Cell-free protein synthesis systems derived from cultured mammalian cells

Vesicle-based cell-free synthesis of short and long unspecific peroxygenases

Unspecific peroxygenases (UPOs, EC 1.11.2.1) are fungal enzymes that catalyze the oxyfunctionalization of non-activated hydrocarbons, making them valuable biocatalysts. Despite the increasing interest in UPOs that has led to the identification of thousands of putative UPO genes, only a few of these have been successfully expressed and characterized. There is currently no universal expression system in place to explore their full potential. Cell-free protein synthesis has proven to be a sophisticated technique for the synthesis of difficult-to-express proteins. In this work, we aimed to establish an insect-based cell-free protein synthesis (CFPS) platform to produce UPOs. CFPS relies on translationally active cell lysates rather than living cells. The system parameters can thus be directly manipulated without having to account for cell viability, thereby making it highly adaptable. The insect-based lysate contains translocationally active, ER-derived vesicles, called microsomes. These microsomes have been shown to allow efficient translocation of proteins into their lumen, promoting post-translational modifications such as disulfide bridge formation and N-glycosylations. In this study the ability of a redox optimized, vesicle-based, eukaryotic CFPS system to synthesize functional UPOs was explored. The influence of different reaction parameters as well as the influence of translocation on enzyme activity was evaluated for a short UPO from Marasmius rotula and a long UPO from Agrocybe aegerita. The capability of the CFPS system described here was demonstrated by the successful synthesis of a novel UPO from Podospora anserina, thus qualifying CFPS as a promising tool for the identification and evaluation of novel UPOs and variants thereof.

In Vitro Synthesis and Reconstitution Using Mammalian Cell-Free Lysates Enables the Systematic Study of the Regulation of LINC Complex Assembly

Understanding the structure and structure-function relationships of membrane proteins is a fundamental problem in biomedical research. Given the difficulties inherent to performing mechanistic biochemical and biophysical studies of membrane proteins in vitro, we previously developed a facile HeLa cell-based cell-free expression (CFE) system that enables the efficient reconstitution of full-length (FL) functional inner nuclear membrane Sad1/UNC-84 (SUN) proteins (i.e., SUN1 and SUN2) in supported lipid bilayers. Here, we provide evidence that suggests that the reconstitution of CFE-synthesized FL membrane proteins in supported lipid bilayers occurs primarily through the fusion of endoplasmic reticulum-derived microsomes present within our CFE reactions with our supported lipid bilayers. In addition, we demonstrate the ease with which our synthetic biology platform can be used to investigate the impact of the chemical environment on the ability of CFE-synthesized FL SUN proteins reconstituted in supported lipid bilayers to interact with the luminal domain of the KASH protein nesprin-2. Moreover, we use our platform to study the molecular requirements for the homo- and heterotypic interactions between SUN1 and SUN2. Finally, we show that our platform can be used to simultaneously reconstitute three different CFE-synthesized FL membrane proteins in a single supported lipid bilayer. Overall, these results establish our HeLa cell-based CFE and supported lipid bilayer reconstitution platform as a powerful tool for performing mechanistic dissections of the oligomerization and function of FL membrane proteins in vitro. While our platform is not a substitute for cell-based studies, it does provide important mechanistic insights into the biology of difficult-to-study membrane proteins.

Cell-free protein synthesis using Chinese hamster ovary cells

Cell-free protein synthesis (CFPS) platforms can be used for rapid and flexible expression of proteins. The use of CFPS platforms from mammalian, specifically Chinese hamster ovary (CHO) cells, offers the possibility of a rapid prototyping platform for recombinant protein production with the capabilities of post-translational modifications. In this chapter, we discuss a refined CFPS system based on CHO cells, including: extract preparation, reaction mix composition, and accessory protein supplementation to enhance expression. Specifically, when the CHO cell extract is combined with a truncated version of GADD34 and K3L, stress-induced eIF2 phosphorylation is reduced and inhibition of translation initiation is relieved, increasing yields. A brief summary of the protocol for running the CFPS reactions is also described. Overall, the method is reliable and leads to a highly reproducible expression system. Finally, the advantages and disadvantages of the platform, in addition to expected outcomes, are also discussed.

