Functional analysis of a putative Bombyx mori cypovirus miRNA BmCPV-miR-10 and its effect on virus replication

Bombyx mori cypovirus (BmCPV) is an important pathogen of silkworm (B. mori), the economically beneficial insect. The mechanism of its interaction with host immune defence system in the process of infection is still not yet completely clear. Researches have demonstrated that virus-encoded microRNAs (miRNA) play a crucial role in regulating host-pathogen interaction, but few reports are available so far on miRNAs encoded by insect viruses, especially the RNA viruses. In this study, a putative miRNA encoded by the 10th segment of BmCPV genomic RNA, BmCPV-miR-10, was identified and functionally analysed. The expression of the putative BmCPV-miR-10 could be detected via stem-loop RT-PCR (reverse transcription-Polymerase Chain Reaction) in the midgut of silkworm larvae infected with BmCPV. BmCSDE1 (B. mori cold shock domain E1 protein) gene was predicted to be a candidate target gene for BmCPV-miR-10 with the miRNA binding site located in 3' untranslated region of its mRNA. The regulation effect of the putative BmCPV-miR-10 on BmCSDE1 was verified in HEK293 cells by lentiviral expression system, in BmN cells by transfecting BmCPV-miR-10 mimics. The qRT-PCR (quantitative real-time PCR) results showed that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1. By injection of BmCPV-miR-10 mimics into the silkworm larvae infected with BmCPV, it was further proved that the putative BmCPV-miR-10 could suppress the expression of BmCSDE1 in vivo, then inhibit the expression of BmApaf-1 (B. mori apoptotic protease activating factor 1), while enhance the replication of BmCPV genomic RNAs to a certain extent. These results implied that the putative BmCPV-miR-10 could down-regulate the expression of BmCSDE1, then suppress the expression of BmApaf-1, thereby created a favourable intracellular environment for virus replication and proliferation.