Mouse-hamster chimeric prion protein (PrP) devoid of N-terminal residues 23-88 restores susceptibility to 22L prions, but not to RML prions in PrP-knockout mice

Prion Propagation is Dependent on Key Amino Acids in Charge Cluster 2 within the Prion Protein

To dissect the N-terminal residues within the cellular prion protein (PrP^C) that are critical for efficient prion propagation, we generated a library of point, double, or triple alanine replacements within residues 23-111 of PrP, stably expressed them in cells silenced for endogenous mouse PrP^C and challenged the reconstituted cells with four common but biologically diverse mouse prion strains. Amino acids (aa) 105-111 of Charge Cluster 2 (CC2), which is disordered in PrP^C, were found to be required for propagation of all four prion strains; other residues had no effect or exhibited strain-specific effects. Replacements in CC2, including aa105-111, dominantly inhibited prion propagation in the presence of endogenous wild type PrP^C whilst other changes were not inhibitory. Single alanine replacements within aa105-111 identified leucine 108 and valine 111 or the cluster of lysine 105, threonine 106 and asparagine 107 as critical for prion propagation. These residues mediate specific ordering of unstructured CC2 into β-sheets in the infectious prion fibrils from Rocky Mountain Laboratory (RML) and ME7 mouse prion strains.

Central Residues in Prion Protein PrPC Are Crucial for Its Conversion into the Pathogenic Isoform

Conformational conversion of the cellular prion protein, PrP^C, into the amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases. However, the conversion mechanism remains to be elucidated. Here, we generated Tg(PrPΔ91-106)-8545/Prnp^0/0 mice, which overexpress mouse PrP lacking residues 91-106. We showed that none of the mice became sick after intracerebral inoculation with RML, 22L, and FK-1 prion strains nor accumulated PrP^ScΔ91-106 in their brains except for a small amount of PrP^ScΔ91-106 detected in one 22L-inoculated mouse. However, they developed disease around 85 days after inoculation with bovine spongiform encephalopathy (BSE) prions with PrP^ScΔ91-106 in their brains. These results suggest that residues 91-106 are important for PrP^C conversion into PrP^Sc in infection with RML, 22L, and FK-1 prions but not BSE prions. We then narrowed down the residues 91-106 by transducing various PrP deletional mutants into RML- and 22L-infected cells and identified that PrP mutants lacking residues 97-99 failed to convert into PrP^Sc in these cells. Our in vitro conversion assay also showed that RML, 22L, and FK-1 prions did not convert PrPΔ97-99 into PrP^ScΔ97-99, but BSE prions did. We further found that PrP mutants with proline residues at positions 97 to 99 or charged residues at positions 97 and 99 completely or almost completely lost their converting activity into PrP^Sc in RML- and 22L-infected cells. These results suggest that the structurally flexible and noncharged residues 97-99 could be important for PrP^C conversion into PrP^Sc following infection with RML, 22L, and FK-1 prions but not BSE prions.

Prion Protein Devoid of the Octapeptide Repeat Region Delays Bovine Spongiform Encephalopathy Pathogenesis in Mice

Conformational conversion of the cellular isoform of prion protein, PrP^C, into the abnormally folded, amyloidogenic isoform, PrP^Sc, is a key pathogenic event in prion diseases, including Creutzfeldt-Jakob disease in humans and scrapie and bovine spongiform encephalopathy (BSE) in animals. We previously reported that the octapeptide repeat (OR) region could be dispensable for converting PrP^C into PrP^Sc after infection with RML prions. We demonstrated that mice transgenically expressing mouse PrP with deletion of the OR region on the PrP knockout background, designated Tg(PrPΔOR)/Prnp^0/0 mice, did not show reduced susceptibility to RML scrapie prions, with abundant accumulation of PrP^ScΔOR in their brains. We show here that Tg(PrPΔOR)/Prnp^0/0 mice were highly resistant to BSE prions, developing the disease with markedly elongated incubation times after infection with BSE prions. The conversion of PrPΔOR into PrP^ScΔOR was markedly delayed in their brains. These results suggest that the OR region may have a crucial role in the conversion of PrP^C into PrP^Sc after infection with BSE prions. However, Tg(PrPΔOR)/Prnp^0/0 mice remained susceptible to RML and 22L scrapie prions, developing the disease without elongated incubation times after infection with RML and 22L prions. PrP^ScΔOR accumulated only slightly less in the brains of RML- or 22L-infected Tg(PrPΔOR)/Prnp^0/0 mice than PrP^Sc in control wild-type mice. Taken together, these results indicate that the OR region of PrP^C could play a differential role in the pathogenesis of BSE prions and RML or 22L scrapie prions.IMPORTANCE Structure-function relationship studies of PrP^C conformational conversion into PrP^Sc are worthwhile to understand the mechanism of the conversion of PrP^C into PrP^Sc We show here that, by inoculating Tg(PrPΔOR)/Prnp^0/0 mice with the three different strains of RML, 22L, and BSE prions, the OR region could play a differential role in the conversion of PrP^C into PrP^Sc after infection with RML or 22L scrapie prions and BSE prions. PrPΔOR was efficiently converted into PrP^ScΔOR after infection with RML and 22L prions. However, the conversion of PrPΔOR into PrP^ScΔOR was markedly delayed after infection with BSE prions. Further investigation into the role of the OR region in the conversion of PrP^C into PrP^Sc after infection with BSE prions might be helpful for understanding the pathogenesis of BSE prions.

