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Relationship: 2444
Title
ASL Height, Decreased leads to Mucus Viscosity, Increased
Upstream event
Downstream event
Key Event Relationship Overview
AOPs Referencing Relationship
Taxonomic Applicability
Sex Applicability
Sex | Evidence |
---|---|
Mixed | Moderate |
Life Stage Applicability
Term | Evidence |
---|---|
All life stages | Low |
Key Event Relationship Description
The phenomenon of ASL volume changes determining mucus viscosity is well described in the cystic fibrosis literature. In patients with this genetic defect, impaired CFTR function results in ASL depletion and mucus hyperviscosity. Mechanistically, the imbalance of Cl– and HCO3– secretion and increased Na+ absorption by the airway epithelium results in dehydration of airway mucus, making it more viscous and adhesive (Knowles and Boucher, 2002; Mall et al., 2004; Puchelle et al., 2002; Tarran, 2004). Studies with transgenic mice overexpressing βENaC in the airways corroborate the link between ASL hydration and mucociliary impairment as evidenced by the increased incidence of airway mucus plugging (Mall et al., 2004; Mall, 2008). Increased mucus viscosity may also play a significant role in asthma and chronic bronchitis, although the mechanisms are less well explored. In a ferret model of cigarette smoke-induced COPD, Lin et al. identified ASL depletion as one of the drivers of increased mucus viscosity and decreased MCC (Lin et al., 2020). The authors also showed that mucus from COPD patients, obtained from 3D organotypic airway epithelial cultures from different smoking donors with COPD, was significantly more viscous than that of healthy, non-smoking individuals and smokers without disease (Lin et al., 2020). Considering the known effects of cigarette smoke exposure on the ASL height (Hassan et al., 2014; Lambert et al., 2014; Raju et al., 2016; Schmid et al., 2015; Xu et al., 2015), this links decreased ASL height to increased mucus viscosity in the context of chronic bronchitis.
Evidence Collection Strategy
Evidence Supporting this KER
In patients with cystic fibrosis, impaired CFTR function results in ASL depletion and mucus hyperviscosity (Knowles and Boucher, 2002; Puchelle et al., 2002; Mall et al., 2004; Tarran, 2004). This has been confirmed experimentally in pig and rat models of this disease (Birket et al., 2014; Birket et al., 2016; Birket et al., 2018). Studies with transgenic mice overexpressing βENaC in the airways also corroborate the link between ASL dehydration and increased mucus viscosity, evidenced by the increased incidence of airway mucus plugging [129, 195]. In a ferret model of cigarette smoke-induced COPD, ASL depletion was shown to be one of the drivers of increased mucus viscosity and decreased MCC (Lin et al., 2020). The same study also showed that mucus from COPD patients, obtained from 3D organotypic airway epithelial cultures from different smoking donors with COPD, is significantly more viscous than that from healthy, non-smoking individuals and smokers without disease (Lin et al., 2020).
Biological Plausibility
There are only few studies that report ASL height and mucus viscosity, and although studies on cystic fibrosis in animal models or human cell cultures show the dependencies between these two KEs, the causal evidence is sparse. However, because the underlying mechanism is well-described and translatable across different species and is amenable to positive modulation by e.g. CFTR drugs, we consider the biological plausibility of this KER to be moderate.
Empirical Evidence
ASL height was inversely related to mucociliary transport (MCT) rate, an outcome of particle tracking measurements and hence an indicator of mucus viscosity, in tracheas of CFTR-deficient piglets. Complementary studies in airway cell monolayer cultures also identified elevated effective viscosity of the ASL in cystic fibrosis compared to non-cystic fibrosis cultures (Birket et al., 2014).
ASL height was decreased and mucus viscosity was increased in human airway epithelial cultures from cystic fibrosis donors grown at the air-liquid interface (Birket et al., 2014). Treatments with CFTR function-restoring drugs improve both ASL height and mucus viscosity (Birket et al., 2016).
Deficiency of Cftr in rats results in decreased ASL height and increased mucus viscosity compared to their wild-type littermates (Birket et al., 2018).
Exposure of rats to nebulized N-acetylcysteine, assumed to function as a mucolytic (i.e., to decrease mucus viscosity), for 20 or 90 min increased the viscoelastic properties of mucus and decreased mucus transport speed (Lorenzi et al., 1992).
Computational modeling of the MCT process indicated that mucus velocity is not only dependent on the cilia beat frequency and the numbers of cilia, but also on ASL height (Jayathilake et al., 2015; Lee et al., 2011).
