Development and optimization of herpes simplex virus vectors for multiple long-term gene delivery to the peripheral nervous system

Technological advances in the use of viral and non-viral vectors for delivering genetic and non-genetic cargos for cancer therapy

The burden of cancer is increasing globally. Several challenges facing its mainstream treatment approaches have formed the basis for the development of targeted delivery systems to carry and distribute anti-cancer payloads to their defined targets. This site-specific delivery of drug molecules and gene payloads to selectively target druggable biomarkers aimed at inducing cell death while sparing normal cells is the principal goal for cancer therapy. An important advantage of a delivery vector either viral or non-viral is the cumulative ability to penetrate the haphazardly arranged and immunosuppressive tumour microenvironment of solid tumours and or withstand antibody-mediated immune response. Biotechnological approaches incorporating rational protein engineering for the development of targeted delivery systems which may serve as vehicles for packaging and distribution of anti-cancer agents to selectively target and kill cancer cells are highly desired. Over the years, these chemically and genetically modified delivery systems have aimed at distribution and selective accumulation of drug molecules at receptor sites resulting in constant maintenance of high drug bioavailability for effective anti-tumour activity. In this review, we highlighted the state-of-the art viral and non-viral drug and gene delivery systems and those under developments focusing on cancer therapy.

Target-specific projections of amygdala somatostatin-expressing neurons to the hypothalamus and brainstem

Somatostatin neurons in the central nucleus of the amygdala (CeA/Sst) can be parsed into subpopulations that project either to the nucleus of the solitary tract (NST) or parabrachial nucleus (PBN). We have shown recently that inhibition of CeA/Sst-to-NST neurons increased the ingestion of a normally aversive taste stimulus, quinine HCl (QHCl). Because the CeA innervates other forebrain areas such as the lateral hypothalamus (LH) that also sends axonal projections to the NST, the effects on QHCl intake could be, in part, the result of CeA modulation of LH-to-NST neurons. To address these issues, the present study investigated whether CeA/Sst-to-NST neurons are distinct from CeA/Sst-to-LH neurons. For comparison purposes, additional experiments assessed divergent innervation of the LH by CeA/Sst-to-PBN neurons. In Sst-cre mice, two different retrograde transported flox viruses were injected into the NST and the ipsilateral LH or PBN and ipsilateral LH. The results showed that 90% or more of retrograde-labeled CeA/Sst neurons project either to the LH, NST, or PBN. Separate populations of CeA/Sst neurons projecting to these different regions suggest a highly heterogeneous population in terms of synaptic target and likely function.

Construction and optimization of herpes simplex virus vectors for central nervous system gene delivery based on CRISPR/Cas9-mediated genome editing

AIMS

We aim to define parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors and optimize the expression cassettes to achieve robust and sustained expression in CNS.

BACKGROUND

Engineered, attenuated Herpes simplex virus (HSV) vectors are promising vehicles for gene delivery to the peripheral and central nervous systems. The virus latent promoter (LAP) is commonly used to drive exogenous gene expression; however, parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors have not been fully understood.

OBJECTIVE

This study aimed to construct attenuated HSV-1 vectors using the CRISPR-Cas9 system and examine the influence of transgene cassette construction and insertion site on transgene expression and vector safety.

METHOD

In this study, we used a CRISPR-Cas9 system to accurately and efficiently edit attenuated HSV-1 strain 1716, and constructed two series of recombinant virus LMR and LMRx with different sets of gene cassettes insertion in Exon1(LAP2) and 2.0 kb intron downstream of LAP, respectively. The transgene expression and viral gene transcriptional kinetics were compared in in-vitro cell lines. The reporter gene expression and safety profiles of each vector were further evaluated in the mouse hippocampus gene transduction model.

RESULT

The in-vitro cell line analysis indicated that the insertion of a gene expression cassette would disrupt virus gene transcription. Mouse hippocampus transducing analysis suggested that complete expression cassette insertion at 2.0 kb intron could achieve robust and longtime gene expression than the other constructs. Recombinants with gene expression cassettes lacked Poly (A), which induced significant neuronal inflammation due to persistent viral antigen expression and microglia activation.

CONCLUSION

Our results indicated that the integrity of LAT transcripts was not necessary for the establishment of long-term latent expression. Exogenous strong promoters (like cBh promoter) could remain active during latency when placed in Exon1 or 2.0 Kb Intron of LAT locus, although their transcriptional activity declined with time. Consistent with previous research, the foreign gene expression would last much longer when the gene cassette was located downstream of Exon1, which suggested a role of LAP2 in maintaining promoter activity during latency. Besides, over-transcription of the downstream part of LAT may induce continuous activation of the attenuated vectors, suggesting an important role of LAT in maintaining viral reactivation potential.

Protocol Optimization for the Production of the Non-Cytotoxic JΔNI5 HSV Vector Deficient in Expression of Immediately Early Genes

Non-toxic herpes simplex virus (HSV) vectors can be generated by functional deletion of all immediate-early (IE) genes, providing a benign vehicle with potential for gene therapy. However, deletion of multiple IE genes raises manufacturing concerns and thus limits clinical application of these vectors. To address this issue, we previously developed a novel production cell line, called U2OS-ICP4/27, by lentiviral transduction of human osteosarcoma U2OS cells with two essential HSV IE genes, ICP4 and ICP27. To optimize the process of vector manufacturing on this platform, we evaluated several cell culture parameters of U2OS-ICP4/27 for high-titer and -quality production of non-toxic HSV vectors, revealing that the yields and functionality of these vectors can be significantly influenced by culturing conditions. We also found that several chemical compounds can enhance the replication of non-toxic HSV vectors and their release from producer cells into the supernatants. Notably, the vector produced by our optimized protocol displayed a greatly improved vector yield and quality and showed elevated transgene expression in cultures of primary dorsal root ganglion neurons. Taken together, our optimized production approach emerges as a relevant protocol for high-yield and high-quality preparation of non-toxic HSV-based gene therapy vectors.

