Proteasome inhibitors promote the sequestration of PrPSc into aggresomes within the cytosol of prion-infected CAD neuronal cells

Dysfunction of the endoplasmic reticulum associated protein degradation/proteasome system is believed to contribute to the initiation or aggravation of neurodegenerative disorders associated with protein misfolding, and there is some evidence to suggest that proteasome dysfunctions might be implicated in prion disease. This study investigated the effect of proteasome inhibitors on the biogenesis of both the cellular (PrPC) and abnormal (PrPSc) forms of prion protein in CAD neuronal cells, a newly introduced prion cell system. In uninfected cells, proteasome impairment altered the intracellular distribution of PrPC, leading to a strong accumulation in the Golgi apparatus. Moreover, a detergent-insoluble and weakly protease-resistant PrP species of 26 kDa, termed Prp(26K), accumulated in the cells, whether they were prion-infected or not. However, no evidence was found that, in infected cells, this Prp26K species converts into the highly proteinase K-resistant PrPSc. In the infected cultures, proteasome inhibition caused an increased intracellular aggregation of PrPSc that was deposited into large aggresomes. These findings strengthen the view that, in neuronal cells expressing wild-type PrPC from the natural promoter, proteasomal impairment may affect both the process of PrPC biosynthesis and the subcellular sites of PrPSc accumulation, despite the fact that these two effects could essentially be disconnected.