Synergistic and strain-specific effects of bovine spongiform encephalopathy and scrapie prions in the cell-free conversion of recombinant prion protein.

This study describes the conversion of murine PrP(C) by PrP(Sc) from three different mouse scrapie strains (ME7, 87V and 22A) and from a mouse-passaged bovine spongiform encephalopathy (BSE) strain (BSE/Bl6). This was demonstrated by a modified, non-radioactive, cell-free conversion assay using bacterial prion protein, which was converted into a proteinase K (PK)-resistant fragment designated PrP(res). Using this assay, newly formed PrP(res) could be detected by an antibody that discriminated de novo PrP(res) and the original PrP(Sc) seed. The results suggested that PrP(res) formation occurs in three phases: the first 48 h when PrP(res) formation is delayed, followed by a period of substantially accelerated PrP(res) formation and a plateau phase when a maximum concentration of PrP(res) is reached after 72 h. The conversion of prokaryotically expressed PrP(C) by ME7 and BSE prions led to unglycosylated, PK-digested, abnormal PrP(res) fragments, which differed in molecular mass by 1 kDa. Therefore, prion strain phenotypes were retained in the cell-free conversion, even when recombinant PrP(C) was used as the substrate. Moreover, co-incubation of ME7 and BSE prions resulted in equal amounts of both ME7- and BSE-derived PrP(res) fragments (as distinguished by their different molecular sizes) and also in a significantly increased total amount of de novo-generated PrP(res). This was found to be more than twice the amount of either strain when incubated separately. This result indicates a synergistic effect of both strains during cell-free conversion. It is not yet known whether such a cooperative action between BSE and scrapie prions also occurs in vivo.