Cytotoxicity of a replication-defective mutant of herpes simplex virus type 1

Genome-wide CRISPR screen for HSV-1 host factors reveals PAPSS1 contributes to heparan sulfate synthesis

Herpes simplex virus type 1 (HSV-1) is a ubiquitous pathogen that causes various diseases in humans, ranging from common mucocutaneous lesions to severe life-threatening encephalitis. However, our understanding of the interaction between HSV-1 and human host factors remains incomplete. Here, to identify the host factors for HSV-1 infection, we performed a human genome-wide CRISPR screen using near-haploid HAP1 cells, in which gene knockout (KO) could be efficiently achieved. Along with several already known host factors, we identified 3′-phosphoadenosine 5′-phosphosulfate synthase 1 (PAPSS1) as a host factor for HSV-1 infection. The KO of PAPSS1 in HAP1 cells reduced heparan sulfate (HepS) expression, consequently diminishing the binding of HSV-1 and several other HepS-dependent viruses (such as HSV-2, hepatitis B virus, and a human seasonal coronavirus). Hence, our findings provide further insights into the host factor requirements for HSV-1 infection and HepS biosynthesis.

A genome-wide CRISPR screen for HSV-1 host factors using near-haploid HAP1 cells revealed PAPSS1 as an essential factor for heparan sulfate biosynthesis and HSV-1 infection, and identified several other host factors also involved in both processes.

Anterograde Neuronal Circuit Tracers Derived from Herpes Simplex Virus 1: Development, Application, and Perspectives

Herpes simplex virus type 1 (HSV-1) has great potential to be applied as a viral tool for gene delivery or oncolysis. The broad infection tropism of HSV-1 makes it a suitable tool for targeting many different cell types, and its 150 kb double-stranded DNA genome provides great capacity for exogenous genes. Moreover, the features of neuron infection and neuron-to-neuron spread also offer special value to neuroscience. HSV-1 strain H129, with its predominant anterograde transneuronal transmission, represents one of the most promising anterograde neuronal circuit tracers to map output neuronal pathways. Decades of development have greatly expanded the H129-derived anterograde tracing toolbox, including polysynaptic and monosynaptic tracers with various fluorescent protein labeling. These tracers have been applied to neuroanatomical studies, and have contributed to revealing multiple important neuronal circuits. However, current H129-derived tracers retain intrinsic drawbacks that limit their broad application, such as yet-to-be improved labeling intensity, potential nonspecific retrograde labeling, and high toxicity. The biological complexity of HSV-1 and its insufficiently characterized virological properties have caused difficulties in its improvement and optimization as a viral tool. In this review, we focus on the current H129-derived viral tracers and highlight strategies in which future technological development can advance its use as a tool.

Overcoming the blood-brain barrier in neurodegenerative disorders and brain tumours

Drug delivery is one of the major challenges in the treatment of central nervous system disorders. The brain needs to be protected from harmful agents, which are done by the capillary network, the so-called blood-brain barrier (BBB). This protective guard also prevents the delivery of therapeutic agents to the brain and limits the effectiveness of treatment. For this reason, various strategies have been explored by scientists for overcoming the BBB from disruption of the BBB to targeted delivery of nanoparticles (NPs) and cells and immunotherapy. In this review, different promising brain drug delivery strategies including disruption of tight junctions in the BBB, enhanced transcellular transport by peptide-based delivery, local delivery strategies, NP delivery, and cell-based delivery have been fully discussed.

Engineering HSV-1 Vectors for Gene Therapy

Virus vectors have been employed as gene transfer vehicles for various preclinical and clinical gene therapy applications and with the approval of Glybera (Alipogene tiparvovec) as the first gene therapy product as a standard medical treatment (Yla-Herttuala, Mol Ther 20:1831-1832, 2013), gene therapy has reached the status of being a part of standard patient care. Replication-competent herpes simplex virus (HSV) vectors that replicate specifically in actively dividing tumor cells have been used in Phase I-III human trials in patients with glioblastoma multiforme (GBM), a fatal form of brain cancer, and in malignant melanoma. In fact, Imlygic^® (T-VEC, Talimogene laherparepvec, formerly known as OncoVex GM-CSF), displayed efficacy in a recent Phase-III trial when compared to standard GM-CSF treatment alone (Andtbacka et al., J Clin Oncol 31:sLBA9008, 2013), and has since become the first FDA-approved viral gene therapy product used in standard patient care (October 2015) (Pol et al., Oncoimmunology 5:e1115641, 2016). Moreover, increased efficacy was observed when Imlygic^® was combined with checkpoint inhibitory antibodies as a frontline therapy for malignant melanoma (Ribas et al., Cell 170:1109-1119.e1110, 2017; Dummer et al., Cancer Immunol Immunother 66:683-695, 2017). In addition to the replication-competent oncolytic HSV vectors like T-VEC, replication-defective HSV vectors have been employed in Phase I-II human trials and have been explored as delivery vehicles for disorders such as pain, neuropathy and other neurodegenerative conditions. Research during the last decade on the development of HSV vectors has resulted in the engineering of recombinant vectors that are completely replication defective, nontoxic, and capable of long-term transgene expression in neurons. This chapter describes methods for the construction of recombinant genomic HSV vectors based on the HSV-1 replication-defective vector backbones, steps in their purification, and their small-scale production for use in cell culture experiments as well as preclinical animal studies.

Herpes Simplex Virus Vectors for Gene Transfer to the Central Nervous System

Neurodegenerative diseases (NDs) have a profound impact on human health worldwide and their incidence is predicted to increase as the population ages. ND severely limits the quality of life and leads to early death. Aside from treatments that may reduce symptoms, NDs are almost completely without means of therapeutic intervention. The genetic and biochemical basis of many NDs is beginning to emerge although most have complex etiologies for which common themes remain poorly resolved. Largely relying on progress in vector design, gene therapy is gaining increasing support as a strategy for genetic treatment of diseases. Here we describe recent developments in the engineering of highly defective herpes simplex virus (HSV) vectors suitable for transfer and long-term expression of large and/or multiple therapeutic genes in brain neurons in the complete absence of viral gene expression. These advanced vector platforms are safe, non-inflammatory, and persist in the nerve cell nucleus for life. In the near term, it is likely that HSV can be used to treat certain NDs that have a well-defined genetic cause. As further information on disease etiology becomes available, these vectors may take on an expanded role in ND therapies, including gene editing and repair.

