CS-SELEX generates high-affinity ssDNA aptamers as molecular probes for hepatitis C virus envelope glycoprotein E2. PLoS One. 2009;4:e8142. [PMID: 19997645 DOI: 10.1371/journal.pone.0008142]

Small-molecule oligonucleotides as smart modality for antiviral therapy: a medicinal chemistry perspective

Small-molecule oligonucleotides could be exploited therapeutically to silence the expression of viral infection-causing genes, and a few of them are now in clinical trials for the management of viral infections. The most challenging aspect of these oligonucleotides' therapeutic success involves their delivery. Thus medicinal chemistry strategies are inevitable to avoid degradation by serum nucleases, avoid kidney clearance and improve cellular uptake. Recently small-molecule oligonucleotide design has opened up new avenues to improve the treatment of drug-resistant viral infections, along with the development of COVID-19 medicines. This review is directed toward the recent advances in rational design, mechanism of action, structure-activity relationships and future perspective of the small-molecule oligonucleotides targeting viral infections, including COVID-19.

Aptamers targeting SARS-COV-2: a promising tool to fight against COVID-19

SARS-CoV-2, the causative agent of COVID-19, remains among the main causes of global mortality. Although antigen/antibody-based immunoassays and neutralizing antibodies targeting SARS-CoV-2 have been successfully developed over the past 2 years, they are often inefficient and unreliable for emerging SARS-CoV-2 variants. Novel approaches against SARS-CoV-2 and its variants are therefore urgently needed. Aptamers have been developed for the detection and inhibition of several different viruses such as HIV, influenza viruses, Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV. Aptamers targeting SARS-CoV-2 represent a promising tool in the fight against COVID-19, which is of paramount importance for the current and any future pandemics. This review presents recent advances and future trends in the development of aptamer-based approaches for SARS-CoV-2 diagnosis and treatment.

Aptamer-based biosensors for virus protein detection

Virus threatens life health seriously. The accurate early diagnosis of the virus is vital for clinical control and treatment of virus infection. Aptamers are small single-stranded oligonucleotides (DNAs or RNAs). In this review, we summarized aptasensors for virus detection in recent years according to the classification of the viral target protein, and illustrated common detection mechanisms in the aptasensors (colorimetry, fluorescence assay, surface plasmon resonance (SPR), surface-enhanced raman spectroscopy (SERS), electrochemical detection, and field-effect transistor (FET)). Furthermore, aptamers against different target proteins of viruses were summarized. The relationships between the different biomarkers of the viruses and the detection methods, and their performances were revealed. In addition, the challenges and future directions of aptasensors were discussed. This review will provide valuable references for constructing on-site aptasensors for detecting viruses, especially the SARS-CoV-2.

Aptamers: A prospective tool for infectious diseases diagnosis

It is well known that people's health is seriously threatened by various pathogens (such as Mycobacterium tuberculosis, Treponema pallidum, Novel coronavirus, HIV, Mucor, etc.), which leads to heavy socioeconomic burdens. Therefore, early and accurate pathogen diagnosis is essential for timely and effective therapies. Up to now, diagnosing human contagious diseases at molecule and nano levels is remarkably difficult owing to insufficient valid probes when it comes to determining the biological markers of pathogens. Aptamers are a set of high‐specificity and high‐sensitivity plastic oligonucleotides screened in vitro via the selective expansion of ligands by exponential enrichment (SELEX). With the advent of aptamer‐based technologies, their merits have aroused mounting academic interest. In recent years, as new detection and treatment tools, nucleic acid aptamers have been extensively utilized in the field of biomedicine, such as pathogen detection, new drug development, clinical diagnosis, nanotechnology, etc. However, the traditional SELEX method is cumbersome and has a long screening cycle, and it takes several months to screen out aptamers with high specificity. With the persistent development of SELEX‐based aptamer screening technologies, the application scenarios of aptamers have become more and more extensive. The present research briefly reviews the research progress of nucleic acid aptamers in the field of biomedicine, especially in the diagnosis of contagious diseases.

MIP-based electrochemical sensor for direct detection of hepatitis C virus via E2 envelope protein

Hepatitis C is the most common liver disease caused by Hepatitis C virus (HCV), and can evolve into serious health problems e.g. cirrhosis and hepatocellular carcinoma. Nowadays, the initial stage of the disease cannot be practically diagnosed, representing thus an extremely important problem of modern public health care. This study is aimed at the development of a sensor for direct detection of HCV. The sensor utilizes a synthetic recognition element prepared by the technology of molecular imprinting and representing a molecularly imprinted polymer (MIP) having molecular recognition sites of HCV envelope protein E2 (E2-MIP). E2-MIP integrated into an electrochemical sensor platform allows quantitative evaluation of binding of free E2 protein as well as HCV-mimetic particles (HCV-MPs) in human plasma with LOD value of 4.6 × 10^-4 ng/mL (for HCV-MPs). The developed electrochemical HCV sensor represents a simple, fast and inexpensive alternative for the existing methods of HCV detection and paves the way for the point-of care diagnostics of Hepatitis C.

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.

