Abstract
The enormous bacterial load and mechanical forces in colon create a special requirement for protection of the epithelium. In the distal colon, this problem is largely solved by separation of the bacteria from the epithelium by a firmly attached inner mucus layer. In addition, an outer mucus layer entraps bacteria to be cleared by distal transport. The mucus layers contain a network of Muc2 mucins as the main structural component. Here, the renewal rate of the inner protective mucus layer was studied as well as the production and secretion of Muc2 mucin in the distal colon. This was performed by intraperitoneal injection of N-azidoacetyl-galactosamine (GalNAz) that was in vivo incorporated during biosynthesis of O-glycosylated glycoproteins. The only gel-forming mucin produced in the colon is the Muc2 mucin and as it carries numerous O-glycans, the granulae of the goblet cells producing Muc2 mucin were intensely stained. The GalNAz-labeled glycoproteins were first observed in the Golgi apparatus of most cells. Goblet cells in the luminal surface epithelium had the fastest biosynthesis of Muc2 and secreted material already three hours after labeling. This secreted GalNAz-labeled Muc2 mucin formed the inner mucus layer. The goblet cells along the crypt epithelium accumulated labeled mucin vesicles for a longer period and secretion of labeled Muc2 mucin was first observed after 6 to 8 h. This study reveals a fast turnover (1 h) of the inner mucus layer in the distal colon mediated by goblet cells of the luminal surface epithelium.
Highlights
Mucus, the protective cover of all epithelial surfaces, has gelforming mucins as its major structural component
The N-acetylgalactosamine (GalNAc) analogue was first, as expected, detected within the Golgi apparatus throughout the crypt where it is being used as a substrate by the peptidyl-GalNAc transferases to produce O-linked glycoproteins (Figure 1A, 1 h)
The goblet cells located in the luminal surface epithelium were intensely stained after 2 h and this staining could be observed during the following 3 h
Summary
The protective cover of all epithelial surfaces, has gelforming mucins as its major structural component. The mucus layer covering the intestinal epithelium has the MUC2 mucin as the core molecule in this structural network. Dimerization occurs via the C-terminal ends prior to translocation into the Golgi apparatus [1,2] where the O-linked glycans are attached to the abundant serine and threonine residues making up the two central mucin domains. After release and volume expansion, the MUC2 mucin replenishes the luminal mucus. The mucus protects and forms an inner dense mucus layer in the colon that separates the epithelium from the bacteria [4]. On the luminal side of this inner mucus layer, the expanded soluble outer mucus forms the habitat for the colonic bacteria [4,5]. One general function of the intestinal mucus is to trap microorganisms and to facilitate their distal transport
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