Introduction

Primary infection with HSV-1 or HSV-2 results in productive replication of the virus at the site of infection, following the pattern of gene expression described elsewhere in this volume. During this initial phase, virus enters sensory neurons via their termini and retrograde transport takes the genome to the neuronal nuclei in the sensory ganglia that innervate the infected dermatome. At early times after infection, virus replication occurs in ganglionic neurons but within a few days no virus can be detected. The genome, however, persists in neurons in a latent state from which it reactivates periodically to resume replication and produce infectious virus. This reactivation event may be “spontaneous” but is generally thought to be provoked by stress stimuli that act on the neuron, or at a peripheral site innervated by the infected ganglion, or systemically. Three phases of latency are recognized. Establishment occurs during the period following primary infection, and although virus replication can be detected in a proportion of neurons during this phase, the initiation and normal progression of productive infection and cell death is arrested in those neurons destined to become latently infected. Unravelling the way in which the seemingly inexorable progression of the gene expression program is blocked constitutes a major challenge for the molecular virologist. The maintenance phase of latency is characterized by the lifelong retention of the HSV genome in a silent state, characterized by repression of all viral lytic genes. One region, encoding the latency-associated transcripts (LATs), remains active during latency. Questions relating to the maintenance of the latent state focus on the structure of the genome, the mechanisms that silence it, and the specific properties of the LAT transcription unit that enable it to remain active. During the reactivation phase the silent genome responds to cellular signals that provoke the resumption of viral gene expression. The molecular basis for this dramatic functional reversal is poorly understood and is the subject of considerable research effort. In view of the specific association of LAT with the latent state, questions concerning the role of this transcript pervade all aspects of latency.

Model systems to study latency

Animal systems provide the most relevant means of studying HSV latency. In mice, latency can be established efficiently and relatively reproducibly after inoculation with HSV-1 in the cornea, ear, footpad, or at other peripheral sites. The virus replicates at the site of inoculation, and as a consequence the exact dose applied to nerve termini is undefined and changing over the first few days. These factors complicate quantitative aspects of latency studies. Reactivation is difficult to achieve in mice and most investigations have relied on explantation of ganglia, with subsequent culture in the laboratory, to recover virus (Stevens and Cook, 1971). An in vivo protocol has been developed in which reactivation is achieved by exposure of mice to transient hyperthermia (Sawtell and Thompson, 1992b). While this method is inefficient in terms of the number of reactivation events per animal, it represents the most relevant mouse model currently available for paralleling latency in humans. In the rabbit, inoculation of the cornea results in latency that, for certain strains of HSV-1 such as McKrea, is characterized by long-term periodic virus shedding, described as spontaneous reactivation. Production of virus can be enhanced by procedures such as iontophoresis of epinephrine into the eye. For HSV-2, inoculation into the guinea pig vagina results in latency in which virus is periodically shed to form lesions that can be scored. Therefore, the rabbit and guinea pig provide reasonable models for HSV-1 and HSV-2, respectively, in humans but they are more difficult and expensive to use than the mouse. All animal systems entail inoculation with relatively high virus doses, often just short of fatal, and it is by no means clear how this relates to natural infection. A further problem is the considerable variation in the efficiency of establishment of latency and reactivation between strains of HSV, a factor that frequently causes confusion when comparisons are made between the results from different laboratories.

As a more tractable system, infection of cultured neurons has been investigated (Wilcox and Johnson, 1988; Arthur et al., 2001). These cells are susceptible to lytic infection with HSV, but if measures are taken to prevent virus replication, long-term retention of the viral genome can be achieved. LAT can be detected in a proportion of neurons, but all other viral genes are repressed. Production of virus can be induced by removal of nerve growth factor (NGF), inhibition of histone deacetylases, or various treatments that activate signal transduction pathways (Smith et al., 1992). Alternatively, cultures may be made from ganglia dissected from mice harboring latent HSV; in this case, heat shock or treatment with dexamethasone are the most effective reactivation stimuli (Halford et al., 1996).

The final type of model involves the infection of standard tissue culture cells, usually human fibroblasts, with HSV-1 mutants that are impaired for immediate early (IE) gene expression and thus do not kill cells (Preston and Nicholl, 1997; Samaniego et al., 1998). The viral genome is retained in a quiescent state in which all gene expression, including that of LAT, is repressed. The only known way of reactivating quiescent virus is to provide the HSV IE protein ICP0 by superinfection of cultures. Fibroblast systems may mimic some, but certainly not all, aspects of latency.

