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

Protocells programmed through artificial reaction networks

As the smallest unit of life, cells attract interest due to their structural complexity and functional reliability. Protocells assembled by inanimate components are created as an artificial entity to mimic the structure and some essential properties of a natural cell, and artificial reaction networks are used to program the functions of protocells. Although the bottom-up construction of a protocell that can be considered truly ‘alive’ is still an ambitious goal, these man-made constructs with a certain degree of ‘liveness’ can offer effective tools to understand fundamental processes of cellular life, and have paved the new way for bionic applications. In this review, we highlight both the milestones and recent progress of protocells programmed by artificial reaction networks, including genetic circuits, enzyme-assisted non-genetic circuits, prebiotic mimicking reaction networks, and DNA dynamic circuits. Challenges and opportunities have also been discussed.

In this review, the milestones and recent progress of protocells programmed by various types of artificial reaction networks are highlighted.

Self-Propulsion Strategies for Artificial Cell-Like Compartments

Reconstitution of life-like properties in artificial cells is a current research frontier in synthetic biology. Mimicking metabolism, growth, and sensing are active areas of investigation; however, achieving motility and directional taxis are also challenging in the context of artificial cells. To tackle this problem, recent progress has been made that leverages the tools of active matter physics in synthetic biology. This review surveys the most significant achievements in designing motile cell-like compartments. In this context, strategies for self-propulsion are summarized, including, compartmentalization of catalytically active particles, phoretic propulsion of vesicles and emulsion droplet motion driven by Marangoni flows. This work showcases how the realization of motile protocells may impact biomedical engineering while also aiming at answering fundamental questions in locomotion of prebiotic cells.

Toward long-lasting artificial cells that better mimic natural living cells

Chemical communication is ubiquitous in biology, and so efforts in building convincing cellular mimics must consider how cells behave on a population level. Simple model systems have been built in the laboratory that show communication between different artificial cells and artificial cells with natural, living cells. Examples include artificial cells that depend on purely abiological components and artificial cells built from biological components and are driven by biological mechanisms. However, an artificial cell solely built to communicate chemically without carrying the machinery needed for self-preservation cannot remain active for long periods of time. What is needed is to begin integrating the pathways required for chemical communication with metabolic-like chemistry so that robust artificial systems can be built that better inform biology and aid in the generation of new technologies.

Artificial cells containing sustainable energy conversion engines

Living cells naturally maintain a variety of metabolic reactions via energy conversion mechanisms that are coupled to proton transfer across cell membranes, thereby producing energy-rich compounds. Until now, researchers have been unable to maintain continuous biochemical reactions in artificially engineered cells, mainly due to the lack of mechanisms that generate energy-rich resources, such as adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). If these metabolic activities in artificial cells are to be sustained, reliable energy transduction strategies must be realized. In this perspective, this article discusses the development of an artificially engineered cell containing a sustainable energy conversion process.

De novo synthesized Min proteins drive oscillatory liposome deformation and regulate FtsA-FtsZ cytoskeletal patterns

The Min biochemical network regulates bacterial cell division and is a prototypical example of self-organizing molecular systems. Cell-free assays relying on purified proteins have shown that MinE and MinD self-organize into surface waves and oscillatory patterns. In the context of developing a synthetic cell from elementary biological modules, harnessing Min oscillations might allow us to implement higher-order cellular functions. To convey hereditary information, the Min system must be encoded in a DNA molecule that can be copied, transcribed, and translated. Here, the MinD and MinE proteins are synthesized de novo from their genes inside liposomes. Dynamic protein patterns and accompanying liposome shape deformation are observed. When integrated with the cytoskeletal proteins FtsA and FtsZ, the synthetic Min system is able to dynamically regulate FtsZ patterns. By enabling genetic control over Min protein self-organization and membrane remodeling, our methodology offers unique opportunities towards directed evolution of bacterial division processes in vitro.

The Min biochemical network regulates bacterial cell division and is a prototypical example of self-organizing molecular systems. Here authors synthesize Min proteins from their genes inside liposomes and observe dynamic protein patterns and liposome shape deformation.

A synthetic metabolic network for physicochemical homeostasis

One of the grand challenges in chemistry is the construction of functional out-of-equilibrium networks, which are typical of living cells. Building such a system from molecular components requires control over the formation and degradation of the interacting chemicals and homeostasis of the internal physical-chemical conditions. The provision and consumption of ATP lies at the heart of this challenge. Here we report the in vitro construction of a pathway in vesicles for sustained ATP production that is maintained away from equilibrium by control of energy dissipation. We maintain a constant level of ATP with varying load on the system. The pathway enables us to control the transmembrane fluxes of osmolytes and to demonstrate basic physicochemical homeostasis. Our work demonstrates metabolic energy conservation and cell volume regulatory mechanisms in a cell-like system at a level of complexity minimally needed for life.

Functional out-of-equilibrium networks are typical of living cells. Here the authors report the construction of a sustained ATP production system in vesicles with controlled energy dissipation and physicochemical homeostasis.

