Peptidoglycan in Mycobacteria: chemistry, biology and intervention

The fall of the mycobacterial cell wall: interrogating peptidoglycan synthesis for novel anti-TB agents

Tuberculosis (TB) caused by Mycobacterium tuberculosis has been a threat to human health for thousands of years and still leads to millions of deaths each year. TB is a disease that is refractory to treatment, partially due to its capacity for in-host persistence. The cell wall of mycobacteria, rich in mycolic acid, is broadly associated with bacterial persistence together with antimicrobial and immunological resistance. Enzymes for the biosynthesis of bacterial peptidoglycan, an essential component of the cell wall, have been addressed and considered as appealing drug targets in pathogens. Significant effort has been dedicated to finding inhibitors that hinder peptidoglycan biosynthesis, many with demonstrated enzymatic inhibition in vitro being published. One family of critical biosynthetic enzymes are the Mur enzymes, with many enzyme specific inhibitors having been reported. However, a lesser developed strategy which may have positive clinical implications is to take advantage of the common structural and catalytic characteristics among Mur enzymes and to allow simultaneous, multiple Mur inhibition, and avert the development of drug resistance. M. tuberculosis relies on these essential Mur enzymes, with the best-known subset being Mur ligases, but also utilizes unique functions of atypical transpeptidases resulting in peptidoglycan peptide cross-linking beneficial to the bacteria's capacity for chronic persistence in humans. A systematic review is now needed, with an emphasis on M. tuberculosis. The urgent development of novel anti-TB agents to counter rapidly developing drug resistance requires a revisit of the literature, past successes and failures, in an attempt to reveal liabilities in critical cellular functions and drive innovation.

Mycobacterium tuberculosis suppresses host antimicrobial peptides by dehydrogenating L-alanine

Antimicrobial peptides (AMPs), ancient scavengers of bacteria, are very poorly induced in macrophages infected by Mycobacterium tuberculosis (M. tuberculosis), but the underlying mechanism remains unknown. Here, we report that L-alanine interacts with PRSS1 and unfreezes the inhibitory effect of PRSS1 on the activation of NF-κB pathway to induce the expression of AMPs, but mycobacterial alanine dehydrogenase (Ald) Rv2780 hydrolyzes L-alanine and reduces the level of L-alanine in macrophages, thereby suppressing the expression of AMPs to facilitate survival of mycobacteria. Mechanistically, PRSS1 associates with TAK1 and disruptes the formation of TAK1/TAB1 complex to inhibit TAK1-mediated activation of NF-κB pathway, but interaction of L-alanine with PRSS1, disables PRSS1-mediated impairment on TAK1/TAB1 complex formation, thereby triggering the activation of NF-κB pathway to induce expression of AMPs. Moreover, deletion of antimicrobial peptide gene β-defensin 4 (Defb4) impairs the virulence by Rv2780 during infection in mice. Both L-alanine and the Rv2780 inhibitor, GWP-042, exhibits excellent inhibitory activity against M. tuberculosis infection in vivo. Our findings identify a previously unrecognized mechanism that M. tuberculosis uses its own alanine dehydrogenase to suppress host immunity, and provide insights relevant to the development of effective immunomodulators that target M. tuberculosis.

In vitro evaluation of the binding activity of novel mouse IgG1 opsonic monoclonal antibodies to Mycobacterium tuberculosis and other selected mycobacterial species

Antimicrobial resistance alongside other challenges in tuberculosis (TB) therapeutics have stirred renewed interest in host-directed interventions, including the role of antibodies as adjunct therapeutic agents. This study assessed the binding efficacy of two novel IgG1 opsonic monoclonal antibodies (MABs; GG9 & JG7) at 5, 10, and 25 µg/mL to live cultures of Mycobacterium tuberculosis, M. avium, M. bovis, M. fortuitum, M. intracellulare, and M. smegmatis American Type Culture Collection laboratory reference strains, as well as clinical susceptible, multi-drug resistant, and extensively drug resistant M. tuberculosis strains using indirect enzyme-linked immunosorbent assays. These three MAB concentrations were selected from a range of concentrations used in previous optimization (binding and functional) assays. Both MABs bound to all mycobacterial species and sub-types tested, albeit to varying degrees. Statistically significant differences in MAB binding activity were observed when comparing the highest and lowest MAB concentrations (p < 0.05) for both MABs GG9 and JG7, irrespective of the M. tuberculosis resistance profile. Binding affinity increased with an increase in MAB concentration, and optimal binding was observed at 25 µg/mL. JG7 showed better binding activity than GG9. Both MABs also bound to five MOTT species, albeit at varied levels. This non-selective binding to different mycobacterial species suggests a potential role for GG9 and JG7 as adjunctive agents in anti-TB chemotherapy with the aim to enhance bacterial killing.

