Disruption of dopamine homeostasis underlies selective neurodegeneration mediated by α‐synuclein

A key challenge in Parkinson's disease research is to understand mechanisms underlying selective degeneration of dopaminergic neurons mediated by genetic factors such as α‐synuclein (α‐Syn). The present study examined whether dopamine (DA)‐dependent oxidative stress underlies α‐Syn‐mediated neurodegeneration using Drosophila primary neuronal cultures. Green fluorescent protein (GFP) was used to identify live dopaminergic neurons in primary cultures prepared on a marked photoetched coverslip, which allowed us to repeatedly access preidentified dopaminergic neurons at different time points in a non‐invasive manner. This live tracking of GFP‐marked dopaminergic neurons revealed age‐dependent neurodegeneration mediated by a mutant human α‐Syn (A30P). Degeneration was rescued when α‐Syn neuronal cultures were incubated with 1 m m glutathione from Day 3 after culturing. Furthermore, depletion of cytoplasmic DA by 100 µ m α‐methyl‐ p ‐tyrosine completely rescued the early stage of α‐Syn‐mediated dopaminergic cell loss, demonstrating that DA plays a major role in oxidative stress‐dependent neurodegeneration mediated by α‐Syn. In contrast, overexpression of a Drosophila tyrosine hydroxylase gene (dTH1) alone caused DA neurodegeneration by enhanced DA synthesis in the cytoplasm. Age‐dependent dopaminergic cell loss was comparable in α‐Syn vs dTH1‐overexpressed neuronal cultures, indicating that increased DA levels in the cytoplasm is a critical change downstream of mutant α‐Syn function. Finally, overexpression of a Drosophila vesicular monoamine transporter rescued α‐Syn‐mediated neurodegeneration through enhanced sequestration of cytoplasmic DA into synaptic vesicles, further indicating that a main cause of selective neurodegeneration is α‐Syn‐induced disruption of DA homeostasis. All of these results demonstrate that elevated cytoplasmic DA is a main factor underlying the early stage of α‐Syn‐mediated neurodegeneration.