Chordin affects pronephros development in Xenopus embryos by anteriorizing presomitic mesoderm

Spemann's organizer emits signals that pattern the mesodermal germ layer during Xenopus embryogenesis. In a previous study, we demonstrated that FGFR1 activity within the organizer is required for the production of both the somitic muscle- and pronephros-patterning signals by the organizer and the expression of chordin, an organizer-specific secreted protein (Mitchell and Sheets [2001] Dev. Biol. 237:295-305). Studies from others in both chicken and Xenopus embryos provide compelling evidence that pronephros forms by means of secondary induction signals emitted from anterior somites (Seufert et al. [1999] Dev. Biol. 215:233-242; Mauch et al. [2000] Dev. Biol. 220:62-75). Here we provide several lines of evidence in support of the hypothesis that chordin influences pronephros development by directing the formation of anterior somites. Chordin mRNA was absent in ultraviolet (UV) -irradiated embryos lacking pronepheros (average DAI<2) but was always found in UV-irradiated embryos that retain pronepheros (average DAI>2). Furthermore, ectopic expression of chordin in embryos and in tissue explants leads to the formation of anterior somites and pronephros. In these experiments, pronephros was only observed in association with muscle. Chordin diverted somatic muscle cells to more anterior positions within the somite file in chordin-induced secondary trunks and induced the expression of the anterior myogenic gene myf5. Finally, depletion of chordin mRNA with DEED antisense oligonucleotides substantially reduced somitic muscle and pronephric tubule and duct formation in whole embryos. These data and previous studies on ectoderm and endoderm (Sasai et al. [1995] Nature 377:757) support the idea that chordin functions as an anteriorizing signal in patterning the germ layers during vertebrate embryogenesis. Our data support the hypothesis that chordin directs the formation of anterior somites that in turn are necessary for pronephros development.