Synthetic translational regulation by an L7Ae–kink-turn RNP switch

RNA-based logic for selective protein expression in senescent cells

Cellular senescence is a cellular state characterized by irreversible growth arrest, resistance to apoptosis and secretion of inflammatory molecules, which is causally linked to the pathogenesis of many age-related diseases. Besides, there is accumulating evidence that selective removal of senescent cells can benefit therapies for cancer and fibrosis by modulating the inflammatory microenvironment. While the field of so-called senolytics has spawned promising small molecules and peptides for the selective removal of senescent cells, there is still no effective means to detect senescent cells in vivo, a prerequisite for understanding the role of senescence in pathophysiology and to assess the effectiveness of treatments aimed at removing senescent cells. Here, we present a strategy based on an mRNA logic circuit, that yields mRNA-dependent protein expression only when a senescence-specific miRNA signature is present. Following a validation of radiation-induced senescence induction in primary human fibroblasts, we identify miRNAs up- and downregulated in association with cellular senescence using RT-qPCR. Incorporating binding sites to these miRNAs into the 3' untranslated regions of the mRNA logic circuit, we demonstrate the senescence-specific expression of EGFP for detection of senescent cells and of a constitutively active caspase-3 for selective removal. Altogether, our results pave the way for a novel approach to execute an mRNA-based programme specifically in senescent cells aimed at their detection or selective removal.

Genetic switch selectively kills hepatocellular carcinoma cell based on microRNA and tissue-specific promoter

The clinical treatment of hepatocellular carcinoma (HCC) is still a heavy burden worldwide. Intracellular microRNAs (miRNAs) commonly express abnormally in cancers, thus they are potential therapeutic targets for cancer treatment. miR-21 is upregulated in HCC whereas miR-122 is enriched in normal hepatocyte but downregulated in HCC. In our study, we first generated a reporter genetic switch compromising of miR-21 and miR-122 sponges as sensor, green fluorescent protein (GFP) as reporter gene and L7Ae:K-turn as regulatory element. The reporter expression was turned up in miR-21 enriched environment while turned down in miR-122 enriched environment, indicating that the reporter switch is able to respond distinctly to different miRNA environment. Furthermore, an AAT promoter, which is hepatocyte-specific, is applied to increase the specificity to hepatocyte. A killing switch with AAT promoter and an apoptosis-inducing element, Bax, in addition to miR-21 and miR-122 significantly inhibited cell viability in Huh-7 by 70 % and in HepG2 by 60 %. By contrast, cell viability was not affected in five non-HCC cells. Thus, we provide a novel feasible strategy to improve the safety of miRNA-based therapeutic agent to cancer.

Advancing in vivo reprogramming with synthetic biology

Reprogramming cells will play a fundamental role in shaping the future of cell therapies by developing new strategies to engineer cells for improved performance and higher-order physiological functions. Approaches in synthetic biology harness cells' natural ability to sense diverse signals, integrate environmental inputs to make decisions, and execute complex behaviors based on the health of the organism or tissue. In this review, we highlight strategies in synthetic biology to reprogram cells, and discuss how recent approaches in the delivery of modified mRNA have created new opportunities to alter cell function in vivo. Finally, we discuss how combining concepts from synthetic biology and the delivery of mRNA in vivo could provide a platform for innovation to advance in vivo cellular reprogramming.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

A Novel Cargo Delivery System-AnCar-ExoLaIMTS Ameliorates Arthritis via Specifically Targeting Pro-Inflammatory Macrophages

Macrophages are heterogenic phagocytic cells that play distinct roles in physiological and pathological processes. Targeting different types of macrophages has shown potent therapeutic effects in many diseases. Although many approaches are developed to target anti-inflammatory macrophages, there are few researches on targeting pro-inflammatory macrophages, which is partially attributed to their non-s pecificity phagocytosis of extracellular substances. In this study, a novel recombinant protein is constructed that can be anchored on an exosome membrane with the purpose of targeting pro-inflammatory macrophages via antigen recognition, which is named AnCar-ExoLaIMTS . The data indicate that the phagocytosis efficiencies of pro-inflammatory macrophages for different AnCar-ExoLaIMTS show obvious differences. The AnCar-ExoLaIMTS3 has the best targeting ability for pro-inflammatory macrophages in vitro and in vivo. Mechanically, AnCar-ExoLaIMTS3 can specifically recognize the leucine-rich repeat domain of the TLR4 receptor, and then enter into pro-inflammatory macrophages via the TLR4-mediated receptor endocytosis pathway. Moreover, AnCar-ExoLaIMTS3 can efficiently deliver therapeutic cargo to pro-inflammatory macrophages and inhibit the synovial inflammatory response via downregulation of HIF-1α level, thus ameliorating the severity of arthritis in vivo. Collectively, the work established a novel gene/drug delivery system that can specifically target pro-inflammatory macrophages, which may be beneficial for the treatments of arthritis and other inflammatory diseases.

