Cross-species comparative analysis of Dicer proteins during Sindbis virus infection

The role of RNA silencing as a defense mechanism against viruses remains to be formerly established in mammalian somatic cells. Here, we determined the antiviral properties of human and Drosophila Dicer proteins in a heterologous setup. We expressed human Dicer (hDicer) in Drosophila, and Drosophila Dicer-2 in human cells, and measured the impact on the response to Sindbis virus (SINV) infection. In flies, hDicer presents a low processing activity, but partially rescues a Dcr2 null mutation in flies. Expression of Dicer-2 in HEK293 cells allows the processing of SINV RNA into 21-nt-long small RNAs. Nevertheless, instead of conferring a protective effect against SINV, Dicer-2 expression increases viral replication in HEK293 cells. We present evidence that this effect is due to a competition with the interferon pathway. Our results therefore suggest that adding functionial RNA silencing machinery in IFN-competent differentiated mammalian cells can be detrimental for antiviral defense. Small RNA profiles of SINV-infected transgenic flies expressing RFP tagged hDicer or Dicer-2 in a Dicer-2 null background, 5 dpi (Df = Dicer-2 Deficiency = control, RFP-hDicer, RFP-Dicer-2) and of SINV-infected HEK293 stable cell lines, expressing or not some D. melanogaster RNAi components, 6 and 16 hpi (empty (e) = control, Dicer-2, DAR = Dicer-2 + R2D2) were generated by deep-sequencing, without replicate, using Illumina HiSeq 2500.