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

A viral E3 ubiquitin ligase produced by herpes simplex virus 1 inhibits the NLRP1 inflammasome

Guard proteins initiate defense mechanisms upon sensing pathogen-encoded virulence factors. Successful viral pathogens likely inhibit guard protein activity, but these interactions have been largely undefined. Here, we demonstrate that the human pathogen herpes simplex virus 1 (HSV-1) stimulates and inhibits an antiviral pathway initiated by NLRP1, a guard protein that induces inflammasome formation and pyroptotic cell death when activated. Notably, HSV-1 infection of human keratinocytes promotes posttranslational modifications to NLRP1, consistent with MAPK-dependent NLRP1 activation, but does not result in downstream inflammasome formation. We identify infected cell protein 0 (ICP0) as the critical HSV-1 protein that is necessary and sufficient for inhibition of the NLRP1 pathway. Mechanistically, ICP0's cytoplasmic localization and function as an E3 ubiquitin ligase prevents proteasomal degradation of the auto-inhibitory NT-NLRP1 fragment, thereby preventing inflammasome formation. Further, we demonstrate that inhibiting this inflammasome is important for promoting HSV-1 replication. Thus, we have established a mechanism by which HSV-1 overcomes a guard-mediated antiviral defense strategy in humans.

A 77 Amino Acid Region in the N-Terminal Half of the HSV-1 E3 Ubiquitin Ligase ICP0 Contributes to Counteracting an Established Type 1 Interferon Response

Herpes simplex virus 1 (HSV-1) is a human pathogen capable of establishing lifelong latent infections that can reactivate under stress conditions. A viral immediate early protein that plays important roles in the HSV-1 lytic and latent infections is the viral E3 ubiquitin ligase, ICP0. ICP0 transactivates all temporal classes of HSV-1 genes and facilitates viral gene expression. ICP0 also impairs the antiviral effects of interferon (IFN)-β, a component of host innate defenses known to limit viral replication. To begin to understand how ICP0 allows HSV-1 to disarm the IFN-β response, we performed genetic analyses using a series of ICP0 truncation mutants in the absence and presence of IFN-β in cell culture. We observed that IFN-β pretreatment of cells significantly impaired the replication of the ICP0 truncation mutants, n212 and n312, which code for the first 211 and 311 amino acids of ICP0, respectively; this effect of IFN-β correlated with decreased HSV-1 early and late gene expression. This increased sensitivity to IFN-β was not as apparent with the ICP0 mutant, n389. Our mapping studies indicate that loss of 77 amino acids from residues 312 to 388 in the N-terminal half of ICP0 resulted in a virus that was significantly more sensitive to cells pre-exposed to IFN-β. This 77 amino acid region contains a phospho-SUMO-interacting motif or -SIM, which we propose participates in ICP0's ability to counteract the antiviral response established by IFN-β. IMPORTANCE Interferons (IFNs) are secreted cellular factors that are induced by viral infection and limit replication. HSV-1 is largely refractory to the antiviral effects of type 1 IFNs, which are synthesized shortly after viral infection, in part through the activities of the viral regulatory protein, ICP0. To understand how ICP0 impedes the antiviral effects of type 1 IFNs, we used a series of HSV-1 ICP0 mutants and examined their viral replication and gene expression levels in cells stimulated with IFN-β (a type 1 IFN). Our mapping data identifies a discrete 77 amino acid region in the N-terminal half of ICP0 that facilitates HSV-1 resistance to IFN-β. This region of ICP0 is modified by phosphorylation and binds to the posttranslational modification SUMO, suggesting that HSV, and potentially other viruses, may counteract type 1 IFN signaling by altering SUMO and/or SUMO modified cellular proteins.

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.

Advancing Our Understanding of Corneal Herpes Simplex Virus-1 Immune Evasion Mechanisms and Future Therapeutics

Herpes stromal keratitis (HSK) is a disease that commonly affects the cornea and external eye and is caused by Herpes Simplex Virus type 1 (HSV-1). This virus infects approximately 66% of people worldwide; however, only a small portion of these people will develop symptoms in their lifetime. There is no cure or vaccine available for HSV-1; however, there are treatments available that aim to control the inflammation caused by the virus and prevent its recurrence. While these treatments are beneficial to those suffering with HSK, there is a need for more effective treatments to minimise the need for topical steroids, which can have harmful effects, and to prevent bouts of disease reactivation, which can lead to progressive corneal scarring and visual impairment. This review details the current understanding of HSV-1 infection and discusses potential novel treatment options including microRNAs, TLRs, mAbs, and aptamers.

The Role of ND10 Nuclear Bodies in Herpesvirus Infection: A Frenemy for the Virus?

Nuclear domains 10 (ND10), a.k.a. promyelocytic leukemia nuclear bodies (PML-NBs), are membraneless subnuclear domains that are highly dynamic in their protein composition in response to cellular cues. They are known to be involved in many key cellular processes including DNA damage response, transcription regulation, apoptosis, oncogenesis, and antiviral defenses. The diversity and dynamics of ND10 residents enable them to play seemingly opposite roles under different physiological conditions. Although the molecular mechanisms are not completely clear, the pro- and anti-cancer effects of ND10 have been well established in tumorigenesis. However, in herpesvirus research, until the recently emerged evidence of pro-viral contributions, ND10 nuclear bodies have been generally recognized as part of the intrinsic antiviral defenses that converge to the incoming viral DNA to inhibit the viral gene expression. In this review, we evaluate the newly discovered pro-infection influences of ND10 in various human herpesviruses and analyze their molecular foundation along with the traditional antiviral functions of ND10. We hope to shed light on the explicit role of ND10 in both the lytic and latent cycles of herpesvirus infection, which is imperative to the delineation of herpes pathogenesis and the development of prophylactic/therapeutic treatments for herpetic diseases.

Comparative proteomics identifies Schlafen 5 (SLFN5) as a herpes simplex virus restriction factor that suppresses viral transcription

Intrinsic antiviral host factors confer cellular defence by limiting virus replication and are often counteracted by viral countermeasures. We reasoned that host factors that inhibit viral gene expression could be identified by determining proteins bound to viral DNA (vDNA) in the absence of key viral antagonists. Herpes simplex virus 1 (HSV-1) expresses E3 ubiquitin-protein ligase ICP0 (ICP0), which functions as an E3 ubiquitin ligase required to promote infection. Cellular substrates of ICP0 have been discovered as host barriers to infection but the mechanisms for inhibition of viral gene expression are not fully understood. To identify restriction factors antagonized by ICP0, we compared proteomes associated with vDNA during HSV-1 infection with wild-type virus and a mutant lacking functional ICP0 (ΔICP0). We identified the cellular protein Schlafen family member 5 (SLFN5) as an ICP0 target that binds vDNA during HSV-1 ΔICP0 infection. We demonstrated that ICP0 mediates ubiquitination of SLFN5, which leads to its proteasomal degradation. In the absence of ICP0, SLFN5 binds vDNA to repress HSV-1 transcription by limiting accessibility of RNA polymerase II to viral promoters. These results highlight how comparative proteomics of proteins associated with viral genomes can identify host restriction factors and reveal that viral countermeasures can overcome SLFN antiviral activity.

"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.

Herpes simplex virus 1 targets IRF7 via ICP0 to limit type I IFN induction

Herpes simplex keratitis (HSK), caused by herpes simplex virus type 1 (HSV-1) infection, is the commonest cause of infectious blindness in the developed world. Following infection the virus is initially suspended in the tear film, where it encounters a multi-pronged immune response comprising enzymes, complement, immunoglobulins and crucially, a range of anti-viral and pro-inflammatory cytokines. However, given that HSV-1 can overcome innate immune responses to establish lifelong latency throughout a susceptible individual's lifetime, there is significant interest in understanding the mechanisms employed by HSV-1 to downregulate the anti-viral type I interferon (IFN) mediated immune responses. This study aimed to investigate the interactions between infected cell protein (ICP)0 and key elements of the IFN pathway to identify possible novel targets that contribute to viral immune evasion. Reporter gene assays demonstrated the ability of ICP0 to inhibit type I IFN activity downstream of pathogen recognition receptors (PRRs) which are known to be involved in host antiviral defences. Further experiments identified interferon regulatory factor (IRF)7, a driver of type I IFN, as a potential target for ICP0. These findings increase our understanding of the pathogenesis of HSK and suggest IRF7 as a potential therapeutic target.

The HSV-1 ubiquitin ligase ICP0: Modifying the cellular proteome to promote infection

•

ICP0 is a viral E3 ubiquitin ligase that promotes HSV-1 infection.

•

ICP0 interacts with multiple component proteins of the ubiquitin pathway.

•

ICP0 disrupts multiple cellular processes activated in response to infection

•

ICP0 remodels the SUMO proteome to counteract host immune defences to infection.

•

ICP0 is an attractive drug target for the development of antiviral HSV-1 therapeutics.

Herpes simplex virus 1 (HSV-1) hijacks ubiquitination machinery to modify the cellular proteome to create an environment permissive for virus replication. HSV-1 encodes its own RING-finger E3 ubiquitin (Ub) ligase, Infected Cell Protein 0 (ICP0), that directly interfaces with component proteins of the Ub pathway to inactivate host immune defences and cellular processes that restrict the progression of HSV-1 infection. Consequently, ICP0 plays a critical role in the infectious cycle of HSV-1 that is required to promote the efficient onset of lytic infection and productive reactivation of viral genomes from latency. This review will describe the current knowledge regarding the biochemical properties and known substrates of ICP0 during HSV-1 infection. We will highlight the gaps in the characterization of ICP0 function and propose future areas of research required to understand fully the biological properties of this important HSV-1 regulatory protein.

