Comparative efficacy of expression of genes delivered to mouse sensory neurons with herpes virus vectors

Efficient establishment of reactivatable latency by an acyclovir-resistant herpes simplex virus 1 thymidine kinase substitution mutant with reduced neuronal replication

Herpes simplex virus 1 causes recurrent diseases by reactivating from latency, which requires the viral thymidine kinase (TK) gene. An acyclovir-resistant mutation in TK, V204G, was previously repeatedly identified in a patient with recurrent herpetic keratitis. We found that compared with its parental strain KOS, a laboratory-derived V204G mutant virus was impaired in replication in cultured neurons despite little defect in non-neuronal cells. After corneal inoculation of mice, V204G exhibited defects in ocular replication that were modest over the first three days but severe afterward. Acute replication of V204G in trigeminal ganglia was significantly impaired. However, V204G established latency with viral loads as high as KOS and reactivated with high frequency albeit reduced kinetics. Acyclovir treatment that drastically decreased ocular and ganglionic replication of KOS had little effect on V204G. Thus, despite reduced neuronal replication due to impaired TK activity, this clinically relevant drug-resistant mutant can efficiently establish reactivatable latency.

Thymidine Kinase-Negative Herpes Simplex Virus 1 Can Efficiently Establish Persistent Infection in Neural Tissues of Nude Mice

Herpes simplex virus 1 (HSV-1) establishes latency in neural tissues of immunocompetent mice but persists in both peripheral and neural tissues of lymphocyte-deficient mice. Thymidine kinase (TK) is believed to be essential for HSV-1 to persist in neural tissues of immunocompromised mice, because infectious virus of a mutant with defects in both TK and UL24 is detected only in peripheral tissues, but not in neural tissues, of severe combined immunodeficiency mice (T. Valyi-Nagy, R. M. Gesser, B. Raengsakulrach, S. L. Deshmane, B. P. Randazzo, A. J. Dillner, and N. W. Fraser, Virology 199:484-490, 1994, https://doi.org/10.1006/viro.1994.1150). Here we find infiltration of CD4 and CD8 T cells in peripheral and neural tissues of mice infected with a TK-negative mutant. We therefore investigated the significance of viral TK and host T cells for HSV-1 to persist in neural tissues using three genetically engineered mutants with defects in only TK or in both TK and UL24 and two strains of nude mice. Surprisingly, all three mutants establish persistent infection in up to 100% of brain stems and 93% of trigeminal ganglia of adult nude mice at 28 days postinfection, as measured by the recovery of infectious virus. Thus, in mouse neural tissues, host T cells block persistent HSV-1 infection, and viral TK is dispensable for the virus to establish persistent infection. Furthermore, we found 30- to 200-fold more virus in neural tissues than in the eye and detected glycoprotein C, a true late viral antigen, in brainstem neurons of nude mice persistently infected with the TK-negative mutant, suggesting that adult mouse neurons can support the replication of TK-negative HSV-1.

IMPORTANCE

Acyclovir is used to treat herpes simplex virus 1 (HSV-1)-infected immunocompromised patients, but treatment is hindered by the emergence of drug-resistant viruses, mostly those with mutations in viral thymidine kinase (TK), which activates acyclovir. TK mutants are detected in brains of immunocompromised patients with persistent infection. However, answers to the questions as to whether TK-negative (TK^-) HSV-1 can establish persistent infection in brains of immunocompromised hosts and whether neurons in vivo are permissive for TK^- HSV-1 remain elusive. Using three genetically engineered HSV-1 TK^- mutants and two strains of nude mice deficient in T cells, we found that all three HSV-1 TK^- mutants can efficiently establish persistent infection in the brain stem and trigeminal ganglion and detected glycoprotein C, a true late viral antigen, in brainstem neurons. Our study provides evidence that TK^- HSV-1 can persist in neural tissues and replicate in brain neurons of immunocompromised hosts.

Efficacy of Herpes Simplex Virus Vector Encoding the Human Preproenkephalin Gene for Treatment of Facial Pain in Mice

AIMS

To determine whether herpes simplex virus-based vectors can efficiently transduce mouse trigeminal ganglion (TG) neurons and attenuate preexisting nerve injury-induced whisker pad mechanical hypersensitivity in a trigeminal inflammatory compression (TIC) neuropathic pain model.

METHODS

Tissue transduction efficiencies of replication-conditional and replication-defective vectors to mouse whisker pads after topical administration and subcutaneous injection were assessed using quantitative real-time PCR (qPCR). Tissue tropism and transgene expression were assessed using qPCR and reverse-transcriptase qPCR following topical application of the vectors. Whisker pad mechanical sensitivities of TIC-injured mice were determined using graduated von Frey fibers before and after application of human preproenkephalin expressing replication-conditional vector (KHPE). Data were analyzed using one-way analysis of variance (ANOVA) and post hoc tests.

RESULTS

Transduction of target TGs was 8- to 50-fold greater after topical application than subcutaneous injection and ≥ 100-fold greater for replication-conditional than replication-defective vectors. Mean KHPE loads remained constant in TGs (4.5-9.8 × 10(4) copies/TG) over 3 weeks but were below quantifiable levels (10 copies/tissue) within 2 weeks of application in other nontarget cephalic tissues examined. Transgene expression in TGs was maximal during 2 weeks after topical application (100-200 cDNA copies/mL) and was below quantifiable levels (1 cDNA copy/mL) in all nontarget tissues. Topical KHPE administration reduced TIC-related mechanical hypersensitivity on whisker pads 4-fold (P < .05) for at least 1 week.

CONCLUSION

Topically administered KHPE produced a significant antinociceptive effect in the TIC mouse model of chronic facial neuropathic pain. This is the first report in which a gene therapeutic approach reduced trigeminal pain-related behaviors in an established pain state in mice.

