Characterization of herpes simplex virus 1 alpha proteins 0, 4, and 27 with monoclonal antibodies

"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.

hnRNPA2B1 Associated with Recruitment of RNA into Exosomes Plays a Key Role in Herpes Simplex Virus 1 Release from Infected Cells

hnRNPA2B1, an abundant cellular protein, has been reported to recruit RNAs bearing a specific sequence (EXO motif) into exosomes. We characterized an exosome population averaging 100 ± 50 nm in diameter and containing a defined set of constitutive exosome markers. This population packages microRNAs (miRNAs) and can be directed to block targeted gene expression in a dose-dependent fashion. The objective of this study was to characterize the role of hnRNPA2B1 in the recruitment of miRNA. We report the following four key findings. (i) hnRNPA2B1 is not a component of exosomes produced in HEp-2 or HEK293T cells. Hence, hnRNPA2B1 carries its cargo, at most, to the site of exosome assembly, but it is not itself incorporated into exosomes. (ii) The accumulation of exosomes produced by cells in which the gene encoding hnRNPA2B1 has been knocked out (ΔhnRNPA2B1 cells) was reduced 3-fold. (iii) In uninfected HEp-2 cells, hnRNPA2B1 is localized in the nucleus. In cells infected with herpes simplex virus 1 (HSV-1), hnRNPA2B1 was quantitatively exported to the cytoplasm and at least a fraction of hnRNPA2B1 colocalized with a Golgi marker. (iv) Lastly, in ΔhnRNPA2B1 cells, there was a 2- to 3-fold reduction in virus yield but a significant (>10-fold) reduction in HSV-1 released through the apical surface into the extracellular environment. The absence of hnRNPA2B1 had no significant impact on the basolateral export of HSV-1 from infected to uninfected cells by direct cell-to-cell contact. The results suggest that hnRNPA2B1 plays a key role in the transport of enveloped virus from its site of assembly to the extracellular environment.IMPORTANCE In this report, we show that hnRNPA2B1 is not a component of exosomes produced in HEp-2 or HEK293T cells. In herpes simplex virus 1 (HSV-1)-infected cells, hnRNPA2B1 was quantitatively translocated from the nucleus into the cytoplasm. In infected ΔhnRNPA2B1 cells, Golgi-dependent transport of virus from the apical surface to the extracellular medium was significantly reduced. In essence, this report supports the hypothesis that hnRNPA2B1 plays a key role in the egress of exosomes and HSV-1 from infected cells.

Degradation of Herpes Simplex Virus-1 Viral miRNA H11 by Vaccinia Virus Protein VP55 Attenuates Viral Replication

Among 29 distinct miRNAs expressed by the herpes simplex virus-1 (HSV-1) during lytic infection, miR-H11, together with miR-H1 to miR-H8 are reported to locate in the RNA-induced silencing complex (RISC). miR-H11 is encoded within viral origins of replication and lies entirely within the origins of replication. However, the roles of this miRNA derived from lytic infection with HSV-1 remain unclear. Using the advantage of vaccinia virus protein VP55 (VP55)-mediated degradation of miRNAs, we constructed a recombinant virus expressing VP55 (R5502) to demonstrate that: (1) accumulation of miR-H11 from R5502 was reduced by 540-fold versus that in cells infected with wild-type HSV-1, but miR-H1 to miR-H8 which also located in the RISC were not reduced significantly from R5502 compare with wild-type HSV-1; (2) downregulation of miR-H11 from R5502 infected cells results in markedly lower viral DNA synthesis compared with wild-type HSV-1; and (3) downregulation of miR-H11 also restricted viral spreading, and resulted in low accumulation of representative viral proteins and viral yields. The findings were confirmed through either using of a miR-H11 inhibitor or pre-transfection of a plasmid expressing VP55. These data suggest that miR-H11 plays a currently unidentified role in maintaining sufficient viral DNA synthesis during the course of viral infection.

Herpes Simplex Virus 1 MicroRNA miR-H28 Exported to Uninfected Cells in Exosomes Restricts Cell-to-Cell Virus Spread by Inducing Gamma Interferon mRNA

An earlier report showed that herpes simplex virus 1 (HSV-1) expresses two microRNAs (miRNAs), miR-H28 and miR-H29, late in the infectious cycle. The miRNAs are packed in exosomes and, in recipient cells, restrict the transmission of virus from infected cells to uninfected cells. We now report that (i) miR-H28 induced the synthesis of gamma interferon (IFN-γ) in both infected cells and cells transfected with miR-H28, (ii) IFN-γ accumulated concurrently with viral proteins in infected cells, (iii) IFN-γ was produced in HEp-2 cells derived from cancer tissue and in HEK293T cells derived from normal tissue, and (iv) HSV-1 replication was affected by exposure to IFN-γ before infection but not during or after infection. The results presented in this report support the growing body of evidence indicating that HSV-1 encodes functions designed to reduce the spread of infection from infected cells to uninfected cells, possibly in order to maximize the transmission of virus from infected individuals to uninfected individuals.IMPORTANCE In this report, we show that IFN-γ is produced by HSV-1 viral miR-H28 and viral replication is blocked in cells exposed to IFN-γ before infection but not during or after infection. The inevitable conclusion is that HSV-1 induces IFN-γ to curtail its spread from infected cells to uninfected cells. In essence, this report supports the hypothesis that HSV-1 encodes functions that restrict the transmission of virus from cell to cell.

Effect of Loss-of-function of the Herpes Simplex Virus-1 microRNA H6-5p on Virus Replication

To date, 29 distinct microRNAs (miRNAs) have been reported to be expressed during herpes simplex virus infections. Sequence analysis of mature herpes simplex virus-1 (HSV-1) miRNAs revealed five sets of miRNAs that are complementary to each other: miR-H6-5p/H1-3p, miR-H6-3p/H1-5p, H2-5p/H14-3p, miR-H2-3p/H14-5p, and miR-H7/H27. However, the roles of individual miRNAs and consequences of this complementarity remain unclear. Here, we focus on two of these complementary miRNAs, miR-H6-5p and miR-H1-3p, using loss-of-function experiments in vitro and in a mouse model of infection using an miRNA sponge approach, including tandem multiplex artificial miRNA-binding sequences that do not match perfectly to the target miRNA inserted downstream of a green fluorescent protein reporter gene. Infection with recombinant virus expressing the miR-H6-5p sponge reduced viral protein levels and virus yield. Decreased accumulation of viral proteins was also observed at early stages of infection in the presence of both an miR-H6-5p inhibitor and plasmid-expressed miR-H1-3p. Moreover, establishment of latency and reactivation did not differ between the recombinant virus expressing the miR-H6-5p sponge and wild-type HSV-1. Taken together, these data suggest that miR-H6-5p has an as-yet-unidentified role in the early stages of viral infection, and its complement miR-H1-3p suppresses this role in later stages of infection. This report extends understanding of the roles of miRNAs in infection by herpes simplex viruses, supporting a model of infection in which the production of virus and its virulent effects are tightly controlled to maximize persistence in the host and population.

GADD45γ Activated Early in the Course of Herpes Simplex Virus 1 Infection Suppresses the Activation of a Network of Innate Immunity Genes

The stress response genes encoding GADD45γ, and to a lesser extent GADD45β, are activated early in infection with herpes simplex virus 1 (HSV-1). Cells that had been depleted of GADD45γ by transfection of short hairpin RNA (shRNA) or in which the gene had been knocked out (ΔGADD45γ) yielded significantly less virus than untreated infected cells. Consistent with lower virus yields, the ΔGADD45γ cells (either uninfected or infected with HSV-1) exhibited significantly higher levels of transcripts of a cluster of innate immunity genes, including those encoding IFI16, IFIT1, MDA5, and RIG-I. Members of this cluster of genes were reported by this laboratory to be activated concurrently with significantly reduced virus yields in cells depleted of LGP2 or HDAC4. We conclude that innate immunity to HSV-1 is normally repressed in unstressed cells and repression appears to be determined by two mechanisms. The first, illustrated here, is through activation by HSV-1 infection of the gene encoding GADD45γ. The second mechanism requires constitutively active expression of LGP2 and HDAC4.IMPORTANCE Previous studies from our laboratory reported that knockout of some innate immunity genes was associated with increases in the expression of overlapping networks of genes and significant loss of the ability to support the replication of HSV-1; knockout of other genes was associated with decreases in the expression of overlapping networks of genes and had no effect on virus replication. In this report, we document that depletion of GADD45γ reduced virus yields concurrently with significant upregulation of the expression of a cluster of innate immunity genes comprising IFI16, IFIT1, MDA5, and RIG-I. This report differs from the preceding study in an important respect; i.e., the preceding study found no evidence to support the hypothesis that HSV-1 maintained adequate levels of LGP2 or HDAC4 to block upregulation of the cluster of innate immunity genes. We show that HSV-1 causes upregulation of the GADD45γ gene to prevent the upregulation of innate immunity genes.