Analysis of Virus and Host Proteomes During Productive HSV-1 and VZV Infection in Human Epithelial Cells

Herpes simplex virus 1 (HSV-1) and varicella-zoster virus (VZV) are two closely related human alphaherpesviruses that persistently infect most adults worldwide and cause a variety of clinically important diseases. Herpesviruses are extremely well adapted to their hosts and interact broadly with cellular proteins to regulate virus replication and spread. However, it is incompletely understood how HSV-1 and VZV interact with the host proteome during productive infection. This study determined the temporal changes in virus and host protein expression during productive HSV-1 and VZV infection in the same cell type. Results demonstrated the temporally coordinated expression of HSV-1 and VZV proteins in infected cells. Analysis of the host proteomes showed that both viruses affected extracellular matrix composition, transcription, RNA processing and cell division. Moreover, the prominent role of epidermal growth factor receptor (EGFR) signaling during productive HSV-1 and VZV infection was identified. Stimulation and inhibition of EGFR leads to increased and decreased virus replication, respectively. Collectively, the comparative temporal analysis of viral and host proteomes in productively HSV-1 and VZV-infected cells provides a valuable resource for future studies aimed to identify target(s) for antiviral therapy development.

Host Intrinsic and Innate Intracellular Immunity During Herpes Simplex Virus Type 1 (HSV-1) Infection

When host cells are invaded by viruses, they deploy multifaceted intracellular defense mechanisms to control infections and limit the damage they may cause. Host intracellular antiviral immunity can be classified into two main branches: (i) intrinsic immunity, an interferon (IFN)-independent antiviral response mediated by constitutively expressed cellular proteins (so-called intrinsic host restriction factors); and (ii) innate immunity, an IFN-dependent antiviral response conferred by IFN-stimulated gene (ISG) products, which are (as indicated by their name) upregulated in response to IFN secretion following the recognition of pathogen-associated molecular patterns (PAMPs) by host pattern recognition receptors (PRRs). Recent evidence has demonstrated temporal regulation and specific viral requirements for the induction of these two arms of immunity during herpes simplex virus type 1 (HSV-1) infection. Moreover, they exert differential antiviral effects to control viral replication. Although they are distinct from one another, the words "intrinsic" and "innate" have been interchangeably and/or simultaneously used in the field of virology. Hence, the aims of this review are to (1) elucidate the current knowledge about host intrinsic and innate immunity during HSV-1 infection, (2) clarify the recent advances in the understanding of their regulation and address the distinctions between them with respect to their induction requirements and effects on viral infection, and (3) highlight the key roles of the viral E3 ubiquitin ligase ICP0 in counteracting both aspects of immunity. This review emphasizes that intrinsic and innate immunity are temporally and functionally distinct arms of host intracellular immunity during HSV-1 infection; the findings are likely pertinent to other clinically important viral infections.

The ICP0 Protein of Herpes Simplex Virus 1 (HSV-1) Downregulates Major Autophagy Adaptor Proteins Sequestosome 1 and Optineurin during the Early Stages of HSV-1 Infection

Herpes simplex virus 1 (HSV-1) infects mucosal epithelial cells and establishes lifelong infections in sensory neurons. Following reactivation, the virus is transferred anterograde to the initial site of infection or to sites innervated by infected neurons, causing vesicular lesions. Upon immunosuppression, frequent HSV-1 reactivation can cause severe diseases, such as blindness and encephalitis. Autophagy is a process whereby cell components are recycled, but it also serves as a defense mechanism against pathogens. HSV-1 is known to combat autophagy through the functions of the γ₁34.5 protein, which prevents formation of the autophagophore by binding to Beclin 1, a key factor involved in the elongation of the isolation membrane, and by redirecting the protein phosphatase 1α (PP1α) to dephosphorylate the translation initiation factor 2α (eIF2α) to prevent host translational shutoff. Other viral proteins that counteract innate immunity negatively impact autophagy. Here, we present a novel strategy of HSV-1 to evade the host through the downregulation of the autophagy adaptor protein sequestosome (p62/SQSTM1) and of the mitophagy adaptor optineurin (OPTN). This down-modulation occurs during the early steps of the infection. We also found that infected cell protein 0 (ICP0) of the virus mediates the down-modulation of the two autophagy adaptors in a mechanism independent of its E3 ubiquitin ligase activity. Cells depleted of either p62 or OPTN were able to mount greater antiviral responses, whereas cells expressing exogenous p62 displayed decreased virus yields. We conclude that downregulation of p62/SQSTM1 and OPTN is a viral strategy to counteract the host.IMPORTANCE Autophagy is a homeostatic mechanism of cells to recycle components, as well as a defense mechanism to get rid of pathogens. Strategies that HSV-1 has developed to counteract autophagy have been described and involve inhibition of autophagosome formation or indirect mechanisms. Here, we present a novel mechanism that involves downregulation of two major autophagy adaptor proteins, sequestosome 1 (p62/SQSTM1) and optineurin (OPTN). These findings generate the question of why the virus targets two major autophagy adaptors if it has mechanisms to block autophagosome formation. P62/SQSTM1 and OPTN proteins have pleiotropic functions, including regulation of innate immunity, inflammation, protein sorting, and chromatin remodeling. The decrease in virus yields in the presence of exogenous p62/SQSTM1 suggests that these adaptors have an antiviral function. Thus, HSV-1 may have developed multiple strategies to incapacitate autophagy to ensure replication. Alternatively, the virus may target another antiviral function of these proteins.

Charge-Mediated Pyrin Oligomerization Nucleates Antiviral IFI16 Sensing of Herpesvirus DNA

The formation of multimerized protein assemblies has emerged as a core component of immune signal amplification, yet the biochemical basis of this phenomenon remains unclear for many mammalian proteins within host defense pathways. The interferon-inducible protein 16 (IFI16) is a viral DNA sensor that oligomerizes upon binding to nuclear viral DNA and induces downstream antiviral responses. Here, we identify the pyrin domain (PYD) residues that mediate IFI16 oligomerization in a charge-dependent manner. Based on structure modeling, these residues are predicted to be surface exposed within distinct α-helices. By generating oligomerization-deficient mutants, we demonstrate that IFI16 homotypic clustering is necessary for its assembly onto parental viral genomes at the nuclear periphery upon herpes simplex virus 1 (HSV-1) infection. Preventing oligomerization severely hampered the capacity of IFI16 to induce antiviral cytokine expression, suppress viral protein levels, and restrict viral progeny production. Restoring oligomerization via residue-specific charge mimics partially rescued IFI16 antiviral roles. We show that pyrin domains from PYHIN proteins are functionally interchangeable, facilitating cooperative assembly with the IFI16 HINs, highlighting an inherent role for pyrin domains in antiviral response. Using immunoaffinity purification and targeted mass spectrometry, we establish that oligomerization promotes IFI16 interactions with proteins involved in transcriptional regulation, including PAF1C, UBTF, and ND10 bodies. We further discover PAF1C as an HSV-1 restriction factor. Altogether, our study uncovers intrinsic properties that govern IFI16 oligomerization, which serves as a signal amplification platform to activate innate immune responses and to recruit transcriptional regulatory proteins that suppress HSV-1 replication.IMPORTANCE The ability of mammalian cells to detect the genomes of nuclear-replicating viruses via cellular DNA sensors is fundamental to innate immunity. Recently, mounting evidence is supporting the universal role of polymerization in these host defense factors as a signal amplification strategy. Yet, what has remained unclear are the intrinsic properties that govern their immune signal transmission. Here, we uncover the biochemical basis for oligomerization of the nuclear DNA sensor, IFI16. Upon infection with herpes simplex virus 1 (HSV-1) in human fibroblasts, we characterize the contribution of IFI16 oligomerization to downstream protein interactions and antiviral functions, including cytokine induction and suppression of HSV-1 replication. Until now, the global characterization of oligomerization-dependent protein interactions for an immune receptor has never been explored. Our integrative quantitative proteomics, molecular CRISPR/Cas9-based assays, mutational analyses, and confocal microscopy shed light on the dynamics of immune signaling cascades activated against pathogens.

