XB-ART-7947Dev Dyn 2001 Nov 01;2223:368-76. doi: 10.1002/dvdy.1189.
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Xiro-1 controls mesoderm patterning by repressing bmp-4 expression in the Spemann organizer.
The Iroquois genes code for homeodomain proteins that have been implicated in the neural development of Drosophila and vertebrates. We show here for the first time that Xiro-1, one of the Xenopus Iroquois genes, is expressed in the Spemann organizer from the start of gastrulation and that its overexpression induces a secondary axis as well as the ectopic expression of several organizer genes, such as chordin, goosecoid, and Xlim-1. Our results also indicate that Xiro-1 normally functions as a transcriptional repressor in the mesoderm. Overexpression of Xiro-1 or a chimeric form fused to the repressor domain of Engrailed cause similar phenotypes while overexpression of functional derivatives of Xiro-1 fused with transactivation domains (VP16 or E1A) produce the opposite effects. Finally, we show that Xiro-1 works as a repressor of bmp-4 transcription and that its effect on organizer development is dependent on BMP-4 activity. We propose that the previously observed down regulation of bmp-4 in the dorsal mesoderm during gastrulation can be explained by the repressor activity of Xiro-1 described here. Thus, Xiro-1 seems to have at least two different functions: control of neural plate and organizer development, both of which could be mediated by repression of bmp-4 transcription.
PubMed ID: 11747072
Article link: Dev Dyn
Species referenced: Xenopus
Genes referenced: bmp4 cer1 chrd.1 gsc irx1 lhx1 mst1
Article Images: [+] show captions
|Figure 1. Xiro-1 expression. A: Saggital sections of a stage 10+ embryos were analyzed by in situ hybridization for the expression of Xiro-1 (arrow). Arrowhead: blastopore; asterisk: ectodermal expression. B-E: Embryos were fixed at stage 10+ and after in situ hybridization, were sectioned into halves to examine internal gene expression. The border of the blastocoel cavity is indicated by a black line (top); the arrowhead indicates the dorsal blastopore lip (bottom). B: Xiro-1 expression in the involuting marginal zone is indicated by a white line; asterisk: ectodermal expression. C: Chd expression; D: Gsc expression; E: Cer expression. The expression of Xiro-1 is indicated in all three cases (C-E) to show its overlapping expression with Chd, Gsc, and Cer. F: After gastrulation, Xiro-1 expression is detected in the ectoderm (arrow), notochord (n, arrowhead), (saggital section stage 17). G: At stage 25, Xiro-1 is located in the neural tube and can easily be seen in the somites (arrow). H: A section from a stage-25 embryo following in situ hybridization for Xiro-1. Expression is observed in the notochord (arrowhead), somites and neural tube (n: notochord; NT: neural tube).|
|Figure 3. Xiro-1 regulates the expression of mesodermal genes by repression. When injected into one blastomere of 2-cell stage embryos, in the presumptive ventro-lateral mesodermal region, 2 ng of either Xiro-1 (A,C,E,G) or EnR-Xiro (B,D,F,H) produced the ectopic expression of chordin (A and B: arrowheads, A: 37% n = 104 and B: 38% n = 132) and goosecoid (C and D: arrowheads, C: 34% n = 95 and D: 54% n = 131). Ventral genes were inhibited by these treatments. Inhibition of Xvent-1 (E) and Xwnt-8 (G) by Xiro-1 was observed in 89 and 91% of cases, respectively (E and G: arrowheads, E: n = 162 and G: n = 172) while EnR-Xiro caused an inhibitory effect in 92% (F: arrowhead, n = 158) and 97% (H: arrowhead, n = 129) of cases, respectively. All figures shown are vegetal views of stage 10.25 embryos with the dorsal region at the top. Dexametasone treatment was started at stage 5 until the embryos were fixed.|
|Figure 6. Xiro-1 represses bmp-4 expression. Embryos were analyzed for bmp-4 expression (A,B: vegetal view; C,D: animal view). Overexpression of 2 ng of Xiro-1 or EnR-Xiro repressed the expression of bmp-4 when injected into one blastomere of 4-cell stage embryos. Xiro-1 inhibited bmp-4 expression in the ventro-lateral mesoderm and ectoderm (A and C: arrowheads, 95% n = 41). EnR-Xiro behaved in the same manner, repressing bmp-4 expression in 98% of cases (B and D: arrowheads, n = 53).|