miRNAs Targeting ICP4 and Delivered to Susceptible Cells in Exosomes Block HSV-1 Replication in a Dose-Dependent Manner

miRNAs are potent tools that in principle can be used to control the replication of infectious agents. The objectives of the studies reported here were to design miRNAs that can block the replication of herpes simplex virus 1 and which could be delivered to infected cells via exosomes. We report the following: (1) We designed three miRNAs targeting the mRNA encoding ICP4, an essential viral regulatory protein. Of the three miRNAs, one miRNA401 effectively blocked ICP4 accumulation and viral replication on transfection into susceptible cells. (2) To facilitate packaging of the miRNA into exosomes, we incorporated into the sequence of miRNA401 an exosome-packaging motif. miRNA401 was shown to be packaged into exosomes and successfully delivered by exosomes to susceptible cells, where it remained stable for at least 72 hr. Finally, the results show that miRNA401 delivered to cells via exosomes effectively reduced virus yields in a miRNA401 dose-dependent fashion. The protocol described in this report can be applied to study viral gene functions without actually deleting or mutagenizing the gene.

PUM1 is a biphasic negative regulator of innate immunity genes by suppressing LGP2

PUM1 is an RNA binding protein shown to regulate the stability and function of mRNAs bearing a specific sequence. We report the following: (i) A key function of PUM1 is that of a repressor of key innate immunity genes by repressing the expression of LGP2. Thus, between 12 and 48 hours after transfection of human cells with siPUM1 RNA there was an initial (phase 1) upsurge of transcripts encoding LGP2, CXCL10, IL6, and PKR. This was followed 24 hours later (phase 2) by a significant accumulation of mRNAs encoding RIG-I, SP100, MDA5, IFIT1, PML, STING, and IFNβ. The genes that were not activated encoded HDAC4 and NF-κB1. (ii) Simultaneous depletion of PUM1 and LGP2, CXCL10, or IL6 revealed that up-regulation of phase 1 and phase 2 genes was the consequence of up-regulation of LGP2. (iii) IFNβ produced 48-72 hours after transfection of siPUM1 was effective in up-regulating LGP2 and phase 2 genes and reducing the replication of HSV-1 in untreated cells. (iv) Because only half of genes up-regulated in phase 1 and 2 encode mRNAs containing PUM1 binding sites, the upsurge in gene expression could not be attributed solely to stabilization of mRNAs in the absence of PUM1. (v) Lastly, depletion of PUM2 does not result in up-regulation of phase 1 or phase 2 genes. The results of the studies presented here indicate that PUM1 is a negative regulator of LGP2, a master regulator of innate immunity genes expressed in a cascade fashion.

miR-H28 and miR-H29 expressed late in productive infection are exported and restrict HSV-1 replication and spread in recipient cells

We report on the properties and function of two herpes simplex virus-1 (HSV-1) microRNAs (miRNAs) designated "miR-H28" and "miR-H29." Both miRNAs accumulate late in productive infection at a time when, for the most part, viral DNA and proteins have been made. Ectopic expression of miRNA mimics in human cells before infection reduced the accumulation of viral mRNAs and proteins, reduced plaque sizes, and at vey low multiplicities of infection reduced viral yields. The specificity of the miRNA mimics was tested in two ways. First, ectopic expression of mimics carrying mutations in the seed sequence was ineffective. Second, in similar tests two viral miRNAs made early in productive infection also had no effect. Both miR-H28 and miR-H29 are exported from infected cells in exosomes. A noteworthy finding is that both miR-H28 and miR-H29 were absent from murine ganglia harboring latent virus but accumulated in ganglia in which the virus was induced to reactivate. The significance of these findings rests on the principle that the transmission of HSV from person to person is by physical contact between the infected tissues of the donor and those of uninfected recipient. Diminished size of primary or recurrent lesions could be predicted to enhance person-to-person transmission. Reduction in the amount of reactivating latent virus would reduce the risk of retrograde transport to the CNS but would not interfere with anterograde transport to a site at or near the site of initial infection.

Higher Throughput Quantification of Neutralizing Antibody to Herpes Simplex Viruses

We report a rapid, higher throughput method for measuring neutralizing antibody to herpes simplex virus (HSV) in human sera. Clinical isolates and sera from the Herpevac Trial for Women were used in a colorimetric assay in which infection of tissue culture (lack of neutralization) was indicated by substrate metabolism by beta-galactosidase induced in the ELVIS cell line. The neutralization assay was optimized by addition of guinea pig complement, which particularly enhanced neutralizing antibody titers to HSV-2. Higher neutralizing antibody titers were also achieved using virus particles isolated from the supernatant of infected cells rather than lysate of infected cells as the source of virus. The effect of assay incubation time and incubation time with substrate were also optimized. We found that incubating with substrate until a standard optical density of 1.0 was reached permitted a better comparison among virus isolates, and achieved reliable measurement of neutralizing antibody activity. Interestingly, in contrast to results in the absence of complement, addition of complement allowed sera from HSV-2 gD-vaccinated subjects to neutralize HSV-1 and HSV-2 clinical and laboratory isolates with equal potency.

N-terminal phosphorylation sites of herpes simplex virus 1 ICP0 differentially regulate its activities and enhance viral replication

The herpes simplex virus 1 (HSV-1) infected cell protein 0 (ICP0) is an immediate-early phosphoprotein that transactivates viral gene expression. Evidence suggests that phosphorylation regulates the functions of ICP0, and three regions (termed regions I, II, and III) in the protein are known to be phosphorylated. Mutation of the putative phosphorylation sites within region I, termed Phos 1, which lies in the N-terminal portion of ICP0, impairs the E3 ubiquitin (Ub) ligase and ND10-disrupting activities of ICP0 in cell culture and diminishes viral replication. To identify the specific phosphorylation site(s) or residues responsible for the phenotypes observed with Phos 1, individual residues within region I were mutated to alanine (S224A, T226A, T231A, and T232A) and one double mutant S224A/T226A was constructed. Tissue culture studies demonstrated that the S224A, S224A/T226A, T231A, and T232A mutants were unable to dissociate the cellular protein PML from ND10 and that the S224/T226A mutant was defective in its ability to dissociate the cellular protein Sp100 from ND10. Additionally, the transactivation activity of ICP0 was impaired in the S224A and S224A/T226A mutants. The S224A and S224A/T226A mutant forms were more stable than wild-type ICP0, suggesting that their ability to autoubiquitinate was limited. Moreover, one ICP0 ubiquitination target, USP-7, was also more stable after infection with these two mutants. Lastly, the replication of the S224A and S224A/T226A mutant viruses was reduced in cell culture and in vivo. Overall, our data suggest that specific phosphorylation sites within region I differentially regulate the activities of ICP0, which are required for efficient viral replication.

ICP27 protein encoded by bovine herpesvirus type 1 (bICP27) interferes with promoter activity of the bovine genes encoding beta interferon 1 (IFN-β1) and IFN-β3

Bovine herpes virus 1 (BHV-1) infection leads to upper respiratory tract infections, conjunctivitis, and the infection predisposes cattle to secondary bacterial infections. The infected cell protein 0 (bICP0) encoded by BHV-1 suppresses antiviral innate immune signaling by interfering with expression of interferon beta (IFN-β). In contrast to humans or mice, cattle contain three IFN-β genes that have distinct transcriptional promoters. We previously cloned and characterized all three bovine IFN-β promoters. In this study, we provide evidence that bICP27; a viral early protein that shuttles between the nucleus and cytoplasm inhibits transcriptional activity of two bovine IFN-β gene promoters (IFN-β1 and IFN-β3). Conversely, the BHV-1 infected cell protein 0 (bICP0) early promoter was not inhibited by bICP27. C-terminal mutants lacking the bICP27 zinc RING finger-like motif did not efficiently inhibit IFN-β3 promoter activity but inhibited IFN-β1 promoter activity as efficiently as wild type bICP27. An N-terminal mutant lacking the nuclear localization signal (NLS) and nucleolar localization signal (NoLS) was localized to the cytoplasm and this mutant had no effect on IFN-β promoter activity. In summary, these studies provided evidence that bICP27 inhibited IFN-β1 and IFN-β3 promoter activity in transiently transfected cells.