Discovery of Small-Molecule Inhibitors Targeting the E3 Ubiquitin Ligase Activity of the Herpes Simplex Virus 1 ICP0 Protein Using an In Vitro High-Throughput Screening Assay

Herpes simplex virus 1 (HSV-1) has infected more than 80% of the population. Reactivation of the virus causes diseases ranging in severity from benign cold sores to fatal encephalitis. Current treatments involve viral DNA replication inhibitors, but the emergence of drug-resistant mutants is observed frequently, highlighting the need for novel antiviral therapies. Infected cell protein 0 (ICP0) of HSV-1 is encoded by an immediate early gene and plays a fundamental role during infection, because it enables viral gene expression and blocks antiviral responses. One mechanism by which ICP0 functions is through an E3 ubiquitin ligase activity that induces the degradation of targeted proteins. A ΔICP0 virus or mutants with deficiencies in E3 ligase activity cannot counteract beta interferon (IFN-β)-induced restriction of viral infection, are highly immunogenic, are avirulent, and fail to spread. Thus, small molecules interfering with essential and conserved ICP0 functions are expected to compromise HSV-1 infection. We have developed a high-throughput screening assay, based on the autoubiquitination properties of ICP0, to identify small-molecule inhibitors of ICP0 E3 ubiquitin ligase activity. Through a pilot screening procedure, we identified nine compounds that displayed dose-dependent inhibitory effects on ICP0 but not on Mdm2, a control E3 ubiquitin ligase. Following validation, one compound displayed ICP0-dependent inhibition of HSV-1 infection. This compound appeared to bind ICP0 in a cellular thermal shift assay, it blocked ICP0 self-elimination, and it blocked wild-type but not ICP0-null virus gene expression. This scaffold displays specificity and could be used to develop optimized ICP0 E3 ligase inhibitors.IMPORTANCE Since acyclovir and its derivatives were launched for herpesviruses control almost four decades ago, the search for novel antivirals has waned. However, as human life expectancy has increased, so has the number of immunocompromised individuals who receive prolonged treatment for HSV recurrences. This has led to an increase in unresponsive patients due to acquired viral drug resistance. Thus, novel treatments need to be explored. Here we explored the HSV-1 ICP0 E3 ligase as a potential antiviral target because (i) ICP0 is expressed before virus replication, (ii) it is essential for infection in vivo, (iii) it is required for efficient reactivation of the virus from latency, (iv) inhibition of its E3 ligase activity would sustain host immune responses, and (v) it is shared by other herpesviruses. We report a compound that inhibits HSV-1 infection in an ICP0-dependent manner by inhibiting ICP0 E3 ligase activity.

Biogenesis of Extracellular Vesicles during Herpes Simplex Virus 1 Infection: Role of the CD63 Tetraspanin

Herpes simplex virus 1 (HSV-1) infections afflict more than 80% of the population worldwide. The virus primarily infects mucoepithelial cells and establishes latent reservoirs in neurons in sensory ganglia. Frequent reactivation has been linked to severe diseases, especially in immunocompromised individuals. Earlier, we reported that viral and host factors are packaged in extracellular vesicles (EVs) and delivered to uninfected cells, where they activate antiviral responses and restrict virus infection. Here, we interrogated the effect of HSV-1 infection on EV biogenesis. We found that HSV-1 infection causes a decrease in the amount of intracellular CD63 protein with a concomitant increase in extracellular CD63. This observation correlates with our previous finding that infected cells release more CD63-positive EVs than uninfected cells. The stimulation of CD63 exocytosis requires virus replication. CD63 is a member of the tetraspanin family of proteins that traffics between the plasma membrane and endosomal compartments and has a role in sorting cargo into the EVs. Previously, we reported that in cells depleted of CD63, HSV-1 virus yields increased, and here we provide data showing that in cells overexpressing CD63, HSV-1 virus yields decreased. Taken together, our data indicate that CD63 negatively impacts HSV-1 infection and that the CD63-positive EVs could control the dissemination of the virus in the host. Perhaps EV release by HSV-1-infected cells is a mechanism that controls virus dissemination.IMPORTANCE Intercellular communication, especially in neurons, largely relies on EVs, and modulation of EVs is known to impact physiological processes. Here, we present evidence that HSV-1 infection causes major alterations in the biogenesis of EVs, including an increase in their number and an increase in the CD63-positive population of EVs. These alterations result in an enrichment of the milieu of infection with EVs carrying signatures from infected cells. In addition to changes in the origin and type, EVs released by infected cells have differences in cargo, as they carry viral and host factors determined by the virus. The tetraspanin CD63 negatively impacts the infection, as demonstrated by CD63-knockdown and overexpression assays. A proposed mechanism involves the activation of antiviral responses in cells receiving CD63-positive EVs released by infected cells. Overall, HSV-1 causes major alterations in EVs that could contribute to HSV-1 persistence and pathogenesis.

Characterization of Elements Regulating the Nuclear-to-Cytoplasmic Translocation of ICP0 in Late Herpes Simplex Virus 1 Infection

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an immediate early protein containing a RING-type E3 ubiquitin ligase. It targets several host factors for proteasomal degradation and subsequently activates viral expression. ICP0 has a nuclear localization sequence and functions in the nucleus early during infection. However, later in infection, ICP0 is found solely in the cytoplasm. The molecular mechanism and biological function of the ICP0 nuclear-to-cytoplasmic translocation are not well understood. In this study, we sought to characterize elements important for this translocation. We found that (i) in human embryonic lung fibroblast (HEL) cells, ICP0 C-terminal residues 741 to 775 were necessary but not sufficient for the nuclear-to-cytoplasmic translocation; (ii) the loss of ICP0 E3 ubiquitin ligase activity, which led to defective viral replication in nonpermissive cells, also caused mutant ICP0 to be retained in the nucleus of HEL cells; (iii) in permissive U2OS cells, however, ICP0 lacking E3 ligase activity was translocated to the cytoplasm at a pace faster than that of wild-type ICP0, suggesting that nuclear retention of ICP0 occurs in an ICP0 E3 ligase-dependent manner; and (iv) the ICP0 C terminus and late viral proteins cooperate in order to overcome nuclear retention and stimulate ICP0 cytoplasmic translocation. Taken together, less ICP0 nuclear retention may contribute to the permissiveness of U2OS cells to HSV-1 in the absence of functional ICP0.IMPORTANCE A distinct characteristic for eukaryotes is the compartmentalization of cell metabolic pathways, which allows greater efficiency and specificity of cellular functions. ICP0 of HSV-1 is a multifunctional viral protein that travels through different compartments as infection progresses. Its main regulatory functions are carried out in the nucleus, but it is translocated to the cytoplasm late during HSV-1 infection. To understand the biological significance of cytoplasmic ICP0 in HSV-1 infection, we investigated the potential players involved in this nuclear-to-cytoplasmic translocation. We found that there is a nuclear retention force in an ICP0 E3 ubiquitin ligase-dependent manner. In addition, we identified the C terminus of ICP0 as a cis element cooperating with late viral proteins to overcome the nuclear retention and stimulate the nuclear-to-cytoplasmic translocation of ICP0.

Human Antiviral Protein IFIX Suppresses Viral Gene Expression during Herpes Simplex Virus 1 (HSV-1) Infection and Is Counteracted by Virus-induced Proteasomal Degradation

The interferon-inducible protein X (IFIX), a member of the PYHIN family, was recently recognized as an antiviral factor against infection with herpes simplex virus 1 (HSV-1). IFIX binds viral DNA upon infection and promotes expression of antiviral cytokines. How IFIX exerts its host defense functions and whether it is inhibited by the virus remain unknown. Here, we integrated live cell microscopy, proteomics, IFIX domain characterization, and molecular virology to investigate IFIX regulation and antiviral functions during HSV-1 infection. We find that IFIX has a dynamic localization during infection that changes from diffuse nuclear and nucleoli distribution in uninfected cells to discrete nuclear puncta early in infection. This is rapidly followed by a reduction in IFIX protein levels. Indeed, using immunoaffinity purification and mass spectrometry, we define IFIX interactions during HSV-1 infection, finding an association with a proteasome subunit and proteins involved in ubiquitin-proteasome processes. Using synchronized HSV-1 infection, microscopy, and proteasome-inhibition experiments, we demonstrate that IFIX co-localizes with nuclear proteasome puncta shortly after 3 h of infection and that its pyrin domain is rapidly degraded in a proteasome-dependent manner. We further demonstrate that, in contrast to several other host defense factors, IFIX degradation is not dependent on the E3 ubiquitin ligase activity of the viral protein ICP0. However, we show IFIX degradation requires immediate-early viral gene expression, suggesting a viral host suppression mechanism. The IFIX interactome also demonstrated its association with transcriptional regulatory proteins, including the 5FMC complex. We validate this interaction using microscopy and reciprocal isolations and determine it is mediated by the IFIX HIN domain. Finally, we show IFIX suppresses immediate-early and early viral gene expression during infection. Altogether, our study demonstrates that IFIX antiviral functions work in part via viral transcriptional suppression and that HSV-1 has acquired mechanisms to block its functions via proteasome-dependent degradation.

A Tale of Two PMLs: Elements Regulating a Differential Substrate Recognition by the ICP0 E3 Ubiquitin Ligase of Herpes Simplex Virus 1

Infected cell protein 0 (ICP0) of herpes simplex virus 1 (HSV-1) is an α gene product required for viral replication at low multiplicities of infection. Upon entry, nuclear domain 10 (ND10) converges at the incoming DNA and represses viral gene expression. ICP0 contains a RING-type E3 ubiquitin ligase that degrades the ND10 organizer PML and disperses ND10 to alleviate the repression. In the present study, we focused on understanding the regulation of ICP0 E3 ligase activity in the degradation of different ICP0 substrates. We report the following. (i) A SUMO interaction motif located at ICP0 residues 362 to 364 is required for the degradation of PML isoforms II, IV, and VI but not isoform I. This differentiation mechanism exists in both HEp-2 and U2OS cells, regardless of the cell's permissiveness to the ICP0-null virus. (ii) Physical interaction between SIM_362-364 and PML II is necessary but not sufficient for PML II degradation. Both proximal sequences surrounding SIM_362-364 and distal sequences located at the ICP0 C terminus enhance the degradation of PML II. (iii) The ICP0 C terminus is dispensable for PML I degradation. Instead, bipartite PML I binding domains located in the N-terminal half of ICP0 coordinate to promote the degradation of PML I. (iv) The stability of ICP0, but not its ND10 fusion ability, affects the rate of PML I degradation. Taken together, our results show that ICP0 uses at least two regulatory mechanisms to differentiate its substrates. The disparate recognition of the ICP0 E3 substrates may be related to the different roles these substrates may play in HSV-1 infection.

