Mechanical phenotyping of cells by atomic force microscopy (AFM) was proposed as a novel tool in cancer cell research as cancer cells undergo massive structural changes, comprising remodelling of the cytoskeleton and changes of their adhesive properties. In this work, we focused on the mechanical properties of human breast cell lines with different metastatic potential by AFM-based microrheology experiments. Using this technique, we are not only able to quantify the mechanical properties of living cells in the context of malignancy, but we also obtain a descriptor, namely the loss tangent, which provides model-independent information about the metastatic potential of the cell line. Including also other cell lines from different organs shows that the loss tangent (G″/G′) increases generally with the metastatic potential from MCF-10A representing benign cells to highly malignant MDA-MB-231 cells.
The mechanical behavior of lipid bilayers spanning the pores of highly ordered porous silicon substrates was scrutinized by local indentation experiments as a function of surface functionalization, lipid composition, solvent content, indentation velocity, and pore radius. Solvent-containing nano black lipid membranes (nano-BLMs) as well as solvent-free pore-spanning bilayers were imaged by fluorescence and atomic force microscopy prior to force curve acquisition, which allows distinguishing between membrane-covered and uncovered pores. Force indentation curves on pore-spanning bilayers attached to functionalized hydrophobic porous silicon substrates reveal a predominately linear response that is mainly attributed to prestress in the membranes. This is in agreement with the observation that indentation leads to membrane lysis well below 5% area dilatation. However, membrane bending and lateral tension dominate over prestress and stretching if solvent-free supported membranes obtained from spreading giant liposomes on hydrophilic porous silicon are indented. An elastic regime diagram is presented that readily allows determining the dominant contribution to the mechanical response upon indentation as a function of load and pore radius.
Epithelial cells usually form a dense continuous cobblestone-like sheet that is frequently exposed to a variety of mechanical challenges encompassing osmotic stress and external forces. The response to external forces was investigated and the question of how individual polar epithelial cells organized in confluent monolayers respond to pharmaceutical stimuli targeting the key players of cellular mechanics was answered. In particular, we ask how epithelial cells respond to changes in cortical and membrane tension by surface area regulation if challenged by diverse chemical and mechanical cues. Here, a tension-based model is used that allows capturing the relevant modes of cell deformation. Together with independent measurements of membrane tension, cortical tension and excess surface area of confluent MDCK II cells it is possible to draw a mechanistic picture of how confluent cells respond to mechanical stimuli in general. Changes in tension are provoked by external stimuli directed towards the contractile actomyosin cortex (cytochalasin D, blebbistatin), and changes in the excess surface area are produced by cholesterol extraction (methyl-b-cyclodextrin) or inhibition of dynamin (dynasore). A combination of AFM-indentation experiments with membrane-tether pulling at the same position allowed us to simultaneously monitor changes in membrane tension, cortical tension and excess surface area. Generally, we observed that removing or producing excess surface area of the plasma membrane readily adjusts membrane tension that is pivotal for the mechanical response of confluent cells. We found that isolated apical membranes from confluent MDCK II monolayers display similar mechanical properties as the apical side of living MDCK II cells in a confluent monolayer confirming that membrane mechanics in conjunction with cytoskeletal adhesion dominates the elastic response of confluent epithelial cells at large strain.
Bipolar phospholipids (bolalipids) represent an exciting class of amphiphilic molecules as they self-assemble in water to distinct structures of nanoscopic dimensions. Reported here are structural details of helical nanofibers, composed of achiral, symmetrical single-chain bolalipids with phosphocholine headgroups. These nanofibers are used as template for the fixation of gold nanoparticles (AuNPs) without prior functionalization. This realization of a metal array on bolalipid nanofibers is one of the rare examples of one-dimensional AuNP arrangements in solution. The loading and the heat of binding of AuNPs are determined applying transmission electron microscopy and isothermal titration calorimetry.
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