Synthesis of Fluorescently Labeled Antibodies Using Non-Canonical Amino Acids in Eukaryotic Cell-Free Systems

Cell-free protein synthesis (CFPS) enables the development of antibody conjugates, such as fluorophore conjugates and antibody-drug conjugates (ADCs), in a rapid and straightforward manner. In the first part, we describe the cell-free synthesis of antibodies containing fluorescent non-canonical amino acids (ncaa) by using pre-charged tRNA. In the second part, we describe the cell-free synthesis of antibodies containing ncaa by using an orthogonal system, followed by the site-specific conjugation of the fluorescent dye DyLight 650-phosphine. The expression of the antibodies containing ncaa was analyzed by SDS-PAGE, followed by autoradiography and the labeling by in-gel fluorescence. Two different fluorescently labeled antibodies could be generated.

Easy Synthesis of Complex Biomolecular Assemblies: Wheat Germ Cell-Free Protein Expression in Structural Biology

Cell-free protein synthesis (CFPS) systems are gaining more importance as universal tools for basic research, applied sciences, and product development with new technologies emerging for their application. Huge progress was made in the field of synthetic biology using CFPS to develop new proteins for technical applications and therapy. Out of the available CFPS systems, wheat germ cell-free protein synthesis (WG-CFPS) merges the highest yields with the use of a eukaryotic ribosome, making it an excellent approach for the synthesis of complex eukaryotic proteins including, for example, protein complexes and membrane proteins. Separating the translation reaction from other cellular processes, CFPS offers a flexible means to adapt translation reactions to protein needs. There is a large demand for such potent, easy-to-use, rapid protein expression systems, which are optimally serving protein requirements to drive biochemical and structural biology research. We summarize here a general workflow for a wheat germ system providing examples from the literature, as well as applications used for our own studies in structural biology. With this review, we want to highlight the tremendous potential of the rapidly evolving and highly versatile CFPS systems, making them more widely used as common tools to recombinantly prepare particularly challenging recombinant eukaryotic proteins.

Design, Development and Optimization of a Functional Mammalian Cell-Free Protein Synthesis Platform

In this paper, we describe the stepwise development of a cell-free protein synthesis (CFPS) platform derived from cultured Chinese hamster ovary (CHO) cells. We provide a retrospective summary of the design challenges we faced, and the optimized methods developed for the cultivation of cells and the preparation of translationally active lysates. To overcome low yields, we developed procedures to supplement two accessory proteins, GADD34 and K3L, into the reaction to prevent deactivation of the translational machinery by phosphorylation. We compared different strategies for implementing these accessory proteins including two variants of the GADD34 protein to understand the potential trade-offs between yield and ease of implementation. Addition of the accessory proteins increased yield of turbo Green Fluorescent Protein (tGFP) by up to 100-fold depending on which workflow was used. Using our optimized protocols as a guideline, users can successfully develop their own functional CHO CFPS system, allowing for broader application of mammalian CFPS.

Cell-free synthesis of the hirudin variant 1 of the blood-sucking leech Hirudo medicinalis

Synthesis and purification of peptide drugs for medical applications is a challenging task. The leech-derived factor hirudin is in clinical use as an alternative to heparin in anticoagulatory therapies. So far, recombinant hirudin is mainly produced in bacterial or yeast expression systems. We describe the successful development and application of an alternative protocol for the synthesis of active hirudin based on a cell-free protein synthesis approach. Three different cell lysates were compared, and the effects of two different signal peptide sequences on the synthesis of mature hirudin were determined. The combination of K562 cell lysates and the endogenous wild-type signal peptide sequence was most effective. Cell-free synthesized hirudin showed a considerably higher anti-thrombin activity compared to recombinant hirudin produced in bacterial cells.

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.

Synthetic Biology Goes Cell-Free

Cell-free systems (CFS) have recently evolved into key platforms for synthetic biology applications. Many synthetic biology tools have traditionally relied on cell-based systems, and while their adoption has shown great progress, the constraints inherent to the use of cellular hosts have limited their reach and scope. Cell-free systems, which can be thought of as programmable liquids, have removed many of these complexities and have brought about exciting opportunities for rational design and manipulation of biological systems. Here we review how these simple and accessible enzymatic systems are poised to accelerate the rate of advancement in synthetic biology and, more broadly, biotechnology.

