Herpes simplex virus immediate-early protein ICP22 is required for viral modification of host RNA polymerase II and establishment of the normal viral transcription program

The opportunities and challenges of epigenetic approaches to manage herpes simplex infections

INTRODUCTION

Despite the existence of antivirals that potently and efficiently inhibit the replication of herpes simplex virus 1 and 2 (HSV-1, -2), their ability to establish and maintain, and reactivate from, latency has precluded the development of curative therapies. Several groups are exploring the opportunities of targeting epigenetic regulation to permanently silence latent HSV genomes or induce their simultaneous reactivation in the presence of antivirals to flush the latent reservoirs, as has been explored for HIV.

AREAS COVERED

This review covers the basic principles of epigenetic regulation with an emphasis on those mechanisms relevant to the regulation of herpes simplex viruses, as well as the current knowledge on the regulation of lytic infections and the establishment and maintenance of, and reactivation from, latency, with an emphasis on epigenetic regulation. The differences with the epigenetic regulation of viral and cellular gene expression are highlighted as are the effects of known epigenetic regulators on herpes simplex viruses. The major limitations of current models to the development of novel antiviral strategies targeting latency are highlighted.

EXPERT OPINION

We provide an update on the epigenetic regulation during lytic and latent HSV-1 infection, highlighting the commonalities and differences with cellular gene expression and the potential of epigenetic drugs as antivirals, including the opportunities, challenges, and potential future directions.

Impact of the interaction between herpes simplex virus 1 ICP22 and FACT on viral gene expression and pathogenesis

Facilitates chromatin transcription (FACT) interacts with nucleosomes to promote gene transcription by regulating the dissociation and reassembly of nucleosomes downstream and upstream of RNA polymerase II (Pol II). A previous study reported that herpes simplex virus 1 (HSV-1) regulatory protein ICP22 interacted with FACT and was required for its recruitment to the viral DNA genome in HSV-1-infected cells. However, the biological importance of interactions between ICP22 and FACT in relation to HSV-1 infection is unclear. Here, we mapped the minimal domain of ICP22 required for its efficient interaction with FACT to a cluster of five basic amino acids in ICP22. A recombinant virus harboring alanine substitutions in this identified cluster led to the decreased accumulation of viral mRNAs from UL54, UL38, and UL44 genes, reduced Pol II occupancy of these genes in MRC-5 cells, and impaired HSV-1 virulence in mice following ocular or intracranial infection. Furthermore, the treatment of mice infected with wild-type HSV-1 with CBL0137, a FACT inhibitor currently being investigated in clinical trials, significantly improved the survival rate of mice. These results suggested that the interaction between ICP22 and FACT was required for efficient HSV-1 gene expression and pathogenicity. Therefore, FACT might be a potential therapeutic target for HSV-1 infection.IMPORTANCEICP22 is a well-known regulatory factor of HSV-1 gene expression, but its mechanism(s) are poorly understood. Although the interaction of FACT with ICP22 was reported previously, its significance in HSV-1 infection is unknown. Given that FACT is involved in gene transcription, it is of interest to investigate this interaction as it relates to HSV-1 gene expression. To determine a direct link between the interaction and HSV-1 infection, we mapped a minimal domain of ICP22 required for its efficient interaction with FACT and generated a recombinant virus carrying mutations in the identified domain. Using the recombinant virus, we obtained evidence suggesting that the interaction between ICP22 and FACT promoted Pol II transcription from HSV-1 genes and viral virulence in mice. In addition, CBL0137, an inhibitor of FACT, effectively protected mice from lethal HSV-1 infection, suggesting FACT might be a potential target for the development of novel anti-HSV drugs.

Aberrant RNA polymerase initiation and processivity on the genome of a herpes simplex virus 1 mutant lacking ICP27

Within the first 15 minutes of infection, herpes simplex virus 1 immediate early proteins repurpose cellular RNA polymerase (Pol II) for viral transcription. An important role of the viral-infected cell protein 27 (ICP27) is to facilitate viral pre-mRNA processing and export viral mRNA to the cytoplasm. Here, we use precision nuclear run-on followed by deep sequencing (PRO-seq) to characterize transcription of a viral ICP27 null mutant. At 1.5 and 3 hours post infection (hpi), we observed increased total levels of Pol II on the mutant viral genome and accumulation of Pol II downstream of poly A sites indicating increased levels of initiation and processivity. By 6 hpi, Pol II accumulation on specific mutant viral genes was higher than that on wild-type virus either at or upstream of poly A signals, depending on the gene. The PRO-seq profile of the ICP27 mutant on late genes at 6 hpi was similar but not identical to that caused by treatment with flavopiridol, a known inhibitor of RNA processivity. This pattern was different from PRO-seq profiles of other α gene mutants and upon inhibition of viral DNA replication with PAA. Together, these results indicate that ICP27 contributes to the repression of aberrant viral transcription at 1.5 and 3 hpi by inhibiting initiation and decreasing RNA processivity. However, ICP27 is needed to enhance processivity on most late genes by 6 hpi in a mechanism distinguishable from its role in viral DNA replication.IMPORTANCEWe developed and validated the use of a processivity index for precision nuclear run-on followed by deep sequencing data. The processivity index calculations confirm infected cell protein 27 (ICP27) induces downstream of transcription termination on certain host genes. The processivity indices and whole gene probe data implicate ICP27 in transient immediate early gene-mediated repression, a process that also requires ICP4, ICP22, and ICP0. The data indicate that ICP27 directly or indirectly regulates RNA polymerase (Pol II) initiation and processivity on specific genes at specific times post infection. These observations support specific and varied roles for ICP27 in regulating Pol II activity on viral genes in addition to its known roles in post transcriptional mRNA processing and export.

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.

The HSV-1 ICP22 protein selectively impairs histone repositioning upon Pol II transcription downstream of genes

Herpes simplex virus 1 (HSV-1) infection and stress responses disrupt transcription termination by RNA Polymerase II (Pol II). In HSV-1 infection, but not upon salt or heat stress, this is accompanied by a dramatic increase in chromatin accessibility downstream of genes. Here, we show that the HSV-1 immediate-early protein ICP22 is both necessary and sufficient to induce downstream open chromatin regions (dOCRs) when transcription termination is disrupted by the viral ICP27 protein. This is accompanied by a marked ICP22-dependent loss of histones downstream of affected genes consistent with impaired histone repositioning in the wake of Pol II. Efficient knock-down of the ICP22-interacting histone chaperone FACT is not sufficient to induce dOCRs in ΔICP22 infection but increases dOCR induction in wild-type HSV-1 infection. Interestingly, this is accompanied by a marked increase in chromatin accessibility within gene bodies. We propose a model in which allosteric changes in Pol II composition downstream of genes and ICP22-mediated interference with FACT activity explain the differential impairment of histone repositioning downstream of genes in the wake of Pol II in HSV-1 infection.

Cellular state landscape and herpes simplex virus type 1 infection progression are connected

Prediction, prevention and treatment of virus infections require understanding of cell-to-cell variability that leads to heterogenous disease outcomes, but the source of this heterogeneity has yet to be clarified. To study the multimodal response of single human cells to herpes simplex virus type 1 (HSV-1) infection, we mapped high-dimensional viral and cellular state spaces throughout the infection using multiplexed imaging and quantitative single-cell measurements of viral and cellular mRNAs and proteins. Here we show that the high-dimensional cellular state scape can predict heterogenous infections, and cells move through the cellular state landscape according to infection progression. Spatial information reveals that infection changes the cellular state of both infected cells and of their neighbors. The multiplexed imaging of HSV-1-induced cellular modifications links infection progression to changes in signaling responses, transcriptional activity, and processing bodies. Our data show that multiplexed quantification of responses at the single-cell level, across thousands of cells helps predict infections and identify new targets for antivirals.

HSV: The scout and assault for digestive system tumors

More than 25% of all malignant tumors are digestive system tumors (DSTs), which mostly include esophageal cancer, gastric cancer, pancreatic cancer, liver cancer, gallbladder cancer and cholangiocarcinoma, and colorectal cancer. DSTs have emerged as one of the prominent reasons of morbidity and death in many nations and areas around the world, posing a serious threat to human life and health. General treatments such as radiotherapy, chemotherapy, and surgical resection can poorly cure the patients and have a bad prognosis. A type of immunotherapy known as oncolytic virus therapy, have recently shown extraordinary anti-tumor effectiveness. One of the viruses that has been the subject of the greatest research in this field, the herpes simplex virus (HSV), has shown excellent potential in DSTs. With a discussion of HSV-1 based on recent studies, we outline the therapeutic effects of HSV on a number of DSTs in this review. Additionally, the critical function of HSV in the detection of cancers is discussed, and some HSV future possibilities are shown.

