A new role for bicarbonate in mucus formation

Area of Science:

• Biochemistry
• Cell Biology
• Physiology

Background:

• The role of small anions, particularly bicarbonate (HCO3-), in regulating gel-forming mucin properties is poorly understood.
• Abnormalities in HCO3- secretion and mucus in cystic fibrosis (CF) suggest a link between HCO3- and normal mucus formation.
• Previous hypotheses proposed HCO3- sequesters calcium (Ca2+) from mucins, but direct evidence was lacking.

Purpose of the Study:

• To investigate the direct impact of bicarbonate (HCO3-) on the physical properties of gel-forming mucin.
• To determine if HCO3- influences mucin swelling, hydration, and viscosity.
• To explore the mechanism by which HCO3- affects mucin structure and Ca2+ association.

Main Methods:

• Directly measured exocytosed mucin-swelling kinetics from airway cells.
• Quantified mucin diffusivity (inversely related to viscosity) at varying extracellular HCO3- concentrations.
• Assessed Ca2+ levels associated with mucins under different HCO3- conditions.

Main Results:

• Mucin diffusivity significantly increased with rising extracellular HCO3- concentration, showing a ~300% increase at 20 mM.
• HCO3- was shown to reduce free Ca2+ concentration and the amount of Ca2+ bound to mucins.
• HCO3- demonstrated Ca2+ chelating properties, dispersing mucin aggregates, similar to EGTA.

Conclusions:

• Bicarbonate (HCO3-) enhances mucin swelling and hydration by reducing Ca2+ cross-linking, thereby decreasing viscosity and improving transportability.
• The findings suggest that insufficient HCO3- in cystic fibrosis (CF) contributes to aggregated and viscous mucus.
• This study provides fundamental insights into CF pathogenesis and the general mechanisms of gel mucus formation and secretion.