Robust measurement of membrane bending moduli using light sheet fluorescence imaging of vesicle fluctuations

A review on the measurement of the bending rigidity of lipid membranes

This review provides an overview of the latest developments in both experimental and simulation techniques used to assess the bending rigidity of lipid membranes. It places special emphasis on experimental methods that utilize model vesicles to manipulate lipid compositions and other experimental parameters to determine the bending rigidity of the membrane. It also describes two commonly used simulation methods for estimating bending rigidity. The impact of various factors on membrane bending rigidity is summarized, including cholesterol, lipids, salt concentration, surface charge, membrane phase state, peptides, proteins, and polyethylene glycol. These factors are shown to influence the bending rigidity, contributing to a better understanding of the biophysical properties of membranes and their role in biological processes. Furthermore, the review discusses future directions and potential advancements in this research field, highlighting areas where further investigation is required.

Molecular mechanism of claudin-15 strand flexibility: A computational study

Claudins are one of the major components of tight junctions that play a key role in the formation and maintenance of the epithelial barrier function. Tight junction strands are dynamic and capable of adapting their structure in response to large-scale tissue rearrangement and cellular movement. Here, we present molecular dynamics simulations of claudin-15 strands of up to 225 nm in length in two parallel lipid membranes and characterize their mechanical properties. The persistence length of claudin-15 strands is comparable with those obtained from analyses of freeze-fracture electron microscopy. Our results indicate that lateral flexibility of claudin strands is due to an interplay of three sets of interfacial interaction networks between two antiparallel double rows of claudins in the membranes. In this model, claudins are assembled into interlocking tetrameric ion channels along the strand that slide with respect to each other as the strands curve over submicrometer-length scales. These results suggest a novel molecular mechanism underlying claudin-15 strand flexibility. It also sheds light on intermolecular interactions and their role in maintaining epithelial barrier function.

Identifying and Manipulating Giant Vesicles: Review of Recent Approaches

Giant vesicles (GVs) are closed bilayer membranes that primarily comprise amphiphiles with diameters of more than 1 μm. Compared with regular vesicles (several tens of nanometers in size), GVs are of greater scientific interest as model cell membranes and protocells because of their structure and size, which are similar to those of biological systems. Biopolymers and nano-/microparticles can be encapsulated in GVs at high concentrations, and their application as artificial cell bodies has piqued interest. It is essential to develop methods for investigating and manipulating the properties of GVs toward engineering applications. In this review, we discuss current improvements in microscopy, micromanipulation, and microfabrication technologies for progress in GV identification and engineering tools. Combined with the advancement of GV preparation technologies, these technological advancements can aid the development of artificial cell systems such as alternative tissues and GV-based chemical signal processing systems.

Optical Membrane Control with Red Light Enabled by Red-Shifted Photolipids

Photoswitchable phospholipids, or "photolipids", that harbor an azobenzene group in their lipid tails are versatile tools to manipulate and control lipid bilayer properties with light. So far, the limited ultraviolet-A/blue spectral range in which the photoisomerization of regular azobenzene operates has been a major obstacle for biophysical or photopharmaceutical applications. Here, we report on the synthesis of nano- and micrometer-sized liposomes from tetra-ortho-chloro azobenzene-substituted phosphatidylcholine (termed red-azo-PC) that undergoes photoisomerization on irradiation with tissue-penetrating red light (≥630 nm). Photoswitching strongly affects the fluidity and mechanical properties of lipid membranes, although small-angle X-ray scattering and dynamic light scattering measurements reveal only a minor influence on the overall bilayer thickness and area expansion. By controlling the photostationary state and the photoswitching efficiency of red-azo-PC for specific wavelengths, we demonstrate that shape transitions such as budding or pearling and the division of cell-sized vesicles can be achieved. These results emphasize the applicability of red-azo-PC as a nanophotonic tool in synthetic biology and for biomedical applications.