Herpes simplex virus type 2 inhibits TNF-α-induced NF-κB activation through viral protein ICP22-mediated interaction with p65

Herpes simplex virus type 2 (HSV-2) is a prevalent human pathogen and the main cause of genital herpes. After initial infection, HSV-2 can establish lifelong latency within dorsal root ganglia by evading the innate immunity of the host. NF-κB has a crucial role in regulating cell proliferation, inflammation, apoptosis, and immune responses. It is known that inhibition of NF-κB activation by a virus could facilitate it to establish infection in the host. In the current study, we found that HSV-2 inhibited TNF-α-induced activation of NF-κB-responsive promoter in a dose-dependent manner, while UV-inactivated HSV-2 did not have such capability. We further identified the immediate early protein ICP22 of HSV-2 as a vital viral element in inhibiting the activation of NF-κB-responsive promoter. The role of ICP22 was confirmed in human cervical cell line HeLa and primary cervical fibroblasts in the context of HSV-2 infection, showing that ICP22 deficient HSV-2 largely lost the capability in suppressing NF-κB activation. HSV-2 ICP22 was further shown to suppress the activity of TNF receptor-associated factor 2 (TRAF2)-, IκB kinase α (IKK α)-, IKK β-, IKK γ-, or p65-induced activation of NF-κB-responsive promoter. Mechanistically, HSV-2 ICP22 inhibited the phosphorylation and nuclear translocation of p65 by directly interacting with p65, resulting in the blockade of NF-κB activation. Furthermore, ICP22 from several alpha-herpesviruses could also inhibit NF-κB activation, suggesting the significance of ICP22 in herpesvirus immune evasion. Findings in this study highlight the importance of ICP22 in inhibiting NF-κB activation, revealing a novel mechanism by which HSV-2 evades the host antiviral responses.

Pseudorabies Virus Infection Results in a Broad Inhibition of Host Gene Transcription

Pseudorabies virus (PRV) is a porcine alphaherpesvirus that belongs to the Herpesviridae family. We showed earlier that infection of porcine epithelial cells with PRV triggers activation of the nuclear factor κB (NF-κB) pathway, a pivotal signaling axis in the early immune response. However, PRV-induced NF-κB activation does not lead to NF-κB-dependent gene expression. Here, using electrophoretic mobility shift assays (EMSAs), we show that PRV does not disrupt the ability of NF-κB to interact with its κB target sites. Assessing basal cellular transcriptional activity in PRV-infected cells by quantitation of prespliced transcripts of constitutively expressed genes uncovered a broad suppression of cellular transcription by PRV, which also affects the inducible expression of NF-κB target genes. Host cell transcription inhibition was rescued when viral genome replication was blocked using phosphonoacetic acid (PAA). Remarkably, we found that host gene expression shutoff in PRV-infected cells correlated with a substantial retention of the NF-κB subunit p65, the TATA box binding protein, and RNA polymerase II-essential factors required for (NF-κB-dependent) gene transcription-in expanding PRV replication centers in the nucleus and thereby away from the host chromatin. This study reveals a potent mechanism used by the alphaherpesvirus PRV to steer the protein production capacity of infected cells to viral proteins by preventing expression of host genes, including inducible genes involved in mounting antiviral responses. IMPORTANCE Herpesviruses are highly successful pathogens that cause lifelong persistent infections of their host. Modulation of the intracellular environment of infected cells is imperative for the success of virus infections. We reported earlier that a DNA damage response in epithelial cells infected with the alphaherpesvirus pseudorabies virus (PRV) results in activation of the hallmark proinflammatory NF-κB signaling axis but, remarkably, that this activation does not lead to NF-κB-induced (proinflammatory) gene expression. Here, we report that PRV-mediated inhibition of host gene expression stretches beyond NF-κB-dependent gene expression and in fact reflects a broad inhibition of host gene transcription, which correlates with a substantial recruitment of essential host transcription factors in viral replication compartments in the nucleus, away from the host chromatin. These data uncover a potent alphaherpesvirus mechanism to interfere with production of host proteins, including proteins involved in antiviral responses.

The Nuclear DNA Sensor IFI16 Indiscriminately Binds to and Diminishes Accessibility of the HSV-1 Genome to Suppress Infection

Human cells identify invading pathogens and activate immune signaling pathways through a wide array of pattern recognition receptors, including DNA sensors. The interferon-inducible protein 16 (IFI16) is a nuclear DNA sensor that recognizes double-stranded DNA from a number of viral sources, including genomes of nuclear-replicating viruses. Among these is the prevalent human pathogen herpes simplex virus 1 (HSV-1). Upon binding to the HSV-1 DNA genome, IFI16 both induces antiviral cytokine expression and suppresses virus gene expression. Here, we used a multiomics approach of DNA sequencing techniques paired with targeted mass spectrometry to obtain an extensive view of the interaction between IFI16 and the HSV-1 genome and how this binding affects the viral DNA structure and protein expression. Through chromatin immunoaffinity purification coupled with next-generation DNA sequencing (ChIP-seq), we found that IFI16 binds to the HSV-1 genome in a sequence-independent manner while simultaneously exhibiting broad enrichment at two loci: UL30, the viral DNA polymerase gene, and US1 to US7. The assay for transposase-accessible chromatin with sequencing (ATAC-seq) revealed that these two regions are among the most accessible stretches of DNA on the genome, thereby facilitating IFI16 binding. Accessibility of the entire HSV-1 genome is elevated upon IFI16 knockout, indicating that expression of IFI16 globally induces chromatinization of viral DNA. Deletion of IFI16 also results in a global increase in the expression of HSV-1 proteins, as measured by parallel reaction monitoring-mass spectrometry of viral proteins representing 80% of the HSV-1 genome. Altogether, we demonstrate that IFI16 interacts with the HSV-1 genome in a sequence-independent manner, coordinating epigenetic silencing of the viral genome and decreasing protein expression and virus replication. IMPORTANCE Mammalian host defense against viral infection includes broad-acting cellular restriction factors, as well as effectors of intrinsic and innate immunity. IFI16 is a critical nuclear host defense factor and intrinsic immune protein involved in binding viral DNA genomes, thereby repressing the replication of nucleus-replicating viruses, including the human herpes simplex virus 1. What has remained unclear is where on the viral genome IFI16 binds and how binding affects both viral DNA structural accessibility and viral protein expression. Our study provides a global view of where and how a nuclear restriction factor of DNA viruses associates with viral genomes to exert antiviral functions during early stages of an acute virus infection. Our study can additionally serve as a systems-level model to evaluate nuclear DNA sensor interactions with viral genomes, as well as the antiviral outcomes of transcriptionally silencing pathogen-derived DNA.

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.

Manipulation of RNA polymerase III by Herpes Simplex Virus-1

RNA polymerase III (Pol III) transcribes noncoding RNA, including transfer RNA (tRNA), and is commonly targeted during cancer and viral infection. We find that Herpes Simplex Virus-1 (HSV-1) stimulates tRNA expression 10-fold. Perturbation of host tRNA synthesis requires nuclear viral entry, but not synthesis of specific viral transcripts. tRNA with a specific codon bias were not targeted—rather increased transcription was observed from euchromatic, actively transcribed loci. tRNA upregulation is linked to unique crosstalk between the Pol II and III transcriptional machinery. While viral infection results in depletion of Pol II on host mRNA promoters, we find that Pol II binding to tRNA loci increases. Finally, we report Pol III and associated factors bind the viral genome, which suggests a previously unrecognized role in HSV-1 gene expression. These findings provide insight into mechanisms by which HSV-1 alters the host nuclear environment, shifting key processes in favor of the pathogen.

RNA Polymerase III (Pol III) transcribes non-coding RNA, including tRNAs. Applying different RNA-Seq techniques, Dremel et al. provide the Pol III transcriptional landscape of Herpes simplex virus 1 (HSV-1) infected cells. Infection leads to an increase in tRNA expression from host euchromatin and Pol II re-localization to tRNA loci. They also find that Pol III – associated factors bind to the viral genome.

ICP22/IE63 Mediated Transcriptional Regulation and Immune Evasion: Two Important Survival Strategies for Alphaherpesviruses

In the process of infecting the host, alphaherpesviruses have derived a series of adaptation and survival strategies, such as latent infection, autophagy and immune evasion, to survive in the host environment. Infected cell protein 22 (ICP22) or its homologue immediate early protein 63 (IE63) is a posttranslationally modified multifunctional viral regulatory protein encoded by all alphaherpesviruses. In addition to playing an important role in the efficient use of host cell RNA polymerase II, it also plays an important role in the defense process of the virus overcoming the host immune system. These two effects of ICP22/IE63 are important survival strategies for alphaherpesviruses. In this review, we summarize the complex mechanism by which the ICP22 protein regulates the transcription of alphaherpesviruses and their host genes and the mechanism by which ICP22/IE63 participates in immune escape. Reviewing these mechanisms will also help us understand the pathogenesis of alphaherpesvirus infections and provide new strategies to combat these viral infections.

