Study of the novel Nodal target genes : molecular mechanism of inhibition of the FGF signaling and involvement of mesoderm formation during Xenopus development.

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The XSPR2b gene was isolated as the genes induced by FAST-1, a major transcriptional regulator of the Nodal signaling, from Xenopus laevis. It was shown that the expression of the XSPR2b gene was induced not only by the Nodal/FAST-1 signaling, but also the FGF signaling by using animal cap assay. This gene encodes transcription factor and is expressed mainly in the mesodermal region during the early Xenopus development. The expression domain is overlapped with those of Xbra gene, a major gene of mesoderm formation. Overexpression of the XSPR2b gene induced the ectopic expression of the Xbra gene in the embryos, suggesting the functional interaction between these genes. In the animal cap cells, overexpression of the Xbra gene itself could induce some mesodermal genes, whereas XSPR2b gene could not. When both genes were co-expressed in the cap cells, induction of the Xbra-target genes were up regulated. However, the expression of the none-Xbra target genes was not induced. These results suggested that XSPR2b gene has a role in mesoderm formation of Xenopus, through Xbra gene. By the reporter gene assay, wild type XSPR2b gene was shown to encode transcriptional activator. The molecular mechanism of the XSPR2b gene to induce the Xbra gene is currently under investigation.