Oxidative stress and α-synuclein aggregation both drive neurodegeneration in Parkinson's disease, and the protein kinase c-Abl provides a potential amplifying link between these pathogenic factors. Suppressing interactions between these factors may thus be a viable therapeutic approach for this disorder. To evaluate this possibility, pre-formed α-synuclein fibrils (PFFs) were used to induce α-synuclein aggregation in neuronal cultures. Exposure to PFFs induced oxidative stress and c-Abl activation in wild-type neurons. By contrast, α-synuclein -deficient neurons, which cannot form α-synuclein aggregates, failed to exhibit either oxidative stress or c-Abl activation. N-acetyl cysteine, a thiol repletion agent that supports neuronal glutathione metabolism, suppressed the PFF -induced redox stress and c-Abl activation in the wild-type neurons, and likewise suppressed α-synuclein aggregation. Parallel findings were observed in mouse brain: PFF-induced α-synuclein aggregation in the substantia nigra was associated with redox stress, c-Abl activation, and dopaminergic neuronal loss, along with microglial activation and motor impairment, all of which were attenuated with oral N-acetyl cysteine. Similar results were obtained using AAV-mediated α-synuclein overexpression as an alternative means of driving α-synuclein aggregation in vivo. These findings show that α-synuclein aggregates induce c-Abl activation by a redox stress mechanism. c-Abl activation in turn promotes α-synuclein aggregation, in a feed-forward interaction. The capacity of N-acetyl cysteine to interrupt this interaction adds mechanistic support its consideration as a therapeutic in Parkinson's disease.

Both genetic and environmental factors increase risk for Parkinson’s disease. Many of the known genetic factors influence α-synuclein aggregation or degradation, whereas most of the identified environmental factors produce oxidative stress. Studies using in vitro approaches have identified mechanisms by which oxidative stress can accelerate the formation of α-synuclein aggregates, but there is a paucity of evidence supporting the importance of these processes over extended time periods in brain. To assess this issue, we evaluated α-synuclein aggregates in brains of three transgenic mouse strains: hSyn mice, which overexpress human α-synuclein in neurons and spontaneously develop α-synuclein aggregates; EAAT3−/− mice, which exhibit a neuron-specific impairment in cysteine uptake and resultant neuron-selective chronic oxidative stress; and double-transgenic hSyn/EAAT3−/− mice. Aggregate formation was evaluated by quantitative immunohistochemistry for phosphoserine 129 α-synuclein and by an α-synuclein proximity ligation assay. Both methods showed that the double transgenic hSyn/EAAT3−/− mice exhibited a significantly higher α-synuclein aggregate density than littermate hSyn mice in each brain region examined. Negligible aggregate formation was observed in the EAAT3−/− mouse strain, suggesting a synergistic rather than additive interaction between the two genotypes. A similar pattern of results was observed in assessments of motor function: the pole test and rotarod test. Together, these observations indicate that chronic, low-grade neuronal oxidative stress promotes α-synuclein aggregate formation in vivo. This process may contribute to the mechanism by which environmentally induced oxidative stress contributes to α-synuclein pathology in idiopathic Parkinson’s disease.

Objective: Redox stress, c-Abl activation, and α -synuclein aggregates each independently contribute to neurodegeneration in Parkinson's disease. Interactions between these factors may underlie convergent and feed-forward mechanisms of disease progression. Methods:α -synuclein aggregate formation was induced in neuronal cultures and mouse substantia nigra by exposure to pre-formed human α -synuclein fibrils or by AAV-mediated over-expression of α -synuclein. Aggregate formation, c-Abl activation, redox stress, and neurodegeneration were evaluated by immunohistochemistry and Western blots, and mouse motor function was evaluated using the rota-rod and pole tests. To suppress redox stress, cultures and mice were treated with N-acetyl cysteine, a thiol repletion agent that supports neuronal glutathione metabolism. Results: In primary neuron cultures, the formation of α -synuclein aggregates led to redox stress and c-Abl activation. Redox stress alone, in the absence of α -synuclein aggregates, was also sufficient to induced c-Abl activation. N-acetyl cysteine suppressed redox stress, and likewise suppressed both c-Abl activation and α -synuclein aggregation. A similar pattern was observed in the two mouse models of Parkinson's disease. In both models, α synuclein aggregates in the substantia nigra were accompanied by redox stress, c-Abl activation, dopaminergic neurodegeneration and motor impairment, all of which were attenuated in mice treated with oral N-acetyl cysteine.Interpretation: These results indicate that α -synuclein aggregates induce c-Abl activation by a redox stress mechanism. c-Abl in turn promotes α -synuclein aggregation, and this potentially feed-forward process can be blocked by N-acetyl cysteine. The findings thus add mechanistic support for N-acetyl cysteine as a therapeutic for Parkinson's disease.