RNA Switches Using Cas Proteins

Expanding the number of available RNA-binding proteins (RBPs) is vital to establishing posttranscriptional circuits in mammalian cells. We focused on CRISPR-Cas systems and exploited Cas proteins for their versatility as RBPs. The translation of genes encoded in an mRNA becomes regulatable by a Cas protein by inserting a crRNA/sgRNA sequence recognizable by the specific Cas protein into its 5'UTR. These Cas protein-responsive switches vastly expand the available tools in synthetic biology because of the wide range of Cas protein orthologs that can be used as trigger proteins.Here, we describe the design principle of Cas protein-responsive switches, both plasmid and RNA versions, using Streptococcus pyogenes Cas9 (SpCas9) as an example and show an example of its use in mammalian cells, HEK293FT cells.

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.

Engineered Protease-Responsive RNA-Binding Proteins (RBPs) to Expand the Toolbox of Synthetic Circuits in Mammalian Cells

Genetically encoded sensor-actuator circuits aim at reprogramming cellular functions and are inspired by intracellular networks: from the input signal (sensor) to the desired output response (actuator). In the last years, circuits with posttranscriptional regulation of gene expression have aroused great interest for their potential in the biomedical space. Posttranscriptional modulation can be achieved with ribozymes, riboswitches (simple regulatory elements based on RNA secondary structures), noncoding RNAs, and RNA-binding proteins (RBPs). RBPs are proteins that recognize specific motifs on the mRNA target inducing mRNA decay or translation inhibition. The use of RBPs deriving from different species in mammalian cells has allowed to create sophisticated and multilayered regulatory networks, addressing the previous limitation of regulatory orthogonal parts that can be assembled in synthetic devices. In this chapter, we describe the engineering and tests of protease-responsive RNA-binding proteins (L7Ae and MS2-cNOT7) to expand the toolbox of synthetic circuits in mammalian cells.

mRNA Delivery Platform Based on Bacterial Outer Membrane Vesicles for Tumor Vaccine

The rapid display and delivery method for customized tumor mRNA vaccines is limited. Herein, bacteria-derived outer membrane vesicles (OMVs) are employed as an mRNA delivery platform by surface engineering of an RNA-binding protein, L7Ae. OMV-L7Ae can rapidly adsorb boxC/D sequence-labeled mRNA antigens through L7Ae-boxC/D binding and deliver them into HEK-293T and dendritic cells. This platform provides an mRNA delivery technology distinct from lipid nanoparticles (LNPs) for personalized mRNA tumor vaccination and with a Plug-and-Display strategy suitable for rapid preparation of the personalized mRNA tumor vaccine against varied tumor antigens. Key features OMVs are employed as an mRNA delivery platform through L7Ae-boxC/D binding.

Archaeal Kink-Turn Binding Protein Mediates Inhibition of Orthomyxovirus Splicing Biology

Despite lacking a DNA intermediate, orthomyxoviruses complete their replication cycle in the nucleus and generate multiple transcripts by usurping the host splicing machinery. This biology results in dynamic changes of relative viral transcripts over time and dictates the replicative phase of the infection. Here, we demonstrate that the family of archaeal L7Ae proteins uniquely inhibit the splicing biology of influenza A virus, influenza B virus, and Salmon isavirus, revealing a common strategy utilized by Orthomyxoviridae members to achieve this dynamic. L7Ae-mediated inhibition of virus biology was lost with the generation of a splicing-independent strain of influenza A virus and attempts to select for an escape mutant resulted in variants that conformed to host splicing biology at significant cost to their overall fitness. As L7Ae recognizes conventional kink turns in various RNAs, these data implicate the formation of a similar structure as a shared strategy adopted by this virus family to coordinate their replication cycle. IMPORTANCE Here, we demonstrate that a family of proteins from archaea specifically inhibit this splicing biology of all tested members of the Orthomyxoviridae family. We show that this inhibition extends to influenza A virus, influenza B virus, and isavirus genera, while having no significant impact on the mammalian transcriptome or proteome. Attempts to generate an escape mutant against L7Ae-mediated inhibition resulted in mutations surrounding the viral splice sites and a significant loss of viral fitness. Together, these findings reveal a unique biology shared among diverse members of the Orthomyxoviridae family that may serve as a means to generate future universal therapeutics.

