Cell-Selective Bioorthogonal Metabolic Labeling of RNA

Cell-selective bioorthogonal labeling

In classic bioorthogonal labeling experiments, the cell's biosynthetic machinery incorporates bioorthogonal tags, creating tagged biomolecules that are subsequently reacted with a corresponding bioorthogonal partner. This two-step approach labels biomolecules throughout the organism indiscriminate of cell type, which can produce background in applications focused on specific cell populations. In this review, we cover advances in bioorthogonal chemistry that enable targeting of bioorthogonal labeling to a desired cell type. Such cell-selective bioorthogonal labeling is achieved in one of three ways. The first approach restricts labeling to specific cells by cell-selective expression of engineered enzymes that enable the bioorthogonal tag's incorporation. The second approach preferentially localizes the bioorthogonal reagents to the desired cell types to restrict their uptake to the desired cells. Finally, the third approach cages the reactivity of the bioorthogonal reagents, allowing activation of the reaction in specific cells by uncaging the reagents selectively in those cell populations.

Determining RNA Natural Modifications and Nucleoside Analog-Labeled Sites by a Chemical/Enzyme-Induced Base Mutation Principle

The natural chemical modifications of messenger RNA (mRNA) in living organisms have shown essential roles in both physiology and pathology. The mapping of mRNA modifications is critical for interpreting their biological functions. In another dimension, the synthesized nucleoside analogs can enable chemical labeling of cellular mRNA through a metabolic pathway, which facilitates the study of RNA dynamics in a pulse-chase manner. In this regard, the sequencing tools for mapping both natural modifications and nucleoside tags on mRNA at single base resolution are highly necessary. In this work, we review the progress of chemical sequencing technology for determining both a variety of naturally occurring base modifications mainly on mRNA and a few on transfer RNA and metabolically incorporated artificial base analogs on mRNA, and further discuss the problems and prospects in the field.

An Optimized Enzyme-Nucleobase Pair Enables In Vivo RNA Metabolic Labeling with Improved Cell-Specificity

Current transcriptome-wide analyses have identified a growing number of regulatory RNA with expression that is characterized in a cell-type-specific manner. Herein, we describe RNA metabolic labeling with improved cell-specificity utilizing the in vivo expression of an optimized uracil phosphoribosyltransferase (UPRT) enzyme. We demonstrate improved selectivity for metabolic incorporation of a modified nucleobase (5-vinyuracil) into nascent RNA, using a battery of tests. The selective incorporation of vinyl-U residues was demonstrated in 3xUPRT LM2 cells through validation with dot blot, qPCR, LC-MS/MS and microscopy analysis. We also report using this approach in a metastatic human breast cancer mouse model for profiling cell-specific nascent RNA.

Probing Nascent RNA with Metabolic Incorporation of Modified Nucleosides

The discovery of previously unknown functional roles of RNA in biological systems has led to increased interest in revealing novel RNA molecules as therapeutic targets and the development of tools to better understand the role of RNA in cells. RNA metabolic labeling broadens the scope of studying RNA by incorporating of unnatural nucleobases and nucleosides with bioorthogonal handles that can be utilized for chemical modification of newly synthesized cellular RNA. Such labeling of RNA provides access to applications including measurement of the rates of synthesis and decay of RNA, cellular imaging for RNA localization, and selective enrichment of nascent RNA from the total RNA pool. Several unnatural nucleosides and nucleobases have been shown to be incorporated into RNA by endogenous RNA synthesis machinery of the cells. RNA metabolic labeling can also be performed in a cell-specific manner, where only cells expressing an essential enzyme incorporate the unnatural nucleobase into their RNA. Although several discoveries have been enabled by the current RNA metabolic labeling methods, some key challenges still exist: (i) toxicity of unnatural analogues, (ii) lack of RNA-compatible conjugation chemistries, and (iii) background incorporation of modified analogues in cell-specific RNA metabolic labeling. In this Account, we showcase work done in our laboratory to overcome these challenges faced by RNA metabolic labeling.To begin, we discuss the cellular pathways that have been utilized to perform RNA metabolic labeling and study the interaction between nucleosides and nucleoside kinases. Then we discuss the use of vinyl nucleosides for metabolic labeling and demonstrate the low toxicity of 5-vinyluridine (5-VUrd) compared to other widely used nucleosides. Next, we discuss cell-specific RNA metabolic labeling with unnatural nucleobases, which requires the expression of a specific phosphoribosyl transferase (PRT) enzyme for incorporation of the nucleobase into RNA. In the course of this work, we discovered the enzyme uridine monophosphate synthase (UMPS), which is responsible for nonspecific labeling with modified uracil nucleobases. We were able to overcome this background labeling by discovering a mutant uracil PRT (UPRT) that demonstrates highly specific RNA metabolic labeling with 5-vinyluracil (5-VU). Furthermore, we discuss the optimization of inverse-electron-demand Diels-Alder (IEDDA) reactions for performing chemical modification of vinyl nucleosides to achieve covalent conjugation of RNA without transcript degradation. Finally, we highlight our latest endeavor: the development of mutually orthogonal chemical reactions for selective labeling of 5-VUrd and 2-vinyladenosine (2-VAdo), which allows for potential use of multiple vinyl nucleosides for simultaneous investigation of multiple cellular processes involving RNA. We hope that our methods and discoveries encourage scientists studying biological systems to include RNA metabolic labeling in their toolkit for studying RNA and its role in biological systems.