Cell Culture Evolution of a Herpes Simplex Virus 1 (HSV-1)/Varicella-Zoster Virus (VZV) UL34/ORF24 Chimeric Virus Reveals Novel Functions for HSV Genes in Capsid Nuclear Egress

Herpes simplex virus (HSV) and varicella-zoster virus (VZV) are both members of the alphaherpesvirus subfamily but belong to different genera. Substitution of the HSV-1 UL34 coding sequence with that of its VZV homolog, open reading frame 24 (ORF24), results in a virus that has defects in viral growth, spread, capsid egress, and nuclear lamina disruption very similar to those seen in a UL34-null virus despite normal interaction between ORF24 protein and HSV pUL31 and proper localization of the nuclear egress complex at the nuclear envelope. Minimal selection for growth in cell culture resulted in viruses that grew and spread much more efficiently that the parental chimeric virus. These viruses varied in their ability to support nuclear lamina disruption, normal nuclear egress complex localization, and capsid de-envelopment. Single mutations that suppress the growth defect were mapped to the coding sequences of ORF24, ICP22, and ICP4, and one virus carried single mutations in each of the ICP22 and US3 coding sequences. The phenotypes of these viruses support a role for ICP22 in nuclear lamina disruption and a completely unexpected role for the major transcriptional regulator, ICP4, in capsid nuclear egress. IMPORTANCE Interactions among virus proteins are critical for assembly and egress of virus particles, and such interactions are attractive targets for antiviral therapy. Identification of critical functional interactions can be slow and tedious. Capsid nuclear egress of herpesviruses is a critical event in the assembly and egress pathway and is mediated by two proteins, pUL31 and pUL34, that are conserved among herpesviruses. Here, we describe a cell culture evolution approach to identify other viral gene products that functionally interact with pUL34.

HSV-1 ICP22 Is a Selective Viral Repressor of Cellular RNA Polymerase II-Mediated Transcription Elongation

The Herpes Simplex Virus (HSV-1) immediate-early protein ICP22 interacts with cellular proteins to inhibit host cell gene expression and promote viral gene expression. ICP22 inhibits phosphorylation of Ser2 of the RNA polymerase II (pol II) carboxyl-terminal domain (CTD) and productive elongation of pol II. Here we show that ICP22 affects elongation of pol II through both the early-elongation checkpoint and the poly(A)-associated elongation checkpoint of a protein-coding gene model. Coimmunoprecipitation assays using tagged ICP22 expressed in human cells and pulldown assays with recombinant ICP22 in vitro coupled with mass spectrometry identify transcription elongation factors, including P-TEFb, additional CTD kinases and the FACT complex as interacting cellular factors. Using a photoreactive amino acid incorporated into ICP22, we found that L191, Y230 and C225 crosslink to both subunits of the FACT complex in cells. Our findings indicate that ICP22 interacts with critical elongation regulators to inhibit transcription elongation of cellular genes, which may be vital for HSV-1 pathogenesis. We also show that the HSV viral activator, VP16, has a region of structural similarity to the ICP22 region that interacts with elongation factors, suggesting a model where VP16 competes with ICP22 to deliver elongation factors to viral genes.

A Review of the Multipronged Attack of Herpes Simplex Virus 1 on the Host Transcriptional Machinery

During lytic infection, herpes simplex virus (HSV) 1 induces a rapid shutoff of host RNA synthesis while redirecting transcriptional machinery to viral genes. In addition to being a major human pathogen, there is burgeoning clinical interest in HSV as a vector in gene delivery and oncolytic therapies, necessitating research into transcriptional control. This review summarizes the array of impacts that HSV has on RNA Polymerase (Pol) II, which transcribes all mRNA in infected cells. We discuss alterations in Pol II holoenzymes, post-translational modifications, and how viral proteins regulate specific activities such as promoter-proximal pausing, splicing, histone repositioning, and termination with respect to host genes. Recent technological innovations that have reshaped our understanding of previous observations are summarized in detail, along with specific research directions and technical considerations for future studies.

Multifaceted Roles of ICP22/ORF63 Proteins in the Life Cycle of Human Herpesviruses

Herpesviruses are extremely successful parasites that have evolved over millions of years to develop a variety of mechanisms to coexist with their hosts and to maintain host-to-host transmission and lifelong infection by regulating their life cycles. The life cycle of herpesviruses consists of two phases: lytic infection and latent infection. During lytic infection, active replication and the production of numerous progeny virions occur. Subsequent suppression of the host immune response leads to a lifetime latent infection of the host. During latent infection, the viral genome remains in an inactive state in the host cell to avoid host immune surveillance, but the virus can be reactivated and reenter the lytic cycle. The balance between these two phases of the herpesvirus life cycle is controlled by broad interactions among numerous viral and cellular factors. ICP22/ORF63 proteins are among these factors and are involved in transcription, nuclear budding, latency establishment, and reactivation. In this review, we summarized the various roles and complex mechanisms by which ICP22/ORF63 proteins regulate the life cycle of human herpesviruses and the complex relationships among host and viral factors. Elucidating the role and mechanism of ICP22/ORF63 in virus-host interactions will deepen our understanding of the viral life cycle. In addition, it will also help us to understand the pathogenesis of herpesvirus infections and provide new strategies for combating these infections.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

The Herpes Simplex Virus 1 Immediate Early Protein ICP22 Is a Functional Mimic of a Cellular J Protein

Molecular chaperones and cochaperones are the most abundant cellular effectors of protein homeostasis, assisting protein folding and preventing aggregation of misfolded proteins. We have previously shown that herpes simplex virus 1 (HSV-1) infection results in the drastic spatial reorganization of the cellular chaperone Hsc70 into nuclear domains called VICE (Virus Induced Chaperone Enriched) domains and that this recruitment is dependent on the viral immediate early protein ICP22. Here, we present several lines of evidence supporting the notion that ICP22 functions as a virally encoded cochaperone (J-protein/Hsp40) functioning together with its Hsc70 partner to recognize and manage aggregated and misfolded proteins. We show that ICP22 results in (i) nuclear sequestration of nonnative proteins, (ii) reduction of cytoplasmic aggresomes in cells expressing aggregation-prone proteins, and (iii) thermoprotection against heat inactivation of firefly luciferase, and (iv) sequence homology analysis indicated that ICP22 contains an N-terminal J domain and a C-terminal substrate binding domain, similar to type II cellular J proteins. ICP22 may thus be functionally similar to J-protein/Hsp40 cochaperones that function together with their HSP70 partners to prevent aggregation of nonnative proteins. This is not the first example of a virus hijacking a function of a cellular chaperone, since simian immunodeficiency virus T antigen was previously shown to contain a J domain; however, this the first known example of the acquisition of a functional J-like protein by a virus and suggests that HSV has taken advantage of the adaptable nature of J proteins to evolve a multifunctional cochaperone that functions with Hsc70 to promote lytic infection.IMPORTANCE Viruses have evolved a variety of strategies to succeed in a hostile environment. The herpes simplex virus 1 (HSV-1) immediate early protein ICP22 plays several roles in the virus life cycle, including downregulation of cellular gene expression, upregulation of late viral gene expression, inhibition of apoptosis, prevention of aggregation of nonnative proteins, and the recruitment of a cellular heat shock protein, Hsc70, to nuclear domains. We present evidence that ICP22 functionally resembles a cellular J-protein/HSP40 family cochaperone, interacting specifically with Hsc70. We suggest that HSV has taken advantage of the adaptable nature of J proteins to evolve a multifunctional cochaperone that functions with Hsc70 to promote lytic infection.

MiR-155-5p modulates HSV-1 replication via the epigenetic regulation of SRSF2 gene expression

A previous study reported that miR-155-5p knockout mice were more resistant to herpes simplex virus type I (HSV-1) infection. However, the exact underlying molecular mechanism remains to be elucidated. Here, we demonstrated that HSV-1 infection upregulates miR-155-5p expression. By binding to the promoter of serine/arginine-rich splicing factor 2 (SRSF2), which is an important transcriptional activator of HSV-1 genes that was previously reported by our group, and altering the histone modification located near the transcription start site (TSS) of the SRSF2 gene, miR-155-5p promotes the transcription of the SRSF2 gene, ultimately increasing viral replication and viral gene expression. Our results provide insight for an understanding of the roles and molecular mechanism of miR-155-5p in HSV-1 replication and the epigenetic control of SRSF2 gene expression.

