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

Identifying Conditions for Protein Synthesis Inside Giant Vesicles Using Microfluidics toward Standardized Artificial Cell Production

To expand the range of practical applications of artificial cells, it is important to standardize the production process of giant (cell-sized) vesicles that encapsulate reconstituted biochemical reaction systems. For this purpose, a rapidly developing microfluidics-based giant vesicle generation system is a promising approach, similar to the droplet assay systems that are already widespread in the market. In this study, we examined the composition of the solutions used to generate vesicles encapsulating the in vitro transcription-translation (IVTT) system. We show that tuning of the lipid composition and adding poly(vinyl alcohol) to the outer solution improved the stability of the transition process into the lipid membrane so that protein synthesis proceeded in vesicles. The direct integration of α-hemolysin nanopores synthesized in situ was also demonstrated. These protein-synthesizing monodisperse giant vesicles can be prepared by using a simple microfluidic fabrication/operation with a commercial IVTT system.

Cell-free expression of RuBisCO for ATP production in the synthetic cells

Recent advances in bottom-up synthetic biology have made it possible to reconstitute cellular systems from non-living components, yielding artificial cells with potential applications in industry, medicine and basic research. Although a variety of cellular functions and components have been reconstituted in previous studies, sustained biological energy production remains a challenge. ATP synthesis via ribulose-1,5-diphosphate carboxylase/oxygenase (RuBisCO), a central enzyme in biological CO2 fixation, holds potential as an energy production system, but its feasibility in a cell-free expression system has not yet been tested. In this study, we test RuBisCO expression and its activity-mediated ATP synthesis in a reconstituted Escherichia coli-based cell-free translation system. We then construct a system in which ATP is synthesized by RuBisCO activity in giant vesicles and used as energy for translation reactions. These results represent an advance toward independent energy production in artificial cells. Graphical Abstract.

Formation of Giant Unilamellar Vesicles Assisted by Fluorinated Nanoparticles

In the quest to produce artificial cells, one key challenge that remains to be solved is the recreation of a complex cellular membrane. Among the existing models, giant unilamellar vesicles (GUVs) are particularly interesting due to their intrinsic compartmentalisation ability and their resemblance in size and shape to eukaryotic cells. Many techniques have been developed to produce GUVs all having inherent advantages and disadvantages. Here, the authors show that fluorinated silica nanoparticles (FNPs) used to form Pickering emulsions in a fluorinated oil can destabilise lipid nanosystems to template the formation of GUVs. This technique enables GUV production across a broad spectrum of buffer conditions, while preventing the leakage of the encapsulated components into the oil phase. Furthermore, a simple centrifugation process is sufficient for the release of the emulsion-trapped GUVs, bypassing the need to use emulsion-destabilising chemicals. With fluorescent FNPs and transmission electron microscopy, the authors confirm that FNPs are efficiently removed, producing contaminant-free GUVs. Further experiments assessing the lateral diffusion of lipids and unilamellarity of the GUVs demonstrate that they are comparable to GUVs produced via electroformation. Finally, the ability of incorporating transmembrane proteins is demonstrated, highlighting the potential of this method for the production of GUVs for artificial cell applications.

Spontaneous Formation of Uniform Cell-Sized Microgels through Water/Water Phase Separation

In this study, a one-step method is discussed for producing uniform cell-sized microgels using glass capillaries filled with a binary polymer blend of polyethylene glycol (PEG) and gelatin. Upon decreasing temperature, phase separation of the PEG/gelatin blends and gelation of gelatin occur, and then the polymer blend forms linearly aligned, uniformly sized gelatin microgels in the glass capillary. When DNA is added to the polymer solution, gelatin microgels entrapping DNA are spontaneously formed, and the DNA prevents the coalescence of the microdroplets even at temperatures above the melting point. This novel method to form uniform cell-sized microgels may be applicable to other biopolymers. This method is expected to contribute to diverse materials science via biopolymer microgels and biophysics and synthetic biology through cellular models containing biopolymer gels.

