A minimal biochemical route towards de novo formation of synthetic phospholipid membranes

Biosynthesis of isonitrile lipopeptides

Isonitrile lipopeptides discovered from Actinobacteria have attracted wide attention due to their fascinating biosynthetic pathways and relevance to the virulence of many human pathogens including Mycobacterium tuberculosis. Specifically, the identification of the new class of isonitrile-forming enzymes that belong to non-heme iron (II) and α-ketoglutarate dependent dioxygenases has intrigued several research groups to investigate their catalytic mechanism. Here we summarize the recent studies on the biosynthesis of isonitrile lipopeptides from Streptomyces and Mycobacterium. The latest research on the core and tailoring enzymes involved in the pathway as well as the isonitrile metabolic enzymes are discussed in this review.

Iterative design of training data to control intricate enzymatic reaction networks

Kinetic modeling of in vitro enzymatic reaction networks is vital to understand and control the complex behaviors emerging from the nonlinear interactions inside. However, modeling is severely hampered by the lack of training data. Here, we introduce a methodology that combines an active learning-like approach and flow chemistry to efficiently create optimized datasets for a highly interconnected enzymatic reactions network with multiple sub-pathways. The optimal experimental design (OED) algorithm designs a sequence of out-of-equilibrium perturbations to maximize the information about the reaction kinetics, yielding a descriptive model that allows control of the output of the network towards any cost function. We experimentally validate the model by forcing the network to produce different product ratios while maintaining a minimum level of overall conversion efficiency. Our workflow scales with the complexity of the system and enables the optimization of previously unobtainable network outputs.

Rapid Formation of Non-canonical Phospholipid Membranes by Chemoselective Amide-Forming Ligations with Hydroxylamines

There has been increasing interest in methods to generate synthetic lipid membranes as key constituents of artificial cells or to develop new tools for remodeling membranes in living cells. However, the biosynthesis of phospholipids involves elaborate enzymatic pathways that are challenging to reconstitute in vitro. An alternative approach is to use chemical reactions to non-enzymatically generate natural or non-canonical phospholipids de novo. Previous reports have shown that synthetic lipid membranes can be formed in situ using various ligation chemistries, but these methods lack biocompatibility and/or suffer from slow kinetics at physiological pH. Thus, it would be valuable to develop chemoselective strategies for synthesizing phospholipids from water-soluble precursors that are compatible with synthetic or living cells Here, we demonstrate that amide-forming ligations between lipid precursors bearing hydroxylamines and α-ketoacids (KAs) or potassium acyltrifluoroborates (KATs) can be used to prepare non-canonical phospholipids at physiological pH conditions. The generated amide-linked phospholipids spontaneously self-assemble into cell-like micron-sized vesicles similar to natural phospholipid membranes. We show that lipid synthesis using KAT ligation proceeds extremely rapidly, and the high selectivity and biocompatibility of the approach facilitates the in situ synthesis of phospholipids and associated membranes in living cells.

Chemoselective Esterification of Natural and Prebiotic 1,2-Amino Alcohol Amphiphiles in Water

In cells, a vast number of membrane lipids are formed by the enzymatic O-acylation of polar head groups with acylating agents such as fatty acyl-CoAs. Although such ester-containing lipids appear to be a requirement for life on earth, it is unclear if similar types of lipids could have spontaneously formed in the absence of enzymatic machinery at the origin of life. There are few examples of enzyme-free esterification of amphiphiles in water and none that can occur in water at physiological pH using biochemically relevant acylating agents. Here we report the unexpected chemoselective O-acylation of 1,2-amino alcohol amphiphiles in water directed by Cu(II) and several other transition metal ions. In buffers containing Cu(II) ions, mixing biological 1,2-amino alcohol amphiphiles such as sphingosylphosphorylcholine with biochemically relevant acylating agents, namely, acyl adenylates and acyl-CoAs, leads to the formation of the O-acylation product with high selectivity. The resulting O-acylated sphingolipids self-assemble into vesicles with markedly different biophysical properties than those formed from their N-acyl counterparts. We also demonstrate that Cu(II) can direct the O-acylation of alternative 1,2-amino alcohols, including prebiotically relevant 1,2-amino alcohol amphiphiles, suggesting that simple mechanisms for aqueous esterification may have been prevalent on earth before the evolution of enzymes.

Biomimetic construction of phospholipid membranes by direct aminolysis ligations

Construction of artificial cells requires the development of straightforward methods for mimicking natural phospholipid membrane formation. Here we describe the use of direct aminolysis ligations to spontaneously generate biomimetic phospholipid membranes from water-soluble starting materials. Additionally, we explore the suitability of such biomimetic approaches for driving the in situ formation of native phospholipid membranes. Our studies suggest that non-enzymatic ligation reactions could have been important for the synthesis of phospholipid-like membranes during the origin of life, and might be harnessed as simplified methods to enable the generation of lipid compartments in artificial cells.

