Recent developments in the field of bending rigidity measurements on membranes

Hepatic endoplasmic reticulum-derived nanodiscs for broad-spectrum drug detoxification

Drug overdose is a pressing global public health challenge, with current detoxification treatments often lacking the broad-spectrum efficacy needed for emergency applications. Inspired by the unique advantages of cell membrane-derived nanodiscs (CNDs), including their compact size, rapid distribution, and preservation of native cell membrane functions, we developed endoplasmic reticulum (ER)-derived nanodiscs (ER-NDs) from the ER membranes of mouse hepatic cells for broad-spectrum drug detoxification. ER-NDs retain natural cytochrome P450 (CYP) enzymes, enabling effective detoxification of three model drugs: bupropion, haloperidol, and propranolol. Cell-based assays demonstrated ER-NDs' ability to mitigate drug-induced cytotoxicity, reduce oxidative stress, and restore antioxidant defenses. In mouse models of drug intoxication, ER-ND treatment significantly improved survival rates and alleviated drug-induced oxidative damage. Importantly, ER-NDs showed no evidence of acute toxicity in vivo. These findings underscore the potential of ER-NDs as a versatile platform for broad-spectrum drug detoxification and as a promising tool for managing drug intoxication in emergency and clinical settings.

Sterols in plant biology - Advances in studying membrane dynamics

Plants sense their environment at the cell surface, i.e. the plasma membrane, where extracellular signals are perceived and transduced. Together with the cortical cytoskeleton and the cell wall, membrane lipids can influence these processes by acting on protein dynamics at the plasma membrane. Among these lipids, sterols regulate membrane fluidity and thus, protein functions. However, plant sterols are diverse in structure and particularly difficult to study due to technical limitations. Nevertheless, advances in sterol imaging, sterol-protein interaction studies, and sterol perturbation methods have resulted in a better understanding of their functions in plant development and physiology. Here we summarize the current knowledge and the latest breakthroughs, and discuss future challenges, in the field of plant sterol biology and cell surface organization.

Cholesterol Affects the Pore Formation and the Membrane-Membrane Interaction Induced by an Antimicrobial Peptide, NK-2, in Phospholipid Vesicles

Antimicrobial peptides are part of the innate immune response and show their antimicrobial activity by forming pores, followed by disintegration of the membrane. Cholesterol in the membrane can affect the pore formation process, as cholesterol is known to alter the permeability and elastic properties of the membrane. The present research systematically explores the role of cholesterol in modulating the interaction of the antimicrobial peptide NK-2 with phospholipid membranes, as well as the processes of pore formation induced by NK-2 within the membrane. Large unilamellar vesicles (LUVs) and giant unilamellar vesicles (GUVs) made from DOPC-DOPG and Egg PC with varying cholesterol concentrations have been studied using a variety of experimental techniques. The present study revealed that both the magnitude of zeta potential and surface charge density diminished as cholesterol concentrations increased at an intermediate NK-2 concentration. The proliferation of the size distributions of LUVs containing cholesterol when exposed to NK-2 indicates the occurrence of vesicle aggregation. The phase contrast micrographs of GUVs as well as the calcein release experiments on LUVs show evidence of pores. Notably, the incorporation of cholesterol into the membrane was found to have a significant effect on both the permeability of the membrane and the kinetics of the pore formation process. This biophysical research contributes essential knowledge regarding the role of cholesterol in influencing the antimicrobial efficacy of the membrane.

Curvature-sensing and generation by membrane proteins: a review

Membrane proteins are crucial in regulating biomembrane shapes and controlling the dynamic changes in membrane morphology during essential cellular processes. These proteins can localize to regions with their preferred curvatures (curvature sensing) and induce localized membrane curvature. Thus, this review describes the recent theoretical development in membrane remodeling performed by membrane proteins. The mean-field theories of protein binding and the resulting membrane deformations are reviewed. The effects of hydrophobic insertions on the area-difference elasticity energy and that of intrinsically disordered protein domains on the membrane bending energy are discussed. For the crescent-shaped proteins, such as Bin/Amphiphysin/Rvs superfamily proteins, anisotropic protein bending energy and orientation-dependent excluded volume significantly contribute to curvature sensing and generation. Moreover, simulation studies of membrane deformations caused by protein binding are reviewed, including domain formation, budding, and tubulation.

Fusion of asymmetric membranes: the emergence of a preferred direction

The fusion of lipid membranes progresses through a series of intermediate steps with two significant energy barriers: hemifusion-stalk formation and fusion-pore expansion. The cell's ability to tune these energy barriers is crucial as they determine the rate of many biological processes involving membrane fusion. However, a mechanism that allows the cell to manipulate both barriers in the same direction remains elusive, since membrane properties that the cell could dynamically tune during its life cycle, such as the lipids' spontaneous curvatures and membrane tension, have an opposite effect on the two barriers: tension inhibits stalk formation while promoting fusion-pore expansion. In contrast, increasing the total membrane concentration of lipids with negative intrinsic curvatures, such as cholesterol, promotes hemifusion-stalk formation while inhibiting pore expansion, and vice versa for lipids with positive intrinsic curvatures. Therefore, changes in these membrane properties increase one energy barrier at the expense of the other, resulting in a mixed effect on the fusion reaction. A possible mechanism to change both barriers in the same direction is by inducing lipid composition asymmetry, which results in tension and spontaneous curvature differences between the monolayers. To test the feasibility of this mechanism, a continuum elastic model was used to simulate the fusion intermediates and calculate the changes in the energy barriers. The calculations showed that a reasonable lipid composition asymmetry could lead to a 10-20kBT difference in both energy barriers, depending on the direction from which fusion occurs. We further provide experimental support to the model predictions, demonstrating changes in the time to hemifusion upon asymmetry introduction. These results indicate that biological membranes, which are asymmetric, have a preferred direction for fusion.

Interaction of lipid domains originating from differential domain-monolayer contact energy

We consider a flat membrane containing pure lipid domains located in the membrane monolayers and separated in the membrane plane. We assume the energy of contact along the membrane mid-surface between a domain and the underlying monolayer to be different from that between the two monolayers. We theoretically analyse the effect of the differential contact energy on the elastic deformations of tilt and splay in the membrane monolayers and the resulting interaction between two domains situated in the apposed monolayers. We demonstrate that the character of this interaction depends on the ratio, η, between the domain rigidity and that of a regular membrane monolayer. For the rigidity ratio smaller than a critical value, η < η* ≈ 3, the domain interaction is predicted to be attractive for all inter-monolayer distances. For the super-critical values of the rigidity ratio, η > η*, the interaction is repulsive for small distances and attractive for large distances with a certain equilibrium inter-domain separation corresponding to a vanishing interaction force. The predicted attractive interaction is proposed to favor the registration in the membrane plane of apposed domains as observed in most domain-containing membranes.

