A molecular switch controls interspecies prion disease transmission in mice
Christina J. Sigurdson1,2,3, K. Peter R. Nilsson3, Simone Hornemann4, Giuseppe Manco3, Natalia Fernández-Borges5, Petra Schwarz3, Joaquín Castilla5,6, Kurt Wüthrich4,7 and Adriano Aguzzi3
1Department of Pathology and Department of Medicine, University of California, San Diego, La Jolla, California, USA. 2Department of Pathology, Immunology, and Microbiology, University of California, Davis, Davis, California, USA. 3UniversitätsSpital Zürich, Institute of Neuropathology, Zürich, Switzerland. 4Institut für Molekularbiologie und Biophysik, ETH Zürich, Zürich, Switzerland. 5CIC BioGUNE, Parque tecnológico de Bizkaia, Bizkaia, Spain. 6Ikerbasque, Basque Foundation for Science, Bizkaia, Spain. 7Department of Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, California, USA.
Address correspondence to: Adriano Aguzzi, UniversitätsSpital Zürich, Institute of Neuropathology, Department of Pathology, Schmelzbergstrasse 12, CH–8091 Zürich, Switzerland. Phone: 22.214.171.1247; Fax: 126.96.36.19902; E-mail: firstname.lastname@example.org. Or to: Christina Sigurdson, Department of Pathology, University of California, San Diego, 9500 Gilman Dr., La Jolla, California 92093, USA. Phone: 858.534.0978; Fax: 858.246.0523; E-mail: email@example.com.
Published June 14, 2010 Received for publication December 15, 2009, and accepted in revised form April 28, 2010.
Transmissible spongiform encephalopathies are lethal neurodegenerative disorders that present with aggregated forms of the cellular prion protein (PrPC), which are known as PrPSc. Prions from different species vary considerably in their transmissibility to xenogeneic hosts. The variable transmission barriers depend on sequence differences between incoming PrPSc and host PrPC and additionally, on strain-dependent conformational properties of PrPSc. The ß2-a2 loop region within PrPC varies substantially between species, with its structure being influenced by the residue types in the 2 amino acid sequence positions 170 (most commonly S or N) and 174 (N or T). In this study, we inoculated prions from 5 different species into transgenic mice expressing either disordered-loop or rigid-loop PrPC variants. Similar ß2-a2 loop structures correlated with efficient transmission, whereas dissimilar loops correlated with strong transmission barriers. We then classified literature data on cross-species transmission according to the 170S/N polymorphism. Transmission barriers were generally low between species with the same amino acid residue in position 170 and high between those with different residues. These findings point to a triggering role of the local ß2-a2 loop structure for prion transmissibility between different species.
Discussion 170 and 174 are positions that can modulate prion transmission. The S170N and N174T substitutions in the ß2-a2 loop of PrPC erected a complete barrier to TSEs derived from 2 different host species, cattle and sheep (Table 3). Conversely, the very strong barrier of mice against hamster prions was completely ablated, as hamster prions infected all mice expressing RL-PrPC (Table 3). The potent impact on species barriers caused by 2 amino acid substitutions indicates that prion species barriers may be profoundly altered in humans or animals expressing polymorphisms or mutant PrP molecules. Thus, human or animal TSEs may be replicated by mutant molecules, even when otherwise strong species barriers are known to exist.
TSE infection in the tg1020 mice. We have previously reported that tg1020 mice spontaneously develop a new strain of TSE (53). The resulting disease may confound the assessment of TSE transmission to prion-inoculated tg1020 mice. On the other hand, several traits allowed for unequivocal differentiation between transmitted and spontaneous prion disease. First, immunoblots showed the canonical “shift” in electrophoretic mobility in PrPSc recovered from TSE-infected tg1020 mice after PK digestion. This shift has never been detected in spontaneously sick tg1020 mice. Second, inoculated mice showed widespread deposition of PrPSc in the brain, which extended to regions, such as the thalamus, in which aggregates have never been observed in spontaneous disease. Third, the staining profiles of specific LCP dyes and their fluorescence emission spectra differed vastly between inoculated and spontaneously sick tg1020 mice. For example, PTAA failed to stain any brain sections of RL-RML2 and RL hamster strains in tg1020 mice, whereas it consistently labeled spontaneously appearing prion aggregates. Taken together, these features support that the instances of disease reported in this study represent bona fide transmission of TSE rather than spontaneous disease typical of noninoculated tg1020 mice.
tga20 mice express 2-fold more PrPC than the tg1020 mice and would therefore have been expected to develop prion disease after a shorter ip if there was no transmission barrier induced by the S170N and N174T substitutions. However, the tga20 mice only developed prion disease with a shorter ip after inoculation with RML prions and actually had a longer ip after inoculation with CWD prions. With RML infection, the tg1020 mice showed clear evidence of a transmission barrier, in that the ip exceeded that of lower expressing WT mice and shortened by 50% upon second passage. Together, the data clearly support that the differences in prion susceptibility are due to the S170N and N174T substitutions and not due to the differences in PrPC expression.
Consistency of in vivo and in vitro studies of species barriers. Two decades of published studies indicate that the penetrance of cross-species transmission is affected by the sequence similarity between the host PrPC and the incoming PrPSc (61). Even single–amino acid polymorphisms can radically alter the transmissibility profiles of individual prion strains (62–69). Scrapie infection of cells expressing variant sequences of PrPC has also revealed the striking effects of mutations on the efficiency of conversion of PrPC to PrPSc (70).
