Introduction and objectives Surfactant protein C (SFTPC) is secreted by type II pneumocytes to reduce alveolar lining fluid surface tension and thus prevent alveolar collapse at low lung volumes. The immature form of SFTPC must undergo proteolytic processing before being secreted as the mature form, but several pathogenic SFTPC mutations associated with familial interstitial lung disease impede this process. Mutations in the C-terminal BRICHOS domain of SFTPC (ΔEx4 and L188Q) lead to retention of the protein within the endoplasmic reticulum (ER), while other mutations (e.g. I73T) cause SFTPC mis-trafficking.
Methods To study these mutants in vivo in a genetically tractable organism, we generated lines of Drosophila melanogaster expressing wild type or mutant human SFTPC. The transgenic proteins could be tagged with green fluorescent protein (GFP) to facilitate in vivo visualisation. These fusion proteins were expressed under the control of tissue-specific drivers. Components of the ER associated degradation (ERAD) machinery or of the autophagy pathway were depleted in those tissues by RNA interference. Lines expressing an ER stress reporter or autophagy reporter were used as readouts for these phenomena.
Results Expression of the BRICHOS mutants ΔEx4 and L188Q led to the progressive deposition of protein aggregates when expressed in the fly eye. In contrast, the I73T mutant accumulated in a more diffuse distribution. When expressed in the larval salivary gland, the BRICHOS mutants where retained within the cell, in contrast to the wild type protein that was trafficked to the cell surface. The I73T mutant showed low-level cell surface and weak intracellular fluorescence. Depletion of the ERAD E3 ubiquitin ligase Hrd1 or its associated E2 ligases failed to affect mutant protein levels arguing against an important role of ERAD in the degradation of SFTPC in this model. In contrast, inhibition of autophagy by depletion of Atg8 enhanced the accumulation of L188Q SFTPC. Accordingly, robust activation of autophagy was detected in L188Q SFTPC-expressing tissue. Interestingly, ER stress was not detected.
Conclusion In a Drosophila model of hSFTPC trafficking, autophagy was the major degradation pathway for L188Q mutant SFTPC.