Factors required for neural crest induction in Xenopus laevis embryos: from screening to characterisation

Neural crest cells are a transient population of pluripotent cells unique to vertebrate embryos. Their importance in vertebrate development is highlighted by the diversity of their final fates, which include cells forming the cranialfacial cartilage, the peripheral nervous system, and melanocytes, amongst others. Neural crest cells maintain pluripotency until after they delaminate from the surrounding epithelium and migrate to their sites of final differentiation. This multi-step process allowing neural crest delamination and migration is known as epithelial-mesenchymal transition (EMT). Key molecular events of EMT include the downregulation of E-cadherin and upregulation of many transcription factors, including Snail and Slug. Snail and Slug are key regulators of EMT, and they are also markers for neural crest identity. This intimate link between neural crest fate and EMT provides a system whereby cell fate specification and EMT can be studied together. Many signalling pathways are known to affect neural crest induction and EMT. However, the molecular details of how these signals mediate this developmental program are poorly understood. I have carried out an overexpression screen in Xenopus laevis embryos to look for factors affecting neural crest induction and identified several novel candidates of interest. Included in the hits identified in the screen are known genes that have already been implicated in neural crest induction, which validates the screen. After some preliminary experiments, I focused my studies on a regulator of gene expression, SNW domain-containing 1 (SNW1). I discovered that SNW1 regulates the BMP gradient in Xenopus embryos responsible for dorsal-ventral patterning. Manipulation of SNW1 protein level in whole embryos alters the BMP gradient so that the graded response responsible for patterning the ectoderm is disrupted, with severe consequences for neural crest induction. Two downstream targets of SNW1 were identified using microarray analysis and these may mediate the effect of SNW1 on the BMP gradient, and they may also have direct effects on neural crest induction. Neural crest cells undergo EMT to migrate to their final sites of differentiation, and require transcription factors of the Snail family to drive this process. This is reminiscent of cases of EMT observed in some metastatic cancer cells. Loss of SNW1 prevents neural crest induction and expression of Snail, hence SNW1 may play an important role in cancer progression as well