Malaria parasites on the move

In numbers terms, malaria is probably the world’s most important tropical disease. But its lifecycle - moving from a mosquito’s blood meal into the insect’s salivary glands and then back into human blood via a liver cell - makes studying some aspects of malaria biology very difficult. Dennis Klug has been studying what goes on mid-way along the mosquito’s intestine where it forms a structure called an oocyst where the infecting parasites multiply. Speaking with Chris Smith, he’s found a critical gene that releases the progeny into the mosquito’s blood...

Dennis - The parasite has to colonise first the midgut of the mosquito and develops into a stage that is called the oocyst where it multiplies. From this oocyst, parasites are released into the blood of the mosquito. These parasites are called sporozoites and these sporozoites then invade the salivary glands and can then be transmitted with the next bite to a new host. We are interested how the sporozoites are able to do that. It egress from the oocysts and to invade the salivary glands. So what we do is we take infected mosquitoes and then we extract the salivary glands and then we can extract these sporozoites and then we just imaged some. These sporozoites are highly motile cells and make continuous circles on this microscopic slide which makes some very easy to image.

Chris - What about the oocyst though, because that’s a different beast entirely, the one that’s sitting in the midgut of mosquito and needs to get into the blood of the mosquito? It can't be very easy to see in there.

Dennis - What we are really interested in is the motility of these sporozoites. We know that there are certain surface proteins called adhesins that play a crucial role in this interaction of the sporozoite with its environment. We found a new protein that was not previously investigated. By making a gene deletion of this protein, we found that the sporozoites are no longer able to egress from this oocyst. Probably because of a motility defect that makes it unable to break free from this rigid membrane.

Chris - What's the protein and how did you find it in the first place?

Dennis - We believe the protein is a surface protein and we found this protein because the adhesins that I described already in the literature, they have a very specific main organisation and we looked out for proteins that have a similar domain organisation and we found the specific one, and then we investigated this one by different genetic approaches – taking the protein to fluorescent tag or by gene deletion.

Chris - So you knock it out and the sporozoites can no longer escape from this oocyst. Have you done the experiment to put it back in and prove just by discretely putting that protein back in, you can restore the ability of the sporozoites to get where they need to go? Thus, proving it must be that protein that’s responsible.

Dennis - Yeah, exactly. We did two different gene knockouts and the next step, we rescued these by putting the gene back and once the gene is back in place, sporozoites are again able to egress from the oocysts and also to invade the salivary glands.

Chris - Do you have a feel for what these proteins do yet or do you know that they're only involved in helping the sporozoites to move?

Dennis - It’s not known exactly what these proteins are actually doing. There are some speculations that some of these adhesins have adhesive properties, parasites needs them to adhere to substrates. It could also be that some of these proteins play a role as a sensor. The parasites need them to interact with the environment and to sense where it is or if it is in close proximity to other parasites.

Chris - Are they used exclusively in the escape from the oocyst or does the malaria parasite use these same genes or the same protein at any other point in its life cycle because it’s got to go through a lot of transitions between different tissues and different cells in switching between the different hosts it infects, hasn’t it?

Dennis - This specific protein that I investigated in the study was also important for the invasion of salivary glands because we found that in some mosquitoes, some of these sporozoites are able to egress from the oocyst probably because of mechanical stress and these sporozoites are then also not able to invade the salivary glands. It is known that other adhesins play a role in salivary gland invasion for example in motility but also in migration through the skin once these sporozoites are deposited in the skin of a bitten human for example.