Budding and fission of vesicles

Light-Activated Synthetic Rotary Motors in Lipid Membranes Induce Shape Changes Through Membrane Expansion

Membranes are the key structures to separate and spatially organize cellular systems. Their rich dynamics and transformations during the cell cycle are orchestrated by specific membrane-targeted molecular machineries, many of which operate through energy dissipation. Likewise, man-made light-activated molecular rotary motors have previously shown drastic effects on cellular systems, but their physical roles on and within lipid membranes remain largely unexplored. Here, the impact of rotary motors on well-defined biological membranes is systematically investigated. Notably, dramatic mechanical transformations are observed in these systems upon motor irradiation, indicative of motor-induced membrane expansion. The influence of several factors on this phenomenon is systematically explored, such as motor concentration and membrane composition., Membrane fluidity is found to play a crucial role in motor-induced deformations, while only minor contributions from local heating and singlet oxygen generation are observed. Most remarkably, the membrane area expansion under the influence of the motors continues as long as irradiation is maintained, and the system stays out-of-equilibrium. Overall, this research contributes to a comprehensive understanding of molecular motors interacting with biological membranes, elucidating the multifaceted factors that govern membrane responses and shape transitions in the presence of these remarkable molecular machines, thereby supporting their future applications in chemical biology.

Concepts of a synthetic minimal cell: Information molecules, metabolic pathways, and vesicle reproduction

How do the living systems emerge from non-living molecular assemblies? What physical and chemical principles supported the process? To address these questions, a promising strategy is to artificially reconstruct living cells in a bottom-up way. Recently, the authors developed the "synthetic minimal cell" system showing recursive growth and division cycles, where the concepts of information molecules, metabolic pathways, and cell reproduction were artificially and concisely redesigned with the vesicle-based system. We intentionally avoided using the sophisticated molecular machinery of the biological cells and tried to redesign the cells in the simplest forms. This review focuses on the similarities and differences between the biological cells and our synthetic minimal cell concerning each concept of cells. Such comparisons between natural and artificial cells will provide insights on how the molecules should be assembled to create living systems to the wide readers in the field of synthetic biology, artificial cells, and protocells research. This review article is an extended version of the Japanese article "Growth and division of vesicles coupled with information molecules," published in SEIBUTSU-BUTSURI vol. 61, p. 378-381 (2021).

Simulated dynamic cholesterol redistribution favors membrane fusion pore constriction

Endo- and exocytosis proceed through a highly strained membrane fusion pore topology regardless of the aiding protein machinery. The membrane's lipid components bias fusion pores toward expansion or closure, modifying the necessary work done by proteins. Cholesterol, a key component of plasma membranes, promotes both inverted lipid phases with concave leaflets (i.e., negative total curvature, which thins the leaflet) and flat bilayer phases with thick, ordered hydrophobic interiors. We demonstrate by theory and simulation that both leaflets of nascent catenoidal fusion pores have negative total curvature. Furthermore, the hydrophobic core of bilayers with strong negative Gaussian curvature is thinned. Therefore, it is an open question whether cholesterol will be enriched in these regions because of the negative total curvature or depleted because of the membrane thinning. Here, we compare all-atom molecular dynamics simulations (built using a procedure to create specific fusion pore geometries) and theory to understand the underlying reasons for lipid redistribution on fusion pores. Our all-atom molecular dynamics simulations resolve this question by showing that cholesterol is strongly excluded from the thinned neck of fusion and fission pores, revealing that thickness (and/or lipid order) influences cholesterol distributions more than curvature. The results imply that cholesterol exclusion can drive fusion pore closure by creating a small, cholesterol-depleted zone in the neck. This model agrees with literature evidence that membrane reshaping is connected to cholesterol-dependent lateral phase separation.

A Note on Vestigial Osmotic Pressure

Recent experiments have indicated that at least a part of the osmotic pressure across the giant unilamellar vesicle (GUV) membrane was balanced by the rapid formation of the monodisperse daughter vesicles inside the GUVs through an endocytosis-like process. Therefore, we investigated a possible osmotic role played by these daughter vesicles for the maintenance of osmotic regulation in the GUVs and, by extension, in living cells. We highlighted a mechanism whereby the daughter vesicles acted as osmotically active solutes (osmoticants), contributing an extra vestigial osmotic pressure component across the membrane of the parent vesicle, and we showed that the consequences were consistent with experimental observations. Our results highlight the significance of osmotic regulation in cellular processes, such as fission/fusion, endocytosis, and exocytosis.

Exploring the Lipid World Hypothesis: A Novel Scenario of Self-Sustained Darwinian Evolution of the Liposomes

According to the Lipid World hypothesis, life on Earth originated with the emergence of amphiphilic assemblies in the form of lipid micelles and vesicles (liposomes). However, the mechanism of appearance of the information molecules (ribozymes/RNA) accompanying that process, considered obligatory for Darwinian evolution, is unclear. We propose a novel scenario of self-sustained Darwinian evolution of the liposomes driven by ever-present natural phenomena: solar UV radiation, day/night cycle, gravity, and the formation of liposomes in an aqueous media. The central tenet of this scenario is the liposomes' encapsulation of the heavy solutes, followed by their gravitational submerging in the water. The submerged liposomes, being protected from the damaging UV radiation, acquire the longevity necessary for autocatalytic replication of amphiphiles, their mutation, and the selection of those amphiphilic assemblies that provide the greatest membrane stability. These two sets of adaptive compositional information (heavy content and amphiphilic assemblies design) generate a population of liposomes with self-replication/reproduction properties, which are amendable to mutation, inheritance, and selection, thereby establishing Darwinian progression. Temporary and spatial expansion of this liposomal population will provide the basis for the next evolutionary step-a transition of accidentally entrapped RNA precursor molecules into complex functional molecules, such as ribozymes/RNA.

