Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Viral Suppressors of RNA Silencing Hinder Exogenous and Endogenous Small RNA Pathways in Drosophila

Viral Suppressors of RNA Silencing Hinder Exogenous and Endogenous Small RNA Pathways in Drosophila

  • Bassam Berry, 
  • Safia Deddouche, 
  • Doris Kirschner, 
  • Jean-Luc Imler, 
  • Christophe Antoniewski
PLOS
x

Abstract

Background

In plants and insects, RNA interference (RNAi) is the main responder against viruses and shapes the basis of antiviral immunity. Viruses counter this defense by expressing viral suppressors of RNAi (VSRs). While VSRs in Drosophila melanogaster were shown to inhibit RNAi through different modes of action, whether they act on other silencing pathways remained unexplored.

Methodology/Principal Findings

Here we show that expression of various plant and insect VSRs in transgenic flies does not perturb the Drosophila microRNA (miRNA) pathway; but in contrast, inhibits antiviral RNAi and the RNA silencing response triggered by inverted repeat transcripts, and injection of dsRNA or siRNA. Strikingly, these VSRs also suppressed transposon silencing by endogenous siRNAs (endo-siRNAs).

Conclusions/Significance

Our findings identify VSRs as tools to unravel small RNA pathways in insects and suggest a cosuppression of antiviral RNAi and endo-siRNA silencing by viruses during fly infections.

Introduction

RNA silencing is a eukaryotic gene regulation mechanism by which RNA expression is shut down in a sequence specific manner through the intervention of homologous small non coding RNAs [1]. Three types of small RNAs, miRNAs, piRNAs and siRNAs, accumulate in different tissues and developmental stages of Drosophila.

The ∼22 nucleotides (nt) miRNAs derive from stem-loop precursor transcripts through the action of the Drosha/Pasha and Dicer-1/Loqs complexes. In D. melanogaster, they are mostly loaded into the Argonaute-1 (Ago1) protein and guide translational repression of mRNAs [2], [3]. The ∼25-nt piRNAs are produced from the cleavage of selfish and repetitive genetic element transcripts by the Piwi Argonautes. They are restricted to the gonads where they silence Transposable Elements (TEs) [4]. The ∼21 nt siRNAs originate from the processing by Dicer-2 (Dcr2) of long dsRNA precursors, such as those produced by inverted-repeat (IR) transgenes. They load into Argonaute-2 (Ago2) and guide the cleavage of target mRNAs with perfect complementary sequence matches [5]. siRNAs produced from endogenous precursors (endo-siRNAs) were recently described in flies. Beside piRNAs, they contribute to the silencing of TEs and some endogenes in both gonads and somatic tissues [1], [4], [6].

siRNAs are also produced by Dcr-2 from dsRNA viral intermediates upon viral infection [7][9], playing an essential role in the fly's defense against viruses. To counterattack, viruses evolved to encode proteins that suppress their host's antiviral silencing. Viral suppressors of RNAi (VSRs) have been reported in all types of plant viruses [10]. More recently, insect viral suppressors, the Flock House virus (FHV) B2, the Cricket Paralysis virus (CrPV) and the Drosophila C virus (DCV) 1A proteins where shown to act as potent inhibitors of antiviral RNAi in Drosophila [8], [9], [11]. VSRs are very diverse in sequence and structure across viral kingdoms, but operate through a few evolutionarily convergent strategies: they most commonly bind to long dsRNAs to inhibit their processing by Dicer proteins [12][14]; sequester siRNA duplexes to prevent their loading to Argonaute complexes [15][17]; or may directly interact with Dicers or Argonautes to impair their antiviral activities [18], [19].

Alongside inhibiting antiviral RNAi, plant VSRs including HcPro, P21, P19, P15 and P0 were shown to suppress miRNA- or endo-siRNA-mediated silencing, therefore corrupting development or homeostasis of their host and contributing to viral pathogenicity [18], [20][23]. In contrast, the effect of VSRs on endo-siRNAs in animals and its potential contribution to host disorders has not been explored in detail. To address this issue, we established Drosophila transgenic lines expressing a panel of 9 VSRs from plant and insect viruses (Table 1) and performed a comparative analysis of their effects on RNAi induced by (i) viral infection, (ii) inverted repeat transgenes, (iii) in vitro transcribed dsRNA, (iv) synthetic siRNAs, (v) endogenous siRNAs and (vi) miRNAs.

Our study reveals that none of these VSRs perturb the miRNA pathway. In contrast, the insect VSRs FHV B2 and DCV 1A, as well as the plant VSRs P21, P19 and P15, inhibited anti-viral RNAi in flies and suppressed silencing induced by siRNAs either exogenously supplied or endogenously produced, strongly supporting the relevance of RNAi mechanisms throughout evolution and kingdoms. In addition, B2, 1A and P19 suppressed the silencing of TEs in somatic tissues and in gonads, suggesting that effects of VSRs on endo-siRNA-mediated silencing may account for viral pathogenicity in insects. Our results demonstrate the effectiveness of using viral silencing suppressor proteins as a tool to dissect small RNA pathways in animals, as previously established in plants.

