A general design strategy for protein-responsive riboswitches in mammalian cells

CHiTA: A scarless high-throughput pipeline for characterization of ribozymes

Small self-cleaving ribozymes are catalytic RNAs that cleave their phosphodiester backbone rapidly and site-specifically, without the assistance of proteins. Their catalytic properties make them ideal targets for applications in RNA pharmaceuticals and bioengineering. Consequently, computational pipelines that predict or design thousands of self-cleaving ribozyme candidates have been developed. Traditional experimental techniques for verifying the activity of these putative ribozymes, however, are low-throughput and time intensive. High-throughput (HT) pipelines that employ next-generation sequencing (NGS) analyze the activity of these thousands of ribozymes simultaneously. Until recently, the application of these HT pipelines has been limited to studying all single and double mutants of a select representative ribozyme. Unfortunately, this prevents the exploration of candidates having different lengths, circular permutations, and auxiliary stem-loops. Moreover, pipelines that analyze ribozymes en masse often include transcription of non-native flanking sequences that preclude accurate assessment of the intrinsic rate of ribozyme self-cleavage. To overcome these limitations, we developed a HT pipeline, "Cleavage High-Throughput Assay (CHiTA)", which employs NGS and massively parallel oligonucleotide synthesis (MPOS) to characterize ribozyme activity for thousands of candidates in a scarless fashion. Herein, we describe detailed strategies and protocols to implement CHiTA to measure the activity of putative ribozymes from a wide range of ribozyme classes.

Designing RNA switches for synthetic biology using inverse-RNA-folding

RNA switches respond to specific ligands to control gene expression. They are widely used in synthetic biology applications and hold potential for future RNA-based therapeutic breakthroughs. However, the crux is their precise design. Here, we will discuss how inverse-RNA-folding could be utilized for the accurate design of RNA switches.

Manipulating gene expression levels in mammalian cell factories: An outline of synthetic molecular toolboxes to achieve multiplexed control

Mammalian cells have developed dedicated molecular mechanisms to tightly control expression levels of their genes where the specific transcriptomic signature across all genes eventually determines the cell's phenotype. Modulating cellular phenotypes is of major interest to study their role in disease or to reprogram cells for the manufacturing of recombinant products, such as biopharmaceuticals. Cells of mammalian origin, for example Chinese hamster ovary (CHO) and Human embryonic kidney 293 (HEK293) cells, are most commonly employed to produce therapeutic proteins. Early genetic engineering approaches to alter their phenotype have often been attempted by "uncontrolled" overexpression or knock-down/-out of specific genetic factors. Many studies in the past years, however, highlight that rationally regulating and fine-tuning the strength of overexpression or knock-down to an optimum level, can adjust phenotypic traits with much more precision than such "uncontrolled" approaches. To this end, synthetic biology tools have been generated that enable (fine-)tunable and/or inducible control of gene expression. In this review, we discuss various molecular tools used in mammalian cell lines and group them by their mode of action: transcriptional, post-transcriptional, translational and post-translational regulation. We discuss the advantages and disadvantages of using these tools for each cell regulatory layer and with respect to cell line engineering approaches. This review highlights the plethora of synthetic toolboxes that could be employed, alone or in combination, to optimize cellular systems and eventually gain enhanced control over the cellular phenotype to equip mammalian cell factories with the tools required for efficient production of emerging, more difficult-to-express biologics formats.

Deep generative design of RNA family sequences

RNA engineering has immense potential to drive innovation in biotechnology and medicine. Despite its importance, a versatile platform for the automated design of functional RNA is still lacking. Here, we propose RNA family sequence generator (RfamGen), a deep generative model that designs RNA family sequences in a data-efficient manner by explicitly incorporating alignment and consensus secondary structure information. RfamGen can generate novel and functional RNA family sequences by sampling points from a semantically rich and continuous representation. We have experimentally demonstrated the versatility of RfamGen using diverse RNA families. Furthermore, we confirmed the high success rate of RfamGen in designing functional ribozymes through a quantitative massively parallel assay. Notably, RfamGen successfully generates artificial sequences with higher activity than natural sequences. Overall, RfamGen significantly improves our ability to design functional RNA and opens up new potential for generative RNA engineering in synthetic biology.

An RNA Motif That Enables Optozyme Control and Light-Dependent Gene Expression in Bacteria and Mammalian Cells

The regulation of gene expression by light enables the versatile, spatiotemporal manipulation of biological function in bacterial and mammalian cells. Optoribogenetics extends this principle by molecular RNA devices acting on the RNA level whose functions are controlled by the photoinduced interaction of a light-oxygen-voltage photoreceptor with cognate RNA aptamers. Here light-responsive ribozymes, denoted optozymes, which undergo light-dependent self-cleavage and thereby control gene expression are described. This approach transcends existing aptamer-ribozyme chimera strategies that predominantly rely on aptamers binding to small molecules. The optozyme method thus stands to enable the graded, non-invasive, and spatiotemporally resolved control of gene expression. Optozymes are found efficient in bacteria and mammalian cells and usher in hitherto inaccessible optoribogenetic modalities with broad applicability in synthetic and systems biology.

