Promoter sequences of varicella-zoster virus glycoprotein I targeted by cellular transactivating factors Sp1 and USF determine virulence in skin and T cells in SCIDhu mice in vivo

Creating the "Dew Drop on a Rose Petal": the Molecular Pathogenesis of Varicella-Zoster Virus Skin Lesions

Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes varicella (chicken pox) as the primary infection in a susceptible host. Varicella is very contagious through its transmission by direct contact with vesicular skin lesions that contain high titers of infectious virus and respiratory droplets. While the clinical manifestations of primary VZV infection are well recognized, defining the molecular mechanisms that drive VZV pathogenesis in the naive host before adaptive antiviral immunity is induced has been a challenge due to species specificity. This review focuses on advances made in identifying the differentiated human host cells targeted by VZV to cause varicella, the processes involved in viral takeover of these heterogenous cell types, and the host cell countermeasures that typically culminate in a benign illness. This work has revealed many unexpected and multifaceted mechanisms used by VZV to achieve its high prevalence and persistence in the human population.

Insights into the pathogenesis of varicella viruses

Purpose of review

Varicella zoster virus (VZV) is a highly contagious, neurotropic alpha herpes virus that causes varicella (chickenpox). VZV establishes lifelong latency in the sensory ganglia from which it can reactivate to induce herpes zoster (HZ), a painful disease that primarily affects older individuals and those who are immune-suppressed. Given that VZV infection is highly specific to humans, developing a reliable in vivo model that recapitulates the hallmarks of VZV infection has been challenging. Simian Varicella Virus (SVV) infection in nonhuman primates reproduces the cardinal features of VZV infections in humans and allows the study of varicella virus pathogenesis in the natural host. In this review, we summarize our current knowledge about genomic and virion structure of varicelloviruses as well as viral pathogenesis and antiviral immune responses during acute infection, latency and reactivation. We also examine the immune evasion mechanisms developed by varicelloviruses to escape the host immune responses and the current vaccines available for protecting individuals against chickenpox and herpes zoster.

Recent findings

Data from recent studies suggest that infected T cells are important for viral dissemination to the cutaneous sites of infection as well as site of latency and that a viral latency-associated transcript might play a role in the transition from lytic infection to latency and then reactivation.

Summary

Recent studies have provided exciting insights into mechanisms of varicelloviruses pathogenesis such as the critical role of T cells in VZV/SVV dissemination from the respiratory mucosa to the skin and the sensory ganglia; the ability of VZV/SVV to interfere with host defense; and the identification of VLT transcripts in latently infected ganglia. However, our understanding of these phenomena remains poorly understood. Therefore, it is critical that we continue to investigate host-pathogen interactions during varicelloviruses infection. These studies will lead to a deeper understanding of VZV biology as well as novel aspects of cell biology.

Mutational analysis of varicella-zoster virus (VZV) immediate early protein (IE62) subdomains and their importance in viral replication

VZV IE62 is an essential, immediate-early, tegument protein and consists of five domains. We generated recombinant viruses carrying mutations in the first three IE62 domains and tested their influence on VZV replication kinetics. The mutations in domain I did not affect replication kinetics while domain II mutations, disrupting the DNA binding and dimerization domain (DBD), were lethal for VZV replication. Mutations in domain III of the nuclear localization signal (NLS) and the two phosphorylation sites S686A/S722A resulted in slower growth in early and late infection respectively and were associated with IE62 accumulation in the cytoplasm and nucleus respectively. This study mapped the functional domains of IE62 in context of viral infection, indicating that DNA binding and dimerization domain is essential for VZV replication. In addition, the correct localization of IE62, whether nuclear or cytoplasmic, at different points in the viral life cycle, is important for normal progression of VZV replication.

Full trans-activation mediated by the immediate-early protein of equine herpesvirus 1 requires a consensus TATA box, but not its cognate binding sequence

The immediate-early protein (IEP) of equine herpesvirus 1 (EHV-1) has extensive homology to the IEP of alphaherpesviruses and possesses domains essential for trans-activation, including an acidic trans-activation domain (TAD) and binding domains for DNA, TFIIB, and TBP. Our data showed that the IEP directly interacted with transcription factor TFIIA, which is known to stabilize the binding of TBP and TFIID to the TATA box of core promoters. When the TATA box of the EICP0 promoter was mutated to a nonfunctional TATA box, IEP-mediated trans-activation was reduced from 22-fold to 7-fold. The IEP trans-activated the viral promoters in a TATA motif-dependent manner. Our previous data showed that the IEP is able to repress its own promoter when the IEP-binding sequence (IEBS) is located within 26-bp from the TATA box. When the IEBS was located at 100 bp upstream of the TATA box, IEP-mediated trans-activation was very similar to that of the minimal IE(nt -89 to +73) promoter lacking the IEBS. As the distance from the IEBS to the TATA box decreased, IEP-mediated trans-activation progressively decreased, indicating that the IEBS located within 100 bp from the TATA box sequence functions as a distance-dependent repressive element. These results indicated that IEP-mediated full trans-activation requires a consensus TATA box of core promoters, but not its binding to the cognate sequence (IEBS).

Differential effects of Sp cellular transcription factors on viral promoter activation by varicella-zoster virus (VZV) IE62 protein

The immediate early (IE) 62 protein is the major varicella-zoster virus (VZV) regulatory factor. Analysis of the VZV genome revealed 40 predicted GC-rich boxes within 36 promoters. We examined effects of ectopic expression of Sp1-Sp4 on IE62- mediated transactivation of three viral promoters. Ectopic expression of Sp3 and Sp4 enhanced IE62 activation of ORF3 and gI promoters while Sp3 reduced IE62 activation of ORF28/29 promoter and VZV DNA replication. Sp2 reduced IE62 transactivation of gI while Sp1 had no significant influence on IE62 activation with any of these viral promoters. Electrophoretic mobility shift assays (EMSA) confirmed binding of Sp1 and Sp3 but not Sp2 and Sp4 to the gI promoter. Sp1-4 bound to IE62 and amino acids 238-258 of IE62 were important for the interaction with Sp3 and Sp4 as well as Sp1. This work shows that Sp family members have differential effects on IE62-mediated transactivation in a promoter-dependent manner.