Viral Vector Biosafety in Laboratory Animal Research

Viral vector research presents unique occupational health and safety challenges to institutions due to the rapid development of both in vivo and in vitro gene-editing technologies. Risks to human and animal health make it incumbent on institutions to appropriately evaluate viral vector usage in research on the basis of available information and governmental regulations and guidelines. Here we review the factors related to risk assessment regarding viral vector usage in animals and the relevant regulatory documents associated with this research, and we highlight the most commonly used viral vectors in research today. This review is particularly focused on the background, use in research and associated health and environmental risks related to adenoviral, adeno-associated viral, lentiviral, and herpesviral vectors.

Toxicity and biodistribution of the serotype 2 recombinant adeno-associated viral vector, encoding Aquaporin-1, after retroductal delivery to a single mouse parotid gland

In preparation for testing the safety of using serotype 2 recombinant adeno-associated vector, encoding Aquaporin-1 to treat radiation-induced salivary gland damage in a phase 1 clinical trial, we conducted a 13 week GLP biodistribution and toxicology study using Balb/c mice. To best assess the safety of rAAV2hAQP1 as well as resemble clinical delivery, vector (10(8), 10(9), 10(10), or 4.4 × 10(10) vector particles/gland) or saline was delivered to the right parotid gland of mice via retroductal cannulation. Very mild surgically induced inflammation was caused by this procedure, seen in 3.6% of animals for the right parotid gland, and 5.3% for the left parotid gland. Long term distribution of vector appeared to be localized to the site of cannulation as well as the right and left draining submandibular lymph nodes at levels >50 copies/μg in some animals. As expected, there was a dose-related increase in neutralizing antibodies produced by day 29. Overall, animals appeared to thrive, with no differences in mean body weight, food or water consumption between groups. There were no significant adverse effects due to treatment noted by clinical chemistry and pathology evaluations. Hematology assessment of serum demonstrated very limited changes to the white blood cell, segmented neutrophils, and hematocrit levels and were concluded to not be vector-associated. Indicators for liver, kidney, cardiac functions and general tissue damage showed no changes due to treatment. All of these indicators suggest the treatment is clinically safe.

Activation of H2AX and ATM in varicella-zoster virus (VZV)-infected cells is associated with expression of specific VZV genes

Mammalian cells activate DNA damage response pathways in response to virus infections. Activation of these pathways can enhance replication of many viruses, including herpesviruses. Activation of cellular ATM results in phosphorylation of H2AX and recruits proteins to sites of DNA damage. We found that varicella-zoster (VZV) infected cells had elevated levels of phosphorylated H2AX and phosphorylated ATM and that these levels increased in cells infected with VZV deleted for ORF61 or ORF63, but not deleted for ORF67. Expression of VZV ORF61, ORF62, or ORF63 alone did not result in phosphorylation of H2AX. While BGLF4, the Epstein-Barr virus homolog of VZV ORF47 protein kinase, phosphorylates H2AX and ATM, neither VZV ORF47 nor ORF66 protein kinase phosphorylated H2AX or ATM. Cells lacking ATM had no reduction in VZV replication. Thus, VZV induces phosphorylation of H2AX and ATM and this effect is associated with the presence of specific VZV genes in virus-infected cells.

Engineering HSV-1 vectors for gene therapy

Virus vectors have been employed as gene transfer vehicles for various preclinical and clinical gene therapy applications, and with the approval of Glybera (alipogene tiparvovec) as the first gene therapy product as a standard medical treatment (Yla-Herttuala, Mol Ther 20: 1831-1832, 2013), gene therapy has reached the status of being a part of standard patient care. Replication-competent herpes simplex virus (HSV) vectors that replicate specifically in actively dividing tumor cells have been used in Phase I-III human trials in patients with glioblastoma multiforme, a fatal form of brain cancer, and in malignant melanoma. In fact, T-VEC (talimogene laherparepvec, formerly known as OncoVex GM-CSF) displayed efficacy in a recent Phase III trial when compared to standard GM-CSF treatment alone (Andtbacka et al. J Clin Oncol 31: sLBA9008, 2013) and may soon become the second FDA-approved gene therapy product used in standard patient care. In addition to the replication-competent oncolytic HSV vectors like T-VEC, replication-defective HSV vectors have been employed in Phase I-II human trials and have been explored as delivery vehicles for disorders such as pain, neuropathy, and other neurodegenerative conditions. Research during the last decade on the development of HSV vectors has resulted in the engineering of recombinant vectors that are totally replication defective, nontoxic, and capable of long-term transgene expression in neurons. This chapter describes methods for the construction of recombinant genomic HSV vectors based on the HSV-1 replication-defective vector backbones, steps in their purification, and their small-scale production for use in cell culture experiments as well as preclinical animal studies.

Generation of high-titer defective HSV-1 vectors

Basic Protocol 1 describes the generation of helper virus stocks. Preparation of recombinant amplicon vector particles by transfection of amplicon and superinfection of helper virus into cells, and harvesting of packaged particles, is delineated in Basic Protocol 3. Thorough characterization of each amplicon viral vector stock involves measuring (1) the helper virus plaque-forming units per ml (pfu/ml) on 2-2 cells and (2) the amplicon stock infectious units per ml (iu/ml) on PC12 cells. The Support Protocols detail methods for determining titers of helper virus by plaque assay, and of amplicon stocks by vector assay.

HSV-1 as a Model for Emerging Gene Delivery Vehicles

The majority of viral vectors currently used possess modest cargo capability (up to 40 kb) being based on retroviruses, lentiviruses, adenoviruses, and adenoassociated viruses. These vectors have made the most rapid transition from laboratory to clinic because their small genomes have simplified their characterization and modification. However, there is now an increasing need both in research and therapy to complement this repertoire with larger capacity vectors able to deliver multiple transgenes or to encode complex regulatory regions, constructs which can easily span more than 100 kb. Herpes Simplex Virus Type I (HSV-1) is a well-characterized human virus which is able to package about 150 kb of DNA, and several vector systems are currently in development for gene transfer applications, particularly in neurons where other systems have low efficiency. However, to reach the same level of versatility and ease of use as that of smaller genome viral vectors, simple systems for high-titer production must be developed. This paper reviews the major HSV-1 vector systems and analyses the common elements which may be most important to manipulate to achieve this goal.

