reMelting curve analysis as a tool for enrichment monitoring in the SELEX process

IMPATIENT-qPCR: monitoring SELEX success during in vitro aptamer evolution

SELEX (Systematic Evolution of Ligands by Exponential enrichment) processes aim on the evolution of high-affinity aptamers as binding entities in diagnostics and biosensing. Aptamers can represent game-changers as constituents of diagnostic assays for the management of instantly occurring infectious diseases or other health threats. Without in-process quality control measures SELEX suffers from low overall success rates. We present a quantitative PCR method for fast and easy quantification of aptamers bound to their targets. Simultaneous determination of melting temperatures (Tm) of each SELEX round delivers information on the evolutionary success via the correlation of increasing GC content and Tm alone with a round-wise increase of aptamer affinity to the respective target. Based on nine successful and published previous SELEX processes, in which the evolution/selection of aptamer affinity/specificity was demonstrated, we here show the functionality of the IMPATIENT-qPCR for polyclonal aptamer libraries and resulting individual aptamers. Based on the ease of this new evolution quality control, we hope to introduce it as a valuable tool to accelerate SELEX processes in general. IMPATIENT-qPCR SELEX success monitoring. Selection and evolution of high-affinity aptamers using SELEX technology with direct aptamer evolution monitoring using melting curve shifting analyses to higher Tm by quantitative PCR with fluorescence dye SYBR Green I. KEY POINTS: • Fast and easy analysis. • Universal applicability shown for a series of real successful projects.

Generation of epitope-specific hCG aptamers through a novel targeted selection approach

Human chorionic gonadotropin (hCG) is a glycoprotein hormone used as a biomarker for several medical conditions, including pregnancy, trophoblastic and nontrophoblastic cancers. Most commercial hCG tests rely on a combination of antibodies, one of which is usually specific to the C-terminal peptide of the β-subunit. However, cleavage of this region in many hCG degradation variants prevents rapid diagnostic tests from quantifying all hCG variants in serum and urine samples. An epitope contained within the core fragment, β1, represents an under-researched opportunity for developing immunoassays specific to most variants of hCG. In the study described here, we report on a SELEX procedure tailored towards the identification of two pools of aptamers, one specific to the β-subunit of hCG and another to the β1 epitope within it. The described SELEX procedure utilized antibody-blocked targets, which is an underutilized strategy to exert negative selection pressure and in turn direct aptamer enrichment to a specific epitope. We report on the first aptamers, designated as R4_64 and R6_5, each capable of recognising two distinct sites of the hCG molecule-the β-subunit and the (presumably) β1-epitope, respectively. This study therefore presents a new SELEX approach and the generation of novel aptamer sequences that display potential hCG-specific biorecognition.

Development of a Novel DNA Aptamer Targeting Colorectal Cancer Cell-Derived Small Extracellular Vesicles as a Potential Diagnostic and Therapeutic Agent

Colorectal cancer (CRC) as the second leading cause of global cancer deaths poses critical challenges in clinical settings. Cancer-derived small extracellular vesicles (sEVs), which are secreted by cancer cells, have been shown to mediate tumor development, invasion, and even metastasis, and have thus received increasing attention for the development of cancer diagnostic or therapeutic platforms. In the present study, the sEV-targeted systematic evolution of ligands by exponential enrichment (E-SELEX) is developed to generate a high-quality aptamer (CCE-10F) that recognizes and binds to CRC-derived sEVs. Via an in-depth investigation, it is confirmed that this novel aptamer possesses high affinity (Kd = 3.41 nm) for CRC-derived sEVs and exhibits a wide linear range (2.0 × 104 -1.0 × 106 particles µL-1 ) with a limit of detection (LOD) of 1.0 × 103 particles µL-1 . Furthermore, the aptamer discriminates CRC cell-derived sEVs from those derived from normal colon cell, human serum, and other cancer cells, showing high specificity for CRC cell-derived sEVs and significantly suppresses the critical processes of metastasis, including cellular migration, invasion, and angiogenesis, which are originally induced by sEVs themselves. These findings are highly encouraging for the potential use of the aptamer in sEV-based diagnostic and therapeutic applications.

