Viral DNA synthesis is required for the efficient expression of specific herpes simplex virus type 1 mRNA species

Integrator Complex Subunit 3 Knockdown Has Minimal Effect on Lytic Herpes Simplex Virus Type-1 Infection in Fibroblast Cells

Proteomic analysis of viral and cellular proteins that copurify with the herpes simplex virus type-1 (HSV-1) genome revealed that the cellular Integrator complex associates with viral DNA throughout infection. The Integrator complex plays a key role in the regulation of transcription of cellular coding and non-coding RNAs. We therefore predicted that it may regulate transcription of viral genes. Here, we demonstrate that knockdown of the Integrator complex subunit, Ints3, has minimal effect on HSV-1 infection. Despite reducing viral yield during low multiplicity infection, Ints3 knockdown had no effect on viral DNA replication, mRNA expression, or yield during high multiplicity infection.

A Revision of Herpes Simplex Virus Type 1 Transcription: First, Repress; Then, Express

The herpes virus genome bears more than 80 strong transcriptional promoters. Upon entry into the host cell nucleus, these genes are transcribed in an orderly manner, producing five immediate-early (IE) gene products, including ICP0, ICP4, and ICP22, while non-IE genes are mostly silent. The IE gene products are necessary for the transcription of temporal classes following sequentially as early, leaky late, and true late. A recent analysis using precision nuclear run-on followed by deep sequencing (PRO-seq) has revealed an important step preceding all HSV-1 transcription. Specifically, the immediate-early proteins ICP4 and ICP0 enter the cell with the incoming genome to help preclude the nascent antisense, intergenic, and sense transcription of all viral genes. VP16, which is also delivered into the nucleus upon entry, almost immediately reverses this repression on IE genes. The resulting de novo expression of ICP4 and ICP22 further repress antisense, intergenic, and early and late viral gene transcription through different mechanisms before the sequential de-repression of these gene classes later in infection. This early repression, termed transient immediate-early protein-mediated repression (TIEMR), precludes unproductive, antisense, intergenic, and late gene transcription early in infection to ensure the efficient and orderly progression of the viral cascade.

Herpes simplex virus infected cell protein 8 is required for viral inhibition of the cGAS pathway

DNA virus infection triggers an antiviral type I interferon (IFN) response in cells that suppresses infection of surrounding cells. Consequently, viruses have evolved mechanisms to inhibit the IFN response for efficient replication. The cellular cGAS protein binds to double-stranded DNA and synthesizes the small molecule cGAMP to initiate DNA-dependent type I IFN production. We showed previously that cGAMP production is relatively low during HSV-1 infection compared to plasmid DNA transfection. Therefore, we hypothesized that HSV-1 produces antagonists of the cGAS DNA sensing pathway. In this study, we found that the HSV-1 ICP8 protein is required for viral inhibition of the cGAS pathway by reducing cGAMP levels stimulated by double-stranded DNA transfection. ICP8 alone inhibited the cGAMP response and may inhibit cGAS action by direct interaction with DNA, cGAS, or other infected cell proteins. Our results reveal another cGAS antiviral pathway inhibitor and highlight the importance of countering IFN for efficient viral replication.

ICP22 of Herpes Simplex Virus 1 Decreases RNA Polymerase Processivity

To determine the role of ICP22 in transcription, we performed precise nuclear run-on followed by deep sequencing (PRO-seq) and global nuclear run-on with sequencing (GRO-seq) in cells infected with a viral mutant lacking the entire ICP22-encoding α22 (US1/US1.5) gene and a virus derived from this mutant bearing a restored α22 gene. At 3 h postinfection (hpi), the lack of ICP22 reduced RNA polymerase (Pol) promoter proximal pausing (PPP) on the immediate early α4, α0, and α27 genes. Diminished PPP at these sites accompanied increased Pol processivity across the entire herpes simplex virus 1 (HSV-1) genome in GRO-seq assays, resulting in substantial increases in antisense and intergenic transcription. The diminished PPP on α gene promoters at 3 hpi was distinguishable from effects caused by treatment with a viral DNA polymerase inhibitor at this time. The ICP22 mutant had multiple defects at 6 hpi, including lower viral DNA replication, reduced Pol activity on viral genes, and increased Pol activity on cellular genes. The lack of ICP22 also increased PPP release from most cellular genes, while a minority of cellular genes exhibited decreased PPP release. Taken together, these data indicate that ICP22 acts to negatively regulate transcriptional elongation on viral genes in part to limit antisense and intergenic transcription on the highly compact viral genome. This regulatory function directly or indirectly helps to retain Pol activity on the viral genome later in infection. IMPORTANCE The longstanding observation that ICP22 reduces RNA polymerase II (Pol II) serine 2 phosphorylation, which initiates transcriptional elongation, is puzzling because this phosphorylation is essential for viral replication. The current study helps explain this apparent paradox because it demonstrates significant advantages in negatively regulating transcriptional elongation, including the reduction of antisense and intergenic transcription. Delays in elongation would be expected to facilitate the ordered assembly and functions of transcriptional initiation, elongation, and termination complexes. Such limiting functions are likely to be important in herpesvirus genomes that are otherwise highly transcriptionally active and compact, comprising mostly short, intronless genes near neighboring genes of opposite sense and containing numerous 3'-nested sets of genes that share transcriptional termination signals but differ at transcriptional start sites on the same template strand.

RNA Polymerase II Promoter-Proximal Pausing and Release to Elongation Are Key Steps Regulating Herpes Simplex Virus 1 Transcription

Herpes simplex virus 1 (HSV-1) genes are transcribed by cellular RNA polymerase II (Pol II). Expression of viral immediate early (α) genes is followed sequentially by early (β), late (γ₁), and true late (γ₂) genes. We used precision nuclear run-on with deep sequencing to map and to quantify Pol II on the HSV-1(F) genome with single-nucleotide resolution. Approximately 30% of total Pol II relocated to viral genomes within 3 h postinfection (hpi), when it occupied genes of all temporal classes. At that time, Pol II on α genes accumulated most heavily at promoter-proximal pause (PPP) sites located ∼60 nucleotides downstream of the transcriptional start site, while β genes bore Pol II more evenly across gene bodies. At 6 hpi, Pol II increased on γ₁ and γ₂ genes while Pol II pausing remained prominent on α genes. At that time, average cytoplasmic mRNA expression from α and β genes decreased, relative to levels at 3 hpi, while γ₁ relative expression increased slightly and γ₂ expression increased more substantially. Cycloheximide treatment during the first 3 h reduced the amount of Pol II associated with the viral genome and confined most of the remaining Pol II to α gene PPP sites. Inhibition of both cyclin-dependent kinase 9 activity and viral DNA replication reduced Pol II on the viral genome and restricted much of the remaining Pol II to PPP sites.IMPORTANCE These data suggest that viral transcription is regulated not only by Pol II recruitment to viral genes but also by control of elongation into viral gene bodies. We provide a detailed map of Pol II occupancy on the HSV-1 genome that clarifies features of the viral transcriptome, including the first identification of Pol II PPP sites. The data indicate that Pol II is recruited to late genes early in infection. Comparing α and β gene occupancy at PPP sites and gene bodies suggests that Pol II is released more efficiently into the bodies of β genes than α genes at 3 hpi and that repression of α gene expression late in infection is mediated by prolonged promoter-proximal pausing. In addition, DNA replication is required to maintain full Pol II occupancy on viral DNA and to promote elongation on late genes later in infection.

Viral, bacterial, and fungal infections of the oral mucosa: Types, incidence, predisposing factors, diagnostic algorithms, and management

For millions of years, microbiota residing within us, including those in the oral cavity, coexisted in a harmonious symbiotic fashion that provided a quintessential foundation for human health. It is now clear that disruption of such a healthy relationship leading to microbial dysbiosis causes a wide array of infections, ranging from localized, mild, superficial infections to deep, disseminated life-threatening diseases. With recent advances in research, diagnostics, and improved surveillance we are witnessing an array of emerging and re-emerging oral infections and orofacial manifestations of systemic infections. Orofacial infections may cause significant discomfort to the patients and unnecessary economic burden. Thus, the early recognition of such infections is paramount for holistic patient management, and oral clinicians have a critical role in recognizing, diagnosing, managing, and preventing either new or old orofacial infections. This paper aims to provide an update on current understanding of well-established and emerging viral, bacterial, and fungal infections manifesting in the human oral cavity.

