Prion diseases are caused by conversion of a normal cell-surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is infectious in the absence of nucleic acid. Although a great deal has been learned about PrP(Sc) and its role in prion propagation, much less is known about the physiological function of PrP(C). In this review, we will summarize some of the major proposed functions for PrP(C), including protection against apoptotic and oxidative stress, cellular uptake or binding of copper ions, transmembrane signaling, formation and maintenance of synapses, and adhesion to the extracellular matrix. We will also outline how loss or subversion of the cytoprotective or neuronal survival activities of PrP(C) might contribute to the pathogenesis of prion diseases, and how similar mechanisms are probably operative in other neurodegenerative disorders.

Background: C1 is the main physiological cleavage fragment of PrP, but its role in disease is unknown. Results: C1 is not toxic when expressed in mice and delays the onset of disease and PrP Sc formation when co-expressed with WT PrP. Conclusion: C1 is a dominant-negative inhibitor of PrP Sc formation. Significance: Modulation of C1 cleavage may represent a therapeutic strategy for combating PrP Sc infection.The cellular prion protein (PrP C ) undergoes constitutive proteolytic cleavage between residues 111/112 to yield a soluble N-terminal fragment (N1) and a membrane-anchored C-terminal fragment (C1). The C1 fragment represents the major proteolytic fragment of PrP C in brain and several cell types. To explore the role of C1 in prion disease, we generated Tg(C1) transgenic mice expressing this fragment (PrP(⌬23-111)) in the presence and absence of endogenous PrP. In contrast to several other N-terminally deleted forms of PrP, the C1 fragment does not cause a spontaneous neurological disease in the absence of endogenous PrP. Tg(C1) mice inoculated with scrapie prions remain healthy and do not accumulate protease-resistant PrP, demonstrating that C1 is not a substrate for conversion to PrP Sc (the disease-associated isoform). Interestingly, Tg(C1) mice coexpressing C1 along with wild-type PrP (either endogenous or encoded by a second transgene) become ill after scrapie inoculation, but with a dramatically delayed time course compared with mice lacking C1. In addition, accumulation of PrP Sc was markedly slowed in these animals. Similar effects were produced by a shorter C-terminal fragment of PrP(⌬23-134). These results demonstrate that C1 acts as dominant-negative inhibitor of PrP Sc formation and accumulation of neurotoxic forms of PrP. Thus, C1, a naturally occurring fragment of PrP C , might play a modulatory role during the course of prion diseases. In addition, enhancing production of C1, or exogenously administering this fragment, represents a potential therapeutic strategy for the treatment of prion diseases.Transmissible spongiform encephalopathies such as Creutzfeldt-Jakob disease and bovine spongiform encephalopathy are fatal neurodegenerative disorders whose pathology is associated with propagation of prions, novel infectious agents whose transmission is based on changes in protein conformation rather than inheritance of nucleic acid sequence (1). Prion propagation depends on conversion of an endogenous cellular glycoprotein (PrP C ) 2 into an aggregated, protease-resistant isoform (PrP Sc ) that is rich in ␤-sheet structure (1-4). PrP C is synthesized on endoplasmic reticulum-attached ribosomes and transits the secretory pathway to the cell surface, where most molecules are attached to the outer leaflet of the lipid bilayer via a C-terminal glycosylphosphatidylinositol (GPI) anchor (5). Most of the protein resides in lipid rafts on the plasma membrane, although some molecules are constitutively endocytosed via clathrin-coated pits and are then recycled back to the cell surface (6 -9).After its synth...

Transgenic mice expressing PrP molecules with several different internal deletions display spontaneous neurodegenerative phenotypes that can be dose-dependently suppressed by co-expression of wild-type PrP. Each of these deletions, including the largest one (Δ32–134), retains nine amino acids immediately following the signal peptide cleavage site (residues 23–31; KKRPKPGGW). These residues have been implicated in several biological functions of PrP, including endocytic trafficking and binding of glycosaminoglycans. We report here on our experiments to test the role of this domain in the toxicity of deleted forms of PrP. We find that transgenic mice expressing Δ23–134 PrP display no clinical symptoms or neuropathology, in contrast to mice expressing Δ32–134 PrP, suggesting that residues 23–31 are essential for the toxic phenotype. Using a newly developed cell culture assay, we narrow the essential region to amino acids 23–26, and we show that mutant PrP toxicity is not related to the role of the N-terminal residues in endocytosis or binding to endogenous glycosaminoglycans. However, we find that mutant PrP toxicity is potently inhibited by application of exogenous glycosaminoglycans, suggesting that the latter molecules block an essential interaction between the N-terminus of PrP and a membrane-associated target site. Our results demonstrate that a short segment containing positively charged amino acids at the N-terminus of PrP plays an essential role in mediating PrP-related neurotoxicity. This finding identifies a protein domain that may serve as a drug target for amelioration of prion neurotoxicity.

Several lines of evidence suggest that the normal form of the prion protein, PrPC, exerts a neuroprotective activity against cellular stress or toxicity. One of the clearest examples of such activity is the ability of wild-type PrPC to suppress the spontaneous neurodegenerative phenotype of transgenic mice expressing a deleted form of PrP (Δ32–134, called F35). To define domains of PrP involved in its neuroprotective activity, we have analyzed the ability of several deletion mutants of PrP (Δ23–31, Δ23–111, and Δ23–134) to rescue the phenotype of Tg(F35) mice. Surprisingly, all of these mutants displayed greatly diminished rescue activity, although Δ23–31 PrP partially suppressed neuronal loss when expressed at very high levels. Our results pinpoint the N-terminal, polybasic domain as a critical determinant of PrPC neuroprotective activity, and suggest that identification of molecules interacting with this region will provide important clues regarding the normal function of the protein. Small molecule ligands targeting this region may also represent useful therapeutic agents for treatment of prion diseases.

Summary Variation in amyloid structures profoundly influences a wide array of pathological phenotypes in mammalian protein conformation disorders and dominantly inherited phenotypes in yeast. Here, we describe, for the first time, naturally occurring, self-propagating, structural variants of a prion protein isolated from wild strains of the yeast Saccharomyces cerevisiae. Variants of the [RNQ+] prion propagating in a variety of wild yeast differ biochemically, in their intracellular distributions, and in their ability to promote formation of the [PSI+] prion. [PSI+] is an epigenetic regulator of cellular phenotype and adaptability. Strikingly, we find that most natural [RNQ+] variants induced [PSI+] at high frequencies and the majority of [PSI+] variants elicited strong cellular phenotypes. We hypothesize that the presence of an efficient [RNQ+] template primes the cell for [PSI+] formation in order to induce [PSI+] in conditions where it would be advantageous. These studies utilize naturally occurring structural variants to expand our understanding of the consequences of diverse prion conformations on cellular phenotypes.