Results

VSRs do not inhibit the Drosophila miRNA pathway

Drosophila transgenic lines were established to express a panel of 9 Flag-tagged VSRs (Table 1) under the control of a GAL4 inducible UAS promoter [24]. Two VSRs, B2 and 1A, derive from the insect viruses FHV and DCV, respectively. The remaining VSRs P0, HcPro, P25, P38, P21, P15 and P19 derive from plant viruses. The VSR transgenic lines were crossed with a daughterless-GAL4 driver line, which ubiquitously expresses GAL4 throughout development. Western blot assay using a Flag antibody indicated that all suppressors are expressed at a high level in the progenies from these crosses (Fig. S1).

In plants, HcPro, P21, P19, P15 and P14 interfere with miRNAs, causing developmental defects that resemble those observed in mutant plants deficient in their miRNA pathway [20][23]. In addition, it has been reported that the P19 VSR might interfere with the silencing activity of miR-32 in human cells [25]. Thus, we assessed the effect of the overexpression of the 9 VSRs on miRNA activity. Expression of the VSRs in the posterior compartment of the wing imaginal discs did not suppress the silencing of a GFP reporter targeted by the bantam miRNA (Fig. S2 and Movies S1 and S2). Additionally, the transgenic stocks ubiquitously expressing each of the 9 VSRs progressed normally from embryogenesis to adulthood, showing neither developmental defects nor altered viability. These results indicate that the VSRs here tested do not appreciably perturb the miRNA pathway in Drosophila.

Suppression of antiviral RNAi.

A FHV RNA1 transgenic construct was previously engineered to autonomously replicate in the presence of the viral RNA-dependent RNA polymerase (RdRp) encoded by RNA1 [7]. Non-sense mutations in the B2 ORF of RNA1 (Fig. 1A) strongly impair the replication of a derivative RNA1ΔB2 transgenic construct (Fig. 1B, compare RNA1 and RNA1ΔB2 controls), an effect that was attributed to the inability of RNA1ΔB2 to limit the RNAi host response through the activity of the B2 VSR [7]. Accordingly, RNA1ΔB2 expression was restored in flies expressing the B2 transgene (Fig. 1B). We therefore decided to monitor the ability of our panel of VSRs to restore the replication level of RNA1ΔB2 (Fig. 1B).

thumbnail
Figure 1. Flies expressing insect and plant VSRs accumulate high levels of FHV RNA1ΔB2 transcripts.

(A) RNA1 and RNA1ΔB2 constructs. The GAL4 inducible UAS-RNA1 transgene (left hand side) expresses a RNA1 of (+) polarity. RNA1 encodes an RdRp protein, which directs autonomous replication of RNA1 of (−) polarity, then of both RNA1 and RNA3 of (+) polarity. Two point mutations (triangles) interrupt the B2 ORF in the RNA1ΔB2 (right hand side) without affecting the RdRp ORF [7]. (B) Fold changes in RNA1+RNA3 levels. The RNA1 transgenic line was crossed with the da>GAL4 line (control). The RNA1ΔB2 transgenic line was crossed with the da>GAL4 line (control) or with da>GAL4; UAS-VSR lines as indicated. RNA1+RNA3 expression was measured by quantitative RT-PCR from the progenies of these crosses. Error bars indicate the standard deviation from triplicate qPCR experiments.

http://dx.doi.org/10.1371/journal.pone.0005866.g001

RNA1ΔB2 replication was restored in DCV 1A transgenic flies at levels similar to those observed in B2 transgenic flies. High levels of RNA1ΔB2 replication were also measured in flies expressing P15, P21 or P19, suggesting that these proteins, in a similar way to B2 and 1A, suppress fly antiviral RNAi. In contrast, no significant increase of RNA1ΔB2 replication was observed in flies expressing P38, P25, HcPro or P0.

To further characterize the impact of the suppressors on antiviral RNAi, we tested their effect on fly susceptibility to infection by the Drosophila C virus (DCV), a common Drosophila pathogen. Upon DCV inoculation of wild type flies, 50% of animals died after 6 to 7 days (Fig. 2A). The survival was similar for flies expressing P25, P38, HcPro or P0 under the control of the ubiquitous Actin5C-GAL4 driver (data not shown), paralleling the lack of effect of these proteins on the replication of the FHV RNA1ΔB2. In contrast, flies expressing B2, 1A, P19, P21 and P15 under the control of the ubiquitous Actin5C>GAL4 driver showed a more rapid course of disease and died faster after inoculation with DCV (Fig. 2A and data not shown). Similar results were obtained when these suppressors were expressed under the control of the heat-shock>GAL4 driver after heat shock induction (Fig. 2B). These data show that the B2 and 1A insect VSRs, as well as the P19, P15 and P21 plant VSRs act as potent inhibitors of antiviral RNAi in flies.

thumbnail
Figure 2. Hypersensitivity to DCV infection.

(A) An Act5C>GAL4 driver line was crossed to UAS>VSR transgenic lines (▪) or to w1118 control strain (□). 20 females from the progeny of these crosses were challenged by an intrathoracic injection of a control Tris solution (---) or a DCV suspension corresponding to 102 LD50 (–) and survival was monitored daily. (B) An hsp>GAL4 driver line was crossed to UAS>VSR transgenic lines. 20 females from the progeny of these crosses were challenged by an injection of the control Tris solution (---) or a the DCV suspension (–) under heat shock (▪) or non heat shock (□) conditions. Plotted values represent the mean±SEM (standard error to the mean) of three independent experiments.

http://dx.doi.org/10.1371/journal.pone.0005866.g002

Suppression of silencing induced by exogenous dsRNA.

