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 c oat' of the intestinal epithelium is composed in rabbits of a 400 kDa mucin-type glycoprotein, which was purified using the 3A4 monoclonal a ntibody, This monoclonal antibody recognizes a filamentous brush borde r glycocalyx-specific glycosidic structure containing an O-acetylated sialic acid, which is absent from all the other glycoproteins in the e pithelium, with the exception of certain goblet cell mucins, Here we e stablish that only 50% of the rabbits tested synthesized this glycosid ic structure, Upon immunolabeling surface epithelia and sections of je junum from these rabbits, the carbohydrate epitope recognized by the 3 A4 mAb was found to be present on the filamentous brush border glycoca lyx of a variable number of enterocytes, which were patchily distribut ed over all the villi, This heterogeneous expression of 3A4 antigenici ty, which was also observed in the crypts, suggests the existence of d ifferences between the patterns of differentiation of enterocytes, whi ch results in the expression of different pools of glycosyltransferase s and/or acetyl transferases, In mature enterocytes, the 3A4 determina nts were present only on the filamentous brush border glycocalyx, whic h is anchored solely to the membrane microdomain at the tip of brush b order microvilli, However, expression of 3A4 antigenicity begins in th e median third of crypts, in enterocytes with a short, thin brush bord er devoid of apical filamentous brush border glycocalyx, Here the 3A4 epitopes were present over the whole brush border membrane, A few cell s 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 t he lateral membrane of microvilli gradually decreased and then disappe ared 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 e nterocytes, whereas no labeled goblet cells were ever observed at high er cell locations.