Lipooligosaccharides from Mycobacteria: Structure, Function, and Synthesis

Total Synthesis of a Trehalose-Containing Lipooligosaccharide Analogue from Mycobacterium linda

A short and efficient methodology has been developed to synthesize an analogue of a lipooligosaccharide from Mycobacterium linda isolated from Crohn's disease. The total synthesis of the tetrasaccharide was achieved via a convergent [2 + 2] glycosylation approach. The key features of the synthesis involve the selective functionalization of a trehalose core via highly regioselective acylations and regioselective glycosylations. The synthesis was completed via a longest linear sequence of 14 steps in a 14.2% overall yield.

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

The surface of cells is coated with a dense layer of glycans, known as the cell glycocalyx. The complex glycans in the glycocalyx are involved in various biological events, such as bacterial pathogenesis, protection of bacteria from environmental stresses, etc. Polysaccharides on the bacterial cell surface are highly conserved and accessible molecules, and thus they are excellent immunological targets. Consequently, bacterial polysaccharides and their repeating units have been extensively studied as antigens for the development of antibacterial vaccines. This Review surveys the recent developments in the synthetic and immunological investigations of bacterial polysaccharide repeating unit-based conjugate vaccines against several human pathogenic bacteria. The major challenges associated with the development of functional carbohydrate-based antibacterial conjugate vaccines are also considered.

Synthesis of the Mycobacterium tuberculosis Canetti Lipooligosaccharide II Nonasaccharide

A route for preparing lipooligosaccharide (LOS) glycans from Mycobacterium tuberculosis Canetti was developed and applied to the most complex of these structures, LOS II. The synthesis of the target nonasaccharide was achieved via a convergent [3+3+3] approach. Key features of the strategy include the stereoselective synthesis of an asymmetrically substituted trehalose moiety from two protected glucose residues and several chemoselective glycosylations involving thioglycoside donors.

Rationalizing the Stereoelectronic Influence of Interglycosidic Bond Conformations on the Reactivity of 1,4-O-Linked Disaccharide Donors

Disaccharide donors are key precursors in convergent glycan synthesis strategies. Unexpectedly, we observed that disaccharide thioglycosyl donors containing 1,4-O-linked α-glycosidic bonds are much more reactive than their β-analogues with the same protecting group pattern. Herein, we rationalized that such a difference in their reactivity is attributed to the conformation of the 1,4-O-interglycosidic bond which is controlled by anomeric and exo-anomeric effects. Moreover, the conformational preferences of these donors are dictated by the dihedral angles ϕ and ψ of their interglycosidic linkages and the torsional angle ω of their side chain along the C5-C6 bond. This fundamental research clarifies how the long-range stereoelectronic effects from the nonreducing end sugar can influence the reactivity of the leaving group at the reducing end and the behavior of disaccharide donors thereof.

Synthesis of a Tridecasaccharide Lipooligosaccharide Antigen from the Opportunistic Pathogen Mycobacterium kansasii

The outer surfaces of mycobacteria, including the organism that causes tuberculosis, are decorated with an array of immunomodulatory glycans. Among these are lipooligosaccharides (LOSs), a class of molecules for which the function remains poorly understood. We describe the chemical synthesis of the glycan portion of a tridecasaccharide LOS from the opportunistic pathogen Mycobacterium kansasii. The target contains a number of unusual structural motifs that complicate its assembly and is the most complex mycobacterial LOS glycan to be synthesized to date when considering size and number of unique monosaccharides and glycosidic linkages. These studies not only provide a roadmap for the preparation of additional members of this family of glycans, but also provides a valuable probe for use in structure-activity relationship investigations.

