Herpes simplex virus type 1 immediate early gene expression is stimulated by inhibition of protein synthesis

The ORF45 Protein of Kaposi's Sarcoma-Associated Herpesvirus and Its Critical Role in the Viral Life Cycle

Kaposi's sarcoma-associated herpesvirus (KSHV) protein ORF45 is a virion-associated tegument protein that is unique to the gammaherpesvirus family. Generation of KSHV ORF45-knockout mutants and their subsequent functional analyses have permitted a better understanding of ORF45 and its context-specific and vital role in the KSHV lytic cycle. ORF45 is a multifaceted protein that promotes infection at both the early and late phases of the viral life cycle. As an immediate-early protein, ORF45 is expressed within hours of KSHV lytic reactivation and plays an essential role in promoting the lytic cycle, using multiple mechanisms, including inhibition of the host interferon response. As a tegument protein, ORF45 is necessary for the proper targeting of the viral capsid for envelopment and release, affecting the late stage of the viral life cycle. A growing list of ORF45 interaction partners have been identified, with one of the most well-characterized being the association of ORF45 with the host extracellular-regulated kinase (ERK) p90 ribosomal s6 kinase (RSK) signaling cascade. In this review, we describe ORF45 expression kinetics, as well as the host and viral interaction partners of ORF45 and the significance of these interactions in KSHV biology. Finally, we discuss the role of ORF45 homologs in gammaherpesvirus infections.

Dewormer drug fenbendazole has antiviral effects on BoHV-1 productive infection in cell cultures

BACKGROUND

Fenbendazole, a dewormer drug, is used widely in the clinical treatment of parasite infections in animals. Recent studies have shown that fenbendazole has substantial effects on tumor growth, immune responses, and inflammatory responses, suggesting that fenbendazole is a pluripotent drug. Nevertheless, the antiviral effects have not been reported. Fenbendazole can disrupt microtubules, which are essential for multiple viruses infections, suggesting that fenbendazole might have antiviral effects.

OBJECTIVES

This study examined whether fenbendazole could inhibit bovine herpesvirus 1 (BoHV-1) productive infection in cell cultures.

METHODS

The effects of fenbendazole on viral production, transcription of the immediate early (IE) genes, viron-associated protein expression, and the cellular signaling PLC-γ1/Akt pathway were assessed using distinct methods.

RESULTS

Fenbendazole could inhibit BoHV-1 productive infections significantly in MDBK cells in a dose-dependent manner. A time-of-addition assay indicated that fenbendazole affected both the early and late stages in the virus replication cycles. The transcription of IE genes, including BoHV-1 infected cell protein 0 (bICP0), bCP4, and bICP22, as well as the synthesis of viron-associated proteins, were disrupted differentially by the fenbendazole treatment. The treatment did not affect the cellular signaling pathway of PLC-γ1/Akt, a known cascade playing important roles in virus infection.

CONCLUSIONS

Overall, fenbendazole has antiviral effects on BoHV-1 replication.

Human cytomegalovirus major immediate early transcripts arise predominantly from the canonical major immediate early promoter in reactivating progenitor-derived dendritic cells

Human cytomegalovirus latency and reactivation is a major source of morbidity in immune-suppressed patient populations. Lifelong latent infections are established in CD34+progenitor cells in the bone marrow, which are hallmarked by a lack of major lytic gene expression, genome replication and virus production. A number of studies have shown that inhibition of the major immediate early promoter (MIEP) – the promoter that regulates immediate early (IE) gene expression – is important for the establishment of latency and that, by extension, reactivation requires reversal of this repression of the MIEP. The identification of novel promoters (termed ip1 and ip2) downstream of the MIEP that can drive IE gene expression has led to speculation over the precise role of the MIEP in reactivation. In this study we show that IE transcripts arise from both the MIEP and ip2 promoter in the THP1 cell macrophage cell line and also CD14+monocytes stimulated with phorbol ester. In contrast, we show that in in vitro generated dendritic cells or macrophages that support HCMV reactivation IE transcripts arise predominantly from the MIEP and not the intronic promoters. Furthermore, inhibition of histone modifying enzyme activity confirms the view that the MIEP is predominantly regulated by the activity of cellular chromatin. Finally, we observe that ip2-derived IE transcription is cycloheximide-sensitive in reactivating DCs, behaviour consistent with an early gene designation. Taken together, these data argue that MIEP activity is still important for HCMV reactivation but ip2 activity could play cell-type-specific roles in reactivation.

Promyelocytic leukemia (PML) nuclear bodies (NBs) induce latent/quiescent HSV-1 genomes chromatinization through a PML NB/Histone H3.3/H3.3 Chaperone Axis

Herpes simplex virus 1 (HSV-1) latency establishment is tightly controlled by promyelocytic leukemia (PML) nuclear bodies (NBs) (or ND10), although their exact contribution is still elusive. A hallmark of HSV-1 latency is the interaction between latent viral genomes and PML NBs, leading to the formation of viral DNA-containing PML NBs (vDCP NBs), and the complete silencing of HSV-1. Using a replication-defective HSV-1-infected human primary fibroblast model reproducing the formation of vDCP NBs, combined with an immuno-FISH approach developed to detect latent/quiescent HSV-1, we show that vDCP NBs contain both histone H3.3 and its chaperone complexes, i.e., DAXX/ATRX and HIRA complex (HIRA, UBN1, CABIN1, and ASF1a). HIRA also co-localizes with vDCP NBs present in trigeminal ganglia (TG) neurons from HSV-1-infected wild type mice. ChIP and Re-ChIP show that vDCP NBs-associated latent/quiescent viral genomes are chromatinized almost exclusively with H3.3 modified on its lysine (K) 9 by trimethylation, consistent with an interaction of the H3.3 chaperones with multiple viral loci and with the transcriptional silencing of HSV-1. Only simultaneous inactivation of both H3.3 chaperone complexes has a significant impact on the deposition of H3.3 on viral genomes, suggesting a compensation mechanism. In contrast, the sole depletion of PML significantly impacts the chromatinization of the latent/quiescent viral genomes with H3.3 without any overall replacement with H3.1. vDCP NBs-associated HSV-1 genomes are not definitively silenced since the destabilization of vDCP NBs by ICP0, which is essential for HSV-1 reactivation in vivo, allows the recovery of a transcriptional lytic program and the replication of viral genomes. Consequently, the present study demonstrates a specific chromatin regulation of vDCP NBs-associated latent/quiescent HSV-1 through an H3.3-dependent HSV-1 chromatinization involving the two H3.3 chaperones DAXX/ATRX and HIRA complexes. Additionally, the study reveals that PML NBs are major actors in latent/quiescent HSV-1 H3.3 chromatinization through a PML NB/histone H3.3/H3.3 chaperone axis.