Programmable mammalian translational modulators by CRISPR-associated proteins

Translational modulation based on RNA-binding proteins can be used to construct artificial gene circuits, but RNA-binding proteins capable of regulating translation efficiently and orthogonally remain scarce. Here we report CARTRIDGE (Cas-Responsive Translational Regulation Integratable into Diverse Gene control) to repurpose Cas proteins as translational modulators in mammalian cells. We demonstrate that a set of Cas proteins efficiently and orthogonally repress or activate the translation of designed mRNAs that contain a Cas-binding RNA motif in the 5'-UTR. By linking multiple Cas-mediated translational modulators, we designed and built artificial circuits like logic gates, cascades, and half-subtractor circuits. Moreover, we show that various CRISPR-related technologies like anti-CRISPR and split-Cas9 platforms could be similarly repurposed to control translation. Coupling Cas-mediated translational and transcriptional regulation enhanced the complexity of synthetic circuits built by only introducing a few additional elements. Collectively, CARTRIDGE has enormous potential as a versatile molecular toolkit for mammalian synthetic biology.

Exosomes and ultrasound: The future of theranostic applications

Biomaterials and pertaining formulations have been very successful in various diagnostic and therapeutic applications because of its ability to overcome pharmacological limitations. Some of them have gained significant focus in the recent decade for their theranostic properties. Exosomes can be grouped as biomaterials, since they consist of various biological micro/macromolecules and possess all the properties of a stable biomaterial with size in nano range. Significant research has gone into isolation and exploitation of exosomes as potential theranostic agent. However, the limitations in terms of yield, efficacy, and target specificity are continuously being addressed. On the other hand, several nano/microformulations are responsive to physical or chemical alterations and were successfully stimulated by tweaking the physical characteristics of the surrounding environment they are in. Some of them are termed as photodynamic, sonodynamic or thermodynamic therapeutic systems. In this regard, ultrasound and acoustic systems were extensively studied for its ability towards altering the properties of the systems to which they were applied on. In this review, we have detailed about the diagnostic and therapeutic applications of exosomes and ultrasound separately, consisting of their conventional applications, drawbacks, and developments for addressing the challenges. The information were categorized into various sections that provide complete overview of the isolation strategies and theranostic applications of exosomes in various diseases. Then the ultrasound-based disease diagnosis and therapy were elaborated, with special interest towards the use of ultrasound in enhancing the efficacy of nanomedicines and nanodrug delivery systems, Finally, we discussed about the ability of ultrasound in enhancing the diagnostic and therapeutic properties of exosomes, which could be the future of theranostics.

A translational repression reporter assay for the analysis of RNA-binding protein consensus sites

RNA-binding proteins are essential regulators of RNA processing and function. Translational repression assays can be used to study how they interact with specific RNA sequences by insertion of such a consensus sequence into the 5' untranslated region of a reporter mRNA and measuring reporter protein translation. The straightforward set-up of these translational repression assays avoids the need for the isolation of the protein or the RNA providing speed, robustness and a low-cost method. Here, we report the optimization of the assay to function with linear RNA sequences instead of the previously reported hairpin type sequences to allow the study of a wider variety of RNA-binding proteins. Multiplication of a consensus sequence strongly improves the signal allowing analysis by both fluorescence intensity measurements and flow cytometry.

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.

