Preparation of a Millimeter-Sized Supergiant Liposome That Allows for Efficient, Eukaryotic Cell-Free Translation in the Interior by Spontaneous Emulsion Transfer

A Self-Regenerating Artificial Cell, that is One Step Closer to Living Cells: Challenges and Perspectives

Controllable, self-regenerating artificial cells (SRACs) can be a vital advancement in the field of synthetic biology, which seeks to create living cells by recombining various biological molecules in the lab. This represents, more importantly, the first step on a long journey toward creating reproductive cells from rather fragmentary biochemical mimics. However, it is still a difficult task to replicate the complex processes involved in cell regeneration, such as genetic material replication and cell membrane division, in artificially created spaces. This review highlights recent advances in the field of controllable, SRACs and the strategies to achieve the goal of creating such cells. Self-regenerating cells start by replicating DNA and transferring it to a location where proteins can be synthesized. Functional but essential proteins must be synthesized for sustained energy generation and survival needs and function in the same liposomal space. Finally, self-division and repeated cycling lead to autonomous, self-regenerating cells. The pursuit of controllable, SRACs will enable authors to make bold advances in understanding life at the cellular level, ultimately providing an opportunity to use this knowledge to understand the nature of life.

Simple Method for the Creation of a Bacteria-Sized Unilamellar Liposome with Different Proteins Localized to the Respective Sides of the Membrane

Artificial cells are membrane vesicles mimicking cellular functions. To date, giant unilamellar vesicles made from a single lipid membrane with a diameter of 10 μm or more have been used to create artificial cells. However, the creation of artificial cells that mimic the membrane structure and size of bacteria has been limited due to technical restrictions of conventional liposome preparation methods. Here, we created bacteria-sized large unilamellar vesicles (LUVs) with proteins localized asymmetrically to the lipid bilayer. Liposomes containing benzylguanine-modified phospholipids were prepared by combining the conventional water-in-oil emulsion method and the extruder method, and green fluorescent protein fused with SNAP-tag was localized to the inner leaflet of the lipid bilayer. Biotinylated lipid molecules were then inserted externally, and the outer leaflet was modified with streptavidin. The resulting liposomes had a size distribution in the range of 500-2000 nm with a peak at 841 nm (the coefficient of variation was 10.3%), which was similar to that of spherical bacterial cells. Fluorescence microscopy, quantitative evaluation using flow cytometry, and western blotting proved the intended localization of different proteins on the lipid membrane. Cryogenic electron microscopy and quantitative evaluation by α-hemolysin insertion revealed that most of the created liposomes were unilamellar. Our simple method for the preparation of bacteria-sized LUVs with asymmetrically localized proteins will contribute to the creation of artificial bacterial cells for investigating functions and the significance of their surface structure and size.

Coupled in vitro transcription/translation based on wheat germ extract for efficient expression from PCR-generated templates in short-time batch reactions

We successfully constructed a coupled in vitro transcription/translation (cIVTT) system based on wheat germ extract (WGE) for efficient expression from PCR-generated DNA templates in short-time (∼3-h) batch reactions. The productivity of this system under optimized conditions was 85 μg (2.8 nmol) per 1 mL of reaction solution (corresponding to 425 μg per 1 mL of WGE), which was about 9-fold higher than that by the conventional batch method using mRNA as a template. The DNA template concentration required for efficient cIVTT was as low as 2.5 nM, which is much lower than those required for other eukaryotic cIVTT systems to maximize their productivity (30-50 nM). The productivity of the present system with a 2.5 nM template was 80-fold and 4-fold higher than that of a commercially available WGE-based cIVTT system with a 2.5 nM and a 40 nM template, respectively. In addition, the present system functioned well in a liposome (i.e., in an artificial cell) without a loss of productivity. Given that WGE-based systems have the advantage of being suitable for the expression of a broad range of proteins, the present cIVTT system is expected to be widely used in future cell-free synthetic biology.

Controlled metabolic cascades for protein synthesis in an artificial cell

In recent years, researchers have been pursuing a method to design and to construct life forms from scratch - in other words, to create artificial cells. In many studies, artificial cellular membranes have been successfully fabricated, allowing the research field to grow by leaps and bounds. Moreover, in addition to lipid bilayer membranes, proteins are essential factors required to construct any cellular metabolic reaction; for that reason, different cell-free expression systems under various conditions to achieve the goal of controlling the synthetic cascades of proteins in a confined area have been reported. Thus, in this review, we will discuss recent issues and strategies, enabling to control protein synthesis cascades that are being used, particularly in research on artificial cells.

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.

A Detailed Protocol for Preparing Millimeter-sized Supergiant Liposomes that Permit Efficient Eukaryotic Cell-free Translation in the Interior

Liposomes have been used as a pseudo cell membrane for encapsulating biomolecules and creating an artificial cell in the interior where biochemical reactions can occur. Among the several methods used to prepare biomolecule-encapsulating liposomes, the spontaneous emulsion transfer method is superior to others in that it allows us to readily prepare relatively large liposomes whose sizes are controlled (from micrometer- to millimeter-sized liposomes) without special equipment. However, conventional protocols for this method require liposomes to contain a considerably high concentration of sucrose (high-density solute), which severely inhibits gene expression, one of the most important biochemical reactions. Thus, we optimized the preparation conditions to develop a wheat germ extract (WGE)-based protocol that requires a much lower concentration of sucrose and has almost no effect on eukaryotic cell-free translation. Our protocol allows us to successfully prepare millimeter-sized, moderately stable, WGE-encapsulating liposomes in which WGE translation takes place efficiently. Since a broad range of genes derived from various types of organisms can be efficiently translated in a WGE-based translation system, liposomes prepared using our protocol would be useful as a versatile research tool for artificial cells.