Herpes simplex virus 1 ICP0 phosphorylation site mutants are attenuated for viral replication and impaired for explant-induced reactivation

In cell culture experiments, phosphorylation appears to be a critical regulator of the herpes simplex virus 1 (HSV-1) immediate-early (IE) protein, ICP0, which is an E3 ubiquitin ligase that transactivates viral gene expression. Three major regions of phosphorylation in ICP0 (amino acids 224 to 232, 365 to 371, and 508 to 518) have been identified, and mutant viruses that block phosphorylation sites within each region (termed Phos 1, 2, and 3, respectively) have been constructed. Previous studies indicated that replication of Phos 1 is significantly reduced compared to that of wild-type virus in cell culture (C. Boutell, et al., J. Virol. 82:10647-10656, 2008). To determine the effects these phosphorylation site mutations have on the viral life cycle in vivo, mice were ocularly infected with wild-type HSV-1, the Phos mutants, or their marker rescue counterparts. Subsequently, viral replication, establishment of latency, and viral explant-induced reactivation of these viruses were examined. Relative to wild-type virus, Phos 1 eye titers were reduced as much as 7- and 18-fold on days 1 and 5 postinfection, respectively. Phos 2 eye titers showed a decrease of 6-fold on day 1 postinfection. Titers of Phos 1 and 2 trigeminal ganglia were reduced as much as 16- and 20-fold, respectively, on day 5 postinfection. Additionally, the reactivation efficiencies of Phos 1 and 2 were impaired relative to wild-type HSV-1, although both viruses established wild-type levels of latency in vivo. The acute replication, latency, and reactivation phenotypes of Phos 3 were similar to those of wild-type HSV-1. We conclude from these studies that phosphorylation is likely a key modulator of ICP0's biological activities in a mouse ocular model of HSV-1 infection.

Development of a glycoprotein D-expressing dominant-negative and replication-defective herpes simplex virus 2 (HSV-2) recombinant viral vaccine against HSV-2 infection in mice

Using the T-REx (Invitrogen, California) gene switch technology and a dominant-negative mutant polypeptide of herpes simplex virus 1 (HSV-1)-origin binding protein UL9, we previously constructed a glycoprotein D-expressing replication-defective and dominant-negative HSV-1 recombinant viral vaccine, CJ9-gD, for protection against HSV infection and disease. It was demonstrated that CJ9-gD is avirulent following intracerebral inoculation in mice, cannot establish detectable latent infection following different routes of infection, and offers highly effective protective immunity against primary HSV-1 and HSV-2 infection and disease in mouse and guinea pig models of HSV infections. Given these favorable safety and immunological profiles of CJ9-gD, aiming to maximize levels of HSV-2 glycoprotein D (gD2) expression, we have constructed an ICP0 null mutant-based dominant-negative and replication-defective HSV-2 recombinant, CJ2-gD2, that contains 2 copies of the gD2 gene driven by the tetracycline operator (tetO)-bearing HSV-1 major immediate-early ICP4 promoter. CJ2-gD2 expresses gD2 as efficiently as wild-type HSV-2 infection and can lead to a 150-fold reduction in wild-type HSV-2 viral replication in cells coinfected with CJ2-gD2 and wild-type HSV-2 at the same multiplicity of infection. CJ2-gD2 is avirulent following intracerebral injection and cannot establish a detectable latent infection following subcutaneous (s.c.) immunization. CJ2-gD2 is a more effective vaccine than HSV-1 CJ9-gD and a non-gD2-expressing dominant-negative and replication-defective HSV-2 recombinant in protection against wild-type HSV-2 genital disease. Using recall response, we showed that immunization with CJ2-gD2 elicited strong HSV-2-specific memory CD4(+) and CD8(+) T-cell responses. Collectively, given the demonstrated preclinical immunogenicity and its unique safety profiles, CJ2-gD2 represents a new class of HSV-2 replication-defective recombinant viral vaccines in protection against HSV-2 genital infection and disease.

ICP0 dismantles microtubule networks in herpes simplex virus-infected cells

Infected-cell protein 0 (ICP0) is a RING finger E3 ligase that regulates herpes simplex virus (HSV) mRNA synthesis, and strongly influences the balance between latency and replication of HSV. For 25 years, the nuclear functions of ICP0 have been the subject of intense scrutiny. To obtain new clues about ICP0's mechanism of action, we constructed HSV-1 viruses that expressed GFP-tagged ICP0. To our surprise, both GFP-tagged and wild-type ICP0 were predominantly observed in the cytoplasm of HSV-infected cells. Although ICP0 is exclusively nuclear during the immediate-early phase of HSV infection, further analysis revealed that ICP0 translocated to the cytoplasm during the early phase where it triggered a previously unrecognized process; ICP0 dismantled the microtubule network of the host cell. A RING finger mutant of ICP0 efficiently bundled microtubules, but failed to disperse microtubule bundles. Synthesis of ICP0 proved to be necessary and sufficient to disrupt microtubule networks in HSV-infected and transfected cells. Plant and animal viruses encode many proteins that reorganize microtubules. However, this is the first report of a viral E3 ligase that regulates microtubule stability. Intriguingly, several cellular E3 ligases orchestrate microtubule disassembly and reassembly during mitosis. Our results suggest that ICP0 serves a dual role in the HSV life cycle, acting first as a nuclear regulator of viral mRNA synthesis and acting later, in the cytoplasm, to dismantle the host cell's microtubule network in preparation for virion synthesis and/or egress.

Transcriptional and translational dual-regulated oncolytic herpes simplex virus type 1 for targeting prostate tumors

The aim of this project was to demonstrate that an oncolytic herpes simplex virus type 1 (HSV-1) can replicate in a tissue- and tumor-specific fashion through both transcriptional (prostate-specific promoter, ARR(2)PB) and translational (5'-untranslated regions (5'UTRs) of rFGF-2) regulation of an essential viral gene, ICP27. We generated two recombinant viruses, ARR(2)PB-ICP27 (A27) and ARR(2)PB-5'UTR-ICP27 (AU27) and tested their efficacy and toxicity both in vitro and in vivo. The ARR(2)PB promoter caused overexpression of ICP27 gene in the presence of activated androgen receptors (ARs) and increased viral replication in prostate cells. However, this transcriptional upregulation was effectively constrained by the 5'UTR-mediated translational regulation. Mice bearing human prostate LNCaP tumors, treated with a single intravenous injection of 5 x 10(7) plaque-forming units (pfu) of AU27 virus exhibited a >85% reduction in tumor size at day 28 after viral injection. Although active viral replication was readily evident in the tumors, no viral DNA was detectable in normal organs as measured by real-time PCR analyses. In conclusion, a transcriptional and translational dual-regulated (TTDR) viral essential gene expression can increase both viral lytic activity and tumor specificity, and this provides a basis for the development of a novel tumor-specific oncolytic virus for systemic treatment of locally advanced and metastatic prostate cancers.

Recruitment of herpes simplex virus type 1 immediate-early protein ICP0 to the virus particle

Although the herpes simplex virus type 1 (HSV-1) tegument is comprised of a large number of viral and cellular proteins, how and where in the cell these proteins are recruited into the virus structure is poorly understood. We have shown previously that the immediate-early gene product ICP0 is packaged by a mechanism dependent on the major tegument protein VP22, while others have shown a requirement for ICP27. We now extend our studies to show that ICP0 packaging correlates directly with the ability of ICP0 to complex with VP22 in infected cells. ICP27 is not, however, present in this VP22-ICP0 complex but is packaged into the virion in a VP22- and ICP0-independent manner. Biochemical fractionation of virions indicated that ICP0 associates tightly with the virus capsid, but intranuclear capsids contained no detectable ICP0. The RING finger domain of ICP0 and the N terminus of VP22 were both shown to be essential but not sufficient for ICP0 packaging and complex formation. Strikingly, however, the N-terminal region of VP22, while unable to form a complex with ICP0, inhibited its translocation from the nucleus to the cytoplasm. PML degradation by ICP0 was efficient in cells infected with this VP22 mutant virus, confirming that ICP0 retains activity. Hence, we would suggest that VP22 is an important molecular partner of ICP0 that controls at least one of its activities: its assembly into the virion. Moreover, we propose that the pathway by which VP22 recruits ICP0 to the virion may begin in the nucleus prior to ICP0 translocation to its final site of assembly in the cytoplasm.

