Animal Model to Study Retinoic Acid Function in Postnatal and Adult Tissues

Duester, Gregg

Abstract

Among the various secreted intercellular signaling factors, retinoic acid (RA) is unique in that it is a small molecule that directly regulates gene expression by entering the nucleus of target cells and binding to target genes by acting as a ligand for nuclear receptors. Vitamin A (retinol) is an essential nutrient that serves as a precursor for RA synthesis through a two step metabolic pathway in which retinol is converted to retinaldehyde which is then converted to RA. Gene knockout studies in mice have determined that RA signaling is controlled by three RA receptors and three retinaldehyde dehydrogenases that perform the last step of RA synthesis. Major success in unraveling the physiological roles of RA has come only for analysis of RA function during embryonic development since elimination of RA signaling leads to embryonic lethality. Administration of RA or drugs that inhibit RA synthesis or receptor activity suggests that RA may also control many adult functions including hippocampal neurogenesis, immune function, spermatogenesis, stem cell function, skin and hair follicle regeneration, and prevention of cancer. However, these approaches are more prone to artifacts and misinterpretation compared with gene knockout approaches, and may not reveal an accurate view of postnatal or adult RA function. Thus, the goal of this project is to develop a successful genetic model for analysis of RA function postnatally. Pursuant to this goal, we will develop a knockout mouse lacking retinaldehyde synthesis. In contrast to metabolism of retinaldehyde to RA, which is irreversible and tissue-specific, interconversion of retinol and retinaldehyde is reversible and occurs widely throughout the organism. Therefore, we propose that a model organism lacking retinaldehyde synthesis will be able to survive postnatally if maintained on a retinaldehyde-supplemented diet. Gene knockout studies support the existence of three enzymes that convert retinol to retinaldehyde needed for RA synthesis and survival, i.e. retinol dehydrogenase-10 (RDH10) and two alcohol dehydrogenases (ADH3 and ADH4). Rdh10 knockout mice die during gestation due to lack of RA synthesis, and Adh3 or Adh4 knockout mice die postnatally when placed on a vitamin A deficient diet. In order to generate a model for adult RA function, we propose to: (1) Establish a dietary retinaldehyde treatment that will allow postnatal survival of Rdh10 knockouts and Rdh10;Adh-del compound knockouts lacking RDH10 and all forms of ADH;(2) Withdraw retinaldehyde from Rdh10 and Rdh10;Adh-del knockout mice postnatally to examine survival and determine what tissues have a reduction or elimination of RA activity;(3) Perform microarray studies on Rdh10 and Rdh10;Adh-del knockout mice ( retinaldehyde) to identify RA target genes in a variety of tissues that are of broad interest to the research community.

Public Health Relevance

The essential nature of the vitamin A metabolite retinoic acid has been known for many years, and although we know quite a lot about its function during embryogenesis, we know little about how it functions postnatally and in the adult. We predict that many reported adult actions of retinoic acid based on drug studies will not be supported by genetic loss-of-function studies proposed here, but other functions of RA will be discovered, thus having a major impact on a large community of researchers including neuroscientists, immunologists, reproductive biologists, stem cell biologists, and cancer researchers. Sharing of the model mice generated in this research project will thus greatly accelerate the ability to determine how retinoic acid contributes to physiological processes in a wide variety of postnatal and adult organs, and will lead to an understanding of how defects in the vitamin A pathway contribute to disease.