Structural Insights into the Inhibition of Amyloid Fibril Formation by Fibrinogen via Interaction with Prefibrillar Intermediates

Tracking the Structural Development of Amyloid Precursors in the Insulin B Chain and the Inhibition Effect by Fibrinogen

Amyloid fibrils are abnormal protein aggregates associated with several amyloidoses and neurodegenerative diseases. Prefibrillar intermediates, which emerge before amyloid fibril formation, play an important role in structure formation. Therefore, to prevent fibril formation, the mechanisms underpinning the structural development of prefibrillar intermediates must be elucidated. An insulin-derived peptide, the insulin B chain, is known for its stable accumulation of prefibrillar intermediates. In this study, the structural development of B chain prefibrillar intermediates and their inhibition by fibrinogen (Fg) were monitored by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) combined with solid-state nuclear magnetic resonance spectroscopy (NMR) and size exclusion chromatography. TEM images obtained in a time-lapse manner demonstrated that prefibrillar intermediates were wavy rod-like structures emerging from initial non-rod-like aggregates, and their bundling was responsible for protofilament formation. Time-resolved SAXS revealed that the prefibrillar intermediates became thicker and longer as a function of time. Solid-state NMR measurement suggested a β-sheet formation around Ala14 residue was crucial for the structural conversion from prefibrillar intermediates to amyloid fibril. These observations suggested that prefibrillar intermediates serve as reaction fields for amyloid nucleation and its structural propagation. Time-resolved SAXS also demonstrated that Fg prevented elongation of the prefibrillar intermediates by forming specific complexes together, which implied that regulation of the length of prefibrillar intermediates upon Fg binding was the factor suppressing the prefibrillar intermediate elongation. The fibril formation mechanism and the inhibition strategy found in this study will be helpful in seeking appropriate methods against amyloid-related diseases.

Polyisobutylene-based glycopolymers as potent inhibitors for in vitro insulin aggregation

A family of amphiphilic diblock copolymers containing a hydrophobic polyisobutylene (PIB, M_n = 1000 g mol^-1) segment and a hydrophilic block with sugar pendants has been synthesized by combining living cationic and reversible addition-fragmentation chain transfer (RAFT) polymerization techniques; to explore their potential in insulin fibrillation inhibition. The glucose content in the hydrophilic segment has been tailor-made from 20 to 57 units to prepare block copolymers. The removal of the acetates from the pendent glucose units resulted in amphiphilic block copolymers that generated micellar aggregates in aqueous media. The treatment of insulin with these block copolymers affected the fibril formation process which was demonstrated using an array of biophysical techniques, namely, thioflavin T (ThT) fluorescence, tyrosine (Tyr) fluorescence, Nile red (NR) fluorescence, isothermal titration calorimetry (ITC), etc. The Tyr fluorescence assay and NR fluorescence study revealed the crucial role of hydrophobic interaction in the inhibition process, whereas ITC measurements confirmed the importance of polar interaction. Thus, the block copolymers exhibit potent inhibition of insulin fibrillation owing to hydrophobic (from PIB segment) and glycosidic cluster effect (from sugar pendant block).

Molecular Mechanisms of Inhibition of Protein Amyloid Fibril Formation: Evidence and Perspectives Based on Kinetic Models

Inhibition of fibril formation is considered a possible treatment strategy for amyloid-related diseases. Understanding the molecular nature of inhibitor action is crucial for the design of drug candidates. In the present review, we describe the common kinetic models of fibril formation and classify known inhibitors by the mechanism of their interactions with the aggregating protein and its oligomers. This mechanism determines the step or steps of the aggregation process that become inhibited and the observed changes in kinetics and equilibrium of fibril formation. The results of numerous studies indicate that possible approaches to antiamyloid inhibitor discovery include the search for the strong binders of protein monomers, cappers blocking the ends of the growing fibril, or the species absorbing on the surface of oligomers preventing nucleation. Strongly binding inhibitors stabilizing the native state can be promising for the structured proteins while designing the drug candidates targeting disordered proteins is challenging.