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.

MicroRNA regulation of oncolytic herpes simplex virus-1 for selective killing of prostate cancer cells

PURPOSE

Advanced castration-resistant prostate cancer, for which there are few treatment options, remains one of the leading causes of cancer death. MicroRNAs (miRNA) have provided a new opportunity for more stringent regulation of tumor-specific viral replication. The purpose of this study was to provide a proof-of-principle that miRNA-regulated oncolytic herpes simplex virus-1 (HSV-1) virus can selectively target cancer cells with reduced toxicity to normal tissues.

EXPERIMENTAL DESIGN

We incorporated multiple copies of miRNA complementary target sequences (for miR-143 or miR-145) into the 3'-untranslated region (3'-UTR) of an HSV-1 essential viral gene, ICP4, to create CMV-ICP4-143T and CMV-ICP4-145T amplicon viruses and tested their targeting specificity and efficacy both in vitro and in vivo.

RESULTS

Although miR-143 and miR-145 are highly expressed in normal tissues, they are significantly down-regulated in prostate cancer cells. We further showed that miR-143 and miR-145 inhibited the expression of the ICP4 gene at the translational level by targeting the corresponding 3'-UTR in a dose-dependent manner. This enabled selective viral replication in prostate cancer cells. When mice bearing LNCaP human prostate tumors were treated with these miRNA-regulated oncolytic viruses, a >80% reduction in tumor volume was observed, with significantly attenuated virulence to normal tissues in comparison with control amplicon viruses not carrying these 3'-UTR sequences.

CONCLUSION

Our study is the first to show that inclusion of specific miRNA target sequences into the 3'-UTR of an essential HSV-1 gene is a viable strategy for restricting viral replication and oncolysis to cancer cells while sparing normal tissues.

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.

Role for herpes simplex virus 1 ICP27 in the inhibition of type I interferon signaling

Host cells respond to viral infection by many mechanisms, including the production of type I interferons which act in a paracrine and autocrine manner to induce the expression of antiviral interferon-stimulated genes (ISGs). Viruses have evolved means to inhibit interferon signaling to avoid induction of the innate immune response. Herpes simplex virus 1 (HSV-1) has several mechanisms to inhibit type I interferon production, the activities of ISGs, and the interferon signaling pathway itself. We report that the inhibition of the Jak/STAT pathway by HSV-1 requires viral gene expression and that viral immediate-early protein ICP27 plays a role in downregulating STAT-1 phosphorylation and in preventing the accumulation of STAT-1 in the nucleus. We also show that expression of ICP27 by transfection causes an inhibition of IFN-induced STAT-1 nuclear accumulation. Therefore, ICP27 is necessary and sufficient for at least some of the effects of HSV infection on STAT-1.

Herpes simplex virus type 1 immediate-early protein ICP27 is required for efficient incorporation of ICP0 and ICP4 into virions

Early in infection, herpes simplex virus type 1 (HSV-1) immediate-early (IE) proteins ICP0 and ICP4 localize to the nucleus, where they stimulate viral transcription. Later in infection, ICP0 and to a lesser extent ICP4 accumulate in the cytoplasm, but their biological role there is unknown. Previously, it was shown that the cytoplasmic localization of ICP0/4 requires the multifunctional IE protein ICP27, which is itself an activator of viral gene expression. Here, we identify a viral ICP27 mutant, d3-4, which is unable to efficiently localize ICP0 and ICP4 to the cytoplasm but which otherwise resembles wild-type HSV-1 in its growth and viral gene expression phenotypes. These results genetically separate the function of ICP27 that affects ICP0/4 localization from its other functions, which affect viral growth and gene expression. As both ICP0 and ICP4 are known to be minor virion components, we used d3-4 to test the hypothesis that the cytoplasmic localization of these proteins is required for their incorporation into viral particles. Consistent with this conjecture, d3-4 virions were found to lack ICP0 in their tegument and to have greatly reduced levels of ICP4. Thus, the cytoplasmic localization of ICP0 and ICP4 appears to be a prerequisite for the assembly of these important transcriptional regulatory proteins into viral particles. Furthermore, our results show that ICP27 plays a previously unrecognized role in determining the composition of HSV-1 virions.

Protein kinase B/Akt is present in activated form throughout the entire replicative cycle of deltaU(S)3 mutant virus but only at early times after infection with wild-type herpes simplex virus 1

The product of the herpes simplex virus 1 (HSV-1) US3 gene is a multifunctional serine-threonine protein kinase that can block apoptosis induced by proapoptotic cellular proteins, exogenous agents, or replication-defective viruses. Earlier studies showed that the U(S)3 kinase activates and functionally overlaps cellular protein kinase A (PKA). In this study we examined the status of phosphatidylinositol 3-kinase [PI3K] and of its effector, protein kinase B/Akt (PKB/Akt), a component of a major pathway of mammalian antiapoptotic signaling systems. We report the following. (i) Infection of target cells with HSV-1 induces transient phosphorylation of serine 473 of PKB/Akt early in infection, with a mechanism that is dependent on PI3K. Inhibition of PI3K induced apoptosis in mock-infected or deltaU(S)3 mutant-virus-infected but not in wild-type-virus-infected cells and reduced the accumulation of specific viral gene products, including the U(S)3 protein kinase, but had a marginal effect on virus yields. (ii) At later times after infection, the total amounts of PKB/Akt decreased and phosphorylated PKB/Akt forms disappeared in a U(S)3-dependent and protein phosphatase 2A-independent manner. (iii) Activation of PKA by forskolin did not mediate significant dephosphorylation of PKB/Akt. Our results are consistent with the model that PKB/Akt is activated early in infection and acts to block apoptosis in infected cells prior to the accumulation of U(S)3 protein kinase and that it persists and continues to function as an antiapoptotic protein in the absence of U(S)3 but becomes redundant or even inimical once U(S)3 protein kinase accumulates in effective amounts.

Construction and properties of a herpes simplex virus 1 designed to enter cells solely via the IL-13alpha2 receptor

Current design of genetically engineered viruses for selective destruction of cancer cells is based on the observation that attenuated viruses replicate better in tumor cells than in normal cells. The ideal virus, however, is one that can infect only cancer cells by virtue of altered host range. Such a virus can be made more robust than the highly attenuated viruses used in clinical trials. Earlier, we reported the construction of a recombinant herpes simplex virus 1 (R5111) in which the capacity to bind heparan sulfate was disabled and which contained a chimeric IL-13-glycoprotein D that enabled the virus to infect cells expressing the IL-13alpha2 receptor (IL-13Ralpha2) commonly found on the surface of malignant glioblastomas or high-grade astrocytomas. In the earlier report, we showed that the recombinant R5111 was able to enter and infect cells via the interaction of the chimeric glycoprotein D with IL-13Ralpha2 but that the virus retained the capacity to bind and replicate in cells expressing the natural viral receptors HveA or nectin-1. Here, we report the construction of a recombinant virus (R5141) that can only enter and replicate in cells that express the IL-13Ralpha2. The recombinant R5141 does not depend on endocytosis to infect cells. It does not infect cells expressing HveA or nectin-1 receptors or cells expressing IL-13Ralpha2 that had been exposed to soluble IL-13 before infection. The studies described here show that the host range of herpes simplex viruses can be altered by genetic manipulation to specifically target cancer cells.

Herpes simplex virus type 1 immediate-early protein ICP0 diffuses out of infected rabbit corneas

Herpes stromal keratitis (HSK) results from infection of herpes simplex virus (HSV) in the cornea. Recurrent HSV infection is a leading cause of corneal scarring and visual loss. Although it is generally thought that HSK is the result of an immune response to one or more viral proteins, no viral proteins have been detected in HSK corneas. Thus, the viral proteins involved in HSK, if any, remain undetermined. In contrast, it is reported here that when HSK corneal buttons from latently infected rabbits were fixed using standard procedures, the important immediate-early HSV-1 protein ICP0 was readily detected in the fixative by Western blotting. Similarly, when HSK corneal buttons were soaked in buffer (rather than fixative), ICP0 was readily detected in the soaking buffer. Other HSV-1 proteins were not detected either in the fixative or in the soaking buffer. It is also reported here that ICP0 was consistently detected in virus-free tears from the eyes of rabbits acutely infected with HSV-1. These results suggest that ICP0 rapidly diffuses out of the cornea and may explain why ICP0 was detected in the fixative of HSK corneas and in the soaking buffer of acutely infected corneas.

