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

Deletion of Double Copies of the US1 Gene Reduces the Infectivity of Recombinant Duck Plague Virus In Vitro and In Vivo

Duck plague caused by duck plague virus (DPV) is one of the main diseases that seriously endangers the production of waterfowl. DPV possesses a large genome consisting of 78 open reading frames (ORFs), and understanding the function and mechanism of each encoded protein in viral replication and pathogenesis is the key to controlling duck plague outbreaks. US1 is one of the two genes located in the repeat regions of the DPV genome, but the function of its encoded protein in DPV replication and pathogenesis remains unclear. Previous studies found that the US1 gene or its homologs exist in almost all alphaherpesviruses, but the loci, functions, and pathogenesis of their encoded proteins vary among different viruses. Here, we aimed to define the roles of US1 genes in DPV infection and pathogenesis by generating a double US1 gene deletion mutant and its revertant without any mini-F cassette retention. In vitro and in vivo studies found that deletion of both copies of the US1 gene significantly impaired the replication, gene expression, and virulence of DPV, which could represent a potential candidate vaccine strain for the prevention of duck plague. IMPORTANCE Duck plague virus contains nearly 80 genes, but the functions and mechanisms of most of the genes have not yet been elucidated, including those of the newly identified immediate early gene US1. Here, we found that US1 deletion reduces viral gene expression, replication, and virus production both in vitro and in vivo. This insight defines a fundamental role of the US1 gene in DPV infection and indicates its involvement in DPV transcription. These results provide clues for the study of the pathogenesis of the US1 gene and the development of attenuated vaccines targeting this gene.

ICP22/IE63 Mediated Transcriptional Regulation and Immune Evasion: Two Important Survival Strategies for Alphaherpesviruses

In the process of infecting the host, alphaherpesviruses have derived a series of adaptation and survival strategies, such as latent infection, autophagy and immune evasion, to survive in the host environment. Infected cell protein 22 (ICP22) or its homologue immediate early protein 63 (IE63) is a posttranslationally modified multifunctional viral regulatory protein encoded by all alphaherpesviruses. In addition to playing an important role in the efficient use of host cell RNA polymerase II, it also plays an important role in the defense process of the virus overcoming the host immune system. These two effects of ICP22/IE63 are important survival strategies for alphaherpesviruses. In this review, we summarize the complex mechanism by which the ICP22 protein regulates the transcription of alphaherpesviruses and their host genes and the mechanism by which ICP22/IE63 participates in immune escape. Reviewing these mechanisms will also help us understand the pathogenesis of alphaherpesvirus infections and provide new strategies to combat these viral infections.

Interactions between the transcription and replication machineries regulate the RNA and DNA synthesis in the herpesviruses

The temporal coordination of viral gene expression is imperative for the regulation of the herpesvirus replication cycle. While the main factors of this transcriptional coordination are known, the subtler control mechanisms of gene expression remain elusive. Recent long read sequencing-based approached have revealed an intricate meshwork of overlaps between the herpesvirus transcripts and the overlap of the replication origins with noncoding RNAs. It has been shown that the transcriptional apparatuses can physically interfere with one another while transcribing overlapping regions. We hypothesize that transcriptional interference regulates the global gene expression across the herpesvirus genome. Additionally, an overall decrease in transcriptional activity in individual viral genes has been observed following the onset of DNA replication. An overlap of the replication origins with specific transcripts has also been described in several herpesviruses. The genome-wide interactions between the transcriptional apparatuses and between the replication and transcriptional machineries suggest the existence of novel layers of genetic regulation.

GaHV-2 ICP22 protein is expressed from a bicistronic transcript regulated by three GaHV-2 microRNAs

Herpesviruses have a lifecycle consisting of successive lytic, latent and reactivation phases. Only three infected cell proteins (ICPs) have been described for the oncogenic Marek's disease virus (or Gallid herpes virus 2, GaHV-2): ICP4, ICP22 and ICP27. We focus here on ICP22, confirming its cytoplasmic location and showing that ICP22 is expressed during productive phases of the lifecycle, via a bicistronic transcript encompassing the US10 gene. We also identified the unique promoter controlling ICP22 expression, and its core promoter, containing functional responsive elements including E-box, ETS-1 and GATA elements involved in ICP22 transactivation. ICP22 gene expression was weakly regulated by DNA methylation and activated by ICP4 or ICP27 proteins. We also investigated the function of GaHV-2 ICP22. We found that this protein repressed transcription from its own promoter and from those of IE ICP4 and ICP27, and the late gK promoter. Finally, we investigated posttranscriptional ICP22 regulation by GaHV-2 microRNAs. We found that mdv1-miR-M5-3p and -M1-5p downregulated ICP22 mRNA expression during latency, whereas, unexpectedly, mdv1-miR-M4-5p upregulated the expression of the protein ICP22, indicating a tight regulation of ICP22 expression by microRNAs.

