Chatterjee Suman, Elinson Richard P
Department of Biological Sciences, Duquesne University, Pittsburgh, Pennsylvania.
J Exp Zool B Mol Dev Evol. 2014 Jan;322(1):27-44. doi: 10.1002/jez.b.22543. Epub 2013 Nov 5.
The vegetal cells of a Xenopus laevis embryo commit to mesendoderm via the Nodal-signaling pathway. In the direct developing frog Eleutherodactylus coqui, mesendoderm is specified at the marginal zone of the early gastrula, and vegetal core cells transform into nutritional endoderm. Nutritional endoderm, a novel tissue, consists of transient, yolky cells that provide nutrition but remain undifferentiated. We report a dual regulation for the generation of nutritional endoderm. First, differential expressions of the Nodal-signal transducers Smad2 and Smad4 were observed during early gastrulation between the marginal zone and the vegetal core cells. Although EcSmad2 RNA as well as total and activated Smad2 protein were detected in the vegetal core, Smad4 protein was expressed less in vegetal core during early gastrulation. Only 12% and 50% of vegetal core cells were positive for nuclear Smad2 and Smad4 signals respectively compared to 100% of marginal zone cells. These results suggest a signaling disruption in the vegetal core. Second, vegetal core cells were transcriptionally repressed. At the blastula stage, both marginal zone and vegetal core cells were transcriptionally silent, but during early gastrulation, only marginal zone cells became transcriptionally active. This indicates the occurrence of a mid-blastula transition in the marginal zone by early gastrulation, but global transcriptional repression persisted in the vegetal core and its derivative, nutritional endoderm, throughout development. We have described a novel mechanism, which prevents differentiation of the vegetal core through differential Nodal-signaling and global transcriptional repression.
非洲爪蟾胚胎的植物性细胞通过Nodal信号通路分化为中内胚层。在直接发育的蛙类——寇氏雨蛙中，中内胚层在原肠胚早期的边缘区被特化，植物性核心细胞转变为营养内胚层。营养内胚层是一种新型组织，由短暂存在的富含卵黄的细胞组成，这些细胞提供营养但仍未分化。我们报道了营养内胚层生成的双重调控机制。首先，在原肠胚早期，边缘区和植物性核心细胞之间观察到Nodal信号转导蛋白Smad2和Smad4的差异表达。虽然在植物性核心中检测到了EcSmad2 RNA以及总Smad2蛋白和活化的Smad2蛋白，但在原肠胚早期，植物性核心中Smad4蛋白的表达较少。与100%的边缘区细胞相比，植物性核心细胞中分别只有12%和50%的细胞核Smad2和Smad4信号呈阳性。这些结果表明植物性核心中存在信号传导中断。其次，植物性核心细胞受到转录抑制。在囊胚期，边缘区和植物性核心细胞均处于转录沉默状态，但在原肠胚早期，只有边缘区细胞变得转录活跃。这表明在原肠胚早期边缘区发生了囊胚中期转变，但在整个发育过程中，植物性核心及其衍生物营养内胚层持续存在全局转录抑制。我们描述了一种新机制，该机制通过差异Nodal信号传导和全局转录抑制来阻止植物性核心的分化。