Accomplishment of one-step specific PCR and evaluated SELEX process by a dual-microfluidic amplified system

One of the main obstacles for systematic evolution of ligands by exponential enrichment (SELEX) failure is the generation of a non-specific product, as selection-inherent amplification procedures tend to form by-products, which prevents the enrichment of target-binding aptamers. Herein, we reported a dual-microfluidic amplified system (dual-MAS) based on the real-time polymerase chain reaction (PCR) detection chip and the large volume PCR chip for one-step specific PCR and for evaluating the SELEX process. First, it is a simple method to accomplish analytical PCR and amplification PCR in one step, and the optimal number of cycles for generating the specific PCR product is the cycles when the slope of the linear amplification period of the real-time PCR curve begins to decrease. Second, the time used by the dual-MAS for generating a specific PCR product is reduced to 30 min, and the multi-functional dual-MAS can simultaneously evaluate the SELEX process by providing important information on the amounts of enriched sequences and the library diversity in every round of SELEX. In addition, pollution contamination and fragment loss can be significantly avoided in the closed chip. Last, the specific PCR product, the amounts of enriched sequences, and the library diversity can be obtained for every single SELEX in just 30 min. Compared with current methods, this system can reduce the time for generating a specific PCR product and SELEX, and it is easier to choose the optimal number of cycles for a specific PCR product. In a word, it is a sensitive, simple, and rapid strategy to improve the specificity of the PCR product and make the process of SELEX in a controlled way.

DNA aptamers against bacterial cells can be efficiently selected by a SELEX process using state-of-the art qPCR and ultra-deep sequencing

DNA aptamers generated by cell-SELEX against bacterial cells have gained increased interest as novel and cost-effective affinity reagents for cell labelling, imaging and biosensing. Here we describe the selection and identification of DNA aptamers for bacterial cells using a combined approach based on cell-SELEX, state-of-the-art applications of quantitative real-time PCR (qPCR), next-generation sequencing (NGS) and bioinformatic data analysis. This approach is demonstrated on Enterococcus faecalis (E. faecalis), which served as target in eleven rounds of cell-SELEX with multiple subtractive counter-selections against non-target species. During the selection, we applied qPCR-based analyses to evaluate the ssDNA pool size and remelting curve analysis of qPCR amplicons to monitor changes in pool diversity and sequence enrichment. Based on NGS-derived data, we identified 16 aptamer candidates. Among these, aptamer EF508 exhibited high binding affinity to E. faecalis cells (K_D-value: 37 nM) and successfully discriminated E. faecalis from 20 different Enterococcus and non-Enterococcus spp. Our results demonstrate that this combined approach enabled the rapid and efficient identification of an aptamer with both high affinity and high specificity. Furthermore, the applied monitoring and assessment techniques provide insight into the selection process and can be highly useful to study and improve experimental cell-SELEX designs to increase selection efficiency.

Therapeutic Interventions into Innate Immune Diseases by Means of Aptamers

The immune system plays a crucial role in the body's defense system against various pathogens, such as bacteria, viruses, and parasites, as well as recognizes non-self- and self-molecules. The innate immune system is composed of special receptors known as pattern recognition receptors, which play a crucial role in the identification of pathogen-associated molecular patterns from diverse microorganisms. Any disequilibrium in the activation of a particular pattern recognition receptor leads to various inflammatory, autoimmune, or immunodeficiency diseases. Aptamers are short single-stranded deoxyribonucleic acid or ribonucleic acid molecules, also termed "chemical antibodies," which have tremendous specificity and affinity for their target molecules. Their features, such as stability, low immunogenicity, ease of manufacturing, and facile screening against a target, make them preferable as therapeutics. Immune-system-targeting aptamers have a great potential as a targeted therapeutic strategy against immune diseases. This review summarizes components of the innate immune system, aptamer production, pharmacokinetic characteristics of aptamers, and aptamers related to innate-immune-system diseases.

