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The Mesp bHLH genes play a conserved role during segmental patterning of the mesoderm in the vertebrate embryo by specifying segmental boundaries and anteroposterior (A-P) segmental polarity. Here we use a xenotransgenic approach to compare the transcriptional enhancers that drive expression of the Mesp genes within segments of the presomitic mesoderm (PSM) of different vertebrate species. We find that the genomic sequences upstream of the mespb gene in the pufferfish Takifugu rubripes (Tr-mespb) are able to drive segmental expression in transgenic Xenopus embryos while those from the Xenopus laevis mespb (Xl-mespb) gene drive segmental expression in transgenic zebrafish. In both cases, the anterior segmental boundary of transgene expression closely matches the expression of the endogenous Mesp genes, indicating that many inputs into segmental gene expression are highly conserved. By contrast, we find that direct retinoic acid (RA) regulation of endogenous Mesp gene expression is variable among vertebrate species. Both Tr-mespb and Xl-mespb are directly upregulated by RA, through a complex, distal element. By contrast, RA represses the zebrafish Mesp genes. We show that this repression is mediated, in part, by RA-mediated activation of the Ripply genes, which together with Mesp genes form an RA-responsive negative feedback loop. These observations suggest that variations in a direct response to RA input may allow for changes in A-P patterning of the segments in different vertebrate species.
Fig. 1. Mesp genes in Xenopus. (A) Genomic organization of the Mesp genes. Shown are syntenic genomic regions from mouse, Xenopus tropicalis, Takifugu rubripes and Danio rerio. Chromosome number is given when available, otherwise, scaffold number is indicated. Boxing indicates synteny of genes. X. tropicalis genes ENSXETG00000017725 and ENSXETG00000027628 are designated as mespb and mespa respectively. Green triangles indicate a syntenic block of 6 genes that are conserved between X. tropicalis and mouse, but in frog this block is between mespa and the AnPep gene, while in mouse the block is located 4 megabases (mb) away. (B, C) Expression of Xl-mespa and Xl-mespb in the PSM of X. laevis embryos by whole mount in situ hybridization. Dotted lines indicate approximate locations of somitomere boundaries. (D) Double in situ hybridization with Xl-mespb (red) and Xl-mespa (purple). (E–L) Shown is the expression of Xenopus Mesp genes after treatment with CHX and RA, as indicated. Treatment was for 1.5 h at RT prior to fixation. (M) Schematic representation of the PSM showing the relative locations of the somitomeres, transition zone (TZ) and tailbud (TBD) with X. laevis Mesp expression patterns indicated. Anterior is to the left in all panels.
Xenopus mespa gene expression in paraxial mesoderm
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