The latent genome

Latent HSV DNA does not contain detectable termini and almost certainly exists as a circular episome, in contrast to the linear state in the virus particle (Rock and Fraser, 1983; Efstathiou et al., 1986). Quiescent genomes stably retained in fibroblasts are also circular (Jamieson et al., 1995; Jackson and DeLuca, 2003). Various methods of quantifying viral DNA load revealed that latently infected neurons must contain, on average, many more than one HSV genome copy per infected cell. This conclusion has been verified by the use of “contextual analysis” (CXA), in which individual neurons or small groups of cells are separated and analyzed by polymerase chain reaction (PCR) (Sawtell, 1997). The latent viral genome copy number varied generally between 1 and 100, but a small proportion of neurons contained more than 1000 viral DNA molecules per cell. Likely, the retention of such high copy numbers has an influence on neuronal physiology, and recent studies have shown that latently infected ganglia contain increased levels of certain cellular gene products (Kramer et al., 2003). Furthermore, analysis of latent DNA at a gross level is skewed towards the few neurons containing thousands of viral genomes. Surprisingly, for reasons that are not understood, latent viral DNA cannot be detected by in situ hybridization (ISH), therefore in situ PCR has been applied to investigate the number of neurons that harbor HSV genomes. This approach shows that many more cells contain DNA than are detected by ISH for LAT; thus, LAT is not an unambiguous marker for latent HSV. Laser capture microdissection, in which individual neurons are excised and analyzed by PCR, confirmed that viral genomes can be isolated from LAT-negative (LAT-) neurons and essentially agreed with the quantification from CXA (Chen et al., 2002). The conclusion that there is a population of latently infected neurons that does not express LAT may depend on the sensitivity of ISH, since a study using in situ RT-PCR suggested that LAT was present in all HSV DNA-containing neurons, albeit at low concentration in many (Ramakrishnan et al., 1996).

In cells, silencing of large gene blocks occurs at the level of chromatin structure, and it is therefore suspected that an organization of this type applies to the latent viral genome. One study has addressed this issue and found that all regions of HSV DNA examined, including the LAT region, exhibit a regular nucleosomal pattern in mouse brain stem (Deshmane and Fraser, 1989). Interpretation of this result is complicated by the fact that reactivation from brain stem is inefficient and, unfortunately, it was not possible to obtain sufficient material from trigeminal ganglia for similar analyses. More recently, the application of chromatin immunoprecipitation (ChiP) assays has demonstrated the importance of histone modifications in the maintenance of latency. It is well established that post-translational modification of the amino terminal tails of histones is involved in the regulation of transcription. Thus hyperacetylation of histones is generally associated with an “open chromatin” conformation and transcriptional activity, whilst histone hypoacetylation is associated with condensed chromatin and gene silencing. Recent work on HSV-1 suggests that chromatinization of the viral genome and certain accompanying histone modifications offer a means to regulate virus gene expression during lytic infection (Herrera and Triezenberg, 2004; Kent et al., 2004). In the context of latency it is of particular significance that ChiP assays have shown the LAT promoter to be enriched with acetylated histone H3 whilst representative lytic cycle promoters exhibit a decreased association with acetylated histones (Kubat et al., 2004a,b). The demonstration that enrichment of acetylated histones on the ICP0 promoter following the application of a reactivation stimulus by ganglionic explantation strongly supports the view that genome de-repression is linked to the acetylation status of histones positioned on lytic cycle promoters (Amelio et al., 2006). Furthermore, it has been shown that a LAT-mutant exhibits enrichment of histone modifications associated with transcriptional activation during latency, suggesting that LAT-encoded functions facilitate maintenance of a repressed chromatinized genome (Wang et al., 2005). Since HSV replication in ganglia precedes latency, it has long been suspected that some viral genomes are derived from residual replication intermediates rather than from virions delivered from the periphery. During infection with TK-mutants, which replicate at the site of inoculation but not in neurons, high copy number retention of TK-virus genomes is possible and therefore some neurons can receive hundreds of viruses from the periphery (Thompson and Sawtell, 2000). In general, however, TK-mutants deposit less latent DNA than wild type virus. However, depending on the site of inoculation, TK-mutants replicate less efficiently peripherally. Thus, apparently normal latency can be established without viral replication in neurons.

The latency-associated transcripts

The only transcripts detectable during latency are the LATs, which map to the viral repeats flanking U_L (Fig. 33.1). These have been detected in latently infected neuronal tissues from experimentally infected animals and following natural infection in humans (Stevens et al., 1987). Similar transcripts are synthesized during latent infection by HSV-2 and other alphaherpesviruses such as bovine herpesvirus-1 (BHV-1) and pseudorabies virus.

Fig. 33.1

Location and organization of HSV-1 LATs. The IR_L/IR_S region of the genome is expanded in part (a). In addition to the LATs, the positions of lytic cycle genes and a set of transcripts known as L/STs, which specify ORFs O and P, are shown. The functions (more...)

Establishment of latency

Specific features of the neuron must be crucial for the interruption of the normal gene expression program during the establishment of latency. The point of arrest in neurons is not known at present, but a number of possibilities, not necessarily mutually exclusive, have been proposed. A description that includes all of the current data and hypotheses in a consistent manner cannot be presented, thus the concepts that are currently favored will be considered separately.

Maintenance of latency

The stability of the latent state, together with the failure to detect viral gene expression apart from that of LAT, supports the concept that the majority of the genome is in a silent state that can be reversed only by specific triggers. Studies in the mouse using sensitive RT-PCR, however, demonstrated that transcripts from the ICP4 and TK regions of the genome could be detected in ganglia during latency (Kramer et al., 1998). This observation is supported by experiments in which sections from many mouse ganglia were analyzed by ISH (Feldman et al., 2002). Approximately one neuron per 10 sections was found to be positive for transcripts representing the lytic genes ICP4, TK and glycoprotein C. In addition, antigen positive neurons were detected at approximately the same frequency, and these cells were surrounded by an immune infiltrate. The most reasonable explanation for the results is that a few neurons support viral gene expression in the mouse, a view that is supported by the finding that interferon gamma and CD8+ T cells are present in murine ganglia at latent times, suggesting that active immune surveillance may operate to maintain latency (Cantin et al., 1995; Khanna et al., 2003). Further discussion of these results is given elsewhere in this volume. Therefore, although the majority of latent genomes are retained in an untranscribed state, the possibility exists that some neurons express HSV-specific proteins and are prevented from producing virus by host immune responses.

