Herpes simplex virus gene expression in neurons: viral DNA synthesis is a critical regulatory event in the branch point between the lytic and latent pathways

Reversing thyroid-hormone-mediated repression of a HSV-1 promoter via computationally guided mutagenesis

Thyroid hormones (THs) and their DNA-binding nuclear receptors (TRs) direct transcriptional regulation in diverse ways depending on the host cell environment and specific promoter characteristics of TH-sensitive genes. This study sought to elucidate the impact on transcriptional repression of nucleotide sequence or orientation within TR binding sites - the TH response elements (TREs) of TH-sensitive promoters - to better understand ligand-dependent transcriptional repression of wild-type promoters. Computational analysis of the HSV-1 thymidine kinase (TK) gene TRE bound by TR and retinoid X receptor (RXR) revealed a single TRE point mutation sufficient to reverse the TRE orientation. In vitro experiments showed that the TRE point mutation had distinct impacts on promoter activity, sufficient to reverse the TH-dependent negative regulation in neuroendocrine differentiated cells. This point mutation altered the promoter's regulatory mechanism by discrete changes in transcription factor TR occupancy and altered enrichment of the repressive chromatin modification of histone-3-lysine-9-trimethyl (H3K9Me3). Insights relating to this negative TRE (nTRE) mechanism aids our understanding of other nTREs and TRE mutations associated with TH and herpes diseases.

Plaque2.0-A High-Throughput Analysis Framework to Score Virus-Cell Transmission and Clonal Cell Expansion

Classical plaque assay measures the propagation of infectious agents across a monolayer of cells. It is dependent on cell lysis, and limited by user-specific settings and low throughput. Here, we developed Plaque2.0, a broadly applicable, fluorescence microscopy-based high-throughput method to mine patho-biological clonal cell features. Plaque2.0 is an open source framework to extract information from chemically fixed cells by immuno-histochemistry or RNA in situ hybridization, or from live cells expressing GFP transgene. Multi-parametric measurements include infection density, intensity, area, shape or location information at single plaque or population levels. Plaque2.0 distinguishes lytic and non-lytic spread of a variety of DNA and RNA viruses, including vaccinia virus, adenovirus and rhinovirus, and can be used to visualize simultaneous plaque formation from co-infecting viruses. Plaque2.0 also analyzes clonal growth of cancer cells, which is relevant for cell migration and metastatic invasion studies. Plaque2.0 is suitable to quantitatively analyze virus infections, vector properties, or cancer cell phenotypes.

Thyroid hormone-dependent epigenetic suppression of herpes simplex virus-1 gene expression and viral replication in differentiated neuroendocrine cells

A global HSV-1 gene repression occurs during latency in sensory neurons where most viral gene transcriptions are suppressed. The molecular mechanisms of gene silencing and how stress factors trigger the reactivation are not well understood. Thyroid hormones are known to be altered due to stress, and with its nuclear receptor impart transcriptional repression or activation depending upon the hormone level. Therefore we hypothesized that triiodothyronine (T3) treatment of infected differentiated neuron like cells would reduce the ability of HSV-1 to produce viral progeny compared to untreated infected cells. Previously we identified putative thyroid hormone receptor elements (TREs) within the promoter regions of HSV-1 thymidine kinase (TK) and other key genes. Searching for a human cell line that can model neuronal HSV-1 infection, we performed HSV-1 infection experiments on differentiated human neuroendocrine cells, LNCaP. Upon androgen deprivation these cells undergo complete differentiation and exhibit neuronal-like morphology and physiology. These cells were readily infected by our HSV-1 recombinant virus, expressing GFP and maintaining many processes iconic of dendritic morphology. Our results demonstrated that differentiated LNCaP cells produced suppressive effects on HSV-1 gene expression and replication compared to its undifferentiated counterpart and T3 treatment has further decreased the viral plaque counts compared to untreated cells. Upon washout of the T3 viral plaque counts were restored, indicating an increase of viral replication. The qRT-PCR experiments using primers for TK showed reduced expression under T3 treatment. ChIP assays using a panel of antibodies for H3 lysine 9 epigenetic marks showed increased repressive marks on the promoter regions of TK. In conclusion we have demonstrated a T3 mediated quiescent infection in differentiated LNCaP cells that has potential to mimic latent infection. In this HSV-1 infection model thyroid hormone treatment caused decreased viral replication, repressed TK expression and increased repressive histone tail marks on the TK promoter.

Intrinsic innate immunity fails to control herpes simplex virus and vesicular stomatitis virus replication in sensory neurons and fibroblasts

Herpes simplex virus 1 (HSV-1) establishes lifelong latent infections in the sensory neurons of the trigeminal ganglia (TG), wherein it retains the capacity to reactivate. The interferon (IFN)-driven antiviral response is critical for the control of HSV-1 acute replication. We therefore sought to further investigate this response in TG neurons cultured from adult mice deficient in a variety of IFN signaling components. Parallel experiments were also performed in fibroblasts isolated concurrently. We showed that HSV-1 replication was comparable in wild-type (WT) and IFN signaling-deficient neurons and fibroblasts. Unexpectedly, a similar pattern was observed for the IFN-sensitive vesicular stomatitis virus (VSV). Despite these findings, TG neurons responded to IFN-β pretreatment with STAT1 nuclear localization and restricted replication of both VSV and an HSV-1 strain deficient in γ34.5, while wild-type HSV-1 replication was unaffected. This was in contrast to fibroblasts in which all viruses were restricted by the addition of IFN-β. Taken together, these data show that adult TG neurons can mount an effective antiviral response only if provided with an exogenous source of IFN-β, and HSV-1 combats this response through γ34.5. These results further our understanding of the antiviral response of neurons and highlight the importance of paracrine IFN-β signaling in establishing an antiviral state.

IMPORTANCE

Herpes simplex virus 1 (HSV-1) is a ubiquitous virus that establishes a lifelong latent infection in neurons. Reactivation from latency can cause cold sores, blindness, and death from encephalitis. Humans with deficiencies in innate immunity have significant problems controlling HSV infections. In this study, we therefore sought to elucidate the role of neuronal innate immunity in the control of viral infection. Using neurons isolated from mice, we found that the intrinsic capacity of neurons to restrict virus replication was unaffected by the presence or absence of innate immunity. In contrast, neurons were able to mount a robust antiviral response when provided with beta interferon, a molecule that strongly stimulates innate immunity, and that HSV-1 can combat this response through the γ34.5 viral gene. Our results have important implications for understanding how the nervous system defends itself against virus infections.

Transcription of the herpes simplex virus 1 genome during productive and quiescent infection of neuronal and nonneuronal cells

Herpes simplex virus 1 (HSV-1) can undergo a productive infection in nonneuronal and neuronal cells such that the genes of the virus are transcribed in an ordered cascade. HSV-1 can also establish a more quiescent or latent infection in peripheral neurons, where gene expression is substantially reduced relative to that in productive infection. HSV mutants defective in multiple immediate early (IE) gene functions are highly defective for later gene expression and model some aspects of latency in vivo. We compared the expression of wild-type (wt) virus and IE gene mutants in nonneuronal cells (MRC5) and adult murine trigeminal ganglion (TG) neurons using the Illumina platform for cDNA sequencing (RNA-seq). RNA-seq analysis of wild-type virus revealed that expression of the genome mostly followed the previously established kinetics, validating the method, while highlighting variations in gene expression within individual kinetic classes. The accumulation of immediate early transcripts differed between MRC5 cells and neurons, with a greater abundance in neurons. Analysis of a mutant defective in all five IE genes (d109) showed dysregulated genome-wide low-level transcription that was more highly attenuated in MRC5 cells than in TG neurons. Furthermore, a subset of genes in d109 was more abundantly expressed over time in neurons. While the majority of the viral genome became relatively quiescent, the latency-associated transcript was specifically upregulated. Unexpectedly, other genes within repeat regions of the genome, as well as the unique genes just adjacent the repeat regions, also remained relatively active in neurons. The relative permissiveness of TG neurons to viral gene expression near the joint region is likely significant during the establishment and reactivation of latency.

