Murine cytomegalovirus infection of cultured mouse cells induces expression of miR-7a.

One goal of virus infection is to reprogramme the host cell to optimize virus replication. As part of this process, viral microRNAs (miRNAs) may compete for components of the miRNA/small interfering RNA pathway, as well as regulate cellular targets. Murine cytomegalovirus (MCMV) has been described to generate large numbers of viral miRNAs during lytic infection and was therefore used to analyse the impact of viral miRNAs on the host-cell small-RNA system, as well as to check for sorting of viral small RNAs into specific Argonaute (Ago) proteins. Deep-sequencing analysis of MCMV-infected cells revealed that viral miRNAs represented only ~13% of all detected miRNAs. All previously described MCMV miRNAs with the exception of miR-m88-1* were confirmed, and for the MCMV miR-m01-1 hairpin, an additional miRNA, designated miR-m01-1-3p, was found. Its presence was confirmed by quantitative real-time PCR and Northern blotting. Deep sequencing after RNA-induced silencing complex (RISC) immunoprecipitation with antibodies specific for either Ago1 or Ago2 showed that all MCMV miRNAs were loaded into both RISCs. The ratio of MCMV to mouse miRNAs was not increased after immunoprecipitation of Ago proteins. Viral miRNAs therefore did not overwhelm the host miRNA processing system, nor were they incorporated preferentially into RISCs. Three mouse miRNAs were found that showed altered expression as a result of MCMV infection. Downregulation of miR-27a, as described previously, could be confirmed. In addition, miR-26a was downregulated, and upregulation of miR-7a dependent on viral protein expression could be observed.