Evidence against efficient spontaneous disassembly of prions into small oligomers

Infectious prion assemblies must fragment to replicate, spread, and trigger disease. However, the extent to which various types of amyloid fibrils fragment on their own versus being driven by other cellular processes is unclear. In the case of highly infectious, tissue-derived prion (PrP^Sc) preparations, over 40 years of previous studies have yielded starkly contradictory indications on this question. Many have reported high stability of PrP^Sc multimers in even strong detergents. However, others using nondisinfecting detergents and size-exclusion chromatography combined with light scattering measurements have described complete spontaneous disassembly into dimeric-tetrameric units. In attempting to replicate the latter experiments, we determined that PrP^Sc size-exclusion elution behavior was dominated by binding to the column matrix, not particle size. The light scattering behavior of fractions containing PrP^Sc was dominated by the coelution of detergent micelles similar in size to hypothetical PrP^Sc dimers-trimers. Furthermore, sedimentation velocity centrifugation and electron microscopy indicated that most detergent-treated PrP^Sc particles remained larger than 70-mers. When added to live cells that lacked prion protein and were therefore incapable of new PrP^Sc assembly, most PrP^Sc remained in the form of large multimers for ≥24 h, confirming substantial stability in a cellular model. Thus, we found no evidence that the much larger assemblies that predominate in brain homogenates or purified PrP^Sc preparations fragment spontaneously into small oligomers. Moreover, our identification of prion-associated size-exclusion chromatography artifacts reconciles previously disparate reports about prion disassembly.