Medina A, Wendler S R, Steinbeisser H
Max-Planck-Institute for Developmental Biology, Dept. of Cell Biology, Tübingen, Germany.
Int J Dev Biol. 1997 Oct;41(5):741-5.
Beta-catenin, a component of the wnt-signal-transduction pathway, is essential for the formation of the dorsal axis in Xenopus laevis embryos. On the dorsal side of the embryo, beta-catenin is translocated into the nuclei via a process linked to cortical rotation. When cortical rotation is blocked by UV-irradiation, nuclear beta-catenin is found in the vegetal pole of the embryo. Here we show that overexpression of beta-catenin in animal cap explants, in the absence of mesoderm induction, is sufficient to activate the expression of genes with dorsalizing activity such as siamois (sia) and nodal-related 3 (nr3) but not goosecoid (gsc). In embryos ventralized by UV-treatment, the expression of the dorsal-specific genes sia, nr3 and gsc is induced at the vegetal pole after the Mid-Blastula-Transition (MBT). While nr3 and sia expression continues in these embryos until gastrula stages, gsc transcription cannot be maintained. We propose that the spatial separation of the expression domains of genes with dorsalizing activities and the prospective mesodermal region results in the loss of dorsal structures in the embryo. The role of cortical rotation is to generate an overlap of the region with dorsal axis-forming activity, indicated by nuclear translocation of beta-catenin, and the prospective mesoderm in the marginal zone to assure the correct positioning of the Spemann organizer.
β-连环蛋白是Wnt信号转导通路的一个组成部分，对非洲爪蟾胚胎背轴的形成至关重要。在胚胎的背侧，β-连环蛋白通过与皮层旋转相关的过程转运到细胞核中。当皮层旋转被紫外线照射阻断时，核β-连环蛋白出现在胚胎的植物极。我们在此表明，在没有中胚层诱导的情况下，动物帽外植体中β-连环蛋白的过表达足以激活具有背化活性的基因的表达，如暹罗蛋白（sia）和结节相关蛋白3（nr3），但不能激活鹅膏蕈氨酸（gsc）。在经紫外线处理而腹化的胚胎中，背侧特异性基因sia、nr3和gsc的表达在中囊胚转换（MBT）后在植物极被诱导。虽然nr3和sia的表达在这些胚胎中持续到原肠胚阶段，但gsc转录不能维持。我们提出，具有背化活性的基因的表达域与预期中胚层区域的空间分离导致胚胎中背侧结构的丧失。皮层旋转的作用是使由β-连环蛋白的核转运所指示的具有背轴形成活性的区域与边缘区的预期中胚层产生重叠，以确保施佩曼组织者的正确定位。
