No abstract

Recently, the transfer method has been shown to be useful for preparing cell-sized phospholipid bilayer vesicles, within which desired substances at desired concentrations can be encapsulated, with a desired asymmetric lipid composition. Here, we investigated the transfer process of water-in-oil (W/O) droplets coated by phospholipid monolayers across an oil/water interface by both experimental observation and theoretical modeling. Real-time experimental observation of the transfer revealed that the transfer process is characterized by three kinetic regimes: a precontact process (approaching regime), an early fast process (entering regime), and a late slow process (relaxation regime). In addition, bigger droplets require much more time to transfer than smaller droplets. We propose a theoretical model to interpret this kinetic process. Our theoretical model reproduces the essential aspects of the transfer kinetics, including its size-dependence.

We investigated the effect of charge on the membrane morphology of giant unilamellar vesicles (GUVs) composed of various mixtures containing charged lipids. We observed the membrane morphologies by fluorescent and confocal laser microscopy in lipid mixtures consisting of a neutral unsaturated lipid [dioleoylphosphatidylcholine (DOPC)], a neutral saturated lipid [dipalmitoylphosphatidylcholine (DPPC)], a charged unsaturated lipid [dioleoylphosphatidylglycerol (DOPG (-) )], a charged saturated lipid[dipalmitoylphosphatidylglycerol (DPPG (-) )], and cholesterol (Chol). In binary mixtures of neutral DOPC/DPPC and charged DOPC/DPPG (-) , spherical vesicles were formed. On the other hand, pore formation was often observed with GUVs consisting of DOPG (-) and DPPC.In a DPPC/DPPG (-) /Chol ternary mixture, pore-formed vesicles were also frequently observed. The percentage of pore-formed vesicles increased with the DPPG (-) concentration.Moreover, when the head group charges of charged lipids were screened by the addition of salt, pore-formed vesicles were suppressed in both the binary and ternary charged lipid mixtures. We discuss the mechanisms of pore formation in charged lipid mixtures and the relationship between phase separation and the membrane morphology. Finally, we reproduce the results seen in experimental systems by using coarse-grained molecular dynamics simulations. Ⅰ. INTRODUCTIONThe basic structure of a biomembrane is a lipid bilayer that is composed of various types of phospholipids. Biomembranes not only separate the inner and outer environments of living cells, but also play a role in a wide range of life-related phenomena through dynamic structural changes. In biomembranes, the components are not uniformly dispersed, and it is believed that such compositional heterogeneity emerges spontaneously. This heterogeneous structure is known as a "lipid raft" [1][2][3]. Lipid rafts, which are enriched with saturated lipids, cholesterol, or various membrane proteins, are expected to function as platforms to which proteins are attached during signal transduction and membrane trafficking [4,5].Synthetic lipid vesicles consisting of several lipid molecules are commonly used as models of biomembranes to investigate the physicochemical properties of lipid membranes. In particular, ternary lipid mixtures consisting of a saturated lipid, unsaturated lipid, and cholesterol exhibit phase separation between the saturated lipid and the cholesterol-rich phase (the liquid-ordered (Lo) phase) and the unsaturated lipid-rich phase (the liquid-disordered (Ld) phase) [6,7]. The spontaneous domain formation that results from this phase separation has attracted great attention in connection to raft formation in biomembranes.Most previous studies have investigated the primary physical properties of lipid membranes composed of electrically neutral lipids [6,8]. However, biomembranes also contain negatively charged lipids. For instance, the membranes of prokaryotes such asStaphylococcus aureus and Escherichia co...

