Prion subcellular fractionation reveals infectivity spectrum, with a high titre-low PrP res level disparity

Abstract

Background: Prion disease transmission and pathogenesis are linked to misfolded, typically protease resistant (PrP res) conformers of the normal cellular prion protein (PrP C), with the former posited to be the principal constituent of the infectious 'prion'. Unexplained discrepancies observed between detectable PrP res and infectivity levels exemplify the complexity in deciphering the exact biophysical nature of prions and those host cell factors, if any, which contribute to transmission efficiency. In order to improve our understanding of these important issues, this study utilized a bioassay validated cell culture model of prion infection to investigate discordance between PrP res levels and infectivity titres at a subcellular resolution. Findings. Subcellular fractions enriched in lipid rafts or endoplasmic reticulum/mitochondrial marker proteins were equally highly efficient at prion transmission, despite lipid raft fractions containing up to eight times the levels of detectable PrP res. Brain homogenate infectivity was not differentially enhanced by subcellular fraction-specific co-factors, and proteinase K pre-treatment of selected fractions modestly, but equally reduced infectivity. Only lipid raft associated infectivity was enhanced by sonication. Conclusions: This study authenticates a subcellular disparity in PrP res and infectivity levels, and eliminates simultaneous divergence of prion strains as the explanation for this phenomenon. On balance, the results align best with the concept that transmission efficiency is influenced more by intrinsic characteristics of the infectious prion, rather than cellular microenvironment conditions or absolute PrP res levels. © 2012 Lewis et al; licensee BioMed Central Ltd.