Construction of an excisable bacterial artificial chromosome containing a full-length infectious clone of herpes simplex virus type 1: viruses reconstituted from the clone exhibit wild-type properties in vitro and in vivo

Herpes simplex virus type 1-derived recombinant and amplicon vectors

Herpes simplex virus type 1 (HSV-1) is a human pathogen whose lifestyle is based on a long-term dual interaction with the infected host, being able to establish both lytic and latent infections. The virus genome is a 153 kbp double-stranded DNA molecule encoding more than 80 genes. The interest of HSV-1 as gene transfer vector stems from its ability to infect many different cell types, both quiescent and proliferating cells, the very high packaging capacity of the virus capsid, the outstanding neurotropic adaptations that this virus has evolved, and the fact that it never integrates into the cellular chromosomes, thus avoiding the risk of insertional mutagenesis. Two types of vectors can be derived from HSV-1, recombinant vectors and amplicon vectors, and different methodologies have been developed to prepare large stocks of each type of vector. This chapter summarizes (1) the two approaches most commonly used to prepare recombinant vectors through homologous recombination, either in eukaryotic cells or in bacteria, and (2) the two methodologies currently used to generate helper-free amplicon vectors, either using a bacterial artificial chromosome (BAC)-based approach or a Cre/loxP site-specific recombination strategy.

A mutation in UL15 of herpes simplex virus 1 that reduces packaging of cleaved genomes

Herpesvirus genomic DNA is cleaved from concatemers that accumulate in infected cell nuclei. Genomic DNA is inserted into preassembled capsids through a unique portal vertex. Extensive analyses of viral mutants have indicated that intact capsids, the portal vertex, and all components of a tripartite terminase enzyme are required to both cleave and package viral DNA, suggesting that DNA cleavage and packaging are inextricably linked. Because the processes have not been functionally separable, it has been difficult to parse the roles of individual proteins in the DNA cleavage/packaging reaction. In the present study, a virus bearing the deletion of codons 400 to 420 of U(L)15, encoding a terminase component, was analyzed. This virus, designated vJB27, failed to replicate on noncomplementing cells but cleaved concatemeric DNA to ca. 35 to 98% of wild-type levels. No DNA cleavage was detected in cells infected with a U(L)15-null virus or a virus lacking U(L)15 codons 383 to 385, comprising a motif proposed to couple ATP hydrolysis to DNA translocation. The amount of vJB27 DNA protected from DNase I digestion was reduced compared to the wild-type virus by 6.5- to 200-fold, depending on the DNA fragment analyzed, thus indicating a profound defect in DNA packaging. Capsids containing viral DNA were not detected in vJB27-infected cells, as determined by electron microscopy. These data suggest that pU(L)15 plays an essential role in DNA translocation into the capsid and indicate that this function is separable from its role in DNA cleavage.

A functional role for TorsinA in herpes simplex virus 1 nuclear egress

Herpes simplex virus 1 (HSV-1) capsids leave the nucleus by a process of envelopment and de-envelopment at the nuclear envelope (NE) that is accompanied by structural alterations of the NE. As capsids translocate across the NE, transient primary enveloped virions form in the perinuclear space. Here, we provide evidence that torsinA (TA), a ubiquitously expressed ATPase, has a role in HSV-1 nuclear egress. TA resides within the lumen of the endoplasmic reticulum (ER)/NE and functions in maintaining normal NE architecture. We show that perturbation of TA normal function by overexpressing torsinA wild type (TAwt) inhibits HSV-1 production. Ultrastructural analysis of infected cells overexpressing TAwt revealed reduced levels of surface virions in addition to accumulation of novel, double-membrane structures called virus-like vesicles (VLVs). Although mainly found in the cytoplasm, VLVs resemble primary virions in their size, by the appearance of the inner membrane, and by the presence of pUL34, a structural component of primary virions. Collectively, our data suggest a model in which interference of TA normal function by overexpression impairs de-envelopment of the primary virions leading to their accumulation in a cytoplasmic membrane compartment. This implies novel functions for TA at the NE.

Herpes simplex virus 1 protein kinase Us3 and major tegument protein UL47 reciprocally regulate their subcellular localization in infected cells

Us3 is a serine-threonine protein kinase encoded by herpes simplex virus 1 (HSV-1). We have identified UL47, a major virion protein, as a novel physiological substrate of Us3. In vitro kinase assays and systematic analysis of mutations at putative Us3 phosphorylation sites near the nuclear localization signal of UL47 showed that serine at residue 77 (Ser-77) was required for Us3 phosphorylation of UL47. Replacement of UL47 Ser-77 by alanine produced aberrant accumulation of UL47 at the nuclear rim and impaired the nuclear localization of UL47 in a significant fraction of infected cells. The same defect in UL47 localization was produced by an amino acid substitution in Us3 that inactivated its protein kinase activity. In contrast, a phosphomimetic mutation at UL47 Ser-77 restored wild-type nuclear localization. The UL47 S77A mutation also reduced viral replication in the mouse cornea and the development of herpes stromal keratitis in mice. In addition, UL47 formed a stable complex with Us3 in infected cells, and nuclear localization of Us3 was significantly impaired in the absence of UL47. These results suggested that Us3 phosphorylation of UL47 Ser-77 promoted the nuclear localization of UL47 in cell cultures and played a critical role in viral replication and pathogenesis in vivo. Furthermore, UL47 appeared to be required for efficient nuclear localization of Us3 in infected cells. Therefore, Us3 protein kinase and its substrate UL47 demonstrated a unique regulatory feature in that they reciprocally regulated their subcellular localization in infected cells.

Screening and identification of host factors interacting with UL14 of herpes simplex virus 1

The UL14 protein of herpes simplex virus type 1 (HSV-1) is highly conserved in herpesvirus family. However, its exact function during the HSV-1 replication cycle is little known. In the present study, a high throughput yeast two-hybrid system was employed to screen the cellular factors interacting with UL14, and five target candidates were yielded: (1) TSC22 domain family protein 3 (TSC22D3); (2) Mediator of RNA polymerase II transcription subunit 8 isoform 1(MED8); (3) Runt-related transcription factor 3 (RUNX3); (4) Arrestin beta-2 (ARRB2); (5) Cereblon (CRBN). Indirect immunofluorescent assay showed that both TSC22D3 and MED8 co-localized with UL14. Co-immunoprecipitation assay demonstrated that UL14 could be immunoprecipitated by TSC22D3, suggesting that UL14 interacted with TSC22D3 under physiological condition. In summary, this study opened up new avenues toward delineating the function and physiological significance of UL14 during the HSV-1 replication cycle.

Anterograde transport of herpes simplex virus capsids in neurons by both separate and married mechanisms

Anterograde transport of herpes simplex virus (HSV) from neuronal cell bodies into, and down, axons is a fundamentally important process for spread to other hosts. Different techniques for imaging HSV in axons have produced two models for how virus particles are transported in axons. In the Separate model, viral nucleocapsids devoid of the viral envelope and membrane glycoproteins are transported in axons. In the Married model, enveloped HSV particles (with the viral glycoproteins) encased within membrane vesicles are transported in the anterograde direction. Earlier studies of HSV-infected human neurons involving electron microscopy (EM) and immunofluorescence staining of glycoproteins and capsids supported the Separate model. However, more-recent live-cell imaging of rat, chicken, and mouse neurons produced evidence supporting the Married model. In a recent EM study, a mixture of Married (75%) and Separate (25%) HSV particles was observed. Here, we studied an HSV recombinant expressing a fluorescent form of the viral glycoprotein gB and a fluorescent capsid protein (VP26), observing that human SK-N-SH neurons contained both Separate (the majority) and Married particles. Live-cell imaging of rat superior cervical ganglion (SCG) neuronal axons in a chamber system (which oriented the axons) also produced evidence of Separate and Married particles. Together, our results suggest that one can observe anterograde transport of both HSV capsids and enveloped virus particles depending on which neurons are cultured and how the neurons are imaged.

