Structural details of amyloid ß oligomers in complex with human prion protein as revealed by solid-state MAS NMR spectroscopy.

Human PrP (huPrP) is a high-affinity receptor for oligomeric amyloid ß (Aß) protein aggregates. Binding of Aß oligomers to membrane-anchored huPrP has been suggested to trigger neurotoxic cell signaling in Alzheimer's disease, while an N-terminal soluble fragment of huPrP can sequester Aß oligomers and reduce their toxicity. Synthetic oligomeric Aß species are known to be heterogeneous, dynamic, and transient, rendering their structural investigation particularly challenging. Here, using huPrP to preserve Aß oligomers by coprecipitating them into large heteroassemblies, we investigated the conformations of Aß(1-42) oligomers and huPrP in the complex by solid-state MAS NMR spectroscopy. The disordered N-terminal region of huPrP becomes immobilized in the complex and therefore visible in dipolar spectra without adopting chemical shifts characteristic of a regular secondary structure. Most of the well-defined C-terminal part of huPrP is part of the rigid complex, and solid-state NMR spectra suggest a loss in regular secondary structure in the two C-terminal α-helices. For Aß(1-42) oligomers in complex with huPrP, secondary chemical shifts reveal substantial ß-strand content. Importantly, not all Aß(1-42) molecules within the complex have identical conformations. Comparison with the chemical shifts of synthetic Aß fibrils suggests that the Aß oligomer preparation represents a heterogeneous mixture of ß-strand-rich assemblies, of which some have the potential to evolve and elongate into different fibril polymorphs, reflecting a general propensity of Aß to adopt variable ß-strand-rich conformers. Taken together, our results reveal structural changes in huPrP upon binding to Aß oligomers that suggest a role of the C terminus of huPrP in cell signaling. Trapping Aß(1-42) oligomers by binding to huPrP has proved to be a useful tool for studying the structure of these highly heterogeneous ß-strand-rich assemblies.

A d-enantiomeric peptide interferes with heteroassociation of amyloid-ß oligomers and prion protein.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects millions of people worldwide. One AD hallmark is the aggregation of ß-amyloid (Aß) into soluble oligomers and insoluble fibrils. Several studies have reported that oligomers rather than fibrils are the most toxic species in AD progression. Aß oligomers bind with high affinity to membrane-associated prion protein (PrP), leading to toxic signaling across the cell membrane, which makes the Aß-PrP interaction an attractive therapeutic target. Here, probing this interaction in more detail, we found that both full-length, soluble human (hu) PrP(23-230) and huPrP(23-144), lacking the globular C-terminal domain, bind to Aß oligomers to form large complexes above the megadalton size range. Following purification by sucrose density-gradient ultracentrifugation, the Aß and huPrP contents in these heteroassemblies were quantified by reversed-phase HPLC. The Aß:PrP molar ratio in these assemblies exhibited some limited variation depending on the molar ratio of the initial mixture. Specifically, a molar ratio of about four Aß to one huPrP in the presence of an excess of huPrP(23-230) or huPrP(23-144) suggested that four Aß units are required to form one huPrP-binding site. Of note, an Aß-binding all-d-enantiomeric peptide, RD2D3, competed with huPrP for Aß oligomers and interfered with Aß-PrP heteroassembly in a concentration-dependent manner. Our results highlight the importance of multivalent epitopes on Aß oligomers for Aß-PrP interactions and have yielded an all-d-peptide-based, therapeutically promising agent that competes with PrP for these interactions.

Clustering of human prion protein and α-synuclein oligomers requires the prion protein N-terminus.

The interaction of prion protein (PrP) and α-synuclein (αSyn) oligomers causes synaptic impairment that might trigger Parkinson's disease and other synucleinopathies. Here, we report that αSyn oligomers (αSynO) cluster with human PrP (huPrP) into micron-sized condensates. Multivalency of αSyn within oligomers is required for condensation, since clustering with huPrP is not observed for monomeric αSyn. The stoichiometry of the heteroassemblies is well defined with an αSyn:huPrP molar ratio of about 1:1. The αSynO-huPrP interaction is of high affinity, signified by slow dissociation. The huPrP region responsible for condensation of αSynO, residues 95-111 in the intrinsically disordered N-terminus, corresponds to the region required for αSynO-mediated cognitive impairment. HuPrP, moreover, achieves co-clustering of αSynO and Alzheimer's disease-associated amyloid-ß oligomers, providing a case of a cross-interaction of two amyloidogenic proteins through an interlinking intrinsically disordered protein region. The results suggest that αSynO-mediated condensation of huPrP is involved in the pathogenesis of synucleinopathies.