Molecular mechanisms of varicella zoster virus pathogenesis

Varicella zoster virus (VZV) is the causative agent of varicella (chickenpox) and zoster (shingles). Investigating VZV pathogenesis is challenging as VZV is a human-specific virus and infection does not occur, or is highly restricted, in other species. However, the use of human tissue xenografts in mice with severe combined immunodeficiency (SCID) enables the analysis of VZV infection in differentiated human cells in their typical tissue microenvironment. Xenografts of human skin, dorsal root ganglia or foetal thymus that contains T cells can be infected with mutant viruses or in the presence of inhibitors of viral or cellular functions to assess the molecular mechanisms of VZV-host interactions. In this Review, we discuss how these models have improved our understanding of VZV pathogenesis.

Cellular transcription factor YY1 mediates the varicella-zoster virus (VZV) IE62 transcriptional activation

Several cellular transcription factors have been shown to be involved in IE62-mediated activation. The YY1 cellular transcription factor has activating and repressive effects on gene transcription. Analysis of the VZV genome revealed 19 postulated YY1 binding sites located within putative promoters of 16 VZV genes. Electrophoretic mobility shift assays (EMSA) confirmed the binding of YY1 to ORF10, ORF28/29 and gI promoters and the mutation of these binding sites inhibited YY1 binding and the promoter activation by IE62 alone or following VZV infection. Mutation of the ORF28/29 YY1 site in the VZV genome displayed insignificant influence on virus growth in melanoma cells; but it inhibited the virus replication significantly at day 5 and 6 post infection in HELF cells. This work suggests a novel role for the cellular factor YY1 in VZV replication through the mediation of IE62 activation of viral gene expression.

Regulation of the varicella-zoster virus ORF3 promoter by cellular and viral factors

The varicella zoster virus (VZV) immediate early 62 protein (IE62) activates most if not all identified promoters of VZV genes and also some minimum model promoters that contain only a TATA box element. Analysis of the DNA elements that function in IE62 activation of the VZV ORF3 promoter revealed that the 100 nucleotides before the translation start site of the ORF3 gene contains the promoter elements. This promoter lacks any functional TATA box element. Cellular transcription factors Sp1, Sp3 and YY1 bind to the promoter, and mutation of their binding sites inhibited ORF3 gene expression. VZV regulatory proteins, IE63 and ORF29, ORF61 and ORF10 proteins inhibited IE62-mediated activation of this promoter. Mutation of the Sp1/Sp3 binding site in the VZV genome did not alter VZV replication kinetics. This work suggests that Sp family proteins contribute to the activation of VZV promoters by IE62 in the absence of functional TATA box.

An Sp1/Sp3 site in the downstream region of varicella-zoster virus (VZV) oriS influences origin-dependent DNA replication and flanking gene transcription and is important for VZV replication in vitro and in human skin

The distribution and orientation of origin-binding protein (OBP) sites are the main architectural contrasts between varicella-zoster virus (VZV) and herpes simplex virus (HSV) origins of DNA replication (oriS). One important difference is the absence of a downstream OBP site in VZV, raising the possibility that an alternative cis element may replace its function. Our previous work established that Sp1, Sp3, and YY1 bind to specific sites within the downstream region of VZV oriS; we hypothesize that one or both of these sites may be the alternative cis element(s). Here, we show that the mutation of the Sp1/Sp3 site decreases DNA replication and transcription from the adjacent ORF62 and ORF63 promoters following superinfection with VZV. In contrast, in the absence of DNA replication or in transfection experiments with ORF62, only ORF63 transcription is affected. YY1 site mutations had no significant effect on either process. Recombinant viruses containing these mutations were then constructed. The Sp1/Sp3 site mutant exhibited a significant decrease in virus growth in MeWo cells and in human skin xenografts, while the YY1 site mutant virus grew as well as the wild type in MeWo cells, even showing a late increase in VZV replication in skin xenografts following infection. These results suggest that the Sp1/Sp3 site plays an important role in both VZV origin-dependent DNA replication and ORF62 and ORF63 transcription and that, in contrast to HSV, these events are linked during virus replication.

Investigation of varicella-zoster virus neurotropism and neurovirulence using SCID mouse-human DRG xenografts

Varicella-zoster virus (VZV) is a medically important human alphaherpesvirus. Investigating pathogenic mechanisms that contribute to VZV neurovirulence are made difficult by a marked host restriction. Our approach to investigating VZV neurotropism and neurovirulence has been to develop a mouse-human xenograft model in which human dorsal root ganglia (DRG) are maintained in severe compromised immunodeficient (SCID) mice. In this review, we will describe our key findings using this model in which we have demonstrated that VZV infection of SCID DRG xenograft results in rapid and efficient spread, enabled by satellite cell infection and polykaryon formation, which facilitates robust viral replication and release of infectious virus. In neurons that persist following this acute replicative phase, VZV genomes are present at low frequency with limited gene transcription and no protein synthesis, a state that resembles VZV latency in the natural human host. VZV glycoprotein I and interaction between glycoprotein I and glycoprotein E are critical for neurovirulence. Our work demonstrates that the DRG model can reveal characteristics about VZV replication and long-term persistence of latent VZV genomes in human neuronal tissues, in vivo, in an experimental system that may contribute to our knowledge of VZV neuropathogenesis.

A sequence within the varicella-zoster virus (VZV) OriS is a negative regulator of DNA replication and is bound by a protein complex containing the VZV ORF29 protein

The architecture of the varicella-zoster virus (VZV) origin of DNA replication (OriS) differs significantly from that of the herpes simplex virus (HSV) DNA replication origin. Novel aspects of the VZV OriS include a GA-rich region, three binding sites for the VZV origin-binding protein (OBP) all on the same strand and oriented in the same direction, and a partial OBP binding site of unknown function. We have designated this partial binding site Box D and have investigated the role it plays in DNA replication and flanking gene expression. This has been done with a model system using a replication-competent plasmid containing OriS and a replication- and transcription-competent dual-luciferase reporter plasmid containing both the OriS and the intergenic region between VZV open reading frames (ORFs) 62 and 63. We have found that (i) Box D is a negative regulator of DNA replication independent of flanking gene expression, (ii) the mutation of Box D results in a decrease in flanking gene expression, thus a sequence within the VZV OriS affects transcription, which is in contrast to results reported for HSV-1, (iii) there is a specific Box D complex formed with infected cell extracts in electrophoretic mobility shift assay experiments, (iv) supershift assays show that this complex contains the VZV ORF29 single-strand DNA-binding protein, and (v) the formation of this complex is dependent on the presence of CGC motifs in Box D and its downstream flanking region. These findings show that the VZV ORF29 protein, while required for DNA replication, also plays a novel role in the suppression of that process.

