Normal mucus formation requires cAMP‐dependent HCO ₃ ⁻ secretion and Ca ²⁺ ‐mediated mucin exocytosis

Key points HCO 3 − is required for gel‐forming mucins to form normal mucus, but how? Two apparently separate signalling pathways are activated concurrently to bring mucus formation to completion: a Ca 2+ ‐mediated pathway mainly directs goblet cell exocytosis, and an independent cAMP‐mediated pathway stimulates HCO 3 − secretion to help discharge exocytosed mucus. cAMP‐dependent HCO 3 − secretion fails, disrupting the normal formation and discharge of mucins in cystic fibrosis (CF) leading to pathologically viscous and tenacious mucus in affected organs. This work advances our understanding of the role of cAMP (CFTR)‐dependent HCO 3 − secretion in forming normal mucus and underscores a new importance of addressing the defect in HCO 3 − secretion as a critical new therapeutic target in CF. Abstract Evidence from the pathology in cystic fibrosis (CF) and recent results in vitro indicate that HCO 3 − is required for gel‐forming mucins to form the mucus that protects epithelial surfaces. Mucus formation and release is a complex process that begins with an initial intracellular phase of synthesis, packaging and apical granule exocytosis that is followed by an extracellular phase of mucin swelling, transport and discharge into a lumen. Exactly where HCO 3 − becomes crucial in these processes is unknown, but we observed that in the presence of HCO 3 − , stimulating dissected segments of native mouse intestine with 5‐hydroxytryptamine (5–HT) and prostaglandin E 2 (PGE 2 ) induced goblet cell exocytosis followed by normal mucin discharge in wild‐type (WT) intestines. CF intestines that inherently lack cystic fibrosis transmembrane conductance regulator (CFTR)‐dependent HCO 3 − secretion also demonstrated apparently normal goblet cell exocytosis, but in contrast, this was not followed by similar mucin discharge. Moreover, we found that even in the presence of HCO 3 − , when WT intestines were stimulated only with a Ca 2+ ‐mediated agonist (carbachol), exocytosis was followed by poor discharge as with CF intestines. However, when the Ca 2+ ‐mediated agonist was combined with a cAMP‐mediated agonist (isoproterenol (isoprenaline) or vasoactive intestinal peptide) in the presence of HCO 3 − both normal exocytosis and normal discharge was observed. These results indicate that normal mucus formation requires concurrent activation of a Ca 2+ ‐mediated exocytosis of mucin granules and an independent cAMP‐mediated, CFTR‐dependent, HCO 3 − secretion that appears to mainly enhance the extracellular phases of mucus excretion.