Selection and Characterization of ssDNA Aptamers Targeting Largemouth Bass Virus Infected Cells With Antiviral Activities

Largemouth bass virus (LMBV) is one of the most devastating viral pathogens in farmed Largemouth bass. Aptamers are novel molecule probes and have been widely applied in the field of efficient therapeutic and diagnostic agents development. LMBV-infected fathead minnow cells (LMBV-FHM) served as target cells in this study, and three DNA aptamers (LBVA1, LBVA2, and LBVA3) were generated against target cells by SELEX technology. The selected aptamers could specifically bind to LMBV-FHM cells, with rather high calculated dissociation constants (Kd) of 890.09, 517.22, and 249.31 nM for aptamers LBVA1, LBVA2, and LBVA3, respectively. Three aptamers displayed efficient antiviral activities in vitro. It indicates that the selected aptamers have great potentials in developing efficient anti-viruses treatments. The targets of aptamers LBVA1, LBVA2, and LBVA3 could be membrane proteins on host cells. The targets of aptamers (LBVA1, LBVA2, and LBVA3) come out on the cells surface at 8, 10, 8 h post-infection. As novel molecular probes for accurate recognition, aptamer LBVA3 could detect LMBV infection in vitro and in vivo, it indicates that the selected aptamers could be applied in the development of rapid detective technologies, which are characterized by high sensitivity, accuracy, and easy operation.

Aptamers for Anti-Viral Therapeutics and Diagnostics

Viral infections cause a host of fatal diseases and seriously affect every form of life from bacteria to humans. Although most viral infections can receive appropriate treatment thereby limiting damage to life and livelihood with modern medicine and early diagnosis, new types of viral infections are continuously emerging that need to be properly and timely treated. As time is the most important factor in the progress of many deadly viral diseases, early detection becomes of paramount importance for effective treatment. Aptamers are small oligonucleotide molecules made by the systematic evolution of ligands by exponential enrichment (SELEX). Aptamers are characterized by being able to specifically bind to a target, much like antibodies. However, unlike antibodies, aptamers are easily synthesized, modified, and are able to target a wider range of substances, including proteins and carbohydrates. With these advantages in mind, many studies on aptamer-based viral diagnosis and treatments are currently in progress. The use of aptamers for viral diagnosis requires a system that recognizes the binding of viral molecules to aptamers in samples of blood, serum, plasma, or in virus-infected cells. From a therapeutic perspective, aptamers target viral particles or host cell receptors to prevent the interaction between the virus and host cells or target intracellular viral proteins to interrupt the life cycle of the virus within infected cells. In this paper, we review recent attempts to use aptamers for the diagnosis and treatment of various viral infections.

An electrochemical biosensor for direct detection of hepatitis C virus

This paper is aimed at the development of a biosensor for direct detection of Hepatitis C virus (HCV) surface antigen: envelope protein (E2). A recombinant LEL fragment of biological cell receptor CD81 and two short synthetic peptides imitating the fragment of LEL sequence of CD81 (linear and loop-like peptides) capable of specific binding to E2 were tested as molecular recognition elements of the biosensor. For this purpose the selected ligands were immobilized to the surface of a screen-printed electrode utilized as an electrochemical sensor platform. The immobilization parameters such as the ligand concentration and the immobilization time were carefully optimized for each ligand. Differential pulse voltammetry used to evaluate quantitatively binding of E2 to the ligands revealed their similar binding affinity towards E2. Thus, the linear peptide was selected as a less expensive and easily prepared ligand for the HCV biosensor preparation. The resulting HCV biosensor demonstrated selectivity towards E2 in the presence of interfering protein, conalbumin. Moreover, it was found that the prepared biosensor effectively detected E2 bound to hepatitis C virus-mimetic particles (HC VMPs) at LOD value of 2.1∙10^-5 mg/mL both in 0.01 M PBS solution (pH 7.4) and in simulated blood plasma.

Selection of DNA Aptamers for Subcellular Localization of RBSDV P10 Protein in the Midgut of Small Brown Planthoppers by Emulsion PCR-Based SELEX

Rice black-streaked dwarf virus (RBSDV), classified under the Reoviridae, Fijivirus genus, caused an epidemic in the eastern provinces of China and other East Asian countries and resulted in severe yield loss in rice and wheat production. RBSDV is transmitted by the small brown planthopper (SBPH, Laodelphax striatellus Fallén) in a persistent manner. In order to provide a stable and cost-effective detection probe, in this study we selected three DNA aptamers (R3, R5 and R11) by an optimized, standardized and time saving emulsion PCR-based SELEX, for the detection of RBSDV outer-shell P10 protein for in situ localization studies in the midgut of SBPH. The specificity of these three DNA aptamers was tested through detection of the P10 protein using an enzyme-linked oligonucleotide assay (ELONA) and aptamer-based dot-blot ELISA. All three DNA aptamers can be used to detect RBSDV P10 protein by immunofluorescent labeling in the midgut of RBSDV-infected SBPH. These data show that the selected aptamers can be used for the detection of RBSDV P10 protein in vitro and in vivo. This is the first report of aptamers being selected for detection of a rice virus capsid protein.

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.

Aptamer-nanobody based ELASA for detection of Vibrio cholerae O1

Background and Objectives

In recent years, the prevalence of diseases caused by Vibrio spp. is increasing in the world, and among them species, Vibrio cholerae is the most important Vibrio associated with pandemic and epidemic cholera outbreaks. Therefore, the development of a reliable method for early and accurate detection of V. cholerae for management of diseases is a real need. Aptamers with the ability to detect targets with high specificity and accuracy can be one of the candidates used for the whole cell and thereby V. cholerae detection.

Materials and Methods

In this research high-affinity DNA aptamers against with two major serotypes of Inaba (ATCC 39315) and Ogawa (clinical sample) were selected from DNA aptamer library through 12 rounds of Systematic Evolution of Ligands by Exponential (SELEX) enrichment procedure using live cells as a target which monitored with flow cytometry.