On biochemical constructors and synthetic cells

Is it possible to build life? More specifically, is it possible to create a living synthetic cell from inanimate building blocks? This question precipitated into one of the most significant grand challenges in biochemistry and synthetic biology, with several large research consortia forming around this endeavour in Europe (European Synthetic Cell Initiative), the USA (Build-a-Cell Initiative) and Japan (Japanese Society for Cell Synthesis Research). The mature field of biochemistry, the advent of synthetic biology in the early 2000s, and the burgeoning field of cell-free synthetic biology made it feasible to tackle this grand challenge.

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

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

Minimal Out-of-Equilibrium Metabolism for Synthetic Cells: A Membrane Perspective

Life-like systems need to maintain a basal metabolism, which includes importing a variety of building blocks required for macromolecule synthesis, exporting dead-end products, and recycling cofactors and metabolic intermediates, while maintaining steady internal physical and chemical conditions (physicochemical homeostasis). A compartment, such as a unilamellar vesicle, functionalized with membrane-embedded transport proteins and metabolic enzymes encapsulated in the lumen meets these requirements. Here, we identify four modules designed for a minimal metabolism in a synthetic cell with a lipid bilayer boundary: energy provision and conversion, physicochemical homeostasis, metabolite transport, and membrane expansion. We review design strategies that can be used to fulfill these functions with a focus on the lipid and membrane protein composition of a cell. We compare our bottom-up design with the equivalent essential modules of JCVI-syn3a, a top-down genome-minimized living cell with a size comparable to that of large unilamellar vesicles. Finally, we discuss the bottlenecks related to the insertion of a complex mixture of membrane proteins into lipid bilayers and provide a semiquantitative estimate of the relative surface area and lipid-to-protein mass ratios (i.e., the minimal number of membrane proteins) that are required for the construction of a synthetic cell.

Metabolic changes underlying drug resistance in the multiple myeloma tumor microenvironment

Multiple myeloma (MM) is characterized by the clonal expansion of malignant plasma cells in the bone marrow (BM). MM remains an incurable disease, with the majority of patients experiencing multiple relapses from different drugs. The MM tumor microenvironment (TME) and in particular bone-marrow stromal cells (BMSCs) play a crucial role in the development of drug resistance. Metabolic reprogramming is emerging as a hallmark of cancer that can potentially be exploited for cancer treatment. Recent studies show that metabolism is further adjusted in MM cells during the development of drug resistance. However, little is known about the role of BMSCs in inducing metabolic changes that are associated with drug resistance. In this Perspective, we summarize current knowledge concerning the metabolic reprogramming of MM, with a focus on those changes associated with drug resistance to the proteasome inhibitor Bortezomib (BTZ). In addition, we present proof-of-concept fluxomics (glucose isotope-tracing) and Seahorse data to show that co-culture of MM cells with BMSCs skews the metabolic phenotype of MM cells towards a drug-resistant phenotype, with increased oxidative phosphorylation (OXPHOS), serine synthesis pathway (SSP), TCA cycle and glutathione (GSH) synthesis. Given the crucial role of BMSCs in conveying drug resistance, insights into the metabolic interaction between MM and BMSCs may ultimately aid in the identification of novel metabolic targets that can be exploited for therapy.

Synthesising a minimal cell with artificial metabolic pathways

A "synthetic minimal cell" is considered here as a cell-like artificial vesicle reproduction system in which a chemical and physico-chemical transformation network is regulated by information polymers. Here we synthesise such a minimal cell consisting of three units: energy production, information polymer synthesis, and vesicle reproduction. Supplied ingredients are converted to energy currencies which trigger the synthesis of an information polymer, where the vesicle membrane plays the role of a template. The information polymer promotes membrane growth. By tuning the membrane composition and permeability to osmolytes, the growing vesicles show recursive reproduction over several generations. Our "synthetic minimal cell" greatly simplifies the scheme of contemporary living cells while keeping their essence. The chemical pathways and the vesicle reproduction pathways are well described by kinetic equations and by applying the membrane elasticity model, respectively. This study provides new insights to better understand the differences and similarities between non-living forms of matter and life.

Pseudopeptosomes: non-lipidated vesicular assemblies from bispidine-appended pseudopeptides

We report a novel molecular topology-based approach for creating reproducible vesicular assemblies in different solvent environments (including aqueous) using specifically designed pseudopeptides. Deviating from the classical "polar head group and hydrophobic tail" model of amphiphiles, we showed (reversible) self-assembly of synthesized pseudopeptides into vesicles. Naming these new type/class of vesicles "pseudopetosomes", we characterized them by high-resolution microscopy (scanning electron, transmission electron, atomic force, epifluorescence and confocal) along with dynamic light scattering. While accounting for hydropathy index of the constituent amino acids (side chains) of pseudopeptides, we probed molecular interactions, resulting in assembly of pseudopeptosomes by spectroscopy (fourier-transform infrared and fluorescence). Molecular characterization by X-ray crystallography and circular dichroism revealed "tryptophan (Trp)-Zip" arrangements and/or hydrogen-bonded one-dimensional assembly depending on specific pseudopeptides and solvent environments. Our data indicated that pseudopeptosomes are formed in solutions by self-assembly of bispidine pseudopeptides (of Trp, leucine and alanine amino-acid constituents) into sheets that transform into vesicular structures. Thus, we showed that assembly of pseudopeptosomes utilizes the full spectrum of all four weak interactions essential in biological systems. Our findings have direct implications in chemical and synthetic biology, but may also provide a new avenue of investigations on origins of life via pseudopeptosome-like assemblies. We also showed that these designer peptides can act as carriers for cellular transport.

