Stage-dependent responses of the developing lung to retinoic acid signaling

Morphological analysis of vitamin A-deficient rat fetuses and of retinoic a cid receptor (RAR and RXR) mutant mice have demonstrated that retinoic acid (RA) is essential for lung development. To gain further insight into RA si gnaling pathways during primary lung bud formation and lung branching, we h ave investigated the effects of RA and of a pan-RAR antagonist in cultures of whole embryos and lung explants, Treatment of E8.0 embryos with the pan- RAR antagonist inhibits the formation of the primitive respiratory system. On the other hand, treatment of E11.75 and E12.5 lung explants with RA inhi bits branching morphogenesis, whereas treatment with the pan-RAR antagonist at the same developmental stages stimulates formation of distal buds. The inhibitory effect of RA on branching is strongly decreased in RAR beta null lungs, while enhancement of budding by the pan-RAR antagonist is not affec ted by an RAR gamma null mutation. Additionally, cellular retinol binding p rotein one (CRBPI) null lungs are more sensitive than wild type lungs to th e pan-RAR antagonist-induced stimulation of branching. These data indicate that retinoid signaling is indispensable for the formation of primary lung buds and the oesophagotracheal septum from the primitive foregut. They also suggest that at the pseudoglandular stage, RA signaling through RAR beta, but not RAR gamma, inhibits distal bud formation thereby promoting the form ation of conducting airways. Moreover, the level of CRBPI in the pseudoglan dular lung appears to participate in the control of branching morphogenesis .