Specific transcriptional and post-transcriptional regulation of the major immediate early ICP4 gene of GaHV-2 during the lytic, latent and reactivation phases

Transcriptional and post-transcriptional mechanisms are involved in the switch between the lytic, latent and reactivation phases of the viral cycle in herpesviruses. During the productive phases, herpesvirus gene expression is characterized by a temporally regulated cascade of immediate early (IE), early (E) and late (L) genes. In alphaherpesviruses, the major product of the IE ICP4 gene is a transcriptional regulator that initiates the cascade of gene expression that is essential for viral replication. In this study, we redefine the infected cell protein 4 (ICP4) gene of the oncogenic Marek's disease virus (MDV or gallid herpesvirus 2) as a 9438 nt gene ended with four alternative poly(A) signals and controlled by two alternative promoters containing essentially ubiquitous functional response elements (GC, TATA and CCAAT boxes). The distal promoter is associated with ICP4 gene expression during the lytic and the latent phases, whereas the proximal promoter is associated with the expression of this gene during the reactivation phase. Both promoters are regulated by DNA methylation during the viral cycle and are hypermethylated during latency. Transcript analyses showed ICP4 to consist of three exons and two introns, the alternative splicing of which is associated with five predicted nested ICP4ORFs. We show that the ICP4 gene is highly and specifically regulated by transcriptional and post-transcriptional mechanisms during the three phases of the GaHV-2 viral cycle, with a clear difference in expression between the lytic phase and reactivation from latency in our model.

Vaccine-type mutations identified in Varicella zoster virus passaged in cell culture

Varicella-zoster virus (VZV) is a causative agent for chickenpox and shingles. Comparative genomic sequence analysis of clinical and vaccine strains suggested potential sites responsible for attenuation. In this study, low and high passages of two VZV clinical strains cultured in human fibroblast cells were compared for genomic DNA sequences and growth characteristics. Mutations were detected at 187 and 162 sites in the strain YC01 and YC02, respectively. More than 86% of mutations were found in open reading frames, and ORF62 exhibited highest frequency of mutations. T to C and A to G transitions accounted for more 90% of all possible substitutions. Forty mutations were common to two strains, including 27 in ORF62. Mutations found in attenuated vaccine strains were also detected at 7 positions. Both high and low passage strains were infectious and grew similarly in human fibroblast cells. In guinea pig cells, however, high passage strain remained infectious while low passage strain lost infectivity. This study may provide new insight into the attenuating mutations associated with in vitro passaging of VZV.

ORF7 of Varicella-Zoster Virus Is Required for Viral Cytoplasmic Envelopment in Differentiated Neuronal Cells

Although a varicella-zoster virus (VZV) vaccine has been used for many years, the neuropathy caused by VZV infection is still a major health concern. Open reading frame 7 (ORF7) of VZV has been recognized as a neurotropic gene in vivo, but its neurovirulent role remains unclear. In the present study, we investigated the effect of ORF7 deletion on VZV replication cycle at virus entry, genome replication, gene expression, capsid assembly and cytoplasmic envelopment, and transcellular transmission in differentiated neural progenitor cells (dNPCs) and neuroblastoma SH-SY5Y (dSY5Y) cells. Our results demonstrate that the ORF7 protein is a component of the tegument layer of VZV virions. Deleting ORF7 did not affect viral entry, viral genome replication, or the expression of typical viral genes but clearly impacted cytoplasmic envelopment of VZV capsids, resulting in a dramatic increase of envelope-defective particles and a decrease in intact virions. The defect was more severe in differentiated neuronal cells of dNPCs and dSY5Y. ORF7 deletion also impaired transmission of ORF7-deficient virus among the neuronal cells. These results indicate that ORF7 is required for cytoplasmic envelopment of VZV capsids, virus transmission among neuronal cells, and probably the neuropathy induced by VZV infection.IMPORTANCE The neurological damage caused by varicella-zoster virus (VZV) reactivation is commonly manifested as clinical problems. Thus, identifying viral neurovirulent genes and characterizing their functions are important for relieving VZV related neurological complications. ORF7 has been previously identified as a potential neurotropic gene, but its involvement in VZV replication is unclear. In this study, we found that ORF7 is required for VZV cytoplasmic envelopment in differentiated neuronal cells, and the envelopment deficiency caused by ORF7 deletion results in poor dissemination of VZV among neuronal cells. These findings imply that ORF7 plays a role in neuropathy, highlighting a potential strategy to develop a neurovirulence-attenuated vaccine against chickenpox and herpes zoster and providing a new target for intervention of neuropathy induced by VZV.

