Noncompetitive Determination of Small Analytes by Sandwich-Type Lateral Flow Assay Based on an Aptamer Kissing Complex.

Here, we present the proof-of-concept of a lateral flow assay (LFA) that is capable of detecting small-molecule targets in a noncompetitive manner by deploying a sandwich-type format based on the aptamer kissing complex (AKC) strategy. A fluorescently labeled hairpin aptamer served as the signaling agent, while a specific RNA hairpin grafted onto the strip served as the capture element. The hairpin aptamer switched from an unfolded to a folded form in the presence of the target, resulting in kissing interactions between the loops of the reporter and the capture agents. This design triggered a target-dependent fluorescent signal at the test line. The AKC-based LFA was developed for the detection of adenosine, achieving a detection limit in the micromolar range. The assay revealed the presence of the same analyte in urine. The method also proved effective with another small molecule (theophylline). We believe that the AKC-based LFA approach could overcome many of the shortcomings associated with conventional signal-off methods and competitive processes.

Nile blue as reporter dye in salt aggregation based-colorimetric aptasensors for peptide, small molecule and metal ion detection.

Herein, we report a novel approach for the design of a colorimetric aptasensor, relying on a Dye Salt Aggregation-based Colorimetric Oligonucleotide assay (DYSACO assay). This method is based on the use of an intercalating agent, Nile Blue (NB), whose aggregation capacities (and thus modification of its absorption spectrum) are drastically amplified by adding salts to the working solution. The presence of an aptamer could protect NB from such aggregation process due to its intercalation into double-stranded DNA and/or interaction with nucleobases. In response to the addition of the specific ligand, the competition between NB and the target for binding to the aptamer occurs, resulting in an increase in the dye salt aggregation and then in the blue-to-blank color change of the solution. The proof-of-principle was demonstrated by employing the anti-l-tyrosinamide aptamer and the assay was successfully applied to the trace enantiomer detection, allowing the detection of an enantiomeric impurity down to approximately 2% in a non-racemic sample. Through a reversed mechanism based on the increased capture of NB by DNA upon analyte binding, the sensing platform was further demonstrated for the Hg(II) detection. Water samples of different origin were spiked with Hg(II) analyte at final range concentrations comprised between (0.5-15 µM). An excellent overall recovery of 122 ± 14%; 105 ± 14%; 99 ± 9%; was respectively obtained from river, tap and mineral water, suggesting that the sensor can be used under real sample conditions. The assay was also shown to work for sensing the ochratoxin A and d-arginine vasopressin compounds, revealing its simplicity and generalizability potentialities.

Detection of small molecules by fluorescence intensity using single dye labeled aptamers and quencher transition metal ions.

We describe herein an aptamer-based sensing approach that signal the presence of small-molecule targets when fluorescent DNA probes are challenged with the Ni2+ or Co2+ quencher metal ions. Functional oligonucleotides targeting L-tyrosinamide (L-Tym), adenosine (Ade) or cocaine (Coc) were end-labeled by the Texas-Red fluorophore. A fluorescence quenching occurred upon association of these transition metal ions with the free conjugates. The formation of the target-probe complex, by the way of variations in the overall binding of quencher metal ions along the DNA strands, led to a partial restoration (for the Ade and Coc systems) or a further attenuation (for the L-Tym system) of the fluorescence intensity. The absolute signal gain varied from 40 to 180% depending on the target-probe pair investigated. The approach was also used to detect the compound Ade in a spiked biological matrix in 1 min or less. The transition metal ion-based quenching strategy is characterized by its very simple implementation, low cost, and rapid signaling.

Anti-pesticide DNA aptamers fail to recognize their targets with asserted micromolar dissociation constants.

