Prion protein is ubiquitinated after developing protease resistance in the brains of scrapie-infected mice

Although the key event in the pathology of prion diseases is thought to be the conversion of cellular prion protein (PrPC) to the protease-resistant scrapie species termed PrPSc, the factors that contribute to neurodegeneration in scrapie-infected animals are poorly understood. One probable determinant could be when the accumulation of PrPSc in infected brain overwhelms the ubiquitin–proteasome system and triggers the degenerative cascade. In the present study, it was found that in mouse brains infected with the ME7 scrapie strain, the level of ubiquitin protein conjugates increased significantly at ∼144 days post-infection (pi) when clinical signs first become apparent. This elevation correlated with the detection of protease-resistant PrPSc and a decline in two endopeptidase activities associated with proteasome function. However, ubiquitination of PrP was only detected at the terminal stage, 3 weeks after the development of clinical symptoms (∼165 days pi). These results suggest that ubiquitination of PrP is a late event phenomenon and this conjugation occurs after the formation of protease-resistant PrPSc. Whether this post-translational modification and the impairment of proteasome function are pivotal events in the pathogenesis of prion diseases remains to be determined.