Down-Regulation of MHC Class I Expression in Human Keratinocytes Using Viral Vectors Containing US11 Gene of Human Cytomegalovirus and Cultivation on Bovine Collagen-Elastin Matrix (Matriderm®): Potential Approach for an Immune-Privileged Skin Substitute

Skin transplantation, especially in burn patients, is still challenging because surgeons are faced with limited disposability of autologous donor side material. The in vitro culture of keratinocytes has become an important reconstructive option. However, only non-immunogenic allogenic keratinocytes offer the opportunity to develop a skin graft that can overcome rejection. The purpose of the study was to develop targeted gene modification of keratinocytes in order to reduce immunogenicity for the use as allogenic transplantable skin graft by decreasing the expression of MHC class I. To reduce MHC class I expression, viral vectors containing the US11 gene of human cytomegalovirus were generated and tested on their functionality using Western blotting, indirect immunofluorescence staining, and flow cytometry. Transfected keratinocytes were seeded on commercially available bovine collagen-elastin matrices and further cultured for histological and cell survival assays. Results showed transient down-regulation of MHC class I after 24 h post-transfection, with recovery of MHC class I expression after 48 h. Histological assessments showed long-term cell survival as well as histological patterns comparable to epidermal layers of healthy human skin. The data postulates the potential application of US11 transfected keratinocytes as an approach towards an immune-privileged skin substitute. Nevertheless, further studies and data are needed.

On the development of optical peripheral nerve interfaces

Limb loss and spinal cord injury are two debilitating conditions that continue to grow in prevalence. Prosthetic limbs and limb reanimation present two ways of providing affected individuals with means to interact in the world. These techniques are both dependent on a robust interface with the peripheral nerve. Current methods for interfacing with the peripheral nerve tend to suffer from low specificity, high latency and insufficient robustness for a chronic implant. An optical peripheral nerve interface may solve some of these problems by decreasing invasiveness and providing single axon specificity. In order to implement such an interface three elements are required: (1) a transducer capable of translating light into a neural stimulus or translating neural activity into changes in fluorescence, (2) a means for delivering said transducer and (3) a microscope for providing the stimulus light and detecting the fluorescence change. There are continued improvements in both genetically encoded calcium and voltage indicators as well as new optogenetic actuators for stimulation. Similarly, improvements in specificity of viral vectors continue to improve expression in the axons of the peripheral nerve. Our work has recently shown that it is possible to virally transduce axons of the peripheral nerve for recording from small fibers. The improvements of these components make an optical peripheral nerve interface a rapidly approaching alternative to current methods.

Cellular Antisilencing Elements Support Transgene Expression from Herpes Simplex Virus Vectors in the Absence of Immediate Early Gene Expression

Inactivation of all herpes simplex virus (HSV) immediate early (IE) genes to eliminate vector cytotoxicity results in rapid silencing of the viral genome, similar to the establishment of HSV latency. We recently reported that silencing of a nonviral reporter cassette could be overcome in nonneuronal cells by positioning the cassette in the viral latency (LAT) locus between resident chromatin boundary elements. Here, we tested the abilities of the chicken hypersensitive site 4 insulator and the human ubiquitous chromatin opening element A2UCOE to promote transgene expression from an IE-gene-inactivated HSV vector. We found that A2UCOE was particularly active in nonneuronal cells and reduced reporter promoter occupancy by a repressive histone mark. We determined whether multiple transgenes could be expressed under the control of different promoters from different loci of the same virus. The results showed abundant coexpression of LAT-embedded and A2UCOE-flanked genes in nonneuronal cells. In addition, a third reporter gene without known protective elements was active in cultured rat sensory neurons. These findings indicate that cellular antisilencing sequences can contribute to the expression of multiple genes from separate promoters in fully IE gene-disabled HSV vectors, providing an opportunity for therapeutic applications requiring mutually independent expression of different gene products from a single vector.IMPORTANCE Gene therapy has now entered a phase of development in which a growing number of recessive single gene defects can be successfully treated by vector-mediated introduction of a wild-type copy of the gene into the appropriate tissue. However, many disease conditions, such as neurodegeneration, cancer, and inflammatory processes, are more complex, requiring either multiple gene corrections or provision of coordinated gene activities to achieve a therapeutic outcome. Although herpes simplex virus (HSV) vectors have the capacity to meet this need, the challenge has been to genetically engineer the HSV genome in a manner to prevent expression of any viral genes while retaining the ability to express multiple therapeutic transgenes under independent transcriptional control. Here, we show that non-HSV insulator elements can be applied to retain at least transient transgene activity from multiple viral loci, thereby opening the door for more complex gene therapy applications in the future.

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.

Deletion of the Virion Host Shut-off Gene Enhances Neuronal-Selective Transgene Expression from an HSV Vector Lacking Functional IE Genes

The ability of herpes simplex virus (HSV) to establish lifelong latency in neurons suggests that HSV-derived vectors hold promise for gene delivery to the nervous system. However, vector toxicity and transgene silencing have created significant barriers to vector applications to the brain. Recently, we described a vector defective for all immediate-early gene expression and deleted for the joint region between the two unique genome segments that proved capable of extended transgene expression in non-neuronal cells. Sustained expression required the proximity of boundary elements from the latency locus. As confirmed here, we have also found that a transgene cassette introduced into the ICP4 locus is highly active in neurons but silent in primary fibroblasts. Remarkably, we observed that removal of the virion host shutoff (vhs) gene further improved transgene expression in neurons without inducing expression of viral genes. In rat hippocampus, the vhs-deleted vector showed robust transgene expression exclusively in neurons for at least 1 month without evidence of toxicity or inflammation. This HSV vector design holds promise for gene delivery to the brain, including durable expression of large or complex transgene cassettes.

Regulable Transgene Expression in Dorsal Root Ganglia of a Replication-Defective Herpes Simplex Virus Type 1 Vector by Means of Sciatic Nerve Injection

BACKGROUND

Targeted and controllable gene delivery to neurons is essential to efforts to facilitate peripheral nerve regeneration. The authors investigated both the in vitro and in vivo expression profiles of a tetracycline-controlled, replication-defective, herpes simplex virus type 1-based vector.

METHODS

Mouse primary dorsal root ganglia cells were infected with QR9TO-LacZ in the absence or presence of tetracycline. LacZ gene expression was examined. It was also injected into sciatic nerves in CD-1 mice fed with and without tetracycline. LacZ expression in the upstream dorsal root ganglia was examined.

RESULTS

Following inoculation with QR9TO-LacZ, approximately 40 percent of the cultured primary dorsal root ganglia cells exhibited strong LacZ activity in the presence of tetracycline at 48 and 72 hours, whereas little was detected in those in the absence of tetracycline. Quantitative analysis revealed that the β-galactosidase activity within cells exposed to tetracycline increased 181-fold at 48 hours (p < 0.001) and 47-fold at 72 hours after infection (p < 0.05) compared with those without tetracycline. However, this LacZ transgene activity in the presence of tetracycline tapered off to less than sevenfold over baseline 168 hours after infection (p < 0.05). Furthermore, successful uptake of this replication-defective viral vector was evident in upstream dorsal root ganglia after sciatic nerve injection in mice. In addition, its expression profile was similar to that in vitro, as strong β-galactosidase activity was evident only in mice fed with a doxycycline-containing diet, and it tapered off by 168 hours.

