Discovering ligands for a microRNA precursor with peptoid microarrays

Synthetic Peptides: Promising Modalities for the Targeting of Disease-Related Nucleic Acids

DNA and RNA play pivotal roles in life processes by storing and transferring genetic information, modulating gene expression, and contributing to essential cellular machinery such as ribosomes. Dysregulation and mutations in nucleic acid-related processes are implicated in numerous diseases. Despite the critical impact on health of nucleic acid mutations or dysregulation, therapeutic compounds addressing these biomolecules remain limited. Peptides have emerged as a promising class of molecules for biomedical research, offering potential solutions for challenging drug targets. This review focuses on the use of synthetic peptides to target disease-related nucleic acids. We discuss examples of peptides targeting double-stranded DNA, including the clinical candidate Omomyc, and compounds designed for regulatory G-quadruplexes. Further, we provide insights into both library-based screenings and the rational design of peptides to target regulatory human RNA scaffolds and viral RNAs, emphasizing the potential of peptides in addressing nucleic acid-related diseases.

Drug-Like Small Molecules That Inhibit Expression of the Oncogenic MicroRNA-21

We report the discovery of drug-like small molecules that bind specifically to the precursor of the oncogenic and pro-inflammatory microRNA-21 with mid-nanomolar affinity. The small molecules target a local structure at the Dicer cleavage site and induce distinctive structural changes in the RNA, which correlate with specific inhibition of miRNA processing. Structurally conservative single nucleotide substitutions eliminate the conformational change induced by the small molecules, which is also not observed in other miRNA precursors. The most potent of these compounds reduces cellular proliferation and miR-21 levels in cancer cell lines without inhibiting kinases or classical receptors, while closely related compounds without this specific binding activity are inactive in cells. These molecules are highly ligand-efficient (MW < 330) and display specific biochemical and cellular activity by suppressing the maturation of miR-21, thereby providing an avenue toward therapeutic development in multiple diseases where miR-21 is abnormally expressed.

Trading places: Peptide and small molecule alternatives to oligonucleotide-based modulation of microRNA expression

It is well established that microRNA (miRNA) dysregulation is involved in the development and progression of various diseases, especially cancer. Emerging evidence suggests that small molecule and peptide agents can interfere with miRNA disease pathways. Despite this, very little is known about structural features that drive drug-miRNA interactions and subsequent inhibition. In this review, we highlight the advances made in the development of small molecule and peptide inhibitors of miRNA processing. Specifically, we attempt to draw attention to peptide features that may be critical for interaction with the miRNA secondary structure to regulate miRNA expression. We hope that this review will help to establish peptides as exciting miRNA expression modulators and will contribute towards the development of the first miRNA-targeting peptide therapy.

Rational Approach to Identify RNA Targets of Natural Products Enables Identification of Nocathiacin as an Inhibitor of an Oncogenic RNA

The discovery of biofunctional natural products (NPs) has relied on the phenotypic screening of extracts and subsequent laborious work to dereplicate active NPs and define cellular targets. Herein, NPs present as crude extracts, partially purified fractions, and pure compounds were screened directly against molecular target libraries of RNA structural motifs in a library-versus-library fashion. We identified 21 hits with affinity for RNA, including one pure NP, nocathiacin I (NOC-I). The resultant data set of NOC-I-RNA fold interactions was mapped to the human transcriptome to define potential bioactive interactions. Interestingly, one of NOC-I's most preferred RNA folds is present in the nuclease processing site in the oncogenic, noncoding microRNA-18a, which NOC-I binds with low micromolar affinity. This affinity for the RNA translates into the selective inhibition of its nuclease processing in vitro and in prostate cancer cells, in which NOC-I also triggers apoptosis. In principle, adaptation of this combination of experimental and predictive approaches to dereplicate NPs from the other hits (extracts and partially purified fractions) could fundamentally transform the current paradigm and accelerate the discovery of NPs that bind RNA and their simultaneous correlation to biological targets.

DNA-encoded library versus RNA-encoded library selection enables design of an oncogenic noncoding RNA inhibitor

Drug discovery generally investigates one target at a time, in sharp contrast to living organisms, which mold ligands and targets by evolution of highly complex molecular interaction networks. We recapitulate this modality of discovery by encoding drug structures in DNA, allowing the entire DNA-encoded library to interact with thousands of RNA fold targets, and then decoding both drug and target by sequencing. This information serves as a filter to identify human RNAs aberrantly produced in cancer that are also binding partners of the discovered ligand, leading to a precision medicine candidate that selectively ablates an oncogenic noncoding RNA, reversing a disease-associated phenotype in cells.