Results

The binding efficiency and dissociation constant of the isolated aptamers V.ch47 and V.ch27 were 56.4%, 53.3% and 15.404 ± 4.776 pM, 20.186 ± 3.655 pM, respectively. A sandwich Enzyme-linked aptamer sorbent assay (ELASA) was developed with the biotinylated V.ch47 aptamer and our previously developed nanobody anti-Lipopolysaccharides (LPS). We optimized this system with V. cholerae O1 and analyzed their cross reactivity with close physiological bacteria. The threshold of detection was obtained 10⁴ CFU/ml in the sandwich ELASA process.

Conclusion

Our results showed that the sandwich ELASA is sensitive enough for the rapid detection of V. cholerae from other bacteria.

Nanotechnology: A reality for diagnosis of HCV infectious disease

Hepatitis C virus (HCV) is the primary etiologic agent of liver cirrhosis or hepatocellular carcinoma. HCV elevated infection rates are mostly due to the lack of an accurate and accessible screening and diagnosis, especially in low- and middle-income countries. Conventional HCV diagnostic algorithm consists of a serological test followed by a nucleic acid test. This sequence of tests is time consuming and not affordable for low-resource settings. Nanotechnology have introduced new promising tests for the diagnose of infectious diseases. Based on the employment of nanoparticles and other nanomaterials which lead to highly sensitive and specific nanoscale tests, most of them target pathogen genome. Implementation of nanoscale tests, which are affordable, portable and easy to use by non-specialized personal, would improve HCV diagnosis algorithm. In this review, we have summed up the current emerging nanotechnology tools, which will improve actual screening and treatment programs, and help to reach HCV elimination proposal.

Rapid selection of single-stranded DNA aptamers binding Staphylococcus epidermidis in platelet concentrates

Staphylococcus epidermidis is the most common transfusion-associated pathogen contaminating platelet concentrates. Methods to reduce or eliminate contaminating bacteria from platelet units are critical for improving the safety of blood transfusions. We used rapid isolation of DNA aptamers (RIDA) to identify single-stranded (ss)DNA aptamers as ligands that specifically bind to S. epidermidis. Five target-specific ssDNA aptamers (76 mer) were obtained under stringent selection conditions. Aptamer SE43 demonstrated higher binding affinity compared with scrambled control. Furthermore, when binding assays were conducted in platelet concentrate, there was a twofold increase in binding affinity compared with the SE43 binding in buffer alone. Our data identified an aptamer that may be useful as a ligand to capture, detect or remove S. epidermidis contaminant from platelet concentrates.

Application of Aptamers in Virus Detection and Antiviral Therapy

Viral infections can cause serious diseases for humans and animals. Accurate and early detection of viruses is often crucial for clinical diagnosis and therapy. Aptamers are mostly single-stranded nucleotide sequences that are artificially synthesized by an in vitro technology known as the Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Similar to antibodies, aptamers bind specifically to their targets. However, compared with antibody, aptamers are easy to synthesize and modify and can bind to a broad range of targets. Thus, aptamers are promising for detecting viruses and treating viral infections. In this review, we briefly introduce aptamer-based biosensors (aptasensors) and describe their applications in rapid detection of viruses and as antiviral agents in treating infections. We summarize available data about the use of aptamers to detect and inhibit viruses. Furthermore, for the first time, we list aptamers specific to different viruses that have been screened out but have not yet been used for detecting viruses or treating viral infections. Finally, we analyze barriers and developing perspectives in the application of aptamer-based virus detection and therapeutics.

Binding of cellular nucleolin with the viral core RNA G-quadruplex structure suppresses HCV replication

Hepatitis C virus (HCV) infection is a major cause of human chronic liver disease and hepatocellular carcinoma. G-quadruplex (G4) is an important four-stranded secondary structure of nucleic acids. Recently, we discovered that the core gene of HCV contains a G4 RNA structure; however, the interaction between the HCV core RNA G4 and host cellular proteins, and the roles of the HCV core RNA G4 in HCV infection and pathogenesis remain elusive. Here, we identified a cellular protein, nucleolin (NCL), which bound and stabilized the HCV core RNA G4 structure. We demonstrated the direct interaction and colocalization between NCL and wild-type core RNA G4 at both in vitro and in cell physiological conditions of the alive virus; however no significant interaction was found between NCL and G4-modified core RNA. NCL is also associated with HCV particles. HCV infection induced NCL mRNA and protein expression, while NCL suppressed wild-type viral replication and expression, but not G4-modified virus. Silencing of NCL greatly enhanced viral RNA replication. Our findings provide new insights that NCL may act as a host factor for anti-viral innate immunity, and binding of cellular NCL with the viral core RNA G4 structure is involved in suppressing HCV replication.

Oligonucleotide aptamers: promising and powerful diagnostic and therapeutic tools for infectious diseases

The entire human population is at risk of infectious diseases worldwide. Thus far, the diagnosis and treatment of human infectious diseases at the molecular and nanoscale levels have been extremely challenging tasks because of the lack of effective probes to identify and recognize biomarkers of pathogens. Oligonucleotide aptamers are a class of small nucleic acid ligands that are composed of single-stranded DNA (ssDNA) or RNA and act as affinity probes or molecular recognition elements for a variety of targets. These aptamers have an exciting potential for diagnose and/or treatment of specific diseases. In this review, we highlight areas where aptamers have been developed as diagnostic and therapeutic agents for both bacterial and viral infectious diseases as well as aptamer-based detection.