Role for a Filamentous Nuclear Assembly of IFI16, DNA, and Host Factors in Restriction of Herpesviral Infection

Mammalian cells exhibit numerous strategies to recognize and contain viral infections. The best-characterized antiviral responses are those that are induced within the cytosol by receptors that activate interferon responses or shut down translation. Antiviral responses also occur in the nucleus, yet these intranuclear innate immune responses are poorly defined at the receptor-proximal level. In this study, we explored the ability of cells to restrict infection by assembling viral DNA into transcriptionally silent heterochromatin within the nucleus. We found that the IFI16 restriction factor forms filaments on DNA within infected cells. These filaments recruit antiviral restriction factors to prevent viral replication in various cell types. Mechanistically, IFI16 filaments inhibit the recruitment of RNA polymerase II to viral genes. We propose that IFI16 filaments with associated restriction factors constitute a “restrictosome” structure that can signal to other parts of the nucleus where foreign DNA is located that it should be silenced.

Several host cell nuclear factors are known to restrict herpes simplex virus 1 (HSV-1) replication, but their mechanisms of action remain to be defined. Interferon-inducible protein 16 (IFI16) and the nuclear domain 10-associated proteins, such as promyelocytic leukemia (PML) protein, localize to input viral genomes, but they are also capable of restricting progeny viral transcription. In this study, we used structured illumination microscopy to show that after HSV DNA replication, IFI16 forms nuclear filamentous structures on DNA within a subset of nuclear replication compartments in HSV-1 ICP0-null mutant virus-infected human cells. The ability to form filaments in different cell types correlates with the efficiency of restriction, and the kinetics of filament formation and epigenetic changes are similar. Thus, both are consistent with the filamentous structures being involved in epigenetic silencing of viral progeny DNA. IFI16 filaments recruit other restriction factors, including PML, Sp100, and ATRX, to aid in the restriction. Although the filaments are only in a subset of the replication compartments, IFI16 reduces the levels of elongation-competent RNA polymerase II (Pol II) in all replication compartments. Therefore, we propose that IFI16 filaments with associated restriction factors that form in replication compartments constitute a “restrictosome” structure that signals in cis and trans to silence the progeny viral DNA throughout the infected cell nucleus. The IFI16 filamentous structure may constitute the first known nuclear supramolecular organizing center for signaling in the cell nucleus.

Modulation of bovine herpesvirus 1 infection by virally encoded microRNAs

Bovine herpesvirus 1 (BoHV-1), is a member of the subfamily Alphaherpesvirinae in the order Herpesviridae and is a ubiquitous pathogen of cattle responsible for significant economic loss worldwide. The BoHV-1 genome encodes at least 10 BoHV-1 microRNA (miRNA) genes, whose functions remain poorly understood. This study sought to understand the role of three BoHV-1 miRNA genes, Bhv1-miR-B6, Bhv1-miR-B8 and Bhv1-miR-B9, which are located proximal to the BoHV-1 origins of replication (OriS). Therefore, plasmids expressing the precursor miRNA hairpins for the Bhv1-miR-B6, Bhv1-miR-B8, and Bhv1-miR-B9 genes were constructed and transfected into Madin-Darby bovine kidney cells prior to BoHV-1 infection. Interestingly, transient expression of either Bhv1-miR-B8 or Bhv1-miR-B9 in Madin-Darby bovine kidney cells prior to infection resulted in partial suppression of BoHV-1 replication, quantified through estimating levels of glycoprotein C mRNA and protein levels. Putative interactions between the mature miRNA bhv1-miR-B8-3p and bhv1-miR-B9 and BoHV-1 transcripts were identified providing plausible pathways for these molecules to affect virus replication. Therefore, these two miRNAs are implicated in the post-transcriptional regulation of BoHV-1 transcripts important for virus replication and could be used to limit BoHV-1 replication.

GaHV-2 ICP22 protein is expressed from a bicistronic transcript regulated by three GaHV-2 microRNAs

Herpesviruses have a lifecycle consisting of successive lytic, latent and reactivation phases. Only three infected cell proteins (ICPs) have been described for the oncogenic Marek's disease virus (or Gallid herpes virus 2, GaHV-2): ICP4, ICP22 and ICP27. We focus here on ICP22, confirming its cytoplasmic location and showing that ICP22 is expressed during productive phases of the lifecycle, via a bicistronic transcript encompassing the US10 gene. We also identified the unique promoter controlling ICP22 expression, and its core promoter, containing functional responsive elements including E-box, ETS-1 and GATA elements involved in ICP22 transactivation. ICP22 gene expression was weakly regulated by DNA methylation and activated by ICP4 or ICP27 proteins. We also investigated the function of GaHV-2 ICP22. We found that this protein repressed transcription from its own promoter and from those of IE ICP4 and ICP27, and the late gK promoter. Finally, we investigated posttranscriptional ICP22 regulation by GaHV-2 microRNAs. We found that mdv1-miR-M5-3p and -M1-5p downregulated ICP22 mRNA expression during latency, whereas, unexpectedly, mdv1-miR-M4-5p upregulated the expression of the protein ICP22, indicating a tight regulation of ICP22 expression by microRNAs.

A Herpesviral Immediate Early Protein Promotes Transcription Elongation of Viral Transcripts

Herpes simplex virus 1 (HSV-1) genes are transcribed by cellular RNA polymerase II (RNA Pol II). While four viral immediate early proteins (ICP4, ICP0, ICP27, and ICP22) function in some capacity in viral transcription, the mechanism by which ICP22 functions remains unclear. We observed that the FACT complex (comprised of SSRP1 and Spt16) was relocalized in infected cells as a function of ICP22. ICP22 was also required for the association of FACT and the transcription elongation factors SPT5 and SPT6 with viral genomes. We further demonstrated that the FACT complex interacts with ICP22 throughout infection. We therefore hypothesized that ICP22 recruits cellular transcription elongation factors to viral genomes for efficient transcription elongation of viral genes. We reevaluated the phenotype of an ICP22 mutant virus by determining the abundance of all viral mRNAs throughout infection by transcriptome sequencing (RNA-seq). The accumulation of almost all viral mRNAs late in infection was reduced compared to the wild type, regardless of kinetic class. Using chromatin immunoprecipitation sequencing (ChIP-seq), we mapped the location of RNA Pol II on viral genes and found that RNA Pol II levels on the bodies of viral genes were reduced in the ICP22 mutant compared to wild-type virus. In contrast, the association of RNA Pol II with transcription start sites in the mutant was not reduced. Taken together, our results indicate that ICP22 plays a role in recruiting elongation factors like the FACT complex to the HSV-1 genome to allow for efficient viral transcription elongation late in viral infection and ultimately infectious virion production.

HSV-1 interacts with many cellular proteins throughout productive infection. Here, we demonstrate the interaction of a viral protein, ICP22, with a subset of cellular proteins known to be involved in transcription elongation. We determined that ICP22 is required to recruit the FACT complex and other transcription elongation factors to viral genomes and that in the absence of ICP22 viral transcription is globally reduced late in productive infection, due to an elongation defect. This insight defines a fundamental role of ICP22 in HSV-1 infection and elucidates the involvement of cellular factors in HSV-1 transcription.

Serine/Arginine-rich Splicing Factor 2 Modulates Herpes Simplex Virus Type 1 Replication via Regulating Viral Gene Transcriptional Activity and Pre-mRNA Splicing

Once it enters the host cell, herpes simplex virus type 1 (HSV-1) recruits a series of host cell factors to facilitate its life cycle. Here, we demonstrate that serine/arginine-rich splicing factor 2 (SRSF2), which is an important component of the splicing speckle, mediates HSV-1 replication by regulating viral gene expression at the transcriptional and posttranscriptional levels. Our results indicate that SRSF2 functions as a transcriptional activator by directly binding to infected cell polypeptide 0 (ICP0), infected cell polypeptide 27 (ICP27), and thymidine kinase promoters. Moreover, SRSF2 participates in ICP0 pre-mRNA splicing by recognizing binding sites in ICP0 exon 3. These findings provide insight into the functions of SRSF2 in HSV-1 replication and gene expression.

Infection by Herpes Simplex Virus 1 Causes Near-Complete Loss of RNA Polymerase II Occupancy on the Host Cell Genome

Lytic infection by herpes simplex virus 1 (HSV-1) triggers a change in many host cell programs as the virus strives to express its own genes and replicate. Part of this process is repression of host cell transcription by RNA polymerase II (Pol II), which also transcribes the viral genome. Here, we describe a global characterization of Pol II occupancy on the viral and host genomes in response to HSV-1 infection using chromatin immunoprecipitation followed by deep sequencing (ChIP-seq). The data reveal near-complete loss of Pol II occupancy throughout host cell mRNA genes, in both their bodies and promoter-proximal regions. Increases in Pol II occupancy of host cell genes, which would be consistent with robust transcriptional activation, were not observed. HSV-1 infection induced a more potent and widespread repression of Pol II occupancy than did heat shock, another cellular stress that widely represses transcription. Concomitant with the loss of host genome Pol II occupancy, we observed Pol II covering the HSV-1 genome, reflecting a high level of viral gene transcription. Interestingly, the positions of the peaks of Pol II occupancy at HSV-1 and host cell promoters were different.

