Cellular prion protein (PrP(C)) is a mammalian glycoprotein which is usually found anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. The precise function of PrP(C) remains elusive but may depend upon its cellular localization. PrP(C) misfolds to a pathogenic isoform PrP(Sc), the causative agent of neurodegenerative prion diseases. Nonetheless some forms of prion disease develop in the apparent absence of infectious PrP(Sc), suggesting that molecular species of PrP distinct from PrP(Sc) may represent the primary neurotoxic culprits. Indeed, in some inherited cases of human prion disease, the predominant form of PrP detectable in the brain is not PrP(Sc) but rather (Ctm)PrP, a transmembrane form of the protein. The relationship between the neurodegeneration occurring in prion diseases involving PrP(Sc) and that associated with (Ctm)PrP remains unclear. However, the different membrane topology of the PrP mutants, as well as the presence of the GPI anchor, could influence both the function and the intracellular localization and trafficking of the protein, all being potentially very important in the pathophysiological mechanism that ultimately causes the disease. Here, we review the latest findings on the fundamental aspects of prions biology, from the PrP(C) biosynthesis, function, and structure up to its intracellular traffic and analyze the possible roles of the different topological isoforms of the protein, as well as the GPI anchor, in the pathogenesis of the disease.
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