By David N. Leff
Living things come in two persuasions: Eukaryotes are the higher - more complex - forms; they have cell nuclei. Prokaryotes are simpler - unnucleated.
Nowadays, in the laudable campaigns to rescue living organisms from imminent extinction - whales, pandas, spotted owls, vanishing birds and plants, insects and other forms of life - there is one eukaryote that scientists around the world are heaven-bent on rendering extinct.
No animal-righteous group is demanding the rescue and protection of this threatened species, eukaryotic though it be. Its name: Plasmodium falciparum - the parasitic pathogen of human malaria. This deadly nose-cone payload rides a guided missile called Anopheles gambiae - i.e., the mosquito that sows disease and death throughout tropical Africa.
One researcher intent on rendering P. falciparum's coup de grace is parasitologist Oumou Niare, at the Malaria Research and Training Center in the Republic of Mali's capital, Bamako. She is a co-author of a paper in the current Proceedings of the National Academy of Sciences (PNAS), dated Oct. 10, 2000. Its title: "Genes identified by an expression screen of the vector mosquito Anopheles gambiae display differential molecular immune response to malaria parasites and bacteria."
Niare's close collaborator, Kenneth Vernick, who directs a molecular parasitology laboratory at New York University, is the article's senior author. Just back from a field trip to Mali, Vernick told BioWorld Today: "I have a long-term collaboration with the group there. We've made a revolving door between laboratory and field. Things we find in the lab we can test in the field - look at populations, and run a reality check to see if this is a laboratory artifact or does it really happen. How much influence does it have in the natural malaria transmission system? Things we find in the field," Vernick added, "we can take to the lab and study in a defined setting."
African Mosquitos Sleep Off Their Bloodmeal
"It's highly organized," he went on. "There are remote villages that participate in different studies organized by the Malians. We go out to villagers' houses and collect mosquitos that are resting. In the morning what we catch are mosquitos that bit people the previous night. They're so full of blood that they can't fly out the window. So they rest in the clothing or under the table. We find a wealth of information in these sated insects: We can tell how many times they've taken blood. How many times they've been infected with malaria parasites. And we can analyze by molecular genetic strategies genotypes that may be involved in blocking parasite development." These strategies rely largely on an original improvement in subtractive hybridization technique.
As he reported, Vernick's first goal was "casting the nets widely to screen for genes that are regulated during the immune response of this Anopheles gambiae mosquito. Our second goal was more specific. Among the fish that came back in the net," he recounted, "we explored the specificity of immune response in this insect to various pathogens. It's never been shown before that mosquitos - or for that matter, any insects - can distinguish between infection with bacteria and eukaryotic parasites, in this case malarial parasites. Invertebrate immunity still remains a little-understood black box.
"Parenthetically," he observed, "this should interest people who study the molecular interactions between mosquito vectors and malarial parasites from the potential point of view of developing novel strategies to control malaria transmission."
Vernick made the point that the invertebrate immune system is the evolutionary precursor of the mammalian immune system. "In mammalian immunity," he pointed out, "immunologists study antibodies and T cells - the arm called adaptive immunity. There's also been a murky part of the immune response called 'innate immunity.' That was never very well understood. It's now clear that there's a whole innate immune system that invertebrates developed. Vertebrates have the same innate thing as a foundation, but at a certain point they added the adaptive immune system on top, which insects don't have - as far as we know.
"But to the extent that we can understand molecular signals of the parasite that trigger an immune response by the mosquito," Vernick continued, "it could conceivably help us to design potential malaria vaccines. These would induce an antibody response and also drag into the mix an efficient innate immune reaction.
"When Plasmodium parasites infect their mosquito hosts," he explained, "those insects get sick. Their immunity costs them in decreased longevity, fecundity, less egg-laying, decreased flight distance and time. There's an evolutionary dance between Anopheles and Plasmodium. On the one hand, very large numbers of the parasite impact the infected mosquito. On the other side, zero parasite per mosquito clearly impacts the parasite because it's not transmitted. So their evolutionary compromise is basically, 'You don't kill me and I won't kill you.'
"There may be ways that we can subvert that evolutionary agreement," Vernick speculated. "One possible strategy is - instead of allowing the parasite to slip by undetected with small numbers in a mosquito - we could conceivably cause an immune gene to be expressed in response to a bloodmeal, but one that normally doesn't trigger that response. For example, we found a gene that responds pretty specifically to malaria parasite infection - and not to bacterial infection."
Specific Gene Nudges Parasite Toward Oblivion
"So a promoter from such a gene could be used to drive a distinct parasiticidal gene that kills parasites very efficiently. Conversely, we could drive a mosquito's anti-parasite gene with a different promoter, so every time Anopheles takes a bloodmeal, it releases a kind of sterilizing factor into the blood that kills parasites.
"One reason that there's an evolutionary agreement between these two organisms," he observed, "is that neither one of them wants to go extinct, and it would be our job to drive one of them extinct - the parasite in a strategy like this. Typically in nature, almost all infected mosquitos carry fewer than 10 oocysts in their midgut, and mostly in the range of one to three. So the problem for the parasite," Vernick concluded, "is that they're actually close to the edge of reproduction. Which means it wouldn't take a very strong push, or a very long push, to really topple them."