Because one of the modes of action of VSR is the binding to long dsRNA species, sequestering them from Dicer access, or to siRNAs hindering their loading into the RISC complex, we next tested the ability of the suppressors to inhibit RNAi induced by injection of dsRNA or siRNA in embryos. 500-nt dsRNAs or 21-nt siRNAs matching the fushi tarazu gene (ftz) induced a high frequency (>80%) of cuticle defects in wild type survivors, but not in ago2−/− survivors, indicating that the two types of molecules elicited potent RNAi against the ftz gene (Fig. 3). Injection of long dsRNAs or siRNAs in transgenic embryos P0, HcPro, P25 or P38 also induced a strong ftz RNAi, with a ∼80% frequency of cuticule defects (Fig. 3B–C). In contrast, only 10–30% of cuticule defects were observed in transgenic embryos P21, P15, P19 or B2, indicating that maternal stocks and/or embryonic expression of these proteins efficiently suppressed ftz RNAi. In contrast to B2 transgenic embryos, 1A transgenic embryos were able to mount an efficient ftz silencing when injected with short siRNAs but not with long dsRNAs (Compare Fig. 3B and C). This result is fully consistent with previous data indicating that DCV-1A specifically binds long dsRNAs in vitro [8], while B2 bind to both long dsRNA [14], [26] and short siRNA [12], [27].

thumbnail
Figure 3. Suppression of silencing induced by injection of long dsRNAs or siRNAs in embryos.

RNAi of the fushi tarazu gene (ftz) in wild type syncytial embryos results in loss of denticle belts in the cuticle of prehatching larvae (A). Early embryos homozygous for the ago2414 mutant allele or expressing the indicated VSR under the control of the da>GAL4 driver were injected with long dsRNAs (B) or siRNAs (C) targeted to the ftz gene and scored 48 h after injection for the ftz mutant phenotype. Confidence intervals (alpha risk 0.05) for the observed ratio are indicated.

http://dx.doi.org/10.1371/journal.pone.0005866.g003

Suppression of silencing induced by endogenous siRNAs.

To establish the role of VSRs on endo-siRNAs we sought to use a GMR>IR[w] transgenic construct expressing an RNA hairpin in eyes [28]. This construct triggers the Dcr-2-dependent production of siRNAs [29], [30] and silences the white gene (Fig. 4, control panels). When co-expressed under the control of a GMR>GAL4 driver with the RNAi trigger GMR>IR[w] transgene, B2, 1A and to a lesser extent, P19, P15 and P21, suppressed this white silencing (Fig. 4). This result indicates that in addition to antiviral and exogenously induced RNAi, these 5 VSRs can block RNAi induced by an transgenic hairpin locus. In contrast, P38, P25, HcPro and P0 failed to inhibit the white silencing, paralleling their lack of effect on antiviral or exogenously induced RNAi.

thumbnail
Figure 4. VSRs inhibit silencing induced a white inverse-repeat transgene.

Silencing of the white gene induced by an IR[w] inverted-repeat transgene in flies expressing insect and plant VSRs in eyes. (A) In all heterozygous combinations GMR>GAL4; UAS>GFP or GMR>GAL4; UAS>VSR, the mini-white markers of the two transgenes produce equivalent strong red eye pigmentation. (B) Silencing of the mini-white markers by one copy of the GMR>IR[w] transgene (note that the GMR>IR[w] construct has no mini-white marker [28]). The effect of the GMR>IR[w] transgene on the two mini-white markers in GMR>GAL4; UAS>VSR genetic combinations depends on the efficiency of the suppressor to inhibit white silencing. Eye pigment levels were measured in 3 separate experiments from 2 days-old adult eyes and are expressed as a percentage of pigment levels measured in the GMR>GAL4; UAS>GFP combination. Error bars correspond to standard deviations.

http://dx.doi.org/10.1371/journal.pone.0005866.g004

A class of endogenous siRNAs is produced from sense and antisense transcription of transposable elements. In somatic tissues, these endo-siRNAs were shown to silence TEs from where they originate in a negative feedback loop [4], [6]. To further characterize the activity of B2, 1A and P19 on the fly endo-siRNA pathway, we analyzed the effects of their overexpression on two retrotransposons in female adult heads and ovaries (Fig. 5).

thumbnail
Figure 5. VSRs inhibit retrotransposon silencing by endo-siRNAs.

297 and ZAM retrotransposon silencing is impaired by B2, 1A and P19 VSRs. RNA levels of 297 and ZAM were measured in heads and ovaries of 1 day-old female flies heterozygous for the da>GAL4 driver and the indicated UAS>VSR transgene. Fold changes in RNA levels were calculated relative to the 297 and ZAM RNA levels measured in heterozygous da>GAL4 control flies. Fold changes in homozygous ago2414 mutants were calculated relative to the 297 and ZAM RNA levels measured in heterozygous ago2414 flies. Therefore, the weaker effect of the ago2 mutation on 297 expression observed in adult heads may result from variations in copy number of 297 between the ago2 mutant stock and the VSR transgenic stocks. Error bars indicate the standard deviation from triplicate qPCR experiments.

http://dx.doi.org/10.1371/journal.pone.0005866.g005

As expected, the steady-state levels of the LTR retrotransposons 297 and ZAM increased in heads from ago2−/− mutant control flies. The ago2 mutation also derepressed ZAM and to a lesser extent, 297, in ovaries, agreeing with previous expression level analyses in this tissue [31], [32].

