Retinoic acid (RA), a metabolic derivative of vitamin A (retinol), is essential for axial patterning of the eye during vertebrate development. Retinoid signaling occurs when retinol is metabolized to retinal and then to RA which serves as a ligand for nuclear retinoid receptors that regulate gene expression. Disruption of retinoid signaling either by gestational vitamin A deficiency or by creation of retinoid receptor null mutations results in abnormal dorsoventral axial development of the eye. The mechanism for generation of RA locally in the eye as well as the mechanism for how RA regulates dorsoventral patterning of the eye are not yet understood. Our understanding of how retinol is physiologically activated to form RA is still vague, but has been improved by discovery of three retinoid dehydrogenases expressed in the eye that metabolize retinal to RA, i.e. RALDH1, RALDH2, and RALDH3, all members of the aldehyde dehydrogenase (ALDH) enzyme family. Raldh2 null mutant mice generated in this laboratory and another have revealed phenotypes that are lethal at midgestation with massive trunk and forelimb developmental defects plus abnormal hindbrain patterning, but no optic vesicle defects were observed. Maternal RA administration rescues many defects in Raldh2 mutants, thus allowing further analysis by conditional rescue. Raldh1 null mutant mice recently generated in this laboratory are viable, but embryos suffer a lack of RA synthesis in the dorsal retina suggesting that retinal defects will be discovered upon further analysis. There have been no genetic studies reported on Raldh3. Further analysis of RALDHs should provide key information needed to understand human developmental eye defects. The overall goals of this project are to use null mutant mice to establish roles for RALDH1, RALDH2, and RALDH3 in eye RA synthesis, plus use these mutant mice as tools to further examine the mechanism of retinoid signaling in eye development. Mutant mice will be examined for developmental eye defects histologically and by in situ hybridization to detect disrupted gene expression in eye tissues. Mice carrying the RARE-lacZ marker gene will be used to detect endogenous RA in mutant embryos. Specific goals for this project will be as follows: (1) Use Raldh1 mutants to examine the effect of a lack of RA in the dorsal retina on retinal development and axonal pathfinding for retinofugal projections; (2) Examine the contribution of RALDH1 and RALDH2 to RA synthesis for eye development by comparison of conditionally RA-rescued Raldh2 mutants and Raldh1-Raldh2 double mutants; (3) Generate Raldh3 null mutant mice to examine the role of this gene in eye development.
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