Several retrospective epidemiological reports have indicated an inverse correlation between smoking and development of Parkinson's disease (PD). This has mostly been attributed to the neuroprotective role of nicotine in stimulating nicotinic acetylcholine receptors and dopaminergic neurons which are damaged in PD. One of the characteristic features of PD is the intraneuronal deposition of globular inclusions of the intrinsically disordered protein α-synuclein as Lewy bodies. Using in vitro and the well-validated yeast cell models, we show that nicotine also exerts a beneficial effect on aggregation of α-synuclein. The alkaloid increases the lag time of the nucleation step and reduces the build-up of the more toxic oligomeric species in a concentration-dependent manner. This results in lower oxidative stress in the cell, reduced cytotoxicity and increased cell survival. Structural studies using CD spectroscopy and fluorescence quenching showed that α-synuclein forms a transient complex with nicotine, distorting its native structure and altering its aggregation landscape such that the formation of oligomers is inhibited. As soluble oligomers are believed to modulate the mechanism of PD pathogenesis mainly by formation of pores in neuronal membranes, resulting in leaching of vital components of the cytoplasm with deleterious effects for the cell, our results provide a mechanistic rationale for the observed beneficial role of nicotine on the progression of the disease.

The neurotransmitter dopamine has been shown to inhibit fibrillation of α‐synuclein by promoting the formation of nonamyloidogenic oligomers. Fibrillation of α‐synuclein is accelerated in the presence of pesticides and the neurotoxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). The aim of this study was to determine whether dopamine continues to have an adverse effect on the fibrillation of α‐synuclein in the presence of MPTP and its metabolite 1‐methyl‐4‐phenylpyridinum ion (MPP+). We also attempted to answer the ambiguous question of whether conversion of MPTP to MPP+ is required for the fibrillation of α‐synuclein. For this, α‐synuclein was incubated in the presence of MPTP and MPP+ along with dopamine. The fibrillation of α‐synuclein was monitored by Thioflavin T fluorescence and immunoblotting. The morphology of the aggregates formed was observed using scanning electron microscopy. The concentrations of the neurotoxin and its metabolite were estimated by reverse phase HPLC. We found definitive evidence that the conversion of MPTP to MPP+ is not required for aggregation of α‐synuclein. MPP+ was found to accelerate the rate of α‐synuclein aggregation even in the absence of components of mitochondrial complex I. In contrast to the effect of dopamine on the aggregation of α‐synuclein alone, in the presence of MPTP or MPP+, the aggregates formed are Thioflavin T‐positive and amyloidogenic. Thus, the effect of dopamine on the nature of aggregates formed in case of α‐synuclein alone and in the presence of MPTP/MPP+ is different.

Epidemiological studies report a beneficial relationship between drinking coffee and the risk of developing Parkinson's disease (PD). This is likely due to caffeine, a constituent of coffee, acting as an adenosine A2A receptor antagonist. This study was planned to investigate whether caffeine has any effect on the aggregation of α-synuclein, present in Lewy bodies, the pathological hallmark of PD, which may account for this positive association. Aggregation of recombinant α-synuclein was followed in vitro and in a well-validated yeast proteotoxicity model of PD. Caffeine was found to have twin effects: it accelerated the process of aggregation and also altered the nature of mature aggregates. Aggregates formed in the presence of caffeine displayed amorphous as well as fibrillar morphology. In the presence of caffeine, the toxicity of oligomers and aggregates was diminished, with concomitant reduction in intracellular oxidative stress, decreased oxidative proteome damage, and increased cell survival. Caffeine-treated samples showed improved binding to phospholipids, a property likely to be important in cellular functioning of α-synuclein. Far-UV CD spectroscopy and fluorescence quenching analysis revealed that caffeine induced transient changes in this intrinsically disordered protein, forming a non-native species that enhanced the rate of aggregation of α-synuclein and modified the population of mature aggregates, introducing a higher fraction of amorphous, less toxic species. Increasingly, it is felt that the process of fibrillation itself, along with the nature of mature aggregates, dictates the cytotoxicity of the process. Our results provide a rationale for the observed epidemiological link between drinking coffee and developing PD.

Despite the significant amount of experimental data available on trehalose, the molecular mechanism responsible for its intracellular stabilising properties has not emerged yet. The repair of cellular homeostasis in many protein-misfolding diseases by trehalose is credited to the disaccharide being an inducer of autophagy, a mechanism by which aggregates of misfolded proteins are cleared by the cell. In this work, we expressed the pathogenic N-terminal fragment of huntingtin in Δnth1 mutant (unable to degrade trehalose) of Saccharomyces cerevisiae BY4742 strain. We show that the presence of trehalose resulted in the partitioning of the mutant huntingtin in the soluble fraction of the cell. This led to reduced oxidative stress and improved cell survival. The beneficial effect was independent of the expression of the major cellular antioxidant enzyme, superoxide dismutase. Additionally, trehalose led to the overexpression of the heat shock protein, Hsp104p, in mutant huntingtin-expressing cells, and resulted in rescue of the endocytotic defect in the yeast cell. We propose that at least in the initial stages of aggregation, trehalose functions as a stabiliser, increasing the level of monomeric mutant huntingtin protein, with its concomitant beneficial effects, in addition to its role as an inducer of autophagy.

Fibrillation of α-synuclein proceeds through distinct stages, with oligomers combining to form the seed or the nucleus, followed by exponential and saturation phases. Osmolytes are considered to act as protein stabilizers by virtue of their ability to inhibit protein aggregation. Trehalose, a non-reducing disaccharide which is conventionally used as a stabilizer, was found to order α-synuclein, a natively disordered protein, into a non-native conformation such that the protein folding pathway is driven towards aggregation. Thus, by ordering the pathway intermediates, the osmolyte trehalose exerts variable effect on an intrinsically disordered protein when compared with its effect on natively folded proteins.