Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis

The filamentous brush border glycocalyx forming the “enteric surface coat” of the intestinal epithelium is composed in rabbits of a 400 kDa mucin-type glycoprotein, which was purified using the 3A4 monoclonal antibody. This monoclonal antibody recognizes a filamentous brush border glycocalyx-specific glycosidic structure containing an O-acetylated sialic acid, which is absent from all the other glycoproteins in the epithelium, with the exception of certain goblet cell mucins. Here we establish that only 50% of the rabbits tested synthesized this glycosidic structure. Upon immunolabeling surface epithelia and sections of jejunum from these rabbits, the carbohydrate epitope rec ognized by the 3A4 mAb was found to be present on the fil amentous brush border glycocalyx of a variable number of enterocytes, which were patchily distributed over all the villi. This heterogeneous expression of 3A4 antigenicity, which was also observed in the crypts, suggests the existence of differences between the patterns of differen tiation of enterocytes, which results in the expression of different pools of glycosyltransferases and/or acetyl trans ferases. In mature enterocytes, the 3A4 determinants were present only on the filamentous brush border glycocalyx, which is anchored solely to the membrane microdomain at the tip of brush border microvilli. However, expression of 3A4 antigenicity begins in the median third of crypts, in enterocytes with a short, thin brush border devoid of apical filamentous brush border glycocalyx. Here the 3A4 epitopes were present over the whole brush border membrane. A few cells higher up, the hyperpolarized expression of filamentous brush border glycocalyx, i.e. its segregation at the tip of the microvilli, began and both the apical filamentous brush border glycocalyx and the whole brush border membrane were labeled with the 3A4 mAb. The labeling of the lateral membrane of microvilli gradually decreased and then disappeared during the migration of the cells to the upper part of the crypts. The 3A4 structure was also detected in some of the granules of some rare goblet cells in the vicinity of the most deeply located labeled enterocytes, whereas no labeled goblet cells were ever observed at higher cell locations.