Elasticity mapping of pore-suspending native cell membranes

Viscoelastic properties of epithelial cells

Epithelial cells form tight barriers that line both the outer and inner surfaces of organs and cavities and therefore face diverse environmental challenges. The response to these challenges relies on the cells' dynamic viscoelastic properties, playing a pivotal role in many biological processes such as adhesion, growth, differentiation, and motility. Therefore, the cells usually adapt their viscoelastic properties to mirror the environment that determines their fate and vitality. Albeit not a high-throughput method, atomic force microscopy is still among the dominating methods to study the mechanical properties of adherent cells since it offers a broad range of forces from Piconewtons to Micronewtons at biologically significant time scales. Here, some recent work of deformation studies on epithelial cells is reviewed with a focus on viscoelastic models suitable to describe force cycle measurements congruent with the architecture of the actin cytoskeleton. The prominent role of the cortex in the cell's response to external forces is discussed also in the context of isolated cortex extracts on porous surfaces.

Viscoelasticity of basal plasma membranes and cortices derived from MDCK II cells

The mechanical properties of cells are largely determined by the architecture and dynamics of their viscoelastic cortex, which consists of a contractile, cross-linked actin mesh attached to the plasma membrane via linker proteins. Measuring the mechanical properties of adherent, polarized epithelial cells is usually limited to the upper, i.e., apical side, of the cells because of their accessibility on culture dishes. Therefore, less is known about the viscoelastic properties of basal membranes. Here, I investigate the viscoelastic properties of basolateral membranes derived from polarized MDCK II epithelia in response to external deformation and compare them to living cells probed at the apical side. MDCK II cells were grown on porous surfaces to confluence, and the upper cell body was removed via a squirting-lysis protocol. The free-standing, defoliated basal membranes were subject to force indentation and relaxation experiments permitting a precise assessment of cortical viscoelasticity. A new theoretical framework to describe the force cycles is developed and applied to obtain the time-dependent area compressibility modulus of cell cortices from adherent cells. Compared with the viscoelastic response of living cells, the basolateral membranes are substantially less fluid and stiffer but obey to the same universal scaling law if excess area is taken correctly into account.

Pulling lipid tubes from supported bilayers unveils the underlying substrate contribution to the membrane mechanics

Cell processes like endocytosis, membrane resealing, signaling and transcription involve conformational changes which depend on the chemical composition and the physicochemical properties of the lipid membrane. The better understanding of the mechanical role of lipids in cell membrane force-triggered and sensing mechanisms has recently become the focus of attention. Different membrane models and experimental methodologies are commonly explored. While general approaches involve controlled vesicle deformation using micropipettes or optical tweezers, due to the local and dynamic nature of the membrane, high spatial resolution atomic force microscopy (AFM) has been widely used to study the mechanical compression and indentation of supported lipid bilayers (SLBs). However, the substrate contribution remains unkown. Here, we demonstrate how pulling lipid tubes with an AFM out of model SLBs can be used to assess the nanomechanics of SLBs through the evaluation of the tube growing force (Ftube), allowing for very local evaluation with high spatial and force resolution of the lipid membrane tension. We first validate this approach to determine the contribution of different phospholipids, by varying the membrane composition, in both one-component and phase-segregated membranes. Finally, we successfully assess the contribution of the underlying substrate to the membrane mechanics, demonstrating that SLB models may represent an intermediate scenario between a free membrane (blebs) and a cytoskeleton supported membrane.

Continuous Pore-Spanning Lipid Bilayers on Silicon Oxide-Coated Porous Substrates

A number of techniques has been developed and analyzed in recent years to generate pore-spanning membranes (PSMs). While quite a number of methods rely on nanoporous substrates, only a few use micrometer-sized pores to be able to individually resolve suspending membranes by means of fluorescence microscopy. To be able to produce PSMs on pores that are micrometer in size, an orthogonal functionalization strategy resulting in a hydrophilic surface is highly desirable. Here, we report on a method to prepare PSMs based on the evaporation of a thin layer of silicon monoxide on top of the porous substrate. PM-IRRAS experiments demonstrate that the final surface is composed of SiO_x with 1 < x < 2. The hydrophilic surface turned out to be well suited to spread giant unilamellar vesicles forming PSMs. As the method does not rely on a gold coating as frequently used for orthogonal functionalization, fluorescence micrographs provide information not only from the freestanding membrane areas but also from the supported ones. The observation of the entire PSM area enabled us to observe phase-separation in these membranes on the freestanding and supported parts as well as protein binding and possible lipid reorganization of the membranes induced by binding of the protein Shiga toxin.

Mechanics of lipid bilayers: What do we learn from pore-spanning membranes?

The mechanical properties of biological membranes have become increasingly important not only from a biophysical viewpoint but also as they play a substantial role in the information transfer in cells and tissues. This minireview summarizes some of our recent understanding of the mechanical properties of artificial model membranes with particular emphasis on membranes suspending an array of pores, so called pore-spanning membranes. A theoretical description of the mechanical properties of these membranes might pave the way to biophysically describe and understand the complex behavior of native biological membranes. This article is part of a Special Issue entitled: Mechanobiology.