Probing and characterizing the high specific sequences of ssDNA aptamer against SGIV-infected cells

As the major viral pathogen of grouper aquaculture, Singapore grouper iridovirus (SGIV) has caused great economic losses in China and Southeast Asia. In the previous study, we have generated highly specific ssDNA aptamers against SGIV-infected grouper spleen cells (GS) by Systematic Evolution of Ligands by Exponential Enrichment technology (SELEX), in which Q2 had the highest binding affinity of 16.43 nM. In this study, we would try to identify the specific sequences in the aptamer Q2 that exhibited the high binding affinity to SGIV-infected cells by truncating the original Q2 into some different specific segments. We first evaluated the specificity and binding affinity of these truncated aptamers to SGIV-infected cells by flow cytometry, fluorescent imaging of cells and aptamer-based enzyme-linked apta-sorbent assay (ELASA). We then performed cytotoxicity analysis, assessment of the inhibitory effects upon SGIV infection and the celluar internalization kinetics of each truncated aptamer. Compared to the initial Q2, one of the truncated aptamer Q2-C5 showed a 3-fold increase in the binding affinity for SGIV-infected cells, and held more effective inhibitory effects, higher internalization kinetics and stability. Hence, the aptamer's truncated methods could be applied in the research of identifying aptamer's key sequences. The shorter, structure optimizing aptamer showed more excellent performance over the originally selected aptamer, which could potentially be applied in developing commercial detection probes for the early and rapid diagnosis of SGIV infection, and highly specific therapeutic drugs against SGIV infection.

Application of aptamers in diagnostics, drug-delivery and imaging

Aptamers are small, single-stranded oligonucleotides (DNA or RNA) that bind to their target with high specificity and affinity. Although aptamers are analogous to antibodies for a wide range of target recognition and variety of applications, they have significant advantages over antibodies. Since aptamers have recently emerged as a class of biomolecules with an application in a wide array of fields, we need to summarize the latest developments herein. In this review we will discuss about the latest developments in using aptamers in diagnostics, drug delivery and imaging. We begin with diagnostics, discussing the application of aptamers for the detection of infective agents itself, antigens/ toxins (bacteria), biomarkers (cancer), or a combination. The ease of conjugation and labelling of aptamers makes them a potential tool for diagnostics. Also, due to the reduced off-target effects of aptamers, their use as a potential drug delivery tool is emerging rapidly. Hence, we discuss their use in targeted delivery in conjugation with siRNAs, nanoparticles, liposomes, drugs and antibodies. Finally, we discuss about the conjugation strategies applicable for RNA and DNA aptamers for imaging. Their stability and self-assembly after heating makes them superior over protein-based binding molecules in terms of labelling and conjugation strategies.

A novel method for detection of H9N2 influenza viruses by an aptamer-real time-PCR

H9N2 Influenza subtype has emerged in Tunisia causing epidemics in poultry and resulting in major economic losses. New mutations in their hemagglutinin and neuraminidase proteins were acquired, suggesting their potential to directly infect humans. Effective surveillance tools should be implemented to help prevent potential spillover of the virus across species. We have developed a highly sensitive real time immuno-polymerase chain reaction (RT-I-PCR) method for detecting H9N2 virus. The assay applies aptamers as ligands to capture and detect the virus. First, a panel of specific ssDNA aptamers was selected via a one step high stringency protocol. Next, the panel of selected aptamers was characterized for their affinities and their specificity to H9N2 virus. The aptamer showing the highest binding affinity to the virus was used as ligand to develop a highly sensitive sandwich Aptamer I-PCR. A 3-log increase in analytical sensitivity was achieved as compared to a routinely used ELISA antigen test, highlighting the potential of this approach to detect very low levels of virus particles. The test was validated using clinical samples and constitutes a rapid and a label-free platform, opening a new venue for the development of aptamer -based viability sensing for a variety of microorganisms of economic importance in Tunisia and surrounding regions.

Oligonucleotide aptamers: potential novel molecules against viral hepatitis

Viral hepatitis, as an international public health concern, seriously affects communities and health system. In recent years, great strides have been taken for development of new potential tools against viral hepatitis. Among these efforts, a valuable strategy introduced new molecules called “aptamers”. Aptamers as potential alternatives for antibodies could be directed against any protein in infected cells and any components of viral particles. In this review, we will focus on recent advances in the diagnosis and treatment of viral hepatitis based on aptamer technology. In recent years, various types of aptamers including RNA and DNA were introduced against viral hepatitis. Some of these aptamers can be utilized for early and precise diagnosis of hepatitis infections and other group selected as therapeutic tools against viral targets. Designing diagnostic and therapeutic platforms based on aptamer technology is a promising approach in viral infections. The obtained aptamers in the recent years showed obvious potential for use as diagnostic and therapeutic tools against viral hepatitis. Although some modifications to increase the biostability and half-life of aptamers are underway, it seems these molecules will be a favorable substitute for monoclonal antibody in near future.

Use of Aptamers as Diagnostics Tools and Antiviral Agents for Human Viruses

Appropriate diagnosis is the key factor for treatment of viral diseases. Time is the most important factor in rapidly developing and epidemiologically dangerous diseases, such as influenza, Ebola and SARS. Chronic viral diseases such as HIV-1 or HCV are asymptomatic or oligosymptomatic and the therapeutic success mainly depends on early detection of the infective agent. Over the last years, aptamer technology has been used in a wide range of diagnostic and therapeutic applications and, concretely, several strategies are currently being explored using aptamers against virus proteins. From a diagnostics point of view, aptamers are being designed as a bio-recognition element in diagnostic systems to detect viral proteins either in the blood (serum or plasma) or into infected cells. Another potential use of aptamers is for therapeutics of viral infections, interfering in the interaction between the virus and the host using aptamers targeting host-cell matrix receptors, or attacking the virus intracellularly, targeting proteins implicated in the viral replication cycle. In this paper, we review how aptamers working against viral proteins are discovered, with a focus on recent advances that improve the aptamers' properties as a real tool for viral infection detection and treatment.