Nature evolves molecular interaction networks through persistent perturbation and selection, in stark contrast to drug discovery, which evaluates candidates one at a time by screening. Here, nature’s highly parallel ligand-target search paradigm is recapitulated in a screen of a DNA-encoded library (DEL; 73,728 ligands) against a library of RNA structures (4,096 targets). In total, the screen evaluated ∼300 million interactions and identified numerous bona fide ligand–RNA three-dimensional fold target pairs. One of the discovered ligands bound a 5′GAG/3′CCC internal loop that is present in primary microRNA-27a (pri-miR-27a), the oncogenic precursor of microRNA-27a. The DEL-derived pri-miR-27a ligand was cell active, potently and selectively inhibiting pri-miR-27a processing to reprogram gene expression and halt an otherwise invasive phenotype in triple-negative breast cancer cells. By exploiting evolutionary principles at the earliest stages of drug discovery, it is possible to identify high-affinity and selective target–ligand interactions and predict engagements in cells that short circuit disease pathways in preclinical disease models.

Interbase-FRET binding assay for pre-microRNAs

The aberrant expression of microRNAs (miRs) has been linked to several human diseases. A promising approach for targeting these anomalies is the use of small-molecule inhibitors of miR biogenesis. These inhibitors have the potential to (i) dissect miR mechanisms of action, (ii) discover new drug targets, and (iii) function as new therapeutic agents. Here, we designed Förster resonance energy transfer (FRET)-labeled oligoribonucleotides of the precursor of the oncogenic miR-21 (pre-miR-21) and used them together with a set of aminoglycosides to develop an interbase-FRET assay to detect ligand binding to pre-miRs. Our interbase-FRET assay accurately reports structural changes of the RNA oligonucleotide induced by ligand binding. We demonstrate its application in a rapid, qualitative drug candidate screen by assessing the relative binding affinity between 12 aminoglycoside antibiotics and pre-miR-21. Surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) were used to validate our new FRET method, and the accuracy of our FRET assay was shown to be similar to the established techniques. With its advantages over SPR and ITC owing to its high sensitivity, small sample size, straightforward technique and the possibility for high-throughput expansion, we envision that our solution-based method can be applied in pre-miRNA–target binding studies.

Small Molecule-Induced Dimerization of Hairpin RNA Interfered with the Dicer Cleavage Reaction

MicroRNAs are potential targets for drug development. Small molecules that can inhibit or promote a specific miRNA's biogenesis would be useful for regulating its target genes. Various types of small molecules have been investigated so far for their potential application in modulating miRNA biogenesis. They bind to the target primary or precursor miRNAs and inhibit the processing of these precursors by Drosha or Dicer. However, the binding site that effectively interferes with the Dicer cleavage reaction is still undetermined. Here we report that our designed small molecule restricted naphthyridine dimer (RND) binds to the hairpin loop of a hairpin RNA and induces its dimerization. This study shows that the binding of the RND to the hairpin loop was not effective in interfering with the Dicer cleavage reaction, but dimerization of the hairpin RNA by RND binding effectively interfered with the Dicer cleavage reaction.

Recognizing and stabilizing miR-21 by chiral ruthenium(II) complexes

MiR-21, a non-coding miRNA with 22 nucleotides, plays an important part in the proliferation, invasion, and metastasis of tumor cells. The present study demonstrates that isomers of chiral ruthenium(II) complexes with alkynes (Λ-1 and Δ-1) were synthesized by Songogashira coupling reaction by using microwave-assisted synthetic technology. The isomers can recognize and stabilize miR-21, with the Λ-isomer showing a stronger binding capacity than the Δ-isomer. Further studies showed that both isomers can be uptaken by MDA-MB-231 cells and enriched in the nucleus. Treatment with the Λ-/Δ-isomer downregulated the expression of miR-21. In a word, the development of chiral ruthenium(II) complexes act as potential inhibitors against tumor cells by recognizing, stabilizing, and regulating the expression of miR-21.

Decrypting noncoding RNA interactions, structures, and functional networks

The world of noncoding RNAs (ncRNAs) is composed of an enormous and growing number of transcripts, ranging in length from tens of bases to tens of kilobases, involved in all biological processes and altered in expression and/or function in many types of human disorders. The premise of this review is the concept that ncRNAs, like many large proteins, have a multidomain architecture that organizes them spatially and functionally. As ncRNAs are beginning to be imprecisely classified into functional families, we review here how their structural properties might inform their functions with focus on structural architecture–function relationships. We will describe the properties of “interactor elements” (IEs) involved in direct physical interaction with nucleic acids, proteins, or lipids and of “structural elements” (SEs) directing their wiring within the “ncRNA interactor networks” through the emergence of secondary and/or tertiary structures. We suggest that spectrums of “letters” (ncRNA elements) are assembled into “words” (ncRNA domains) that are further organized into “phrases” (complete ncRNA structures) with functional meaning (signaling output) through complex “sentences” (the ncRNA interactor networks). This semiotic analogy can guide the exploitation of ncRNAs as new therapeutic targets through the development of IE-blockers and/or SE-lockers that will change the interactor partners’ spectrum of proteins, RNAs, DNAs, or lipids and consequently influence disease phenotypes.