Reactivation

Since viral gene products characteristic of the lytic cycle cannot, in general, be detected in latently infected neurons, cellular mechanisms must be important for reactivation. The crucial cellular events are not understood at the molecular level and are still vaguely described as applying ‘stress’ to the neuron. Furthermore, the models for reactivation may rely on very different cellular stimuli and hence the mechanisms involved may vary between both animals and systems. For instance, explantation is probably a more severe stress than in vivo treatments. A further serious complication arises from the fact that reactivation is an inefficient process with only a small proportion of the neurons that harbor viral genomes responding by production of virus. This means that the genomes detected at a gross level during latency may not represent those able to reactivate, with the latter possibly forming a small subset of the total. In addition, in a comparison between HSV-1 strains that differ in their abilities to respond to hyperthermia in vivo, the efficiency of reactivation correlated with the genome copy number distribution but not the number of neurons harboring latent virus (Sawtell, 1998). Therefore, the neurons containing large amounts of HSV DNA may be more susceptible to reactivation stimuli in vivo.

Models for reactivation depend critically on understanding the mechanism of establishment of latency. Thus, if the view is taken that a block in IE transcription leads to establishment, the route to reactivation can be subdivided into two basic concepts, depending on the consequences of the IE block. If, as in fibroblast models, failure of IE gene expression results in conversion of the genome into a quiescent state that is disrupted by ICP0 but is unresponsive to changes in cell physiology such as activation of signal transduction pathways, it follows that reactivation must be provoked either by the action of cellular proteins that mimic the activity of ICP0 or by induction of ICP0 synthesis. An alternative, more popular, view is that viral promoters are not repressed thoroughly, as in fibroblasts, but are inactive and potentially responsive to cellular signals provided by reactivation stimuli. The two models overlap if the genome is generally repressed but the ICP0 promoter specifically escapes repression. In this case, reactivation stimuli would initially be targeted to the ICP0 promoter, with the subsequent reversal of genome repression by the ICP0 protein. A role for LAT in reactivation is suggested by a number of experimental observations, although the interpretation of the data again depends on the events leading to establishment.

The ICP0 promoter as a possible target for reactivation signals

The ICP0 promoter has many motifs, in addition to the TAATGARATs, that bind transcription factors, and these sites might be targets for reactivation stimuli (Fig. 33.2). Nucleotides –79 to –97 bind Olf-1, a neuron-specific factor that activates transcription (Devireddy and Jones, 2000), and sequences between –74 and –89 are recognized by two proteins, NF-Y and one of unknown identity (named F1), in the human neuroblastoma line IMR-32 (O’Rourke and O’Hare, 1993). Gene array analysis demonstrated that explantation of ganglia induces the synthesis of Bcl-3 (as well as other gene products) in neurons (Tsavachidou et al., 2001). This is interesting because Bcl-3 associates with a dimer of the p50 subunit of NF-κB, and the ICP0 promoter contains NF-κB consensus binding sites at –51 and –273. Phosphorylation influences the binding of Bcl-3 to p50, thus explantation may activate kinases that promote the interaction of these proteins. Bcl-3 also interacts with Tip60, a histone acetylase, and therefore may mediate its effects by recruiting this protein and modifying chromatin structure at the ICP0 promoter. In ganglia, HCF is found in the cytoplasm of neurons but is transported to the nucleus within 20 minutes of explantation (Kristie et al., 1999). If the HSV genome, or strategic regions such as the ICP0 promoter, is available for transcription then HCF, by virtue of its intrinsic activation domain, could trigger the viral gene expression program. This hypothesis requires that a mechanism exists for localizing HCF to the promoter, and binding to Oct-1 is the obvious candidate. However, HCF interacts with the ETS family member GABP, which binds to motifs of consensus CGGAAR (Vogel and Kristie, 2000). There are GABP recognition sites in the ICP0 promoter, and these might direct activation of transcription through a GABP/HCF complex. Another HCF-binding protein, Luman, is cytoplasmic in tissue culture cells but is, in essence, a basic leucine zipper transcription factor of the ATF/CREB family that can bind to CREs and activate transcription in an HCF-dependent manner (Lu and Misra, 2000). In tissue culture cells, Luman is released from the ER by the action of the site 1 protease, an enzyme that catalyzes the regulated intramembrane proteolysis (RIP) of membrane-bound transcription factors, releasing them for transport to the nucleus and activation of transcription (Raggo et al., 2002). Various stress stimuli trigger RIP, thus reactivation signals may result in the release of Luman from the ER of neurons, thereby relocating a complex of this protein plus HCF to the nucleus. The proposed site of action is a CRE at position –67 in the ICP0 promoter.

Fig. 33.2

Binding sites for transcription factors in the ICP0 promoter. The region to nucleotide −400 is shown, with the −100 to −30 sequences expanded. The major TAATGARAT element (TG), and two TAATGARAT homologies that have not been demonstrated (more...)

