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 g allstone formation and cytoprotection, it is essential to identify HGB M 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 det ected a mucin precursor (similar to 470 kDa) in the gallbladder and co lon, but not in the small intestine. In the gallbladder, Western blott ing using specific anti-MUC5B antibodies showed that this mucin precur sor represented an identical mucin, MUC5B. RT-PCR experiments demonstr ated a similar tissue distribution pattern of MUC5B mRNA. Immunohistoc hemistry with anti-HGBM and anti-MUC5B showed staining in gallbladder epithelial cells and colonic goblet cells in the crypt base, but not i n the small intestine; double labeling showed that HGBM was located in small granules within goblet cells, colocalizing to MUC2-containing g oblet cells. Metabolic labeling demonstrated the secretion of mature M UC5B in the colon. Conclusively, MUC5B is identified as the prominent HGBM and is also expressed and secreted in the colon.