IMPORTANCE

We investigated the effect of herpes simplex virus 1 (HSV-1) infection on transcription of host cell and viral genes by RNA polymerase II (Pol II). The approach we used was to determine how levels of genome-bound Pol II changed after HSV-1 infection. We found that HSV-1 caused a profound loss of Pol II occupancy across the host cell genome. Increases in Pol II occupancy were not observed, showing that no host genes were activated after infection. In contrast, Pol II occupied the entire HSV-1 genome. Moreover, the pattern of Pol II at HSV-1 genes differed from that on host cell genes, suggesting a unique mode of viral gene transcription. These studies provide new insight into how HSV-1 causes changes in the cellular program of gene expression and how the virus coopts host Pol II for its own use.

A comparison of herpes simplex virus type 1 and varicella-zoster virus latency and reactivation

Herpes simplex virus type 1 (HSV-1; human herpesvirus 1) and varicella-zoster virus (VZV; human herpesvirus 3) are human neurotropic alphaherpesviruses that cause lifelong infections in ganglia. Following primary infection and establishment of latency, HSV-1 reactivation typically results in herpes labialis (cold sores), but can occur frequently elsewhere on the body at the site of primary infection (e.g. whitlow), particularly at the genitals. Rarely, HSV-1 reactivation can cause encephalitis; however, a third of the cases of HSV-1 encephalitis are associated with HSV-1 primary infection. Primary VZV infection causes varicella (chickenpox) following which latent virus may reactivate decades later to produce herpes zoster (shingles), as well as an increasingly recognized number of subacute, acute and chronic neurological conditions. Following primary infection, both viruses establish a latent infection in neuronal cells in human peripheral ganglia. However, the detailed mechanisms of viral latency and reactivation have yet to be unravelled. In both cases latent viral DNA exists in an 'end-less' state where the ends of the virus genome are joined to form structures consistent with unit length episomes and concatemers, from which viral gene transcription is restricted. In latently infected ganglia, the most abundantly detected HSV-1 RNAs are the spliced products originating from the primary latency associated transcript (LAT). This primary LAT is an 8.3 kb unstable transcript from which two stable (1.5 and 2.0 kb) introns are spliced. Transcripts mapping to 12 VZV genes have been detected in human ganglia removed at autopsy; however, it is difficult to ascribe these as transcripts present during latent infection as early-stage virus reactivation may have transpired in the post-mortem time period in the ganglia. Nonetheless, low-level transcription of VZV ORF63 has been repeatedly detected in multiple ganglia removed as close to death as possible. There is increasing evidence that HSV-1 and VZV latency is epigenetically regulated. In vitro models that permit pathway analysis and identification of both epigenetic modulations and global transcriptional mechanisms of HSV-1 and VZV latency hold much promise for our future understanding in this complex area. This review summarizes the molecular biology of HSV-1 and VZV latency and reactivation, and also presents future directions for study.

HSV-2 immediate-early protein US1 inhibits IFN-β production by suppressing association of IRF-3 with IFN-β promoter

HSV-2 is the major cause of genital herpes, and its infection increases the risk of HIV-1 acquisition and transmission. After initial infection, HSV-2 can establish latency within the nervous system and thus maintains lifelong infection in humans. It has been suggested that HSV-2 can inhibit type I IFN signaling, but the underlying mechanism has yet to be determined. In this study, we demonstrate that productive HSV-2 infection suppresses Sendai virus (SeV) or polyinosinic-polycytidylic acid-induced IFN-β production. We further reveal that US1, an immediate-early protein of HSV-2, contributes to such suppression, showing that US1 inhibits IFN-β promoter activity and IFN-β production at both mRNA and protein levels, whereas US1 knockout significantly impairs such capability in the context of HSV-2 infection. US1 directly interacts with DNA binding domain of IRF-3, and such interaction suppresses the association of nuclear IRF-3 with the IRF-3 responsive domain of IFN-β promoter, resulting in the suppression of IFN-β promoter activation. Additional studies demonstrate that the 217-414 aa domain of US1 is critical for the suppression of IFN-β production. Our results indicate that HSV-2 US1 downmodulates IFN-β production by suppressing the association of IRF-3 with the IRF-3 responsive domain of IFN-β promoter. Our findings highlight the significance of HSV-2 US1 in inhibiting IFN-β production and provide insights into the molecular mechanism by which HSV-2 evades the host innate immunity, representing an unconventional strategy exploited by a dsDNA virus to interrupt type I IFN signaling pathway.

Role of herpes simplex virus 1 immediate early protein ICP22 in viral nuclear egress

In order to investigate the novel function(s) of the herpes simplex virus 1 (HSV-1) immediate early protein ICP22, we screened for ICP22-binding proteins in HSV-1-infected cells. Our results were as follows. (i) Tandem affinity purification of ICP22 from infected cells, coupled with mass spectrometry-based proteomics and subsequent analyses, demonstrates that ICP22 forms a complex(es) with the HSV-1 proteins UL31, UL34, UL47 (or VP13/14), and/or Us3. All these proteins were previously reported to be important for viral egress through the nuclear membrane. (ii) ICP22 colocalizes with UL31 and UL34 at the nuclear membrane in wild-type HSV-1-infected cells. (iii) The UL31-null mutation prevents the targeting of ICP22 to the nuclear membrane. (iv) The ICP22-null mutation resulted in UL31 and UL34 being mislocalized in the endoplasmic reticulum (in addition to the nuclear membrane) and significantly reduced numbers of primary enveloped virions in the perinuclear space, although capsids accumulated in the nuclei. Collectively, these results suggest that (i) ICP22 interacts with HSV-1 regulators of nuclear egress, including UL31, UL34, UL47, and Us3 in HSV-1-infected cells; (ii) UL31 mediates the recruitment and anchorage of ICP22 at the nuclear membrane; and (iii) ICP22 plays a regulatory role in HSV-1 primary envelopment, probably by interacting with and regulating UL31 and UL34. Here we report a previously unknown function for ICP22 in the regulation of HSV-1 nuclear egress.

IMPORTANCE

The herpes simplex virus 1 (HSV-1) immediate early protein ICP22 is recognized primarily as a regulator of viral gene expression. In this study, we show that ICP22 interacts with the HSV-1 proteins UL31 and UL34, which play crucial roles at the nuclear membrane in HSV-1 primary envelopment during viral nuclear egress. We also demonstrate that UL31 is required for the targeting of ICP22 to the nuclear membrane and that ICP22 is required for the correct localization of UL31 and/or UL34. Furthermore, we confirm that ICP22 is required for efficient HSV-1 primary envelopment during viral nuclear egress. Thus, we report, for the first time, that ICP22 plays a regulatory role in HSV-1 nuclear egress.

Inhibition of cdk9 during herpes simplex virus 1 infection impedes viral transcription

During herpes simplex virus 1 (HSV-1) infection there is a loss of the serine-2 phosphorylated form of RNA polymerase II (RNAP II) found in elongation complexes. This occurs in part because RNAP II undergoes ubiquitination and proteasomal degradation during times of highly active viral transcription, which may result from stalled elongating complexes. In addition, a viral protein, ICP22, was reported to trigger a loss of serine-2 RNAP II. These findings have led to some speculation that the serine-2 phosphorylated form of RNAP II may not be required for HSV-1 transcription, although this form is required for cellular transcription elongation and RNA processing. Cellular kinase cdk9 phosphorylates serine-2 in the C-terminal domain (CTD) of RNAP II. To determine if serine-2 phosphorylated RNAP II is required for HSV-1 transcription, we inhibited cdk9 during HSV-1 infection and measured viral gene expression. Inhibition was achieved by adding cdk9 inhibitors 5,6-dichlorobenzimidazone-1-β-D-ribofuranoside (DRB) or flavopiridol (FVP) or by expression of a dominant–negative cdk9 or HEXIM1, which in conjunction with 7SK snRNA inhibits cdk9 in complex with cyclin 1. Here we report that inhibition of cdk9 resulted in decreased viral yields and levels of late proteins, poor formation of viral transcription-replication compartments, reduced levels of poly(A)+ mRNA and decreased RNA synthesis as measured by uptake of 5-bromouridine into nascent RNA. Importantly, a global reduction in viral mRNAs was seen as determined by microarray analysis. We conclude that serine-2 phosphorylation of the CTD of RNAP II is required for HSV-1 transcription.