Hypotheses on the significance of transcription factor binding to the ICP0 promoter must include the possibility that such binding may be relevant to replication in neurons at the early stages of infection rather than to reactivation. In addition, all of the above ideas must take account of studies on an HSV-1 mutant deleted for nucleotides –70 to –420 in the ICP0 promoter, thus lacking most of the factor-binding sequences mentioned above (Davido and Leib, 1996). This mutant established latency and displayed normal reactivation efficiency in the explantation system even though replication in cell culture was impaired, suggesting that the region between –70 and –420 in the ICP0 promoter, which includes most of the important known elements controlling expression in cell culture systems, does not contain critical target sequences for explant reactivation of latent virus. Notably, the NF-κB binding site at –51 and the CRE at –67 lie outwith the dispensable region.

Concluding remarks

Latency is clearly very complex at the molecular level, and the difficulties inherent in the model systems ensure that it will not be unraveled easily. The non-uniformity of latency, in terms of viral genome copy number, LAT expression and nature of reactivation stimulus, may be of fundamental benefit to the virus. If latency was uniform, a single stimulus might induce reactivation in the entire latent reservoir and result in clearance of the virus from the host. Perhaps the different virus/cell interactions respond to different host signals, explaining why it has been so difficult to arrive at a simple model for the molecular basis of latency.

Acknowledgments

We thank Valerie Preston for constructive comments on the manuscript, and Robin Lachmann for help with Figure 33.1.

