Effect of the E200K mutation on prion protein metabolism. Comparative study of a cell model and human brain

Mutant prion proteins are partially retained in the endoplasmic reticulum

Familial prion diseases are linked to point and insertional mutations in the prion protein (PrP) gene that are presumed to favor conversion of the cellular isoform of PrP to the infectious isoform. In this report, we have investigated the subcellular localization of PrP molecules carrying pathogenic mutations using immunofluorescence staining, immunogold labeling, and PrP-green fluorescent protein chimeras. To facilitate visualization of the mutant proteins, we have utilized a novel Sindbis viral replicon engineered to produce high protein levels without cytopathology. We demonstrate that several different pathogenic mutations have a common effect on the trafficking of PrP, impairing delivery of the molecules to the cell surface and causing a portion of them to accumulate in the endoplasmic reticulum. These observations suggest that protein quality control in the endoplasmic reticulum may play an important role in prion diseases, as it does in some other inherited human disorders. Our experiments also show that chimeric PrP molecules with the sequence of green fluorescent protein inserted adjacent to the glycolipidation site are post-translationally modified and localized normally, thus documenting the utility of these constructs in cell biological studies of PrP.

Prion diseases: what is the neurotoxic molecule?

A great deal of effort has been devoted during the past 20 years to defining the chemical nature of prions, the infectious agents responsible for transmissible spongiform encephalopathies. In contrast, much less attention has been paid to elucidating how prions actually damage the central nervous system. Although it is commonly assumed that PrP(Sc), the protein constituent of infectious prions, is the primary culprit, increasing evidence indicates that this may not be the case. Several alternative molecular forms of PrP are reasonable candidates for the neurotoxic species in prion diseases, although it is still too early to tell whether these or other ones will turn out to be the true instigating factors. The cellular pathways activated by neurotoxic forms of PrP that ultimately result in neuronal death are also being investigated, and several possible mechanisms have been uncovered, including the operation of quality control processes in the endoplasmic reticulum. Elucidating the distinction between the infectious and neurotoxic forms of PrP has important implications for designing therapy of prion diseases, as well as for understanding pathogenic mechanisms operative in other neurodegenerative disorders and the role of prion-like states in biology.

Proteasomes and ubiquitin are involved in the turnover of the wild-type prion protein

Prion diseases propagate by converting a normal glycoprotein of the host, PrP(C), into a pathogenic "prion" conformation. Several misfolding mutants of PrP(C) are degraded through the ER-associated degradation (ERAD)-proteasome pathway. In their infectious form, prion diseases such as bovine spongiform encephalopathy involve PrP(C) of wild-type sequence. In contrast to mutant PrP, wild-type PrP(C) was hitherto thought to be stable in the ER and thus immune to ERAD. Using proteasome inhibitors, we now show that approximately 10% of nascent PrP(C) molecules are diverted into the ERAD pathway. Cells incubated with N-acetyl-leucinal-leucinal-norleucinal (ALLN), lactacystin or MG132 accumulated both detergent-soluble and insoluble PrP species. The insoluble fraction included an unglycosylated 26 kDa PrP species with a protease-resistant core, and a M(r) "ladder" that contained ubiquitylated PrP. Our results show for the first time that wild-type PrP(C) molecules are subjected to ERAD, in the course of which they are dislocated into the cytosol and ubiquitylated. The presence of wild-type PrP molecules in the cytosol may have potential pathogenic implications.

N-terminal truncation of prion protein affects both formation and conformation of abnormal protease-resistant prion protein generated in vitro

Transmissible spongiform encephalopathy diseases are characterized by conversion of the normal protease-sensitive host prion protein, PrP-sen, to an abnormal protease-resistant form, PrP-res. In the current study, deletions were introduced into the flexible tail of PrP-sen (23) to determine if this region was required for formation of PrP-res in a cell-free assay. PrP-res formation was significantly reduced by deletion of residues 34-94 relative to full-length hamster PrP. Deletion of another nineteen amino acids to residue 113 further reduced the amount of PrP-res formed. Furthermore, the presence of additional proteinase K cleavage sites indicated that deletion to residue 113 generated a protease-resistant product with an altered conformation. Conversion of PrP deletion mutants was also affected by post-translational modifications to PrP-sen. Conversion of unglycosylated PrP-sen appeared to alter both the amount and the conformation of protease-resistant PrP-res produced from N-terminally truncated PrP-sen. The N-terminal region also affected the ability of hamster PrP to block mouse PrP-res formation in scrapie-infected mouse neuroblastoma cells. Thus, regions within the flexible N-terminal tail of PrP influenced interactions required for both generating and disrupting PrP-res formation.