Gene-Directed FtsZ Ring Assembly Generates Constricted Liposomes with Stable Membrane Necks

Mimicking bacterial cell division in well-defined cell-free systems has the potential to elucidate the minimal set of proteins required for cytoskeletal formation, membrane constriction, and final abscission. Membrane-anchored FtsZ polymers are often regarded as a sufficient system to realize this chain of events. By using purified FtsZ and its membrane-binding protein FtsA or the gain-of-function mutant FtsA* expressed in PURE (Protein synthesis Using Reconstituted Elements) from a DNA template, it is shown in this study that cytoskeletal structures are formed, and yield constricted liposomes exhibiting various morphologies. However, the resulting buds remain attached to the parental liposome by a narrow membrane neck. No division events can be monitored even after long-time tracking by fluorescence microscopy, nor when the osmolarity of the external solution is increased. The results provide evidence that reconstituted FtsA-FtsZ proto-rings coating the membrane necks are too stable to enable abscission. The prospect of combining a DNA-encoded FtsZ system with assisting mechanisms to achieve synthetic cell division is discussed.

Bottom-Up Synthetic Biology Using Cell-Free Protein Synthesis

Technical advances in biotechnology have greatly accelerated the development of bottom-up synthetic biology. Unlike top-down approaches, bottom-up synthetic biology focuses on the construction of a minimal cell from scratch and the application of these principles to solve challenges. Cell-free protein synthesis (CFPS) systems provide minimal machinery for transcription and translation, from either a fractionated cell lysate or individual purified protein elements, thus speeding up the development of synthetic cell projects. In this review, we trace the history of the cell-free technique back to the first in vitro fermentation experiment using yeast cell lysate. Furthermore, we summarized progresses of individual cell mimicry modules, such as compartmentalization, gene expression regulation, energy regeneration and metabolism, growth and division, communication, and motility. Finally, current challenges and future perspectives on the field are outlined.

Activation energy and force fields during topological transitions of fluid lipid vesicles

Topological transitions of fluid lipid membranes are fundamental processes for cell life. For example, they are required for endo- and exocytosis or to enable neurotransmitters to cross the neural synapses. Here, inspired by the idea that fusion and fission proteins could have evolved in Nature in order to carry out a minimal work expenditure, we evaluate the minimal free energy pathway for the transition between two spherical large unilamellar vesicles and a dumbbell-shaped one. To address the problem, we propose and successfully use a Ginzburg-Landau type of free energy, which allows us to uniquely describe without interruption the whole, full-scale topological change. We also compute the force fields needed to overcome the involved energy barriers. The obtained forces are in excellent agreement, in terms of intensity, scale, and spatial localization with experimental data on typical fission protein systems, whereas they suggest the presence of additional features in fusion proteins.

From vesicles toward protocells and minimal cells

In contrast to ordinary condensed matter systems, "living systems" are unique. They are based on molecular compartments that reproduce themselves through (i) an uptake of ingredients and energy from the environment, and (ii) spatially and timely coordinated internal chemical transformations. These occur on the basis of instructions encoded in information molecules (DNAs). Life originated on Earth about 4 billion years ago as self-organised systems of inorganic compounds and organic molecules including macromolecules (e.g. nucleic acids and proteins) and low molar mass amphiphiles (lipids). Before the first living systems emerged from non-living forms of matter, functional molecules and dynamic molecular assemblies must have been formed as prebiotic soft matter systems. These hypothetical cell-like compartment systems often are called "protocells". Other systems that are considered as bridging units between non-living and living systems are called "minimal cells". They are synthetic, autonomous and sustainable reproducing compartment systems, but their constituents are not limited to prebiotic substances. In this review, we focus on both membrane-bounded (vesicular) protocells and minimal cells, and provide a membrane physics background which helps to understand how morphological transformations of vesicle systems might have happened and how vesicle reproduction might be coupled with metabolic reactions and information molecules. This research, which bridges matter and life, is a great challenge in which soft matter physics, systems chemistry, and synthetic biology must take joined efforts to better understand how the transformation of protocells into living systems might have occurred at the origin of life.