Vitamin E Acetate Causes Softening of Pulmonary Surfactant Membrane Models

The popularity of electronic cigarettes and vaping products has launched the outbreak of a condition affecting the respiratory system of users, known as electronic-cigarette/vaping-associated lung injury (EVALI). The build-up of vitamin E acetate (VEA), a diluent of some illicit vaping oils, in the bronchoalveolar lavage of patients with EVALI provided circumstantial evidence as a target for investigation. In this work, we provide a fundamental characterization of the interaction of VEA with lung cells and pulmonary surfactant (PS) models to explore the mechanisms by which vaping-related lung injuries may be present. We first confirm the localization and uptake of VEA in pulmonary epithelial cells. Further, as PS is vitally responsible for the biophysical functions of the lungs, we explore the effect of added VEA on three increasingly complex models of PS: dipalmitoylphosphatidylcholine (DPPC), a lipid-only synthetic PS, and the biologically derived extract Curosurf. Using high-resolution techniques of small-angle X-ray scattering, small-angle neutron scattering, neutron spin-echo spectroscopy, and neutron reflectometry, we compare the molecular-scale behaviors of these membranes to the bulk viscoelastic properties of surfactant monolayer films as studied by Langmuir monolayer techniques. While VEA does not obviously alter the structure or organization of PS membranes, a consistent softening of membrane systems─regardless of compositional complexity─provides a biophysical explanation for the respiratory distress associated with EVALI and yields a new perspective on the behavior of the PS system.

Membrane-mediated interactions between arc-shaped particles strongly depend on membrane curvature

Besides direct molecular interactions, proteins and nanoparticles embedded in or adsorbed to membranes experience indirect interactions that are mediated by the membranes. Membrane-mediated interactions between curvature-inducing proteins or nanoparticles can lead to assemblies of particles that generate highly curved spherical or tubular membrane shapes, but have mainly been quantified for planar or weakly curved membranes. In this article, we systematically investigate the membrane-mediated interactions of arc-shaped particles adsorbed to a variety of tubular and spherical membrane shapes with coarse-grained modelling and simulations. These arc-shaped particles induce membrane curvature by binding to the membrane with their inner, concave side akin to N-BAR domain proteins. We determine both the pairwise interaction free energy, which includes entropic contributions due to rotational entropy loss at close particle distances, and the pairwise interaction energy without entropic components from particle distributions observed in the simulations. For membrane shapes with small curvature, the membrane-mediated interaction free energies of particle pairs exceed the thermal energy kBT and can lead to particle ordering and aggregation. The interactions strongly decrease with increasing curvature of the membrane shape and are minimal for tubular shapes with membrane curvatures close to the particle curvature.

Picofluidic Electro-Osmosis Measurement of Cell Membrane Mechanical Properties

Cells connect with their internal and external environments through plasma membranes, and the mechanical properties of cell membranes govern numerous biological events. Membrane detection techniques such as optical or magnetic tweezers have revealed mechanical strength by membrane-anchored modifications, but it remains challenging to develop label-free methods to reduce the influence of exogenous interference. Here picofluidic electro-osmosis measurement (PEOM), which enables direct and efficient sensing of cell membrane mechanical properties by using a glass nanopipette without labeling, is presented. By generating a picoliter electroosmotic fluid at the nanopipette tip, periodic cell membrane vibration modes are observed from current traces, which carry information on membrane mechanical properties to indicate its biological state. Based on characteristic peaks in the frequency domain, a theoretical framework to describe the vibration modes, which contains two ideal spring vibrator models corresponding to stretching and bending vibrations of cell membrane respectively, is developed. Notably, the PEOM strategy represents a label-free approach to reveal the mechanical properties of living cell membranes from two dimensions, which is completely different from other methods. Additionally, the exciting potential of PEOM is demonstrated for label-free observation of membrane mechanical property changes during different bioprocesses, including cytoskeletal alteration, membrane tension change, and mechanical polarization.

Multiscale Simulations of Membrane Adhesion Mediated by CD47-SIRPα Complexes

Adhesion of biological cells is essential for various processes, including tissue formation, immune responses, and signaling. It involves multiple length scales, ranging from nanometers to micrometers, which are characteristic of (a) the intercellular receptor-ligand binding that mediates the cell adhesion, (b) the spatial distribution of the receptor and ligand proteins in the membranes of adhering cells, (c) adhesion-induced deformations and thermal undulations of the membranes, (d) the overall size of the interface between adhering cells. Therefore, computer simulations of cell membrane adhesion require multiscale modeling and suitable approximations that capture the essential physics of the system under study. Here, we introduce such a multiscale approach to study membrane adhesion mediated by the CD47-SIRPα binding, which is an immunologically relevant process. The synergetic use of coarse-grained molecular dynamics simulations and mesoscale kinetic Monte Carlo simulations allows us to explore both equilibrium properties and dynamical behavior of adhering membranes on the relevant length scales between 1 nm and 1 μm on time scales ranging from 0.1 ns all the way up to about 20 s. The multiscale simulations not only reproduce available experimental data but also give quantitative predictions on binding-induced conformational changes of SIRPα and membrane-mediated cooperativity of the CD47-SIRPα binding as well as fluctuation-induced interactions between the CD47-SIRPα complexes. Our approach is applicable to various membrane proteins and provides invaluable data for comparison with experimental findings.

Small-Angle and Quasi-Elastic Neutron Scattering from Polydisperse Oligolamellar Vesicles Containing Glycolipids

Glycolipids are known to stabilize biomembrane multilayers through preferential sugar-sugar interactions that act as weak transient membrane cross-links. Here, we use small-angle and quasi-elastic neutron scattering on oligolamellar phospholipid vesicles containing defined glycolipid fractions in order to elucidate the influence of glycolipids on membrane mechanics and dynamics. Small-angle neutron scattering (SANS) reveals that the oligolamellar vesicles (OLVs) obtained by extrusion are polydisperse with regard to the number of lamellae, n, which renders the interpretation of the quasi-elastic neutron spin echo (NSE) data nontrivial. To overcome this problem, we propose a method to model the NSE data in a rigorous fashion based on the obtained histograms of n and on their q-dependent intensity-weighted contribution. This procedure yields meaningful values for the bending rigidity of individual lipid membranes and insights into the mechanical coupling between adjacent membrane lamellae including the effect of the glycolipids.

Spatiotemporal pattern formation of membranes induced by surface molecular binding/unbinding

Nonequilibrium membrane pattern formation is studied using meshless membrane simulation. We consider that molecules bind to either surface of a bilayer membrane and move to the opposite leaflet by flip-flop. When binding does not modify the membrane properties and the transfer rates among the three states are cyclically symmetric, the membrane exhibits spiral-wave and homogeneous-cycling modes at high and low binding rates, respectively, as in an off-lattice cyclic Potts model. When binding changes the membrane spontaneous curvature, these spatiotemporal dynamics are coupled with microphase separation. When two symmetric membrane surfaces are in thermal equilibrium, the membrane domains form 4.8.8 tiling patterns in addition to stripe and spot patterns. In nonequilibrium conditions, moving biphasic domains and time-irreversible fluctuating patterns appear. The domains move ballistically or diffusively depending on the conditions.

