Structural optimization of an aptamer generated from Ligand-Guided Selection (LIGS) resulted in high affinity variant toward mIgM expressed on Burkitt's lymphoma cell lines

Tools and techniques for the discovery of therapeutic aptamers: recent advances

INTRODUCTION

The pursuit of novel therapeutic agents for serious diseases such as cancer has been a global endeavor. Aptamers characteristic of high affinity, programmability, low immunogenicity, and rapid permeability hold great promise for the treatment of diseases. Yet obtaining the approval for therapeutic aptamers remains challenging. Consequently, researchers are increasingly devoted to exploring innovative strategies and technologies to advance the development of these therapeutic aptamers.

AREAS COVERED

The authors provide a comprehensive summary of the recent progress of the SELEX (Systematic Evolution of Ligands by EXponential enrichment) technique, and how the integration of modern tools has facilitated the identification of therapeutic aptamers. Additionally, the engineering of aptamers to enhance their functional attributes, such as inhibiting and targeting, is discussed, demonstrating the potential to broaden their scope of utility.

EXPERT OPINION

The grand potential of aptamers and the insufficient development of relevant drugs have spurred countless efforts for stimulating their discovery and application in the therapeutic field. While SELEX techniques have undergone significant developments with the aid of advanced analysis instruments and ingeniously updated aptameric engineering strategies, several challenges still impede their clinical translation. A key challenge lies in the insufficient understanding of binding conformation and susceptibility to degradation under physiological conditions. Despite the hurdles, our opinion is optimistic. With continued progress in overcoming these obstacles, the widespread utilization of aptamers for clinical therapy is envisioned to become a reality soon.

The Research Advances of Aptamers in Hematologic Malignancies

Currently, research for hematological malignancies is very intensive, with many breakthroughs. Among them, aptamer-based targeted therapies could be counted. Aptamer is a targeting tool with many unique advantages (easy synthesis, low toxicity, easy modification, low immunogenicity, nano size, long stability, etc.), therefore many experts screened corresponding aptamers in various hematological malignancies for diagnosis and treatment. In this review, we try to summarize and provide the recent progress of aptamer research in the diagnosis and treatment of hematologic malignancies. Until now, 29 aptamer studies were reported in hematologic malignancies, of which 12 aptamers were tested in vivo and the remaining 17 aptamers were only tested in vitro. In this case, 11 aptamers were combined with chemotherapeutic drugs for the treatment of hematologic malignancies, 4 aptamers were used in combination with nanomaterials for the diagnosis and treatment of hematologic malignancies, and some studies used aptamers for the targeted transportation of siRNA and miRNA for targeted therapeutic effects. Their research provides multiple approaches to achieve more targeted goals. These findings show promising and encouraging future for both hematological malignancies basic and clinical trials research.

Systematic bio-fabrication of aptamers and their applications in engineering biology

Aptamers are single-stranded DNA or RNA molecules that have high affinity and selectivity to bind to specific targets. Compared to antibodies, aptamers are easy to in vitro synthesize with low cost, and exhibit excellent thermal stability and programmability. With these features, aptamers have been widely used in biology and medicine-related fields. In the meantime, a variety of systematic evolution of ligands by exponential enrichment (SELEX) technologies have been developed to screen aptamers for various targets. According to the characteristics of targets, customizing appropriate SELEX technology and post-SELEX optimization helps to obtain ideal aptamers with high affinity and specificity. In this review, we first summarize the latest research on the systematic bio-fabrication of aptamers, including various SELEX technologies, post-SELEX optimization, and aptamer modification technology. These procedures not only help to gain the aptamer sequences but also provide insights into the relationship between structure and function of the aptamers. The latter provides a new perspective for the systems bio-fabrication of aptamers. Furthermore, on this basis, we review the applications of aptamers, particularly in the fields of engineering biology, including industrial biotechnology, medical and health engineering, and environmental and food safety monitoring. And the encountered challenges and prospects are discussed, providing an outlook for the future development of aptamers.

