Coexpression of Escherichia coli RNase III in silkworm cells improves the efficiency of RNA interference induced by long hairpin dsRNAs

Over-expression of RNA interference (RNAi) core machinery improves susceptibility to RNAi in silkworm larvae

RNA interference (RNAi), one of the strategies that organisms use to defend against invading viruses, is an important tool for functional genomic analysis. In insects, the efficacy of RNAi varies amongst taxa. Lepidopteran insects are, in large part, recalcitrant to RNAi. The overall goal of this study is to overcome such insensitivity in lepidopterans to RNAi. We hypothesize that over-expression of core RNAi machinery enzymes can improve RNAi efficacy in traditionally recalcitrant species. A transgenic Bombyx mori strain, Baculovirus Immediate-Early Gene, ie1, promoter driven expression of silkworm Dicer2 coding sequence (IE1-BmDicer2), which over-expresses BmDicer2, was generated by piggyBac transposon-mediated transgenesis. Two indexes, the ratio of animals that showed a silencing phenotype and the duration of silencing, were used to evaluate silencing efficiency. Significant knockdown of target gene expression was observed at 48 h postinjection at both the transcriptional and translational levels. Furthermore, we coexpressed B. mori Argonaute 2 BmAgo2）and BmDicer 2 and found that 22% of the animals (n = 18) showed an obvious silencing effect even at 72 h, suggesting that coexpression of these two RNAi core machinery enzymes further increased the susceptibility of B. mori to injected double-stranded RNAs. This study offers a new strategy for functional genomics research in RNAi-refractory insect taxa in general and for lepidopterans in particular.

Transport of orally delivered dsRNA in southern green stink bug, Nezara viridula

The southern green stink bug (SGSB, Nezara viridula) is an emerging polyphagous pest in many regions of the world. RNA interference (RNAi) is a valuable method for understanding gene function and holds great potential for pest management. However, RNAi efficiency is variable among insects and the differences in transport of double-stranded RNA (dsRNA) are one of the major factors that contribute to this variability. In this study, Cy3 labeled dsRNA was used to track the transport of dsRNA in SGSB tissues. Cy3_dsRNA was detected in the hemocytes, fat body (FB), epidermis, and midgut tissues at 24-72 hr after injection. Orally delivered Cy3_dsRNA or Cypher-5E labeled dsRNA was mostly detected in the midgut and a few signals were detected in parts of the FB and epidermis. Both injected and fed Cy3_dsRNA showed stronger signals in SGSB tissues when compared to Cy3_siRNA (small interfering RNA) or Cy3_shRNA (short hairpin RNA). dsRNA targeting the gene for a vacuolar-sorting protein, SNF7, induced higher knockdown of the target gene and greater SGSB mortality compared to siRNA or shRNA targeting this gene. ³² P-labeled dsRNA injected into SGSB was processed into siRNA, but fed ³² P-labeled dsRNA was not efficiently processed into siRNA. These data suggest that transport of orally delivered dsRNA across the midgut epithelium is not efficient in SGSB which may contribute to variable RNAi efficiency. Targeting genes expressed in the midgut rather than other tissues and using dsRNA instead of siRNA or shRNA would be more effective for RNAi-mediated control of this pest.

Influence of various stressors on the expression of core genes of the small interfering RNA pathway in the oriental fruit fly, Bactrocera dorsalis

RNA interference (RNAi)-based technology has emerged as a potential tool for controlling insect pests, however, previous studies found that the efficiency of RNAi in Bactrocera dorsalis was variable. In nature, insects often meet various challenges, such as pathogen infections, extreme temperatures, lack of nutrition and heavy metals. To better understand the association of the stressors with efficiency of RNAi, in the current study we tested the expression of three core genes, dicer2 (Bddcr2), r2d2 (Bdr2d2) and argonaute2 (Bdago2), of the small interfering RNA (siRNA) pathway of B. dorsalis upon various stressors. Our results showed that all three genes were upregulated by the infection of invertebrate iridescent virus 6, which suggested a function of the siRNA pathway against viral infection. The loading of FeCl₃ could also increase the expression of Bddcr2. The treatments of Escherichia coli, extremely high (40°C) and low (0°C) temperatures, as well as starvation, could negatively influence the expression of Bddcr2 and/or Bdago2. In total, our results showed that various stressors could influence the expression of core components of B. dorsalis siRNA pathway. This highlights further speculation on the RNAi efficiency upon these stressors. Considering the complexity and variation of RNAi efficiency in different conditions, these results provide initial aspects in possible environmental stressors to influence the activity of the siRNA pathway, but the real impact of RNAi efficiency posed by these stressors requires further studies.

RNA interference for the identification of ectoparasite vaccine candidates

Ectoparasites present a major challenge for disease management globally. With drug resistance increasingly observed in many disease-causing species, the need for novel control measures is pressing. Ever-expanding genomic resources from 'next generation' sequencing are now available for a number of arthropod ectoparasites, necessitating an effective means of screening these data for novel candidates for vaccine antigens or targets for chemotherapeutics. Such in vitro screening methods must be developed if we are to make discoveries in a timely and cost-effective manner. This review will discuss the potential that RNA interference (RNAi) has demonstrated thus far in the context of arthropod ectoparasites and the potential roles for this technology in the development of novel methods for parasite control.

Enhancement of larval RNAi efficiency by over-expressing Argonaute2 in Bombyx mori

RNA interference has been described as a powerful genetic tool for gene functional analysis and a promising approach for pest management. However, RNAi efficiency varies significantly among insect species due to distinct RNAi machineries. Lepidopteran insects include a large number of pests as well as model insects, such as the silkworm, Bombyx mori. However, only limited success of in vivo RNAi has been reported in lepidoptera, particularly during the larval stages when the worms feed the most and do the most harm to the host plant. Enhancing the efficiency of larval RNAi in lepidoptera is urgently needed to develop RNAi-based pest management strategies. In the present study, we investigate the function of the conserved RNAi core factor, Argonaute2 (Ago2), in mediating B. mori RNAi efficiency. We demonstrate that introducing BmAgo2 dsRNA inhibits the RNAi response in both BmN cells and embryos. Furthermore, we establish several transgenic silkworm lines to assess the roles of BmAgo2 in larval RNAi. Over-expressing BmAgo2 significantly facilitated both dsRNA-mediated larval RNAi when targeting DsRed using dsRNA injection and shRNA-mediated larval RNAi when targeting BmBlos2 using transgenic shRNA expression. Our results show that BmAgo2 is involved in RNAi in B. mori and provides a promising approach for improving larval RNAi efficiency in B. mori and in lepidopteran insects in general.