Role of the herpes simplex virus 1 Us3 kinase phosphorylation site and endocytosis motifs in the intracellular transport and neurovirulence of envelope glycoprotein B

Herpes simplex virus 1 (HSV-1) Us3 protein kinase phosphorylates threonine at position 887 (Thr-887) in the cytoplasmic tail of envelope glycoprotein B (gB) in infected cells. This phosphorylation downregulates cell surface expression of gB and plays a role in viral pathogenesis in the mouse herpes stromal keratitis model. In the present study, we demonstrated that Us3 phosphorylation of gB Thr-887 upregulated the accumulation of endocytosed gB from the surfaces of infected cells. We also showed that two motifs in the cytoplasmic tail of gB, tyrosine at position 889 (Tyr-889) and dileucines at positions 871 and 872, were required for efficient downregulation of gB cell surface expression and upregulation of accumulation of endocytosed gB in infected cells. A systematic analysis of mutations in these three sequences in gB suggested that the expression of gB on the surfaces of infected cells was downregulated in part by the increase in the accumulation of endocytosed gB, which was coordinately and tightly regulated by the three gB trafficking signals. Tyr-889 appeared to be of predominant importance in regulating the intracellular transport of gB and was linked to HSV-1 neurovirulence in mice following intracerebral infection. These observations support the hypothesis that HSV-1 evolved the three gB sequences for proper regulation of gB intracellular transport and that this regulation plays a critical role in diverse aspects of HSV-1 pathogenesis.

Recent advances in cloning herpesviral genomes as infectious bacterial artificial chromosomes

Herpesviruses are common but important pathogens in humans and animals. These viruses have large complex genomes encoding genes with diverse functions in different phases of their life cycle and associated diseases. In the last decade, genomes of herpesviruses cloned as infectious bacterial artificial chromosomes (BACs) have become powerful tools for delineating the functions of viral genes and understanding the pathogenesis of their associated diseases. Here we review the history of herpesviral genetics and recent advances in methods for cloning herpesviral genomes as infectious BACs.

Human herpesvirus 6 major immediate early promoter has strong activity in T cells and is useful for heterologous gene expression

BACKGROUND

Human herpesvirus-6 (HHV-6) is a beta-herpesvirus. HHV-6 infects and replicates in T cells. The HHV-6-encoded major immediate early gene (MIE) is expressed at the immediate-early infection phase. Human cytomegalovirus major immediate early promoter (CMV MIEp) is commercially available for the expression of various heterologous genes. Here we identified the HHV-6 MIE promoter (MIEp) and compared its activity with that of CMV MIEp in various cell lines.

METHODS

The HHV-6 MIEp and some HHV-6 MIEp variants were amplified by PCR from HHV-6B strain HST. These fragments and CMV MIEp were subcloned into the pGL-3 luciferase reporter plasmid and subjected to luciferase reporter assay. In addition, to investigate whether the HHV-6 MIEp could be used as the promoter for expression of foreign genes in a recombinant varicella-zoster virus, we inserted HHV-6 MIEp-DsRed expression casette into the varicella-zoster virus genome.

RESULTS

HHV-6 MIEp showed strong activity in T cells compared with CMV MIEp, and the presence of intron 1 of the MIE gene increased its activity. The NF-κB-binding site, which lies within the R3 repeat, was critical for this activity. Moreover, the HHV-6 MIEp drove heterologous gene expression in recombinant varicella-zoster virus-infected cells.

CONCLUSIONS

These data suggest that HHV-6 MIEp functions more strongly than CMV MIEp in various T-cell lines.

Equine herpesvirus 4: recent advances using BAC technology

The equine herpesviruses are major infectious pathogens that threaten equine health. Equine herpesvirus 4 (EHV-4) is an important equine pathogen that causes respiratory tract disease, known as rhinopneumonitis, among horses worldwide. EHV-4 genome manipulation with subsequent understanding of the viral gene functions has always been difficult due to the limited number of susceptible cell lines and the absence of small-animal models of the infection. Efficient generation of mutants of EHV-4 would significantly contribute to the rapid and accurate characterization of the viral genes. This problem has been solved recently by the cloning of the genome of EHV-4 as a stable and infectious bacterial artificial chromosome (BAC) without any deletions of the viral genes. Very low copy BAC vectors are the mainstay of present genomic research because of the high stability of inserted clones and the possibility of mutating any gene target in a relatively short time. Manipulation of EHV-4 genome is now feasible using the power of BAC technology, and should aid greatly in assessing the role of viral genes in the virus-host interaction.

Strategies for the rapid construction of conditionally-replicating HSV-1 vectors expressing foreign genes as anticancer therapeutic agents

Conditionally replication-competent Herpes Simplex Virus Type 1 (HSV-1) vectors expressing foreign genes have been developed as experimental therapeutic agents. Traditional methods of virus construction, including growth selection based on thymidine kinase gene expression, and color selection based on a reporter gene expression are often time-consuming and relatively inefficient. This review summarizes the various strategies developed in recent years for the rapid and efficient construction of novel conditionally replication-competent mutant HSV expressing multiple foreign genes. Additionally, two new modifications of existing strategies, which have not been previously reported, are discussed.

Epstein-Barr virus genetics: talking about the BAC generation

Genetic mutant organisms pervade all areas of Biology. Early on, herpesviruses (HV) were found to be amenable to genetic analysis using homologous recombination techniques in eukaryotic cells. More recently, HV genomes cloned onto a bacterial artificial chromosome (BAC) have become available. HV BACs can be easily modified in E.coli and reintroduced in eukaryotic cells to produce infectious viruses. Mutants derived from HV BACs have been used both to understand the functions of all types of genetic elements present on the virus genome, but also to generate mutants with potentially medically relevant properties such as preventative vaccines. Here we retrace the development of the BAC technology applied to the Epstein-Barr virus (EBV) and review the strategies available for the construction of mutants. We expand on the appropriate controls required for proper use of the EBV BACs, and on the technical hurdles researchers face in working with these recombinants. We then discuss how further technological developments might successfully overcome these difficulties. Finally, we catalog the EBV BAC mutants that are currently available and illustrate their contributions to the field using a few representative examples.

Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-low-producer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action.

Characterization of the subcellular localization of herpes simplex virus type 1 proteins in living cells

In this study, we presented the construction of a library of expression clones for the herpes simplex virus type 1 (HSV-1) proteome and subcellular localization map of HSV-1 proteins in living cells using yellow fluorescent protein (YFP) fusion proteins. As a result, 21 proteins showed cytoplasmic or subcytoplasmic localization, 16 proteins showed nuclear or subnuclear localization, and others were present both in the nucleus and cytoplasm. Interestingly, most capsid proteins showed enriched or exclusive localization in the nucleus, and most of the envelope proteins showed cytoplasmic localization, suggesting that subcellular localization of the proteins correlated with their functions during virus replication. These results present a subcellular localization map of HSV-1 proteins in living cells, which provide useful information to further characterize the functions of these proteins.

