Structure, Regulation, and Inhibition of the Quorum-Sensing Signal Integrator LuxO.

In a process called quorum sensing, bacteria communicate with chemical signal molecules called autoinducers to control collective behaviors. In pathogenic vibrios, including Vibrio cholerae, the accumulation of autoinducers triggers repression of genes responsible for virulence factor production and biofilm formation. The vibrio autoinducer molecules bind to transmembrane receptors of the two-component histidine sensor kinase family. Autoinducer binding inactivates the receptors' kinase activities, leading to dephosphorylation and inhibition of the downstream response regulator LuxO. Here, we report the X-ray structure of LuxO in its unphosphorylated, autoinhibited state. Our structure reveals that LuxO, a bacterial enhancer-binding protein of the AAA+ ATPase superfamily, is inhibited by an unprecedented mechanism in which a linker that connects the catalytic and regulatory receiver domains occupies the ATPase active site. The conformational change that accompanies receiver domain phosphorylation likely disrupts this interaction, providing a mechanistic rationale for LuxO activation. We also determined the crystal structure of the LuxO catalytic domain bound to a broad-spectrum inhibitor. The inhibitor binds in the ATPase active site and recapitulates elements of the natural regulatory mechanism. Remarkably, a single inhibitor molecule may be capable of inhibiting an entire LuxO oligomer.

The structural basis for tight control of PP2A methylation and function by LCMT-1.

Proper formation of protein phosphatase 2A (PP2A) holoenzymes is essential for the fitness of all eukaryotic cells. Carboxyl methylation of the PP2A catalytic subunit plays a critical role in regulating holoenzyme assembly; methylation is catalyzed by PP2A-specific methyltransferase LCMT-1, an enzyme required for cell survival. We determined crystal structures of human LCMT-1 in isolation and in complex with PP2A stabilized by a cofactor mimic. The structures show that the LCMT-1 active-site pocket recognizes the carboxyl terminus of PP2A, and, interestingly, the PP2A active site makes extensive contacts to LCMT-1. We demonstrated that activation of the PP2A active site stimulates methylation, suggesting a mechanism for efficient conversion of activated PP2A into substrate-specific holoenzymes, thus minimizing unregulated phosphatase activity or formation of inactive holoenzymes. A dominant-negative LCMT-1 mutant attenuates the cell cycle without causing cell death, likely by inhibiting uncontrolled phosphatase activity. Our studies suggested mechanisms of LCMT-1 in tight control of PP2A function, important for the cell cycle and cell survival.

A quorum-sensing inhibitor blocks Pseudomonas aeruginosa virulence and biofilm formation.

Quorum sensing is a chemical communication process that bacteria use to regulate collective behaviors. Disabling quorum-sensing circuits with small molecules has been proposed as a potential strategy to prevent bacterial pathogenicity. The human pathogen Pseudomonas aeruginosa uses quorum sensing to control virulence and biofilm formation. Here, we analyze synthetic molecules for inhibition of the two P. aeruginosa quorum-sensing receptors, LasR and RhlR. Our most effective compound, meta-bromo-thiolactone (mBTL), inhibits both the production of the virulence factor pyocyanin and biofilm formation. mBTL also protects Caenorhabditis elegans and human lung epithelial cells from killing by P. aeruginosa. Both LasR and RhlR are partially inhibited by mBTL in vivo and in vitro; however, RhlR, not LasR, is the relevant in vivo target. More potent antagonists do not exhibit superior function in impeding virulence. Because LasR and RhlR reciprocally control crucial virulence factors, appropriately tuning rather than completely inhibiting their activities appears to hold the key to blocking pathogenesis in vivo.

Modulating Vibrio cholerae quorum-sensing-controlled communication using autoinducer-loaded nanoparticles.

The rise of bacterial antibiotic resistance has created a demand for alternatives to traditional antibiotics. Attractive possibilities include pro- and anti-quorum sensing therapies that function by modulating bacterial chemical communication circuits. We report the use of Flash NanoPrecipitation to deliver the Vibrio cholerae quorum-sensing signal CAI-1 ((S)-3-hydroxytridecan-4-one) in a water dispersible form as nanoparticles. The particles activate V. cholerae quorum-sensing responses 5 orders of magnitude higher than does the identically administered free CAI-1 and are diffusive across in vivo delivery barriers such as intestinal mucus. This work highlights the promise of combining quorum-sensing strategies with drug delivery approaches for the development of next-generation medicines.