Exploiting Endogenous Enzymes for Cancer-Cell Selective Metabolic Labeling of RNA in Vivo

Tissues and organs are composed of many diverse cell types, making cell-specific gene expression profiling a major challenge. Herein we report that endogenous enzymes, unique to a cell of interest, can be utilized to enable cell-specific metabolic labeling of RNA. We demonstrate that appropriately designed "caged" nucleosides can be rendered active by serving as a substrate for cancer-cell specific enzymes to enable RNA metabolic labeling, only in cancer cells. We envision that the ease and high stringency of our approach will enable expression analysis of tumor cells in complex environments.

Total RNA Synthesis and its Covalent Labeling Innovation

RNA plays critical roles in a wide range of physiological processes. For example, it is well known that RNA plays an important role in regulating gene expression, cell proliferation, and differentiation, and many other chemical and biological processes. However, the research community still suffers from limited approaches that can be applied to readily visualize a specific RNA-of-interest (ROI). Several methods can be used to track RNAs; these rely mainly on biological properties, namely, hybridization, aptamer, reporter protein, and protein binding. With respect to covalent approaches, very few cases have been reported. Happily, several new methods for efficient labeling studies of ROIs have been demonstrated successfully in recent years. Additionally, methods employed for the detection of ROIs by RNA modifying enzymes have also proved feasible. Several approaches, namely, phosphoramidite chemistry, in vitro transcription reactions, co-transcription reactions, chemical post-modification, RNA modifying enzymes, ligation, and other methods targeted at RNA labeling have been revealed in the past decades. To illustrate the most recent achievements, this review aims to summarize the most recent research in the field of synthesis of RNAs-of-interest bearing a variety of unnatural nucleosides, the subsequent RNA labeling research via biocompatible ligation, and beyond.

Labeling and sequencing nucleic acid modifications using bio-orthogonal tools

The bio-orthogonal reaction is a type of reaction that can occur within a cell without interfering with the active components of the cell. Bio-orthogonal reaction techniques have been used to label and track the synthesis, metabolism, and interactions of distinct biomacromolecules in cells. Thus, it is a handy tool for analyzing biological macromolecules within cells. Nucleic acid modifications are widely distributed in DNA and RNA in cells and play a critical role in regulating physiological and pathological cellular activities. Utilizing bio-orthogonal tools to study modified bases is a critical and worthwhile research direction. The development of bio-orthogonal reactions focusing on nucleic acid modifications has enabled the mapping of nucleic acid modifications in DNA and RNA. This review discusses the recent advances in bio-orthogonal labeling and sequencing nucleic acid modifications in DNA and RNA.

Labeling nucleic acid modifications using bio-orthogonal tools, then sequencing and imaging the labeled modifications in DNA and RNA.

Mutually Orthogonal Bioconjugation of Vinyl Nucleosides for RNA Metabolic Labeling

We report a strategy for the orthogonal conjugation of the vinyl nucleosides, 5-vinyluridine (5-VU) and 2-vinyladenosine (2-VA), via selective reactivity with maleimide and tris(2-carboxyethyl)phosphine (TCEP), respectively. The orthogonality was investigated using density functional theory (DFT) and confirmed by reactions with vinyl nucleosides. Further, these chemistries were used to modify RNA for fluorescent cell imaging. These reactions allow for the expanded use of RNA metabolic labeling to study nascent RNA expression within different RNA populations.

