Breaking an absolute species barrier: transgenic mice expressing the mink PrP gene are susceptible to transmissible mink encephalopathy

Abstract

Transmissible mink encephalopathy (TME) is a rare disease of the North American mink, which has never been successfully transmitted to laboratory mice. We generated transgenic mice expressing the mink prion protein (PrP) and inoculated them with TME or the mouse-adapted scrapie strain 79A. TME infected mink PrP-transgenic mice on a murine PrP knockout background. The absolute species barrier between the infectious agent of TME and mice was therefore broken. Following TME and 79A infection of mice carrying both mink and murine PrP(C), only proteinase-resistant PrP homologous to the incoming agent was detectable. The presence of the murine PrP(C) prolonged the incubation time of TME substantially.

FIG. 1.

(A) Sequence comparison between murine and mink PrP. The sequence of the murine Prnp^a allele (36) is shown and differences in the mink PrP sequence (17) are indicated underneath. The numbering is according to the murine PrP. The single letter code for amino acids is used and only the sequences of the mature PrP devoid of the N- and C-terminal signal peptides are shown. Amino acids proposed to be involved in protein X-binding (15) are underlined and amino acids proposed to be involved in the PrP^Sc/PrP^C interface (28) are highlighted in bold and italics. (B) Schematic representation of cosMink. Exonic sequences of the hamster PrP gene are represented by white boxes and the coding region of the mink PrP gene is highlighted in gray.

FIG. 2.

Western blot analysis of transgenic mouse lines carrying the mink PrP gene. Brain homogenates of F1 animals of the four different transgenic lines carrying the mink PrP gene, MK7, MK27, MK39, and MK45 were analyzed alongside of North American mink and tg20, a transgenic mouse line overexpressing the murine Prnp (9). Antibody L42 was used as primary antibody in the Western blot analysis following SDS-PAGE. Molecular size markers are indicated to the right of the blot and estimates of the amount of mink PrP in the different transgenic lines compared to the mink are indicated underneath.

FIG. 3.

Histopathological analysis of infected MK7/Prnp^0/0 mice. (A) Cerebral cortex showing numerous delicate vacuoles (spongiform change) stained with hematoxylin and eosin Original magnification, ×20. (B) Cerebellum showing strong immunostaining for PrP^Sc in red in the molecular and internal granule cell layer using L42 as primary antibody. Original magnification, ×10.

FIG. 4.

Western blot analysis of PK-resistant PrP in brain homogenates of infected animals. (A) Brain homogenates of transgenic mice (MK45/FVB, MK7/FVB) infected with 79A were compared to normal mice (C57/Black6) infected with the same agent as well as to Mk7/Prnp^0/0 mice and mink infected with TME. Homogenates were either treated with PK (+) or left untreated (−). The upper panel was immunostained using L42 as primary antibody, while the lower panel was stained using Ra3153. A molecular marker was loaded in the left lane, and the molecular sizes are indicated. (B) Brain homogenates of normal mice (FVB and C57/Black6), transgenic MK7/FVB and mink, all infected with TME, were compared using either L42 (upper panel) or Ra3153 (lower panel) as the primary antibody.