Asymmetric repression of the Nodal inhibitor Coco by cilia-driven leftward flow in Xenopus

纤毛驱动的细胞外液流动打破了早期神经胚青蛙胚胎的双侧对称性。在非洲爪蟾中，这种从右到左的定向流动发生在所谓的胃腔顶板（GRP），一种中胚层命运的上皮细胞，其暂时嵌入原肠顶板的内胚层层中。左旋血流代表了脊椎动物的祖先特征，尽管确实存在变异。作为流动的结果，Nodal信号级联在左侧板中胚层（LPM）中变得不对称地激活。这一级联反应随后指导心脏等内脏器官的不对称形态发生。我们的初步实验首次确定了流动的关键目标：Nodal抑制剂Coco，它在GRP左侧边缘的mRNA和蛋白质水平上下调，从而提高了对共表达Nodal蛋白的抑制。上位性实验表明，Nodal和Coco作用于Flow下游，Nodal作用于Coco下游。这一提议解决了心流如何实现这种压抑的问题。该问题从两个方面来解决：（1）流量输入的分析，（2）Coco mRNA和蛋白质水平调节的研究。后者将在目前讨论的两种模型的基础上进行研究，即（2a）形态原被运输到左侧，或者（2b）左侧边缘的感觉纤毛接收和处理流动的方向性。我们的目标是解开Coco介导的对GRP左侧Nodal抑制的流动驱动释放机制。

A cilia-driven flow of extracellular fluid breaks the bilateral symmetry of the early neurula frog embryo. In Xenopus, this directional flow from right to left occurs at the so-called gastrocoel roof plate (GRP), an epithelium of mesodermal fate which is transiently embedded in the endodermal layer of the archenteron roof. Leftward flow represents an ancestral character of the vertebrates, although variations do exist. As a consequence of flow, the Nodal signaling cascade becomes asymmetrically activated in the left lateral plate mesoderm (LPM). This cascade then directs the asymmetric morphogenesis of inner organs such as the heart. Our preliminary experiments identify for the first time the critical target of flow: the Nodal inhibitor Coco, which becomes down-regulated at the mRNA and protein level on the left margin of the GRP and thus lifts repression of the coexpressed Nodal protein. Epistasis experiments show that Nodal and Coco act downstream of flow, and Nodal downstream of Coco. This proposal addresses the question how flow achieves this repression. The problem is addressed from two sides: (1) analysis of the input of flow, and (2) investigation of the regulation of Coco mRNA and protein levels. The latter will be investigated on the basis of the two currently discussed models, namely that (2a) a morphogen is transported to the left side, or that (2b) sensory cilia at the left margin receive and process the directionality of flow. Our goal is to unravel the mechanism of flow-driven release of Coco-mediated repression of Nodal on the left side of the GRP.