DNA aptamer-based bioanalysis of IgE by fluorescence anisotropy

Exploring modified chitosan-based gene delivery technologies for therapeutic advancements

One of the critical steps in gene therapy is the successful delivery of the genes. Immunogenicity and toxicity are major issues for viral gene delivery systems. Thus, non-viral vectors are explored. A cationic polysaccharide like chitosan could be used as a nonviral gene delivery vector owing to its significant interaction with negatively charged nucleic acid and biomembrane, providing effective cellular uptake. However, the native chitosan has issues of targetability, unpacking ability, and solubility along with poor buffer capability, hence requiring modifications for effective use in gene delivery. Modified chitosan has shown that the "proton sponge effect" involved in buffering the endosomal pH results in osmotic swelling owing to the accumulation of a greater amount of proton and chloride along with water. The major challenges include limited exploration of chitosan as a gene carrier, the availability of high-purity chitosan for toxicity reduction, and its immunogenicity. The genetic drugs are in their infancy phase and require further exploration for effective delivery of nucleic acid molecules as FDA-approved marketed formulations soon.

Assessing Aptamer-Analyte Binding Kinetics by Microfluidic Fluorescence Microscopy

Kinetic measurement plays a crucial role in understanding aptamer binding mechanisms and identifying appropriate aptamers for clinical and research applications. Current techniques, while well established, generally require large sample volumes, bulky and expensive instruments operated by trained personnel, and are hence not readily accessible to resource-limited research laboratories. This paper presents a fluorescence microscopy-based microfluidic assay for measuring aptamer-analyte binding kinetics in a simple and cost-effective manner. Kinetic measurements are achieved by monitoring time-course fluorescence of fluorescently labeled aptamers as they bind to the targets trapped in a microfluidic chip. Fluorescence measurements are performed on a standard fluorescence microscope and are accessible to laboratories with only modest resources. Moreover, microfluidic technology allows efficient and cost-effective immobilization of small amounts of target molecules or live cells as well as flow-based manipulation of aptamers for the measurements. Kinetic measurements of aptamer binding to immunoglobulin E protein and CCRF-CEM cells have yielded results consistent with those obtained from established methods, demonstrating the potential utility of our method for exploring aptamer-target interactions and identifying aptamers that best suit specific given biomedical applications.

Characterization and Identification of Aptamers against CD49c for the Detection, Capture, and Release of Cancer Cells

As a kind of recognition molecule, aptamers can be inserted into some regulatory sequences for the smart response of their targets. However, the molecular engineering might lead to the change of the binding affinity. Here, we present a stable aptamer ZAJ-2c and an environmentally sensitive aptamer ZAJ-2d optimized from an original cell-binding aptamer ZAJ-2, and the molecular target was further identified as CD49c on the cell membrane. ZAJ-2c was characterized with high binding ability independent of the presence of divalent cations at a temperature range from 4 to 37 °C, showing promise for measuring the expression of CD49c on cancer cells. Moreover, ZAJ-2d had a nanomolar binding affinity in the binding buffer at 4 °C, the same as ZAJ-2c, but lost the binding ability in a PBS buffer supplemented with 5 mM EDTA at 37 °C. This aptamer variant proved to selectively capture and release the CD49c positive cells by simply adjusting the temperatures and divalent cations. This set of aptamers might provide a toolbox for monitoring and operating of a wide range of cancer cells with CD49c expression on the surface, which will be helpful for the studying the heterogeneity of rare cells.

Recent advances in fluorescence anisotropy/polarization signal amplification

Fluorescence anisotropy/polarization is an attractive and versatile technique based on molecular rotation in biochemical/biophysical systems. Traditional fluorescence anisotropy/polarization assays showed relatively low sensitivity for molecule detection, because widespread molecular masses are too small to produce detectable changes in fluorescence anisotropy/polarization value. In this review, we discuss in detail how the potential of fluorescence anisotropy/polarization signal approach considerably expanded through the implementation of mass amplification, recycle the target amplification, fluorescence probes structure-switching amplification, resonance energy transfer amplification, and provide perspectives at future directions and applications.

Assays to Estimate the Binding Affinity of Aptamers

Aptamers have become coming-of-age molecular recognition elements in both diagnostic and therapeutic applications. Generated by SELEX, the 'quality control' of aptamers, which involves the validation of their binding affinity against their respective targets is pivotal to ascertain their potency prior to use in any downstream assays or applications. Several aptamers have been isolated thus far, however, the usage of inappropriate validation assays renders some of these aptamers dubitable in terms of their binding capabilities. Driven by this need, we provide an up-to-date critical review of the various strategies used to determine the aptamer-target binding affinity with the aim of providing researchers a better comprehension of the different analytical approaches in respect to the molecular properties of aptamers and their intended targets. The techniques reported have been classified as label-based techniques such as fluorescence intensity, fluorescence anisotropy, filter-binding assays, gel shift assays, ELISA; and label-free techniques such as UV-Vis spectroscopy, circular dichroism, isothermal titration calorimetry, native electrospray ionization-mass spectrometry, quartz crystal microbalance, surface plasmon resonance, NECEEM, backscattering interferometry, capillary electrophoresis, HPLC, and nanoparticle aggregation assays. Hybrid strategies combining the characteristics of both categories such as microscale thermophoresis have been also additionally emphasized. The fundamental principles, complexity, benefits, and challenges under each technique are elaborated in detail.