Bioorthogonal chemistry

Bioorthogonal chemistry represents a class of high-yielding chemical reactions that proceed rapidly and selectively in biological environments without side reactions towards endogenous functional groups. Rooted in the principles of physical organic chemistry, bioorthogonal reactions are intrinsically selective transformations not commonly found in biology. Key reactions include native chemical ligation and the Staudinger ligation, copper-catalysed azide-alkyne cycloaddition, strain-promoted [3 + 2] reactions, tetrazine ligation, metal-catalysed coupling reactions, oxime and hydrazone ligations as well as photoinducible bioorthogonal reactions. Bioorthogonal chemistry has significant overlap with the broader field of 'click chemistry' - high-yielding reactions that are wide in scope and simple to perform, as recently exemplified by sulfuryl fluoride exchange chemistry. The underlying mechanisms of these transformations and their optimal conditions are described in this Primer, followed by discussion of how bioorthogonal chemistry has become essential to the fields of biomedical imaging, medicinal chemistry, protein synthesis, polymer science, materials science and surface science. The applications of bioorthogonal chemistry are diverse and include genetic code expansion and metabolic engineering, drug target identification, antibody-drug conjugation and drug delivery. This Primer describes standards for reproducibility and data deposition, outlines how current limitations are driving new research directions and discusses new opportunities for applying bioorthogonal chemistry to emerging problems in biology and biomedicine.

Chemical methods for measuring RNA expression with metabolic labeling

Tracking the expression of RNA in a cell-specific manner is a major challenge in basic and disease research. Herein we outline the current state of employing chemical approaches for cell-specific RNA expression studies. We define the utility of metabolic labels for tracking RNA synthesis, the approaches for characterizing metabolic incorporation and enrichment of labeled RNAs, and finally outline how these approaches have been used to study biological systems by providing mechanistic insights into transcriptional dynamics. Further efforts on this front will be the continued development of novel chemical handles for RNA enrichment and profiling as well as innovative approaches to control cell-specific incorporation of chemically modified metabolic probes. These advancements in RNA metabolic labeling techniques permit sensitive detection of RNA expression dynamics within relatively small subsets of cells in living tissues and organisms that are critical to performing complex developmental and pathological processes. This article is categorized under: RNA Methods > RNA Analyses in Cells RNA Evolution and Genomics > Ribonomics RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry.

Posttranscriptional Suzuki-Miyaura Cross-Coupling Yields Labeled RNA for Conformational Analysis and Imaging

Chemical labeling of RNA by using chemoselective reactions that work under biologically benign conditions is increasingly becoming valuable in the in vitro and in vivo analysis of RNA. Here, we describe a modular RNA labeling method based on a posttranscriptional Suzuki-Miyaura coupling reaction, which works under mild conditions and enables the direct installation of various biophysical reporters and tags. This two-part procedure involves the incorporation of a halogen-modified UTP analog (5-iodouridine-5'-triphosphate) by a transcription reaction. Subsequent posttranscriptional coupling with boronic acid/ester substrates in the presence of a palladium catalyst provides access to RNA labeled with (a) fluorogenic environment-sensitive nucleosides for probing nucleic acid structure and recognition, (b) fluorescent probes for microscopy, and (3) affinity tags for pull-down and immunoassays. It is expected that this method could also become useful for imaging nascent RNA transcripts in cells if the nucleotide analog can be metabolically incorporated and coupled with reporters by metal-assisted cross-coupling reactions.

A Bump-Hole Strategy for Increased Stringency of Cell-Specific Metabolic Labeling of RNA

Profiling RNA expression in a cell-specific manner continues to be a grand challenge in biochemical research. Bioorthogonal nucleosides can be utilized to track RNA expression; however, these methods currently have limitations due to background and incorporation of analogs into undesired cells. Herein, we design and demonstrate that uracil phosphoribosyltransferase can be engineered to match 5-vinyluracil for cell-specific metabolic labeling of RNA with exceptional specificity and stringency.

In Cellulo Generation of Fluorescent Probes for Live-Cell Imaging of Cylooxygenase-2

Live-cell imaging with fluorescent probes is an essential tool in chemical biology to visualize the dynamics of biological processes in real-time. Intracellular disease biomarker imaging remains a formidable challenge due to the intrinsic limitations of conventional fluorescent probes and the complex nature of cells. This work reports the in cellulo assembly of a fluorescent probe to image cyclooxygenase-2 (COX-2). We developed celecoxib-azide derivative 14, possessing favorable biophysical properties and excellent COX-2 selectivity profile. In cellulo strain-promoted fluorogenic click chemistry of COX-2-engaged compound 14 with non/weakly-fluorescent compounds 11 and 17 formed fluorescent probes 15 and 18 for the detection of COX-2 in living cells. Competitive binding studies, biophysical, and comprehensive computational analyses were used to describe protein-ligand interactions. The reported new chemical toolbox enables precise visualization and tracking of COX-2 in live cells with superior sensitivity in the visible range.

