Great progress has been made identifying signaling factor pathways controlling differentiation of progenitor cells during embryogenesis. However, we still have a rudimentary understanding of what developmental processes and genes these signaling factors regulate. Retinoic acid (RA) is a secreted signaling factor derived from retinol, an essential nutrient that is converted first to retinaldehyde and then to RA by specific enzymes. The tissue-specific location and timing of RA synthesis during vertebrate embryogenesis provides intercellular signaling information needed to stimulate differentiation of progenitor cells, thus generating mature tissues and organs. RA synthesis initiates during the early stages of body axis extension through the sequential actions of retinol dehydrogenase (Rdh10) and retinaldehyde dehydrogenase (Raldh2) that together generate RA in trunk mesoderm just anterior to the caudal progenitor zone. Body axis extension requires FGF signaling and Wnt signaling in progenitor cells at the caudal tip of the embryo, and studies on Raldh2-/- mouse embryos suggest that RA signaling sets the anterior limit of this progenitor zone by acting in the developing trunk to down-regulate FGF and Wnt signaling. While doing this, RA signaling also ensures that somites are generated in a bilaterally symmetric fashion. However, the mechanism of caudal RA action is still unclear. RA directly regulates transcription of key genes by serving as a ligand for nuclear RA receptors bound to RA response elements (RAREs). RA has traditionally been associated with induction of gene expression, but some studies suggest that RA controls body axis extension and somitogenesis through RA-mediated repression of Fgf8 and Wnt8a, and that RA acts in newly generated posterior neuroectoderm or the node rather than presomitic mesoderm. Chromatin immunoprecipitation (ChIP) studies on mouse embryos have identified RAREs upstream of the Fgf8 and Wnt8a promoters that bind RA receptors, enabling a deeper examination of the caudal RA mechanism. In this project we plan to use several mouse and zebrafish genetic models to eliminate or reduce RA, FGF, and Wnt signaling, as well as transgenic and ChIP approaches to examine Fgf8 and Wnt8a promoters. The goal of this project is to understand the mechanism through which caudal RA represses FGF and Wnt signaling to ensure normal body axis extension. Specifically, we propose to: (1) Determine the target tissue for RA repression of FGF signaling during somitogenesis and body axis extension using several genetic models; (2) Reduce Wnt signaling in RA-deficient mouse and zebrafish embryos to rescue defects in body axis extension and to examine crosstalk between RA and Wnt signaling; (3) Validate repressive functions of Fgf8 and Wnt8a RA response elements through in vivo and in vitro studies.
Studies focused on understanding how tissues normally develop within the embryo provide important information useful in the rational design of regenerative treatments for various human diseases. Key to this understanding is the use of genetic studies in mouse and zebrafish to learn how embryonic progenitor cells communicate with one another via secreted signaling molecules during generation of tissues and organs. By determining how the signaling molecule retinoic acid functions to regulate two other important signaling agents in progenitor cells (Fgf and Wnt), this project will help us better understand the process of embryogenesis and will provide important clues about the potential usefulness of these signaling factors in the search for effective regenerative treatments that can be used to combat human disease or aging.
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