Modulation of proteinase K-resistant prion protein in cells and infectious brain homogenate by redox iron: Implications for prion replication and disease pathogenesis

The principal infectious and pathogenic agent in all prion disorders is a beta-sheet-rich isoform of the cellular prion protein (PrPC) termed PrP-scrapie (PrPSc). Once initiated, PrPSc is self-replicating and toxic to neuronal cells, but the underlying mechanisms remain unclear. In this report, we demonstrate that PrPC binds iron and transforms to a PrPSc-like form (*PrPSc) when human neuroblastoma cells are exposed to an inorganic source of redox iron. The *PrPSc thus generated is itself redox active, and it induces the transformation of additional PrPc, simulating *PrPSc propagation in the absence of brain-derived PrPSc. Moreover, limited depletion of iron from prion disease-affected human and mouse brain homogenates and scrapie-infected mouse neuroblastoma cells results in 4- to 10-fold reduction in proteinase K (PK)-resistant PrPSc, implicating redox iron in the generation, propagation, and stability of PK-resistant PrPSc. Furthermore, we demonstrate increased redox-active ferrous iron levels in prion disease-affected brains, suggesting that accumulation of PrPSc is modulated by the combined effect of imbalance in brain iron homeostasis and the redox-active nature of PrPSc. These data provide information on the mechanism of replication and toxicity by PrPSc, and they evoke predictable and therapeutically amenable ways of modulating PrPSc load.