Herpes simplex virus type 1 latency-associated transcript (LAT) promoter deletion mutants can express a 2-kilobase transcript mapping to the LAT region

The latency-associated transcript locus of herpes simplex virus 1 is a virulence determinant in human skin

Herpes simplex virus 1 (HSV-1) infects skin and mucosal epithelial cells and then travels along axons to establish latency in the neurones of sensory ganglia. Although viral gene expression is restricted during latency, the latency-associated transcript (LAT) locus encodes many RNAs, including a 2 kb intron known as the hallmark of HSV-1 latency. Here, we studied HSV-1 infection and the role of the LAT locus in human skin xenografts in vivo and in cultured explants. We sequenced the genomes of our stock of HSV-1 strain 17syn⁺ and seven derived viruses and found nonsynonymous mutations in many viral proteins that had no impact on skin infection. In contrast, deletions in the LAT locus severely impaired HSV-1 replication and lesion formation in skin. However, skin replication was not affected by impaired intron splicing. Moreover, although the LAT locus has been implicated in regulating gene expression in neurones, we observed only small changes in transcript levels that were unrelated to the growth defect in skin, suggesting that its functions in skin may be different from those in neurones. Thus, although the LAT locus was previously thought to be dispensable for lytic infection, we show that it is a determinant of HSV-1 virulence during lytic infection of human skin.

Herpes simplex virus type 1 (HSV-1) infects and destroys the outer layer of skin cells, producing lesions known as cold sores. Although these lesions heal, the virus persists in the host for the lifetime and can reactivate to cause new lesions. This is possible because the virus enters the axons of neurones in the skin and moves to their cell bodies located in spinal or cranial nerve bundles called ganglia, where the virus becomes dormant (latent). The most abundant viral RNAs expressed during this state are the latency associated transcripts (LATs), which have been considered a hallmark of HSV-1 latency. Here, we studied HSV-1 infection and spread in human skin. Unexpectedly, we found that the LAT locus is necessary for lesion formation in skin. HSV-1 viruses that were genetically mutated to delete the start of the locus could not spread in skin, whereas viruses with many other genetic mutations had this capacity. Our results suggest that an antiviral drug that inhibits transcripts from this region of the viral genome could block viral spread in skin, or a vaccine could possibly be produced by genetically modifying the virus at the LAT locus and by doing so, limit the virus’ ability become latent in neurones.

Termination of Transcription of LAT Increases the Amounts of ICP0 mRNA but Does Not Alter the Course of HSV-1 Infection in Latently Infected Murine Ganglia

On entering sensory ganglia, herpes simplex viruses 1 (HSV-1) establishes a latent infection with the synthesis of a latency associated transcript (LAT) or initiates productive infection with expression of a set of immediate early viral proteins. The precise mechanisms how expression of α genes is suppressed during the latency are unknown. One mechanism that has been proposed is illustrated in the case of ICP0, a key immediate early viral regulatory protein. Specifically, the 2 kb LAT intron is complementary to the 3' terminal portion of ICP0 mRNA. To test the hypothesis that accumulation of LAT negatively affects the accumulation of ICP0 mRNA, we inserted a DNA fragment encoding two poly(A) sequences into LAT to early terminate LAT transcript without interrupting the complementary sequence of ICP0 transcript (named as SR1603). Comparisons of the parent (SR1601) and mutant (SR1603) HSV-1 viruses showed the following: Neurons harboring latent SR1603 virus accumulated equivalent amounts of viral DNA but higher amounts of ICP0 mRNA and lower amounts of LAT, when compared to neurons harboring the SR1601 virus. One notable difference between the two viruses is that viral RNA accumulation in explanted ganglia harboring SR1603 virus initiated significantly sooner than that in neurons harboring SR1601 virus, suggesting that ICP0 may act as an activator of viral gene expression in permissive cells. Collectively, these data suggest that increased ICP0 mRNA by suppressed LAT did not affect the establishment of latency in latently infected murine ganglia.

HSV1 latent transcription and non-coding RNA: A critical retrospective

Virologists have invested great effort into understanding how the herpes simplex viruses and their relatives are maintained dormant over the lifespan of their host while maintaining the poise to remobilize on sporadic occasions. Piece by piece, our field has defined the tissues in play (the sensory ganglia), the transcriptional units (the latency-associated transcripts), and the responsive genomic region (the long repeats of the viral genomes). With time, the observed complexity of these features has compounded, and the totality of viral factors regulating latency are less obvious. In this review, we compose a comprehensive picture of the viral genetic elements suspected to be relevant to herpes simplex virus 1 (HSV1) latent transcription by conducting a critical analysis of about three decades of research. We describe these studies, which largely involved mutational analysis of the notable latency-associated transcripts (LATs), and more recently a series of viral miRNAs. We also intend to draw attention to the many other less characterized non-coding RNAs, and perhaps coding RNAs, that may be important for consideration when trying to disentangle the multitude of phenotypes of the many genetic modifications introduced into recombinant HSV1 strains.