IMPORTANCE

During productive infection, the genes of HSV-1 are transcribed in an ordered cascade. HSV can also establish a more quiescent or latent infection in peripheral neurons. HSV mutants defective in multiple immediate early (IE) genes establish a quiescent infection that models aspects of latency in vivo. We simultaneously quantified the expression of all the HSV genes in nonneuronal and neuronal cells by RNA-seq analysis. The results for productive infection shed further light on the nature of genes and promoters of different kinetic classes. In quiescent infection, there was greater transcription across the genome in neurons than in nonneuronal cells. In particular, the transcription of the latency-associated transcript (LAT), IE genes, and genes in the unique regions adjacent to the repeats persisted in neurons. The relative activity of this region of the genome in the absence of viral activators suggests a more dynamic state for quiescent genomes persisting in neurons.

Stress-induced cellular transcription factors expressed in trigeminal ganglionic neurons stimulate the herpes simplex virus 1 ICP0 promoter

Alphaherpesvirinae family members can reactivate from latency following stress. The synthetic corticosteroid dexamethasone induces certain cellular transcription factors in murine and bovine trigeminal ganglionic neurons. Three dexamethasone-induced transcription factors, Krüppel-like factor 15, Slug, and SPDEF, stimulated the herpes simplex virus type 1-infected cell protein 0 (ICP0) promoter more than 150-fold. Conversely, other viral promoters (VP16 and ICP4) were not strongly stimulated, suggesting that the ICP0 promoter is preferentially activated by dexamethasone-simulated stress.

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.

Targeting the JMJD2 histone demethylases to epigenetically control herpesvirus infection and reactivation from latency

Chromatin and the chromatin modulation machinery not only provide a regulatory matrix for enabling cellular functions such as DNA replication and transcription but also regulate the infectious cycles of many DNA viruses. Elucidation of the components and mechanisms involved in this regulation is providing targets for the development of new antiviral therapies. Initiation of infection by herpes simplex virus (HSV) requires the activity of several cellular chromatin modification enzymes including the histone demethylases LSD1 and the family of JMJD2 proteins that promote transcriptional activation of the initial set of viral genes. Depletion of the JMJD2 members or inhibition of their activity with a new drug results in repression of expression of viral immediate early genes and abrogation of infection. This inhibitor also represses the reactivation of HSV from the latent state in sensory neurons. Like HSV, the β-herpesvirus human cytomegalovirus also requires the activity of LSD1 and the JMJD2s to initiate infection, thus demonstrating the potential of this chromatin-based inhibitor to be useful against a variety of different viruses.

A Novel Thyroid Hormone Mediated Regulation of HSV-1 Gene Expression and Replication is Specific to Neuronal Cells and Associated with Disruption of Chromatin Condensation

Previously we showed that thyroid hormone (T3) regulated the Herpes Simplex Virus Type -1 (HSV-1) gene expression and replication through its nuclear receptor TR via histone modification and chromatin remodeling in a neuroblastoma cell line neuro-2a cells (N2a). This observation suggested that T3 regulation may be neuron-specific and have implication in HSV-1 latency and reactivation. In this study, our in vitro latency/reactivation model demonstrated that removal of T3 can de-repress the HSV-1 replication and favor reactivation. Transfection studies and infection assays indicated that HSV-1 thymidine kinase (TK), a key viral gene during reactivation, was repressed by TR/T3 in cells with neuronal origin but not in non-neuronal cells. Additional studies showed that RCC1 (Regulator of Chromosome Condensation 1) was sequestered but efficiently detected upon viral infection in N2a cells. Western blot analyses indicated that addition of T3 repressed the RCC1 expression upon infection. It is likely that diminution of RCC1 upon infection in neuronal cells under the influence of TR/T3 may lead to repression of viral replication/gene expression thus promote latency. Together these results demonstrated that TR/T3 mediated regulation is specific to neuronal cells and differential chromosome condensation may play a critical role in this process.

C-terminal trans-activation sub-region of VP16 is uniquely required for forskolin-induced herpes simplex virus type 1 reactivation from quiescently infected-PC12 cells but not for replication in neuronally differentiated-PC12 cells

The HSV-1 tegument protein VP16 contains a trans-activation domain (TAD) that is required for induction of immediate early (IE) genes during lytic infection and induced reactivation from latency. Here we report the differential contributions of the two sub-regions of the TAD in neuronal and non-neuronal cells during activation of IE gene expression, virus replication, and reactivation from quiescently infected (QIF)-PC12 cells. Our studies show that VP16- and chemical (hexamethylenebisacetamide)-induced IE gene activation is attenuated in neuronal cells. Irrespective of neuronal or non-neuronal cell backgrounds, IE gene activation demonstrated a greater requirement for the N-terminal sub-region of VP16 TAD (VP16N) than the C-terminal sub-region (VP16C). In surprising contrast to these findings, a recombinant virus (RP4) containing the VP16N deletion was capable of modest forskolin-induced reactivation whereas a recombinant (RP3) containing a deletion of VP16C was incapable of stress-induced reactivation from QIF-PC12 cells. These unique process-dependent functions of the VP16 TAD sub-regions may be important during particular stages of the virus life cycle (lytic, entrance, and maintenance of a quiescent state and reactivation) when viral DNA would be expected to be differentially modified.

Transient reversal of episome silencing precedes VP16-dependent transcription during reactivation of latent HSV-1 in neurons

Herpes simplex virus type-1 (HSV-1) establishes latency in peripheral neurons, creating a permanent source of recurrent infections. The latent genome is assembled into chromatin and lytic cycle genes are silenced. Processes that orchestrate reentry into productive replication (reactivation) remain poorly understood. We have used latently infected cultures of primary superior cervical ganglion (SCG) sympathetic neurons to profile viral gene expression following a defined reactivation stimulus. Lytic genes are transcribed in two distinct phases, differing in their reliance on protein synthesis, viral DNA replication and the essential initiator protein VP16. The first phase does not require viral proteins and has the appearance of a transient, widespread de-repression of the previously silent lytic genes. This allows synthesis of viral regulatory proteins including VP16, which accumulate in the cytoplasm of the host neuron. During the second phase, VP16 and its cellular cofactor HCF-1, which is also predominantly cytoplasmic, concentrate in the nucleus where they assemble an activator complex on viral promoters. The transactivation function supplied by VP16 promotes increased viral lytic gene transcription leading to the onset of genome amplification and the production of infectious viral particles. Thus regulated localization of de novo synthesized VP16 is likely to be a critical determinant of HSV-1 reactivation in sympathetic neurons.

Herpes simplex virus is a widespread human pathogen that establishes permanent infections in nerves innervating the lips, eyes and other surfaces. The viral DNA genome is transported to the neuronal nucleus located in the nerve ganglia, where it establishes a semi-dormant state known as latency. Periodically, latent viruses undergo reactivation, a process that leads to the production of infectious particles, allowing for person-to-person transmission and acting as the major source for painful lesions (cold sores) and other more severe pathological outcomes. How latency and reactivation are controlled is not well understood. Using cultured nerve cells, we show that reactivation involves a unique, two stage program of viral gene expression. We find that the essential control protein VP16 is synthesized during the first stage but accumulates in the cytoplasm rather than the nucleus where it functions. Nuclear entry is determined by host signaling and marks the onset of the second reactivation stage. This work provides important new insights into the virus-host interaction and reveals a natural control point that could be used in innovative therapies that for the first time target the latent virus.

Cellular transcription factors induced in trigeminal ganglia during dexamethasone-induced reactivation from latency stimulate bovine herpesvirus 1 productive infection and certain viral promoters

Bovine herpesvirus 1 (BHV-1), an alphaherpesvirinae subfamily member, establishes latency in sensory neurons. Elevated corticosteroid levels, due to stress, reproducibly triggers reactivation from latency in the field. A single intravenous injection of the synthetic corticosteroid dexamethasone (DEX) to latently infected calves consistently induces reactivation from latency. Lytic cycle viral gene expression is detected in sensory neurons within 6 h after DEX treatment of latently infected calves. These observations suggested that DEX stimulated expression of cellular genes leads to lytic cycle viral gene expression and productive infection. In this study, a commercially available assay-Bovine Gene Chip-was used to compare cellular gene expression in the trigeminal ganglia (TG) of calves latently infected with BHV-1 versus DEX-treated animals. Relative to TG prepared from latently infected calves, 11 cellular genes were induced more than 10-fold 3 h after DEX treatment. Pentraxin three, a regulator of innate immunity and neurodegeneration, was stimulated 35- to 63-fold after 3 or 6 h of DEX treatment. Two transcription factors, promyelocytic leukemia zinc finger (PLZF) and Slug were induced more than 15-fold 3 h after DEX treatment. PLZF or Slug stimulated productive infection 20- or 5-fold, respectively, and Slug stimulated the late glycoprotein C promoter more than 10-fold. Additional DEX-induced transcription factors also stimulated productive infection and certain viral promoters. These studies suggest that DEX-inducible cellular transcription factors and/or signaling pathways stimulate lytic cycle viral gene expression, which subsequently leads to successful reactivation from latency in a small subset of latently infected neurons.

