Die chemische Biologie von Aptameren

Strategies for Covalent Labeling of Long RNAs

The introduction of chemical modifications into long RNA molecules at specific positions for visualization, biophysical investigations, diagnostic and therapeutic applications still remains challenging. In this review, we present recent approaches for covalent internal labeling of long RNAs. Topics included are the assembly of large modified RNAs via enzymatic ligation of short synthetic oligonucleotides and synthetic biology approaches preparing site-specifically modified RNAs via in vitro transcription using an expanded genetic alphabet. Moreover, recent approaches to employ deoxyribozymes (DNAzymes) and ribozymes for RNA labeling and RNA methyltransferase based labeling strategies are presented. We discuss the potentials and limits of the individual methods, their applicability for RNAs with several hundred to thousands of nucleotides in length and indicate future directions in the field.

Protein Labeling and Crosslinking by Covalent Aptamers

We developed a new approach to selectively modify native proteins in their biological environment using electrophilic covalent aptamers. These aptamers are generated through introduction of a proximity-driven electrophile at specific nucleotide sites. Using thrombin as a proof-of-concept, we demonstrate that covalent aptamers can selectively transfer a variety of functional handles and/or irreversibly crosslink to the target protein. This approach offers broad programmability and high target specificity. Furthermore, it addresses issues common to aptamers such as instability towards endogenous nucleases and residence times during target engagement. Covalent aptamers are new tools that enable specific protein modification and sensitive protein detection. Moreover, they provide prolonged, nuclease-resistant enzyme inhibition.

Characterization of New DNA Aptamers for Anti-HIV-1 Reverse Transcriptase

HIV-1 RT is a necessary enzyme for retroviral replication, which is the main target for antiviral therapy against AIDS. Effective anti-HIV-1 RT drugs are divided into two groups; nucleoside inhibitors (NRTI) and non-nucleoside inhibitors (NNRTI), which inhibit DNA polymerase. In this study, new DNA aptamers were isolated as anti-HIV-1 RT inhibitors. The selected DNA aptamer (WT62) presented with high affinity and inhibition against wild-type (WT) HIV-1 RT and gave a KD value of 75.10±0.29 nM and an IC₅₀ value of 84.81±8.54 nM. Moreover, WT62 decreased the DNA polymerase function of K103 N/Y181 C double mutant (KY) HIV-1 RT by around 80 %. Furthermore, the ITC results showed that this aptamer has small binding enthalpies with both WT and KY HIV-1 RTs through which the complex might form a hydrophobic interaction or noncovalent bonding. The NMR result also suggested that the WT62 aptamer could bind with both WT and KY mutant HIV-1 RTs at the connection domain.

The Diversity of a Polyclonal FluCell-SELEX Library Outperforms Individual Aptamers as Emerging Diagnostic Tools for the Identification of Carbapenem Resistant Pseudomonas aeruginosa

Textbook procedures require the use of individual aptamers enriched in SELEX libraries which are subsequently chemically synthesized after their biochemical characterization. Here we show that this reduction of the available sequence space of large libraries and thus the diversity of binding molecules reduces the labelling efficiency and fidelity of selected single aptamers towards different strains of the human pathogen Pseudomonas aeruginosa compared to a polyclonal aptamer library enriched by a whole-cell-SELEX involving fluorescent aptamers. The library outperformed single aptamers in reliable and specific targeting of different clinically relevant strains, allowed to inhibit virulence associated cellular functions and identification of bound cell surface targets by aptamer based affinity purification and mass spectrometry. The stunning ease of this FluCell-SELEX and the convincing performance of the P. aeruginosa specific library may pave the way towards generally new and efficient diagnostic techniques based on polyclonal aptamer libraries not only in clinical microbiology.

The Structure of an Archaeal B-Family DNA Polymerase in Complex with a Chemically Modified Nucleotide

Archaeal B-family DNA polymerases (DNA pols) are the driving force of cutting-edge biotechnological applications like next-generation sequencing. The acceptance of chemically modified nucleotides by DNA pols is key to these technologies. Until now, no structural data have been available for these DNA pols in complex with modified substrates, which could build the basis for understanding interactions between the enzyme and the chemically modified nucleotide and for the further development of next-generation nucleotides. For the first time, we crystallized an exonuclease-deficient variant of the wild-type B-family KOD DNA pol with a modified nucleotide in a closed, ternary complex. We also crystalized the A-family DNA pol KlenTaq with the same nucleotide. The reported structural data reveal how the protein and the DNA modulate two distinct conformations of the appended moiety in the A- and B-family DNA pols and how these influence the processing of the modified nucleotide. Overall, this study provides first insight into the interplay between B-family DNA pols and relevant modified substrates.

