Mechanical response of adherent giant liposomes to indentation with a conical AFM-tip

Mechanics of biomimetic free-standing lipid membranes: insights into the elasticity of complex lipid compositions

The creation of free-standing lipid membranes has been so far of remarkable interest to investigate processes occurring in the cell membrane since its unsupported part enables studies in which it is important to maintain cell-like physicochemical properties of the lipid bilayer, that nonetheless depend on its molecular composition. In this study, we prepare pore-spanning membranes that mimic the composition of plasma membranes and perform force spectroscopy indentation measurements to unravel mechanistic insights depending on lipid composition. We show that this approach is highly effective for studying the mechanical properties of such membranes. Furthermore, we identify a direct influence of cholesterol and sphingomyelin on the elasticity of the bilayer and adhesion between the two leaflets. Eventually, we explore the possibilities of imaging in the unsupported membrane regions. For this purpose, we investigate the adsorption and movement of a peripheral protein, the fibroblast growth factor 2, on the complex membrane.

A simple method to make, trap and deform a vesicle in a gel

We present a simple method to produce giant lipid pseudo-vesicles (vesicles with an oily cap on the top), trapped in an agarose gel. The method can be implemented using only a regular micropipette and relies on the formation of a water/oil/water double droplet in liquid agarose. We characterize the produced vesicle with fluorescence imaging and establish the presence and integrity of the lipid bilayer by the successful insertion of [Formula: see text]-Hemolysin transmembrane proteins. Finally, we show that the vesicle can be easily mechanically deformed, non-intrusively, by indenting the surface of the gel.

Methods to mechanically perturb and characterize GUV-based minimal cell models

Cells shield organelles and the cytosol via an active boundary predominantly made of phospholipids and membrane proteins, yet allowing communication between the intracellular and extracellular environment. Micron-sized liposome compartments commonly known as giant unilamellar vesicles (GUVs) are used to model the cell membrane and encapsulate biological materials and processes in a cell-like confinement. In the field of bottom-up synthetic biology, many have utilized GUVs as substrates to study various biological processes such as protein-lipid interactions, cytoskeletal assembly, and dynamics of protein synthesis. Like cells, it is ideal that GUVs are also mechanically durable and able to stay intact when the inner and outer environment changes. As a result, studies have demonstrated approaches to tune the mechanical properties of GUVs by modulating membrane composition and lumenal material property. In this context, there have been many different methods developed to test the mechanical properties of GUVs. In this review, we will survey various perturbation techniques employed to mechanically characterize GUVs.

Actomyosin-Driven Division of a Synthetic Cell

One of the major challenges of bottom-up synthetic biology is rebuilding a minimal cell division machinery. From a reconstitution perspective, the animal cell division apparatus is mechanically the simplest and therefore attractive to rebuild. An actin-based ring produces contractile force to constrict the membrane. By contrast, microbes and plant cells have a cell wall, so division requires concerted membrane constriction and cell wall synthesis. Furthermore, reconstitution of the actin division machinery helps in understanding the physical and molecular mechanisms of cytokinesis in animal cells and thus our own cells. In this review, we describe the state-of-the-art research on reconstitution of minimal actin-mediated cytokinetic machineries. Based on the conceptual requirements that we obtained from the physics of the shape changes involved in cell division, we propose two major routes for building a minimal actin apparatus capable of division. Importantly, we acknowledge both the passive and active roles that the confining lipid membrane can play in synthetic cytokinesis. We conclude this review by identifying the most pressing challenges for future reconstitution work, thereby laying out a roadmap for building a synthetic cell equipped with a minimal actin division machinery.

