A headless zebrafish larva 120 hours after fertilization, obtained by ectopic expression of ΔNp63, which blocks neural specification in the anterior neuroectoderm. Courtesy of Matthias Hammerschmidt, Max Planck Institute for Immunobiology, Freiburg, Germany.

The central tenet of the default model of neural induction states that ectodermal cells are fated to become neural unless they are instructed by bone morphogenetic proteins (BMPs) to take on an epidermal fate. This indicates that there must be transcriptional repressors that act downstream of the BMPs to inhibit the expression of neural-specification genes, and Bakkers and colleagues have now identified a strong candidate for such a repressor. The zebrafish ΔNp63 protein is a homologue of the mammalian p63, which is a close relative of the tumour-suppressor protein p53. Alternative splicing generates at least six different isoforms of ΔNp63, all of which function as transcriptional activators or repressors. Mutation of p63 in mice revealed defects in epithelial development and indicated a possible role in the maintenance of epithelial stem cells, but these new studies in zebrafish imply a much earlier role in ectodermal cell-fate choice.

First, the authors looked for evidence that ΔNp63 acts downstream of BMP signalling. They showed that the promoter region of the ΔNp63 gene contains binding sites for the BMP-signalling mediators Smad4 and Smad5. They also showed that ΔNp63 expression could be upregulated by increasing the level of BMP signalling, an effect that was abolished if the Smad binding sites were mutated.

Next, the authors examined the role of ΔNp63 in dorsoventral patterning. In the gastrulating zebrafish embryo, ΔNp63 transcripts are confined to the ventral ectoderm, which gives rise to epidermis. Bakkers et al. inactivated ΔNp63 at this stage of development using antisense oligonucleotides. In the resulting embryos, the neuroectoderm was expanded, and there was a concomitant reduction in expression of non-neural ectodermal markers. Inhibition of BMP signalling also causes expansion of the neuroectoderm, and the authors found that this phenotype could be rescued by forcing the expression of ΔNp63. Overexpression of ΔNp63 in wild-type embryos, by contrast, led to a reduction in the amount of neural tissue.

So, ΔNp63 acts downstream of BMP signalling and it seems to be involved in ectodermal cell-fate specification, but how does it work? Bakkers et al. found that if the DNA-binding domain of ΔNp63 was linked to the repressor domain of a different protein, overexpression of the resulting chimeric protein could produce the same phenotype as ΔNp63 overexpression. This implies that ΔNp63 normally functions as a transcriptional repressor, with its DNA-binding domain conferring target specificity.

Taken together, these lines of evidence point towards ΔNp63 acting in the ventral ectoderm, downstream of BMP signalling, to repress genes that promote neural cell-fate specification. To complete the picture, it will be necessary to identify the target genes of ΔNp63, and to find out more about the mechanisms by which it achieves this repression.