Rescue of SP-B Knockout Mice with a Truncated SP-B Proprotein: FUNCTION OF THE C-TERMINAL PROPEPTIDE

The function of the 102-amirno acid C-terminal propeptide of surfactant protein B (SP-B) was analyzed by characterizing the phenotype associated with loss of expression of Shia peptide domain in transgenic mice. A construct encoding the signal peptide, N-terminal propeptide, and mature peptide of human SP-B (hSP-B-Delta c) was cloned under the control of the 3.7-kilobase human SP-C promoter and injected into fertilized eggs of the FVB/N mouse strain. Founder mice expressing the hSP-B-Delta c transgene were bred with heterozygous SP-B knockout mice (SP-B +/-). Offspring containing the transgene and one allele al mouse SP-B were identified and subsequently crossed to generate a transgenic line that expressed SP-B-Delta c in a null background (SP-B(-/-)/hSP-B-Delta c(+/+)). Expression of hSP-B-Delta c in SP-B(-/-) mice was restricted to type TP cells and resulted in a 2-fold Increase in mature SF-B relative to wild type littermates, These mice survived without any evidence of respiratory problems and had normal lung function, normal alveolar surfactant phospholipid pool sizes, and typical tubular myelin indicating that the 102-residue C-terminal propeptide of SF-B is mot required for normal structure and function of extracellular surfactant. However, proteolytic processing of the SP-C proprotein was perturbed resulting in the accumulation of a processing intermediate, M(r) = 11,000, similar to the phenotype detected in SP-B(-/-) mice; furthermore, lamellar bodies ire type II cells of SP-B(-/-)hSP-B-Delta c(+/+) mice were much larger than in the wild type animal and saturated phosphatidylcholine content in lung tissue was significantly increased although the incorporation of choline into saturated phosphatidylcholine was normal, Collectively, these results demonstrate a role for the C-terminal propeptide of SP-B in SP-I: proprotein processing and the maintenance of lamellar body size, The C-terminal propeptide may be an important determinant of intracellular surfactant pool size.