A bug's life in the granuloma

Conditional protein splicing of the Mycobacterium tuberculosis RecA intein in its native host

The recA gene, encoding Recombinase A (RecA) is one of three Mycobacterium tuberculosis (Mtb) genes encoding an in-frame intervening protein sequence (intein) that must splice out of precursor host protein to produce functional protein. Ongoing debate about whether inteins function solely as selfish genetic elements or benefit their host cells requires understanding of interplay between inteins and their hosts. We measured environmental effects on native RecA intein splicing within Mtb using a combination of western blots and promoter reporter assays. RecA splicing was stimulated in bacteria exposed to DNA damaging agents or by treatment with copper in hypoxic, but not normoxic, conditions. Spliced RecA was processed by the Mtb proteasome, while free intein was degraded efficiently by other unknown mechanisms. Unspliced precursor protein was not observed within Mtb despite its accumulation during ectopic expression of Mtb recA within E. coli. Surprisingly, Mtb produced free N-extein in some conditions, and ectopic expression of Mtb N-extein activated LexA in E. coli. These results demonstrate that the bacterial environment greatly impacts RecA splicing in Mtb, underscoring the importance of studying intein splicing in native host environments and raising the exciting possibility of intein splicing as a novel regulatory mechanism in Mtb.

Host-directed therapy against mycobacterium tuberculosis infections with diabetes mellitus

Tuberculosis (TB) is caused by the bacterial pathogen Mycobacterium tuberculosis (MTB) and is one of the principal reasons for mortality and morbidity worldwide. Currently, recommended anti-tuberculosis drugs include isoniazid, rifampicin, ethambutol, and pyrazinamide. TB treatment is lengthy and inflicted with severe side-effects, including reduced patient compliance with treatment and promotion of drug-resistant strains. TB is also prone to other concomitant diseases such as diabetes and HIV. These drug-resistant and complex co-morbid characteristics increase the complexity of treating MTB. Host-directed therapy (HDT), which effectively eliminates MTB and minimizes inflammatory tissue damage, primarily by targeting the immune system, is currently an attractive complementary approach. The drugs used for HDT are repositioned drugs in actual clinical practice with relative safety and efficacy assurance. HDT is a potentially effective therapeutic intervention for the treatment of MTB and diabetic MTB, and can compensate for the shortcomings of current TB therapies, including the reduction of drug resistance and modulation of immune response. Here, we summarize the state-of-the-art roles and mechanisms of HDT in immune modulation and treatment of MTB, with a special focus on the role of HDT in diabetic MTB, to emphasize the potential of HDT in controlling MTB infection.

Inhibition of host PARP1 contributes to the anti-inflammatory and antitubercular activity of pyrazinamide

The antibiotic pyrazinamide (PZA) is a cornerstone of tuberculosis (TB) therapy that shortens treatment durations by several months despite being only weakly bactericidal. Intriguingly, PZA is also an anti-inflammatory molecule shown to specifically reduce inflammatory cytokine signaling and lesion activity in TB patients. However, the target and clinical importance of PZA's host-directed activity during TB therapy remain unclear. Here, we identify the host enzyme Poly(ADP-ribose) Polymerase 1 (PARP1), a pro-inflammatory master regulator strongly activated in TB, as a functionally relevant host target of PZA. We show that PZA inhibits PARP1 enzymatic activity in macrophages and in mice where it reverses TB-induced PARP1 activity in lungs to uninfected levels. Utilizing a PZA-resistant mutant, we demonstrate that PZA's immune-modulatory effects are PARP1-dependent but independent of its bactericidal activity. Importantly, PZA's bactericidal efficacy is impaired in PARP1-deficient mice, suggesting that immune modulation may be an integral component of PZA's antitubercular activity. In addition, adjunctive PARP1 inhibition dramatically reduces inflammation and lesion size in mice and may be a means to reduce lung damage and shorten TB treatment duration. Together, these findings provide insight into PZA's mechanism of action and the therapeutic potential of PARP1 inhibition in the treatment of TB.

