Selection of DNA aptamers using atomic force microscopy

Aptamer Technologies in Neuroscience, Neuro-Diagnostics and Neuro-Medicine Development

Aptamers developed using in vitro Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technology are single-stranded nucleic acids 10-100 nucleotides in length. Their targets, often with specificity and high affinity, range from ions and small molecules to proteins and other biological molecules as well as larger systems, including cells, tissues, and animals. Aptamers often rival conventional antibodies with improved performance, due to aptamers' unique biophysical and biochemical properties, including small size, synthetic accessibility, facile modification, low production cost, and low immunogenicity. Therefore, there is sustained interest in engineering and adapting aptamers for many applications, including diagnostics and therapeutics. Recently, aptamers have shown promise as early diagnostic biomarkers and in precision medicine for neurodegenerative and neurological diseases. Here, we critically review neuro-targeting aptamers and their potential applications in neuroscience research, neuro-diagnostics, and neuro-medicine. We also discuss challenges that must be overcome, including delivery across the blood-brain barrier, increased affinity, and improved in vivo stability and in vivo pharmacokinetic properties.

TBA for Sensing Toxic Cations: A Critical Analysis of Structural and Electrical Properties

Food and drinks can be contaminated with pollutants such as lead and strontium, which poses a serious danger to human health. For this reason, a number of effective sensors have been developed for the rapid and highly selective detection of such contaminants. TBA, a well-known aptamer developed to selectively target and thereby inhibit the protein of clinical interest α-thrombin, is receiving increasing attention for sensing applications, particularly for the sensing of different cations. Indeed, TBA, in the presence of these cations, folds into the stable G-quadruplex structure. Furthermore, different cations produce small but significant changes in this structure that result in changes in the electrical responses that TBA can produce. In this article, we give an overview of the expected data regarding the use of TBA in the detection of lead and strontium, calculating the expected electrical response using different measurement techniques. Finally, we conclude that TBA should be able to detect strontium with a sensitivity approximately double that achievable for lead.

Electrochemical aptasensors for pathogenic detection toward point-of-care diagnostics

A biosensor system refers to a biomedical device, which detects biological, chemical, or biochemical components by converting those signals to an electrical signal by utilizing and uniting physical or chemical transducer with biorecognition elements. An electrochemical biosensor is generally based on the reaction of either production or consumption of electrons under a three-electrode system. Biosensor systems are exploited in a wide range of areas, such as medicine, agriculture, husbandry, food, industry, environment protection, quality control, waste disposal, and the military. Pathogenic infections are the third leading cause of death worldwide after cardiovascular diseases and cancer. Therefore, there is an urgent need for effective diagnostic tools to control food, water, and soil contamination result in protecting human life and health. Aptamers are peptide or oligonucleotide-based molecules that show very high affinity to their targets that are produced from large pools of random amino acid or oligonucleotide sequences. Generally, aptamers have been utilized for fundamental sciences and clinical implementations for their target-specific affinity and have been intensely exploited for different kinds of biosensor applications for approximately 30 years. The convergence of aptamers with biosensor systems enabled the construction of voltammetric, amperometric, and impedimetric biosensors for the detection of specific pathogens. In this review, electrochemical aptamer biosensors were evaluated by discussing the definition, types, and production techniques of aptamers, the advantages of aptamers as a biological recognition element against their alternatives, and a wide range of aptasensor examples from literature in the detection of specific pathogens.

A Genosensor Based on the Modification of a Microcantilever: A Review

When the free end of a microcantilever is modified by a genetic probe, this sensor can be used for a wider range of applications, such as for chemical analysis, biological testing, pharmaceutical screening, and environmental monitoring. In this paper, to clarify the preparation and detection process of a microcantilever sensor with genetic probe modification, the core procedures, such as probe immobilization, complementary hybridization, and signal extraction and processing, are combined and compared. Then, to reveal the microcantilever's detection mechanism and analysis, the influencing factors of testing results, the theoretical research, including the deflection principle, the establishment and verification of a detection model, as well as environmental influencing factors are summarized. Next, to demonstrate the application results of the genetic-probe-modified sensors, based on the classification of detection targets, the application status of other substances except nucleic acid, virus, bacteria and cells is not introduced. Finally, by enumerating the application results of a genetic-probe-modified microcantilever combined with a microfluidic chip, the future development direction of this technology is surveyed. It is hoped that this review will contribute to the future design of a genetic-probe-modified microcantilever, with further exploration of the sensitive mechanism, optimization of the design and processing methods, expansion of the application fields, and promotion of practical application.

