Aptameric enzyme subunit for homogeneous DNA sensing

G-quadruplex-forming aptamer enhances the peroxidase activity of myoglobin against luminol

Aptamers can control the biological functions of enzymes, thereby facilitating the development of novel biosensors. While aptamers that inhibit catalytic reactions of enzymes were found and used as signal transducers to sense target molecules in biosensors, no aptamers that amplify enzymatic activity have been identified. In this study, we report G-quadruplex (G4)-forming DNA aptamers that upregulate the peroxidase activity in myoglobin specifically for luminol. Using in vitro selection, one G4-forming aptamer that enhanced chemiluminescence from luminol by myoglobin's peroxidase activity was discovered. Through our strategy—in silico maturation, which is a genetic algorithm-aided sequence manipulation method, the enhancing activity of the aptamer was improved by introducing mutations to the aptamer sequences. The best aptamer conserved the parallel G4 property with over 300-times higher luminol chemiluminescence from peroxidase activity more than myoglobin alone at an optimal pH of 5.0. Furthermore, using hemin and hemin-binding aptamers, we demonstrated that the binding property of the G4 aptamers to heme in myoglobin might be necessary to exert the enhancing effect. Structure determination for one of the aptamers revealed a parallel-type G4 structure with propeller-like loops, which might be useful for a rational design of aptasensors utilizing the G4 aptamer-myoglobin pair.

Nucleic acid aptamer-based methods for diagnosis of infections

Infectious diseases are a serious global problem, which not only take an enormous human toll but also incur tremendous economic losses. In combating infectious diseases, rapid and accurate diagnostic tests are required for pathogen identification at the point of care (POC). In this review, investigations of diagnostic strategies for infectious diseases that are based on aptamers, especially nucleic acid aptamers, oligonucleotides that have high affinities and specificities toward their targets, are described. Owing to their unique features including low cost of production, easy chemical modification, high chemical stability, reproducibility, and low levels of immunogenicity and toxicity, aptamers have been widely utilized as bio-recognition elements (bio-receptors) for the development of infection diagnostic systems. We discuss nucleic acid aptamer-based methods that have been developed for diagnosis of infections using a format that organizes discussion according to the target pathogenic analytes including toxins or proteins, whole cells and nucleic acids. Also included is, a summary of recent advances made in the sensitive detection of pathogenic bacteria utilizing the isothermal nucleic acid amplification method. Lastly, a nucleic acid aptamer-based POC system is described and future directions of studies in this area are discussed.

Target DNA induced switches of DNA polymerase activity

A novel concept that target DNA can induce switching of DNA polymerase activity is devised. The method relies on the finding that a DNA aptamer can undergo conformational change upon hybridization with a complementary target DNA, which leads to activation or inactivation of DNA polymerase. This strategy is utilized to identify the presence of target DNA with high levels of sensitivity and selectivity.

Emerging techniques employed in aptamer-based diagnostic tests

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

Aptameric sensors based on structural change for diagnosis

Aptamers are nucleic acids that can bind to various molecules. Because they have some features that are lacking in antibodies, aptamers could serve as alternatives to antibodies. For the purpose of biosensing, we focused on aptamers that undergo structural changes on binding to their target molecules. We constructed an aptamer-based bound/free (B/F) separation system that uses a designed aptamer named the "capturable aptamer". The capturable aptamer changes its structure upon recognizing its target molecule thereby exposing a specific single-strand region. The oligonucleotide that is complementary to this exposed region, named the "capture DNA" is immobilized on a support. This design permits the exclusive capture by the capture DNA of the aptamer bound to its target, and subsequent removal of any unbound aptamer and contaminants by B/F separation. The removal of unbound contaminants or aptamers results in highly sensitive detection at similar levels to those achievable by sandwich-based immunoassay. We describe the construction of a thrombin-detection system by using a capturable aptamer, and we discuss the potential of capturable aptamers in clinical diagnostics.