Synthetic Gene Circuits for Regulation of Next-Generation Cell-Based Therapeutics

Arming human cells with synthetic gene circuits enables to expand their capacity to execute superior sensing and response actions, offering tremendous potential for innovative cellular therapeutics. This can be achieved by assembling components from an ever-expanding molecular toolkit, incorporating switches based on transcriptional, translational, or post-translational control mechanisms. This review provides examples from the three classes of switches, and discusses their advantages and limitations to regulate the activity of therapeutic cells in vivo. Genetic switches designed to recognize internal disease-associated signals often encode intricate actuation programs that orchestrate a reduction in the sensed signal, establishing a closed-loop architecture. Conversely, switches engineered to detect external molecular or physical cues operate in an open-loop fashion, switching on or off upon signal exposure. The integration of such synthetic gene circuits into the next generation of chimeric antigen receptor T-cells is already enabling precise calibration of immune responses in terms of magnitude and timing, thereby improving the potency and safety of therapeutic cells. Furthermore, pre-clinical engineered cells targeting other chronic diseases are gathering increasing attention, and this review discusses the path forward for achieving clinical success. With synthetic biology at the forefront, cellular therapeutics holds great promise for groundbreaking treatments.

Engineered poly(A)-surrogates for translational regulation and therapeutic biocomputation in mammalian cells

Here, we present a gene regulation strategy enabling programmable control over eukaryotic translational initiation. By excising the natural poly-adenylation (poly-A) signal of target genes and replacing it with a synthetic control region harboring RNA-binding protein (RBP)-specific aptamers, cap-dependent translation is rendered exclusively dependent on synthetic translation initiation factors (STIFs) containing different RBPs engineered to conditionally associate with different eIF4F-binding proteins (eIFBPs). This modular design framework facilitates the engineering of various gene switches and intracellular sensors responding to many user-defined trigger signals of interest, demonstrating tightly controlled, rapid and reversible regulation of transgene expression in mammalian cells as well as compatibility with various clinically applicable delivery routes of in vivo gene therapy. Therapeutic efficacy was demonstrated in two animal models. To exemplify disease treatments that require on-demand drug secretion, we show that a custom-designed gene switch triggered by the FDA-approved drug grazoprevir can effectively control insulin expression and restore glucose homeostasis in diabetic mice. For diseases that require instantaneous sense-and-response treatment programs, we create highly specific sensors for various subcellularly (mis)localized protein markers (such as cancer-related fusion proteins) and show that translation-based protein sensors can be used either alone or in combination with other cell-state classification strategies to create therapeutic biocomputers driving self-sufficient elimination of tumor cells in mice. This design strategy demonstrates unprecedented flexibility for translational regulation and could form the basis for a novel class of programmable gene therapies in vivo.

Customizing cellular signal processing by synthetic multi-level regulatory circuits

As synthetic biology permeates society, the signal processing circuits in engineered living systems must be customized to meet practical demands. Towards this mission, novel regulatory mechanisms and genetic circuits with unprecedented complexity have been implemented over the past decade. These regulatory mechanisms, such as transcription and translation control, could be integrated into hybrid circuits termed "multi-level circuits". The multi-level circuit design will tremendously benefit the current genetic circuit design paradigm, from modifying basic circuit dynamics to facilitating real-world applications, unleashing our capabilities to customize cellular signal processing and address global challenges through synthetic biology.

Sensing intracellular signatures with synthetic mRNAs

The bottom-up assembly of biological components in synthetic biology has contributed to a better understanding of natural phenomena and the development of new technologies for practical applications. Over the past few decades, basic RNA research has unveiled the regulatory roles of RNAs underlying gene regulatory networks; while advances in RNA biology, in turn, have highlighted the potential of a wide variety of RNA elements as building blocks to construct artificial systems. In particular, synthetic mRNA-based translational regulators, which respond to signals in cells and regulate the production of encoded output proteins, are gaining attention with the recent rise of mRNA therapeutics. In this Review, we discuss recent progress in RNA synthetic biology, mainly focusing on emerging technologies for sensing intracellular protein and RNA molecules and controlling translation.

Eukaryotic artificial ON-riboswitches that respond efficiently to mid-sized short peptides

We chose two types of mid-sized Arg-rich peptides (Rev-pep and Tat-pep) as ligands and used their aptamers to construct efficient eukaryotic ON-riboswitches (ligand-dependently upregulating riboswitches). Due to the aptamers' high affinities, the best Rev-pep-responsive and Tat-pep-responsive riboswitches obtained showed much higher switching efficiencies at low ligand concentrations than small ligand-responsive ON-riboswitches in the same mechanism. In addition, despite the high sequence similarity of Rev-pep and Tat-pep, the two best riboswitches were almost insensitive to each other's peptide ligand. Considering the high responsiveness and specificity along with the versatility of the expression platform used and the applicability of Arg-rich peptides, this orthogonal pair of riboswitches would be widely useful eukaryotic gene regulators or biosensors.