Overview of gene delivery into cells using HSV-1-based vectors

This overview describes the considerations involved in the preparation and use of a herpes simplex virus type 1 (HSV-1) amplicon as a vector for gene transfer into neurons. Strategies for gene delivery into neurons, either to study the molecular biology of brain function or for gene therapy, must utilize vectors that persist stably in postmitotic cells and that can be targeted both spatially and temporally in the nervous system in vivo. This unit describes the biology of HSV-1 along with a discussion covering development of amplicon and genomic HSV-1 vectors. Advantages and disadvantages of current HSV-1 vectors are presented, and HSV-1 vectors are compared with other vectors for gene transfer into neurons.

Gene delivery with viral vectors for cerebrovascular diseases

Recent achievements in the understanding of molecular events involved in the pathogenesis of central nervous system (CNS) injury have made gene transfer a promising approach for various neurological disorders, including cerebrovascular diseases. However, special obstacles, including the post-mitotic nature of neurons and the blood-brain barrier (BBB), constitute key challenges for gene delivery to the CNS. Despite the various limitations in current gene delivery systems, a spectrum of viral vectors has been successfully used to deliver genes to the CNS. Furthermore, recent advancements in vector engineering have improved the safety and delivery of viral vectors. Numerous viral vector-based clinical trials for neurological disorders have been initiated. This review will summarize the current implementation of viral gene delivery in the context of cerebrovascular diseases including ischemic stroke, hemorrhagic stroke and subarachnoid hemorrhage (SAH). In particular, we will discuss the potentially feasible ways in which viral vectors can be manipulated and exploited for use in neural delivery and therapy.

DNA viruses and the cellular DNA-damage response

It is clear that a number of host-cell factors facilitate virus replication and, conversely, a number of other factors possess inherent antiviral activity. Research, particularly over the last decade or so, has revealed that there is a complex inter-relationship between viral infection and the host-cell DNA-damage response and repair pathways. There is now a realization that viruses can selectively activate and/or repress specific components of these host-cell pathways in a temporally coordinated manner, in order to promote virus replication. Thus, some viruses, such as simian virus 40, require active DNA-repair pathways for optimal virus replication, whereas others, such as adenovirus, go to considerable lengths to inactivate some pathways. Although there is ever-increasing molecular insight into how viruses interact with host-cell damage pathways, the precise molecular roles of these pathways in virus life cycles is not well understood. The object of this review is to consider how DNA viruses have evolved to manage the function of three principal DNA damage-response pathways controlled by the three phosphoinositide 3-kinase (PI3K)-related protein kinases ATM, ATR and DNA-PK and to explore further how virus interactions with these pathways promote virus replication.

Generation of replication-competent and -defective HSV vectors

INTRODUCTION

Engineering effective vectors has been crucial to the efficient delivery and expression of therapeutic gene products in vivo. Among these, HSV-1 represents an excellent candidate vector for delivery to the peripheral and central nervous systems. The natural biology of HSV-1 includes the establishment of a lifelong latent state in neurons in which the viral genome persists as an episomal molecule. Genomic HSV vectors can be produced that are completely replication-defective, nontoxic, and capable of long-term transgene expression. Herpes simplex virus (HSV) vectors are constructed by using a replication-deficient vector backbone (TOZ.1) for homologous recombination with a shuttle plasmid containing a cassette expressing the gene of interest inserted into the UL41 gene sequence. The TOZ.1 vector expresses a reporter gene (lacZ) in the UL41 locus, such that recombination of the transgenic cassette into the UL41 locus results in the loss of the reporter gene activity. The TOZ.1 vector also contains a unique PacI endonuclease site for digestion of parental viral DNA that substantially reduces the nonrecombinant background. Following homologous recombination of the shuttle plasmid into the PacI-digested TOZ.1 genome, the recombinants are identified as clear plaques. After three rounds of limiting dilution analysis, the structure of the recombinants can be confirmed by Southern blot or by polymerase chain reaction (PCR) analysis.

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

HSV as a vector in vaccine development and gene therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), major human pathogen whose lifestyle is based on a long-term dual interaction with the infected host characterized by the existence of lytic and latent infections, has allowed the development of potential vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous system, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases and targeted infection of specific tissues or organs. Three different classes of vectors can be derived from HSV-1: replication-competent attenuated vectors, replication-incompetent recombinant vectors and defective helper-dependent vectors known as amplicons. This chapter highlights the current knowledge concerning design, construction and recent applications, as well as the potential and current limitations of the three different classes of HSV-1-based vectors.

Recognition of virus infection and innate host responses to viral gene therapy vectors

The innate immune and inflammatory response represents one of the key stumbling blocks limiting the efficacy of viral-based therapies. Numerous human diseases could be corrected or ameliorated if viruses were harnessed to safely and effectively deliver therapeutic genes to diseased cells and tissues in vivo. Recent studies have shown that host cells recognize viruses using an elaborate network of sensor proteins localized at the plasma membrane, in endosomes, or in the cytosol. Three classes of sensors have been implicated in sensing viruses in mammalian cells-Toll-like receptors (TLRs), retinoid acid-inducible gene (RIG)-I-like receptors (RLRs), and nucleotide oligomerization domain (NOD)-like receptors (NLRs). The interaction of virus-associated nucleic acids with these sensor molecules triggers a signaling cascade that activates the principal host defense program aimed to limit or eliminate virus infection and restore tissue homeostasis. In addition, recent data strongly suggest that host cells can mount innate immune responses to viruses without prior recognition of their nucleic acids. To deliver therapeutic genes into the nuclei of diseased cells, viral gene therapy vectors must be efficient at penetrating either the plasma or endosomal membrane. The therapeutic use of high numbers of virus particles disturbs cellular homeostasis, triggering cell damage and stress pathways, or "sensing of modified self". Accumulating data indicate that the sensing of modified self might represent a powerful framework explaining the innate immune response activation by viral gene therapy vectors.

