Mycobacterium tuberculosis antigen 85A and 85C structures confirm binding orientation and conserved substrate specificity

Antigen 85C-mediated acyl-transfer between synthetic acyl donors and fragments of the arabinan

Antigen 85 (ag85) is a complex of acyltransferases (ag85A-C) known to play a role in the mycolation of the D-arabino-D-galactan (AG) component of the mycobacterial cell wall. In order to better understand the chemistry and substrate specificity of ag85, a trehalose monomycolate mimic p-nitrophenyl 6-O-octanoyl-beta-D-glucopyranoside (1) containing an octanoyl moiety in lieu of a mycolyl moiety was synthesized as an acyl donor. Arabinofuranoside acceptors, methyl alpha-D-arabinofuranoside (2), methyl beta-D-arabinofuranoside (3), and methyl 2-O-beta-D-arabinofuranosyl-alpha-D-arabinofuranoside (9) were synthesized to mimic the terminal saccharides found on the AG. The acyl transfer reaction between acyl donor 1 and acceptors 2, 3, and 9 in the presence of ag85C from Mycobacterium tuberculosis (M. tuberculosis) resulted in the formation of esters, methyl 2, 5-di-O-octanoyl-alpha-D-arabinofuranoside (10), methyl 5-O-octanoyl-beta-D-arabinofuranoside (11), and methyl 2-O-(5-O-octanoyl-beta-D-arabinofuranosyl)-5-O-octanoyl-alpha-D-arabinofuranoside (12) in 2 h, 2 h and 8 h, respectively. The initial velocities of the reactions were determined with a newly developed assay for acyltransferases. As expected, the regioselectivity corresponds to mycolylation patterns found at the terminus of the AG in M. tuberculosis. The study shows that D-arabinose-based derivatives are capable of acting as substrates for ag85C-mediated acyl-transfer and the acyl glycoside 1 can be used in lieu of TMM extracted from bacteria to study ag85-mediated acyl-transfer and inhibition leading to the better understanding of the ag85 protein class.

A coupled assay measuring Mycobacterium tuberculosis antigen 85C enzymatic activity

The prevalence of drug-resistant strains of Mycobacterium tuberculosis (M. tb) emphasizes the need for new antitubercular drugs. An essential component of the drug discovery process is the development of tools to rapidly screen potential drug libraries against important biological targets. Similarly to well-documented M. tb targets, the antigen 85 (Ag85) enzymes are involved in the maintenance of the mycobacterial cell wall. The products synthesized by these mycolyltransferases are the cell wall components most responsible for the reduced permeability of drugs into the bacterial cell, thereby linking Ag85 activity directly with drug resistance. This article presents the development of a high-throughput colorimetric assay suitable for direct monitoring of the enzymatic activity. The assay uses a synthetic substrate containing three chemical moieties: an octanoyl fatty acid, beta-D-glucose, and p-nitrophenyl. In the context of the assay, Ag85 catalyzes the removal of the fatty acid and releases p-nitrophenyl-beta-D-glucoside. The glucoside is hydrolyzed by beta-glucosidase to release the p-nitrophenolate chromophore. With this assay, the K(M) and k(cat) values of Ag85C were determined to be 0.047 +/- 0.008 mM and 0.062 s(-1), respectively. In addition, the assay exhibits a Z' value of 0.81 +/- 0.06, indicating its suitability for high-throughput screening applications and drug development.

The Delta fbpA mutant derived from Mycobacterium tuberculosis H37Rv has an enhanced susceptibility to intracellular antimicrobial oxidative mechanisms, undergoes limited phagosome maturation and activates macrophages and dendritic cells