An understanding of the molecular mechanisms contributing to the persistence of a virus in its host is essential to be able to control viral reactivation and its associated diseases. Herpes simplex virus 1 (HSV-1) is a human pathogen that remains latent in the PNS and CNS of the infected host. The latency is unstable, and frequent reactivations of the virus are responsible for PNS and CNS pathologies. It is thus crucial to understand the physiological, immunological and molecular levels of interplay between latent HSV-1 and the host. Promyelocytic leukemia (PML) nuclear bodies (NBs) control viral infections by preventing the onset of lytic infection. In previous studies, we showed a major role of PML NBs in favoring the establishment of a latent state for HSV-1. A hallmark of HSV-1 latency establishment is the formation of PML NBs containing the viral genome, which we called “viral DNA-containing PML NBs” (vDCP NBs). The genome entrapped in the vDCP NBs is transcriptionally silenced. This naturally occurring latent/quiescent state could, however, be transcriptionally reactivated. Therefore, understanding the role of PML NBs in controlling the establishment of HSV-1 latency and its reactivation is essential to design new therapeutic approaches based on the prevention of viral reactivation.

Latency Entry of Herpes Simplex Virus 1 Is Determined by the Interaction of Its Genome with the Nuclear Environment

Herpes simplex virus 1 (HSV-1) establishes latency in trigeminal ganglia (TG) sensory neurons of infected individuals. The commitment of infected neurons toward the viral lytic or latent transcriptional program is likely to depend on both viral and cellular factors, and to differ among individual neurons. In this study, we used a mouse model of HSV-1 infection to investigate the relationship between viral genomes and the nuclear environment in terms of the establishment of latency. During acute infection, viral genomes show two major patterns: replication compartments or multiple spots distributed in the nucleoplasm (namely “multiple-acute”). Viral genomes in the “multiple-acute” pattern are systematically associated with the promyelocytic leukemia (PML) protein in structures designated viral DNA-containing PML nuclear bodies (vDCP-NBs). To investigate the viral and cellular features that favor the acquisition of the latency-associated viral genome patterns, we infected mouse primary TG neurons from wild type (wt) mice or knock-out mice for type 1 interferon (IFN) receptor with wt or a mutant HSV-1, which is unable to replicate due to the synthesis of a non-functional ICP4, the major virus transactivator. We found that the inability of the virus to initiate the lytic program combined to its inability to synthesize a functional ICP0, are the two viral features leading to the formation of vDCP-NBs. The formation of the “multiple-latency” pattern is favored by the type 1 IFN signaling pathway in the context of neurons infected by a virus able to replicate through the expression of a functional ICP4 but unable to express functional VP16 and ICP0. Analyses of TGs harvested from HSV-1 latently infected humans showed that viral genomes and PML occupy similar nuclear areas in infected neurons, eventually forming vDCP-NB-like structures. Overall our study designates PML protein and PML-NBs to be major cellular components involved in the control of HSV-1 latency, probably during the entire life of an individual.

Establishment of latency of herpes simplex virus 1 (HSV-1) at the cellular level results from the combination of a series of complex molecular events involving cellular and viral-associated features. HSV-1 establishes latency in trigeminal ganglia (TG) sensory neurons. HSV-1 genomes remain as extrachromosomal DNA; their initial interaction with the nuclear architecture is likely to determine commitment toward the lytic or the latent transcriptional program. Among the major nuclear components that influence the infection process the promyelocytic leukemia (PML) nuclear bodies (NBs) play a major role as nuclear relays of the intrinsic antiviral response. In this study, using infected mice and cultured mouse primary TG neuron models, as well as human TGs, we investigated the interaction between HSV-1 genomes and the nuclear environment in individual neurons. We found that the inability of HSV-1 to initiate a lytic program at the initial stages of infection led to the formation of latency-associated viral DNA-containing PML-NBs (vDCP-NBs), or another pattern if the type 1 interferon pathway was activated prior to infection. vDCP-NB–like structures were also present in neurons of latently infected human TGs, designating PML-NBs as major nuclear components involved in the control of HSV-1 latency for the entire life of an individual.

Herpes simplex viral-vector design for efficient transduction of nonneuronal cells without cytotoxicity

The design of highly defective herpes simplex virus (HSV) vectors for transgene expression in nonneuronal cells in the absence of toxic viral-gene activity has been elusive. Here, we report that elements of the latency locus protect a nonviral promoter against silencing in primary human cells in the absence of any viral-gene expression. We identified a CTCF motif cluster 5' to the latency promoter and a known long-term regulatory region as important elements for vigorous transgene expression from a vector that is functionally deleted for all five immediate-early genes and the 15-kb internal repeat region. We inserted a 16.5-kb expression cassette for full-length mouse dystrophin and report robust and durable expression in dystrophin-deficient muscle cells in vitro. Given the broad cell tropism of HSV, our design provides a nontoxic vector that can accommodate large transgene constructs for transduction of a wide variety of cells without vector integration, thereby filling an important void in the current arsenal of gene-therapy vectors.

Expression of herpes simplex virus 1 microRNAs in cell culture models of quiescent and latent infection

To facilitate studies of herpes simplex virus 1 latency, cell culture models of quiescent or latent infection have been developed. Using deep sequencing, we analyzed the expression of viral microRNAs (miRNAs) in two models employing human fibroblasts and one using rat neurons. In all cases, the expression patterns differed from that in productively infected cells, with the rat neuron pattern most closely resembling that found in latently infected human or mouse ganglia in vivo.