A limited innate immune response is induced by a replication-defective herpes simplex virus vector following delivery to the murine central nervous system

Herpes simplex virus type 1 (HSV-1)-based vectors readily transduce neurons and have a large payload capacity, making them particularly amenable to gene therapy applications within the central nervous system (CNS). Because aspects of the host responses to HSV-1 vectors in the CNS are largely unknown, we compared the host response of a nonreplicating HSV-1 vector to that of a replication-competent HSV-1 virus using microarray analysis. In parallel, HSV-1 gene expression was tracked using HSV-specific oligonucleotide-based arrays in order to correlate viral gene expression with observed changes in host response. Microarray analysis was performed following stereotactic injection into the right hippocampal formation of mice with either a replication-competent HSV-1 or a nonreplicating recombinant of HSV-1, lacking the ICP4 gene (ICP4-). Genes that demonstrated a significant change (P < .001) in expression in response to the replicating HSV-1 outnumbered those that changed in response to mock or nonreplicating vector by approximately 3-fold. Pathway analysis revealed that both the replicating and nonreplicating vectors induced robust antigen presentation but only mild interferon, chemokine, and cytokine signaling responses. The ICP4- vector was restricted in several of the Toll-like receptor-signaling pathways, indicating reduced stimulation of the innate immune response. These array analyses suggest that although the nonreplicating vector induces detectable activation of immune response pathways, the number and magnitude of the induced response is dramatically restricted compared to the replicating vector, and with the exception of antigen presentation, host gene expression induced by the nonreplicating vector largely resembles mock infection.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

MicroRNA regulation of oncolytic herpes simplex virus-1 for selective killing of prostate cancer cells

PURPOSE

Advanced castration-resistant prostate cancer, for which there are few treatment options, remains one of the leading causes of cancer death. MicroRNAs (miRNA) have provided a new opportunity for more stringent regulation of tumor-specific viral replication. The purpose of this study was to provide a proof-of-principle that miRNA-regulated oncolytic herpes simplex virus-1 (HSV-1) virus can selectively target cancer cells with reduced toxicity to normal tissues.

EXPERIMENTAL DESIGN

We incorporated multiple copies of miRNA complementary target sequences (for miR-143 or miR-145) into the 3'-untranslated region (3'-UTR) of an HSV-1 essential viral gene, ICP4, to create CMV-ICP4-143T and CMV-ICP4-145T amplicon viruses and tested their targeting specificity and efficacy both in vitro and in vivo.

RESULTS

Although miR-143 and miR-145 are highly expressed in normal tissues, they are significantly down-regulated in prostate cancer cells. We further showed that miR-143 and miR-145 inhibited the expression of the ICP4 gene at the translational level by targeting the corresponding 3'-UTR in a dose-dependent manner. This enabled selective viral replication in prostate cancer cells. When mice bearing LNCaP human prostate tumors were treated with these miRNA-regulated oncolytic viruses, a >80% reduction in tumor volume was observed, with significantly attenuated virulence to normal tissues in comparison with control amplicon viruses not carrying these 3'-UTR sequences.

CONCLUSION

Our study is the first to show that inclusion of specific miRNA target sequences into the 3'-UTR of an essential HSV-1 gene is a viable strategy for restricting viral replication and oncolysis to cancer cells while sparing normal tissues.

HERPES SIMPLEX VIRUS INFECTION AND APOPTOSIS

Consequences of human herpes simplex virus (HSV) infection include the induction of apoptosis and the concomitant synthesis of proteins which act to block this process from killing the infected cell. Recent data has clarified our current understanding of the mechanisms of induction and prevention of apoptosis by HSV. These findings emphasize the fact that modulation of apoptosis by HSV during infection is a multicomponent phenomenon. We review recent evidence showing how this important human pathogen modulates the fundamental cell death process.

Improved long-term expression from helper virus-free HSV-1 vectors packaged using combinations of mutated HSV-1 proteins that include the UL13 protein kinase and specific components of the VP16 transcriptional complex

BACKGROUND

Herpes Simplex Virus (HSV-1) gene expression is thought to shut off recombinant gene expression from HSV-1 vectors; however, in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. These results raise the paradox that recombinant gene expression remains short-term even in the absence of almost all (approximately 99%) of the HSV-1 genome, HSV-1 genes, and HSV-1 gene expression. To resolve this paradox, we hypothesized that specific proteins in the HSV-1 virus particle shut off recombinant gene expression. In two earlier studies, we examined the effects on recombinant gene expression of packaging vectors using specific mutated HSV-1 proteins. We found that vectors packaged using mutated UL13 (a protein kinase), or VP16, or UL46 and/or UL47 (components of the VP16 transcriptional complex) supported improved long-term expression, and vectors packaged using mutated UL46 and/or UL47 also supported improved gene transfer (numbers of cells at 4 days). These results suggested the hypothesis that specific proteins in the HSV-1 particle act by multiple pathways to reduce recombinant gene expression. To test this hypothesis, we examined combinations of mutated proteins that included both UL13 and specific components of the VP16 transcriptional complex.

RESULTS

A HSV-1 vector containing a neuronal-specific promoter was packaged using specific combinations of mutated proteins, and the resulting vector stocks were tested in the rat striatum. For supporting long-term expression, the preferred combination of mutated HSV-1 proteins was mutated UL13, UL46, and UL47. Vectors packaged using this combination of mutated proteins supported a higher efficiency of gene transfer and high levels expression for 3 months, the longest time examined.

CONCLUSION

Vector particles containing this combination of mutated HSV-1 proteins improve recombinant gene expression. Implications of these results for strategies to further improve long-term expression are discussed. Moreover, long-term expression will benefit specific gene therapy applications.

High-level expression of glycoprotein D by a dominant-negative HSV-1 virus augments its efficacy as a vaccine against HSV-1 infection

Using the T-REx (Invitrogen, Carlsbad, CA) gene switch technology, we previously generated a dominant-negative herpes simplex virus (HSV)-1 recombinant, CJ83193, capable of inhibiting its own replication as well as that of wild-type HSV-1 and HSV-2. It has been further demonstrated that CJ83193 is an effective vaccine against HSV-1 infection in a mouse ocular model. To ensure its safety and augment its efficacy, we generated an improved CJ83193-like HSV-1 recombinant, CJ9-gD, which contains a deletion in an HSV-1 essential gene and encodes an extra copy of gene-encoding glycoprotein D (gD) driven by the tetO-bearing human cytomegalovirus major immediate-early promoter. Unlike CJ83193, which exhibits limited plaque-forming capability in Vero cells and expresses little gD in infected cells, CJ9-gD is completely replication defective, yields high-level expression of gD following infection, and cannot establish detectable infection in mouse trigeminal ganglia following intranasal and ocular inoculation. Mice immunized with CJ9-gD produced 3.5-fold higher HSV-1 neutralizing antibody titer than CJ83193-immunized mice, and were completely protected from herpetic ocular disease following corneal challenge with wild-type HSV-1. Moreover, immunization of mice with CJ9-gD elicited a strong HSV-1-specific T-cell response and led to an 80% reduction in latent infection by challenge wild-type HSV-1 compared with the mock-immunized control.