Aptamer‐Array‐Guided Protein Assembly Enhances Synthetic mRNA Switch Performance

Synthetic messenger RNA (mRNA) switches are powerful synthetic biological tools that can sense cellular molecules to manipulate cell fate. However, their performances are limited by high output signal noise due to leaky output protein expression. Here, we designed a readout control module that disables protein leakage from generating signal. Aptamer array on the switch guides the inactive output protein to self‐assemble into functional assemblies that generate output signal. Leaky protein expression fails to saturate the array, thus produces marginal signal. In this study, we demonstrated that switches with this module exhibit substantially lower signal noise and, consequently, higher input sensitivity and wider output range. Such switches are applicable for different types of input molecules and output proteins. The work here demonstrates a new type of spatially guided protein self‐assembly, affording novel synthetic mRNA switches that promise accurate cell manipulation for biomedical applications.

A genetic mammalian proportional-integral feedback control circuit for robust and precise gene regulation

To survive in the harsh environments they inhabit, cells have evolved sophisticated regulatory mechanisms that can maintain a steady internal milieu or homeostasis. This robustness, however, does not generally translate to engineered genetic circuits, such as the ones studied by synthetic biology. Here, we introduce an implementation of a minimal and universal gene regulatory motif that produces robust perfect adaptation for mammalian cells, and we improve on it by enhancing the precision of its regulation.

The processes that keep a cell alive are constantly challenged by unpredictable changes in its environment. Cells manage to counteract these changes by employing sophisticated regulatory strategies that maintain a steady internal milieu. Recently, the antithetic integral feedback motif has been demonstrated to be a minimal and universal biological regulatory strategy that can guarantee robust perfect adaptation for noisy gene regulatory networks in Escherichia coli. Here, we present a realization of the antithetic integral feedback motif in a synthetic gene circuit in mammalian cells. We show that the motif robustly maintains the expression of a synthetic transcription factor at tunable levels even when it is perturbed by increased degradation or its interaction network structure is perturbed by a negative feedback loop with an RNA-binding protein. We further demonstrate an improved regulatory strategy by augmenting the antithetic integral motif with additional negative feedback to realize antithetic proportional–integral control. We show that this motif produces robust perfect adaptation while also reducing the variance of the regulated synthetic transcription factor. We demonstrate that the integral and proportional–integral feedback motifs can mitigate the impact of gene expression burden, and we computationally explore their use in cell therapy. We believe that the engineering of precise and robust perfect adaptation will enable substantial advances in industrial biotechnology and cell-based therapeutics.

Synthetic Translational Regulation by Protein-Binding RNA Origami Scaffolds

Rational design approaches for the regulation of gene expression are expanding the synthetic biology toolbox. However, only a few tools for regulating gene expression at the translational level have been developed. Here, we devise an approach for translational regulation using the MS2 and PP7 aptamer and coat-protein pairs in Escherichia coli. The aptamers are used as operators in transcription units that encode proteins fused to their cognate coat proteins, which leads to self-repression. RNA origami scaffolds that contain up to four aptamers serve as an alternate binder to activate translation. With this system, we demonstrate that the increase in expression of a reporter protein is dependent on both the concentration and number of aptamers on RNA origami scaffolds. We also demonstrate regulation of multiple proteins using a single MS2 coat protein fusion and apply this method to regulate the relative expression of enzymes of the branched pathway for deoxyviolacein biosynthesis.

PERSIST platform provides programmable RNA regulation using CRISPR endoRNases

Regulated transgene expression is an integral component of gene therapies, cell therapies and biomanufacturing. However, transcription factor-based regulation, upon which most applications are based, suffers from complications such as epigenetic silencing that limit expression longevity and reliability. Constitutive transgene transcription paired with post-transcriptional gene regulation could combat silencing, but few such RNA- or protein-level platforms exist. Here we develop an RNA-regulation platform we call "PERSIST" which consists of nine CRISPR-specific endoRNases as RNA-level activators and repressors as well as modular OFF- and ON-switch regulatory motifs. We show that PERSIST-regulated transgenes exhibit strong OFF and ON responses, resist silencing for at least two months, and can be readily layered to construct cascades, logic functions, switches and other sophisticated circuit topologies. The orthogonal, modular and composable nature of this platform as well as the ease in constructing robust and predictable gene circuits promises myriad applications in gene and cell therapies.

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.