Selection of DNA aptamers using atomic force microscopy

Atomic force microscopy (AFM) can detect the adhesion or affinity force between a sample surface and cantilever, dynamically. This feature is useful as a method for the selection of aptamers that bind to their targets with very high affinity. Therefore, we propose the Systematic Evolution of Ligands by an EXponential enrichment (SELEX) method using AFM to obtain aptamers that have a strong affinity for target molecules. In this study, thrombin was chosen as the target molecule, and an ‘AFM-SELEX’ cycle was performed. As a result, selected cycles were completed with only three rounds, and many of the obtained aptamers had a higher affinity to thrombin than the conventional thrombin aptamer. Moreover, one type of obtained aptamer had a high affinity to thrombin as well as the anti-thrombin antibody. AFM-SELEX is, therefore, considered to be an available method for the selection of DNA aptamers that have a high affinity for their target molecules.

Selection of a DNA aptamer that binds 8-OHdG using GMP-agarose

DNA aptamers, which bind specific molecule, such as 8-OHdG, with high affinity were investigated using an in vitro selection strategy called systematic evolution of ligands by exponential enrichment (SELEX). However, 8-OHdG was difficult to immobilize on a carrier for SELEX. Therefore, a DNA aptamer binding to 8-OHdG was selected using GMP-agarose as an analogue from a library of about 4(60) random ssDNA sources. As a result, three aptamer candidates were selected. Among the selected DNA aptamers, the No. 22 DNA aptamer exhibited a high affinity for 8-OHdG. The dissociation constant, K(D), of No. 22 DNA aptamer was on the order of 0.1micromol/L. This result suggests that using an analogue will be a useful new SELEX method for obtaining various aptamers that are difficult to immobilize on a matrix.

Detection system based on the conformational change in an aptamer and its application to simple bound/free separation

Aptamers are good molecular recognition elements for biosensors. Especially, their conformational change, which is induced by the binding to the target molecule, enables the development of several types of useful detection systems. We applied this property to bound/free separation, which is a crucial process for highly sensitive detection. We designed aptamers which change their conformation upon binding to the target molecule and thereby expose a single-strand bearing the complementary sequence to the capture probe immobilized onto the support. We named the designed aptamers "capturable aptamers" and the capture probe "capture DNA". Three capturable aptamers were designed based on the PrP aptamer, which binds to prion protein. One of these capturable aptamers was demonstrated to recognize prion protein and change its conformation upon binding to it. A detection system using this designed capturable aptamer for prion protein was developed. Capturable aptamers and capture DNA allow us to perform simple bound/free separation with only one target ligand.

Selection of DNA aptamers against insulin and construction of an aptameric enzyme subunit for insulin sensing

We selected DNA aptamers against insulin and developed an aptameric enzyme subunit (AES) for insulin sensing. The insulin-binding aptamers were identified from a single-strand DNA library which was expected to form various kinds of G-quartet structures. In vitro selection was carried out by means of aptamer blotting, which visualizes the oligonucleotides binding to the target protein at each round. After the 6th round of selection, insulin-binding aptamers were identified. These identified insulin-binding aptamers had a higher binding ability than the insulin-linked polymorphic region (ILPR) oligonucleotide, which can be called a "natural" insulin-binding DNA aptamer. The circular-dichroism (CD) spectrum measurement of the identified insulin-binding DNA aptamers indicated that the aptamers would fold into a G-quartet structure. We also developed an AES by connecting the best identified insulin-binding aptamer with the thrombin-inhibiting aptamer. Using this AES, we were able to detect insulin by measuring the thrombin enzymatic activity without bound/free separation.

Label-free homogeneous detection of immunoglobulin E by an aptameric enzyme subunit

We have developed an aptameric enzyme subunit (AES) for immunoglobulin E (IgE) sensing. AES is an artificial enzyme subunit constructed from two different aptamers and does not require any modification. Using the AES, the target molecule can be detected by measuring enzymatic activity in homogeneous solution. We connected IgE-binding aptamer and its complementary strand to split thrombin-inhibiting aptamer. The hybrid of these two oligonucleotides inhibited thrombin activity and it decreased in the presence of IgE. We were able to detect IgE by using this AES in homogeneous solution with a detection limit of 50 pmol.