Epithelial cells sacrifice excess area to preserve fluidity in response to external mechanical stress

Viscoelastic properties of epithelial cells subject to shape changes were monitored by indentation-retraction/relaxation experiments. MDCK II cells cultured on extensible polydimethylsiloxane substrates were laterally stretched and, in response, displayed increased cortex contractility and loss of excess surface area. Thereby, the cells preserve their fluidity but inevitably become stiffer. We found similar behavior in demixed cell monolayers of ZO-1/2 double knock down (dKD) cells, cells exposed to different temperatures and after removal of cholesterol from the plasma membrane. Conversely, the mechanical response of single cells adhered onto differently sized patches displays no visible rheological change. Sacrificing excess surface area allows the cells to respond to mechanical challenges without losing their ability to flow. They gain a new degree of freedom that permits resolving the interdependence of fluidity β on stiffness [Formula: see text]. We also propose a model that permits to tell apart contributions from excess membrane area and excess cell surface area.

Reconstitution of contractile actomyosin rings in vesicles

One of the grand challenges of bottom-up synthetic biology is the development of minimal machineries for cell division. The mechanical transformation of large-scale compartments, such as Giant Unilamellar Vesicles (GUVs), requires the geometry-specific coordination of active elements, several orders of magnitude larger than the molecular scale. Of all cytoskeletal structures, large-scale actomyosin rings appear to be the most promising cellular elements to accomplish this task. Here, we have adopted advanced encapsulation methods to study bundled actin filaments in GUVs and compare our results with theoretical modeling. By changing few key parameters, actin polymerization can be differentiated to resemble various types of networks in living cells. Importantly, we find membrane binding to be crucial for the robust condensation into a single actin ring in spherical vesicles, as predicted by theoretical considerations. Upon force generation by ATP-driven myosin motors, these ring-like actin structures contract and locally constrict the vesicle, forming furrow-like deformations. On the other hand, cortex-like actin networks are shown to induce and stabilize deformations from spherical shapes.

Cytoskeletal networks support and direct cell shape and guide intercellular transport, but relatively little is understood about the self-organization of cytoskeletal components on the scale of an entire cell. Here, authors use an in vitro system and observe the assembly of different types of actin networks and the condensation of membrane-bound actin into single rings.

Protein Reconstitution Inside Giant Unilamellar Vesicles

Giant unilamellar vesicles (GUVs) have gained great popularity as mimicries for cellular membranes. As their sizes are comfortably above the optical resolution limit, and their lipid composition is easily controlled, they are ideal for quantitative light microscopic investigation of dynamic processes in and on membranes. However, reconstitution of functional proteins into the lumen or the GUV membrane itself has proven technically challenging. In recent years, a selection of techniques has been introduced that tremendously improve GUV-assay development and enable the precise investigation of protein-membrane interactions under well-controlled conditions. Moreover, due to these methodological advances, GUVs are considered important candidates as protocells in bottom-up synthetic biology. In this review, we discuss the state of the art of the most important vesicle production and protein encapsulation methods and highlight some key protein systems whose functional reconstitution has advanced the field.

Quantification of nanoscale forces in lectin-mediated bacterial attachment and uptake into giant liposomes

Interactions of the bacterial lectin LecA with the host cells glycosphingolipid Gb3 have been shown to be crucial for the cellular uptake of the bacterium Pseudomonas aeruginosa. LecA-induced Gb3 clustering, referred to as lipid zipper mechanism, leads to full membrane engulfment of the bacterium. Here, we aim for a nanoscale force characterization of this mechanism using two complementary force probing techniques, atomic force microscopy (AFM) and optical tweezers (OT). The LecA-Gb3 interactions are reconstituted using giant unilamellar vesicles (GUVs), a well-controlled minimal system mimicking the plasma membrane and nanoscale forces between either bacteria (PAO1 wild-type and LecA-deletion mutant strains) or LecA-coated probes (as minimal, synthetic bacterial model) and vesicles are measured. LecA-Gb3 interactions strengthen the bacterial attachment to the membrane (1.5-8-fold) depending on the membrane tension and the applied technique. Moreover, significantly less energy (reduction up to 80%) is required for the full uptake of LecA-coated beads into Gb3-functionalized vesicles. This quantitative approach highlights that lectin-glycolipid interactions provide adequate forces and energies to drive bacterial attachment and uptake.