Recent Advances in Aptasensors For Rapid and Sensitive Detection of Staphylococcus Aureus

The infection of Staphylococcus aureus (S.aureus) and the spread of drug-resistant bacteria pose a serious threat to global public health. Therefore, timely, rapid and accurate detection of S. aureus is of great significance for food safety, environmental monitoring, clinical diagnosis and treatment, and prevention of drug-resistant bacteria dissemination. Traditional S. aureus detection methods such as culture identification, ELISA, PCR, MALDI-TOF-MS and sequencing, etc., have good sensitivity and specificity, but they are complex to operate, requiring professionals and expensive and complex machines. Therefore, it is still challenging to develop a fast, simple, low-cost, specific and sensitive S. aureus detection method. Recent studies have demonstrated that fast, specific, low-cost, low sample volume, automated, and portable aptasensors have been widely used for S. aureus detection and have been proposed as the most attractive alternatives to their traditional detection methods. In this review, recent advances of aptasensors based on different transducer (optical and electrochemical) for S. aureus detection have been discussed in details. Furthermore, the applications of aptasensors in point-of-care testing (POCT) have also been discussed. More and more aptasensors are combined with nanomaterials as efficient transducers and amplifiers, which appears to be the development trend in aptasensors. Finally, some significant challenges for the development and application of aptasensors are outlined.

Aptamers for Viral Detection and Inhibition

Recent times have experienced more than ever the impact of viral infections in humans. Viral infections are known to cause diseases not only in humans but also in plants and animals. Here, we have compiled the literature review of aptamers selected and used for detection and inhibition of viral infections in all three categories: humans, animals, and plants. This review gives an in-depth introduction to aptamers, different types of aptamer selection (SELEX) methodologies, the benefits of using aptamers over commonly used antibody-based strategies, and the structural and functional mechanism of aptasensors for viral detection and therapy. The review is organized based on the different characterization and read-out tools used to detect virus-aptasensor interactions with a detailed index of existing virus-targeting aptamers. Along with addressing recent developments, we also discuss a way forward with aptamers for DNA nanotechnology-based detection and treatment of viral diseases. Overall, this review will serve as a comprehensive resource for aptamer-based strategies in viral diagnostics and treatment.

Oligonucleotide aptamers: Recent advances in their screening, molecular conformation and therapeutic applications

Aptamers are single stranded oligonucleotides with specific recognition and binding ability to target molecules, which can be obtained by Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Aptamers have the advantages of low molecular weight, low immunogenicity, easy modification and high stability. They play promising role in promoting food safety, monitoring the environment and basic research, especially in clinical diagnosis and therapeutic drugs. To date, great achievements regarding the selection, modifications and application of aptamers have been made. However, since it is still a challenge to obtain aptamers with high affinity in a more effective way, few aptamer-based products have already successfully entered into clinical use. This review aims to provide a thorough overview of the latest advances in this rapidly developing field, focusing on aptamer screening methods for different targets, the structure of the interaction between aptamers and target substances, and the challenges and potential of current therapeutic aptamers.

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.

SELEX-based DNA Aptamer Selection: A Perspective from the Advancement of Separation Techniques

DNA aptamers, which are short, single-stranded DNA sequences that selectively bind to target substances (proteins, cells, small molecules, metal ions), can be acquired by means of the systematic evolution of ligands by exponential enrichment (SELEX) methodology. In the SELEX procedure, one of the keys for the effective acquisition of high-affinity and functional aptamer sequences is the separation stage to isolate target-bound DNA from unbound DNA in a randomized DNA library. In this review, various remarkable advancements in separation techniques for SELEX-based aptamer selection developed in this decade, are described and discussed, including CE-, microfluidic chip-, solid phase-, and FACS-based SELEX along with other methods.

A single-molecule study reveals novel rod-like structures formed by a thrombin aptamer repeat sequence

Thrombin aptamers (TBAs) have attracted much attention due to their various applications. The structures and properties of long ssDNA chains with multiple TBA repeat sequences are interesting and distinct from those of their monomers. Due to the complexity of the sample system, it is quite difficult to reveal the structure of such a long-chain ssDNA using traditional methods. In this work, we investigated the repeated ssDNA by using single-molecule magnetic tweezers and AFM imaging. To do that we developed the polymerase change-rolling circle amplification (PC-RCA) synthetic method and prepared two-end modified repeated ssDNA. The rod-like G4 structures formed by intramolecular stacking of the repeat sequence were for the first time identified. This novel structure is different from those higher-order quadruplex structures formed by G-tetrads or loop-mediated interactions. It is also quite interesting to find that the increase of the TBA copy number can unitize the diversity of TBA conformation to the best-fit binding structure for thrombin. The methodology developed in this work can be used for studying other repeat sequences in the genome, such as telomeric DNA as well as interactions of ssDNA with the binding molecule.

A DNA-Based Biosensor Assay for the Kinetic Characterization of Ion-Dependent Aptamer Folding and Protein Binding

Therapeutic and diagnostic nucleic acid aptamers are designed to bind tightly and specifically to their target. The combination of structural and kinetic analyses of aptamer interactions has gained increasing importance. Here, we present a fluorescence-based switchSENSE aptasensor for the detailed kinetic characterization of aptamer-analyte interaction and aptamer folding, employing the thrombin-binding aptamer (TBA) as a model system. Thrombin-binding aptamer folding into a G-quadruplex and its binding to thrombin strongly depend on the type and concentration of ions present in solution. We observed conformational changes induced by cations in real-time and determined the folding and unfolding kinetics of the aptamer. The aptamer's affinity for K⁺ was found to be more than one order of magnitude higher than for other cations (K⁺ > NH₄⁺ >> Na⁺ > Li⁺). The aptamer's affinity to its protein target thrombin in the presence of different cations followed the same trend but differed by more than three orders of magnitude (K_D = 0.15 nM to 250 nM). While the stability (k_OFF) of the thrombin-TBA complex was similar in all conditions, the cation type strongly influenced the association rate (k_ON). These results demonstrated that protein-aptamer binding is intrinsically related to the correct aptamer fold and, hence, to the presence of stabilizing ions. Because fast binding kinetics with on-rates exceeding 10⁸ M^-1s^-1 can be quantified, and folding-related phenomena can be directly resolved, switchSENSE is a useful analytical tool for in-depth characterization of aptamer-ion and aptamer-protein interactions.

