An HSV-1 vector expressing tyrosine hydroxylase causes production and release of L-dopa from cultured rat striatal cells

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

Methods for gene transfer to the central nervous system

Gene transfer is an increasingly utilized approach for research and clinical applications involving the central nervous system (CNS). Vectors for gene transfer can be as simple as an unmodified plasmid, but more commonly involve complex modifications to viruses to make them suitable gene delivery vehicles. This chapter will explain how tools for CNS gene transfer have been derived from naturally occurring viruses. The current capabilities of plasmid, retroviral, adeno-associated virus, adenovirus, and herpes simplex virus vectors for CNS gene delivery will be described. These include both focal and global CNS gene transfer strategies, with short- or long-term gene expression. As is described in this chapter, an important aspect of any vector is the cis-acting regulatory elements incorporated into the vector genome that control when, where, and how the transgene is expressed.

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.

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.

Coexpression of tyrosine hydroxylase, GTP cyclohydrolase I, aromatic amino acid decarboxylase, and vesicular monoamine transporter 2 from a helper virus-free herpes simplex virus type 1 vector supports high-level, long-term biochemical and behavioral correction of a rat model of Parkinson's disease

Parkinson's disease is due to the selective loss of nigrostriatal dopaminergic neurons. Consequently, many therapeutic strategies have focused on restoring striatal dopamine levels, including direct gene transfer to striatal cells, using viral vectors that express specific dopamine biosynthetic enzymes. The central hypothesis of this study is that coexpression of four dopamine biosynthetic and transporter genes in striatal neurons can support the efficient production and regulated, vesicular release of dopamine: tyrosine hydroxylase (TH) converts tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA), GTP cyclohydrolase I (GTP CH I) is the rate-limiting enzyme in the biosynthesis of the cofactor for TH, aromatic amino acid decarboxylase (AADC) converts L-DOPA to dopamine, and a vesicular monoamine transporter (VMAT-2) transports dopamine into synaptic vesicles, thereby supporting regulated, vesicular release of dopamine and relieving feedback inhibition of TH by dopamine. Helper virus-free herpes simplex virus type 1 vectors that coexpress the three dopamine biosynthetic enzymes (TH, GTP CH I, and AADC; 3-gene-vector) or these three dopamine biosynthetic enzymes and the vesicular monoamine transporter (TH, GTP CH I, AADC, and VMAT-2; 4-gene-vector) were compared. Both vectors supported production of dopamine in cultured fibroblasts. These vectors were microinjected into the striatum of 6-hydroxydopamine-lesioned rats. These vectors carry a modified neurofilament gene promoter, and gamma-aminobutyric acid (GABA)-ergic neuron-specific gene expression was maintained for 14 months after gene transfer. The 4-gene-vector supported higher levels of correction of apomorphine-induced rotational behavior than did the 3-gene-vector, and this correction was maintained for 6 months. Proximal to the injection sites, the 4-gene-vector, but not the 3-gene-vector, supported extracellular levels of dopamine and dihydroxyphenylacetic acid (DOPAC) that were similar to those observed in normal rats, and only the 4-gene-vector supported significant K(+)-dependent release of dopamine.

Production of functional recombinant tyrosine hydroxylase by the BPV-1 expression plasmids in the cell cultures

Bovine papilloma virus type-1 (BPV-1)-based expression plasmids TkBPVTH and CGalBPVTH encoding the rat tyrosine hydroxylase (TH) enzyme have been designed for the development of gene therapy for experimental Parkinson's disease. The aim of the present work was to examine the transfection of BPVTH plasmids to express a dopaminergic transgene in the monkey CV1-P fibroblast, rat C6 glioma and human NHA astrocyte cell cultures. The biological function of the transgene was estimated by analyzing the production of recombinant TH mRNA and protein, and the synthesis of L-dopa and dopamine. The highest transfection efficiency was obtained using TkBPVTH plasmids (5 microg). Furthermore, the expression of TkBPVTH plasmids was associated with significant synthesis of TH enzyme and L-dopa in the C6 and NHA cell cultures.