Herein, we originally aimed at developing fluorescence anisotropy biosensor platforms devoted to the homogeneous-phase detection of isocarbophos and phorate pesticides by using previously isolated DNA aptamers. To achieve this, two reporting approaches displaying very high generalizability features were implemented, based on either the complementary strand or the SYBR green intercalator displacement strategies. Unfortunately, none of the transduction methods led to phorate-dependent signals. Only the SYBR green displacement method provided a small output in the presence of isocarbophos, but at an analyte concentration greater than 100 µM. In order to identify the origin of such data, isothermal titration calorimetry (ITC) experiments were subsequently performed. It was shown that aptamers bind neither isocarbophos nor phorate in free solution with the claimed micromolar dissociation constants. This work puts forward some doubts about the previously described aptasensors that rely on the use of these functional DNA molecules. It also highlights the need to carefully investigate the binding capabilities of aptamers after their isolation and to include appropriate control experiments with scrambled or mutated oligonucleotides.

Aptamer Switches Regulated by Post-Transition/Transition Metal Ions.

We introduced an aptamer switch design that relies on the ability of post-transition/transition metal ions to trigger, through their coordination to nucleobases, substantial DNA destabilization. In the absence of molecular target, the addition of one such metal ion to usual aptamer working solutions promotes the formation of an alternative, inert DNA state. Upon exposure to the cognate compound, the equilibrium is shifted towards the competent DNA form. The switching process was preferentially activated by metal ions of intermediate base over phosphate complexation preference (i.e. Pb2+ , Cd2+ ) and operated with diversely structured DNA molecules. This very simple aptamer switch scheme was applied to the detection of small organics using the fluorescence anisotropy readout mode. We envision that the approach could be adapted to a variety of signalling methods that report on changes in the surface charge density of DNA receptors.

Kissing interactions for the design of a multicolour fluorescence anisotropy chiral aptasensor.

The development of enantioselective assays and sensors has received much attention for the determination of enantiomeric impurities. Herein, we demonstrated that the previously reported aptamer kissing complex (AKC) assay strategy can be implemented for designing a chiral tool that allows both the simultaneous enantiomer quantification and the enantiopurity analysis. D- and L-arginine vasopressin (AVP) were employed as model enantiomeric targets. The D- and L-AVP engineered aptamers (aptaswitch) were used as recognition units whereas the Fluorescein or Texas Red labelled D- and L-hairpin probes (aptakiss) served as probes of the enantiomer-dependent AKC formation. The orthogonal fluorescence anisotropy signaling scheme at two different emission wavelengths permitted the concomitant sensing of the AVP enantiomers in a single sample, under a high-throughput microplate format. It was also shown that the AKC-based enantioselective sensor allowed the enantiomeric impurity detection at a level as low as 0.01%.

Photochemistry on the Space Station-Aptamer Resistance to Space Conditions: Particles Exposure from Irradiation Facilities and Real Exposure Outside the International Space Station.

Some microarray-based instruments that use bioaffinity receptors such as antibodies or aptamers are under development to detect signatures of past or present life on planetary bodies. Studying the resistance of such instruments against space constraints and cosmic rays in particular is a prerequisite. We used several ground-based facilities to study the resistance of aptamers to various types of particles (protons, electrons, neutrons, and carbon ions) at different energies and fluences. We also tested the resistance of aptamers during the EXPOSE-R2 mission outside the International Space Station (ISS). The accumulated dose measured after the 588 days of this mission (220 mGy) corresponds to the accumulated dose that can be expected during a mission to Mars. We found that the recognition ability of fluorescently labeled aptamers was not significantly affected during short-term exposure experiments taking into account only one type of radiation at a time. However, we demonstrated that the same fluorescent dye was significantly affected by temperature variations (-21°C to +58°C) and storage throughout the entirety of the ISS experiment (60% of signal loss). This induced a large variability of aptamer signal in our analysis. However, we found that >50% of aptamers were still functional after the whole EXPOSE-R2 mission. We conclude that aptamer-based instruments are well suited for in situ analysis on planetary bodies, but the detection step requires additional investigations.

Aptamer Efficacies for In Vitro and In Vivo Modulation of αC-Conotoxin PrXA Pharmacology.

