Illustration : Purabi Deshpande / Research Matters
When Confucius said, “don’t use a cannon to kill a mosquito”, little did he know how irksome these insects turn to be! In fact, we have almost lost our war with mosquitoes by employing strategies dangerous than the cannons – more to us than to the mosquitoes – like insecticides. Yet, we find ourselves infected with malaria, dengue and others that do not yet have a vaccine. Mosquitoes, being highly adaptive, have developed resistance to most of these insecticides and our overuse of them is partly to be blamed. So, where do we look to in our fight against the mosquitoes?
The good news is there is hope. Scientists have devised a recent approach called ‘RNA interference’ (RNAi), a Nobel winning discovery, where RNAs (Ribonucleic acid) are artificially introduced into cells. Now, recent research have proposed introducing specific RNAs into the cells of mosquitoes that can kill them and help us control their population, addressing the needs of grass root level in health care. In a recent review, a team of researchers led by Dr. A. Balakrishna Pillai from the Central Inter-Disciplinary Research Facility (CIDRF) at Sri Balaji Vidyapeeth University, Puducherry, in collaboration with other researchers, has discussed the mechanism of action, effective methods to deliver these RNA into the bodies of mosquitoes and current trends in the usage of this method on a large scale.
RNA interference is a widely used technique in the field of molecular biology to study basic molecular processes. In this method, a specific gene can be ‘silenced’ or ‘knocked out’ at will by injecting small bits of double stranded RNA molecules into cells (also called siRNAs or interference RNAs). By ‘knocking out’ a specific gene, scientists can tell what function the gene performs by looking at what ‘character’ is lost. Now researchers are using this method to cripple mosquitoes, either adults or larvae, by ‘knocking out’ some essential genes that are vital for the survival of mosquitoes or the pathogens they host.
“RNAi is employed to decipher the role of genes involved in mosquito immune response towards disease causing pathogens. This would allow us to identify the vector molecules that may resist the development of pathogens inside mosquito hosts,” says Dr. Agiesh Kumar, the lead author of this study published in the journal Insect Molecular Biology. So far, RNA interference has been used to study the interaction between the mosquito Anopheles gambiae and the malaria causing pathogen Plasmodium falciparum.
RNA interference evolved as an arm of immune system to fight against foreign double stranded RNA (including RNA viruses). When a foreign RNA enters our cells either by infection or by artificially insertion, it is first broken down into small pieces of a specific length called ‘small interfering RNAs (siRNA)’ by an enzyme called dicer. These siRNAs are then stripped into single stranded RNAs and loaded into a protein complex called RISC (RNA-Induced Silencing Complex), which specializes in cutting double stranded RNA.
But how does the siRNA ‘silence’ a gene? The single stranded siRNA inside the RISC recognizes its target RNA, which is usually a ‘messenger RNA’ (mRNAs) and binds to it. The bounded entity with the target RNA and mRNA will be cut by RISC, resulting in the destruction of mRNAs, and non-production of concomitant protein , which may prove fatal to the cell or the organism. This process is as good as ‘silencing’ the gene, even though the gene itself is expressed into mRNA copies and there is no protein production in the end. Today, scientists use this technique to study or silence a particular gene by deliberately introducing double stranded RNA (dsRNA), which contains the same sequence as that of the target RNA of a desired gene into the cell or an entire organism.
Mosquitoes also raise immune response against the viral pathogen. “RNAi could be used to understand such molecules involved in mosquito immune response that may resist or facilitate the development of pathogens,” says Dr. Agiesh Kumar. By systematically silencing genes that are suspected to be involved in vital interactions, we could gain valuable insights about these interactions that might help us in our fight against mosquito-borne diseases. So far, RNA interference has been used to study the interaction between mosquito Anopheles gambiae and the malaria causing pathogen Plasmodium falciparum. It has also been used to control dengue virus in the mosquito Aedes aegypti. Thus far, through RNAi, we have come to know that mosquitoes also have a complex immune system, which can raise their own RNAi mechanism in an attempt to get rid of these pathogens.
One major impediment that we have been facing in our fight against mosquito-borne diseases is the fact that pathogens almost always develop drug resistance. Mosquitoes also have developed resistance against widely used insecticides like DDT. RNA interference, say the scientists, could alleviate this problem. When genes like Glutathione-S-Transferase in Aedes aegypti and Cytochrome P450 reductase in Anopheles gambiae were targeted and silenced by RNAi, the mosquitoes showed increased susceptibility to permethrin, an insecticide to which they have been resistant hitherto.
With such promising results how can these dsRNA- the initiators of RNAi, be delivered into mosquitoes? For adults, dsRNA can be directly injected into their body using a micro syringe. But this method cannot be applied on a larger scale where the goal is to control or clear mosquito population from an infested habitat. A more effective and practical way is to introduce dsRNA along with larval feed in the water body were the larvae are developing. The hapless larvae feeding on these tiny fragments unwittingly trigger RNAi mechanism in their own bodies against crucial single or multiple genes thus dooming themselves to death. Other methods have also been evolved where dsRNA is packed in lipid vesicles or in nano-particles for a targeted delivery.
RNA based pesticides would have some crucial advantages over traditional chemical pesticides including safety, environment friendliness, minimal toxicity to undesired target animals, and minimization of resistance problem. There are still some hurdles to be overcome though. “A cost effective method needs to be devised and the stability of dsRNA must be ensured before large scale release,” says Dr. Kumar on the future of this technique.
Have we finally found a magic wand in RNAi to rid ourselves of the most irritating pests of all? Only time can tell!