Quadruplex-Flanking Stem Structures Modulate the Stability and Metal Ion Preferences of RNA Mimics of GFP

Development of a paper-based transcription aptasensor for convenient urinary uric acid self-testing

The abnormal uric acid (UA) level in urine can serve as warning signals of many diseases, such as gout and metabolic cardiovascular diseases. The current methods for detecting UA face limitations of instrument dependence and the requirement for non-invasiveness, making it challenging to fulfill the need for home-based application. In this study, we designed an aptasensor that combined UA-specific transcriptional regulation and a fluorescent RNA aptamer for convenient urinary UA testing. The concentration of UA can be translated into the intensity of fluorescent signals. The aptasensor showed higher sensitivity and more robust anti-interference performance. UA levels in the urine of different volunteers could be accurately tested using this method. In addition, a paper-based aptasensor for UA self-testing was manufactured, in which the urinary UA levels could be determined using a smartphone-based colorimetric approach. This work not only demonstrates a new approach for the design of disease-associated aptasensor, but also offers promising ideas for home-based detection of UA.

Quantification of all 12 canonical ribonucleotides by real-time fluorogenic in vitro transcription

Enzymatic methods to quantify deoxyribonucleoside triphosphates have existed for decades. In contrast, no general enzymatic method to quantify ribonucleoside triphosphates (rNTPs), which drive almost all cellular processes and serve as precursors of RNA, exists to date. ATP can be measured with an enzymatic luminometric method employing firefly luciferase, but the quantification of other ribonucleoside mono-, di-, and triphosphates is still a challenge for a non-specialized laboratory and practically impossible without chromatography equipment. To allow feasible quantification of ribonucleoside phosphates in any laboratory with typical molecular biology and biochemistry tools, we developed a robust microplate assay based on real-time detection of the Broccoli RNA aptamer during in vitro transcription. The assay employs the bacteriophage T7 and SP6 RNA polymerases, two oligonucleotide templates encoding the 49-nucleotide Broccoli aptamer, and a high-affinity fluorogenic aptamer-binding dye to quantify each of the four canonical rNTPs. The inclusion of nucleoside mono- and diphosphate kinases in the assay reactions enabled the quantification of the mono- and diphosphate counterparts. The assay is inherently specific and tolerates concentrated tissue and cell extracts. In summary, we describe the first chromatography-free method to quantify ATP, ADP, AMP, GTP, GDP, GMP, UTP, UDP, UMP, CTP, CDP and CMP in biological samples.

Double-stemmed and split structural variants of fluorescent RNA Mango aptamers

Aptamers with fluorogenic ligands are emerging as useful tools to quantify and track RNA molecules. The RNA Mango family of aptamers have a useful combination of tight ligand binding, bright fluorescence, and small size. However, the simple structure of these aptamers, with a single base-paired stem capped by a G-quadruplex, can limit the sequence and structural modifications needed for many use-inspired designs. Here we report new structural variants of RNA Mango that have two base-paired stems attached to the quadruplex. Fluorescence saturation analysis of one of the double-stemmed constructs showed a maximum fluorescence that is ∼75% brighter than the original single-stemmed Mango I. A small number of mutations to nucleotides in the tetraloop-like linker of the second stem were subsequently analyzed. The effect of these mutations on the affinity and fluorescence suggested that the nucleobases of the second linker do not directly interact with the fluorogenic ligand (TO1-biotin), but may instead induce higher fluorescence by indirectly altering the ligand properties in the bound state. The effects of the mutations in this second tetraloop-like linker indicate the potential of this second stem for rational design and reselection experiments. Additionally, we demonstrated that a bimolecular mango designed by splitting the double-stemmed Mango can function when two RNA molecules are cotranscribed from different DNA templates in a single in vitro transcription. This bimolecular Mango has potential application in detecting RNA-RNA interactions. Together, these constructs expand the designability of the Mango aptamers to facilitate future applications of RNA imaging.

