McClay D R, Peterson R E, Range R C, Winter-Vann A M, Ferkowicz M J
Department of Biology, DCMB Group, Box 91000, Duke University, Durham, NC 27708, USA.
Development. 2000 Dec;127(23):5113-22. doi: 10.1242/dev.127.23.5113.
At fourth cleavage of sea urchin embryos four micromeres at the vegetal pole separate from four macromeres just above them in an unequal cleavage. The micromeres have the capacity to induce a second axis if transplanted to the animal pole and the absence of micromeres at the vegetal pole results in the failure of macromere progeny to specify secondary mesenchyme cells (SMCs). This suggests that micromeres have the capacity to induce SMCs. We demonstrate that micromeres require nuclear beta-catenin to exhibit SMC induction activity. Transplantation studies show that much of the vegetal hemisphere is competent to receive the induction signal. The micromeres induce SMCs, most likely through direct contact with macromere progeny, or at most a cell diameter away. The induction is quantitative in that more SMCs are induced by four micromeres than by one. Temporal studies show that the induction signal is passed from the micromeres to macromere progeny between the eighth and tenth cleavage. If micromeres are removed from hosts at the fourth cleavage, SMC induction in hosts is rescued if they later receive transplanted micromeres between the eighth and tenth cleavage. After the tenth cleavage addition of induction-competent micromeres to micromereless embryos fails to specify SMCs. For macromere progeny to be competent to receive the micromere induction signal, beta-catenin must enter macromere nuclei. The macromere progeny receive the micromere induction signal through the Notch receptor. Signaling-competent micromeres fail to induce SMCs if macromeres express dominant-negative Notch. Expression of an activated Notch construct in macromeres rescues SMC specification in the absence of induction-competent micromeres. These data are consistent with a model whereby beta-catenin enters the nuclei of micromeres and, as a consequence, the micromeres produce an inductive ligand. Between the eighth and tenth cleavage micromeres induce SMCs through Notch. In order to be receptive to the micromere inductive signal the macromeres first must transport beta-catenin to their nuclei, and as one consequence the Notch pathway becomes competent to receive the micromere induction signal, and to transduce that signal. As Notch is maternally expressed in macromeres, additional components must be downstream of nuclear beta-catenin in macromeres for these cells to receive and transduce the micromere induction signal.
在海胆胚胎的第四次卵裂时，植物极的四个小分裂球与它们上方的四个大分裂球以不等分裂的方式分离。如果将小分裂球移植到动物极，它们有能力诱导形成第二条轴，而植物极缺少小分裂球会导致大分裂球后代无法分化出次级间充质细胞（SMC）。这表明小分裂球有诱导SMC的能力。我们证明小分裂球需要核β-连环蛋白来展现SMC诱导活性。移植研究表明植物半球的大部分区域都有能力接收诱导信号。小分裂球诱导SMC，很可能是通过与大分裂球后代直接接触，或者最多相隔一个细胞直径的距离。这种诱导是定量的，即四个小分裂球比一个小分裂球诱导出更多的SMC。时间研究表明，诱导信号在第八次和第十次卵裂之间从小分裂球传递到大分裂球后代。如果在第四次卵裂时从小分裂球宿主中移除小分裂球，那么在第八次和第十次卵裂之间再给宿主移植小分裂球，宿主中的SMC诱导就能恢复。第十次卵裂后，向无小分裂球的胚胎中添加有诱导能力的小分裂球无法分化出SMC。为了使大分裂球后代有能力接收小分裂球诱导信号，β-连环蛋白必须进入大分裂球细胞核。大分裂球后代通过Notch受体接收小分裂球诱导信号。如果大分裂球表达显性负性Notch，有信号传导能力的小分裂球就无法诱导SMC。在没有有诱导能力的小分裂球的情况下，在大分裂球中表达活化的Notch构建体可挽救SMC的分化。这些数据与一个模型一致，即β-连环蛋白进入小分裂球的细胞核，结果是小分裂球产生一种诱导配体。在第八次和第十次卵裂之间，小分裂球通过Notch诱导SMC。为了能够接收小分裂球诱导信号，大分裂球首先必须将β-连环蛋白转运到它们的细胞核中，结果是Notch途径有能力接收小分裂球诱导信号并转导该信号。由于Notch在大分裂球中是母源表达的，对于这些细胞接收和转导小分裂球诱导信号来说，大分裂球中必须有其他成分位于核β-连环蛋白的下游。
