Nano-scale viscometry reveals an inherent mucus defect in cystic fibrosis

Abnormally viscous and thick mucus is a hallmark of cystic fibrosis (CF). How the genetic defect causes abnormal mucus in CF remains unanswered and a question of paramount interest. Mucus is produced by hydration of gel-forming mucin macromolecules that are stored in secretory granules prior to release. Current understanding of mucin/mucus structure before and after secretion remains limited and contradictory models exist. Here we used a molecular viscometer and fluorescence lifetime imaging of primary epithelial cells (Normal and CF) to measure nanometer-scale viscosity. We found significantly elevated intraluminal nanoviscosity in a population of CF mucin granules, indicating an intrinsic, pre-secretory, mucin defect. Validation experiments showed that high nanoviscosity in cellular environments is mainly due to the low mobility of water that hydrates macromolecules. Nanoviscosity influences protein conformational dynamics and function. Its elevation along the protein secretory pathway indicates molecular overcrowding and is expected to alter mucins post-translational processing, hydration, and mucus rheology after release. The nanoviscosity of extracellular CF mucus was elevated compared to non-CF mucus. Remarkably, it was higher after secretion than in granules, which suggests mucins have a weakly-ordered state in granules and adopt a highly-ordered, nematic crystalline structure extracellularly. This challenges the classical view of mucus as a porous agarose-like gel and suggests an alternative model for mucin organization before and after secretion. Our study also suggests that endoplasmic reticulum stress due to molecular overcrowding contributes to mucus pathogenesis in CF cells. It encourages the development of therapeutics that target pre-secretory mechanisms in CF and other muco-obstructive lung diseases. ### Competing Interest Statement The authors have declared no competing interest.