Virion factors that target Daxx to overcome intrinsic immunity

PML nuclear bodies and their associated functions are part of an intrinsic cellular mechanism aimed at maintaining transcriptional control over viral gene expression and preventing replication of invading viruses. To overcome these barriers, many viruses express early nonstructural, multifunctional proteins to support the viral replication cycle or modulate host immune responses. Virion proteins constituting the invading particle are traditionally investigated for their role in transport during entry or egress and in the assembly of new virions. The additional functions of virion proteins have largely been ignored, in contrast to those of their nonstructural counterparts. A number of recent reports suggest that several virion proteins may also play vital roles in gene activation processes, in particular by counteracting intrinsic immune mechanisms mediated by the PML nuclear body-associated cellular factors Daxx, ATRX, and Sp100. These virion proteins share several features with their more potent nonstructural counterparts, and they may serve to bridge the gap in the early phase of an infection until immediate early viral gene expression is established. In this review, we discuss how virion proteins are an integral part of gene regulation among several viral families and to what extent structural proteins of incoming virions may contribute to species barrier, latency, and oncogenesis.

Early induction of autophagy in human fibroblasts after infection with human cytomegalovirus or herpes simplex virus 1

The infection of human fetal foreskin fibroblasts (HFFF2) with human cytomegalovirus (HCMV) resulted in the induction of autophagy. This was demonstrated by the increased lipidation of microtubule-associated protein 1 light chain 3 (LC3), a hallmark of autophagy, and by the visualization of characteristic vesicles within infected cells. The response was detected first at 2 h postinfection and persisted for at least 3 days. De novo protein synthesis was not required for the effect, since HCMV that was irradiated with UV light also elicited the response, and furthermore the continuous presence of cycloheximide did not prevent induction. Infection with herpes simplex virus type 1 (HSV-1) under conditions that inhibited viral gene expression provoked autophagy, whereas UV-irradiated respiratory syncytial virus did not. The induction of autophagy occurred when cells were infected with HCMV or HSV-1 that was gradient purified, but HCMV dense bodies and HSV-1 light particles, each of which lack nucleocapsids and genomes, were inactive. The depletion of regulatory proteins Atg5 and Atg7, which are required for autophagy, reduced LC3 modification in response to infection but did not result in any detectable difference in viral or cellular gene expression at early times after infection. The electroporation of DNA into HFFF2 cultures induced the lipidation of LC3 but double-stranded RNA did not, even though both agents stimulated an innate immune response. The results show a novel, early cellular response to the presence of the incoming virion and additionally demonstrate that autophagy can be induced by the presence of foreign DNA within cells.

ICP0 antagonizes ICP4-dependent silencing of the herpes simplex virus ICP0 gene

ICP0 is a regulatory protein that plays a critical role in the replication-latency balance of herpes simplex virus (HSV). Absence of ICP0 renders HSV prone to establish quiescent infections, and thus cellular repressor(s) are believed to silence HSV mRNA synthesis when ICP0 fails to accumulate. To date, an ICP0-antagonized repressor has not been identified that restricts HSV mRNA synthesis by more than 2-fold. We report the unexpected discovery that HSV's major transcriptional regulator, ICP4, meets the criteria of a bona fide ICP0-antagonized repressor of viral mRNA synthesis. Our study began when we noted a repressive activity that restricted ICP0 mRNA synthesis by up to 30-fold in the absence of ICP0. When ICP0 accumulated, the repressor only restricted ICP0 mRNA synthesis by 3-fold. ICP4 proved to be necessary and sufficient to repress ICP0 mRNA synthesis, and did so in an ICP4-binding-site-dependent manner. ICP4 co-immunoprecipitated with FLAG-tagged ICP0; thus, a physical interaction likely explains how ICP0 antagonizes ICP4's capacity to silence the ICP0 gene. These findings suggest that ICP0 mRNA synthesis is differentially regulated in HSV-infected cells by the virus-encoded repressor activity embedded in ICP4, and a virus-encoded antirepressor, ICP0. Bacteriophage λ relies on a similar repression-antirepression regulatory scheme to “decide” whether a given infection will be productive or silent. Therefore, our findings appear to add to the growing list of inexplicable similarities that point to a common evolutionary ancestry between the herpesviruses and tailed bacteriophage.

Properties of virion transactivator proteins encoded by primate cytomegaloviruses

Background

Human cytomegalovirus (HCMV) is a betaherpesvirus that causes severe disease in situations where the immune system is immature or compromised. HCMV immediate early (IE) gene expression is stimulated by the virion phosphoprotein pp71, encoded by open reading frame (ORF) UL82, and this transactivation activity is important for the efficient initiation of viral replication. It is currently recognized that pp71 acts to overcome cellular intrinsic defences that otherwise block viral IE gene expression, and that interactions of pp71 with the cell proteins Daxx and ATRX are important for this function. A further property of pp71 is the ability to enable prolonged gene expression from quiescent herpes simplex virus type 1 (HSV-1) genomes. Non-human primate cytomegaloviruses encode homologs of pp71, but there is currently no published information that addresses their effects on gene expression and modes of action.

Results

The UL82 homolog encoded by simian cytomegalovirus (SCMV), strain Colburn, was identified and cloned. This ORF, named S82, was cloned into an HSV-1 vector, as were those from baboon, rhesus monkey and chimpanzee cytomegaloviruses. The use of an HSV-1 vector enabled expression of the UL82 homologs in a range of cell types, and permitted investigation of their abilities to direct prolonged gene expression from quiescent genomes. The results show that all UL82 homologs activate gene expression, and that neither host cell type nor promoter target sequence has major effects on these activities. Surprisingly, the UL82 proteins specified by non-human primate cytomegaloviruses, unlike pp71, did not direct long term expression from quiescent HSV-1 genomes. In addition, significant differences were observed in the intranuclear localization of the UL82 homologs, and in their effects on Daxx. Strikingly, S82 mediated the release of Daxx from nuclear domain 10 substructures much more rapidly than pp71 or the other proteins tested. All UL82 homologs stimulated the early release of ATRX from nuclear domain 10.

Conclusion

All of the UL82 homolog proteins analysed activated gene expression, but surprising differences in other aspects of their properties were revealed. The results provide new information on early events in infection with cytomegaloviruses.

