Metal ion coordination delays amyloid-β peptide self-assembly by forming an aggregation–inert complex

Abstract

A detailed understanding of the molecular pathways for amyloid-β (Aβ) peptide aggregation from monomers into amyloid fibrils, a hallmark of Alzheimer's disease, is crucial for the development of diagnostic and therapeutic strategies. We investigate the molecular details of peptide fibrillization in vitro by perturbing this process through addition of differently charged metal ions. Here, we used a monovalent probe, the silver ion, that, similarly to divalent metal ions, binds to monomeric Aβ peptide and efficiently modulates Aβ fibrillization. On the basis of our findings, combined with our previous results on divalent zinc ions, we propose a model that links the microscopic metal-ion binding to Aβ monomers to its macroscopic impact on the peptide self-assembly observed in bulk experiments. We found that substoichiometric concentrations of the investigated metal ions bind specifically to the N-terminal region of Aβ, forming a dynamic, partially compact complex. The metal-ion bound state appears to be incapable of aggregation, effectively reducing the available monomeric Aβ pool for incorporation into fibrils. This is especially reflected in a decreased fibril-end elongation rate. However, because the bound state is significantly less stable than the amyloid state, Aβ peptides are only transiently redirected from fibril formation, and eventually almost all Aβ monomers are integrated into fibrils. Taken together, these findings unravel the mechanistic consequences of delaying Aβ aggregation via weak metal-ion binding, quantitatively linking the contributions of specific interactions of metal ions with monomeric Aβ to their effects on bulk aggregation.

The amyloid-inhibiting NCAM-PrP peptide targets Aβ peptide aggregation in membrane-mimetic environments

“…However, the reason for the loss of signal due to increasing concentrations of NCAM-PrP is not fully clear. We performed CPMG relaxation dispersion experiments ( Carr and Purcell 1954 ; Meiboom and Gill 1958 ; Tollinger et al, 2001 ; Wallin et al, 2020 ) to be able to detect chemical exchange effects between Aβ and Aβ:NCAM-PrP complexes ( Figure S8 ). No exchange was detected in the relaxation dispersion profiles.…”

“…At physiological pH, the net charge of the Aβ peptide is about −2.7, whereas the NCAM-PrP peptide is relatively highly positively charged (+6). Attenuation of Aβ amyloid formation by interactions with positively charged proteins and molecules are common ( Assarsson et al, 2014 ; Wallin et al, 2018 , Wallin et al, 2020 ; Österlund et al, 2018 ). The NCAM-PrP inhibitory effect on Aβ fibrillization was neither affected by an increase in ionic strength nor in the presence of seeds ( Figure 4 ).…”

“…Prior to all ThT amyloid aggregation kinetics experiments, the Aβ peptides, after sonication in an ice water bath for 1-3 min in 10 mM NaOH, were subject to one further purification step using size exclusion chromatography (SEC) using a Superdex 75 10/300 GL column ( GE Healthcare , USA), to remove aggregated peptides from the monomeric peptides according to previously published protocols ( Wallin et al, 2020 ). Prepared Aβ 40 (12 μM) and Aβ 42 peptide (5 μM) solutions were supplemented with ThT (40 μM for Aβ 40 and 10 μM for Aβ 42 ) in 20 mM phosphate buffer (pH 7.4 for Aβ 40 and pH 8 for Aβ 42 ) and used for the aggregation kinetics measurements.…”

“…15 N-Carr-Purcell-Meiboom-Gill (CPMG) relaxation dispersion experiments ( Carr and Purcell 1954 ; Meiboom and Gill 1958 ; Tollinger et al, 2001 ; Wallin et al, 2020 ) were performed to study exchange dynamics on the ms-μs timescale of Aβ 40 upon addition of NCAM-PrP peptide. The relaxation dispersion experiments were performed on the sample containing 70 μM 15 N-Aβ 40 peptides, 30 μM NCAM-PrP in 50 mM NaP buffer pH 7.3.…”

An Overview of Several Inhibitors for Alzheimer’s Disease: Characterization and Failure

“…Wallin et al [ 117 ] used Fluorescence spectroscopy, solid-state AFM imaging, circular dichroism spectroscopy, and NMR spectroscopy to probe the Aβ fibrillization pathways in the presence of silver (Ag + ) metal ions. The Aβ peptide from the fibril incorporates other monomeric Aβ to elongate the fibril end.…”