Structurally Related Receptors and Antagonists Compete for Secreted Wnt Ligands

Since Frzb can block the signaling activity of ectopic Xwnt-8, this raises the question of whether Frzb and Wnts are ever expressed in a manner consistent with their normally functioning in an antagonistic manner. In mouse embryos, Frzb is expressed in the primitive streak during gastrulation, and in mouse and human adults Frzb is expressed in a variety of adult organs, including the heart, brain, and skeletal muscle, pancreas, and kidney (Leyns et al. 1997xLeyns, L., Bouwmeester, T., Kim, S.-H., Piccolo, S., and DeRobertis, E.M. Cell. 1997; 88Abstract | Full Text | Full Text PDF | PubMed | Scopus (505)See all ReferencesLeyns et al. 1997). In Xenopus embryos, Frzb is also expressed during gastrulation, in a remarkable region of the embryo known as the gastrula organizer, or Spemann's organizer (Figure 1Figure 1) (8xLeyns, L., Bouwmeester, T., Kim, S.-H., Piccolo, S., and DeRobertis, E.M. Cell. 1997; 88Abstract | Full Text | Full Text PDF | PubMed | Scopus (505)See all References, 16xWang, S., Krinks, M., Lin, K., Luyten, F.P., and Moos, M. Jr. Cell. 1997; 88Abstract | Full Text | Full Text PDF | Scopus (371)See all References). This dorsal region, when transplanted to the ventral side of a host embryo, is able to induce the formation of a new body axis, complete with notochord, neural structures, and muscle (Spemann 1938xSee all ReferencesSpemann 1938; reviewed byLemaire and Kodjabachian 1996xLemaire, P. and Kodjabachian, L. Trends Genet. 1996; 12: 525–531Abstract | Full Text PDF | PubMed | Scopus (138)See all ReferencesLemaire and Kodjabachian 1996). Like Frzb, Wnts are also expressed in vertebrate embryos and adult tissues, including some of the tissues that express Frzb (9xMcMahon, A.P. Trends Genet. 1992; 8: 236–242Abstract | Full Text PDF | Scopus (89)See all References, 10xNusse, R. and Varmus, H.E. Cell. 1992; 69: 1073–1087Abstract | Full Text PDF | PubMed | Scopus (656)See all References). While careful mapping of the spatial patterns of expression of Frzb, Frizzled homologs, and Wnts has not yet been conducted, the complementary patterns of Frzb and Xwnt-8 in Xenopus (Figure 1Figure 1) are certainly consistent with these secreted factors working in an antagonistic manner in the future mesoderm.The ability of ectopic Frzb to block embryonic responses to ectopic Xwnt-8 does not establish that Frzb normally works to counteract the functions of endogenous Xwnt-8; however, further experiments by Leyns et al. 1997xLeyns, L., Bouwmeester, T., Kim, S.-H., Piccolo, S., and DeRobertis, E.M. Cell. 1997; 88Abstract | Full Text | Full Text PDF | PubMed | Scopus (505)See all ReferencesLeyns et al. 1997 and Wang et al. 1997xWang, S., Krinks, M., Lin, K., Luyten, F.P., and Moos, M. Jr. Cell. 1997; 88Abstract | Full Text | Full Text PDF | Scopus (371)See all ReferencesWang et al. 1997 support this likelihood. As noted above, endogenous Xwnt-8 is expressed in prospective ventro-lateral mesoderm of the gastrula, and its overexpression in regions of the embryo in which it is not normally expressed can divert the fate of those cells to a ventro-lateral mesodermal fate (Christian and Moon 1993xChristian, J.L. and Moon, R.T. Genes Dev. 1993; 7: 13–28CrossRef | PubMedSee all ReferencesChristian and Moon 1993). If a normal function of Xwnt-8 were to participate in forming ventro-lateral mesoderm, then its loss-of-function should perturb the somites and skeletal muscle, which are derived from this region (Figure 1Figure 1). Supporting this role, a dominant negative Xwnt-8 blocks induction of MyoD in the embryo, and inhibits formation of skeletal muscle (Hoppler et al. 1996xHoppler, S., Brown, J.D., and Moon, R.T. Genes Dev. 1996; 10: 2805–2817CrossRef | PubMedSee all ReferencesHoppler et al. 1996). Consistent with Frzb functioning to antagonize endogenous Xwnt-8, ectopic expression of Frzb throughout early Xenopus embryos also blocks induction of MyoD, and inhibits formation of skeletal muscle (8xLeyns, L., Bouwmeester, T., Kim, S.