Synthesis of mannopyranose disaccharides as photoaffinity probes for mannosyltransferases in Mycobacterium tuberculosis

Synthesis of a multivalent α-1,2-mannobiose ligand for targeting C-type lectins

The importance of lectins in biological processes such as pathogen recognition, cell adhesion, and cell recognition is well documented. C-Type lectins, which require calcium for binding, play an important role in the innate immune response by engaging carbohydrates presented as part of the human and pathogen glycocalyx. For example, lectins such as MBL, Dectin-2, langerin and DC-SIGN selectively recognize mannose rich (high-mannose) structures presented as part of the glycocalyx. One common sugar binding motif that is recognized by these lectins on the pathogen glycocalyx is α-1,2-mannobiose, a disaccharide that consists of two mannose units connected via a α-1,2-linkage. To study the binding of these motifs in different contexts, synthetic replicas of α-1,2-mannobiose that can be presented in a multivalent fashion mimicking their presentation on the glycocalyx are required. Here we present the synthesis of a novel α-1,2-mannobiose analog bearing an azido linker from known precursors using a split and combine approach guided by neighboring group participation. Our approach makes it possible to achieve comparatively high yields and stereoselectivities while reducing the number of steps required to prepare such structures. We also introduce, for the first time, the trivalent presentation of our α-1,2-mannobiose ligand on a precision glycomacromolecule using copper-catalyzed azide-alkyne cycloaddition (CuAAC) to create high-mannose mimetics. Such structures have the potential to serve as probes for unlocking the rules of engagement between high-mannose glycans and C-type lectins like langerin and DC-SIGN.

Neutral Re(I) Complex Platform for Live Intracellular Imaging

Luminescent metal complexes are a valuable platform for the generation of cell imaging agents. However, many metal complexes are cationic, a factor that can dominate the intracellular accumulation to specific organelles. Neutral Re(I) complexes offer a more attractive platform for the development of bioconjugated imaging agents, where charge cannot influence their intracellular distribution. Herein, we report the synthesis of a neutral complex (ReAlkyne), which was used as a platform for the generation of four carbohydrate-conjugated imaging agents via Cu(I)-catalyzed azide-alkyne cycloaddition. A comprehensive evaluation of the physical and optical properties of each complex is provided, together with a determination of their utility as live cell imaging agents in H9c2 cardiomyoblasts. Unlike their cationic counterparts, many of which localize within mitochondria, these neutral complexes have localized within the endosomal/lysosomal network, a result consistent with examples of dinuclear carbohydrate-appended neutral Re(I) complexes that have been reported. This further demonstrates the utility of these neutral Re(I) complex imaging platforms as viable imaging platforms for the development of bioconjugated cell imaging agents.

Mycobacterium tuberculosis curli pili (MTP) deficiency is associated with alterations in cell wall biogenesis, fatty acid metabolism and amino acid synthesis

INTRODUCTION

In an effort to find alternative therapeutic interventions to combat tuberculosis, a better understanding of the pathophysiology of Mycobacterium tuberculosis is required. The Mycobacterium tuberculosis curli pili (MTP) adhesin, present on the surface of this pathogen, has previously been shown using functional genomics and global transcriptomics, to play an important role in establishing infection, bacterial aggregation, and modulating host response in vitro and in vivo.

OBJECTIVE

This investigation aimed to determine the role of MTP in modulating the metabolism of M. tuberculosis, using mtp gene-knockout mutant and complemented strains.

METHODS

Untargeted two-dimensional gas chromatography time-of-flight mass spectrometry, and bioinformatic analyses, were used to identify significant differences in the metabolite profiles among the wild-type, ∆mtp mutant and mtp-complemented strains, and validated with results generated by real-time quantitative PCR.

RESULTS

A total of 28 metabolites were found to be significantly altered when comparing the ∆mtp mutant and the wild-type strains indicating a decreased utilisation of metabolites in cell wall biogenesis, a reduced efficiency in the breakdown of fatty acids, and decreased amino acid biosynthesis in the former strain. Comparison of the wild-type to mtp-complement, and ∆mtp to mtp-complemented strains revealed 10 and 16 metabolite differences, respectively. Real-time quantitative PCR results supported the metabolomics findings. Complementation of the ∆mtp mutant resulted in a partial restoration of MTP function.

CONCLUSION

The lack of the MTP adhesin resulted in various bacterial cell wall alterations and related metabolic changes. This study highlights the importance of MTP as a virulence factor and further substantiates its potential use as a suitable biomarker for the development of diagnostic tools and intervention therapeutics against TB.