Exploring the Lipid World Hypothesis: A Novel Scenario of Self-Sustained Darwinian Evolution of the Liposomes

According to the Lipid World hypothesis, life on Earth originated with the emergence of amphiphilic assemblies in the form of lipid micelles and vesicles (liposomes). However, the mechanism of appearance of the information molecules (ribozymes/RNA) accompanying that process, considered obligatory for Darwinian evolution, is unclear. We propose a novel scenario of self-sustained Darwinian evolution of the liposomes driven by ever-present natural phenomena: solar UV radiation, day/night cycle, gravity, and the formation of liposomes in an aqueous media. The central tenet of this scenario is the liposomes' encapsulation of the heavy solutes, followed by their gravitational submerging in the water. The submerged liposomes, being protected from the damaging UV radiation, acquire the longevity necessary for autocatalytic replication of amphiphiles, their mutation, and the selection of those amphiphilic assemblies that provide the greatest membrane stability. These two sets of adaptive compositional information (heavy content and amphiphilic assemblies design) generate a population of liposomes with self-replication/reproduction properties, which are amendable to mutation, inheritance, and selection, thereby establishing Darwinian progression. Temporary and spatial expansion of this liposomal population will provide the basis for the next evolutionary step-a transition of accidentally entrapped RNA precursor molecules into complex functional molecules, such as ribozymes/RNA.

Cation and buffer specific effects on the DNA-lipid interaction

Knowledge of DNA - lipid layer interactions is key for the development of biosensors, synthetic nanopores, scaffolds, and gene-delivery systems. These interactions are strongly affected by the ionic composition of the solvent. We have combined quartz crystal microbalance (QCM) and ellipsometry measurements to reveal how pH, buffers and alkali metal chloride salts affect the interaction of DNA with lipid bilayers (DOTAP/DOPC 30:70 in moles). We found that the thickness of the DNA layer adsorbed onto the lipid bilayer decreased in the order citrate > phosphate > Tris > HEPES. The effect of cations on the thickness of the DNA layer decreased in the order (K⁺ > Na⁺ > Cs⁺ ∼ Li⁺). Rationalization of the experimental results requires that adsorption, due to cation specific charge screening, is driven by the simultaneous action of two mechanisms namely, the law of matching water affinities for kosmotropes (Li⁺) and ion dispersion forces for chaotropes (Cs⁺). The outcome of these two opposing mechanisms is a ^"bell-shaped" specific cations sequence. Moreover, a superimposed buffer specificity, which goes beyond the simple effect of pH regulation, further modulated cation specificity. In summary, DNA-lipid bilayer interactions are maximized if citrate buffer (50 mM, pH 7.4) and KCl (100 mM) are used.

Self-assembly and biophysical properties of archaeal lipids

Archaea constitute one of the three fundamental domains of life. Archaea possess unique lipids in their cell membranes which distinguish them from bacteria and eukaryotes. This difference in lipid composition is referred to as 'Lipid Divide' and its origins remain elusive. Chemical inertness and the highly branched nature of the archaeal lipids afford the membranes stability against extremes of temperature, pH, and salinity. Based on the molecular architecture, archaeal polar lipids are of two types - monopolar and bipolar. Both monopolar and bipolar lipids have been shown to form vesicles and other well-defined membrane architectures. Bipolar archaeal lipids are among the most unique lipids found in nature because of their membrane-spanning nature and mechanical stability. The majority of the self-assembly studies on archaeal lipids have been carried out using crude polar lipid extracts or molecular mimics. The complexity of the archaeal lipids makes them challenging to synthesize chemically, and as a result, studies on pure lipids are few. There is an ongoing effort to develop simplified routes to synthesize complex archaeal lipids to facilitate diverse biophysical studies and pharmaceutical applications. Investigation on archaeal lipids may help us understand how life survives in extreme conditions and therefore unlock some of the mysteries surrounding the origins of cellular life.

