Comparison of SHAPE reagents for mapping RNA structures inside living cells

Tick-borne flavivirus exoribonuclease-resistant RNAs contain a double loop structure

Viruses from the Flaviviridae family contain human relevant pathogens that generate subgenomic noncoding RNAs during infection using structured exoribonuclease resistant RNAs (xrRNAs). These xrRNAs block progression of host cell's 5' to 3' exoribonucleases. The structures of several xrRNAs from mosquito-borne and insect-specific flaviviruses reveal a conserved fold in which a ring-like motif encircles the 5' end of the xrRNA. However, the xrRNAs found in tick-borne and no known vector flaviviruses have distinct characteristics, and their 3-D fold was unsolved. Here, we verify the presence of xrRNAs in the encephalitis-causing tick-borne Powassan Virus. We characterize their secondary structure and obtain a mid-resolution map of one of these xrRNAs using cryo-EM, revealing a unique double-loop ring element. Integrating these results with covariation analysis, biochemical data, and existing high-resolution structural information yields a model in which the core of the fold matches the previously solved xrRNA fold, but the expanded double loop ring is remodeled upon encountering the exoribonuclease. These results are representative of a broad class of xrRNAs and reveal a conserved strategy of structure-based exoribonuclease resistance achieved through a unique topology across a viral family of importance to global health.

The Druggable Transcriptome Project: From Chemical Probes to Precision Medicines

RNA presents abundant opportunities as a drug target, offering significant potential for small molecule medicine development. The transcriptome, comprising both coding and noncoding RNAs, is a rich area for therapeutic innovation, yet challenges persist in targeting RNA with small molecules. RNA structure can be predicted with or without experimental data, but discrepancies with the actual biological structure can impede progress. Prioritizing RNA targets supported by genetic or evolutionary evidence enhances success. Further, small molecules must demonstrate binding to RNA in cells, not solely in vitro, to validate both the target and compound. Effective small molecule binders modulate functional sites that influence RNA biology, as binding to nonfunctional sites requires recruiting effector mechanisms, for example degradation, to achieve therapeutic outcomes. Addressing these challenges is critical to unlocking RNA's vast potential for small molecule medicines, and a strategic framework is proposed to navigate this promising field, with a focus on targeting human RNAs.

Identification of RNA structures and their roles in RNA functions

The development of high-throughput RNA structure profiling methods in the past decade has greatly facilitated our ability to map and characterize different aspects of RNA structures transcriptome-wide in cell populations, single cells and single molecules. The resulting high-resolution data have provided insights into the static and dynamic nature of RNA structures, revealing their complexity as they perform their respective functions in the cell. In this Review, we discuss recent technical advances in the determination of RNA structures, and the roles of RNA structures in RNA biogenesis and functions, including in transcription, processing, translation, degradation, localization and RNA structure-dependent condensates. We also discuss the current understanding of how RNA structures could guide drug design for treating genetic diseases and battling pathogenic viruses, and highlight existing challenges and future directions in RNA structure research.

Chemical diversity of reagents that modify RNA 2'-OH in water: a review

Electrophilic water-soluble compounds have proven versatile in reacting selectively with 2'-OH groups in RNA, enabling structure mapping, probing, caging, labeling, crosslinking, and conjugation of RNAs in vitro and in living cells. While early work focused on one or two types of reagents with limited properties, recent studies have greatly diversified the structure, properties, and applications of these reagents. Here we review the scope of documented RNA hydroxyl-reactive species reported to date, with an eye to the effects of chemical structure on reactivity with RNA and other useful properties. Multiple forms of carbonyl electrophiles are now known to react at the 2'-OH, and recently, sulfonyl and aryl electrophiles have also been documented to form bonds there in high yields as well. In addition to electrophilicity, data also point to significant effects of reagent stability, steric bulk, and chirality on reaction yields and selectivity. Finally, we outline reagent properties and principles that define utility in applications with RNA, with an eye to the design of future reagents.

A new reagent for in vivo structure probing of RNA G and U residues that improves RNA structure prediction alone and combined with DMS

A key to understanding the roles of RNA in regulating gene expression is knowing their structures in vivo. One way to obtain this information is through probing the structures of RNA with chemicals. To probe RNA structure directly in cells, membrane-permeable reagents that modify the Watson-Crick (WC) face of unpaired nucleotides can be used. Although dimethyl sulfate (DMS) has led to substantial insight into RNA structure, it has limited nucleotide specificity in vivo, with WC face reactivity only at adenine (A) and cytosine (C) at neutral pH. The reagent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) was recently shown to modify the WC face of guanine (G) and uracil (U). Although useful at lower concentrations in experiments that measure chemical modifications by reverse transcription (RT) stops, at higher concentrations necessary for detection by mutational profiling (MaP), EDC treatment leads to degradation of RNA. Here, we demonstrate EDC-stimulated degradation of RNA in Gram-negative and Gram-positive bacteria. In an attempt to overcome these limitations, we developed a new carbodiimide reagent, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide methiodide (ETC), which we show specifically modifies unpaired Gs and Us in vivo without substantial degradation of RNA. We establish ETC as a probe for MaP and optimize the RT conditions and computational analysis in Escherichia coli Importantly, we demonstrate the utility of ETC as a probe for improving RNA structure prediction both alone and with DMS.

Probing RNA structure and dynamics using nanopore and next generation sequencing

It has become increasingly evident that the structures RNAs adopt are conformationally dynamic; the various structured states that RNAs sample govern their interactions with other nucleic acids, proteins, and ligands to regulate a myriad of biological processes. Although several biophysical approaches have been developed and used to study the dynamic landscape of structured RNAs, technical limitations have limited their application to all classes of RNA due to variable size and flexibility. Recent advances combining chemical probing experiments with next-generation- and direct sequencing have emerged as an alternative approach to exploring the conformational dynamics of RNA. In this review, we provide a methodological overview of the sequencing-based techniques used to study RNA conformational dynamics. We discuss how different techniques have enabled us to better understand the propensity of RNAs from a variety of different classes to sample multiple conformational states. Finally, we present examples of the ways these techniques have reshaped how we think about RNA structure.

