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

Significance of in situ quantitative membrane property-morphology relation (QmPMR) analysis

Deformation of the cell membrane is well understood from the viewpoint of protein interactions and free energy balance. However, the various dynamic properties of the membrane, such as lipid packing and hydrophobicity, and their relationship with cell membrane deformation are unknown. Therefore, the deformation of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and oleic acid (OA) giant unilamellar vesicles (GUVs) was induced by heating and cooling cycles, and time-lapse analysis was conducted based on the membrane hydrophobicity and physical parameters of "single-parent" and "daughter" vesicles. Fluorescence ratiometric analysis by simultaneous dual-wavelength detection revealed the variation of different hydrophilic GUVs and enabled inferences of the "daughter" vesicle composition and the "parent" membrane's local composition during deformation; the "daughter" vesicle composition of OA was lower than that of the "parents", and lateral movement of OA was the primary contributor to the formation of the "daughter" vesicles. Thus, our findings and the newly developed methodology, named in situ quantitative membrane property-morphology relation (QmPMR) analysis, would provide new insights into cell deformation and accelerate research on both deformation and its related events, such as budding and birthing.

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).

Synthesising a minimal cell with artificial metabolic pathways

A "synthetic minimal cell" is considered here as a cell-like artificial vesicle reproduction system in which a chemical and physico-chemical transformation network is regulated by information polymers. Here we synthesise such a minimal cell consisting of three units: energy production, information polymer synthesis, and vesicle reproduction. Supplied ingredients are converted to energy currencies which trigger the synthesis of an information polymer, where the vesicle membrane plays the role of a template. The information polymer promotes membrane growth. By tuning the membrane composition and permeability to osmolytes, the growing vesicles show recursive reproduction over several generations. Our "synthetic minimal cell" greatly simplifies the scheme of contemporary living cells while keeping their essence. The chemical pathways and the vesicle reproduction pathways are well described by kinetic equations and by applying the membrane elasticity model, respectively. This study provides new insights to better understand the differences and similarities between non-living forms of matter and life.

Light-Switchable Membrane Permeability in Giant Unilamellar Vesicles

In this work, giant unilamellar vesicles (GUVs) were synthesized by blending the natural phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with a photoswitchable amphiphile (1) that undergoes photoisomerization upon irradiation with UV-A (E to Z) and blue (Z to E) light. The mixed vesicles showed marked changes in behavior in response to UV light, including changes in morphology and the opening of pores. The fine control of membrane permeability with consequent cargo release could be attained by modulating either the UV irradiation intensity or the membrane composition. As a proof of concept, the photocontrolled release of sucrose from mixed GUVs is demonstrated using microscopy (phase contrast) and confocal studies. The permeability of the GUVs to sucrose could be increased to ~4 × 10^-2 μm/s when the system was illuminated by UV light. With respect to previously reported systems (entirely composed of synthetic amphiphiles), our findings demonstrate the potential of photosensitive GUVs that are mainly composed of natural lipids to be used in medical and biomedical applications, such as targeted drug delivery and localized topical treatments.

Evolution of Proliferative Model Protocells Highly Responsive to the Environment

In this review, we discuss various methods of reproducing life dynamics using a constructive approach. An increase in the structural complexity of a model protocell is accompanied by an increase in the stage of reproduction of a compartment (giant vesicle; GV) from simple reproduction to linked reproduction with the replication of information molecules (DNA), and eventually to recursive proliferation of a model protocell. An encounter between a plural protic catalyst (C) and DNA within a GV membrane containing a plural cationic lipid (V) spontaneously forms a supramolecular catalyst (C@DNA) that catalyzes the production of cationic membrane lipid V. The local formation of V causes budding deformation of the GV and equivolume divisions. The length of the DNA strand influences the frequency of proliferation, associated with the emergence of a primitive information flow that induces phenotypic plasticity in response to environmental conditions. A predominant protocell appears from the competitive proliferation of protocells containing DNA with different strand lengths, leading to an evolvable model protocell. Recently, peptides of amino acid thioesters have been used to construct peptide droplets through liquid–liquid phase separation. These droplets grew, owing to the supply of nutrients, and were divided repeatedly under a physical stimulus. This proposed chemical system demonstrates a new perspective of the origins of membraneless protocells, i.e., the “droplet world” hypothesis. Proliferative model protocells can be regarded as autonomous supramolecular machines. This concept of this review may open new horizons of “evolution” for intelligent supramolecular machines and robotics.