Nucleic Acid Conformation Influences Postsynthetic Suzuki-Miyaura Labeling of Oligonucleotides

Chemoselective transformations that work under physiological conditions have emerged as powerful tools to label nucleic acids in cell-free and cellular environments. However, detailed studies investigating the influence of nucleic acid conformation on the performance of such chemoselective nucleic labeling methods are less explored. Given that nucleic acids adopt complex structures, it is highly important to study the scope of the chemical modification method in the context of nucleic acid conformations. Here we report a systematic study on the effect of local conformation on the postsynthetic Suzuki-Miyaura functionalization of human telomeric (H-Telo) DNA repeat oligonucleotide (ON) sequences, which form multiple G-quadruplex (GQ) structures. 5-Iodo-2'-deoxyuridine (IdU)-modified H-Telo ONs were synthesized by the solid-phase method, and when subjected to Suzuki-Miyaura cross-coupling reaction, its efficiency was found to depend on the type of conformation and the position of IdU label in different loops of the GQ structure. IdU-labeled GQs gave better yields as compared to single-stranded random coil structures. However, the IdU-labeled duplex under different ionic conditions did not undergo the coupling reaction. Further, using this method, we directly installed an environment-sensitive fluorescent probe, which photophysically reported the formation as well as distinguished different GQ topologies of telomeric repeat. Collectively, this systematic study underscores the influence of nucleic acid conformation, which has to be taken into account when establishing postsynthetic chemoselective functionalization strategies.

An optimized chemical-genetic method for cell-specific metabolic labeling of RNA

Tissues and organs are composed of diverse cell types, which poses a major challenge for cell-specific gene expression profiling. Current metabolic labeling methods rely on the inability of mammalian cells to incorporate exogenous pyrimidine analogs, which are then co-opted by ectopically-expressed enzymes. We demonstrate that mammalian cells can incorporate uracil analogs and characterize the enzymatic pathways responsible for high background incorporation. To overcome these limitations, we developed a novel small-molecule/enzyme pair consisting of uridine-cytidine kinase 2 (UCK2) and 2’-azidouridine (2’AzUd). We demonstrate that 2’AzUd is only incorporated in UCK2-expressing cells and characterize selectivity mechanisms using molecular dynamics and X-ray crystallography. Furthermore, this pair can be used to purify and track RNA from specific cellular populations, making it ideal for high-resolution cell-specific RNA labeling. Overall, these results reveal novel aspects of mammalian salvage pathways and serve as a new benchmark for designing, characterizing and evaluating cell-specific biomolecule labeling methodologies.

Bioorthogonal Metabolic Labeling Utilizing Protein Biosynthesis for Dynamic Visualization of Nonenveloped Enterovirus 71 Infection

Bioorthogonal metabolic labeling through the endogenous cellular metabolic pathways (e.g., phospholipid and sugar) is a promising approach for effectively labeling live viruses. However, it remains a big challenge to label nonenveloped viruses due to lack of host-derived envelopes. Herein, a novel bioorthogonal labeling strategy is developed utilizing protein synthesis pathway to label and trace nonenveloped viruses. The results show that l-azidohomoalanine (Aha), an azido derivative of methionine, is more effective than azido sugars to introduce azido motifs into viral capsid proteins by substituting methionine residues during viral protein biosynthesis and assembly. The azide-modified EV71 (N₃-EV71) particles are then effectively labeled with dibenzocyclooctyl (DBCO)-functionalized fluorescence probes through an in situ bioorthogonal reaction with well-preserved viral infectivity. Dual-labeled imaging clearly clarifies that EV71 virions primarily bind to scavenger receptors and are internalized through clathrin-mediated endocytosis. The viral particles are then transported into early and late endosomes where viral RNA is released in a low-pH dependent manner at about 70 min postinfection. These results first reveal viral trafficking and uncoating mechanisms, which may shed light on the pathogenesis of EV71 infection and contribute to antiviral drug discovery.

Expanding the Scope of RNA Metabolic Labeling with Vinyl Nucleosides and Inverse Electron-Demand Diels-Alder Chemistry

Optimized and stringent chemical methods to profile nascent RNA expression are still in demand. Herein, we expand the toolkit for metabolic labeling of RNA through application of inverse electron demand Diels-Alder (IEDDA) chemistry. Structural examination of metabolic enzymes guided the design and synthesis of vinyl-modified nucleosides, which we systematically tested for their ability to be installed through cellular machinery. Further, we tested these nucleosides against a panel of tetrazines to identify those which are able to react with a terminal alkene, but are stable enough for selective conjugation. The selected pairings then facilitated RNA functionalization with biotin and fluorophores. We found that this chemistry not only is amenable to preserving RNA integrity but also endows the ability to both tag and image RNA in cells. These key findings represent a significant advancement in methods to profile the nascent transcriptome using chemical approaches.

