Reversible Morphological Control of Tubulin-Encapsulating Giant Liposomes by Hydrostatic Pressure

A Self-Regenerating Artificial Cell, that is One Step Closer to Living Cells: Challenges and Perspectives

Controllable, self-regenerating artificial cells (SRACs) can be a vital advancement in the field of synthetic biology, which seeks to create living cells by recombining various biological molecules in the lab. This represents, more importantly, the first step on a long journey toward creating reproductive cells from rather fragmentary biochemical mimics. However, it is still a difficult task to replicate the complex processes involved in cell regeneration, such as genetic material replication and cell membrane division, in artificially created spaces. This review highlights recent advances in the field of controllable, SRACs and the strategies to achieve the goal of creating such cells. Self-regenerating cells start by replicating DNA and transferring it to a location where proteins can be synthesized. Functional but essential proteins must be synthesized for sustained energy generation and survival needs and function in the same liposomal space. Finally, self-division and repeated cycling lead to autonomous, self-regenerating cells. The pursuit of controllable, SRACs will enable authors to make bold advances in understanding life at the cellular level, ultimately providing an opportunity to use this knowledge to understand the nature of life.

Exploring Giant Unilamellar Vesicle Production for Artificial Cells - Current Challenges and Future Directions

Creating an artificial cell from the bottom up is a long-standing challenge and, while significant progress has been made, the full realization of this goal remains elusive. Arguably, one of the biggest hurdles that researchers are facing now is the assembly of different modules of cell function inside a single container. Giant unilamellar vesicles (GUVs) have emerged as a suitable container with many methods available for their production. Well-studied swelling-based methods offer a wide range of lipid compositions but at the expense of limited encapsulation efficiency. Emulsion-based methods, on the other hand, excel at encapsulation but are only effective with a limited set of membrane compositions and may entrap residual additives in the lipid bilayer. Since the ultimate artificial cell will need to comply with both specific membrane and encapsulation requirements, there is still no one-method-fits-all solution for GUV formation available today. This review discusses the state of the art in different GUV production methods and their compatibility with GUV requirements and operational requirements such as reproducibility and ease of use. It concludes by identifying the most pressing issues and proposes potential avenues for future research to bring us one step closer to turning artificial cells into a reality.

Engineering Tissue-Scale Properties with Synthetic Cells: Forging One from Many

In metazoans, living cells achieve capabilities beyond individual cell functionality by assembling into multicellular tissue structures. These higher-order structures represent dynamic, heterogeneous, and responsive systems that have evolved to regenerate and coordinate their actions over large distances. Recent advances in constructing micrometer-sized vesicles, or synthetic cells, now point to a future where construction of synthetic tissue can be pursued, a boon to pressing material needs in biomedical implants, drug delivery systems, adhesives, filters, and storage devices, among others. To fully realize the potential of synthetic tissue, inspiration has been and will continue to be drawn from new molecular findings on its natural counterpart. In this review, we describe advances in introducing tissue-scale features into synthetic cell assemblies. Beyond mere complexation, synthetic cells have been fashioned with a variety of natural and engineered molecular components that serve as initial steps toward morphological control and patterning, intercellular communication, replication, and responsiveness in synthetic tissue. Particular attention has been paid to the dynamics, spatial constraints, and mechanical strengths of interactions that drive the synthesis of this next-generation material, describing how multiple synthetic cells can act as one.

A Practical Guide to Preparation and Applications of Giant Unilamellar Vesicles Formed via Centrifugation of Water-in-Oil Emulsion Droplets

Giant vesicles (GVs), which are closed lipid bilayer membranes with a diameter of more than 1 μm, have attracted attention not only as model cell membranes but also for the construction of artificial cells. For encapsulating water-soluble materials and/or water-dispersible particles or functionalizing membrane proteins and/or other synthesized amphiphiles, giant unilamellar vesicles (GUVs) have been applied in various fields, such as supramolecular chemistry, soft matter physics, life sciences, and bioengineering. In this review, we focus on a preparation technique for GUVs that encapsulate water-soluble materials and/or water-dispersible particles. It is based on the centrifugation of a water-in-oil emulsion layered on water and does not require special equipment other than a centrifuge, which makes it the first choice for laboratory use. Furthermore, we review recent studies on GUV-based artificial cells prepared using this technique and discuss their future applications.

