Vesicular monoamine transporter-2 and aromatic L-amino acid decarboxylase enhance dopamine delivery after L-3, 4-dihydroxyphenylalanine administration in Parkinsonian rats

Imbalanced Dopaminergic Transmission Mediated by Serotonergic Neurons in L-DOPA-Induced Dyskinesia

L-DOPA-induced dyskinesias (LIDs) are one of the main motor side effects of L-DOPA therapy in Parkinson's disease. The review will consider the biochemical evidence indicating that the serotonergic neurons are involved in the dopaminergic effects of L-DOPA in the brain. The consequences are an ectopic and aberrant release of dopamine that follows the serotonergic innervation of the brain. After mid- to long-term treatment with L-DOPA, the pattern of L-DOPA-induced dopamine release is modified. In several brain regions, its effect is dramatically reduced while, in the striatum, its effect is quite preserved. LIDs could appear when the dopaminergic effects of L-DOPA fall in brain areas such as the cortex, enhancing the subcortical impact of dopamine and promoting aberrant motor responses. The consideration of the serotonergic system in the core mechanism of action of L-DOPA opens an important reserve of possible strategies to limit LIDs.

Consideration of gene therapy for paediatric neurotransmitter diseases

The paediatric neurotransmitter diseases (PNDs) are a group of inborn errors of metabolism characterized by abnormalities of neurotransmitter synthesis or metabolism. Although some children may react favourably to neurotransmitter augmentation treatment, optimal response is not universal and other modes of treatment should be sought. The genes involved in many of the currently known monoamine PNDs have been utilized in pre-clinical and in phase I clinical trials in Parkinson disease (PD) and the basic principles could be applied to the therapy of PNDs with some modifications regarding the targeting and distribution of vectors. However, issues that go beyond neurotransmitter replacement are important considerations in PD and even more so in PNDs. Understanding the pathophysiology of PNDs including abnormal development resulting from the neurotransmitter deficiency will be critical for rational therapeutic approaches. Better animal models of PNDs are necessary to test gene therapy before clinical trials can be attempted.

Chronic cocaine administration modulates the expression of transcription factors involved in midbrain dopaminergic neuron function

Chronic cocaine use leads to pronounced alterations in neuronal functions in brain circuits associated with reward. In the present study, we examined in the rat midbrain the effects of acute, subchronic (5 days) and chronic cocaine treatments (14 days) on the gene expression of transcription factors involved in the development and maintenance of dopaminergic neurons. We show that chronic, but not acute or subchronic, cocaine administration downregulates Nurr1 and Pitx3 transcripts whereas En1 transcripts are upregulated. Conversely, Lmx1b and En2 transcripts are not affected by the drug treatment, indicating that the modulation of the midbrain transcription factors analyzed is highly selective. Interestingly, modification of the gene expression for these transcription factors persists in midbrain as long as two weeks after the last drug administration, suggesting that it may account for some of the enduring alterations in midbrain dopaminergic circuits associated with chronic cocaine use.

Transplantation of autologous sympathetic neurons as a potential strategy to restore metabolic functions of the damaged nigrostriatal dopamine nerve terminals in Parkinson's disease

Grafting of catecholamine-producing cells can be a possible therapeutic strategy for attenuating motor symptoms in Parkinson's disease (PD). The potential of autologous sympathetic neurons has been investigated as a donor for cell therapy of PD. The clinical trials of autotransplantation of sympathetic ganglion cells in PD have revealed that the grafts increase the duration of L-DOPA (L-dihydroxy phenyl alanine)-induced beneficial effects, and that the graft-mediated effect is detectable during a follow-up period of at least 1 year postgrafting. In an in vitro analysis of the ability of human sympathetic neurons to release catecholamines, although DA was not detectable under basal conditions, DA levels were significantly increased upon exposure to exogenous L-DOPA. Furthermore, animal experiments with xenografting of human sympathetic ganglionic neurons in the DA-denervated striatum of rats demonstrated that a significant increase in striatal DA levels is noted after systemic L-DOPA treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals with sham surgery. The L-DOPA-induced rise of striatal DA levels was significantly attenuated when given reserpine pretreatment. This suggests that DA derived from exogenously administered L-DOPA is subjected to, at least in part, vesicular storage in grafted sympathetic neurons. Histological examinations indeed showed that the grafts express aromatic-L-amino acid decarboxylase and vesicular monoamine transporter-2, both of which are important molecules for the synthesis and the storage of DA, respectively. Taken together, grafted sympathetic neurons can provide a site for both the conversion of exogenous L-DOPA to DA and the storage of the synthesized DA in the DA-denervated striatum. This might be an explanation for a mechanism by which sympathetic neuron autografts can increase the duration of L-DOPA effects in PD patients. This review article summarizes the clinical effect of transplantation of autologous sympathetic neurons in PD and discusses the underlying mechanism for the effect based on experimental evidence previously obtained.