A Combined Cell-Free Protein Synthesis and Fluorescence-Based Approach to Investigate GPCR Binding Properties

Fluorescent labeling of de novo synthesized proteins is in particular a valuable tool for functional and structural studies of membrane proteins. In this context, we present two methods for the site-specific fluorescent labeling of difficult-to-express membrane proteins in combination with cell-free protein synthesis. The cell-free protein synthesis system is based on Chinese Hamster Ovary Cells (CHO) since this system contains endogenous membrane structures derived from the endoplasmic reticulum. These so-called microsomes enable a direct integration of membrane proteins into a biological membrane. In this protocol the first part describes the fluorescent labeling by using a precharged tRNA, loaded with a fluorescent amino acid. The second part describes the preparation of a modified aminoacyl-tRNA-synthetase and a suppressor tRNA that are applied to the CHO cell-free system to enable the incorporation of a non-canonical amino acid. The reactive group of the non-canonical amino acid is further coupled to a fluorescent dye. Both methods utilize the amber stop codon suppression technology. The successful fluorescent labeling of the model G protein-coupled receptor adenosine A_2A (Adora2a) is analyzed by in-gel-fluorescence, a reporter protein assay, and confocal laser scanning microscopy (CLSM). Moreover, a ligand-dependent conformational change of the fluorescently labeled Adora2a was analyzed by bioluminescence resonance energy transfer (BRET).

Improving the recombinant human erythropoietin glycosylation using microsome supplementation in CHO cell-free system

Cell-Free Protein Synthesis (CFPS) offers many advantages for the production of recombinant therapeutic proteins using the CHO cell-free system. However, many complex proteins are still difficult to express using this method. To investigate the current bottlenecks in cell-free glycoprotein production, we chose erythropoietin (40% glycosylated), an essential endogenous hormone which stimulates the development of red blood cells. Here, we report the production of recombinant erythropoietin (EPO) using CHO cell-free system. Using this method, EPO was expressed and purified with a twofold increase in yield when the cell-free reaction was supplemented with CHO microsomes. The protein was purified to near homogeneity using an ion-metal affinity column. We were able to analyze the expressed and purified products (glycosylated cell-free EPO runs at 25-28 kDa, and unglycosylated protein runs at 20 kDa on an SDS-PAGE), identifying the presence of glycan moieties by PNGase shift assay. The purified protein was predicted to have ∼2,300 IU in vitro activity. Additionally, we tested the presence and absence of sugars on the cell-free EPO using a lectin-based assay system. The results obtained in this study indicate that microsomes augmented in vitro production of the glycoprotein is useful for the rapid production of single doses of a therapeutic glycoprotein drug and to rapidly screen glycoprotein constructs in the development of these types of drugs. CFPS is useful for implementing a lectin-based method for rapid screening and detection of glycan moieties, which is a critical quality attribute in the industrial production of therapeutic glycoproteins.

A synthetic biology platform for the reconstitution and mechanistic dissection of LINC complex assembly

The linker of nucleoskeleton and cytoskeleton (LINC) is a conserved nuclear envelope-spanning molecular bridge that is responsible for the mechanical integration of the nucleus with the cytoskeleton. LINC complexes are formed by a transluminal interaction between the outer and inner nuclear membrane KASH and SUN proteins, respectively. Despite recent structural insights, our mechanistic understanding of LINC complex assembly remains limited by the lack of an experimental system for its in vitro reconstitution and manipulation. Here, we describe artificial nuclear membranes (ANMs) as a synthetic biology platform based on mammalian cell-free expression for the rapid reconstitution of SUN proteins in supported lipid bilayers. We demonstrate that SUN1 and SUN2 are oriented in ANMs with solvent-exposed C-terminal KASH-binding SUN domains. We also find that SUN2 possesses a single transmembrane domain, while SUN1 possesses three. Finally, SUN protein-containing ANMs bind synthetic KASH peptides, thereby reconstituting the LINC complex core. This work represents the first in vitro reconstitution of KASH-binding SUN proteins in supported lipid bilayers using cell-free expression, which will be invaluable for testing proposed models of LINC complex assembly and its regulation.