An Epigenetic Journey: Epstein-Barr Virus Transcribes Chromatinized and Subsequently Unchromatinized Templates during Its Lytic Cycle

The Epstein-Barr virus (EBV) lytic phase, like those of all herpesviruses, proceeds via an orderly cascade that integrates DNA replication and gene expression. EBV early genes are expressed independently of viral DNA amplification, and several early gene products facilitate DNA amplification. On the other hand, EBV late genes are defined by their dependence on viral DNA replication for expression. Recently, a set of orthologous genes found in beta- and gammaherpesviruses have been determined to encode a viral preinitiation complex (vPIC) that mediates late gene expression. The EBV vPIC requires an origin of lytic replication in cis, implying that the vPIC mediates transcription from newly replicated DNA. In agreement with this implication, EBV late gene mRNAs localize to replication factories. Notably, these factories exclude canonical histones. In this review, we compare and contrast the mechanisms and epigenetics of EBV early and late gene expression. We summarize recent findings, propose a model explaining the dependence of EBV late gene expression on lytic DNA amplification, and suggest some directions for future study.

Evasion of early antiviral responses by herpes simplex viruses

Besides overcoming physical constraints, such as extreme temperatures, reduced humidity, elevated pressure, and natural predators, human pathogens further need to overcome an arsenal of antimicrobial components evolved by the host to limit infection, replication and optimally, reinfection. Herpes simplex virus-1 (HSV-1) and herpes simplex virus-2 (HSV-2) infect humans at a high frequency and persist within the host for life by establishing latency in neurons. To gain access to these cells, herpes simplex viruses (HSVs) must replicate and block immediate host antiviral responses elicited by epithelial cells and innate immune components early after infection. During these processes, infected and noninfected neighboring cells, as well as tissue-resident and patrolling immune cells, will sense viral components and cell-associated danger signals and secrete soluble mediators. While type-I interferons aim at limiting virus spread, cytokines and chemokines will modulate resident and incoming immune cells. In this paper, we discuss recent findings relative to the early steps taking place during HSV infection and replication. Further, we discuss how HSVs evade detection by host cells and the molecular mechanisms evolved by these viruses to circumvent early antiviral mechanisms, ultimately leading to neuron infection and the establishment of latency.

Barrier to auto integration factor becomes dephosphorylated during HSV-1 Infection and Can Act as a host defense by impairing viral DNA replication and gene expression

BAF (Barrier to Autointegration Factor) is a highly conserved DNA binding protein that senses poxviral DNA in the cytoplasm and tightly binds to the viral genome to interfere with DNA replication and transcription. To counteract BAF, a poxviral-encoded protein kinase phosphorylates BAF, which renders BAF unable to bind DNA and allows efficient viral replication to occur. Herein, we examined how BAF phosphorylation is affected by herpes simplex virus type 1 (HSV-1) infection and tested the ability of BAF to interfere with HSV-1 productive infection. Interestingly, we found that BAF phosphorylation decreases markedly following HSV-1 infection. To determine whether dephosphorylated BAF impacts HSV-1 productive infection, we employed cell lines stably expressing a constitutively unphosphorylated form of BAF (BAF-MAAAQ) and cells overexpressing wild type (wt) BAF for comparison. Although HSV-1 production in cells overexpressing wtBAF was similar to that in cells expressing no additional BAF, viral growth was reduced approximately 80% in the presence of BAF-MAAAQ. Experiments were also performed to determine the mechanism of the antiviral activity of BAF with the following results. BAF-MAAAQ was localized to the nucleus, whereas wtBAF was dispersed throughout cells prior to infection. Following infection, wtBAF becomes dephosphorylated and relocalized to the nucleus. Additionally, BAF was associated with the HSV-1 genome during infection, with BAF-MAAAQ associated to a greater extent than wtBAF. Importantly, unphosphorylated BAF inhibited both viral DNA replication and gene expression. For example, expression of two regulatory proteins, ICP0 and VP16, were substantially reduced in cells expressing BAF-MAAAQ. However, other viral genes were not dramatically affected suggesting that expression of certain viral genes can be differentially regulated by unphosphorylated BAF. Collectively, these results suggest that BAF can act in a phosphorylation-regulated manner to impair HSV-1 transcription and/or DNA replication, which is similar to the antiviral activity of BAF during vaccinia infection.

Development and evaluation of SYBR Green-I based quantitative PCR assays for herpes simplex virus type 1 whole transcriptome analysis

There is an emerging need for viral gene specific quantitative PCR (qPCR) assays that validate and complement whole transcriptome level technologies, including microarray and next generation sequencing. Therefore, a compilation of qPCR assays that represented the breadth of the entire Herpes simplex virus type 1 (HSV-1) genome were developed and evaluated. SYBR Green-I-based quantitation of each of the 74 HSV-1 lytic genes enabled accurate and reproducible detection of viral genes using a minimal number of reaction conditions. The amplification specificity of these assays for HSV-1 target genes was confirmed by amplicon size and purity determination on agarose gels, melt temperature dissociation curve analysis, and direct DNA sequencing of amplified products. Analysis of representative target genes demonstrated that these assays accurately and reproducibly quantified target gene expression across a wide and linear range of detection. In addition, minimal intra- and inter-assay variability was observed with significant well-to-well and plate-to-plate/assay-to-assay precision. To evaluate the utility of the developed qPCR assay system, kinetic profiles of viral gene expression were determined for an array of representative genes from all HSV-1 transcriptional gene classes. Collectively, these data demonstrate that the compiled optimized qPCR assays is a scalable and cost-effective method to assess HSV-1 gene expression with broad application potential, including investigation of pathogenesis and antiviral therapies. In addition, they can be employed to validate and complement evolving technologies for genome-wide transcriptome analysis.

Murine cytomegalovirus protein pM92 is a conserved regulator of viral late gene expression

In this study, we report that murine cytomegalovirus (MCMV) protein pM92 regulates viral late gene expression during virus infection. Previously, we have shown that MCMV protein pM79 and its human cytomegalovirus (HCMV) homologue pUL79 are required for late viral gene transcription. Identification of additional factors involved is critical to dissecting the mechanism of this regulation. We show here that pM92 accumulated abundantly at late times of infection in a DNA synthesis-dependent manner and localized to nuclear viral replication compartments. To investigate the role of pM92, we constructed a recombinant virus SMin92, in which pM92 expression was disrupted by an insertional/frameshift mutation. During infection, SMin92 accumulated representative viral immediate-early gene products, early gene products, and viral DNA sufficiently but had severe reduction in the accumulation of late gene products and was thus unable to produce infectious progeny. Coimmunoprecipitation and mass spectrometry analysis revealed an interaction between pM92 and pM79, as well as between their HCMV homologues pUL92 and pUL79. Importantly, we showed that the growth defect of pUL92-deficient HCMV could be rescued in trans by pM92. This study indicates that pM92 is an additional viral regulator of late gene expression, that these regulators (represented by pM92 and pM79) may need to complex with each other for their activity, and that pM92 and pUL92 share a conserved function in CMV infection. pM92 represents a potential new target for therapeutic intervention in CMV disease, and a gateway into studying a largely uncharted viral process that is critical to the viral life cycle.

Bovine herpesvirus 1 regulatory proteins bICP0 and VP16 are readily detected in trigeminal ganglionic neurons expressing the glucocorticoid receptor during the early stages of reactivation from latency

Bovine herpesvirus 1 (BHV-1) establishes a lifelong latent infection in sensory neurons following acute infection. Increased corticosteroid levels, due to stress, increases the incidence of reactivation from latency. Within minutes, corticosteroids activate the glucocorticoid receptor and transcription of promoters containing a glucocorticoid receptor element. A single intravenous injection of the synthetic corticosteroid dexamethasone consistently induces reactivation from latency in calves. Lytic cycle viral gene expression is detected within 6 h after dexamethasone treatment of calves latently infected with BHV-1. Cellular transcription factors are induced by dexamethasone in trigeminal ganglionic neurons within 1.5 h after dexamethasone treatment, suggesting they promote viral gene expression during the early phases of reactivation from latency, which we operationally defined as the escape from latency. In this study, immunohistochemistry was utilized to examine viral protein expression during the escape from latency. Within 1.5 h after dexamethasone treatment, bICP0 and a late protein (VP16) were consistently detected in a subset of trigeminal ganglionic neurons. Most neurons expressing bICP0 also expressed VP16. Additional studies revealed that neurons expressing the glucocorticoid receptor also expressed bICP0 or VP16 at 1.5 h after dexamethasone treatment. Two other late proteins, glycoprotein C and D, were not detected until 6 h after dexamethasone treatment and were detected in only a few neurons. These studies provide evidence that VP16 and the promiscuous viral trans-activator (bICP0) are expressed during the escape from latency, suggesting they promote the production of infectious virus in a small subset of latently infected neurons.