Induction of cellular stress overcomes the requirement of herpes simplex virus type 1 for immediate-early protein ICP0 and reactivates expression from quiescent viral genomes

Herpes simplex virus type 1 (HSV-1) mutants impaired in the activities of the structural protein VP16 and the immediate-early (IE) proteins ICP0 and ICP4 establish a quiescent infection in human fibroblasts, with most cells retaining an inactive, repressed viral genome for sustained periods in culture. To date, the quiescent state has been considered stable, since it has been reversed only by provision of herpesviral proteins, such as ICP0, not by alteration of the cell physiological state. We report that the interaction of HSV-1 with human fibroblasts can be altered significantly by transient treatment of cultures with sodium arsenite, an inducer of heat shock and oxidative stress, or gramicidin D, a toxin that selectively permeabilizes cell membranes, prior to infection. These regimens stimulated gene expression from IE-deficient HSV-1 mutants in a promoter sequence-independent manner and also overcame the replication defect of ICP0-null mutants. Reactivation of gene expression from quiescent HSV-1 genomes and the resumption of virus replication were observed following addition of arsenite or gramicidin D to cultures. Both agents induced reorganization of nuclear domain 10 structures, the sites of quiescent genomes, but appeared to do so through different mechanisms. The results demonstrate that the physiological state of the cell is important in determining the outcome of infection with IE-deficient HSV-1 and show novel methods for reactivating quiescent HSV-1 in fibroblasts with a high efficiency.

The UL25 gene product of herpes simplex virus type 1 is involved in uncoating of the viral genome

Studies on the herpes simplex virus type 1 UL25-null mutant KUL25NS have shown that the capsid-associated UL25 protein is required at a late stage in the encapsidation of viral DNA. Our previous work on UL25 with the UL25 temperature-sensitive (ts) mutant ts1204 also implicated UL25 in a role at very early times in the virus growth cycle, possibly at the stage of penetration of the host cell. We have reexamined this mutant and discovered that it had an additional ts mutation elsewhere in the genome. The ts1204 UL25 mutation was transferred into wild-type (wt) virus DNA, and the UL25 mutant ts1249 was isolated and characterized to clarify the function of UL25 at the initial stages of virus infection. Indirect immunofluorescence assays and in situ hybridization analysis of virus-infected cells revealed that the mutant ts1249 was not impaired in penetration of the host cell but had an uncoating defect at the nonpermissive temperature. When ts1249-infected cells were incubated initially at the permissive temperature to allow uncoating of the viral genome and subsequently transferred to the restrictive temperature, a DNA-packaging defect was evident. The results suggested that ts1249, like KUL25NS, had a block at a late stage of DNA packaging and that the packaged genome was shorter than the full-length genome. Examination of ts1249 capsids produced at the nonpermissive temperature revealed that, in comparison with wt capsids, they contained reduced amounts of UL25 protein, thereby providing a possible explanation for the failure of ts1249 to package full-length viral DNA.

Reactivation of expression from quiescent herpes simplex virus type 1 genomes in the absence of immediate-early protein ICP0

Model systems have previously been developed in which herpes simplex virus (HSV) is retained in human fibroblasts in a nonreplicating state known as quiescence. The HSV type 1 (HSV-1) immediate-early (IE) protein ICP0, an important activator of gene expression, reactivates the quiescent genome and promotes the resumption of virus replication. Previous studies reported that infection with ICP0-null HSV-1 mutants fails to reactivate quiescent HSV, even when the mutant itself undergoes productive replication, leading to the hypothesis that quiescent genomes exist in a silent configuration in which they are shielded from trans-acting factors. I reinvestigated these findings, using HSV-1 mutants with lesions in the transcription activators VP16, ICP0, and ICP4 to establish quiescent infection at high efficiency. Superinfection with ICP0-null HSV-1 mutants at a low multiplicity of infection (MOI), so that individual plaques were formed, reactivated expression from the quiescent genome, demonstrating that the requirement for ICP0 is not absolute. The previously reported failure to observe reactivation by ICP0-null mutants was shown to be a consequence of either a low initial MOI or a high superinfecting MOI. Competition between viral genomes at the level of gene expression and virus replication, especially when ICP0 was absent, was demonstrated during reactivation and also during normal infection of human fibroblasts. The results show that the multiplicity-dependent phenotype of ICP0-null mutants limits the efficiency of reactivation at low MOIs and that competition between genomes occurs at high MOIs. The conclusion that quiescent HSV genomes are extensively silenced and intrinsically insensitive to trans-acting factors must be reevaluated.

Herpes simplex virus type 1 genomes are associated with ND10 nuclear substructures in quiescently infected human fibroblasts

Herpes simplex virus type 1 (HSV-1) genomes become associated with structures related to cellular nuclear substructures known as ND10 or promyelocytic leukemia nuclear bodies during the early stages of lytic infection. This paper describes the relationship between HSV-1 genomes and ND10 in human fibroblasts that maintain the viral genomes in a quiescent state. We report that quiescent HSV-1 genomes detected by fluorescence in situ hybridization (FISH) are associated with enlarged ND10-like structures, frequently such that the FISH-defined viral foci are apparently enveloped within a sphere of PML and other ND10 proteins. The number of FISH viral foci in each quiescently infected cell is concordant with the input multiplicity of infection, with each structure containing no more than a small number of viral genomes. A proportion of the enlarged ND10-like foci in quiescently infected cells contain accumulations of the heterochromatin protein HP1 but not other common markers of heterochromatin such as histone H3 di- or trimethylated on lysine residue 9. Many of the virally induced enlarged ND10-like structures also contain concentrations of conjugated ubiquitin. Quiescent infections can be established in cells that are highly depleted for PML. However, during the initial stages of establishment of a quiescent infection in such cells, other ND10 proteins (Sp100, hDaxx, and ATRX) are recruited into virally induced foci that are likely to be associated with HSV-1 genomes. These observations illustrate that the intimate connections between HSV-1 genomes and ND10 that occur during lytic infection also extend to quiescent infections.

