LAUSR

RATIONALE MUC5AC and MUC5B are the predominant secreted polymeric mucins in mammalian airways. They contribute differently to the pathogenesis of various muco-obstructive and interstitial lung diseases, and their genes are separately regulated, but whether they are packaged together or in separate secretory granules is not known. OBJECTIVES To determine the packaging of MUC5AC and MUC5B within individual secretory granules in mouse and human airways under varying conditions of inflammation and along the proximal-distal axis. METHODS Lung tissue was obtained from mice stimulated to upregulate mucin production by the cytokines IL-1β and IL-13 or by porcine pancreatic elastase. Human lung tissue was obtained from donated normal lungs, biopsies of transplanted lungs, and explanted COPD lungs. MUC5AC and MUC5B were labelled with antibodies from different animal species or, in mice only, by transgenic chimeric mucin-fluorescent proteins, and imaged by widefield deconvolution or Airyscan fluorescence microscopy. MEASUREMENTS AND MAIN RESULTS In both mouse and human airways, most secretory granules contained both mucins interdigitating within the granules. Smaller numbers of granules contained MUC5B alone, and even fewer contained MUC5AC alone. CONCLUSIONS MUC5AC and MUC5B are variably stored both in the same and in separate secretory granules of both mice and humans. The high fraction of granules containing both mucins under a variety of conditions makes it unlikely that their secretion can be differentially controlled as a therapeutic strategy. This work also advances knowledge of the packaging of mucins within secretory granules to understand mechanisms of epithelial stress in the pathogenesis of chronic lung diseases.