A Multi-domain Protein System Based on the HCFragment of Tetanus Toxin for Targeting DNA to Neuronal Cells

One goal of gene therapy is the targeted delivery of therapeutic genes to defined tissues. One attractive target is the central nervous system as there are several neuronal degenerative diseases which may be amenable to gene therapy. At present there is a lack of delivery systems that are able to target genes specifically to neuronal cells. Multi-domain proteins were designed and constructed to facilitate the delivery of exogenous genes to neuronal cells. Neuronal targeting activity of the proteins was achieved by inclusion of the HC fragment of tetanus toxin (TeNT), a protein with well-characterised tropism for the central nervous system. The yeast Gal4 DNA-binding domain enabled specific binding of DNA while the translocation domain from diphtheria toxin (DT) was included to facilitate crossing of the endosomal vesicle. One multi-domain protein, containing all three of these domains, was found to transfect up to 8% of neuroblastoma N18-RE105 cells with marker genes. Monitoring the transfection by confocal microscopy indicated that this protein-DNA transfection complex is to some extent localised at the cell surface, suggesting that further improvements to translocating this membrane barrier may yield higher transfection levels. The demonstration that this multi-domain protein can target genes specifically to neuronal cells is a first step in the development of novel vectors for the delivery of genes with therapeutic potential to diseased neuronal tissues.

Construction and production of recombinant herpes simplex virus vectors

Virus vectors have been employed as gene transfer vehicles for various pre-clinical and clinical gene therapy applications. Replication-competent herpes simplex virus (HSV) vectors that replicate specifically in actively dividing glial tumor cells have been used in Phase I-II human trials in patients with glioblastoma multiforme (GBM), a fatal form of brain cancer. Research during the last decade on the development of HSV vectors has resulted in the engineering of recombinant vectors that are totally replication defective, non-toxic, and capable of long-term transgene expression. This chapter describes methods for the construction of recombinant genomic HSV vectors based on the HSV-1 replication-defective vector backbones, steps in their purification, and their small-scale production for use in cell culture experiments as well as studies in animals.

Herpes simplex virus downregulates secretory leukocyte protease inhibitor: a novel immune evasion mechanism

Secretory leukocyte protease inhibitor (SLPI), an anti-inflammatory mediator of mucosal immunity, inhibits human immunodeficiency virus (HIV) and herpes simplex virus (HSV) in cell culture. Epidemiological studies demonstrate that higher concentrations of SLPI in mucosal secretions are associated with a reduced risk of HIV transmission. The current studies were designed to test the hypothesis that HSV triggers a loss of SLPI to evade innate immunity and that this response may contribute to the increased risk of HIV infection in the setting of HSV infection. Exposure of human cervical epithelial cells to HSV-1 or HSV-2, but not HIV or vesicular stomatitis virus, triggered a significant and sustained reduction in SLPI levels. The reduction persisted when cells were infected in the presence of acyclovir but not following infection with UV-inactivated virus, indicating that viral gene expression, but not replication, is required. Reverse transcriptase PCR studies demonstrated that the loss of SLPI is mediated by downregulation of gene expression. SLPI downregulation was associated with activation of NF-kappaB signaling pathways and upregulation of proinflammatory cytokines, consistent with the known inhibitor effects of SLPI on NF-kappaB pathways. The downregulation mapped to viral early-gene expression, as variants impaired in expression of the ICP4 or ICP0 immediate-early gene failed to downregulate SLPI or activate NF-kappaB. Together, these results identify a novel role for HSV immediate-early-gene expression in regulating mucosal immune responses.

Overview of gene delivery into cells using HSV-1-based vectors

This overview describes the considerations involved in the preparation and use of herpes simplex virus type 1 (HSV-1) as a vector for gene transfer into neurons. Strategies for gene delivery into neurons, either to study the molecular biology of brain function or for gene therapy, must utilize vectors that persist stably in postmitotic cells and that can be targeted both spatially and temporally in the nervous system in vivo. This unit describes the biology of HSV-1 along with a discussion covering development of amplicon and genomic HSV-1 vectors. Advantages and disadvantages of current HSV-1 vectors are presented, and HSV-1 vectors are compared with other vectors for gene transfer into neurons.

Generation of high-titer defective HSV-1 vectors

There are two types of replication-deficient herpes simplex virus type 1 (HSV-1) vectors: those in which the foreign DNA of interest is cloned into the viral genome itself, and those that are comprised of a plasmid (amplicon) carrying minimal HSV-1 sequences that allow it to be packaged into virus particles with the aid of a helper virus. This unit describes the generation of helper virus stocks. Preparation of recombinant amplicon vector particles by transfection of amplicon and superinfection of helper virus into cells, and harvesting of packaged particles, is also delineated. Thorough characterization of the viral vector stock involves measuring (1) the helper virus plaque-forming units per ml (pfu/ml) on 2-2 cells, (2) the wild-type HSV-1 pfu/ml on Vero cells, and (3) the amplicon stock infectious vector units per ml (ivu/ml) on PC12 cells. Support protocols detail methods for determining titers of helper virus and wild-type virus by plaque assay, and of amplicon stocks by vector assay.

Susceptibility of cancer cells to herpes simplex virus-dependent apoptosis

Apoptosis has recently been associated with herpes simplex virus 1 (HSV-1) latency and disease severity. There is an intricate balance between pro- and anti-apoptotic processes during HSV-1 infection. When anti-apoptotic pathways are suppressed, this balance is upset and the cells die by apoptosis, referred to here as HSV-1-dependent apoptosis (HDAP). It has been observed previously that HeLa cancer cells exhibit an enhanced sensitivity to HDAP. Here, a series of specific patient-derived cancer cells was utilized to investigate the cell-type specificity of HDAP. The results showed that a human mammary tumour cell line was sensitive to HDAP, whilst syngeneic normal cells were resistant. Furthermore, low-passage-number primary human mammary epithelial cells were resistant to HDAP. When the susceptibility of human colon, brain, breast and cervical cancer cells was assessed, the only cells insensitive to HDAP were those resistant to all environmental stimuli tested. This implies that the HDAP resistance was probably due to mutations in the cellular apoptotic machinery. Thus, the susceptibility of cancer cells to HDAP requires that they possess a functional ability to undergo programmed cell death.

