LOCALIZATION AND DEVELOPMENTAL EXPRESSION OF SURFACTANT PROTEIN-D ANDPROTEIN-A IN THE RESPIRATORY-TRACT OF THE MOUSE

Surfactant protein D (SP-D),is synthesized and secreted by pulmonary e pithelial cells. Like surfactant protein A (SP-A). SP-D is a collagen- like glycoprotein belonging to the ''collectin'' class of C-type lecti ns that may play an important role in pulmonary host defense. To begin studies on SP-D gene regulation and function using the mouse as all a nimal model, we identified the cellular sites of SP-D gene expression in adult mouse lung and trachea and characterized the developmental ex pression of SP-D mRNA in murine fetal and newborn lungs. We compared t hese findings with similar studies for murine SP-A, which has all esta blished rule in surfactant function and metabolism and a probable role in pulmonary host defense. SP-D mRNA and protein wen readily detected by in situ hybridization and immunocytochemistry in alveolar type II and nonciliated bronchiolar epithelial cells of the lung, as well as i n cells of the tracheal epithelium and tracheal submucosal glands of t he adult mouse. Although SP-A mRNA and protein were also localized to alveolar and nonciliated bronchiolar epithelial cells of the murine lu ng, there was no delectable labeling for either SP-A mRNA or protein i n the murine trachea. Expression of murine SP-D mRNA was first detecte d by Northern blot analysis on d 16 of gestation in limed-pregnant mic e, with an average gestational period of 17 d, and this increased dram atically before birth and during the immediate postnatal period. The d evelopmental expression of murine SP-A mRNA paralleled that of SP-D ex cept that there was. small decrease iu mRNA content on postnatal d 5. These studies provide the first description of the cellular distributi on and developmental expression of SP-D in mouse lung, which will be i mportant For interpreting future studies of SP-D gene expression in tr ansgenic animal models. In addition, these studies provide the first d ocumentation that, unlike SP-A, SP-D is synthesized not only in the lu ng but also in submucosal glands of the trachea.