Curvature-Regulated Lipid Membrane Softening of Nano-Vesicles

The physico-mechanical properties of nanoscale lipid vesicles (e.g., natural nano-vesicles and artificial nano-liposomes) dictate their interaction with biological systems. Understanding the interplay between vesicle size and stiffness is critical to both the understanding of the biological functions of natural nano-vesicles and the optimization of nano-vesicle-based diagnostics and therapeutics. It has been predicted that, when vesicle size is comparable to its membrane thickness, the effective bending stiffness of the vesicle increases dramatically due to both the entropic effect as a result of reduced thermal undulation and the nonlinear curvature elasticity effect. Through systematic molecular dynamics simulations, we show that the vesicle membrane thins and softens with the decrease in vesicle size, which effectively counteracts the stiffening effects as already mentioned. Our simulations indicate that the softening of nano-vesicles results from a change in the bilayer's interior structure - a decrease in lipid packing order - as the membrane curvature increases. Our work thus leads to a more complete physical framework to understand the physico-mechanical properties of nanoscale lipid vesicles, paving the way to further advances in the biophysics of nano-vesicles and their biomedical applications.

Membrane tension may define the deadliest virus infection

This manuscript describes the potentially significant role of interfacial tension in viral infection. Our hypothesis is based on evidence from drop coalescence hydrodynamics. A change in membrane tension can trigger fusion between the vesicle and cell such that genetic material, like viral RNA, can subsequently be transported to the cell interior. In other cases, RNA may reside near the cell membrane inside the cell, which could make their removal energetically unfavorable because of hydrodynamic interactions between membrane and RNA. Interfacial tension of the virus membrane can be modulated by temperature, among many other factors, of the mucosa layer. We discuss our hypothesis within the scope of recent SARS-CoV-2 studies where temperature-dependent membrane surface tension could be impacted through different atmospheric conditions, air conditioning systems, and the use of masks.

Determination of the Mechanical Properties of Model Lipid Bilayers Using Atomic Force Microscopy Indentation

By conducting a systematic study of model lipid membranes using the atomic force microscopy (AFM) indentation, we demonstrate the importance of an experimental protocol on the determination of their mechanical parameters. We refine the experimental approach by analyzing the influence of the contact mechanics models used to process the data, substrate preparation, and indenter geometry. We show that both bending rigidity and area compressibility can be determined from a single AFM indentation measurement.

Detachment of giant liposomes - coupling of receptor mobility and membrane shape

Cellular adhesion is an intricate physical process controlled by ligand-receptor affinity, density, mobility, and external forces transmitted through the elastic properties of the cell. As a model for cellular adhesion we study the detachment of cell-sized liposomes and membrane-coated silica beads from supported bilayers using atomic force microscopy. Adhesion between the two surfaces is mediated by the interaction between the adhesive lipid anchored saccharides lactosylceramide and the ganglioside G_M3. We found that force-distance curves of liposome detachment have a very peculiar, partially concave shape, reminiscent of the nonlinear extension of polymers. By contrast, detachment of membrane coated beads led to force-distance curves similar to the detachment of living cells. Theoretical modelling of the enforced detachment suggests that the non-convex force curve shape arises from the mobility of ligands provoking a switch of shapes from spherical to unduloidal during detachment.

Microfluidic Formation of Honeycomb-Patterned Droplets Bounded by Interface Bilayers via Bimodal Molecular Adsorption

