Selective recognition of parallel and anti-parallel thrombin-binding aptamer G-quadruplexes by different fluorescent dyes

Structural Antagonism-Aided Conformational Regulation Enables an Aptamer-Loop G-Quadruplex Modular Sensor of β-Lactoglobulin

A simple, reliable method for identifying β-lactoglobulin (β-LG) in dairy products is needed to protect those with β-LG allergies. A common, practical strategy for target detection is designing simplified nucleic acid nanodevices by integrating functional components. This work presents a label-free modular β-LG aptasensor consisting of an aptamer-loop G-quadruplex (G4), the working conformation of which is regulated by conformational antagonism to ensure respective module functionality and the related signal transduction. The polymorphic conformations of the module-fused sequence are systematically characterized, and the cause is revealed as shifting antagonistic equilibrium. Combined with conformational folding dynamics, this helped regulate functional conformations by fine-tuning the sequences. Furthermore, the principle of specific β-LG detection by parallel G4 topology is examined as binding on the G4 aptamer loop by β-LG to reinforce the G4 topology and fluorescence. Finally, a label-free, assembly-free, succinct, and turn-on fluorescent aptasensor is established, achieving excellent sensitivity across five orders of magnitude, rapidly detecting β-LG within 22-min. This study provides a generalizable approach for the conformational regulation of module-fused G4 sequences and a reference model for creating simplified sensing devices for a variety of targets.

Regulation of thrombin activity by ligand-induced topological alteration in a thrombin-binding aptamer

Thrombin-binding aptamer (TBA), which forms a G-quadruplex (G4) structure with anti-parallel topology, interacts with thrombin to inhibit its enzymatic activity. Here we show that the G4-topology-altering ligand L2H2-2M2EA-6LCO (6LCO) changes the anti-parallel topology of TBA G4 to the parallel topology, thereby abrogating the thrombin-inhibitory activity of TBA. This finding suggests that G4 ligands that alter topology may be promising drug candidates for diseases involving G4-binding proteins.

Newly found K+-Thioflavin T competitive binding to DNA G-quadruplexes and the development of a label-free fluorescent biosensor with extra low detection limit for K+ determination in urine samples

The determination of potassium ion K⁺ in body fluids is important in health monitoring and diagnoses. One of the interesting and simple methods for K⁺ detection is the use of label-free biosensors based on DNA G-quadruplexes (GQs) coupled with a specific fluorescent probe, such as Thioflavin T (ThT), which lights up when bound with K⁺-stabilized GQs. However, these biosensors are not generally sensitive. In this work, we found a solution: at a low concentration, K⁺ competes with ThT in binding to a bimolecular GQ or a tetramolecular GQ, resulting in a decrease in ThT fluorescence emission with increasing K⁺. Therefore, we developed a label-free turn-off fluorescent K⁺ sensor. The sensor provides a very low detection limit of 21.87 ± 0.59 nM. Other possible interfering components in urine did not exert any effect even at quantities that were 10-fold greater than their upper limit of normal concentrations found in urine samples. With its only requirement of diluting samples, the developed low-cost label-free probe and simple sensor was successfully applied to the direct detection of K⁺ in normal urine samples with high accuracy (recoveries ranged from 90% to 100%).

Solution structure of thrombin binding aptamer complex with a non-planar platinum(II) compound

Thrombin Binding Aptamer (TBA) is a monomolecular well-defined two G-tetrads antiparallel G-quadruplex DNA that inhibits the activity of human α-thrombin. In this report, we synthesized a quasi-cross-shaped platinum(II) compound (L'2LPt)...

Quantification of ATP in cell by fluorescence spectroscopy based on generalized ratio quantitative analysis model

In this contribution, fluorescence spectroscopy based on G-quadruplex formation was developed for the quantification of ATP in cell. In the absence of ATP, thioflavin T (ThT) dye can combine with the G-rich ATP aptamer to form an ATP aptamer-ThT G-quandruplex complex, resulting in the enhancement of fluorescence intensity; otherwise, fluorescence intensity of the system will weaken with the addition of ATP, because ATP has a strong affinity with G-rich ATP aptamer and can replace ThT to form an ATP aptamer-ATP complex. A calibration model based on generalized ratio quantitative analysis model was employed to mitigate the influence of scatterers and background absorbers in cell suspensions. The proposed fluorescence method was applied to the quantitative analysis of ATP in two type of cell lines, and achieved satisfactory quantitative results with accuracy comparable to that of the reference method-ATP detection kit. The limit of detection and limit of quantification of the proposed method for ATP in cell were estimated to be 0.22 and 0.66 μM, respectively. This proposed fluorescence method is highly simple and rapid, and does not require the use of fluorescent labeling.

