A conserved control module for cleft-like tissue boundary formation in the vertebrate embryo

组织边界的形成和维持是所有动物的基本功能，涉及胚胎发育和组织稳态。它阻止来自不同区域的细胞混合，它的失败与病理状况有关，如肿瘤转移，癌细胞越过边界侵入邻近组织。我们提出了一类以裂隙形态为特征的组织边界，由一个保守的超级模块控制，该模块将几个已知的信号通路或子模块集成到一个独特的过程中。通过对爪蟾和斑马鱼胚胎外胚层和中胚层组织分离的研究，我们发现Eph/ephrin信号通路、snail1激活的旁轴原钙粘蛋白（paraxial protocadherin， PAPC）功能以及PAPC对平面细胞极性信号通路的抑制是形成裂状外胚层-中胚层边界的必要条件。根据自己的初步数据和已发表的结果，我们提出了其他具有裂缝状结构的边界，如some -somite和noto索-somite边界，也受到这个超级模块的控制。作为第一步，我们将研究爪蟾和斑马鱼胚胎中脊索体（N-S）边界的形成。我们将研究PAPC和Snail1在N-S边界形成中的作用的明显逆转是否仅仅是一个实验伪产物，这是由于在转录后水平上对这些因子表达的不同控制，还是基于它们相互作用的实际重新连接。作为我们的第二个目标，我们将研究超级模块的组成部分是否在N-S边界形成中表现出与外胚层-中胚层边界相同的上位关系。此外，N-S边界形成发生在不同的组织环境中，即在背后中胚层内，而不是在中胚层和外胚层之间，我们将询问调节如何可能适应这种新的环境。第三，我们将研究上述特征的调控模块在斑马鱼N-S边界形成中是否保守。实验方法和工具将与我们对爪蟾和斑马鱼的外胚层-中胚层边界的研究一样。证明所提出的控制超级模块的存在将简化包括人类在内的脊椎动物组织边界的分类、分析和操作。此外，当我们比较两种不同脊椎动物的两种不同组织界面的相同过程时，我们的结果将揭示边界形成的进化。

The formation and maintenance of tissue boundaries is an essential function in all animals, and involved in embryonic development and tissue homeostasis. It prevents the intermingling of cells from different regions, and its failure is associated with pathological conditions such as tumour metastasis where cancer cells cross boundaries and invade adjacent tissues. We propose that a whole class of tissue boundaries, characterized by a cleft-like morphology, are controlled by a conserved super-module that integrates several known signaling pathways or sub-modules into a unique process. Studying tissue separation between ectoderm and mesoderm germ layers in the Xenopus and zebrafish embryo, we found that a combination of Eph/ephrin signaling, Snail1-activated paraxial protocadherin (PAPC) function, and inhibition of planar cell polarity signaling by PAPC is necessary for the formation of the cleft-like ectoderm-mesoderm boundary. Based on own preliminary data and on published results, we propose that other boundaries with a cleft-like structure, the somite-somite and the notochord-somite boundaries, are also controlled by this super-module. As a first step, we will study notochord-somite (N-S) boundary formation in Xenopus and zebrafish embryos. We will examine whether an apparent reversal of the roles of PAPC and Snail1 in N-S boundary formation is only an experimental artifact, due to a different control of the expression of these factors at the post-transcriptional level, or whether it is based on an actual re-wiring of their interaction. As our second aim, we will examine whether the components of the super-module show the same epistatic relations in N-S boundary formation as at the ectoderm-mesoderm boundary. Moreover, N-S boundary formation occurs in a different tissue context, i.e. within the dorsal posterior mesoderm instead of between mesoderm and ectoderm, and we will ask how regulation is possibly adapted to this new context. Third, we will examine whether the regulatory modules characterized above are conserved in zebrafish N-S boundary formation. Experimental approaches and tools will be as in our studies of the ectoderm-mesoderm boundary in Xenopus and zebrafish. Demonstrating the existence of the proposed control super-module would simplify the categorization, analysis and manipulation of tissue boundaries in vertebrates including humans. In addition, our results will shed light on the evolution of boundary formation as we compare the same process at two different tissue interfaces in two different vertebrates.