Molecular analysis of the abnormal prion protein during coinfection of mice by bovine spongiform encephalopathy and a scrapie agent.

Molecular features of the proteinase K-resistant prion protein (PrP res) may discriminate among prion strains, and a specific signature could be found during infection by the infectious agent causing bovine spongiform encephalopathy (BSE). To investigate the molecular basis of BSE adaptation and selection, we established a model of coinfection of mice by both BSE and a sheep scrapie strain (C506M3). We now show that the PrP res features in these mice, characterized by glycoform ratios and electrophoretic mobilities, may be undistinguishable from those found in mice infected with scrapie only, including when mice were inoculated by both strains at the same time and by the same intracerebral inoculation route. Western blot analysis using different antibodies against sequences near the putative N-terminal end of PrP res also demonstrated differences in the main proteinase K cleavage sites between mice showing either the BSE or scrapie PrP res profile. These results, which may be linked to higher levels of PrP res associated with infection by scrapie, were similar following a challenge by a higher dose of the BSE agent during coinfection by both strains intracerebrally. Whereas PrP res extraction methods used allowed us to distinguish type 1 and type 2 PrP res, differing, like BSE and scrapie, by their electrophoretic mobilities, in the same brain region of some patients with Creutzfeldt-Jakob disease, analysis of in vitro mixtures of BSE and scrapie brain homogenates did not allow us to distinguish BSE and scrapie PrP res. These results suggest that the BSE agent, the origin of which remains unknown so far but which may have arisen from a sheep scrapie agent, may be hidden by a scrapie strain during attempts to identify it by molecular studies and following transmission of the disease in mice.