Human herpesvirus 6 encoded glycoprotein Q1 gene is essential for virus growth

Human herpesvirus 6 (HHV-6) glycoprotein Q1 (gQ1), a unique gene in HHV-6, forms a complex with glycoproteinH (gH) and gL, which is the viral ligand for its cellular receptor, CD46. However, whether gQ1 is essential for virus growth is unknown, because a system is lacking for making gene knockouts for HHV-6. Recently, bacterial artificial chromosome (BAC) and E. coli mutagenesis techniques have been applied to herpesvirus investigation. Here we successfully inserted the HHV-6A genome into a BAC, and obtained reconstituted infectious virus from the HHV-6A-containing BAC DNA. Using this system, we generated a gQ1 mutant virus genome, which failed to yield reconstituted infectious virus, whereas its revertant virus could be produced, indicating that the HHV-6 gQ1 gene is essential for virus growth. Therefore, we successfully applied BAC and E. coli mutagenesis techniques to the study of HHV-6, and discovered that HHV-6 gQ1 is an essential gene for virus growth.

Resolving the assembly state of herpes simplex virus during axon transport by live-cell imaging

Neurotropic herpesviruses depend on long-distance axon transport for the initial establishment of latency in peripheral ganglia (retrograde transport) and for viral spread in axons to exposed body surfaces following reactivation (anterograde transport). Images of neurons infected with herpes simplex virus type 1 (HSV-1), acquired using electron microscopy, have led to a debate regarding why different types of viral structures are seen in axons and which of these particles are relevant to the axon transport process. In this study, we applied time-lapse fluorescence microscopy to image HSV-1 virion components actively translocating to distal axons in primary neurons and neuronal cell lines. Key to these findings, only a small fraction of viral particles were engaged in anterograde transport during the egress phase of infection at any given time. By selective analysis of the composition of the subpopulation of actively transporting capsids, a link between transport of fully assembled HSV-1 virions and the neuronal secretory pathway was identified. Last, we have evaluated the seemingly opposing findings made in previous studies of HSV-1 axon transport in fixed cells and demonstrate a limitation to assessing the composition of individual HSV-1 particles using antibody detection methods.

HSV as a vector in vaccine development and gene therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), major human pathogen whose lifestyle is based on a long-term dual interaction with the infected host characterized by the existence of lytic and latent infections, has allowed the development of potential vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous system, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases and targeted infection of specific tissues or organs. Three different classes of vectors can be derived from HSV-1: replication-competent attenuated vectors, replication-incompetent recombinant vectors and defective helper-dependent vectors known as amplicons. This chapter highlights the current knowledge concerning design, construction and recent applications, as well as the potential and current limitations of the three different classes of HSV-1-based vectors.

A single-amino-acid substitution in herpes simplex virus 1 envelope glycoprotein B at a site required for binding to the paired immunoglobulin-like type 2 receptor alpha (PILRalpha) abrogates PILRalpha-dependent viral entry and reduces pathogenesis

Paired immunoglobulin-like type 2 receptor α (PILRα) is a herpes simplex virus 1 (HSV-1) entry receptor that associates with O-glycans on HSV-1 envelope glycoprotein B (gB). Two threonine residues (Thr-53 and Thr-480) in gB, which are required for the addition of the principal gB O-glycans, are essential for binding to soluble PILRα. However, the role of the two threonines in PILRα-dependent viral entry remains to be elucidated. Therefore, we constructed a recombinant HSV-1 carrying an alanine replacement of gB Thr-53 alone (gB-T53A) or of both gB Thr-53 and Thr-480 (gB-T53/480A) and demonstrated that these mutations abrogated viral entry in CHO cells expressing PILRα. In contrast, the mutations had no effect on viral entry in CHO cells expressing known host cell receptors for HSV-1 gD, viral entry in HL60 cells expressing myelin-associated glycoprotein (MAG) (another HSV-1 gB receptor), viral attachment to heparan sulfate, and viral replication in PILRα-negative cells. These results support the hypothesis that gB Thr-53 and Thr-480 as well as gB O-glycosylation, probably at these sites, are critical for PILRα-dependent viral entry. Interestingly, following corneal inoculation in mice, the gB-T53A and gB-T53/480A mutations significantly reduced viral replication in the cornea, the development of herpes stroma keratitis, and neuroinvasiveness. The abilities of HSV-1 to enter cells in a PILRα-dependent manner and to acquire specific carbohydrates on gB are therefore linked to an increase in viral replication and virulence in the experimental murine model.

Molecular anatomy of subcellular localization of HSV-1 tegument protein US11 in living cells

The herpes simplex virus type I (HSV-1) US11 protein is an RNA-binding multifunctional regulator that specifically and stably associates with nucleoli. Although the C-terminal part of US11 was responsible for its nucleolar localization, the precise nucleolar localization signal (NoLS) and nuclear export signal (NES) of US11 and its nuclear import and export mechanisms are still elusive. In this study, fluorescence microscopy was employed to investigate the subcellular localization of US11 and characterize its transport mechanism in living cells. By constructing a series of deletion mutants fused with enhanced yellow fluorescent protein (EYFP), three novel NoLSs of US11 were for the first time mapped to amino acids 84-125, 126-152, and 89-146, respectively. Additionally, the NES was identified to locate between amino acids 89 and 119. Furthermore, the US11 protein was demonstrated to target to the cytoplasm through the NES by chromosomal region maintenance 1 (CRM1)-independent pathway, and to the nucleolus through Ran and importin beta-dependent mechanism that does not require importin alpha 5.

Characterization of varicella-zoster virus-encoded ORF0 gene--comparison of parental and vaccine strains

The varicella-zoster virus (VZV) Oka vaccine strain (vOka) differs from the parental strain (pOka) at several amino acid positions, but the mutations responsible for the attenuation of vOka have not been clearly defined. The ORF0 of vOka carries some of the mutations. Although we found that the ORF0 of both strains was incorporated into virus particles, the C-terminal region of vOka ORF0 was presented on the virion surface and was N-glycosylated, suggesting that the mutation in vOka ORF0 changes it into a novel envelope glycoprotein. In a mutant virus in which pOka ORF0 was replaced by vOka ORF0, the molecular weight of ORF0 was altered, but the plaque size was not. In addition, a pOka recombinant virus lacking the hydrophobic domain of ORF0 grew equally well as the wild-type virus, indicating that the mutation in ORF0 is not by itself sufficient for the attenuation of the vOka virus.

Herpes simplex virus type 1 ICP27 protein: its expression, purification and specific antiserum production

Herpes simplex virus type 1 (HSV-1) is the causative agent of cold sores and other more serious diseases. HSV-1 infected-cell protein 27 (ICP27) is an immediate-early regulatory phosphoprotein homologous to gene products identified in all classes of herpesviruses so far. To raise the antiserum to ICP27 for further characterization of its biological function, the ICP27 gene was cloned into the pET-28a (+) vector, then ICP27 protein was expressed in E. coli and purified by nickel-nitrilotriacetic acid (Ni(2+)-NTA) affinity resin column, finally the purified protein was used to raise antiserum. Western blot analysis demonstrated that the antiserum recognized the recombinant protein, and the antiserum was able to probe the ICP27 in HSV-1 infected cells with high specificity by immunofluorescence assay (IFA). Therefore, the specific antiserum will provide a valuable tool for further studies investigating ICP27's biological function during HSV-1 infection.