IMPORTANCE

Viruses have a limited genetic coding capacity but must encounter a multilayered comprehensive host defense. To establish a successful infection, viruses usually produce multifunctional proteins to coordinate the counteractions. Here we report that an HSV-1 protein, ICP0, can recognize individual host factors and target them differently for destruction. We identified elements that are important for the ICP0 E3 ubiquitin ligase to differentially recognize two of its substrates, PML I and PML II. This is the first study that has systematically investigated how ICP0 discriminates two similar molecules by very different mechanisms. This work lays the foundation for understanding the role of host defensive factors and the mechanisms viruses use to take advantage of some host proteins while destroying others.

Role of ND10 nuclear bodies in the chromatin repression of HSV-1

Herpes simplex virus (HSV) is a neurotropic virus that establishes lifelong latent infection in human ganglion sensory neurons. This unique life cycle necessitates an intimate relation between the host defenses and virus counteractions over the long course of infection. Two important aspects of host anti-viral defense, nuclear substructure restriction and epigenetic chromatin regulation, have been intensively studied in the recent years. Upon viral DNA entering the nucleus, components of discrete nuclear bodies termed nuclear domain 10 (ND10), converge at viral DNA and place restrictions on viral gene expression. Meanwhile the infected cell mobilizes its histones and histone-associated repressors to force the viral DNA into nucleosome-like structures and also represses viral transcription. Both anti-viral strategies are negated by various HSV countermeasures. One HSV gene transactivator, infected cell protein 0 (ICP0), is a key player in antagonizing both the ND10 restriction and chromatin repression. On one hand, ICP0 uses its E3 ubiquitin ligase activity to target major ND10 components for proteasome-dependent degradation and thereafter disrupts the ND10 nuclear bodies. On the other hand, ICP0 participates in de-repressing the HSV chromatin by changing histone composition or modification and therefore activates viral transcription. Involvement of a single viral protein in two seemingly different pathways suggests that there is coordination in host anti-viral defense mechanisms and also cooperation in viral counteraction strategies. In this review, we summarize recent advances in understanding the role of chromatin regulation and ND10 dynamics in both lytic and latent HSV infection. We focus on the new observations showing that ND10 nuclear bodies play a critical role in cellular chromatin regulation. We intend to find the connections between the two major anti-viral defense pathways, chromatin remodeling and ND10 structure, in order to achieve a better understanding of how host orchestrates a concerted defense and how HSV adapts with and overcomes the host immunity.

Interactions of the Antiviral Factor Interferon Gamma-Inducible Protein 16 (IFI16) Mediate Immune Signaling and Herpes Simplex Virus-1 Immunosuppression

The interferon-inducible protein IFI16 has emerged as a critical antiviral factor and sensor of viral DNA. IFI16 binds nuclear viral DNA, triggering expression of antiviral cytokines during infection with herpesviruses. The knowledge of the mechanisms and protein interactions through which IFI16 exerts its antiviral functions remains limited. Here, we provide the first characterization of endogenous IFI16 interactions following infection with the prominent human pathogen herpes simplex virus 1 (HSV-1). By integrating proteomics and virology approaches, we identified and validated IFI16 interactions with both viral and host proteins that are involved in HSV-1 immunosuppressive mechanisms and host antiviral responses. We discover that during early HSV-1 infection, IFI16 is recruited to sub-nuclear puncta and subsequently targeted for degradation. We observed that the HSV-1 E3 ubiquitin ligase ICP0 is necessary, but not sufficient, for the proteasom e-mediated degradation of IFI16 following infection. We substantiate that this ICP0-mediated mechanism suppresses IFI16-dependent immune responses. Utilizing an HSV-1 strain that lacks ICP0 ubiquitin ligase activity provided a system for studying IFI16-dependent cytokine responses to HSV-1, as IFI16 levels were maintained throughout infection. We next defined temporal IFI16 interactions during this immune signaling response. We discovered and validated interactions with the viral protein ICP8 and cellular ND10 nuclear body components, sites at which HSV-1 DNA is present during infection. These interactions may be critical for IFI16 to bind to nuclear viral DNA. Altogether, our results provide critical insights into both viral inhibition of IFI16 and interactions that can contribute to IFI16 antiviral functions.

Advances in targeting nucleocapsid-nucleic acid interactions in HIV-1 therapy

The continuing challenge of HIV-1 treatment resistance in patients creates a need for the development of new antiretroviral inhibitors. The HIV nucleocapsid (NC) protein is a potential therapeutic target. NC is necessary for viral RNA packaging and in the early stages of viral infection. The high level of NC amino acid conservation among all HIV-1 clades suggests a low tolerance for mutations. Thus, NC mutations that could arise during inhibitor treatment to provide resistance may render the virus less fit. Disruption of NC function provides a unique opportunity to strongly dampen replication at multiple points during the viral life cycle with a single inhibitor. Although NC exhibits desirable features for a potential antiviral target, the structural flexibility, size, and the presence of two zinc fingers makes small molecule targeting of NC a challenging task. In this review, we discuss the recent advances in strategies to develop inhibitors of NC function and present a perspective on potential novel approaches that may help to overcome some of the current challenges in the field.

Identification of TRIM27 as a novel degradation target of herpes simplex virus 1 ICP0

The herpes simplex virus 1 (HSV-1) immediate early protein ICP0 performs many functions during infection, including transactivation of viral gene expression, suppression of innate immune responses, and modification and eviction of histones from viral chromatin. Although these functions of ICP0 have been characterized, the detailed mechanisms underlying ICP0's complex role during infection warrant further investigation. We thus undertook an unbiased proteomic approach to identifying viral and cellular proteins that interact with ICP0 in the infected cell. Cellular candidates resulting from our analysis included the ubiquitin-specific protease USP7, the transcriptional repressor TRIM27, DNA repair proteins NBN and MRE11A, regulators of apoptosis, including BIRC6, and the proteasome. We also identified two HSV-1 early proteins involved in nucleotide metabolism, UL39 and UL50, as novel candidate interactors of ICP0. Because TRIM27 was the most statistically significant cellular candidate, we investigated the relationship between TRIM27 and ICP0. We observed rapid, ICP0-dependent loss of TRIM27 during HSV-1 infection. TRIM27 protein levels were restored by disrupting the RING domain of ICP0 or by inhibiting the proteasome, arguing that TRIM27 is a novel degradation target of ICP0. A mutant ICP0 lacking E3 ligase activity interacted with endogenous TRIM27 during infection as demonstrated by reciprocal coimmunoprecipitation and supported by immunofluorescence data. Surprisingly, ICP0-null mutant virus yields decreased upon TRIM27 depletion, arguing that TRIM27 has a positive effect on infection despite being targeted for degradation. These results illustrate a complex interaction between TRIM27 and viral infection with potential positive or negative effects of TRIM27 on HSV under different infection conditions.

IMPORTANCE

During productive infection, a virus must simultaneously redirect multiple cellular pathways to replicate itself while evading detection by the host's defenses. To orchestrate such complex regulation, viruses, including herpes simplex virus 1 (HSV-1), rely on multifunctional proteins such as the E3 ubiquitin ligase ICP0. This protein regulates various cellular pathways concurrently by targeting a diverse set of cellular factors for degradation. While some of these targets have been previously identified and characterized, we undertook a proteomic screen to identify additional targets of this activity to further characterize ICP0's role during infection. We describe a set of candidate interacting proteins of ICP0 identified through this approach and our characterization of the most statistically significant result, the cellular transcriptional repressor TRIM27. We present TRIM27 as a novel degradation target of ICP0 and describe the relationship of these two proteins during infection.

Novel roles of cytoplasmic ICP0: proteasome-independent functions of the RING finger are required to block interferon-stimulated gene production but not to promote viral replication

The immediate-early protein ICP0 from herpes simplex virus 1 (HSV-1) plays pleiotropic roles in promoting viral lytic replication and reactivation from latency. Most of the known actions of ICP0 occur in the nucleus and are thought to involve the E3 ubiquitin ligase activity of its RING finger domain, which targets proteins for degradation via the proteasome. Although ICP0 translocates to the cytoplasm as the infection progresses, little is known about its activities in this location. Here, we show that cytoplasmic ICP0 has two distinct functions. In primary cell cultures and in an intravaginal mouse model, cytoplasmic ICP0 promotes viral replication in the absence of an intact RING finger domain. Additionally, ICP0 blocks the activation of interferon regulatory factor 3 (IRF3), a key transcription factor of the innate antiviral response, in a mechanism that requires the RING finger domain but not the proteasome. To our knowledge, this is the first observation of a proteasome-independent function of the RING finger domain of ICP0. Collectively, these results underscore the importance of cytoplasm-localized ICP0 and the diverse nature of its activities. Importance: Despite ICP0 being a well-studied viral protein, the significance of its cytoplasmic localization has been largely overlooked. This is, in part, because common experimental manipulations result in the restriction of ICP0 to the nucleus. By overcoming this constraint, we both further characterize the ability of cytoplasmic ICP0 to inhibit antiviral signaling and show that ICP0 at this site has unexpected activities in promoting viral replication. This demonstrates the importance of considering location when analyzing protein function and adds a new perspective to our understanding of this multifaceted protein.