Real-Time Tunable Dynamic Range for Calibration-Free Biomolecular Measurements with a Temperature-Modulated Electrochemical Aptamer-Based Sensor in an Unprocessed Actual Sample

The sensing technologies for monitoring molecular analytes in biological fluids with high frequency and in real time could enable a broad range of applications in personalized healthcare and clinical diagnosis. However, due to the limited dynamic range (less than 81-fold), real-time analysis of biomolecular concentration varying over multiple orders of magnitude is a severe challenge faced by this class of analytical platforms. For the first time, we describe here that temperature-modulated electrochemical aptamer-based sensors with a dynamically adjustable calibration-free detection window could enable continuous, real-time, and accurate response for the several-hundredfold target concentration changes in unprocessed actual samples. Specifically, we could regulate the electrode surface temperature of sensors to obtain the corresponding dynamic range because of the temperature-dependent affinity variations. This temperature modulation method relies on an alternate hot and cold electrode reported by our group, whose surface could actively be heated and cooled without the need for altering ambient temperature, thus likewise applying for the flowing system. We then performed dual-frequency calibration-free measurements at different interface temperatures, thus achieving an extended detection window from 25 to 2500 μM for procaine in undiluted urine, 1-500 μM for adenosine triphosphate, and 5-2000 μM for adenosine in undiluted serum. The resulting sensor architecture could drastically expand the real-time response range accessible to these continuous, reagent-less biosensors.

Selection and characterization of DNA aptamers for the rat major urinary protein 13 (MUP13) as selective biorecognition elements for sensitive detection of rat pests

Among invasive mammalian predators, rats represent a major threat, endangering ecosystem functioning worldwide. After rat-control operations, detecting their continued presence or reinvasion requires more sensitive and lower cost detection technologies. Here, we develop a new sensing paradigm by using a specific rat urine biomarker (MUP13) to unambiguously signal the presence of rats. As the first step towards a new remote surveillance technology, aptamers were selected to MUP13 using the Flu-Mag SELEX method. Six aptamer candidates were initially screened by dot blot and two of them (Apt-2.5 and Apt-1.4) exhibited high affinity and specificity. Both aptamers were further characterized by bead-based assay to confirm affinity and selectivity. The lead aptamer candidates were then applied to fluorescence anisotropy (FA) and surface plasmon resonance (SPR)-based biosensor platforms, showing dissociation constants in the nanomolar range and high specificity towards their target. The SPR biosensor had limits of detection of 13.8 and 7.5 nM for Apt-2.5 and Apt-1.4, respectively, which are more than three orders of magnitude lower than the physiological concentrations found in rat urine. Selectivity of the aptamers, when comparing with other major urinary proteins, was excellent, indicating strong efficacy in specific detection of rats. In order to validate the aptamer Apt-2.5 for use with real world samples a FA-based assay was performed on a rat urine sample. The assay showed that the aptamer could detect recombinant MUP13 spiked in filtered urine and the natural MUP13 in unfiltered urine, as a first step into translation to real world application. These are the first known assays to detect and quantify a MUP biomarker of rats.

Acoustofluidic Separation of Proteins Using Aptamer-Functionalized Microparticles

We propose an acoustofluidic method for the triseparation of proteins conjugated with aptamer-coated microparticles inside a microchannel. Traveling surface acoustic waves (TSAWs) produced from a slanted-finger interdigital transducer (SFIT) are used to separate the protein-loaded microparticles of different sizes via the TSAW-driven acoustic radiation force (ARF). The acoustofluidic device consists of an SFIT deposited onto a piezoelectric lithium niobate substrate and a polydimethylsiloxane (PDMS) microfluidic channel on top of the substrate. The TSAWs propagating on the substrate penetrate into the sample fluid flow, where the human protein-conjugated microparticles are suspended, inside the PDMS microchannel. The microparticles are subjected to the TSAW-driven ARF with varying magnitude depending on their size and thus flow along different streamlines, leading to triseparation of the proteins. In this work, we used two different-sized streptavidin-functionalized polystyrene (PS) microparticles to capture two kinds of aptamers (apt15 and aptD17.4), which were labeled with a respective biotin molecule at one end. The biotin ends of the aptamers were attached to the microparticles through streptavidin-biotin linkage, whereas the free ends of the aptamers were used to capture their target proteins of thrombin (th) and immunoglobulin E (IgE). The resultant PS-apt15-th and PS-aptD17.4-IgE complexes, as well as mCardinal2, were used for experimental demonstration of acoustofluidic triseparation of the human proteins. We achieved simultaneous separation of proteins of three kinds (th, IgE, and mCardinal2) for the first time via the TSAW-driven ARF in the proposed acoustofluidic device.

Characterization of DNA aptamer-protein binding using fluorescence anisotropy assays in low-volume, high-efficiency plates

Aptamers have many useful attributes including specific binding to molecular targets. After aptamers are identified, their target binding must be characterized. Fluorescence anisotropy (FA) is one technique that can be used to characterize affinity and to optimize aptamer-target interactions. Efforts to make FA assays more efficient by reducing assay volume and time from mixing to measurement may save time and resources by minimizing consumption of costly reagents. Here, we use thrombin and two thrombin-binding aptamers as a model system to show that plate-based FA experiments can be performed in volumes as low as 2 μL per well with 20 minute incubations with minimal loss in assay precision. We demonstrate that the aptamer-thrombin interaction is best modelled with the Hill equation, indicating cooperative binding. The miniaturization of this assay has implications in drug development, as well as in the efficiency of aptamer selection workflows by allowing for higher throughput aptamer analysis.

