Cell-free Protein Synthesis Platform Research Articles

Cell‐Free Protein Synthesis: A Cell‐Free Platform Based on Nisin Biosynthesis for Discovering Novel Lanthipeptides and Guiding their Overproduction In Vivo (Adv. Sci. 17/2020)

In article number 2001616, Tiangang Liu and co-workers develop a cell-free protein synthesis (CFPS) platform that enables rapid discovery of novel lanthipeptides and guides lanthipeptide overproduction in vivo. This study extends the use of cell-free systems to address the issue facing lanthipeptide research, while providing the possibility for developing CFPS platforms for other ribosomally synthesized and post-translationally modified peptides studies.

A Cell-Free Platform Based on Nisin Biosynthesis for Discovering Novel Lanthipeptides and Guiding their Overproduction In Vivo.

Lanthipeptides have extensive therapeutic and industrial applications. However, because many are bactericidal, traditional in vivo platforms are limited in their capacity to discover and mass produce novel lanthipeptides as bacterial organisms are often critical components in these systems. Herein, the development of a cell‐free protein synthesis (CFPS) platform that enables rapid genome mining, screening, and guided overproduction of lanthipeptides in vivo is described. For proof‐of‐concept studies, a type I lanthipeptide, nisin, is selected. Four novel lanthipeptides with antibacterial activity are identified among all nisin analogs in the National Center for Biotechnology Information (NCBI) database in a single day. Further, the CFPS platform is coupled with a screening assay for anti‐gram‐negative bacteria growth, resulting in the identification of a potent nisin mutant, M5. The titers of nisin and the nisin analog are found to be improved with CFPS platform guidance. Owing to the similarities in biosynthesis, the CFPS platform is broadly applicable to other lanthipeptides, thereby providing a universal method for lanthipeptide discovery and overproduction.

Establishing a Eukaryotic Pichia pastoris Cell-Free Protein Synthesis System.

In recent years, cell-free protein synthesis (CFPS) systems have been used to synthesize proteins, prototype genetic elements, manufacture chemicals, and diagnose diseases. These exciting, novel applications lead to a new wave of interest in the development of new CFPS systems that are derived from prokaryotic and eukaryotic organisms. The eukaryotic Pichia pastoris is emerging as a robust chassis host for recombinant protein production. To expand the current CFPS repertoire, we report here the development and optimization of a eukaryotic CFPS system, which is derived from a protease-deficient strain P. pastoris SMD1163. By developing a simple crude extract preparation protocol and optimizing CFPS reaction conditions, we were able to achieve superfolder green fluorescent protein (sfGFP) yields of 50.16 ± 7.49 μg/ml in 5 h batch reactions. Our newly developed P. pastoris CFPS system fits to the range of the productivity achieved by other eukaryotic CFPS platforms, normally ranging from several to tens of micrograms protein per milliliter in batch mode reactions. Looking forward, we believe that our P. pastoris CFPS system will not only expand the CFPS toolbox for synthetic biology applications, but also provide a novel platform for cost-effective, high-yielding production of complex proteins that need post-translational modification and functionalization.

Redesigned Upstream Processing Enables a 24‐hour Workflow from E. coli Cells to Cell‐free Protein Synthesis

