RNAi in Arthropods: Insight into the Machinery and Applications for Understanding the Pathogen-Vector Interface

Abstract

The availability of genome sequencing data in combination with knowledge of expressed genes via transcriptome and proteome data has greatly advanced our understanding of arthropod vectors of disease. Not only have we gained insight into vector biology, but also into their respective vector-pathogen interactions. By combining the strengths of postgenomic databases and reverse genetic approaches such as RNAi, the numbers of available drug and vaccine targets, as well as number of transgenes for subsequent transgenic or paratransgenic approaches, have expanded. These are now paving the way for in-field control strategies of vectors and their pathogens. Basic scientific questions, such as understanding the basic components of the vector RNAi machinery, is vital, as this allows for the transfer of basic RNAi machinery components into RNAi-deficient vectors, thereby expanding the genetic toolbox of these RNAi-deficient vectors and pathogens. In this review, we focus on the current knowledge of arthropod vector RNAi machinery and the impact of RNAi on understanding vector biology and vector-pathogen interactions for which vector genomic data is available on VectorBase.

Figure 1

The RNAi process and biochemical machinery involved. Adapted from [30,31]. dsRNA is processed into short pieces (siRNA) by the endonuclease Dicer. The siRNA is loaded into the RNA-induced silencing complex (RISC) via the RISC loading complex (RLC), followed by cleavage and release of the passenger strand. The guide strand then associates with a homologous mRNA strand by conventional base paring, and the mRNA strand is cleaved by RISC and released for further degradation within the cytoplasm.

Figure 2

A schematic representation of a putative RNAi pathway for invertebrate pathogens and vectors. The proposed model might either use a multi trans-membrane protein (similar to SID-1) for dsRNA uptake or an endocytosis-mediated process which may include scavenger receptors. Once the dsRNA is in the cytoplasma it is processed into siRNAs ~ 21–23 nt in length, by a Dicer homologue. The siRNA are then presented to RISC which incorporates the siRNA, targets and degrades any homologous mRNA. RISC includes Ago, TudorSN and FmRp homologues. The proposed activity of RdRP is indicated as amplifying either trigger dsRNA, cleaved siRNA or using primary siRNA to prime synthesis of secondary siRNAs utilizing native mRNA as template. This causes 5’ amplification and spreading of the initial RNAi signal, and is known as transitive RNAi.