Protective Action of Monomeric and Fibrillar α-Synuclein on Neuronal Cells Exposed to the Dopaminergic Toxins Salsolinol and DOPAL.

The aggregation of α-synuclein (aSyn) is believed to be mechanistically linked to the degeneration of dopamine (DA)-producing neurons in Parkinson's disease (PD). In this respect, one crucial question that yet remains unsolved is whether aSyn aggregation is associated with either a gain- or loss-of-function of the protein in neuronal cells. Herein, we investigated the effect of monomeric versus fibrillar aSyn on mesencephalic dopaminergic neurons in primary cultures challenged with the neurotoxic catechols: salsolinol (SALSO; 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) and 3,4-dihydroxyphenylacetaldehyde (DOPAL). aSyn monomer protected cells against either SALSO- or DOPAL-induced toxicity via inhibition of caspase-3-mediated apoptosis. While fibrillar aSyn failed in attenuating SALSO neurotoxicity, it increased the viability of DOPAL-treated cells, which was apparently not associated with the inhibition of caspase-3 cleavage. The fact that DOPAL-derived aSyn adducts exhibit lower toxicity compared with DOPAL itself raises the question of whether the generation of these adducts could be part of or a collateral effect of aSyn-mediated protection in neurons exposed to DOPAL. Overall, our work provides important evidence on the impact of the fibrillation of aSyn on its protective role in neuronal cells exposed to the toxic catechols SALSO and DOPAL.