Cell‐free protein synthesis (CFPS) platforms have undergone numerous workflow improvements to enable diverse applications in research, biomanufacturing, point‐of‐care detection, therapeutics, and education using affordable laboratory equipment and reagents. The Escherichia coli cell extract‐based platform, being one of the most affordable and versatile CFPS platforms, has been broadly adopted. In spite of the promise of simplicity, the cell‐free platform remains technically nuanced, posing challenges to reproducible implementation and broad adoption. Additionally, while the CFPS reaction itself can be implemented on‐demand, the upstream processing of cells to generate crude cell lysate remains time‐intensive, representing one of the largest sources of cost associated with the biotechnology. To circumvent the lengthy and tedious upstream workflow, we have redesigned the processes by developing a long‐lasting autoinduction media formulation for cell‐free that obviates human intervention between inoculation and harvest. Cell‐free autoinduction (CFAI) media supports these advantages through the production of highly robust cell extracts from high cell density cultures nearing stationary phase of growth. Growth of cells to high density and autoinduction of T7 RNAP expression can be achieved by incubation overnight, eliminating the need for user intervention for the entirety of the process. The total mass of cells obtained is substantially increased, which directly results in a 400% increase in total extract volume obtained compared to past workflows. Based on these advances, we outline a new upstream processing workflow allowing researchers to go from cells on a streak plate to completing CFPS reactions within 24 hours while maintaining robust reaction yields of sfGFP (>1 mg/ml). We hope this advance improves the time and cost‐efficiency for existing CFPS researchers, increase the simplicity and reproducibility, and reduce the barrier‐to‐entry for new researchers interested in implementing CFPS. Lastly, we have completed preliminary metabolomics analysis to elucidate the mechanisms by which CFAI‐grown cells are capable of producing robust extracts.Support or Funding InformationAuthors would like to acknowledge Dr. Jennifer VanderKelen and Andrea Laubscher for technical support, Wesley Kao, Nicole Gregorio, Phillip Smith, and Logan Burrington for helpful discussions. Authors also acknowledge funding support from the Bill and Linda Frost Fund, Center for Applications in Biotechnology’s Chevron Biotechnology Applied Research Endowment Grant, Cal Poly Research, Scholarly, and Creative Activities Grant Program (RSCA 2017), and the National Science Foundation (NSF‐1708919). MZL would like to acknowledge the California State University Graduate Grant.

Flexible on-demand cell-free protein synthesis platform based on a tube-in-tube reactor

A flexible on-demand cell-free protein synthesis platform using a tube-in-tube reactor is established for continuous synthesis of different protein drugs.

A Highly Productive, One-Pot Cell-Free Protein Synthesis Platform Based on Genomically Recoded Escherichia coli

The site-specific incorporation of non-canonical amino acids (ncAAs) into proteins via amber suppression provides access to novel protein properties, structures, and functions. Historically, poor protein expression yields resulting from release factor 1 (RF1) competition has limited this technology. To address this limitation, we develop a high-yield, one-pot cell-free platform for synthesizing proteins bearing ncAAs based on genomically recoded Escherichia coli lacking RF1. A key feature of this platform is the independence on the addition of purified T7 DNA-directed RNA polymerase (T7RNAP) to catalyze transcription. Extracts derived from our final strain demonstrate high productivity, synthesizing 2.67± 0.06 g/L superfolder GFP in batch mode without supplementation of purified T7RNAP. Using an optimized one-pot platform, we demonstrate multi-site incorporation of the ncAA p-acetyl-L-phenylalanine into an elastin-like polypeptide with high accuracy of incorporation and yield. Our work has implications for chemical and synthetic biology.

Abstract CT160: A Phase I, open-label, safety, pharmacokinetic, and preliminary efficacy study of STRO-002, an anti-folate receptor alpha (FolRα) antibody drug conjugate (ADC), in patients with advanced epithelial ovarian cancer (including fallopian tube or primary peritoneal cancers) and endometrial cancers