CONCLUSION

The replication-defective herpes simplex virus type 1-based vector, which provides tightly regulated transgene expression in dorsal root ganglia by means of peripheral nerve injection, represents an appealing approach to improve peripheral nerve regeneration.

Establishment of a novel therapeutic vector targeting the trigeminal ganglion in rats

Background

In the pathogenesis of herpes simplex keratitis, herpes simplex virus type 1 (HSV-1) infection begins in corneal epithelium cells and then progresses through the sensory nerve endings and finally travels up forward to the trigeminal ganglion (TG), where it remains as latent virus. The available anti-HSV therapies do not completely suppress the recurrence of active HSV-1 infection. The aim of this study was to establish a novel replication-defective (rd) HSV-1 (rdHSV) vector (rdHSV-interferon gamma [IFNγ]) that could effectively target the TG.

Methods

Recombinant HSV-1 virus was inserted into a shuttle plasmid carrying IFNγ to establish the rdHSV-IFNγ vector. Safety was evaluated in vitro by 50% cellular cytotoxicity in transfected SH-SY5Y neuroblastoma cells and in vivo by Kaplan–Meier survival estimate and infection rate. Wistar rats were immunized with rdHSV-IFNγ to evaluate the TG targeting efficiency. Real-time polymerase chain reaction and Western blot assays were used to evaluate IFNγ mRNA and protein expression and rdHSV-IFNγ localization.

Results

The rdHSV-IFNγ vector was successfully constructed and showed high in vitro safety and overall survival and a corneal infection rate similar to that of control rats immunized with saline (control group; P>0.05). Real-time polymerase chain reaction and immunohistochemistry assays confirmed IFNγ expression and effective TG targeting on days 14 and 21, which increased with postimmunization time. Moreover, IFNγ was expressed sufficiently in the TG tissues.

Conclusion

The rdHSV-IFNγ can act as an effective gene transporting vector that carries the therapeutic genes to the TG and triggers its expression.

Herpes simplex viral-vector design for efficient transduction of nonneuronal cells without cytotoxicity

The design of highly defective herpes simplex virus (HSV) vectors for transgene expression in nonneuronal cells in the absence of toxic viral-gene activity has been elusive. Here, we report that elements of the latency locus protect a nonviral promoter against silencing in primary human cells in the absence of any viral-gene expression. We identified a CTCF motif cluster 5' to the latency promoter and a known long-term regulatory region as important elements for vigorous transgene expression from a vector that is functionally deleted for all five immediate-early genes and the 15-kb internal repeat region. We inserted a 16.5-kb expression cassette for full-length mouse dystrophin and report robust and durable expression in dystrophin-deficient muscle cells in vitro. Given the broad cell tropism of HSV, our design provides a nontoxic vector that can accommodate large transgene constructs for transduction of a wide variety of cells without vector integration, thereby filling an important void in the current arsenal of gene-therapy vectors.

Replication-defective HSV-1 effectively targets trigeminal ganglion and inhibits viral pathopoiesis by mediating interferon gamma expression in SH-SY5Y cells

It has been widely believed that recurrence of herpes simplex keratitis (HSK) is due to the reactivation of herpes simplex virus type 1 (HSV-1) from latent sites in trigeminal ganglion (TG). However, there are also not effective vectors which could target TG for therapy. Replication-defective HSV-1 vector (rdHSV-IFNγ) was established by calcium phosphate co-transfection of complementing cells. We firstly infected rdHSV-IFNγ to SH-SY5Y, and detected IFNγ expression by western blot, evaluated 50 % cellular cytotoxicity (CC(50)) by ELISA. Antiviral activity of rdHSV-IFNγ was examined by immunofluorescence and antiviral concentration of 50 % effectiveness (EC(50)) assay. The rdHSV-IFNγ vector was immunized to Wistar rats to observe targeting function to TG. Kaplan-Meier survival analysis was utilized to assess security of rdHSV-IFNγ. RT-PCR and immunohistochemistry assay were employed to detect rdHSV-IFNγ localization in TG. Western blot was employed to detect IFNγ expression. rdHSV-IFNγ was successfully established, and performed an effective antiviral activity and higher security in SH-SY5Y. There were no significant differences of survival and corneal infection rate of rdHSV-IFNγ immunized rats among groups (P > 0.05). RT-PCR and immunohistochemistry indicated that expression of glycoprotein D (gD) in TG could target TG and decreased following with times post immunization. Furthermore, IFNγ was expressed effectively in TG tissues. Our findings indicated that established rdHSV-IFNγ vector effectively transported therapeutic gene into TG tissues. The administration of replication-defective vector carrying therapeutic genes may become a promising tool in inhibition or reoccurrence of HSK in clinical.

Modulating pain in the periphery: gene-based therapies to enhance peripheral opioid analgesia: Bonica lecture, ASRA 2010

This article provides a brief overview of earlier work of our group on the peripheral signaling of pain, summarizes more recent studies on the role of opioids in chronic neuropathic pain, and speculates on the future of gene-based therapies as novel strategies to enhance the peripheral modulation of pain. Neurophysiologic and psychophysical studies have revealed features of primary afferent activity from somatic tissue that led to improved understanding of the physiology and pathophysiology of pain signaling by nociceptive and nonnociceptive fibers. The demonstration of peripheral opioid mechanisms in neuropathic pain suggests a potential role for these receptors in the modulation of pain at its initiation site. Our work has focused on characterizing this peripheral opioid analgesia in chronic neuropathic pain such that it can be exploited to develop novel and potent peripheral analgesics for its treatment. Ongoing research on virus-mediated gene transfer strategies to enhance peripheral opioid analgesia is presented.

