Long-term doxycycline-controlled expression of human tyrosine hydroxylase after direct adenovirus-mediated gene transfer to a rat model of Parkinson's disease

Pharmacological approach using doxycycline and tocopherol in rotenone induced oxidative stress, neuroinflammation and Parkinson's like symptoms

BACKGROUND

Parkinson's disease (PD) is a second most common neurodegenerative disorder characterized by the selective and progressive degeneration of dopaminergic neurons in substantia nigra pars compacta. Rotenone is a neurotoxin which selectively degenerate dopaminergic neurons in striatum, leading to cause PD like symptoms.

METHOD

Rotenone was administered at a dose of 1.5 mg/kg, i.p. from day 1 to day 40. Treatment with doxycycline (50 and 100 mg/kg, p.o), tocopherol (5 mg and 10 mg/kg, p.o) alone, doxycycline (50 mg/kg, p.o) in combination with tocopherol (10 mg/kg, p.o), and ropinirole (0.5 mg/kg, i.p.) was given for 40 days 1 h prior to administration of rotenone. All behavioral parameters were analyzed on weekly basis. On day 41, animals were sacrificed and the striatum region was isolated for neurotransmitters estimation (dopamine, serotonin, norepinephrine, GABA and glutamate), biochemical analysis (GSH, nitrite, LPO, mitochondrial complexes I and IV), inflammatory markers estimation (IL-6, IL-1β and TNF-α) and activity of MAO-A, MAO-B.

RESULT

Doxycycline and tocopherol in combination significantly attenuated behavioral, neurotransmitters and biochemical alterations induced by rotenone in experimental rats as compared to alone treatment with DOX and TOCO. Similarly, DOX and TOCO combination significantly reduced the level of inflammatory markers, prevented the biochemical changes, decreased MAO-A and MAO-B and improved complex-I, complex-IV, cAMP levels significantly.

CONCLUSION

The current study revealed that a combination of doxycycline with tocopherol contributed to the prevention of PD like symptoms in rats by antioxidant, anti-inflammatory, MAO inhibitory and neuromodulatory mechanisms.

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Brain targeted oral delivery of doxycycline hydrochloride encapsulated Tween 80 coated chitosan nanoparticles against ketamine induced psychosis: behavioral, biochemical, neurochemical and histological alterations in mice

To develop statistically optimized brain targeted Tween 80 coated chitosan nanoparticulate formulation for oral delivery of doxycycline hydrochloride for the treatment of psychosis and to evaluate its protective effect on ketamine induced behavioral, biochemical, neurochemical and histological alterations in mice. 32 full factorial design was used to optimize the nanoparticulate formulation to minimize particle size and maximize entrapment efficiency, while independent variables chosen were concentration of chitosan and Tween 80. The optimized formulation was characterized by particle size, drug entrapment efficiency, Fourier transform infrared, Transmission electron microscopy analysis and drug release behavior. Pure doxycycline hydrochloride (25 and 50 mg/kg, p.o.) and optimized doxycycline hydrochloride encapsulated Tween 80 coated chitosan nanoparticles (DCNPopt) (equivalent to 25 mg/kg doxycycline hydrochloride, p.o.) were explored against ketamine induced psychosis in mice. The experimental studies for DCNPopt, with mean particle size 237 nm and entrapment efficiency 78.16%, elucidated that the formulation successfully passed through blood brain barrier and exhibited significant antipsychotic activity. The underlying mechanism of action was further confirmed by behavioral, biochemical, neurochemical estimations and histopathological study. Significantly enhanced GABA and GSH level and diminished MDA, TNF-α and dopamine levels were observed after administration of DCNPopt at just half the dose of pure doxycycline hydrochloride, showing better penetration of doxycyline hydrochloride in the form of Tween 80 coated nanoparticles through blood brain barrier. This study demonstrates the hydrophilic drug doxycycline hydrochloride, loaded in Tween 80 coated chitosan nanoparticles, can be effectively brain targeted through oral delivery and therefore represents a suitable approach for the treatment of psychotic symptoms.

Regulated Gene Therapy

Gene therapy represents a promising approach for the treatment of monogenic and multifactorial neurological disorders. It can be used to replace a missing gene and mutated gene or downregulate a causal gene. Despite the versatility of gene therapy, one of the main limitations lies in the irreversibility of the process: once delivered to target cells, the gene of interest is constitutively expressed and cannot be removed. Therefore, efficient, safe and long-term gene modification requires a system allowing fine control of transgene expression.Different systems have been developed over the past decades to regulate transgene expression after in vivo delivery, either at transcriptional or post-translational levels. The purpose of this chapter is to give an overview on current regulatory system used in the context of gene therapy for neurological disorders. Systems using external regulation of transgenes using antibiotics are commonly used to control either gene expression using tetracycline-controlled transcription or protein levels using destabilizing domain technology. Alternatively, specific promoters of genes that are regulated by disease mechanisms, increasing expression as the disease progresses or decreasing expression as disease regresses, are also examined. Overall, this chapter discusses advantages and drawbacks of current molecular methods for regulated gene therapy in the central nervous system.

Controlled Striatal DOPA Production From a Gene Delivery System in a Rodent Model of Parkinson's Disease

Conventional symptomatic treatment for Parkinson's disease (PD) with long-term L-3,4-dihydroxyphenylalanine (DOPA) is complicated with development of drug-induced side effects. In vivo viral vector-mediated gene expression encoding tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) provides a drug delivery strategy of DOPA with distinct advantages over pharmacotherapy. Since the brain alterations made with current gene transfer techniques are irreversible, the therapeutic approaches taken to the clinic should preferably be controllable to match the needs of each individual during the course of their disease. We used a recently described tunable gene expression system based on the use of destabilized dihydrofolate reductase (DD) and generated a N-terminally coupled GCH1 enzyme (DD-GCH1) while the TH enzyme was constitutively expressed, packaged in adeno-associated viral (AAV) vectors. Expression of DD-GCH1 was regulated by the activating ligand trimethoprim (TMP) that crosses the blood-brain barrier. We show that the resulting intervention provides a TMP-dose-dependent regulation of DOPA synthesis that is closely linked to the magnitude of functional effects. Our data constitutes the first proof of principle for controlled reconstitution of dopamine capacity in the brain and suggests that such next-generation gene therapy strategies are now mature for preclinical development toward use in patients with PD.