Abstract Background: Folate receptor alpha (FolRα) is overexpressed in 80% of epithelial ovarian cancer (EOC) and endometrial adenocarcinoma (EC), with minimal expression on normal tissues and is thus a relevant target for cancer therapy using antibody drug conjugates (ADCs). STRO-002 is a novel, site-specific FolRα-targeting ADC that was generated using Sutro’s XpressCF+™ cell free protein synthesis platform. STRO-002 has highly specific cytotoxic activity on cells expressing FolRα. STRO-002 demonstrated reproducible and significant therapeutic efficacy in multiple preclinical models in vivo. STRO-002 activity was evaluated in EC patient-derived xenograft (PDX) models with a range of FolRα expression. Statistically significant efficacy was observed in 60% of the FolRα-positive PDX models tested, with response appearing to positively correlate with FolRα expression levels. Toxicology studies in cynomolgus monkeys demonstrate dose-dependent and reversible neutropenia when STRO-002 is administered at up to 9 mg/kg, with no evidence of ocular toxicity (Solis et al. AACR 2019). Methods: This study is a first-in-human Phase 1, open-label, multicenter, dose escalation (Part 1) study with dose expansion (Part 2) to identify the maximum tolerated dose (MTD), recommended phase 2 doses (RP2D) and to evaluate the safety, tolerability, and preliminary anti-tumor activity of STRO-002 in adults with ovarian cancer and endometrial cancer who are refractory to, or intolerant of, therapies known to provide clinical benefit. STRO-002 is given to all patients on study via intravenous infusion on Day 1of each 21-day cycle until disease progression. Dose limiting toxicities will be assessed in the first cycle (Days 1-21) of dose escalation. Part 1 will enroll about 30 ovarian cancer patients to determine the MTD and RP2D for expansion while Part 2 will enroll 2 cohorts based on disease subtypes (ovarian or endometrial). Efficacy will be evaluated per RECIST 1.1 criteria. Key inclusion criteria include relapsed and or progressive disease, adequate bone marrow and renal function, and ability to comply with treatment, testing and pharmacokinetic (PK) schedules. Key exclusion criteria include clear cell, mucinous or sarcomatous ovarian carcinoma, prior treatment with a FolRα-targeting ADC, need for ongoing immunotherapy including systemic corticosteroids and a history of CNS involvement. Samples will be collected to assess FolRα expression levels, PK and immunogenicity. No formal statistical hypothesis testing will be conducted in this study. This study is currently open for enrollment in the US with the ClinicalTrials.gov Identifier: NCT03748186.: Citation Format: Robert Wendel Naumann, Denise S. Uyar, Russell J. Schilder, Michael A. Palumbo, Venita DeAlmeida, Shannon L. Matheny, Arturo Molina. A Phase I, open-label, safety, pharmacokinetic, and preliminary efficacy study of STRO-002, an anti-folate receptor alpha (FolRα) antibody drug conjugate (ADC), in patients with advanced epithelial ovarian cancer (including fallopian tube or primary peritoneal cancers) and endometrial cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr CT160.

Towards On-Demand E. coli-Based Cell-Free Protein Synthesis of Tissue Plasminogen Activator

Stroke is the leading cause of death with over 5 million deaths worldwide each year. About 80% of strokes are ischemic strokes caused by blood clots. Tissue plasminogen activator (tPa) is the only FDA-approved drug to treat ischemic stroke with a wholesale price over $6000. tPa is now off patent although no biosimilar has been developed. The production of tPa is complicated by the 17 disulfide bonds that exist in correctly folded tPA. Here, we present an Escherichia coli-based cell-free protein synthesis platform for tPa expression and report conditions which resulted in the production of active tPa. While the activity is below that of commercially available tPa, this work demonstrates the potential of cell-free expression systems toward the production of future biosimilars. The E. coli-based cell-free system is increasingly becoming an attractive platform for low-cost biosimilar production due to recent developments which enable production from shelf-stable lyophilized reagents, the removal of endotoxins from the reagents to prevent the risk of endotoxic shock, and rapid on-demand production in hours.

Optimizing Cell-Free Protein Synthesis for Increased Yield and Activity of Colicins.

Colicins are antimicrobial proteins produced by Escherichia coli that hold great promise as viable complements or alternatives to antibiotics. Cell-free protein synthesis (CFPS) is a useful production platform for toxic proteins because it eliminates the need to maintain cell viability, a common problem in cell-based production. Previously, we demonstrated that colicins produced by CFPS based on crude Escherichia coli lysates are effective in eradicating antibiotic-tolerant bacteria known as persisters. However, we also found that some colicins have poor solubility or low cell-killing activity. In this study, we improved the solubility of colicin M from 16% to nearly 100% by producing it in chaperone-enriched E. coli extracts, resulting in enhanced cell-killing activity. We also improved the cytotoxicity of colicin E3 by adding or co-expressing the E3 immunity protein during the CFPS reaction, suggesting that the E3 immunity protein enhances colicin E3 activity in addition to protecting the host strain. Finally, we confirmed our previous finding that active colicins can be rapidly synthesized by observing colicin E1 production over time in CFPS. Within three hours of CFPS incubation, colicin E1 reached its maximum production yield and maintained high cytotoxicity during longer incubations up to 20 h. Taken together, our findings indicate that colicin production can be easily optimized for improved solubility and activity using the CFPS platform.

A User's Guide to Cell-Free Protein Synthesis.

