TENASCIN IN RAT LUNG DEVELOPMENT - IN-SITU LOCALIZATION AND CELLULAR SOURCES

Tenascin (TN) is a hexameric extracellular matrix glycoprotein that ma y play an important role during lung development. TN protein is tempor ally and spatially restricted during lung organogenesis. The temporo-s patial and cellular expression of TN mRNA in lung remains unclear. Loc alization of message expression of TN in rat lung tissue was first inv estigated by using in situ hybridization performed with an antisense R NA probe. TN mRNA was present primarily within the mesenchyme of day 2 6 gestational age fetal rat lung tissue, whereas immunoreactive TN pro tein was found along the basement membrane. In postnatal day 3 rat lun g tissue, TN mRNA was detected along alveolar septal walls and was con centrated at secondary septal tips. Expression of TN message was consi stent with localization of immunoreactive TN protein. Accumulation of TN mRNA in alveolar septal tips suggests that mesenchyme may be the ma jor source of TN mRNA. To investigate the cellular source of TN in rat lung, we studied the expression of TN in cultured rat lung fibroblast s, endothelial cells, and alveolar epithelial cells. Two TN isoforms h aving molecular mass of 230 and 180 kDa were in conditioned medium and in cellular extracts of lung fibroblasts and endothelial cells. TN wa s secreted and deposited in the extracellular matrix closely associate d with the surface of lung fibroblasts and endothelial cells. Lung alv eolar epithelial cells showed undetectable or barely detectable amount s of TN. These studies demonstrated that TN isoforms are expressed not only by lung fibroblasts but also by lung endothelial cells. The uniq ue spatial localization of TN mRNA during lung development and express ion of TN by different lung cell types suggested TN may be involved in matrix organization and cell-cell interactions during lung developmen t.