Structural conversion of α-synuclein at the mitochondria induces neuronal toxicity

Aggregation of alpha-synuclein (α-Syn) drives Parkinson’s disease, although the initial stages of
self-assembly and structural conversion have not been captured inside neurons. We track the
intracellular conformational states of α-Syn utilizing a single-molecule Förster resonance energy
transfer (smFRET) biosensor and show that α-Syn converts from its monomeric state to form two
distinct oligomeric states in neurons in a concentration-dependent and sequence-specific
manner. 3D FRET-Correlative light and electron microscopy (FRET- CLEM) reveals the structural
organization and location of aggregation hotspots inside the neuron. Notably, multiple
intracellular seeding events occur preferentially on membrane surfaces, especially at the
mitochondrial membranes. The mitochondrial lipid, cardiolipin, triggers rapid oligomerization of
A53T α-Syn, and cardiolipin is sequestered within aggregating lipid-protein complexes.
Mitochondrial aggregates impair complex I activity and increase mitochondrial ROS generation,
which accelerates the oligomerization of A53T α-Syn, and ultimately causes permeabilization of
mitochondrial membranes and cell death. Patient iPSC-derived neurons harboring A53T
mutations exhibit accelerated α-Syn oligomerization that is dependent on mitochondrial ROS,
early mitochondrial permeabilization and neuronal death. Our study highlights a mechanism of
de novo oligomerization at the mitochondria and its induction of neuronal toxicity.