How The Malaria Parasite Hijacks Human Red Blood Cells

"It's a nice piece of biology revealing how the parasite survives in and totally changes red blood cells," said Alan Cowman of The Walter and Eliza Hall Institute of Medical Research in Australia. Now that those players have been found, "there may be some way of inhibiting these processes by drugs or possibly a live vaccine."

Plasmodium falciparum causes the most severe form of malaria in humans with one to three million deaths annually, the researchers said. Once in the blood, multiplication of the parasite inside red blood cells (also known as erythrocytes) is responsible for its severity and mortality associated with the disease. After the parasite invades, the red cells undergo profound structural and morphological changes, dramatically altering their physical properties and impairing circulation. In contrast to normal red blood cells, parasitized cells are rigid and adhere to the lining of the blood vessels and other cell types.

Those changes are known to be caused by proteins the parasite produces inside the cells of its host and exports across several membranes out to the red cell itself. Earlier studies showed two important ingredients: P. falciparum erythrocyte membrane protein (PfEMP1), which allows infected cells to stick to blood vessels, and knobs made up of a second protein (knob associated histidine-rich protein or KAHRP) that anchor PfEMP1 at the red cell surface.

In the new study, the researchers were particularly interested in finding proteins required for "trafficking" PfEMP1 to the infected erythrocyte surface, correct assembly of the anchoring knobs, and those involved in making infected red cells rigid, all processes associated with virulence in malaria infection, they said. In search of those players, the researchers developed mutant strains of P. falciparum each lacking one of 83 genes known or predicted to play a role in the red cell remodeling process.

Their screen turned up eight genes encoding proteins required for export of the PfEMP1 and assembly of knobs. Additionally, they show that multiple proteins play a role in generating increased rigidity of infected erythrocytes.

"In summary," the researchers wrote, "we have used a gene knockout approach on a scale not previously attempted in this organism to address the role of P. falciparum proteins that are exported into the parasite-infected erythrocyte. Collectively these proteins act like the secretion systems seen in bacteria in which pathogenicity arises from secreted proteins that interact with host cells by direct injection or by their presence in the extracellular milieu. ...It may be valuable to adopt approaches being tested in bacteria in which these systems are the target of new therapeutic approaches aimed at minimizing pathogen virulence."

However, Cowman added, while they now know the identity of the several proteins involved, they still don't know what those proteins actually do. Those are questions Cowman's team now plans to pursue in greater detail.

The researchers include Alexander G. Maier, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Melanie Rug, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Matthew T. O'Neill, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Monica Brown, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Srabasti Chakravorty, Liverpool School of Tropical Medicine, Liverpool, UK; Tadge Szestak, Liverpool School of Tropical Medicine, Liverpool, UK; Joanne Chesson, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Yang Wu, Liverpool School of Tropical Medicine, Liverpool, UK; Katie Hughes, Liverpool School of Tropical Medicine, Liverpool, UK; Ross L. Coppel, Monash University, Department of Microbiology, Clayton, Australia; Chris Newbold, University of Oxford, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, UK; James G. Beeson, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; Alister Craig, Liverpool School of Tropical Medicine, Liverpool, UK; Brendan S. Crabb, The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia; and Alan F. Cowman; The Walter and Eliza Hall Institute of Medical Research, Melbourne, Australia.