Flexible freestanding graphene paper-based potentiometric enzymatic aptasensor for ultrasensitive wireless detection of kanamycin

Flexible sensing devices have drawn tremendous attention in the past decades due to their potential applications in future hand-held, potable consumer, and wearable electronics. Here, we firstly developed an ultrasensitive wireless potentiometric aptasensor based on flexible freestanding graphene paper for kanamycin detection. Flexible graphene paper made from a simple vacuum filtration method was used as a biocompatible platform for effective immobilization of aptamer. A nuclease-assisted amplification strategy was introduced into this potentiometric biosensing system in order to significantly improve the detection sensitivity through a classic catalytic recycling reaction of target induced by the nuclease (DNase I). As expected, an ultra-low detection limit of 30.0 fg/mL for kanamycin was achieved. Furthermore, the developed potentiometric enzymatic aptasensor exhibits high selectivity, favorable flexibility, excellent stability and reproducibility, which holds great promising for its routine sensing application.

Structural Basis for the KlenTaq DNA Polymerase Catalysed Incorporation of Alkene- versus Alkyne-Modified Nucleotides

Efficient incorporation of modified nucleotides by DNA polymerases is essential for many cutting-edge biomolecular technologies. The present study compares the acceptance of either alkene- or alkyne-modified nucleotides by KlenTaq DNA polymerase and provides structural insights into how 7-deaza-adenosine and deoxyuridine with attached alkene-modifications are incorporated into the growing DNA strand. Thereby, we identified modified nucleotides that prove to be superior substrates for KlenTaq DNA polymerase compared with their natural analogues. The knowledge can be used to guide future design of functionalized nucleotide building blocks.

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.

A Versatile Approach Towards Nucleobase-Modified Aptamers

A novel and versatile method has been developed for modular expansion of the chemical space of nucleic acid libraries, thus enabling the generation of nucleobase-modified aptamers with unprecedented recognition properties. Reintroduction of the modification after enzymatic replication gives broad access to many chemical modifications. This wide applicability, which is not limited to a single modification, will rapidly advance the application of in vitro selection approaches beyond what is currently feasible and enable the generation of aptamers to many targets that have so far not been addressable.

Optimal DNA templates for rolling circle amplification revealed by in vitro selection

Rolling circle amplification (RCA) has been widely used as an isothermal DNA amplification technique for diagnostic and bioanalytical applications. Because RCA involves repeated copying of the same circular DNA template by a DNA polymerase thousands of times, we hypothesized there exist DNA sequences that can function as optimal templates and produce more DNA amplicons within an allocated time. Herein we describe an in vitro selection effort conducted to search from a random sequence DNA pool for such templates for phi29 DNA polymerase, a frequently used polymerase for RCA. Diverse DNA molecules were isolated and they were characterized by richness in adenosine (A) and cytidine (C) nucleotides. The top ranked sequences exhibit superior RCA efficiency and the use of these templates for RCA results in significantly improved detection sensitivity. AC-rich sequences are expected to find useful applications for setting up effective RCA assays for biological sensing.

Reversible photoswitching of RNA hybridization at room temperature with an azobenzene C-nucleoside

Photoregulation of RNA remains a challenging task as the introduction of a photoswitch entails changes in the shape and the stability of the duplex that strongly depend on the chosen linker strategy. Herein, the influence of a novel nucleosidic linker moiety on the photoregulation efficiency of azobenzene is investigated. To this purpose, two azobenzene C-nucleosides were stereoselectively synthesized, characterized, and incorporated into RNA oligonucleotides. Spectroscopic characterization revealed a reversible and fast switching process, even at 20 °C, and a high thermal stability of the respective cis isomers. The photoregulation efficiency of RNA duplexes upon trans-to-cis isomerization was investigated by using melting point studies and compared with the known D-threoninol-based azobenzene system, revealing a photoswitching amplitude of the new residues exceeding 90 % even at room temperature. Structural changes in the duplexes upon photoisomerization were investigated by using MM/MD calculations. The excellent photoswitching performance at room temperature and the high thermal stability make these new azobenzene residues promising candidates for in-vivo and nanoarchitecture photoregulation applications of RNA.

