The herpes simplex virus type 1 regulatory protein ICP0 enhances virus replication during acute infection and reactivation from latency

Resistance to pseudorabies virus infection in transgenic mice expressing the chimeric transgene that represses the immediate-early gene transcription

A chimeric gene encoding a fusion protein consisting of the DNA-binding domain of the immediate-early (IE) protein of pseudorabies virus (PRV) and a tail-truncated VP16 of herpes simplex virus 1, lacking the transcription activation domain, has been shown to repress transcription of the PRV IE gene, resulting in the inhibition of PRV growth in vitro. To assess the antiviral potential of the fusion protein in vivo, transgenic mice containing the chimeric gene under the control of the virus- and interferon-inducible Mx 1 promoter were generated. A transgenic mouse line showed marked resistance to PRV infection when the mice were challenged intranasally with PRV. Inhibition of PRV replication was also observed in monolayers of embryonic cells prepared from the transgenic mice. In the cells infected with PRV, transcription of the PRV IE gene was repressed. The present results indicate that the chimeric gene is able to exert a significant antiviral effect against PRV infection in vivo.

A single amino acid substitution in the cyclin D binding domain of the infected cell protein no. 0 abrogates the neuroinvasiveness of herpes simplex virus without affecting its ability to replicate

The infected cell protein no. 0 (ICP0) of herpes simplex virus 1 is a promiscuous transactivator shown to enhance the expression of genes introduced into cells by infection or transfection. The protein interacts with several viral and cellular proteins. Earlier studies have shown that ICP0 binds and stabilizes cyclin D3 but does interfere with the phosphorylation of retinoblastoma protein, its major function. Cyclin D3 plays a key role in the transition from G1 to S phase. To define the role of cyclin D3 in productive infection, the ICP0 binding site for cyclin D3 was mapped and mutagenized by substitution of aspartic acid codon 199 with the alanine codon. We report that the substitution precluded the interaction of this protein with cyclin D3 in the yeast two-hybrid system and the stabilization of cyclin D3 in infected cells. A recombinant virus carrying this mutation could not be differentiated from wild-type parent with respect to replication in dividing cells but yielded 10-fold less progeny from infected resting cells and serum-deprived or contact-inhibited human fibroblasts. In mice, the mutant was only slightly less pathogenic than the wild-type parent by intracerebral route but was significantly less neuroinvasive after peripheral inoculation. Replacement of the mutated amino acid with aspartic acid restored wild-type phenotype. Stabilization of cyclin D3 therefore is linked to higher virus yields in nondividing cells and potentially higher virulence in experimental and natural hosts. One function of ICP0 is to scavenge the cell for proteins that could bolster viral replication.

Genetic studies exposing the splicing events involved in herpes simplex virus type 1 latency-associated transcript production during lytic and latent infection

Herpes simplex virus type 1 (HSV-1) establishes latency in sensory neurons, a state in which the viral lytic genes are silenced and only the latency locus is transcriptionally active, producing the 2. 0- and 1.5-kb latency-associated transcripts (LATs). Previous experimental evidence indicates that the LATs are stable introns, and it has been reported that LAT formation is abolished by debilitating substitution mutations in the predicted splice sites during lytic infection but not latency (J. L. Arthur et al., J. Gen. Virol. 79:107-116, 1998). We have independently studied a set of deletion mutations to explore the roles of the proposed splice sites during lytic and latent infection. HSV-1 mutant viruses missing the invariant intron-terminal 5'-G(T/C) or 3'-AG dinucleotides were analyzed for LAT formation during lytic infection in vitro, when only the 2-kb LAT is produced, and during latency in mouse trigeminal ganglia, where both LATs are expressed. Northern blot analysis of total RNAs from different productively infected cell lines showed that the lytic (2-kb) LAT was not expressed by the various splice site deletion mutants. In vivo studies using a mouse eye model of latency similarly showed that the latent (2- and 1. 5-kb) LATs were not expressed by the mutants. PCR analysis with primers flanking the LAT sequence revealed the expected splice junction for LAT excision in RNA from sensory neurons latently infected with wild-type but not mutant virus. Using a virus mutant deleted in the splicing signals flanking the 556-bp region of LAT whose absence distinguishes the 1.5- and 2-kb LATs, we observed selective elimination of 1.5-kb LAT expression in latency, supporting previous suggestions that the internal region is removed by splicing. Taken together, these results demonstrate that the 2-kb LAT is formed during both lytic and latent infection by splicing at the predicted splice sites and that an additional splicing event is involved in the latency-restricted production of the 1.5-kb LAT. We have also mapped the 3' end of the lytic 2-kb LAT and discuss our results in the context of previous models addressing the unusual stability of the LATs.

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

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

The ability of herpes simplex virus type 1 immediate-early protein Vmw110 to bind to a ubiquitin-specific protease contributes to its roles in the activation of gene expression and stimulation of virus replication

Herpes simplex virus type 1 immediate-early protein Vmw110 stimulates the onset of virus infection and is required for efficient reactivation from latency. In transfection assays, Vmw110 is a potent activator of gene expression, but its mode of action has yet to be determined. Previous work has shown that Vmw110 localizes to specific intranuclear structures known as ND10, PML bodies, or PODs and causes the disruption of these domains. The ability of Vmw110 to disrupt ND10 correlates with its biological activities in infected and transfected cells. It has also been found that Vmw110 binds strongly and specifically to a ubiquitin-specific protease known as HAUSP, itself a component of a subset of ND10. In this study we have investigated the role of HAUSP in Vmw110 activity; single amino acid residues of Vmw110 required for the interaction were identified, and the effects of mutation of these residues in infected and transfected cells were then assayed. The results indicate that the ability to bind to HAUSP contributes to the functional activities of Vmw110.

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

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

Molecular aspects of herpes simplex virus I latency, reactivation, and recurrence

The application of molecular biology in the study of the pathogenesis of herpes simplex virus type 1 (HSV-1) has led to significant advances in our understanding of mechanisms that regulate virus behavior in sensory neurons and epithelial tissue. Such study has provided insight into the relationship of host and viral factors that regulate latency, reactivation, and recurrent disease. This review attempts to distill decades of information involving human, animal, and cell culture studies of HSV-1 with the goal of correlating molecular events with the clinical and laboratory behavior of the virus during latency, reactivation, and recurrent disease. The purpose of such an attempt is to acquaint the clinician/scientist with the current thinking in the field, and to provide key references upon which current opinions rest.

Functional conservations of the alkaline nuclease of herpes simplex type 1 and human cytomegalovirus

The herpes simplex virus type 1 UL12 gene product, alkaline nuclease (AN), appears to be involved in viral DNA processing and capsid egress from the nucleus (Shao, L., Rapp, L. M., and Weller, S. K., Virology 196, 146-162, 1993). Although the HSV-1 AN is not absolutely essential for viral replication in tissue culture, conservation of the AN gene in all herpesviruses suggests an important role in the life cycle of herpesviruses. The counterpart of HSV-1 AN for human cytomegalovirus (HCMV) is the UL98 gene product. To examine whether the HCMV AN could substitute for HSV-1 AN, we performed trans-complementation experiments using a HSV-1 amplicon plasmid carrying the HCMV UL98 gene. Our results indicate (i) HCMV AN can complement the growth of the HSV-1 AN deletion mutant UL12lacZ virus in trans; (ii) a new recombinant virus, UL12laZcUL98/99, appears to be generated by the integration of the HCMV UL98 gene into the HSV-1 UL12lacZ viral genome; (iii) in contrast to its parental HSV-1 UL12lacZ virus, capsids formed in UL12lacZUL98/99-infected Vero cells were able to transport from the nucleus to the cytoplasm and mature into infectious viruses. Our results demonstrate a functional conservation of AN between HSV-1 and HCMV.