Evaluation of different direct and indirect SELEX monitoring methods and implementation of melt-curve analysis for rapid discrimination of variant aptamer sequences

Systematic Evolution of Ligands by Exponential enrichment (SELEX) is an iterative method for in vitro selection of aptamers from a random synthetic oligonucleotide library. Successful retrieving of aptamers by SELEX relies on optimization of various steps including target immobilization, aptamer partitioning, amplification, and ssDNA generation, which all require spending considerable effort and cost. Furthermore, due to the random nature of the initial library, SELEX may redirect toward the selection of low-affinity aptamers that are over-represented in the ssDNA population due to PCR bias. Thus, precise monitoring of the SELEX process is crucial to ensure the selection of target-specific aptamers. In the present study, we investigated the reliability and simplicity of different direct and indirect monitoring methods including UV-Vis spectroscopy, real-time PCR quantification and melt-curve analysis, electrophoretic mobility shift assay (EMSA) and enzyme-linked oligonucleotide assay (ELONA) for selection of DNA aptamers for a protein target. All the examined methods were capable of illustrating the gradual evolution of specific aptamers by the progression of SELEX and showed almost similar results regarding the identification of the enriched round of selection. Moreover, we describe the use of melt-curve analysis in the colony real-time PCR method as a simple, robust, and repeatable tool for pre-sequencing separation of distinct aptamer clones.

SELEX tool: a novel and convenient gel-based diffusion method for monitoring of aptamer-target binding

BACKGROUND

Aptamers, single-stranded DNAs or RNAs, can be selected from a library containing random sequences using a method called Systematic Evolution of Ligands by EXponential Enrichment (SELEX). In SELEX, monitoring the enriching statuses of aptamer candidates during the process is a key step until today. Conformational change of an aptamer caused by target-binding in gel can be used to indicate its statuses of binding.

RESULTS

In this study, an easy-to-implement gel-based diffusion method (GBDM) was developed to monitor the interaction between enriched aptamer candidates and their targets. In order to prove the concept, characterization of aptamers targeting their targets including protein (thrombin) and non-protein molecules (acetamiprid, ATP, atrazine, profenofos and roxithromycin), respectively, were performed using mini gels. Our method has advantages over the common methods including easy performed with labor- and time- saving in experimental operation. The concept has been proven by monitoring enrichment of dynamic aptamer candidate libraries targeting a small molecule 2,2-bis(4-chlorophenyl) acetic acid (DDA) during SELEX process. A mini gel cassette was designed and fabricated by our laboratory to make mini agarose gels for diffusion with different directions.

CONCLUSIONS

These results indicate that GBDM, in particular, chasing diffusion is suitable for monitoring the interaction between enriched aptamer candidates and their targets. These pioneering efforts are helpful for novel aptamer selection by breaking through the technical bottleneck of aptamer development and helpful for development of novel aptasensors.

[Evaluation of the diversity of random DNA-libraries by the shape of amplification curves for estimation of the efficiency of aptamer selection]

Using random (combinatorial) DNA-libraries with various degrees of diversity, it was shown that their amplification by polymerase chain reaction in real time resulted in appearance of a maximum on amplification curves. The relative decrease of fluorescence after passing the maximum was directly proportional to the logarithm of the number of oligonucleotide sequence variants in the random DNA-library provided that this number was within in the interval from 1 to 104 and remained practically unaltered when the number of variants was in the interval from 105 to 108. The obtained dependence was used in the course of SELEX to evaluate changes in the diversity of random DNA-libraries from round to round in selection of DNA-aptamers to the recombinant SMAD4 protein. As a result, oligonucleotides containing sequences able to form a site of SMAD4-DNA interactions known as SBE (SMAD-binding element) have been selected thus indicating that the SMAD4-SBE interaction dominates the aptamer selection.

Inside the Black Box: What Makes SELEX Better?

Aptamers are small oligonucleotides that are capable of binding specifically to a target, with impressive potential for analysis, diagnostics, and therapeutics applications. Aptamers are isolated from large nucleic acid combinatorial libraries using an iterative selection process called SELEX (Systematic Evolution of Ligands by EXponential enrichment). Since being implemented 30 years ago, the SELEX protocol has undergone many modifications and improvements, but it remains a laborious, time-consuming, and costly method, and the results are not always successful. Each step in the aptamer selection protocol can influence its results. This review discusses key technical points of the SELEX procedure and their influence on the outcome of aptamer selection.

