In chronic bronchitis, mucociliary clearance is dysfunctional, especially during disease exacerbation. Chronic bronchitis exacerbations are commonly associated with increased production of reactive oxygen species (ROS) and airway acidification. This project will examine how ROS and intracellular acidification may regulate and/or dysregulate mucociliary function. At least two endogenous and paracrine mediators control key components of the mucociliary transport system: extracellular ATP and hyaluronan fragments. We propose that pannexins, proteins related to but different from connexins, form channels to the outside of cells (called pannexons) and are responsible, at least in part, for the release of apical ATP in the airway epithelium. We hypothesize that early in disease exacerbation, the production of ROS not only increases intracellular calcium concentrations ([Ca2+]i) to stimulate ciliary beat frequency (CBF) and apical ATP release through pannexons but also degrades apical hyaluronan which in turn stimulates CBF and increases airway surface liquid (ASL) volume. Subsequent intracellular acidification, possibly mediated by cytokine-mediated upregulation of an NADPH oxidase activity expressed in the airway (Duox), leads to mucociliary dysfunction by inhibiting ciliary activity and pannexins. This hypothesis will be tested with three specific aims. Specific Aim 1 will test the hypothesis that exogenous ROS activate a signaling cascade that includes an initial [Ca2+]i increase to stimulate CBF, despite a mild and temporary intracellular acidification due to H+ production by Duox;in addition, direct apical hyaluronan degradation activates RHAMM and RON to stimulate CBF and possibly increase ASL volume. Specific Aim 2 will test the hypothesis that pannexins are responsible for releasing ATP to the apical surface of airway epithelial cells where ATP plays an important role in regulating mucociliary functions including CBF and ASL volume. Specific Aim 3 will test the hypothesis that persistent intracellular acidification inhibits mucociliary clearance by a direct action on ciliary beating as well as by preventing ATP release through pannexons. The results of the proposed experiments, using novel and state-of-the-art methods, will provide new and important mechanistic insights into the regulation of mucociliary clearance in chronic bronchitis.