Phase transition induced fission in lipid vesicles

Phase behaviour and structural properties of SOPC model lipid system in a sucrose solution

Lipid membranes are an important component of the biological cell. The profound understanding of their structure and functionality, as well as, the influence of various biologically relevant admixtures on their main characteristics is of great importance for research and development in medicine and pharmacology. The effect of sugars on the behaviour of the membrane cell enjoys an ever-increasing interest as they are biologically significant substances. We have studied the influence of the disaccharide sucrose on the physicochemical properties of SOPC (1-stearoyl-2-oleoyl-sn- glycero-3-phosphocholine) lipid system aiming to gain better understanding of the mechanisms of the interaction between both substances. For that purpose, we have used differential scanning calorimetry and Fourier-transform infrared spectroscopy. Our results show that adding sugar up to 300 mM concentration substantially alters the thermodynamic and structural properties of SOPC. The DSC thermograms at heating reveal a general lowering of the SOPC transition temperature Tm from gel to liquid crystalline phase (main phase transition, ordered-disordered phase transition) in the presence of sugar. The corresponding peaks are smeared and harder to trace. In agreement with this, a gradual decrease of the enthalpy values up to 300 mM was measured. The IR spectroscopy study provided spectral evidence for two states of hydration of the phosphate groups in the sugar-SOPC model systems suggesting a mechanism of interaction where only part of the phospholipid headgroups are hydrogen bonded to the sugar molecules. The obtained results are in good agreement with various earlier data including results about the bending elasticity moduli, as well as, some theoretical simulations on the sugar-lipid interactions. The current results also reinforce the potential of sucrose to be used as a cell protector against drought at, both, high and low temperatures.

Hydrophobicity-Adaptive Polymers Trigger Fission of Tumor-Cell-Derived Microparticles for Enhanced Anticancer Drug Delivery

Tumor-cell-derived microparticles (MPs) can function as anticancer drug-delivery carriers. However, short blood circulation time, large-size-induced insufficient tumor accumulation and penetration into tumor parenchyma, as well as limited cellular internalization by tumor cells and cancer stem cells (CSCs), and difficult intracellular drug release restrict the anticancer activity of tumor-cell-derived MP-based drug-delivery systems. In this work, hydrophobicity-adaptive polymers based on poly(N-isopropylacrylamide) are anchored to tumor-cell-derived MPs for enhanced delivery of the anticancer drug doxorubicin (DOX). The polymers are hydrophilic in blood to prolong the circulation time of DOX-loaded MPs (DOX@MPs), while rapidly switching to hydrophobic at the tumor acidic microenvironment. The hydrophobicity of polymers drives the fission of tumor-cell-derived MPs to form small vesicles, facilitating tumor accumulation, deep tumor penetration, and efficient internalization of DOX@MPs into tumor cells and CSCs. Subsequently, the hydrophobicity of polymers in acidic lysosomes further promotes DOX release to nuclei for strong cytotoxicity against tumor cells and CSCs. The work provides a facile and simple strategy for improved anticancer drug delivery of tumor-cell-derived MPs.

Vesicle formation induced by thermal fluctuations

The process of fission and vesicle formation depends on the geometry of the membrane that will split. For instance, a flat surface finds it difficult to form vesicles because of the lack of curved regions where to start the process. Here we show that vesicle formation can be promoted by temperature, by using a membrane phase field model with Gaussian curvature. We find a phase transition between fluctuating and vesiculation phases that depends on temperature, spontaneous curvature, and the ratio between bending and Gaussian moduli. We analysed the energy dynamical behaviour of these processes and found that the main driving ingredient is the Gaussian energy term, although the curvature energy term usually helps with the process as well. We also found that the chemical potential can be used to investigate the temperature of the system. Finally we address how temperature changes the condition for spontaneous vesiculation for all geometries, making it happen in a wider range of values of the Gaussian modulus.

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.