Expression of ZAM increased in heads and to a lesser extent, in ovaries, from B2, 1A and P19 transgenic animals driven by the ubiquitous da>GAL4 driver. Expression of 297 also increased in heads but not in ovaries of all three transgenic lines. These data indicate that B2, 1A and P19 impair transposon silencing by endo-siRNAs in somatic tissues. The modest effect of these VSRs in ovaries suggests that they do not interfere with piRNA silencing in this tissue.

Discussion

Here we analyzed the ability of 9 VSRs to interfere with small RNA silencing pathways in adult flies. We found that 2 insect VSRs - B2 and 1A - and 3 plant VSRs - P19, P21 and P15 - inhibit in vivo antiviral RNAi as well as RNAi induced by both exogenously and endogenously supplied dsRNAs.

None of the VSRs tested in this study significantly interfered with miRNA activity. Biogenesis of miRNAs involves primary miRNA processing by Drosha/Pasha in the nucleus, exportin-5 dependent export of pre-miRNAs in the cytoplasm, pre-miRNA processing by Dicer-1 and miRNA loading into RISC complexes. It is possible that VSRs cannot access the miRNA machinery in insect cells during this coordinated sequence of events. Alternatively, the imperfect duplex structure of miRNAs may preclude their binding by VSRs. In any case, the absence of effects of VSRs on miRNAs might represent an adaptation of insect hosts during evolution to limit the perturbation of gene expression upon viral infection. Of note, the VSR B2 was recently shown to suppress an inducible antiviral response in Drosophila, in addition to its role in suppressing RNAi [33]. It will be interesting to test if other VSRs can also block the upregulation of the antiviral gene Vago in infected flies.

The B2 and 1A proteins are encoded by FHV and DCV respectively, which belong to distinct families of (+) single stranded RNA viruses naturally infecting Drosophila. Both proteins inhibit the silencing of FHVΔB2 and sensitize flies to infection with DCV. Synergistic viral diseases in plants, namely the co-infection of a host plant with two unrelated viruses which elicits disease symptoms that are more severe than the sum of those induced in either single infection, have been shown to be dependent in some instances on VSRs [34]. Our data suggest that such viral synergism may also exist in insects and that VSRs could act as “synergism genes” becoming permissive agents for the development of infection.

The plant VSRs P19, P15 and P21 inhibit the antiviral silencing machinery of their host by binding to 21 nt siRNAs [35]. P19 also inhibits RNAi in mammalian cultured cells [12] and in vitro studies demonstrated that P19 and P21 proteins block siRNA-directed target RNA cleavage in Drosophila embryonic extracts by sequestering siRNA duplexes [17]. Here, we show that in vivo P19, P21 and P15 promote the accumulation of the FHVΔB2 replicon, sensitize flies to DCV infection and inhibit ftz silencing triggered either by long dsRNA or siRNA. Altogether, these data indicate that, in flies, these three VSRs mimic their function in plants by sequestering siRNAs. It is worthy to note that in a number of cases, plant viruses are transmitted by insect vectors [36] and may then have to face the antiviral RNAi response in distant organisms. Sequestering siRNAs, whose structure is conserved throughout evolution, may indeed represent an optimal strategy to adapt to otherwise evolutionarily divergent antiviral responses.

Although P0, P25, P38 and HcPro were characterized as potent VSRs in plants [37], they did not show activity in flies in any of our assays. We cannot exclude that this lack of activity results from imperfect biosynthesis of the viral suppressors in flies. Nevertheless, it is unlikely due to a deleterious effect of the Flag epitope as non-Flagged versions of these suppressors gave identical results (data not shown). P0 is an F-box like protein that induces the ubiquitinylation and the degradation of plant Argonautes [38]. Although the mechanisms by which P25 and P38 inhibit RNA silencing remain unclear, these two proteins do not appear to act by sequestering siRNAs [21]. It is therefore likely that the lack of activity of P0, P25 and P38 in flies reflects interactions of these VSR with host plant proteins whose structure and/or function are not conserved in flies. The HcPro protein from Tobacco etch virus (TEV) was shown to inhibit dsRNA-induced silencing in Drosophila S2 cells [39] and sequester siRNA from embryonic extracts, although less efficiently than P19 [17]. The lack of activity of HcPro from the ZYMV that we observed in our study may be due a divergence of sequence between HcPro proteins from TEV and ZYMV.

B2, 1A, P19, P15 and P21 VSRs that inhibit silencing induced by exogenously supplied dsRNAs were also capable of inhibiting the silencing mediated by siRNAs produced from an inverse repeat transgene. In addition, the insect VSRs B2 and 1A, and the plant VSR P19 inhibited the silencing of the Drosophila 297 and ZAM retrotransposons in heads and ovaries of adult flies, indicating that these suppressors interfere with the endo-siRNA pathway in flies [4], [6]. Their modest effect in gonads also suggests that they do not interfere with the piRNA-mediated silencing of TEs in this tissue.

The DCV-1A protein specifically binds long dsRNAs and does not interfere with RNAi when directly triggered by siRNAs [8]. Immunoprecipitation experiments showed that P19 and to a lesser extent B2 bind to TE-derived endo-siRNA in S2 cultured cell; additionally, B2 was specifically able to bind longer RNA species (Fagegaltier et al, in preparation). Together, these data suggest that the effects of VSRs on TE silencing is achieved through sequestration of TE-derived endo-siRNAs by P19, of long double-stranded TE RNA precursors by 1A and of both endo-siRNAs and their double-stranded precursors by B2.