Recent Advances in Aptamer Discovery and Applications

Aptamers are short, single-stranded DNA, RNA, or synthetic XNA molecules that can be developed with high affinity and specificity to interact with any desired targets. They have been widely used in facilitating discoveries in basic research, ensuring food safety and monitoring the environment. Furthermore, aptamers play promising roles as clinical diagnostics and therapeutic agents. This review provides update on the recent advances in this rapidly progressing field of research with particular emphasis on generation of aptamers and their applications in biosensing, biotechnology and medicine. The limitations and future directions of aptamers in target specific delivery and real-time detection are also discussed.

Regulatory-sequence mechanical biosensor: A versatile platform for investigation of G-quadruplex/label-free protein interactions and tunable protein detection

Mechanical biosensors can be used to quantitatively explore DNA-protein binding mechanisms by detecting targets at low concentrations or measuring force in single-molecule force spectroscopy. However, restrictions in single-molecule manipulation and labelling protocols have hindered the application for bulk analysis of label-free protein detection. Here, we present the integration of molecular force measurement and finely tunable detection of label-free proteins into one mechanical sensor. Regulatory-sequence force spectroscopy was obtained to investigate the binding force of DNA G-quadruplexes (GQ) and label-free protein. The dual control of regulatory sequences and mechanical forces induces the structure switching from DNA duplex to GQ/protein complex. It exhibits a synergistic effect, enabling the rational fine-tuning of the dynamic range for biosensing protein concentrations over eight orders of magnitude. Furthermore, this method was exploited to estimate the stability of the human telomeric DNA GQ by Ku protein and ligand methylpyridostatin. The results revealed that human telomeric GQ has two different binding sites for Ku protein and ligand. Force spectroscopy integrating label-free force measurement and tunable target detection holds great promise for use in biosensing, drug screening, targeted therapies, DNA nanotechnology, and fields in which GQ are of rapidly increasing importance.

Glycan-Imprinted Magnetic Nanoparticle-Based SELEX for Efficient Screening of Glycoprotein-Binding Aptamers

Nucleic acid aptamers, as useful alternatives of antibodies, have found a large range of promising applications such as affinity separation and bioassays. The screening of aptamers is critical for their applications. Aptamers are often screened by an in vitro methodology called SELEX (systematic evolution of ligands by exponential enrichment). Although numerous SELEX methods have been established to facilitate the selection, new efficient selection methods are still much needed. Molecularly imprinted polymers, which are antibody alternatives at the material level and competitors of aptamers, have not been used as a platform for aptamer selection yet so far. In this study, a glycan-imprinted magnetic nanoparticles (MNPs)-based SELEX was developed to efficiently screen aptamers against glycoproteins. Glycan-imprinted MNPs were used as an affinity interface to bind target glycoprotein, and then the target glycoprotein-bound MNPs were used as an affinity substrate for aptamer selection. The glycan-imprinted MNPs were synthesized by a state-of-the-art imprinting approach called boronate affinity controllable oriented surface imprinting. The glycan-imprinted MNPs exhibited high affinity and specificity and therefore allowed preferential binding toward target glycoproteins while excluding unwanted species. Two representative glycoproteins, including RNase B and transferrin, were employed as target glycoproteins, and aptamers with high affinity and specificity toward the two target glycoproteins were screened out in 3 rounds. This method exhibited some merits, such as high affinity, fast speed, and avoiding negative screening. Therefore, the glycan-imprinted MNP-based SELEX approach holds great values for the efficient screening of high-performance aptamers.

Recent Advances in SELEX Technology and Aptamer Applications in Biomedicine

Aptamers are short DNA/RNA oligonucleotides capable of binding to target molecules with high affinity and specificity. The process of selecting an aptamer is called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Thanks to the inherit merits, aptamers have been used in a wide range of applications, including disease diagnosis, targeted delivery agents and therapeutic uses. To date, great achievements regarding the selection, modifications and application of aptamers have been made. However, few aptamer-based products have already successfully entered into clinical and industrial use. Besides, it is still a challenge to obtain aptamers with high affinity in a more efficient way. Thus, it is important to comprehensively review the current shortage and achievement of aptamer-related technology. In this review, we first present the limitations and notable advances of aptamer selection. Then, we compare the different methods used in the kinetic characterization of aptamers. We also discuss the impetus and developments of the clinical application of aptamers.

