An epitope within the DNA-binding domain of the herpes simplex virus immediate early protein Vmw175 is conserved in the varicella-zoster virus gene 62 protein

Herpes simplex virus type 1 genomes are associated with ND10 nuclear substructures in quiescently infected human fibroblasts

Herpes simplex virus type 1 (HSV-1) genomes become associated with structures related to cellular nuclear substructures known as ND10 or promyelocytic leukemia nuclear bodies during the early stages of lytic infection. This paper describes the relationship between HSV-1 genomes and ND10 in human fibroblasts that maintain the viral genomes in a quiescent state. We report that quiescent HSV-1 genomes detected by fluorescence in situ hybridization (FISH) are associated with enlarged ND10-like structures, frequently such that the FISH-defined viral foci are apparently enveloped within a sphere of PML and other ND10 proteins. The number of FISH viral foci in each quiescently infected cell is concordant with the input multiplicity of infection, with each structure containing no more than a small number of viral genomes. A proportion of the enlarged ND10-like foci in quiescently infected cells contain accumulations of the heterochromatin protein HP1 but not other common markers of heterochromatin such as histone H3 di- or trimethylated on lysine residue 9. Many of the virally induced enlarged ND10-like structures also contain concentrations of conjugated ubiquitin. Quiescent infections can be established in cells that are highly depleted for PML. However, during the initial stages of establishment of a quiescent infection in such cells, other ND10 proteins (Sp100, hDaxx, and ATRX) are recruited into virally induced foci that are likely to be associated with HSV-1 genomes. These observations illustrate that the intimate connections between HSV-1 genomes and ND10 that occur during lytic infection also extend to quiescent infections.

TATA-binding protein and TBP-associated factors during herpes simplex virus type 1 infection: localization at viral DNA replication sites

Host RNA polymerase II (RNAP II) is responsible for viral transcription of the herpes simplex virus type 1 (HSV-1) genome and is relocalized to viral DNA replication compartments. Thus, we investigated whether TATA-binding protein (TBP) and TBP-associated factors (TAFs) are recruited to sites of viral transcription and replication and whether TBP/TAF expressions are influenced upon infection. The protein levels of TBP, hsTAF1/TAF(II)250, hsTAF4/TAF(II)135, and hsTAF5/TAF(II)100 were constant during the early phase of infection and started to decrease late during infection. Only for hsTAF7/TAF(II)55 we sometimes observed a decrease already at 4-8h postinfection (p.i.). Concomitantly with the relocalization of RNAP II, TBP and hsTAFs were redistributed to sites of viral DNA replication and transcription. In the absence of viral DNA replication TBP/hsTAFs were present in distinct nuclear dots, however, enlargement of the nuclear structures did not take place. Our results show that HSV-1 infection has no influence on the protein levels of TFIID components and leads to a redistribution of TBP and hsTAFs to prereplicative sites that enlarge to viral DNA replication compartments.

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.

A RING finger ubiquitin ligase is protected from autocatalyzed ubiquitination and degradation by binding to ubiquitin-specific protease USP7

Herpes simplex virus type 1 immediate-early regulatory protein ICP0 stimulates lytic infection and reactivation from latency, processes that require the ubiquitin E3 ligase activity mediated by the RING finger domain in the N-terminal portion of the protein. ICP0 stimulates the production of polyubiquitin chains by the ubiquitin-conjugating enzymes UbcH5a and UbcH6 in vitro, and in infected and transfected cells it induces the proteasome-dependent degradation of a number of cellular proteins including PML, the major constituent protein of PML nuclear bodies. However, ICP0 binds strongly to the cellular ubiquitin-specific protease USP7, a member of a family of proteins that cleave polyubiquitin chains and/or ubiquitin precursors. The region of ICP0 that is required for its interaction with USP7 has been mapped, and mutations in this domain reduce the functionality of ICP0. These findings pose the question: why does ICP0 include domains that are associated with the potentially antagonistic functions of ubiquitin conjugation and deconjugation? Here we report that although neither protein affected the intrinsic activities of the other in vitro, USP7 protected ICP0 from autoubiquitination in vitro, and their interaction can greatly increase the stability of ICP0 in vivo. These results demonstrate that RING finger-mediated autoubiquitination of ICP0 is biologically relevant and can be regulated by interaction with USP7. This principle may extend to a number of cellular RING finger E3 ubiquitin ligase proteins that have analogous interactions with ubiquitin-specific cleavage enzymes.

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.