Vesicular monoamine transporter-2 and aromatic L-amino acid decarboxylase gene therapy prevents development of motor complications in parkinsonian rats after chronic intermittent L-3,4-dihydroxyphenylalanine administration

Motor complications after chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy occur partly because of the sensitization to dopaminergic agents resulting from pulsatile dopaminergic stimulation. The loss of presynaptic storage contributes to short duration of action by dopamine. Vesicular monoamine transporter-2 (VMAT-2) controls intraneuronal dopamine storage by packaging dopamine into synaptic vesicles, thereby allowing exocytotic release of dopamine. Using primary fibroblast doubly transduced with VMAT-2 and aromatic L-amino acid decarboxylase (AADC) genes, we previously demonstrated the beneficial effects of such double gene transduction in the production, storage, and gradual release of dopamine in vitro and in vivo. In this study, we further evaluate the effect of achieving sustained level of dopamine within the striata by VMAT-2 gene on behavioral response of parkinsonian rats after chronic intermittent L-DOPA administration. Primary fibroblast (PF) cells were genetically modified with AADC and VMAT-2 genes. We grafted primary fibroblast cells, PF with AADC (PFAADC), or doubly transduced PF with AADC and VMAT-2 (PFVMAA) (n = 6 for each group) into parkinsonian rat striata and administered L-DOPA (25 mg/kg/day) intermittently for 4 weeks. For behavioral study, we employed a model of akinesia using forepaw adjusting steps (FAS) that have been well characterized to reflect the effect of the lesion and the antiparkinsonian effect of dopaminergic drugs and transplants. The duration of FAS response to L-DOPA was sustained for a longer duration in rats grafted with PFVMAA cells than in those grafted with either control cells or cells with AADC alone. In PFVMAA-grafted animals, prolonged duration of FAS responses to L-DOPA was sustained even 6 weeks after discontinuation of 4-week intermittent L-DOPA treatment. These findings suggest that the restoration of dopamine storage capacity could enhance the efficacy of L-DOPA therapy and attenuate the motor fluctuations that result from chronic intermittent L-DOPA administration. The gene therapy expressing AADC and VMAT-2 along with systemic L-DOPA therapy could provide a novel treatment strategy to prevent motor fluctuations.

Biological effects of pramipexole on dopaminergic neuron-associated genes: relevance to neuroprotection

Pramipexole (PRX) is a non-ergot dopamine (DA) D2/D3 receptor agonist. Experimental studies have provided evidence that PRX may exert neuroprotective effects on the nigro-striatal system. Recent studies have demonstrated a slower decline of DAT density in Parkinson's disease patients treated with PRX as measured by SPECT. The aim of this study is to determine whether PRX has direct biological effects on DAergic neuron-associated genes expression, including DAT, VMAT2, and Nurr1. The human neuroblastoma SH-SY5Y cells were treated with PRX for various time periods and harvested to measure the mRNA and protein products of these genes. Treatment with PRX at 10 microM significantly increased DAT mRNA levels by 54-130% in 4-8 h, VMAT2 mRNA levels by 34% in 4 h, and Nurr1 mRNA levels by 31-39% in 2-4 h, which was the earliest induction among these three genes. The protein levels of DAT, VMAT2, and Nurr1 were markedly increased after PRX treatment, among which the increase of Nurr1 protein level was the highest at first 2 h treatment of PRX. Nafadotride, a D3 DA receptor antagonist, blocked the increase of Nurr1 gene expression induced by PRX, while eticlopride, a D2 DA receptor antagonist, didn't show this effect. Our findings that PRX has biological regulatory effects on DAergic neuron-associated genes may explain both the slower decline of imaged DAT and the neuroprotective effect of PRX. Furthermore, our results suggest that the induction of Nurr1 gene expression by PRX may be mediated by D3 DA receptor.