Self-consistent field modeling of mesomorphic phase changes of monoolein and phospholipids in response to additives

Mapping the topological phase behaviour of lipids in aqueous solution is time consuming and finding the ideal lipid system for a desired application is often a matter of trial and error. Modelling techniques that can accurately predict the mesomorphic phase behaviour of lipid systems are therefore of paramount importance. Here, the self-consistent field theory of Scheutjens and Fleer (SF-SCF) in which a lattice refinement has been implemented, is used to scrutinize how various additives modify the self-assembled phase behaviour of monoolein (MO) and 1,2-dioleoyl-phosphatidylcholine (DOPC) lipids in water. The mesomorphic behaviour is inferred from trends in the mechanical properties of equilibrium lipid bilayers with increasing additive content. More specifically, we focus on the Helfrich parameters, that is, the mean and Gaussian bending rigidities (κ and [small kappa, Greek, macron], respectively) supplemented with the spontaneous curvature of the monolayer (Jm0). We use previously established interaction parameters that position the unperturbed DOPC system in the lamellar Lα phase ([small kappa, Greek, macron] < 0, κ > 0 and Jm0 ≈ 0). Similar interaction parameters position the MO system firmly in a bicontinuous cubic phase ([small kappa, Greek, macron] > 0). In line with experimental data, a mixture of MO and DOPC tends to be in one of these two phases, depending on the mixing ratio. Moreover we find good correlations between predicted trends and experimental data concerning the phase changes of MO in response to a wide range of additives. These correlations give credibility to the use of SF-SCF modelling as a valuable tool to quickly explore the mesomorphic phase space of (phospho)lipid bilayer systems including additives.

Structural and mechanical parameters of lipid bilayer membranes using a lattice refined self-consistent field theory

The self-consistent field theory of Scheutjens and Fleer is implemented on a grid with (lattice) sites that are smaller than the segment size. In this quasi lattice-free implementation we consider united atom-like molecular models and study bilayer self-assembly of phospholipids in a selective solvent (water). We find structural as well as mechanical parameters for these bilayers. The mean (κ) and Gaussian ([small kappa, Greek, macron]) bending moduli, as well as the spontaneous curvature of the monolayer (Jm0), are computed for the first time following a grand canonical ensemble route. Results are in line with previous estimates for mechanical parameters that at the time could not be made following this correct route. This proves that the mean bending modulus is only a very weak function of the membrane tension. We performed a systematic study on the effects of model parameter variations. The mean bending modulus generally grows with increasing bilayer thickness. As expected Jm0 and [small kappa, Greek, macron] behave oppositely with respect to each other and for classical phospholipids assumes values near zero. As an example, an increase in the lipophilic to hydrophilic ratio in the lipids, may cause the Gaussian bending rigidity to switch sign from negative to positive, while - not necessarily at the same point - the spontaneous curvature of the monolayer may switch sign from positive to negative. Together with other investigated trends, these results point to mechanisms of how topological phase transitions of the lipid bilayer membranes may be regulated in the biological context, which correlates with known lipid phase behaviour.

Membrane Shape Dynamics-Based Analysis of the Physical Properties of Giant Unilamellar Vesicles Prepared by Inverted Emulsion and Hydration Techniques

The giant unilamellar vesicle (GUV) is a basic model of the cell membrane that allows for the modulation and control of membrane shape dynamics, which play essential roles in the functions of living cell membranes. However, to properly use these artificial cell-like model systems, we need to understand their physical properties. GUV generation techniques are key technologies in the synthesis of artificial cell-like model systems. However, it is unclear whether GUVs produced by different techniques have the same physical properties. Here, we have investigated the physical properties of GUVs prepared by inverted emulsion and hydration techniques by examining the membrane shape deformation induced by external stimulation with a nonionic surfactant. We reveal differences in the spontaneous curvature of the membrane, the preferred differential area between the inner and outer leaflets of the membrane, and the edge tension of membrane pores between the GUVs prepared using the two distinct techniques.

