Pathway complexity of prion protein assembly into amyloid

In vivo under pathological conditions, the normal cellular form of the prion protein, PrP(C) (residues 23-231), misfolds to the pathogenic isoform PrP(Sc), a beta-rich aggregated pathogenic multimer. Proteinase K digestion of PrP(Sc) leads to a proteolytically resistant core, PrP 27-30 (residues 90-231), that can form amyloid fibrils. To study the kinetic pathways of amyloid formation in vitro, we used unglycosylated recombinant PrP corresponding to the proteinase K-resistant core of PrP(Sc) and found that it can adopt two non-native abnormal isoforms, a beta-oligomer and an amyloid fibril. Several lines of kinetic data suggest that the beta-oligomer is not on the pathway to amyloid formation. The preferences for forming either a beta-oligomer or amyloid can be dictated by experimental conditions, with acidic pH similar to that seen in endocytic vesicles favoring the beta-oligomer and neutral pH favoring amyloid. Although both abnormal isoforms have high beta-sheet content and bind 1-anilinonaphthalene-8-sulfonate, they are dissimilar structurally. Multiple pathways of misfolding and the formation of distinct beta-sheet-rich abnormal isoforms may explain the difficulties in refolding PrP(Sc) in vitro, the need for a PrP(Sc) template, and the significant variation in disease presentation and neuropathology.