Drugs targeting the ribosome

RNA-Small-Molecule Interaction: Challenging the "Undruggable" Tag

RNA targeting, specifically with small molecules, is a relatively new and rapidly emerging avenue with the promise to expand the target space in the drug discovery field. From being "disregarded" as an "undruggable" messenger molecule to FDA approval of an RNA-targeting small-molecule drug Risdiplam, a radical change in perspective toward RNA has been observed in the past decade. RNAs serve important regulatory functions beyond canonical protein synthesis, and their dysregulation has been reported in many diseases. A deeper understanding of RNA biology reveals that RNA molecules can adopt a variety of structures, carrying defined binding pockets that can accommodate small-molecule drugs. Due to its functional diversity and structural complexity, RNA can be perceived as a prospective target for therapeutic intervention. This perspective highlights the proof of concept of RNA-small-molecule interactions, exemplified by targeting of various transcripts with functional modulators. The advent of RNA-oriented knowledge would help expedite drug discovery.

Probing Bioactive Chemical Space to Discover RNA-Targeted Small Molecules

Small molecules have become increasingly recognized as invaluable tools to study RNA structure and function and to develop RNA-targeted therapeutics. To rationally design RNA-targeting ligands, a comprehensive understanding and explicit testing of small molecule properties that govern molecular recognition is crucial. To date, most studies have primarily evaluated properties of small molecules that bind RNA in vitro, with little to no assessment of properties that are distinct to selective and bioactive RNA-targeted ligands. Therefore, we curated an RNA-focused library, termed the Duke RNA-Targeted Library (DRTL), that was biased towards the physicochemical and structural properties of biologically active and non-ribosomal RNA-targeted small molecules. The DRTL represents one of the largest academic RNA-focused small molecule libraries curated to date with more than 800 small molecules. These ligands were selected using computational approaches that measure similarity to known bioactive RNA ligands and that diversify the molecules within this space. We evaluated DRTL binding in vitro to a panel of four RNAs using two optimized fluorescent indicator displacement assays, and we successfully identified multiple small molecule hits, including several novel scaffolds for RNA. The DRTL has and will continue to provide insights into biologically relevant RNA chemical space, such as the identification of additional RNA-privileged scaffolds and validation of RNA-privileged molecular features. Future DRTL screening will focus on expanding both the targets and assays used, and we welcome collaboration from the scientific community. We envision that the DRTL will be a valuable resource for the discovery of RNA-targeted chemical probes and therapeutic leads.

Targeting Riboswitches with Beta-Axial-Substituted Cobalamins

RNA-targeting small-molecule therapeutics is an emerging field hindered by an incomplete understanding of the basic principles governing RNA-ligand interactions. One way to advance our knowledge in this area is to study model systems where these interactions are better understood, such as riboswitches. Riboswitches bind a wide array of small molecules with high affinity and selectivity, providing a wealth of information on how RNA recognizes ligands through diverse structures. The cobalamin-sensing riboswitch is a particularly useful model system, as similar sequences show highly specialized binding preferences for different biological forms of cobalamin. This riboswitch is also widely dispersed across bacteria and therefore holds strong potential as an antibiotic target. Many synthetic cobalamin forms have been developed for various purposes including therapeutics, but their interaction with cobalamin riboswitches is yet to be explored. In this study, we characterize the interactions of 11 cobalamin derivatives with three representative cobalamin riboswitches using in vitro binding experiments (both chemical footprinting and a fluorescence-based assay) and a cell-based reporter assay. The derivatives show productive interactions with two of the three riboswitches, demonstrating simultaneous plasticity and selectivity within these RNAs. The observed plasticity is partially achieved through a novel structural rearrangement within the ligand binding pocket, providing insight into how similar RNA structures can be targeted. As the derivatives also show in vivo functionality, they serve as several potential lead compounds for further drug development.

The Time and Place for Nature in Drug Discovery

The case for a renewed focus on Nature in drug discovery is reviewed; not in terms of natural product screening, but how and why biomimetic molecules, especially those produced by natural processes, should deliver in the age of artificial intelligence and screening of vast collections both in vitro and in silico. The declining natural product-likeness of licensed drugs and the consequent physicochemical implications of this trend in the context of current practices are noted. To arrest these trends, the logic of seeking new bioactive agents with enhanced natural mimicry is considered; notably that molecules constructed by proteins (enzymes) are more likely to interact with other proteins (e.g., targets and transporters), a notion validated by natural products. Nature's finite number of building blocks and their interactions necessarily reduce potential numbers of structures, yet these enable expansion of chemical space with their inherent diversity of physical characteristics, pertinent to property-based design. The feasible variations on natural motifs are considered and expanded to encompass pseudo-natural products, leading to the further logical step of harnessing bioprocessing routes to access them. Together, these offer opportunities for enhancing natural mimicry, thereby bringing innovation to drug synthesis exploiting the characteristics of natural recognition processes. The potential for computational guidance to help identifying binding commonalities in the route map is a logical opportunity to enable the design of tailored molecules, with a focus on "organic/biological" rather than purely "synthetic" structures. The design and synthesis of prototype structures should pay dividends in the disposition and efficacy of the molecules, while inherently enabling greener and more sustainable manufacturing techniques.

A small-molecule fluorescence probe ANP77 for sensing RNA internal loop of C, U and A/CC motifs and their binding molecules

Small-molecules interacting with particular RNAs and modulating their functions are vital tools for RNA-targeting drug discovery. Considering the substantial distribution of the internal loops involving two contiguous cytosines opposite to a single-nucleotide base (Y/CC; Y = C, U or A) within the biologically significant functional RNAs, developing small-molecule probes targeting Y/CC sites should provide profound insight into their functions and roles in biochemical processes. Herein, we report ANP77 as the small-molecule probe for sensing RNA internal loop of Y/CC motifs and molecules binding to the motifs. The Y/CC motifs interact with ANP77 via the formation of a 1:1 complex and quench the fluorescence of ANP77. The flanking sequence-dependent binding to C/CC and U/CC sites was assessed by fluorometric screening, provided the binding heat maps. The quenching phenomena of ANP77 fluorescence was confirmed with intrinsic potential drug target pre-miR-1908. Finally, the binding-dependent fluorescence quenching of ANP77 was utilized in the fluorescence indicator displacement assay to demonstrate the potential of ANP77 as an indicator by using the RNA-binding drugs, risdiplam and branaplam.

