Artificial cells: building bioinspired systems using small-scale biology

Towards Synthetic Cells with Self-Producing Energy

Autonomous generation of energy, specifically adenosine triphosphate (ATP), is critical for sustaining the engineered functionalities of synthetic cells constructed from the bottom-up. In this mini-review, we categorize studies on ATP-producing synthetic cells into three different approaches: photosynthetic mechanisms, mitochondrial respiration mimicry, and utilization of non-conventional approaches such as exploiting synthetic metabolic pathways. Within this framework, we evaluate the strengths and limitations of each approach and provide directions for future research endeavors. We also introduce a concept of building ATP-generating synthetic organelle that will enable us to mimic cellular respiration in a simpler way than current strategies. This review aims to highlight the importance of energy self-production in synthetic cells, providing suggestions and ideas that may help overcome some longstanding challenges in this field.

Constructing artificial gap junctions to mediate intercellular signal and mass transport

Natural gap junctions are a type of channel protein responsible for intercellular signalling and mass communication. However, the scope of applications for these proteins is limited as they cannot be prepared at a large scale and are unable to spontaneously insert into cell membranes in vitro. The construction of artificial gap junctions may provide an alternative strategy for preparing analogues of the natural proteins and bottom-up building blocks necessary for the synthesis of artificial cells. Here we show the construction of artificial gap junction channels from unimolecular tubular molecules consisting of alternately arranged positively and negatively charged pillar[5]arene motifs. These molecules feature a hydrophobic-hydrophilic-hydrophobic triblock structure that allows them to efficiently insert into two adjacent plasma membranes and stretch across the gap between the two membranes to form gap junctions. Similar to natural gap junction channels, the synthetic channels could mediate intercellular signal coupling and reactive oxygen species transmission, leading to cellular activity.

Controlled exchange of protein and nucleic acid signals from and between synthetic minimal cells

Synthetic minimal cells are a class of bioreactors that have some, but not all, functions of live cells. Here, we report a critical step toward the development of a bottom-up minimal cell: cellular export of functional protein and RNA products. We used cell-penetrating peptide tags to translocate payloads across a synthetic cell vesicle membrane. We demonstrated efficient transport of active enzymes and transport of nucleic acid payloads by RNA-binding proteins. We investigated influence of a concentration gradient alongside other factors on the efficiency of the translocation, and we show a method to increase product accumulation in one location. We demonstrate the use of this technology to engineer molecular communication between different populations of synthetic cells, to exchange protein and nucleic acid signals. The synthetic minimal cell production and export of proteins or nucleic acids allows experimental designs that approach the complexity and relevancy of natural biological systems. A record of this paper's transparent peer review process is included in the supplemental information.

DNA-Origami Line-Actants Control Domain Organization and Fission in Synthetic Membranes

Cells can precisely program the shape and lateral organization of their membranes using protein machinery. Aiming to replicate a comparable degree of control, here we introduce DNA-origami line-actants (DOLAs) as synthetic analogues of membrane-sculpting proteins. DOLAs are designed to selectively accumulate at the line-interface between coexisting domains in phase-separated lipid membranes, modulating the tendency of the domains to coalesce. With experiments and coarse-grained simulations, we demonstrate that DOLAs can reversibly stabilize two-dimensional analogues of Pickering emulsions on synthetic giant liposomes, enabling dynamic programming of membrane lateral organization. The control afforded over membrane structure by DOLAs extends to three-dimensional morphology, as exemplified by a proof-of-concept synthetic pathway leading to vesicle fission. With DOLAs we lay the foundations for mimicking, in synthetic systems, some of the critical membrane-hosted functionalities of biological cells, including signaling, trafficking, sensing, and division.