Mapping of functional regions conferring nuclear localization and RNA-binding activity of pseudorabies virus early protein UL54

Pseudorabies virus (PRV) is an alphaherpesvirus, and its gene organization and regulation are similar to the well-characterized human herpes simplex virus (HSV). The PRV early protein UL54 consists of 363 amino acids with homology to the HSV ICP27 immediate-early protein. Previously, we have demonstrated the nuclear accumulation and poly(G) RNA-binding activity of UL54 protein. In the present study, we have identified further the functional regions within UL54 conferring for nuclear localization and RNA-binding activity. Several recombinant expression plasmids containing various coding regions of UL54 gene were constructed for producing a series of C-terminally truncated or internally deleted forms of UL54 mutants in Escherichia coli or porcine kidney (PK-15) cells. RNA-binding activity of E. coli-expressed UL54 mutants was characterized by the binding ability to poly(G) RNA homopolymer in dot blot hybridization assay and the results have shown that the N-terminal 83 residues were responsible for RNA-binding, and the region of residues 35-82 containing an RGG box was necessary for its function. Furthermore, the region responsible for nuclear localization was investigated by transient expression of various deletion mutants in PK-15 cells followed by detection of their subcellular distribution. The results showed that C-terminal deletion beyond the amino acid residue 83 or internal deletion containing the RGG box sequence could restrict UL54 mutants in the cytoplasm. The ability of the N-terminal 83 residues to target the green fluorescence protein to the nucleus confirmed further its role as a functional nuclear localization signal (NLS). The utmost N-terminal 83 residues portion of UL54 contains two important functional domains, NLS and RNA-binding, and thus it would play an indispensable role in UL54 regulatory function.

Evidence for a direct interaction between HSV-1 ICP27 and ICP8 proteins

Herpes simplex virus 1 (HSV-1) ICP27 and ICP8 proteins have both been implicated in the transcription of late genes and regulation of viral gene expression. We showed previously that ICP27 and ICP8 associate with the RNAP II holoenzyme (Zhou and Knipe, J. Virol. 76, 5893-5904). Here, we demonstrate that ICP27 and ICP8 coprecipitate from lysates of HSV-1-infected HEp2 cells and from lysates of insect cells expressing ICP27 and ICP8, the latter being in the absence of other HSV-1 proteins. By expressing and purifying hexahistidine-tagged ICP8 (His-ICP8) and maltose binding protein (MBP)-tagged ICP27 (MBP-27) proteins and performing in vitro immunoprecipitation and pull-down assays, we also demonstrate that ICP27 and ICP8 coprecipitate in the absence of other viral or cellular proteins. Taken together, these data provide evidence that ICP27 and ICP8 interact directly in vitro and in infected cells. We hypothesize that the ICP27-ICP8 interaction plays a role in the stimulation of late gene transcription.

Phosphorylation site mutations affect herpes simplex virus type 1 ICP0 function

The herpes simplex virus type 1 (HSV-1) immediate-early (IE) regulatory protein infected-cell protein 0 (ICP0) is a strong and global transactivator of both viral and cellular genes. In a previous study, we reported that ICP0 is highly phosphorylated and contains at least seven distinct phosphorylation signals as determined by phosphotryptic peptide mapping (D. J. Davido et al., J. Virol. 76:1077-1088, 2002). Since phosphorylation affects the activities of many viral regulatory proteins, we sought to determine whether the phosphorylation of ICP0 affects its functions. To address this question, it was first necessary to identify the regions of ICP0 that are phosphorylated. For this purpose, ICP0 was partially purified, and phosphorylation sites were mapped by microcapillary high-pressure liquid chromatography tandem mass spectrometry. Three phosphorylated regions containing 11 putative phosphorylation sites, all within or adjacent to domains important for the transactivating activity of ICP0, were identified. The 11 sites were mutated to alanine as clusters in each of the three regions by site-directed mutagenesis, generating plasmids expressing mutant forms of ICP0: Phos 1 (four mutated sites), Phos 2 (three mutated sites), and Phos 3 (four mutated sites). One-dimensional phosphotryptic peptide analysis confirmed that the phosphorylation state of each Phos mutant form of ICP0 is altered relative to that of wild-type ICP0. In functional assays, the ICP0 phosphorylation site mutations affected the subcellular and subnuclear localization of ICP0, its ability to alter the staining pattern of the nuclear domain 10 (ND10)-associated protein PML, and/or its transactivating activity in Vero cells. Only mutations in Phos 1, however, impaired the ability of ICP0 to complement the replication of an ICP0 null mutant in Vero cells. This study thus suggests that phosphorylation is an important regulator of ICP0 function.

Proteomics of herpes simplex virus infected cell protein 27: association with translation initiation factors

The herpes simplex virus (HSV) immediate early ICP27 protein plays an essential role in stimulating viral early and late gene expression. ICP27 appears to be multifunctional in that it has been reported to stimulate viral late gene transcription, polyadenylation site usage, and RNA export. We report here on proteomic studies involving immunoprecipitation of ICP27 and mass spectrometric identification of co-precipitated proteins. These studies show an association of ICP27 with the cellular translation initiation factors poly A binding protein (PABP), eukaryotic initiation factor 3 (eIF3), and eukaryotic initiation factor 4G (eIF4G) in infected cells. Immunoprecipitation-western blot studies confirmed these associations. Finally, purified MBP-tagged ICP27 (MBP-27) can interact with eIF3 subunits p47 and p116 in vitro. These results suggest that ICP27 may also play a role in stimulating translation of certain viral and host mRNAs and/or in inhibiting host mRNA translation.

Cells lacking NF-kappaB or in which NF-kappaB is not activated vary with respect to ability to sustain herpes simplex virus 1 replication and are not susceptible to apoptosis induced by a replication-incompetent mutant virus

Earlier we reported that NF-kappaB is activated by protein kinase R (PKR) in herpes simplex virus 1-infected cells. Here we report that in PKR(-/-) cells the yields of wild-type virus are 10-fold higher than in PKR(+/+) cells. In cells lacking NF-kappaB p50 (nfkb1), p65 (relA), or both p50 and p65, the yields of virus were reduced 10-fold. Neither wild-type nor mutant cells undergo apoptosis following infection with wild-type virus. Whereas PKR(+/+) and NF-kappaB(+/+) control cell lines undergo apoptosis induced by the d120 (Deltaalpha4) mutant of HSV-1, the mutant PKR(-/-) and NF-kappaB(-/-) cell lines were resistant. The evidence suggests that the stress-induced apoptosis resulting from d120 infection requires activation of NF-kappaB and that this proapoptotic pathway is blocked in cells in which NF-kappaB is not activated or absent. Activation of NF-kappaB in the course of viral infection may have dual roles of attempting to curtain viral replication by rendering the cell susceptible to apoptosis induced by the virus and by inducing the synthesis of proteins that enhance viral replication.

Characterization and expression of the pseudorabies virus early gene UL54

Pseudorabies virus (PRV) is an alphaherpesvirus, and its gene organization and regulation are similar to the well-characterized human simplex virus (HSV). Sequence analysis of the complete coding region of PRV UL54 gene revealed that the UL54 gene consisted of 1092 nucleotides encoding a protein of 363 amino acids and the gene showed homology to HSV immediate-early protein ICP27. Detection of the UL54 transcript in infected cells by reverse transcription-polymerase chain reaction (RT-PCR) demonstrated that the UL54 gene belonged to the early kinetic class based on sensitivity to cycloheximide and insensitivity to phosphonoacetic acid (PAA). To study the structure and function of UL54 protein, this gene was subcloned on Escherichia coli expression vector pET28b for overexpression, and the expressed product was applied to generate specific antibody against UL54 protein. The specificity of the mouse immuneserum was confirmed by its ability to react with a 40kDa viral protein present in the PRV infected cells in Western immunblotting assay, detected as early as 4h after infection. In addition, immunoperoxidasing staining of PRV infected cells undertaken with this antibody demonstrated mainly nuclear staining pattern. Furthermore, the RNA binding potential of UL54 protein was demonstrated by its binding activity to poly(G) RNA homopolymer in Northwestern blotting assay.

