Binding of Cellular Export Factor REF/Aly by Kaposi's Sarcoma-Associated Herpesvirus (KSHV) ORF57 Protein Is Not Required for Efficient KSHV Lytic Replication

Krueger

et al. 2015

J Virol

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The Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57 gene product is essential for lytic KSHV replication and virion production. Recombinant ORF57-null mutants fail to accumulate several lytic cycle mRNAs at wild-type levels, leading to decreased production of lytic proteins necessary for efficient replication. Several mechanisms by which ORF57 may enhance expression of lytic KSHV mRNAs have been proposed, including mRNA stabilization, mRNA nuclear export, increased polyadenylation, and transcriptional activation. ORF57 activity is also gene specific, with some genes being highly dependent on ORF57, whereas others are relatively independent. Most experiments have utilized transfection models for ORF57 and have not systematically examined the gene specificity and potential mechanisms of action of ORF57 in the context of KSHV-infected cells. In this study, the KSHV genes that are most highly upregulated by ORF57 during KSHV lytic replication were identified by a combination of high-throughput deep RNA sequencing, quantitative PCR, Northern blotting, and rapid amplification of cDNA ends methods. Comparison of gene expression from a ⌬ORF57 KSHV recombinant, a rescued ⌬ORF57 KSHV recombinant, and wild-type KSHV revealed that two clusters of lytic genes are most highly dependent on ORF57 for efficient expression. Despite contiguous location in the genome and shared polyadenylation of several of the ORF57-dependent genes, ORF57 regulation was promoter and polyadenylation signal independent, suggesting that the mRNAs are stabilized by ORF57. The eight genes identified to critically require ORF57 belong to both early and late lytic temporal classes, and seven are involved in DNA replication, virion assembly, or viral infectivity, explaining the essential role of ORF57 in infectious KSHV production. IMPORTANCEKaposi's sarcoma-associated herpesvirus (KSHV) is a human herpesvirus involved in the causation of several human cancers. The KSHV ORF57 protein is required for KSHV to replicate and produce infectious virus. We have identified several KSHV genes whose expression is highly dependent on ORF57 and shown that ORF57 increases expression of these genes specifically. These genes code for proteins that are required for the virus to replicate its DNA and to infect other cells. Identifying the targets and mechanism of action of ORF57 provides further approaches to discover antiviral therapy. K aposi's sarcoma associated herpesvirus (KSHV; human herpesvirus 8) is a human gammaherpesvirus associated with the development of Kaposi's sarcoma and several pathogenic lymphoproliferative syndromes (for a review, see reference 1). Lytic KSHV replication plays a role in pathogenesis by maintaining the infected cell reservoir, and lytic-phase KSHV proteins contribute to cell proliferation, inflammation, and immune evasion (2, 3). Understanding the mechanism of action of proteins required for lytic herpesvirus replication is therefore critical for further understanding of oncogenic mechanisms and the development of new antivi...

Section: Methodsmentioning

confidence: 99%

Identification of the Physiological Gene Targets of the Essential Lytic Replicative Kaposi's Sarcoma-Associated Herpesvirus ORF57 Protein

Krueger

et al. 2015

J Virol

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“…EBV lytic replication in AGS and AGSiZ cells was induced by TPA or doxycycline treatment, respectively, and also treated with either DMSO control or with SPR, CAN, TMS, or EPR for 5 d. Filtered cell supernatants were used to infect 293T cells, and the numbers of GFP-positive 293T cells representing infectious viral particles were quantitated by flow cytometry as described previously (14,33). For P3HR1-ZHT cells, cell supernatants were harvested at 5 d after induction of replication, and EBV DNA released in the supernatant was measured by qPCR.…”

Section: Methodsmentioning

confidence: 99%

Spironolactone blocks Epstein–Barr virus production by inhibiting EBV SM protein function

Proc. Natl. Acad. Sci. U.S.A.

