High Enrichment of Nucleobase-modified Aptamers in Early Selection Rounds by Microbeads-assisted Capillary Electrophoresis SELEX

Proficiently partitioning of bioactive peptide-ssDNA conjugates by microbead-assisted capillary electrophoresis (MACE)

Low-molecular drug discovery using DNA-encoded chemical library (DEL) is a powerful technology, although improving the partitioning efficiency of affinity ligands from DEL remains a challenge. Here, we assessed the usefulness of microbead-assisted capillary electrophoresis (MACE) for partitioning peptide-oligonucleotide conjugates (POCs), in which high selection pressure is applied because of different mobility of target-modified beads and POCs during CE. Despite their different charge characteristics, all POCs were well separated from the beads. When bead extraction was performed, the tagged DNA amplification was observed only in the couple of a ligand/target, suggesting proficiently specific partitioning of peptide ligands was accomplished using MACE.

Recent progress of SELEX methods for screening nucleic acid aptamers

Nucleic acid aptamers are oligonucleotide sequences screened by an in vitro methodology called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Known as "chemical antibodies", aptamers can achieve specific recognition towards the targets through conformational changes with high affinity, and possess multiple attractive features including, but not limited to, easy and inexpensive to prepare by chemical synthesis, relatively stable and low batch-to-batch variability, easy modification and signal amplification, and low immunogenicity. Now, aptamers are attracting researchers' attentions from more than 25 disciplines, and have showed great potential for application and economic benefits in disease diagnosis, environmental detection, food security, drug delivery and discovery. Although some aptamers exist naturally as the ligand-binding elements of riboswitches, SELEX is a recognized method for aptamers screening. After thirty-two years of development, a series of SELEX methods have been investigated and developed, as well as have shown unique advantages to improve sequence performances or to explore screening mechanisms. This review would mainly focus on the novel or improved SELEX methods that are available in the past five years. Firstly, we present a clear overview of the aptamer's history, features, and SELEX development. Then, we highlight the specific examples to emphasize the recent progress of SELEX methods in terms of carrier materials, technical improvements, real sample-improved screening, post-SELEX and other methods, as well as their respects of screening strategies, implementation features, screening parameters. Finally, we discuss the remaining challenges that have the potential to hinder the success of SELEX and aptamers in practical applications, and provide the suggestions and future directions for developing more convenient, efficient, and stable SELEX methods in the future.

Modified nucleic acid aptamers: development, characterization, and biological applications

Aptamers are single-stranded oligonucleotides that bind to their targets via specific structural interactions. To improve the properties and performance of aptamers, modified nucleotides are incorporated during or after a selection process such as systematic evolution of ligands by exponential enrichment (SELEX). We summarize the latest modified nucleotides and strategies used in modified (mod)-SELEX and post-SELEX to develop modified aptamers, highlight the methods used to characterize aptamer-target interactions, and present recent progress in modified aptamers that recognize different targets. We discuss the challenges and perspectives in further advancing the methodologies and toolsets to accelerate the discovery of modified aptamers, improve the throughput of aptamer-target characterization, and expand the functional diversity and complexity of modified aptamers.

Steric hindrance and structural flexibility shape the functional properties of a guanine-rich oligonucleotide

Ligand/protein molecular recognition involves a dynamic process, whereby both partners require a degree of structural plasticity to regulate the binding/unbinding event. Here, we present the characterization of the interaction between a highly dynamic G-rich oligonucleotide, M08s-1, and its target protein, human α-thrombin. M08s-1 is the most active anticoagulant aptamer selected thus far. Circular dichroism and gel electrophoresis analyses indicate that both intramolecular and intermolecular G-quadruplex structures are populated in solution. The presence of thrombin stabilises the antiparallel intramolecular chair-like G-quadruplex conformation, that provides by far the main contribution to the biological activity of the aptamer. The crystal structure of the thrombin-oligonucleotide complex reveals that M08s-1 adopts a kinked structural organization formed by a G-quadruplex domain and a long duplex module, linked by a stretch of five purine bases. The quadruplex motif hooks the exosite I region of thrombin and the duplex region is folded towards the surface of the protein. This structural feature, which has never been observed in other anti-exosite I aptamers with a shorter duplex motif, hinders the approach of a protein substrate to the active site region and may well explain the significant increase in the anticoagulant activity of M08s-1 compared to the other anti-exosite I aptamers.

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.

Accelerated Discovery of Potent Bioactive anti-TNFα Aptamers by Microbead-Assisted Capillary Electrophoresis (MACE)-SELEX

Dysregulation of tumor necrosis factor-α (TNFα), a pro-inflammatory cytokine, causes several diseases, making it an important therapeutic target. Here, we identified a novel DNA aptamer against human TNFα using in vitro selection, which included a high exclusion pressure process against non-binding and weak binders through microbead-assisted capillary electrophoresis (MACE) in only three rounds. Among the 15 most enriched aptamers, Apt14 exhibited the highest inhibitory activity for the interaction between TNFα and its cognate receptor in mouse L929 cells. For further improving the bioactivity of the aptamer, dimerization programed by hybridization was evaluated, resulting in the Apt14 dimer exhibited a twofold higher binding affinity and stronger inhibition compared to the monomer counterpart. Rapid identification of bioactive aptamers using MACE in combination with facile dimerization by hybridization accelerates the discovery of novel bioactive aptamers, paving the way toward replacing current monoclonal antibody therapy with the less expensive and non-immunogenic aptamer therapy.

