6-Hydroxydopamine but not 1-methyl-4-phenylpyridinium abolishes α-synuclein anti-apoptotic phenotype by inhibiting its proteasomal degradation and by promoting its aggregation

We established previously that α-synuclein displayed a protective anti-apoptotic phenotype in neurons, mainly by down-regulating p53-dependent caspase-3 activation (Alves da Costa, C., Ancolio, K., and Checler, F. (2000) J. Biol. Chem. 275, 24065-24069; Alves da Costa, C., Paitel, E., Vincent, B., and Checler, F. (2002) J. Biol. Chem. 277, 50980-50984). This function was abolished by Parkinson disease-linked pathogenic mutations and by the dopaminergic toxin, 6-hydroxydopamine (6OH-DOPA) (Alves da Costa, C., Paitel, E., Vincent, B., and Checler, F. (2002) J. Biol. Chem. 277, 50980-50984). However, the mechanisms by which 6OH-DOPA interfered with α-synuclein function remained unclear. Here we showed that 6OH-DOPA prevents α-synuclein-mediated anti-apoptotic function by altering its degradation. Thus, 6OH-DOPA treatment of TSM1 neurons and SH-SY5Y neuroblastoma cells enhances endogenous α-synuclein-like immunoreactivity and inhibits the catabolism of endogenous and recombinant α-synucleins by purified 20 S proteasome. Furthermore, we demonstrated that 6OH-DOPA directly inhibits endogenous proteasomal activity inTSM1 and SH-SY5Y cells and also blocks purified proteasome activity in vitro. This inhibitory effect can be prevented by the anti-oxidant phenyl-N-butylnitrone. We also established that 6OH-DOPA triggers the aggregation of recombinant α-synuclein in vitro. Therefore, we conclude that 6OH-DOPA abolishes α-synuclein anti-apoptotic phenotype by inhibiting its proteasomal degradation, thereby increasing its intracellular concentration and potential propensity to aggregation, the latter phenomenon being directly exacerbated by 6OH-DOPA itself. Interestingly, 1-methyl-4-phenylpyridinium (MPP⁺), another toxin inducer of Parkinson disease-like pathology, does not affect α-synuclein protective function and fails to trigger aggregation of recombinant α-synuclein. Furthermore, MPP⁺ does not alter cellular proteasomal activity, and only high concentrations of the toxin affect purified 20 S proteasome by a mechanism that remains insensitive to phenyl-N-butylnitrone. The drastically distinct effects of 6OH-DOPA and MPP ⁺ on α-synuclein function are discussed with respect to Parkinson disease pathology and animal models mimicking this pathology.