A next step in adeno-associated virus-mediated gene therapy for neurological diseases: regulation and targeting

Recombinant adeno-associated virus (rAAV) vectors mediating long term transgene expression are excellent gene therapy tools for chronic neurological diseases. While rAAV2 was the first serotype tested in the clinics, more efficient vectors derived from the rh10 serotype are currently being evaluated and other serotypes are likely to be tested in the near future. In addition, aside from the currently used stereotaxy-guided intraparenchymal delivery, new techniques for global brain transduction (by intravenous or intra-cerebrospinal injections) are very promising. Various strategies for therapeutic gene delivery to the central nervous system have been explored in human clinical trials in the past decade. Canavan disease, a genetic disease caused by an enzymatic deficiency, was the first to be approved. Three gene transfer paradigms for Parkinson's disease have been explored: converting L-dopa into dopamine through AADC gene delivery in the putamen; synthesizing GABA through GAD gene delivery in the overactive subthalamic nucleus and providing neurotrophic support through neurturin gene delivery in the nigro-striatal pathway. These pioneer clinical trials demonstrated the safety and tolerability of rAAV delivery in the human brain at moderate doses. Therapeutic effects however, were modest, emphasizing the need for higher doses of the therapeutic transgene product which could be achieved using more efficient vectors or expression cassettes. This will require re-addressing pharmacological aspects, with attention to which cases require either localized and cell-type specific expression or efficient brain-wide transgene expression, and when it is necessary to modulate or terminate the administration of transgene product. The ongoing development of targeted and regulated rAAV vectors is described.

Continuous DOPA synthesis from a single AAV: dosing and efficacy in models of Parkinson's disease

We used a single adeno-associated viral (AAV) vector co-expressing tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) to investigate the relationship between vector dose, and the magnitude and rate of recovery in hemi-parkinsonian rats. Intrastriatal injections of >1E10 genomic copies (gc) of TH-GCH1 vector resulted in complete recovery in drug-naïve behavior tests. Lower vector dose gave partial to no functional improvement. Stereological quantification revealed no striatal NeuN+ cell loss in any of the groups, whereas a TH-GCH1 dose of >1E11 gc resulted in cell loss in globus pallidus. Thus, a TH-GCH1 dose of 1E10 gc gave complete recovery without causing neuronal loss. Safety and efficacy was also studied in non-human primates where the control vector resulted in co-expression of the transgenes in caudate-putamen. In the TH-GCH1 group, GCH1 expression was robust but TH was not detectable. Moreover, TH-GCH1 treatment did not result in functional improvement in non-human primates.

Subthalamic hGAD65 gene therapy and striatum TH gene transfer in a Parkinson's disease rat model

The aim of the present study is to detect a combination method to utilize gene therapy for the treatment of Parkinson’s disease (PD). Here, a PD rat model is used for the in vivo gene therapy of a recombinant adeno-associated virus (AAV2) containing a human glutamic acid decarboxylase 65 (rAAV2-hGAD65) gene delivered to the subthalamic nucleus (STN). This is combined with the ex vivo gene delivery of tyrosine hydroxylase (TH) by fibroblasts injected into the striatum. After the treatment, the rotation behavior was improved with the greatest efficacy in the combination group. The results of immunohistochemistry showed that hGAD65 gene delivery by AAV2 successfully led to phenotypic changes of neurons in STN. And the levels of glutamic acid and GABA in the internal segment of the globus pallidus (GPi) and substantia nigra pars reticulata (SNr) were obviously lower than the control groups. However, hGAD65 gene transfer did not effectively protect surviving dopaminergic neurons in the SNc and VTA. This study suggests that subthalamic hGAD65 gene therapy and combined with TH gene therapy can alleviate symptoms of the PD model rats, independent of the protection the DA neurons from death.

Continuous dopaminergic stimulation: clinical aspects and experimental bases

In patients with Parkinson disease, pulsatile administration of dopaminergic drugs is associated with motor fluctuations and dyskinesias. By contrast, treatments that provide more continuous dopaminergic stimulation are associated with less intense motor complications. This can be achieved by using drugs with longer half-lives, delayed release formulations, and routes of administration that permit continuous delivery. The mechanisms by which different modes of dopaminergic treatment (pulsatile or continuous) determine the motor response are not fully understood. However, the use of experimental models of parkinsonism has helped understand the motor complications associated with pulsatile dopamine replacement. These studies have provided important insights into the biochemical and molecular changes in the basal ganglia in response to continuous stimulation. In addition, these models have facilitated the development of new treatments that may stabilize the motor response and the biochemical alterations in the basal ganglia to provide more efficient forms of continuous dopaminergic stimulation in patients with Parkinson disease.