Engineering Cyborg Bacteria Through Intracellular Hydrogelation

Natural and artificial cells are two common chassis in synthetic biology. Natural cells can perform complex tasks through synthetic genetic constructs, but their autonomous replication often causes safety concerns for biomedical applications. In contrast, artificial cells based on nonreplicating materials, albeit possessing reduced biochemical complexity, provide more defined and controllable functions. Here, for the first time, the authors create hybrid material-cell entities termed Cyborg Cells. To create Cyborg Cells, a synthetic polymer network is assembled inside each bacterium, rendering them incapable of dividing. Cyborg Cells preserve essential functions, including cellular metabolism, motility, protein synthesis, and compatibility with genetic circuits. Cyborg Cells also acquire new abilities to resist stressors that otherwise kill natural cells. Finally, the authors demonstrate the therapeutic potential by showing invasion into cancer cells. This work establishes a new paradigm in cellular bioengineering by exploiting a combination of intracellular man-made polymers and their interaction with the protein networks of living cells.

Modularize and Unite: Toward Creating a Functional Artificial Cell

An artificial cell is a simplified model of a living system, bringing breakthroughs into both basic life science and applied research. The bottom-up strategy instructs the construction of an artificial cell from nonliving materials, which could be complicated and interdisciplinary considering the inherent complexity of living cells. Although significant progress has been achieved in the past 2 decades, the area is still facing some problems, such as poor compatibility with complex bio-systems, instability, and low standardization of the construction method. In this review, we propose creating artificial cells through the integration of different functional modules. Furthermore, we divide the function requirements of an artificial cell into four essential parts (metabolism, energy supplement, proliferation, and communication) and discuss the present researches. Then we propose that the compartment and the reestablishment of the communication system would be essential for the reasonable integration of functional modules. Although enormous challenges remain, the modular construction would facilitate the simplification and standardization of an artificial cell toward a natural living system. This function-based strategy would also broaden the application of artificial cells and represent the steps of imitating and surpassing nature.

Amphiphilic DNA nanostructures for bottom-up synthetic biology

DNA nanotechnology enables the construction of sophisticated biomimetic nanomachines that are increasingly central to the growing efforts of creating complex cell-like entities from the bottom-up. DNA nanostructures have been proposed as both structural and functional elements of these artificial cells, and in many instances are decorated with hydrophobic moieties to enable interfacing with synthetic lipid bilayers or regulating bulk self-organisation. In this feature article we review recent efforts to design biomimetic membrane-anchored DNA nanostructures capable of imparting complex functionalities to cell-like objects, such as regulated adhesion, tissue formation, communication and transport. We then discuss the ability of hydrophobic modifications to enable the self-assembly of DNA-based nanostructured frameworks with prescribed morphology and functionality, and explore the relevance of these novel materials for artificial cell science and beyond. Finally, we comment on the yet mostly unexpressed potential of amphiphilic DNA-nanotechnology as a complete toolbox for bottom-up synthetic biology - a figurative and literal scaffold upon which the next generation of synthetic cells could be built.

Injectable Biocatalytic Nanocomposite Hydrogel Factories for Focal Enzyme-Prodrug Cancer Therapy

Systemic enzyme-prodrug therapy (EPT) using nanofactories, nanoparticles encapsulating prodrug-activating enzymes, is a promising concept for anticancer therapy. However, systemic delivery systems can be problematic. As nanofactories are typically carried by the blood circulation to tissues throughout the body, conversion of anticancer drugs in normal tissues can cause severe side effects. To overcome this problem, we developed a novel focal EPT approach utilizing nanocomposite hydrogels composed of a poly(dl-lactide-co-glycolide)-b-poly(ethylene glycol)-b-poly(dl-lactide-co-glycolide) (PLGA-PEG-PLGA) copolymer, LAPONITE, and β-galactosidase (β-gal). The nanocomposite gels can be easily injected locally, and the inherent enzyme activity of β-gal can be preserved long-term. Prodrug 5-FU-β-gal readily permeated into the interior space of gels and was converted into the active anticancer drug 5-FU. Importantly, a single local injection of nanocomposite gels and prodrug 5-FU-β-gal provided long-lasting antitumor activity in vivo without observable side effects, demonstrating the potential utility of injectable biocatalytic hydrogel factories for novel focal EPT systems.