Recruitment of herpes simplex virus type 1 transcriptional regulatory protein ICP4 into foci juxtaposed to ND10 in live, infected cells

At the early stages of herpes simplex virus type 1 (HSV-1) infection, parental viral genomes have a tendency to become juxtaposed to cellular nuclear structures known as PML (promyelocytic leukemia) nuclear bodies or ND10, while the immediate-early (IE) protein ICP0 precisely colocalizes with these structures. Previous indirect-immunofluorescence studies observed that the HSV-1 transcriptional regulator ICP4 has a mainly diffuse nuclear distribution early in infection and is later recruited into viral replication compartments. We have constructed HSV-1 variants expressing ICP4 and ICP0 linked to ECFP and EYFP, respectively, both singly and in combination. Coupled with an efficient method of expressing autofluorescent PML in ND10, we have studied the dynamics of ICP0, ICP4, and ND10 in live, infected cells. The greater sensitivity and lower background signals in live cells revealed that early in infection, ICP4 forms discrete foci, some of which are juxtaposed to ND10, while ICP0 was found to colocalize precisely with PML. As expected from these results, using a double-labeled virus, we observed that foci of ICP0 and ICP4 were also juxtaposed but not colocalized early in infection. Some of the ICP4 foci must have contained parental viral genomes, because they developed into replication compartments. We propose that a proportion of the ND10-associated ICP4 foci represent ICP4 molecules being recruited onto parental viral genomes, a process likely to be a critical step early in lytic infection. These results may be analogous to the localization of IE1 and IE2 during human cytomegalovirus infection, suggesting a principle common to the alpha- and betaherpesviruses.

Human neuron-committed teratocarcinoma NT2 cell line has abnormal ND10 structures and is poorly infected by herpes simplex virus type 1

Herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP0 stimulates the initiation of lytic infection and reactivation from quiescence in human fibroblast cells. These functions correlate with its ability to localize to and disrupt centromeres and specific subnuclear structures known as ND10, PML nuclear bodies, or promyelocytic oncogenic domains. Since the natural site of herpesvirus latency is in neurons, we investigated the status of ND10 and centromeres in uninfected and infected human cells with neuronal characteristics. We found that NT2 cells, a neuronally committed human teratocarcinoma cell line, have abnormal ND10 characterized by low expression of the major ND10 component PML and no detectable expression of another major ND10 antigen, Sp100. In addition, PML is less extensively modified by the ubiquitin-like protein SUMO-1 in NT2 cells compared to fibroblasts. After treatment with retinoic acid, NT2 cells differentiate into neuron-like hNT cells which express very high levels of both PML and Sp100. Infection of both NT2 and hNT cells by HSV-1 was poor compared to human fibroblasts, and after low-multiplicity infection yields of virus were reduced by 2 to 3 orders of magnitude. ICP0-deficient mutants were also disabled in the neuron-related cell lines, and cells quiescently infected with an ICP0-null virus could be established. These results correlated with less-efficient disruption of ND10 and centromeres induced by ICP0 in NT2 and hNT cells. Furthermore, the ability of ICP0 to activate gene expression in transfection assays in NT2 cells was poor compared to Vero cells. These results suggest that a contributory factor in the reduced HSV-1 replication in the neuron-related cells is inefficient ICP0 function; it is possible that this is pertinent to the establishment of latent infection in neurons in vivo.

Alphaherpesvirus proteins related to herpes simplex virus type 1 ICP0 affect cellular structures and proteins

The herpes simplex virus type 1 (HSV-1) immediate-early protein ICP0 interacts with several cellular proteins and induces the proteasome-dependent degradation of others during infection. In this study we show that ICP0 is required for the proteasome-dependent degradation of the ND10 protein Sp100 and, as with the other target proteins, the ICP0 RING finger domain is essential. Further, comparison of the kinetics and ICP0 domain requirements for the degradation of PMI and Sp100 suggests that a common mechanism is involved. Homologues of ICP0 are encoded by other members of the alphaherpesvirus family. These proteins show strong sequence homology to ICP0 within the RING finger domain but limited similarity elsewhere. Using transfection assays, we have shown that all the ICP0 homologues that we tested have significant effects on the immunofluorescence staining character of at least one of the proteins destabilized by ICP0, and by using a recombinant virus, we found that the equine herpesvirus ICP0 homologue induced the proteasome-dependent degradation of endogenous CENP-C and modified forms of PML and Sp100. However, in contrast to ICP0, the homologue proteins had no effect on the distribution of the ubiquitin-specific protease USP7 within the cell, consistent with their lack of a USP7 binding domain. We also found that ICP0 by itself could induce the abrogation of SUMO-1 conjugation and then the proteasome-dependent degradation of unmodified exogenous PML in transfected cells, thus demonstrating that other HSV-1 proteins are not required. Surprisingly, the ICP0 homologues were unable to cause these effects. Overall, these data suggest that the members of the ICP0 family of proteins may act via a similar mechanism or pathway involving their RING finger domain but that their intrinsic activities and effects on endogenous and exogenous proteins differ in detail.