Assembled water-in-oil droplets bounded by lipid bilayers are used in synthetic biology as minimal models of cell tissue. Microfluidic devices successfully generate monodispersed droplets and assemble them via droplet interface bilayesr (DIB) formation. However, a honeycomb pattern of DIB-bounded droplets, similar to epithelial tissues, remains unrealized because the rapid DIB formation between the droplets hinders their ability to form the honeycomb pattern. In this paper, we demonstrate the microfluidic formation of a honeycomb pattern of DIB-bounded droplets using two surfactants with different adsorption rates on the droplet surface. A non-DIB forming surfactant (sorbitan monooleate, Span 80) was mixed with a lipid (1,2-dioleoyl-sn-glycero-3-phosphocholine, PC), whose adsorption rate on the droplet surface and saturated interfacial tension were lower than those of Span 80. By changing the surfactant composition, we established the conditions under which the droplets initially form a honeycomb pattern and subsequently adhere to each other via DIB formation to minimize the interfacial energy. In addition, the reconstituted membrane protein nanopores at the DIBs were able to transport molecules. This new method, using the difference in the adsorption rates of two surfactants, allows the formation of a honeycomb pattern of DIB-bounded droplets in a single step, and thus facilitates research using DIB-bounded droplet assemblies.

Liposomal adhesion via electrostatic interactions and osmotic deflation increase membrane tension and lipid diffusion coefficient

Membrane adhesion is a ubiquitous phenomenon in cells and is related to various biological events such as migration, morphogenesis, and differentiation. To understand the physicochemical aspects of membrane adhesion, liposome-liposome adhesion and liposome-substrate adhesion have been studied. Although membrane adhesion has been shown to increase membrane tension and inhibit lipid diffusion, the relationship between these changes and the degree of membrane adhesion have not been quantified. Here, we analyzed the dependence of membrane tension and lipid diffusion on the degree of membrane adhesion, i.e., area fraction of the adherent region. For this purpose, we developed a simple method to prepare adhered liposomes by simple electrostatic interactions between the membranes and by osmotic deflation. We found that the membrane tension of the adhered liposomes increases slightly with an increase in the area fraction of the adherent region. In addition, the lipid diffusion coefficient of the adhered liposomes is larger than that of isolated liposomes, which is consistent with the theoretical prediction. The analysis provides a framework to understand the correlation between cell adhesion and bio-membrane properties such as membrane tension and molecular diffusion.

Liposome Drug Delivery System across Endothelial Plasma Membrane: Role of Distance between Endothelial Cells and Blood Flow Rate

This paper discusses specific features of the interactions of small-diameter liposomes with the cytoplasmic membrane of endothelial cells using in silico methods. The movement pattern of the liposomal drug delivery system was modeled in accordance with the conditions of the near-wall layer of blood flow. Our simulation results show that the liposomes can become stuck in the intercellular gaps and even break down when the gap is reduced. Liposomes stuck in the gaps are capable of withstanding a shell deformation of ~15% with an increase in liposome energy by 26%. Critical deformation of the membrane gives an impetus to drug release from the liposome outward. We found that the liposomes moving in the near-wall layer of blood flow inevitably stick to the membrane. Liposome sticking on the membrane is accompanied by its gradual splicing with the membrane bilayer. This leads to a gradual drug release inside the cell.

Observations of Membrane Domain Reorganization in Mechanically Compressed Artificial Cells

Giant unilamellar vesicles (GUVs) are considered to be the gold standard for assembling artificial cells from the bottom up. In this study, we investigated the behavior of such biomimetic vesicles as they were subjected to mechanical compression. A microfluidic device is presented that comprises a trap to capture GUVs and a microstamp that is deflected downwards to mechanically compress the trapped vesicle. After characterization of the device, we show that single-phase GUVs can be controllably compressed to a high degree of deformation (D=0.40) depending on the pressure applied to the microstamp. A permeation assay was implemented to show that vesicle bursting is prevented by water efflux. Next, we mechanically compressed GUVs with co-existing liquid-ordered and liquid-disordered membrane phases. Upon compression, we observed that the normally stable lipid domains reorganized themselves across the surface and fused into larger domains. This phenomenon, observed here in a model membrane system, not only gives us insights into how the multicomponent membranes of artificial cells behave, but might also have interesting consequences for the role of lipid rafts in biological cells that are subjected to compressive forces in a natural environment.