Sharp, localized phase transitions in single neuronal cells

Phase transitions in materials are accompanied by drastic changes in their properties. Systems abruptly become softer, more conductive, have better heat storage, or support chemical reactions more efficiently. Since changes take place over small variations in external conditions (tension, temperature, pH, calcium), they appear like an on/off switch. Here, we provide experimental evidence that membrane patches of single living cells can go through a reversible phase transition. It is extremely “sharp” (highly nonlinear), and from a thermodynamic point of view we conclude it cannot only be triggered by temperature but also by pH changes (as produced by enzymes). The results strongly support the idea that phase transitions may be a tool for living systems to control their functions even specifically.

The origin of nonlinear responses in cells has been suggested to be crucial for various cell functions including the propagation of the nervous impulse. In physics, nonlinear behavior often originates from phase transitions. Evidence for such transitions on the single-cell level, however, has so far not been provided, leaving the field unattended by the biological community. Here, we demonstrate that single cells of a human neuronal cell line display all optical features of a sharp, highly nonlinear phase transition within their membrane. The transition is reversible and does not originate from protein denaturation. Triggered by temperature and modified by pH here, a thermodynamic approach strongly suggests that similar nonlinear state changes can be induced by other variables such as calcium or mechanical stress. At least in lipid membranes, such state changes are accompanied by significant changes in permeability, enzyme activity, elastic, and electrical properties.

Anisotropic Particles through Multilayer Assembly

The anisotropy in the shape of polymeric particles has been demonstrated to have many advantages over spherical particulates, including bio-mimetic behavior, shaped-directed flow, deformation, surface adhesion, targeting, motion, and permeability. The layer-by-layer (LbL) assembly is uniquely suited for synthesizing anisotropic particles as this method allows for simple and versatile replication of diverse colloid geometries with precise control over their chemical and physical properties. This review highlights recent progress in anisotropic particles of micrometer and nanometer sizes produced by a templated multilayer assembly of synthetic and biological macromolecules. Synthetic approaches to produce capsules and hydrogels utilizing anisotropic templates such as biological, polymeric, bulk hydrogel, inorganic colloids, and metal-organic framework crystals as sacrificial templates are overviewed. Structure-property relationships controlled by the anisotropy in particle shape and surface are discussed and compared with their spherical counterparts. Advances and challenges in controlling particle properties through varying shape anisotropy and surface asymmetry are outlined. The perspective applications of anisotropic colloids in biomedicine, including programmed behavior in the blood and tissues as artificial cells, nano-motors/sensors, and intelligent drug carriers are also discussed.

Prebiological Membranes and Their Role in the Emergence of Early Cellular Life

Membrane compartmentalization is a fundamental feature of contemporary cellular life. Given this, it is rational to assume that at some stage in the early origins of life, membrane compartments would have potentially emerged to form a dynamic semipermeable barrier in primitive cells (protocells), protecting them from their surrounding environment. It is thought that such prebiological membranes would likely have played a crucial role in the emergence and evolution of life on the early Earth. Extant biological membranes are highly organized and complex, which is a consequence of a protracted evolutionary history. On the other hand, prebiotic membrane assemblies, which are thought to have preceded sophisticated contemporary membranes, are hypothesized to have been relatively simple and composed of single chain amphiphiles. Recent studies indicate that the evolution of prebiotic membranes potentially resulted from interactions between the membrane and its physicochemical environment. These studies have also speculated on the origin, composition, function and influence of environmental conditions on protocellular membranes as the niche parameters would have directly influenced their composition and biophysical properties. Nonetheless, the evolutionary pathways involved in the transition from prebiological membranes to contemporary membranes are largely unknown. This review critically evaluates existing research on prebiotic membranes in terms of their probable origin, composition, energetics, function and evolution. Notably, we outline new approaches that can further our understanding about how prebiotic membranes might have evolved in response to relevant physicochemical parameters that would have acted as pertinent selection pressures on the early Earth.

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.

