Small Protease Sensitive Oligomers of PrPSc in Distinct Human Prions Determine Conversion Rate of PrPC

The mammalian prions replicate by converting cellular prion protein (PrPC) into pathogenic conformational isoform (PrPSc). Variations in prions, which cause different disease phenotypes, are referred to as strains. The mechanism of high-fidelity replication of prion strains in the absence of nucleic acid remains unsolved. We investigated the impact of different conformational characteristics of PrPSc on conversion of PrPC in vitro using PrPSc seeds from the most frequent human prion disease worldwide, the Creutzfeldt-Jakob disease (sCJD). The conversion potency of a broad spectrum of distinct sCJD prions was governed by the level, conformation, and stability of small oligomers of the protease-sensitive (s) PrPSc. The smallest most potent prions present in sCJD brains were composed only of∼20 monomers of PrPSc. The tight correlation between conversion potency of small oligomers of human sPrPSc observed in vitro and duration of the disease suggests that sPrPSc conformers are an important determinant of prion strain characteristics that control the progression rate of the disease.