References

• Ahmed M., Fraser N. W. Herpes simplex virus type 1 2-kilobase latency-associated transcript intron associates with ribosomal proteins and splicing factors. J. Virol. 2001;75:12070–12080. [PMC free article: PMC116102] [PubMed: 11711597]
• Ahmed M., Lock M., Miller C. G., Fraser N. W. Regions of the herpes simplex virus type 1 latency-associated transcript that protect cells from apoptosis in vitro and protect neuronal cells in vivo. J. Virol. 2002;76:717–729. [PMC free article: PMC136840] [PubMed: 11752162]
• Akhova O., Bainbridge M., Misra V. The neuronal host cell factor-binding protein Zhangfei inhibits herpes simplex virus replication. J. Virol. 2005;79:14708–14718. [PMC free article: PMC1287584] [PubMed: 16282471]
• Amelio A. L., Giordani N. V., Kubat N. J., O’Neil J. E., Bloom D. C. Deacetylation of the herpes simplex virus type 1 latency-associated transcript (LAT) enhancer and a decrease in LAT abundance precede an increase in ICP0 transcriptional permissiveness at early times postexplant. J. Virol. 2006;80:2063–2068. [PMC free article: PMC1367155] [PubMed: 16439563]
• Arthur J., Efstathiou S., Simmons A. Intranuclear foci containing low abundance herpes simplex virus latency-associated transcripts visualized by non-isotopic in situ hybridization. J. Gen. Virol. 1993;74:1363–1370. [PubMed: 8393077]
• Arthur J. L., Scarpini C. G., Connor V., Lachmann R. H., Tolkovsky A. M., Efstathiou S. Herpes simplex virus type 1 promoter activity during latency establishment, maintenance, and reactivation in primary dorsal root neurons in vitro. J. Virol. 2001;75:3885–3895. [PMC free article: PMC114879] [PubMed: 11264377]
• Berthomme H., Thomas J., Texier P., Epstein A., Feldman L. T. Enhancer and long-term expression functions of herpes simplex virus type 1 latency-associated promoter are both located in the same region. J. Virol. 2001;75:4386–4393. [PMC free article: PMC114183] [PubMed: 11287587]
• Bloom D. C., Hill J. M., Devi-Rao G. B., Wagner E. K., Feldman L. T., Stevens J. G. A 348-base-pair region in the latency-associated transcript facilitates herpes simplex virus type 1 reactivation. J. Virol. 1996;70:2449–2459. [PMC free article: PMC190088] [PubMed: 8642650]
• Bloom D. C., Stevens J. G., Hill J. M., Tran R. K. Mutagenesis of a cAMP response element within the latency-associated transcript promoter of HSV-1 reduces adrenergic reactivation. Virology. 1997;236:202–207. [PubMed: 9299632]
• Boutell C., Sadis S., Everett R. D. Herpes simplex virus type 1 immediate-early protein ICP0 and its isolated RING finger domain act as ubiquitin E3 ligases in vitro. J. Virol. 2002;76:841–850. [PMC free article: PMC136846] [PubMed: 11752173]
• Burton E. A., Hong C. S., Glorioso J. C. The stable 2.0-kilobase intron of the herpes simplex virus type 1 latency-associated transcript does not function as an antisense repressor of ICP0 in nonneuronal cells. J. Virol. 2003;77:3516–3530. [PMC free article: PMC149500] [PubMed: 12610127]
• Cantin E. M., Hinton D. R., Chen J. D., Openshaw H. Gamma interferon expression during acute and latent nervous system infection by herpes simplex virus type 1. J. Virol. 1995;69:4898–4905. [PMC free article: PMC189304] [PubMed: 7609058]
• Chen X., Schmidt M. C., Goins W. F., Glorioso J. C. Two herpes simplex virus type 1 latency-active promoters differ in their contributions to latency-associated transcript expression during lytic and latent infections. J. Virol. 1995;69:7899–7908. [PMC free article: PMC189734] [PubMed: 7494302]
• Chen X.-P., Li J., Mata M., Goss J., Wolfe D., Glorioso J. C., Fink D. J. Herpes simplex virus type 1 ICP0 protein does not accumulate in the nucleus of primary neurons in culture. J. Virol. 2000;74:10132–10141. [PMC free article: PMC102052] [PubMed: 11024142]
• Chen X.-P., Mata M., Kelley M., Glorioso J. C., Fink D. J. The relationship of herpes simplex virus latency associated transcript expression to genome copy number: a quantitative study using laser capture microdissection. Journal of Neurovirology. 2002;8:204–210. [PubMed: 12053275]
• Cleary M. A., Stern S., Tanaka M., Herr W. Differential positive control by Oct-1 and Oct-2 – activation of a transcriptionally silent motif through Oct-1 and VP16 recruitment. Genes Dev. 1993;7:72–83. [PubMed: 8422989]
• Cui C., Griffiths A., Li G. L., Silva L. M., Kramer M. F., Gaasterland T., Wang X. J., Coen D. M. Prediction and identification of herpes simplex virus 1-encoded microRNAs. J. Virol. 2006;80:5499–5508. [PMC free article: PMC1472173] [PubMed: 16699030]
• Colgin M. A., Smith R. L., Wilcox C. L. Inducible cyclic AMP early repressor produces reactivation of latent herpes simplex virus type 1 in neurons in vitro. J. Virol. 2001;75:2912–2920. [PMC free article: PMC115917] [PubMed: 11222716]
• Davido D. J., Leib D. A. Role of cis-acting sequences of the ICP0 promoter of herpes simplex virus type 1 in viral pathogenesis, latency and reactivation. J. Gen. Virol. 1996;77:1853–1863. [PubMed: 8760437]
• Davido D. J., Leib D. A., Schaffer P. A. The cyclin-dependent kinase inhibitor roscovitine inhibits the transactivating activity and alters the posttranslational modification of herpes simplex virus type 1 ICP0. J. Virol. 2002;76:1077–1088. [PMC free article: PMC135868] [PubMed: 11773384]
• Deshmane S. L., Fraser N. W. During latency, herpes simplex virus type 1 DNA is associated with nucleosomes in a chromatin structure. J. Virol. 1989;63:943–947. [PMC free article: PMC247770] [PubMed: 2536115]
• Devireddy L. R., Jones C. J. Olf-1, a neuron-specific transcription factor, can activate the herpes simplex virus type 1-infected cell protein 0 promoter. J. Biol. Chem. 2000;275:77–81. [PubMed: 10617588]