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.

The ability of grafted human sympathetic neurons to synthesize and store dopamine: a potential mechanism for the clinical effect of sympathetic neuron autografts in patients with Parkinson's disease

We have investigated the potential of autologous sympathetic neurons as a donor for cell therapy of Parkinson's disease (PD). Our recent study demonstrated that sympathetic neuron autografts increase the duration of levodopa-induced "on" periods with consequent reduction in the percent time spent in "off" phase. We also found that human sympathetic neurons grown in culture have the ability to convert exogenous levodopa to dopamine and to store the synthesized dopamine. This may explain the clinically observed prolongation in the duration of levodopa effects. To further analyze the mechanism for the graft-mediated effect, the present study investigated the metabolic function of human sympathetic ganglionic neurons xenografted into the dopamine (DA)-denervated striatum of rats by monitoring striatal levels of DA and its primary metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), after systemic administration of levodopa. We also explored whether the graft-mediated effect above may last in four PD patients who had been given the grafts and followed for 12-36 months postgrafting. Clinical evaluations showed that an increase in the duration of levodopa-induced "on" phase is detected during a follow-up period of 12-36 months postgrafting in all the four patients tested. Accordingly, the percent time spent in "off" phase exhibited a 30-40% reduction as compared to the pregrafting values. The animal experiment showed that a significant increase in striatal DA levels is noted after systemic levodopa treatment, and that the DA levels remain high for longer periods of time in the grafted rats than in control animals. When given reserpine pretreatment, the levodopa-induced rise of striatal DA levels was significantly attenuated with concomitant increase in DOPAC levels. Histological examinations demonstrated that the grafts contain some tyrosine hydroxylase (TH)-positive cells. These cells were also found to express aromatic-l-amino acid decarboxylase (AADC) and vesicular monoamine transporter-2 (VMAT), both of which are important molecules for the synthesis and the storage of DA, respectively. These results indicate that grafted sympathetic neurons can provide a site for both the conversion of exogenous levodopa to DA and the storage of the synthesized DA in the DA-denervated striatum, explaining a mechanism by which sympathetic neuron autografts can increase the duration of levodopa-induced "on" phase in PD patients.

Gene transfer to the nervous system: prospects for novel treatments directed at diseases of the aging nervous system

In the past 3 decades, gene therapy has moved from a theoretical construct to an active field of basic research, animal studies, and clinical trials. In this article, we describe the conceptual basis underlying the use of gene therapy for diseases of the aging nervous system, the principal techniques used for gene delivery, and review preclinical animal studies in 4 different classes of neurologic dysfunction: 1) focal neuronal degeneration in the central nervous system; 2) global neuronal dysfunction in the central nervous system; 3) degenerative disease affecting components of the peripheral nervous system; and 4) intractable focal pain. The full potential of this approach will not be established until the human trials are completed.

The effects of chronic L-DOPA therapy on pharmacodynamic parameters in a rat model of motor response fluctuations