Curvature expansion method for polymer self-assembled membranes

Polymer vesicles consist of a thin bilayer membrane, the shape of which can be precisely controlled by adjusting the composition of block copolymers. These self-assembled structures provide a powerful method for mimicking biological cell membranes. A key characteristic of these membranes is their elastic moduli, which play a crucial role in determining their mechanical properties. Traditionally, these moduli are derived by fitting the free energy of different membrane shapes to Helfrich's model. In this paper, we present a curvature expansion method that yields an analytical expression for the moduli within the framework of self-consistent field theory. A significant advantage of our approach is its ability to easily extract higher-order moduli, a task that presents substantial challenges for existing methods.

Biallelic variants in SREBF2 cause autosomal recessive spastic paraplegia

Hereditary spastic paraplegias (HSPs) refer to a genetically and clinically heterogeneous group of neurodegenerative disorders characterized by the degeneration of motor neurons. To date, a significant number of patients still have not received a definite genetic diagnosis. Therefore, identifying unreported causative genes continues to be of great importance. Here, we perform whole exome sequencing in a cohort of Chinese HSP patients. Three homozygous variants (p.L604W, p.S517F, and p.T984A) within the sterol regulatory element-binding factor 2 (SREBF2) gene are identified in one autosomal recessive family and two sporadic patients, respectively. Co-segregation is confirmed by Sanger sequencing in all available members. The three variants are rare in the public or in-house database and are predicted to be damaging. The biological impacts of variants in SREBF2 are examined by functional experiments in patient-derived fibroblasts and Drosophila. We find that the variants upregulate cellular cholesterol due to the overactivation of SREBP2, eventually impairing the autophagosomal and lysosomal functions. The overexpression of the mature form of SREBP2 leads to locomotion defects in Drosophila. Our findings identify SREBF2 as a causative gene for HSP and highlight the impairment of cholesterol as a critical pathway for HSP.

Microtube self-assembly leads to conformational freezing point depression

HYPOTHESIS

Multi-walled tubular aggregates formed by hierarchical self-assembly of beta-cyclodextrin (β-CD) and sodium dodecylsulfate (SDS) hold a great potential as microcarriers. However, the underlying mechanism for this self-assembly is not well understood. To advance the application of these structures, it is essential to fine-tune the cavity size and comprehensively elucidate the energetic balance driving their formation: the bending modulus versus the microscopic line tension.

EXPERIMENTS

We investigated temperature-induced changes in the hierarchical tubular aggregates using synchrotron small-angle X-ray scattering across a broad concentration range. Detailed analysis of the scattering patterns enabled us to determine the structural parameters of the microtubes and to construct a phase diagram of the system.

FINDINGS

The microtubes grow from the outside in and melt from the inside out. We relate derived structural parameters to enthalpic changes driving the self-assembly process on the molecular level in terms of their bending modulus and microscopic line tension. We find that the conformation of the crystalline bilayer affects the saturation concentration, providing an example of a phenomenon we call conformational freezing point depression. Inspired by the colligative phenomenon of freezing point depression, well known from undergraduate physics, we model this system by including the membrane conformation, which can describe the energetics of this hierarchical system and give access to microscopic properties without free parameters.

Cellular and Nuclear Forces: An Overview

Biological cells sample their surrounding microenvironments using nanoscale force sensors on the cell surfaces. These surface-based force and stress sensors generate physical and chemical responses inside the cell. The inherently well-connected cytoskeleton and its physical contacts with the force elements on the nuclear membrane lead these physicochemical responses to cascade all the way inside the cell nucleus, physically altering the nuclear state. These physical alterations of the cell nucleus, through yet-unknown complex steps, elicit physical and functional responses from the chromatin in the form of altered gene expression profiles. This mechanism of force/stress sensing by the cell and then its nuclear response has been shown to play a vital role in maintaining robust cellular homeostasis, controlling gene expression profiles during developmental phases as well as cell differentiation. In the last few years, there has been appreciable progress toward the identification of the molecular players responsible for force sensing. However, the actual sensing mechanism of cell surface-bound force sensors and more importantly cascading of the signals, both physical (via cytosolic force sensing elements such as microtubule and actin framework) as well as chemical (cascade of biochemical signaling from cell surface to nuclear surface and further to the chromatin), inside the cell is poorly understood. In this chapter, we present a review of the currently known molecular players in cellular as well as nuclear force sensing repertoire and their possible mechanistic aspects. We also introduce various biophysical concepts and review some frequently used techniques that are used to describe the force/stress sensing and response of a cell. We hope that this will help in asking clearer questions and designing pointed experiments for better understanding of the force-dependent design principles of the cell surface, nuclear surface, and gene expression.

Exploration of lipid bilayer mechanical properties using molecular dynamics simulation

Important biological structures known for their exceptional mechanical qualities, lipid bilayers are essential to many cellular functions. Fluidity, elasticity, permeability, stiffness, tensile strength, compressibility, shear viscosity, line tension, and curvature elasticity are some of the fundamental characteristics affecting their behavior. The purpose of this review is to examine these characteristics in more detail by molecular dynamics simulation, elucidating their importance and the elements that lead to their appearance in lipid bilayers. Comprehending the mechanical characteristics of lipid bilayers is critical for creating medications, drug delivery systems, and biomaterials that interact with biological membranes because it allows one to understand how these materials respond to different stresses and deformations. The influence of mechanical characteristics on important lipid bilayer properties is examined in this review. The mechanical properties of lipid bilayers were clarified through the use of molecular dynamics simulation analysis techniques, including bilayer thickness, stress-strain analysis, lipid bilayer area compressibility, membrane bending rigidity, and time- or ensemble-averaged the area per lipid evaluation. We explain the significance of molecular dynamics simulation analysis methods, providing important new information about the stability and dynamic behavior of the bilayer. In the end, we hope to use molecular dynamics simulation to provide a comprehensive understanding of the mechanical properties and behavior of lipid bilayers, laying the groundwork for further studies and applications. Taken together, careful investigation of these mechanical aspects deepens our understanding of the adaptive capacities and functional roles of lipid bilayers in biological environments.