Reversible Modulation of Aptamer-Ligand Binding in RNA Light-Up Aptamers Containing G-Quadruplex Using Chemical Stimuli

Regulating a system in equilibrium transiently to out-of-equilibrium by using certain stimuli is the strategy used by natural biomolecules to function. Herein, we showed that the interaction of synthetic RNA aptamers, having a G-quadruplex core structure, with their corresponding ligands could be regulated from their equilibrium state to non-equilibrium state in a reversible manner using simple chemical stimuli (Ag⁺ and cysteine). The approach would be useful for designing aptamer regulators that work in a dynamic nucleic acid network, where a strict control on aptamer-ligand interaction is needed. In addition, to the best of our knowledge, this is the first report which shows that RNA G-quadruplexes can be disrupted by the addition of silver ions. This would be useful not only in designing RNA-based sensors or regulators but would also be useful for understanding the role of metal ions in RNA folding and catalysis.

Ligands with polyfluorophenyl moieties promote a local structural rearrangement in the Spinach2 and Broccoli aptamers that increases ligand affinities

The interaction of nucleic acids with their molecular targets often involves structural reorganization that may traverse a complex folding landscape. With the more recent recognition that many RNAs, both coding and noncoding, may regulate cellular activities by interacting with target molecules, it becomes increasingly important to understand how nucleic acids interact with their targets and how drugs might be developed that can influence critical folding transitions. We have extensively investigated the interaction of the Spinach2 and Broccoli aptamers with a library of small molecule ligands modified by various extensions from the imido nitrogen of DFHBI [(Z)-5-(3,5-difluoro-4-hydroxybenzylidene)-2,3-dimethyl-3,5-dihydro-4H-imidazol-4-one] that reach out from the Spinach2 ligand binding pocket. Studies of the interaction of these compounds with the aptamers revealed that polyfluorophenyl-modified ligands initiate a slow change in aptamer affinity that takes an extended time (half-life of ∼40 min) to achieve. The change in affinity appears to involve an initial disruption of the entrance to the ligand binding pocket followed by a gradual transition to a more defined structure for which the most likely driving force is an interaction of the gateway adenine with a nearby 2'OH group. These results suggest that polyfluorophenyl modifications might increase the ability of small molecule drugs to disrupt local structure and promote RNA remodeling.

A novel fluorescent probe with a pyrazolo[4,3-c]quinoline core selectively recognizes c-MYC promoter G-quadruplexes

An aptamer combined with a c-MYC-selective fluorophore could work as the fluorescent core of nucleic acid mimics of fluorescent proteins to locate and image functional biomolecules in cells.

Large Stokes shift fluorescence activation in an RNA aptamer by intermolecular proton transfer to guanine

Fluorogenic RNA aptamers are synthetic functional RNAs that specifically bind and activate conditional fluorophores. The Chili RNA aptamer mimics large Stokes shift fluorescent proteins and exhibits high affinity for 3,5-dimethoxy-4-hydroxybenzylidene imidazolone (DMHBI) derivatives to elicit green or red fluorescence emission. Here, we elucidate the structural and mechanistic basis of fluorescence activation by crystallography and time-resolved optical spectroscopy. Two co-crystal structures of the Chili RNA with positively charged DMHBO⁺ and DMHBI⁺ ligands revealed a G-quadruplex and a trans-sugar-sugar edge G:G base pair that immobilize the ligand by π-π stacking. A Watson-Crick G:C base pair in the fluorophore binding site establishes a short hydrogen bond between the N7 of guanine and the phenolic OH of the ligand. Ultrafast excited state proton transfer (ESPT) from the neutral chromophore to the RNA was found with a time constant of 130 fs and revealed the mode of action of the large Stokes shift fluorogenic RNA aptamer.

