Elastogenesis in the developing chick lung is transcriptionally regulated

The overall goals of this study were to establish the level at which elastin gene expression is regulated during chick lung embryogenesis and to identify the temporal and spatial relationships among elastogenesis, smooth muscle cell differentiation, and cell proliferation. A comparison of lung elastin mRNA and transcriptional levels during embryogenesis shows that elastin expression is developmentally regulated at the transcriptional level. The increase in elastogenic activity occurs during the late stages of lung embryogenesis and coincides with terminal maturation of the tertiary bronchi. In situ hybridization analysis demonstrates that the increase in elastin mRNA expression is confined to the tertiary bronchial respiratory subunits, connective tissue septa, and supporting vasculature of the lung parenchyma. Immunohistochemical localization of smooth muscle cell alpha-actin and tropoelastin suggests that alpha-actin-immunoreactive cells of the lung parenchyma are a major contributor to the increase in elastin expression during embryogenesis. This observation is also reflected by Northern blot analysis, which demonstrates a temporal coincidence in the increase of both alpha-actin and elastin mRNA levels. Histone mRNA expression, which was used as an index of cellular proliferation, reveals a level and spatial pattern inversely related to that of the elastin transcript. Tissue transfections of chick lungs isolated from 18-day embryos with various elastin gene deletion/reporter constructs illustrate that the elastin promoter is not promiscuous within a tissue environment and that sequences spanning the -500 to +2 region are capable of directing promoter activity spatially comparable to the endogenous elastin gene.