Synthetic RNA-based post-transcriptional expression control methods and genetic circuits

RNA-based synthetic genetic circuits provide an alternative for traditional transcription-based circuits in applications where genomic integration is to be avoided. Incorporating various post-transcriptional control methods into such circuits allows for controlling the behaviour of the circuit through the detection of certain biomolecular inputs or reconstituting defined circuit behaviours, thus manipulating cellular functions. In this review, recent developments of various types of post-transcriptional control methods in mammalian cells are discussed as well as auxiliary components that allow for the creation and development of mRNA-based switches. How such post-transcriptional switches are combined into synthetic circuits as well as their applications in biomedical and preclinical settings are also described. Finally, we examine the challenges that need to be surmounted before RNA-based synthetic circuits can be reliably deployed into clinical settings.

Identification of Sclareol As a Natural Neuroprotective Cav 1.3-Antagonist Using Synthetic Parkinson-Mimetic Gene Circuits and Computer-Aided Drug Discovery

Parkinson's disease (PD) results from selective loss of substantia nigra dopaminergic (SNc DA) neurons, and is primarily caused by excessive activity-related Ca²⁺ oscillations. Although L-type voltage-gated calcium channel blockers (CCBs) selectively inhibiting Ca_v 1.3 are considered promising candidates for PD treatment, drug discovery is hampered by the lack of high-throughput screening technologies permitting isoform-specific assessment of Cav-antagonistic activities. Here, a synthetic-biology-inspired drug-discovery platform enables identification of PD-relevant drug candidates. By deflecting Cav-dependent activation of nuclear factor of activated T-cells (NFAT)-signaling to repression of reporter gene translation, they engineered a cell-based assay where reporter gene expression is activated by putative CCBs. By using this platform in combination with in silico virtual screening and a trained deep-learning neural network, sclareol is identified from a essential oils library as a structurally distinctive compound that can be used for PD pharmacotherapy. In vitro studies, biochemical assays and whole-cell patch-clamp recordings confirmed that sclareol inhibits Ca_v 1.3 more strongly than Ca_v 1.2 and decreases firing responses of SNc DA neurons. In a mouse model of PD, sclareol treatment reduced DA neuronal loss and protected striatal network dynamics as well as motor performance. Thus, sclareol appears to be a promising drug candidate for neuroprotection in PD patients.

Chimeric RNA-binding protein-based killing switch targeting hepatocellular carcinoma cells

Cancer cell-specific killing switches are synthetic circuits developed as an intelligent weapon to specifically eliminate malignant cells. RNA-delivered synthetic circuits provide safer means to control oncolytic functions, in which proteolysis-responding capsid-cNOT7 is developed to enable logic computation and modular design. Unfortunately, although circuits containing these capsid-cNOT7s exhibited good performance when introduced as replicons, in modified mRNA (modRNA) delivery, the performance was not quite as good. To improve this situation, alternative modules suitable for modRNA delivery need to be developed. An attractive option is RNA-binding protein (RBP)/riboswitches. In this study, RBPs were engineered by fusing with degron and cleavage sites. The compatibility of these chimeric RBPs with proteolysis-based sensing units were tested. Eight two-input logic gates and four three-input logic gates were implemented. After building this chimeric RBP-based system, we constructed a hepatocellular carcinoma (HCC) cell-specific killing circuit using two proteolysis-based sensing units, a two-input logic OR gate, and a leakproof apoptosis-inducing actuator, which distinguished HCC cells and induced apoptosis in a mixed IMR90-PLC/PRF/5 population.

Circularly-Permuted Pistol Ribozyme: A Synthetic Ribozyme Scaffold for Mammalian Riboswitches

A small molecule-responsive self-cleaving ribozyme (aptazyme) embedded in the untranslated region of an mRNA functions as a riboswitch that allows chemical regulation of gene expression in mammalian cells. Aptazymes are engineered by fusing a self-cleaving ribozyme with an RNA aptamer that recognizes a small molecule so that the ribozyme is either activated or inhibited in the presence of the small molecule. However, the variety of aptamers, ribozymes, and aptazyme design strategies suitable for mammalian riboswitch applications is still limited. This work focuses on a new ribozyme scaffold for engineering aptazymes and riboswitches that function in mammalian cells. We investigated circularly permuted variants of the pistol ribozyme class (CPP) as a synthetic ribozyme scaffold for mammalian riboswitch applications. Through semirational design and high-throughput screening, we designed guanine and tetracycline activated riboswitches based on three distinct aptazyme architectures, resulting in riboswitches with ON/OFF ratios as high as 8.6. Our work adds CPP to the limited ribozyme scaffold toolbox for mammalian synthetic biology applications and highlights the opportunities in exploring ribozymes beyond natural motifs.