Regulation of the latency-reactivation cycle by products encoded by the bovine herpesvirus 1 (BHV-1) latency-related gene

Like other α-herpesvirinae subfamily members, the primary site for bovine herpesvirus 1 (BHV-1) latency is ganglionic sensory neurons. Periodically BHV-1 reactivates from latency, virus is shed, and consequently virus transmission occurs. Transcription from the latency-related (LR) gene is readily detected in neurons of trigeminal ganglia (TG) of calves or rabbits latently infected with BHV-1. Two micro-RNAs and a transcript encompassing a small open reading frame (ORF-E) located within the LR promoter can also be detected in TG of latently infected calves. A BHV-1 mutant that contains stop codons near the beginning of the first open reading frame (ORF2) within the major LR transcript (LR mutant virus) has been characterized. The LR mutant virus does not express ORF2, a reading frame that lacks an initiating ATG (reading frame B), and has reduced expression of ORF1 during productive infection. The LR mutant virus does not reactivate from latency following dexamethasone treatment suggesting that LR protein expression regulates the latency-reactivation cycle. Higher levels of apoptosis occur in TG neurons of calves infected with the LR mutant viruses when compared to wild-type BHV-1 indicating that the anti-apoptotic properties of the LR gene is necessary for the latency-reactivation cycle. ORF2 inhibits apoptosis and regulates certain viral promoters, in part, because it interacts with three cellular transcription factors (C/EBP-alpha, Notch1, and Notch3). Although ORF2 is important for the latency-reactivation cycle, we predict that other LR gene products play a supportive role during life-long latency in cattle.

HSV-1 gene expression from reactivated ganglia is disordered and concurrent with suppression of latency-associated transcript and miRNAs

In cell cultures, HSV-1 replication is initiated by recruitment by virion protein 16 of transcriptional factors and histone-modifying enzymes to immediate early (α) gene promoters. HSV establishes latent infections characterized by suppression of viral gene expression except for latency-associated transcripts (LATs) and miRNAs. The latent virus reactivates in stressed neurons. A fundamental question is how reactivation initiates in the absence of virion protein 16. We report the following findings in the ganglion explant model. (i) Anti-nerve growth factor antibody accelerated the reactivation of latent virus. Viral mRNAs were detected as early as 9 h after explantation. (ii) After explantation the amounts of viral mRNAs increased whereas amounts of miRNAs and LATs decreased. The decrease in miRNAs and LATs required ongoing protein synthesis, raising the possibility that LAT and miRNAs were degraded by a viral gene product. (iii) The expression of viral genes in explanted ganglia was disordered rather than sequentially ordered as in infected cells in culture. These findings suggest that in reactivating ganglia gene expression is totally derepressed and challenge the current models in that establishment of or exit from latency could not be dependent on the suppression or activation of single or small clusters of viral genes. Finally, miRNAs and LATs reached peak levels 9-11 d after corneal inoculation, thus approximating the pattern of virus replication in these ganglia. These findings suggest that the patterns of accumulation of LATs and miRNAs reflect many different stages in the infection of neurons.

Effects of thyroid hormone on HSV-1 gene regulation: implications in the control of viral latency and reactivation

Thyroid hormone (TH) is involved in many biological functions such as animal development, cell differentiation, etc. Variation and/or disruption of plasma TH level often led to abnormalities and physiological disorders. TH exerts the effects through its nuclear receptors (TR). Literature showed that procedures resulted in TH alteration also linked to reactivation of several viruses including Herpes Simplex Virus Type -1 (HSV-1). Bioinformatic analyses revealed a number of putative TH responsive elements (TRE) located in the critical regulatory regions of HSV-1 genes such as thymidine kinase (TK), latency associated transcript (LAT), etc. Studies using neuronal cell lines have provided evidences demonstrating that liganded TR regulated viral gene expression via chromatin modification and controlled viral replication. The removal of TH reversed the inhibition and induced the viral replication previously blocked by TH. These results suggest that TH may have implication to participate in the control of reactivation during HSV-1 latency.

Human cytomegalovirus early protein pUL21a promotes efficient viral DNA synthesis and the late accumulation of immediate-early transcripts

We have previously reported that a newly annotated gene of human cytomegalovirus (HCMV), UL21a, encodes an early viral protein termed pUL21a. Most notably, the virions of a UL21a deletion virus had markedly reduced infectivity, indicating that UL21a is required to establish an efficient productive infection. In this study, we infected fibroblasts with equal numbers of DNA-containing viral particles and identified where in the viral life cycle pUL21a acted. The UL21a deletion virus entered cells and initiated viral gene expression efficiently; however, it synthesized viral DNA poorly and accumulated several immediate-early (IE) transcripts at reduced levels at late times of infection. The defect in viral DNA synthesis preceded that in gene expression, and inhibition of viral DNA synthesis reduced the late accumulation of IE transcripts in both wild-type and mutant virus-infected cells to equivalent levels. This suggests that reduced viral DNA synthesis is the cause of reduced IE gene expression in the absence of UL21a. The growth of UL21a deletion virus was similar to that of recombinant HCMV in which pUL21a expression was abrogated by stop codon mutations, and the defect was rescued in pUL21a-expressing fibroblasts. pUL21a expression in trans was sufficient to restore viral DNA synthesis and gene expression of mutant virus produced from normal fibroblasts, whereas mutant virus produced from complementing cells still exhibited the defect in normal fibroblasts. Thus, pUL21a does not promote the functionality of HCMV virions; rather, its de novo synthesis facilitates viral DNA synthesis, which is necessary for the late accumulation of IE transcripts and establishment of a productive infection.

Thyroid hormone controls the gene expression of HSV-1 LAT and ICP0 in neuronal cells

Various factors/pathways including hormonal regulation have been suggested to control HSV-1 latency and reactivation. Our computer analysis identified a DNA repeat containing thyroid hormone response elements (TRE) in the regulatory region of HSV-1 LAT. Thyroid hormone (T₃) exerts its function via its receptor (TR), a transcriptional factor. Present study investigated the roles of TR and T₃ on HSV-1 gene expression using cultured neuoroblastoma cell lines. We demonstrated that liganded TR activated LAT transcription but repressed ICP0 transcription in the presence of LAT TRE. The chromatin immunoprecipitation (ChIP) assays showed that TRs were recruited to LAT TREs independently of T₃ and hyperacetylated H4 was associated with promoters that were transcriptionally active. In addition, ChIP results showed that a chromatin insulator protein CTCF was enriched at the LAT TREs in the presence of TR and T₃. In addition, chromatin remodeling factor BRG1 complex is found to participate in the T₃/TR-mediated LAT activation since overexpression of BRG1 enhanced the LAT transcription and the dominant negative mutant K785R abolished the activation. This is the first report revealing that TR exerted epigenetic regulation on HSV-1 ICP0 expression in neuronal cells and could have a role in the complex processes of HSV-1 latency/reactivation.

