Intracellular accumulation of a mild-denatured monomer of the human PrP fragment 90–231, as possible mechanism of its neurotoxic effects

Because of high tendency of the prion protein (PrP) toaggregate, the exact PrP isoform responsible for prion diseasesas well as the pathological mechanism that it activatesremains still controversial. In this study, we show that a prefibrillar,monomeric or small oligomeric conformation of thehuman PrP fragment 90–231 (hPrP90–231), rather than solubleor fibrillar large aggregates, represents the neurotoxicspecies. In particular, we demonstrate that monomeric milddenaturedhPrP90–231 (incubated for 1 h at 53C) inducesSH-SY5Y neuroblastoma cell death, while, when structured inlarge aggregates, it is ineffective. Using spectroscopic andcellular techniques we demonstrate that this toxic conformer ischaracterized by a high exposure of hydrophobic regions thatfavors the intracellular accumulation of the protein. Inside the cells hPrP90–231 is mainly compartmentalized into the lysosomeswhere it may trigger pro-apoptotic ‘cell death’ signals.The PrP toxic conformation, which we have obtained inducinga controlled in vitro conformational change of the protein,might mimic mild-unfolding events occurring in vivo, in thepresence of specific mutations, oxidative reactions or proteolysis.Thus, in light of this model, we propose that noveltherapeutic strategies, designed to inhibit the interaction of thetoxic PrP with the plasmamembrane, could be beneficial toprevent the formation of intracellular neurotoxic aggregatesand ultimately the neuronal death.[...]