Surfactant Protein C (SP-C), a hydrophobic protein exclusively synthesized and secreted by alveolar type II (AT2) cells, is important for reducing alveolar surface tension in the distal lung. Chronic interstitial pulmonary diseases have been associated with SFTPC mutations. However, a detailed understanding of SP-C maturation in the secretory pathway and disruptions caused by mutations has remained incomplete. In vivo, the 21-kDa proSP-C undergoes sequential C-term and N-term cleavages through 16-, 7-, and 6-kDa intermediates to generate mature 4.2-kDa SP-C. While Cathepsin H and AT2-specific enzymes such as Napsin/Pepsinogen C are implicated in late N-terminal processing within lamellar bodies (LBs), no enzyme has been identified for the initial C-terminal cleavage step that precedes multivesicular body (MVB)–associated maturation. This study aimed to define trafficking and post-translational processing differences between wild-type and disease-associated SP-C mutants using doxycycline-inducible mouse lung epithelial (MLE-12) cell lines expressing either SP-CWT or the common clinical variant SP-CI73T, validated using primary AT2 cells isolated from a murine SP-CI73T pulmonary fibrosis model and induced pluripotent stem cell (iPSC)-derived human alveolar type 2 cells (iATs) expressing the same mutant. Immunogold electron microscopy of SP-CWT mouse lung revealed robust WT proSP-C labeling in the Golgi, MVBs, in contrast, SP-CI73T expressing AT2 cells exhibited a fragmented Golgi morphology, characterized by dispersed, swollen cisternae and unstacking. In conclusion, we have identified a previously uncharacterized early proteolytic processing step for SP-CWT shown to be essential to exclude aberrant transport of the proprotein to the plasma membrane, refining the model of SP-C trafficking, and highlighting the intricate regulation of protein processing in the Golgi. This more granular understanding of SP-C and lung epithelial biology will open new avenues for linking AT2 cargo defects and cell quality control dysfunction to diseases such as Idiopathic Pulmonary Fibrosis. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Bui et al. (Fri,) studied this question.