Generating Bona Fide Mammalian Prions with Internal Deletions

PrP C as a Transducer of Physiological and Pathological Signals

After the discovery of prion phenomenon, the physiological role of the cellular prion protein (PrP ^C ) remained elusive. In the past decades, molecular and cellular analysis has shed some light regarding interactions and functions of PrP ^C in health and disease. PrP ^C , which is located mainly at the plasma membrane of neuronal cells attached by a glycosylphosphatidylinositol (GPI) anchor, can act as a receptor or transducer from external signaling. Although the precise role of PrP ^C remains elusive, a variety of functions have been proposed for this protein, namely, neuronal excitability and viability. Although many issues must be solved to clearly define the role of PrP ^C , its connection to the central nervous system (CNS) and to several misfolding-associated diseases makes PrP ^C an interesting pharmacological target. In a physiological context, several reports have proposed that PrP ^C modulates synaptic transmission, interacting with various proteins, namely, ion pumps, channels, and metabotropic receptors. PrP ^C has also been implicated in the pathophysiological cell signaling induced by β-amyloid peptide that leads to synaptic dysfunction in the context of Alzheimer's disease (AD), as a mediator of Aβ-induced cell toxicity. Additionally, it has been implicated in other proteinopathies as well. In this review, we aimed to analyze the role of PrP ^C as a transducer of physiological and pathological signaling.

A seven-residue deletion in PrP leads to generation of a spontaneous prion formed from C-terminal C1 fragment of PrP

Prions result from a drastic conformational change of the host-encoded cellular prion protein (PrP), leading to the formation of β-sheet-rich, insoluble, and protease-resistant self-replicating assemblies (PrP^Sc). The cellular and molecular mechanisms involved in spontaneous prion formation in sporadic and inherited human prion diseases or equivalent animal diseases are poorly understood, in part because cell models of spontaneously forming prions are currently lacking. Here, extending studies on the role of the H2 α-helix C terminus of PrP, we found that deletion of the highly conserved ¹⁹⁰HTVTTTT¹⁹⁶ segment of ovine PrP led to spontaneous prion formation in the RK13 rabbit kidney cell model. On long-term passage, the mutant cells stably produced proteinase K (PK)-resistant, insoluble, and aggregated assemblies that were infectious for naïve cells expressing either the mutant protein or other PrPs with slightly different deletions in the same area. The electrophoretic pattern of the PK-resistant core of the spontaneous prion (Δ^Spont) contained mainly C-terminal polypeptides akin to C1, the cell-surface anchored C-terminal moiety of PrP generated by natural cellular processing. RK13 cells expressing solely the Δ190-196 C1 PrP construct, in the absence of the full-length protein, were susceptible to Δ^Spont prions. Δ^Spont infection induced the conversion of the mutated C1 into a PK-resistant and infectious form perpetuating the biochemical characteristics of Δ^Spont prion. In conclusion, this work provides a unique cell-derived system generating spontaneous prions and provides evidence that the 113 C-terminal residues of PrP are sufficient for a self-propagating prion entity.

Probing the early stages of prion protein (PrP) aggregation with atomistic molecular dynamics simulations

Prions are self-replicating infectious proteinaceous agents whose conformations are capable of forming amyloid-like aggregate fibrils. Here we present molecular dynamics simulations aimed at investigating the aggregation process of the β-rich H2H3 domain of the ovine prion protein (H2H3-OvPrPSc), known to be the portion of prion protein carrying oligomerization activity.

Determining the Relative Susceptibility of Four Prion Protein Genotypes to Atypical Scrapie

Atypical scrapie is a sheep prion (PrP^Sc) disease whose epidemiology is consistent with a sporadic origin and is associated with specific polymorphisms of the normal cellular prion protein (PrP^C). To determine the relative amounts of PrP polymorphisms present in atypical scrapie, total PrP was digested with chymotrypsin to generate characteristic peptides spanning relevant polymorphisms at positions 136, 141, 154, 171, and 172 of sheep PrP^C. A multiple reaction monitoring method (MRM), employing ¹⁵N-labeled internal standards, was used to detect and quantify these polymorphisms present in both the PrP^Sc and PrP^C from heterozygous (ALRRY and ALHQY or ALRQD or AFRQY) atypical scrapie-infected or uninfected control sheep. Both polymorphisms of the full length and truncated (C1) natively expressed PrP^C are produced in equal amounts. The overall amount of PrP^C present in the infected or uninfected animals was similar. PrP^Sc isolated from heterozygotes was composed of significant amounts of both PrP polymorphisms, including the ALRRY polymorphism which is highly resistant to classical scrapie. Thus, an atypical scrapie infection does not result from an overexpression of sheep PrP^C. The replication of all atypical scrapie prions occurs at comparable rates, despite polymorphisms at positions 141, 154, 171, or 172.

Prion Strains and Transmission Barrier Phenomena

Several experimental evidences show that prions are non-conventional pathogens, which physical support consists only in proteins. This finding raised questions regarding the observed prion strain-to-strain variations and the species barrier that happened to be crossed with dramatic consequences on human health and veterinary policies during the last 3 decades. This review presents a focus on a few advances in the field of prion structure and prion strains characterization: from the historical approaches that allowed the concept of prion strains to emerge, to the last results demonstrating that a prion strain may in fact be a combination of a few quasi species with subtle biophysical specificities. Then, we will focus on the current knowledge on the factors that impact species barrier strength and species barrier crossing. Finally, we present probable scenarios on how the interaction of strain properties with host characteristics may account for differential selection of new conformer variants and eventually species barrier crossing.