Cell-free protein synthesis (CFPS) is a platform technology that provides new opportunities for protein expression, metabolic engineering, therapeutic development, education, and more. The advantages of CFPS over in vivo protein expression include its open system, the elimination of reliance on living cells, and the ability to focus all system energy on production of the protein of interest. Over the last 60 years, the CFPS platform has grown and diversified greatly, and it continues to evolve today. Both new applications and new types of extracts based on a variety of organisms are current areas of development. However, new users interested in CFPS may find it challenging to implement a cell-free platform in their laboratory due to the technical and functional considerations involved in choosing and executing a platform that best suits their needs. Here we hope to reduce this barrier to implementing CFPS by clarifying the similarities and differences amongst cell-free platforms, highlighting the various applications that have been accomplished in each of them, and detailing the main methodological and instrumental requirement for their preparation. Additionally, this review will help to contextualize the landscape of work that has been done using CFPS and showcase the diversity of applications that it enables.

Escherichia coli-Based Cell-Free Protein Synthesis: Protocols for a robust, flexible, and accessible platform technology.

Over the last 50 years, Cell-Free Protein Synthesis (CFPS) has emerged as a powerful technology to harness the transcriptional and translational capacity of cells within a test tube. By obviating the need to maintain the viability of the cell, and by eliminating the cellular barrier, CFPS has been foundational to emerging applications in biomanufacturing of traditionally challenging proteins, as well as applications in rapid prototyping for metabolic engineering, and functional genomics. Our methods for implementing an E. coli-based CFPS platform allow new users to access many of these applications. Here, we describe methods to prepare extract through the use of enriched media, baffled flasks, and a reproducible method of tunable sonication-based cell lysis. This extract can then be used for protein expression capable of producing 900 µg/mL or more of super folder green fluorescent protein (sfGFP) in just 5 h from experimental setup to data analysis, given that appropriate reagent stocks have been prepared beforehand. The estimated startup cost of obtaining reagents is $4,500 which will sustain thousands of reactions at an estimated cost of $0.021 per µg of protein produced or $0.019 per µL of reaction. Additionally, the protein expression methods mirror the ease of the reaction setup seen in commercially available systems due to optimization of reagent pre-mixes, at a fraction of the cost. In order to enable the user to leverage the flexible nature of the CFPS platform for broad applications, we have identified a variety of aspects of the platform that can be tuned and optimized depending on the resources available and the protein expression outcomes desired.

Biosensor-assisted engineering of a high-yield Pichia pastoris cell-free protein synthesis platform.

Cell-free protein synthesis (CFPS) has recently undergone a resurgence partly due to the proliferation of synthetic biology. The variety of hosts used for cell-free extract production has increased, which harnesses the diversity of cellular biosynthetic, protein folding, and posttranslational modification capabilities available. Here we describe a CFPS platform derived from Pichia pastoris, a popular recombinant protein expression host both in academia and the biopharmaceutical industry. A novel ribosome biosensor was developed to optimize the cell extract harvest time. Using this biosensor, we identified a potential bottleneck in ribosome content. Therefore, we undertook strain engineering to overexpress global regulators of ribosome biogenesis to increase in vitro protein production. CFPS extracts from the strain overexpressing FHL1 had a three-fold increase in recombinant protein yield compared with those from the wild-type X33 strain. Furthermore, our novel CFPS platform can produce complex therapeutic proteins, as exemplified by the production of human serum albumin to a final yield of 48.1 μg ml -1 . Therefore, this study not only adds to the growing number of CFPS systems from diverse organisms but also provides a blueprint for rapidly engineering new strains with increased productivity in vitro that could be applied to other organisms.

Deconstructing Cell-Free Extract Preparation for in Vitro Activation of Transcriptional Genetic Circuitry.