Development of Phosphorothioate DNA and DNA Thioaptamers

Nucleic acid aptamers are short RNA- or DNA-based affinity reagents typically selected from combinatorial libraries to bind to a specific target such as a protein, a small molecule, whole cells or even animals. Aptamers have utility in the development of diagnostic, imaging and therapeutic applications due to their size, physico-chemical nature and ease of synthesis and modification to suit the application. A variety of oligonucleotide modifications have been used to enhance the stability of aptamers from nuclease degradation in vivo. The non-bridging oxygen atoms of the phosphodiester backbones of RNA and DNA aptamers can be substituted with one or two sulfur atoms, resulting in thioaptamers with phosphorothioate or phosphorodithioate linkages, respectively. Such thioaptamers are known to have increased binding affinity towards their target, as well as enhanced resistance to nuclease degradation. In this review, we discuss the development of phosphorothioate chemistry and thioaptamers, with a brief review of selection methods.

Aptamers as targeted therapeutics: current potential and challenges

Nucleic acid aptamers, often termed 'chemical antibodies', are functionally comparable to traditional antibodies, but offer several advantages, including their relatively small physical size, flexible structure, quick chemical production, versatile chemical modification, high stability and lack of immunogenicity. In addition, many aptamers are internalized upon binding to cellular receptors, making them useful targeted delivery agents for small interfering RNAs (siRNAs), microRNAs and conventional drugs. However, several crucial factors have delayed the clinical translation of therapeutic aptamers, such as their inherent physicochemical characteristics and lack of safety data. This Review discusses these challenges, highlighting recent clinical developments and technological advances that have revived the impetus for this promising class of therapeutics.

ABCs of DNA aptamer and related assay development

This review is intended to guide the novice in aptamer research and development to understand virtually all of the aptamer development options and currently available assay modalities. Aptamer development topics range from discussions of basic and advanced versions of Systematic Evolution of Ligands by EXponential Enrichment (SELEX) and SELEX variations involving incorporation of exotic unnatural nucleotides to expand library diversity for even greater aptamer affinity and specificity to improved next generation methods of DNA sequencing, screening and tracking aptamer development throughout the SELEX process and characterization of lead aptamer candidates. Aptamer assay development topics include descriptions of various colorimetric and fluorescent assays in microplates or on membranes including homogeneous beacon and multiplexed Fluorescence Resonance Energy Transfer (FRET) assays. Finally, a discussion of the potential for marketing successful aptamer-based assays or test kits is included.

Aptamers as a promising approach for the control of parasitic diseases

Aptamers are short single-stranded RNA or DNA oligonucleotides that are capable of binding various biological targets with high affinity and specificity. Their identification initially relies on a molecular process named SELEX (Systematic Evolution of Ligands by EXponential enrichment) that has been later modified in order to improve aptamer sensitivity, minimize duration and cost of the assay, as well as increase target types. Several biochemical modifications can help to enhance aptamer stability without affecting significantly target interaction. As a result, aptamers have generated a large interest as promising tools to compete with monoclonal antibodies for detection and inhibition of specific markers of human diseases. One aptamer-based drug is currently authorized and several others are being clinically evaluated. Despite advances in the knowledge of parasite biology and host–parasite interactions from “omics” data, protozoan parasites still affect millions of people around the world and there is an urgent need for drug target discovery and novel therapeutic concepts. In this context, aptamers represent promising tools for pathogen identification and control. Recent studies have reported the identification of “aptasensors” for parasite diagnosis, and “intramers” targeting intracellular proteins. Here we discuss various strategies that have been employed for intracellular expression of aptamers and expansion of their possible application, and propose that they may be suitable for the clinical use of aptamers in parasitic infections.

Aptamers and Their Significant Role in Cancer Therapy and Diagnosis

Aptamers are nucleic acid/peptide molecules that can be generated by a sophisticated, well-established technique known as Systematic Evolution of Ligands by EXponential enrichment (SELEX). Aptamers can interact with their targets through structural recognition, as in antibodies, though with higher specificity. With this added advantage, they can be made useful for clinical applications such as targeted therapy and diagnosis. In this review, we have discussed the steps involved in SELEX process and modifications executed to attain high affinity nucleic acid aptamers. Moreover, our review also highlights the therapeutic applications of aptamer functionalized nanoparticles and nucleic acids as chemo-therapeutic agents. In addition, we have described the development of "aptasensor" in clinical diagnostic application for detecting cancer cells and the use of aptamers in different routine imaging techniques, such as Positron Emission Tomography/Computed Tomography, Ultrasound, and Magnetic Resonance Imaging.

Highly stable aptamers selected from a 2'-fully modified fGmH RNA library for targeting biomaterials

When developed as targeting ligands for the in vivo delivery of biomaterials to biological systems, RNA aptamers immediately face numerous obstacles, in particular nuclease degradation and post-selection 2' modification. This study aims to develop a novel class of highly stable, 2'-fully modified RNA aptamers that are ideal for the targeted delivery of biomaterials. We demonstrated the facile transcription of a fGmH (2'-F-dG, 2'-OMe-dA/dC/dU) RNA library with unexpected hydrophobicity, the direct selection of aptamers from a fGmH RNA library that bind Staphylococcus aureus Protein A (SpA) as a model target, and the superior nuclease and serum stability of these aptamers compared to 2'-partially modified RNA variants. Characterizations of fGmH RNA aptamers binding to purified SpA and to endogenous SpA present on the surface of S. aureus cells demonstrate fGmH RNA aptamer selectivity and stability. Significantly, fGmH RNA aptamers were able to functionalize, stabilize, and specifically deliver aggregation-prone silver nanoparticles (AgNPs) to S. aureus with SpA-dependent antimicrobial effects. This study describes a novel aptamer class with considerable potential to improve the in vivo applicability of nucleic acid-based affinity molecules to biomaterials.

