Manganese Upregulates Cellular Prion Protein and Contributes to Altered Stabilization and Proteolysis: Relevance to Role of Metals in Pathogenesis of Prion Disease

Abstract

Prion diseases are fatal neurodegenerative diseases resulting from misfolding of normal cellular prion (PrP(C)) into an abnormal form of scrapie prion (PrP(Sc)). The cellular mechanisms underlying the misfolding of PrP(C) are not well understood. Since cellular prion proteins harbor divalent metal-binding sites in the N-terminal region, we examined the effect of manganese on PrP(C) processing in in vitro models of prion disease. Exposure to manganese significantly increased PrP(C) levels both in cytosolic and in membrane-rich fractions in a time-dependent manner. Manganese-induced PrP(C) upregulation was independent of messenger RNA transcription or stability. Additionally, manganese treatment did not alter the PrP(C) degradation by either proteasomal or lysosomal pathways. Interestingly, pulse-chase analysis showed that the PrP(C) turnover rate was significantly altered with manganese treatment, indicating increased stability of PrP(C) with the metal exposure. Limited proteolysis studies with proteinase-K further supported that manganese increases the stability of PrP(C). Incubation of mouse brain slice cultures with manganese also resulted in increased prion protein levels and higher intracellular manganese accumulation. Furthermore, exposure of manganese to an infectious prion cell model, mouse Rocky Mountain Laboratory-infected CAD5 cells, significantly increased prion protein levels. Collectively, our results demonstrate for the first time that divalent metal manganese can alter the stability of prion proteins and suggest that manganese-induced stabilization of prion protein may play a role in prion protein misfolding and prion disease pathogenesis.