Photoinducible Azobenzene trimethylammonium bromide (AzoTAB)-mediated giant vesicle fusion compatible with synthetic protein translation reactions

Lipid giant vesicles represent a versatile minimal model system to study the physicochemical basis of lipid membrane fusion. Membrane fusion processes are also of interest in synthetic cell research, where cell-mimicking behavior often requires dynamically interacting compartments. For these applications, triggered fusion compatible with transcription-translation systems is key in achieving complexity. Recently, a photosensitive surfactant, azobenzene trimethylammonium bromide (AzoTAB), has been reported to induce membrane fusion by a photoinduced conformational change. Using imaging flow cytometer (IFC) and confocal microscopy we quantitatively investigated photoinduced AzoTAB-mediated fusion of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine vesicles. The IFC analysis result showed that the fusion rate could reach about 40% following AzoTAB addition and UV irradiation in optimized conditions. We confirmed the compatibility between AzoTAB-induced vesicle fusion and a synthetic cell-free protein translation system using green fluorescent protein as reporter. With the techniques presented, cell-sized vesicle fusion can be quantitatively analyzed and optimized, paving the way to controllable synthetic cells with fundamental biological functions like the ability to express proteins from encapsulated plasmids.

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.

Vesicle deformation and division induced by flip-flops of lipid molecules

We investigated the deformation of small unilamellar vesicles (SUVs) induced by flip-flops of lipids using coarse-grained molecular dynamics simulations. In the case of single-component SUVs composed of zero spontaneous curvature lipids (ZLs), the flip-flop of ZLs deformed stomatocyte-shaped SUVs into an oblate shape, whereas pear-shaped SUVs were deformed into a prolate shape. These two equilibrium shapes comply with the local minima of elastic energy. In the case of binary vesicles composed of ZLs and negative spontaneous curvature lipids (NLs), the vesicle deformation pathway depended on the initial NL distribution in the bilayer. If the initial difference in the NL concentration between the outer and inner leaflets was small, the flip-flop of ZLs and NLs rapidly deformed pear-shaped SUVs into an equilibrium prolate shape. On the other hand, when NLs were localised in the inner leaflet, the flip-flop of ZLs and NLs deformed pear-shaped SUVs into a limiting shape and then induced vesicle division. Because the flip-flop rate of NLs is much faster than that of ZLs, the total free energy was first relaxed by the flip-flop of NLs and then by that of ZLs. This kinetic effect is responsible for the observed vesicle division induced by flip-flops.

Communication is key: Extracellular vesicles as mediators of infection and defence during host-microbe interactions in animals and plants

Extracellular vesicles (EVs) are now understood to be ubiquitous mediators of cellular communication. In this review, we suggest that EVs have evolved into a highly regulated system of communication with complex functions including export of wastes, toxins and nutrients, targeted delivery of immune effectors and vectors of RNA silencing. Eukaryotic EVs come in different shapes and sizes and have been classified according to their biogenesis and size distributions. Small EVs (or exosomes) are released through fusion of endosome-derived multivesicular bodies with the plasma membrane. Medium EVs (or microvesicles) bud off the plasma membrane as a form of exocytosis. Finally, large EVs (or apoptotic bodies) are produced as a result of the apoptotic process. This review considers EV secretion and uptake in four eukaryotic kingdoms, three of which produce cell walls. The impacts cell walls have on EVs in plants and fungi are discussed, as are roles of fungal EVs in virulence. Contributions of plant EVs to development and innate immunity are presented. Compelling cases are sporophytic self-incompatibility and cellular invasion by haustorium-forming filamentous pathogens. The involvement of EVs in all of these eukaryotic processes is reconciled considering their evolutionary history.

Non-Polar Lipids as Regulators of Membrane Properties in Archaeal Lipid Bilayer Mimics

The modification of archaeal lipid bilayer properties by the insertion of apolar molecules in the lipid bilayer midplane has been proposed to support cell membrane adaptation to extreme environmental conditions of temperature and hydrostatic pressure. In this work, we characterize the insertion effects of the apolar polyisoprenoid squalane on the permeability and fluidity of archaeal model membrane bilayers, composed of lipid analogues. We have monitored large molecule and proton permeability and Laurdan generalized polarization from lipid vesicles as a function of temperature and hydrostatic pressure. Even at low concentration, squalane (1 mol%) is able to enhance solute permeation by increasing membrane fluidity, but at the same time, to decrease proton permeability of the lipid bilayer. The squalane physicochemical impact on membrane properties are congruent with a possible role of apolar intercalants on the adaptation of Archaea to extreme conditions. In addition, such intercalant might be used to cheaply create or modify chemically resistant liposomes (archeaosomes) for drug delivery.

Division and Regrowth of Phase-Separated Giant Unilamellar Vesicles*

Success in the bottom‐up assembly of synthetic cells will depend on strategies for the division of protocellular compartments. Here, we describe the controlled division of phase‐separated giant unilamellar lipid vesicles (GUVs). We derive an analytical model based on the vesicle geometry, which makes four quantitative predictions that we verify experimentally. We find that the osmolarity ratio required for division is 2 , independent of the GUV size, while asymmetric division happens at lower osmolarity ratios. Remarkably, we show that a suitable osmolarity change can be triggered by water evaporation, enzymatic decomposition of sucrose or light‐triggered uncaging of CMNB‐fluorescein. The latter provides full spatiotemporal control, such that a target GUV undergoes division whereas the surrounding GUVs remain unaffected. Finally, we grow phase‐separated vesicles from single‐phased vesicles by targeted fusion of the opposite lipid type with programmable DNA tags to enable subsequent division cycles.