Aptamer-based biosensors for biomedical diagnostics

Aptamers are single-stranded nucleic acids that selectively bind to target molecules. Most aptamers are obtained through a combinatorial biology technique called SELEX. Since aptamers can be isolated to bind to almost any molecule of choice, can be readily modified at arbitrary positions and they possess predictable secondary structures, this platform technology shows great promise in biosensor development. Over the past two decades, more than one thousand papers have been published on aptamer-based biosensors. Given this progress, the application of aptamer technology in biomedical diagnosis is still in a quite preliminary stage. Most previous work involves only a few model aptamers to demonstrate the sensing concept with limited biomedical impact. This Critical Review aims to summarize progress that might enable practical applications of aptamers for biological samples. First, general sensing strategies based on the unique properties of aptamers are summarized. Each strategy can be coupled to various signaling methods. Among these, a few detection methods including fluorescence lifetime, flow cytometry, upconverting nanoparticles, nanoflare technology, magnetic resonance imaging, electronic aptamer-based sensors, and lateral flow devices have been discussed in more detail since they are more likely to work in a complex sample matrix. The current limitations of this field include the lack of high quality aptamers for clinically important targets. In addition, the aptamer technology has to be extensively tested in a clinical sample matrix to establish reliability and accuracy. Future directions are also speculated to overcome these challenges.

Bovine thrombin enhances the efficiency and specificity of polymerase chain reaction

The polymerase chain reaction (PCR) has become one of the central techniques in molecular biology since its invention. However, PCR can be fraught with difficulties in various situations, and it is desirable to find novel PCR enhancers suitable for universal applications. Here we show that bovine thrombin (BT), a well-known coagulation protein, is exceptionally effective at preventing the formation of primer dimers and enhancing the formation of the desired PCR products. The PCR enhancement effects of BT were demonstrated by testing various types of samples, including low-copy synthetic single-stranded DNAs (ssDNAs), synthetic ssDNA pools, human genomic DNA, and hepatitis B virus genomic DNA. In addition, BT was also able to effectively relieve PCR inhibition by nanomaterial inhibitors such as gold nanoparticles (AuNPs) and graphene oxide (GO). Compared with BSA, one of the most popular PCR enhancers, BT was more effective and required concentrations 18-178 times less than that of BSA to achieve a similar level of PCR enhancement.

Cell-specific aptamers and their conjugation with nanomaterials for targeted drug delivery

INTRODUCTION

Aptamers are short, single-stranded DNA or RNA sequences that can fold into complex secondary and tertiary structures and bind to various target molecules with high affinity and specificity. These properties, as well as rapid tissue penetration and ease of chemical modification, make aptamers ideal recognition elements for in vivo targeted drug delivery and attractive molecules for use in disease diagnosis and therapy.

AREAS COVERED

The general properties of aptamers as well as advantages over their counterpart antibodies are briefly discussed. Next, aptamer selection by cell- systematic evolution of ligands by exponential enrichment is described in detail. Finally, the review summarizes recent progress in the field of targeted drug delivery based on aptamers and their conjugation to liposomes, micelles and other nanomaterials.

EXPERT OPINION

Advances in nanotechnology have led to new and improved nanomaterials for biomedical applications. Conjugation of nanoparticles (NPs) with aptamers exploits both technologies, making aptamer-NP conjugates ideal agents for drug delivery with proven therapeutic effects and the reduction of toxicity to normal tissue. The use of multivalent aptamer-conjugated nanomaterials represents one of the new directions for drug development in the future; as such, continuing studies of these multivalent aptamers and bioconjugates should result in important clinical applications in targeted drug delivery.

Structural evaluation of the DNA aptamer for ATP DH25.42 by AFM

Atomic force microscopy (AFM) can dynamically detect the adhesion or affinity force between a sample surface and a cantilever. This feature could be used to analyze bio-molecular interactions between a DNA aptamer and a target molecule. In this study, the binding force between adenosine triphosphate (ATP) and the anti-ATP DNA aptamer DH25.42, based on structural changes was measured using AFM. In addition, the relationship between the cations in the binding buffer, and the affinity and structure formation of the DNA aptamer was also evaluated using AFM and circular dichroism (CD) spectrum analysis. As a result, the specific force between DH25.42 and ATP could be measured by AFM. Moreover, it was suggested that Mg(2+) in the binding buffer was critical to the binding function of DH25.42 to ATP.

