Human prion diseases are neurodegenerative disorders caused by prion protein. Although infectivity was historically detected only in the central nervous system and lymphoreticular tissues of patients with sporadic Creutzfeldt-Jakob disease, recent reports suggest that the seeding activity of Creutzfeldt-Jakob disease prions accumulates in various non-neuronal organs including the liver, kidney, and skin. Therefore, we reanalyzed autopsy samples collected from patients with sporadic and genetic human prion diseases and found that seeding activity exists in almost all digestive organs. Unexpectedly, activity in the esophagus reached a level of prion seeding activity close to that in the central nervous system in some CJD patients, indicating that the safety of endoscopic examinations should be reconsidered.

Prions are composed solely of the pathological isoform (PrP^Sc) of the normal cellular prion protein (PrP^C). Identification of different PrP^Sc structures is crucially important for understanding prion biology because the pathogenic properties of prions are hypothesized to be encoded in the structures of PrP^Sc. However, these structures remain yet to be identified, because of the incompatibility of PrP^Sc with conventional high-resolution structural analysis methods. Previously, we reported that the region between the first and the second α-helix (H1∼H2) of PrP^C might cooperate with the more C-terminal side region for efficient interactions with PrP^Sc. From this starting point, we created a series of PrP variants with two cysteine substitutions (C;C-PrP) forming a disulfide-crosslink between H1∼H2 and the distal region of the third helix (Ctrm). We then assessed the conversion capabilities of the C;C-PrP variants in N2a cells infected with mouse-adapted scrapie prions (22L-ScN2a). Specifically, Cys substitutions at residues 165, 166, or 168 in H1∼H2 were combined with cysteine scanning along Ctrm residues 220–229. We found that C;C-PrPs are expressed normally with glycosylation patterns and subcellular localization similar to WT PrP, albeit differing in expression levels. Interestingly, some C;C-PrPs converted to protease-resistant isoforms in the 22L-ScN2a cells, but not in Fukuoka1 prion-infected cells. Crosslink patterns of convertible C;C-PrPs indicated a positional change of H1∼H2 toward Ctrm in PrP^Sc–induced conformational conversion. Given the properties of the C;C-PrPs reported here, we propose that these PrP variants may be useful tools for investigating prion strain–specific structures and structure–phenotype relationships of PrP^Sc.

The mechanism of prion strain diversity remains unsolved. Investigation of inheritance and diversification of protein-based pathogenic information demands the identification of the detailed structures of abnormal isoforms of the prion protein (PrP^Sc); however, achieving purification is difficult without affecting infectivity. Similar prion-like properties are recognized also in other disease-associated in-register parallel β-sheet amyloids including Tau and α-synuclein (αSyn) amyloids. Investigations into structures of those amyloids via solid-state nuclear magnetic resonance spectroscopy and cryo-electron microscopy recently made remarkable advances due to their relatively small sizes and lack of post-translational modifications. Herein, we review advances regarding pathogenic amyloids, particularly Tau and αSyn, and discuss implications about strain diversity mechanisms of prion/PrP^Sc from the perspective that PrP^Sc is an in-register parallel β-sheet amyloid. Additionally, we present our recent data of molecular dynamics simulations of αSyn amyloid, which suggest significance of compatibility between β-sheet propensities of the substrate and local structures of the template for stability of amyloid structures. Detailed structures of αSyn and Tau amyloids are excellent models of pathogenic amyloids, including PrP^Sc, to elucidate strain diversity and pathogenic mechanisms.

Prions are unique infectious agents, consisting solely of abnormally-folded prion protein (PrP^Sc). However, they possess virus-like features, including strain diversity, the ability to adapt to new hosts and to be altered evolutionarily. Because prions lack genetic material (DNA and RNA), these biological phenomena have been attributed to the structural properties of PrP^Sc. Therefore, many structural models of the structure of PrP^Sc have been proposed based on the limited structural information available, regardless of the incompatibility with high-resolution structural analysis. Recently hypothesized models consist solely of β-sheets and intervening loops/kinks; i.e. parallel in-register β-sheet and β-solenoid models. Owing to the relative simplicity of these structural models of PrP^Sc, we hypothesized that numerical conversion of the primary structures with a relevant algorithm would enable quantitative comparison between PrPs of distinct primary structures. We therefore used the theoretical values of β-sheet (Pβ) and random-coil (Pc) propensity calculated by secondary structure prediction with a neural network, to analyze interspecies transmission of prions. By reviewing experiments in the literature, we ascertained the biological relevance of Pβ and Pc and found that these classical parameters surprisingly carry substantial information of amyloid structures. We also demonstrated how these parameters could aid in interpreting and explaining phenomena in interspecies transmissions. Our approach can lead to the development of a versatile tool for investigating not only prions but also other amyloids.

Diversity of prion strains is one of the most mysterious traits of prions because they are mere aggregates of abnormally-folded forms of single protein species, prion protein (PrP ^Sc ), without genome. Although the strain-specific properties are hypothesized to be enciphered in the strain-specific structures of PrP ^Sc instead of nucleotide genome, specifically what structure can code the information remains an enigma due to the incompatibility of PrP ^Sc with structural analyses. Although the strain diversity was regarded as unique to prions, recently other disease-associated amyloids of α-synuclein (αSyn) or tau are also reported to have “strains”. As detailed structures of αSyn amyloid are already identified and the properties of mutant αSyn associated with familial Parkinson’s diseases, e.g. A53T, H50Q, and G51D, have been characterized, structure-phenotype relations of this type of amyloid could be investigated by using the αSyn amyloid as a model. Here we intensively investigated the mutant αSyn amyloids by molecular dynamics simulation to characterize influences of mutations on the structures of homo- or hetero-oligomer stacks of the amyloid. The simulations revealed directionality of the amyloid stack, remote effects of the mutations on distant β-sheets, existence of at least two switchable interfaces/amyloid cores, and distinct effects of hetero-oligomerization depending on mutation types. Collectively, those findings implied a possible mechanism of the strain diversity of the amyloids which have multiple in-register parallel β-sheets side-by-side, and support the view that their prion-like properties are inherent in the characteristic structures. We expect that the notion is also applicable to PrP ^Sc .