Scalable Synthesis of Planar Macroscopic Lipid-Based Multi-Compartment Structures

As life evolved, the path from simple single cell organisms to multicellular enabled increasingly complex functionalities. The spatial separation of reactions at the micron scale achieved by cellular structures allowed diverse and scalable implementation in biomolecular systems. Mimicking such spatially separated domains in a scalable approach could open a route to creating synthetic cell-like structured systems. Here, we report a facile and scalable method to create multicellular-like, multi-compartment (MC) structures. Aqueous droplet-based compartments ranging from 50 to 400 μm were stabilized and connected together by hydrophobic layers composed of phospholipids and an emulsifier. Planar centimeter-scale MC structures were formed by droplet deposition on a water interface. Further, the resulting macroscopic shapes were shown to be achieved by spatially controlled deposition. To demonstrate configurability and potential versatility, MC assemblies of both homogeneous and mixed compartment types were shown. Notably, magnetically heterogeneous systems were achieved by the inclusion of magnetic nanoparticles in defined sections. Such structures demonstrated actuated motion with structurally imparted directionality. These novel and functionalized structures exemplify a route toward future applications including compartmentally assembled "multicellular" molecular robots.

Enhancement of polyethylene glycol-cell fusion efficiency by novel application of transient pressure using a jet injector

Cell-cell fusion involves the fusion of somatic cells into a single hybrid cell. It is not only a physiological process but also an important cell engineering technology which can be applied to various fields, such as regenerative medicine, antibody engineering, genetic engineering, and cancer therapy. There are three major methods of cell fusion: electrical cell fusion, polyethylene glycol (PEG) cell fusion, and virus-mediated cell fusion. Although PEG cell fusion is the most economical approach and does not require expensive instrumentation, it has a poor fusion rate and induces a high rate of cell cytotoxicity. To improve the fusion rate of the PEG method, we combined it with the pyro-drive jet injector (PJI). PJI provides instant pressure instead of cell agitation to increase the probability of cell-to-cell contact and shorten the distance between cells in the process of cell fusion. Here, we report that this improved fusion method not only decreased cell cytotoxicity during the fusion process, but also increased fusion rate compared with the conventional PEG method. Furthermore, we tested the functionality of cells fused using the PJI-PEG method and found them to be comparable to those fused using the conventional PEG method in terms of their application for dendritic cell (DC)-tumor cell fusion vaccine production; in addition, the PJI-PEG method demonstrated excellent performance in hybridoma cell preparation. Taken together, our data indicate that this method improves cell fusion efficiency as compared to the PEG method and thus has the potential for use in various applications that require cell fusion technology.

Methods to mechanically perturb and characterize GUV-based minimal cell models

Cells shield organelles and the cytosol via an active boundary predominantly made of phospholipids and membrane proteins, yet allowing communication between the intracellular and extracellular environment. Micron-sized liposome compartments commonly known as giant unilamellar vesicles (GUVs) are used to model the cell membrane and encapsulate biological materials and processes in a cell-like confinement. In the field of bottom-up synthetic biology, many have utilized GUVs as substrates to study various biological processes such as protein-lipid interactions, cytoskeletal assembly, and dynamics of protein synthesis. Like cells, it is ideal that GUVs are also mechanically durable and able to stay intact when the inner and outer environment changes. As a result, studies have demonstrated approaches to tune the mechanical properties of GUVs by modulating membrane composition and lumenal material property. In this context, there have been many different methods developed to test the mechanical properties of GUVs. In this review, we will survey various perturbation techniques employed to mechanically characterize GUVs.

