Bidirectional regulation of mRNA translation in mammalian cells by using PUF domains

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.

Nature-inspired engineering of an artificial ligase enzyme by domain fusion

The function of most proteins is accomplished through the interplay of two or more protein domains and fine-tuned by natural evolution. In contrast, artificial enzymes have often been engineered from a single domain scaffold and frequently have lower catalytic activity than natural enzymes. We previously generated an artificial enzyme that catalyzed an RNA ligation by >2 million-fold but was likely limited in its activity by low substrate affinity. Inspired by nature's concept of domain fusion, we fused the artificial enzyme to a series of protein domains known to bind nucleic acids with the goal of improving its catalytic activity. The effect of the fused domains on catalytic activity varied greatly, yielding severalfold increases but also reductions caused by domains that previously enhanced nucleic acid binding in other protein engineering projects. The combination of the two better performing binding domains improved the activity of the parental ligase by more than an order of magnitude. These results demonstrate for the first time that nature's successful evolutionary mechanism of domain fusion can also improve an unevolved primordial-like protein whose structure and function had just been created in the test tube. The generation of multi-domain proteins might therefore be an ancient evolutionary process.

Sequence-specific targeting of RNA

Post-transcriptional modifications play an important role in several processes, including translation, splicing, and RNA degradation in eukaryotic cells. To investigate the function of specific modifications it is of high interest to develop tools for sequence-specific RNA-targeting. This work focuses on two abundant modifications of eukaryotic mRNA, namely methylation of the guanine-N7 position of the 5'-cap and internal N⁶-methyladenosine (m⁶A). We describe the sequence-specific targeting of model RNA transcripts via RNA-binding proteins, such as nuclease-deficient RNA-targeting Cas9 (RCas9) and the Pumilio homology domain (PumHD) fused to two different effector enzymes, the dioxygenase FTO and the guanine-N7 methyltransferase Ecm1. With this tool, we were able to install and remove the methylation at the respective positions with high specificity.

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.

A Novel Research Technology to Explore the Mystery of Traditional Chinese Medicine: Optogenetics

Traditional Chinese medicine (TCM) is gaining increasing popularity worldwide for the function of health promotion and adjuvant therapy. However, the world's understanding of TCM is far from enough, which seriously limits the modernization and internationalization of TCM. Therefore, modern and efficient analytical methods are urgently needed to understand the mechanism of TCM. Optogenetics is one of the most prevalent technologies in the 21st century and has been used to explore life science, especially neuroscience. It already has had great influences in the study of neural circuits and animal models of mental diseases and was named “Method of the Year” by the Nature Methods journal in 2010. Increased interests occurred in the applications of optogenetics to explore a myriad of medical and mental health disorders. However, it has not so far been noticed by TCM researchers. We elaborated on an idea that introducing this technique into the field of TCM research to improve diagnosis, treatments, and evaluating the therapeutic effects. In this review, we made a systematic prospect in the theory, feasibility, and application of TCM optogenetics. We mainly focused on applying optogenetic methodologies to make a more comprehensive understanding of TCM.

A light way for nuclear cell biologists

The nucleus is a very complex organelle present in eukaryotic cells. Having the crucial task to safeguard, organize and manage the genetic information, it must tightly control its molecular constituents, its shape and its internal architecture at any given time. Despite our vast knowledge of nuclear cell biology, much is yet to be unravelled. For instance, only recently we came to appreciate the existence of a dynamic nuclear cytoskeleton made of actin filaments that regulates processes such as gene expression, DNA repair and nuclear expansion. This suggests further exciting discoveries ahead of us. Modern cell biologists embrace a new methodology relying on precise perturbations of cellular processes that require a reversible, highly spatially confinable, rapid, inexpensive and tunEable external stimulus: light. In this review, we discuss how optogenetics, the state-of-the-art technology that uses genetically encoded light-sensitive proteins to steer biological processes, can be adopted to specifically investigate nuclear cell biology.