Caenorhabditis elegans recognizes a bacterial quorum-sensing signal molecule through the AWCON neuron.

In a process known as quorum sensing, bacteria use chemicals called autoinducers for cell-cell communication. Population-wide detection of autoinducers enables bacteria to orchestrate collective behaviors. In the animal kingdom detection of chemicals is vital for success in locating food, finding hosts, and avoiding predators. This behavior, termed chemotaxis, is especially well studied in the nematode Caenorhabditis elegans. Here we demonstrate that the Vibrio cholerae autoinducer (S)-3-hydroxytridecan-4-one, termed CAI-1, influences chemotaxis in C. elegans. C. elegans prefers V. cholerae that produces CAI-1 over a V. cholerae mutant defective for CAI-1 production. The position of the CAI-1 ketone moiety is the key feature driving CAI-1-directed nematode behavior. CAI-1 is detected by the C. elegans amphid sensory neuron AWC(ON). Laser ablation of the AWC(ON) cell, but not other amphid sensory neurons, abolished chemoattraction to CAI-1. These analyses define the structural features of a bacterial-produced signal and the nematode chemosensory neuron that permit cross-kingdom interaction.

Synthesis and biological evaluation of bis-CNB-GABA, a photoactivatable neurotransmitter with low receptor interference and chemical two-photon uncaging properties.

Photoactivatable "caged" neurotransmitters allow optical control of neural tissue with high spatial and temporal precision. However, the development of caged versions of the chief vertebrate inhibitory neurotransmitter, γ-amino butyric acid (GABA), has been limited by the propensity of caged GABAs to interact with GABA receptors. We describe herein the synthesis and application of a practically useful doubly caged GABA analog, termed bis-α-carboxy-2-nitrobenzyl-GABA (bis-CNB-GABA). Uncaging of bis-CNB-GABA evokes inward GABAergic currents in cerebellar molecular layer interneurons with rise times of 2 ms, comparable to flash duration. Response amplitudes depend on the square of flash intensity, as expected for a chemical two-photon uncaging effect. Importantly, prior to uncaging, bis-CNB-GABA is inactive at the GABAA receptor, evoking no changes in holding current in voltage-clamped neurons and showing an IC50 of at least 2.5 mM as measured using spontaneous GABAergic synaptic currents. Bis-CNB-GABA is stable in solution, with an estimated half-life of 98 days in the light. We expect that bis-CNB-GABA will prove to be an effective tool for high-resolution chemical control of brain circuits.

Broad spectrum pro-quorum-sensing molecules as inhibitors of virulence in vibrios.

Quorum sensing (QS) is a bacterial cell-cell communication process that relies on the production and detection of extracellular signal molecules called autoinducers. QS allows bacteria to perform collective activities. Vibrio cholerae, a pathogen that causes an acute disease, uses QS to repress virulence factor production and biofilm formation. Thus, molecules that activate QS in V. cholerae have the potential to control pathogenicity in this globally important bacterium. Using a whole-cell high-throughput screen, we identified eleven molecules that activate V. cholerae QS: eight molecules are receptor agonists and three molecules are antagonists of LuxO, the central NtrC-type response regulator that controls the global V. cholerae QS cascade. The LuxO inhibitors act by an uncompetitive mechanism by binding to the pre-formed LuxO-ATP complex to inhibit ATP hydrolysis. Genetic analyses suggest that the inhibitors bind in close proximity to the Walker B motif. The inhibitors display broad-spectrum capability in activation of QS in Vibrio species that employ LuxO. To the best of our knowledge, these are the first molecules identified that inhibit the ATPase activity of a NtrC-type response regulator. Our discovery supports the idea that exploiting pro-QS molecules is a promising strategy for the development of novel anti-infectives.

The major Vibrio cholerae autoinducer and its role in virulence factor production.

