It might not seem like corals and humans have much in common. Corals can’t see or hear; they don’t have brains or bones or lungs or hearts. Just like humans, though, corals need to protect their soft tissues from a world full of microbial invaders. Both species, and indeed, most animal species on planet Earth, have a similar first line of defense: thick, slimy mucus (1).
The fact that mucus exists in some of humanity’s most distant relatives corroborates its ancient origins, yet this sticky goo remains underappreciated and poorly understood. “People really thought that mucus was a waste product for a very long time,” said Jessica Kramer, a biomedical engineer at the University of Utah. In recent years, scientists have begun to appreciate mucus. Often, it protects us from harmful infections, but in some cases, it may actually increase our vulnerability. Learning more about these processes and how they are regulated could yield important benefits.
“With antibiotic resistance becoming a threat to our current ways of treating infections, I think it is imperative to find alternative treatments,” Sara Lindén, a microbiologist at the University of Gothenburg, wrote. “Enhancing and mimicking the body's natural defenses, such as the mucin-based defenses, appear to be plausible paths forward.”
Know your mucus
Mucus protects the delicate epithelial tissues of our eyes, digestive systems, respiratory systems, and urogenital tracts. It is a highly heterogenous substance. “The structure is unique to each species and actually varies between different people,” said Kramer. “Even on one person's bodily surfaces like the eyes versus the lungs, or the reproductive tract versus the lungs, the composition of the mucus is different.”
Mucus is largely composed of mucins, macromolecules with protein backbones and sugars called glycans sticking out in all directions like bottlebrushes. A single mucin can have more than 200 different types of glycan structures, making mucins difficult to study and recreate in the lab (2).
Our internal surfaces are generally covered with two distinct layers of mucins. “There's the mucus that everybody is quite familiar with that’s made of mucins that have been secreted by cells. They are freely floating; we blow them into a tissue,” said Kramer. “However, I think most people are unaware of a structure called the glycocalyx, which is a layer of mucins that are actually attached to our cell surfaces.” In order to infect a cell, pathogens have to penetrate the layer of free floating mucins as well as the transmembrane mucins of the glycocalyx.
In a recent preprint, Karin Strijbis, an infection biologist at Utrecht University, reported the role of a transmembrane mucin called MUC1. In the course of her research, she incubated a layer of MUC1-expressing lung cells with the SARS-CoV-2 virus. She removed the MUC1 layer from a different set of lung cells before adding virus to these cells as well. Without MUC1 present, the SARS-CoV-2 virus ran amok, infecting far more cells than the unaltered population.
“The mucin is making a barrier that prevents the virus from reaching its receptor,” said Strijbis. “Mucins are very large, so they're sticking out quite far from the membrane. And the receptor that the virus needs to bind to is much closer to the membrane. The mucin is like a little umbrella that's just there in between the virus and the receptor that prevents it from connecting. If you cleave off the whole umbrella, then the virus can reach the ACE2 receptor and enter the cell.”
The mucin is like a little umbrella that's just there in between the virus and the receptor that prevents it from connecting.
- Karin Strijbis, Utrecht University
Mucins can run interference in other ways as well. One example is mucins’ interaction with Helicobacter pylori, a relatively common bacteria that occasionally causes gastritis, stomach ulcers, and even stomach cancer. Early work showed that MUC1 protected against H. pylori infection in mice; without MUC1, mice had higher bacterial loads in their stomach linings and more severe gastritis (5).
Lindén showed that MUC1 helps protect host cells by tricking the bacteria. In order to infect cells, bacteria need to attach themselves to the cell, which they do with proteins called adhesins. Lindén demonstrated that H. pylori bound to MUC1 using these adhesins and that once this binding occurred, the extracellular part of the mucin detached from the cell.
“Mucins can act as releasable decoys, thereby hindering attachment to the epithelial surface,” she explained. “Mucins that have strong abilities to bind to Helicobacter pylori lead to fewer bacteria reaching the epithelial surface since mucin-bound H. pylori gets shed from the mucosal surface and transported away from the stomach with the gastric emptying.”
Mucins also appear to regulate microbial behavior in complex and poorly understood ways. One of the microbes that mucins manipulate is Candida albicans. This single cell fungus usually lives harmlessly on human mucus membranes, but it occasionally causes bothersome infections such as thrush or a life-threatening infection known as invasive candidiasis, which kills thousands every year (6). Rachel Hevey, a biochemist at the University of Basel, along with collaborators at the Massachusetts Institute of Technology (MIT), showed that mucus-associated glycans may be key for keeping this microbe tame.
“When Candida starts to form filaments, this is associated with its pathogenic form,” said Hevey. “Filaments allow it to attach to surfaces like epithelial surfaces and to penetrate and establish infections. The glycans prevent them from being able to form these filaments. So, they stay in what we call a yeast form, but it's essentially just a round form with no filaments. Then it can't really establish an infection as easily. The glycans force it to stay in a healthy form and not the pathogenic form.”
The ability of mucins to manipulate the behavior of pathogens is not limited to C. albicans. When Pseudomonas aeruginosa bacteria, which can cause deadly lung and blood infections, mix with mucins, they reduced the expression of virulence genes, including those related to toxin secretion (7). Mucins in saliva suppress pathways related to quorum sensing, the means by which bacteria communicate with each other and coordinate behaviors, including those that promote infection (8).