In situ synthesis of artificial lipids

Lipids constitute one of the most enigmatic family of biological molecules. Although the importance of lipids as basic units of compartmental structure and energy storage is well-acknowledged, deciphering the biosynthesis and precise roles of specific lipid species has been challenging. To better understand the structure and function of these biomolecules, there is a burgeoning interest in developing strategies to produce noncanonical lipids in a controlled manner. This review covers recent advances in the area of in situ generation of synthetic lipids. Specifically, we report several approaches that constitute a powerful toolbox for achieving noncanonical lipid synthesis. We describe how these methodologies enable the direct construction of synthetic lipids, helping to address fundamental questions related to the cell biology of lipid biosynthesis, trafficking, and signaling. We envision that highlighting the current advances in artificial lipid synthesis will pave the way for broader interest into this emerging class of biomimetic molecules.

Bioconjugation Strategies for Revealing the Roles of Lipids in Living Cells

The structural boundaries of living cells are composed of numerous membrane-forming lipids. Lipids not only are crucial for the cellular compartmentalization but also are involved in cell signaling as well as energy storage. Abnormal lipid levels have been linked to severe human diseases such as cancer, multiple sclerosis, neurodegenerative diseases, as well as lysosomal storage disorders. Given their biological significance, there is immense interest in studying lipids and their effect on cells. However, limiting factors include the low solubility of lipids, their structural complexity, and the challenge of using genetic techniques to directly manipulate lipid structure. Current methods to study lipids rely mostly on lipidomics, which analyzes the composition of lipid extracts using mass spectrometry. Although, these efforts have successfully catalogued and profiled a great number of lipids in cells, many aspects about their exact functional role and subcellular distribution remain enigmatic.In this Account, we outline how our laboratory developed and applied different bioconjugation strategies to study the role of lipids and lipid modifications in cells. Inspired by our ongoing work on developing lipid bioconjugation strategies to generate artificial cell membranes, we developed a ceramide synthesis method in live cells using a salicylaldehyde ester that readily reacts with sphingosine in form of a traceless ceramide ligation. Our study not only confirmed existing knowledge about the association of ceramides with cell death, but also gave interesting new findings about the structure-function relationship of ceramides in apoptosis. Our initial efforts led us to investigate probes that detect endogenous sphingolipids using live cell imaging. We describe the development of a fluorogenic probe that reacts chemoselectively with sphingosine in living cells, enabling the detection of elevated endogenous levels of this biomarker in human disease. Building on our interest in the fluorescence labeling of lipids, we have also explored the use of bioorthogonal reactions to label chemically synthesized lipid probes. We discuss the development of photocaged dihydrotetrazine lipids, where the initiation of the bioorthogonal reaction can be triggered by visible light, allowing for live cell modification of membranes with spatiotemporal control.Finally, proteins are often post-translationally modified by lipids, which have important effects on protein subcellular localization and function. Controlling lipid modifications with small molecule probes could help reveal the function of lipid post-translational modifications and could potentially inspire novel therapeutic strategies. We describe how our previous studies on synthetic membrane formation inspired us to develop an amphiphilic cysteine derivative that depalmitoylates membrane-bound S-acylated proteins in live cells. Ultimately, we applied this amphiphile mediated depalmitoylation (AMD) in studies investigating the palmitoylation of cancer relevant palmitoylated proteins in healthy and diseased cells.

HiChIPdb: a comprehensive database of HiChIP regulatory interactions

Elucidating the role of 3D architecture of DNA in gene regulation is crucial for understanding cell differentiation, tissue homeostasis and disease development. Among various chromatin conformation capture methods, HiChIP has received increasing attention for its significant improvement over other methods in profiling of regulatory (e.g. H3K27ac) and structural (e.g. cohesin) interactions. To facilitate the studies of 3D regulatory interactions, we developed a HiChIP interactions database, HiChIPdb (http://health.tsinghua.edu.cn/hichipdb/). The current version of HiChIPdb contains ∼262M annotated HiChIP interactions from 200 high-throughput HiChIP samples across 108 cell types. The functionalities of HiChIPdb include: (i) standardized categorization of HiChIP interactions in a hierarchical structure based on organ, tissue and cell line and (ii) comprehensive annotations of HiChIP interactions with regulatory genes and GWAS Catalog SNPs. To the best of our knowledge, HiChIPdb is the first comprehensive database that utilizes a unified pipeline to map the functional interactions across diverse cell types and tissues in different resolutions. We believe this database has the potential to advance cutting-edge research in regulatory mechanisms in development and disease by removing the barrier in data aggregation, preprocessing, and analysis.

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.

