Ligand design for riboswitches, an emerging target class for novel antibiotics

Assessment of Nucleobase Protomeric and Tautomeric States in Nucleic Acid Structures for Interaction Analysis and Structure-Based Ligand Design

With increasing interest in RNA as a therapeutic and a potential target, the role of RNA structures has become more important. Even slight changes in nucleobases, such as modifications or protomeric and tautomeric states, can have a large impact on RNA structure and function, while local environments in turn affect protonation and tautomerization. In this work, the application of empirical tools for pKa and tautomer prediction for RNA modifications was elucidated and compared with ab initio quantum mechanics (QM) methods and expanded toward macromolecular RNA structures, where QM is no longer feasible. In this regard, the Protonate3D functionality within the molecular operating environment (MOE) was expanded for nucleobase protomer and tautomer predictions and applied to reported examples of altered protonation states depending on the local environment. Overall, observations of nonstandard protomers and tautomers were well reproduced, including structural C+G:C(A) and A+GG motifs, several mismatches, and protonation of adenosine or cytidine as the general acid in nucleolytic ribozymes. Special cases, such as cobalt hexamine-soaked complexes or the deprotonation of guanosine as the general base in nucleolytic ribozymes, proved to be challenging. The collected set of examples shall serve as a starting point for the development of further RNA protonation prediction tools, while the presented Protonate3D implementation already delivers reasonable protonation predictions for RNA and DNA macromolecules. For cases where higher accuracy is needed, like following catalytic pathways of ribozymes, incorporation of QM-based methods can build upon the Protonate3D-generated starting structures. Likewise, this protonation prediction can be used for structure-based RNA-ligand design approaches.

Riboswitches as therapeutic targets: promise of a new era of antibiotics

INTRODUCTION

The growth of antibiotic resistance among bacterial pathogens is an impending global threat that can only be averted through the development of novel antibacterial drugs. A promising answer could be the targeting of riboswitches, structured RNA elements found almost exclusively in bacteria.

AREAS COVERED

This review examines the potential of riboswitches as novel antibacterial drug targets. The limited mechanisms of action of currently available antibiotics are summarized, followed by a delineation of the functional mechanisms of riboswitches. We then discuss the potential for developing novel approaches that target paradigmatic riboswitches in the context of their bacterial gene expression machinery.

EXPERT OPINION

We highlight potential advantages of targeting riboswitches in their functional form, embedded within gene expression complexes critical for bacterial survival. We emphasize the benefits of this approach, including potentially higher species specificity and lower side effects.

Protein-Based Virtual Screening Tools Applied for RNA-Ligand Docking Identify New Binders of the preQ1-Riboswitch

Targeting RNA with small molecules is an emerging field. While several ligands for different RNA targets are reported, structure-based virtual screenings (VSs) against RNAs are still rare. Here, we elucidated the general capabilities of protein-based docking programs to reproduce native binding modes of small-molecule RNA ligands and to discriminate known binders from decoys by the scoring function. The programs were found to perform similar compared to the RNA-based docking tool rDOCK, and the challenges faced during docking, namely, protomer and tautomer selection, target dynamics, and explicit solvent, do not largely differ from challenges in conventional protein-ligand docking. A prospective VS with the Bacillus subtilis preQ₁-riboswitch aptamer domain performed with FRED, HYBRID, and FlexX followed by microscale thermophoresis assays identified six active compounds out of 23 tested VS hits with potencies between 29.5 nM and 11.0 μM. The hits were selected not solely based on their docking score but for resembling key interactions of the native ligand. Therefore, this study demonstrates the general feasibility to perform structure-based VSs against RNA targets, while at the same time it highlights pitfalls and their potential solutions when executing RNA-ligand docking.

