BioWorld International Correspondent
LONDON - Manipulating the immune system of the mosquito could provide a completely novel way of reducing the transmission of the malaria parasite, new research suggests.
The discovery of mosquito proteins that either protect the malaria parasite from killing by the mosquito immune system, or which play a role in killing the parasites, could allow the development of new chemical sprays for mosquitoes, or transgenic mosquitoes with altered sets of immune proteins.
Mike Osta, postdoctoral fellow at the European Molecular Biology Laboratory in Heidelberg, Germany, told BioWorld International: "This work shows us that the mosquito itself can be a battleground against malaria. We have found that shutting off certain genes helps the insects to rid themselves of most of the parasites. This opens up new strategies to kill malaria parasites."
The life cycle of the malaria parasite, Plasmodium, begins when an Anopheles mosquito bites an infected animal. As the insect feeds on the animal's blood, it ingests the Plasmodium, which enters its gut. There, the parasite divides and reproduces, before passing across the insect's gut epithelium and traveling to the salivary glands. When the insect bites another animal, it injects the parasites into the new host, so starting its life cycle all over again.
One feature that had puzzled malaria researchers was that some mosquitoes transmitted malaria, while others of the same species did not. Scientists suspected that proteins of the mosquito's immune system might be responsible for that difference.
Research into the area was greatly helped by the availability in 2002 of the genome sequence of the Anopheles mosquito, which allowed comparison with that of Drosophila. The team of Fotis Kafatos, the director general of the European Molecular Biology Laboratory, concentrated on those genes of Anopheles known to have roles in the immune system, screening them for any effect on Plasmodium infection.
Their strategy was to silence each gene of interest with double-stranded RNA (dsRNA) in a laboratory model of malaria. Researchers in Kafatos' lab developed a method of using microinjection to deliver the dsRNA into the thorax of adult mosquitoes.
After four days, the targeted gene was completely shut down. The mosquitoes were then allowed to feed on mice infected with Plasmodium berghei, the rodent malaria parasite. After another seven days, the midguts of the insects could be dissected to check on the survival of the parasites.
The team used a transgenic parasite containing the gene for green fluorescent protein, which makes it easy to identify live parasites. Live parasites fluoresce green, while those killed by a process called melanization show up under the microscope as black.
An account of the study appears in a paper in the March 26, 2004, issue of Science titled "Effects of Mosquito Genes on Plasmodium Development." Osta is joint first author with George Christophides, also at the EMBL.
The group discovered that when they silenced the mosquito gene called LRIM1, there was a fourfold increase in parasite numbers. "This means that the protein encoded by this gene is normally involved in killing the parasite," Osta said. "When we knocked out two other genes, called CTL4 and CTLMA2, most of the parasites were dead, so we can conclude that they are normally involved in some kind of protection of the parasite."
The team also found that the expression of LRIM was strongly and transiently up-regulated in the mosquito midgut 24 hours after the insects had taken a meal of infected blood.
The group now is trying to replicate its experiments using the human malaria parasite, Plasmodium falciparum. "We are also working on finding out the exact function of the three proteins, and what other proteins they interact with," Osta said.
Commenting on the paper in the same issue of Science, Janet Hemingway and Alister Craig of the Liverpool School of Tropical Medicine in the UK wrote: "[These results] give optimism that new methods of malaria control through blocking transmission in the mosquito vector will be possible. This achievement, however, is only the first small step along the difficult road to practical implementation of such strategies." For example, they said researchers will have to find a way of delivering dsRNA to mosquitoes that is less invasive and that requires minimal human intervention.