Analysis of the functions of glycoproteins E and I and their promoters during VZV replication in vitro and in skin and T-cell xenografts in the SCID mouse model of VZV pathogenesis

The two VZV glycoproteins, gE and gI, are encoded by genes that are designated open reading frames, ORF67 and ORF68, located in the short unique region of the VZV genome. These proteins have homologs in the other alphaherpesviruses. Like their homologues, VZV gE and gI exhibit prominent co-localization in infected cells and form heterodimers. However, VZV gE is much larger than its homologues because it has a unique N-terminal domain, consisting of 188 amino acids that are not present in these other gene products. VZV gE also differs from the related gE proteins, in that it is essential for viral replication. Targeted mutations of gE that are compatible with VZV replication in cultured cells have varying phenotypes in skin and T-cell xenografts in the SCID mouse model of VZV pathogenesis in vivo. While gI is dispensable for growth in cultured cells in vitro, this glycoprotein is essential for VZV infection of differentiated human skin and T cells in vivo. The promoter regions of gE and gI are regulated by the cellular transactivator, specificity protein factor 1 (Sp1) in combination with the major VZV transactivator in reporter construct experiments and some Sp1 promoter elements are important for VZV virulence in vivo. Further analysis of VZV gE and gI functions and their interactions with other viral and host cell proteins are important areas for studies of VZV replication and pathogenesis.

Varicella-zoster virus neurotropism in SCID mouse-human dorsal root ganglia xenografts

Varicella-zoster virus (VZV) is a neurotropic human alphaherpesvirus and the causative agent of varicella and herpes zoster. VZV reactivation from latency in sensory nerve ganglia is a direct consequence of VZV neurotropism. Investigation of VZV neuropathogenesis by infection of human dorsal root ganglion xenografts in immunocompromised (SCID) mice has provided a novel system in which to examine VZV neurotropism. Experimental infection with recombinant VZV mutants with targeted deletions or mutations of specific genes or regulatory elements provides an opportunity to assess gene candidates that may mediate neurotropism and neurovirulence. The SCID mouse-human DRG xenograft model may aid in the development of clinical strategies in the management of herpes zoster as well as in the development of "second generation" neuroattenuated vaccines.

Roles of cellular transcription factors in VZV replication

Varicella zoster virus (VZV) is the causative agent of chickenpox and shingles. During productive infection the complete VZV proteome consisting of some 68 unique gene products is expressed through interaction of a small number of viral transcriptional activators with the general transcription apparatus of the host cell. Recent work has shown that the major viral transactivator, commonly designated the IE62 protein, interacts with the human Mediator of transcription. This interaction requires direct contact between the MED25 subunit of Mediator and the acidic N-terminal transactivation domain of IE62. A second cellular factor, host cell factor-1, has been shown to be the common element in two mechanisms of activation of the promoter driving expression of the gene encoding IE62. Finally, the ubiquitous cellular transcription factors Sp1, Sp3, and YY1 have been shown to interact with sequences near the VZV origin of DNA replication and in the case of Sp1/Sp3 to influence replication efficiency.

Regulation of the expression of the varicella-zoster virus open reading frame 66 gene

The varicella-zoster virus (VZV) open reading frame (ORF) 66 encodes a serine/threonine kinase that phosphorylates the major viral transactivator protein, immediate-early (IE) 62, preventing its nuclear importation. Cytoplasmic sequestration of IE62 may alter viral gene transcription and could serve as a mechanism for maintaining VZV latency. We examined the regulation of expression of the ORF66 gene by mapping the promoter region, which was localized to within 150 bases of the start codon. The ORF66 promoter was activated by two viral regulatory proteins, IE62 and IE63. We evaluated the binding of viral regulatory proteins and cellular transcription factors based on recognized cellular transcription factor binding sites identified within the ORF66 promoter. These included Sp1 and TBP binding sites, several of which were essential for optimal promoter activity. Site-directed mutations in Sp1 and TBP binding sites led to varying degrees of impairment of ORF66 gene expression in the context of VZV infection. We also examined the effect of Sp1 and TBP mutations on IE62, Sp1, and TBP binding. These studies reveal that host cell-derived and viral factors contribute to and cooperate in the expression of this important viral kinase gene.

Role of the IE62 consensus binding site in transactivation by the varicella-zoster virus IE62 protein

The varicella-zoster virus (VZV) IE62 protein is the major transcriptional activator. IE62 is capable of associating with DNA both nonspecifically and in a sequence-specific manner via a consensus binding site (5'-ATCGT-3'). However, the function of the consensus site is poorly understood, since IE62 efficiently transactivates promoter elements lacking this sequence. In the work presented here, sequence analysis of the VZV genome revealed the presence of 245 IE62 consensus sites throughout the genome. Some 54 sites were found to be present within putative VZV promoters. Electrophoretic mobility shift assay (EMSA) experiments using an IE62 fragment containing the IE62 DNA-binding domain and duplex oligonucleotides that did or did not contain the IE62 consensus binding sequence yielded K(D) (equilibrium dissociation constant) values in the nanomolar range. Further, the IE62 DNA binding domain was shown to have a 5-fold-increased affinity for its consensus site compared to nonconsensus sequences. The effect of consensus site presence and position on IE62-mediated activation of native VZV and model promoters was examined using site-specific mutagenesis and transfection and superinfection reporter assays. In all promoters examined, the consensus sequence functioned as a distance-dependent repressive element. Protein recruitment assays utilizing the VZV gI promoter indicated that the presence of the consensus site increased the recruitment of IE62 but not Sp1. These data suggest a model where the IE62 consensus site functions to down-modulate IE62 activation, and interaction of IE62 with this sequence may result in loss or decrease of the ability of IE62 to recruit cellular factors needed for full promoter activation.

