Parasitic infections are particularly difficult diseases to fight and have always represented a quandary to researchers, as these small, seemingly simple organisms are actually quite complex. Many parasites have multiple hosts with complex lifecycles, are extremely adept at immune system evasion, and all are eukaryotic—which presents a particular problem when trying to uncover new therapeutic targets, as they are evolutionarily similar to their mammalian hosts.
African sleeping sickness, which is caused by the parasite Trypanosoma brucei and transmitted by the tsetse fly, is an example of a parasitic infection that has been notoriously problematic to treat due to the organism’s exceptional ability to dodge immune system bullets.
Yet now, researchers from Rockefeller University have developed a method to manipulate trypanosomes in the mammalian bloodstream causing them to change their characteristics so that it is easier for the host immune system to eliminate the insidious invader. The research team found that inhibiting specific proteins that interact with chromatin can “trick” the parasite into differentiating to a different stage of its lifecycle.
“By blocking these chromatin-interacting proteins, we have found a way to make the parasite visible to the immune system,” explained co-senior author Nina Papavasiliou, Ph.D., head of the laboratory of lymphocyte biology at Rockefeller University. “The bloodstream form of the parasite is constantly switching protein coats, so the immune system can't recognize and eliminate it. This new method makes the parasite think it's in the fly, where it doesn't need to worry about the immune system attacking it.”
The findings from this study were published recently in PLOS Biology through an article entitled “Bromodomain Proteins Contribute to Maintenance of Bloodstream Form Stage Identity in the African Trypanosome.”
Like many parasites, T. brucei has a two-stage life cycle: one-half takes place within the tsetse fly and the other half in the mammalian host. While inhabiting the fly, the parasite is protected from immune system attacks and is covered with proteins called procyclins. Upon entering the bloodstream of a mammal, they acquire a dense layer of glycoproteins that continually change, allowing the parasite to dodge an attack from the host's immune system.
Expanding off of previous studies, which found that bromodomains on some regulatory proteins are involved in cell differentiation, the Rockefeller team hypothesized that such epigenetic mechanisms may drive the trypanosome to change from one form to another.
“The changes in gene expression that accompany the transition between the different parasite forms had been well established,” noted lead author Danae Schulz, Ph.D., postdoctoral fellow in Dr. Papavasiliou’s laboratory. “But we didn't understand if there was some type of regulation happening at DNA, at the level of chromatin. Whether chromatin-altering mechanisms might be important for differentiation hadn't really been studied before.”
The investigators inhibited bromodomain proteins in cells by introducing genetic mutations into their DNA or by exposing the cells to a small-molecule drug called I-BET151, which is known to block bromodomains in mammals. When these perturbations were made, the investigators observed changes in gene expression levels that resembled those seen in cells differentiating from the bloodstream form to the fly form. They also saw that the parasites developed a procyclin coat normally found on the fly form.
“When bromodomains are inhibited, the variant protein coat is replaced with an unvarying coat on the surface of the trypanosome cell,” said Dr. Schulz. “This means that the parasite surface is no longer a moving target, giving the immune system enough time to eliminate it.”
While a compound like I-BET151 is not effective enough to be used in the clinic, the researchers are looking intensely at the molecule’s structure to see if it could provide clues to developing more optimized drugs.
“Current treatments for this disease are limited, and they have substantial side effects, including very high mortality rates,” Dr. Papavasiliou stated. “This study and recent work by others demonstrates that targeting chromatin-interacting proteins offer a promising new avenue to develop therapeutics.”