Fractionation of Single-stranded DNAs with/without Stable Preorganized Structures Using Capillary Sieving Electrophoresis for Aptamer Selection

Aptamers, single-stranded DNAs/RNAs with a strong and specific interaction towards a target molecule, have wide applications in the fields of medicine and biosensors. In conventional aptamer selection methods, it is difficult to obtain "preorganized" and/or "induced-fit" type of aptamers selectively. In this study, separation and fractionation of single-stranded DNAs with/without stable preorganized structures were carried out using capillary sieving electrophoresis. The fractionated DNAs showed different mobilities and thermodynamic stabilities of their secondary structures; this outcome is deemed to be necessary for the synthesis of novel aptasensors with a desirable sensing mechanism.

Selection of Ovalbumin-specific Binding Peptides through Instant Translation in Ribosome Display Using E. coli Extract

In vitro selection has been widely used to generate molecular-recognition elements in analytical sciences. Although reconstituted types of in vitro transcription and translation (IVTT) system, such as PURE system, are nowadays widely used for ribosome display and mRNA/cDNA display, use of E. coli extract is often avoided, presumably because it contains unfavorable contaminants, such as ribonuclease. Nevertheless, the initial speed of protein translation in E. coli extract is markedly faster than that of PURE system. We thus hypothesized that E. coli extract is more appropriate for instant translation in ribosome display than PURE system. Here, we first revisit the potency of E. coli extract for ribosome display by shortening the translation time, and then applied the optimized condition for selecting peptide aptamers for ovalbumin (OVA). The OVA-binding peptides selected using E. coli extract exhibited specific binding to OVA, even in the presence of 50% serum. We conclude that instant translation in ribosome display using E. coli extract has the potential to generate easy-to-use and economical molecular-recognition elements in analytical sciences.

Capillary electrophoresis-based methodology for screening of oligonucleotide aptamers

As a new molecular recognition element, oligonucleotide aptamer not only has higher affinity and specificity to target molecules, but also has the advantages of wide recognition range, in vitro synthesis and chemical stability compared with conventional antibodies. Since a kind of screening method termed systematic evolution of ligands by exponential enrichment (SELEX) was reported, scientists have extensively researched the methodology of how to highly and efficiently screen out aptamers from a library consisting of a large number of random oligonucleotides. Certainly capillary electrophoresis-based screening methodologies, including nonequilibrium capillary electrophoresis of equilibrium mixtures, equilibrium capillary electrophoresis of equilibrium mixtures, non-SELEX, ideal-filter capillary electrophoresis, capillary transient isotachophoresis, etc., are revolutionary. Compared with conventional SELEX, these capillary electrophoresis-based methodologies show incomparable advantages such as the single-round screening of aptamers and increased successful screening rate. Methodology studies on the screening process of aptamers are comprehensively reviewed.

Graphene-Based FRET Aptasensors

Graphene-based FRET aptasensors can be realized only by unique combinations of aptamer that can be freely functionalized by chemical modification, and graphene/graphene oxide that works as an excellent fluorescence acceptor at the same time as aptamer adsorbates. This review describes the principles of the sensor, several applications to microchannel devices, improvement of the sensing performance by molecular design of the aptamer and remarks on future prospects based on an introduction of recent works and achievements, including the author's paper. The sensor employs DNA modified with graphene/graphene oxide at the terminal as the molecular probe. This system is supported by the excellent property of DNA that does not lose the molecular recognition ability even due to a chemical modification at the terminal. I hope that this review will be useful for developing research on higher performance of graphene aptasensors in the future.

Evolution of multi-functional capillary electrophoresis for high-efficiency selection of aptamers

Aptamers have drawn considerable attention as newly emerging molecular recognition elements in clinical diagnostics, drug delivery, therapeutics, environmental monitoring, and food safety analyses. As the in vitro screening antibody analogs, aptamers are enabled to recognize various types of targets with high affinity and specificity like or even superior to antibodies. However, the restrictions and inefficiency of selection have been hampering their wider application. Among various modified systematic evolution of ligands by exponential enrichment (SELEX) methods, capillary electrophoresis (CE)-SELEX holds multiple functions and advantages with the powerful qualitative and quantitative analysis capabilities, less consumption of sample and analytical reagent, natural binding environment, higher screening efficiency, and availability in multiple modes. This review summarizes the key developments in the area of CE-SELEX by leading research groups, including our teams' ten years of research and experience to help researchers fully understand and utilize CE-SELEX. Aptamers' history, applications, as well as the SELEX developments, have been briefly described; the advantages of CE-SELEX are highlighted compared with the conventional SELEX methods. Further, we describe some essential CE-SELEX models and provide an overview of the CE-SELEX, including the targets and ssDNA library, every technical point in the selection process, and post-SELEX protocol. We expect this review will inspire more researchers to have insight into the screening problems from CE-SELEX viewpoint and will help to improve the selection efficiency and probability of success to meet the growing needs of aptamers' discovery in bioanalytical and medical fields.