Regulation of the ORF61 promoter and ORF61 functions in varicella-zoster virus replication and pathogenesis

Varicella-zoster virus (VZV) open reading frame 61 (ORF61) encodes a protein that transactivates viral and cellular promoters in transient-transfection assays and is the ortholog of herpes simplex virus ICP0. In this report, we mapped the ORF61 promoter and investigated its regulation by viral and cellular proteins in transient-expression experiments and by mutagenesis of the VZV genome (parent Oka strain). The 5' boundary of the minimal ORF61 promoter required for IE62 transactivation was mapped to position -95 relative to the mRNA start site, and three noncanonical GT-rich Sp1-binding sites were documented to occur within the region comprising positions -95 to -45. Contributions of the three Sp1-binding-site motifs, designated Sp1a, Sp1b, and Sp1c, to ORF61 expression and viral replication were varied despite their similar sequences. Two sites, Sp1a and Sp1c, functioned synergistically. When both sites were mutated in the pOka genome to produce pOka-61proDeltaSp1ac, the mutant virus expressed significantly less ORF61 protein. Using this mutant to investigate ORF61 functions resulted in reductions in the expression levels of IE proteins, viral kinases ORF47 and ORF66, and the major glycoprotein gE, with the most impact on gE. Virion morphogenesis appeared to be intact despite minimal ORF61 expression. Pretreating melanoma cells with sodium butyrate enhanced titers of pOka-61proDeltaSp1ac but not pOka, suggesting that ORF61 has a role in histone deacetylase inhibition. Growth of pOka-61proDeltaSp1ac was impaired in SCIDhu skin xenografts, indicating that the regulation of the ORF61 promoter by Sp1 family proteins is important for ORF61 expression in vivo and that ORF61 contributes to VZV virulence at skin sites of replication.

Deletion of the first cysteine-rich region of the varicella-zoster virus glycoprotein E ectodomain abolishes the gE and gI interaction and differentially affects cell-cell spread and viral entry

Varicella-zoster virus (VZV) glycoprotein E (gE) is the most abundant glycoprotein in infected cells and, in contrast to those of other alphaherpesviruses, is essential for viral replication. The gE ectodomain contains a unique N-terminal region required for viral replication, cell-cell spread, and secondary envelopment; this region also binds to the insulin-degrading enzyme (IDE), a proposed VZV receptor. To identify new functional domains of the gE ectodomain, the effect of mutagenesis of the first cysteine-rich region of the gE ectodomain (amino acids 208 to 236) was assessed using VZV cosmids. Deletion of this region was compatible with VZV replication in vitro, but cell-cell spread of the rOka-DeltaCys mutant was reduced significantly. Deletion of the cysteine-rich region abolished the binding of the mutant gE to gI but not to IDE. Preventing gE binding to gI altered the pattern of gE expression at the plasma membrane of infected cells and the posttranslational maturation of gI and its incorporation into viral particles. In contrast, deletion of the first cysteine-rich region did not affect viral entry into human tonsil T cells in vitro or into melanoma cells infected with cell-free VZV. These experiments demonstrate that gE/gI heterodimer formation is essential for efficient cell-cell spread and incorporation of gI into viral particles but that it is dispensable for infectious varicella-zoster virion formation and entry into target cells. Blocking gE binding to gI resulted in severe impairment of VZV infection of human skin xenografts in SCIDhu mice in vivo, documenting the importance of cell fusion mediated by this complex for VZV virulence in skin.

DNA vaccines: ready for prime time?

Since the discovery, over a decade and a half ago, that genetically engineered DNA can be delivered in vaccine form and elicit an immune response, there has been much progress in understanding the basic biology of this platform. A large amount of data has been generated in preclinical model systems, and more sustained cellular responses and more consistent antibody responses are being observed in the clinic. Four DNA vaccine products have recently been approved, all in the area of veterinary medicine. These results suggest a productive future for this technology as more optimized constructs, better trial designs and improved platforms are being brought into the clinic.

Cellular transcription factors Sp1 and Sp3 suppress varicella-zoster virus origin-dependent DNA replication

The varicella-zoster virus (VZV) origin of DNA replication (oriS) contains a 46-bp AT-rich palindrome and three consensus binding sites for the VZV origin binding protein (OBP) encoded by VZV ORF51. All three OBP binding sites are upstream of the palindrome in contrast to the sequence of the herpes simplex virus oriS, which has required OBP binding sites upstream and downstream of the AT-rich region. We are investigating the roles that sequences downstream of the palindrome play in VZV oriS-dependent DNA replication. Computer analysis identified two GC boxes, GC box 1 and GC box 2, in the downstream region which were predicted to be binding sites for the cellular transcription factor Sp1. Electrophoretic mobility shift assay and supershift assays showed that two members of the Sp family (Sp1 and Sp3) stably bind to GC box 1, but not to GC box 2. A predicted binding site for the cellular factor Yin Yang 1 (YY1) that overlaps with GC box 2 was also identified. Supershift and mutational analyses confirmed the binding of YY1 to this site. Mutation of GC box 1 resulted in loss of Sp1 and Sp3 binding and an increase in origin-dependent replication efficiency in DpnI replication assays. In contrast, mutation of the YY1 site had a statistically insignificant effect. These results suggest a model where origin-dependent DNA replication and viral transcription are coupled by the binding of Sp1 and Sp3 to the downstream region of the VZV replication origin during lytic infection. They may also have implications regarding establishment or reactivation of viral latency.

The phosphorylation profile of protein kinase A substrates is modulated during Varicella-zoster virus infection

The cAMP-dependent protein kinase A (PKA) is a key enzyme for many cellular mechanisms. In this study, we investigated the importance of this kinase for the replication of the alphaherpesvirus Varicella-zoster virus (VZV). We report that the expression of the catalytic subunit of PKA was strongly increased at the beginning of the viral cycle. The presence of a peptide inhibitor of PKA had no consequence on viral replication in a melanoma cell line whereas in fibroblasts, it resulted in a drastic decrease of replication. An overall analysis of PKA substrates phosphorylation patterns during VZV replication showed that the phosphorylation of PKA substrates was modulated. These results were completed by investigating the accumulation and phosphorylation patterns of the PKA target cAMP response element binding protein (CREB). This transcription factor remained available throughout the VZV replication, but its phosphorylation decreased in the early phase of infection before it rose later on. These results indicate that the PKA signalling plays a cell-type dependent role for VZV replication and that the infection resulted in a regulated CREB-dependent gene expression.

Aberrant infection and persistence of varicella-zoster virus in human dorsal root ganglia in vivo in the absence of glycoprotein I

Varicella-zoster virus (VZV) causes varicella, establishes latency in sensory ganglia, and reactivates as herpes zoster. Human dorsal root ganglia (DRGs) xenografts in immunodeficient mice provide a model for evaluating VZV neuropathogenesis. Our investigation of the role of glycoprotein I (gI), which is dispensable in vitro, examines the functions of a VZV gene product during infection of human neural cells in vivo. Whereas intact recombinant Oka (rOka) initiated a short replicative phase followed by persistence in DRGs, the gI deletion mutant, rOkaDeltagI, showed prolonged replication with no transition to persistence up to 70 days after infection. Only a few varicella-zoster nucleocapsids and cytoplasmic virions were observed in neurons, and the major VZV glycoprotein, gE, was retained in the rough endoplasmic reticulum in the absence of gI. VZV neurotropism was not disrupted when DRG xenografts were infected with rOka mutants lacking gI promoter elements that bind cellular transactivators, specificity factor 1 (Sp1) and upstream stimulatory factor (USF). Because gI is essential and Sp1 and USF contribute to VZV pathogenesis in skin and T cells in vivo, these DRG experiments indicate that the genetic requirements for VZV infection are less stringent in neural cells in vivo. The observations demonstrate that gI is important for VZV neurotropism and suggest that a strategy to reduce neurovirulence by deleting gI could prolong active infection in human DRGs.

