Lateral Diffusion of a Submicrometer Particle on a Lipid Bilayer Membrane

In past decades, nanoparticles and nanomaterials have been actively used for applications such as visualizing nano/submicrometer cell structure, killing cancer cells, and using drug delivery systems. It is important to understand the physicochemical mechanisms that govern the motion of nanoparticles on a plasma membrane surface. However, the motion of small particles of <1000 nm on lipid membranes is poorly understood. In this study, we investigated the diffusion of particles with a diameter of 200–800 nm on a lipid membrane using cell-sized liposomes. Particle-associated liposomes were obtained by applying centrifugal force to a mixture of liposomes and particle solutions. We measured the thermal motion of the particles by phase-contrast microscopy. We found that (i) the particle-size dependence of the diffusion of particles adhering to membranes was better described by the DADL model rather than the Einstein–Stokes model, (ii) the diffusion coefficient of a particle strongly depends on the adsorption state of the particle, such as fully or partially wrapped by the membrane, and (iii) anomalous diffusion was induced by the localization of particles on the neck of budded vesicles

Photoinduced Fusion of Lipid Bilayer Membranes

We have developed a novel system for photocontrol of the fusion of lipid vesicles through the use of a photosensitive surfactant containing an azobenzene moiety (AzoTAB). Real-time microscopic observations clarified a change in both the surface area and internal volume of vesicles during fusion. We also determined the optimal cholesterol concentrations and temperature for inducing fusion. The mechanism of fusion can be attributed to a change in membrane tension, which is caused by the solubilization of lipids through the isomerization of AzoTAB. We used a micropipet technique to estimate membrane tension and discuss the mechanism of fusion in terms of membrane elastic energy. The obtained results regarding this novel photoinduced fusion could lead to a better understanding of the mechanism of membrane fusion in living cells and may also see wider applications, such as in drug delivery and biomimetic material design

Biomimetic Microdroplet Membrane Interface: Detection of the Lateral Localization of Amyloid Beta Peptides

Lateral membrane organization into domains, such as lipid rafts, plays an important role in the selective association of biological and nonbiological materials on heterogeneous membrane surfaces. The localization of such materials has profound influence on cellular responses. We constructed a biomimetic water-in-oil microdroplet membrane to study the lateral localization of these materials at heterogeneous biological interfaces. As a case study, we studied selective association of amyloid β peptide on the constructed membrane surface. Amyloid β peptide has attracted much attention as one of these membrane-associating proteins because of its “role” in Alzheimer’s disease pathology. Ternary lipid membranes covering microdroplets successfully produced lipid ordered structures, which mimicked biological lipid rafts. We revealed that amyloid β peptide selectively localizes within nonraft fluid membrane regions. The successful lateral organization in microdroplet membrane systems may lead to new opportunities for the study of molecular associations within heterogeneous membranes

Physicochemical Profiling of Surfactant-Induced Membrane Dynamics in a Cell-Sized Liposome

We used a cell-sized model system, giant liposomes, to investigate the interaction between lipid membranes and surfactants, and the membrane transformation during the solubilization process was captured in real time. We found that there are four distinct dynamics in surfactant-induced membrane deformation: an episodic increase in the membrane area prior to pore-forming associated shrinkage (Dynamics A), fission into many small liposomes (Dynamics B), the formation of multilamellar vesicles and peeling (Dynamics C), and bursting (Dynamics D). Classification of the diversity of membrane dynamics may contribute to a better understanding of the physicochemical mechanism of membrane solubilization induced by various surfactants

Mechano-Sensitive Synthetic Ion Channels

Mechanical stress is a ubiquitous stimulus sensed by membrane proteins, but rarely by synthetic molecules. Inspired by mechano-sensitive ion channels found in cell membranes, tension-responsive transmembrane multiblock amphiphiles were developed. In membranes, a single-transmembrane amphiphile responds to both expanding and contracting tensions to weaken and strengthen the stacking of membrane-spanning units, respectively, and ion transportation is triggered by expanding tension to form a supramolecular channel, while little transportation is observed under a tensionless condition. In contrast, a three-transmembrane amphiphile showed little spectroscopic response to tensions, likely due to weaker stacking of membrane-spanning units than in the single-transmembrane amphiphile. Nevertheless, the three-transmembrane amphiphile shows ion transportation by forming a unimolecular channel even under a tensionless condition, and the ion-transporting activity decreased with expanding tension. Interestingly, the estimated operating force of these synthetic systems was comparable to that of the mechano-sensitive proteins. This study opens the door toward new mechano-sensitive molecular devices

Size-Dependent Partitioning of Nano/Microparticles Mediated by Membrane Lateral Heterogeneity

It is important that we understand the physical, chemical, and biological mechanisms that govern the interaction between nanoparticles (NPs) and heterogeneous cellular surfaces because of the possible cytotoxicity of engineered nanomaterials. In this study, we investigated the lateral localization of nano/microparticles within a biomimetic heterogeneous membrane interface using cell-sized two-phase liposomes. We found that lateral heterogeneity in the membrane mediates the partitioning of nano/microparticles in a size-dependent manner: small particles with a diameter of ≤200 nm were localized in an ordered phase, whereas large particles preferred a fluidic disordered phase. This partitioning behavior was verified by temperature-controlled membrane miscibility transition and laser-trapping of associated particles. In terms of the membrane elastic energy, we present a physical model that explains this localization preference of nano/microparticles. The calculated threshold diameter of particles that separates the particle-partitioning phase was 260 nm, which is in close agreement with our observation (200 nm). These findings may lead to a better understanding of the basic mechanisms that underlie the association of nanomaterials within a cell surface

