Development of Artificial Cell Models Using Microfluidic Technology and Synthetic Biology

Molecular Transportation Conversion of Membrane Tension Using a Mechanosensitive Channel in Asymmetric Lipid-Protein Vesicles

Giant lipid vesicles composed of a lipid bilayer form complex membrane structures and enzyme network reactions that can be used to construct well-defined artificial cell models based on microfluidic technologies and synthetic biology. As a different approach to cell-mimicking systems, we formed an asymmetric lipid-amphiphilic protein (oleosin) vesicle containing a lipid and an oleosin monolayer in the outer and inner leaflets, respectively. These asymmetric vesicles enabled the reconstitution and function of β-barrel types of membrane proteins (OmpG) and the fission of vesicles stimulated by lysophospholipids. These applications combine the advantages of the high stability of lipids and oleosin leaflets in asymmetric lipid-oleosin vesicles. In this study, to evaluate the versatility of this asymmetric lipid-oleosin vesicle, the molecular transport of the mechanosensitive channel of large conductance (MscL) with an α-helix was evaluated by changing the tension of the asymmetric vesicle membrane with lysophospholipid. A nanopore of MscL assembled as a pentamer of MscLs transports small molecules of less than 10 kDa by sensing physical stress at the lipid bilayer. The amount and maximum size of the small molecules transported via MscL in the asymmetric lipid-oleosin vesicles were compared to those in the lipid vesicles. We revealed the existence of the C- and N-terminal regions (cytoplasmic side) of MscL on the inner leaflet of the asymmetric lipid-oleosin vesicles using an insertion direction assay. Furthermore, the change in the tension of the lipid-oleosin membrane activated the proteins in these vesicles, inducing their transportation through MscL nanopores. Therefore, asymmetric lipid-oleosin vesicles containing MscL can be used as substrates to study the external environment response of complex artificial cell models.

Efficiency of transcription and translation of cell-free protein synthesis systems in cell-sized lipid vesicles with changing lipid composition determined by fluorescence measurements

To develop artificial cell models that mimic living cells, cell-sized lipid vesicles encapsulating cell-free protein synthesis (CFPS) systems are useful for protein expressions or artificial gene circuits for vesicle-vesicle communications. Therefore, investigating the transcriptional and translational properties of CFPS systems in lipid vesicles is important for maximizing the synthesis and functions of proteins. Although transcription and translation using CFPS systems inside lipid vesicles are more important than that outside lipid vesicles, the former processes are not investigated by changing the lipid composition of lipid vesicles. Herein, we investigated changes in transcription and translation using CFPS systems inside giant lipid vesicles (approximately 5-20 μm in diameter) caused by changing the lipid composition of lipid vesicles containing neutral, positively, and negatively charged lipids. After incubating for 30 min, 1 h, 2 h, and 4 h, the transcriptional and translational activities in these lipid vesicles were determined by detecting the fluorescence intensities of the fluorogenic RNA aptamer on the 3'-untranslated region of mRNA (transcription) and the fluorescent protein sfCherry (translation), respectively. The results revealed that transcriptional and translational activities in a lipid vesicle containing positively charged lipids were high when the protein was synthesized using the CFPS system inside the lipid vesicle. Thus, the present study provides an experimental basis for constructing complex artificial cell models using bottom-up approaches.

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.

β-defensin 2 synthesized by a cell-free protein synthesis system and encapsulated in liposomes inhibits adhesion of Porphyromonas gingivalis to oral epithelial cells

β-defensin 2 (BD-2), an antimicrobial peptide (AMP), is expressed by oral epithelial cells and plays an important role in innate immunity of the oral cavity. Cell-free protein synthesis (CFPS) systems have been studied for the synthesis of various proteins, however, the synthesis of BD-2 by a CFPS system has not been extensively explored. Liposomes have been developed as tools for drug delivery. A delivery of liposome-encapsulated AMP to oral epithelium may be useful to prevent oral infectious diseases. In the present study, we investigated the antimicrobial activity of the BD-2 protein, artificially synthesized using a CFPS system and encapsulated in liposomes. BD-2 protein was artificially synthesized using template DNA and a reconstituted CFPS system and was identified by western blotting. Bilayer liposomes were prepared using 1,2-dioleoyl-sn-glycero-3-phospho-choline and 3-sn-phosphatidylcholine from egg yolk. The artificially synthesized BD-2 was encapsulated in liposomes, collected by ultrafiltration, and detected by western blotting. Human oral epithelial cells were cultured with the liposome-encapsulated BD-2 and the concentration of BD-2 in the cell lysate of the culture with the synthesized BD-2 was higher than that of the control cultures. The antimicrobial activity of the synthesized BD-2 was investigated by an adhesion assay of Porphyromonas gingivalis to oral epithelial cells. The artificially synthesized BD-2 and its liposome significantly inhibited adhesion of P. gingivalis to oral epithelial cells. These results suggest that artificially synthesized BD-2 and liposome-encapsulated BD-2 show antimicrobial activity and can potentially play a role in oral healthcare for periodontal diseases.