Translational recoding by chemical modification of non-AUG start codon ribonucleotide bases

In contrast to prokaryotes wherein GUG and UUG are permissive start codons, initiation frequencies from non-AUG codons are generally low in eukaryotes, with CUG being considered as strongest. Here, we report that combined 5-cytosine methylation (5mC) and pseudouridylation (Ψ) of near-cognate non-AUG start codons convert GUG and UUG initiation strongly favored over CUG initiation in eukaryotic translation under a certain context. This prokaryotic-like preference is attributed to enhanced NUG initiation by Ψ in the second base and reduced CUG initiation by 5mC in the first base. Molecular dynamics simulation analysis of tRNAiMet anticodon base pairing to the modified codons demonstrates that Ψ universally raises the affinity of codon:anticodon pairing within the ribosomal preinitiation complex through partially mitigating discrimination against non-AUG codons imposed by eukaryotic initiation factor 1. We propose that translational control by chemical modifications of start codon bases can offer a new layer of proteome diversity regulation and therapeutic mRNA technology.

Versatile Design of Intracellular Protein-Responsive Translational Regulation System for Synthetic mRNA

Synthetic mRNA (mRNA) enables transgene expression without the necessity of nuclear import and the risk of insertional mutagenesis, which makes it an attractive tool for medical applications such as vaccination and protein replacement therapy. For further improvement of mRNA therapeutics, cell-selective translation is desirable, because transgene expression in nontarget cells sometimes causes adverse effects. In this study, we developed an intracellular protein-responsive translational regulation system based on Caliciviral VPg-based translational activator (CaVT) combined with inteins and target protein-binding nanobodies. This system enabled both translational activation and repression in a target protein-dependent manner. Importantly, the target protein can be altered by simply exchanging the nanobodies. The versatile design for target protein-responsive translational regulation holds promise for producing mRNA therapeutics with high safety.

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.

RNP-Based Control Systems for Genetic Circuits in Synthetic Biology Beyond CRISPR

Ribonucleoproteins (RNPs) are RNA-protein complexes utilized natively in both prokaryotes and eukaryotes to regulate essential processes within the cell. Over the past few years, many of these native systems have been adapted to provide control over custom genetic targets. Engineered RNP-based control systems allow for fine-tune regulation of desired targets, by providing customizable nucleotide-nucleotide interactions. However, as there have been several engineered RNP systems developed recently, identifying an optimal system for various bioprocesses is challenging. Here, we review the most successful engineered RNP systems and their applications to survey the current state of the field. Additionally, we provide selection criteria to provide users a streamlined method for identifying an RNP control system most useful to their own work. Lastly, we discuss future applications of RNP control systems and how they can be utilized to address the current grand challenges of the synthetic biology community.

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.

Regulated control of gene therapies by drug-induced splicing

So far, gene therapies have relied on complex constructs that cannot be finely controlled^1,2. Here we report a universal switch element that enables precise control of gene replacement or gene editing after exposure to a small molecule. The small-molecule inducers are currently in human use, are orally bioavailable when given to animals or humans and can reach both peripheral tissues and the brain. Moreover, the switch system, which we denote X^on, does not require the co-expression of any regulatory proteins. Using X^on, the translation of the desired elements for controlled gene replacement or gene editing machinery occurs after a single oral dose of the inducer, and the robustness of expression can be controlled by the drug dose, protein stability and redosing. The ability of X^on to provide temporal control of protein expression can be adapted for cell-biology applications and animal studies. Additionally, owing to the oral bioavailability and safety of the drugs used, the X^on switch system provides an unprecedented opportunity to refine and tailor the application of gene therapies in humans.

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.

An RNA Triangle with Six Ribozyme Units Can Promote a Trans-Splicing Reaction through Trimerization of Unit Ribozyme Dimers

Ribozymes are catalytic RNAs that are attractive platforms for the construction of nanoscale objects with biological functions. We designed a dimeric form of the Tetrahymena group I ribozyme as a unit structure in which two ribozymes were connected in a tail-to-tail manner with a linker element. We introduced a kink-turn motif as a bent linker element of the ribozyme dimer to design a closed trimer with a triangular shape. The oligomeric states of the resulting ribozyme dimers (kUrds) were analyzed biochemically and observed directly by atomic force microscopy (AFM). Formation of kUrd oligomers also triggered trans-splicing reactions, which could be monitored with a reporter system to yield a fluorescent RNA aptamer as the trans-splicing product.

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.

