| Pulmonary surfactant, a mixture of phospholipids and surfactant proteins, lines the alveolar space and functions to reduce surface tension at the air-liquid interface. Alveolar type II epithelial cells synthesize surfactant protein C (SP-C) as a type II integral membrane protein which is proteolytic processed in multivesicular bodies to remove the N- and C-terminal peptides liberating the mature peptide. SP-C mature peptide, consisting of a transmembrane and short extramembrane domain, is stored in lamellar bodies and secreted with surfactant phospholipids into the alveolar space. The overall goal of this dissertation was to define how SP-C, an integral membrane protein, is secreted by type II cells. The hypothesis underlying these studies is that secretion of SP-C occurs in two distinct steps involving sorting to multivesicular bodies and internalization into the inner vesicles. Deletion analyses indicated that the SP-C mature peptide was sorted to the regulated secretory pathway in the absence of its flanking peptides. The SP-C mature peptide was secreted by type II cells and associated with large aggregate surfactant, the biologically active form of surfactant, in wildtype but not SP-C null mice. In the absence of endogenous SP-C, the SP-C N-terminal peptide was required for secretion of the mature peptide or a heterologous transmembrane domain into the alveolar space. These studies indicate that the N-terminal peptide contains the information required to sort and internalize SP-C into the inner vesicles of multivesicular bodies.; SP-C deficient mice survive with only slightly changes in lung function and have normal levels of mature SP-B peptide. However, SP-B deficient mice have reduced levels of SP-C mature peptide in the airspace and develop neonatal lethal respiratory distress syndrome. To test the hypothesis that SP-B and SP-C are functionally redundant in their ability to maintain the active surface film, SP-C mature peptide was expressed in transgenic mice to restore lung function in SP-B deficient mice. However, expression of the SP-C mature peptide in the lungs of transgenic mice perturbed lung development resulting in respiratory distress and neonatal lethality. These studies suggest that aberrant folding, sorting and secretion of SP-C may have profound effects on lung development and function. |