Alphaherpesvirus Latency: A Dynamic State of Transcription and Reactivation

Alphaherpesviruses infect a variety of species from sea turtles to man and can cause significant disease in mammals including humans and livestock. These viruses are characterized by a lytic and latent state in nerve ganglia, with the ability to establish a lifelong latent infection that is interrupted by periodic reactivation. Previously, it was accepted that latency was a dominant state and that only during relatively infrequent reactivation episodes did latent genomes within ganglia become transcriptionally active. Here, we review recent data, focusing mainly on Herpes Simplex Virus type 1 which indicate that the latent state is more dynamic than recently appreciated.

Increased neurovirulence and reactivation of the herpes simplex virus type 1 latency-associated transcript (LAT)-negative mutant dLAT2903 with a disrupted LAT miR-H2

At least six microRNAs (miRNAs) appear to be encoded by the latency-associated transcript (LAT) of herpes simplex virus type 1 (HSV-1). The gene for ICP0, an important immediate early (IE) viral protein, is anti-sense to, and overlaps with, the region of LAT from which miRNA H2 (miR-H2) is derived. We recently reported that a mutant (McK-ΔH2) disrupted for miR-H2 on the wild-type HSV-1 strain McKrae genomic background has increased ICP0 expression, increased neurovirulence, and slightly more rapid reactivation. We report here that HSV-1 mutants deleted for the LAT promoter nonetheless make significant amounts of miR-H2 during lytic tissue culture infection, presumably via readthrough transcription from an upstream promoter. To determine if miR-H2 might also play a role in the HSV-1 latency/reactivation cycle of a LAT-negative mutant, we constructed dLAT-ΔH2, in which miR-H2 is disrupted in dLAT2903 without altering the predicted amino acid sequence of the overlapping ICP0 open reading frame. Similar to McK-ΔH2, dLAT-ΔH2 expressed more ICP0, was more neurovirulent, and had increased reactivation in the mouse TG explant-induced reactivation model of HSV-1 compared with its parental virus. Interestingly, although the increased reactivation of McK-ΔH2 compared with its parental wild-type (wt) virus was subtle and only detected at very early times after explant TG induced reactivation, the increased reactivation of dLAT-ΔH2 compared with its dLAT2903 parental virus appeared more robust and was significantly increased even at late times after induction. These results confirm that miR-H2 plays a role in modulating the HSV-1 reactivation phenotype.

Genome wide nucleosome mapping for HSV-1 shows nucleosomes are deposited at preferred positions during lytic infection

HSV is a large double stranded DNA virus, capable of causing a variety of diseases from the common cold sore to devastating encephalitis. Although DNA within the HSV virion does not contain any histone protein, within 1 h of infecting a cell and entering its nucleus the viral genome acquires some histone protein (nucleosomes). During lytic infection, partial micrococcal nuclease (MNase) digestion does not give the classic ladder band pattern, seen on digestion of cell DNA or latent viral DNA. However, complete digestion does give a mono-nucleosome band, strongly suggesting that there are some nucleosomes present on the viral genome during the lytic infection, but that they are not evenly positioned, with a 200 bp repeat pattern, like cell DNA. Where then are the nucleosomes positioned? Here we perform HSV-1 genome wide nucleosome mapping, at a time when viral replication is in full swing (6 hr PI), using a microarray consisting of 50mer oligonucleotides, covering the whole viral genome (152 kb). Arrays were probed with MNase-protected fragments of DNA from infected cells. Cells were not treated with crosslinking agents, thus we are only mapping tightly bound nucleosomes. The data show that nucleosome deposition is not random. The distribution of signal on the arrays suggest that nucleosomes are located at preferred positions on the genome, and that there are some positions that are not occupied (nucleosome free regions -NFR or Nucleosome depleted regions -NDR), or occupied at frequency below our limit of detection in the population of genomes. Occupancy of only a fraction of the possible sites may explain the lack of a typical MNase partial digestion band ladder pattern for HSV DNA during lytic infection. On average, DNA encoding Immediate Early (IE), Early (E) and Late (L) genes appear to have a similar density of nucleosomes.

Construction of replication-defective herpes simplex virus vectors

Advances in identification and characterization of gene products responsible for specific diseases of the nervous system have opened opportunities for novel therapies using gene transfer vectors for gene replacement. Herpes simplex virus (HSV)-based vectors are particularly well suited for gene delivery to neurons of the central and peripheral nervous systems. The authors have developed methods to delete HSV-1 IE gene functions and to subsequently introduce foreign genes into the HSV-1 genome using homologous recombination. This unit describes methods for generating cell lines that complement multiple essential gene deletion mutants as well for generating such replication-defective virus recombinants and inserting foreign DNA sequences into replication-defective viral genomes, the last step in preparing a vector. Three support protocols describe methods for preparing virus stocks, titering virus, and preparing viral DNA.

Stable Levels of Long-Term Transgene Expression Driven by the Latency-Associated Transcript Promoter in a Herpes Simplex Virus Type 1 Vector

Previous gene transfer studies of the herpes simplex virus type 1 (HSV-1) using the latency-associated transcript (LAT) promoter have reported a decrease in transgene expression in the brain over time, but the extent of this decrease has not been measured and it is unknown if expression eventually stabilizes. We examined LAT promoter-mediated transgene expression in the mouse brain for 1 year following intracranial injection with a HSV-1 vector expressing human beta-glucuronidase (GUSB). The vector genome copy number remained stable from 2 to 52 weeks. Quantitative reverse transcriptase PCR detected a peak of LAT intron expression at 2 weeks (corresponding to the end of the acute phase of viral infection), followed by stable expression during latency (13-52 weeks). The number of GUSB-positive cells also had a peak in the acute phase and then was stable during latency (13-52 weeks). GUSB enzymatic activity was maintained at 11% of normal at 6 and 12 months, indicating that the LAT promoter is capable of driving stable transgene expression in the brain.