• Dobson A., Sederati F., Devi-Rao G., Flanagan W. M., Farrell M., Stevens J., Wagner E., Feldman L. T. Identification of the latency-associated transcript promoter by expression of rabbit beta-globin mRNA in sensory nerve ganglia latently infected with a recombinant herpes simplex virus. J. Virol. 1989;63:3844–3851. [PMC free article: PMC250978] [PubMed: 2474674]
• Drolet B. S., Perng G. C., Cohen J., et al. The region of herpes simplex virus type 1 LAT gene involved in spontaneous reactivation does not encode a functional protein. Virology. 1998;242:221–232. [PubMed: 9501054]
• Efstathiou S., Minson A. C., Field H. J., Anderson J. R., Wildy P. Detection of herpes simplex virus-specific DNA sequences in latently infected mice and in humans. J. Virol. 1986;57:446–455. [PMC free article: PMC252756] [PubMed: 3003377]
• Everett R. D. ICP0, a regulator of herpes simplex virus during lytic and latent infection. Bioessays. 2000;22:761–770. [PubMed: 10918307]
• Feldman L. T., Ellison A. R., Voytek C. C., Yang L., Krause P., Margolis T. Spontaneous molecular reactivation of herpes simplex virus type 1 latency in mice. Proc. Natl Acad. Sci. USA. 2002;99:978–983. [PMC free article: PMC117416] [PubMed: 11773630]
• Frazier D. P., Cox D., Godshalk E. M., Schaffer P. A. Identification of cis-acting sequences in the promoter of the herpes simplex virus type 1 latency-associated transcripts required for activation by nerve growth factor and sodium butyrate in PC12 cells. J Virol. 1996;70:7433–7444. [PMC free article: PMC190810] [PubMed: 8892861]
• Freiman R. N., Herr W. Viral mimicry: common mode of association with HCF by VP16 and the cellular protein LZIP. Genes Dev. 1997;11:3122–3127. [PMC free article: PMC316754] [PubMed: 9389645]
• Garber D. A., Schaffer P. A., Knipe D. M. A LAT-associated function reduces productive-cycle gene expression during acute infection of murine sensory neurons with herpes simplex virus type 1. J. Virol. 1997;71:5885–5893. [PMC free article: PMC191844] [PubMed: 9223478]
• Gu H., Liang Y., Mandel G., Roizman B. Components of the REST/CoREST/histone deacetylase repressor complex arre disrupted, modified, and translocated in HSV-1-infected cells. Proc Natl Acad Sci U S A. 2005;102:7571–7576. [PMC free article: PMC1140450] [PubMed: 15897453]
• Gupta A., Gartner J. J., Sethupathy P., Hatzigeorgiou A. G., Fraser N. W. Anti-apoptotic function of a microRNA encoded by the HSV-1 latency-associated transcript. Nature. 2006;442:82–85. [PubMed: 16738545]
• Hagmann M., Georgiev O., Schaffner W., Douville P. Transcription factors interacting with herpes simplex virus α gene promoters in sensory neurons. Nucl. Acids Res. 1995;23:4978–4985. [PMC free article: PMC307502] [PubMed: 8559654]
• Halford W. P., Schaffer P. A. ICP0 is required for efficient reactivation of herpes simplex virus type 1 from neuronal latency. J. Virol. 2001;75:3240–3249. [PMC free article: PMC114117] [PubMed: 11238850]
• Halford W. P., Gebhardt B. M., Carr D. J. Mechanisms of herpes simplex virus type 1 reactivation. J. Virol. 1996;70:5051–5060. [PMC free article: PMC190459] [PubMed: 8764012]
• Herrera F. J., Triezenberg S. J. VP16-dependent association of chromatin-modifying coactivators and underrepresentation of histones at immediate-early gene promoters during herpes simplex virus infection. J. Virol. 2004;78:9689–9696. [PMC free article: PMC515004] [PubMed: 15331701]
• Inman M., Perng G. -C., Henderson G., Ghiasi H., Nesburn A. B., Wechsler S. L., Jones C. Region of herpes simplex virus type 1 latency-associated transcript sufficient for wild-type spontaneous reactivation promotes cell survival in tissue culture. J. Virol. 2001;75:3636–3646. [PMC free article: PMC114855] [PubMed: 11264353]
• Jackson S. A., Luca N. A. and De2003Relationship of herpes simplex virus genome configuration to productive and persistent infections Proc. Natl Acad. Sci. USA 1007871–7876. [PMC free article: PMC164680] [PubMed: 12796511]
• Jamieson D. R. S., Robinson L. H., Daksis J. I., Nicholl M. J., Preston C. M. Quiescent viral genomes in human fibroblasts after infection with herpes simplex virus Vmw65 mutants. J. Gen. Virol. 1995;76:1417–1431. [PubMed: 7782770]
• Jin L., Perng G. C., Mott K. R., et al. A herpes simplex virus type 1 mutant expressing a baculovirus inhibitor of apoptosis gene in place of latency-associated transcript has a wild-type reactivation phenotype in the mouse. J. Virol. 2005;79:12286–12295. [PMC free article: PMC1211541] [PubMed: 16160155]
• Kent J. R., Zeng P.-Y., Atanasiu D., Gardner J., Fraser N. W., Berger S. L. During lytic infection herpes simplex virus type 1 is associated with histones bearing modifications that correlate with active transcription. J. Virol. 2004;78:10178–10186. [PMC free article: PMC514973] [PubMed: 15331750]
• Khanna K. M., Bonneau R. H., Kinchington P. R., Hendricks R. L. Herpes simplex virus -specific memory CD8+ T cells are selectively activated and retained in latently infected sensory ganglia. Immunity. 2003;18:593–603. [PMC free article: PMC2871305] [PubMed: 12753737]
• Kramer M. F., Chen S. -H., Knipe D. M., Coen D. M. Accumulation of viral transcripts and DNA during establishment of latency by herpes simplex virus. J. Virol. 1998;72:1177–1185. [PMC free article: PMC124594] [PubMed: 9445016]
• Kramer M. F., Cook W. J., Roth F. P., et al. Latent herpes simplex virus infection of sensory neurons alters neuronal gene expression. J. Virol. 2003;77:9533–9541. [PMC free article: PMC187408] [PubMed: 12915567]
