Head induction by simultaneous repression of Bmp and Wnt signalling in Xenopus

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Abstract
The Spemann organizer of the amphibian embryo can be subdivided into two discrete activities, namely trunk organizer and head organizer1. Several factors secreted from the organizer that are involved in trunk organization are thought to act by repressing Bmp signalling2,3,4. With the exception of the secreted factor cerberus5, little is known about head-organizer inducers. Here we show that co-expression of a dominant-negative Bmp receptor with inhibitors of the Wnt-signalling pathway in Xenopus leads to the induction of complete secondary axes, including a head. This induction does not require expression of the siamois marker of Nieuwkoop centre signalling, suggesting that cells are directly shifting to head-organizer fate. Furthermore, we find that cerberus is a potent inhibitor of Wnt signalling. Our results indicate that head-organizer activity results from the simultaneous repression of Bmp and Wnt signalling and they suggest a mechanism for region-specific induction by the organizer.