Rapid and efficient synthesis of α(1–2)mannobiosides

A very rapid, versatile and high yield synthesis of Manα1,2Man derivatives using a common orthogonal benzoyl/acetyl protection strategy compatible with the presence of azido groups and the use of CuAAC for conjugating the α(1–2)mannobiosides to different scaffolds, reducing the cost and time of the synthesis and improving the overall yield.

Epimeric and amino disaccharide analogs as probes of an alpha-(1-->6)-mannosyltransferase involved in mycobacterial lipoarabinomannan biosynthesis

Mycobacterial lipoarabinomannan (LAM) is an important, immunologically active glycan found in the cell wall of mycobacteria, including the human pathogen Mycobacterium tuberculosis. At the core of LAM is a mannan domain comprised of alpha-(1-->6)-linked-mannopyranose (Manp) residues. Previously, we and others have demonstrated that alpha-Manp-(1-->6)-alpha-Manp disaccharides (e.g., Manp-(1-->6)-alpha-ManpOctyl, ) are the minimum acceptor substrates for enzymes involved in the assembly of the LAM mannan core. We report here the synthesis five epimeric and three amino analogs of , and their subsequent biochemical evaluation against an alpha-(1-->6)-ManT activity present in a membrane preparation from M. smegmatis. Changing the manno- configuration of either residue of to talo- or gluco- led to a reduction or loss of activity, thus confirming earlier work showing that the C-2 and C-4 hydroxyl groups of each monosaccharide were important for enzymatic recognition. Characterization of the products formed from these analogs was done using a combination of mass spectrometry and glycosidase digestion, and full substrate kinetics were also performed. The analogs in which the acceptor hydroxyl group had been replaced with an amino group were, as expected, not substrates for the enzyme, but were weak inhibitors.

An Improved Synthetic Route to the Potent Angiogenesis Inhibitor Benzyl Manα(1→3)-Manα(1→3)-Manα(1→3)-Manα(1→2)-Man Hexadecasulfate

An improved synthetic route to α(1→3)/α(1→2)-linked mannooligosaccharides has been developed and applied to a more efficient preparation of the potent anti-angiogenic sulfated pentasaccharide, benzyl Manα(1→3)-Manα(1→3)-Manα(1→3)-Manα(1→2)-Man hexadecasulfate, using only two monosaccharide building blocks. Of particular note are improvements in the preparation of both building blocks and a simpler, final deprotection strategy. The route also provides common intermediates for the introduction of aglycones other than benzyl, either at the building block stage or after oligosaccharide assembly. The anti-angiogenic activity of the synthesized target compound was confirmed via the rat aortic assay.

Disaccharide analogs as probes for glycosyltransferases in Mycobacterium tuberculosis

Glycosyltransferases (GTs) play a crucial role in mycobacterial cell wall biosynthesis and are necessary for the survival of mycobacteria. Hence, these enzymes are potential new drug targets for the treatment of tuberculosis (TB), especially multiple drug-resistant TB (MDR-TB). Herein, we report the efficient syntheses of Araf(alpha 1-->5)Araf, Galf(beta 1-->5)Galf, and Galf(beta 1-->6)Galf disaccharides possessing a 5-N,N-dimethylaminonaphthalene-1-sulfonamidoethyl (dansyl) unit that were prepared as fluorescent disaccharide acceptors for arabinosyl- and galactosyl-transferases, respectively. Such analogs may offer advantages relative to radiolabeled acceptors or donors for studying the enzymes and for assay development and compound screening. Additionally, analogs possessing a 5-azidonaphthalene-1-sulfonamidoethyl unit were prepared as photoaffinity probes for their potential utility in studying active site labeling of the GTs (arabinosyl and galactosyl) in Mycobacterium tuberculosis (MTB). Beyond their preparation, initial biological testing and kinetic analysis of these disaccharides as acceptors toward glycosyltransferases are also presented.

Synthesis of deoxy and methoxy analogs of octyl alpha-D-mannopyranosyl-(1-->6)-alpha-D-mannopyranoside as probes for mycobacterial lipoarabinomannan biosynthesis

A panel of analogs of the disaccharide alpha-D-Manp-(1-->6)-alpha-D-Manp-OOctyl, a known acceptor substrate for a polyprenol monophosphomannose-dependent alpha-(1-->6)-mannosyltransferase involved in the assembly of the alpha-(1-->6)-linked mannan core of mycobacterial lipoarabinomannan, has been synthesized. Described are synthetic routes to the target deoxy and methoxy analogs in which one of the hydroxyl groups of the parent disaccharide has been modified. All glycosylation reactions involved the use of octyl glycoside acceptors and thioglycoside donors using iodonium-ion activation, and the stereochemistry of the mannopyranoside bond formed was established by measurement of the 1J(C-1,H-1). Depending on the target, the key methylation or deoxygenation reactions were carried out on either mono- or disaccharide substrates. This series of analogs will be useful for probing the substrate specificity of the enzyme, in particular, its steric and hydrogen-bonding requirements.