Gene therapy for the treatment of chronic peripheral nervous system pain

Chronic pain is a major health concern affecting 80 million Americans at some time in their lives with significant associated morbidity and effects on individual quality of life. Chronic pain can result from a variety of inflammatory and nerve damaging events that include cancer, infectious diseases, autoimmune-related syndromes and surgery. Current pharmacotherapies have not provided an effective long-term solution as they are limited by drug tolerance and potential abuse. These concerns have led to the development and testing of gene therapy approaches to treat chronic pain. The potential efficacy of gene therapy for pain has been reported in numerous pre-clinical studies that demonstrate pain control at the level of the spinal cord. This promise has been recently supported by a Phase-I human trial in which a replication-defective herpes simplex virus (HSV) vector was used to deliver the human pre-proenkephalin (hPPE) gene, encoding the natural opioid peptides met- and leu-enkephalin (ENK), to cancer patients with intractable pain resulting from bone metastases (Fink et al., 2011). The study showed that the therapy was well tolerated and that patients receiving the higher doses of therapeutic vector experienced a substantial reduction in their overall pain scores for up to a month post vector injection. These exciting early clinical results await further patient testing to demonstrate treatment efficacy and will likely pave the way for other gene therapies to treat chronic pain.

Functionalized nanostructures with application in regenerative medicine

In the last decade, both regenerative medicine and nanotechnology have been broadly developed leading important advances in biomedical research as well as in clinical practice. The manipulation on the molecular level and the use of several functionalized nanoscaled materials has application in various fields of regenerative medicine including tissue engineering, cell therapy, diagnosis and drug and gene delivery. The themes covered in this review include nanoparticle systems for tracking transplanted stem cells, self-assembling peptides, nanoparticles for gene delivery into stem cells and biomimetic scaffolds useful for 2D and 3D tissue cell cultures, transplantation and clinical application.

Hepatocyte growth factor incorporated into herpes simplex virus vector accelerates facial nerve regeneration after crush injury

Hepatocyte growth factor (HGF) promotes regeneration of the central nervous system, but its effects on the peripheral nervous system remain unclear. This study was conducted to elucidate the effect of HGF on regeneration of the murine facial nerve after crush injury. To do so, a replication-defective herpes simplex virus vector that incorporated HGF was prepared (HSV-HGF). The main trunk of the facial nerve was compressed by mosquito hemostats, and HSV-HGF, control vector or medium was then applied to the compressed nerve. We found that mice in the HGF group required significantly fewer days for complete recovery from nerve compression. Furthermore, the amplitude of the evoked buccinator muscle compound action potential increased following HSV-HGF application. HGF expression in and around the compressed nerve was demonstrated by enzyme-linked immunoassay and immunohistochemistry. In addition, HSV-HGF introduction around the damaged nerve significantly accelerated recovery of function of the facial nerve. These data suggest a possible role of HGF in promoting facial nerve regeneration after nerve damage. Furthermore, this viral delivery method may be applied clinically for many types of severe facial palsy during facial nerve decompression surgery.

Plasmid-encapsulated polyethylene glycol-grafted polyethylenimine nanoparticles for gene delivery into rat mesenchymal stem cells

Background:

Mesenchymal stem cell transplantation is a promising method in regenerative medicine. Gene-modified mesenchymal stem cells possess superior characteristics of specific tissue differentiation, resistance to apoptosis, and directional migration. Viral vectors have the disadvantages of potential immunogenicity, carcinogenicity, and complicated synthetic procedures. Polyethylene glycol-grafted polyethylenimine (PEG-PEI) holds promise in gene delivery because of easy preparation and potentially targeting modification.

Methods:

A PEG8k-PEI25k graft copolymer was synthesized. Agarose gel retardation assay and dynamic light scattering were used to determine the properties of the nanoparticles. MTT reduction, wound and healing, and differentiation assays were used to test the cytobiological characteristics of rat mesenchymal stem cells, fluorescence microscopy and flow cytometry were used to determine transfection efficiency, and atomic force microscopy was used to evaluate the interaction between PEG-PEI/plasmid nanoparticles and mesenchymal stem cells.

Results:

After incubation with the copolymer, the bionomics of mesenchymal stem cells showed no significant change. The mesenchymal stem cells still maintained high viability, resettled the wound area, and differentiated into adipocytes and osteoblasts. The PEG-PEI completely packed plasmid and condensed plasmid into stable nanoparticles of 100–150 nm diameter. After optimizing the N/P ratio, the PEG-PEI/plasmid microcapsules delivered plasmid into mesenchymal stem cells and obtained an optimum transfection efficiency of 15%–21%, which was higher than for cationic liposomes.

Conclusion:

These data indicate that PEG-PEI is a valid gene delivery agent and has better transfection efficiency than cationic liposomes in mesenchymal stem cells.

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.

Delivery of RNA interference triggers to sensory neurons in vivo using herpes simplex virus

IMPORTANCE OF THE FIELD

Pain is a hugely important area of research attracting considerable academic and commercial interest. However, the application of RNA interference (RNAi) to the study of nociceptive processes and the development of new analgesics has been limited by the specific challenges associated with the delivery of RNAi triggers to the cell bodies of sensory neurons in the dorsal root ganglia (DRG).

AREAS COVERED IN THIS REVIEW

In the past five years, delivery of small-interfering RNA (siRNA) to the DRG and spinal cord has achieved effective and specific silencing of targeted genes in various animal models of pain. However, delivery of short-hairpin RNA (shRNA) or artificial microRNA (miRNA) to sensory neurons in vivo has not been feasible using most delivery systems currently available.

WHAT THE READER WILL GAIN

Replication-defective vectors based on herpes simplex virus (HSV), which are particularly efficient at targeting DRG neurons, have been recently engineered to express shRNA and artificial miRNA. Whilst silencing induced by siRNA is transient and requires relatively high doses of silencing triggers, HSV-mediated expression of shRNA/miRNA in sensory neurons allows silencing of targeted genes for at least one week following a single injection.

TAKE HOME MESSAGE

The potential to use inducible or tissue-specific promoters and to simultaneously silence multiple gene targets, in addition to recent studies suggesting that artificial miRNAs may have improved safety profiles, hold clear advantages for the use of miRNA-based vectors for gene silencing in sensory neurons.

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

Gene-based approaches in the study of pathological pain

Chronic pathological pain is characterized by extensive plasticity of the systems involved in pain signal transmission and modulation and tissue remodeling in several CNS structures. These long-lasting alterations are mediated by, or associated with, changes in the production of key molecules of nociceptive processing. Gene-based approaches offer the unique possibility of using local or even cell-type specific interventions to correct the abnormal production of some of these proteins, modulate the activity of signal transduction pathways, or overproduce various therapeutic secreted proteins. We showed that certain viral-derived vectors are particularly suitable for mediating gene transfer highly preferential for instance into the primary sensory neurons or into the spinal cord glial cells that represent particularly pertinent targets in the search for new therapeutic strategies of pathological pain.