An In Vitro Selection Platform to Identify Multiple Aptamers against Multiple Cell-Surface Markers Using Ligand-Guided Selection

Aptamer ligand discovery against multiple molecules expressed on whole cells is an essential component in molecular tool development. However, owing to their intrinsic structural characteristics, cell-surface receptors have proven to be challenging targets in ligand discovery. Several variants to systematic evolution of ligands by exponential enrichment (SELEX) have been introduced to address the ″target problem″ for aptamer screening. To this end, we introduced a variant of SELEX, termed ligand-guided selection (LIGS), to identify highly specific aptamers against complex cell-surface markers in their native state. So far, the application of LIGS has been aimed at identifying aptamers against the most dominant receptors on the cell surface. Here, we report that LIGS can be expanded to identify two receptors on the same cell surface, paving the way to generate a multiplexed ligand discovery platform based on SELEX-targeting membrane receptors in their native functional state. Using CD19 and CD20 expressed on Toledo cells as a model system, multiple aptamer families were evolved against Toledo cells. We then utilized two monoclonal antibodies (mAbs) against CD20 and CD19 to selectively partition specific aptamers against CD19 and CD20. Following biochemical characterization, we introduce two specific aptamers against CD19 and two specific aptamers against CD20 with high affinity. Multi-target LIGS, as reported here, demonstrates a successful combinatorial approach for nucleic acid library screening to generate multiple artificial nucleic acid ligands against multiple receptors expressed on a single cell.

Utility of Multivalent Aptamers to Develop Nanoscale DNA Devices against Surface Receptors

DNA nanotechnology is undergoing rapid progress in the assembly of functional devices with biological relevance. In particular, currently, the research attention is more focused on the application of nanodevices at the interface of chemistry and biology, on the cell membrane where protein receptors communicate with the extracellular environment. This review explores the use of multivalent nucleic acid ligands termed aptamers in the design of DNA-based nanodevices to probe cellular interactions followed by a perspective on the untapped utility of XNA and UBP nanotechnology in designing functional nanomaterials with broader structural space.

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.

Dimeric and Multimeric DNA Aptamers for Highly Effective Protein Recognition

Multivalent interactions frequently occur in biological systems and typically provide higher binding affinity and selectivity in target recognition than when only monovalent interactions are operative. Thus, taking inspiration by nature, bivalent or multivalent nucleic acid aptamers recognizing a specific biological target have been extensively studied in the last decades. Indeed, oligonucleotide-based aptamers are suitable building blocks for the development of highly efficient multivalent systems since they can be easily modified and assembled exploiting proper connecting linkers of different nature. Thus, substantial research efforts have been put in the construction of dimeric/multimeric versions of effective aptamers with various degrees of success in target binding affinity or therapeutic activity enhancement. The present review summarizes recent advances in the design and development of dimeric and multimeric DNA-based aptamers, including those forming G-quadruplex (G4) structures, recognizing different key proteins in relevant pathological processes. Most of the designed constructs have shown improved performance in terms of binding affinity or therapeutic activity as anti-inflammatory, antiviral, anticoagulant, and anticancer agents and their number is certainly bound to grow in the next future.

Aptamers against Immunoglobulins: Design, Selection and Bioanalytical Applications

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

Ligand-Guided Selection with Artificially Expanded Genetic Information Systems against TCR-CD3ε

Here we are reporting, for the first time, a ligand-guided selection (LIGS) experiment using an artificially expanded genetic information system (AEGIS) to successfully identify an AEGIS-DNA aptamer against T cell receptor-CD3ε expressed on Jurkat.E6 cells. Thus, we have effectively combined the enhanced diversity of an AEGIS DNA library with LIGS to develop a superior screening platform to discover superior aptamers. Libraries of DNA molecules from highly diversified building blocks will provide better ligands due to more functional diversity and better-controlled folding. Thus, a DNA library with AEGIS components (dZ and dP) was used in LIGS experiments against TCR-CD3ε in its native state using two clinically relevant monoclonal antibodies to identify an aptamer termed JZPO-10, with nanomolar affinity. Multiple specificity assays using knockout cells, and competition experiments using monoclonal antibodies utilized in LIGS, show unprecedented specificity of JZPO-10, suggesting that the combination of LIGS with AEGIS-DNA libraries will provide a superior screening platform to discover artificial ligands against critical cellular targets.

