Résumé
The molecular nature of the primary dorsalizing inducing event in Xenopus is controversial and several secreted factors have been proposed as potential candidates: Wnts, Vg1, Activin and Noggin. Recent studies, however, have provided new insight into the activity of the dorsalizing region, called the Nieuwkoop Center. (1) The activity of this dorsalizing center involves an entire signal transduction pathway that requires maternal β-catenin (Heasman, J., Crawford, A., Goldstone, K., Garner-Hamrick, P., Gumbiner, B., McCrea, P., Kintner, C., Noro, C. Y. and Wylie, C. (1994)Cell 79, 791-803). (2) A transcription factor with potent dorsalizing activity, Siamois, is expressed within the Nieuwkoop Center (Lemaire, P., Garrett, N. and Gurdon, J. B. (1995)Cell 81, 85-94). We have used these two properties of the Nieuwkoop Center to evaluate the dorsalizing activity of the four secreted factors Wnt8, Vg1, Activin and Noggin. The requirement for β-catenin was tested by coexpressing a cadherin, which sequesters β-catenin at the cell membrane and specifically blocks its intracellular signaling activity (Fagotto, F., Funayama, N., Glück, U. and Gumbiner, B. M. (1996)J. Cell Biol. 132, 1105-1114). Induction of Siamois expression was detected by RT-PCR. Of the four growth factors, only Wnt was sensitive to inhibition of β-catenin activity and only Wnt could induce Siamois expression. Therefore, Wnt is able to induce a bonafide Nieuwkoop Center, while Vg1, Activin and Noggin probably induce dorsal structures by a different mechanism.
To order the steps in the Nieuwkoop Center signaling cascade, we have tested the relationship between β-catenin and GSK, a serine-threonine kinase that has been implicated in axis formation in a step downstream of Wnt. We found that GSK acts upstream of β-catenin, similar to the order of these components in the Wingless pathway in Drosophila. We have also examined the relationship between the Wnt/β-catenin pathway and Siamois. We show that β-catenin induces expression of Siamois and that the free signaling pool of β-catenin is required for normal expression of endogenous Siamois. We conclude that the sequence of steps in the signaling pathway is Wnt ⊣ GSK ⊣ β-catenin→ Siamois.