Hamster prions were successfully transmitted only to tg1020 mice and not to tga20 mice. The hamster PrP sequence is homologous to RL-PrP at position 170 and to WT-PrP at position 174, suggesting that homology at 170 was critical for transmission. Studies of prion transmission in bank voles and PMCA experiments with CWD in a panel of mammalian species also implicate residue 170 as a key position for influencing species barriers (37, 71). These results can be rationalized by classifying TSE hosts into 2 groups: 170S animals that include cattle, sheep, and mice and 170N animals that include elk, deer, hamsters, and RL mice.
Here, we show that susceptibility to a number of prion strains seemed to be driven by homology at position 170, although further experiments with single mutants would be necessary to fully clarify the relative contributions of residues at positions 170 and 174. tga20 mice could readily convert other 170S prions, such as cattle BSE and classical sheep scrapie, but not the 170N prions, such as hamster scrapie. While the overexpressing tga20 mice could convert 170N mule deer CWD prions after a long ip (59), WT mice are resistant to CWD (57, 58). Conversely, tg1020 mice could convert 170N hamster scrapie and mule deer CWD but completely resisted infection with 170S sheep scrapie and BSE (Table 3). While tg1020 mice may have developed spontaneous infectivity, which would have been detected on further passages, this possibility would not have interfered with the above interpretation.
The above results prompted us to reinterpret published prion transmission experiments and epidemiologic studies in the context of the 170S/170N transmission barrier. In accordance with our predictions, prions are readily transmitted within 170S mammals: sheep scrapie prions infect cattle (72–74) and mice (75, 76), and cattle BSE prions readily infect sheep (77–79) and mice (14, 80). BSE (170S) has also been transmitted to other 170S species, including greater kudu, nyala, eland, scimitar-horned oryx, bison, rhesus macaque, domestic cat, and mink (13, 16, 81, 82). Conversely, prions generated in 170S animals are poorly transmissible to 170N recipients and vice versa. Transgenic mice overexpressing bovine or ovine PrP completely resisted infection to 9 different CWD isolates (58), and WT mice do not develop clinical prion disease after infection with elk CWD or hamster 263K scrapie (57, 58, 60). Furthermore, cattle and sheep are poorly infectible with mule deer CWD, since few animals develop disease after prolonged ips of 6 years, and only after intracerebral inoculation (83, 84). Among 170N animals, prions are also transmissible, as elk CWD is infectious to hamsters (85), albeit not very efficiently. Collectively, most historical data from studies on species barriers support the model that similarity at the loop region, and particularly the 170S/N switch, impacts transmission barriers in a broad variety of species. As a possible exception to these observations, cattle may be susceptible to CWD from white-tailed deer (86). The latter finding suggests that specific prion strains can overrule the codon 170 homology requirement.
Does the ß2-a2 loop direct prion strain conformation? What are the structural consequences of the substitutions at residues 170 and 174, and how might they explain the important role of these substitutions in interspecies prion transmission? Crystallographic investigations of PrP peptides revealed that adjacent ß2-a2 loops can engage in a “dry steric zipper” interface, which was proposed to represent the elemental backbone of many amyloids. Peptide crystal structures encompassing the ß2-a2 loops bearing the 170S/174N and 170N/174T substitutions were arranged in a P1 or a P21 crystal space group, respectively (87). These observations suggest striking differences in the ß-sheet alignment of PrPSc aggregates between prion-infected 170S and 170N animals and may provide a plausible starting point for clarifying the structural basis of prion species barriers that are highly relevant to public health, including the potential transmissibility of bovine and cervid prions to humans.
JCI online early table of contents: June 14, 2010 Featured In: Disease Research
Monday, June 14, 2010
EDITOR'S PICK: Sequence and structure key to prion disease transmission
Prion diseases are lethal neurodegenerative disorders that include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE; commonly known as mad cow disease) in cows. A team of researchers, led by Adriano Aguzzi and Christina Sigurdson, at UniversitätsSpital Zürich, Switzerland, has generated data in mice that provides greater understanding of the factors that determine how easy it is for prion diseases to be transmitted to a new host species. This information provides new insight into a highly important food safety issue; dietary exposure to beef contaminated with the BSE agent is believed to have caused nearly 200 cases of variant CJD in humans.
The key infectious agent in prion diseases is PrPSc, a highly aggregated form of the cellular prion protein (PrPC). The ease with which prions from different species can be transmitted to a new host species varies dramatically. The team found that transmission between species with the same protein building block at position 170 in PrPC was relatively easy while it was relatively difficult between those species with different building blocks at that position. As this protein building block influences the structure of the PrPC protein, the authors suggest that local structure of PrPC affected by the protein building block at position 170 might have a triggering role in prion transmissibility between different species.
TITLE: A molecular switch controls interspecies prion disease transmission in mice
>>> These observations suggest striking differences in the ß-sheet alignment of PrPSc aggregates between prion-infected 170S and 170N animals and may provide a plausible starting point for clarifying the structural basis of prion species barriers that are highly relevant to public health, including the potential transmissibility of bovine and cervid prions to humans.<<<
Friday, May 14, 2010
Prion Strain Mutation Determined by Prion Protein Conformational Compatibility and Primary Structure
Published Online May 13, 2010 Science DOI: 10.1126/science.1187107 Science Express Index