Effective RNA Regulation by Combination of Multiple Programmable RNA-Binding Proteins

RNAs play important roles in gene expression through translation and RNA splicing. Regulation of specific RNAs is useful to understand and manipulate specific transcripts. Pumilio and fem-3 mRNA-binding factor (PUF) proteins, programmable RNA-binding proteins, are promising tools for regulating specific RNAs by fusing them with various functional domains. The key question is: How can PUF-based molecular tools efficiently regulate RNA functions? Here, we show that the combination of multiple PUF proteins, compared to using a single PUF protein, targeting independent RNA sequences at the 3′ untranslated region (UTR) of a target transcript caused cooperative effects to regulate the function of the target RNA by luciferase reporter assays. It is worth noting that a higher efficacy was achieved with smaller amounts of each PUF expression vector introduced into the cells compared to using a single PUF protein. This strategy not only efficiently regulates target RNA functions but would also be effective in reducing off-target effects due to the low doses of each expression vector.

Controlling gene expression with light: a multidisciplinary endeavour

The expression of a gene to a protein is one of the most vital biological processes. The use of light to control biology offers unparalleled spatiotemporal resolution from an external, orthogonal signal. A variety of methods have been developed that use light to control the steps of transcription and translation of specific genes into proteins, for cell-free to in vivo biotechnology applications. These methods employ techniques ranging from the modification of small molecules, nucleic acids and proteins with photocages, to the engineering of proteins involved in gene expression using naturally light-sensitive proteins. Although the majority of currently available technologies employ ultraviolet light, there has been a recent increase in the use of functionalities that work at longer wavelengths of light, to minimise cellular damage and increase tissue penetration. Here, we discuss the different chemical and biological methods employed to control gene expression, while also highlighting the central themes and the most exciting applications within this diverse field.

Engineering REST-Specific Synthetic PUF Proteins to Control Neuronal Gene Expression: A Combined Experimental and Computational Study

Regulation of gene transcription is an essential mechanism for differentiation and adaptation of organisms. A key actor in this regulation process is the repressor element 1 (RE1)-silencing transcription factor (REST), a transcriptional repressor that controls more than 2000 putative target genes, most of which are neuron-specific. With the purpose of modulating REST expression, we exploited synthetic, ad hoc designed, RNA binding proteins (RBPs) able to specifically target and dock to REST mRNA. Among the various families of RBPs, we focused on the Pumilio and FBF (PUF) proteins, present in all eukaryotic organisms and controlling a variety of cellular functions. Here, a combined experimental and computational approach was used to design and test 8- and 16-repeat PUF proteins specific for REST mRNA. We explored the conformational properties and atomic features of the PUF-RNA recognition code by Molecular Dynamics simulations. Biochemical assays revealed that the 8- and 16-repeat PUF-based variants specifically bind the endogenous REST mRNA without affecting its translational regulation. The data also indicate a key role of stacking residues in determining the binding specificity. The newly characterized REST-specific PUF-based constructs act as excellent RNA-binding modules and represent a versatile and functional platform to specifically target REST mRNA and modulate its endogenous expression.

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.

Programmable RNA manipulation in living cells

RNAs are responsible for mediating genetic information flow within the cell. RNA splicing, modification, trafficking, translation, and stability are all controlled at the transcript level. However, biological tools to study and manipulate them in a programmable fashion are currently limited. In this review, we summarize recent advances regarding available RNA-targeting systems discovered so far, including CRISPR-based technologies-Cas9 and Cas13, and programmable RNA-binding proteins-PUF and PPR. These tools allow transcript-specific manipulation in gene expression.

The potential of engineered eukaryotic RNA-binding proteins as molecular tools and therapeutics

Eukaroytic RNA-binding proteins (RBPs) recognize and process RNAs through recognition of their sequence motifs via RNA-binding domains (RBDs). RBPs usually consist of one or more RBDs and can include additional functional domains that modify or cleave RNA. Engineered RBPs have been used to answer basic biology questions, control gene expression, locate viral RNA in vivo, as well as many other tasks. Given the growing number of diseases associated with RNA and RBPs, engineered RBPs also have the potential to serve as therapeutics. This review provides an in depth description of recent advances in engineered RBPs and discusses opportunities and challenges in the field. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Methods > RNA Nanotechnology RNA in Disease and Development > RNA in Disease.