Role of immediate early protein ICP27 in the differential sensitivity of herpes simplex viruses 1 and 2 to leptomycin B

Leptomycin B (LMB) is a highly specific inhibitor of CRM1, a cellular karyopherin-β that transports nuclear export signal-containing proteins from the nucleus to the cytoplasm. Previous work has shown that LMB blocks herpes simplex virus 1 (HSV-1) replication in Vero cells and that certain mutations in viral immediate early protein ICP27 can confer LMB resistance. However, little is known of the molecular mechanisms involved. Here we report that HSV-2, a close relative of HSV-1, is naturally resistant to LMB. To see whether the ICP27 gene determines this phenotypic difference, we generated an HSV-1 mutant that expresses the HSV-2 ICP27 instead of the HSV-1 protein. This recombinant was fully sensitive to LMB, indicating that one or more other viral genes must be important in determining HSV-2's LMB-resistant phenotype. In additional work, we report several findings that shed light on how HSV-1 ICP27 mutations can confer LMB resistance. First, we show that LMB treatment of HSV-1-infected cells leads to suppression of late viral protein synthesis and a block to progeny virion release. Second, we identify a novel type of ICP27 mutation that can confer LMB resistance, that being the addition of a 100-residue amino-terminal affinity purification tag. Third, by studying infections where both LMB-sensitive and LMB-resistant forms of ICP27 are present, we show that HSV-1's sensitivity to LMB is dominant to its resistance. Together, our results suggest a model in which the N-terminal portion of ICP27 mediates a nonessential activity that interferes with HSV-1 replication when CRM1 is inactive. We suggest that LMB resistance mutations weaken or abrogate this activity.

The ULb' region of the human cytomegalovirus genome confers an increased requirement for the viral protein kinase UL97

We report a requirement for the viral protein kinase UL97 in human cytomegalovirus (HCMV) replication that maps to the ULb' region of the viral genome. A UL97-null (Δ97) mutant of strain TB40/E, which encodes a full-length ULb' region, exhibited replication defects, particularly in production of cell-free virus, that were more severe than those seen with a Δ97 mutant of laboratory strain AD169, which harbors extensive deletions in its ULb' region. These differences were recapitulated with additional HCMV strains by treatment with a UL97 kinase inhibitor, 1-(β-L-ribofuranosyl)-2-isopropylamino-5,6-dichlorobenzimidazole (maribavir). We observed lower levels of viral DNA synthesis and an increased requirement for UL97 in viral late gene expression in strains with full-length ULb' regions. Analysis of UL97-deficient TB40/E infections by electron microscopy revealed fewer C-capsids in nuclei, unusual viral particles in the cytoplasmic assembly compartment, and defective viral nuclear egress. Partial inhibition of viral DNA synthesis caused defects in production of cell-free virus that were up to ≈ 100-fold greater than those seen with cell-associated virus in strains TB40/E and TR, suggesting that UL97-dependent defects in cell-free virus production in strains with full-length ULb' regions were secondary to DNA synthesis defects. Accordingly, a chimeric virus in which the ULb' region of TB40/E was replaced with that of AD169 showed reduced effects of UL97 inhibition on viral DNA synthesis, late gene expression, and production of cell-free virus compared to parental TB40/E. Together, these results argue that the ULb' region encodes a factor(s) which invokes an increased requirement for UL97 during viral DNA synthesis.

Cytomegalovirus replicon-based regulation of gene expression in vitro and in vivo

There is increasing evidence for a connection between DNA replication and the expression of adjacent genes. Therefore, this study addressed the question of whether a herpesvirus origin of replication can be used to activate or increase the expression of adjacent genes. Cell lines carrying an episomal vector, in which reporter genes are linked to the murine cytomegalovirus (MCMV) origin of lytic replication (oriLyt), were constructed. Reporter gene expression was silenced by a histone-deacetylase-dependent mechanism, but was resolved upon lytic infection with MCMV. Replication of the episome was observed subsequent to infection, leading to the induction of gene expression by more than 1000-fold. oriLyt-based regulation thus provided a unique opportunity for virus-induced conditional gene expression without the need for an additional induction mechanism. This principle was exploited to show effective late trans-complementation of the toxic viral protein M50 and the glycoprotein gO of MCMV. Moreover, the application of this principle for intracellular immunization against herpesvirus infection was demonstrated. The results of the present study show that viral infection specifically activated the expression of a dominant-negative transgene, which inhibited viral growth. This conditional system was operative in explant cultures of transgenic mice, but not in vivo. Several applications are discussed.

All herpesviruses show a precisely regulated gene expression profile, including true-late genes, which are turned on only after the onset of DNA replication. We used this intrinsic viral mechanism to generate a versatile conditional gene expression system that exploits the activity of the murine cytomegalovirus (MCMV) viral origin of lytic replication (oriLyt). Upon virus infection, replication of the viral genome also led to the replication and activation of the oriLyt-coupled episomal transgene. The oriLyt-based replicons were silenced in all stable cell lines and transgenic mice; however, virus infection liberated the plasmids from histone-deacetylase-induced inactivation. As maximum gene expression relied on relief from silencing via replication of the episomal constructs, very strong induction of the reporter gene was achieved. We showed that this system can be used for trans-complementation of late, toxic viral genes, to block virus production by activating dominant-negative (DN) transgenes, and to provide a new tool to study the principles of viral replication.

Genome-wide analysis of Epstein-Barr virus Rta DNA binding

The Epstein-Barr virus (EBV) lytic transactivator Rta activates promoters through direct binding to cognate DNA sites termed Rta response elements (RREs). Rta also activates promoters that apparently lack Rta binding sites, notably Zp and Rp. Chromatin immunoprecipitation (ChIP) of endogenous Rta expressed during early replication in B95-8 cells was performed to identify Rta binding sites in the EBV genome. Quantitative PCR (qPCR) analysis showed strong enrichment for known RREs but little or no enrichment for Rp or Zp, suggesting that the Rta ChIP approach enriches for direct Rta binding sites. Rta ChIP combined with deep sequencing (ChIP-seq) identified most known RREs and several novel Rta binding sites. Rta ChIP-seq peaks were frequently upstream of Rta-responsive genes, indicating that these Rta binding sites are likely functioning as RREs. Unexpectedly, the BALF5 promoter contained an Rta binding peak. To assess whether BALF5 might be activated by an RRE-dependent mechanism, an Rta mutant (Rta K156A), deficient for DNA binding and RRE activation but competent for Zp/Rp activation, was used. Rta K156A failed to activate BALF5p, suggesting this promoter can be activated by an RRE-dependent mechanism. Rta binding to late gene promoters was not seen at early time points but was specifically detected at later times within the Rta-responsive BLRF2 and BFRF3 promoters, even when DNA replication was inhibited. Our results represent the first characterization of Rta binding to the EBV genome during replication, identify previously unknown RREs, such as one in BALF5p, and highlight the complexity of EBV late gene promoter activation by Rta.

Herpesvirus replication compartments originate with single incoming viral genomes

Previously we described a method to estimate the average number of virus genomes expressed in an infected cell. By analyzing the color spectrum of cells infected with a mixture of isogenic pseudorabies virus (PRV) recombinants expressing three fluorophores, we estimated that fewer than seven incoming genomes are expressed, replicated, and packaged into progeny per cell. In this report, we expand this work and describe experiments demonstrating the generality of the method, as well as providing more insight into herpesvirus replication. We used three isogenic PRV recombinants, each expressing a fluorescently tagged VP26 fusion protein (VP26 is a capsid protein) under the viral VP26 late promoter. We calculated a similar finite limit on the number of expressed viral genomes, indicating that this method is independent of the promoter used to transcribe the fluorophore genes, the time of expression of the fluorophore (early versus late), and the insertion site of the fluorophore gene in the PRV genome (UL versus US). Importantly, these VP26 fusion proteins are distributed equally in punctate virion assembly structures in each nucleus, which improves the signal-to-noise ratio when determining the color spectrum of each cell. To understand how the small number of genomes are distributed among the replication compartments, we used a two-color fluorescent in situ hybridization assay. Most viral replication compartments in the nucleus occupy unique nuclear territories, implying that they arose from single genomes. Our experiments suggest a correlation between the small number of expressed viral genomes and the limited number of replication compartments.