Mechanical characterization of freestanding lipid bilayers with temperature-controlled phase

Coexistence of lipid domains in cell membranes is associated with vital biological processes. Here, we investigate two such membranes: a multi-component membrane composed of DOPC and DPPC lipids with gel and fluid separated domains, and a single component membrane composed of PMPC lipids forming ripples. We characterize their mechanical properties below their melting point, where ordered and disordered regions coexist, and above their melting point, where they are in fluid phase. To conduct these inquiries, we create lipid bilayers in a microfluidic chip interfaced with a heating system and optical tweezers. The chip features a bubble trap and enables high-throughput formation of planar bilayers. Optical tweezers experiments reveal interfacial hydrodynamics (fluid-slip) and elastic properties (membrane tension and bending rigidity) at various temperatures. For PMPC bilayers, we demonstrate a higher fluid slip at the interface in the fluid-phase compared to the ripple phase, while for the DOPC:DPPC mixture, similar fluid slip is measured below and above the transition point. Membrane tension for both compositions increases after thermal fluidization. Bending rigidity is also measured using the forces required to extend a lipid nanotube pushed out of the freestanding membranes. This novel temperature-controlled microfluidic platform opens numerous possibilities for thermomechanical studies on freestanding planar membranes.

Effects of Normal and Lateral Electric Fields on Membrane Mechanical Properties

As a core component of biological and synthetic membranes, lipid bilayers are key to compartmentalizing chemical processes. Bilayer morphology and mechanical properties are heavily influenced by electric fields, such as those caused by biological ion concentration gradients. We present atomistic simulations exploring the effects of electric fields applied normally and laterally to lipid bilayers. We find that normal fields decrease membrane tension, while lateral fields increase it. Free energy perturbation calculations indicate the importance of dipole-dipole interactions to these tension changes, especially for lateral fields. We additionally show that membrane area compressibilities can be related to their cohesive energies, allowing us to estimate changes in membrane bending rigidity under applied fields. We find that normal and lateral fields decrease and increase bending rigidity, respectively. These results point to the use of directed electric fields to locally control membrane stiffness, thereby modulating associated cellular processes.

Compound giant unilamellar vesicles as a bio-mimetic model for electrohydrodynamics of a nucleate cell

The understanding obtained by studies on the electrohydrodynamics (EHD) of single giant unilamellar vesicles (sGUVs) has contributed significantly towards a better comprehension of the response of biological cells to electric fields. This has subsequently helped in developing technologies such as cell dielectrophoresis and cell electroporation. For nucleate eukaryotic cells though, a vesicle-in-vesicle compound giant unilamellar vesicle (cGUV) is a more appropriate bio-mimic than a sGUV. In this work, we present an improvised method for the formation of cGUVs, wherein the electrical conductivities of the inner, annular and outer regions of the cGUVs can be modified. A comprehensive experimental study is presented on the EHD of these cGUVs under weak AC fields over a wide range of frequencies, and an encouraging agreement is observed between the experiments and earlier published theoretical studies on concentric cGUVs. The spherical, prolate or oblate spheroidal deformations of a cGUV under AC electric fields depend upon the membrane electromechanical properties as well as the magnitude and direction of the electric traction at the membrane produced by the Maxwell stress that varies with the relative timescales associated with the frequency of the applied AC electric field and that of the membrane charging time and the Maxwell-Wagner relaxation time. This work establishes cGUVs as appropriate bio-mimics for conducting EHD studies relevant to eukaryotic cells.

Dynamins combine mechano-constriction and membrane remodeling to enable two-step mitochondrial fission via a 'snap-through' instability

Mitochondrial fission is controlled by dynamin proteins, the dysregulation of which is correlated with diverse diseases. Fission dynamins are GTP hydrolysis-driven mechanoenzymes that self-oligomerize into helical structures that constrict membrane to achieve fission, but details are not well understood. However, dynamins can also remodel membranes by inducing negative Gaussian curvature, the type of curvature required for completion of fission. Here, we examine how these drastically different mechanisms synergistically exert their influences on a membrane, via a mechanical model calibrated with small-angle X-ray scattering structural data. We find that free dynamin can trigger a "snap-through instability" that enforces a shape transition from an oligomer-confined cylindrical membrane to a drastically narrower catenoid-shaped neck within the spontaneous hemi-fission regime, in a manner that depends critically on the length of the confined tube. These results indicate how the combination of dynamin assembly, and paradoxically disassembly, can lead to diverse pathways to scission.

Interaction of anionic Fe3O4 nanoparticles with lipid vesicles: a review on deformation and poration under various conditions

This review focuses on the deformation and poration of lipid vesicles caused by the interaction of anionic magnetite nanoparticles (MNPs). Effects of various factors, such as surface charge density, salt and sugar concentrations in buffer, membrane cholesterol content, polymer-grafted phospholipid, and membrane potential have been discussed for the interaction of MNPs with lipid vesicles. To quantify these effects on the vesicles, compactness, fraction of deformation and poration, dynamics of membrane permeation, and kinetics of membrane permeation have been critically evaluated. The review explores the potential advancements as well as future directions of the research field in the biomedical application of MNPs.

Roles of membrane mechanics-mediated feedback in membrane traffic

Synthesizing the recent progresses, we present our perspectives on how local modulations of membrane curvature, tension, and bending energy define the feedback controls over membrane traffic processes. We speculate the potential mechanisms of, and the control logic behind, the different membrane mechanics-mediated feedback in endocytosis and exo-endocytosis coupling. We elaborate the path forward with the open questions for theoretical considerations and the grand challenges for experimental validations.

The diffuse interface description of fluid lipid membranes captures key features of the hemifusion pathway and lateral stress profile

Topological transitions of lipid membranes are ubiquitous in key biological processes for cell life, like neurotransmission, fertilization, morphogenesis, and viral infections. Despite this, they are not well understood due to their multiscale nature, which limits the use of molecular models and calls for a mesoscopic approach such as the celebrated Canham-Helfrich one. Unfortunately, such a model cannot handle topological transitions, hiding the crucial involved forces and the appearance of the experimentally observed hemifused intermediates. In this work, we describe the membrane as a diffuse interface preserving the Canham-Helfrich elasticity. We show that pivotal features of the hemifusion pathway are captured by this mesoscopic approach, e.g. a (meta)stable hemifusion state and the fusogenic behavior of negative monolayer spontaneous curvatures. The membrane lateral stress profile is calculated as a function of the elastic rigidities, yielding a coarse-grained version of molecular models findings. Insights into the fusogenic mechanism are reported and discussed.

Temperature change elicits lipidome adaptation in the simple organisms Mycoplasma mycoides and JCVI-syn3B

Cell membranes mediate interactions between life and its environment, with lipids determining their properties. Understanding how cells adjust their lipidomes to tune membrane properties is crucial yet poorly defined due to the complexity of most organisms. We used quantitative shotgun lipidomics to study temperature adaptation in the simple organism Mycoplasma mycoides and the minimal cell JCVI-syn3B. We show that lipid abundances follow a universal logarithmic distribution across eukaryotes and bacteria, with comparable degrees of lipid remodeling for adaptation regardless of lipidomic or organismal complexity. Lipid features analysis demonstrates head-group-specific acyl chain remodeling as characteristic of lipidome adaptation; its deficiency in Syn3B is associated with impaired homeoviscous adaptation. Temporal analysis reveals a two-stage cold adaptation process: swift cholesterol and cardiolipin shifts followed by gradual acyl chain modifications. This work provides an in-depth analysis of lipidome adaptation in minimal cells, laying a foundation to probe the design principles of living membranes.

