"These findings reveal an important new detail about the molecular mechanism Toxoplasma uses to penetrate cells," says study leader L. David Sibley, Ph.D., professor of molecular microbiology. "Identifying the molecular interactions that govern motility may allow development of small molecules that block this mechanism and perhaps lead to new treatments for diseases such as toxoplasmosis and malaria. That's an ambitious undertaking, and there are many hurdles to overcome, but given the importance of these diseases, it is a possibility we must explore."
About 35 million people in the United States - and up to a quarter of the world's population - are thought to be infected with Toxoplasma. But only those with weakened immunity typically develop severe toxoplasmosis, which can lead to birth defects, brain inflammation and vision problems. People usually acquire the infection by accidentally swallowing spores from contaminated soil, water, cat litter or objects that have had contact with cat feces or by eating raw or undercooked meat, especially chicken, pork, lamb or venison.
Once consumed, the parasite bores into cells, where it reproduces asexually. These organisms then bore into other cells and reproduce again.
Previous research had shown that Toxoplasma uses a protein known as MIC2 (micronemal protein 2) to recognize and attach to host cells. MIC2 molecules bind to receptors on the host cell, thus gripping the membrane. The parasite moves these adhesin-receptor complexes over its surface from front to back like a tractor tread. This concerted action propels the host membrane around the parasite as it enters the cell.
Scientists also knew that the parasite uses its elaborate cytoskeleton as a mini-muscle--a tiny motor of actin and myosin proteins--to power the tractor tread. But it was unclear how the motor was connected to the tractor tread. Some scientists believed that MIC2 was linked directly to the mini-muscle. Sibley and other scientists, however, could find no evidence of a direct interaction.
That's when Sibley and first-author Travis J. Jewett, a doctoral student in the Program in Molecular Microbiology and Microbial Pathogenesis, went fishing with the tail of the MIC2 molecule. They wanted to find what that tail piece - which projects through the cell membrane to the inside of the one-celled parasite - would bind to. And that led them to aldolase.
The Sibley laboratory now is studying the interaction between MIC2 and aldolase in greater detail to determine the specific residues in each protein that are involved. Knowing the details of how Toxoplasma penetrates cells also will improve the understanding of how related parasites cause infection. These include parasites like malaria that enter host cells using an analogous mechanism.
"We were surprised to discover that aldolase served this function in Toxoplasma," says Sibley. "It shows that aldolase and perhaps other proteins can hold dual roles in the cell. We should keep that possibility in mind when assessing the possible function of proteins."
Jewett TJ, Sibley LD. Aldolase forms a bridge between cell surface adhesins and the actin cytoskeleton in apicomplexan parasites. Molecular Cell, April, 2003.
Funding from the National Institute of Allergy and Infectious Diseases supported this research.