Mucin-microbe interactions are not only related to infection, but may also influence the transmissibility of the pathogen (9). For example, some viruses can spread through contact with contaminated objects or surfaces. Kramer showed that a virus related to SARS-CoV-2 was still highly infectious after virus-laden droplets of saline dried on a surface. But when these droplets also contained mucins, as droplets expelled during a cough or sneeze would, the virus’s ability to infect cells was dramatically impaired, indicating that mucins likely reduce viral transmission through touching contaminated surfaces for at least some types of viruses (9).
Not all mucins are created equal. A study of 28 people identified more than 600 gastric mucin-associated glycans with only six shared by all participants. These different glycosylated mucins also seem to vary in their interactions with pathogens (10).
More research into this field, “could improve our understanding of how people's compositions of mucus affect their vulnerability to contracting serious disease,” Kramer said. “Of course, there are many other factors, such as comorbidities and immune function and all that, but it could be one piece of the puzzle. And then, on the flip side, it might be helpful for identifying who is more likely to spread the virus.”
Mucins could do much more than help assess risk, however. Many researchers look at mucin manipulation or mimicry as a way to prevent or treat disease. One strategy is to develop ways to increase production of specific mucins but, said Strijbis, this needs to be done carefully.
For example, MUC1 seems to protect against SARS-CoV-2. On the other hand, overproduction of secreted mucins associates with worse COVID-19 outcomes. MUC1 overexpression also associates with some forms of cancer (11). “There’s always a fine line with mucins,” Strijbis said.
A ready made way to manipulate mucins may already exist. Just as mucins manipulate microbes, some microbes alter the composition of mucus (12). Strijbis and her team are currently screening a library of probiotic bacteria to determine their effects on mucins. “We want to find out how good bacteria can stimulate healthy mucosal barriers and see how we then can prevent pathogen invasion.”
Other researchers want to create drugs based on mucins or elements of the mucins. A team at the University of Copenhagen genetically engineered cells to produce artificial mucins and is now exploring the interactions of these mucins with bacteria and viruses (13). Researchers at MIT have created mucin-inspired polymers that bind to cholera toxin (14).
Hevey thinks that it might not be necessary to mimic the entire glycoprotein, which is large and complex. Her team studies the glycans. The unattached glycan molecules aren’t very good as drugs, said Hevey, since they get excreted too quickly, but they may be valuable for informing drug design.
“A big focus of our research is understanding the chemical properties of the molecule and which are important for biological activity and which aren't,” she said. “And then [we use] our synthetic methodologies to make molecules that have better drug-like properties but still mimic the natural activity of the glycan.”
An evolutionary arms race
As mucins evolved to protect us against pathogens, the pathogens also evolved. Some came up with ways to overcome mucin-mediated defenses. For example, some mucins bind to viruses, using their glycans like a sponge to mop them up.
“But the flu virus is really sneaky,” Kramer said. “It has co-evolved mechanisms to chew off those sugars, releasing itself from the mucin.”
The flu virus is really sneaky.
- Jessica Kramer, University of Utah
Some microbes go beyond simply evading mucin-based defenses; they evolved ways to use mucins to their advantage. In a study of the pathogen Salmonella enterica, for example, Strijbis found that bacteria exploited the normally protective MUC1 and used it to help them enter the cell. Removing MUC1 reduced bacterial invasion of gut epithelial cells (15).
More research is needed to determine the role of mucins in mediating susceptibility to different infectious agents, and this will influence scientists when designing mucin-inspired therapeutics.
Into the future
People often ask Kramer how to make their mucus more protective. While genetics are important, factors like diet and environment may also affect mucus composition. Unfortunately, there simply isn’t enough evidence yet to make solid recommendations about how to optimize mucus-mediated protection.
However, as researchers begin to unravel the complex contributions of mucus, therapeutics targeting or inspired by this often-overlooked substance may help give humanity the upper hand in the constant battle against microbial adversaries.
- Bakshani, C. R. et al. Evolutionary conservation of the antimicrobial function of mucus: a first defense against infection. npj Biofilms Microbiomes 4, 1–12 (2018).
- McShane, A. et al. Mucus. Current Biology 31, R938–R945 (2021).
- Biering, S. B. et al. Genome-wide bidirectional CRISPR screens identify mucins as host factors modulating SARS-CoV-2 infection. Nat Genet 54, 1078–1089 (2022).
- Chatterjee, M. et al. The glycosylated extracellular domain of MUC1 protects against SARS-CoV-2 infection at the respiratory surface. 2021.10.29.466408 Preprint at https://doi.org/10.1101/2021.10.29.466408 (2021)
- McGuckin, M. A. et al. Muc1 Mucin Limits Both Helicobacter pylori Colonization of the Murine Gastric Mucosa and Associated Gastritis. Gastroenterology 133, 1210–1218 (2007).
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- Chahal, G. et al. A Complex Connection Between the Diversity of Human Gastric Mucin O-Glycans, Helicobacter pylori Binding, Helicobacter Infection and Fucosylation. Molecular & Cellular Proteomics 21, 100421 (2022).
- Chen, W. et al. MUC1: Structure, Function, and Clinic Application in Epithelial Cancers. International Journal of Molecular Sciences 22, 6567 (2021).
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- Nason, R. et al. Display of the human mucinome with defined O-glycans by gene engineered cells. Nat Commun 12, 4070 (2021).
- Kruger, A. G. et al. Stereochemical Control Yields Mucin Mimetic Polymers. ACS Cent. Sci. 7, 624–630 (2021).
- Li, X. et al. MUC1 is a receptor for the Salmonella SiiE adhesin that enables apical invasion into enterocytes. PLOS Pathogens 15, e1007566 (2019).