Biochemical differentiation of the plasma membrane of the intestinal epithelial cell.

Abstract

The brush border and basolateral regions of the surface of the intestinal epithelial cell (enterocyte) are exposed to environments which differ even more than those which bathe the different functional regions of the surface of the hepatocyte or the renal tubular cell, and therefore the biochemical differentiation of the enterocyte plasma membrane is also more striking. In addition, it has been easier to study, since the highly adapted brush-border membrane of the enterocyte is one of the easiest of plasma membranes to isolate pure and in quantity (see Isselbacher, 1974). From the study of brush-border preparations a picture has emerged of a membrane with a variety of terminal digestive hydrolases (especially aminopeptidase, disaccharidases and alkaline phosphatase) attached to its luminal surface (Louvard et al., 1975). These are large proteins (Maestracci et al., 1975), which are heavily glycosylated (Kelly & Alpers, 1973; Weiser, 1973; Quaroni et al., 1974) and are moored to the membrane by relatively small hydrophobic polypeptide segments, probably at their N-terminal ends (Maroux & Louvard, 1976). Some of the end-products of digestion that these enzymes generate are then passed across the membrane by transport systems which are closely coupled to the hydrolases and are Na+-independent (Ramaswamy et al., 1976). Some enter the cell via one of several electrogenically driven Na+/solute symport systems (Crane, 1965,1974; Schultz & Curran, 1970; Berger et a[., 1972; Murer & Hopfer, 1974; Hopfer et al., 1973, 1976; Sigrist-Nelson et al., 1976; Alvarado, 1976) and others are taken into the cell by specific transport systems that are independent of Na+ (e.g. fructose; Sigrist-Nelson & Hopfer, 1974). At the cytoplasmic surface of the brush-border membrane, and attached to it by aactinin bridges, is an actin array which forms the microvillus core that also interacts with myosin-like proteins in the terminal web (Mooseker & Tilney, 1975). This raises the exciting, but still largely unexplored, possibility that the brush border is motile (Thuneberg & Rostegaard, 1969; Sandstrom, 1971; Mooseker, 1974) and that this somehow facilitates its digestive and absorptive function, perhaps by preventing the formation of unstirred layers of fluids at either surface of the brush-border membrane. The remainder of the enterocyte plasma membrane (the basolateral region) is separated from the brush border by the terminal bar. Thus, in contrast with the brush border, it interacts with the protective and controlled internal environment of the body and might be expected to be much more similar to the plasma membranes of other cells. It is this region of the plasma membrane which plays the key role in such processes as generating * Present address: Department of Biochemistry, University of Bristol, Medical School, Bristol BS8 1TD. U.K.