Behavioral correction of Parkinsonian rats following the transplantation of immortalized fibroblasts genetically modified with TH and GCH genes

Eukaryotic plasmid vectors encoding the tyrosine hydroxylase (TH) gene and GTP cyclohydrolase-1 (GCH) gene were constructed and introduced into immortalized fibroblasts obtained from SV40 large antigen (LT(AG)) transformed rat primary fibroblasts. TH and GCH positive clones were selected and identified by immunohistochemistry and RT-PCR, respectively. Hemi-parkinsonian rats created using 6-hydroxydopamine (6-OHDA) were used to assess the therapeutic effect created by the co-implantation of immortalized fibroblasts genetically modified by TH or GCH genes. Animal behavior was significantly improved two weeks following implantation and behavioral correction was maintained for over 14 weeks. Behavioral improvement was paralleled by exogenous TH gene expression, identified by TH immunohistochemistry and RT-PCR analyses. The transplanted cells survived for at least 38 weeks as demonstrated by fibronectin immunohistochemical staining. Tumor formation or host reaction was not seen, although TH expression was negative for 20 weeks after the implantation. This work demonstrates that the co-transplantation of immortalized fibroblasts genetically modified by TH and GCH genes may be developed as a valuable approach to the treatment of Parkinson's disease.

Neurovirological methods and their applications

Over the last 30 years neurovirology has emerged as a major discipline which has much relevance to both human disease and many aspects of neuroscience. This overview of the field aims to define briefly most of the major neurovirological techniques, both "classical" and more recent, and to indicate how these have been used to gain knowledge about the pathogenesis, clinical investigation, and treatment of viral infections of the central nervous system.

Application of conditionally replicating herpes vector for gene therapy treatment of urologic neoplasms

Herpes vector has been widely used for experimental gene therapy. We herein review the strategies of such therapy for the treatment of urologic neoplasms. Most experimental studies of genetically altered viruses have employed replication-incompetent vectors. However, such viruses are unable to infect additional cells subsequent to the initial infection event. Therefore, this strategy has relied heavily on the bystander effect because a large number of noninfected tumor cells remain. Conditionally replicating herpes vector G207 has been developed in order to overcome potential problems of safety and tumor specificity for human use. It has been used to treat malignant brain tumors because of its neural tropism. In the last few years, applications of G207 for non-neural tumors have been reported. Because G207 may be useful for the treatment of urologic malignant tumors, we evaluated the antitumor effect against several types of tumor cells both in vitro and in vivo. Our data suggest that G207 may be applicable for the treatment of urologic malignant tumors.

Gene therapy of Parkinson's disease using adeno-associated virus (AAV) vectors

Parkinson's disease (PD) is characterized by the progressive loss of the dopaminergic neurons in the substantia nigra and a severe decrease in dopamine in the striatum. A promising approach to the gene therapy of PD is intrastriatal expression of dopamine-synthesizing enzymes [tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC)]. The most appropriate gene-delivery vehicles for neurons are adeno-associated virus (AAV) vectors, which are derived from non-pathogenic virus. Therefore, TH and AADC genes were introduced into the striatum in the lesioned side using separate AAV vectors in parkinsonian rats, and the coexpression of TH and AADC resulted in better behavioral recovery compared with TH alone. Another strategy for gene therapy of PD is the protection of dopaminergic neurons in the substantia nigra using an AAV vector containing a glial cell line-derived neurotrophic factor (GDNF) gene. Combination of dopamine-supplement gene therapy and GDNF gene therapy would be a logical approach to the treatment of PD.

The localization and functional contribution of striatal aromatic L-amino acid decarboxylase to L-3,4-dihydroxyphenylalanine decarboxylation in rodent parkinsonian models

L-3,4-Dihydroxyphenylalanine (L-dopa) is the mainstay of therapy for patients with Parkinson's disease (PD), and mediates its primary effects through conversion into dopamine by aromatic L-amino acid decarboxylase (AADC). Given the loss of AADC-containing nigrostriatal dopaminergic neurons in PD, however, the location of residual AADC that converts L-dopa into dopamine remains controversial. The first objective of this study was to establish the presence of AADC expression in striatal neurons and glia using reverse transcriptase and PCR. Transcripts for the neuronal but not nonneuronal forms of AADC were detected in striatal tissue, cultured striatal neurons, and glia. We then examined whether this striatal AADC expression represents a physiologically significant source of dopaine production. No dopamine release was detected following incubation of striatal cultures with L-dopa or transduction with adenovirus expressing tyrosine hydoxylase. Our data establish the presence of AADC expression in the striatum both in vivo and in vitro, but suggest that striatal components do not represent a primary source of L-dopa decarboxylation following nigrostriatal denervation in rats. Understanding the source and localization of AADC is important in understanding the complications of L-dopa therapy and in designing rational therapeutic strategies for PD, including cellular transplantation and gene therapy.