Thioflavin T fluorescence and NMR spectroscopy suggesting a non-G-quadruplex structure for a sodium binding aptamer embedded in DNAzymes

Recently, a Na+-binding aptamer was reported to be embedded in a few RNA-cleaving DNAzymes, including NaA43, Ce13d, and NaH1. The Na+ aptamer consists of multiple GG stretches, which is a prerequisite for the formation of G-quadruplex (G4) structures. These DNAzymes require Na+ for activity but show no activity in the presence of K+ or other metal ions. Given that DNA can selectively bind K+ by forming a G4 structure, this work aims to answer whether this Na+ aptamer also uses a G4 to bind Na+. Through comparative ThT fluorescence spectrometry studies, while a control G4 DNA exhibited notable fluorescence enhancement up to 5 mM K+ with a Kd of 0.28 ± 0.06 mM, the Ce13d DNAzyme fluorescence was negligibly perturbed with similar concentrations of K+. Opposed to this, Ce13d displayed specific remarkable fluorescence decrease with low millimolar concentrations of Na+. NMR experiments at two different pH values suggest that Ce13d adopts a significantly different conformation or equilibrium of conformations in the presence of Na+ versus K+ and has a more stable structure in the presence of Na+. Additionally, absence of characteristic G4 peaks in one-dimensional 1H NMR suggest that G4 is not responsible for the Na+ binding. This hypothesis is confirmed by the absence of characteristic peaks in the CD spectra of this sequence. Therefore, we concluded that the aptamer must be selective for Na+ and that it binds Na+ using a structural element that does not contain G4.

TERRA G-quadruplex RNA interaction with TRF2 GAR domain is required for telomere integrity

Telomere dysfunction causes chromosomal instability which is associated with many cancers and age-related diseases. The non-coding telomeric repeat-containing RNA (TERRA) forms a structural and regulatory component of the telomere that is implicated in telomere maintenance and chromosomal end protection. The basic N-terminal Gly/Arg-rich (GAR) domain of telomeric repeat-binding factor 2 (TRF2) can bind TERRA but the structural basis and significance of this interaction remains poorly understood. Here, we show that TRF2 GAR recognizes G-quadruplex features of TERRA. We show that small molecules that disrupt the TERRA-TRF2 GAR complex, such as N-methyl mesoporphyrin IX (NMM) or genetic deletion of TRF2 GAR domain, result in the loss of TERRA, and the induction of γH2AX-associated telomeric DNA damage associated with decreased telomere length, and increased telomere aberrations, including telomere fragility. Taken together, our data indicates that the G-quadruplex structure of TERRA is an important recognition element for TRF2 GAR domain and this interaction between TRF2 GAR and TERRA is essential to maintain telomere stability.

Influence of core extension and side chain nature in targeting G-quadruplex structures with perylene monoimide derivatives

A structure-activity relationship (SAR) study in terms of G-quadruplex binding ability and antiproliferative activity of six fluorescent perylenemonoimide (PMIs) derivatives is reported. A positive charge seems to be the key to target G4. This study also reveals the importance of the element substitution in the potential biological activity of PMIs, being the polyethylene glycol (PEG) chains in the peri position responsible for their antiproliferative activity. Among them, the cationic PMI6 with two PEG chains is the most promising compound since its fluorescence is enhanced in the presence of G-quadruplex structures. Moreover, PMI6 binds to the human telomeric G-quadruplex hTelo with high affinity and displays a high antiproliferative potential towards HeLa (cervical adenocarcinoma), A549 (lung adenocarcinoma) and A2780 (ovarian adenocarcinoma) cells. Its fate can be followed inside cells thanks to its fluorescent properties: the compound is found to accumulate in the mitochondria.

Molecular Rotors as Guest Fluorophores Probing the Local Environment inside Host G4 Supramolecular Hydrogels

Fluorescent molecular rotors with a high binding affinity toward the guanosine quartet (G4) were incorporated as guest fluorophores into host supramolecular hydrogels based on the self-assembly of G4 units, to probe the local environment. Torsional dynamics of the rotors were severely inhibited inside the hydrogels in comparison with aqueous solutions, although the hydrogels were composed of >95% water. Moreover, even though all the gels were rigid bodies with no spontaneous deformation or flow property at room temperature, torsional dynamics in G4 borate gels was found to be consistently several orders of magnitude slower than those in the other G4 gels, irrespective of the identity of the molecular rotor probe. This clear difference in the molecular mobilities of the guest fluorophore could be attributed to systematic differences in the internal structure between the two categories of host G4 hydrogels. In specific terms, the borate groups in G4 borate hydrogels serve as bridging units between separate G4 quadruplex strands, generating additional cross-links that reinforce the network structure of the gel. The results demonstrate that molecular rotors act as efficient fluorescent probes for the quantitative assessment of the molecular-level environment and dynamics inside the hydrogels, an aspect that is missed out by most other analytical methods that are routinely employed for studying them.

G-quadruplex specific thioflavin T-based label-free fluorescence aptasensor for rapid detection of tetracycline

A label-free fluorescence aptasensor was developed for the rapid detection of tetracycline (TET) based on G-quadruplex structure of TET aptamers and G-quadruplex specific dye Thioflavin T (ThT). The fluorescence of free ThT is essentially weak in aqueous solution, whereas it selectively identifies the G-quadruplex of aptamers to form the G-quadruplex/ThT conjugates, resulting in an enormous increase of the fluorescence intensity. However, the fluorescence intensity of G-quadruplex/ThT conjugates was drastically suppressed due to the release of free ThT from G-quadruplex/ThT conjugates after the addition of TET via specific binding with TET aptamers. The key factors affecting sensitivity and selectivity including the reaction medium, binding time of ThT to TET aptamers, incubation time between TET aptamers and TET, concentration of ThT and TET aptamers were investigated in detail. The optimal conditions were as follows: ultrapure water as reaction medium, binding time of 5 min, incubation time of 1 min, 9.0 μmol/L ThT and 0.03 μmol/L aptamers. A good linear relationship (correlation coefficient of 0.9973) was obtained between the fluorescence quenching efficiency (F₀ - F) / F₀ and the logarithm of TET concentration in the range of 0.01-1.0 μmol/L. The limit of detection was 0.001 μmol/L (S/N = 3). The proposed assay was applied for the detection of TET in the spiked honey and milk samples with recoveries ranging from 93.5% to 106.9%. The developed label-free fluorescence aptasensor showed advantages of high specificity, low cost and short time-consuming, illustrating potential application for on-site detection of TET in foodstuffs.

