Structural plasticity of the cellular prion protein and implications in health and disease

Abstract

Two lines of transgenic mice expressing mouse/elk and mouse/ horse prion protein (PrP) hybrids, which both form a well-structured β2-α2 loop in the NMR structures at 20°C termed rigid-loop cellular prion proteins (RL-PrP C ), presented with accumulation of the aggregated scrapie form of PrP in brain tissue, and the mouse/ elk hybrid has also been shown to develop a spontaneous transmissible spongiform encephalopathy. Independently, there is in vitro evidence for correlations between the amino acid sequence in the β2-α2 loop and the propensity for conformational transitions to disease-related forms of PrP. To further contribute to the structural basis for these observations, this paper presents a detailed characterization of RL-PrP C conformations in solution. A dynamic local conformational polymorphism involving the β2-α2 loop was found to be evolutionarily preserved among all mammalian species, including those species for which the WT PrP forms an RL-PrP C . The interconversion between two ensembles of PrP C conformers that contain, respectively, a 3 10 -helix turn or a type I β-turn structure of the β2-α2 loop, exposes two different surface epitopes, which are analyzed for their possible roles in the still evasive function of PrP C in healthy organisms and/or at the onset of a transmissible spongiform encephalopathy. Creutzfeldt-Jakob disease in humans, scrapie in sheep and goats, bovine spongiform encephalopathy in cattle, and chronic wasting disease (CWD) in elk and deer (1-3). A common feature of these diseases is the conversion of the cellular form of the prion protein (PrP C ) found in healthy organisms into aggregated isoforms (PrP Sc ), which are deposited primarily in the brain of the diseased individuals (1, 4). Despite extensive investigations, the physiological function of PrP C in healthy organisms as well as the mechanistic aspects of its pathophysiological role remain elusive (4-9). Although the PrP Sc form found in diseased tissue has been intensively studied, other approaches underline the importance of PrP C as a potential target for TSE prevention and medical intervention after outbreak of the disease (10-12), with a special focus on rigid-loop cellular prion proteins (RL-PrP C s) (11,(13)(14)(15), which are investigated in this paper.A common PrP C fold for a globular domain formed by the polypeptide segment of residues 125-228 in mouse PrP (mPrP) [see Schätzl et al. (16) for the numeration in different species], with three α-helices and a short two-stranded antiparallel β-sheet, has been observed for the cellular prion proteins of all mammalian species studied so far (17-27). For WT PrP of most species, parts of the backbone amide group NMR signals of residues in a loop between a β-strand, β2, and a helix, α2, are not observable in NMR spectra recorded with aqueous solutions at pH 4.5 and 20°C at a 1 H resonance frequency of 500 MHz (or higher) because of line broadening by conformational exchange; therefore, the β2-α2 loop in these PrP C s is poorly defined in NMR structures determined un...

Prion Protein—Antibody Complexes Characterized by Chromatography-Coupled Small-Angle X-Ray Scattering

“…The interaction of Fab-R1 with both isoforms of PrP C and PrP Sc might sterically inhibit a crucial interaction between PrP C and PrP Sc . In addition to the known Fab-R1 epitope of PrP C , our structural model revealed putative contacts between Fab-R1 and PrP residues 165-175, the a2-b2 loop of PrP C (41)(42)(43). This loop region in murine PrP exists in two structural states, a major state that forms a 3 10 -helical turn and a minor state that forms a type-I b-turn (41,43) (41)(42)(43).…”

“…Specifically, Fab-P binds to a region that is affected by structural reorganization (residues 95-105), and thus may prevent the PrP C -to-PrP Sc transition. Our model of the recPrP-Fab-R1 complex identified an additional interaction site (residues 165-175) that is known to influence the misfolding of PrP (41)(42)(43); R1 binding to this site may inhibit misfolding of PrP.…”

A Proposed Mechanism for the Promotion of Prion Conversion Involving a Strictly Conserved Tyrosine Residue in the β2-α2 Loop of PrPC

“…The strictly conserved Tyr-169 has a key role in maintaining the 3 10 -helical turn in the ␤2-␣2 loop, and the Y169G substitution results in the loss of a -stacking interaction with 175F and a switch to a type I ␤-turn, forming a well defined loop (35,50,51). The new loop orientation may obstruct PrP C -PrP Sc interactions, preventing efficient binding and conversion.…”

Negative Purifying Selection Drives Prion and Doppel Protein Evolution

“…Selection against changes that could favor the pathogenic conformations would be advantageous. Existing polymorphisms in prion genes can lead to a higher probability of seeding conversion to PrP Sc (Christen et al 2013). It has also been suggested that variants conferring susceptibility to TSEs are in generally conserved regions and that such variation is generally derived (Martin et al 2009).…”