RNAi in mosquitoes

The recognition and destruction of dsRNA has recently been recognized as an intracellular antiviral response against insect viruses of medical importance, and is the subject of a number of arbovirus-vector competence studies. In addition, mosquito cells support RNAi and homologues of the Drosophila RNAi components are encoded by the genome of Anopheles gambiae, which transmits the O'nyong nyong alphavirus and Aedes aegypti, the yellow fever virus vector (Waterhouse et al., 2007). It has been shown that replicating viral RNAs are naturally targeted for destruction by RNAi in mosquito cells (Li et al, 2004), and increased viral loads have been reported in A. gambiae upon silencing of the gene encoding the AGO-2 protein (Keene et al, 2004).

Previous experiments indicated that RNA-based strategies could be used to control vector-borne viruses of public health importance. For example, expression of dengue virus genome fragments in mosquito cells or in vivo inhibited later dengue virus infection and replication (Olson et al., 1996; Adelman et al., 2001). Similarly, A. aegypti were genetically modified to express an inverted repeat RNA

that derives from the dengue type 2 virus (DENV2) prM gene, under the control of a midgut-specific promoter. These transgenic mosquitoes express a dsRNA product corresponding to prM sequences after they imbibe a bloodmeal, thus triggering anti-DENV2 RNAi and thereby keeping the virus from disseminating to the host salivary glands (Franz et al., 2006). Thus, by genetically manipulating A. aegypti to enhance RNAi, the competence of such mosquitoes to serve as vectors for dengue viruses becomes greatly reduced.

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