Dynamic Duo: DISC1 and Dixdc1 Team Up to Regulate Brain Development
19 July 2010. The protein encoded by DISC1, the disrupted in schizophrenia 1 gene, partners with the Wnt signaling protein Dixdc1 in mice according to a study published on July 15 in Neuron. This finding adds a new player to the roster of proteins known to associate with DISC1, a major suspect in schizophrenia and other psychiatric disorders. Depending on the complex formed, this interaction can promote either neural proliferation or migration during brain development.
The study, from Li-Huei Tsai's lab at Massachusetts Institute of Technology, Cambridge, builds on earlier work connecting DISC1 to neural proliferation (see Mao et al., 2009 and SRF related news story) through Wnt signaling. The Wnt pathway controls cell fate and can trigger cancer when inappropriately activated. DISC1 directly inhibits glycogen synthase kinase 3β (GSK3β), which keeps levels of the transcription activator β-catenin high, spurring the birth of neurons.
The new study finds that Dixdc1's role in neural proliferation is so similar to that of DISC1 that each protein can stand in for the other. This means that DISC1 and Dixdc1 (short for "DIX domain-containing-1") may offer redundant ways of ensuring proper neural proliferation, and suggests that schizophrenia and other psychiatric diseases may disrupt the Wnt pathway. Consistent with this, lithium, a drug commonly used to treat mood disorders and to augment antipsychotic medication for schizophrenia, indirectly inhibits GSK3β (Beaulieu et al., 2008 and see SRF related news story).
For neural migration, however, the results were different. Dixdc1 could not correct migration defects induced by inhibiting DISC1. Instead, the researchers found evidence for a three-part complex among DISC1, Dixdc1, and another schizophrenia suspect, the protein called nuclear distribution factor E homolog like-1 (Ndel1), which regulated neuron migration without relying on the Wnt pathway. Overall, the study reveals a complex dance between DISC1 and Dixdc1 in regulating neuron birth and migration. It reinforces the idea that perturbations of brain development could lead to the disorganized brain circuitry suspected in schizophrenia and other psychiatric disorders.
A neuron is born
Interested in how DISC1 is regulated during brain development, first author Karun Singh and colleagues began by looking for proteins that bind DISC1 early on when new neurons are proliferating. Co-immunoprecipitation of embryonic mouse brain tissue taken on embryonic day E14 showed that DISC1 bound to Dixdc1. Other experiments showed that Dixdc1 was highly expressed throughout development, and was found in both neural progenitor cells and in neurons.
Having tied Dixdc1 to the right place at the right time, the researchers turned to manipulating the protein’s levels in vivo to probe its role in brain development. The results mirrored their previous DISC1 findings in many ways. Dixdc1 knockdown, in which a Dixdc1-inhibiting shRNA was introduced into the brains of embryonic mice via in utero electroporation, disrupted the normal distribution of neurons in the brain by prematurely halting the proliferation of neural progenitors. This result looked similar to the 2009 DISC1 knockdown experiments by Tsai and colleagues.
Singh and colleagues also found that, like DISC1, Dixdc1 worked through the Wnt pathway. Knockdown of Dixdc1 reduced the pathway’s activity in vitro, as reported by a construct sensitive to β-catenin levels in the cells. Overexpression of Dixdc1 boosted this activity, and the sizes of these effects were similar to those induced by DISC1 manipulations. Further experiments showed synergy between DISC1 and Dixdc1: for example, downregulating both lowered Wnt pathway activity more than downregulating either protein alone. These effects seemed to require an interaction between the two proteins, and other experiments placed Dixdc1, like DISC1, upstream of GSK3β and β-catenin.
Dixdc1 and DISC1 acted so similarly that the researchers tested whether one could compensate for the other. Strikingly, the low levels of Wnt activity induced by knockdown of DISC1 could be fully rescued by overexpression of Dixdc1 and vice versa.
The mechanics of this partnership held in vivo, with Dixdc1 activating the Wnt pathway in neural progenitors and compensating for a loss of DISC1. This translated into real results for developing neurons in the brain: knocking down Dixdc1 and DISC1 together led to fewer proliferating cells than inhibiting either protein alone, and overexpression of one protein could fully rescue neural proliferation that had been compromised by knockdown of the other.
