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
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.
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
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.