Alveolar surfactant is a mixture of lipids, proteins, and carbohydrates, which is synthesized in alveolar type II cells where it is stored in lamellar bodies until it is secreted into the alveoli. Its main function is to reduce surface tension in the lungs. An inadequate amount of functional surfactant, such as occurs in the respiratory distress syndrome of the newborn, results in impaired gas exchange. Although the synthesis and secretion of surfactant have been extensively studied, far less is known about its intra-alveolar metabolism and clearance. Available data suggest that a major route for clearance is the uptake of surfactant phospholipids by type II cells. The principal goal of the proposed work is to study the mechanisms involved in surfactant uptake. Studies performed in this laboratory over the past few years suggest that subfractions of surfactant, which are obtained by differential centrifugation of lavage fluid and which have different physical and biochemical properties, are taken up from the alveoli into lamellar bodies of type II cells at different rates. The two fractions which were most rapidly taken up contained the major surfactant apoprotein (apo 36), large multilamellar vesicles and tubular myelin, and they were highly surface active. The other major surfactant apoprotein group, apo 10, was present in only one of the fractions. The more slowly taken up fraction contained very little apoprotein or tubular myelin and was minimally surface active. I am proposing that there is a specific mechanism of uptake, mediated by some property of the rapidly taken up fractions. The experiments I am proposing to test this hypothesis are directed at 3 specific aims. 1) I will determine which properties of the preferentially taken up subfractions are responsible for their increased uptake. I will determine if apo 36 or some other property, determined by using better purified fractions, augment uptake of phospholipids by isolated type II cells. 2) I will determine if proteins are internalized and I will study the metabolism of lipids and proteins after uptake. Phospholipids will be analyzed by thin-layer chromatography; proteins will be analyzed by slab gel electrophoresis. 3) I will attempt to determine the mechanism of uptake (i.e. endocytosis, fusion or lipid transfer).
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