The medical staff is often powerless to treat patients affected by drug abuse or misuse and poisoning. In the case of envenomation, the treatment of choice remains horse sera administration that poses a wealth of other medical conditions and threats. Previously, we have demonstrated that DNA-based aptamers represent powerful neutralizing tools for lethal animal toxins of venomous origin. Herein, we further pursued our investigations in order to understand whether all toxin-interacting aptamers possessed equivalent potencies to neutralize αC-conotoxin PrXA in vitro and in vivo. We confirmed the high lethality in mice produced by αC-conotoxin PrXA regardless of the mode of injection and further characterized myoclonus produced by the toxin. We used high-throughput patch-clamp technology to assess the effect of αC-conotoxin PrXA on ACh-mediated responses in TE671 cells, responses that are carried by muscle-type nicotinic receptors. We show that 2 out of 4 aptamers reduce the affinity of the toxin for its receptor, most likely by interfering with the pharmacophore. In vivo, more complex responses on myoclonus and mice lethality are observed depending on the type of aptamer and mode of administration (concomitant or differed). Concomitant administration always works better than differed administration indicating the stability of the complex in vivo. The most remarkable conclusion is that an aptamer that has no or a limited efficacy in vitro may nevertheless be functional in vivo probably owing to an impact on the biodistribution or pharmacokinetics of the toxin in vivo. Overall, the results highlight that a blind selection of aptamers against toxins leads to efficient neutralizing compounds in vivo regardless of the mode of action. This opens the door to the use of aptamer mixtures as substitutes to horse sera for the neutralization of life-threatening animal venoms, an important WHO concern in tropical areas.

Non-SELEX isolation of DNA aptamers for the homogeneous-phase fluorescence anisotropy sensing of tau Proteins.

Herein, we report for the first time the isolation of DNA aptamers directed against the whole tau protein, an important Alzheimer's disease (AD) biomarker. Non-SELEX approach based on the capillary electrophoresis partitioning technique was employed to isolate a high-affinity DNA sequence pool towards the target in only three rounds and one working day. High-throughput sequencing was next performed and the recognition ability of five selected aptamers was preliminary evaluated by surface plasmon resonance using the protein target immobilized on the chip. Finally, the analytical potential of the most affine aptamer was demonstrated through the design of a homogeneous-phase fluorescence anisotropy assay. This DNA aptamer was found to be able to recognize not only the whole τ-441 but also the τ-381, τ-352, τ-383 isoforms. The sensing platform allowed the determination of these four targets with a detection limit of 28 nM, 3.2 nM, 6.3 nM and 22 nM, respectively.

Mirror-image aptamer kissing complex for arginine-vasopressin sensing.

The recently reported aptamer kissing complex (AKC) strategy has allowed for the development of a new kind of sandwich-like sensing tools. Currently AKC assays have been only applied to low molecular weight molecules and their functionality in complex matrices remains challenging. The objective of the present study broken down into two sub-aims; exploring the propensity to broaden the scope of detectable analytes and designing a more robust system for potential applications to realistic samples. An all L-configuration aptaswitch module derived from a hairpin spiegelmer specific to a larger target, i.e. the arginine-vasopressin (AVP) hormone, was elaborated. The target-induced AKC formation in presence of a specific mirror-image RNA hairpin (L-aptakiss) probe were analyzed by using fluorescence anisotropy. The mirror-image kissing complex was successfully formed when the L-AVP target bound to the engineered L-aptaswitch element. It was also established that the use of methanol as cosolvent significantly improved the assay sensitivity through the stabilization of the ternary complex. Finally, the capability of the mirror-image method to operate in 10-fold diluted, untreated human serum was illustrated. The current work revealed that the AKC concept can be expanded to a wider range of targets and converted to a L-configuration sensing platform especially suitable for bioanalysis purposes.

Efficient functional neutralization of lethal peptide toxins in vivo by oligonucleotides.

Medical means to save the life of human patients affected by drug abuse, envenomation or critical poisoning are currently limited. While the compounds at risks are most often well identified, particularly for bioterrorism, chemical intervention to counteract the toxic effects of the ingested/injected compound(s) is restricted to the use of antibodies. Herein, we illustrate that DNA aptamers, targeted to block the pharmacophore of a poisonous compound, represent a fast-acting and reliable method of neutralization in vivo that possesses efficient and long-lasting life-saving properties. For this proof of concept, we used one putative bioweapon, αC-conotoxin PrXA, a marine snail ultrafast-killing paralytic toxin, to identify peptide-binding DNA aptamers. We illustrate that they can efficiently neutralize the toxin-induced (i) displacement of [125I]-α-bungarotoxin binding onto nicotinic receptors, (ii) inhibition of diaphragm muscle contraction, and (iii) lethality in mice. Our results demonstrate the preclinical value of DNA aptamers as fast-acting, safe and cheap antidotes to lethal toxins at risk of misuse in bioterrorism and offer hope for an alternative method than donor sera to treat cases of envenomation.