Invited review: festschrift edition of neurosurgery peripheral nervous system as a conduit for delivering therapies for diabetic neuropathy, amyotrophic lateral sclerosis, and nerve regeneration

In this review, we describe how therapies that promote axonal regeneration and neuronal protection can complement surgery for a successful functional restoration in peripheral nerve disorders. We discuss the advantages of peripheral drug delivery and the role of the neurosurgeon in the precise delivery of molecular therapies to surgically inaccessible structures. Strategies for enhancing uptake and retrograde transport of therapeutics, including gene therapy, are emphasized as conduits for delivery of therapeutics. Finally, candidate therapeutic proteins and genes are discussed in the context of application to degenerative disorders of the nervous system, including nerve injury, peripheral neuropathy, and amyotrophic lateral sclerosis.

Delivery of RNA interference to peripheral neurons in vivo using herpes simplex virus

RNA interference (RNAi) has become a powerful tool for modulating gene expression. While delivery of small interfering RNAs (siRNAs) has achieved silencing of pain-related genes in various animal models of nociception, delivery of short-hairpin RNA (shRNA) or artificial miRNA (miRNA) to dorsal root ganglia (DRG) has proven particularly challenging. This chapter describes a highly efficient method for in vivo gene silencing in sensory neurons using replication-defective vectors based on herpes simplex virus (HSV). This method can be utilised to obtain a better understanding of gene function, validate novel gene targets in drug discovery and potentially develop new RNAi-mediated approaches to achieve analgesia.

Recombinant adeno-associated virus serotype 6 (rAAV2/6)-mediated gene transfer to nociceptive neurons through different routes of delivery

Background

Gene transfer to nociceptive neurons of the dorsal root ganglia (DRG) is a promising approach to dissect mechanisms of pain in rodents and is a potential therapeutic strategy for the treatment of persistent pain disorders such as neuropathic pain. A number of studies have demonstrated transduction of DRG neurons using herpes simplex virus, adenovirus and more recently, adeno-associated virus (AAV). Recombinant AAV are currently the gene transfer vehicles of choice for the nervous system and have several advantages over other vectors, including stable and safe gene expression. We have explored the capacity of recombinant AAV serotype 6 (rAAV2/6) to deliver genes to DRG neurons and characterized the transduction of nociceptors through five different routes of administration in mice.

Results

Direct injection of rAAV2/6 expressing green fluorescent protein (eGFP) into the sciatic nerve resulted in transduction of up to 30% eGFP-positive cells of L4 DRG neurons in a dose dependant manner. More than 90% of transduced cells were small and medium sized neurons (< 700 μm²), predominantly colocalized with markers of nociceptive neurons, and had eGFP-positive central terminal fibers in the superficial lamina of the spinal cord dorsal horn. The efficiency and profile of transduction was independent of mouse genetic background. Intrathecal administration of rAAV2/6 gave the highest level of transduction (≈ 60%) and had a similar size profile and colocalization with nociceptive neurons. Intrathecal administration also transduced DRG neurons at cervical and thoracic levels and resulted in comparable levels of transduction in a mouse model for neuropathic pain. Subcutaneous and intramuscular delivery resulted in low levels of transduction in the L4 DRG. Likewise, delivery via tail vein injection resulted in relatively few eGFP-positive cells within the DRG, however, this transduction was observed at all vertebral levels and corresponded to large non-nociceptive cell types.

Conclusion

We have found that rAAV2/6 is an efficient vector to deliver transgenes to nociceptive neurons in mice. Furthermore, the characterization of the transduction profile may facilitate gene transfer studies to dissect mechanisms behind neuropathic pain.

Gene therapy for the treatment of diabetic neuropathy

Neuropathy is a common, untreatable complication of type 1 and type 2 diabetes. In animal models peptide neurotrophic factors can be used to protect against the development of neuropathy, but the combination of short half-life and off-target effects of these potent pleiotropic peptides has limited translation to human therapy. Gene transfer is a promising strategy that may circumvent these limitations. In this article, we review the basic methods of gene transfer and the -preclinical data in rodent models that support the use of this approach in the treatment of diabetic neuropathy. The path to clinical applications and potential pitfalls in developing gene therapy for the treatment of diabetic neuropathy are considered.

Efficient delivery of RNA Interference to peripheral neurons in vivo using herpes simplex virus

Considerable interest has been focused on inducing RNA interference (RNAi) in neurons to study gene function and identify new targets for disease intervention. Although small interfering RNAs (siRNAs) have been used to silence genes in neurons, in vivo delivery of RNAi remains a major challenge limiting its applications. We have developed a highly efficient method for in vivo gene silencing in dorsal root ganglia (DRG) using replication-defective herpes simplex viral (HSV-1) vectors. HSV-mediated delivery of short-hairpin RNA (shRNA) targeting reporter genes resulted in highly effective and specific silencing in neuronal and non-neuronal cells in culture and in the DRG of mice in vivo including in a transgenic mouse model. We further establish proof of concept by demonstrating in vivo silencing of the endogenous trpv1 gene. These data are the first to show silencing in DRG neurons in vivo by vector-mediated delivery of shRNA. Our results support the utility of HSV vectors for gene silencing in peripheral neurons and the potential application of this technology to the study of nociceptive processes and in pain gene target validation studies.

Transduction of brain by herpes simplex virus vectors

An imposing obstacle to gene therapy is the inability to transduce all of the necessary cells in a target organ. This certainly applies to gene transfer to the brain, especially when one considers the challenges involved in scaling up transduction from animal models to use in the clinic. Non-neurotropic viral gene transfer vectors (e.g., adenovirus, adeno-associated virus, and lentivirus) do not spread very far in the nervous system, and consequently these vectors transduce brain regions mostly near the injection site in adult animals. This indicates that numerous, well-spaced injections would be required to achieve widespread transduction in a large brain with these vectors. In contrast, herpes simplex virus type 1 (HSV-1) is a promising vector for widespread gene transfer to the brain owing to the innate ability of the virus to spread through the nervous system and form latent infections in neurons that last for the lifetime of the infected individual. In this review, we summarize the published literature of the transduction patterns produced by attenuated HSV-1 vectors in small animals as a function of the injection site, and discuss the implications of the distribution for widespread gene transfer to the large animal brain.