Relaxed repression of herpes simplex virus type 1 genomes in Murine trigeminal neurons

The expression of herpes simplex virus (HSV) genomes in the absence of viral regulatory proteins in sensory neurons is poorly understood. Previously, our group reported an HSV immediate early (IE) mutant (d109) unable to express any of the five IE genes and encoding a model human cytomegalovirus immediate early promoter-green fluorescent protein (GFP) transgene. In cultured cells, GFP expressed from this mutant was observed in only a subset of infected cells. The subset exhibited cell type dependence, as the fractions of GFP-expressing cells varied widely among the cell types examined. Herein, we characterize this mutant in murine embryonic trigeminal ganglion (TG) cultures. We found that d109 was nontoxic to neural cultures and persisted in the cultures throughout their life spans. Unlike with some of the cultured cell lines and strains, expression of the GFP transgene was observed in a surprisingly large subset of neurons. However, very few nonneuronal cells expressed GFP. The abilities of ICP0 and an inhibitor of histone deacetylase, trichostatin A (TSA), to activate GFP expression from nonexpressing cells were also compared. The provision of ICP0 by infection with d105 reactivated quiescent genomes in nearly every cell, whereas reactivation by TSA was much more limited and restricted to the previously nonexpressing neurons. Moreover, we found that d109, which does not express ICP0, consistently reactivated HSV type 1 (KOS) in latently infected adult TG cultures. These results suggest that the state of persisting HSV genomes in some TG neurons may be more dynamic and more easily activated than has been observed with nonneuronal cells.

Point mutations in herpes simplex virus type 1 oriL, but not in oriS, reduce pathogenesis during acute infection of mice and impair reactivation from latency

In vitro studies of herpes simplex virus type 1 (HSV-1) viruses containing mutations in core sequences of the viral origins of DNA replication, oriL and oriS, that eliminate the ability of these origins to initiate viral-DNA synthesis have demonstrated little or no effect on viral replication in cultured cells, leading to the conclusion that the two types of origins are functionally redundant. It remains unclear, therefore, why origins that appear to be redundant are maintained evolutionarily in HSV-1 and other neurotropic alphaherpesviruses. To test the hypothesis that oriL and oriS have distinct functions in the HSV-1 life cycle in vivo, we determined the in vivo phenotypes of two mutant viruses, DoriL-I(LR) and DoriS-I, containing point mutations in oriL and oriS site I, respectively, that eliminate origin DNA initiation function. Following corneal inoculation of mice, tear film titers of DoriS-I were reduced relative to wild-type virus. In all other tests, however, DoriS-I behaved like wild-type virus. In contrast, titers of DoriL-I(LR) in tear film, trigeminal ganglia (TG), and hindbrain were reduced and mice infected with DoriL-I(LR) exhibited greatly reduced mortality relative to wild-type virus. In the TG explant and TG cell culture models of reactivation, DoriL-I(LR) reactivated with delayed kinetics and, in the latter model, with reduced efficiency relative to wild-type virus. Rescuant viruses DoriL-I(LR)-R and DoriS-I-R behaved like wild-type virus in all tests. These findings demonstrate that functional differences exist between oriL and oriS and reveal a prominent role for oriL in HSV-1 pathogenesis.

Herpes simplex virus DNA synthesis is not a decisive regulatory event in the initiation of lytic viral protein expression in neurons in vivo during primary infection or reactivation from latency

The herpes simplex virus genome can enter a repressed transcriptional state (latency) in sensory neurons of the host nervous system. Although reduced permissiveness of the neuronal environment is widely accepted as a causal factor, the molecular pathway(s) directing and maintaining the viral genome in the latent state remains undefined. Over the past decade, the field has been strongly influenced by the observations of Kosz-Vnenchak et al., which have been interpreted to indicate that, in sensory neurons in vivo, a critical level of viral DNA synthesis within the neuron is required for sufficient viral immediate-early (IE) and early (E) gene expression (M. Kosz-Vnenchak, J. Jacobson, D. M. Coen, and D. M. Knipe, J. Virol. 67:5383-5393, 1993). The levels of IE and E genes are, in turn, thought to regulate the decision to enter the lytic cycle or latency. We have reexamined this issue using new strategies for in situ detection and quantification of viral gene expression in whole tissues. Our results using thymidine kinase-null and rescued mutants as well as wild-type strains in conjunction with viral DNA synthesis blockers demonstrate that (i) despite inhibition of viral DNA replication, many neurons express lytic viral proteins, including IE proteins, during acute infection in the ganglion; (ii) at early times postinoculation, the number of neurons expressing viral proteins in the ganglion is not reduced by inhibition of viral DNA replication; and (iii) following a reactivation stimulus, the numbers of neurons and apparent levels of lytic viral proteins, including IE proteins, are not reduced by inhibition of viral DNA replication. We conclude that viral DNA replication in the neuron per se does not regulate IE gene expression or entry into the lytic cycle.

ICP0 is not required for efficient stress-induced reactivation of herpes simplex virus type 1 from cultured quiescently infected neuronal cells

Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17+ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17+ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (deltaICP0), KD6 (deltaICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17+ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.

Herpes simplex virus 1 immediate-early and early gene expression during reactivation from latency under conditions that prevent infectious virus production

The program of gene expression exhibited by herpes simplex virus during productive infection of cultured cells is well established; however, less is known about the regulatory controls governing reactivation from latency in neurons. One difficulty in examining gene regulation during reactivation lies in distinguishing between events occurring in initial reactivating cells versus events occurring in secondarily infected cells. Thus, two inhibitors were employed to block production of infectious virus: acyclovir, which inhibits viral DNA synthesis, and WAY-150138, which permits viral DNA synthesis but inhibits viral DNA encapsidation. Latently infected murine ganglia were explanted in the presence of either inhibitor, and then amounts of RNA, DNA, or infectious virus were quantified. In ganglia explanted for 48 h, the levels of five immediate-early and early RNAs did not exhibit meaningful differences between acyclovir and WAY-150138 treatments when analyzed by in situ hybridization or quantitative reverse transcription-PCR. However, comparative increases in viral DNA and RNA content in untreated ganglia suggested that virus was produced before 48 h postexplant. This was confirmed by the detection of infectious virus as early as 14 h postexplant. Together, these results suggest that high levels of viral gene expression at 48 h postexplant are due largely to the production of infectious virus and subsequent spread through the tissue. These results lead to a reinterpretation of previous results indicating a role for DNA replication in immediate-early and early viral gene expression; however, it remains possible that viral gene expression is regulated differently in neurons than in cultured cells.

Site-directed mutagenesis of large DNA palindromes: construction and in vitro characterization of herpes simplex virus type 1 mutants containing point mutations that eliminate the oriL or oriS initiation function

Technical challenges associated with mutagenesis of the large oriL palindrome have hindered comparisons of the functional roles of the herpes simplex virus type 1 (HSV-1) origins of DNA replication, oriL and oriS, in viral replication and pathogenesis. To address this problem, we have developed a novel PCR-based strategy to introduce site-specific mutations into oriL and other large palindromes. Using this strategy, we generated three plasmids containing mutant forms of oriL, i.e., pDoriL-I(L), pDoriL-I(R), and pDoriL-I(LR), containing point mutations in the left, right, and both copies, respectively, of the origin binding protein (OBP) binding site (site I) which eliminate OBP binding. In in vitro DNA replication assays, plasmids with mutations in only one arm of the palindrome supported origin-dependent DNA replication, whereas plasmids with symmetrical mutations in both arms of the palindrome were replication incompetent. An analysis of the cloned mutant plasmids used in replication assays revealed that a fraction of each plasmid mutated in only one arm of the palindrome had lost the site I mutation. In contrast, plasmids containing symmetrical mutations in both copies of site I retained both mutations. These observations demonstrate that the single site I mutations in pDoriL-I(L) and pDoriL-I(R) are unstable upon propagation in bacteria and suggest that functional forms of both the left and right copies of site I are required to initiate DNA replication at oriL. To examine the role of oriL and oriS site I in virus replication, we introduced the two site I mutations in pDoriL-I(LR) into HSV-1 DNA to yield the mutant virus DoriL-I(LR) and the same point mutations into the single site I sequence present in both copies of oriS to yield the mutant virus DoriS-I. In Vero cells and primary rat embryonic cortical neurons (PRN) infected with either mutant virus, viral DNA synthesis and viral replication were efficient, confirming that the two origins can substitute functionally for one another in vitro. Measurement of the levels of oriL and oriS flanking gene transcripts revealed a modest alteration in the kinetics of ICP8 transcript accumulation in DoriL-I(LR)-infected PRN, but not in Vero cells, implicating a cell-type-specific role for oriL in regulating ICP8 transcription.