Function Investigations and Applications of Membrane Proteins on Artificial Lipid Membranes

Membrane proteins play an important role in key cellular functions, such as signal transduction, apoptosis, and metabolism. Therefore, structural and functional studies of these proteins are essential in fields such as fundamental biology, medical science, pharmacology, biotechnology, and bioengineering. However, observing the precise elemental reactions and structures of membrane proteins is difficult, despite their functioning through interactions with various biomolecules in living cells. To investigate these properties, methodologies have been developed to study the functions of membrane proteins that have been purified from biological cells. In this paper, we introduce various methods for creating liposomes or lipid vesicles, from conventional to recent approaches, as well as techniques for reconstituting membrane proteins into artificial membranes. We also cover the different types of artificial membranes that can be used to observe the functions of reconstituted membrane proteins, including their structure, number of transmembrane domains, and functional type. Finally, we discuss the reconstitution of membrane proteins using a cell-free synthesis system and the reconstitution and function of multiple membrane proteins.

A Guide to Your Desired Lipid-Asymmetric Vesicles

Liposomes are prevalent model systems for studies on biological membranes. Recently, increasing attention has been paid to models also representing the lipid asymmetry of biological membranes. Here, we review in-vitro methods that have been established to prepare free-floating vesicles containing different compositions of the classic two-chain glycero- or sphingolipids in their outer and inner leaflet. In total, 72 reports are listed and assigned to four general strategies that are (A) enzymatic conversion of outer leaflet lipids, (B) re-sorting of lipids between leaflets, (C) assembly from different monolayers and (D) exchange of outer leaflet lipids. To guide the reader through this broad field of available techniques, we attempt to draw a road map that leads to the lipid-asymmetric vesicles that suit a given purpose. Of each method, we discuss advantages and limitations. In addition, various verification strategies of asymmetry as well as the role of cholesterol are briefly discussed. The ability to specifically induce lipid asymmetry in model membranes offers insights into the biological functions of asymmetry and may also benefit the technical applications of liposomes.

Cell-sized asymmetric phospholipid-amphiphilic protein vesicles with growth, fission, and molecule transportation

Lipid vesicles, which mimic cell membranes in structure and components, have been used to study the origin of life and artificial cell construction. A different approach to developing cell-mimicking systems focuses on the formation of protein- or polypeptide-based vesicles. However, micro-sized protein vesicles that are similar in membrane dynamics to the cell and that reconstitute membrane proteins are difficult to form. In this study, we generated cell-sized asymmetric phospholipid-amphiphilic protein (oleosin) vesicles that allow the reconstitution of membrane proteins and the growth and fission of vesicles. These vesicles are composed of a lipid membrane on the outer leaflet and an oleosin membrane on the inner leaflet. Further, we elucidated a mechanism for the growth and fission of cell-sized asymmetric phospholipid-oleosin vesicles by feeding phospholipid micelles. Our asymmetric phospholipid-oleosin vesicles with the advantages of the lipid leaflet and the protein leaflet will potentially promote understanding of biochemistry and synthetic biology.

Artificial cell design: reconstructing biology for life science applications

Artificial cells are developed to redesign novel biological functions in a programmable and tunable manner. Although it aims to reconstitute living cell features and address 'origin of life' related questions, rapid development over the years has transformed artificial cells into an engineering tool with huge potential in applied biotechnology. Although the application of artificial cells was introduced decades ago as drug carriers, applications in other sectors are relatively new and could become possible with the technological advancement that can modulate its designing principles. Artificial cells are non-living system that includes no prerequisite designing modules for their formation and therefore allow freedom of assembling desired biological machinery within a physical boundary devoid of complex contemporary living-cell counterparts. As stimuli-responsive biomimetic tools, artificial cells are programmed to sense the surrounding, recognise their target, activate its function and perform the defined task. With the advantage of their customised design, artificial cells are being studied in biosensing, drug delivery, anti-cancer therapeutics or artificial photosynthesis type fields. This mini-review highlights those advanced fields where artificial cells with a minimalistic setup are developed as user-defined custom-made microreactors, targeting to reshape our future 'life'.

