Mechanism of Wnt signalling and its role in neural patterning and differentiation in vertebrate embryos

Lead Research Organisation: MRC National Inst for Medical Research

Abstract

Our brain is one of the most amazing organs because of its complexity, accuracy, and plasticity. Studies on the central nervous system (CNS) are of great advantage to us not only to understand the mechanism of how it functions, but also to solve problems of malformation and disorder, and even to utilise the mechanism in artificial intelligence. Although extraordinarily complex in construction, the primitive structure of the CNS in the early embryo is rather simple and provides us with an opportunity to see how it is formed. In order to organise the development of the CNS, neural cells know where they are and what cell types they should differentiate into. This process, called regional specification, occurs at very early stages of embryogenesis. Some dietary conditions such as an excessive intake of vitamin A during pregnancy, for example, can affect embryos during this process and can result in abnormal craniofacial development. This is due to the fact that vitamin A up-regulates some genes such as the Hox gene cluster which is responsible for the regional specification of the developing CNS. We are investigating the mechanism that regulates the correct expression of Hox genes using chick and mouse embryos. We have found that cell-cell interactions by signalling mechanisms, especially by the Wnt and BMP signalling pathways, are important for this process. ||While analysing the molecular mechanism of signalling cascades in detail, we found a molecule that modulates the readout of the Wnt signalling pathway. More precisely, the molecule regulates the susceptibility of cells to the signal by altering the amount of the receptor available on the cell surface. We are currently studying the detailed mechanism of this novel type of signal regulation in cultured cells. Clarifying the mechanism of Wnt pathway regulation is beneficial not only to elucidate the neural development but also to provide more information to the field of oncology, since cancers often show an elevated activity of the Wnt pathway..

Technical Summary

Our primary interest is the development of the central nervous system (CNS), especially the process of primitive neural cells to acquire positional identities. Positional identities are specified by the combinatorial gene expressions along both the anterior-posterior and dorsal-ventral axes, and are required for the precise neuronal connections at later stages of development. ||We have found that the positional identity of neural cells along the anterior-posterior axis is not cell-autonomously programmed when the cells are born; rather, it is determined through tissue-interactions with non-neural cells. By functional screening and candidate tests, the importance of the Wnt and BMP signalling pathways has been raised. Currently, we are studying how these signalling pathways are involved in the up-regulation of genes responsible for neural identity. ||A novel gene, Wise, was isolated by a functional screen in our lab. While Wise is able to up-regulate a set of genes that give posterior positional identities in neural explants, the in vivo expression pattern of Wise suggests that it may also be involved in maintaining proliferation of neural stem cells at the posterior end of the axis. We are currently studying the in vivo function of Wise in axis elongation where production of neural stem cells and their differentiation is continuously occurring. ||Molecular analysis of Wise function in vitro using HEK 293 cells has revealed a novel mechanism which regulates the readout of the Wnt signalling pathway. The Wnt pathway plays crucial roles in cell proliferation and differentiation during embryogenesis. Aberrant activation of the Wnt pathway is often a cause of cancer. Wise appears to alter the cells susceptibility to Wnt signals by altering the sub-cellular localisation of the Wnt receptor, LRP6. We are further investigating the molecular mechanism of this process. Since many cancers arise from elevated Wnt pathway activity, clarifying the mechanism of Wnt pathway regulation will be beneficial to this field. |In summary, we are studying the patterning and proliferation of neural cells focusing on the role of the Wnt pathway. In vivo function of Wise and other Wnt pathway components is studied mainly using chick embryos which are easily accessible by electroporation for gene over-expression and silencing. For genetic deletion and transgenic analyses, mouse embryos are also used. In the scope of potential therapeutic use of Wise, a patent has been applied in the USA with collaborators

Publications

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Ellies DL (2006) Bone density ligand, Sclerostin, directly interacts with LRP5 but not LRP5G171V to modulate Wnt activity. in Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research

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Shigetani Y (2007) Expression of Wise in chick embryos. in Developmental dynamics : an official publication of the American Association of Anatomists