Development and Applications of Fluorogen/Light-Up RNA Aptamer Pairs for RNA Detection and More

The central role of RNA in living systems made it highly desirable to have noninvasive and sensitive technologies allowing for imaging the synthesis and the location of these molecules in living cells. This need motivated the development of small pro-fluorescent molecules called "fluorogens" that become fluorescent upon binding to genetically encodable RNAs called "light-up aptamers." Yet, the development of these fluorogen/light-up RNA pairs is a long and thorough process starting with the careful design of the fluorogen and pursued by the selection of a specific and efficient synthetic aptamer. This chapter summarizes the main design and the selection strategies used up to now prior to introducing the main pairs. Then, the vast application potential of these molecules for live-cell RNA imaging and other applications is presented and discussed.

Revisiting T7 RNA polymerase transcription in vitro with the Broccoli RNA aptamer as a simplified real-time fluorescent reporter

Methods for rapid and high-throughput screening of transcription in vitro to examine reaction conditions, enzyme mutants, promoter variants, and small molecule modulators can be extremely valuable tools. However, these techniques may be difficult to establish or inaccessible to many researchers. To develop a straightforward and cost-effective platform for assessing transcription in vitro, we used the "Broccoli" RNA aptamer as a direct, real-time fluorescent transcript readout. To demonstrate the utility of our approach, we screened the effect of common reaction conditions and components on bacteriophage T7 RNA polymerase (RNAP) activity using a common quantitative PCR instrument for fluorescence detection. Several essential conditions for in vitro transcription by T7 RNAP were confirmed with this assay, including the importance of enzyme and substrate concentrations, covariation of magnesium and nucleoside triphosphates, and the effects of several typical additives. When we used this method to assess all possible point mutants of a canonical T7 RNAP promoter, our results coincided well with previous reports. This approach should translate well to a broad variety of bacteriophage in vitro transcription systems and provides a platform for developing fluorescence-based readouts of more complex transcription systems in vitro.

Fundamental studies of functional nucleic acids: aptamers, riboswitches, ribozymes and DNAzymes

This review aims at juxtaposing common versus distinct structural and functional strategies that are applied by aptamers, riboswitches, and ribozymes/DNAzymes. Focusing on recently discovered systems, we begin our analysis with small-molecule binding aptamers, with emphasis on in vitro-selected fluorogenic RNA aptamers and their different modes of ligand binding and fluorescence activation. Fundamental insights are much needed to advance RNA imaging probes for detection of exo- and endogenous RNA and for RNA process tracking. Secondly, we discuss the latest gene expression-regulating mRNA riboswitches that respond to the alarmone ppGpp, to PRPP, to NAD⁺, to adenosine and cytidine diphosphates, and to precursors of thiamine biosynthesis (HMP-PP), and we outline new subclasses of SAM and tetrahydrofolate-binding RNA regulators. Many riboswitches bind protein enzyme cofactors that, in principle, can catalyse a chemical reaction. For RNA, however, only one system (glmS ribozyme) has been identified in Nature thus far that utilizes a small molecule - glucosamine-6-phosphate - to participate directly in reaction catalysis (phosphodiester cleavage). We wonder why that is the case and what is to be done to reveal such likely existing cellular activities that could be more diverse than currently imagined. Thirdly, this brings us to the four latest small nucleolytic ribozymes termed twister, twister-sister, pistol, and hatchet as well as to in vitro selected DNA and RNA enzymes that promote new chemistry, mainly by exploiting their ability for RNA labelling and nucleoside modification recognition. Enormous progress in understanding the strategies of nucleic acids catalysts has been made by providing thorough structural fundaments (e.g. first structure of a DNAzyme, structures of ribozyme transition state mimics) in combination with functional assays and atomic mutagenesis.