Effects of Charged Lipids on Giant Unilamellar Vesicle Fusion and Inner Content Mixing via Freeze-Thawing

Fusion between giant unilamellar vesicles (GUVs) can incorporate and mix components of biochemical reactions. Recently, GUV fusion induced by freeze-thawing (F/T) was employed to construct artificial cells that can easily and repeatedly fuse GUVs with efficient content mixing. However, GUVs were ruptured during F/T, and the inner contents leaked. Herein, we investigated the effects of charged lipids on GUV fusion via F/T. The presence of 10 %-50 % (w/w%) negatively charged lipids in GUV membranes, mainly composed of the neutral charged lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), improved resistance to GUV rupture and decreased inner content leakage. Furthermore, we found that the presence of positively charged lipids in GUV membranes elevated GUV rupture compared with F/T between GUVs containing POPC alone. Modified GUVs may better incorporate nutrients and lipid membranes with less damage following GUV fusion via F/T, providing an improved artificial model.

Effects of Sugars on Giant Unilamellar Vesicle Preparation, Fusion, PCR in Liposomes, and Pore Formation

The water-in-oil emulsion transfer method was developed for preparing giant unilamellar vesicles (GUVs) and is useful for studying cellular functions under conditions that mimic cellular environments. A shortcoming of this method for encapsulating biochemical reactions is that it requires high sugar concentrations to enable the density effect to transverse the oil-water interface. In this study, we investigated the effects of sugars on GUV preparation and several biochemical reactions. We found that changing the sugar in the inner solution from sucrose to maltose or trehalose improved GUV formation. The fusion ratio of the freeze-thaw method was better in the traditional glucose-sucrose condition compared with the other examined conditions. For the inner biochemical reaction, we performed PCR in liposomes. The presence of maltose in the inner solution improved the stability of GUVs against damage caused by thermal cycles. Finally, fructose in the outer solution reduced leakage of the inner solution via pores on the membranes of GUVs. Our findings provide new insight for optimizing sugar conditions for preparing GUVs and inner GUV reactions. This could increase the utilization of GUVs as artificial cell compartment models.

Thermo-Statistical Effects of Inclusions on Vesicles: Division into Multispheres and Polyhedral Deformation

The construction of simple cellular models has attracted much attention as a way to explore the origin of life or elucidate the mechanisms of cell division. In the absence of complex regulatory systems, some bacteria spontaneously divide through thermostatistically elucidated mechanisms, and incorporating these simple physical principles could help to construct primitive or artificial cells. Because thermodynamic interactions play an essential role in such mechanisms, this review discusses the thermodynamic aspects of spontaneous division models of vesicles that contain a high density of inclusions, with their membrane serving as a boundary. Vesicles with highly dense inclusions are deformed according to the volume-to-area ratio. The phase separation of beads at specific intermediate volume fractions and the associated polyhedral deformation of the membrane are considered in relation to the Alder transition. Current advances in the development of a membrane-growth vesicular model are summarized. The thermostatistical understanding of these mechanisms could become a cornerstone for the construction of vesicular models that display spontaneous cell division.

Shaping Liposomes by Cell-Free Expressed Bacterial Microtubules

Genetic control over a cytoskeletal network inside lipid vesicles offers a potential route to controlled shape changes and DNA segregation in synthetic cell biology. Bacterial microtubules (bMTs) are protein filaments found in bacteria of the genus Prosthecobacter. They are formed by the tubulins BtubA and BtubB, which polymerize in the presence of GTP. Here, we show that the tubulins BtubA/B can be functionally expressed from DNA templates in a reconstituted transcription-translation system, thus providing a cytosol-like environment to study their biochemical and biophysical properties. We found that bMTs spontaneously interact with lipid membranes and display treadmilling. When compartmentalized inside liposomes, de novo synthesized BtubA/B tubulins self-organize into cytoskeletal structures of different morphologies. Moreover, bMTs can exert a pushing force on the membrane and deform liposomes, a phenomenon that can be reversed by a light-activated disassembly of the filaments. Our work establishes bMTs as a new building block in synthetic biology. In the context of creating a synthetic cell, bMTs could help shape the lipid compartment, establish polarity or directional transport, and assist the division machinery.