From vesicles toward protocells and minimal cells

In contrast to ordinary condensed matter systems, "living systems" are unique. They are based on molecular compartments that reproduce themselves through (i) an uptake of ingredients and energy from the environment, and (ii) spatially and timely coordinated internal chemical transformations. These occur on the basis of instructions encoded in information molecules (DNAs). Life originated on Earth about 4 billion years ago as self-organised systems of inorganic compounds and organic molecules including macromolecules (e.g. nucleic acids and proteins) and low molar mass amphiphiles (lipids). Before the first living systems emerged from non-living forms of matter, functional molecules and dynamic molecular assemblies must have been formed as prebiotic soft matter systems. These hypothetical cell-like compartment systems often are called "protocells". Other systems that are considered as bridging units between non-living and living systems are called "minimal cells". They are synthetic, autonomous and sustainable reproducing compartment systems, but their constituents are not limited to prebiotic substances. In this review, we focus on both membrane-bounded (vesicular) protocells and minimal cells, and provide a membrane physics background which helps to understand how morphological transformations of vesicle systems might have happened and how vesicle reproduction might be coupled with metabolic reactions and information molecules. This research, which bridges matter and life, is a great challenge in which soft matter physics, systems chemistry, and synthetic biology must take joined efforts to better understand how the transformation of protocells into living systems might have occurred at the origin of life.

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

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

Protocells: Milestones and Recent Advances

The origin of life is still one of humankind's great mysteries. At the transition between nonliving and living matter, protocells, initially featureless aggregates of abiotic matter, gain the structure and functions necessary to fulfill the criteria of life. Research addressing protocells as a central element in this transition is diverse and increasingly interdisciplinary. The authors review current protocell concepts and research directions, address milestones, challenges and existing hypotheses in the context of conditions on the early Earth, and provide a concise overview of current protocell research methods.

Synthesis of Phospholipids Under Plausible Prebiotic Conditions and Analogies with Phospholipid Biochemistry for Origin of Life Studies

Phospholipids are essential components of biological membranes and are involved in cell signalization, in several enzymatic reactions, and in energy metabolism. In addition, phospholipids represent an evolutionary and non-negligible step in life emergence. Progress in the past decades has led to a deeper understanding of these unique hydrophobic molecules and their most pertinent functions in cell biology. Today, a growing interest in "prebiotic lipidomics" calls for a new assessment of these relevant biomolecules.

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.

ATP Recycling Fuels Sustainable Glycerol 3-Phosphate Formation in Synthetic Cells Fed by Dynamic Dialysis

The bottom-up construction of an autonomously growing, self-reproducing cell represents a great challenge for synthetic biology. Synthetic cellular systems are envisioned as out-of-equilibrium enzymatic networks encompassed by a selectively open phospholipid bilayer allowing for protein-mediated communication; internal metabolite recycling is another key aspect of a sustainable metabolism. Importantly, gaining tight control over the external medium is essential to avoid thermodynamic equilibrium due to nutrient depletion or waste buildup in a closed compartment (e.g., a test tube). Implementing a sustainable strategy for phospholipid biosynthesis is key to expanding the cellular boundaries. However, phospholipid biosynthesis is currently limited by substrate availability, e.g., of glycerol 3-phosphate, the essential core of phospholipid headgroups. Here, we reconstitute an enzymatic network for sustainable glycerol 3-phosphate synthesis inside large unilamellar vesicles. We exploit the Escherichia coli glycerol kinase GlpK to synthesize glycerol 3-phosphate from externally supplied glycerol. We fuel phospholipid headgroup formation by sustainable l-arginine breakdown. In addition, we design and characterize a dynamic dialysis setup optimized for synthetic cells, which is used to control the external medium composition and to achieve sustainable glycerol 3-phosphate synthesis.

Core-shell microparticles: From rational engineering to diverse applications

Core-shell microparticles, composed of solid, liquid, or gas bubbles surrounded by a protective shell, are gaining considerable attention as intelligent and versatile carriers that show great potential in biomedical fields. In this review, an overview is given of recent developments in design and applications of biodegradable core-shell systems. Several emerging methodologies including self-assembly, gas-shearing, and coaxial electrospray are discussed and microfluidics technology is emphasized in detail. Furthermore, the characteristics of core-shell microparticles in artificial cells, drug release and cell culture applications are discussed and the superiority of these advanced multi-core microparticles for the generation of artificial cells is highlighted. Finally, the respective developing orientations and limitations inherent to these systems are addressed. It is hoped that this review can inspire researchers to propel the development of this field with new ideas.

Demolish and Rebuild: Controlling Lipid Self-Assembly toward Triggered Release and Artificial Cells

The ability to modulate the structures of lipid membranes, predicated on our nuanced understanding of the properties that drive and alter lipid self-assembly, has opened up many exciting biological applications. In this Perspective, we focus on two endeavors in which the same principles are invoked to achieve completely opposite results. On one hand, controlled liposome decomposition enables triggered release of encapsulated cargo through the development of synthetic lipid switches that perturb lipid packing in the presence of disease-associated stimuli. In particular, recent approaches have utilized artificial lipid switches designed to undergo major conformational changes in response to a range of target conditions. On the other end of the spectrum, the ability to drive the in situ formation of lipid bilayer membranes from soluble precursors is an important component in the establishment of artificial cells. This work has culminated in chemoenzymatic strategies that enable lipid manufacturing from simple components. Herein, we describe recent advancements in these two unique undertakings that are linked by their reliance on common principles of lipid self-assembly.