Computational identification of putative common genomic drug and vaccine targets in Mycoplasma genitalium

Mycoplasma genitalium is an obligate intracellular bacterium that is responsible for several sexually transmitted infections, including non-gonococcal urethritis in men and several inflammatory reproductive tract syndromes in women. Here, we applied subtractive genomics and reverse vaccinology approaches for in silico prediction of potential vaccine and drug targets against five strains of M. genitalium. We identified 403 genes shared by all five strains, from which 104 non-host homologous proteins were selected, comprising of 44 exposed/secreted/membrane proteins and 60 cytoplasmic proteins. Based on the essentiality, functionality, and structure-based binding affinity, we finally predicted 19 (14 novel) putative vaccine and 7 (2 novel) candidate drug targets. The docking analysis showed six molecules from the ZINC database as promising drug candidates against the identified targets. Altogether, both vaccine candidates and drug targets identified here may contribute to the future development of therapeutic strategies to control the spread of M. genitalium worldwide.

Evidence for ligandable sites in structured RNA throughout the Protein Data Bank

RNA has attracted considerable attention as a target for small molecules. However, methods to identify, study, and characterize suitable RNA targets have lagged behind strategies for protein targets. One approach that has received considerable attention for protein targets has been to utilize computational analysis to investigate ligandable "pockets" on proteins that are amenable to small molecule binding. These studies have shown that selected physical properties of pockets are important parameters that govern the ability of a structure to bind to small molecules. This work describes a similar analysis to study pockets on all RNAs in the Protein Data Bank (PDB). Using parameters such as buriedness, hydrophobicity, volume, and other properties, the set of all RNAs is analyzed and compared to all proteins. Considerable overlap is observed between the properties of pockets on RNAs and proteins. Thus, many RNAs are capable of populating conformations with pockets that are likely suitable for small molecule binding. Further, principal moment of inertia (PMI) calculations reveal that liganded RNAs exist in diverse structural space, much of which overlaps with protein structural space. Taken together, these results suggest that complex folded RNAs adopt unique structures with pockets that may represent viable opportunities for small molecule targeting.

Molecular Mechanism of Antibiotic Resistance: The Untouched Area of Future Hope

The treatment of bacterial infections is becoming increasingly ineffective due to rapid mutation which leads to antibiotic resistant and resistant bacteria become more prevalent. As a result the existing antibiotics are gradually obsolete and again new drugs are needed to be designed for the same threat. However, the prediction of evolutionary processes/antibiotic resistance is uncertain. Still, the understanding of mode of evolution of resistance in bacteria is a determining step in the preclinical development of new antibiotics, because drug developers assess the risk of resistance arising against a drug during preclinical development. Multidrug efflux pump systems play an important role for making multidrug resistance to a range of clinically important antibiotics in gram-negative bacteria like Pseudomonas aeruginosa, which lower the intracellular drug concentration by exporting incoming antibiotics across the membranes. We tried to show that the wild type susceptible bacteria P. aeruginosa modified its genetic makeup at mutational hotspots under stress. This strain may either become multidrug resistant or remain susceptible depending on position of amino acid changes in regulatory proteins of efflux pump. Multidrug resistant strain made significant changes at the amino acid positions, 103rd (G → A) and 126th (E → V) through the mutation on the nucleotide position of 308th (G → C); both 377th (A → T) and 378th (G → T), respectively in mexR, a repressor of mexAB-oprM efflux pump. This mutant protein showed low affinity with their operator. But the alteration at 103th position (G → A) in mexR may provide almost similar structural and functional stability as wild type. It was found that mutation was seemed to be well regulated within the limit and position specific under stress which might be back to its original form by supplying counter stress unless addition or deletion takes place.

Synthesis of Glycosidic (β-1''→6, 3' and 4') Site Isomers of Neomycin B and their Effect on RNA and DNA Triplex Stability

Glycosidic (β-1''→6, 3' and 4') site isomers of neomycin B (i.e., neobiosamine (β-1''→6, 3' and 4') neamines) have been synthesized in a straightforward manner. Peracetylated neomycin azide was used as a common starting material to obtain neobiosamine glycosyl donor and 6, 3',4'-tri-O-acetyl neamine azide that after simple protecting group manipulation was converted to three different glycosyl acceptors (i.e., 5,6,4'-, 5,3',4'- and 5,6,3'-tri-O-acetyl neamine azide). Glycosylation between the neobiosamine glycosyl donor and the neamine-derived acceptors gave the protected pseudo-tetrasaccharides, which were converted, via global deprotection (deacetylation and reduction of the azide groups), to the desired site isomers of neomycin. The effect of these aminoglycosides on the RNA and DNA triplex stability was studied by UV-melting profile analysis.

FRET Assay for Ligands Targeting the Bacterial A-Site RNA

A robust, fluorescence-based analysis and discovery platform is described for bacterial A-site binders. The assay relies on an incorporated isomorphic fluorescent uridine analog, which substitutes the A-site's U1406 and serves as a FRET donor to an A-site bound coumarin-labeled aminoglycoside that serves as the FRET acceptor. Binding efficiency of unlabeled A-site ligands can be determined by competition experiments, where the acceptor-labeled aminoglycoside is displaced. The replacement efficiency is gauged by the concentration-dependent loss of the sensitized FRET acceptor's signal with concomitant restoration of the donor's emission. Plotting the relative emission intensity of both the donor and acceptor as a function of ligand concentration followed by fitting of the data points to a dose-response curve yields IC₅₀ values, one possible measure of the antibiotic potency of new A-site binders.