Design of a single AAV vector for coexpression of TH and GCH1 to establish continuous DOPA synthesis in a rat model of Parkinson's disease

Preclinical efficacy of continuous delivery of 3,4-dihydroxyphenylalanine (DOPA) with adeno-associated viral (AAV) vectors has recently been documented in animal models of Parkinson's disease (PD). So far, all studies have utilized a mix of two monocistronic vectors expressing either of the two genes, tyrosine hydroxylase (TH) and GTP cyclohydrolase-1 (GCH1), needed for DOPA production. Here, we present a novel vector design that enables efficient DOPA production from a single AAV vector in rats with complete unilateral dopamine (DA) lesions. Functional efficacy was assessed with drug-induced and spontaneous motor behavioral tests where vector-treated animals showed near complete and stable recovery within 1 month. Recovery of motor function was associated with restoration of extracellular DA levels as assessed by online microdialysis. Histological analysis showed robust transgene expression not only in the striatum but also in overlying cortical areas. In globus pallidus, we noted loss of NeuN staining, which might be due to different sensitivity in neuronal populations to transgene expression. Taken together, we present a single AAV vector design that result in efficient DOPA production and wide-spread transduction. This is a favorable starting point for continued translation toward a therapeutic application, although future studies need to carefully review target region, vector spread and dilution with this approach.

Testing the therapeutic potential of doxycycline in a Drosophila melanogaster model of Alzheimer disease

Therapies for Alzheimer disease that reduce the production of pathogenic amyloid β (Aβ) peptides have been associated with a range of unwanted effects. For this reason, alternative strategies that promote the clearance of the peptide by preventing its aggregation and deposition in the brain have been favored. In this context we have studied doxycycline, a member of the tetracycline family of antibiotics that has shown neuroprotective effects in a number of models of neurodegenerative disease. We investigated the neuroprotective potential of doxycycline in a Drosophila model of Aβ toxicity and sought to correlate any effects with the aggregation state of the peptide. We found that administration of doxycycline to Aβ42-expressing flies did not improve their lifespan but was able to slow the progression of their locomotor deficits. We also measured the rough eye phenotype of transgenic flies expressing the E22G variant of Aβ42 and showed that doxycycline administration partially rescued the toxicity of Aβ in the developing eye. We correlated these in vivo effects with in vitro observations using transmission electron microscopy, dynamic light scattering, and thioflavin T binding. We found that doxycycline prevents Aβ fibrillization and favors the generation of smaller, non-amyloid structures that were non-toxic as determined by the lack of caspase 3 activation in a neuroblastoma cell line. Our confirmation that doxycycline can prevent amyloid β toxicity both in vitro and in vivo supports its therapeutic potential in AD.

Development of advanced therapies based on viral vector-mediated overexpression of therapeutic molecules and knockdown of disease-related genes for Parkinson’s disease

The last decade witnessed the translation of several gene-based therapeutic approaches from experimental studies to early clinical trials. Studies targeting the treatment of Parkinson’s disease (PD) were among the forefront of trials in the CNS. In this article, we overview three major strategies for the treatment of PD: the enzyme-replacement strategies are based on well-defined principles of functional restoration and are well suited for treatment of patients with advanced disease who would typically experience complications due to side effects of pharmacotherapy. Neurotrophic factor delivery, on the other hand, aims to delay the disability and eventually modifiy disease progression. Finally, we present an outlook to a completely new way of interfering with the disease process, which is taking advantage of recently discovered RNAi mechanisms in cells. Gene therapy is now becoming a reality in the clinics and developments in the next decade will help uncover the true potential of this approach for not only the treatment of PD patients, but also many other neurological disorders.

The vesicular glutamate transporter-1 upstream promoter and first intron each support glutamatergic-specific expression in rat postrhinal cortex

Multiple applications of direct gene transfer into neurons require restricting expression to glutamatergic neurons, or specific subclasses of glutamatergic neurons. Thus, it is desirable to develop and analyze promoters that support glutamatergic-specific expression. The three vesicular glutamate transporters (VGLUTs) are found in different populations of neurons, and VGLUT1 is the predominant VGLUT in the neocortex, hippocampus, and cerebellar cortex. We previously reported on a plasmid (amplicon) Herpes Simplex Virus vector that contains a VGLUT1 promoter. This vector supports long-term expression in VGLUT1-containing glutamatergic neurons in rat postrhinal (POR) cortex, but does not support expression in VGLUT2-containing glutamatergic neurons in the ventral medial hypothalamus. This VGLUT1 promoter contains both the VGLUT1 upstream promoter and the VGLUT1 first intron. In this study, we begin to isolate and analyze the glutamatergic-specific regulatory elements in this VGLUT1 promoter. We show that the VGLUT1 upstream promoter and first intron each support glutamatergic-specific expression. We isolated a small, basal VGLUT1 promoter that does not support glutamatergic-specific expression. Next, we fused either the VGLUT1 upstream promoter or the first intron to this basal promoter. The VGLUT1 upstream promoter or the first intron, fused to the basal promoter, each supported glutamatergic-specific expression in POR cortex.

Gene therapy for Parkinson's disease

The once fantastic theoretical concept that patients with Parkinson's disease (PD) would receive gene therapy in an attempt to alleviate their symptoms and potentially modify the course of their disease has become a reality. On the basis of positive preclinical data, four different gene therapy approaches are currently in Phase I or Phase II clinical trials. Some approaches are intended to increase levels of endogenous dopamine or enhance the function of the prodrug levodopa. Others are intended to normalize basal ganglia circuitry by reducing the PD-related overactivity of specific brain structures such as the subthalamic nucleus. Each is intended for symptomatic benefit. Finally, gene delivery of trophic factors that not only augment dopaminergic function but are potentially disease modifying has a strong preclinical database and are also in clinical trials. Each of these approaches is discussed in the present review.

