Functional analysis of the non-coding RNAs of murine gammaherpesvirus 68

Abstract

Murine gammaherpesvirus 68 (MHV-68) is used as a model for the study of gammaherpesvirus infection and pathogenesis. In the left region of the genome MHV-68 encodes four unique genes, eight viral tRNA-like molecules (vtRNAs) and nine miRNAs. The vtRNAs have a predicted cloverleaf-like secondary structure like cellular tRNAs and are processed into mature tRNAs with the addition of 3’ CCA termini, but are not aminoacylated. Their function is unknown; however they have been found to be expressed at high levels during both lytic and latent infection and are packaged in the virion. The miRNAs are expressed from the vtRNA primary transcripts during latent infection. All herpesviruses examined to date have been found to express miRNAs. These are thought to aid the viruses in avoiding the host immune response and to establish and maintain latency. The aim of this project was to investigate the functions of the vtRNAs and miRNAs of MHV-68. MHV-76 is a natural deletant mutant lacking the unique genes, vtRNAs and miRNAs. This virus was previously used in our lab to construct two insertion viruses encoding vtRNAs1-5 and miRNAs1-6. The only difference between MHV-76 and the insertion viruses is therefore the vtRNAs and miRNAs. The B-cell line NS0 was latently infected with the various viruses and the infected cells characterised. In situ hybridisation and antibody staining showed that all viruses infect the same proportion of cells. The insertion viruses were confirmed to express the vtRNAs during latency by RT-PCR. In addition, using Northern blot analysis the insertion viruses were shown to express miRNA1 during lytic infection of fibroblast cells; however, not during latent infection of NS0 cells. The lack of miRNA1 expression during latency was confirmed using qRT-PCR and miRNAs3-6 were found to be expressed at a lower level than in MHV-68 infected cells. Replication and reactivation kinetics of latently infected NS0 cells showed that introduction of vtRNAs and miRNAs into MHV-76 causes a reduction in reactivation and production of lytic virus. To determine if the reduction in reactivation was caused by the miRNAs, they were introduced into infected cells by transfection. Transfection of miRNAs1-6 into MHV-76 infected cells or miRNA1 into insertion virus infected cells did not lead to an increase or decrease in reactivation. It was confirmed by qRT-PCR that the transfection did result in miRNA levels higher than in insertion virus infected cells. Further, down-regulation of miRNAs using a siRNA against DICER did not lead to a reduction in reactivation. This supports the hypothesis that the vtRNAs rather than the miRNAs are responsible for the reduction of reactivation seen in insertion virus latently infected cells. To determine the effect of the non-coding RNAs on protein expression, NS0 cells latently infected with MHV-76 and insertion virus were analysed using cleavable ICAT and 1-D PAGE cleavable ICAT. In an ICAT analysis the proteins are labelled and the levels of individual proteins in two samples can be compared using mass spectrometry. These techniques were optimised and several proteins with differences in expression between the viruses were identified. It was, however, difficult to determine any specific functions of the non-coding RNAs from the data.