Zygotic Wnt activity is required for Brachyury expression in the early Xenopus laevis embryo.
The canonical, beta-catenin-dependent Wnt pathway is a crucial player in the early events of Xenopus development. Dorsal axis formation and mesoderm patterning are accepted effects of this pathway, but the regulation of expression of genes involved in mesoderm specification is not. This conclusion is based largely on the inability of the Wnt pathway to induce mesoderm in animal cap explants. Using injections of inhibitors of canonical Wnt signaling, we demonstrate that expression of the general mesodermal marker Brachyury (Xbra) requires a zygotic, ligand-dependent Wnt activity throughout the marginal zone. Analysis of the Xbra promoter reveals that putative TCF-binding sites mediate Wnt activation, the first sites in this well-studied promoter to which an activation role can be ascribed. However, established mesoderm inducers like eFGF and activin can bypass the Wnt requirement for Xbra expression. Another mesoderm promoting factor, VegT, activates Xbra in a Wnt-dependent manner. We also show that the activin/nodal signaling is necessary for ectopic Xbra induction by the Wnt pathway, but not by VegT. Our data significantly change the understanding of Brachyury regulation in Xenopus, implying the existence of an unknown zygotic Wnt ligand in Spemann's organizer. Since Brachyury is considered to have a major role in mesoderm formation, it is possible that Wnts might play a role in mesoderm specification, in addition to patterning.
Vonica
Alin
A
Cellular Biochemistry and Biophysics Program, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
Gumbiner
Barry M
BM
D028341:Activins / Q000378:metabolism; D000818:Animals; D015820:Cadherins / Q000235:genetics / Q000378:metabolism; D015870:Gene Expression; D006023:Glycoproteins; D024002:HMGB Proteins / Q000235:genetics / Q000378:metabolism*; D018398:Homeodomain Proteins / Q000235:genetics; D036341:Intercellular Signaling Peptides and Proteins; D008024:Ligands; D011401:Promoter Regions, Genetic; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D015398:Signal Transduction; D020825:T-Box Domain Proteins / Q000235:genetics* / Q000378:metabolism; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000378:metabolism; D051153:Wnt Proteins; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology / Q000235:genetics / Q000378:metabolism; D029961:Zebrafish Proteins; D015053:Zygote
C074260:CDH3 protein, Xenopus; D015820:Cadherins; D006023:Glycoproteins; D024002:HMGB Proteins; D018398:Homeodomain Proteins; D036341:Intercellular Signaling Peptides and Proteins; D008024:Ligands; D011506:Proteins; D011518:Proto-Oncogene Proteins; C093133:SIA1 protein, Xenopus; D020825:T-Box Domain Proteins; C116765:TBXT protein, Xenopus; D051820:TCF Transcription Factors; D058125:Transcription Factor 7-Like 1 Protein; D014157:Transcription Factors; D016212:Transforming Growth Factor beta; C103966:VegT protein, Xenopus; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; C094620:nodal3.1 protein, Xenopus; D028341:Activins; C054234:chordin