Transporters Involved in the Biogenesis and Functionalization of the Mycobacterial Cell Envelope

The biology of mycobacteria is dominated by a complex cell envelope of unique composition and structure and of exceptionally low permeability. This cell envelope is the basis of many of the pathogenic features of mycobacteria and the site of susceptibility and resistance to many antibiotics and host defense mechanisms. This review is focused on the transporters that assemble and functionalize this complex structure. It highlights both the progress and the limits of our understanding of how (lipo)polysaccharides, (glyco)lipids, and other bacterial secretion products are translocated across the different layers of the cell envelope to their final extra-cytoplasmic location. It further describes some of the unique strategies evolved by mycobacteria to import nutrients and other products through this highly impermeable barrier.

Reactivity-Stereoselectivity Mapping for the Assembly of Mycobacterium marinum Lipooligosaccharides

The assembly of complex bacterial glycans presenting rare structural motifs and cis-glycosidic linkages is significantly obstructed by the lack of knowledge of the reactivity of the constituting building blocks and the stereoselectivity of the reactions in which they partake. We here report a strategy to map the reactivity of carbohydrate building blocks and apply it to understand the reactivity of the bacterial sugar, caryophyllose, a rare C12-monosaccharide, containing a characteristic tetrasubstituted stereocenter. We mapped reactivity-stereoselectivity relationships for caryophyllose donor and acceptor glycosides by a systematic series of glycosylations in combination with the detection and characterization of different reactive intermediates using experimental and computational techniques. The insights garnered from these studies enabled the rational design of building blocks with the required properties to assemble mycobacterial lipooligosaccharide fragments of M. marinum.

Synthesis of trehalose glycolipids

Chemical synthesis of trehalose glycolipids such as DAT, TDM, SL-1, SL-3, and Ac₂SGL from MTb, emmyguyacins from fungi, succinoyl trehalose from rhodococcus, and maradolipids from worms, as well as mycobacterial oligosaccharides is reviewed.

Insights into Interactions of Mycobacteria with the Host Innate Immune System from a Novel Array of Synthetic Mycobacterial Glycans

An array of homogeneous glycans representing all the major carbohydrate structures present in the cell wall of the human pathogen Mycobacterium tuberculosis and other mycobacteria has been probed with a panel of glycan-binding receptors expressed on cells of the mammalian innate immune system. The results provide an overview of interactions between mycobacterial glycans and receptors that mediate uptake and survival in macrophages, dendritic cells, and sinusoidal endothelial cells. A subset of the wide variety of glycan structures present on mycobacterial surfaces interact with cells of the innate immune system through the receptors tested. Endocytic receptors, including the mannose receptor, DC-SIGN, langerin, and DC-SIGNR (L-SIGN), interact predominantly with mannose-containing caps found on the mycobacterial polysaccharide lipoarabinomannan. Some of these receptors also interact with phosphatidyl-myo-inositol mannosides and mannose-containing phenolic glycolipids. Many glycans are ligands for overlapping sets of receptors, suggesting multiple, redundant routes by which mycobacteria can enter cells. Receptors with signaling capability interact with two distinct sets of mycobacterial glycans: targets for dectin-2 overlap with ligands for the mannose-binding endocytic receptors, while mincle binds exclusively to trehalose-containing structures such as trehalose dimycolate. None of the receptors surveyed bind furanose residues, which often form part of the epitopes recognized by antibodies to mycobacteria. Thus, the innate and adaptive immune systems can target different sets of mycobacterial glycans. This array, the first of its kind, represents an important new tool for probing, at a molecular level, biological roles of a broad range of mycobacterial glycans, a task that has not previously been possible.

The Emergence of Phenolic Glycans as Virulence Factors in Mycobacterium tuberculosis

Tuberculosis is the leading infectious cause of mortality worldwide. The global epidemic, caused by Mycobacterium tuberculosis, has prompted renewed interest in the development of novel vaccines for disease prevention and control. The cell envelope of M. tuberculosis is decorated with an assortment of glycan structures, including glycolipids, that are involved in disease pathogenesis. Phenolic glycolipids and the structurally related para-hydroxybenzoic acid derivatives display potent immunomodulatory activities and have particular relevance for both understanding the interaction of the bacterium with the host immune system and also in the design of new vaccine and therapeutic candidates. Interest in glycobiology has grown exponentially over the past decade, with advancements paving the way for effective carbohydrate based vaccines. This review highlights recent advances in our understanding of phenolic glycans, including their biosynthesis and role as virulence factors in M. tuberculosis. Recent chemical synthesis approaches and biochemical analysis of synthetic glycans and their conjugates have led to fundamental insights into their roles in host-pathogen interactions. The applications of these synthetic glycans as potential vaccine candidates are discussed.

