Cell membranes are typically very complex, consisting of a multitude of different lipids and proteins. Supported lipid bilayers are widely used as model systems to study biological membranes. Atomic force microscopy and force spectroscopy techniques are nanoscale methods that are successfully used to study supported lipid bilayers. These methods, especially force spectroscopy, require the reliable preparation of supported lipid bilayers with extended coverage. The unreliability and a lack of a complete understanding of the vesicle fusion process though have held back progress in this promising field. We document here robust protocols for the formation of fluid phase DOPC and gel phase DPPC bilayers on mica. Insights into the most crucial experimental parameters and a comparison between DOPC and DPPC preparation are presented. Finally, we demonstrate force spectroscopy measurements on DOPC surfaces and measure rupture forces and bilayer depths that agree well with X-ray diffraction data. We also believe our approach to decomposing the force-distance curves into depth sub-components provides a more reliable method for characterising the depth of fluid phase lipid bilayers, particularly in comparison with typical image analysis approaches.

Using atomic force microscopy (AFM) we investigated the interaction of amyloid beta (Aβ) (1–42) peptide with chemically modified surfaces in order to better understand the mechanism of amyloid toxicity, which involves interaction of amyloid with cell membrane surfaces. We compared the structure and density of Aβ fibrils on positively and negatively charged as well as hydrophobic chemically-modified surfaces at physiologically relevant conditions. We report that due to the complex distribution of charge and hydrophobicity amyloid oligomers bind to all types of surfaces investigated (CH₃, COOH, and NH₂) although the charge and hydrophobicity of surfaces affected the structure and size of amyloid deposits as well as surface coverage. Hydrophobic surfaces promote formation of spherical amorphous clusters, while charged surfaces promote protofibril formation. We used the nonlinear Poisson-Boltzmann equation (PBE) approach to analyze the electrostatic interactions of amyloid monomers and oligomers with modified surfaces to complement our AFM data.

Metal ions, including copper and zinc, have been implicated in the pathogenesis of Alzheimer’s disease through a variety of mechanisms including increased amyloid-β affinity and redox effects. Recent reports have demonstrated that the amyloid-β monomer does not necessarily travel through a definitive intermediary en-route to a stable amyloid fibril structure. Rather, amyloid-β misfolding may follow a variety of pathways resulting in a fibrillar end-product or a variety of oligomeric end-products with a diversity of structures and sizes. The presence of metal ions has been demonstrated to alter the kinetic pathway of the amyloid-β peptide which may lead to more toxic oligomeric end-products. In this work, we review the contemporary literature supporting the hypothesis that metal ions alter the reaction pathway of amyloid-β misfolding leading to more neurotoxic species.

The link between metals, Alzheimer's disease (AD) and its implicated protein, amyloid-β (Aβ), is complex and highly studied. AD is believed to occur as a result of the misfolding and aggregation of Aβ. The dyshomeostasis of metal ions and their propensity to interact with Aβ has also been implicated in AD. In this work, we use single molecule atomic force spectroscopy to measure the rupture force required to dissociate two Aβ (1–42) peptides in the presence of copper ions, Cu²⁺. In addition, we use atomic force microscopy to resolve the aggregation of Aβ formed. Previous research has shown that metal ions decrease the lag time associated with Aβ aggregation. We show that with the addition of copper ions the unbinding force increases notably. This suggests that the reduction of lag time associated with Aβ aggregation occurs on a single molecule level as a result of an increase in binding forces during the very initial interactions between two Aβ peptides. We attribute these results to copper ions acting as a bridge between the two peptide molecules, increasing the stability of the peptide-peptide complex.

Amyloid-beta peptides (Aβ), implicated in Alzheimer’s disease (AD), interact with the cellular membrane and induce amyloid toxicity. The composition of cellular membranes changes in aging and AD. We designed multi-component lipid models to mimic healthy and diseased states of the neuronal membrane. Using atomic force microscopy (AFM), Kelvin probe force microscopy (KPFM) and black lipid membrane (BLM) techniques, we demonstrated that these model membranes differ in their nanoscale structure and physical properties, and interact differently with Aβ_1–42. Based on our data, we propose a new hypothesis that changes in lipid membrane due to aging and AD may trigger amyloid toxicity through electrostatic mechanisms, similar to the accepted mechanism of antimicrobial peptide action. Understanding the role of the membrane changes as a key activating amyloid toxicity may aid in the development of a new avenue for the prevention and treatment of AD.

Amyloid fibril formation is generally associated with many neurodegenerative disorders, including Alzheimer's disease (AD). Although fibril plaque formation is associated with biological membranes in vivo, the role of the cell surfaces in amyloid fibril formation and the molecular mechanism of amyloid toxicity are not well understood. Understanding the details of amyloid interaction with lipid membrane may shed light on the mechanism of amyloid toxicity. Using atomic force microscopy, we investigated aggregation of amyloid-β _1-42 (Aβ _1-42 ) on model phospholipid membranes as a function of time and membrane composition. Neutral, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), anionic - 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (sodium salt) (DOPG), and cationic - 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), were used to study the effect of lipid type on amyloid binding. We showed that both the charge on the lipid head group and lipid phase affect the interaction of amyloid oligomers with the membrane surface changing the rate of adsorption and causing changes in membrane structure and structure of amyloid deposits. We observed that amyloid aggregates progressively accumulate in a similar manner on the surface of neutral DPPC gel phase membrane and on the surface of fluid phase negatively charged DOPG membrane. In contrast to DPPC and DOPG, positively charged fluid DOTAP membrane and neutral fluid phase DOPC membrane contain amyloid deposits with reduced height, which suggests fusing of Aβ _1-42 into the lipid membrane surface.