Researchers at the Massachusetts Institute of Technology (MIT) have recently discovered that components in mucus can interact with Candida albicans and prevent it from causing infection. These molecules, called glycans, make up a large portion of mucin, the gel-like polymers that make up mucus.
Mucins contain a variety of different carbohydrates and are complex carbohydrate molecules. Growing research suggests that glycans can be specialized to help ‘tame’ specific pathogens — not just Candida albicans, but others such as Pseudomonas aeruginosa and Staphylococcus aureus, Massachusetts Andrew and Elner-Viterbi Professor Katharina Ribbeck of the Institute of Technology said.
Ribbeck, who led the research team, said: “What is emerging is that mucus shows a broad library of small molecules, with many virulence inhibitors against a variety of problematic pathogens, ready to be discovered and exploited.”
Harnessing these mucus proteins could help researchers design new antifungal drugs, or make disease-causing fungi more sensitive to existing drugs. Currently, such drugs are rare, and some types of disease-causing fungi have developed resistance to them.
Key members of the research team also include Rachel Hevey, research associate at the University of Basel; Micheal Tiemeyer, professor of biochemistry and molecular biology at the University of Georgia; Richard Cummings, professor of surgery at Harvard Medical School; and Clarissa Nobile, associate professor of molecular and cell biology at the University of California, Merced. ; and Daniel Wozniak, professor of microbial infection and immunity and microbiology at Ohio State University.
MIT graduate student Julie Takagi is the lead author of the paper, which will be published on June 6, 2022 innature chemical biology” magazine.
Over the past decade, Ribbeck and others have discovered that mucus, far from being an inert waste, plays an active role in controlling potentially harmful microbes. In the mucus in most areas of the body, there is a dense population of different microbial communities, many of which are beneficial, but some that are harmful.
Candida albicans is one of those microbes that can be harmful if left unchecked, causing a mouth and throat infection called thrush, or a vaginal yeast infection. These infections can usually be cleared up with antifungal drugs, but invasive Candida albicans infects the blood or internal organs, which can occur in people with weakened immune systems, and has a mortality rate of up to 40 percent.
Ribbeck’s previous work showed that mucins prevent Candida albicans cells from switching from their round yeast form to multicellular filaments called hyphae, the microbe’s harmful version. Hyphae can secrete toxins that damage the immune system and underlying tissues, and are also necessary for the formation of biofilms, a hallmark of infection.
“Most Candida infections are caused by pathogenic biofilms that are inherently resistant to the host’s immune system and antifungal therapeutics, posing a significant clinical challenge to treatment,” Takagi said.
In the mucus, yeast cells continue to grow and thrive, but they do not become pathogenic.
“These pathogens don’t seem to cause harm to healthy people,” Ribbeck said. “There is something in the slime that has evolved over millions of years and appears to keep pathogens in check.”
Mucins are composed of hundreds of glycans attached to a long protein backbone, forming a bottle-like structure. In this study, Ribbeck and her students wanted to explore whether glycans could detach from the mucin backbone to disarm C. albicans on their own, or if the entire mucin molecule was necessary.
After separating the glycans from the backbone, the researchers exposed them to C. albicans and found that these collections of sugars prevented the single-celled C. albicans from forming filaments. They can also inhibit adhesion and biofilm formation and alter the dynamics of C. albicans interactions with other microorganisms. The same goes for mucin sugars from human saliva and animal gastrointestinal mucus.
Isolating single glycans from these collections is very difficult, so researchers from Hevey’s group at the University of Basel synthesized six different sugars that are most abundant on mucosal surfaces and used them to test whether individual glycans Can disarm Candida albicans.
“Isolating individual glycans from mucus samples is nearly impossible with current techniques,” Hevey said. “The only way to characterize individual glycans is to synthesize them, which involves extremely complex and lengthy chemical procedures.” She and her colleagues are One of the few research groups worldwide that are developing methods to synthesize these complex molecules.
Tests in Ribbeck’s lab found that each of these glycans showed at least some ability to stop filamentation alone, and some were as potent as pools of multiple glycans that the researchers had previously tested.
Analysis of gene expression in Candida found that more than 500 genes were either up- or down-regulated after interacting with glycans. These genes include not only those involved in hyphae and biofilm formation, but also other roles such as amino acid synthesis and other metabolic functions. Many of these genes appear to be controlled by a transcription factor called NRG1, a master regulator activated by sugars.
“These glycans really seem to go into physiological pathways and reconnect these microbes,” Ribbeck said. “It’s a huge library of molecules that can facilitate host compatibility.”
The analysis performed in this study also allowed the researchers to associate specific mucin samples with the glycan structures found in them, which should allow them to further explore how these structures relate to microbial behavior, Tiemeyer said.
“Using state-of-the-art glycology methods, we have begun to comprehensively define the rich diversity of mucin sugars and annotate this diversity as patterns that have functional implications for both the host and the microbe,” he said.
This research, combined with Ribbeck’s previous work on Pseudomonas aeruginosa and ongoing research on Staphylococcus aureus and Vibrio cholerae, shows that different sugars are specifically designed to disable different kinds of microbes.
She hopes that by harnessing these different glycans, researchers will be able to develop new treatments for different infectious diseases. As an example, glycans could be used to stop Candida albicans infection, or to help sensitize it to existing antifungal drugs, by breaking the filaments they form in their disease-causing state.
“Glycans alone have the potential to reverse the infection and transform Candida into a less harmful growth state for the body,” Ribbeck said. “They may also sensitize microbes to antifungal drugs because they individualize them, which in turn sensitizes them, too. more easily controlled by immune cells.”
Ribbeck is now working with collaborators who specialize in drug delivery to find ways to deliver mucin sugars in the body or on surfaces like the skin. She also has several ongoing studies investigating how glycans affect a variety of different microbes. “We’re working with different pathogens to learn how to take advantage of this amazing set of natural regulatory molecules,” she said.
“I’m very excited about this new work because I think it has important implications for how we develop new antimicrobial therapies in the future,” Nobile said. “If we figure out how to therapeutically deliver or increase these protective mucin sugars into the human mucosal layer, we have the potential to prevent and treat infections in humans by maintaining a symbiotic form of the microbe.”
