Silencing Sexually Transmitted Infections: Topical siRNA-Based Interventions for the Prevention of HIV and HSV

Figure 1

Cellular machinery for RNAi-mediated gene silencing. (a) miRNA-mediated gene silencing. miRNAs (endogenous) and shRNAs (exogenous—lentiviral vectors) are expressed from Pol II-derived primary transcripts (pri-miRNA/pri-shRNA). The pri-miRNA/shRNA is cleaved by Drosha, an RNase III-type enzyme, producing the precursor miRNA/shRNA (pre-miRNA/shRNA), which is exported into the cytoplasm by exportin-5. Once in the cytoplasm, the loop of the pre-miRNA is cleaved off by Dicer, producing the transiently double-stranded miRNA, which is taken up by the miRNA-containing RISC (miRISC). The passenger strand rapidly dissociates, leaving the single-stranded mature miRNA to guide the sequence-specific inhibition of translation or cleavage of the target mRNA. shRNA is processed in a similar way but has been introduced to the host genome exogenously, often by lentiviral vector. (b) siRNAs, 19–21 nucleotide duplexes, are processed by Dicer to generate 2-3 nucleotide overhangs at their 3′ ends and phosphate groups on their 5′ termini., thereby activating the siRNAs. Active siRNAs are then incorporated into the RISC. The passenger strand of the duplexed siRNA is cleaved and released from the complex, leaving the single-stranded guide strand to direct RISC to the complimentary site on the target mRNA. Ago2 catalyzes the cleavage of target mRNA in trans, and the cleaved mRNA is released and the active strand containing RISC can direct the cleavage of additional target mRNAs. Ago: argonaute; RISC: RNA-induced silencing complex. Figure is adapted from [105].