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

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

Genetic analysis of the role of protein kinase C signaling pathways in behaviors by direct gene transfer with HSV-1 vectors

A genetic intervention strategy is described to elucidate the specific biochemical pathways in identified types of neurons that underlie behavioral adaptations. This strategy contains three parts: A Herpes simplex virus (HSV-1) vector is used to obtain localized gene transfer, a cell type-specific promoter is used to target expression to a particular type of neuron, and a constitutively active signal transduction enzyme is expressed to alter neuronal physiology. To enable this approach, a constitutively active protein kinase C (PKC) was developed which causes a long-lasting, activation-dependent increase in neurotransmitter release from cultured sympathetic neurons. This genetic intervention strategy was tested using the nigrostriatal system: Microinjection of HSV-1 vectors that contain the tyrosine hydroxylase promoter targeted expression to dopaminergic nigrostriatal neurons. Expression of the constitutively active PKC in a small percentage of nigrostriatal neurons (approximately 0.1-2%) produced a long-term (> or = 1 month) change in apomorphine-induced rotational behavior, the amount of rotational behavior correlated with the number of affected nigrostriatal neurons, and D2-like dopamine receptor levels were elevated in the striatal regions innervated by the affected nigrostriatal neurons. The strengths and limitations of this genetic intervention strategy are discussed.

Somatic gene therapy in animal models of Parkinson's disease

Gene therapy in Parkinson's disease (PD) emerged about 10 years ago but until now, no clinical trials are under way, because most approaches have failed to show long-term therapeutic effects in PD animal models and because safety concerns precluded the use in humans so far. This review tries to give an overview on the development of different strategies in gene therapy in PD animal models and point out new and possibly more successful directions, including the transplantation of neural precursor cells and pig tissue.

Behavioral recovery in 6-hydroxydopamine-lesioned rats by cotransduction of striatum with tyrosine hydroxylase and aromatic L-amino acid decarboxylase genes using two separate adeno-associated virus vectors

Parkinson's disease (PD) is characterized by the progressive loss of the dopaminergic neurons in the substantia nigra and a severe decrease in dopamine in the striatum. A promising approach to the gene therapy of PD is intrastriatal expression of enzymes in the biosynthetic pathway for dopamine. Tyrosine hydroxylase (TH) catalyzes the synthesis of L-dopa, which must be converted to dopamine by aromatic L-amino acid decarboxylase (AADC). Since the endogenous AADC activity in the striatum is considered to be low, coexpression of both TH and AADC in the same striatal cells would increase the dopamine production and thereby augment the therapeutic effects. In the present study, the TH gene and also the AADC gene were simultaneously transduced into rat striatal cells, using two separate adeno-associated virus (AAV) vectors, AAV-TH and AAV-AADC. Immunostaining showed that TH and AADC were coexpressed efficiently in the same striatal cells in vitro and in vivo. Moreover, cotransduction with these two AAV vectors resulted in more effective dopamine production and more remarkable behavioral recovery in 6-hydroxydopamine (6-OHDA)-lesioned rats, compared with rats receiving AAV-TH alone (p < 0.01). These findings suggest an alternative strategy for gene therapy of PD and indicate that the simultaneous transduction with two AAV vectors can extend their utility for potential gene therapy applications.

Direct gene transfer into human epileptogenic hippocampal tissue with an adeno-associated virus vector: implications for a gene therapy approach to epilepsy

PURPOSE

Virus vectors capable of transferring genetic information into human cells provide hope for improved therapy in several neurological diseases, including epilepsy. We evaluated the ability of an adeno-associated virus (AAV) vector to transfer and cause expression of a lacZ marker gene in brain slices obtained from patients undergoing temporal lobectomy for control of medically intractable seizures.

METHODS

Human brain slices were injected with an AAV vector (AAVlacZ) encoding Escherichia coli beta-galactosidase and incubated for as long as 24 h. The presence of lacZ mRNA. beta-galactosidase protein and enzymatic activity were assayed by reverse transcriptase polymerase chain reaction (rtPCR), immunocytochemistry, and the X-Gal technique, respectively.

RESULTS

AAVlacZ directed the expression in human epileptogenic brain of E. coli beta-galactosidase that had functional activity. Expression was observed in < or =5 h and was sustained for as long as the slices were viable. Morphological analysis indicated that neurons were preferentially transfected, and there was no evidence of cytotoxicity.