Versatile Cell-Free Protein Synthesis Systems Based on Chinese Hamster Ovary Cells

We present an alternative production platform for the synthesis of complex proteins. Apart from conventionally applied protein production using engineered mammalian cell lines, this protocol describes the preparation and principle of cell-free protein synthesis systems based on CHO cell lysates. The CHO cell-free system contains endogenous microsomes derived from the endoplasmic reticulum, which enables a direct integration of membrane proteins into a nature like milieu and the introduction of posttranslational modifications. Different steps of system development are described including the cultivation of CHO cells, cell harvesting and cell disruption to prepare translationally active CHO cell lysates. The requirements for DNA templates and the generation of linear DNA templates suitable for the CHO cell-free reaction is further depicted to underline the opportunity to produce different protein variants in a short period. This experimental setup provides a basis for high-throughput applications. The productivity of the CHO cell-free systems is further increased by using a non-canonical translation initiation due to the attachment of an internal ribosomal entry site of the Cricket paralysis virus (CRPV IRES) to the 5´ UTR of the desired gene. In this way, a direct interaction of the IRES structure with the ribosome facilitates a translation factor independent initiation of translation. Cell-free reactions were performed in fast and efficient batch reactions leading to protein yields up to 40 μg/mL. The reaction format was further adjusted to a continuous exchange CHO cell-free reaction (CHO CECF) to prolong reaction time and thereby increase the productivity of the cell-free systems. Finally, protein yields up to 1 g/L were obtained. The CHO CECF system represents a sophisticated resource to address structural and functional aspects of difficult-to-express proteins in fundamental and applied research.

Cell-free synthesis of functional antibodies using a coupled in vitro transcription-translation system based on CHO cell lysates

Antibodies are indispensable tools for basic research as well as diagnostic and therapeutic applications. Consequently, the development of alternative manufacturing strategies which circumvent the hurdles connected to conventional antibody production technologies is of enormous interest. To address this issue, we demonstrate the synthesis of complex antibody formats, in particular immunoglobulin G (IgG) and single-chain variable fragment Fc fusion (scFv-Fc), in a microsome-containing cell-free system based on translationally active chinese hamster ovary (CHO) cell lysates. To mimic the environment for antibody folding and assembly present in living cells, antibody genes were fused to an endoplasmic reticulum (ER)-specific signal sequence. Signal-peptide induced translocation of antibody polypeptide chains into the lumen of ER microsomes was found to be the prerequisite for antibody chain assembly and functionality. In this context, we show the rapid synthesis of antibody molecules in different reaction formats, including batch and continuous-exchange cell-free (CECF) reactions, depending on the amount of protein needed for further analysis. In addition, we demonstrate site-specific and residue-specific labeling of antibodies with fluorescent non-canonical amino acids. In summary, our study describes a novel antibody production platform which combines the highly efficient mammalian protein folding machinery of CHO cells with the benefits of cell-free protein synthesis.

Comparison of cell-based versus cell-free mammalian systems for the production of a recombinant human bone morphogenic growth factor

The human bone morphogenetic protein-2 (hBMP2) is a glycoprotein, which induces de novo bone formation. Here, recombinant production in stably transfected Chinese Hamster Ovary (CHO) cells is compared to transient expression in Human Embryo Kidney (HEK) cells and cell-free synthesis in CHO cell lysates containing microsomal structures as sites of post-translational processing. In case of the stably transfected cells, growth rates and viabilities were similar to those of the parent cells, while entry into the death phase of the culture was delayed. The maximum achievable rhBMP2 concentration in these cultures was 153 pg/mL. Up to 280 ng/mL could be produced in the transient expression system. In both cases the rhBMP-2 was found to interact with the producer cells, which presumably contributed to the low yields. In the cell-free system, hBMP2 yields could be increased to almost 40 μg/mL, reached within three hours. The cell-free system thus approached productivities for the active (renatured) protein previously only recorded for bacterial hosts, while assuring comprehensive post-translational processing.

