13-P069 A synthetic enhancer screen to elucidate the role of the Wnt/β-catenin network in vertebrate embryonic development

nal tissues, and their contributions to signaling during pulmonary development; however, are unknown. a5 was assessed by immunohistochemistry in mouse lungs from embryonic day (E) 13.5 to post-natal day (PN) 10. Transcriptional control of a5 was determined by transfection of murine pulmonary epithelial cell lines with reporter constructs containing 2.0 kb, 850 bp, or 450 bp of the mouse a5 promoter. TTF-1, a key transcription factor that controls pulmonary morphogenesis and Egr-1, a prevalent factor expressed at mid-gestation, were used to evaluate a5 regulation. a5 was initially detected in the most proximal primitive tubules at E15.5. a5 expression followed the proximal-distal axis and was detected throughout the lung until PN5, an early stage of alveologenesis. From PN5 to PN10, a5 expression decreased in the proximal airways and was exclusively observed in the peripheral lung by PN10. Co-localizing staining revealed that a5 was expressed in Clara cells in the proximal lung and type II alveolar epithelial cells in the periphery. Promoter mutagenesis revealed that both TTF-1 and Egr-1 individually and additively induced the transcription of a5. Exogenous TTF-1/Egr-1 also significantly induced a5 transcription. These data demonstrate that a5 is specifically controlled in a temporal and spatial manner during pulmonary morphogenesis. Ongoing research may demonstrate that specific regulation of a5 in differentiating pulmonary epithelial cells is involved in normal organogenesis.