Le 3-O-méthyl-l-rhamnose, sucre isolé du mycoside G de Mycobacterium marinum

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Mycobacterial envelope lipids fingerprint from direct MALDI-TOF MS analysis of intact bacilli

Mycobacterium tuberculosis (Mtb) lipids including glycolipids and lipoglycans play a crucial role in the modulation of the host immune response by targeting the innate receptors C-type lectins, TLRs and the CD1 proteins of class 1. Glycolipids have been shown to be biomarkers of M. tuberculosis strains and also of opportunistic mycobacteria called non-tuberculous mycobacteria. Most of the structural and functional work of the Mtb lipids has been done using lipids arising from M. tuberculosis cell growth in vitro. However it is likely that lipid structures can change during infection or among the M. tuberculosis or opportunistic clinical strains. Here we describe a new, rapid and sensitive analysis of lipids directly on whole mycobacteria which can be done in few minutes and on less than 1000 mycobacteria by direct matrix-assisted laser desorption/ionization mass spectrometry using an unusual solvent matrix. By this new methodology, which does not require extraction or purification steps, we are able to discriminate mycobacteria belonging to the Mtb complex as well as opportunistic and non-pathogenic mycobacteria. This method was also found to be successful for identification of an envelope lipid mutant. This work opens a new analytical route for in vivo analysis of mycobacterial lipids.

Mycobacterium leprae phenolglycolipid-1 expressed by engineered M. bovis BCG modulates early interaction with human phagocytes

The species-specific phenolic glycolipid 1 (PGL-1) is suspected to play a critical role in the pathogenesis of leprosy, a chronic disease of the skin and peripheral nerves caused by Mycobacterium leprae. Based on studies using the purified compound, PGL-1 was proposed to mediate the tropism of M. leprae for the nervous system and to modulate host immune responses. However, deciphering the biological function of this glycolipid has been hampered by the inability to grow M. leprae in vitro and to genetically engineer this bacterium. Here, we identified the M. leprae genes required for the biosynthesis of the species-specific saccharidic domain of PGL-1 and reprogrammed seven enzymatic steps in M. bovis BCG to make it synthesize and display PGL-1 in the context of an M. leprae-like cell envelope. This recombinant strain provides us with a unique tool to address the key questions of the contribution of PGL-1 in the infection process and to study the underlying molecular mechanisms. We found that PGL-1 production endowed recombinant BCG with an increased capacity to exploit complement receptor 3 (CR3) for efficient invasion of human macrophages and evasion of inflammatory responses. PGL-1 production also promoted bacterial uptake by human dendritic cells and dampened their infection-induced maturation. Our results therefore suggest that M. leprae produces PGL-1 for immune-silent invasion of host phagocytic cells.

The dimycocerosate ester polyketide virulence factors of mycobacteria

Recent advances in the study of mycobacterial lipids indicate that the class of outer membrane lipids known as dimycocerosate esters (DIMs) are major virulence factors of clinically relevant mycobacteria including Mycobacterium tuberculosis and Mycobacterium leprae. DIMs are a structurally intriguing class of polyketide synthase-derived wax esters discovered over seventy years ago, yet, little was known until recently about their biosynthesis. Availability of several mycobacterial genomes has accelerated progress toward clarifying steps in the DIM biosynthetic pathway and it is our belief that reviewing the bases of our current knowledge will clarify outstanding issues and help direct future endeavors.

Thin-layer chromatography systems for the identification of Mycobacterium tuberculosis, M. bovis BCG, M. kansasii, M. gastri and M. marinum

Knowledge of mycobacterial glycolipid antigens and the study of their specificity have resulted in their utilization as species markers. We describe a thin-layer chromatography method which could serve as a useful adjunct for the identification of Mycobacterium tuberculosis, M. bovis BCG, M. kansasii, M. gastri and M. marinum.