“Click saccharides”: novel separation materials for hydrophilic interaction liquid chromatography

Bonded mono-, di- and oligosaccharides were developed as novel separation materials for HILIC via click chemistry and proven to have excellent chromatographic properties for separation of polar compounds.

Rapid, iterative assembly of octyl alpha-1,6-oligomannosides and their 6-deoxy equivalents

Mycobacterium tuberculosis is the cause of the deadly human disease tuberculosis. In studies over the last 40 years it has been revealed that this organism possesses a complex cell wall including glycophospholipids such as the phosphatidylinositiol mannosides (PIMs), lipomannan (LM) and lipoarabinomannan (LAM). These glycolipids all contain a common alpha-1,6-linked mannoside core, and the higher PIMs and LAM possess alpha-1,2-linked mannosyl residues. It has been shown that simple alpha-1,6-linked oligomannosides can act as substrates for alpha-1,6-mannosyltransferases in mycobacteria. Here we report a simple iterative synthesis of a series of hydrophobic octyl alpha-1,6-linked oligomannosides from mono- through to tetrasaccharides. We have utilized a single thioglycoside donor and alcohol acceptor. Further, we have developed conditions for the conversion of each of these compounds to the 6-deoxy congeners. Deoxygenation of the 6-position of the terminal mannosyl residue should prevent these compounds acting as substrates for the abundant alpha-1,6-mannosyltransferases in mycobacteria and should permit detection of the elusive alpha-1,2-mannosyltransferase activity responsible for elaboration of LM to mature LAM and the biosynthesis of the higher PIMs.

Modified mannose disaccharides as substrates and inhibitors of a polyprenol monophosphomannose-dependent α-(1→6)-mannosyltransferase involved in mycobacterial lipoarabinomannan biosynthesis

A panel of alpha-(1-->6)-linked mannose disaccharides (5-8) in which the 2'-OH group has been replaced, independently, by deoxy, fluoro, amino, and methoxy functionalities has been synthesized. Evaluation of these compounds as potential substrates or inhibitors of a polyprenol monophosphomannose-dependent alpha-(1-->6)-mannosyltransferase involved in mycobacterial LAM biosynthesis demonstrated that the enzyme is somewhat tolerant substitution at this site. The enzyme recognizes the disaccharides with groups similar or smaller in size than the native hydroxyl (6-8), but not the disaccharide with the more sterically demanding methoxy group (5). The 2'-OH appears not form a critical hydrogen bonding interaction with the protein as the 2'-deoxy analog is a substrate for the enzyme.

Mycobacterial lipoarabinomannan and related lipoglycans: from biogenesis to modulation of the immune response

The cell wall component lipoarabinomannan (ManLAM) from Mycobacterium tuberculosis is involved in the inhibition of phagosome maturation, apoptosis and interferon (IFN)-gamma signalling in macrophages and interleukin (IL)-12 cytokine secretion of dendritic cells (DC). All these processes are important for the host to mount an efficient immune response. Conversely, LAM isolated from non-pathogenic mycobacteria (PILAM) have the opposite effect, by inducing a potent proinflammatory response in macrophages and DCs. LAMs from diverse mycobacterial species differ in the modification of their terminal arabinose residues. The strong proinflammatory response induced by PILAM correlates with the presence of phospho-myo-inositol on the terminal arabinose. Interestingly, recent work indicates that the biosynthetic precursor of LAM, lipomannan (LM), which is also present in the cell wall, displays strong proinflammatory effects, independently of which mycobacterial species it is isolated from. Results from in vitro assays and knock-out mice suggest that LM, like PILAM, mediates its biological activity via Toll-like receptor 2. We hypothesize that the LAM/LM ratio might be a crucial factor in determining the virulence of a mycobacterial species and the outcome of the infection. Recent progress in the identification of genes involved in the biosynthesis of LAM is discussed, in particular with respect to the fact that enzymes controlling the LAM/LM balance might represent targets for new antitubercular drugs. In addition, inactivation of these genes may lead to attenuated strains of M. tuberculosis for the development of new vaccine candidates.