Mucous granule exocytosis and CFTR expression in gallbladder epithelium

A mechanistic model of mucous granule exocytosis by columnar epithelial cells must take into account the unique physical-chemical properties of mucin glycoproteins and the resultant mucus gel. In particular, any model must explain the intracellular packaging and the kinetics of release of these large, heavily charged species. We studied mucous granule exocytosis in gallbladder epithelium, a model system for mucus secretion by columnar epithelial cells. Mucous granules released mucus by merocrine exocytosis in mouse gallbladder epithelium when examined by transmission electron microscopy. Spherules of secreted mucus larger than intracellular granules were noted on scanning electron microscopy. Electron probe microanalysis demonstrated increased calcium concentrations within mucous granules. Immunofluorescence microscopic studies revealed intracellular colocalization of mucins and the cystic fibrosis transmembrane conductance regulator (CFTR). Confocal laser immunofluorescence microscopy confirmed colocalization. These observations suggest that calcium in mucous secretory granules provides cationic shielding to keep mucus tightly packed. The data also suggests CFTR chloride channels are present in granule membranes. These observations support a model in which influx of chloride ions into the granule disrupts cationic shielding, leading to rapid swelling, exocytosis and hydration of mucus. Such a model explains the physical-chemical mechanisms involved in mucous granule exocytosis.