Herpes simplex virus type 1 infection induces the stabilization of p53 in a USP7- and ATM-independent manner

The major oncoprotein p53 regulates several cellular antiproliferation pathways that can be triggered in response to a variety of cellular stresses, including viral infection. The stabilization of p53 is a key factor in the ability of cells to initiate an efficient transcriptional response after cellular stress. Here we present data demonstrating that herpes simplex virus type 1 (HSV-1) infection of HFFF-2 cells, a low-passage-number nontransformed human primary cell line, results in the stabilization of p53. This process required viral immediate-early gene expression but occurred independently of the viral regulatory protein ICP0 and viral DNA replication. No specific viral protein could be identified as being solely responsible for the effect, which appears to be a cellular response to developing HSV-1 infections. HSV-1 infection also induced the phosphorylation of p53 at residues Ser15 and Ser20, which have previously been implicated in its stabilization in response to DNA damage. However, an HSV-1 infection of ATM(-/-) cells, which lack a kinase implicated in these phosphorylation events, did not lead to the phosphorylation of p53 at these residues, but nonetheless p53 was stabilized. We also show that the wild-type p53 expressed by osteosarcoma U2OS cells can be stabilized in response to DNA damage induced by UV irradiation, but not in response to HSV-1 infection. These data suggest that multiple cellular mechanisms are initiated to stabilize p53 during an HSV-1 infection. These mechanisms occur independently of ICP0 and its ability to sequester USP7 and may differ from those initiated in response to DNA damage.

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

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

Phenotype of a herpes simplex virus type 1 mutant that fails to express immediate-early regulatory protein ICP0

Herpes simplex virus type 1 (HSV-1) immediate-early (IE) regulatory protein ICP0 is required for efficient progression of infected cells into productive lytic infection, especially in low-multiplicity infections of limited-passage human fibroblasts. We have used single-cell-based assays that allow detailed analysis of the ICP0-null phenotype in low-multiplicity infections of restrictive cell types. The major conclusions are as follows: (i) there is a threshold input multiplicity above which the mutant virus replicates normally; (ii) individual cells infected below the threshold multiplicity have a high probability of establishing a nonproductive infection; (iii) such nonproductively infected cells have a high probability of expressing IE products at 6 h postinfection; (iv) even at 24 h postinfection, IE protein-positive nonproductively infected human fibroblast cells exceed the number of cells that lead to plaque formation by up to 2 orders of magnitude; (v) expression of individual IE proteins in a proportion of the nonproductively infected cells is incompletely coordinated; (vi) the nonproductive cells can also express early gene products at low frequencies and in a stochastic manner; and (vii) significant numbers of human fibroblast cells infected at low multiplicity by an ICP0-deficient virus are lost through cell death. We propose that in the absence of ICP0 expression, HSV-1 infected human fibroblasts can undergo a great variety of fates, including quiescence, stalled infection at a variety of different stages, cell death, and, for a minor population, initiation of formation of a plaque.

Formation of nuclear foci of the herpes simplex virus type 1 regulatory protein ICP4 at early times of infection: localization, dynamics, recruitment of ICP27, and evidence for the de novo induction of ND10-like complexes

Herpes simplex virus type 1 (HSV-1) has an intricate association with cellular nuclear structures known as ND10 or promyelocytic leukemia protein (PML) nuclear bodies. Parental viral genomes initially become juxtaposed to ND10, and then viral replication compartments develop from the ND10-associated genomes. Viral immediate-early (IE) regulatory protein ICP0 colocalizes with ND10 and then induces the degradation of critical ND10 component protein PML and therefore the release and dispersal of other ND10 proteins. The IE transcriptional regulatory protein ICP4 also forms foci at early times of infection, many of which are juxtaposed to ND10 and later develop into replication compartments, indicating that at least some of the initial ICP4 foci contain parental viral genomes. Here we report that the ICP4 foci also contain ICP27 and that their formation occurs extremely rapidly at locations just inside the nuclear envelope. By examining developing plaques or thinly seeded cells infected at high multiplicity, we found evidence to suggest that at least some of the ND10-viral nucleoprotein complex association could be attributed to de novo formation of ND10-like structures in response to incoming viral genomes. The ICP4 complexes associated efficiently with ND10 in cells infected with an ICP0-null mutant virus at high but not at low multiplicity, and the degree of association was reduced by the proteasome inhibitor MG132. Therefore, the interaction between viral nucleoprotein complexes and ND10 is in part due to a dynamic response by the cell. This response is modulated by functional ICP0, and cells that are productively or nonproductively infected in the absence of functional ICP0 can be distinguished by the relative locations of ICP4 foci and ND10 proteins.

The differential requirement for cyclin-dependent kinase activities distinguishes two functions of herpes simplex virus type 1 ICP0

Herpes simplex virus type 1 (HSV-1) ICP0 directs the degradation of cellular proteins associated with nuclear structures called ND10, a function thought to be closely associated with its broad transactivating activity. Roscovitine (Rosco), an inhibitor of cyclin-dependent kinases (cdks), inhibits the replication of HSV-1, HSV-2, human cytomegalovirus, varicella-zoster virus, and human immunodeficiency virus type 1 by inhibiting specific steps or activities of viral regulatory proteins, indicating the broad and pleiotropic effects that cdks have on the replication of these viruses. We previously demonstrated that Rosco inhibits the transactivating activity of ICP0. In the present study, we asked whether Rosco also affects the ability of ICP0 to direct the degradation of ND10-associated proteins. For this purpose, WI-38 cells treated with cycloheximide (CHX) were mock infected or infected with wild-type HSV-1 or an ICP0(-) mutant (7134). After release from the CHX block, the infections were allowed to proceed for 2 h in the presence or absence of Rosco at a concentration known to inhibit ICP0's transactivating activity. The cells were then examined for the presence of ICP0 and selected ND10-associated antigens (promyelocytic leukemia antigen [PML], sp100, hDaxx, and NDP55) by immunofluorescence. Staining for the ND10-associated antigens was detected in </=20% of KOS-infected cells in the presence or absence of Rosco, demonstrating that Rosco-sensitive kinases are not required for ICP0's ability to direct the dispersal or degradation of these antigens. In contrast, >90% of 7134- and mock-infected cells stained positive for all ND10-associated antigens in the presence or absence of Rosco. Similar results were obtained with a non-ND10-associated antigen, DNA-PK(cs), a known target of ICP0-directed degradation. The results of the PML and DNA-PK(cs) immunofluorescence studies correlated with a decrease in the levels of these proteins as determined by Western blotting. Thus, the differential requirement for Rosco-sensitive cdk activities distinguishes ICP0's ability to direct the dispersal or degradation of cellular proteins from its transactivating activity.

Herpes simplex virus 1 mutant in which the ICP0 HUL-1 E3 ubiquitin ligase site is disrupted stabilizes cdc34 but degrades D-type cyclins and exhibits diminished neurotoxicity

Herpes simplex virus type 1 (HSV-1) infected cell protein 0 (ICP0) is a multifunctional protein that functions as a promiscuous transactivator and promotes the degradation of multiple cellular proteins. In vitro studies indicated that it encodes two physically separated functional E3 ubiquitin ligase domains. One, designated herpesvirus ubiquitin ligase 1 (HUL-1), maps to a region encoded by exon 3 and is contained between residues 543 and 680. Deletion of amino acids 621 to 625 abolishes this activity. The second, designated HUL-2, maps to the RING finger domain present in ICP0 encoded by exon 2. Earlier studies have shown that ICP0 stabilizes cyclins D1 and D3, and several lines of investigation led to the hypothesis that this function of ICP0 is the consequence of degradation of the E2 enzyme cdc34, known to be involved in the proteasome-dependent degradation of D-type cyclins. Consistent with this hypothesis, we have previously shown that cdc34 physically interacts with ICP0 at or near aspartate 199 and at amino acids 621 to 625 and that the former site is required for effective ubiquitylation and degradation of cdc34. Furthermore, the ICP0 HUL-1 domain promotes the polyubiquitination of cdc34 in vitro. If the mechanism by which D-type cyclins are salvaged in wild-type-infected cells is dependent on polyubiquitination and consequent destruction of cdc34, than the mutant virus R6701, which was constructed for these studies and lacks ICP0 residues 621 to 625, should destabilize the D cyclins and preclude the degradation of cdc34. We report that ICP0 residues 621 to 625 are essential for degradation of cdc34 in infected cells and for the ICP0-mediated stabilization of D-type cyclins, that a mutation that specifically disrupted the ring finger domain of the HUL-2 site had no effect on the degradation of cdc34 in infected cells, and that deletion of ICP0 residues 621 to 625 decreased the replicative capacity of the virus in growth-arrested but not in dividing cells and resulted in diminished pathogenicity on intracerebral inoculation of mice. We conclude that the ICP0 HUL-1 domain acts in infected cells to degrade cdc34 and that this function requires the interaction of cdc34 with sequences in exons 2 and 3 but does not involve the HUL-2 RING finger E3 domain.