Towards a synthetic cell cycle

Recent developments in synthetic biology may bring the bottom-up generation of a synthetic cell within reach. A key feature of a living synthetic cell is a functional cell cycle, in which DNA replication and segregation as well as cell growth and division are well integrated. Here, we describe different approaches to recreate these processes in a synthetic cell, based on natural systems and/or synthetic alternatives. Although some individual machineries have recently been established, their integration and control in a synthetic cell cycle remain to be addressed. In this Perspective, we discuss potential paths towards an integrated synthetic cell cycle.

Effects of high-pressure treatment on the structure and function of myofibrils

The effects of high pressure (40-70 MPa) on the structure and function of myofibrils were investigated by high pressure microscopy. When this pressure was applied to myofibrils immersed in relaxing solution, the sarcomere length remained almost unchanged, and the A band became shorter and wider. The higher the applied pressure, the faster the change. However, shortening and widening of the A band were not observed when pressure was applied to myofibrils immersed in a solution obtained by omitting ATP from the relaxing solution. However, even under these conditions, structural loss, such as loss of the Z-line structure, occurred. In order to evaluate the consequences of this pressure-treated myofibril, the oscillatory movement of sarcomere (sarcomeric oscillation) was evoked and observed. It was possible to induce sarcomeric oscillation even in pressure-treated myofibrils whose structure was destroyed. The pressurization reduced the total power of the sarcomeric oscillation, but did not change the average frequency. The average frequency did not change even when a pressure of about 40 MPa was applied during sarcomeric oscillation. The average frequency returned to the original when the pressure was returned to the original value after applying stronger pressure to prevent the sarcomere oscillation from being observed. This result suggests that the decrease in the number of myosin molecules forming the crossbridge does not affect the average frequency of sarcomeric oscillation. This fact will help build a mechanical hypothesis for sarcomeric oscillation. The pressurization treatment is a unique method for controlling the structure of myofibrils as described above.

Protein Reconstitution Inside Giant Unilamellar Vesicles

Giant unilamellar vesicles (GUVs) have gained great popularity as mimicries for cellular membranes. As their sizes are comfortably above the optical resolution limit, and their lipid composition is easily controlled, they are ideal for quantitative light microscopic investigation of dynamic processes in and on membranes. However, reconstitution of functional proteins into the lumen or the GUV membrane itself has proven technically challenging. In recent years, a selection of techniques has been introduced that tremendously improve GUV-assay development and enable the precise investigation of protein-membrane interactions under well-controlled conditions. Moreover, due to these methodological advances, GUVs are considered important candidates as protocells in bottom-up synthetic biology. In this review, we discuss the state of the art of the most important vesicle production and protein encapsulation methods and highlight some key protein systems whose functional reconstitution has advanced the field.

Membrane Shape Dynamics-Based Analysis of the Physical Properties of Giant Unilamellar Vesicles Prepared by Inverted Emulsion and Hydration Techniques

The giant unilamellar vesicle (GUV) is a basic model of the cell membrane that allows for the modulation and control of membrane shape dynamics, which play essential roles in the functions of living cell membranes. However, to properly use these artificial cell-like model systems, we need to understand their physical properties. GUV generation techniques are key technologies in the synthesis of artificial cell-like model systems. However, it is unclear whether GUVs produced by different techniques have the same physical properties. Here, we have investigated the physical properties of GUVs prepared by inverted emulsion and hydration techniques by examining the membrane shape deformation induced by external stimulation with a nonionic surfactant. We reveal differences in the spontaneous curvature of the membrane, the preferred differential area between the inner and outer leaflets of the membrane, and the edge tension of membrane pores between the GUVs prepared using the two distinct techniques.

Compartmentalizing Cell-Free Systems: Toward Creating Life-Like Artificial Cells and Beyond

Building an artificial cell is a research area that is rigorously studied in the field of synthetic biology. It has brought about much attention with the aim of ultimately constructing a natural cell-like structure. In particular, with the more mature cell-free platforms and various compartmentalization methods becoming available, achieving this aim seems not far away. In this review, we discuss the various types of artificial cells capable of hosting several cellular functions. Different compartmental boundaries and the mature and evolving technologies that are used for compartmentalization are examined, and exciting recent advances that overcome or have the potential to address current challenges are discussed. Ultimately, we show how compartmentalization and cell-free systems have, and will, come together to fulfill the goal to assemble a fully synthetic cell that displays functionality and complexity as advanced as that in nature. The development of such artificial cell systems will offer insight into the fundamental study of evolutionary biology and the sea of applications as a result. Although several challenges remain, emerging technologies such as artificial intelligence also appear to help pave the way to address them and achieve the ultimate goal.

