β-Sheet-rich α-synuclein (αS) aggregates characterize Parkinson's disease (PD). αS was long believed to be a natively unfolded monomer, but recent work suggests it also occurs in α-helix-rich tetramers. Crosslinking traps principally tetrameric αS in intact normal neurons, but not after cell lysis, suggesting a dynamic equilibrium. Here we show that freshly biopsied normal human brain contains abundant αS tetramers. The PD-causing mutation A53T decreases tetramers in mouse brain. Neurons derived from an A53T patient have decreased tetramers. Neurons expressing E46K do also, and adding 1-2 E46K-like mutations into the canonical αS repeat motifs (KTKEGV) further reduces tetramers, decreases αS solubility and induces neurotoxicity and round inclusions. The other three fPD missense mutations likewise decrease tetramer:monomer ratios. The destabilization of physiological tetramers by PD-causing missense mutations and the neurotoxicity and inclusions induced by markedly decreasing tetramers suggest that decreased α-helical tetramers and increased unfolded monomers initiate pathogenesis. Tetramer-stabilizing compounds should prevent this.

Background: ␣Syn is central to Parkinsonism, but its native state is unsettled. Results: A new, facile method for cross-linking ␣Syn in living cells, including neurons, reveals a major 60-kDa form consistent with a tetramer. Cell lysis destabilizes it, yielding mostly monomers. Conclusion: ␣Syn exists principally as a metastable tetramer in vivo. Significance: Models of native ␣Syn as an unfolded monomer should be reconsidered.

Background: Mitochondrial dysfunction and aggregation of ␣-synuclein both contribute to Parkinson disease. Results: Prefibrillar ␣-synuclein oligomers reduce the Ca 2ϩ retention time of isolated mitochondria respiring with complex I but not II substrates. Conclusion: Oligomeric ␣-synuclein promotes mitochondrial dysfunction in a Ca