Fibrillar seeds alleviate amyloid-ß cytotoxicity by omitting formation of higher-molecular-weight oligomers.

Amyloid-ß (Aß) peptides can exist in distinct forms including monomers, oligomers and fibrils, consisting of increased numbers of monomeric units. Among these, Aß oligomers are implicated as the primary toxic species as pointed by multiple lines of evidence. It has been suggested that toxicity could be rendered by the soluble higher-molecular-weight (high-n) Aß oligomers. Yet, the most culpable form in the pathogenesis of Alzheimer's disease (AD) remains elusive. Moreover, the potential interaction among the insoluble fibrils that have been excluded from the responsible aggregates in AD development, Aß monomers and high-n oligomers is undetermined. Here, we report that insoluble Aß fibrillar seeds can interact with Aß monomers at the stoichiometry of 1:2 (namely, each Aß molecule of seed can bind to two Aß monomers at a time) facilitating the fibrillization by omitting the otherwise mandatory formation of the toxic high-n oligomers during the fibril maturation. As a result, the addition of exogenous Aß fibrillar seeds is seen to rescue neuronal cells from Aß cytotoxicity presumably exerted by high-n oligomers, suggesting an unexpected protective role of Aß fibrillar seeds.

Prevention and promotion effects of apolipoprotein E4 on amylin aggregation.

The misfolding of islet amyloid polypeptide (IAPP, amylin) results in the formation of islet amyloid, which is one of the most common pathological features of type 2 diabetes (T2D). Amylin, a 37-amino-acid peptide co-secreted with insulin and apolipoprotein E (ApoE) from the beta-cells of pancreatic islets, is thought to be responsible for the reduced mass of insulin-producing beta-cells. However, neither the relationship between amylin and ApoE nor the biological consequence of amylin misfolding is known. Here we have characterized the interaction between ApoE4 and amylin in vitro. We found that ApoE4 can strongly bind to amylin, and insulin can hardly inhibit amylin-ApoE binding. We further found that amylin fibrillization can be prevented by low concentration of ApoE4 and promoted by high concentration of ApoE4. Taken together, we propose that under physiological conditions ApoE4 efficiently binds and sequesters amylin, preventing its aggregation, and in T2D the enhanced ApoE4-amylin binding leads to the critical accumulation of amylin, facilitating islet amyloid formation.

TiO2 nanoparticles promote beta-amyloid fibrillation in vitro.

Alzheimer's disease (AD) is the most common neurodegenerative disease in the world. The pathogenesis of AD is associated with beta-amyloid (Abeta) fibrillation. Nanoparticles have large surface area and can access the brain. But no investigation has been made to study the relationship between nanoparticles and AD. In our study, we observed Abeta fibril formation in the presence of six kinds of nanoparticles and found that TiO2 nanoparticles can promote Abeta fibrillation by shortening nucleation process, which is the key rate-determining step of fibrillation. Hereby the interaction between Abeta and nanoparticles may contribute to AD etiology.

Characterizing the assembly behaviors of human amylin: a perspective derived from C-terminal variants.

The differences in the C-terminal domains of human amylin peptide variants initiate different aggregation processes and differences in the composition of the aggregates by affecting the hydrophobic cores, conformations, and intra-sheet interactions of the peptides, which have distinct effects on the cytotoxicity of the peptides.

Influence of hydrophobicity on the surface-catalyzed assembly of the islet amyloid polypeptide.