Lineage-specific insertions in T-box riboswitches modulate antibiotic binding and action

T-box riboswitches (T-boxes) are essential RNA regulatory elements with a remarkable structural diversity, especially among bacterial pathogens. In staphylococci, all glyS T-boxes synchronize glycine supply during synthesis of nascent polypeptides and cell wall formation and are characterized by a conserved and unique insertion in their antiterminator/terminator domain, termed stem Sa. Interestingly, in Staphylococcus aureus the stem Sa can accommodate binding of specific antibiotics, which in turn induce robust and diverse effects on T-box-mediated transcription. In the present study, domain swap mutagenesis and probing analysis were performed to decipher the role of stem Sa. Deletion of stem Sa significantly reduces both the S. aureus glyS T-box-mediated transcription readthrough levels and the ability to discriminate among tRNAGly isoacceptors, both in vitro and in vivo. Moreover, the deletion inverted the previously reported stimulatory effects of specific antibiotics. Interestingly, stem Sa insertion in the terminator/antiterminator domain of Geobacillus kaustophilus glyS T-box, which lacks this domain, resulted in elevated transcription in the presence of tigecycline and facilitated discrimination among proteinogenic and nonproteinogenic tRNAGly isoacceptors. Overall, stem Sa represents a lineage-specific structural feature required for efficient staphylococcal glyS T-box-mediated transcription and it could serve as a species-selective druggable target through its ability to modulate antibiotic binding.

Fragment-Based Drug Discovery for RNA Targets

Rapid development within the fields of both fragment-based drug discovery (FBDD) and medicinal targeting of RNA provides possibilities for combining technologies and methods in novel ways. This review provides an overview of fragment-based screening (FBS) against RNA targets, including a discussion of the most recently used screening and hit validation methods such as NMR spectroscopy, X-ray crystallography, and virtual screening methods. A discussion of fragment library design based on research from small-molecule RNA binders provides an overview on both the currently limited guidelines within RNA-targeting fragment library design, and future possibilities. Finally, future perspectives are provided on screening and hit validation methods not yet used in combination with both fragment screening and RNA targets.

DrugPred_RNA-A Tool for Structure-Based Druggability Predictions for RNA Binding Sites

RNA is an emerging target for drug discovery. However, like for proteins, not all RNA binding sites are equally suited to be addressed with conventional drug-like ligands. To this end, we have developed the structure-based druggability predictor DrugPred_RNA to identify druggable RNA binding sites. Due to the paucity of annotated RNA binding sites, the predictor was trained on protein pockets, albeit using only descriptors that can be calculated for both RNA and protein binding sites. DrugPred_RNA performed well in discriminating druggable from less druggable binding sites for the protein set and delivered predictions for selected RNA binding sites that agreed with manual assignment. In addition, most drug-like ligands contained in an RNA test set were found in pockets predicted to be druggable, further adding confidence to the performance of DrugPred_RNA. The method is robust against conformational and sequence changes in the binding sites and can contribute to direct drug discovery efforts for RNA targets.

Tying the knot in the tetrahydrofolate (THF) riboswitch: A molecular basis for gene regulation

Effective gene regulation by the tetrahydrofolate riboswitch depends not only on ligand affinity but also on the kinetics of ligand association, which involves two cooperative binding sites. We have determined a 1.9-Å resolution crystal structure of the ligand-free THF riboswitch aptamer. The pseudoknot binding site 'unwinds' in the absence of ligand, whereby the adjacent helical domains (P1, P2, and P3) become disjointed, resulting in rotation and misalignment of the gene-regulatory P1 helix with respect to P3. In contrast, the second binding site at the three-way junction, which is the first to fold, is structurally conserved between apo and holo forms. This suggests a kinetic role for this site, in which binding of the first ligand molecule to the stably folded three-way junction promotes formation of the regulatory pseudoknot site and subsequent binding of the second molecule. As such, these findings provide a molecular basis for both conformational switching and kinetic control.

Riboswitches as Drug Targets for Antibiotics

Riboswitches reside in the untranslated region of RNA and regulate genes involved in the biosynthesis of essential metabolites through binding of small molecules. Since their discovery at the beginning of this century, riboswitches have been regarded as potential antibacterial targets. Using fragment screening, high-throughput screening and rational ligand design guided by X-ray crystallography, lead compounds against various riboswitches have been identified. Here, we review the current status and suitability of the thiamine pyrophosphate (TPP), flavin mononucleotide (FMN), glmS, guanine, and other riboswitches as antibacterial targets and discuss them in a biological context. Further, we highlight challenges in riboswitch drug discovery and emphasis the need to develop riboswitch specific high-throughput screening methods.