miRNA inhibition by proximity-enabled Dicer inactivation

microRNAs (miRNAs) are considered as master regulators of biological processes. Dysregulation of miRNA expression has been implicated in many human diseases. Driven by the key biological roles and the therapeutic potential, developing methods for miRNA regulation has become an intense research area. Due to favorable pharmacological properties, small molecule-based miRNA inhibition emerges as a promising strategy and significant progresses have been made. However, it remains challenging to regulate miRNA using small molecules because of the inherent difficulty in RNA targeting and inhibition. Herein we outline the workflow of generating bifunctional small molecule inhibitors blocking miRNA biogenesis through proximity-enabled inactivation of Dicer, an enzyme required for the processing of precursor miRNA (pre-miRNA) into mature miRNA. By conjugating a weak Dicer inhibitor with a pre-miRNA binder, the inhibitor can be delivered to the Dicer processing site associated with the targeted pre-miRNA, and as a result inhibiting Dicer-mediated pre-miRNA processing. This protocol can be applicable in producing bifunctional inhibitors for different miRNAs.

Tetracyclines as Inhibitors of Pre-microRNA Maturation: A Disconnection between RNA Binding and Inhibition

In a high-throughput screening campaign, we recently discovered the rRNA-binding tetracyclines, methacycline and meclocycline, as inhibitors of Dicer-mediated processing of microRNAs. Herein, we describe our biophysical and biochemical characterization of these compounds. Interestingly, although direct, albeit weak, binding to the pre-microRNA hairpins was observed, the inhibitory activity of these compounds was not due to RNA binding. Through additional biochemical and chemical studies, we revealed that metal chelation likely plays a principle role in their mechanism of inhibition. By exploring the activity of other known RNA-binding scaffolds, we identified additional disconnections between direct RNA interaction and inhibition of Dicer processing. Thus, the results presented within provide a valuable case study in the complexities of targeting RNA with small molecules, particularly with weak binding and potentially promiscuous scaffolds.

Triple-helix molecular-switch-actuated exponential rolling circular amplification for ultrasensitive fluorescence detection of miRNAs

We have developed a rapid and high-efficiency fluorescent biosensing platform based on triple-helix molecular-switch (THMS)-actuated exponential rolling circular amplification (RCA) strategy for the ultrasensitive detection of miR-21.

Expanding chemical space of DNA-binding molecules with three base-binding units

A new molecule NC3-3 designed to expand chemical space of parent molecule NCD by adding the third base-binding unit was reported. NC3-3 bound to the G-G mismatch in the 5'-CGG-3'/5'-CGG-3' motif but not to that in 5'-GGC-3'/5'-GGC-3'. This binding selectivity is similar to that reported for NCD. Fluorimetric screening of NCD and NC3-3 to dsDNA library containing yGw/xGz motifs showed that NC3-3 still kept the sequence selectivity as we observed for NCD-binding. The third naphthyridine heterocycle in NC3-3 affected the mode of the binding, but a little effect on the sequence selectivity.

Targeting RNA in mammalian systems with small molecules

The recognition of RNA functions beyond canonical protein synthesis has challenged the central dogma of molecular biology. Indeed, RNA is now known to directly regulate many important cellular processes, including transcription, splicing, translation, and epigenetic modifications. The misregulation of these processes in disease has led to an appreciation of RNA as a therapeutic target. This potential was first recognized in bacteria and viruses, but discoveries of new RNA classes following the sequencing of the human genome have invigorated exploration of its disease-related functions in mammals. As stable structure formation is evolving as a hallmark of mammalian RNAs, the prospect of utilizing small molecules to specifically probe the function of RNA structural domains and their interactions is gaining increased recognition. To date, researchers have discovered bioactive small molecules that modulate phenotypes by binding to expanded repeats, microRNAs, G-quadruplex structures, and RNA splice sites in neurological disorders, cancers, and other diseases. The lessons learned from achieving these successes both call for additional studies and encourage exploration of the plethora of mammalian RNAs whose precise mechanisms of action remain to be elucidated. Efforts toward understanding fundamental principles of small molecule-RNA recognition combined with advances in methodology development should pave the way toward targeting emerging RNA classes such as long noncoding RNAs. Together, these endeavors can unlock the full potential of small molecule-based probing of RNA-regulated processes and enable us to discover new biology and underexplored avenues for therapeutic intervention in human disease. This article is categorized under: RNA Methods > RNA Analyses In Vitro and In Silico RNA Interactions with Proteins and Other Molecules > Small Molecule-RNA Interactions RNA in Disease and Development > RNA in Disease.

