Recombinant adeno-associated viral vector-mediated glial cell line-derived neurotrophic factor gene transfer protects nigral dopamine neurons after onset of progressive degeneration in a rat model of Parkinson's disease

Molecular Signature of Astrocytes for Gene Delivery by the Synthetic Adeno-Associated Viral Vector rAAV9P1

Astrocytes have crucial functions in the central nervous system (CNS) and are major players in many CNS diseases. Research on astrocyte-centered diseases requires efficient and well-characterized gene transfer vectors. Vectors derived from the Adeno-associated virus serotype 9 (AAV9) target astrocytes in the brains of rodents and nonhuman primates. A recombinant (r) synthetic peptide-displaying AAV9 variant, rAAV9P1, that efficiently and selectively transduces cultured human astrocytes, has been described previously. Here, it is shown that rAAV9P1 retains astrocyte-targeting properties upon intravenous injection in mice. Detailed analysis of putative receptors on human astrocytes shows that rAAV9P1 utilizes integrin subunits αv, β8, and either β3 or β5 as well as the AAV receptor AAVR. This receptor pattern is distinct from that of vectors derived from wildtype AAV2 or AAV9. Furthermore, a CRISPR/Cas9 genome-wide knockout screening revealed the involvement of several astrocyte-associated intracellular signaling pathways in the transduction of human astrocytes by rAAV9P1. This study delineates the unique receptor and intracellular pathway signatures utilized by rAAV9P1 for targeting human astrocytes. These results enhance the understanding of the transduction biology of synthetic rAAV vectors for astrocytes and can promote the development of advanced astrocyte-selective gene delivery vehicles for research and clinical applications.

Adeno-Associated Viral Delivery of GDNF Promotes Recovery of Dopaminergic Phenotype following a Unilateral 6-Hydroxydopamine Lesion

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor for dopamine neurons that has been proposed for use in the treatment of Parkinson's disease (PD). Previous studies using viral vectors to deliver GDNF in rodent models of PD have entailed administering the virus either prior to or immediately after neurotoxin-induced lesions, when the nigrostriatal pathway is largely intact, a paradigm that does not accurately reflect the clinical situation encountered with Parkinson's patients. In this study, recombinant adeno-associated virus carrying the gene encoding GDNF (rAAV-GDNF) was administered to animals bearing a maximal lesion in the nigrostriatal system, more closely resembling fully developed PD. Rats were treated with 6-hydroxydopamine into the medial forebrain bundle and assessed by apomorphine-induced rotational behavior for 5 weeks prior to virus administration. Within 4 weeks of a single intrastriatal injection of rAAV-GDNF, unilaterally lesioned animals exhibited significant behavioral recovery, which correlated with increased expression of dopaminergic markers in the substantia nigra, the medial forebrain bundle, and the striatum. Our findings demonstrate that rAAV-GDNF is capable of rescuing adult dopaminergic neurons from near complete phenotypic loss following a neurotoxic lesion, effectively restoring a functional dopaminergic pathway and diminishing motoric deficits. These data provide further support for the therapeutic potential of rAAV-GDNF-based gene therapy in the treatment of PD.

Glial Cell Line-Derived Neurotrophic Factor Gene Delivery in Parkinson's Disease: A Delicate Balance between Neuroprotection, Trophic Effects, and Unwanted Compensatory Mechanisms

Glial cell line-derived neurotrophic factor (GDNF) and Neurturin (NRTN) bind to a receptor complex consisting of a member of the GDNF family receptor (GFR)-α and the Ret tyrosine kinase. Both factors were shown to protect nigro-striatal dopaminergic neurons and reduce motor symptoms when applied terminally in toxin-induced Parkinson's disease (PD) models. However, clinical trials based on intraputaminal GDNF protein administration or recombinant adeno-associated virus (rAAV)-mediated NRTN gene delivery have been disappointing. In this review, several factors that could have limited the clinical benefits are discussed. Retrograde transport of GDNF/NRTN to the dopaminergic neurons soma is thought to be necessary for NRTN/GFR-α/Ret signaling mediating the pro-survival effect. Therefore, the feasibility of treating advanced patients with neurotrophic factors is questioned by recent data showing that: (i) tyrosine hydroxylase-positive putaminal innervation has almost completely disappeared at 5 years post-diagnosis and (ii) in patients enrolled in the rAAV-NRTN trial more than 5 years post-diagnosis, NRTN was almost not transported to the substantia nigra pars compacta. In addition to its anti-apoptotic and neurotrophic properties, GDNF also interferes with dopamine homeostasis via time and dose-dependent effects such as: stimulation of dopamine neuron excitability, inhibition of dopamine transporter activity, tyrosine hydroxylase phosphorylation, and inhibition of tyrosine hydroxylase transcription. Depending on the delivery parameters, the net result of this intricate network of regulations could be either beneficial or deleterious. In conclusion, further unraveling of the mechanism of action of GDNF gene delivery in relevant animal models is still needed to optimize the clinical benefits of this new therapeutic approach. Recent developments in the design of regulated viral vectors will allow to finely adjust the GDNF dose and period of administration. Finally, new clinical studies in less advanced patients are warranted to evaluate the potential of AAV-mediated neurotrophic factors gene delivery in PD. These will be facilitated by the demonstration of the safety of rAAV administration into the human brain.

Gene therapy of the central nervous system: general considerations on viral vectors for gene transfer into the brain

The last decade has nourished strong doubts on the beneficial prospects of gene therapy for curing fatal diseases. However, this climate of reservation is currently being transcended by the publication of several successful clinical protocols, restoring confidence in the appropriateness of therapeutic gene transfer. A strong sign of this present enthusiasm for gene therapy by clinicians and industrials is the market approval of the therapeutic viral vector Glybera, the first commercial product in Europe of this class of drug. This new field of medicine is particularly attractive when considering therapies for a number of neurological disorders, most of which are desperately waiting for a satisfactory treatment. The central nervous system is indeed a very compliant organ where gene transfer can be stable and successful if provided through an appropriate strategy. The purpose of this review is to present the characteristics of the most efficient virus-derived vectors used by researchers and clinicians to genetically modify particular cell types or whole regions of the brain. In addition, we discuss major issues regarding side effects, such as genotoxicity and immune response associated to the use of these vectors.

