Multiple regulatory effects of varicella-zoster virus (VZV) gL on trafficking patterns and fusogenic properties of VZV gH

Preliminary investigation and analysis of nucleotide site variability of nine glycoproteins on varicella-zoster virus envelope, Jilin Province, China, 2010-March 2024

Varicella is endemic worldwide. In China, varicella has not yet been included in the list of legal infectious diseases, nor has a unified national surveillance program been established. And the live attenuated varicella vaccine has not been included in routine immunization. In this study, we analyzed for the first time the varicella epidemiology in Jilin Province in the past 20 years, and the nucleotide site, amino acid site and N-glycosylation site variation of glycoprotein in varicella-zoster virus (VZV) surface 9 in the past 15 years. The results showed that the reported incidence of varicella in Jilin Province in the last 20 years was fluctuating above and below 20/100,000, especially after the epidemic of the COVID-19, and fatal cases appeared in individual years. The genotypic branching of VZV was monitored as Clade 2 in the last 15 years. 9 glycogen nucleotide sites of VZV had different degrees of variability, and the variability had specificity. Therefore, it gives us the idea that in order to reduce the incidence of varicella and herpes zoster, a provincial or even national surveillance program should be introduced as early as possible, and the dynamic monitoring of the variability of the nucleotide sites of VZV should be strengthened at the same time as the vaccine immunization strategy is introduced.

Membrane fusion, potential threats, and natural antiviral drugs of pseudorabies virus

Pseudorabies virus (PrV) can infect several animals and causes severe economic losses in the swine industry. Recently, human encephalitis or endophthalmitis caused by PrV infection has been frequently reported in China. Thus, PrV can infect animals and is becoming a potential threat to human health. Although vaccines and drugs are the main strategies to prevent and treat PrV outbreaks, there is no specific drug, and the emergence of new PrV variants has reduced the effectiveness of classical vaccines. Therefore, it is challenging to eradicate PrV. In the present review, the membrane fusion process of PrV entering target cells, which is conducive to revealing new therapeutic and vaccine strategies for PrV, is presented and discussed. The current and potential PrV pathways of infection in humans are analyzed, and it is hypothesized that PrV may become a zoonotic agent. The efficacy of chemically synthesized drugs for treating PrV infections in animals and humans is unsatisfactory. In contrast, multiple extracts of traditional Chinese medicine (TCM) have shown anti-PRV activity, exerting its effects in different phases of the PrV life-cycle and suggesting that TCM compounds may have great potential against PrV. Overall, this review provides insights into developing effective anti-PrV drugs and emphasizes that human PrV infection should receive more attention.

Common characteristics and unique features: A comparison of the fusion machinery of the alphaherpesviruses Pseudorabies virus and Herpes simplex virus

Membrane fusion is a fundamental biological process that allows different cellular compartments delimited by a lipid membrane to release or exchange their respective contents. Similarly, enveloped viruses such as alphaherpesviruses exploit membrane fusion to enter and infect their host cells. For infectious entry the prototypic human Herpes simplex viruses 1 and 2 (HSV-1 and -2, collectively termed HSVs) and the porcine Pseudorabies virus (PrV) utilize four different essential envelope glycoproteins (g): the bona fide fusion protein gB and the regulatory heterodimeric gH/gL complex that constitute the "core fusion machinery" conserved in all members of the Herpesviridae; and the subfamily specific receptor binding protein gD. These four components mediate attachment and fusion of the virion envelope with the host cell plasma membrane through a tightly regulated sequential activation process. Although PrV and the HSVs are closely related and employ the same set of glycoproteins for entry, they show remarkable differences in the requirements for fusion. Whereas the HSVs strictly require all four components for membrane fusion, PrV can mediate cell-cell fusion without gD. Moreover, in contrast to the HSVs, PrV provides a unique opportunity for reversion analyses of gL-negative mutants by serial cell culture passaging, due to a limited cell-cell spread capacity of gL-negative PrV not observed in the HSVs. This allows a more direct analysis of the function of gH/gL during membrane fusion. Unraveling the molecular mechanism of herpesvirus fusion has been a goal of fundamental research for years, and yet important mechanistic details remain to be uncovered. Nevertheless, the elucidation of the crystal structures of all key players involved in PrV and HSV membrane fusion, coupled with a wealth of functional data, has shed some light on this complex puzzle. In this review, we summarize and discuss the contemporary knowledge on the molecular mechanism of entry and membrane fusion utilized by the alphaherpesvirus PrV, and highlight similarities but also remarkable differences in the requirements for fusion between PrV and the HSVs.

Outer nuclear membrane fusion of adjacent nuclei in varicella-zoster virus-induced syncytia

Syncytia formation has been considered important for cell-to-cell spread and pathogenesis of many viruses. As a syncytium forms, individual nuclei often congregate together, allowing close contact of nuclear membranes and possibly fusion to occur. However, there is currently no reported evidence of nuclear membrane fusion between adjacent nuclei in wild-type virus-induced syncytia. Varicella-zoster virus (VZV) is one typical syncytia-inducing virus that causes chickenpox and shingles in humans. Here, we report, for the first time, an interesting observation of apparent fusion of the outer nuclear membranes from juxtaposed nuclei that comprise VZV syncytia both in ARPE-19 human epithelial cells in vitro and in human skin xenografts in the SCID-hu mouse model in vivo. This work reveals a novel aspect of VZV-related cytopathic effect in the context of multinucleated syncytia. Additionally, the information provided by this study could be helpful for future studies on interactions of viruses with host cell nuclei.

Functional Relevance of the N-Terminal Domain of Pseudorabies Virus Envelope Glycoprotein H and Its Interaction with Glycoprotein L

Several envelope glycoproteins are involved in herpesvirus entry into cells, direct cell-to-cell spread, and induction of cell fusion. The membrane fusion protein glycoprotein B (gB) and the presumably gB-activating heterodimer gH/gL are essential for these processes and conserved throughout the Herpesviridae However, after extended cell culture passage of gL-negative mutants of the alphaherpesvirus pseudorabies virus (PrV), phenotypic revertants could be isolated which had acquired spontaneous mutations affecting the gL-interacting N-terminal part of the gH ectodomain (gDH and gH^B4.1) (B. G. Klupp and T. C. Mettenleiter, J Virol 73:3014-3022, 1999; C. Schröter, M. Vallbracht, J. Altenschmidt, S. Kargoll, W. Fuchs, B. G. Klupp, and T. C. Mettenleiter, J Virol 90:2264-2272, 2016). To investigate the functional relevance of this part of gH in more detail, we introduced an in-frame deletion of 66 codons at the 5' end of the plasmid-cloned gH gene (gH^32/98). The N-terminal signal peptide was retained, and the deletion did not affect expression or processing of gH but abrogated its function in in vitro fusion assays. Insertion of the engineered gH gene into the PrV genome resulted in a defective mutant (pPrV-gH^32/98K), which was incapable of entry and spread. Interestingly, in vitro activity of mutated gH^32/98 was restored when it was coexpressed with hyperfusogenic gB^B4.1, obtained from a passaged gL deletion mutant of PrV. Moreover, the entry and spread defects of pPrV-gH^32/98K were compensated by the mutations in gB^B4.1 in cis, as well as in trans, independent of gL. Thus, PrV gL and the gL-interacting domain of gH are not strictly required for function.IMPORTANCE Membrane fusion is crucial for infectious entry and spread of enveloped viruses. While many enveloped viruses require only one or two proteins for receptor binding and membrane fusion, herpesvirus infection depends on several envelope glycoproteins. Besides subfamily-specific receptor binding proteins, the core fusion machinery consists of the conserved fusion protein gB and the gH/gL complex. The role of the latter is unclear, but it is hypothesized to interact with gB for fusion activation. Using isogenic virus recombinants, we demonstrate here that gL and the gL-binding domain of PrV gH are not strictly required for membrane fusion during virus entry and spread when concomitantly mutations in gB are present which increase its fusogenicity. Thus, our results strongly support the notion of a functional gB-gH interaction during the fusion process.