• Kristie T. M., Roizman B. Differentiation and DNA contact points of the host proteins binding at the cis site for virion-mediated induction of herpes simplex virus 1 α genes. J. Virol. 1988;62:1145–1157. [PMC free article: PMC253122] [PubMed: 2831377]
• Kristie T. M., Vogel J. L., Sears A. E. Nuclear localization of the C1 factor (host cell factor) in sensory neurons correlates with reactivation of herpes simplex virus from latency. Proc. Natl Acad. Sci. USA. 1999;96:1229–1233. [PMC free article: PMC15445] [PubMed: 9990006]
• Kubat N. J., Amelio A. L., Giordani N. V., Bloom D. C. The herpes simplex virus type 1 latency-associated transcript (LAT) enhancer/rcr is hyperacetylated during latency independently of LAT transcription. J. Virol. 2004a;78:12508–12518. [PMC free article: PMC525101] [PubMed: 15507638]
• Kubat N. J., Tran R. K., McAnany P. K., Bloom D. C. Specific histone tail modification and not DNA methylation is a determinant of herpes simplex virus type 1 latent gene expression. J. Virol. 2004b;78:1139–1149. [PMC free article: PMC321404] [PubMed: 14722269]
• Lachmann R. H., Efstathiou S. Utilization of the herpes simplex virus type 1 latency-associated regulatory region to drive stable reporter gene expression in the nervous system. J. Virol. 1997;71:3197–3207. [PMC free article: PMC191452] [PubMed: 9060683]
• Latchman D. S. POU family transcription factors in the nervous system. J. Cell. Physiol. 1999;80:1271–1282. [PubMed: 10199551]
• Lociano R. L., Wilson A. C. An activation domain in the C-terminal subunit of HCF-1 is important for transactivation by VP16 and LZIP. Proc. Natl Acad. Sci. USA. 2002;99:13403–13408. [PMC free article: PMC129685] [PubMed: 12271126]
• Lu R., Misra V. Potential role for luman, the cellular homologue of herpes simplex virus VP16 (alpha gene trans-inducing factor), in herpesvirus latency. J. Virol. 2000;74:934–943. [PMC free article: PMC111614] [PubMed: 10623756]
• Lomonte P., Thomas J., Texier P., Caron C., Khochbin S., Epstein A. L. Functional interaction between class Ⅱ histone deacetylases and ICP0 of herpes simplex type 1. J. Virol. 2004;78:6744–6757. [PMC free article: PMC421675] [PubMed: 15194749]
• Mador N., Goldenberg D., Cohen O., Panet A., and Steiner I., (1998Herpes simplex virus type 1 latency-associated transcripts suppress viral replication and reduce immediate-early gene mRNA levels in a neuronal cell line J. Virol. 725067–5075. [PMC free article: PMC110070] [PubMed: 9573277]
• Margolis T., Sederati F., Dobson A. T., Feldman L. T., Stevens J. G. Pathways of viral gene expression during acute neuronal infection with HSV-1. Virology. 1992;189:150–160. [PubMed: 1604806]
• Marquart M. E., Zheng X., Tran R. K., Thompson H. W., Bloom D. C., Hill J. M. A cAMP response element within the latency-associated transcript promoter of HSV-1 facilitates induced ocular reactivation in a mouse hyperthermia model. Virology. 2001;284:62–69. [PubMed: 11352668]
• Marshall K. R., Lachmann R. H., Efstathiou S., Rinaldi A., Preston C. M. Long-term transgene expression in mice infected with a herpes simplex virus type 1 mutant severely impaired for immediate-early gene expression. J. Virol. 2000;74:956–964. [PMC free article: PMC111616] [PubMed: 10623758]
• Nicosia M., Deshmane S. L., Zabolotny J. M., Valyi-Nagy T., Fraser N. W. Herpes simplex virus type 1 latency-associated transcript (LAT) promoter deletion mutants can express a 2-kilobase transcript mapping to the LAT region. J. Virol. 1993;67:7276–7283. [PMC free article: PMC238191] [PubMed: 8230451]
• O’Reilly D., Hanscombe O., O’Hare P. A single serine residue at position 375 of VP16 is critical for complex assembly with Oct-1 and HCF and is a target of phosphorylation by casein kinase Ⅱ EMBO J. 1997;16:2420–2430. [PMC free article: PMC1169842] [PubMed: 9171355]
• O’Rourke D., O’Hare P. Mutually exclusive binding of two cellular factors within a critical promoter region of the gene for the IE110k protein of herpes simplex virus. J. Virol. 1993;67:7201–7214. [PMC free article: PMC238182] [PubMed: 8230442]
• Perng G., Ghiasi H., Slanina S., Nesburn A., Wechsler S. The spontaneous reactivation function of the herpes simplex virus type 1 LAT gene resides completely within the first 1.5 kilobases of the 8.3- kilobase primary transcript. J. Virol. 1996;70:976–984. [PMC free article: PMC189902] [PubMed: 8551638]
• Perng G., Jones C., Ciacci-Zanella J., et al. Virus-induced neuronal apoptosis blocked by the herpes simplex virus latency-associated transcript. Science. 2000;287:1500–1503. [PubMed: 10688801]
• Pfeffer S., Sewer A., Lagos-Quintana M., Sheridan R., Sander C., Grasser F. A., Dyk, Ho C. K., Shuman S., Chien M., Fusso J. J., Ju J., Randall G., Lindenbach B. D., Rice C. M., Simon V., Ho D. D., Zavolan M., Tuschl T. Identification of microRNAs of the herpesvirus family. Nature Methods. 2005;2:269–276. [PubMed: 15782219]
• Preston C. M., Nicholl M. J. Repression of gene expression upon infection of cells with herpes simplex virus type 1 mutants impaired for immediate-early protein synthesis. J. Virol. 1997;71:7807–7813. [PMC free article: PMC192134] [PubMed: 9311867]
• Raggo C., Rapin N., Stirling J., et al. Luman, the cellular counterpart of herpes simplex virus VP16, is processed by regulated intramembrane proteolysis. Mol. Cell. Biol. 2002;22:5639–5649. [PMC free article: PMC133973] [PubMed: 12138176]
• Ramakrishnan R., Poliani P. L., Levine M., Glorioso J. C., Fink D. J. Detection of herpes simplex virus type 1 latency-associated transcript expression in trigeminal ganglia by in situ reverse transcriptase PCR. J. Virol. 1996;70:6519–6523. [PMC free article: PMC190691] [PubMed: 8709293]