Electric stimulation and decimeter wave therapy improve the recovery of injured sciatic nerves

Drug treatment, electric stimulation and decimeter wave therapy have been shown to promote the repair and regeneration of the peripheral nerves at the injured site. This study prepared a Mackinnon's model of rat sciatic nerve compression. Electric stimulation was given immediately after neurolysis, and decimeter wave radiation was performed at 1 and 12 weeks post-operation. Histological observation revealed that intraoperative electric stimulation and decimeter wave therapy could improve the local blood circulation of repaired sites, alleviate hypoxia of compressed nerves, and lessen adhesion of compressed nerves, thereby decreasing the formation of new entrapments and enhancing compressed nerve regeneration through an improved microenvironment for regeneration. Immunohistochemical staining results revealed that intraoperative electric stimulation and decimeter wave could promote the expression of S-100 protein. Motor nerve conduction velocity and amplitude, the number and diameter of myelinated nerve fibers, and sciatic functional index were significantly increased in the treated rats. These results verified that intraoperative electric stimulation and decimeter wave therapy contributed to the regeneration and the recovery of the functions in the compressed nerves.

Silencing neurodegenerative disease: bringing RNA interference to the clinic

RNA interference (RNAi) is a recently described conserved biological pathway where non-coding RNAs suppress the expression of specific genes. Research efforts in the RNAi field aim to gain a better understanding of how its underlying machinery is orchestrated, to define the biological role of this conserved pathway, determine how to effectively manipulate RNAi in the laboratory and to integrate all this knowledge to develop novel therapies for human disease. This review summarizes the advances in the design of therapeutic RNAi for neurodegenerative diseases and discusses some of the experimental steps required to bring this therapy to human clinical trials.

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.

Efficient gene delivery and selective transduction of astrocytes in the mammalian brain using viral vectors

Astrocytes are now considered as key players in brain information processing because of their newly discovered roles in synapse formation and plasticity, energy metabolism and blood flow regulation. However, our understanding of astrocyte function is still fragmented compared to other brain cell types. A better appreciation of the biology of astrocytes requires the development of tools to generate animal models in which astrocyte-specific proteins and pathways can be manipulated. In addition, it is becoming increasingly evident that astrocytes are also important players in many neurological disorders. Targeted modulation of protein expression in astrocytes would be critical for the development of new therapeutic strategies. Gene transfer is valuable to target a subpopulation of cells and explore their function in experimental models. In particular, viral-mediated gene transfer provides a rapid, highly flexible and cost-effective, in vivo paradigm to study the impact of genes of interest during central nervous system development or in adult animals. We will review the different strategies that led to the recent development of efficient viral vectors that can be successfully used to selectively transduce astrocytes in the mammalian brain.

Glial cell line-derived neurotrophic factor partially ameliorates motor symptoms without slowing neurodegeneration in mice with respiratory chain-deficient dopamine neurons

Degeneration of midbrain dopamine neurons causes the striatal dopamine deficiency responsible for the hallmark motor symptoms of Parkinson's disease (PD). Intraparenchymal delivery of neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF), is a possible future therapeutic approach. In animal PD models, GDNF can both ameliorate neurodegeneration and promote recovery of the dopamine system following a toxic insult. However, clinical studies have generated mixed results, and GDNF has not been efficacious in genetic animal models based on α-synuclein overexpression. We have tested the response to GDNF in a genetic mouse PD model with progressive degeneration of dopamine neurons caused by mitochondrial impairment. We find that GDNF, delivered to the striatum by either an adeno-associated virus or via miniosmotic pumps, partially alleviates the progressive motor symptoms without modifying the rate of neurodegeneration. These behavioral changes are accompanied by increased levels of dopamine in the midbrain, but not in striatum. At high levels, GDNF may instead reduce striatal dopamine levels. These results demonstrate the therapeutic potential of GDNF in a progressively impaired dopamine system.

Construction and functional activity of a recombinant vector expressing rat glutamic acid decarboxylase 65

OBJECTIVE

Glutamic acid decarboxylase 2 (GAD65) is a gamma-aminobutyric acid (GABA) synthetase. This study aimed to construct a recombinant lentivirus-rGAD65 (rLV-rGAD65) vector containing the cDNA of rat GAD65 (rGAD65) and assess its functional activity in vitro and in vivo.

METHODS

cDNA of rGAD65 was amplified by RT-PCR and subcloned into the LV vector, forming the rLV-GFP-rGAD65 plasmid. The recombinant lentivirus particles (rLV-rGAD65) were packaged by the LV Helper-Free System and the titer was measured. Primary rat lung fibroblasts were transfected with rLV-rGAD65. The expression of rGAD65 in fibroblasts was detected by immunocytochemistry and western blot and the level of GABA in the medium was assessed by high-performance liquid chromatograph (HPLC). In vivo, rLV-rGAD65 was injected into the subthalamic nucleus (STN) of Sprague-Dawley rats using stereotaxic methods, and rGAD65 protein levels in the STN were assessed by immunohistochemistry and Western blot, while the GABA concentration in the substantia nigra pars reticulata (SNr) was assayed by HPLC.

RESULTS

The sequence of rGAD65 cDNA was in accord with that in GenBank. The amino-acid sequence of rGAD65 had no mutations and the titer of rLV-rGAD65 reached 6.8 × 10⁸/mL. The efficiency of infection of fibroblasts was 80%, and the concentration of GABA in the medium was (48.14 +/- 9.35) nmol/L. In vivo, rGAD65 expression was detected in the STN, and the concentration of GABA in the SNr increased from (5.95 +/- 1.09) to (12.44 +/- 3.79) nmol/g tissue.

