Small Protease Sensitive Oligomers of PrP ^Sc in Distinct Human Prions Determine Conversion Rate of PrP ^C

Abstract

The mammalian prions replicate by converting cellular prion protein (PrP ^C) into pathogenic conformational isoform (PrP ^Sc). 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 PrP ^Sc on conversion of PrP ^C in vitro using PrP ^Sc 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) PrP ^Sc. The smallest most potent prions present in sCJD brains were composed only of∼20 monomers of PrP ^Sc. The tight correlation between conversion potency of small oligomers of human sPrP ^Sc observed in vitro and duration of the disease suggests that sPrP ^Sc conformers are an important determinant of prion strain characteristics that control the progression rate of the disease.

Author Summary

Mammalian prion diseases were originally characterized by accumulation of protease-resistant prion protein (PrP ^Sc), often forming large amyloid deposits and fibrils. However, the apparent absence of protease-resistant PrP ^Sc or amyloid fibrils in growing number of prion diseases raised several fundamental questions; specifically, whether presumably protease-sensitive forms of PrP ^Sc exist as distinct conformers; and whether they comprise the initial steps in prion replication or are related to the alternative misfolding pathway generating noninfectious aggregates. We investigated the conformational characteristics of protease sensitive conformers of PrP ^Sc and their role in the pathogenesis of sporadic Creutzfeldt-Jakob disease (sCJD). Using two different in vitro prion protein (PrP ^C) conversion techniques in tandem with biophysical methods, we identified small oligomers of protease sensitive PrP ^Sc present in sCJD brains as the most potent initiators of PrP ^C conversion. Their concentration and conformational stability determine the distinctly different replication potency of PrP ^Sc in individual isolates of sCJD and each of these characteristics correlates tightly with duration of the disease. These features argue for a broad range of distinct prion strains causing the sCJD and imply that small oligomers of protease sensitive conformers of pathogenic prion protein are encoding incubation time and progression rate of the disease.