USE OF ARTIFICIAL CELLS AS DRUG CARRIERS

Cells are the fundamental functional units of biological systems and mimicking their size, function and complexity is a primary goal in the development of new therapeutic strategies. Recent advances in chemistry, synthetic biology and material science have enabled the development of cell membrane-based drug delivery systems (DDSs), often referred to as "artificial cells" or protocells. Artificial cells can be made by removing functions from natural systems in a top-down manner, or assembly from synthetic, organic or inorganic materials, through a bottom-up approach where simple units are integrated to form more complex structures. This review covers the latest advances in the development of artificial cells as DDSs, highlighting how their designs have been inspired by natural cells or cell membranes. Advancement of artificial cell technologies has led to a set of drug carriers with effective and controlled release of a variety of therapeutics for a range of diseases, and with increasing complexity they will have a greater impact on therapeutic designs.

Towards proteinoid computers. Hypothesis paper

Proteinoids - thermal proteins - are produced by heating amino acids to their melting point and initiation of polymerisation to produce polymeric chains. Proteinoids swell in aqueous solution into hollow microspheres. The proteinoid microspheres produce endogenous burst of electrical potential spikes and change patterns of their electrical activity in response to illumination. The microspheres can interconnect by pores and tubes and form networks with a programmable growth. We speculate on how ensembles of the proteinoid microspheres can be developed into unconventional computing devices.

Bottom-Up Synthesis of Artificial Cells: Recent Highlights and Future Challenges

The bottom-up approach in synthetic biology aims to create molecular ensembles that reproduce the organization and functions of living organisms and strives to integrate them in a modular and hierarchical fashion toward the basic unit of life-the cell-and beyond. This young field stands on the shoulders of fundamental research in molecular biology and biochemistry, next to synthetic chemistry, and, augmented by an engineering framework, has seen tremendous progress in recent years thanks to multiple technological and scientific advancements. In this timely review of the research over the past decade, we focus on three essential features of living cells: the ability to self-reproduce via recursive cycles of growth and division, the harnessing of energy to drive cellular processes, and the assembly of metabolic pathways. In addition, we cover the increasing efforts to establish multicellular systems via different communication strategies and critically evaluate the potential applications.

A Sustainable and Efficient Artificial Microgel System: Toward Creating a Configurable Synthetic Cell

Artificial cells are a powerful platform in the study of synthetic biology and other valuable fields. They share a great potential in defining and utilizing the superiority of the living system. Here, a protein synthesis system based on thermal responsive hydrogels with porous structure is reported. The hydrogels can immobilize plasmids on the surface inside their porous structure through a volume phase transition upon 34 °C, forming an aggregation state of DNAs as in nature conditions. The artificial microgels can carry out bioreactions in cell-free systems and exhibit a sustainable and efficient performance for protein translation. The protein synthesis level reaches a maximum of twice more than that in a conventional solution system when the plasmid concentration is 10-20 ng µL^-1 , along with a doubled effective interval. This is perhaps attributed to confined transcription and translation processes in the near-surface area of hydrogels. Summarily, the research provides an easy-handling approach in fabricating effective microgels for cell-free synthesis and also inspirations for constructing a configurable artificial cell.

Artificial Cells Capable of Long-Lived Protein Synthesis by Using Aptamer Grafted Polymer Hydrogel

Herein we report a new type of artificial cells capable of long-term protein expression and regulation. We constructed the artificial cells by grafting anti-His-tag aptamer into the polymer backbone of the hydrogel particles, and then immobilizing the His-tagged proteinaceous factors of the transcription and translation system into the hydrogel particles. Long-term protein expression for at least 16 days was achieved by continuously flowing feeding buffer through the artificial cells. The effect of various metal ions on the protein expression in the artificial cells was investigated. Utilizing the lac operator-repressor system, we could regulate the expression level of eGFP in the artificial cells by controlling the β-D-1-thiogalatopyranoside (IPTG) concentration in the feeding buffer. The artificial cells based on the aptamer grafted hydrogel provide a useful platform for gene circuit engineering, metabolic engineering, drug delivery, and biosensors.