Vibrio cholerae, the causative agent of the human disease cholera, uses cell-to-cell communication to control pathogenicity and biofilm formation. This process, known as quorum sensing, relies on the secretion and detection of signalling molecules called autoinducers. At low cell density V. cholerae activates the expression of virulence factors and forms biofilms. At high cell density the accumulation of two quorum-sensing autoinducers represses these traits. These two autoinducers, cholerae autoinducer-1 (CAI-1) and autoinducer-2 (AI-2), function synergistically to control gene regulation, although CAI-1 is the stronger of the two signals. V. cholerae AI-2 is the furanosyl borate diester (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran borate. Here we describe the purification of CAI-1 and identify the molecule as (S)-3-hydroxytridecan-4-one, a new type of bacterial autoinducer. We provide a synthetic route to both the R and S isomers of CAI-1 as well as simple homologues, and we evaluate their relative activities. Synthetic (S)-3-hydroxytridecan-4-one functions as effectively as natural CAI-1 in repressing production of the canonical virulence factor TCP (toxin co-regulated pilus). These findings suggest that CAI-1 could be used as a therapy to prevent cholera infection and, furthermore, that strategies to manipulate bacterial quorum sensing hold promise in the clinical arena.

Probing bacterial transmembrane histidine kinase receptor-ligand interactions with natural and synthetic molecules.

Bacterial histidine kinases transduce extracellular signals into the cytoplasm. Most stimuli are chemically undefined; therefore, despite intensive study, signal recognition mechanisms remain mysterious. We exploit the fact that quorum-sensing signals are known molecules to identify mutants in the Vibrio cholerae quorum-sensing receptor CqsS that display altered responses to natural and synthetic ligands. Using this chemical-genetics approach, we assign particular amino acids of the CqsS sensor to particular roles in recognition of the native ligand, CAI-1 (S-3 hydroxytridecan-4-one) as well as ligand analogues. Amino acids W104 and S107 dictate receptor preference for the carbon-3 moiety. Residues F162 and C170 specify ligand head size and tail length, respectively. By combining mutations, we can build CqsS receptors responsive to ligand analogues altered at both the head and tail. We suggest that rationally designed ligands can be employed to study, and ultimately to control, histidine kinase activity.

Processing the interspecies quorum-sensing signal autoinducer-2 (AI-2): characterization of phospho-(S)-4,5-dihydroxy-2,3-pentanedione isomerization by LsrG protein.

The molecule (S)-4,5-dihydroxy-2,3-pentanedione (DPD) is produced by many different species of bacteria and is the precursor of the signal molecule autoinducer-2 (AI-2). AI-2 mediates interspecies communication and facilitates regulation of bacterial behaviors such as biofilm formation and virulence. A variety of bacterial species have the ability to sequester and process the AI-2 present in their environment, thereby interfering with the cell-cell communication of other bacteria. This process involves the AI-2-regulated lsr operon, comprised of the Lsr transport system that facilitates uptake of the signal, a kinase that phosphorylates the signal to phospho-DPD (P-DPD), and enzymes (like LsrG) that are responsible for processing the phosphorylated signal. Because P-DPD is the intracellular inducer of the lsr operon, enzymes involved in P-DPD processing impact the levels of Lsr expression. Here we show that LsrG catalyzes isomerization of P-DPD into 3,4,4-trihydroxy-2-pentanone-5-phosphate. We present the crystal structure of LsrG, identify potential catalytic residues, and determine which of these residues affects P-DPD processing in vivo and in vitro. We also show that an lsrG deletion mutant accumulates at least 10 times more P-DPD than wild type cells. Consistent with this result, we find that the lsrG mutant has increased expression of the lsr operon and an altered profile of AI-2 accumulation and removal. Understanding of the biochemical mechanisms employed by bacteria to quench signaling of other species can be of great utility in the development of therapies to control bacterial behavior.

The free and bound forms of Lpp occupy distinct subcellular locations in Escherichia coli.

The lipoprotein Lpp is the most numerically abundant protein in Escherichia coli, has been investigated for over 40 years, and has served as the paradigmatic bacterial lipoprotein since its initial discovery. It exists in two distinct forms: a 'bound-form', which is covalently bound to the cell's peptidoglycan layer, and a 'free-form', which is not. Although it is known that the carboxyl-terminus of bound-form Lpp is located in the periplasm, the precise location of free-form Lpp has never been determined. For decades, it has been widely assumed that free-form Lpp is associated with bound-form. In this work, we show that the free and bound forms of Lpp are not largely associated with each other, but are found in distinct subcellular locations. Our results indicate that free-form Lpp spans the outer membrane and is surface-exposed, whereas bound-form Lpp resides in the periplasm. Thus, Lpp represents a novel example of a single lipoprotein that is able to occupy distinct subcellular locations, and challenges models in which the free and bound forms of Lpp are assumed to be associated with each other.