Aptamer-Based Detection of Circulating Targets for Precision Medicine

The past decade has witnessed ongoing progress in precision medicine to improve human health. As an emerging diagnostic technique, liquid biopsy can provide real-time, comprehensive, dynamic physiological and pathological information in a noninvasive manner, opening a new window for precision medicine. Liquid biopsy depends on the sensitive and reliable detection of circulating targets (e.g., cells, extracellular vesicles, proteins, microRNAs) from body fluids, the performance of which is largely governed by recognition ligands. Aptamers are single-stranded functional oligonucleotides, capable of folding into unique tertiary structures to bind to their targets with superior specificity and affinity. Their mature evolution procedure, facile modification, and affinity regulation, as well as versatile structural design and engineering, make aptamers ideal recognition ligands for liquid biopsy. In this review, we present a broad overview of aptamer-based liquid biopsy techniques for precision medicine. We begin with recent advances in aptamer selection, followed by a summary of state-of-the-art strategies for multivalent aptamer assembly and aptamer interface modification. We will further describe aptamer-based micro-/nanoisolation platforms, aptamer-enabled release methods, and aptamer-assisted signal amplification and detection strategies. Finally, we present our perspectives regarding the opportunities and challenges of aptamer-based liquid biopsy for precision medicine.

Construction of an Enzymatically-Conjugated DNA Aptamer–Protein Hybrid Molecule for Use as a BRET-Based Biosensor

DNA-protein conjugates are useful molecules for construction of biosensors. Herein, we report the development of an enzymatically-conjugated DNA aptamer–protein hybrid molecule for use as a bioluminescence resonance energy transfer (BRET)-based biosensor. DNA aptamers were enzymatically conjugated to a fusion protein via the catalytic domain of porcine circovirus type 2 replication initiation protein (PCV2 Rep) comprising residues 14–109 (tpRep), which was truncated from the full catalytic domain of PCV2 Rep comprising residues 1–116 by removing the flexible regions at the N- and C-terminals. For development of a BRET-based biosensor, we constructed a fusion protein in which tpRep was positioned between NanoLuc luciferase and a fluorescent protein and conjugated to single-stranded DNA aptamers that specifically bind to either thrombin or lysozyme. We demonstrated that the BRET ratios depended on the concentration of the target molecules.

Aptamers against Immunoglobulins: Design, Selection and Bioanalytical Applications

Nucleic acid aptamers show clear promise as diagnostic reagents, as highly specific strands were reported against a large variety of biomarkers. They have appealing benefits in terms of reproducible generation by chemical synthesis, controlled modification with labels and functionalities providing versatile means for detection and oriented immobilization, as along with high biochemical and temperature resistance. Aptamers against immunoglobulin targets-IgA, IgM, IgG and IgE-have a clear niche for diagnostic applications, therefore numerous aptamers have been selected and used in combination with a variety of detection techniques. The aim of this review is to overview and evaluate aptamers selected for the recognition of antibodies, in terms of their design, analytical properties and diagnostic applications. Aptamer candidates showed convincing performance among others to identify stress and upper respiratory tract infection through SIgA detection, for cancer cell recognition using membrane bound IgM, to detect and treat hemolytic transfusion reactions, autoimmune diseases with IgG and detection of IgE for allergy diseases. However, in general, their use still lags significantly behind what their claimed benefits and the plethora of application opportunities would forecast.

Functional nucleic acid-based fluorescence polarization/anisotropy biosensors for detection of biomarkers

The sensitive and selective detection of biomarkers plays a crucial role in disease diagnostics, drug discovery, and early screening of cancers. The achievement of this goal highly depends on the continuous development of biosensing technologies. Among them, fluorescence anisotropy/polarization (FA/FP) analysis receives increasing interest due to the advantage of simple operation, fast response, and no background interference. In recent decades, great progress has been achieved in FA/FP sensors thanks to the development of functional nucleic acids (FNAs) including aptamers and nucleic acid enzymes. This review focuses on FNA-based FA/FP sensors for the quantitative detection of biomarkers, such as nucleic acid, small molecules, and proteins. The design strategies, recognition elements, and practical applications are fully highlighted. The article also discusses the challenges of applying FNA-based FA/FP sensors in the next generation and the potential solutions along with future prospects. Graphical abstract.

Aptamer binding assays and molecular interaction studies using fluorescence anisotropy - A review

Binding of nucleic acid aptamers to specific targets and detection with fluorescence anisotropy (FA) or fluorescence polarization (FP) take advantage of the complementary features of aptamers and the fluorescence techniques. We review recent advances in affinity binding assays using aptamers and FA/FP, with an emphasis on studies of molecular interactions and identification of binding sites. Aptamers provide several benefits, including the ease of labelling fluorophores on specific sites, binding-induced changes in aptamer structures, hybridization of the aptamers to complementary sequences, changes in molecular volume upon binding of the aptamer to its target, and adsorption of aptamers onto nanomaterials. Some of these benefits have been utilized for FA/FP assays. Once the aptamer binds to its target, the resulting changes in molecular volume (size), structure, local rotation of the fluorophore, and/or the fluorescence lifetime influence changes to the FA/FP values. Measurements of these fluorescence anisotropy/polarization changes have provided insights into the molecular interactions, such as the binding affinity and the site of binding. Studies of molecular interactions conducted in homogeneous solutions, as well as those with separations, e.g., capillary electrophoresis, have been summarized in this review. Studies on mapping the position of binding in aptamers at the single nucleotide level have demonstrated a unique benefit of the FA/FP techniques and pointed to an exciting direction for future research.