Insights into the G-rich VEGF-binding aptamer V7t1: when two G-quadruplexes are better than one!

The G-quadruplex-forming VEGF-binding aptamer V7t1 was previously found to be highly polymorphic in a K⁺-containing solution and, to restrict its conformational preferences to a unique, well-defined form, modified nucleotides (LNA and/or UNA) were inserted in its sequence. We here report an in-depth biophysical characterization of V7t1 in a Na⁺-rich medium, mimicking the extracellular environment in which VEGF targeting should occur, carried out combining several techniques to analyse the conformational behaviour of the aptamer and its binding to the protein. Our results demonstrate that, in the presence of high Na⁺ concentrations, V7t1 behaves in a very different way if subjected or not to annealing procedures, as evidenced by native gel electrophoresis, size exclusion chromatography and dynamic light scattering analysis. Indeed, not-annealed V7t1 forms both monomeric and dimeric G-quadruplexes, while the annealed oligonucleotide is a monomeric species. Remarkably, only the dimeric aptamer efficiently binds VEGF, showing higher affinity for the protein compared to the monomeric species. These findings provide new precious information for the development of improved V7t1 analogues, allowing more efficient binding to the cancer-related protein and the design of effective biosensors or theranostic devices based on VEGF targeting.

Ligand Guided Selection (LIGS) of Artificial Nucleic Acid Ligands against Cell Surface Targets

With the success of RNA-based therapeutic drugs, the demand has increased for sophisticated nucleic-acid-based targeting agents. Nucleic acid aptamers (NAAs), in this regard, represent a suitable class of molecules with synthetic versatility. Aptamers are composed of single-stranded RNA/DNA/XNA molecules, which can be identified using a method called systematic evolution of ligands by exponential enrichment (SELEX) against any molecule. This Spotlight summarizes the recent introduction of ligand guided selection (LIGS), which will permit the identification of a wide range of functional aptamers against complex targets such as cell surface receptors while maintaining their native functional state. Aptamers identified from LIGS will allow researchers to develop aptamers in biomedicine as low-cost, stable therapeutic agents and diagnostic molecules or biochemical devices.

Integrating Ligand-Receptor Interactions and In Vitro Evolution for Streamlined Discovery of Artificial Nucleic Acid Ligands

To discover DNA ligands against a predetermined receptor protein complex, we introduce a comprehensive version of ligand-guided selection (LIGS). LIGS is, itself, a variant of systematic evolution of ligands by exponential enrichment (SELEX). Herein, we have optimized LIGS to identify higher affinity aptamers with high specificity. In addition, we demonstrate the expandability of LIGS by performing specific aptamer elution at 25°C, utilizing multiple monoclonal antibodies (mAbs) against cultured cells and primary cells obtained from human donors expressing the same receptor. Eluted LIGS libraries obtained through Illumina high-throughput (HT) DNA sequencing were analyzed by bioinformatics tools to discover five DNA aptamers with apparent affinities ranging from 3.06 ± 0.485 nM to 325 ± 62.7 nM against the target, T cell receptor-cluster of differentiation epsilon (TCR-CD3ε) expressed on human T cells. The specificity of the aptamers was validated utilizing multiple strategies, including competitive binding analysis and a double-knockout Jurkat cell line generated by CRISPR technology. The cross-competition experiments using labeled and unlabeled aptamers revealed that all five aptamers compete for the same binding site. Collectively, the data in this report introduce a modified LIGS strategy as a universal platform to identify highly specific multiple aptamers toward multi-component receptor proteins in their native state without changing the cell-surface landscape.

