Researchers determine how mosquitoes survive dengue virus infection - PLoS

[Sally Furniss]

Researchers determine how mosquitoes survive dengue virus infection

Colorado State University researchers have discovered that mosquitoes that transmit deadly viruses such as dengue avoid becoming ill by mounting an immediate, potent immune response. Because their immune system does not eliminate the virus, however, they are able to pass it on to a new victim. In a study published February 13 in the open-access journal PLoS Pathogens, the researchers show that RNA interference ? the mosquito immune response -- is initiated immediately after they ingest blood containing dengue virus, but the virus multiplies in the mosquitoes nevertheless.
Dengue fever and dengue hemorrhagic fever are major global public health burdens, with up to 100 million cases occurring annually, yet no vaccines or specific preventative medicines are currently available. The Aedes aegypti mosquito transmits dengue virus. Determining how the virus evades the mosquito's defense is an important next step in research that aims to fight disease by interrupting the growth of dengue virus within the mosquito before it can be transmitted.
RNA interference is an evolutionarily ancient antiviral defense used by mosquitoes and other invertebrates to destroy the RNA of many invading arthropod-borne viruses. This team of researchers previously showed that ramping up the RNA interference response in mosquitoes prevented dengue infection, and now they show that temporarily impairing this immune response increased virus transmission.
The investigators analyzed RNA from adult mosquitoes, finding that both the trigger and initiator molecules for RNA interference were formed after infection, yet viral RNA could readily be detected in the same mosquitoes. They also measured infectious virus rates in the mosquitoes' saliva, which revealed levels whereby the mosquitoes could transmit the disease to humans.
These findings indicate that genetic manipulation of RNA interference could be a significant weapon in stopping dengue virus transmission by Aedes aegypti.

http://www.eurekalert.org/pub_releases/2009-02/plos-rdh020909.php

[Sally Furniss]

Re: Researchers determine how mosquitoes survive dengue virus infection - PLoS

Dengue Virus Type 2 Infections of Aedes aegypti Are Modulated by the Mosquito's RNA Interference Pathway

Irma S?nchez-Vargas, Jaclyn C. Scott, B. Katherine Poole-Smith, Alexander W. E. Franz, Val?rie Barbosa-Solomieu, Jeffrey Wilusz, Ken E. Olson, Carol D. Blair^*
Arthropod-borne and Infectious Diseases Laboratory, Department of Microbiology, Immunology and Pathology, Colorado State University, Fort Collins, Colorado, United States of America
Abstract

A number of studies have shown that both innate and adaptive immune defense mechanisms greatly influence the course of human dengue virus (DENV) infections, but little is known about the innate immune response of the mosquito vector Aedes aegypti to arbovirus infection. We present evidence here that a major component of the mosquito innate immune response, RNA interference (RNAi), is an important modulator of mosquito infections. The RNAi response is triggered by double-stranded RNA (dsRNA), which occurs in the cytoplasm as a result of positive-sense RNA virus infection, leading to production of small interfering RNAs (siRNAs). These siRNAs are instrumental in degradation of viral mRNA with sequence homology to the dsRNA trigger and thereby inhibition of virus replication. We show that although dengue virus type 2 (DENV2) infection of Ae. aegypti cultured cells and oral infection of adult mosquitoes generated dsRNA and production of DENV2-specific siRNAs, virus replication and release of infectious virus persisted, suggesting viral circumvention of RNAi. We also show that DENV2 does not completely evade RNAi, since impairing the pathway by silencing expression of dcr2, r2d2, or ago2, genes encoding important sensor and effector proteins in the RNAi pathway, increased virus replication in the vector and decreased the extrinsic incubation period required for virus transmission. Our findings indicate a major role for RNAi as a determinant of DENV transmission by Ae. aegypti.

Author Summary

Dengue viruses, globally the most prevalent arboviruses, are transmitted to humans by persistently infected Aedes aegypti mosquitoes. Understanding the mechanisms mosquitoes use to modulate infections by these agents of serious human diseases should give us critical insights into virus?vector interactions leading to transmission. RNA interference (RNAi) is an innate defense mechanism used by invertebrates to inhibit RNA virus infections; however, little is known about the antiviral role of RNAi in mosquitoes. RNAi is triggered by double-stranded RNA, leading to degradation of RNA with sequence homology to the dsRNA trigger. We show that dengue virus type 2 (DENV2) infection of Ae. aegypti by the natural route generates dsRNA and DENV2-specific small interfering RNAs, hallmarks of the RNAi response; nevertheless, persistent infection of mosquitoes occurs, suggesting that DENV2 circumvents RNAi. We also show that DENV2 infection is modulated by RNAi, since impairment by silencing expression of genes encoding important sensor and effector proteins in the RNAi pathway increases virus replication in the vector and decreases the incubation period before virus transmission. Our findings indicate a significant role for RNAi in determining the mosquito vector's potential for transmitting human diseases.

http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1000299