Recent advances in cell-free gene expression (CFE) systems have enabled their use for a host of synthetic biology applications, particularly for rapid prototyping of genetic circuits and biosensors. Despite the proliferation of cell-free protein synthesis platforms, the large number of currently existing protocols for making CFE extracts muddles the collective understanding of how the extract preparation method affects its functionality. A key aspect of extract performance relevant to many applications is the activity of the native host transcriptional machinery that can mediate protein synthesis. However, protein yields from genes transcribed in vitro by the native Escherichia coli RNA polymerase are variable for different extract preparation techniques, and specifically low in some conventional crude extracts originally optimized for expression by the bacteriophage transcriptional machinery. Here, we show that cell-free expression of genes under bacterial σ70 promoters is constrained by the rate of transcription in crude extracts, and that processing the extract with a ribosomal runoff reaction and subsequent dialysis alleviates this constraint. Surprisingly, these processing steps only enhance protein synthesis in genes under native regulation, indicating that the translation rate is unaffected. We further investigate the role of other common extract preparation process variants on extract performance and demonstrate that bacterial transcription is inhibited by including glucose in the growth culture but is unaffected by flash-freezing the cell pellet prior to lysis. Our final streamlined and detailed protocol for preparing extract by sonication generates extract that facilitates expression from a diverse set of sensing modalities including protein and RNA regulators. We anticipate that this work will clarify the methodology for generating CFE extracts that are active for biosensing using native transcriptional machinery and will encourage the further proliferation of cell-free gene expression technology for new applications.

Establishing a High-Yielding Cell-Free Protein Synthesis Platform Derived from Vibrio natriegens.

A new wave of interest in cell-free protein synthesis (CFPS) systems has shown their utility for producing proteins at high titers, establishing genetic regulatory element libraries ( e.g., promoters, ribosome binding sites) in nonmodel organisms, optimizing biosynthetic pathways before implementation in cells, and sensing biomarkers for diagnostic applications. Unfortunately, most previous efforts have focused on a select few model systems, such as Escherichia coli. Broadening the spectrum of organisms used for CFPS promises to better mimic host cell processes in prototyping applications and open up new areas of research. Here, we describe the development and characterization of a facile CFPS platform based on lysates derived from the fast-growing bacterium Vibrio natriegens, which is an emerging host organism for biotechnology. We demonstrate robust preparation of highly active extracts using sonication, without specialized and costly equipment. After optimizing the extract preparation procedure and cell-free reaction conditions, we show synthesis of 1.6 ± 0.05 g/L of superfolder green fluorescent protein in batch mode CFPS, making it competitive with existing E.coli CFPS platforms. To showcase the flexibility of the system, we demonstrate that it can be lyophilized and retain biosynthesis capability, that it is capable of producing antimicrobial peptides, and that our extract preparation procedure can be coupled with the recently described Vmax Express strain in a one-pot system. Finally, to further increase system productivity, we explore a knockout library in which putative negative effectors of CFPS are genetically removed from the source strain. Our V.natriegens-derived CFPS platform is versatile and simple to prepare and use. We expect it will facilitate expansion of CFPS systems into new laboratories and fields for compelling applications in synthetic biology.

Cell-free protein synthesis from genomically recoded bacteria enables multisite incorporation of noncanonical amino acids

Cell-free protein synthesis has emerged as a powerful approach for expanding the range of genetically encoded chemistry into proteins. Unfortunately, efforts to site-specifically incorporate multiple non-canonical amino acids into proteins using crude extract-based cell-free systems have been limited by release factor 1 competition. Here we address this limitation by establishing a bacterial cell-free protein synthesis platform based on genomically recoded Escherichia coli lacking release factor 1. This platform was developed by exploiting multiplex genome engineering to enhance extract performance by functionally inactivating negative effectors. Our most productive cell extracts enabled synthesis of 1,780 ± 30 mg/L superfolder green fluorescent protein. Using an optimized platform, we demonstrated the ability to introduce 40 identical p-acetyl-l-phenylalanine residues site specifically into an elastin-like polypeptide with high accuracy of incorporation ( ≥ 98%) and yield (96 ± 3 mg/L). We expect this cell-free platform to facilitate fundamental understanding and enable manufacturing paradigms for proteins with new and diverse chemistries.

Development of a Pseudomonas putida cell-free protein synthesis platform for rapid screening of gene regulatory elements.

