Symposium Summaries

Mucociliary clearance is an essential innate defense of the airway, as shown by loss-of-function experiments involving MUC5B, CFTR, and axonemal genes. Effective mucociliary clearance requires mucin production and secretion, mucus hydration and expansion, and forceful and coordinated ciliary beating. This talk focuses on mucin production and secretion, starting with a brief summary of mucin production, then moving to the role of mucin secretion in CF pathogenesis. Two major polymeric secreted mucins are produced in the airways – MUC5AC and MUC5B. MUC5B expression is widespread throughout the airways, found both in submucosal gland mucous cells and surface epithelial secretory cells. In surface cells, expression is strongest proximally and gradually tapers off by the terminal bronchiole. The expression of MUC5B is mostly constitutive at a cellular level, though overall production is increased in CF by submucosal gland hypertrophy and hyperplasia. The importance of MUC5B expression in microbial defense is indicated by the death from bacterial infection of mice in which the gene has been deleted, the dose-dependency of mucociliary clearance on MUC5B expression, and the high prevalence of a gain-offunction allele that causes MUC5B overexpression despite the fact that it increases the risk of idiopathic pulmonary fibrosis 6-fold, suggesting strong positive selection. MUC5AC is expressed at baseline in proximal surface epithelial secretory cells of humans, and its expression can be dramatically upregulated by T2 inflammation in surface epithelium throughout the airways in diseases such as asthma. Its primary function appears to be in defense against helminths, and it also functions in fungal and viral defense. In CF pathogenesis, the central importance of impaired mucus hydration and expansion due to the underlying genetic defect in anion transport is clear. However increasing evidence indicates that the sudden release of stored intracellular mucins interacts with the deficiency in anion transport to result in mucus adhesion to the airway wall and mucus obstruction of the airway lumen. In the CF pig model, mucociliary clearance was normal at baseline, but the addition of a cholinergic agonist to stimulate mucin secretion from submucosal glands resulted in the emergence of mucus strands that remained anchored at the gland necks. These strands often became entangled with one another and impeded the clearance of instilled particles. In a mouse allergic asthma model, exposure to a cholinergic agonist resulted in the sudden release of hyperproduced MUC5AC to cause widespread airway lumen occlusion and, together with smooth muscle contraction, impede airflow. The role of mucin release from surface epithelial cells in CF is less clear than it is in asthma. However it is plausible that MUC5AC is hyperproduced by surface epithelial cells in response to the presence of fungi and bacteria that accumulate due to the defect in microbial clearance, and that sudden release of the large intracellular stores leads to small airway obstruction as it does in asthma. Mucins are secreted from both submucosal gland mucous cells and surface epithelial cells at either a low baseline rate or a high stimulated rate. The molecular mechanism of mucin exocytosis has been better studied in surface secretory cells than gland mucous cells. In surface cells, both baseline and stimulated secretion occur via a regulated mechanism as indicated by the requirement for Munc13-2, which responds to the second messengers calcium and diacylglycerol. These second messengers are generated in response to extracellular agonists such as ATP, adenosine, and acetylcholine that are present at low levels at baseline and at high levels in pathologic conditions. The rate of stimulated mucin secretion in surface cells exceeds the rate of baseline secretion thousands-fold. Mucin secretion is mediated by a molecular machinery that involves a core SNARE complex, a Munc18 scaffold, and the regulatory proteins Munc13-2 and synaptotagmin-2 (see Figure). Some components of the exocytic machinery are conserved between baseline and stimulated secretion, whereas others differ. The requirement for a subset of distinct exocytic proteins in stimulated secretion suggests that these proteins might be targeted therapeutically to inhibit pathologic stimulated secretion while preserving homeostatic baseline secretion. Another underappreciated role of mucin secretion in CF is in the use of aerosolized hypertonic saline solution. Hypertonic solutions in the range of 500 mM are routinely used to induce maximal secretion in experimental cellular systems. The molarity of 7% sodium chloride is approximately 1200 mM. We hypothesized that part of the therapeutic benefit of treatment with 7% saline solution is the induction of mucin degranulation into a well-hydrated airway lumen to form mucus with normal viscoelasticity. We additionally hypothesized that the consequent depletion of intracellular mucin stores