Mucus Matters: Mucociliary Physiology in Chronic Lung Diseases

Abstract

The mucociliary escalator is an innate defense mechanism in the lung comprised of the ciliated, pseudostratified epithelium that lines the conducting airways and the mucus that sits atop the cilia. Mucus traps inhaled particulate matter/pathogens, and the cilia brush works in concert to propel the mucin gel proximally. Therefore, proper mucociliary physiology is critical to maintaining lung health.

A metachronal wave propagated across the epithelium can occur when the phase of the ciliary stroke is shifted in time relative to its neighbors. Metachrony has been hypothesized to help overcome the viscoelastic forces required for the propulsion of mucus. Still, its effect on mucociliary transport rate has not been characterized due to previous limitations in imaging technology. The first portion of my thesis was to describe the influence of metachronal beating on mucociliary transport (MCT) rates in human bronchi and ferret tracheae and investigate the role of calcium signaling on metachronal wave generation using μOCT. We found that mucociliary transport was significantly faster when metachrony was present. Treatment of ferret tracheae with either cell-permeant chelator of intracellular Ca²⁺, a nonpermeant Ca²⁺ channel competitive antagonist, or an inhibitor of calaxin was sufficient to abolish metachrony and had a deleterious effect on MCT.

A gain-of-function promoter variant for MUC5B (rs35705950) is a significant risk factor for developing idiopathic pulmonary fibrosis (IPF); yet, the role of MUC5B mucin or abnormal mucociliary physiology in IPF pathogenesis is unknown. Bleomycin (BLM)-exposed rodent models do not develop sustained fibrosis or exhibit IPF-like airway remodeling. Ferrets, unlike mice, have a human-like distribution of Muc5b in the lung and natively express the riskconferring variant that induces high MUC5B expression in humans. Therefore, I established a BLM-ferret model for the second portion of my thesis. Our BLMinduced pulmonary fibrosis ferret demonstrated sustained fibrosis by μCT, hallmarks of restrictive physiology by flexivent, and pulse oxygenation decrement through 12-weeks following a single dose of BLM. Additionally, BLM ferrets recapitulate features of human IPF not observed in lower species, such as aberrant bronchiolization of the distal lung associated with fibrotic remodeling and pathologic lesions including fibroblastic foci and honeycomb cysts.

These studies are significant for 1) adding fundamental knowledge about how metachronal waves influence mucociliary physiology and 2) developing the ferret model of pulmonary fibrosis, which will be a valuable tool to elucidate the impact of a human-like mucin microenvironment on fibrosis pathogenesis.