A viral nuclear noncoding RNA binds re-localized poly(A) binding protein and is required for late KSHV gene expression

During the lytic phase of infection, the gamma herpesvirus Kaposi's Sarcoma-Associated Herpesvirus (KSHV) expresses a highly abundant, 1.1 kb nuclear noncoding RNA of unknown function. We observe that this polyadenylated nuclear (PAN) RNA avidly binds host poly(A)-binding protein C1 (PABPC1), which normally functions in the cytoplasm to bind the poly(A) tails of mRNAs, regulating mRNA stability and translation efficiency. During the lytic phase of KSHV infection, PABPC1 is re-localized to the nucleus as a consequence of expression of the viral shutoff exonuclease (SOX) protein; SOX also mediates the host shutoff effect in which host mRNAs are downregulated while viral mRNAs are selectively expressed. We show that whereas PAN RNA is not required for the host shutoff effect or for PABPC1 re-localization, SOX strongly upregulates the levels of PAN RNA in transient transfection experiments. This upregulation is destroyed by the same SOX mutation that ablates the host shutoff effect and PABPC1 nuclear re-localization or by removal of the poly(A) tail of PAN. In cells induced into the KSHV lytic phase, depletion of PAN RNA using RNase H-targeting antisense oligonucleotides reveals that it is necessary for the production of late viral proteins from mRNAs that are themselves polyadenylated. Our results add to the repertoire of functions ascribed to long noncoding RNAs and suggest a mechanism of action for nuclear noncoding RNAs in gamma herpesvirus infection.

Almost all eukaryotic messenger RNAs (mRNAs) have a string of 150–200 adenylates at the 3′ end. This poly(A) tail has been implicated as important for regulating mRNA translation, stability and export. During the lytic phase of infection of Kaposi's Sarcoma-Associated Herpesvirus (KSHV), a noncoding viral RNA is synthesized that resembles an mRNA in that it is transcribed by RNA polymerase II, is methyl-G capped at the 5′ end, and is polyadenylated at the 3′ end; yet this RNA is never exported to the cytoplasm for translation. Rather, it builds up in the nucleus to exceedingly high levels. We present evidence that the function of this abundant, polyadenylated nuclear (PAN) RNA is to bind poly(A) binding protein, which normally binds poly(A) tails of mRNAs in the cytoplasm but is re-localized into the nucleus during lytic KSHV infection. The interaction between PAN RNA and re-localized poly(A) binding protein is important for formation of new virus, in particular for the synthesis of proteins made late in infection. Our study provides new insight into the function of this noncoding RNA during KSHV infection and expands recent discoveries regarding re-localization of poly(A) binding protein during many viral infections.

Tpl2/AP-1 enhances murine gammaherpesvirus 68 lytic replication

How cellular factors regulate gammaherpesvirus lytic replication is not well understood. Here, through functional screening of a cellular kinase expression library, we identified mitogen-activated protein kinase kinase kinase 8 (MAP3K8/Tpl2) as a positive regulator of murine gammaherpesvirus 68 (MHV-68 or gammaHV-68) lytic gene expression and replication. Tpl2 enhances MHV-68 lytic replication by upregulating lytic gene expression and promoter activities of viral lytic genes, including RTA and open reading frame 57 (ORF57). By screening a cellular transcription factor library, we identified the Fos AP-1 transcription factor as a downstream factor that is both necessary and sufficient for mediating the enhancement of MHV-68 lytic replication by Tpl2. In addition, Tpl2 stimulates the promoter activities of key viral lytic genes, including RTA and ORF57, in an AP-1-dependent manner. We identified an AP-1-responsive element on the MHV-68 RTA promoter as the cis element mediating the upregulation of RTA promoter activity by Tpl2. MHV-68 lytic infection upregulates Fos expression, AP-1 activity, and RTA promoter activity in a Tpl2-dependent manner. We constructed a mutant MHV-68 virus that abolished this AP-1-responsive element. This mutant virus exhibited attenuated lytic replication kinetics, indicative of a critical role of this AP-1-responsive element during lytic replication. Moreover, Tpl2 knockdown inhibited the lytic replication of wild-type MHV-68 (MHV-68-WT) but not that of the MHV-68 mutant virus, indicating that endogenous Tpl2 promotes efficient virus lytic replication through AP-1-dependent upregulation of RTA expression. In summary, through tandem functional screens, we identified the Tpl2/AP-1 signaling transduction pathway as a positive regulator of MHV-68 lytic replication.

The histone variant H3.3 regulates gene expression during lytic infection with herpes simplex virus type 1

It has been proposed that incorporation of the histone variant H3.3 within actively transcribed regions of a genome helps to facilitate transcription. In this report we use lytic infection by herpes simplex virus type 1 (HSV-1) as a model to examine the temporal profile of histone H3 incorporation and to determine whether the variant histone H3.3 has a direct effect on transcription. We find that canonical H3.1 and variant H3.3 exhibit distinct temporal associations with the genome in cell lines expressing equal amounts of epitope-tagged H3 variants. At the earliest times examined after infection, the HSV-1 genome is incorporated into chromatin that predominantly contains the variant H3.3, whereas incorporation of canonical H3.1 occurs later in infection and is dependent on replication of the HSV-1 genome. Further, inhibition of H3.3 association, via reduced expression of the H3.3 chaperone HIRA, significantly reduces the levels of HSV-1 mRNA. These findings show that incorporation of H3.3 facilitates transcription, and they provide new evidence for a regulatory role of chromatin composition during HSV-1 acute infection.

Herpes simplex virus type 1 ICP27 regulates expression of a variant, secreted form of glycoprotein C by an intron retention mechanism

We previously showed that herpes simplex virus type 1 (HSV-1) immediate-early (IE) protein ICP27 can posttranscriptionally stimulate mRNA accumulation from a transfected viral late gene encoding glycoprotein C (gC) (K. D. Perkins, J. Gregonis, S. Borge, and S. A. Rice, J. Virol. 77:9872-9884, 2003). We began this study by asking whether ICP27 homologs from other herpesviruses can also mediate this activity. Although the homologs from varicella-zoster virus (VZV) and human cytomegalovirus (HCMV) were inactive, the homolog from bovine herpesvirus 4 (BHV-4), termed HORF1/2, was a very efficient transactivator. Surprisingly, most of the mRNA produced via HORF1/2 transactivation was 225 nucleotides shorter than expected due to the removal of a previously undescribed intron from the gC transcript. We found that the gC mRNA produced in the absence of transactivation was also mostly spliced. In contrast, gC mRNA produced by ICP27 transactivation was predominantly unspliced. Based on these results, we conclude that ICP27 has two distinct effects on the transfected gC gene: it (i) stimulates mRNA accumulation and (ii) promotes the retention of an intron. Interestingly, the spliced transcript encodes a variant of gC that lacks its transmembrane domain and is secreted from transfected cells. As the gC splicing signals are conserved among several HSV-1 strains, we investigated whether the variant gC is expressed during viral infection. We report here that both the spliced transcript and its encoded protein are readily detected in Vero cells infected with three different laboratory strains of wild-type HSV-1. Moreover, the variant gC is efficiently secreted from infected cells. We have designated this alternate form of the protein as gCsec. As the extracellular domain of gC is known to bind heparan sulfate-containing proteoglycans and to inhibit the complement cascade via an interaction with complement component C3b, we speculate that gCsec could function as a secreted virulence factor.

The interaction of herpes simplex virus 1 regulatory protein ICP22 with the cdc25C phosphatase is enabled in vitro by viral protein kinases US3 and UL13

Earlier studies have shown that ICP22 and the U(L)13 protein kinase but not the U(S)3 kinase are required for optimal expression of a subset of late (gamma(2)) genes exemplified by U(L)38, U(L)41, and U(S)11. In primate cells, ICP22 mediates the disappearance of inactive isoforms of cdc2 and degradation of cyclins A and B1. Active cdc2 acquires a new partner, the viral DNA synthesis processivity factor U(L)42. The cdc2-U(L)42 complex recruits and phosphorylates topoisomerase IIalpha for efficient expression of the gamma(2) genes listed above. In uninfected cells, the cdc25C phosphatase activates cdc2 by removing two inhibitory phosphates. The accompanying report shows that in the absence of cdc25C, the rate of degradation of cyclin B1 is similar to that occurring in infected wild-type mouse embryo fibroblast cells but the levels of cdc2 increase, and the accumulation of a subset of late proteins and virus yields are reduced. This report links ICP22 with cdc25C. We show that in infected cells, ICP22 and U(S)3 protein kinase mediate the phosphorylation of cdc25C at its C-terminal domain. In in vitro assays with purified components, both U(L)13 and U(S)3 viral kinases phosphorylate cdc25C and ICP22. cdc25C also interacts with cdc2. However, in infected cells, the ability of cdc25C to activate cdc2 by dephosphorylation of the inactive cdc2 protein is reduced. Coupled with the phosphorylation of cdc25C by the U(S)3 kinase, the results raise the possibility that herpes simplex virus 1 diverts cdc25C to perform functions other than those performed in uninfected cells.