Identification of Vigilin as a Potential Ischemia Biomarker by Brain Slice-Based Systematic Evolution of Ligands by Exponential Enrichment

Stroke is one of the leading causes of disability and death among adults worldwide and results in numerous biochemical alterations. However, few efficient biomarkers are clinically available to diagnose stroke because of the limitations of biomarkers and their probes. In this work, we utilized frozen brain slices of middle cerebral artery occlusion (MCAO) in a mouse model of ischemia to select a specific binding aptamer, termed LCW17, by tissue-based SELEX (systematic evolution of ligands by exponential enrichment). LCW17 was enhanced in binding in ischemic brain slices compared to sham control. We identified the binding target of LCW17 as vigilin. Vigilin is increased in ischemia brain slices and exhibits enhanced release from cultured hippocampal neurons after oxygen glucose deprivation in vitro. Taken together, ischemic brain slice-based aptamer selection will enable identification of more probes and potential target molecules for diagnosis and therapy of ischemic stroke. Aptamer LCW17 and vigilin may potentially be applied to define the molecular mechanism underlying ischemic stroke, as well as its diagnosis.

Development of α4 integrin DNA aptamer as a potential therapeutic tool for multiple sclerosis

One of the most important molecules for multiple sclerosis pathogenesis is α4 integrin, which is responsible for autoreactive leukocytes migration into the brain. The monoclonal antibody, natalizumab, was introduced to market for blocking the extravasation of autoreactive leukocytes via inhibition of α4 integrin. However, the disadvantages of antibodies provided a suitable background for other agents to be replaced with antibodies. Considering the profound advantages of aptamers over antibodies, aptamer isolation against α4 integrin was intended in the current study. The α4 integrin-specific aptamers were selected using cell-systematic evolution of ligands by exponential enrichment (SELEX) method with human embryonic kidney (HEK)-293T overexpressing α4 integrin and HEK-293T as target and control cells, respectively. Evaluation of selected aptamer was performed through flow cytometric analysis. The selected clones were then sequenced and analyzed for any possible secondary structure and affinity. The results of this study led to isolation of 13 different single-stranded DNA clones in 11 rounds of selection which were categorized to three clusters based on common structural motifs and the equilibrium dissociation constant (K _d ) of the most stable structure was calculated. The evaluation of SELEX progress showed growth in aptamer affinity with increasing of the number of cycles. Taken together, the findings of this study demonstrated the isolation of α4-specific single-stranded DNA aptamers with suitable affinity for ligand, which can further be replaced with natalizumab.

Focusing aptamer selection on the glycan structure of prostate-specific antigen: Toward more specific detection of prostate cancer

The development of chemical sensors capable of detecting the specific glycosylation patterns of proteins offers a powerful mean for the early detection of cancer. Unfortunately, this strategy is scarcely explored because receptors recognizing the glycans linked to proteins are challenging to discover. In this work, we describe a simple method for directing the selection of aptamers toward the glycan structure of the glycoproteins, with prostate-specific antigen (PSA) as a model target. Using this strategy, we identified one aptamer (PSA-1) that binds the glycan moiety of PSA with reasonable affinity (a dissociation constant of 177 ± 65 nM). Interestingly, an electrochemical sensor with a sandwich format employing the identified aptamer as a signaling receptor, provides a tool of discriminating human PSA from the unglycosylated protein, with a limit of detection of 0.66 ng/mL. The sensor responds to different levels of PSA in serum, correlating well with chemiluminescence ELISA used in hospitals even with higher potential to discriminate clinically meaningful prostate cancer. Although validation on a larger cohort is needed, this is the first demonstration of an aptamer-based sensor to detect PSA by focusing in its glycan moiety.

Developing a combined strategy for monitoring the progress of aptamer selection

The development of a simple, low-cost, time-saving and universally applicable method to monitor the progression of aptamer selection is particularly challenging. Herein, a combined strategy dependent on quantitative polymerase chain reaction amplification curve (AC) and melting curve analysis (MCA) is developed to monitor the convergence of the aptamer species during selection progress. As a parallel and complementary method to affinity tests and binding analyses, the AC-MCA method can be used to achieve the DNA complexity assay when affinity is undetectable. It is independent of the target properties and SELEX methods. Therefore, it has been demonstrated as a universal monitoring tool in different SELEX methods towards different targets (small molecules, proteins, bacteria and cancer cells). The AC-MCA method will facilitate current new aptamer discovery and aptamer-based wide application.