Chronic L-3,4-dihydroxyphenylalanine (L-DOPA) therapy in Parkinson's disease (PD) is complicated by motor response fluctuations and dyskinesia. The relative contributions of disease severity and chronic L-DOPA therapy to the development of motor fluctuation are not well defined clinically. Experimental studies have been limited partly because models for the antiparkinsonian effects on akinesia have not been employed. Therefore, we employed a model of akinesia using forepaw adjusting steps that have been well characterized to reflect the effect of lesions and the antiparkinsonian effect of dopaminergic drugs and transplants. We administered L-DOPA (12.5 mg/kg) intermittently for 4 weeks to rats with severe nigrostriatal lesions produced by injecting 6-hydroxydopamine into the medial forebrain bundle. The peak magnitude responses to L-DOPA increased after treatment compared to the pretreatment baseline. The latency to peak response to L-DOPA became shorter and reversed after the discontinuation of treatment. The duration of response showed minor changes. The pattern of changes in response to apomorphine was similar to that of L-DOPA except that the peak magnitude did not increase despite chronic L-DOPA treatment. The changes in D1 and D2 receptor binding did not correlate with behavioral changes. In summary, long-term intermittent L-DOPA treatment resulted in priming of antiparkinsonian effects on improving akinesia in a rat model of severe PD. These observed changes do not mirror all aspects of motor response fluctuations in advanced PD patients and suggest differential contributions of dopaminergic treatment and lesion severity to motor complication patterns.

Potential of gene therapy for pediatric neurotransmitter diseases: lessons from Parkinson's disease

Gene therapy methods have continued to develop rapidly, and many initial limitations that hampered clinical application have been overcome. Thus serious consideration of clinical application of gene therapy is warranted for selected disorders in which the pathogenesis is well defined. Parkinson's disease has been the most extensively studied target of gene therapy for central nervous system disorders and shares many features with pediatric neurotransmitter diseases. Neurotransmitter replacement therapy using catecholamine-synthesizing genes and delivery of neurotrophic factors such as glial cell line-derived neurotrophic factors has been successful in animal models of Parkinson's disease. Application of gene therapy for pediatric neurotransmitter diseases will require delineating the optimal set of genes to correct the consequences of the deficiencies. The optimal anatomical targets and proper timing of the gene replacement must be understood. Safety of gene therapy vehicles and the ability to regulate gene expression will be essential for eventual clinical application.

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.

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.

[Pharmacology of monoamine neurotransmitter transporters]

Following exocytotic release, the biogenic amine neurotransmitters, norepinephrine, dopamine, and serotonin are removed from the synaptic cleft by the respective transporter, NET, DAT, and SERT, located on the plasma membrane and then re-stored into synaptic vesicles by vesicular monoamine transporter, VMAT. The molecular cloning of these transporters revealed that NET, DAT, and SERT are members of a sodium-dependent neurotransmitter transporter gene family, while VMATs arise from proton-dependent transporter gene family. Structural features common to NET, DAT, and SERT reveal a putative 12 transmembrane-spanning domain structure with cytosolic N- and C-terminal regions. Recent evidence suggest the regulation of the functional expression of these transporters via phosphorylation, which include direct phosphorylation of transporter proteins and/or of associated proteins that may control transporter function/expression. In addition, the substrates and inhibitors for these transporters appear capable of regulating transporter cell surface expression, thereby suggesting both activity-dependent and pharmacological regulatory mechanisms for transporter expression. Analyses of the genes provide new insight into their relation to neuronal diseases since NET, DAT and SERT are the molecular targets for many antidepressants as well as drugs of abuse such as cocaine and amphetamine. The availability of cDNAs of these and vesicular transporters has permitted detailed pharmacological studies in heterologous expression systems, and thus would promise the development of novel drugs with diverse chemical structures.

A site-specific mutation of tyrosine hydroxylase reduces feedback inhibition by dopamine in genetically modified cells grafted in parkinsonian rats