The cellular response to herpes simplex virus type 1 (HSV-1) during latency and reactivation

In order to learn more about the cellular response to viral gene activity during latency and reactivation of herpes simplex virus type 1 (HSV-1), the authors have employed microarray analysis. On an array of about 1200 cellular genes, approximately 56 genes were found to be differentially regulated in infected trigeminal ganglia of mice, compared to uninfected mice, during latency and reactivation. Of these genes, 10 were examined more closely using quantitative real-time polymerase chain reaction (PCR) to confirm the microarray results. Genes involved in interferon and other signaling pathways appeared to predominate in response to a latent or reactivating HSV infection. Interestingly, some genes found to be differentially regulated in latently infected ganglia are neuronal-specific genes (pro-opiomelanocortinin; zinc finger proteins of the cerebellum 1 and 2). During reactivation, the involvement of several cell signaling molecules that may be important for the initiation of an HSV infection was observed, including various receptors and molecules involved in cell-cell spread.

Histone deacetylase inhibitors induce reactivation of herpes simplex virus type 1 in a latency-associated transcript-independent manner in neuronal cells

Histone acetylation is implicated in the regulation of herpes simplex virus type 1 (HSV-1) latency. However, the role of histone acetylation in HSV-1 reactivation is less clear. In this study, the well-established model system, quiescently infected, neuronally differentiated PC12 (QIF-PC12) cells, was used to address the participation of histone acetylation in HSV-1 reactivation. In this model, sodium butyrate and trichostatin A (TSA), two histone deacetylase inhibitors, stimulated production of infectious HSV-1 progeny from a quiescent state. To identify viral genes responsive to TSA, the authors analyzed representative alpha, beta, and gamma viral genes using quantitative real-time polymerase chain reaction. Only the latency-associated transcript (LAT) accumulated in response to TSA treatment, under culture conditions that restricted virus replication and spread. This led the authors to evaluate the importance of LAT expression on TSA-induced reactivation. In QIF-PC12 cells, the LAT deletion mutant virus dLAT2903 reactivated equivalently with its wild-type parental strain (McKrae) after TSA treatment, as well as forskolin and heat stress treatment. Both viruses also reactivated equivalently from latently infected trigeminal ganglia explants from rabbits. In contrast, there was a marked reduction in the recovery of dLAT2903, as compared to wild-type virus, from the eyes of latently infected rabbits following epinephrine iontophoresis. These combined in vitro, ex vivo, and in vivo data suggest that LAT is not required for reactivation from latently infected neuronal cells per se, but may enhance processes that allow for the arrival of virus at, or close to, the site of original inoculation (i.e., recrudescence).

Ocular and neuronal cell apoptosis during HSV-1 infection: a review

HSV-1 may activate or suppress the apoptotic pathway in various cells. This review will discuss this apparent dichotomy and place particular emphasis on the different strategies HSV-1 uses to block or suppress the apoptotic pathway in various cell lines and tissues.

Tetracycline-regulated gene expression in replication-incompetent herpes simplex virus vectors

Although herpes simplex virus (HSV) vectors appear to have great potential as gene delivery vectors both in vitro and in vivo, the expression of foreign genes in such vectors cannot be easily regulated. Of the known eukaryotic regulatory systems, the tetracycline-inducible gene expression system is perhaps the most widely used because of its induction characteristics and because of the well-known pharmacological properties of tetracycline (Tet) and analogs such as doxycycline. Here, we describe the adaptation of the Tet-inducible system for use in replication-incompetent HSV vectors. HSV vectors were constructed that contained several types of Tet-inducible promoters for foreign gene expression. These promoters contained a tetracycline response element (TRE) linked to either a minimal cytomegalovirus (CMV) immediate-early promoter, a minimal HSV ICP0 promoter, or a truncated HSV ICP0 promoter containing one copy of the HSV TAATGARAT cis-acting immediate-early regulatory element (where R represents a prime base). All three promoter constructs were regulated appropriately by doxycycline, as shown by the expression of the marker gene lacZ in cell lines engineered to express Tet transactivators. The ICP0 promoter constructs expressed the highest and most sustained levels of lacZ, but the CMV promoter construct had the highest relative level of induction, suggesting their use in different applications. To extend the utility of Tet-regulated HSV vectors, vectors were constructed that coexpressed an inducible Tet transactivator in addition to the inducible lacZ marker gene. This modification resulted in tetracycline-inducible gene expression that was not restricted to specific cell lines, and this vector was capable of inducible expression in irreversibly differentiated NT2 cells (NT-neurons) for several days. Finally, HSV vectors were constructed that expressed modified Tet transactivators, resulting in improved induction properties and indicating the flexibility of the Tet-regulated system for regulation of foreign gene expression in HSV vector-infected cells.

The R1 subunit of herpes simplex virus ribonucleotide reductase protects cells against apoptosis at, or upstream of, caspase-8 activation

The R1 subunit of herpes simplex virus (HSV) ribonucleotide reductase, which in addition to its C-terminal reductase domain possesses a unique N-terminal domain of about 400 amino acids, is thought to have an additional, as yet unknown, function. Here, we report that the full-length HSV-2 R1 has an anti-apoptotic function able to protect cells against death triggered by expression of R1(Delta2-357), an HSV-2 R1 subunit with its first 357 amino acids deleted. We further substantiate the R1 anti-apoptotic activity by showing that its accumulation at low level could completely block apoptosis induced by TNF-receptor family triggering. Activation of caspase-8 induced either by TNF or by Fas ligand expression was prevented by the R1 protein. As HSV R1 did not inhibit cell death mediated by several agents acting via the mitochondrial pathway (Bax overexpression, etoposide, staurosporine and menadione), it is proposed that it functions to interrupt specifically death receptor-mediated signalling at, or upstream of, caspase-8 activation. The N-terminal domain on its own did not exhibit anti-apoptotic activity, suggesting that both domains of R1 or part(s) of them are necessary for this new function. Evidence for the importance of HSV R1 in protecting HSV-infected cells against cytokine-induced apoptosis was obtained with the HSV-1 R1 deletion mutants ICP6Delta and hrR3. These results show that, in addition to its ribonucleotide reductase function, which is essential for virus reactivation, HSV R1 could contribute to virus propagation by preventing apoptosis induced by the immune system.

Susceptibility of sensory neurons to apoptosis following infection by bovine herpesvirus type 1

Like other members of the alpha subfamily of herpesviruses, bovine herpesvirus type 1 (BHV-1) establishes latent infections in sensory neurons. BHV-1 induces apoptosis in lymphoid cells in vivo and in epithelial cell lines, but the ability of BHV-1 to induce apoptosis in sensory neurons remains unknown. In this report, the susceptibility of rabbit ganglionic neurons to infection by BHV-1 was examined in vitro and in vivo. Following infection of cultured neurons with BHV-1, hallmarks of apoptosis such as chromatin condensation, DNA fragmentation and membrane blebbing were detected. The appearance of these changes was preceded by active viral DNA replication as determined by in situ hybridization. When viral DNA replication was blocked by treatment of cultures with an inhibitor of eukaryotic DNA polymerases, apoptosis but not virus attachment to neurons or bICP0 gene expression was completely prevented. Taken together, these results demonstrate that sensory neurons are not intrinsically resistant to BHV-1-induced apoptosis and that viral DNA replication plays a role in triggering the apoptotic programme. Infection of rabbits with BHV-1 resulted in pathological changes in the trigeminal ganglia (TG) which included mononuclear cell infiltration and neuronophagia. Morphological evidence of apoptosis was not detected in neurons, even in cells with advanced cytophatology. Furthermore, whereas DNA fragmentation was common in infiltrating cells, it was very rare and sporadic in neurons. Therefore, mechanisms in the TG should exist to prevent neuronal apoptosis upon BHV-1 infection.