Label-free one-step fluorescent method for the detection of endonuclease activity based on thioflavin T/G-quadruplex

Endonucleases, one of the basic tool enzymes of modern molecular biology and medical genetics, have also been clarified as the potential targets for antimicrobial and antiviral drugs screening. However, traditional assays to monitor endonuclease activity can be expensive, time-consuming, or laborious. In order to provide new detective platform, we proposed a novel label-free one-step fluorescent method for the detection of endonuclease activity based on cleavage-induced G-quadruplex formation. In this detection system, a simple DNA probe can spontaneously form a duplex structure with recognition sites of EcoRI and prevent the generation of the G-quadruplex. Once EcoRI is present, the recognition sites in the duplex DNA are cleavage, producing a free guanine-rich sequence to form G-quadruplex. When thioflavin T (ThT) is added, a strong fluorescence signal is given by ThT/G-quadruplex, and therefore the EcoRI activity can be detected. After systematic investigation and optimization, this method has gained a sensitive limit of detection at 0.75 U/mL, and a wide detection range between 0.75 U/mL and 120 U/mL. Furthermore, the inhibitory effect of 5-fluorouracil on EcoRI activity was verified and IC50 was calculated. Taken together, these experimental results have proven that this turn-on fluorescent method has considerable analytical performances. As far as we are concerned, this method is the first reported EcoRI assay based on ThT/G-quadruplex, and only one kind of probe and one kind of dye are involved, providing one of the simplest detective strategies on EcoRI. More importantly, the convenience and cost make this one-step method quite attractive for application transformation. Therefore, we hope this method could be a hopeful option for EcoRI activity determination, and further to help monitoring the quality of tool enzymes and promote the development of high-through automatic drug screening system.

N-methyl mesoporphyrin IX as a highly selective light-up probe for G-quadruplex DNA

N-methyl mesoporphyrin IX (NMM) is a water-soluble, non-symmetric porphyrin with excellent optical properties and unparalleled selectivity for G-quadruplex (GQ) DNA. G-quadruplexes are non-canonical DNA structures formed by guanine-rich sequences. They are implicated in genomic stability, longevity, and cancer. The ability of NMM to selectively recognize GQ structures makes it a valuable scaffold for designing novel GQ binders. In this review, we survey the literature describing the GQ-binding properties of NMM as well as its wide utility in chemistry and biology. We start with the discovery of the GQ-binding properties of NMM and the development of NMM-binding aptamers. We then discuss the optical properties of NMM, focusing on the light-switch effect - high fluorescence of NMM induced upon its binding to GQ DNA. Additionally, we examine the affinity and selectivity of NMM for GQs, as well as its ability to stabilize GQ structures and favor parallel GQ conformations. Furthermore, a portion of the review is dedicated to the applications of NMM-GQ complexes as biosensors for heavy metals, small molecules (e.g. ATP and pesticides), DNA, and proteins. Finally and importantly, we discuss the utility of NMM as a probe to investigate the roles of GQs in biological processes.

Identification of optimal fluorescent probes for G-quadruplex nucleic acids through systematic exploration of mono- and distyryl dye libraries

A library of 52 distyryl and 9 mono-styryl cationic dyes was synthesized and investigated with respect to their optical properties, propensity to aggregation in aqueous medium, and capacity to serve as fluorescence “light-up” probes for G-quadruplex (G4) DNA and RNA structures. Among the 61 compounds, 57 dyes showed preferential enhancement of fluorescence intensity in the presence of one or another G4-DNA or RNA structure, while no dye displayed preferential response to double-stranded DNA or single-stranded RNA analytes employed at equivalent nucleotide concentration. Thus, preferential fluorimetric response towards G4 structures appears to be a common feature of mono- and distyryl dyes, including long-known mono-styryl dyes used as mitochondrial probes or protein stains. However, the magnitude of the G4-induced “light-up” effect varies drastically, as a function of both the molecular structure of the dyes and the nature or topology of G4 analytes. Although our results do not allow to formulate comprehensive structure–properties relationships, we identified several structural motifs, such as indole- or pyrrole-substituted distyryl dyes, as well as simple mono-stryryl dyes such as DASPMI [2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide] or its 4-isomer, as optimal fluorescent light-up probes characterized by high fluorimetric response (I/I₀ of up to 550-fold), excellent selectivity with respect to double-stranded DNA or single-stranded RNA controls, high quantum yield in the presence of G4 analytes (up to 0.32), large Stokes shift (up to 150 nm) and, in certain cases, structural selectivity with respect to one or another G4 folding topology. These dyes can be considered as promising G4-responsive sensors for in vitro or imaging applications. As a possible application, we implemented a simple two-dye fluorimetric assay allowing rapid topological classification of G4-DNA structures.