A Metabolic Engineering Approach to Incorporate Modified Pyrimidine Nucleosides into Cellular RNA

The incorporation of modified nucleotides into RNA is a powerful strategy to probe RNA structure and function. While a wide variety of modified nucleotides can be incorporated into RNA in vitro using chemical or enzymatic synthesis, strategies for the metabolic incorporation of artificial nucleotides into cellular RNA are limited, largely due to the incompatibility of modified nucleobases and nucleosides with nucleotide salvage pathways. In this work, we develop a metabolic engineering strategy to facilitate the labeling of cellular RNA with noncanonical pyrimidine nucleosides. First, we use structure-based protein engineering to alter the substrate specificity of uridine-cytidine kinase 2 (UCK2), a key enzyme in the pyrimidine nucleotide salvage pathway. Next, we show that expression of mutant UCK2 in HeLa and U2OS cells is sufficient to enable the incorporation of 5-azidomethyl uridine (5-AmU) into cellular RNA and promotes RNA labeling by other C5-modified pyrimidines. Finally, we apply UCK2-mediated RNA labeling with 5-AmU to study RNA trafficking and turnover during normal and stress conditions and find diminished RNA localization in the cytosol during arsenite stress. Taken together, our study provides a general strategy for the incorporation of modified pyrimidine nucleosides into cellular RNA and expands the chemical toolkit of modified bases for studying dynamic RNA behavior in living cells.

Click Chemistry (CuAAC) and Detection of Tagged de novo Synthesized Proteins in Drosophila

Copper-catalyzed azide-alkyne-cycloaddition (CuAAC), also known as 'click chemistry' serves as a technique for bio-orthogonal, that is, bio-compatible labeling of macromolecules including proteins or lipids. Click chemistry has been widely used to covalently, selectively, and efficiently attach probes such as fluorophores or biotin to small bio-orthogonal chemical reporter groups introduced into macromolecules. In bio-orthogonal non-canonical amino acid tagging (BONCAT) and fluorescent non-canonical amino acid tagging (FUNCAT) proteins are metabolically labeled with a non-canonical, azide-bearing amino acid and subsequently CuAAC-clicked either to an alkyne-bearing biotin (BONCAT) for protein purification, Western blot, or mass spectrometry analyses or to an alkyne-bearing fluorophore (FUNCAT) for immunohistochemistry. In combination with mass spectrometry, these kinds of labeling and tagging strategies are a suitable option to identify and characterize specific proteomes in living organisms without the need of prior cell sorting. Here, we provide detailed protocols for FUNCAT and BONCAT click chemistry and the detection of tagged de novo synthesized proteins in Drosophila melanogaster.

Electron-Mediated Aminyl and Iminyl Radicals from C5 Azido-Modified Pyrimidine Nucleosides Augment Radiation Damage to Cancer Cells

Two classes of azido-modified pyrimidine nucleosides were synthesized as potential radiosensitizers; one class is 5-azidomethyl-2'-deoxyuridine (AmdU) and cytidine (AmdC), while the second class is 5-(1-azidovinyl)-2'-deoxyuridine (AvdU) and cytidine (AvdC). The addition of radiation-produced electrons to C5-azido nucleosides leads to the formation of π-aminyl radicals followed by facile conversion to σ-iminyl radicals either via a bimolecular reaction involving intermediate α-azidoalkyl radicals in AmdU/AmdC or by tautomerization in AvdU/AvdC. AmdU demonstrates effective radiosensitization in EMT6 tumor cells.

Photosensitizers with Aggregation-Induced Emission: Materials and Biomedical Applications

Photodynamic therapy is arising as a noninvasive treatment modality for cancer and other diseases. One of the key factors to determine the therapeutic function is the efficiency of photosensitizers (PSs). Opposed to traditional PSs, which show quenched fluorescence and reduced singlet oxygen production in the aggregate state, PSs with aggregation-induced emission (AIE) exhibit enhanced fluorescence and strong photosensitization ability in nanoparticles. Here, the design principles of AIE PSs and their biomedical applications are discussed in detail, starting with a summary of traditional PSs, followed by a comparison between traditional and AIE PSs to highlight the various design strategies and unique features of the latter. Subsequently, the applications of AIE PSs in photodynamic cancer cell ablation, bacteria killing, and image-guided therapy are discussed using charged AIE PSs, AIE PS molecular probes, and AIE PS nanoparticles as examples. These studies have demonstrated the great potential of AIE PSs as effective theranostic agents to treat tumor or bacterial infection. This review hopefully will spur more research interest in AIE PSs for future translational research.