Isoform-specific RNA structure determination using Nano-DMS-MaP

RNA structure determination is essential to understand how RNA carries out its diverse biological functions. In cells, RNA isoforms are readily expressed with partial variations within their sequences due, for example, to alternative splicing, heterogeneity in the transcription start site, RNA processing or differential termination/polyadenylation. Nanopore dimethyl sulfate mutational profiling (Nano-DMS-MaP) is a method for in situ isoform-specific RNA structure determination. Unlike similar methods that rely on short sequencing reads, Nano-DMS-MaP employs nanopore sequencing to resolve the structures of long and highly similar RNA molecules to reveal their previously hidden structural differences. This Protocol describes the development and applications of Nano-DMS-MaP and outlines the main considerations for designing and implementing a successful experiment: from bench to data analysis. In cell probing experiments can be carried out by an experienced molecular biologist in 3-4 d. Data analysis requires good knowledge of command line tools and Python scripts and requires a further 3-5 d.

Bioorthogonal masked acylating agents for proximity-dependent RNA labelling

RNA localization is highly regulated, with subcellular organization driving context-dependent cell physiology. Although proximity-based labelling technologies that use highly reactive radicals or carbenes provide a powerful method for unbiased mapping of protein organization within a cell, methods for unbiased RNA mapping are scarce and comparably less robust. Here we develop α-alkoxy thioenol and chloroenol esters that function as potent acylating agents upon controlled ester unmasking. We pair these probes with subcellular-localized expression of a bioorthogonal esterase to establish a platform for spatial analysis of RNA: bioorthogonal acylating agents for proximity labelling and sequencing (BAP-seq). We demonstrate that, by selectively unmasking the enol probe in a locale of interest, we can map RNA distribution in membrane-bound and membrane-less organelles. The controlled-release acylating agent chemistry and corresponding BAP-seq method expand the scope of proximity labelling technologies and provide a powerful approach to interrogate the cellular organization of RNAs.

Mapping of the influenza A virus genome RNA structure and interactions reveals essential elements of viral replication

Influenza A virus (IAV) represents a constant public health threat. The single-stranded, segmented RNA genome of IAV is replicated in host cell nuclei as a series of 8 ribonucleoprotein complexes (vRNPs) with RNA structures known to exert essential function to support viral replication. Here, we investigate RNA secondary structures and RNA interactions networks of the IAV genome and construct an in vivo structure model for each of the 8 IAV genome segments. Our analyses reveal an overall in vivo and in virio resemblance of the IAV genome conformation but also wide disparities among long-range and intersegment interactions. Moreover, we identify a long-range RNA interaction that exerts an essential role in genome packaging. Disrupting this structure displays reduced infectivity, attenuating virus pathogenicity in mice. Our findings characterize the in vivo RNA structural landscape of the IAV genome and reveal viral RNA structures that can be targeted to develop antiviral interventions.

Analysis of Telomerase RNA Structure in Physcomitrium patens Indicates Functionally Relevant Transitions Between OPEN and CLOSED Conformations

Telomerase RNA (TR) conformation determines its function as a template for telomere synthesis and as a scaffold for the assembly of the telomerase nucleoprotein complex. Experimental analyses of TR secondary structure using DMS-Map Seq and SHAPE-Map Seq techniques show its CLOSED conformation as the consensus structure where the template region cannot perform its function. Our data show that the apparent discrepancy between experimental results and predicted TR functional conformation, mostly ignored in published studies, can be explained using data analysis based on single-molecule structure prediction from individual sequencing reads by the recently established DaVinci method. This method results in several clusters of secondary structures reflecting the structural dynamics of TR, possibly related to its multiple functional states. Interestingly, the presumed active (OPEN) conformation of TR corresponds to a minor fraction of TR under in vivo conditions. Therefore, structural polymorphism and dynamic TR transitions between CLOSED and OPEN conformations may be involved in telomerase activity regulation as a switch that functions independently of total TR transcript levels.

RNA Folding, Mutation, and Detection

The structure of RNA molecules is absolutely critical to their functions in a biological system. RNA structure is dynamic and changes in response to cellular needs. Within the last few decades, there has been an increased interest in studying the structure of RNA molecules and how they change to support the needs of the cell in different conditions. Selective 2'-hydroxyl acylation-based mutational profiling using high-throughput sequencing is a powerful method to predict the secondary structure of RNA molecules both in vivo and in immunopurified samples. Selective 2'-hydroxyl acylation-based mutational profiling using high-throughput sequencing works by adding bulky groups onto accessible "flexible" bases in an RNA molecule that are not involved in any base-pairing or RNA-protein interactions. When the RNA is reverse transcribed into cDNA, the bulky groups are incorporated as base mutations, which can be compared to an unmodified control to identify the locations of flexible bases. The comparison of sequence data between modified and unmodified samples allows the computer software program (developed to generate reactivity profiles) to generate RNA secondary structure models. These models can be compared in a variety of conditions to determine how specific stimuli influence RNA secondary structures.

Led-Seq: ligation-enhanced double-end sequence-based structure analysis of RNA

Structural analysis of RNA is an important and versatile tool to investigate the function of this type of molecules in the cell as well as in vitro. Several robust and reliable procedures are available, relying on chemical modification inducing RT stops or nucleotide misincorporations during reverse transcription. Others are based on cleavage reactions and RT stop signals. However, these methods address only one side of the RT stop or misincorporation position. Here, we describe Led-Seq, a new approach based on lead-induced cleavage of unpaired RNA positions, where both resulting cleavage products are investigated. The RNA fragments carrying 2', 3'-cyclic phosphate or 5'-OH ends are selectively ligated to oligonucleotide adapters by specific RNA ligases. In a deep sequencing analysis, the cleavage sites are identified as ligation positions, avoiding possible false positive signals based on premature RT stops. With a benchmark set of transcripts in Escherichia coli, we show that Led-Seq is an improved and reliable approach based on metal ion-induced phosphodiester hydrolysis to investigate RNA structures in vivo.

