Transgenic Mice Expressing Porcine Prion Protein Resistant to Classical Scrapie but Susceptible to Sheep Bovine Spongiform Encephalopathy and Atypical Scrapie

Atypical scrapie strain phenotypes may shift when transmitted to a new host

Estudios "in vivo" de la transmisión inter-especie de priones y desarrollo de un modelo de replicación "ex vivo"

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología Molecular. Fecha de lectura: 11-01-2008Barriers limiting prion transmission between two species are not absolutely defined. In one way, it is known that intra-species prion transmission is a highly efficient process, affecting 100% individual with relative sort survival times, while inter-species prion transmission is a low efficient process accompanied with extended incubation periods. Moreover, it is demonstrated that inter-species transmission can give rise to the emergency of new characteristics in the generated prions. With regard to BSE, natural and experimental inter-species transmission to sheep and goat had been described, including genotypes traditionally considered as resistant, besides the subsequent intra-species transmission of the ovine BSE generated in sheep. As inter-species transmission can alter the biochemical and biological properties of prions the possibility of having circulating BSE in other species than cattle constitutes an animal and human health risk. In this work, we have deeply evaluated the changes in the BSE prion properties when transmitted to sheep. For that purpose, we performed a comparative study of the biochemical, biological and neuropathological properties of BSE before (cattle-BSE) and after propagation in sheep (sheep-BSE). For the biological characterization of these two prions we used transgenic mouse models expressing either bovine PrP (boPrP-Tg110) or porcine PrP (poPrP-Tg001). Results show that the BSE prion after crossing through the ovine species barriers maintains most of its biochemical and neuropathological characteristics which are different from Sheep-Scrapie prions. However, the comparative analysis of cattle-BSE and sheep-BSE prion transmission in the transgenic mice models showed an alteration in the biological characteristics of BSE after propagation in sheep. This alteration constitutes an increased virulence of the sheep-BSE prion in both transgenic models when compared to the original cattle-BSE prion. The high infectivity of sheep-BSE in the porcine transgenic model and the absence of many data about porcine species susceptibility to scrapie prions justified the study of the transmissibility of a panel of different Sheep-Scrapie strains into the poPrPTg001 mice. Results show the inability of different classical scrapie isolates to infect the porcine model suggesting a high transmission barrier, if not total, to those ovine prion strains in pig. Meanwhile, the porcine model shows a high susceptibility to sheep- BSE prions and moderated to the atypical scrapie Sc-152 (Nor98 like) which reveal the risk of porcine species to ovine prion infection and its implication in animal and human health. In another hand, this results show the first inter-species transmission for an atypical scrapie prion. Finally, we try to overcome the practical handicaps of animal experimentation in this work by establishing neurosphere cultures from two lines of wild type mice (129/ola and FVB), one line of mice over-expessing murine PrP (Tga20) and mice knocked out for the Prnp gene (KO). Murine neurospheres inoculated during their differentiation have result susceptible to prion infection with the two murine strains tested. Independently of the line of mice origin of the neurospheres, those cells propagate the inoculated prions at detectable levels. Those results allow us to consider neurospheres inoculated during differentiation as a valuable model for prion replication and propagation

Sheep-Passaged Bovine Spongiform Encephalopathy Agent Exhibits Altered Pathobiological Properties in Bovine-PrP Transgenic Mice

Sheep can be experimentally infected with bovine spongiform encephalopathy (BSE), and the ensuing disease is similar to scrapie in terms of pathogenesis and clinical signs. BSE infection in sheep is an animal and human health concern. In this study, the transmission in BoPrP-Tg110 mice of prions from BSE-infected sheep was examined and compared to the transmission of original cattle BSE in cattle and sheep scrapie prions. Our results indicate no transmission barrier for sheep BSE prions to infect BoPrP-Tg110 mice, but the course of the disease is accelerated compared to the effects of the original BSE isolate. The shortened incubation period of sheep BSE in the model was conserved in subsequent passage in BoPrP-Tg110 mice, indicating that it is not related to infectious titer differences. Biochemical signature, lesion profile, and PrP(Sc) deposition pattern of both cattle and sheep BSE were similar. In contrast, all three sheep scrapie isolates tested showed an evident transmission barrier and further adaptation in subsequent passage. Taken together, those data indicate that BSE agent can be altered by crossing a species barrier, raising concerns about the virulence of this new prion towards other species, including humans. The BoPrP-Tg110 mouse bioassay should be considered as a valuable tool for discriminating scrapie and BSE in sheep

Elements modulating the prion species barrier and its passage consequences.