Mutation of UL24 impedes the dissemination of acute herpes simplex virus 1 infection from the cornea to neurons of trigeminal ganglia

Herpes simplex virus 1 (human herpesvirus 1) initially infects epithelial cells of the mucosa and then goes on to infect sensory neurons leading ultimately to a latent infection in trigeminal ganglia (TG). UL24 is a core herpesvirus gene that has been identified as a determinant of pathogenesis in several Alphaherpesvirinae, although the underlying mechanisms are unknown. In a mouse model of ocular infection, a UL24-deficient virus exhibited a reduction in viral titres in tear films of 1 log10, whilst titres in TG are often below the level of detection. Moreover, the efficiency of reactivation from latency was also severely reduced. Herein, we investigated how UL24 contributed to acute infection of TG. Our results comparing the impact of UL24 on viral titres in eye tissue versus in tear films did not reveal a general defect in virus release from the cornea. We also found that the impairment of replication seen in mouse primary embryonic neurons with a UL24-deficient virus was not more severe than that observed in an epithelial cell line. Rather, in situ histological analyses revealed that infection with a UL24-deficient virus led to a significant reduction in the number of acutely infected neurons at 3 days post-infection (p.i.). Moreover, there was a significant reduction in the number of neurons positive for viral DNA at 2 days p.i. for the UL24-deficient virus as compared with that observed for WT or a rescue virus. Our results supported a model whereby UL24 functions in the dissemination of acute infection from the cornea to neurons in TG.

Tranylcypromine reduces herpes simplex virus 1 infection in mice

Herpes simplex virus 1 (HSV-1) infects the majority of the human population and establishes latency by maintaining viral genomes in neurons of sensory ganglia. Latent virus can undergo reactivation to cause recurrent infection. Both primary and recurrent infections can cause devastating diseases, including encephalitis and corneal blindness. Acyclovir is used to treat patients, but virus resistance to acyclovir is frequently reported. Recent in vitro findings reveal that pretreatment of cells with tranylcypromine (TCP), a drug widely used in the clinic to treat neurological disorders, restrains HSV-1 gene transcription by inhibiting the histone-modifying enzyme lysine-specific demethylase 1. The present study was designed to examine the anti-HSV-1 efficacy of TCP in vivo because of the paucity of reports on this issue. Using the murine model, we found that TCP decreased the severity of wild-type-virus-induced encephalitis and corneal blindness, infection with the acyclovir-resistant (thymidine kinase-negative) HSV-1 mutant, and tissue viral loads. Additionally, TCP blocked in vivo viral reactivation in trigeminal ganglia. These results support the therapeutic potential of TCP for controlling HSV-1 infection.

Suppression of transcription factor early growth response 1 reduces herpes simplex virus 1-induced corneal disease in mice

Herpes simplex virus 1 replication initiates angiogenesis and inflammation in the cornea. This can result in herpetic stromal keratitis (HSK), which is a leading cause of infection-induced corneal blindness. Host cellular factors mediate the progression of HSK, but little is known about these cellular factors and their mechanisms of action. We show here that the expression of the cellular transcription factor early growth response 1 (Egr-1) in HSV-1-infected mouse corneas was enhanced. Enhanced Egr-1 expression aggravated HSK by increasing viral replication and subsequent neovascularization with high levels of potent angiogenic factors, fibroblast growth factor 2, and vascular endothelial growth factor. Furthermore, Egr-1 deficiency due to a targeted disruption of the gene or knockdown of Egr-1 expression topically using a DNA-based enzyme significantly reduced HSK by decreasing both viral replication and the angiogenic response. The present study provides the first evidence that endogenous Egr-1 aggravates HSK and that blocking Egr-1 reduces corneal damage.

Slipping and sliding: frameshift mutations in herpes simplex virus thymidine kinase and drug-resistance

Some of the most successful antiviral agents currently available are effective against herpes simplex virus. However, resistance to these drugs is frequently associated with significant morbidity, particularly in immunocompromised patients. In addition to the clinical implications of drug resistance, the range of biological processes exploited by the virus to attain resistance while maintaining pathogenicity is proving to be surprising. These mechanisms, which include ribosomal frameshifting, induced infidelity of the DNA polymerase, and internal ribosome entry, are discussed.

Differential importance of highly conserved residues in UL24 for herpes simplex virus 1 replication in vivo and reactivation

The UL24 gene of herpes simplex virus 1 (HSV-1) is widely conserved among all subfamilies of the Herpesviridae. It is one of only four HSV-1 genes for which mutations have been mapped that confer a syncytial plaque phenotype. In a mouse model of infection, UL24-deficient viruses exhibit reduced titres, particularly in neurons, and an apparent defect in reactivation from latency. There are several highly conserved residues in UL24; however, their importance in the role of UL24 in vivo is unknown. In this study, we compared virus strains with substitution mutations corresponding to the PD-(D/E)XK endonuclease motif of UL24 (vUL24-E99A/K101A) or a substitution of another highly conserved residue (vUL24-G121A). Both mutant viruses cause the formation of syncytial plaques at 39 degrees C; however, we found that the viruses differed dramatically when tested in a mouse model of infection. vUL24-E99A/K101A exhibited titres in the eye that were 10-fold lower than those of the wild-type virus KOS, and titres in trigeminal ganglia (TG) that were more than 2 log10 lower. Clinical signs were barely detectable with vUL24-E99A/K101A. Furthermore, the percentage of TG from which virus reactivated was also significantly lower for this mutant than for KOS. In contrast, vUL24-G121A behaved similarly to the wild-type virus in mice. These results are consistent with the endonuclease motif being important for the role of UL24 in vivo and also imply that the UL24 temperature-dependent syncytial plaque phenotype can be separated genetically from several in vivo phenotypes.

Investigation of the mechanism by which herpes simplex virus type 1 LAT sequences modulate preferential establishment of latent infection in mouse trigeminal ganglia

We previously demonstrated that herpes simplex virus type 1 (HSV-1) preferentially establishes latent infection in monoclonal antibody (MAb) A5-positive ganglionic neurons and that a 2.8-kb portion of the HSV-1 genome, corresponding to the 5' end of the LAT (latency-associated transcript) coding region, is responsible for this phenotype (38, 65). In the current study we carried out further genetic mapping of this latency phenotype and investigated some of the mechanisms that might be responsible. Studies with the chimeric virus HSV-1 17syn+/LAT2, an HSV-1 virus engineered to express HSV-2 LAT, demonstrated that this virus exhibited an HSV-2 latency phenotype, preferentially establishing latency in MAb KH10-positive neurons. This result is complementary to that previously described for the chimeric virus HSV-2 333/LAT1 and indicate that the HSV-1 latency phenotype can be changed to that of HSV-2 by substitution of a 2.8-kb piece of complementary viral DNA. Sequential studies in which we evaluated the pattern of HSV-1 latent infection of the mouse trigeminal ganglion following ocular inoculation with viruses with deletions of functional thymidine kinase, glycoprotein E, ICP0, and US9 protein demonstrate that preferential establishment of HSV-1 latent infection in A5-positive neurons is not a consequence of (i) differential access of HSV-1 to A5-positive neurons,(ii) differential cell-to-cell spread of HSV-1 to A5-positive neurons, (iii) differential "round-trip" spread of HSV-1 to A5-positive neurons, or (iv) expression of ICP0. Additional mapping studies with the HSV-1 LAT deletion viruses dLAT371, 17DeltaSty, and 17Delta348 indicate that most of the LAT 5' exon is not required for HSV-1 to preferentially establish latent infection in A5-positive neurons.

