Johns Hopkins researchers have discovered a new metabolic pathway in a parasite that could lead to drugs for treating so-called African sleeping sickness. The discovery, reported in the April 7 issue of Science, provides hope for the estimated 500,000 people who are fatally infected with the African trypanosome.
"There are no good drugs to treat this disease," says Paul T. Englund, Ph.D., a professor of biological chemistry at Johns Hopkins School of Medicine and principal investigator of the study. "The one drug out there is so toxic that it sometimes even kills people. The newly discovered pathway is a potential target for development of new drugs."
The immune system is particularly bad at fighting sleeping sickness, a major health problem in sub-Saharan Africa, because the parasite responsible is a master of disguise, Englund says. Throughout its lifetime, the parasite coats its surface with as many as 1,000 different proteins. By the time the immune system recognizes the intruder and starts making antibodies to destroy it, the parasite is already changing its coat so it can hide. This constant coat swapping makes it impossible to develop a vaccine for the disease, and researchers also have not been successful in developing drugs.
For several years, researchers have known that the protein coats are anchored to the parasite's cell surface by GPI anchors (glycosyl phosphatidylinositol anchors). These anchors contain myristate, a rather short fatty acid. Researchers haven't figured out, however, where the parasite obtains this anchor material. This fatty acid is rare in blood, and previous work had shown that it is not made in the parasite. Secondly, the researchers wondered, why did the anchor contain myristate, an uncommon fatty acid?
To examine the question, Yasu S. Morita, a predoctoral fellow in Englund's lab, tested whether the parasite would make GPI anchors with a shorter fatty acid. In test tube studies, he discovered that when he offered a shorter fatty acid to the African trypanosome, the parasite efficiently elongated it and made it into myristate. The researchers realized that the long-held assumption that the parasite did not make this material might be false. Otherwise, why would the parasite have such an efficient elongating system?
Using radioactive labels, the researchers reanalyzed the parasite's biochemical makeup and found that indeed the parasite did make myristate but not in the conventional way. While most cells make a variety of fatty acids for general use in all types of membranes as well as for energy storage, trypanosomes only make myristate, and they only use it to make GPI anchors. In another experiment, the researchers tested the effects of a fatty acid inhibitor, thiolactomycin, on the parasite. In test tube studies, they found that thiolactomycin successfully inhibited myristate synthesis and killed the parasite.
"We are not going to be able to use thiolactomycin as a drug to treat sleeping sickness because it is needed in high concentrations to have an effect," says Englund. "But this is a new and very interesting pathway in a parasite that is a potential drug target. Derivatives of thiolactomycin might have potential."
Sleeping sickness is a fatal disease that is prevalent in much of tropical Africa. Depending upon the trypanosome type, an individual dies in a few months or a couple of years. Fatigue, tremors, fever, inflammation of the lymph nodes, and involvement of the brain and spinal cord leading to profound lethargy, paralysis, coma and death characterize the disease. Human infections are transmitted by the tsetse fly.
The other author of the study is Kimberly S. Paul, Ph.D., a postdoctoral fellow in the Department of Biological Chemistry at Johns Hopkins School of Medicine.