Optimization of continuous in vivo DOPA production and studies on ectopic DA synthesis using rAAV5 vectors in Parkinsonian rats

Viral vector-mediated gene transfer is emerging as a novel therapeutic approach with clinical utility in treatment of Parkinson's disease. Recombinant adeno-associated viral (rAAV) vector in particular has been utilized for continuous l-3,4 dihydroxyphenylalanine (DOPA) delivery by expressing the tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) genes which are necessary and sufficient for efficient synthesis of DOPA from dietary tyrosine. The present study was designed to determine the optimal stoichiometric relationship between TH and GCH1 genes for ectopic DOPA production and the cellular machinery involved in its synthesis, storage, and metabolism. For this purpose, we injected a fixed amount of rAAV5-TH vector and increasing amounts of rAAV5-GCH1 into the striatum of rats with complete unilateral dopamine lesion. After 7 weeks the animals were killed for either biochemical or histological analysis. We show that increasing the availability of 5,6,7,8-tetrahydro-l-biopterin (BH4) in the same cellular compartment as the TH enzyme resulted in better efficiency in DOPA synthesis, most likely by hindering inactivation of the enzyme and increasing its stability. Importantly, the BH4 synthesis from ectopic GCH1 expression was saturable, yielding optimal TH enzyme functionality between GCH1 : TH ratios of 1 : 3 and 1 : 7.

In vivo gene regulation using tetracycline-regulatable systems

Numerous preclinical studies have demonstrated the efficacy of viral gene delivery vectors, and recent clinical trials have shown promising results. However, the tight control of transgene expression is likely to be required for therapeutic applications and in some instances, for safety reasons. For this purpose, several ligand-dependent transcription regulatory systems have been developed. Among these, the tetracycline-regulatable system is by far the most frequently used and the most advanced towards gene therapy trials. This review will focus on this system and will describe the most recent progress in the regulation of transgene expression in various organs, including the muscle, the retina and the brain. Since the development of an immune response to the transactivator was observed following gene transfer in the muscle of nonhuman primate, focus will be therefore, given on the immune response to transgene products of the tetracycline inducible promoter.

Scientific rationale for the development of gene therapy strategies for Parkinson's disease

The ever-evolving understanding of the neuronal systems involved in Parkinson's disease together with the recent advances in recombinant viral vector technology has led to the development of several gene therapy applications that are now entering into clinical testing phase. To date, four fundamentally different approaches have been pursued utilizing recombinant adeno-associated virus and lentiviruses as vectors for delivery. These strategies aim either to restore the lost brain functions by substitution of enzymes critical for synthesis of neurotransmitters or neurotrophic factors as a means to boost the function of remaining neurons in the diseased brain. In this review we discuss the differences in mechanism of action and describe the scientific rationale behind the currently tested gene therapy approaches for Parkinson's disease in some detail and pinpoint their individual unique strengths and weaknesses.

Lentiviral vectors mediate nonimmunosuppressive rapamycin analog-induced production of secreted therapeutic factors in the brain: regulation at the level of transcription and exocytosis

Gene transfer may become a powerful clinical tool for the delivery of secreted therapeutic polypeptides, provided that the in situ production of these peptides can be tightly regulated by the administration of a small inducer molecule. Particularly efficient control may be achieved by simultaneously using two regulation systems that interfere with the biosynthesis of the therapeutic factor at two different levels. Therefore, we have developed a set of two lentiviral vectors containing two regulation systems. These systems are induced by nonimmunosuppressive derivatives of rapamycin ("rapalogs") and allow simultaneous control of expression and of exocytosis of secreted therapeutic polypeptides. The set of vectors was used to produce green fluorescent protein (GFP) and glial cell line-derived neurotrophic factor (GDNF); GFP served as a model factor to demonstrate expression and entry into the exocytotic pathway in transduced cells. The constructs allowed robust in vitro expression and secretion of the polypeptides in the presence of rapalog AP21967. Withdrawal of the inducer resulted in efficient downregulation. In vivo, tightly regulated production of GFP and GDNF was observed after injection of the constructs into the striata of mice. The vectors thus fulfill key requirements for application in gene therapy.

Enhancement of cell-specific transgene expression from a Tet-Off regulatory system using a transcriptional amplification strategy in the rat brain

Background

The Tet-Off system uses a tetracycline-controlled transactivator protein (tTA) and a tetracycline-responsive promoter element (TRE) to regulate expression of a target gene. This system can be used to achieve regulatable transgene expression in specific cell types by employing a cell-specific promoter to drive tTA expression. Wide applications of this attractive approach are, however, hindered by relatively weak transcriptional activity of most cell-specific promoters. We report here the feasibility of using a transcriptional amplification strategy to overcome the problem.

Methods and results

In the developed cell-type-specific, Tet-inducible lentiviral system, two distinct cellular promoters were tested, a human synapsin-1 promoter for neurons and a compact glial fibrillary acidic protein promoter for astroglial cells. Lentiviral vectors were constructed that contained two copies of one or the other of these two promoters. One copy was used to drive the expression of a chimeric transactivator consisting of a part of the transcriptional activation domain of the NF-κB p65 protein fused to the DNA-binding domain of the yeast GAL4 protein. The second copy of the cell-specific promoter was modified by introduction of the GAL4 binding sequences at its 5′ end. This copy was used to drive expression of tTA. A gene encoding a red fluorescent protein was cloned into another lentiviral vector under transcriptional control of TRE. Co-transduction with the two types of viral vectors provided doxycycline-regulated transgene expression in a neuron- or astrocyte-specific manner. Compared to control viruses without transcriptional amplification, our enhanced systems were approximately 8-fold more potent in cultured neurons and astroglial cells and at least 8- to 12-fold more potent in the rat brain in vivo.

Conclusions

Our results demonstrate the effectiveness of the transcriptional amplification strategy in developing viral gene delivery systems that combine the advantages of specific cell type targeting and Tet-inducible expression. Copyright © 2008 John Wiley & Sons, Ltd.

Tetracycline-controlled genetic switches

Unlike recombinase-mediated gene manipulations, tetracycline (Tet)-controlled genetic switches permit reversible control of gene expression in the mouse. Trancriptional activation can be induced by activators termed tTA (Tet-Off) or rtTA (Tet-On) in the absence and presence of Tet, respectively. The Tet-Off and Tet-On systems are complementary, and the decision to choose one over the other depends on the particular experimental strategy. Both systems were optimized over the years and can now be used to develop mouse models.