Uncertainties and Inconsistencies
At least one study in primary cultures of human bronchial epithelial cells grown at the air-liquid interface found mucus viscosity to be higher in cultures from cystic fibrosis donors compared to those from healthy donors, but did not observe differences in ASL height (Derichs et al., 2011). In asthmatics, airway mucus is very viscous, and its viscosity increases even more in acute exacerbations (Innes et al., 2009). However, unlike in individuals with cystic fibrosis or chronic bronchitis, changes in ASL height because of impaired anion transport might not be the primary cause for increased mucus viscosity in patients with asthma. Instead a more acidic ASL may lead to improper unpacking of secreted mucins and tethering of mucins to the epithelial surface, where they form protein tangles that result in mucus airway plugs (Abdullah et al., 2017; Bonser and Erle, 2017; Bonser et al., 2016; Evans et al., 2015; Shimura et al., 1988; Tang et al., 2016). Alternatively, the composition of mucus may be altered by production of more acidic mucins thereby skewing the pH balance of mucus itself (Gearhart and Schlesinger, 1989; Holma, 1989; Kim et al., 2014). In addition, mucus viscosity is also affected by the presence of DNA, lipids and proteins other than mucins, such as lactoferrin, albumin and immunoglobulins, all of which may be present in higher amounts in the presence of airway inflammation (Puchelle et al., 2002; Rogers, 2004).
Known modulating factors
Unknown
Quantitative Understanding of the Linkage
There are a small number of studies reporting on ASL and mucus viscosity that provide insights into the quantitative relationship between these two KEs. Some of these studies use pharmacological agents with well described effects on ASL height and mucus viscosity. However, most studies evaluated the KEs in parallel, without interrogating dose responses and/or time responses. Therefore, we only have a rudimentary quantitative understanding of this KER, leading us to judge this as weak.
Response-response Relationship
In primary human bronchial epithelial cultures grown at the air-liquid interface, the ASL depth of cystic fibrosis cells was significantly lower than that of non-cystic fibrosis cells (2.4±0.6 µm vs. 6.7±0.2 µm) and mucus viscosity was significantly higher (80.6±26.5 cP vs. 12.0±3.6 mm/min) by particle-tracking microrheology. FRAP indicated an increased half-life of fluorescence recovery (evidence of increased viscosity) of approx. 16 s in cystic fibrosis cultures vs ca. 11 s in non-cystic fibrosis cultures (Birket et al., 2014).
ASL height in the tracheas of -/- CFTR piglets was significantly lower than in wild-type littermates (3.2±0.8 µm vs. 6.5±0.2 µm) and mucus viscosity was significantly higher by FRAP, which indicated an increased half-life of approx. 13 s in CFTR-deficient tracheas vs ca. 9.5 s in wild-type controls (Birket et al., 2014). Treatment of normal adult pig trachea with 100 µM bumetanide to block HCO3– transport reduced ASL height from 7.8±0.5 µm (untreated) to 6.4±0.4 µm and mucus viscosity from approx. 600 cP (untreated) to 2000 cP at frequencies between 0.5 and 10 Hz (Birket et al., 2014).
In tracheas of Cftr-deficient rats, ASL depths were diminished in both basal and stimulated conditions, from weaning until at least 6 months of age (WT 1 month 22.9 ± 4.6 μm, 6 months 43.6 ± 12.5 μm vs. KO 1 month 6.9 ± 0.7 μm, 6 month 19.5 ± 4.8 μm). Under baseline conditions at 1 month of age, effective viscosity was no different in KO tracheae compared with WT tracheae (1.76 ± 1.0 cP WT vs. 1.90 ± 1.7 cP KO). At 3 months, effective viscosity of KO airway mucus increased slightly but was still no different than that of WT airway mucus (5.12 ± 1.0 cP WT vs. 10.72 ± 2.7 cP KO). By 6 months of age, KO tracheal mucus was 20-fold more viscous compared with WT littermates (2.91 ± 0.9 cP WT vs. 65.09 ± 3.6 cP KO) (Birket et al., 2018).
Treatment of primary bronchial epithelial cell monolayer cultures from G551D/F508del cystic fibrosis patients with ivacaftor, a CFTR potentiator, at concentrations ≥ 100 nM for 24 h increased ASL height from ca. 6 to 17.5 µm, which was similar to the ASL height of non-cystic fibrosis cultures (18.03 ± 1.6 µm). The half-life of time to recovery measured by FRAP shortened from 12.39 ± 1.3 to 7.57 ± 0.8 s, indicative of decreased mucus viscosity. Effective viscosity of cells treated with ivacaftor (600 cP) was significantly lower than control (2,600 cP) at the physiological frequency of 0.9 Hz (Birket et al., 2016).
Treatment of primary bronchial epithelial cell monolayer cultures from homozygous F508del cystic fibrosis patients with the combination of 10 µM ivacaftor and 3 µM C18 (a ivacaftor homolog) with 20 µM forskolin significantly increased ASL height (23.4 ± 2.6 vs. 9.01 ± 1.4 µm C18 + forskolin, 10.99 ± 1.7 µm ivacaftor + forskolin; 13.03 ± 2.8 µm forskolin alone, and 12.53 ± 2.3 µm vehicle). Effective mucus viscosity effective in situ was reduced from ca. 1200 cP (vehicle) to ca. 100 cP by 48-h treatment only with the combination of ivacaftor and C18 (Birket et al., 2016).
Time-scale
No data
Known Feedforward/Feedback loops influencing this KER
Unknown
Domain of Applicability
References
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