Thiamine pyrophosphate riboswitch regulation: a new possible mechanism involved in the action of nalidixic acid

Objectives

The development of novel antibiotic compounds requires riboswitches; in fact, riboswitches are RNA elements present in the 5′ untranslated region of bacterial mRNA and have a metabolite-binding aptamer domain and an expression platform regulating the expression of vital genes. In the present research, one riboswitch, namely thi-box riboswitch with distinct regulatory mechanisms, was studied. It recognizes Thiamine Pyrophosphates (TPP) regulating TPP-biosynthesis genes in Escherichia coli.

Methods

First, the compounds similar to riboswitch ligands were studied, and their binding with the riboswitch and nucleosides was investigated by molecular docking. Then, compounds containing high binding energy were chosen, and their minimum inhibitory concentration in E. coli was determined by the MIC test. Finally, the binding of compounds to nucleotides and RNA was investigated by measuring the absorbance spectrum through NanoDrop and circular dichroism (CD).

Results

In the thi-box riboswitch, nalidixic acid was found to have the best binding energy (−5.31 kJ/mol), and it inhibited E. coli growth at the minimum inhibitory concentration of 125 μg/mL, and it could bind to ribonucleosides and RNA in vitro.

Conclusions

One possible mechanism involved in the action of nalidixic acid in inhibiting the E. coli growth is to influence thi-box riboswitch.

Employing a ZTP Riboswitch to Detect Bacterial Folate Biosynthesis Inhibitors in a Small Molecule High-Throughput Screen

Various riboswitch classes are being discovered that precisely monitor the status of important biological processes, including metabolic pathway function, signaling for physiological adaptations, and responses to toxic agents. Biochemical components for some of these processes might make excellent targets for the development of novel antibacterial molecules, which can be broadly sought by using phenotypic drug discovery (PDD) methods. However, PDD data do not normally provide clues regarding the target for each hit compound. We have developed and validated a robust fluorescent reporter system based on a ZTP riboswitch that identifies numerous folate biosynthesis inhibitors with high sensitivity and precision. The utility of the riboswitch-based PDD strategy was evaluated using Escherichia coli bacteria by conducting a 128 310-compound high-throughput screen, which identified 78 sulfanilamide derivatives among the many initial hits. Similarly, representatives of other riboswitch classes could be employed to rapidly match antibacterial hits with the biological processes they target.

Insights Into Non-coding RNAs as Novel Antimicrobial Drugs

Multidrug resistant bacteria are a serious worldwide problem, especially carbapenem-resistant Enterobacteriaceae (such as Klebsiella pneumoniae and Escherichia coli), Acinetobacter baumannii and Pseudomonas aeruginosa. Since the emergence of extensive and pan-drug resistant bacteria there are few antibiotics left to treat patients, thus novel RNA-based strategies are being considered. Here, we examine the current situation of different non-coding RNAs found in bacteria as well as their function and potential application as antimicrobial agents. Furthermore, we discuss the factors that may contribute in the efficient development of RNA-based drugs, the limitations for their implementation and the use of nanocarriers for delivery.

Superior cellular activities of azido- over amino-functionalized ligands for engineered preQ1 riboswitches in E.coli

For this study, we utilized class-I and class-II preQ1-sensing riboswitches as model systems to decipher the structure-activity relationship of rationally designed ligand derivatives in vitro and in vivo. We found that synthetic preQ1 ligands with amino-modified side chains that protrude from the ligand-encapsulating binding pocket, and thereby potentially interact with the phosphate backbone in their protonated form, retain or even increase binding affinity for the riboswitches in vitro. They, however, led to significantly lower riboswitch activities in a reporter system in vivo in E. coli. Importantly, when we substituted the amino- by azido-modified side chains, the cellular activities of the ligands were restored for the class-I conditional gene expression system and even improved for the class-II counterpart. Kinetic analysis of ligand binding in vitro revealed enhanced on-rates for amino-modified derivatives while they were attenuated for azido-modified variants. This shows that neither high affinities nor fast on-rates are necessarily translated into efficient cellular activities. Taken together, our comprehensive study interconnects in vitro kinetics and in vitro thermodynamics of RNA-ligand binding with the ligands' in vivo performance and thereby encourages azido- rather than amino-functionalized design for enhanced cellular activity.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.