Cell adhesion molecule CEACAM7
apical plasma membrane
Four carcinoembryonic antigen subfamily members, CEA, NCA, BGP and CGM2, selectively expressed in the normal human colonic epithelium, are integral components of the fuzzy coat.
To elucidate which of the seven transcriptionally active genes of the carcinoembryonic antigen (CEA) subfamily are expressed in human colon, we first examined mRNA expression using reverse transcriptase PCR. The result showed the CEA, nonspecific crossreacting antigen 50/90 (NCA), biliary glycoprotein (BGP), and carcinoembryonic antigen gene family member 2 (CGM2) mRNAs were expressed in the colon. To determine the cellular sources of these members within normal colonic mucosa, in situ hybridization and immunocytochemistry were then performed. CEA and NCA mRNAs were clearly detectable in the cytoplasm of columnar and goblet cells at the free luminal surface and the upper crypts with low hybridization in the mid crypt and the crypt base. In contrast, BGP and CGM2 mRNAs were restricted only to columnar cells at the upper third of the crypts and the luminal surface. Colon epithelium expression of CEA, NCA, BGP and CGM2 coincided with that of corresponding mRNAs. Ultrastructurally, CEA, NCA, BGP and CGM2 were localized mainly to the apical surface glycocalyx, the fuzzy coat, of columnar cells. Interestingly, these molecules were localized in different microdomains within the fuzzy coat. Furthermore, BGP was highly expressed in the fuzzy coat of cryptal caveolated cells. As integral components of the fuzzy coat, CEA, NCA, BGP and CGM2 can hardly function as intercellular adhesion molecules; they possibly play an important role in epithelial-microbial interactions.