• Rock D. L., Fraser N. W. Detection of HSV-1 genome in central nervous system of latently infected mice. Nature. 1983;302:523–525. [PubMed: 6300686]
• Samaniego L. A., Neiderhiser L., Luca N. A., and De1998Persistence and expression of the herpes simplex virus genome in the absence of immediate-early proteins J. Virol. 723307–3320. [PMC free article: PMC109808] [PubMed: 9525658]
• Sawtell N. M. Comprehensive quantification of herpes simplex virus latency at the single-cell level. J. Virol. 1997;71:5423–5431. [PMC free article: PMC191782] [PubMed: 9188614]
• Sawtell N. M. The probability of in vivo reactivation of herpes simplex virus type 1 increases with the number of latently infected neurons in the ganglia. J. Virol. 1998;72:6888–6892. [PMC free article: PMC109900] [PubMed: 9658140]
• Sawtell N. M., Thompson R. L. Herpes simplex virus type 1 latency-associated transcription unit promotes anatomical site-dependent establishment and reactivation from latency. J. Virol. 1992a;66:2157–2169. [PMC free article: PMC289008] [PubMed: 1312626]
• Sawtell N. M., Thompson R. L. Rapid in vivo reactivation of herpes simplex virus in latently infected murine ganglionic neurons after transient hyperthermia. J. Virol. 1992b;66:2150–2156. [PMC free article: PMC289007] [PubMed: 1312625]
• Simmons A., Slobedman B., Speck P., Arthur J., Efstathiou S. Two patterns of persistence of herpes simplex virus DNA sequences in the nervous system of latently infected mice. J. Gen. Virol. 1992;73:1287–1291. [PubMed: 1316947]
• Smith R. L., Escudero J. M., Wilcox C. L. Activation of second messenger pathways activates latent herpes simplex virus in neuronal cultures. Virology. 1992;188:311–318. [PubMed: 1314458]
• Stevens J., Wagner E., Devi-Rao G. B., Cook M. L., Feldman L. T. RNA complementary to a herpesvirus alpha gene mRNA is predominant in latently infected neurons. Science. 1987;235:1056–1059. [PubMed: 2434993]
• Stevens J. G., Cook M. L. Latent herpes simplex virus in spinal ganglia of mice. Science. 1971;173:843–845. [PubMed: 4328483]
• Thomas S. K., Lilley C. E., Latchman D. S., Coffin R. S. A protein encoded by the herpes simplex virus (HSV) type 1 2-kilobase latency-associated transcript is phosphorylated, localized to the nucleus, and overcomes the repression of expression from exogenous promoters when inserted into the quiescent HSV genome. J. Virol. 2002;76:4056–4067. [PMC free article: PMC136061] [PubMed: 11907244]
• Thompson R. L., Sawtell N. M. The herpes simplex virus type 1 latency-associated transcript gene regulates the establishment of latency. J. Virol. 1997;71:5432–5440. [PMC free article: PMC191783] [PubMed: 9188615]
• Thompson R. L., Sawtell N. M. Replication of herpes simplex virus type 1 within trigeminal ganglia is required for high frequency but not high viral genome copy number latency. J. Virol. 2000;74:965–974. [PMC free article: PMC111617] [PubMed: 10623759]
• Thompson R. L., Sawtell N. M. Herpes simplex virus type 1 latency-associated transcript gene promotes neuronal survival. J. Virol. 2001;75:6660–6675. [PMC free article: PMC114389] [PubMed: 11413333]
• Tsavachidou D., Podrzucki W., Seykora J., Berger S. L. Gene array analysis reveals changes in peripheral nervous system gene expression following stimuli that result in reactivation of latent herpes simplex virus type 1: induction of transcription factor Bcl-3. J. Virol. 2001;75:9909–9917. [PMC free article: PMC114562] [PubMed: 11559823]
• Sant, Hagglund R., Lopez P., Roizman B. The infected cell protein 0 of herpes simplex virus 1 dynamically interacts with proteasomes, binds and activates the cdc34 E2 ubiquitin-conjugating enzyme, and possesses in vitro E3 ubiquitin ligase activity. Proc. Natl Acad. Sci. USA. 2001;98:8815–8820. [PMC free article: PMC37518] [PubMed: 11447293]
• Vogel J. L., Kristie T. M. The novel coactivator C1 (HCF) coordinates multiprotein enhancer formation and mediates transcription activation by GABP. EMBO J. 2000;19:683–690. [PMC free article: PMC305606] [PubMed: 10675337]
• Wald A., Corey L., Cone R., Hobson A., Davis G., Zeh J. Frequent genital herpes simplex virus 2 shedding in immunocompetent women. Effect of acyclovir treatment. J. Clin. Investig. 1997;99:1092–1097. [PMC free article: PMC507918] [PubMed: 9062368]
• Wang Q. Y., Zhou C. H., Johnson K. E., Colgrove R. C., Coen D. M., Knipe D. M. Herpesviral latency-associated transcript gene promotes assembly of heterochromatin on viral lytic-gene promoters in latent infection. Proc. Natl. Acad. Sci. USA. 2005;102:16055–16059. [PMC free article: PMC1266038] [PubMed: 16247011]
• Wilcox C. L., Johnson E. Characterization of nerve growth factor-dependent herpes simplex virus latency in neurons in vitro. J. Virol. 1988;62:393–399. [PMC free article: PMC250548] [PubMed: 2826804]
• Wilson A. C., LaMarco K., Peterson M. G., Herr W. The VP16 accessory protein HCF-1 is a family of polypeptides processed from a large precursor protein. Cell. 1993;74:115–125. [PubMed: 8392914]
• Yoshikawa T., Hill J. M., Stanberry L. R., Bourne N., Kurawadwala J. F., Krause P. R. The characteristic site-specific reactivation phenotypes of HSV-1 and HSV-2 depend upon the latency-associated transcript region. J. Exp. Med. 1996;184:659–664. [PMC free article: PMC2192722] [PubMed: 8760819]
• Zabolotny J. M., Krummenacher C., Fraser N. W. The herpes simplex virus type 1 2.0-kilobase latency-associated transcript is a stable intron which branches at a guanosine. J. Virol. 1997;71:4199–4208. [PMC free article: PMC191634] [PubMed: 9151806]