References:

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. Behavioral phenotypes of Disc1 missense mutations in mice. Neuron. 2007 May 3;54(3):387-402. Abstract

Hikida T, Jaaro-Peled H, Seshadri S, Oishi K, Hookway C, Kong S, Wu D, Xue R, Andradé M, Tankou S, Mori S, Gallagher M, Ishizuka K, Pletnikov M, Kida S, Sawa A. Dominant-negative DISC1 transgenic mice display schizophrenia-associated phenotypes detected by measures translatable to humans. Proc Natl Acad Sci U S A. 2007 Sep 4;104(36):14501-6. Abstract

Ishizuka K, Chen J, Taya S, Li W, Millar JK, Xu Y, Clapcote SJ, Hookway C, Morita M, Kamiya A, Tomoda T, Lipska BK, Roder JC, Pletnikov M, Porteous D, Silva AJ, Cannon TD, Kaibuchi K, Brandon NJ, Weinberger DR, Sawa A. Evidence that many of the DISC1 isoforms in C57BL/6J mice are also expressed in 129S6/SvEv mice. Mol Psychiatry. 2007 Oct 1;12(10):897-9. Abstract

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 Mar 7;103(10):3693-7. Abstract

Li W, Zhou Y, Jentsch JD, Brown RA, Tian X, Ehninger D, Hennah W, Peltonen L, Lönnqvist J, Huttunen MO, Kaprio J, Trachtenberg JT, Silva AJ, Cannon TD. Specific developmental disruption of disrupted-in-schizophrenia-1 function results in schizophrenia-related phenotypes in mice. Proc Natl Acad Sci U S A. 2007 Nov 13;104(46):18280-5. Abstract

Millar JK, James R, Christie S, Porteous DJ. Disrupted in schizophrenia 1 (DISC1): subcellular targeting and induction of ring mitochondria. Mol Cell Neurosci. 2005 Dec 1;30(4):477-84. Abstract

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

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

Pletnikov MV, Ayhan Y, Nikolskaia O, Xu Y, Ovanesov MV, Huang H, Mori S, Moran TH, Ross CA. Inducible expression of mutant human DISC1 in mice is associated with brain and behavioral abnormalities reminiscent of schizophrenia. Mol Psychiatry. 2007 Sep 11; [Epub ahead of print] Abstract

Although DISC1 is multifunctional, its role for neurite outgrowth has been substantially characterized for the past couple of years (Ozeki et al., 2003; Miyoshi et al., 2003; Kamiya et al., 2006). These studies indicated that DISC1 is involved in neurite outgrowth by more than one mechanism, such as interactions with NUDEL/NDEL1 and FEZ1.

These two papers from Kaibuchi’s lab provide further understanding of how DISC1 is involved in neuronal outgrowth. Kaibuchi’s group identified kinesin heavy chain of kinesin-1 as a novel interactor of DISC1. In their papers, a novel role for DISC1, to link kinesin-1 (microtubule-dependent and plus-end directed motor) to several cellular molecules, including NUDEL, LIS1, 14-3-3, and Grb2, is reported. DISC1 and kinesin-1 are, therefore, responsible to sort Grb2 to the distal part of axons where Grb2...  Read more

Although DISC1 is multifunctional, its role for neurite outgrowth has been substantially characterized for the past couple of years (Ozeki et al., 2003; Miyoshi et al., 2003; Kamiya et al., 2006). These studies indicated that DISC1 is involved in neurite outgrowth by more than one mechanism, such as interactions with NUDEL/NDEL1 and FEZ1.

These two papers from Kaibuchi’s lab provide further understanding of how DISC1 is involved in neuronal outgrowth. Kaibuchi’s group identified kinesin heavy chain of kinesin-1 as a novel interactor of DISC1. In their papers, a novel role for DISC1, to link kinesin-1 (microtubule-dependent and plus-end directed motor) to several cellular molecules, including NUDEL, LIS1, 14-3-3, and Grb2, is reported. DISC1 and kinesin-1 are, therefore, responsible to sort Grb2 to the distal part of axons where Grb2 functions as an adaptor and plays a role in NT-3-induced phosphorylation of ERK1/2. This mechanism well explains our previous work, led by Ryota Hashimoto, reporting that knockdown of DISC1 expression results in decreased levels of phosphorylation of ERK1/2 and Akt in primary cortical neurons (Hashimoto et al., 2006).

The interaction of DISC1 and kinesin-1 may also be interesting from the perspective of psychiatric genetics. First, the mechanism proposed in one of the papers (Taya et al., 2007) supports the notion that the C-terminal truncated DISC1 fragment—that occurs due to the balanced translocation in an extended Scottish family—functions as a dominant-negative. Second, the domain of DISC1 responsible for kinesin-1 is overlapped with the haplotype block region(s) that are positive in more than one association study of DISC1 and major mental illnesses.

Two recent back-to-back papers, published this month in Journal of Neuroscience, highlight the value of protein-protein interactions in determining the biological role of a key schizophrenia risk factor, DISC1, in processes that are important for the proper development of neurons.