CONCLUSIONS

Our results confirm the feasibility of using AAV vectors to transfer genes into the human CNS and in particular, into neurons. Replacement of the lacZ gene with a functional gene modulating hippocampal neuronal physiology, might allow a localized genetic intervention for focal seizures based on the stereotaxic or endovascular delivery of such a vector system into the appropriate brain region.

Helper virus-free herpes simplex virus-1 plasmid vectors for gene therapy of Parkinson's disease and other neurological disorders

Vectors based on herpes simplex virus type 1 (HSV-1) have potential for gene therapy of neurological disorders. HSV-1 plasmid vectors (amplicons) contain only approximately 1% of the 150-kb HSV-1 genome and have been packaged into virus particles by using a helper virus. We have demonstrated that HSV-1 plasmid vectors which express tyrosine hydroxylase can cause long-term biochemical and behavioral recovery in the 6-hydroxydopamine rat model of Parkinson's disease. Furthermore, we and others have used HSV-1 plasmid vectors which express a wide range of genes that affect neuronal physiology. Because of the pathogenicity of the HSV-1 helper virus, however, the use of this vector system has been limited to studies in animal models or primary cultures of neural cells. Thus, to increase the safety of HSV-1 plasmid vectors, we recently developed a helper virus-free packaging system that may facilitate studies on neuronal physiology and potential therapeutic applications.

Development of an HSV-based vector for the treatment of Parkinson's disease

The restricted pattern of neurodegeneration seen in Parkinson's disease, and the identification of trophic factors that prevent toxin-induced degeneration of dopaminergic neurons, has spurred research into potential gene therapy for this disease. Herpes simplex virus (HSV-1) is a neurotrophic virus which naturally establishes latency in neurons. HSV-based vectors have been demonstrated to transfer and transiently express transgenes in neurons in brain in vivo. Recent experiment have shown that deletion of multiple immediate-early HSV genes reduces the potential cytotoxicity of these vectors, and in addition results in altered patterns of transgene expression that may allow for long-term expression required for human gene therapy applications.

Defective viral vectors as agents for gene transfer in the nervous system

Viral vectors have attracted great interest as vehicles for gene therapy. Due to concerns regarding continued viral gene expression in several systems, new approaches have been sought for gene transfer in the nervous system. This article reviews the general concepts and basic biology of defective viral vectors. These are vectors which can package into a viral coat but contain no viral genes, thereby allowing efficient gene transfer in the absence of viral gene expression in target cells. The defective herpes simplex virus (HSV) vector has been applied to numerous interesting questions in neurobiology. The inability to completely eliminate helper viruses has raised concern regarding the application of this vector to human disease. The adeno-associated virus (AAV) vector has recently been introduced into the nervous system. This vector harbors no viral genes, however helper viruses can also be completely eliminated from the system. Although the smaller size may limit the range of applications for this vector, it has received great interest as a potential agent for gene therapy in the nervous system. Potential future directions are discussed as well.

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

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

Prospects for gene therapy in Parkinson's disease

Numerous advances in in vivo and ex vivo gene-therapy approaches to Parkinson's disease offer promise for direct clinical trials in patients in the next several years. These systems are predicated on introducing gene that encode enzymes responsible for dopamine biosynthesis or neurotrophic factors that may delay nigrostriatal degeneration or facilitate regeneration. We review the current status of experimental approaches to gene therapy for Parkinson's disease. Comparative advantages and disadvantages of each system are enumerated, and preclinical trials of some of the systems are evaluated. Although the specific in vivo or ex vivo methods used for gene transfer into the brain are likely to be supplanted by newer technology over the next decade, the principles and approaches developed in current studies likely will remain the same.

