A Cis-regulatory Model for Neural Border Induction

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DESCRIPTION (provided by applicant): The neural crest is a population of multipotent progenitor cells that forms at the border of the developing central nervous system and epidermis. During early development, neural crest cells migrate from this neural border domain to form diverse derivatives throughout the embryo, including bones and cartilage in the head, and portions of the cranial ganglia and heart. Disruptions in neural crest formation cause numerous human developmental abnormalities in the head, face, heart, and nervous system, as well as neural tube closure defects. The critical first step in neural crest development is the segregation of the neural border from adjacent neural and epidermal ectoderm. This process has been shown to involve Wnt and BMP signals secreted from adjacent ectoderm and underlying paraxial mesoderm. These signals induce neural border formation by activating the expression of the 'neural border specifier' genes, Zic, Pax3/7, and tfap2. While Wnts and BMPs are essential for neural border induction, how these pathways are integrated to drive expression of the neural border specifiers is unknown. Wnt and BMP signals can regulate transcription through activation of their canonical downstream effectors, TCF/LEF and SMAD1/5/8 transcription factors, or through several non- canonical effectors. Thus, neural border specifier expression could involve direct regulation by TCF/LEF and SMAD1/5/8, direct regulation by non-canonical effectors, or indirect regulation by unknown intermediate factors. To evaluate these possibilities, we are characterizing 8 evolutionarily conserved cis-regulatory elements (CREs) from the Pax3, Pax7, Zic2/5, Zic3/6 and tfap2a loci that recapitulate early neural border expression of these genes. Preliminary analyses of these CREs support a general mechanism for neural border induction in which the Wnt and BMP signaling pathways are integrated directly on evolutionarily conserved CREs through TCF/LEFs and SMAD1/5/8. The proposed work will test this model using mRNA injections, mutagenesis, and chromatin immunoprecipitation (ChIP). The experimental aims of the proposed work are; 1) Test for regulation of the neural border specifiers by TCF/LEFs and SMAD1/5/8 using hormone- inducible, activated forms of these factors, and mutagenesis of evolutionarily conserved TCF/LEF and SMAD binding sites and, 2) Test for direct binding of the TCF/LEF-ss-catenin complex and SMAD1/5/8 to CREs using ChIP. In addition to testing our model, the proposed work will validate ss-catenin and SMAD antibodies for use in zebrafish embryo ChIP, and establish strategies for rapidly testing the BMP and Wnt responsiveness of novel CREs. Future work will leverage these results to test if neural border induction via direct, combinatorial action of canonical BMP and Wnt signaling pathways is a genome-wide mechanism for achieving neural border gene expression. These studies will also identify novel direct targets of BMP and Wnt signaling in the neural border, adding both breadth and depth to our understanding of neural border formation. The proposed work will support the training of one postdoctoral scholar for two years.