Structure-based functional analyses of domains II and III of pseudorabies virus glycoprotein H

Enveloped viruses utilize membrane fusion for entry into, and release from, host cells. For entry, members of the Herpesviridae require at least three envelope glycoproteins: the homotrimeric gB and a heterodimer of gH and gL. The crystal structures of three gH homologues, including pseudorabies virus (PrV) gH, revealed four conserved domains. Domain II contains a planar β-sheet ("fence") and a syntaxin-like bundle of three α-helices (SLB), similar to those found in eukaryotic fusion proteins, potentially executing an important role in gH function. To test this hypothesis, we introduced targeted mutations into the PrV gH gene, which either disrupt the helices of the SLB by introduction of proline residues or covalently join them by artificial intramolecular disulfide bonds between themselves, to the adjacent fence region, or to domain III. Disruption of either of the three α-helices of the SLB (A250P, V275P, V298P) severely affected gH function in in vitro fusion assays and replication of corresponding PrV mutants. Considerable defects in fusion activity of gH, as well as in penetration kinetics and cell-to-cell spread of PrV mutants, were also observed after disulfide linkage of two α-helices within the SLB (A284C-S291C) or between SLB and domain III (H251C-L432C), as well as by insertions of additional cysteine pairs linking fence, SLB, and domain III. In vitro fusion activity of mutated gH could be partly restored by reduction of the artificial disulfide bonds. Our results indicate that the structure and flexibility of the SLB are relevant for the function of PrV gH in membrane fusion.

IMPORTANCE

Mutational analysis based on crystal structures of proteins is a powerful tool to understand protein function. Here, we continued our study of pseudorabies virus gH, a part of the core fusion machinery of herpesviruses. We previously showed that the "flap" region in domain IV of PrV gH is important for its function. We now demonstrate that mutations within domain II that interfere with integrity or flexibility of a syntaxin-like three-helix bundle also significantly impair gH function during fusion. These studies provide important insights into the structural requirements of gH for function in fusion.

Genetic analysis of clinical VZV isolates collected in China reveals a more homologous profile

Forty-four varicella-zoster virus (VZV) isolates from China were genotyped by using a scattered single nucleotide polymorphism (SNP) method, including open reading frames (ORFs) 1, 22, 31, 37, 60, 62, 67, and 68. Based on the analysis of the polymorphic markers in the 8 ORFs, all of the 44 isolates can be placed in genotype J defined by the SNP profiles in ORF22 or clade B defined by the SNP profiles in ORFs 31, 37, 60, 62, 67, and 68. The three consecutive nucleotide (CGG) in-frame insertions in ORF 1 were found in 8 (18.2%) isolates, which has not been described in VZV strains from any other part of the world. A novel synonymous A>G substitution in ORF60 was revealed in 4 (9.1%) of the isolates. In addition, a previously described three consecutive nucleotide (ATC) insertion in ORF 60 was found in all the Chinese isolates but not in the US isolate MLS. The results showed all the 44 strains that belong to genotype J/clade B with significantly high homogeneity, and phylogenetic analysis suggested that the 44 Chinese isolates consist of 4 clusters, but interstrain variations also exist. Overall, VZV isolates obtained in China showed significantly higher genetic homogeneity than isolates reported from other countries.

Structure-function analysis of varicella-zoster virus glycoprotein H identifies domain-specific roles for fusion and skin tropism

Enveloped viruses require membrane fusion for cell entry and replication. For herpesviruses, this event is governed by the multiprotein core complex of conserved glycoproteins (g)B and gH/gL. The recent crystal structures of gH/gL from herpes simplex virus 2, pseudorabies virus, and Epstein-Barr virus revealed distinct domains that, surprisingly, do not resemble known viral fusogens. Varicella-zoster virus (VZV) causes chicken pox and shingles. VZV is an α-herpesvirus closely related to herpes simplex virus 2, enabling prediction of the VZV gH structure by homology modeling. We have defined specific roles for each gH domain in VZV replication and pathogenesis using structure-based site-directed mutagenesis of gH. The distal tip of domain (D)I was important for skin tropism, entry, and fusion. DII helices and a conserved disulfide bond were essential for gH structure and VZV replication. An essential (724)CXXC(727) motif was critical for DIII structural stability and membrane fusion. This assignment of domain-dependent mechanisms to VZV gH links elements of the glycoprotein structure to function in herpesvirus replication and virulence.

Overview of varicella-zoster virus glycoproteins gC, gH and gL

The VZV genome is smaller than the HSV genome and only encodes nine glycoproteins. This chapter provides an overview of three VZV glycoproteins: gH (ORF37), gL (ORF60), and gC (ORF14). All three glycoproteins are highly conserved among the alpha herpesviruses. However, VZV gC exhibits unexpected differences from its HSV counterpart gC. In particular, both VZV gC transcription and protein expression are markedly delayed in cultured cells. These delays occur regardless of the virus strain or the cell type, and may account in part for the aberrant assembly of VZV particles. In contrast to VZV gC, the general properties of gH and gL more closely resemble their HSV homologs. VZV gL behaves as a chaperone protein to facilitate the maturation of the gH protein. The mature gH protein in turn is a potent fusogen. Its fusogenic activity can be abrogated when infected cultures are treated with monoclonal anti-gH antibodies.

Genome-wide mutagenesis reveals that ORF7 is a novel VZV skin-tropic factor

The Varicella Zoster Virus (VZV) is a ubiquitous human alpha-herpesvirus that is the causative agent of chicken pox and shingles. Although an attenuated VZV vaccine (v-Oka) has been widely used in children in the United States, chicken pox outbreaks are still seen, and the shingles vaccine only reduces the risk of shingles by 50%. Therefore, VZV still remains an important public health concern. Knowledge of VZV replication and pathogenesis remains limited due to its highly cell-associated nature in cultured cells, the difficulty of generating recombinant viruses, and VZV's almost exclusive tropism for human cells and tissues. In order to circumvent these hurdles, we cloned the entire VZV (p-Oka) genome into a bacterial artificial chromosome that included a dual-reporter system (GFP and luciferase reporter genes). We used PCR-based mutagenesis and the homologous recombination system in the E. coli to individually delete each of the genome's 70 unique ORFs. The collection of viral mutants obtained was systematically examined both in MeWo cells and in cultured human fetal skin organ samples. We use our genome-wide deletion library to provide novel functional annotations to 51% of the VZV proteome. We found 44 out of 70 VZV ORFs to be essential for viral replication. Among the 26 non-essential ORF deletion mutants, eight have discernable growth defects in MeWo. Interestingly, four ORFs were found to be required for viral replication in skin organ cultures, but not in MeWo cells, suggesting their potential roles as skin tropism factors. One of the genes (ORF7) has never been described as a skin tropic factor. The global profiling of the VZV genome gives further insights into the replication and pathogenesis of this virus, which can lead to improved prevention and therapy of chicken pox and shingles.

The Varicella Zoster Virus (VZV) is the causative agent of chicken pox and shingles. The long-term efficacy of the current chickenpox vaccine is yet to be determined, and the current shingles vaccine fails to provide protective immunity for a substantial number of individuals. Shingles can also lead to post-herpetic neuralgia (PHN), a debilitating condition associated with an intractable pain that can linger for life. Therefore, VZV remains an important public health concern. We use growth-rate analysis of our genome-wide deletion library to determine the essentiality of all known VZV genes, including novel annotations for 51% of the VZV proteome. We also discovered a novel skin-tropic factor encoded by ORF7. Overall, our identification of genes essential for VZV replication and pathogenesis will serve as the basis for multiple in-depth genetic studies of VZV, which can lead to improved prevention and therapy of chicken pox and shingles. For example, essential genes may be appealing drug targets and genes whose deletion causes a substantial growth defect may be prospective candidates for novel live attenuated vaccines.

