Post-SELEX modification of DNA aptamers is an established strategy to improve their affinity or inhibitory characteristics. In this study, we examined the possibility of increasing the recognition interface between the thrombin-binding aptamer HD1 (TBA) and thrombin by adding a chemically modified side chain to selected nucleotide residues. A panel of 22 TBA variants with N3-modified residues T3 and T12 was prepared by a two-step modification procedure. Aptamers were characterized by a combination of biophysical and biochemical methods. We identified mutants with enhanced affinity and improved anticoagulant activity. The crystal structures of thrombin complexes with three selected modified variants revealed that the modified pyrimidine base invariably allocates in proximity to thrombin residues Tyr76 and Ile82 due to the directing role of the unmodified TT loop. The modifications induced an increase in the contact areas between thrombin and the modified TBAs. Comparative analysis of the structural, biochemical, and biophysical data suggests that the non-equivalent binding modes of the mutants with thrombin in the T3and T12-modified series account for the observed systematic differences in their affinity characteristics. In this study, we show that extending the recognition surface between the protein and modified aptamers is a promising approach that may improve characteristics of aptamer ligands.

In this study we describe new modified analogs of the thrombin binding aptamer (TBA) containing 5-nitroindole residues. It has been shown that all modified TBAs form an anti-parallel G-quadruplex structure and retain the ability to inhibit thrombin. The most advanced TBA variant (TBA-N8) has a substantially increased clotting time and two-fold lower IC50 value compared to the unmodified prototype. Molecular modelling studies suggest that the improved anticoagulant properties of TBA-N8 result from changes in the binding mode of the analog. A modified central loop in TBA-N8 is presumed to participate in the binding of the target protein. Studies of FAM labelled TBA and TBA-N8 showed an improved binding affinity of the modified aptamer and provided evidence of a direct interaction between the modified central loop and thrombin. Our findings have implications for the design of new aptamers with improved binding affinities.

Oligonucleotide–peptide conjugates (OPCs) are a promising class of biologically active compounds with proven potential for improving nucleic acid therapeutics. OPCs are commonly recognized as an efficient instrument to enhance the cellular delivery of therapeutic nucleic acids. In addition to this application field, OPCs have an as yet unexplored potential for the post-SELEX optimization of DNA aptamers. In this paper, we report the preparation of designer thrombin aptamer OPCs with peptide side chains anchored to a particular thymidine residue of the aptamer. The current conjugation strategy utilizes unmodified short peptides and support-bound protected oligonucleotides with activated carboxyl functionality at the T3 thymine nucleobase. The respective modification of the oligonucleotide strand was implemented using N3-derivatized thymidine phosphoramidite. Aptamer OPCs retained the G-quadruplex architecture of the parent DNA structure and showed minor to moderate stabilization. In a series of five OPCs, conjugates bearing T3–Ser–Phe–Asn (SFN) or T3–Tyr–Trp–Asn (YWN) side chains exhibited considerably improved anticoagulant characteristics. Molecular dynamics studies of the aptamer OPC complexes with thrombin revealed the roles of the amino acid nature and sequence in the peptide subunit in modulating the anticoagulant activity.

Recent advances in G-quadruplex (GQ) studies have provided evidence for their important role in key biological processes (replication, transcription, genome stability, and epigenetics). These findings imply highly specific interactions between GQ structures and cellular proteins. The details of the interaction between GQs and cellular proteins remain unknown. It is now accepted that GQ loop elements play a major role in protein recognition. It remains unclear whether and to what extent the GQ core contributes to maintaining the recognition interface. In the current paper, we used the thrombin binding aptamer as a model to study the effect of modification in the quadruplex core on the ability of aptamer to interact with thrombin. We used alpha-2′-deoxyguanosine and 8-bromo-2′-deoxyguanosine to reconfigure the core or to affect syn–anti preferences of selected dG-residues. Our data suggest that core guanines not only support a particular type of GQ architecture, but also set structural parameters that make GQ protein recognition sensitive to quadruplex topology.

A simple and stereoselective synthesis of a protected 4-(aminomethyl)-1-(2-deoxy-β-D-ribofuranosyl)-1,2,3-triazole cyanoethyl phosphoramidite was developed for the modification of synthetic oligonucleotides. The configuration of the 1,2,3-triazolyl moiety with respect to the deoxyribose was unambiguously determined in ROESY experiments. The aminomethyl group of the triazolyl nucleotide was fully functional in labelling reactions. Furthermore, the hybridization behavior of 5' triazole-terminated oligonucleotide was similar to that of 5' aminohexyl-terminated oligomer with the same sequence. Internal modifications of the oligonucleotide strands resulted in significant decrease of duplex stability.

No abstract

Replicative strand slippage is a biological phenomenon, ubiquitous among different organisms. However, slippage events are also relevant to non-natural replication models utilizing synthetic polymerase substrates. Strand slippage may notably affect the outcome of the primer extension reaction with repetitive templates in the presence of non-natural nucleoside triphosphates. In the current paper, we studied the ability of Taq, Vent (exo-), and Deep Vent (exo-) polymerases to produce truncated, full size, or expanded modified strands utilizing non-natural 2′-deoxyuridine nucleotide analogues and different variants of the homopolymer template. Our data suggest that the slippage of the primer strand is dependent on the duplex fluttering, incorporation efficiency for a particular polymerase-dNTP pair, rate of non-templated base addition, and presence of competing nucleotides.

Currently, oligonucleotide therapy has emerged as a new paradigm in the treatment of human diseases. In many cases, however, therapeutic oligonucleotides cannot be used directly without modification. Chemical modification or the conjugation of therapeutic oligonucleotides is required to increase their stability or specificity, improve their affinity or inhibitory characteristics, and address delivery issues. Recently, we proposed a conjugation strategy for a 15-nt G-quadruplex thrombin aptamer aimed at extending the recognition interface of the aptamer. In particular, we have prepared a series of designer peptide conjugates of the thrombin aptamer, showing improved anticoagulant activity. Herein, we report a new series of aptamer–peptide conjugates with optimized peptide sequences. The anti-thrombotic activity of aptamer conjugates was notably improved. The lead conjugate, TBA–GLE, was able to inhibit thrombin-induced coagulation approximately six-fold more efficiently than the unmodified aptamer. In terms of its anticoagulant activity, the TBA–GLE conjugate approaches NU172, one of the most potent G-quadruplex thrombin aptamers. Molecular dynamics studies have confirmed that the principles applied to the design of the peptide side chain are efficient instruments for improving aptamer characteristics for the proposed TBA conjugate model.