Mechanisms of Cilia-Driven Transport in the Airways in the Absence of Mucus

Abstract

Airway mucus is thought to be required for the clearance of inhaled particles by mucociliary transport, but this view has recently been challenged. To test if mucus is necessary for cilia-driven particle transport, we removed mucus from murine and human ex vivo airway preparations by thorough rinsing with buffer with or without additional dithiothreitol washing. The transport of particles with diameters of 4.5 μm, 200 nm, and 40 nm and of bacteria was analyzed by video microscopy. Complete removal of mucus was verified by wheat germ agglutinin staining and by scanning electron microscopy. In the absence of mucus, we observed efficient transport of particles and bacteria by direct cilia-mediated propulsion or via fluid flow generated by ciliary beating. Virus-sized particles had the tendency to attach to cilia. Because direct contact of particles with ciliated cells occurs in the absence of mucus, we examined if this direct interaction changes epithelial function. Neither bacteria- nor LPS-induced nuclear translocation of NF-κB p65 in ciliated cells occurred, indicating that mere contact between ciliated cells and bacteria during transport does not activate the epithelium. Attachment of virus-sized particles to cilia could induce mucus release and/or increase the ciliary beat frequency. Our results indicate that cilia-driven transport of particles with various sizes is possible in murine and human airways without the presence of mucus. If mucus-free transport fails, the epithelium can react by releasing mucus or increasing the ciliary beat frequency to maintain particle transport.

Epithelial tethering of MUC5AC-rich mucus impairs mucociliary transport in asthma

“…Con sistent with our previous results, IL-13 markedly reduced the velocity of microspheres prior to removal of the gel. It was recently reported that removal of the mucus gel from healthy airways did not abolish particle transport but instead modestly increased transport velocity (21). We obtained similar results after removal of gels from unstim ulated cultures.…”

Biological effects of carbon black nanoparticles are changed by surface coating with polycyclic aromatic hydrocarbons

“…These aggregates bound to ciliated cells and increased ciliary beat frequency. Furthermore, microscopically visible nanoparticle agglomerates have been shown to increase ciliary beat frequency in the murine trachea [72]. A similar effect has been observed in the bovine trachea where polystyrene particles >200 nm in diameter that attach to cilia increase ciliary beat frequency due to mechanical stimulation of axonema possibly involving second messengers such as cAMP or cGMP [73], both of which are known to increase ciliary beat frequency [74].…”

“…The strength of short-term organ culture models is that they can be adapted to screen for specific organ functions that are not routinely examined in standardized in vivo experiments. This was demonstrated by using our ex vivo trachea model that allowed examining the complex interplay between mucus production, ciliary beating and cell death [72, 89]. …”

Microscale imaging of cilia-driven fluid flow

“…It has been used to study the effects of mucus removal on flow [108,109], the effects of physiological stressors [135], and development of directional flow in the trachea [136]. It also became the primary method of estimating mucus velocity in bronchoepithelial cell culture [23,137–139].…”