Gammaherpesviral Gene Expression and Virion Composition Are Broadly Controlled by Accelerated mRNA Degradation

Abstract

Lytic gammaherpesvirus infection restricts host gene expression by promoting widespread degradation of cytoplasmic mRNA through the activity of the viral endonuclease SOX. Though generally assumed to be selective for cellular transcripts, the extent to which SOX impacts viral mRNA stability has remained unknown. We addressed this issue using the model murine gammaherpesvirus MHV68 and, unexpectedly, found that all stages of viral gene expression are controlled through mRNA degradation. Using both comprehensive RNA expression profiling and half-life studies we reveal that the levels of the majority of viral mRNAs but not noncoding RNAs are tempered by MHV68 SOX (muSOX) activity. The targeting of viral mRNA by muSOX is functionally significant, as it impacts intracellular viral protein abundance and progeny virion composition. In the absence of muSOX-imposed gene expression control the viral particles display increased cell surface binding and entry as well as enhanced immediate early gene expression. These phenotypes culminate in a viral replication defect in multiple cell types as well as in vivo, highlighting the importance of maintaining the appropriate balance of viral RNA during gammaherpesviral infection. This is the first example of a virus that fails to broadly discriminate between cellular and viral transcripts during host shutoff and instead uses the targeting of viral messages to fine-tune overall gene expression.

Murine Gammaherpesvirus 68 LANA and SOX Homologs Counteract ATM-Driven p53 Activity during Lytic Viral Replication

“…Along these lines, it is reasonable to suggest that p53 hyperactivation in the absence of muSOX host shutoff activity contributes to the altered virion protein composition phenotype. This would agree with our previous demonstration that p53 drives viral gene expression (33), perhaps compounding the increased presence of viral transcripts due to the lack of degradation by muSOX (74). Thus, it will be of interest to determine if ORF37.⌬HS viruses produced in p53-deficient cells exhibit ratios of virion proteins that more closely resemble those of WT virus.…”

“…These phenotypes correlate with attenuation in vivo (74). Although muSOX host shutoff function was not a necessity for protection from cell death induced by exogenous p53-activating treatments, it remains possible that muSOX hindrance of p53 signaling is important in other cell types or settings.…”

“…ORF37.⌬HS MHV68 exhibits cell type-specific replication defects and altered virion protein composition (74). These phenotypes correlate with attenuation in vivo (74).…”

“…It is well established that muSOX functions to regulate the levels of both viral and host transcripts during viral replication (58,74,75). This includes host factors involved in antiviral processes, signaling, cellular responses to stress, and cell death (74)(75)(76).…”

Murine Gammaherpesvirus 68 Pathogenesis Is Independent of Caspase-1 and Caspase-11 in Mice and Impairs Interleukin-1β Production upon Extrinsic Stimulation in Culture

“…Like KSHV, MHV68 encodes an alkaline nuclease (ORF37/vSox) that mediates the reduction of a large number of host genes and influences virion protein composition (80)(81)(82). vSox may play a role in the general repression of cytokines observed late during infection (Fig.…”