Positron emission tomography of herpes simplex virus 1 oncolysis

Viral oncolysis, the destruction of cancer cells by replicating viruses, is under clinical investigation for cancer therapy. Lytic viral replication in cancer cells both destroys the cells and liberates progeny virion to infect adjacent cancer cells. The safety and efficacy of this approach are dependent on selective and robust viral replication in cancer cells rather than in normal cells. Methods to detect and quantify viral replication in tissues have relied on organ sampling for molecular analyses. Preclinical and clinical studies of viral oncolysis will benefit significantly from development of a noninvasive method to repetitively measure viral replication. We have shown that positron emission tomography (PET) allows for in vivo detection of herpes simplex virus (HSV)-1 replication in tumor cells using 9-(4-[(18)F]-fluoro-3-[hydroxymethyl]butyl)guanine ([(18)F]FHBG) as the substrate for HSV thymidine kinase (HSV-TK). As expected, phosphorylated [(18)F]FHBG is initially trapped within HSV-1-infected tumor cells and is detectable as early as 2 h following virus administration. MicroPET images reveal that [(18)F]FHBG accumulation in HSV-1-infected tumors peaks at 6 h. However, despite progressive accumulation of HSV-1 titers and HSV-TK protein in the tumor as viral oncolysis proceeds, tumor cell degradation resulting from viral oncolysis increases over time, which limits intracellular retention of [(18)F]FHBG. These observations have important consequences with regard to strategies to use [(18)F]FHBG PET for monitoring sites of HSV-TK expression during viral oncolysis.

An HSV-1 amplicon system for prostate-specific expression of ICP4 to complement oncolytic viral replication for in vitro and in vivo treatment of prostate cancer cells

The aim of the present study was to determine whether a prostate-specific amplicon, containing a probasin-derived promoter (ARR(2)PB) upstream of an essential Herpes simplex virus-1 (HSV-1) viral gene, infected-cell polypeptide 4 (ICP4), could complement an HSV-1 helper virus with this gene deleted (ICP4-) and cause lytic replication specifically in prostate cancer cells. Two amplicon constructs, CMV-ICP4 and ARR(2)PB-ICP4, were packaged by a replication-deficient ICP4- helper virus. The amplicon viruses could complement ICP4- helper viruses to efficiently replicate and cause cell lysis in prostate cancer cells. Intratumoral injection of LNCaP human prostate cancer xenografts with either amplicon/helper virus resulted in >75% reduction in tumor volume and serum prostate specific antigen (PSA). Histological and Q-PCR (quantitative PCR) analyses indicated that the toxicity in nontumor tissues was much lower with ARR(2)PB-ICP4 than with CMV-ICP4 amplicon/helper virus. In conclusion, a replication-deficient HSV-1 virus could be complemented by an amplicon virus to restore its oncolytic activity in a tissue-specific and low toxicity fashion, illustrating that this approach could be a potentially useful strategy for developing an oncolytic viral therapy for prostate cancer.

ICP0 gene expression is a herpes simplex virus type 1 apoptotic trigger

Apoptosis is a highly regulated programmed cell death process which is activated during normal development and by various stimuli, such as viral infection, which disturb cellular metabolism and physiology. That herpes simplex virus type 1 (HSV-1) induces apoptosis but then prevents its killing of infected cells is well-established. However, little is known about the viral factor/event which triggers the apoptotic process. We previously reported that infections with either (i) a temperature-sensitive virus at its nonpermissive temperature which does not inject viral DNA into nuclei or (ii) various UV-inactivated wild-type viruses do not result in the induction of apoptosis (C. M. Sanfilippo, F. N. W. Chirimuuta, and J. A. Blaho, J. Virol. 78:224-239, 2004). This indicates that virus receptor binding/attachment to cells, membrane fusion, virion disassembly/tegument dispersal, virion RNAs, and capsid translocation to nuclei are not responsible for induction and implicates viral immediate-early (IE) gene expression in the process. Here, we systematically evaluated the contribution of each IE gene to the stimulation of apoptosis. Using a series of viruses individually deleted for alpha27, alpha4, and alpha22, we determined that these genes are not required for apoptosis induction but rather that their products play roles in its prevention, likely through regulatory effects. Sole expression of alpha0 acted as an "apoptoxin" that was necessary and sufficient to trigger the cell death cascade. Importantly, results using a recombinant virus which contains a stop codon in alpha0 showed that it was not the ICP0 protein which acted as the apoptotic inducer. Based on these findings, we propose that alpha0 gene expression acts as an initial inducer of apoptosis during HSV-1 infection. This represents the first description of apoptosis induction in infected cells triggered as a result of expression of a single viral gene. Expression of apoptotic viral genes is a unique mechanism through which human pathogens may modulate interactions with their host cells.

ICP0 is not required for efficient stress-induced reactivation of herpes simplex virus type 1 from cultured quiescently infected neuronal cells

Viral genes sufficient and required for herpes simplex virus type 1 (HSV-1) reactivation were identified using neuronally differentiated PC12 cells (ND-PC12 cells) in which quiescent infections with wild-type and recombinant strains were established. In this model, the expression of ICP0, VP16, and ICP4 from adenovirus vectors was sufficient to reactivate strains 17+ and KOS. The transactivators induced similar levels of reactivation with KOS; however, 17+ responded more efficiently to ICP0. To identify viral transactivators required for reactivation, we examined quiescently infected PC12 cell cultures (QIF-PC12 cell cultures) established with HSV-1 deletion mutants R7910 (deltaICP0), KD6 (deltaICP4), and in1814, a virus containing an insertion mutation in VP16. Although growth of these mutant viruses was impaired in ND-PC12 cells, R7910 and in1814 reactivated at levels equivalent to or better than their respective parental controls following stress (i.e., heat or forskolin) treatment. After treatment with trichostatin A, in1814 and 17+ reactivated efficiently, whereas the F strain and R7910 reactivated inefficiently. In contrast, KD6 failed to reactivate. In experiments with the recombinant KM100, which contains the in1814 mutation in VP16 and the n212 mutation in ICP0, spontaneous and stress-induced reactivation was observed. However, two strains, V422 and KM110, which lack the acidic activation domain of VP16, did not reactivate above low spontaneous levels after stress. These results demonstrate that in QIF-PC12 cells ICP0 is not required for efficient reactivation of HSV-1, the acidic activation domain of VP16 is essential for stress-induced HSV-1 reactivation, and HSV-1 reactivation is modulated uniquely by different treatment constraints and phenotypes.