A lifetime-sensitive fluorescence anisotropy probe for DNA-based bioassays: The case of SYBR Green.

In standard steady-state fluorescence anisotropy (FA) DNA-based assays, the ligand binding to a given receptor is typically signalled by the rotational correlation time changes of the tracer. Herein, we report a radically different strategy that relies on the peculiar excited state lifetime features of the SYBR Green (SG) dye. This DNA-binding probe exhibits a drastically short lifetime in solution, leading to a high FA signal. Its complexation to oligonucleotides determines a singular and very large depolarization depending on the concerted effects of extreme lifetime enhancement and resonance energy homotransfer. On the basis of ligand-induced changes in the molar fractions of bound and free forms of SG, the approach provides an unprecedented means for the FA monitoring of the ligand binding to short DNA molecules, allowing the elaboration of a variety of intercalator displacement assays and label-free biosensors that involve diverse DNA structures (duplex, hairpin, G-quadruplex and single-stranded), ligand types (ion, small organic molecule and protein) and binding modes (intercalation, minor groove, allosteric switch). These findings open up promising avenues in the design of a new generation of FA assays.

Multianalytical Study of the Binding between a Small Chiral Molecule and a DNA Aptamer: Evidence for Asymmetric Steric Effect upon 3'- versus 5'-End Sequence Modification.

Nucleic acid aptamers are involved in a broad field of applications ranging from therapeutics to analytics. Deciphering the binding mechanisms between aptamers and small ligands is therefore crucial to improve and optimize existing applications and to develop new ones. Particularly interesting is the enantiospecific binding mechanism involving small molecules with nonprestructured aptamers. One archetypal example is the chiral binding between l-tyrosinamide and its 49-mer aptamer for which neither structural nor mechanistic information is available. In the present work, we have taken advantage of a multiple analytical characterization strategy (i.e., using electroanalytical techniques such as kinetic rotating droplet electrochemistry, fluorescence polarization, isothermal titration calorimetry, and quartz crystal microbalance) for interpreting the nature of binding process. Screening of the binding thermodynamics and kinetics with a wide range of aptamer sequences revealed the lack of symmetry between the two ends of the 23-mer minimal binding sequence, showing an unprecedented influence of the 5' aptamer modification on the bimolecular binding rate constant kon and no significant effect on the dissociation rate constant koff. The results we have obtained lead us to conclude that the enantiospecific binding reaction occurs through an induced-fit mechanism, wherein the ligand promotes a primary nucleation binding step near the 5'-end of the aptamer followed by a directional folding of the aptamer around its target from 5'-end to 3'-end. Functionalization of the 5'-end position by a chemical label, a polydA tail, a protein, or a surface influences the kinetic/thermodynamic constants up to 2 orders of magnitude in the extreme case of a surface immobilized aptamer, while significantly weaker effect is observed for a 3'-end modification. The reason is that steric hindrance must be overcome to nucleate the binding complex in the presence of a modification near the nucleation site.

A combinatorial approach to the repertoire of RNA kissing motifs; towards multiplex detection by switching hairpin aptamers.