Pulsed Electric Fields Promote Liposome Buddings

Background: Liposomes have been a useful tool to analyze membrane behavior. Various studies have attempted to induce biological activities, for example, buddings, divisions, and endocytosis, on liposomes, focusing on lipid rafts that move along electric fields. Materials and Methods: Liposomes consisting of soybean lecithin, phosphatidylcholine, and cholesterol were prepared, with inner and outer liquid conductivities of 0.595 and 1.564 S/m, respectively. Results: We tried to induce buddings by pulsed electric fields (PEFs) on liposomes. Results demonstrated that 1.248 kV/cm, 400 μs PEF promoted postpulse liposome buddings, which were preceded by a membrane relaxation. Although a transient thick area (a lipid raft-like area) on the membrane just after PEF application preceded buddings, it was not the sufficient factor for buddings. Conclusion: We established a brief model as follows: 1.248 kV/cm, 400 μs PEF induced the lipid membrane relaxation without electroporation to trigger buddings. The current results could be a new frontier in bioelectrics.

Active shape oscillations of giant vesicles with cyclic closure and opening of membrane necks

Reaction-diffusion systems encapsulated within giant unilamellar vesicles (GUVs) can lead to shape oscillations of these vesicles as recently observed for the bacterial Min protein system. This system contains two Min proteins, MinD and MinE, which periodically attach to and detach from the GUV membranes, with the detachment being driven by ATP hydrolysis. Here, we address these shape oscillations within the theoretical framework of curvature elasticity and show that they can be understood in terms of a spontaneous curvature that changes periodically with time. We focus on the simplest case provided by a attachment-detachment kinetics that is laterally uniform along the membrane. During each oscillation cycle, the vesicle shape is transformed from a symmetric dumbbell with two subcompartments of equal size to an asymmetric dumbbell with two subcompartments of different size, followed by the reverse, symmetry-restoring transformation. This sequence of shapes is first analyzed within the spontaneous curvature model which is then extended to the area-difference-elasticity model by decomposing the spontaneous curvature into a local and nonlocal component. For both symmetric and asymmetric dumbbells, the two subcompartments are connected by a narrow membrane neck with a circular waistline. The radius of this waistline undergoes periodic oscillations, the time dependence of which can be reasonably well fitted by a single Fourier mode with an average time period of 56 s.

Light-triggered explosion of lipid vesicles

Lipid vesicles have received considerable interest because of their applications to in vitro reductionist cell membrane models as well as therapeutic delivery vehicles. In these contexts, the mechanical response of vesicles in nonequilibrium environments plays a key role in determining the corresponding dynamics. A common understanding of the response of lipid vesicles upon exposure to a hypotonic solution is a characteristic pulsatile behavior. Recent experiments, however, have shown vesicles exploding under an osmotic shock generated by photo-reactions, yet the explanatory mechanism is unknown. Here we present a generalized biophysical model incorporating a stochastic account of membrane rupture to describe both swell-burst-reseal cycling and exploding dynamics. This model agrees well with experimental observations, and it unravels that the sudden osmotic shock strains the vesicle at an extreme rate, driving the vesicle into buckling instabilities responsible for membrane fragmentation, i.e. explosion. Our work not only advances the fundamental framework for non-equilibrium vesicle dynamics under osmotic stress, but also offers design guidelines for programmable vesicle-encapsulated substance release in therapeutic carriers.

Cholesterol Alters the Orientation and Activity of the Influenza Virus M2 Amphipathic Helix in the Membrane

The influenza virus M2 amphipathic helix (M2AH) alters membrane curvature in a cholesterol-dependent manner, mediating viral membrane scission during influenza virus budding. Here, we have investigated the biophysical effects of cholesterol on the ability of an M2AH peptide to manipulate membrane properties. We see that the ability of the M2AH to interact with membranes and form an α-helix is independent of membrane cholesterol concentration; however, cholesterol affects the angle of the M2AH peptide within the membrane. This change in membrane orientation affects the ability of the M2AH to alter lipid order. In low-cholesterol membranes, the M2AH is inserted near the level of the lipid head groups, increasing lipid order, which may contribute to generation of the membrane curvature. As the cholesterol content increases, the M2AH insertion becomes flatter and slightly deeper in the membrane below the lipid headgroups, where the polar face can continue to interact with the headgroups while the hydrophobic face binds cholesterol. This changed orientation minimizes lipid packing defects and lipid order changes, likely reducing the generation of membrane curvature. Thus, cholesterol regulates M2 membrane scission by precisely modulating M2AH positioning within the membrane. This has implications for the understanding of many of amphipathic-helix-driven cellular budding processes that occur in specific lipid environments.

Synergy between plant phenols and carotenoids in stabilizing lipid-bilayer membranes of giant unilamellar vesicles against oxidative destruction