Infectious laryngotracheitis herpesvirus expresses a related pair of unique nuclear proteins which are encoded by split genes located at the right end of the UL genome region

Avian infectious laryngotracheitis virus (ILTV) possesses an alphaherpesvirus type D DNA genome of ca. 155 kbp. Completion of our previous sequence analyses (W. Fuchs and T. C. Mettenleiter, J. Gen. Virol. 77:2221-2229, 1996) of the right end of the unique long (UL) genome region revealed the presence of two adjacent, presumably ILTV-specific genes, which were named UL0 and UL[-1] because of their location upstream of the conserved UL1 (glycoprotein L) gene. Transcriptional analyses showed that both genes are abundantly expressed during the late phase of the viral replication cycle and that both mRNAs are spliced by the removal of short introns close to their 5' ends. Furthermore, the deduced gene products exhibit a moderate but significant homology of 28% to each other. The newly identified ILTV genes encode proteins of 63 kDa (UL0) and 73 kDa (UL[-1]), which both are predominantly localized in the nuclei of virus infected chicken cells. In summary, our results indicate that duplication of a spliced ILTV-specific gene encoding a nuclear protein has occurred during evolution of ILTV.

Human thymidine kinase can functionally replace herpes simplex virus type 1 thymidine kinase for viral replication in mouse sensory ganglia and reactivation from latency upon explant

Herpes simplex virus type 1 thymidine kinase exhibits a strikingly broad substrate specificity. It is capable of phosphorylating deoxythymidine and deoxyuridine as does human thymidine kinase, deoxycytidine as does human deoxycytidine kinase, the cytosolic kinase whose amino acid sequence it most closely resembles, and thymidylate as does human thymidylate kinase. Following peripheral inoculation of mice, viral thymidine kinase is ordinarily required for viral replication in ganglia and for reactivation from latency following ganglionic explant. To determine which activity of the viral kinase is important for replication and reactivation in mouse ganglia, recombinant viruses lacking viral thymidine kinase but expressing individual human kinases were constructed. Each recombinant virus expressed the appropriate kinase activity with early kinetics following infection of cultured cells. The virus expressing human thymidine kinase exhibited thymidine phosphorylation activity equivalent to approximately 5% of that of wild-type virus in a quantitative plaque autoradiography assay. Nevertheless, it was competent for ganglionic replication and reactivation following corneal inoculation of mice. The virus expressing human thymidylate kinase was partially competent for these activities despite failing to express detectable thymidine kinase activity. The virus expressing human deoxycytidine kinase failed to replicate acutely in neurons or to reactivate from latency. Therefore, it appears that low levels of thymidine phosphorylation suffice to fulfill the role of the viral enzyme in ganglia and that this role can be partially fulfilled by thymidylate kinase activity alone.

The disruption of ND10 during herpes simplex virus infection correlates with the Vmw110- and proteasome-dependent loss of several PML isoforms

The small nuclear structures known as ND10 or PML nuclear bodies have been implicated in a variety of cellular processes including response to stress and interferons, oncogenesis, and viral infection, but little is known about their biochemical properties. Recently, a ubiquitin-specific protease enzyme (named HAUSP) and a ubiquitin-homology family protein (PIC1) have been found associated with ND10. HAUSP binds strongly to Vmw110, a herpesvirus regulatory protein which has the ability to disrupt ND10, while PIC1 was identified as a protein which interacts with PML, the prototype ND10 protein. We have investigated the role of ubiquitin-related pathways in the mechanism of ND10 disruption by Vmw110 and the effect of virus infection on PML stability. The results show that the disruption of ND10 during virus infection correlates with the loss of several PML isoforms and this process is dependent on active proteasomes. The PML isoforms that are most sensitive to virus infection correspond closely to those which have recently been identified as being covalently conjugated to PIC1. In addition, a large number of PIC1-protein conjugates can be detected following transfection of a PIC1 expression plasmid, and many of these are also eliminated in a Vmw110-dependent manner during virus infection. These observations provide a biochemical mechanism to explain the observed effects of Vmw110 on ND10 and suggest a simple yet powerful mechanism by which Vmw110 might function during virus infection.

The probability of in vivo reactivation of herpes simplex virus type 1 increases with the number of latently infected neurons in the ganglia

The purpose of this study was to define the relationship between herpes simplex virus (HSV) latency and in vivo ganglionic reactivation. Groups of mice with numbers of latently infected neurons ranging from 1.9 to 24% were generated by varying the input titer of wild-type HSV type 1 strain 17syn+. Reactivation of the virus in mice from each group was induced by hyperthermic stress. The number of animals that exhibited virus reactivation was positively correlated with the number of latently infected neurons in the ganglia over the entire range examined (r = 0.9852, P < 0. 0001 [Pearson correlation]).

Varicella-zoster virus ORF61 deletion mutants replicate in cell culture, but a mutant with stop codons in ORF61 reverts to wild-type virus

Varicella-zoster virus (VZV) ORF61 encodes a phosphoprotein that transactivates VZV promoters. Transfection of cells with cosmid DNAs, including a cosmid with a large deletion in ORF61, resulted in a VZV ORF61 deletion mutant that was impaired for growth in vitro and could be partially complemented by growth in neuroblastoma or osteosarcoma cell lines. Cells infected with the VZV ORF61 deletion mutant expressed normal levels of an immediate-early VZV protein, but had reduced levels of a late protein and showed abnormal syncytia. Carboxy terminal truncation mutants of VZV ORF61 protein have a transrepressing phenotype and inhibit the infectivity of cotransfected wild-type viral DNA. Transfection of cells with cosmid DNAs, including a cosmid with stop codons that should result in an ORF61 truncation mutant expressing a transrepressing protein that retains the RING finger domain, resulted in a viral genome which reverted back to the wild-type sequence. BAL-31 exonuclease was used to produce deletions at the site of the stop codons in ORF61 of the cosmid, resulting in loss of the RING finger domain. Transfection of tissue culture cells with the ORF61 BAL-31 deletion mutants and other cosmid DNAs yielded viable viruses. Thus, while deletion mutants lacking the RING finger domain of ORF61 replicate in cell culture, a mutant with stop codons that retains this domain could not be propagated and reverted to wild-type virus.