Interplay of Three G-Quadruplex Units in the KIT Promoter

The proto-oncogene KIT encodes for a tyrosine kinase receptor, which is a clinically validated target for treating gastrointestinal stromal tumors. The KIT promoter contains a G-rich domain within a relatively long sequence potentially able to form three adjacent G-quadruplex (G4) units, namely, K2, SP, and K1. These G4 domains have been studied mainly as single quadruplex units derived from short truncated sequences and are currently considered promising targets for anticancer drugs, alternatively to the encoded protein. Nevertheless, the information reported so far does not contemplate the interplay between those neighboring G4s in the context of the whole promoter, possibly thwarting drug-discovery efforts. Here we report the structural and functional study of the KIT promoter core sequence, in both single- and double-stranded forms, which includes all three predicted G4 units. By preventing the formation of alternatively one or two G4 units and by combining biophysical techniques and biological assays, we show for the first time that these quadruplexes cannot be analyzed independently, but they are correlated to each other. Our data suggest that, while K2 and K1 G-rich sequences retain the ability to fold into parallel G4 motifs within a long sequence, the SP G-rich domain contributes to G4 structure only together with K2. Remarkably, we have found that, in the context of a dynamic equilibrium between the three G4 units, the G4 formed by K1 has the most significant influence on the structure stability and on the biological role of the whole promoter.

Thioflavin T Monitoring of Guanine Quadruplex Formation in the rs689-Dependent INS Intron 1

The human proinsulin gene (INS) contains a thymine-to-adenine variant (rs689) located in the 3′ splice site (3′ ss) recognition motif of the first intron. The adenine at rs689 is strongly associated with type 1 diabetes. By weakening the polypyrimidine tract, the adenine allele reduces the efficiency of intron 1 splicing, which can be ameliorated by antisense oligonucleotides blocking a splicing silencer located upstream of the 3′ ss. The silencer is surrounded by guanine-rich tracts that may form guanine quadruplexes (G4s) and modulate the accessibility of the silencer. Here, we employed thioflavin T (ThT) to monitor G4 formation in synthetic DNAs and RNAs derived from INS intron 1. We show that the antisense target is surrounded by ThT-positive segments in each direction, with oligoribonucleotides exhibiting consistently higher fluorescence than their DNA counterparts. The signal was reduced for ThT-positive oligonucleotides that were extended into the silencer, indicating that flanking G4s have a potential to mask target accessibility. Real-time monitoring of ThT fluorescence during INS transcription in vitro revealed a negative correlation with ex vivo splicing activities of corresponding INS constructs. Together, these results provide a better characterization of antisense targets in INS primary transcripts for restorative strategies designed to improve the INS splicing defect associated with type 1 diabetes.

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.

A rapid fluorometric method for determination of aflatoxin B1 in plant-derived food by using a thioflavin T-based aptasensor

A fluorometric aptamer-based method is described for the determination of aflatoxin B₁ (AFB₁). The fluorescent dye thioflavin T (ThT) forms a complex with the aptamer against AFB₁ (aptamer/ThT), and the fluorescence of the complex is strongly enhanced. On addition of AFB₁, it will bind to the aptamer and release ThT. The fluorescence of free ThT is much weaker. The fluorescence of the system, best measured at excitation/emission wavelengths of 440/487 nm, drops gradually in the AFB₁ concentration range from 0.2 to 200 ng·mL^-1, exhibiting good linearity. The detection limits are 0.2 ng·mL^-1 in buffer solution, and 1 ng·mL^-1 when applied to plant-derived food. The recovery of AFB₁ from spiked foodstuff ranges from 74.7% to 121%. The assay can be performed within 20 min. Graphical abstract Schematic presentation of label-free thioflavin T (ThT)-based fluorescence aptasensor using aflatoxin B₁(AFB₁) aptamer/ThT G-quadruplex complex and the AFB1 aptamer/AFB1 complex fluorescent signal system for the rapid and sensitive detection of AFB1 in soy sauce, spirits, rice, corn and peanuts.

Thioflavin T specifically brightening “Guanine Island” in duplex-DNA: a novel fluorescent probe for single-nucleotide mutation

Here, we found that Thioflavin T (ThT) could specifically bind with a G-GGG unit (named as “Guanine Island”) in double stranded DNA (ds-DNA).

A simple, label-free, electrochemical DNA parity generator/checker for error detection during data transmission based on "aptamer-nanoclaw"-modulated protein steric hindrance

The first electrochemical DNA parity generator/checker system for error detection during data transmission was constructed based on “aptamer-nanoclaw”-modulated protein steric hindrance.

