Bastian Rouven Brückner

Actin filament turnover drives leading edge growth during myelin sheath formation in the central nervous system

During CNS development, oligodendrocytes wrap their plasma membrane around axons to generate multilamellar myelin sheaths. To drive growth at the leading edge of myelin at the interface with the axon, mechanical forces are necessary, but the underlying mechanisms are not known. Using an interdisciplinary approach that combines morphological, genetic, and biophysical analyses, we identified a key role for actin filament network turnover in myelin growth. At the onset of myelin biogenesis, F-actin is redistributed to the leading edge, where its polymerization-based forces push out non-adhesive and motile protrusions. F-actin disassembly converts protrusions into sheets by reducing surface tension and in turn inducing membrane spreading and adhesion. We identified the actin depolymerizing factor ADF/cofilin1, which mediates high F-actin turnover rates, as an essential factor in this process. We propose that F-actin turnover is the driving force in myelin wrapping by regulating repetitive cycles of leading edge protrusion and spreading.

Membrane tension homeostasis of epithelial cells through surface area regulation in response to osmotic stress

Osmotic stress poses one of the most fundamental challenges to living cells. Particularly, the largely inextensible plasma membrane of eukaryotic cells easily ruptures under in-plane tension calling for sophisticated strategies to readily respond to osmotic stress. We describe how epithelial cells react and adapt mechanically to the exposure to hypotonic and hypertonic solutions in the context of a confluent monolayer. Site-specific indentation experiments in conjunction with tether pulling on individual cells have been carried out with an atomic force microscope to reveal spatio-temporal changes in membrane tension and surface area. We found that cells compensate for an increase in lateral tension due to hypoosmotic stress by sacrificing excess of membrane area stored in protrusions and invaginations such as microvilli and caveolae. At mild hypotonic conditions lateral tension increases partly compensated by surface are regulation, i.e. the cell sacrifices some of its membrane reservoirs. A loss of membrane-actin contacts occurs upon exposure to stronger hypotonic solutions giving rise to a drop in lateral tension. Tension release recovers on longer time scales by an increasing endocytosis, which efficiently removes excess membrane from the apical side to restore the initial pre-stress. Hypertonic solutions lead to shrinkage of cells and collapse of the apical membrane onto the cortex. Exposure to distilled water leads to stiffening of cells due to removal of excess surface area and tension increase due to elevated osmotic pressure across the plasma membrane.

Ezrin is a Major Regulator of Membrane Tension in Epithelial Cells

Plasma membrane tension is responsible for a variety of cellular functions such as motility, cell division, and endocytosis. Since membrane tension is dominated by the attachment of the actin cortex to the inner leaflet of the plasma membrane, we investigated the importance of ezrin, a major cross-linker of the membrane-cytoskeleton interface, for cellular mechanics of confluent MDCK II cells. For this purpose, we carried out ezrin depletion experiments and also enhanced the number of active ezrin molecules at the interface. Mechanical properties were assessed by force indentation experiments followed by membrane tether extraction. PIP2 micelles were injected into individual living cells to reinforce the linkage between plasma membrane and actin-cortex, while weakening of this connection was reached by ezrin siRNA and administration of the inhibitors neomycin and NSC 668394, respectively. We observed substantial stiffening of cells and an increase in membrane tension after addition of PIP2 micelles. In contrast, reduction of active ezrin led to a decrease of membrane tension accompanied by loss of excess surface area, increase in cortical tension, remodelling of actin cytoskeleton, and reduction of cell height. The data confirm the importance of the ezrin-mediated connection between plasma membrane and cortex for cellular mechanics and cell morphology.

Elastic properties of cells in the context of confluent cell monolayers: impact of tension and surface area regulation

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-β-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.

Phosphatidylinositol 4,5-Bisphosphate Alters the Number of Attachment Sites between Ezrin and Actin Filaments A COLLOIDAL PROBE STUDY

Background: Ezrin can establish a dynamic linkage between plasma membrane and cytoskeleton. Results: The individual bond strength between ezrin and F-actin is small, but the number of attachment sites is significantly altered by phosphatidylinositol 4,5-bisphosphate (PIP2). Conclusion: PIP2 activates ezrin to establish multiple weak ezrin/F-actin interactions. Significance: Plasma membrane tension is maintained by ezrin/F-actin interactions. Direct linkage between the plasma membrane and the actin cytoskeleton is controlled by the protein ezrin, a member of the ezrin-radixin-moesin protein family. To function as a membrane-cytoskeleton linker, ezrin needs to be activated in a process that involves binding of ezrin to phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphorylation of a conserved threonine residue. Here, we used colloidal probe microscopy to quantitatively analyze the interaction between ezrin and F-actin as a function of these activating factors. We show that the measured individual unbinding forces between ezrin and F-actin are independent of the activating parameters, in the range of approximately 50 piconewtons. However, the cumulative adhesion energy greatly increases in the presence of PIP2 demonstrating that a larger number of bonds between ezrin and F-actin has formed. In contrast, the phosphorylation state, represented by phosphor-mimetic mutants of ezrin, only plays a minor role in the activation process. These results are in line with in vivo experiments demonstrating that an increase in PIP2 concentration recruits more ezrin to the apical plasma membrane of polarized cells and significantly increases the membrane tension serving as a measure of the adhesion sites between the plasma membrane and the F-actin network.

