The Collagen-like Region of Surfactant Protein A (SP-A) Is Required for Correction of Surfactant Structural and Functional Defects in the SP-A Null Mouse

Pulmonary surfactant isolated from gene-targeted surfactant protein A null mice (SP-A−/−) is deficient in the surfactant aggregate tubular myelin and has surface tension-lowering activity that is easily inhibited by serum proteins in vitro. To further elucidate the role of SP-A and its collagen-like region in surfactant function, we used the human SP-C promoter to drive expression of rat SP-A (rSPA) or SP-A containing a deletion of the collagen-like domain (ΔG8-P80) in the Clara cells and alveolar type II cells of SP-A−/− mice. The level of the SP-A in the alveolar wash of the SP-A−/−,rSP-A and SP-A−/−,ΔG8-P80mice was 6.1-and 1.3-fold higher, respectively, than in the wild type controls. Tissue levels of saturated phosphatidylcholine were slightly reduced in the SP-A−/−,rSP-A mice compared with SP-A−/− littermates. Tubular myelin was present in the large surfactant aggregates isolated from the SP-A−/−,rSP-A lines but not in the SP-A−/−,ΔG8-P80 mice or SP-A−/− controls. The equilibrium and minimum surface tensions of surfactant from the SP-A−/−,rSP-A mice were similar to SP-A−/−controls, but both were markedly elevated in the SP-A−/−,ΔG8-P80 mice. There was no defect in the surface tension-lowering activity of surfactant from SP-A+/+,ΔG8-P80 mice, indicating that the inhibitory effect of ΔG8-P80 on surface activity can be overcome by wild type levels of mouse SP-A. The surface activity of surfactant isolated from the SP-A−/−,rSP-A but not the SP-A−/−,ΔG8-P80 mice was more resistant than SP-A−/− littermate control animals to inhibition by serum proteins in vitro. Pressure volume relationships of lungs from the SP-A−/−, SP-A−/−,rSP-A, and SP-A−/−,ΔG8-P80 lines were very similar. These data indicate that expression of SP-A in the pulmonary epithelium of SP-A−/− animals restores tubular myelin formation and resistance of isolated surfactant to protein inhibition by a mechanism that is dependent on the collagen-like region.