Herpes simplex virus 1 (HSV-1) for cancer treatment

Cancer remains a serious threat to human health, causing over 500 000 deaths each year in US alone, exceeded only by heart diseases. Many new technologies are being developed to fight cancer, among which are gene therapies and oncolytic virotherapies. Herpes simplex virus type 1 (HSV-1) is a neurotropic DNA virus with many favorable properties both as a delivery vector for cancer therapeutic genes and as a backbone for oncolytic viruses. Herpes simplex virus type 1 is highly infectious, so HSV-1 vectors are efficient vehicles for the delivery of exogenous genetic materials to cells. The inherent cytotoxicity of this virus, if harnessed and made to be selective by genetic manipulations, makes this virus a good candidate for developing viral oncolytic approach. Furthermore, its large genome size, ability to infect cells with a high degree of efficiency, and the presence of an inherent replication controlling mechanism, the thymidine kinase gene, add to its potential capabilities. This review briefly summarizes the biology of HSV-1, examines various strategies that have been used to genetically modify the virus, and discusses preclinical as well as clinical results of the HSV-1-derived vectors in cancer treatment.

Long-term inducible expression in striatal neurons from helper virus-free HSV-1 vectors that contain the tetracycline-inducible promoter system

Direct gene transfer into neurons in the brain via a virus vector system has potential for both examining neuronal physiology and for developing gene therapy treatments for neurological diseases. Many of these applications require precise control of the levels of recombinant gene expression. The preferred method for controlling the levels of expression is by use of an inducible promoter system, and the tetracycline (tet)-inducible promoter system is the preferred system. Helper virus-free Herpes Simplex Virus (HSV-1) vectors have a number of the advantages, including their large size and efficient gene transfer. Also, we have reported long-term (14 months) expression from HSV-1 vectors that contain a modified neurofilament heavy gene promoter. A number of studies have reported short-term, inducible expression from helper virus-containing HSV-1 vector systems. However, long-term, inducible expression has not been reported using HSV-1 vectors. The goal of this study was to obtain long-term, inducible expression from helper virus-free HSV-1 vectors. We examined two different vector designs for adapting the tet promoter system to HSV-1 vectors. One design was an autoregulatory design; one transcription unit used a tet-regulated promoter to express the tet-regulated transcription factor tet-off, and another transcription unit used a tet-regulated promoter to express the Lac Z gene. In the other vector design, one transcription unit used the modified neurofilament heavy gene promoter to express tet-off, and another transcription unit used a tet-regulated promoter to express the Lac Z gene. The results showed that both vector designs supported inducible expression in cultured fibroblast or neuronal cell lines and for a short time (4 days) in the rat striatum. Of note, only the vector design that used the modified neurofilament promoter to express tet-off supported long-term (2 months) inducible expression in striatal neurons.

Herpes simplex virus type-1 amplicon vectors for vaccine generation in acute lymphoblastic leukemia

For leukemia vaccine generation, high-efficiency gene transfer is required to express immunomodulatory molecules that stimulate potent antileukemic immune responses. In this context, herpes simplex virus type-1 (HSV-1)-derived vectors have proven to be a promising tool for genetic modification of lymphoblastic leukemia cells. Yet, vector-associated viral protein expression might inadvertently modulate vaccine efficacy facilitating both immune evasion and immune stimulation. To explore the issue of immune-stimulation versus immune-suppression in immature lymphoblastic leukemia cells, two types of HSV-1 amplicon vectors, helper virus-dependent and helper virus-free that express the immunomodulatory molecules CD70 and IL-2, were compared with regard to their vector-associated immunomodulatory potential. We first established that lymphoblastic cell lines and primary acute lymphoblastic leukemia (ALL) cells express HSV receptor genes. Lymphoblastic cell lines were transduced with high efficiency, and in primary ALL cells high gene transfer rates of 47+/-15 and 42+/-14% were obtained with helper virus-dependent and -free HSV-1 amplicon vectors, respectively. The efficacy of the two amplicon vectors to induce antineoplastic responses was assessed in a vaccine setting in mice with pre-existing highly malignant lymphoblastic disease. Treatment of mice with vaccine cells transgenically expressing CD70+IL2 significantly suppressed lymphoblastic cell proliferation and improved survival. Of note, when helper virus-dependent HSV-1 amplicon vectors were used for vaccine preparation, the high immunogenic potential of the vector itself, in the absence of transgenic CD70+IL2 expression, seemed to be sufficient to mediate protection comparable to the antineoplastic response achieved by expression of immunomodulatory molecules. Thus for vaccine generation in B lymphoblastic leukemia, the immunogenic potential of HSV-1 helper virus-dependent amplicon vectors does provide additional benefit to the high transduction efficiency of HSV-1-derived vectors.

HSV-1-based amplicon vectors: design and applications

Amplicons are defective, helper-dependent herpes simplex virus type 1 (HSV-1)-based vectors able to convey more than 100 kbp of foreign DNA to the nucleus of mammalian cells. This unique feature make amplicons very appealing for preventive or therapeutic gene transfer requiring the transduction of very large pieces of DNA, as well as for upstream fundamental studies, such as functional genomics. Several recent achievements in amplicon technology have allowed to produce relatively large amounts of essentially helper-free amplicons, as well as to expand the host range of these vectors. In this review, we will update the current know-how concerning design, construction, and recent applications, as well as the potential and current limitations, of this interesting and promising class of vectors.

Improved HSV-1 amplicon packaging using virion host shutoff mutants lacking mRNAse activity

Given their generous transgene capacity and inherent neurotropism, herpes simplex virus (HSV-1)-based viral vectors are promising tools for gene delivery to the central nervous system. Despite their widespread pre-clinical use, vector toxicity remains a concern with regard to the use of herpes vectors in humans. One potential source of toxicity stems from the tegument-associated virion host shutoff protein (vhs), which induces translational arrest in the host cell through non-specific mRNAse activity. In the current study we utilized a series of HSV-1 viruses containing a deletion in the U(L)41 open reading frame to investigate: (1) the requirement of intact vhs function in amplicon packaging and (2) whether vhs influences the post-transduction survival of dissociated cortical neurons. Our results demonstrate that while amplicon yield was reduced an order of magnitude, U(L)41 deletion was associated with reduced vector toxicity. Furthermore, partial reconstitution of vhs function using mRNAse-inactive point mutants improved amplicon titers without imparting the toxicity observed with wild-type controls. These findings offer a novel approach to improving the titer and toxicity profiles of HSV-based viral vectors.