Biomolecular Condensates in Contact with Membranes

Biomolecular condensates are highly versatile membraneless organelles involved in a plethora of cellular processes. Recent years have witnessed growing evidence of the interaction of these droplets with membrane-bound cellular structures. Condensates' adhesion to membranes can cause their mutual molding and regulation, and their interaction is of fundamental relevance to intracellular organization and communication, organelle remodeling, embryogenesis, and phagocytosis. In this article, we review advances in the understanding of membrane-condensate interactions, with a focus on in vitro models. These minimal systems allow the precise characterization and tuning of the material properties of both membranes and condensates and provide a workbench for visualizing the resulting morphologies and quantifying the interactions. These interactions can give rise to diverse biologically relevant phenomena, such as molecular-level restructuring of the membrane, nano- to microscale ruffling of the condensate-membrane interface, and coupling of the protein and lipid phases.

Mesoscopic elasticity controls dynamin-driven fission of lipid tubules

Mesoscale physics bridges the gap between the microscopic degrees of freedom of a system and its large-scale continuous behavior and highlights the role of a few key quantities in complex and multiscale phenomena, like dynamin-driven fission of lipid membranes. The dynamin protein wraps the neck formed during clathrin-mediated endocytosis, for instance, and constricts it until severing occurs. Although ubiquitous and fundamental for life, the cooperation between the GTP-consuming conformational changes within the protein and the full-scale response of the underlying lipid substrate is yet to be unraveled. In this work, we build an effective mesoscopic model from constriction to fission of lipid tubules based on continuum membrane elasticity and implicitly accounting for ratchet-like power strokes of dynamins. Localization of the fission event, the overall geometry, and the energy expenditure we predict comply with the major experimental findings. This bolsters the idea that a continuous picture emerges soon enough to relate dynamin polymerization length and membrane rigidity and tension with the optimal pathway to fission. We therefore suggest that dynamins found in in vivo processes may optimize their structure accordingly. Ultimately, we shed light on real-time conductance measurements available in literature and predict the fission time dependency on elastic parameters.

Clathrin mediates membrane fission and budding by constricting membrane pores

Membrane budding, which underlies fundamental processes like endocytosis, intracellular trafficking, and viral infection, is thought to involve membrane coat-forming proteins, including the most observed clathrin, to form Ω-shape profiles and helix-forming proteins like dynamin to constrict Ω-profiles' pores and thus mediate fission. Challenging this fundamental concept, we report that polymerized clathrin is required for Ω-profiles' pore closure and that clathrin around Ω-profiles' base/pore region mediates pore constriction/closure in neuroendocrine chromaffin cells. Mathematical modeling suggests that clathrin polymerization at Ω-profiles' base/pore region generates forces from its intrinsically curved shape to constrict/close the pore. This new fission function may exert broader impacts than clathrin's well-known coat-forming function during clathrin (coat)-dependent endocytosis, because it underlies not only clathrin (coat)-dependent endocytosis, but also diverse endocytic modes, including ultrafast, fast, slow, bulk, and overshoot endocytosis previously considered clathrin (coat)-independent in chromaffin cells. It mediates kiss-and-run fusion (fusion pore closure) previously considered bona fide clathrin-independent, and limits the vesicular content release rate. Furthermore, analogous to results in chromaffin cells, we found that clathrin is essential for fast and slow endocytosis at hippocampal synapses where clathrin was previously considered dispensable, suggesting clathrin in mediating synaptic vesicle endocytosis and fission. These results suggest that clathrin and likely other intrinsically curved coat proteins are a new class of fission proteins underlying vesicle budding and fusion. The half-a-century concept and studies that attribute vesicle-coat contents' function to Ω-profile formation and classify budding as coat-protein (e.g., clathrin)-dependent or -independent may need to be re-defined and re-examined by considering clathrin's pivotal role in pore constriction/closure.

The Role of Lipid Intrinsic Curvature in the Droplet Interface Bilayer

Model bilayers are constructed from lipids having different intrinsic curvatures using the droplet interface bilayer (DIB) method, and their static physicochemical properties are determined. Geometrical and tensiometric measurements are used to derive the free energy of formation (ΔF) of a two-droplet DIB relative to a pair of isolated aqueous droplets, each decorated with a phospholipid monolayer. The lipid molecules employed have different headgroup sizes but identical hydrophobic tail structure, and each is characterized by an intrinsic curvature value (c0) that increases in absolute value with decreasing size of headgroup. Mixtures of lipids at different ratios were also investigated. The role of curvature stress on the values of ΔF of the respective lipid bilayers in these model membranes is discussed and is illuminated by the observation of a decrement in ΔF that scales as a near linear function of c02. Overall, the results reveal an association that should prove useful in studies of ion channels and other membrane proteins embedded in model droplet bilayer systems that will impact the understanding of protein function in cellular membranes composed of lipids of high and low curvature.

Glycerophospholipid polyunsaturation modulates resveratrol action on biomimetic membranes

The phytoalexin resveratrol has received increasing attention for its potential to prevent oxidative damages in human organism. To shed further light on molecular mechanisms of its interaction with lipid membranes we study resveratrol influence on the organisation and mechanical properties of biomimetic lipid systems composed of synthetic phosphatidylcholines with mixed aliphatic chains and different degree of unsaturation at sn-2 position (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC, and 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine, PDPC). High-sensitivity isothermal titration calorimetric measurements reveal stronger spontaneous resveratrol association to polyunsaturated phosphatidylcholine bilayers compared to the monounsaturated ones resulting from hydrophobic interactions, conformational changes of the interacting species and desolvation of molecular surfaces. The latter is supported by the results from Laurdan spectroscopy of large unilamellar vesicles providing data on hydration at the glycerol backbones of glycerophospholipides. Higher degree of lipid order is reported for POPC membranes compared to PDPC. While resveratrol mostly enhances the hydration of PDPC membranes, increasing POPC dehydration is reported upon treatment with the polyphenol. Dehydration of the polyunsaturated lipid bilayers is measured only at the highest phytoalexin content studied (resveratrol/lipid 0.5 mol/mol) and is less pronounced than the effect reported for POPC membranes. The polyphenol effect on membrane mechanics is probed by thermal shape fluctuation analysis of quasispherical giant unilamellar vesicles. Markedly different trend of the bending elasticity with increasing resveratrol concentration is reported for the two types of phospholipid bilayers studied. POPC membranes become more rigid in the presence of resveratrol, whereas PDPC-containing bilayers exhibit softening at lower concentrations of the polyphenol followed by a slight growth without bilayer stiffening even at the highest resveratrol content explored. The new data on the structural organization and membrane properties of resveratrol-treated phosphatidylcholine membranes may underpin the development of future liposomal applications of the polyphenol in medicinal chemistry.