Enhanced regulation of prokaryotic gene expression by a eukaryotic transcriptional activator

Expanding the genetic toolbox for prokaryotic synthetic biology is a promising strategy for enhancing the dynamic range of gene expression and enabling new engineered applications for research and biomedicine. Here, we reverse the current trend of moving genetic parts from prokaryotes to eukaryotes and demonstrate that the activating eukaryotic transcription factor QF and its corresponding DNA-binding sequence can be moved to E. coli to introduce transcriptional activation, in addition to tight off states. We further demonstrate that the QF transcription factor can be used in genetic devices that respond to low input levels with robust and sustained output signals. Collectively, we show that eukaryotic gene regulator elements are functional in prokaryotes and establish a versatile and broadly applicable approach for constructing genetic circuits with complex functions. These genetic tools hold the potential to improve biotechnology applications for medical science and research.

Expanded toolkits for prokaryotic synthetic biology can enhance the dynamic range of gene expression. Here the authors move the eukaryotic transcription factor QF into E. coli and integrate it into genetic devices.

Directed evolution of orthogonal RNA-RBP pairs through library-vs-library in vitro selection

RNA-binding proteins (RBPs) and their RNA ligands play many critical roles in gene regulation and RNA processing in cells. They are also useful for various applications in cell biology and synthetic biology. However, re-engineering novel and orthogonal RNA-RBP pairs from natural components remains challenging while such synthetic RNA-RBP pairs could significantly expand the RNA-RBP toolbox for various applications. Here, we report a novel library-vs-library in vitro selection strategy based on Phage Display coupled with Systematic Evolution of Ligands by EXponential enrichment (PD-SELEX). Starting with pools of 1.1 × 1012 unique RNA sequences and 4.0 × 108 unique phage-displayed L7Ae-scaffold (LS) proteins, we selected RNA-RBP complexes through a two-step affinity purification process. After six rounds of library-vs-library selection, the selected RNAs and LS proteins were analyzed by next-generation sequencing (NGS). Further deconvolution of the enriched RNA and LS protein sequences revealed two synthetic and orthogonal RNA-RBP pairs that exhibit picomolar affinity and >4000-fold selectivity.

Riboswitches for Controlled Expression of Therapeutic Transgenes Delivered by Adeno-Associated Viral Vectors

Vectors developed from adeno-associated virus (AAV) are powerful tools for in vivo transgene delivery in both humans and animal models, and several AAV-delivered gene therapies are currently approved for clinical use. However, AAV-mediated gene therapy still faces several challenges, including limited vector packaging capacity and the need for a safe, effective method for controlling transgene expression during and after delivery. Riboswitches, RNA elements which control gene expression in response to ligand binding, are attractive candidates for regulating expression of AAV-delivered transgene therapeutics because of their small genomic footprints and non-immunogenicity compared to protein-based expression control systems. In addition, the ligand-sensing aptamer domains of many riboswitches can be exchanged in a modular fashion to allow regulation by a variety of small molecules, proteins, and oligonucleotides. Riboswitches have been used to regulate AAV-delivered transgene therapeutics in animal models, and recently developed screening and selection methods allow rapid isolation of riboswitches with novel ligands and improved performance in mammalian cells. This review discusses the advantages of riboswitches in the context of AAV-delivered gene therapy, the subsets of riboswitch mechanisms which have been shown to function in human cells and animal models, recent progress in riboswitch isolation and optimization, and several examples of AAV-delivered therapeutic systems which might be improved by riboswitch regulation.

A guide to the design of synthetic gene networks in mammalian cells

Synthetic biology aims to harness natural and synthetic biological parts and engineering them in new combinations and systems, producing novel therapies, diagnostics, bioproduction systems, and providing information on the mechanism of function of biological systems. Engineering cell function requires the rewiring or de novo construction of cell information processing networks. Using natural and synthetic signal processing elements, researchers have demonstrated a wide array of signal sensing, processing and propagation modules, using transcription, translation, or post-translational modification to program new function. The toolbox for synthetic network design is ever-advancing and has still ample room to grow. Here, we review the diversity of synthetic gene networks, types of building modules, techniques of regulation, and their applications.

