Ectoderm formation in the early Xenopus embryo

描述(申请人提供)：形态发生的一个基本组成部分是细胞相互分离形成新的组织或器官的能力。脊椎动物发育过程中最早的例子是初级胚层的形成。在非洲爪哇，每一层胚层都来自球形囊胚的一个单独区域。外胚层来自动物细胞，中胚层来自边缘细胞，内胚层来自植物细胞。谱系分析、区域标记的表达、组织重组实验、细胞分选实验和细胞移植实验都表明，胚层的特化发生在胚泡的中后期，在胚层明显分离之前。植物细胞继承一种母性编码的转录因子--Vegt。这激活了启动内胚层分化的其他转录因子的表达，以及诱导邻近边缘带细胞形成中胚层的信号分子。导致外胚层形成的途径尚不清楚。初步数据表明，它也是由在卵母细胞中转录并由卵子遗传的mRNAs启动的。我们发现，LIM同源结构域蛋白XLIM5只在外胚层中表达，是母体转录因子(S)的直接靶标，当在早期植物细胞中过度表达时，会导致它们的后代分离到其他生殖层。这个项目的目标是以此为起点来确定导致外胚层特化的途径，以及动物细胞分离形成外胚层胚层的过程。我们将确定激活XLIM5表达的上游母体因子和赋予动物细胞外胚层类型黏附的下游基因，并通过简单的功能得失分析来测试它们各自的功能。我们还将研究外胚层和内胚层之间的相互作用，这种相互作用导致囊胚期的不同黏附行为。

DESCRIPTION (provided by applicant): One of the fundamental components of morphogenesis is the ability of cells to segregate from each other to form new tissues or organs. The earliest example of this during vertebrate development is the formation of the primary germ layers. In Xenopus, each germ layer arises from a separate area of the spherical blastula. The ectoderm arises from animal cells, the mesoderm from marginal cells, and the endoderm from vegetal cells. Lineage analysis, the expression of regional markers, tissue recombination experiments, cell sorting experiments, and cell transplantation experiments all suggest that specification of the germ layers occurs during the mid- to late-blastula stages, before overt segregation of the germ layers. Vegetal cells inherit a maternally encoded transcription factor, VegT. This activates the expression of other transcription factors that initiate endoderm differentiation, and signaling molecules that induce the neighboring marginal zone cells to form mesoderm. The pathway leading to ectoderm formation is unknown. Preliminary data suggests that it is also initiated by mRNAs transcribed in the oocyte and inherited by the egg. We have found that a LIM homeodomain protein, XLIM5, already known to be expressed only in the ectoderm, is an immediate target of maternal transcription factor(s), and, when over expressed in early vegetal cells, causes their descendants to segregate to other germ layers. The goals of this project are to use this as a starting point to identify the pathway leading to ectoderm specification and the segregation of animal cells to form the ectoderm germ layer. We will identify the upstream maternal factors that activate XLIM5 expression, and the downstream genes which confer ectoderm-type adhesion on animal cells, and test their individual functions by simple gain- and loss-of function assays. We will also investigate the interaction between the ectoderm and endoderm pathways that leads to differential adhesion behaviour at the blastula stage.