Mechanical Anisotropy in GNNQQNY Amyloid Crystals

Dynamically Mapping the Topography and Stiffness of the Leading Edge of Migrating Cells Using AFM in Fast-QI Mode

Cell migration profoundly influences cellular function, often resulting in adverse effects in various pathologies including cancer metastasis. Directly assessing and quantifying the nanoscale dynamics of living cell structure and mechanics has remained a challenge. At the forefront of cell movement, the flat actin modules─the lamellipodium and the lamellum─interact to propel cell migration. The lamellipodium extends from the lamellum and undergoes rapid changes within seconds, making measurement of its stiffness a persistent hurdle. In this study, we introduce the fast-quantitative imaging (fast-QI) mode, demonstrating its capability to simultaneously map both the lamellipodium and the lamellum with enhanced spatiotemporal resolution compared with the classic quantitative imaging (QI) mode. Specifically, our findings reveal nanoscale stiffness gradients in the lamellipodium at the leading edge, where it appears to be slightly thinner and significantly softer than the lamellum. Additionally, we illustrate the fast-QI mode's accuracy in generating maps of height and effective stiffness through a streamlined and efficient processing of force-distance curves. These results underscore the potential of the fast-QI mode for investigating the role of motile cell structures in mechanosensing.

The Protein Folding Problem: The Role of Theory

The protein folding problem was first articulated as question of how order arose from disorder in proteins: How did the various native structures of proteins arise from interatomic driving forces encoded within their amino acid sequences, and how did they fold so fast? These matters have now been largely resolved by theory and statistical mechanics combined with experiments. There are general principles. Chain randomness is overcome by solvation-based codes. And in the needle-in-a-haystack metaphor, native states are found efficiently because protein haystacks (conformational ensembles) are funnel-shaped. Order-disorder theory has now grown to encompass a large swath of protein physical science across biology.

Evolution of Conformation, Nanomechanics, and Infrared Nanospectroscopy of Single Amyloid Fibrils Converting into Microcrystals

Nanomechanical properties of amyloid fibrils and nanocrystals depend on their secondary and quaternary structure, and the geometry of intermolecular hydrogen bonds. Advanced imaging methods based on atomic force microscopy (AFM) have unravelled the morphological and mechanical heterogeneity of amyloids, however a full understanding has been hampered by the limited resolution of conventional spectroscopic methods. Here, it is shown that single molecule nanomechanical mapping and infrared nanospectroscopy (AFM-IR) in combination with atomistic modelling enable unravelling at the single aggregate scale of the morphological, nanomechanical, chemical, and structural transition from amyloid fibrils to amyloid microcrystals in the hexapeptides, ILQINS, IFQINS, and TFQINS. Different morphologies have different Young's moduli, within 2-6 GPa, with amyloid fibrils exhibiting lower Young's moduli compared to amyloid microcrystals. The origins of this stiffening are unravelled and related to the increased content of intermolecular β-sheet and the increased lengthscale of cooperativity following the transition from twisted fibril to flat nanocrystal. Increased stiffness in Young's moduli is correlated with increased density of intermolecular hydrogen bonding and parallel β-sheet structure, which energetically stabilize crystals over the other polymorphs. These results offer additional evidence for the position of amyloid crystals in the minimum of the protein folding and aggregation landscape.

Atomic Force Microscopy Imaging and Nanomechanical Properties of Six Tau Isoform Assemblies

The amyloid fibrillar form of the protein Tau is involved in a number of neurodegenerative diseases, also known as tauopathies. In this work, six different fibrillar Tau isoforms were assembled in vitro. The morphological and nanomechanical properties of these isoforms were studied using atomic force microscopy at high resolution in air and buffer. Our results demonstrate that all Tau isoform fibrils exhibit paired-helical-filament-like structures consisting of two protofibrils separated by a shallow groove. Interestingly, whereas the N-terminal inserts do not contribute to any morphological or mechanical difference between the isoforms with the same carboxyl-terminal microtubule-binding domain repeats, isoforms with four microtubule repeats (4R) exhibited a persistence length ranging from 2.0 to 2.8 μm, almost twofold higher than those with three repeats (3R). In addition, the axial Young's modulus values derived from the persistence lengths, as well as their radial ones determined via nanoindentation experiments, were very low compared to amyloid fibrils made of other proteins. This sheds light on the weak intermolecular interaction acting between the paired β-sheets within Tau fibrils. This may play an important role in their association into high molecular weight assemblies, their dynamics, their persistence, their clearance in cells, and their propagation.

Quantifying the Mechanical Anisotropy in Poly(3-hexylthiophene) Nanofibers

Correlating the structure with nanomechanical property of semicrystalline conjugated-polymer crystal is of essential importance for the performance improvement and design of flexible electronic devices. Although it is well-known that the semicrystalline conjugated-polymer crystal exhibits anisotropic structure owing to the π-π and layer stacking of highly coplanar conjugated backbones, the structure-nanomechanical property relationship is missing. Here, we investigated the axial mechanical anisotropy of the P3HT nanofiber by using thermal shape-fluctuation analysis and a three-point bending test based on atomic force microscopy. Our results show that Young's modulus in the layer-stacking direction (E_L) is 1-2 orders of magnitude greater than that in the π-conjugated backbone direction (E_B). We attribute this mechanical anisotropy to the π-stacking of the P3HT backbone, but the layer stacking will decrease E_L, which weakens the mechanical anisotropy. Moreover, we demonstrated that the P3HT nanofiber shows a loading-rate-independent Young's modulus and deformation-dependent resilience in the layer-stacking direction.

Amyloid-like peptide nanofibrils as scaffolds for tissue engineering: Progress and challenges (Review)

Networks of amyloid-like nanofibrils assembled from short peptide sequences have the ability to form scaffolds that can encapsulate clinically relevant stem cells encouraging their attachment, growth, and differentiation into various lineages which can be used in tissue engineering applications to treat a range of diseases and traumas. In this review, the author highlights a selection of important proof-of-principle papers that show how this class of self-assembled networks is highly suited to biomaterial scaffold development. The author highlights recent studies which have shown that these scaffolds can be used to promote cell and tissue regeneration both in vitro and in vivo. The author also presents some fundamental knowledge gaps which are preventing the widespread translation of such scaffolds. Finally, the author outlines a selection of studies that elucidate molecular assembly mechanisms and biophysical properties of amyloid-like peptide nanofibrils and suggests how studies like these might lead to the ability to generate nanofibril scaffolds with bespoke properties for tissue engineering.

Inverse Correlation between Amyloid Stiffness and Size

We reveal that the axial stiffness of amyloid fibrils is inversely correlated with their cross-sectional area. Because amyloid fibrils' stiffness is determined by hydrogen bond (H-bond) density with a linear correlation, our finding implies that amyloid fibrils with larger radial sizes are generally softer and have lower density H-bond networks. In silico calculations show that the stiffness-size relationship of amyloid fibrils is, indeed, driven by the packing densities of residues and H-bonds. Our results suggest that polypeptide chains which form amyloid fibrils with narrow cross sections can optimize packing densities in the fibrillar core structure, in contrast to those forming wide amyloid fibrils. Consequently, the density of residues and H-bonds that contribute to mechanical stability is higher in amyloid fibrils with narrow cross sections. This size dependence of nanomechanics appears to be a global property of amyloid fibrils, just like the well-known cross-β sheet topology.