Calcium Signaling in ß-cell Physiology and Pathology: A Revisit

Pancreatic beta (β) cell dysfunction results in compromised insulin release and, thus, failed regulation of blood glucose levels. This forms the backbone of the development of diabetes mellitus (DM), a disease that affects a significant portion of the global adult population. Physiological calcium (Ca²⁺) signaling has been found to be vital for the proper insulin-releasing function of β-cells. Calcium dysregulation events can have a dramatic effect on the proper functioning of the pancreatic β-cells. The current review discusses the role of calcium signaling in health and disease in pancreatic β-cells and provides an in-depth look into the potential role of alterations in β-cell Ca²⁺ homeostasis and signaling in the development of diabetes and highlights recent work that introduced the current theories on the connection between calcium and the onset of diabetes.

What can we learn from the construction of in vitro replication systems?

Replication is a central function of living organisms. Several types of replication systems have been constructed in vitro from various molecules, including peptides, DNA, RNA, and proteins. In this review, I summarize the progress in the construction of replication systems over the past few decades and discuss what we can learn from their construction. I introduce various types of replication systems, supporting the feasibility of the spontaneous appearance of replication early in Earth's history. In the latter part of the review, I focus on parasitic replicators, one of the largest obstacles for sustainable replication. Compartmentalization is discussed as a possible solution.

Long-lived protein expression in hydrogel particles: towards artificial cells

Herein we report a new type of cell-mimic particle capable of long-lived protein expression. We constructed the cell-mimic particles by immobilizing the proteinaceous factors of the cell-free transcription and translation system on the polymer backbone of hydrogel particles and encapsulating the plasmid template and ribosome inside the hydrogel. With the continuous supply of nutrients and energy, the protein expression in the cell-mimic particles remained stable for at least 11 days. We achieved the regulation of protein expression in the cell-mimic particles by the usage of lac operon. The cell-mimic particles quickly responded to the concentration change of isopropyl β-d-1-thiogalactopyranoside (IPTG) in the feeding buffer to regulate the mCherry expression level. We also constructed an in vitro genetic oscillator in the cell-mimic particles. Protein LacI provided a negative feedback to the expression of both LacI itself and eGFP, and the expression level change of eGFP presented an oscillation. We expect the cell-mimic particles to be a useful platform for gene circuit engineering, metabolic engineering, and biosensors.

Constructing vesicle-based artificial cells with embedded living cells as organelle-like modules

There is increasing interest in constructing artificial cells by functionalising lipid vesicles with biological and synthetic machinery. Due to their reduced complexity and lack of evolved biochemical pathways, the capabilities of artificial cells are limited in comparison to their biological counterparts. We show that encapsulating living cells in vesicles provides a means for artificial cells to leverage cellular biochemistry, with the encapsulated cells serving organelle-like functions as living modules inside a larger synthetic cell assembly. Using microfluidic technologies to construct such hybrid cellular bionic systems, we demonstrate that the vesicle host and the encapsulated cell operate in concert. The external architecture of the vesicle shields the cell from toxic surroundings, while the cell acts as a bioreactor module that processes encapsulated feedstock which is further processed by a synthetic enzymatic metabolism co-encapsulated in the vesicle.

Engineering the Surface of Therapeutic "Living" Cells

Biological cells are complex living machines that have garnered significant attention for their potential to serve as a new generation of therapeutic and delivery agents. Because of their secretion, differentiation, and homing activities, therapeutic cells have tremendous potential to treat or even cure various diseases and injuries that have defied conventional therapeutic strategies. Therapeutic cells can be systemically or locally transplanted. In addition, with their ability to express receptors that bind specific tissue markers, cells are being studied as nano- or microsized drug carriers capable of targeted transport. Depending on the therapeutic targets, these cells may be clustered to promote intercellular adhesion. Despite some impressive results with preclinical studies, there remain several obstacles to their broader development, such as a limited ability to control their transport, engraftment, secretion and to track them in vivo. Additionally, creating a particular spatial organization of therapeutic cells remains difficult. Efforts have recently emerged to resolve these challenges by engineering cell surfaces with a myriad of bioactive molecules, nanoparticles, and microparticles that, in turn, improve the therapeutic efficacy of cells. This review article assesses the various technologies developed to engineer the cell surfaces. The review ends with future considerations that should be taken into account to further advance the quality of cell surface engineering.

