Polysaccharide length affects mycobacterial cell shape and antibiotic susceptibility

Maintenance of cell wall remodeling and vesicle production are connected in Mycobacterium tuberculosis

Pathogenic and nonpathogenic mycobacteria secrete extracellular vesicles (EVs) under various conditions. EVs produced by Mycobacterium tuberculosis ( Mtb ) have raised significant interest for their potential in cell communication, nutrient acquisition, and immune evasion. However, the relevance of vesicle secretion during tuberculosis infection remains unknown due to the limited understanding of mycobacterial vesicle biogenesis. We have previously shown that a transposon mutant in the LCP-related gene virR ( virR mut ) manifested a strong attenuated phenotype during experimental macrophage and murine infections, concomitant to enhanced vesicle release. In this study, we aimed to understand the role of VirR in the vesicle production process in Mtb . We employ genetic, transcriptional, proteomics, ultrastructural and biochemical methods to investigate the underlying processes explaining the enhanced vesiculogenesis phenomenon observed in the virR mutant. Our results establish that VirR is critical to sustain proper cell permeability via regulation of cell envelope remodeling possibly through the interaction with similar cell envelope proteins, which control the link between peptidoglycan and arabinogalactan. These findings advance our understanding of mycobacterial extracellular vesicle biogenesis and suggest that these set of proteins could be attractive targets for therapeutic intervention.

GlfT1 down-regulation affects Mycobacterium tuberculosis biofilm formation and its in-vitro and in-vivo survival

Mycobacterial galactan biosynthesis is critical for cell viability and growth, therefore an effort was made to study galactofuranosyl transferase 1, encoded by MRA_3822 in Mycobacterium tuberculosis H37Ra (Mtb-Ra). Galactofuranosyl transferases are involved in the biosynthesis of mycobacterial cell wall galactan chain and have been shown to be essential for in-vitro growth of Mycobacterium tuberculosis. In Mtb-Ra and Mycobacterium tuberculosis H37Rv (Mtb-Rv), two galactofuranosyl transferases are present, GlfT1 acts as initiator of galactan biosynthesis and GlfT2 continues with the subsequent polymerization events. GlfT2 has been well studied however GlfT1 inhibition/down-regulation and its effect on mycobacterial survival fitness has not been evaluated. To study the Mtb-Ra survival after GlfT1 silencing, Mtb-Ra knockdown and complemented strains were developed. In this study we show that GlfT1 down-regulation leads to increased susceptibility to ethambutol. Expression of glfT1 was up-regulated in the presence of ethambutol, and also in the presence of oxidative and nitrosative stress and upon exposure to low pH. Also, reduced biofilm formation, increased accumulation of ethidium bromide, and reduced tolerance to peroxide, nitric oxide and acid stress, were observed. The present study also demonstrates that GlfT1 down-regulation leads to reduced survival of Mtb-Ra in macrophages and in mice.

Mycobacterium tuberculosis cell-wall and antimicrobial peptides: a mission impossible?

Mycobacterium tuberculosis (Mtb) is one of the most important infectious agents worldwide and causes more than 1.5 million deaths annually. To make matters worse, the drug resistance among Mtb strains has risen substantially in the last few decades. Nowadays, it is not uncommon to find patients infected with Mtb strains that are virtually resistant to all antibiotics, which has led to the urgent search for new molecules and therapies. Over previous decades, several studies have demonstrated the efficiency of antimicrobial peptides to eliminate even multidrug-resistant bacteria, making them outstanding candidates to counterattack this growing health problem. Nevertheless, the complexity of the Mtb cell wall makes us wonder whether antimicrobial peptides can effectively kill this persistent Mycobacterium. In the present review, we explore the complexity of the Mtb cell wall and analyze the effectiveness of antimicrobial peptides to eliminate the bacilli.