Atomic force microscopy fishing and mass spectrometry identification of gp120 on immobilized aptamers

Atomic force microscopy (AFM) was applied to carry out direct and label-free detection of gp120 human immunodeficiency virus type 1 envelope glycoprotein as a target protein. This approach was based on the AFM fishing of gp120 from the analyte solution using anti-gp120 aptamers immobilized on the AFM chip to count gp120/aptamer complexes that were formed on the chip surface. The comparison of image contrasts of fished gp120 against the background of immobilized aptamers and anti-gp120 antibodies on the AFM images was conducted. It was shown that an image contrast of the protein/aptamer complexes was two-fold higher than the contrast of the protein/antibody complexes. Mass spectrometry identification provided an additional confirmation of the target protein presence on the AFM chips after biospecific fishing to avoid any artifacts.

New Technologies Provide Quantum Changes in the Scale, Speed, and Success of SELEX Methods and Aptamer Characterization

Single-stranded oligonucleotide aptamers have attracted great attention in the past decade because of their diagnostic and therapeutic potential. These versatile, high affinity and specificity reagents are selected by an iterative in vitro process called SELEX, Systematic Evolution of Ligands by Exponential Enrichment. Numerous SELEX methods have been developed for aptamer selections; some that are simple and straightforward, and some that are specialized and complicated. The method of SELEX is crucial for selection of an aptamer with desired properties; however, success also depends on the starting aptamer library, the target molecule, aptamer enrichment monitoring assays, and finally, the analysis and characterization of selected aptamers. Here, we summarize key recent developments in aptamer selection methods, as well as other aspects of aptamer selection that have significant impact on the outcome. We discuss potential pitfalls and limitations in the selection process with an eye to aid researchers in the choice of a proper SELEX strategy, and we highlight areas where further developments and improvements are desired. We believe carefully designed multiplexed selection methods, when complemented with high-throughput downstream analysis and characterization assays, will yield numerous high-affinity aptamers to protein and small molecule targets, and thereby generate a vast array of reagents for probing basic biological mechanisms and implementing new diagnostic and therapeutic applications in the near future.

Directly investigating the interaction between aptamers and thrombin by atomic force microscopy

Aptamers are single-stranded nucleic acid molecules that can be used for protein recognition, detection, and inhibition. Over the past decades, two thrombin-binding aptamers (15apt and 27apt) were reported by systemic evolution of ligands by exponential enrichment technique. Though many studies have been reported about the interactions between the aptamers and thrombin by atomic force microscopy, the thrombins in those studies were all immobilized by chemical agents. Recently, we developed a new method using atomic force microscopy to directly investigate the specific interactions between thrombin and its two aptamers without immobilizing the thrombin. Furthermore, the unbinding dynamics and dissociation energy landscapes of aptamer/thrombin were discussed. The results indicate that the underlying interaction mechanisms of thrombin with its two aptamers will be similar despite that the structures of 15apt and 27apt are different in buffer solution.

Screening of DNA aptamers against myoglobin using a positive and negative selection units integrated microfluidic chip and its biosensing application

An aptamer screening method using a positive and negative selection units integrated microfluidic chip was introduced. Here, myoglobin (Myo), one of the early markers to increase after acute myocardial infarction, was used as the model. After 7-round selection, the aptamers, which exhibited dissociation constants (K(d)) in the nanomolar range (from 4.93 to 6.38 nM), were successfully obtained using a positive and negative selection units integrated microfluidic chip. The aptamer with the highest affinity (K(d) = 4.93 nM) was then used for the fabrication of a label-free supersandwich electrochemical biosensor for Myo detection based on target-induced aptamer displacement. The detection limit of this aptamer-based electrochemical biosensor was 10 pM, which was significantly lower than that of those previous antibody-based biosensors for Myo detection. This work may not only develop a strategy for screening aptamer but also offer promising alternatives to the traditional analytical and immunological methods for Myo detection.

Emerging techniques employed in aptamer-based diagnostic tests

Since aptamers were reported in 1990, research into the applications of aptamers, particularly diagnostic applications, has been growing. Aptamers can act as recognition elements instead of antibodies. In this regard, aptamers have unique characteristics because they are composed of nucleic acids. Intra- and intermolecular interactions of nucleic acids can be easily tailored following straightforward hybridization rules. Nucleic acids can be enzymatically replicated and their sequences can be determined using high-throughput methods. Using these properties, ligand-induced structural change-based aptamer sensors for homogeneous assays, polymerase- and/or nuclease-combined aptamer sensors for ultrasensitive assays, and microarray/next-generation sequencing-based aptamer sensors for multiplexed assays have been developed. This article reviews these unique aptamer sensors, demonstrating their great potential for diagnostic applications.

Upregulation of SIRT1 by 17β-estradiol depends on ubiquitin-proteasome degradation of PPAR-γ mediated by NEDD4-1

17β-estradiol (E2) treatment of cells results in an upregulation of SIRT1 and a down-regulation of PPARγ. The decrease in PPARγ expression is mediated by increased degradation of PPARγ. Here we report that PPARγ is ubiquitinated by HECT E3 ubiquitin ligase NEDD4-1 and degraded, along with PPARγ, in response to E2 stimulation. The PPARγ interacts with ubiquitin ligase NEDD4-1 through a conserved PPXY-WW binding motif. The WW3 domain in NEDD4-1 is critical for binding to PPARΓ. NEDD4-1 overexpression leads to PPARγ ubiquitination and reduced expression of PPARγ. Conversely, knockdown of NEDD4-1 by specific siRNAs abolishes PPARΓ ubiquitination. These data indicate that NEDD4-1 is the E3 ubiquitin ligase responsible for PPARγ ubiquitination. Here, we show that NEDD4-1 delays cellular senescence by degrading PPARΓ expression. Taken together, our data show that E2 could upregulate SIRT1 expression via promoting the PPARΓ ubiquitination-proteasome degradation pathway to delay the process of cell senescence.

