Retinoic Acid Signaling and Development of the Respiratory System.

Retinoic acid (RA), the bioactive metabolite of vitamin A (VA), has long been recognized as a critical regulator of the development of the respiratory system. During embryogenesis, RA signaling is involved in the development of the trachea, airways, lung, and diaphragm. During postnatal life, RA continues to impact respiratory health. Disruption of RA activity during embryonic development produces dramatic phenotypes in animal models and human diseases, including tracheoesophageal fistula, tracheomalacia, congenital diaphragmatic hernia (CDH), and lung agenesis or hypoplasia. Several experimental methods have been used to target RA pathways during the formation of the embryonic lung. These have been performed in different animal models using gain- and loss-of-function strategies and dietary, pharmacologic, and genetic approaches that deplete retinoid stores or disrupt retinoid signaling. Experiments utilizing these methods have led to a deeper understanding of RA's role as an important signaling molecule that influences all stages of lung development. Current research is uncovering RA cross talk interactions with other embryonic signaling factors, such as fibroblast growth factors, WNT, and transforming growth factor-beta.