| Retinoids regulate, through a complex receptor network, the activities of many genes. They play important roles in the development of all vetebrates. Despite much progress made during the last few years, it still remains unclear how retinoids are synthesized and metabolized, and how the signals are transduced through their receptors. This thesis describes: (1) identification and functional expression of a cDNA encoding a retinoic acid synthase, RAsyn-1, (2) isolation and functional characterization of rat 9-cis-retinoic acid receptor gamma (RXR;Mouse embryonal carcinoma cells (P19 cells) were used to examine the differentiation-induction potencies of 9-cis, 13-cis and all-trans isomers of retinal and retinoic acid. It was found that the cis isomers of retinoic acid are 5-100 times more potent than the all-trans retinoic acid (0.5-1 mM). All isomers of retinaldehyde are also capable of inducing differentiation, but their potencies are 5-100 times lower than their oxidized products, retinoic acid. This indicates that retinaldehyde may be metabolized inside the cell, and that it is the retinoic acid that bears the differentiation signal to the genome. Using a sensitive zymographic analysis, a novel enzyme that converts retinal to retinoic acid was detected in the differentiated P19 cells. Using a PCR-based strategy, a cDNA encoding this enzyme, retinoic acid synthase-1 (RAsyn-1), was isolated and sequenced. It belongs to the aldehyde dehydrogenase super family. RAsyn-1 is expressed in a discrete fashion in the developing embryo and adult mouse. Its role in the induction of pattern formation in vertebrates is discussed.;To elucidate further the signal transduction pathways of retinoids, a rat striatum cDNA library was screened with an oligonucleotide probe derived from the sequences of the DNA-binding domains of the known retinoid receptors and thyroid hormone receptors (TRs). Two novel nuclear receptor cDNAs were isolated. These receptors interact with the known retinoid receptors and TRs. Their role in retinoid signal transduction regulation is discussed. |