Characterization of the varicella-zoster virus ORF50 gene, which encodes glycoprotein M

The ORF50 gene of the varicella-zoster virus (VZV) encodes glycoprotein M (gM), which is conserved among all herpesviruses and is important for the cell-to-cell spread of VZV. However, few analyses of ORF50 gene expression or its posttranscriptional and translational modifications have been published. Here we found that in VZV-infected cells, ORF50 encoded four transcripts: a full-size transcript, which was translated into the gM, and three alternatively spliced transcripts, which were not translated. Using a splicing-negative mutant virus, we showed that the alternative transcripts were nonessential for viral growth in cell culture. In addition, we found that two amino acid mutations of gM, V42P and G301M, blocked gM's maturation and transport to the trans-Golgi network, which is generally recognized as the viral assembly complex. We also found that the mutations disrupted gM's interaction with glycoprotein N (gN), revealing their interaction through a bond that is otherwise unreported for herpesviruses. Using this gM maturation-negative virus, we found that immature gM and gN were incorporated into intracellularly isolated virus particles and that mature gM was required for efficient viral growth via cell-to-cell spread but not for virion morphogenesis. The virus particles were more abundant at the abnormally enlarged perinuclear cisternae than those of the parental virus, but they were also found at the cell surface and in the culture medium. Additionally, in the gM maturation-negative mutant virus-infected melanoma cells, typical syncytium formation was rarely seen, again indicating that mature gM functions in cell-to-cell spread via enhancement of syncytium formation.

Anti-glycoprotein H antibody impairs the pathogenicity of varicella-zoster virus in skin xenografts in the SCID mouse model

Varicella-zoster virus (VZV) infection is usually mild in healthy individuals but can cause severe disease in immunocompromised patients. Prophylaxis with varicella-zoster immunoglobulin can reduce the severity of VZV if given shortly after exposure. Glycoprotein H (gH) is a highly conserved herpesvirus protein with functions in virus entry and cell-cell spread and is a target of neutralizing antibodies. The anti-gH monoclonal antibody (MAb) 206 neutralizes VZV in vitro. To determine the requirement for gH in VZV pathogenesis in vivo, MAb 206 was administered to SCID mice with human skin xenografts inoculated with VZV. Anti-gH antibody given at 6 h postinfection significantly reduced the frequency of skin xenograft infection by 42%. Virus titers, genome copies, and lesion size were decreased in xenografts that became infected. In contrast, administering anti-gH antibody at 4 days postinfection suppressed VZV replication but did not reduce the frequency of infection. The neutralizing anti-gH MAb 206 blocked virus entry, cell fusion, or both in skin in vivo. In vitro, MAb 206 bound to plasma membranes and to surface virus particles. Antibody was internalized into vacuoles within infected cells, associated with intracellular virus particles, and colocalized with markers for early endosomes and multivesicular bodies but not the trans-Golgi network. MAb 206 blocked spread, altered intracellular trafficking of gH, and bound to surface VZV particles, which might facilitate their uptake and targeting for degradation. As a consequence, antibody interference with gH function would likely prevent or significantly reduce VZV replication in skin during primary or recurrent infection.

Comparative analyses of the 9 glycoprotein genes found in wild-type and vaccine strains of varicella-zoster virus

The complete DNA sequences of wild-type and vaccine strains of varicella-zoster virus have been published and listed in GenBank. In this comparative genomic analysis, the sequences of the 9 glycoprotein open reading frames (ORFs) were compared. They included gE (ORF68), gI (ORF 67), gC (ORF14), gH (ORF37), gL (ORF60), gB (ORF31), gK (ORF5), gM (ORF50), and gN (ORF8 or ORF9A). After realignment on the basis of newer data, the corrected gB sequence was lengthened to include 931 residues. The data showed that there were glycoprotein polymorphisms that differentiated North American/European strains from Japanese strains-for example, an additional ATG codon in the gL of all Oka strains. Also, there were a small number of coding single-nucleotide polymorphisms present only in glycoproteins of vaccine strains. Because these changes were highly conserved, the structure of the glycoprotein was unlikely to be altered.

Mutagenic analysis of herpes simplex virus type 1 glycoprotein L reveals the importance of an arginine-rich region for function

Herpes simplex virus type 1 (HSV-1) glycoproteins H and L (gH and gL) are required for virus-induced membrane fusion. Expression of gH at the virion or infected cell surface is mediated by the chaperone-like activity of gL. We have previously shown that a region between amino acids 155 and 161 is critical for gL chaperone-like activity. Here, we conducted Ala substitution mutagenesis of residues in this region and found that substitution of Cys160, Arg156, Arg158, or Arg156/158/159 with Ala resulted in a gL mutant that bound gH but displayed a reduced ability in gH trafficking and membrane fusion. Substitution of Arg156 with another positively charged amino acid, Lys, restored function. Substitution of Arg158 with Lys restored function in gH trafficking and cell fusion but not virus entry. These results indicate that an arginine-rich region of gL is critical for function.

Functional homology of gHs and gLs from EBV-related gamma-herpesviruses for EBV-induced membrane fusion

Epstein-Barr virus (EBV) is a human gamma-herpesvirus that primarily infects B lymphocytes and epithelial cells. Entry of EBV into B cells requires the viral glycoproteins gp42, gH/gL and gB, while gp42 is not necessary for infection of epithelial cells. In EBV, gH and gL form two distinct complexes, a bipartite complex that contains only gH and gL, used for infection of epithelial cells, and a tripartite complex that additionally includes gp42, used for infection of B cells. The gH/gL complex is conserved within the herpesvirus family, but its exact role in entry and mechanism of fusion is not yet known. To understand more about the functionality of EBVgH/gL, we investigated the functional homology of gHs and gLs from human herpesvirus 8 (HHV8) and two primate (rhesus and marmoset) gamma-herpesviruses in EBV-mediated virus-free cell fusion assay. Overall, gHs and gLs from the more homologous primate herpesviruses were better at complementing EBV gH and gL in fusion than HHV8 gH and gL. Interestingly, marmoset gH was able to complement fusion with epithelial cells, but not B cells. Further investigation of this led to the discovery that EBVgH is the binding partner of gp42 in the tripartite complex and the absence of fusion with B cells in the presence of marmoset gH/gL is due to its inability to bind gp42.

Postentry events are responsible for restriction of productive varicella-zoster virus infection in Chinese hamster ovary cells

Productive infection of varicella-zoster virus (VZV) in vitro is restricted almost exclusively to cells derived from humans and other primates. We demonstrate that the restriction of productive VZV infection in CHO-K1 cells occurs downstream of virus entry. Entry of VZV into CHO-K1 cells was characterized by utilizing an ICP4/beta-galactosidase reporter gene that has been used previously to study herpes simplex virus type 1 entry. Entry of VZV into CHO-K1 cells involved cell surface interactions with heparan sulfate glycosaminoglycans and a cation-independent mannose-6-phosphate receptor. Lysosomotropic agents inhibited the entry of VZV into CHO-K1 cells, consistent with a low-pH-dependent endocytic mechanism of entry. Infection of CHO-K1 cells by VZV resulted in the production of both immediate early and late gene products, indicating that a block to progeny virus production occurs after the initiation of virus gene expression.

Induction of cell-cell fusion from without by human herpesvirus 6B

Human herpesvirus (HHV) 6A induce fusion from without (FFWO), whereas HHV-6B is believed to be ineffective in this process. Here, we demonstrate that HHV-6B induces rapid fusion in both epithelial cells and lymphocytes. The fusion was identified 1 h postinfection, could be inhibited by antibodies to HHV-6B gH and to the cellular receptor CD46, and was dependent on virus titer but independent of de novo protein synthesis and UV inactivation of the virus. Comparisons indicate that HHV-6A is only 10-fold more effective in inducing FFWO than HHV-6B. These data demonstrate that HHV-6B can induce FFWO in epithelial cells and lymphocytes.

Varicella

Varicella-zoster virus, a herpesvirus, causes varicella (chickenpox) and, after endogenous reactivation, herpes zoster (shingles). Varicella, which is recognised by a characteristic vesicular rash, arises mainly in young children, although older individuals can be affected. In immunocompetent patients, symptoms are usually mild to moderate, but an uncomplicated severe case can have more than 1000 lesions and severe constitutional symptoms. Serious complications--including central nervous system involvement, pneumonia, secondary bacterial infections, and death--are sometimes seen. Varicella can be prevented by vaccination. Vaccine is about 80-85% effective against all disease and highly (more than 95%) effective in prevention of severe disease. In the USA, a routine childhood immunisation programme has reduced disease incidence, complications, hospital admissions, and deaths in children and in the general population, indicating strong herd immunity. Similar immunisation programmes have been adopted by some other countries, including Uruguay, Germany, Taiwan, Canada, and Australia, and are expected to be implemented more widely in future.