Control of Enzyme Reaction Initiation inside Giant Unilamellar Vesicles by the Cell-Penetrating Peptide-Mediated Translocation of Cargo Proteins

Cell-penetrating peptides (CPPs) play important roles in directly delivering biomolecules, such as DNA, proteins, and peptides, into living cells. In artificial lipid membranes, such as planar lipid bilayers, the direct membrane translocation of β-galactosidase via Pep-1 (one of the CPPs) is dependent upon a voltage gradient between the inner and outer leaflets of the lipid membranes. Giant unilamellar vesicles (GUVs) with asymmetric lipid distributions, which are recently generated using microfluidic technologies, can be observed by optical microscopy. Therefore, interactions between CPPs and asymmetric lipid bilayers in different kinds of lipids and the translocation mechanism of proteins via CPPs into GUVs can be investigated at the level of a single asymmetric GUV. This CPP-based system for transporting proteins into GUVs will be applied to control the start of enzyme reactions in GUVs. This study aimed to explore efficient protein translocation into GUVs via CPP and demonstrate that enzymatic reactions start in GUVs using a CPP-mediated direct translocation. The interactions and the enzyme reactions between the CPP (Pep-1 or penetratin)-DNase I complexes and the asymmetric or symmetric GUV membranes containing the negatively or neutrally charged lipids were observed by confocal laser-scanning microscopy. The asymmetric GUVs containing phosphatidylserine (PS) in the inner leaflet showed efficient DNase I translocation into GUVs via penetratin. Finally, the formation of a cross-linked actin network was observed in asymmetric PS GUVs incubated with Pep-1-streptavidin complexes. The CPP-mediated direct translocation can contribute to developing artificial cell models with the capacity to control the initiation of enzymatic reactions.

Ordered Domain (Raft) Formation in Asymmetric Vesicles and Its Induction upon Loss of Lipid Asymmetry in Artificial and Natural Membranes

Lipid asymmetry, the difference in the lipid composition in the inner and outer lipid monolayers (leaflets) of a membrane, is an important feature of eukaryotic plasma membranes. Investigation of the biophysical consequences of lipid asymmetry has been aided by advances in the ability to prepare artificial asymmetric membranes, especially by use of cyclodextrin-catalyzed lipid exchange. This review summarizes recent studies with artificial asymmetric membranes which have identified conditions in which asymmetry can induce or suppress the ability of membranes to form ordered domains (rafts). A consequence of the latter effect is that, under some conditions, a loss of asymmetry can induce ordered domain formation. An analogous study in plasma membrane vesicles has demonstrated that asymmetry can also suppress domain formation in natural membranes. Thus, it is possible that a loss of asymmetry can induce domain formation in vivo.

Binding of His-tagged fluorophores to lipid bilayers of giant vesicles

His-tagged molecules can be attached to lipid bilayers via certain anchor lipids, a method that has been widely used for the biofunctionalization of membranes and vesicles. To observe the membrane-bound molecules, it is useful to consider His-tagged molecules that are fluorescent as well. Here, we study two such molecules, green fluorescence protein (GFP) and green-fluorescent fluorescein isothiocyanate (FITC), both of which are tagged with a chain of six histidines (6H) that bind to the anchor lipids within the bilayers. The His-tag 6H is much smaller than the GFP molecule but somewhat larger than the FITC dye. The lipid bilayers form giant unilamellar vesicles (GUVs), the behavior of which can be directly observed in the optical microscope. We apply and compare three well-established preparation methods for GUVs: electroformation on platinum wire, polyvinyl alcohol (PVA) hydrogel swelling, and electroformation on indium tin oxide (ITO) glass. Microfluidics is used to expose the GUVs to a constant fluorophore concentration in the exterior solution. The brightness of membrane-bound 6H-GFP exceeds the brightness of membrane-bound 6H-FITC, in contrast to the quantum yields of the two fluorophores in solution. In fact, 6H-FITC is observed to be strongly quenched by the anchor lipids which bind the fluorophores via Ni²⁺ ions. For both 6H-GFP and 6H-FITC, the membrane fluorescence is measured as a function of the fluorophores' molar concentration. The theoretical analysis of these data leads to the equilibrium dissociation constants K_d = 37.5 nM for 6H-GFP and K_d = 18.5 nM for 6H-FITC. We also observe a strong pH-dependence of the membrane fluorescence.