Gene delivery to differentiated neurotypic cells with RGD and HIV Tat peptide functionalized polymeric nanoparticles

A number of neurodegenerative disorders may potentially be treated by the delivery of therapeutic genes to neurons. Nonviral gene delivery systems, however, typically provide low transfection efficiency in post-mitotic differentiated neurons. To uncover mechanistic reasons for this observation, we compared gene transfer to undifferentiated and differentiated SH-SY5Y cells using polyethylenimine (PEI)/DNA nanocomplexes. Differentiated cells exhibited substantially lower uptake of gene vectors. To overcome this bottleneck, RGD or HIV-1 Tat peptides were attached to PEI/DNA nanocomplexes via poly(ethylene glycol) (PEG) spacer molecules. Both RGD and Tat improved the cellular uptake of gene vectors and enhanced gene transfection efficiency of primary neurons up to 14-fold. RGD functionalization resulted in a statistically significant increase in vector escape from endosomes, suggesting it may improve gene delivery by more than one mechanism.

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.

Control of dopamine-secretion by Tet-Off system in an in vivo model of parkinsonian rat

We established a PC12 cell line (PC12TH Tet-Off) in which human tyrosine hydroxylase (TH) expression can be negatively controlled by Doxycycline (Dox). First, dopamine (DA)-secretion from PC12TH Tet-Off cells was controlled by Dox-administration in a dose-responsive manner ranging from 0 to 100 ng/ml for 70 days in vitro. Furthermore, Parkinson's disease model of rats receiving encapsulated PC12TH Tet-Off cells displayed a significant decrease of dopamine concentration in the cerebrospinal fluid (CSF) and increase of the number of apomorphine-induced rotations by Dox-administration, as compared to transplanted rats without Dox-administration, although the significant decrease of the reduction ratio of DA concentration in the CSF with Dox-administration was recognized over time. At 2 months post-implantation, concentration of dopamine in the implanted striatum and from the retrieved capsules demonstrated that the control of DA-secretion could be partially achieved for 2 months in vivo. Our results support both the value of cell therapy using Tet-Off system and the technique of encapsulation might be a feasible option for Parkinson's disease especially in resolving the problem of dopamine oversupply in the future, although a more efficient way to control DA-secretion with quicker regulation and much titration of dose should be explored before clinical application.

Gene therapy for Parkinson's disease using recombinant adeno-associated viral vectors

Existing strategies for gene therapy in the treatment of Parkinson's disease include the delivery of genes encoding dopamine (DA)-synthesising enzymes, leading to localised production of DA in the striatum; genes encoding factors that protect nigral neurons against ongoing degeneration, such as glial cell line-derived neurotrophic factor; and genes encoding proteins that produce the inhibitory transmitter gamma-aminobutylic acid (GABA) in the subthalamic nucleus (STN), thus suppressing the hyperactive STN. Recombinant adeno-associated viral (rAAV) vectors, which are derived from non-pathogenic viruses, have been shown to be suitable for clinical trials. These rAAVs have been found to transduce substantial numbers of neurons efficiently and to express transgenes in mammalian brains for long periods of time, with minimum inflammatory and immunological responses. In vivo imaging using positron emission tomography is useful for monitoring transgene expression and for assessing the functional effects of gene delivery. Vector systems that regulate transgene expression are necessary to increase safety in clinical applications, and the development of such systems is in progress.

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

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

Recombinant adeno-associated viral vectors as therapeutic agents to treat neurological disorders

Recombinant adeno-associated virus (rAAV) is derived from a small human parvovirus with an excellent safety profile. In addition, this viral vector efficiently transduces and supports long-term transgene expression in the nervous system. These properties make rAAV a reasonable candidate vector for treating neurological disorders. Indeed, rAAV is currently being used in five early stage clinical trials for various neurodegenerative disorders. Therefore, we will review the currently available preclinical data using rAAV in animal models of central nervous system (CNS) disorders. Moreover, potential caveats for rAAV-based gene therapy in the CNS are also presented.

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.

Viral-based modelling and correction of neurodegenerative diseases by RNA interference

Experimental recapitulation of recessive human genetic neurodegenerative disease in rodents can be classically addressed through genetic disruption of the related gene. Although very informative, this specific gene targeting is restricted to mice and precludes a species scale-up towards non-human primates. Concomitantly, this requirement to silence a specific gene in a broad range of animal models is important in the design of therapeutic approaches to dominantly inherited neurodegenerative diseases. The emergence of RNA interference (RNAi), a highly specific mechanism of post-translational gene silencing, has opened a plethora of biological application ranging from reverse genetic analysis to therapeutic schemes. Recombinant viral vectors, by promoting a long-lasting delivery of genetic instructions in a broad range of cellular types of different species origins, represent potential platforms mandating silencing of specific gene products through RNAi. This review aims at providing an overview of the different viral systems engineered so far for efficient in vitro and in vivo delivery of RNAi instructions. Additionally, the potential of RNAi for functional analysis and therapy for polyglutamine disorders or amyotrophic lateral sclerosis is discussed.

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.

Gene therapy in Parkinson?s disease

Gene therapy in Parkinson's disease appears to be at the brink of the clinical study phase. Future gene therapy protocols will be based on a substantial amount of preclinical data regarding the use of ex vivo and in vivo genetic modifications with the help of viral or non-viral vectors. To date, the supplementation of neurotrophic factors and substitution for the dopaminergic deficit have formed the focus of trials to achieve relief in animal models of Parkinson's disease. Newer approaches include attempts to influence detrimental cell signalling pathways and to inhibit overactive basal ganglia structures. Nevertheless, current models of Parkinson's disease do not mirror all aspects of the human disease, and important issues with respect to long-term protein expression, choice of target structures and transgenes and safety remain to be solved. Here, we thoroughly review available animal data of gene transfer in models of Parkinson's disease.