The capsid protein encoded by U(L)17 of herpes simplex virus 1 interacts with tegument protein VP13/14

The U(L)17 protein (pU(L)17) of herpes simplex virus 1 (HSV-1) likely associates with the surfaces of DNA-containing capsids in a heterodimer with pU(L)25. pU(L)17 is also associated with viral light particles that lack capsid proteins, suggesting its presence in the tegument of the HSV-1 virion. To help determine how pU(L)17 becomes incorporated into virions and its functions therein, we identified pU(L)17-interacting proteins by immunoprecipitation with pU(L)17-specific IgY at 16 h postinfection, followed by mass spectrometry. Coimmunoprecipitated proteins included cellular histone proteins H2A, H3, and H4; the intermediate filament protein vimentin; the major HSV-1 capsid protein VP5; and the HSV tegument proteins VP11/12 (pU(L)46) and VP13/14 (pU(L)47). The pU(L)17-VP13/14 interaction was confirmed by coimmunoprecipitation in the presence and absence of intact capsids and by affinity copurification of pU(L)17 and VP13/14 from lysates of cells infected with a recombinant virus encoding His-tagged pU(L)17. pU(L)17 and VP13/14-HA colocalized in the nuclear replication compartment, in the cytoplasm, and at the plasma membrane between 9 and 18 h postinfection. One possible explanation of these data is that pU(L)17 links the external face of the capsid to VP13/14 and associated tegument components.

Rapid and efficient introduction of a foreign gene into bacterial artificial chromosome-cloned varicella vaccine by Tn7-mediated site-specific transposition

Using a rapid and reliable system based on Tn7-mediated site-specific transposition, we have successfully constructed a recombinant Oka varicella vaccine (vOka) expressing the mumps virus (MuV) fusion protein (F). The backbone of the vector was our previously reported vOka-BAC (bacterial artificial chromosome) genome. We inserted the transposon Tn7 attachment sequence, LacZalpha-mini-attTn7, into the region between ORF12 and ORF13 to generate a vOka-BAC-Tn genome. The MuV-F expressing cassette was transposed into the vOka-BAC genome at the mini-attTn7 transposition site. MuV-F protein was expressed in recombinant virus, rvOka-F infected cells. In addition, the MuV-F protein was cleaved in the rvOka-F infected cells as in MuV-infected cells. The growth of rvOka-F was similar to that of the original recombinant vOka without the F gene. Thus, we show that Tn7-mediated transposition is an efficient method for introducing a foreign gene expression cassette into the vOka-BAC genome as a live virus vector.

Expression, purification of the UL3 protein of herpes simplex virus type 1, and production of UL3 polyclonal antibody

Herpes simplex virus type 1 (HSV-1) is a common pathogen which causes infections of the mucocutaneous membranes. The UL3 protein belongs to a group of HSV-1 late proteins. To date, the function of the UL3 protein in cell culture, animal models, and natural infection is unknown. To investigate further the function of the UL3 protein, this study was undertaken to express the UL3 protein and raise a polyclonal antibody. The UL3 gene was cloned in the prokaryotic expression vector pET-28a (+) to yield pET-28a (+)-UL3. The His6-tagged UL3 protein was expressed in Escherichia coli (E. coli) BL21 (DE3) cells and analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). After purification by nickel affinity chromatography and refolding, the recombinant protein was used to raise the anti-UL3 polyclonal antibody. Western blot analysis demonstrated that the UL3 protein was recognized by the polyclonal antibody, and immunofluorescent assay also showed that the antibody was able to recognize the UL3 protein in the cells infected with HSV-1.

Nucleolin is required for efficient nuclear egress of herpes simplex virus type 1 nucleocapsids

Herpesvirus nucleocapsids assemble in the nucleus and must cross the nuclear membrane for final assembly and maturation to form infectious progeny virions in the cytoplasm. It has been proposed that nucleocapsids enter the perinuclear space by budding through the inner nuclear membrane, and these enveloped nucleocapsids then fuse with the outer nuclear membrane to enter the cytoplasm. Little is known about the mechanism(s) for nuclear egress of herpesvirus nucleocapsids and, in particular, which, if any, cellular proteins are involved in the nuclear egress pathway. UL12 is an alkaline nuclease encoded by herpes simplex virus type 1 (HSV-1) and has been suggested to be involved in viral DNA maturation and nuclear egress of nucleocapsids. Using a live-cell imaging system to study cells infected by a recombinant HSV-1 expressing UL12 fused to a fluorescent protein, we observed the previously unreported nucleolar localization of UL12 in live infected cells and, using coimmunoprecipitation analyses, showed that UL12 formed a complex with nucleolin, a nucleolus marker, in infected cells. Knockdown of nucleolin in HSV-1-infected cells reduced capsid accumulation, as well as the amount of viral DNA resistant to staphylococcal nuclease in the cytoplasm, which represented encapsidated viral DNA, but had little effect on these viral components in the nucleus. These results indicated that nucleolin is a cellular factor required for efficient nuclear egress of HSV-1 nucleocapsids in infected cells.

Analysis of a charge cluster mutation of herpes simplex virus type 1 UL34 and its extragenic suppressor suggests a novel interaction between pUL34 and pUL31 that is necessary for membrane curvature around capsids

Interaction between pUL34 and pUL31 is essential for targeting both proteins to the inner nuclear membrane (INM). Sequences mediating the targeting interaction have been mapped by others with both proteins. We have previously reported identification of charge cluster mutants of herpes simplex virus type 1 UL34 that localize properly to the inner nuclear membrane, indicating interaction with UL31, but fail to complement a UL34 deletion. We have characterized one mutation (CL04) that alters a charge cluster near the N terminus of pUL34 and observed the following. (i) The CL04 mutant has a dominant-negative effect on pUL34 function, indicating disruption of some critical interaction. (ii) In infections with CL04 pUL34, capsids accumulate in close association with the INM, but no perinuclear enveloped viruses, cytoplasmic capsids, or virions or cell surface virions were observed, suggesting that CL04 UL34 does not support INM curvature around the capsid. (iii) Passage of UL34-null virus on a stable cell line that expresses CL04 resulted in selection of extragenic suppressor mutants that grew efficiently using the mutant pUL34. (iv) All extragenic suppressors contained an R229-->L mutation in pUL31 that was sufficient to suppress the CL04 phenotype. (v) Immunolocalization and coimmunoprecipitation experiments with truncated forms of pUL34 and pUL31 confirm that N-terminal sequences of pUL34 and a C-terminal domain of pUL31 mediate interaction but not nuclear membrane targeting. pUL34 and pUL31 may make two essential interactions-one for the targeting of the complex to the nuclear envelope and another for nuclear membrane curvature around capsids.

