Ex Vivo Sputum Analysis Reveals Impairment of Protease-dependent Mucus Degradation by Plasma Proteins in Acute Asthma

Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California, USA.
American Journal of Respiratory and Critical Care Medicine (Impact Factor: 13). 06/2009; 180(3):203-10. DOI: 10.1164/rccm.200807-1056OC
Source: PubMed


Airway mucus plugs, composed of mucin glycoproteins mixed with plasma proteins, are an important cause of airway obstruction in acute severe asthma, and they are poorly treated with current therapies.
To investigate mechanisms of airway mucus clearance in health and in acute severe asthma.
We collected airway mucus from patients with asthma and nonasthmatic control subjects, using sputum induction or tracheal aspiration. We used rheological methods complemented by centrifugation-based mucin size profiling and immunoblotting to characterize the physical properties of the mucus gel, the size profiles of mucins, and the degradation products of albumin in airway mucus.
Repeated ex vivo measures of size and entanglement of mucin polymers in airway mucus from nonasthmatic control subjects showed that the mucus gel is normally degraded by proteases and that albumin inhibits this degradation. In airway mucus collected from patients with asthma at various time points during acute asthma exacerbation, protease-driven mucus degradation was inhibited at the height of exacerbation but was restored during recovery. In immunoblots of human serum albumin digested by neutrophil elastase and in immunoblots of airway mucus, we found that albumin was a substrate of neutrophil elastase and that products of albumin degradation were abundant in airway mucus during acute asthma exacerbation.
Rheological methods complemented by centrifugation-based mucin size profiling of airway mucins in health and acute asthma reveal that mucin degradation is inhibited in acute asthma, and that an excess of plasma proteins present in acute asthma inhibits the degradation of mucins in a protease-dependent manner. These findings identify a novel mechanism whereby plasma exudation may impair airway mucus clearance.

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Available from: Wilfred Raymond, Mar 14, 2014
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    • "The dense glycosylation of these molecules (up to 80% in mass) with neutral and negatively charged O-glycans stiffens the mucin polypeptide, resulting in a high volume occupancy in solution which is important for gel-formation (Gerken, 1993). Furthermore, the extensive glycosylation protects the underlying mucin polypeptide from degradation by host and pathogen-proteases produced during inflammation and infection (Hasnain et al., 2012; Henke et al., 2011; Innes et al., 2009) and provides specific ligands for bacterial adhesins that influence the interaction of the host with commensal and pathogenic organisms (McGuckin et al., 2011). Flanking the central glycosylated region are the N-and Cterminal cysteine-rich domains of the mucin monomers, which are important for intracellular disulphide-linked polymerisation (Asker et al., 1995, 1998a,b; Axelsson et al., 1998; Perez-Vilar and Hill, 1999); their role in this process will be discussed in more detail below. "
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    • "The importance of neutrophil elastase and cathepsin G for intracellular and extracellular killing of S. pneumoniae by neutrophils was demonstrated recently and may be relevant for colonization (Standish & Weiser, 2009). Extracellular neutrophil protease is present on the conjunctival and nasal mucosa as it can be demonstrated in tear fluid and nasal secretions (Sakata et al., 1997; Innes et al., 2009). "
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    • "HNE as well as leukotriene B 4 (LTB 4 ) and proteinase 3 can act as important secretagogues for goblet cells and submucosal gland cells [58] [59], so that neutrophils may play an additional role in the prominent mucus hypersecretion seen in acute and severe asthma [29]. HNE has also been implicated in impaired mucus clearing observed in patients suffering from acute severe asthma [60]. "
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