CONCLUSION

The recombinant LV-GFP-rGAD65 vector was successfully constructed. rLV-rGAD65-infected primary fibroblasts in vitro and the expressed rGAD65 catalyzed the formation of GABA from glutamic acid. In vivo, the concentration of GABA in the SNr was increased after rLV-rGAD65 injection into the STN.

Response of aged parkinsonian monkeys to in vivo gene transfer of GDNF

This study assessed the potential for functional and anatomical recovery of the diseased aged primate nigrostriatal system, in response to trophic factor gene transfer. Aged rhesus monkeys received a single intracarotid infusion of MPTP, followed one week later by MRI-guided stereotaxic intrastriatal and intranigral injections of lentiviral vectors encoding for glial derived neurotrophic factor (lenti-GDNF) or beta-galactosidase (lenti-LacZ). Functional analysis revealed that the lenti-GDNF, but not lenti-LacZ treated monkeys displayed behavioral improvements that were associated with increased fluorodopa uptake in the striatum ipsilateral to lenti-GDNF treatment. GDNF ELISA of striatal brain samples confirmed increased GDNF expression in lenti-GDNF treated aged animals that correlated with functional improvements and preserved nigrostriatal dopaminergic markers. Our results indicate that the aged primate brain challenged by MPTP administration has the potential to respond to trophic factor delivery and that the degree of neuroprotection depends on GDNF levels.

Long-term transgene expression in the central nervous system using DNA nanoparticles

This study demonstrates proof of concept for delivery and expression of compacted plasmid DNA in the central nervous system. Plasmid DNA was compacted with polyethylene glycol substituted lysine 30-mer peptides, forming rod-like nanoparticles with diameters between 8 and 11 nm. Here we show that an intracerebral injection of compacted DNA can transfect both neurons and glia, and can produce transgene expression in the striatum for up to 8 weeks, which was at least 100-fold greater than intracerebral injections of naked DNA plasmids. Bioluminescent imaging (BLI) of injected animals at the 11th postinjection week revealed significantly higher transgene activity in animals receiving compacted DNA plasmids when compared to animals receiving naked DNA. There was minimal evidence of brain inflammation. Intrastriatal injections of a compacted plasmid encoding for glial cell line-derived neurotrophic factor (pGDNF) resulted in a significant overexpression of GDNF protein in the striatum 1-3 weeks after injection.

Functional models of Parkinson's disease: a valuable tool in the development of novel therapies

Functional models of Parkinson's disease (PD) have led to effective treatment for the motor symptoms. Toxin-based models, such as the 6-hydroxydopamine-lesioned rat and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primate, have resulted in novel dopaminergic therapies and new therapeutic strategies. They have also been used to study processes underlying motor complications, particularly dyskinesia, and for developing pharmacological approaches to dyskinesia avoidance and suppression. Symptomatic models of PD based on nigrostriatal degeneration have a high degree of predictability of clinical effect of dopaminergic drugs on motor symptoms in humans. However, the effects of nondopaminergic drugs in these models do not translate effectively into clinical efficacy. Newer experimental models of PD have attempted to reproduce the pathogenic process and to involve all areas of the brain pathologically affected in humans. In addition, models showing progressive neuronal death have been sought but so far unsuccessfully. Pathogenic modeling has been attempted using a range of toxins, as well as through the use of transgenic models of gene defects in familial PD and mutant rodent strains. However, there are still no accepted progressive models of PD that mimic the processes known to occur during cell death and that result in the motor deficits, pathology, biochemistry, and drug responsiveness as seen in humans. Nevertheless, functional models of PD have led to many advances in treating the motor symptoms of the disorder, and we have been fortunate to have them available. They are an important reason the treatment of PD is so much better compared with treatments for related illnesses.

Adeno-associated virus-mediated gene delivery approaches for the treatment of CNS disorders

Over the last few years, a large number of preclinical and clinical studies have demonstrated the potential of gene therapy applications using adeno-associated viral (AAV) vectors. Gene transfer via AAV vectors has been particularly successful for the treatment or adjunct therapy of several CNS disorders. The present review summarizes the progress on AAV gene delivery models for three different CNS disorders. In particular, we discuss advances in AAV-mediated gene transfer strategies in animal models of Parkinson's disease, Alzheimer's disease and spinal cord trauma and summarize the results from the first clinical studies using AAV systems.

Emerging restorative treatments for Parkinson's disease

Several exciting approaches for restorative therapy in Parkinson's disease have emerged over the past two decades. This review initially describes experimental and clinical data regarding growth factor administration. We focus on glial cell line-derived neurotrophic factor (GDNF), particularly its role in neuroprotection and in regeneration in Parkinson's disease. Thereafter, we discuss the challenges currently facing cell transplantation in Parkinson's disease and briefly consider the possibility to continue testing intrastriatal transplantation of fetal dopaminergic progenitors clinically. We also give a more detailed overview of the developmental biology of dopaminergic neurons and the potential of certain stem cells, i.e. neural and embryonic stem cells, to differentiate into dopaminergic neurons. Finally, we discuss adult neurogenesis as a potential tool for restoring lost dopamine neurons in patients suffering from Parkinson's disease.

Preclinical development of gene therapy for Parkinson's disease

Multiple targets and pathways may be amenable to the development of gene therapy approaches for Parkinson's disease. This article discusses some of the cellular and brain circuit pathways relevant to Parkinson's disease that would be clinically amenable to gene therapy. Approaches could be classified according to two main categories, i.e. symptomatic vs. neuroprotective/neurorestorative strategies. Examples of the different possibilities currently in development are given and feature both dopaminergic and non-dopaminergic symptomatic treatments of parkinsonian symptoms and/or L-DOPA-induced side effects, anti-apoptotic neuroprotective strategies and growth-factor delivery for neuroprotection/neurorestoration. While gene therapy has been mostly used so far for enhancing the expression of the target gene, the use of dominant negative or siRNA opens new possibilities. This, combined with the key feature of gene delivery that offers access to intracellular signalling pathways, is likely to further expand the number of proposed targets to be studied.

