PNAS: changes in intestinal mucus may trigger ulcerative colitis

Ulcerative colitis is the most common type of inflammatory bowel disease characterized by
chronic ulceration and inflammation of the colon and rectum.
Symptoms can be lifelong, ranging from mild to life-threatening
.
There is no known treatment for
this disease.

Researchers suspect that immune system failure, in which immune cells attack colonic epithelial cells, could explain the recurrent inflammation
characteristic of the disease.
However, what makes the immune system attack healthy cells in the first place remains a mystery
.

A study published in the Proceedings of the National Academy of Sciences suggests a range of conditions
that could serve as a starting point for the development of ulcerative colitis.
The researchers combined experimental data from patient biopsies, mouse models, and cell lines to suggest that the disease may be caused
by elevated levels of α-1-6 focused transferases, also known as FUT8, in the colon.
Levels of this enzyme in healthy people are usually low
.

The main function of FUT8 is to modify the physical properties
of many different types of proteins through a process called focusing.
In 2016, a study led by Dr.
Naoyuki Taniguchi of the Osaka International Cancer Research Institute showed that mice lacking FUT8 did not develop ulcerative colitis, but the cause was unknown
.
This caught the attention of Vivek Malhotra's team at the Centre for Genome Regulation (CRG) in Barcelona, which discovered back in 2013 that FUT8 could play a role
in the secretion of mucous proteins, the proteins that make up the main component of the mucus layer.
The researchers decided to work together to further elucidate the role of
FUT8 in ulcerative colitis.

Mucin is a large protein that expands hundreds of times
in volume when they are released into additional cellular space.
It is estimated that the average human produces one liter of mucin per day, which combines with other molecules to form mucus – a thick, slippery fluid that covers, lubricates and protects the entire gastrointestinal tract and has a surface area about half the size
of a badminton court.

The researchers, in collaboration with Ivo Gut's research team, at the Análisis Genómico National Center (CNAG-CRG), provided gene expression data
collected from colon tissue biopsies from 24 patients with ulcerative colitis and the same type of tissue from 16 healthy patients.
The results showed that FUT8 levels in samples from patients with ulcerative colitis were 3.
5 times higher
than those from patients with non-ulcerative colitis.
They also found that in the inflammatory colon of patients with ulcerative colitis, there were increases
in levels of several types of mucin.

The researchers then examined whether the mucosal layer of the mice with FUT8 was altered, and FUT8 has a protective effect
against ulcerative colitis.
They found that FUT8 knockout mice had a thinner
mucus layer compared to mice with high expression of FUT8.

Next, the researchers explored why these changes in the mucus layer occur at the
molecular level.
Using colon cell lines, they found that different levels of FUT8 altered the ratio of two different types of mucin secreted by the cells—mucin2 and Mucin5AC—that make up the scaffold
of the mucus layer.
They also found that it altered the expression of mucin 1, which is the "anchor"
of bacteria on the cell surface.

"We found that when FUT8 was overexpressed, as seen in patients with ulcerative colitis, the mucin secreted by epithelial cells was less dense
than normal mucus.
Because the mucus layer is less compact, it is more permeable, which we think makes it easier for bacteria to reach epithelial cells
in the colon.
The mucus layer will also be stickier, making it more resistant to washdown
.
This, in turn, traps bacteria and other pathogens in the mucus, giving them a longer time to invade the underlying cells," explains Dr.
Gerard Cantero-Recasens, who carried out the work in the laboratory of Dr.
Marhotra at CRG and is now a junior principal investigator
at the Wald Hebron Institute (VHIR) in Barcelona.

"Instead, when FUT8 is depleted, epithelial cells secrete mucin, forming a denser mucus
.
This makes the mucus layer less permeable and easily washed away
.
As cells continue to secrete more mucin, bacteria trapped in the mucus are constantly removed
.
We believe that the fewer bacteria, the less contact there is with epithelial cells, which can prevent or reduce inflammation
.
This explains why mice lacking FUT8 were able to resist ulcerative colitis," Dr.
Cantero-Recasens added
.

A healthy person's distal colon has two layers of mucus
.
The outer layer is where many different types of microorganisms live, both harmful and beneficial
.
The inner layer binds tightly to the epithelial layer and is usually sterile, preventing harmful bacteria from the intestine or outer layer from reaching the cells of the large intestine, preventing inflammation or infection
.
One theory is that FUT8 levels in healthy people's colons are intentionally kept low so that mucin can form a mucus layer
that bacteria cannot penetrate.
While researchers have shown that FUT8 alters the mucosal layer, which may lead to the onset of ulcerative colitis, why this enzyme is dysregulated in patients remains an open question
.

Another unknown factor is what happens
after the initial onset of inflammation in ulcerative colitis.
The researchers believe that the gut microbiome may have played a role
.
"The mucosal layer is the first line
of defense against dangerous microorganisms, toxins and harmful by-products of digestion.
It also has a large number of beneficial microorganisms
that are essential for human health.
The changes we observe in mucin are likely to affect the gut microbiota, which in turn triggers an immune response
.
Assessment of the gut microbiota in the gut of normal people and patients should help with this," said Vivek Malhotra, ICREA research professor and senior author of the study and coordinator of
the CRG Cell and Developmental Biology Project.

The discovery paves the way for the development of new tests to detect ulcerative colitis before symptoms appear and to gauge the prognosis
of the condition.
"We can measure the level
of focus or MUC1 release in the mucus of the large intestine by measuring it from a fecal sample from a patient.
It will be a relatively simple test and a routine blood test," explains
Dr.
Cantero-Recasens.

Dr Malhotra concluded: "This is a good start to solving a vital problem and yet another example
of how basic science can still be the best means of solving a medical problem.
"