Interface Engineering in Multiphase Systems toward Synthetic Cells and Organelles: From Soft Matter Fundamentals to Biomedical Applications

Synthetic cells have a major role in gaining insight into the complex biological processes of living cells; they also give rise to a range of emerging applications from gene delivery to enzymatic nanoreactors. Living cells rely on compartmentalization to orchestrate reaction networks for specialized and coordinated functions. Principally, the compartmentalization has been an essential engineering theme in constructing cell-mimicking systems. Here, efforts to engineer liquid-liquid interfaces of multiphase systems into membrane-bounded and membraneless compartments, which include lipid vesicles, polymer vesicles, colloidosomes, hybrids, and coacervate droplets, are summarized. Examples are provided of how these compartments are designed to imitate biological behaviors or machinery, including molecule trafficking, growth, fusion, energy conversion, intercellular communication, and adaptivity. Subsequently, the state-of-art applications of these cell-inspired synthetic compartments are discussed. Apart from being simplified and cell models for bridging the gap between nonliving matter and cellular life, synthetic compartments also are utilized as intracellular delivery vehicles for nuclei acids and nanoreactors for biochemical synthesis. Finally, key challenges and future directions for achieving the full potential of synthetic cells are highlighted.

Recent Advances in Liposome-Based Molecular Robots

A molecular robot is a microorganism-imitating micro robot that is designed from the molecular level and constructed by bottom-up approaches. As with conventional robots, molecular robots consist of three essential robotics elements: control of intelligent systems, sensors, and actuators, all integrated into a single micro compartment. Due to recent developments in microfluidic technologies, DNA nanotechnologies, synthetic biology, and molecular engineering, these individual parts have been developed, with the final picture beginning to come together. In this review, we describe recent developments of these sensors, actuators, and intelligence systems that can be applied to liposome-based molecular robots. First, we explain liposome generation for the compartments of molecular robots. Next, we discuss the emergence of robotics functions by using and functionalizing liposomal membranes. Then, we discuss actuators and intelligence via the encapsulation of chemicals into liposomes. Finally, the future vision and the challenges of molecular robots are described.

High pressure inhibits signaling protein binding to the flagellar motor and bacterial chemotaxis through enhanced hydration

High pressure below 100 MPa interferes inter-molecular interactions without causing pressure denaturation of proteins. In Escherichia coli, the binding of the chemotaxis signaling protein CheY to the flagellar motor protein FliM induces reversal of the motor rotation. Using molecular dynamics (MD) simulations and parallel cascade selection MD (PaCS-MD), we show that high pressure increases the water density in the first hydration shell of CheY and considerably induces water penetration into the CheY-FliM interface. PaCS-MD enabled us to observe pressure-induced dissociation of the CheY-FliM complex at atomic resolution. Pressure dependence of binding free energy indicates that the increase of pressure from 0.1 to 100 MPa significantly weakens the binding. Using high-pressure microscopy, we observed that high hydrostatic pressure fixes the motor rotation to the counter-clockwise direction. In conclusion, the application of pressure enhances hydration of the proteins and weakens the binding of CheY to FliM, preventing reversal of the flagellar motor.

Evidence that reduction in volume protects in situ articular chondrocytes from mechanical impact