Developing Aptamers by Cell-Based SELEX

The reliable targeting of cell surface disease-associated proteins is a major challenge in chemical biology and molecular medicine. In this regard, aptamers represent a very attractive and innovative class of ligand molecules. Aptamers are generated by a reiterated in vitro procedure, named SELEX (Systematic Evolution of Ligands by Exponential enrichment). In order to generate aptamers for heavily modified cell surface-bound proteins and transmembrane receptors, the SELEX procedure has been recently adapted to the use of living cells as complex targets (referred as "cell-SELEX"). Here we give an overview on the most recent advances in the field of cell-SELEX technology, providing a detailed description of the differential cell-SELEX approach that has been developed in our laboratory to identify specific signatures for human malignant glioma and non-small-cell lung cancer. The procedures used for the evaluation of binding specificity and for the preliminary identification of potential target receptors will be also described.

Recent Progress in Aptamer-Based Functional Probes for Bioanalysis and Biomedicine

Nucleic acid aptamers are short synthetic DNA or RNA sequences that can bind to a wide range of targets with high affinity and specificity. In recent years, aptamers have attracted increasing research interest due to their unique features of high binding affinity and specificity, small size, excellent chemical stability, easy chemical synthesis, facile modification, and minimal immunogenicity. These properties make aptamers ideal recognition ligands for bioanalysis, disease diagnosis, and cancer therapy. This review highlights the recent progress in aptamer selection and the latest applications of aptamer-based functional probes in the fields of bioanalysis and biomedicine.

Aptamer-nanobody based ELASA for specific detection of Acinetobacter baumannii isolates

Acinetobacter baumannii has turned into an important threat in nosocomial outbreak infections and multidrug resistance leading to high mortality rates in the 21st century. In recent years its mortality has increased by 15% which in part could be due to lack of a rapid and sensitive diagnostic test. In this work we introduced a new detection test for A. baumannii with two highly specific aptamer and nanobody molecules. High binding affinity DNA oligonucleotide aptamers toward A. baumannii were selected through 12 rounds of whole cell System Evolution of Ligands by EXponential enrichment process (SELEX). The SELEX procedures was monitored by flow cytometry. The dissociation constant and binding efficiency of the selected aptamer Aci49 was 7.547±1:353pM and 47.50%, respectively. A sandwich enzyme linked aptamer sorbent assay (ELASA) was designed with the biotinylated Aci49 aptamer and our previously developed nanobody against biofilm associated protein (Bap). The assay system was optimized with A. baumannii (ATCC 19606) and 47 clinical isolates of A. baumannii were tested. The threshold of detection in sandwich ELASA process was10(3) CFU/ml. The sensitivity of test toward the clinical isolates was 95.47%. Our results reveal that the sandwich ELASA is sensitive and specific enough for the rapid detection of A. baumannii from clinical isolates.

Aptasensors for viral diagnostics

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We discuss progress in aptamer-based detection of viruses.

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We consider the use of aptasensors for point-of-care diagnostics of viruses.

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Aptamers have distinct advantages over antibodies for virus recognition.

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There is strong demand for multiplexed diagnostic measurement of pathogens.

Novel viral diagnostic tools need to be affordable, fast, accurate and easy to use with sensitivity and specificity equivalent or superior to current standards. At present, viral diagnostics are based on direct detection of viral components or indirect detection by measuring antibodies generated in response to viral infection. While sensitivity of detection and quantification are still important challenges, we expect major advances from new assay formats and synthetic binding molecules, such as aptamers. Compared to traditional antibody-based detection, aptamers could provide faster adaptation to continuously evolving virus strains and higher discriminating capacity between specific virus serotypes. Aptamers are very stable and easily modifiable, so are ideal molecules for detection and chemical sensing applications. Here, we review the use of aptasensors for detection of viral pathogens and consider the feasibility of aptasensors to become standard devices for point-of-care diagnostics of viruses.

Aptamers against pathogenic microorganisms

An important current issue of modern molecular medicine and biotechnology is the search for new approaches to early diagnostic assays and adequate therapy of infectious diseases. One of the promising solutions to this problem might be a development of nucleic acid aptamers capable of interacting specifically with bacteria, protozoa, and viruses. Such aptamers can be used for the specific recognition of infectious agents as well as for blocking of their functions. The present review summarizes various modern SELEX techniques used in this field, and of several currently identified aptamers against viral particles and unicellular organisms, and their applications. The prospects of applying nucleic acid aptamers for the development of novel detection systems and antibacterial and antiviral drugs are discussed.

Single-Stranded DNA Aptamers against Pathogens and Toxins: Identification and Biosensing Applications

Molecular recognition elements (MREs) can be short sequences of single-stranded DNA, RNA, small peptides, or antibody fragments. They can bind to user-defined targets with high affinity and specificity. There has been an increasing interest in the identification and application of nucleic acid molecular recognition elements, commonly known as aptamers, since they were first described in 1990 by the Gold and Szostak laboratories. A large number of target specific nucleic acids MREs and their applications are currently in the literature. This review first describes the general methodologies used in identifying single-stranded DNA (ssDNA) aptamers. It then summarizes advancements in the identification and biosensing application of ssDNA aptamers specific for bacteria, viruses, their associated molecules, and selected chemical toxins. Lastly, an overview of the basic principles of ssDNA aptamer-based biosensors is discussed.