Role of the cellular protein hDaxx in human cytomegalovirus immediate-early gene expression

Human cytomegalovirus (HCMV) immediate-early (IE) transcription is stimulated by virion phosphoprotein pp71, the product of gene UL82. It has previously been shown that pp71 interacts with the cellular protein hDaxx and, in the studies presented here, the significance of this interaction was investigated for HCMV IE gene expression. In co-transfection experiments, the presence of hDaxx increased the transcriptional response of the HCMV major IE promoter (MIEP) to pp71, but it was not possible to determine whether the effect was due to an interaction between the two proteins or to stimulation of hDaxx synthesis by pp71. The use of small interfering RNA (siRNA) in long- and short-term transfection approaches reduced intracellular hDaxx levels to no more than 3 % of normal. Infection of hDaxx-depleted cells with herpes simplex virus recombinants containing the HCMV MIEP revealed significantly greater promoter activity when hDaxx levels were minimal. Similarly, reducing intracellular hDaxx amounts resulted in greater IE gene expression during infection with an HCMV mutant lacking pp71, but had no effect on IE transcription during infection with wild-type HCMV. The results suggest that hDaxx is not important as a positive-acting factor for the stimulation of HCMV IE transcription by pp71. Instead, it appears that hDaxx acts as a repressor of IE gene expression, and it is proposed here that the interaction of pp71 with hDaxx is important to relieve repression and permit efficient initiation of productive replication.

Herpes simplex virus-based vectors

Herpes simplex virus (HSV)-based vectors have primarily been developed for neuronal gene delivery, taking advantage of the virus' natural neurotropism. Two types of vector are available: replication defective viruses, whose cytotoxicity has been abolished by deleting viral gene products, and amplicon vectors, which are plasmids packaged into HSV particles with the aid of a helper virus. In this review I discuss how the cytotoxicity of the wild-type virus has been abolished, the progress which has been made toward defining promoter elements capable of directing long-term transgene expression form the latent viral genome and some of the potential clinical uses of these versatile vectors.

Human cytomegalovirus tegument protein pp71 directs long-term gene expression from quiescent herpes simplex virus genomes

The human cytomegalovirus tegument protein pp71 is important for transactivation of immediate-early (IE) gene expression and for the efficient initiation of virus replication. We have analyzed the properties of pp71 by assaying its effects on gene expression from the genome of in1312, a herpes simplex virus type 1 (HSV-1) mutant devoid of functional VP16, ICP0, and ICP4. Upon infection of human fibroblasts, in1312-derived viruses are repressed and retained in a quiescent state, but the presence of pp71 prevented the quiescent state from being attained. Reporter gene cassettes cloned into the in1312 genome, in addition to the endogenous IE promoters, remained active for at least 12 days postinfection, and infected cells were viable and morphologically normal. Cells expressing pp71 remained responsive to the HSV-1 transactivating factors VP16 and ICP4 and to trichostatin A. The C-terminal 61 amino acids, but not the LACSD motif, were required for pp71 activity. In addition to preventing attainment of quiescence, pp71 was able to disrupt the quiescent state of in1312 derivatives and promote the resumption of viral gene expression after a lag of approximately 3 days. The results extend the functional analysis of pp71 and suggest a degree of similarity with the HSV-1 IE protein ICP0. The ability to provoke slow reactivation of quiescent genomes, in conjunction with cell survival, represents a novel property for a viral structural protein.

Inhibition of the stress-activated kinase, p38, does not affect the virus transcriptional program of herpes simplex virus type 1

To investigate the impact of stress kinase p38 activation on HSV-1 transcription, we performed a global transcript profile analysis of viral mRNA using an oligonucleotide-based DNA microarray. RNA was isolated from Vero cells infected with the KOS strain of HSV-1 in the presence or absence of SB203580, a pyridinyl imidazole inhibitor of p38. Under conditions that eliminated ATF2 activation but had no effect on c-Jun, and reduced virus yield by 85-90%, no effect on accumulation of viral IE, DE, or L transcripts was observed by array analysis or selected Northern blot analysis at 2, 4, and 6 h post infection. Results of array data from cells infected with the ICP27 mutant d27-1 in the presence or absence of SB203580 only reflected the known restricted transcription phenotype of the ICP27 mutant. This result is consistent with a role for p38 activation on virus replication lying downstream of the essential role of ICP27 in DE and perhaps late transcription regulation. No effect of SB203580 on transcription was detected after infection with the ICP0 mutant 7134, at 0.5 or 5.0 PFU/cell, though decreases in the rate of accumulation of all kinetic classes of mRNA could be detected, relative to wt virus. These results indicate that inhibiting p38 activity in Vero cells, while significantly reducing wt virus yield, demonstrated no obvious impact on the program of viral transcription.

Herpes simplex virus 1 infection is required to produce ICP27 immunoreactive triplet forms when ribosomal aminoacyl-tRNA translocation is blocked by cycloheximide

Infected cell protein (ICP) 27 is an essential herpes simplex virus type 1 (HSV-1) phosphoprotein required for optimal viral DNA and early or late gene synthesis. Three slow-migrating immunoreactive species were detected using multiple anti-ICP27 antibodies following HSV-1 infection of HEp-2 and Vero cells in the presence of cycloheximide (CHX). Generation of the protein triplet moieties required transcription of the alpha27 gene. These forms were observed following infection with a series of recombinant viruses that produce truncated ICP27 polypeptides, suggesting that alternative splicing is not involved in the process. These ICP27 species were not observed following translation inhibition by puromycin (PUR). Synthesis of the triplet occurred by 6 hpi and CHX addition as late as 3 hpi still enabled their production. That the ICP27 species were detected in uninfected ICP27-expressing cells without CHX, but not in its presence, suggests a mechanism in which virus infection is required to produce the forms when ribosomal aminoacyl-transfer RNA (tRNA) translocation is blocked.

Herpes simplex virus type 1 immediate-early gene expression is required for the induction of apoptosis in human epithelial HEp-2 cells

Wild-type herpes simplex virus type 1 (HSV-1) induces apoptosis in human epithelial HEp-2 cells, but infected cell proteins produced later in infection block the process from killing the cells. Thus, HSV-1 infection in the presence of the translational inhibitor cycloheximide (CHX) results in apoptosis. Our specific goal was to gain insight as to the viral feature(s) responsible for triggering apoptosis during HSV-1 infection. We now report the following. (i) No viral protein synthesis or death factor processing was detected after infection with HSV-1(HFEMtsB7) at 39.5 degrees C; this mutant virus does not inject its virion DNA into the nucleus at this nonpermissive temperature. (ii) No death factor processing or apoptotic morphological changes were detected following infection with UV-irradiated, replication-defective viruses possessing transcriptionally active incoming VP16. (iii) Addition of the transcriptional inhibitor actinomycin D prevented death factor processing upon infection with the apoptotic, ICP27-deletion virus HSV-1(vBSDelta27). (iv) Apoptotic morphologies and death factor processing were not observed following infection with HSV-1(d109), a green fluorescent protein-expressing recombinant virus possessing deletions of all five immediate-early (IE) (or alpha) genes. (v) Finally, complete death factor processing was observed upon infection with the VP16 transactivation domain-mutant HSV-1(V422) in the presence of CHX. Based on these findings, we conclude that (vi) the expression of HSV-1 alpha/IE genes is required for the viral induction of apoptosis and (vii) the transactivation activity of VP16 is not necessary for this induction.