It is remarkable that plants as well as insect VSRs can, besides their well-known roles in viral counterdefense mechanisms, consistently interfere with endo-siRNA pathway. Whether viruses can manipulate this pathway to improve their interaction with the host during the course of a natural infection and what the physiological implications would be are open questions.

Materials and Methods

Suppressor transgenic constructs

VSR cDNAs were first PCR amplified using the following templates : pBWDi-FHV-B2 kindly provided by C. Vaury (B2, Swiss-Prot P68831); pAC-DCV1A (1A, [8]; pBIN-P15, -P19, -P21, -P25 and -P38 constructs kindly provided by O. Voinnet (P15, P19, P21, P25 and P38, [21]; Zucchini yellows mosaic virus HcPro cDNA kindly provided by C. Desbiez (HcPro, Swiss-Prot P18479); pGBKT7-P0CAB (P0, [38]. PCR primers were as follows: B2-F, 5′-CACCATGCCAAGCAAAC-3′; B2-R, 5′-CAGTTTTGCGGGTGG-3′; 1A-F, 5′-CACCATGGAATCTGATAAAAGTAT-3′; 1A-R, 5′-CTTGTCATCGTCATCCTTGTAAT-3′; P19-F, 5′-CACCATGGAACGAGC-3′; P19-R, 5′-CTCGCTTTCTTTTTCG-3′; P15-F, 5′-CACCATGCCTAAGTCG-3′; P15-R, 5′-CAGTTTAGAACGAAG-3′; P21-F 5′-CACCATGAAGTTTTTCTTTAATGA-3′; P21-R 5′-TACAGCTATACCGAGGATTT-3′; P25-F, 5′-CACCATGGATATTCTC-3′; P25-R, 5′-TGGCCCTGCGCGGAC-3′; P38-F, 5′-CACCATGGAAAATGATC-3′; P38-R, 5′-AATTCTGAGTGCTTGC-3′; HcPro-F, 5′-CACCATGTCGTCACAAC-3′; HcPro-R, 5′-CACCATGTCGTCACAAC-3′; P0-F, 5′-ATGGAAATTGAGTCTGTCAAGC-3′; P0-R, 5′-GCGTTGTAGCTCCTTTTG-3′

VSR cDNAs were cloned into the pENTR-D-TOPO vector (Invitrogen) and were subsequently recombined into pUASp vectors [24] adapted for the Gateway system (Drosophila Gateway® Vector collection, Carnegie Institution) to give C-terminal Flag tagged pPWF-VSR and non tagged pPW-VSR transgenesis vectors. VSR sequences in these constructs were verified by sequencing before establishing UAS>VSR transgenic stocks by standard injection procedure. For each VSR construct, three independent transgenic lines were tested in western blot assay as well as in GMR>IR[w] desilencing test. One highly expressed transgene insertion for each suppressor (Fig. S1) was chosen for further analysis.

Transgenic and mutant stocks

The FHV RNA1 and RNA1ΔB2 flies are described in [7]. We obtained the GMR>IR[w] transgenic line from R. Carthew [28], the Ago2414 mutant stock from H. Siomi [5], the engrailed>GAL4, Tubulin>GFP and Tubulin>GFP-ban transgenic stocks from S. Cohen [40], the GMR>GAL4, da>GAL4, hsp>GAL4 and UAS>GFP lines from the Bloomington Stock Center and the Act5C>GAL4 17a driver line (n°U192) from the Fly stocks of National Institute for Genetics from Japan (http://www.shigen.nig.ac.jp/fly/download/nigflyfiles/mutant.txt).

Genetic crosses

To study the effect of the VSRs on the replication of FHV RNA1ΔB2, we genetically recombined the da>GAL4 driver with the UAS>VSR transgenic insertions. The resulting UAS>VSR; da>GAL4 stocks were then crossed to the UAS>RNA1ΔB2 transgenic line. For control, the UAS>RNA1ΔB2 line was crossed to the da>GAL4 driver line.

To monitor fly sensitivity to infection by DCV, the UAS>VSRs transgenic lines were crossed to the Act5C>GAL4 or hsp>GAL4 driver lines.

To analyze the effect of the suppressors on white silencing, we constructed an homozygous w1118, GMR>IR[w]; GMR>GAL4 stock by genetic recombination and crossed it to the homozygous UAS>GFP and UAS>VSR stocks. For controls, the UAS>GFP line, as well as all the UAS>VSR transgenic stocks were crossed to the homozygous w1118; GMR>GAL4 stock.

To analyze the effect of suppressors of bantam silencing, we recombined the Tubulin>GFP-ban sensor transgene with the engrailed>GAL4 driver on the third chromosome and crossed the resulting homozygous stock to the UAS>VSR lines.

Quantitative RT PCR

To measure the steady state level of the FHV RNA1ΔB2, ten 6-days old female flies were collected from the progeny of each UASp>VSR; da>GAL4 stock crossed to homozygous UAS>RNA1ΔB2 flies. Same aged UAS>RNA1; da>GAL4 or UAS>RNA1ΔB2; da>GAL4 heterozygous flies were collected as controls. Total RNA extracts were prepared with Trizol Reagent (Invitrogen) according to the manufacturer's instructions, treated with DNAse I then reverse transcribed with iScript cDNA synthesis kit (Biorad). FHV RNA1 levels were then analyzed by real-time PCR on an Opticon2 Instrument (Biorad) using the Faststart Sybr-Green Mix (Roche) and the 5′ACCTCGATGGCAGGGTTT 3′ and 5′CTTGAACCATGGCCTTTTG 3′ primers. Template concentrations were normalized to endogenous reference RpL23 and to the heterozygous UAS>RNA1ΔB2; da>GAL4 control calibrator using the ΔΔCT method.