Hepatocyte growth factor promotes endogenous repair and functional recovery after spinal cord injury

Many therapeutic interventions using neurotrophic factors or pharmacological agents have focused on secondary degeneration after spinal cord injury (SCI) to reduce damaged areas and promote axonal regeneration and functional recovery. Hepatocyte growth factor (HGF), which was identified as a potent mitogen for mature hepatocytes and a mediator of inflammatory responses to tissue injury, has recently been highlighted as a potent neurotrophic and angiogenic factor in the central nervous system (CNS). In the present study, we revealed that the extent of endogenous HGF up-regulation was less than that of c-Met, an HGF receptor, during the acute phase of SCI and administered exogenous HGF into injured spinal cord using a replication-incompetent herpes simplex virous-1 (HSV-1) vector to determine whether HGF exerts beneficial effects and promotes functional recovery after SCI. This treatment resulted in the significant promotion of neuron and oligodendrocyte survival, angiogenesis, axonal regrowth, and functional recovery after SCI. These results suggest that HGF gene delivery to the injured spinal cord exerts multiple beneficial effects and enhances endogenous repair after SCI. This is the first study to demonstrate the efficacy of HGF for SCI.

Construction and characterization of a herpes simplex virus type I recombinant expressing green fluorescent protein: acute phase replication and reactivation in mice

A recombinant HSV-1 virus expressing EGFP from the HCMV major immediate early promoter (KOS-CMVGFP) was constructed to monitor viral replication and spread in vitro and in mice. KOS-CMVGFP replicated as efficiently as wild-type virus, strain KOS, in single cycle growth experiments in Vero cells indicating that the recombinant virus has no significant growth defects in vitro. Following ocular inoculation of mice, KOS-CMVGFP exhibited slight but statistically significant reductions in mouse tear film titers relative to wild-type virus. Progression of virus infection of the eyes, periocular tissue, and snout was readily followed by fluorescence microscopy. Insertion of the EGFP expression cassette into the KOS genome had no effect on the efficiency of establishment of latency as determined by quantitative competitive PCR of viral genomes in latently infected TG. KOS-CMVGFP reactivated with wild-type kinetics and efficiency by explant cocultivation, but exhibited a significant delay in the kinetics and a modest reduction in the efficiency of reactivation compared to KOS in the more sensitive TG cell culture model. Notably, EGFP expression preceded the detection of infectious virus by greater than 24 h in both ex vivo models and thus is a useful marker of the early stages in the induction of reactivation.

Regulatable gene expression systems for gene therapy

It is feasible to restrict transgene expression to a tissue or region in need of therapy by using promoters that respond to focusable physical stimuli. The most extensively investigated promoters of this type are radiation-inducible promoters and heat shock protein gene promoters that can be activated by directed, transient heat. Temporal regulation of transgenes can be achieved by various two- or three-component gene switches that are triggered by an appropriate small molecule inducer. The most commonly considered gene switches that are reviewed herein are based on small molecule-responsive transactivators derived from bacterial tetracycline repressor, insect or mammalian steroid receptors, or mammalian FKBP12/FRAP. A new generation of gene switches combines a heat shock protein gene promoter and a small molecule-responsive gene switch and can provide for both spatial and temporal regulation of transgene activity.

Herpes simplex virus type-1 latency inhibits dendritic growth in sympathetic neurons

Herpes simplex virus type-1 (HSV-1) initially infects mucoepithelial tissues of the orofacial region, the eye and to a lesser extent the genitalia. Subsequently, the virus is retrogradely transported through the axons of the sensory and sympathetic neurons to their nuclei, where the virus establishes a life-long latent infection. During this latency period, the viral genome is transcriptionally silent except for a single region encoding the latency-associated transcript (LAT). LAT has been shown to affect apoptosis, but little else is known regarding its effects on neurons. To understand how HSV-1 latency might affect dendrites in sympathetic neurons, we transfected primary cultures of sympathetic neurons obtained from rat embryos, with LAT expressing plasmids. LAT inhibited initial dendritic growth and induced dendritic retraction in sympathetic neurons. Latent HSV-1 infection of cultured sympathetic neurons inhibited dendritic growth indicating that this is likely also a consequence of natural infection.

Injection of the sciatic nerve with TMEV: a new model for peripheral nerve demyelination

Demyelination of the human peripheral nervous system (PNS) can be caused by diverse mechanisms including viral infection. Despite association of several viruses with the development of peripheral demyelination, animal models of the condition have been limited to disease that is either autoimmune or genetic in origin. We describe here a model of PNS demyelination based on direct injection of sciatic nerves of mice with the cardiovirus, Theiler's murine encephalomyelitis virus (TMEV). Sciatic nerves of FVB mice develop inflammatory cell infiltration following TMEV injection. Schwann cells and macrophages are infected with TMEV. Viral replication is observed initially in the sciatic nerves and subsequently the spinal cord. Sciatic nerves are demyelinated by day 5 post-inoculation (p.i.). Injecting sciatic nerves of scid mice resulted in increased levels of virus recovered from the sciatic nerve and spinal cord relative to FVB mice. Demyelination also occurred in scid mice and by 12 days p.i., hindlimbs were paralyzed. This new model of virus-induced peripheral demyelination may be used to dissect processes involved in protection of the PNS from viral insult and to study the early phases of lesion development.

Novel therapeutic strategy for stroke in rats by bone marrow stromal cells and ex vivo HGF gene transfer with HSV-1 vector

Occlusive cerebrovascular disease leads to brain ischemia that causes neurological deficits. Here we introduce a new strategy combining mesenchymal stromal cells (MSCs) and ex vivo hepatocyte growth factor (HGF) gene transferring with a multimutated herpes simplex virus type-1 vector in a rat transient middle cerebral artery occlusion (MCAO) model. Gene-transferred MSCs were intracerebrally transplanted into the rats' ischemic brains at 2 h (superacute) or 24 h (acute) after MCAO. Behavioral tests showed significant improvement of neurological deficits in the HGF-transferred MSCs (MSC-HGF)-treated group compared with the phosphate-buffered saline (PBS)-treated and MSCs-only-treated group. The significant difference of infarction areas on day 3 was detected only between the MSC-HGF group and the PBS group with the superacute treatment, but was detected among each group on day 14 with both transplantations. After the superacute transplantation, we detected abundant expression of HGF protein in the ischemic brain of the MSC-HGF group compared with others on day 1 after treatment, and it was maintained for at least 2 weeks. Furthermore, we determined that the increased expression of HGF was derived from the transferred HGF gene in gene-modified MSCs. The percentage of apoptosis-positive cells in the ischemic boundary zone (IBZ) was significantly decreased, while that of remaining neurons in the cortex of the IBZ was significantly increased in the MSC-HGF group compared with others. The present study shows that combined therapy is more therapeutically efficient than MSC cell therapy alone, and it may extend the therapeutic time window from superacute to acute phase.