Herpes simplex virus type 1 origins of DNA replication play no role in the regulation of flanking promoters

Herpes simplex virus (HSV) exhibits altered gene regulation in neuronal compared to nonneuronal tissues. It has been hypothesized that initiation of DNA synthesis at the viral origins of replication (oriS and oriL) is a critical step in the upregulation of transcriptional activity of flanking divergent promoters, thereby increasing productive gene expression in neurons. Notably, oriS is flanked by the immediate-early (IE) ICP4 and ICP22/47 promoters, and oriL is flanked by the early (E) UL29 and UL30 promoters. To test this hypothesis further, a series of constructs were generated in which these promoters were placed upstream of luciferase genes. In addition, DNA replication origins were deleted in the context of these promoter constructs. All cassettes were recombined into the viral genome of HSV type 1 strain KOS at a site distal to its native origins. Recombinant reporter expression was monitored in vitro and in vivo to determine the role of viral origins of DNA replication in the regulation of their flanking promoters. Reporter gene expression was unaffected by the presence or absence of oriS or oriL, with the exception of a twofold increase in ICP22/47 promoter activity in the absence of oriS. DNA synthesis inhibitors resulted in a decrease of both IE- and E-promoter activity in primary cells but not continuous cell cultures. Reporter activity was readily assayed in vivo during acute infection and reactivation from latency and was also sensitive to DNA synthesis inhibitors. In all assays, reporter gene expression was unaffected by the presence or absence of either oriS or oriL. These data support the requirement of DNA synthesis for full viral gene expression in vivo but suggest that the origin elements play no role in the regulation of their flanking promoters.

Latency associated promoter transgene expression in the central nervous system after stereotaxic delivery of replication-defective HSV-1-based vectors

The herpes simplex virus type 1 (HSV-1) latency associated promoter (LAP) has been shown to sustain long-term reporter gene expression within sensory neurones. Its activity within the CNS is, however, less well understood. In this study we characterise the activity of the LAP after stereotaxic delivery of recombinant HSV-1-based vectors to the brain. Two classes of vectors were utilised in these studies: (1) a replication-defective vector lacking the glycoprotein H and thymidine kinase genes, designated CS1, and (2) a virus mutant severely impaired for immediate-early (IE) gene expression which lacks functional VP16, ICP4 and ICP0 genes, designated in1388. Both vectors contain the LacZ gene under the control of the LAP. Following delivery of either vector to the striatum, beta-gal expression was detected within anatomically related CNS regions distal to the site of injection. At these sites the number of beta-gal-positive cells increased with time and remained stable up to 4 weeks p.i. beta-Gal expression could not be detected at the site of injection after delivery of CS1 but beta-gal expression within neurones located at this site was observed after delivery of in1388, indicating reduced toxicity of this severely disabled virus. Transgene expression decreased dramatically with both vectors at later time-points (>4 weeks after delivery), but PCR analysis demonstrated that viral genomes were stably maintained for up to 180 days following delivery, indicating that the loss of beta-gal-positive neurones was not likely to be due to a loss of vector-transduced cells. Moreover, after delivery of an equivalent virus to the rat striatum in situ hybridisation analysis showed a similar decrease in the number of neurones expressing the endogenous LATs with time. These data indicate that although the HSV-1 LAP can drive the expression of foreign genes in a variety of CNS neurones, in these cells there is a slow down-regulation of the viral promoter which eventually results in the loss of detectable transgene expression.

ICP0, ICP4, or VP16 expressed from adenovirus vectors induces reactivation of latent herpes simplex virus type 1 in primary cultures of latently infected trigeminal ganglion cells

In a previous study, we demonstrated that infected-cell polypeptide 0 (ICP0) is necessary for the efficient reactivation of herpes simplex virus type 1 (HSV-1) in primary cultures of latently infected trigeminal ganglion (TG) cells (W. P. Halford and P. A. Schaffer, J. Virol. 75:3240-3249, 2001). The present study was undertaken to determine whether ICP0 is sufficient to trigger HSV-1 reactivation in latently infected TG cells. To test this hypothesis, replication-defective adenovirus vectors that express wild-type and mutant forms of ICP0 under the control of a tetracycline response element (TRE) promoter were constructed. Similar adenovirus vectors encoding wild-type ICP4, wild-type and mutant forms of the HSV-1 origin-binding protein (OBP), and wild-type and mutant forms of VP16 were also constructed. The TRE promoter was induced by coinfection of Vero cells with the test vector and an adenovirus vector that expresses the reverse tetracycline-regulated transactivator in the presence of doxycycline. Northern blot analysis demonstrated that transcription of the OBP gene in the adenovirus expression vector increased as a function of doxycycline concentration over a range of 0.1 to 10 microM. Likewise, Western blot analysis demonstrated that addition of 3 microM doxycycline to adenovirus vector-infected Vero cells resulted in a 100-fold increase in OBP expression. Wild-type forms of ICP0, ICP4, OBP, and VP16 expressed from adenovirus vectors were functional based on their ability to complement plaque formation in Vero cells by replication-defective HSV-1 strains with mutations in these genes. Adenovirus vectors that express wild-type forms of ICP0, ICP4, or VP16 induced reactivation of HSV-1 in 86% +/- 5%, 86% +/- 5%, and 97% +/- 5% of TG cell cultures, respectively (means +/- standard deviations). In contrast, vectors that express wild-type OBP or mutant forms of ICP0, OBP, or VP16 induced reactivation in 5% +/- 5%, 8% +/- 0%, 0% +/- 0%, and 13% +/- 6% of TG cell cultures, respectively. In control infections, an adenovirus vector expressed green fluorescent protein efficiently in TG neurons but did not induce HSV-1 reactivation. Therefore, expression of ICP0, ICP4, or VP16 is sufficient to induce HSV-1 reactivation in latently infected TG cell cultures. We conclude that this system provides a powerful tool for determining which cellular and viral proteins are sufficient to induce HSV-1 reactivation from neuronal latency.

Purification of a bacterially expressed herpes simplex virus type 1 origin binding protein for use in posttranslational processing studies

The origin binding protein (OBP) encoded by the UL9 open reading frame of herpes simplex virus type 1 (HSV-1) plays an essential role in productive infection by promoting the initiation of viral DNA synthesis. In this study, OBP was inducibly expressed in Escherichia coli and purified to homogeneity using a two-step chromatographic separation procedure. The properties of this recombinant OBP (rOBP) were found to be indistinguishable from those of the virus-encoded protein. Since rOBP was synthesized in bacterial cells, it lacked the posttranslational processing which normally occurs in OBP produced in HSV-1-infected mammalian cells and could therefore be exploited in experiments which addressed the effects of protein modification on OBP function. As an initial study, the impact of phosphorylation on enzymatic activity was examined using rOBP which had been treated with a panel of purified cellular kinases. rOBP was found to act as a substrate for nearly all of the kinases tested in (32)P-labeled phosphate transfer assays. However, only phosphorylation by protein kinase A (PKA, or cAMP-dependent protein kinase) was shown to significantly alter the enzymatic properties of rOBP, as it increased by five- to eightfold the ATPase activity associated with this protein. Activation of this critical viral DNA replication enzyme by a cAMP-dependent kinase such as PKA may be of some relevance in the natural course of HSV-1 infections, since reactivation of latent virus is thought to involve both signal transduction events and the induction of viral DNA synthesis. Thus, the expression and purification strategy outlined in this work provides an economical source of unmodified HSV-1 OBP that should prove useful in future in vitro studies.

Infection of human NT2 cells and differentiated NT-neurons with herpes simplex virus and replication-incompetent herpes simplex virus vectors

The human embryonal carcinoma cell line NT2 differentiates irreversibly into postmitotic neuron-like cells following treatment with retinoic acid. These differentiated NT-neurons resemble central nervous system (CNS) neurons and are characterized by development of dendrites and axons and the expression of neuron-specific markers. Because of their unique biological characteristics, NT-neurons were investigated for their utility as a system for studying the replication of herpes simplex virus (HSV) in the neuron and for evaluating characteristics of HSV vectors designed for gene delivery to the neuron. Virus replication in differentiated NT-neurons was significantly reduced and delayed relative to replication in undifferentiated NT2 cells. Replication of thymidine-kinase (tk) deficient HSV was further impaired in NT-neurons, reflecting the behavior of tk-negative virus in primary neurons in vitro and ganglia in vivo. Furthermore, replication-incompetent HSV vectors were capable of infecting NT-neurons, expressing a foreign gene, and persisting in a recoverable state for at least 2 weeks following delivery. These results suggest that differentiated NT-neurons can provide a continuous source of human, post-mitotic neurons-like cells for the study of HSV biology and HSV vector development.

