Murine Polyomavirus encodes a microRNA that cleaves early RNA transcripts but is not essential for experimental infection

Abstract

MicroRNAs are small regulatory RNAs that post-transcriptionally regulate gene expression and can be encoded by viral as well as cellular genomes. The functions of most viral miRNAs are unknown and few have been studied in an in vivo context. Here we show that the murine polyomavirus (PyV) encodes a precursor microRNA that is processed into two mature microRNAs, both of which are active at directing the cleavage of the early PyV mRNAs. Furthermore, we identify a deletion mutant of polyomavirus that is defective in encoding the microRNAs. This mutant replicates normally and transforms cultured cells with efficiencies comparable to wildtype PyV. The miRNA mutant is competent to establish a transient infection of mice following parenteral inoculation, and is cleared post infection at approximately the same rate as the wildtype virus. In addition, under these laboratory conditions, we observe no differences in anti-viral CD8 T cell responses. These results indicate that PyV miRNA expression is not essential for infection of cultured cells or experimentally inoculated mice, and raise the possibility that its role in natural infection might involve aspects of acquisition or spread that are not recapitulated by experimental inoculation.

The role of RNAi and microRNAs in animal virus replication and antiviral immunity

“…All herpesviruses examined to date have been found to encode miRNAs, as have human adenovirus, Heliothis virescens ascovirus (HvAc), and several members of the polyomavirus family, (Table 1; Cantalupo et al 2005;Sullivan et al , 2009Aparicio et al 2006;Hussain et al 2008;Seo et al 2008Seo et al , 2009. That these are all nuclear DNA viruses likely reflects the fact that Drosha and DGCR8, required for the initial pre-miRNA excision event, localize to the nucleus-viruses that replicate exclusively in the cytoplasm would not have ready access to this machinery.…”

“…The function of these miRNAs also appears to be conserved as several also down-regulate TAg expression in their cognate viruses (Cantalupo et al 2005;Seo et al 2008Seo et al , 2009Sullivan et al 2009). Experimental infection of mice with a mutant form of mouse polyomavirus that does not express this miRNA did not, however, result in an appreciable decrease in replication when compared with wild-type virus; infection was established with equal efficiency by both viruses, and both were cleared from the host with approximately the same kinetics ).…”

miRNA regulation of BK polyomavirus replication during early infection

“…However, the miRNA mutant SV40 produces the same amount of infectious progeny as wild-type (WT) virus. A naturally occurring deletion mutant of the MPyV miRNA has no defects with respect to replication, transformation efficiency, or establishment or clearance of infection in vivo (10). Consequently, the miRNA was deemed to be nonessential for infection in vitro and in vivo.…”

Host Response to Polyomavirus Infection Is Modulated by RNA Adenosine Deaminase ADAR1 but Not by ADAR2

“…Interestingly, mouse polyomavirus has been shown to encode two microRNAs that direct the cleavage of early PyV mRNAs (44). The inhibition of the PyV microRNAs increases early protein levels, but interestingly the PyV microRNAs appear to be dispensable for the virus infection of cultured cells and intact mice, as mutant virus that fails to make the microRNAs replicates and transforms well (44). Our results suggest that ADAR1 impairs cell-destructive processes following PyV infection.…”

“…Examples of cellular microRNAs are known whose production or targeting is altered by the action of ADARs (14,19,33). Interestingly, mouse polyomavirus has been shown to encode two microRNAs that direct the cleavage of early PyV mRNAs (44). The inhibition of the PyV microRNAs increases early protein levels, but interestingly the PyV microRNAs appear to be dispensable for the virus infection of cultured cells and intact mice, as mutant virus that fails to make the microRNAs replicates and transforms well (44).…”