Structure-Activity Relationship of Flavin Analogues That Target the Flavin Mononucleotide Riboswitch

The flavin mononucleotide (FMN) riboswitch is an emerging target for the development of novel RNA-targeting antibiotics. We previously discovered an FMN derivative, 5FDQD, that protects mice against diarrhea-causing Clostridium difficile bacteria. Here, we present the structure-based drug design strategy that led to the discovery of this fluoro-phenyl derivative with antibacterial properties. This approach involved the following stages: (1) structural analysis of all available free and bound FMN riboswitch structures; (2) design, synthesis, and purification of derivatives; (3) in vitro testing for productive binding using two chemical probing methods; (4) in vitro transcription termination assays; and (5) resolution of the crystal structures of the FMN riboswitch in complex with the most mature candidates. In the process, we delineated principles for productive binding to this riboswitch, thereby demonstrating the effectiveness of a coordinated structure-guided approach to designing drugs against RNA.

Transfer RNA-derived fragments target and regulate ribosome-associated aminoacyl-transfer RNA synthetases

Ribosome-associated noncoding (ranc) RNAs are a novel class of short regulatory RNAs with functions and origins that have not been well studied. In this present study, we functionally characterized the molecular activity of Saccharomyces cerevisiae transfer RNA (tRNA)-derived fragments (tRFs) during protein biosynthesis. Our results indicate ribosome-associated tRFs derived from both 5' (ranc-5'-tRFs) and 3'-part of tRNAs (ranc-3'-tRFs) have regulatory roles during translation. We demonstrated five 3'-tRFs and one 5'-tRF associate with a small ribosomal subunit and aminoacyl-tRNA synthetases (aa-RSs) in yeast. Furthermore, we discovered that four yeast aa-RSs interact directly with yeast ribosomes. tRFs interactions with ribosome-associated aa-RSs correlate with impaired efficiency of tRNA aminoacylation.

Superior HIV-1 TAR Binders with Conformationally Constrained R52 Arginine Mimics in the Tat(48-57) Peptide

We report a 100-fold increase in binding affinity of the Tat(48-57) peptide to HIV-1 transcriptional activator-responsive element (TAR) RNA by replacing Arg52, an essential and critical residue for Tat's specific binding, with (2S,4S)-4-guanidinoproline. The resulting αTat1M peptide is a far superior binder than γTat1M, a peptide containing another conformationally constrained arginine mimic, (2S,4S)-4-amino-N-(3-guanidinopropyl)proline, or even the control Tat peptide (CtrlTat) itself. Our observations are supported by circular dichroism (CD), isothermal titration calorimetry (ITC), gel electrophoresis and UV spectroscopy studies. Molecular dynamics simulations suggest increased interactions between the more compact αTat1M and TAR RNA, relative to CtrlTat. The CD signature of the RNA itself remains largely unchanged upon binding of the peptides. The Tat mimetics further have better cell uptake properties than the control Tat peptide, thus increasing their potential application as specific TAR-binding molecules.

Pseudouridine modifications influence binding of aminoglycosides to helix 69 of bacterial ribosomes

Development of antibiotics that target new regions of functionality is a possible way to overcome antibiotic resistance. In this study, the interactions of aminoglycoside antibiotics with helix 69 of the E. coli 23S rRNA in the context of complete 70S ribosomes or the isolated 50S subunit were investigated by using chemical probing and footprinting analysis. Helix 69 is a dynamic RNA motif that plays major roles in bacterial ribosome activity. Neomycin, paromomycin, and gentamicin interact with the stem region of helix 69 in complete 70S ribosomes, but have diminished binding to the isolated 50S subunit. Pseudouridine modifications in helix 69 were shown to impact the aminoglycoside interactions. These results suggest a requirement for a specific conformational state of helix 69 for efficient aminoglycoside binding, and imply that this motif may be a suitable target for mechanism-based therapeutics.

Eukaryotic Ribosomal Expansion Segments as Antimicrobial Targets

Diversity in eukaryotic rRNA structure and function offers possibilities of therapeutic targets. Unlike ribosomes of prokaryotes, eukaryotic ribosomes contain species-specific rRNA expansion segments (ESs) with idiosyncratic structures and functions that are essential and specific to some organisms. Here we investigate expansion segment 7 (ES7), one of the largest and most variable expansions of the eukaryotic ribosome. We hypothesize that ES7 of the pathogenic fungi Candida albicans (ES7_CA) could be a prototypic drug target. We show that isolated ES7_CA folds reversibly to a native-like state. We developed a fluorescence displacement assay using an RNA binding fluorescent probe, F-neo. F-neo binds tightly to ES7_CA with a K_d of 2.5 × 10^-9 M but binds weakly to ES7 of humans (ES7_HS) with a K_d estimated to be greater than 7 μM. The fluorescence displacement assay was used to investigate the affinities of a library of peptidic aminosugar conjugates (PAs) for ES7_CA. For conjugates with highest affinities for ES7_CA (NeoRH, NeoFH, and NeoYH), the lowest dose needed to induce mortality in C. albicans (minimum inhibitory concentration, MIC) was determined. PAs with the lowest MIC values were tested for cytotoxicity in HEK293T cells. Molecules with high affinity for ES7_CA in vitro induce mortality in C. albicans but not in HEK293T cells. The results are consistent with the hypothesis that ESs represent useful targets for chemotherapeutics directed against eukaryotic pathogens.

Halogen Bonding in Nucleic Acid Complexes

Halogen bonding (X-bonding) has attracted notable attention among noncovalent interactions. This highly directional attraction between a halogen atom and an electron donor has been exploited in knowledge-based drug design. A great deal of information has been gathered about X-bonds in protein-ligand complexes, as opposed to nucleic acid complexes. Here we provide a thorough analysis of nucleic acid complexes containing either halogenated building blocks or halogenated ligands. We analyzed close contacts between halogens and electron-rich moieties. The phosphate backbone oxygen is clearly the most common halogen acceptor. We identified 21 X-bonds within known structures of nucleic acid complexes. A vast majority of the X-bonds is formed by halogenated nucleobases, such as bromouridine, and feature excellent geometries. Noncovalent ligands have been found to form only interactions with suboptimal interaction geometries. Hence, the first X-bonded nucleic acid binder remains to be discovered.