The specific characteristics of Transmissible Spongiform Encephalopathy (TSE) strains may be altered during passage across a species barrier. In this study we investigated the biochemical and biological characteristics of Bovine Spongiform Encephalopathy (BSE) after transmission in both natural host species (cattle, sheep, pigs and mice) and in transgenic mice overexpressing the corresponding cellular prion protein (PrPC) in comparison with other non-BSE related prions from the same species. After these passages, most features of the BSE agent remained unchanged. BSE-derived agents only showed slight modifications in the biochemical properties of the accumulated PrPSc, which were demonstrated to be reversible upon re-inoculation into transgenic mice expressing bovine-PrPC. Transmission experiments in transgenic mice expressing bovine, porcine or human-PrP revealed that all BSE-derived agents were transmitted with no or a weak transmission barrier. In contrast, a high species barrier was observed for the non-BSE related prions that harboured an identical PrP amino acid sequence, supporting the theory that the prion transmission barrier is modulated by strain properties (presumably conformation-dependent) rather than by PrP amino acid sequence differences between host and donor. As identical results were observed with prions propagated either in natural hosts or in transgenic mouse models, we postulate that the species barrier and its passage consequences are uniquely governed by the host PrPC sequence and not influenced by other host genetic factors. The results presented herein reinforce the idea that the BSE agent is highly promiscuous, infecting other species, maintaining its properties in the new species, and even increasing its capabilities to jump to other species including humans. These data are essential for the development of an accurate risk assessment for BSE

PrP^res in BoPrP-Tg110 mice.

<p>Electrophoretic profiles and antibody labelling of PrP^res as detected by mAbs Sha31 (A) and 12B2 (B) in brain extracts from BoPrP-Tg110 mice inoculated with the different BSE-derived prions: cattle-BSE (lane 1), sheep-BSE (lane 2), OvTg-BSE (lane 3), pig-BSE (lane 4), PoTg-BSE (lane 5), mouse-BSE (lane 6), MoTga20-BSE (lane 7), human-vCJD (lane 8), HuTg-BSE (lane 9). Brain extracts from BoPrP-Tg110 mice inoculated with atypical cattle-BSE H (lane 10), sheep-scrapie (lane 11) and mouse-RML (lane 12) were included as a control non-BSE related prion propagated in the same mouse model. Panels A and B were loaded with the same quantities of PrP^res extracted from each sample. MW, molecular weight in kilodaltons.</p

Electrophoretic profiles and antibody labelling of PrP^res in BSE-derived prions.

<p>PrP^res was detected by western blot using the mAbs Sha31 (A) and 12B2 (B) in the different BSE-derived prions propagated either in natural hosts or in transgenic mouse models over-expressing their corresponding PrP^C sequence: cattle-BSE (lanes 1 and 11), BoTg-BSE (lanes 2 and 12), sheep-BSE (lane 3), OvTg-BSE (lane 4), pig-BSE (lane 5), PoTg-BSE (lane 6), mouse-BSE (lane 7), MoTga20-BSE (lane 8), human-vCJD (lane 13), HuTg-BSE (lane 14). Sheep-scrapie (lane 9), mouse-RML (lane 10), human-sCJD (lane 15) and atypical cattle-BSE H (lane 16) were included as control non-BSE related prions. Panels A and B were loaded with the same quantities of PrP^res extracted from each sample. MW, molecular weight in kilodaltons.</p

Glycoform ratios of PrP^res detected by Western blotting.

<p>PrP^res was detected with the Sha31 monoclonal antibody in the different BSE-derived prions propagated either in the natural hosts (A) or in transgenic mouse models over-expressing their corresponding PrP^C sequence (B). Atypical cattle-BSE H, sheep-scrapie, mouse-RML and human-sCJD isolates (C) are included for comparison purposes. Values are the normalized means from at least six repeated runs. Arrows indicate the reading direction of the axis.</p

PrP^Sc proteinase K resistance ELISA test in BSE-derived prions.

<p>A) BSE-derived prions propagated in several host species (cattle-BSE, sheep-BSE, pig-BSE, mouse-BSE and human-vCJD); mouse-RML, sheep-scrapie and human-sCJD isolates are included for comparison purposes. B) BSE-derived prions propagated in transgenic mice expressing PrP^C of these species (BoTg-BSE, OvTg-BSE, PoTg-BSE, MoTga20-BSE and HuTg-BSE).</p