Recent Advancements in the Loading and Modification of Therapeutic Exosomes

Exosomes have a rapid development of bio-nanoparticles for drug delivery and confluent advances in next-generation diagnostics, monitoring the progression of several diseases, and accurate guidance for therapy. Based on their prominent stability, cargo-carriage properties, stable circulating capability, and favorable safety profile, exosomes have great potential to regulate cellular communication by carrying variable cargoes into specific site. However, the specific loading strategies and modification methods for engineered exosomes to enhance the targeting ability are unclear. The clinical application of exosomes is still limited. In this review, we discuss both original and modified exosomes for loading specific therapeutic molecules (proteins, nucleic acids, and small molecules) and the design strategies used to target specific cells. This review can be used as a reference for further loading and modification strategies as well as for the therapeutic applications of exosomes.

In Vivo Validation of a Reversible Small Molecule-Based Switch for Synthetic Self-Amplifying mRNA Regulation

Synthetic mRNA therapeutics have the potential to revolutionize healthcare, as they enable patients to produce therapeutic proteins inside their own bodies. However, convenient methods that allow external control over the timing and magnitude of protein production after in vivo delivery of synthetic mRNA are lacking. In this study, we validate the in vivo utility of a synthetic self-amplifying mRNA (RNA replicon) whose expression can be turned off using a genetic switch that responds to oral administration of trimethoprim (TMP), a US Food and Drug Administration (FDA)-approved small-molecule drug. After intramuscular electroporation, the engineered RNA replicon exhibited dose-dependent and reversible expression of its encoded protein upon TMP administration. The TMP serum level needed for maximal downregulation of protein translation was approximately 45-fold below that used in humans for therapeutic purposes. To demonstrate the therapeutic potential of the technology, we injected mice with a TMP-responsive RNA replicon encoding erythropoietin (EPO) and successfully controlled the timing and magnitude of EPO production as well as changes in hematocrit. This work demonstrates the feasibility of controlling mRNA kinetics in vivo, thereby broadly expanding the clinical versatility of mRNA therapeutics.

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.

Programmable Synthetic Protein Circuits for the Identification and Suppression of Hepatocellular Carcinoma

Precisely identifying and killing tumor cells are diligent pursuits in oncotherapy. Synthesized gene circuits have emerged as an intelligent weapon to solve these problems. Gene circuits based on post-transcriptional regulation enable a faster response than systems based on transcriptional regulation, which requires transcription and translation, showing superior safety. In this study, synthetic-promoter-free gene circuits possessing two control layers were constructed to improve the specific recognition of tumor cells. Using split-TEV, we designed and verified the basic control layer of protein-protein interaction (PPI) sensing. Another orthogonal control layer was built to sense specific proteins. Two layers were integrated to generate gene circuits sensing both PPI and specific proteins, forming 10 logic gates. To demonstrate the utility of this system, the circuit was engineered to sense alpha-fetoprotein (AFP) expression and the PPI between YAP and 14-3-3σ, the matching profile of hepatocellular carcinoma (HCC). Gene-circuit-loaded cells distinguished HCC from other cells and released therapeutic antibodies, exhibiting in vitro and in vivo therapeutic effects.

Molecular swarm robots: recent progress and future challenges

Recent advancements in molecular robotics have been greatly contributed by the progress in various fields of science and technology, particularly in supramolecular chemistry, bio- and nanotechnology, and informatics. Yet one of the biggest challenges in molecular robotics has been controlling a large number of robots at a time and employing the robots for any specific task as flocks in order to harness emergent functions. Swarming of molecular robots has emerged as a new paradigm with potentials to overcome this hurdle in molecular robotics. In this review article, we comprehensively discuss the latest developments in swarm molecular robotics, particularly emphasizing the effective utilization of bio- and nanotechnology in swarming of molecular robots. Importance of tuning the mutual interaction among the molecular robots in regulation of their swarming is introduced. Successful utilization of DNA, photoresponsive molecules, and natural molecular machines in swarming of molecular robots to provide them with processing, sensing, and actuating ability is highlighted. The potentials of molecular swarm robots for practical applications by means of their ability to participate in logical operations and molecular computations are also discussed. Prospects of the molecular swarm robots in utilizing the emergent functions through swarming are also emphasized together with their future perspectives.

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.

