Protein “tuning” system lets scientists uncloak dangerous parasite

Researchers at Washington University School of Medicine in St. Louis have found a way to dress and undress Leishmania, a parasite that causes death and disfigurement in developing countries.

Scientists showed that they could control the parasite’s ability to put on its carbohydrate coat, causing it to put on the whole coat, a lighter version or to forego the coat entirely. The coat helps Leishmania invade host cells and may also help it evade antimicrobial drugs and the host’s immune system.

Leishmania’s carbohydrate coat is important to the parasite’s survival when it infects sand flies, the insects that transmit it, and in the early stages of human infections,” says senior author Stephen Beverley, Ph.D., the Marvin A. Brennecke Professor and head of Molecular Microbiology. “By manipulating the coat’s presence, we hope to learn more about how it helps the parasite survive and potentially provide new targets for drug development.”

The findings were published online in April in the Proceedings of the National Academy of Sciences.

If left untreated, the most serious forms of Leishmania infection or leishmaniasis can cause large, disfiguring skin sores, damage internal organs and kill. While some infections yield disease pathology, more often there are few if any symptoms, rendering such individuals healthy carriers. In the United States, these asymptomatic infections most often occur in travelers returning from endemic regions such as the Middle East.

“If they’re lucky, the infection stays asymptomatic,” Beverley says. “But if their immune systems become weakened by age, disease or drug treatments, the parasite can pop up again and cause illness.”

Drugs are available to treat the more serious forms of leishmaniasis, but the parasite is developing resistance to the pharmaceuticals commonly used in developing nations.

Beverley’s laboratory used a protein “tuning” system developed by researchers at Stanford University to manipulate Leishmania. The technique involves genetically adding a special segment of DNA to a gene of interest. The protein cells produce from the altered gene contains new material that prevents proper folding of the protein, which is normally one of the final steps of production. This in turn causes the cell to mark the protein as defective and dispose of it.

The cells’ ability to make the protein is therefore blocked, but scientists can modify this blockage by adding a compound to the cell that binds to the new segment of the protein and allows the folding process to proceed.

“This is where the ‘tunability’ part comes in,” Beverley says. “Add more of the compound, and the cell can make more of the protein you’re studying. Reduce the compound’s availability, and levels of the protein go down.”

Beverley believes the new study is the first instance of the tuning technique being applied to manipulate the production of carbohydrates, which are assembled through the action of groups of enzymes. His lab had earlier identified a protein called UGM that plays an essential role in the synthesis of an unusual sugar required to make the Leishmania parasite’s coat. When postdoctoral fellow Luciana Madeira da Silva, Ph.D., implanted the tuning system in the UGM gene, researchers found they could use it to adjust carbohydrate production by proxy, changing the amount of the parasite’s carbohydrate coat by altering levels of the UGM protein.

“This opens up many new doors to studying Leishmania, because we haven’t previously had a way to manipulate the parasite in such a rapid and direct fashion,” Beverley says. “We are currently implanting this tuning system in several interesting proteins and using it to further study the carbohydrate coat.”

Beverley notes that his colleague, Dan Goldberg, M.D., Ph.D., has put the “tuning” system to work in Plasmodium falciparum, the parasite that causes malaria. He thinks it will also be useful in studies of many other parasites and disease-causing microorganisms.

da Silva LM, Owens KL, Murta SFM, Beverley SM. Regulated expression of the Leishmania major surface virulence factor lipophosphogylcan using conditionally destabilized fusion proteins. Proceedings of the National Academy of Sciences, online week of April 20, 2009.

Funding from the National Institutes of Health and the Washington University Infectious Diseases Scholar Program supported this research.

Washington University School of Medicine’s 2,100 employed and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children’s hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked third in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children’s hospitals, the School of Medicine is linked to BJC HealthCare.