Neural stem cell transplantation and melatonin treatment in a 6-hydroxydopamine model of Parkinson's disease

Melatonin has multiple roles including neuroprotection. Melatonin signaling involves diverse targets including two G-protein-coupled receptors, MT(1) and MT(2), which have both been localized to the nigrostriatal pathway. Previous studies in our laboratory demonstrated preservation of tyrosine hydroxylase immunoreactivity, following chronic treatment with a physiological dose of melatonin, in the 6-hydroxydopamine rat model of Parkinson's disease. Additionally, we reported the presence of the melatonin MT(1) receptor subtype in cultured C17.2 neural stem cells (NSCs). In the present study, we examined the effects of C17.2 NSC transplantation on dopaminergic denervation following 6-hydroxydopamine lesioning in the rat striatum. Moreover, based on our detection of the MT(1) in these cells, we examined the effects of combined C17.2 NSC transplantation and melatonin treatment, following striatal lesioning. Behavioral studies indicated a marked inhibition of apomorphine-induced rotations in lesioned animals that received C17.2 NSC transplantation, melatonin, or the combined regimen. In addition, these treatments resulted in a significant protection of tyrosine hydroxylase immunoreactivity in the striatum and substantia nigra of lesioned animals, when compared with untreated controls. Lesioned animals treated with C17.2 NSCs, melatonin or a combination of both agents exhibited no significant differences in the number of tyrosine hydroxylase-positive cells in the substantia nigra or ventral tegmental area ipsilateral or contralateral to the lesioned striatum. These findings suggest that stem cell therapy and concomitant use of neuroprotective agents such as melatonin could be a viable approach in Parkinson's disease.

AAV2-mediated delivery of human neurturin to the rat nigrostriatal system: Long-term efficacy and tolerability of CERE-120 for Parkinson’s disease

Neurturin (NTN) is a neurotrophic factor with known potential to protect and restore the function of dopaminergic substantia nigra neurons whose degeneration has been most closely linked to the major motor deficits in Parkinson's disease (PD). CERE-120, an adeno-associated virus serotype 2 (AAV2)-based gene delivery vector encoding human NTN, is being developed as a potential therapeutic for PD. In a series of preclinical studies reported herein, CERE-120 delivery to the striatum produced a dose-related neuroprotection of nigrostriatal neurons in the rat 6-hydroxydopamine (6-OHDA) lesion model. Long-lasting efficacy of CERE-120 was evidenced by substantia nigra cell protection, preserved fiber innervation of the striatum, and behavioral recovery for at least 6 months. In addition, striatal infusion of CERE-120 was found to have a safety and tolerability profile devoid of side effects or toxicological responses, for at least 12 months post-treatment, even at dose multiples 125 times that of the lowest efficacious dose tested. These results support the ongoing CERE-120 clinical program in PD patients.

Time course of transgene expression after intrastriatal pseudotyped rAAV2/1, rAAV2/2, rAAV2/5, and rAAV2/8 transduction in the rat

In vivo recombinant adeno-associated viral vector (rAAV)-mediated transduction of various tissues including brain has been characterized by slow onset and gradual increase in gene expression before reaching stable long-term protein levels. The early time course of transgene expression has not been quantified using newly available rAAV capsid serotypes. In this experiment, the onset of expression of green fluorescent protein (GFP) after intrastriatal injection of rAAV2-based pseudotyped vectors (rAAV1, rAAV5, and rAAV8 capsids) was quantified. Native GFP fluorescence displayed a delayed onset of expression of at least 7 days for all the pseudotyped rAAV vectors. However, GFP immunohistochemical staining revealed significant transgene expression by 4 days after transduction for all serotypes and stable GFP(+) neuronal populations mediated by all serotypes within 14 days post transduction at the latest. rAAV2/1 and rAAV2/2 displayed no time-dependent increase of GFP(+) striatal neurons; reaching maximal striatal cell GFP(+) counts at 4 days after injection. All serotypes displayed peak transgene expression by 4 weeks post injection where native GFP(+) neurons were equal to immunostained striatal GFP(+) neurons. The inflammatory response to these rAAV vectors was present up to 4 weeks after transduction but was not apparent 9 months post injection. Thus, rAAV-mediated transgene expression begins earlier than previously thought.

Prospects for gene therapy in inherited neurodegenerative diseases

PURPOSE OF REVIEW

The use of gene therapy to correct or replace deficient genes has been a long-standing aspiration.

RECENT FINDINGS

Recent findings from basic and applied research suggest that at last it may be possible to translate experimental procedures into effective patient therapies for genetic diseases. Therapies for neurodegenerative diseases potentially include, as their targets, both monogenic conditions (e.g. lysosomal storage disorders) and more genetically complex diseases (such as Alzheimer's and Parkinson's disorders).

SUMMARY

The use of gene therapy to target the central nervous system presents specific technical and biological challenges. These may be overcome by using novel gene vector delivery strategies. Current research should illuminate the temporal window required to achieve a successful therapy. As greater knowledge is accumulated about gene therapy, correlations will be made between the level of gene expression from the therapeutic vector, the extent of correction after treatment, and the stage of disease progression when therapy is initiated.

Neurotrophic factors in neurodegeneration

Neurotrophic factors (NTFs) have the unique potential to support neuronal survival and to augment neuronal function in the injured and diseased nervous system. Numerous studies conducted over the last 20 years have provided evidence for the potent therapeutic potential of NTFs in animal models of neurodegenerative diseases. However, major obstacles for the therapeutic use of NTFs are the inability to deliver proteins across the blood-brain-barrier, and dose-limiting adverse effects resulting from the broad exposure of nontargeted structures to NTFs. Two recent developments have allowed NTFs' promise to be truly tested for the first time: first, recent improvements in viral vectors that allow the targeted delivery of NTFs while providing a long-lasting supply and sufficient therapeutic doses of NTFs; and second, improved animal models developed in recent years. In this review, we will discuss some of the potential therapeutic applications of NTFs in neurodegenerative diseases and the potential contribution of disturbed neurotrophic factor signaling to neurodegenerative diseases.

