Difference in XTcf-3 dependency accounts for change in response to beta-catenin-mediated Wnt signalling in Xenopus blastula.

Wnt signalling functions in many tissues and during different stages of animal development to produce very specific responses. In early Xenopus embryos there is a dramatic change in response to Wnt signalling within only a few hours of development. Wnt signalling in very early embryos leads to a dorsalising response, which establishes the endogenous dorsal axis. Only a few hours later in development, almost the opposite happens: Xwnt-8 functions to pattern the embryonic mesoderm by promoting ventral and lateral mesoderm. The specificity of the response could conceivably be carried out by differential use of different signal transduction pathways, many of which have recently been described. We have found, however, that this dramatic shift in response to Wnt signalling in early Xenopus is not brought about by differential use of distinct signal transduction pathways. In fact beta-catenin, a downstream component of the canonical Wnt signal transduction pathway, functions not only in the early dorsalising response but also in the later ventrolateral-promoting response. Interaction of beta-catenin with the XTcf-3 transcription factor is required for the early dorsalising activity. In contrast, our experiments suggest that late Wnt signalling in the ventrolateral mesoderm does not require a similar dependency of beta-catenin function on XTcf-3. Our results highlight the potential versatility of the canonical Wnt pathway to interact with tissue-specific factors downstream of beta-catenin, in order to achieve tissue-specific effects.