Cell-free protein synthesis (CFPS) systems enable the production of protein without the use of living, intact cells. An emerging area of interest is to use CFPS systems to characterize individual elements for genetic programs [e.g. promoters, ribosome binding sites (RBS)]. To enable this research area, robust CFPS systems must be developed from new chassis organisms. One such chassis is the Gram-negative Pseudomonas bacteria, which have been studied extensively for their diverse metabolism with promises in the field of bioremediation and biosynthesis. Here, we report the development and optimization of a high-yielding (198 ± 5.9 µg/ml) batch CFPS system from Pseudomonas putida ATCC 12633. Importantly, both circular and linear DNA templates can be applied directly to the CFPS reaction to program protein synthesis. Therefore, it is possible to prepare hundreds or even thousands of DNA templates without time-consuming cloning work. This opens the possibility to rapidly assess and validate genetic part performance in vitro before performing experiments in cells. To validate the P. putida CFPS system as a platform for prototyping genetic parts, we designed and constructed a library consisting of 15 different RBSs upstream of the reporter protein sfGFP, which covered an order of magnitude range in expression. Looking forward, our P. putida CFPS platform will not only expand the protein synthesis toolkit for synthetic biology but also serve as a platform in expediting the screening and prototyping of gene regulatory elements.

A cell-free platform for rapid synthesis and testing of active oligosaccharyltransferases.

Protein glycosylation, or the attachment of sugar moieties (glycans) to proteins, is important for protein stability, activity, and immunogenicity. However, understanding the roles and regulations of site-specific glycosylation events remains a significant challenge due to several technological limitations. These limitations include a lack of available tools for biochemical characterization of enzymes involved in glycosylation. A particular challenge is the synthesis of oligosaccharyltransferases (OSTs), which catalyze the attachment of glycans to specific amino acid residues in target proteins. The difficulty arises from the fact that canonical OSTs are large (>70 kDa) and possess multiple transmembrane helices, making them difficult to overexpress in living cells. Here, we address this challenge by establishing a bacterial cell-free protein synthesis platform that enables rapid production of a variety of OSTs in their active conformations. Specifically, by using lipid nanodiscs as cellular membrane mimics, we obtained yields of up to 420 μg/ml for the single-subunit OST enzyme, "Protein glycosylation B" (PglB) from Campylobacter jejuni, as well as for three additional PglB homologs from Campylobacter coli, Campylobacter lari, and Desulfovibrio gigas. Importantly, all of these enzymes catalyzed N-glycosylation reactions in vitro with no purification or processing needed. Furthermore, we demonstrate the ability of cell-free synthesized OSTs to glycosylate multiple target proteins with varying N-glycosylation acceptor sequons. We anticipate that this broadly applicable production method will advance glycoengineering efforts by enabling preparative expression of membrane-embedded OSTs from all kingdoms of life.

Expanding the palette of Streptomyces-based cell-free protein synthesis systems with enhanced yields

Cell-free protein synthesis (CFPS) has emerged as a powerful approach to recombinant protein biosynthesis for applications in biochemical engineering and synthetic biology. To date, CFPS systems have been most commonly derived from a variety of organism sources including microbes (e.g., Escherichia coli and yeast), plants (e.g., wheat germ and tobacco), and eukaryotic cells (e.g., rabbit reticulocytes and Chinese Hamster Ovary cells), each with their own advantages and opportunities. To expand the palette of CFPS platforms, we recently established a Streptomyces lividans-based cell-free system for the expression, especially, of high GC-content genes that are involved in the biosynthesis of natural products. Unfortunately, batch protein expression yields were limited to ∼50μg/mL of a model enhanced green fluorescent protein (EGFP). Here, we sought to address this limitation and improve protein biosynthesis yields. By increasing the total extract protein concentration in the CFPS reaction, which increases the concentration of catalyst proteins available for protein biosynthesis and energy regeneration, and modifying our extract preparation procedure, we enhanced batch protein biosynthesis yields of EGFP more than 2-fold, to 116.9±8.2μg/mL. Then, we demonstrated that our simple and robust approach could be applied to six other Streptomyces strains. Looking forward, we expect that our more highly productive and efficient Streptomyces CFPS systems can be used to synthesize, study, and discover natural product biosynthesis pathways in vitro.

