Ionic Strength Modulation of the Free Energy Landscape of Aβ₄₀ Peptide Fibril Formation

Abstract

Protein misfolding and formation of cross-β structured amyloid fibrils are linked to many neurodegenerative disorders. Although recently developed quantitative approaches have started to reveal the molecular nature of self-assembly and fibril formation of proteins and peptides, it is yet unclear how these self-organization events are precisely modulated by microenvironmental factors, which are known to strongly affect the macroscopic aggregation properties. Here, we characterize the explicit effect of ionic strength on the microscopic aggregation rates of amyloid β peptide (Aβ40) self-association, implicated in Alzheimer's disease. We found that physiological ionic strength accelerates Aβ40 aggregation kinetics by promoting surface-catalyzed secondary nucleation reactions. This promoted catalytic effect can be assigned to shielding of electrostatic repulsion between monomers on the fibril surface or between the fibril surface itself and monomeric peptides. Furthermore, we observe the formation of two different β-structured states with similar but distinct spectroscopic features, which can be assigned to an off-pathway immature state (Fβ*) and a mature stable state (Fβ), where salt favors formation of the Fβ fibril morphology. Addition of salt to preformed Fβ* accelerates transition to Fβ, underlining the dynamic nature of Aβ40 fibrils in solution. On the basis of these results we suggest a model where salt decreases the free-energy barrier for Aβ40 folding to the Fβ state, favoring the buildup of the mature fibril morphology while omitting competing, energetically less favorable structural states.

A spidroin‐derived solubility tag enables controlled aggregation of a designed amyloid protein

“…We suggest that the high calcium concentration in the ER can promote very fast fibril formation of β17 and thereby reduce its ability to form toxic intermediates. Our results are in accordance with the reports showing that Aβ1‐40 and insulin aggregations are promoted by various salts, and α‐synuclein aggregation is accelerated by calcium . The mechanisms by which salts promote aggregation are not well understood.…”

Secondary nucleation in amyloid formation

“…These changes can take the form of a change in pH 95 or an increase in ionic strength. 93,94 In addition to increasing the overall rate of aggregation, these changes often also affect the balance of the individual steps in multistep processes, such as secondary nucleation. As secondary nucleation involves an attachment step, followed by a rearrangement/detachment step, decrease in charge repulsion often speeds up the initial attachment step more than the subsequent steps, thus leading to a saturation of secondary nucleation.…”

Ligand–Receptor Interaction Modulates the Energy Landscape of Enzyme-Instructed Self-Assembly of Small Molecules

“…51 However, control over peptide concentration and initial state remains difficult. 51 While it remains to prove that ligand-receptor interactions may modulate the aggregation of Aβ 50 this study on how interaction with a ligand significantly alters supramolecular assembly of small molecules should provide useful insights. Because the formation of Aβ results from enzymatic reactions, 64–65 the study of ligand-receptor interactions to modulate EISA of small molecules is more relevant to the disease condition than using hexafluoroisopropanol (HFIP) or dimethylsulfoxide (DMSO) to generate Aβ amyloids.…”