Functional RNA structures throughout the Hepatitis C Virus genome

The single-stranded Hepatitis C Virus (HCV) genome adopts a set of elaborate RNA structures that are involved in every stage of the viral lifecycle. Recent advances in chemical probing, sequencing, and structural biology have facilitated analysis of RNA folding on a genome-wide scale, revealing novel structures and networks of interactions. These studies have underscored the active role played by RNA in every function of HCV and they open the door to new types of RNA-targeted therapeutics.

Synthesis of Aminoglycoside-2'-O-Methyl Oligoribonucleotide Fusions

Phosphoramidite building blocks of ribostamycin (3 and 4), that may be incorporated at any position of the oligonucleotide sequence, were synthesized. The building blocks, together with a previously described neomycin-modified solid support, were applied for the preparation of aminoglycoside-2′-O-methyl oligoribonucleotide fusions. The fusions were used to clamp a single strand DNA sequence (a purine-rich strand of c-Myc promoter 1) to form triple helical 2′-O-methyl RNA/DNA-hybrid constructs. The potential of the aminoglycoside moieties to stabilize the triple helical constructs were studied by UV-melting profile analysis.

Amiloride as a new RNA-binding scaffold with activity against HIV-1 TAR

Diversification of RNA-targeted scaffolds offers great promise in the search for selective ligands of therapeutically relevant RNA such as HIV-1 TAR. We herein report the establishment of amiloride as a novel RNA-binding scaffold along with synthetic routes for combinatorial C(5)- and C(6)-diversification. Iterative modifications at the C(5)- and C(6)- positions yielded derivative 24, which demonstrated a 100-fold increase in activity over the parent dimethylamiloride in peptide displacement assays. NMR chemical shift mapping was performed using the 2D SOFAST- [1H-13C] HMQC NMR method, which allowed for facile and rapid evaluation of binding modes for all library members. Cheminformatic analysis revealed distinct differences between selective and non-selective ligands. In this study, we evolved dimethylamiloride from a weak TAR ligand to one of the tightest binding selective TAR ligands reported to date through a novel combination of synthetic methods and analytical techniques. We expect these methods to allow for rapid library expansion and tuning of the amiloride scaffold for a range of RNA targets and for SOFAST NMR to allow unprecedented evaluation of small molecule:RNA interactions.

Probing interaction of a fluorescent ligand with HIV TAR RNA

Trans-activator of Transcription (Tat) antagonists could block the interaction between Tat protein and its target, trans-activation responsive region (TAR) RNA, to inhibit Tat function and prevent human immunodeficiency virus type 1 (HIV-1) replication. For the first time, a small fluorescence ligand, ICR 191, was found to interact with TAR RNA at the Tat binding site and compete with Tat. It was also observed that the fluorescence of ICR 191 could be quenched when binding to TAR RNA and recovered when discharged via competition with Tat peptide or a well-known Tat inhibitor, neomycin B. The binding parameters of ICR 191 to TAR RNA were determined through theoretical calculations. Mass spectrometry, circular dichroism and molecular docking were used to further confirm the interaction of ICR 191 with TAR RNA. Inspired by these discoveries, a primary fluorescence model for the discovery of Tat antagonists was built using ICR 191 as a fluorescence indicator and the feasibility of this model was evaluated. This ligand-RNA interaction could provide a new strategy for research aimed at discovering Tat antagonists.

Analysis of ribosomal inter-subunit sites as targets for complementary oligonucleotides

The bacterial ribosome has many functional ribosomal RNA (rRNA) sites. We have computationally analyzed the rRNA regions involved in the interactions between the 30S and 50S subunits. Various properties of rRNA such as solvent accessibility, opening energy, hydrogen bonding pattern, van der Waals energy, thermodynamic stability were determined. Based on these properties we selected rRNA targets for hybridization with complementary 2'-O-methyl oligoribonucleotides (2'-OMe RNAs). Further, the inhibition efficiencies of the designed ribosome-interfering 2'-OMe RNAs were tested using a β-galactosidase assay in a translation system based on the E. coli extract. Several of the oligonucleotides displayed IC₅₀ values below 1 μM, which were in a similar range as those determined for known ribosome inhibitors, tetracycline and pactamycin. The calculated opening and van der Waals stacking energies of the rRNA targets correlated best with the inhibitory efficiencies of 2'-OMe RNAs. Moreover, the binding affinities of several oligonucleotides to both 70S ribosomes and isolated 30S and 50S subunits were measured using a double-filter retention assay. Further, we applied heat-shock chemical transformation to introduce 2'-OMe RNAs to E. coli cells and verify inhibition of bacterial growth. We observed high correlation between IC₅₀ in the cell-free extract and bacterial growth inhibition. Overall, the results suggest that the computational analysis of potential rRNA targets within the conformationally dynamic regions of inter-subunit bridges can help design efficient antisense oligomers to probe the ribosome function.

A Numbers Game: Ribosome Densities, Bacterial Growth, and Antibiotic-Mediated Stasis and Death

We postulate that the inhibition of growth and low rates of mortality of bacteria exposed to ribosome-binding antibiotics deemed bacteriostatic can be attributed almost uniquely to these drugs reducing the number of ribosomes contributing to protein synthesis, i.e., the number of effective ribosomes. We tested this hypothesis with Escherichia coli K-12 MG1655 and constructs that had been deleted for 1 to 6 of the 7 rRNA (rrn) operons. In the absence of antibiotics, constructs with fewer rrn operons have lower maximum growth rates and longer lag phases than those with more ribosomal operons. In the presence of the ribosome-binding “bacteriostatic” antibiotics tetracycline, chloramphenicol, and azithromycin, E. coli strains with 1 and 2 rrn operons are killed at a substantially higher rate than those with more rrn operons. This increase in the susceptibility of E. coli with fewer rrn operons to killing by ribosome-targeting bacteriostatic antibiotics is not reflected in their greater sensitivity to killing by the bactericidal antibiotic ciprofloxacin, which does not target ribosomes, but also to killing by gentamicin, which does. Finally, when such strains are exposed to these ribosome-targeting bacteriostatic antibiotics, the time before these bacteria start to grow again when the drugs are removed, referred to as the post-antibiotic effect (PAE), is markedly greater for constructs with fewer rrn operons than for those with more rrn operons. We interpret the results of these other experiments reported here as support for the hypothesis that the reduction in the effective number of ribosomes due to binding to these structures provides a sufficient explanation for the action of bacteriostatic antibiotics that target these structures.