Sequence-specific m6A demethylation in RNA by FTO fused to RCas9

N⁶-methyladenosine (m⁶A) is the most common internal modification in eukaryotic mRNA and associated with numerous cellular processes in health and disease. Up- and down-regulation of its "writer" or "eraser" proteins alter the global m⁶A level; however, modifying distinct m⁶A sites has remained elusive. We genetically fused the dioxygenase FTO responsible for m⁶A demethylation to RCas9 as an RNA-targeting module. The resulting RCas9-FTO retained demethylation activity and bound to RNA in a sequence-specific manner depending on the sgRNA and PAMmer. Using SCARLET analysis, we quantified the m⁶A level at a specific site and analyzed the effect of the PAM-to-m⁶A distance on activity. Sequence-specific demethylation by RCas9-FTO was tested on different RNA combinations and showed up to 15-fold sequence preference for target RNA compared to off-target RNA. Taken together, RCas9-FTO represents a new tool for sequence-specific demethylation of m⁶A in RNA that can be readily adapted to any given RNA sequence and opens the door to studying the function of distinct m⁶A sites.

Modulation of miRNA function by natural and synthetic RNA-binding proteins in cancer

RNA-binding proteins (RBPs) and microRNAs (miRNAs) are the most important regulators of mRNA stability and translation in eukaryotic cells; however, the complex interplay between these systems is only now coming to light. RBPs and miRNAs regulate a unique set of targets in either a positive or negative manner and their regulation is mainly opposed to each other on overlapping targets. In some cases, the levels of RBPs or miRNAs regulate the cellular levels of one another and decreased levels of either results in changes in translation of their targets. There is growing evidence that these regulatory circuits are crucial in the development and progression of cancer; however, the rules underlying synergism and antagonism between miRNAs and RNA-binding proteins remain unclear. Synthetic biology seeks to develop artificial systems to better understand their natural counterparts and to develop new, useful technologies for manipulation of gene expression at the RNA level. The recent development of artificial RNA-binding proteins promises to enable a much greater understanding of the importance of the functional interactions between RNA-binding proteins and miRNAs, as well as enabling their manipulation for therapeutic purposes.

A Yeast System for Discovering Optogenetic Inhibitors of Eukaryotic Translation Initiation

The precise spatiotemporal regulation of protein synthesis is essential for many complex biological processes such as memory formation, embryonic development, and tumor formation. Current methods used to study protein synthesis offer only a limited degree of spatiotemporal control. Optogenetic methods, in contrast, offer the prospect of controlling protein synthesis noninvasively within minutes and with a spatial scale as small as a single synapse. Here, we present a hybrid yeast system where growth depends on the activity of human eukaryotic initiation factor 4E (eIF4E) that is suitable for screening optogenetic designs for the down-regulation of protein synthesis. We used this system to screen a diverse initial panel of 15 constructs designed to couple a light switchable domain (PYP, RsLOV, AsLOV, Dronpa) to 4EBP2 (eukaryotic initiation factor 4E binding protein 2), a native inhibitor of translation initiation. We identified cLIPS1 (circularly permuted LOV inhibitor of protein synthesis 1), a fusion of a segment of 4EBP2 and a circularly permuted version of the LOV2 domain from Avena sativa, as a photoactivated inhibitor of translation. Adapting the screen for higher throughput, we tested small libraries of cLIPS1 variants and found cLIPS2, a construct with an improved degree of optical control. We show that these constructs can both inhibit translation in yeast harboring a human eIF4E in vivo, and bind human eIF4E in vitro in a light-dependent manner. This hybrid yeast system thus provides a convenient way for discovering optogenetic constructs that can regulate human eIF4E-dependent translation initiation in a mechanistically defined manner.

Live Cell Imaging of Endogenous RNAs Using Pumilio Homology Domain Mutants: Principles and Applications

Recently, dynamic changes in the location of RNA in space and time in living cells have become a target of interest in biology because of their essential roles in controlling physiological phenomena. To visualize RNA, methods for the fluorescent labeling of RNA in living cells have been developed. For RNA labeling, oligonucleotide-based RNA probes have mainly been used because of their high selectivity for target RNAs. By contrast, protein-based RNA probes have not been used widely because of their lack of design flexibility, although they have various potential advantages compared with nucleotide-based probes, such as controllability of intracellular localization, high detectability, and ease of introduction into cells and transgenic organisms in a cell type and tissue specific manner by genetic engineering techniques. This Perspective focuses on a possible approach to the development of protein-based RNA probes using Pumilio homology domain (PUM-HD) mutants. The PUM-HD is a domain of an RNA binding protein that allows custom-made modifications to recognize a given eight-base RNA sequence. PUM-HD-based RNA probes have been applied to visualize various RNAs in living cells. Here, the techniques and RNA imaging results obtained using the PUM-HD are introduced.