297 and ZAM transposon expression levels were measured in double heterozygous UASp>VSR; da>GAL4 and control heterozygous da>GAL4 2-days old females. Fifty heads and ovaries were separated manually and processed for total RNA extraction and reverse transcription as described above. FHV RNA1 levels were then analyzed by real-time PCR using the 297F 5′-AAAGGGCGCTCATACAAATG-3′, 297R 5′-TGTGCACATAAAATGGTTCG-3′ [30], ZAM-F 5′-ACTTGACCTGGATACACTCACAAC-3′ and ZAM-R 5′-GAGTATTACGGCGACTAGGGATAC-3′ [31] primers. Template concentrations were normalized to endogenous reference RpL23 and to the heterozygous da>GAL4 control calibrator using the ΔΔCT method.

In both experiments, RpL23 levels were measured using the RpL-F, 5′-CGGATCGATATGCTAAGCTGT-3′ and RpL-R, 5′-GCGCTTGTTCGATCCGTA-3′ primer pair.

Viral infection

4–6 days old double heterozygous Act5C>GAL4; UAS>VSR or heat-shocked hsp>GAL4; UAS>VSR and control heterozygous Act5C>GAL4 or hsp>GAL4 flies were used in infection experiments. Viral stocks were prepared in 10 mM Tris-HCl, pH 7.5. Infections were done as described in [41] by injection (Nanoject II apparatus; Drummond Scientific) of 4.6 nL of a viral suspension (DCV, 2×105 LD50 (50% lethal dose)/mL) into the thorax of flies. Injection of the same volume of 10 mM Tris-HCl, pH 7.5, was used as a control. For heat-shock induction of transgene expression, flies were incubated for 20 min at 37°C, followed by 30 min at 18°C and another 20 min at 37°C. After the treatment, flies were allowed to recover for 6 h at 25°C before infection. All survival experiments were performed at 22°C.

ftz dsRNA and siRNA injection in embryos

A region of the fushi tarazu (ftz) gene was amplified with the T7-ftz-F 5′-GAATTGTAATACGACTCACTATAGGGCCAACATGTATCACCCCCA-3′ and T7-ftz-R 5′-GAATTGTAATACGACTCACTATAGGGCTGGCAAAGTCGCCATTCT-3′ primers using a w1118 fly genomic DNA template. The PCR product was then used for dsRNA synthesis, using MEGAScript® RNAi Kit (Ambion®) according to the manufacturer's instructions. 5′ phosphorylated siRNA (21 bp) with dTdT 3′overhangs corresponding to the ftz gene (siFTZ 5′-UGCCUACUAUCAGAACACC-3′) was purchased from Eurogentec.

Embryos were collected from UAS>VSR; da>GAL4 stocks over a 30 min period at 25°C, hand dechorionated, and attached to a coverslip coated with double-stick tape. Embryos were then desiccated at room temperature, covered in 700 halocarbon oil and injected with a 1 mg/ml solution of dsRNA or siRNA using Eppendorf's sterile Femtotips® needles. After 36 hr recovery, the surviving embryos were scored for the number of ventral denticle belts. Embryos with four, five, or six denticle belts were scored as ftz.

Eye pigment dosage

Assays were performed on 3 days old virgin females. Five heads per genotype were manually collected and homogenized in 50 µl of a freshly prepared solution of acidified methanol (0.1% HCl). Pigment was extracted by rocking tubes for 36 hr at 4°C. Homogenates were then incubated at 50°C for 5 min, clarified by centrifugation and optical density of each sample was read at 480 nm. Three independent extractions were performed for each genotype and the mean values of the absorption per extraction were calculated.

Western-Blot analysis

For protein analysis, equal amounts of proteins were extracted from heterozygous UAS>VSR; da>GAL4 L3 larvae, boiled in Laemmli buffer and loaded on 15% SDS-PAGE. After transfer onto nitrocellulose membrane, equal loading was verified by Ponceau staining. Membranes were blocked in 5% milk, 1× PBS, 0.1% Tween, and incubated overnight with mouse HRP coupled anti-Flag antibody (1∶10000, Sigma). Detection was performed using Chemiluminescent Substrate (Pierce).

Supporting Information

Figure S1.

Expression of VSRs in transgenic animals. Third larval instar protein extract were prepared from transgenic larvae double heterozygous for the da>GAL4 driver and the indicated UAS>VSR transgene and analyzed by western blot using an anti-Flag antibody. Ponceau staining (bottom) shows equal protein loading between lanes.

doi:10.1371/journal.pone.0005866.s001

(0.98 MB EPS)

Figure S2.