Herpes simplex virus immediate-early protein ICP22 triggers loss of serine 2-phosphorylated RNA polymerase II

During eukaryotic mRNA transcription, the synthetic activity and mRNA processing factor interactions of RNA polymerase II (RNAP II) are regulated by phosphorylation of its carboxyl-terminal domain (CTD), with modification occurring primarily on serines 2 and 5 of the CTD. We previously showed that herpes simplex virus type 1 (HSV-1) infection rapidly triggers the loss of RNAP II forms bearing serine 2 phosphorylation (Ser-2P RNAP II). Here we show that the HSV-1 immediate-early (IE) protein ICP22 is responsible for this effect during the IE phase of infection. This activity does not require the viral UL13 protein kinase, which is required for several other regulatory functions of ICP22. Additionally, we show that transient expression of ICP22 can trigger the loss of Ser-2P RNAP II in transfected cells. Thus, the ability of ICP22 to cause the loss of Ser-2 RNAP II does not require other viral factors or the context of the infected cell. Expression of the HSV-1 ICP22-related protein US1.5, which corresponds to residues 147 to 420 of ICP22, also triggers a loss of Ser-2P RNAP II in transfected cells, whereas expression of the varicella-zoster virus ICP22 homolog, ORF63, does not. Our study also provides evidence for a second, viral late gene-dependent pathway that triggers loss of Ser-2P RNAP II in infected cells, consistent with the recent work of Dai-Ju et al. (J. Q. Dai-Ju, L. Li, L. A. Johnson, and R. M. Sandri-Goldin, J. Virol. 80:3567-3581, 2006). Therefore, it appears that HSV-1 has evolved redundant mechanisms for triggering the loss of a specific phosphorylated form of RNAP II.

ICP22 is required for wild-type composition and infectivity of herpes simplex virus type 1 virions

The immediate-early regulatory protein ICP22 is required for efficient replication of herpes simplex virus type 1 in some cell types (permissive) but not in others (restrictive). In mice infected via the ocular route, the pathogenesis of an ICP22- virus, 22/n199, was altered relative to that of wild-type virus. Specifically, tear film titers of 22/n199-infected mice were significantly reduced at 3 h postinfection relative to those of mice infected with wild-type virus. Further, 22/n199 virus titers were below the level of detection in trigeminal ganglia (TG) during the first 9 days postinfection. On day 30 postinfection, TG from 22/n199-infected mice contained reduced viral genome loads and exhibited reduced expression of latency-associated transcripts and reduced reactivation efficiency relative to TG from wild-type virus-infected mice. Notably, the first detectable alteration in the pathogenesis of 22/n199 in these tests occurred in the eye prior to the onset of nascent virus production. Thus, ICP22- virions appeared to be degraded, cleared, or adsorbed more rapidly than wild-type virions, implying potential differences in the composition of the two virion types. Analysis of the protein composition of purified extracellular virions indicated that ICP22 is not a virion component and that 22/n199 virions sediment at a reduced density relative to wild-type virions. Although similar to wild-type virions morphologically, 22/n199 virions contain reduced amounts of two gamma2 late proteins, US11 and gC, and increased amounts of two immediate-early proteins, ICP0 and ICP4, as well as protein species not detected in wild-type virions. Although ICP22- viruses replicate to near-wild-type levels in permissive cells, the virions produced in these cells are biochemically and physically different from wild-type virions. These virion-specific differences in ICP22- viruses add a new level of complexity to the functional analysis of this immediate-early viral regulatory protein.

ORF18 is a transfactor that is essential for late gene transcription of a gammaherpesvirus

Lytic replication of the tumor-associated human gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus has important implications in pathogenesis and tumorigenesis. Herpesvirus lytic genes have been temporally classified as exhibiting immediate-early (IE), early, and late expression kinetics. Though the regulation of IE and early gene expression has been studied extensively, very little is known regarding the regulation of late gene expression. Late genes, which primarily encode virion structural proteins, require viral DNA replication for their expression. We have identified a murine gammaherpesvirus 68 (MHV-68) early lytic gene, ORF18, essential for viral replication. ORF18 is conserved in both beta- and gammaherpesviruses. By generating an MHV-68 ORF18-null virus, we characterized the stage of the virus lytic cascade that requires the function of ORF18. Gene expression profiling and quantitation of viral DNA synthesis of the ORF18-null virus revealed that the expression of early genes and viral DNA replication were not affected; however, the transcription of late genes was abolished. Hence, we have identified a gammaherpesvirus-encoded factor essential for the expression of late genes independently of viral DNA synthesis.

HSV trafficking and development of gene therapy vectors with applications in the nervous system

Herpes simplex virus type 1 (HSV-1) is a neurotropic double-stranded DNA virus that causes cold sores, keratitis, and rarely encephalitis in humans. Nonpathogenic HSV-1 gene transfer vectors have been generated by elimination of viral functions necessary for replication. The life cycle of the native virus includes replication in epithelial cells at the site of initial inoculation followed by retrograde axonal transport to the nuclei of sensory neurons innervating the area of cutaneous primary infection. In this review, we summarize the current understanding of the molecular basis for HSV cell entry, nuclear transport of the genome, virion egress following replication, and retrograde and anterograde axonal transport in neurons. We discuss how each of these properties has been exploited or modified to allow the generation of gene transfer vectors with particular utility for neurological applications. Recent advances in engineering virus entry have provided proof of principle that vector targeting is possible. Furthermore, significant and potentially therapeutic modifications to the pathological responses to various noxious insults have been demonstrated in models of peripheral nerve disease. These applications exploit the natural axonal transport mechanism of HSV, allowing transgene expression in the cell nucleus within the inaccessible trigeminal ganglion or dorsal root ganglion, following the noninvasive procedure of subcutaneous vector inoculation. These findings demonstrate the importance of understanding basic virology in the design of vector systems and the powerful approach of exploiting favorable properties of the parent virus in the generation of gene transfer vectors.

Differential cellular requirements for activation of herpes simplex virus type 1 early (tk) and late (gC) promoters by ICP4

The herpes simplex virus type 1 immediate-early protein, ICP4, activates the transcription of viral early and late genes and is essential for viral growth. It has been shown to bind DNA and interact with components of the general transcription machinery to activate or repress viral transcription, depending upon promoter context. Since early and late gene promoters have different architectures and cellular metabolism may be very different at early and late times after infection, the cellular requirements for ICP4-mediated activation of early and late genes may differ. This hypothesis was tested using tk and gC as representative early and late promoters, respectively. Nuclear extracts and phosphocellulose column fractions derived from nuclear extracts were able to reconstitute basal and ICP4-activated transcription of both promoters in vitro. When examining the contribution of the general transcription factors on the ability of ICP4 to activate transcription, the fraction containing the general transcription factor TFIIA was not essential for ICP4 activation of the gC promoter, but it was required for efficient activation of the tk promoter. The addition of recombinant TFIIA restored the ability of ICP4 to efficiently activate the tk promoter, but it had no net effect on activation of the gC promoter. The dispensability of TFIIA for ICP4 activation of the gC promoter required an intact INR element. In addition, microarray and Northern blot analysis indicated that TFIIA abundance may be reduced at late times of infection. This decrease in TFIIA expression during infection and its dispensability for activation of late but not early genes suggest one of possibly many mechanisms for the transition from viral early to late gene expression.

Requirement of a 12-base-pair TATT-containing sequence and viral lytic DNA replication in activation of the Kaposi's sarcoma-associated herpesvirus K8.1 late promoter

Kaposi's sarcoma-associated herpesvirus (KSHV) K8.1 late promoter consists of a minimal 24-bp sequence, with a TATA-like, 12-bp promoter core, AATATTAAAGGG, and is active on a reporter only in butyrate-induced KSHV-infected cells. The activity of the K8.1 promoter can be enhanced (>15-fold) by the KSHV left-end lytic origin of DNA replication (oriLyt-L) sequence while providing inefficient replication of plasmid DNA and is inhibited by viral DNA replication inhibitors, suggesting that activation of the K8.1 promoter on the reporter is involved in KSHV lytic DNA replication largely by trans.

