NMR studies of interactions between periplasmic chaperones from uropathogenic E. coli and pilicides that interfere with chaperone function and pilus assembly

Crystal Growth, Single Crystal Structure, and Biological Activity of Thiazolo-Pyridine Dicarboxylic Acid Derivatives

Four novel TPDCA derivatives were prepared via a supersaturation method combining TPDCA with water, N-methyl-2-pyrrolidone (NMP), Na(PO₂H₂), and ammonia solution: 2(C₉H₇NO₅S)H₂O (1), (C₉H₇NO₅S)C₅H₉NO (2), (C₉H₇NO₅S)Na(PO₂H₂) (3), and (C₉H₅NO₅S)(NH₄)₂(H₂O) (4). Their crystal structures were determined by single-crystal X-ray diffraction. Compounds (1) and (2) crystallize in the monoclinic space groups P2₁ and P2₁/c, respectively, whereas compounds (3) and (4) crystallize in the triclinic space group P1̅. Weak and moderate hydrogen bonds were detected in the four compounds. In the biological tests, (1) and (3) exhibited significant antibacterial activity against Escherichia coli and Staphylococcus aureus; in addition, (1) was cytotoxic against leukemia HL-60 cells with the IC₅₀ value of 158.5 ± 12.5 μM.

Small Molecule Anti-biofilm Agents Developed on the Basis of Mechanistic Understanding of Biofilm Formation

Microbial biofilms are the cause of persistent infections associated with various medical implants and distinct body sites such as the urinary tract, lungs, and wounds. Compared with their free living counterparts, bacteria in biofilms display a highly increased resistance to immune system activities and antibiotic treatment. Therefore, biofilm infections are difficult or impossible to treat with our current armory of antibiotics. The challenges associated with biofilm infections have urged researchers to pursue a better understanding of the molecular mechanisms that are involved in the formation and dispersal of biofilms, and this has led to the identification of several steps that could be targeted in order to eradicate these challenging infections. Here we describe mechanisms that are involved in the regulation of biofilm development in Pseudomonas aeruginosa, Escherichia coli, and Acinetobacter baumannii, and provide examples of chemical compounds that have been developed to specifically inhibit these processes. These compounds include (i) pilicides and curlicides which inhibit the initial steps of biofilm formation by E. coli; (ii) compounds that interfere with c-di-GMP signaling in P. aeruginosa and E. coli; and (iii) compounds that inhibit quorum-sensing in P. aeruginosa and A. baumannii. In cases where compound series have a defined molecular target, we focus on elucidating structure activity relationship (SAR) trends within the particular compound series.

Development of Protein-Protein Interaction Inhibitors for the Treatment of Infectious Diseases

Protein-protein interaction (PPI) inhibitors are a rapidly expanding class of therapeutics. Recent advances in our understanding of PPIs and success of early examples of PPI inhibitors demonstrate the feasibility of targeting PPIs. This review summarizes the techniques used for the discovery and optimization of a diverse set PPI inhibitors, focusing on the development of PPI inhibitors as new antibacterial and antiviral agents. We close with a summary of the advances responsible for making PPI inhibitors realistic targets for therapeutic intervention and brief outlook of the field.

Interactions in bacterial biofilm development: a structural perspective

A community-based life style is the normal mode of growth and survival for many bacterial species. These cellular accretions or biofilms are initiated upon recognition of solid phases by cell surface exposed adhesive moieties. Further cell-cell interactions, cell signalling and bacterial replication leads to the establishment of dense populations encapsulated in a mainly self-produced extracellular matrix; this comprises a complex mixture of macromolecules. These fascinating architectures protect the inhabitants from radiation damage, dehydration, pH fluctuations and antimicrobial compounds. As such they can cause bacterial persistence in disease and problems in industrial applications. In this review we discuss the current understandings of these initial biofilm-forming processes based on structural data. We also briefly describe latter biofilm maturation and dispersal events, which although lack high-resolution insights, are the present focus for many structural biologists working in this field. Finally we give an overview of modern techniques aimed at preventing and disrupting problem biofilms.

Island biogeography effects on microbial evolution may contribute to Crohn's disease

Inflammatory bowel diseases (IBDs), such as Crohn's disease (CD), involve a poorly understood and complex immune response to both the biota of the human gut and the gut itself. The role of the gut biota in human health has been ill defined and attitudes toward the intestinal flora have ranged from judging them largely irrelevant to declaring them a human organ system. A better way to view the intestinal flora is as a group of evolutionarily self-interested species that form large, potentially interbreeding populations that utilize human beings as a series of semi-isolated habitats, like islands in an archipelago. Here we propose that the imposition of modern sanitation and hygiene standards has drastically attenuated the connection between the "islands" inhabited by the gut flora, and that existing work drawn from evolutionary biology studies of island ecosystems, rather than medicine, predicts that the evolution of gut flora should now be pushed toward limited-dispersion forms of intestinal microorganisms - a proposition borne out by the discovery of so-called "adherent invasive Escherichia coli." This pathogenic variant of the gut bacterium E. coli clings to and invades the intestinal epithelium and has been implicated in CD. Gut flora and diseases of the gut should arguably be studied as ecology as much as medicine, and treated within this context.