Mycobacterium tuberculosis H37Rv (Mtb) excludes phagocyte oxidase (phox) and inducible nitric oxide synthase (iNOS) while preventing lysosomal fusion in macrophages (MPhis). The antigen 85A deficient (Delta fbpA) mutant of Mtb was vaccinogenic in mice and the mechanisms of attenuation were compared with MPhis infected with H37Rv and BCG. Delta fbpA contained reduced amounts of trehalose 6, 6, dimycolate and induced minimal levels of SOCS-1 in MPhis. Blockade of oxidants enhanced the growth of Delta fbpA in MPhis that correlated with increased colocalization with phox and iNOS. Green fluorescent protein-expressing strains within MPhis or purified phagosomes were analysed for endosomal traffick with immunofluorescence and Western blot. Delta fbpA phagosomes were enriched for rab5, rab11, LAMP-1 and Hck suggesting enhanced fusion with early, recycling and late endosomes in MPhis compared with BCG or H37Rv. Delta fbpA phagosomes were thus more mature than H37Rv or BCG although, they failed to acquire rab7 and CD63 preventing lysosomal fusion. Finally, Delta fbpA infected MPhis and dendritic cells (DCs) showed an enhanced MHC-II and CD1d expression and primed immune T cells to release more IFN-gamma compared with those infected with BCG and H37Rv. Delta fbpA was thus more immunogenic in MPhis and DCs because of an enhanced susceptibility to oxidants and increased maturation.

The genetics of cell wall biosynthesis in Mycobacterium tuberculosis

Despite an available vaccine and effective antibiotics, Mycobacterium tuberculosis is still the causative agent of almost 2 million deaths every year. The cell wall of M. tuberculosis is composed of sugars and lipids of exotic structure, many of which contribute to its pathogenicity. The majority of the enzymes responsible for building this structure are essential. However, they share very little homology with well-characterized enzymes, which makes their identification in the genome difficult. Despite this, our knowledge of the structure of the cell wall of M. tuberculosis is fairly complete and an increasing number of genes have been identified that are involved in its biosynthesis. By contrast, data concerning regulation of the expression of these genes and control of the cell wall composition are restricted. This review summarizes current information on the genetics of cell wall biosynthesis in M. tuberculosis, incorporating available data on gene organization and regulation.

Synthesis of methyl 5-S-alkyl-5-thio-D-arabinofuranosides and evaluation of their antimycobacterial activity

The emergence of drug resistant tuberculosis necessitates a search for new antimycobacterial compounds. The antigen 85 (ag85) complex is a family of mycolyl transferases involved in the synthesis of trehalose-6,6'-dimycolate and the mycolated hexasaccharide motif found at the terminus of the arabinogalactan in mycobacterium. Enzymes involved in the synthesis of cell wall structures like these are potential targets for the development of new antiinfectives. To potentially inhibit the ag85 complex, methyl 5-S-alkyl-5-thio-arabinofuranoside analogues were designed based on docking studies with ag85C derived from Mycobacterium tuberculosis. The target arabinofuranosides were then synthesized and the antibacterial activity evaluated against Mycobacterium smegmatis ATCC 14468. Two of the compounds, 5-S-octyl-5-thio-alpha-d-arabinofuranoside (8) and 5-S-octyl-5-thio-beta-d-arabinofuranoside (11), showed MICs of 256 and 512microg/mL, respectively. Attempts to directly evaluate acyltransferase inhibitory activity of the arabinofuranosides against ag85C are also described. In conclusion, a new class of antimycobacterial arabinofuranosides has been discovered.

Synthesis of the Docosanasaccharide Arabinan Domain of Mycobacterial Arabinogalactan and a Proposed Octadecasaccharide Biosynthetic Precursor

Two major components of the cell wall in mycobacteria, including Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), are polysaccharides containing arabinofuranose residues. In one of these polysaccharides, arabinogalactan, this arabinan domain consists of three identical motifs of 22 arabinofuranose residues, which are in turn attached to an underlying galactofuranan backbone. Recent studies have proposed that this docosanasaccharide motif, and a structurally related arabinan present in another cell wall polysaccharide, lipoarabinomannan, are biosynthesized from a common octadecasaccharide precursor. To facilitate the testing of this hypothesis, we report here the first total syntheses of these 18- and 22-residue oligosaccharides both functionalized with an aminooctyl linker arm. The route to the target compounds involved the preparation of four tri- to heptasaccharide building blocks possessing only benzoyl protecting groups that were coupled in a highly convergent manner via glycosyl trichloroacetimidate donors. Each of the targets could be prepared in only six steps from these intermediates, and in both cases more than 10 mg of material was obtained. These compounds are expected to be useful tools in probing the biosynthesis of these arabinan-containing polysaccharides. Such studies are essential prerequisites for the identification of novel anti-TB agents that target arabinan assembly.