Reduction in severity of a herpes simplex virus type 1 murine infection by treatment with a ribozyme targeting the UL20 gene RNA

Hammerhead ribozymes were designed to target mRNA of several essential herpes simplex virus type 1 (HSV-1) genes. A ribozyme specific for the late gene U(L)20 was packaged in an adenovirus vector (Ad-U(L)20 Rz) and evaluated for its capacity to inhibit the viral replication of several HSV-1 strains, including that of the wild-type HSV-1 (17syn+ and KOS) and several acycloguanosine-resistant strains (PAAr5, tkLTRZ1, and ACGr4) in tissue culture. The Ad-U(L)20 Rz was also tested for its ability to block an HSV-1 infection, using the mouse footpad model. Mouse footpads were treated with either the Ad-U(L)20 Rz or an adenoviral vector expressing green fluorescent protein (Ad-GFP) and then infected immediately thereafter with 10(4) PFU of HSV-1 strain 17syn+. Ad-U(L)20 ribozyme treatment consistently led to a 90% rate of protection for mice from lethal HSV-1 infection, while the survival rate in the control groups was less than 45%. Consistent with this protective effect, treatment with the Ad-U(L)20 Rz reduced the viral DNA load in the feet, the dorsal root ganglia, and the spinal cord relative to that of the Ad-GFP-treated animals. This study suggests that ribozymes targeting essential genes of the late kinetic class may represent a new therapeutic strategy for inhibiting HSV infection.

Expression of extremely low levels of thymidine kinase from an acyclovir-resistant herpes simplex virus mutant supports reactivation from latently infected mouse trigeminal ganglia

A single-cytosine-deletion in the herpes simplex virus gene encoding thymidine kinase (TK) was previously found in an acyclovir-resistant clinical isolate. A laboratory strain engineered to carry this mutation did not generate sufficient TK activity for detection by plaque autoradiography, which detected 0.25% wild-type activity. However, a drug sensitivity assay suggested that extremely low levels of TK are generated by this virus. The virus was estimated to express 0.09% of wild-type TK activity via a ribosomal frameshift 24 nucleotides upstream of the mutation. Remarkably, this appeared to be sufficient active TK to support a low level of reactivation from latently infected mouse trigeminal ganglia.

Involvement of UL24 in herpes-simplex-virus-1-induced dispersal of nucleolin

UL24 of herpes simplex virus 1 is important for efficient viral replication, but its function is unknown. We generated a recombinant virus, vHA-UL24, encoding UL24 with an N-terminal hemagglutinin tag. By indirect immunofluorescence at 9 h post-infection (hpi), we detected HA-UL24 in nuclear foci and in cytoplasmic speckles. HA-UL24 partially co-localized with nucleolin, but not with ICP8 or coilin, markers for nucleoli, viral replication compartments, and Cajal bodies respectively. HA-UL24 staining was often juxtaposed to that of another nucleolar protein, fibrillarin. Analysis of HSV-1-induced nucleolar modifications revealed that by 18 hpi, nucleolin staining had dispersed, and fibrillarin staining went from clusters of small spots to a few separate but prominent spots. Fibrillarin redistribution appeared to be independent of UL24. In contrast, cells infected with a UL24-deficient virus retained foci of nucleolin staining. Our results demonstrate involvement of UL24 in dispersal of nucleolin during infection.

Competition and complementation between thymidine kinase-negative and wild-type herpes simplex virus during co-infection of mouse trigeminal ganglia

Laboratory strains of herpes simplex virus lacking thymidine kinase (TK) cannot replicate acutely to detectable levels in mouse trigeminal ganglia and do not reactivate from latency. However, many pathogenic clinical isolates that are resistant to the antiviral drug acyclovir are heterogeneous populations of TK-negative (TK(-)) and TK-positive (TK(+)) viruses. To recapitulate this in vivo, mice were infected with mixtures of wild-type virus and a recombinant TK(-) mutant in various ratios. Following co-infection, the replication, number of latent viral genomes and reactivation efficiency of TK(+) virus in trigeminal ganglia were reduced in a manner related to the amount of TK(-) virus in the inoculum. TK(+) virus did not always complement the acute replication or increase the number of latent viral genomes of TK(-) mutant in mouse ganglia. Even so, TK(+) virus could still confer the pathogenic phenotype to a TK(-) mutant, somehow providing sufficient TK activity in trans to permit a TK(-) mutant to reactivate from latently infected ganglia.

Neurotropism of herpes simplex virus type 1 in brain organ cultures

The mechanism of herpes simplex virus type 1 (HSV-1) penetration into the brain and its predilection to infect certain neuronal regions is unknown. In order to study HSV-1 neurotropism, an ex vivo system of mice organotypic brain slices was established and the tissue was infected with HSV-1 vectors. Neonate tissues showed restricted infection confined to leptomeningeal, periventricular and cortical brain regions. The hippocampus was the primary parenchymatous structure that was also infected. Infection was localized to early progenitor and ependymal cells. Increasing viral inoculum increased the intensity and enlarged the infected territory, but the distinctive pattern of infection was maintained and differed from that observed with adenovirus and Vaccinia virus. Neonate brain tissues were much more permissive for HSV-1 infection than adult mouse brain tissues. Taken together, these results indicate a complex interaction of HSV-1 with different brain-cell types and provide a useful vehicle to elucidate the mechanisms of viral neurotropism.

