(362e) Asymmetric Amyloid Fibril Elongation: A New Perspective On a Symmetric World

Amyloids are insoluble, fibrous proteins formed through the aggregation of mis-folded proteins. They accumulate in the tissue of individuals with neurodegenerative disease, such as Parkinson's disease, Huntington's disease, and most notably Alzheimer's disease. Many models have been proposed for the structure of amyloid fibrils, but there has been little experimental evidence demonstrating the growth of fibrils. We used dual-color fluorescent tagging of insulin fibrils to determine whether fibril formation from an initial fibril seed is a uni- or bi-directional process. The prevailing theory on amyloid formation is that a symmetric fibril elongates equally from both ends. This study provides evidence to the contrary; the process may occur uni-directionally, demonstrating that amyloid fibrils may be asymmetric and propagate mostly in one direction. Alexa Fluor 568 labeled insulin fibrils were seeded into a native insulin solution and allowed to aggregate at 65°C while the elongation kinetics was monitored by light absorbance at 600 nm. The resulting elongated fibrils were labeled with thioflavin-T, a fluorescent amyloid dye, for visualization using fluorescence microscopy. The resulting images show that a large majority of the elongated fibrils propagated along only one end of the seed, with the remaining labeled fibrils having bi-directional elongation or no elongation. This result was confirmed by using two different Alexa Fluor dyes that bind to different functional groups on the insulin fibril. Thus, instead of the current view that fibrils grow symmetrically from both ends of the fibril, this is the first evidence that insulin amyloid fibrils are asymmetric and appear to grow from only one end in solution.