Biomimetic nanobubbles for triple-negative breast cancer targeted ultrasound molecular imaging

Triple-negative breast cancer (TNBC) is a highly heterogeneous breast cancer subtype with poor prognosis. Although anatomical imaging figures prominently for breast lesion screening, TNBC is often misdiagnosed, thus hindering early medical care. Ultrasound (US) molecular imaging using nanobubbles (NBs) capable of targeting tumor cells holds great promise for improved diagnosis and therapy. However, the lack of conventional biomarkers in TNBC impairs the development of current targeted agents. Here, we exploited the homotypic recognition of cancer cells to synthesize the first NBs based on TNBC cancer cell membrane (i.e., NB_CCM) as a targeted diagnostic agent. We developed a microfluidic technology to synthesize NB_CCM based on the self-assembly property of cell membranes in aqueous solutions. In vitro, optimal NB_CCM had a hydrodynamic diameter of 683 ± 162 nm, showed long-lasting US contrast enhancements and homotypic affinity. In vivo, we demonstrated that NB_CCM showed increased extravasation and retention in a TNBC mouse model compared to non-targeted NBs by US molecular imaging. Peak intensities and areas under the curves from time-intensity plots showed a significantly enhanced signal from NB_CCM compared to non-targeted NBs (2.1-fold, P = 0.004, and, 3.6-fold, P = 0.0009, respectively). Immunofluorescence analysis further validated the presence of NB_CCM in the tumor microenvironment. Circumventing the challenge for universal cancer biomarker identification, our approach could enable TNBC targeting regardless of tumor tissue heterogeneity, thus improving diagnosis and potentially gene/drug targeted delivery. Ultimately, our approach could be used to image many cancer types using biomimetic NBs prepared from their respective cancer cell membranes.

Natural Polyether Ionophores and Their Pharmacological Profile

This review is devoted to the study of the biological activity of polyether ionophores produced by bacteria, unicellular marine algae, red seaweeds, marine sponges, and coelenterates. Biological activities have been studied experimentally in various laboratories, as well as data obtained using QSAR (Quantitative Structure-Activity Relationships) algorithms. According to the data obtained, it was shown that polyether toxins exhibit strong antibacterial, antimicrobial, antifungal, antitumor, and other activities. Along with this, it was found that natural polyether ionophores exhibit such properties as antiparasitic, antiprotozoal, cytostatic, anti-mycoplasmal, and antieczema activities. In addition, polyethers have been found to be potential regulators of lipid metabolism or inhibitors of DNA synthesis. Further study of the mechanisms of action and the search for new polyether ionophores and their derivatives may provide more effective therapeutic natural polyether ionophores for the treatment of cancer and other diseases. For some polyether ionophores, 3D graphs are presented, which demonstrate the predicted and calculated activities. The data presented in this review will be of interest to pharmacologists, chemists, practical medicine, and the pharmaceutical industry.

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.

Liposome Preparation by 3D-Printed Microcapillary-Based Apparatus

Liposomal encapsulation serves as the basis for the engineering of biomimetic and novel synthetic cells. Liposomes are normally formed using such methods as thin film rehydration (TFH), density-mediated reverse emulsion encapsulation (REE), or one of many microfluidics-based approaches-with the latter of these two methods being used mainly for the encapsulation of various lumen constituents such as cell-free protein expression reactions. Here, we describe the simultaneous formation and encapsulation of liposomes and various cell-mimetic lumen chemistries, respectively, using a 3D-printable microcapillary-based microfluidics device based off of the droplet-shooting and size-filtration (DSSF) liposome preparation method.

Bioinspired Networks of Communicating Synthetic Protocells

The bottom-up synthesis of cell-like entities or protocells from inanimate molecules and materials is one of the grand challenges of our time. In the past decade, researchers in the emerging field of bottom-up synthetic biology have developed different protocell models and engineered them to mimic one or more abilities of biological cells, such as information transcription and translation, adhesion, and enzyme-mediated metabolism. Whilst thus far efforts have focused on increasing the biochemical complexity of individual protocells, an emerging challenge in bottom-up synthetic biology is the development of networks of communicating synthetic protocells. The possibility of engineering multi-protocellular systems capable of sending and receiving chemical signals to trigger individual or collective programmed cell-like behaviours or for communicating with living cells and tissues would lead to major scientific breakthroughs with important applications in biotechnology, tissue engineering and regenerative medicine. This mini-review will discuss this new, emerging area of bottom-up synthetic biology and will introduce three types of bioinspired networks of communicating synthetic protocells that have recently emerged.

