Caenorhabditis elegans RIG-I Homolog Mediates Antiviral RNA Interference Downstream of Dicer-Dependent Biogenesis of Viral Small Interfering RNAs.

Dicer enzymes process virus-specific double-stranded RNA (dsRNA) into small interfering RNAs (siRNAs) to initiate specific antiviral defense by related RNA interference (RNAi) pathways in plants, insects, nematodes, and mammals. Antiviral RNAi in Caenorhabditis elegans requires Dicer-related helicase 1 (DRH-1), not found in plants and insects but highly homologous to mammalian retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), intracellular viral RNA sensors that trigger innate immunity against RNA virus infection. However, it remains unclear if DRH-1 acts analogously to initiate antiviral RNAi in C. elegans Here, we performed a forward genetic screen to characterize antiviral RNAi in C. elegans Using a mapping-by-sequencing strategy, we uncovered four loss-of-function alleles of drh-1, three of which caused mutations in the helicase and C-terminal domains conserved in RLRs. Deep sequencing of small RNAs revealed an abundant population of Dicer-dependent virus-derived small interfering RNAs (vsiRNAs) in drh-1 single and double mutant animals after infection with Orsay virus, a positive-strand RNA virus. These findings provide further genetic evidence for the antiviral function of DRH-1 and illustrate that DRH-1 is not essential for the sensing and Dicer-mediated processing of the viral dsRNA replicative intermediates. Interestingly, vsiRNAs produced by drh-1 mutants were mapped overwhelmingly to the terminal regions of the viral genomic RNAs, in contrast to random distribution of vsiRNA hot spots when DRH-1 is functional. As RIG-I translocates on long dsRNA and DRH-1 exists in a complex with Dicer, we propose that DRH-1 facilitates the biogenesis of vsiRNAs in nematodes by catalyzing translocation of the Dicer complex on the viral long dsRNA precursors.IMPORTANCE The helicase and C-terminal domains of mammalian RLRs sense intracellular viral RNAs to initiate the interferon-regulated innate immunity against RNA virus infection. Both of the domains from human RIG-I can substitute for the corresponding domains of DRH-1 to mediate antiviral RNAi in C. elegans, suggesting an analogous role for DRH-1 as an intracellular dsRNA sensor to initiate antiviral RNAi. Here, we developed a forward genetic screen for the identification of host factors required for antiviral RNAi in C. elegans Characterization of four distinct drh-1 mutants obtained from the screen revealed that DRH-1 did not function to initiate antiviral RNAi. We show that DRH-1 acted in a downstream step to enhance Dicer-dependent biogenesis of viral siRNAs in C. elegans As mammals produce Dicer-dependent viral siRNAs to target RNA viruses, our findings suggest a possible role for mammalian RLRs and interferon signaling in the biogenesis of viral siRNAs.

Homologous RIG-I-like helicase proteins direct RNAi-mediated antiviral immunity in C. elegans by distinct mechanisms.

RNAi-mediated antiviral immunity in Caenorhabditis elegans requires Dicer-related helicase 1 (DRH-1), which encodes the helicase and C-terminal domains homologous to the mammalian retinoic acid inducible gene I (RIG-I)-like helicase (RLH) family of cytosolic immune receptors. Here we show that the antiviral function of DRH-1 requires the RIG-I homologous domains as well as its worm-specific N-terminal domain. We also demonstrate that the helicase and C-terminal domains encoded by either worm DRH-2 or human RIG-I can functionally replace the corresponding domains of DRH-1 to mediate antiviral RNAi in C. elegans. Notably, substitutions in a three-residue motif of the C-terminal regulatory domain of RIG-I that physically interacts with viral double-stranded RNA abolish the antiviral activity of C-terminal regulatory domains of both RIG-I and DRH-1 in C. elegans. Genetic analysis revealed an essential role for both DRH-1 and DRH-3 in C. elegans antiviral RNAi targeting a natural viral pathogen. However, Northern blot and small RNA deep sequencing analyses indicate that DRH-1 acts to enhance production of viral primary siRNAs, whereas DRH-3 regulates antiviral RNAi by participating in the biogenesis of secondary siRNAs after Dicer-dependent production of primary siRNAs. We propose that DRH-1 facilitates the acquisition of viral double-stranded RNA by the worm dicing complex for the subsequent processing into primary siRNAs. The strong parallel for the antiviral function of RLHs in worms and mammals suggests that detection of viral double-stranded RNA may activate completely unrelated effector mechanisms or, alternatively, that the mammalian RLHs have a conserved activity to stimulate production of viral siRNAs for antiviral immunity by an RNAi effector mechanism.

