The Aβ peptides form self-assembled fibrillar structures in Alzheimer's disease. The Aβ25-35 peptide used in our experiments is the biologically active, toxic fragment of the full-length Aβ peptide. We constructed a highly ordered, stable network from Aβ25-35 peptides showing trigonal orientation on mica. The same peptide can assemble itself into amyloid fibrils in solution as well. We mapped the morphological and nanomechanical features of the oriented network used later as an experimental model system.
We were the first to compare the properties (morphology, nanomechanics and secondary structure) of epitaxially-grown and solution-grown amyloid fibrils by means of AFM, force spectroscopy and FTIR spectroscopy. Both types of fibrils displayed underlying fibrillar morphology. Nevertheless, in contrast to the homogeneity structure of epitaxially-grown fibrils, solution-grown fibrils showed higher structural hierarchy, polymorphism and topographical periodicity. Epitaxially grown fibrils formed linear, sheet-like structures. The fibrils differed in their growth kinetics: while fibril assembly in solution occurred on a time scale of hours to days, on mica surface fibrils appeared within a few minutes. The mica surface might act as a catalyzer accelerating the process of fibrillogenesis. Because of the accelerated fibrillogenesis, the effect of physical and chemical parameters on amyloid fibrillogenesis can be easily explored. Both types of fibrils displayed identical nanomechanical responses in form of force plateaus; the elementary force plateau was ∼30 pN. Both types of fibrils displayed an underlying β-sheet structure. The FTIR spectra also showed that fibrils assembled in solution have a more compact structure. We demonstrated that the epitaxially grown oriented network be considered as a good amyloid-fibril model.
The stable, ordered network of Aβ25-35 fibrils on mica is a simple, reproducible system on the nanoscale. Studying on this model the effect of different factors interfering with fibrillogenesis, the changes of structural, nanomechanical properties of the Ab25-35 fibril network can be easily monitored by AFM. The effect of the BSB peptide LPFFD was investigated in an amyloid model system of oriented Aβ25-35 fibrils on mica. From force spectroscopy measurements we concluded that the LPFFD peptide may weaken the interaction between protofilaments. Our results indicate that the LPFFD peptide does not have a generalized beta-sheet-breaking effect in the amyloid assembly system utilized in the present experiment.
By using mutant Aβ25-35 peptides containing amino acid residues with specific chemical reactivity the labeling of the oriented network with various biomolecules or conducting metals can be accomplished. For future applications the fine-tuning of the network's properties (e.g., average fibril length, number of branching points) was needed. We succeded in fine-tuning the properties of the mutant network by varying the cation-concentration. We demonstrated that the sulfhydryl group of Cys27 within the oriented fibril network is indeed exposed to the solution and is chemically accessible and reactive. We therefore demonstrated that the properties of the mutant Aβ25-35 network may provide important tools for future nanotechnological applications
