General Method of Quantifying the Extent of Methionine Oxidation in the Prion Protein

The normal cellular prion protein (PrP^C) and its infectious conformer, PrP^Sc, possess a disproportionately greater amount of methionines than would be expected for a typical mammalian protein. The thioether of methionine can be readily oxidized to the corresponding sulfoxide, which means that oxidation of methionine can be used to map the surface of the conformation of PrP^C or PrP^Sc, as covalent changes are retained after denaturation. We identified a set of peptides (TNMK, MLGSAMSR, LLGSAMSR, PMIHFGNDWEDR, ENMNR, ENMYR, IMER, MMER, MIER, VVEQMCVTQYQK, and VVEQMCITQYQR) that contains every methionine in sheep, cervid, mouse, and bank vole PrP. Each is the product of a tryptic digestion and is suitable for a multiple reaction monitoring (MRM) based analysis. The peptides chromatograph well. The oxidized and unoxidized peptides containing one methionine readily separate. The unoxidized, two singly oxidized, and doubly oxidized forms of the MLGSAMSR and MMER peptides are also readily distinguishable. This approach can be used to determine the surface exposure of each methionine by measuring its oxidation after reaction with added hydrogen peroxide.

A Mass Spectrometry-Based Method of Quantifying the Contribution of the Lysine Polymorphism at Position 171 in Sheep PrP

In sheep, the transmissibility and progression of scrapie, a sheep prion (PrPSc) disease, is strongly dependent upon specific amino acid polymorphisms in the natively expressed prion protein (PrPC). Sheep expressing PrPC with lysine (K) polymorphism at position 171 (K171) are partially resistant to oronasal dosing of classical sheep scrapie. In addition, scrapie infected sheep expressing the K171 polymorphism show a longer incubation period compared to sheep homozygous (glutamine (Q)) at position 171. Quantitating the amount of the K171 polymorphism in a sheep scrapie sample can provide important information on the composition of PrPSc. A tryptic peptide, 159R.YPNQVYYRPVDK.Y172, derived from the digestion of 171K recombinant PrP, was identified as an analyte peptide suitable for a multiple reaction monitoring-based analysis. This method, using 15N-labeled analogs and another internal peptide from the proteinase K-resistant core, permits the simultaneous quantitation of the total amount of PrP and the proportion of K171 polymorphism in the sample. Background molecules with similar retention times and transitions were present in samples from scrapie-infected sheep. Proteinase K digestion followed by ultracentrifugation-based isolation or phosphotungstic acid-based isolation were employed to minimize the contribution of those background molecules, making this approach suitable for quantitating the amount of the K171 polymorphism in heterozygous scrapie infected sheep.

Cellular and Molecular Mechanisms Mediated by recPrPC Involved in the Neuronal Differentiation Process of Mesenchymal Stem Cells

Human Dental Pulp Stem Cells (hDPSCs) represent a type of adult mesenchymal stem cells that have the ability to differentiate in vitro in several lineages such as odontoblasts, osteoblasts, chondrocytes, adipocytes and neurons. In the current work, we used hDPSCs as the experimental model to study the role of recombinant prion protein 23⁻231 (recPrPC) in the neuronal differentiation process, and in the signal pathway activation of ERK 1/2 and Akt. We demonstrated that recPrPC was able to activate an intracellular signal pathway mediated by extracellular-signal-regulated kinase 1 and 2 (ERK 1/2) and protein kinase B (Akt). Moreover, in order to understand whether endogenous prion protein (PrPC) was necessary to mediate the signaling induced by recPrPC, we silenced PrPC, demonstrating that the presence of endogenous PrPC was essential for ERK 1/2 and Akt phosphorylation. Since endogenous PrPC is a well-known lipid rafts component, we evaluated the role of these structures in the signal pathway induced by recPrPC. Our results suggest that lipid rafts integrity play a key role in recPrPC activity. In fact, lipid rafts inhibitors, such as fumonisin B1 and MβCD, significantly prevented ERK 1/2 and Akt phosphorylation induced by recPrPC. In addition, we investigated the capacity of recPrPC to induce hDPSCs neuronal differentiation process after long-term stimulation through the evaluation of typical neuronal markers expression such as B3-Tubulin, neurofilament-H (NFH) and growth associated protein 43 (GAP43). Accordingly, when we silenced endogenous PrPC, we observed the inhibition of neuronal differentiation induced by recPrPC. The combined data suggest that recPrPC plays a key role in the neuronal differentiation process and in the activation of specific intracellular signal pathways in hDPSCs.

High-Level Production of High-Purity Human and Murine Recombinant Prion Proteins Functionally Compatible to In Vitro Seeding Assay

Recombinant (rec) prion protein (PrP) is an extremely useful resource for studying protein misfolding and subsequent protein aggregation events. Here, we report mass production of high-purity rec-polypeptide encoding the C-terminal globular domain of PrP; (90-230) for human and (89-231) for murine PrP. These proteins were expressed as His-tagged fusion proteins in E. coli cultured by a high cell-density aerobic fermentation method. RecPrPs recovered from inclusion bodies were slowly refolded under reducing conditions. Purification was performed by a sequence of metal-affinity, cation-exchange, and reverse-phase chromatography. The current procedure yielded several dozens of milligrams of recPrP per liter with ＞95% purity. The purified recPrPs predominantly adopted an α-helix-rich conformation and were functionally sufficient as substrates to measure the seeding activity of human and animal prions. Establishment of a procedure for high-level production of high-purity recPrP supports the advancement of in vitro investigations of PrP including diagnosis for prion diseases.

Quiescin-sulfhydryl oxidase inhibits prion formation in vitro

Prions are infectious proteins that cause a group of fatal transmissible diseases in animals and humans. The scrapie isoform (PrP^Sc) of the cellular prion protein (PrP^C) is the only known component of the prion. Several lines of evidence have suggested that the formation and molecular features of PrP^Sc are associated with an abnormal unfolding/refolding process. Quiescin-sulfhydryl oxidase (QSOX) plays a role in protein folding by introducing disulfides into unfolded reduced proteins. Here we report that QSOX inhibits human prion propagation in protein misfolding cyclic amplification reactions and murine prion propagation in scrapie-infected neuroblastoma cells. Moreover, QSOX preferentially binds PrP^Sc from prion-infected human or animal brains, but not PrP^C from uninfected brains. Surface plasmon resonance of the recombinant mouse PrP (moPrP) demonstrates that the affinity of QSOX for monomer is significantly lower than that for octamer (312 nM vs 1.7 nM). QSOX exhibits much lower affinity for N-terminally truncated moPrP (PrP89-230) than for the full-length moPrP (PrP23-231) (312 nM vs 2 nM), suggesting that the N-terminal region of PrP is critical for the interaction of PrP with QSOX. Our study indicates that QSOX may play a role in prion formation, which may open new therapeutic avenues for treating prion diseases.