At Light Speed: Advances in Optogenetic Systems for Regulating Cell Signaling and Behavior

Cells are bombarded by extrinsic signals that dynamically change in time and space. Such dynamic variations can exert profound effects on behaviors, including cellular signaling, organismal development, stem cell differentiation, normal tissue function, and disease processes such as cancer. Although classical genetic tools are well suited to introduce binary perturbations, new approaches have been necessary to investigate how dynamic signal variation may regulate cell behavior. This fundamental question is increasingly being addressed with optogenetics, a field focused on engineering and harnessing light-sensitive proteins to interface with cellular signaling pathways. Channelrhodopsins initially defined optogenetics; however, through recent use of light-responsive proteins with myriad spectral and functional properties, practical applications of optogenetics currently encompass cell signaling, subcellular localization, and gene regulation. Now, important questions regarding signal integration within branch points of signaling networks, asymmetric cell responses to spatially restricted signals, and effects of signal dosage versus duration can be addressed. This review summarizes emerging technologies and applications within the expanding field of optogenetics.

PPR-SMR protein SOT1 has RNA endonuclease activity

Numerous attempts have been made to identify and engineer sequence-specific RNA endonucleases, as these would allow for efficient RNA manipulation. However, no natural RNA endonuclease that recognizes RNA in a sequence-specific manner has been described to date. Here, we report that SUPPRESSOR OF THYLAKOID FORMATION 1 (SOT1), an Arabidopsis pentatricopeptide repeat (PPR) protein with a small MutS-related (SMR) domain, has RNA endonuclease activity. We show that the SMR moiety of SOT1 performs the endonucleolytic maturation of 23S and 4.5S rRNA through the PPR domain, specifically recognizing a 13-nucleotide RNA sequence in the 5' end of the chloroplast 23S-4.5S rRNA precursor. In addition, we successfully engineered the SOT1 protein with altered PPR motifs to recognize and cleave a predicted RNA substrate. Our findings point to SOT1 as an exciting tool for RNA manipulation.

A FACS-based screening strategy to assess sequence-specific RNA-binding of Pumilio protein variants in E. coli

Sequence-specific and programmable binding of proteins to RNA bears the potential to detect and manipulate target RNAs. Applications include analysis of subcellular RNA localization or post-transcriptional regulation but require sequence-specificity to be readily adjustable to any target RNA. The Pumilio homology domain binds an eight nucleotide target sequence in a predictable manner allowing for rational design of variants with new specificities. We describe a high-throughput system for screening Pumilio variants based on fluorescence-activated cell sorting of E. coli. Our approach should help optimizing variants obtained from rational design regarding folding and stability or identifying new variants with alternative binding modes.

Engineering reprogrammable RNA-binding proteins for study and manipulation of the transcriptome

With the expanding interest in RNA biology, interest in artificial RNA-binding proteins (RBPs) is likewise increasing. RBPs can be designed in a modular fashion, whereby effector and RNA-binding domains are combined in chimeric proteins that exhibit both functions and can be applied for regulation of a broad range of biological processes. The elucidation of the RNA recognition code for Pumilio and fem-3 mRNA-binding factor (PUF) homology proteins allowed engineering of artificial RBPs for targeting endogenous mRNAs. In this review, we will focus on the recent advances in elucidating and reprogramming PUF domain specificity, update on several promising applications of PUF-based designer RBPs, and discuss some other domains that hold the potential to be used as the RNA-binding scaffolds for designer RBP engineering.