Directing a rational design of aptamer-based fluorescence anisotropy assay for sensitive detection of immunoglobulin E by site-specific binding study

Mapping aptamer-protein interactions is important for characterization and applications of aptamers against proteins. We describe here probing affinity interactions between aptamer and immunoglobulin E (IgE) with a fluorescence anisotropy (FA) approach using a series of aptamer probes having single fluorescein (FAM) label at individual nucleotide (A, C, T). Studies of binding between IgE and aptamer probes revealed several possible close-contact sites, e.g., T9, T10, T11, T13, C15, and T17 of a 37-nt aptamer with a stem-loop secondary structure. FAM labeling on these sites resulted in much higher FA values (higher than 0.230 for T10, T11, T13 and C15) of aptamer-IgE complexes than the distant sites (e.g., terminals) of aptamer probably because the bound IgE close to these sites significantly restricted local rotation of FAM. Close-contact site labeled aptamer probes with high affinity allowed to develop a more sensitive FA assay for IgE than distant site labeled aptamers. The FA assay using T10-labeled aptamer with a dissociation constant (K_d) about 0.8 nM enabled selective detection of IgE at 20 pM and large FA increase upon IgE addition. We also found A12, C14, A25, and T27 were important for IgE-aptamer binding as FAM labeling at these sites significantly reduced aptamer affinity. FA study showed the loop region of this stem-loop aptamer was crucial for affinity binding, and IgE bound to the loop. This FA method will be helpful for understanding aptamer-protein binding and making a rational design of aptamer affinity assays for proteins.

Fluorescence Polarization-Based Rapid Detection System for Salivary Biomarkers Using Modified DNA Aptamers Containing Base-Appended Bases

The field of care testing toward the analysis of blood and saliva lacks nowadays simple test techniques for biomarkers. In this study, we have developed a novel nucleobase analog, U^gu, which is a uracil derivative bearing a guanine base at the 5-position. Moreover, we attempted the development of aptamers that can bind to secretory immunoglobulin A (SIgA), which has been examined as a stress marker in human saliva. It was observed that the acquired aptamer binds strongly and selectively to the SIgA dimer (K_d = 13.6 nM) without binding to the IgG and IgA monomers of human serum. Reduction of the aptamer length (41 mer) successfully improved 4-fold the binding affinity (K_d = 3.7 nM), compared to the original, longer aptamer (78 mer). Furthermore, the development of a simple detection system for human saliva samples by fluorescence polarization was investigated, using the reported human salivary α-amylase (sAA) and the SIgA-binding aptamer. Comparison of the present method with conventional enzyme-linked immunosorbent assay techniques highlighted a significant Pearson's correlation of 0.94 and 0.83 when targeting sAA and SIgA, respectively. It is thus strongly suggested that a new simple test of stress markers in human saliva can be quantified quickly without bound/free (B/F) separation.

Development of DNA Aptamers against the Nucleocapsid Protein of Severe Fever with Thrombocytopenia Syndrome Virus for Diagnostic Application: Catalytic Signal Amplification using Replication Protein A-Conjugated Liposomes

Most prevalent infectious diseases worldwide are caused by mediators such as insects and characterized by high mortality and morbidity, thereby creating a global public health concern. Therefore, a sensitive, selective detection platform for diagnosing diseases in the early stages of infection is needed to prevent disease spread and to protect public health. Here, we developed novel DNA aptamers specific to the nucleocapsid protein (NP) of the severe fever with thrombocytopenia syndrome (SFTS) virus and synthesized ssDNA-binding protein-conjugated liposomes encapsulated with horseradish peroxidase (HRP) for application in a simple and universal platform. This platform achieved highly sensitive detection of the NP by measuring the colorimetric signal following lysis of the HRP encapsulated liposomes, mediated by a mixture of 3,3',5,5'-tetramethylbenzidine and H₂O₂ solution. The limit of detection was 0.009 ng·mL^-1, and NP was successfully detected in diluted human serum with a high recovery rate. Moreover, this method was specific and did not exhibit cross-reactivity among NPs of other virus types. These results demonstrated the efficacy of the proposed method as a highly sensitive, specific, and universal diagnostic tool for potential application in monitoring of the early stages of infectious diseases.

Self-Assembly of Functional Nucleic Acid-Based Colorimetric Competition Assay for the Detection of Immunoglobulin E

In this work, we have developed a simple and rapid colorimetric assay for the detection of immunoglobulin E (IgE) using functional nucleic acids (FNAs) and a solid-phase competition enzyme-linked immunosorbent assay (ELISA). The FNAs including aptamer of recombinant IgE, G-quadruplex and its complementary fragments were immobilized on 96-well microplates to achieve recognition and detection of IgE in biological samples. The G-quadruplex DNAzyme catalyzed 2,2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid (ABTS)-hemin-H2O2 system was used to improve the sensitivity of colorimetric assay. In the presence of IgE, the hairpin structure and G-quadruplex would be destroyed, resulting in the inactivation of DNAzyme and subsequent reduction of its absorbance. This cost-effective approach detected IgE in the linear range from 5.0 pg/mL to 500 ng/mL, with the limit of detection (LOD) of 2.0 pg/mL, under optimal conditions. Moreover, the developed method was successfully applied to the rapid detection of IgE in human urine, indicating a great potentiality of this approach in clinical diagnosis and other biomedical applications.

Development of a DNA aptamer selection method based on the heterogeneous sandwich form and its application in a colorimetric assay for influenza A virus detection

In this paper, we introduce an effective method for selecting aptamer that increases the signal-to-noise ratio in a heterogenous sandwich-type immunosensor and confirm the efficiency of selected aptamer candidates in the colorimetric assay. Using the proposed method, four aptamer candidates withK_dvalues ranging from 77.6 nM to 125.7 nM were obtained.