Riboswitches based on kissing complexes for the detection of small ligands

Biosensors derived from aptamers were designed for which folding into a hairpin shape is triggered by binding of the cognate ligand. These aptamers (termed aptaswitches) thus switch between folded and unfolded states in the presence and absence of the ligand, respectively. The apical loop of the folded aptaswitch is recognized by a second hairpin called the aptakiss through loop-loop or kissing interactions, whereas the aptakiss does not bind the unfolded aptaswitch. Therefore, the formation of a kissing complex signals the presence of the ligand. Aptaswitches were designed that enable the detection of GTP and adenosine in a specific and quantitative manner by surface plasmon resonance when using a grafted aptakiss or in solution by anisotropy measurement with a fluorescently labeled aptakiss. This approach is generic and can potentially be extended to the detection of any molecule for which hairpin aptamers have been identified, as long as the apical loop is not involved in ligand binding.

Structures of KOD and 9°N DNA polymerases complexed with primer template duplex

Replicate it: Structures of KOD and 9°N DNA polymerases, two enzymes that are widely used to replicate DNA with highly modified nucleotides, were solved at high resolution in complex with primer/template duplex. The data elucidate substrate interaction of the two enzymes and pave the way for further optimisation of the enzymes and substrates.

Nucleic acid and peptide aptamers: fundamentals and bioanalytical aspects

In recent years new nucleic acid and protein-based combinatorial molecules have attracted the attention of researchers working in various areas of science, ranging from medicine to analytical chemistry. These molecules, called aptamers, have been proposed as alternatives to antibodies in many different applications. The aim of this Review is to illustrate the peculiarities of these combinatorial molecules which have initially been explored for their importance in molecular medicine, but have enormous potential in other biotechnological fields historically dominated by antibodies, such as bioassays. A description of these molecules is given, and the methods for their selection and production are also summarized. Moreover, critical aspects related to these molecules are discussed.

Aptamers can discriminate alkaline proteins with high specificity

Aptamers are single-stranded nucleic acids that fold into stable three-dimensional structures with ligand binding sites that are complementary in shape and charge to a desired target. Aptamers are generated by an iterative process known as in vitro selection, which permits their isolation from pools of random sequences. While aptamers have been selected to bind a wide range of targets, it is generally thought that these molecules are incapable of discriminating strongly alkaline proteins due to the attractive forces that govern oppositely charged polymers (e.g., polyelectrolyte effect). Histones, eukaryotic proteins that make up the core structure of nucleosomes are attractive targets for exploring the binding properties of aptamers because these proteins have positively charged surfaces that bind DNA through noncovalent sequence-independent interactions. Previous selections by our lab and others have yielded DNA aptamers with high affinity but low specificity to individual histone proteins. Whether this is a general limitation of aptamers is an interesting question with important practical implications in the future development of protein affinity reagents. Here we report the in vitro selection of a DNA aptamer that binds to histone H4 with a K(d) of 13 nM and distinguishes other core histone proteins with 100 to 480-fold selectivity, which corresponds to a ΔΔG of up to 3.4 kcal mol(-1) . This result extends our fundamental understanding of aptamers and their ability to fold into shapes that selectively bind alkaline proteins.

Plug and play with RNA

Retooling RNA: RNA aptamers are high-affinity ligands that can be assembled with other structures to yield multivalent molecules. These properties have been addressed in two recent studies: One describes a GFP-like RNA reporter used to study the dynamics of endogenous RNA; the other study reports on an aptamer-templated assembly of multi-enzyme complexes in bacteria for the controlled production of secondary molecules (see picture).