Perspective on the Future Role of Aptamers in Analytical Chemistry

It has been almost 30 years since the invention of Systematic Evolution of Ligands by Exponential Enrichment (SELEX) methodology and the description of the first aptamers. In retrospect over the past 30 years, advances in aptamer development and application have demonstrated that aptamers are potentially useful reagents that can be employed in diverse areas within analytical chemistry, biotechnology, biomedicine, and molecular biology. While often touted as artificial antibodies with an ability to be selected for any target, aptamer development, unfortunately, lags behind development of analytical methodologies that employ aptamers, hindering deeper integration into the application of analytical tool development. This perspective covers recent advances in SELEX methodology for improving efficiency of the SELEX procedure and enhancing affinity and specificity of the selected aptamers, what we view as a critical barrier in the future role of aptamers in analytical chemistry. We discuss postselection modifications that can be used for enhancing performance of the selected aptamers in an analytical device by including understanding intermolecular interaction forces in the binding domain. While highlighting promising properties of aptamers that enable several analytical advances, we provide discussion on the challenges of penetration of aptamers in the analytical field.

Nanomechanics and co-transcriptional folding of Spinach and Mango

Recent advances in fluorogen-binding “light-up” RNA aptamers have enabled protein-free detection of RNA in cells. Detailed biophysical characterization of folding of G-Quadruplex (GQ)-based light-up aptamers such as Spinach, Mango and Corn is still lacking despite the potential implications on their folding and function. In this work we employ single-molecule fluorescence-force spectroscopy to examine mechanical responses of Spinach2, iMangoIII and MangoIV. Spinach2 unfolds in four discrete steps as force is increased to 7 pN and refolds in reciprocal steps upon force relaxation. In contrast, GQ-core unfolding in iMangoIII and MangoIV occurs in one discrete step at forces >10 pN and refolding occurred at lower forces showing hysteresis. Co-transcriptional folding using superhelicases shows reduced misfolding propensity and allowed a folding pathway different from refolding. Under physiologically relevant pico-Newton levels of force, these aptamers may unfold in vivo and subsequently misfold. Understanding of the dynamics of RNA aptamers will aid engineering of improved fluorogenic modules for cellular applications.

Light-up aptamers are widely used for fluorescence visualization of non-coding RNA in vivo. Here the authors employ single-molecule fluorescence-force spectroscopy to characterize the mechanical responses of the G-Quadruplex based light-up aptamers Spinach2, iMangoIII and MangoIV, which is of interest for the development of improved fluorogenic modules for imaging applications.

Programmable RNA detection with a fluorescent RNA aptamer using optimized three-way junction formation

RNAs play essential roles in various cellular processes and can be used as biomarkers. Hence, it is important to detect endogenous RNA for understanding diverse cellular functions and diagnosing diseases. To construct a low-cost and easy-to-use RNA detection probe, a chemically unmodified RNA aptamer that binds to a pro-fluorophore to increase its fluorescence is desirable. Here, we focused on Broccoli, a superior variant of Spinach, which is a well-known fluorescent RNA aptamer that binds to DFHBI-1T and emits green fluorescence. We experimentally characterized Broccoli and predicted that it forms a G-quadruplex-based DFHBI-1T recognition region sandwiched between two stems. Based on this, we designed a Broccoli-based RNA detection probe (BRD probe) composed of a sequence of destabilized Broccoli fused with complementary sequences against target RNA. The resulting probe with its target RNA formed a stable three-way junction, named the MT2 three-way junction, which contributed to efficient refolding of the Broccoli structure and allowed for programmable RNA detection with high signal-to-noise ratio and sensitivity. Interestingly, the MT2 three-way junction also could be applied to probe construction of a truncated form of Spinach (Baby Spinach). The BRD and Baby Spinach-based RNA detection probes (BSRD probe) exhibited up to 48- and 140-fold fluorescence enhancements in the presence of their target RNAs and detected small amounts of target RNA that were as low as 160 and 5 nM, respectively. Thus, we experimentally characterized the higher order structure of Broccoli and developed structure-switching aptamer probes for highly sensitive, programmable, RNA detection using an MT2 three-way junction.

