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.

The mechanism of sublethal chlorantraniliprole exposure causing silkworm pupation metamorphosis defects

BACKGROUND

Chlorantraniliprole (CAP) is widely used in agriculture and forestry to prevent and control pests. The effects of environmental CAP residue on non-target insect metamorphosis have not been reported. Our research aimed to investigate the sublethal effect of CAP on larva-pupa transformation in silkworm, and explore the mechanism of sublethal CAP exposure-mediated pupation metamorphosis defects.

RESULT

Sublethal CAP exposure affected the growth and development of silkworm larvae and caused defects in pupation metamorphosis. After CAP exposure, formation the of prepupa procuticle, ecdysial membrane and new epidermis was inhibited. Also, the level of 20-hydroxyecdysone (20E) and mRNA levels of the 20E signaling pathway-related genes EcR, USP, E74, E75 and Ftz-f1 were significantly reduced. Moreover, genes involved in chitin synthesis, such as ChsA, CDA1 and CDA2, were downregulated. Injection of 20E led to the upregulation of chitin synthesis-related genes and increased formation of new epidermis in CAP-treated silkworm. However, injection of 20E failed to prevent downregulation of Ftz-f1 and the defects in pupation metamorphosis.

CONCLUSION

Our results suggested that 20E is a target hormone of CAP exposure-mediated epidermis formation phenotype. Ftz-f1 was silenced by CAP and might be a direct target gene of sublethal CAP exposure. Our study provided new evidence of the effects of sublethal CAP exposure on insect development and metamorphosis. © 2020 Society of Chemical Industry.

mori

Heat shock protein 90 (Hsp90) plays a very important role in facilitating the replication of many viruses. Until now, little has been known about the role of Hsp90 in Bombyx mori virus infection. In this study, we explored the role of BmHsp90 in B. mori nucleopolyhedrovirus (BmNPV) replication. We found that BmHsp90 inhibition by geldanamycin (GA) significantly reduced the BmNPV titre, the protein expression level of BmNPV nucleocapsid protein 39 (VP39) and the transcript level of BmNPV genes. Silencing the hsp90 gene in BmN cells by small interfering RNA suppressed BmNPV replication whereas overexpression of hsp90 promoted the replication of BmNPV. After inhibition of Hsp90, the expression of three key genes [signal transducing activator of transcription (stat), suppressor of cytokine signalling protein 2 (socs2), socs6] involved in the Janus kinase/STAT pathway significantly changed, with up-regulation of stat and down-regulation of socs2 and socs6. In addition, the expression of two antiapoptosis genes, BmNPV inhibitor of apoptosis protein1 (BmNPV-iap1) and Bmiap2, was greatly decreased in GA-treated cells, whereas their expression was significantly increased in hsp90-overexpressed silkworm larvae. Our results indicated that inhibition of Hsp90 can suppress BmNPV proliferation in B. mori. Our findings may provide new clues to elucidate the molecular mechanisms of silkworm-virus interactions.

The effect of acetylation on the protein stability of BmApoLp-III in the silkworm, Bombyx mori

Acetylation is an important, reversible posttranslational modification to a protein. In a previous study, we found that there were a large number of acetylated sites in various nutrient storage proteins of the silkworm haemolymph. In this study, we confirmed that acetylation can affect the stability of nutrient storage protein Bombyx mori apolipophorin-III (BmApoLp-III). First, the expression of BmApoLp-III could be upregulated when BmN cells were treated with the deacetylase inhibitor panobinostat (LBH589); similarly, the expression was downregulated when the cells were treated with the acetylase inhibitor C646. Furthermore, the increase in acetylation by LBH589 could inhibit the degradation and improve the accumulation of BmApoLp-III in BmN cells treated with cycloheximide and MG132 respectively. Moreover, we found that an increase in acetylation could decrease the ubiquitination of BmApoLp-III and vice versa; therefore, we predicted that acetylation could improve the stability of BmApoLp-III by competing for ubiquitination and inhibiting the protein degradation pathway mediated by ubiquitin. Additionally, BmApoLp-III had an antiapoptosis function that increased after LBH589 treatment, which might have been due to the improved protein stability after acetylation. These results have laid the foundation for further study on the mechanism of acetylation in regulating the storage and utilization of silkworm nutrition.

The mechanism of damage by trace amounts of acetamiprid to the midgut of the silkworm, Bombyx mori

Acetamiprid is widely used for agricultural pest control. However, it remains poorly understood whether the environmental residues of acetamiprid have the potential effects on economic insect. In this study, we evaluated the effects of acetamiprid on silkworm growth and development. The exposure to trace amounts of acetamiprid significantly decreased body weight, viability, and spinning ability. In addition, the activity of trypsin in the midgut was decreased after exposure. DGE and KEGG pathway enrichment analysis revealed that the significantly differentially expressed genes were mainly involved in nutrient metabolism, stress responses, and inflammation pathways. These results, in combination with hematoxylin-eosin staining and transmission electron microscopy, indicated that acetamiprid could cause oxidative damage to midgut, lead to inflammatory responses, and affect the activities of midgut digestive enzymes, thus resulting in abnormal growth and development. Our findings greatly contributed to the evaluation of the effects of acetamiprid residues on other nontarget beneficial insect.