Versatile DNA logic devices have exhibited magical power in molecular-level computing and data processing. During any type of data transmission, the appearance of erroneous bits (which have severe impacts on normal computing) is unavoidable. Luckily, the erroneous bits can be detected via placing a parity generator (pG) at the sending module and a parity checker (pC) at the receiving module. However, all current DNA pG/pC systems use optical signals as outputs. In comparison, sensitive, facilely operated, electric-powered electrochemical outputs possess inherent advantages in terms of potential practicability and future integration with semiconductor transistors. Herein, taking an even pG/pC as a model device, we construct the first electrochemical DNA pG/pC system so far. Innovatively, a thrombin aptamer is integrated into the input-strand and it functions as a “nanoclaw” to selectively capture thrombin; the electrochemical impedance changes induced by the “nanoclaw/thrombin” complex are used as label-free outputs. Notably, this system is simple and can be operated within 2 h, which is comparable with previous fluorescent ones, but avoids the high-cost labeled-fluorophore and tedious nanoquencher. Moreover, taking non-interfering poly-T strands as additional inputs, a cascade logic circuit (OR-2 to 1 encoder) and a parity checker that could distinguish even/odd numbers from natural numbers (0 to 9) is also achieved based on the same system. This work not only opens up inspiring horizons for the design of novel electrochemical functional devices and complicated logic circuits, but also lays a solid foundation for potential logic-programmed target detection.

Fluorescence-based tools to probe G-quadruplexes in cell-free and cellular environments

Biophysical and biochemical investigations provide compelling evidence connecting the four-stranded G-quadruplex (GQ) structure with its role in regulating multiple cellular processes. Hence, modulating the function of GQs by using small molecule binders is being actively pursued as a strategy to develop new chemotherapeutic agents. However, sequence diversity and structural polymorphism of GQs have posed immense challenges in terms of understanding what conformation a G-rich sequence adopts inside the cell and how to specifically target a GQ motif amidst several other GQ-forming sequences. In this context, here we review recent developments in the applications of biophysical tools that use fluorescence readout to probe the GQ structure and recognition in cell-free and cellular environments. First, we provide a detailed discussion on the utility of covalently labeled environment-sensitive fluorescent nucleoside analogs in assessing the subtle difference in GQ structures and their ligand binding abilities. Furthermore, a detailed discussion on structure-specific antibodies and small molecule probes used to visualize and confirm the existence of DNA and RNA GQs in cells is provided. We also highlight the open challenges in the study of tetraplexes (GQ and i-motif structures) and how addressing these challenges by developing new tools and techniques will have a profound impact on tetraplex-directed therapeutic strategies.

Affinity Biosensors for Detection of Mycotoxins in Food

This chapter reviews recent achievements in methods of detection of mycotoxins in food. Special focus is on the biosensor technology that utilizes antibodies and nucleic acid aptamers as receptors. Development of biosensors is based on the immobilization of antibodies or aptamers onto various conventional supports like gold layer, but also on nanomaterials such as graphene oxide, carbon nanotubes, and quantum dots that provide an effective platform for achieving high sensitivity of detection using various physical methods, including electrochemical, mass sensitive, and optical. The biosensors developed so far demonstrate high sensitivity typically in subnanomolar limit of detection. Several biosensors have been validated in real samples. The sensitivity of biosensors is similar and, in some cases, even better than traditional analytical methods such as ELISA or chromatography. We believe that future trends will be focused on improving biosensor properties toward practical application in food industry.

Iron Redox Chemistry Promotes Antiparallel Oligomerization of α-Synuclein

Brain metal dyshomeostasis and altered structural dynamics of the presynaptic protein α-synuclein (αS) are both implicated in the pathology of Parkinson's disease (PD), yet a mechanistic understanding of disease progression in the context of αS structure and metal interactions remains elusive. In this Communication, we detail the influence of iron, a prevalent redox-active brain biometal, on the aggregation propensity and secondary structure of N-terminally acetylated αS (^NAcαS), the physiologically relevant form in humans. We demonstrate that under aerobic conditions, Fe(II) commits ^NAcαS to a PD-relevant oligomeric assembly, verified by the oligomer-selective A11 antibody, that does not have any parallel β-sheet character but contains a substantial right-twisted antiparallel β-sheet component based on CD analyses and descriptive deconvolution of the secondary structure. This ^NAcαS-Fe^II oligomer does not develop into the β-sheet fibrils that have become hallmarks of PD, even after extended incubation, as verified by TEM imaging and the fibril-specific OC antibody. Thioflavin T (ThT), a fluorescent probe for β-sheet fibril formation, also lacks coordination to this antiparallel conformer. We further show that this oligomeric state is not observed when O₂ is excluded, indicating a role for iron(II)-mediated O₂ chemistry in locking this dynamic protein into a conformation that may have physiological or pathological implications.

Thermodynamic, Anticoagulant, and Antiproliferative Properties of Thrombin Binding Aptamer Containing Novel UNA Derivative

Thrombin is a serine protease that plays a crucial role in hemostasis, fibrinolysis, cell proliferation, and migration. Thrombin binding aptamer (TBA) is able to inhibit the activity of thrombin molecule via binding to its exosite I. This 15-nt DNA oligonucleotide forms an intramolecular, antiparallel G-quadruplex structure with a chair-like conformation. In this paper, we report on our investigations on the influence of certain modified nucleotide residues on thermodynamic stability, folding topology, and biological properties of TBA variants. In particular, the effect of single incorporation of a novel 4-thiouracil derivative of unlocked nucleic acid (UNA), as well as single incorporation of 4-thiouridine and all four canonical UNAs, was evaluated. The studies presented herein have shown that 4-thiouridine in RNA and UNA series, as well as all four canonical UNAs, can efficiently modulate G-quadruplex thermodynamic and biological stability, and that the effect is strongly position dependent. Interestingly, TBA variants containing the modified nucleotide residues are characterized by unchanged folding topology. Thrombin time assay revealed that incorporation of certain UNA residues may improve G-quadruplex anticoagulant properties. Noteworthy, some TBA variants, characterized by decreased ability to inhibit thrombin activity, possess significant antiproliferative properties reducing the viability of the HeLa cell line even by 95% at 10 μM concentration.