Diverse Reagent Scaffolds Provide Differential Selectivity of 2'-OH Acylation in RNA

RNA 2'-OH acylation is widely used both for mapping structure and for conjugating RNA, generally relying on selective reactions with unpaired nucleotides over paired ones. Common reagents for this acylation have been chiefly restricted to two similar aryl scaffolds, leaving open the question of how more broadly varied reagent structure might affect selectivity. Here, we prepared a set of 10 structurally diverse acylimidazole reagents and employed deep sequencing to profile their reactivity and selectivity in an RNA library of systematically varied structure. We show that structure-directed reactivity profiles vary significantly with the reagent scaffold, and we document new acylating agents that have altered selectivity profiles, including reagents that show elevated selectivity within loops, as well as compounds with reduced off-target reactivity in loop closing base pairs. Interestingly, we also show that the simplest reagent (acetylimidazole) is cell permeable and is small enough to map RNA structure in the presence of protein contacts that block other reagents. Finally, we describe reagents that show elevated selectivity within small loops, with applications in site-selective labeling. The results provide new tools for improved conjugation and mapping of RNA.

RNA-Responsive gRNAs for Controlling CRISPR Activity: Current Advances, Future Directions, and Potential Applications

CRISPR-Cas9 has emerged as a major genome manipulation tool. As Cas9 can cause off-target effects, several methods for controlling the expression of CRISPR systems were developed. Recent studies have shown that CRISPR activity could be controlled by sensing expression levels of endogenous transcripts. This is particularly interesting, as endogenous RNAs could harbor important information about the cell type, disease state, and environmental challenges cells are facing. Single-guide RNA (sgRNA) engineering played a major role in the development of RNA-responsive CRISPR systems. Following further optimizations, RNA-responsive sgRNAs could enable the development of novel therapeutic and research applications. This review introduces engineering strategies that could be employed to modify Streptococcus pyogenes sgRNAs with a focus on recent advances made toward the development of RNA-responsive sgRNAs. Future directions and potential applications of these technologies are also discussed.

Cell Compartment-Specific Folding of Ty1 Long Terminal Repeat Retrotransposon RNA Genome

The structural transitions RNAs undergo during trafficking are not well understood. Here, we used the well-developed yeast Ty1 retrotransposon to provide the first structural model of genome (g) RNA in the nucleus from a retrovirus-like transposon. Through a detailed comparison of nuclear Ty1 gRNA structure with those established in the cytoplasm, virus-like particles (VLPs), and those synthesized in vitro, we detected Ty1 gRNA structural alterations that occur during retrotransposition. Full-length Ty1 gRNA serves as the mRNA for Gag and Gag-Pol proteins and as the genome that is reverse transcribed within VLPs. We show that about 60% of base pairs predicted for the nuclear Ty1 gRNA appear in the cytoplasm, and active translation does not account for such structural differences. Most of the shared base pairs are represented by short-range interactions, whereas the long-distance pairings seem unique for each compartment. Highly structured motifs tend to be preserved after nuclear export of Ty1 gRNA. In addition, our study highlights the important role of Ty1 Gag in mediating critical RNA-RNA interactions required for retrotransposition.

Acylation probing of "generic" RNA libraries reveals critical influence of loop constraints on reactivity

The reactivity of RNA 2'-OH acylation is broadly useful both in probing structure and in preparing conjugates. To date, this reactivity has been analyzed in limited sets of biological RNA sequences, leaving open questions of how reactivity varies inherently without regard to sequence in structured contexts. We constructed and probed "generic" structured RNA libraries using homogeneous loop sequences, employing deep sequencing to carry out a systematic survey of reactivity. We find a wide range of RNA reactivities among single-stranded sequences, with nearest neighbors playing substantial roles. Remarkably, certain small loops are found to be far more reactive on average (up to 4,000-fold) than single-stranded RNAs, due to conformational constraints that enhance reactivity. Among loops, we observe large variations in reactivity based on size, type, and position. The results lend insights into RNA designs for achieving high-efficiency local conjugation and provide new opportunities to refine structure analysis.

Lessons Learned and Yet-to-Be Learned on the Importance of RNA Structure in SARS-CoV-2 Replication

SARS-CoV-2, the etiological agent responsible for the COVID-19 pandemic, is a member of the virus family Coronaviridae, known for relatively extensive (~30-kb) RNA genomes that not only encode for numerous proteins but are also capable of forming elaborate structures. As highlighted in this review, these structures perform critical functions in various steps of the viral life cycle, ultimately impacting pathogenesis and transmissibility. We examine these elements in the context of coronavirus evolutionary history and future directions for curbing the spread of SARS-CoV-2 and other potential human coronaviruses. While we focus on structures supported by a variety of biochemical, biophysical, and/or computational methods, we also touch here on recent evidence for novel structures in both protein-coding and noncoding regions of the genome, including an assessment of the potential role for RNA structure in the controversial finding of SARS-CoV-2 integration in “long COVID” patients. This review aims to serve as a consolidation of previous works on coronavirus and more recent investigation of SARS-CoV-2, emphasizing the need for improved understanding of the role of RNA structure in the evolution and adaptation of these human viruses.

Control of RNA with quinone methide reversible acylating reagents

Caging RNA by polyacylation (cloaking) has been developed recently as a simple and rapid method to control the function of RNAs. Previous approaches for chemical reversal of acylation (uncloaking) made use of azide reduction followed by amine cyclization, requiring ∼2-4 h for the completion of cyclization. In new studies aimed at improving reversal rates and yields, we have designed novel acylating reagents that utilize quinone methide (QM) elimination for reversal. The QM de-acylation reactions were tested with two bioorthogonally cleavable motifs, azide and vinyl ether, and their acylation and reversal efficiencies were assessed with NMR and mass spectrometry on model small-molecule substrates as well as on RNAs. Successful reversal both with phosphines and strained alkenes was documented. Among the compounds tested, the azido-QM compound A-3 displayed excellent de-acylation efficiency, with t1/2 for de-acylation of less than an hour using a phosphine trigger. To test its function in RNA caging, A-3 was successfully applied to control EGFP mRNA translation in vitro and in HeLa cells. We expect that this molecular caging strategy can serve as a valuable tool for biological investigation and control of RNAs both in vitro and in cells.

