By David N. Leff
From assault rifles to machetes to clubs, genocide and military coups have turned the subSaharan midriff of Africa into a vast killing field. Aerial warfare, too, compounds the massacre ¿ piloted by mosquitoes shooting up victims with malarial parasites, and tsetse flies that inflict sleeping sickness.
That disease, once bad-mouthed by European colonizers as ¿African lethargy,¿ now known as trypanosomiasis, threatens an estimated half a million men, women and children with certain death. Besides these hapless humans, the lethal tsetse-fly-borne affliction destroys huge numbers of domestic cattle, which are the staff of life for millions of Africans. (See BioWorld Today, Dec. 3, 1998, p. 1.)
The trypanosome parasites air-freighted by the tsetse flies are long, slender, curvaceous cells about 15 micrometers long. Some 300 of these fast-moving pathogens could span the diameter of this dot (.)
A single bite by the saw-toothed proboscis of the blood-sucking tsetse (Glossina species) ¿can inject perhaps a few hundred trypanosomes into its victim¿s skin,¿ said biological chemist Paul Englund, of Johns Hopkins School of Medicine in Baltimore. ¿But you can actually get the infection with only one parasite,¿ he added, ¿because they are highly efficient pathogens.¿
Englund and his research team are zeroing in on how the parasite wreaks its havoc on the human system. ¿One of their amazing properties that we have been studying,¿ he recounted, ¿is called antigenic variation. The invading trypanosomes live in the blood stream,¿ he explained, ¿so they¿re very susceptible to the immune system. What happens is that antibodies of the immune defenses will temporarily eliminate the infection, because they recognize most of the cells as foreign invaders. But what happens next is that a few of the cells will undergo variation will change their surface coat.
¿They disguise themselves,¿ Englund went on, ¿by putting on a new coat, to evade immune recognition. The immune defenses see through this deception, and destroy the cells a week or 10 days later. But meanwhile, new coat variants will come up, so the parasite is able to go on living in its victim¿s blood, which is normally a very hostile environment for a foreign invader. By continually changing their coat, they can evade the immune counterattack.¿
Coat Protein: Now-You-See-it, Now You Don¿t
That constantly replaced surface coat consists of 10 million copies of this quick-change barrier protein. ¿If you look at them by the electron microscope,¿ Englund went on, ¿you can see that this surface coat completely, seamlessly covers the whole parasite. If the coat had breaks in it, then the immune system could attack structures underneath the coat. So the only thing the immune system sees is this array of surface-coat proteins. They all adhere tightly to the parasite cell¿s plasma membrane, to which they cling by a complex glycolipid (GPI) anchor.¿
The Hopkins researchers perceived that GPI anchorage as the parasite¿s potential Achilles heel. The cell surface membrane and GPI stick together via a sticky, fatty acid 14 carbons long, named myristate. Scientists seeking to break trypanosome¿s hammerlock on the human immune system have long assumed that the parasite could not synthesize myristate, but scavenged this crucial component from its victim¿s blood. In today¿s issue of Science, dated April 7, 2000, the Hopkins group disproved this supposition in a paper titled, ¿Specialized fatty acid synthesis in African trypanosomes: Myristate for GPI anchors.¿
¿It was done by my predoctoral fellow, Yasu Morita,¿ Englund told BioWorld Today. ¿One of the questions we were asking is why the parasites use myristate, for which they have an enormous appetite. GPI anchors are very common in all cells, including human ones, but none of them have myristate. So Morita gave the parasite¿s cells radioactive lauric acid, a shorter, 12-carbon fatty acid.
¿In that cell-free in vitro system,¿ Englund recounted, ¿he found that the short fatty acid was incorporated into GPIs. So we thought we¿d try it in a cell, and then we could make a trypanosome that had 12-carbon fatty acids in its GPI anchor. When Morita did this experiment, he found that the 12-fatty acid was very efficiently elongated, from 12 to 14 carbons ¿ that is, to myristate.
¿We had no idea why the trypanosomes would have this elongation mechanism, when in fact they¿d never seen 12-carbon fatty acids in their own cells or the blood of their victims. Then we thought that perhaps this elongation is part of a synthetic reaction, and maybe trypanosomes really can synthesize fatty acids, including myristate. So Morita tried the experiment and it worked.
¿He also found,¿ Englund continued, ¿that an experimental drug called thiolactomycin will inhibit this fatty acid synthesis pathway, and kill the parasite. So that raises the possibility,¿ he observed, ¿that this pathway is a target for developing new drugs for trypanosomiasis.¿
Drug Targets: Versatile, Unaffordable
¿Thiolactomycin is an antibiotic, developed by a Japanese pharmaceutical company about 20 years ago,¿ Englund continued. ¿In preliminary tests it actually cured mice of bacterial infections, but the problem with thiolactomycin is that when it goes into the blood, it clears very quickly; it doesn¿t stay around long enough. So it¿s not being used any more.¿ However, Englund¿s Science paper noted, ¿a recent finding that thiolactomycin inhibits growth of the malaria parasite raises the possibility that fatty acid synthesis can be a versatile drug target in diverse parasitic infections.¿
But on the down side, the Hopkins researcher pointed out that, ¿pharmaceutical companies are not very interested in developing drugs for tropical diseases. The people who have this disease are not in a position to pay for expensive drugs. One of the frustrating things about trypanosomiasis,¿ he added, ¿is that the company that developed one of the best drugs for late-stage sleeping sickness, called difluoromethyl ornithine, transferred its license to Ilex Pharmaceuticals Inc., of San Antonio, Texas, which has said it can¿t provide it for less than $750 per patient.
¿There¿s a tremendous need for drugs to treat this disease, which is getting much worse in Africa,¿ Englund stated. ¿What with civil unrest and wars, and the government infrastructure collapsing, trypanosomiasis becomes very rampant. And unless the 500,000 people who have this disease," he concluded, "are treated, all of them will die. If they are treated, then they can all be saved."