Gene therapy for the treatment of sensory neuropathy

Sensory polyneuropathy can be a serious problem, but for the majority of clinically important neuropathies there are no available therapies. Neurotrophic and neuroprotective peptide factors have been identified that prevent or reverse neuropathy in rodent models of disease, but delivery of these highly pleiotropic peptides has posed an obstacle for translation into effective human therapies. Gene transfer into muscle using viral or non-viral vectors, or into neurons of the dorsal root ganglion using herpes simplex virus-based vectors, provides an alternative means to achieve this end. Studies in animal models have been promising, and the first human trial, using a plasmid to transfer the gene coding for vascular endothelial growth factor into muscle for the treatment of diabetic neuropathy, is now underway. Evidence supporting the trial and the challenges facing this therapy are reviewed.

Highly efficient regulation of gene expression by tetracycline in a replication-defective herpes simplex viral vector

Employing the tetracycline repressor tetR and the wild-type hCMV major immediate-early promoter, we have developed a highly sensitive tetracycline-inducible transcription switch in mammalian cells (T-REx; Invitrogen, Carlsbad, CA, USA). In view of the previous difficulty in achieving regulatable gene expression in recombinant HSV vector systems, we constructed a T-REx-encoding replication-defective HSV-1 recombinant, QR9TO-lacZ, that encodes two copies of the tetR gene controlled by the HSV-1 immediate-early ICP0 promoter and a reporter, the LacZ gene, under the control of the tetO-bearing hCMV major immediate-early promoter. Infection of cells, such as Vero, PC12, and NGF-differentiated PC12 cells, with QR9TO-lacZ led to 300- to 1000-fold tetracycline-regulated gene expression. Moreover, the expression of the LacZ gene by QR9TO-lacZ can be finely controlled by tetracycline in a dose-dependent fashion. Efficiently regulated gene expression can also be achieved in vivo following intracerebral and footpad inoculations in mice. The demonstrated capability of T-REx for achieving high levels of sensitively regulated gene expression in the context of the HSV-1 genome will significantly expand the utility of HSV-based vector systems for studying gene function in the nervous system and delivering regulated gene expression in therapeutic applications, particularly in the treatment of CNS diseases.

Neuroprotective effects of virally delivered HSPs in experimental stroke

Heat shock proteins (HSPs) are molecular chaperones with essential roles in modulating the proteolytic machinery and accelerating cell repair. Heat shock protein overexpression has been observed in vivo and in vitro under stresses including heat, nutrient deprivation and ischemia. Experiments in in vivo models of stroke indicate that transgenically overexpressed or virally delivered HSPs can enhance cell survival, but cannot always reduce lesion size. This study aims to assess the effects of virally delivered HSPs in a rat middle cerebral artery occlusion model of reversible focal cerebral ischemia using noninvasive magnetic resonance imaging. Attenuated herpes simplex virus carrying HSP27, HSP70, or a LacZ control was microinjected into the striatum 3 days before ischemia. Multislice T(2)-weighted images at 24 h after ischemia indicated that lesion volume was reduced by 44% in HSP27-treated animals compared with controls (P = 0.019). No significant differences were found between HSP70-treated and control animals (P = 0.88). Immunohistochemistry and Western blots revealed that HSP27 and HSP70 expression levels were equally high in injected hemispheres, but only the former had an effect on lesion size. This is the first evidence of the efficacy of gene therapy with any viral vector expressing HSP27 in an experimental model of stroke.

Replication-competent herpes simplex vectors: design and applications

Replication-competent vectors are derived from attenuated viruses whose genes, that are nonessential for replication in cultured cells in vitro, are either mutated or deleted. The removal of one or more nonessential genes may reduce pathogenicity without requiring a cell line to complement growth. Herpes simplex viruses (HSV) are potential vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. This review highlights the progress in creating attenuated genetically engineered HSV vectors.

Development and application of replication-incompetent HSV-1-based vectors

The replication-incompetent HSV-1-based vectors are herpesviruses in which genes that are 'essential' for viral replication have been either mutated or deleted. These deletions have substantially reduced their cytotoxicity by preventing early and late viral gene expression and, together with other deletions involving 'nonessential' genes, have also created space to introduce distinct and independently regulated expression cassettes for different transgenes. Therapeutic effects in gene therapy applications requiring simultaneous and synergic expression of multiple gene products are easily achievable with these vectors. A number of different HSV-1-based nonreplicative vectors for specific gene therapy applications have been developed so far. They have been tested in different gene therapy animal models of neuropathies (Parkinson's disease, chronic pain, spinal cord injury pain) and lysosomal storage disorders. Many replication-incompetent HSV-1-based vectors have also been used either as potential anti-herpes vaccines, as well as vaccine vectors for other pathogens in murine and simian models. Anticancer gene therapy approaches have also been successfully set up; gene therapy to other targets by using these vectors is feasible.

Gene-based treatment of motor neuron diseases

Motor neuron diseases (MND), such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), are progressive neurodegenerative diseases that share the common characteristic of upper and/or lower motor neuron degeneration. Therapeutic strategies for MND are designed to confer neuroprotection, using trophic factors, anti-apoptotic proteins, as well as antioxidants and anti-excitotoxicity agents. Although a large number of therapeutic clinical trials have been attempted, none has been shown satisfactory for MND at this time. A variety of strategies have emerged for motor neuron gene transfer. Application of these approaches has yielded therapeutic results in cell culture and animal models, including the SOD1 models of ALS. In this study we describe the gene-based treatment of MND in general, examining the potential viral vector candidates, gene delivery strategies, and main therapeutic approaches currently attempted. Finally, we discuss future directions and potential strategies for more effective motor neuron gene delivery and clinical translation.

Adenoviral gene transfer in the peripheral nervous system

BACKGROUND

Viral vectors have gained widespread use as vehicles for somatic gene transfer, and the targeted expression of foreign proteins by these vectors offers advantages over the systemic administration of the drugs in some therapeutic situations. Selective virus-mediated gene transfer to the peripheral nervous system (PNS), however, remains to be established. There are no data showing efficiency of protein transduction in the PNS, which consists of a variety of cell types, many of which are postmitotic.