Highly specific real-time quantification of diverse microRNAs in human samples using universal primer set frame

In this study, one group of universal primer set frame, composed by one reverse transcription (RT) primer frame and a pair of quantitative real-time polymerase chain reaction (qRT-PCR) primer frames, was elaborately screened and designed by homebuilt software for sensitive and specific quantification of diverse miRNAs. The universal primer set frame can be applied for multiplex miRNAs detection by simply changing the RT-X part of RT primer frame and RP-Y part of qRT-PCR reverse primer frame based on target sequence. The maximum similarity of RT-Y, RT-Z and qRT-PCR forward primer to the human genome and human transcriptome is less than 76%, ensuring the high specificity in human sample detection. The high sensitivity and broad dynamic linear range of the developed approaches by using designed primer set frame were demonstrated on the in vitro detection of miR-21 and miR-155, with dynamic range of 10 fM to 10 nM and detection limit of 3.74 × 10^-15 M and 5.81 × 10^-15 M for miR-21 and miR-155, respectively. In particular, the developed assays also have high sequence specificity which could clearly discriminate a single base difference in miRNA sequence. The contents of miR-21 and miR-155 in tissue and serum samples have been successfully detected using the developed assays. Results indicated that miR-21 and miR-155 were elevated in cancer tissue and serum specimens than that of normal samples, implying the developed assays hold a great promise for further application in biomedical research and early clinical diagnosis. More importantly, the primer set frame can be universally used in any miRNA investigation.

Development and Implementation of an HTS-Compatible Assay for the Discovery of Selective Small-Molecule Ligands for Pre-microRNAs

microRNAs (miRNAs) are small gene regulatory RNAs, and their expression has been found to be dysregulated in a number of human diseases. To facilitate the discovery of small molecules capable of selectively modulating the activity of a specific miRNA, we have utilized new high-throughput screening technology targeting Dicer-mediated pre-miRNA maturation. Pilot screening of ~50,000 small molecules and ~33,000 natural product extract libraries against pre-miR-21 processing indicated the potential of our assay for this goal, yielding a campaign Z' factor of 0.52 and an average plate signal-to-background (S/B) ratio of 13. Using two-dimensional screening against a second pre-miRNA, pre-let-7d, we evaluated the selectivity of confirmed hits. The results presented demonstrate how high-throughput screening can be used to identify selective small molecules for a target RNA.

A Macrocyclic Peptide Ligand Binds the Oncogenic MicroRNA-21 Precursor and Suppresses Dicer Processing

MicroRNAs (miRNAs) help orchestrate cellular growth and survival through post-transcriptional mechanisms. The dysregulation of miRNA biogenesis can lead to cellular growth defects and chemotherapeutic resistance and plays a direct role in the development of many chronic diseases. Among these RNAs, miR-21 is consistently overexpressed in most human cancers, leading to the down-regulation of key tumor-suppressing and pro-apoptotic factors, suggesting that inhibition of miR-21 biogenesis could reverse these negative effects. However, targeted inhibition of miR-21 using small molecules has had limited success. To overcome difficulties in targeting RNA secondary structure with small molecules, we developed a class of cyclic β-hairpin peptidomimetics which bind to RNA stem-loop structures, such as miRNA precursors, with potent affinity and specificity. We screened an existing cyclic peptide library and discovered a lead structure which binds to pre-miR21 with K_D = 200 nM and prefers it over other pre-miRNAs. The NMR structure of the complex shows that the peptide recognizes the Dicer cleavage site and alters processing of the precursor to the mature miRNA in vitro and in cultured cells. The structure provides a rationale for the peptide binding activity and clear guidance for further improvements in affinity and targeting.

Discovery of Inhibitors of MicroRNA-21 Processing Using Small Molecule Microarrays

The identification of small molecules that bind to and perturb the function of microRNAs is an attractive approach for the treatment for microRNA-associated pathologies. However, there are only a few small molecules known to interact directly with microRNAs. Here, we report the use of a small molecule microarray (SMM) screening approach to identify low molecular weight compounds that directly bind to a pre-miR-21 hairpin. Compounds identified using this approach exhibit good affinity for the RNA (ranging from 0.8-2.0 μM) and are not composed of a polycationic scaffold. Several of the highest affinity compounds inhibit Dicer-mediated processing, while in-line probing experiments indicate that the compounds bind to the apical loop of the hairpin, proximal to the Dicer site. This work provides evidence that small molecules can be developed to bind directly to and inhibit miR-21.

