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By Jeff Austin
Researchers at Indiana University and the University of Pennsylvania have discovered what they hope will be a new target in the battle against some single-celled parasites, including the malaria-causing Plasmodium and Toxoplasma, which often cause AIDS-related infections.
In the March 7 issue of Science, molecular evolutionist Jeff Palmer and botanist Charles Delwiche of IU Bloomington, along with parasitologists Sabine Kohler and David Roos of the University of Pennsylvania and their colleagues, report that chloroplast-like plastids recently found in a group of single-celled parasites apparently originated in algal cells that were engulfed as the parasite was evolving.
The functions of the plastids in the parasites are unknown (they no longer perform photosynthesis) but the fact they remained after being engulfed suggests they play an important role in this particular class of parasites. And because the cells of humans and other mammalian hosts of the parasites don't have such chloroplast-like plastids, they are likely targets for drug therapies.
It is commonly understood that eukaryotic cells (those of animals, plants, fungi and protists) acquire some of their most important components, such as the photosynthesizing chloroplast, when they engulf simpler bacterial cells. The study conducted by Palmer, Roos and six co-authors supports the idea that a more complex cell, in this case an algal cell, can be taken up by, and became an integral part of, another cell.
In addition to the implications for new drug treatments, the findings also suggest that the process of cells engulfing other cells may have been more common in molecular evolution than previously suspected.
The researchers first suspected they were looking at a case of microbiological thievery when they discovered DNA in the parasites that closely resembled DNA within the chloroplasts of green algae.
They next determined that the purloined DNA's new home was within a separate organelle that appears to be surrounded by four membranes.
The number of membranes would support the idea that the DNA came from an algal cell's chloroplast. The chloroplast has two membranes of its own, in addition to the algal cell's outer membrane. When engulfed, these three membranes would be retained in addition to a remnant of the parasite cell's own engulfing membrane, for a total of four.
While the number of membranes alone is not conclusive evidence of the researchers' theory, genetic analysis conducted by Palmer's lab lends further support.
Understanding the origin and genetic structure of these novel plastids in parasitic cells is the first step in understanding how to eradicate them. Biomedical researchers must now work to determine the critical functions of the plastids to make it possible to design drugs that inhibit these functions and shut down the parasite without harming the host.
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