On the existence of endocytosis driven by membrane phase separations

Large endocytic responses can occur rapidly in diverse cell types without dynamins, clathrin, or actin remodeling. Our experiments suggest that membrane phase separations are crucial with more ordered plasma membrane domains being internalized. Not only do these endocytic processes rely on coalescence of membrane domains, they are promoted by participation of membrane proteins in such domains, one important regulatory influence being palmitoylation. Membrane actin cytoskeleton in general resists membrane phase transitions, and its remodeling may play many roles. Besides membrane 'caging' and 'pinching' roles, typically ascribed to clathrin and dynamins, cytoskeleton remodeling may modify local membrane tension and buckling, as well as the presence and location of actin- and tension-free membrane patches. Endocytosis that depends on membrane phase separations becomes activated in metabolic stress and in response to Ca and PI3 kinase signaling. Internalized membrane traffics normally, and the secretory pathway eventually resupplies membrane to the plasmalemma or directs internalized membrane to other locations, including the extracellular space as exosomes. We describe here that endocytosis driven by membrane phase transitions is regulated by the same signaling mechanisms that regulate macropinocytosis, and it may play diverse roles in cells from nutrient assimilation to membrane recycling, cell migration, and the initiation of quiescent or hibernating cell states. Membrane ordering and phase separations have been shown to promote endocytosis in diverse cell types, including fibroblasts, myocytes, glial cells, and immune cells. We propose that clathrin/dynamin-independent endocytosis represents a continuum of related mechanisms with variable but universal dependence on membrane ordering and actin remodeling. This article is part of a Special Issue entitled: Molecular biophysics of membranes and membrane proteins.

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.

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.

Surface deformation during an action potential in pearled cells

Electric pulses in biological cells (action potentials) have been reported to be accompanied by a propagating cell-surface deformation with a nanoscale amplitude. Typically, this cell surface is covered by external layers of polymer material (extracellular matrix, cell wall material, etc.). It was recently demonstrated in excitable plant cells (Chara braunii) that the rigid external layer (cell wall) hinders the underlying deformation. When the cell membrane was separated from the cell wall by osmosis, a mechanical deformation, in the micrometer range, was observed upon excitation of the cell. The underlying mechanism of this mechanical pulse has, to date, remained elusive. Herein we report that Chara cells can undergo a pearling instability, and when the pearled fragments were excited even larger and more regular cell shape changes were observed (∼10-100μm in amplitude). These transient cellular deformations were captured by a curvature model that is based on three parameters: surface tension, bending rigidity, and pressure difference across the surface. In this paper these parameters are extracted by curve-fitting to the experimental cellular shapes at rest and during excitation. This is a necessary step to identify the mechanical parameters that change during an action potential.

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.

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.

Cell-Free Phospholipid Biosynthesis by Gene-Encoded Enzymes Reconstituted in Liposomes

The goal of bottom-up synthetic biology culminates in the assembly of an entire cell from separate biological building blocks. One major challenge resides in the in vitro production and implementation of complex genetic and metabolic pathways that can support essential cellular functions. Here, we show that phospholipid biosynthesis, a multiple-step process involved in cell membrane homeostasis, can be reconstituted starting from the genes encoding for all necessary proteins. A total of eight E. coli enzymes for acyl transfer and headgroup modifications were produced in a cell-free gene expression system and were co-translationally reconstituted in liposomes. Acyl-coenzyme A and glycerol-3-phosphate were used as canonical precursors to generate a variety of important bacterial lipids. Moreover, this study demonstrates that two-step acyl transfer can occur from enzymes synthesized inside vesicles. Besides clear implications for growth and potentially division of a synthetic cell, we postulate that gene-based lipid biosynthesis can become instrumental for ex vivo and protein purification-free production of natural and non-natural lipids.

