The protein ␣-synuclein has a central role in Parkinson disease, but the mechanism by which it contributes to neural degeneration remains unknown. We now show that the expression of ␣-synuclein in mammalian cells, including neurons in vitro and in vivo, causes the fragmentation of mitochondria. The effect is specific for synuclein, with more fragmentation by ␣-than ␤-or ␥-isoforms, and it is not accompanied by changes in the morphology of other organelles or in mitochondrial membrane potential. However, mitochondrial fragmentation is eventually followed by a decline in respiration and neuronal death. The fragmentation does not require the mitochondrial fission protein Drp1 and involves a direct interaction of synuclein with mitochondrial membranes. In vitro, synuclein fragments artificial membranes containing the mitochondrial lipid cardiolipin, and this effect is specific for the small oligomeric forms of synuclein. ␣-Synuclein thus exerts a primary and direct effect on the morphology of an organelle long implicated in the pathogenesis of Parkinson disease.Many observations have implicated mitochondria in the pathogenesis of PD.2 Mitochondria from the substantia nigra of affected patients show a selective reduction in the activity of respiratory chain complex I (1). Somatic mutations also accumulate with age and PD in the mitochondrial DNA of substantia nigra neurons (2). In addition, the neurotoxins MPTP and rotenone, which produce models of PD, both act by disrupting mitochondrial function. Genetic evidence further supports a primary role for mitochondria in the pathogenesis of PD. Mutations in parkin and the mitochondrial kinase PINK1 both cause autosomal recessive PD (3), and these genes appear required for the normal clearance of defective mitochondria by autophagy (4). However, the molecular mechanisms responsible for mitochondrial dysfunction in the much more common sporadic forms of PD have remained unclear. Several observations suggest a central role for the protein ␣-synuclein in the pathogenesis of sporadic PD. Point mutations in synuclein produce a rare autosomal dominant form of PD (5-7), indicating a causative role for the protein. ␣-Synuclein also accumulates in the Lewy bodies and dystrophic neurites of essentially all patients with idiopathic PD (8), implicating the protein in sporadic as well as familial forms of the disease. Furthermore, duplication and particularly triplication of the SNCA (␣-synuclein) gene cause a severe, highly penetrant form of PD (9, 10), indicating a dose-dependent pathogenic role for the wild type protein when overexpressed and suggesting that the accumulation of synuclein in sporadic PD is the primary pathogenic event. However, the mechanism by which ␣-synuclein causes PD remains poorly understood. Expressed in yeast and Drosophila, human ␣-synuclein produces severe toxicity (11-14), but these model organisms lack endogenous synuclein, and the overexpression of wild type synuclein in mammalian systems causes remarkably little if any consistent toxicity (15-18).Althou...

Intracellular proteinaceous inclusions (Lewy bodies and Lewy neurites) of alpha-synuclein are pathological hallmarks of neurodegenerative diseases such as Parkinson's disease, dementia with Lewy bodies (DLB), and multiple systemic atrophy. The molecular mechanisms underlying the aggregation of alpha-synuclein into such filamentous inclusions remain unknown, although many factors have been implicated, including interactions with lipid membranes. To model the effects of membrane fields on alpha-synuclein, we analyzed the structural and fibrillation properties of this protein in mixtures of water with simple and fluorinated alcohols. All solvents that were studied induced folding of alpha-synuclein, with the common first stage being formation of a partially folded intermediate with an enhanced propensity to fibrillate. Protein fibrillation was completely inhibited due to formation of beta-structure-enriched oligomers with high concentrations of methanol, ethanol, and propanol and moderate concentrations of trifluoroethanol (TFE), or because of the appearance of a highly alpha-helical conformation at high TFE and hexafluoro-2-propanol concentrations. At least to some extent, these conformational effects mimic those observed in the presence of phospholipid vesicles, and can explain some of the observed effects of membranes on alpha-synuclein fibrillation.

Macromolecular crowding is expected to have several significant effects on protein aggregation; the major effects will be those due to excluded volume and increased viscosity. In this report we summarize data demonstrating that macromolecular crowding may lead to a dramatic acceleration in the rate of protein aggregation and formation of amyloid fibrils, using the protein alpha-synuclein. The aggregation of alpha-synuclein has been implicated as a critical factor in development of Parkinson's disease. Various types of polymers, from neutral polyethylene glycols and polysaccharides (Ficolls, dextrans) to inert proteins, are shown to accelerate alpha-synuclein fibrillation. The stimulation of fibrillation increases with increasing length of polymer, as well as increasing polymer concentration. At lower polymer concentrations (typically up to approximately 100 mg/ml) the major effect is ascribed to excluded volume, whereas at higher polymer concentrations evidence of opposing viscosity effects become apparent. Pesticides and metals, which are linked to increased risk of Parkinson's disease by epidemiological studies, are shown to accelerate alpha-synuclein fibrillation under conditions of molecular crowding.

Deposition of misfolded alpha-synuclein is a critical factor in several neurodegenerative disorders. Filamentous alpha-synuclein is the major component of Lewy bodies and Lewy neurites, the intracellular inclusions in the dopaminergic neurons of the substantia nigra, which are considered the pathological hallmark of Parkinson's disease. We show here that anions induce partial folding of alpha-synuclein at neutral pH, forming a critical amyloidogenic intermediate, which leads to significant acceleration of the rate of fibrillation. The magnitude of the accelerating effect generally followed the position of the anions in the Hofmeister series, indicating a major role of protein-water-anion interactions in the process at salt concentrations above 10 mM. Below this concentration, electrostatic effects dominated in the mechanism of anion-induced fibrillation. The acceleration of fibrillation by anions was also dependent on the cation. Moderate concentrations of anions affected both the rates of nucleation and the elongation of alpha-synuclein fibrillation, primarily via their effect on the interaction of the protein with water.