A new glycolipid from Mycobacterium avium--Mycobacterium intracellulare complex

Synthesis of a Di-Mycoloyl Tri-Arabinofuranosyl Glycerol Fragment of the Mycobacterial Cell Wall, Based on Synthetic Mycolic Acids

Fragments of mycobacterial cell walls such as arabinoglycerol mycolate and dimycoloyl diarabinoglycerol, comprising complex mixtures of mycolic acids, have immunostimulatory and antigenic properties. A related di-mycoloyl tri-arabinofuranosyl glycerol fragment has been isolated from cell wall hydrolysates. An effective stereoselective synthesis of tri-arabinofuranosyl glycerol, followed by coupling with stereochemically defined mycolic acids of different structural classes, to provide unique di-mycoloyl tri-arabinofuranosyl glycerols is now described.

The synthesis of mycobacterial dimycoloyl diarabinoglycerol based on defined synthetic mycolic acids

Complex mixtures of natural dimycoloyl diarabinoglycerols isolated from mycobacteria have been shown to be both potent immune signalling agents and potentially valuable antigens in the serodiagnosis of mycobacterial infections. We now report the highly stereocontrolled synthesis of diacyl l-glycerol-(1'→1)-β-d-arabinofuranosyl-α-d-arabinofuranosides based on simple fatty acids and single defined synthetic mycolic acids. NMR analysis confirmed that the synthetic core was identical to that in natural mixtures.

Structural determination and Toll-like receptor 2-dependent proinflammatory activity of dimycolyl-diarabino-glycerol from Mycobacterium marinum

Although it was identified in the cell wall of several pathogenic mycobacteria, the biological properties of dimycolyl-diarabino-glycerol have not been documented yet. In this study an apolar glycolipid, presumably corresponding to dimycolyl-diarabino-glycerol, was purified from Mycobacterium marinum and subsequently identified as a 5-O-mycolyl-β-Araf-(1→2)-5-O-mycolyl-α-Araf-(1→1')-glycerol (designated Mma_DMAG) using a combination of nuclear magnetic resonance spectroscopy and mass spectrometry analyses. Lipid composition analysis revealed that mycolic acids were dominated by oxygenated mycolates over α-mycolates and devoid of trans-cyclopropane functions. Highly purified Mma_DMAG was used to demonstrate its immunomodulatory activity. Mma_DMAG was found to induce the secretion of proinflammatory cytokines (TNF-α, IL-8, IL-1β) in human macrophage THP-1 cells and to trigger the expression of ICAM-1 and CD40 cell surface antigens. This activation mechanism was dependent on TLR2, but not on TLR4, as demonstrated by (i) the use of neutralizing anti-TLR2 and -TLR4 antibodies and by (ii) the detection of secreted alkaline phosphatase in HEK293 cells co-transfected with the human TLR2 and secreted embryonic alkaline phosphatase reporter genes. In addition, transcriptomic analyses indicated that various genes encoding proinflammatory factors were up-regulated after exposure of THP-1 cells to Mma_DMAG. Importantly, a wealth of other regulated genes related to immune and inflammatory responses, including chemokines/cytokines and their respective receptors, adhesion molecules, and metalloproteinases, were found to be modulated by Mma_DMAG. Overall, this study suggests that DMAG may be an active cell wall glycoconjugate driving host-pathogen interactions and participating in the immunopathogenesis of mycobacterial infections.

Exposure of mycobacteria to cell wall-inhibitory drugs decreases production of arabinoglycerolipid related to Mycolyl-arabinogalactan-peptidoglycan metabolism

The "cell wall core" consisting of a mycolyl-arabinogalactan-peptidoglycan (mAGP) complex represents the hallmark of the mycobacterial cell envelope. It has been the focus of intense research at both structural and biosynthetic levels during the past few decades. Because it is essential, mAGP is also regarded as a target for several antitubercular drugs. Herein, we demonstrate that exposure of Mycobacterium bovis Bacille Calmette-Guérin or Mycobacterium marinum to thiacetazone, a second line antitubercular drug, is associated with a severe decrease in the level of a major apolar glycolipid. This inhibition requires MmaA4, a methyltransferase reported to participate in the activation process of thiacetazone. Following purification, this glycolipid was subjected to detailed structural analyses, combining gas-liquid chromatography, mass spectrometry, and nuclear magnetic resonance. This allowed to identify it as a 5-O-mycolyl-β-Araf-(1→2)-5-O-mycolyl-α-Araf-(1→1)-Gro, designated dimycolyl diarabinoglycerol (DMAG). The presence of DMAG was subsequently confirmed in other slow growing pathogenic species, including Mycobacterium tuberculosis. DMAG production was stimulated in the presence of exogenous glycerol. Interestingly, DMAG appears structurally identical to the terminal portion of the mycolylated arabinosyl motif of mAGP, and the metabolic relationship between these two components was provided using antitubercular drugs such as ethambutol or isoniazid known to inhibit the biosynthesis of arabinogalactan or mycolic acid, respectively. Finally, DMAG was identified in the cell wall of M. tuberculosis. This opens the possibility of a potent biological function for DMAG that may be important to mycobacterial pathogenesis.

Structure of the major glycolipid from Rothia dentocariosa

Structural studies of the major glycolipid isolated from Rothia dentocariosa were carried out by specific chemical degradation and nuclear magnetic resonance spectroscopy. The glycolipid was found to be a dimannosylacylmonoglyceride in which the carbohydrate part was the glycerol-linked dimannoside alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->3)-sn-Gro, and the internal mannose was esterified at C-6 by fatty acid residue. The other fatty acyl chain substituted the primary methylene position of glycerol. The occurrence of this glycolipid is limited to the related microorganisms. The structural characteristics can facilitate the differentiation of some genera.

