Giant vesicles: a powerful tool to reconstruct bacterial division assemblies in cell-like compartments

Membrane Manipulation of Giant Unilamellar Polymer Vesicles with a Temperature-Responsive Polymer

Understanding the complex behavior and dynamics of cellular membranes is integral to gain insight into cellular division and fusion processes. Bottom-up synthetic cells are as a platform for replicating and probing cellular behavior. Giant polymer vesicles are more robust than liposomal counterparts, as well as having a broad range of chemical functionalities. However, the stability of the membrane can prohibit dynamic processes such as membrane phase separation and division. Here, we present a method for manipulating the membrane of giant polymersomes using a temperature responsive polymer. Upon elevation of temperature deformation and phase separation of the membrane was observed. Upon cooling, the membrane relaxed and became homogeneous again, with infrequent division of the synthetic cells.

Use of giant unilamellar lipid vesicles as antioxidant carriers in in vitro culture medium of bovine embryos

Giant unilamellar vesicles (GUVs) are composed of lipophilic layers and are sensitive to the action of reactive oxygen species (ROS). The use of GUVs as microcarriers of biological macromolecules is particularly interesting since ROS produced by gametes or embryos during in vitro culture can induce the opening of pores in the membrane of these vesicles and cause the release of their content. This study investigated the behavior of GUVs [composed of 2-dioleoyl-sn-glycero-3-phosphocholine and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl)] in co-culture with in vitro produced bovine embryos, as well as their embryotoxicity and effectiveness as cysteine carriers in culture medium. Embryonic developmental rates were unaffected, demonstrating the absence of toxicity of GUVs co-cultured with the embryos. No increase of intracellular ROS levels was observed in the embryos co-cultured with GUVs, indicating that the higher lipid content of the culture environment resulting from the lipid composition of the GUV membrane itself did not increase oxidative stress. Variations in the diameter and number of GUVs demonstrated their sensitivity to ROS produced by embryos cultured under conditions that generate oxidative stress. Encapsulation of cysteine in GUVs was found to be more effective in controlling the production of ROS in embryonic cells than direct dilution of this antioxidant in the medium. In conclusion, the use of GUVs in in vitro culture was found to be safe since these vesicles did not promote toxic effects nor did they increase intracellular ROS concentrations in the embryos. GUVs were sensitive to oxidative stress, which resulted in structural changes in response to the action of ROS. The possible slow release of cysteine into the culture medium by GUV rupture would therefore favor the gradual supply of cysteine, prolonging its presence in the medium. Thus, the main implication of the use of GUVs as cysteine microcarriers is the greater effectiveness in preventing the intracytoplasmic increase of ROS in in vitro produced bovine embryos.

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.

Transcription and Translation in Cytomimetic Protocells Perform Most Efficiently at Distinct Macromolecular Crowding Conditions

The formation of cytomimetic protocells that capture the physicochemical aspects of living cells is an important goal in bottom-up synthetic biology. Here, we recreated the crowded cytoplasm in liposome-based protocells and studied the kinetics of cell-free gene expression in these crowded containers. We found that diffusion of key components is affected not only by macromolecular crowding but also by enzymatic activity in the protocell. Surprisingly, size-dependent diffusion in crowded conditions yielded two distinct maxima for protein synthesis, reflecting the differential impact of crowding on transcription and translation. Our experimental data show, for the first time, that macromolecular crowding induces a switch from reaction to diffusion control and that this switch depends on the sizes of the macromolecules involved. These results highlight the need to control the physical environment in the design of synthetic cells.

Dynamic Synthetic Cells Based on Liquid-Liquid Phase Separation

Living cells have long been a source of inspiration for chemists. Their capacity of performing complex tasks relies on the spatiotemporal coordination of matter and energy fluxes. Recent years have witnessed growing interest in the bottom-up construction of cell-like models capable of reproducing aspects of such dynamic organisation. Liquid-liquid phase-separation (LLPS) processes in water are increasingly recognised as representing a viable compartmentalisation strategy through which to produce dynamic synthetic cells. Herein, we highlight examples of the dynamic properties of LLPS used to assemble synthetic cells, including their biocatalytic activity, reversible condensation and dissolution, growth and division, and recent directions towards the design of higher-order structures and behaviour.

Using Imaging Flow Cytometry to Quantify and Optimize Giant Vesicle Production by Water-in-oil Emulsion Transfer Methods

Many biologists, biochemists, and biophysicists study giant vesicles, which have a diameter of >1 μm, owing to their ease of characterization using standard optical methods. More recently, there has been interest in using giant vesicles as model systems for living cells and for the construction of artificial cells. In fact, there have been a number of reports about functionalizing giant vesicles using membrane-bound pore proteins and encapsulating biochemical reactions. Among the various methods for preparing giant vesicles, the water-in-oil emulsion transfer method is particularly well established. However, the giant vesicles prepared by this method have complex and heterogeneous properties, such as particle size and membrane structure. Here, we demonstrate the characterization of giant vesicles by imaging flow cytometry to provide quantitative and qualitative information about the vesicle products prepared by the water-in-oil emulsion transfer method. Through image-based analyses, several kinds of protocol byproducts, such as oil droplets and vesicles encapsulating no target molecules, were identified and successfully quantified. Further, the optimal agitation conditions for the water-in-oil emulsion transfer method were found from detailed analysis of imaging flow cytometry data. Our results indicate that a sonication-based water-in-oil emulsion transfer method exhibited a higher efficiency in producing giant vesicles, about 10 times or higher than that of vortex and rumble strip-based methods. It is anticipated that these approaches will be useful for fine-tuning giant vesicle production and subsequent applications.

