Raised calcium promotes α-synuclein aggregate formation

Parkinson's and Parkinson's-plus diseases are associated with abnormal, aggregated forms of the protein, α-synuclein. We have investigated the effects of calcium on α-synuclein aggregation in vitro and in vivo. We treated monomeric α-synuclein with calcium in vitro and used fluorescence imaging, fluorescence correlation and scanning electron microscopy to investigate protein aggregation. Incubation of fluorescent-labelled monomeric α-synuclein (24 h) at low concentration (10 μM) with calcium resulted in surface aggregates (1.5 ± 0.7 μm2) detected by fluorescence microscopy saturating at a half-maximum calcium concentration of 80 μM, whilst incubations without calcium showed few protein aggregates. Scanning electron microscopy revealed that α-synuclein surface plaques (0.5-1 μm) form in the presence of calcium and comprise 10-20 nm globular particles. Incubation of α-synuclein at high concentration (75 μM; 6 h) resulted in soluble oligomeric aggregates detected by fluorescence correlation spectroscopy in a calcium dependent process, saturating at a half maximum calcium concentration of 180 μM. In cell culture experiments, we used thapsigargin or calcium ionophore A23187 to induce transient increases of intracellular free calcium in human 1321N1 cells expressing an α-synuclein-GFP construct and observed calcium flux and α-synuclein aggregation by fluorescence microscopy. The cell culture data shows that a transient increase in intracellular free calcium significantly increased the proportion of cells bearing cytoplasmic α-synuclein aggregates 6 and 12 h post-treatment (P, 0.01). Our data indicates that calcium accelerates α-synuclein aggregation on surfaces, in free solution and in cultured cells and suggests that surface adsorption may play an important role in the calcium-dependent aggregation mechanism.