Structure of a new glycolipid from the Mycobacterium avium-Mycobacterium intracellulare complex

From the lipid fraction of a freeze-dried cell mass of a strain of the Mycobacterium avium-Mycobacterium intracellulare complex, a new glycolipid was isolated and was characterized as 5-mycoloyl-alpha-arabinofuranosyl (1-->1')-glycerol, mainly on the basis of nuclear magnetic resonance spectroscopy studies.

Structures of phenolic glycolipids from Mycobacterium kansasii

From the lipid fraction of cells of Mycobacterium kansasii, four phenolic glycolipids K5, K6, K7 and K8, were isolated, in addition to three known phenolic glycolipids KI, KII and KIV. K5 was identified as a tetraglycosyl phenolic glycolipid whose sugar moiety was 2,6-dideoxy-4-O-methyl-L-alpha-arabinohexopyranosyl(1-->3)-4-O-pro pionyl-2-O-methyl-L-alpha-fucopyranosyl(1-->3)-2-O-methyl-L-alpha-rhamno pyranosyl-(1-->3)-2,4-di-O-methyl-L-alpha-rhamnopyranosyl(1-->) and phenolic glycolipid K6 as deacetyl-KI. Glycolipids K7 and K8 were triglycosyl phenolic glycolipids having the sugar moieties of 2-O-methyl-L-alpha-fucopyranosyl(1-->3)-2-O-methyl-L-alpha-rhamnopyranos yl(1-->3)-2,4-di-O-methy-L-alpha-rhamnopyranosyl(1-->) and 2-O-methyl-L-alpha-fucopyranosyl(1-->3)-2-O-methyl-L-alpha-rhamnopyranos yl(1-->3)-2-O-methyl-L-alpha-rhamnopyranosyl(1-->), respectively. Phenolic glycolipids K6 and K7 have been referred to as controlled degradation products of phenolic glycolipid KI previously. Also isolated was 5-mycolyl-beta-arabinofuranosyl(1-->2)-5-mycolyl-alpha-ar abinofurnosyl(1-->1')-glycerol, an analogue of glycolipid ai, originally isolated from Mycobacterium avium-Mycobacterium intracellulare complex, having mycolic acids of M. kansasii.

Structural studies of the major glycolipid from Saccharopolyspora genus

A major glycolipid was isolated from the well characterized Saccharopolyspora species, S. hirsuta, S. rectivirgula, S. erythraea and one not completely identified strain (Saccharopolyspora sp.). On the basis of sugar and methylation analysis, specific enzymatic and chemical degradations of the carbohydrate moiety, its FAB mass spectrometry and NMR spectroscopy characterizations, the carbohydrate part was shown to be the glycerol linked dimannoside alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->1/3)Gro. The internal mannose residue is esterified at C-6 by one fatty acid residue, whereas another fatty acyl chain substitutes the primary methylene position of glycerol. The main fatty acyl residues are anteiso-branched heptadecanoic acid and the iso-branched fatty acids iso-17:0, iso-16:0, and iso-18:0, with the former species being predominant. The major glycolipid has potential value for taxonomic and diagnostic purposes, especially in the specific diagnosis of farmer's lung disease.

The presence of beta-D-ribosyl-1-monophosphodecaprenol in mycobacteria

Biosynthesis of the cell envelope in mycobacteria is largely unknown; however, several antituberculosis drugs apparently interfere with this process. Recently, we described a lipid intermediate for the biosynthesis of the cell wall arabinogalactan/arabinomannan of Mycobacterium smegmatis: beta-D-arabinofuranosyl-1-monophosphodecaprenol (Wolucka, B. A., McNeil, M. R., de Hoffmann, E., Chojnacki, T., and Brennan, P. J. (1994) J. Biol. Chem. 269, 23328-23335). In the present work, by means of gas chromatography-mass spectrometry, fast atom bombardment tandem mass spectrometry, and proton NMR, the major pentose-containing component of the polyprenyl-P-sugar family from M. smegmatis was characterized as beta-D-ribosyl-1-monophosphodecaprenol (decaprenyl-P-ribose). Additionally, the structure of a minor arabinose-containing compound, beta-D-arabinosyl-1-monophosphooctahydroheptaprenol, could be deduced. In vivo labeling experiments with [14C]glucose demonstrated unequivocally that decaprenyl-P-ribose is actively synthesized in Mycobacterium tuberculosis H37Ra and Mycobacterium avium serovar 4. It is proposed that decaprenyl-P-ribose could be a precursor for the biosynthesis of either some unknown ribose-containing cell envelope polymers of mycobacteria or the arabinan part of the cell wall arabinogalactan/arabinomannan due to the presence of a 2'-epimerase activity at some late stages of the arabinogalactan/arabinomannan biosynthesis.

Evaluation of the use of 5-mycoloyl-beta-arabinofuranosyl-(1-->2)-5-mycoloyl- alpha-arabinofuranosyl-(1-->1')-glycerol in serodiagnosis of Mycobacterium avium-intracellulare complex infection

5-Mycoloyl-beta-arabinofuranosyl-(1-->2)-5-mycoloyl-alpha-ar abinofuranosyl-(1-->1')-glycerol, an antigenic glycolipid from the Mycobacterium avium-intracellulare complex (MAC) was examined for its applicability to the serodiagnosis of MAC infection by ELISA. Serum IgM antibody titres against this glycolipid in healthy controls, pulmonary tuberculosis patients and sputum-MAC-culture-negative MAC patients were generally below the cut-off point (ELISA-negative), whereas most of the MAC-culture-positive MAC patient sera were ELISA-positive (93.5%) and their titres were often very high. Thus, high serum IgM titres against this glycolipid may be said to imply that the MAC disease is in an active phase.