Focused ultrasound influence on calcein-loaded thermosensitive stealth liposomes

Focused ultrasound (FUS) is a versatile technology for non-invasive thermal therapies in oncology. Indeed, this technology has great potential for local heat-mediated drug delivery from thermosensitive liposomes (TSLs), thus improving therapeutic efficacy and reducing toxicity profiles. In the present study we evaluated the influence of FUS parameters on the release of calcein from TSLs used to model a hydrophilic drug. Quantitative calcein release from TSLs (DPPC/CHOL/DSPE-PEG2000: 90/5/5) and non-thermosensitive liposomes (NTSLs) (DPPC/CHOL/DSPE-PEG2000: 65/30/5) was measured by spectrofluorimetry after both water bath and FUS-induced in vitro heating. The heating of TSLs at 42 °C in a water bath resulted in a maximum calcein release of 45%. No additional calcein release was observed at temperatures above 42 °C. A similar percentage of calcein release was achieved when TSLs were exposed to 1 MHz sinusoidal waves at peak negative pressure of 1.5 MPa, 40% duty cycle, for 10 min (i.e. above 42 °C). No release was detected when NTSLs were heated in a water bath. For both TSLs and NTSLs, the calcein release was increased by more than 10% for acoustic pressures ranging from 1.5 MPa to 2 MPa. This additional release was attributed to the mechanical stress generated by FUS, which was sufficient to disrupt the liposomal membrane. Furthermore, analysis of cryo-TEM images showed a significant decrease in liposome size (14%) induced by the thermal effect, whereas the liposome diameter remained unaffected by the FUS-triggered non-thermal effects.

Evidence for two-dimensional solitary sound waves in a lipid controlled interface and its implications for biological signalling

Biological membranes by virtue of their elastic properties should be capable of propagating localized perturbations analogous to sound waves. However, the existence and the possible role of such waves in communication in biology remain unexplored. Here, we report the first observations of two-dimensional solitary elastic pulses in lipid interfaces, excited mechanically and detected by FRET. We demonstrate that the nonlinearity near a maximum in the susceptibility of the lipid monolayer results in solitary pulses that also have a threshold for excitation. These experiments clearly demonstrate that the state of the interface regulates the propagation of pulses both qualitatively and quantitatively. Finally, we elaborate on the striking similarity of the observed phenomenon to nerve pulse propagation and a thermodynamic basis of cell signalling in general.

Intake of silica nanoparticles by giant lipid vesicles: influence of particle size and thermodynamic membrane state

The uptake of nanoparticles into cells often involves their engulfment by the plasma membrane and a fission of the latter. Understanding the physical mechanisms underlying these uptake processes may be achieved by the investigation of simple model systems that can be compared to theoretical models. Here, we present experiments on a massive uptake of silica nanoparticles by giant unilamellar lipid vesicles (GUVs). We find that this uptake process depends on the size of the particles as well as on the thermodynamic state of the lipid membrane. Our findings are discussed in the light of several theoretical models and indicate that these models have to be extended in order to capture the interaction between nanomaterials and biological membranes correctly.

Liposome-based liquid handling platform featuring addition, mixing, and aliquoting of femtoliter volumes

This paper describes the utilization of giant unilamellar vesicles (GUVs) as a platform for handling chemical and biochemical reagents. GUVs with diameters of 5 to 10 µm and containing chemical/biochemical reagents together with inert polymers were fused with electric pulses (electrofusion). After reagent mixing, the fused GUVs spontaneously deformed to a budding shape, separating the mixed solution into sub-volumes. We utilized a microfluidic channel and optical tweezers to select GUVs of interest, bring them into contact, and fuse them together to mix and aliquot the reaction product. We also show that, by lowering the ambient temperature close to the phase transition temperature T_m of the lipid used, daughter GUVs completely detached (fission). This process performs all the liquid-handing features used in bench-top biochemistry using the GUV, which could be advantageous for the membrane-related biochemical assays.

Dynamics and instabilities of lipid bilayer membrane shapes

Biological membranes undergo constant shape remodeling involving the formation of highly curved structures. The lipid bilayer represents the fundamental architecture of the cellular membrane with its shapes determined by the Helfrich curvature bending energy. However, the dynamics of bilayer shape transitions, especially their modulation by membrane proteins, and the resulting shape instabilities, are still not well understood. Here, we review in a unifying manner several theories that describe the fluctuations (i.e. undulations) of bilayer shapes as well as their local coupling with lipid or protein density variation. The coupling between local membrane curvature and lipid density gives rise to a 'slipping mode' in addition to the conventional 'bending mode' for damping the membrane fluctuation. This leads to a number of interesting experimental phenomena regarding bilayer shape dynamics. More importantly, curvature-inducing proteins can couple with membrane shape and eventually render the membrane unstable. A criterion for membrane shape instability is derived from a linear stability analysis. The instability criterion reemphasizes the importance of membrane tension in regulating the stability and dynamics of membrane geometry. Recent progresses in understanding the role of membrane tension in regulating dynamical cellular processes are also reviewed. Protein density is emphasized as a key factor in regulating membrane shape transitions: a threshold density of curvature coupling proteins is required for inducing membrane morphology transitions.