Design of Multipartite Transcription Factors for Multiplexed Logic Genome Integration Control in Mammalian Cells

Synthetic biology relies on rapid and efficient methods to stably integrate DNA payloads encoding for synthetic biological systems into the genome of living cells. The size of designed biological systems increases with their complexity, and novel methods are needed that enable efficient and simultaneous integration of multiple payloads into single cells. By assembling natural and synthetic protein–protein dimerization domains, we have engineered a set of multipartite transcription factors for driving heterologous target gene expression. With the distribution of single parts of multipartite transcription factors on piggyback transposon-based donor plasmids, we have created a logic genome integration control (LOGIC) system that allows for efficient one-step selection of stable mammalian cell lines with up to three plasmids. LOGIC significantly enhances the efficiency of multiplexed payload integration in mammalian cells compared to traditional cotransfection and may advance cell line engineering in synthetic biology and biotechnology.

Artificial Protein-Responsive Riboswitches Upregulate Non-AUG Translation Initiation in Yeast

Artificial control of gene expression is one of the core technologies for engineering biological systems. Riboswitches are cis-acting elements on mRNA that regulate gene expression in a ligand-dependent manner often seen in prokaryotes, but rarely in eukaryotes. Because of the poor variety of such elements available in eukaryotic systems, the number of artificially engineered eukaryotic riboswitches, especially of the upregulation type, is still limited. Here, we developed a design principle for upregulation-type riboswitches that utilize non-AUG initiation induced by ribosomal stalling in a ligand-dependent manner in Saccharomyces cerevisiae. Our design principle simply required the proper positioning of a near-cognate start codon relative to the RNA aptamer. Intriguingly, the CUG codon was the most preferable for non-AUG ON switches in terms of output level and switch performance. This work establishes novel choices for artificial genetic control in eukaryotes with versatile potential for industrial and biomedical applications as well as basic research.

Synthetic mRNA-Based Systems in Mammalian Cells

Living organisms are programmed to perform multiple functions by sensing intra- and extra-cellular environments and by controlling gene expressions. Synthetic biologists aim to program cells by mimicking, designing, and constructing genetic circuits. Synthetic mRNA-based genetic switches and circuits have attracted attention for future therapeutic applications because of their safety and functional diversity. Here, the mRNA-based switches and circuits that detect specific microRNAs or proteins expressed in a target cell to control transgene expression and cell fate are reviewed. Future perspectives of artificial RNA systems for cell engineering will also be addressed.

RNA nanotechnology in synthetic biology

We review recent advances in the design and expression of synthetic RNA sequences inside cells, to regulate gene expression and to achieve spatial localization of components. We focus on approaches that exploit the programmability of the secondary and tertiary structure of RNA to build scalable and modular devices that fold spontaneously and have the capacity to respond to environmental inputs.

Orthogonal Protein-Responsive mRNA Switches for Mammalian Synthetic Biology

The lack of available genetic modules is a fundamental issue in mammalian synthetic biology. Especially, the variety of genetic parts for translational control are limited. Here we report a new set of synthetic mRNA-based translational switches by engineering RNA-binding proteins (RBPs) and RBP-binding RNA motifs (aptamers) that perform strong translational repression. We redesigned the RNA motifs with RNA scaffolds and improved the efficiency of the repression to target RBPs. Using new and previously reported mRNA switches, we demonstrated that the orthogonality of translational regulation was ensured among five different RBP-responsive switches. Moreover, the new switches functioned not only with plasmid introduction, but also with RNA-only delivery, which provides a transient and safer regulation of expression. The translational regulators using RNA-protein interactions provide an alternative strategy to construct complex genetic circuits for future cell engineering and therapeutics.

Massively parallel RNA device engineering in mammalian cells with RNA-Seq

Synthetic RNA-based genetic devices dynamically control a wide range of gene-regulatory processes across diverse cell types. However, the limited throughput of quantitative assays in mammalian cells has hindered fast iteration and interrogation of sequence space needed to identify new RNA devices. Here we report developing a quantitative, rapid and high-throughput mammalian cell-based RNA-Seq assay to efficiently engineer RNA devices. We identify new ribozyme-based RNA devices that respond to theophylline, hypoxanthine, cyclic-di-GMP, and folinic acid from libraries of ~22,700 sequences in total. The small molecule responsive devices exhibit low basal expression and high activation ratios, significantly expanding our toolset of highly functional ribozyme switches. The large datasets obtained further provide conserved sequence and structure motifs that may be used for rationally guided design. The RNA-Seq approach offers a generally applicable strategy for developing broad classes of RNA devices, thereby advancing the engineering of genetic devices for mammalian systems.

Designing cell function: assembly of synthetic gene circuits for cell biology applications

Synthetic biology is the discipline of engineering application-driven biological functionalities that were not evolved by nature. Early breakthroughs of cell engineering, which were based on ectopic (over)expression of single sets of transgenes, have already had a revolutionary impact on the biotechnology industry, regenerative medicine and blood transfusion therapies. Now, we require larger-scale, rationally assembled genetic circuits engineered to programme and control various human cell functions with high spatiotemporal precision in order to solve more complex problems in applied life sciences, biomedicine and environmental sciences. This will open new possibilities for employing synthetic biology to advance personalized medicine by converting cells into living therapeutics to combat hitherto intractable diseases.