Selective recognition of c-MYC Pu22 G-quadruplex by a fluorescent probe

Nucleic acid mimics of fluorescent proteins can be valuable tools to locate and image functional biomolecules in cells. Stacking between the internal G-quartet, formed in the mimics, and the exogenous fluorophore probes constitutes the basis for fluorescence emission. The precision of recognition depends upon probes selectively targeting the specific G-quadruplex in the mimics. However, the design of probes recognizing a G-quadruplex with high selectivity in vitro and in vivo remains a challenge. Through structure-based screening and optimization, we identified a light-up fluorescent probe, 9CI that selectively recognizes c-MYC Pu22 G-quadruplex both in vitro and ex vivo. Upon binding, the biocompatible probe emits both blue and green fluorescence with the excitation at 405 nm. With 9CI and c-MYC Pu22 G-quadruplex complex as the fluorescent response core, a DNA mimic of fluorescent proteins was constructed, which succeeded in locating a functional aptamer on the cellular periphery. The recognition mechanism analysis suggested the high selectivity and strong fluorescence response was attributed to the entire recognition process consisting of the kinetic match, dynamic interaction, and the final stacking. This study implies both the single stacking state and the dynamic recognition process are crucial for designing fluorescent probes or ligands with high selectivity for a specific G-quadruplex structure.

Spectral Tuning by a Single Nucleotide Controls the Fluorescence Properties of a Fluorogenic Aptamer

Fluorogenic aptamers are genetically encoded RNA aptamers that bind and induce the fluorescence of otherwise nonfluorescent small molecule dyes. These RNA-fluorophore complexes can be highly fluorescent and useful for RNA visualization and genetically encoded biosensors. Notably, different RNA aptamers can bind the same fluorophore, resulting in complexes that exhibit spectrally distinct fluorescence properties. The basis for spectral tuning of small molecule fluorophores has not yet been studied. Here we explore the mechanism of spectral tuning in three highly related RNA aptamers, Broccoli, Orange Broccoli, and Red Broccoli, each of which binds the DFHO (3,5-difluoro-4-hydroxybenzylidene imidazolinone-2-oxime) fluorophore and generates distinct spectral emissions. We show that DFHO fluorescence spectral tuning is controlled by interaction of the oxime moiety of the fluorophore and one specific nucleotide that is different in each RNA aptamer. Our finding presents, for the first time, a mechanism by which RNA can control the properties of a bound small molecule fluorophore. More broadly, our finding can guide further development of fluorogenic aptamers with novel spectral properties.

Light-Up RNA Aptamers and Their Cognate Fluorogens: From Their Development to Their Applications

An RNA-based fluorogenic module consists of a light-up RNA aptamer able to specifically interact with a fluorogen to form a fluorescent complex. Over the past decade, significant efforts have been devoted to the development of such modules, which now cover the whole visible spectrum, as well as to their engineering to serve in a wide range of applications. In this review, we summarize the different strategies used to develop each partner (the fluorogen and the light-up RNA aptamer) prior to giving an overview of their applications that range from live-cell RNA imaging to the set-up of high-throughput drug screening pipelines. We then conclude with a critical discussion on the current limitations of these modules and how combining in vitro selection with screening approaches may help develop even better molecules.

Crystal structure and fluorescence properties of the iSpinach aptamer in complex with DFHBI

Fluorogenic RNA aptamers are short nucleic acids able to specifically interact with small molecules and strongly enhance their fluorescence upon complex formation. Among the different systems recently introduced, Spinach, an aptamer forming a fluorescent complex with the 3,5-difluoro-4-hydroxybenzylidene imidazolinone (DFHBI), is one of the most promising. Using random mutagenesis and ultrahigh-throughput screening, we recently developed iSpinach, an improved version of the aptamer, endowed with an increased folding efficiency and thermal stability. iSpinach is a shorter version of Spinach, comprising five mutations for which the exact role has not yet been deciphered. In this work, we cocrystallized a reengineered version of iSpinach in complex with the DFHBI and solved the X-ray structure of the complex at 2 Å resolution. Only a few mutations were required to optimize iSpinach production and crystallization, underlying the good folding capacity of the molecule. The measured fluorescence half-lives in the crystal were 60% higher than in solution. Comparisons with structures previously reported for Spinach sheds some light on the possible function of the different beneficial mutations carried by iSpinach.