Oct-1 is posttranslationally modified and exhibits reduced capacity to bind cognate sites at late times after infection with herpes simplex virus 1

In herpes simplex virus 1-infected cells, a high level of alpha gene expression requires the transactivation of the genes by a complex containing the viral alpha transinducing factor (alphaTIF) and two cellular proteins. The latter two, HCF-1 and octamer binding protein Oct-1, are transcriptional factors regulated in a cell cycle-dependent manner. alphaTIF is a protein made late in infection but packaged with the virion to transactivate viral genes in newly infected cells. In light of the accumulation of large amounts of alphaTIF, the absence of alpha gene expression late in infection suggested the possibility that one or more transcriptional factors required for alpha gene expression is modified late in infection. Here we report that Oct-1 is posttranscriptionally modified late in infection, that the modification is mediated by the virus but does not involve viral protein kinases or cdc2 kinase activated by the virus late in infection, and that the modified Oct-1 has a reduced affinity for its cognate DNA site. These results are consistent with the hypothesis that modification of Oct-1 transcriptional factor could account at least in part for the shutoff of alpha gene expression late in infection.

Activation of NF-kappaB in cells productively infected with HSV-1 depends on activated protein kinase R and plays no apparent role in blocking apoptosis

Microarray data reported elsewhere indicated that herpes simplex virus 1 induces the up-regulation of nuclear factor kappaB (NF-kappaB)-regulated genes, including that of its inhibitor, IkappaBalpha, consistent with the reports that wild-type virus induces the activation of NF-kappaB. In this report we show that activation of NF-kappaB in infected cells is linked to the activation of protein kinase R (PKR). Specifically: (i) PKR is activated in infected cells although the effects of the activated enzyme on protein synthesis are negated by the viral gene gamma134.5, which encodes a protein phosphatase 1alpha accessory factor that enables the dephosphorylation of the alpha subunit of eukaryotic translation initiation factor 2. NF-kappaB is activated in wild-type murine embryonic fibroblasts but not in related PKR-null cells. (ii) In cells infected with a replication-competent Deltagamma134.5 mutant (R5104), but carrying a US11 gene expressed early in infection, eukaryotic translation initiation factor 2alpha is not phosphorylated, and in in vitro assays, PKR bound to the US11 protein is not phosphorylated on subsequent addition of double-stranded RNA. Here we report that this mutant does not activate PKR, has no effect on the accumulation of IkappaBalpha, and does not cause the translocation of NF-kappaB in infected cells. (iii) One hypothesis advanced for the activation of NF-kappaB is that it blocks apoptosis induced by viral gene products. The replication-competent R5104 mutant does not induce the programmed cell's death. We conclude that in herpes simplex virus 1-infected cells, activation of NF-kappaB depends on activation of PKR and that NF-kappaB is not required to block apoptosis in productively infected cells.

Promyelocytic leukemia protein mediates interferon-based anti-herpes simplex virus 1 effects

Herpes simplex virus (HSV) 1 disaggregates the nuclear domain 10 (ND10) nuclear structures and disperses its organizing promyelocytic leukemia protein (PML). An earlier report showed that ectopic overexpression of PML precludes the disaggregation of ND10 but has no effect on viral replication. PML has been reported to mediate the effects of interferon (IFN) and viral mutants lacking ICP0 (Delta alpha 0 mutants). To test the hypothesis that HSV disaggregates ND10 structures and disperses PML to preclude IFN-mediated antiviral effects, we tested the accumulation of viral proteins and virus yields from murine PML(+/+) and PML(-/-) cells mock treated or exposed to IFN-alpha, IFN-gamma, or both and infected with the wild-type or Delta alpha 0 mutant virus. We report the following results. (i) The levels of growth of wild-type and mutant viruses and of accumulation of viral proteins were not significantly different in untreated PML(+/+) and PML(-/-) cells. (ii) Major effects of IFN-alpha and -gamma were observed in PML(+/+) cells infected with the Delta alpha 0 mutant virus, and more minor effects were observed in cells infected with the wild-type virus. The effects of the IFNs on either wild-type or the mutant virus in PML(-/-) cells were minimal. (iii) The mixture of IFN-alpha and -gamma was more effective than either IFN alone, but again, the effect was more drastic in PML(+/+) cells than in PML(-/-) cells. We concluded that the anti-HSV state induced by exogenous IFN is mediated by PML and that the virus targets the ND10 structures and disseminates PML in order to preclude the establishment of the antiviral state induced by IFNs.

A novel apoptotic interaction between HSV-1 and human corneal epithelial cells

PURPOSE

Herpes simplex virus type 1 (HSV-1) infects the cornea possibly causing blindness. The specific mechanisms of herpetic keratitis are unclear. We aimed to investigate whether HSV-1 would up- or down-regulate the apoptotic pathway of human corneal epithelial (HCE) cells.

METHODS

HSV-1 infection of HCE and Vero cells was demonstrated (immunofluorescence) and apoptotic gene expression was quantified (ribonuclease protection assay). Caspase 8 protein activity (colorimetric assay) was quantified and compared to caspase 8 mRNA amounts from RPA experiments. The apoptotic index of HSV-1 infected HCE and Vero cells (apoptotic index = % of apoptotic cells in infected samples/mock treated samples) was obtained and compared to gene expression.

RESULTS

A down-regulation in apoptotic gene expression was observed in HSV-1 infected HCE cells in contrast to Vero cells (infected and mock treated). Caspase 8 protein levels mirrored caspase 8 mRNA levels in HSV-1 infected HCE cells. The apoptotic index also supports this down-regulation. HSV-1 infected human corneal epithelial cells and Vero cells at similar rates.

CONCLUSION

HSV-1 down-regulates the apoptotic pathway of human corneal epithelial cells. This down-regulation of apoptotic gene expression seems to be cell specific. Also infectivity is excluded in playing a role in regulation of the apoptotic pathway because HSV-1 replicated at similar rates in HCE and Vero cells.

Transmission of human cytomegalovirus from infected uterine microvascular endothelial cells to differentiating/invasive placental cytotrophoblasts

Analysis of placentas infected with human cytomegalovirus (CMV) suggested that viral transmission could involve differentiating/invasive cytotrophoblasts in villi that attach the placenta to the uterine wall. To parse the cellular components in this process, we developed a coculture system of polarized uterine microvascular endothelial cell (UtMVEC) infection with an endothelial cell-tropic pathogenic strain of CMV. Then we evaluated the potential role of neutrophils and endothelial cells in the spread of infection to differentiating cytotrophoblasts. As shown by immunocytochemistry and analysis of viral replication, CMV preferentially infected endothelial cells via apical membranes and disrupted cell junction proteins, thereby altering paracellular permeability and cell polarity. Neutralizing antibodies to CMV glycoprotein B, an envelope component that facilitates virion penetration, blocked plaque formation in polarized UtMVEC. Neutrophils transmitted CMV infection to UtMVEC, which in turn infected cytotrophoblasts. However, neutrophils did not directly infect cytotrophoblasts. These findings implicate endothelial cells from the uterine microvasculature as a potential source for CMV infection of endovascular cytotrophoblasts of the anchoring villi. Possibly the cytokine/chemokine milieu in the pregnant uterus could attract immune cells that infect endothelial cells in hybrid fetal-maternal vessels. In turn, these cells could infect endovascular cytotrophoblasts, one possible initiation point of a cascade that results in retrograde placental CMV infection.

Antiviral agents alter ability of HSV-2 to disrupt gap junctional intercellular communication between mammalian cells in vitro

In cultured mammalian cells (Vero), different antiviral agents change to differing degrees the ability of HSV2 to down-regulate gap junctions, each agent having a specific effect. Measured by intracellular electrodes, control cell populations showed 49-51% coupling, uninfected populations treated with acyclovir or SDS averaged 43-51% coupling while populations infected with HSV2 had coupling reduced to 8%. The antiviral agent acyclovir (1 microg/ml), which suppresses viral replication, failed to prevent this down regulation (final coupling ratio of 11%). A plant extract (250 microg/ml) from Pilostigma thonningii offered slightly more protection (final coupling ratio of 22%), while sodium dodecyl sulfate (SDS) (50 microM) afforded nearly complete protection (final coupling ratio of 40%). With SDS there was an initial down regulation to only 16% coupling by 3 h post infection, followed by a recovery of intercellular communication to near control levels by 24 h. While SDS was originally believed to alter the viral coat and prevent entry into the cell, our data are in agreement with recent studies which indicate that SDS treated viruses can enter into host cells, but in a severely diminished condition. Our data also suggest that the gap junction antagonist is brought into the cells as part of the entering virus.