Synthetic switch to minimize CRISPR off-target effects by self-restricting Cas9 transcription and translation

CRISPR/Cas9 is a powerful genome editing system but uncontrolled Cas9 nuclease expression triggers off-target effects and even in vivo immune responses. Inspired by synthetic biology, here we built a synthetic switch that self-regulates Cas9 expression not only in the transcription step by guide RNA-aided self-cleavage of cas9 gene, but also in the translation step by L7Ae:K-turn repression system. We showed that the synthetic switch enabled simultaneous transcriptional and translational repression, hence stringently attenuating the Cas9 expression. The restricted Cas9 expression induced high efficiency on-target indel mutation while minimizing the off-target effects. Furthermore, we unveiled the correlation between Cas9 expression kinetics and on-target/off-target mutagenesis. The synthetic switch conferred detectable Cas9 expression and concomitant high frequency on-target mutagenesis at as early as 6 h, and restricted the Cas9 expression and off-target effects to minimal levels through 72 h. The synthetic switch is compact enough to be incorporated into viral vectors for self-regulation of Cas9 expression, thereby providing a novel 'hit and run' strategy for in vivo genome editing.

Numerical operations in living cells by programmable RNA devices

Integrated bioengineering systems can make executable decisions according to the cell state. To sense the state, multiple biomarkers are detected and processed via logic gates with synthetic biological devices. However, numerical operations have not been achieved. Here, we show a design principle for messenger RNA (mRNA) devices that recapitulates intracellular information by multivariate calculations in single living cells. On the basis of this principle and the collected profiles of multiple microRNA activities, we demonstrate that rationally programmed mRNA sets classify living human cells and track their change during differentiation. Our mRNA devices automatically perform multivariate calculation and function as a decision-maker in response to dynamic intracellular changes in living cells.

Synthetic 5' UTRs Can Either Up- or Downregulate Expression upon RNA-Binding Protein Binding

The construction of complex gene-regulatory networks requires both inhibitory and upregulatory modules. However, the vast majority of RNA-based regulatory "parts" are inhibitory. Using a synthetic biology approach combined with SHAPE-seq, we explored the regulatory effect of RNA-binding protein (RBP)-RNA interactions in bacterial 5' UTRs. By positioning a library of RNA hairpins upstream of a reporter gene and co-expressing them with the matching RBP, we observed a set of regulatory responses, including translational stimulation, translational repression, and cooperative behavior. Our combined approach revealed three distinct states in vivo: in the absence of RBPs, the RNA molecules can be found in either a molten state that is amenable to translation or a structured phase that inhibits translation. In the presence of RBPs, the RNA molecules are in a semi-structured phase with partial translational capacity. Our work provides new insight into RBP-based regulation and a blueprint for designing complete gene-regulatory circuits at the post-transcriptional level.

Protein-based parts and devices that respond to intracellular and extracellular signals in mammalian cells

Synthetic biology aims to rewire cellular activities and functionality by implementing genetic circuits with high biocomputing capabilities. Recent efforts led to the development of smart sensing interfaces which integrate multiple inputs to activate desired outputs in a highly specific and sensitive manner. In this review, we highlight protein-based interfaces that sense intracellular or extracellular cues providing information about dynamic environmental changes and cellular state. We will also discuss different mechanisms of regulation of gene expression connected to the sensors to develop diagnostic and therapeutic devices. We conclude discussing challenges and opportunities for biomedical applications of synthetic mammalian protein-based devices.

Advances in engineered trans-acting regulatory RNAs and their application in bacterial genome engineering

Small noncoding RNAs, a large class of ancient posttranscriptional regulators, are increasingly recognized and utilized as key modulators of gene expression in a broad range of microorganisms. Owing to their small molecular size and the central role of Watson-Crick base pairing in defining their interactions, structure and function, numerous diverse types of trans-acting RNA regulators that are functional at the DNA, mRNA and protein levels have been experimentally characterized. It has become increasingly clear that most small RNAs play critical regulatory roles in many processes and are, therefore, considered to be powerful tools for genetic engineering and synthetic biology. The trans-acting regulatory RNAs accelerate this ability to establish potential framework for genetic engineering and genome-scale engineering, which allows RNA structure characterization, easier to design and model compared to DNA or protein-based systems. In this review, we summarize recent advances in engineered trans-acting regulatory RNAs that are used in bacterial genome-scale engineering and in novel cellular capabilities as well as their implementation in wide range of biotechnological, biological and medical applications.