A migration trio
The researchers then asked whether Dixdc1 would continue to mimic DISC1 in the realm of neural migration, which begins after neural proliferation. When they downregulated Dixdc1 in mouse brains at a time when new neurons migrate to their ultimate locations in the brain, Singh and colleagues found a striking migration defect four days later: cells were stuck in the intermediate zone of the developing cortex, rather than making their way to the cortical plate. Further experiments indicated that this was an effect on migration, rather than changes in cell fate.
But here's where a different kind of DISC1-Dixdc1 partnership emerges. Adding extra DISC1 could not compensate for these migration deficits, nor could extra Dixdc1 rescue the migration problems induced by DISC1 knockdown. The Wnt pathway did not seem to be involved either, because throwing it into overdrive with degradation-resistant β-catenin did not reverse the migration deficits induced by either DISC1 or Dixdc1 knockdown.
To identify the pathway involved, the researchers turned to Ndel1, which binds DISC1 to regulate neuron migration and has itself been associated with schizophrenia risk (Kamiya et al., 2005). They found that Dixdc1 also bound Ndel1, and that Dixdc1 can form a three-part complex with DISC1 and Ndel1. Interestingly, Ndel1 bound to the same Dixdc1 region as DISC1 does, suggesting that Dixdc1 may facilitate the interaction between DISC1 and Ndel1.
The researchers found that formation of the Dixdc1-DISC1-Ndel1 complex depended on phosphorylation of Dixdc1. Cyclin-dependent kinase 5 (Cdk5), also involved in migration, phosphorylated Dixdc1 in the same region where Ndel1 and DISC1 bind. Using a mutant Dixdc1 construct that could not be phosphorylated at this site, the researchers found reduced Ndel1, but not DISC1, binding to Dixdc1. Introducing this mutant Dixdc1 into embryonic mouse brains disrupted migration, indicating that phosphorylation of Dixdc1 at this site was crucial. Likewise, inhibiting the formation of the Dixdc1-DISC1-Ndel1 complex by using a peptide fragment that interfered with DISC1 and Ndel1 binding to Dixdc1 also stalled migration in vivo.
An intriguing idea
The similarity of Dixdc1 to DISC1, particularly in neuron proliferation, might help explain why some members of the Scottish DISC1 family who carry the DISC1 translocation do not have a psychiatric diagnosis (Blackwood et al., 2001). Singh and colleagues speculate that Dixdc1 may be compensating for a disrupted DISC1 in these people and that, therefore, certain single nucleotide polymorphisms (SNPs) in the Dixdc1 gene may well segregate with disease. This provides a concrete candidate for those murky compensatory factors that are often invoked to explain different outcomes in people who harbor the same mutation.
Together, these findings emphasize how multiple molecules team up to govern brain development. Teasing out how each molecule works, whether it acts alone or in partnership with others, is essential to understanding how problems with any one molecule may wreak havoc with brain development in ways that lead to psychiatric disease.—Michele Solis.
Singh KK, Ge X, Mao Y, Drane L, Meletis K, Samuels BA, Tsai LH. Dixdc1 is a critical regulator of DISC1 and embryonic cortical development. Neuron. 2010 July 15; 67: 33-48. Abstract
Comments on News and Primary Papers
Comment by: Kevin J. Mitchell
Submitted 19 July 2010
Posted 19 July 2010
The paper by Singh and colleagues adds to the growing list of proteins that interact with DISC1 and deepens our understanding of the biochemical pathways through which DISC1 modulates various neurodevelopmental processes. They demonstrate that the Dixdc1 protein interacts biochemically with DISC1, and that it functions together with DISC1 in two separable processes: neuronal proliferation and migration.
Interestingly, the nature of the interaction between Dixdc1 and DISC1 differs in these two processes. Knockdown of either Dixdc1 or DISC1 reduces proliferation, but the effects of knocking both down together are additive, indicating the absence of any epistatic interaction. Moreover, the effects of knockdown of either gene alone can be rescued by overexpressing the other gene. This suggests a partial redundancy in their functions rather than an intimate relationship where they necessarily work together.