Divided we stand: splitting synthetic cells for their proliferation

With the recent dawn of synthetic biology, the old idea of man-made artificial life has gained renewed interest. In the context of a bottom-up approach, this entails the de novo construction of synthetic cells that can autonomously sustain themselves and proliferate. Reproduction of a synthetic cell involves the synthesis of its inner content, replication of its information module, and growth and division of its shell. Theoretical and experimental analysis of natural cells shows that, whereas the core synthesis machinery of the information module is highly conserved, a wide range of solutions have been realized in order to accomplish division. It is therefore to be expected that there are multiple ways to engineer division of synthetic cells. Here we survey the field and review potential routes that can be explored to accomplish the division of bottom-up designed synthetic cells. We cover a range of complexities from simple abiotic mechanisms involving splitting of lipid-membrane-encapsulated vesicles due to physical or chemical principles, to potential division mechanisms of synthetic cells that are based on prokaryotic division machineries.

The free energy of nanopores in tense membranes

Membrane nanopores are central players for a range of important cellular membrane remodeling processes as well as membrane rupture. Understanding pore formation in tense membranes requires comprehension of the molecular mechanism of pore formation and the associated free energy change as a function of the membrane tension. Here we propose a scheme to calculate the free energy change associated with the formation of a nanometer sized pore in molecular dynamics simulations as a function of membrane tension, which requires the calculation of only one computationally expensive potential of mean force. We show that membrane elastic theory can be used to estimate the pore formation free energy at different tension values from the free energy change in a relaxed membrane and the area expansion curves of the membranes. We have computed the pore formation free energy for a dipalmitoyl-phosphatidylcholine (DPPC) membrane at two different lateral pressure values, 1 bar and -40 bar, by calculating the potential of mean force acting on the head group of a single lipid molecule. Unrestrained simulations of the closing process confirm that the intermediate states along this reaction coordinate are reasonable and show that hydrophilic indentations spanning half the bilayer connected by a hydrophobic pore segment represent the corresponding high energy transition state. A comparison of the stability of simulated membranes to experiment at high loading rates show that, contrary to expectation, pores form too easily in small simulated membrane patches. This discrepancy originates from a combination of the absence of ions in the simulations and the small membrane size.

Chemical and mechanical impact of silica nanoparticles on the phase transition behavior of phospholipid membranes in theory and experiment

The interaction of nanoparticles (NPs) with lipid membranes is an integral step in the interaction of NPs and living cells. During particle uptake, the membrane has to bend. Due to the nature of their phase diagram, the modulus of compression of these membranes can vary by more than one order of magnitude, and thus both the thermodynamic and mechanical aspects of the membrane have to be considered simultaneously. We demonstrate that silica NPs have at least two independent effects on the phase transition of phospholipid membranes: 1), a chemical effect resulting from the finite instability of the NPs in water; and 2), a mechanical effect that originates from a bending of the lipid membrane around the NPs. Here, we report on recent experiments that allowed us to clearly distinguish both effects, and present a thermodynamic model that includes the elastic energy of the membranes and correctly predicts our findings both quantitatively and qualitatively.

Replication of simulated prebiotic amphiphile vesicles controlled by experimental lipid physicochemical properties

We present a new embodiment of the graded autocatalysis replication domain (GARD) for the growth, replication and evolution of lipid vesicles based on a semi-empirical foundation using experimentally measured kinetic values of selected extant lipid species. Extensive simulations using this formalism elucidated the details of the dependence of the replication and properties of the vesicles on the physicochemical properties and concentrations of the lipids, both in the environment and in the vesicle. As expected, the overall concentration and number of amphiphilic components strongly affect average replication time. Furthermore, variations in acyl chain length and unsaturation of vesicles also influence replication rate, as do the relative concentrations of individual lipid types. Understanding of the dependence of replication rates on physicochemical parameters opens a new direction in the study of prebiotic vesicles and lays the groundwork for future studies involving the competition between lipid vesicles for available amphiphilic monomers.

