MRC National Institute for Medical Research, London
Science for Health
Understanding malaria parasite egress
14 May 2010
Malaria kills over 1 million people each year, mostly children below the age of five. It is an important cause of poverty across the developing world, because people who are ill with the disease cannot work and need to be looked after by dependents or health care workers. Malaria is also a constant threat to travellers; there are thousands of cases of severe malaria each year in people returning from malaria-endemic areas.
The most dangerous form of malaria is caused by Plasmodium falciparum, a single-celled parasite which invades and grows within a membrane-bound vacuole (called the parasitophorous vacuole) inside red blood cells. The parasite finally bursts the vacuole and red cell in a process called 'egress', releasing invasive forms called merozoites which quickly invade new red cells. We know very little about how egress works, although previous work from NIMR and elsewhere has shown that the parasite uses at least two enzymes called proteases during egress.
Mike Blackman (pictured) in NIMR’s Division of Parasitology, together with collaborators from the USA and the London School of Hygiene and Tropical Medicine, has discovered two previously unknown additional steps in the pathway leading to egress, involving enzymes called kinases. The researchers used two main approaches. In the first they used a drug called Compound 1 which very specifically inhibits a parasite kinase called PKG. It has previously been shown that Compound 1 blocks egress very efficiently and that egress requires discharge of a parasite protease called SUB1 into the parasitophorous vacuole. This new research has shown that PKG activity is needed for SUB1 discharge, nicely explaining the block in egress caused by Compound 1. Interestingly, when parasites treated with Compound 1 were mechanically disrupted, the released merozoites were not invasive, showing that they need exposure to SUB1 to become invasive.
In the second approach parasites were genetically modified using a recently developed technique that enables a single defined parasite gene to be 'switched off' in the presence of a particular drug called Shield-1. It was found that when Shield-1 was added to cultures of the modifed P. falciparum, SUB1 discharge occurred normally but the parasites still became 'stuck' in their host red blood cells. This indicates that a kinase called CDPK5 is required late in the egress pathway. In contrast to the situation with Compound 1-treated parasites, when these parasites were mechanically released from the red cells they could still invade, showing that although CDPK5 is required for egress, it is not required for invasion of a new cell.
Producing a vaccine against malaria has proven difficult, so currently prevention and treatment of the disease is largely dependent on antimalarial drugs. As resistance to these existing drugs spreads they are becoming less useful, so many researchers are urgently seeking new ways to attack the disease. One way to do this is to identify essential biochemical pathways that the parasite needs to complete its life cycle in infected patients, then find drugs that selectively target these pathways. Taken together, these new pieces of information provide exciting new insights into the pathway leading to egress of the parasite, information that is invaluable for the development of drugs designed to prevent egress and prevent the progression of malarial disease in an infected person.
The malaria parasite is only one of dozens of important bacterial and protozoal pathogens that grow inside cells of their hosts. All these microrganisms eventually egress from their host cells, but we generally know very little about how egress is regulated. Our work may provide clues to egress in these other pathogens too.
Mike Blackman
Knockdown of CDPK5 results in a block in egress
Microscopy of synchronized parasites grown [+] or [-] Shield-1 at indicated hours post invasion (h.p.i.). Pie charts show relative number of rings, trophozoites, and schizonts (the mature, multinucleated intracellular parasite). The latter accumulate in the cultures without Shield-1, indicating a block in egress.
Original article
The research findings are published in full in:
Jeffrey D. Dvorin, Derek C. Martyn, Saurabh D. Patel, Joshua S. Grimley, Christine R. Collins, Christine S. Hopp, A. Taylor Bright, Scott Westenberger, Elizabeth Winzeler, Michael J. Blackman, David A. Baker, Thomas J. Wandless, Manoj T. Duraisingh. (2010)
A plant-like kinase in Plasmodium falciparum regulates parasite egress from erythrocytes.
Science, 326(5980): 910-12. Publisher abstract
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