Proteomic Studies reveal Disrupted in Schizophrenia 1 as a key regulator unifying neurodevelopment and synaptic function

Background:
The Disrupted in Schizophrenia 1 (DISC1) gene has gained increasing recognition as studies have revealed its significance in regulating a number of processes in synapse function and regulation. These findings stem from studies on a diverse range of pathways, including those on DISC1 chromosomal translocation, gene truncation and molecular studies on its binding partners. An example of the role of DISC1 as a molecular bridge in neuronal development is a study on components involved in regulation of neurogenesis in mice. The study highlights the interplay between intrinsic DISC1 and extrinsic GABA signaling which are pathways critical for regulating neurogenesis and influence risk of development of mental disorders such as schizophrenia (1). The revelation of chromosomal translocation underlying neurogenesis hindrance arises from another study by Millar et al, in which a balanced translocation of chromosome 1 in humans was found to cause disruption in gene function, and thus structural functionality, of DISC1 leading to psychosis (2).This further demonstrates the importance of the DISC1 gene in neural and synaptic function.
In this preprint, the authors have revealed the involvement of DISC1 in various mechanisms such as regulation of CRMP proteins in neuronal differentiation and axonal development (3), among many others. The discovery of such pathways which are dependent on DISC1 provides further useful insight to its role in the landscape of mental diseases and could drive the discernment of novel therapeutic targets.

Key Findings:
Firstly, a proteomic analysis was carried out on both wild-type and DISC1 knockout cells using 4 2D gel electrophoreses each, in which spots on each gel were compared. 48 proteins differentially expressed between the two cell types were identified with mass spectrometry, most of which have reported functions related to neurodevelopment and synaptic function. In particular, several proteins amongst these have also been reported to be binding partners of DISC1 such as 14-3-3 proteins and LIS1 (4) as well as CRMP-2, a potential DISC1 interactor. This is interesting as it could indicate that, in addition to binding, such proteins are also regulated by DISC1 (either over-expressed or down regulated) in pathways contributing to neurodevelopment.
Secondly, the Ingenuity Pathway Analysis software was used to verify common pathways shared amongst the proteins identified as seen in the table below.

Following the identification of common pathways, it was worthwhile to further scrutinize the CRMPs (collapsin response mediator proteins) in particular, as Semaphorin signaling is an integral component of axonal growth. Interestingly, the truncation of the DISC1 gene (leading to altered protein structure) was previously found to cause disruption of the Semaphorin pathway in neurons (5) which further implicates the potential of CRMP regulation by DISC1. All CRMPs were up-regulated in DISC1 silenced cells, and particularly 1 in 3 isoforms of CRMP-2 was up-regulated while the other 2 were down-regulated; this suggests that wild-type DISC1 could play a role in suppressing CRMP expression and thus appropriately regulating Semaphorin signaling.

Another key finding was the morphological changes of neurite growth exhibited in SH-SY5Y neuroblastoma cells with silenced DISC1. As SH-SY5Y cells are commonly used for the study of
neuronal differentiation and function, those with silenced DISC1 are useful for demonstrating whether DISC1 is involved in such pathways. Fluorescence microscopy images comparing control and DISC1-silenced cells showed significant growth impairments in the latter upon retinoic acid- induced differentiation as seen in the figure below. Frequency of cells by neurite length was also measured for control and silenced in which the latter showed around 20 and 50 micrometers shorter neurite at the 50 percentile at 7 and 14 days RA exposure respectively.

Why I chose this article:
The elucidation of the molecular mechanisms of mental illnesses is highly challenging, despite the prominence of patients suffering from such diseases as schizophrenia and depression. The findings in this preprint form a useful foundation for the development of new treatment strategies. DISC1 appears to be a molecular scaffold regulating various signaling pathways and many possibilities as drug targets are presented for further research.

Questions for the authors:
1. In addition to neurite length and synapse formation, what other morphological changes in SH-SY5Y (or other) cells could be used to probe impairment of neurodevelopment?
2. Considering the opposite expression patterns of the different 14-3-3 proteins in DISC1 knockdowns, what could be the molecular mechanism of DISC1 in binding and signaling of the 3 14-3-3 proteins causing such opposing results in the context of neurite growth?

References
1. Kim, J., Liu, C., Zhang, F., Duan, X., Wen, Z., Song, J., Feighery, E., Lu, B., Rujescu, D., St Clair, D., Christian, K., Callicott, J., Weinberger, D., Song, H. and Ming, G. (2012). Interplay between DISC1 and GABA Signaling Regulates Neurogenesis in Mice and Risk for Schizophrenia. Cell, 148(5), pp.1051-1064.
2. Millar, J. (2000). Disruption of two novel genes by a translocation co-segregating with schizophrenia. Human Molecular Genetics, 9(9), pp.1415-1423.
3. Nagai, J., Baba, R. and Ohshima, T. (2016). CRMPs Function in Neurons and Glial Cells: Potential Therapeutic Targets for Neurodegenerative Diseases and CNS Injury. Molecular Neurobiology, 54(6), pp.4243-4256.
4. Taya, S., Shinoda, T., Tsuboi, D., Asaki, J., Nagai, K., Hikita, T., Kuroda, S., Kuroda, K., Shimizu, M., Hirotsune, S., Iwamatsu, A. and Kaibuchi, K. (2007). DISC1 Regulates the Transport of the NUDEL/LIS1/14-3-3 Complex through Kinesin-1. Journal of Neuroscience, 27(1), pp.15-26.
5. Sialana, F., Wang, A., Fazari, B., Kristofova, M., Smidak, R., Trossbach, S., Korth, C., Huston, J., de Souza Silva, M. and Lubec, G. (2018). Quantitative Proteomics of Synaptosomal Fractions in a Rat Overexpressing Human DISC1 Gene Indicates Profound Synaptic Dysregulation in the Dorsal Striatum. Frontiers in Molecular Neuroscience, 11.

 

Posted on: 19 December 2018

doi: https://doi.org/10.1242/prelights.6605

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