Controlled delivery of glial cell line-derived neurotrophic factor by a single tetracycline-inducible AAV vector

An autoregulated tetracycline-inducible recombinant adeno-associated viral vector (rAAV-pTet(bidi)ON) utilizing the rtTAM2 reverse tetracycline transactivator (rAAV-rtTAM2) was used to conditionally express the human GDNF cDNA. Doxycycline, a tetracycline analog, induced a time- and dose-dependent release of GDNF in vitro in human glioma cells infected with rAAV-rtTAM2 serotype 2 virus. Introducing the Woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) downstream to the rtTAM2 coding sequence, resulted in a more rapid induction and a higher basal expression level. In vivo, 8 weeks after a single injection of the rAAV-rtTAM2-GDNF vector encapsidated into AAV serotype 1 capsids in the rat striatum, the GDNF protein level was 60 pg/mg tissue in doxycycline-treated animals whereas in untreated animals, it was undistinguishable from the endogenous level ( approximately 4 pg/mg tissue). However, a residual GDNF expression in the uninduced animals was evidenced by a sensitive immunohistochemical staining. As compared to rAAV1-rtTAM2-GDNF, the rAAV1-rtTAM2-WPRE-GDNF vector expressed a similar concentration of GDNF in the induced state (with doxycycline) but a basal level (without doxycycline) approximately 2.5-fold higher than the endogenous striatal level. As a proof for biological activity, for both vectors, downregulation of tyrosine hydroxylase was evidenced in dopaminergic terminals of doxycycline-treated but not untreated animals. In conclusion, the rAAV1-rtTAM2 vector which expressed biologically relevant doses of GDNF in the striatum in response to doxycycline with a basal level undistinguishable from the endogenous striatal level, as measured by quantitative ELISA assay, constitutes an interesting tool for local conditional transgenesis.

Treatment of human disease by adeno-associated viral gene transfer

During the past decade, in vivo administration of viral gene transfer vectors for treatment of numerous human diseases has been brought from bench to bedside in the form of clinical trials, mostly aimed at establishing the safety of the protocol. In preclinical studies in animal models of human disease, adeno-associated viral (AAV) vectors have emerged as a favored gene transfer system for this approach. These vectors are derived from a replication-deficient, non-pathogenic parvovirus with a single-stranded DNA genome. Efficient gene transfer to numerous target cells and tissues has been described. AAV is particularly efficient in transduction of non-dividing cells, and the vector genome persists predominantly in episomal forms. Substantial correction, and in some instances complete cure, of genetic disease has been obtained in animal models of hemophilia, lysosomal storage disorders, retinal diseases, disorders of the central nervous system, and other diseases. Therapeutic expression often lasted for months to years. Treatments of genetic disorders, cancer, and other acquired diseases are summarized in this review. Vector development, results in animals, early clinical experience, as well as potential hurdles and challenges are discussed.

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.

Therapeutic potential of neurotrophic factors in neurodegenerative diseases

There is a vast amount of evidence indicating that neurotrophic factors play a major role in the development, maintenance, and survival of neurons and neuron-supporting cells such as glia and oligodendrocytes. In addition, it is well known that alterations in levels of neurotrophic factors or their receptors can lead to neuronal death and contribute to the pathogenesis of neurodegenerative diseases such as Parkinson disease, Alzheimer disease, Huntington disease, amyotrophic lateral sclerosis, and also aging. Although various treatments alleviate the symptoms of neurodegenerative diseases, none of them prevent or halt the neurodegenerative process. The high potency of neurotrophic factors, as shown by many experimental studies, makes them a rational candidate co-therapeutic agent in neurodegenerative disease. However, in practice, their clinical use is limited because of difficulties in protein delivery and pharmacokinetics in the central nervous system. To overcome these disadvantages and to facilitate the development of drugs with improved pharmacotherapeutic profiles, research is underway on neurotrophic factors and their receptors, and the molecular mechanisms by which they work, together with the development of new technologies for their delivery into the brain.

Treatment of Parkinson's disease : what's on the horizon?