A graphene aptasensor for biomarker detection in human serum

This paper presents an affinity graphene nanosensor for detection of biomarkers in undiluted and non-desalted human serum. The affinity nanosensor is a field-effect transistor in which graphene serves as the conducting channel. The graphene surface is sequentially functionalized with a nanolayer of the polymer polyethylene glycol (PEG) and a biomarker-specific aptamer. The aptamer is able to specifically bind with and capture unlabeled biomarkers in serum. A captured biomarker induces a change in the electric conductivity of the graphene, which is measured in a buffer of optimally chosen ionic strength to determine the biomarker concentration. The PEG layer effectively rejects nonspecific adsorption of background molecules in serum while still allowing the aptamer to be readily accessible to serum-borne biomarkers and increases the effective Debye screening length on the graphene surface. Thus, the aptamer-biomarker binding sensitively changes the graphene conductivity, thereby achieving specific and label-free detection of biomarkers with high sensitivity and without the need to dilute or desalt the serum. Experimental results demonstrate that the graphene nanosensor is capable of specifically capturing human immunoglobulin E (IgE), used as a representative biomarker, in human serum in the concentration range of 50 pM-250 nM, with a resolution of 14.5 pM and a limit of detection of 47 pM.

Highly Stable Conjugates of Carbon Nanoparticles with DNA Aptamers

Conjugates of carbon nanoparticles and aptamers have great potential in many areas of biomedicine. In order to be implemented in practice, such conjugates should keep their properties throughout long storage period in commonly available conditions. In this work, we prepared conjugates of carbon nanoparticles (CNP) with DNA aptamers using streptavidin-biotin reaction. Obtained conjugates possess superior stability and kept their physical-chemical and functional properties during 30 days at +4 °C and -20 °C. Proposed approach to conjugation allows loading of about 100-120 pM of biotinylated aptamer per 1 mg of streptavidin-coated CNP (CNP-Str). Aptamer-functionalized CNP-Str have zeta potential of -34 mV at pH 7, mean diameter of 168-177 nm, and polydispersity index of 0.080-0.140. High reproducibility of functionalization was confirmed by preparation of several batches of CNP-aptamer with the same size distribution and aptamer loading using independently synthesized parent CNP-Str nanoparticles. Stability of CNP-aptamer conjugates was significantly enhanced by postsynthesis addition of EDTA that prevents nuclease degradation of immobilized aptamers. Obtained nanoparticles were stable at pH ranging from 6 to 10. Optical properties of CNP-aptamer nanoparticles were also studied and their ability to quench fluorescence via Förster resonance energy transfer was shown. Taking into account properties of CNP-aptamer conjugates, we suppose they may be used in both homo- and heterogeneous colorimetric, fluorescent, and aggregation-based assays.

Comprehensive Regression Model for Dissociation Equilibria of Cell-Specific Aptamers

A comprehensive nonlinear regression model for dissociation equilibria of cell-specific aptamers is proposed by considering the effect of receptor expression level. Benefiting from the global regression of simultaneous equations, the fitted parameters reach a very significant level, indicating the statistical validity of this updated model. According to the fitting results, we found that dissociation constants fitted using the previous model are obviously larger than the updated values, which can be explained by the effect of receptor number on curve fitting. In addition, equivalent receptor density can be estimated using the updated model, which may lead to some new judgments about reported results of cell-SELEX.

Magnetic nanoparticles and polydopamine amplified FP aptasensor for the highly sensitive detection of rHuEPO-α

In this paper, an amplified fluorescence polarization (FP) aptasensor based on magnetic nanoparticles @polydopamine (MNP@PDA) was innovatively developed for sensitive detection of recombinant human erythropoietin-alpha(rHuEPO-α). The amplified FP signal was due to the large mass of protein and MNP@PDA. And this assay can be utilized for target separation or recycling based on the magnetic property of MNP@PDA through magnetic separation. Briefly, rHuEPO-α and MNP@PDA were added successively to react with the labeled aptamer (FAM-P1), which both contributed to the increase of FP signal via the formation of FAM-P1-rHuEPO-α and particularly FAM-P1-MNP@PDA complex. The strong interaction between MNP@PDA and FAM-P1 ensured the high efficiency of mass amplification and magnetic separation. As a result, the detection limit for rHuEPO-α was 0.12 pM, 4 orders of magnitude lower than original assay. Besides, three kinds of rHuEPO-α injections, NuPIAO, Epogen and ESPO were used to evaluate the selectivity of this assay in complex matrix with reasonable standard deviation. In a word, this work provides a simple, rapid, non-modified, highly sensitive and selective sensing platform for the detection of rHuEPO-α.

Panoply of Fluorescence Polarization/Anisotropy Signaling Mechanisms for Functional Nucleic Acid-Based Sensing Platforms

Fluorescence polarization/anisotropy is a very popular technique that is widely used in homogeneous-phase immunoassays for the small molecule quantification. In the present Feature, we discuss how the potential of this signaling approach considerably expanded during the last 2 decades through the implementation of a myriad of original transducing strategies that use functional nucleic acid recognition elements as a promising alternative to antibodies.

Gas-generating reactions for point-of-care testing

Gas generation-based measurement is an attractive alternative approach for POC (Point-of-care) testing, which relies on the amount of generated gas to detect the corresponding target concentrations.

Electrochemical aptasensor for human osteopontin detection using a DNA aptamer selected by SELEX

A DNA aptamer with affinity and specificity for human osteopontin (OPN), a potential breast cancer biomarker, was selected using the SELEX process, considering its homology rate and the stability of its secondary structures. This aptamer exhibited a satisfactory affinity towards OPN, showing dissociation constants lower than 2.5 nM. It was further used to develop a simple, label-free electrochemical aptasensor against OPN. The aptasensor showed good sensitivity towards OPN in standard solutions, being the square wave voltammetry (SWV), compared to the cyclic voltammetry, the most sensitive technique with detection and quantification limits of 1.4 ± 0.4 nM and 4.2 ± 1.1 nM, respectively. It showed good reproducibility and acceptable selectivity, exhibiting low signal interferences from other proteins, as thrombin, with 2.6-10 times lower current signals-off than for OPN. The aptasensor also successfully detected OPN in spiked synthetic human plasma. Using SWV, detection and quantification limits (1.3 ± 0.1 and 3.9 ± 0.4 nM) within the OPN plasma levels reported for patients with breast cancer (0.4-4.5 nM) or with metastatic or recurrent breast cancer (0.9-8.4 nM) were found. Moreover, preliminary assays, using a sample of human plasma, showed that the aptasensor and the standard ELISA method quantified similar OPN levels (2.2 ± 0.7 and 1.7 ± 0.1 nM, respectively). Thus, our aptasensor coupled with SWV represents a promising alternative for the detection of relevant breast cancer biomarkers.