Histone deacetylase inhibitors induce reactivation of herpes simplex virus type 1 in a latency-associated transcript-independent manner in neuronal cells

Histone acetylation is implicated in the regulation of herpes simplex virus type 1 (HSV-1) latency. However, the role of histone acetylation in HSV-1 reactivation is less clear. In this study, the well-established model system, quiescently infected, neuronally differentiated PC12 (QIF-PC12) cells, was used to address the participation of histone acetylation in HSV-1 reactivation. In this model, sodium butyrate and trichostatin A (TSA), two histone deacetylase inhibitors, stimulated production of infectious HSV-1 progeny from a quiescent state. To identify viral genes responsive to TSA, the authors analyzed representative alpha, beta, and gamma viral genes using quantitative real-time polymerase chain reaction. Only the latency-associated transcript (LAT) accumulated in response to TSA treatment, under culture conditions that restricted virus replication and spread. This led the authors to evaluate the importance of LAT expression on TSA-induced reactivation. In QIF-PC12 cells, the LAT deletion mutant virus dLAT2903 reactivated equivalently with its wild-type parental strain (McKrae) after TSA treatment, as well as forskolin and heat stress treatment. Both viruses also reactivated equivalently from latently infected trigeminal ganglia explants from rabbits. In contrast, there was a marked reduction in the recovery of dLAT2903, as compared to wild-type virus, from the eyes of latently infected rabbits following epinephrine iontophoresis. These combined in vitro, ex vivo, and in vivo data suggest that LAT is not required for reactivation from latently infected neuronal cells per se, but may enhance processes that allow for the arrival of virus at, or close to, the site of original inoculation (i.e., recrudescence).

The herpes simplex virus type 1 latency-associated transcript (LAT) enhancer/rcr is hyperacetylated during latency independently of LAT transcription

During herpes simplex virus type 1 (HSV-1) latency, only one region of the viral genome is actively transcribed: the region encoding the latency-associated transcript (LAT). A previous study demonstrated that during latency the LAT promoter is hyperacetylated at histone H3 (K9, K14) relative to lytic genes examined. In the present study, we examine the acetylation profile of regions downstream of the LAT promoter during a latent infection of murine dorsal root ganglia. These analyses revealed the following: (i) the region of the genome containing the 5' exon of the LAT primary transcript was at least as enriched in acetylated H3 as the LAT promoter, and (ii) the region of hyperacetylation does not extend to the ICP0 promoter. In order to assess the contribution of LAT transcription to the acetylation of the 5' exon region, the acetylation profile of KOS/29, a recombinant with a deletion of the LAT promoter, was examined. The region containing the 5' exon of KOS/29 was hyperacetylated relative to lytic gene regions in the absence of detectable LAT transcription. These results indicate that the region containing the 5' exon of LAT, known to contain enhancer activities and to be critical for induced reactivation (rcr), exists in a chromatin structure during latency that is distinct from other lytic gene regions. This result suggests a role for the 5' exon LAT enhancer region as a cis-acting regulator of transcription that maintains a transcriptionally permissive chromatin domain in the HSV-1 latent episome.

Wide variations in herpes simplex virus type 1 inoculum dose and latency-associated transcript expression phenotype do not alter the establishment of latency in the rabbit eye model

The latency-associated transcript (LAT) is required for efficient reactivation of herpes simplex virus type 1 from latent infection in the rabbit eye model, but LAT's mechanism of action is unknown. In addition to reactivation, the LAT region seems to correspond to multiple functions, with some LAT deletion mutants exhibiting increased virulence, increased neuronal death, and restricted establishment of latency. While a LAT promoter deletion mutant (17DeltaPst) seems to be primarily restricted in reactivation in the rabbit, subtle effects on virulence or the establishment of latency cannot be precluded at the normal high levels of virus inoculum used in the rabbit model. Since such additional LAT phenotypes may be more evident with lower doses of virus, we evaluated the influence of initial viral inoculum and LAT expression on the progression of acute infection and the establishment of latency. We have assayed both virus recovery rates and viral genome loads in rabbit corneas and trigeminal ganglia. Our results show that (i) in the corneas and trigeminal ganglia, the maximum amount of virus present during acute infection is independent of the LAT genotype and inoculum dose, although greater viral yields are obtained earlier with higher inoculum doses, and (ii) the range in numbers of latent genomes detected in the ganglia is independent of the inoculum dose and the LAT genotype and therefore no difference in establishment of latency is observed.