Acoustic deformation for the extraction of mechanical properties of lipid vesicle populations

We use an ultrasonic standing wave to simultaneously trap and deform thousands of soft lipid vesicles immersed in a liquid solution. In our device, acoustic radiation stresses comparable in magnitude to those generated in optical stretching devices are achieved over a spatial extent of more than ten acoustic wavelengths. We solve the acoustic scattering problem in the long-wavelength limit to obtain the radiation stress. The result is then combined with thin-shell elasticity theory to form expressions that relate the deformed geometry to the applied acoustic field intensity. Using observation of the deformed geometry and this model, we rapidly extract mechanical properties, such as the membrane Young's modulus, from populations of lipid vesicles.

Response of an actin network in vesicles under electric pulses

We study the role of a biomimetic actin network during the application of electric pulses that induce electroporation or electropermeabilization, using giant unilamellar vesicles (GUVs) as a model system. The actin cortex, a subjacently attached interconnected network of actin filaments, regulates the shape and mechanical properties of the plasma membrane of mammalian cells, and is a major factor influencing the mechanical response of the cell to external physical cues. We demonstrate that the presence of an actin shell inhibits the formation of macropores in the electroporated GUVs. Additionally, experiments on the uptake of dye molecules after electroporation show that the actin network slows down the resealing process of the permeabilized membrane. We further analyze the stability of the actin network inside the GUVs exposed to high electric pulses. We find disruption of the actin layer that is likely due to the electrophoretic forces acting on the actin filaments during the permeabilization of the GUVs. Our findings on the GUVs containing a biomimetic network provide a step towards understanding the discrepancies between the electroporation mechanism of a living cell and its simplified model of the empty GUV.

Glycan-decorated protocells: novel features for rebuilding cellular processes

In synthetic biology approaches, lipid vesicles are widely used as protocell models. While many compounds have been encapsulated in vesicles (e.g. DNA, cytoskeleton and enzymes), the incorporation of glycocalyx components in the lipid bilayer has attracted much less attention so far. In recent years, glycoconjugates have been integrated in the membrane of giant unilamellar vesicles (GUVs). These minimal membrane systems have largely contributed to shed light on the molecular mechanisms of cellular processes. In this review, we first introduce several preparation and biophysical characterization methods of GUVs. Then, we highlight specific applications of protocells investigating glycolipid-mediated endocytosis of toxins, viruses and bacteria. In addition, we delineate how prototissues have been assembled from glycan-decorated protocells by using lectin-mediated cross-linking of opposed glycoreceptors (e.g. glycolipids and glycopeptides). In future applications, glycan-decorated protocells might be useful for investigating cell-cell interactions (e.g. adhesion and communication). We also speculate about the implication of lectin-glycoreceptor interactions in membrane fusion processes.

Adhesion strategies of Dictyostelium discoideum- a force spectroscopy study

Biological adhesion is essential for all motile cells and generally limits locomotion to suitably functionalized substrates displaying a compatible surface chemistry. However, organisms that face vastly varying environmental challenges require a different strategy. The model organism Dictyostelium discoideum (D.d.), a slime mould dwelling in the soil, faces the challenge of overcoming variable chemistry by employing the fundamental forces of colloid science. To understand the origin of D.d. adhesion, we realized and modified a variety of conditions for the amoeba comprising the absence and presence of the specific adhesion protein Substrate Adhesion A (sadA), glycolytic degradation, ionic strength, surface hydrophobicity and strength of van der Waals interactions by generating tailored model substrates. By employing AFM-based single cell force spectroscopy we could show that experimental force curves upon retraction exhibit two regimes. The first part up to the critical adhesion force can be described in terms of a continuum model, while the second regime of the curve beyond the critical adhesion force is governed by stochastic unbinding of individual binding partners and bond clusters. We found that D.d. relies on adhesive interactions based on EDL-DLVO (Electrical Double Layer-Derjaguin-Landau-Verwey-Overbeek) forces and contributions from the glycocalix and specialized adhesion molecules like sadA. This versatile mechanism allows the cells to adhere to a large variety of natural surfaces under various conditions.