VSRs do not suppress silencing by the bantam miRNA. Confocal microscopy of wing imaginal discs expressing the engrailed-GAL4 driver in their posterior half compartment (P) and either a control Tubulin>GFP or a Tubulin>GFP-ban sensor with bantam miRNA target sites in 3′ UTR. The quadrant pattern of GFP silencing by bantam in the wing pouch is not affected by expression of the indicated UAS>VSR transgenes in imaginal disc posterior compartment, indicating no obvious interference with the miRNA pathway by either VSR.

doi:10.1371/journal.pone.0005866.s002

(4.43 MB TIF)

Movie S1.

B2 confocal set. Confocal slice sets of UAS>B2; engrailed-GAL4, Tubulin>GFP-ban wing imaginal discs shown in Fig. S2

doi:10.1371/journal.pone.0005866.s003

(56.46 MB AVI)

Movie S2.

P15 confocal set. Confocal slice set of UAS>P15; engrailed>GAL4, Tubulin>GFP-ban wing imaginal discs shown in Fig. S2

doi:10.1371/journal.pone.0005866.s004

(34.75 MB AVI)

Acknowledgments

We thank O. Voinnet, C. Vaury, C. Desbiez, H. Siomi, R. Carthew and S. Cohen for providing materials, the Bestgene Inc for transgenesis, the PFID for assistance on microscopy, and C. Saleh and M. Vignuzzi for discussions and critical reading of the manuscript.

Author Contributions

Conceived and designed the experiments: BB SD JLI CA. Performed the experiments: BB SD. Analyzed the data: BB SD JLI CA. Contributed reagents/materials/analysis tools: DK. Wrote the paper: BB CA.