Knockdown of either gene also disrupts neuronal migration in the cortex, but in this case the defects cannot be rescued by overexpression of the other gene, suggesting they may work more closely together in this context. These genes also act in distinct biochemical pathways in each context—through the Wnt/β-catenin pathway in proliferation and through a Cdk5-modulated Ndel1 pathway in cell migration, which impinges on the cytoskeleton.
These findings have several implications for the ongoing quest to understand the pathogenic mechanisms by which disruption of DISC1 pathways can result in psychiatric disease. First, as with PDE4B, NDE1, and PCM1, for example, they provide—through guilt by association—another viable candidate gene, Dixdc1, that may be analyzed for mutations in upcoming whole-genome sequencing efforts. More importantly, perhaps, they provide the means to dissociate the various functions of DISC1 during neurodevelopment.
One of the major difficulties in moving from gene identification to pathogenic mechanism has been that many of the genes with mutations linked to schizophrenia have highly pleiotropic effects; they play roles in many different processes, any one of which, or all of which together, might be responsible for the pathogenic effects. By dissecting the biochemical pathways mediating the different cellular functions of genes like DISC1, we can hope to develop animal models that can dissociate these functions and begin to dissect their contributions to pathogenesis.
View all comments by Kevin J. MitchellComment by: David J. Porteous, SRF Advisor
Submitted 21 July 2010
Posted 21 July 2010
The high prevalence of schizophrenia and related major mental illness, including bipolar disorder, in the Scottish family with the chromosome 1;11 translocation told us that the breakpoint gene DISC1 was an important key to unlocking the door on the molecular mechanisms underlying psychiatric illness (Millar et al., 2000; Blackwood et al., 2001). And so it has turned out to be (see review by Chubb et al., 2008). DISC1 is a scaffold protein that binds to and regulates other proteins critical in neurodevelopment and neurosignaling. We know the identity of several DISC1 interactors—PDE4, NDE1, NDEL1, PCM1, and Girdin amongst them—but at every turn, a new interactor seems to turn up.
Just last year, Li-Huei Tsai’s group identified GSK3β as a fascinating addition to the pantheon (Mao et al., 2009). GSK3β is interesting on two major counts: first, for its role in Wnt signalling and neuronal transcription; second, as a target for the action of lithium, the front-line treatment for bipolar disorder. Now, her group reports on another novel DISC1 interactor, Dixdc1. I won’t attempt to summarize the whole story—see the SRF news story for more background and details and then seek out the original for the full story—but hers is a “must-read” study. The key points are that 1) Dixdc1 is a novel and potent DISC1 interactor; 2) Dixdc1, like DISC1, modulates GSK3β and Wnt signalling; 3) the Wnt pathway regulates neuronal progenitor proliferation; 4) the effects of DISC1 and Dixdc1 are additive and compensatory; 5) the same is true for their effect in neuronal migration, which occurs not through Wnt signaling, but rather through Cdk5-mediated, phosphorylation-dependent tripartite interaction with NDEL1.
This important new work highlights yet again the insights emerging from the DISC1 complex and its manifold consequences. It needs to be integrated with other recent important findings, including the link through Girdin to AKT signalling (Kim et al., 2009; Enomoto et al., 2009), which in turn links back to GSK3β and Wnt signaling. It also connects to other recent work suggesting a convergent link between DISC1 and a second, strong genetic candidate risk factor for major mental illness, NRG1, mediated via Erb2/3 and P13K/AKT (Seshadri et al., 2010) and to glutamatergic neurotransmission (Hayashi-Takagi et al., 2010).
It is worth reflecting whether or how these connections would have been made without the start point of DISC1 as an unambiguous causal link to psychiatric illness (Porteous, 2008). The new study raises important questions about the potential contribution to genetic risk from variation in each of these DISC1 pathway genes (Porteous, 2008; Hennah and Porteous, 2009). It begs the question of which proteins bind DISC1 in developmental time and cellular space and how this all affects neurodevelopment, neurosignaling, and brain circuitry (Porteous and Millar, 2009).