Physico-chemical studies of elastic compliance and adsorption of DOPC vesicles and its mixture with charged lipids at fluid/solid interface

Lipid bilayer mechanics is crucial to membrane dynamics and in design of liposomes for delivery applications. In this work, vesicles of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (size from 50 nm to 1 μm) and its mixtures with anionic 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) sodium salt (DOPG) and cationic dimethyldioctadecylammonium bromide (DODAB), have been studied under shear stress at fluid/solid interface and their elastic compliance evaluated. Results show that the rate of spreading of the smaller vesicles (∼70 nm) is about 1.4 times slower than those of larger ones (∼1 μ) and that DOPC has the highest elastic compliance compared with DOPC + DOPG and DOPC + DODAB vesicles. A direct correlation between the elastic compliance and the size of the vesicles shows larger vesicles are more structurally labile during adsorption and subsequent adhesion to solid surfaces than the smaller ones. Specific role of bound water in DODAB is reflected in the lowest elastic compliance of DODAB compared to other lipids. Results show that during the process of adhesion at the fluid/air interface, the vesicles undergo contraction, thereby transmitting mechanical stresses to their microenvironment, which matches the SAXS electron density profiles that indicates larger vesicles have thicker bilayer membranes with larger volume of water compared to the smaller sized ones.

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.

Wrapping of Microparticles by Floppy Lipid Vesicles

Lipid membranes, the barrier defining living cells and many of their subcompartments, bind to a wide variety of nano- and micrometer sized objects. In the presence of strong adhesive forces, membranes can strongly deform and wrap the particles, an essential step in crossing the membrane for a variety of healthy and disease-related processes. A large body of theoretical and numerical work has focused on identifying the physical properties that underly wrapping. Using a model system of micron-sized colloidal particles and giant unilamellar lipid vesicles with tunable adhesive forces, we measure a wrapping phase diagram and make quantitative comparisons to theoretical models. Our data are consistent with a model of membrane-particle interactions accounting for the adhesive energy per unit area, membrane bending rigidity, particle size, and vesicle radius.

Fluctuation spectroscopy of giant unilamellar vesicles using confocal and phase contrast microscopy

A widely used method to measure the bending rigidity of bilayer membranes is fluctuation spectroscopy, which analyses the thermally-driven membrane undulations of giant unilamellar vesicles recorded with either phase-contrast or confocal microscopy. Here, we analyze the fluctuations of the same vesicle using both techniques and obtain consistent values for the bending modulus. We discuss the factors that may lead to discrepancies.

Non-contact microfluidic mechanical property measurements of single apoptotic bodies

BACKGROUND

Cells exchange information by secreting micro- and nanosized extracellular vesicles (EVs), ranging from exosomes (30-100 nm) to apoptotic bodies (ABs, 1-5 μm). There is still much to understand about fundamental EV biological, physical, and chemical properties before clinical applications can be developed. EV mechanical properties have only been measured with atomic force microscopy (AFM) with its problematic adhesion and hard substrate effects. To understand EV mechanical behavior in less extreme mechanical conditions relevant to blood flow and many soft tissue environments, a non-contact measurement technique is needed.

METHODS

We measured the mechanical properties of single microscale ABs derived from human blood plasma using non-contact microfluidics. EVs were gently stretched in extensional flow, similar to a traditional tensile test, and a linear mechanical model was applied to estimate mechanical stiffnesses from the observed stretching.

RESULTS

The effective shear elastic modulus of ABs in non-contact flow conditions is approximately 5.6 ± 0.5 Pa, 7 orders of magnitude lower than previously reported AFM-measured biological exosome stiffnesses and 200 times smaller than suspended cells.

CONCLUSIONS

Apoptotic bodies are very soft in fluid environments and exhibit lower effective stiffnesses than suspended cells. By measuring ABs in a natural fluid environment and low-force regime without hard probes and surfaces, we achieved closer agreement with linear mechanical theory and therefore more accurate stiffness measurements.

GENERAL SIGNIFICANCE

AFM manufacturers and users should consider implementing new mechanical models to interpret AFM force indentation curves so that accurate extracellular vesicle mechanical properties can be extracted.