Multiple 5′ ends of human cytomegalovirus UL57 transcripts identify a complex, cycloheximide-resistant promoter region that activates oriLyt

The human cytomegalovirus (HCMV) UL57 gene lies adjacent to HCMV oriLyt, from which it is separated by an organizationally conserved, mostly noncoding region that is thought to both regulate UL57 expression and activate oriLyt function. However, the UL57 promoter has not been studied. We determined the 5' ends of UL57 transcripts toward an understanding of the potential relationship between UL57 expression and oriLyt activation. The results presented here identified three distinct 5' ends spread over 800 bp, at nt 90302, 90530, and 91138; use of these sites exhibited differential sensitivity to phosphonoformic acid treatment. Interestingly, a 10-kb UL57 transcript accumulated in cycloheximide-treated infected cells, even though other early transcripts were not detectable. However, the 10-kb transcript did not accumulate in cells treated with the more stringent translation inhibitor anisomycin. Consistent with the notion that the identified 5' ends arise from distinct transcription start sites, the sequences upstream of sites I and II functioned as promoters responsive to HCMV infection in transient assays. However, the origin-proximal promoter region III required downstream sequences for transcriptional activity. Mutation of candidate core promoter elements suggested that promoter III is regulated by an initiator region (Inr) and a downstream promoter element. Finally, a 42-bp sequence containing the candidate Inr activated a minimal oriLyt core construct in transient replication assays. Thus, these studies showed that a large, complex promoter region with novel features controls UL57 expression, and identified a sequence that regulates both UL57 transcription and oriLyt activation.

Viral oncoapoptosis of human tumor cells

Many cancer cells refractory to radiation treatment and chemotherapy proliferate because of loss of intrinsic programmed cell death (apoptosis) regulation. Consequently, the resolution of these cancers are many times outside the management capabilities of conventional therapeutics. We now report that replication-defective delta27 herpes simplex virus (rd delta27) triggers apoptosis in three representative transformed human cell lines. Susceptibility to virus-induced cell death is dependent on the abundance and distribution of modified p53 protein in the tumor cells indicating specific targeting of the treatment. Primary human and mouse fibroblast cells that produce modified p53 are resistant to rd delta27 killing but not to apoptosis induced by nonviral environmental factors. These results suggest that induction of apoptosis by nonreplicating virus is a feasible genetic therapy approach for killing human cancer cells. Our findings may have important implications in designing novel virus-based anticancer strategies in appropriate animal model systems.

General and specific alterations in programming of global viral gene expression during infection by VP16 activation-deficient mutants of herpes simplex virus type 1

During productive infection by herpes simplex virus 1 (HSV-1), viral gene expression occurs in a temporally regulated cascade in which transcription of the viral immediate-early (IE) genes is strongly stimulated by the virion protein VP16. We have employed an oligonucleotide microarray to examine the effect of VP16 mutations on the overall pattern of viral gene expression following infection of HeLa cells. This microarray detects essentially all HSV-1 transcripts with relative and absolute levels correlating well with known kinetics of expression. This analysis revealed that deletion of the VP16 activation domain sharply reduced overall viral gene expression; moreover, the pattern of this reduced expression varied greatly from the pattern of a wild-type (wt) infection. However, when this mutant virus was delivered at a high multiplicity of infection or in the presence of the cellular stress inducer hexamethylene bisacetamide, expression was largely restored to the wt levels and pattern. Infection with virions that deliver wt VP16 protein at the start of infection but synthesize only truncated VP16 resulted in a normal kinetic cascade. This suggests that newly synthesized VP16 does not play a significant role in the expression of later classes of transcripts. The VP16 activation domain comprises two subregions. Deletion of the C-terminal subregion resulted in minimal changes in the level and profile of gene expression compared to a normal (wt) cascade. In contrast, deletion of the N-terminal subregion reduced the overall expression levels and skewed the relative levels of IE transcripts but did not significantly alter the kinetic pattern of early and late transcript expression. We conclude that the general activation of IE gene transcription by VP16, but not the specific ratios of IE transcripts, is necessary for the subsequent ordered expression of viral genes. Moreover, this report establishes the feasibility of microarray analysis for globally assessing viral gene expression programs as a function of the conditions of infection.

Activity and intracellular localization of the human cytomegalovirus protein pp71

The human cytomegalovirus (HCMV) tegument phosphoprotein pp71 activates viral immediate early (IE) transcription and thus has a role in initiating lytic infection. Protein pp71 stimulates expression from a range of promoters in a sequence-independent manner, and in this respect behaves similarly to the herpes simplex virus type 1 (HSV-1) IE protein ICP0. The intracellular localization of pp71 was investigated after its expression from transfected plasmids or from HSV-1 mutants constructed to produce pp71 transiently. The protein colocalized with the cell promyelocytic leukaemia (PML) protein at nuclear domain 10 (ND10) structures but, unlike ICP0, pp71 did not induce disruption of ND10. The activity of pp71 in mouse sensory neurons in vivo was investigated after co-inoculation of animals with pairs of HSV-1 mutants, one expressing pp71 and the second containing the E. coli lacZ gene controlled by various promoters. In this system, pp71 stimulated beta-galactosidase expression from a range of viral IE promoters when mice were analysed at 4 days postinoculation. At later times, expression of pp71 resulted in a reduction in numbers of neurons containing beta-galactosidase, indicating cytotoxicity or promoter shutoff. The HSV-1 latency-active promoter was not responsive to pp71, demonstrating specificity in the activity of the protein. Pp71 was as active in mice lacking both copies of the PML gene (PML-/-) as in control animals, and in PML-/- fibroblasts pp71 stimulated gene expression as effectively as in other cell types. Therefore, neither the PML protein nor the normal ND10 structure is necessary for pp71 to stimulate gene expression.