Herpes simplex virus type 1 infection stimulates p38/c-Jun N-terminal mitogen-activated protein kinase pathways and activates transcription factor AP-1

Cells respond to environmental stress and proinflammatory cytokines by stimulating the Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and the p38 mitogen-activated protein kinase cascades. Infection of eukaryotic cells with herpes simplex virus type 1 (HSV-1) resulted in stimulation of both JNK/SAPK and p38 mitogen-activated protein kinase after 3 h of infection, and activation reached a maximum of 4-fold by 9 h post-infection. By using a series of mutant viruses, we showed that the virion transactivator protein VP16 stimulates p38/JNK, whereas no immediate-early, early, or late viral expressed gene is involved. We identified the stress-activated protein kinase kinase 1 as an upstream activator of p38/JNK, and we demonstrated that activation of AP-1 binding proceeded p38/JNK stimulation. During infection, the activated AP-1 consisted mainly of JunB and JunD with a simultaneous decrease in the cellular levels of Jun protein. We suggest that activation of the stress pathways by HSV-1 infection either represents a cascade triggered by the virus to facilitate the lytic cycle or a defense mechanism of the host cell against virus invasion.

Herpes simplex virus type 1 immediate-early protein vmw110 induces the proteasome-dependent degradation of the catalytic subunit of DNA-dependent protein kinase

Herpes simplex virus type 1 (HSV-1) infection causes the active degradation of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs), and this process is reliant on the expression of the HSV-1 immediate-early protein Vmw110. In this study we investigated in more detail the mechanism by which the degradation occurs, the domains of Vmw110 which are required, and whether Vmw110 is by itself sufficient for the effect. We found that proteasome inhibitors prevented the degradation of DNA-PKcs, indicating the involvement of a proteasome pathway. Furthermore, the continued activity of DNA-PK during infection in the presence of these inhibitors indicated that Vmw110 does not directly alter the enzyme activity of DNA-PKcs prior to its degradation in a normal infection. Indeed, Vmw110 was found to bind to neither the catalytic nor Ku subunits of DNA-PK. Using mutant Vmw110 viruses we show that the RING finger domain of Vmw110 is essential for the induced degradation of DNA-PKcs but that the ability of Vmw110 to bind to a cellular ubiquitin-specific protease (HAUSP) is not required. When expressed in the absence of other viral proteins, Vmw110 was sufficient to cause the degradation of DNA-PKcs, indicating that the effect on the stability of DNA-PKcs was a direct consequence of Vmw110 activity and not an indirect Vmw110-dependent effect of virus infection. Finally, the Vmw110-induced degradation of DNA-PKcs and loss in DNA-PK activity appears to be beneficial to HSV-1 infection, as virus replication was more efficient in cells lacking DNA-PKcs, especially at low multiplicities of infection.

A viral activator of gene expression functions via the ubiquitin-proteasome pathway

The ability of herpes simplex virus type 1 (HSV-1) to attain a latent state in sensory neurones and reactivate periodically is crucial for its biological and clinical properties. The active transcription of the entire 152 kb viral genome during lytic replication contrasts with the latent state, which is characterized by the production of a single set of nuclear-retained transcripts. Reactivation of latent genomes to re-initiate the lytic cycle therefore involves a profound change in viral transcriptional activity, but the mechanisms by which this fundamentally important process occurs are yet to be well understood. In this report we show that the stimulation of the onset of viral lytic infection mediated by the viral immediate-early (IE) protein Vmw110 is strikingly inhibited by inactivation of the ubiquitin-proteasome pathway. Similarly, the Vmw110-dependent reactivation of quiescent viral genomes in cultured cells is also dependent on proteasome activity. These results constitute the first demonstration that the transcriptional activity of a viral genome can be regulated by protein stability control pathways.

The DNA binding domains of the varicella-zoster virus gene 62 and herpes simplex virus type 1 ICP4 transactivator proteins heterodimerize and bind to DNA

The product of varicella-zoster virus gene 62 (VZV 140k) is the functional counterpart of the major transcriptional regulatory protein of herpes simplex virus type 1 (HSV-1), ICP4. We have found that the purified bacterially expressed DNA binding domain of VZV 140k (residues 417-647) is a stable dimer in solution. As demonstrated by the appearance of a novel protein--DNA complex of intermediate mobility in gel retardation assays, following in vitro co-translation of a pair of differently sized VZV 140k DNA binding domain peptides, the 140k DNA binding domain peptide binds to DNA as a dimer. In addition, the DNA binding domain peptide of HSV-1 ICP4 readily heterodimerizes with the VZV 140k peptide on co-translation, indicating that HSV-1 ICP4 and VZV 140k possess very similar dimerization interfaces. It appears that only one fully wild type subunit of the dimer is sufficient to mediate sequence specific DNA recognition in certain circumstances. Co-immunoprecipitation analysis of mutant DNA binding domain peptides, co-translated with an epitope-tagged ICP4 DNA binding domain, shows that the sequence requirements for dimerization are lower than those necessary for DNA binding.