The effect of lipid phase on liposome stability upon exposure to the mechanical stress

Mechanical properties of a lipid bilayer are parameters determined mainly for giant unilamellar vesicles (GUVs). It is not clear if values obtained on the GUV model can be directly translated to submicron large unilamellar vesicles (LUVs). This ambiguity is a major obstacle in exploring the effect of lipid bilayer mechanics on membrane associated processes and effectiveness of liposome-based targeted drug delivery systems. In presented work extrusion, which is a common method to prepare LUVs, was used to study liposomes preparation and stability upon exposure to mechanical stress. The effect of parameters of the extrusion process (temperature, membrane pore size, extrusion force and volumetric flux) on the properties of liposome suspension (average liposome size, polydispersity index and lipid recovery ratio) was determined for model liposomes composed of DPPC lipid. The state of the DPPC lipid bilayer depends on temperature, therefore, the effect of lipid bilayer mechanics on the extrusion process can be quantitated without altering membrane composition. The extrusion process was carried out with the automated extruder delivering quantitative data on the extrusion force and volumetric flux. Obtained results have been interpreted in terms of mechanical properties of the lipid bilayer. Determined mechanical properties of the lipid bilayer and its dependence on temperature are in good agreement with the literature results determined for GUVs. This shows that mechanical properties of the lipid bilayer does not depend on the liposome size in the range from 100 nm to hundreds of microns.

The Role of the Optical Stretcher Is Crucial in the Investigation of Cell Mechanics Regulating Cell Adhesion and Motility

The mechanical properties of cells, tissues, and the surrounding extracellular matrix environment play important roles in the process of cell adhesion and migration. In physiological and pathological processes of the cells, such as wound healing and cancer, the capacity to migrate through the extracellular matrix is crucial. Hence biophysical techniques were used to determine the mechanical properties of cells that facilitate the various migratory capacities. Since the field of mechanobiology is rapidly growing, the reliable and reproducible characterization of cell mechanics is required that facilitates the adhesion and migration of cells. One of these cell mechanical techniques is the optical stretching device, which was originally developed to investigate the mechanical properties of cells, such as the deformation of single cells in suspension. After discussing the strengths and weaknesses of the technology, the latest findings in optical stretching-based cell mechanics are presented in this review. Finally, the mechanical properties of cells are correlated with their migratory potential and it is pointed out how the inhibition of biomolecules that contribute to the to the maintenance of cytoskeletal structures in cells affect their mechanical deformability.

Variations in carotenoid content and acyl chain composition in exponential, stationary and biofilm states of Staphylococcus aureus, and their influence on membrane biophysical properties

Bacteria are often found in close association with surfaces, resulting in the formation of biofilms. In Staphylococcus aureus (S. aureus), biofilms are implicated in the resilience of chronic infections, presenting a serious clinical problem world-wide. Here, S. aureus biofilms are grown under flow within clinical catheters at 37 °C. The lipid composition and biophysical properties of lipid extracts from these biofilms are compared with those from exponential growth and stationary phase cells. Biofilms show a reduction in iso and anteiso branching compensated by an increase in saturated fatty acids compared to stationary phase. A drastic reduction in carotenoid levels is also observed during biofilm formation. Thermotropic measurements of Laurdan GP and DPH polarization, show a reduction of lipid packing at 37 °C for biofilms compared to stationary phase. We studied the effects of carotenoid content on DMPG and DPPG model membranes showing trends in thermotropic behavior consistent with those observed in bacterial isolates, indicating that carotenoids participate in modulating lipid packing. Additionally, bending elastic constant (k_c) measurements using vesicle fluctuation analysis (VFA) show that the presence of carotenoids can increase membrane bending rigidity. The antimicrobial peptide Magainin H2 was less activity on liposomes composed of stationary phase compared to biofilms or exponential growth isolates. This study contributes to an understanding of how Staphylococcus aureus modulates the composition of its membrane lipids, and how those changes affect the biophysical properties of membranes, which in turn may play a role in its virulence and its resistance to different membrane-active antimicrobial agents.