Association of herpes simplex virus type 1 ICP8 and ICP27 proteins with cellular RNA polymerase II holoenzyme

Herpes simplex virus 1 (HSV-1) infection causes the shutoff of host gene transcription and the induction of a transcriptional program of viral gene expression. Cellular RNA polymerase II is responsible for transcription of all the viral genes, but several viral proteins stimulate viral gene transcription. ICP4 is required for all delayed-early and late gene transcription, ICP0 stimulates transcription of viral genes, and ICP27 stimulates expression of some early genes and transcription of at least some late viral genes. The early DNA-binding protein, ICP8, also stimulates late gene transcription. We therefore investigated which HSV proteins interact with RNA polymerase II. Using immunoprecipitation and Western blotting methods, we observed the coprecipitation of ICP27 and ICP8 with RNA polymerase II holoenzyme. The association of ICP27 with RNA polymerase II was detectable as early as 3 h postinfection, while ICP8 association became evident by 5 h postinfection, and the association of both was independent of viral DNA synthesis. Infections with ICP27 gene mutant viruses revealed that ICP27 is required for the association of ICP8 with RNA polymerase II, while studies with ICP8 gene deletion mutants showed no apparent role for ICP8 in the association of ICP27 with RNA polymerase II. The association of ICP27 and ICP8 with RNA polymerase II holoenzyme appeared to be independent of nucleic acids. We hypothesize that the interaction of ICP27 with RNA polymerase II holoenzyme reflects its role in stimulating early and late gene expression and/or its role in inhibiting host transcription and that the interaction of ICP8 with RNA polymerase II holoenzyme reflects its role in stimulating late gene transcription.

Splicing inhibition at the level of spliceosome assembly in the presence of herpes simplex virus protein ICP27

Herpes simplex virus (HSV) immediate-early protein ICP27 is a multifunctional regulator of viral and cellular gene expression. It has previously been shown that ICP27 directly or indirectly modulates several posttranscriptional processes, such as pre-mRNA splicing and polyadenylation. We show here that pre-mRNA splicing is inhibited in nuclear extracts prepared from cells in which ICP27 has been transiently expressed. Our results show that splicing inhibition in ICP27 extracts is manifested at early stages of the splicing process. Furthermore, our results suggest that an enzymatic activity in ICP27-containing extracts causes the splicing inhibition.

The cyclin-dependent kinase inhibitor roscovitine inhibits the transactivating activity and alters the posttranslational modification of herpes simplex virus type 1 ICP0

The cyclin-dependent kinase (cdk) inhibitor Roscovitine (Rosco) reduces transcription of herpes simplex virus early genes significantly, even in the presence of wild-type levels of immediate-early (IE) viral proteins, suggesting that the transactivating functions of IE proteins may require the activities of one or more Rosco-sensitive cdk (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Based on this observation, we sought to determine whether Rosco alters the transactivating activity and posttranslational modification state of the IE protein, infected cell protein 0 (ICP0), in KOS6beta-infected Vero cells. KOS6beta is a KOS-derived recombinant virus containing an ICP0-inducible ICP6 promoter::lacZ cassette. To monitor ICP0's transactivating activity, KOS6beta-infected cells were released from a cycloheximide (CHX)-mediated protein synthesis block into medium with or without Rosco, and beta-galactosidase activity was measured. Rosco inhibited the ability of ICP0 to transactivate the ICP6 promoter by 50-fold. This inhibition was shown not to be a consequence of inhibition of ICP6 basal promoter activity or aberrant nuclear localization of ICP0. Rosco also altered the electrophoretic mobility of a portion of ICP0 molecules derived from KOS-infected cells following reversal of a CHX block. Notably, however, Rosco had only a minimal effect on the phosphorylation state of ICP0. We conclude that ICP0's transactivating activity requires Rosco-sensitive cdks and hypothesize that these cdks regulate the functions of cellular enzymes which modify ICP0, and are, consequently, required for its transactivating activity. Thus, we propose that Rosco regulates ICP0's posttranslational state by mechanisms other than, or in addition to, phosphorylation.

Herpes simplex virus 1-infected cell protein 0 contains two E3 ubiquitin ligase sites specific for different E2 ubiquitin-conjugating enzymes

Infected cell protein 0 (ICP0) of herpes simplex virus 1, a multifunctional ring finger protein, enhances the expression of genes introduced into cells by infection or transfection, interacts with numerous cellular and viral proteins, and is associated with the degradation of several cellular proteins. Sequences encoded by exon 2 of ICP0 (residues 20-241) bind the UbcH3 (cdc34) ubiquitin-conjugating enzyme, and its carboxy terminus expresses a ubiquitin ligase activity demonstrable by polyubiquitylation of cdc34 in vitro. We report that: (i) The physical interaction of cdc34 and ICP0 leads to its degradation. Thus, substitution of ICP0 aspartate 199 with alanine attenuates the degradation of cdc34 and its binding to the ICP0 ring finger domain. (ii) Substitution of residue 620 reported to abolish the interaction with a ubiquitin-specific protease has no effect on the function of ubiquitin ligase. (iii) ICP0 contains an additional distinct E3 ligase activity specific for the UbcH5a- and UbcH6 E2-conjugating enzymes mapping to the ring finger domain. This is, to our knowledge, the first identification of a viral protein with at least two physically separated E3 ligase activities with different E2 specificities. The results suggest that each activity may target different proteins.

Posttranslational processing of infected cell proteins 0 and 4 of herpes simplex virus 1 is sequential and reflects the subcellular compartment in which the proteins localize

The herpes simplex virus 1 (HSV-1) infected cell proteins 0 and 4 (ICP0 and ICP4) are multifunctional proteins extensively posttranscriptionally processed by both cellular and viral enzymes. We examined by two-dimensional separations the posttranslational forms of ICP0 and ICP4 in HEp-2 cells and in human embryonic lung (HEL) fibroblasts infected with wild-type virus, mutant R325, lacking the sequences encoding the U(S)1.5 protein and the overlapping carboxyl-terminal domain of ICP22, or R7914, in which the aspartic acid 199 of ICP0 was replaced by alanine. We report the following (i) Both ICP0 and ICP4 were sequentially posttranslationally modified at least until 12 h after infection. In HEL fibroblasts, the processing of ICP0 shifted from A+B forms at 4 h to D+G forms at 8 h and finally to G, E, and F forms at 12 h. The ICP4 progression was from the A' form noted at 2 h to B' and C' forms noted at 4 h to the additional D' and E' forms noted at 12 h. The progression tended to be toward more highly charged forms of the proteins. (ii) Although the overall patterns were similar, the mobility of proteins made in HEp-2 cells differed from those made in HEL fibroblasts. (iii) The processing of ICP0 forms E and F was blocked in HEL fibroblasts infected with R325 or with wild-type virus and treated with roscovitine, a specific inhibitor of cell cycle-dependent kinases cdc2, cdk2, and cdk5. R325-infected HEp-2 cells lacked the D' form of ICP4, and roscovitine blocked the appearance of the most highly charged E' form of ICP4. (iv) A characteristic of ICP0 is that it is translocated into the cytoplasm of HEL fibroblasts between 5 and 9 h after infection. Addition of MG132 to the cultures late in infection resulted in rapid relocation of cytoplasmic ICP0 back into the nucleus. Exposure of HEL fibroblasts to MG132 late in infection resulted in the disappearance of the highly charged ICP0 G isoform. The G form of ICP0 was also absent in cells infected with R7914 mutant. In cells infected with this mutant, ICP0 is not translocated to the cytoplasm. (v) Last, cdc2 was active in infected cells, and this activity was inhibited by roscovitine. In contrast, the activity of cdk2 exhibited by immunoprecipitated protein was reduced and resistant to roscovitine and may represent a contaminating kinase activity. We conclude from these results that the ICP0 G isoform is the cytoplasmic form, that it may be phosphorylated by cdc2, consistent with evidence published earlier (S. J., Advani, R. R. Weichselbaum, and B. Roizman, Proc. Natl. Acad. Sci. USA 96:10996-11001, 2000), and that the processing is reversed upon relocation of the G isoform from the cytoplasm into the nucleus. The processing of ICP4 is also affected by R325 and roscovitine. The latter result suggests that ICP4 may also be a substrate of cdc2 late in infection. Last, additional modifications are superimposed by cell-type-specific enzymes.