Effects of phosphorylation of herpes simplex virus 1 envelope glycoprotein B by Us3 kinase in vivo and in vitro

We recently reported that the herpes simplex virus 1 (HSV-1) Us3 protein kinase phosphorylates threonine at position 887 (Thr-887) in the cytoplasmic tail of envelope glycoprotein B (gB) (A. Kato, J. Arii, I. Shiratori, H. Akashi, H. Arase, and Y. Kawaguchi, J. Virol. 83:250-261, 2009; T. Wisner, C. C. Wright, A. Kato, Y. Kawaguchi, F. Mou, J. D. Baines, R. J. Roller and D. C. Johnson, J. Virol. 83:3115-3126, 2009). In the studies reported here, we examined the effect(s) of this phosphorylation on viral replication and pathogenesis in vivo and present data showing that replacement of gB Thr-887 by alanine significantly reduced viral replication in the mouse cornea and development of herpes stroma keratitis and periocular skin disease in mice. The same effects have been reported for mice infected with a recombinant HSV-1 carrying a kinase-inactive mutant of Us3. These observations suggested that Us3 phosphorylation of gB Thr-887 played a critical role in viral replication in vivo and in HSV-1 pathogenesis. In addition, we generated a monoclonal antibody that specifically reacted with phosphorylated gB Thr-887 and used this antibody to show that Us3 phosphorylation of gB Thr-887 regulated subcellular localization of gB, particularly on the cell surface of infected cells.

Mutagenesis of the repeat regions of herpesviruses cloned as bacterial artificial chromosomes

Cloning of infectious and pathogenic herpesvirus genomes in a bacterial artificial chromosome (BAC) vector greatly facilitates genetic manipulation of their genomes. BAC-based mutagenesis strategies of viruses can advance our understanding of the viral gene functions and determinants of pathogenicity, and can ultimately help to develop molecularly defined improved vaccines against virus diseases. Unlike the virus stocks, where continuous passage in tissue culture can lead to phenotypic alterations such as loss of virulence or immunogenicity, viral genomes can be stably maintained with high fidelity as BAC clones in bacteria. Thanks to the "RecA" or the inducible phage "lambda Red" homologous recombination systems and a variety of positive and negative selection strategies, viral genomes cloned as BAC can be efficiently manipulated in E. coli. All the manipulations, including DNA fragment deletion or insertion, point mutations, or even multiple modifications in repeat regions can be carried out accurately in E. coli, and the mutated DNA can be used directly to reconstitute mutant viruses in transfected host cells. Furthermore, using self-excision strategies, the non-viral bacterial replicon sequence can be excised automatically during virus reconstitution, thus generating recombinant viruses virtually identical to the wild-type parent viruses. Here, we describe the various technologies of manipulating the infectious BAC clones of a group E herpesvirus as an example through a combination of different approaches.

Medical application of herpes simplex virus

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are important human pathogens that cause a variety of diseases from mild skin diseases such as herpes labialis and herpes genitalis to life-threatening diseases such as herpes encephalitis and neonatal herpes. A number of studies have elucidated the roles of this virus in viral replication and pathogenicity, the regulation of gene expression, interaction with the host cell and immune evasion from the host system. This research has allowed the development of potential therapeutic agents and vectors for human diseases. This review focuses on the basic functions and roles of HSV gene products and reviews the current knowledge of medical applications of genetically engineered HSV mutants using different strategies. These major HSV-derived vectors include: (i) amplicons for gene delivery vectors; (ii) replication-defective HSV recombinants for vaccine vectors; (iii) replication-attenuated HSV recombinants for oncolytic virotherapy.

Retrograde axon transport of herpes simplex virus and pseudorabies virus: a live-cell comparative analysis

Upon entry, neuroinvasive herpesviruses traffic from axon terminals to the nuclei of neurons resident in peripheral ganglia, where the viral DNA is deposited. A detailed analysis of herpes simplex virus type 1 (HSV-1) transport dynamics in axons following entry is currently lacking. Here, time lapse fluorescence microscopy was used to compare the postentry viral transport of two neurotropic herpesviruses: HSV-1 and pseudorabies virus (PRV). HSV-1 capsid transport dynamics were indistinguishable from those of PRV and did not differ in neurons of human, mouse, or avian origin. Simultaneous tracking of capsids and tegument proteins demonstrated that the composition of actively transporting HSV-1 is remarkably similar to that of PRV. This quantitative assessment of HSV-1 axon transport following entry demonstrates that HSV-1 and PRV share a conserved mechanism for postentry retrograde transport in axons and provides the foundation for further studies of the retrograde transport process.

Expression, purification of herpes simplex virus type 1 UL4 protein, and production and characterization of UL4 polyclonal antibody

The herpes simplex virus type 1 (HSV-1) UL4 protein is a late protein encoded by the UL4 gene. To date, the function of this protein is poorly understood. To aid further investigation of the function of this protein, the UL4 gene was cloned into the vector pET28a (+) to express His-tagged UL4 protein in Escherichia coli. The recombinant fusion protein was purified from inclusion body by histidine selected nickel affinity chromatography under denaturing conditions. After refolding, the purified recombinant protein was used to produce anti-UL4 polyclonal antibody. Western blot analysis demonstrated that the polyclonal sera could recognize the purified UL4 protein specifically, and in the immunofluorescence assay, the antibody was able to probe the UL4 protein with a punctate staining in HSV-1 infected cells.

Effects of major capsid proteins, capsid assembly, and DNA cleavage/packaging on the pUL17/pUL25 complex of herpes simplex virus 1

The U(L)17 and U(L)25 proteins (pU(L)17 and pU(L)25, respectively) of herpes simplex virus 1 are located at the external surface of capsids and are essential for DNA packaging and DNA retention in the capsid, respectively. The current studies were undertaken to determine whether DNA packaging or capsid assembly affected the pU(L)17/pU(L)25 interaction. We found that pU(L)17 and pU(L)25 coimmunoprecipitated from cells infected with wild-type virus, whereas the major capsid protein VP5 (encoded by the U(L)19 gene) did not coimmunoprecipitate with these proteins under stringent conditions. In addition, pU(L)17 (i) coimmunoprecipitated with pU(L)25 in the absence of other viral proteins, (ii) coimmunoprecipitated with pU(L)25 from lysates of infected cells in the presence or absence of VP5, (iii) did not coimmunoprecipitate efficiently with pU(L)25 in the absence of the triplex protein VP23 (encoded by the U(L)18 gene), (iv) required pU(L)25 for proper solubilization and localization within the viral replication compartment, (v) was essential for the sole nuclear localization of pU(L)25, and (vi) required capsid proteins VP5 and VP23 for nuclear localization and normal levels of immunoreactivity in an indirect immunofluorescence assay. Proper localization of pU(L)25 in infected cell nuclei required pU(L)17, pU(L)32, and the major capsid proteins VP5 and VP23, but not the DNA packaging protein pU(L)15. The data suggest that VP23 or triplexes augment the pU(L)17/pU(L)25 interaction and that VP23 and VP5 induce conformational changes in pU(L)17 and pU(L)25, exposing epitopes that are otherwise partially masked in infected cells. These conformational changes can occur in the absence of DNA packaging. The data indicate that the pU(L)17/pU(L)25 complex requires multiple viral proteins and functions for proper localization and biochemical behavior in the infected cell.

Analysis of herpesvirus host specificity determinants using herpesvirus genomes as bacterial artificial chromosomes

Almost all mammalian alphaherpesviruses can grow in cells derived from several types of animals in vitro. However, FHV-1 can only infect feline cell lines. For this reason, FHV-1 should be a good model to investigate species barriers to herpesviruses in vivo. To apply bacterial mutagenesis of FHV-1, we cloned the FHV-1 genome as a BAC. Using lambda and flp recombinations, we introduced a monomeric red fluorescence protein into the C-terminus of glycoprotein D. Although GFP in the constructed recombinant FHV-1, a transfectant of the bacmid of FHV-1 that possessed the GFP, acted in non-feline cell lines, the virus could not enter non-feline cell lines, demonstrating that the host specificity of FHV-1 was restricted in an early step of infection. The host range of canine herpesvirus is limited to dogs in vitro and in vivo; it cannot enter non-canine cell lines as a result of infection but the GFP is active by transfection, revealing the same result that the restriction step is at an early stage of infection. These results suggest the possibility of breaking species barriers of FHV-1 and CHV by modifying the gene(s) that act at the early stage of infection.