Intracellular transport and stability of varicella-zoster virus glycoprotein K

VZV gK, an essential glycoprotein that is conserved among the alphaherpesviruses, is believed to participate in membrane fusion and cytoplasmic virion morphogenesis based on analogy to its HSV-1 homolog. However, the production of VZV gK-specific antibodies has proven difficult presumably due to its highly hydrophobic nature and, therefore, VZV gK has received limited study. To overcome this obstacle, we inserted a FLAG epitope into gK near its amino terminus and produced VZV recombinants expressing epitope-tagged gK (VZV gK-F). These recombinants grew indistinguishably from native VZV, and FLAG-tagged gK could be readily detected in VZV gK-F-infected cells. FACS analysis established that gK is transported to the plasma membrane of infected cells, while indirect immunofluorescence demonstrated that gK accumulates predominately in the Golgi. Using VZV gK-F-infected cells we demonstrated that VZV gK, like several other herpesvirus glycoproteins, is efficiently endocytosed from the plasma membrane. However, pulse-labeling experiments revealed that the half-life of gK is considerably shorter than that of other VZV glycoproteins including gB, gE and gH. This finding suggests that gK may be required in lower abundance than other viral glycoproteins during virion morphogenesis or viral entry.

Molecular studies of Varicella zoster virus

VZV is a highly cell-associated member of the Herpesviridae family and one of the eight herpesviruses to infect humans. The virus is ubiquitous in most populations worldwide, primary infection with which causes varicella, more commonly known as chickenpox. Characteristic of members of the alphaherpesvirus sub-family, VZV is neurotropic and establishes latency in sensory neurones. Reactivation from latency, usually during periods of impaired cellular immunity, causes herpes zoster (shingles). Despite being one of the most genetically stable human herpesviruses, nucleotide alterations in the virus genome have been used to classify VZV strains from different geographical regions into distinct clades. Such studies have also provided evidence that, despite pre-existing immunity to VZV, subclinical reinfection and reactivation of reinfecting strains to cause zoster is also occurring. During both primary infection and reactivation, VZV infects several PBMC and skin cell lineages. Difficulties in studying the pathogenesis of VZV because of its high cell association and narrow host range have been overcome through the development of the VZV severe combined immunodeficient mouse model carrying human tissue implants. This model has provided a valuable tool for studying the importance of individual viral proteins during both the complex intracellular replication and assembly of new virions and for understanding the underlying mechanism of attenuation of the live varicella vaccine. In addition, a rat model has been developed and successfully used to uncover which viral proteins are important for both the establishment and maintenance of latent VZV infection.

Herpes simplex virus type 1 glycoprotein L mutants that fail to promote trafficking of glycoprotein H and fail to function in fusion can induce binding of glycoprotein L-dependent anti-glycoprotein H antibodies

The herpes simplex virus type 1 (HSV-1) glycoproteins H (gH) and L (gL) form a heterodimer and efficient expression of gH at the virion or cell surface is dependent upon gL. Five carboxy-terminal deletion mutants of gL were created and their ability to interact with and mediate cell-surface expression of gH, to promote binding of gL-dependent anti-gH antibodies and to contribute to cell fusion was analysed. All of the gL mutants bound gH, but only two mutants, containing the amino-terminal 161 or 168 aa of gL, mediated cell-surface expression of gH, and only gL161 and gL168 functioned in cell fusion. The binding of gL to gH, therefore, was not sufficient to ensure gH cell-surface expression and it was not possible to separate the gH-trafficking role of gL from gL function in fusion. Co-expression of gH with any gL mutant conferred binding of the anti-gH mAbs 53S and LP11. If the acquisition of 53S and LP11 binding to gH reflects a gL-induced conformational change, such a change is not sufficient to mediate trafficking of the gH-gL heterodimer.

Mannose 6-phosphate receptor dependence of varicella zoster virus infection in vitro and in the epidermis during varicella and zoster

Varicella zoster virus (VZV) is a highly infectious human pathogen; nevertheless, infectious virions are not released in vitro where infection is cell associated. Four VZV envelope glycoproteins contain mannose 6-phosphate (Man 6-P), and Man 6-P blocks infection of cells by cell-free VZV. Expression of antisense cDNA or siRNA-like transcripts were used to generate five stable human cell lines deficient in cation-independent mannose 6-phosphate receptors (MPRci). All 5 MPRci-deficient lines resisted infection by cell-free, but not cell-associated, VZV, secreted lysosomal enzymes, and released infectious virions when infected by cell-associated VZV. Intracellular MPRci thus appear to divert newly enveloped VZV to late endosomes, and plasmalemmal MPRci are necessary for entry by cell-free VZV. Biopsies from VZV-infected human skin supported the idea that because MPRci expression is naturally lost in maturing superficial epidermal cells, these cells do not divert VZV to endosomes and constitutively secrete infectious VZV.

Incorporation of three endocytosed varicella-zoster virus glycoproteins, gE, gH, and gB, into the virion envelope

The cytoplasmic tails of all three major varicella-zoster virus (VZV) glycoproteins, gE, gH, and gB, harbor functional tyrosine-based endocytosis motifs that mediate internalization. The aim of the present study was to examine whether endocytosis from the plasma membrane is a cellular route by which VZV glycoproteins are delivered to the final envelopment compartment. In this study, we demonstrated that internalization of the glycoproteins occurred in the first 24 h postinfection but was reduced later in infection. Using surface biotinylation of VZV-infected cells followed by a glutathione cleavage assay, we showed that endocytosis was independent of antibody binding to gE, gH, and gB. Subsequently, with this assay, we demonstrated that biotinylated gE, gH, and gB retrieved from the cell surface were incorporated into nascent virus particles isolated after density gradient sedimentation. To confirm and extend this finding, we repeated the above sedimentation step and specifically detected envelopes decorated with Streptavidin-conjugated gold beads on a majority of complete virions through examination by transmission electron microscopy. In addition, a gE-gI complex and a gE-gH complex were found on the virions. Therefore, the above studies established that VZV subsumed a postendocytosis trafficking pathway as one mechanism by which to deliver viral glycoproteins to the site of virion assembly in the cytoplasm. Furthermore, since a recombinant VZV genome lacking only endocytosis-competent gE cannot replicate, these results supported the conclusion that the endocytosis-envelopment pathway is an essential component of the VZV life cycle.

Intracellular processing of human herpesvirus 6 glycoproteins Q1 and Q2 into tetrameric complexes expressed on the viral envelope

Human herpesvirus 6 (HHV-6) glycoproteins H and L (gH and gL, respectively) and the 80-kDa form of glycoprotein Q (gQ-80K) form a heterotrimeric complex that is found on the viral envelope and that is a viral ligand for human CD46. Besides gQ-80K, the gQ gene encodes an additional product whose mature molecular mass is 37 kDa (gQ-37K) and which is derived from a different transcript. Therefore, we designated gQ-80K as gQ1 and gQ-37K as gQ2. We show here that gQ2 also interacts with the gH-gL-gQ1 complex in HHV-6-infected cells and in virions. To examine how these components interact in HHV-6-infected cells, we performed pulse-chase studies. The results demonstrated that gQ2-34K, which is endo-beta-N-acetylglucosaminidase H sensitive and which is the precursor form of gQ2-37K, associates with gQ1-74K, which is the precursor form of gQ1-80K, within 30 min of the pulse period. After a 1-h chase, these precursor forms had associated with the gH-gL dimer. Interestingly, an anti-gH monoclonal antibody coimmunoprecipitated mainly gQ1-80K and gQ2-37K, with little gQ1-74K or gQ2-34K. These results indicate that although gQ2-34K and gQ1-74K interact in the endoplasmic reticulum, the gH-gL-gQ1-80K-gQ2-37K heterotetrameric complex arises in the post-endoplasmic reticulum compartment. The mature complex is subsequently incorporated into viral particles.