Infection of murine keratinocytes with herpes simplex virus type 1 induces the expression of interleukin-10, but not interleukin-1 alpha or tumour necrosis factor-alpha

Herpes simplex virus (HSV) is known to possess several mechanisms whereby it can evade the normal host immune defences. In this study the expression of the immunosuppressive cytokine, interleukin (IL)-10, was monitored following infection of a murine keratinocyte cell line (PAM-212) and compared with the expression of two proinflammatory cytokines: IL-1 alpha and tumour necrosis factor (TNF)-alpha. The PAM-212 cells were infected at a multiplicity of 0.5 with a clinical isolate of HSV type 1, and the mRNA of the three cytokines was assessed by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) over the following 24 hr. By 12 hr postinfection the amount of IL-10 mRNA had increased significantly to five-fold greater than that found in uninfected cells (P < 0.01), and this elevated level was maintained until at least 24 hr postinfection. In contrast, IL-1 alpha and TNF-alpha mRNAs were not significantly up-regulated by the HSV infection. Immunostaining with an IL-10 monoclonal antibody (mAb) revealed that cytoplasmic IL-10 protein had increased by 6-12 hr postinfection. This quantity was further increased at 24 hr postinfection, when the viral cytopathic effect was apparent. Viral replication was necessary, but not sufficient on its own, for IL-10 induction. Experiments with HSV mutants lacking functional transactivating factors suggested that the viral transactivating proteins ICP-0 and VP-16 may be necessary for HSV-induced IL-10 expression. Thus, the up-regulation in the expression of IL-10 mRNA and protein induced by HSV early in the infection of keratinocytes represents a specific response and may be part of the viral strategy to avoid local immune defence mechanisms in the skin.

Activation of cellular interferon-responsive genes after infection of human cells with herpes simplex virus type 1

Previous studies have shown that infection of human fibroblasts with human cytomegalovirus (HCMV) results in activation of cellular interferon-responsive gene expression. We demonstrate here that infection of human fibroblasts with herpes simplex virus type 1 (HSV-1) in the absence of de novo protein synthesis also induces the expression of interferon-responsive genes. Five genes tested (encoding ISG54, IFI56, ISG15, 9-27 and MxA) were activated by infection with HSV-1, although the degree of response varied between the individual genes. HSV-1 was a less efficient inducer than HCMV. The effect was a consequence of binding of the virus particle to the cell surface or of the presence of virion components within the infected cell. Induction was mediated by a pathway other than the mechanism through which interferon-alpha mediates its effects on cellular gene expression.

Early expression of herpes simplex virus (HSV) proteins and reactivation of latent infection

During the last decade, new data accumulated describing the early events during herpes simplex virus 1 (HSV-1) replication occurring before capsid formation and virion envelopment. The HSV virion carries its own specific transcription initiation factor (alpha-TIF), which functions together with other components of the cellular transcriptase complex to mediate virus-specific immediate early (IE) transcription. The virus-coded IE proteins are the transactivator and regulatory elements modulating early transcription and subsequent translation of nonstructural virus-coded proteins needed mainly for viral DNA synthesis and for the supply of corresponding nucleoside components. They also cooperate at the late transcription and translation of the virion (capsid, tegument and envelope) proteins. In addition, the transactivator IE proteins down-regulate their own transcription, while others facilitate viral mRNA processing or interfere with the presentation of newly synthesized virus antigens. Establishment of latency is closely related to the transcription of a separate category of transcripts, termed latency-associated (LAT). Formation of LATs occurs mainly in nondividing neurons which are metabolically less active and express lower levels of cellular transcription factors (nonpermissive cells). Expression of the stable non-spliced (2 kb), and especially of stable spliced (1.5 and 1.45 kb) LATs is a prerequisite for HSV reactivation. Different HSV genomes (from various HSV strains) do not undergo IE transcription at the same rate. Restricted IE transcription and the absence of viral DNA synthesis favors LAT formation and persistence of the silenced genome. Uneven levels of LAT expression and differences in the metabolic state of carrier neurons influence the reactivation competence. Under artificial or natural activation conditions, sufficient amounts of IE transactivator proteins and proteins promoting nucleoside metabolism are synthesized even in the absence of the viral alpha-TIF facilitating reactivation.

Long-term transgene expression in mice infected with a herpes simplex virus type 1 mutant severely impaired for immediate-early gene expression

The role of viral immediate-early (IE) gene expression in herpes simplex virus type 1 (HSV-1) latency was investigated. The HSV-1 multiple mutant in1312, defective for the expression of the virion transactivator VP16 and the IE proteins ICP0 and ICP4, was used as the parent for these studies. The coding sequences of the Escherichia coli lacZ gene, preceded by the encephalomyocarditis virus internal ribosome entry site, were inserted into the region of in1312 that encodes the latency-associated transcripts (LATs) such that transcription of the transgene was controlled by the LAT promoter. This insert has previously been shown to direct long-term latent-phase expression of beta-galactosidase in a wild-type HSV-1 genome (R. H. Lachmann and S. Efstathiou, J. Virol. 71, 3197-3207, 1997). The resulting recombinant, in1388, was apathogenic after inoculation into mice via the footpad and did not detectably replicate in dorsal root ganglia (DRG) or footpads. Mutant in1388 established latency in DRG, and beta-galactosidase was expressed in increasing numbers of neurons over the first 25 days of infection. During latency, more than 1% of neurons in ganglia that innervate the footpad expressed beta-galactosidase, with the number of positive cells remaining constant for at least 5 months. Rescue of the VP16, ICP0, or ICP4 mutations of in1388 did not affect the number of beta-galactosidase-expressing neurons detected during latency. The results demonstrate that HSV-1 mutants severely impaired for IE gene expression are capable of establishing latency and efficiently expressing a foreign gene product under control of the LAT promoter.

