Synthesis and molecular modeling provide insight into a Pseudomonas aeruginosa quorum sensing conundrum

Abiotic Small Molecule Inhibitors and Activators of the LasR Quorum Sensing Receptor in Pseudomonas aeruginosa with Potencies Comparable or Surpassing N-Acyl Homoserine Lactones

The opportunistic pathogen Pseudomonas aeruginosa controls almost 10% of its genome, including myriad virulence genes, via a cell-to-cell chemical communication system called quorum sensing (QS). Small molecules that either inhibit or activate QS in P. aeruginosa represent useful research tools to study the role of this signaling pathway in infection and interrogate its viability as an antivirulence target. However, despite active research in this area over the past 20+ years, there are relatively few synthetic compounds known to strongly inhibit or activate QS in P. aeruginosa. Most reported QS modulators in this pathogen are of low potency or have structural liabilities that limit their application in biologically relevant environments such as mimics of the native N-acyl l-homoserine lactone (AHL) signals. Here, we report the results of a high-throughput screen for abiotic small molecules that target LasR, a key QS regulator in P. aeruginosa. We screened a 25,000-compound library and discovered four new structural classes of abiotic LasR modulators. These compounds include antagonists that surpass the potency of all known AHL-type compounds and mimetics thereof, along with an agonist with potency approaching that of LasR's native ligand. The novel structures of this compound set, along with their anticipated robust physicochemical profiles, underscore their potential value as probe molecules to interrogate the roles of QS in this formidable pathogen.

Palladium-Catalyzed Direct Esterification via C-H Bond Activation of Aldehydes

A concise and highly efficient synthesis method of direct esterification of aldehydes via Pd-catalyzed C-H bond activation of aldehyde group has been developed. The strategy avoids the preoxidation step of aldehyde or use of condensing agents in ester synthesis, which is not only applicable to various alcohols but also suitable for the esterification of phenolics which are usually difficult to be esterified. The methodology has the significant advantages of broad substrate scope, mild reaction conditions, and nonrequirement of additional oxidants.

Design, Synthesis, and Biochemical Characterization of Non-Native Antagonists of the Pseudomonas aeruginosa Quorum Sensing Receptor LasR with Nanomolar IC50 Values

Quorum sensing (QS), a bacterial cell-to-cell communication system mediated by small molecules and peptides, has received significant interest as a potential target to block infection. The common pathogen Pseudomonas aeruginosa uses QS to regulate many of its virulence phenotypes at high cell densities, and the LasR QS receptor plays a critical role in this process. Small molecule tools that inhibit LasR activity would serve to illuminate its role in P. aeruginosa virulence, but we currently lack highly potent and selective LasR antagonists, despite considerable research in this area. V-06-018, an abiotic small molecule discovered in a high-throughput screen, represents one of the most potent known LasR antagonists but has seen little study since its initial report. Herein, we report a systematic study of the structure-activity relationships (SARs) that govern LasR antagonism by V-06-018. We synthesized a focused library of V-06-018 derivatives and evaluated the library for bioactivity using a variety of cell-based LasR reporter systems. The SAR trends revealed by these experiments allowed us to design probes with 10-fold greater potency than that of V-06-018 and 100-fold greater potency than other commonly used N-acyl-l-homoserine lactone (AHL)-based LasR antagonists, along with high selectivities for LasR. Biochemical experiments to probe the mechanism of antagonism by V-06-018 and its analogues support these compounds interacting with the native ligand-binding site in LasR and, at least in part, stabilizing an inactive form of the protein. The compounds described herein are the most potent and efficacious antagonists of LasR known and represent robust probes both for characterizing the mechanisms of LuxR-type QS and for chemical biology research in general in the growing QS field.