Low-level expression and reversion both contribute to reactivation of herpes simplex virus drug-resistant mutants with mutations on homopolymeric sequences in thymidine kinase

Many acyclovir-resistant herpes simplex virus isolates from patients contain insertions or deletions in homopolymeric sequences in the thymidine kinase (TK) gene (tk). Viruses that have one (G8) or two (G9) base insertions in a run of seven G's (G string) synthesize low levels of active TK (TK-low phenotype), evidently via ribosomal frameshifting. These levels of TK can suffice to permit reactivation from latently infected mouse ganglia, but in a majority of ganglia, especially with the G9 virus, reactivation of virus that has reverted to the TK-positive phenotype predominates. To help address the relative contributions of translational mechanisms and reversion in reactivation, we generated viruses with a base either inserted or deleted just downstream of the G string. Both of these viruses had a TK-low phenotype similar to that of the G8 and G9 viruses but with less reversion. Both of these viruses reactivated from latently infected trigeminal ganglia, albeit inefficiently, and most viruses that reactivated had a uniformly TK-low phenotype. We also generated viruses that have one insertion in a run of six C's or one deletion in a run of five C's. These viruses lack measurable TK activity. However, they reactivated from latently infected ganglia, albeit inefficiently, with the reactivating viruses having reverted to the wild-type TK phenotype. Therefore, for G-string mutants, levels of active TK as low as 0.25% generated by translational mechanisms can suffice for reactivation, but reversion can also contribute. For viruses that lack TK activity due to mutations on other homopolymeric sequences, reactivation can occur via reversion.

Herpes simplex virus type 1 recombinants without the oriL sequence replicate DNA with increased fidelity

Herpes simplex virus type 1 (HSV-1) contains three DNA replication origins (ori) of two types. The oriL is located in the center of the unique long sequences, whereas two copies of oriS, which are structurally different from oriL, are within the reiterated sequences flanking the unique short sequences. Recombinant viruses were constructed from ts+7, which contains a deletion of oriL sequences, to have either the beta-galactosidase gene or the supF amplicon integrated into the thymidine kinase locus. Rescue recombinants also were constructed from the supF-containing recombinant to restore the deleted oriL to the wild type sequences. These recombinants were subjected to mutagenesis assays. Results demonstrated that ts+7 viruses with the deletion in oriL sequences replicated both target genes with higher fidelity compared to those derived from the parental strain KOS. Possible mechanisms leading to the high fidelity of DNA replication mediated by viruses without intact oriL sequences are discussed.

Solid tissues can be manipulated ex vivo and used as vehicles for gene therapy

BACKGROUND

Organ fragments can be cultured for weeks in vitro if they are prepared of microscopic thickness and if the basic organ structure is preserved. Such organ fragments, which we termed micro-organs (MOs), express in culture endogenous tissue-specific gene products. We have exploited this methodology to engineer MOs ex vivo by gene transfer.

METHODS

MOs prepared from spleen, lung, colon and skin were infected using: herpes simplex type-1, adeno virus, vaccinia virus and murine leukemia virus (MuLV), carrying the reporter gene beta-galactosidase.

RESULTS

All four viral vectors infected MOs in culture, with adeno infection giving significantly higher values. After optimization, high levels of expression (> 15% positive cells), comparable to those obtained with the adeno construct, were also obtained using the MuLV construct both in vitro and after implantation into syngeneic hosts. After implantation, the engineered tissue was found to remain localized, become vascularized, and to express the transduced gene for several months.

CONCLUSIONS

The system can be used to study interactions between viruses and tissues both ex vivo and in vivo. Furthermore, the approach proposes a novel platform for ex vivo gene therapy. Such engineered structures could be used as autologous biological pumps for continuous secretion in vivo of gene products of clinical importance.

An unusual internal ribosome entry site in the herpes simplex virus thymidine kinase gene

We have investigated a herpes simplex virus mutant that expresses low levels of thymidine kinase (TK), a phenotype associated with drug resistance and pathogenicity, despite a single-base deletion in the gene. Using a dual-reporter system, a 39-nt sequence including the mutation was shown to direct expression of the downstream reporter gene in reticulocyte lysate. Translation of the downstream reporter was not impaired when the mRNA lacked a 5' cap or had a stable stem loop 5' of the upstream reporter and was relatively resistant to edeine, an antibiotic that prevents AUG codon recognition by the 40S-eIF2-GTP/Met-tRNAi complex. Twelve nucleotides were as active as the original sequence for translation of the downstream reporter. Surprisingly, this sequence lacks an AUG codon. Analysis of point mutations showed that a CUG codon in the sequence was important. However, many single-base changes had only limited effects, and introduction of AUG codons did not increase translation. A mutant virus containing both the single-base deletion and a mutation that reduced downstream translation in vitro had significantly less TK activity than a virus with the single-base deletion alone. Thus, a remarkably short internal ribosome entry site (IRES) that lacks an AUG codon resides in the viral tk gene. The IRES appears to be responsible for TK expression from a drug-resistant mutant that would otherwise express no TK, which may contribute to pathogenicity. Because we found numerous short sequences with IRES activity, there might be many hitherto unrecognized polypeptides expressed at low levels from eukaryotic mRNAs.

Failure of thymidine kinase-negative herpes simplex virus to reactivate from latency following efficient establishment

Thymidine kinase-negative mutants of herpes simplex virus did not reactivate from latency in mouse trigeminal ganglia, even when their latent viral loads were comparable to those that permitted reactivation by wild-type virus. Thus, reduced establishment of latency does not suffice to account for the failure to reactivate.

Translational compensation of a frameshift mutation affecting herpes simplex virus thymidine kinase is sufficient to permit reactivation from latency

Herpes simplex virus thymidine kinase is important for reactivation of virus from its latent state and is a target for the antiviral drug acyclovir. Most acyclovir-resistant isolates have mutations in the thymidine kinase gene; however, how these mutations confer clinically relevant resistance is unclear. Reactivation from explanted mouse ganglia was previously observed with a patient-derived drug-resistant isolate carrying a single guanine insertion within a run of guanines in the thymidine kinase gene. Despite this mutation, low levels of active enzyme were synthesized following an unusual ribosomal frameshift. Here we report that a virus, generated from a pretherapy isolate from the same patient, engineered to lack thymidine kinase activity, was competent for reactivation. This suggested that the clinical isolate contains alleles of other genes that permit reactivation in the absence of thymidine kinase. Therefore, to establish whether thymidine kinase synthesized via a ribosomal frameshift was sufficient for reactivation under conditions where reactivation requires this enzyme, we introduced the mutation into the well-characterized strain KOS. This mutant virus reactivated from latency, albeit less efficiently than KOS. Plaque autoradiography revealed three phenotypes of reactivating viruses: uniformly low thymidine kinase activity, mixed high and low activity, and uniformly high activity. We generated a recombinant thymidine kinase-null virus from a reactivating virus expressing uniformly low activity. This virus did not reactivate, confirming that mutations in other genes that would influence reactivation had not arisen. Therefore, in strains that require thymidine kinase for reactivation from latency, low levels of enzyme synthesized via a ribosomal frameshift can suffice.