Synthetic beta cells for fusion-mediated dynamic insulin secretion

Generating artificial pancreatic beta cells by using synthetic materials to mimic glucose-responsive insulin secretion in a robust manner holds promise for improving clinical outcomes in people with diabetes. Here, we describe the construction of artificial beta cells (AβCs) with a multicompartmental 'vesicles-in-vesicle' superstructure equipped with a glucose-metabolism system and membrane-fusion machinery. Through a sequential cascade of glucose uptake, enzymatic oxidation and proton efflux, the AβCs can effectively distinguish between high and normal glucose levels. Under hyperglycemic conditions, high glucose uptake and oxidation generate a low pH (<5.6), which then induces steric deshielding of peptides tethered to the insulin-loaded inner small liposomal vesicles. The peptides on the small vesicles then form coiled coils with the complementary peptides anchored on the inner surfaces of large vesicles, thus bringing the membranes of the inner and outer vesicles together and triggering their fusion and insulin 'exocytosis'.

Biotransporting Self-Assembled Nanofactories Using Polymer Vesicles with Molecular Permeability for Enzyme Prodrug Cancer Therapy

As "biotransporting nanofactories", in vivo therapeutic biocatalyst nanoreactors would enable encapsulated enzymes to transform inert prodrugs or neutralize toxic compounds at target disease sites. This would offer outstanding potential for next-generation therapeutic platforms, such as enzyme prodrug therapy. Designing such advanced materials has, however, proven challenging. Here, it is shown that self-assembled nanofactories formulate with polymeric vesicles with an intrinsically permeable membrane. The vesicles, CAPsomes, are composed of carbohydrate-b-poly(propylene glycol) and show molecular-weight-depended permeability. This property enables CAPsomes to act as biocatalyst nanoreactors, protecting encapsulated enzymes from degradation while acting on low-molecular-weight substrates. In tumor bearing mice, combined treatment with enzyme-loaded CAPsomes and doxorubicin prodrug inhibit tumor growth in these mice without any observable toxicity. The results demonstrate, for the first time, in vivo therapeutic efficacy of CAPsomes as nanofactories for enzyme prodrug cancer therapy.

Enzymes as key features in therapeutic cell mimicry

Cell mimicry is a nature inspired concept that aims to substitute for missing or lost (sub)cellular function. This review focuses on the latest advancements in the use of enzymes in cell mimicry for encapsulated catalysis and artificial motility in synthetic bottom-up assemblies with emphasis on the biological response in cell culture or more rarely in animal models. Entities across the length scale from nano-sized enzyme mimics, sub-micron sized artificial organelles and self-propelled particles (swimmers) to micron-sized artificial cells are discussed. Although the field remains in its infancy, the primary aim of this review is to illustrate the advent of nature-mimicking artificial molecules and assemblies on their way to become a complementary alternative to their role models for diverse biomedical purposes.

Construction of membrane-bound artificial cells using microfluidics: a new frontier in bottom-up synthetic biology

The quest to construct artificial cells from the bottom-up using simple building blocks has received much attention over recent decades and is one of the grand challenges in synthetic biology. Cell mimics that are encapsulated by lipid membranes are a particularly powerful class of artificial cells due to their biocompatibility and the ability to reconstitute biological machinery within them. One of the key obstacles in the field centres on the following: how can membrane-based artificial cells be generated in a controlled way and in high-throughput? In particular, how can they be constructed to have precisely defined parameters including size, biomolecular composition and spatial organization? Microfluidic generation strategies have proved instrumental in addressing these questions. This article will outline some of the major principles underpinning membrane-based artificial cells and their construction using microfluidics, and will detail some recent landmarks that have been achieved.

Liposomes in Polymersomes: Multicompartment System with Temperature-Triggered Release

Multicompartmentalization is a key feature of eukaryotic cells, allowing separation and protection of species within the membrane walls. During the last years, several methods have been reported to afford synthetic multicompartment lipidic or polymeric vesicles that mimic biological cells and that allow cascade chemical or enzymatic reactions within their lumen. We hereby report on the preparation and study of liposomes in polymersomes (LiPs) systems. We discuss on the loading and coloading of lipidic nanovesicles made of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipentadecanoyl-sn-glycero-3-phosphocholine (diC15-PC), or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) inside the lumen of giant poly(butadiene)-b-poly(ethylene oxide) (PBut-b-PEO) polymersomes. These LiPs systems were characterized by confocal microscopy and UV-visible spectroscopy. We further demonstrate that we can achieve controlled sequential release of dyes from diC15-PC and DPPC liposomes at defined temperatures inside the giant PBut-b-PEO polymersomes. This controlled release could be used as a means to initiate cascade reactions on demand in confined microreactors.