Key questions need to be addressed once having established a set of interactors for a given protein. First, where do these proteins interact on the target molecule? Second, do these interactions take place at the same time (i.e., do they form a complex)? Third, in what context do these interactions occur (temporal, tissue/cell compartment, signaling), and, fourth, are the biological processes of the interacting molecules affected/regulated by the protein of interest? The Kaibuchi lab, as exemplified in the works by Taya et al. and Shinoda et al., elegantly address some of these questions in the context of DISC1 interactions with Grb2, Nudel (NDEL1), 14-3-3ε, and kinesin-1. The key findings of these papers are as follows:

1. Identification of the interaction sites, or more importantly,...  Read more

Two recent back-to-back papers, published this month in Journal of Neuroscience, highlight the value of protein-protein interactions in determining the biological role of a key schizophrenia risk factor, DISC1, in processes that are important for the proper development of neurons.

Key questions need to be addressed once having established a set of interactors for a given protein. First, where do these proteins interact on the target molecule? Second, do these interactions take place at the same time (i.e., do they form a complex)? Third, in what context do these interactions occur (temporal, tissue/cell compartment, signaling), and, fourth, are the biological processes of the interacting molecules affected/regulated by the protein of interest? The Kaibuchi lab, as exemplified in the works by Taya et al. and Shinoda et al., elegantly address some of these questions in the context of DISC1 interactions with Grb2, Nudel (NDEL1), 14-3-3ε, and kinesin-1. The key findings of these papers are as follows:

1. Identification of the interaction sites, or more importantly, which part of DISC1 is involved in particular processes, for example, that axon elongation is dependent on the N-terminal, but not the C-terminal portion of DISC1. This suggests that the DISC1 role in axon elongation is mediated by interactions with the N-terminal portion of DISC1 that could be competed for by the truncated protein in a dominant negative fashion (Camargo et al., 2007).

2. Although a protein may have many interacting partners, such as DISC1, these interactions may not occur at the same time. For example, DISC1 is able to form a ternary complex with kinesin-1 and NDEL1 or with kinesin-1 and Grb2. However, a ternary complex of DISC1-Grb2-NDEL1 is not possible as Grb2 and NDEL1 may be competing for the same interaction site on DISC1.

3. Protein interactions may occur in certain cellular compartments, in the case of DISC1, the cell body and the distal part of axons.

4. Neurotrophin-induced axon elongation requires DISC1.

These papers confirm some of the hypotheses raised by the interactions that we have recently derived for DISC1 and some of its interacting partners (see Camargo et al., 2007). From the DISC1 interactome, we concluded that DISC1 may affect key intracellular transport mechanisms, such as those regulated by kinesins, and that DISC1 may be downstream of neurotrophin receptors, via its interaction with SH3BP5, an adaptor protein, which we found to interact with SOS1, a guanine exchange factor that binds Grb2 and responds to signaling of neurotrophin receptors. These observations have been validated by Taya et al. and Shinoda et al. and demonstrate the value of the DISC1 interactome in understanding the role of DISC1, and as a valuable resource to the wider community.

The molecular function of DISC1, as defined by its structure, still remains elusive, requiring a more dedicated effort on this front. The good news is that, via its protein-protein interactions, significant progress on the role of DISC1 in key biological processes has been achieved, as illustrated by the work of different labs (Brandon et. al., 2004; Millar et al., 2005; Kamiya et al., 2005; and now by Shinoda et al. and Taya et al.).

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

The elegant research by Faulkner and colleagues, along with their previous work (Duan et al., 2007), clearly demonstrates a role for DISC1 in regulating the timing of neuronal development in the adult brain. The loss of Disc1 in adult-born dentate granule cells resulted in aberrant axonal targeting and accelerated mossy fiber maturation. Although it is hypothesized that the hippocampus is involved in the pathophysiology of schizophrenia, the cellular and molecular underpinnings of hippocampal dysfunction are unknown. However, it is becoming apparent that Disc1 is a regulator of granule cell integration and maturation in the adult hippocampus. The function of adult-born granule cells and the contribution they make to hippocampal function is, of course, yet to be fully elucidated. In the context of schizophrenia, though, it may be that abnormal incorporation of newborn granule cells into the hippocampal network—perhaps caused by mutations in key genes such as Disc1—is a post-developmental trigger which leads to the onset...  Read more

The elegant research by Faulkner and colleagues, along with their previous work (Duan et al., 2007), clearly demonstrates a role for DISC1 in regulating the timing of neuronal development in the adult brain. The loss of Disc1 in adult-born dentate granule cells resulted in aberrant axonal targeting and accelerated mossy fiber maturation. Although it is hypothesized that the hippocampus is involved in the pathophysiology of schizophrenia, the cellular and molecular underpinnings of hippocampal dysfunction are unknown. However, it is becoming apparent that Disc1 is a regulator of granule cell integration and maturation in the adult hippocampus. The function of adult-born granule cells and the contribution they make to hippocampal function is, of course, yet to be fully elucidated. In the context of schizophrenia, though, it may be that abnormal incorporation of newborn granule cells into the hippocampal network—perhaps caused by mutations in key genes such as Disc1—is a post-developmental trigger which leads to the onset of disease symptoms.

The finding that Disc1 regulates mossy fiber targeting and synapse formation is also intriguing on a more general level. There is much research suggesting that schizophrenia is a disease of the synapse, likely involving several neurotransmitter systems. A role for Disc1 in axon outgrowth and synapse formation would certainly be a means through which Disc1 disruption could affect the hippocampal trisynaptic circuit. Next, we as researchers will have to determine the molecular mechanisms by which Disc1 is regulating neuronal development and axonal targeting. Based on the work in the developing and adult brain, Disc1 has multiple cellular roles involving a long list of interactors. The challenge is determining which Disc1 functions and pathways are relevant to the schizophrenia phenotype.

References:

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