DESCRIPTION: (Scanned from the applicant's description): Although many studies implicate retinoic acid (RA) in lung development, whether and how RA-dependent pathways regulate lung bud induction from the endodermal foregut and airway branching remain unclear. Our preliminary data show that RA synthesis and utilization are active when lung buds are forming in the foregut endoderm, but both synthesis and utilization are downregulated once airways start to branch. This downregulation may be due to the appearance of local mechanisms that control RA availability and retinoid receptor (RAR) activation. In this proposal, we plan to investigate the role of RA signaling in the initiation of lung bud from the endodermal foregut (Specific Aim 1), and the role of RA in regulating airway branching during branching morphogenesis (Specific Aim 2). We also propose to study the role of the RA metabolizing enzyme, P45ORAI, in controlling RA availability during branching morphogenesis (Specific Aim 3). To accomplish Aim 1, we have developed and are characterizing an in vitro system of foregut cultures that support initiation of the primitive lung bud. We will explore the role that RA plays in this process by enhancing or antagonizing RA signaling, and examining the effects on lung bud induction and expression of candidate pattern forming genes. For Aim 2, we will study the effect of expressing throughout the period of airway branching a constitutively active form of RAR (RARca) in distal lung epithelia of transgenic mice. We will characterize the resulting Jung phenotype and identify epithelial and mesenchymal RA target genes using in situ hybridization, semiquantitative PCR and differential gene expression microarrays. To define the developmental role of P45ORAI, we will misexpress P45ORAI in lung cultures using adenoviral gene transduction, and will disrupt P45ORAI expression in the developing lung epithelium using an SP-C-driven Cre-loxP system. These studies will help to define the role of RA signaling at different stages of lung development and to identify RA-dependent pathways involved in lung morphogenesis.