Aptamer-based sensing of β-casomorphin-7

β-Casomorphin-7 (BCM-7), a seven amino acid peptide, is released during digestion of β-casein A1 variant of milk which is speculated to be associated with certain diseases. Fifteen ssDNA aptamers having high affinity toward BCM-7 were identified from a 72 nt long random library after ten rounds of systematic evolution of ligands by exponential enrichment. Dissociation constant values of selected aptamers were in the range of 7.7-156.7 nM. Seq6 aptamer exhibited the lowest Kd value. Nine aptamers were evaluated for their binding toward BCM-7, BCM-9A1, and BCM-9A2 peptides, and binding was variable. SeqU5 exhibited the lowest binding with BCM-9A1 and BCM-9A2. Aptamer-coated gold nanoparticles (GNPs) resulted in color change of GNPs in the presence of BCM-7, thereby establishing recognition of BCM-7 by aptamers. The enzyme-linked aptamer-sorbent assay (ELASA) was evaluated as an assay of BCM-7 in biological fluids. BCM-7-peroxidase competed with BCM-7 in ELASA, performed with BCM-7 solution and BCM-7 spiked urine pretreated with urease, plasma, and β-casein digest samples.

Selection of aptamers for aflatoxin M1 and their characterization

In the present work, aptamers against aflatoxin M1 and aflatoxin B1 were generated and tested for creating proof of principle of recognition of aflatoxin M1 by generated aptamers. The aptamers were selected through the process referred as systematic evolution of ligands by exponential enrichment. A total of 41 different aptamer (36 aptamers for aflatoxin M1 and 5 for aflatoxin B1) sequences were obtained. The determination of dissociation constant (Kd ) values revealed that aptamers generated against aflatoxin M1 exhibited Kd values in the range of 35-1515 nM. Selected aptamers were grouped on the basis of the presence of common motifs or G-quadruplex. We find it interesting that one aptamer with no conserved motif or G-quadruplex had lowest Kd value (Kd = 35 nM). This structural motif is very distinct from motifs present in other aptamers. The Kd values of selected aptamers for aflatoxin B1 were in the range of 96-221 nM. One aptamer from each group was further tested for its ability to be used in aptasensor. The aptamer recognized aflatoxin M1 as indicated by color change (red to purple or blue) of aptamer-coated gold nanoparticles in the presence of 250-500 nM aflatoxin M1. The aptamers can be used in developing methods for detection/estimation/separation of aflatoxin or antidote for aflatoxin toxicity.

Aptamers in diagnostics and treatment of viral infections

Aptamers are in vitro selected DNA or RNA molecules that are capable of binding a wide range of nucleic and non-nucleic acid molecules with high affinity and specificity. They have been conducted through the process known as SELEX (Systematic Evolution of Ligands by Exponential Enrichment). It serves to reach specificity and considerable affinity to target molecules, including those of viral origin, both proteins and nucleic acids. Properties of aptamers allow detecting virus infected cells or viruses themselves and make them competitive to monoclonal antibodies. Specific aptamers can be used to interfere in each stage of the viral replication cycle and also inhibit its penetration into cells. Many current studies have reported possible application of aptamers as a treatment or diagnostic tool in viral infections, e.g., HIV (Human Immunodeficiency Virus), HBV (Hepatitis B Virus), HCV (Hepatitis C Virus), SARS (Severe Acute Respiratory Syndrome), H5N1 avian influenza and recently spread Ebola. This review presents current developments of using aptamers in the diagnostics and treatment of viral diseases.

Screening and characterization of a novel RNA aptamer that specifically binds to human prostatic acid phosphatase and human prostate cancer cells

Prostatic acid phosphatase (PAP) expression increases proportionally with prostate cancer progression, making it useful in prognosticating intermediate to high-risk prostate cancers. A novel ligand that can specifically bind to PAP would be very helpful for guiding prostate cancer therapy. RNA aptamers bind to target molecules with high specificity and have key advantages such as low immunogenicity and easy synthesis. Here, human PAP-specific aptamers were screened from a 2'-fluoropyrimidine (FY)-modified RNA library by SELEX. The candidate aptamer families were identified within six rounds followed by analysis of their sequences and PAP-specific binding. A gel shift assay was used to identify PAP binding aptamers and the 6N aptamer specifically bound to PAP with a Kd value of 118 nM. RT-PCR and fluorescence labeling analyses revealed that the 6N aptamer bound to PAP-positive mammalian cells, such as PC-3 and LNCaP. IMR-90 negative control cells did not bind the 6N aptamer. Systematic minimization analyses revealed that 50 nucleotide sequences and their two hairpin structures in the 6N 2'-FY RNA aptamer were equally important for PAP binding. Renewed interest in PAP combined with the versatility of RNA aptamers, including conjugation of anti-cancer drugs and nano-imaging probes, could open up a new route for early theragnosis of prostate cancer.

Aptamer-conjugated magnetic nanoparticles for the efficient removal of HCV particles from human plasma samples

Described here is a methodology for selectively capturing HCV particles from human plasma samples using aptamer-conjugated magnetic nanoparticles. The aptamers were specifically bound to the E1E2 glycoprotein of HCV viruses.

Identification of aptamer-binding sites in hepatitis C virus envelope glycoprotein e2

Hepatitis C Virus (HCV) encodes two envelope glycoproteins, E1 and E2. Our previous work selected a specific aptamer ZE2, which could bind to E2 with high affinity, with a great potential for developing new molecular probes as an early diagnostic reagents or therapeutic drugs targeting HCV. In this study, the binding sites between E2 and aptamer ZE2 were further explored. E2 was truncated to 15 peptides (P1 to P15) and these peptides were used to detect the affinity with ZE2 by ELISA respectively. The peptide with high affinity was then further truncated, detected and compared with six kinds of HCV genotypes. The basic amino acid in 500 aa bound to ZE2 with high affinity, while acidic amino acid in 501 aa reduced the reaction between E2 and ZE2. The results showed the 500 aa and 501 aa of E2 were the key sites that bound to ZE2.