Blackwood DH, Fordyce A, Walker MT, St. Clair DM, Porteous DJ, Muir WJ. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet. 2001;69:428-33. Abstract
Chubb JE, Bradshaw NJ, Soares DC, Porteous DJ, Millar JK. The DISC locus in psychiatric illness. Mol Psychiatry. 2008;13:36-64. Abstract
Enomoto A, Asai N, Namba T, Wang Y, Kato T, Tanaka M, Tatsumi H, Taya S, Tsuboi D, Kuroda K, Kaneko N, Sawamoto K, Miyamoto R, Jijiwa M, Murakumo Y, Sokabe M, Seki T, Kaibuchi K, Takahashi M. Roles of disrupted-in-schizophrenia 1-interacting protein girdin in postnatal development of the dentate gyrus. Neuron. 2009;63:774-87. Abstract
Hayashi-Takagi A, Takaki M, Graziane N, Seshadri S, Murdoch H, Dunlop AJ, Makino Y, Seshadri AJ, Ishizuka K, Srivastava DP, Xie Z, Baraban JM, Houslay MD, Tomoda T, Brandon NJ, Kamiya A, Yan Z, Penzes P, Sawa A. Disrupted-in-Schizophrenia 1 (DISC1) regulates spines of the glutamate synapse via Rac1. Nat Neurosci. 2010;13:327-32. Abstract
Hennah W, Porteous D. The DISC1 pathway modulates expression of neurodevelopmental, synaptogenic and sensory perception genes. PLoS One. 2009;4:e4906. Abstract
Kim JY, Duan X, Liu CY, Jang MH, Guo JU, Pow-anpongkul N, Kang E, Song H, Ming GL. DISC1 regulates new neuron development in the adult brain via modulation of AKT-mTOR signaling through KIAA1212. Neuron. 2009;63:761-73. Abstract
Mao Y, Ge X, Frank CL, Madison JM, Koehler AN, Doud MK, Tassa C, Berry EM, Soda T, Singh KK, Biechele T, Petryshen TL, Moon RT, Haggarty SJ, Tsai LH. Disrupted in schizophrenia 1 regulates neuronal progenitor proliferation via modulation of GSK3beta/beta-catenin signaling. Cell. 2009;136:1017-31. Abstract
Millar JK, Wilson-Annan JC, Anderson S, Christie S, Taylor MS, Semple CA, Devon RS, St. Clair DM, Muir WJ, Blackwood DH, Porteous DJ. Disruption of two novel genes by a translocation co-segregating with schizophrenia. Hum Mol Genet. 2000;9:1415-23. Abstract
Porteous D. Genetic causality in schizophrenia and bipolar disorder: out with the old and in with the new. Curr Opin Genet Dev. 2008;18:229-34. Abstract
Porteous D, Millar K. How DISC1 regulates postnatal brain development: girdin gets in on the AKT. Neuron. 2009;63:711-3. Abstract
Seshadri S, Kamiya A, Yokota Y, Prikulis I, Kano S, Hayashi-Takagi A, Stanco A, Eom TY, Rao S, Ishizuka K, Wong P, Korth C, Anton ES, Sawa A. Disrupted-in-Schizophrenia-1 expression is regulated by beta-site amyloid precursor protein cleaving enzyme-1-neuregulin cascade. Proc Natl Acad Sci U S A. 2010;107:5622-7. Abstract
View all comments by David J. PorteousComment by: Fengquan Zhou
Submitted 3 August 2010
Posted 3 August 2010
I recommend the Primary Papers
Last year, an interesting paper (Mao et al., 2009) demonstrated that DISC1 regulates neurogenesis via directly interacting with and inhibiting GSK3, which subsequently activates the canonical Wnt pathway via stabilization of β-cantenin. Now a paper from the same group has identified a DISC1 binding protein named Dixdc1, which functions together with DISC1 to regulate neurogenesis and neuronal migration.