Chemotherapeutic agents, including antibiotics, have been used for more than a century; nevertheless, there are still major gaps in our understanding of how these drugs operate which limit future advances in antibacterial chemotherapy. Although the molecular mechanisms by which antibiotics bind to their target structures are largely known, fundamental questions about how these drugs actually kill and/or inhibit the replication of bacteria remain unanswered and subjects of controversy. We postulate that for the broad class of ribosome-binding bacteriostatic antibiotics, their reducing the number of active (functional) ribosomes per cell provides a sufficient explanation for the abatement of replication and the low rate of decline in densities of viable cells of bacteria exposed to these drugs. Using E. coli K-12 constructs with deletions of from one to six of the seven ribosome-RNA operons and the ribosome-binding bacteriostatic antibiotics tetracycline, chloramphenicol, and azithromycin, we tested this hypothesis. The results of our experiments are consistent with this “numbers game” hypothesis.

Structural Elucidation and Molecular Docking of a Novel Antibiotic Compound from Cyanobacterium Nostoc sp. MGL001

Cyanobacteria are rich source of array of bioactive compounds. The present study reports a novel antibacterial bioactive compound purified from cyanobacterium Nostoc sp. MGL001 using various chromatographic techniques viz. thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). Further characterization was done using electrospray ionization mass spectroscopy (ESIMS) and nuclear magnetic resonance (NMR) and predicted structure of bioactive compound was 9-Ethyliminomethyl-12-(morpholin - 4 - ylmethoxy) -5, 8, 13, 16–tetraaza–hexacene - 2, 3 dicarboxylic acid (EMTAHDCA). Structure of EMTAHDCA clearly indicated that it is a novel compound that was not reported in literature or natural product database. The compound exhibited growth inhibiting effects mainly against the gram negative bacterial strains and produced maximum zone of inhibition at 150 μg/mL concentration. The compound was evaluated through in silico studies for its ability to bind 30S ribosomal fragment (PDB ID: 1YRJ, 1MWL, 1J7T, and 1LC4) and OmpF porin protein (4GCP, 4GCQ, and 4GCS) which are the common targets of various antibiotic drugs. Comparative molecular docking study revealed that EMTAHDCA has strong binding affinity for these selected targets in comparison to a number of most commonly used antibiotics. The ability of EMTAHDCA to bind the active sites on the proteins and 30S ribosomal fragments where the antibiotic drugs generally bind indicated that it is functionally similar to the commercially available drugs.

The development of peptide ligands that target helix 69 rRNA of bacterial ribosomes

Antibiotic resistance prevents successful treatment of common bacterial infections, making it clear that new target locations and drugs are required to resolve this ongoing challenge. The bacterial ribosome is a common target for antibacterials due to its essential contribution to cell viability. The focus of this work is a region of the ribosome called helix 69 (H69), which was recently identified as a secondary target site for aminoglycoside antibiotics. H69 has key roles in essential ribosomal processes such as subunit association, ribosome recycling, and tRNA selection. Conserved across phylogeny, bacterial H69 also contains two pseudouridines and one 3-methylpseudouridine. Phage display revealed a heptameric peptide sequence that targeted H69. Using solid-phase synthesis, peptide variants with higher affinity and improved selectivity to modified H69 were generated. Electrospray ionization mass spectrometry was used to determine relative apparent dissociation constants of the RNA-peptide complexes.

The Emerging Role of RNA as a Therapeutic Target for Small Molecules

Recent advances in understanding different RNAs and unique features of their biology have revealed a wealth of information. However, approaches to identify small molecules that target these newly discovered regulatory elements have been lacking. The application of new biochemical screening and design-based technologies, coupled with a resurgence of interest in phenotypic screening, has resulted in several compelling successes in targeting RNA. A number of recent advances suggest that achieving the long-standing goal of developing drug-like, biologically active small molecules that target RNA is possible. This review highlights advances and successes in approaches to targeting RNA with diverse small molecules, and the potential for these technologies to pave the way to new types of RNA-targeted therapeutics.

From Erythromycin to Azithromycin and New Potential Ribosome-Binding Antimicrobials

Macrolides, as a class of natural or semisynthetic products, express their antibacterial activity primarily by reversible binding to the bacterial 50S ribosomal subunits and by blocking nascent proteins’ progression through their exit tunnel in bacterial protein biosynthesis. Generally considered to be bacteriostatic, they may also be bactericidal at higher doses. The discovery of azithromycin from the class of macrolides, as one of the most important new drugs of the 20th century, is presented as an example of a rational medicinal chemistry approach to drug design, applying classical structure-activity relationship that will illustrate an impressive drug discovery success story. However, the microorganisms have developed several mechanisms to acquire resistance to antibiotics, including macrolide antibiotics. The primary mechanism for acquiring bacterial resistance to macrolides is a mutation of one or more nucleotides from the binding site. Although azithromycin is reported to show different, two-step process of the inhibition of ribosome function of some species, more detailed elaboration of that specific mode of action is needed. New macrocyclic derivatives, which could be more potent and less prone to escape bacterial resistance mechanisms, are also continuously evaluated. A novel class of antibiotic compounds—macrolones, which are derived from macrolides and comprise macrocyclic moiety, linker, and either free or esterified quinolone 3-carboxylic group, show excellent antibacterial potency towards key erythromycin-resistant Gram-positive and Gram-negative bacterial strains, with possibly decreased potential of bacterial resistance to macrolides.

rRNA Binding Sites and the Molecular Mechanism of Action of the Tetracyclines

The tetracycline antibiotics are known to be effective in the treatment of both infectious and noninfectious disease conditions. The 16S rRNA binding mechanism currently held for the antibacterial action of the tetracyclines does not explain their activity against viruses, protozoa that lack mitochondria, and noninfectious conditions. Also, the mechanism by which the tetracyclines selectively inhibit microbial protein synthesis against host eukaryotic protein synthesis despite conservation of ribosome structure and functions is still questionable. Many studies have investigated the binding of the tetracyclines to the 16S rRNA using the small ribosomal subunit of different bacterial species, but there seems to be no agreement between various reports on the exact binding site on the 16S rRNA. The wide range of activity of the tetracyclines against a broad spectrum of bacterial pathogens, viruses, protozoa, and helminths, as well as noninfectious conditions, indicates a more generalized effect on RNA. In the light of recent evidence that the tetracyclines bind to various synthetic double-stranded RNAs (dsRNAs) of random base sequences, suggesting that the double-stranded structures may play a more important role in the binding of the tetracyclines to RNA than the specific base pairs, as earlier speculated, it is imperative to consider possible alternative binding modes or sites that could help explain the mechanisms of action of the tetracyclines against various pathogens and disease conditions.