The stability of herpes simplex virus 1 ICP0 early after infection is defined by the RING finger and the UL13 protein kinase

Herpes simplex virus 1 (HSV-1)-infected cell protein 0 (ICP0) is a multifunctional protein that plays a key role in overcoming numerous facets of host innate immunity. A key function of ICP0 that requires an intact RING finger domain is that of an ubiquitin E3 ligase: ICP0 interacts with at least three ubiquitin-conjugating enzymes of which one, UbcH5a, is required for degradation of PML and SP100. A preceding report showed that ICP0 is highly unstable at very early times after infection but becomes stable at later times. We report here that (i) the degradation of ICP0 is not infected cell specific, (ii) the degradation does not require the interaction of ICP0 with either UbcH5a, UbcH6, or UbcH9, (iii) ICP0 is degraded both early and late in cells infected with a mutant lacking the UL13 protein kinase, (iv) ICP0 encoded by wild-type virus or the ΔUL13 mutant is stable in cells transfected with a plasmid encoding UL13 before infection, (v) ICP0 carrying mutations in the RING finger domain is stable both early and late in infection, and, finally, (vi) in cells infected with both wild type and RING finger mutant only the wild-type ICP0 is rapidly degraded at early times. The results suggest that the stability of ICP0 is mediated by the UL13 protein kinase and that the target of proteolysis is a site at or near the RING domain of ICP0.

IMPORTANCE

ICP0, a major regulatory protein of HSV-1, turns over rapidly early in infection but becomes stable at late times. We report that stabilization requires the presence of UL13 protein kinase and that an ICP0 with mutations in RING finger is stable. In mixed infections mutant ICP0 is stable, whereas the wild-type ICP0 is degraded. Our findings suggest that the lifestyle of HSV-1 requires an ICP0 that turns over rapidly if late proteins are absent.

HSV-1 degrades, stabilizes, requires, or is stung by STING depending on ICP0, the US3 protein kinase, and cell derivation

STING (stimulator of IFN genes) activates the IFN pathway in response to cytosolic DNA. Knockout of STING in mice was reported to exacerbate the pathogenicity of herpes simplex virus 1 (HSV-1). Here we report the following: (i) STING is stable in cancer-derived HEp-2 or HeLa cells infected with wild-type HSV-1 but is degraded in cells infected with mutants lacking the genes encoding functional infected cell protein 0 (ICP0), ICP4, or the US3 protein kinase (US3-PK). In HEp-2 cells, depletion of STING by shRNA results in a decrease in the yields of wild-type or ΔICP0 viruses. (ii) STING is stable throughout infection with either wild-type or ICP0 mutant viruses in human embryonic lung cells (HEL) or HEK293T cells derived from normal tissues. In these cells, depletion of STING results in higher yields of both wild-type and ΔICP0 viruses. (iii) The US3-PK is also required for stabilization of IFI16, a nuclear DNA sensor. However, the stability of IFI16 does not correlate positively or negatively with that of STING. IFI16 is stable in STING-depleted HEL cells infected with wild-type virus. In contrast to HEL cells, IFI16 was undetectable in STING-depleted HEp-2 cells, and hence the role of HSV-1 in maintaining IFI16 could not be ascertained. The results indicate that in HSV-1-infected cells the stability of IFI16 and the function and stability of STING are dependent on cell derivation, the functional integrity of ICP0, and US3-PK, an indication that in wild-type virus-infected cells both proteins are actively stabilized. In HEp-2 cells, the stability of IFI16 requires STING.

A proteomic perspective of inbuilt viral protein regulation: pUL46 tegument protein is targeted for degradation by ICP0 during herpes simplex virus type 1 infection

Much like the host cells they infect, viruses must also regulate their life cycles. Herpes simples virus type 1 (HSV-1), a prominent human pathogen, uses a promoter-rich genome in conjunction with multiple viral trans-activating factors. Following entry into host cells, the virion-associated outer tegument proteins pUL46 and pUL47 act to increase expression of viral immediate-early (α) genes, thereby helping initiate the infection life cycle. Because pUL46 has gone largely unstudied, we employed a hybrid mass spectrometry-based approach to determine how pUL46 exerts its functions during early stages of infection. For a spatio-temporal characterization of pUL46, time-lapse microscopy was performed in live cells to define its dynamic localization from 2 to 24 h postinfection. Next, pUL46-containing protein complexes were immunoaffinity purified during infection of human fibroblasts and analyzed by mass spectrometry to investigate virus-virus and virus-host interactions, as well as post-translational modifications. We demonstrated that pUL46 is heavily phosphorylated in at least 23 sites. One phosphorylation site matched the consensus 14-3-3 phospho-binding motif, consistent with our identification of 14-3-3 proteins and host and viral kinases as specific pUL46 interactions. Moreover, we determined that pUL46 specifically interacts with the viral E3 ubiquitin ligase ICP0. We demonstrated that pUL46 is partially degraded in a proteasome-mediated manner during infection, and that the catalytic activity of ICP0 is responsible for this degradation. This is the first evidence of a viral protein being targeted for degradation by another viral protein during HSV-1 infection. Together, these data indicate that pUL46 levels are tightly controlled and important for the temporal regulation of viral gene expression throughout the virus life cycle. The concept of a structural virion protein, pUL46, performing nonstructural roles is likely to reflect a theme common to many viruses, and a better understanding of these functions will be important for developing therapeutics.

Interaction of herpes simplex virus ICP0 with ND10 bodies: a sequential process of adhesion, fusion, and retention

On entry into the nucleus, herpes simplex virus 1 (HSV-1) DNA localizes to nuclear bodies known as ND10. Gene repression imposed by ND10 is released by a viral protein, ICP0, via degradation of the ND10 constituents promyelocytic leukemia protein (PML) and Sp100 and the subsequent dispersal of ND10 bodies. In order to understand the dynamic interaction between ICP0 and ND10, we carried out deletion mapping to identify the domains of ICP0 responsible for its association with ND10. Here, we report the following. (i) An ND10 entry signal (ND10-ES), located between residues 245 and 474, is required for ICP0 to penetrate and fuse with ND10. ICP0 lacking ND10-ES adheres to the surface of ND10 but fails to enter. (ii) In the absence of ND10-ES, the E3 ubiquitin ligase of ICP0 facilitates the transient adhesion of the truncated ICP0 to the ND10 surface, whereas the presence of ND10-ES in ICP0 renders ND10 fusion regardless of the E3 ligase activity. (iii) The C terminus of ICP0 is required for retention of ICP0 in ND10 but plays no role in the recruitment process. (iv) The adverse effects of an inactive RING domain on viral replication are partially reversed by deleting either ND10-ES or the C-terminal retention domain, suggesting that additional ICP0 functions require the release of ICP0 from ND10. Based on these results, we conclude that association of ICP0 and ND10 is a dynamic process, in which three sequential steps--adhesion, fusion, and retention--are adopted to stabilize the interaction. A faithful execution of these steps defines the ultimate productivity of the virus.

Regulation of alphaherpesvirus infections by the ICP0 family of proteins

Immediate-early protein ICP0 of herpes simplex virus type 1 (HSV-1) is important for the regulation of lytic and latent viral infection. Like the related proteins expressed by other alphaherpesviruses, ICP0 has a zinc-stabilized RING finger domain that confers E3 ubiquitin ligase activity. This domain is essential for the core functions of ICP0 and its activity leads to the degradation of a number of cellular proteins, some of which are involved in cellular defences that restrict viral infection. The article reviews recent advances in ICP0-related research, with an emphasis on the mechanisms by which ICP0 and related proteins counteract antiviral restriction and the roles in this process of cellular nuclear substructures known as ND10 or PML nuclear bodies. We also summarize recent advances in the understanding of the biochemical aspects of ICP0 activity. These studies highlight the importance of the SUMO conjugation pathway in both intrinsic resistance to HSV-1 infection and in substrate targeting by ICP0. The topics discussed in this review are relevant not only to HSV-1 infection, but also to cellular intrinsic resistance against herpesviruses more generally and the mechanisms by which viruses can evade this restriction.

Nuclear IFI16 induction of IRF-3 signaling during herpesviral infection and degradation of IFI16 by the viral ICP0 protein

Innate sensing of microbial components is well documented to occur at many cellular sites, including at the cell surface, in the cytosol, and in intracellular vesicles, but there is limited evidence of nuclear innate signaling. In this study we have defined the mechanisms of interferon regulatory factor-3 (IRF-3) signaling in primary human foreskin fibroblasts (HFF) infected with herpes simplex virus 1 (HSV-1) in the absence of viral gene expression. We found that the interferon inducible protein 16 (IFI16) DNA sensor, which is required for induction of IRF-3 signaling in these cells, is nuclear, and its localization does not change detectably upon HSV-1 d109 infection and induction of IRF-3 signaling. Consistent with the IFI16 sensor being nuclear, conditions that block viral DNA release from incoming capsids inhibit IRF-3 signaling. An unknown factor must be exported from the nucleus to activate IRF-3 through cytoplasmic STING, which is required for IRF-3 activation and signaling. However, when the viral ICP0 protein is expressed in the nucleus, it causes the nuclear relocalization and degradation of IFI16, inhibiting IRF-3 signaling. Therefore, HSV-1 infection is sensed in HFF by nuclear IFI16 upon release of encapsidated viral DNA into the nucleus, and the viral nuclear ICP0 protein can inhibit the process by targeting IFI16 for degradation. Together these results define a pathway for nuclear innate sensing of HSV DNA by IFI16 in infected HFF and document a mechanism by which a virus can block this nuclear innate response.