Cellular and viral factors regulate the varicella-zoster virus gE promoter during viral replication

Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for viral replication and is involved in cell-to-cell spread, secondary envelopment, and entry. We created a set of mutations in the gE promoter to investigate the role of viral and cellular transcriptional factors in regulation of the gE promoter. Deletion or point mutation of the two Sp1 sites in the gE promoter abolished Sp1 binding and IE62-mediated transactivation of the gE promoter in vitro. Incorporation of the deletion or the point mutations disrupting both of the Sp1 binding sites into the VZV genome was not compatible with viral replication. A point mutation altering the atypical Sp1 binding site was lethal, while altering the second site impaired VZV replication significantly, indicating functional differences between the two Sp1 binding sites. Deletions in the gE promoter that abolished putative binding sites for cellular transcriptional factors other than Sp1, identified by bioinformatics analysis, were inserted in the VZV genome. Replication of the viruses with mutations of the gE promoter did not differ from control recombinants in melanoma cells or primary human tonsil T cells in vitro. These deletions did not affect infection of human skin xenografts in SCIDhu mice. These results indicate that Sp1 is required for IE62-mediated transactivation of the gE promoter and that this transcriptional factor appears to be the only cellular factor essential for regulation of the gE promoter.

Inhibition of the NF-kappaB pathway by varicella-zoster virus in vitro and in human epidermal cells in vivo

Varicella-zoster virus (VZV) is an alphaherpesvirus that causes varicella and herpes zoster. Using human cellular DNA microarrays, we found that many nuclear factor kappa B (NF-kappaB)-responsive genes were down-regulated in VZV-infected fibroblasts, suggesting that VZV infection inhibited the NF-kappaB pathway. The activation of this pathway causes a cellular antiviral response, including the production of alpha/beta interferon, cytokines, and other proteins that restrict viral infection. In these experiments, we demonstrated that VZV interferes with NF-kappaB activation in cultured fibroblasts and in differentiated epidermal cells in skin xenografts of SCIDhu mice infected in vivo. VZV infection of fibroblasts caused a transient nuclear translocation of p50 and p65, the canonical NF-kappaB family members. In a process that was dependent upon the presence of infectious VZV, these proteins rapidly became sequestered in the cytoplasm of VZV-infected cells. Exclusion of NF-kappaB proteins from nuclei was associated with the continued presence of IkappaBalpha, which binds p50 and p65 and prevents their nuclear accumulation. IkappaBalpha levels did not diminish even though the protein became phosphorylated and ubiquitinated, as determined based on detection of the characteristic high-molecular-weight form of the protein, and the 26S proteasome remained functional in VZV-infected cells. VZV infection also inhibited the characteristic degradation of IkappaBalpha that is induced by exposure of fibroblasts to tumor necrosis factor alpha. As expected, herpes simplex virus 1 caused the persistent nuclear translocation of NF-kappaB proteins, which has been shown to facilitate its replication, whereas VZV infection progressed without persistent NF-kappaB nuclear localization. We suggest that VZV has evolved a mechanism to limit host cell antiviral defenses by sequestering NF-kappaB proteins in the cytoplasm, a strategy that appears to be unique among the herpesviruses.

The ubiquitous cellular transcriptional factor USF targets the varicella-zoster virus open reading frame 10 promoter and determines virulence in human skin xenografts in SCIDhu mice in vivo

Varicella-zoster virus (VZV) open reading frame 10 (ORF10) is a determinant of virulence in SCIDhu skin xenografts but not in human T cells in vivo. In this analysis of the regulation of ORF10 transcription, we have identified four ORF10-related transcripts, including a major 1.3-kb RNA spanning ORF10 only and three other read-through transcripts. Rapid-amplification-of-cDNA-ends experiments indicated that the 1.3-kb transcript of ORF10 has single initiation and termination sites. In transient expression assays, the ORF10 promoter was strongly stimulated by the major VZV transactivator, IE62. Deletion analyses revealed approximate boundaries for the full ORF10 promoter activity between -75 and -45 and between +5 and -8, relative to the ORF10 transcription start site. The recombinant virus POKA10-Deltapro, with the ORF10 promoter deletion, blocked transcription of ORF10 and also of ORF9A and ORF9 mRNAs, whereas expression of read-through ORF9A/9/10 and ORF9/10 transcripts was increased, compensating for the loss of the monocistronic mRNAs. The cellular factor USF bound specifically to its consensus site within the ORF10 promoter and was required for IE62 transactivation, whereas disrupting the predicted TATA boxes or Oct-1 binding elements had no effect. The USF binding site was disrupted in the recombinant virus, POKA10-proDeltaUSF, and no ORF10 protein was produced. Both ORF10 promoter mutants reduced VZV replication in SCIDhu skin xenografts. These observations provided further evidence of the contribution of the ORF10 protein to VZV pathogenesis in skin and demonstrated that VZV depends upon the cellular transcriptional factor USF to support its virulence in human skin in vivo.

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.

Promoter activation by the varicella-zoster virus major transactivator IE62 and the cellular transcription factor USF

The varicella-zoster virus major transactivator, IE62, can activate expression from homologous and heterologous promoters. High levels of IE62-mediated activation appear to involve synergy with cellular transcription factors. The work presented here focuses on functional interactions of IE62 with the ubiquitously expressed cellular factor USF. We have found that USF can synergize with IE62 to a similar extent on model minimal promoters and the complex native ORF28/29 regulatory element, neither of which contains a consensus IE62 binding site. Using Gal4 fusion constructs, we have found that the activation domain of USF1 is necessary and sufficient for synergistic activation with IE62. We have mapped the regions of USF and IE62 required for direct physical interaction. Deletion of the required region within IE62 does not ablate synergistic activation but does influence its efficiency depending on promoter architecture. Both proteins stabilize/increase binding of TATA binding protein/TFIID to promoter elements. These findings suggest a novel mechanism for the observed synergistic activation which requires neither site-specific IE62 binding to the promoter nor a direct physical interaction with USF.