High-frequency phenotypic reversion and pathogenicity of an acyclovir-resistant herpes simplex virus mutant

A double-guanine-insertion mutation within a run of guanines in the herpes simplex virus gene encoding thymidine kinase (TK) was previously found in an acyclovir-resistant clinical isolate. This mutation was engineered into strain KOS, and stocks were generated from single plaques. Plaque autoradiography revealed that most plaques in such stocks exhibited low levels of TK activity, while approximately 3% of plaques exhibited high levels of TK activity, indicating a remarkably high frequency of phenotypic reversion. This virus was able to reactivate from latency in mouse ganglia; a fraction of the reactivating virus expressed a high level of TK activity due to an additional G insertion, suggesting that the observed genetic instability contributed to pathogenicity.

Thymidine kinase of herpes simplex virus type 1 strain KOS lacks mutator activity

The effect of thymidine kinase (TK) encoded by herpes simplex virus type 1(HSV-1) strain KOS in DNA replication fidelity was examined by two different mutagenesis assays. Mutagenesis assay of the LacZ reporter gene present in recombinant tkLTRZ1, which contained the integrated LacZ gene in the tk locus, revealed a less than 0.05% mutation frequency of the LacZ gene regardless of whether the viruses were propagated in TK-expressing cells or control cells, conflicting an earlier report that a HSV-1 TK(+) strain replicated a 0.5% mutation frequency of the LacZ gene (R. B. Pyles and R. L. Thompson, 1994, J. Virol. 68, 4514-4524). Furthermore, TK-proficient and -deficient recombinant viruses replicated with similar mutation frequencies (0.027 and 0.026%, respectively) of the LacZ gene, which was integrated in the polymerase locus. Results of SupF mutagenesis assay demonstrated that neither the spectra of mutation nor the mutation frequencies of SupF gene, which was integrated in the tk locus of recombinant, were significantly different (P > 0.05) in progeny viruses grown in TK-expressing cells and control cells. Therefore, both LacZ and SupF mutagenesis assays demonstrated that TK of the HSV-1 strain KOS did not have detectable mutator activity.

Gene transfer mediated by different viral vectors following direct cannulation of mouse submandibular salivary glands

The salivary gland has been suggested as an accessible organ for gene transfer to express recombinant proteins locally in the saliva, as well as for secretion to the blood circulation. The aim of this study was to evaluate the efficiency of gene transfer to salivary glands using different viral vectors: adenovirus, vaccinia, herpes simplex type 1 (HSV), and two retroviral vectors (murine leukemia virus (MuLV) and lentivirus). We show, by in situ staining and beta-galactosidase reporter activity assay, that the adenoviral and vaccinia vectors were able to deliver the reporter gene efficiently to acinar and duct cells. The HSV vector was less efficient and infected only the acinar cells. The lentiviral vector infected acinar and duct cells, but at a relatively low efficiency. The MuLV vector did not infect the salivary gland unless cell proliferation was induced. Host immune responses to viral infection, inflammation, apoptosis and lymphocyte infiltration, in the transduced glands, were assessed. The DNA viral vectors induced local lymphocyte infiltration and apoptosis. In contrast, the retroviral vectors did not induce an immune response. Our results describe the outcome of salivary gland infection with each of the five different viral vectors and indicate their advantages and limitations for transferring genes to the salivary glands.

Models of repair mechanisms for future treatment modalities of Parkinson's disease

Parkinson's disease is one of the most likely neurological disorders to be fully treatable by drugs and new therapeutic modalities. The age-dependent and multifactorial nature of its pathogenesis allows for many strategies of intervention and repair. Most data indicate that the selectively vulnerable dopaminergic neurons in the substantia nigra of patients that have developed Parkinson's disease can be modified by protective and reparative therapies. First, the oxidative stress, protein abnormalities, and cellular inclusions typically seen could be dealt with by anti-oxidants, trophic factors, and proteolytic enhancements. Secondly, if the delay of degeneration is not sufficient, then immature dopamine neurons can be placed in the parkinsonian brain by transplantation. Such neurons can be derived from stem cell sources or even stimulated to repair from endogenous stem cells. Novel molecular and cellular treatments provide new tools to prevent and alleviate Parkinson's disease.

Replication fidelity of the supF gene integrated in the thymidine kinase locus of herpes simplex virus type 1

Recombinant viruses were constructed to have an Escherichia coli replicon containing a mutagenesis marker, the supF gene, integrated within the thymidine kinase locus (tk) of herpes simplex virus type 1. These viruses expressed either wild-type or mutant DNA polymerase (Pol) and were tested in a mutagenesis assay for the fidelity of their replication of the supF gene. A mutation frequency of approximately 10(-4) was observed for wild-type strain KOS-derived recombinants in their replication of the supF gene. However, recombinants derived from the PAA(r)5 Pol mutant, which has been demonstrated to have an antimutator phenotype in replicating the tk gene, had three- to fourfold increases in supF mutation frequency (P < 0.01), a result similar to that exhibited when the supF gene was induced to replicate as episomal DNA (Y. T. Hwang, B.-Y. Liu, C.-Y. Hong, E. J. Shillitoe, and C. B. C. Hwang, J. Virol. 73:5326-5332, 1999). Thus, the PAA(r)5 Pol mutant had an antimutator function in replicating the tk gene and was less accurate in replicating the supF gene than was the wild-type strain. The spectra of mutations and distributions of substituted bases within the supF genes that replicated as genomic DNA were different from those in the genes that replicated as episomal DNA. Therefore, the differences in sequence contents between the two target genes influenced the accuracy of the Pol during viral replication. Furthermore, the replication mode of the target gene also affected the mutational spectrum.

Therapeutic efficacy in experimental polyarthritis of viral-driven enkephalin overproduction in sensory neurons

Rheumatoid arthritis is characterized by erosive inflammation of the joints, new bone proliferation, and ankylosis, leading to severely reduced locomotion and intense chronic pain. In a model of this disease, adjuvant-induced polyarthritis in the rat, neurons involved in pain transmission and control undergo plastic changes, especially at the spinal level. These changes affect notably neurons that contain opioids, such as enkephalins deriving from preproenkephalin A (PA) precursor protein. Using recombinant herpes simplex virus containing rat PA cDNA, we enhanced enkephalin synthesis in sensory neurons of polyarthritic rats. This treatment markedly improved locomotion and reduced hyperalgesia. Furthermore, the progression of bone destruction slowed down, which is the most difficult target to reach in the treatment of patients suffering from arthritis. These data demonstrate the therapeutic efficacy of enkephalin overproduction in a model of systemic inflammatory and painful chronic disorder.

