Simultaneous improvement of specificity and affinity of aptamers against Streptococcus mutans by in silico maturation for biosensor development.

In silico evolution with an in vitro system can facilitate the development of functional aptamers with high specificity and affinity. Although a general technique known as systematic evolution of ligand by exponential enrichment (SELEX) is an efficient method for aptamer selection, it sometimes fails to identify aptamers with sufficient binding properties. We have previously developed in silico maturation (ISM) to improve functions of aptamers based on genetic algorithms. ISM represents an intelligent exploitation of a random search within a defined sequence space to optimize aptamer sequences and improve their function of interest. Here we demonstrated a successful application of ISM of aptamers to simultaneously improve specificity and affinity for Streptococcus mutans with discovery of a core sequence, which was required to form a polymerized guanine quadruplex structure for target binding. We applied ISM to aptamers selected by whole-cell SELEX and identified an aptamer with up to 16-fold improvement in affinity compared to its parent aptamers, and specificity was increased to show 12-fold more binding to S. mutans than to Lactobacillus acidophilus. Furthermore, we demonstrated a specific flow-through detection of S. mutans at a concentration range of 1 × 10(5) -10(8) CFU/mL using the evolved aptamer immobilized on gold colloids.

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.

Selection of DNA aptamers that inhibit enzymatic activity of PQQGDH and its application.

We screened DNA aptamers against the water-soluble quinoprotein glucose dehydrogenase (PQQGDH) with a competitive selection method. PQQGDH is a dimeric enzyme that consists of 50-kDa subunits, and has high catalytic activity for glucose (about 5000 U/mg), therefore is used for glucose sensors in the market. PQQGDH is an excellent molecular recognition device for biosensor for diagnosis. The SELEX was performed using a competitive selection method which enables us to select aptamer having high specificity for a target molecule. After 6 rounds screening, we obtained 2 aptamers which bind to PQQGDH with high affinity and specificity. One aptamer inhibited PQQGDH activity. In addition, we constructed a dimer aptamer linked by linker sequences expecting high affinity compared to the monomer aptamer. As the result of Aptamer Blotting, the dimer aptamer showed higher affinity than the monomer.