Reversible unfolding of infectious prion assemblies reveals the existence of an oligomeric elementary brick

Mammalian prions, the pathogens that cause transmissible spongiform encephalopathies, propagate by self-perpetuating the structural information stored in the abnormally folded, aggregated conformer (PrP^Sc) of the host-encoded prion protein (PrP^C). To date, no structural model related to prion assembly organization satisfactorily describes how strain-specified structural information is encoded and by which mechanism this information is transferred to PrP^C. To achieve progress on this issue, we correlated the PrP^Sc quaternary structural transition from three distinct prion strains during unfolding and refolding with their templating activity. We reveal the existence of a mesoscopic organization in PrP^Sc through the packing of a highly stable oligomeric elementary subunit (suPrP), in which the strain structural determinant (SSD) is encoded. Once kinetically trapped, this elementary subunit reversibly loses all replicative information. We demonstrate that acquisition of the templating interface and infectivity requires structural rearrangement of suPrP, in concert with its condensation. The existence of such an elementary brick scales down the SSD support to a small oligomer and provide a basis of reflexion for prion templating process and propagation.

Prions are self-propagating assemblies with all necessary and sufficient replicative information stored in the 3D structure of the misfolded form of PrP called PrP^Sc. Since the emergence of the prion theory in the 80s, many attempts have been done to identify prion replicative information at molecular scale. Different models have been constructed based on a broad panel of experimental observations and some of them predict the existence of periodic elements constituting prion assemblies. Here, by using partial unfolding approaches, we trapped an oligomeric conformer that we called suPrP, which could constitute the elementary brick of prion assemblies. Once isolated, this elementary brick is devoid of infectivity. However, it becomes infectious once condensated into larger assemblies. The identification of the elementary PrP building block provides a new structural basis for understanding prion replicative information storage and spreading.

A stretch of residues within the protease-resistant core is not necessary for prion structure and infectivity

Mapping out regions of PrP influencing prion conversion remains a challenging issue complicated by the lack of prion structure. The portion of PrP associated with infectivity contains the α-helical domain of the correctly folded protein and turns into a β-sheet-rich insoluble core in prions. Deletions performed so far inside this segment essentially prevented the conversion. Recently we found that deletion of the last C-terminal residues of the helix H2 was fully compatible with prion conversion in the RK13-ovPrP cell culture model, using 3 different infecting strains. This was in agreement with preservation of the overall PrP^C structure even after removal of up to one-third of this helix. Prions with internal deletion were infectious for cells and mice expressing the wild-type PrP and they retained prion strain-specific characteristics. We thus identified a piece of the prion domain that is neither necessary for the conformational transition of PrP^C nor for the formation of a stable prion structure.

Quantitating PrP Polymorphisms Present in Prions from Heterozygous Scrapie-Infected Sheep

Scrapie is a prion (PrP^Sc) disease of sheep. The incubation period of sheep scrapie is strongly influenced by polymorphisms at positions 136, 154, and 171 of a sheep's normal cellular prion protein (PrP^C). Chymotrypsin was used to digest sheep recombinant PrP to identify a set of characteristic peptides [M₁₃₂LGSXMSRPL₁₄₁ (X = A or V), Y₁₅₃XENMY₁₅₈ (X,= H or R), and Y₁₆₆RPVDXY₁₇₂ (X = H, K, Q, or R)] that could be used to detect and quantitate polymorphisms at positions 136, 154, and 171 of sheep PrP^C or PrP^Sc. These peptides were used to develop a multiple reaction monitoring method (MRM) to detect the amounts of a particular polymorphism in a sample of PrP^Sc isolated from sheep heterozygous for their PrP^C proteins. The limit of detection for these peptides was less than 50 attomole. Spinal cord tissue from heterozygous (ARQ/VRQ or ARH/ARQ) scrapie-infected Rasa Aragonesa sheep was analyzed using this MRM method. Both sets of heterozygotes show the presence of both polymorphisms in PrP^Sc. This was true for samples containing both proteinase K (PK)-sensitive and PK-resistant PrP^Sc and samples containing only the PK-resistant PrP^Sc. These results show that heterozygous animals contain PrP^Sc that is composed of significant amounts of both PrP polymorphisms.

The role of the unusual threonine string in the conversion of prion protein

The conversion of normal prion protein (PrP) into pathogenic PrP conformers is central to prion disease, but the mechanism remains unclear. The α-helix 2 of PrP contains a string of four threonines, which is unusual due to the high propensity of threonine to form β-sheets. This structural feature was proposed as the basis for initiating PrP conversion, but experimental results have been conflicting. We studied the role of the threonine string on PrP conversion by analyzing mouse Prnp^a and Prnp^b polymorphism that contains a polymorphic residue at the beginning of the threonine string, and PrP mutants in which threonine 191 was replaced by valine, alanine, or proline. The PMCA (protein misfolding cyclic amplification) assay was able to recapitulate the in vivo transmission barrier between PrP^a and PrP^b. Relative to PMCA, the amyloid fibril growth assay is less restrictive, but it did reflect certain properties of in vivo prion transmission. Our results suggest a plausible theory explaining the apparently contradictory results in the role of the threonine string in PrP conversion and provide novel insights into the complicated relationship among PrP stability, seeded conformational change, and prion structure, which is critical for understanding the molecular basis of prion infectivity.