Characterization of a nerve growth factor-inducible cellular activity that enhances herpes simplex virus type 1 gene expression and replication of an ICP0 null mutant in cells of neural lineage

Herpes simplex virus type 1 (HSV-1) ICP0 is required for efficient viral gene expression during lytic infection, especially at low multiplicities. A series of cellular activities that can substitute for ICP0 has been identified, suggesting that when the activity of ICP0 is limiting, these activities can substitute for ICP0 to activate viral gene expression. The cellular activities may be especially important during reactivation of HSV from neuronal latency when viral gene expression is initiated in the absence of prior viral protein synthesis. Consistent with this hypothesis, we have identified an inducible activity in cells of neural lineage (PC12) that can complement the low-multiplicity growth phenotype of an ICP0 null mutant, n212. Pretreatment of PC12 cells with nerve growth factor (NGF) or fibroblast growth factor (FGF) prior to infection produced a 10- to 20-fold increase in the 24-h yield of n212 but only a 2- to 4-fold increase in the yield of wild-type virus relative to mock treatment. Slot blot analysis of nuclear DNA isolated from infected cells treated or mock treated with NGF indicated that NGF treatment does not significantly affect viral entry. The NGF-induced activity in PC12 cells was expressed transiently, with peak complementing activity observed when cells were treated with NGF 12 h prior to infection. Addition of NGF 3 h after infection had little effect on virus yield. The NGF-induced cellular activity was inhibited by pretreatment of PC12 cells with kinase inhibitors that have high specificity for kinases involved in NGF/FGF-dependent signal transduction. RNase protection assays demonstrated that the NGF-inducible PC12 cell activity, like that of ICP0, functions to increase the level of viral mRNA during low-multiplicity infection. These results suggest that activation of viral transcription by ICP0 and transcriptional activation of cellular genes by NGF and FGF utilize common signal transduction pathways in PC12 cells.

The latent herpes simplex virus type 1 genome copy number in individual neurons is virus strain specific and correlates with reactivation

The viral genetic elements that determine the in vivo reactivation efficiencies of fully replication competent wild-type herpes simplex virus (HSV) strains have not been identified. Among the common laboratory strains, KOS reactivates in vivo at a lower efficiency than either strain 17syn+ or strain McKrae. An important first step in understanding the molecular basis for this observation is to distinguish between viral genetic factors that regulate the establishment of latency from those that directly regulate reactivation. Reported here are experiments performed to determine whether the reduced reactivation of KOS was associated with a reduced ability to establish or maintain latent infections. For comparative purposes, latent infections were quantified by (i) quantitative PCR on DNA extracted from whole ganglia, (ii) the number of latency-associated transcript (LAT) promoter-positive neurons, using KOS and 17syn+ LAT promoter-beta-galactosidase reporter mutants, and (iii) contextual analysis of DNA. Mice latently infected with 17syn+-based strains contained more HSV type 1 (HSV-1) DNA in their ganglia than those infected with KOS strains, but this difference was not statistically significant. The number of latently infected neurons also did not differ significantly between ganglia latently infected with either the low- or high-reactivator strains. In addition to the number of latent sites, the number of viral genome copies within the individual latently infected neurons has recently been demonstrated to be variable. Interestingly, neurons latently infected with KOS contained significantly fewer viral genome copies than those infected with either 17syn+ or McKrae. Thus, the HSV-1 genome copy number profile is viral strain specific and positively correlates with the ability to reactivate in vivo. This is the first demonstration that the number of HSV genome copies within individual latently infected neurons is regulated by viral genetic factors. These findings suggest that the latent genome copy number may be an important parameter for subsequent induced reactivation in vivo.

Herpes simplex virus type 1 latency-associated transcripts suppress viral replication and reduce immediate-early gene mRNA levels in a neuronal cell line

During herpes simplex virus type 1 (HSV-1) latent infection in human dorsal root ganglia, limited viral transcription, which has been linked to HSV-1 reactivation ability, takes place. To study the involvement of this transcription in HSV-1 replication in neuronal cells and consequently in viral latency, we constructed stably transfected neuronal cell lines containing (i) the entire HSV-1 latency transcriptionally active DNA fragment, (ii) the same DNA sequence with deletions of the latency-associated transcript (LAT) promoters, or (iii) the DNA coding sequence of the LAT domain. Replication of HSV-1 or a LAT-negative mutant was markedly repressed in the LAT-expressing cells, a phenomenon mediated by the LATs. To study the mechanism responsible for this effect, we examined LAT influence upon expression of HSV-1 immediate-early (IE) genes ICP0, ICP4, and ICP27, by Northern blot analysis. Following infection of a LAT-expressing neuronal cell line with a LAT-negative mutant, the steady-state levels of all three IE mRNAs were reduced compared to those for control cells. Transient transfections into a neuronal cell line indicated that the LAT suppressive effect upon ICP0 mRNA was mediated directly and was not due to the LAT effect upon the ICP0 promoter. We therefore propose that the LATs may repress viral replication in neuronal cells by reducing IE gene mRNA levels and thus facilitate the establishment of HSV-1 latency in nervous tissue.

Genomic organization and evolution of the human herpesviruses

Members of the Herpesviridae family have been isolated from most animal species examined. Of approximately 100 individual virus species, eight have been isolated from humans, and three of these only within the last 10 years. Fortunately, there is now an enormous amount of sequence data from many of these viruses, particularly the eight human herpesviruses. This wealth of sequence information from such a diverse group of related viruses provides a unique resource for studies of viral gene evolution, comparative gene function, and virus identification.

Persistence and expression of the herpes simplex virus genome in the absence of immediate-early proteins

The immediate-early (IE) proteins of herpes simplex virus (HSV) function on input genomes and affect many aspects of host cell metabolism to ensure the efficient expression and regulation of the remainder of the genome and, subsequently, the production of progeny virions. Due to the many and varied effects of IE proteins on host cell metabolism, their expression is not conducive to normal cell function and viability. This presents a major impediment to the use of HSV as a vector system. In this study, we describe a series of ICP4 mutants that are defective in different subsets of the remaining IE genes. One mutant, d109, does not express any of the IE proteins and carries a green fluorescent protein (GFP) transgene under the control of the human cytomegalovirus IE promoter (HCMVIEp). d109 was nontoxic to Vero and human embryonic lung (HEL) cells at all multiplicities of infection tested and was capable of establishing persistent infections in both of these cell types. Paradoxically, the genetic manipulations that were required to eliminate toxicity and allow the genome to persist in cells for long periods of time also dramatically lowered the level of transgene expression. Efficient expression of the HCMVIEp-GFP transgene in the absence of ICP4 was dependent on the ICP0 protein. In d109-infected cells, the level of transgene expression was very low in most cells but abundant in a small subpopulation of cells. However, expression of the transgene could be induced in cells containing quiescent d109 genomes weeks after the initial infection, demonstrating the functionality of the persisting genomes.

Promoter activity of sequence located upstream of the pseudorabies virus early protein 0 gene

Promoter activity of the 5'-flanking region of the pseudorabies virus (PRV) early protein 0 (EP0) gene was analysed by transient transfection assays employing chloramphenicol acetyl transferase (CAT) reporter constructs. We identified a 213 bp segment of the viral genome that was capable of efficiently driving expression of the EPO gene and a linked reporter gene upon transient transfection into Vero cells. This segment lacked the typical TATA element, and possessed an initiator element and the putative binding sites for the transcription factor Sp1 and immediate-early protein IE180, a strong transactivator of PRV. By analysing 5'-deletion mutants of the segment, a 48 bp segment (from nucleotide positions -65 to -17), which possessed three Sp1 binding sites, was identified to be critical for the promoter activity. Cotransfection of Vero cells with the mutant constructs and an IE180 expression plasmid resulted in transactivation of only those constructs in which the Sp1 sites were present. These results indicate that the EP0 gene may be transcribed from the TATA-less promoter that responds to Sp1.