Encoded particle microfluidic platform for rapid multiplexed screening and characterization of aptamers against influenza A nucleoprotein

Aptamers represent interesting bioreceptor alternatives to antibodies when developing a bioassay and are selected by the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) process. After selection, an extensive characterization process is essential to verify the binding affinity and specificity of aptamer candidates, which is the most time-consuming and costly step. In this study, we assessed a new microfluidic platform, namely Evalution™, as a rapid and high throughput aptamer characterization platform. To do this, we first selected aptamers against influenza A nucleoprotein (infA NP) by performing magnetic bead-based SELEX. The selected aptamer candidates were subsequently screened using Evalution™ for their binding kinetics and specificity towards infA NP. All aptamers showed dissociation constants (K_D) in the low nanomolar range (from 13 to 41 nM), and differential binding behavior towards control proteins, such as BSA and influenza B nucleoprotein (infB NP). Among 5 selected candidates, one aptamer (NP5) exhibited a significant discrimination between infA NP and infB NP and was further used to benchmark the kinetic analysis of Evalution™ (K_D = 41 nM) with an SPR platform (K_D = 17 nM). These results suggested that NP5 has the potential to be used for developing sensitive and infA NP specific aptamer-based assay. Moreover, the presented platform proved to be an efficient aptamer characterization tool for performing typical aptamer characterization experiments like binding kinetics (due to the real-time monitoring feature) and specificity assessment in a high-throughput manner due to the multiplexing capacity.

Selection and Identification of Novel Aptamers Specific for Clenbuterol Based on ssDNA Library Immobilized SELEX and Gold Nanoparticles Biosensor

We describe a multiple combined strategy to discover novel aptamers specific for clenbuterol (CBL). An immobilized ssDNA library was used for the selection of specific aptamers using the systematic evolution of ligands by exponential enrichment (SELEX). Progress was monitored using real-time quantitative PCR (Q-PCR), and the enriched library was sequenced by high-throughput sequencing. Candidate aptamers were picked and preliminarily identified using a gold nanoparticles (AuNPs) biosensor. Bioactive aptamers were characterized for affinity, circular dichroism (CD), specificity and sensitivity. The Q-PCR amplification curve increased and the retention rate was about 1% at the eighth round. Use of the AuNPs biosensor and CD analyses determined that six aptamers had binding activity. Affinity analysis showed that aptamer 47 had the highest affinity (Kd = 42.17 ± 8.98 nM) with no cross reactivity to CBL analogs. Indirect competitive enzyme linked aptamer assay (IC-ELAA) based on a 5′-biotin aptamer 47 indicated the limit of detection (LOD) was 0.18 ± 0.02 ng/L (n = 3), and it was used to detect pork samples with a mean recovery of 83.33–97.03%. This is the first report of a universal strategy including library fixation, Q-PCR monitoring, high-throughput sequencing, and AuNPs biosensor identification to select aptamers specific for small molecules.

Selection of Anti-gluten DNA Aptamers in a Deep Eutectic Solvent

Herein, we show the feasibility of using deep eutectic solvents as a faster way of selecting aptamers targeting poorly water-soluble species. This unexplored concept is illustrated for gluten proteins. In this way, aptamer-based gluten detection can be performed directly in the extraction media with improved detectability. We envision deep implications for applications not only in food safety control but also in biomedicine.

In Vitro Selection of Diversely Functionalized Aptamers

We describe the application of T4 DNA ligase-catalyzed DNA templated oligonucleotide polymerization toward the evolution of a diversely functionalized nucleic acid aptamer for human α-thrombin. Using a 256-membered ANNNN comonomer library comprising 16 sublibraries modified with different functional groups, a highly functionalized aptamer for thrombin was raised with a dissociation constant of 1.6 nM. The aptamer was found to be selective for thrombin and required the modifications for binding affinity. This study demonstrates the most differentially functionalized nucleic acid aptamer discovered by in vitro selection and should enable the future exploration of functional group dependence during the evolution of nucleic acid polymer activity.