Complete budding and asymmetric division of primitive model cells to produce daughter vesicles with different interior and membrane compositions

Asymmetric cell division is common in biology and plays critical roles in differentiation and development. Unicellular organisms are often used as model systems for understanding the origins and consequences of asymmetry during cell division. Although basic as compared to mammalian cells, these are already quite complex. We report complete budding and asymmetric fission of very simple nonliving model cells to produce daughter vesicles that are chemically distinct in both interior and membrane compositions. Our model cells are based on giant lipid vesicles (GVs, 10-30 μm) encapsulating a polyethylene glycol (PEG)/dextran aqueous two-phase system (ATPS) as a crowded and compartmentalized cytoplasm mimic. Ternary lipid compositions were used to provide coexisting micrometer-scale liquid disordered (L(d)) and liquid ordered (L(o)) domains in the membranes. ATPS-containing vesicles formed buds when sucrose was added externally to provide increased osmotic pressure, such that they became not only morphologically asymmetric but also asymmetric in both their interior and their membrane compositions. Further increases in osmolality drove formation of two chemically distinct daughter vesicles, which were in some cases connected by a lipid nanotube (complete budding), and in others were not (fission). In all cases, separation occurred at the aqueous-aqueous phase boundary, such that one daughter vesicle contained the PEG-rich aqueous phase and the other contained the dextran-rich aqueous phase. PEGylated lipids localized in the L(o) domain resulted in this membrane domain preferentially coating the PEG-rich bud prior to division, and subsequently the PEG-rich daughter vesicle. Varying the mole ratio of lipids resulted in excess surface area of L(o) or L(d) membrane domains such that, upon division, this excess portion was inherited by one of the daughter vesicles. In some cases, a second "generation" of aqueous phase separation and budding could be induced in these daughter vesicles. Asymmetric fission of a simple self-assembled model cell, with production of daughter vesicles that harbored different protein concentrations and lipid compositions, is an example of the seemingly complex behavior possible for simple molecular assemblies. These compartmentalized and asymmetrically dividing ATPS-containing GVs could serve as a test bed for investigating possible roles for spatial and organizational cues in asymmetric cell division and inheritance.

Giant vesicles with membranous microcompartments

Incubation of a cell-sized lipid membrane vesicle (giant vesicle, GV) in a diluted aqueous solution of neutral phosphate buffer salts or glucose transformed the GV to an oligovesicular vesicle (OVV) that encapsulates one or more smaller GVs. During the incubation, the membrane of flaccid vesicle invaginated and closed to form the inner vesicle of an OVV engulfing a part of the bulk aqueous phase. Using the GV-to-OVV transformation, an OVV that has different aqueous contents in each membranous microcompartment was constructed.

Thermomechanic-electrical coupling in phospholipid monolayers near the critical point

Lipid monolayers have been shown to represent a powerful tool in studying mechanical and thermodynamic properties of lipid membranes as well as their interaction with proteins. Using Einstein's theory of fluctuations we here demonstrate that an experimentally derived linear relationship both between transition entropy S and area A as well as between transition entropy and charge q implies a linear relationships between compressibility κT, heat capacity cπ, thermal expansion coefficient αT, and electric capacity CT. We demonstrate that these couplings have strong predictive power as they allow calculating electrical and thermal properties from mechanical measurements. The precision of the prediction increases as the critical point TC is approached.

Vesicle budding and the origin of cellular life

This Minireview provides an appropriate opportunity to demonstrate the connection between the results of some early experimental and theoretical investigations of vesicle budding and the more recent application of the concepts developed there to the process of vesicle self-reproduction. Herein, we also explain why vesicle budding could have preceded the establishment of cellular life.