Gene delivery using herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus with many favorable properties as a gene delivery vector. HSV is highly infectious, so HSV vectors are efficient vehicles for the delivery of exogenous genetic material to cells. Viral replication is readily disrupted by null mutations in immediate early genes that in vitro can be complemented in trans, enabling straightforward production of high-titre pure preparations of non-pathogenic vector. The genome is large (152 Kb) and many of the viral genes are dispensable for replication in vitro, allowing their replacement with large or multiple transgenes. Latent infection with wild-type virus results in episomal viral persistence in sensory neuronal nuclei for the duration of the host lifetime. Transduction with replication-defective vectors causes a latent-like infection in both neural and non-neural tissue; the vectors are non-pathogenic, unable to reactivate and persist long-term. The latency active promoter complex can be exploited in vector design to achieve long-term stable transgene expression in the nervous system. HSV vectors transduce a broad range of tissues because of the wide expression pattern of the cellular receptors recognized by the virus. Increasing understanding of the processes involved in cellular entry has allowed preliminary steps to be taken towards targeting the tropism of HSV vectors. Using replication-defective HSV vectors, highly encouraging results have emerged from recent pre-clinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration. Consequently, HSV vectors encoding appropriate transgenes to tackle these pathogenic processes are poised to enter clinical trials.

Multiple applications for replication-defective herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus. The viral genome is large (152 kb), and many genes are dispensable for viral function, allowing insertion of multiple or large transgene expression cassettes. The virus life cycle includes a latent phase, during which the viral genome remains as a stable episomal element within neuronal nuclei for the lifetime of the host, without disturbing normal function. We have exploited these features of HSV to construct a series of nonpathogenic gene therapy vectors that efficiently deliver therapeutic and experimental transgenes to neural and non-neural tissue. Importantly, transgene expression may be sustained long term; reporter gene expression has been demonstrated for over a year in the nervous system. This article discusses the generation of replication-defective HSV vectors and reviews recent studies investigating their use in several animal models of human disease. We have demonstrated correction or prevention of a number of important neurological phenotypes, including neurodegeneration, chronic pain, peripheral neuropathy, and malignancy. In addition, HSV-mediated transduction of non-neurological tissues allows their use as depot sites for synthesis of circulating and locally acting secreted proteins. New applications for this vector system include the genetic modification of stem cell populations; this may become an important means to direct cellular differentiation or deliver therapeutic genes systemically. Replication-defective HSV vectors are an effective and flexible vehicle for the delivery of transgenes to numerous tissues, with multiple applications.

Analysis of protein expression from within the region encoding the 2.0-kilobase latency-associated transcript of herpes simplex virus type 1

During latent infections of sensory neurons, herpes simplex virus type 1 gene expression is restricted to the latency-associated transcripts (LATs). The association of the stable 2.0-kb LAT intron with polysomes has suggested that it might represent a novel mRNA. In this work, we investigated expression of 2.0-kb LAT open reading frames (ORFs) by inserting the gene for green fluorescent protein (GFP) within the 2.0-kb LAT sequence, both within a LAT expression plasmid and in the context of the virus. Upon transient transfection of cells of both neuronal and nonneuronal origin with LAT-GFP expression vectors, low-level GFP fluorescence was distributed over the cell cytoplasm and likely resulted from infrequent initiation at a GFP AUG codon, on either unspliced or alternately spliced LAT RNAs. A second nucleolar GFP expression pattern which resulted from fusion of GFP to a conserved ORF in exon 1 of the LAT gene was also observed. However, the abundant expression of this fusion protein was dependent upon an artificially added translation initiation codon. Expression was much reduced and restricted to a small subset of transfected cells when this initiator codon was removed. Neither the 2.0-kb LAT-GFP intron itself nor transcripts originating from the latency-associated promoter 2 (LAP2) were responsible for GFP expression. Abundant alternate splicing involving the 1.5-kb LAT splice acceptor and including splicing between the 1.5-kb LAT splice donor and acceptor, was observed in the nonneuronal Cos-1 cell line. Contrary to the results of our transfection studies, GFP expression could not be detected from a LAT-GFP virus at any stage of the infection cycle. Our results suggest that the inhibition of LAT ORF expression during viral infection occurred primarily at the level of translation.

Identification of a novel 0.7-kb polyadenylated transcript in the LAT promoter region of HSV-1 that is strain specific and may contribute to virulence

Herpes Simplex virus expresses latency-associated transcripts (LATs) the function of which remains obscure despite increasing knowledge of their structure and expression. Upstream of the LAT coding region is a region of the genome that is poorly characterized although it lies in an area that is responsible for modulation of reactivation efficiency in two different animal models. Transcript mapping with strains 17, McKrae, KOS, and F has revealed strain differences in this region of the viral genome. Strain 17 and McKrae expressed a novel polyadenylated 0.7-kb transcript that is absent from KOS and F. This transcript is expressed in the LAT direction and has the kinetics of a true late gene during the lytic cycle of infection. A deletion mutant, 17DeltaBsa, which does not express the 0.7-kb RNA, is less virulent than the parental strain 17. A rescuant with F sequence (17DeltaBsa/RF) shows virulence similar to F, whereas a rescuant with strain 17 sequence (17DeltaBsa/R17) is similar to strain 17. Virulence is altered by deletion or substitution in the region encoding the 0.7-kb transcript (BsaI-BsaI); however, reactivation in the mouse explant cocultivation assay or the adrenergically induced rabbit reactivation model remained unchanged. The importance of this region for virulence is discussed.

HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part I. HSV-1 structure, replication and pathogenesis

The design of effective gene therapy strategies for brain tumors and other neurological disorders relies on the understanding of genetic and pathophysiological alterations associated with the disease, on the biological characteristics of the target tissue, and on the development of safe vectors and expression systems to achieve efficient, targeted and regulated, therapeutic gene expression. The herpes simplex virus type 1 (HSV-1) virion is one of the most efficient of all current gene transfer vehicles with regard to nuclear gene delivery in central nervous system-derived cells including brain tumors. HSV-1-related research over the past decades has provided excellent insight into the structure and function of this virus, which, in turn, facilitated the design of innovative vector systems. Here, we review aspects of HSV-1 structure, replication and pathogenesis, which are relevant for the engineering of HSV-1-based vectors.

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 vector-mediated expression of nerve growth factor protects dorsal root ganglion neurons from peroxide toxicity

Nerve growth factor beta subunit (beta-NGF) transgene delivery and expression by herpes simplex virus type 1 (HSV-1) vectors was examined in a cell culture model of neuroprotection from hydrogen peroxide toxicity. Replication-competent (tk- K mutant background) and replication-defective (ICP4(-);tk- S mutant background) vectors were engineered to contain the murine beta-NGF cDNA under transcriptional control of either the human cytomegalovirus immediate-early gene promoter (HCMV IEp) (e.g., KHN and SHN) or the latency-active promoter 2 (LAP2) (e.g., KLN and SLN) within the viral thymidine kinase (tk) locus. Infection of rat B103 and mouse N2A neuronal cell lines, 9L rat glioma cells, and Vero cells with the KHN or SHN vectors resulted in the production of beta-NGF-specific transcripts and beta-NGF protein reaching a maximum at 3 days postinfection (p.i.). NGF protein was released into the culture media in amounts ranging from 10.83 to 352.86 ng/ml, with the highest levels being achieved in B103 cells, and was capable of inducing neurite sprouting of PC-12 cells. The same vectors produced high levels of NGF in primary dorsal root ganglion (DRG) cultures at 3 days. In contrast to HCMV IEp-mediated expression, the LAP2-NGF vectors showed robust expression in primary DRG neurons at 14 days. The neuroprotective effect of vector produced NGF was assessed by its ability to inhibit hydrogen peroxide-induced neuron toxicity in primary DRG cultures. Consistent with the kinetics of vector-mediated NGF expression, HCMV-NGF vectors were effective in abrogating the toxic effects of peroxide at 3 but not 14 days p.i. whereas LAP2-NGF vector transduction inhibited apoptosis in DRG neurons at 14 days p.i. but was ineffective at 3 days p.i. Similar kinetics of NGF expression were observed with the KHN and KLN vectors in latently infected mouse trigeminal ganglia, where high levels of beta-NGF protein expression were detected at 4 wks p.i. only from the LAP2; HCMV-NGF-driven expression peaked at 3 days but could not be detected during HSV latency at 4 weeks. Together, these results indicate that (i) NGF vector-infected cells produce and secrete mature, biologically active beta-NGF; (ii) vector-synthesized NGF was capable of blocking peroxide-induced apoptosis in primary DRG cultures; and (iii) the HCMV-IEp functioned to produce high levels of NGF for several days; but (iv) only the native LAP2 was capable of long-term expression of a therapeutic gene product in latently infected neurons in vivo.

The herpes simplex virus type 1 2.0-kilobase latency-associated transcript is a stable intron which branches at a guanosine

We have used a minigene construct of the herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene to analyze its transcripts in transient transfection assays. A 2.8-kb fragment of the approximately 8.5-kb LAT gene encompassing the 2.0-kb LAT was cloned into a eukaryotic expression vector downstream of the cytomegalovirus immediate-early gene promoter. Northern hybridization of RNA isolated from transfected COS-1 cells identified three LAT-specific transcripts, 3.4, 2.0, and 1.4 kb in size. Mapping of these transcripts by Northern hybridization indicated that the 1.4- and 2.0-kb RNAs are nonoverlapping, while the 3.4-kb RNA overlaps both smaller RNAs. Reverse transcription-PCR (RT-PCR) and partial sequencing of the 1.4-kb RNA revealed that this RNA is the spliced exons of the 3.4-kb primary transcript. The 2.0-kb LAT appears to be an intron accumulating after splicing of the minor LAT (mLAT) pre-mRNA. The splice donor and acceptor sites for the 2.0-kb LAT identified in transfected and HSV-1-infected cells are identical. Mapping of the branch point of this intron by RT-PCR in transfected and HSV-1-infected cells, as well as in latently infected murine trigemial ganglia, shows that it is a guanosine. This branch site does not bear homology to consensus mammalian branch site sequences. These data provide evidence that the 2.0-kb LAT is an intron of the mLAT pre-mRNA with a unique branch point.