Bivalent Aptasensor Based on Silver-Enhanced Fluorescence Polarization for Rapid Detection of Lactoferrin in Milk

Here we report a novel type of bivalent aptasensor based on silver-enhanced fluorescence polarization (FP) for detection of lactoferrin (Lac) in milk powder with high sensitivity and specificity. The novel two split aptamers were obtained from the aptamer reported in our previous SELEX (systematic evolution of ligands by exponential enrichment) selection, and their minimal structural units were optimized on the basis of their affinity and specificity. Also, dual binding sites of split aptamers were verified. The bivalent aptamers were modified to be linked with signal-molecule fluorescein isothiocyanate (FITC) and enhancer silver decahedral nanoparticles (Ag₁₀NPs). The split aptamers could bind to different sites of Lac and assemble into a split-aptamers-target complex, narrowing the distance between Ag₁₀NPs and FITC dye. As a result, Ag₁₀NPs could produce a mass-augmented and metal-enhanced fluorescence (MEF) effect. In general, ternary amplification based on Ag₁₀NPs, split aptamers, and the MEF effect all contributed to the significant increase of FP values. It was proved that the sensitivity of this assay was about 3 orders of magnitude over traditional aptamer-based homogeneous assays with a detection limit of 1.25 pM. Furthermore, this design was examined by actual milk powder with rapid and high-throughout detection.

Recent Advances in Aptamers Targeting Immune System

The immune system plays important role in protecting the organism by recognizing non-self molecules from pathogen such as bacteria, parasitic worms, and viruses. When the balance of the host defense system is disturbed, immunodeficiency, autoimmunity, and inflammation occur. Nucleic acid aptamers are short single-stranded DNA (ssDNA) or RNA ligands that interact with complementary molecules with high specificity and affinity. Aptamers that target the molecules involved in immune system to modulate their function have great potential to be explored as new diagnostic and therapeutic agents for immune disorders. This review summarizes recent advances in the development of aptamers targeting immune system. The selection of aptamers with superior chemical and biological characteristics will facilitate their application in the diagnosis and treatment of immune disorders.

Replacing antibodies with modified DNA aptamers in vaccine potency assays

Vaccine in vitro potency assays are vital regulatory tests that are used to confirm the presence and concentration of an antigen of interest in a form that directly or indirectly relates to protective activity in patients. Current assays come in many forms, but they almost exclusively use antibody reagents for selective detection of the target antigen. Antibodies provide specific recognition of vaccine antigens but also exhibit drawbacks such as stability limitations, cost, and lot-to-lot variation, which can make it challenging to maintain the reagent throughout the lifetime of the vaccine. We explored replacing antibodies with aptamers. Aptamers are macromolecules, such as nucleic acids, which can bind to their targets with high specificity and affinity, similar to that of antibodies. Some of the advantages of using aptamers over antibodies is that aptamers can be more stable, smaller, less expensive to produce, synthesized in vitro, and logistically easier to supply throughout the multi-decade lifespan of a commercial vaccine. We created modified DNA aptamers against the common vaccine carrier protein, CRM₁₉₇. Several aptamers were discovered and one was chosen for further characterization. The binding kinetics of the aptamer revealed an off-rate 16-fold slower than anti-CRM₁₉₇ antibodies used for comparison. The aptamers were more sensitive than available antibodies in some assay formats and comparable in others. The aptamer epitope was mapped to the receptor-binding domain of CRM₁₉₇, a site adjacent to a known antibody binding site. These data address some key aspects for a path forward in replacing antibodies with aptamers for use as critical reagents in vaccine assays. We further highlight the possibility of using nucleic acid reagents to develop next generation potency assays.

DNA-stabilized silver nanoclusters and carbon nanoparticles oxide: A sensitive platform for label-free fluorescence turn-on detection of HIV-DNA sequences

Based on the remarkable difference between the interactions of carbon nanoparticles (CNPs) oxide with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), and the fact that fluorescence of DNA-stabilized silver nanoclusters (AgNCs) can be quenched by CNPs oxide, DNA-functionalized AgNCs were applied as label-free fluorescence probes and a novel fluorescence resonance energy transfer (FRET) sensor was successfully constructed for the detection of human immunodeficiency virus (HIV) DNA sequences. CNPs oxide were prepared with the oxidation of candle soot, hence it is simple, time-saving and low-cost. The strategy of dual AgNCs probes was applied to improve the detection sensitivity by using dual- probe capturing the same target DNA in a sandwich mode and as the fluorescence donor, and using CNPs oxide as the acceptor. In the presence of target DNA, a dsDNA hybrid forms, leading to the desorption of the ssDNA-AgNCs probes from CNPs oxide, and the recovering of fluorescence of the AgNCs in a HIV-DNA concentration-dependent manner. The results show that HIV-DNA can be detected in the range of 1-50nM with a detection limit of 0.40nM in aqueous buffer. The method is simple, rapid and sensitive with no need of labeled fluorescent probes, and moreover, the design of fluorescent dual-probe makes full use of the excellent fluorescence property of AgNCs and further improves the detection sensitivity.

