Transgenic indicator mice for studying activated retinoic acid receptors during development

Retinoic acid (RA) receptors (RARs) are ligand-inducible transcription factors that bind to specific DNA sequences associated with the regulatory regions of RA-regulatable genes. Since RA has been implicated as an important factor both in normal development and in teratological studies, one would like to have a model system that detects the presence of activated receptors during development. We have constructed a recombinant reporter gene that has three copies of the RA response element (RARE) from the RAR beta-2 promoter 5' to the herpes simplex virus thymidine kinase promoter; this regulatory region is coupled to the bacterial beta-galactosidase reporter gene. This construct was RA inducible in transient transfection assays in F9 embryonal carcinoma cells. Transgenic embryos with this reporter gene construct exhibited restricted and reproducible patterns of beta-galactosidase activity during embryogenesis, beginning between gestational ages day 7.5 and 8.5. At day 8.5, beta-galactosidase activity was detected in the closed neurotube and somites. Day 8.5 embryos, from pregnant females fed RA 14 hr earlier, exhibited a greater intensity and distribution of beta-galactosidase activity. Similarly, at later stages of gestation, maternal RA exposure resulted in enhanced embryonic beta-galactosidase expression. This type of transgenic indicator mouse should be useful in detailing the role of activated RARs during embryonic development.