Aromatic L-amino acid decarboxylase (AADC) is necessary for conversion of L-DOPA to dopamine. Therefore, AADC gene therapy has been proposed to enhance pharmacological or gene therapies delivering L-DOPA. However, addition of AADC to the grafts of genetically modified cells expressing tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1), which produce L-DOPA in parkinsonian rats, resulted in decreased production of L-DOPA and dopamine owing to feedback inhibition of TH by dopamine. End-product feedback inhibition has been shown to be mediated by the regulatory domain of TH, and site-specific mutation of serine 40 makes TH less susceptible to dopamine inhibition. Therefore, we investigated the efficacy of using TH with serine 40 mutated to leucine (mTH) in an ex vivo gene-therapy paradigm. Primary fibroblasts (PF) from Fischer 344 rats were transduced with retrovirus to express mTH or wild-type rat TH cDNA (wtTH). Both cell types were also transduced with GCH1 to provide the obligate TH cofactor, tetrahydrobiopterin. PF transfected with AADC were used as coculture and cografting partners. TH activities and L-DOPA production in culture were comparable between PFwtTHGC and PFmTHGC cells. In cocultures with PFAADC cells, PFmTHGC cells showed significant reduction in the inhibitory effect of dopamine compared with PFwtTHGC cells. In vivo microdialysis measurement showed that cografting PFAADC cells with PFmTHGC cells resulted in smaller decreases in L-DOPA and no reduction in dopamine levels compared with cografts of PFAADC cells with PFwtTHGC cells, which decreased both L-DOPA and dopamine levels. Maintenance of dopamine levels with lower levels of L-DOPA would result in more focused local delivery of dopamine and less potential side-effects arising from L-DOPA diffusion into other structures. These data support the hypothesis that mutation of serine 40 attenuates TH end-product inhibition in vivo and illustrates the importance of careful consideration of biochemical pathways and interactions between multiple genes in gene therapy.

Comparison of epidermal keratinocytes and dermal fibroblasts as potential target cells for somatic gene therapy of phenylketonuria

Phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase (PAH) and increased levels of phenylalanine. PAH requires the cofactor BH(4) to function and the rate-limiting step in the synthesis of BH(4) is GTP cyclohydrolase I (GTP-CH). The skin is a potential target tissue for PKU gene therapy. We have previously shown that overexpression of PAH and GTP-CH in primary human keratinocytes leads to high levels of phenylalanine clearance without BH(4) supplementation [Gene Ther. 7 (2000) 1971]. Here, we investigate the capacity of fibroblasts, another cell type from the skin, to metabolize phenylalanine. After retroviral gene transfer of PAH and GTP-CH both normal and PKU patient fibroblasts were able to metabolize phenylalanine, however, in lower amounts compared to genetically modified keratinocytes. Further comparative analyses between keratinocytes and fibroblasts revealed a higher copy number of transgenes in keratinocytes and also a higher metabolic capacity.

Gene therapy for Parkinson's disease

Significant progress has been made in the field of gene therapy for Parkinson's disease (PD). Successful vehicles for gene transfer into the central nervous system have been developed and clinical efficacy and safety have both been shown in various animal models of PD. Further optimisation of dosing, timing and location of gene therapy delivery as well as the ability to regulate and prolong gene expression will be important for the commencement of human trials. Current gene therapy models for PD have focused on two treatment strategies. One is the replacement of biosynthetic enzymes for dopamine synthesis and the second strategy is the addition of neurotrophic factors for protection and restoration of dopaminergic neurones. Concepts of neuroprotection and restoration of the nigrostriatal pathway will become important themes for future genetic treatment strategies for PD and may include, in addition to neurotrophic factors, genes to prevent apoptosis or detoxify free radical species. This review will highlight the recent literature on gene therapy for PD and summarise general approaches to gene therapy.

Functional effect of adeno-associated virus mediated gene transfer of aromatic L-amino acid decarboxylase into the striatum of 6-OHDA-lesioned rats

In animal models of Parkinson's disease, gene transfer of aromatic L-amino acid decarboxylase (AADC) leads to an increase in the capacity of the striatum to decarboxylate exogenous L-DOPA. However, the functional effects of enhanced L-DOPA to dopamine conversion have not been explored. Here, we show that following adeno-associated virus (AAV)-AADC transduction, the transgenic AADC is able to decarboxylate exogenous L-DOPA more efficiently so that a dose of L-DOPA ineffective before gene transfer elicits a motor asymmetry (rotational behavior) following gene transfer. Furthermore, rotation scores showed a strong correlation with AADC activity in the lesioned striatum, thus allowing for behavioral screening of successful gene transfer in the brain. In animals receiving AAV2-AADC, dopamine production was restored to 50% of normal levels 12 weeks after the infusion. Microdialysis experiments demonstrated an in vivo enhanced conversion of L-DOPA to dopamine, but no storage capacity as dopamine was released to the extracellular space in a continuous, nonregulated fashion. In addition to the potential clinical benefit of improving decarboxylation efficiency in Parkinson's disease, our approach may be relevant for the treatment of AADC deficiency, a rare, autosomal recessive disorder causing a severe movement disorder and progressive cognitive impairment.