Discovery and Analysis of Natural-Product Compounds Inhibiting Protein Synthesis in Pseudomonas aeruginosa

Bacterial protein synthesis is the target for numerous natural and synthetic antibacterial agents. We have developed a poly(U) mRNA-directed aminoacylation/translation (A/T) protein synthesis system composed of phenylalanyl-tRNA synthetases (PheRS), ribosomes, and ribosomal factors from Pseudomonas aeruginosa This system has been used for high-throughput screening of a natural-compound library. Assays were developed for each component of the system to ascertain the specific target of inhibitory compounds. In high-throughput screens, 13 compounds were identified that inhibit protein synthesis with 50% inhibitory concentrations ranging from 0.3 to >80 μM. MICs were determined for the compounds against the growth of a panel of pathogenic organisms, including Enterococcus faecalis, Escherichia coli, Haemophilus influenzae, Moraxella catarrhalis, P. aeruginosa, Staphylococcus aureus, and Streptococcus pneumoniae Three of the compounds were observed to have broad-spectrum activity and inhibited a hypersensitive strain of P. aeruginosa with MICs of 8 to 16 μg/ml. The molecular target of each of the three compounds was determined to be PheRS. One compound was found to be bacteriostatic, and one compound was bactericidal against both Gram-positive and Gram-negative pathogens. The third compound was observed to be bacteriostatic against Gram-positive and bactericidal against Gram-negative bacteria. All three compounds were competitive with the substrate ATP; however, one compound was competitive, one was uncompetitive, and one noncompetitive with the amino acid substrate. Macromolecular synthesis assays confirm the compounds inhibit protein synthesis. The compounds were shown to be more than 25,000-fold less active than the control staurosporine in cytotoxicity MTT testing in human cell lines.

Small molecules targeting viral RNA

Highly conserved noncoding RNA (ncRNA) elements in viral genomes and transcripts offer new opportunities to expand the repertoire of drug targets for the development of antiinfective therapy. Ligands binding to ncRNA architectures are able to affect interactions, structural stability or conformational changes and thereby block processes essential for viral replication. Proof of concept for targeting functional RNA by small molecule inhibitors has been demonstrated for multiple viruses with RNA genomes. Strategies to identify antiviral compounds as inhibitors of ncRNA are increasingly emphasizing consideration of drug‐like properties of candidate molecules emerging from screening and ligand design. Recent efforts of antiviral lead discovery for RNA targets have provided drug‐like small molecules that inhibit viral replication and include inhibitors of human immunodeficiency virus (HIV), hepatitis C virus (HCV), severe respiratory syndrome coronavirus (SARS CoV), and influenza A virus. While target selectivity remains a challenge for the discovery of useful RNA‐binding compounds, a better understanding is emerging of properties that define RNA targets amenable for inhibition by small molecule ligands. Insight from successful approaches of targeting viral ncRNA in HIV, HCV, SARS CoV, and influenza A will provide a basis for the future exploration of RNA targets for therapeutic intervention in other viral pathogens which create urgent, unmet medical needs. Viruses for which targeting ncRNA components in the genome or transcripts may be promising include insect‐borne flaviviruses (Dengue, Zika, and West Nile) and filoviruses (Ebola and Marburg). WIREs RNA 2016, 7:726–743. doi: 10.1002/wrna.1373

This article is categorized under: 1

RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems

2

RNA Interactions with Proteins and Other Molecules > Small Molecule–RNA Interactions

3

Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs

Translational readthrough-promoting drugs enhance pseudoknot-mediated suppression of the stop codon at the Moloney murine leukemia virus gag–pol junction

Translational readthrough-promoting drugs enhance the incorporation of amino acids at stop codons and can thus bypass premature termination during protein synthesis. The polymerase (Pol) proteins of Moloney murine leukemia virus (MoMLV) are synthesized as a large Gag–Pol fusion protein, formed by the readthrough of a stop codon at the end of the gag ORF. The downstream pol ORF lacks its own start codon, and Pol protein synthesis is wholly dependent on translation of the upstream gag gene and the readthrough event for expression. Here, we explored the effects of readthrough-promoting drugs – aminoglycoside antibiotics and the small molecule ataluren – on the efficiency of readthrough of the stop codon in the context of the MoMLV genome. We showed that these compounds increased readthrough of the stop codon at the MoMLV gag–pol junction in vivo above the already high basal level and that the resulting elevated gag–pol readthrough had deleterious effects on virus replication. We also showed that readthrough efficiency could be driven to even higher levels in vitro, and that the combination of the small molecules and the RNA structure at the MoMLV stop codon could achieve extremely high readthrough efficiencies.

Bridgehead-Substituted Triptycenes for Discovery of Nucleic Acid Junction Binders

Recently, the utility of triptycene as a scaffold for targeting nucleic acid three-way junctions was demonstrated. A rapid, efficient route for the synthesis of bridgehead-substituted triptycenes is reported, in addition to solid-phase diversification to a new class of triptycene peptides. The triptycene peptides were evaluated for binding to a d(CAG)·(CTG) repeat DNA junction exhibiting potent affinities. The bridgehead-substituted triptycenes provide new building blocks for rapid access to diverse triptycene ligands with novel architectures.