Nanomaterial-based cocaine aptasensors

Up to now, many different methods have been developed for detection of cocaine, but most of these methods are usually time-consuming, tedious and require special or expensive equipment. Therefore, the development of simple, sensitive and rapid detection methods is necessary. In the last decade, aptamers have been used as a new biosensor platform for detection of cocaine in different samples. Aptamers are artificial single-stranded DNA or RNA oligonucleotides capable of binding to specific molecular targets with high affinity and if integrated to nanomaterials, it may lead in precise methods for cocaine detection in the common laboratories. In this review, recent advances and applications of aptamer-based biosensors and nanobiosensors, have been updated, paying attention to the use of fluorescence, colorimetric and electrochemical techniques for the detection and quantitative determination of cocaine.

Aptamers as a replacement for antibodies in enzyme-linked immunosorbent assay

The application of antibodies in enzyme-linked immunosorbent assay (ELISA) is the basis of this diagnostic technique which is designed to detect a potpourri of complex target molecules such as cell surface antigens, allergens, and food contaminants. However, development of the systematic evolution of Ligands by Exponential Enrichment (SELEX) method, which can generate a nucleic acid-based probe (aptamer) that possess numerous advantages compared to antibodies, offers the possibility of using aptamers as an alternative molecular recognition element in ELISA. Compared to antibodies, aptamers are smaller in size, can be easily modified, are cheaper to produce, and can be generated against a wide array of target molecules. The application of aptamers in ELISA gives rise to an ELISA-derived assay called enzyme-linked apta-sorbent assay (ELASA). As with the ELISA method, ELASA can be used in several different configurations, including direct, indirect, and sandwich assays. This review provides an overview of the strategies involved in aptamer-based ELASA.

An aptamer targets HBV core protein and suppresses HBV replication in HepG2.2.15 cells

Hepatitis B virus (HBV)-related hepatitis is a major health concern worldwide. As current anti-HBV therapies are limited, it is essential to develop new strategies. Aptamer, a newly developed adaptive molecule (single-strand DNA or RNA also known as nucleotide antibody), is a new strategy for clinical diagnosis and therapy due to its high affinity and specificity. In the present study, by systematic evolution of ligand by exponential enrichment (SELEX), aptamers were screened against the core protein of HBV (HBc) from a random ssDNA library. Quantitative PCR (qPCR) results showed that the binding proportions of the SELEX-enriched aptamer pools were increased with the SELEX rounds, until round seven. Thus, the pool of round seven was cloned. Following the sequence analysis of a total of 90 clones by Macaw software, five aptamer candidates were selected and their affinity to HBc was tested by dot blot. Apt.No.28, which had sequence replicates in the clones, was shown to have a high affinity. Furthermore, by agarose gel electrophoresis-capillary transfer-blotting and qPCR, Apt.No.28 was shown to inhibit the assembly of the nucleocapsid, reducing extracellular HBV DNA whose synthesis relied on the formation of the nucleocapsid, indicating its role in suppressing HBV replication. The results provided a new ideal targeting molecule and may facilitate the strategy for targeted therapy as well as drug development of HBV-related diseases.

Ficolin-2 inhibits hepatitis C virus infection, whereas apolipoprotein E3 mediates viral immune escape

Human ficolin-2 (L-ficolin/p35) is a lectin-complement pathway activator that is present in normal human plasma and is associated with infectious diseases; however, little is known regarding the roles and mechanisms of ficolin-2 during chronic hepatitis C virus (HCV) infection. In this study, we found that ficolin-2 inhibits the entry of HCV at an early stage of viral infection, regardless of the viral genotype. Ficolin-2 neutralized and inhibited the initial attachment and infection of HCV by binding to the HCV envelope surface glycoproteins E1 and E2, blocking HCV attachment to low-density lipoprotein receptor (LDLR) and scavenger receptor B1, and weakly interfering with CD81 receptor attachment. However, no interference with claudin-1 and occludin receptor attachment was observed. The C-terminal fibrinogen domain (201-313 aa) of ficolin-2 was identified as the critical binding region for the HCV-E1-E2 N-glycans, playing a critical role in the anti-HCV activity. More importantly, we found that apolipoprotein E (ApoE)3, which is enriched in the low-density fractions of HCV RNA-containing particles, promotes HCV infection and inhibits ficolin-2-mediated antiviral activity. ApoE3, but not ApoE2 and ApoE4, blocked the interaction between ficolin-2 and HCV-E2. Our data suggest that the HCV entry inhibitor ficolin-2 is a novel and promising antiviral innate immune molecule, whereas ApoE3 blocks the effect of ficolin-2 and mediates an immune escape mechanism during chronic HCV infection. HCV may be neutralized using compounds directed against the lipoprotein moiety of the viral particle, and ApoE3 may be a new target to combat HCV infection.

