Ablation of the metal ion-induced endocytosis of the prion protein by disease-associated mutation of the octarepeat region

Abstract

The neurodegenerative spongiform encephalopathies, or prion diseases, are characterized by the conversion of the normal cellular form of the prion protein PrP(C) to a pathogenic form, PrP(Sc) [1]. There are four copies of an octarepeat PHGG(G/S)WGQ that specifically bind Cu(2+) ions within the N-terminal half of PrP(C) [2--4]. This has led to proposals that prion diseases may, in part, be due to abrogation of the normal cellular role of PrP(C) in copper homeostasis [5]. Here, we show that murine PrP(C) is rapidly endocytosed upon exposure of neuronal cells to physiologically relevant concentrations of Cu(2+) or Zn(2+), but not Mn(2+). Deletion of the four octarepeats or mutation of the histidine residues (H68/76 dyad) in the central two repeats abolished endocytosis, indicating that the internalization of PrP(C) is governed by metal binding to the octarepeats. Furthermore, a mutant form of PrP that contains nine additional octarepeats and is associated with familial prion disease [6] failed to undergo Cu(2+)-mediated endocytosis. For the first time, these results provide evidence that metal ions can promote the endocytosis of a mammalian prion protein in neuronal cells and that neurodegeneration associated with some prion diseases may arise from the ablation of this function due to mutation of the octarepeat region.