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We have isolated two Xenopus relatives of murine Sox17 expressed in gastrula presumptive endoderm. Xsox17alpha and -beta expression can be induced in animal caps by activin, but not by FGF. Ectopic expression of these genes in animal caps induces the expression of endoderm markers; this induction is blocked by overexpression of a fusion of the Xsox17beta HMG domain to the Drosophila Engrailed repressor domain, as is induction of endoderm markers by activin and the expression of endodermal markers in whole embryos and isolated vegetal poles. These experiments, as well as the effects of the mRNAs on embryo phenotypes, suggest that the Xsox17 genes mediate an activin-induced endoderm differentiation pathway in animal caps and are involved in normal endoderm differentiation in embryos.
Figure 2. In Situ Hybridizations to Xenopus Embryos(A) Hybridization of Xsox17β to stage 10.5 gastrulae (uncleared), showing hybridization throughout the vegetal pole and superficially around the blastopore.(B) Fate map of the early gastrula. AP and VP, animal and vegetal poles; dl and vl, dorsal and ventral blastopore lips; blue, ectoderm; red, mesoderm; yellow, endoderm (Keller 1991).(C) shows an optical section through a cleared embryo as in (A). It is possible to see hybridization throughout the presumptive endoderm, but the mesoderm and ectoderm are unstained.(D) This is confirmed by a section of the mesodermal region; lp, blastopore lip.(E) For comparison, Xbra hybridization is shown; Xbra expression ceases when the cells have rolled over the lip.(F and G) Hybridization of Xsox17α to a stage 26 embryo (F); shown in section in (G).(H) Hybridization of Xsox17α to a cleared stage 35 embryo. At this stage, there is staining in the posterior gut and in an annulus where the hepatic diverticulum opens into the submesodermal space; this is where the gall bladder develops (Nieuwkoop and Faber 1994). The annular appearance of hybridization in the liver region is shown by the inset uncleared embryo.(I) Rescue of Xsox17β::EnR by Xsox-17β mRNA. Embryos are shown at stage 33. (I) Embryos injected at the 2-cell stage with 240pg Xsox17β::EnR mRNA. (J) Control embryo. (K) Xsox17β::EnR mRNA (240 pg) plus 60 pg Xsox17β mRNA.(L) Phenotypic effects of mRNAs. (L) Effect of Xsox17α mRNA. At the top is a stage 42 control embryo, at the bottom is one injected with 120 pg Xsox17α mRNA. (M) Stage 33 embryo injected with 120 pg Xsox17β::EnR mRNA. A control at this stage is shown in (J). (N) 120 pg Xsox17β::EnR mRNA. Note the reduced gut coiling compared to a control (O).