Localization and developmental expression of surfactant proteins D and A in the respiratory tract of the mouse

Surfactant protein D (SP-D) is synthesized and secreted by pulmonary epithelial cells. Like surfactant protein A (SP-A), SP-D is a collagen-like glycoprotein belonging to the "collectin" class of C-type lectins that may play an important role in pulmonary host defense. To begin studies on SP-D gene regulation and function using the mouse as an animal model, we identified the cellular sites of SP-D gene expression in adult mouse lung and trachea and characterized the developmental expression of SP-D mRNA in murine fetal and newborn lungs. We compared these findings with similar studies for murine SP-A, which has an established role in surfactant function and metabolism and a probable role in pulmonary host defense. SP-D mRNA and protein were readily detected by in situ hybridization and immunocytochemistry in alveolar type II and nonciliated bronchiolar epithelial cells of the lung, as well as in cells of the tracheal epithelium and tracheal submucosal glands of the adult mouse. Although SP-A mRNA and protein were also localized to alveolar and nonciliated bronchiolar epithelial cells of the murine lung, there was no detectable labeling for either SP-A mRNA or protein in the murine trachea. Expression of murine SP-D mRNA was first detected by Northern blot analysis on d 16 of gestation in timed-pregnant mice, with an average gestational period of 17 d, and this increased dramatically before birth and during the immediate postnatal period. The developmental expression of murine SP-A mRNA paralleled that of SP-D except that there was a small decrease in mRNA content on postnatal d 5. These studies provide the first description of the cellular distribution and developmental expression of SP-D in mouse lung, which will be important for interpreting future studies of SP-D gene expression in transgenic animal models. In addition, these studies provide the first documentation that, unlike SP-A, SP-D is synthesized not only in the lung but also in submucosal glands of the trachea.