α-Synuclein occurs physiologically as a helically folded tetramer that resists aggregation

Abstract

Pathogenic aggregation of α-synuclein (αSyn) is implicated in Parkinson's disease and related disorders. αSyn has long been regarded as a natively unfolded monomer that acquires secondary structure only when it binds to its target. This model is the basis for a number of published studies, but Bartels et al. report that endogenous αSyn isolated under entirely non-denaturing conditions from brain tissue, cell lines and living human cells occurs principally as a folded tetramer. This finding suggests that destabilization of the αSyn tetramer precedes misfolding and aggregation in synucleinopathies, and that agents that stabilize the normal tetramer may reduce αSyn pathogenicity. Parkinson’s disease is the second most common neurodegenerative disorder1,2. Growing evidence indicates a causative role of misfolded forms of the protein α-synuclein in the pathogenesis of Parkinson’s disease3,4. Intraneuronal aggregates of α-synuclein occur in Lewy bodies and Lewy neurites5, the cytopathological hallmarks of Parkinson’s disease and related disorders called synucleinopathies4. α-Synuclein has long been defined as a ‘natively unfolded’ monomer of about 14 kDa (ref. 6) that is believed to acquire α-helical secondary structure only upon binding to lipid vesicles7. This concept derives from the widespread use of recombinant bacterial expression protocols for in vitro studies, and of overexpression, sample heating and/or denaturing gels for cell culture and tissue studies. In contrast, we report that endogenous α-synuclein isolated and analysed under non-denaturing conditions from neuronal and non-neuronal cell lines, brain tissue and living human cells occurs in large part as a folded tetramer of about 58 kDa. Several methods, including analytical ultracentrifugation, scanning transmission electron microscopy and in vitro cell crosslinking confirmed the occurrence of the tetramer. Native, cell-derived α-synuclein showed α-helical structure without lipid addition and had much greater lipid-binding capacity than the recombinant α-synuclein studied heretofore. Whereas recombinantly expressed monomers readily aggregated into amyloid-like fibrils in vitro, native human tetramers underwent little or no amyloid-like aggregation. On the basis of these findings, we propose that destabilization of the helically folded tetramer precedes α-synuclein misfolding and aggregation in Parkinson’s disease and other human synucleinopathies, and that small molecules that stabilize the physiological tetramer could reduce α-synuclein pathogenicity.