Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders that include kuru and variant Creutzfeldt-Jakob disease (vCJD) in humans, chronic wasting disease (CWD) in deer and elk, scrapie in sheep and goats, and bovine spongiform encephalopathy (BSE). A critical event in TSE pathogenesis is the conversion of the normal, protease-sensitive host prion protein (PrP-sen) to an abnormal, partially protease-resistant form (PrP-res) that is closely associated with TSE. PrP-sen is a glycoprotein with an apparent molecular mass of 33 to 37 kDa, which is anchored to the cell membrane by a glycosylphosphatidylinositol anchor (
3). No characteristic sequence differences or chemical modifications between PrP-sen and PrP-res have been identified (
13), suggesting that the two isoforms differ mainly in their conformation (
8,
23,
31,
33).
Initial transmission of the TSE agent from one species to another is usually associated with prolonged incubation times, which decrease as the TSE agent adapts over multiple passages through the new species. Understanding this so-called “species barrier” is of particular importance, because the BSE agent has crossed the human species barrier to cause vCJD and there are concerns that the CWD agent in North America could cross species barriers and pose a threat to human health. Transmission of TSE agents to laboratory animals such as mice, hamsters, and rats has greatly enhanced our understanding of the TSE species barrier. These animals are susceptible to a variety of TSE agents, including those from sheep, humans, and cattle. Rabbits are the only mammalian species reported to be resistant to TSE agents isolated from different species. Rabbits do not develop signs of TSE disease after inoculation with the CJD, kuru, or scrapie agent (
10). Additionally, no clinical signs were observed when rabbits were challenged with scrapie agent that had been previously passaged in mice (
1). These experiments suggest that resistance to infection by the TSE agent is a characteristic of the host species rather than the strain of TSE agent used for infection.
Previous studies with transgenic mice, scrapie-infected tissue cultures, or cell-free conversion assays for PrP-res formation have demonstrated that the PrP amino acid sequence strongly influences both PrP-res formation and interspecies transmission of the TSE agent (
16,
25,
28,
34). The transmission of TSE agent from one species to another appears to be dependent on amino acid sequence homology between the host PrP-sen and the PrP-res in the inoculum. In several model systems, both species-specific formation of PrP-res and the transmission of the TSE agent across species barriers have been mapped to the central region of the PrP molecule comprising residues 108 to 171 (
16,
25,
35). Recently, experiments with transgenic mice suggest that amino acid residues in the C terminus can also influence interspecies transmission of the TSE agent (
36). Thus, the apparent resistance of rabbits to various TSE agents may reside in specific amino acid residues within the same regions of the rabbit PrP-sen molecule (
1,
10,
19).
We have studied the ability of rabbit PrP-sen to be converted to PrP-res in mouse neuroblastoma cells persistently infected with mouse-adapted scrapie (Sc+-MNB cells). Here we show that rabbit PrP-sen was not converted to PrP-res in these cells. Multiple amino acid residues throughout the rabbit PrP sequence correlated with the inability of rabbit PrP-sen to form PrP-res. However, in one instance, the negative effect of a specific amino acid residue on the formation of PrP-res could be overcome by altering an adjacent amino acid residue. Thus, the inability of rabbit PrP to be converted to PrP-res is likely due to the overall structural characteristics of the rabbit PrP molecule.
DISCUSSION
Previous studies have demonstrated that amino acid residues that can affect the interspecies induction of PrP-res are located in the central region (
16,
25,
35) or C terminus (
36) of the PrP molecule. Here we show that amino acid residues that interfered with the formation of PrP-res in scrapie-infected mouse neuroblastoma cells were present throughout the rabbit PrP-sen molecule. Thus, these results suggest that PrP amino acid differences in the N terminus, the C terminus, and the central part can all potentially influence the TSE species barrier.