We have investigated the synergism between plant phenols and carotenoids in protecting the phosphatidylcholine (PC) membranes of giant unilamellar vesicles (GUVs) from oxidative destruction, for which chlorophyll-a (Chl-a) was used as a lipophilic photosensitizer. The effect was examined for seven different combinations of β-carotene (β-CAR) and plant phenols. The light-induced change in GUV morphology was monitored via conventional optical microscopy, and quantified by a dimensionless image-entropy parameter, ΔE. The ΔE-t time evolution profiles exhibiting successive lag phase, budding phase and ending phase could be accounted for by a Boltzmann model function. The length of the lag phase (LP in s) for the combination of syringic acid and β-CAR was more than seven fold longer than for β-CAR alone, and those for other different combinations followed the order: salicylic acid < vanillic acid < syringic acid > rutin > caffeic acid > quercetin > catechin, indicating that moderately reducing phenols appeared to be the most efficient membrane co-stabilizers. The same order held for the residual contents of β-CAR in membranes after light-induced oxidative degradation as determined by resonance Raman spectroscopy. The dependence of LP on the reducing power of phenols coincided with the Marcus theory plot for the rate of electron transfer from phenols to the radical cation β-CAR˙⁺ as a primary oxidative product, suggesting that the plant phenol regeneration of β-CAR plays an important role in stabilizing the GUV membranes, as further supported by the involvement of CAR˙⁺ and the distinct shortening of its lifetime as shown by transient absorption spectroscopy.

Fission of Lipid-Vesicles by Membrane Phase Transitions in Thermal Convection

Unilamellar lipid vesicles can serve as model for protocells. We present a vesicle fission mechanism in a thermal gradient under flow in a convection chamber, where vesicles cycle cold and hot regions periodically. Crucial to obtain fission of the vesicles in this scenario is a temperature-induced membrane phase transition that vesicles experience multiple times. We model the temperature gradient of the chamber with a capillary to study single vesicles on their way through the temperature gradient in an external field of shear forces. Starting in the gel-like phase the spherical vesicles are heated above their main melting temperature resulting in a dumbbell-deformation. Further downstream a temperature drop below the transition temperature induces splitting of the vesicles without further physical or chemical intervention. This mechanism also holds for less cooperative systems, as shown here for a lipid alloy with a broad transition temperature width of 8 K. We find a critical tether length that can be understood from the transition width and the locally applied temperature gradient. This combination of a temperature-induced membrane phase transition and realistic flow scenarios as given e.g. in a white smoker enable a fission mechanism that can contribute to the understanding of more advanced protocell cycles.

De novo synthesized Min proteins drive oscillatory liposome deformation and regulate FtsA-FtsZ cytoskeletal patterns

The Min biochemical network regulates bacterial cell division and is a prototypical example of self-organizing molecular systems. Cell-free assays relying on purified proteins have shown that MinE and MinD self-organize into surface waves and oscillatory patterns. In the context of developing a synthetic cell from elementary biological modules, harnessing Min oscillations might allow us to implement higher-order cellular functions. To convey hereditary information, the Min system must be encoded in a DNA molecule that can be copied, transcribed, and translated. Here, the MinD and MinE proteins are synthesized de novo from their genes inside liposomes. Dynamic protein patterns and accompanying liposome shape deformation are observed. When integrated with the cytoskeletal proteins FtsA and FtsZ, the synthetic Min system is able to dynamically regulate FtsZ patterns. By enabling genetic control over Min protein self-organization and membrane remodeling, our methodology offers unique opportunities towards directed evolution of bacterial division processes in vitro.

The Min biochemical network regulates bacterial cell division and is a prototypical example of self-organizing molecular systems. Here authors synthesize Min proteins from their genes inside liposomes and observe dynamic protein patterns and liposome shape deformation.

Reproduction of vesicles coupled with a vesicle surface-confined enzymatic polymerisation

Molecular assembly systems that have autonomous reproduction and Darwinian evolution abilities can be considered as minimal cell-like systems. Here we demonstrate the reproduction of cell-sized vesicles composed of AOT, i.e., sodium bis-(2-ethylhexyl) sulfosuccinate, coupled with an enzymatic polymerisation reaction occurring on the surface of the vesicles. The particular reaction used is the horseradish peroxidase-catalysed polymerisation of aniline with hydrogen peroxide as oxidant, which yields polyaniline in its emeraldine salt form (PANI-ES). If AOT micelles are added during this polymerisation reaction, the AOT - PANI-ES vesicles interact with the AOT molecules in the external solution and selectively incorporate them in their membrane, which leads to a growth of the vesicles. If the AOT vesicles also contain cholesterol, the vesicles not only show growth, but also reproduction. An important characteristic of this reproduction system is that the AOT-based vesicles encourage the synthesis of PANI-ES and PANI-ES promotes the growth of AOT vesicles.

Beating Vesicles: Encapsulated Protein Oscillations Cause Dynamic Membrane Deformations

The bacterial Min protein system was encapsulated in giant unilamellar vesicles (GUVs). Using confocal fluorescence microscopy, we identified several distinct modes of spatiotemporal patterns inside spherical GUVs. For osmotically deflated GUVs, the vesicle shape actively changed in concert with the Min oscillations. The periodic relocation of Min proteins from the vesicle lumen to the membrane and back is accompanied by drastic changes in the mechanical properties of the lipid bilayer. In particular, two types of oscillating membrane-shape changes are highlighted: 1) GUVs that repeatedly undergo fission into two connected compartments and fusion of these compartments back into a dumbbell shape and 2) GUVs that show periodic budding and subsequent merging of the buds with the mother vesicle, accompanied by an overall shape change of the vesicle reminiscent of a bouncing ball. These findings demonstrate how reaction-diffusion-based protein self-organization can directly yield visible mechanical effects on membrane compartments, even up to autonomous division, without the need for coupling to cytoskeletal elements.