Thompson

Swaminathan

2016

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Clinically available drugs active against Epstein-Barr virus (EBV) and other human herpesviruses are limited to those targeting viral DNA replication. To identify compounds directed against other steps in the viral life cycle, we searched for drugs active against the EBV SM protein, which is essential for infectious virus production. SM has a highly gene-specific mode of action and preferentially enhances expression of several late lytic cycle EBV genes. Here we demonstrate that spironolactone, a mineralocorticoid receptor antagonist approved for clinical use, inhibits SM function and infectious EBV production. Expression of EBV viral capsid antigen is highly SM dependent, and spironolactone inhibits viral capsid antigen synthesis and capsid formation, blocking EBV virion production at a step subsequent to viral DNA replication. In addition, spironolactone inhibits expression of other SM-dependent genes necessary for infectious virion formation. We further demonstrate that molecules structurally related to spironolactone with similar antimineralocorticoid blocking activity do not inhibit EBV production. These findings pave the way for development of antiherpesvirus drugs with new mechanisms of action directed against SM and homologous essential proteins in other herpesviruses. . EBV infects the majority of humans worldwide and establishes a persistent, lifelong latent infection in memory B cells. Occasional reactivation from latency occurs, and EBV enters a lytic phase of replication, during which a tightly regulated series of lytic genes is expressed, culminating in lysis of the host cell and release of infectious viral progeny. All herpesviruses share these abilities to establish asymptomatic latent infection and reactivate intermittently.All herpesviruses also express a regulatory protein early in lytic replication that is essential for efficient gene expression and infectious virion production. The EBV member of this family, which includes herpes simplex virus ICP27, cytomegalovirus (CMV) UL69, and Kaposi's sarcoma-associated herpesvirus (KSHV) ORF57, is known as SM (2). SM acts posttranscriptionally to enhance accumulation of many lytic EBV transcripts. SM activity is highly gene-specific and preferentially enhances expression of several late lytic transcripts, including those encoding the major viral capsid antigen (VCA) and gp350, which is required for infection of B lymphocytes (3-5). SM and its homologs in other herpesviruses may also enhance transcription and translation of specific viral genes (6).Currently, all available antiherpesvirus drugs target viral DNA polymerases and are usually highly effective, but toxicity and development of resistance limits their use (7). To discover potential novel therapeutic compounds, we applied a cell-based screening assay to identify compounds that would specifically target SM function (8). Using this assay, spironolactone (SPR), a drug in clinical use for over 50 y, was identified as inhibiting SM function and EBV virion production. SPR is an aldosterone antagonis...

Section: Importancementioning

confidence: 99%

“…Similarly, the relative binding affinity of EBV transcripts to SM has been assessed by immunoprecipitation of SMbound mRNAs, and while preferential binding to specific transcripts was demonstrated, these have not been correlated to SM responsiveness during lytic replication (7). While several mechanisms of action have been proposed for SM's ability to enhance mRNA accumulation, including roles in nuclear RNA stabilization and export (9,(12)(13)(14)(15)(16)(17)(18)(19), the exact nature of SM target gene specificity remains to be fully defined.A previous study described inefficient expression of a few late genes from recombinant EBV in which the SM gene had been inactivated (20). We subsequently performed a study using an oligonucleotide hybridization array to compare lytic EBV gene expression in wild-type (WT) EBV and SM-deleted virus, which revealed a widespread decrease in EBV lytic gene expression in the absence of SM (6).…”

mentioning

confidence: 99%

Identification and Characterization of the Physiological Gene Targets of the Essential Lytic Replicative Epstein-Barr Virus SM Protein

Thompson

et al. 2016

J Virol

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Epstein-Barr virus (EBV) SM protein is an essential lytic cycle protein with multiple posttranscriptional mechanisms of action. SM binds RNA and increases accumulation of specific EBV transcripts. Previous studies using microarrays and PCR have shown that SM-null mutants fail to accumulate several lytic cycle mRNAs and proteins at wild-type levels. However, the complete effect of SM on the EBV transcriptome has been incompletely characterized. Here we precisely identify the effects of SM on all EBV transcripts by high-throughput RNA sequencing, quantitative PCR (qPCR), and Northern blotting. The effect of SM on EBV mRNAs was highly skewed and was most evident on 13 late genes, demonstrating why SM is essential for infectious EBV production. EBV DNA replication was also partially impaired in SM mutants, suggesting additional roles for SM in EBV DNA replication. While it has been suggested that SM specificity is based on recognition of either RNA sequence motifs or other sequence properties, no such unifying property of SM-responsive targets was discernible. The binding affinity of mRNAs for SM also did not correlate with SM responsiveness. These data suggest that while target RNA binding by SM may be required for its effect, specific activation by SM is due to differences in inherent properties of individual transcripts. We therefore propose a new model for the mechanism of action and specificity of SM and its homologs in other herpesviruses: that they bind many RNAs but only enhance accumulation of those that are intrinsically unstable and poorly expressed. IMPORTANCEThis study examines the mechanism of action of EBV SM protein, which is essential for EBV replication and infectious virus production. Since SM protein is not similar to any cellular protein and has homologs in all other human herpesviruses, it has potential importance as a therapeutic target. Here we establish which EBV RNAs are most highly upregulated by SM, allowing us to understand why it is essential for EBV replication. By comparing and characterizing these RNA transcripts, we conclude that the mechanism of specific activity is unlikely to be based simply on preferential recognition of a target motif. Rather, SM binding to its target RNA may be necessary but not sufficient for enhancing accumulation of the RNA. Preferential effects of SM on its most responsive RNA targets may depend on other inherent characteristics of these specific mRNAs that require SM for efficient expression, such as RNA stability. E pstein-Barr virus (EBV) SM protein is an essential regulatory protein expressed by EBV during the lytic phase of replication (for reviews, see references 1 and 2). SM protein is detectable after immediate early gene expression and prior to expression of other early gene products (3, 4). When the SM gene is insertionally inactivated in recombinant EBV genomes, the resultant virus is incapable of completing the lytic cycle and producing infectious virions (5, 6). The exact reasons that SM is essential for lytic replication and virus pro...