Discovery of RNA Binding Small Molecules Using Small Molecule Microarrays

New methods to identify RNA-binding small molecules open yet unexplored opportunities for the pharmacological modulation of RNA-driven biology and disease states. One such approach is the use of small molecule microarrays (SMMs). Typically, SMMs are generated by spatially arraying and covalently linking a library of small molecules to a glass surface. Next, incubation of the arrays with a fluorescently labeled RNA reveals binding interactions that are detected upon slide imaging. The relative ease with which SMMs are manufactured enables the screening of multiple oligonucleotides in parallel against tens of thousands of small molecules, providing information about both binding and selectivity of identified RNA-small molecule interactions. This approach is useful for screening a broad variety of structurally and functionally diverse RNAs. Here, we present a general method for the preparation and use of SMMs to rapidly identify small molecules that selectively bind to an RNA of interest.

BzDANP, a Small-Molecule Modulator of Pre-miR-29a Maturation by Dicer

We here report the synthesis of novel molecule BzDANP having a three-ring benzo[c][1,8]naphthyridine system, the evaluation of its binding properties to a single nucleotide bulge in RNA duplexes, and BzDANP-induced suppression of pre-miR-29a processing by Dicer. BzDANP showed much increased affinity to the bulged RNAs as compared with the parent molecule DANP, which possesses the same hydrogen-bonding surface as BzDANP but in a two-ring [1,8]naphthyridine system. Melting temperature analysis of bulged RNAs showed that BzDANP most effectively stabilized the C-bulged RNA. Dicer-mediated processing of pre-miR-29a was suppressed by BzDANP in a concentration dependent manner. The presence of the C-bulge at the Dicer cleavage site was effective for the suppression of pre-miR-29a processing by BzDANP. These results demonstrated that the small molecule binding to the bulged site in the vicinity of the Dicer cleavage site could be a potential modulator for the maturation of pre-miRNA.

Microarray-based technologies for the discovery of selective, RNA-binding molecules

The identification of small molecules that bind specifically to RNA is a challenge. However, the recent explosion in knowledge about the role RNA plays in a number of physiological processes apart from coding for protein sequences makes it a highly interesting target for chemical probes and therapeutics. One technology that has played an important role in the discovery of RNA-binding molecules is microarrays. Microarrays have been broadly employed to screen, profile, and quantify RNA interactions, and will likely play an important role in the discovery of new classes of ligands going forward. Here, we discuss the development of microarray technologies, including aminoglycoside, peptide, peptoid, and small molecule microarrays, and their use in studying RNA-interacting molecules.

Small-molecule approaches toward the targeting of oncogenic miRNAs: roadmap for the discovery of RNA modulators

miRNAs are a recently discovered class of small noncoding RNAs implicated in the regulation of gene expression. The deregulation of miRNAs levels has been linked to the development of various cancers where oncogenic miRNAs are overexpressed and tumor suppressor miRNAs are underexpressed. Here we report the three main strategies developed in order to discover small-molecule drugs able to selectively interfere with oncogenic miRNAs: the high throughput screening of large libraries of compounds, the focused screening of small libraries of molecules that are known to be able to interact with RNA thus being supposed modulators of miRNAs pathway and the design of small molecules based on the secondary structure of targeted RNA and/or three-dimensional structure of enzymes involved in miRNAs pathway.

Identification of Spermidine Binding Site in T-box Riboswitch Antiterminator RNA

The T-box transcription antitermination riboswitch controls bacterial gene expression by structurally responding to uncharged, cognate tRNA. Previous studies indicated that cofactors, such as the polyamine spermidine, might serve a specific functional role in enhancing riboswitch efficacy. As riboswitch function depends on key RNA structural changes involving the antiterminator element, the interaction of spermidine with the T-box riboswitch antiterminator element was investigated. Spermidine binds antiterminator model RNA with high affinity (micromolar Kd ) based on isothermal titration calorimetry and fluorescence-monitored binding assays. NMR titration studies, molecular modeling, and inline and enzymatic probing studies indicate that spermidine binds at the 3' portion of the highly conserved seven-nucleotide bulge in the antiterminator. Together, these results support the conclusion that spermidine binds the T-box antiterminator RNA preferentially in a location important for antiterminator function. The implications of these findings are significant both for better understanding of the T-box riboswitch mechanism and for antiterminator-targeted drug discovery efforts.

Synthesis and cell-based screening of one-bead-one-compound peptide libraries

Combinatorial one-bead-one-compound (OBOC) peptide library screening has proven to be a powerful tool for identification of small molecules, peptides, or peptidomimetics against a variety of specific targets such as cell surface receptors, protein kinases, proteases, and phosphatases. With each bead displaying many copies of a single chemical entity, millions of compounds can be rapidly synthesized and screened with whole-cell binding on-bead functional assays. Here we describe the methodology for the synthesis, screening, and sequence deconvolution of an OBOC peptide library analyzed for affinity to a cancer cell line.