A strict-late viral promoter is a strong tumor-specific promoter in the context of an oncolytic herpes simplex virus

Confinement of transgene expression to target cells is highly desirable in gene therapy. Current strategies of transcriptional targeting to tumors usually rely on tissue-specific promoters to control gene expression. However, such promoters generally have much lower activity than the constitutive viral promoters. We have explored an alternative approach, using a strict-late viral promoter (UL38p) in the context of an oncolytic herpes simplex virus (HSV) for tumor-selective gene expression. As with many DNA viruses, the genomic transcription of HSV is a tightly regulated molecular cascade in which early and late phases of gene expression are separated by viral DNA replication. In particular, some of the late transcripts are categorized as strict-late, whose expression depends rigorously on the initiation of viral DNA replication. Our in vitro and in vivo characterization showed that in normal nondividing cells, where the oncolytic HSV has limited ability to replicate, the UL38p has minimal activity. However, in tumor or cycling cells where the virus can fully replicate, transgene expression from UL38p was almost as high as from the cytomegalovirus immediate-early promoter. These results suggest that delivery of therapeutic genes driven by UL38p through an oncolytic HSV may be an effective approach to gene therapy for malignant diseases.

Gene delivery using herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus with many favorable properties as a gene delivery vector. HSV is highly infectious, so HSV vectors are efficient vehicles for the delivery of exogenous genetic material to cells. Viral replication is readily disrupted by null mutations in immediate early genes that in vitro can be complemented in trans, enabling straightforward production of high-titre pure preparations of non-pathogenic vector. The genome is large (152 Kb) and many of the viral genes are dispensable for replication in vitro, allowing their replacement with large or multiple transgenes. Latent infection with wild-type virus results in episomal viral persistence in sensory neuronal nuclei for the duration of the host lifetime. Transduction with replication-defective vectors causes a latent-like infection in both neural and non-neural tissue; the vectors are non-pathogenic, unable to reactivate and persist long-term. The latency active promoter complex can be exploited in vector design to achieve long-term stable transgene expression in the nervous system. HSV vectors transduce a broad range of tissues because of the wide expression pattern of the cellular receptors recognized by the virus. Increasing understanding of the processes involved in cellular entry has allowed preliminary steps to be taken towards targeting the tropism of HSV vectors. Using replication-defective HSV vectors, highly encouraging results have emerged from recent pre-clinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration. Consequently, HSV vectors encoding appropriate transgenes to tackle these pathogenic processes are poised to enter clinical trials.

Selective internalization of sodium channels in rat dorsal root ganglion neurons infected with herpes simplex virus-1

The neurotropic virus, herpes simplex type 1 (HSV-1), inhibits the excitability of peripheral mammalian neurons, but the molecular mechanism of this effect has not been identified. Here, we use voltage-clamp measurement of ionic currents and an antibody against sodium channels to show that loss of excitability results from the selective, precipitous, and complete internalization of voltage-activated sodium channel proteins from the plasma membrane of neurons dissociated from rat dorsal root ganglion. The internalization process requires viral protein synthesis but not viral encapsulation, and does not alter the density of voltage-activated calcium or potassium channels. However, internalization is blocked completely when viruses lack the neurovirulence factor, infected cell protein 34.5, or when endocytosis is inhibited with bafilomycin A(1) or chloroquine. Although it has been recognized for many years that viruses cause cell pathology by interfering with signal transduction pathways, this is the first example of viral pathology resulting from selective internalization of an integral membrane protein. In studying the HSV-induced redistribution of sodium channels, we have uncovered a previously unknown pathway for the rapid and dynamic control of excitability in sensory neurons by internalization of sodium channels.

Effect of genetic background and culture conditions on the production of herpesvirus-based gene therapy vectors

Herpes simplex virus type-1 (HSV-1) represents a unique vehicle for the introduction of foreign DNA to cells of a variety of tissues. The nature of the vector, the cell line used for propagation of the vector, and the culture conditions will significantly impact vector yield. An ideal vector should be deficient in genes essential for replication as well as those that contribute to its cytotoxicity. Advances in the engineering of replication-defective HSV-1 vectors to reduce vector-associated cytotoxicity and attain sustained expression of target genes make HSV-1 an attractive gene-delivery vehicle. However, the yield of the less-cytotoxic vectors produced in standard tissue-culture systems is at least three order of magnitudes lower than that achieved with wild-type virus. The low overall yield and the complexity involved in the preparation of HSV vectors at high concentrations represent major obstacles in use of replication-defective HSV-derived vectors in gene therapy applications. In this work, the dependence of the vector yield on the genetic background of the virus is examined. In addition, we investigated the production of the least toxic, lowest-yield vector in a CellCube bioreactor system. After initial optimization of the operational parameters of the cellcube system, we were able to attain virus yields similar to or better than those values attained using the tissue culture flask system for vector production with significant savings with respect to time, labor, and cost.

The initiator element in a herpes simplex virus type 1 late-gene promoter enhances activation by ICP4, resulting in abundant late-gene expression

The start site regions of late genes of herpes simplex virus type 1 are similar to the eukaryotic initiator sequence (Inr), have been shown to affect the levels of expression, and may also play a role in transcription activation by the viral activator ICP4. A series of linker-scanning mutations spanning the start site of transcription and several downstream mutations in the true late gC promoter were analyzed in reconstituted in vitro transcription reactions with and without ICP4, as well as in the context of the viral genome during infection. The nucleotide contacts previously found to be important for Inr function were also found to be important for optimal induction by ICP4. While the Inr had a substantial effect on the accumulation of gC RNA during infection, no other sequence downstream of the TATA box to +124 had a significant effect on levels of expression during infection. Therefore, these studies suggest that TATA box and the Inr are the only cis-acting elements required to achieve optimal expression of gC, and that the high levels of late-gene transcription may be largely due to the induction by ICP4, functioning through the Inr element.

The Epstein-Barr virus lytic program is controlled by the co-operative functions of two transactivators

The propagation of herpesviruses has long been viewed as a temporally regulated sequential process that results from the consecutive expression of specific viral transactivators. As a key step in this process, lytic viral DNA replication is considered as a checkpoint that controls the expression of the late structural viral genes. In a novel genetic approach, we show that both hypotheses do not hold true for the Epstein-Barr virus (EBV). The study of viral mutants of EBV in which the early genes BZLF1 and BRLF1 are deleted allowed a precise assignment of the function of these proteins. Both transactivators were absolutely essential for viral DNA replication. Both BZLF1 and BRLF1 were required for full expression of the EBV proteins expressed during the lytic program, although the respective influence of these molecules on the expression of various viral target genes varied greatly. In replication-defective viral mutants, neither early gene expression nor DNA replication was a prerequisite for late gene expression. This work shows that BRLF1 and BZLF1 harbor distinct but complementary functions that influence all stages of viral production.

Herpesvirus mRNAs are sorted for export via Crm1-dependent and -independent pathways

Cellular pre-mRNA splicing is inhibited by ICP27, a herpes simplex virus regulatory protein, resulting in the shutoff of host protein synthesis. Here we reveal that ICP27 also mediates the export of some virus RNAs via a Crm1-dependent pathway and present evidence that independent domains are required for these functions. Sorting of some viral mRNAs for nuclear export requires Crm1, while other virus mRNAs are exported via another pathway.

Herpes simplex virus vector-mediated dystrophin gene transfer and expression in MDX mouse skeletal muscle

BACKGROUND

Duchenne muscular dystrophy (DMD) results from mutations that prevent the expression of functional dystrophin in muscle fibers. Herpes simplex virus type-1 (HSV-1) represents a potentially useful vector for treatment of DMD because it has the capacity to accommodate the 14-kb full-length dystrophin cDNA and can efficiently transduce muscle cells. We have tested the ability of first- and second-generation replication-defective HSV vectors to deliver full-length dystrophin to dystrophin-deficient mdx muscle cells in vitro and in vivo.

METHODS

First-generation replication-defective HSV vectors harboring full-length or truncated (Becker) dystrophin expression cassettes and lacking a single viral immediate-early (IE) gene were constructed and tested by immunofluorescence and immunoblotting for their ability to direct dystrophin expression in infected mdx cells in culture. To reduce vector cytotoxicity and safety concerns, a second-generation dystrophin vector missing additional IE genes was constructed and tested in vitro and in vivo.