A light-up probe targeting for Bcl-2 2345 G-quadruplex DNA with carbazole TO

As its significant role, the selective recognition of G-quadruplex with specific structures and functions is important in biological and medicinal chemistry. Carbazole derivatives have been reported as a kind of fluorescent probe with many excellent optical properties. In the present study, the fluorescence of the dye (carbazole TO) increased almost 70 fold in the presence of bcl-2 2345 G4 compared to that alone in aqueous buffer condition with almost no fluorescence and 10-30 fold than those in the presence of other DNAs. The binding study results by activity inhibition of G4/Hemin peroxidase experiment, NMR titration and molecular docking simulation showed the high affinity and selectivity to bcl-2 2345 G4 arises from its end-stacking interaction with G-quartet. It is said that a facile approach with excellent sensitive, good selectivity and quick response for bcl-2 2345 G-quadruplex was developed and may be used for antitumor recognition or antitumor agents.

Thioflavin T binds dimeric parallel-stranded GA-containing non-G-quadruplex DNAs: a general approach to lighting up double-stranded scaffolds

A molecular rotor thioflavin T (ThT) is usually used as a fluorescent ligand specific for G-quadruplexes. Here, we demonstrate that ThT can tightly bind non-G-quadruplex DNAs with several GA motifs and dimerize them in a parallel double-stranded mode, accompanied by over 100-fold enhancement in the fluorescence emission of ThT. The introduction of reverse Watson–Crick T-A base pairs into these dimeric parallel-stranded DNA systems remarkably favors the binding of ThT into the pocket between G•G and A•A base pairs, where ThT is encapsulated thereby restricting its two rotary aromatic rings in the excited state. A similar mechanism is also demonstrated in antiparallel DNA duplexes where several motifs of two consecutive G•G wobble base pairs are incorporated and serve as the active pockets for ThT binding. The insight into the interactions of ThT with non-G-quadruplex DNAs allows us to introduce a new concept for constructing DNA-based sensors and devices. As proof-of-concept experiments, we design a DNA triplex containing GA motifs in its Hoogsteen hydrogen-bonded two parallel strands as a pH-driven nanoswitch and two GA-containing parallel duplexes as novel metal sensing platforms where C–C and T–T mismatches are included. This work may find further applications in biological systems (e.g. disease gene detection) where parallel duplex or triplex stretches are involved.

Specific Recognition of G-Quadruplexes Over Duplex-DNA by a Macromolecular NIR Two-Photon Fluorescent Probe

The implication of guanine-rich DNA sequences in biologically important roles such as telomerase dysfunction and the regulation of gene expression has prompted the search for structure-specific G-quadruplex agents for targeted diagnostic and therapeutic applications. Herein, we report on a near-infrared (NIR) two-photon poly(cationic) anthracene-based macromolecule able to selectively target G-quadruplexes (G4s) over genomic double-stranded DNA. In particular, the striking changes in its linear and third-order nonlinear optical properties, combined with the emergence of a strong induced electronic circular dichroism (ECD) signal upon binding to canonical and noncanonical DNA secondary structures allowed for a highly specific detection of several different G4s. Furthermore, through a detailed computational analysis we bring compelling evidence that our probe intercalation within G4s is a thermodynamically favored event, and we fully rationalize the spectroscopic evolution resulting from this complexation event by providing a reasonable explanation regarding the origin of the peculiar ECD effect that accompanies it.

Dual functional Phi29 DNA polymerase-triggered exponential rolling circle amplification for sequence-specific detection of target DNA embedded in long-stranded genomic DNA

An exonucleolytic digestion-assisted exponential rolling circle amplification (RCA) strategy was developed for sensitive and sequence-specific detection of target DNA embedded in long-stranded genomic DNA. Herein, Phi29 DNA polymerase plays two important roles as exonuclease and polymerase. Long-stranded genomic DNAs can be broken into small DNA fragments after ultrasonication. The fragments that contain target DNA, hybridize with a linear padlock probe to trigger the formation of a circular RCA template. The tails protruding from the 3'-end of the target DNA sequences are then digested by the 3' → 5' exonuclease activity of Phi29 DNA polymerase even if they fold into a double-stranded structure. The digested DNA fragments can then initiate subsequent RCA reaction. RCA products, which are designed to fold into G-quadruplex structures, exponentially accumulate when appropriate nicking endonuclease recognition sites are introduced rationally into the RCA template. This method is demonstrated to work well for real genomic DNA detection using human pathogen Cryptococcus neoformans as a model. In addition, this work has two other important discoveries: First, the presence of a 3'-tail can protect the RCA primer from degradation by Phi29 DNA polymerase. Second, 3' → 5' exonucleolytic activity of Phi29 DNA polymerase can work for both single- and double-stranded DNA.