Double transduction with GTP cyclohydrolase I and tyrosine hydroxylase is necessary for spontaneous synthesis of L-DOPA by primary fibroblasts

Gene transfer of tyrosine hydroxylase (TH) in animal models of Parkinson's disease (PD), using either genetically modified cells or recombinant virus vectors, has produced partial restoration of behavioral and biochemical deficits. The limited success of this approach may be related to the availability of the cofactor, tetrahydrobiopterin (BH4), because neither the dopamine-depleted striatum nor the cells used for gene transfer possess a sufficient amount of BH4 to support TH activity. To determine the role of BH4 in gene therapy, fibroblast cells transduced with the gene for TH were additionally modified with the gene for GTP cyclohydrolase l; an enzyme critical for BH4 synthesis. In contrast to cells transduced with only TH, doubly transduced fibroblasts spontaneously produced both BH4 and 3, 4-dihydroxy-L-phenylalanine. To examine further the importance of GTP cyclohydrolase I in gene therapy for PD, in vivo micro-dialysis was used to assess the biochemical changes in the dopamine-denervated striatum containing grafts of genetically modified fibroblasts. Only denervated striata grafted with fibro-blasts possessing both TH and GTP cyclohydrolase I genes displayed biochemical restoration. However, no significant differences from controls were observed in apomorphine-induced rotation. This is partly attributable to a limited duration of gene expression in vivo. These differences between fibroblasts transduced with TH alone and those additionally modified with the GTP cyclohydrolase I gene indicate that BH4 is critical for biochemical restoration in a rat model of PD and that GTP cyclohydrolase I is sufficient for production of BH4.

Double transduction with GTP cyclohydrolase I and tyrosine hydroxylase is necessary for spontaneous synthesis of L-DOPA by primary fibroblasts

Gene transfer of tyrosine hydroxylase (TH) in animal models of Parkinson's disease (PD), using either genetically modified cells or recombinant virus vectors, has produced partial restoration of behavioral and biochemical deficits. The limited success of this approach may be related to the availability of the cofactor, tetrahydrobiopterin (BH4), because neither the dopamine-depleted striatum nor the cells used for gene transfer possess a sufficient amount of BH4 to support TH activity. To determine the role of BH4 in gene therapy, fibroblast cells transduced with the gene for TH were additionally modified with the gene for GTP cyclohydrolase l; an enzyme critical for BH4 synthesis. In contrast to cells transduced with only TH, doubly transduced fibroblasts spontaneously produced both BH4 and 3, 4-dihydroxy-L-phenylalanine. To examine further the importance of GTP cyclohydrolase I in gene therapy for PD, in vivo micro-dialysis was used to assess the biochemical changes in the dopamine-denervated striatum containing grafts of genetically modified fibroblasts. Only denervated striata grafted with fibro-blasts possessing both TH and GTP cyclohydrolase I genes displayed biochemical restoration. However, no significant differences from controls were observed in apomorphine-induced rotation. This is partly attributable to a limited duration of gene expression in vivo. These differences between fibroblasts transduced with TH alone and those additionally modified with the GTP cyclohydrolase I gene indicate that BH4 is critical for biochemical restoration in a rat model of PD and that GTP cyclohydrolase I is sufficient for production of BH4.

Long-term expression in sensory neurons in tissue culture from herpes simplex virus type 1 (HSV-1) promoters in an HSV-1-derived vector

Amplicons, defective herpes simplex virus type 1 (HSV-1) vectors, were constructed to use four HSV-1 promoters, from the immediate-early (IE) 1 IE 3, IE 4/5, and late glycoprotein C (gC) genes, to regulate expression of the Escherichia coli lacZ gene, encoding beta-galactosidase, and packaged into infectious particles. Infection of sensory neurons in vitro with amplicons containing the IE 1, IE 3, or IE 4/5 promoter resulted in stable long-term expression of beta-galactosidase from 2 to 10 weeks after gene transfer. The number of neurons expressing beta-galactosidase was not changed by treatments previously shown to produce reactivation of latent HSV-1. In addition, the latency-associated transcript was detected in many of the same neurons that expressed beta-galactosidase, indicating that the viral IE promoters in the amplicons can function in the same neurons that harbor latent virus. Delivery of beta-galactosidase protein directly into neurons by microinjection indicated that the half-life for histochemical detection of beta-galactosidase was between 24 and 48 h, suggesting that the persistence of beta-galactosidase histochemical staining cannot be explained by the stability of the reporter protein alone. In contrast to the IE promoters, the gC promoter of the late gene class did not support long-term expression of beta-galactosidase; instead, beta-galactosidase was detected in only a few neurons per culture at 2 weeks after infection, and superinfection with wild-type HSV-1 did not increase the level of expression from the gC promoter. These results suggest that the HSV-1 IE promoters in the amplicons are not subject to the promoter inactivation that occurs with many types of virus vectors and that the IE promoters in the context of the amplicon avoid the promoter inactivation observed from the same promoters in the HSV-1 genome during latency.