Malaria researchers take closer look at pathogen inside red blood cell

October 27, 1995

By Lynn Yarris,

The Advanced Light Source is providing a revealing look at one of the oldest and most persistent of all human diseases -- malaria. Cathie Magowan, a parasitologist in the Life Sciences Division (LSD), is using the x-ray microscopy beamline at the ALS to obtain never before seen views of the malarial parasite inside a red blood cell.

According to the World Health Organization, each year 300 to 500 million people living in the tropics and subtropics become infected with malaria, suffering burning fever and severe pain. Nearly three million -- mostly children -- die. Medical researchers have been unable to stamp out a scourge described in 4 B.C. by Hippocrates, Magowan says, because the parasite's complex life-cycle makes it an extremely tough opponent.

Malaria is caused by the plasmodium parasite, which is transmitted to humans through the bite of an infected female anopheles mosquito. The parasite enters the bloodstream and travels straight to the liver, where it is safe from any counteraction.

"When the parasite is introduced into the bloodstream, there is less than 30 minutes of vulnerability before it gets into the liver," says Magowan, who has been studying Plasmodium falciparum, the deadliest strain.

Once in the liver, it replicates itself many thousands of times before being released back into the bloodstream in a form that invades red blood cells. Once inside the red blood cell, it matures into a form that can offer up a shifty defense against the body's immune system.

"The parasite synthesizes a family of proteins that affect the adhesive properties of the host red blood cell," says Magowan. These proteins act in concert with tiny protrusions, called "knobs," which the parasite induces on the surface of the red blood cell. The combined effect is to anchor the cell to the wall of a capillary and prevent it from being transported to the spleen for destruction--the fate that normally befalls damaged blood cells.

While the red blood cell is in the blood stream, the parasite feeds on hemoglobin and multiplies. After 48 hours, the cell ruptures and the new parasites invade more red blood cells and repeat the cycle.

"Once inside the red blood cell, the parasite can keep changing the configuration of the new surface proteins so that the immune system doesn't have time to identify each new variant," Magowan says. Untreated, a malaria infection can last up to 100 days."

Electron microscopes have been used to obtain valuable information about the parasite inside a cell, but the cells had to be dehydrated and sliced into thin sections before they could be imaged. Consequently, there have been few detailed images of the malaria parasite within an intact cell.

Using ALS beamline 6.1, also known as the x-ray microscope, or XM-1, Magowan plans to record the entire cycle of the parasite in the red blood. She is working on the project with LSD's Mohan Narla, plus Werner Meyer-Ilse, John Brown, and John Heck at the Center for X-ray Optics, and researchers from Monash University in Australia.

"Until recently, no one had ever studied the malaria parasite inside the red blood cell with an x-ray microscope," says Magowan. "XM-1 gives us five times the resolution of light microscopes."

Already, Magowan and her colleagues have produced the clearest, most detailed images ever obtained of a malaria-infected red blood cell. One of the first areas being investigated is the interaction between the respective membranes of the parasite and the host cell. Previous studies by Magowan and her colleagues lead them to believe that the malaria parasite's ability to survive depends upon this interaction.

Membranes are the interfaces of the biological world, the site where cells interact with the exterior environment. Magowan and colleagues are studying the association between a protein expressed by the parasite called MESA (mature-parasite-infected erythrocyte surface antigen) and a protein in the red blood cell membrane called protein 4.1.

"It is known that MESA binds to protein 4.1 but no one has determined the consequence of that association," Magowan says. "Our findings suggest that MESA binding to protein 4.1 plays a major role in the parasite's viability."

Magowan is also using XM-1 to study other important proteins, including one that is in the knobs on the surface of an infected red blood cell, and one that binds infected cells to non-infected cells. In the future, when a scanning x-ray microscope comes on line at the ALS, she will use a fluorescent labeling technique being developed by LSD's Mario Moronne to locate and track the movements of various antigens in and out of both parasite and red blood cell membranes.

"X-ray microscopy should provide a better understanding of the ways in which infection with a malaria parasite changes a red blood cell," says Magowan. Such knowledge could help researchers finally tame this ancient beast.