A facile label-free G-quadruplex based fluorescent aptasensor method for rapid detection of ATP

The present work demonstrates a simple, rapid and label-free ATP detection method using a fluorescent aptasensor that is based on G-quadruplex formation. In the absence of ATP, the Thioflavin T (ThT) dye binds to the G-rich ATP aptamer and forms an ATP aptamer/ThT G-quadruplex complex, which results in high fluorescence intensity. Upon addition of ATP, the ATP aptamer/ThT complex will be replaced by the formation of an ATP aptamer/ATP complex. During this process, separation of the ThT dye from the ATP aptamer/ThT complex decreases the fluorescence intensity of the reaction mixture dramatically. This fluorescence aptasensor is highly sensitive and rapid, with a detection limit of 18nM and a total reaction time of only 10min. Furthermore, this method is cost-effective and simple, removing the requirement for labeling the detection reagents with a fluorophore-quencher pair.

Novel fluorescent cationic benzothiazole dye that responds to G-quadruplex aptamer as a novel K+ sensor

A fluorescent cationic benzothiazole dye that selectively targets a G-quadruplex aptamer was designed and synthesized as a K⁺ sensor.

Sensitive fluorescent detection of DNA methyltransferase using nicking endonuclease-mediated multiple primers-like rolling circle amplification

Sensitive and reliable detection of DNA methyltransferase (MTase) is of great significance for both early tumor diagnosis and therapy. In this study, a simple, label-free and sensitive DNA MTase-sensing method was developed on the basis of a nicking endonuclease-mediated multiple primers-like rolling circle amplification (RCA) strategy. In this method, a dumbbell RCA template was prepared by blunt-end ligation of two molecules of hairpin DNA. In addition to the primer-binding sequence, the dumbbell template contained another three important parts: 5'-CCGG-3' sequences in double-stranded stems, nicking endonuclease recognition sites and C-rich sequences in single-stranded loops. The introduction of 5'-CCGG-3' sequences allows the dumbbell template to be destroyed by the restriction endonuclease, HpaII, but is not destroyed in the presence of the target MTase-M.SssI MTase. The introduction of nicking endonuclease recognition sites makes the M.SssI MTase-protected dumbbell template-mediated RCA proceed in a multiple primers-like exponential mode, thus providing the RCA with high amplification efficiency. The introduction of C-rich sequences may promote the folding of amplification products into a G-quadruplex structure, which is specifically recognized by the commercially available fluorescent probe thioflavin T. Improved RCA amplification efficiency and specific fluorescent recognition of RCA products provide the M.SssI MTase-sensing platform with high sensitivity. When a dumbbell template containing four nicking endonuclease sites is used, highly specific M.SssI MTase activity detection can be achieved in the range of 0.008-50U/mL with a detection limit as low as 0.0011U/mL. Simple experimental operation and mix-and-detection fluorescent sensing mode ensures that M.SssI MTase quantitation works well in a real-time RCA mode, thus further simplifying the sensing performance and making high throughput detection possible. The proposed MTase-sensing strategy was also demonstrated to be applicable for screening and evaluating the inhibitory activity of MTase inhibitors.

Label-free monitoring of DNA polymerase activity based on a thrombin-binding aptamer G-quadruplex

We have developed a label-free assay for the detection of DNA polymerase activity based on a thrombin-binding aptamer (TBA) G-quadruplex. In the presence of DNA polymerase, the 3'-OH termini of the hairpin substrate are immediately elongated to replace the TBA, which can be recognized quickly by the ThT dye and results in an increase of fluorescence. This method is highly sensitive with a detection limit of 0.1 U/mL. It is simple and cost-effective without any requirement of labeling with a fluorophore-quencher pair. Furthermore, the proposed method can also be applied to analyze the inhibition of DNA polymerase, which clearly indicates that the proposed method can be applied for screening of potential DNA polymerase inhibitors.

Thioflavin T behaves as an efficient fluorescent ligand for label-free ATP aptasensor

Here, we for the first time demonstrated thioflavin T (ThT) as an efficient fluorescent ligand for 27-mer ATP-binding aptamer (ABA27), providing a novel signal readout mode for label-free selective ATP detection. ABA27 can promote the fluorescence emission of ThT with an unprecedentedly high efficiency, attributed to the specific structure of ABA27 rather than the G-tracts. Polyacrylamide gel electrophoresis, fluorescence spectroscopy, and fluorometric titration reveal that ThT interacts with ABA27 with a lower binding affinity (Kd ~89 μM) than ATP, which allows ATP to easily compete with ThT for the DNA binder. In the presence of ThT, adding ATP induces ABA27 to undergo a structural change, thereby not favoring the binding to ThT, verified by circular dichroism and UV-Vis absorption spectroscopy. As a result, the fluorescence intensity of ThT decreases dramatically, enabling the sensitive detection of ATP with high selectivity over other analogs. Such a sensing strategy may make ThT able to serve as a facile signal reporter for DNA nanomechanical devices fueled with ATP. Graphical Abstract The principle of the displacement of ThT by ATP.

