The importance of glycosylation for protein drugs and vaccines has been well established, as companies spend millions of dollars either isolating or creating the appropriate glycosylated form of their desired compound—for increasing efficacy and extend the half-life of the drug. Yet, for the malaria parasite Plasmodium falciparum, researchers have often discounted carbohydrate moieties, since the parasite lacks much of the machinery to create the complex glycosylation patterns similar to other eukaryotic organisms.
However now, an international team of researchers led by investigators at the Walter and Eliza Hall Institute (WEHI) has just shown that carbohydrates on the surface of malaria parasites play a critical role in malaria's ability to infect both mosquito and human hosts. Moreover, the researchers suggest steps that may improve the only malaria vaccine approved to protect people against Plasmodium falciparum—the deadliest form of malaria. Findings from the new study were published today in Nature Communications in an article entitled “Protein O-Fucosylation in Plasmodium falciparum Ensures Efficient Infection of Mosquito and Vertebrate Hosts.”
“Malaria parasites have a complex lifecycle that involves constant shapeshifting to evade detection and infect humans and subsequently mosquitoes,” explained senior study investigator Justin Boddey, Ph.D., associate professor and laboratory head at WEHI. “We found that the parasite's ability to 'tag' key proteins with carbohydrates is important for two stages of the malaria life cycle.”
“It is critical for the earliest stages of human infection when the parasite migrates through the body and invades in the liver, and later when it is transmitted back to the mosquito from an infected human, enabling the parasite to be spread among people,” Dr. Body added. “Interfering with the parasite's ability to attach these carbohydrates to its proteins hinders liver infection and transmission to the mosquito, and weakens the parasite to the point that it cannot survive in the host.”
With close to half of the world’s population living in areas endemic for malaria—causing 200 million cases annually and 650,000 deaths—efforts to eradicate the disease require the development of new therapeutics, particularly an effective malaria vaccine. The first malaria vaccine approved for human use—RTS,S/AS01—was approved by European regulators in July 2015, but has not been as successful as hoped, with marginal efficacy that wanes over time.
“The protein used in the RTS,S vaccine mimics one of the proteins we've been studying on the surface of the malaria parasite that is readily recognized by the immune system,” noted co-senior study investigator Ethan Goddard-Borger, Ph.D., laboratory head in the division of chemical biology at WEHI. “It was hoped that the vaccine would generate a good antibody response that protected against the parasite. However, it has unfortunately not been as effective at evoking protective immunity as hoped. With this study, we've shown that the parasite protein is tagged with carbohydrates, making it slightly different to the vaccine, so the antibodies produced may not be optimal for recognizing target parasites.”
In the current study, the research team looked closely at the O-glycosylation patterns of Plasmodium parasites, specifically at the surface proteins CSP and TRAP (circumsporozoite protein and thrombospondin-related anonymous protein, respectively).
“We identify the Plasmodium protein O-fucosyltransferase (POFUT2) responsible for O-glycosylating CSP and TRAP,” the authors wrote. “Genetic disruption of POFUT2 in Plasmodium falciparum results in ookinetes that are attenuated for colonizing the mosquito midgut, an essential step in malaria transmission. Some POFUT2-deficient parasites mature into salivary gland sporozoites although they are impaired for gliding motility, cell traversal, hepatocyte invasion, and production of exoerythrocytic forms in humanized chimeric liver mice. These defects can be attributed to destabilization and incorrect trafficking of proteins bearing thrombospondin repeats (TSRs).”
With the data from this study and the many previously documented cases where attaching carbohydrates to a protein improved its efficacy as a vaccine, the researchers extrapolated that improvements to current and future malaria vaccines should consider glycosylation patterns.
“It may be that a version of RTS,S with added carbohydrates will perform better than the current vaccine,” Dr. Goddard-Borger concluded. “Now that we know how important these carbohydrates are to the parasite, we can be confident that the malaria parasite cannot 'escape' vaccination pressure by doing away with its carbohydrates.”