Watching amyloid fibrils grow by time-lapse atomic force microscopy

Late-onset diabetes is typically associated with amyloid deposits of fibrillar amylin in the pancreatic islets. Aqueous synthetic human amylin spontaneously forms polymorphic fibrils in vitro, and this system was used to examine the dynamics of fibril assembly. By time-lapse atomic force microscopy (AFM), the growth of individual amylin fibrils on a mica surface was observed over several hours. Prominent was the assembly of a protofibril with an elongation rate in these experiments of 1.1(+/-0.5) nm/minute. The assembly of higher order polymorphic fibrils was also observed. Growth of the protofibrils was bidirectional, i.e. it occurred by elongation at both ends. This ability of AFM to continuously monitor growth, directionality, and changes in morphology for individual fibrils, provides a significant advantage over spectroscopy-based bulk methods which average the growth of many fibrils and typically require 100 to 1000-fold more protein. The time-lapse AFM procedure used for human amylin here is thus likely to be applicable to fibril formation from other amyloid proteins and peptides.