-H., Piccolo, S., and DeRobertis, E.M. Cell. 1997; 88Abstract | Full Text | Full Text PDF | PubMed | Scopus (505)See all References, 16xWang, S., Krinks, M., Lin, K., Luyten, F.P., and Moos, M. Jr. Cell. 1997; 88Abstract | Full Text | Full Text PDF | Scopus (371)See all References). Local ectopic expression of Frzb in ventral blastomeres at the 4-cell stage results in partial dorsalized phenotypes, also likely by antagonizing the ventral signal of endogenous Xwnt-8 (Wang et al. 1997xWang, S., Krinks, M., Lin, K., Luyten, F.P., and Moos, M. Jr. Cell. 1997; 88Abstract | Full Text | Full Text PDF | Scopus (371)See all ReferencesWang et al. 1997). These experiments support the conclusion that endogenous Frzb is a novel antagonist of the functions of some endogenous Wnts, including Xwnt-8.The presence of Frzb in the gastrula organizer may serve several functions, depending upon the largely unknown extent of diffusion of both Frzb and Xwnts. Within the extracellular space of the gastrula organizer, Frzb may locally block the instructive signal of Xwnt-8 which promotes ventro-lateral mesoderm formation, thereby allowing the development of dorsal cell fates. Second, there may be a region at the boundary of expression of Xwnt-8 and Frzb where the extracellular environment contains both proteins, which could yield a graded response to Xwnt-8 in the prospective lateral mesoderm. Third, since signals from the organizer induce and pattern the nervous system (Hemmati-Brivanlou and Melton 1997xHemmati-Brivanlou, A. and Melton, D. Cell. 1997; 88: 13–17Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesHemmati-Brivanlou and Melton 1997), it is possible that Frzb is involved in modulating neural cell fate, or endodermal cell fate, in addition to its likely roles in the mesoderm. Further testing of the functions of Frzb requires both a better understanding of the range of action of Frzb and Wnt signals, as well as genetic loss-of-function data. The loss-of-function studies would presumably need to come from another organism, since methods for interfering with gene expression in Xenopus embryos are not very precise, and the approach of interfering with protein function through design of a dominant negative protein is probably not suitable when the protein in question is essentially a dominant negative protein to begin with.While showing that the gastrula organizer is the source of a Wnt antagonist was completely unexpected, the new work on Frzb also adds to the growing evidence that the gastrula organizer is the surprising source of multiple secreted antagonists. Previously, the gastrula organizer has been shown to be a source of noggin, follistatin, and chordin that can bind to bone morphogenetic proteins (BMPs), thereby preventing their activating receptor-mediated signaling pathways (reviewed by7xLemaire, P. and Kodjabachian, L. Trends Genet. 1996; 12: 525–531Abstract | Full Text PDF | PubMed | Scopus (138)See all References, 3xHemmati-Brivanlou, A. and Melton, D. Cell. 1997; 88: 13–17Abstract | Full Text | Full Text PDF | PubMedSee all References). The consequence of antagonizing BMPs is that some of the dorsal ectodermal tissue then follows a default developmental pathway and becomes the nervous system. These examples underscore the important role of inhibitory signals, along with receptor-activating signals, in establishing regions of distinct cell fate and function in the embryo.In developmental biology one is constantly reminded that modern cell and molecular studies are often filling the gaps in processes identified or suspected many years ago by the embryologists and philosophers who asked how eggs exploit adults to make more eggs. The idea that different regions of embryos can be the sources of different signals that work in gradients, or in synergistic or antagonistic combinations, was well developed from the work of Boveri, Child, Holtfreter, and others (reviewed bySpemann 1938xSee all ReferencesSpemann 1938). Indeed, “the inducing effect of the organizer was, from the beginning, characterized as a releasing one” (Spemann 1938xSee all ReferencesSpemann 1938). One suspects that these embryologists would have been satisfied to learn of the complementary patterns of expression of Xwnt-8 and Frzb and to learn of their antagonistic functions in the Xenopus embryo.sc