We have shown that rabbit PrP-sen is not converted to PrP-res in tissue culture cells persistently infected with mouse-adapted scrapie. Since conversion of PrP-sen to PrP-res is believed to be the fundamental process underlying TSE agent pathogenesis, the resistance of rabbits to mouse TSE agent could be due to the inability of mouse PrP-res to convert rabbit PrP-sen to rabbit PrP-res. Recent studies suggest that the amino acid residues critical for species-specific PrP-res formation can differ, depending upon the species from which PrP-sen and PrP-res originate. For example, in the case of the species barrier between mice and hamsters, sequence homology at either amino acid residue 138 or 154 is important, depending upon whether or not PrP-res has been isolated from mice or hamsters, respectively (
26). Thus, it is possible that within the rabbit PrP sequence, amino acid residues other than those identified here could influence the TSE species barriers that exist between rabbits and other mammals besides mice (
1,
10). However, the fact that amino acid residues negatively affecting PrP-res formation in scrapie-infected mouse neuroblastoma cells are located throughout the rabbit PrP molecule suggests that the three-dimensional structure of rabbit PrP either cannot easily refold to PrP-res or is converted to PrP-res only at an extremely low efficiency by PrP-res molecules originating from different species. This may explain why rabbits are among the few mammalian species resistant to challenge with several different TSE agents.
Two rabbit-specific amino acid residues (99G and 108M) that inhibited the formation of PrP-res in scrapie infected mouse neuroblastoma cells reside within the N-terminal domain of the PrP-sen molecule. While this polypeptide segment appears to be relatively flexible and unstructured, it may be involved in intra- or intermolecular interactions with the protein core, the cell surface, or with a natural ligand (
5). Amino acid residues 90 to 120 within this region have also been demonstrated to be crucial for both PrP-res formation (
12,
18,
32) and intraspecies transmission of TSE agent (
9,
39). During PrP-res formation, the N-terminal domain appears to undergo major structural changes to a more structured conformation and is thus partially protected from proteolytic cleavage within the PrP-res aggregate (
24,
37). Therefore, one explanation for the detrimental effect of residues 99G and 108M on PrP-res formation is that an N-terminal region containing these amino acid residues would be unable to undergo this conformational change.
An unexpected finding of this study was that within the N terminus, a rabbit-specific methionine at position 108 in MoL42 PrP prevented PrP-res formation, while MoL42 PrP molecules with rabbit-specific residues at positions 108 and 107 were readily converted to PrP-res. This demonstrates that the negative effect of one amino acid residue can be compensated for by an additional amino acid residue substitution. Again, these data suggest that it is the overall tertiary structure of PrP-sen rather than sequence homology that is the critical determinant in the induction of PrP-res formation. A compensatory effect could also explain why PrP molecules lacking amino acid residues 23 to 88 and 141 to 176 are conversion competent, although they lack regions shown to be important in PrP-res formation (
38,
41).
We have also identified amino acid residues that inhibit the conversion of rabbit PrP-sen to its abnormal isoform within the central region and the C-terminal portion of the PrP molecule (residues 173S and 214I). Residue 173 is located in the turn leading into the second α-helix (Fig.
3B), an area that has been implicated in the species barrier between mink and ferrets (
2). Amino acid mismatches in this region appear to also critically affect the intraspecies transmission of scrapie between different breeds of sheep or mice (
22,
43,
44) and the formation of sheep and mouse PrP-res (
6,
7,
30). Both amino acid residues 173 and 214 are part of a surface region of PrP formed by the loop between the second β-strand and the second α-helix and part of the C terminus (Fig.
3B) that could potentially function as an initial interaction site between PrP-sen and PrP-res (residues 164 to 173 and 205 to 222) (
14). Substitutions at positions 173 (N→S) and 214 (V→I) are not expected to alter the global structure of this interaction site. However, they could possibly modify its specificity for both long-range electrostatic interactions and short-range hydrogen bonding with other proteins (
5). Interestingly, amino acid residues within this surface region (residues 167, 171, 214, and 218) have also been proposed to be involved in the binding of an as-yet-unidentified factor, protein X, hypothesized to be involved in the conversion process (
15) The identification of the protein X binding site was based on tissue culture experiments demonstrating that the expression of mouse PrP mutated at these residues could interfere with PrP-res formation. However, as shown here and by others, amino acid residues other than the proposed protein X binding residues also interfere with PrP-res formation (
12,
25,
27,
41). Additionally, cell-free conversion assays using purified PrP-res and PrP-sen have demonstrated that heterologous PrP-sen molecules bind to PrP-res in regions containing the putative protein X site, thus blocking PrP-res formation (
14). Therefore, the effect of changes at residue 214 on PrP-res formation is most likely due to direct competition between different PrP-sen molecules for a common binding site on PrP-res.