Intracellular Processing of Human Secreted Polymeric Airway Mucins

Mucociliary clearance is a crucial component of innate defense of the lung. In respiratory diseases, such as asthma, chronic obstructive pulmonary disease, and cystic fibrosis, mucus with abnormal properties contributes to obstruction of the airways. The failure in function of mucus in airway clearance and pathogen protection leads to chronic infection and risk of death. Polymeric mucins (MUC5AC and MUC5B) provide the structural framework of the airway mucus gel. The intracellular synthesis and assembly of these enormous, polymeric O-linked glycoproteins is a complex, multistage process involving intra- and intermolecular disulfide bond formation and extensive addition of O-glycan chains. The fully formed polymers are packaged in a highly organized and condensed form within secretory granules inside specialized secretory cells, and after the appropriate stimulus, mucins are released and expand to form mucus. This short article brings together the current knowledge on the different steps in the production of mucin polymers and the molecular mechanisms that condense them into a packaged form in secretory granules. It is by unraveling the molecular mechanisms that control intracellular mucin supramolecular structure that we might gain new insight into what determines mucus gel properties in health and disease.