CFP10 and ESAT6 aptamers as effective Mycobacterial antigen diagnostic reagents

The development of effective Mycobacterial antigen diagnostic reagents remains a high priority. The 6-kDa early secreted antigenic target (ESAT6) and 10-kDa culture filtrate protein (CFP10) are secreted early by virulent Mycobacterium tuberculosis (M. tb) and are not present in the non-virulent Bacillus Calmette-Guerin (BCG). In this study, we used a Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technique to screen for a functional ssDNA aptamer "antibody" that specifically bound to ESAT6-CFP10 (CE) protein. The selected single ssDNA aptamers (CE24 and CE15) demonstrated the highest specificity and binding affinity to CFP10 (CE24: Kd = 3.75 × 10(-7) M) and ESAT6 (CE15: Kd = 1.6 × 10(-7) M). We further detected CFP10 and ESAT6 proteins in serum samples from active pulmonary tuberculosis (TB) patients, extrapulmonary TB patients and healthy donors by using an enzyme-linked oligonucleotide assay (ELONA). The results showed that the sensitivity and specificity were 100% and 94.1% (using CE24 aptamer-based ELONA) and 89.6% and 94.1% (using CE15 aptamer-based ELONA), respectively. A good correlation was observed between aptamer-based ELONA and T-SPOT TB assay. Thus, our study suggests that CE24 and CE15 have potentially broad applications as early antigen diagnostic agents not only for active pulmonary TB, extrapulmonary TB, but also possibly for latent TB infection and TB with immune-deficiency.

DNA Aptamer Technology for Personalized Medicine

This review highlights recent progress in developing DNA aptamers for personalized medicine, with more focus on in vivo studies for potential clinical applications. Examples include design of aptamers in combination with DNA nanostructures, nanomaterials, or microfluidic devices as diagnostic probes or therapeutic agents for cancers and other diseases. The use of aptamers as targeting agents in drug delivery is also covered. The advantages and future directions of such DNA aptamer-based technology for the continued development of personalized medicine are discussed.

Nucleic Acid aptamers: new methods for selection, stabilization, and application in biomedical science

The adoption of oligonucleotide aptamer is well on the rise, serving an ever increasing demand for versatility in biomedical field. Through the SELEX (Systematic Evolution of Ligands by EXponential enrichment), aptamer that can bind to specific target with high affinity and specificity can be obtained. Aptamers are single-stranded nucleic acid molecules that can fold into complex threedimensional structures, forming binding pockets and clefts for the specific recognition and tight binding of any given molecular target. Recently, aptamers have attracted much attention because they not only have all of the advantages of antibodies, but also have unique merits such as thermal stability, ease of synthesis, reversibility, and little immunogenicity. The advent of novel technologies is revolutionizing aptamer applications. Aptamers can be easily modified by various chemical reactions to introduce functional groups and/or nucleotide extensions. They can also be conjugated to therapeutic molecules such as drugs, drug containing carriers, toxins, or photosensitizers. Here, we discuss new SELEX strategies and stabilization methods as well as applications in drug delivery and molecular imaging.

Aptamers: novel molecules as diagnostic markers in bacterial and viral infections?

Worldwide the entire human population is at risk of infectious diseases of which a high degree is caused by pathogenic protozoans, worms, bacteria, and virus infections. Moreover the current medications against pathogenic agents are losing their efficacy due to increasing and even further spreading drug resistance. Therefore, there is an urgent need to discover novel diagnostic as well as therapeutic tools against infectious agents. In view of that, the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) represents a powerful technology to target selectively pathogenic factors as well as entire bacteria or viruses. SELEX uses a large combinatorial oligonucleic acid library (DNA or RNA) which is processed a by high-flux in vitro screen of iterative cycles. The selected ligands, termed aptamers, are characterized by high specificity and affinity to their target molecule, which are already exploited in diagnostic and therapeutic applications. In this minireview we will discuss the current status of the SELEX technique applied on bacterial and viral pathogens.

Antimicrobial aptamers for detection and inhibition of microbial pathogen growth

Discovery of alternative sources of antimicrobial agents are essential in the ongoing battle against microbial pathogens. Legislative and scientific challenges considerably hinder the discovery and use of new antimicrobial drugs, and new approaches are in urgent demand. On the other hand, rapid, specific and sensitive detection of airborne pathogens is becoming increasingly critical for public health. In this respect affinity oligonucleotides, aptamers, provide unique opportunities for the development of nanotechnological solutions for such medical applications. In recent years, aptamers specifically recognizing microbial cells and viruses showed great potential in a range of analytical and therapeutic applications. This article describes the significant advances in the development of aptamers targeting specific pathogens. Therapeutic application of aptamers as neutralizing agents demonstrates great potential as a future source of antimicrobial agent.

Dual-aptamer-based biosensing of toxoplasma antibody

A panel of seven aptamers to antitoxoplasma IgG is first discovered in this report. The aptamers are selected using systematic evolution of ligands by exponential enrichment (SELEX) technology, cloned, and identified by sequencing and affinity assay. Among them, two aptamers (TGA6 and TGA7) with the highest affinities are employed as capture probe and detection probe in developing a quantum dots-labeled dual aptasensor (Q-DAS). In the presence of antitoxoplasma IgG, an aptamer-protein-aptamer sandwich complex (TGA6-IgG-TGA7) is formed and captured on a multiwell microplate, whose fluorescence can be read out using quantum dots as the fluorescence label, ensuring highly sensitive and specific sensing of antitoxoplasma IgG. The operating characteristics of the proposed assay are guaranteed using dual aptamers as the recognizing probes when compared with antibody-based immunoassay. Q-DAS has a linearity within the range of 0.5-500 IU with a lowest detection of 0.1 IU. Receiver operating curves of 212 clinical samples show a 94.8% sensitivity and 95.7% specificity when the cutoff value is set as 6.5 IU, indicating the proposed Q-DAS is a promising assay in large-scale screening of toxoplasmosis.