Importance of the herpes simplex virus UL24 gene for productive ganglionic infection in mice

The UL24 gene of herpes simplex virus overlaps the viral thymidine kinase (tk) gene. Most previous studies of UL24 have examined UL24 mutants that have also contained tk and sometimes other mutations. To address the importance of UL24 for viral replication in cell culture and in infections of a mammalian host, we constructed a mutant virus containing a UL24 nonsense mutation that does not affect TK activity and a second mutant that contains clustered point mutations in UL24 and a mutation in tk that does not by itself affect the ability of the virus to replicate acutely in mouse ganglia or to reactivate from latent infection following corneal inoculation of mice. Both mutant viruses replicated in cells in culture and in the mouse eye, albeit less efficiently than wild type or control viruses. Both mutants were much more severely impaired for acute replication in trigeminal ganglia and for reactivation from latency following explant of these ganglia. Viral DNA and latency-associated transcripts were present, albeit at lower levels in ganglia infected with the nonsense mutant. These results indicate that UL24 is especially important for productive infection of mouse sensory ganglia and may have implications for the behaviors of certain tk mutants in pathogenesis.

Activation of gene expression by herpes simplex virus type 1 ICP0 occurs at the level of mRNA synthesis

ICP0 is a nuclear phosphoprotein involved in the activation of herpes simplex virus type 1 (HSV-1) gene expression during lytic infection and reactivation from viral latency. Although available evidence suggests that ICP0 acts at the level of transcription, definitive studies specifically addressing this issue have not been reported. In the present study we measured the ability of ICP0 to activate gene expression (i) from promoters representing the major kinetic classes of viral genes in transient expression assays and (ii) from the same promoters during viral infection at multiplicities of infection ranging from 0.1 to 5.0 PFU/cell. The levels of synthesis and steady-state accumulation of mRNA, mRNA stability, and levels of protein synthesis were compared in cells transfected with a reporter plasmid in the presence and absence of ICP0 and in cells infected with wild-type HSV-1 or an ICP0 null mutant, n212. In transient expression assays and during viral infection at all multiplicities tested, the levels of steady-state mRNA and protein were significantly lower in the absence of ICP0, indicating that ICP0 activates gene expression at the level of mRNA accumulation. In transient expression assays and during infection at low multiplicities (< 1 PFU/cell) in the presence or absence of ICP0, marked increases in the levels of viral mRNAs accompanied by proportional increases in the levels of protein synthesis were observed with increasing multiplicity. At a high multiplicity (5 PFU/cell) in the presence or absence of ICP0, mRNA levels did not increase as a function of multiplicity and changes in the levels of protein were no longer related to changes in the levels of mRNA. Collectively, these tests indicate that transcription of viral genes is rate limiting at low multiplicities and that translation is rate limiting at high multiplicities, independent of ICP0. Consistent with the lower levels of mRNA detected in the absence of ICP0, the rates of transcription initiation measured by nuclear run-on assays were uniformly lower in cells infected with the ICP0 null mutant at all multiplicities tested, implying that ICP0 enhances transcription at or before initiation or both. No evidence was found of posttranscriptional effects of ICP0 (i.e., effects on the stability of mRNA, nuclear-cytoplasmic distribution, polyribosomal mRNA distribution, or rates of protein synthesis). Taken together, these results suggest that ICP0 activates gene expression prior to or at the level of initiation of mRNA synthesis in transient expression assays and during viral infection. Based on these findings; we hypothesize that the exaggerated multiplicity-dependent growth phenotype characteristic of ICP0 null mutants reflects the requirement for ICP0 under conditions where the steady-state level of mRNA is rate limiting, such as during low-multiplicity infection and reactivation from latency.

The herpes simplex virus type 1 immediate-early protein ICP0 is necessary for the efficient establishment of latent infection

The immediate-early protein ICP0 of herpes simplex virus type 1 (HSV-1) is not essential for viral replication. However, ICP0 is important for efficient viral replication during the productive infection and for reactivation of latent HSV-1 in vivo. The in vitro model of HSV-1 latency in dorsal root ganglia neurons was used to examine the role of ICP0 in the individual steps that could lead to the appearance of a decreased reactivation phenotype of ICP0 mutant viruses. After establishment of latent infections in the neuronal cultures, induction of reactivation by nerve growth factor (NGF) deprivation resulted in the production of infectious virus with delayed kinetics and a burst size that was significantly decreased for the ICP0 mutants compared with wild-type HSV-1. The efficiency of establishment of latency with the ICP0 mutants was similarly decreased at least 10-fold, as measured by three criteria: (i) the percentage of neurons expressing the major latency-associated transcript during the latent infection, (ii) the amount of viral DNA detected in the neuronal cultures, and (iii) the percentage of neurons expressing ICP4 immunoreactivity after the induction of reactivation. The most striking finding was that ICP0 supplied by an adenovirus vector significantly restored the ability of an ICP0 mutant to establish latency and reactivation. These results strongly indicate a critical role for ICP0 in the establishment of the latent HSV-1 infection in the in vitro neuronal model.

A viral function represses accumulation of transcripts from productive-cycle genes in mouse ganglia latently infected with herpes simplex virus

Latent infections of neurons by herpes simplex virus form reservoirs of recurrent viral infections that resist cure. In latently infected neurons, viral gene expression is severely repressed; only the latency-associated transcripts (LATs) are expressed abundantly. Using sensitive reverse transcriptase PCR assays, we analyzed the effects of a deletion mutation in the LAT locus on viral gene expression in latently infected mouse trigeminal ganglia. The deletion mutation, which reduced expression of the major LATs 10(5)-fold, resulted in a approximately 5-fold increase in accumulation of transcripts from the immediate-early gene encoding ICP4, an essential transactivator of viral gene expression. The LAT deletion also resulted in a >10-fold increase in the accumulation of transcripts from the early gene encoding thymidine kinase, whose expression during productive infection stringently depends on ICP4, and positively affected the correlation of the levels of these transcripts with the levels of ICP4 transcripts. We also detected transcripts antisense to ICP4 RNA, which were in substantial excess to ICP4 transcripts in ganglia latently infected with wild-type virus. In contrast to its effects on productive-cycle transcripts, the LAT deletion reduced the accumulation of these antisense transcripts approximately 15-fold. Thus, a viral function associated with the LAT locus represses the accumulation of transcripts from at least two productive-cycle genes in latently infected mouse ganglia. We discuss possible mechanisms and consequences of this repression.