Anti-tuberculosis drug development via targeting the cell envelope of Mycobacterium tuberculosis

Mycobacterium tuberculosis possesses a dynamic cell envelope, which consists of a peptidoglycan layer, a mycolic acid layer, and an arabinogalactan polysaccharide. This envelope possesses a highly complex and unique structure representing a barrier that protects and assists the growth of M. tuberculosis and allows its adaptation to the host. It regulates the immune response of the host cells, causing their damage. Therefore, the cell envelope of M. tuberculosis is an attractive target for vaccine and drug development. The emergence of multidrug-resistant as well as extensively drug resistant tuberculosis and co-infection with HIV prevented an effective control of this disease. Thus, the discovery and development of new drugs is a major keystone for TB treatment and control. This review mainly summarizes the development of drug enzymes involved in the biosynthesis of the cell wall in M. tuberculosis, and other potential drug targets in this pathway, to provide more effective strategies for the development of new drugs.

Arabinogalactan enhances Mycobacterium marinum virulence by suppressing host innate immune responses

Arabinogalactan (AG) participates in forming the cell wall core of mycobacteria, a structure known as the mAGP complex. Few studies have reported the virulence of inartificial AG or its interaction with the host immune system. Using clustered regularly interspaced short palindromic repeats interference gene editing technology, conditional Mycobacterium marinum mutants were constructed with a low expression of embA or glfT2 (EmbA_KD or GlfT2_KD), which are separately involved in the biosynthesis of AG arabinose and galactose domains. High-performance gel permeation chromatography and high-performance liquid chromatography assays confirmed that the EmbA_KD strain showed a remarkable decrease in AG content with fragmentary arabinose chains, and the GlfT2_KD strain displayed less reduction in content with cut-down galactose chains. Based on transmission and scanning electron microscopy observations, the cell walls of the two mutants were found to be dramatically thickened, and the boundaries of different layers were more distinct. Phenotypes including the over-secretion of extracellular substances and enhanced spreading motility with a concomitant decreased resistance to ethambutol appeared in the EmbA_KD strain. The EmbA_KD and GlfT2_KD strains displayed limited intracellular proliferation after infecting murine J774A.1 macrophages. The disease progression infected with the EmbA_KD or GlfT2_KD strain significantly slowed down in zebrafish/murine tail infection models as well. Through transcriptome profiling, macrophages infected by EmbA_KD/GlfT2_KD strains showed enhanced oxidative metabolism. The cell survival measured using the CCK8 assay of macrophages exposed to the EmbA_KD strain was upregulated and consistent with the pathway enrichment analysis of differentially expressed genes in terms of cell cycle/apoptosis. The overexpression of C/EBPβ and the increasing secretion of proinflammatory cytokines were validated in the macrophages infected by the EmbA_KD mutant. In conclusion, the AG of Mycobacterium appears to restrain the host innate immune responses to enhance intracellular proliferation by interfering with oxidative metabolism and causing macrophage death. The arabinose chains of AG influence the Mycobacterium virulence and pathogenicity to a greater extent.

Cell Envelope Proteomics of Mycobacteria

The World Health Organization (WHO) estimates that Mycobacterium tuberculosis, the most pathogenic mycobacterium species to humans, has infected up to a quarter of the world's population, with the occurrence of multidrug-resistant strains on the rise. Research into the detailed composition of the cell envelope proteome in mycobacteria over the last 20 years has formed a key part of the efforts to understand host-pathogen interactions and to control the current tuberculosis epidemic. This is due to the great importance of the cell envelope proteome during infection and during the development of antibiotic resistance as well as the search of surface-exposed proteins that could be targeted by therapeutics and vaccines. A variety of experimental approaches and mycobacterial species have been used in proteomic studies thus far. Here we provide for the first time an extensive summary of the different approaches to isolate the mycobacterial cell envelope, highlight some of the limitations of the studies performed thus far, and comment on how the recent advances in membrane proteomics in other fields might be translated into the field of mycobacteria to provide deeper coverage.

L-Alanine specifically potentiates fluoroquinolone efficacy against Mycobacterium persisters via increased intracellular reactive oxygen species

Tuberculosis caused by Mycobacterium tuberculosis remains a major global health concern; M. tuberculosis drug resistance and persistence further fueled the situation. Nutrient supportive therapy was intensively pursued to complement the conventional treatment, as well as their synergy with current antibiotics. To explore whether L-alanine can synergize with fluoroquinolones against M. tuberculosis, M. smegmatis was used as a surrogate in this study. We found that L-alanine can boost the bactericidal efficacy of fluoroquinolones, increasing the production of intracellular reactive oxygen species. This effect is very significant for persisters. Accelerated tricarboxylic acid cycle and/or nucleotide metabolism were observed after the addition of L-alanine. M. smegmatis MSMEG2660 is a homolog of the alanine dehydrogenase (Rv2780, MSMEG2659) negative regulator Rv2779c and involved in the L-alanine potentiation of fluoroquinolone via funneling more alanine into tricarboxylic acid. Deletion mutant of the MSMEG2660 (∆Ms2660) became more susceptible, and more readily revived from persistence. We firstly found that L-alanine can synergize with fluoroquinolones against Mycobacterium, especially the persisters via promoting metabolism. This will inspire new avenue to eliminate Mycobacterium persisters.

Breaking down walls

A better understanding of the mechanisms underpinning the growth of mycobacteria could help identify targets for new antibiotics.