Phosphorylation of the herpes simplex virus type 1 origin binding protein

The herpes simplex virus type 1 (HSV-1) origin binding protein (OBP), the product of the UL9 gene, is one of seven HSV-encoded proteins required for viral DNA replication. OBP performs multiple functions characteristic of a DNA replication initiator protein, including origin-specific DNA binding and ATPase and helicase activities, as well as the ability to interact with viral and cellular proteins involved in DNA replication. Replication initiator proteins in other systems, including those of other DNA viruses, are known to be regulated by phosphorylation; however, the role of phosphorylation in OBP function has been difficult to assess due to the low level of OBP expression in HSV-infected cells. Using a metabolic labeling and immunoprecipitation approach, we obtained evidence that OBP is phosphorylated during HSV-1 infection. Kinetic analysis of metabolically labeled cells indicated that the levels of OBP expression and phosphorylation increased at approximately 4 h postinfection. Notably, when expressed from a transfected plasmid, a recombinant baculovirus, or a recombinant adenovirus (AdOBP), OBP was phosphorylated minimally, if at all. In contrast, superinfection of AdOBP-infected cells with an OBP-null mutant virus increased the level of OBP phosphorylation approximately threefold, suggesting that HSV-encoded viral or HSV-induced cellular factors enhance the level of OBP phosphorylation. Using HSV mutants inhibited at sequential stages of the viral life cycle, we demonstrated that this increase in OBP phosphorylation is dependent on early protein synthesis and is independent of viral DNA replication. Based on gel mobility shift assays, phosphorylation does not appear to affect the ability of OBP to bind to the HSV origins.

A dominant-negative herpesvirus protein inhibits intranuclear targeting of viral proteins: effects on DNA replication and late gene expression

The d105 dominant-negative mutant form of the herpes simplex virus 1 (HSV-1) single-stranded DNA-binding protein, ICP8 (d105 ICP8), inhibits wild-type viral replication, and it blocks both viral DNA replication and late gene transcription, although to different degrees (M. Gao and D. M. Knipe, J. Virol. 65:2666-2675, 1991; Y. M. Chen and D. M. Knipe, Virology 221:281-290, 1996). We demonstrate here that this protein is also capable of preventing the formation of intranuclear prereplicative sites and replication compartments during HSV infection. We defined three patterns of ICP8 localization using indirect immunofluorescence staining of HSV-1-infected cells: large replication compartments, small compartments, and no specific intranuclear localization of ICP8. Cells that form large replication compartments replicate viral DNA and express late genes. Cells that form small replication compartments replicate viral DNA but do not express late genes, while cells without viral replication compartments are incapable of both DNA replication and late gene expression. The d105 ICP8 protein blocks formation of prereplicative sites and large replication compartments in 80% of infected cells and formation of large replication compartments in the remaining 20% of infected cells. The phenotype of d105 suggests a correlation between formation of large replication compartments and late gene expression and a role for intranuclear rearrangement of viral DNA and bound proteins in activation of late gene transcription. Thus, these results provide evidence for specialized machinery for late gene expression within replication compartments.

Herpes simplex virus type 1 ICP0 protein does not accumulate in the nucleus of primary neurons in culture

Infected-cell protein 0 (ICP0), the product of the herpes simplex virus (HSV) immediate-early (IE) alpha0 gene, is a promiscuous transactivator of viral early (E) and late (L) gene expression. HSV mutants lacking ICP0 function are severely deficient in viral growth and protein synthesis, particularly at low multiplicities of infection. Early in the infectious process in vitro, ICP0 protein accumulates in distinct domains within the nucleus to form characteristic structures active in the transcription of viral genes. However, following infection of primary trigeminal ganglion cells in vitro with a recombinant HSV mutant that expresses only ICP0, we observed that ICP0 protein accumulated in the characteristic intranuclear distribution only in the nuclei of Schwann cells; neurons in the culture did not accumulate ICP0 despite expression of ICP0 RNA in those cells. The same phenomenon was observed in PC12 cells differentiated to assume a neuronal phenotype. In primary neurons in culture, the amount of ICP0 protein could be increased by pharmacologic inhibition of calcium-activated protease (calpain) activity or by inhibition of protein phosphatase 2B (calcineurin). The failure of ICP0 protein to accumulate in the nucleus of neurons suggests that one mechanism which may impair efficient replication of the virus in neurons, and thus favor the establishment of viral latency in those cells, may be found in the cell-specific processing of that IE gene product.

Neural precursor cells for delivery of replication-conditional HSV-1 vectors to intracerebral gliomas

Cellular delivery of a replication-conditional herpes simplex virus type 1 (HSV-1) vector provides a means for gene therapy of invasive tumor cells. LacZ-bearing neural precursor cells, which can migrate and differentiate in the brain, were infected with a ribonucleotide reductase-deficient HSV-1 mutant virus (rRp450) that replicates only in dividing cells. Replication of rRp450 in neural precursor cells was blocked prior to implantation into the tumor by growth arrest in late G1 phase through treatment with mimosine. Viral titers in the medium of mimosine-treated, rRp450-infected neural precursor cells were below detection levels 3 days after infection. In culture, after removal of mimosine and passaging, cells resumed growth and replication of rRp450 so that, 7 days later, virus was present in the medium and cell death was evident. Mimosine-treated neural precursor cells injected into established intracerebral CNS-1 gliomas in nude mice migrated extensively throughout the tumor and into the surrounding parenchyma beyond the tumor over 3 days. Mimosine-treated neural precursor cells, infected with rRp450 and injected into intracerebral CNS-1 tumors, also migrated within the tumor with the appearance of foci of HSV-thymidine kinase-positive (TK+) cells, presumably including tumor cells, distributed throughout the tumor and in the surrounding parenchyma over a similar period. This migratory cell delivery method has the potential to expand the range of delivery of HSV-1 vectors to tumor cells in the brain.

Roscovitine, a specific inhibitor of cellular cyclin-dependent kinases, inhibits herpes simplex virus DNA synthesis in the presence of viral early proteins

We have previously shown that two inhibitors specific for cellular cyclin-dependent kinases (cdks), Roscovitine (Rosco) and Olomoucine (Olo), block the replication of herpes simplex virus (HSV). Based on these results, we demonstrated that HSV replication requires cellular cdks that are sensitive to these drugs (L. M. Schang, J. Phillips, and P. A. Schaffer. J. Virol. 72:5626-5637, 1998). We further established that at least two distinct steps in the viral replication cycle require cdks: transcription of immediate-early (IE) genes and transcription of early (E) genes (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Since Rosco inhibits HSV replication efficiently even when added to infected cells at 6 h postinfection, we postulated that cdks may also be required for viral functions that occur after E gene expression. In the study presented herein, we tested this hypothesis directly by measuring the efficiency of viral replication, viral DNA synthesis, and expression of several viral genes during infections in which Rosco was added after E proteins had already been synthesized. Rosco inhibited HSV replication, and specifically viral DNA synthesis, when the drug was added at the time of release from a 12-h phosphonoacetic acid (PAA)-induced block in viral DNA synthesis. Inhibition of DNA synthesis was not a consequence of inhibition of expression of IE or E genes in that Rosco had no effect on steady-state levels of two E transcripts under the same conditions in which it inhibited viral DNA synthesis. Moreover, viral DNA synthesis was inhibited by Rosco even in the absence of protein synthesis. In a second series of experiments, the replication of four HSV mutants harboring temperature-sensitive mutations in genes essential for viral DNA replication was inhibited when Rosco was added at the time of shift-down from the nonpermissive to the permissive temperature. Viral DNA synthesis was inhibited by Rosco under these conditions, whereas expression of viral E genes was not affected. We conclude that cellular Rosco-sensitive cdks are required for replication of viral DNA in the presence of viral E proteins. This requirement may indicate that HSV DNA synthesis is functionally linked to transcription, which requires cdks, or that both viral transcription and DNA replication, independently, require viral or cellular factors activated by Rosco-sensitive cdks.

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.