The herpes simplex virus immediate-early protein ICP0 affects transcription from the viral genome and infected-cell survival in the absence of ICP4 and ICP27

ICP4, ICP0, and ICP27 are the immediate-early (IE) regulatory proteins of herpes simplex virus that have the greatest effect on viral gene expression and growth. Comparative analysis of viral mutants defective in various subsets of these IE genes should help elucidate how these proteins affect cellular and viral processes. This study focuses on the mutant d97, which is defective for the genes encoding ICP4, ICP0, and ICP27 and expresses the bacterial beta-galactosidase (beta-gal) gene from the ICP0 promoter. Together with the d92 virus (ICP4- ICP27-) and the ICP0-complementing cell line L7, d97 provided a unique opportunity to evaluate ICP0 function in the absence of the regulatory activities specified by ICP4 and ICP27. The pattern of protein synthesis in d97-infected cells was unique relative to other IE gene mutants in that it was similar to that seen in the absence of prior viral protein synthesis, possibly approximating the effect of cellular factors and virion components alone. Inactivation of ICP0 in the absence of ICP4 produced a significant decrease in the levels of the early mRNAs ICP6 and thymidine kinase (tk). There was also a marginal reduction in the levels of the IE ICP22 mRNA, and this was most notable at low multiplicity of infection (MOI). In d97-infected L7 cells, the levels of the viral mRNAs were mostly restored to those observed in infections with d92. Nuclear runoff transcription analysis demonstrated that the presence of ICP0 resulted in an increase in the transcription rates of the analyzed genes. The transcription rates of the early genes were dramatically reduced in the absence of ICP0. At low MOI, the transcription rates of ICP6 and tk were comparable to the rate of transcription of a cellular gene. Relevant to the potential use of d97 as a transfer vector, it was also determined that the absence of ICP0 reduced the cellular toxicity of the virus compared to that of d92. The beta-gal transgene expressed from an IE promoter was detected for up to 14 days postinfection; however, the level of beta-gal expression declined dramatically after 1 day postinfection. In the presence of ICP0, the level of expression of beta-gal was increased; however the infected monolayer was destroyed by 3 days postinfection. Therefore, deletion of ICP0 in the absence of ICP4 and ICP27 reduces toxicity and lowers the level of expression of genes from the viral genome.

The major immediate-early proteins IE1 and IE2 of human cytomegalovirus colocalize with and disrupt PML-associated nuclear bodies at very early times in infected permissive cells

The major immediate-early (MIE) gene products of human cytomegalovirus (HCMV) are nuclear phosphoproteins that are thought to play key roles in initiating lytic cycle gene regulation pathways. We have examined the intranuclear localization pattern of both the IE1 and IE2 proteins in virus-infected and DNA-transfected cells. When HCMV-infected human diploid fibroblast (HF) cells were stained with specific monoclonal antibodies, IE1 localized as a mixture of nuclear diffuse and punctate patterns at very early times (2 h) but changed to an exclusively nuclear diffuse pattern at later times. In contrast, IE2 was distributed predominantly in nuclear punctate structures continuously from 2 to at least 12 h after infection. These punctate structures resembled the preexisting PML-associated nuclear bodies (ND10 or PML oncogenic domains [PODs]) that are disrupted and dispersed by the IE110 protein as a very early event in herpes simplex virus (HSV) infection. However, HCMV differed from HSV by leading instead to a change in both the PML and SP100 protein distribution from punctate bodies to uniform diffuse patterns, a process that was complete in 50% of the cells at 2 h and in 90% of the cells by 4 h after infection. Confocal double-label indirect immunofluorescence assay analysis confirmed that both IE1 and IE2 colocalized transiently with PML in punctate bodies at very early times after infection. In transient expression assays, introduction of IE1-encoding plasmid DNA alone into Vero or HF cells produced the typical total redistribution of PML into a uniform nuclear diffuse pattern together with the IE1 protein, whereas introduction of IE2-encoding plasmid DNA alone resulted in stable colocalization of the IE2 protein with PML in the PODs. A truncated mutant form of IE1 gave large nuclear aggregates and failed to redistribute PML, and similarly a deleted mutant form of IE2 failed to colocalize with the punctate PML bodies, confirming the specificity of these effects. Furthermore, both Vero and U373 cell lines constitutively expressing IE1 also showed total PML relocalization together with the IE1 protein into a nuclear diffuse pattern, although a very small percentage of the cells which failed to express IE1 reverted to a punctate PML pattern. Finally, the PML redistribution activity of IE1 and the direct association of IE2 with PML punctate bodies were both confirmed by infection with E1A-negative recombinant adenovirus vectors expressing either IE1 or IE2 alone. These results confirm that transient colocalization with and disruption of PML-associated nuclear bodies by IE1 and continuous targeting to PML-associated nuclear bodies by IE2 are intrinsic properties of these two MIE regulatory proteins, which we suggest may represent critical initial events for efficient lytic cycle infection by HCMV.

A novel ubiquitin-specific protease is dynamically associated with the PML nuclear domain and binds to a herpesvirus regulatory protein

Herpes simplex virus type 1 immediate-early protein Vmw110 is a non-specific activator of gene expression and is required for efficient initiation of the viral lytic cycle. Since Vmw110-deficient viruses reactivate inefficiently in mouse latency models it has been suggested that Vmw110 plays a role in the balance between the latent and lytic states of the virus. The mechanisms by which Vmw110 achieves these functions are poorly understood. Vmw110 migrates to discrete nuclear structures (ND10) which contain the cellular PML protein, and in consequence PML and other constituent proteins are dispersed. In addition, Vmw110 binds to a cellular protein of approximately 135 kDa, and its interactions with the 135 kDa protein and ND10 contribute to its ability to stimulate gene expression and viral lytic growth. In this report we identify the 135 kDa protein as a novel member of the ubiquitin-specific protease family. The protease is distributed in the nucleus in a micropunctate pattern with a limited number of larger discrete foci, some of which co-localize with PML in ND10. At early times of virus infection, the presence of Vmw110 increases the proportion of ND10 which contain the ubiquitin-specific protease. These results identify a novel, transitory component of ND10 and implicate a previously uncharacterized ubiquitin-dependent pathway in the control of viral gene expression.

Construction and characterization of herpes simplex virus type 1 mutants with conditional defects in immediate early gene expression

The herpes simplex virus type 1 (HSV-1) mutant in 1814 contains an insertion mutation in the coding sequence for the virion transactivator protein VP16 and is thus impaired for the activation of immediate early (IE) gene expression. This virus was modified further by introducing the Moloney murine leukemia virus LTR promoter in place of the upstream sequences controlling expression of the IE regulatory protein ICPO, to yield mutant in 1820. In almost all cell types tested, in 1820 initiated infection less efficiently than in 1814, behaving as if lacking both VP16 and ICPO functions, but in BHK cells in 1820 was less impaired than in 1814. A rescuant of in 1820 at the VP16 locus, in 1825, also exhibited a host range phenotype, initiating replication as efficiently as wild-type HSV-1 in BHK cells but inefficiently in other cell types. In 1825 was unable to complement an ICPO null mutant in restricted cells, demonstrating that the promoter exchange prevented the expression of ICPO protein in functionally significant amounts. The novel host range properties of in 1820 provided a basis for the construction of additional viruses conditionally impaired for IE gene expression and assessment of their value as prototype vectors. Production of an HSV-1 mutant multiply defective in the expression of IE gene products was achieved by introduction of the temperature-sensitive mutation of HSV-1 tsK, which inactivates the IE transcription activator ICP4 at nonpermissive temperatures, into in 1820 to produce in 1820K. This mutant could be propagated effectively in BHK cells at 31 degrees but was effectively devoid of the major regulators ICPO, ICP4, and VP16 in other cells types at 38.5 degrees. Cultures could withstand infection with 5 PFU of in 1820K per cell without detectable cytopathology and could be reseeded to form colonies at approximately 90% efficiency. A derivative of in 1820K containing the Escherichia coli lacZ gene controlled by the human cytomegalovirus (HCMV) major IE promoter expressed low but detectable levels of beta-galactosidase in almost all cells after infection of cultures at 5 PFU per cell and incubation at 38.5 degrees. Cultures infected with 5 PFU per cell of an in 1820K derivative expressing neomycin phosphotransferase (npt) controlled by the HCMV IE promoter were resistant to killing by the antibiotic G418 for up to 3 days, and cell survival correlated with the retention of functional levels of npt. Mutants based on in 1820K can thus express foreign gene products in virtually all cells in a culture under conditions in which cytotoxicity is eliminated, demonstrating that progressive reduction of IE gene expression is an important step in the design of HSV-1-derived vectors.