Neomycin-dependent hammerhead ribozymes for the direct control of gene expression in Saccharomyces cerevisiae

Hammerhead ribozyme-based RNA switches have been proven to be powerful tools for conditional gene regulation in various organisms. We present neomycin-dependent hammerhead ribozymes (HHR) that influence gene expression in a ligand- and dose-dependent manner in S. cerevisiae. We utilized a novel design of fusing the aptamer domain to the HHR enabling for the first time the identification of genetic ON- and OFF-switches within the same library. For this purpose a neomycin aptamer was fused to stem 1 of a type 3 hammerhead ribozyme via an addressable three-way junction that shows high flexibility at the connection site. An in vivo screening approach identified sequences that allow to induce or repress gene expression 2- to 3-fold in response to neomycin addition. The ribozyme switches operate at neomycin concentrations that show no toxic effect on cell growth and pose powerful genetic tools to study and modulate cellular function in yeast.

Plant synthetic biology could drive a revolution in biofuels and medicine

Population growth, climate change, and dwindling finite resources are amongst the major challenges which are facing the planet. Requirements for food, materials, water, and energy will soon exceed capacity. Green biotechnology, fueled by recent plant synthetic biology breakthroughs, may offer solutions. This review summarizes current progress towards robust and predictable engineering of plants. I then discuss applications from the lab and field, with a focus on bioenergy, biomaterials, and medicine.

Synthetic switch-based baculovirus for transgene expression control and selective killing of hepatocellular carcinoma cells

Baculovirus (BV) holds promise as a vector for anticancer gene delivery to combat the most common liver cancer-hepatocellular carcinoma (HCC). However, in vivo BV administration inevitably results in BV entry into non-HCC normal cells, leaky anticancer gene expression and possible toxicity. To improve the safety, we employed synthetic biology to engineer BV for transgene expression regulation. We first uncovered that miR-196a and miR-126 are exclusively expressed in HCC and normal cells, respectively, which allowed us to engineer a sensor based on distinct miRNA expression signature. We next assembled a synthetic switch by coupling the miRNA sensor and RNA binding protein L7Ae for translational repression, and incorporated the entire device into a single BV. The recombinant BV efficiently entered HCC and normal cells and enabled cis-acting transgene expression control, by turning OFF transgene expression in normal cells while switching ON transgene expression in HCC cells. Using pro-apoptotic hBax as the transgene, the switch-based BV selectively killed HCC cells in separate culture and mixed culture of HCC and normal cells. These data demonstrate the potential of synthetic switch-based BV to distinguish HCC and non-HCC normal cells for selective transgene expression control and killing of HCC cells.

Synthetic gene circuits for the detection, elimination and prevention of disease

In living organisms, naturally evolved sensors that constantly monitor and process environmental cues trigger corrective actions that enable the organisms to cope with changing conditions. Such natural processes have inspired biologists to construct synthetic living sensors and signalling pathways, by repurposing naturally occurring proteins and by designing molecular building blocks de novo, for customized diagnostics and therapeutics. In particular, designer cells that employ user-defined synthetic gene circuits to survey disease biomarkers and to autonomously re-adjust unbalanced pathological states can coordinate the production of therapeutics, with controlled timing and dosage. Furthermore, tailored genetic networks operating in bacterial or human cells have led to cancer remission in experimental animal models, owing to the network's unprecedented specificity. Other applications of designer cells in infectious, metabolic and autoimmune diseases are also being explored. In this Review, we describe the biomedical applications of synthetic gene circuits in major disease areas, and discuss how the first genetically engineered devices developed on the basis of synthetic-biology principles made the leap from the laboratory to the clinic.

Engineering modular intracellular protein sensor-actuator devices

Understanding and reshaping cellular behaviors with synthetic gene networks requires the ability to sense and respond to changes in the intracellular environment. Intracellular proteins are involved in almost all cellular processes, and thus can provide important information about changes in cellular conditions such as infections, mutations, or disease states. Here we report the design of a modular platform for intrabody-based protein sensing-actuation devices with transcriptional output triggered by detection of intracellular proteins in mammalian cells. We demonstrate reporter activation response (fluorescence, apoptotic gene) to proteins involved in hepatitis C virus (HCV) infection, human immunodeficiency virus (HIV) infection, and Huntington’s disease, and show sensor-based interference with HIV-1 downregulation of HLA-I in infected T cells. Our method provides a means to link varying cellular conditions with robust control of cellular behavior for scientific and therapeutic applications.

Synthetic biology principles are often used to design circuits that tune gene expression in response to changes in intracellular environments. Here the authors design a modular platform for intracellular protein sensing devices with transcriptional output.

Exploring the Potential Application of Short Non-Coding RNA-Based Genetic Circuits in Chinese Hamster Ovary Cells

The majority of cell engineering for recombinant protein production to date has relied on traditional genetic engineering strategies, such as gene overexpression and gene knock-outs, to substantially improve the production capabilities of Chinese Hamster Ovary (CHO) cells. However, further improvements in cellular productivity or control over product quality is likely to require more sophisticated rational approaches to coordinate and balance cellular pathways. For these strategies to be implemented, novel molecular tools need to be developed to facilitate more refined control of gene expression. Multiple gene control strategies are developed over the last decades in the field of synthetic biology, including DNA and RNA-based systems, which allows tight and timely control over gene expression. microRNAs has received a lot of attention over the last decade in the CHO field and are used to engineer and improve CHO cells. In this review we focus on microRNA-based gene control systems and discuss their potential use as tools rather than targets in order to gain better control over gene expression.