Regulation of varicella-zoster virus-induced cell-to-cell fusion by the endocytosis-competent glycoproteins gH and gE

The gH glycoprotein of varicella-zoster virus (VZV) is a major fusogen. The realigned short cytoplasmic tail of gH (18 amino acids) harbors a functional endocytosis motif (YNKI) that mediates internalization in both VZV-infected and transfected cells (T. J. Pasieka, L. Maresova, and C. Grose, J. Virol. 77: 4194-4202, 2003). During subsequent confocal microscopy studies of endocytosis-deficient gH mutants, we observed that cells transfected with the gH tail mutants exhibited marked fusion. Therefore, we postulated that VZV gH endocytosis served to regulate cell-to-cell fusion. Subsequent analyses of gH+gL transfection fusion assays by the Kolmogorov-Smirnov statistical test demonstrated that expression of the endocytosis-deficient gH mutants resulted in a statistically significant enhancement of cell-to-cell fusion (P < 0.0001) compared to wild-type gH. On the other hand, coexpression of VZV gE, another endocytosis-competent VZV glycoprotein, was able to temper the fusogenicity of the gH endocytosis mutants by facilitating internalization of the mutant gH protein from the cell surface. When the latter results were similarly analyzed, there was no longer any enhanced fusion by the endocytosis-deficient gH mutant protein. In summary, these studies support a role for gH endocytosis in regulating the cell surface expression of gH and thereby regulating gH-mediated fusion. The data also confirm and extend prior observations of a gE-gH interaction during viral glycoprotein trafficking in a VZV transfection system.

Membrane fusion mediated by herpesvirus glycoproteins: the paradigm of varicella-zoster virus

Varicella-zoster virus (VZV) is well known for its propensity to cause polykaryons (syncytia) in the vesicles within infected skin. Similarly in cultured cells, VZV induces extensive syncytial formation by virus-mediated cell-to-cell fusion. Statistical analyses of fusion parameters demonstrated three-way interactive effects among all three tested variables (incubation temperature, cell type and virus strain). For example, fusion was greatly enhanced at 33 degrees C vs 37 degrees C; also fusion was pronounced in epidermal cells but negligible in fibroblast cells. As with all herpesviruses, VZV gH was a major fusogen. VZV cell fusion was inhibited by antibody to gH, but surprisingly was enhanced by antibody to gE. Other evidence implicating a role for VZV gE in the fusion process was provided by two mutant viruses, in which gE cell surface expression was enhanced. Under transfection conditions, VZV fusion formation occurred after expression of the gH/gL complex; in contrast, pseudorabies virus requires expression of gH, gL and gB, while the herpes simplex virus (HSV) types 1 and 2 require the quartet of gH, gL, gB and gD. VZV has no gD gene and no apparent gD functional homologue. On the other hand, VZV gE exerts a greater effect than HSV gE on membrane fusion. Taken together, the data in this review suggest that VZV has evolved viral glycoprotein machinery more geared toward cell-to-cell fusion (fusion-from-within) than toward virus-to-cell fusion (entry/fusion-from-without), as a means for syncytium formation within the human epidermis.

A functional YNKI motif in the short cytoplasmic tail of varicella-zoster virus glycoprotein gH mediates clathrin-dependent and antibody-independent endocytosis

The trafficking of varicella-zoster virus (VZV) gH was investigated under both infection and transfection conditions. In initial endocytosis assays performed in infected cells, the three glycoproteins gE, gI, and gB served as positive controls for internalization from the plasma membrane. Subsequently, we discovered that gH in VZV-infected cells was also internalized and followed a similar trafficking pattern. This observation was unexpected because all herpesvirus gH homologues have short endodomains not known to contain trafficking motifs. Further investigation demonstrated that VZV gH, when expressed alone with its chaperone gL, was capable of endocytosis in a clathrin-dependent manner, independent of gE, gI, or gB. Upon inspection of the short gH cytoplasmic tail, we discovered a putative tyrosine-based endocytosis motif (YNKI). When the tyrosine was replaced with an alanine, endocytosis of gH was blocked. Utilizing an endocytosis assay dependent on biotin labeling, we further documented that endocytosis of VZV gH was antibody independent. In control experiments, we showed that gE, gI, and gB also internalized in an antibody-independent manner. Alignment analysis of the VZV gH cytoplasmic tail to other herpesvirus gH homologues revealed two important findings: (i) herpes simplex virus type 1 and 2 homologues lacked an endocytosis motif, while all other alphaherpesvirus gH homologues contained a potential motif, and (ii) the VZV gH and simian varicella virus gH cytoplasmic tails were likely longer in length (18 amino acids) than predicted in the original sequence analyses (12 and 16 amino acids, respectively). The longer tails provided the proper context for a functional endocytosis motif.

The out of Africa model of varicella-zoster virus evolution: single nucleotide polymorphisms and private alleles distinguish Asian clades from European/North American clades

Until 1998, varicella-zoster virus (VZV) was generally considered sufficiently stable to allow the use of a single sequenced virus (VZV-Dumas) as a consensual representation of the world VZV genotype. But recent investigations have uncovered a gE mutant virus called VZV-MSP with a second genotype and a distinguishable accelerated cell spread phenotype. A subsequent study suggested that single nucleotide polymorphisms (SNPs) could be applied toward the genetic analysis of the VZV genome. To further assess the scope of genetic variation in the VZV genome on a worldwide basis, we carried out an extensive SNP analysis of structural glycoprotein genes gB, gE, gH, gI, gL, as well as the IE62 regulatory gene in viruses collected from Western Europe, North America and Asia, including the VZV vaccine strain. The SNP data showed segregation of viral isolates of Asian origin from those of Western ancestry into distinct phylogenetic clades. Unexpectedly, however, VZV from Thailand segregated with VZV from Iceland and the United States, i.e. it was more Western than Asian in nature. Further, SNP analysis disclosed strikingly unusual genotypes, e.g. gH genes with up to five missense mutations and gL genes with insertions of an in-frame methionine codon. In summary, these VZV genomic analyses have shown that individual VZV strains, like closely related human beings, have distinctive SNP profiles containing private alleles within just five VZV genes (gB, gH, gE, gL and IE62) that provide a fingerprint to localize ancestry of the viral strain.

Induction of cellular immunity to varicella-zoster virus glycoproteins tested with pernasal coadministration of Escherichia coli enterotoxin in mice

A mutant of Escherichia coli enterotoxin promotes the induction of cellular immunity to a live varicella vaccine (the Oka strain) as a mucosal adjuvant in mice. An investigation was carried out to determine which of the purified glycoproteins of the virus among three induced cellular immunity with a single nasal administration. Spleen cells from mice immunized nasally with the vaccine and toxin produced interleukin-2 (IL-2) at the same level on restimulation in vitro with glycoprotein H: glycoprotein L (gH:gL), gB, and gE:gI, but not IL-4. The spleen cells from mice immunized with gH:gL, gB, or gE:gI and toxin produced IL-2 on restimulation with gH:gL, gB, or gE:gI, respectively, and the vaccine, but not IL-4. Immunization with gH:gL and the toxin showed increased thymidine uptake and production of IL-2 and interferon-gamma (IFN-gamma) of the spleen cells, but not IL-4, depending on the dose of gH:gL used for immunization and restimulation in vitro. Purified gE:gI and gB have been reported to be the strongest stimulators of cellular immunity to varicella upon subcutaneous injection and are useful as a subunit vaccine. All the glycoproteins tested are excellent stimulators of cellular immunity to the virus and itself on nasal co-immunization with the toxin.

Phosphorylation by the varicella-zoster virus ORF47 protein serine kinase determines whether endocytosed viral gE traffics to the trans-Golgi network or recycles to the cell membrane

Like all alphaherpesviruses, varicella-zoster virus (VZV) infection proceeds by both cell-cell spread and virion production. Virions are enveloped within vacuoles located near the trans-Golgi network (TGN), while in cell-cell spread, surface glycoproteins fuse cells into syncytia. In this report, we delineate a potential role for serine/threonine phosphorylation of the cytoplasmic tail of the predominant VZV glycoprotein, gE, in these processes. The fact that VZV gE (formerly called gpI) is phosphorylated has been documented (E. A. Montalvo and C. Grose, Proc. Natl. Acad. Sci. USA 83:8967-8971, 1986), although respective roles of viral and cellular protein kinases have never been delineated. VZV ORF47 is a viral serine protein kinase that recognized a consensus sequence similar to that of casein kinase II (CKII). During open reading frame 47 (ORF47)-specific in vitro kinase assays, ORF47 phosphorylated four residues in the cytoplasmic tail of VZV gE (S593, S595, T596, and T598), thus modifying the known phosphofurin acidic cluster sorting protein 1 domain. CKII phosphorylated gE predominantly on the two threonine residues. In wild-type-virus-infected cells, where ORF47-mediated phosphorylation predominated, gE endocytosed and relocalized to the TGN. In cells infected with a VZV ORF47-null mutant, internalized VZV gE recycled to the plasma membrane and did not localize to the TGN. The mutant virus also formed larger syncytia than the wild-type virus, linking CKII-mediated gE phosphorylation with increased cell-cell spread. Thus, ORF47 and CKII behaved as "team players" in the phosphorylation of VZV gE. Taken together, the results showed that phosphorylation of VZV gE by ORF47 or CKII determined whether VZV infection proceeded toward a pathway likely involved with either virion production or cell-cell spread.