Characterizing the Structure and Interactions of Model Lipid Membranes Using Electrophysiology

The cell membrane is a protective barrier whose configuration determines the exchange both between intracellular and extracellular regions and within the cell itself. Consequently, characterizing membrane properties and interactions is essential for advancements in topics such as limiting nanoparticle cytotoxicity. Characterization is often accomplished by recreating model membranes that approximate the structure of cellular membranes in a controlled environment, formed using self-assembly principles. The selected method for membrane creation influences the properties of the membrane assembly, including their response to electric fields used for characterizing transmembrane exchanges. When these self-assembled model membranes are combined with electrophysiology, it is possible to exploit their non-physiological mechanics to enable additional measurements of membrane interactions and phenomena. This review describes several common model membranes including liposomes, pore-spanning membranes, solid supported membranes, and emulsion-based membranes, emphasizing their varying structure due to the selected mode of production. Next, electrophysiology techniques that exploit these structures are discussed, including conductance measurements, electrowetting and electrocompression analysis, and electroimpedance spectroscopy. The focus of this review is linking each membrane assembly technique to the properties of the resulting membrane, discussing how these properties enable alternative electrophysiological approaches to measuring membrane characteristics and interactions.

Investigation of Fusion between Nanosized Lipid Vesicles and a Lipid Monolayer Toward Formation of Giant Lipid Vesicles with Various Kinds of Biomolecules

We determined the properties of fusion between large unilamellar vesicles (LUVs) and the lipid monolayer by measuring the fluorescence intensity of rhodamine-conjugated phospholipids in cell-sized lipid vesicles. The charge of LUVs (containing cationic lipids) and lipid droplets (containing anionic lipids) promoted lipid membrane fusion. We also investigated the formation of cell-sized lipid vesicles with asymmetric lipid distribution using this fusion method. Moreover, cell-sized asymmetric ganglioside vesicles can be generated from the planar lipid bilayer formed at the interface between the lipid droplets with/without LUVs containing ganglioside. The flip-flop dynamics of ganglioside were observed on the asymmetric ganglioside vesicles. This fusion method can be used to form asymmetric lipid vesicles with poor solubility in n-decane or lipid vesicles containing various types of membrane proteins for the development of complex artificial cell models.

Therapeutic Nanobodies Targeting Cell Plasma Membrane Transport Proteins: A High-Risk/High-Gain Endeavor

Cell plasma membrane proteins are considered as gatekeepers of the cell and play a major role in regulating various processes. Transport proteins constitute a subclass of cell plasma membrane proteins enabling the exchange of molecules and ions between the extracellular environment and the cytosol. A plethora of human pathologies are associated with the altered expression or dysfunction of cell plasma membrane transport proteins, making them interesting therapeutic drug targets. However, the search for therapeutics is challenging, since many drug candidates targeting cell plasma membrane proteins fail in (pre)clinical testing due to inadequate selectivity, specificity, potency or stability. These latter characteristics are met by nanobodies, which potentially renders them eligible therapeutics targeting cell plasma membrane proteins. Therefore, a therapeutic nanobody-based strategy seems a valid approach to target and modulate the activity of cell plasma membrane transport proteins. This review paper focuses on methodologies to generate cell plasma membrane transport protein-targeting nanobodies, and the advantages and pitfalls while generating these small antibody-derivatives, and discusses several therapeutic nanobodies directed towards transmembrane proteins, including channels and pores, adenosine triphosphate-powered pumps and porters.

Efficient Lipid Bilayer Formation by Dipping Lipid-Loaded Microperforated Sheet in Aqueous Solution

This paper describes a method for a bilayer lipid membrane (BLM) formation using a perforated sheet along with an open chamber. Microscopic observation of the formed membrane showed a typical droplet interface bilayer. We proved that the formed membrane was a BLM based on electrical measurements of the membrane protein α-hemolysin, which produces nanopores in BLMs. Unlike the conventional approach for BLM formation based on the droplet contact method, this method provides aqueous surfaces with no organic solvent coating layer. Hence, this method is suitable for producing BLMs that facilitate the direct addition of chemicals into the aqueous phase.