Surfactant Protein (SP)-A Benefits Over SP-A Mutant: A Preliminary Study for Interstitial Lung Disease Treatment

Surfactant protein (SP)-A, an octadecamer composed of SP-A1 and SP-A2, is secreted into the alveolar space. Heterozygous variations in SFTPA1 and SFTPA2, reported to impair protein secretion, have been associated with interstitial lung disease and lung adenocarcinoma. To date, no specific treatment is available. Here, the impact of wild-type (WT) SP-A1 or SP-A2 on the localization, oligomerization, and secretion of deleterious SP-A1 or SP-A2 variants is investigated. To achieve this, we used expression vectors carrying four previously described variations as well as a newly identified variation in SFTPA1 and SFTPA2 or WT sequences. Proteins were transiently expressed in HEK293T, and, after extraction, SP-A1 and SP-A2 were analyzed by Western blotting to assess their stability, ability to form oligomers, and secretion. In addition, the subcellular localization of these proteins in HEK293 cells was examined using immunofluorescence microscopy. Consistent with previous reports, we observed that all the variations impair SP-A1 or SP-A2 secretion. Localization of mutated proteins was also disrupted. Furthermore, all variations in SFTPA1 and SFTPA2 exhibited defects in oligomerization of mutated proteins, together with lower expression levels. Interestingly, coexpression of SP-A1 or SP-A2 WT resulted in an increased expression of the mutated proteins, restored a proper oligomerization profile, and partially restored SP-A secretion. This study reveals the beneficial effect of SP-A WT on oligomerization and secretion of mutant SP-A, suggesting that SP-A may be studied as a potential targeted treatment in interstitial lung disease linked to SP-A molecular variations.