Statistical Analysis of Bending Rigidity Coefficient Determined Using Fluorescence-Based Flicker-Noise Spectroscopy

Bending rigidity coefficient describes propensity of a lipid bilayer to deform. In order to measure the parameter experimentally using flickering noise spectroscopy, the microscopic imaging is required, which necessitates the application of giant unilamellar vesicles (GUV) lipid bilayer model. The major difficulty associated with the application of the model is the statistical character of GUV population with respect to their size and the homogeneity of lipid bilayer composition, if a mixture of lipids is used. In the paper, the bending rigidity coefficient was measured using the fluorescence-enhanced flicker-noise spectroscopy. In the paper, the bending rigidity coefficient was determined for large populations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphocholine vesicles. The quantity of obtained experimental data allows to perform statistical analysis aiming at the identification of the distribution, which is the most appropriate for the calculation of the value of the membrane bending rigidity coefficient. It has been demonstrated that the bending rigidity coefficient is characterized by an asymmetrical distribution, which is well approximated with the gamma distribution. Since there are no biophysical reasons for that we propose to use the difference between normal and gamma fits as a measure of the homogeneity of vesicle population. In addition, the effect of a fluorescent label and types of instrumental setups on determined values has been tested. Obtained results show that the value of the bending rigidity coefficient does not depend on the type of a fluorescent label nor on the type of microscope used.

On the origin of shape fluctuations of the cell nucleus

The nuclear envelope (NE) presents a physical boundary between the cytoplasm and the nucleoplasm, sandwiched in between two highly active systems inside the cell: cytoskeleton and chromatin. NE defines the shape and size of the cell nucleus, which increases during the cell cycle, accommodating for chromosome decondensation followed by genome duplication. In this work, we study nuclear shape fluctuations at short time scales of seconds in human cells. Using spinning disk confocal microscopy, we observe fast fluctuations of the NE, visualized by fluorescently labeled lamin A, and of the chromatin globule surface (CGS) underneath the NE, visualized by fluorescently labeled histone H2B. Our findings reveal that fluctuation amplitudes of both CGS and NE monotonously decrease during the cell cycle, serving as a reliable cell cycle stage indicator. Remarkably, we find that, while CGS and NE typically fluctuate in phase, they do exhibit localized regions of out-of-phase motion, which lead to separation of NE and CGS. To explore the mechanism behind these shape fluctuations, we use biochemical perturbations. We find the shape fluctuations of CGS and NE to be both thermally and actively driven, the latter caused by forces from chromatin and cytoskeleton. Such undulations might affect gene regulation as well as contribute to the anomalously high rates of nuclear transport by, e.g., stirring of molecules next to NE, or increasing flux of molecules through the nuclear pores.

Cholesterol-Enriched Domain Formation Induced by Viral-Encoded, Membrane-Active Amphipathic Peptide

The α-helical (AH) domain of the hepatitis C virus nonstructural protein NS5A, anchored at the cytoplasmic leaflet of the endoplasmic reticulum, plays a role in viral replication. However, the peptides derived from this domain also exhibit remarkably broad-spectrum virocidal activity, raising questions about their modes of membrane association. Here, using giant lipid vesicles, we show that the AH peptide discriminates between membrane compositions. In cholesterol-containing membranes, peptide binding induces microdomain formation. By contrast, cholesterol-depleted membranes undergo global softening at elevated peptide concentrations. Furthermore, in mixed populations, the presence of ∼100 nm vesicles of viral dimensions suppresses these peptide-induced perturbations in giant unilamellar vesicles, suggesting size-dependent membrane association. These synergistic composition- and size-dependent interactions explain, in part, how the AH domain might on the one hand segregate molecules needed for viral assembly and on the other hand furnish peptides that exhibit broad-spectrum virocidal activity.

Membrane stiffness is modified by integral membrane proteins

The ease with which a cell membrane can bend and deform is important for a wide range of biological functions. Peripheral proteins that induce curvature in membranes (e.g. BAR domains) have been studied for a number of years. Little is known, however, about the effect of integral membrane proteins on the stiffness of a membrane (characterised by the bending rigidity, Kc). We demonstrate by computer simulation that adding integral membrane proteins at physiological densities alters the stiffness of the membrane. First we establish that the coarse-grained MARTINI forcefield is able to accurately reproduce the bending rigidity of a small patch of 1500 phosphatidyl choline lipids by comparing the calculated value to both experiment and an atomistic simulation of the same system. This enables us to simulate the dynamics of large (ca. 50 000 lipids) patches of membrane using the MARTINI coarse-grained description. We find that altering the lipid composition changes the bending rigidity. Adding integral membrane proteins to lipid bilayers also changes the bending rigidity, whilst adding a simple peripheral membrane protein has no effect. Our results suggest that integral membrane proteins can have different effects, and in the case of the bacterial outer membrane protein, BtuB, the greater the density of protein, the larger the reduction in stiffness.