Programmable RNA-binding protein composed of repeats of a single modular unit

The ability to monitor and perturb RNAs in living cells would benefit greatly from a modular protein architecture that targets unmodified RNA sequences in a programmable way. We report that the RNA-binding protein PumHD (Pumilio homology domain), which has been widely used in native and modified form for targeting RNA, can be engineered to yield a set of four canonical protein modules, each of which targets one RNA base. These modules (which we call Pumby, for Pumilio-based assembly) can be concatenated in chains of varying composition and length, to bind desired target RNAs. The specificity of such Pumby-RNA interactions was high, with undetectable binding of a Pumby chain to RNA sequences that bear three or more mismatches from the target sequence. We validate that the Pumby architecture can perform RNA-directed protein assembly and enhancement of translation of RNAs. We further demonstrate a new use of such RNA-binding proteins, measurement of RNA translation in living cells. Pumby may prove useful for many applications in the measurement, manipulation, and biotechnological utilization of unmodified RNAs in intact cells and systems.

Development of Genome Engineering Tools from Plant-Specific PPR Proteins Using Animal Cultured Cells

The pentatricopeptide repeat (PPR) motif is a sequence-specific RNA/DNA-binding module. Elucidation of the RNA/DNA recognition mechanism has enabled engineering of PPR motifs as new RNA/DNA manipulation tools in living cells, including for genome editing. However, the biochemical characteristics of PPR proteins remain unknown, mostly due to the instability and/or unfolding propensities of PPR proteins in heterologous expression systems such as bacteria and yeast. To overcome this issue, we constructed reporter systems using animal cultured cells. The cell-based system has highly attractive features for PPR engineering: robust eukaryotic gene expression; availability of various vectors, reagents, and antibodies; highly efficient DNA delivery ratio (>80 %); and rapid, high-throughput data production. In this chapter, we introduce an example of such reporter systems: a PPR-based sequence-specific translational activation system. The cell-based reporter system can be applied to characterize plant genes of interested and to PPR engineering.

Regulation of neural gene transcription by optogenetic inhibition of the RE1-silencing transcription factor

Optogenetics provides new ways to activate gene transcription; however, no attempts have been made as yet to modulate mammalian transcription factors. We report the light-mediated regulation of the repressor element 1 (RE1)-silencing transcription factor (REST), a master regulator of neural genes. To tune REST activity, we selected two protein domains that impair REST-DNA binding or recruitment of the cofactor mSin3a. Computational modeling guided the fusion of the inhibitory domains to the light-sensitive Avena sativa light-oxygen-voltage-sensing (LOV) 2-phototrophin 1 (AsLOV2). By expressing AsLOV2 chimeras in Neuro2a cells, we achieved light-dependent modulation of REST target genes that was associated with an improved neural differentiation. In primary neurons, light-mediated REST inhibition increased Na(+)-channel 1.2 and brain-derived neurotrophic factor transcription and boosted Na(+) currents and neuronal firing. This optogenetic approach allows the coordinated expression of a cluster of genes impinging on neuronal activity, providing a tool for studying neuronal physiology and correcting gene expression changes taking place in brain diseases.

Multilevel Regulation and Translational Switches in Synthetic Biology

In contrast to the versatility of regulatory mechanisms in natural systems, synthetic genetic circuits have been so far predominantly composed of transcriptionally regulated modules. This is about to change as the repertoire of foundational tools for post-transcriptional regulation is quickly expanding. We provide an overview of the different types of translational regulators: protein, small molecule and ribonucleic acid (RNA) responsive and we describe the new emerging circuit designs utilizing these tools. There are several advantages of achieving multilevel regulation via translational switches and it is likely that such designs will have the greatest and earliest impact in mammalian synthetic biology for regenerative medicine and gene therapy applications.

A universal strategy for regulating mRNA translation in prokaryotic and eukaryotic cells

We describe a simple strategy to control mRNA translation in both prokaryotic and eukaryotic cells which relies on a unique protein–RNA interaction. Specifically, we used the Pumilio/FBF (PUF) protein to repress translation by binding in between the ribosome binding site (RBS) and the start codon (in Escherichia coli), or by binding to the 5′ untranslated region of target mRNAs (in mammalian cells). The design principle is straightforward, the extent of translational repression can be tuned and the regulator is genetically encoded, enabling the construction of artificial signal cascades. We demonstrate that this approach can also be used to regulate polycistronic mRNAs; such regulation has rarely been achieved in previous reports. Since the regulator used in this study is a modular RNA-binding protein, which can be engineered to target different 8-nucleotide RNA sequences, our strategy could be used in the future to target endogenous mRNAs for regulating metabolic flows and signaling pathways in both prokaryotic and eukaryotic cells.