Herpes simplex virus 1 ICP22 but not US 1.5 is required for efficient acute replication in mice and VICE domain formation

The herpes simplex virus 1 (HSV-1) immediate-early protein, infected cell protein 22 (ICP22), is required for efficient replication in restrictive cells, for virus-induced chaperone-enriched (VICE) domain formation, and for normal expression of a subset of viral late proteins. Additionally, ICP22 is important for optimal acute viral replication in vivo. Previous studies have shown that the US1 gene that encodes ICP22, produces an in-frame, N-terminally truncated form of ICP22, known as US1.5. To date, studies conducted to characterize the functions of ICP22 have not separated its functions from those of US1.5. To determine the individual roles of ICP22 and US1.5, we made viral mutants that express either ICP22 with an M90A mutation in the US1.5 initiation codon (M90A) or US1.5 with three stop codons introduced upstream of the US1.5 start codon (3×stop). Our studies showed that, in contrast to M90A, 3×stop was unable to replicate efficiently in the eyes and trigeminal ganglia of mice during acute infection, to efficiently establish a latent infection, or to induce VICE domain formation and was only mildly reduced in its replication in restrictive HEL-299 cells and murine embryonic fibroblasts (MEFs). Both mutants enhanced the expression of the late viral proteins virion host shutoff (vhs) and glycoprotein C (gC) and inhibited viral gene expression mediated by HSV-1 infected cell protein 0 (ICP0). When we tested our mutants' sensitivity to type I interferon (beta interferon [IFN-β]) in restrictive cells, we noticed that the plating of the ICP22 null (d22) and 3×stop mutants was reduced by the addition of IFN-β. Overall, our data suggest that US1.5 partially complements the functions of ICP22.

HSV-1 ICP22: hijacking host nuclear functions to enhance viral infection

During its productive infection, HSV-1 dramatically remodels the architecture and physiology of the host cell nucleus. The immediate-early proteins, the first viral proteins to be expressed during infection, are key players in this process. Here, we review the known properties and functions of immediate-early protein ICP22. Although this polypeptide has received less attention than other immediate-early proteins, the published evidence indicates that it mediates several striking changes to important host nuclear systems, including those involved in RNA polymerase II transcription, cell cycle regulation and protein quality control. Recent genetic analyses suggest that these alterations can promote HSV-1 productive infection. Thus, future work on ICP22 is likely to reveal novel mechanisms by which herpesviruses, and possibly other DNA viruses, manipulate the host cell nucleus to enhance their replication.

Roles of p53 in herpes simplex virus 1 replication

p53 is a critical factor in the cellular response to a broad range of stress factors through its ability to regulate various cellular pathways. In this study, tandem affinity purification of transiently expressed herpes simplex virus 1 (HSV-1) regulatory protein ICP22 coupled with mass spectrometry-based proteomics technology and subsequent analyses showed that ICP22 interacted with p53 in HSV-1-infected cells. In p53(-/-) cells, replication of wild-type HSV-1 was reduced compared to that in parental p53(+/+) cells, indicating that p53 had a positive effect on HSV-1 replication. In contrast, the levels of viral replication of an ICP22-null mutant virus were similar in both p53(-/-) and p53(+/+) cells. At 2 h postinfection, the level of expression of ICP27, an essential viral regulatory protein, in p53(-/-) cells infected with wild-type HSV-1 or the ICP22-null mutant virus was lower than in p53(+/+) cells. In contrast, at 18 h postinfection, the level of expression of ICP0, a critical viral regulatory protein, in p53(-/-) cells infected with the ICP22-null mutant virus was higher than in p53(+/+) cells, although the levels of ICP0 expression in p53(-/-) and p53(+/+) cells infected with wild-type HSV-1 were almost identical. These results suggested that p53 overall promoted HSV-1 replication and that p53 played both positive and negative roles in HSV-1 replication: upregulating ICP27 expression very early in infection and downregulating ICP0 expression later in infection, which was antagonized by ICP22.

Bovine Herpes Virus 1 (BHV-1) and Herpes Simplex Virus Type 1 (HSV-1) Promote Survival of Latently Infected Sensory Neurons, in Part by Inhibiting Apoptosis

α-Herpesvirinae subfamily members, including herpes simplex virus type 1 (HSV-1) and bovine herpes virus 1 (BHV-1), initiate infection in mucosal surfaces. BHV-1 and HSV-1 enter sensory neurons by cell-cell spread where a burst of viral gene expression occurs. When compared to non-neuronal cells, viral gene expression is quickly extinguished in sensory neurons resulting in neuronal survival and latency. The HSV-1 latency associated transcript (LAT), which is abundantly expressed in latently infected neurons, inhibits apoptosis, viral transcription, and productive infection, and directly or indirectly enhances reactivation from latency in small animal models. Three anti-apoptosis genes can be substituted for LAT, which will restore wild type levels of reactivation from latency to a LAT null mutant virus. Two small non-coding RNAs encoded by LAT possess anti-apoptosis functions in transfected cells. The BHV-1 latency related RNA (LR-RNA), like LAT, is abundantly expressed during latency. The LR-RNA encodes a protein (ORF2) and two microRNAs that are expressed in certain latently infected neurons. Wild-type expression of LR gene products is required for stress-induced reactivation from latency in cattle. ORF2 has anti-apoptosis functions and interacts with certain cellular transcription factors that stimulate viral transcription and productive infection. ORF2 is predicted to promote survival of infected neurons by inhibiting apoptosis and sequestering cellular transcription factors which stimulate productive infection. In addition, the LR encoded microRNAs inhibit viral transcription and apoptosis. In summary, the ability of BHV-1 and HSV-1 to interfere with apoptosis and productive infection in sensory neurons is crucial for the life-long latency-reactivation cycle in their respective hosts.

The ICP22 protein selectively modifies the transcription of different kinetic classes of pseudorabies virus genes

BACKGROUND

Pseudorabies virus (PRV), an alpha-herpesvirus of swine, is a widely used model organism in investigations of the molecular pathomechanisms of the herpesviruses. This work is the continuation of our earlier studies, in which we investigated the effect of the abrogation of gene function on the viral transcriptome by knocking out PRV genes playing roles in the coordination of global gene expression of the virus. In this study, we deleted the us1 gene encoding the ICP22, an important viral regulatory protein, and analyzed the changes in the expression of other PRV genes.

RESULTS

A multi-timepoint real-time RT-PCR technique was applied to evaluate the impact of deletion of the PRV us1 gene on the overall transcription kinetics of viral genes. The mutation proved to exert a differential effect on the distinct kinetic classes of PRV genes at the various stages of lytic infection. In the us1 gene-deleted virus, all the kinetic classes of the genes were significantly down-regulated in the first hour of infection. After 2 to 6 h of infection, the late genes were severely suppressed, whereas the early genes were unaffected. In the late stage of infection, the early genes were selectively up-regulated. In the mutant virus, the transcription of the ie180 gene, the major coordinator of PRV gene expression, correlated closely with the transcription of other viral genes, a situation which was not found in the wild-type (wt) virus. A 4-h delay was observed in the commencement of DNA replication in the mutant virus as compared with the wt virus. The rate of transcription from a gene normalized to the relative copy number of the viral genome was observed to decline drastically following the initiation of DNA replication in both the wt and mutant backgrounds. Finally, the switch between the expressions of the early and late genes was demonstrated not to be controlled by DNA replication, as is widely believed, since the switch preceded the DNA replication.

CONCLUSIONS

Our results show a strong dependence of PRV gene expression on the presence of functional us1 gene. ICP22 is shown to exert a differential effect on the distinct kinetic classes of PRV genes and to disrupt the close correlation between the transcription kinetics of ie180 and other PRV transcripts. Furthermore, DNA replication exerts a severe constraint on the viral transcription.

MHC class I cross-presentation by dendritic cells counteracts viral immune evasion

DCs very potently activate CD8(+) T cells specific for viral peptides bound to MHC class I molecules. However, many viruses have evolved immune evasion mechanisms, which inactivate infected DCs and might reduce priming of T cells. Then MHC class I cross-presentation of exogenous viral Ag by non-infected DCs may become crucial to assure CD8(+) T cell responses. Although many vital functions of infected DCs are inhibited in vitro by many different viruses, the contributions of cross-presentation to T cell immunity when confronted with viral immune inactivation in vivo has not been demonstrated up to now, and remains controversial. Here we show that priming of Herpes Simplex Virus (HSV)-, but not murine cytomegalovirus (mCMV)-specific CD8(+) T cells was severely reduced in mice with a DC-specific cross-presentation deficiency. In contrast, while CD8(+) T cell responses to mutant HSV, which lacks crucial inhibitory genes, also depended on CD8α(+) DCs, they were independent of cross-presentation. Therefore HSV-specific CTL-responses entirely depend on the CD8α(+) DC subset, which present via direct or cross-presentation mechanisms depending on the immune evasion equipment of virus. Our data establish the contribution of cross-presentation to counteract viral immune evasion mechanisms in some, but not all viruses.