Twenty Years of Mycobacterial Glycans: Furanosides and Beyond

The cell surface (or cell wall) of bacteria is coated with carbohydrate (or glycan) structures that play a number of important roles. These include providing structural integrity, serving as a permeability barrier to extracellular compounds (e.g., drugs) and modulating the immune system of the host. Of interest to this Account is the cell wall structure of mycobacteria. There are a host of different mycobacterial species, some of which cause human disease. The most well-known is Mycobacterium tuberculosis, the causative agent of tuberculosis. The mycobacterial cell wall is characterized by the presence of unusual carbohydrate structures that fulfill the roles described above. However, in many cases, a molecular-level understanding of how mycobacterial cell wall glycans mediate these processes is lacking.

Inspired by a seminar he heard as a postdoctoral fellow, the author began his independent research program with a focus on the chemical synthesis of mycobacterial glycans. The goals were not only to develop synthetic approaches to these unique structures but also to provide molecules that could be used to probe their biological function. Initial work addressed the preparation of fragments of two key polysaccharides, arabinogalactan and lipoarabinomannan, which contain large numbers of sugar residues in the furanose (five-membered) ring form. At the time these investigations began, there were few methods reported for the synthesis of oligosaccharides containing furanose rings. Thus, early in the program, a major area of interest was methodology development, particularly for the preparation of 1,2-cis-furanosides. To solve this challenge, a range of conformationally restricted donors have been developed, both in the author’s group and others, which provide 1,2-cis-furanosidic linkages with high stereoselectivity.

These investigations were followed by application of the developed methods to the synthesis of a range of target molecules containing arabinofuranose and galactofuranose residues. These molecules have now found application in biochemical, immunological, and structural biology investigations, which have shed light on their biosynthesis and how these motifs are recognized by both the innate and adaptive immune systems.

More recently, attention has been directed toward the synthesis of another class of immunologically active mycobacterial cell wall glycans, the extractable glycolipids. In this case, efforts have been primarily on phenolic glycolipids, and the compounds synthesized have been used to evaluate their ability to modulate cytokine release. Over the past 20 years, the use of chemical synthesis to provide increasingly complex glycan structures has provided significant benefit to the burgeoning field of mycobacterial glycobiology. Through the efforts of groups from around the globe, access to these compounds is now possible via relatively straightforward methods. As the pool of mycobacterial glycans continues to grow, so too will our understanding of their role in disease, which will undoubtedly lead to new strategies to prevent or treat mycobacterial infections.

First total synthesis of trehalose containing tetrasaccharides from Mycobacterium smegmatis

Total synthesis of three important trehalose containing tetrasaccharides isolated fromMycobacterium smegmatisis reported for the first time, using regioselective opening of benzylidene acetals and stereoselective glycosylations as key steps.

Synthesis of Mycobacterium tuberculosis Sulfolipid-3 Analogues and Total Synthesis of the Tetraacylated Trehaloglycolipid of Mycobacterium paraffinicum

A novel methodology for the regioselective O6 acylation of the 2,3-diacyl trehaloses to access Mycobacterium tuberculosis sulfolipid SL-3 and related 2,3,6-triester glycolipid analogues is reported for the first time. The methodology was successfully extended to achieve the first total synthesis of the tetraacylated trehalose glycolipid from Mycobacterium paraffinicum. The corresponding 2,3,6'-triesters trehalose glycolipids were also synthesized starting from the common 2,3-diacyl trehalose. These synthetic glycolipids are potential candidates for serodiagnosis and vaccine development for tuberculosis.