Circulating anti-wild-type adeno-associated virus type 2 (AAV2) antibodies inhibit recombinant AAV2 (rAAV2)-mediated, but not rAAV5-mediated, gene transfer in the brain

Epidemiological studies report that 80% of the population maintains antibodies (Ab) to wild-type (wt) adeno-associated virus type 2 (AAV2), with 30% expressing neutralizing Ab (NAb). The blood-brain barrier (BBB) provides limited immune privilege to brain parenchyma, and the immune response to recombinant AAV (rAAV) administration in the brain of a naive animal is minimal. However, central nervous system transduction in preimmunized animals remains unstudied. Vector administration may disrupt the BBB sufficiently to promote an immune response in a previously immunized animal. We tested the hypothesis that intracerebral rAAV administration and readministration would not be affected by the presence of circulating Ab to wt AAV2. Rats peripherally immunized with live wt AAV2 and naive controls were tested with single intrastriatal injections of rAAV2 encoding human glial cell line-derived neurotrophic factor (GDNF) or green fluorescent protein (GFP). Striatal readministration of rAAV2-GDNF was also tested in preimmunized and naive rats. Finally, serotype specificity of the immunization against wt AAV2 was examined by single injections of rAAV5-GFP. Preimmunization resulted in high levels of circulating NAb and prevented transduction by rAAV2 as assessed by striatal GDNF levels. rAAV2-GFP striatal transduction was also prevented by immunization, while rAAV5-GFP-mediated transduction, as assessed by stereological cell counting, was unaffected. Additionally, inflammatory markers were present in those animals that received repeated administrations of rAAV2, including markers of a cell-mediated immune response and cytotoxic damage. A live virus immunization protocol generated the circulating anti-wt-AAV Ab seen in this experiment, while human titers are commonly acquired via natural infection. Regardless, the data show that the presence of high levels of NAb against wt AAV can reduce rAAV-mediated transduction in the brain and should be accounted for in future experiments utilizing this vector.

Targeting conditional gene modification into the serotonin neurons of the dorsal raphe nucleus by viral delivery of the Cre recombinase

Delivery of viral vectors encoding the Cre recombinase is showing promise to target gene modification in specific brain regions. Here we describe the targeting of the dorsal raphe nucleus (DRN), which contains the majority of the serotonin (5-HT) neurons projecting to the forebrain. First, we demonstrate successful transgene expression in the mouse DRN by stereotaxic delivery of the AdnlslacZ adenoviral vector. Second, we show that expression of the Cre recombinase can be achieved in the 5-HT neurons by optimized injection of the Adcre vector. Using reporter mice in which Cre activity induces beta-galactosidase (beta-gal) expression, we demonstrate efficient Cre-mediated recombination and persistence of beta-gal positive 5-HT neurons at least 1 month postinjection. Together, these results demonstrate that viral delivery provides a valuable method to target Cre recombination throughout the murine DRN and thus to study 5-HT neurotransmission by conditional gene modification.

Minocycline-induced activation of tetracycline-responsive promoter

Minocycline has been suggested to be an anti-apoptotic compound and an anti-inflammatory agent in various models of neurodegeneration. In the present study, using a stable cell line expressing green fluorescent protein under the control of a tetracycline-responsive promoter, we demonstrate that minocycline is able to promote tetracycline-controlled gene expression although it needs longer time and higher concentration to reach the effect obtained with the classical inducer doxycycline. Furthermore, the extinction of the system after antibiotics removal is faster when using minocycline. Interestingly, minocycline displays lower cytotoxicity than doxycycline. It is thus tempting to speculate that combining the intrinsic neuroprotective activity of minocycline with its ability to induce tetracycline-regulatable promoters would be greatly beneficial for neuroprotective/neurorestaurative gene therapy.

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.

Future and current surgical therapies in Parkinson's disease

PURPOSE OF REVIEW

The mainstay of treatment for Parkinson's disease remains medical therapy. With improved surgical precision and decreased morbidity, stereotactic lesioning and deep brain stimulation have become more popular. New therapies currently in clinical trials include gene therapy and direct drug delivery to the brain. The present review discusses surgical therapies for the treatment of Parkinson's disease and the status of experimental strategies currently in preclinical and clinical testing.

RECENT FINDINGS

Both stereotactic ablation and deep brain stimulation of the thalamus, globus pallidus interna, and subthalamic nucleus are discussed and compared in the current literature. New therapies such as drug infusions into the brain, gene therapy, and neural transplantation are in clinical trials and have been tested extensively in animals. Safety and efficacy of such therapies are discussed in recent literature.

SUMMARY

Although medication remains the first and main line of treatment and the mainstay for Parkinson's disease, advances in techniques and safety of operations have made surgical interventions more popular. Thalamic surgery remains helpful only in a limited subset of patients with predominent tremor that is unresponsive to medication. Bilateral subthalamic nucleus DBS holds the most promising results for patients with tremor, severe motor fluctuations and dyskinesias from L-dopa, with the best improvements seen in daily activities and quality of life. Newer therapies currently in clinical trial include gene therapy to replace lost gamma-aminobutyric acid inputs to the subthalamic nucleus and globus pallidus interna/substantia nigra pars reticulata, and infusion of recombinant glial derived neurotrophic factor to support at-risk nigrostriatal neurons. Further developments in these areas, along with evolution in stem cell science that hopefully will permit replacement of lost neurons, may alter the nature of surgical practice in Parkinson's disease patients in the not too distant future.