Apoptosis During Herpes Simplex Virus Infection

Herpes simplex virus (HSV) infection triggers apoptosis in infected cells. However, proteins synthesized later in infected cells prevent apoptotic cell death from ensuing. In vivo data showing that apoptosis accompanies herpes stromal keratitis and encephalitis suggest that apoptotic modulation plays a role in the development of herpetic disease. Tremendous progress has been made toward identifying the viral factors that are responsible for inducing and inhibiting apoptosis during infection. However, the mechanisms whereby they act are still largely unknown. Recent studies have illustrated a wide diversity in the cellular response to HSV-triggered apoptosis, emphasizing the importance of host factors in this process. Together, these findings indicate that apoptosis during HSV infection represents an important virus-host interaction process, which likely influences viral pathogenesis.

Characterization of antiproliferative and cytotoxic properties of the HSV-1 immediate-early ICPo protein

BACKGROUND

The identification of novel proteins displaying cytostatic and/or cytotoxic actions that could eventually be used for gene therapy is a major issue in cancer research. Data from the literature suggested that the immediate-early ICP0 protein from herpes simplex virus type 1 (HSV-1) could fulfill these properties as it had been observed that this protein is involved in arrest of cell growth at the G1/S and G2/M phases of the cell cycle and that deletion of ICP0 from HSV-1 or HSV-1-recombinant vectors significantly reduced their cytotoxicity. The molecular basis of its action is likely related to the ability of ICP0, which possesses E3-ubiquitin ligase activity, to target destruction of key cellular proteins, including centromeric proteins, resulting in abnormal chromosome segregation, unusual cytokinesis, and emergence of nuclear morphological aberrations. However, neither the gene therapy potential of ICP0 on its own nor its action on primary quiescent cells has been assessed to date. The aim of this work was therefore to evaluate the antiproliferative and cytotoxic properties of ICP0 on a human glioblastoma cell line and on quiescent primary cells, and to explore whether this protein has a potential for gene therapy of cancer.

METHODS

HSV-1-based amplicon particles were generated following a recently described method that produces relatively high titers of vector stocks that are essentially free of helper virus. These vectors express either wild-type ICP0 or FXE, a RING finger mutated inactive form of ICP0, together with reporter green fluorescent protein (GFP). The vectors were used to infect Gli36 cells, which derive from a human glioblastome, and cultured rat primary cardiomyocytes and brain cells, two well-established models of non-dividing cells. Expression and localization of ICP0 and FXE, as well as their action on centromeres and nuclear morphology, were evaluated by Western blotting, indirect immune fluorescence, and confocal microscopy using specific antibodies and DAPI labeling. The impact of ICP0 on cell growth, toxicity, and viability was evaluated in the different cells using a variety of methods, including FACS analysis after propidium iodide and AnnexinV staining, crystal violet staining, clonogenic capability, caspase 3 activation, MTT tests, and release of lactate dehydrogenase, after infection with the different vectors.

RESULTS

The three cell types under study showed high levels of transduction by amplicons and strong expression of GFP, ICP0, and FXE transgenic proteins. Wild-type ICP0, but not FXE, induced centromeric disruption, appearance of micronuclei, arrest of proliferation, and significant cell death in glioblastoma Gli36 cells. In contrast, neither micronuclei formation nor any other sign of cell toxicity could be observed in cultured primary cardiomyocytes or brain cells, as evaluated by MTT tests and crystal violet staining. Furthermore, in the case of cardiomyocytes, expression of ICP0 did not interfere with beating as cells continued to beat at the same frequency as non-infected cells for several days post-infection. Neither AnnexinV early staining nor caspase 3 activation was observed in dying infected Gli36 cells, suggesting that these cells were not entering apoptosis. In contrast, release of lactate dehydrogenase by infected Gli36 cells suggested a necrotic way of death.

CONCLUSIONS

ICP0 induced a strong cytostatic action followed by significant cell death on the glioblastoma Gli36 cell line. In contrast, neither cell death nor any other evidence of ICP0-induced toxicity affecting major physiological parameters was observed in primary cultured cardiomyoctes and brain cells, two models of primary non-cycling cells. These results suggest that ICP0 has gene therapy potential and could represent the first member of a novel family of directly acting proteins that could be used to treat cancers.

Enhanced long-term expression from helper virus-free HSV-1 vectors packaged in the presence of deletions in genes that modulate the function of VP16, UL46 and UL47

Herpes simplex virus (HSV-1) gene expression is hypothesized to shut off recombinant gene expression from HSV-1 vectors, but in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. Thus paradoxically, recombinant gene expression remains short-term in the absence of almost all (approximately 99%) of the HSV-1 genome. To resolve this paradox, we hypothesize that specific HSV-1 proteins that affect the virion can shut off recombinant gene expression. In an earlier study, we examined the effects on recombinant gene expression of five different proteins that affect the HSV-1 virion. We found that vectors packaged in the presence of mutated vhs or U S 11 exhibited minimal changes in gene expression, vectors packaged in the presence of a mutated U S 3 supported improved gene transfer (numbers of cells at 4 days), and vectors packaged in the presence of mutated U L 13 or VP16 supported improved long-term expression. The capability of the VP16 transcriptional complex to reduce gene expression deserves additional study because VP16 is a powerful enhancer that interacts with a number of cellular and viral proteins. In particular, U L 46 and U L 47 are known to modulate the effects of VP16 on immediate early promoters. In this study, we examined expression from a HSV-1 vector that contains a neuronal-specific promoter and was packaged in the presence of deletions in U L 46, or U L 47, or both U L 46 and U L 47. In the rat striatum, each of these vector stocks supported both improved gene transfer (numbers of cells at 4 days) and improved long-term expression (2 months). Vectors packaged in the presence of a deletion in both U L 46 and U L 47 supported larger improvements in gene expression compared to vectors packaged in the presence of deletions in either gene alone. The implications of these results for strategies to improve long-term expression are discussed.