SHAPER: A Web Server for Fast and Accurate SHAPE Reactivity Prediction

Selective 2′-hydroxyl acylation analyzed by primer extension (SHAPE) chemical probing serves as a convenient and efficient experiment technique for providing information about RNA local flexibility. The local structural information contained in SHAPE reactivity data can be used as constraints in 2D/3D structure predictions. Here, we present SHAPE predictoR (SHAPER), a web server for fast and accurate SHAPE reactivity prediction. The main purpose of the SHAPER web server is to provide a portal that uses experimental SHAPE data to refine 2D/3D RNA structure selection. Input structures for the SHAPER server can be obtained through experimental or computational modeling. The SHAPER server can accept RNA structures with single or multiple conformations, and the predicted SHAPE profile and correlation with experimental SHAPE data (if provided) for each conformation can be freely downloaded through the web portal. The SHAPER web server is available at http://rna.physics.missouri.edu/shaper/.

The RNA structurome in the asexual blood stages of malaria pathogen plasmodium falciparum

Plasmodium falciparum is a deadly human pathogen responsible for the devastating disease called malaria. In this study, we measured the differential accumulation of RNA secondary structures in coding and non-coding transcripts from the asexual developmental cycle in P. falciparum in human red blood cells. Our comprehensive analysis that combined high-throughput nuclease mapping of RNA structures by duplex RNA-seq, SHAPE-directed RNA structure validation, immunoaffinity purification and characterization of antisense RNAs collectively measured differentially base-paired RNA regions throughout the parasite's asexual RBC cycle. Our mapping data not only aligned to a diverse pool of RNAs with known structures but also enabled us to identify new structural RNA regions in the malaria genome. On average, approximately 71% of the genes with secondary structures are found to be protein coding mRNAs. The mapping pattern of these base-paired RNAs corresponded to all regions of mRNAs, including the 5' UTR, CDS and 3' UTR as well as the start and stop codons. Histone family genes which are known to form secondary structures in their mRNAs and transcripts from genes which are important for transcriptional and post-transcriptional control, such as the unique plant-like transcription factor family, ApiAP2, DNA-/RNA-binding protein, Alba3 and proteins important for RBC invasion and malaria cytoadherence also showed strong accumulation of duplex RNA reads in various asexual stages in P. falciparum. Intriguingly, our study determined stage-specific, dynamic relationships between mRNA structural contents and translation efficiency in P. falciparum asexual blood stages, suggesting an essential role of RNA structural changes in malaria gene expression programs.

In Vivo RNA Chemical Footprinting Analysis in Archaea

RNA structural conformation and dynamics govern the functional properties of all RNA/RNP. Accordingly, defining changes of RNA structure and dynamics in various conditions may provide detailed insight into how RNA structural properties regulate the function of RNA/RNP. Traditional chemical footprinting analysis using chemical modifiers allows to sample the dynamics and conformation landscape of diverse RNA/RNP. However, many chemical modifiers are limited in their capacity to provide unbiased information reflecting the in vivo RNA/RNP structural landscape. In the recent years, the development of selective-2'-hydroxyl acylation analyzed by primer extension (SHAPE) methodology that uses powerful new chemical modifiers has significantly improved in vitro and in vivo structural probing of secondary and tertiary interactions of diverse RNA species at the single nucleotide level.Although the original discovery of Archaea as an independent domain of life is intimately linked to the technological development of RNA analysis, our understanding of in vivo RNA structural conformation and dynamics in this domain of life remains scarce.This protocol describes the in vivo use of SHAPE chemistry in two evolutionary divergent model Archaea, Sulfolobus acidocaldarius and Haloferax volcanii.

Mapping the RNA Chaperone Activity of the T. brucei Editosome Using SHAPE Chemical Probing

Mitochondrial pre-mRNAs in African trypanosomes adopt intricately folded, highly stable 2D and 3D structures. The RNA molecules are substrates of a U-nucleotide-specific insertion/deletion-type RNA editing reaction, which is catalyzed by a 0.8 MDa protein complex known as the editosome. RNA binding to the editosome is followed by a chaperone-mediated RNA remodeling reaction. The reaction increases the dynamic of specifically U-nucleotides to lower their base-pairing probability and as a consequence generates a simplified RNA folding landscape that is critical for the progression of the editing reaction cycle. Here we describe a chemical mapping method to quantitatively monitor the chaperone-driven structural changes of pre-edited mRNAs upon editosome binding. The method is known as selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE). SHAPE is based on the differential electrophilic modification of ribose 2'-hydroxyl groups in structurally constraint (double-stranded) versus structurally unconstrained (single-stranded) nucleotides. Electrophilic anhydrides such as 1-methyl-7-nitroisatoic anhydride are used as probing reagents, and the ribose 2'-modified nucleotides are mapped as abortive cDNA synthesis products. As a result, SHAPE allows the identification of all single-stranded and base-paired regions in a given RNA, and the data are used to compute experimentally derived RNA 2D structures. A side-by-side comparison of the RNA 2D folds in the pre- and post-chaperone states finally maps the chaperone-induced dynamic of the different pre-mRNAs with single-nucleotide resolution.

In Vivo and In Vitro Genome-Wide Profiling of RNA Secondary Structures Reveals Key Regulatory Features in Plasmodium falciparum

It is widely accepted that the structure of RNA plays important roles in a number of biological processes, such as polyadenylation, splicing, and catalytic functions. Dynamic changes in RNA structure are able to regulate the gene expression programme and can be used as a highly specific and subtle mechanism for governing cellular processes. However, the nature of most RNA secondary structures in Plasmodium falciparum has not been determined. To investigate the genome-wide RNA secondary structural features at single-nucleotide resolution in P. falciparum, we applied a novel high-throughput method utilizing the chemical modification of RNA structures to characterize these structures. Structural data from parasites are in close agreement with the known 18S ribosomal RNA secondary structures of P. falciparum and can help to predict the in vivo RNA secondary structure of a total of 3,396 transcripts in the ring-stage and trophozoite-stage developmental cycles. By parallel analysis of RNA structures in vivo and in vitro during the Plasmodium parasite ring-stage and trophozoite-stage intraerythrocytic developmental cycles, we identified some key regulatory features. Recent studies have established that the RNA structure is a ubiquitous and fundamental regulator of gene expression. Our study indicate that there is a critical connection between RNA secondary structure and mRNA abundance during the complex biological programme of P. falciparum. This work presents a useful framework and important results, which may facilitate further research investigating the interactions between RNA secondary structure and the complex biological programme in P. falciparum. The RNA secondary structure characterized in this study has potential applications and important implications regarding the identification of RNA structural elements, which are important for parasite infection and elucidating host-parasite interactions and parasites in the environment.

