Prion Protein Endoproteolysis: Cleavage Sites, Mechanisms and Connections to Prion Disease

Highly abundant in neurons, the cellular prion protein (PrP^C) is an obligatory precursor to the disease-associated misfolded isoform denoted PrP^Sc that accumulates in the rare neurodegenerative disorders referred to either as transmissible spongiform encephalopathies (TSEs) or as prion diseases. The ability of PrP^C to serve as a substrate for this template-mediated conversion process depends on several criteria but importantly includes the presence or absence of certain endoproteolytic events performed at the cell surface or in acidic endolysosomal compartments. The major endoproteolytic events affecting PrP^C are referred to as α- and β-cleavages, and in this review we outline the sites within PrP^C at which the cleavages occur, the mechanisms potentially responsible and their relevance to pathology. Although the association of α-cleavage with neuroprotection is well-supported, we identify open questions regarding the importance of β-cleavage in TSEs and suggest experimental approaches that could provide clarification. We also combine findings from in vitro cleavage assays and mass spectrometry-based studies of prion protein fragments in the brain to present an updated view in which α- and β-cleavages may represent two distinct clusters of proteolytic events that occur at multiple neighbouring sites rather than at single positions. Furthermore, we highlight the candidate proteolytic mechanisms best supported by the literature; currently, despite several proteases identified as capable of processing PrP^C in vitro, in cell-based models and in some cases, in vivo, none have been shown conclusively to cleave PrP^C in the brain. Addressing this knowledge gap will be crucial for developing therapeutic interventions to drive PrP^C endoproteolysis in a neuroprotective direction. Finally, we end this review by briefly addressing other cleavage events, specifically ectodomain shedding, γ-cleavage, the generation of atypical pathological fragments in the familial prion disorder Gerstmann-Sträussler-Scheinker syndrome and the possibility of an additional form of endoproteolysis close to the PrP^C N-terminus.