Architects at the bacterial surface - sortases and the assembly of pili with isopeptide bonds

The cell wall envelope of Gram-positive bacteria can be thought of as a surface organelle for the assembly of macromolecular structures that enable the unique lifestyle of each microorganism. Sortases - enzymes that cleave the sorting signals of secreted proteins to form isopeptide (amide) bonds between the secreted proteins and peptidoglycan or polypeptides - function as the principal architects of the bacterial surface. Acting alone or with other sortase enzymes, sortase construction leads to the anchoring of surface proteins at specific sites in the envelope or to the assembly of pili, which are fibrous structures formed from many protein subunits. The catalysis of intermolecular isopeptide bonds between pilin subunits is intertwined with the assembly of intramolecular isopeptide bonds within pilin subunits. Together, these isopeptide bonds endow these sortase products with adhesive properties and resistance to host proteases.

The application of NMR techniques to bacterial adhesins

Extracellular adhesins frequently compose large, highly-ordered structural assemblies that project away from the bacterial surface. These assemblies, known as pili or fimbriae, are rod-like polymeric structures that in some cases can extend up to several micrometers from the cell surface. Because these adhesin structures are critical to bacterial colonization of host cell surfaces, there is an incentive to understand their structure, assembly and mechanism of host cell attachment. Various methods in Nuclear Magnetic Resonance (NMR) spectroscopy have been used to address these topics, yielding structural information at the atomic level. Also, new methods in solid-state NMR spectroscopy have thus far been under-utilized in the study of large adhesin structures and offer a powerful approach to overcoming problems with crystallization to better understand the structures of these complexes. The following is a brief overview of the contributions of NMR to the study of bacterial adhesins with an emphasis on the future potential of solid-state NMR.

Structural biology of the chaperone-usher pathway of pilus biogenesis

The chaperone-usher (CU) pathway of pilus biogenesis is the most widespread of the five pathways that assemble adhesive pili at the surface of Gram-negative bacteria. Recent progress in the study of the structural biology of the CU pathway has unravelled the molecular basis of chaperone function and elucidated the mechanisms of fibre assembly at the outer membrane, leading to a comprehensive description of each step in the biogenesis pathway. Other studies have provided the molecular basis of host recognition by CU pili. The knowledge that has been gathered about both the assembly of and host recognition by CU pili has been harnessed to design promising antibiotic compounds.

Importance of the ebp (endocarditis- and biofilm-associated pilus) locus in the pathogenesis of Enterococcus faecalis ascending urinary tract infection

BACKGROUND

We recently demonstrated that the ubiquitous Enterococcus faecalis ebp (endocarditis- and biofilm-associated pilus) operon is important for biofilm formation and experimental endocarditis. Here, we assess its role in murine urinary tract infection (UTI) by use of wild-type E. faecalis OG1RF and its nonpiliated, ebpA allelic replacement mutant (TX5475).

METHODS

OG1RF and TX5475 were administered transurethrally either at an ~1 : 1 ratio (competition assay) or individually (monoinfection). Kidney pairs and urinary bladders were cultured 48 h after infection. These strains were also tested in a peritonitis model.

RESULTS

No differences were observed in the peritonitis model. In mixed UTIs, OG1RF significantly outnumbered TX5475 in kidneys (P=.0033) and bladders (P< or =.0001). More OG1RF colony-forming units were also recovered from the kidneys of monoinfected mice at the 4 inocula tested (P=.015 to P=.049), and 50% infective doses of OG1RF for kidneys and bladder (9.1x10(1) and 3.5x10(3) cfu, respectively) were 2-3 log(10) lower than those of TX5475. Increased tropism for the kidney relative to the bladder was observed for both OG1RF and TX5475.

CONCLUSION

The ebp locus, part of the core genome of E. faecalis, contributes to infection in an ascending UTI model and is the first such enterococcal locus shown to be important in this site.

Pilicides-small molecules targeting bacterial virulence

In a time of emerging bacterial resistance there is a vital need for new targets and strategies in antibacterial therapy. Using uropathogenic Escherichia coli as a model pathogen we have developed a class of compounds, pilicides, which inhibit the formation of virulence-associated organelles termed pili. The pilicides interfere with a highly conserved bacterial assembly and secretion system called the chaperone-usher pathway, which is abundant in a vast number of Gram-negative pathogens and serves to assemble multi-protein surface fibers (pili/fimbriae). This class of compounds provides a platform to gain insight into important biological processes such as the molecular mechanisms of the chaperone-usher pathway and the sophisticated function of pili. Pili are primarily involved in bacterial adhesion, invasion and persistence to host defenses. On this basis, pilicides can aid the development of new antibacterial agents.