T-cell tropism and the role of ORF66 protein in pathogenesis of varicella-zoster virus infection

The pathogenesis of varicella-zoster virus (VZV) involves a cell-associated viremia during which infectious virus is carried from sites of respiratory mucosal inoculation to the skin. We now demonstrate that VZV infection of T cells is associated with robust virion production and modulation of the apoptosis and interferon pathways within these cells. The VZV serine/threonine protein kinase encoded by ORF66 is essential for the efficient replication of VZV in T cells. Preventing ORF66 protein expression by stop codon insertion (pOka66S) impaired the growth of the parent Oka (pOka) strain in T cells in SCID-hu T-cell xenografts in vivo and reduced formation of VZV virions. The lack of ORF66 protein also increased the susceptibility of infected T cells to apoptosis and reduced the capacity of the virus to interfere with induction of the interferon (IFN) signaling pathway following exposure to IFN-gamma. However, preventing ORF66 protein expression only slightly reduced growth in melanoma cells in culture and did not diminish virion formation in these cells. The pOka66S virus showed only a slight defect in growth in SCID-hu skin implants compared with intact pOka. These observations suggest that the ORF66 kinase plays a unique role during infection of T cells and supports VZV T-cell tropism by contributing to immune evasion and enhancing survival of infected T cells.

Replication of varicella-zoster virus in human skin organ culture

Varicella-zoster virus (VZV) infection is restricted to humans, which hinders studies of its pathogenesis in rodent models of disease. To facilitate the study of VZV skin tropism, we developed an ex vivo system using human fetal skin organ culture (SOC). VZV replication was analyzed by plaque assay, transmission electron microscopy, and histology. The yield of infectious VZV from SOC increased approximately 100-fold over 6 days, virions were abundant, and lesions developed that contained VZV antigens and resembled varicella and zoster lesions. The SOC system for VZV replication has applications for testing virus mutants and antiviral drugs.

Role of the varicella-zoster virus gene product encoded by open reading frame 35 in viral replication in vitro and in differentiated human skin and T cells in vivo

Although genes related to varicella-zoster virus (VZV) open reading frame 35 (ORF35) are conserved in the herpesviruses, information about their contributions to viral replication and pathogenesis is limited. Using a VZV cosmid system, we deleted ORF35 to produce two null mutants, designated rOkaDelta35(#1) and rOkaDelta35(#2), and replaced ORF35 at a nonnative site, generating two rOkaDelta35/35@Avr mutants. ORF35 Flag-tagged recombinants were made by inserting ORF35-Flag at the nonnative Avr site as the only copy of ORF35, yielding rOkaDelta35/35Flag@Avr, or as a second copy, yielding rOka35Flag@Avr. Replication of rOkaDelta35 viruses was diminished in melanoma and Vero cells in a 6-day analysis of growth kinetics. Plaque sizes of rOkaDelta35 mutants were significantly smaller than those of rOka in melanoma cells. Infection of melanoma cells with rOkaDelta35 mutants was associated with disrupted cell fusion and polykaryocyte formation. The small plaque phenotype was not corrected by growth of rOkaDelta35 mutants in melanoma cells expressing the major VZV glycoprotein E, gE. The rOkaDelta35/35@Avr viruses displayed growth kinetics and plaque morphologies that were indistinguishable from those of rOka. Analysis with ORF35-Flag recombinants showed that the ORF35 gene product localized predominantly to the nuclei of infected cells. Evaluations in the SCIDhu mouse model demonstrated that ORF35 was required for efficient VZV infection of skin and T-cell xenografts, although the decrease in infectivity was most significant in skin. These mutagenesis experiments indicated that ORF35 was dispensable for VZV replication, but deleting ORF35 diminished growth in cultured cells and was associated with attenuated VZV infection of differentiated human skin and T cells in vivo.

Analysis of varicella zoster virus attenuation by evaluation of chimeric parent Oka/vaccine Oka recombinant viruses in skin xenografts in the SCIDhu mouse model

Varicella-zoster virus (VZV) is the only human herpes virus for which a vaccine has been licensed. A clinical VZV isolate, designated the parent Oka (pOka) strain was passed in human and non-human fibroblasts to produce vaccine Oka (vOka). The pOka and vOka viruses exhibit similar infectivity in cultured cells but healthy susceptible individuals given vaccines derived from vOka rarely develop the cutaneous vesicular lesions characteristic of varicella. Inoculation of skin xenografts in the SCIDhu mouse model of VZV pathogenesis demonstrated that vOka had a reduced capacity to replicate in differentiated human epidermal cells in vivo (Moffat, J.F., Zerboni, L., Kinchington, P.R., Grose, C., Kaneshima, H., Arvin A.M., 1998a. Attenuation of the vaccine Oka strain of varicella-zoster virus and role of glycoprotein C in alphaherpesvirus virulence demonstrated in the SCID-hu mouse. J Virol. 72:965-74). In order to investigate the attenuation of vOka in skin, we made chimeric pOka and vOka recombinant viruses from VZV cosmids. Six chimeric pOka/vOka viruses were generated using cosmid sets that incorporate linear overlapping fragments of VZV DNA from cells infected with pOka or vOka. The cosmid sets consist of pOka and vOka DNA segments that have identical restriction sites. As expected, the growth kinetics and plaque morphologies of the six chimeric pOka/vOka viruses were indistinguishable in vitro. However, the chimeric viruses exhibited varying capacities to replicate when evaluated in skin xenografts in vivo. The presence of ORFs 30-55 from the pOka genome was sufficient to maintain wild-type infectivity in skin. Chimeric viruses containing different vOka components retained the attenuation phenotype, suggesting that vOka attenuation is multi-factorial and can be produced by genes from different regions of the vOka genome.

Replication of varicella-zoster virus is influenced by the levels of JNK/SAPK and p38/MAPK activation

Stimulation of the Jun NH(2)-terminal kinase/stress-activated protein kinase (JNK/SAPK) and the p38 mitogen-activated protein kinase (p38/MAPK) is part of the stress-related signal transduction pathways conveying signals from the cell surface into the nucleus in order to initiate programmes of gene expression. Here, it was shown that infection by varicella-zoster virus (VZV) caused a 34-fold increase in activation of JNK/SAPK in the early phase of infection and a 2-fold increase in activation of p38/MAPK in the later phase. The phosphorylation of downstream targets c-Jun and ATF-2 was also increased; subsequent cascades to induce pro-inflammatory responses were significantly activated whereas cascades to activate apoptotic events were not. In the late phase of infection, both JNK/SAPK and p38/MAPK activities were reduced to basal levels. The use of specific inhibitors demonstrated that inhibition of JNK/SAPK resulted in a 2-fold increase in VZV replication whereas a strong decrease in virus replication was observed after inhibition of p38/MAPK. In contrast, constitutive activation of JNK/SAPK resulted in a decline in VZV replication. Blocking gene expression by treating cells with actinomycin D or cycloheximide prior to infection resulted in activation of neither JNK/SAPK nor p38/MAPK. It was assumed that the presence of tegument proteins was not sufficient to activate stress pathways, but that expression of viral genes was necessary. This suggests that activation of stress pathways by VZV infection represents a finely regulated system that activates cellular transcription factors for transregulation of VZV-encoded genes, but prevents activation of cellular defence mechanisms.

