Xenopus msx-1 regulates dorso-ventral axis formation by suppressing the expression of organizer genes

We demonstrated previously that Xmsx-1 is involved in mesoderm patterning a long the dorso-ventral axis, under the regulation of BMP-4 signaling. When Xmsx-1 RNA was injected into the dorsal blastomeres, a mass of muscle tissu e formed instead of notochord. This activity was similar to that of Xwnt-8 reported previously. In this study, we investigated whether the activity of Xmsx-1 is related to the ventralizing signal and myogenesis promoting fact or, Xwnt-8. Whole-mount in situ hybridization showed that Xmsx-1, Xwnt-8, a nd XmyoD were expressed in overlapping areas, including the ventro-lateral marginal zone at mid-gastrula stage. The expression of XmyoD was induced by the ectopic expression of either Xmsx-1 or Xwnt-8 in dorsal blastomeres, a nd Xwnt-8 was induced by the ectopic expression of Xmsx-1. On the other han d, the expression of Xmsx-1 was not affected by the loading of pCSKA-Xwnt-8 or dominant-negative Xwnt-8 (DN-Xwnt-8) RNA. In addition, Xmsx-1 RNA did n ot abrogate the formation of notochord if coinjected with DN-Xwnt-8 RNA. Th ese results suggest that Xmsx-1 functions upstream of the Xwnt-8 signal. Fu rthermore, the antagonistic function of Xmsx-1 to the expression of organiz er genes, such as Xlim-1 and goosecoid, was shown by in situ hybridization analysis and luciferase reporter assay using the goosecoid promoter constru ct. Finally if Xmsx-1/VP-16 fusion RNA, which was expected to function as a dominant-negative Xmsx-1, was injected into ventral blastomeres, a partial secondary axis formed in a significant number of embryos. In such embryos, the activity of luciferase, under the control of goosecoid promoter sequen ce, was significantly elevated at gastrula stage. These results led us to c onclude that Xmsx-1 plays a central role in establishing dorso-ventral axis in gastrulating embryo, by suppressing the expression of organizer genes. (C) 2000 Elsevier Science Inc. All rights reserved.