Mucus Viscosity, Increased

Various mucosal surfaces, such as the luminal sides of the urogenital, gastrointestinal, reproductive and respiratory tracts, are lined by mucus, a complex biopolymer that forms a barrier to environmental insults and maintains lubrication (Lai Samuel K. et al., 2009). Chemically, mucus consists of large glycoproteins called mucins (MUCs). Both secreted, gel-forming mucins (e.g. MUC2, MUC5AC, MUC5B and MUC19) and membrane-bound mucins (e.g. MUC1, MUC4, MUC13, MUC16, MUC20, MUC21 and MUC22) are found in the lungs (Atanasova and Reznikov, 2019). Mucins are very heterogeneous, with protein backbones and carbohydrates making up approx. 20% and 80% of their molecular weight, respectively. Cysteine residues in the carboxy and amino terminals of mucin backbones facilitate end-to-end disulfide bonding, resulting in dimerization and multimerization (Ma et al., 2018; Rose and Voynow, 2006). This results in a complex hydrated porous molecular network or gel aggregates that, together with secreted host defense proteins, DNA, lipids, cellular debris and immune cells, make up airway mucus (Atanasova and Reznikov, 2019; Thornton and Sheehan, 2004). 

According to Girod et al., “Mucus is a highly non-Newtonian viscoelastic material. Under a discontinuous stress, induced by ciliary motion during active stroke or by cough, the mucus starts to instantaneously deform and, once the stress is removed (as during the recovery period of beating or after cessation of coughing), the mucus relaxes…” (Girod et al., 1992). Mucin content in mucus typically accounts for 2–5% of mucus, with MUC5AC and MUC5AB being the most abundant mucins in airway mucus, whereas water accounts for between 90–98% of mucus mass. Increased mucin production, differential mucin glycosylation or a change in the proportions of the various mucins as is seen in many pulmonary diseases (e.g. cystic fibrosis, asthma, COPD) can therefore increase mucus viscosity. Water availability in and ionic composition of the immediate environment also influence the physical properties of mucus (Hill et al., 2018; Thornton et al., 2008). For example, there is a 5- to 10-fold greater mucin-to-water ratio in patients with cystic fibrosis than in healthy subjects. This results from the CFTR defect-induced inadequate airway hydration and imbalances in ASL ion concentrations that lead to increased mucus viscosity, causes mucus impaction to the consistency of rubber and hence hampers effective mucociliary transport (Fahy and Dickey, 2010; Gheber et al., 1998; Lai et al., 2009).

There is no standardized method to determine increased mucus viscosity. In their recent review “Strategies for measuring airway mucus and mucins”, Atanasova and Reznikov as well as Chen and colleagues describe the most widely applied methods for collection and analysis of mucus as well as the associated challenges (Atanasova and Reznikov, 2019; Chen et al., 2019).  Because both physical and chemical properties of mucus are dependent on its composition, many studies examine mucin content of or the contributions of the various mucin proteins to a given mucus sample, by using for example chromatography (historically) and, more recently, mass spectrometry (Atanasova and Reznikov, 2019). The latter not only permits the distinction between the different mucin proteins and their abundances but also facilitates the analysis of glycosylation patterns (Jensen et al., 2010; Mulagapati et al., 2017). This may provide useful insights into mucus viscoelastic properties, because mucin backbone O-glycosylation was linked to higher molecular rigidity, extended conformation and increased hydration (Gum, 1992; Verdugo, 2012). Simpler methods that do not require mucus or sputum collection include imaging after staining of histological specimens with special stains, such as Alcian Blue and Periodic Acid–Schiff, lectins and mucin antibodies and subsequent quantitative image analysis (Atanasova and Reznikov, 2019). This approach can be applied to both in vitro systems (e.g. 3D organotypic airway cultures) and ex vivo tissues.  In addition to focusing on mucins, direct rheometry–still considered the gold standard to determine viscosity and elasticity–is performed to characterize the physical properties of mucus (Atanasova and Reznikov, 2019). There are dynamic and non-dynamic techniques that can be used and, independent of the method chosen, two parameters are normally examined: (i) viscosity or loss modulus (G″), which is the extent to which the gel resists the tendency to flow, and (ii) elasticity or storage modulus (G′), which measures the tendency for the gel to recover its original shape following stress-induced deformation (Girod et al., 1992; Lai et al., 2009). The most common rheometers are the rotational rheometers, which measure the macrorheological properties of a sample. In a rotational rheometer, a continuous shear motion is applied to the material of interest by the relative rotative motion of two surfaces (https://lsinstruments.ch/en/theory/rheology/rheometers; accessed 4 June 2021). However, other types of rheometers, such as a cone-and-plate rheometer or a capillary viscometer can also be used (Chen et al., 2019; Lai et al., 2014). All these techniques draw on the behavior of mucus when subjected to different shear rates, torques, strains or tractions (Atanasova and Reznikov, 2019). The choice will depend on both the availability of the equipment and the sample size. Because rheometry requires rather large sample volumes, more novel techniques utilizing fluorescence imaging have been developed to interrogate mucus viscoelasticity in, for example, in vitro studies. One of these techniques involves the tracking of particle movement, another fluorescence recovery after photobleaching (FRAP) (Lai et al., 2009).  Current knowledge indicates that airway mucus has intermediate viscoelasticity, with a viscosity in the range of 12–15 Pa ∙s, a relaxation time of ca. 40 s and an elastic modulus of 1 Pa (Chen et al., 2019; Lai et al., 2009). In comparison, mucus from cystic fibrosis patients, which exhibits altered mucin glycosylation patterns (Boat et al., 1976) as well as lower water and salt content (Baconnais et al., 1999; Kopito et al., 1973), has a much higher viscosity that can reach up to 110 Pa ∙s at shear rates of 0.1 s⁻¹ (Carlson et al., 2018; Rose and Voynow, 2006; Rubin, 2007).  

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