Infectious prions propagate from peripheral entry sites into the central nervous system (CNS), where they cause progressive neurodegeneration that ultimately leads to death. Yet the pathogenesis of prion disease can vary dramatically depending on the strain, or conformational variant of the aberrantly folded and aggregated protein, PrPSc. Although most prion strains invade the CNS, some prion strains cannot gain entry and do not cause clinical signs of disease. The conformational basis for this remarkable variation in the pathogenesis among strains is unclear. Using mouse-adapted prion strains, here we show that highly neuroinvasive prion strains primarily form diffuse aggregates in brain and are noncongophilic, conformationally unstable in denaturing conditions, and lead to rapidly lethal disease. These neuroinvasive strains efficiently generate PrPSc over short incubation periods. In contrast, the weakly neuroinvasive prion strains form large fibrillary plaques and are stable, congophilic, and inefficiently generate PrPSc over long incubation periods. Overall, these results indicate that the most neuroinvasive prion strains are also the least stable, and support the concept that the efficient replication and unstable nature of the most rapidly converting prions may be a feature linked to their efficient spread into the CNS.

Zoonotic prion transmission was reported after the bovine spongiform encephalopathy (BSE) epidemic, when Ͼ200 cases of prion disease in humans were diagnosed as variant Creutzfeldt-Jakob disease. Assessing the risk of cross-species prion transmission remains challenging. We and others have studied how specific amino acid residue differences between species impact prion conversion and have found that the ␤2-␣2 loop region of the mouse prion protein (residues 165-175) markedly influences infection by sheep scrapie, BSE, mouse-adapted scrapie, deer chronic wasting disease, and hamster-adapted scrapie prions. The tyrosine residue at position 169 is strictly conserved among mammals and an aromatic side chain in this position is essential to maintain a 3 10 -helical turn in the ␤2-␣2 loop. Here we examined the impact of the Y169G substitution together with the previously described S170N, N174T "rigid loop" substitutions on cross-species prion transmission in vivo and in vitro. We found that transgenic mice expressing mouse PrP containing the triple-amino acid substitution completely resisted infection with two strains of mouse prions and with deer chronic wasting disease prions. These studies indicate that Y169 is important for prion formation, and they provide a strong indication that variation of the ␤2-␣2 loop structure can modulate interspecies prion transmission.

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Background: Single residue differences can block conversion of the cellular prion protein (PrP) to the pathogenic conformation. Results: Prion conversion was reduced by non-aromatic amino acids at PrP position 169 in the ␤ 2 -␣ 2 loop. Conclusion: The conserved tyrosine side chain at PrP position 169 promotes efficient prion formation. Significance: These findings are consistent with a steric zipper model of prion conversion.

Prions are misfolded, aggregated conformers of the prion protein that can be transmitted between species. The precise determinants of interspecies transmission remain unclear, although structural similarity between the infectious prion and host prion protein is required for efficient conversion to the misfolded conformer. The β2-α2 loop region of endogenous prion protein, PrP(C), has been implicated in barriers to prion transmission. We recently discovered that conversion was efficient when incoming and host prion proteins had similar β2-α2 loop structures; however, the roles of primary vs. secondary structural homology could not be distinguished. Here we uncouple the effect of primary and secondary structural homology of the β2-α2 loop on prion conversion. We inoculated prions from animals having a disordered or an ordered β2-α2 loop into mice having a disordered loop or an ordered loop due to a single residue substitution (D167S). We found that prion conversion was driven by a homologous primary structure and occurred independently of a homologous secondary structure. Similarly, cell-free conversion using PrP(C) from mice with disordered or ordered loops and prions from 5 species correlated with primary but not secondary structural homology of the loop. Thus, our findings support a model in which efficient interspecies prion conversion is determined by small stretches of the primary sequence rather than the secondary structure of PrP.