Temporal-spatial expression and transcriptional regulation of α7 nicotinic acetylcholine receptor by thyroid transcription factor-1 and early growth response factor-1 during murine lung development

Abstract

Nicotinic acetylcholine receptors are ligand-gated ion channels formed by five homologous subunits that are involved in processes including signal transduction, proliferation, and apoptosis. The developmental role of these receptors, however, is unclear. In the present investigation, α7 nicotinic acetylcholine receptor expression was assessed by immunohistochemistry in mouse lungs from embryonic day (E)13.5 to postnatal day (PN)20. Transcriptional mechanisms that regulate α7 were assessed by the transfection of murine bronchiolar cells with a reporter containing 1.1 kb of the mouse α7 promoter, TTF-1, and Egr-1. α7 was initially detected at E13.5 in pulmonary mesenchymal cells and in the epithelium of the primitive tubules at E15.5. From E18.5 to PN1, α7 was expressed in conducting airway and saccule epithelial cells. By PN10, expression was observed in the peripheral epithelium and on luminal membranes of bronchiolar epithelial cells in the proximal lung, a pattern that continued through PN20. From E15.5 to PN20, type II alveolar cells expressed both prosurfactant protein C and α7. From E18.5 to PN20, Clara cells in the bronchiolar epithelium co-expressed Clara cell secretory protein and α7. TTF-1 dose-dependently activated α7 transcription in vitro by binding specific TTF-1 regulatory elements in the mouse α7 promoter. Furthermore, α7 was not detected in TTF-1-null mice and markedly increased in TTF-1-overexpressing mice. Conversely, Egr-1 inhibited α7 expression. Temporal-spatial α7 expression supports the concept that these receptors function during normal pulmonary morphogenesis. A model is also supported whereby α7 is induced by the essential pulmonary transcription factor TTF-1 and suppressed by Egr-1 during pulmonary development. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.