Prions amplify through degradation of the VPS10P sorting receptor sortilin

Prion diseases are a group of fatal neurodegenerative disorders caused by prions, which consist mainly of the abnormally folded isoform of prion protein, PrP^Sc. A pivotal pathogenic event in prion disease is progressive accumulation of prions, or PrP^Sc, in brains through constitutive conformational conversion of the cellular prion protein, PrP^C, into PrP^Sc. However, the cellular mechanism by which PrP^Sc is progressively accumulated in prion-infected neurons remains unknown. Here, we show that PrP^Sc is progressively accumulated in prion-infected cells through degradation of the VPS10P sorting receptor sortilin. We first show that sortilin interacts with PrP^C and PrP^Sc and sorts them to lysosomes for degradation. Consistently, sortilin-knockdown increased PrP^Sc accumulation in prion-infected cells. In contrast, overexpression of sortilin reduced PrP^Sc accumulation in prion-infected cells. These results indicate that sortilin negatively regulates PrP^Sc accumulation in prion-infected cells. The negative role of sortilin in PrP^Sc accumulation was further confirmed in sortilin-knockout mice infected with prions. The infected mice had accelerated prion disease with early accumulation of PrP^Sc in their brains. Interestingly, sortilin was reduced in prion-infected cells and mouse brains. Treatment of prion-infected cells with lysosomal inhibitors, but not proteasomal inhibitors, increased the levels of sortilin. Moreover, sortilin was reduced following PrP^Sc becoming detectable in cells after infection with prions. These results indicate that PrP^Sc accumulation stimulates sortilin degradation in lysosomes. Taken together, these results show that PrP^Sc accumulation of itself could impair the sortilin-mediated sorting of PrP^C and PrP^Sc to lysosomes for degradation by stimulating lysosomal degradation of sortilin, eventually leading to progressive accumulation of PrP^Sc in prion-infected cells.

Once prions consisting mainly of PrP^Sc infect hosts, they constitutively propagate in their brains. Progressive production of PrP^Sc through the constitutive conformational conversion of PrP^C into PrP^Sc underlies prion propagation. However, the mechanism enabling progressive production of PrP^Sc in prion-infected cells remains unknown. We here found that the VPS10P sorting receptor sortilin is involved in degradation of PrP^C and PrP^Sc in infected cells by binding to both molecules and subsequently trafficking them to the lysosomal protein degradation pathway. Interestingly, we also found that degradation of sortilin was stimulated in lysosomes in prion-infected cells possibly as a result of the sortilin-PrP^C or -PrP^Sc complexes being trafficked to lysosomes. Our findings indicate that PrP^Sc itself impairs the sortilin-mediated degradation of PrP^C and PrP^Sc by stimulating degradation of sortilin in lysosomes. This eventually results in progressive production of PrP^Sc in prion-infected cells by increasing the opportunity of PrP^C to convert into PrP^Sc and by accumulating the already produced PrP^Sc. This mechanism was confirmed in sortilin-KO mice infected with prions. The mice had exacerbated prion disease with earlier accumulation of PrP^Sc in their brains.

Assessment of the PrPc Amino-Terminal Domain in Prion Species Barriers

Chronic wasting disease (CWD) in cervids and bovine spongiform encephalopathy (BSE) in cattle are prion diseases that are caused by the same protein-misfolding mechanism, but they appear to pose different risks to humans. We are interested in understanding the differences between the species barriers of CWD and BSE. We used real-time, quaking-induced conversion (RT-QuIC) to model the central molecular event in prion disease, the templated misfolding of the normal prion protein, PrP^c, to a pathogenic, amyloid isoform, scrapie prion protein, PrP^Sc We examined the role of the PrP^c amino-terminal domain (N-terminal domain [NTD], amino acids [aa] 23 to 90) in cross-species conversion by comparing the conversion efficiency of various prion seeds in either full-length (aa 23 to 231) or truncated (aa 90 to 231) PrP^c We demonstrate that the presence of white-tailed deer and bovine NTDs hindered seeded conversion of PrP^c, but human and bank vole NTDs did the opposite. Additionally, full-length human and bank vole PrP^cs were more likely to be converted to amyloid by CWD prions than were their truncated forms. A chimera with replacement of the human NTD by the bovine NTD resembled human PrP^c The requirement for an NTD, but not for the specific human sequence, suggests that the NTD interacts with other regions of the human PrP^c to increase promiscuity. These data contribute to the evidence that, in addition to primary sequence, prion species barriers are controlled by interactions of the substrate NTD with the rest of the substrate PrP^c molecule.

IMPORTANCE

We demonstrate that the amino-terminal domain of the normal prion protein, PrP^c, hinders seeded conversion of bovine and white-tailed deer PrP^cs to the prion forms, but it facilitates conversion of the human and bank vole PrP^cs to the prion forms. Additionally, we demonstrate that the amino-terminal domain of human and bank vole PrP^cs requires interaction with the rest of the molecule to facilitate conversion by CWD prions. These data suggest that interactions of the amino-terminal domain with the rest of the PrP^c molecule play an important role in the susceptibility of humans to CWD prions.