Toward synthetic life: Biomimetic synthetic cell communication

Engineering synthetic minimal cells provide a controllable chassis for studying the biochemical principles of natural life, increasing our understanding of complex biological processes. Recently, synthetic cell engineering has enabled communication between both natural live cells and other synthetic cells. A system such as these enable studying interactions between populations of cells, both natural and artificial, and engineering small molecule cell communication protocols for a variety of basic research and practical applications. In this review, we summarize recent progress in engineering communication between synthetic and natural cells, and we speculate about the possible future directions of this work.

Towards a synthetic cell cycle

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

Synthesis of lipid membranes for artificial cells

A major goal of synthetic biology is to understand the transition between non-living matter and life. The bottom-up development of an artificial cell would provide a minimal system with which to study the border between chemistry and biology. So far, a fully synthetic cell has remained elusive, but chemists are progressing towards this goal by reconstructing cellular subsystems. Cell boundaries, likely in the form of lipid membranes, were necessary for the emergence of life. In addition to providing a protective barrier between cellular cargo and the external environment, lipid compartments maintain homeostasis with other subsystems to regulate cellular processes. In this Review, we examine different chemical approaches to making cell-mimetic compartments. Synthetic strategies to drive membrane formation and function, including bioorthogonal ligations, dissipative self-assembly and reconstitution of biochemical pathways, are discussed. Chemical strategies aim to recreate the interactions between lipid membranes, the external environment and internal biomolecules, and will clarify our understanding of life at the interface of chemistry and biology.

Expression of Fatty Acyl-CoA Ligase Drives One-Pot De Novo Synthesis of Membrane-Bound Vesicles in a Cell-Free Transcription-Translation System

Despite the central importance of lipid membranes in cellular organization, it is challenging to reconstitute their formation de novo from minimal chemical and biological elements. Here, we describe a chemoenzymatic route to membrane-forming noncanonical phospholipids in which cysteine-modified lysolipids undergo spontaneous coupling with fatty acyl-CoA thioesters generated enzymatically by a fatty acyl-CoA ligase. Due to the high efficiency of the reaction, we were able to optimize phospholipid formation in a cell-free transcription-translation (TX-TL) system. Combining DNA encoding the fatty acyl-CoA ligase with suitable lipid precursors enabled one-pot de novo synthesis of membrane-bound vesicles. Noncanonical sphingolipid synthesis was also possible by using a cysteine-modified lysosphingomyelin as a precursor. When the sphingomyelin-interacting protein lysenin was coexpressed alongside the acyl-CoA ligase, the in situ assembled membranes were spontaneously decorated with protein. Our strategy of coupling gene expression with membrane lipid synthesis in a one-pot fashion could facilitate the generation of proteoliposomes and brings us closer to the bottom-up generation of synthetic cells using recombinant synthetic biology platforms.

Building protein networks in synthetic systems from the bottom-up

The recent development of synthetic biology has expanded the capability to design and construct protein networks outside of living cells from the bottom-up. The new capability has enabled us to assemble protein networks for the basic study of cellular pathways, expression of proteins outside cells, and building tissue materials. Furthermore, the integration of natural and synthetic protein networks has enabled new functions of synthetic or artificial cells. Here, we review the underlying technologies for assembling protein networks in liposomes, water-in-oil droplets, and biomaterials from the bottom-up. We cover the recent applications of protein networks in biological transduction pathways, energy self-supplying systems, cellular environmental sensors, and cell-free protein scaffolds. We also review new technologies for assembling protein networks, including multiprotein purification methods, high-throughput assay screen platforms, and controllable fusion of liposomes. Finally, we present existing challenges towards building protein networks that rival the complexity and dynamic response akin to natural systems. This review addresses the gap in our understanding of synthetic and natural protein networks. It presents a vision towards developing smart and resilient protein networks for various biomedical applications.

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.

Membrane Mimetic Chemistry in Artificial Cells

Lipid membranes in cells are fluid structures that undergo constant synthesis, remodeling, fission, and fusion. The dynamic nature of lipid membranes enables their use as adaptive compartments, making them indispensable for all life on Earth. Efforts to create life-like artificial cells will likely involve mimicking the structure and function of lipid membranes to recapitulate fundamental cellular processes such as growth and division. As such, there is considerable interest in chemistry that mimics the functional properties of membranes, with the express intent of recapitulating biological phenomena. We suggest expanding the definition of membrane mimetic chemistry to capture these efforts. In this Perspective, we discuss how membrane mimetic chemistry serves the development of artificial cells. By leveraging recent advances in chemical biology and systems chemistry, we have an opportunity to use simplified chemical and biochemical systems to mimic the remarkable properties of living membranes.

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.