During intraerythrocytic development, the human malaria parasite Plasmodium falciparum alters the mechanical deformability of its host cell. The underpinning biological processes involve gain in parasite mass, changes in the membrane protein compositions, reorganization of the cytoskeletons and its coupling to the plasma membrane, and formation of membrane protrusions, termed knobs. The hemoglobinopathies S and C are known to partially protect carriers from severe malaria, possibly through additional changes in the erythrocyte biomechanics, but a detailed quantification of cell mechanics is still missing. Here, we combined flicker spectroscopy and a mathematical model and demonstrated that knob formation strongly suppresses membrane fluctuations by increasing membrane-cytoskeleton coupling. We found that the confinement increased with hemoglobin S but decreases with hemoglobin C in spite of comparable knob densities and diameters. We further found that the membrane bending modulus strongly depends on the hemoglobinopathetic variant, suggesting increased amounts of irreversibly oxidized hemichromes bound to membranes.

Biomembranes, which are mainly composed of neutral and charged lipids, exhibit a large variety of functional structures and dynamics. Here, we report a coarse-grained molecular dynamics (MD) simulation of the phase separation and morphological dynamics in charged lipid bilayer vesicles. The screened long-range electrostatic repulsion among charged head groups delays or inhibits the lateral phase separation in charged vesicles compared with neutral vesicles, suggesting the transition of the phase-separation mechanism from spinodal decomposition to nucleation or homogeneous dispersion. Moreover, the electrostatic repulsion causes morphological changes, such as pore formation, and further transformations into disk, string, and bicelle structures, which are spatiotemporally coupled to the lateral segregation of charged lipids. Based on our coarse-grained MD simulation, we propose a plausible mechanism of pore formation at the molecular level. The pore formation in a charged-lipid-rich domain is initiated by the prior disturbance of the local molecular orientation in the domain.

No abstract

Physical interactions of four major green tea catechin derivatives with cell membrane models were systemically investigated. Catechins with the galloyl moiety caused the aggregation of small unilamellar vesicles and an increase in the surface pressure of lipid monolayers, while those without did not. Differential scanning calorimetry revealed that, in a low concentration regime (≤10 μM), catechin molecules are not significantly incorporated into the hydrophobic core of lipid membranes as substitutional impurities. Partition coefficient measurements revealed that the galloyl moiety of catechin and the cationic quaternary amine of lipids dominate the catechin-membrane interaction, which can be attributed to the combination of electrostatic and cation-π interactions. Finally, we shed light on the mechanical consequence of catechin-membrane interactions using the Fourier-transformation of the membrane fluctuation. Surprisingly, the incubation of cell-sized vesicles with 1 μM galloyl catechins, which is comparable to the level in human blood plasma after green tea consumption, significantly increased the bending stiffness of the membranes by a factor of more than 60, while those without the galloyl moiety had no detectable influence. Atomic force microscopy and circular dichroism spectroscopy suggest that the membrane stiffening is mainly attributed to the adsorption of galloyl catechin aggregates to the membrane surfaces. These results contribute to our understanding of the physical and thus the generic functions of green tea catechins in therapeutics, such as cancer prevention.

The deformability of a red blood cell (RBC) is one of the most important biological parameters affecting blood flow, both in large arteries and in the microcirculation, and hence it can be used to quantify the cell state. Despite numerous studies on the mechanical properties of RBCs, including cell rigidity, much is still unknown about the relationship between deformability and the configuration of flowing cells, especially in a confined rectangular channel. Recent computer simulation techniques have successfully been used to investigate the detailed behavior of RBCs in a channel, but the dynamics of a translating RBC in a narrow rectangular microchannel have not yet been fully understood. In this study, we numerically investigated the behavior of RBCs flowing at different velocities in a narrow rectangular microchannel that mimicked a microfluidic device. The problem is characterized by the capillary number C a , which is the ratio between the fluid viscous force and the membrane elastic force. We found that confined RBCs in a narrow rectangular microchannel maintained a nearly unchanged biconcave shape at low C a , then assumed an asymmetrical slipper shape at moderate C a , and finally attained a symmetrical parachute shape at high C a . Once a RBC deformed into one of these shapes, it was maintained as the final stable configurations. Since the slipper shape was only found at moderate C a , measuring configurations of flowing cells will be helpful to quantify the cell state.