Utilization of the herpes simplex virus type 1 latency-associated regulatory region to drive stable reporter gene expression in the nervous system

The ability of herpes simplex virus type 1 (HSV-1) to establish a lifelong, transcriptionally active, latent infection in neurons has led to much interest in developing HSV-based vectors for gene delivery to the nervous system. A prerequisite of such vectors is that they should be capable of directing long-term transgene expression in latently infected neurons. The continued transcription of HSV-1 latency-associated transcripts (LATs) during neuronal latency suggests that regulatory sequences which mediate expression of LATs could be utilized for long-term expression of heterologous genes in the mammalian nervous system. In addition to upstream regulatory elements which are important for LAT promoter-mediated transcription during neuronal latency, there is growing evidence that sequences downstream of the LAT transcription start site play an important role in facilitating long-term latent-phase transcription. In order to maintain the integrity of both upstream and downstream regulatory elements of the LAT promoter, we constructed viruses which contained the lacZ and lacZ-neo reporter genes linked to the encephalomyocarditis virus internal ribosomal entry site (IRES) (viruses LbetaA and LbetaB, respectively) inserted approximately 1.5 kb downstream of the LAT transcription start site. These viruses expressed low levels of beta-galactosidase in lytically infected Vero cells and in cervical dorsal root ganglion neurons during the acute stage of infection in vivo. In contrast, at later times postinfection and consistent with the establishment of latency, increases both in the numbers of neurons expressing beta-galactosidase and in the intensity of staining were observed. Examination of the brain stems and spinal cords of animals latently infected with LbetaA, sampled at time points from 72 to 307 days postinfection, revealed the stable expression of beta-galactosidase within neurons located in facial and hypoglossal nerve nuclei and the upper cervical spinal cord. We conclude that the insertion of an IRES linked to a reporter gene 1.5 kb downstream from the LAT transcription start site does not disrupt elements of the LAT promoter necessary for long-term gene expression and, in both the peripheral and central nervous systems, facilitates beta-galactosidase expression in a wide variety of neurons.

The herpes simplex virus type 1 latency-associated transcript promoter is activated through Ras and Raf by nerve growth factor and sodium butyrate in PC12 cells

Herpes simplex virus establishes latent infections in the nuclei of sensory neurons. These infections are characterized by the abundant expression of a series of 5' coterminal transcripts termed the latency-associated transcripts (LATs). Available evidence indicates that LAT expression is specifically regulated in latently infected neurons. Although previous studies have examined the regulation of LAT expression in neuronal and nonneuronal cells, the mechanism of regulation of LAT expression in neuronal cells in response to external factors has not been investigated. To address this question, we characterized the activity of LAT promoter fusion constructs in PC12 cells following treatment with nerve growth factor (NGF) and/or sodium butyrate (NaB), agents that affect expression of cell cycle-associated genes. Expression from the LAT promoter was induced 8- to 12-fold by either NGF or NaB alone and 40- to 60-fold when the two agents were added simultaneously. Fibroblast growth factor also induced expression from the LAT promoter but to a lesser extent than NGF. Treatment with factors such as epidermal growth factor, phorbol myristate acetate, cyclic AMP, or KCI had no significant effect on LAT promoter activity. Notably, promoter-reporter constructs containing immediate-early (ICP0 and ICP4), early (ICP8 and UL9), and late (UL10 and UL30) viral promoters were induced only two- to fourfold by NGF, suggesting that the LAT promoter may be unusual among herpes simplex virus genes in the magnitude of its response to this factor. To identify pathways leading to LAT activation in vitro, we characterized the response of the LAT promoter to NGF and/or NaB in PC12-derived cell lines containing mutations in specific signal transduction pathways. We found that activation of the LAT promoter requires Ras activation and that activation of the serine/threonine kinase, Raf, is sufficient to induce LAT expression. Together, these results indicate that the LAT promoter is regulated via the Ras/Raf signal transduction pathway in response to external factors such as NGF and NaB and that LAT expression may be regulated by NGF in latently infected neurons.

cis-acting elements involved in transcriptional regulation of the herpes simplex virus type 1 latency-associated promoter 1 (LAP1) in vitro and in vivo

Latency-associated promoter 1 (LAP1) of herpes simplex virus type 1 is required to generate a series of latency-associated transcripts (LATs) in sensory neurons of latently infected animals. Sequence analysis and DNA binding studies have suggested the existence of several cis-acting elements within LAP1 that are potentially important for promoter function, although their role in LAT gene expression during latency is largely unexplored. In this report, we present evidence that the LAP1 TATA box is essential for transcription initiation in vitro. A reduction in LAT synthesis measured by in situ hybridization and reverse transcription-PCR (RT-PCR) of rat brain tissue latently infected with a LAP1 TATA substitution virus demonstrated that this sequence was required for full LAP1 activity in vivo. Analysis of additional site-directed and 5'-deletion mutants of LAP1 by in vitro transcription-primer extension assays showed that upstream elements including the USF and cyclic AMP response element (CRE) site specifically contributed to LAP1 function and that sequences beginning at position -620 relative to the transcription start site were essential for full promoter activity. The combination of deleting USF, CRE, and TATA completely abolished LAT expression in the brain, identifying these as essential elements for the neuron-specific functioning of LAP1 during latency. Mutation of the transcription start site did not abolish transcription, suggesting the absence of an initiator element. However, one of the most exciting findings from this study is that the region downstream of the TATA box appears to contain a true enhancer that is not only essential for transcription, but also functional when positioned 1.6 kb downstream of the start site of transcription. It was concluded that (i) the TATA box was essential for full transcriptional activity from LAP1 both in vitro and in vivo, (ii) the USF element and CRE contribute to LAP1 function during latency in combination with the TATA element, (iii) multiple trans-acting factors besides the USF- and CRE-binding proteins were required for full promoter activity in vitro, and (iv) sequences downstream of the TATA box enhanced promoter activity in vitro.

