Ronald van Rij will give a keynote lecture at the annual scientific meeting of the Netherlands Center for One Health in which Radboud university medical center participates.

What makes the mosquito such a good vector of viruses? To answer this question, Ronald van Rij of Radboud university medical center is carrying out a detailed study of the defense system of the mosquito. In particular, he is looking at the role of small RNA fragments that are produced when the defense system attacks a virus.

“If we want to know why mosquitoes spread disease, we need to understand the immune system.”

Mosquitoes are important vectors of viruses, such as dengue, Zika and yellow fever. But we still know very little about what happens in the mosquito when it is infected with a virus. What determines which mosquito species is a good carrier of a specific virus? Ronald van Rij hopes to answer this question with his study: “We address this from a mechanistic perspective: how exactly does the defense system of a mosquito react to a virus infection?”

What specifically are you looking at?

“We are mainly working on small RNA fragments. The genetic material of a virus (RNA) is recognized by the mosquito as foreign and broken down into small fragments. There are two variants of viral small RNAs. The first class consists of small interfering RNAs (siRNAs; see box). It is now clear that these play an important role in the host defense. If you inactivate the mechanism for the production of these siRNAs in the mosquito, up to a hundred times more virus is produced. The other class of small RNAs are Piwi-interacting RNAs (piRNAs; see box). There is much less information on the role of these fragments in antiviral defense.”

What could this role be?

“One of the most exciting recent findings is that there are fragments of genetic material of different viruses in the genome of mosquitoes. Apparently this material became part of the mosquito’s DNA at some point during evolution. We hypothesize that these fragments are the basis for a form of inherited immunity, which prepares the mosquito to recognize specific viruses. This is however still speculative at this stage.”

So these virus fragments in the mosquito’s DNA strengthen the defense?

“Yes, that is possible. We found that these virus fragments have the potential to make small RNAs, even without virus infection. The mechanism would then already be prepared to destroy incoming viruses. This all needs to be proved, but it’s an exciting hypothesis.”

And do these RNA fragments determine how well the mosquito spreads viruses?

“We find virus fragments in the DNA of several different species of mosquitoes, but mainly in Aedes mosquitoes that spread human viruses, such as the dengue mosquito and the tiger mosquito. As a comparison, in fruit flies we did not find any virus fragments in the genome, in Aedes mosquitoes there are more than one hundred. This may be chance, but there may also be a biological reason for it.”

Is this then related to the risk of virus transmission by Aedes mosquitoes?

“Strikingly, we find more virus fragments in the DNA of those mosquito species that spread human epidemic viruses. Of course, it’s difficult to establish cause and effect. We find a lot of these integrations in the genome, but this still doesn’t mean that this is the reason that they spread more viruses. Perhaps it’s the other way round. Or maybe it’s just chance.”

If it is chance, does this mean our native mosquitoes spread viruses too?

“Yes. And they do, which isn’t all that surprising. The Culex mosquito, for example, spreads the Usutu virus that mainly occurs in birds and is similar to the West Nile virus. You may wonder why these mosquitoes don’t spread Zika or dengue. Probably a large number factors play a role in this. Our work is one piece of the puzzle.”

What do you hope to achieve with your research in the long term?

“My research group is working on virus interactions with both the human host and the vector mosquito. On the human side, for example, we are trying to contribute to the development of inhibitors of mosquito-transmitted viruses. The work on the mosquito has two objectives: to understand how this intriguing class of small RNAs works and to obtain fundamental insight into the spread of mosquito-transmitted viruses. In the long term this may provide us with a basis to make risk assessments. If you understand properly how the mosquito spreads a virus, you may be able to predict which viruses can be transmitted by local mosquitoes in the Netherlands.”

And why one mosquito more efficiently transmits viruses than another?

“Perhaps. If you look at the genome of mosquitoes from different geographic areas, the repertoire of these integrated virus elements is not the same. This may partly explain why some mosquito populations spread viruses better than others. For example, Aedes is a good vector of dengue and Zika, but at the same time we see that one population does this more efficiently than another. Why is this? We still don’t know.”

Does your research also have applications in the long term?

“Possibly. The kind of fundamental work that we are doing has shown, for example, that the symbiotic Wolbachia bacteria protects mosquitoes from virus infection. Mosquitoes infected with Wolbachia are now being released at twelve locations in the world in the hope that the wild mosquito populations are replaced with a Wolbachia-infected population, thus reducing the spread of viruses. Clearly, very fundamental work can have important practical implications.”

siRNA and piRNA

Both small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs) are small pieces of RNA about 20 to 30 nucleotides in length. As the name siRNA implies, these fragments interfere with the replication of viral RNA, thus reducing or eliminating virus infection. This mechanism plays an important role in the defense of plants and invertebrates (including flies and mosquitoes). It is already known that piRNAs play a role in keeping reproductive cells stable. The research group of Ronald van Rij demonstrated only recently that they also play a role in the defense against viruses.

Associate professor Ronald van Rij is head of the laboratory of Experimental Virology at Radboud university medical center’s Department of Medical Microbiology. His research is funded by several large national and international grants, including a Consolidator Grant from the European Research Council (ERC), a VICI grant from the Netherlands Organisation for Scientific Research (NWO), and a research grant from the Human Frontiers Science Program.