Construction and characterization of a recombinant herpes simplex virus type 1 which overexpresses the transrepressor protein ICPOR

ICPOR is a truncated form of the herpes simplex virus type 1 (HSV-1) transactivator protein ICPO that is synthesized at low levels during infection through an alternative splicing mechanism. In transient expression assays, ICPOR has been shown to inhibit the transactivation function of several HSV-1 regulatory proteins, suggesting that an antiviral strategy which alters normal ICPO mRNA splicing and thereby stimulates the synthesis of ICPOR protein may have potential in suppressing HSV-1 infections. To explore the feasibility of this approach, a recombinant virus was constructed which expressed high levels of ICPOR instead of ICPO. Surprisingly, overexpression of the ICPOR protein in this virus, HSV-KST, had no detectable effect on virus replication, since the growth properties of HSV-KST were indistinguishable from those of the ICPO/ICPOR null mutant dl 1403, and HSV-KST was no more efficient than dl 1403 at inhibiting the replication of an ICPO-expressing wild-type virus. The absence of a demonstrable phenotype in HSV-KST was not due to the acquisition of an inactivating mutation in the gene encoding ICPOR, since copies of the gene rescued from this virus retained full transrepression capability in transient expression assays. These results indicate that the ability of ICPOR to act as a transrepressor is significantly reduced if not completely eliminated in the context of a productive HSV-1 infection and suggest that this protein may not represent an exploitable target for the development of novel antiviral therapies.

A novel ubiquitin-specific protease is dynamically associated with the PML nuclear domain and binds to a herpesvirus regulatory protein

Herpes simplex virus type 1 immediate-early protein Vmw110 is a non-specific activator of gene expression and is required for efficient initiation of the viral lytic cycle. Since Vmw110-deficient viruses reactivate inefficiently in mouse latency models it has been suggested that Vmw110 plays a role in the balance between the latent and lytic states of the virus. The mechanisms by which Vmw110 achieves these functions are poorly understood. Vmw110 migrates to discrete nuclear structures (ND10) which contain the cellular PML protein, and in consequence PML and other constituent proteins are dispersed. In addition, Vmw110 binds to a cellular protein of approximately 135 kDa, and its interactions with the 135 kDa protein and ND10 contribute to its ability to stimulate gene expression and viral lytic growth. In this report we identify the 135 kDa protein as a novel member of the ubiquitin-specific protease family. The protease is distributed in the nucleus in a micropunctate pattern with a limited number of larger discrete foci, some of which co-localize with PML in ND10. At early times of virus infection, the presence of Vmw110 increases the proportion of ND10 which contain the ubiquitin-specific protease. These results identify a novel, transitory component of ND10 and implicate a previously uncharacterized ubiquitin-dependent pathway in the control of viral gene expression.

Recombinant bovine herpesvirus-1 (BHV-1) lacking transactivator protein BICPO entails lack of glycoprotein C and severely reduced infectivity

The immediate-early transactivator protein BICPO is a key regulatory element of bovine herpesvirus 1 (BHV-1) replication based on transient expression assays. To examine BICPO function in the context of the viral genome, we created recombinant BHV-1 expressing beta-galactosidase instead of BICPO. To complement the defect, a neomycin resistant MDBK cell line (M164) expressing BICPO was established, permitting selection of a blue-staining BHV-1 recombinant (A2G2). Southern blot and PCR analysis confirmed that the BICPO gene was interrupted by the beta-galactosidase gene and that wt progeny was absent. Compared with wt BHV-1, A2G2 reached lower titers in M164 cells but replicated with similar kinetics. Once isolated, A2G2 also grew in MDBK cells although the titer was reduced a further 10-fold and the virus remained strongly cell-associated. Thus, BICPO is not absolutely required for replication in cell culture. Gene expression of A2G2 was investigated by Western blots and immunofluorescence. Surprisingly, not only was BICPO absent, but glycoprotein C (gC) was also missing. Other viral genes were expressed normally. Semiquantitative PCR showed that A2G2 produced similar amounts of viral DNA as wt but a much smaller number of infectious particles. Cotransfection of A2G2 DNA and a plasmid containing the BICPO gene yielded revertant virus with fully restored wt properties. We conclude that BICPO is required for gC expression, and that the missing gC partly accounts for the reduced A2G2 infectivity.

Attenuation of DNA-dependent protein kinase activity and its catalytic subunit by the herpes simplex virus type 1 transactivator ICP0

The DNA-dependent protein kinase (DNA-PK) is involved in several fundamental nuclear processes, including DNA double-strand break repair, V(D)J recombination, and transcription by RNA polymerases I and II. In this study, we show that infection of mammalian cells with herpes simplex virus type 1 attenuates DNA-PK activity by specifically depleting the p350/DNA-PKcs catalytic subunit. The half-life of the p350/DNA-PKcs protein decreases from greater than 24 h to less than 4 h following infection. The depletion of DNA-PK activity and p350/DNA-PKcs abundance is dependent on expression of the viral immediate-early protein ICP0. As ICP0 acts as a promoter-independent transactivator of gene expression, these data suggest that ICP0 may function by directly or indirectly targeting the p350/DNA-PKcs subunit of DNA-PK, thereby altering the inhibitory effects of DNA-PK on RNA polymerase II transcription.

Mutational analysis of ICP0R, a transrepressor protein created by alternative splicing of the ICP0 gene of herpes simplex virus type 1

The immediate-early protein ICP0 (infected-cell polypeptide 0) of herpes simplex virus type 1 (HSV-1) is a promiscuous transactivator of both viral and nonviral promoters in transient expression assays. Failure to splice the second of two introns in the ICP0 gene results in the utilization of an alternate stop codon that generates a truncated form of ICP0 called ICP0R. This protein exists in low levels in HSV-1-infected cells and functions as a dominant negative repressor of ICP0-mediated transactivation in transient expression assays. To conduct a detailed structure-function analysis of ICP0R, a series of insertion and deletion mutants of this protein were generated and analyzed in transfection assays. These studies indicated that segments of ICP0R that were rich in acidic amino acid residues (amino acids 9 to 76 and 233 to 241) or glycine residues (amino acids 242 to 262) were dispensable for the dominant negative phenotype. In contrast, the RING finger domain (amino acids 116 to 156) and surprisingly the sequences carboxy terminal to it (amino acids 157 to 232) were absolutely essential for transdominant repression. Consistent with these findings, the amino acid sequences of these two regions were conserved among other alphaherpesvirus ICP0 homologs. A construct containing only amino acids 76 to 232 inhibited ICP0-mediated transactivation almost as efficiently as wild-type ICP0R and represented the minimal sequences necessary for the dominant negative phenotype. These results demonstrated that the critical functional domain shared by both ICP0R and ICP0 is much more complex than a simple RING finger motif. Western blot (immunoblot) analyses of transfected cell lysates revealed that nearly all of the mutant constructs directed the expression of stable ICP0R proteins of the predicted molecular weight. However, there was a striking inverse correlation between the ability of a mutant construct to mediate transrepression and the amount of protein that it synthesized, indicating that dominant negative inhibition is achieved through the action of very little ICP0R protein.