Carbon Nanotube Porins in Amphiphilic Block Copolymers as Fully Synthetic Mimics of Biological Membranes

Biological membranes provide a fascinating example of a separation system that is multifunctional, tunable, precise, and efficient. Biomimetic membranes, which mimic the architecture of cellular membranes, have the potential to deliver significant improvements in specificity and permeability. Here, a fully synthetic biomimetic membrane is reported that incorporates ultra-efficient 1.5 nm diameter carbon nanotube porin (CNTPs) channels in a block-copolymer matrix. It is demonstrated that CNTPs maintain high proton and water permeability in these membranes. CNTPs can also mimic the behavior of biological gap junctions by forming bridges between vesicular compartments that allow transport of small molecules.

Mathematical modeling of dynamic behavior of fluid bilayer membranes under the effect of density asymmetry

Shape transformations in biological membranes are crucial in a variety of cellular processes such as transport in the Golgi apparatus and endoplasmic reticulum, shaping the cell organelles and signaling in neuronal synapses. Dynamic analysis of lipid bilayer membranes is popular among researchers as valuable information about cell functions can be retrieved. There are several limitations in experimental tests and simulations such as computational and implementation cost while in theoretical studies, different phenomena can be modeled and the effect of each parameter can be investigated. In this paper, a continuum model including elastic energies and dissipation functions is utilized with energy approach to obtain the governing equations of an enclosed lipid bilayer membrane. The governing equations are solved numerically for vesicles initially disturbed and the relaxation dynamics is studied. The stationary shape of the vesicles for different values of reduced volume and reduced area difference is obtained to explore the phase diagram and verify the governing equations. Then, the density asymmetry in bilayers caused by the change in the density or the equilibrium density of the outer monolayer is studied. This leads to the formation of buds, tubules, and pearls. This can be observed in the recruitment of proteins to the outer monolayer or pH gradients of the environment of a vesicle. The effect of density difference and curvature on creation and growth of tubules are investigated. An interesting metastable state in the adsorption of the final bud due to the increase in the density of the outer monolayer is observed in which the shape of the vesicle is almost unchanged. A prolate vesicle relaxes toward an oblate or a stomatocyte vesicle when the equilibrium density of the outer monolayer increases.

Osmotically Induced Membrane Fission in Giant Polymer Vesicles: Multilamellarity and Effect of the Amphiphilic Block Lengths

Amphiphilic block co-polymers are used to form large spherical vesicles. A sudden change in the osmotic pressure across the polymer membrane is used to induce the fission of the polymer vesicle. The membrane area to volume ratio, as expected, is observed to be a parameter suitable to describe the process and even mark the critical points along this transition. The effect of the length of the hydrophobic and hydrophilic chains on the fission process is analyzed. The effects of membrane lamellarity and initial polydispersity are thoroughly analyzed from the experimental data following mathematical models, and the phenomenon of fission in these polymer vesicles is understood via measurements characterizing the membrane, i.e., area stretch modulus.

Ectopic Neo-Formed Intracellular Membranes in Escherichia coli: A Response to Membrane Protein-Induced Stress Involving Membrane Curvature and Domains

Bacterial cytoplasmic membrane stress induced by the overexpression of membrane proteins at high levels can lead to formation of ectopic intracellular membranes. In this review, we report the various observations of such membranes in Escherichia coli, compare their morphological and biochemical characterizations, and we analyze the underlying molecular processes leading to their formation. Actually, these membranes display either vesicular or tubular structures, are separated or connected to the cytoplasmic membrane, present mono- or polydispersed sizes and shapes, and possess ordered or disordered arrangements. Moreover, their composition differs from that of the cytoplasmic membrane, with high amounts of the overexpressed membrane protein and altered lipid-to-protein ratio and cardiolipin content. These data reveal the importance of membrane domains, based on local specific lipid⁻protein and protein⁻protein interactions, with both being crucial for local membrane curvature generation, and they highlight the strong influence of protein structure. Indeed, whether the cylindrically or spherically curvature-active proteins are actively curvogenic or passively curvophilic, the underlying molecular scenarios are different and can be correlated with the morphological features of the neo-formed internal membranes. Delineating these molecular mechanisms is highly desirable for a better understanding of protein⁻lipid interactions within membrane domains, and for optimization of high-level membrane protein production in E. coli.

Molecular mechanism of vesicle division induced by coupling between lipid geometry and membrane curvatures

We investigated the effects of lipid geometry on vesicle division using coarse grained molecular dynamics simulations. When the vesicle is composed of zero and negative spontaneous curvature lipids (ZSLs and NSLs), the difference in their molecular spontaneous curvatures destabilizes the neck of the limiting shape vesicle. In the vesicle division pathway, the neck developed into the stalk intermediates. The stalk was broken when the NSLs were expelled from the stalk. Free energy analysis shows that the coupling between the lipid geometry and the Gaussian rigidity is responsible for the observed vesicle division.