Few neurological diseases have received as much attention and investment in research as Parkinson's disease. Although great strides have been made in the development of agents to treat this neurodegenerative disease, none yet address the underlying problem associated with it, the progressive loss of dopaminergic neurons. Current therapeutic strategies for Parkinson's disease focus primarily on reducing the severity of its symptoms using dopaminergic medications. Although providing substantial benefit, these agents are burdened by adverse effects and long-term complications. This review highlights new and emerging therapies for Parkinson's disease, categorised as symptomatic, neuroprotective and neurorestorative, although at times, this distinction is not easily made. Novel symptomatic treatments target nondopaminergic areas in the hope of avoiding the motor complications seen with dopaminergic therapies. Two emerging treatment approaches under investigation are adenosine A(2A) receptor antagonists (such as istradefylline [KW-6002]) and glutamate AMPA receptor antagonists (such as talampanel [LY-300164]). In 2003, the results from two studies using istradefylline in patients with Parkinson's disease were published, with both showing a positive benefit of the study drug when used as adjunctive therapy to levodopa. In non-human primate models of Parkinson's disease, talampanel has been found to have antiparkinsonian effects when administered as high-dose monotherapy and antidyskinetic effects on levodopa-induced dyskinesias. NS-2330, another drug currently undergoing clinical trials, is a triple monoamine reuptake inhibitor that has therapeutic potential in both Parkinson's and Alzheimer's disease. A phase II proof-of-concept study is currently underway in early Parkinson's disease. However, a recently published study in advanced Parkinson's disease showed no therapeutic benefit of NS-2330 in this patient population. Even more exciting are agents that have a neuroprotective or neurorestorative role. These therapies aim to prevent disease progression by targeting the mechanisms involved in the pathogenesis of Parkinson's disease. Several lines of investigation for neuroprotective therapies have been taken, including the antioxidant coenzyme Q10 (ubidecarenone) and anti-apoptotic agents such as CEP-1347. Studies in patients with Parkinson's disease with coenzyme Q10 have suggested that it slows down functional decline. The PRECEPT study is currently in progress to assess the neuroprotective role of CEP-1347 in the early phase of the disease. Gene therapy is another exciting arena and includes both potentially neuroprotective and neurorestorative agents. Novel methods include subthalamic glutamic acid decarboxylase gene therapy and the use of glial cell line-derived neurotrophic factor (GDNF). Eleven of 12 patients have been enrolled in the first FDA-approved phase I subthalamic glutamic acid decarboxylase gene therapy trial for Parkinson's disease, with currently no evidence of adverse events. GDNF delivered intracerebroventricularly was studied in a small population of patients with Parkinson's disease, but unfortunately did not reveal positive results. Other methods of administering GDNF include direct delivery via infusions into the basal ganglia and the use of viral vectors; thus far, these approaches have shown promising results. This is an exciting and rewarding time for research into Parkinson's disease. With so many therapies currently under investigation, the time is ripe for the beginning of a new phase of treatment strategies.

Functional reinnervation from remaining DA terminals induced by GDNF lentivirus in a rat model of early Parkinson's disease

Glial cell-line derived neurotrophic factor (GDNF) is a good candidate agent for restoring functional reinnervation and/or neuroprotection of dopamine (DA) nigrostriatal system and thus for the treatment of Parkinson's disease (PD). Viral delivery is currently the most likely in vivo strategy for delivery of the therapeutic protein into the brain for treatment of neurological diseases. However, one of the important unresolved issues for this strategy is the threshold number of DA nigral neurons and/or of striatal DA terminals necessary for optimal benefit from GDNF therapy. In this study, we examined the intrastriatal neurotrophic effects of long-term GDNF delivery using a lentiviral vector in a new rat model of early PD. Lenti-GDNF was injected into the striatum 4 weeks after partial substantia nigra pars compacta 6-hydroxydopamine-induced lesion. Striatal denervation was evaluated by assessing tyrosine hydroxylase-positive DA fiber density and corroborated by testing motor deficit by means of a staircase test. GDNF treatment restored complete striatal DA innervation in the previously denervated area and this was associated with significant behavioral improvements.

Delayed gene therapy of glial cell line-derived neurotrophic factor is efficacious in a rat model of Parkinson's disease

Gene transfer of glial cell line-derived neurotrophic factor (GDNF) in rodent models of Parkinson's disease (PD) has been shown to protect against neurodegeneration either prior to or immediately after neurotoxin-induced lesions; however, the nigrostriatal pathway was largely intact when gene delivery was completed in these models, which may not accurately reflect the clinical situation encountered with Parkinson's patients. In this study, replication-incompetent adenoviral vectors encoding the rat GDNF gene were administered into the striatum 4 weeks following 6-hydroxydopamine (6-OHDA) injection in the unilateral striatum, more closely resembling fully developed PD. Apomorphine-induced rotational behavior testing was performed every week following 6-OHDA injection. At the 10th week after gene transfer, the striatal dopamine concentrations were measured by HPLC with an electrochemical detector and the number of tyrosine hydroxylase (TH)-positive dopamine neurons in the substantia nigra (SN) was determined by immunohistochemistry. Injection of 6-OHDA into the striatum produced stable increases in rotation, which reached a plateau between 4 and 5 weeks post-injection. The number of TH-positive neuron in the SN and dopamine levels in the striatum was significantly lower in the 6-OHDA group compared to the normal group. Gene transfer of GDNF, but not beta-galactosidase, significantly increased the number of TH-positive neurons and dopamine levels, with a subsequent behavioral recovery between 5 and 10 weeks following GDNF transduction. These findings demonstrate that adenovirus-mediated gene transfer of GDNF is efficacious even in the late stages of 6-OHDA-induced PD rats. They also provide further evidence on the effectiveness of GDNF-based gene therapy for experimental Parkinson's disease.

Recombinant AAV viral vectors pseudotyped with viral capsids from serotypes 1, 2, and 5 display differential efficiency and cell tropism after delivery to different regions of the central nervous system

Recombinant adeno-associated virus 2 (rAAV2) has been shown to deliver genes to neurons effectively in the brain, retina, and spinal cord. The characterization of new AAV serotypes has revealed that they have different patterns of transduction in diverse tissues. We have investigated the tropism and transduction frequency in the central nervous system (CNS) of three different rAAV vector serotypes. The vectors contained AAV2 terminal repeats flanking a green fluorescent protein expression cassette under the control of the synthetic CBA promoter, in AAV1, AAV2, or AAV5 capsids, producing the pseudotypes rAAV2/1, rAAV2/2, and rAAV2/5. Rats were injected with rAAV2/1, rAAV2/2, or rAAV2/5 into selected regions of the CNS, including the hippocampus (HPC), substantia nigra (SN), striatum, globus pallidus, and spinal cord. In all regions injected, the three vectors transduced neurons almost exclusively. All three vectors transduced the SN pars compacta with high efficiency, but rAAV2/1 and rAAV2/5 also transduced the pars reticulata. Moreover, rAAV2/1 showed widespread distribution throughout the entire midbrain. In the HPC, rAAV2/1 and rAAV2/5 targeted the pyramidal cell layers in the CA1-CA3 regions, whereas AAV2/2 primarily transduced the hilar region of the dentate gyrus. In general, rAAV2/1 and rAAV2/5 exhibited higher transduction frequencies than rAAV2/2 in all regions injected, although the differences were marginal in some regions. Retrograde transport of rAAV1 and rAAV5 was also observed in particular CNS areas. These results suggest that vectors based on distinct AAV serotypes can be chosen for specific applications in the nervous system.