Multimodal Detection of a Small Molecule Target Using Stimuli-Responsive Liposome Triggered by Aptamer-Enzyme Conjugate

Nanomaterials which can respond to external stimuli have attracted considerable attention due to their potential applications in sensing and biomedicine. One of the most promising classes of such materials is the stimuli-responsive liposome that can release its contents in response to a specific target. Despite recent progress, development of liposomes responsive to small molecular targets remains a challenge, due to the difficulty in designing the transduction process to link between target binding and triggered release, even though small molecular metabolites play important roles in many biological processes. Herein, we demonstrate a combination of an aptamer (apt) for target recognition and enzyme phosphatidylcholine 2-acetylhydrolase (PLA2) for rupture of liposome. As a proof-of-concept, cocaine molecules were used to trigger the release of the enzyme. The exposure to DNA-PLA2 conjugates induced the rupture of liposome containing uranin and gadopentetic acid (Gd-DTPA), allowing multimodal fluorescent and MRI detection of cocaine. The strategy demonstrated in this work can be generally applied to other imaging modalities by loading different imaging agents, as well as other targets by using different functional DNAs.

Use of aptamers in immunoassays

Aptamers, short single-chain DNA or RNA oligonucleotides, react specifically with small molecules, as well as with proteins. Unlike antibodies, they may be obtained relatively easily. Aptamers are now widely employed in immunological studies and could replace antibodies in immunoassays. In this short review, methods for immobilizing aptamers on various insoluble materials (so-called apta-sorbents) are described. Recent findings on their use in the detection and isolation of immunoglobulins and their application in various immunoassays are also discussed.

Aptamers Against Immunologic Targets: Diagnostic and Therapeutic Prospects

The concept of in vitro selection of nucleic acid aptamers emerged 25 years ago, and since then tremendous progress has been achieved in the development of different aptamers and their applications for various bioanalytical and therapeutic purposes. Among other protein targets of aptamers, immune system proteins are of particular interest both as diagnostic markers and therapeutic targets. The present review summarizes up-to-date articles concerning the selection and design of DNA and RNA aptamers against immunologic targets such as antibodies, cytokines, and T-cell and B-cell receptors. We also discuss the prospects of employing aptamers as recognizing modules of diagnostic aptasensors, potential therapeutic candidates for the treatment of autoimmune diseases and cancer, and specific tools for functional studies of immune system proteins.

Ensemble and single-molecule biophysical characterization of D17.4 DNA aptamer-IgE interactions

BACKGROUND

The IgE-binding DNA aptamer 17.4 is known to inhibit the interaction of IgE with the high-affinity IgE Fc receptor FcεRI. While this and other aptamers have been widely used and studied, there has been relatively little investigation of the kinetics and energetics of their interactions with their targets, by either single-molecule or ensemble methods.

METHODS

The dissociation kinetics of the D17.4/IgE complex and the effects of temperature and ionic strength were studied using fluorescence anisotropy and single-molecule spectroscopy, and activation parameters calculated.

RESULTS

The dissociation of D17.4/IgE complex showed a strong dependence on temperature and salt concentration. The koff of D17.4/IgE complex was calculated to be (2.92±0.18)×10(-3) s(-1) at 50 mM NaCl, and (1.44±0.02)×10(-2) s(-1) at 300 mM NaCl, both in 1 mM MgCl2 and 25°C. The dissociation activation energy for the D17.4/IgE complex, Ea, was 16.0±1.9 kcal mol(-1) at 50 mM NaCl and 1 mM MgCl2. Interestingly, we found that the C19A mutant of D17.4 with stabilized stem structure showed slower dissociation kinetics compared to D17.4. Single-molecule observations of surface-immobilized D17.4/IgE showed much faster dissociation kinetics, and heterogeneity not observable by ensemble techniques.

CONCLUSIONS

The increasing koff value with increasing salt concentration is attributed to the electrostatic interactions between D17.4/IgE. We found that both the changes in activation enthalpy and activation entropy are insignificant with increasing NaCl concentration. The slower dissociation of the mutant C19A/IgE complex is likely due to the enhanced stability of the aptamer.

GENERAL SIGNIFICANCE

The activation parameters obtained by applying transition state analysis to kinetic data can provide details on mechanisms of molecular recognition and have applications in drug design. Single-molecule dissociation kinetics showed greater kinetic complexity than was observed in the ensemble in-solution systems, potentially reflecting conformational heterogeneity of the aptamer. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Fluorescence anisotropy (polarization): from drug screening to precision medicine

INTRODUCTION

Fluorescence anisotropy (FA) is one of the major established methods accepted by industry and regulatory agencies for understanding the mechanisms of drug action and selecting drug candidates utilizing a high-throughput format.

AREAS COVERED

This review covers the basics of FA and complementary methods, such as fluorescence lifetime anisotropy and their roles in the drug discovery process. The authors highlight the factors affecting FA readouts, fluorophore selection and instrumentation. Furthermore, the authors describe the recent development of a successful, commercially valuable FA assay for long QT syndrome drug toxicity to illustrate the role that FA can play in the early stages of drug discovery.

EXPERT OPINION

Despite the success in drug discovery, the FA-based technique experiences competitive pressure from other homogeneous assays. That being said, FA is an established yet rapidly developing technique, recognized by academic institutions, the pharmaceutical industry and regulatory agencies across the globe. The technical problems encountered in working with small molecules in homogeneous assays are largely solved, and new challenges come from more complex biological molecules and nanoparticles. With that, FA will remain one of the major work-horse techniques leading to precision (personalized) medicine.