Poly (lactide-co-glycolide) microspheres in respirable sizes enhance an in vitro T cell response to recombinant Mycobacterium tuberculosis antigen 85B

PURPOSE

To investigate the use of poly (lactide-co-glycolide) (PLGA) microparticles in respirable sizes as carriers for Antigen 85B (Ag85B), a secreted protein of Mycobacterium tuberculosis, with the ultimate goal of employing them in pulmonary delivery of tuberculosis vaccine.

MATERIALS AND METHODS

Recombinant Ag85B was expressed from two Escherichia coli strains and encapsulated by spray-drying in PLGA microspheres with/without adjuvants. These microspheres containing rAg85B were assessed for their ability to deliver antigen to macrophages for subsequent processing and presentation to the specific CD4 T-hybridoma cells DB-1. DB-1 cells recognize the Ag85B(97-112) epitope presented in the context of MHC class II and secrete IL-2 as the cytokine marker.

RESULTS

Microspheres suitable for aerosol delivery to the lungs (3.4-4.3 microm median diameter) and targeting alveolar macrophages were manufactured. THP-1 macrophage-like cells exposed with PLGA-rAg85B microspheres induced the DB-1 cells to produce IL-2 at a level that was two orders of magnitude larger than the response elicited by soluble rAg85B. This formulation demonstrated extended epitope presentation.

CONCLUSIONS

PLGA microspheres in respirable sizes were effective in delivering rAg85B in an immunologically relevant manner to macrophages. These results are a foundation for further investigation into the potential use of PLGA particles for delivery of vaccines to prevent M. tuberculosis infection.

Tuberculosis subunit vaccine development: Impact of physicochemical properties of mycobacterial test antigens

Tuberculosis caused by Mycobacterium tuberculosis continues to be one of the major public health problems in the world. The eventual control of this disease will require the development of a safe and effective vaccine. One of the approaches receiving a great deal of attention recently is subunit vaccination. An efficacious antituberculous subunit vaccine requires the identification and isolation of key components of the pathogen that are capable of inducing a protective immune response. Clues to identify promising subunit vaccine candidates may be found in their physicochemical and immunobiological properties. In this article, we review the evidence that the physicochemical properties of mycobacterial components can greatly impact the induction of either protective or deleterious immune response and consequently influence the potential utility as an antituberculous subunit vaccine.

Antituberculosis drugs: ten years of research

Tuberculosis is today amongst the worldwide health threats. As resistant strains of Mycobacterium tuberculosis have slowly emerged, treatment failure is too often a fact, especially in countries lacking the necessary health care organisation to provide the long and costly treatment adapted to patients. Because of lack of treatment or lack of adapted treatment, at least two million people will die of tuberculosis this year. Due to this concern, this infectious disease was the focus of renewed scientific interest in the last decade. Regimens were optimized and much was learnt on the mechanisms of action of the antituberculosis drugs used. Moreover, the quest for original drugs overcoming some of the problems of current regimens also became the focus of research programmes and many new series of M. tuberculosis growth inhibitors were reported. This review presents the drugs currently used in antituberculosis treatments and the most advanced compounds undergoing clinical trials. We then provide a description of their mechanism of action along with other series of inhibitors known to act on related biochemical targets. This is followed by other inhibitors of M. tuberculosis growth, including recently reported compounds devoid of a reported mechanism of action.

New lipophilic phthalimido- and 3-phenoxybenzyl sulfonates: inhibition of antigen 85C mycolyltransferase activity and cytotoxicity

Four new sulfonates were prepared as potential inhibitors of antigen 85C, a mycolyl transferase involved in the biosynthesis of the mycobacterial cell wall being designed on the basis of the proposed catalytic mechanism and antigen 85C crystal structure. The inhibitors contained a sulfonate moiety, 3-phenoxybenzyl alcohol or N-(hydroxyethyl)phthalimide as trehalose mimetics, and an alkyl chain of different length mimicking either the mycolate (alpha-chain or the mycolic acid (beta-branch. One compound displayed promising activity in a mycolyltransferase inhibition assay (compound 2b, IC50 = 4.3 microM). The two compounds containing a phthalimide moiety (compounds 3a and 3b) showed significant and selective cytotoxicity against the breast cancer cell line MDA-MB231.