A non-cytotoxic herpes simplex virus vector which expresses Cre recombinase directs efficient site specific recombination

The coding sequences for the bacteriophage P1 recombinase Cre were cloned into the genome of a herpes simplex virus type 1 (HSV-1) mutant which is severely impaired for the synthesis of immediate early (IE) proteins. The resulting recombinant, virus in1372, expressed functional Cre which mediated the excision in trans of loxP-flanked sequences located in the HSV-1 genome, both in tissue culture cells and in vivo in mouse sensory neurons. Infection with in1372 also resulted in recombination, at high efficiency, between loxP sequences in the cellular genome without causing detectable cytotoxicity. Mutant in1372 is a versatile vector for the delivery of Cre in tissue culture and in vivo.

Truncation of the C-terminal acidic transcriptional activation domain of herpes simplex virus VP16 renders expression of the immediate-early genes almost entirely dependent on ICP0

The herpes simplex virus (HSV) proteins VP16 and ICP0 play key roles in stimulating the onset of the viral lytic cycle. We sought to explore the regulatory links between these proteins by studying the phenotypes of viral mutants in which the activation functions of both were simultaneously inactivated. This analysis unexpectedly revealed that truncation of the C-terminal transcriptional activation domain of VP16 (allele V422) in an ICP0-deficient background almost completely eliminated immediate-early gene expression and virus replication in Vero and HEL cells. The doubly mutant viral genome persisted in a quiescent state for at least 10 days in HEL cells infected at high multiplicity and could be reactivated by superinfection with wild-type HSV. In contrast, the in1814 VP16 mutation produced a markedly less severe phenotype in the same ICP0-deficient background. These data demonstrate that expression of the immediate-early genes requires ICP0 when the C-terminal activation domain of VP16 is deleted and raise the possibility that the in1814 form of VP16 retains a residual ability to stimulate gene expression during virus infection.

Activation of herpesvirus gene expression by the human cytomegalovirus protein pp71

The activation of gene expression by the human cytomegalovirus (HCMV) particle was investigated. The HCMV major immediate-early (IE) promoter was cloned upstream of the Escherichia coli lacZ coding sequences, and the resulting cassette was introduced into the genome of a herpes simplex virus type 1 (HSV-1) mutant lacking functional VP16. Upon infection with the HSV-1 recombinant in the presence of cycloheximide, to block de novo protein synthesis, expression of lacZ-specific transcripts was increased by fivefold when HCMV was included in the inoculum. Accumulation of HSV-1 IE RNAs was also stimulated by coinfection with HCMV, as was expression of the adenovirus 5 VAI transcript when the VAI gene was cloned into the HSV-1 genome. Coinfection with HCMV did not alter mRNA stability or uncoating of the HSV-1 genome. The coding sequences for the HCMV phosphoprotein pp71, controlled by the HCMV IE promoter, were cloned into an HSV-1 recombinant impaired for the production of the three major transactivators (VP16, ICP0, and ICP4) to yield a recombinant (in1324) which expressed pp71 but did not cause significant cytotoxicity. Infection with in1324 resulted in stimulation of HCMV IE, HSV-1 IE, and VAI expression, demonstrating that pp71 is responsible for the effects we observed when using the entire HCMV particle. Therefore, HCMV pp71 exhibits novel properties in its ability to stimulate gene expression from a range of promoters present in a herpesvirus genome.

A viral activator of gene expression functions via the ubiquitin-proteasome pathway

The ability of herpes simplex virus type 1 (HSV-1) to attain a latent state in sensory neurones and reactivate periodically is crucial for its biological and clinical properties. The active transcription of the entire 152 kb viral genome during lytic replication contrasts with the latent state, which is characterized by the production of a single set of nuclear-retained transcripts. Reactivation of latent genomes to re-initiate the lytic cycle therefore involves a profound change in viral transcriptional activity, but the mechanisms by which this fundamentally important process occurs are yet to be well understood. In this report we show that the stimulation of the onset of viral lytic infection mediated by the viral immediate-early (IE) protein Vmw110 is strikingly inhibited by inactivation of the ubiquitin-proteasome pathway. Similarly, the Vmw110-dependent reactivation of quiescent viral genomes in cultured cells is also dependent on proteasome activity. These results constitute the first demonstration that the transcriptional activity of a viral genome can be regulated by protein stability control pathways.

Molecular aspects of herpes simplex virus I latency, reactivation, and recurrence

The application of molecular biology in the study of the pathogenesis of herpes simplex virus type 1 (HSV-1) has led to significant advances in our understanding of mechanisms that regulate virus behavior in sensory neurons and epithelial tissue. Such study has provided insight into the relationship of host and viral factors that regulate latency, reactivation, and recurrent disease. This review attempts to distill decades of information involving human, animal, and cell culture studies of HSV-1 with the goal of correlating molecular events with the clinical and laboratory behavior of the virus during latency, reactivation, and recurrent disease. The purpose of such an attempt is to acquaint the clinician/scientist with the current thinking in the field, and to provide key references upon which current opinions rest.

Cytodifferentiating agents affect the replication of herpes simplex virus type 1 in the absence of functional VP16

The herpes simplex virus type 1 (HSV-1) mutant in1814 encodes an altered form of the virion protein VP16 that is unable to transactivate immediate-early (IE) transcription. As a consequence of the mutation, in1814 initiates productive replication inefficiently after infection of tissue culture cells. Previous studies showed that this defect could be overcome by the inclusion in the culture medium of hexamethylene bisacetamide (HMBA), a compound that promotes the differentiation of murine erythroleukemia cells (MELCs). The effects of additional agents known to induce differentiation of MELCs were investigated. N'-Methylnicotinamide, at concentrations optimal for the induction of MELCs, complemented the replication of in1814 and stimulated IE gene expression. Suberoyl bishydroxamic acid and suberoylanilide hydroxamic acid, which induce differentiation of MELCs at micromolar concentrations, did not complement in1814 but specifically blocked the action of HMBA. The histone deacetylase inhibitor trichostatin A, which also induces differentiation of MELCs, antagonized the effect of HMBA in a manner similar to that of suberoyl bishydroxamic acid and suberoylanilide hydroxamic acid. The results demonstrate that the requirement for VP16 activity is dependent on the metabolic state of the host cell and that the pathways leading to complementation of in1814 and differentiation of MELCs are overlapping but not identical.