G-quadruplex fluorescent probe-mediated real-time rolling circle amplification strategy for highly sensitive microRNA detection

Real-time PCR has revolutionized PCR from qualitative to quantitative. As an isothermal DNA amplification technique, rolling circular amplification (RCA) has been demonstrated to be a versatile tool in many fields. Development of a simple, highly sensitive, and specific strategy for real-time monitoring of RCA will increase its usefulness in many fields. The strategy reported here utilized the specific fluorescence response of thioflavin T (ThT) to G-quadruplexes formed by RCA products. Such a real-time monitoring strategy works well in both traditional RCA with linear amplification efficiency and modified RCA proceeded in an exponential manner, and can be readily performed in commercially available real-time PCR instruments, thereby achieving high-throughput detection and making the proposed technique more suitable for biosensing applications. As examples, real-time RCA-based sensing platforms were designed and successfully used for quantitation of microRNA over broad linear ranges (8 orders of magnitude) with a detection limit of 4 aM (or 0.12 zmol). The feasibility of microRNA analysis in human lung cancer cells was also demonstrated. This work provides a new method for real-time monitoring of RCA by using unique nucleic acid secondary structures and their specific fluorescent probes. It has the potential to be extended to other isothermal single-stranded DNA amplification techniques.

Thioflavin T as a fluorescence light-up probe for both parallel and antiparallel G-quadruplexes of 29-mer thrombin binding aptamer

A wide range of pathologies have been targeted with bimodular aptamers that contain both G-quadruplex (G4) and duplex motifs, while the structures and functions are poorly understood. G4-selective fluorescent dyes have served as facile tools to probe G4s, but not for bimodular aptamers, yet. Here, taking the 29-mer thrombin binding aptamer (TBA29) as an example, we demonstrated that 3,6-dimethyl-2-(4-dimethylaminophenyl)-benzothiazolium (ThT) was the most effective dye compared to NMM and PPIX in recognizing TBA29. Binding studies indicate that ThT recognized TBA29 via distinct buffer-dependent mechanisms. Specifically, ThT induced the formation of a bimolecular parallel G4 in cation-deficient buffer, showing 341-fold fluorescent enhancement. The competitive binding of thrombin disrupted the complex, leading to the monotonic fluorescence decrease. A similar mechanism was previously reported for the interaction between ThT and the 15-mer thrombin binding aptamer (TBA15). However, TBA29 bound with ThT in a more favorable state than TBA15, showing hyperchromic effects and two times stronger fluorescence enhancement. Differently, ThT bound with antiparallel TBA29/TBA15 in an intercalating/groove binding mode in 100 mM KCl, generating 181/28-fold fluorescence enhancement, respectively. These results revealed that ThT recognized both parallel and antiparallel G4s of TBA29 more efficiently than it recognized TBA15. The duplex structure of TBA29 may play an important role in its interaction with ThT. Our study broadens the application of ThT in screening G4 to bimodular aptamers and provides some insights into the structures of TBA29, along with the interaction between ThT and TBA29. Our study also is useful for the development of structure-switching-based biosensors using bimodular aptamers. Graphical abstract The buffer-dependent binding mechanisms of ThT with TBA29, and the competitive (top)/noncompetitive (bottom) binding of thrombin with TBA29-ThT complex.

Three label-free thrombin aptasensors based on aptamers and [Ru(bpy)2(o-mopip)]2+

The label free aptasensor based on an aptamer pair and [Ru(bpy)₂(o-mopip)]²⁺–GO has been successfully applied to sandwich assays for thrombin detection.

A Universal Base in a Specific Role: Tuning up a Thrombin Aptamer with 5-Nitroindole

In this study we describe new modified analogs of the thrombin binding aptamer (TBA) containing 5-nitroindole residues. It has been shown that all modified TBAs form an anti-parallel G-quadruplex structure and retain the ability to inhibit thrombin. The most advanced TBA variant (TBA-N8) has a substantially increased clotting time and two-fold lower IC₅₀ value compared to the unmodified prototype. Molecular modelling studies suggest that the improved anticoagulant properties of TBA-N8 result from changes in the binding mode of the analog. A modified central loop in TBA-N8 is presumed to participate in the binding of the target protein. Studies of FAM labelled TBA and TBA-N8 showed an improved binding affinity of the modified aptamer and provided evidence of a direct interaction between the modified central loop and thrombin. Our findings have implications for the design of new aptamers with improved binding affinities.

Conformation-sensitive nucleoside analogues as topology-specific fluorescence turn-on probes for DNA and RNA G-quadruplexes

Development of probes that can discriminate G-quadruplex (GQ) structures and indentify efficient GQ binders on the basis of topology and nucleic acid type is highly desired to advance GQ-directed therapeutic strategies. In this context, we describe the development of minimally perturbing and environment-sensitive pyrimidine nucleoside analogues, based on a 5-(benzofuran-2-yl)uracil core, as topology-specific fluorescence turn-on probes for human telomeric DNA and RNA GQs. The pyrimidine residues of one of the loop regions (TTA) of telomeric DNA and RNA GQ oligonucleotide (ON) sequences were replaced with 5-benzofuran-modified 2′-deoxyuridine and uridine analogues. Depending on the position of modification the fluorescent nucleoside analogues distinguish antiparallel, mixed parallel-antiparallel and parallel stranded DNA and RNA GQ topologies from corresponding duplexes with significant enhancement in fluorescence intensity and quantum yield. Further, these GQ sensors enabled the development of a simple fluorescence binding assay to quantify topology- and nucleic acid-specific binding of small molecule ligands to GQ structures. Together, our results demonstrate that these nucleoside analogues are useful GQ probes, which are anticipated to provide new opportunities to study and discover efficient G-quadruplex binders of therapeutic potential.