Herpes simplex virus 1 ICP22 inhibits the transcription of viral gene promoters by binding to and blocking the recruitment of P-TEFb

ICP22 is a multifunctional herpes simplex virus 1 (HSV-1) immediate early protein that functions as a general repressor of a subset of cellular and viral promoters in transient expression systems. Although the exact mechanism of repression remains unclear, this protein induces a decrease in RNA polymerase II Serine 2 (RNAPII Ser-2) phosphorylation, which is critical for transcription elongation. To characterize the mechanism of transcriptional repression by ICP22, we established an in vivo transient expression reporter system. We found that ICP22 inhibits transcription of the HSV-1 α, β and γ gene promoters. The viral tegument protein VP16, which plays vital roles in initiation of viral gene expression and viral proliferation, can overcome the inhibitory effect of ICP22 on α-gene transcription. Further immunoprecipitation studies indicated that both ICP22 and VP16 bind to positive transcription elongation factor b (P-TEFb) and form a complex with it in vivo. We extended this to show that P-TEFb regulates transcription of the viral α-gene promoters and affects transcriptional regulation of ICP22 and VP16 on the α-genes. Additionally, ChIP assays demonstrated that ICP22 blocks the recruitment of P-TEFb to the viral promoters, while VP16 reverses this blocking effect by recruiting P-TEFb to the viral α-gene promoters through recognition of the TAATGARAT motif. Taken together, our results suggest that ICP22 interacts with and blocks the recruitment of P-TEFb to viral promoter regions, which inhibits transcription of the viral gene promoters. The transactivator VP16 binds to and induces the recruitment of P-TEFb to viral α-gene promoters, which counteracts the transcriptional repression of ICP22 on α-genes by recruiting p-TEFb to the promoter region.

Herpes simplex virus 1 DNA is in unstable nucleosomes throughout the lytic infection cycle, and the instability of the nucleosomes is independent of DNA replication

Herpes simplex virus 1 (HSV-1) DNA is chromatinized during latency and consequently regularly digested by micrococcal nuclease (MCN) to nucleosome-size fragments. In contrast, MCN digests HSV-1 DNA in lytically infected cells to mostly heterogeneous sizes. Yet HSV-1 DNA coimmunoprecipitates with histones during lytic infections. We have shown that at 5 h postinfection, most nuclear HSV-1 DNA is in particularly unstable nucleoprotein complexes and consequently is more accessible to MCN than DNA in cellular chromatin. HSV-1 DNA was quantitatively recovered at this time in complexes with the biophysical properties of mono- to polynucleosomes following a modified MCN digestion developed to detect potential unstable intermediates. We proposed that most HSV-1 DNA is in unstable nucleosome-like complexes during lytic infections. Physiologically, nucleosome assembly typically associates with DNA replication, although DNA replication transiently disrupts nucleosomes. It therefore remained unclear whether the instability of the HSV-1 nucleoprotein complexes was related to the ongoing viral DNA replication. Here we tested whether HSV-1 DNA is in unstable nucleosome-like complexes before, during, or after the peak of viral DNA replication or when HSV-1 DNA replication is inhibited. HSV-1 DNA was quantitatively recovered in complexes fractionating as mono- to polynucleosomes from nuclei harvested at 2, 5, 7, or 9 h after infection, even if viral DNA replication was inhibited. Therefore, most HSV-1 DNA is in unstable nucleosome-like complexes throughout the lytic replication cycle, and the instability of these complexes is surprisingly independent of HSV-1 DNA replication. The specific accessibility of nuclear HSV-1 DNA, however, varied at different times after infection.

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.

Sequence variation in the herpes simplex virus U(S)1 ocular virulence determinant

PURPOSE

The herpes simplex virus type 1 (HSV-1) U(S)1 gene encodes host-range and ocular virulence determinants. Mutations in U(S)1 affecting virulence are known in strain OD4, but the genomic variation across several strains is not known. The goal was to determine the degree of sequence variation in the gene from several ocular HSV isolates.

METHODS

The U(S)1 gene from six ocular HSV-1 isolates, as well as strains KOS and F, were sequenced, and bioinformatics analyses were applied to the data.

RESULTS

Strains 17, F, CJ394, and CJ311 had identical amino acid sequences. With the other strains, most of the variability was concentrated in the amino-terminal third of the protein. MEME analysis identified a 63-residue core sequence (motif 1) present in all α-herpesvirus U(S)1 homologs that were located in a region identified as structured. Ten amino acids were absolutely conserved in all the α-herpesvirus U(S)1 homologs and were all located in the central core. Consensus-binding motifs for cyclin-dependent kinases and pocket proteins were also identified.

CONCLUSIONS

These results suggest that significant sequence variation exists in the U(S)1 gene, that the α22 protein contains a conserved central core region with structurally variable regions at the amino- and carboxyl termini, that 10 amino acids are conserved in α-herpes U(S)1 homologs, and that additional host proteins may interact with the HSV-1 U(S)1 and U(S)1.5 proteins. This information will be valuable in designing further studies on structure-function relationships and on the role these play in host-range determination and keratitis.

The herpes simplex virus type 1 infected cell protein 22

As one of the immediate-early (IE) proteins of herpes simplex virus type 1 (HSV-1), ICP22 is a multifunctional viral regulator that localizes in the nucleus of infected cells. It is required in experimental animal systems and some nonhuman cell lines, but not in Vero or HEp-2 cells. ICP22 is extensively phosphorylated by viral and cellular kinases and nucleotidylylated by casein kinase II. It has been shown to be required for efficient expression of early (E) genes and a subset of late (L) genes. ICP22, in conjunction with the UL13 kinase, mediates the phosphorylation of RNA polymerase II. Both ICP22 and UL13 are required for the activation of cdc2, the degradation of cyclins A and B and the acquisition of a new cdc2 partner, the UL42 DNA polymerase processivity factor. The cdc2-UL42 complex mediates postranscriptional modification of topoisomerase IIα in an ICP22-dependent manner to promote L gene expression. In addition, ICP22 interacts with cdk9 in a Us3 kinase dependent fashion to phosphorylate RNA polymerase II.

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

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

Impaired expression and function of toll-like receptor 7 in hepatitis C virus infection in human hepatoma cells

Hepatitis C virus (HCV) interferes with interferon (IFN)-mediated innate immune defenses. Toll-like receptor (TLR) 7 agonists robustly inhibit HCV infection. We hypothesize that HCV infection may interfere with the expression and/or function of TLR7, a sensor of single-stranded RNA. We identified reduced TLR7 RNA and protein levels in hepatoma cells expressing HCV (full-length, BB7-subgenomic, and JFH-1 clone) compared with control HCV-naïve cells. The biological relevance of this finding was confirmed by the observation of decreased TLR7 RNA in livers of HCV-infected patients compared with controls. HCV clearance, by IFN-alpha treatment or restrictive culture conditions, restored the decreased TLR7 expression. Treatment with RNA polymerase inhibitors revealed a shorter TLR7 half-life in HCV-replicating cells compared with controls. Downstream of TLR7, an increased baseline IRF7 nuclear translocation was observed in HCV-positive cells compared with controls. Stimulation with the TLR7 ligand R837 resulted in significant IRF7 nuclear translocation in control cells. In contrast, HCV-replicating cells showed attenuated TLR7 ligand-induced IRF7 activation.

CONCLUSION

Reduced TLR7 expression, due to RNA instability, directly correlates with HCV replication and alters TLR7-induced IRF7-mediated cell activation. These results suggest a role for TLR7 in HCV-mediated evasion of host immune surveillance.

Herpes simplex virus type 1 immediate-early protein ICP22 is required for VICE domain formation during productive viral infection

During productive infection, herpes simplex virus type 1 (HSV-1) induces the formation of discrete nuclear foci containing cellular chaperone proteins, proteasomal components, and ubiquitinated proteins. These structures are known as VICE domains and are hypothesized to play an important role in protein turnover and nuclear remodeling in HSV-1-infected cells. Here we show that VICE domain formation in Vero and other cells requires the HSV-1 immediate-early protein ICP22. Since ICP22 null mutants replicate efficiently in Vero cells despite being unable to induce VICE domain formation, it can be concluded that VICE domain formation is not essential for HSV-1 productive infection. However, our findings do not exclude the possibility that VICE domain formation is required for viral replication in cells that are nonpermissive for ICP22 mutants. Our studies also show that ICP22 itself localizes to VICE domains, suggesting that it could play a role in forming these structures. Consistent with this, we found that ICP22 expression in transfected cells is sufficient to reorganize the VICE domain component Hsc70 into nuclear inclusion bodies that resemble VICE domains. An N-terminal segment of ICP22, corresponding to residues 1 to 146, is critical for VICE domain formation in infected cells and Hsc70 reorganization in transfected cells. We previously found that this portion of the protein is dispensable for ICP22's effects on RNA polymerase II phosphorylation. Thus, ICP22 mediates two distinct regulatory activities that both modify important components of the host cell nucleus.