Signal production and detection specificity in Vibrio CqsA/CqsS quorum-sensing systems.

Quorum sensing is a process of bacterial cell-cell communication that enables populations of cells to carry out behaviours in unison. Quorum sensing involves detection of the density-dependent accumulation of extracellular signal molecules called autoinducers that elicit population-wide changes in gene expression. In Vibrio species, CqsS is a membrane-bound histidine kinase that acts as the receptor for the CAI-1 autoinducer which is produced by the CqsA synthase. In Vibrio cholerae, CAI-1 is (S)-3-hydroxytridecan-4-one. The C170 residue of V. cholerae CqsS specifies a preference for a ligand with a 10-carbon tail length. However, a phenylalanine is present at this position in Vibrio harveyi CqsS and other homologues, suggesting that a shorter CAI-1-like molecule functions as the signal. To investigate this, we purified the V. harveyi CqsS ligand, and determined that it is (Z)-3-aminoundec-2-en-4-one (Ea-C8-CAI-1) carrying an 8-carbon tail. The V. harveyi CqsA/CqsS system is exquisitely selective for production and detection of this ligand, while the V. cholerae CqsA/CqsS counterparts show relaxed specificity in both production and detection. We isolated CqsS mutants in each species that display reversed specificity for ligands. Our analysis provides insight into how fidelity is maintained in signal transduction systems.

Identification of small molecules that antagonize diguanylate cyclase enzymes to inhibit biofilm formation.

Bacterial biofilm formation is responsible for numerous chronic infections, causing a severe health burden. Many of these infections cannot be resolved, as bacteria in biofilms are resistant to the host's immune defenses and antibiotic therapy. New strategies to treat biofilm-based infections are critically needed. Cyclic di-GMP (c-di-GMP) is a widely conserved second-messenger signal essential for biofilm formation. As this signaling system is found only in bacteria, it is an attractive target for the development of new antibiofilm interventions. Here, we describe the results of a high-throughput screen to identify small-molecule inhibitors of diguanylate cyclase (DGC) enzymes that synthesize c-di-GMP. We report seven small molecules that antagonize these enzymes and inhibit biofilm formation by Vibrio cholerae. Moreover, two of these compounds significantly reduce the total concentration of c-di-GMP in V. cholerae, one of which also inhibits biofilm formation by Pseudomonas aeruginosa in a continuous-flow system. These molecules represent the first compounds described that are able to inhibit DGC activity to prevent biofilm formation.

Identification of a novel benzimidazole that inhibits bacterial biofilm formation in a broad-spectrum manner.

Bacterial biofilm formation causes significant industrial economic loss and high morbidity and mortality in medical settings. Biofilms are defined as multicellular communities of bacteria encased in a matrix of protective extracellular polymers. Because biofilms have a high tolerance for treatment with antimicrobials, protect bacteria from immune defense, and resist clearance with standard sanitation protocols, it is critical to develop new approaches to prevent biofilm formation. Here, a novel benzimidazole molecule, named antibiofilm compound 1 (ABC-1), identified in a small-molecule screen, was found to prevent bacterial biofilm formation in multiple Gram-negative and Gram-positive bacterial pathogens, including Pseudomonas aeruginosa and Staphylococcus aureus, on a variety of different surface types. Importantly, ABC-1 itself does not inhibit the growth of bacteria, and it is effective at nanomolar concentrations. Also, coating a polystyrene surface with ABC-1 reduces biofilm formation. These data suggest ABC-1 is a new chemical scaffold for the development of antibiofilm compounds.

The Vibrio cholerae quorum-sensing autoinducer CAI-1: analysis of the biosynthetic enzyme CqsA.