Small molecules targeting microRNA for cancer therapy: Promises and obstacles

Aberrant expression of miRNAs is critically implicated in cancer initiation and progression. Therapeutic approaches focused on regulating miRNAs are therefore a promising approach for treating cancer. Antisense oligonucleotides, miRNA sponges, and CRISPR/Cas9 genome editing systems are being investigated as tools for regulating miRNAs. Despite the accruing insights in the use of these tools, delivery concerns have mitigated clinical application of such systems. In contrast, little attention has been given to the potential of small molecules to modulate miRNA expression for cancer therapy. In these years, many researches proved that small molecules targeting cancer-related miRNAs might have greater potential for cancer treatment. Small molecules targeting cancer related miRNAs showed significantly promising results in different cancer models. However, there are still several obstacles hindering the progress and clinical application in this area. This review discusses the development, mechanisms and application of small molecules for modulating oncogenic miRNAs (oncomiRs). Attention has also been given to screening technologies and perspectives aimed to facilitate clinical translation for small molecule-based miRNA therapeutics.

Factors that influence T box riboswitch efficacy and tRNA affinity

The T box riboswitch is an intriguing potential target for antibacterial drug discovery. Found primarily in Gram-positive bacteria, the riboswitch regulates gene expression by selectively responding to uncharged tRNA to control transcription readthrough. Polyamines and molecular crowding are known to specifically affect RNA function, but their effect on T box riboswitch efficacy and tRNA affinity have not been fully characterized. A fluorescence-monitored in vitro transcription assay was developed to readily quantify these molecular interactions and to provide a moderate-throughput functional assay for a comprehensive drug discovery screening cascade. The polyamine spermidine specifically enhanced T box riboswitch readthrough efficacy with an EC50 = 0.58 mM independent of tRNA binding. Molecular crowding, simulated by the addition of polyethylene glycol, had no effect on tRNA affinity for the riboswitch, but did reduce the efficacy of tRNA-induced readthrough. These results indicate that the T box riboswitch tRNA affinity and readthrough efficacy are intricately modulated by environmental factors.

Tethering in RNA: an RNA-binding fragment discovery tool

Tethering has been extensively used to study small molecule interactions with proteins through reversible disulfide bond forming reactions to cysteine residues. We describe the adaptation of Tethering to the study of small molecule binding to RNA using a thiol-containing adenosine analog (A_SH). Among 30 disulfide-containing small molecules screened for efficient Tethering to A_SH-bearing RNAs derived from pre-miR21, a benzotriazole-containing compound showed prominent adduct formation and selectivity for one of the RNAs tested. The results of this screen demonstrate the viability of using thiol-modified nucleic acids to discover molecules with binding affinity and specificity for the purpose of therapeutic compound lead discovery.

Structure based approaches for targeting non-coding RNAs with small molecules

The increasing appreciation of the central role of non-coding RNAs (miRNAs and long non-coding RNAs) in chronic and degenerative human disease makes them attractive therapeutic targets. This would not be unprecedented: the bacterial ribosomal RNA is a mainstay for antibacterial treatment, while the conservation and functional importance of viral RNA regulatory elements has long suggested they would constitute attractive targets for new antivirals. Oligonucleotide-based chemistry has obvious appeals but also considerable pharmacological limitations that are yet to be addressed satisfactorily. Recent studies identifying small molecules targeting non-coding RNAs may provide an alternative approach to oligonucleotide methods. Here we review recent work investigating new structural and chemical principles for targeting RNA with small molecules.

High-throughput platform assay technology for the discovery of pre-microrna-selective small molecule probes

MicroRNAs (miRNA) play critical roles in human development and disease. As such, the targeting of miRNAs is considered attractive as a novel therapeutic strategy. A major bottleneck toward this goal, however, has been the identification of small molecule probes that are specific for select RNAs and methods that will facilitate such discovery efforts. Using pre-microRNAs as proof-of-concept, herein we report a conceptually new and innovative approach for assaying RNA-small molecule interactions. Through this platform assay technology, which we term catalytic enzyme-linked click chemistry assay or cat-ELCCA, we have designed a method that can be implemented in high throughput, is virtually free of false readouts, and is general for all nucleic acids. Through cat-ELCCA, we envision the discovery of selective small molecule ligands for disease-relevant miRNAs to promote the field of RNA-targeted drug discovery and further our understanding of the role of miRNAs in cellular biology.

Nonconventional chemical inhibitors of microRNA: therapeutic scope

MicroRNAs (miRNAs) are a class of genomically encoded small RNA molecules (∼22nts in length), which regulate gene expression post transcriptionally. miRNAs are implicated in several diseases, thus modulation of miRNA is of prime importance. Small molecules offer a non-conventional alternative to do so.