Tryptophan residues in the portal protein of herpes simplex virus 1 critical to the interaction with scaffold proteins and incorporation of the portal into capsids

Incorporation of the herpes simplex virus 1 (HSV-1) portal vertex into the capsid requires interaction with a 12-amino-acid hydrophobic domain within capsid scaffold proteins. The goal of this work was to identify domains and residues in the UL6-encoded portal protein pUL6 critical to the interaction with scaffold proteins. We show that whereas the wild-type portal and scaffold proteins readily coimmunoprecipitated with one another in the absence of other viral proteins, truncation beyond the first 18 or last 36 amino acids of the portal protein precluded this coimmunoprecipitation. The coimmunoprecipitation was also precluded by mutation of conserved tryptophan (W) residues to alanine (A) at positions 27, 90, 127, 163, 241, 262, 532, and 596 of UL6. All of these W-to-A mutations precluded the rescue of a viral deletion mutant lacking UL6, except W163A, which supported replication poorly, and W596A, which fully rescued replication. A recombinant virus bearing the W596A mutation replicated and packaged DNA normally, and scaffold proteins readily coimmunoprecipitated with portal protein from lysates of infected cells. Thus, viral functions compensated for the W596A mutation's detrimental effects on the portal-scaffold interaction seen during transient expression of portal and scaffold proteins. In contrast, the W27A mutation precluded portal-scaffold interactions in infected cell lysates, reduced the solubility of pUL6, decreased incorporation of the portal into capsids, and abrogated viral-DNA cleavage and packaging.

Differences in the regulatory and functional effects of the Us3 protein kinase activities of herpes simplex virus 1 and 2

Us3 protein kinases encoded by herpes simplex virus 1 (HSV-1) and 2 (HSV-2) are serine/threonine protein kinases and play critical roles in viral replication and pathogenicity in vivo. In the present study, we investigated differences in the biological properties of HSV-1 and HSV-2 Us3 protein kinases and demonstrated that HSV-2 Us3 did not have some of the HSV-1 Us3 kinase functions, including control of nuclear egress of nucleocapsids, localization of UL31 and UL34, and cell surface expression of viral envelope glycoprotein B. In agreement with the observations that HSV-2 Us3 was less important for these functions, the effect of HSV-2 Us3 kinase activity on virulence in mice following intracerebral inoculation was much lower than that of HSV-1 Us3. Furthermore, we showed that alanine substitution in HSV-2 Us3 at a site (aspartic acid at position 147) corresponding to one that can be autophosphorylated in HSV-1 Us3 abolished HSV-2 Us3 kinase activity. Thus, the regulatory and functional effects of Us3 kinase activity are different between HSV-1 and HSV-2.

Herpesvirus protein ICP27 switches PML isoform by altering mRNA splicing

Viruses use alternative splicing to produce a broad series of proteins from small genomes by utilizing the cellular splicing machinery. Since viruses use cellular RNA binding proteins for viral RNA processing, it is presumable that the splicing of cellular pre-mRNAs is affected by viral infection. Here, we showed that herpes simplex virus type 2 (HSV-2) modifies the expression of promyelocytic leukemia (PML) isoforms by altering pre-mRNA splicing. Using a newly developed virus-sensitive splicing reporter, we identified the viral protein ICP27 as an alternative splicing regulator of PML isoforms. ICP27 was found to bind preferentially to PML pre-mRNA and directly inhibit the removal of PML intron 7a in vitro. Moreover, we demonstrated that ICP27 functions as a splicing silencer at the 3' splice site of the PML intron 7a. The switching of PML isoform from PML-II to PML-V as induced by ICP27 affected HSV-2 replication, suggesting that the viral protein modulates the splicing code of cellular pre-mRNA(s) governing virus propagation.

Proline and tyrosine residues in scaffold proteins of herpes simplex virus 1 critical to the interaction with portal protein and its incorporation into capsids

Previous results showed that amino acids 449 to 457 of pU(L)26, a component of the scaffold of herpes simplex virus 1 capsids, were critical for interaction with the portal protein encoded by U(L)6 and for incorporation of the portal into capsids. To identify residues in this scaffold domain critical for the interaction with pU(L)6, the two proteins were coexpressed in the absence of other viral proteins and subjected to immunoprecipitation with scaffold-specific antibody. Coimmunoprecipitation of pU(L)6 was precluded by pU(L)26 mutations Y451A, P452A, and E454A but not by P449A, R456A, or Y450A. In infected cells, Y451A and P452A diminished solubilization of pU(L)6, reduced incorporation of the portal into the capsid, and precluded viral replication and DNA packaging. In contrast, E454A did not affect these parameters despite the fact that E454 is invariant in a number of different alphaherpesvirus scaffold proteins. These data suggest that the interaction between the scaffold E454A mutant and portal protein is rescued by other viral functions. Finally, we show that amino acids 448 to 459 of pU(L)26 are sufficient to interact with pU(L)6 in a glutathione S-transferase pulldown assay in the absence of other viral proteins and that this interaction is inhibited with excess peptide containing pU(L)26 amino acids 443 to 462. Together, these observations suggest that a direct interaction between this scaffold domain and portal protein mediates incorporation of the portal into the capsid.

TRAF6 establishes innate immune responses by activating NF-kappaB and IRF7 upon sensing cytosolic viral RNA and DNA

Background

In response to viral infection, the innate immune system recognizes viral nucleic acids and then induces production of proinflammatory cytokines and type I interferons (IFNs). Toll-like receptor 7 (TLR7) and TLR9 detect viral RNA and DNA, respectively, in endosomal compartments, leading to the activation of nuclear factor κB (NF-κB) and IFN regulatory factors (IRFs) in plasmacytoid dendritic cells. During such TLR signaling, TNF receptor-associated factor 6 (TRAF6) is essential for the activation of NF-κB and the production of type I IFN. In contrast, RIG-like helicases (RLHs), cytosolic RNA sensors, are indispensable for antiviral responses in conventional dendritic cells, macrophages, and fibroblasts. However, the contribution of TRAF6 to the detection of cytosolic viral nucleic acids has been controversial, and the involvement of TRAF6 in IRF activation has not been adequately addressed.

Principal Findings

Here we first show that TRAF6 plays a critical role in RLH signaling. The absence of TRAF6 resulted in enhanced viral replication and a significant reduction in the production of IL-6 and type I IFNs after infection with RNA virus. Activation of NF-κB and IRF7, but not that of IRF3, was significantly impaired during RLH signaling in the absence of TRAF6. TGFβ-activated kinase 1 (TAK1) and MEKK3, whose activation by TRAF6 during TLR signaling is involved in NF-κB activation, were not essential for RLH-mediated NF-κB activation. We also demonstrate that TRAF6-deficiency impaired cytosolic DNA-induced antiviral responses, and this impairment was due to defective activation of NF-κB and IRF7.

Conclusions/Significance

Thus, TRAF6 mediates antiviral responses triggered by cytosolic viral DNA and RNA in a way that differs from that associated with TLR signaling. Given its essential role in signaling by various receptors involved in the acquired immune system, TRAF6 represents a key molecule in innate and antigen-specific immune responses against viral infection.

