Patterning the neural plate border: Slug & SOXs

Cranial neural crest cells migrate to form a substantial proportion of the bones and cartilages of the skull and face,and the parts of the heart.

Neural crest defects, known as neurocristopathies, underlie a number of severe birth defects. 

A particularly dramatic example of epithelial to mesenchymal transition occurs in those cells that lie between the embryonic epidermis and neural tube:

the neural crest. Induction of the neural crest is initiatied by interactions between the embryonic epidermis, which expresses Bone Morphogenic Proteins (BMPs) and the neural plate, which express inhibitors of BMPs (e.g. chordin). 

The formation of the nervous system is directed by a number of interacting/intersecting signaling pathways. Bone morphogenic proteins (BMPs), BMP antagonists, Wnts, fibroblasts growth factors, notch, sonic hedgehog etc combine to differentiate neural ectoderm and neural crest from the embryonic epidermis.

With the discoveries that certain SOX-type transcription factors can modulate TCF-mediated Wnt signaling and that the different catenin-binding TCF-type transcription factors have different effects on target genes activities it has become increasingly clear that the response of a system to an external Wnt signal can be moduled.

The neural crest is a tissue unique to vertebrates.  It forms at the boundary of the epidermis and neural tube.

During neurulation, neural crest cells delaminate from the neuroectoderm, migrate throughout the embryo and differentiate into a number of different cell types.

Cranial neural crest cells, for example, migrate to form a substantial proportion of the bones and cartilages of the skull and face, and the parts of the heart. Neural crest defects, known as neurocristopathies, underlie a number of severe birth defects. 

Many studies indicate that initially neural crest cells are multipotent, and that their final destination plays an critical role in directing their differentiation.  There is also evidence, however, that there is pre-patterning within the crest population.  In particular, we are interested in the role of Wnts and Wnt-like signaling in the development of the neural crest in Xenopus.

Preliminary results:  To down-regulate slug expression in Xenopus, we have used the injection of "anti-sense" slug RNA, together with tissue transplantation.  We find that depletion of slug RNA leads to the inhibition of neural crest migration in Xenopus (Carl et al, 1999. Devel. Biol., in press).  We are currently examining anti-sense SLUG RNA injected embryos to determine whether depletion of slug leads to changes in the patterns of cadherin expression in neural crest cells, whether it blocks the disassembly of desmosomes, or whether desmosome assembly is stimulated.

        To target Wnt signaling agonists and antagonists to the neural crest, we have mastered the art of tissue transplantation (see animation at the top of the page).  Tissues to be transplanted are marked by injection of RNA encoding the green fluorescent protein (GFP).  We are now able to routinely transplant pre-migratory neural crest from one embryo to another and then follow neural crest migration in living embryos.

        In Xenopus, the most reliable method to regulate gene expression is through the injection of capped RNAs into the fertilized egg.  To regulate the activity of the translated product, we have constructed chimeric proteins containing hormone regulable repressor elements and such regulable forms of the Wnt-regulated transcription factors XTcf-3 and LEF-1 and the vertebrate armadillo-like proteins b-catenin and plakoglobin (g-catenin) are currently being tested for their activity in the Xenopus embryo.