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.

Identifying and Manipulating Giant Vesicles: Review of Recent Approaches

Giant vesicles (GVs) are closed bilayer membranes that primarily comprise amphiphiles with diameters of more than 1 μm. Compared with regular vesicles (several tens of nanometers in size), GVs are of greater scientific interest as model cell membranes and protocells because of their structure and size, which are similar to those of biological systems. Biopolymers and nano-/microparticles can be encapsulated in GVs at high concentrations, and their application as artificial cell bodies has piqued interest. It is essential to develop methods for investigating and manipulating the properties of GVs toward engineering applications. In this review, we discuss current improvements in microscopy, micromanipulation, and microfabrication technologies for progress in GV identification and engineering tools. Combined with the advancement of GV preparation technologies, these technological advancements can aid the development of artificial cell systems such as alternative tissues and GV-based chemical signal processing systems.

Rapid and Facile Preparation of Giant Vesicles by the Droplet Transfer Method for Artificial Cell Construction

Giant vesicles have been widely used for the bottom-up construction of artificial (or synthetic) cells and the physicochemical analysis of lipid membranes. Although methods for the formation of giant vesicles and the encapsulation of molecules within them have been established, a standardized protocol has not been shared among researchers including non-experts. Here we proposed a rapid and facile protocol that allows the formation of giant vesicles within 30 min. The quality of the giant vesicles encapsulating a cell-free protein expression system was comparable to that of the ones formed using a conventional method, in terms of the synthesis of both soluble and membrane proteins. We also performed protein synthesis in artificial cells using a lyophilized cell-free mixture and showed an equivalent level of protein synthesis. Our method could become a standard method for giant vesicle formation suited for artificial cell research.

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.

Liposomes: Structure, Biomedical Applications, and Stability Parameters With Emphasis on Cholesterol

Liposomes are essentially a subtype of nanoparticles comprising a hydrophobic tail and a hydrophilic head constituting a phospholipid membrane. The spherical or multilayered spherical structures of liposomes are highly rich in lipid contents with numerous criteria for their classification, including structural features, structural parameters, and size, synthesis methods, preparation, and drug loading. Despite various liposomal applications, such as drug, vaccine/gene delivery, biosensors fabrication, diagnosis, and food products applications, their use encounters many limitations due to physico-chemical instability as their stability is vigorously affected by the constituting ingredients wherein cholesterol performs a vital role in the stability of the liposomal membrane. It has well established that cholesterol exerts its impact by controlling fluidity, permeability, membrane strength, elasticity and stiffness, transition temperature (Tm), drug retention, phospholipid packing, and plasma stability. Although the undetermined optimum amount of cholesterol for preparing a stable and controlled release vehicle has been the downside, but researchers are still focused on cholesterol as a promising material for the stability of liposomes necessitating explanation for the stability promotion of liposomes. Herein, the prior art pertaining to the liposomal appliances, especially for drug delivery in cancer therapy, and their stability emphasizing the roles of cholesterol.

Effect of the Membrane Composition of Giant Unilamellar Vesicles on Their Budding Probability: A Trade-Off between Elasticity and Preferred Area Difference

The budding and division of artificial cells engineered from vesicles and droplets have gained much attention in the past few decades due to an increased interest in designing stimuli-responsive synthetic systems. Proper control of the division process is one of the main challenges in the field of synthetic biology and, especially in the context of the origin of life studies, it would be helpful to look for the simplest chemical and physical processes likely at play in prebiotic conditions. Here we show that pH-sensitive giant unilamellar vesicles composed of mixed phospholipid/fatty acid membranes undergo a budding process, internally fuelled by the urea–urease enzymatic reaction, only for a given range of the membrane composition. A gentle interplay between the effects of the membrane composition on the elasticity and the preferred area difference of the bilayer is responsible for the existence of a narrow range of membrane composition yielding a high probability for budding of the vesicles.

Shape changes and budding of giant vesicles induced by an internal chemical trigger: an interplay between osmosis and pH change

Shape transformation and budding of phospholipid/fatty acid giant hybrid vesicles can be induced by an internal chemical stimulus (pH change) when coupled with an osmotic shock. In particular, an autocatalytic enzymatic reaction set (urea-urease system), confined in the lumen of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/oleic acid (HOA) vesicles, can force the budding of the hosting vesicle, when properly fed by a trans-membrane substrate infusion. Herein, we elucidate the budding mechanism by simulating the shape changes of a vesicle during the enzymatic reaction. The area-difference-elasticity (ADE) theory is thus implemented to minimize the surface elastic energy and obtain the equilibrium shape at different values of the reduced volume and different values of the reduced preferred area difference (Δa0). Simulations, together with control experiments, unambiguously show that to obtain an effective vesicle shape transformation, the osmotic stress and the pH change in the lumen of the vesicle must act in synergy at the same timescale. Osmotic pressure induces a vesicle deflation (volume loss), while the pH change affects the preferred area difference between the outer and the inner membrane leaflets.

