Surfactant is a complex protein-phospholipid that reduced surface tension in the lung and prevents collapse of alveoli at low volumes. Inadequate amounts of surfactant are associated with the respiratory distress syndrome in premature infants. While the secretion of surfactant by pulmonary alveolar type-2 cells is an essential component of pulmonary function, the mechanism leading to surfactant release are still poorly understood. Nevertheless, it is clear that the process of secretion is a calcium-dependent phenomenon triggered by a transient rise in intracellular calcium and mediated by enzymes and factors that undergo alterations in the presence of calcium. The goal of this research project is to attempt to identify the enzymes and factors that link cellular calcium fluxes with secretion of surfactant by alveolar type-2 cells. The study will test the hypotheses that (1) the endogenous protease, calpain, will be activated when type-2 cells are stimulated by secretagogues and that (2) calpain then modifies two key signal transduction enzymes, cAMP-dependent protein kinase and calcium/phospholipid-dependent protein kinase. These protein kinases are known to be involved in secretory process of membrane. They interface calcium-dependent cytosolic proteases activated at the cell enzymes, cytoskeletal proteins, and receptors. The study will follow the secretagogue-induced, calpain-mediate proteolysis of protein kinases by monitoring the formation of protein kinase fragments and the appearance of co-factor independent phosphorylation activities. The study will also test the hypotheses that (3) these protein kinases specifically phosphorylate the fusogenic protein annexin, which may then mediate fusion submembranous cytoskeletal protein spectrin leads to a loss of membrane-integrated competent. Finally, alterations in calpain gene expression in type-2 cells in response to secretagogues will be monitored for changes in protein synthesis, mRNA level and rate of transcription. Identification of the factor that regulate surfactant secretion may provide fundamental information and insight into a key determinant of pulmonary function and dysfunction.