Overexpression of the ubiquitin-specific protease 7 resulting from transfection or mutations in the ICP0 binding site accelerates rather than depresses herpes simplex virus 1 gene expression

Earlier studies reported that ICP0, a key regulatory protein encoded by herpes simplex virus 1 (HSV-1), binds ubiquitin-specific protease 7 (USP7). The fundamental conclusion of these studies is that depletion of USP7 destabilized ICP0, that ICP0 mediated the degradation of USP7, and that amino acid substitutions in ICP0 that abolished binding to USP7 significantly impaired the ability of HSV-1 to replicate. We show here that, indeed, depletion of USP7 leads to reduction of ICP0 and that USP7 is degraded in an ICP0-dependent manner. However, overexpression of USP7 or substitution in ICP0 of a single amino acid to abolish binding to USP7 accelerated the accumulation of viral mRNAs and proteins at early times after infection and had no deleterious effect on virus yields. A clue as to why USP7 is degraded emerged from the observation that, notwithstanding the accelerated expression of viral genes, the plaques formed by the mutant virus were very small, implying a defect in virus transmission from cell to cell.

Herpes simplex virus 1 infection activates poly(ADP-ribose) polymerase and triggers the degradation of poly(ADP-ribose) glycohydrolase

Herpes simplex virus 1 infection triggers multiple changes in the metabolism of host cells, including a dramatic decrease in the levels of NAD(+). In addition to its role as a cofactor in reduction-oxidation reactions, NAD(+) is required for certain posttranslational modifications. Members of the poly(ADP-ribose) polymerase (PARP) family of enzymes are major consumers of NAD(+), which they utilize to form poly(ADP-ribose) (PAR) chains on protein substrates in response to DNA damage. PAR chains can subsequently be removed by the enzyme poly(ADP-ribose) glycohydrolase (PARG). We report here that the HSV-1 infection-induced drop in NAD(+) levels required viral DNA replication, was associated with an increase in protein poly(ADP-ribosyl)ation (PARylation), and was blocked by pharmacological inhibition of PARP-1/PARP-2 (PARP-1/2). Neither virus yield nor the cellular metabolic reprogramming observed during HSV-1 infection was altered by the rescue or further depletion of NAD(+) levels. Expression of the viral protein ICP0, which possesses E3 ubiquitin ligase activity, was both necessary and sufficient for the degradation of the 111-kDa PARG isoform. This work demonstrates that HSV-1 infection results in changes to NAD(+) metabolism by PARP-1/2 and PARG, and as PAR chain accumulation can induce caspase-independent apoptosis, we speculate that the decrease in PARG levels enhances the auto-PARylation-mediated inhibition of PARP, thereby avoiding premature death of the infected cell.

Functional characterization of residues required for the herpes simplex virus 1 E3 ubiquitin ligase ICP0 to interact with the cellular E2 ubiquitin-conjugating enzyme UBE2D1 (UbcH5a)

The viral ubiquitin ligase ICP0 is required for efficient initiation of herpes simplex virus 1 (HSV-1) lytic infection and productive reactivation of viral genomes from latency. ICP0 has been shown to target a number of specific cellular proteins for proteasome-dependent degradation during lytic infection, including the promyelocytic leukemia protein (PML) and its small ubiquitin-like modified (SUMO) isoforms. We have shown previously that ICP0 can catalyze the formation of unanchored polyubiquitin chains and mediate the ubiquitination of specific substrate proteins in vitro in the presence of two E2 ubiquitin-conjugating enzymes, namely, UBE2D1 (UbcH5a) and UBE2E1 (UbcH6), in a RING finger-dependent manner. Using homology modeling in conjunction with site-directed mutagenesis, we identify specific residues required for the interaction between the RING finger domain of ICP0 and UBE2D1, and we report that point mutations at these residues compromise the ability of ICP0 to induce the colocalization of conjugated ubiquitin and the degradation of PML and its SUMO-modified isoforms. Furthermore, we show that RING finger mutants that are unable to interact with UBE2D1 fail not only to complement the plaque-forming defect of an ICP0-null mutant virus but also to mediate the derepression of quiescent HSV-1 genomes in cell culture. These data demonstrate that the ability of ICP0 to interact with cellular E2 ubiquitin-conjugating enzymes is fundamentally important for its biological functions during HSV-1 infection.

Depletion of intracellular zinc inhibits the ubiquitin ligase activity of viral regulatory protein ICP0 and restricts herpes simplex virus 1 replication in cell culture

The viral ubiquitin ligase ICP0 stimulates the onset of HSV-1 lytic infection and productive reactivation of viral genomes from latency. In order to mediate these processes, it requires its C3HC4 RING finger domain, a tertiary structural fold that is coordinated by the binding of two zinc (Zn(2+)) atoms. Here we formally demonstrate that Zn(2+) binding and intracellular Zn(2+) levels are critical for ICP0's biochemical activity and that depletion of intracellular Zn(2+) severely attenuates HSV-1 replication.

A viral ubiquitin ligase has substrate preferential SUMO targeted ubiquitin ligase activity that counteracts intrinsic antiviral defence

Intrinsic antiviral resistance represents the first line of intracellular defence against virus infection. During herpes simplex virus type-1 (HSV-1) infection this response can lead to the repression of viral gene expression but is counteracted by the viral ubiquitin ligase ICP0. Here we address the mechanisms by which ICP0 overcomes this antiviral response. We report that ICP0 induces the widespread proteasome-dependent degradation of SUMO-conjugated proteins during infection and has properties related to those of cellular SUMO-targeted ubiquitin ligases (STUbLs). Mutation of putative SUMO interaction motifs within ICP0 not only affects its ability to degrade SUMO conjugates, but also its capacity to stimulate HSV-1 lytic infection and reactivation from quiescence. We demonstrate that in the absence of this viral countermeasure the SUMO conjugation pathway plays an important role in mediating intrinsic antiviral resistance and the repression of HSV-1 infection. Using PML as a model substrate, we found that whilst ICP0 preferentially targets SUMO-modified isoforms of PML for degradation, it also induces the degradation of PML isoform I in a SUMO modification-independent manner. PML was degraded by ICP0 more rapidly than the bulk of SUMO-modified proteins in general, implying that the identity of a SUMO-modified protein, as well as the presence of SUMO modification, is involved in ICP0 targeting. We conclude that ICP0 has dual targeting mechanisms involving both SUMO- and substrate-dependent targeting specificities in order to counteract intrinsic antiviral resistance to HSV-1 infection.

Viruses must evade several antiviral defences in order to establish a productive infection. These include antibody- and cell-mediated acquired immunity and interferon-regulated innate immunity. Recently, a third arm of antiviral defence has been discovered, so called intrinsic immunity. This aspect of antiviral resistance represents the first line of intracellular defence against virus infection and is mediated by pre-existing cellular factors that attempt to repress viral replication during the initial stages of infection. Like acquired and innate immunity, viruses have evolved mechanisms that overcome intrinsic defence. Here we show that in response to herpes simplex virus type-1 (HSV-1) infection an important aspect of intrinsic immunity is regulated by the small ubiquitin-like modifier (SUMO) conjugation pathway. In response to this defence, the virus induces rapid degradation of specific SUMO-conjugated proteins, followed by widespread loss of SUMO-conjugated species in general. Inactivation of the SUMO pathway inhibits the cell’s ability to efficiently repress viral replication in the absence of this viral countermeasure. Our data identifies an important regulatory pathway that mediates intrinsic resistance to HSV-1 infection and describes the biochemical mechanism that the virus utilizes in order to counteract this antiviral defence.

Cytoplasmic localized infected cell protein 0 (bICP0) encoded by bovine herpesvirus 1 inhibits β interferon promoter activity and reduces IRF3 (interferon response factor 3) protein levels

Bovine herpesvirus 1 (BHV-1), an alpha-herpesvirinae subfamily member, establishes a life-long latent infection in sensory neurons. Periodically, BHV-1 reactivates from latency, infectious virus is spread, and consequently virus transmission occurs. BHV-1 acute infection causes upper respiratory track infections and conjunctivitis in infected cattle. As a result of transient immune-suppression, BHV-1 infections can also lead to life-threatening secondary bacterial pneumonia that is referred to as bovine respiratory disease. The infected cell protein 0 (bICP0) encoded by BHV-1 reduces human β-interferon (IFN-β) promoter activity, in part, by inducing degradation of interferon response factor 3 (IRF3) and interacting with IRF7. In contrast to humans, cattle contain three IFN-β genes. All three bovine IFN-β proteins have anti-viral activity: but each IFN-β gene has a distinct transcriptional promoter. We have recently cloned and characterized the three bovine IFN-β promoters. Relative to the human IFN-β promoter, each of the three IFN-β promoters contain differences in the four positive regulatory domains that are required for virus-induced activity. In this study, we demonstrate that bICP0 effectively inhibits bovine IFN-β promoter activity following transfection of low passage bovine cells with interferon response factor 3 (IRF3) or IRF7. A bICP0 mutant that localizes to the cytoplasm inhibits bovine IFN-β promoter activity as efficiently as wt bICP0. The cytoplasmic localized bICP0 protein also induced IRF3 degradation with similar efficiency as wt bICP0. In summary, these studies suggested that cytoplasmic localized bICP0 plays a role in inhibiting the IFN-β response during productive infection.