Fluorescent DNA-Protected Silver Nanoclusters for Ligand-HIV RNA Interaction Assay

Studying ligand-biomacromolecule interactions provides opportunities for creating new compounds that can efficiently regulate specific biological processes. Ribonucleic acid (RNA) molecules have become attractive drug targets since the discovery of their roles in modulating gene expression, while only a limited number of studies have investigated interactions between ligands and functional RNA molecules, especially those based on nanotechnology. DNA-protected silver nanoclusters (AgNCs) were used to investigate ligand-RNA interactions for the first time in this study. The anthracycline anticancer drug mitoxantrone (MTX) was found to quench the fluorescence of AgNCs. After adding human immunodeficiency virus trans-activation responsive region (TAR) RNA or Rev-response element (RRE) RNA to AgNCs-MTX mixtures, the fluorescence of the AgNCs recovered due to interactions between MTX with RNAs. The binding constants and number of binding sites of MTX to TAR and RRE RNA were determined through theoretical calculations. MTX-RNA interactions were further confirmed in fluorescence polarization and mass spectrometry experiments. The mechanism of MTX-based fluorescence quenching of the AgNCs was also explored. This study provides a new strategy for ligand-RNA binding interaction assay.

Cisplatin Targeting of Bacterial Ribosomal RNA Hairpins

Cisplatin is a clinically important chemotherapeutic agent known to target purine bases in nucleic acids. In addition to major deoxyribonucleic acid (DNA) intrastrand cross-links, cisplatin also forms stable adducts with many types of ribonucleic acid (RNA) including siRNA, spliceosomal RNAs, tRNA, and rRNA. All of these RNAs play vital roles in the cell, such as catalysis of protein synthesis by rRNA, and therefore serve as potential drug targets. This work focused on platination of two highly conserved RNA hairpins from E. coli ribosomes, namely pseudouridine-modified helix 69 from 23S rRNA and the 790 loop of helix 24 from 16S rRNA. RNase T1 probing, MALDI mass spectrometry, and dimethyl sulfate mapping revealed platination at GpG sites. Chemical probing results also showed platination-induced RNA structural changes. These findings reveal solvent and structural accessibility of sites within bacterial RNA secondary structures that are functionally significant and therefore viable targets for cisplatin as well as other classes of small molecules. Identifying target preferences at the nucleotide level, as well as determining cisplatin-induced RNA conformational changes, is important for the design of more potent drug molecules. Furthermore, the knowledge gained through studies of RNA-targeting by cisplatin is applicable to a broad range of organisms from bacteria to human.

Biophysical insights into the intercalative interaction of surfactant cobalt(III) complexes of certain diimine ligands bound to yeast tRNA: Effects of hydrophobicity

The interaction of two surfactant cobalt(III) complexes, cis-[Co(ip)₂(DA)₂](ClO₄)₃ 1 and cis-[Co(dpq)₂(DA)₂](ClO₄)₃ 2 where ip=imidazo[4,5-f][1,10]phenanthroline and dpq=dipyrido[3,2-d:2'-3'-f]quinoxaline with yeast tRNA have been explored by using electronic absorption, competitive binding, electrochemical studies and viscosity measurements. The results suggest that these complexes can bind to tRNA by intercalation. The presence of hydrophobic diimine ligand and the long aliphatic double chains of these complexes facilitate its intercalative interaction with the hydrophobic interior of the tRNA. The extent of tRNA binding of complex 2 has greater affinity than that of complex containing imidazo[4,5-f][1,10]phenanthroline ligands.

HIV-1 drug discovery: targeting folded RNA structures with branched peptides

Human immunodeficiency virus type 1 (HIV-1) is an RNA virus that is prone to high rates of mutation. While the disease is managed with current antiretroviral therapies, drugs with a new mode of action are needed. A strategy towards this goal is aimed at targeting the native three-dimensional fold of conserved RNA structures. This perspective highlights medium-sized peptides and peptidomimetics used to target two conserved RNA structures of HIV-1. In particular, branched peptides have the capacity to bind in a multivalent fashion, utilizing a large surface area to achieve the necessary affinity and selectivity toward the target RNA.

The slow dissociation rate of K-1602 contributes to the enhanced inhibitory activity of this novel alkyl-aryl-bearing fluoroketolide

Ketolides belong to the latest generation of macrolides and are not only effective against macrolide susceptible bacterial strains but also against some macrolide resistant strains. Here we present data providing insights into the mechanism of action of K-1602, a novel alkyl-aryl-bearing fluoroketolide. According to our data, the K-1602 interacts with the ribosome as a one-step slow binding inhibitor, displaying an association rate constant equal to 0.28 × 10(4) M(-1) s(-1) and a dissociation rate constant equal to 0.0025 min(-1). Both constants contribute to produce an overall inhibition constant Ki equal to 1.49 × 10(-8) M, which correlates very well with the superior activity of this compound when compared with many other ketolides or fluoroketolides.

Multivalent Amino Sugars to Recognize Different TAR RNA Conformations

Neomycin dimers synthesized using "click chemistry" with varying functionality and length in the linker region have been shown to be effective in targeting the HIV-1 TAR RNA region of the HIV virus. TAR (Transactivation Response) RNA region, a 59 base pair stem loop structure located at the 5'-end of all nascent viral transcripts interacts with its target, a key regulatory protein, Tat, and necessitates the replication of HIV-1 virus. Ethidium bromide displacement and FRET competition assays have revealed nanomolar binding affinity between neomycin dimers and wildtype TAR RNA while in case of neomycin, only a weak binding was detected. Here, NMR and FID-based comparisons reveal an extended binding interface for neomycin dimers involving the upper stem of the TAR RNA thereby offering an explanation for increased affinities. To further explore the potential of these modified aminosugars we have extended binding studies to include four TAR RNA mutants that display conformational differences with minimal sequence variation. The differences in binding between neomycin and neomycin dimers is characterized with TAR RNA mutants that include mutations to the bulge region, hairpin region, and both the bulge and hairpin regions. Our results demonstrate the effect of these mutations on neomycin binding and our results show that linker functionalities between dimeric units of neomycin can distinguish between the conformational differences of mutant TAR RNA structures.