Correction of a rat model of Parkinson's disease by coexpression of tyrosine hydroxylase and aromatic amino acid decarboxylase from a helper virus-free herpes simplex virus type 1 vector

We previously reported long-term biochemical and behavioral correction of the 6-hydroxydopamine (6-OHDA) rat model of Parkinson's disease (PD) by expression of tyrosine hydroxylase (TH) in the partially denervated striatum, using a herpes simplex virus type 1 (HSV-1) vector. This study had a number of limitations, including the use of a helper virus packaging system, limited long-term expression, and expression of only TH. To address these issues, we developed a helper virus-free packaging system, a modified neurofilament gene promoter that supports long-term expression in forebrain neurons, and a vector that coexpresses TH and aromatic amino acid decarboxylase (AADC). Coexpression of TH and AADC supported high-level (80%), behavioral correction of the 6-OHDA rat model of PD for 5 weeks. Biochemical correction included increases in extracellular dopamine and DOPAC concentrations between 2 and 4 months after gene transfer. Histologic analyses demonstrated neuronal-specific coexpression of TH and AADC at 4 days to 7 months after gene transfer, and cell counts revealed 1000 to 10,000 TH positive cells per rat at 2 months after gene transfer. This improved system efficiently corrects the rat model of PD.

Highly suppressible expression of single-chain interleukin-12 by doxycycline following adenoviral infection with a single-vector Tet-regulatory system

BACKGROUND

Adenoviral vectors have been shown to efficiently transfer DNA into a wide variety of eukaryotic cells in vitro and in vivo. However, the therapeutic benefit of this approach is limited by severe side effects as a result of uncontrolled transgene expression.

METHODS

A bi-directional promoter that controls the desired transgene as well as a tetracycline-suppressible transactivator (tTA) was cloned into the E1-region of E1-deleted recombinant adenoviral vectors. Autoregulation within this construct was obtained by tTA expression under control of the operator, to which tTA binds in the absence of tetracycline. Consequently, binding of tetracycline to tTA results in downregulation of tTA as well as the co-expressed transgene in the infected cell.

RESULTS

We were able to suppress luciferase-reporter gene expression by up to 16 000-fold in the presence of doxycycline (dox, 2 micro g/ml). Under control of this tetracycline-regulated system, single-chain interleukin-12 (scIL12) was expressed. Adenovirally mediated expression of this potentially lethal cytokine with strong activation of antitumoral immune response was downregulated by up to 6000-fold in the presence of dox. Subsequently, this downregulation also resulted in a highly significant reduction of interferon-gamma secretion by stimulated splenocytes. These mainly contribute to the toxicity of this immunotherapeutic approach.

CONCLUSIONS

With expression levels exceeding those of the cytomegalovirus (CMV) promoter in almost all cell lines tested, these new vectors will also contribute to the safety of adenoviral approaches by controlled expression without compromising on maximum expression levels.

Virus-based expression systems facilitate rapid target in vivo functionality validation and high-throughput screening

Target validation is one of rate-limiting steps in the modern drug discovery. The authors developed a strategy of combining adenovirus-mediated gene transfer for efficient target functionality validation, both in vivo and in vitro, with baculovirus expression to produce sufficient quantities of protein for high-throughput screening (HTS). The incorporation of green fluorescent protein (GFP) in the adenovirus vectors accelerates recombinant adenovirus plaque purification, whereas the use of epitope and affinity tags facilitates the identification and purification of recombinant protein. In this generalized scheme, the flexible modular design of viral vectors facilitates the transition between target validation and HTS. In the example presented, functional target validation in vivo was achieved by overexpressing the target gene in cell-based models and in the mouse cortex following adenovirus-mediated gene delivery. In this context, target overexpression resulted in the accumulation of a disease-related biomarker both in vitro and in vivo. A baculovirus-based expressional system was then generated to produce enough target protein for HTS. Thus, the use of these viral expression systems represents a generalized method for rapid target functionality validation and HTS assay development, which could be applied to numerous target candidates being elucidated in gene discovery programs.

Gene therapy for Parkinson's disease: current status and future potential

Parkinson's disease appears to be a good candidate for gene therapy. The primary biochemical defect associated with the disease has been clearly determined as an absence of dopamine in the caudate-putamen, and the anatomical region where the neuropathologic hallmark of the disease, death of the nigral dopamine-producing neurons, occurs, remains circumscribed. Based on the biochemical and anatomical information gathered on Parkinson's disease, different gene therapy strategies have been devised to treat it. The first, and most explored strategy so far, consists in engineering cells to produce levodopa or dopamine so they will replace dopaminergic neurotransmission. Several types of cells have been employed in these experiments, and behavioral recovery has been reported in animal models of the disease. However, this approach cannot prevent neuronal death, nor reconstruct brain circuits. Another strategy is to protect cells by transferring genes that encode neurotrophic factors. Effort is now being concentrated into this research area, and promising results have recently been reported. Finally, an additional strategy aims at generating cells with a dopaminergic phenotype so they will be capable of replacing the missing dopaminergic neurons in biochemical, anatomical and functional terms. This has the potential to become an important constituent for an effective cure. Gene therapy holds significant promise for the treatment of neurodegenerative disorders, and Parkinson's disease treatment will benefit greatly from the knowledge and information arising from gene therapy research.

The degradation of amyloid beta as a therapeutic strategy in Alzheimer's disease and cerebrovascular amyloidoses

The deposition of 4-kDa amyloid beta peptide in the brain is a prominent feature of several human diseases. Such process is heterogeneous in terms of causative factors, biochemical phenotype, localization and clinical manifestations. Amyloid beta accumulates in the neuropil or within the walls of cerebral vessels, and associates with dementia or stroke, both hereditary and sporadic. Amyloid beta is normally released by cells as soluble monomeric-dimeric species yet, under pathological conditions, it self-aggregates as soluble oligomers or insoluble fibrils that may be toxic to neurons and vascular cells. Lowering amyloid beta levels may be achieved by inhibiting its generation from the amyloid beta-precursor protein or by promoting its clearance by transport or degradation. We will summarize recent findings on brain proteases capable of degrading amyloid beta with a special focus on those enzymes for which there is genetic, transgenic or biochemical evidence suggesting that they may participate in the proteolysis of amyloid beta in vivo. We will also put in perspective their possible utilization as therapeutic agents in amyloid beta diseases.