Gene transfer by viral vectors into blood vessels in a rat model of retinopathy of prematurity

AIMS

To test the feasibility of gene transfer into hyaloid blood vessels and into preretinal neovascularisation in a rat model of retinopathy of prematurity (ROP), using different viral vectors.

METHODS

Newborn rats were exposed to alternating hypoxic and hyperoxic conditions in order to induce ocular neovascularisation (ROP rats). Adenovirus, herpes simplex, vaccinia, and retroviral (MuLV based) vectors, all carrying the beta galactosidase (beta-gal) gene, were injected intravitreally on postnatal day 18 (P18). Two sets of controls were also examined: P18 ROP rats injected with saline and P18 rats that were raised in room air before the viral vectors or saline were injected. Two days after injection, the rats were killed, eyes enucleated, and beta-gal expression was examined by X-gal staining in whole mounts and in histological sections.

RESULTS

Intravitreal injection of the adenovirus and vaccinia vectors yielded marked beta-gal expression in hyaloid blood vessels in the rat ROP model. Retinal expression of beta-gal with these vectors was limited almost exclusively to the vicinity of the injection site. Injection of herpes simplex yielded a punctuate pattern of beta-gal expression in the retina but not in blood vessels. No significant beta-gal expression occurred in rat eyes injected with the retroviral vector.

CONCLUSIONS

Adenovirus is an efficient vector for gene transfer into blood vessels in an animal model of ROP. This may be a first step towards utilising gene transfer as a tool for modulating ocular neovascularisation for experimental and therapeutic purposes.

Gene therapy in the CNS

Gene therapy for neurological disorder is currently an experimental concept. The goals for clinical utilization are the relief of symptoms, slowing of disease progression, and correction of genetic abnormalities. Experimental studies are realizing these goals in the development of gene therapies in animal models. Discoveries of the molecular basis of neurological disease and advances in gene transfer systems have allowed focal and global delivery of therapeutic genes for a wide variety of CNS disorders. Limitations are still apparent, such as stability and regulation of transgene expression, and safety of both vector and expressed transgene. In addition, the brain adds several challenges not seen in peripheral gene therapy paradigms, such as post-mitotic cells, heterogeneity of cell types and circuits, and limited access. Moreover, it is likely that several modes of gene delivery will be necessary for successful gene therapies of the CNS. Collaborative efforts between clinicians and basic researchers will likely yield effective gene therapy in the CNS.

Gene transfer to the nigrostriatal system by hybrid herpes simplex virus/adeno-associated virus amplicon vectors

To improve gene transfer to CNS neurons, critical elements of herpes simplex virus 1 (HSV-1) amplicons and recombinant adeno-associated virus (AAV) vectors were combined to construct a hybrid amplicon vector, and then packaged via a helper virus-free system. We tested the HSV/AAV hybrid amplicon vectors for transduction efficiency and stability of transgene expression (green fluorescent protein) in primary neuronal cultures from rat fetal ventral mesencephalon, in comparison with traditional HSV amplicon, AAV, or adenovirus (Ad) vectors at the same multiplicity of infection. The HSA/AAV hybrid vectors transduced the highest number of primary neurons in culture 2 days after infection. As compared with all other vectors tested, only hybrid vectors containing the AAV rep gene maintained the 2-day level of transgene expression over 12 days in culture. This rep-containing hybrid vector was then tested for efficiency and safety in the brain. One month after injection into adult rat striatum (1 x 10(6) transducing units injected), transgene expression was observed within the striatum (ranging from 564 to 8610 cells) and the substantia nigra (via retrograde transport, ranging from 130 to 809 neurons). The HSV/AAV hybrid amplicon vectors transduced predominantly neurons within the striatum, and showed transduction efficacy similar to and in many cases higher than that of HSV amplicon vectors. No immune response was observed in the HSA/AAV hybrid vector-injected brains, as determined by immune markers specific for helper T lymphocytes, cytotoxic T lymphocytes, and microglia. This HSV/AAV hybrid system shows high transduction efficiency and stability in culture. The effective and safe transgene delivery into the nigrostriatal system illustrates its potential for therapeutic application for neurologic disorders, such as Parkinson and Huntington disease.

Antihyperalgesic effects of infection with a preproenkephalin-encoding herpes virus

To test the utility of gene therapeutic approaches for the treatment of pain, a recombinant herpes simplex virus, type 1, has been engineered to contain the cDNA for an opioid peptide precursor, human preproenkephalin, under control of the human cytomegalovirus promoter. This virus and a similar recombinant containing the Escherichia coli lacZ gene were applied to the abraded skin of the dorsal hindpaw of mice. After infection, the presence of beta-galactosidase in neuronal cell bodies of the relevant spinal ganglia (lacZ-containing virus) and of human proenkephalin (preproenkephalin-encoding virus) in the central terminals of these neurons indicated appropriate gene delivery and expression. Baseline foot withdrawal responses to noxious radiant heat mediated by Adelta and C fibers were similar in animals infected with proenkephalin-encoding and beta-galactosidase-encoding viruses. Sensitization of the foot withdrawal response after application of capsaicin (C fibers) or dimethyl sulfoxide (Adelta fibers) observed in control animals was reduced or eliminated in animals infected with the proenkephalin-encoding virus for at least 7 weeks postinfection. Hence, preproenkephalin cDNA delivery selectively blocked hyperalgesia without disrupting baseline sensory neurotransmission. This blockade of sensitization was reversed by administration of the opioid antagonist naloxone, apparently acting in the spinal cord. The results demonstrate that the function of sensory neurons can be selectively altered by viral delivery of a transgene. Because hyperalgesic mechanisms may be important in establishing and maintaining neuropathic and other chronic pain states, this approach may be useful for treatment of chronic pain and hyperalgesia in humans.

Recent advances in the pharmacology of nerve-injury pain

Nerve injury pain remains a complex clinical challenge. Although the development of animal models of nerve injury pain has aided our understanding of potential pathophysiologic mechanisms for this condition, effective treatment still remains beyond our reach. Several classes of agents appear to block pain behavior in these animal models and humans, but they are often limited in their use by low efficacy, or undesirable side-effects. A prerequisite for the improvement of nerve injury pain includes the development of clinically-relevant animal models in which therapeutic targets can be identified.

