Oral biologists at the University at Buffalo have shown for the first time how histatin, the naturally occurring antifungal agent in saliva, kills the oral pathogen Candida albicans, the fungus responsible for most HIV-related oral infections.
Researchers led by Mira Edgerton, D.D.S., Ph.D., discovered that histatin binds to a specific membrane protein called TRK1p, which regulates potassium ion flow through the cell membrane of the pathogenic fungus Candida albicans and allows the cell to regulate its volume.
The binding action of histatin acts like a "foot in the door," said Edgerton, UB research associate professor of oral biology in the UB School of Dental Medicine and senior author on the study. "Blocking the channel open allows a lethal unregulated flow of potassium and other essential molecules into, and out of, the cell.
"This is the first identification of a specific target for histatin," she said. "The finding paves the way for eventually developing a better therapeutic drug for candidiasis."
Results of the research were presented today (March 11, 2005) at the International Association on Dental Research General Session being held in Baltimore.
Candidiasis also is known as thrush, a disease characterized by whitish spots and ulcers on the membranes of the mouth, tongue and throat. It affects primarily people with weakened immune systems caused by antibiotics, chemotherapy or by diseases such as AIDS. Thrush also affects many denture wearers.
The condition can be treated with antifungal medication in otherwise healthy people, said Edgerton, but it is difficult to treat in persons with compromised immunity and can be deadly if it infects vital organs.
Researchers were aware that histatin usually can keep Candida albicans in check in persons with enough saliva and a healthy immune system, but they did not know precisely how histatin accomplished this.
"There are many types of naturally occurring proteins, such as venoms and toxins, that kill cells by creating holes in the cell membrane," said Edgerton, "but we thought that wasn't the case with histatin. We didn't think it acted in the same way. And we wanted to know why it acts on yeast (fungus is a type of yeast) and not on other types of cells."
Through a series of studies, the researchers identified the target-binding protein on Candida albicans by creating mutant strains of the fungus without the target and exposing the mutants to histatin. Results showed that histatin was significantly less active when the suspect target was missing.
Further research indicated that histatin binding to the target protein killed the fungal cells by preventing it from regulating its ions, the positive and negative charged molecules that move into, and out of, cells. Ions regulate electrostatic pressure between the cells' internal and external environments, which, in turn, regulates their volume and water content. Cells that lose their water content without being able to regulate its re-uptake die rapidly, Edgerton said.
"Now that the target for histatin has been identified, we can design a better protein that will be even more effective in binding and holding the channel open, causing even better and more rapid killing of the fungus," said Edgerton.
"In addition, many other pathogenic fungi that cause disease in elderly individuals or AIDS patients also should be able to be killed by histatins or drugs designed to target their potassium channels."
Additional researchers on the studies were Slavena Vylkova and Jianing Sun, oral biology doctoral students; Nedelcho Baev, Ph.D., oral biology postdoctoral fellow, and Clifford Slayman, Ph.D., and Alberta Rivetta, Ph.D., from Yale University.
The research was funded by the National Institute for Dental Research.
The University at Buffalo is a premier research-intensive public university, the largest and most comprehensive campus in the State University of New York.