Translocation of an Incompressible Vesicle through a Pore

We have derived the free energy landscape for the translocation of a single vesicle through a narrow pore by accounting for bending and stretching of the vesicle, and the deformation of the vesicle by the pore. Emergence of a free energy barrier for translocation is a general result, and the magnitude of the barrier is calculated in terms of the various material parameters. The extent of the reduction in the barrier by the presence of an external constant force is calculated. Using the Fokker-Planck formalism, we have calculated the average translocation time corresponding to the various free energy landscapes representing different parameter sets. The dependencies of the average translocation time on the strength of the external force, vesicle size, bending and stretching moduli of the vesicle, and radius and length of the pore are derived, and the computed results are discussed.

Membrane remodeling and mechanics: Experiments and simulations of α-Synuclein

We review experimental and simulation approaches that have been used to determine curvature generation and remodeling of lipid bilayers by membrane-bending proteins. Particular emphasis is placed on the complementary approaches used to study α-Synuclein (αSyn), a major protein involved in Parkinson's disease (PD). Recent cellular and biophysical experiments have shown that the protein 1) deforms the native structure of mitochondrial and model membranes; and 2) inhibits vesicular fusion. Today's advanced experimental and computational technology has made it possible to quantify these protein-induced changes in membrane shape and material properties. Collectively, experiments, theory and multi-scale simulation techniques have established the key physical determinants of membrane remodeling and rigidity: protein binding energy, protein partition depth, protein density, and membrane tension. Despite the exciting and significant progress made in recent years in these areas, challenges remain in connecting biophysical insights to the cellular processes that lead to disease. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

Bud-neck scaffolding as a possible driving force in ESCRT-induced membrane budding

Membrane budding is essential for processes such as protein sorting and transport. Recent experimental results with ESCRT proteins reveal a novel budding mechanism, with proteins emerging in bud necks but separated from the entire bud surface. Using an elastic model, we show that ESCRT protein shapes are sufficient to spontaneously create experimentally observed structures, with protein-membrane interactions leading to protein scaffolds in bud-neck regions. Furthermore, the model reproduces experimentally observed budding directions and bud sizes. Finally, our results reveal that membrane-mediated sorting has the capability of creating structures more complicated than previously assumed.

Recent developments in the field of bending rigidity measurements on membranes

This review gives a brief overview of experimental approaches used to assess the bending rigidity of membranes. Emphasis is placed on techniques based on the use of giant unilamellar vesicles. We summarize the effect on the bending rigidity of membranes as a function of membrane composition, presence of various inclusions in the bilayer and molecules and ions in the bathing solutions. Examples for the impact of temperature, cholesterol, some peptides and proteins, sugars and salts are provided and the literature data are discussed critically. Future directions, open questions and possible developments in this research field are also included.

Measuring lipid membrane viscosity using rotational and translational probe diffusion

The two-dimensional fluidity of lipid bilayers enables the motion of membrane-bound macromolecules and is therefore crucial to biological function. Microrheological methods that measure fluid viscosity via the translational diffusion of tracer particles are challenging to apply and interpret for membranes, due to uncertainty about the local environment of the tracers. Here, we demonstrate a new technique in which determination of both the rotational and translational diffusion coefficients of membrane-linked particles enables quantification of viscosity, measurement of the effective radii of the tracers, and assessment of theoretical models of membrane hydrodynamics. Surprisingly, we find a wide distribution of effective tracer radii, presumably due to a variable number of lipids linked to each tracer particle. Furthermore, we show for the first time that a protein involved in generating membrane curvature, the vesicle trafficking protein Sar1p, dramatically increases membrane viscosity. Using the rheological method presented here, therefore, we are able to reveal a class of previously unknown couplings between protein activity and membrane mechanics.