Aptamer-Mediated Reversible Transactivation of Gene Expression by Light

Induction of bacterial expression at the mRNA level by light

Vital organismal processes, including development, differentiation and adaptation, involve altered gene expression. Although expression is frequently controlled at the transcriptional stage, various regulation mechanisms operate at downstream levels. Here, we leverage the photoreceptor NmPAL to optogenetically induce RNA refolding and the translation of bacterial mRNAs. Blue-light-triggered NmPAL binding disrupts a cis-repressed mRNA state, thereby relieves obstruction of translation initiation, and upregulates gene expression. Iterative probing and optimization of the circuit, dubbed riboptoregulator, enhanced induction to 30-fold. Given action at the mRNA level, the riboptoregulator can differentially regulate individual structural genes within polycistronic operons. Moreover, it is orthogonal to and can be wed with other gene-regulatory circuits for nuanced and more stringent gene-expression control. We thus advance the pAurora2 circuit that combines transcriptional and translational mechanisms to optogenetically increase bacterial gene expression by >1000-fold. The riboptoregulator strategy stands to upgrade numerous regulatory circuits and widely applies to expression control in microbial biotechnology, synthetic biology and materials science.

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.

Spatiotemporal control of RNA metabolism and CRISPR-Cas functions using engineered photoswitchable RNA-binding proteins

RNA molecules perform various crucial roles in diverse cellular processes, from translating genetic information to decoding the genome, regulating gene expression and catalyzing chemical reactions. RNA-binding proteins (RBPs) play an essential role in regulating the diverse behaviors and functions of RNA in live cells, but techniques for the spatiotemporal control of RBP activities and RNA functions are rarely reported yet highly desirable. We recently reported the development of LicV, a synthetic photoswitchable RBP that can bind to a specific RNA sequence in response to blue light irradiation. LicV has been used successfully for the optogenetic control of RNA localization, splicing, translation and stability, as well as for the photoswitchable regulation of transcription and genomic locus labeling. Compared to classical genetic or pharmacologic perturbations, LicV-based light-switchable effectors have the advantages of large dynamic range between dark and light conditions and submicron and millisecond spatiotemporal resolutions. In this protocol, we provide an easy, efficient and generalizable strategy for engineering photoswitchable RBPs for the spatiotemporal control of RNA metabolism. We also provide a detailed protocol for the conversion of a CRISPR-Cas system to optogenetic control. The protocols typically take 2-3 d, including transfection and results analysis. Most of this protocol is applicable to the development of novel LicV-based photoswitchable effectors for the optogenetic control of other RNA metabolisms and CRISPR-Cas functions.

Establishing artificial gene connections through RNA displacement-assembly-controlled CRISPR/Cas9 function

Construction of synthetic circuits that can reprogram genetic networks and signal pathways is a long-term goal for manipulation of biosystems. However, it is still highly challenging to build artificial genetic communications among endogenous RNA species due to their sequence independence and structural diversities. Here we report an RNA-based synthetic circuit that can establish regulatory linkages between expression of endogenous genes in both Escherichiacoli and mammalian cells. This design employs a displacement-assembly approach to modulate the activity of guide RNA for function control of CRISPR/Cas9. Our experiments demonstrate the great effectiveness of this RNA circuit for building artificial connections between expression of originally unrelated genes. Both exogenous and naturally occurring RNAs, including small/microRNAs and long mRNAs, are capable of controlling expression of another endogenous gene through this approach. Moreover, an artificial signal pathway inside mammalian cells is also successfully established to control cell apoptosis through our designed synthetic circuit. This study provides a general strategy for constructing synthetic RNA circuits, which can introduce artificial connections into the genetic networks of mammalian cells and alter the cellular phenotypes.

Allosteric Regulation of Aptamer Affinity through Mechano-Chemical Coupling

The capacity to precisely modulate aptamer affinity is important for a wide variety of applications. However, most such engineering strategies entail laborious trial-and-error testing or require prior knowledge of an aptamer's structure and ligand-binding domain. We describe here a simple and generalizable strategy for allosteric modulation of aptamer affinity by employing a double-stranded molecular clamp that destabilizes aptamer secondary structure through mechanical tension. We demonstrate the effectiveness of the approach with a thrombin-binding aptamer and show that we can alter its affinity by as much as 65-fold. We also show that this modulation can be rendered reversible by introducing a restriction enzyme cleavage site into the molecular clamp domain and describe a design strategy for achieving even more finely-tuned affinity modulation. This strategy requires no prior knowledge of the aptamer's structure and binding mechanism and should thus be generalizable across aptamers.

