MUC5B is the prominent mucin in human gallbladder and is also expressed in a subset of colonic goblet cells

To elucidate the roles of human gallbladder mucin (HGBM), such as in gallstone formation and cytoprotection, it is essential to identify HGBM and study its expression. This was performed by metabolic labeling, Western blotting, immunohistochemistry, and RT-PCR. In a large number of individuals, antibodies against purified HGBM and against MUC5B detected a mucin precursor (similar to 470 kDa) in the gallbladder and colon, but not in the small intestine. In the gallbladder, Western blotting using specific anti-MUC5B antibodies showed that this mucin precursor represented an identical mucin, MUC5B. RT-PCR experiments demonstrated a similar tissue distribution pattern of MUC5B mRNA. Immunohistochemistry with anti-HGBM and anti-MUC5B showed staining in gallbladder epithelial cells and colonic goblet cells in the crypt base, but not in the small intestine; double labeling showed that HGBM was located in small granules within goblet cells, colocalizing to MUC2-containing goblet cells. Metabolic labeling demonstrated the secretion of mature MUC5B in the colon. Conclusively, MUC5B is identified as the prominent HGBM and is also expressed and secreted in the colon.