They are very fine threads of a protein present in mucous and salivary secretions.
Mucin is a glycoprotein mucopolysaccharide, which is one of the main constituents of mucous secretion.
All mucous membranes secrete mucus, and therefore mucin, but where it has a greater importance is in the gastrointestinal tract, as a protective element of its mucosa.
Health Effects of Mucin Filaments
“Mucus, or mucin, is something that has such powerful effects on our health,” says Katharina Ribbeck, a biophysicist at MIT who along with her colleagues described the many roles of mucus in the 2018 Cell and Developmental Biology Annual Review.
Most of those functions come from the 5 percent of the non-water substance: various salts, lipids, and proteins, especially mucins, that give mucus its gelatinous qualities: long, thread-like polypeptides coated with sugar chains covalently called glucans.
Scientists have discovered many ways that mucin proteins work to keep body surfaces clean and protected, and they continue to analyze the complex interactions that molecules have with microbes.
Here’s some of what they’ve learned so far, and where the research is heading.
Mucous coatings vary considerably throughout the body, in line with necessary functions. The eye, for example, is covered with a thin film of not particularly viscous mucus, enough to keep it hydrated.
The inside of the colon, in contrast, has a thick, rubbery coating that prevents bacteria from escaping.
The key to these physical properties are the mucins themselves. Produced by specialized cells in tissues that line body cavities and surfaces.
They are some of the largest molecules we make and come in two main types: secreted mucins, which exude to form large mesh-like networks, and bound mucins, which remain attached to cells.
The manufacture of mucin changes with place and circumstances. “There’s a lot of cell specificity,” says biophysicist Brian Button of the University of North Carolina School of Medicine.
For example, the gelatinous mucus that keeps the respiratory tract clean is made up of the secreted mucins MUC5B and MUC5AC.
MUC5B is generally dominant – a 2017 analysis in the New England Journal of Medicine found that it is about 10 times more abundant than MUC5AC, for example.
But during infections and other medical conditions, MUC5AC levels rise abruptly, creating a much tougher and stickier mucus that is more difficult to remove from the airways.
More MUC5AC can be good, because the more viscous mucus prevents bacteria from sticking to cells in the body and causing damage, Button says.
But in conditions like asthma, cystic fibrosis, and chronic obstructive pulmonary disease, the overproduction of MUC5AC can cause a harmful buildup of mucus in the airways.
In other parts of the body, such as the stomach, high levels of MUC5AC are standard and help protect the lining from acidic digestive juices. And in the gut, another mucin, MUC2, is the main player.
In the colon, MUC2 forms two layers of mucus: a loose outer lining that harbors bacteria and a densely packed inner barrier that prevents those microorganisms from penetrating the cells of the tissue below.
Unlike the airways, “in the gut, you really don’t have a problem with too much,” says Gunnar Hansson, a mucin biologist at the University of Gothenburg in Sweden. “You’d rather have too much than too little.”
Today it is increasingly appreciated that the trillions of microbes that live in our intestines, the gut microbiome, play a vital role in health and disease. And where do they live? In layers of nourishing mucus.
In fact, it has become clear that many of these commensal bacteria use the glycans that plug mucin molecules as their primary source of energy. To that end, their genomes carry codes for enzymes that can cleave these carbohydrates and digest them.
The bacteria also release metabolites like short-chain fatty acid butyrate that intestinal cells use to further fuel mucin production.
“They are using energy to feed themselves, but they are also producing energy that they send back to us and making it possible to produce these large amounts of mucins,” says Hansson. “They benefit from this, and we benefit too.”
Unlike the natural inhabitants of our intestines, bacterial pathogens tend to lack the machinery necessary to harness the glucans in mucins, and have developed other ways to expand their populations.
Managing the behavior of microbes
Mucins do more than serve as physical barriers and food for microbes.
Scientists have discovered that the glycans that decorate the surface of these molecules can influence the behavior and physiology of pathogenic microbes and reduce their ability to spread and cause damage.
In a 2012 study in Current Biology, for example, Ribbeck and colleagues discovered in test tube experiments that mucins could stop the bacterium Pseudomonas aeruginosa, the cause of many dangerous hospital-acquired infections, from forming biofilms, tight-knit microbial communities. that are difficult to eradicate.
Ribbeck’s lab then showed that the mucins could prevent the formation of biofilms of other pathogens, including those of Streptococcus mutans, the bacteria responsible for tooth decay.
In addition to controlling foreign invaders, mucins can also help control our body’s resident microbes.
Ribbeck’s team discovered that the molecules reverse the transition from Candida albicans, a fungus that normally resides peacefully within the healthy microbiome, to a pathogenic form.
They do this by suppressing Candida’s ability to form filaments, adhere to surfaces, and develop other traits that allow it to cause harm.
Mucin filaments can also act as decoys to prevent infection.
In 2009, Mike McGuckin, a molecular biologist at the University of Melbourne in Australia, and his colleagues reported in PLOS Pathogens that when Helicobacter pylori, a bacterium that can cause peptic ulcers and gastric cancer, attempts to attach a cell on the surface of the stomach , a mucin can bind to the pathogen instead.
The mucin then breaks away from the cell membrane and carries the potential invader into the acidic gastric juice.
“Many bacteria and viruses recognize certain sugars on the cell surface, and this is how they know they are reaching the cell,” Button explains. “Mucins can replicate these glycosylation patterns and act as molecular decoys.”
But some disease-causing bacteria use mucins to their advantage.
In another study, McGuckin and his colleagues showed that Campylobacter jejuni, a microbe that lives harmlessly in chickens but can cause food poisoning in people, recognizes human mucins and uses their presence as a signal to increase the activity of the genes involved. in pathogenicity.
Scientific Advances: Synthetic Mucins
Scientists hope to one day create synthetic mucins for both research and treatment and have done a lot of work developing tissue culture for their production.
But a better understanding of molecule structure and biophysical properties will be needed before synthetic substitutes become a clinical reality, says molecular biologist Christopher Evans of the University of Colorado Denver, who is working with colleagues to separate some of those details.
Once this is done, synthetic mucins could be used in laboratory studies and eventually for healthcare applications such as controlling problem pathogens, restoring damaged or defective mucosal linings, and improving drug delivery through creation of coatings that can avoid mucosal barriers.