Acidic pH increases airway surface liquid viscosity in cystic fibrosis

Abstract

Cystic fibrosis (CF) disrupts respiratory host defenses, allowing bacterial infection, inflammation, and mucus accumulation to progressively destroy the lungs. Our previous studies revealed that mucus with abnormal behavior impaired mucociliary transport in newborn CF piglets prior to the onset of secondary manifestations. To further investigate mucus abnormalities, here we studied airway surface liquid (ASL) collected from newborn piglets and ASL on cultured airway epithelia. Fluorescence recovery after photobleaching revealed that the viscosity of CF ASL was increased relative to that of non-CF ASL. CF ASL had a reduced pH, which was necessary and sufficient for genotype-dependent viscosity differences. The increased viscosity of CF ASL was not explained by pH-independent changes in HCO3- concentration, altered glycosylation, additional pH-induced disulfide bond formation, increased percentage of nonvolatile material, or increased sulfation. Treating acidic ASL with hypertonic saline or heparin largely reversed the increased viscosity, suggesting that acidic pH influences mucin electrostatic interactions. These findings link loss of cystic fibrosis transmembrane conductance regulator-dependent alkalinization to abnormal CF ASL. In addition, we found that increasing Ca2+ concentrations elevated ASL viscosity, in part, independently of pH. The results suggest that increasing pH, reducing Ca2+ concentration, and/or altering electrostatic interactions in ASL might benefit early CF.

Relationships among CFTR expression, HCO 3 − secretion, and host defense may inform gene- and cell-based cystic fibrosis therapies

“…As previously reported (32,33), CFTR −/− epithelia had a lower ASL pH than CFTR +/+ epithelia (Fig. 3A).…”

“…CF mucus has altered properties that impair mucociliary transport (31). Recent studies showed that CF ASL has an increased viscosity due to a reduced ASL pH (32,35). In epithelia composed of mixtures of CFTR −/− and CFTR +/+ cells, the relationship between the percentage of WT cells and ASL viscosity (τ ASL /τ saline ) approximately paralleled that of ASL pH (Fig.…”

“…The * indicates the difference from epithelia with 100% WT cells by repeated measures ANOVA, P < 0.05. epithelium are WT or overexpress CFTR, Cl − secretion rises to the level of WT epithelia (10)(11)(12)(13)(14). However, knowledge that CFTR conducts HCO 3 − (48), that HCO 3 − is the major pH buffer of ASL (49), and that HCO 3 − secretion plays a key role in airway host defense (31)(32)(33)(34)(35)50) led us to test the relationship between CFTR expression and HCO 3 − secretion. We found that, unlike Cl − secretion, HCO 3 − secretion was directly proportional to CFTR expression.…”

“…By studying newborn CF piglets, we identified at least two airway host defense defects (2). Mucociliary transport is impaired by mucus with abnormal biophysical properties (31,32), and the activity of airway surface liquid (ASL) antimicrobials and synergism between antimicrobials are impaired (33,34). CF ASL has an abnormally low pH, which reduces the activity of ASL antimicrobials and increases the viscosity of ASL.…”

Burkholderia cenocepacia J2315-mediated destruction of Staphylococcus aureus NRS77 biofilms.

“…However, destruction of S. aureus biofilm is not likely occurring due to decreasing the pH. The CF lung, which S. aureus frequently infects, is acidic in comparison to a healthy lung due to the improper transport of bicarbonate ions [117].…”