A deletion mutant in the human cytomegalovirus gene encoding IE1(491aa) is replication defective due to a failure in autoregulation

Human cytomegalovirus (CMV) replication begins with the expression of two regulatory proteins, IE1(491aa) and IE2(579aa), produced from differentially spliced transcripts under control of the ie1/ie2 promoter-enhancer. A deletion mutation removing all 406 IE1(491aa)-specific amino acids was engineered into the viral genome and this mutant (RC303 delta Acc) was propagated on an IE1(491aa)-expressing human fibroblast cell line (ihfie1.3). RC303 delta Acc failed to replicate on normal human fibroblasts at low multiplicities of infection (mois). At mois > 3 plaque-forming units per cell, virus replication and production of progeny were comparable to wild type. However, at mois between 0.01 and 1, mutant virus replicated slowly on normal fibroblasts, a pattern that suggested initiation of productive infection required multiple hits. Replication of RC303 delta Acc correlated with the ability to express IE2(579aa), consistent with a role for IE1(491aa) in positive autoregulation of the ie1/ie2 promoter-enhancer and with data suggesting that virion transactivators compensate for the lack of IE1(491aa) under high moi conditions. ie1-deficient CMV should be completely avirulent, suggesting its utility as a gene therapy vector for hematopoietic progenitors that are normal sites of CMV latency.

Prolonged gene expression and cell survival after infection by a herpes simplex virus mutant defective in the immediate-early genes encoding ICP4, ICP27, and ICP22

Very early in infection, herpes simplex virus (HSV) expresses four immediate-early (IE) regulatory proteins, ICP4, ICP0, ICP22, and ICP27. The systematic inactivation of sets of the IE proteins in cis, and the subsequent phenotypic analysis of the resulting mutants, should provide insights into how these proteins function in the HSV life cycle and also into the specific macromolecular events that are altered or perturbed in cells infected with virus strains blocked very early in infection. This approach may also provide a rational basis to assess the efficacy and safety of HSV mutants for use in gene transfer experiments. In this study, we generated and examined the phenotype of an HSV mutant simultaneously mutated in the ICP4, ICP27, and ICP22 genes of HSV. Unlike mutants deficient in ICP4 (d120), ICP4 and ICP27 (d92), and ICP4 and ICP22 (d96), mutants defective in ICP4, ICP27, and ICP22 (d95) were visually much less toxic to Vero and human embryonic lung cells. Cells infected with d95 at a multiplicity of infection of 10 PFU per cell retained a relatively normal morphology and expressed genes from the viral and cellular genomes for at least 3 days postinfection. The other mutant backgrounds were too toxic to allow examination of gene expression past 1 day postinfection. However, when cell survival was measured by the capacity of the infected cells to form colonies, d95 inhibited colony formation similarly to d92. This apparent paradox was reconciled by the observation that host cell DNA synthesis was inhibited in cells infected with d120, d92, d96, and d95. In addition, all of the mutants exhibited pronounced and distinctive alterations in nuclear morphology, as determined by electron microscopy. The appearance of d95-infected cells deviated from that of uninfected cells in that large circular structures formed in the nucleus. d95-infected cells abundantly expressed ICP0, which accumulated in fine punctate structures in the nucleus at early times postinfection and coalesced or grew to the large circular objects that were revealed by electron microscopy. Therefore, while the abundant accumulation of ICPO in the absence of ICP4, ICP22, and ICP27 may allow for prolonged gene expression, cell survival is impaired, in part, as a result of the inhibition of cellular DNA synthesis.

Point mutations in the herpes simplex virus type 1 Vmw110 RING finger helix affect activation of gene expression, viral growth, and interaction with PML-containing nuclear structures

Herpes simplex virus type 1 immediate-early protein Vmw110 (also known as ICP0) has been implicated in the control of the balance between the lytic and latent states, but the precise mechanisms by which it exerts its effects are unknown. Vmw110 includes a characteristic zinc binding domain, termed the C3HC4 domain or RING finger, which is essential for its function. The solution structure of a related herpesvirus RING finger domain suggested that an amphipathic alpha helix might be an important functional component of the RING finger. In this paper, we show that the equivalent region of Vmw110 is important for virus growth in tissue culture and for the normal interaction of Vmw110 with nuclear structures which include the PML protein.

Herpes simplex virus immediate-early protein ICP22 is required for viral modification of host RNA polymerase II and establishment of the normal viral transcription program

Infection of cells with herpes simplex virus type 1 (HSV-1) results in a rapid alteration of phosphorylation on the large subunit of cellular RNA polymerase II (RNAP II), most likely on its C-terminal domain (S. A. Rice, M. C. Long, V. Lam, C. A. Spencer, J. Virol. 68:988-1001, 1994). This phosphorylation modification generates a novel form of the large subunit which we have designed IIi. In this study, we examine roles that HSV-1 gene products play in this process. An HSV-1 mutant defective in the immediate-early transcriptional activator protein ICP4 is able to efficiently induce IIi. Viruses having mutations in the genes for the ICP0, ICP6, or ICP27 proteins are also competent for IIi formation. In contrast, 22/n199, an HSV-1 mutant which contains a nonsense mutation in the gene encoding the immediate-early protein ICP22, is significantly deficient in IIi induction. This effect is seen in Vero cells, where 22/n199 grows relatively efficiently, and in human embryonic lung (HEL) cells, where 22/n199 growth in more restricted. RNAP II is recruited into viral replication compartments in 22/n199-infected cells, indicating that altered phosphorylation of RNAP II is not a prerequisite for nuclear relocalization of RNAP II. In addition, we show by nuclear run-on transcription analysis that viral gene transcription is deficient in HEL cells infected with 22/n199. Viral late gene transcription does not occur efficiently, and antisense transcription throughout the genome is diminished compared with that of the wild-type HSV-1 infection. These transcriptional effects cannot be explained by differences in viral DNA replication, since 22/n199 replicates its DNA efficiently in HEL cells. Our results demonstrated that ICP22 is necessary for virus-induced aberrant phosphorylation of RNAP II and for normal patterns of viral gene transcription in certain cell lines.