Protected pyrimidine nucleosides for cell-specific metabolic labeling of RNA

RNA molecules can perform a myriad of functions, from the regulation of gene expression to providing the genetic blueprint for protein synthesis. Characterizing RNA expression dynamics, in a cell-specific manner, still remains a great challenge in biology. Herein we present a new set of protected alkynyl nucleosides for cell-specific metabolic labeling of RNA. We anticipate these analogs will find wide spread utility toward the goal of understanding RNA expression in complex cellular and tissue environments, even within living animals.

In Vivo Tracking of Multiple Tumor Exosomes Labeled by Phospholipid-Based Bioorthogonal Conjugation

Exosomes are cell-secreted nanoscale membrane vesicles that play critical roles in many pathophysiological processes. The clinical value of exosomes is under intense investigation, yet current knowledge regarding their in vivo properties is very limited because of the lack of efficient labeling techniques. Here, we report a phospholipid-based bioorthogonal labeling strategy to endow exosomes with optical probes without influencing their native biological functions. We investigated the dynamic in vivo biodistribution and organotropic uptake of multiple tumor exosomes in a single mouse. The results indicate that the exosomes derived from different cell lines show specific organotropic uptake. This phospholipid-based labeling strategy opens a new window to directly visualize and monitor exosome trafficking in living systems and holds great promise for exploring exosome-involved biological events such as cancer metastasis.

"Clicking" Gene Therapeutics: A Successful Union of Chemistry and Biomedicine for New Solutions

The use of nucleic acid, DNA and RNA, based strategies to disrupt gene expression as a therapeutic is quickly emerging. Indeed, synthetic oligonucleotides represent a major component of modern gene therapeutics. However, the efficiency and specificity of intracellular uptake for nonmodified oligonucleotides is rather poor. Utilizing RNA based oligonucleotides as therapeutics is even more challenging to deliver, due to extremely fast enzymatic degradation of the RNAs. RNAs get rapidly degraded in vivo and demonstrate large off-target binding events when they can reach and enter the desired target cells. One approach that holds much promise is the utilization of "click chemistry" to conjugate receptor or cell specific targeting molecules directly to the effector oligonucleotides. We discuss here the applications of the breakthrough technology of CuAAC click chemistry and the immense potential in utilizing "click chemistry" in the development of new age targeted oligonucleotide therapeutics.

Bioorthogonal Metabolic Labeling of Nascent RNA in Neurons Improves the Sensitivity of Transcriptome-Wide Profiling

Transcriptome-wide expression profiling of neurons has provided important insights into the underlying molecular mechanisms and gene expression patterns that transpire during learning and memory formation. However, there is a paucity of tools for profiling stimulus-induced RNA within specific neuronal cell populations. A bioorthogonal method to chemically label nascent (i.e., newly transcribed) RNA in a cell-type-specific and temporally controlled manner, which is also amenable to bioconjugation via click chemistry, was recently developed and optimized within conventional immortalized cell lines. However, its value within a more fragile and complicated cellular system such as neurons, as well as for transcriptome-wide expression profiling, has yet to be demonstrated. Here, we report the visualization and sequencing of activity-dependent nascent RNA derived from neurons using this labeling method. This work has important implications for improving transcriptome-wide expression profiling and visualization of nascent RNA in neurons, which has the potential to provide valuable insights into the mechanisms underlying neural plasticity, learning, and memory.

Post-transcriptional labeling by using Suzuki-Miyaura cross-coupling generates functional RNA probes

Pd-catalyzed C-C bond formation, an important vertebra in the spine of synthetic chemistry, is emerging as a valuable chemoselective transformation for post-synthetic functionalization of biomacromolecules. While methods are available for labeling protein and DNA, development of an analogous procedure to label RNA by cross-coupling reactions remains a major challenge. Herein, we describe a new Pd-mediated RNA oligonucleotide (ON) labeling method that involves post-transcriptional functionalization of iodouridine-labeled RNA transcripts by using Suzuki-Miyaura cross-coupling reaction. 5-Iodouridine triphosphate (IUTP) is efficiently incorporated into RNA ONs at one or more sites by T7 RNA polymerase. Further, using a catalytic system made of Pd(OAc)2 and 2-aminopyrimidine-4,6-diol (ADHP) or dimethylamino-substituted ADHP (DMADHP), we established a modular method to functionalize iodouridine-labeled RNA ONs in the presence of various boronic acid and ester substrates under very mild conditions (37°C and pH 8.5). This method is highly chemoselective, and offers direct access to RNA ONs labeled with commonly used fluorescent and affinity tags and new fluorogenic environment-sensitive nucleoside probes in a ligand-controlled stereoselective fashion. Taken together, this simple approach of generating functional RNA ON probes by Suzuki-Miyaura coupling will be a very important addition to the resources and tools available for analyzing RNA motifs.