Duplex/quadruplex oligonucleotides: Role of the duplex domain in the stabilization of a new generation of highly effective anti-thrombin aptamers

Recently, mixed duplex/quadruplex oligonucleotides have attracted great interest for use as biomedical aptamers. In the case of anti-thrombin aptamers, the addition of duplex-forming sequences to a G-quadruplex module identical or very similar to the best-known G-quadruplex of the Thrombin Binding Aptamer (HD1) results in new or improved biological properties, such as higher activity or different recognition properties with respect to HD1. Remarkably, this bimodular fold was hypothesized, based on its sequence, for the only anti-thrombin aptamer in advanced clinical trial, NU172. Whereas cation modulation of G-quadruplex conformation and stability is well characterized, only few data from similar analysis on duplex/quadruplex oligonucleotides exist. Here we have performed a characterization of structure and stability of four different duplex/quadruplex anti-thrombin aptamers, including NU172, in the presence of different cations and in physiological-mimicking conditions in comparison to HD1, by means of spectroscopic techniques (UV and circular dichroism) and differential scanning calorimetry. Our data show a strong reciprocal influence of each domain on the stability of the other and in particular suggest a stabilizing effect of the duplex region in the presence of solutions mimicking the physiological conditions, strengthening the idea that bimodular aptamers present better therapeutic potentialities than those containing a single G-quadruplex domain.

A Fluorescent Split Aptamer for Visualizing RNA-RNA Assembly In Vivo

RNA–RNA assembly governs key biological processes and is a powerful tool for engineering synthetic genetic circuits. Characterizing RNA assembly in living cells often involves monitoring fluorescent reporter proteins, which are at best indirect measures of underlying RNA–RNA hybridization events and are subject to additional temporal and load constraints associated with translation and activation of reporter proteins. In contrast, RNA aptamers that sequester small molecule dyes and activate their fluorescence are increasingly utilized in genetically encoded strategies to report on RNA-level events. Split-aptamer systems have been rationally designed to generate signal upon hybridization of two or more discrete RNA transcripts, but none directly function when expressed in vivo. We reasoned that the improved physiological properties of the Broccoli aptamer enable construction of a split-aptamer system that could function in living cells. Here we present the Split-Broccoli system, in which self-assembly is nucleated by a thermostable, three-way junction RNA architecture and fluorescence activation requires both strands. Functional assembly of the system approximately follows second-order kinetics in vitro and improves when cotranscribed, rather than when assembled from purified components. Split-Broccoli fluorescence is digital in vivo and retains functional modularity when fused to RNAs that regulate circuit function through RNA–RNA hybridization, as demonstrated with an RNA Toehold switch. Split-Broccoli represents the first functional split-aptamer system to operate in vivo. It offers a genetically encoded and nondestructive platform to monitor and exploit RNA–RNA hybridization, whether as an all-RNA, stand-alone AND gate or as a tool for monitoring assembly of RNA–RNA hybrids.

Illuminating Messengers: An Update and Outlook on RNA Visualization in Bacteria

To be able to visualize the abundance and spatiotemporal features of RNAs in bacterial cells would permit obtaining a pivotal understanding of many mechanisms underlying bacterial cell biology. The first methods that allowed observing single mRNA molecules in individual cells were introduced by Bertrand et al. (1998) and Femino et al. (1998). Since then, a plethora of techniques to image RNA molecules with the aid of fluorescence microscopy has emerged. Many of these approaches are useful for the large eukaryotic cells but their adaptation to study RNA, specifically mRNA molecules, in bacterial cells progressed relatively slow. Here, an overview will be given of fluorescent techniques that can be used to reveal specific RNA molecules inside fixed and living single bacterial cells. It includes a critical evaluation of their caveats as well as potential solutions.