Recombinant mouse prion protein (recMoPrP) produced in Escherichia coli was polymerized into amyloid fibrils that represent a subset of β sheet–rich structures. Fibrils consisting of recMoPrP(89–230) were inoculated intracerebrally into transgenic (Tg) mice expressing MoPrP(89–231). The mice developed neurologic dysfunction between 380 and 660 days after inoculation. Brain extracts showed protease-resistant PrP by Western blotting; these extracts transmitted disease to wild-type FVB mice and Tg mice overexpressing PrP, with incubation times of 150 and 90 days, respectively. Neuropathological findings suggest that a novel prion strain was created. Our results provide compelling evidence that prions are infectious proteins.

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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 ␤-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 ␤-oligomer and an amyloid fibril. Several lines of kinetic data suggest that the ␤-oligomer is not on the pathway to amyloid formation. The preferences for forming either a ␤-oligomer or amyloid can be dictated by experimental conditions, with acidic pH similar to that seen in endocytic vesicles favoring the ␤-oligomer and neutral pH favoring amyloid. Although both abnormal isoforms have high ␤-sheet content and bind 1-anilinonaphthalene-8-sulfonate, they are dissimilar structurally. Multiple pathways of misfolding and the formation of distinct ␤-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.

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