Docking of phosphonate and trehalose analog inhibitors into M. tuberculosis mycolyltransferase Ag85C: Comparison of the two scoring fitness functions GoldScore and ChemScore, in the GOLD software

The Ag85 family enzymes are responsible for the synthesis of cell wall components in mycobacterial species. Inhibitors to these enzymes are potential antimycobacterial agents. We have carried out the docking of phoshonate and trehalose analog inhibitors into the three dimensional structure of mycolyltransferase enzyme, Ag85C of M. tuberculosis using the GOLD software. The inhibitor binding positions and affinity were evaluated using both the scoring fitness functions- GoldScore and ChemScore. We observed that the inhibitor binding position identified using the GoldScore was marginally better than the ChemScore. A qualitative agreement between the reported experimental biological activities (IC50) and the GoldScore was observed. We identified that amino acid residues Arg541, Trp762 are important for inhibitor recognition via hydrogen bonding interactions. This information can be exploited to design Ag85C specific inhibitors.

Design, synthesis, biochemical evaluation and antimycobacterial action of phosphonate inhibitors of antigen 85C, a crucial enzyme involved in biosynthesis of the mycobacterial cell wall

Phosphonate inhibitors of antigen 85C were prepared. The inhibitors, comprising a phosphonate moiety, mycolic acid mimetic and a trehalose surrogate, contain substituted benzyl alcohols, N-(omega-hydroxyalky)phthalimide, 2-phenylethanol or 4-(phthalimido)butanol as trehalose mimetics, and an alkyl chain of different lengths mimicking the mycolic acid side chain. The best compounds inhibited the mycolyltransferase activity of antigen 85C with IC(50) in the low micromolar range and inhibited the growth of Mycobacterium avium in culture. The best compounds in the 3-phenoxybenzyl- and omega-(phthalimido)alkoxy series, ethyl 3-phenoxybenzyl butylphosphonate (4a) and (1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl ethyl heptylphosphonate (5c) displayed IC(50) values of 2.0 and 1.3 microM, respectively, in a mycolyltransferase inhibition assay. In a M. avium growth inhibition assay MIC of 4a and (1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl ethyl nonylphosphonate (5d) were 248.8 and 84.5 microg/mL, respectively.

Analysis and modeling of mycolyl-transferases in the CMN group

Mycolyl-transferases are a family of proteins that are specifically present in the CMN (Corynebacterium, Mycobacterium and Nocardia) genera and are responsible for the synthesis of cell wall components. We modeled the three-dimensional structures of mycolyl-transfersases from Corynebacterium and Nocardia using homology modeling methods based on the crystal structures of mycolyl-transferases from M. tuberculosis. Comparison of the models revealed significant differences in their substrate binding site. Some mycolyl-transferases identified by the following Gene Ids: Nfa25110, Nfa45560, Nfa7210, Nfa38260, Nfa32420, Nfa23770, Nfa43800, Nfa30260, Dip0365, Ncgl0987, Ce1488, Ncgl0885, Ce0984, Ncgl2101, Ncgl0336, Ce0356 are associated with a relatively larger substrate binding site and amino acid residue mutations (D40N, R43D/G, S236N/A) are likely to affect binding to trehalose.

The potential impact of structural genomics on tuberculosis drug discovery

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB) in humans, is a devastating infectious organism that kills approximately two million people annually. The current suite of antibiotics used to treat TB faces two main difficulties: (i) the emergence of multidrug-resistant (MDR) strains of M. tuberculosis, and (ii) the persistent state of the bacterium, which is less susceptible to antibiotics and causes very long antibiotic treatment regimes. The complete genome sequences of a laboratory strain (H37Rv) and a clinical strain (CDC1551) of M. tuberculosis and the concurrent identification of all the open reading frames that encode proteins within this organism, present structural biologists with a wide array of protein targets for structure determination. Comparative genomics of the species that make up the M. tuberculosis complex has also added an array of genomic information to our understanding of these organisms. In response to this, structural genomics consortia have been established for targeting proteins from M. tuberculosis. This review looks at the progress of these major initiatives and the potential impact of large scale structure determination efforts on the development of inhibitors to many proteins. Increasing sophistication in structure-based drug design approaches, in combination with increasing numbers of protein structures and inhibitors for TB proteins, will have a significant impact on the downstream development of TB antibiotics.