A regulatory loop between the fibroblast growth factors FGF-8 and FGF-10 plays a key role in limb initiation and AER induction in vertebrate embryos. Here, we show that three WNT factors signaling through β-catenin act as key regulators of the FGF-8/FGF-10 loop. The Wnt-2b gene is expressed in the intermediate mesoderm and the lateral plate mesoderm in the presumptive chick forelimb region. Cells expressing Wnt-2b are able to induce Fgf-10 and generate an extra limb when implanted into the flank. In the presumptive hindlimb region, another Wnt gene, Wnt-8c, controls Fgf-10 expression, and is also capable of inducing ectopic limb formation in the flank. Finally, we also show that the induction of Fgf-8 in the limb ectoderm by FGF-10 is mediated by the induction of Wnt-3a. Thus, three WNT signals mediated by β-catenin control both limb initiation and AER induction in the vertebrate embryo.
Bibliographical noteFunding Information:
We thank Dr. Frank Costantini (Columbia University, NY) for the mouse Axin clone, Dr. Tohru Tsukui (Salk Institute, La Jolla, CA, currently at Saitama Medical School, Saitama, Japan) for constructing the adenoviral-Axin construct, May Chu for excellent technical assistance, and Lorraine Hooks for help in preparing the manuscript. Y. K. was supported by a grant from the National Science Foundation (USA) (to J. C. I. B.). T. I. was supported by a Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists. C. R. E., D. B., J. C., and J. C. I. B. were supported by the NIH and the G. Harold and Leila Y. Mathers Charitable Foundation.