Varicella-zoster Virus gB and gE coexpression, but not gB or gE alone, leads to abundant fusion and syncytium formation equivalent to those from gH and gL coexpression

Varicella-zoster virus (VZV) is distinguished from herpes simplex virus type 1 (HSV-1) by the fact that cell-to-cell fusion and syncytium formation require only gH and gL within a transient-expression system. In the HSV system, four glycoproteins, namely, gH, gL, gB, and gD, are required to induce a similar fusogenic event. VZV lacks a gD homologous protein. In this report, the role of VZV gB as a fusogen was investigated and compared to the gH-gL complex. First of all, the VZV gH-gL experiment was repeated under a different set of conditions; namely, gH and gL were cloned into the same vaccinia virus (VV) genome. Surprisingly, the new expression system demonstrated that a recombinant VV-gH+gL construct was even more fusogenic than seen in the prior experiment with two individual expression plasmids containing gH and gL (K. M. Duus and C. Grose, J. Virol. 70:8961-8971, 1996). Recombinant VV expressing VZV gB by itself, however, effected the formation of only small syncytia. When VZV gE and gB genes were cloned into one recombinant VV genome and another fusion assay was performed, extensive syncytium formation was observed. The degree of fusion with VZV gE-gB coexpression was comparable to that observed with VZV gH-gL: in both cases, >80% of the cells in a monolayer were fused. Thus, these studies established that VZV gE-gB coexpression greatly enhanced the fusogenic properties of gB. Control experiments documented that the fusion assay required a balance between the fusogenic potential of the VZV glycoproteins and the fusion-inhibitory effect of the VV infection itself.

Mutational analysis of the repeated open reading frames, ORFs 63 and 70 and ORFs 64 and 69, of varicella-zoster virus

Varicella-zoster virus (VZV) open reading frame 63 (ORF63), located between nucleotides 110581 and 111417 in the internal repeat region, encodes a nuclear phosphoprotein which is homologous to herpes simplex virus type 1 (HSV-1) ICP22 and is duplicated in the terminal repeat region as ORF70 (nucleotides 118480 to 119316). We evaluated the role of ORFs 63 and 70 in VZV replication, using recombinant VZV cosmids and PCR-based mutagenesis to make single and dual deletions of these ORFs. VZV was recovered within 8 to 10 days when cosmids with single deletions were transfected into melanoma cells along with the three intact VZV cosmids. In contrast, VZV was not detected in transfections carried out with a dual deletion cosmid. Infectious virus was recovered when ORF63 was cloned into a nonnative AvrII site in this cosmid, confirming that failure to generate virus was due to the dual ORF63/70 deletion and that replication required at least one gene copy. This requirement may be related to our observation that ORF63 interacts directly with ORF62, the major immediate-early transactivating protein of VZV. ORF64 is located within the inverted repeat region between nucleotides 111565 and 112107; it has some homology to the HSV-1 Us10 gene and is duplicated as ORF69 (nucleotides 117790 to 118332). ORF64 and ORF69 were deleted individually or simultaneously using the VZV cosmid system. Single deletions of ORF64 or ORF69 yielded viral plaques with the same kinetics and morphology as viruses generated with the parental cosmids. The dual deletion of ORF64 and ORF69 was associated with an abnormal plaque phenotype characterized by very large, multinucleated syncytia. Finally, all of the deletion mutants that yielded recombinants retained infectivity for human T cells in vitro and replicated efficiently in human skin in the SCIDhu mouse model of VZV pathogenesis.

Varicella-zoster virus retains major histocompatibility complex class I proteins in the Golgi compartment of infected cells

We sought to examine the effects of varicella-zoster virus (VZV) infection on the expression of major histocompatibility complex class I (MHC I) molecules by human fibroblasts and T lymphocytes. By flow cytometry, VZV infection reduced the cell surface expression of MHC I molecules on fibroblasts significantly, yet the expression of transferrin receptor was not affected. Importantly, when human fetal thymus/liver implants in SCID-hu mice were inoculated with VZV, cell surface MHC I expression was downregulated specifically on VZV-infected human CD3+ T lymphocytes, a prominent target that sustains VZV viremia. The stage in the MHC I assembly process that was disrupted by VZV in fibroblasts was examined in pulse-chase and immunoprecipitation experiments in the presence of endoglycosidase H. MHC I complexes continued to be assembled in VZV-infected cells and were not retained in the endoplasmic reticulum. In contrast, immunofluorescence and confocal microscopy showed that VZV infection resulted in an accumulation of MHC I molecules which colocalized to the Golgi compartment. Inhibition of late viral gene expression by treatment of infected fibroblasts with phosphonoacetic acid did not influence the modulation of MHC I expression, nor did transfection of cells with plasmids expressing immediate early viral proteins. However, cells transfected with a plasmid carrying the early gene ORF66 did result in a significant downregulation of MHC I expression, suggesting that this gene encodes a protein with an immunomodulatory function. Thus, VZV downregulates MHC I expression by impairing the transport of MHC I molecules from the Golgi compartment to the cell surface; this effect may enable the virus to evade CD8+ T-cell immune recognition during VZV pathogenesis, including the critical phase of T-lymphocyte-associated viremia.

Glycoprotein E of varicella-zoster virus enhances cell-cell contact in polarized epithelial cells

Varicella-zoster virus (VZV) infection involves the cell-cell spread of virions, but how viral proteins interact with the host cell membranes that comprise intercellular junctions is not known. Madin-Darby canine kidney (MDCK) cells were constructed to express the glycoproteins gE, gI, or gE/gI constitutively and were used to examine the effects of these VZV glycoproteins in polarized epithelial cells. At low cell density, VZV gE induced partial tight junction (TJ) formation under low-calcium conditions, whether expressed alone or with gI. Although most VZV gE was intracellular, gE was also shown to colocalize with the TJ protein ZO-1 with or without concomitant expression of gI. Freeze fracture electron microscopy revealed normal TJ strand morphology in gE-expressing MDCK cells. Functionally, the expression of gE was associated with a marked acceleration in the establishment of maximum transepithelial electrical resistance (TER) in MDCK-gE cells; MDCK-gI and MDCK-gE/gI cells exhibited a similar pattern of early TER compared to MDCK cells, although peak resistances were lower than those of gE alone. VZV gE expression altered F-actin organization and lipid distribution, but coexpression of gI modulated these effects. Two regions of the gE ectodomain, amino acids (aa) 278 to 355 and aa 467 to 498, although lacking Ca(2+) binding motifs, exhibit similarities with corresponding regions of the cell adhesion molecules, E-cadherin and desmocollin. These observations suggest that VZV gE and gE/gI may contribute to viral pathogenesis by facilitating epithelial cell-cell contacts.