Mechanical Division of Cell-Sized Liposomes

Liposomes, self-assembled vesicles with a lipid-bilayer boundary similar to cell membranes, are extensively used in both fundamental and applied sciences. Manipulation of their physical properties, such as growth and division, may significantly expand their use as model systems in cellular and synthetic biology. Several approaches have been explored to controllably divide liposomes, such as shape transformation through temperature cycling, incorporation of additional lipids, and the encapsulation of protein division machinery. However, so far, these methods lacked control, exhibited low efficiency, and yielded asymmetric division in terms of volume or lipid composition. Here, we present a microfluidics-based strategy to realize mechanical division of cell-sized (∼6 μm) liposomes. We use octanol-assisted liposome assembly (OLA) to produce liposomes on chip, which are subsequently flowed against the sharp edge of a wedge-shaped splitter. Upon encountering such a Y-shaped bifurcation, the liposomes are deformed and, remarkably, are able to divide into two stable daughter liposomes in just a few milliseconds. The probability of successful division is found to critically depend on the surface area-to-volume ratio of the mother liposome, which can be tuned through osmotic pressure, and to strongly correlate to the mother liposome size for given microchannel dimensions. The division process is highly symmetric (∼3% size variation between the daughter liposomes) and is accompanied by a low leakage. This mechanical division of liposomes may constitute a valuable step to establish a growth-division cycle of synthetic cells.

Thermally-induced lateral assembly of a PEG-containing amphiphile triggering vesicle budding

A macrocyclic amphiphile consisting of a thermo-responsive octaethylene glycol chain with hydrophobic aromatic and aliphatic units undergoes lateral self-assembly in a liquid-disordered-state phospholipid bilayer membrane upon heating, which further leads to vesicle budding.

Modeling the Mechanics of Cell Division: Influence of Spontaneous Membrane Curvature, Surface Tension, and Osmotic Pressure

Many cell division processes have been conserved throughout evolution and are being revealed by studies on model organisms such as bacteria, yeasts, and protozoa. Cellular membrane constriction is one of these processes, observed almost universally during cell division. It happens similarly in all organisms through a mechanical pathway synchronized with the sequence of cytokinetic events in the cell interior. Arguably, such a mechanical process is mastered by the coordinated action of a constriction machinery fueled by biochemical energy in conjunction with the passive mechanics of the cellular membrane. Independently of the details of the constriction engine, the membrane component responds against deformation by minimizing the elastic energy at every constriction state following a pathway still unknown. In this paper, we address a theoretical study of the mechanics of membrane constriction in a simplified model that describes a homogeneous membrane vesicle in the regime where mechanical work due to osmotic pressure, surface tension, and bending energy are comparable. We develop a general method to find approximate analytical expressions for the main descriptors of a symmetrically constricted vesicle. Analytical solutions are obtained by combining a perturbative expansion for small deformations with a variational approach that was previously demonstrated valid at the reference state of an initially spherical vesicle at isotonic conditions. The analytic approximate results are compared with the exact solution obtained from numerical computations, getting a good agreement for all the computed quantities (energy, area, volume, constriction force). We analyze the effects of the spontaneous curvature, the surface tension and the osmotic pressure in these quantities, focusing especially on the constriction force. The more favorable conditions for vesicle constriction are determined, obtaining that smaller constriction forces are required for positive spontaneous curvatures, low or negative membrane tension and hypertonic media. Conditions for spontaneous constriction at a given constriction force are also determined. The implications of these results for biological cell division are discussed. This work contributes to a better quantitative understanding of the mechanical pathway of cellular division, and could assist the design of artificial divisomes in vesicle-based self-actuated microsystems obtained from synthetic biology approaches.

Light-Controlled Membrane Mechanics and Shape Transitions of Photoswitchable Lipid Vesicles

Giant unilamellar vesicles (GUVs) represent a versatile model system to emulate the fundamental properties and functions associated with the plasma membrane of living cells. Deformability and shape transitions of lipid vesicles are closely linked to the mechanical properties of the bilayer membrane itself and are typically difficult to control under physiological conditions. Here, we developed a protocol to form cell-sized vesicles from an azobenzene-containing phosphatidylcholine (azo-PC), which undergoes photoisomerization on irradiation with UV-A and visible light. Photoswitching within the photolipid vesicles enabled rapid and precise control of the mechanical properties of the membrane. By varying the intensity and dynamics of the optical stimulus, controlled vesicle shape changes such as budding transitions, invagination, pearling, or the formation of membrane tubes were achieved. With this system, we could mimic the morphology changes normally seen in cells, in the absence of any molecular machines associated with the cytoskeleton. Furthermore, we devised a mechanism to utilize photoswitchable lipid membranes for storing mechanical energy and then releasing it on command as locally usable work.

General neck condition for the limit shape of budding vesicles

The shape equation and linking conditions for a vesicle with two phase domains are derived. We refine the conjecture on the general neck condition for the limit shape of a budding vesicle proposed by Jülicher and Lipowsky [Phys. Rev. Lett. 70, 2964 (1993)PRLTAO0031-900710.1103/PhysRevLett.70.2964; Phys. Rev. E 53, 2670 (1996)1063-651X10.1103/PhysRevE.53.2670], and then we use the shape equation and linking conditions to prove that this conjecture holds not only for axisymmetric budding vesicles, but also for asymmetric ones. Our study reveals that the mean curvature at any point on the membrane segments adjacent to the neck satisfies the general neck condition for the limit shape of a budding vesicle when the length scale of the membrane segments is much larger than the characteristic size of the neck but still much smaller than the characteristic size of the vesicle.