Chondrocytes, the resident cells in articular cartilage, carry the burden of producing and maintaining the extracellular matrix (ECM). However, as these cells have a low proliferative capacity and are not readily replaced, chondrocyte death due to extreme forces may contribute to the pathogenesis of osteoarthritis (OA) after injury or may inhibit healing after osteochondral transplantation, a restorative procedure for damaged cartilage that requires a series of mechanical impacts to insert the graft. Consequently, there is a need to understand what factors influence the vulnerability of in situ chondrocytes to mechanical trauma. To this end, the objective of this study was to investigate how altering cell volume by different means (hydrostatic pressure, uniaxial load, and osmotic challenge with and without inhibition of regulatory volume decrease) affects the vulnerability of in situ chondrocytes to extreme mechanical forces. Using a custom experimental platform enabling testing of viable and intact murine cartilage-on-bone explants, we established a strong correlation between chondrocyte volume and vulnerability to impact injury wherein reduced volume was protective. Moreover, we found that the volume-perturbing interventions did not affect cartilage ECM mechanical properties, suggesting that their effects on chondrocyte vulnerability occurred at the cellular level. The findings of this study offer new avenues for novel strategies aimed at preventing chondrocyte loss during osteochondral grafting or to halting the progression of cell death after a joint destabilizing injury.

Encapsulation of the cytoskeleton: towards mimicking the mechanics of a cell

The cytoskeleton of a cell controls all the aspects of cell shape changes and motility from its physiological functions for survival to reproduction to death. The structure and dynamics of the cytoskeletal components: actin, microtubules, intermediate filaments, and septins - recently regarded as the fourth member of the cytoskeleton family - are conserved during evolution. Such conserved and effective control over the mechanics of the cell makes the cytoskeletal components great candidates for in vitro reconstitution and bottom-up synthetic biology studies. Here, we review the recent efforts in reconstitution of the cytoskeleton in and on membrane-enclosed biomimetic systems and argue that co-reconstitution and synergistic interplay between cytoskeletal filaments might be indispensable for efficient mechanical functionality of active minimal cells. Further, mechanical equilibrium in adherent eukaryotic cells is achieved by the formation of integrin-based focal contacts with extracellular matrix (ECM) and the transmission of stresses generated by actomyosin contraction to ECM. Therefore, a minimal mimic of such balance of forces and quasi-static kinetics of the cell by bottom-up reconstitution requires a careful construction of contractile machineries and their link with adhesive contacts. In this review, in addition to cytoskeletal crosstalk, we provide a perspective on reconstruction of cell mechanical equilibrium by reconstitution of cortical actomyosin networks in lipid membrane vesicles adhered on compliant substrates and also discuss future perspectives of this active research area.

Molecular dynamics simulation of proteins under high pressure: Structure, function and thermodynamics

BACKGROUND

Molecular dynamics (MD) simulation is well-recognized as a powerful tool to investigate protein structure, function, and thermodynamics. MD simulation is also used to investigate high pressure effects on proteins. For conducting better MD simulation under high pressure, the main issues to be addressed are: (i) protein force fields and water models were originally developed to reproduce experimental properties obtained at ambient pressure; and (ii) the timescale to observe the pressure effect is often much longer than that of conventional MD simulations.

SCOPE OF REVIEW

First, we describe recent developments in MD simulation methodologies for studying the high-pressure structure and dynamics of protein molecules. These developments include force fields for proteins and water molecules, and enhanced simulation techniques. Then, we summarize recent studies of MD simulations of proteins in water under high pressure.

MAJOR CONCLUSIONS

Recent MD simulations of proteins in solution under pressure have reproduced various phenomena identified by experiments using high pressure, such as hydration, water penetration, conformational change, helix stabilization, and molecular stiffening.

GENERAL SIGNIFICANCE

MD simulations demonstrate differences in the properties of proteins and water molecules between ambient and high-pressure conditions. Comparing the results obtained by MD calculations with those obtained experimentally could reveal the mechanism by which biological molecular machines work well in collaboration with water molecules.

Interrogating the Structural Dynamics and Energetics of Biomolecular Systems with Pressure Modulation

High hydrostatic pressure affects the structure, dynamics, and stability of biomolecular systems and is a key parameter in the context of the exploration of the origin and the physical limits of life. This review lays out the conceptual framework for exploring the conformational fluctuations, dynamical properties, and activity of biomolecular systems using pressure perturbation. Complementary pressure-jump relaxation studies are useful tools to study the kinetics and mechanisms of biomolecular phase transitions and structural transformations, such as membrane fusion or protein and nucleic acid folding. Finally, the advantages of using pressure to explore biomolecular assemblies and modulate enzymatic reactions are discussed.