Differential requirement for cell fusion and virion formation in the pathogenesis of varicella-zoster virus infection in skin and T cells

The protein product of varicella-zoster virus (VZV) ORF47 is a serine/threonine protein kinase and tegument component. Evaluation of two recombinants of the Oka strain, rOka47DeltaC, with a C-terminal truncation of ORF47, and rOka47D-N, with a point mutation in the conserved kinase motif, showed that ORF47 kinase function was necessary for optimal VZV replication in human skin xenografts in SCID mice but not in cultured cells. We now demonstrate that rOka47DeltaC and rOka47D-N mutants do not infect human T-cell xenografts. Differences in the growth of kinase-defective ORF47 mutants allowed an examination of requirements for VZV pathogenesis in skin and T cells in vivo. Although virion assembly was reduced and no virion transport to cell surfaces was observed, epidermal cell fusion persisted, and VZV polykaryocytes were generated by rOka47DeltaC and rOka47D-N in skin. Virion assembly was also impaired in vitro, but VZV-induced cell fusion continued to cause syncytia in cultured cells infected with rOka47DeltaC or rOka47D-N. Intracellular trafficking of envelope glycoprotein E and the ORF47 and IE62 proteins, components of the tegument, was aberrant without ORF47 kinase activity. In summary, normal VZV virion assembly appears to require ORF47 kinase function. Cell fusion was induced by ORF47 mutants in skin, and cell-cell spread occurred even though virion formation was deficient. VZV-infected T cells do not undergo cell fusion, and impaired virion assembly by ORF47 mutants was associated with a complete elimination of T-cell infectivity. These observations suggest a differential requirement for cell fusion and virion formation in the pathogenesis of VZV infection in skin and T cells.

Functions of the C-terminal domain of varicella-zoster virus glycoprotein E in viral replication in vitro and skin and T-cell tropism in vivo

Varicella-zoster virus (VZV) glycoprotein E (gE) is essential for VZV replication. To further analyze the functions of gE in VZV replication, a full deletion and point mutations were made in the 62-amino-acid (aa) C-terminal domain. Targeted mutations were introduced in YAGL (aa 582 to 585), which mediates gE endocytosis, AYRV (aa 568 to 571), which targets gE to the trans-Golgi network (TGN), and SSTT, an "acid cluster" comprising a phosphorylation motif (aa 588 to 601). Substitutions Y582G in YAGL, Y569A in AYRV, and S593A, S595A, T596A, and T598A in SSTT were introduced into the viral genome by using VZV cosmids. These experiments demonstrated a hierarchy in the contributions of these C-terminal motifs to VZV replication and virulence. Deletion of the gE C terminus and mutation of YAGL were lethal for VZV replication in vitro. Mutations of AYRV and SSTT were compatible with recovery of VZV, but the AYRV mutation resulted in rapid virus spread in vitro and the SSTT mutation resulted in higher virus titers than were observed for the parental rOka strain. When the rOka-gE-AYRV and rOka-gE-SSTT mutants were evaluated in skin and T-cell xenografts in SCIDhu mice, interference with TGN targeting was associated with substantial attenuation, especially in skin, whereas the SSTT mutation did not alter VZV infectivity in vivo. These results provide the first information about how targeted mutations of this essential VZV glycoprotein affect viral replication in vitro and VZV virulence in dermal and epidermal cells and T cells within intact tissue microenvironments in vivo.

Varicella-zoster virus infection of human neural cells in vivo

Varicella-zoster virus (VZV) establishes latency in sensory ganglia and causes herpes zoster upon reactivation. These investigations in a nonobese diabetic severe combined immunodeficient mouse-human neural cell model showed that VZV infected both neurons and glial cells and spread efficiently from cell to cell in vivo. Neural cell morphology and protein synthesis were preserved, in contrast to destruction of epithelial cells by VZV. Expression of VZV genes in neural cells was characterized by nuclear retention of the major viral transactivating protein and a block in synthesis of the predominant envelope glycoprotein. The attenuated VZV vaccine strain retained infectivity for neurons and glial cells in vivo. VZV gene expression in differentiated human neural cells in vivo differs from neural infection by herpes simplex virus, which is characterized by latency-associated transcripts, and from lytic VZV replication in skin. The chimeric nonobese diabetic severe combined immunodeficient mouse model may be useful for investigating other neurotropic human viruses.

Complete DNA sequence analyses of the first two varicella-zoster virus glycoprotein E (D150N) mutant viruses found in North America: evolution of genotypes with an accelerated cell spread phenotype

Varicella-zoster virus (VZV) is considered to be one of the most genetically stable of all the herpesviruses. Yet two VZV strains with a D150N missense mutation within the gE glycoprotein were isolated in North America in 1998 and 2002. The mutant strains have an accelerated cell spread phenotype, which distinguishes them from all wild-type and laboratory viruses. Since the VZV genome contains 70 additional open reading frames (ORFs), the possibility existed that the phenotypic change was actually due to an as-yet-undiscovered mutation or deletion elsewhere in the genome. To exclude this hypothesis, the entire genomes of the two mutant viruses were sequenced and found to contain 124,883 (VZV-MSP) and 125,459 (VZV-BC) nucleotides. Coding single-nucleotide polymorphisms (SNPs) were identified in 14 ORFs. One missense mutation was discovered in gH, but none was found in gB, gI, gL, or gK. There were no coding SNPs in the major regulatory protein ORF 62. One polymorphism was discovered which could never have been anticipated based on current knowledge of herpesvirus genomics, namely, the origins of replication differed from those in the prototype strain but not in a manner expected to affect cell spread. When the two complete mutant VZV sequences were surveyed in their entirety, the most reasonable conclusion was that the increased cell spread phenotype was dependent substantially or solely on the single D150N polymorphism in glycoprotein gE. The genomic results also expanded the evolutionary database by identifying which VZV ORFs were more likely to mutate over time.

