Mucus, mucins, and cystic fibrosis

Abstract

Cystic fibrosis (CF) is both the most common and most lethal genetic disease in the Caucasian population. CF is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and is characterized by the accumulation of thick, adherent mucus plaques in multiple organs, of which the lungs, gastrointestinal tract and pancreatic ducts are the most commonly affected. A similar pathogenesis cascade is observed in all of these organs: loss of CFTR function leads to altered ion transport, consisting of decreased chloride and bicarbonate secretion via the CFTR channel and increased sodium absorption via epithelial sodium channel upregulation. Mucosa exposed to changes in ionic concentrations sustain severe pathophysiological consequences. Altered mucus biophysical properties and weakened innate defense mechanisms ensue, furthering the progression of the disease. Mucins, the high-molecular-weight glycoproteins responsible for the viscoelastic properties of the mucus, play a key role in the disease but the actual mechanism of mucus accumulation is still undetermined. Multiple hypotheses regarding the impact of CFTR malfunction on mucus have been proposed and are reviewed here. (a) Dehydration increases mucin monomer entanglement, (b) defective Ca²⁺ chelation compromises mucin expansion, (c) ionic changes alter mucin interactions, and (d) reactive oxygen species increase mucin crosslinking. Although one biochemical change may dominate, it is likely that all of these mechanisms play some role in the progression of CF disease. This article discusses recent findings on the initial cause(s) of aberrant mucus properties in CF and examines therapeutic approaches aimed at correcting mucus properties.

Keywords: biochemical interactions; cftr; mucins; mucus; polymeric network; viscoelastic properties.

Figure 1.. Mucin domains govern polymeric network organization.

Mucin structure and assembly rely on von Willerbrand Factor (vWF) D and C domains in the N- and C-terminal regions and cysteine-rich domains (CysD) scattered throughout the protein backbone including the cysteine knot (CK) at the C-terminal end. The protein core, which is rich in serine and threonine residues, undergoes post-translational O-glycosylation (O-glycans) to form mature mucin monomers. Dimer formation ensues in the endoplasmic reticulum and is mediated by disulfide linkage of the CK domains to form linear, interwoven polymeric networks.

Figure 2.. Model of normal vs. CF airway mucus layer illustrating changes within the mucus network (i.e., polymer entanglement, mucin compaction, and/or changes in molecular interactions) in response to altered ionic fluxes.

In normal individuals, CFTR function ensures proper Cl⁻ and HCO₃⁻ secretion as well as regulates Na⁺ absorption via the downregulation of the ENaC channel, controlling water flux through the epithelium. A thin mucus layer is produced by airway goblet cells with optimal biophysical properties (e.g., loose transportable) for airway clearance. In CF, reduced Cl⁻ and HCO₃⁻ secretion and increased Na⁺ absorption can alter the biochemical interface of the mucin network in different ways. Mucus layer hyperconcentration causes decrease in mucus mesh size (e.g., entanglement). Impaired Ca²⁺ sequestering prevents mucin expansion (e.g., compaction). Changes in hydration, pH, and oxidative stress can introduce additional ionic, hydrogen, hydrophobic, and disulfide bonds (e.g., interactions).

Figure 3.. Images of mucus flakes collected via bronchoalveolar lavage (BAL) from a CF preschooler and treated ex vivo with PBS or TCEP.

Top images show the mucin polymeric network of a mucus flake treated with PBS or TCEP (5 mM for 10 min). BAL samples were stained with anti-MUC5B (green) and anti-MUC5AC (red) antibodies. DAPI (blue) was used to stain the nuclei of entrapped inflammatory cells. Bottom images reveal the organization of the mucin mesh via scanning electron microscopy (SEM) following treatment with PBS or TCEP. Scale bar is 4 μm.