Unraveling topology-induced shape transformations in dendrimersomes

The vital functions of cell membranes require their ability to quickly change shape to perform complex tasks such as motion, division, endocytosis, and apoptosis. Membrane curvature in cells is modulated by very complex processes such as changes in lipid composition, the oligomerization of curvature-scaffolding proteins, and the reversible insertion of protein regions that act like wedges in the membrane. But, could much simpler mechanisms support membrane shape transformation? In this work, we demonstrate how the change of amphiphile topology in the bilayer can drive shape transformations of cell membrane models. To tackle this, we have designed and synthesized new types of amphiphiles-Janus dendrimers-that self-assemble into uni-, multilamellar, or smectic-ordered vesicles, named dendrimersomes. We synthesized Janus dendrimers containing a photo-labile bond that upon UV-Vis irradiation cleavage lose a part of the hydrophilic dendron. This leads to a change from a cylindrically to a wedge-shaped amphiphile. The high mobility of these dendrimers allows for the concentration of the wedge-shaped amphiphiles and the generation of transmembrane asymmetries. The concentration of the wedges and their rate of segregation allowed control of the budding and generation of structures such as tubules and high genus vesicles.

In situ formation of a biomimetic lipid membrane triggered by an aggregation-enhanced photoligation chemistry

Nature or synthetic systems that can self-assemble into biomimetic membranes and form compartments in aqueous solution have received extensive attention. However, these systems often have the problems of requiring complex processes or lacking of control in simulating lipid synthesis and membrane formation of cells. This paper demonstrates a conceptually new strategy that uses a photoligation chemistry to convert nonmembrane molecules to yield liposomes. Lysosphingomyelin (Lyso) and 2-nitrobenzyl alcohol derivatives (NBs) are used as precursors and the amphiphilic character of Lyso promotes the formation of mixed aggregates with NBs, bringing the lipid precursors into close proximity. Light irradiation triggers the conversion of NBs into reactive aldehyde intermediates, and the preassembly facilitates the efficient and specific ligation between aldehyde and Lyso amine over other biomolecules, thereby accelerating the synthesis of phospholipids and forming membrane compartments similar to natural lipids. The light-controllable transformation represents the use of an external energy stimulus to form a biomimetic phospholipid membrane, which has a wide range of applications in medicinal chemistry, synthetic biological and abiogenesis.

Engineering spatiotemporal organization and dynamics in synthetic cells

Constructing synthetic cells has recently become an appealing area of research. Decades of research in biochemistry and cell biology have amassed detailed part lists of components involved in various cellular processes. Nevertheless, recreating any cellular process in vitro in cell-sized compartments remains ambitious and challenging. Two broad features or principles are key to the development of synthetic cells-compartmentalization and self-organization/spatiotemporal dynamics. In this review article, we discuss the current state of the art and research trends in the engineering of synthetic cell membranes, development of internal compartmentalization, reconstitution of self-organizing dynamics, and integration of activities across scales of space and time. We also identify some research areas that could play a major role in advancing the impact and utility of engineered synthetic cells. This article is categorized under: Biology-Inspired Nanomaterials > Lipid-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

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

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

Cell mimicry as a bottom-up strategy for hierarchical engineering of nature-inspired entities

Artificial biology is an emerging concept that aims to design and engineer the structure and function of natural cells, organelles, or biomolecules with a combination of biological and abiotic building blocks. Cell mimicry focuses on concepts that have the potential to be integrated with mammalian cells and tissue. In this feature article, we will emphasize the advancements in the past 3-4 years (2017-present) that are dedicated to artificial enzymes, artificial organelles, and artificial mammalian cells. Each aspect will be briefly introduced, followed by highlighting efforts that considered key properties of the different mimics. Finally, the current challenges and opportunities will be outlined. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Dynamics of the vesicles composed of fatty acids and other amphiphile mixtures: unveiling the role of fatty acids as a model protocell membrane

Fundamental research at the interface of chemistry and biology has the potential to shine light on the question of how living cells can be synthesized from inanimate matter thereby providing plausible pathways for the emergence of cellular life. Compartmentalization of different biochemical reactions within a membrane bound water environment is considered an essential first step in any origin of life pathway. It has been suggested that fatty acid-based vesicles can be considered a model protocell having the potential for change via Darwinian evolution. As such, protocell models have the potential to assist in furthering our understanding of the origin of life in the laboratory. Fatty acids, both by themselves and in mixtures with other amphiphiles, can form different self-assembled structures depending on their surroundings. Recent studies of fatty acid-based membranes have suggested likely pathways of protocell growth, division and membrane permeabilisation for the transport of different nutrients, such as nucleotides across the membrane. In this review, different dynamic processes related to the growth and division of the protocell membrane are discussed and possible pathways for transition of the protocell to the modern cell are explored. These areas of research may lead to a better understanding of the synthesis of artificial cell-like entities and thus herald the possibility of creating new form of life distinct from existing biology. Graphical Abstract Table of Content (TOC) only.