A novel SHAPE reagent enables the analysis of RNA structure in living cells with unprecedented accuracy

Due to the mounting evidence that RNA structure plays a critical role in regulating almost any physiological as well as pathological process, being able to accurately define the folding of RNA molecules within living cells has become a crucial need. We introduce here 2-aminopyridine-3-carboxylic acid imidazolide (2A3), as a general probe for the interrogation of RNA structures in vivo. 2A3 shows moderate improvements with respect to the state-of-the-art selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) reagent NAI on naked RNA under in vitro conditions, but it significantly outperforms NAI when probing RNA structure in vivo, particularly in bacteria, underlining its increased ability to permeate biological membranes. When used as a restraint to drive RNA structure prediction, data derived by SHAPE-MaP with 2A3 yields more accurate predictions than NAI-derived data. Due to its extreme efficiency and accuracy, we can anticipate that 2A3 will rapidly take over conventional SHAPE reagents for probing RNA structures both in vitro and in vivo.

In vivo structure of the Ty1 retrotransposon RNA genome

Long terminal repeat (LTR)-retrotransposons constitute a significant part of eukaryotic genomes and influence their function and evolution. Like other RNA viruses, LTR-retrotransposons efficiently utilize their RNA genome to interact with host cell machinery during replication. Here, we provide the first genome-wide RNA secondary structure model for a LTR-retrotransposon in living cells. Using SHAPE probing, we explore the secondary structure of the yeast Ty1 retrotransposon RNA genome in its native in vivo state and under defined in vitro conditions. Comparative analyses reveal the strong impact of the cellular environment on folding of Ty1 RNA. In vivo, Ty1 genome RNA is significantly less structured and more dynamic but retains specific well-structured regions harboring functional cis-acting sequences. Ribosomes participate in the unfolding and remodeling of Ty1 RNA, and inhibition of translation initiation stabilizes Ty1 RNA structure. Together, our findings support the dual role of Ty1 genomic RNA as a template for protein synthesis and reverse transcription. This study also contributes to understanding how a complex multifunctional RNA genome folds in vivo, and strengthens the need for studying RNA structure in its natural cellular context.

Probing RNA Conformational Equilibria within the Functional Cellular Context

Low-abundance short-lived non-native conformations referred to as excited states (ESs) are increasingly observed in vitro and implicated in the folding and biological activities of regulatory RNAs. We developed an approach for assessing the relative abundance of RNA ESs within the functional cellular context. Nuclear magnetic resonance (NMR) spectroscopy was used to estimate the degree to which substitution mutations bias conformational equilibria toward the inactive ES in vitro. The cellular activity of the ES-stabilizing mutants was used as an indirect measure of the conformational equilibria within the functional cellular context. Compensatory mutations that restore the ground-state conformation were used to control for changes in sequence. Using this approach, we show that the ESs of two regulatory RNAs from HIV-1, the transactivation response element (TAR) and the Rev response element (RRE), likely form in cells with abundances comparable to those measured in vitro, and their targeted stabilization may provide an avenue for developing anti-HIV therapeutics.

Mapping circular RNA structures in living cells by SHAPE-MaP

Circular RNAs are produced from back-splicing of exons of precursor mRNAs (pre-mRNAs). The sequences of exons in circular RNAs are identical to their linear cognate mRNAs, but the circular format may confer constraints on their folding and conformation, leading to potentially different functions from their linear RNA cognates. Here, we describe experimental and computational steps that optimize the selective 2'-hydroxyl acylation analyzed by primer extension and mutational profiling (SHAPE-MaP) to probe circular RNA secondary structure at single-nucleotide resolution in living cells.

Sieving RNA 3D Structures with SHAPE and Evaluating Mechanisms Driving Sequence-Dependent Reactivity Bias

Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) chemical probing provides local RNA flexibility information at single-nucleotide resolution. In general, SHAPE is thought of as a secondary structure (2D) technology, but we find evidence that robust tertiary structure (3D) information is contained in SHAPE data. Here, we report a new model that achieves a higher correlation between SHAPE data and native RNA 3D structures than the previous 3D structure-SHAPE relationship model. Furthermore, we demonstrate that the new model improves our ability to discern between SHAPE-compatible and -incompatible structures on model decoys. After identifying sequence-dependent bias in SHAPE experiments, we propose a mechanism driving sequence-dependent bias in SHAPE experiments, using replica-exchange umbrella sampling simulations to confirm that the SHAPE sequence bias is largely explained by the stability of the unreacted SHAPE reagent in the binding pocket. Taken together, this work represents multiple practical advances in our mechanistic and predictive understanding of SHAPE technology.

From structure to function: Route to understanding lncRNA mechanism

RNAs have emerged as a major target for diagnostics and therapeutics approaches. Regulatory nonprotein-coding RNAs (ncRNAs) in particular display remarkable versatility. They can fold into complex structures and interact with proteins, DNA, and other RNAs, thus modulating activity, localization, or interactome of multi-protein complexes. Thus, ncRNAs confer regulatory plasticity and represent a new layer of regulatory control. Interestingly, long noncoding RNAs (lncRNAs) tend to acquire complex secondary and tertiary structures and their function-in many cases-is dependent on structural conservation rather than primary sequence conservation. Whereas for many proteins, structure and its associated function are closely connected, for lncRNAs, the structural domains that determine functionality and its interactome are still not well understood. Numerous approaches for analyzing the structural configuration of lncRNAs have been developed recently. Here, will provide an overview of major experimental approaches used in the field, and discuss the potential benefit of using combinatorial strategies to analyze lncRNA modes of action based on structural information.