Cell-Free Synthetic Biology: Engineering Beyond the Cell

Cell-free protein synthesis (CFPS) technologies have enabled inexpensive and rapid recombinant protein expression. Numerous highly active CFPS platforms are now available and have recently been used for synthetic biology applications. In this review, we focus on the ability of CFPS to expand our understanding of biological systems and its applications in the synthetic biology field. First, we outline a variety of CFPS platforms that provide alternative and complementary methods for expressing proteins from different organisms, compared with in vivo approaches. Next, we review the types of proteins, protein complexes, and protein modifications that have been achieved using CFPS systems. Finally, we introduce recent work on genetic networks in cell-free systems and the use of cell-free systems for rapid prototyping of in vivo networks. Given the flexibility of cell-free systems, CFPS holds promise to be a powerful tool for synthetic biology as well as a protein production technology in years to come.

In vitro flow cytometry-based screening platform for cellulase engineering

Ultrahigh throughput screening (uHTS) plays an essential role in directed evolution for tailoring biocatalysts for industrial applications. Flow cytometry-based uHTS provides an efficient coverage of the generated protein sequence space by analysis of up to 10⁷ events per hour. Cell-free enzyme production overcomes the challenge of diversity loss during the transformation of mutant libraries into expression hosts, enables directed evolution of toxic enzymes, and holds the promise to efficiently design enzymes of human or animal origin. The developed uHTS cell-free compartmentalization platform (InVitroFlow) is the first report in which a flow cytometry-based screened system has been combined with compartmentalized cell-free expression for directed cellulase enzyme evolution. InVitroFlow was validated by screening of a random cellulase mutant library employing a novel screening system (based on the substrate fluorescein-di-β-D-cellobioside), and yielded significantly improved cellulase variants (e.g. CelA2-H288F-M1 (N273D/H288F/N468S) with 13.3-fold increased specific activity (220.60 U/mg) compared to CelA2 wildtype: 16.57 U/mg).

Advancing translational research with next-generation protein microarrays

Protein microarrays are a high-throughput technology used increasingly in translational research, seeking to apply basic science findings to enhance human health. In addition to assessing protein levels, posttranslational modifications, and signaling pathways in patient samples, protein microarrays have aided in the identification of potential protein biomarkers of disease and infection. In this perspective, the different types of full-length protein microarrays that are used in translational research are reviewed. Specific studies employing these microarrays are presented to highlight their potential in finding solutions to real clinical problems. Finally, the criteria that should be considered when developing next-generation protein microarrays are provided.

Cell-Free Protein Synthesis: Pros and Cons of Prokaryotic and Eukaryotic Systems

From its start as a small-scale in vitro system to study fundamental translation processes, cell-free protein synthesis quickly rose to become a potent platform for the high-yield production of proteins. In contrast to classical in vivo protein expression, cell-free systems do not need time-consuming cloning steps, and the open nature provides easy manipulation of reaction conditions as well as high-throughput potential. Especially for the synthesis of difficult to express proteins, such as toxic and transmembrane proteins, cell-free systems are of enormous interest. The modification of the genetic code to incorporate non-canonical amino acids into the target protein in particular provides enormous potential in biotechnology and pharmaceutical research and is in the focus of many cell-free projects. Many sophisticated cell-free systems for manifold applications have been established. This review describes the recent advances in cell-free protein synthesis and details the expanding applications in this field.

Cotranslational incorporation of non-standard amino acids using cell-free protein synthesis

Over the last years protein engineering using non-standard amino acids has gained increasing attention. As a result, improved methods are now available, enabling the efficient and directed cotranslational incorporation of various non-standard amino acids to equip proteins with desired characteristics. In this context, the utilization of cell-free protein synthesis is particularly useful due to the direct accessibility of the translational machinery and synthesized proteins without having to maintain a vital cellular host. We review prominent methods for the incorporation of non-standard amino acids into proteins using cell-free protein synthesis. Furthermore, a list of non-standard amino acids that have been successfully incorporated into proteins in cell-free systems together with selected applications is provided.