Fluorescence assays for monitoring RNA-ligand interactions and riboswitch-targeted drug discovery screening

Riboswitches and other noncoding regulatory RNA are intriguing targets for the development of therapeutic agents. A significant challenge in the drug discovery process, however, is the identification of potent compounds that bind the target RNA specifically and disrupt its function. Essential to this process is an effectively designed cascade of screening assays. A screening cascade for identifying compounds that target the T box riboswitch antiterminator element is described. In the primary assays, moderate to higher throughput screening of compound libraries is achieved by combining the sensitivity of fluorescence techniques with functionally relevant assays. Active compounds are then validated and the binding to target RNA further characterized in secondary assays. The cascade of assays monitor ligand-induced changes in the steady-state fluorescence of an attached dye or internally incorporated 2-aminopurine; the fluorescence anisotropy of an RNA complex; and, the thermal denaturation fluorescence profile of a fluorophore-quencher labeled RNA. While the assays described have been developed for T box riboswitch-targeted drug discovery, the fluorescence methods and screening cascade design principles can be applied to drug discovery efforts targeted toward other medicinally relevant noncoding RNA.

Oligonucleotide analogues as modulators of the expression and function of noncoding RNAs (ncRNAs): emerging therapeutics applications

ncRNAs are emerging as key regulators of physiological and pathological processes and therefore have been identified as pharmacological targets and as markers for some diseases. Oligonucleotide analogues represent so far the most widely employed tool for the modulation of the expression of ncRNAs. In this perspective we briefly describe most of the known classes of ncRNAs and then we discuss the design and the applications of oligonucleotide analogues for their targeting. The effects of modifications of the chemical structure of the oligonucleotides on properties such as the binding affinity toward targets and off targets, and the stability to degradation and their biological effects (when known) are discussed. Examples of molecules currently used in clinical trials are also reported.

NMR characterization of an oligonucleotide model of the miR-21 pre-element

We have used NMR spectroscopy to characterize an oligonucleotide stem loop structure based on the pre-element of an oncogenic microRNA, miR-21. This predicted stem-loop structure is cleaved from the precursor of miR-21 (pre-miR-21) by the nuclease Dicer. It is also a critical feature recognized by the protein complex that converts the primary transcript (pri-miR-21) into the pre-miRNA. The secondary structure of the native sequence is poorly defined by NMR due to rapid exchange of imino protons with solvent; however, replacement of two adjacent putative G•U base pairs with G•C base pairs retains the conformation of the hairpin observed by chemical probing and stabilizes it sufficiently to observe most of the imino proton resonances of the molecule. The observed resonances are consistent with the predicted secondary structure. In addition, a peak due to a loop uridine suggests an interaction between it and a bulged uridine in the stem. Assignment of non-exchangeable proton resonances and characterization of NOEs and coupling constants allows inference of the following features of the structure: extrahelicity of a bulged adenosine, deviation from A-form geometry in a base-paired stem, and consecutive stacking of the adenosines in the 5′ side of the loop, the guanosine of the closing base pair, and a cross-strand adenosine. Modeling of the structure by restrained molecular dynamics suggests a basis for the interaction between the loop uridine, the bulged uridine in the stem, and an A•U base pair in the stem.

Immobilized OBOC combinatorial bead array to facilitate multiplicative screening

One-bead-one-compound (OBOC) combinatorial library screening has been broadly utilized for the last two decades to identify small molecules, peptides or peptidomimetics targeting variable screening probes such as cell surface receptors, bacteria, protein kinases, phosphatases, proteases etc. In previous screening methods, library beads were suspended in solution and screened against one single probe. Only the positive beads were tracked and isolated for additional screens and finally selected for chemical decoding. During this process, the remaining negative beads were not tracked and discarded. Here we report a novel bead immobilization method such that a bead library array can be conveniently prepared and screened in its entirety, sequentially many times with a series of distinct probes. This method not only allows us to increase the screening efficiency but also permits us to determine the binding profile of each and every library bead against a large number of target receptors. As proof of concept, we serially screened a random OBOC disulfide containing cyclic heptapeptide library with three water soluble dyes as model probes: malachite green, bromocresol purple and indigo carmine. This multiplicative screening approach resulted in a rapid determination of the binding profile of each and every bead respective to each of the three dyes. Beads that interacted with malachite green only, bromocresol purple only, or both indigo carmine and bromocresol purple were isolated, and their peptide sequences were determined with microsequencer. Ultimately, the novel OBOC multiplicative screening approach could play a key role in the enhancement of existing on-bead assays such as whole cell binding, bacteria binding, protein binding, posttranslational modifications etc. with increased efficiency, capacity, and specificity.

