Definition of a Molecular Pathway Mediating α-Synuclein Neurotoxicity

alpha-Synuclein physiologically chaperones SNARE-complex assembly at the synapse but pathologically misfolds into neurotoxic aggregates that are characteristic for neurodegenerative disorders, such as Parkinson's disease, and that may spread from one neuron to the next throughout the brain during Parkinson's disease pathogenesis. In normal nerve terminals, alpha-synuclein is present in an equilibrium between a cytosolic form that is natively unfolded and monomeric and a membrane-bound form that is composed of an alpha-helical multimeric species that chaperones SNARE-complex assembly. Although the neurotoxicity of alpha-synuclein is well established, the relationship between the native conformations of alpha-synuclein and its pathological aggregation remain incompletely understood; most importantly, it is unclear whether alpha-synuclein aggregation originates from its monomeric cytosolic or oligomeric membrane-bound form. Here, we address this question by introducing into alpha-synuclein point mutations that block membrane binding and by then assessing the effect of blocking membrane binding on alpha-synuclein aggregation and neurotoxicity. We show that membrane binding inhibits alpha-synuclein aggregation; conversely, blocking membrane binding enhances alpha-synuclein aggregation. Stereotactic viral expression of wild-type and mutant alpha-synuclein in the substantia nigra of mice demonstrated that blocking alpha-synuclein membrane binding significantly enhanced its neurotoxicity in vivo. Our data delineate a folding pathway for alpha-synuclein that ranges from a physiological multimeric, alpha-helical, and membrane- bound species that acts as a SNARE-complex chaperone over a monomeric, natively unfolded form to an amyloid-like aggregate that is neurotoxic in vivo.