A Conformational Restriction Strategy for the Control of CRISPR/Cas Gene Editing with Photoactivatable Guide RNAs

The CRISPR/Cas system is one of the most powerful tools for gene editing. However, approaches for precise control of genome editing and regulatory events are still desirable. Here, we report the spatiotemporal and efficient control of CRISPR/Cas9- and Cas12a-mediated editing with conformationally restricted guide RNAs (gRNAs). This approach relied on only two or three pre-installed photo-labile substituents followed by an intramolecular cyclization, representing a robust synthetic method in comparison to the heavily modified linear gRNAs that often require extensive screening and time-consuming optimization. This tactic could direct the precise cleavage of the genes encoding green fluorescent protein (GFP) and the vascular endothelial growth factor A (VEGFA) protein within a predefined cutting region without notable editing leakage in live cells. We also achieved light-mediated myostatin (MSTN) gene editing in embryos, wherein a new bow-knot-type gRNA was constructed with excellent OFF/ON switch efficiency. Overall, our work provides a significant new strategy in CRISPR/Cas editing with modified circular gRNAs to precisely manipulate where and when genes are edited.

Light-Dependent Control of Bacterial Expression at the mRNA Level

Sensory photoreceptors mediate numerous light-dependent adaptations across organisms. In optogenetics, photoreceptors achieve the reversible, non-invasive, and spatiotemporally precise control by light of gene expression and other cellular processes. The light-oxygen-voltage receptor PAL binds to small RNA aptamers with sequence specificity upon blue-light illumination. By embedding the responsive aptamer in the ribosome-binding sequence of genes of interest, their expression can be downregulated by light. We developed the pCrepusculo and pAurora optogenetic systems that are based on PAL and allow to down- and upregulate, respectively, bacterial gene expression using blue light. Both systems are realized as compact, single plasmids that exhibit stringent blue-light responses with low basal activity and up to several 10-fold dynamic range. As PAL exerts light-dependent control at the RNA level, it can be combined with other optogenetic circuits that control transcription initiation. By integrating regulatory mechanisms operating at the DNA and mRNA levels, optogenetic circuits with emergent properties can thus be devised. As a case in point, the pEnumbra setup permits to upregulate gene expression under moderate blue light whereas strong blue light shuts off expression again. Beyond providing novel signal-responsive expression systems for diverse applications in biotechnology and synthetic biology, our work also illustrates how the light-dependent PAL-aptamer interaction can be harnessed for the control and interrogation of RNA-based processes.

Small-Molecule-Mediated Split-Aptamer Assembly for Inducible CRISPR-dCas9 Transcription Activation

Inducible CRISPR-dCas9 transcription system has become a powerful tool for transcription regulation and sensing. Here, we develop a new concept of small-molecule-mediated split-aptamer assembly for inducible CRISPR-dCas9 transcription activation, allowing quantitative detection and imaging of S-adenosyl methionine (SAM) in live cells. This inducible transcription system is designed by integrating one fragment of a split SAM aptamer to guide RNA (gRNA) and the other to MS2 arrays. SAM-mediated reassembly of the split fragments recruits an MCP-fused transcription activator to the gRNA-dCas9 complex, activating the expression of a near-infrared fluorescent protein for imaging. We demonstrate that this inducible transcription system achieves quantitative detection of SAM with high sensitivity in live cells. Our system shows that methionine adenosyltransferase 1A (MAT1A) and MAT2A can both catalyze SAM production in live cells and the SAM levels in cancer cells can be increased via upregulation of MAT1A mRNA by epigenetic inhibitors. This split-aptamer assembly strategy could afford a new approach for controlling the CRISPR-dCas9 system, enabling conditional transcription regulation in response to endogenous metabolites in live cells.

Photoactivatable nanoCRISPR/Cas9 System Based on crRNA Reversibly Immobilized on Carbon Nanoparticles

Here, we proposed a new approach to engineering a photoactivatable CRISPR/Cas9 gene-editing system. The novel nanoCRISPR/Cas9 system is based on the use of auxiliary photocleavable oligodeoxyribonucleotides (PC-DNAs) complementary to crRNA. PC-DNAs contained up to three UV-sensitive linkers made of 1-(2-nitrophenyl)-1,2-ethanediol inside the oligonucleotide chain. Immobilizing PC-DNAs on the surface of carbon nanoparticles through 3′-terminal pyrene residue provided sufficient blocking of crRNA (and corresponding Cas9 activity) before UV irradiation and allows for crRNA release after UV irradiation at 365 nm, which restores Cas9 activity. We optimized the length of blocking photocleavable oligonucleotide, number of linkers, time of irradiation, and the type of carbon nanoparticles. Based on the results, we consider the nanoCRISPR/Cas9 system involving carbon-encapsulated iron nanoparticles the most promising. It provides the greatest difference of functional activity before/after irradiation and can be used in prospective for magnetic field-controlled delivery of CRISPR system into the target cells or tissues and spatiotemporal gene editing induced by UV irradiation.