Therapeutic suppression of premature termination codons: mechanisms and clinical considerations (review)

An estimated one-third of genetic disorders are the result of mutations that generate premature termination codons (PTCs) within protein coding genes. These disorders are phenotypically diverse and consist of diseases that affect both young and old individuals. Various small molecules have been identified that are capable of modulating the efficiency of translation termination, including select antibiotics of the aminoglycoside family and multiple novel synthetic molecules, including PTC124. Several of these agents have proved their effectiveness at promoting nonsense suppression in preclinical animal models, as well as in clinical trials. In addition, it has recently been shown that box H/ACA RNA-guided peudouridylation, when directed to modify PTCs, can also promote nonsense suppression. In this review, we summarize our current understanding of eukaryotic translation termination and discuss various methods for promoting the read-through of disease-causing PTCs, as well as the current obstacles that stand in the way of using the discussed agents broadly in clinical practice.

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.

Themes and variations in riboswitch structure and function

The complexity of gene expression control by non-coding RNA has been highlighted by the recent progress in the field of riboswitches. Discovered a decade ago, riboswitches represent a diverse group of non-coding mRNA regions that possess a unique ability to directly sense cellular metabolites and modulate gene expression through formation of alternative metabolite-free and metabolite-bound conformations. Such protein-free metabolite sensing domains utilize sophisticated three-dimensional folding of RNA molecules to discriminate between a cognate ligand from related compounds so that only the right ligand would trigger a genetic response. Given the variety of riboswitch ligands ranging from small cations to large coenzymes, riboswitches adopt a great diversity of structures. Although many riboswitches share structural principles to build metabolite-competent folds, form precise ligand-binding pockets, and communicate a ligand-binding event to downstream regulatory regions, virtually all riboswitch classes possess unique features for ligand recognition, even those tuned to recognize the same metabolites. Here we present an overview of the biochemical and structural research on riboswitches with a major focus on common principles and individual characteristics adopted by these regulatory RNA elements during evolution to specifically target small molecules and exert genetic responses. This article is part of a Special Issue entitled: Riboswitches.

A yeast-based assay identifies drugs that interfere with immune evasion of the Epstein-Barr virus

Epstein-Barr virus (EBV) is tightly associated with certain human cancers, but there is as yet no specific treatment against EBV-related diseases. The EBV-encoded EBNA1 protein is essential to maintain viral episomes and for viral persistence. As such, EBNA1 is expressed in all EBV-infected cells, and is highly antigenic. All infected individuals, including individuals with cancer, have CD8(+) T cells directed towards EBNA1 epitopes, yet the immune system fails to detect and destroy cells harboring the virus. EBV immune evasion depends on the capacity of the Gly-Ala repeat (GAr) domain of EBNA1 to inhibit the translation of its own mRNA in cis, thereby limiting the production of EBNA1-derived antigenic peptides presented by the major histocompatibility complex (MHC) class I pathway. Here we establish a yeast-based assay for monitoring GAr-dependent inhibition of translation. Using this assay we identify doxorubicin (DXR) as a compound that specifically interferes with the GAr effect on translation in yeast. DXR targets the topoisomerase-II-DNA complexes and thereby causes genomic damage. We show, however, that the genotoxic effect of DXR and various analogs thereof is uncoupled from the effect on GAr-mediated translation control. This is further supported by the observation that etoposide and teniposide, representing another class of topoisomerase-II-DNA targeting drugs, have no effect on GAr-mediated translation control. DXR and active analogs stimulate, in a GAr-dependent manner, EBNA1 expression in mammalian cells and overcome GAr-dependent restriction of MHC class I antigen presentation. These results validate our approach as an effective high-throughput screening assay to identify drugs that interfere with EBV immune evasion and, thus, constitute candidates for treating EBV-related diseases, in particular EBV-associated cancers.

SMMRNA: a database of small molecule modulators of RNA

We have developed SMMRNA, an interactive database, available at http://www.smmrna.org, with special focus on small molecule ligands targeting RNA. Currently, SMMRNA consists of ∼770 unique ligands along with structural images of RNA molecules. Each ligand in the SMMRNA contains information such as Kd, Ki, IC50, ΔTm, molecular weight (MW), hydrogen donor and acceptor count, XlogP, number of rotatable bonds, number of aromatic rings and 2D and 3D structures. These parameters can be explored using text search, advanced search, substructure and similarity-based analysis tools that are embedded in SMMRNA. A structure editor is provided for 3D visualization of ligands. Advance analysis can be performed using substructure and OpenBabel-based chemical similarity fingerprints. Upload facility for both RNA and ligands is also provided. The physicochemical properties of the ligands were further examined using OpenBabel descriptors, hierarchical clustering, binning partition and multidimensional scaling. We have also generated a 3D conformation database of ligands to support the structure and ligand-based screening. SMMRNA provides comprehensive resource for further design, development and refinement of small molecule modulators for selective targeting of RNA molecules.

Synthesis of 4′-deoxy-4′-fluoro neamine and 4′-deoxy-4′-fluoro 4′-epi neamine

The syntheses of 4′-deoxy-4′-fluoro neamine and 4′-deoxy-4′-fluoro 4′-epi neamine from the readily available neamine and paromamine are described.

Influence of dimethylsulfoxide on RNA structure and ligand binding

Dimethyl sulfoxide (DMSO) is widely used as a cosolvent to solubilize hydrophobic compounds in RNA-ligand binding assays. Although it is known that high concentrations of DMSO (>75%) can significantly affect RNA structure and folding energetics, a thorough analysis of how lower concentrations (<10%) of DMSO typically used in binding assays affects RNA structure and ligand binding has not been undertaken. Here, we use NMR and 2-aminopurine fluorescence spectroscopy to examine how DMSO affects the structure, dynamics, and ligand binding properties of two flexible hairpin RNAs: the transactivation response element from HIV-1 and bacterial ribosomal A-site. In both cases, 5-10% DMSO decreased stacking interactions and increased local disorder in noncanonical residues within bulges and loops and resulted in 0.3-4-fold reduction in the measured binding affinities for different small molecules, with the greatest reduction observed for an intercalating compound that binds RNA nonspecifically. Our results suggest that, by competing for hydrophobic interactions, DMSO can have a small but significant effect on RNA structure and ligand binding. These effects should be considered when developing ligand binding assays and high throughput screens.