<italic>In vitro</italic> synthetic biology: Cell-free protein synthesis

Synthetic biology is an emerging interdisciplinary field covering the subjects of biology, chemistry and engineering, which promotes the progress from understanding life to creating living systems. As one of important forefront directions of synthetic biology, in vitro synthetic biology (IVSB) focuses on studying the phenomenon and rules of life activities in testing tubes, not relying on living organisms. Cell-free protein synthesis (CFPS) is an effective research platform for IVSB, which utilizes exogenous DNA or mRNA as genetic information for synthesizing protein in vitro , instead of traditional protein expression in cells. CFPS possesses superior advantages over cell-based systems including convenience, simplicity, rapidness, excellent controllability and green economy. Moreover, CFPS is a robust research model for exploration of life evolution and creation of artificial living system. Since 1958, it has been demonstrated for the first time that proteins could be synthesized from cell extract; and afterwards, CFPS has emerged to not only a promising platform for proteomics and pharmaceutics, but also a significant tool for revelation of chemical basis of cells and exploration of origins of life. Cell extracts contained basic transcription and translation machineries, energy regeneration substrates, amino acids, nucleotides and cofactors. A variety of cell extracts have been created from different cell sources such as Escherichia coli ( E.coli ), wheat germ, yeast and rabbit reticulocyte. E.coli was the earliest prokaryotic system and most widely used due to its high production yield, short production cycle and low cost. The protein yields of E. coli system could reach up to several milligrams per milliliter of reaction. Eukaryotic systems such as wheat germ and rabbit reticulocyte although less productive, provided more robust platform for studying protein functions, in particular for post-translationally modified proteins. Different production formats of CFPS have been developed and classified according to how the reactions were fed, including batch, continuous-flow, continuous-exchange, bilayer and hydrogel. Batch was the simplest format, but the key limitation was the short lifetimes and the consequent low yield. This was primarily because of the rapid depletion of the high-energy phosphate pool, leading to accumulation of free phosphates and further inhibition of protein synthesis. Continuous-flow overcame the problem in batch format because of the continuous supply of energy and substrates and the continuous removal of the by-products. As a result, the reaction time could be extended to 20 hours, and the product yield was increased by two orders of magnitude in comparison with batch format. In continuous-exchange format, passive exchange of substrates and by-products further extended the reaction lifetime. However, continuous formats were not easily applicable to high-throughput processes, in which miniaturization and automation were required. Bilayer format was simple, efficient and high-throughput-friendly. Very recently, hydrogel formats have been developed because of the gel niche resembling the physicochemical nature of cells, subsequently leading to high yield. One of the promising applications of CFPS was to synthesize biologically active proteins. CFPS could produce proteins that were difficult or impossible to express with cell-based systems, such as membrane proteins, cell-toxic proteins, isotope-labeling proteins and proteins with unnatural amino acids incorporated. The open nature and high versatility of the CFPS platform enabled high-throughput tools for genomics and proteomics, allowing for biopharmaceutical applications. Through the high- throughput screening of proteins in CFPS the efficacy of protein drugs could be significantly improved by addition of unnatural amino acids, which provided excellent pharmacological properties and prolonged the half-life of drugs in plasma. In this review, we therefore summarize the development history, compositions, categories and production formats of CFPS, as well as applications in the synthesis of functional proteins, high throughput synthesis and screening of protein, and production of protein drugs.

Development of a CHO-Based Cell-Free Platform for Synthesis of Active Monoclonal Antibodies.

Chinese Hamster Ovary (CHO) cells are routinely optimized to stably express monoclonal antibodies (mAbs) at high titers. At the early stages of lead isolation and optimization, hundreds of sequences for the target protein of interest are screened. Typically, cell-based transient expression technology platforms are used for expression screening, but these can be time- and resource-intensive. Here, we have developed a cell-free protein synthesis (CFPS) platform utilizing a commercially available CHO extract for the rapid in vitro synthesis of active, aglycosylated mAbs. Specifically, we optimized reaction conditions to maximize protein yields, established an oxidizing environment to enable disulfide bond formation, and demonstrated the importance of temporal addition of heavy chain and light chain plasmids for intact mAb production. Using our optimized platform, we demonstrate for the first time to our knowledge the cell-free synthesis of biologically active, intact mAb at >100 mg/L using a eukaryotic-based extract. We then explored the utility of our system as a tool for ranking yields of candidate antibodies. Unlike stable or transient transfection-based screening, which requires a minimum of 7 days for setup and execution, results using our CHO-based CFPS platform are attained within 2 days and it is well-suited for automation. Further development would provide a tool for rapid, high-throughput prediction of mAb expression ranking to accelerate design-build-test cycles required for antibody expression and engineering. Looking forward, the CHO-based CFPS platform could facilitate the synthesis of toxic proteins as well.