Downstream regulatory elements increase acute and latent herpes simplex virus type 2 latency-associated transcript expression but do not influence recurrence phenotype or establishment of latency

The role of putative promoter or activator sequences downstream of the herpes simplex virus type 2 latency-associated transcript (LAT) promoter and upstream of the LAT intron was investigated in vivo by constructing and evaluating mutant viruses with deletions in this region. The deletion of LAT promoter sequences upstream of the primary LAT transcript reduced levels of LAT expression during productive infections, compared with the LAT expression level of wild-type virus, and abolished LAT expression during latency. The deletion of the putative downstream regulatory elements reduced but did not eliminate LAT expression during productive and latent infections. The deletion of both regions almost completely eliminated acute LAT transcription, although additional acute LAT-region transcription directed by sequences upstream of either region was detected by reverse transcriptase PCR. The deletion of the downstream elements did not influence the ability of the virus to reactivate from latently infected guinea pigs relative to the ability of the wild-type virus to reactivate; thus, decreased LAT expression did not affect the frequency of recurrence. The deletion of both regions did not affect the ability of the virus to establish latency. We conclude that downstream regulatory elements are necessary for maximal acute LAT expression but do not constitute an independent promoter during latency and do not play an obvious role in the establishment of our reactivation from latency.

Two herpes simplex virus type 1 latency-active promoters differ in their contributions to latency-associated transcript expression during lytic and latent infections

Herpes simplex virus type 1 (HSV-1) establishes latency in human sensory ganglia, during which time the viral genome is transcriptionally silent with the exception of the latency-associated transcripts (LATs). The most abundant LAT is a 2-kb RNA whose biosynthesis is poorly characterized. The 2-kb LAT may be a primary transcript, or its synthesis may involve splicing and/or other forms of processing. Two potential RNA polymerase II promoters (LAP1 and LAP2) upstream of the 2-kb LAT 5' end have been identified. To investigate the role played by LAP1 and LAP2 in the synthesis of the 2-kb LAT under lytic and latent conditions, we analyzed HSV-1 mutants which contain deletions of one or both of these promoters. During lytic infection in cell culture, the cis elements critical for the normal accumulation of the 2-kb LAT were mapped to LAP2, while LAP1 sequences were largely dispensable. The 5' ends of the major 2-kb LATs produced by the wild-type and LAP deletion viruses were examined by primer extension analysis and were all found to be identical (+/- 2 bp). The accumulation of the 2-kb LAT during latent infections of murine trigeminal ganglia was examined by Northern (RNA) blot and by reverse transcription-PCR. In contrast to the results found in lytic infections, the critical cis elements needed for 2-kb LAT accumulation during latency were mapped to LAP1. Deletion of LAP1 resulted in a 500-fold reduction in 2-kb LAT accumulation, whereas deletion of LAP2 resulted in only a 2- to 3-fold reduction. Deletion of both LAP1 and LAP2 resulted in undetectable levels of the 2-kb LAT. Our results indicate that both LAP1 and LAP2 are critical for 2-kb LAT expression but under different conditions. LAP1 is essential for LAT expression during latency, while LAP2 is primarily responsible for LAT expression in lytic infections in cell culture. LAP1 and LAP2 may prove to be functionally independent promoter elements that control 2-kb LAT expression during different stages of HSV-1 infections.

HSV as a gene transfer vector for the nervous system

Gene therapy for diseases of the nervous system requires vectors capable of delivering the therapeutic gene into postmitotic cells in vivo. Herpes simplex virus type 1 is a neurotropic virus that naturally establishes latency in neurons of the peripheral nervous system. Replication defective HSV vectors have been developed; these are deleted for at least one essential immediate early regulatory gene, rendering the virus less cytotoxic, incapable of reactivation, but still capable of establishing latency. Foreign genes can be vigorously expressed from an HSV-based vector in a transient manner in brain and other tissues. Long-term but weak foreign gene expression may be achieved in the nervous system by exploiting the transcriptional control mechanisms of the natural viral latency active promoter. To meet the needs of specific applications, either highly active long-term or regulatable transgene expression will be needed, requiring further studies in order to design the appropriate latency-based promoter systems.

Analysis of the promoter and cis-acting elements regulating expression of herpes simplex virus type 2 latency-associated transcripts

In latently infected neurons, herpes simplex virus type 2 (HSV-2) expresses one abundant family of transcripts, the latency-associated transcripts (LATs). We demonstrate here that the sequence lying about 700 bp upstream of the 5' end of the HSV-2 major LAT acts as a very strong promoter in transient expression assays in both neuronal and nonneuronal cells. Transcription starts about 27 to 32 bp downstream of a functional TATA box. The proximal fragment from -102 to +34 includes the basal promoter and accounts for constitutive transcriptional activity in various cell lines. The distal region from -392 to -103 contributes to particularly strong promoter activity in neuronal cell lines and involves multiple cis-acting elements. A functional activating transcription factor/cyclic AMP (cAMP) response element binding protein motif lies just upstream of the TATA. By DNase I footprint and methylation protection assays, we identified several additional protein-binding sites upstream of the activating transcription factor/cAMP response element binding protein motif. A GC-rich element, termed LAT-3, was located between bases -128 to -102. A 2-bp substitution in LAT-3 markedly reduced promoter activity and abolished protein-binding ability in vitro. Gel retardation assay showed no competition for protein binding to LAT-3 by other GC-rich elements. LAT-3 appears to be a novel cis-acting element that may contribute to the neuronal responsiveness of the HSV-2 LAT promoter.