HSV-1 ICP0: paving the way for viral replication

Herpes simplex virus type 1 (HSV-1) has two distinct phases of its viral life cycle: lytic and latent. One viral immediate-early protein that is responsible for determining the balance between productive lytic replication and reactivation from latency is infected cell protein 0 (ICP0). ICP0 is a 775-amino acid really interesting new gene (RING)-finger-containing protein that possesses E3 ubiquitin ligase activity, which is required for ICP0 to activate HSV-1 gene expression, disrupt nuclear domain (ND) 10 structures, mediate the degradation of cellular proteins, and evade the host cell's intrinsic and innate antiviral defenses. This article examines our current understanding of ICP0's transactivating, E3 ubiquitin ligase, and antihost defense activities and their inter-relationships to one another. Lastly, we will discuss how these properties of ICP0 may be utilized as possible targets for HSV-1 antiviral therapies.

Activities of ICP0 involved in the reversal of silencing of quiescent herpes simplex virus 1

ICP0 is a transcriptional activating protein required for the efficient replication and reactivation of latent herpes simplex virus 1 (HSV-1). Multiple regions of ICP0 contribute its activity, the most prominent of which appears to be the RING finger, which confers E3 ubiquitin ligase activity. A region in the C terminus of ICP0 has also been implicated in several activities, including the disruption of a cellular repressor complex, REST/CoREST/HDAC1/2/LSD1. We used quiescent infection of MRC-5 cells with a virus that does not express immediate-early proteins, followed by superinfection with various viral mutants to quantify the ability of ICP0 variants to reactivate gene expression and alter chromatin structure. Superinfection with wild-type virus resulted in a 400-fold increase in expression from the previously quiescent d109 genome, the removal of heterochromatin and histones from the viral genome, and an increase in histone marks associated with activated transcription. RING finger mutants were unable to reactivate transcription or remove heterochromatin from d109, while mutants that are unable to bind CoREST activate gene expression from quiescent d109, albeit to a lesser degree than the wild-type virus. One such mutant, R8507, resulted in the partial removal of heterochromatin. Infection with R8507 did not result in the hyperacetylation of H3 and H4. The results demonstrate that (i) consistent with previous findings, the RING finger domain of ICP0 is required for the activation of quiescent genomes, (ii) the RF domain is also crucial for the ultimate removal of repressive chromatin, (iii) activities or interactions specified by the carboxy-terminal region of ICP0 significantly contribute to activation, and (iv) while the effects of the R8507 on chromatin are consistent with a role for REST/CoREST/HDAC1/2/LSD1 in the repression of quiescent genomes, the mutation may also affect other activities involved in derepression.

Regulation of promyelocytic leukemia (PML) protein levels and cell morphology by bovine herpesvirus 1 infected cell protein 0 (bICP0) and mutant bICP0 proteins that do not localize to the nucleus

BHV-1 is an important pathogen of cattle. The infected cell protein 0 (bICP0) encoded by BHV-1 is an important regulatory protein because it is constitutively expressed and can activate all viral promoters. The mechanism by which bICP0 activates viral promoters is not well understood because bICP0 does not appear to be a sequence specific binding protein. A C(3)HC(4) zinc RING (really interesting novel gene) motif at the N-terminus of bICP0 has E3 ubiquitin ligase activity, which is important for activating viral gene expression and inhibiting interferon dependent transcription. Like other alpha-herpesvirinae ICP0 homologues, bICP0 is associated with promyelocytic leukemia (PML) protein-containing nuclear domains. During productive infection of cultured cells, BHV-1 induces degradation of the PML protein, which correlates with efficient productive infection. In this study, we demonstrated that a plasmid expressing bICP0 reduces steady state levels of the PML protein, and the C(3)HC(4) zinc RING finger is important for PML degradation. Surprisingly, bICP0 mutants with an intact C(3)HC(4) zinc RING finger that lack a nuclear localization signal also reduces steady PML protein levels. In addition, mutant bICP0 proteins that primarily localize to the cytoplasm induced morphological changes in transfected cells. During productive infection, bICP0 was detected in the cytoplasm of low-passage bovine kidney, but not established bovine kidney cells. These studies demonstrated that bICP0, even when not able to efficiently localize to the nucleus, was able to induce degradation of the PML protein and alter the morphology of transfected cells.

Herpes simplex virus immediate-early ICP0 protein inhibits Toll-like receptor 2-dependent inflammatory responses and NF-kappaB signaling

The discovery of the Toll-like receptors (TLRs) and their importance in the regulation of host responses to infection raised attention to the complex interplay between viral gene products and the host innate immune responses in determining the outcome of virus infection. Robust inflammatory cytokine responses are observed in herpes simplex virus (HSV)-infected animals and cells. Our studies have demonstrated that Toll-like receptor 2 (TLR2) activation by HSV results in NF-κB activation with concomitant inflammatory cytokine production and that TLR2 activation plays a critical role in HSV-induced pathology and mortality. Here we demonstrate that the HSV-1 immediate-early ICP0 protein reduces the TLR2-mediated inflammatory response to HSV 1 (HSV-1) infection. Expression of ICP0 alone is sufficient to block TLR2-driven responses to both viral and nonviral ligands at or downstream of the MyD88 adaptor and upstream of p65. ICP0 alone can also reduce the levels of MyD88 and Mal (TIRAP). In HSV-infected cells, the E3 ligase function of ICP0 and cellular proteasomal activity are required for the inhibitory activity. Our results argue for a model in which ICP0 promotes the degradation of TLR adaptor molecules and inhibition of the inflammatory response, much as it inhibits the interferon response by sequestration and degradation of interferon regulatory factor 3 (IRF-3).

During its nuclear phase the multifunctional regulatory protein ICP0 undergoes proteolytic cleavage characteristic of polyproteins

ICP0 is a multifunctional herpes simplex virus protein known primarily as a promiscuous transactivator. In the course of productive infection, it is localized during the first 5-7 h in the nucleus and later in the cytoplasm. In the nucleus, its primary activities are to suppress the silencing of viral DNA by host proteins, activate cdk4 through recruitment of cyclin D3 to the sites of formation of replication compartments, and degrade several cellular proteins including PML and Sp100, key components of the ND10 nuclear bodies. ICP0 is not translocated to the cytoplasm in cells infected with mutants incapable of performing these tasks. We report the unexpected finding that ICP0 is cleaved into several discrete polypeptides by a proteasome-independent process. The products of this cleavage accumulate in cells infected with ICP0 mutants incapable of degrading PML and therefore are retained in the nucleus. In the second step, the products of the initial cleavage of wild-type virus-infected cells are themselves subject to proteasome-dependent degradation. The average half life of intact ICP0 during the nuclear phase is approximately 1 h. The proteasome-independent cleavage products are no longer detected at late times corresponding to the cytoplasmic phase of ICP0. The results are consistent with the hypothesis that the cleavage products of ICP0 function in topologically distinct domains during its nuclear phase.

The transactivating effect of HSV-1 ICP0 is enhanced by its interaction with the PCAF component of histone acetyltransferase

ICP0 is a multifunctional protein that plays diverse roles in herpes simplex virus type 1 (HSV-1) infection. It can promote the lytic replication of HSV-1 and activate a variety of viral or cellular genes when introduced into cells by transfection or infection. However, the exact mechanism of ICP0 action is not fully understood. In the present study, we observed the co-localization of ICP0 and PCAF (P300/CBP-associated factor), a component of histone acetyltransferase (HAT), in the ND10 (nuclear dot 10) nuclear body. We further confirmed the interaction between ICP0 and PCAF via yeast two-hybrid assay, co-immunoprecipitation, and histone acetyltransferase assays. Analysis of the functional significance of this interaction suggested that PCAF improved the ability of ICP0 to activate transcription of viral genes. Using chromatin immunoprecipitation (ChIP) assays, we observed ICP0-enhanced histone acetylation levels in both viral and cellular gene promoters. Our study suggests that ICP0 regulates transcription through specific interaction with PCAF.

Complementation of a herpes simplex virus ICP0 null mutant by varicella-zoster virus ORF61p

The herpes simplex virus (HSV) ICP0 protein acts to overcome intrinsic cellular defenses that repress viral alpha gene expression. In that vein, viruses that have mutations in ICP0's RING finger or are deleted for the gene are sensitive to interferon, as they fail to direct degradation of promyelocytic leukemia protein (PML), a component of host nuclear domain 10s. While varicella-zoster virus is also insensitive to interferon, ORF61p, its ICP0 ortholog, failed to degrade PML. A recombinant virus with each coding region of the gene for ICP0 replaced with sequences encoding ORF61p was constructed. This virus was compared to an ICP0 deletion mutant and wild-type HSV. The recombinant degraded only Sp100 and not PML and grew to higher titers than its ICP0 null parental virus, but it was sensitive to interferon, like the virus from which it was derived. This analysis permitted us to compare the activities of ICP0 and ORF61p in identical backgrounds and revealed distinct biologic roles for these proteins.

