Modulation of the conformation, fibrillation, and fibril morphologies of human brain α-, β-, and γ-synuclein proteins by the disaccharide chemical chaperone trehalose

Human α-, β-, and γ-synuclein (syn) are natively unfolded proteins present in the brain. Deposition of aggregated α-syn in Lewy bodies is associated with Parkinson's disease (PD) and γ-syn is known to be involved in both neurodegeneration and breast cancer. At physiological pH, while α-syn has the highest propensity for fibrillation followed by γ-syn, β-syn does not form any fibrils. Fibril formation in these proteins could be modulated by protein structure stabilizing osmolytes such as trehalose which has an exceptional stabilizing effect for globular proteins. We present a comprehensive study of the effect of trehalose on the conformation, aggregation, and fibril morphology of α-, β-, and γ-syn proteins. Rather than stabilizing the intrinsically disordered state of the synucleins, trehalose accelerates the rate of fibril formation by forming aggregation-competent partially folded intermediate structures. Fibril morphologies are also strongly dependent on the concentration of trehalose with ≤ 0.4M favoring the formation of mature fibrils in α-, and γ-syn with no effect on the fibrillation of β-syn. At ≥ 0.8M, trehalose promotes the formation of smaller aggregates that are more cytotoxic. Live cell imaging of preformed aggregates of a labeled A90C α-syn shows their rapid internalization into neural cells which could be useful in reducing the load of aggregated species of α-syn. The findings throw light on the differential effect of trehalose on the conformation and aggregation of disordered synuclein proteins with respect to globular proteins and could help in understanding the effect of osmolytes on intrinsically disordered proteins under cellular stress conditions.