The development of a μ-biomimetic uncooled IR-Sensor inspired by the infrared receptors of Melanophila acuminata

The beetle Melanophila acuminata uses a specialized organ to detect infrared radiation. The organ consists of about 100 individual sensilla. The main component of the sensillum is a pressure chamber. Upon absorption of radiation, the pressure increases, and the tip of a dendrite is deformed. A unique feature of the organ is a compensation mechanism that prevents large pressures. The beetle uses this organ to detect forest fires and to navigate inside burning woods. However, the sensitivity is part of a long-lasting discussion, providing thresholds between [Formula: see text] and [Formula: see text]. To end the decade-long discussion and to provide a novel type of infrared sensor, we are developing an uncooled μ-biomimetic infrared (IR) sensor inspired by Melanophila acuminata using MEMS technology. Here, we present the development of a μ-capacitor that is used to detect pressure changes and the characterization of the compensation mechanism. We describe the microtechnological fabrication process for air-filled capacitors with a ratio of diameter-to-electrode distance of 1000 and a technique to fill the sensor bubble-free with water. Finally, we estimate the sensitivity of the beetle using a theoretical model of the sensillum.

Sum frequency generation image reconstruction: aliphatic membrane under spherical cap geometry

The article explores an opportunity to approach structural properties of phospholipid membranes using Sum Frequency Generation microscopy. To establish the principles of sum frequency generation image reconstruction in such systems, at first approach, we may adopt an idealistic spherical cap uniform assembly of hydrocarbon molecules. Quantum mechanical studies for decanoic acid (used here as a representative molecular system) provide necessary information on transition dipole moments and Raman tensors of the normal modes specific to methyl terminal - a typical moiety in aliphatic (and phospholipid) membranes. Relative degree of localization and frequencies of the normal modes of methyl terminals make nonlinearities of this moiety to be promising in structural analysis using Sum Frequency Generation imaging. Accordingly, the article describes derivations of relevant macroscopic nonlinearities and suggests a mapping procedure to translate amplitudes of the nonlinearities onto microscopy image plane according to geometry of spherical assembly, local molecular orientation, and optical geometry. Reconstructed images indicate a possibility to extract local curvature of bilayer envelopes of spherical character. This may have practical implications for structural extractions in membrane systems of practical relevance.

Electrochemical impedance spectroscopy and atomic force microscopic studies of electrical and mechanical properties of nano-black lipid membranes and size dependence

We present electrochemical impedance spectroscopic (EIS) and two-chamber AFM investigations of the electrical and mechanical properties of solvent-containing nano-BLMs suspended on chip-based nanopores of diameter of 200, 400, and 700 nm. The chips containing nanoporous silicon nitride membranes are fabricated based on low-cost colloidal lithography with low aspect ratio of the nanopores. BLMs of DPhPC lipid molecules are constructed across the nanopores by the painting method. Two equivalent circuits are compared in view of their adequacy in description of the EIS performances of the nano-BLMs and more importantly the structures associated with the nano-BLMs systems. The BLM resistance and capacitance as well as their size and time dependence are studied by EIS. The breakthrough forces, elasticity in terms of apparent spring constant, and lateral tension of the solvent-containing nano-BLMs are investigated by AFM force measurements. The exact relationship of the breakthrough force of the nano-BLM as a function of pore size is revealed. Both EIS and AFM studies show increasing lifetime and mechanical stability of the nano-BLMs with decreasing pore size. Finally, the robust 200 nm diameter nanopores are used to accommodate functional BLMs containing DPhPC lipid molecules and gramicidins by using a painting method with drop of mixture solutions of DPhPC and gramicidins. EIS investigation of the functional nano-BLMs is also performed.

Membrane nanotubes drawn by optical tweezers transmit electrical signals between mammalian cells over long distances

Biological cells continuously change shape allowing essential functions such as cell motility, vesicle-mediated release/uptake of soluble and membrane components or nanotube-mediated cell-cell communications. Here we use single cell micromanipulation to induce functional changes of cell shape for nanobiotechnological applications. Optical tweezers are focused on the plasma membrane of living cells to pull membrane nanotubes of approximately 200 nanometre diameters and 100 micrometre lengths. Upon switching off the laser tweezer membrane nanotubes relax back to the cell surface. Single-exponential relaxation times deliver local mechanical properties of cells' plasma membrane. Nanotubes pulled beyond 100 micrometre tear off and form micrometre-sized vesicles carrying functional membrane receptors and cytoplasmic signaling components. Membrane nanotubes from one cell can be contacted to adjacent cells forming via connexins intercellular electrical connections within seconds in all directions. Our method opens broad applications for multiplexing single-cell analytics to submicrometer/subfemtoliter ranges and for creating artificial intercellular signaling networks, both not attainable by current methodologies.

Surface plasmon resonance for high-throughput ligand screening of membrane-bound proteins

Technologies based on surface plasmon resonance (SPR) have allowed rapid, label-free characterization of protein-protein and protein-small molecule interactions. SPR has become the gold standard in industrial and academic settings, in which the interaction between a pair of soluble binding partners is characterized in detail or a library of molecules is screened for binding against a single soluble protein. In spite of these successes, SPR is only beginning to be adapted to the needs of membrane-bound proteins which are difficult to study in situ but represent promising targets for drug and biomarker development. Existing technologies, such as BIAcoreTM, have been adapted for membrane protein analysis by building supported lipid layers or capturing lipid vesicles on existing chips. Newer technologies, still in development, will allow membrane proteins to be presented in native or near-native formats. These include SPR nanopore arrays, in which lipid bilayers containing membrane proteins stably span small pores that are addressable from both sides of the bilayer. Here, we discuss current SPR instrumentation and the potential for SPR nanopore arrays to enable quantitative, high-throughput screening of G protein coupled receptor ligands and applications in basic cellular biology.