Regulation of Innate Immune Responses by Bovine Herpesvirus 1 and Infected Cell Protein 0 (bICP0)

Bovine herpesvirus 1 (BoHV-1) infected cell protein 0 (bICP0) is an important transcriptional regulatory protein that stimulates productive infection. In transient transfection assays, bICP0 also inhibits interferon dependent transcription. bICP0 can induce degradation of interferon stimulatory factor 3 (IRF3), a cellular transcription factor that is crucial for activating beta interferon (IFN-β) promoter activity. Recent studies also concluded that interactions between bICP0 and IRF7 inhibit trans-activation of IFN-β promoter activity. The C3HC4 zinc RING (really important new gene) finger located near the amino terminus of bICP0 is important for all known functions of bICP0. A recombinant virus that contains a single amino acid change in a well conserved cysteine residue of the C3HC4 zinc RING finger of bICP0 grows poorly in cultured cells, and does not reactivate from latency in cattle confirming that the C3HC4 zinc RING finger is crucial for viral growth and pathogenesis. A bICP0 deletion mutant does not induce plaques in permissive cells, but induces autophagy in a cell type dependent manner. In summary, the ability of bICP0 to stimulate productive infection, and repress IFN dependent transcription plays a crucial role in the BoHV-1 infection cycle.

The infected cell protein 0 encoded by bovine herpesvirus 1 (bICP0) associates with interferon regulatory factor 7 and consequently inhibits beta interferon promoter activity

The bICP0 protein encoded by bovine herpesvirus 1 stimulates productive infection and viral gene expression but inhibits interferon (IFN)-dependent transcription. bICP0 inhibits beta IFN (IFN-beta) promoter activity and induces degradation of IFN regulatory factor 3 (IRF3). Although bICP0 inhibits the trans-activation activity of IRF7, IRF7 protein levels are not reduced. In this study, we demonstrate that bICP0 is associated with IRF7. Furthermore, bICP0 inhibits the ability of IRF7 to trans-activate the IFN-beta promoter in the absence of IRF3 expression. The interaction between bICP0 and IRF7 correlates with reduced trans-activation of the IFN-beta promoter by IRF7.

The co-chaperone BAG3 regulates Herpes Simplex Virus replication

Viruses as obligate intracellular parasites use host cell proteins to ensure efficient replication and spread. Cellular proteins are required for several stages of a virus life cycle. Here, we identify BAG3, a co-chaperone, as a regulator of herpes virus immediate early gene expression. We report that a herpes simplex virus lacking the gene encoding a potent transcriptional activator, ICP0, is compromised for replication in cells silenced for BAG3 in a multiplicity of infection-dependent manner. We also show a requirement for BAG3 to augment virus gene expression and demonstrate that the co-chaperone acts independently of promyelocytic leukemia to increase herpes simplex virus replication.

The two functions of herpes simplex virus 1 ICP0, inhibition of silencing by the CoREST/REST/HDAC complex and degradation of PML, are executed in tandem

ICP0, an alpha (immediate-early) protein of herpes simplex virus 1, performs at least two key functions. It blocks inhibition of viral-gene expression by interferon, a function dependent on the degradation of the ND10 components PML and SP100 by the ubiquitin ligase expressed by the RING finger (RF), and it blocks silencing of viral DNA mediated by the HDAC1/2-CoREST-REST complex. In the latter case, a mutant CoREST lacking the HDAC1 binding site compensates totally or in part for the absence of ICP0 in a cell-type-dependent manner. Here, we compare the phenotypes of an ICP0 mutant containing disabling amino acid substitutions in the RF with those of a mutant with substitutions in the CoREST binding site (R8507). We report the following: (i) the onset of replication of both mutants was delayed, but the RF mutant yields did not reach wild-type virus levels even as late as 48 h after infection, and (ii) in infected cells, PML is rapidly degraded by wild-type virus, with some delay by the R8507 mutant, and is spared by the RF mutant. The translocation of ICP0 to the cytoplasm is impaired in cells infected with the RF mutant or delayed in cells infected with the R8507 mutant. Finally, in contrast to wild-type viruses, both mutants are inhibited by alpha or gamma interferon. The results indicate that both sets of events, the degradation of PML and the blocking of silencing, are interdependent and in large measure dependent on events in the ND10 nuclear bodies.

The zinc RING finger of bovine herpesvirus 1-encoded bICP0 protein is crucial for viral replication and virulence

Bovine herpesvirus 1 (BHV-1) infected cell protein 0 (bICP0) stimulates productive infection, in part by activating viral gene expression. The C(3)HC(4) zinc RING finger of bICP0 is crucial for activating viral transcription and productive infection. In this study, we used a bacterial artificial chromosome containing a wild-type (wt) virulent BHV-1 strain to generate a single amino acid mutation in the C(3)HC(4) zinc RING finger of bICP0. This virus (the 51g mutant) contains a cysteine-to-glycine mutation (51st amino acid) in the C(3)HC(4) zinc RING finger of bICP0. A plasmid expressing the 51g mutant protein did not transactivate viral promoter activity as efficiently as wt bICP0. The 51g mutant virus expressed higher levels of the bICP0 protein than did the 51g rescued virus (51gR) but yielded reduced virus titers following infection of permissive bovine cells. The 51g mutant virus, but not the 51gR virus, grew poorly in bovine cells pretreated with imiquimod to stimulate interferon production. During acute infection of calves, levels of infectious virus were 2 to 3 logs lower in ocular or nasal swabs with 51g than with 51gR. Calves latently infected with the 51g mutant did not reactivate from latency because virus shedding did not occur in ocular or nasal cavities. As expected, calves latently infected with 51gR reactivated from latency following dexamethasone treatment. These studies demonstrate that mutation of a single well-conserved cysteine residue in the C(3)HC(4) zinc RING finger of bICP0 has dramatic effects on the growth properties of BHV-1.

Herpes simplex virus type 1 ICP0 phosphorylation mutants impair the E3 ubiquitin ligase activity of ICP0 in a cell type-dependent manner

Herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) is a 110-kDa nuclear phosphoprotein that is required for both the efficient initiation of lytic infection and the reactivation of quiescent viral genomes from latency. The ability of ICP0 to act as a potent viral transactivator is mediated by its N-terminal zinc-binding RING finger domain. This domain confers E3 ubiquitin ligase activity to ICP0 and is required for the proteasome-dependent degradation of a number of cellular proteins during infection, including the major nuclear domain 10 (ND10) constituent protein promyelocytic leukemia. In previous work we mapped three phosphorylation regions within ICP0, two of which directly affected its transactivation capabilities in transient transfection assays (Davido et al., J. Virol. 79:1232-1243, 2005). Because ICP0 is a phosphoprotein, we initially sought to test the hypothesis that phosphorylation regulates the E3 ubiquitin ligase activity of ICP0. Although none of the mutations affected ICP0 E3 ligase activity in vitro, transient transfection analysis indicated that mutations within one or more of the phosphorylated regions impaired the ability of ICP0 to form foci with colocalizing conjugated ubiquitin and to disrupt ND10. Mutations within one of the regions also affected ICP0 stability, and all of these phenomena occurred in a cell type-dependent manner. In the context of viral infection, only one ICP0 phosphorylation mutant (P1) showed a significant defect in viral replication and enhanced protein stability compared to all the other viruses tested. This study suggests that specific cellular environments and context of expression (transfection versus infection) differentially regulate several activities of ICP0 related to its E3 ubiquitin ligase activity via phosphorylation.

Bcl-2 blocks accretion or depletion of stored calcium but has no effect on the redistribution of IP3 receptor I mediated by glycoprotein E of herpes simplex virus 1

We examined the status of stable, resting intracellular Ca2+ ([Ca2+]i) and the calcium that can be released from intracellular stores in HEp-2 or VAX-3 cells overexpressing Bcl-2 after infection with wild-type or mutant herpes simplex viruses. The mutants included viruses lacking ICP4 or ICP27 and known to induce apoptosis. We report the following. Stable Ca2+ levels decrease after infection with wild-type or mutant viruses in both HEp-2 and VAX-3 cells. The histamine-sensitive calcium stores became depleted in wild-type and mutant virus-infected cells late in infection but increased significantly in DeltaICP4- or DeltaICP27-infected cells prior to depletion. In VAX-3 cells, the depletion in calcium stores did not take place as late as 24 h after infection, concomitant with lack of visually detectable cytopathic effects. Concurrent analyses showed that the amounts of IP3 Ca2+-receptor type I (IP3R-I) remained stable throughout infection, but the intensity of the signal increased and intracellular distribution changed dramatically in both HEp-2 and VAX-3 cells infected with the wild-type and all mutant viruses, except for the mutant lacking glycoprotein E (DeltagE). In transfected HEp-2 cells, gE and gI were more effective at augmenting the signal intensity and redistribution of IP3R-I than gE or gI alone. We conclude the following. (i) Depleted histamine-sensitive calcium stores correlate with appearance of cytopathic effects. (ii) Apoptosis, the calcium stores, and cytopathic effects are regulated by Bcl-2. (iii) The changes in the distribution of IP3R-I are mediated by the viral Fc receptor complex, but the redistribution is not related to changes in stored calcium.