Identification of biologically active, HIV TAR RNA-binding small molecules using small molecule microarrays

Identifying small molecules that selectively bind to structured RNA motifs remains an important challenge in developing potent and specific therapeutics. Most strategies to find RNA-binding molecules have identified highly charged compounds or aminoglycosides that commonly have modest selectivity. Here we demonstrate a strategy to screen a large unbiased library of druglike small molecules in a microarray format against an RNA target. This approach has enabled the identification of a novel chemotype that selectively targets the HIV transactivation response (TAR) RNA hairpin in a manner not dependent on cationic charge. Thienopyridine 4 binds to and stabilizes the TAR hairpin with a Kd of 2.4 μM. Structure-activity relationships demonstrate that this compound achieves activity through hydrophobic and aromatic substituents on a heterocyclic core, rather than cationic groups typically required. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis was performed on a 365-nucleotide sequence derived from the 5' untranslated region (UTR) of the HIV-1 genome to determine global structural changes in the presence of the molecule. Importantly, the interaction of compound 4 can be mapped to the TAR hairpin without broadly disrupting any other structured elements of the 5' UTR. Cell-based anti-HIV assays indicated that 4 inhibits HIV-induced cytopathicity in T lymphocytes with an EC50 of 28 μM, while cytotoxicity was not observed at concentrations approaching 1 mM.

Association of a peptoid ligand with the apical loop of pri-miR-21 inhibits cleavage by Drosha

We have found a small molecule that specifically inhibits cleavage of a precursor to the oncogenic miRNA, miR-21, by the microprocessor complex of Drosha and DGCR8. We identified novel ligands for the apical loop of this precursor from a screen of 14,024 N-substituted oligoglycines (peptoids) in a microarray format. Eight distinct compounds with specific affinity were obtained, three having affinities for the targeted loop in the low micromolar range and greater than 15-fold discrimination against a closely related hairpin. One of these compounds completely inhibits microprocessor cleavage of a miR-21 primary transcript at concentrations at which cleavage of another miRNA primary transcript, pri-miR-16, is little affected. The apical loop of pri-miR-21, placed in the context of pri-miR-16, is sufficient for inhibition of microprocessor cleavage by the peptoid. This compound also inhibits cleavage of pri-miR-21 containing the pri-miR-16 apical loop, suggesting an additional site of association within pri-miR-21. The reported peptoid is the first example of a small molecule that inhibits microprocessor cleavage by binding to the apical loop of a pri-miRNA.

A liquid array platform for the multiplexed analysis of synthetic molecule-protein interactions

Synthetic molecule microarrays, consisting of many different compounds spotted onto a planar surface such as modified glass or cellulose, have proven to be useful tools for the multiplexed analysis of small molecule- and peptide-protein interactions. However, these arrays are technically difficult to manufacture and use with high reproducibility and require specialized equipment. Here we report a more convenient alternative composed of color-encoded beads that display a small molecule protein ligand on the surface. Quantitative, multiplexed assay of protein binding to up to 24 different ligands can be achieved using a common flow cytometer for the readout. This technology should be useful for evaluating hits from library screening efforts, the determination of structure activity relationships, and certain types of serological analyses.

Approaches to the modulation of miRNA maturation

The therapeutic importance of microRNA (miRNA) regulation has recently been realized as these small, noncoding RNAs have been demonstrated to be involved with a plethora of diseases and disorders. Due to the complex miRNA maturation process, the expression of these important biomolecules can be manipulated at various stages of the pathway. This review examines both in vivo and in vitro mechanisms and assays that have been developed to regulate miRNA levels. Modulation of miRNA maturation can be accomplished via several therapeutic agents, including small molecules and oligonucleotides, in both specific and nonspecific fashions. Due to the relevance of miRNAs, these novel therapeutic approaches represent new tools for the treatment of various cancers and other deleterious disorders.

Fluorescent indicator displacement assay of ligands targeting 10 microRNA precursors

Fluorescent indicator displacement (FID) assay is a rapid and convenient assay for identifying new ligands that bind to the target molecules. In our previous studies, we have shown that a series of 2,7-diaminoalkoxy xanthone and thioxanthone derivatives can be used as fluorescent indicators for detecting the interaction between RNA and a ligand. The xanthone and thioxanthone fluorochromes showed efficient fluorescence quenching upon binding to target RNA. Subsequent displacement of the bound-fluorochrome with a ligand that binds more strongly to the target RNA led to the recovery of the fluorescence by releasing the fluorochrome from RNA. Here we report a pilot screening of a chemical library that contains 9600 structurally diverse compounds for molecules that bind to a specific miRNA precursor using the FID assay.