Identification of multiple sites suitable for insertion of foreign genes in herpes simplex virus genomes

Information on sites in HSV genomes at which foreign gene(s) can be inserted without disrupting viral genes or affecting properties of the parental virus are important for basic research on HSV and development of HSV-based vectors for human therapy. The intergenic region between HSV-1 UL3 and UL4 genes has been reported to satisfy the requirements for such an insertion site. The UL3 and UL4 genes are oriented toward the intergenic region and, therefore, insertion of a foreign gene(s) into the region between the UL3 and UL4 polyadenylation signals should not disrupt any viral genes or transcriptional units. HSV-1 and HSV-2 each have more than 10 additional regions structurally similar to the intergenic region between UL3 and UL4. In the studies reported here, it has been demonstrated that insertion of a reporter gene expression cassette into several of the HSV-1 and HSV-2 intergenic regions has no effect on viral growth in cell culture or virulence in mice, suggesting that these multiple intergenic regions may be suitable HSV sites for insertion of foreign genes.

[Visualization of viruses in living cells]

Live-cell imaging of cells infected with recombinant viruses expressing fluorescently tagged structural viral proteins enables to visualize virion maturation pathways at a virion particle level in the same cells as infection progresses. This technology have gradually unveiled previously unreported aspects of the pathways. This article focuses on live-cell imaging technology of herpes simplex virus 1 (HSV-1) and reviews the up-to-date topics of HSV-1 maturation pathway including our recent progress.

The herpes simplex virus type 1 glycoprotein D (gD) cytoplasmic terminus and full-length gE are not essential and do not function in a redundant manner for cytoplasmic virion envelopment and egress

Herpes simplex virus type 1 (HSV-1) acquires its final envelope by budding into cytoplasmic vesicles thought to be derived from trans-Golgi network membranes. This process is facilitated by interactions among the carboxyl termini of viral glycoproteins and tegument proteins. To directly investigate the relative importance of the carboxyl terminus of glycoprotein D (gD) in the presence or absence of gE, a recombinant virus (gDDeltact) was constructed to specify a truncated gD lacking the carboxy-terminal 29 amino acids. Furthermore, two additional recombinant viruses were constructed by mutating from ATG to CTG the initiation codons of gE (gEctg) or both gE and gM (gEctg+gMctg), causing lack of expression of gE or both gE and gM, respectively. A fourth mutant virus was constructed to specify the gEctg+gDDeltact mutations. The replication properties of these viruses were compared to those of a newly constructed recombinant virus unable to express UL20 due to alteration of the two initiation codons of UL20 (UL20ctgctg). All recombinant viruses were constructed by using the double-Red, site-directed mutagenesis system implemented on the HSV-1(F) genome cloned into a bacterial artificial chromosome. The gEctg, gEctg+gMctg, gDDeltact, and gEctg+gDDeltact viruses produced viral plaques on African monkey kidney cells (Vero), as well as other cells, that were on average approximately 30 to 50% smaller than those produced by the wild-type virus HSV-1(F). In contrast, the UL20ctgctg virus produced very small plaques containing three to five cells, as reported previously for the DeltaUL20 virus lacking the entire UL20 gene. Viral replication kinetics of intracellular and extracellular viruses revealed that all recombinant viruses produced viral titers similar to those produced by the wild-type HSV-1(F) virus intracellularly and extracellularly at late times postinfection, with the exception of the UL20ctgctg and DeltaUL20 viruses, which replicated more than two-and-a-half logs less efficiently than HSV-1(F). Electron microscopy confirmed that all viruses, regardless of their different gene mutations, efficiently produced enveloped virions within infected cells, with the exception of the UL20ctgctg and DeltaUL20 viruses, which accumulated high levels of unenveloped virions in the cytoplasm. These results show that the carboxyl terminus of gD and the full-length gE, either alone or in a redundant manner, are not essential in cytoplasmic virion envelopment and egress from infected cells. Similarly, gM and gE do not function alone or in a redundant manner in cytoplasmic envelopment and virion egress, confirming previous findings.

Regulation of the catalytic activity of herpes simplex virus 1 protein kinase Us3 by autophosphorylation and its role in pathogenesis

Us3 is a serine/threonine protein kinase encoded by herpes simplex virus 1 (HSV-1). We recently identified serine at Us3 position 147 (Ser-147) as a physiological phosphorylation site of Us3 (A. Kato, M. Tanaka, M. Yamamoto, R. Asai, T. Sata, Y. Nishiyama, and Y. Kawaguchi, J. Virol. 82:6172-6189, 2008). In the present study, we investigated the effects of phosphorylation of Us3 Ser-147 on regulation of Us3 catalytic activity in infected cells and on HSV-1 pathogenesis. Our results were as follows. (i) Only a small fraction of Us3 purified from infected cells was phosphorylated at Ser-147. (ii) Us3 phosphorylated at Ser-147 purified from infected cells had significantly higher kinase activity than Us3 not phosphorylated at Ser-147. (iii) Phosphorylation of Us3 Ser-147 in infected cells was dependent on Us3 kinase activity. (iv) Replacement of Us3 Ser-147 by alanine significantly reduced viral replication in the mouse cornea and the development of herpes stromal keratitis and periocular skin disease in mice. These results indicated that Us3 catalytic activity is tightly regulated by autophosphorylation of Ser-147 in infected cells and that regulation of Us3 activity by autophosphorylation appeared to play a critical role in viral replication in vivo and in HSV-1 pathogenesis.

Phosphorylation of the U(L)31 protein of herpes simplex virus 1 by the U(S)3-encoded kinase regulates localization of the nuclear envelopment complex and egress of nucleocapsids

Herpes simplex virus 1 nucleocapsids bud through the inner nuclear membrane (INM) into the perinuclear space to obtain a primary viral envelope. This process requires a protein complex at the INM composed of the U(L)31 and U(L)34 gene products. While it is clear that the viral kinase encoded by the U(S)3 gene regulates the localization of pU(L)31/pU(L)34 within the INM, the molecular mechanism by which this is accomplished remains enigmatic. Here, we have determined the following. (i) The N terminus of pU(L)31 is indispensable for the protein's normal function and contains up to six serines that are phosphorylated by the U(S)3 kinase during infection. (ii) Phosphorylation at these six serines was not essential for a productive infection but was required for optimal viral growth kinetics. (iii) In the presence of active U(S)3 kinase, changing the serines to alanine caused the pU(L)31/pU(L)34 complex to aggregate at the nuclear rim and caused some virions to accumulate aberrantly in herniations of the nuclear membrane, much as in cells infected with a U(S)3 kinase-dead mutant. (iv) The replacement of the six serines of pU(L)31 with glutamic acid largely restored the smooth distribution of pU(L)34/pU(L)31 at the nuclear membrane and precluded the accumulation of virions in herniations whether or not U(S)3 kinase was active but also precluded the optimal primary envelopment of nucleocapsids. These observations indicate that the phosphorylation of pU(L)31 by pU(S)3 represents an important regulatory event in the virion egress pathway that can account for much of pU(S)3's role in nuclear egress. The data also suggest that the dynamics of pU(L)31 phosphorylation modulate both the primary envelopment and the subsequent fusion of the nascent virion envelope with the outer nuclear membrane.