Monodisperse Liposomes with Femtoliter Volume Enable Quantitative Digital Bioassays of Membrane Transporters and Cell-Free Gene Expression

Digital bioassays have emerged as a new category of bioanalysis. However, digital bioassays for membrane transporter proteins have not been well established yet despite high demands in molecular physiology and molecular pharmacology due to the lack of biologically functional monodisperse liposomes with femtoliter volumes. Here, we established a simple and robust method to produce femtoliter-sized liposomes (femto-liposomes). We prepared 10⁶ monodispersed water-in-oil droplets stabilized by a lipid monolayer using a polyethylene glycol-coated femtoliter reactor array device. Droplets were subjected to the optimized emulsion transfer process for femto-liposome production. Liposomes were monodispersed (coefficient of variation = 5-15%) and had suitable diameter (0.6-5.3 μm) and uniform volumes of subfemtoliter or a few femtoliters; thus, they were termed uniform femto-liposomes. The unilamellarity of uniform femto-liposomes allowed quantitative single-molecule analysis of passive and active transporter proteins: α-hemolysin and F_oF₁-ATPase. Digital gene expression in uniform femto-liposomes (cell-free transcription and translation from single DNA molecules) was also demonstrated, showing the versatility of digital assays for membrane transporter proteins and cell-free synthetic biology.

Genetically controlled membrane synthesis in liposomes

Lipid membranes, nucleic acids, proteins, and metabolism are essential for modern cellular life. Synthetic systems emulating the fundamental properties of living cells must therefore be built upon these functional elements. In this work, phospholipid-producing enzymes encoded in a synthetic minigenome are cell-free expressed within liposome compartments. The de novo synthesized metabolic pathway converts precursors into a variety of lipids, including the constituents of the parental liposome. Balanced production of phosphatidylethanolamine and phosphatidylglycerol is realized, owing to transcriptional regulation of the activity of specific genes combined with a metabolic feedback mechanism. Fluorescence-based methods are developed to image the synthesis and membrane incorporation of phosphatidylserine at the single liposome level. Our results provide experimental evidence for DNA-programmed membrane synthesis in a minimal cell model. Strategies are discussed to alleviate current limitations toward effective liposome growth and self-reproduction.

The New Age of Cell-Free Biology

The cell-free molecular synthesis of biochemical systems is a rapidly growing field of research. Advances in the Human Genome Project, DNA synthesis, and other technologies have allowed the in vitro construction of biochemical systems, termed cell-free biology, to emerge as an exciting domain of bioengineering. Cell-free biology ranges from the molecular to the cell-population scales, using an ever-expanding variety of experimental platforms and toolboxes. In this review, we discuss the ongoing efforts undertaken in the three major classes of cell-free biology methodologies, namely protein-based, nucleic acids–based, and cell-free transcription–translation systems, and provide our perspectives on the current challenges as well as the major goals in each of the subfields.

Microfluidics for Biosynthesizing: from Droplets and Vesicles to Artificial Cells

Fabrication of artificial biomimetic materials has attracted abundant attention. As one of the subcategories of biomimetic materials, artificial cells are highly significant for multiple disciplines and their synthesis has been intensively pursued. In order to manufacture robust "alive" artificial cells with high throughput, easy operation, and precise control, flexible microfluidic techniques are widely utilized. Herein, recent advances in microfluidic-based methods for the synthesis of droplets, vesicles, and artificial cells are summarized. First, the advances of droplet fabrication and manipulation on the T-junction, flow-focusing, and coflowing microfluidic devices are discussed. Then, the formation of unicompartmental and multicompartmental vesicles based on microfluidics are summarized. Furthermore, the engineering of droplet-based and vesicle-based artificial cells by microfluidics is also reviewed. Moreover, the artificial cells applied for imitating cell behavior and acting as bioreactors for synthetic biology are highlighted. Finally, the current challenges and future trends in microfluidic-based artificial cells are discussed. This review should be helpful for researchers in the fields of microfluidics, biomaterial fabrication, and synthetic biology.

Light-responsive polymersomes with a charge-switch for targeted drug delivery

Unlike the traditional block amphiphilic polymersomes, we herein report a lipid-like amphiphilic polymer that self-assembles into photo-responsive polymersomes for drug delivery. The introduction of a quaternary ammonium moiety not only provides a hydrophilic segment of the polymersomes, but also enables electrostatic adsorption with folic acid, thus achieving the targeting of cancer cells with overexpression of folate receptor. Upon light irradiation, the photocleavage reaction of the o-nitrobenzyl moiety disintegrates polymersomes by changing the polymer structure from cationic amphiphilic state to zwitterionic hydrophilic state, thus realizing photo-triggered drug release. The data showed that anticancer drugs (doxorubicin hydrochloride, DOX·HCl) can be loaded into the hydrophilic cavity of polymersomes and controllably released by photo-induced disintegration of polymersomes. Cellular assay showed that the active targeting of folic acid and photo-triggered release endowed the DOX-loaded polymersomes with a higher cytotoxicity to HeLa cells. Such cationic polymersomes provide a novel strategy for designing effective and intelligent drug carriers, and have potential application as a novel integrated platform for targeted drug delivery.