Molecular Viscometer Identified a Population of Mucin Granules of Higher Viscosity in Cystic Fibrosis Cells

The cystic fibrosis (CF) disease is characterized by abnormally thick and sticky mucus. Normal airway mucus has bulk viscosity (macroviscosity) in the range of 12–15×10 3 cP increasing to 14–110×10 3 cP for CF patients. At micro‐length scale (≤1 μm), due to the presence of fluid‐filled spaces between mucin fibers, mucus microrheological properties are similar to that of water (~1 cP). However, they are only slightly higher in CF mucus (~3 cP). Our study was designed to verify if similar viscosity differences exist between normal and CF mucin granules and secreted mucus. We used a custom‐designed membrane‐permeant molecular viscometer (phenyl‐BODIPY) and fluorescence lifetime imaging microscopy (FLIM) to study the microviscosity of mucin granules and secreted mucus. FLIM microviscosity measurements were validated in calibration experiments with sucrose (low microviscosity range), glycerol (high microviscosity range), agarose (gel‐like structure) and reconstituted mucus of isolated bovine mucin proteins in solution. Human bronchial epithelial (HBE) cells, isolated from tissues of 4 normal and 5 CF donors, were seeded on filters and cultured at the air‐liquid interface. FLIM measurements revealed that median values of microviscosity for normal granules are 41±7 cP (n filters =30) and 49±11 cP (n filters =31) for CF granules (p<0.001, Mann–Whitney U test). Next, we classified mucin granules in three categories based on their microviscosity: low (1–30 cP), medium (30–60 cP) and high (>60 cP). The quantity of granules with medium microviscosity does not differ between normal (74%) and CF (77%) donors. However, normal phenotype was characterized by the presence of a larger fraction of low microviscosity mucin granules (19%) compared to CF (4%), while CF phenotype displayed a larger proportion of high microviscosity granules (19%) compared to normal (7%). Raising the pH inside granules by blocking H + ‐ATPase with 10 nM bafilomycin lowered microviscosity values of CF granules towards normal phenotype. Microviscosity of secreted mucus covering HBE cells was measured in situ and reached values of 147±54 cP (n filters =7) for normal donors and 198±75 cP (n filters =7) for CF donors. Our study demonstrates the presence of a subpopulation of mucin granules with higher microviscosity in CF that is less pronounced in normal patients. FLIM microviscosity measurements of secreted mucus show much higher values than expected for a water‐filled interfiber spaces in a gel. This may indicate that mucus covering HBE cells resembles an amorphous colloid rather than a well‐structured hydrogel. Relative differences in mucus microviscosity between normal and CF, however, are much smaller than those reported for a bulk viscosity of mucus collected from patients. Support or Funding Information Supported by the Cystic Fibrosis Canada (R.G.) and CIHR PJT148593 (E.B.).