Advances in RNA 3D Structure Modeling Using Experimental Data

RNA is a unique bio-macromolecule that can both record genetic information and perform biological functions in a variety of molecular processes, including transcription, splicing, translation, and even regulating protein function. RNAs adopt specific three-dimensional conformations to enable their functions. Experimental determination of high-resolution RNA structures using x-ray crystallography is both laborious and demands expertise, thus, hindering our comprehension of RNA structural biology. The computational modeling of RNA structure was a milestone in the birth of bioinformatics. Although computational modeling has been greatly improved over the last decade showing many successful cases, the accuracy of such computational modeling is not only length-dependent but also varies according to the complexity of the structure. To increase credibility, various experimental data were integrated into computational modeling. In this review, we summarize the experiments that can be integrated into RNA structure modeling as well as the computational methods based on these experimental data. We also demonstrate how computational modeling can help the experimental determination of RNA structure. We highlight the recent advances in computational modeling which can offer reliable structure models using high-throughput experimental data.

The molecular structure of long non-coding RNAs: emerging patterns and functional implications

Long non-coding RNAs (lncRNAs) are recently-discovered transcripts that regulate vital cellular processes and are crucially connected to diseases. Despite their unprecedented molecular complexity, it is emerging that lncRNAs possess distinct structural motifs. Remarkably, the 3D shape and topology of full-length, native lncRNAs have been visualized for the first time in the last year. These studies reveal that lncRNA structures dictate lncRNA functions. Here, we review experimentally determined lncRNA structures and emphasize that lncRNA structural characterization requires synergistic integration of computational, biochemical and biophysical approaches. Based on these emerging paradigms, we discuss how to overcome the challenges posed by the complex molecular architecture of lncRNAs, with the goal of obtaining a detailed understanding of lncRNA functions and molecular mechanisms in the future.

RNAProbe: a web server for normalization and analysis of RNA structure probing data

RNA molecules play key roles in all living cells. Knowledge of the structural characteristics of RNA molecules allows for a better understanding of the mechanisms of their action. RNA chemical probing allows us to study the susceptibility of nucleotides to chemical modification, and the information obtained can be used to guide secondary structure prediction. These experimental results can be analyzed using various computational tools, which, however, requires additional, tedious steps (e.g., further normalization of the reactivities and visualization of the results), for which there are no fully automated methods. Here, we introduce RNAProbe, a web server that facilitates normalization, analysis, and visualization of the low-pass SHAPE, DMS and CMCT probing results with the modification sites detected by capillary electrophoresis. RNAProbe automatically analyzes chemical probing output data and turns tedious manual work into a one-minute assignment. RNAProbe performs normalization based on a well-established protocol, utilizes recognized secondary structure prediction methods, and generates high-quality images with structure representations and reactivity heatmaps. It summarizes the results in the form of a spreadsheet, which can be used for comparative analyses between experiments. Results of predictions with normalized reactivities are also collected in text files, providing interoperability with bioinformatics workflows. RNAProbe is available at https://rnaprobe.genesilico.pl.

Challenges and perspectives for structural biology of lncRNAs-the example of the Xist lncRNA A-repeats

Following the discovery of numerous long non-coding RNA (lncRNA) transcripts in the human genome, their important roles in biology and human disease are emerging. Recent progress in experimental methods has enabled the identification of structural features of lncRNAs. However, determining high-resolution structures is challenging as lncRNAs are expected to be dynamic and adopt multiple conformations, which may be modulated by interaction with protein binding partners. The X-inactive specific transcript (Xist) is necessary for X inactivation during dosage compensation in female placental mammals and one of the best-studied lncRNAs. Recent progress has provided new insights into the domain organization, molecular features, and RNA binding proteins that interact with distinct regions of Xist. The A-repeats located at the 5′ end of the transcript are of particular interest as they are essential for mediating silencing of the inactive X chromosome. Here, we discuss recent progress with elucidating structural features of the Xist lncRNA, focusing on the A-repeats. We discuss the experimental and computational approaches employed that have led to distinct structural models, likely reflecting the intrinsic dynamics of this RNA. The presence of multiple dynamic conformations may also play an important role in the formation of the associated RNPs, thus influencing the molecular mechanism underlying the biological function of the Xist A-repeats. We propose that integrative approaches that combine biochemical experiments and high-resolution structural biology in vitro with chemical probing and functional studies in vivo are required to unravel the molecular mechanisms of lncRNAs.

The how and why of lncRNA function: An innate immune perspective

Next-generation sequencing has provided a more complete picture of the composition of the human transcriptome indicating that much of the "blueprint" is a vastness of poorly understood non-protein-coding transcripts. This includes a newly identified class of genes called long noncoding RNAs (lncRNAs). The lack of sequence conservation for lncRNAs across species meant that their biological importance was initially met with some skepticism. LncRNAs mediate their functions through interactions with proteins, RNA, DNA, or a combination of these. Their functions can often be dictated by their localization, sequence, and/or secondary structure. Here we provide a review of the approaches typically adopted to study the complexity of these genes with an emphasis on recent discoveries within the innate immune field. Finally, we discuss the challenges, as well as the emergence of new technologies that will continue to move this field forward and provide greater insight into the biological importance of this class of genes. This article is part of a Special Issue entitled: ncRNA in control of gene expression edited by Kotb Abdelmohsen.

The chemistry and applications of RNA 2'-OH acylation

RNA is a versatile biomolecule with a broad range of biological functions that go far beyond its initially described role as a simple information carrier. The development of chemical methods to control, manipulate and modify RNA has the potential to yield new insights into its many functions and properties. Traditionally, most of these methods involved the chemical modification of RNA structure using solid-state synthesis or enzymatic transformations. However, over the past 15 years, the direct functionalization of RNA by selective acylation of the 2'-hydroxyl (2'-OH) group has emerged as a powerful alternative that enables the simple modification of both synthetic and transcribed RNAs. In this Review, we discuss the chemical properties and design of effective reagents for RNA 2'-OH acylation, highlighting the unique problem of 2'-OH reactivity in the presence of water. We elaborate on how RNA 2'-OH acylation is being exploited to develop selective chemical probes that enable interrogation of RNA structure and function, and describe new developments and applications in the field.

