Formic acid impairs α-synuclein seeding activity in human and mouse brains

α-Synuclein (α-syn) is an abundant monomeric protein that can aggregate into fibrils and form neuropathological inclusions in the brains of patients with synucleinopathies. New evidence suggests that the mouse-human transmission barrier of α-syn is lower than previously reported, emphasizing the need for improved biosafety procedures when working with α-syn aggregates. Histopathology of α-syn-infected brain represents a significant potential source of occupational exposure, and current methods for tissue fixation do not inactivate the ability of pathologic α-syn to seed the conversion of endogenous, monomeric α-syn into fibrils. In this study, we tested whether 96% formic acid treatment could reduce the seeding activity of α-syn aggregates in paraformaldehyde-fixed brain samples from dementia with Lewy bodies (DLB) patients and α-syn pre-formed fibrils (PFF)-injected mouse brains. Using real-time quaking-induced conversion (RT-QuIC), we found that formic acid treatment reduced α-syn seeding dose (picograms of α-syn seeds per ml of brain homogenate) in DLB and mouse brain by 6 and 8 logarithms, respectively. RT-QuIC reactions seeded with formic acid-treated brain homogenates showed significantly longer lag phase, and decreased total thioflavin T fluorescence compared to untreated samples, indicating that formic acid treatment impairs the ability of pathological α-syn to seed monomeric α-syn. Importantly, the α-syn pathologic features and the immunostaining quality were preserved in formic acid-treated tissues. Our results demonstrate that formic acid treatment is a quick and efficient procedure for reducing α-syn seeding activity in fixed brain samples, thereby lowering the risk of accidental exposure in laboratories without compromising the quality of histopathological analysis. Summary Formic acid treatment drastically reduces α -synuclein seeding activity in fixed human and mouse brain samples while preserving histopathological quality.