An ultra-sensitive detection of a whole virus using dual aptamers developed by immobilization-free screening

In this study, we successfully developed a ssDNA aptamer pairs by using an advanced immobilization-free SELEX method with affinity-based selection and counter-screening process at every round. By implementing this method, two different aptamers specifically binding to bovine viral diarrhea virus type 1(BVDV type 1) with high affinity were successfully screened. This aptamer pair was applied to ultrasensitive detection platform for BVDV type 1 in a sandwich manner. The ultrasensitive detection of BVDV type 1 using one of aptamers conjugated with gold nanoparticles was obtained in aptamer-aptamer sandwich type sensing format, with the limit of detection of 800 copies/ml, which is comparable to a real-time PCR method.

Mechanism studies on nanoPCR and applications of gold nanoparticles in genetic analysis

Recently, the applications of nanomaterial-assisted polymerase chain reaction (nanoPCR) have received considerable attention. Several potential mechanisms have been proposed, but mainly according to the results of PCR assays under specific conditions and lacking direct and general evidence. The mechanism of nanoPCR has not been elucidated yet. Here, taking gold nanoparticles (AuNPs) as an example, we report the three general effects of AuNPs: (1) AuNPs adsorb polymerase and modulate the amount of active polymerase in PCR, which was directly demonstrated by a simple and straightforward colorimetric assay and the dynamic light scattering measurements. (2) AuNPs adsorb primers and decrease the melting temperatures (Tm) of the duplexes formed with perfectly matched and mismatched primers and increase the Tm difference between them. (3) AuNPs adsorb PCR products and facilitate the dissociation of them in the denaturing step. All these effects were confirmed by addition of a rationally selected surface adsorbent, bovine thrombin, to highly efficiently modulate the surface adsorption of PCR components. These findings suggested that AuNPs should have multiple effects on PCR: (1) to regulate PCR in a case-by-case way via modulating the amount of active polymerase in PCR; (2) to improve PCR specificity in the annealing step via increasing the Tm difference between the perfectly matched and mismatched primers; (3) to improve PCR efficiency via speeding up the dissociation of the PCR products in the denaturing step. Taken together, we proposed the mechanism of nanoPCR is that the surface interaction of PCR components (polymerase, primers, and products) with AuNPs regulates nanoPCR. We further demonstrated that the applications of these findings improve the PCR of the amelogenin genes and Hepatitis B virus gene for genetic analysis. These findings could also provide helpful insight for the applications of other nanomaterials in nanoPCR.

The selection of DNA aptamers for two different epitopes of thrombin was not due to different partitioning methods

Nearly all aptamers identified so far for any given target molecule have been specific for the same binding site (epitope). The most notable exception to the 1 aptamer per target molecule rule is the pair of DNA aptamers that bind to different epitopes of thrombin. This communication refutes the suggestion that these aptamers exist because different partitioning methods were used when they were selected. The possibility that selection of these aptamers was biased by conflicting secondary structures was also investigated and found not to contribute. The preparation of protein-coated magnetic beads for systematic evolution of ligands by exponential enrichment (SELEX) and the different specificities of the thrombin aptamers for the α and β forms of thrombin are also reported.

Challenges and opportunities for small molecule aptamer development

Aptamers are single-stranded oligonucleotides that bind to targets with high affinity and selectivity. Their use as molecular recognition elements has emerged as a viable approach for biosensing, diagnostics, and therapeutics. Despite this potential, relatively few aptamers exist that bind to small molecules. Small molecules are important targets for investigation due to their diverse biological functions as well as their clinical and commercial uses. Novel, effective molecular recognition probes for these compounds are therefore of great interest. This paper will highlight the technical challenges of aptamer development for small molecule targets, as well as the opportunities that exist for their application in biosensing and chemical biology.

Challenges and opportunities for small molecule aptamer development

Aptamers are single-stranded oligonucleotides that bind to targets with high affinity and selectivity. Their use as molecular recognition elements has emerged as a viable approach for biosensing, diagnostics, and therapeutics. Despite this potential, relatively few aptamers exist that bind to small molecules. Small molecules are important targets for investigation due to their diverse biological functions as well as their clinical and commercial uses. Novel, effective molecular recognition probes for these compounds are therefore of great interest. This paper will highlight the technical challenges of aptamer development for small molecule targets, as well as the opportunities that exist for their application in biosensing and chemical biology.

Strategies for the discovery of therapeutic aptamers

IMPORTANCE OF THE FIELD

Therapeutic aptamers are synthetic, structured oligonucleotides that bind to a very broad range of targets with high affinity and specificity. They are an emerging class of targeting ligand that show great promise for treating a number of diseases. A series of aptamers currently in various stages of clinical development highlights the potential of aptamers for therapeutic applications.