RESULTS

Dystrophin expression was observed in infected mdx myotubes in vitro in all cases. Confocal microscopy showed exclusive localization of full-length dystrophin to the cell membrane whereas the Becker variant was also found abundantly throughout the cytoplasm. Dystrophin expression in mdx mice was restored in muscle cells near the site of vector injection.

CONCLUSION

Highly defective HSV-1 vectors which lack the ability to spread systemically and are greatly reduced in toxicity for infected cells, thus removing an impediment to prolonged transgene expression, can direct the delivery and proper expression of full-length dystrophin whose considerable size is compatible with few other modes of delivery. These vectors may offer a legitimate opportunity toward the development of effective gene therapy treatments for DMD.

Mutational analysis of the herpes simplex virus type 1 ICP0 C3HC4 zinc ring finger reveals a requirement for ICP0 in the expression of the essential alpha27 gene

The herpes simplex virus type 1 (HSV-1) immediate-early (IE) protein ICP0 has been implicated in the regulation of viral gene expression and the reactivation of latent HSV-1. Evidence demonstrates that ICP0 is an activator of viral gene expression yet does not distinguish between a direct or indirect role in this process. To further our understanding of the function of ICP0 in the context of the virus life cycle, site-directed mutagenesis of the consensus C3HC4 zinc finger domain was performed, and the effects of these mutations on the growth and replication of HSV-1 were assessed. We demonstrate that alteration of any of the consensus C3HC4 cysteine or histidine residues within this domain abolishes ICP0-mediated transactivation, alters the intranuclear localization of ICP0, and significantly increases its stability. These mutations result in severe defects in the growth and DNA replication of recombinant herpesviruses and in their ability to initiate lytic infections at low multiplicities of infection. These viruses, at low multiplicities of infection, synthesize wild-type levels of the IE proteins ICP0 and ICP4 at early times postinfection yet exhibit significant decreases in the synthesis of the essential IE protein ICP27. These findings reveal a role for ICP0 in the expression of ICP27 and suggest that the multiplicity-dependent growth of alpha0 mutant viruses results partially from reduced levels of ICP27.

Progress in development of herpes simplex virus gene vectors for treatment of rheumatoid arthritis

Arthritis is presently incurable and poorly treatable, but there are good grounds for expecting gene therapy to improve matters considerably. Although local ex vivo delivery of anti-arthritic genes to the synovial lining of joints has shown considerable promise, intraarticular gene delivery may be desirable. Herpes simplex virus (HSV) may be a viable vector for in vivo transfer of anti-arthritic genes to joints. HSV has the advantages of high infectivity, large carrying capacity and high titer. The large packaging capacity would permit the inclusion of multiple anti-arthritic genes and necessary regulatory elements. Recombinant vectors produced by this laboratory infect synovial cells efficiently, permitting prolonged expression of transgenes in vitro and in vivo without evidence of cytotoxicity. Further improvements to this vector system include taking advantage of an endogenous HSV 'stealthing' gene, ICP47, which interferes with formation of antigen-class I complexes. Inclusion of inducible promoters to appropriately regulate expression of anti-arthritic genes should further improve this system.

Suppression of apoptotic DNA fragmentation in herpes simplex virus type 1-infected cells

HEp-2 cells underwent apoptosis when the cells were incubated in medium containing sorbitol. Infection with herpes simplex virus type 1 (HSV-1) prior to the sorbitol treatment suppressed this apoptosis completely, indicating that HSV-1 carries an antiapoptosis gene. In addition, HSV-1 multiplication was restricted in these apoptotic HEp-2 cells.

Herpes simplex: evolving concepts

A large body of molecular biologic research has begun to clarify some basic aspects of viral latency and reactivation. The clinical definition of herpes simplex virus infection is expanding, with the recognition that the disease is largely asymptomatic and that most transmission occurs during periods of asymptomatic viral shedding. With this awareness, serologic diagnosis has become increasingly important. New treatment modalities are now available, and other promising treatments are in development.

The herpes simplex virus major regulatory protein ICP4 blocks apoptosis induced by the virus or by hyperthermia

Cells infected with herpes simplex virus 1 (HSV-1) undergo productive or latent infection without exhibiting features characteristic of apoptosis. In this report, we show that HSV-1 induces apoptosis but has evolved a function that blocks apoptosis induced by infection as well as by other means. Specifically, (i) Vero cells infected with a HSV-1 mutant deleted in the regulatory gene alpha 4 (that encodes repressor and transactivating functions), but not those infected with wild-type HSV-1(F), exhibit cytoplasmic blebbing, chromatin condensation, and fragmented DNA detected as a ladder in agarose gels or by labeling free DNA ends with terminal transferase; (ii) Vero cells infected with wild-type HSV-1(F) or cells expressing the alpha 4 gene and infected with the alpha 4- virus did not exhibit apoptosis; (iii) fragmentation of cellular DNA was observed in Vero cells that were mock-infected or infected with the alpha 4- virus and maintained at 39.5 degrees C, but not in cells infected with wild-type virus and maintained at the same temperature. Wild-type strains of HSV-1 with limited extrahuman passages, such as HSV-1 (F), carry a temperature-sensitive lesion in the alpha 4 gene and at 39.5 degrees C only alpha genes are expressed. These results indicate that the product of the alpha 4 gene is able to suppress apoptosis induced by the virus as well by other means.

Repression of the alpha0 gene by ICP4 during a productive herpes simplex virus infection

During a productive infection by herpes simplex virus type 1 (HSV-1), ICP4, the major regulatory protein encoded by the alpha4 gene, binds to its transcription initiation site and represses the accumulation of alpha4 RNA. Evidence suggests that the degree of repression by ICP4 is a function of the absolute distance of an ICP4 binding site 3' from a TATA box. However, repression of HSV-1 gene expression by ICP4 through binding sites located 5' of TATA boxes, as in the case of the alpha0 gene, has not been adequately addressed. To this end, recombinant alpha0 promoters with various arrays of ICP4 binding sites flanking the alpha0 TATA box were constructed and recombined into the HSV-1 genome. Our results demonstrate the following. (i) Destruction of the endogenous alphaO ICP4 binding site, located 5' of the TATA box, results in derepression of alpha0 protein and RNA accumulation in infected Vero cells. (ii) The degree of alpha0 derepression is equivalent to that reported for the alpha4 gene following destruction of the ICP4 binding site at the alpha4 mRNA cap site in HSV-1. (iii) Introduction of an ICP4 binding site at the alpha0 mRNA cap site represses the accumulation of alpha0 RNA greater than threefold relative to the wild type. (iv) Changes in the abundance of alpha0 protein and RNA in infected cells do not affect replication or growth of HSV-1 in tissue culture. Our findings are consistent with the conclusion that alpha0 transcription is repressed by ICP4. These results demonstrate that repression by ICP4 can occur through binding sites located 5' of virus gene TATA boxes in HSV-1. Thus, models addressing repression of HSV-1 gene expression by ICP4 should incorporate the role of binding sites located 5', as well as 3', of virus gene TATA boxes.

Two herpes simplex virus type 1 latency-active promoters differ in their contributions to latency-associated transcript expression during lytic and latent infections

Herpes simplex virus type 1 (HSV-1) establishes latency in human sensory ganglia, during which time the viral genome is transcriptionally silent with the exception of the latency-associated transcripts (LATs). The most abundant LAT is a 2-kb RNA whose biosynthesis is poorly characterized. The 2-kb LAT may be a primary transcript, or its synthesis may involve splicing and/or other forms of processing. Two potential RNA polymerase II promoters (LAP1 and LAP2) upstream of the 2-kb LAT 5' end have been identified. To investigate the role played by LAP1 and LAP2 in the synthesis of the 2-kb LAT under lytic and latent conditions, we analyzed HSV-1 mutants which contain deletions of one or both of these promoters. During lytic infection in cell culture, the cis elements critical for the normal accumulation of the 2-kb LAT were mapped to LAP2, while LAP1 sequences were largely dispensable. The 5' ends of the major 2-kb LATs produced by the wild-type and LAP deletion viruses were examined by primer extension analysis and were all found to be identical (+/- 2 bp). The accumulation of the 2-kb LAT during latent infections of murine trigeminal ganglia was examined by Northern (RNA) blot and by reverse transcription-PCR. In contrast to the results found in lytic infections, the critical cis elements needed for 2-kb LAT accumulation during latency were mapped to LAP1. Deletion of LAP1 resulted in a 500-fold reduction in 2-kb LAT accumulation, whereas deletion of LAP2 resulted in only a 2- to 3-fold reduction. Deletion of both LAP1 and LAP2 resulted in undetectable levels of the 2-kb LAT. Our results indicate that both LAP1 and LAP2 are critical for 2-kb LAT expression but under different conditions. LAP1 is essential for LAT expression during latency, while LAP2 is primarily responsible for LAT expression in lytic infections in cell culture. LAP1 and LAP2 may prove to be functionally independent promoter elements that control 2-kb LAT expression during different stages of HSV-1 infections.