Mechanically activated artificial cell by using microfluidics

All living organisms sense mechanical forces. Engineering mechanosensitive artificial cell through bottom-up in vitro reconstitution offers a way to understand how mixtures of macromolecules assemble and organize into a complex system that responds to forces. We use stable double emulsion droplets (aqueous/oil/aqueous) to prototype mechanosensitive artificial cells. In order to demonstrate mechanosensation in artificial cells, we develop a novel microfluidic device that is capable of trapping double emulsions into designated chambers, followed by compression and aspiration in a parallel manner. The microfluidic device is fabricated using multilayer soft lithography technology, and consists of a control layer and a deformable flow channel. Deflections of the PDMS membrane above the main microfluidic flow channels and trapping chamber array are independently regulated pneumatically by two sets of integrated microfluidic valves. We successfully compress and aspirate the double emulsions, which result in transient increase and permanent decrease in oil thickness, respectively. Finally, we demonstrate the influx of calcium ions as a response of our mechanically activated artificial cell through thinning of oil. The development of a microfluidic device to mechanically activate artificial cells creates new opportunities in force-activated synthetic biology.

Droplet-based microfluidics for artificial cell generation: a brief review

Artificial cells are best defined as micrometre-sized structures able to mimic many of the morphological and functional characteristics of a living cell. In this mini-review, we describe progress in the application of droplet-based microfluidics for the generation of artificial cells and protocells.

Spontaneous vesiculation: a mechanistic insight from the study of hybrid peptide molecules

We report a series of hybrid peptide molecules that display spontaneous vesiculation. A closer look at their vesicle formation revealed a toroidal intermediate en route to the final vesicular form.

Vesicle encapsulation of a nonbiological photochemical system capable of reducing NAD(+) to NADH

One of the fundamental structures of a cell is the membrane. Self-assembling lipid bilayer vesicles can form the membrane of an artificial cell and could also have plausibly assembled prebiotically for the origin of life. Such cell-like structures, that encapsulate some basic subset of the functions of living cells, are important for research to infer the minimum chemistry necessary for a cell, to help understand the origin of life, and to allow the production of useful species in microscopic containers. We show that the encapsulation of TiO2 particles has the potential to provide the basis for an energy transduction system inside vesicles which can be used to drive subsequent chemistry. TiO2 encapsulated in vesicles can be used to produce biochemical species such as NADH. The NADH is formed from NAD(+) reduction and is produced in a form that is able to drive further enzymatic chemistry. This allows us to link a mineral-based, nonbiological photosystem to biochemical reactions. This is a fundamental step toward being able to use this mineral photosystem in a protocell/artificial cell.

Modulation of reactivity in the cavity of liposomes promotes the formation of peptide bonds

In living cells, reactions take place in membrane-bound compartments, often in response to changes in the environment. Learning how the reactions are influenced by this compartmentalization will help us gain an optimal understanding of living organisms at the molecular level and, at the same time, will offer vital clues on the behavior of simple compartmentalized systems, such as prebiotic precursors of cells and cell-inspired artificial systems. In this work we show that a reactive building block (an activated amino acid derivative) trapped in the cavity of a liposome is protected against hydrolysis and reacts nearly quantitatively with another building block, which is membrane-permeable and free in solution, to form the dipeptide. By contrast, when the activated amino acid is found outside the liposome, hydrolysis is the prevalent reaction, showing that the cavity of the liposomes promotes the formation of peptide bonds. We attribute this result to the large lipid concentration in small compartments from the point of view of a membrane-impermeable molecule. Based on this result, we show how the outcome of the reaction can be predicted as a function of the size of the compartment. The implications of these results on the behavior of biomolecules in cell compartments, abiogenesis, and the design of artificial cell-inspired systems are considered.