The islet amyloid polypeptide (IAPP) is a hormonal factor secreted by the ß-cells in the pancreas. Aggregation of misfolded IAPP molecules and subsequent assembly of amyloid nanofibrils are critical for the development of type 2 diabetes mellitus. In the physiological environment, amyloid aggregation is affected by the presence of interfaces such as cell membranes. The physicochemical properties of the interface dictates the interaction of the peptide with the surface, i.e., electrostatic and hydrophobic interactions on hydrophilic and hydrophobic surfaces, respectively. We have studied the influence of hydrophobicity on the surface-catalyzed assembly of IAPP on ultrasmooth hydrocarbon films grown on ion-beam-modified mica surfaces by atomic force microscopy. The contact angle θ of these surfaces can be tuned continuously in the range from ≤20° to ∼90° by aging the samples without significant changes of the chemical composition or the topography of the surface. On hydrophilic surfaces with a θ of ∼20°, electrostatic interactions induce the assembly of IAPP nanofibrils, whereas aggregation of large (∼2.6 nm) oligomers is observed at hydrophobic surfaces with a θ of ∼90°. At intermediate contact angles, the interplay between electrostatic and hydrophobic substrate interactions dictates the pathway of aggregation with fibrillation getting continuously delayed when the contact angle is increased. In addition, the morphology of the formed protofibrils and mature fibrils at intermediate contact angles differs from those observed at more hydrophilic surfaces. These results might contribute to the understanding of the surface-catalyzed assembly of different amyloid aggregates and may also have implications for the technologically relevant controlled synthesis of amyloid nanofibrils of desired morphology.

Copper-induced cytotoxicity: reactive oxygen species or islet amyloid polypeptide oligomer formation.

Copper enhances amyloid cytotoxicity and mediates human islet amyloid polypeptide (hIAPP) oligomerization; nickel, a redox inactive metal with similar protein binding affinity to copper, also mimics this effect, thereby demonstrating copper-mediated hIAPP cytotoxicity is due mainly to granular oligomer generation rather than ROS accumulation in type 2 diabetes.

Cyclen-hybrid compound captures copper to protect INS-1 cells from islet amyloid polypeptide cytotoxicity by inhibiting and lysing effects.

Human islet amyloid polypeptide (hIAPP) deposit is the hallmark of type 2 diabetes pathology. Here, we report that apo-cyclen, attached to a specific hIAPP recognition motif (NYGAIL), captured copper ions and became proteolytically active. This cyclen-NYGAIL-copper complex was able to interfere with hIAPP aggregation and cleave hIAPP. These activities rescued INS-1 cells from hIAPP induced cytotoxicity.

Insulin is a kinetic but not a thermodynamic inhibitor of amylin aggregation.

One of the most important pathological features of type 2 diabetes is the formation of islet amyloid, of which the major component is amylin peptide. However, the presence of a natural inhibitor such as insulin may keep amylin stable and physiologically functional in healthy individuals. Some previous studies demonstrated that insulin was a potent inhibitor of amylin fibril formation in vitro, but others obtained contradictory results. Hence, it is necessary to elucidate the effects of insulin on amylin aggregation. Here we report that insulin is a kinetic inhibitor of amylin aggregation, only keeping its inhibitory effect for a limited time period. Actually, insulin promotes amylin aggregation after long-term incubation. Furthermore, we found that this promotional effect could be attributed to the copolymerization of insulin and amylin. We also found that insulin copolymerized with amylin monomer or oligomer rather than preformed amylin fibrils. These results suggest that the interaction between insulin and amylin may contribute not only to the inhibition of amylin aggregation but also to the coaggregation of both peptides in type 2 diabetes.

Hybrid peptides attenuate cytotoxicity of beta-amyloid by inhibiting its oligomerization: implication from solvent effects.

Abnormal assembly of monomeric beta-amyloid (Abeta) in Alzheimer's disease leads to the formation of most neurotoxic oligomers in vivo. In this study, we explored a linking strategy to design hybrid peptides, by combining the Abeta recognition motif and the solvent disruptive sequences. We found that in vitro all synthetic peptides with the recognition motif can affect Abeta fibrillization and alter the morphology of Abeta aggregates variously, different from those without the recognition motif. The effects of peptides containing recognition motif on Abeta aggregation correlate with their abilities to change the surface tension of solutions. In addition, compounds with the recognition motif, not those without such motif, can inhibit cytotoxicity of Abeta in cell culture probably by decreasing the amount of toxic Abeta oligomers. These results indicate that recognition domain and solvent effect should be considered as important factors when designing molecules to target Abeta aggregation.