Computational analysis and optimization of carcinoembryonic antigen aptamers and experimental evaluation

Carcinoembryonic antigen (CEA), a highly glycosylated protein, overexpresses in many cancers. In this study, computational methods were used to optimize CEA aptamers. Experimental evaluvation of selected aptamers were conducted through electrochemical impedance spectroscopy. After two and three-dimensional structure modeling, the complexes of twelve reported aptamers against CEA were simulated using the ZDOCK server. Based on docking scores, two aptamer sequences (CSR59 and CSR57.1) were selected and used to create a new library. This ssDNA aptamer library consisting of 91 sequences was created using diverse in silico mutational methods. We obtained seventeen sequences having higher binding scores than reported sequences. Based on ZDOCK scores, the interaction domain of CEA, and steric hindrance due to glycosylation, two aptamer sequences (G3S1.5 and G2S2.2) were selected. An impedimetric aptasensor was designed, and selected aptamers were used as biorecognition elements. Resistance to charge transfer (Rct) quantities confirmed the bioinformatic approach and molecular docking scores. The result showed that the interaction ability of selected aptamers was about 13.5 fold higher than the control. It can be concluded that the selected aptamers have good potential for detection of carcinoembryonic antigen biomarker.

The role of G-quadruplex structures of LIGS-generated aptamers R1.2 and R1.3 in IgM specific recognition

Exploiting a variant of SELEX called "Ligand-Guided Selection" (LI-GS), we recently identified two novel truncated G-rich aptamers, called R1.2 and R1.3, specific for membrane-bound IgM (mIgM), the hallmark of B cells. Herein, the conformational behaviour of these aptamers has been analysed by multiple biophysical methods. In order to investigate their functional secondary structures, these studies have been carried out in pseudo-physiological buffers mimicking different cellular environments. Both aptamers proved to be highly polymorphic, folding into stable, unimolecular G-quadruplex structures in K⁺-rich buffers. In turn, in buffered solutions containing Na⁺/Mg²⁺ ions, R1.2 and R1.3 formed mainly duplex structures. Remarkably, these aptamers were able to effectively bind mIgM on B-cell lymphoma exclusively in the presence of potassium ions. These findings demonstrate the key role of G-quadruplex folding in the molecular recognition and efficient binding of R1.2 and R1.3 to mIgM expressed in lymphoma and leukemia cells, providing a precious rational basis for the design of effective aptamer-based biosensors potentially useful for the detection of cancer-relevant biomarkers.

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.

Dimerization of an aptamer generated from Ligand-guided selection (LIGS) yields a high affinity scaffold against B-cells

Nucleic Acid Aptamers (NAAs) are a class of synthetic DNA or RNA molecules that bind specifically to their target. We recently introduced an aptamer termed R1.2 against membrane Immunoglobulin M (mIgM) expressing B-cell neoplasms using Ligand Guided Selection (LIGS). While LIGS-generated aptamers are highly specific, their lower affinity prevents aptamers from being used for translational applications. Highly specific aptamers with higher affinity can increase targetability, boosting the application of aptamers as diagnostic and therapeutic molecules. Herein, we report that dimerization of R1.2, an aptamer generated from LIGS, leads to high affinity variants without compromising the specificity. Three dimeric aptamer analogues with variable linker lengths were designed to evaluate the effect of linker length in affinity. The optimized dimeric R1.2 against cultured B-cell neoplasms, four donor B-cell samples and mIgM-positive Waldenström's Macroglobulinemia (WM) showed specificity. Furthermore, confocal imaging of dimeric aptamer and anti-IgM antibody in purified B-cells suggests co-localization. Binding assays against IgM knockout Burkitt's Lymphoma cells utilizing CRISPR/Cas9 further validated specificity of dimeric R1.2. Collectively, our findings show that LIGS-generated aptamers can be re-engineered into dimeric aptamers with high specificity and affinity, demonstrating wide-range of applicability of LIGS in developing clinically practical diagnostic and therapeutic aptamers.