Induction of transcription by a viral regulatory protein depends on the relative strengths of functional TATA boxes

The mechanisms by which viral regulatory proteins activate the cellular transcription apparatus without binding to specific DNA elements are not fully understood. Several lines of evidence suggest that activation by one such regulatory protein, herpes simplex virus ICP4, could be mediated, at least in part, by TFIID. To test this model, we replaced the TATA box of the ICP4-responsive viral thymidine kinase gene with functional TATA boxes that displayed different apparent affinities for TATA-box-binding protein as measured by DNase I footprinting. We measured the effects of these TATA boxes on ICP4 induction by constructing ICP4-deficient recombinant viruses containing the different TATA alleles and comparing their expression in cells lacking or expressing ICP4. Overall, ICP4 induced weak TATA boxes (those that displayed low apparent affinity for TATA-box-binding protein and low basal expression) the most (18- to 41-fold) and strong TATA boxes the least (7- to 10-fold). Therefore, ICP4 induction correlated inversely with TATA box strength. Using a reconstituted in vitro transcription assay, we determined that the relative levels of induction by ICP4 of the different TATA alleles were similar to those measured in vivo, suggesting that ICP4 was the only viral protein required for induction. These results fit a model in which ICP4 acts in part to enhance binding of TFIID to the TATA box. We compare and contrast these results with those observed with the viral regulatory proteins adenovirus E1a and simian virus 40 large T antigen and the cellular coactivator PC4.

Arginine-rich regions succeeding the nuclear localization region of the herpes simplex virus type 1 regulatory protein ICP27 are required for efficient nuclear localization and late gene expression

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP27 is an essential regulatory protein that localizes to the nuclei of infected cells. The strong nuclear localization signal (NLS) of ICP27 was identified recently and shown to reside in the amino-terminal portion of the polypeptide from residues 110 to 137 (W.E. Mears, V. Lam, and S.A. Rice, J. Virol. 69:935-947, 1995). There are also two arginine-rich regions directly succeeding the NLS. The first of these arginine-rich sequences (residues 141 to 151), together with the NLS, has been shown by Mears et al. to form the nucleolar localization signal. Arginine-rich motifs are common in domains involved in nuclear localization and RNA binding. To analyze the role of the arginine-rich regions in ICP27, we constructed stably transformed cell lines containing ICP27 mutants with deletions of all or parts of the NLS and arginine-rich regions. We also constructed mutants in which these regions were replaced with heterologous NLSs or RNA-binding domains. Characterization of these mutants indicated that the arginine-rich regions were required but not sufficient for wild-type localization of ICP27. More importantly, the NLS and arginine-rich regions were also essential to the function of ICP27. Mutants lacking these sequences were defective in late gene expression during infection even when ICP27 was properly localized to the nucleus by substitution of the NLS from simian virus 40 large T antigen. Further, the defect in late gene expression could not be overcome by replacement with the highly basic RNA-binding domain of human immunodeficiency virus type 1 Tat. The deficiency in late gene expression was independent of ICP27's role in stimulating viral DNA replication. In addition, localization of the HSV-1 proteins ICP4, ICP0, and ICP8 was unaffected by ICP27 mutants in this region. These results suggest that the arginine-rich regions are required for efficient nuclear localization and for the regulatory activity of ICP27 involved in viral late gene expression.

Repression of the herpes simplex virus 1 alpha 4 gene by its gene product (ICP4) within the context of the viral genome is conditioned by the distance and stereoaxial alignment of the ICP4 DNA binding site relative to the TATA box

Infected cell protein no. 4 (ICP4), the major regulatory protein encoded by the alpha 4 gene of herpes simplex virus 1, binds to a site (alpha 4-2) at the transcription initiation site of the alpha 4 gene. An earlier report described the construction of recombinant viruses that contained chimeric genes (alpha 4-tk) that consisted of the 5' untranscribed and transcribed noncoding domains of the alpha 4 gene fused to the coding sequences of the thymidine kinase gene and showed that disruption of the alpha 4-2 binding site by mutagenesis derepressed transcription of this gene (N. Michael and B. Roizman, Proc. Natl. Acad. Sci. USA 90:2286-2290, 1993). This experimental design was used to determine the effect of displacement of the alpha 4-2 binding site on the repression of alpha 4 gene transcription by ICP4. We report the following findings. (i) In the absence of the alpha 4-2 binding site, at 4 h after infection, alpha 4-tk RNA levels increased 10-fold relative to the corresponding RNA levels of a gene that contained the alpha 4-2 site at its natural location. Displacement of the alpha 4-2 binding site by approximately one, two, and three turns of the DNA helix, i.e., by 10, 21, and 30 nucleotides downstream of the original site, increased the concentration of alpha 4-tk RNA 2.4-, 3.5-, and 5.8-fold, respectively. (ii) Displacement of 16 nucleotides, i.e., approximately 1.5 helical turns, increased the accumulation of alpha 4-tk by 5.3-fold, i.e., more than predicted by displacement alone. (iii) At 8 h after infection in the absence of the binding site, the accumulation of alpha 4-tk RNA increased 13.6-fold. However, in cells infected with recombinants that carried displaced alpha 4-2 binding sites, RNA accumulation decreased relative to the levels seen at 4 h after infection. The insertion of DNA sequences in order to displace the alpha 4-2 binding site had no effect on accumulation of RNA in the presence of cycloheximide, i.e., in the absence of ICP4, or on maximum accumulation of alpha 4-tk RNA in the absence of the alpha 4-2 binding site.(ABSTRACT TRUNCATED AT 400 WORDS)

Transcriptional activation of the herpes simplex virus type 1 UL38 promoter conferred by the cis-acting downstream activation sequence is mediated by a cellular transcription factor

The herpes simplex virus (HSV) type 1 strict late (gamma) UL38 promoter contains three cis-acting transcriptional elements: a TATA box, a specific initiator element, and the downstream activation sequence (DAS). DAS is located between positions +20 and +33 within the 5' untranslated leader region and strongly influences transcript levels during productive infection. In this communication, we further characterize DAS and investigate its mechanism of action. DAS function has a strict spacing requirement, and DAS contains an essential 6-bp core element. A similarly positioned element from the gamma gC gene (UL44) has partial DAS function within the UL38 promoter context, and the promoter controlling expression of the gamma US11 transcript contains an identically located element with functional and sequence similarity to UL38 DAS. These data suggest that downstream elements are a common feature of many HSV gamma promoters. Results with recombinant viruses containing modifications of the TATA box or initiator element of the UL38 promoter suggest that DAS functions to increase transcription initiation and not the efficiency of transcription elongation. In vitro transcription assays using uninfected HeLa nuclear extracts show that, as in productive infection with recombinant viruses, the deletion of DAS from the UL38 promoter dramatically decreases RNA expression. Finally, electrophoretic mobility shift assays and UV cross-linking experiments show that DAS DNA forms a specific, stable complex with a cellular protein (the DAS-binding factor) of approximately 35 kDa. These data strongly suggest that the interaction of cellular DAS-binding factor with DAS is required for efficient expression of UL38 and other HSV late genes.

Herpes simplex virus vectors for gene therapy

Herpes simplex virus (HSV) has a number of advantages as a vector for delivering specific genes to the nervous system. These include its large size, wide host range, and its ability to establish long-lived asymptomatic infections in neuronal cells in which a specific region of the viral genome continues to be expressed. Unfortunately, the large size of this virus and difficulty in manipulating it has led to its use as a vector lagging behind that of other, smaller viruses such as the retroviruses. In addition, the virus's ability to replicate lytically in the brain, under some circumstances, causing encephalitis, has led to fears about its potential safety for ultimate use in humans. This review will discuss a number of new approaches that are aimed at rendering simpler the insertion of foreign genes into the virus and making it as safe as possible. Ultimately, these advances offer real hope for the use of HSV vectors in gene therapy procedures.