Shaping up synthetic cells

How do the cells in our body reconfigure their shape to achieve complex tasks like migration and mitosis, yet maintain their shape in response to forces exerted by, for instance, blood flow and muscle action? Cell shape control is defined by a delicate mechanical balance between active force generation and passive material properties of the plasma membrane and the cytoskeleton. The cytoskeleton forms a space-spanning fibrous network comprising three subsystems: actin, microtubules and intermediate filaments. Bottom-up reconstitution of minimal synthetic cells where these cytoskeletal subsystems are encapsulated inside a lipid vesicle provides a powerful avenue to dissect the force balance that governs cell shape control. Although encapsulation is technically demanding, a steady stream of advances in this technique has made the reconstitution of shape-changing minimal cells increasingly feasible. In this topical review we provide a route-map of the recent advances in cytoskeletal encapsulation techniques and outline recent reports that demonstrate shape change phenomena in simple biomimetic vesicle systems. We end with an outlook toward the next steps required to achieve more complex shape changes with the ultimate aim of building a fully functional synthetic cell with the capability to autonomously grow, divide and move.

Investigation on the Nanomechanics of Liposome Adsorption on Titanium Alloys: Temperature and Loading Effects

The mechanical properties of liposomes, determined by the lipid phase state at ambient temperature, have a close relationship with their physiological activities. Here, atomic force microscopy (AFM) was used to produce images and perform force measurements on titanium alloys at two adsorbed temperatures. The mechanical properties were evaluated under repeated loading and unloading, suggesting a better reversibility and resistance of gel phase liposomes. The liquid phase liposomes were irreversibly damaged during the first approach while the gel phase liposomes could bear more iterations, resulting from water flow reversibly going across the membranes. The statistical data offered strong evidence that the lipid membranes in the gel phase are robust enough to resist the tip penetration, mainly due to their orderly organization and strong hydrophobic interactions between lipid molecules. This work regarding the mechanical properties of liposomes with different phases provides guidance for future clinical applications, such as artificial joints.

Relaxation of a simulated lipid bilayer vesicle compressed by an atomic force microscope

Using coarse-grained molecular dynamics simulations, we study the relaxation of bilayer vesicles, uniaxially compressed by an atomic force microscope cantilever. The relaxation time exhibits a strong force dependence. Force-compression curves are very similar to recent experiments wherein giant unilamellar vesicles were compressed in a nearly identical manner.

Epsin N-terminal Homology Domain (ENTH) Activity as a Function of Membrane Tension

The epsin N-terminal homology domain (ENTH) is a major player in clathrin-mediated endocytosis. To investigate the influence of initial membrane tension on ENTH binding and activity, we established a bilayer system based on adhered giant unilamellar vesicles (GUVs) to be able to control and adjust the membrane tension σ covering a broad regime. The shape of each individual adhered GUV as well as its adhesion area was monitored by spinning disc confocal laser microscopy. Control of σ in a range of 0.08-1.02 mN/m was achieved by altering the Mg(2+) concentration in solution, which changes the surface adhesion energy per unit area of the GUVs. Specific binding of ENTH to phosphatidylinositol 4,5-bisphosphate leads to a substantial increase in adhesion area of the sessile GUV. At low tension (<0.1 mN/m) binding of ENTH can induce tubular structures, whereas at higher membrane tension the ENTH interaction deflates the sessile GUV and thereby increases the adhesion area. The increase in adhesion area is mainly attributed to a decrease in the area compressibility modulus KA We propose that the insertion of the ENTH helix-0 into the membrane is largely responsible for the observed decrease in KA, which is supported by the observation that the mutant ENTH L6E shows a reduced increase in adhesion area. These results demonstrate that even in the absence of tubule formation, the area compressibility modulus and, as such, the bending rigidity of the membrane is considerably reduced upon ENTH binding. This renders membrane bending and tubule formation energetically less costly.

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.