[Research advances in the role of aptamers in the diagnosis and targeted therapy of pediatric cancer]

Aptamers are single-stranded DNA or RNA which are isolated from synthesized random oligonucleotide library in vitro via systematic evolution of ligands by exponential enrichment (SELEX) and can bind to metal ions, small molecules, carbohydrates, lipids, proteins, and others targets with high affinity and specificity. Aptamers have the advantages of simple preparation, good thermal stability, and low immunogenicity and have great potential in the medical fields such as molecular imaging, biosensing, early diagnosis of diseases, and targeted therapy. Aptamer technology may be useful for early diagnosis and targeted therapy of pediatric cancer, and may avoid the side effects of conventional chemotherapy, such as growth and development disorders and long-term organ dysfunction. This article reviews the latest research advances in the selection and application of aptamers for pediatric cancer.

[Research advances in the role of aptamers in the diagnosis and targeted therapy of pediatric cancer]

Aptamers are single-stranded DNA or RNA which are isolated from synthesized random oligonucleotide library in vitro via systematic evolution of ligands by exponential enrichment (SELEX) and can bind to metal ions, small molecules, carbohydrates, lipids, proteins, and others targets with high affinity and specificity. Aptamers have the advantages of simple preparation, good thermal stability, and low immunogenicity and have great potential in the medical fields such as molecular imaging, biosensing, early diagnosis of diseases, and targeted therapy. Aptamer technology may be useful for early diagnosis and targeted therapy of pediatric cancer, and may avoid the side effects of conventional chemotherapy, such as growth and development disorders and long-term organ dysfunction. This article reviews the latest research advances in the selection and application of aptamers for pediatric cancer.

Aptamers in Bordeaux 2017: An exceptional "millésime"

About 150 participants attended the symposium organised at the Palais de la Bourse in Bordeaux, France on September 22-23, 2017. Thirty speakers from all over the world delivered lectures covering selection processes, aptamer chemistry and innovative applications of these powerful tools that display major advantages over antibodies. Beyond the remarkable science presented, lively discussion and fruitful exchange between participants made this meeting a great success. A series of lectures were focused on synthetic biology (riboswitches, new synthetic base pairs, mutated polymerases). Innovative selection procedures including functional screening of oligonucleotide pools were described. Examples of aptasensors for the detection of pathogens were reported. The potential of aptamers for the diagnostic and the treatment of diseases was also presented. Brief summaries of the lectures presented during the symposium are given in this report. The third edition of this symposium will take place in Boulder, Colorado in Summer 2018 (information available at http://www.aptamers-in-bordeaux.com/).

Engineered Aptamers to Probe Molecular Interactions on the Cell Surface

Significant progress has been made in understanding the nature of molecular interactions on the cell membrane. To decipher such interactions, molecular scaffolds can be engineered as a tool to modulate these events as they occur on the cell membrane. To guarantee reliability, scaffolds that function as modulators of cell membrane events must be coupled to a targeting moiety with superior chemical versatility. In this regard, nucleic acid aptamers are a suitable class of targeting moieties. Aptamers are inherently chemical in nature, allowing extensive site-specific chemical modification to engineer sensing molecules. Aptamers can be easily selected using a simple laboratory-based in vitro evolution method enabling the design and development of aptamer-based functional molecular scaffolds against wide range of cell surface molecules. This article reviews the application of aptamers as monitors and modulators of molecular interactions on the mammalian cell surface with the aim of increasing our understanding of cell-surface receptor response to external stimuli. The information gained from these types of studies could eventually prove useful in engineering improved medical diagnostics and therapeutics.