Surviving Pulmonary Tuberculosis: Navigating the Long Term Respiratory Effects

Tuberculosis is a transmissible disease caused by the bacteria Mycobacterium tuberculosis, which is a cause of significant morbidity and mortality all over the world. Tuberculosis has a number of risk factors, such as living in a developing country, poor ventilation, smoking, male sex, etc., which not only increase the chance of infection but may be independent factors for impairment in pulmonary function as well. In this review article, we have compiled several studies to learn how tuberculosis causes impaired lung function and further explored the long-term effects of tuberculosis on the same. We studied tuberculosis's effect on the lungs even after appropriate treatment and its relationship with obstructive and restrictive lung disorders. A significant relationship exists between chronic respiratory disorders and tuberculosis even after treatment; hence, we believe prevention is far superior to cure.

Structures of biological heme-based sensors of oxygen

Since their initial discovery some 30 years ago, heme-based O₂ sensors have been extensively studied. Among many other lessons, we have learned that they have adapted a wide variety of folds to bind heme for O₂ sensing, and they can couple those sensory domains to transducer domains with many different activities. There is no question that we have learned a great deal about those systems by solving X-ray structures of the truncated pieces of larger multi-domain proteins. All of the studies have, for example, hinted at the importance of protein residues, which were further investigated, usually by site-directed mutagenesis of the full-length proteins together with physico-chemical measurements and enzymatic studies. The biochemistry has suggested that the sensing functions of heme-based O₂ sensors involve not only the entire proteins but also, and quite often, their associated regulatory partners and targets. Here we critically examine the state of knowledge for some well-studied sensors and discuss outstanding questions regarding their structures. For the near future, we may foresee many large complexes with sensor proteins being solved by cryo-EM, to enhance our understanding of their mechanisms.

Serum biomarkers in patients with unilateral or bilateral active pulmonary tuberculosis: Immunological networks and promising diagnostic applications

The present observational study was designed to characterize the integrative profile of serum soluble mediators to describe the immunological networks associated with clinical findings and identify putative biomarkers for diagnosis and prognosis of active tuberculosis. The study population comprises 163 volunteers, including 84 patients with active pulmonary tuberculosis/(TB), and 79 controls/(C). Soluble mediators were measured by multiplexed assay. Data analysis demonstrated that the levels of CCL3, CCL5, CXCL10, IL-1β, IL-6, IFN-γ, IL-1Ra, IL-4, IL-10, PDGF, VEGF, G-CSF, IL-7 were increased in TB as compared to C. Patients with bilateral pulmonary involvement/(TB-BI) exhibited higher levels of CXCL8, IL-6 and TNF with distinct biomarker signatures (CCL11, CCL2, TNF and IL-10) as compared to patients with unilateral infiltrates/(TB-UNI). Analysis of biomarker networks based in correlation power graph demonstrated small number of strong connections in TB and TB-BI. The search for biomarkers with relevant implications to understand the pathogenetic mechanisms and useful as complementary diagnosis tool of active TB pointed out the excellent performance of single analysis of IL-6 or CXCL10 and the stepwise combination of IL-6 → CXCL10 (Accuracy = 84 %; 80 % and 88 %, respectively). Together, our finding demonstrated that immunological networks of serum soluble biomarkers in TB patients differ according to the unilateral or bilateral pulmonary involvement and may have relevant implications to understand the pathogenetic mechanisms involved in the clinical outcome of Mtb infection.

