Mechanism For Fungal Adherence Found, Possible Key To Disease

In a paper in the latest issue of the journal Science, university researchers reported that the fungi’s ability to adhere to cells lining the mucosal surfaces of the mouth depends on the presence of this protein. If the protein is missing, Candida simply cannot hang on.

Candida albicans is one of many microscopic organisms that live inside mammals, including humans. It normally exists at low levels in the gastrointestinal (GI) tract. But in humans, when the immune system is weakened -- as it is in patients with AIDS, people who are HIV-positive, transplant and chemotherapy patients -- the fungus can proliferate.

Some infections, such as thrush in infants and vaginitis in women, can be mild. But among the serious infections patients acquire while in the hospital -- so-called nosocomial infections -- Candida ranks among the top five pathogens and is potentially fatal.

Occasionally, these outbreaks can breach the GI tract and infiltrate the bloodstream and solid organs, causing the latter to shut down because of tissue damage and the sheer quantity of fungus present.

Paula Sundstrom, an associate professor of medical microbiology and immunology at Ohio State University, and Janet Staab, a postdoctoral fellow in the same department, wanted to discover how Candida can become so prolific.

The organism usually grows in yeast form by budding. But when conditions are right -- as they are in the human body in immune-suppressed patients -- Candida albicans grows by sending off filaments, spreading across and penetrating into epithelial, or skin, cells like ivy across the backyard. Sundstrom focused their research on how these filaments were able to adhere so strongly to the mucosal epithelial cells, fostering the filaments' growth.

“We believe that the purpose of this adhesion is simply so that the organism can stay put and not be washed away by fluids within the GI tract,” she said. ”The invasive behavior of these filaments caused me to start looking for specific proteins in the Candida filaments when I was a graduate student at the University of Washington in Seattle.”

Earlier work had led her to identify a specific gene responsible for producing a protein that can become linked to other proteins using an enzyme called transglutaminase. The protein serves as a substrate, or anchor, upon which the enzyme can build larger, essential molecules.

Transglutaminases are well-known mammalian enzymes, although they are rare or absent in fungi. But in Candida, the researchers found a protein that also bound to transglutaminase.

“We wondered why Candida would have a substrate protein for these mammalian enzymes,” Sundstrom said. ”We’re talking about a fungal protein that is a substrate for a mammalian enzyme. This enzyme is special since it can cross-link proteins together, two different proteins. Most enzymes don’t do that.”

They discovered that when the protein in the fungi filaments uses the enzyme to link to the protein in the epithelial cells, it forms a covalent bond -- the strongest type possible -- giving it great adhesive capabilities. This type of covalent adhesion has not been described previously in any viruses, bacteria, fungi or other microorganisms.

Several years ago, S.D. Bradway, a former assistant professor of periodontology at Ohio State and co-author on this paper, showed that Candida albicans could form stable attachments. However, that work did not identify what proteins were involved.

Sundstrom’s group used a strain of Candida in which the transglutaminase-binding protein had been removed and tested its ability to adhere to human oral mucosal cells. Without the protein, there was a five-fold reduction in the ability of Candida to attach. When the gene was replaced, stable attachments were again formed. This proved that Sundstrom’s newly discovered gene was a key in the fungus’ ability to adhere to mammalian cells.

“That gave us the evidence that these very tight attachments are formed between the Candida filaments and the epithelial cells and that they’re dependent on this gene (and its subsequent protein) being present,” she said.

The researchers wanted to test if this discovery might be important in causing the actual Candidiasis infections, as well as the organism’s enhanced ability to hang on. They injected one group of mice with Candida strains with the transglutaminase-binding protein intact and another group with Candida lacking the protein. Mice in the first group quickly succumbed to the infections while those receiving protein-free Candida fared far better. This showed that the protein’s presence plays an important role in the course of the disease.

Sundstrom hopes that once they understand fully the mechanism of adhesion in these filaments, then they’ll be able to discover inhibitors to alter that process.

“If we can do that, then Candida is going to have a more difficult time colonizing, growing and proliferating on mucosal surfaces,” she explained. “And if it can’t proliferate, it will be less likely to invade and cause more severe infections.”

Sundstrom has filed for a patent based on her research.

Along with Staab, Sundstrom and Bradway, P.L. Fidel of Louisiana State University contributed to the research. The study was supported by both the National Institute of Dental and Craniofacial Research and the National Institute for Allergy and Infectious Diseases.