Antiviral RNA silencing initiated in the absence of RDE-4, a double-stranded RNA binding protein, in Caenorhabditis elegans.

Small interfering RNAs (siRNAs) processed from double-stranded RNA (dsRNA) of virus origins mediate potent antiviral defense through a process referred to as RNA interference (RNAi) or RNA silencing in diverse organisms. In the simple invertebrate Caenorhabditis elegans, the RNAi process is initiated by a single Dicer, which partners with the dsRNA binding protein RDE-4 to process dsRNA into viral siRNAs (viRNAs). Notably, in C. elegans this RNA-directed viral immunity (RDVI) also requires a number of worm-specific genes for its full antiviral potential. One such gene is rsd-2 (RNAi spreading defective 2), which was implicated in RDVI in our previous studies. In the current study, we first established an antiviral role by showing that rsd-2 null mutants permitted higher levels of viral RNA accumulation, and that this enhanced viral susceptibility was reversed by ectopic expression of RSD-2. We then examined the relationship of rsd-2 with other known components of RNAi pathways and established that rsd-2 functions in a novel pathway that is independent of rde-4 but likely requires the RNA-dependent RNA polymerase RRF-1, suggesting a critical role for RSD-2 in secondary viRNA biogenesis, likely through coordinated action with RRF-1. Together, these results suggest that RDVI in the single-Dicer organism C. elegans depends on the collective actions of both RDE-4-dependent and RDE-4-independent mechanisms to produce RNAi-inducing viRNAs. Our study reveals, for the first time, a novel siRNA-producing mechanism in C. elegans that bypasses the need for a dsRNA-binding protein.

Characterization of virus-encoded RNA interference suppressors in Caenorhabditis elegans.

In fungi, plants, and invertebrates, antiviral RNA interference (RNAi) directed by virus-derived small interfering RNAs (siRNAs) represents a major antiviral defense that the invading viruses have to overcome in order to establish infection. As a counterdefense mechanism, viruses of these hosts produce diverse classes of proteins capable of suppressing the biogenesis and/or function of viral siRNAs. This RNA-directed viral immunity (RDVI) in the nematode Caenorhabditis elegans is known to exhibit some unique features. Currently, little is known about viral suppression of RNAi in C. elegans. Here, we show that ectopic expression of the B2 protein encoded by Flock House virus (FHV) suppresses RNAi induced by either long double-stranded RNA (dsRNA) or an FHV-based replicon and facilitates the natural infection of C. elegans by Orsay virus but is not active against RNA silencing mediated by microRNAs. We report the development of an assay for the identification of viral suppressor of RNAi (VSR) in C. elegans based on the suppression of a viral replicon-triggered RDVI by ectopic expression of candidate proteins. No VSR activity was detected for either of the two Orsay viral proteins proposed previously as VSRs. We detected, among the known heterologous VSRs, VSR activity for B2 of Nodamura virus but not for 2b of tomato aspermy virus, p29 of fungus-infecting hypovirus, or p19 of tomato bushy stunt virus. We further show that, unlike that in plants and insects, FHV B2 suppresses worm RDVI mainly by interfering with the function of virus-derived primary siRNAs.

Silencing of host genes directed by virus-derived short interfering RNAs in Caenorhabditis elegans.

Small interfering RNAs (siRNAs) processed from viral replication intermediates by RNase III-like enzyme Dicer guide sequence-specific antiviral silencing in fungi, plants, and invertebrates. In plants, virus-derived siRNAs (viRNAs) can target and silence cellular transcripts and, in some cases, are responsible for the induction of plant diseases. Currently it remains unclear whether viRNAs are also capable of modulating the expression of cellular genes in the animal kingdom, although animal virus-encoded microRNAs (miRNAs) are known to guide efficient silencing of host genes, thereby facilitating virus replication. In this report, we showed that viRNAs derived from a modified nodavirus triggered potent silencing of homologous cellular transcripts produced by the endogenous gene or transgene in the nematode worm Caenorhabditis elegans. Like that found in plants, virus-induced gene silencing (VIGS) in C. elegans also involves RRF-1, a worm RNA-dependent RNA polymerase (RdRP) that is known to produce single-stranded secondary siRNAs in a Dicer-independent manner. We further demonstrated that VIGS in C. elegans is inheritable, suggesting that VIGS has the potential to generate profound epigenetic consequences in future generations. Altogether, these findings, for the first time, confirmed that viRNAs have the potential to modulate host gene expression in the animal kingdom. Most importantly, the success in uncoupling the trigger and the target of the antiviral silencing would allow for the exploration of novel features of virus-host interactions mediated by viRNAs in the animal kingdom.