Bioassay of prion-infected blood plasma in PrP transgenic $\textit{Drosophila}$

Abstract

In pursuit of a tractable bioassay to assess blood prion infectivity, we have generated prion protein (PrP) transgenic $\textit{Drosophila}$, which show a neurotoxic phenotype in adulthood after exposure to exogenous prions at the larval stage. Here, we determined the sensitivity of ovine PrP transgenic $\textit{Drosophila}$ to ovine prion infectivity by exposure of these flies to a dilution series of scrapie-infected sheep brain homogenate. Ovine PrP transgenic $\textit{Drosophila}$ showed a significant neurotoxic response to dilutions of 10$^{−2}$ to 10$^{−10}$ of the original scrapie-infected sheep brain homogenate. Significantly, we determined that this prion-induced neurotoxic response in ovine PrP transgenic $\textit{Drosophila}$ was transmissible to ovine PrP transgenic mice, which is indicative of authentic mammalian prion detection by these flies. As a consequence, we considered that PrP transgenic $\textit{Drosophila}$ were sufficiently sensitive to exogenous mammalian prions to be capable of detecting prion infectivity in the blood of scrapie-infected sheep. To test this hypothesis, we exposed ovine PrP transgenic $\textit{Drosophila}$ to scrapie-infected plasma, a blood fraction notoriously difficult to assess by conventional prion bioassays. Notably, pre-clinical plasma from scrapie-infected sheep induced neurotoxicity in PrP transgenic $\textit{Drosophila}$ and this effect was more pronounced after exposure to samples collected at the clinical phase of disease. The neurotoxic phenotype in ovine PrP transgenic $\textit{Drosophila}$ induced by plasma from scrapie-infected sheep was transmissible since head homogenate from these flies caused neurotoxicity in recipient flies during fly-to-fly transmission. Our data show that PrP transgenic $\textit{Drosophila}$ can be used successfully to bioassay prion infectivity in blood from a prion-diseased mammalian host.