Subclinical Bovine Spongiform Encephalopathy Infection in Transgenic Mice Expressing Porcine Prion Protein

Abstract

The bovine-porcine species barrier to bovine spongiform encephalopathy (BSE) infection was explored by generating transgenic mouse lines expressing the porcine prion protein (PrP) gene. All of the porcine transgenic (poTg) mice showed clinical signs of BSE after intracerebral inoculation with a high-titer BSE inoculum. The protease-resistant PrP (PrP res ) was detected in 14% (3 of 22) of the BSE-infected poTg mice by immunohistochemical or immunoblot analysis. Despite being able to infect 42% (5 of 12) of control mice, a low-dose BSE inoculum failed to penetrate the species barrier in our poTg mouse model. The findings of these infectivity studies suggest that there is a strong species barrier between cows and pigs. However, after second-passage infection of poTg mice using brain homogenates of BSE-inoculated mice scoring negative for the incoming prion protein as inoculum, it was possible to detect the presence of the infectious agent. Thus, porcine-adapted BSE inocula were efficient at infecting poTg mice, giving rise to an incubation period substantially reduced from 300 to 177 d after inoculation and to the presence of PrP res in 100% (21 of 21) of the mice. We were therefore able to conclude that initial exposure to the bovine prion may lead to subclinical infection such that brain homogenates from poTg mice classified as uninfected on the basis of the absence of PrP res are infectious when used to reinoculate poTg mice. Collectively, our findings suggest that these poTg mice could be used as a sensitive bioassay model for prion detection in pigs.

Scrapie in Swine: a Diagnostic Challenge

“…However, mice expressing porcine PrP are susceptible to atypical (Nor-98) scrapie with a low attack rate that adapts to 100% attack rate with a rapid incubation on subsequent passages 24) , similar to the adaptation that was observed in experiments that were conducted with BSE 20,24) . Interestingly, sheep passaged BSE transmits more efficiently to pigs than BSE from cattle and was associated with PrP Sc in variety of peripheral tissues 25) .…”

“…However, compared to sheep, cervids, and humans, the porcine prion protein (PrP) is highly homogenous 17,18) with only four known single nucleotide polymorphisms (G11A, G615C, G684A, T726G) one of which (G615C) that results in a serine to asparagine substitution at codon 4 that is not part of the mature prion protein 19) . As compared to cattle, pigs have 5 unique amino acids that appear in the prion protein sequence of the globular domain and 4 of these appear in helix 3 20) . While epidemiologic evidence would suggest pigs are not susceptible to prion disease, in vitro assays demonstrate that recombinant porcine PrP converts to amyloid fibrils faster than human or bovine sequences 21) .…”

“…However, attempts to transmit BSE to pigs after oral dosing with up to 1.2 kg of brain material from infected cattle was unsuccessful 23) . Studies utilizing transgenic mice expressing the porcine prion protein suggest that initial low dose exposure to BSE prions may result in a subclinical infection that results in a high attack rate and shortened incubation periods on second passage 20) .…”

Rabbits are not resistant to prion infection

“…A raPrPTg mouse line was generated and characterized as previously described (24). These mice express ∼4-to 6-fold the normal rabbit PrP protein under the control of the murine PrP promoter in a murine PrP 0/0 background.…”

Neurobehavioral Testing in Prion Disease Studies

“…Lastly, while this model implicates PrP Sc as a necessary player in the development of prion disease neurodegeneration, there is extensive work implicating that it is unlikely to be singularly sufficient to cause clinical prion disease. To this point, there are numerous studies demonstrating subclinical prion disease in which models accumulate often extensive amounts of PrP Sc without developing clinical disease [5][6][7][8][9][10].…”