Real-Time PCR: an Appropriate Approach to Confirm ssDNA Generation from PCR Product in SELEX Process

Background: Aptamers are single stranded DNA (ssDNA) or RNA molecules. The potential of aptamers for binding to the different targets has made them be widely used as the preferred diagnostic and therapeutic tools. DNA aptamers present several advantages over the RNA oligonucleotides due to their higher stability, easier selection, and production. Selection of DNA aptamers which is facilitated through a systematic evolution of ligand by exponential enrichment (SELEX) method is much dependent on the successful conversion of double stranded DNA (dsDNA) to ssDNA. Objective: There are different methods available for ssDNA generation. While visualization of ssDNA is limited to the gelbased method, the method is not applicable in the initial rounds of SELEX due to more than 1015 different sequences. This study was designed to evaluate the effi ciency of another technique for confi rming the ssDNA generation in comparison to the polyacrylamide electrophoresis (PAGE) analysis. Materials and Methods: Real-time PCR was employed in the present study for PCR amplifi cation of the initial library that was followed by enzymatic digestion of the dsDNA. Subsequently melting curve analysis was carried out to evaluate ssDNA generation from dsDNA. Moreover, PAGE analysis was performed and the results were compared with the melt curve analysis. Results: The melt curves, revealed dsDNA conversion to the ssDNA based on a significant reduction of Tm from 73.8 to 41.5 °C. Applying PAGE analysis, it was not effectively feasible to show ssDNA generation from the corresponding initial dsDNA library, while, it was effi cient enough to confirm ssDNA generation in accordance with the increasing the number of SELEX rounds. Conclusion: The present study has proven the applicability of the real-time PCR as a suitable confirmatory technique for validating ssDNA generation in the DNA aptamer selection process for the initial library preparation.

NMR monitoring of the SELEX process to confirm enrichment of structured RNA

RNA aptamers are RNA molecules that bind to a target molecule with high affinity and specificity using uniquely-folded tertiary structures. RNA aptamers are selected from an RNA pool typically comprising up to 1015 different sequences generated by iterative steps of selection and amplification known as Systematic Evolution of Ligands by EXponential enrichment (SELEX). Over several rounds of SELEX, the diversity of the RNA pool decreases and the aptamers are enriched. Hence, monitoring of the enrichment of these RNA pools is critical for the successful selection of aptamers, and several methods for monitoring them have been developed. In this study, we measured one-dimensional imino proton NMR spectra of RNA pools during SELEX. The spectrum of the initial RNA pool indicates that the RNAs adopt tertiary structures. The structural diversity of the RNA pools was shown to depend highly on the design of the primer-binding sequence. Furthermore, we demonstrate that enrichment of RNA aptamers can be monitored using NMR. The RNA pools can be recovered from the NMR tube after measurement of NMR spectra. We also can monitor target binding in the NMR tubes. Thus, we propose using NMR to monitor the enrichment of structured aptamers during the SELEX process.

Screening and identification of DNA aptamers toward Schistosoma japonicum eggs via SELEX

Schistosomiasis is a major parasitic disease caused by blood flukes of the genus Schistosoma. Several million people all over the world are estimated to suffer from severe morbidity as a consequence of schistosomiasis. The worm's eggs, which cause the symptoms of schistosomiasis, are generally used to diagnose the disease. In this study, we employed egg-based systematic evolution of ligands by exponential enrichment (egg-SELEX) and identified a panel of ssDNA aptamers specifically binding to eggs derived from S. japonicum. Among these, two aptamers LC6 and LC15 exhibited strong binding to and specific recognition of S. japonicum eggs, but not eggs from Fasciolopsis buski, Enterobius, Ascaris or Clonorchis sinensis. Furthermore, tissue imaging results revealed that LC15 could recognize S. japonicum eggs laid in liver tissues with a detection ratio of 80.5%. Collectively, therefore, we obtained useful aptamers specifically recognizing S. japonicum eggs, which will facilitate the development of an effective tool for both schistosomiasis diagnosis and drug delivery.

Hi-Fi SELEX: A High-Fidelity Digital-PCR Based Therapeutic Aptamer Discovery Platform