Liquid-liquid phase separation in artificial cells

Liquid-liquid phase separation (LLPS) in biology is a recently appreciated means of intracellular compartmentalization. Because the mechanisms driving phase separations are grounded in physical interactions, they can be recreated within less complex systems consisting of only a few simple components, to serve as artificial microcompartments. Within these simple systems, the effect of compartmentalization and microenvironments upon biological reactions and processes can be studied. This review will explore several approaches to incorporating LLPS as artificial cytoplasms and in artificial cells, including both segregative and associative phase separation.

Cell-sized lipid vesicles for cell-cell synaptic therapies

Cell-sized lipid vesicles (CLVs) have shown great promise for therapeutic and artificial cell applications, but their fragility and short shelf life has hindered widespread adoption and commercial viability. We present a method to circumvent the storage limitations of CLVs such as giant unilamellar vesicles (GUVs) and single-compartment multisomes (SCMs) by storing them in a double emulsion precursor form. The double emulsions can be stored for at least 8 months and readily converted into either GUVs or SCMs at any time. In this study, we investigate the interfacial parameters responsible for this morphological change, and we also demonstrate the therapeutic potential of CLVs by utilizing them to present a transmembrane protein, neuroligin-2, to pancreatic β-cells, forming cell-cell synapses that stimulate insulin secretion and cellular growth.

Synthetic Biology-The Synthesis of Biology

Synthetic biology concerns the engineering of man-made living biomachines from standardized components that can perform predefined functions in a (self-)controlled manner. Different research strategies and interdisciplinary efforts are pursued to implement engineering principles to biology. The "top-down" strategy exploits nature's incredible diversity of existing, natural parts to construct synthetic compositions of genetic, metabolic, or signaling networks with predictable and controllable properties. This mainly application-driven approach results in living factories that produce drugs, biofuels, biomaterials, and fine chemicals, and results in living pills that are based on engineered cells with the capacity to autonomously detect and treat disease states in vivo. In contrast, the "bottom-up" strategy seeks to be independent of existing living systems by designing biological systems from scratch and synthesizing artificial biological entities not found in nature. This more knowledge-driven approach investigates the reconstruction of minimal biological systems that are capable of performing basic biological phenomena, such as self-organization, self-replication, and self-sustainability. Moreover, the syntheses of artificial biological units, such as synthetic nucleotides or amino acids, and their implementation into polymers inside living cells currently set the boundaries between natural and artificial biological systems. In particular, the in vitro design, synthesis, and transfer of complete genomes into host cells point to the future of synthetic biology: the creation of designer cells with tailored desirable properties for biomedicine and biotechnology.

cDICE method produces giant lipid vesicles under physiological conditions of charged lipids and ionic solutions

Giant unilamellar vesicles are a powerful and common tool employed in biophysical studies of lipid membranes. Here we evaluate a recently introduced method of vesicle formation, "continuous droplet interface crossing encapsulation" (cDICE). This method produces monodisperse giant unilamellar vesicles of controlled sizes and high encapsulation efficiencies, using readily available instrumentation. We find that mixtures of phospholipids within vesicle membranes produced by cDICE undergo phase separation at the same characteristic temperatures as lipids in vesicles formed by a complementary technique. We find that the cDICE method is effective both when vesicles are produced from charged lipids and when the surrounding buffer contains a high concentration of salt. A shortcoming of the technique is that cholesterol is not substantially incorporated into vesicle membranes.

FtsZ Polymers Tethered to the Membrane by ZipA Are Susceptible to Spatial Regulation by Min Waves

Bacterial cell division is driven by an FtsZ ring in which the FtsZ protein localizes at mid-cell and recruits other proteins, forming a divisome. In Escherichia coli, the first molecular assembly of the divisome, the proto-ring, is formed by the association of FtsZ polymers to the cytoplasmic membrane through the membrane-tethering FtsA and ZipA proteins. The MinCDE system plays a major role in the site selection of the division ring because these proteins oscillate from pole to pole in such a way that the concentration of the FtsZ-ring inhibitor, MinC, is minimal at the cell center, thus favoring FtsZ assembly in this region. We show that MinCDE drives the formation of waves of FtsZ polymers associated to bilayers by ZipA, which propagate as antiphase patterns with respect to those of Min as revealed by confocal fluorescence microscopy. The emergence of these FtsZ waves results from the displacement of FtsZ polymers from the vicinity of the membrane by MinCD, which efficiently competes with ZipA for the C-terminal region of FtsZ, a central hub for multiple interactions that are essential for division. The coupling between FtsZ polymers and Min is enhanced at higher surface densities of ZipA or in the presence of crowding agents that favor the accumulation of FtsZ polymers near the membrane. The association of FtsZ polymers to the membrane modifies the response of FtsZ to Min, and comigrating Min-FtsZ waves are observed when FtsZ is free in solution and not attached to the membrane by ZipA. Taken together, our findings show that the dynamic Min patterns modulate the spatial distribution of FtsZ polymers in controlled minimal membranes. We propose that ZipA plays an important role in mid-cell recruitment of FtsZ orchestrated by MinCDE.