The U(L)31 and U(L)34 gene products of herpes simplex virus 1 are required for optimal localization of viral glycoproteins D and M to the inner nuclear membranes of infected cells

U(L)31 and U(L)34 of herpes simplex virus type 1 form a complex necessary for nucleocapsid budding at the inner nuclear membrane (INM). Previous examination by immunogold electron microscopy and electron tomography showed that pU(L)31, pU(L)34, and glycoproteins D and M are recruited to perinuclear virions and densely staining regions of the INM where nucleocapsids bud into the perinuclear space. We now show by quantitative immunogold electron microscopy coupled with analysis of variance that gD-specific immunoreactivity is significantly reduced at both the INM and outer nuclear membrane (ONM) of cells infected with a U(L)34 null virus. While the amount of gM associated with the nuclear membrane (NM) was only slightly (P = 0.027) reduced in cells infected with the U(L)34 null virus, enrichment of gM in the INM at the expense of that in the ONM was greatly dependent on U(L)34 (P < 0.0001). pU(L)34 also interacted directly or indirectly with immature forms of gD (species expected to reside in the endoplasmic reticulum or nuclear membrane) in lysates of infected cells and with the cytosolic tail of gD fused to glutathione S-transferase in rabbit reticulocyte lysates, suggesting a role for the pU(L)34/gD interaction in recruiting gD to the NM. The effects of U(L)34 on gD and gM localization were not a consequence of decreased total expression of gD and gM, as determined by flow cytometry. Separately, pU(L)31 was dispensable for targeting gD and gM to the two leaflets of the NM but was required for (i) the proper INM-versus-ONM ratio of gD and gM in infected cells and (ii) the presence of electron-dense regions in the INM, representing nucleocapsid budding sites. We conclude that in addition to their roles in nucleocapsid envelopment and lamina alteration, U(L)31 and U(L)34 play separate but related roles in recruiting appropriate components to nucleocapsid budding sites at the INM.

Entry of herpes simplex virus 1 and other alphaherpesviruses via the paired immunoglobulin-like type 2 receptor alpha

Herpes simplex virus 1 (HSV-1) enters cells either via fusion of the virion envelope and host cell plasma membrane or via endocytosis, depending on the cell type. In the study reported here, we investigated a viral entry pathway dependent on the paired immunoglobulin-like type 2 receptor alpha (PILRalpha), a recently identified entry coreceptor for HSV-1 that associates with viral envelope glycoprotein B (gB). Experiments using inhibitors of endocytic pathways and ultrastructural analyses of Chinese hamster ovary (CHO) cells transduced with PILRalpha showed that HSV-1 entry into these cells was via virus-cell fusion at the cell surface. Together with earlier observations that HSV-1 uptake into normal CHO cells and those transduced with a receptor for HSV-1 envelope gD is mediated by endocytosis, these results indicated that expression of PILRalpha produced an alternative HSV-1 entry pathway in CHO cells. We also showed that human and murine PILRalpha were able to mediate entry of pseudorabies virus, a porcine alphaherpesvirus, but not of HSV-2. These results indicated that viral entry via PILRalpha appears to be conserved but that there is a PILRalpha preference among alphaherpesviruses.

The putative leucine zipper of the UL6-encoded portal protein of herpes simplex virus 1 is necessary for interaction with pUL15 and pUL28 and their association with capsids

Herpes simplex virus (HSV) type 1 capsids contain a single portal vertex that is composed of 12 copies of the U(L)6 gene product (pU(L)6), which forms a pore through which DNA is inserted during packaging. This unique vertex is also believed to comprise the site with which a molecular motor, termed the terminase, associates during the DNA packaging reaction. In HSV, the terminase likely comprises the U(L)15, U(L)28, and U(L)33 proteins (pU(L)15, pU(L)28, and pU(L)33, respectively). The current study was undertaken to identify portal domains required for interaction with the terminase. Both the amino and carboxyl termini, as well as amino acids 422 to 443 of pU(L)6 forming a putative leucine zipper motif, were critical for coimmunoprecipitation with pU(L)15 in the absence of other viral proteins. Amino acids 422 to 443 were also necessary for interaction with pU(L)28 in the absence of other viral proteins. By using an engineered recombinant virus, it was further determined that although amino acids 422 to 443 were dispensable for interaction with scaffold protein and incorporation of portal protein into capsids, they were necessary for coimmunoprecipitation of pU(L)6 and pU(L)15 from infected cell lysates, association of optimal levels of pU(L)15, pU(L)28, and pU(L)33 with capsids, and DNA cleavage and packaging. These data identify a portal protein domain critical for terminase association with the capsid and suggest that both the pU(L)15- and pU(L)28-bearing terminase subunits mediate docking of the terminase with the portal vertex.

Herpes simplex virus 1 protein kinase Us3 phosphorylates viral envelope glycoprotein B and regulates its expression on the cell surface

Us3 is a serine-threonine protein kinase encoded by herpes simplex virus 1 (HSV-1). As reported here, we attempted to identify the previously unreported physiological substrate of Us3 in HSV-1-infected cells. Our results were as follows. (i) Bioinformatics analysis predicted two putative Us3 phosphorylation sites in the viral envelope glycoprotein B (gB) at codons 557 to 562 (RRVSAR) and codons 884 to 889 (RRNTNY). (ii) In in vitro kinase assays, the threonine residue at position 887 (Thr-887) in the gB domain was specifically phosphorylated by Us3, while the serine residue at position 560 was not. (iii) The phosphorylation of gB Thr-887 in Vero cells infected with wild-type HSV-1 was specifically detected using an antibody that recognized phosphorylated serine or threonine residues with arginine at the -3 and -2 positions. (iv) The phosphorylation of gB Thr-887 in infected cells was dependent on the kinase activity of Us3. (v) The replacement of Thr-887 with alanine markedly upregulated the cell surface expression of gB in infected cells, whereas replacement with aspartic acid, which sometimes mimics constitutive phosphorylation, restored the wild-type phenotype. The upregulation of gB expression on the cell surface also was observed in cells infected with a recombinant HSV-1 encoding catalytically inactive Us3. These results supported the hypothesis that Us3 phosphorylates gB and downregulates the cell surface expression of gB in HSV-1-infected cells.

The product of the Herpes simplex virus 1 UL7 gene interacts with a mitochondrial protein, adenine nucleotide translocator 2

The herpes simplex virus 1 (HSV-1) UL7 gene is highly conserved among herpesviridae. Since the construction of recombinant HSV-1 with a mutation in the UL7 gene has not been reported, the involvement of HSV-1 UL7 in viral replication has been unclear. In this study, we succeeded in generating a UL7 null HSV-1 mutant virus, MT102, and characterized it. Our results were as follows. (i) In Vero cells, MT102 was replication-competent, but formed smaller plaques and yielded 10- to 100-fold fewer progeny than the wild-type virus, depending on the multiplicity of infection. (ii) Using mass spectrometry-based proteomics technology, we identified a cellular mitochondrial protein, adenine nucleotide translocator 2 (ANT2), as a UL7-interacting partner. (iii) When ANT2 was transiently expressed in COS-7 cells infected with HSV-1, ANT2 was specifically co-precipitated with UL7. (iv) Cell fractionation experiments with HSV-1-infected cells detected the UL7 protein in both the mitochondrial and cytosolic fractions, whereas ANT2 was detected only in the mitochondrial fraction. These results indicate the importance of HSV-1 UL7's involvement in viral replication and demonstrate that it interacts with ANT2 in infected cells. The potential biological significance of the interaction between UL7 and ANT2 is discussed.