Structural and Biochemical Studies of Non-native Agonists of the LasR Quorum-Sensing Receptor Reveal an L3 Loop "Out" Conformation for LasR

Chemical strategies to block quorum sensing (QS) could provide a route to attenuate virulence in bacterial pathogens. Considerable research has focused on this approach in Pseudomonas aeruginosa, which uses the LuxR-type receptor LasR to regulate much of its QS network. Non-native ligands that antagonize LasR have been developed, yet we have little understanding of the mode by which these compounds interact with LasR and alter its function, as the receptor is unstable in their presence. Herein, we report an approach to circumvent this challenge through the study of a series of synthetic LasR agonists with varying levels of potency. Structural investigations of these ligands with the LasR ligand-binding domain reveal that certain agonists can enforce a conformation that deviates from that observed for other, often more potent agonists. These results, when combined with cell-based and biophysical analyses, suggest a functional model for LasR that could guide future ligand design.

Flavonoids Suppress Pseudomonas aeruginosa Virulence through Allosteric Inhibition of Quorum-sensing Receptors

Quorum sensing is a process of cell-cell communication that bacteria use to regulate collective behaviors. Quorum sensing depends on the production, detection, and group-wide response to extracellular signal molecules called autoinducers. In many bacterial species, quorum sensing controls virulence factor production. Thus, disrupting quorum sensing is considered a promising strategy to combat bacterial pathogenicity. Several members of a family of naturally produced plant metabolites called flavonoids inhibit Pseudomonas aeruginosa biofilm formation by an unknown mechanism. Here, we explore this family of molecules further, and we demonstrate that flavonoids specifically inhibit quorum sensing via antagonism of the autoinducer-binding receptors, LasR and RhlR. Structure-activity relationship analyses demonstrate that the presence of two hydroxyl moieties in the flavone A-ring backbone are essential for potent inhibition of LasR/RhlR. Biochemical analyses reveal that the flavonoids function non-competitively to prevent LasR/RhlR DNA binding. Administration of the flavonoids to P. aeruginosa alters transcription of quorum sensing-controlled target promoters and suppresses virulence factor production, confirming their potential as anti-infectives that do not function by traditional bacteriocidal or bacteriostatic mechanisms.

Development of a novel series of non-natural triaryl agonists and antagonists of the Pseudomonas aeruginosa LasR quorum sensing receptor

Bacterial chemical communication, through a process called quorum sensing (QS), plays a central role in infection in numerous bacterial pathogens. Quorum sensing in Pseudomonas aeruginosa employs a series of small molecule receptors including the master QS regulator, LasR. In this study we investigate a non-natural triaryl series of LasR ligands using a combination of structure activity relationship studies and computational modeling. These studies have enabled the identification of key structural requirements for ligand binding and have revealed a new strategy for inducing the therapeutically relevant antagonism of LasR.

Chemical probes of quorum sensing: from compound development to biological discovery

Bacteria can utilize chemical signals to coordinate the expression of group-beneficial behaviors in a method of cell-cell communication called quorum sensing (QS). The discovery that QS controls the production of virulence factors and biofilm formation in many common pathogens has driven an explosion of research aimed at both deepening our fundamental understanding of these regulatory networks and developing chemical agents that can attenuate QS signaling. The inherently chemical nature of QS makes studying these pathways with small molecule tools a complementary approach to traditional microbiology techniques. Indeed, chemical tools are beginning to yield new insights into QS regulation and provide novel strategies to inhibit QS. Here, we review the most recent advances in the development of chemical probes of QS systems in Gram-negative bacteria, with an emphasis on the opportunistic pathogen Pseudomonas aeruginosa We first describe reports of novel small molecule modulators of QS receptors and QS signal synthases. Next, in several case studies, we showcase how chemical tools have been deployed to reveal new knowledge of QS biology and outline lessons for how researchers might best target QS to combat bacterial virulence. To close, we detail the outstanding challenges in the field and suggest strategies to overcome these issues.