Two overlapping transcription units which extend across the L-S junction of herpes simplex virus type 1

A region of the herpes simplex virus type 1 genome located upstream of the alpha 0 promoter contains a promoter which regulates transcription in the opposite orientation to that driven by alpha 0. Analyses of mutants from which this promoter, alpha X, was deleted and a mutant in which a fragment that serves as a transcription terminator and polyadenylation signal was inserted upstream of this promoter demonstrate that two distinct transcription units overlap this region of the genome and are transcribed in a direction antisense to the neurovirulence gene gamma (1)34.5. One unit, dependent on the alpha X promoter, is active when cells are infected in the presence of the protein synthesis inhibitor cycloheximide. The second unit, independent of alpha X, is active during the course of productive infection. This transcription unit originates from a promoter upstream of alpha X which is distinct from the latency-associated promoter (LAP). Two polyadenylated transcripts of 0.9 and 4.9 kb accumulate from this region of the genome during productive infection, but no mature transcripts accumulate in infected cells maintained in the presence of cycloheximide. Kinetic analyses demonstrate that the transcripts that accumulate during productive infection fall into the beta class of herpes simplex virus type 1 genes.

Expression of the herpes simplex virus type 2 latency-associated transcript enhances spontaneous reactivation of genital herpes in latently infected guinea pigs

The latency-associated transcript (LAT) is the only herpes simplex virus (HSV) gene product detectable in latently infected humans and animals. In this report, we show that a 624-bp deletion in the promoter of the HSV-2 LAT had no discernable effect on viral growth in tissue culture or in acute genital infection of guinea pigs, but impaired LAT accumulation and led to a marked decrease in spontaneous genital recurrences when compared with the behavior of wild-type and rescuant strains. Differences in the ability of the mutant to replicate, or in how readily it established or maintained latency did not account for this finding. Thus, HSV LAT expression facilitates the spontaneous reactivation of latent virus.

Analysis of a herpes simplex virus type 1 LAT mutant with a deletion between the putative promoter and the 5' end of the 2.0-kilobase transcript

A herpes simplex virus type 1 strain 17 mutant with a deletion between genomic nucleotides 118880 and 119250 was constructed and called 17 delta Sty. The deletion removes most of a putative secondary LAT promoter (called LAPII) as well as 370 of the first 449 nucleotides of the proposed 8.5-kb transcript believed to be the precursor of 2.0-kb LAT. 17 delta Sty was shown to produce major 2.0-kb LATs in tissue culture. Moreover, trigeminal nerves from latently infected mice contained an intact 1.45- to 2.0-kb LAT as well as the minor LATs which are recognized by probes specific for regions downstream of the 2.0-kb LAT. Finally, 17 delta Sty reactivated with normal kinetics from the trigeminal ganglia of latently infected mice in the explant cocultivation assay and egressed from tissue culture cells as efficiently as wild-type virus. These results clearly show that the region deleted in 17 delta Sty is dispensable for intact 2-kb LAT production, viral egress in tissue culture, and normal reactivation from latently infected neurons in mice.

Two open reading frames (ORF1 and ORF2) within the 2.0-kilobase latency-associated transcript of herpes simplex virus type 1 are not essential for reactivation from latency

The herpes simplex virus type 1 (HSV-1) latency-associated transcripts (LATs) are dispensable for establishment and maintenance of latent infection. However, the LATs have been implicated in reactivation of the virus from its latent state. Since the reported LAT deletion and/or insertion variants that are reactivation impaired contain deletions in the putative LAT promoter, it is not known which LAT sequences are involved in reactivation. To examine the role of the 2.0-kb LAT in the process of reactivation and the functional importance of the putative open reading frames (ORF1 and ORF2) contained within the 2.0-kb LAT, we have constructed an HSV-1 variant that contains a precise deletion and insertion within the LAT-specific DNA sequences using site-directed mutagenesis. The HSV-1 variant FS1001K contains an 1,186-bp deletion starting precisely from the 5' end of the 2.0-kb LAT and, for identification, a XbaI restriction endonuclease site insertion. The FS1001K genome contains no other deletions and/or insertions as analyzed by a variety of restriction endonucleases. The deletion in FS1001K removes the entire 556-bp intron within the 2.0-kb LAT, the first 229 nucleotides of ORF1, and the first 159 nucleotides of ORF2 without having an affect on the RL2 (ICP0) gene. Explant cocultivation reactivation assays indicated that this deletion had a minimal effect on reactivation of the variant FS1001K compared with the parental wild-type virus using a mouse eye model. As expected, Northern (RNA) blot analyses have shown that the variant virus (FS1001K) does not produce the 2.0-kb LAT or the 1.45- to 1.5-kb LAT either in vitro or in vivo; however, FS1001K produces an intact RL2 transcript in tissue culture. These data suggest that the 2.0-kb LAT putative ORF1 and ORF2 (or the first 1,186 bp of the 2.0-kb LAT) are dispensable for explant reactivation of latent HSV-1.