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

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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.