Computational design and experimental characterization of a photo-controlled mRNA-cap guanine-N7 methyltransferase

The spatial and temporal control of gene expression at the post-transcriptional level is essential in eukaryotic cells and developing multicellular organisms. In recent years optochemical and optogenetic tools have enabled the manipulation and investigation of many steps in the involved processes. However, examples for light-mediated control of eukaryotic mRNA processing and the responsible enzymes are still rare. In particular, methylation of the 5′ cap of mRNA is required for ribosome assembly, and the responsible guanine-N7 methyltransferase (MTase) from E. cuniculi (Ecm1) proved suitable for activating translation. Here, we report on a photoswitchable MTase obtained by bridging the substrate-binding cleft of Ecm1 with a tetra-ortho-methoxy-azobenzene. This azobenzene derivative is characterized by efficient trans-to-cis isomerization using red light at 615 nm. Starting from a cysteine-free Ecm1 variant (ΔCys), we used a computational approach to identify suitable conjugation sites for the azobenzene moiety. We created and characterized the four best-ranked variants, each featuring two appropriately positioned cysteines close to the substrate-binding cleft. Conjugating and crosslinking the azobenzene between C149/C155 in a designed Ecm1 variant (VAR3-Az) enabled light-dependent modulation of the MTase activity and showed a 50% higher activity for the cis form than the trans-form of the azobenzene conjugated to VAR3-Az.

Guided by computational design, we engineered a light-dependent 5' cap guanine-N7 methyltransferase by bridging the substrate-binding cleft with an azobenzene derivative.

Theophylline-induced synergic activation of guide RNA to control CRISPR/Cas9 function

Ligand-induced activation of CRISPR/Cas9 function is achieved based on a synergic approach through the integration of the theophylline aptamer into protein-unrecognized regions of guide RNA. This design of allosteric regulation opens a new window towards the broad involvement of RNA aptamers for conditional control of CRISPR/Cas9 function.

Inducible CRISPR-dCas9 Transcriptional Systems for Sensing and Genome Regulation

The clustered, regularly interspaced short palindromic repeats-associated protein 9 endonuclease (CRISPR-Cas9) and the nuclease-deactivated Cas9 (dCas9) systems have revolutionized our ability to precisely engineer and regulate genomes. Inducible CRISPR-dCas9-based transcriptional systems have been rapidly developed to conditionally control genetic manipulation. Current strategies mainly focus on conditional control of gRNA function and dCas9 protein using exogenous and endogenous triggers, including external light, small molecules, synthetic and intracellular oligonucleotides. These strategies have established novel platforms for the spatiotemporal regulation of genome activation and repression, epigenome editing, and so on. Herein, we summarize the recent progress in conditionally controlling CRISPR-dCas9 transcriptional systems through gRNA modulation and dCas9 protein engineering.

Light-control of cap methylation and mRNA translation via genetic code expansion of Ecm1

Gene expression is tightly regulated in all domains of life, with post-transcriptional regulation being more pronounced in higher eukaryotes. Optochemical and optogenetic approaches enable the actuation of many underlying processes by light, which is an excellent tool to exert spatio-temporal control. However, light-mediated control of eukaryotic mRNA processing and the respective enzymes has not been reported. We used genetic code expansion to install a photo-caged tyrosine (Y) in the active site of the cap methyltransferase Ecm1. This enzyme is responsible for guanine N7 methylation of the 5' cap, which is required for translation. Substituting Y284 with the photocaged ortho-nitrobenzyl-tyrosine (ONBY) almost completely abrogated the methylation activity of Ecm1. Irradiation with light removed the ONB group, restoring the native tyrosine and Ecm1 activity, yielding up to 97% conversion of the minimal substrate GpppA within 60 min after activation. Using luciferase- and eGFP-mRNAs as reporters, we could show that light actuates translation by inducing activation of Ecm1 ONBY284 in a eukaryotic in vitro translation system.

Aptamer-Mediated Reversible Transactivation of Gene Expression by Light

The investigation and manipulation of cellular processes with subcellular resolution requires non-invasive tools with spatiotemporal precision and reversibility. Building on the interaction of the photoreceptor PAL with an RNA aptamer, we describe a variation of the CRISPR/dCAS9 system for light-controlled activation of gene expression. This platform significantly reduces the coding space required for genetic manipulation and provides a strong on-switch with almost no residual activity in the dark. It adds to the current set of modular building blocks for synthetic biological circuit design and is broadly applicable.