STRUCTURE, FUNCTION AND ASSEMBLY OF MUCINS IN THE GASTROINTESTINAL TRACT IN RELATION THE INFLAMMATORY BOWEL DISEASE ULCERATIVE COLITIS

The group is studying the structure and function of mucins (mucus glycoproteins). This includes most aspects of mucins including also their glycan parts. A special focus is on the gastrointestinal tract and the role of mucins in the protection of the intestine and as part of the intestinal innate immune system. This also includes mucins in relation to cystic fibrosis and colon cancer development.

Mucins are extracellular large highly glycosylated molecules having mucin domains. Usually one also demands that there is more than 50% of the mass that is due to glycans. Mucin or PTS domains are found also in other extracellular proteins, but are dominating in molecules that are usually called mucins. Mucin domains are rich in the amino acids threonine, serine and proline, where the oligosaccharides are linked via N-acetylgalactosamine to the hydroxy amino acids. There are two types of mucins, secreted and membrane bound. Some of the secreted mucins are gel-forming due to their polymeric nature.

The major mucin of the gastrointestinal tract is called MUC2 and is produced by the intestinal goblet cells. The apoprotein of this mucin has two central mucin domains and cysteine-rich domains at both the N- and C-terminal ends. The primary translational product is about 600,000 kDa in mass and is very quickly dimerized in the endoplasmic reticulum of the cell. This dimer is formed in the C-terminus.The addition of O-glycans starts when the dimer enters the Golgi apparatus and when fully glycosylated the mass will increase to about 5 million Da. The MUC2 mucin is forming polymers in the late Golgi - TGN compartment. This involves two different processes. One is the formation of disulfide bonds between the N-termini and the other is the formation of another not yet characterized type of covalent linkage. Thus the MUC2 mucin is forming enormous net-like covalent polymers.

Fig. 1.
The MUC2 mucin build up two mucus layers in colon. A. Domain structure of the MUC2 mucin. B. Assembled MUC2 showing. C. MUC2 stained green in colon. C1. Attached MUC2 to cells. C2. Firm stratified inner MUC2 layer. C3. Outer loose MUC2 layer. D. Composite of MUC2 (green) and bacteria (red). E. Bacteria not found in the inner firm mucus layer, only in the outer mucus layer.


All human beings have more than two pounds of bacteria in their large intestine and the number of bacteria outnumbers the total number of cells in the body by a factor of 10. How we can live with all these bacteria without deleterious effects or diseases have not been understood until we recently have shown (Proc. Natl. Acad. Sci. USA) that the inner of the two mucus layers in colon is a barrier that protects us from the large number of bacteria in our intestine. In the absence of Muc2, the molecule that builds these two mucus layers allows the bacteria can reach into the epithelial cells. These animals got an inflammation and later on colon cancer, a scenario that is similar to the human disease ulcerative colitis. During our studies of the mucus layers, we found a number of other molecules that probably are vital for the correct function and properties of the mucus. We also study the components formed together with the MUC2 mucin as well as its structure, modifications and properties.
The transmembrane mucins are probably acting as signaling molecules. We are addressing such functions in the intestinal tract with a special interest in the role of so called PDZ protein interactions.

The glycan parts of mucins are analyzed after release of the oligosaccharides. Recent developments of methods and equipment has allowed the analysis of O-glycosylation from very small amounts of material, exemplified by the analysis of specific mucin glycosylation from mm-sized biopsies of human colon. In several systems, parasite infection and cystic fibrosis studies in mice and human bronchial cultures it has been shown that mucin glycosylation is very dynamic and can undergo transient alterations due to the expression of glycosyltransferases. We will now study the antibacterial effects of the mucus and how the mucus layers are built up by the use of biochemical methods and the use of various types of gene knock-out animals that are colonized or germ-free. By studies on the growth of the mucus layers from biopsies in vitro we have the possibility to both characterize alterations in ulcerative colitis and to manipulate the mucus properties by adding recombinant mucus proteins and pharmacological agents. Expected results are novel ways to improve the protection of colon and by this treat ulcerative colitis and other inflammation related diseases. An understanding of mucins and the alteration of their properties will be very important for an understanding of ulcerative colitis of the gastrointestinal tract, but also have implications for other diseases like cystic fibrosis of the respiratory tract.