AT A GLANCE COMMENTARY
Scientific Knowledge on the Subject
Airway mucus plugs are an important cause of airway obstruction in acute asthma exacerbations, and are poorly treated with current therapies.
What This Study Adds to the Field
We show that inhibition of protease-dependent mucin digestion occurs in acute asthma and that plasma proteins may mediate this inhibition by competing with mucin substrates for proteolysis. These data provide mechanistic insights into the pathophysiology of mucus impaction of the airways in acute asthma and highlight potential areas for the development of mucolytic therapies.
Autopsy studies of fatal asthma from as early as the 1920s clearly show that intralumenal accumulation of mucus is an important cause of airway obstruction (1
). Dunnill described “numerous grey, glistening, mucus plugs scattered throughout the airway passages,” and he noted that “pathologically the outstanding feature of the asthmatic lung lies in the failure of clearance of the bronchial secretions” (2
). Despite these longstanding insights into the cause of fatal asphyxiation in acute asthma, there has been little progress in understanding the mechanisms of mucus plug formation in acute asthma as well as a lack of specific treatments targeting this pathologic feature.
In acute severe asthma, there is hypersecretion of mucin glycoproteins from airway mucus cells (3
), leakage of plasma from highly permeable bronchial blood vessels (5
), and accumulation of inflammatory cells and inflammatory cell debris (7
). As a consequence, airway mucus in acute asthma is characterized by high concentrations of mucins, plasma proteins, and inflammatory cells. The resultant pathologic mucus is difficult to clear effectively, as demonstrated by studies showing impaired mucociliary clearance during severe asthma exacerbations followed by improvement of clearance during asthma recovery (9
). The cephalad movement of airway mucus propelled by the coordinated, rhythmic beating of epithelial cilia relies not only on ciliary motility but also on the optimal rheological properties of the mucus. Mucus gel elasticity is necessary for cilia to transmit kinetic energy to the mucus layer for forward propulsion, but high elastic recoil would impede mucociliary clearance by the resistance to extrusion from goblet cells as well as the resistance to propulsion by epithelial cilia (10
). A rheological balance between elasticity and viscosity is therefore necessary, and this balance is likely to be perturbed during acute asthma exacerbation, when mucus is produced that is abnormal both in volume and in rheological properties.
The major macromolecular components conferring mucus with its gel properties are mucin glycoproteins, which are large, heavily glycosylated protein polymers (11
). Thus, any mechanism of mucus clearance and turnover in the healthy or asthmatic airway involves degradation of these mucin polymers. Previous studies showed that neutrophil elastase degrades porcine gastric mucin (13
), and we hypothesized that proteolytic degradation of airway mucins might promote mucus clearance in the healthy airway. Inhibition of this mechanism in acute asthma would then decrease mucus clearance and promote mucus plug formation. Testing this hypothesis directly in vivo
poses multiple challenges, so we addressed it in ex vivo
studies of airway mucus from patients in acute asthma exacerbation and from nonasthmatic control subjects. In these ex vivo
studies we used rheological methods complemented by centrifugation-based mucin size profiling to determine physical and biochemical characteristics of the mucus gel in health and disease and under different experimental conditions.
Rheological measurements of viscous and elastic moduli elucidate the microstructure of fluids, including the degree of cross-linking between protein polymers. The viscous and elastic moduli of a fluid are determined by the molecular weight of its components and the architecture formed by intra- and intermolecular interactions. These physical characteristics of airway mucus depend heavily on polymeric, highly glycosylated mucin glycoproteins (mucins). Rheometers probe fluid microstructure by measuring its response to strain (fluid displacement) over a range of oscillatory frequencies. The response of the fluid is measured as the elastic (G′) and viscous (G″) moduli. The elastic modulus can be related to the density of molecular cross-links, whereas the viscous modulus can be related to molecular weight. In our study, we measured the elastic and viscous moduli of airway mucus to determine changes in mucin cross-linking and size. This approach allowed us to determine whether airway mucins are susceptible to proteolytic degradation and to explore whether mucin degradation is altered in acute asthma.
Some of the results of this study have been previously reported in abstract form at American Thoracic Society international conferences (14