Prions are believed to propagate when an assembly of prion protein (PrP) enters a cell and replicates to produce two or more fibrils, leading to an exponential increase in PrP aggregate number with time. However, the molecular basis of this process has not yet been established in detail. Here, we use single-aggregate imaging to study fibril fragmentation and elongation of individual murine PrP aggregates from seeded aggregation in vitro. We found that PrP elongation occurs via a structural conversion from a PK-sensitive to PK-resistant conformer. Fibril fragmentation was found to be length-dependent and resulted in the formation of PK-sensitive fragments. Measurement of the rate constants for these processes also allowed us to predict a simple spreading model for aggregate propagation through the brain, assuming that doubling of the aggregate number is rate-limiting. In contrast, while α-synuclein aggregated by the same mechanism, it showed significantly slower elongation and fragmentation rate constants than PrP, leading to much slower replication rate. Overall, our study shows that fibril elongation with fragmentation are key molecular processes in PrP and α-synuclein aggregate replication, an important concept in prion biology, and also establishes a simple framework to start to determine the main factors that control the rate of prion and prion-like spreading in animals.J. C. S. is supported by a Cambridge Trust Scholarship and a Ministry of Education Technologies Incubation Scholarship, Republic of China (Taiwan). L. H. was supported by the Tsinghua University Initiative Scientific Research Program (Grants 20151080424) and the program of China Scholarships Council (CSC). A. M. T was supported in part by an MRC (NC3Rs) Project (Grant NC/K000462/1). G. M. and T. P. J. K. wish to acknowledge support from Sidney Sussex College Cambridge and the ERC grant PhysProt (337969). A. P. acknowledges funding from EPSRC (Grant EP/L027631/1). D. K. acknowledges funding from the Royal society and an ERC Advanced Grant (669237)