Immunologic and imaging signatures in post tuberculosis lung disease

Post Tuberculosis Lung Disease (PTLD) affects millions of tuberculosis survivors and is a global health burden. The immune mechanisms that drive PTLD are complex and have historically been under investigated. Here, we discuss two immune-mediated paradigms that could drive human PTLD. We review the characteristics of a fibrotic granuloma that favors the development of PTLD via an abundance of T-helper-2 and T-regulatory cells and an upregulation of TGF-β mediated collagen deposition. Next, we discuss the post-primary tuberculosis paradigm and the complex mixture of caseous pneumonia, cavity formation and fibrosis that can also lead to PTLD. We review the delicate balance between cellular subsets and cytokines of the innate and adaptive immune system in conjunction with host-derived proteases that can perpetuate the parenchymal lung damage seen in PTLD. Next, we discuss the role of novel host directed therapies (HDT) to limit the development of PTLD and in particular, the recent repurposing of established medications such as statins, metformin and doxycycline. Finally, we review the emerging role of novel imaging techniques as a non-invasive modality for the early recognition of PTLD. While access to computed tomography imaging is unlikely to be available widely in countries with a high TB burden, its use in research settings can help phenotype PTLD. Due to a lack of disease-specific biomarkers and controlled clinical trials, there are currently no evidence-based recommendations for the management of PTLD. It is likely that an integrated antifibrotic strategy that could simultaneously target inflammatory and pro-fibrotic pathways will probably emerge as a successful way to treat this complex condition. In a disease spectrum as wide as PTLD, a single immunologic or radiographic marker may not be sufficient and a combination is more likely to be a successful surrogate that could aid in the development of successful HDTs.

Recombinant Human Lactoferrin Reduces Inflammation and Increases Fluoroquinolone Penetration to Primary Granulomas During Mycobacterial Infection of C57Bl/6 Mice

Infection with Mycobacterium tuberculosis (Mtb) results in the primary formation of a densely packed inflammatory foci that limits entry of therapeutic agents into pulmonary sites where organisms reside. No current therapeutic regimens exist that modulate host immune responses to permit increased drug penetration to regions of pathological damage during tuberculosis disease. Lactoferrin is a natural iron-binding protein previously demonstrated to modulate inflammation and granuloma cohesiveness, while maintaining control of pathogenic burden. Studies were designed to examine recombinant human lactoferrin (rHLF) to modulate histological progression of Mtb-induced pathology in a non-necrotic model using C57Bl/6 mice. The rHLF was oral administered at times corresponding to initiation of primary granulomatous response, or during granuloma maintenance. Treatment with rHLF demonstrated significant reduction in size of primary inflammatory foci following Mtb challenge, and permitted penetration of ofloxacin fluoroquinolone therapeutic to sites of pathological disruption where activated (foamy) macrophages reside. Increased drug penetration was accompanied by retention of endothelial cell integrity. Immunohistochemistry revealed altered patterns of M1-like and M2-like phenotypic cell localization post infectious challenge, with increased presence of M2-like markers found evenly distributed throughout regions of pulmonary inflammatory foci in rHLF-treated mice.

The H2O2 inherently released by the mycobacterial minor subpopulation enhances the survival of the major kin subpopulation against rifampicin

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Stress survival of mycobacterial minor (SCs) and major (NCs) subpopulations.

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The SCs enhance survival of the NCs against rifampicin.

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H₂O₂ released by the SCs increased KatG levels in the NCs.

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Increased KatG levels neutralised the H₂O₂ formed during rifampicin exposure.

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The enhanced survival was not observed in the furA-katG/katG knockout mutants.

Exposure to antibiotics most often generates oxidative stress in bacteria. Oxidative stress survival mechanisms would facilitate the evolution of antibiotic resistance. As part of an effort to understand oxidative stress survival mechanisms in mycobacteria, here we show that the minor subpopulation (SCs; short-sized cells constituting 10% of the population) of Mycobacterium smegmatis significantly increased the survival of its major kin subpopulation (NCs; normal/long-sized cells constituting 90% of the population) in the mid-log-phase (MLP) cultures against the oxidative stress induced by rifampicin and exogenously added H₂O₂ (positive control). We had earlier shown that the SCs in the MLP cultures inherently and naturally release significantly high levels of H₂O₂ into the medium. Addition of the SCs’ culture supernatant, unlike the supernatant of the dimethylthiourea (H₂O₂ scavenger) exposed SCs, enhanced the survival of NCs. It indicated that NCs’ survival required the H₂O₂ present in the SCs’ supernatant. This H₂O₂ transcriptionally induced high levels of catalase-peroxidase (KatG) in the NCs. The naturally high KatG levels in the NCs significantly neutralised the endogenous H₂O₂ formed upon exposure to rifampicin or H₂O₂, thereby enhancing the survival of NCs against oxidative stress. The absence of such enhanced survival in the furA-katG and katG knockout (KO) mutants of NCs in the presence of wild-type SCs, confirmed the requirement of the H₂O₂ present in the SCs’ supernatant and NCs’ KatG for enhanced oxidative stress survival. The presence of SCs:NCs at 1:9 in the pulmonary tuberculosis patients’ sputum alludes to the clinical significance of the finding.

