LOCAL-SOURCES OF RETINOIC ACID COINCIDE WITH RETINOID-MEDIATED TRANSGENE ACTIVITY DURING EMBRYONIC-DEVELOPMENT

We have assessed whether retinoic acid (RA) comes from local sources o r is available widely to activate gene expression in embryos. We used an RA-responsive indicator cell line, L-C2A5, to localize RA sources. In these cells, an RA-sensitive promoter/lacZ reporter construct used previously by us to produce indicator transgenic mice is induced globa lly by RA in medium or locally by RA released at physiological concent rations (1 nM) from AG-1X2 resin beads. Furthermore, the cells are dif ferentially responsive to the 9-cis and all-trans isomers of RA at low concentrations. Indicator transgenic mice with the same promoter/repo rter construct were used to identify regions of RA-mediated gene activ ation. There are distinct domains of lacZ expression in the cervical a nd lumbar spinal cords of embryonic indicator mice. This pattern might reflect localized RA sources or restricted spatial and temporal expre ssion of RA receptors, binding proteins, or other factors. To resolve this issue we compared the pattern of transgene activation in indicato r cell monolayers cocultured with normal embryonic spinal cords with t hat in transgenic spinal cords. The explants induced reporter gene exp ression in L-C2A5 monolayers in a pattern identical to that in transge nic mice: alar regions of the cervical and lumbar cord were positive w hereas those in the thoracic and sacral regions were not. We conclude that restricted sources of RA in the developing spinal cord mediate th e local activation of RA-inducible genes. Thus, region-specific gene a ctivation in embryos can be mediated by precisely localized sources of inductive molecules like RA.