Wnt/beta-catenin signaling controls Mespo expression to regulate segmentation during Xenopus somitogenesis

The vertebral column is derived from somites, which are transient segments of the paraxial mesoderm that are present in developing vertebrates. The strict spatial and temporal regulation of somitogenesis is of crucial developmental importance. Signals such as Wnt and FGF play roles in somitogenesis, but details regarding how Wnt signaling functions in this process remain unclear. In this study, we report that Wnt/beta-catenin signaling regulates the expression of Mespo, a basic-helix-loop-helix (bHLH) gene critical for segmental patterning in Xenopus somitogenesis. Transgenic analysis of the Mespo promoter identifies Mespo as a direct downstream target of Wnt/beta-catenin signaling pathway. We also demonstrate that activity of Wnt/beta-catenin signaling in somitogenesis can be enhanced by the PI3-K/AKT pathway. Our results illustrate that Wnt/beta-catenin signaling in conjunction with PI3-K/AKT pathway plays a key role in controlling development of the paraxial mesoderm.     

Somite formation: where left meets right

Somites are the bilaterally symmetric embryonic precursors of the vertebrate skeleton and axial muscle. Three recent studies reveal that somites form asymmetrically in the absence of retinoic acid signaling. These results uncover an unexpected relationship between somitogenesis and left-right patterning, and suggest that bilateral somite formation is regulated along the left-right axis.     

Preferential platination of an activated cellular promoter by cis-diamminedichloroplatinum.

This study examines how accessibility to cisplatin on various genomic regions in T47D breast cancer cells, including the retinoic acid receptor beta gene promoter and coding region and the dihydrofolate reductase gene promoter and coding region, is affected by treatment of the cells with 9-cis retinoic acid, a treatment that activates the retinoic acid receptor beta gene promoter in these cells. A PCR-based assay was used to measure cisplatin adduct density based on the inhibition of PCR amplification of templates from cisplatin treated versus untreated cells. Treatment of cells with 9-cis retinoic acid enhanced accessibility to cisplatin on the retinoic acid receptor beta gene promoter region, but not on the coding regions of that gene nor on the dihydrofolate reductase gene promoter or coding regions, where accessibilities to cisplatin remained 2-4 times lower than on the activated retinoic acid receptor beta gene promoter. Examination of smaller regions within this promoter region showed a repression of platination in the 500 bp region surrounding the TATA box in cells prior to 9-cis retinoic acid treatment, which was abolished following promoter activation. Differences in sequence composition between the various regions could not fully account for differences in platination, suggesting that structural features such as bends in retinoic acid receptor beta gene promoter DNA following gene activation, create energetically favorable sites for platination, and contribute to the cytotoxicity of the drug.     

Retinoic-acid signalling in node ectoderm and posterior neural plate directs left-right patterning of somitic mesoderm

Somitogenesis requires bilateral rhythmic segmentation of paraxial mesoderm along the antero-posterior axis. The location of somite segmentation depends on opposing signalling gradients of retinoic acid (generated by retinaldehyde dehydrogenase-2; Raldh2) anteriorly and fibroblast growth factor (FGF; generated by Fgf8) posteriorly. Retinoic-acid-deficient embryos exhibit somite left-right asymmetry, but it remains unclear how retinoic acid mediates left-right patterning. Here, we demonstrate that retinoic-acid signalling is uniform across the left-right axis and occurs in node ectoderm but not node mesoderm. In Raldh2(-/-) mouse embryos, ectodermal Fgf8 expression encroaches anteriorly into node ectoderm and neural plate, but its expression in presomitic mesoderm is initially unchanged. The late stages of somitogenesis were rescued in Raldh2(-/-) mouse embryos when the maternal diet was supplemented with retinoic acid until only the 6-somite stage, demonstrating that retinoic acid is only needed during node stages. A retinoic-acid-reporter transgene marking the action of maternal retinoic acid in rescued Raldh2(-/-) embryos revealed that the targets of retinoic-acid signalling during somitogenesis are the node ectoderm and the posterior neural plate, not the presomitic mesoderm. Our findings suggest that antagonism of Fgf8 expression by retinoic acid occurs in the ectoderm and that failure of this mechanism generates excessive FGF8 signalling to adjacent mesoderm, resulting initially in smaller somites and then left-right asymmetry.