METHODS

We prepared the first-generation replication-deficient recombinant adenovirus vectors engineered to express LacZ. Eight-week-old Wister rats were used in this study. Adenovirus vector (5 microl) containing the LacZ gene (5 x 10(8) pfu) was injected into rat sciatic nerves or the dorsal root ganglia at the level of L5. The sciatic nerves, the dorsal root ganglia, and the spinal cords were obtained 7, 14, 21, and 28 days after injection. Expression of LacZ was assessed by X-gal histochemistry and beta-gal immunohistochemistry.

RESULTS

Following injection of the adenovirus carrying the LacZ gene into the sciatic nerve, LacZ expression was seen mainly in the Schwann cells and the small neurons in the dorsal root ganglion. In contrast, expression was observed in the primary nerve terminals of the spinal dorsal horn and the small to large dorsal root ganglion neurons and the Schwann cells after injection of the vectors into the L5 dorsal root ganglion. There were no side effects in rats with injection in the dorsal root ganglia or the sciatic nerve.

CONCLUSIONS

The present study shows efficient protein transduction by adenovirus vectors in the PNS. It is noted that injection of the virus into the dorsal root ganglia leads to extensive expression of LacZ in the spinal cord, the dorsal root ganglia, and the sciatic nerves.

Regulatable gene expression systems for gene therapy applications: progress and future challenges

Gene therapy aims to revert diseased phenotypes by the use of both viral and nonviral gene delivery systems. Substantial progress has been made in making gene transfer vehicles more efficient, less toxic, and nonimmunogenic and in allowing long-term transgene expression. One of the key issues in successfully implementing gene therapies in the clinical setting is to be able to regulate gene expression very tightly and consistently as and when it is needed. The regulation ought to be achievable using a compound that should be nontoxic, be able to penetrate into the desired target tissue or organ, and have a half-life of a few hours (as opposed to minutes or days) so that when withdrawn or added (depending on the regulatable system used) gene expression can be turned "on" or "off" quickly and effectively. Also, the genetic switches employed should ideally be nonimmunogenic in the host. The ability to switch transgenes on and off would be of paramount importance not only when the therapy is no longer needed, but also in the case of the development of adverse side effects to the therapy. Many regulatable systems are currently under development and some, i.e., the tetracycline-dependent transcriptional switch, have been used successfully for in vivo preclinical applications. Despite this, there are no examples of switches that have been employed in a human clinical trial. In this review, we aim to highlight the main regulatable systems currently under development, the gene transfer systems employed for their expression, and also the preclinical models in which they have been used successfully. We also discuss the substantial challenges that still remain before these regulatable switches can be employed in the clinical setting.

Attenuation of pain-related behavior in a rat model of trigeminal neuropathic pain by viral-driven enkephalin overproduction in trigeminal ganglion neurons

Trigeminal neuropathic pain represents a real challenge to therapy because commonly used drugs are devoid of real beneficial effect or patients frequently become intolerant or refractory to some of these compounds. In a rat model of trigeminal neuropathic pain, which shares numerous similarities with human trigeminal neuralgia and trigeminal neuropathic pain, we used a genomic herpes simplex virus-derived vector (HSVLatEnk) to examine the possible effect of a local overproduction of proenkephalin A (PA) targeted to the trigeminal primary sensory neurons. Unilateral peripheral inoculation of recombinant vectors on the vibrissal pad territory resulted in an about ninefold increase in proenkephalin A mRNA levels in trigeminal ganglion ipsilateral to the infected side. Transgene-derived met-enkephalin accumulated in numerous nerve cell bodies of trigeminal ganglion and was transported through the sensory nerve fibers located in the infraorbital nerve. Bilateral mechanical hyperresponsiveness, which developed 2 weeks after chronic constrictive injury of the left infraorbital nerve, was significantly attenuated in animals overproducing PA in the trigeminal ganglion ipsilateral to the lesioned infraorbital nerve. This antiallodynic effect was reversed by both the opioid receptor antagonist naloxone and the peripherally acting antagonist naloxone methiodide. Our data demonstrate that the local overproduction of PA-derived peptides in trigeminal ganglion sensory neurons evoked a potent antiallodynic effect through the stimulation of mainly peripherally located opioid receptors and suggest that targeted delivery of endogenous opioids may be of interest for the treatment of some severe forms of neuropathic pain.

Reduced immune responses after vaccination with a recombinant herpes simplex virus type 1 vector in the presence of antiviral immunity

Due to the continuous need for new vaccines, viral vaccine vectors have become increasingly attractive. In particular, herpes simplex virus type 1 (HSV-1)-based vectors offer many advantages, such as broad cellular tropism, large DNA-packaging capacity and the induction of pro-inflammatory responses. However, despite promising results obtained with HSV-1-derived vectors, the question of whether pre-existing virus-specific host immunity affects vaccine efficacy remains controversial. For this reason, the influence of pre-existing HSV-1-specific immunity on the immune response induced with a replication-defective, recombinant HSV-1 vaccine was investigated in vivo. It was shown that humoral as well as cellular immune responses against a model antigen encoded by the vaccine were strongly diminished in HSV-1-seropositive mice. This inhibition could be observed in mice infected with wild-type HSV-1 or with a replication-defective vector. Although these data clearly indicate that pre-existing antiviral host immunity impairs the efficacy of HSV-1-derived vaccine vectors, they also show that vaccination under these constraints might still be feasible.

Long-Term Neuroprotection Achieved with Latency-Associated Promoter-Driven Herpes Simplex Virus Gene Transfer to the Peripheral Nervous System

We examined the ability of the herpes simplex virus (HSV) latency-associated promoter (LAP2) to drive biologically relevant prolonged transgene expression in the peripheral nervous system. Rat dorsal root ganglia were transduced in vivo by subcutaneous inoculation of replication-incompetent HSV-based vectors containing nerve growth factor (NGF) or neurotrophin-3 (NT-3) under the control of LAP2 (vectors SLN and QLNT3, respectively) and vector SHN expressing NGF under the control of the human cytomegalovirus immediate early promoter. Twenty-four weeks later a pure sensory neuropathy was induced by overdose of pyridoxine (PDX), and the animals were assessed 6 months after inoculation. Inoculation of SLN, but not SHN, attenuated the nerve damage caused by PDX and protected foot sensory amplitude, H-wave amplitude, and behavioral measures of proprioceptive function. QLNT3 was more effective than SLN in preserving the largest myelinated fibers from degeneration. These results indicate that expression of NGF or NT-3 driven by LAP2 is sufficient to prevent the development of neuropathy 6 months after vector inoculation in rats.