Probing RNA structure in vivo

RNA structure underpins many essential functions in biology. New chemical reagents and techniques for probing RNA structure in living cells have emerged in recent years. High-throughput, genome-wide techniques such as Structure-seq2 and DMS-MaPseq exploit nucleobase modification by dimethylsulfate (DMS) to obtain complete structuromes, and are applicable to multiple domains of life and conditions. New reagents such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), glyoxal, and nicotinoyl azide (NAz) greatly expand the capabilities of nucleobase probing in cells. Additionally, ribose-targeting reagents in selective 2'-hydroxyl acylation and primer extension (SHAPE) detect RNA flexibility in vivo. These techniques, coupled with crosslinking nucleobases in psoralen analysis of RNA interactions and structures (PARIS), provide new and diverse ways to elucidate RNA secondary and tertiary structure in vivo and genome-wide.

RNA Framework: an all-in-one toolkit for the analysis of RNA structures and post-transcriptional modifications

RNA is emerging as a key regulator of a plethora of biological processes. While its study has remained elusive for decades, the recent advent of high-throughput sequencing technologies provided the unique opportunity to develop novel techniques for the study of RNA structure and post-transcriptional modifications. Nonetheless, most of the required downstream bioinformatics analyses steps are not easily reproducible, thus making the application of these techniques a prerogative of few laboratories. Here we introduce RNA Framework, an all-in-one toolkit for the analysis of most NGS-based RNA structure probing and post-transcriptional modification mapping experiments. To prove the extreme versatility of RNA Framework, we applied it to both an in-house generated DMS-MaPseq dataset, and to a series of literature available experiments. Notably, when starting from publicly available datasets, our software easily allows replicating authors' findings. Collectively, RNA Framework provides the most complete and versatile toolkit to date for a rapid and streamlined analysis of the RNA epistructurome. RNA Framework is available for download at: http://www.rnaframework.com.

Polyacetate and Polycarbonate RNA: Acylating Reagents and Properties

Acylation of RNA at 2'-OH groups is widely applied in mapping RNA structure and recently for controlling RNA function. Reactions are described that install the smallest 2-carbon acyl groups on RNA-namely, 2'-O-acetyl and 2'-O-carbonate groups. Hybridization and thermal melting experiments are performed to assess the effects of the acyl groups on duplex formation. Both reagents can be employed at lower concentrations to map RNA secondary structure by reverse transcriptase primer extension (SHAPE) methods.

High-throughput determination of RNA structures

RNA performs and regulates a diverse range of cellular processes, with new functional roles being uncovered at a rapid pace. Interest is growing in how these functions are linked to RNA structures that form in the complex cellular environment. A growing suite of technologies that use advances in RNA structural probes, high-throughput sequencing and new computational approaches to interrogate RNA structure at unprecedented throughput are beginning to provide insights into RNA structures at new spatial, temporal and cellular scales.

Atlas of Subcellular RNA Localization Revealed by APEX-Seq

We introduce APEX-seq, a method for RNA sequencing based on direct proximity labeling of RNA using the peroxidase enzyme APEX2. APEX-seq in nine distinct subcellular locales produced a nanometer-resolution spatial map of the human transcriptome as a resource, revealing extensive patterns of localization for diverse RNA classes and transcript isoforms. We uncover a radial organization of the nuclear transcriptome, which is gated at the inner surface of the nuclear pore for cytoplasmic export of processed transcripts. We identify two distinct pathways of messenger RNA localization to mitochondria, each associated with specific sets of transcripts for building complementary macromolecular machines within the organelle. APEX-seq should be widely applicable to many systems, enabling comprehensive investigations of the spatial transcriptome.

Guidelines for SHAPE Reagent Choice and Detection Strategy for RNA Structure Probing Studies

Chemical probing is an important tool for characterizing the complex folded structures of RNA molecules, many of which play key cellular roles. Electrophilic SHAPE reagents create adducts at the 2'-hydroxyl position on the RNA backbone of flexible ribonucleotides with relatively little dependence on nucleotide identity. Strategies for adduct detection such as mutational profiling (MaP) allow accurate, automated calculation of relative adduct frequencies for each nucleotide in a given RNA or group of RNAs. A number of alternative reagents and adduct detection strategies have been proposed, especially for use in living cells. Here we evaluate five SHAPE reagents: three previously well-validated reagents 1M7 (1-methyl-7-nitroisatoic anhydride), 1M6 (1-methyl-6-nitroisatoic anhydride), and NMIA ( N-methylisatoic anhydride), one more recently proposed NAI (2-methylnicotinic acid imidazolide), and one novel reagent 5NIA (5-nitroisatoic anhydride). We clarify the importance of carefully designed software in reading out SHAPE experiments using massively parallel sequencing approaches. We examine SHAPE modification in living cells in diverse cell lines, compare MaP and reverse transcription-truncation as SHAPE adduct detection strategies, make recommendations for SHAPE reagent choice, and outline areas for future development.

Unveiling the druggable RNA targets and small molecule therapeutics

The increasing appreciation for the crucial roles of RNAs in infectious and non-infectious human diseases makes them attractive therapeutic targets. Coding and non-coding RNAs frequently fold into complex conformations which, if effectively targeted, offer opportunities to therapeutically modulate numerous cellular processes, including those linked to undruggable protein targets. Despite the considerable skepticism as to whether RNAs can be targeted with small molecule therapeutics, overwhelming evidence suggests the challenges we are currently facing are not outside the realm of possibility. In this review, we highlight the most recent advances in molecular techniques that have sparked a revolution in understanding the RNA structure-to-function relationship. We bring attention to the application of these modern techniques to identify druggable RNA targets and to assess small molecule binding specificity. Finally, we discuss novel screening methodologies that support RNA drug discovery and present examples of therapeutically valuable RNA targets.