Highly efficient control of thrombin activity by multivalent nanoparticles

We have demonstrated that the incorporation of sulfated galactose acid (sulf-Gal) into thrombin-binding-aptamer (TBA)-conjugated gold nanoparticles (TBA-AuNPs) enables highly effective inhibition of thrombin activity toward fibrinogen. AuNP bioconjugates (TBA(15)/TBA(29)/sulf-Gal-AuNPs) were prepared from 13 nm AuNPs, 15-mer thrombin-binding aptamer (TBA(15)), 29-mer thrombin-binding aptamer (TBA(29)), and sulf-Gal. The numbers of TBA and sulf-Gal molecules per AuNP proved to have a strong impact on inhibitory potency. The best results were observed for 15-TBA(15)/TBA(29)/sulf-Gal-AuNPs (with 15 TBA(15) and 15 TBA(29) molecules per AuNP), which, because of their particularly flexible conformation and multivalency, exhibited ultrahigh binding affinity toward thrombin (K(d)=3.4×10(-12) M) and thus extremely high anticoagulant (inhibitory) potency. Compared to the case without inhibitors (the "normal" value), their measured thrombin clotting time (TCT) was 91 times longer, whereas for TBA(15) alone it was only 7.2 times longer. Their anticoagulant activity was suppressed by TBA-complementary-sequence (cTBA)-modified AuNPs (cTBA(15)/cTBA(29)-AuNPs) at a rate that was 20 times faster than that of free cTBA(15)/cTBA(29). Thus, easily prepared, low-cost, multivalent AuNPs show great potential for biomedical control of blood clotting.

A G-quadruplex aptamer inhibits the phosphatase activity of oncogenic protein Shp2 in vitro

Shp2 is a member of the protein tyrosine phosphatase (PTP) family, which regulates a variety of cellular processes including cell growth, differentiation, mitotic cycle, and oncogenic transformation. Using a recombinant Shp2-GST protein as the target and GST as a counter target, we have identified two classes of single-stranded DNA aptamers that selectively bind to Shp2 with a K(d) in the nanomolar range. Structural studies of the most abundant sequence in the enriched library, HJ24, revealed a parallel G-quadruplex as the core binding domain. Furthermore, this aptamer was found to be an effective inhibitor of Shp2 phosphatase, an effect which was readily reversed by using the cDNA of HJ24. In view of these characteristics, this aptamer has the potential to be used for further development of Shp2 assays and therapeutics for the treatment of Shp2-dependent cancers and other diseases.

ADLOC: an aptamer-displacement assay based on luminescent oxygen channeling

Functional nucleic acids, such as aptamers and allosteric ribozymes, can sense their ligands specifically, thereby undergoing structural alterations that can be converted into a detectable signal. The direct coupling of molecular recognition to signal generation enables the production of versatile reporters that can be applied as molecular probes for various purposes, including high-throughput screening. Here we describe an unprecedented type of a nucleic acid-based sensor system and show that it is amenable to high-throughput screening (HTS) applications. The approach detects the displacement of an aptamer from its bound protein partner by means of luminescent oxygen channeling. In a proof-of-principle study we demonstrate that the format is feasible for efficient identification of small drug-like molecules that bind to a protein target, in this case to the Sec7 domain of cytohesin. We extended the approach to a new cytohesin-specific single chain DNA aptamer, C10.41, which exhibits a similar binding behavior to cytohesins but has the advantage of being more stable and easier to synthesize and to modify than the RNA-aptamer M69. The results obtained with both aptamers indicate the general suitability of the aptamer-displacement assay based on luminescent oxygen channelling (ADLOC) for HTS. We also analyzed the potential for false positive hits and identified from a library of 18,000 drug-like small molecules two compounds as strong singlet-oxygen quenchers. With full automation and the use of commercially available plate readers, we estimate that the ADLOC-based assay described here could be used to screen at least 100,000 compounds per day.

Semisynthetic DNA-protein conjugates for biosensing and nanofabrication

Conjugation with artificial nucleic acids allows proteins to be modified with a synthetically accessible, robust tag. This attachment is addressable in a highly specific manner by means of molecular recognition events, such as Watson-Crick hybridization. Such DNA-protein conjugates, with their combined properties, have a broad range of applications, such as in high-performance biomedical diagnostic assays, fundamental research on molecular recognition, and the synthesis of DNA nanostructures. This Review surveys current approaches to generate DNA-protein conjugates as well as recent advances in their applications. For example, DNA-protein conjugates have been assembled into model systems for the investigation of catalytic cascade reactions and light-harvesting devices. Such hybrid conjugates are also used for the biofunctionalization of planar surfaces for micro- and nanoarrays, and for decorating inorganic nanoparticles to enable applications in sensing, materials science, and catalysis.