Cellular availability of all-trans-retinoic acid in mice: a pilot study

Retinoic acid signaling is essential for mammalian development and health. The genetic manipulations that enable visualization of this signal have revealed its involvement in many organs, including the eyes, colon, and trunk regions of mouse embryos. The in vivo cellular availability of the ligand, all-trans-retinoic acid (ATRA), is, however, still unclear, even though it, along with receptor expression, constitutes an essential part of signal transduction. In this pilot study, we sought to identify cells with ATRA availability by introducing a fusion protein comprising the mouse retinoic acid receptor alpha ligand-binding domain and the yeast Gal4 DNA-binding domain in mice. By crossing with other reporter mice with Gal4-responsive Cre expression and subsequent tandem DsRed expression to track ATRA availability through adulthood, we labeled the in vivo cellular availability of ATRA in two out of 14 mice from three F1-generation lines. As expected, we observed prior ATRA availability in organs associated with this signaling pathway, including the eye and colon. In addition, we observed prior ATRA availability in gastric epithelial cells and hepatocytes, which originates from the intestinal region where ATRA availability has not been determined, suggesting that ATRA's involvement in development or physiology may be greater than previously thought. Unfortunately, prior ATRA availability was limited to a small population of cells in the animals owing to low expression of the fusion receptor, as determined by quantitative reverse-transcription PCR. In summary, this pilot study suggested extensive effects of ATRA, underscoring its potential importance in developmental biology and clinical nutrition. In addition, this study encourages careful transgene design by reporting our experience with a low success rate in transgene expression.