Analysis of single nucleotide polymorphism among Varicella-Zoster Virus and identification of vaccine-specific sites

Varicella-zoster virus (VZV) is a causative agent for chickenpox and zoster. Live attenuated vaccines have been developed based on Oka and MAV/06 strains. In order to understand the molecular mechanisms of attenuation, complete genome sequences of vaccine and wild-type strains were compared and single nucleotide polymorphism (SNP) was analyzed. ORF22 and ORF62 contained the highest number of SNPs. The detailed analysis of the SNPs suggested 24 potential vaccine-specific sites. All the mutational events found in vaccine-specific sites were transitional, and most of them were substitution of AT to GC pair. Interestingly, 18 of the vaccine-specific sites of the vaccine strains appeared to be genetically heterogeneous. The probability of a single genome of vaccine strain to contain all 24 vaccine-type sequences was calculated to be less than 4%. The average codon adaptation index (CAI) value of the vaccine strains was significantly lower than the CAI value of the clinical strains.

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.

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.

Characterization of cis-acting elements required for autorepression of the equine herpesvirus 1 IE gene

The immediate-early protein (IEP), the major regulatory protein encoded by the IE gene of equine herpesvirus 1 (EHV-1), plays a crucial role as both transcription activator and repressor during a productive lytic infection. To investigate the mechanism by which the EHV-1 IEP inhibits its own promoter, IE promoter-luciferase reporter plasmids containing wild-type and mutant IEP-binding site (IEBS) were constructed and used for luciferase reporter assays. The IEP inhibited transcription from its own promoter in the presence of a consensus IEBS (5'-ATCGT-3') located near the transcription initiation site but did not inhibit when the consensus sequence was deleted. To determine whether the distance between the TATA box and the IEBS affects transcriptional repression, the IEBS was displaced from the original site by the insertion of synthetic DNA sequences. Luciferase reporter assays revealed that the IEP is able to repress its own promoter when the IEBS is located within 26-bp from the TATA box. We also found that the proper orientation and position of the IEBS were required for the repression by the IEP. Interestingly, the level of repression was significantly reduced when a consensus TATA sequence was deleted from the promoter region, indicating that the IEP efficiently inhibits its own promoter in a TATA box-dependent manner. Taken together, these results suggest that the EHV-1 IEP delicately modulates autoregulation of its gene through the consensus IEBS that is near the transcription initiation site and the TATA box.

Varicella zoster virus-induced pain and post-herpetic neuralgia in the human host and in rodent animal models

Pain and post-herpetic neuralgia (PHN) are common and highly distressing complications of herpes zoster that remain a significant public health concern and in need of improved therapies. Zoster results from reactivation of the herpesvirus varicella zoster virus (VZV) from a neuronal latent state established at the primary infection (varicella). PHN occurs in some one fifth to one third of zoster cases with severity, incidence, and duration of pain increasing with rising patient age. While VZV reactivation and the ensuing ganglionic damage trigger the pain response, the mechanisms underlying protracted PHN are not understood, and the lack of an animal model of herpes zoster (reactivation) makes this issue more challenging. A recent preclinical rodent model has developed that opens up the potential to allow the exploration of the underlying mechanisms and treatments for VZV-induced pain. Rats inoculated with live cell-associated human VZV into the hind paw reliably demonstrate thermal hyperalgesia and mechanical allodynia for extended periods and then spontaneously recover. Dorsal root ganglia express a limited VZV gene subset, including the IE62 regulatory protein, and upregulate expression of markers suggesting a neuropathic pain state. The model has been used to investigate treatment modalities and aspects of pain signaling and is under investigation by the authors to delineate VZV genetics involved in the induction of pain. This article compares human zoster-associated pain and PHN to the pain indicators in the rat and poses important questions that, if answered, could be the basis for new treatments.

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.