The conformational change of a host protein, PrP C , into a disease-associated isoform, PrP Sc , appears to play a critical role in the pathogenesis of prion diseases such as Creutzfeldt–Jakob disease and scrapie. However, the fundamental mechanism by which infectious prions are produced in neurons remains unknown. To investigate the mechanism of prion formation biochemically, we conducted a series of experiments using the protein misfolding cyclic amplification (PMCA) technique with a preparation containing only native PrP C and copurified lipid molecules. These experiments showed that successful PMCA propagation of PrP Sc molecules in a purified system requires accessory polyanion molecules. In addition, we found that PrP Sc molecules could be formed de novo from these defined components in the absence of preexisting prions. Inoculation of samples containing either prion-seeded or spontaneously generated PrP Sc molecules into hamsters caused scrapie, which was transmissible on second passage. These results show that prions able to infect wild-type hamsters can be formed from a minimal set of components including native PrP C molecules, copurified lipid molecules, and a synthetic polyanion.

Much evidence supports the hypothesis that the infectious agents of prion diseases are devoid of nucleic acid, and instead are composed of a specific infectious protein. This protein, PrP(Sc), seems to be generated by template-induced conformational change of a normally expressed glycoprotein, PrP(C) (ref. 2). Although numerous studies have established the conversion of PrP(C) to PrP(Sc) as the central pathogenic event of prion disease, it is unknown whether cellular factors other than PrP(C) might be required to stimulate efficient PrP(Sc) production. We investigated the biochemical amplification of protease-resistant PrP(Sc)-like protein (PrPres) using a modified version of the protein-misfolding cyclic amplification method. Here we report that stoichiometric transformation of PrP(C) to PrPres in vitro requires specific RNA molecules. Notably, whereas mammalian RNA preparations stimulate in vitro amplification of PrPres, RNA preparations from invertebrate species do not. Our findings suggest that host-encoded stimulatory RNA molecules may have a role in the pathogenesis of prion disease. They also provide a practical approach to improve the sensitivity of diagnostic techniques based on PrPres amplification.

Prions containing misfolded prion protein (PrP Sc ) can be formed with cofactor molecules using the technique of serial protein misfolding cyclic amplification. However, it remains unknown whether cofactors materially participate in maintaining prion conformation and infectious properties. Here we show that withdrawal of cofactor molecules during serial propagation of purified recombinant prions caused adaptation of PrP Sc structure accompanied by a reduction in specific infectivity of >10 5 -fold, to undetectable levels, despite the ability of adapted "protein-only" PrP Sc molecules to self-propagate in vitro. We also report that changing only the cofactor component of a minimal reaction substrate mixture during serial propagation induced major changes in the strain properties of an infectious recombinant prion. Moreover, propagation with only one functional cofactor (phosphatidylethanolamine) induced the conversion of three distinct strains into a single strain with unique infectious properties and PrP Sc structure. Taken together, these results indicate that cofactor molecules can regulate the defining features of mammalian prions: PrP Sc conformation, infectivity, and strain properties. These findings suggest that cofactor molecules likely are integral components of infectious prions.phospholipid | bioassay | repertoire | convergence | diversity

Infectious prions containing the pathogenic conformer of the mammalian prion protein (PrP Sc ) can be produced de novo from a mixture of the normal conformer (PrP C ) with RNA and lipid molecules. Recent reconstitution studies indicate that nucleic acids are not required for the propagation of mouse prions in vitro, suggesting the existence of an alternative prion propagation cofactor in brain tissue. However, the identity and functional properties of this unique cofactor are unknown. Here, we show by purification and reconstitution that the molecule responsible for the nucleaseresistant cofactor activity in brain is endogenous phosphatidylethanolamine (PE). Synthetic PE alone facilitates conversion of purified recombinant (rec)PrP substrate into infectious recPrP Sc molecules. Other phospholipids, including phosphatidylcholine, phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol, were unable to facilitate recPrP Sc formation in the absence of RNA. PE facilitated the propagation of PrP Sc molecules derived from all four different animal species tested including mouse, suggesting that unlike RNA, PE is a promiscuous cofactor for PrP Sc formation in vitro. Phospholipase treatment abolished the ability of brain homogenate to reconstitute the propagation of both mouse and hamster PrP Sc molecules. Our results identify a single endogenous cofactor able to facilitate the formation of prions from multiple species in the absence of nucleic acids or other polyanions.PrP | scrapie P rions are mechanistically unique infectious agents that contain a misfolded, membrane-bound, glycoprotein (PrP Sc ) formed by the conformational change of a host-encoded conformer (PrP C ) (1). Conversion of PrP C into PrP Sc is the central event in the formation of infectious prions, but the molecular mechanism underlying conformational change remains poorly understood. In particular, the number and identity of endogenous factors other than PrP required for prion formation has not been determined (2).Cell culture and biochemical studies have implicated several classes of macromolecules such as GAGs, nucleic acids, proteins, and lipids as potential cofactors for prion formation (3). Reconstitution experiments with defined substrates (in which purified PrP molecules are mixed with Prnp 0/0 brain homogenate or purified cofactors that facilitate its conversion to PrP Sc ) have suggested that the conversion mechanism may be relatively simple, requiring only a few components (4, 5). Wild-type hamster prions possessing specific infectivity levels similar to those associated with natural scrapie have been formed de novo by using a defined mixture of purified native PrP C , copurified lipid, and RNA molecules (4), and infectious prions have also been formed de novo from bacterially expressed, recombinant PrP substrate in a reaction facilitated by synthetic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) and RNA molecules (5, 6). In summary, infectious prions have not yet been produced either with a single cofactor or in the abs...

Little is currently known about the biochemical mechanism by which induced prion protein (PrP) conformational change occurs during mammalian prion propagation. In this study, we describe the reconstitution of PrPres amplification in vitro using partially purified and synthetic components. Overnight incubation of purified PrP27-30 and PrP C molecules at a molar ratio of 1:250 yielded ϳ2-fold baseline PrPres amplification. Addition of various polyanionic molecules increased the level of PrPres amplification to ϳ10-fold overall. Polyanionic compounds that stimulated purified PrPres amplification to varying degrees included synthetic, homopolymeric nucleic acids such as poly(A) and poly(dT), as well as non-nucleic acid polyanions, such as heparan sulfate proteoglycan. Size fractionation experiments showed that synthetic poly(A) polymers must be >0.2 kb in length to stimulate purified PrPres amplification. Thus, one possible set of minimal components for efficient conversion of PrP molecules in vitro may be surprisingly simple, consisting of PrP27-30, PrP C , and a stimulatory polyanionic compound.