Specific binding of phenolic glycolipid antigens from Mycobacterium bovis BCG with antibodies

We studied the molecular binding specificity of two rabbit polyclonal sera generated against phenolic glycolipid antigens namely PheG1 B and PheG1 B-3 from Mycobacterium bovis BCG. PheG1 B is the well-known mycoside B (2-O-Me-alpha-L-Rhap 1----aglycone), while PheG1 B-3 is a recently found glycolipid (alpha-L-Rhap-(1----3)-2-O-Me-alpha-L-Rhap 1----aglycone). The interaction specificity was mainly explained in terms of the cavity volume of the antibodies paratope. The anti-PheG1 B antibodies paratope fits the 2-O-Me-alpha-L-Rhap ligand, while that of anti-PheG1 B-3 binds the disaccharide moiety of PheG1 B-3, and, with a higher affinity, the monosaccharidic unit localized at the non-reducing end. The B-3 antigen affinity is higher than that of antigen B for their homologous antibodies. This can be explained by the fact that the antibodies against phenolic glycolipid B-3 bind optimally to two sequential glycosyl residues suggesting the presence of two subsites. The immunoglobulin subsite with the major affinity binds the monosaccharidic unit localized at the non-reducing end.

Structural elucidation and antigenicity of a novel phenolic glycolipid antigen from Mycobacterium haemophilum

The structure of a novel antigenic glycolipid that distinguishes the opportunistic pathogen Mycobacterium haemophilum from all other mycobacteria was established by a series of degradation reactions leading to products that were analyzed by gas/liquid chromatography-mass spectrometry. The complete structure of the oligosaccharide unit was determined as 2,3-di-O-CH3-alpha-L-Rhap(1----2)3-O-CH3-alpha-L-Rhap(1----4 )-2,3-di-O-CH3-alpha-L-Rhap(1----. The lipid portion of the phenolic glycolipid was composed of two component phenolphthiocerols differing by two methylene groups, as determined by analysis of their per-O-trideuteriomethylated derivatives. The diol unit of the phenolphthiocerols has a threo relative configuration. The absolute stereochemistry of the asymmetric centers of the phenolphthiocerols is uncertain, but the centers are probably 3R, 4S, 9R, and 11R as found for phthiocerol A from Mycobacterium tuberculosis. The hydroxyl functions of the branched glycolic chain are esterified to a complex mixture of multi-methyl branched mycocerosic acids, C27, C30, C32, C34, and C37 with molecular weights (as methyl esters) of 424, 466, 494, 522, and 564, respectively. The stereochemistry of the methyl branches of the mycocerosates have R absolute configuration. The glycolipid is highly antigenic and appears to be specific for M. haemophilum. There are intriguing similarities between the product from M. haemophilum and the well-known phenolic glycolipid I of Mycobacterium leprae, a matter that is discussed.

Structural and immunological properties of the phenolic glycolipids from Mycobacterium gastri and Mycobacterium kansasii

Mycobacterial species-specific antigens belong to the three following classes: phenolic glycolipids (Phe Gl), acyltrehalose-containing lipooligosaccharides and polar glycopeptidolipids. These antigens have been chemically defined and alkali-labile epitopes were found to characterize the lipooligosaccharide antigen type. In the present study the major Mycobacterium kansasii phenolic glycolipid epitope namely Phe Gl K-I was delineated as the distal monoacetylated disaccharidic residue: 2,6-dideoxy-4-O-methyl-alpha-D-arabino-hexopyranosyl-(1----3)-2-O-methyl -4-O- acetyl-alpha-L-fucopyranose. This acetoxy group is required for K-I epitope recognition demonstrating that alkali-labile epitopes also occur in the phenolic glycolipid antigen class. Using immunoelectron microscopy, the Phe Gl K-I epitope was localized around the electron-transparent layer on the M. kansasii cell-wall surface. Furthermore, two new phenolic glycolipids namely Phe Gl K-III and Phe Gl K-IV were discovered in minute amounts. They were purified and characterized by their retention time in direct-phase column HPLC. These molecules are also M. kansasii antigens, whose epitopes differ from that of Phe Gl K-I. The complete family of phenolic glycolipids Phe Gl K-I, K-II, K-III and K-IV was found in both rough and smooth variants of both M. kansasii and Mycobacterium gastri species.