Controlled transgene expression by E1-E4-defective adenovirus vectors harbouring a "tet-on" switch system

BACKGROUND

The "tet switch system" was originally described under the tet-off configuration with its components encoded by two separate plasmids. Since then, many virus vectors harbouring tet-off components have been designed and their regulation by tetracycline is widely reported. On the contrary, tet-on regulation by viral vectors is poorly documented.

METHODS

E1-E4-defective adenoviruses harbouring either rtTA or the luciferase gene under a minimal inducible promoter (TK* or CMV*) or both components in a single genome were produced. Using either a double or a single virus strategy, induction of luciferase expression was investigated in various cell lines, in mice muscle and in rat brain.

RESULTS

Over 400-fold induction can be reached with PC12 and NHA cells using a double virus strategy. Comparison of the background activity of different minimal inducible promoters revealed a significant difference between TK* and CMV* promoters both with the cell culture and the in vivo experiments. Interestingly, a single virus strategy permitted an induction exceeding 600-fold with human astrocyte primary cells. Moreover, the E1-E4-defective adenovirus-mediated tet-on system can be quickly switched off and turned back on again. Depending on the cell line, the level of rtTA derived by the single virus strategy differed, resulting in different efficiencies. Experiments performed in rat striatum and mouse muscle confirmed the importance of rtTA expression and minimal promoter used on both doxycycline-independent expression and induction efficiency. Under appropriated rtTA expression, a 32-fold induction is observed in mouse muscle.

CONCLUSIONS

In the recombinant adenovirus context, the CMV* but not the TK* promoter is sensitive to transcriptional interference resulting in high doxycycline-independent expression. By paying attention to the rtTA expression, moderate and high induction can be obtained in vivo and in vitro accordingly.

Reversal of motor impairments in parkinsonian rats by continuous intrastriatal delivery of L-dopa using rAAV-mediated gene transfer

Intrastriatal delivery of the tyrosine hydroxylase gene by viral vectors is being explored as a tool for local delivery of L-dopa in animals with lesions of the nigrostriatal pathway. The functional effects reported using this approach have been disappointing, probably because the striatal L-dopa levels attained have been too low. In the present study, we have defined a critical threshold level of L-dopa, 1.5 pmol/mg of tissue, that has to be reached to induce any significant functional effects. Using new generation high-titer recombinant adeno-associated virus vectors, we show that levels of striatal L-dopa production exceeding this threshold can be obtained provided that tyrosine hydroxylase is coexpressed with the cofactor synthetic enzyme, GTP-cyclohydrolase-1. After striatal transduction with this combination of vectors, substantial functional improvement in both drug-induced and spontaneous behavior was observed in rats with either complete or partial 6-hydroxydopamine lesions of the nigrostriatal pathway. However, complete reversal of motor deficits occurred only in animals in which part of the striatal dopamine innervation was left intact. Spared nigrostriatal fibers thus may convert L-dopa to dopamine and store and release dopamine in a more physiologically relevant manner in the denervated striatum to mediate better striatal output-dependent motor function. We conclude that intrastriatal L-dopa delivery may be a viable strategy for treatment and control of adverse side effects associated with oral L-dopa therapy such as on-off fluctuations and drug-induced dyskinesias in patients with Parkinson's disease.

Evaluation of the tetracycline- and ecdysone-inducible systems for expression of neurotransmitter receptors in mammalian cells

Establishing a stable cell line that expresses a particular protein of interest is often a laborious and time-consuming experience. With constitutive expression systems, a gradual loss of the highly expressing clones over a given time span and/or a severe counter-selection due to toxicity of the expressed protein for the host cell line are major drawbacks. In both cases, inducible expression systems offer a valuable alternative. Over the years, many regulated expression systems have been developed and evaluated. In the present study, we compare the efficiency, the advantages and the drawbacks of a tetracycline- and an ecdysone-inducible system for expression of the reporter protein chloramphenicol acetyltransferase and of different G-protein-coupled serotonin (5-HT) receptors. A high level of expression of different 5-HT receptors was obtained with the tetracycline-inducible system. In the cell line L929, which stably expresses the tetracycline-responsive transactivator, a maximum ligand binding of 20,000 and 9500 fmol/mg protein was measured for the h5-HT(1B) and h5-ht(1F) receptors, respectively. In the HEK293rtTA cell line, levels of 15,700, 3000, and 9100 fmol bound ligand/mg protein were obtained for the h5-HT(1B), h5-ht(1F) and h5-HT(4b) receptors, respectively. These high expression levels remained stable for several months of continuous culture. Although the ecdysone-inducible expression system was useful for tightly regulated expression, the levels were far lower than those obtained with the tetracycline system (e.g. 640 fmol bound ligand/mg protein for the h5-ht(1F) receptor in HEK293EcR).

Recombinant proteins for neurodegenerative diseases: the delivery issue

Tackling neurodegenerative diseases represents a formidable challenge for our ageing society. Recently, major achievements have been made in understanding the molecular mechanisms responsible for such diseases, and, simultaneously, numerous proteins such as neurotrophic factors, anti-apoptotic or anti-oxidant have been identified as potential therapeutic agents. Although many neurotrophic factors have been tested on individuals suffering from various neurodegenerative disorders, to date none has shown efficacy. Inadequate protein delivery is believed to be part of the problem. Recent improvements in pump technology, as well as in cell and gene therapy, are providing innovative ways to allow localized, regulatable delivery of proteins in brain parenchyma, opening new avenues for clinical trials in the not so distant future.