Wnt signaling functions repeatedly during embryonic development to induce different but specific responses. What molecular mechanisms ensure that Wnt signaling triggers the correct tissue-specific response in different tissues? Early Xenopus development is an ideal model for addressing this fundamental question, since there is a dramatic change in the response to Wnt signaling at the onset of zygotic gene transcription: Wnt signaling components encoded by maternal mRNA establish the dorsal embryonic axis; zygotically expressed Xwnt-8 causes almost the opposite, by promoting ventral and lateral and restricting dorsal mesodermal development. Although Wnt signaling can function through different signal transduction cascades, the same beta-catenin-dependent, canonical Wnt signal transduction pathway mediates Wnt signaling at both stages of Xenopus development. Here we show that, while the function of the transcription factor XTcf-3 is required for early Wnt signaling to establish the dorsal embryonic axis, closely related XLef-1 is required for Wnt signaling to pattern the mesoderm after the onset of zygotic transcription. Our results show for the first time that different transcription factors of the Lef/Tcf family function in different tissues to bring about tissue-specific responses downstream of canonical Wnt signaling.

The amphibian Xenopus laevis has been successfully used for many years as a model system for studying vertebrate development. Because of technical limitations, however, molecular investigations have mainly concentrated on early stages. We have developed a straightforward method for stage-specific induction of gene expression in transgenic Xenopus embryos [1] [2]. This method is based on the Xenopus heat shock protein 70 (Xhsp70 [3]) promoter driving the expression of desired gene products. We found that ubiquitous expression of the transgene is induced upon relatively mild heat treatment. Green fluorescent protein (GFP) was used as a marker to monitor successful induction of gene expression in transgenic embryos. We used this method to study the stage specificity of Wnt signalling function. Transient ectopic Wnt-8 expression during early neurulation was sufficient to repress anterior head development and this capacity was restricted to early stages of neurulation. By transient over-expression at different stages of development, we show that frizzled-7 disrupted morphogenesis sequentially from anterior to posterior along the dorsal axis as development proceeds. These results demonstrate that this method for inducible gene expression in transgenic Xenopus embryos will be a very powerful tool for temporal analysis of gene function and for studying molecular mechanisms of vertebrate organogenesis.

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 β-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 β-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 β-catenin function on XTcf-3. Our results highlight the potential versatility of the canonical Wnt pathway to interact with tissue-specific factors downstream of β-catenin, in order to achieve tissue-specific effects.