Vibrio cholerae, the bacterium that causes the disease cholera, controls virulence factor production and biofilm development in response to two extracellular quorum-sensing molecules, called autoinducers. The strongest autoinducer, called CAI-1 (for cholera autoinducer-1), was previously identified as (S)-3-hydroxytridecan-4-one. Biosynthesis of CAI-1 requires the enzyme CqsA. Here, we determine the CqsA reaction mechanism, identify the CqsA substrates as (S)-2-aminobutyrate and decanoyl coenzyme A, and demonstrate that the product of the reaction is 3-aminotridecan-4-one, dubbed amino-CAI-1. CqsA produces amino-CAI-1 by a pyridoxal phosphate-dependent acyl-CoA transferase reaction. Amino-CAI-1 is converted to CAI-1 in a subsequent step via a CqsA-independent mechanism. Consistent with this, we find cells release > or =100 times more CAI-1 than amino-CAI-1. Nonetheless, V. cholerae responds to amino-CAI-1 as well as CAI-1, whereas other CAI-1 variants do not elicit a quorum-sensing response. Thus, both CAI-1 and amino-CAI-1 have potential as lead molecules in the development of an anticholera treatment.

Small molecule probes of the receptor binding site in the Vibrio cholerae CAI-1 quorum sensing circuit.

Based on modification of separate structural features of the Vibrio cholerae quorum sensing signal, (S)-3-hydroxytridecan-4-one (CAI-1), three focused compound libraries have been synthesized and evaluated for biological activity. Modifications to the acyl tail and α-hydroxy ketone typically provided agonists with activities correlated to tail length and conservative changes to the hydroxy ketone. Among the molecules identified within this collection of agonists is Am-CAI-1 (B11), which is among the most potent agonists reported to date with an EC(50) of 0.21 µM. Modifications to the ethyl side chain delivered molecules with both agonist and antagonist activity, including m-OH-Ph-CAI-1 (C13) which is the most potent antagonist reported to date with an IC(50) of 36 µM. The molecules described in this manuscript are anticipated to serve as valuable tools in the study of quorum sensing in Vibrio cholerae and provide new leads in the development of an antivirulence therapy against this human pathogen.

Antibiotic Adjuvant Activity Revealed in a Photoaffinity Approach to Determine the Molecular Target of Antipyocyanin Compounds.

Infections with Pseudomonas aeruginosa are a looming threat to public health. New treatment strategies are needed to combat this pathogen, for example, by blocking the production of virulence factors like pyocyanin. A photoaffinity analogue of an antipyocyanin compound was developed to interrogate the inhibitor's molecular mechanism of action. While we sought to develop antivirulence inhibitors, the proteomics results suggested that the compounds had antibiotic adjuvant activity. Unexpectedly, we found that these compounds amplify the bactericidal activity of colistin, a well-characterized antibiotic, suggesting they may represent a first-in-class antibiotic adjuvant therapy. Analogues have the potential not only to widen the therapeutic index of cationic antimicrobial peptides like colistin, but also to be effective against colistin-resistant strains, strengthening our arsenal to combat P. aeruginosa infections.

An expeditious synthesis of DPD and boron binding studies.

A practical synthesis has been developed for DPD (4,5-dihydroxypentane-2,3-dione), an unstable small molecule that is proposed to be the source of universal signaling agents for quorum sensing in bacteria. The synthesis allows preparation of isotopically labeled DPD and ent-DPD as well as detailed studies of spontaneous binding to borate to give the unusual borate complex 6, the signal for marine bacteria such as Vibrio harveyi. [reaction: see text]

Development of potent inhibitors of pyocyanin production in Pseudomonas aeruginosa.

The development of new approaches for the treatment of antimicrobial-resistant infections is an urgent public health priority. The Pseudomonas aeruginosa pathogen, in particular, is a leading source of infection in hospital settings, with few available treatment options. In the context of an effort to develop antivirulence strategies to combat bacterial infection, we identified a series of highly effective small molecules that inhibit the production of pyocyanin, a redox-active virulence factor produced by P. aeruginosa. Interestingly, these new antagonists appear to suppress P. aeruginosa virulence factor production through a pathway that is independent of LasR and RhlR.

Boron binding with the quorum sensing signal AI-2 and analogues.

[reaction: see text] The unstable bacterial metabolic product, DPD, and the related natural product, laurencione, are shown to have a high affinity for borate complexation, through the hydrated analogue. The boron complex of DPD is Vibrio harveyi AI-2, an interspecies quorum sensing signal in bacteria, and an affinity column with a borate resin is effective in providing the first method for concentrating and purifying V. harveyi AI-2 from the biosynthetic product.