Identification of D-arabinan-degrading enzymes in mycobacteria

Bacterial cell growth and division require the coordinated action of enzymes that synthesize and degrade cell wall polymers. Here, we identify enzymes that cleave the D-arabinan core of arabinogalactan, an unusual component of the cell wall of Mycobacterium tuberculosis and other mycobacteria. We screened 14 human gut-derived Bacteroidetes for arabinogalactan-degrading activities and identified four families of glycoside hydrolases with activity against the D-arabinan or D-galactan components of arabinogalactan. Using one of these isolates with exo-D-galactofuranosidase activity, we generated enriched D-arabinan and used it to identify a strain of Dysgonomonas gadei as a D-arabinan degrader. This enabled the discovery of endo- and exo-acting enzymes that cleave D-arabinan, including members of the DUF2961 family (GH172) and a family of glycoside hydrolases (DUF4185/GH183) that display endo-D-arabinofuranase activity and are conserved in mycobacteria and other microbes. Mycobacterial genomes encode two conserved endo-D-arabinanases with different preferences for the D-arabinan-containing cell wall components arabinogalactan and lipoarabinomannan, suggesting they are important for cell wall modification and/or degradation. The discovery of these enzymes will support future studies into the structure and function of the mycobacterial cell wall.

Controlled processivity in glycosyltransferases: A way to expand the enzymatic toolbox

Glycosyltransferases (GT) catalyse the biosynthesis of complex carbohydrates which are the most abundant group of molecules in nature. They are involved in several key mechanisms such as cell signalling, biofilm formation, host immune system invasion or cell structure and this in both prokaryotic and eukaryotic cells. As a result, research towards complete enzyme mechanisms is valuable to understand and elucidate specific structure-function relationships in this group of molecules. In a next step this knowledge could be used in GT protein engineering, not only for rational drug design but also for multiple biotechnological production processes, such as the biosynthesis of hyaluronan, cellooligosaccharides or chitooligosaccharides. Generation of these poly- and/or oligosaccharides is possible due to a common feature of several of these GTs: processivity. Enzymatic processivity has the ability to hold on to the growing polymer chain and some of these GTs can even control the number of glycosyl transfers. In a first part, recent advances in understanding the mechanism of various processive enzymes are discussed. To this end, an overview is given of possible engineering strategies for the purpose of new industrial and fundamental applications. In the second part of this review, we focused on specific chain length-controlling mechanisms, i.e., key residues or conserved regions, and this for both eukaryotic and prokaryotic enzymes.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

AuCl3-Catalyzed Hemiacetal Activation for the Stereoselective Synthesis of 2-Deoxy Trehalose Derivatives

A new practical, catalytic, and highly stereoselective method for directly accessing 1,1-α,α'-linked 2-deoxy trehalose analogues via AuCl₃-catalyzed dehydrative glycosylation using hemiacetal glycosyl donors and acceptors is described. The method relies on the chemoselective Brønsted acid-type activation of tribenzylated 2-deoxy hemiacetals in the presence of other less reactive hemiacetals.

Rv0180c contributes to Mycobacterium tuberculosis cell shape and to infectivity in mice and macrophages

Mycobacterium tuberculosis, the main causative agent of human tuberculosis, is transmitted from person to person via small droplets containing very few bacteria. Optimizing the chance to seed in the lungs is therefore a major adaptation to favor survival and dissemination in the human population. Here we used TnSeq to identify genes important for the early events leading to bacterial seeding in the lungs. Beside several genes encoding known virulence factors, we found three new candidates not previously described: rv0180c, rv1779c and rv1592c. We focused on the gene, rv0180c, of unknown function. First, we found that deletion of rv0180c in M. tuberculosis substantially reduced the initiation of infection in the lungs of mice. Next, we established that Rv0180c enhances entry into macrophages through the use of complement-receptor 3 (CR3), a major phagocytic receptor for M. tuberculosis. Silencing CR3 or blocking the CR3 lectin site abolished the difference in entry between the wild-type parental strain and the Δrv0180c::km mutant. However, we detected no difference in the production of both CR3-known carbohydrate ligands (glucan, arabinomannan, mannan), CR3-modulating lipids (phthiocerol dimycocerosate), or proteins in the capsule of the Δrv0180c::km mutant in comparison to the wild-type or complemented strains. By contrast, we established that Rv0180c contributes to the functionality of the bacterial cell envelope regarding resistance to toxic molecule attack and cell shape. This alteration of bacterial shape could impair the engagement of membrane receptors that M. tuberculosis uses to invade host cells, and open a new perspective on the modulation of bacterial infectivity.