Morphologies of Vesicle Doublets: Competition among Bending Elasticity, Surface Tension, and Adhesion

To study the mechanical laws governing the form of multicellular organisms, we examine the morphology of adhering vesicle doublets as the simplest model system. We monitor the morphological transformations of doublets induced by changes of adhesion strength and volume/area ratio, which are controlled by intermembrane interactions and thermal area expansion, respectively. When we increase the temperature in the weak adhesion regime, a dumbbell flat-contact doublet is transformed to a parallel-prolate doublet, whereas in the strong adhesion regime, heating transforms the dumbbell flat-contact doublet into a spherical sigmoid-contact doublet. We reproduce the observed doublet morphologies by numerically minimizing the total energy, including the contact-potential adhesion term as well as the surface and bending terms, using the Surface Evolver package. From the reproduced morphologies, we extract the adhesion strength, the surface tension, and the volume/area ratio of the vesicles, which reveals the detailed mechanisms of the morphological transitions in doublets.

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.

Limiting shapes of confined lipid vesicles

We theoretically study the shapes of lipid vesicles confined to a spherical cavity, elaborating a framework based on the so-called limiting shapes constructed from geometrically simple structural elements such as double-membrane walls and edges. Partly inspired by numerical results, the proposed non-compartmentalized and compartmentalized limiting shapes are arranged in the bilayer-couple phase diagram which is then compared to its free-vesicle counterpart. We also compute the area-difference-elasticity phase diagram of the limiting shapes and we use it to interpret shape transitions experimentally observed in vesicles confined within another vesicle. The limiting-shape framework may be generalized to theoretically investigate the structure of certain cell organelles such as the mitochondrion.

Migration of Deformable Vesicles Induced by Ionic Stimuli

We have investigated the dynamics of phospholipid vesicles composed of 1,2-dioleoyl- sn-glycero-3-phosphocholine triggered by ionic stimuli using electrolytes such as CaCl₂, NaCl, and NaOH. The ionic stimuli induce two characteristic vesicle dynamics, deformation due to the ion binding to the lipids in the outer leaflet of the vesicle and migration due to the concentration gradient of ions, that is, diffusiophoresis or the interfacial energy gradient mechanism. We examined the deformation pathway for each electrolyte as a function of time and analyzed it based on the surface dissociation model and the area difference elasticity model, which reveals the change of the cross-sectional area of the phospholipid by the ion binding. The metal ions such as Ca²⁺ and Na⁺ encourage inward budding deformation by decreasing the cross-sectional area of a lipid, whereas the hydroxide ion (OH^-) encourages outward budding deformation by increasing the cross-sectional area of a lipid. When we microinjected these electrolytes toward the vesicles, a strong coupling between the deformation and the migration of the vesicle was observed for CaCl₂ and NaOH, whereas for NaCl, the coupling was very weak. This difference probably originates from the binding constants of the ions.

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.

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.

Investigating cell functioning by theoretical analysis of cell-to-cell variability

Here we discuss cell-to-cell variability in isogenic cell populations on the basis of an analogy between the processes of vesicle self-reproduction and cell self-replication. A short review of the theoretical analysis of vesicle self-reproduction is presented to indicate that this process only occurs under the fulfillment of specific criteria: causal relations between the values of vesicle variables involved in its growth and division, and the parameters of the environment. It is shown that when division is asymmetric, both vesicle birth size and interdivision times are variable. We argue that during cell self-replication, the balance between processes of cell growth and division also relies on causal relations between the corresponding cellular variables. A possible method is suggested to unravel previously unidentified causal relations between cell variables from the relationships between their variability parameters such as the widths of their probability distributions and their correlation coefficients. The method is outlined by reviewing the results of the corresponding analysis applied to a population of red blood cells. Some novel research directions are suggested that could lead from the analysis of cell-to-cell variability to a better understanding of the organizational structure of cells and possibly also their evolutionary origin.