Overexpression of human alpha-synuclein (alpha-syn) using recombinant adeno-associated viral (rAAV) vectors provides a novel tool to study neurodegenerative processes seen in Parkinson's disease and other synucleinopathies. We used a pseudotyped rAAV2/5 vector to express human wild-type (wt) alpha-syn, A53T mutated alpha-syn, or the green fluorescent protein (GFP) in the primate ventral midbrain. Twenty-four adult common marmosets (Callithrix jacchus) were followed with regular behavioural tests for 1 year after transduction. alpha-Syn overexpression affected motor behaviour such that all animals remained asymptomatic for at least 9 weeks, then motor bias comprising head position bias and full body rotations were seen in wt-alpha-syn expressing animals between 15 and 27 weeks; in the later phase, the animals overexpressing the A53T alpha -syn, in particular, showed a gradual worsening of motor performance, with increased motor coordination errors. Histological analysis from animals overexpressing either the wt or A53T alpha -syn showed prominent degeneration of dopaminergic fibres in the striatum. In the ventral midbrain, however, the dopaminergic neurodegeneration was more prominent in the A53T group than in the WT group suggesting differential toxicity of these two proteins in the primate brain. The surviving cell bodies and their processes in the substantia nigra were stained by antibodies to the pathological form of alpha-syn that is phosphorylated at Ser position 129. Moreover, we found, for the first time, ubiquitin containing aggregates after overexpression of alpha-syn in the primate midbrain. There was also a variable loss of oligodendroglial cells in the cerebral peduncle. These histological and behavioural data suggest that this model provides unique opportunities to study progressive neurodegeneration in the dopaminergic system and deposition of alpha-syn and ubiquitin similar to that seen in Parkinson's disease, and to test novel therapeutic targets for neuroprotective strategies.

The therapeutic potential of glial cell line-derived neurotrophic factor (GDNF) for Parkinson's disease is likely to depend on sustained delivery of the appropriate amount to the target areas. Recombinant adeno-associated viral vectors (rAAVs) expressing GDNF may be a suitable delivery system for this purpose. The aim of this study was to define a sustained level of GDNF that does not affect the function of the normal dopamine (DA) neurons but does provide anatomical and behavioral protection against an intrastriatal 6-hydroxydopamine (6-OHDA) lesion in the common marmoset. We found that unilateral intrastriatal injection of rAAV resulting in the expression of high levels of GDNF (14 ng/mg of tissue) in the striatum induced a substantial bilateral increase in tyrosine hydroxylase protein levels and activity as well as in DA turnover. Expression of low levels of GDNF (0.04 ng/mg of tissue), on the other hand, produced only minimal effects on DA synthesis and only on the injected side. In addition, the low level of GDNF provided ϳ85% protection of the nigral DA neurons and their projections to the striatum in the 6-OHDA-lesioned hemisphere. Furthermore, the anatomical protection was accompanied by a complete attenuation of sensorimotor neglect, head position bias, and amphetamine-induced rotation. We conclude that when delivered continuously, a low level of GDNF in the striatum (approximately threefold above baseline) is sufficient to provide optimal functional outcome.

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Animal studies investigating the efficacy of neurotrophic factors as treatments for Parkinson's disease (PD) ideally require partial dopamine (DA) lesion models. The intrastriatal 6-hydroxydopamine (6-OHDA) lesion model may be suitable for this purpose. Although this model has been well characterized in rodents, it has not previously been used in monkeys. The goal of the present study was to characterize the behavioral effects of unilateral injections of 6-OHDA in the basal ganglia of common marmoset monkeys (Callithrix jacchus). Cell counts from tyrosine hydroxylase immunochemistry 5 months postlesion revealed DA cell loss in the substantia nigra on the lesioned side to approximately 46% of relative to the unlesioned side. 6-OHDA lesioned monkeys showed a variety of behavioral deficits. Apomorphine induced rotation and simple sensorimotor measures (head position bias and PD disability rating score) were most affected by the lesion. The largest deficits were seen at 1 or 2 weeks postsurgery but had recovered by week 10. 6-OHDA lesioned monkeys took longer to complete a more complex sensorimotor staircase task. At 3.5 months postlesion, 6-OHDA monkeys also showed deficits on an object retrieval task designed to measure sensorimotor planning and skilled hand use. alpha-Methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, reinstated those deficits which had undergone recovery in the lesioned animals and also exacerbated the deficits on the staircase task. This model has potential in assessing treatments for PD aimed at curtailing disease progression such as continuous delivery of neurotrophic factors.

Potential future treatments for Parkinson's disease (PD) include those that not only provide symptomatic relief to patients but are also neuroprotective and/or neurorestorative. Glial cell line-derived neurotrophic factor (GDNF) may be a valuable candidate in this regard. Positive results using monkeys have encouraged the use of GDNF in human trials. These trials have unfortunately shown mixed results, illustrating the influence that various parameters of administration can have on clinical outcome. The aim of this review is to compare the findings of these clinical studies with available primate data. While bolus intraventricular injections of GDNF in primates have shown some behavioural efficacy, there was no clinical benefit in the first human trial using this method, which was most likely a result of inefficient GDNF distribution in the striatal parenchyma. In primates, however, continuous (rather than bolus) delivery of GDNF into the ventricles results in significant distribution in the striatum. While chronic delivery of GDNF into the ventricles has not been assessed in humans, intraputamenal protein delivery in two Phase I trials have demonstrated that GDNF considerably reduces PD symptoms, suggesting that the putamen is the optimal location for delivery. Primate studies have shown that vector mediated delivery of GDNF may provide a suitable means for long-term intraputamenal delivery. However, the possibility of high levels of GDNF resulting in widespread distribution of GDNF to non-targeted areas is a cause of concern, and vectors with transgene regulation are needed. The development of these vectors may be the way forward for GDNF treatment.

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