The epidemic efficiency of tuberculosis bacilli is determined by their capacity to transmit via aerosol. Currently, the bacterial functions that favor Mycobacterium tuberculosis seeding in the lung of naïve host remain mostly unknown. Here we implemented a genome-wide approach to identify M. tuberculosis mutants deficient for seeding and early replication in the lung of mice. In addition to genes known to encode virulence factors, we identified three genes not previously described. We used complementary approaches to characterize the phenotype of a M. tuberculosis mutant with insertion within the rv0180c gene. We found that this mutant is impaired for seeding in the lung of mice and for invasion and replication in human macrophages. In macrophages, the defect relies on a lack of engagement of CR3 receptor. Although we did not detect any difference between the wild type strain and the rv0180c mutant with regard to potential CR3-ligand, we found that the bacterial cell envelope is altered in the rv0180c mutant. Our study provides new insight into bacterial genes required for early interaction of M. tuberculosis with the host and perspective to understand the bacterial functions enhancing infectivity.

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

Molecular Boronic Acid-Based Saccharide Sensors

Boronic acids can reversibly bind diols, a molecular feature that is ubiquitous within saccharides, leading to their use in the design and implementation of sensors for numerous saccharide species. There is a growing understanding of the importance of saccharides in many biological processes and systems; while saccharide or carbohydrate sensing in medicine is most often associated with detection of glucose in diabetes patients, saccharides have proven to be relevant in a range of disease states. Herein the relevance of carbohydrate sensing for biomedical applications is explored, and this review seeks to outline how the complexity of saccharides presents a challenge for the development of selective sensors and describes efforts that have been made to understand the underpinning fluorescence and binding mechanisms of these systems, before outlining examples of how researchers have used this knowledge to develop ever more selective receptors.

An ABC transporter Wzm-Wzt catalyzes translocation of lipid-linked galactan across the plasma membrane in mycobacteria

Mycobacterium tuberculosis, one of the deadliest pathogens in human history, is distinguished by a unique, multilayered cell wall, which offers the bacterium a high level of protection from the attacks of the host immune system. The primary structure of the cell wall core, composed of covalently linked peptidoglycan, branched heteropolysaccharide arabinogalactan, and mycolic acids, is well known, and numerous enzymes involved in the biosynthesis of its components are characterized. The cell wall biogenesis takes place at both cytoplasmic and periplasmic faces of the plasma membrane, and only recently some of the specific transport systems translocating the metabolic intermediates between these two compartments have been characterized [M. Jackson, C. M. Stevens, L. Zhang, H. I. Zgurskaya, M. Niederweis, Chem. Rev., 10.1021/acs.chemrev.0c00869 (2020)]. In this work, we use CRISPR interference methodology in Mycobacterium smegmatis to functionally characterize an ATP-binding cassette (ABC) transporter involved in the translocation of galactan precursors across the plasma membrane. We show that genetic knockdown of the transmembrane subunit of the transporter results in severe morphological changes and the accumulation of an aberrantly long galactan precursor. Based on similarities with structures and functions of specific O-antigen ABC transporters of gram-negative bacteria [C. Whitfield, D. M. Williams, S. D. Kelly, J. Biol. Chem. 295, 10593-10609 (2020)], we propose a model for coupled synthesis and export of the galactan polymer precursor in mycobacteria.

Use of Synthetic Glycolipids to Probe the Number and Position of Arabinan Chains on Mycobacterial Arabinogalactan

The arabinogalactan of Corynebacterianeae is a critical heteropolysaccharide that tethers outer membrane mycolic acids to peptidoglycan thus forming the characteristic cell wall core of these prokaryotes. An essential α-(1→5)-arabinosyltransferase, AftA, is responsible for the transfer of the first arabinofuranosyl (Araf) unit of the arabinan domain to the galactan backbone of arabinogalactan, but the number and precise position at which Araf residue(s) is/are added in mycobacteria remain ill-defined. Using membrane preparations from Mycobacterium smegmatis overexpressing aftA, farnesyl-phospho-arabinose as an Araf donor, and a series of synthetic galactan acceptors of various lengths, we here show that a single priming arabinosyl residue substitutes the C-5 position of a precisely positioned internal 6-linked galactofuranosyl residue of the galactan acceptors, irrespective of their length. This unexpected result suggests that, like the structurally related mycobacterial lipoarabinomannans, the arabinogalactan of mycobacteria may in fact harbor a single arabinan chain.