Pathway Dependence of the Formation and Development of Prefibrillar Aggregates in Insulin B Chain

Amyloid fibrils have been an important subject as they are involved in the development of many amyloidoses and neurodegenerative diseases. The formation of amyloid fibrils is typically initiated by nucleation, whereas its exact mechanisms are largely unknown. With this situation, we have previously identified prefibrillar aggregates in the formation of insulin B chain amyloid fibrils, which have provided an insight into the mechanisms of protein assembly involved in nucleation. Here, we have investigated the formation of insulin B chain amyloid fibrils under different pH conditions to better understand amyloid nucleation mediated by prefibrillar aggregates. The B chain showed strong propensity to form amyloid fibrils over a wide pH range, and prefibrillar aggregates were formed under all examined conditions. In particular, different structures of amyloid fibrils were found at pH 5.2 and pH 8.7, making it possible to compare different pathways. Detailed investigations at pH 5.2 in comparison with those at pH 8.7 have suggested that the evolution of protofibril-like aggregates is a common mechanism. In addition, different processes of evolution of the prefibrillar aggregates have also been identified, suggesting that the nucleation processes diversify depending on the polymorphism of amyloid fibrils.

Multistep growth of amyloid intermediates and its inhibition toward exploring therapeutic way: A case study using insulin B chain and fibrinogen

It is crucial to understand the mechanism of amyloid fibril formation for the development of the therapeutic ways against amyloidoses and neurodegenerative diseases. Prefibrillar intermediates, which emerge prior to the fibril formation, seem to play a key role to the occurrence of nuclei of amyloid fibrils. We have focused on an insulin-derived peptide, B chain, to precisely clarify the mechanism of the fibril formation via prefibrillar intermediates. Various kinds of methods such as circular dichroism spectroscopy, dynamic light scattering, small-angle X-ray scattering, and atomic force microscopy were employed to track the structural changes in prefibrillar intermediates. The prefibrillar intermediates possessing rod-shaped structures elongated as a function of time, which led to fibril formation. We have also found that a blood clotting protein, fibrinogen, inhibits the amyloid fibril formation of B chain. This was caused by the stabilization of prefibrillar intermediates and thus the suppression of their elongation by fibrinogen. These findings have not only shed light on detailed mechanisms about how prefibrillar intermediates convert to the amyloid fibril, but also demonstrated that inhibiting the structural development of prefibrillar intermediates is an effective strategy to develop therapeutic ways against amyloid-related diseases. This review article is an extended version of the Japanese article, Observing Development of Amyloid Prefibrillar Intermediates and their Interaction with Chaperones for Inhibiting the Fibril Formation, published in SEIBUTSU BUTSURI Vol. 61, p. 236–239 (2021).

Human Fibrinogen Inhibits Amyloid Assembly of Most Phenol-Soluble Modulins from Staphylococcus aureus

Functional amyloids are highly organized protein/peptide structures that inter alia promote biofilm formation in different bacteria. One such example is provided by a family of 20-45 residue-long peptides called phenol-soluble modulins (PSMs) from Staphylococcus aureus. External components such as eukaryotic host proteins, which alter self-assembly of bacterial amyloids, can affect the biofilm matrix. Here, we studied the effect of the highly prevalent human plasma protein fibrinogen (Fg) on fibrillation of PSMs. Fg inhibits or suppresses fibrillation of most PSMs tested (PSMα1, PSMβ1, and PSMβ2) except for PSMα3, whose already rapid aggregation is accelerated even further by Fg but leads to amorphous β-rich aggregates rather than fibrils. Fg also induces PSMβ2 to form amorphous aggregates and diverts PSMα1 into off-pathway oligomers which consist of both Fg and PSMα1 and cannot seed fibrillation. Peptide arrays showed that Fg bound to the N-terminus of PSMα1, while it bound to the entire length of PSMα3 (except the C terminus) and to the C-termini of PSMβ1 and PSMβ2. The latter peptides are all positively charged, while Fg is negatively charged at physiological pH. The positive charges complement Fg's net negative charge of -7.6 at pH 7.4. Fg's ability to inhibit PSM fibrillation reveals a potential host-defense mechanism to prevent bacterial biofilm growth and infections in the human body.