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.

Recombinant adeno-associated viral vector (rAAV) delivery of GDNF provides protection against 6-OHDA lesion in the common marmoset monkey (Callithrix jacchus)

Glial cell line-derived neurotrophic factor (GDNF) has shown potential as a treatment for Parkinson's disease. Recombinant adeno-associated viral vectors expressing the GDNF protein (rAAV-GDNF) have been used in rodent models of Parkinson's disease to promote functional regeneration after 6-OHDA lesions of the nigrostriatal system. The goal of the present study was to assess the anatomical and functional efficacy of rAAV-GDNF in the common marmoset monkey (Callithrix jacchus). rAAV-GDNF was injected into the striatum and substantia nigra 4 weeks prior to a unilateral 6-OHDA lesion of the nigrostriatal bundle. Forty percent of the dopamine cells in the lesioned substantia nigra of the rAAV-GDNF-treated monkeys survived, compared with 21% in the untreated monkeys. Fine dopaminergic fibres were observed microscopically in the injected striatum of some rAAV-GDNF-treated monkeys, suggesting that rAAV-GDNF treatment may have prevented, at least in part, the loss of dopaminergic innervation of the striatum. Protection of dopamine cells and striatal fibre innervation was associated with amelioration of the lesion-induced behavioural deficits. rAAV-GDNF-treated monkeys showed partial or complete protection not only in the amphetamine and apomorphine rotation but also in head position and the parkinsonian disability rating scale. Therefore, our study provides evidence for the behavioural and anatomical efficacy of GDNF delivered via an rAAV vector as a possible treatment for Parkinson's disease.

Use of gene therapy in central nervous system repair

Recent advances have increased our molecular understanding of the central nervous system (CNS), in both health and disease. In order to realize the clinical benefits of these findings, new molecular-based therapies need to be developed, such as CNS gene therapy. Although the field has suffered setbacks, it remains an attractive technology for providing new therapies in the post-genomic world. The development of new vectors, and their extensive application in animal models of CNS disease, provides evidence suggesting that gene therapy will eventually become an accepted clinical option. In fact, the first gene therapy clinical trial for Parkinson's disease has recently begun. This review discusses how gene therapy has been applied in animal models, and how it may be used to repair the damage caused by CNS diseases and trauma in human beings. Furthermore, it explores how such treatments may be combined with, and augment, more conventional therapeutic approaches.

Gene therapy: a primer for neurosurgeons

Gene therapy involves the transfer of genes into cells with therapeutic intent. Although several methods can accomplish this, vectors based on viruses still provide the most efficient approach. For neurosurgical purposes, preclinical and clinical applications in the areas of glioma therapy, spinal neurosurgery, and neuroprotection for treatment of Parkinson's disease and cerebral ischemia are reviewed. In general, therapies applied in the neurosurgical realm have proven relatively safe, despite occasional, well-publicized cases of morbidity and death in non-neurosurgical trials. However, continued clinical and preclinical research in this area is critical, to fully elucidate potential toxicities and to generate truly effective treatments that can be applied in neurological diseases.

Adeno-associated virus-mediated aspartoacylase gene transfer to the brain of knockout mouse for canavan disease

Canavan disease (CD) is an autosomal recessive leukodystrophy caused by deficiency of aspartoacylase (ASPA). Deficiency of ASPA leads to elevation of N-acetyl-L-aspartic acid (NAA) in the brain and urine. To explore the feasibility of gene transfer to replace ASPA in CD, we generated a knockout mouse and constructed an AAV vector that encodes human ASPA cDNA (hASPA) followed by green fluorescent protein (GFP) after an intraribosomal entry site. We injected CD mice with rAAV-hASPA-GFP in the striatum and thalamus or injected rAAV-GFP identically into control animals. Three to five months after the injection, we determined the presence of ASPA in the CD mouse brain by ASPA activity assay, GFP expression, and Western blot analysis. While rAAV-GFP-injected animals displayed undetectable levels of ASPA, all detection methods revealed significant ASPA levels in rAAV-hASPA-GFP-injected CD mice. We evaluated the functional effects of rAAV-hASPA-GFP-mediated ASPA expression by standard histological methods, magnetic resonance spectroscopy (MRS) for in vivo NAA levels, and magnetic resonance imaging of CD mice. rAAV-hASPA-injected animals displayed a remarkable lack of spongiform degeneration in the thalamus. However, pathology in sites unrelated to the injected areas showed no improvement in histopathology. The improvement in thalamic neuropathology was also detectable via in vivo MRI. MRS revealed that in vivo NAA levels were also reduced. These data indicate that rAAV-mediated ASPA delivery may be an interesting avenue for the treatment of CD.

In vivo gene delivery of glial cell line--derived neurotrophic factor for Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects approximately 1,000,000 Americans. The cause of the disease remains unknown. The histopathological hallmarks of the disease are dopaminergic striatal insufficiency secondary to a loss of dopaminergic neurons in the substantia nigra pars compacta and intracellular inclusion called Lewy bodies. Currently, only symptomatic treatment for PD is available. Although some treatments are efficacious for many years, all have significant limitations and new therapeutic approaches are needed. Gene therapy is ideal for delivering therapeutic molecules to site-specific regions of the central nervous system. Via gene therapy, a piece or pieces of DNA placed into a carrying vector encoding for a substance of interest can be introduced into specific cells. Although there are several ways that gene therapy can be applied for PD, this review focuses on in vivo gene delivery of glial cell line-derived neurotrophic factor (GDNF) as a neuroprotective strategy for PD.