Parallel on-chip micropipettes enabling quantitative multiplexed characterization of vesicle mechanics and cell aggregates rheology

Micropipette aspiration (MPA) is one of the gold standards for quantifying biological samples' mechanical properties, which are crucial from the cell membrane scale to the multicellular tissue. However, relying on the manipulation of individual home-made glass pipettes, MPA suffers from low throughput and no automation. Here, we introduce the sliding insert micropipette aspiration method, which permits parallelization and automation, thanks to the insertion of tubular pipettes, obtained by photolithography, within microfluidic channels. We show its application both at the lipid bilayer level, by probing vesicles to measure membrane bending and stretching moduli, and at the tissue level by quantifying the viscoelasticity of 3D cell aggregates. This approach opens the way to high-throughput, quantitative mechanical testing of many types of biological samples, from vesicles and individual cells to cell aggregates and explants, under dynamic physico-chemical stimuli.

Methyl-β-cyclodextrin asymmetrically extracts phospholipid from bilayers, granting tunable control over differential stress in lipid vesicles

Lipid asymmetry - that is, a nonuniform lipid distribution between the leaflets of a bilayer - is a ubiquitous feature of biomembranes and is implicated in several cellular phenomena. Differential tension - that is, unequal lateral monolayer tensions comparing the leaflets of a bilayer- is closely associated with lipid asymmetry underlying these varied roles. Because differential tension is not directly measurable in combination with the fact that common methods to adjust this quantity grant only semi-quantitative control over it, a detailed understanding of lipid asymmetry and differential tension are impeded. To overcome these challenges, we leveraged reversible complexation of phospholipid by methyl-β-cyclodextrin (mbCD) to tune the direction and magnitude of lipid asymmetry in synthetic vesicles. Lipid asymmetry generated in our study induced (i) vesicle shape changes and (ii) gel-liquid phase coexistence in 1-component vesicles. By applying mass-action considerations to interpret our findings, we discuss how this approach provides access to phospholipid thermodynamic potentials in bilayers containing lipid asymmetry (which are coupled to the differential tension of a bilayer). Because lipid asymmetry yielded by our approach is (i) tunable and (ii) maintained over minute to hour timescales, we anticipate that this approach will be a valuable addition to the experimental toolbox for systematic investigation into the biophysical role(s) of lipid asymmetry (and differential tension).

Which Coverages of Arc-Shaped Proteins Are Required for Membrane Tubulation?

Arc-shaped BIN/Amphiphysin/Rvs (BAR) domain proteins generate curvature by binding to membranes and induce membrane tubulation at sufficiently large protein coverages. For the amphiphysin N-BAR domain, Le Roux et al., Nat. Commun. 2021, 12, 6550, measured a threshold coverage of 0.44 ± 0.097 for nanotubules emerging from the supported lipid bilayer. In this article, we systematically investigate membrane tubulation induced by arc-shaped protein-like particles with coarse-grained modeling and simulations and determine the threshold coverages at different particle-particle interaction strengths and membrane spontaneous curvatures. In our simulations, the binding of arc-shaped particles induces a membrane shape transition from spherical vesicles to tubules at a particle threshold coverage of about 0.5, which is rather robust to variations of the direct attractive particle interactions or spontaneous membrane curvature in the coarse-grained model. Our study suggests that threshold coverages of around or slightly below 0.5 are a general requirement for membrane tubulation by arc-shaped BAR domain proteins.

Determinative scrolling and folding of membranes through shrinking channels

Flat membranes ubiquitously transform into mysterious complex shapes in nature and artificial worlds. Behind the complexity, clear determinative deformation modes have been continuously found to serve as basic application rules but remain unfulfilled. Here, we decipher two elemental deformation modes of thin membranes, spontaneous scrolling and folding as passing through shrinking channels. We validate that these two modes rule the deformation of membranes of a wide thickness range from micrometer to atomic scale. Their occurrence and the determinative fold number quantitatively correlate with the Föppl-von Kármán number and shrinkage ratio. The unveiled determinative deformation modes can guide fabricating foldable designer microrobots and delicate structures of two-dimensional sheets and provide another mechanical principle beyond genetic determinism in biological morphogens.

Flower-shaped 2D crystals grown in curved fluid vesicle membranes

The morphologies of two-dimensional (2D) crystals, nucleated, grown, and integrated within 2D elastic fluids, for instance in giant vesicle membranes, are dictated by an interplay of mechanics, permeability, and thermal contraction. Mitigation of solid strain drives the formation of crystals with vanishing Gaussian curvature (i.e., developable domain shapes) and, correspondingly, enhanced Gaussian curvature in the surrounding 2D fluid. However, upon cooling to grow the crystals, large vesicles sustain greater inflation and tension because their small area-to-volume ratio slows water permeation. As a result, more elaborate shapes, for instance, flowers with bendable but inextensible petals, form on large vesicles despite their more gradual curvature, while small vesicles harbor compact planar crystals. This size dependence runs counter to the known cumulative growth of strain energy of 2D colloidal crystals on rigid spherical templates. This interplay of intra-membrane mechanics and processing points to the scalable production of flexible molecular crystals of controllable complex shape.

Dynamic framework for large-scale modeling of membranes and peripheral proteins

In this chapter, we present a novel computational framework to study the dynamic behavior of extensive membrane systems, potentially in interaction with peripheral proteins, as an alternative to conventional simulation methods. The framework effectively describes the complex dynamics in protein-membrane systems in a mesoscopic particle-based setup. Furthermore, leveraging the hydrodynamic coupling between the membrane and its surrounding solvent, the coarse-grained model grounds its dynamics in macroscopic kinetic properties such as viscosity and diffusion coefficients, marrying the advantages of continuum- and particle-based approaches. We introduce the theoretical background and the parameter-space optimization method in a step-by-step fashion, present the hydrodynamic coupling method in detail, and demonstrate the application of the model at each stage through illuminating examples. We believe this modeling framework to hold great potential for simulating membrane and protein systems at biological spatiotemporal scales, and offer substantial flexibility for further development and parametrization.

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.

Stationary shapes of axisymmetric vesicles beyond lowest-energy configurations

We conduct a systematic exploration of the energy landscape of vesicle morphologies within the framework of the Helfrich model. Vesicle shapes are determined by minimizing the elastic energy subject to constraints of constant area and volume. The results show that pressurized vesicles can adopt higher-energy spindle-like configurations that require the action of point forces at the poles. If the internal pressure is lower than the external one, multilobed shapes are predicted. We utilize our results to rationalize experimentally observed spindle shapes of giant vesicles in a uniform AC electric field.