Synthetic Biology: A Bridge between Artificial and Natural Cells

Artificial cells are simple cell-like entities that possess certain properties of natural cells. In general, artificial cells are constructed using three parts: (1) biological membranes that serve as protective barriers, while allowing communication between the cells and the environment; (2) transcription and translation machinery that synthesize proteins based on genetic sequences; and (3) genetic modules that control the dynamics of the whole cell. Artificial cells are minimal and well-defined systems that can be more easily engineered and controlled when compared to natural cells. Artificial cells can be used as biomimetic systems to study and understand natural dynamics of cells with minimal interference from cellular complexity. However, there remain significant gaps between artificial and natural cells. How much information can we encode into artificial cells? What is the minimal number of factors that are necessary to achieve robust functioning of artificial cells? Can artificial cells communicate with their environments efficiently? Can artificial cells replicate, divide or even evolve? Here, we review synthetic biological methods that could shrink the gaps between artificial and natural cells. The closure of these gaps will lead to advancement in synthetic biology, cellular biology and biomedical applications.

Positive roles of compartmentalization in internal reactions

Recently, many researchers have attempted to construct artificial cell models using a bottom-up approach in which various biochemical reactions that involve a defined set of molecules are reconstructed in cell-like compartments, such as liposomes and water-in-oil droplets. In many of these studies, the cell-like compartments have acted only as containers for the encapsulated biochemical reactions, whereas other studies have indicated that compartmentalization improves the rates and yields of these reactions. Here, we introduce two ways in which compartmentalization can improve internal reactions: the isolation effect and the condensation effect. These positive effects of compartmentalization might have played an important role in the genesis of the first primitive cell on early Earth.

Mimicking the mechanical properties of the cell cortex by the self-assembly of an actin cortex in vesicles

The composite of the actin cytoskeleton and plasma membrane plays important roles in many biological events. Here, we employed the emulsion method to synthesize artificial cells with biomimetic actin cortex in vesicles and characterized their mechanical properties. We demonstrated that the emulsion method provides the flexibility to adjust the lipid composition and protein concentrations in artificial cells to achieve the desired size distribution, internal microstructure, and mechanical properties. Moreover, comparison of the cortical elasticity measured for reconstituted artificial cells to that of real cells, including those manipulated using genetic depletion and pharmacological inhibition, strongly supports that actin cytoskeletal proteins are dominant over lipid molecules in cortical mechanics. Our study indicates that the assembly of biological systems in artificial cells with purified cellular components provides a powerful way to answer biological questions.

Creating biological nanomaterials using synthetic biology

Synthetic biology is a new discipline that combines science and engineering approaches to precisely control biological networks. These signaling networks are especially important in fields such as biomedicine and biochemical engineering. Additionally, biological networks can also be critical to the production of naturally occurring biological nanomaterials, and as a result, synthetic biology holds tremendous potential in creating new materials. This review introduces the field of synthetic biology, discusses how biological systems naturally produce materials, and then presents examples and strategies for incorporating synthetic biology approaches in the development of new materials. In particular, strategies for using synthetic biology to produce both organic and inorganic nanomaterials are discussed. Ultimately, synthetic biology holds the potential to dramatically impact biological materials science with significant potential applications in medical systems.

Chemical synthetic biology

Although both the most popular form of synthetic biology (SB) and chemical synthetic biology (CSB) share the biotechnologically useful aim of making new forms of life, SB does so by using genetic manipulation of extant microorganism, while CSB utilises classic chemical procedures in order to obtain biological structures which are non-existent in nature. The main query concerning CSB is the philosophical question: why did nature do this, and not that? The idea then is to synthesise alternative structures in order to understand why nature operated in such a particular way. We briefly present here some various examples of CSB, including those cases of nucleic acids synthesised with pyranose instead of ribose, and proteins with a reduced alphabet of amino acids; also we report the developing research on the "never born proteins" (NBP) and "never born RNA" (NBRNA), up to the minimal cell project, where the issue is the preparation of semi-synthetic cells that can perform the basic functions of biological cells.