Spatially Restricting Bioorthogonal Nucleoside Biosynthesis Enables Selective Metabolic Labeling of the Mitochondrial Transcriptome

The cellular RNA pool in animals arises from two separate genomes stored in the nucleus and multiple mitochondria. Chemical methods to track nascent RNA synthesis are unable to distinguish between these two with stringency. Herein, we report that spatially restricting bioorthogonal nucleoside biosynthesis enables, for the first time, selective metabolic labeling of the RNA transcribed in the mitochondria. We envision that this approach could open the door for heretofore-impossible analyses of mitochondrial RNA. Beyond our results revealed herein, our approach provides a roadmap for researchers to begin to design strategies to examine biomolecules within subcellular compartments.

Functional Interplay between Small Non-Coding RNAs and RNA Modification in the Brain

Small non-coding RNAs are essential for transcription, translation and gene regulation in all cell types, but are particularly important in neurons, with known roles in neurodevelopment, neuroplasticity and neurological disease. Many small non-coding RNAs are directly involved in the post-transcriptional modification of other RNA species, while others are themselves substrates for modification, or are functionally modulated by modification of their target RNAs. In this review, we explore the known and potential functions of several distinct classes of small non-coding RNAs in the mammalian brain, focusing on the newly recognised interplay between the epitranscriptome and the activity of small RNAs. We discuss the potential for this relationship to influence the spatial and temporal dynamics of gene activation in the brain, and predict that further research in the field of epitranscriptomics will identify interactions between small RNAs and RNA modifications which are essential for higher order brain functions such as learning and memory.

An uracil-linked hydroxyflavone probe for the recognition of ATP

Background: Nucleotides are essential molecules in living systems due to their paramount importance in various physiological processes. In the past years, numerous attempts were made to selectively recognize and detect these analytes, especially ATP using small-molecule fluorescent chemosensors. Despite the various solutions, the selective detection of ATP is still challenging due to the structural similarity of various nucleotides. In this paper, we report the conjugation of a uracil nucleobase to the known 4'-dimethylamino-hydroxyflavone fluorophore. Results: The complexation of this scaffold with ATP is already known. The complex is held together by stacking and electrostatic interactions. To achieve multi-point recognition, we designed the uracil-appended version of this probe to include complementary base-pairing interactions. The theoretical calculations revealed the availability of multiple complex structures. The synthesis was performed using click chemistry and the nucleotide recognition properties of the probe were evaluated using fluorescence spectroscopy. Conclusions: The first, uracil-containing fluorescent ATP probe based on a hydroxyflavone fluorophore was synthesized and evaluated. A selective complexation with ATP was observed and a ratiometric response in the excitation spectrum.

Bioorthogonal Metabolic DNA Labelling using Vinyl Thioether-Modified Thymidine and o-Quinolinone Quinone Methide

Bioorthogonal metabolic DNA labeling with fluorochromes is a powerful strategy to visualize DNA molecules and their functions. Here, we report the development of a new DNA metabolic labeling strategy enabled by the catalyst-free bioorthogonal ligation using vinyl thioether modified thymidine and o-quinolinone quinone methide. With the newly designed vinyl thioether-modified thymidine (VTdT), we added labeling tags on cellular DNA, which could further be linked to fluorochromes in cells. Therefore, we successfully visualized the DNA localization within cells as well as single DNA molecules without other staining reagents. In addition, we further characterized this bioorthogonal DNA metabolic labeling using DNase I digestion, MS characterization of VTdT as well as VTdT-oQQF conjugate in cell nuclei or mitochondria. This technique provides a powerful strategy to study DNA in cells, which paves the way to achieve future spatiotemporal deciphering of DNA synthesis and functions.

