Isolation of infectious, non-fibrillar and oligomeric prions from a genetic prion disease

Prions are transmissible agents causing lethal neurodegenerative diseases that are composed of aggregates of misfolded cellular prion protein (PrP(Sc)). Despite non-fibrillar oligomers having been proposed as the most infectious prion particles, prions purified from diseased brains usually consist of large and fibrillar PrP(Sc) aggregates, whose protease-resistant core (PrP(res)) encompasses the whole C-terminus of PrP. In contrast, PrP(Sc) from Gerstmann-Sträussler-Scheinker disease associated with alanine to valine substitution at position 117 (GSS-A117V) is characterized by a small protease-resistant core, which is devoid of the C-terminus. We thus aimed to investigate the role of this unusual PrP(Sc) in terms of infectivity, strain characteristics, and structural features. We found, by titration in bank voles, that the infectivity of GSS-A117V is extremely high (10(9.3) ID(50) U/g) and is resistant to treatment with proteinase K (10(9.0) ID(50) U/g). We then purified the proteinase K-resistant GSS-A117V prions and determined the amount of infectivity and PrP(res) in the different fractions, alongside the morphological characteristics of purified PrP(res) aggregates by electron microscopy. Purified pellet fractions from GSS-A117V contained the expected N- and C-terminally cleaved 7 kDa PrP(res), although the yield of PrP(res) was low. We found that this low yield depended on the low density/small size of GSS-A117V PrP(res), as it was mainly retained in the last supernatant fraction. All fractions were highly infectious, thus confirming the infectious nature of the 7 kDa PrP(res), with infectivity levels that directly correlated with the PrP(res) amount detected. Finally, electron microscopy analysis of these fractions showed no presence of amyloid fibrils, but only very small and indistinct, non-fibrillar PrP(res)particles were detected and confirmed to contain PrP via immunogold labelling. Our study demonstrates that purified aggregates of 7 kDa PrP(res), spanning residues ∼90–150, are highly infectious oligomers that encode the biochemical and biological strain features of the original sample. Overall, the autocatalytic behaviour of the prion oligomers reveals their role in the propagation of neurodegeneration in patients with Gerstmann-Sträussler-Scheinker disease and implies that the C-terminus of PrP(Sc) is dispensable for infectivity and strain features for this prion strain, uncovering the central PrP domain as the minimal molecular component able to encode infectious prions. These findings are consistent with the hypothesis that non-fibrillar prion particles are highly efficient propagators of disease and provide new molecular and morphological constraints on the structure of infectious prions.