Advanced subcompartmentalized microreactors: polymer hydrogel carriers encapsulating polymer capsules and liposomes

The design of compartmentalized carriers for advanced drug delivery systems or artificial cells and organelles is of interest for biomedical applications. Herein, a polymer carrier microreactor that contains two different classes of subcompartments, multilayered polymer capsules and liposomes, is presented. 50 nm-diameter liposomes and 300 nm-diameter polymer capsules are encapsulated into a larger polymer carrier capsule, demonstrating control over the spatial positioning of the subcompartments, which are either 'membrane-associated' or 'free-floating' in the aqueous interior. Selective and spatially dependent degradation of the 300 nm-diameter subcompartments (without destroying the structural integrity of the enzyme-loaded liposomes) is also shown, by performing an encapsulated enzymatic reaction using the liposomal subcompartments. These findings cover several important aspects toward the development of engineered compartmentalized carrier vessels for the creation of artificial cell mimics or advanced therapeutic delivery systems.

A review of polypeptide-based polymersomes

Self-assembled systems from biodegradable amphiphilic polymers at the nanometer scale, such as nanotubes, nanoparticles, polymer micelles, nanogels, and polymersomes, have attracted much attention especially in biomedical fields. Among these nano-aggregates, polymersomes have attracted tremendous interests as versatile carriers due to their colloidal stability, tunable membrane properties and ability of encapsulating or integrating a broad range of drugs and molecules. Biodegradable block polymers, especially aliphatic polyesters such as polylactide, polyglycolide and poly (ε-caprolactone) have been widely used as biomedical materials for a long time to well fit the requirement of biomedical drug carriers. To have a precise control of the aggregation behavior of nano-aggregates, the more ordered polypeptide has been used to self-assemble into the drug carriers. In this review we focus on the study of polymersomes which also named pepsomes formed by polypeptide-based copolymers and attempt to clarify the polypeptide-based polymersomes from following aspects: synthesis and characterization of the polypeptide-based copolymers, preparation, multifunction and application of polypeptide-based polymersomes.

Construction of a Ca(2+)-gated artificial channel by fusing alamethicin with a calmodulin-derived extramembrane segment

Using native chemical ligation, we constructed a Ca(2+)-gated fusion channel protein consisting of alamethicin and the C-terminal domain of calmodulin. At pH 5.4 and in the absence of Ca(2+), this fusion protein yielded a burst-like channel current with no discrete channel conductance levels. However, Ca(2+) significantly lengthened the specific channel open state and increased the mean channel current, while Mg(2+) produced no significant changes in the channel current. On the basis of 8-anilinonaphthalene-1-sulfonic acid (ANS) fluorescent measurement, Ca(2+)-stimulated gating may be related to an increased surface hydrophobicity of the extramembrane segment of the fusion protein.

Cholesterol--a biological compound as a building block in bionanotechnology

Cholesterol is a molecule with many tasks in nature but also a long history in science. This feature article highlights the contribution of this small compound to bionanotechnology. We discuss relevant chemical aspects in this context followed by an overview of its self-assembly capabilities both as a free molecule and when conjugated to a polymer. Further, cholesterol in the context of liposomes is reviewed and its impact ranging from biosensing to drug delivery is outlined. Cholesterol is and will be an indispensable player in bionanotechnology, contributing to the progress of this potent field of research.

Biomimetic liposome- and polymersome-based multicompartmentalized assemblies

Liposomes and polymersomes have attracted significant attention and have emerged as versatile materials for therapeutic delivery and in the design of artificial cells and organelles. Through the judicious choice of building blocks, these synthetic carriers can be readily engineered with tailored interfacial properties, offering new possibilities for the design of advanced assemblies with specific permeability, stability, stimuli response, and targeting capabilities. In this feature article, we highlight recent studies on biomimetic liposome- and polymersome-based multicompartmentalized assemblies en route toward the development of artificial cells, microreactors, and therapeutic delivery carriers. The strategies employed to produce these carriers are outlined, and the properties that contribute to their performance are discussed. Applications of these biomimetic assemblies are highlighted, and finally, areas that require additional investigation for the future development of these assemblies as next-generation therapeutic systems are outlined.