Creation of Artificial Cell-Like Structures Promoted by Microfluidics Technologies

The creation of artificial cells is an immensely challenging task in science. Artificial cells contribute to revealing the mechanisms of biological systems and deepening our understanding of them. The progress of versatile biological research fields has clarified many biological phenomena, and various artificial cell models have been proposed in these fields. Microfluidics provides useful technologies for the study of artificial cells because it allows the fabrication of cell-like compartments, including water-in-oil emulsions and giant unilamellar vesicles. Furthermore, microfluidics also allows the mimicry of cellular functions with chip devices based on sophisticated chamber design. In this review, we describe contributions of microfluidics to the study of artificial cells. Although typical microfluidic methods are useful for the creation of artificial-cell compartments, recent methods provide further benefits, including low-cost fabrication and a reduction of the sample volume. Microfluidics also allows us to create multi-compartments, compartments with artificial organelles, and on-chip artificial cells. We discuss these topics and the future perspective of microfluidics for the study of artificial cells and molecular robotics.

Shaping up synthetic cells

How do the cells in our body reconfigure their shape to achieve complex tasks like migration and mitosis, yet maintain their shape in response to forces exerted by, for instance, blood flow and muscle action? Cell shape control is defined by a delicate mechanical balance between active force generation and passive material properties of the plasma membrane and the cytoskeleton. The cytoskeleton forms a space-spanning fibrous network comprising three subsystems: actin, microtubules and intermediate filaments. Bottom-up reconstitution of minimal synthetic cells where these cytoskeletal subsystems are encapsulated inside a lipid vesicle provides a powerful avenue to dissect the force balance that governs cell shape control. Although encapsulation is technically demanding, a steady stream of advances in this technique has made the reconstitution of shape-changing minimal cells increasingly feasible. In this topical review we provide a route-map of the recent advances in cytoskeletal encapsulation techniques and outline recent reports that demonstrate shape change phenomena in simple biomimetic vesicle systems. We end with an outlook toward the next steps required to achieve more complex shape changes with the ultimate aim of building a fully functional synthetic cell with the capability to autonomously grow, divide and move.

Morphological Control of Microtubule-Encapsulating Giant Vesicles by Changing Hydrostatic Pressure

For the development of artificial cell-like machinery, liposomes encapsulating cytoskeletons have drawn much recent attention. However, there has been no report showing isothermally reversible morphological changes of liposomes containing cytoskeletons. We succeeded in reversibly changing the shape of cell-sized giant vesicles by controlling the polymerization/depolymerization state of cytoskeletal microtubules that were encapsulated in the vesicles using pressure changes. The result indicates that it is possible to manipulate artificial cell models composed of molecules such as lipids and proteins. The findings obtained in this study will be helpful in clarifying the details of cooperation between cytoskeletal dynamics and morphogenesis of biological membranes and in improving the design and construction of further advanced artificial cell-like machinery, such as drug-delivery systems. In addition, the experimental system used in this study can be applied to research to elucidate the adaptive strategy of living organisms to external stimuli and extreme conditions such as osmotic stress and high-pressure environments like the deep sea.

Preparation of Giant Vesicles Encapsulating Microspheres by Centrifugation of a Water-in-oil Emulsion

The constructive biology and the synthetic biology approach to creating artificial life involve the bottom-up assembly of biological or nonbiological materials. Such approaches have received considerable attention in research on the boundary between living and nonliving matter and have been used to construct artificial cells over the past two decades. In particular, Giant Vesicles (GVs) have often been used as artificial cell membranes. In this paper, we describe the preparation of GVs encapsulating highly packed microspheres as a model of cells containing highly condensed biomolecules. The GVs were prepared by means of a simple water-in-oil emulsion centrifugation method. Specifically, a homogenizer was used to emulsify an aqueous solution containing the materials to be encapsulated and an oil containing dissolved phospholipids, and the resulting emulsion was layered carefully on the surface of another aqueous solution. The layered system was then centrifuged to generate the GVs. This powerful method was used to encapsulate materials ranging from small molecules to microspheres.