AREAS COVERED IN THIS REVIEW

This review covers in vitro selection of oligonucleotide ligands, called aptamers, from a combinatorial library using the Systematic Evolution of Ligands by Exponential Enrichment process as well as the other known strategies for finding aptamers against various targets.

WHAT THE READER WILL GAIN

Readers will gain an understanding of the highly useful strategies for successful aptamer discovery. They may also be able to combine two or more of the presented strategies for their aptamer discovery projects.

TAKE HOME MESSAGE

Although many processes are available for discovering aptamers, it is not easy to discover an aptamer candidate that is ready to move toward pharmaceutical drug development. It is also apparent that there have been relatively few therapeutic advances and clinical trials undertaken due to the small number of companies that participate in aptamer development.

Single-stranded DNA (ssDNA) production in DNA aptamer generation

The discovery that synthetic short chain nucleic acids are capable of selective binding to biological targets has made them to be widely used as molecular recognition elements. These nucleic acids, called aptamers, are comprised of two types, DNA and RNA aptamers, where the DNA aptamer is preferred over the latter due to its stability, making it widely used in a number of applications. However, the success of the DNA selection process through Systematic Evolution of Ligands by Exponential Enrichment (SELEX) experiments is very much dependent on its most critical step, which is the conversion of the dsDNA to ssDNA. There is a plethora of methods available in generating ssDNA from the corresponding dsDNA. These include asymmetric PCR, biotin-streptavidin separation, lambda exonuclease digestion and size separation on denaturing-urea PAGE. Herein, different methods of ssDNA generation following the PCR amplification step in SELEX are reviewed.

Characterization of enhanced monovalent and bivalent thrombin DNA aptamer binding using single molecule force spectroscopy

Thrombin aptamer binding strength and stability is dependent on sterical parameters when used for atomic force microscopy sensing applications. Sterical improvements on the linker chemistry were developed for high-affinity binding. For this we applied single molecule force spectroscopy using two enhanced biotinylated thrombin aptamers, BFF and BFA immobilized on the atomic force microscopy tip via streptavidin. BFF is a dimer composed of two single-stranded aptamers (aptabody) connected to each other by a complementary sequence close to the biotinylated end. In contrast, BFA consists of a single DNA strand and a complementary strand in the supporting biotinylated part. By varying the pulling velocity in force-distance cycles the formed thrombin-aptamer complexes were ruptured at different force loadings allowing determination of the energy landscape. As a result, BFA aptamer showed a higher binding force at the investigated loading rates and a significantly lower dissociation rate constant, k(off), compared to BFF. Moreover, the potential of the aptabody BFF to form a bivalent complex could clearly be demonstrated.

Investigation of the interaction between a bivalent aptamer and thrombin by AFM

Aptamers are a new class of molecular probes for protein recognition, detection, and inhibition. Multivalent aptamer-protein binding through aptamer assembly has been currently developed as an effective way to achieve higher protein affinity and selectivity. In this study, the specific interaction between bivalent aptamer Bi-8S and thrombin has been measured directly and quantitatively by atomic force microscopy to investigate the unbinding dynamics and dissociation energy landscape of the multivalent interaction. Bivalent aptamer Bi-8S contains thrombin's two aptamers, 15apt and 27apt, which are linked by eight spacer phosphoramidites. The results revealed the sequential dissociation of the two aptamers. Moreover, the dynamic force spectroscopy data revealed that the 27apt's binding to the thrombin remains largely unaffected by the eight-spacer phosphoramidites within Bi-8S. In contrast, the eight-spacer phosphoramidites stabilized the 15apt-thrombin binding.

Challenges and opportunities for small molecule aptamer development

Aptamers are single-stranded oligonucleotides that bind to targets with high affinity and selectivity. Their use as molecular recognition elements has emerged as a viable approach for biosensing, diagnostics, and therapeutics. Despite this potential, relatively few aptamers exist that bind to small molecules. Small molecules are important targets for investigation due to their diverse biological functions as well as their clinical and commercial uses. Novel, effective molecular recognition probes for these compounds are therefore of great interest. This paper will highlight the technical challenges of aptamer development for small molecule targets, as well as the opportunities that exist for their application in biosensing and chemical biology.

Nucleic acid and peptide aptamers: fundamentals and bioanalytical aspects

In recent years new nucleic acid and protein-based combinatorial molecules have attracted the attention of researchers working in various areas of science, ranging from medicine to analytical chemistry. These molecules, called aptamers, have been proposed as alternatives to antibodies in many different applications. The aim of this Review is to illustrate the peculiarities of these combinatorial molecules which have initially been explored for their importance in molecular medicine, but have enormous potential in other biotechnological fields historically dominated by antibodies, such as bioassays. A description of these molecules is given, and the methods for their selection and production are also summarized. Moreover, critical aspects related to these molecules are discussed.

Immobilization-free screening of aptamers assisted by graphene oxide

Graphene oxide (GO) has the ability to separate free short ssDNA in heterogeneous solution. This feature is applied as a label free platform for screening of aptamers that bind to their target with high affinity and specificity. Herein, we report an aptamer selection strategy for Nampt protein based on GO.