Dynamic structure factor of undulating vesicles: finite-size and spherical geometry effects with application to neutron spin echo experiments

We consider the dynamic structure factor (DSF) of quasi-spherical vesicles and present a generalization of an expression that was originally formulated by Zilman and Granek (ZG) for scattering from isotropically oriented quasi-flat membrane plaquettes. The expression is obtained in the form of a multi-dimensional integral over the undulating membrane surface. The new expression reduces to the original stretched exponential form in the limit of sufficiently large vesicles, i.e., in the micron range or larger. For much smaller unilamellar vesicles, deviations from the asymptotic, stretched exponential equation are noticeable even if one assumes that the Seifert-Langer leaflet density mode is completely relaxed and membrane viscosity is neglected. To avoid the need for an exhaustive numerical integration while fitting to neutron spin echo (NSE) data, we provide a useful approximation for polydisperse systems that tests well against the numerical integration of the complete expression. To validate the new expression, we performed NSE experiments on variable-size vesicles made of a POPC/POPS lipid mixture and demonstrate an advantage over the original stretched exponential form or other manipulations of the original ZG expression that have been deployed over the years to fit the NSE data. In particular, values of the membrane bending rigidity extracted from the NSE data using the new approximations were insensitive to the vesicle radii and scattering wavenumber and compared very well with expected values of the effective bending modulus ([Formula: see text]) calculated from results in the literature. Moreover, the generalized scattering theory presented here for an undulating quasi-spherical shell can be easily extended to other models for the membrane undulation dynamics beyond the Helfrich Hamiltonian and thereby provides the foundation for the study of the nanoscale dynamics in more complex and biologically relevant model membrane systems.

The local variation of the Gaussian modulus enables different pathways for fluid lipid vesicle fusion

Viral infections, fertilization, neurotransmission, and many other fundamental biological processes rely on membrane fusion. Straightforward calculations based on the celebrated Canham-Helfrich elastic model predict a large topological energy barrier that prevents the fusion process from being thermally activated. While such high energy is in accordance with the physical barrier function of lipid membranes, it is difficult to reconcile with the biological mechanisms involved in fusion processes. In this work, we use a Ginzburg-Landau type of free energy that recovers the Canham-Helfrich model in the limit of small width-to-vesicle-extension ratio, with the additional ability to handle topological transitions. We show that a local modification of the Gaussian modulus in the merging region both dramatically lowers the elastic energy barrier and substantially changes the minimal energy pathway for fusion, in accordance with experimental evidence. Therefore, we discuss biological examples in which such a modification might play a crucial role.

Forceful patterning: theoretical principles of mechanochemical pattern formation

Biological pattern formation is essential for generating and maintaining spatial structures from the scale of a single cell to tissues and even collections of organisms. Besides biochemical interactions, there is an important role for mechanical and geometrical features in the generation of patterns. We review the theoretical principles underlying different types of mechanochemical pattern formation across spatial scales and levels of biological organization.

The underlying mechanical properties of membranes tune their ability to fuse

Membrane fusion is a ubiquitous process associated with a multitude of biological events. Although it has long been appreciated that membrane mechanics plays an important role in membrane fusion, the molecular interplay between mechanics and fusion has remained elusive. For example, although different lipids modulate membrane mechanics differently, depending on their composition, molar ratio, and complex interactions, differing lipid compositions may lead to similar mechanical properties. This raises the question of whether (i) the specific lipid composition or (ii) the average mesoscale mechanics of membranes acts as the determining factor for cellular function. Furthermore, little is known about the potential consequences of fusion on membrane disruption. Here, we use a combination of confocal microscopy, time-resolved imaging, and electroporation to shed light onto the underlying mechanical properties of membranes that regulate membrane fusion. Fusion efficiency follows a nearly universal behavior that depends on membrane fluidity parameters, such as membrane viscosity and bending rigidity, rather than on specific lipid composition. This helps explaining why the charged and fluid membranes of the inner leaflet of the plasma membrane are more fusogenic than their outer counterparts. Importantly, we show that physiological levels of cholesterol, a key component of biological membranes, has a mild effect on fusion but significantly enhances membrane mechanical stability against pore formation, suggesting that its high cellular levels buffer the membrane against disruption. The ability of membranes to efficiently fuse while preserving their integrity may have given evolutionary advantages to cells by enabling their function while preserving membrane stability.

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.

Photomanipulation of Minimal Synthetic Cells: Area Increase, Softening, and Interleaflet Coupling of Membrane Models Doped with Azobenzene-Lipid Photoswitches

Light can effectively interrogate biological systems in a reversible and physiologically compatible manner with high spatiotemporal precision. Understanding the biophysics of photo-induced processes in bio-systems is crucial for achieving relevant clinical applications. Employing membranes doped with the photolipid azobenzene-phosphatidylcholine (azo-PC), a holistic picture of light-triggered changes in membrane kinetics, morphology, and material properties obtained from correlative studies on cell-sized vesicles, Langmuir monolayers, supported lipid bilayers, and molecular dynamics simulations is provided. Light-induced membrane area increases as high as ≈25% and a ten-fold decrease in the membrane bending rigidity is observed upon trans-to-cis azo-PC isomerization associated with membrane leaflet coupling and molecular curvature changes. Vesicle electrodeformation measurements and atomic force microscopy reveal that trans azo-PC bilayers are thicker than palmitoyl-oleoyl phosphatidylcholine (POPC) bilayers but have higher specific membrane capacitance and dielectric constant suggesting an increased ability to store electric charges across the membrane. Lastly, incubating POPC vesicles with azo-PC solutions results in the insertion of azo-PC in the membrane enabling them to become photoresponsive. All these results demonstrate that light can be used to finely manipulate the shape, mechanical and electric properties of photolipid-doped minimal cell models, and liposomal drug carriers, thus, presenting a promising therapeutic alternative for the repair of cellular disorders.

Measuring the bending rigidity of microbial glucolipid (biosurfactant) bioamphiphile self-assembled structures by neutron spin-echo (NSE): Interdigitated vesicles, lamellae and fibers

Bending rigidity, k, is classically measured for lipid membranes to characterize their nanoscale mechanical properties as a function of composition. Widely employed as a comparative tool, it helps understanding the relationship between the lipid's molecular structure and the elastic properties of its corresponding bilayer. Widely measured for phospholipid membranes in the shape of giant unilamellar vesicles (GUVs), bending rigidity is determined here for three self-assembled structures formed by a new biobased glucolipid bioamphiphile, rather associated to the family of glycolipid biosurfactants than phospholipids. In its oleyl form, glucolipid G-C18:1 can assemble into vesicles or crystalline fibers, while in its stearyl form, glucolipid G-C18:0 can assemble into lamellar gels. Neutron spin-echo (NSE) is employed in the q-range between 0.3 nm-1 (21 nm) and 1.5 nm-1 (4.1 nm) with a spin-echo time in the range of up to 500 ns to characterize the bending rigidity of three different structures (Vesicle suspension, Lamellar gel, Fiber gel) solely composed of a single glucolipid. The low (k = 0.30 ± 0.04 kbT) values found for the Vesicle suspension and high values found for the Lamellar (k = 130 ± 40 kbT) and Fiber gels (k = 900 ± 500 kbT) are unusual when compared to most phospholipid membranes. By attempting to quantify for the first time the bending rigidity of self-assembled bioamphiphiles, this work not only contributes to the fundamental understanding of these new molecular systems, but it also opens new perspectives in their integration in the field of soft materials.