Varicella-zoster virus gE escape mutant VZV-MSP exhibits an accelerated cell-to-cell spread phenotype in both infected cell cultures and SCID-hu mice

Varicella-zoster virus is considered to have one of the most stable genomes of all human herpesviruses. In 1998, we reported the unanticipated discovery of a wild-type virus that had lost an immunodominant B-cell epitope on the gE ectodomain (VZV-MSP); the gE escape mutant virus exhibited an unusual pattern of egress. Further studies have now documented a markedly enhanced cell-to-cell spread by the mutant virus in cell culture. This property was investigated by laser scanning confocal microscopy combined with a software program that allows the measurement of pixel intensity of the fluorescent signal. For this new application of imaging technology, the VZV immediate early protein 62 (IE 62) was selected as the fluoresceinated marker. By 48 h postinfection, the number of IE 62-positive pixels in the VZV-MSP-infected culture was nearly fourfold greater than the number of pixels in a culture infected with a low-passage laboratory strain. Titrations by infectious center assays supported the above image analysis data. Confirmatory studies in the SCID-hu mouse documented that VZV-MSP spread more rapidly than other VZV strains in human fetal skin implants. Generally, the cytopathology and vesicle formation produced by other strains at 21 days postinfection were demonstrable with VZV-MSP at 14 days. To assess whether additional genes were contributing to the unusual VZV-MSP phenotype, approximately 20 kb of the VZV-MSP genome was sequenced, including ORFs 31 (gB), 37 (gH), 47, 60 (gL), 61, 62 (IE 62), 66, 67 (gI), and 68 (gE). Except for a few polymorphisms, as well as the previously discovered mutation within gE, the nucleotide sequences within most open reading frames were identical to the prototype VZV-Dumas strain. In short, VZV-MSP represents a novel variant virus with a distinguishable phenotype demonstrable in both infected cell cultures and SCID-hu mice.

Characterization of interaction of gH and gL glycoproteins of varicella-zoster virus: their processing and trafficking

Varicella-zoster virus (VZV) glycoproteins gH and gL were examined in a recombinant vaccinia virus system. Single expression of glycoprotein gL produced two molecular forms: an 18 kDa form and a 19 kDa form differing in size by one endoglycosidase H-sensitive N-linked oligosaccharide. Coexpression of gL and gH resulted in binding of the 18 kDa gL form with the mature form of gH, while the 19 kDa gL form remained uncomplexed. The glycosylation processing of gL was not dependent on gH; however, gL was required for the conversion of precursor gH (97 kDa) to mature gH (118 kDa). Subsequent analyses indicated that gL (18 kDa) was a more completely processed gL (19 kDa). Screening of the culture media revealed that gH and gL were secreted, but only if coexpressed and complexed together. The secreted form of gL was 18 kDa while that of gH was 114 kDa. The fact that secreted gH was smaller than intracytoplasmic gH suggested a proteolytic processing event prior to secretion. The 19 kDa form of gL was never secreted. These findings support a VZV gL recycling pathway between the endoplasmic reticulum and the cis-Golgi apparatus.

Constitutive expression of human cytomegalovirus (HCMV) glycoprotein gpUL75 (gH) in astrocytoma cells: a study of the specific humoral immune response

The humoral immune response to gpUL75 (gH) was determined in different groups of human cytomegalovirus (HCMV) infected subjects using a full-length glycoprotein constitutively expressed in an astrocytoma cell line. The recombinant molecule consisted of two distinct isoforms resembling the authentic protein of infected cells. Separated from the interactions of other viral gene products gH failed to form an oligomeric complex, thus exhibiting exclusively epitopes present on the monomer. Ninety five percent of serum samples from latently-infected healthy adults revealed the presence of gH-specific IgG. Moreover, examination of sequential sera from immunocompromised and immunocompetent individuals undergoing active HCMV infection demonstrated that antibodies to gH occurred in most cases simultaneously with those to the abundant surface antigen gpUL55 (gB) and at similar titres. Appearance of this response was correlated with a considerable increase of the virus-neutralizing activity and most likely associated with restriction of viral dissemination during subsequent viremic episodes. Together, these results suggest that glycoprotein H of HCMV is like gB, a highly immunogenic component of the infectious particle.

Domains of glycoprotein H of herpes simplex virus type 1 involved in complex formation with glycoprotein L

The complex formation between glycoproteins H (gH) and L (gL) of herpes simplex virus type 1 (HSV-1) was studied by using five recombinant baculoviruses expressing open reading frames that contain deletions in the coding region of the extracellular domain of gH. In addition, the gH-deletion mutants contained a C-terminal tag. Complex formation of gL and the gH-deletion mutants was studied by immunoprecipitations with anti-tag monoclonal antibody (MAb) A16 and with the gH-specific MAbs 37S, 46S, and 52S. All gH-deletion mutants were complexed to gL when analyzed by MAb A16. MAb 37S precipitated complexes between gL and the two gH-deletion mutants that contain the epitope of this MAb. When the gH conformation-dependent MAbs 46S and 52S were used, gL was coprecipitated together with the gH-deletion mutant lacking amino acids 31-299, but gL was not coprecipitated with the gH-deletion mutant lacking amino acids 31-473. The data from the precipitation studies do allow at least two interpretations. There is either one site for gL binding on gH (residue 300-473) or gL contacts multiple regions of gH. We were unable to demonstrate gL-dependent cell surface expression of either of the gH-deletion mutants. This suggests that the coassociation of gH with gL is necessary but not sufficient for transport of gH to the cell surface.

Characterization of Varicella-Zoster virus glycoprotein K (open reading frame 5) and its role in virus growth

Varicella-zoster virus (VZV) is an alphaherpesvirus that is the causative agent of chickenpox and herpes zoster. VZV open reading frame 5 (ORF5) encodes glycoprotein K (gK), which is conserved among alphaherpesviruses. While VZV gK has not been characterized, and its role in viral replication is unknown, homologs of VZV gK in herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PRV) have been well studied. To identify the VZV ORF5 gene product, we raised a polyclonal antibody against a fusion protein of ORF5 codons 25 to 122 with glutathione S-transferase and used it to study the protein in infected cells. A 40,000-molecular-weight protein was detected in cell-free virus by Western blotting. In immunogold electron microscopic studies, VZV gK was in enveloped virions and was evenly distributed in the cytoplasm in infected cells. To determine the function of VZV gK in virus growth, a series of gK deletion mutants were constructed with VZV cosmid DNA derived from the Oka strain. Full and partial deletions in gK prevented viral replication when the gK mutant cosmids were transfected into melanoma cells. Insertion of the HSV-1 (KOS) gK gene into the endogenous VZV gK site did not compensate for the deletion of VZV gK. The replacement of VZV gK at a nonnative AvrII site in the VZV genome restored the phenotypic characteristics of intact recombinant Oka (rOka) virus. Moreover, gK complementing cells transfected with a full gK deletion mutant exhibited viral plaques indistinguishable from those of rOka. Our results are consistent with the studies of gK proteins of HSV-1 and PRV showing that gK is indispensable for viral replication.

Glycoprotein gL-independent infectivity of pseudorabies virus is mediated by a gD-gH fusion protein

Envelope glycoproteins gH and gL, which form a complex, are conserved throughout the family Herpesviridae. The gH-gL complex is essential for the fusion between the virion envelope and the cellular cytoplasmic membrane during penetration and is also required for direct viral cell-to-cell spread from infected to adjacent noninfected cells. It has been proposed for several herpesviruses that gL is required for proper folding, intracellular transport, and virion localization of gH. In pseudorabies virus (PrV), glycoprotein gL is necessary for infectivity but is dispensable for virion localization of gH. A virus mutant lacking gL, PrV-DeltagLbeta, is defective in entry into target cells, and direct cell-to-cell spread is drastically reduced, resulting in only single or small foci of infected cells (B. G. Klupp, W. Fuchs, E. Weiland, and T. C. Mettenleiter, J. Virol. 71:7687-7695, 1997). We used this limited cell-to-cell spreading ability of PrV-DeltagLbeta for serial passaging of cells infected with transcomplemented virus by coseeding with noninfected cells. After repeated passaging, plaque formation was restored and infectivity in the supernatant was observed. One single-plaque isolate, designated PrV-DeltagLPass, was further characterized. To identify the mutation leading to this gL-independent infectious phenotype, Southern and Western blot analyses, radioimmunoprecipitations, and DNA sequencing were performed. The results showed that rearrangement of a genomic region comprising part of the gH gene into a duplicated copy of part of the unique short region resulted in a fusion fragment predicted to encode a protein consisting of the N-terminal 271 amino acids of gD fused to the C-terminal 590 residues of gH. Western blotting and radioimmunoprecipitation with gD- and gH-specific antibodies verified the presence of a gDH fusion protein. To prove that this fusion protein mediates infectivity of PrV-DeltagLPass, cotransfection of PrV-DeltagLbeta DNA with the cloned fusion fragment was performed, and a cell line, Nde-67, carrying the fusion gene was established. After cotransfection, infectious gL-negative PrV was recovered, and propagation of PrV-DeltagLbeta on Nde-67 cells produced infectious virions. Thus, a gDH fusion polypeptide can compensate for function of the essential gL in entry and cell-to-cell spread of PrV.