Loop-loop (also known as kissing) interactions between RNA hairpins are involved in several mechanisms in both prokaryotes and eukaryotes such as the regulation of the plasmid copy number or the dimerization of retroviral genomes. The stability of kissing complexes relies on loop parameters (base composition, sequence and size) and base combination at the loop-loop helix - stem junctions. In order to identify kissing partners that could be used as regulatory elements or building blocks of RNA scaffolds, we analysed a pool of 5.2 × 10(6) RNA hairpins with randomized loops. We identified more than 50 pairs of kissing RNA hairpins. Two kissing motifs, 5'CCNY and 5'RYRY, generate highly stable complexes with KDs in the low nanomolar range. Such motifs were introduced in the apical loop of hairpin aptamers that switch between unfolded and folded state upon binding to their cognate target molecule, hence their name aptaswitch. The aptaswitch-ligand complex is specifically recognized by a second RNA hairpin named aptakiss through loop-loop interaction. Taking advantage of our kissing motif repertoire we engineered aptaswitch-aptakiss modules for purine derivatives, namely adenosine, GTP and theophylline and demonstrated that these molecules can be specifically and simultaneously detected by surface plasmon resonance or by fluorescence anisotropy.

Fluorescence anisotropy-based structure-switching aptamer assay using a peptide nucleic acid (PNA) probe.

This study describes for the first time the feasibility of using peptide nucleic acids (PNAs) as an alternative to the DNA probes in structure-switching aptamer fluorescence polarisation assays. The effects of experimental parameters such as the length of the PNA strand, the nature of dye and the buffer conditions on the assay performances are first explored using two different methodologies based on the competition between the PNA/aptamer hydribridisation and the target/aptamer complexation. D-ATP can be detected from 1 to 25 µM in a linear range and a detection limit (LOD) of 3 µM can be reached. For this target, this lowers by a factor >5 the LOD reported with conventional DNA-based fluorescent structure switching aptamer-based assays and by a factor 3 the LOD observed with non-competitive fluorescent sensing platform indicating the usefulness of the PNA-based approach.

Ultrafast capillary electrophoresis isolation of DNA aptamer for the PCR amplification-based small analyte sensing.

Here, we report a new homogeneous DNA amplification-based aptamer assay for small analyte sensing. The aptamer of adenosine chosen as the model analyte was split into two fragments able to assemble in the presence of target. Primers were introduced at extremities of one fragment in order to generate the amplifiable DNA component. The amount of amplifiable fragment was quantifiable by Real-Time Polymerase Chain Reaction (RT-PCR) amplification and directly reliable on adenosine concentration. This approach combines the very high separation efficiency and the homogeneous format (without immobilization) of capillary electrophoresis (CE) and the sensitivity of real time PCR amplification. An ultrafast isolation of target-bound split aptamer (60 s) was developed by designing a CE input/ouput scheme. Such method was successfully applied to the determination of adenosine with a LOD of 1 µM.

Optimization of Experimental Parameters to Explore Small-Ligand/Aptamer Interactions through Use of (1) H†NMR Spectroscopy and Molecular Modeling.

Aptamers constitute an emerging class of molecules designed and selected to recognize any given target that ranges from small compounds to large biomolecules, and even cells. However, the underlying physicochemical principles that govern the ligand-binding process still have to be clarified. A major issue when dealing with short oligonucleotides is their intrinsic flexibility that renders their active conformation highly sensitive to experimental conditions. To overcome this problem and determine the best experimental parameters, an approach based on the design-of-experiments methodology has been developed. Here, the focus is on DNA aptamers that possess high specificity and affinity for small molecules, L-tyrosinamide, and adenosine monophosphate. Factors such as buffer, pH value, ionic strength, Mg(2+) -ion concentration, and ligand/aptamer ratio have been considered to find the optimal experimental conditions. It was then possible to gain new insight into the conformational features of the two ligands by using ligand-observed NMR spectroscopic techniques and molecular mechanics.

An improved design of the kissing complex-based aptasensor for the detection of adenosine.