Superresolution imaging of individual replication forks reveals unexpected prodrug resistance mechanism

Many drugs require extensive metabolism en route to their targets. High-resolution visualization of prodrug metabolism should therefore utilize analogs containing a small modification that does not interfere with its metabolism or mode of action. In addition to serving as mechanistic probes, such analogs provide candidates for theranostics when applied in both therapeutic and diagnostic modalities. Here a traceable mimic of the widely used anticancer prodrug cytarabine (ara-C) was generated by converting a single hydroxyl group to azide, giving "AzC." This compound exhibited the same biological profile as ara-C in cell cultures and zebrafish larvae. Using azide-alkyne "click" reactions, we uncovered an apparent contradiction: drug-resistant cells incorporated relatively large quantities of AzC into their genomes and entered S-phase arrest, whereas drug-sensitive cells incorporated only small quantities of AzC. Fluorescence microscopy was used to elucidate structural features associated with drug resistance by characterizing the architectures of stalled DNA replication foci containing AzC, EdU, γH2AX, and proliferating cell nuclear antigen (PCNA). Three-color superresolution imaging revealed replication foci containing one, two, or three partially resolved replication forks. Upon removing AzC from the media, resumption of DNA synthesis and completion of the cell cycle occurred before complete removal of AzC from genomes in vitro and in vivo. These results revealed an important mechanism for the low toxicity of ara-C toward normal tissues and drug-resistant cancer cells, where its efficient incorporation into DNA gives rise to highly stable, stalled replication forks that limit further incorporation of the drug, yet allow for the resumption of DNA synthesis and cellular division following treatment.

Uncovering cell type-specific complexities of gene expression and RNA metabolism by TU-tagging and EC-tagging

Cell type-specific transcription is a key determinant of cell fate and function. An ongoing challenge in biology is to develop robust and stringent biochemical methods to explore gene expression with cell type specificity. This challenge has become even greater as researchers attempt to apply high-throughput RNA analysis methods under in vivo conditions. TU-tagging and EC-tagging are in vivo biosynthetic RNA tagging techniques that allow spatial and temporal specificity in RNA purification. Spatial specificity is achieved through targeted expression of pyrimidine salvage enzymes (uracil phosphoribosyltransferase and cytosine deaminase) and temporal specificity is achieved by controlling exposure to bioorthogonal substrates of these enzymes (4-thiouracil and 5-ethynylcytosine). Tagged RNAs can be purified from total RNA extracted from an animal or tissue and used in transcriptome profiling analyses. In addition to identifying cell type-specific mRNA profiles, these techniques are applicable to noncoding RNAs and can be used to measure RNA transcription and decay. Potential applications of TU-tagging and EC-tagging also include fluorescent RNA imaging and selective definition of RNA-protein interactions. TU-tagging and EC-tagging hold great promise for supporting research at the intersection of RNA biology and developmental biology. This article is categorized under: Technologies > Analysis of the Transcriptome.

N6-Allyladenosine: A New Small Molecule for RNA Labeling Identified by Mutation Assay

RNA labeling is crucial for the study of RNA structure and metabolism. Herein we report N⁶-allyladenosine (a⁶A) as a new small molecule for RNA labeling through both metabolic and enzyme-assisted manners. a⁶A behaves like A and can be metabolically incorporated into newly synthesized RNAs inside mammalian cells. We also show that human RNA N⁶-methyladenosine (m⁶A) methyltransferases METTL3/METTL14 can work with a synthetic cofactor, namely allyl-SAM (S-adenosyl methionine with methyl replaced by allyl) in order to site-specifically install an allyl group to the N⁶-position of A within specific sequence to generate a⁶A-labeled RNAs. The iodination of N⁶-allyl group of a⁶A under mild buffer conditions spontaneously induces the formation of N¹,N⁶-cyclized adenosine and creates mutations at its opposite site during complementary DNA synthesis of reverse transcription. The existing m⁶A in RNA is inert to methyltransferase-assisted allyl labeling, which offers a chance to differentiate m⁶A from A at individual RNA sites. Our work demonstrates a new method for RNA labeling, which could find applications in developing sequencing methods for nascent RNAs and RNA modifications.

Photo-controlled cell-specific metabolic labeling of RNA

Elucidating gene expression programs within a cell-specific manner is a grand challenge for biologists. Harder still is the ability to have kinetic control over such experiments. Metabolic labeling with bioorthogonally-functionalized metabolic intermediates provides a means to profile RNA expression in a cell-specific manner, but there is still a lack of kinetic resolution. Herein we present the synthesis and evaluation of photocaged metabolic uracil intermediates. We compare the photo-decaging properties and demonstrate their utility in metabolic labeling experiments in a cell-specific manner. We anticipate that our approach will have far-reaching impact as it provides control over tagging of nascent RNA.