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

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

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

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

Accumulation of viral transcripts and DNA during establishment of latency by herpes simplex virus

Latent infection of mice with wild-type herpes simplex virus is established during an acute phase of ganglionic infection in which there is abundant viral replication and productive-cycle gene expression. Thymidine kinase-negative mutants establish latent infections but are severely impaired for acute ganglionic replication and productive-cycle gene expression. Indeed, by in situ hybridization assays, acute infection by these mutants resembles latency. To assess events during establishment of latency by wild-type and thymidine kinase-negative viruses, we quantified specific viral nucleic acid sequences in mouse trigeminal ganglia during acute ganglionic infection by using sensitive PCR-based assays. Through 32 h postinfection, viral DNA and transcripts representative of the three kinetic classes of productive-cycle genes accumulated to comparable levels in wild-type- and mutant-infected ganglia. At 48 and 72 h, although latency-associated transcripts accumulated to comparable levels in ganglia infected with wild-type or mutant virus, levels of DNA accumulating in wild-type-infected ganglia exceeded those in mutant-infected ganglia by 2 to 3 orders of magnitude. Coincident with this increase in DNA, wild-type-infected ganglia exhibited abundant expression of productive-cycle genes and high titers of infectious progeny. Nevertheless, the levels of productive-cycle RNAs expressed by mutant virus during acute infection greatly exceeded those expressed by wild-type virus during latency. The results thus distinguish acute infection of ganglia by a replication-compromised mutant from latent infection and may have implications for mechanisms of latency.

Gene transfer into sympathetic preganglionic neurons in vivo using a non-replicating thymidine kinase-deficient herpes simplex virus type 1

The suitability of non-replicating thymidine kinase deficient herpes simplex virus type 1 expressing bacterial beta-galactosidase (tk-lacZ HSV-1) as a transfer vehicle into sympathetic preganglionic neurons in vivo was assessed. Many sympathoadrenal preganglionic neurons (451 +/- 105) with normal morphology were identified using beta-galactosidase histochemistry two days after inoculation of tk-lacZ HSV-1 into the adrenal gland of hamsters. Beta-galactosidase activity co-localized with nicotinamide adenine dinucleotide phosphate-diaphorase-positive sympathetic preganglionic neurons in the nucleus intermediolateralus, pars principalis. The maximal number of beta-galactosidase expressing neurons was found two days post-inoculation but this number dropped dramatically after this time. An inflammatory infiltrate was abundant around infected neurons and in the white matter at five days and infected neurons appeared morphologically abnormal. At 26 days, the infiltrate was still present but no infected sympathoadrenal preganglionic neurons were detected. Approximately 25% fewer nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the nucleus intermediolateralis, pars principalis were counted ipsilaterally than contralaterally in animals infected for 14, 21 or 26 days with tk-lacZ HSV-1, compared to the 3% difference in animals mock-infected for 26 days. Approximately 33% of the estimated number of sympathoadrenal preganglionic neurons infected with tk-lacZ HSV-1 at five days were apoptotic or necrotic. About 60% of neurons infected with tk-lacZ HSV-1 at two days no longer expressed nicotinamide adenine dinucleotide phosphate-diaphorase at 14-26 days. In conclusion, the non-replicating thymidine kinase deficient HSV-1 was efficiently retrogradely transported from the adrenal gland to infect sympathoadrenal preganglionic neurons. These gene transfer experiments using tk-lacZ HSV-1 suggest that foreign gene expression in sympathetic preganglionic neurons in vivo may be maximal two days after inoculation when beta-galactosidase was expressed in the greatest number of sympathetic preganglionic neurons. After two days, fewer neurons expressed beta-galactosidase and the presence of tk-lacZ HSV-1 appeared to be altering protein expression in sympathetic preganglionic neurons and/or leading to the demise of the infected neuron.

Transgene expression of plasmid DNAs directed by viral or neural promoters in the rat brain

The use of circular plasmid DNA may be an alternative method for the transfer of genes into the brain and is presumably easier to use than other vectors, such as viruses or genetically engineered cells. The effectiveness and time course of the expression of a reporter gene (LacZ), directed by appropriate promoters, was studied after stereotaxic injection of naked plasmid DNAs into the rat thalamus, cortex or cerebellum. The efficiencies of three different promoters, the human cytomegalovirus (HCMV) promoter and the glial fibrillary acidic protein (GFAP) and neuron-specific enolase (NSE) promoters (specific for astrocytes and neurons, respectively) to drive reporter gene expression were compared. Efficient expression of beta-gal, detected by X-gal histochemistry or immunochemistry, required the use of 50 microg of DNA and was detectable as early as 48 h after injection. Expression increased until day 8, remained stable until day 15, then decreased over 2 months, probably as a result of non-specific degradation of the plasmids within the transfected cells rather than from specific down-regulation of promoters, as the same time course was seen with all three promoters tested. Depending on the promoter used (GFAP or NSE), LacZ was preferentially expressed within astrocytes or neurons, respectively. The GFAP promoter was found to be as efficient as the HCMV promoter, possibly due to the reactive gliosis induced by plasmid injection which is known to up-regulate GFAP expression.

Prolonged in vivo gene expression driven by a tyrosine hydroxylase promoter in a defective herpes simplex virus amplicon vector

A 9.0-kb fragment of the tyrosine hydroxylase (TH) promoter, previously shown to direct tissue-specific expression in transgenic mice, was fused to an Escherichia coli LacZ reporter gene in a defective herpes simplex virus type-1 (HSV-1) amplicon vector (THlac). The HSV immediate early (IE) 4/5 promoter (HSVlac) was used as a control. LacZ gene expression was visualized by X-Gal histochemical and TH immunocytochemical analysis. Two days and 10 weeks after THlac injection into rat caudate nucleus (CN), X-Gal-stained cells were observed in the substantia nigra (SN) and locus ceruleus (LC) ipsilateral to the injection site. These blue cells were TH-positive neurons as evidenced by double labeling with immunocytochemistry. Moreover, the number of X-Gal+, TH+ (double-positive) neurons in the SN increased at 10 weeks as compared to that seen 2 days after THlac injection. In marked contrast, few double-positive nigral neurons were observed either 2 days or 10 weeks after direct injection of THlac into SN. However, neither nigral nor striatal injection of HSVlac resulted in prolonged gene expression. These results suggest that a neuronal, but not a viral, promoter in an HSV vector can produce cell-type-specific, prolonged, and stable gene expression following retrograde transport. In addition, THlac produced infrequent gene expression in TH-negative cells (CN and dorsal to SN) after THlac injection into CN and SN, respectively. Overall, these results suggest that in some in vivo contexts cell-type-preferred expression can be achieved by a cellular promoter in an amplicon vector. Moreover, they underscore the need for the careful and systematic study of neuronal promoters in HSV vectors.