A molecular dynamics study of pilus subunits: insights into pilus biogenesis

Biogenesis of pili in the uropathogenic Echerichia coli, essential to the bacterial pathogenicity, is a complex molecular process, which involves several protein components of the Pap gene cluster. A crucial role in the process is played by the chaperone PapD and by the PapE pilus subunit. Interestingly, PapE exhibits an Ig-like fold with a missing strand. The missing G strand is donated by the chaperone during pilin folding and by adjacent pilus subunits in the final fibre. In order to obtain a detailed picture at atomic level of the molecular events related to this process, we undertook molecular dynamics studies of the non-canonical immuno-globulin-like PapE in its unliganded state. These analyses were extended to the complexes of PapE with the complementary G(1) strand of PapD and with the N-terminal extension of PapK. All three systems investigated were stable in the time interval considered (20 ns). However, significant differences in their local and overall flexibilities were detected. Notably, the equilibrated structure of unliganded PapE, which is difficult to characterise experimentally, displays unexpected features. Indeed, a significant rearrangement of the local structure of the groove, which hosts the complementary strands, is observed. This reorganisation, characterised by the formation of several new hydrogen bonds, leads to a closure of the groove that likely makes pilin polymerisation more difficult. These data suggest that chaperone release and pilin-pilin association must be concerted processes and that chaperone plays an important role in preventing pilin transitions towards states that are not prone to polymerise.

Intervention with bacterial adhesion by multivalent carbohydrates

Bacterial adhesion is often a prelude to infection. In many cases, this process is governed by protein-carbohydrate interactions. Intervention at this early stage of infection is a conceptually highly attractive alternative to conventional antibiotics that are increasingly prone to resistance. The lack of high-affinity inhibitors of adhesion has proven to be a hurdle for further exploitation of this concept; however, new developments indicate a positive change. Structure-based design at the monovalent level and also evaluation of glycodendrimers and glycopolymers have yielded structures of high affinity. In addition to the development of inhibitors, topics of this review include available structural information of adhesion proteins, carbohydrate specificities of the various pathogens and their adhesion proteins. Other new developments aimed at affecting bacterial adhesion and the use of the adhesins for bacterial detection are also discussed.

Rationally designed small compounds inhibit pilus biogenesis in uropathogenic bacteria

A chemical synthesis platform with broad applications and flexibility was rationally designed to inhibit biogenesis of adhesive pili assembled by the chaperone-usher pathway in Gram-negative pathogens. The activity of a family of bicyclic 2-pyridones, termed pilicides, was evaluated in two different pilus biogenesis systems in uropathogenic Escherichia coli. Hemagglutination mediated by either type 1 or P pili, adherence to bladder cells, and biofilm formation mediated by type 1 pili were all reduced by approximately 90% in laboratory and clinical E. coli strains. The structure of the pilicide bound to the P pilus chaperone PapD revealed that the pilicide bound to the surface of the chaperone known to interact with the usher, the outer-membrane assembly platform where pili are assembled. Point mutations in the pilicide-binding site dramatically reduced pilus formation but did not block the ability of PapD to bind subunits and mediate their folding. Surface plasmon resonance experiments confirmed that the pilicide interfered with the binding of chaperone-subunit complexes to the usher. These pilicides thus target key virulence factors in pathogenic bacteria and represent a promising proof of concept for developing drugs that function by targeting virulence factors.

Design, synthesis and evaluation of peptidomimetics based on substituted bicyclic 2-pyridones-targeting virulence of uropathogenic E. coli

Substituted bicyclic 2-pyridones, termed pilicides, are dipeptide mimetics that prevent pilus assembly in uropathogenic Escherichia coli. Here, we apply rational design to produce four classes of extended peptidomimetics based on two bioactive 2-pyridones. The key intermediate in the synthesis was an amino-functionalised 2-pyridone scaffold, which could be obtained via a mild and selective nitration and subsequent reduction. Procedures were then developed to further derivatize this amino-substituted core and a total of 24 extended peptidomimetics were synthesised and evaluated for chaperone affinity and in vivo antivirulence activity in P pili producing E. coli. Enhanced affinities for the target protein were observed within the generated set of compounds, while the ability to prevent pilus assembly in vivo was significantly decreased compared to the parent lead compounds. The results suggest that the limited in vivo potencies of the analogues are either uptake/distribution related or due to loss in binding specificity.

Synthesis of Multi Ring-Fused 2-Pyridones via an Acyl-Ketene Imine Cyclocondensation

Polycyclic ring-fused 2-pyridones (5a-e and 9a-e) have been prepared via a microwave-assisted acyl-ketene imine cyclocondensation. Starting from 3,4-dihydroisoquinolines (4a-b) or 3,4-dihydroharman (8), fused 2-pyridones could be prepared in a one-step procedure. By using either Meldrum's acid derivatives (1a-d) or 1,3-dioxine-4-ones (7a-b) as acyl-ketene sources, mono- or disubstitution of the fused 2-pyridone ring could be accomplished. As an application of the method, a formal synthesis of the indole alkaloid sempervilam was performed.