References

  1. 1. Ghildiyal M, Zamore PD (2009) Small silencing RNAs: an expanding universe. Nat Rev Genet 10: 94–108.
  2. 2. Forstemann K, Horwich MD, Wee L, Tomari Y, Zamore PD (2007) Drosophila microRNAs are sorted into functionally distinct argonaute complexes after production by dicer-1. Cell 130: 287–297.
  3. 3. Iwasaki S, Kawamata T, Tomari Y (2009) Drosophila Argonaute1 and Argonaute2 Employ Distinct Mechanisms for Translational Repression. Mol Cell 34: 58–67.
  4. 4. Siomi H, Siomi MC (2009) On the road to reading the RNA-interference code. Nature 457: 396–404.
  5. 5. Okamura K, Ishizuka A, Siomi H, Siomi MC (2004) Distinct roles for Argonaute proteins in small RNA-directed RNA cleavage pathways. Genes Dev 18: 1655–1666.
  6. 6. Malone CD, Hannon GJ (2009) Small RNAs as guardians of the genome. Cell 136: 656–668.
  7. 7. Galiana-Arnoux D, Dostert C, Schneemann A, Hoffmann JA, Imler JL (2006) Essential function in vivo for Dicer-2 in host defense against RNA viruses in drosophila. Nat Immunol 7: 590–597.
  8. 8. van Rij RP, Saleh MC, Berry B, Foo C, Houk A, et al. (2006) The RNA silencing endonuclease Argonaute 2 mediates specific antiviral immunity in Drosophila melanogaster. Genes Dev 20: 2985–2995.
  9. 9. Wang XH, Aliyari R, Li WX, Li HW, Kim K, et al. (2006) RNA interference directs innate immunity against viruses in adult Drosophila. Science 312: 452–454.
  10. 10. Ding SW, Voinnet O (2007) Antiviral immunity directed by small RNAs. Cell 130: 413–426.
  11. 11. Li H, Li WX, Ding SW (2002) Induction and suppression of RNA silencing by an animal virus. Science 296: 1319–1321.
  12. 12. Sullivan CS, Ganem D (2005) A virus-encoded inhibitor that blocks RNA interference in mammalian cells. J Virol 79: 7371–7379.
  13. 13. Lu R, Maduro M, Li F, Li HW, Broitman-Maduro G, et al. (2005) Animal virus replication and RNAi-mediated antiviral silencing in Caenorhabditis elegans. Nature 436: 1040–1043.
  14. 14. Chao JA, Lee JH, Chapados BR, Debler EW, Schneemann A, et al. (2005) Dual modes of RNA-silencing suppression by Flock House virus protein B2. Nat Struct Mol Biol 12: 952–957.
  15. 15. Vargason JM, Szittya G, Burgyan J, Hall TM (2003) Size selective recognition of siRNA by an RNA silencing suppressor. Cell 115: 799–811.
  16. 16. Ye K, Malinina L, Patel DJ (2003) Recognition of small interfering RNA by a viral suppressor of RNA silencing. Nature 426: 874–878.
  17. 17. Lakatos L, Csorba T, Pantaleo V, Chapman EJ, Carrington JC, et al. (2006) Small RNA binding is a common strategy to suppress RNA silencing by several viral suppressors. Embo J 25: 2768–2780.
  18. 18. Bortolamiol D, Pazhouhandeh M, Marrocco K, Genschik P, Ziegler-Graff V (2007) The Polerovirus F Box Protein P0 Targets ARGONAUTE1 to Suppress RNA Silencing. Curr Biol 17: 1615–1621.
  19. 19. Zhang X, Yuan YR, Pei Y, Lin SS, Tuschl T, et al. (2006) Cucumber mosaic virus-encoded 2b suppressor inhibits Arabidopsis Argonaute1 cleavage activity to counter plant defense. Genes Dev 20: 3255–3268.
  20. 20. Chapman EJ, Prokhnevsky AI, Gopinath K, Dolja VV, Carrington JC (2004) Viral RNA silencing suppressors inhibit the microRNA pathway at an intermediate step. Genes Dev 18: 1179–1186.
  21. 21. Dunoyer P, Lecellier CH, Parizotto EA, Himber C, Voinnet O (2004) Probing the microRNA and small interfering RNA pathways with virus-encoded suppressors of RNA silencing. Plant Cell 16: 1235–1250.
  22. 22. Merai Z, Kerenyi Z, Molnar A, Barta E, Valoczi A, et al. (2005) Aureusvirus P14 is an efficient RNA silencing suppressor that binds double-stranded RNAs without size specificity. J Virol 79: 7217–7226.
  23. 23. Yu B, Chapman EJ, Yang Z, Carrington JC, Chen X (2006) Transgenically expressed viral RNA silencing suppressors interfere with microRNA methylation in Arabidopsis. FEBS Lett 580: 3117–3120.
  24. 24. Rorth P (1998) Gal4 in the Drosophila female germline. Mech Dev 78: 113–118.
  25. 25. Lecellier CH, Dunoyer P, Arar K, Lehmann-Che J, Eyquem S, et al. (2005) A cellular microRNA mediates antiviral defense in human cells. Science 308: 557–560.
  26. 26. Lingel A, Simon B, Izaurralde E, Sattler M (2005) The structure of the flock house virus B2 protein, a viral suppressor of RNA interference, shows a novel mode of double-stranded RNA recognition. EMBO Rep 6: 1149–1155.
  27. 27. Aliyari R, Wu Q, Li HW, Wang XH, Li F, et al. (2008) Mechanism of induction and suppression of antiviral immunity directed by virus-derived small RNAs in Drosophila. Cell Host Microbe 4: 387–397.
  28. 28. Lee YS, Nakahara K, Pham JW, Kim K, He Z, et al. (2004) Distinct roles for Drosophila Dicer-1 and Dicer-2 in the siRNA/miRNA silencing pathways. Cell 117: 69–81.
  29. 29. Vagin VV, Sigova A, Li C, Seitz H, Gvozdev V, et al. (2006) A distinct small RNA pathway silences selfish genetic elements in the germline. Science 313: 320–324.
  30. 30. Ghildiyal M, Seitz H, Horwich MD, Li C, Du T, et al. (2008) Endogenous siRNAs Derived from Transposons and mRNAs in Drosophila Somatic Cells. Science 320: 1077–1081.
  31. 31. Czech B, Malone CD, Zhou R, Stark A, Schlingeheyde C, et al. (2008) An endogenous small interfering RNA pathway in Drosophila. Nature 453: 798–802.
  32. 32. Desset S, Buchon N, Meignin C, Coiffet M, Vaury C (2008) In Drosophila melanogaster the COM locus directs the somatic silencing of two retrotransposons through both Piwi-dependent and -independent pathways. PLoS ONE 3: e1526.
  33. 33. Deddouche S, Matt N, Budd A, Mueller S, Kemp C, et al. (2008) The DExD/H-box helicase Dicer-2 mediates the induction of antiviral activity in drosophila. Nat Immunol 9: 1425–1432.
  34. 34. Pruss G, Ge X, Shi XM, Carrington JC, Bowman Vance V (1997) Plant viral synergism: the potyviral genome encodes a broad-range pathogenicity enhancer that transactivates replication of heterologous viruses. Plant Cell 9: 859–868.
  35. 35. Merai Z, Kerenyi Z, Kertesz S, Magna M, Lakatos L, et al. (2006) Double-stranded RNA binding may be a general plant RNA viral strategy to suppress RNA silencing. J Virol 80: 5747–5756.
  36. 36. Ziegler-Graff V, Brault V (2008) Role of vector-transmission proteins. Methods Mol Biol 451: 81–96.
  37. 37. Voinnet O (2005) Induction and suppression of RNA silencing: insights from viral infections. Nat Rev Genet 6: 206–220.
  38. 38. Pazhouhandeh M, Dieterle M, Marrocco K, Lechner E, Berry B, et al. (2006) F-box-like domain in the polerovirus protein P0 is required for silencing suppressor function. Proc Natl Acad Sci U S A 103: 1994–1999.
  39. 39. Reavy B, Dawson S, Canto T, MacFarlane SA (2004) Heterologous expression of plant virus genes that suppress post-transcriptional gene silencing results in suppression of RNA interference in Drosophila cells. BMC Biotechnol 4: 18.
  40. 40. Brennecke J, Hipfner DR, Stark A, Russell RB, Cohen SM (2003) bantam encodes a developmentally regulated microRNA that controls cell proliferation and regulates the proapoptotic gene hid in Drosophila. Cell 113: 25–36.
  41. 41. Dostert C, Jouanguy E, Irving P, Troxler L, Galiana-Arnoux D, et al. (2005) The Jak-STAT signaling pathway is required but not sufficient for the antiviral response of drosophila. Nat Immunol 6: 946–953.
  42. 42. Moissiard G, Parizotto EA, Himber C, Voinnet O (2007) Transitivity in Arabidopsis can be primed, requires the redundant action of the antiviral Dicer-like 4 and Dicer-like 2, and is compromised by viral-encoded suppressor proteins. Rna 13: 1268–1278.
  43. 43. Deleris A, Gallego-Bartolome J, Bao J, Kasschau KD, Carrington JC, et al. (2006) Hierarchical action and inhibition of plant Dicer-like proteins in antiviral defense. Science 313: 68–71.