Attomolar detection of proteins via cascade strand-displacement amplification and polystyrene nanoparticle enhancement in fluorescence polarization aptasensors

Extremely sensitive and accurate measurements of protein markers for early detection and monitoring of diseases pose a formidable challenge. Herein, we develop a new type of amplified fluorescence polarization (FP) aptasensor based on allostery-triggered cascade strand-displacement amplification (CSDA) and polystyrene nanoparticle (PS NP) enhancement for ultrasensitive detection of proteins. The assay system consists of a fluorescent dye-labeled aptamer hairpin probe and a PS NP-modified DNA duplex (assistant DNA/trigger DNA duplex) probe with a single-stranded part and DNA polymerase. Two probes coexist stably in the absence of target, and the dye exhibits relatively low FP background. Upon recognition and binding with a target protein, the stem of the aptamer hairpin probe is opened, after which the opened hairpin probe hybridizes with the single-stranded part in the PS NP-modified DNA duplex probe and triggers the CSDA reaction through the polymerase-catalyzed recycling of both target protein and trigger DNA. Throughout this CSDA process, numerous massive dyes are assembled onto PS NPs, which results in a substantial FP increase that provides a readout signal for the amplified sensing process. Our newly proposed amplified FP aptasensor enables the quantitative measurement of proteins with the detection limit in attomolar range, which is about 6 orders of magnitude lower than that of traditional homogeneous aptasensors. Moreover, this sensing method also exhibits high specificity for target proteins and can be performed in homogeneous solutions. In addition, the suitability of this method for the quantification of target protein in biological samples has also been shown. Considering these distinct advantages, the proposed sensing method can be expected to provide an ultrasensitive platform for the analysis of various types of target molecules.

Evaluating the dual target binding capabilities of immobilized aptamers using flow cytometry

In the current study, the authors quantify the binding activity of particle-immobilized DNA aptamers to their nucleotide and non-nucleotide targets. For the purposes of this work, DNA and vascular endothelial growth factor (VEGF) binding analysis was carried out for VEGF-binding aptamers and compared to that of an ampicillin-binding aptamer as well as a non-aptamer DNA probe. Binding analysis followed incubation of one target type, coincubation of both DNA and VEGF targets, and serial incubations of each target type. Moreover, recovery of aptamer binding activity following displacement of the DNA target from aptamer:DNA duplexes was also explored. Flow cytometry served as the quantitative tool to directly monitor binding events of both the DNA target and protein target to the various aptamer and non-aptamer functionalized particles. The current work demonstrates how processing steps such as annealing and binding history of particle-immobilized aptamers can affect subsequent binding activity. To this end, the authors demonstrate the ability to fully recover DNA target binding activity capabilities and to partially recover protein target binding activity.

Aptamer fluorescence anisotropy sensors for adenosine triphosphate by comprehensive screening tetramethylrhodamine labeled nucleotides

We previously reported a fluorescence anisotropy (FA) approach for small molecules using tetramethylrhodamine (TMR) labeled aptamer. It relies on target-binding induced change of intramolecular interaction between TMR and guanine (G) base. TMR-labeling sites are crucial for this approach. Only terminal ends and thymine (T) bases could be tested for TMR labeling in our previous work, possibly causing limitation in analysis of different targets with this FA strategy. Here, taking the analysis of adenosine triphosphate (ATP) as an example, we demonstrated a success of conjugating TMR on other bases of aptamer adenine (A) or cytosine (C) bases and an achievement of full mapping various labeling sites of aptamers. We successfully constructed aptamer fluorescence anisotropy (FA) sensors for adenosine triphosphate (ATP). We conjugated single TMR on adenine (A), cytosine (C), or thymine (T) bases or terminals of a 25-mer aptamer against ATP and tested FA responses of 14 TMR-labeled aptamer to ATP. The aptamers having TMR labeled on the 16th base C or 23rd base A were screened out and exhibited significant FA-decreasing or FA-increasing responses upon ATP, respectively. These two favorable TMR-labeled aptamers enabled direct FA sensing ATP with a detection limit of 1 µM and the analysis of ATP in diluted serum. The comprehensive screening various TMR labeling sites of aptamers facilitates the successful construction of FA sensors using TMR-labeled aptamers. It will expand application of TMR-G interaction based aptamer FA strategy to a variety of targets.

Nanomaterial-based biosensors using dual transducing elements for solution phase detection

Biosensors incorporating nanomaterials have demonstrated superior performance compared to their conventional counterparts. Most reported sensors use nanomaterials as a single transducer of signals, while biosensor designs using dual transducing elements have emerged as new approaches to further improve overall sensing performance. This review focuses on recent developments in nanomaterial-based biosensors using dual transducing elements for solution phase detection. The review begins with a brief introduction of the commonly used nanomaterial transducers suitable for designing dual element sensors, including quantum dots, metal nanoparticles, upconversion nanoparticles, graphene, graphene oxide, carbon nanotubes, and carbon nanodots. This is followed by the presentation of the four basic design principles, namely Förster Resonance Energy Transfer (FRET), Amplified Fluorescence Polarization (AFP), Bio-barcode Assay (BCA) and Chemiluminescence (CL), involving either two kinds of nanomaterials, or one nanomaterial and an organic luminescent agent (e.g. organic dyes, luminescent polymers) as dual transducers. Biomolecular and chemical analytes or biological interactions are detected by their control of the assembly and disassembly of the two transducing elements that change the distance between them, the size of the fluorophore-containing composite, or the catalytic properties of the nanomaterial transducers, among other property changes. Comparative discussions on their respective design rules and overall performances are presented afterwards. Compared with the single transducer biosensor design, such a dual-transducer configuration exhibits much enhanced flexibility and design versatility, allowing biosensors to be more specifically devised for various purposes. The review ends by highlighting some of the further development opportunities in this field.