Current technologies for aptamer discovery typically leverage the systematic evolution of ligands by exponential enrichment (SELEX) concept by recursively panning semi-combinatorial ssDNA or RNA libraries against a molecular target. The expectation is that this iterative selection process will be sufficiently stringent to identify a candidate pool of specific high-affinity aptamers. However, failure of this process to yield promising aptamers is common, due in part to (i) limitations in library designs, (ii) retention of non-specific aptamers during screening rounds, (iii) excessive accumulation of amplification artifacts, and (iv) the use of screening criteria (binding affinity) that does not reflect therapeutic activity. We report a new selection platform, High-Fidelity (Hi-Fi) SELEX, that introduces fixed-region blocking elements to safeguard the functional diversity of the library. The chemistry of the target-display surface and the composition of the equilibration solvent are engineered to strongly inhibit non-specific retention of aptamers. Partition efficiencies approaching 10(6) are thereby realized. Retained members are amplified in Hi-Fi SELEX by digital PCR in a manner that ensures both elimination of amplification artifacts and stoichiometric conversion of amplicons into the single-stranded library required for the next selection round. Improvements to aptamer selections are first demonstrated using human α-thrombin as the target. Three clinical targets (human factors IXa, X, and D) are then subjected to Hi-Fi SELEX. For each, rapid enrichment of ssDNA aptamers offering an order-nM mean equilibrium dissociation constant (Kd) is achieved within three selection rounds, as quantified by a new label-free qPCR assay reported here. Therapeutic candidates against factor D are identified.

Putting RNA to work: Translating RNA fundamentals into biotechnological engineering practice

Synthetic biology, in close concert with systems biology, is revolutionizing the field of metabolic engineering by providing novel tools and technologies to rationally, in a standardized way, reroute metabolism with a view to optimally converting renewable resources into a broad range of bio-products, bio-materials and bio-energy. Increasingly, these novel synthetic biology tools are exploiting the extensive programmable nature of RNA, vis-à-vis DNA- and protein-based devices, to rationally design standardized, composable, and orthogonal parts, which can be scaled and tuned promptly and at will. This review gives an extensive overview of the recently developed parts and tools for i) modulating gene expression ii) building genetic circuits iii) detecting molecules, iv) reporting cellular processes and v) building RNA nanostructures. These parts and tools are becoming necessary armamentarium for contemporary metabolic engineering. Furthermore, the design criteria, technological challenges, and recent metabolic engineering success stories of the use of RNA devices are highlighted. Finally, the future trends in transforming metabolism through RNA engineering are critically evaluated and summarized.

More DNA-Aptamers for Small Drugs: A Capture-SELEX Coupled with Surface Plasmon Resonance and High-Throughput Sequencing

To address limitations in the production of DNA aptamers against small molecules, we introduce a DNA-based capture-SELEX (systematic evolution of ligands by exponential enrichment) protocol with long and continuous randomized library for more flexibility, coupled with in-stream direct-specificity monitoring via SPR and high throughput sequencing (HTS). Applying this capture-SELEX on tobramycin shows that target-specificity arises at cycle number 8, which is confirmed by sequence convergence in HTS analysis. Interestingly, HTS also shows that the most enriched sequences are already visible after only two capture-SELEX cycles. The best aptamers displayed K(D) of approximately 200 nM, similar to RNA and DNA-based aptamers previously selected for tobramycin. The lowest concentration of tobramycin detected on label-free SPR experiments with the selected aptamers is 20-fold smaller than the clinical range limit, demonstrating suitability for small-drug biosensing.

Aptamers targeting different functional groups of 17β-estradiol

Aptamers, short synthetic ssDNA or RNA molecules with a specific three-dimensional structure, are promising recognition elements in biosensor technology. In vitro generation of aptamers with high sensitivity and specificity toward a broad range of analytes has been achieved using the systematic evolution of ligands by exponential enrichment (SELEX) process. This iterative pathway of aptamer generation consists of sequential positive and counterselection steps. The present research aimed to select two sets of ssDNA aptamers which both are able to bind to different functional groups on the cyclopentanoperhydrophenanthrene ring of 17β-estradiol (E2). By repetitively switching between positive selection steps using E2 as target molecule and counterselection steps with nortestosterone as countermolecule, aptamers were successfully selected against the hydroxylated aromatic A ring of E2. Additionally, an aptamer which binds the upper segments of the B, C and D ring of the cyclopentanoperhydrophenanthrene ring of E2 was generated after repetitively swapping between positive selection steps with E2 as target molecule and counterselection steps with dexamethasone as countermolecule. Epitope specificity of the aptamers was demonstrated by evaluating their binding responses toward a number of steroid hormones structurally related to E2. The selected aptamers with affinities for different functional groups of E2 can potentially be applied to develop a cross-reactive aptasensor. This aptasensor introduces a promising tool for the future of in-field real-time monitoring of a wide range of steroid hormones.

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.