Nigrostriatal alpha-synucleinopathy induced by viral vector-mediated overexpression of human alpha-synuclein: a new primate model of Parkinson's disease

We used a high-titer recombinant adeno-associated virus (rAAV) vector to express WT or mutant human alpha-synuclein in the substantia nigra of adult marmosets. The alpha-synuclein protein was expressed in 90-95% of all nigral dopamine neurons and distributed by anterograde transport throughout their axonal and dendritic projections. The transduced neurons developed severe neuronal pathology, including alpha-synuclein-positive cytoplasmic inclusions and granular deposits; swollen, dystrophic, and fragmented neuritis; and shrunken and pyknotic, densely alpha-synuclein-positive perikarya. By 16 wk posttransduction, 30-60% of the tyrosine hydroxylase-positive neurons were lost, and the tyrosine hydroxylase-positive innervation of the caudate nucleus and putamen was reduced to a similar extent. The rAAV-alpha-synuclein-treated monkeys developed a type of motor impairment, i.e., head position bias, compatible with this magnitude of nigrostriatal damage. rAAV vector-mediated alpha-synuclein gene transfer provides a transgenic primate model of nigrostriatal alpha-synucleinopathy that is of particular interest because it develops slowly over time, like human Parkinson's disease (PD), and expresses neuropathological features (alpha-synuclein-positive inclusions and dystrophic neurites, in particular) that are similar to those seen in idiopathic PD. This model offers new opportunities for the study of pathogenetic mechanisms and exploration of new therapeutic targets of particular relevance to human PD.

Intramuscular injection of AAV-GDNF results in sustained expression of transgenic GDNF, and its delivery to spinal motoneurons by retrograde transport

Adeno-associated virus (AAV) vector has been developed as an attractive gene delivery system with proven safety. Glial cell line-derived neurotrophic factor (GDNF) is proposed to be a promising therapeutic agent for amyotrophic lateral sclerosis (ALS) and other motor neuron diseases. The purpose of this report was to investigate transgenic GDNF expression at different time points post AAV mediated GDNF intramuscular delivery. An AAV vector was constructed to encode a recombinant fusion of GDNF tagged with a FLAG sequence at the C-terminal (AAV-GDNF) to distinguish it from its endogenous counterpart. A single intramuscular injection of AAV-GDNF led to substantial expression of transgenic GDNF which remained for at least 10 months in transduced gastrocnemius muscle. This transgenic GDNF was distributed in a large number of myofibers, mainly in the vicinity of the sarcolemma and predominantly concentrated at the sites of neuromuscular junctions (NMJs). Furthermore, transgenic GDNF, but not beta-galactosidase expressed as a control, was detected in the motoneurons that projected axons to the injected muscles, thus, indicating retrograde axonal transportation of the transgenic GDNF. This study provides a basis for a strategy of intramuscular AAV-GDNF delivery to protect motoneurons as a possible means of ALS treatment.

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.

Promoters and regulatory elements that improve adeno-associated virus transgene expression in the brain

Since the first demonstration of central nervous system (CNS) transduction with recombinant adeno-associated virus, improvements in vector production and promoter strength have lead to dramatic increases in the number of cells transduced and the level of expression within each cell. The improvements in promoter strength have resulted from a move away from the original cytomegalovirus (CMV) promoter toward the use of hybrid CMV-based promoters and constitutive cellular promoters. This review summarizes and compares different promoters and regulatory elements that have been used with rAAV as a reference toward achieving a high level of rAAV-mediated transgene expression in the CNS.

Gene therapy for treatment of cerebral ischemia using defective recombinant adeno-associated virus vectors

In this review we present our results and experiences in performing gene therapy of cerebral stroke using recombinant adeno-associated virus (rAAV) vectors in a rat model. The methodologies involving the production of AAV vectors, gene transfer to the brain, and a trivessel ligation model of focal ischemic cerebral stroke in rats are described. Furthermore, a brief description of other viral vectors and candidates of therapeutic transgenes used for gene therapy of cerebral stroke are presented. The potential advantages and limitations of stroke gene therapy are also discussed with the intention of outlining the design of more appropriate experiments.

Recombinant adeno-associated virus vector design and gene expression in the mammalian brain

Efficiency and stability of recombinant adeno-associated virus (rAAV)-mediated gene expression within the mammalian brain are determined by several factors. These include the dose of infectious particles, the purity of the vector stock, the serotype of rAAV, the route of administration, and the intrinsic properties, most notably the rAAV receptor density, of the targeted area. Furthermore, the choice of appropriate regulatory elements in rAAV vector design is of fundamental importance to achieve high-level sustained in vivo transcription and translation. This review summarizes the characteristics of various transcriptional and posttranscriptional regulatory elements, and highlights their influence on the expression performance of rAAV vectors in the mammalian brain.

Application of adeno-associated viral vectors in behavioral research

Viral vectors afford the capability of genetically manipulating the expression of neurotransmitters, neuropeptides, hormones, and their receptors in specific brain sites of adult animals of any species. Hence, they are a powerful tool for investigating the neurochemistry underlying complex cognitive processes and behaviors. Here we discuss how the recombinant adeno-associated virus (rAAV) can be engineered for use in neurobehavioral studies, techniques for site-specific delivery of vector into the brain, characterization of expression profiles, and biosafety issues. Finally, we discuss issues of experimental design and interpretation of behavioral results in viral vector studies.

Stem cells in the treatment of Parkinson's disease

Stem cells have been suggested as candidate therapeutic tools for neurodegenerative disorders, given their ability to give rise to the appropriate cell types after grafting in vivo. In this review I summarize some of the evidence currently available concerning two approaches for the treatment of Parkinson's disease: (1) The generation of dopaminergic neurons from embryonic stem cells, multipotent stem cells, and neuronal progenitor cells for cell replacement therapy. (2) The engineering of multipotent stem cells to release glial cell-line derived neurotrophic factor, a potent neurotrophic factor for dopaminergic neurons, in a neuroprotective and neuroregenerative approach to the treatment of Parkinson's disease.