Ion Permeation by a Folded Multiblock Amphiphilic Oligomer Achieved by Hierarchical Construction of Self-Assembled Nanopores

A multiblock amphiphilic molecule <b>1</b>, with a tetrameric alternating sequence of hydrophilic and hydrophobic units, adopts a folded structure in a liposomal membrane like a multipass transmembrane protein, and is able to transport alkali metal cations through the membrane. Hill’s analysis and conductance measurements, analyzed by the Hille equation, revealed that the tetrameric assembly of <b>1</b> forms a 0.53 nm channel allowing for permeation of cations. Since neither <b>3</b>, bearing flexible hydrophobic units and forming no stacked structures in the membrane, nor <b>2</b>, a monomeric version of <b>1</b>, is able to transport cations, the folded conformation of <b>1</b> in the membrane is likely essential for realizing its function. Thus, function and hierarchically formed higher-order structures of <b>1</b>, is strongly correlated with each other like proteins and other biological macromolecules

Ag/FeCo/Ag Core/Shell/Shell Magnetic Nanoparticles with Plasmonic Imaging Capability

Magnetic nanoparticles (NPs) have been used to separate various species such as bacteria, cells, and proteins. In this study, we synthesized Ag/FeCo/Ag core/shell/shell NPs designed for magnetic separation of subcellular components like intracellular vesicles. A benefit of these NPs is that their silver metal content allows plasmon scattering to be used as a tool to observe detection by the NPs easily and semipermanently. Therefore, these NPs are considered a potential alternative to existing fluorescent probes like dye molecules and colloidal quantum dots. In addition, the Ag core inside the NPs suppresses the oxidation of FeCo because of electron transfer from the Ag core to the FeCo shell, even though FeCo is typically susceptible to oxidation. The surfaces of the Ag/FeCo/Ag NPs were functionalized with ε-poly-l-lysine-based hydrophilic polymers to make them water-soluble and biocompatible. The imaging capability of the polymer-functionalized NPs induced by plasmon scattering from the Ag core was investigated. The response of the NPs to a magnetic field using liposomes as platforms and applying a magnetic field during observation by confocal laser scanning microscopy was assessed. The results of the magnetophoresis experiments of liposomes allowed us to calculate the magnetic force to which each liposome was subjected

Shape Effect on Particle-Lipid Bilayer Membrane Association, Cellular Uptake, and Cytotoxicity

Although computer simulation and cell culture experiments have shown that elongated spherical particles can be taken up into cells more efficiently than spherical particles, experimental investigation on effects of these different shapes over the particle–membrane association has never been reported. Therefore, whether the higher cellular uptake of an elongated spherical particles is a result of a better particle–membrane association as suggested by some calculation works or a consequence of its influence on other cellular trans-membrane components involved in particle translocation process, cannot be concluded. Here, we study the effect of particle shape on the particle–membrane interaction by monitoring the association between particles of various shapes and lipid bilayer membrane of artificial cell-sized liposomes. Among the three shaped lanthanide-doped NaYF₄ particles, all with high shape purity and uniformity, similar crystal phase, and surface chemistry, the elongated spherical particle shows the highest level of membrane association, followed by the spherical particle with a similar radius, and the hexagonal prism-shaped particle, respectively. The free energy of membrane curvature calculated based on a membrane indentation induced by a particle association indicates that among the three particle shapes, the elongated spherical particle give the most stable membrane curvature. The elongated spherical particles show the highest cellular uptake into cytosol of human melanoma (A-375) and human liver carcinoma (HepG2) cells when observed through a confocal laser scanning fluorescence microscope. Quantitative study using flow cytometry also gives the same result. The elongated spherical particles also possess the highest cytotoxicity in A-375 and normal skin (WI-38) cell lines, comparing to the other two shaped particles

Micrometer-Size Vesicle Formation Triggered by UV Light

Vesicle formation is a fundamental kinetic process related to the vesicle budding and endocytosis in a cell. In the vesicle formation by artificial means, transformation of lamellar lipid aggregates into spherical architectures is a key process and known to be prompted by e.g. heat, infrared irradiation, and alternating electric field induction. Here we report UV-light-driven formation of vesicles from particles consisting of crumpled phospholipid multilayer membranes involving a photoactive amphiphilic compound composed of 1,4-bis­(4-phenylethynyl)­benzene (BPEB) units. The particles can readily be prepared from a mixture of these components, which is casted on the glass surface followed by addition of water under ultrasonic radiation. Interestingly, upon irradiation with UV light, micrometer-size vesicles were generated from the particles. Neither infrared light irradiation nor heating prompted the vesicle formation. Taking advantage of the benefits of light, we successfully demonstrated micrometer-scale spatiotemporal control of single vesicle formation. It is also revealed that the BPEB units in the amphiphile are essential for this phenomenon