Proteomic analysis of cells in the early stages of herpes simplex virus type-1 infection reveals widespread changes in the host cell proteome

During infection by herpes simplex virus type-1 (HSV-1) the host cell undergoes widespread changes in gene expression and morphology in response to viral replication and release. However, relatively little is known about the specific proteome changes that occur during the early stages of HSV-1 replication prior to the global damaging effects of virion maturation and egress. To investigate pathways that may be activated or utilised during the early stages of HSV-1 replication, 2-DE and LC-MS/MS were used to identify cellular proteome changes at 6 h post infection. Comparative analysis of multiple gels representing whole cell extracts from mock- and HSV-1-infected HEp-2 cells revealed a total of 103 protein spot changes. Of these, 63 were up-regulated and 40 down-regulated in response to infection. Changes in selected candidate proteins were verified by Western blot analysis and their respective cellular localisations analysed by confocal microscopy. We have identified differential regulation and modification of proteins with key roles in diverse cellular pathways, including DNA replication, chromatin remodelling, mRNA stability and the ER stress response. This work represents the first global comparative analysis of HSV-1 infected cells and provides an important insight into host cell proteome changes during the early stages of HSV-1 infection.

Virus-Induced Chaperone-Enriched (VICE) domains function as nuclear protein quality control centers during HSV-1 infection

Virus-Induced Chaperone-Enriched (VICE) domains form adjacent to nuclear viral replication compartments (RC) during the early stages of HSV-1 infection. Between 2 and 3 hours post infection at a MOI of 10, host protein quality control machinery such as molecular chaperones (e.g. Hsc70), the 20S proteasome and ubiquitin are reorganized from a diffuse nuclear distribution pattern to sequestration in VICE domains. The observation that VICE domains contain putative misfolded proteins suggests that they may be similar to nuclear inclusion bodies that form under conditions in which the protein quality control machinery is overwhelmed by the presence of misfolded proteins. The detection of Hsc70 in VICE domains, but not in nuclear inclusion bodies, indicates that Hsc70 is specifically reorganized by HSV-1 infection. We hypothesize that HSV-1 infection induces the formation of nuclear protein quality control centers to remodel or degrade aberrant nuclear proteins that would otherwise interfere with productive infection. Detection of proteolytic activity in VICE domains suggests that substrates may be degraded by the 20S proteasome in VICE domains. FRAP analysis reveals that GFP-Hsc70 is dynamically associated with VICE domains, suggesting a role for Hsc70 in scanning the infected nucleus for misfolded proteins. During 42°C heat shock, Hsc70 is redistributed from VICE domains into RC perhaps to remodel viral replication and regulatory proteins that have become insoluble in these compartments. The experiments presented in this paper suggest that VICE domains are nuclear protein quality control centers that are modified by HSV-1 to promote productive infection.

Protein quality control is a protective cellular mechanism by which damaged proteins are refolded or degraded so that they cannot interfere with essential cellular processes. In the event that protein quality control machinery cannot refold or degrade damaged proteins, sequestration of misfolded protein is an alternative protective mechanism for reducing the toxic effects of misfolded protein. Several neurological diseases result from the accumulation of toxic misfolded proteins that cannot be efficiently refolded or degraded. In neurons from patients afflicted with Huntington's disease, misfolded huntingtin protein is sequestered in large aggregates in the nucleus called inclusion bodies. Inclusion bodies also contain protein quality control machinery including molecular chaperones, the proteasome and ubiquitin. Here we report that analogous structures called Virus-Induced Chaperone-Enriched (VICE) domains form in the nucleus of cells infected with Herpes Simplex Virus type 1 (HSV-1). VICE domains contain misfolded protein, chaperones and protein degradation activity. VICE domain formation is efficient in infected cells taxed with high levels of viral protein production. We hypothesize that misfolded proteins that arise in HSV-1-infected cells are sequestered in VICE domains to promote remodeling of misfolded proteins.

Origin of expression of the herpes simplex virus type 1 protein U(S)1.5

ICP22, an immediate-early protein of herpes simplex virus type 1 (HSV-1), is required for viral replication in nonpermissive cell types and for expression of a class of late viral proteins which includes glycoprotein C. An understanding of the mechanism of ICP22 function has been complicated by the coexpression of the full-length protein with an in-frame, C-terminus-specific protein, U(S)1.5. In this report, we confirm that the U(S)1.5 protein is a bona fide translation product since it is detected during infections with three laboratory strains and two low-passage clinical isolates of HSV-1. To clarify the expression patterns of the ICP22 and U(S)1.5 proteins, we examined their synthesis from plasmids in transient expression assays. Because previous studies had identified two different U(S)1.5 translational start sites, we attempted to determine which is correct by studying the effects of a series of deletion, nonsense, and methionine substitutions on U(S)1.5 expression. First, amino acids 90 to 420 encoded by the ICP22 open reading frame (ORF) migrated at the mobility of U(S)1.5 in sodium dodecyl sulfate-polyacrylamide gels. Second, introduction of a stop codon downstream of M90 ablated expression of both ICP22 and U(S)1.5. Finally, mutation of M90 to alanine (M90A) allowed expression of full-length ICP22 while dramatically reducing expression of U(S)1.5. Levels of U(S)1.5 but not ICP22 protein expression were also reduced in cells infected with an M90A mutant virus. Thus, we conclude that expression of IC22 and that of U(S)1.5 can occur independently of each other and that U(S)1.5 translation initiates at M90 of the ICP22 ORF.

Transient expression of herpes simplex virus type 1 ICP22 represses viral promoter activity and complements the replication of an ICP22 null virus

Of the five herpes simplex virus type 1 immediate early (IE) proteins, the least is known about the function of ICP22 during productive infection and latency. Research characterizing the physical and functional properties of the protein has been limited because ICP22 has proven to be difficult to express in transient assays. In addition, genetic analysis of ICP22 has been complicated by the fact that the C terminus of ICP22 is expressed as a discrete protein product. In order to characterize properties of mutant and wild-type ICP22, we developed a transient expression system. We found that ICP22 can be expressed at detectable levels when placed under the control of the cytomegalovirus IE promoter, confirming recent observations by K. A. Fraser and S. A. Rice (J. Virol. 81:5091-5101, 2007). We extended this analysis to show that ICP22 can also be expressed from its own promoter in the presence of other viral factors, either by coexpression with ICP0 or by infection with an ICP22 null virus. Notably, infection of cells transfected with an ICP22 expression vector yielded ICP22 protein that was modified in a manner similar to that of ICP22 protein detected in wild-type-infected cells. We go on to demonstrate that the failure of ICP22 protein to be expressed in transiently transfected cells was not due to inactivity of the ICP22 promoter, but rather to the ability of ICP22 to inhibit expression of reporter gene activity, including its own, in transient assays. Of special note was the observation that expression of ICP22 was sufficient to prevent transactivation of reporter genes by ICP0. Finally, transient expression of ICP22 was sufficient to complement replication of an ICP22 null virus, demonstrating that this system can be used to study functional properties of ICP22. Collectively, this transient expression system facilitates tests of the physical and functional properties of ICP22 and ICP22 mutants prior to introduction of mutant genes into the viral genome.

Transcriptional regulation of the Herpes Simplex Virus 1alpha-gene by the viral immediate-early protein ICP22 in association with VP16

Herpes Simplex Virus 1 (HSV1) is capable of inducing two forms of infection in individuals, and the establishment of which type of infection occurs is linked to the transcriptional activation of viral alpha genes. One of the HSV1 alpha genes, ICP22, is known to have multiple functions during virus replication, but its distinct roles are still unclear. This study showed that ICP22 functions as a general repressor for certain viral and cellular promoters, and this transcriptional repression by ICP22 is independent of the specific upstream promoter element, as shown using the CAT enzyme assay system. Further work also found that VP16 interfered with ICP22 mediated transcriptional repression of the viral alpha4 gene, through interactions with specific elements upstream of the alpha4 gene promoter. These findings support the possibility that ICP22 and VP16 control transcription of HSV1alpha genes in a common pathway for the establishment of either viral lytic or latent infections.