Long-term expression of a reporter gene from latent herpes simplex virus in the rat hippocampus

A problem in utilizing herpes simplex virus (HSV) as a vector for expression of foreign genes in CNS neurons has been the inability to facilitate long-term expression of the engineered genes. Previously, we showed that the murine moloney leukemia virus LTR would drive beta-galactosidase (beta-gal) transcription for extended periods from the latent viral genome in sensory, but not motor neurons. In this communication we further evaluate the utility of the LTR promoter for use in long-term expression vectors. Following stereotactic injection of 8117/43 (an ICP4 minus, non-replicating virus with the LTR driving the beta-gal gene, or KD6 (an ICP4 minus non-replicating virus not expressing beta-gal) into the hippocampus of rats, polymerase chain reaction (PCR) analysis of viral DNA after 2 months indicated that latent infections were established. Assaying by both x-gal staining and reverse transcriptase PCR we demonstrate that (1) beta-gal can be detected for at least 6 months in hippocampal neurons, and (2) although the number of beta-gal transcripts in these cells drops considerably by 2 weeks, they can be detected during the period studied. These studies indicate that the LTR promoter is active and affords long-term expression in the CNS, albeit at comparatively low levels compared to those observed at acute times.

Delivery of a foreign gene to sympathetic preganglionic neurons using recombinant herpes simplex virus

Two recombinant herpes simplex type 1 viruses expressing beta-galactosidase (encoded by the Escherichia coli lacZ gene) inserted into the unique long 41 (encoding virus host shutoff) or unique short 5 (encoding glycoprotein J) open reading frames were generated. Purified recombinants or wild-type herpes simplex type 1 were injected into the left adrenal gland of hamsters. Three days later, virus-infected neurons were detected in spinal cord sections from all infected hamsters. Neurons were visualized with beta-galactosidase histochemistry in spinal cord sections from hamsters infected with either of the recombinants but not with the wild-type virus. Wild-type virus could only be detected with immunocytochemistry. Insertional mutagenesis into the unique long 41 or unique short 5 regions of the herpes simplex genome by lacZ did not disrupt the neurotropic properties of the virus. Both recombinant viruses labelled the central nervous system sympathoadrenal preganglionic neurons as well as brainstem neurons. Because the virus host shutoff recombinant more readily crossed synapses to reach the brainstem compared to the glycoprotein J recombinant, the presence of glycoprotein J may facilitate cell to cell transmission in vivo. Both recombinants may be useful for the study of synaptic organization of neural circuits. Our recombinant viruses were less lytic yet neurovirulent after mutation of either glycoprotein J or virus host shutoff of herpes simplex virus type 1 wild-type. These recombinant viruses express the bacterial beta-galactosidase which is readily detectable using simple histochemistry. Inoculation of the adrenal gland or kidney with these viruses led to clear labelling of spinal cord cells. These viruses may be useful markers of specific neural circuits.

Quantification of transcripts from the ICP4 and thymidine kinase genes in mouse ganglia latently infected with herpes simplex virus

Herpes simplex virus establishes latency in nervous tissue in which it is maintained for the life of the mammalian host, with occasional reactivation leading to subsequent spread. Latency-associated transcripts are abundant during latency, but viral proteins and productive cycle RNAs have not been detected. Using sensitive, quantitative PCR assays, we have quantified certain viral RNAs specific to productive-cycle genes in mouse ganglia latently infected with herpes simplex virus type 1. Sense-strand RNA specific to the essential immediate-early gene, ICP4, was present in most ganglia in variable amounts relative to the amount of viral DNA, with one to seven molecules of RNA per viral genome in about 20% of ganglia. In contrast, the amount of latency-associated transcripts was much less variable, at an average of 4 x 10(4) molecules per viral genome. The amounts of ICP4-specific RNA were similar at 30 and 60 days postinfection, and at least some of these transcripts initiated within a region consistent with utilization of the ICP4 promoter. RNA specific to the thymidine kinase gene, whose transcription in productive infection is dependent on ICP4, was present in latently infected ganglia at a maximum level of 3.2 x 10(6) molecules per ganglion (500 molecules per viral genome). ICP4-specific and tk-specific RNAs measured from the same samples showed a positive correlation extending over 2 orders of magnitude. We conclude that ICP4-specific RNA is expressed in the absence of detectable reactivation and discuss possible implications of our findings for latent gene expression.

Effect of exogenous nerve growth factor on neurotoxicity of and neuronal gene delivery by a herpes simplex amplicon vector in the rat brain

We have previously shown that local destruction of neural tissue by wild-type herpes simplex virus type 1 (HSV-1) is attenuated by intracerebral infusion of nerve growth factor (NGF). To investigate the effect of NGF on the extent of neurolysis and efficacy of neuronal gene transfer mediated by an HSV-1 amplicon vector system in vivo, rats were stereotaxically injected in the striatum with an amplicon preparation, pHSVlac. This amplicon contains the Escherichia coli lacZ gene under the transcriptional control of the HSV-1 immediate early 4/5 promoter and is packaged by an HSV-1 helper virus carrying a deletion in the immediate early 3 gene. Vector injection was followed by continuous intracerebral infusion of NGF-beta (total dose 5 micrograms) or vehicle solution over 7 days. Animals were sacrificed at the end of the 7-day infusion period for histological analysis of the brains. A distinct zone of inflammation and necrosis surrounded the injection site in all vector-inoculated animals. The volume of striatal tissue destruction was significantly smaller in NGF-treated animals (1.27 +/- 0.19 mm3; mean +/- SEM) than in the vehicle-treated controls (2.16 +/- 0.37 mm3; P < 0.05 by t-test). Immunohistochemical staining for HSV and beta-galactosidase (beta-Gal) in vehicle-treated animals revealed that many striatal cells harbored HSV antigens (3,678 +/- 636), but only a small number expressed the reporter gene at 7 days post-injection (294 +/- 60). NGF infusion did not significantly affect the number of HSV-immunoreactive cells (4,224 +/- 618), or the number of cells expressing beta-Gal (330 +/- 72) at this time.(ABSTRACT TRUNCATED AT 250 WORDS)