Evaluation of colocalization interactions between the IE110, IE175, and IE63 transactivator proteins of herpes simplex virus within subcellular punctate structures

A number of previous studies have implied that three herpes simplex virus-encoded nuclear transactivator proteins, IE175 (ICP4), IE110 (ICP0), and IE63 (ICP27), may cooperate in transcriptional and posttranscriptional stimulation of viral gene expression. Using double-label immunofluorescence assays (IFA) in transient expression assays, we have examined the intracellular localization of these three proteins in DNA-transfected cells. The IE110 protein on its own forms spherical punctate domains within the nucleus, whereas the IE175 and IE63 proteins alone give uniform and speckled diffuse patterns, respectively. In infected cells, the IE110 punctate granules have been shown to correspond to novel preexisting subnuclear structures referred to as ND10 domains or PODs that contain a variety of cellular proteins, including SP100 and the PML proto-oncogene product. Cotransfection experiments with wild-type nuclear forms of both IE175 and IE110 provided direct evidence for partial redistribution of IE175 into the same punctate granules that contained IE110. Surprisingly, nuclear forms of IE110 were found to move a cytoplasmic form of IE175 into nuclear punctate structures, and a cytoplasmic form of IE110 was able to retain nuclear forms of IE175 in cytoplasmic punctate structures. Therefore, the punctate characteristic of IE110 appeared to both dominate the interactions and override the normal nuclear localization signals. The domains responsible for the interaction mapped to between codons 518 and 768 in 1E110 and to between codons 835 and 1029 in IE175. Importantly, a truncated nuclear form of the 1,298-amino-acid IE175 protein, which lacked the C-terminal domain beyond codon 834, was found to be excluded from the IE110 punctate granules. Cotransfection of nuclear or cytoplasmic IE110 with a truncated nuclear form of IE63 also led to partial redistribution of IE63 into either nuclear or cytoplasmic punctate granules containing IE110. Both the IE63-IE110 and IE175-IE110 colocalization interactions were demonstrated in Vero cells but not in 293 cells. Consequently, they differ from IE110 self-interactions, which correlate with in vitro dimerization and occur efficiently in both cell types. These interactions may help to explain the altered promoter target specificity and synergism observed when IE175 is cotransfected with IE110 in transactivation studies.

PCR-based analysis of herpes simplex virus type 1 latency in the rat trigeminal ganglion established with a ribonucleotide reductase-deficient mutant

Competitive quantitative PCR and reverse transcriptase-PCR were used to quantitate DNA and RNA from an attenuated ribonucleotide reductase-deleted herpes simplex virus type 1 (HSV-1) mutant in the rat trigeminal ganglion after peripheral inoculation following corneal scarification. Amplification of ganglionic DNA with oligonucleotide primers specific for the HSV-1 glycoprotein B (gB) gene and for the latency-associated transcript (LAT) gene indicated that there were approximately 2 x 10(5) genome equivalents per ganglion at 2 days, 7 days, and 8 weeks after inoculation. Amplification of ganglionic RNA with primers specific for HSV-1 LAT indicated that the amount of LAT RNA was also stable over 8 weeks, with 10(7) LAT molecules per ganglion at 2 days and at 7 days postinoculation and 1.4 x 10(7) LAT molecules per ganglion at 8 weeks. In situ hybridization with a digoxigenin-labeled riboprobe specific for LAT detected an average of one to two LAT-positive cells in each positive 6-microns section of trigeminal ganglion. In situ PCR detection of HSV-1 genomes in similar sections, using digoxigenin-labeled nucleotides with primers specific for HSV-1 gB, identified as many as 120 genome-positive cells per section. These results indicate that there are approximately 50 LAT molecules per latent HSV-1 genome in the trigeminal ganglion, compared with 15 LAT molecules per latent HSV-1 genome in the central nervous system (R. Ramakrishnan, D. J. Fink, G. Jiang, P. Desai, J. C. Glorioso, and M. Levine, J. Virol. 68:1864-1873, 1994), but that cells with detectable LATs by in situ hybridization represent only a small proportion of those ganglionic neurons containing HSV-1 genomes. The presence of latent HSV-1 genomes in a large number of neurons suggests that HSV-1 may be more efficient in establishing the latent state than would be anticipated from previous reports.

HSV-1 IE protein Vmw110 causes redistribution of PML

Herpes simplex virus immediate-early protein Vmw110 is required for fully efficient viral gene expression and reactivation from latency. At early times of viral infection, Vmw110 localizes to discrete nuclear structures (known as ND10, PODs or Kr bodies) which contain several cellular proteins, including PML. Interestingly, the unregulated growth of promyelocytic leukaemia cells is correlated with disruption of the normal state of ND10. In this paper we show that: (i) Vmw110 affects the distribution of PML in the cell; (ii) Vmw110 proteins lacking a functional RING finger zinc-binding domain cause the production of striking abnormal cytoplasmic and nuclear structures, some of which contain PML and other ND10 antigens; (iii) a mutant form of Vmw110 which is confined to the cytoplasm appears to result in cytoplasmic PML in some cells; (iv) normal interaction with the nuclear structures requires the C-terminal portion of Vmw110; (v) the C-terminal portion of Vmw110, when linked to a heterologous protein, disrupts the normal distribution of PML. The results suggest that, in normal cells, the PML protein migrates between nucleus and cytoplasm. These observations present an unexpected link between processes involved in the control of cell growth and viral infection and latency.

A novel class of transcripts expressed with late kinetics in the absence of ICP4 spans the junction between the long and short segments of the herpes simplex virus type 1 genome

A novel family of transcripts that span the junction between the long and short segments of the herpes simplex virus type 1 genome has been identified. These transcripts, designated L/S junction-spanning transcripts (L/STs), are synthesized in abundance in a variety of cells infected with mutant viruses defective in the gene for ICP4, the major transcriptional regulatory protein of the virus. Transcription of abundant 2.3- and 8.5-kb series of L/STs was shown to initiate within the same sequences as less abundant 4.2-, 7.3-, and > 9.5-kb transcripts by Northern (RNA) blot analysis. S1 nuclease analysis revealed a single 5' terminus 28 bp downstream of a TATA box and 6 bp downstream of a consensus ICP4 binding site. The location of the transcriptional start site indicates that the promoter of the L/STs likely corresponds to the bidirectional promoter described by Bohenzky et al. (R. A. Bohenzky, A. G. Papavassiliou, I. H. Gelman, and S. Silverstein, J. Virol. 67:632-642, 1993). The L/STs accumulate with late kinetics in ICP4 mutant-infected cells and are polyadenylated. Mutant viruses encoding forms of ICP4 unable to bind the consensus site, ATCGTC, exhibited abundant expression of the L/STs, whereas mutants encoding forms of ICP4 able to bind this site expressed no detectable L/STs, suggesting that ICP4 plays a critical role in repressing L/ST expression. Their synthesis in ICP4 mutant-infected cells is inhibited by the protein synthesis inhibitor cycloheximide, indicating that they are induced either by an immediate-early viral protein other than ICP4 or by a virus-induced cellular protein. Preliminary evidence indicates that the L/STs are not present in latently infected ganglia. The abundant expression of the L/STs with late kinetics only in the absence of functional ICP4 and the sensitivity of their synthesis to cycloheximide indicate that they are not members of any of the recognized kinetic classes of herpes simplex virus type 1 transcripts but constitute a new class of viral transcript.