Adaptation and selection of prion protein strain conformations following interspecies transmission of transmissible mink encephalopathy

Abstract

Interspecies transmission of the transmissible spongiform encephalopathies (TSEs), or prion diseases, can result in the adaptation and selection of TSE strains with an expanded host range and increased virulence such as in the case of bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease. To investigate TSE strain adaptation, we serially passaged a biological clone of transmissible mink encephalopathy (TME) into Syrian golden hamsters and examined the selection of distinct strain phenotypes and conformations of the disease-specific isoform of the prion protein (PrP(Sc)). The long-incubation-period drowsy (DY) TME strain was the predominate strain, based on the presence of its strain-specific PrP(Sc) following interspecies passage. Additional serial passages in hamsters resulted in the selection of the hyper (HY) TME PrP(Sc) strain-dependent conformation and its short incubation period phenotype unless the passages were performed with a low-dose inoculum (e.g., 10(-5) dilution), in which case the DY TME clinical phenotype continued to predominate. For both TME strains, the PrP(Sc) strain pattern preceded stabilization of the TME strain phenotype. These findings demonstrate that interspecies transmission of a single cloned TSE strain resulted in adaptation of at least two strain-associated PrP(Sc) conformations that underwent selection until one type of PrP(Sc) conformation and strain phenotype became predominant. To examine TME strain selection in the absence of host adaptation, hamsters were coinfected with hamster-adapted HY and DY TME. DY TME was able to interfere with the selection of the short-incubation HY TME phenotype. Coinfection could result in the DY TME phenotype and PrP(Sc) conformation on first passage, but on subsequent passages, the disease pattern converted to HY TME. These findings indicate that during TSE strain adaptation, there is selection of a strain-specific PrP(Sc) conformation that can determine the TSE strain phenotype.

FIG. 1

Transmission of a biological clone of TME to hamsters. (A) Serial transmission of TME into hamsters is indicated in passage lines A and B. On first passage, the animal numbers refer to Table 1, with incubation period in days (d.) ± standard error of the mean and clinical phenotype, when apparent, noted as either HY, DY, or HD, a combination of both. Incubation periods without a standard error of the mean refer to individual animals that were used for both PrP^Sc analysis and subsequent passages. The PrP^Sc polypeptide migration pattern (19 or 21 kDa) is indicated; numbers 1, 2, and 3 refer to animals whose PrP^Sc patterns were analyzed in panels B and C. (B and C) Western blot analysis of PrP^Sc from first interspecies passage (lanes 1 and 2) and second intraspecies hamster passage (lane 3). Brain samples were resolved by SDS-PAGE (15% gel), and PrP was detected with monoclonal antibody 3F4 as described in Materials and Methods. Lanes HY and DY are the control 21- and 19-kDa patterns, respectively.

FIG. 2

PrP^Sc polypeptide patterns from Syrian hamsters infected with the HY and DY strains of TME. Animals were infected with different combinations and dilutions of HY and DY TME as indicated in each of the trials reported in Table 2 and also illustrated above each lane. Each lane contains an enriched PrP^Sc brain fraction digested with proteinase K from an individual animal and analyzed by SDS-PAGE and Western blot analysis using monoclonal antibody 3F4 (lanes 1 to 3 and 12 to 21) or rabbit polyclonal antibody R20 (lanes 4 to 11). R20 is immunoreactive to a synthetic peptide containing the C-terminal end of the mature prion protein; in addition to recognizing similar-molecular-weight PrP polypeptides as 3F4, it can also detect a 6- to 8-kDa band. Boxed areas refer to the second serial passage, inoculated at a 10⁻² brain dilution, of individual brains from the first passage. HY and DY TME control PrP^Sc banding patterns are indicated in lanes 1, 4, 11, 16, and 21 and in lanes 1 and 11, respectively.