Writing in the March 18 issue of Science, the researchers report that the yeast Candida glabrata use a family of proteins called sirtuins to block access to genes that would otherwise help the yeast stick. The sirtuins, which also help regulate the organism's lifespan, require niacin, or vitamin B3, to work. But urine has only tiny amounts of niacin, so the sirtuins don't work, the genes are exposed, and the yeast can make the proteins that help it stick to cells in the urinary tract, the researchers discovered.
C. glabrata and its cousin C. albicans cause infections in blood and in mucosal tissues such as the urinary tract and vagina. C. glabrata is the second leading cause (behind C. albicans) of yeast infections, or candidiasis, in people with urinary catheters. Unlike some other yeast, C. glabrata can't make niacin and instead has to import it from its surroundings.
"This particular yeast has in some sense committed to living with the human host and so it takes advantage of us to provide certain key nutrients," says Brendan Cormack, Ph.D., professor of molecular biology and genetics in Johns Hopkins' Institute for Basic Biomedical Sciences.
"It turns out that there's enough niacin in blood to keep the yeast's adhesion-promoting genes turned off, we discovered," he adds. "But in urine and perhaps other host environments, there is such a limited amount of niacin that these genes are turned on, allowing the organism to stick to host cells."
The new study builds on the lab's discovery in 1999 that C. glabrata sticks to cells that line mucosal tissues and blood vessels thanks to the products of genes dubbed EPAs by Cormack's team. Then, in 2003, postdoctoral fellows Alejandro De Las Peñas and Irene Castaño discovered that yeast missing the gene for Sir3 were super-sticky.
"Among other things, Sir3 and specific other proteins attach themselves near the tips of chromosomes, obscuring the nearby genes," says Cormack. "It turns out that the yeast's adhesion-promoting genes are near the chromosome tips and are usually silenced by this process. In yeast missing Sir3, the EPA genes were exposed and used."
The researchers' latest work demonstrates that environmental influences -- not just the engineered loss of a gene -- can dictate whether the yeast can use these EPA genes. The environment's effect on these genes helps the organism recognize a good place to colonize, says Cormack.
In the new work, graduate student Renee Domergue studied C. glabrata she had engineered to become permanently drug-resistant if the adhesion-promoting genes got turned on, which would only happen if Sir3 and the other proteins had stopped covering them up for some reason.
No drug resistance developed in blood, but Domergue did detect it in a mouse model of bladder infection that had been developed by collaborator David Johnson of the University of Maryland School of Medicine. Turning to laboratory dishes again, Domergue discovered that the yeast rapidly became drug-resistant (indicating the adhesion-promoting genes had been turned on) when they were grown in artificial urine -- a mix of specific chemicals in known amounts.
"We then tested each of the components of the urine, but none triggered the switch," says Cormack. "So the trigger was actually something that was 'limiting' in the urine, that is, it was present in only tiny amounts -- the vitamin niacin."
Niacin, also known as nicotinic acid, is used by cells to make an important molecule called NAD+ for short. NAD+ binds to sirtuins and is required for them to work properly. (Johns Hopkins scientist Cynthia Wolberger and colleagues recently showed how this happens.) Without niacin, these yeast can't make NAD+, and so the sirtuins don't block access to the EPA genes, the researchers report.
Other researchers have reported that the EPA genes also help C. glabrata stick to plastic, which might explain the organism's propensity to cause catheter infections.
"We don't know whether niacin supplements might help prevent these catheter infections, or whether the plastic could be treated somehow to reduce the organism's ability to bind to it. But there can be significant liver toxicity associated with niacin supplements, so the question would have to be studied very carefully," cautions Cormack.
Not all yeast would be affected by the lack of niacin in the same way, because some, like the popular laboratory yeast S. cerevisiae, can make niacin themselves, and others don't use sirtuins to regulate their adhesion-promoting genes.
The research was funded by the National Institute of Diabetes and Digestive and Kidney Diseases and the National Institute of Allergy and Infectious Disease. Authors on the paper are Domergue, Castaño, De Las Peñas, Cormack and Margaret Zupancic of Johns Hopkins; and Virginia Lockatell, Richard Habel, and Johnson of the University of Maryland School of Medicine. Castaño and De Las Peñas are now at the Instituto Potosino de Investigacion Cientifica y Tecnologica, San Luis Potosi, Mexico.
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