Responsive self-assembly of tectoRNAs with loop-receptor interactions from the tetrahydrofolate (THF) riboswitch

Naturally occurring RNAs are known to exhibit a high degree of modularity, whereby specific structural modules (or motifs) can be mixed and matched to create new molecular architectures. The modular nature of RNA also affords researchers the ability to characterize individual structural elements in controlled synthetic contexts in order to gain new and critical insights into their particular structural features and overall performance. Here, we characterized the binding affinity of a unique loop–receptor interaction found in the tetrahydrofolate (THF) riboswitch using rationally designed self-assembling tectoRNAs. Our work suggests that the THF loop–receptor interaction has been fine-tuned for its particular role as a riboswitch component. We also demonstrate that the thermodynamic stability of this interaction can be modulated by the presence of folinic acid, which induces a local structural change at the level of the loop–receptor. This corroborates the existence of a THF binding site within this tertiary module and paves the way for its potential use as a THF responsive module for RNA nanotechnology and synthetic biology.

Daisy Chain Topology Based Mammalian Synthetic Circuits for RNA-Only Delivery

Owing to superior safety, the RNA-only delivery synthetic circuit is more suitable for cell-based medicine. Modules which possess matching inputs and outputs could be strung by daisy-chaining to compose RNA-only delivery synthetic gene circuits. In this study, we engineered well-characterized biological parts to construct composable modules, each of which could receive signals from the upstream module and transmit the processed signal to the next module using standard interfaces. Capsid-cNOT7, through which logic gates could be changed by merely changing the type of it, was used as the core element for logical process. Daisy chain topology was used to build RNA-only delivery mammalian synthetic circuits which possess validated functions such as fan out, protein sensing, drug sensing, light sensing, 2-input logic gate, 3-input logic gate, and volatile memory, providing a new method to simplify the design of RNA-only delivery synthetic circuits.

Cell-Free Protein Synthesis as a Prototyping Platform for Mammalian Synthetic Biology

The field of mammalian synthetic biology is expanding quickly, and technologies for engineering large synthetic gene circuits are increasingly accessible. However, for mammalian cell engineering, traditional tissue culture methods are slow and cumbersome, and are not suited for high-throughput characterization measurements. Here we have utilized mammalian cell-free protein synthesis (CFPS) assays using HeLa cell extracts and liquid handling automation as an alternative to tissue culture and flow cytometry-based measurements. Our CFPS assays take a few hours, and we have established optimized protocols for small-volume reactions using automated acoustic liquid handling technology. As a proof-of-concept, we characterized diverse types of genetic regulation in CFPS, including T7 constitutive promoter variants, internal ribosomal entry sites (IRES) constitutive translation-initiation sequence variants, CRISPR/dCas9-mediated transcription repression, and L7Ae-mediated translation repression. Our data shows simple regulatory elements for use in mammalian cells can be quickly prototyped in a CFPS model system.

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.

CRISPR technologies for stem cell engineering and regenerative medicine

CRISPR/Cas9 system exploits the concerted action of Cas9 nuclease and programmable single guide RNA (sgRNA), and has been widely used for genome editing. The Cas9 nuclease activity can be abolished by mutation to yield the catalytically deactivated Cas9 (dCas9). Coupling with the customizable sgRNA for targeting, dCas9 can be fused with transcription repressors to inhibit specific gene expression (CRISPR interference, CRISPRi) or fused with transcription activators to activate the expression of gene of interest (CRISPR activation, CRISPRa). Here we introduce the principles and recent advances of these CRISPR technologies, their delivery vectors and review their applications in stem cell engineering and regenerative medicine. In particular, we focus on in vitro stem cell fate manipulation and in vivo applications such as prevention of retinal and muscular degeneration, neural regeneration, bone regeneration, cartilage tissue engineering, as well as treatment of diseases in blood, skin and liver. Finally, the challenges to translate CRISPR to regenerative medicine and future perspectives are discussed and proposed.

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.

