RETINOIC ACID SIGNALING AND TRANSMURAL PRESSURE IN MOUSE LUNG DEVELOPMENT

Abstract

During development, the mammalian lung undergoes several rounds of branching, the rate of which is tuned by the pressure of the fluid within the lumen of the lung. In this dissertation, we carried out bioinformatics analysis of RNA-sequencing of embryonic mouse lungs cultured under physiologic or sub-physiologic transmural pressure and identified transcription-factor binding motifs near genes whose expression changes in response to pressure. Surprisingly, we found retinoic acid (RA) receptor binding sites significantly overrepresented in the promoters and enhancers of pressure-responsive genes. Consistently, increasing luminal pressure activates RA signaling, and pharmacologically inhibiting RA signaling decreases airway epithelial branching and smooth muscle wrapping. Using both pharmacological and transgenic approaches, we found that pressure activates RA signaling through the mechanosensor Yap. Using a computational mechanical model we predicted that mechanical signaling through Yap and RA affects lung development by altering the balance between epithelial proliferation and smooth muscle wrapping around epithelial branches. We further predicted that increasing the rate of epithelial proliferation relative to smooth muscle differentiation would lead to dilated epithelial branches, which we confirmed experimentally. Finally, we found that in lungs from epithelial Yap knockout embryos, there is decreased smooth muscle wrapping relative to the rate of epithelial proliferation, suggesting that decreased smooth muscle differentiation downstream of disrupted RA signaling in these lungs is responsible for the dilated epithelial branches in these lungs. Our results show that transmural pressure signals through RA to balance the relative rates of epithelial growth and smooth muscle differentiation in the developing mouse lung and identify RA as a novel component in the mechanotransduction machinery of embryonic tissues.