Logan C Y, Miller J R, Ferkowicz M J, McClay D R
Developmental, Cellular and Molecular Biology Group and the Department of Zoology, Duke University, Durham, NC 27708, USA.
Development. 1999 Jan;126(2):345-57. doi: 10.1242/dev.126.2.345.
Beta-catenin is thought to mediate cell fate specification events by localizing to the nucleus where it modulates gene expression. To ask whether beta-catenin is involved in cell fate specification during sea urchin embryogenesis, we analyzed the distribution of nuclear beta-catenin in both normal and experimentally manipulated embryos. In unperturbed embryos, beta-catenin accumulates in nuclei that include the precursors of the endoderm and mesoderm, suggesting that it plays a role in vegetal specification. Using pharmacological, embryological and molecular approaches, we determined the function of beta-catenin in vegetal development by examining the relationship between the pattern of nuclear beta-catenin and the formation of endodermal and mesodermal tissues. Treatment of embryos with LiCl, a known vegetalizing agent, caused both an enhancement in the levels of nuclear beta-catenin and an expansion in the pattern of nuclear beta-catenin that coincided with an increase in endoderm and mesoderm. Conversely, overexpression of a sea urchin cadherin blocked the accumulation of nuclear beta-catenin and consequently inhibited the formation of endodermal and mesodermal tissues including micromere-derived skeletogenic mesenchyme. In addition, nuclear beta-catenin-deficient micromeres failed to induce a secondary axis when transplanted to the animal pole of uninjected host embryos, indicating that nuclear beta-catenin also plays a role in the production of micromere-derived signals. To examine further the relationship between nuclear beta-catenin in vegetal nuclei and micromere signaling, we performed both transplantations and deletions of micromeres at the 16-cell stage and demonstrated that the accumulation of beta-catenin in vegetal nuclei does not require micromere-derived cues. Moreover, we demonstrate that cell autonomous signals appear to regulate the pattern of nuclear beta-catenin since dissociated blastomeres possessed nuclear beta-catenin in approximately the same proportion as that seen in intact embryos. Together, these data show that the accumulation of beta-catenin in nuclei of vegetal cells is regulated cell autonomously and that this localization is required for the establishment of all vegetal cell fates and the production of micromere-derived signals.
β-连环蛋白被认为通过定位于细胞核来介导细胞命运决定事件，在细胞核中它调节基因表达。为了探究β-连环蛋白是否参与海胆胚胎发育过程中的细胞命运决定，我们分析了正常胚胎和经实验操作的胚胎中核β-连环蛋白的分布。在未受干扰的胚胎中，β-连环蛋白积聚在内胚层和中胚层前体所在的细胞核中，这表明它在植物极细胞命运决定中发挥作用。我们使用药理学、胚胎学和分子生物学方法，通过研究核β-连环蛋白模式与内胚层和中胚层组织形成之间的关系，确定了β-连环蛋白在植物极发育中的功能。用已知的植物极化剂LiCl处理胚胎，导致核β-连环蛋白水平升高以及核β-连环蛋白模式扩展，这与内胚层和中胚层的增加相吻合。相反，海胆钙黏蛋白的过表达阻止了核β-连环蛋白的积累，从而抑制了包括小分裂球来源的造骨间充质在内的内胚层和中胚层组织的形成。此外，缺乏核β-连环蛋白的小分裂球移植到未注射的宿主胚胎的动物极时，无法诱导形成次生轴，这表明核β-连环蛋白在小分裂球来源信号的产生中也发挥作用。为了进一步研究植物极细胞核中的核β-连环蛋白与小分裂球信号传导之间的关系，我们在16细胞阶段进行了小分裂球的移植和缺失实验，并证明植物极细胞核中β-连环蛋白的积累不需要小分裂球来源的信号。此外，我们证明细胞自主信号似乎调节核β-连环蛋白的模式，因为解离的卵裂球中核β-连环蛋白的比例与完整胚胎中大致相同。总之，这些数据表明植物极细胞细胞核中β-连环蛋白的积累是由细胞自主调节的，并且这种定位对于所有植物极细胞命运的建立和小分裂球来源信号的产生是必需的。
