Function and biosynthetic regulation of retinoic acid in intestinal development and homeostasis

It has been hypothesized for almost 100 years that retinoids could play a role in regulating the differentiation and proliferation of epithelial tissues including the intestine. Some of those original questions still remain as studies to date have been unable to clearly identify the role retinoic acid signaling plays in the intestine. Recent work has also raised questions about how the important process of retinoic acid biosynthesis is regulated. This dissertation presents data which clarify some of mechanisms that regulate retinoic acid biosynthesis in the intestine as well as identify important functions of retinoic acid signaling in intestinal development and homeostasis. The data presented herein identify a requirement for axinl in intestinal differentiation during zebrafish development. In this novel role axinl acts in a Wnt independent manner to positively regulate the expression of rdhll, a retinol dehydrogenase. Furthermore we present data which suggest that axinl is able to promote expression of rdhll by regulating the stability of the transcriptional co-repressor Ctbpl. In this dissertation we utilize the apc"ur mutant as a model of retinoic acid deficiency to show that contrary to the currently accepted model, up-regulation of aldhla2 (raldh2) can be an indication of retinoic acid deficiency rather than an excess. We also provide direct evidence that retinol dehydrogenases are required in the intestine for retinoic acid production and signaling. Further, the data presented here provide genetic evidence that retinoic acid regulates expression of aldhla2 (raldhl) through an indirect negative regulatory loop, and implicates the known stem cell marker pou5fl (•oct4) as an important positive regulator of aldhla2 (raldh2) expression. In addition, the data presented in this dissertation clearly identify the essential role of retinoic acid signaling in establishing intestinal progenitor cell number during development. These findings also clearly identify an important role for retinoic acid in maintaining intestinal homeostasis by regulating epithelial differentiation and proliferation in the adult intestine. Together the data presented in this dissertation establish retinoic acid as an essential regulator of intestinal development and homeostasis and identify two pathways which are important for regulating the biosynthesis of retinoic acid in the intestine.