Nanomechanical mapping is essential as the mechanical properties of whole living cells are closely linked to various pathological and physiological processes.
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Understanding how changes in the mechanical properties of individual subcellular components or organelles can provide insight into the impact on overall cell performance. Scientists have found that identifying the factors that lead to changes in the properties of these components could serve as valuable prognostic markers for diseases.
Nanomechanical properties of living cells
Scientists measure the softness, finite thickness, viscoelasticity and incompressibility of living cells using different techniques. Some mammalian cells exhibit Young’s modulus values below the 1 kPa range. Studies have shown that eukaryotic cells are softer than a single protein.
Living eukaryotic cells are viewed as a solid system consisting of the plasma membrane enclosing many solid elements such as protein filaments, DNA, organelles and a nucleus immersed in an aqueous solution.
Viscoelastic interactions occur when solid elements move in a liquid matrix. Scientists pointed out that viscoelasticity is an intrinsic feature of a cell’s mechanical response.
Measurements based on atomic force microscopy (AFM) describe mechanical properties in terms of the viscoelastic model.
A single cell has been described as anisotropic, heterogeneous with finite thickness. Nanomechanical mapping reveals the structural complexity of a biological component.
The concept of cell incompressibility was confirmed by examining the mass densities of HeLa cells and fibroblasts, which were almost equal to the mass density of water.
Nanomechanical maps characterize the performance of biological molecules. These maps showed that the local spin of a molecule in thin films could be determined based on the temperature dependence of the Young’s modulus from low to high spin states.
Nanomechanical maps developed by bimodal AFM showed that the physical properties of lipid bilayers can be modulated by changing cholesterol concentration.
Scientists reported that areas of low concentration of cholesterol molecules are more elastic than regions of high concentration.
High spatial resolution nanomechanical maps provide data on the adsorption of single alkaline cations to lipid bilayers. The researchers discovered that the development of an ionic network decreases the elastic modulus of the lipid bilayer. These maps also characterize the spatial distribution of magnetic nanoparticles absorbed by liposomes.
Nanomechanical Imaging Techniques
Changes in subcellular components (both molecular and structural) such as the nucleus, nucleolus and cytoskeleton are potential disease markers. Several methods, including non-invasive optics-based Brillouin microscopy, have helped to probe the mechanical properties of subcellular components directly in living plant and mammalian cells at the submicron scale.
Researchers have recently developed a multiharmonic dynamic AFM (dAFM) method to quantitatively image the nanomechanical properties of soft biological samples in a liquid environment at kHz frequencies
The mapping and imaging capabilities of an atomic force microscope have been challenged when examining living cells, particularly eukaryotic cells.
Researchers have reported that living eukaryotic cells are among the softest materials on earth. So far, the images of these cells are best obtained by atomic force microscopy, which contains a spatial resolution in the 100 nm range.
AFM measures the mechanical properties of the cell by measuring the interaction forces related to the AFM tip and the surface distance or interaction force on some parameters of the tip vibration.
Nanomechanical imaging methods are classified according to different characteristics, such as quantitative properties or the ratio between the resonance frequency and the modulation frequency.
Bimodal AFM has been used extensively for nanomechanical mapping. For example, nanomechanical mapping of enucleated cells has helped elucidate the contribution of the nucleus to passive cell mechanics.
Application of nanomechanical imaging
Nanomechanical mapping helps to understand the differences in the results of microsurgery on the eyes.
Studies have shown that many eye diseases are related to changes in the mechanical and physical properties of the eye. Some of the eye disorders associated with mechanical changes include macular holes and wrinkles.
These disorders occur due to abnormal physical traction forces acting on the retina of the eye and can be corrected by the microsurgical procedure that includes mechanical peeling of the retinal inner limiting membrane (ILM).
Scientists performed a nanoscale elastic measurement to understand the differences in the biomechanical response of the ILM in the two diseases, which are also associated with different outcomes after microsurgery.
Nanomechanical mapping of the retina helps ophthalmologists to develop microsurgical protocols based on the ILM material properties in macular holes or folds.
A study of protein nanomechanics is helping scientists understand how mechanical force shapes the free energy of proteins. Researchers studied how nanomechanics are controlled by multiple factors present in living tissue or by mutations.
Identifying specific factors could serve as a target and would greatly benefit drug development for the disease.
High spatial resolution nanomechanical maps characterize the elastic properties of individual proteins, membrane proteins, protein fibrils and DNA. Scientists have also created nanomechanical maps for bones and virus-like particles.
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References and future reading
Efremov, YM, Suter, DM, Timashev, PS et al. (2022) 3D nanomechanical mapping of live cell subcellular and subnuclear structures by multiharmonic AFM with long-tipped micro-cantilevers. Scientific Rep 12, 529 https://doi.org/10.1038/s41598-021-04443-w
Gisbert, GV et al. (2021) High-speed nanomechanical mapping of the early stages of collagen growth by bimodal atomic force microscopy. ACS nano. 15(1). pp. 1850-1857. https://doi.org/10.1021/acsnano.0c10159
Alegre-Cebollada, J. (2021) Protein nanomechanics in a biological context. Biophysical Reviews. 13. pp. 435-454. https://doi.org/10.1007/s12551-021-00822-9
Efremov, MY et al. (2020) Nanomechanical properties of enucleated cells: Contribution of the nucleus to passive cell mechanics. Journal of Nanobiotechnology.18(134). https://doi.org/10.1186/s12951-020-00696-1
Stühn, L. et al. (2019). Nanomechanical subsurface mapping of living biological cells by atomic force microscopy. nanoscale. 11. https://pubs.rsc.org/en/content/articlelanding/2019/nr/c9nr03497h
Efremov, YM, Cartagena-Rivera, AX, Athamneh, AIM et al. (2018) Mapping the heterogeneity of cell mechanics by multiharmonic atomic force microscopy. nat. protocol 13, 2200-2216 (2018). https://doi.org/10.1038/s41596-018-0031-8