The carbohydrate- and lipid-containing cell wall of mycobacteria, phenolic glycolipids: structure and immunological properties

Phenolic glycolipids were first discovered as cell-wall constituents of M. bovis, M. bovis BCG, M. marinum, and M. kansasii. Recently, such compounds were also isolated from M. leprae and have been shown to be specific-species serological markers. Moreover, they seem to be involved, in the case of lepromatous leprosy, in the stimulation of the suppressor T-cells. The functional activities of these phenolic glycolipids over the immune cells stimulation emphasized the role played by these molecules in the mycobacteria pathogenicity. Phenolic glycolipids have also been found in M. gastri and M. tuberculosis strain Canetti. From a structural point of view, these glycolipids contain the same aglycon moiety mainly assigned to phenolphthiocerol diester while the sugar part structure confers to some of these glycolipids their antigenic specificity. The search of immunoreactive glycolipids and their function analysis remain a challenge for chemists and immunologists for the understanding of the mycobacteria pathogenicity.

Mechanisms of pathogenicity in mycobacteria

The purpose of this article is to review current knowledge about the mechanisms of pathogenicity of mycobacteria. The following aspects of the problem are discussed: chemically-defined compounds implicated in the mechanisms of pathogenicity; location in the cell wall of these compounds and their biological activities; mechanisms of intracellular survival of pathogenic mycobacteria as compared to intracellular killing of non-pathogenic mycobacteria; and pathogenesis of mycobacterial infection. The future prospects in the elucidation of the mechanisms of pathogenicity and their possible application for a better control of mycobacterial diseases are briefly discussed.

Analysis of dimycocerosates of glycosylphenolphthiocerols in the identification of some clinically significant mycobacteria

Extracts of representative mycobacteria were examined by thin-layer chromatography for glycosylphenolphthiocerol dimycocerosates. The glycolipid typical of Mycobacterium bovis was also found in Mycobacterium africanum and Mycobacterium microti, but it was absent in Mycobacterium bovis AN 5. Mycobacterium gastri strains contained a glycolipid which was chromatographically similar to that in Mycobacterium kansasii. Representatives of Mycobacterium marinum produced a distinct glycolipid type, and one strain had major amounts of a more polar variant. The sugar moieties of purified lipids, including that from Mycobacterium leprae, were identified by thin-layer chromatography of methyl glycosides in acid methanolysates.

Structure of the major triglycosyl phenol-phthiocerol of Mycobacterium tuberculosis (strain Canetti)

Phenol-phthiocerol glycolipids have been found previously in Mycobacterium leprae, M. kansasii, M. bovis and M. marinum, but not in M. tuberculosis. A search for glycolipids in this latter species showed that the Canetti strains of M. tuberculosis synthesize a major triglycosyl phenol-phthiocerol, accompanied by minor amounts of other glycolipids with a similar aglycone moiety. The triglycoside moiety has the following structure: 2,3,4-tri-O-methyl L-fucopyranosyl(alpha 1----3)L-rhamnopyranosyl(alpha 1----3)2-O-methyl L-rhamnopyranosyl(alpha 1-. The aglycone moiety consists in phenol-phthiocerol (two homologs). Its two secondary alcohol functions are esterified by mycocerosic acids (homologs with 26-32 carbon atoms and with 2-4 methyl branches). The proposed structure differs on several points from the M. leprae glycolipids, but presents some analogy with the major glycolipid of M. kansasii. A minor monoglycosyl phenol-phthiocerol was also studied. Its overall structure is very similar to that of M. bovis, with 2-O-methyl rhamnose as sugar moiety.