Specifically, the paper demonstrates that knocking down either DISC1 or Dixdc1 impairs neural progenitor proliferation and the activation of the canonical Wnt pathway, and double knocking down both proteins has an additive effect. In addition, the effects of knockdown of either gene alone can be fully rescued by overexpressing the other gene. These results suggest that DISC1 and Dixdc1 play redundant roles in regulation of neural progenitor cell proliferation via the GSK3-β-catenin pathway. However, disruption of the interaction between the two proteins also decreases the progenitor proliferation and the activation of the GSK3-β-catenin pathway, suggesting that they coordinate with each other. One potential explanation is that either DISC1 or Dixdc1 can function to inhibit GSK3 and activate the canonical Wnt pathway alone but at less efficiency. The formation of the protein complex may help better recruit GSK3 and thus increase the efficiency of GSK3 inhibition. The rescue results are probably due to overexpression of the proteins, which also increases the access to GSK3.
Very interestingly, the paper shows that knocking down either Dixdc1 or DISC1 also impairs neuronal migration. Unlike the effects in neurogenesis, those on neuronal migration cannot be rescued by overexpression of the other gene. In addition, activation of the canonical Wnt pathway via overexpression of a stabilized β-catenin cannot rescue the defect in neuronal migration. Thus, the authors suggest that DISC1 and Dixdc1 coordinate to regulate neuronal migration independent of the GSK3-β-catenin pathway, possibly via Ndel1, a protein known to regulate neuronal migration. As β-catenin is only one of the many substrates of GSK3 and many known substrates of GSK3 are cytoskeletal proteins (Zhou and Snider, 2005), it is possible that DISC1 and Dixdc1 coordinate to regulate neuronal migration via GSK3 signaling, which has recently been shown to regulate neuronal migration (Asada and Sanada, 2010). A unique feature of GSK3 signaling is the importance of its spatial regulation, which may explain the lack of rescuing effect with overexpression of one gene.
GSK3 signaling has recently been shown to play important roles in many neurodevelopmental processes, including neurogenesis, neuronal polarization, and axon outgrowth (Hur and Zhou, 2010). However, how GSK3 activity is regulated in the developing brain is not clear. This study not only identifies a potential regulator of GSK3, but also provides additional evidence that GSK3 signaling may be involved in neuronal migration.
Asada, N., and Sanada, K. (2010). LKB1-mediated spatial control of GSK3beta and adenomatous polyposis coli contributes to centrosomal forward movement and neuronal migration in the developing neocortex. J Neurosci 30, 8852-8865. Abstract
Hur, E.M., and Zhou, F.Q. (2010). GSK3 signalling in neural development. Nat Rev Neurosci 11, 539-551. Abstract
Mao, Y., Ge, X., Frank, C.L., Madison, J.M., Koehler, A.N., Doud, M.K., Tassa, C., Berry, E.M., Soda, T., Singh, K.K., et al. (2009). Disrupted in schizophrenia 1 regulates neuronal progenitor proliferation via modulation of GSK3beta/beta-catenin signaling. Cell 136, 1017-1031. Abstract
Zhou, F.Q., and Snider, W.D. (2005). Cell biology. GSK-3beta and microtubule assembly in axons. Science 308, 211-214. Abstract
View all comments by Fengquan Zhou
Comments on Related News
Related News: DISC1: A Matter of Life or Death for Neural ProgenitorsComment by: Khaled Rahman
Submitted 26 March 2009
Posted 26 March 2009
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).
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
View all comments by Khaled Rahman
Related News: DISC1: A Matter of Life or Death for Neural Progenitors
Comment by: Simon Lovestone
Submitted 27 March 2009
Posted 27 March 2009
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.
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
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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
View all comments by Simon Lovestone
Related News: DISC1: A Matter of Life or Death for Neural Progenitors
Comment by: Nick Brandon (Disclosure)
Submitted 27 March 2009
Posted 30 March 2009
I recommend the Primary Papers
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).
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
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Related News: DISC1: A Matter of Life or Death for Neural Progenitors
Comment by: Akira Sawa, SRF Advisor
Submitted 8 April 2009
Posted 8 April 2009
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.
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
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