Fibroblasts and dermal gene therapy: a minireview

This review highlights our current understanding of the biology of, survival of, and transgene expression by genetically modified fibroblasts (GMFb) carrying stably integrated transgenes in vivo. Experimental data demonstrate that three elements will enhance expression by and survival of GMFb in vivo: a matrix scaffolding to take the place of the existing dermis, the presence of elements of the extracellular matrix in the construct used to move GMFb to the in vivo setting, and the utilization of immortalized fibroblasts to carry the transgenes. Although moving GMFb to an in vivo setting is an invasive procedure, there are a number of clinical settings where GMFb appear to be the suitable cell for gene therapy.

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

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

Convection-enhanced delivery of AAV vector in parkinsonian monkeys; in vivo detection of gene expression and restoration of dopaminergic function using pro-drug approach

Using an approach that combines gene therapy with aromatic l-amino acid decarboxylase (AADC) gene and a pro-drug (l-dopa), dopamine, the neurotransmitter involved in Parkinson's disease, can be synthesized and regulated. Striatal neurons infected with the AADC gene by an adeno-associated viral vector can convert peripheral l-dopa to dopamine and may therefore provide a buffer for unmetabolized l-dopa. This approach to treating Parkinson's disease may reduce the need for l-dopa/carbidopa, thus providing a better clinical response with fewer side effects. In addition, the imbalance in dopamine production between the nigrostriatal and mesolimbic dopaminergic systems can be corrected by using AADC gene delivery to the striatum. We have also demonstrated that a fundamental obstacle in the gene therapy approach to the central nervous system, i.e., the ability to deliver viral vectors in sufficient quantities to the whole brain, can be overcome by using convection-enhanced delivery. Finally, this study demonstrates that positron emission tomography and the AADC tracer, 6-[(18)F]fluoro-l-m-tyrosine, can be used to monitor gene therapy in vivo. Our therapeutic approach has the potential to restore dopamine production, even late in the disease process, at levels that can be maintained during continued nigrostriatal degeneration.

Catecholamine degradation by monoamine oxidase in locus coeruleus neurons of the rat. An immunohistochemical study

We examined by immunohistochemistry the effects of monoamine oxidase (MAO) inhibition on the content of dopamine (DA) and noradrenaline (NA) in locus coeruleus (LC) neurons of the rat. In normal rats, clusters of DA- and NA-immunopositive neurons were identified in the LC. Rats treated with intraperitoneal injections of pargyline, an MAO inhibitor, showed significantly stronger DA- and NA-staining intensities in LC neurons compared to normal rats. In LC noradrenergic neurons, it is believed that DA is formed in the cytoplasm and then transported into the storage vesicles where it is converted to NA, and the secreted NA is recycled by a reuptake mechanism and transported back into storage vesicles via the cytoplasm. Furthermore, LC neurons of the rat have been shown to contain DA- and NA-degrading MAO activities on the outer membranes of the mitochondria. Therefore, our findings suggest that endogenous MAO degrades not only part of the DA formed in the cytoplasm of LC neurons, but also part of the secreted NA that has been transported back into the cytoplasm.

Gene therapy in the CNS

Gene therapy for neurological disorder is currently an experimental concept. The goals for clinical utilization are the relief of symptoms, slowing of disease progression, and correction of genetic abnormalities. Experimental studies are realizing these goals in the development of gene therapies in animal models. Discoveries of the molecular basis of neurological disease and advances in gene transfer systems have allowed focal and global delivery of therapeutic genes for a wide variety of CNS disorders. Limitations are still apparent, such as stability and regulation of transgene expression, and safety of both vector and expressed transgene. In addition, the brain adds several challenges not seen in peripheral gene therapy paradigms, such as post-mitotic cells, heterogeneity of cell types and circuits, and limited access. Moreover, it is likely that several modes of gene delivery will be necessary for successful gene therapies of the CNS. Collaborative efforts between clinicians and basic researchers will likely yield effective gene therapy in the CNS.