Membrane Tension-Mediated Growth of Liposomes

Recent years have seen a tremendous interest in the bottom-up reconstitution of minimal biomolecular systems, with the ultimate aim of creating an autonomous synthetic cell. One of the universal features of living systems is cell growth, where the cell membrane expands through the incorporation of newly synthesized lipid molecules. Here, the gradual tension-mediated growth of cell-sized (≈10 µm) giant unilamellar vesicles (GUVs) is demonstrated, to which nanometer-sized (≈30 nm) small unilamellar vesicles (SUVs) are provided, that act as a lipid source. By putting tension on the GUV membranes through a transmembrane osmotic pressure, SUV-GUV fusion events are promoted and substantial growth of the GUV is caused, even up to doubling its volume. Thus, experimental evidence is provided that membrane tension alone is sufficient to bring about membrane fusion and growth is demonstrated for both pure phospholipid liposomes and for hybrid vesicles with a mixture of phospholipids and fatty acids. The results show that growth of liposomes can be realized in a protein-free minimal system, which may find useful applications in achieving autonomous synthetic cells that are capable of undergoing a continuous growth-division cycle.

Cell Fuelling and Metabolic Energy Conservation in Synthetic Cells

We are aiming for a blue print for synthesizing (moderately complex) subcellular systems from molecular components and ultimately for constructing life. However, without comprehensive instructions and design principles, we rely on simple reaction routes to operate the essential functions of life. The first forms of synthetic life will not make every building block for polymers de novo according to complex pathways, rather they will be fed with amino acids, fatty acids and nucleotides. Controlled energy supply is crucial for any synthetic cell, no matter how complex. Herein, we describe the simplest pathways for the efficient generation of ATP and electrochemical ion gradients. We have estimated the demand for ATP by polymer synthesis and maintenance processes in small cell-like systems, and we describe circuits to control the need for ATP. We also present fluorescence-based sensors for pH, ionic strength, excluded volume, ATP/ADP, and viscosity, which allow the major physicochemical conditions inside cells to be monitored and tuned.

Stabilization of All-in-Water Emulsions To Form Capsules as Artificial Cells

Building artificial cells through a bottom-up approach is a remarkable challenge that would be of interest for our understanding of the origin of life, research into the minimal conditions required for life, the formation of bioreactors, and for industrial applications. To date, capsules such as liposomes, including polymersomes, are widely used, but the low membrane permeability and method to encapsulate biological materials within these structures hamper their use. By contrast, all-in-water emulsion droplets, including coacervate droplets, are promising compartments, mainly because they can spontaneously sequester chemicals. However, they lack a membrane necessary to control exchange between the inner and outer media. Moreover, droplets tend to coalesce with time, yielding macroscopic phase separation that is deleterious for any use as artificial cells. Recent advances, which are reviewed herein, have shown that such droplets can be stabilized by using lipid membranes, liposomes, polymers, proteins, and particles, and thus, preventing coalescence. Finally, different strategies that could allow the future development of artificial cells from these stabilized all-in-water emulsion droplets are discussed.

Continuous expansion of a synthetic minimal cellular membrane

A critical aspect of a synthetic minimal cell is expansion of the surrounding boundary layer. This layer should consist of phospholipids (mimics) as these molecules assemble into a bilayer, creating a functional barrier with specific phospholipid species that are essential for membrane related processes. As a first step towards synthetic cells, an in vitro phospholipid biosynthesis pathway has been constructed that utilizes fatty acids as precursors to produce a wide variety of phospholipid species, thereby driving membrane growth. This now needs to be developed further into a sustainable expanding system, meanwhile keeping simplicity in mind. The non-enzymatic synthesis of phospholipid-like molecules forms a realistic alternative for natural enzymatic-based pathways, that nowadays can even support functional membrane proteins. Eventually, coupling to in vitro transcription/translation is required, for which efficient mechanisms of insertion and folding of the involved membrane proteins need to be developed. Such an integrated system will form a suitable foundation of a synthetic minimal cell that eventually can be coupled to other cellular processes such as division.

Cell-Free Protein Synthesis: Chassis toward the Minimal Cell

The quest for a minimal cell not only sheds light on the fundamental principles of life but also brings great advances in related applied fields such as general biotechnology. Minimal cell projects came from the study of a plausible route to the origin of life. Later on, research extended and also referred to the construction of artificial cells, or even more broadly, as in vitro synthetic biology. The cell-free protein synthesis (CFPS) techniques harness the central cellular activity of transcription/translation in an open environment, providing the framework for multiple cellular processes assembling. Therefore, CFPS systems have become the first choice in the construction of the minimal cell. In this review, we focus on the recent advances in the quantitative analysis of CFPS and on its advantage for addressing the bottom-up assembly of a minimal cell and illustrate the importance of systemic chassis behavior, such as stochasticity under a compartmentalized micro-environment.

Fluorescent Self-Healing Carbon Dot/Polymer Gels

Multicolor, fluorescent self-healing gels were constructed through reacting carbon dots produced from different aldehyde precursors with branched polyethylenimine. The self-healing gels were formed through Schiff base reaction between the aldehyde units displayed upon the carbon dots' surface and primary amine residues within the polyethylenimine network, generating imine bonds. The dynamic covalent imine bonds between the carbon dots and polymeric matrix endowed the gels with both excellent self-healing properties as well as high mechanical strength. Moreover, the viscoelastic properties of the gels could be intimately modulated by controlling the ratio between the carbon dots and polymer. The distinct fluorescence emissions of the gels, originating from the specific carbon dot constituents, were employed for fabrication of light emitters at different colors, particularly generating white light.