The polyserine tract of herpes simplex virus ICP4 is required for normal viral gene expression and growth in murine trigeminal ganglia

ICP4 of herpes simplex virus (HSV) is essential for productive infection due to its central role in the regulation of HSV transcription. This study identified a region of ICP4 that is not required for viral growth in culture or at the periphery of experimentally inoculated mice but is critical for productive growth in the trigeminal ganglia. This region of ICP4 encompasses amino acids 184 to 198 and contains 13 nearly contiguous serine residues that are highly conserved among the alphaherpesviruses. A mutant in which this region is deleted (DeltaSER) was able to grow on the corneas of mice and be transported back to the trigeminal ganglia. DeltaSER did not grow in the trigeminal ganglia but did express low levels of several immediate-early (ICP4 and ICP27) and early (thymidine kinase [tk] and UL42) genes. It expressed very low levels of the late gC gene and did not appear to replicate DNA. This pattern of gene expression was similar to that observed for a tk mutant, dlsptk. Both DeltaSER and dlsptk expressed higher levels of the latency-associated transcript (LAT) per genome earlier in infected ganglia than did the wild-type virus, KOS. However, infected ganglia from all three viruses accumulated the same level of LAT per genome at 30 days postinfection (during latency). The data suggest that the polyserine tract of ICP4 provides an activity that is required for lytic infection in ganglia to progress to viral DNA synthesis and full lytic gene expression. In the absence of this activity, higher levels of LAT per genome accumulate earlier in infection than with wild-type virus.

Accumulation of viral transcripts and DNA during establishment of latency by herpes simplex virus

Latent infection of mice with wild-type herpes simplex virus is established during an acute phase of ganglionic infection in which there is abundant viral replication and productive-cycle gene expression. Thymidine kinase-negative mutants establish latent infections but are severely impaired for acute ganglionic replication and productive-cycle gene expression. Indeed, by in situ hybridization assays, acute infection by these mutants resembles latency. To assess events during establishment of latency by wild-type and thymidine kinase-negative viruses, we quantified specific viral nucleic acid sequences in mouse trigeminal ganglia during acute ganglionic infection by using sensitive PCR-based assays. Through 32 h postinfection, viral DNA and transcripts representative of the three kinetic classes of productive-cycle genes accumulated to comparable levels in wild-type- and mutant-infected ganglia. At 48 and 72 h, although latency-associated transcripts accumulated to comparable levels in ganglia infected with wild-type or mutant virus, levels of DNA accumulating in wild-type-infected ganglia exceeded those in mutant-infected ganglia by 2 to 3 orders of magnitude. Coincident with this increase in DNA, wild-type-infected ganglia exhibited abundant expression of productive-cycle genes and high titers of infectious progeny. Nevertheless, the levels of productive-cycle RNAs expressed by mutant virus during acute infection greatly exceeded those expressed by wild-type virus during latency. The results thus distinguish acute infection of ganglia by a replication-compromised mutant from latent infection and may have implications for mechanisms of latency.

An intertypic herpes simplex virus helicase-primase complex associated with a defect in neurovirulence has reduced primase activity

R13-1 is an intertypic recombinant virus in which the left-hand 18% of the herpes simplex virus type 1 (HSV-1) genome is replaced by homologous sequences from HSV-2. R13-1 is nonneurovirulent and defective in DNA replication in neurons. The defect was localized to the UL5 open reading frame by using marker rescue analysis (D. C. Bloom and J. G. Stevens, J. Virol. 68:3761-3772, 1994). To provide conclusive evidence that UL5 is the only HSV-2 gene involved in the restricted replication phenotype of R13-1, we have characterized the phenotype of a recombinant virus (IB1) in which only the UL5 gene of HSV-1 was replaced by HSV-2 UL5. Data from 50% lethal dose determinations and the in vivo yields of virus suggested that IB1 has the same phenotypic characteristics as R13-1. UL5 is the helicase component of a complex with helicase and primase activities. All three subunits of this complex (UL5, UL8, and UL52) are required for viral DNA replication in all cell types. The intertypic complex HSV-2 UL5-HSV-1 UL8-HSV-1 UL52 was purified and biochemically characterized. The primase activity of the intertypic complex was 10-fold lower than that of HSV-1 UL5-HSV-1 UL8-HSV-1 UL52. The ATPase activity was comparable to that of the HSV-1 enzyme complex, and although the helicase activity was threefold lower, this did not interfere with the synthesis of leading strands by the HSV polymerase. One explanation for these findings is that the interactions between the subunits of the helicase-primase intertypic complex that are important for the full function of each subunit are inappropriate or weak.

Acyclovir blocks cytokine gene expression in trigeminal ganglia latently infected with herpes simplex virus type 1

We have previously found that interleukin (IL)-2, IL-10, interferon (IFN)-gamma, RANTES, and tumor necrosis factor (TNF)-alpha mRNA transcription remain elevated in the trigeminal ganglia (TG) of herpes simplex virus type 1 (HSV-1) latently infected mice up to 120 days postinoculation (p.i.). To determine if this phenomenon was dependent on HSV-1 DNA replication after the establishment of latency (i.e., reactivation), cytokine gene expression was compared in TG of acyclovir-treated and untreated latently infected mice. Oral acyclovir treatment (begun 16 days p.i.) had no effect on serum levels of total anti-HSV-1 antibodies. However, there was a significant reduction in the titer of antibody specific for glycoprotein D and glycoprotein B but not glycoprotein H/L 120 days PI in the acyclovir-treated compared to vehicle-treated mice. These differences were not significant at earlier time points (i.e., days 34 and 60 p.i.). Consistent with these findings, acyclovir had no effect on cytokine gene expression in latently infected TG 35 and 60 days p.i. However, 120 days p.i., IFN-gamma and TNF-alpha mRNA were approaching baseline levels in TG of acyclovir-treated mice, but remained significantly elevated in untreated controls (i.e., IFN-gamma mRNA levels were sixfold higher in TG of untreated mice). Therefore, viral DNA replication appears to provide an antigenic stimulus for persistent cytokine gene expression in latently infected TG.

Innate and acquired immunity to herpes simplex virus type 1

Immunization with heat-inactivated herpes simplex virus type 1 (HSV-1) 2-5 days before ocular infection reduced the frequency of establishment of latent HSV-1 infection in the trigeminal ganglion (TG); this induction of resistance coincided with reduced expression of IFN-gamma mRNA in the TG. Immunization with unrelated antigens was not protective. In part, this resistance to nervous system invasion correlated with the appearance of serum antibody to HSV-1. Immunization reduced viral replication in the eye and trigeminal ganglion, and prevented HSV-1 spread to the cerebellum. IFN-gamma was detected in immunized mice 4 days postocular infection as determined by plaque reduction using neutralizing Ab to IFN-alpha/beta and IFN-gamma. Injection of antibody (Ab) to IFN-alpha/beta and IFN-gamma administered at the time of immunization did not affect survival. Anti-IFN-gamma-treated mice had significantly reduced levels of IFN in their serum. Treatment with anti-IFN-alpha/beta Ab resulted in an elevation in viral replication as determined by the expression of latency associated transcripts in the TG of mice. Likewise, there was a significant increase in the CD8, IL-12 (p40), and TNF-alpha mRNA levels in the TG of the anti-IFN-alpha/beta-treated mice TG explant cultures demonstrated that viral load was significantly increased in the TG of anti-IFN-alpha/beta-treated mice relative to TG of control mice 7 days after infection. The results suggest that exposure to viral antigens 2-5 days before infection is an important determinant of the extent of HSV-1 spread to the nervous system. Moreover, the data suggest that both an antibody response and IFN-alpha/beta play a role in limiting the progress of infection from the peripheral tissues to the central nervous system.

Gene expression during reactivation of herpes simplex virus type 1 from latency in the peripheral nervous system is different from that during lytic infection of tissue cultures

Herpes simplex virus (HSV) replicates in peripheral tissues and forms latent infections in neurons of the peripheral nervous system. It can be reactivated from latency by various stimuli to cause recurrent disease. During lytic infection in tissue culture cells, there is a well-described temporal pattern of (i) immediate-early, (ii) early, and (iii) late gene expression. However, latency is characterized by little if any expression of genes of the lytic cycle of infection. During reactivation, the pattern of gene expression is presumed to be similar to that during the lytic cycle in tissue culture, though recent work of W. P. Halford et al. (J. Virol. 70:5051-5060, 1996) and P. F. Nichol et al. (J. Virol. 70:5476-5486, 1996) suggests that it is modified in neuronal cell cultures. We have used the mouse trigeminal ganglion explant model and reverse transcription-PCR to determine the pattern of viral and cellular gene expression during reactivation. Surprisingly, the pattern of viral gene expression during lytic infection of cell cultures is not seen during reactivation. During reactivation, early viral transcripts were detected before immediate-early transcripts. The possibility that a cellular factor upregulates early genes during the initial reactivation stimulus is discussed.