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蛙胚胎的两侧对称性。在非洲爪哇，这种从右到左的定向流动发生在所谓的胃腔顶板(GRP)，这是一种中胚层命运的上皮，暂时嵌入到原肠顶层的内胚层中。向左流动代表了脊椎动物的祖先特征，尽管确实存在变异。作为血流的结果，Nodal信号级联在左侧板中胚层(LPM)变得不对称地激活。然后，这种级联反应引导着心脏等内部器官的不对称形态发生。我们的初步实验首次确定了FLOW的关键靶点：节点抑制剂COCO，它在GRP左侧边缘的mRNA和蛋白质水平上发生下调，从而解除了对共表达的节点蛋白的抑制。上位性实验表明，节点和可可作用于水流的下游，节点作用于可可的下游。这项建议解决了流动如何实现这种压抑的问题。本文从两个方面探讨了这一问题：(1)流的输入分析；(2)对Coco基因和蛋白水平调控的研究。后者将根据目前讨论的两个模型进行研究，即(2a)形态原被输送到左侧，或(2b)左侧边缘的感觉纤毛接收和处理流动的方向性。我们的目标是解开流动驱动释放COCO介导的左侧GRP结节抑制的机制。

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.