Visualizing Loss of Plasma Membrane Lipid Asymmetry Using Annexin V Staining

Loss of plasma membrane lipid asymmetry contributes to many cellular functions and responses, including apoptosis, blood coagulation, and cell fusion. In this protocol, we describe the use of fluorescently labeled annexin V to detect loss of lipid asymmetry in the plasma membrane of adherent living cells by fluorescence microscopy. The approach provides a simple, sensitive, and reproducible method to detect changes in lipid asymmetry but is limited by low sample throughput. The protocol can also be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. To validate the lipid binding properties of such probes, we additionally describe here the preparation and use of giant unilamellar vesicles as simple model membrane systems that have a size comparable to cells. Key features Monitoring loss of lipid asymmetry in the plasma membrane via confocal microscopy. Protocol can be applied to any type of cell that is adherent in culture, including primary cells. Assay can be adapted to other fluorescently labeled lipid-binding proteins or peptide probes. Giant unilamellar vesicles serve as a tool to validate the lipid binding properties of such probes. Graphical overview Imaging the binding of fluorescent annexin V to adherent mammalian cells and giant vesicles by confocal microscopy. Annexin V labeling is a useful method for detecting a loss of plasma membrane lipid asymmetry in cells (top image, red); DAPI can be used to identify nuclei (top image, blue). Giant vesicles are used as a tool to validate the lipid binding properties of annexin V to anionic lipids (lower image, red).

Regulated N-Terminal Modification of Proteins Synthesized Using a Reconstituted Cell-Free Protein Synthesis System

The N-terminal modification of nascent proteins, such as acetylation and myristoylation, is one of the most abundant post-translational modifications. To analyze the function of the modification, it is important to compare the modified and unmodified proteins under defined conditions. However, it is technically difficult to prepare unmodified proteins because cell-based systems contain endogenous modification systems. In this study, we developed a cell-free method to conduct N-terminal acetylation and myristoylation of nascent proteins in vitro using a reconstituted cell-free protein synthesis system (PURE system). Proteins synthesized using the PURE system were successfully acetylated or myristoylated in a single-cell-free mixture in the presence of modifying enzymes. Furthermore, we performed protein myristoylation in giant vesicles, which resulted in their partial localization to the membrane. Our PURE-system-based strategy is useful for the controlled synthesis of post-translationally modified proteins.

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.

A lipid membrane supported on an artificial extracellular matrix made of polyelectrolyte multilayers: towards nanoarchitectonics at the cellular interface

To implement a specific function, cells recognize multiple physical and chemical cues and exhibit molecular responses at their interfaces - the boundary regions between the cell lipid-based membrane and the surrounding extracellular matrix (ECM). Mimicking the cellular external microenvironment presents a big challenge in nanoarchitectonics due to the complexity of the ECM and lipid membrane fragility. This study reports an approach for the assembly of a lipid bilayer, mimicking the cellular membrane, placed on top of a polyelectrolyte multilayer cushion made of hyaluronic acid and poly-L-lysine - a nanostructured biomaterial, which represents a 3D artificial ECM. Model proteins, lysozyme and α-lactalbumin, (which have similar molecular masses but carry opposite net charges) have been employed as soluble signalling molecules to probe their interaction with these hybrids. The formation of a lipid bilayer and the intermolecular interactions in the hybrid structure are monitored using a quartz crystal microbalance and confocal fluorescence microscopy. Electrostatic interactions between poly-L-lysine and the externally added proteins govern the transport of proteins into the hybrid. Designed ECM-cell mimicking hybrids open up new avenues for modelling a broad range of cell membranes and ECM and their associated phenomena, which can be used as a tool for synthetic biology and drug screening.

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.

Phospholipid synthesis inside phospholipid membrane vesicles

Construction of living artificial cells from genes and molecules can expand our understanding of life system and establish a new aspect of bioengineering. However, growth and division of cell membrane that are basis of cell proliferation are still difficult to reconstruct because a high-yielding phospholipid synthesis system has not been established. Here, we developed a cell-free phospholipid synthesis system that combines fatty acid synthesis and cell-free gene expression system synthesizing acyltransferases. The synthesized fatty acids were sequentially converted into phosphatidic acids by the cell-free synthesized acyltransferases. Because the system can avoid the accumulation of intermediates inhibiting lipid synthesis, sub-millimolar phospholipids could be synthesized within a single reaction mixture. We also performed phospholipid synthesis inside phospholipid membrane vesicles, which encapsulated all the components, and showed the phospholipids localized onto the mother membrane. Our approach would be a platform for the construction of self-reproducing artificial cells since the membrane can grow sustainably.