Multifrequency AFM reveals lipid membrane mechanical properties and the effect of cholesterol in modulating viscoelasticity

Abstract

The physical properties of lipid bilayers comprising the cell membrane occupy the current spotlight of membrane biology. Their traditional representation as a passive 2D fluid has gradually been abandoned in favor of a more complex picture: an anisotropic time-dependent viscoelastic biphasic material, capable of transmitting or attenuating mechanical forces that regulate biological processes. In establishing new models, quantitative experiments are necessary when attempting to develop suitable techniques for dynamic measurements. Here, we map both the elastic and viscous properties of the model system 1,2-dipalmitoyl--glycero-3-phosphocholine (DPPC) lipid bilayers using multifrequency atomic force microscopy (AFM), namely amplitude modulation-frequency modulation (AM-FM) AFM imaging in an aqueous environment. Furthermore, we investigate the effect of cholesterol (Chol) on the DPPC bilayer in concentrations from 0 to 60%. The AM-AFM quantitative maps demonstrate that at low Chol concentrations, the lipid bilayer displays a distinct phase separation and is elastic, whereas at higher Chol concentration, the bilayer appears homogenous and exhibits both elastic and viscous properties. At low-Chol contents, the modulus (elastic) dominates. As the Chol insertions increases, higher energy is dissipated; and although the bilayer stiffens (increase in), the viscous component dominates (). Our results provide evidence that the lipid bilayer exhibits both elastic and viscous properties that are modulated by the presence of Chol, which may affect the propagation (elastic) or attenuation (viscous) of mechanical signals across the cell membrane.

Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications

“…Regions with a low concentration of cholesterol molecules are predominantly elastic while regions with a high concentration have viscoelastic properties. 249 High-spatial resolution nanomechanical maps characterized the adsorption of single alkali cations on lipid bilayers. 250 It was reported that the formation of an ionic network reduces the elastic modulus of the lipid bilayer.…”

Fast, quantitative and high resolution mapping of viscoelastic properties with bimodal AFM

“…1,6,8,[16][17][18][19][20][21] Bimodal AFM provides a very fast, high resolution and accurate method to map the elastic properties of polymers and biomolecules. 22,23 It has been applied to determine with very high spatial resolution the elastic modulus of a large variety of materials and macromolecules such as antibodies 24 and other proteins, [25][26][27] DNA, 28,29 cells, 30,31 bone microconstituents, 32 lipid bilayers, 33,34 self-assembled monolayers, 35,36 2D materials 37 or organic semiconductor devices. 38 This technique can be operated in air or liquid.…”

Review: Cantilever-Based Sensors for High Speed Atomic Force Microscopy

“…The exhaustive list of AFM imaging modes is very long. Many of the recently developed modes are used for studying a number of biological samples, including proteins; small biological fibrils, like lipid membranes; amyloid fibrils; and viruses with the microcantilever as the sensing element [ 121 , 122 ]. Moreover, it has been demonstrated that malign and benign cell lines present significant differences in their viscoelastic response using MMM [ 123 ].…”