Intravital microscopy of mucus transport in mice provides mechanistic insight into hypertonic saline treatment of Cystic Fibrosis

In clinical studies of cystic fibrosis (CF) patients, inhalation of 7% NaCl was found to be superior compared to 0.9%, which is attributed to increased hydration of mucus. However, mechanistic insight has been hampered by the inability to visualize mucus transport in vivo with microscopic resolution.

To overcome this problem we developed a custom-built optical coherence microscope (OCM) providing a microscopic resolution. Images of mucus transport through the intact trachea were recorded in anesthetized spontaneously breathing wild type (WT) and βENaC-overexpressing mice as a model for CF. NaCl solution (30 µl, 0.9% and 7%, respectively) was applied intranasally.

The airway epithelium of WT and transgenic mice is covered by few µm thick airway surface liquid (ASL). After intranasal application of saline an early fast cough-like fluid transport was observed in both strains. This mechanism cleared a noticeable amount of liquid. After stimulation of WT mice the ASL thickness increased transiently and returned to baseline within minutes. In contrast, in βENaC-overexpressing mice, application of saline induced mucus mobilization and transport over a period of more than 2h. Imaging revealed that treatment of CF-mice with 0.9% saline resulted in an inhomogeneous transport with a high variance of ASL thickness over the tracheal epithelium over time. This also resulted in frequently clogging of the trachea and inability to efficiently remove mucus via the larynx. In contrast, treatment of βENaC-overexpressing mice with 7% NaCl resulted in a more homogeneous mucus transport with lower variance of ASL thickness over the tracheal epithelium over time compared to 0.9% treatment.

Imaging of intravital mucus transport revealed substantial differences in the effects of 7% NaCl vs. 0.9% on mucus properties and transport. These results also demonstrate that OCM imaging is a promising tool for the preclinical evaluation of compounds that influence mucus transport.