Membrane remodeling by the M2 amphipathic helix drives influenza virus membrane scission

Membrane scission is a crucial step in all budding processes, from endocytosis to viral budding. Many proteins have been associated with scission, though the underlying molecular details of how scission is accomplished often remain unknown. Here, we investigate the process of M2-mediated membrane scission during the budding of influenza viruses. Residues 50–61 of the viral M2 protein bind membrane and form an amphipathic α-helix (AH). Membrane binding requires hydrophobic interactions with the lipid tails but not charged interactions with the lipid headgroups. Upon binding, the M2AH induces membrane curvature and lipid ordering, constricting and destabilizing the membrane neck, causing scission. We further show that AHs in the cellular proteins Arf1 and Epsin1 behave in a similar manner. Together, they represent a class of membrane-induced AH domains that alter membrane curvature and fluidity, mediating the scission of constricted membrane necks in multiple biological pathways.

Role of Inverse-Cone-Shape Lipids in Temperature-Controlled Self-Reproduction of Binary Vesicles

We investigate a temperature-driven recursive division of binary giant unilamellar vesicles (GUVs). During the heating step of the heating-cooling cycle, the spherical mother vesicle deforms to a budded limiting shape using up the excess area produced by the chain melting of the lipids and then splits off into two daughter vesicles. Upon cooling, the daughter vesicle opens a pore and recovers the spherical shape of the mother vesicle. Our GUVs are composed of DLPE (1,2-dilauroyl-sn-glycero-3-phosphoethanolamine) and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine). During each cycle, vesicle deformation is monitored by a fast confocal microscope and the images are analyzed to obtain the time evolution of reduced volume and reduced monolayer area difference as the key geometric parameters that quantify vesicle shape. By interpreting the deformation pathway using the area-difference elasticity theory, we conclude that vesicle division relies on (1) a tiny asymmetric distribution of DLPE within the bilayer, which controls the observed deformation from the sphere to the budded shape; and (2) redistribution of DLPE during the deformation-division stage, which ensures that the process is recursive. The spontaneous coupling between membrane curvature and PE lipid distribution is responsible for the observed recursive division of GUVs. These results shed light on the mechanisms of vesicle self-reproduction.

Proliferation of Listeria monocytogenes L-form cells by formation of internal and external vesicles

L-forms are cell wall-deficient bacteria that divide through unusual mechanisms, involving dynamic perturbations of the cellular shape and generation of vesicles, independently of the cell-division protein FtsZ. Here we describe FtsZ-independent mechanisms, involving internal and external vesicles, by which Listeria monocytogenesL-forms proliferate. Using micromanipulation of single cells and vesicles, we show that small vesicles are formed by invagination within larger intracellular vesicles, receive cytoplasmic content, and represent viable progeny. In addition, the L-forms can reproduce by pearling, that is, generation of extracellular vesicles that remain transiently linked to their mother cell via elastic membranous tubes. Using photobleaching and fluorescence recovery, we demonstrate cytoplasmic continuity and transfer through these membranous tubes. Our findings indicate that L-forms' polyploidy and extended interconnectivity through membranous tubes contribute to the generation of viable progeny independently of dedicated division machinery, and further support L-forms as models for studies of potential multiplication mechanisms of hypothetical primitive cells.

L-forms are cell wall-deficient bacteria that divide through unusual mechanisms, potentially resembling those of primitive cells. Here the authors describe how Listeria monocytogenesL-forms proliferate by generation of internal and external vesicles.

Mixing Water, Transducing Energy, and Shaping Membranes: Autonomously Self-Regulating Giant Vesicles

Giant lipid vesicles are topologically closed compartments bounded by semipermeable flexible shells, which isolate femto- to picoliter quantities of the aqueous core from the surrounding bulk. Although water equilibrates readily across vesicular walls (10(-2)-10(-3) cm(3) cm(-2) s(-1)), the passive permeation of solutes is strongly hindered. Furthermore, because of their large volume compressibility (∼10(9)-10(10) N m(-2)) and area expansion (10(2)-10(3) mN m(-1)) moduli, coupled with low bending rigidities (10(-19) N m), vesicular shells bend readily but resist volume compression and tolerate only a limited area expansion (∼5%). Consequently, vesicles experiencing solute concentration gradients dissipate the available chemical energy through the osmotic movement of water, producing dramatic shape transformations driven by surface-area-volume changes and sustained by the incompressibility of water and the flexible membrane interface. Upon immersion in a hypertonic bath, an increased surface-area-volume ratio promotes large-scale morphological remodeling, reducing symmetry and stabilizing unusual shapes determined, at equilibrium, by the minimal bending-energy configurations. By contrast, when subjected to a hypotonic bath, walls of giant vesicles lose their thermal undulation, accumulate mechanical tension, and, beyond a threshold swelling, exhibit remarkable oscillatory swell-burst cycles, with the latter characterized by damped, periodic oscillations in vesicle size, membrane tension, and phase behavior. This cyclical pattern of the osmotic influx of water, pressure, membrane tension, pore formation, and solute efflux suggests quasi-homeostatic self-regulatory behavior allowing vesicular compartments produced from simple molecular components, namely, water, osmolytes, and lipids, to sense and regulate their microenvironment in a negative feedback loop.