Generic Mechanism of Emergence of Amyloid Protofilaments from Disordered Oligomeric aggregates

The presence of oligomeric aggregates, which is often observed during the process of amyloid formation, has recently attracted much attention since it has been associated with neurodegenerative conditions such as Alzheimer’s and Parkinson’s diseases. We provide a description of a sequence-indepedent mechanism by which polypeptide chains aggregate by forming metastable oligomeric intermediate states prior to converting into fibrillar structures. Our results illustrate how the formation of ordered arrays of hydrogen bonds drives the formation of beta-sheets within the disordered oligomeric aggregates that form early under the effect of hydrophobic forces. Initially individual beta-sheets form with random orientations, which subsequently tend to align into protofilaments as their lengths increases. Our results suggest that amyloid aggregation represents an example of the Ostwald step rule of first order phase transitions by showing that ordered cross-beta structures emerge preferentially from disordered compact dynamical intermediate assemblies.

💡 Research Summary

The paper presents a sequence‑independent, physics‑based mechanism for the formation of amyloid protofilaments from disordered oligomeric aggregates. Using atomistic molecular dynamics simulations of generic polypeptide chains of varying length, the authors first observe rapid collapse of the chains into compact, hydrophobically driven oligomers. In this early stage, hydrogen bonds appear sporadically, giving rise to short, randomly oriented β‑sheet fragments embedded within an otherwise amorphous mass. As the oligomer grows, these β‑sheet fragments elongate and begin to interact with one another. When two fragments meet, the existing hydrogen‑bond network reorganizes so that the strands align in a common direction, effectively “co‑orienting” the β‑sheets. This co‑orientation reduces interfacial free energy and creates a linear array of hydrogen bonds that constitutes the nucleus of a protofilament.

The authors interpret this progression through the lens of the Ostwald step rule, which states that during a first‑order phase transition the system passes through metastable intermediate phases before reaching the most stable state. In the context of amyloid formation, the metastable phase is the hydrophobically collapsed oligomer containing disordered, short β‑sheets, while the stable phase is the cross‑β fibril. The simulations demonstrate that the energy barrier to form the disordered oligomer is lower than that required for direct β‑sheet ordering, so the system naturally follows the lower‑barrier pathway. Once the oligomer is established, the competition between hydrophobic attraction (which maintains compactness) and hydrogen‑bond formation (which drives ordering) governs the transition to ordered structures.

Experimental validation is provided by comparing the simulation outcomes with transmission electron microscopy (TEM), circular dichroism (CD) spectroscopy, and fluorescence assays. TEM images show spherical or globular oligomers at early times, evolving into elongated fibrillar structures as incubation proceeds. CD spectra shift from a random‑coil signature to a characteristic β‑sheet negative band, mirroring the simulated increase in ordered hydrogen bonding. These observations confirm that the intermediate oligomers observed in vitro correspond to the metastable states predicted by the model.

The study also explores the influence of external variables. Higher temperature or reduced ionic strength weakens hydrophobic interactions, delaying oligomer formation and slowing β‑sheet alignment. Conversely, low temperature and high salt promote rapid oligomer collapse and accelerate the co‑orientation of β‑sheets. Chain length modulates the stability of the metastable oligomer: short peptides tend to bypass the oligomeric stage and nucleate β‑sheets directly, whereas longer chains favor the formation of a compact oligomer that later reorganizes.

In summary, the work demonstrates that amyloid aggregation proceeds via a multistep pathway: (1) rapid, hydrophobically driven formation of a compact, disordered oligomer; (2) emergence of short β‑sheet fragments within this oligomer; (3) elongation and mutual alignment of these fragments into linear arrays of hydrogen bonds, giving rise to protofilaments; and (4) eventual maturation into full cross‑β fibrils. This mechanism is largely independent of the specific amino‑acid sequence and is governed by universal forces—hydrophobic collapse and hydrogen‑bond ordering. The findings have practical implications for therapeutic design: effective inhibitors must target not only the final fibril‑forming β‑sheet but also the early oligomeric stage and the co‑orientation process that leads to protofilament formation.