The immediate-early 63 protein of Varicella-Zoster virus: analysis of functional domains required for replication in vitro and for T-cell and skin tropism in the SCIDhu model in vivo

The immediate-early 63-kDa (IE63) protein of varicella-zoster virus (VZV) is a phosphoprotein encoded by open reading frame (ORF) ORF63/ORF70. To identify functional domains, 22 ORF63 mutations were evaluated for effects on IE63 binding to the major VZV transactivator, IE62, and on IE63 phosphorylation and nuclear localization in transient transfections, and after insertion into the viral genome with VZV cosmids. The IE62 binding site was mapped to IE63 amino acids 55 to 67, with R59/L60 being critical residues. Alanine substitutions within the IE63 center region showed that S165, S173, and S185 were phosphorylated by cellular kinases. Four mutations that changed two putative nuclear localization signal (NLS) sequences altered IE63 distribution to a cytoplasmic/nuclear pattern. Only three of 22 mutations in ORF63 were compatible with recovery of infectious VZV from our cosmids, but infectivity was restored by inserting intact ORF63 into each mutated cosmid. The viable IE63 mutants had a single alanine substitution, altering T171, S181, or S185. These mutants, rOKA/ORF63rev[T171], rOKA/ORF63rev[S181], and rOKA/ORF63rev[S185], produced less infectious virus and had a decreased plaque phenotype in vitro. ORF47 kinase protein and glycoprotein E (gE) synthesis was reduced, indicating that IE63 contributed to optimal expression of early and late gene products. The three IE63 mutants replicated in skin xenografts in the SCIDhu mouse model, but virulence was markedly attenuated. In contrast, infectivity in T-cell xenografts was not altered. Comparative analysis suggested that IE63 resembled the herpes simplex virus type 1 U(S)1.5 protein, which is expressed colinearly with ICP22 (U(S)1). In summary, most mutations of ORF63 made with our VZV cosmid system were lethal for infectivity. The few IE63 changes that were tolerated resulted in VZV mutants with an impaired capacity to replicate in vitro. However, the IE63 mutants were attenuated in skin but not T cells in vivo, indicating that the contribution of the IE63 tegument/regulatory protein to VZV pathogenesis depends upon the differentiated human cell type which is targeted for infection within the intact tissue microenvironment.

Transcription factor USF, expressed during the entire phase of varicella-zoster virus infection, interacts physically with the major viral transactivator IE62 and plays a significant role in virus replication

The expression of the genes of varicella-zoster virus (VZV) is regulated by self-encoded viral as well as cellular transcription factors. A potential candidate with an ability to influence the transcription of VZV genes is USF (upstream stimulatory factor), which recognizes the consensus E-box motif. Quantitative RT-PCR and immunoblot assays indicate stable expression of both USF1 and USF2 throughout infection. It was also found that USF binds to a variety of E-boxes (consensus and closely related motifs) within the promoters of ORF 8/9 (two elements), ORF 22 and ORF 67. Co-immunoprecipitation experiments and His-tag protein affinity pull-down assays indicate that a direct physical interaction occurs between USF and the major virus transactivator IE62. To study the general effects of USF in the replication of VZV, a cell line expressing a dominant-negative form of USF (A-USF), which inhibits binding of USF to its recognition sites, was created. A significant decrease in virus replication was detected when this cell line was infected with cell-free virus, indicating that USF is an important cellular factor that regulates the transcription of VZV genes.

Interaction between the varicella zoster virus IE62 major transactivator and cellular transcription factor Sp1

The varicella zoster virus (VZV) IE62 protein is involved in the activation of expression of all three kinetic classes of VZV proteins. Analysis of the viral promoter for VZV glycoprotein I has shown that the cellular factor Sp1 is involved in or required for the observed IE62 mediated activation. Co-immunoprecipitation experiments show that the two proteins are present in a complex in VZV-infected cells. Protein affinity pull-down assays using recombinant proteins showed that IE62 and Sp1 interact in the absence of any other viral and cellular proteins. Mapping studies using GST-fusion proteins containing truncations of IE62 and Sp1 have delimited the interacting regions to amino acids 612-778 in Sp1 and amino acids 226-299 in IE62. The region identified in Sp1 is involved in DNA-binding, synergistic Sp1 activation, and Sp1 interaction with cellular transcription factors. The interacting region identified in IE62 overlaps with or borders on sites involved in interactions with the VZV IE4 protein and the cellular factors TBP and TFIIB. Assays using wild-type and mutant promoter elements indicate that Sp1 is involved in recruitment of IE62 to the gI promoter and IE62 enhances Sp1 and TBP binding.

Mutational analysis of open reading frames 62 and 71, encoding the varicella-zoster virus immediate-early transactivating protein, IE62, and effects on replication in vitro and in skin xenografts in the SCID-hu mouse in vivo

The varicella-zoster virus (VZV) genome has unique long (U(L)) and unique short (U(S)) segments which are flanked by internal repeat (IR) and terminal repeat (TR) sequences. The immediate-early 62 (IE62) protein, encoded by open reading frame 62 (ORF62) and ORF71 in these repeats, is the major VZV transactivating protein. Mutational analyses were done with VZV cosmids generated from parent Oka (pOka), a low-passage clinical isolate, and repair experiments were done with ORF62 from pOka and vaccine Oka (vOka), which is derived from pOka. Transfections using VZV cosmids from which ORF62, ORF71, or the ORF62/71 gene pair was deleted showed that VZV replication required at least one copy of ORF62. The insertion of ORF62 from pOka or vOka into a nonnative site in U(S) allowed VZV replication in cell culture in vitro, although the plaque size and yields of infectious virus were decreased. Targeted mutations in binding sites reported to affect interaction with IE4 protein and a putative ORF9 protein binding site were not lethal. Single deletions of ORF62 or ORF71 from cosmids permitted recovery of infectious virus, but recombination events repaired the defective repeat region in some progeny viruses, as verified by PCR and Southern hybridization. VZV infectivity in skin xenografts in the SCID-hu model required ORF62 expression; mixtures of single-copy recombinant Oka Delta 62 (rOka Delta 62) or rOka Delta 71 and repaired rOka generated by recombination of the single-copy deletion mutants were detected in some skin implants. Although insertion of ORF62 into the nonnative site permitted replication in cell culture, ORF62 expression from its native site was necessary for cell-cell spread in differentiated human skin tissues in vivo.