Methylation guide RNA evolution in archaea: structure, function and genomic organization of 110 C/D box sRNA families across six Pyrobaculum species

Archaeal homologs of eukaryotic C/D box small nucleolar RNAs (C/D box sRNAs) guide precise 2′-O-methyl modification of ribosomal and transfer RNAs. Although C/D box sRNA genes constitute one of the largest RNA gene families in archaeal thermophiles, most genomes have incomplete sRNA gene annotation because reliable, fully automated detection methods are not available. We expanded and curated a comprehensive gene set across six species of the crenarchaeal genus Pyrobaculum, particularly rich in C/D box sRNA genes. Using high-throughput small RNA sequencing, specialized computational searches and comparative genomics, we analyzed 526 Pyrobaculum C/D box sRNAs, organizing them into 110 families based on synteny and conservation of guide sequences which determine methylation targets. We examined gene duplications and rearrangements, including one family that has expanded in a pattern similar to retrotransposed repetitive elements in eukaryotes. New training data and inclusion of kink-turn secondary structural features enabled creation of an improved search model. Our analyses provide the most comprehensive, dynamic view of C/D box sRNA evolutionary history within a genus, in terms of modification function, feature plasticity, and gene mobility.

Design strategies and application progress of therapeutic exosomes

Exosomes have great potential to be drug delivery vehicles due to their natural material transportation properties, intrinsic long-term circulatory capability, and excellent biocompatibility, which are suitable for delivering a variety of chemicals, proteins, nucleic acids, and gene therapeutic agents. However, an effective method of loading specific protein agents into exosomes for absorption by target cells is still lacking. The application potential of exosome is still limited. In this review, we discussed the methods for loading specific treating molecules (proteins, nucleic acids and small chemicals) into exosomes, the design strategies for cell and tissue targeting, and the factors for exosome formation. This review can be used as a reference for further research as well as for the development of therapeutic exosomes.

An in Vivo Binding Assay for RNA-Binding Proteins Based on Repression of a Reporter Gene

We study translation repression in bacteria by engineering a regulatory circuit that functions as a binding assay for RNA binding proteins (RBP) in vivo. We do so by inducing expression of a fluorescent protein-RBP chimera, together with encoding its binding site at various positions within the ribosomal initiation region (+11-13 nt from the AUG) of a reporter module. We show that when bound by their cognate RBPs, the phage coat proteins for PP7 (PCP) and Qβ (QCP), strong repression is observed for all hairpin positions within the initiation region. Yet, a sharp transition to no-effect is observed when positioned in the elongation region, at a single-nucleotide resolution. Employing in vivo Selective 2'-hydroxyl acylation analyzed by primer extension followed by sequencing (SHAPE-seq) for a representative construct, established that in the translationally active state the mRNA molecule is nonstructured, while in the repressed state a structured signature was detected. We then utilize this regulatory phenomena to quantify the binding affinity of the coat proteins of phages MS2, PP7, GA, and Qβ to 14 cognate and noncognate binding sites in vivo. Using our circuit, we demonstrate qualitative differences between in vitro to in vivo binding characteristics for various variants when comparing to past studies. Furthermore, by introducing a simple mutation to the loop region for the Qβ-wt site, MCP binding is abolished, creating the first high-affinity QCP site that is completely orthogonal to MCP. Consequently, we demonstrate that our hybrid transcriptional-post-transcriptional circuit can be utilized as a binding assay to quantify RNA-RBP interactions in vivo.

rRNA Mimicry in RNA Regulation of Gene Expression

The rRNA is the largest and most abundant RNA in bacterial and archaeal cells. It is also one of the best-characterized RNAs in terms of its structural motifs and sequence variation. Production of ribosome components including >50 ribosomal proteins (r-proteins) consumes significant cellular resources. Thus, RNA cis-regulatory structures that interact with r-proteins to repress further r-protein synthesis play an important role in maintaining appropriate stoichiometry between r-proteins and rRNA. Classically, such mRNA structures were thought to directly mimic the rRNA. However, more than 30 years of research has demonstrated that a variety of different recognition and regulatory paradigms are present. This review will demonstrate how structural mimicry between the rRNA and mRNA cis-regulatory structures may take many different forms. The collection of mRNA structures that interact with r-proteins to regulate r-protein operons are best characterized in Escherichia coli, but are increasingly found within species from nearly all phyla of bacteria and several archaea. Furthermore, they represent a unique opportunity to assess the plasticity of RNA structure in the context of RNA-protein interactions. The binding determinants imposed by r-proteins to allow regulation can be fulfilled in many ways. Some r-protein-interacting mRNAs are immediately obvious as rRNA mimics from primary sequence similarity, others are identifiable only after secondary or tertiary structure determination, and some show no obvious similarity. In addition, across different bacterial species a host of different mechanisms of action have been characterized, showing that there is no simple one-size-fits-all solution.