Parkinson-Like Neurodegeneration Induced by Targeted Overexpression of α-Synuclein in the Nigrostriatal System

Abstract

Recombinant adeno-associated viral vectors display efficient tropism for transduction of the dopamine neurons of the substantia nigra. Taking advantage of this unique property of recombinant adeno-associated viral vectors, we expressed wild-type and A53T mutated human alpha-synuclein in the nigrostriatal dopamine neurons of adult rats for up to 6 months. Cellular and axonal pathology, including alpha-synuclein-positive cytoplasmic inclusions and swollen, dystrophic neurites similar to those seen in brains from patients with Parkinson's disease, developed progressively over time. These pathological alterations occurred preferentially in the nigral dopamine neurons and were not observed in other nondopaminergic neurons transduced by the same vectors. The degenerative changes were accompanied by a loss of 30-80% of the nigral dopamine neurons, a 40-50% reduction of striatal dopamine, and tyrosine hydroxylase levels that was fully developed by 8 weeks. Significant motor impairment developed in those animals in which dopamine neuron cell loss exceeded a critical threshold of 50-60%. At 6 months, signs of cell body and axonal pathology had subsided, suggesting that the surviving neurons had recovered from the initial insult, despite the fact that alpha-synuclein expression was maintained at a high level. These results show that nigral dopamine neurons are selectively vulnerable to high levels of either wild-type or mutant alpha-synuclein, pointing to a key role for alpha-synuclein in the pathogenesis of Parkinson's disease. Targeted overexpression of alpha-synuclein in the nigrostriatal system may provide a new animal model of Parkinson's disease that reproduces some of the cardinal pathological, neurochemical, and behavioral features of the human disease.

Loss of MicroRNA-7 Regulation Leads to α-Synuclein Accumulation and Dopaminergic Neuronal Loss In Vivo

“…Lentiviral vectors expressing human wild type or mutant forms of α-synuclein have been shown to lead to a progressive loss of nigral dopaminergic neurons by 24-35% in rats over 5 months 37 whereas adenoviruses expressing human α-synuclein have been shown to decrease nigral dopaminergic neurons by up to 80% in as short as 6 weeks, however these results are less consistent. 11,12,[38][39][40][41] In our model, miR-7T-AsRed significantly reduced TH expression by one-third at week 16 and two thirds at week 24. However, staining with VMAT2, another neuronal marker showed that there was only a 30% loss of neurons at 24 weeks suggesting that the nigral neurons had downregulated their activity.…”

“…Downregulation of TH activity has previously been reported after overexpression of human α-synuclein as a protective mechanism of the cell. 38 Interestingly aggregated human α-synuclein has also been shown to reduce TH activity in dopaminergic neurons. 42 As miRs have many targets it is impossible to rule out that miR-7 may be affecting TH expression and dopaminergic neuronal health via another method other than upregulation of α-synuclein.…”

“…43 In addition, previous studies have shown that motor impairments only appear in animals with over 50-60% loss of nigral neurons thus, explaining the lack of motor impairment in our model. 38,44,45 Kim and colleagues found that deleting Dicer in murine ES cell lines led to a near complete loss of production of dopaminergic neurons. Concurrently, mice lacking Dicer in postmitotic dopaminergic neurons developed a 90% loss of midbrain dopaminergic neurons after 8 weeks suggesting that miRs play an important role in neurodegeneration.…”

Tau gene transfer, but not alpha-synuclein, induces both progressive dopamine neuron degeneration and rotational behavior in the rat

“…Viral vector gene transfer offers unique advantages for modeling, particularly for targeting brain regions and ages most associated with specific diseases. While α-synuclein (ASN) vector gene transfer models share features with PD such as loss of SN dopaminergic neurons in our previous study (Klein et al, 2002a) and two others (Kirik et al, 2002;Lo Bianco et al, 2002), motor dysfunction was not readily detected. To determine if the lack of behavioral effect was due to insufficient gene expression, we incorporated an enhancer element (Loeb et al, 1999) to boost ASN expression in this study.…”

Are amyloid diseases caused by protein aggregates that mimic bacterial pore-forming toxins?

“…In vitro studies of a-synuclein aggregation provide possible hints as to the underlying situation : mixtures of human and mouse a-synuclein (as exist in the transgenic mouse brain) accumulate protofibrils, but fibrillize very slowly . Interestingly, virusmediated expression of rat a-synuclein in rat brain results in aggregation, but no neurodegeneration (Kirik et al 2002 ;Lo Bianco et al 2002). Further characterization of the aggregates formed by human and rat a-synuclein could provide important insights into the role of a-synuclein aggregation in the degeneration of dopaminergic neurons.…”