Varicella-zoster virus Fc receptor component gI is phosphorylated on its endodomain by a cyclin-dependent kinase

Varicella-zoster virus (VZV) glycoprotein gI is a type 1 transmembrane glycoprotein which is one component of the heterodimeric gE:gI Fc receptor complex. Like VZV gE, VZV gI was phosphorylated in both VZV-infected cells and gI-transfected cells. Preliminary studies demonstrated that a serine 343-proline 344 sequence located within the gI cytoplasmic tail was the most likely phosphorylation site. To determine which protein kinase catalyzed the gI phosphorylation event, we constructed a fusion protein, consisting of glutathione-S-transferase (GST) and the gI cytoplasmic tail, called GST-gI-wt. When this fusion protein was used as a substrate for gI phosphorylation in vitro, the results demonstrated that GST-gI-wt fusion protein was phosphorylated by a representative cyclin-dependent kinase (CDK) called P-TEFb, a homologue of CDK1 (cdc2). When serine 343 within the serine-proline phosphorylation site was replaced with an alanine residue, the level of phosphorylation of the gI fusion protein was greatly reduced. Subsequent experiments with individually immunoprecipitated mammalian CDKs revealed that the VZV gI fusion protein was phosphorylated best by CDK1, to a lesser degree by CDK2, and not at all by CDK6. Transient-transfection assays carried out in the presence of the specific CDK inhibitor roscovitine strongly supported the prior results by demonstrating a marked decrease in gI phosphorylation while gI protein expression was unaffected. Finally, the possibility that VZV gI contained a CDK phosphorylation site in its endodomain was of further interest because its partner, gE, contains a casein kinase II phosphorylation site in its endodomain; prior studies have established that CDK1 can phosphorylate casein kinase II.

Antigenic variation of varicella zoster virus Fc receptor gE: loss of a major B cell epitope in the ectodomain

Varicella zoster virus (VZV) is considered to possess a genetically stable genome; only one serotype is recognized around the world. The 125-kbp genome contains approximately 70 open reading frames. One that has received particular attention is open reading frame 68, which codes for glycoprotein gE, the predominant 623-residue viral envelope product that harbors both B and T cell epitopes. This report describes the initial characterization of a community-acquired VZV isolate that was a distinguishable second serotype (i.e., it had lost a major B cell epitope defined on the gE ectodomain by a murine monoclonal antibody called mAb 3B3). The mAb 3B3 epitope was found not only on the prototype sequenced Dumas strain from Holland and all previously tested North American isolates but also on the varicella vaccine Oka strain originally attenuated in Japan. Sequencing of the mutated gE ectodomain demonstrated that codon 150 exhibited a single base change that led to an amino acid change (aspartic acid to asparagine). Observation of the monolayers infected with the mutant VZV strain also led to the surprising discovery that the topography of egress was altered. Wild-type VZV emerges along distinctive viral highways, whereas the mutant strain virions were nearly uniformly distributed over the cell surface in a pattern more closely resembling egress of herpes simplex virus 1. The mutant VZV strain was designated VZV-MSP because it was isolated in Minnesota.

Glycoproteins gB, gD, and gHgL of herpes simplex virus type 1 are necessary and sufficient to mediate membrane fusion in a Cos cell transfection system

Herpes simplex virus type 1 glycoproteins gB, gD, and gHgL were expressed by transient transfection of Cos cells. Polykaryocyte formation above the background level seen in untransfected controls was observed only if all three components were expressed. Thus, gB, gD, and gHgL are necessary and sufficient to induce membrane fusion.

Mutational analysis of the role of glycoprotein I in varicella-zoster virus replication and its effects on glycoprotein E conformation and trafficking

The contributions of the glycoproteins gI (ORF67) and gE (ORF68) to varicella-zoster virus (VZV) replication were investigated in deletion mutants made by using cosmids with VZV DNA derived from the Oka strain. Deletion of both gI and gE prevented virus replication. Complete deletion of gI or deletions of 60% of the N terminus or 40% of the C terminus of gI resulted in a small plaque phenotype as well as reduced yields of infectious virus. Melanoma cells infected with gI deletion mutants formed abnormal polykaryocytes with a disrupted organization of nuclei. In the absence of intact gI, gE became localized in patches on the cell membrane, as demonstrated by confocal microscopy. A truncated N-terminal form of gI was transported to the cell surface, but its expression did not restore plaque morphology or infectivity. The fusogenic function of gH did not compensate for gI deletion or the associated disruption of the gE-gI complex. These experiments demonstrated that gI was dispensable for VZV replication in vitro, whereas gE appeared to be required. Although VZV gI was dispensable, its deletion or mutation resulted in a significant decrease in infectious virus yields, disrupted syncytium formation, and altered the conformation and distribution of gE in infected cells. Normal cell-to-cell spread and replication kinetics were restored when gI was expressed from a nonnative locus in the VZV genome. The expression of intact gI, the ORF67 gene product, is required for efficient membrane fusion during VZV replication.

Pseudorabies virus glycoprotein L is necessary for virus infectivity but dispensable for virion localization of glycoprotein H

Herpesviruses contain a number of envelope glycoproteins which play important roles in the interaction between virions and target cells. Although several glycoproteins are not present in all herpesviruses, others, including glycoproteins H and L (gH and gL), are conserved throughout the Herpesviridae. To elucidate common properties and differences in herpesvirus glycoprotein function, corresponding virus mutants must be constructed and analyzed in different herpesvirus backgrounds. Analysis of gH- mutants of herpes simplex virus type 1 (HSV-1) and pseudorabies virus (PrV) showed that in both viruses gH is essential for penetration and cell-to-cell spread and that its presence is required for virion localization of gL. Since gH homologs are found complexed with gL, it was of interest to assess the phenotype of gL- mutant viruses. By using this approach, HSV-1 gL has been shown to be required for entry and for virion localization of gH (C. Roop, L. Hutchinson, and D. Johnson, J. Virol. 67:2285-2297, 1993). To examine whether a similar phenotype is associated with lack of gL in another alphaherpesvirus, PrV, we constructed two independent gL- PrV mutants by insertion and deletion-insertion mutagenesis. The salient findings are as follows: (i) PrV gL is required for penetration of virions and cell-to-cell spread; (ii) unlike HSV-1, PrV gH is incorporated into the virion in the absence of gL; (iii) virion localization of gH in the absence of gL is not sufficient for infectivity; (iv) in the absence of gL, N-glycans on PrV gH are processed to a greater extent than in the presence of gL, indicating masking of N-glycans by association with gL; and (v) an anti-gL polyclonal antiserum is able to neutralize virion infectivity but did not inhibit cell-to-cell spread. Thus, whereas PrV gL is essential for virus replication, as is HSV-1 gL, gL- PrV mutants exhibit properties strikingly different from those of HSV-1. In conclusion, our data show an important functional role for PrV gL in the viral entry process, which is not explained by a chaperone-type mechanism in gH maturation and processing.

Chaperone functions common to nonhomologous Epstein-Barr virus gL and Varicella-Zoster virus gL proteins

Herpesviruses encode the complex-forming, essential glycoproteins gH and gL. Maturation and transport of gH are dependent on coexpression of its chaperone, gL. The gL proteins of alpha herpesviruses and gamma herpesviruses do not have a significant percentage of amino acid sequence homology. Yet, as we report herein, the diverse gL glycoproteins of Epstein-Barr virus (EBV) and varicella-zoster virus (VZV) were functionally interchangeable, although membrane expression and maturation of gH were separate functions for these viruses. In VZV both functions were performed by a single protein. EBV required two separate glycoproteins, one of which can be replaced by its homologous protein from VZV, a distant relative of EBV. Collectively, these results suggested that VZV gL is a simpler form of the gL chaperone protein than EBV gL.