Simultaneous Electrophysiology and Imaging Reveal Changes in Lipid Membrane Thickness and Tension upon Uptake of Amphiphilic Gold Nanoparticles

Amphiphilic gold core nanoparticles (AmNPs) striped with hydrophilic 11-mercapto-1-undecanesulfonate (MUS) and hydrophobic 1-octanethiol (OT) ligands are promising candidates for drug carriers that passively and nondisruptively enter cells. Yet, how they interact with cellular membranes is still only partially understood. Herein, we use electrophysiology and imaging to carefully assess changes in droplet interface bilayer lipid membranes (DIBs) incurred by striped AmNPs added via microinjection. We find that AmNPs spontaneously reduce the steady-state specific capacitance and contact angle of phosphatidylcholine DIBs by amounts dependent on the final NP concentration. These reductions, which are greater for NPs with a higher % OT ligands and membranes containing unsaturated lipids but negligible for MUS-only-coated NPs, reveal that AmNPs passively embed in the interior of the bilayer where they increase membrane thickness and lateral tension through disruption of lipid packing. These results demonstrate the enhanced evaluation of nano-bio interactions possible via electrophysiology and imaging of DIBs.

Membrane Tension Inhibits Lipid Mixing by Increasing the Hemifusion Stalk Energy

Fusion of biological membranes is fundamental in various physiological events. The fusion process involves several intermediate stages with energy barriers that are tightly dependent on the mechanical and physical properties of the system, one of which is membrane tension. As previously established, the late stages of fusion, including hemifusion diaphragm and pore expansions, are favored by membrane tension. However, a current understanding of how the energy barrier of earlier fusion stages is affected by membrane tension is lacking. Here, we apply a newly developed experimental approach combining micropipette-aspirated giant unilamellar vesicles and optically trapped membrane-coated beads, revealing that membrane tension inhibits lipid mixing. We show that lipid mixing is 6 times slower under a tension of 0.12 mN/m compared with tension-free membranes. Furthermore, using continuum elastic theory, we calculate the dependence of the hemifusion stalk formation energy on membrane tension and intermembrane distance and find the increase in the corresponding energy barrier to be 1.6 kBT in our setting, which can explain the increase in lipid mixing time delay. Finally, we show that tension can be a significant factor in the stalk energy if the pre-fusion intermembrane distance is on the order of several nanometers, while for membranes that are tightly docked, tension has a negligible effect.

Engineering DNA-based cytoskeletons for synthetic cells

The development and bottom-up assembly of synthetic cells with a functional cytoskeleton sets a major milestone to understand cell mechanics and to develop man-made machines on the nano- and microscale. However, natural cytoskeletal components can be difficult to purify, deliberately engineer and reconstitute within synthetic cells which therefore limits the realization of multifaceted functions of modern cytoskeletons in synthetic cells. Here, we review recent progress in the development of synthetic cytoskeletons made from deoxyribonucleic acid (DNA) as a complementary strategy. In particular, we explore the capabilities and limitations of DNA cytoskeletons to mimic functions of natural cystoskeletons like reversible assembly, cargo transport, force generation, mechanical support and guided polymerization. With recent examples, we showcase the power of rationally designed DNA cytoskeletons for bottom-up assembled synthetic cells as fully engineerable entities. Nevertheless, the realization of dynamic instability, self-replication and genetic encoding as well as contractile force generating motors remains a fruitful challenge for the complete integration of multifunctional DNA-based cytoskeletons into synthetic cells.

Effects of ionic liquids on biomembranes: A review on recent biophysical studies

Ionic liquids (ILs) have been emerged as a versatile class of compounds that can be easily tuned to achieve desirable properties for various applications. The ability of ILs to interact with biomembranes has attracted significant interest, as they have been shown to modulate membrane properties in ways that may have implications for various biological processes. This review provides an overview of recent studies that have investigated the interaction between ILs and biomembranes. We discuss the effects of ILs on the physical and chemical properties of biomembranes, including changes in membrane fluidity, permeability, and stability. We also explore the mechanisms underlying the interaction of ILs with biomembranes, such as electrostatic interactions, hydrogen bonding, and van der Waals forces. Additionally, we discuss the future prospects of this field.

Effect of cholesterol on the mechanical stability of gel-phase phospholipid bilayers studied by AFM force spectroscopy

The remarkably low sliding friction of articular cartilage in the major joints such as hips and knees, which is crucial for its homeostasis and joint health, has been attributed to lipid bilayers forming lubricious boundary layers at its surface. The robustness of such layers, and thus their lubrication efficiency at joint pressures, depends on the lipids forming them, including cholesterol which is a ubiquitous component, and which may act to strengthen of weaken the bilayer. In this work, a systematic study using an atomic force microscope (AFM) was carried out to understand the effect of cholesterol on the nanomechanical stability of two saturated phospholipids, DSPC (1,2-distearoyl-sn-glycero-3-phosphatidlycholine) and DPPC (1,2-dipalmitoyl-sn-glycero- phosphatidylcholine), that differ in acyl chain lengths. Measurements were carried out both in water and in phosphate buffer solution (PBS). The nanomechanical stability of the lipid bilayers was quantitatively evaluated by measuring the breakthrough force needed to puncture the bilayer by the AFM tip. The molar fractions of cholesterol incorporated in the bilayers were 10% and 40%. We found that for both DSPC and DPPC, cholesterol significantly decreases the mechanical stability of the bilayers in solid-ordered (SO) phase. In accordance with the literature, the strengthening effect of salt on the lipid bilayers was also observed. For DPPC with 10 mol % cholesterol, the effect of tip properties and the experimental procedure parameters on the breakthrough forces were also studied. Tip radius (2-42 nm), material (Si, Si3N4, Au) and loading rate (40-1000 nm/s) were varied systematically. The values of the breakthrough forces measured were not significantly affected by any of these parameters, showing that the weakening effect of cholesterol does not result from such changes in experimental conditions. As we have previously demonstrated that mechanical robustness improves the tribological performance of lipid layers, this study helps to shed light on the mechanism of physiological lubrication. Nanoindentation of SDPC bilayers.