Highly motif- and organism-dependent effects of naturally occurring hammerhead ribozyme sequences on gene expression

Recent bioinformatics studies have demonstrated a wide-spread occurrence of the hammerhead ribozyme (HHR) and similar small endonucleolytic RNA motifs in all domains of life. It is becoming increasingly evident that such ribozyme motifs participate in important genetic processes in diverse organisms. Although the HHR motif has been studied for more than three decades, only little is known about the consequences of ribozyme activity on gene expression. In the present study we analysed eight different naturally occurring HHR sequences in diverse genetic and organismal contexts. We investigated the influence of active ribozymes incorporated into mRNAs in mammalian, yeast and bacterial expression systems. The experiments show an unexpectedly high degree of organism-specific variability of ribozyme-mediated effects on gene expression. The presented findings demonstrate that ribozyme cleavage profoundly affect gene expression. However, the extent of this effect varies and depends strongly on the respective genetic context. The fast-cleaving type 3 HHRs [CChMVd(-) and sLTSV(-)] generally tended to cause the strongest effects on intracellular gene expression. The presented results are important in order to address potential functions of naturally occurring ribozymes in RNA processing and post-transcriptional regulation of gene expression. Additionally, our results are of interest for biotechnology and synthetic biology approaches that aim at the utilisation of self-cleaving ribozymes as widely applicable tools for controlling genetic processes.

RNA-based dynamic genetic controllers: development strategies and applications

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Unique properties of RNA lead to the development of RNA-based dynamic genetic controllers.

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Natural riboswitches are re-engineered to detect new molecules.

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RNA-based regulatory mechanisms are exploited to construct novel dynamic RNA controllers.

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Computational methods and in vitro–in vivo selection enable de novo design of dynamic RNA controllers.

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Dynamic RNA controllers are utilized for metabolic engineering and synthetic biology.

Dynamic regulation of gene expression in response to various molecules is crucial for both basic science and practical applications. RNA is considered an attractive material for creating dynamic genetic controllers because of its specific binding to ligands, structural flexibility, programmability, and small size. Here, we review recent advances in strategies for developing RNA-based dynamic controllers and applications. First, we describe studies that re-engineered natural riboswitches to generate new dynamic controllers. Next, we summarize RNA-based regulatory mechanisms that have been exploited to build novel artificial dynamic controllers. We also discuss computational methods and high-throughput selection approaches for de novo design of dynamic RNA controllers. Finally, we explain applications of dynamic RNA controllers for metabolic engineering and synthetic biology.

Computational design of small transcription activating RNAs for versatile and dynamic gene regulation

A longstanding goal of synthetic biology has been the programmable control of cellular functions. Central to this is the creation of versatile regulatory toolsets that allow for programmable control of gene expression. Of the many regulatory molecules available, RNA regulators offer the intriguing possibility of de novo design-allowing for the bottom-up molecular-level design of genetic control systems. Here we present a computational design approach for the creation of a bacterial regulator called Small Transcription Activating RNAs (STARs) and create a library of high-performing and orthogonal STARs that achieve up to ~ 9000-fold gene activation. We demonstrate the versatility of these STARs-from acting synergistically with existing constitutive and inducible regulators, to reprogramming cellular phenotypes and controlling multigene metabolic pathway expression. Finally, we combine these new STARs with themselves and CRISPRi transcriptional repressors to deliver new types of RNA-based genetic circuitry that allow for sophisticated and temporal control of gene expression.

Synthetic mRNA devices that detect endogenous proteins and distinguish mammalian cells

Synthetic biology has great potential for future therapeutic applications including autonomous cell programming through the detection of protein signals and the production of desired outputs. Synthetic RNA devices are promising for this purpose. However, the number of available devices is limited due to the difficulty in the detection of endogenous proteins within a cell. Here, we show a strategy to construct synthetic mRNA devices that detect endogenous proteins in living cells, control translation and distinguish cell types. We engineered protein-binding aptamers that have increased stability in the secondary structures of their active conformation. The designed devices can efficiently respond to target proteins including human LIN28A and U1A proteins, while the original aptamers failed to do so. Moreover, mRNA delivery of an LIN28A-responsive device into human induced pluripotent stem cells (hiPSCs) revealed that we can distinguish living hiPSCs and differentiated cells by quantifying endogenous LIN28A protein expression level. Thus, our endogenous protein-driven RNA devices determine live-cell states and program mammalian cells based on intracellular protein information.