Dopamine and Cu^+/2+ can induce oligomerization of α-synuclein in the absence of oxygen: Two types of oligomerization mechanisms for α-synuclein and related cell toxicity studies

α-Synuclein oligomers can induce neurotoxicity and are implicated in Parkinson's disease etiology and disease progression. Many studies have reported α-synuclein oligomerization by dopamine (DA) and transition metal ions, but few studies provide insight into joint influences of DA and Cu²⁺. In this study, DA and Cu²⁺ were coadministered aerobically to measure α-synuclein oligomerization under these conditions. In the presence of oxygen, DA induced α-synuclein oligomerization in a dose-dependent manner. Cu^+/2+ did not effect oligomerization in such a manner in the presence of DA. By electrophoresis, Cu²⁺ was found easily to induce oligomerization with DA. This implies that oligomerization invoked by DA is reversible in the presence of Cu²⁺, which appears to be mediated by noncovalent bond interactions. In the absence of oxygen, DA induced less oligomerization of α-synuclein, whereas DA/Cu²⁺ induced aerobic-level amounts of oligomers, suggesting that DA/Cu²⁺ induces oligomerization independent of oxygen concentration. Radical species were detected through electron paramagnetic resonance (EPR) spectroscopic analysis arising from coincubation of DA/Cu²⁺ with α-synuclein. Redox reactions induced by DA/Cu²⁺ were observed in multimer regions of α-synuclein oligomers through NBT assay. Cellular toxicity results confirm that, for normal and hypoxic conditions, copper or DA/Cu²⁺ can induce cell death, which may arise from copper redox chemistry. From these results, we propose that DA and DA/Cu²⁺ induce different mechanisms of α-synuclein oligomerization, cross-linking with noncovalent (or reversible covalent) bonding vs. likely radical-mediated covalent modification.