Mutant PrPSc conformers induced by a synthetic peptide and several prion strains |
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| Authors: | Tremblay Patrick Ball Haydn L Kaneko Kiyotoshi Groth Darlene Hegde Ramanujan S Cohen Fred E DeArmond Stephen J Prusiner Stanley B Safar Jiri G |
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| Affiliation: | Institute for Neurodegenerative Diseases and Department of Neurology, University of California, San Francisco, California 94143, USA. |
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| Abstract: | Gerstmann-Sträussler-Scheinker (GSS) disease is a dominantly inherited, human prion disease caused by a mutation in the prion protein (PrP) gene. One mutation causing GSS is P102L, denoted P101L in mouse PrP (MoPrP). In a line of transgenic mice denoted Tg2866, the P101L mutation in MoPrP produced neurodegeneration when expressed at high levels. MoPrPSc(P101L) was detected both by the conformation-dependent immunoassay and after protease digestion at 4°C. Transmission of prions from the brains of Tg2866 mice to those of Tg196 mice expressing low levels of MoPrP(P101L) was accompanied by accumulation of protease-resistant MoPrPSc(P101L) that had previously escaped detection due to its low concentration. This conformer exhibited characteristics similar to those found in brain tissue from GSS patients. Earlier, we demonstrated that a synthetic peptide harboring the P101L mutation and folded into a β-rich conformation initiates GSS in Tg196 mice (29). Here we report that this peptide-induced disease can be serially passaged in Tg196 mice and that the PrP conformers accompanying disease progression are conformationally indistinguishable from MoPrPSc(P101L) found in Tg2866 mice developing spontaneous prion disease. In contrast to GSS prions, the 301V, RML, and 139A prion strains produced large amounts of protease-resistant PrPSc in the brains of Tg196 mice. Our results argue that MoPrPSc(P101L) may exist in at least several different conformations, each of which is biologically active. Such conformations occurred spontaneously in Tg2866 mice expressing high levels of MoPrPC(P101L) as well as in Tg196 mice expressing low levels of MoPrPC(P101L) that were inoculated with brain extracts from ill Tg2866 mice, with a synthetic peptide with the P101L mutation and folded into a β-rich structure, or with prions recovered from sheep with scrapie or cattle with bovine spongiform encephalopathy.The discovery that brain fractions enriched for prion infectivity contain a protein (rPrPSc) that is resistant to limited proteolytic digestion advanced prion research (8, 37). N-terminal truncation of rPrPSc produced a protease-resistant fragment, denoted PrP 27-30, that is readily measured by Western blotting, enzyme-linked immunosorbent assay, or immunohistochemistry. The measurement of PrPSc was dramatically changed with the development of the conformation-dependent immunoassay (CDI), which permitted detection of full-length rPrPSc as well as previously unrecognized protease-sensitive forms of PrPSc (39).The CDI depends on using anti-PrP antibodies that react with an epitope exposed in native PrPC but that do not bind to native PrPSc. Upon denaturation, the buried epitope in PrPSc becomes exposed and readily reacts with anti-PrP antibodies. Using the CDI, we discovered that most PrPSc is protease sensitive, which we designate sPrPSc. Whether sPrPSc is an intermediate in the formation of rPrPSc remains to be determined. In Syrian hamsters inoculated with eight different strains of prions, the ratio of rPrPSc to sPrPSc was different for each strain and the concentration of sPrPSc was proportional to the length of the incubation time (39).In earlier studies, transgenic (Tg) mice, denoted Tg2866, expressing high levels of PrP(P101L) were used to model Gerstmann-Sträussler-Scheinker (GSS) disease caused by the P102L point mutation. In the brains of several lines of mice expressing high levels of PrP(P101L), no rPrPSc(P101L) was detectable (26, 27, 47). This was particularly perplexing since these Tg mice expressing high levels of PrP(P101L) developed all facets of prion-induced neurodegeneration, including multicentric PrP amyloid plaques. Moreover, brain extracts from ill Tg2866 mice transmitted disease to Tg196 mice expressing low levels of PrP(P101L) that infrequently developed spontaneous neurodegeneration (29).In humans with GSS, several different mutations of the PrP gene (PRNP) resulting in nonconservative amino acid substitutions have been identified (23). In these patients, the clinical presentation, disease course, and amounts of rPrPSc in the brain are variable. Brain extracts from humans who died of GSS were inoculated into apes and monkeys, but the transmission rates were not correlated with the levels of PrPSc in the inoculum (1, 2, 9, 32). In a limited study, GSS(P102L) was transmitted to Tg mice expressing a chimeric mouse-human (MHu2 M) PrP transgene carrying the P102L mutation but not to Tg mice expressing MHu2M PrP without the mutation (47). In another study, GSS(P102L) human prions were transmitted to Tg mice expressing MoPrP(P101L) in which the transgene was incorporated through gene replacement (31). The use of gene replacement permits all of the regulatory elements that control the wild-type (wt) MoPrP gene to modulate the expression of MoPrP(P101L). In these mice, the expression level of MoPrP(P101L) in brain is likely to be similar to that in Tg196 mice.When we synthesized a 55-mer MoPrP peptide composed of residues 89 to 143 containing the P101L mutation and folded it under conditions favoring a β-structure, it induced neurodegeneration in Tg196 mice (29). When the peptide was not folded into a β-structure, it did not produce disease in Tg196 mice. We report here that the peptide-initiated disease in Tg196 mice could be serially transmitted to other Tg196 mice using brain extracts from the peptide-inoculated Tg196 mice. Using procedures derived from the CDI, brain extracts from inoculated Tg196 mice were found to contain sPrPSc(P101L), from which a 22- to 24-kDa PrP fragment was generated by limited digestion with proteinase K (PK) at 4°C and selective precipitation with phosphotungstate (PTA) (25, 39). In the interest of clarity, we have designated digestion at 4°C as “cold PK” and simply refer to standard digestion at 37°C as “PK.” To aid in distinguishing rPrPSc(P101L) from sPrPSc(P101L), their properties based on the work reported here and in other previously published papers are listed in Table (39, 40).TABLE 1.Characteristics of PrP(P101L) isoforms| Characteristic | Isoforma
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| PrPc(P101L) | sPrPSc(P101L) | rPrPSc(P101L) |
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| PrP epitopes (residues 90-125) in native state | Exposed | Buried | Buried | | Precipitatable by PTA | − | + | + | | Digestion with PK at 37°C (“PK”) | Dipeptides, tripeptides | Dipeptides, tripeptides | PrP 27-30 | | Digestion with PK at 4°C (“cold PK”) | Dipeptides, tripeptides | PrP 22-24 | PrP 27-30 | | Infectious | − | ? | + |
Open in a separate windowa?, unknown; +, positive; −, negative.In addition to inoculating Tg196 mice with brain extracts containing sPrPSc(P101L) or with the MoPrP(89-143,P101L) peptide, we inoculated Tg196 with several strains of prions carrying wt MoPrPSc-A or MoPrPSc-B. The 301V strain carrying wt MoPrPSc-B (22) exhibited similar abbreviated incubation times in both Tg196 mice and Prnpb/b mice. In contrast, the RML and 139A strains carrying wt MoPrPSc-A showed prolonged incubation times in both Tg196 and Prnpb/b mice (12, 33). Regardless of the host mouse strain, the 301V, RML, and 139A prion strains produced large amounts of rPrPSc in the brains of inoculated mice. Thus, the discovery of sPrPSc has for the first time provided a molecular signature for GSS prions that either arise spontaneously in mice or are induced by a synthetic peptide carrying the GSS mutation. |
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