Structure-Based Design and Biological Evaluation of Triphenyl Scaffold-Based Hybrid Compounds as Hydrolytically Stable Modulators of a LuxR-Type Quorum Sensing Receptor

Many common bacterial pathogens utilize quorum sensing to coordinate group behaviors and initiate virulence at high cell densities. The use of small molecules to block quorum sensing provides a means of abrogating pathogenic phenotypes, but many known quorum sensing modulators have limitations, including hydrolytic instability and displaying non-monotonic dose curves (indicative of additional targets and/or modes of action). To address these issues, we undertook a structure-based scaffold-hopping approach to develop new chemical modulators of the LasR quorum sensing receptor in Pseudomonas aeruginosa. We combined components from a triphenyl derivative known to strongly agonize LasR with chemical moieties known for LasR antagonism and generated potent LasR antagonists that are hydrolytically stable across a range of pH values. Additionally, many of these antagonists do not exhibit non-monotonic dose effects, delivering probes that inhibit LasR across a wider range of assay conditions relative to known lactone-based ligands.

Design, synthesis and evaluation of N-aryl-glyoxamide derivatives as structurally novel bacterial quorum sensing inhibitors

Bacteria cooperatively regulate the expression of many phenotypes through a mechanism called quorum sensing (QS).

A Comparative Analysis of Synthetic Quorum Sensing Modulators in Pseudomonas aeruginosa: New Insights into Mechanism, Active Efflux Susceptibility, Phenotypic Response, and Next-Generation Ligand Design

Quorum sensing (QS) is a chemical signaling mechanism that allows bacterial populations to coordinate gene expression in response to social and environmental cues. Many bacterial pathogens use QS to initiate infection at high cell densities. Over the past two decades, chemical antagonists of QS in pathogenic bacteria have attracted substantial interest for use both as tools to further elucidate QS mechanisms and, with further development, potential anti-infective agents. Considerable recent research has been devoted to the design of small molecules capable of modulating the LasR QS receptor in the opportunistic pathogen Pseudomonas aeruginosa. These molecules hold significant promise in a range of contexts; however, as most compounds have been developed independently, comparative activity data for these compounds are scarce. Moreover, the mechanisms by which the bulk of these compounds act are largely unknown. This paucity of data has stalled the choice of an optimal chemical scaffold for further advancement. Herein, we submit the best-characterized LasR modulators to standardized cell-based reporter and QS phenotypic assays in P. aeruginosa, and we report the first comprehensive set of comparative LasR activity data for these compounds. Our experiments uncovered multiple interesting mechanistic phenomena (including a potential alternative QS-modulatory ligand binding site/partner) that provide new, and unexpected, insights into the modes by which many of these LasR ligands act. The lead compounds, data trends, and mechanistic insights reported here will significantly aid the design of new small molecule QS inhibitors and activators in P. aeruginosa, and in other bacteria, with enhanced potencies and defined modes of action.

Potent Irreversible Inhibitors of LasR Quorum Sensing in Pseudomonas aeruginosa

Antagonism of quorum sensing represents a promising new antivirulence approach for the treatment of bacterial infection. The development of a novel series of non-natural irreversible antagonists of P. aeruginosa LasR is described. The lead compounds identified (25 and 28) display potent LasR antagonist activity and inhibit expression of the P. aeruginosa virulence factors pyocyanin and biofilm formation in PAO1 and PA14.

Substituted lactam and cyclic azahemiacetals modulate Pseudomonas aeruginosa quorum sensing

Quorum sensing (QS) is a population-dependent signaling process bacteria use to control multiple processes including virulence that is critical for establishing infection. The most common QS signaling molecule used by Gram-negative bacteria are acylhomoserine lactones. The development of non-native acylhomoserine lactone (AHL) ligands has emerged as a promising new strategy to inhibit QS in Gram-negative bacteria. In this work, we have synthesized a set of optically pure γ-lactams and their reduced cyclic azahemiacetal analogues, bearing the additional alkylthiomethyl substituent, and evaluated their effect on the AHL-dependent Pseudomonas aeruginosa las and rhl QS pathways. The concentration of these ligands and the simple structural modification such as the length of the alkylthio substituent has notable effect on activity. The γ-lactam derivatives with nonylthio or dodecylthio chains acted as inhibitors of las signaling with moderate potency. The cyclic azahemiacetal with shorter propylthio or hexylthio substituent was found to strongly inhibit both las and rhl signaling at higher concentrations while the propylthio analogue strongly stimulated the las QS system at lower concentrations.