UPMC Université Paris 06, UMR7009, CNRS, Biologie du Développement, Observatoire Océanologique de Villefranche-sur-Mer, Villefranche-sur-Mer, France.
Development. 2012 Feb;139(4):816-25. doi: 10.1242/dev.072215.
In sea urchins, the nuclear accumulation of β-catenin in micromeres and macromeres at 4th and 5th cleavage activates the developmental gene regulatory circuits that specify all of the vegetal tissues (i.e. skeletogenic mesoderm, endoderm and non-skeletogenic mesoderm). Here, through the analysis of maternal Frizzled receptors as potential contributors to these processes, we found that, in Paracentrotus lividus, the receptor Frizzled1/2/7 is required by 5th cleavage for β-catenin nuclearisation selectively in macromere daughter cells. Perturbation analyses established further that Frizzled1/2/7 signaling is required subsequently for the specification of the endomesoderm and then the endoderm but not for that of the non-skeletogenic mesoderm, even though this cell type also originates from the endomesoderm lineage. Complementary analyses on Wnt6 showed that this maternal ligand is similarly required at 5th cleavage for the nuclear accumulation of β-catenin exclusively in the macromeres and for endoderm but not for non-skeletogenic mesoderm specification. In addition, Wnt6 misexpression reverses Frizzled1/2/7 downregulation-induced phenotypes. Thus, the results indicate that Wnt6 and Frizzled1/2/7 are likely to behave as the ligand-receptor pair responsible for initiating β-catenin nuclearisation in macromeres at 5th cleavage and that event is necessary for endoderm specification. They show also that β-catenin nuclearisation in micromeres and macromeres takes place through a different mechanism, and that non-skeletogenic mesoderm specification occurs independently of the nuclear accumulation of β-catenin in macromeres at the 5th cleavage. Evolutionarily, this analysis outlines further the conserved involvement of the Frizzled1/2/7 subfamily, but not of specific Wnts, in the activation of canonical Wnt signaling during early animal development.
在海胆中，β-连环蛋白在第 4 和第 5 次卵裂时在小分裂球和大分裂球中的核积累激活了指定所有植物组织（即骨骼生成中胚层、内胚层和非骨骼生成中胚层）的发育基因调控回路。在这里，通过分析作为这些过程潜在贡献者的母体 Frizzled 受体，我们发现，在 Paracentrotus lividus 中，受体 Frizzled1/2/7 在第 5 次卵裂时对于β-连环蛋白在大分裂球子细胞中的核化是必需的。扰动分析进一步确立，Frizzled1/2/7 信号随后对于内胚层和内胚层的指定是必需的，但对于非骨骼生成中胚层的指定不是必需的，即使这种细胞类型也源自内胚层中胚层谱系。对 Wnt6 的互补分析表明，这种母体配体在第 5 次卵裂时同样需要β-连环蛋白的核积累，仅在大分裂球中用于内胚层，但不用于非骨骼生成中胚层的指定。此外，Wnt6 过表达逆转了 Frizzled1/2/7 下调诱导的表型。因此，结果表明 Wnt6 和 Frizzled1/2/7 可能作为配体-受体对，负责在第 5 次卵裂时在大分裂球中引发β-连环蛋白的核化，并且该事件对于内胚层的指定是必需的。它们还表明，β-连环蛋白在小分裂球和大分裂球中的核化是通过不同的机制发生的，并且非骨骼生成中胚层的指定发生在第 5 次卵裂时大分裂球中β-连环蛋白的核积累独立。从进化的角度来看，这种分析进一步概述了 Frizzled1/2/7 亚家族的保守参与，而不是特定的 Wnts，在早期动物发育过程中激活经典 Wnt 信号。