A Designed Small Molecule Inhibitor of a Non-Coding RNA Sensitizes HER2 Negative Cancers to Herceptin

A small molecule (1) with overlapping affinity for two microRNA (miRNA) precursors was used to inform design of a dimeric compound (2) selective for one of the miRNAs. In particular, 2 selectively targets the microRNA(miR)-515 hairpin precursor to inhibit production of miR-515 that represses sphingosine kinase 1 (SK1), a key enzyme in the biosynthesis of sphingosine 1-phosphate (S1P). Application of 2 to breast cancer cells enhanced SK1 and S1P levels, triggering a migratory phenotype. Knockout of SK1, forced overexpression of miR-515, and application of a small molecule SK1 inhibitor all ablated 2's effect on phenotype, consistent with its designed mode of action. Target profiling studies via Chem-CLIP showed that 2 bound selectively to the miR-515 hairpin precursor in cells. Global neoprotein synthesis upon addition of 2 to MCF-7 breast cancer cells demonstrated 2's selectivity and upregulation of cancer-associated proteins regulated by S1P. The most upregulated protein was human epidermal growth factor receptor 2 (ERBB2/HER2), which is regulated by the SK1/S1P pathway and is normally not expressed in MCF-7 cells. Like triple negative breast cancer (TNBC) cells, the lack of HER2 renders them insusceptible to Herceptin and its antibody-drug conjugate Kadcyla. In addition to proteomics, an RNA-seq study supports that 2 has limited off target effects and other studies support that 2 is more selective than an oligonucleotide. We therefore hypothesized that 2 could sensitize MCF-7 cells to anti-HER2 therapies. Indeed, application of 2 sensitized cells to Herceptin. These results were confirmed in two other cell lines that express miR-515 and are HER2-, the hepatocellular carcinoma cell line HepG2 and the TNBC line MDA-MB-231. Importantly, normal breast epithelial cells (MCF-10A) that do not express miR-515 are not affected by 2. These observations suggest a precision medicine approach to sensitize HER2- cancers to approved anticancer medicines. This study has implications for broadening the therapeutic utility of known targeted cancer therapeutics by using a secondary targeted approach to render otherwise insensitive cells, sensitive to a targeted therapeutic.

In vivo RNA structural probing of uracil and guanine base-pairing by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)

Many biological functions performed by RNAs arise from their in vivo structures. The structure of the same RNA can differ in vitro and in vivo owing in part to the influence of molecules ranging from protons to secondary metabolites to proteins. Chemical reagents that modify the Watson-Crick (WC) face of unprotected RNA bases report on the absence of base-pairing and so are of value to determining structures adopted by RNAs. Reagents have thus been sought that can report on the native RNA structures that prevail in living cells. Dimethyl sulfate (DMS) and glyoxal penetrate cell membranes and inform on RNA secondary structure in vivo through modification of adenine (A), cytosine (C), and guanine (G) bases. Uracil (U) bases, however, have thus far eluded characterization in vivo. Herein, we show that the water-soluble carbodiimide 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) is capable of modifying the WC face of U and G in vivo, favoring the former nucleobase by a factor of ∼1.5, and doing so in the eukaryote rice, as well as in the Gram-negative bacterium Escherichia coli While both EDC and glyoxal target Gs, EDC reacts with Gs in their typical neutral state, while glyoxal requires Gs to populate the rare anionic state. EDC may thus be more generally useful; however, comparison of the reactivity of EDC and glyoxal may allow the identification of Gs with perturbed pK_as in vivo and genome-wide. Overall, use of EDC with DMS allows in vivo probing of the base-pairing status of all four RNA bases.

The evolution of RNA structural probing methods: From gels to next-generation sequencing

RNA molecules are important players in all domains of life and the study of the relationship between their multiple flexible states and the associated biological roles has increased in recent years. For several decades, chemical and enzymatic structural probing experiments have been used to determine RNA structure. During this time, there has been a steady improvement in probing reagents and experimental methods, and today the structural biologist community has a large range of tools at its disposal to probe the secondary structure of RNAs in vitro and in cells. Early experiments used radioactive labeling and polyacrylamide gel electrophoresis as read-out methods. This was superseded by capillary electrophoresis, and more recently by next-generation sequencing. Today, powerful structural probing methods can characterize RNA structure on a genome-wide scale. In this review, we will provide an overview of RNA structural probing methodologies from a historical and technical perspective. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry RNA Methods > RNA Analyses in vitro and In Silico RNA Methods > RNA Analyses in Cells.

Technique Development for Probing RNA Structure In Vivo and Genome-Wide

How organisms perceive and respond to their surroundings is one of the great questions in biology. It is clear that RNA plays key roles in sensing. Cellular and environmental cues that RNA responds to include temperature, ions, metabolites, and biopolymers. Recent advances in next-generation sequencing and in vivo chemical probing have provided unprecedented insights into RNA folding in vivo and genome-wide. Patterns of chemical reactivity have implicated control of gene expression by RNA and aided prediction of RNA structure. Central to these advances has been development of molecular biological and chemical techniques. Key advances are improvements in the quality, cost, and throughput of library preparation; availability of a wider array of chemicals for probing RNA structure in vivo; and robustness and user friendliness of data analysis. Insights from probing transcriptomes and future directions are provided.

Long Non-coding RNA Structure and Function: Is There a Link?

RNA has emerged as the prime target for diagnostics, therapeutics and the development of personalized medicine. In particular, the non-coding RNAs (ncRNAs) that do not encode proteins, display remarkable biochemical versatility. They can fold into complex structures and interact with proteins, DNA and other RNAs, modulating the activity, DNA targets or partners of multiprotein complexes. Thus, ncRNAs confer regulatory plasticity and represent a new layer of epigenetic control that is dysregulated in disease. Intriguingly, for long non-coding RNAs (lncRNAs, >200 nucleotides length) structural conservation rather than nucleotide sequence conservation seems to be crucial for maintaining their function. LncRNAs tend to acquire complex secondary and tertiary structures and their functions only impose very subtle sequence constraints. In the present review we will discuss the biochemical assays that can be employed to determine the lncRNA structural configurations. The implications and challenges of linking function and lncRNA structure to design novel RNA therapeutic approaches will also be analyzed.