Viscoelastic Properties of Confluent MDCK II Cells Obtained from Force Cycle Experiments

The local mechanical properties of cells are frequently probed by force indentation experiments carried out with an atomic force microscope. Application of common contact models provides a single parameter, the Young’s modulus, to describe the elastic properties of cells. The viscoelastic response of cells, however, is generally measured in separate microrheological experiments that provide complex shear moduli as a function of time or frequency. Here, we present a straightforward way to obtain rheological properties of cells from regular force distance curves collected in typical force indentation measurements. The method allows us to record the stress-strain relationship as well as changes in the weak power law of the viscoelastic moduli. We derive an analytical function based on the elastic-viscoelastic correspondence principle applied to Hertzian contact mechanics to model both indentation and retraction curves. Rheological properties are described by standard viscoelastic models and the paradigmatic weak power law found to interpret the viscoelastic properties of living cells best. We compare our method with atomic force microscopy-based active oscillatory microrheology and show that the method to determine the power law coefficient is robust against drift and largely independent of the indentation depth and indenter geometry. Cells were subject to Cytochalasin D treatment to provoke a drastic change in the power law coefficient and to demonstrate the feasibility of the approach to capture rheological changes extremely fast and precisely. The method is easily adaptable to different indenter geometries and acquires viscoelastic data with high spatiotemporal resolution.

Elastic properties of epithelial cells probed by atomic force microscopy

Cellular mechanics plays a crucial role in many biological processes such as cell migration, cell growth, embryogenesis, and oncogenesis. Epithelia respond to environmental cues comprising biochemical and physical stimuli through defined changes in cell elasticity. For instance, cells can differentiate between certain properties such as viscoelasticity or topography of substrates by adapting their own elasticity and shape. A living cell is a complex viscoelastic body that not only exhibits a shell architecture composed of a membrane attached to a cytoskeleton cortex but also generates contractile forces through its actomyosin network. Here we review cellular mechanics of single cells in the context of epithelial cell layers responding to chemical and physical stimuli. This article is part of a Special Issue entitled: Mechanobiology.

Mechanical properties of MDCK II cells exposed to gold nanorods

Summary Background: The impact of gold nanoparticles on cell viability has been extensively studied in the past. Size, shape and surface functionalization including opsonization of gold particles ranging from a few nanometers to hundreds of nanometers are among the most crucial parameters that have been focussed on. Cytoxicity of nanomaterial has been assessed by common cytotoxicity assays targeting enzymatic activity such as LDH, MTT and ECIS. So far, however, less attention has been paid to the mechanical parameters of cells exposed to gold particles, which is an important reporter on the cellular response to external stimuli. Results: Mechanical properties of confluent MDCK II cells exposed to gold nanorods as a function of surface functionalization and concentration have been explored by atomic force microscopy and quartz crystal microbalance measurements in combination with fluorescence and dark-field microscopy. Conclusion: We found that cells exposed to CTAB coated gold nanorods display a concentration-dependent stiffening that cannot be explained by the presence of CTAB alone. The stiffening results presumably from endocytosis of particles removing excess membrane area from the cell’s surface. Another aspect could be the collapse of the plasma membrane on the actin cortex. Particles coated with PEG do not show a significant change in elastic properties. This observation is consistent with QCM measurements that show a considerable drop in frequency upon administration of CTAB coated rods suggesting an increase in acoustic load corresponding to a larger stiffness (storage modulus).

Cell–Substrate Dynamics of the Epithelial-to-Mesenchymal Transition

The biological process of the epithelial-to-mesenchymal transition (EMT) allows epithelial cells to enhance their migratory and invasive behavior and plays a key role in embryogenesis, fibrosis, wound healing, and metastasis. Among the multiple biochemical changes from an epithelial to a mesenchymal phenotype, the alteration of cellular dynamics in cell-cell as well as cell-substrate contacts is crucial. To determine these variations over the whole time scale of the EMT, we measure the cell-substrate distance of epithelial NMuMG cells during EMT using our newly established metal-induced energy transfer (MIET) microscopy, which allows one to achieve nanometer axial resolution. We show that, in the very first hours of the transition, the cell-substrate distance increases substantially, but later in the process after reaching the mesenchymal state, this distance is reduced again to the level of untreated cells. These findings relate to a change in the number of adhesion points and will help to better understand remodeling processes associated with wound healing, embryonic development, cancer progression, or tissue regeneration.

Importance of integrity of cell-cell junctions for the mechanics of confluent MDCK II cells

Intercellular junctions are important mechanical couplers between cells in epithelial layers providing adhesion and intercellular communication. Regulation of the junctions occurs in cellular processes such as layer formation, epithelial-to-mesenchymal transition, embryogenesis, and cancer progression. Many studies addressed the role of force generation in cells for establishing lateral cell-cell junctions and the role of cellular force transmission in tissue formation and maintenance. Our atomic force microscopy- (AFM) based study shed light on the role of both, tight junctions and adherens junctions for the mechanical properties of individual epithelial cells that are part of a confluent monolayer. We found that tight junctions are important for the establishment of a functional barrier-forming layer but impairing them does not reduce the mechanical integrity of cells. Depletion of ZO-1 results in a weak increase in cortical tension. An opposite effect was observed for disruption of E-cadherin-mediated adherens junctions using DTT. Opening of adherens junctions leads to substantial alterations of cellular mechanics such as reduced overall stiffness, but these changes turned out to be reversible after re-establishing disulfide bridges in E-cadherin by removal of DTT. We found that regulatory mechanisms exist that preserve mechanical integrity during recovery of disrupted adherens junctions.