We very recently reported a novel aptamer biosensing concept based on a dual recognition mechanism originating from the small target-induced formation of a functional nucleic acid assembly. This assembly is constituted of a hairpin aptamer (named aptaswitch) for which the apical loop of the parent aptamer is substituted by a short RNA sequence prone to loop-loop interactions. It can switch between folded and unfolded states in the presence and in the absence of targets, respectively. The apical loop of the folded aptaswitch is then recognized by a second hairpin (called aptakiss), forming a kissing complex that signals the presence of the target. In the present work, we focus on the design improvement of this biosensing platform by using a previously described adenosine-adenoswitch couple as a model system and a fluorophore-labeled aptakiss as a reporting probe for fluorescence anisotropy (FA) detection. In the first step, the initially described adenoswitch was re-engineered to optimally convert the unfolded structure into the active stem-loop form upon adenosine binding. To further improve the assay performance, a blocking DNA oligonucleotide of the adenoswitch sequence was subsequently introduced into the assay scheme. This blocking strategy led to a significant increase in the FA response by reducing the background signal generated by the undesired binding of the free adenoswitch to the aptakiss probe. We obtained a detection limit which is fivefold lower than that observed with the previously reported kissing complex-based sensor. Finally, the optimized biosensing platform was successfully applied under biologically relevant conditions, i.e., diluted human serum, suggesting the potential practical applicability of the kissing sensing approach.

Capillary gel electrophoresis-coupled aptamer enzymatic cleavage protection strategy for the simultaneous detection of multiple small analytes.

This novel, multi small-analyte sensing strategy is the result of combining the target-induced aptamer enzymatic protection approach with the CGE-LIF (capillary gel electrophoresis with laser-induced fluorescence) technique. The implemented assay principle is based on an analysis of the phosphodiesterase I (PDE I)-mediated size variation of a fluorescein-labeled aptamer (FApt), the enzyme catalyzing the removal of nucleotides from DNA in the 3' to 5' direction. In the absence of the target, the unfolded aptamer was enzymatically cleaved into short DNA fragments. Upon target binding, the DNA substrate was partially protected against enzymatic hydrolysis. The amount of bound aptamer remaining after the exonuclease reaction was proportional to the concentration of the target. The CGE technique, which was used to determine the separation of FApt species from DNA digested products, permitted the quantification of adenosine (A), ochratoxin A (O), and tyrosinamide (T) under the same optimized enzymatic conditions. This assay strategy was subsequently applied to the simultaneous detection of A, O, and T in a single capillary under buffered conditions using corresponding FApt probes of different lengths (23, 36, and 49 nucleotides, respectively). Additionally, the detection of these three small molecules was successfully achieved in a complex medium (diluted, heat-treated human serum) showing a good recovery. It is worth noting that the multiplexed analysis was accomplished for targets with different charge states by using aptamers possessing various structural features. This sensing platform constitutes a rationalized and reliable approach with an expanded potential for a high-throughput determination of small analytes in a single capillary.

Single-stranded DNA binding protein-assisted fluorescence polarization aptamer assay for detection of small molecules.

Here, we describe a new fluorescence polarization aptamer assay (FPAA) strategy which is based on the use of the single-stranded DNA binding (SSB) protein from Escherichia coli as a strong FP signal enhancer tool. This approach relied on the unique ability of the SSB protein to bind the nucleic acid aptamer in its free state but not in its target-bound folded one. Such a feature was exploited by using the antiadenosine (Ade)-DNA aptamer (Apt-A) as a model functional nucleic acid. Two fluorophores (fluorescein and Texas Red) were introduced into different sites of Apt-A to design a dozen fluorescent tracers. In the absence of the Ade target, the binding of the labeled aptamers to SSB governed a very high fluorescence anisotropy increase (in the 0.130-0.200 range) as the consequence of (i) the large global diffusion difference between the free and SSB-bound tracers and (ii) the restricted movement of the dye in the SSB-bound state. When the analyte was introduced into the reaction system, the formation of the folded tertiary structure of the Ade-Apt-A complex triggered the release of the labeled nucleic acids from the protein, leading to a strong decrease in the fluorescence anisotropy. The key factors involved in the fluorescence anisotropy change were considered through the development of a competitive displacement model, and the optimal tracer candidate was selected for the Ade assay under buffer and realistic (diluted human serum) conditions. The SSB-assisted principle was found to operate also with another aptamer system, i.e., the antiargininamide DNA aptamer, and a different biosensing configuration, i.e., the sandwich-like design, suggesting the broad usefulness of the present approach. This sensing platform allowed generation of a fluorescence anisotropy signal for aptamer probes which did not operate under the direct format and greatly improved the assay response relative to that of the most previously reported small target FPAA.