Microfluidics as a Novel Technique for Tuberculosis: From Diagnostics to Drug Discovery

Tuberculosis (TB) remains a global healthcare crisis, with an estimated 5.8 million new cases and 1.5 million deaths in 2020. TB is caused by infection with the major human pathogen Mycobacterium tuberculosis, which is difficult to rapidly diagnose and treat. There is an urgent need for new methods of diagnosis, sufficient in vitro models that capably mimic all physiological conditions of the infection, and high-throughput drug screening platforms. Microfluidic-based techniques provide single-cell analysis which reduces experimental time and the cost of reagents, and have been extremely useful for gaining insight into monitoring microorganisms. This review outlines the field of microfluidics and discusses the use of this novel technique so far in M. tuberculosis diagnostics, research methods, and drug discovery platforms. The practices of microfluidics have promising future applications for diagnosing and treating TB.

A bioinorganic chemistry perspective on the roles of metals as drugs and targets against Mycobacterium tuberculosis - a journey of opportunities

Medicinal inorganic chemists have provided many strategies to tackle a myriad of diseases, pushing forward the frontiers of pharmacology. As an example, the fight against tuberculosis (TB), an infectious bacterial disease, has led to the development of metal-based compounds as potential drugs. This disease remains a current health issue causing over 1.4 million of deaths per year. The emergence of multi- (MDR) and extensively-drug resistant (XDR) Mycobacterium tuberculosis (Mtb) strains along with a long dormancy process, place major challenges in developing new therapeutic compounds. Isoniazid is a front-line prodrug used against TB with appealing features for coordination chemists, which have been explored in a series of cases reported here. An isoniazid iron-based compound, called IQG-607, has caught our attention, whose in vitro and in vivo studies are advanced and thoroughly discussed, along with other metal complexes. Isoniazid is inactive against dormant Mtb, a hard to eliminate state of this bacillus, found in one-fourth of the world's population and directly implicated in the lengthy treatment of TB (ca. 6 months). Thus, our understanding of this phenomenon may lead to a rational design of new drugs. Along these lines, we describe how metals as targets can cross paths with metals used as selective therapeutics, where we mainly review heme-based sensors, DevS and DosT, as a key system in the Mtb dormancy process and a current drug target. Overall, we report new opportunities for bioinorganic chemists to tackle this longstanding and current threat.

Immune Response and Apoptosis-Related Pathways Induced by Aeromonas schubertii Infection of Hybrid Snakehead (Channa maculata♀ × Channa argus♂)

Aeromonas schubertii is the etiological pathogen of internal organ nodules in snakehead fish. Infections with A. schubertii produce a significant economic loss in aquaculture. Therefore, it is important to examine the immune mechanisms by which snakeheads defend against A. schubertii infection. In this study, we established a hybrid snakehead infection model by intraperitoneal injection of A. schubertii that produced internal organ nodules. The splenic immune response of infected fish was examined at the transcriptome level by Illumina-seq analysis. Results showed 14,796 differentially expressed genes (DEGs) following A. schubertii infection, including 4441 up-regulated unigenes and 10,355 down-regulated unigenes. KEGG analysis showed 2084 DEGs to be involved in 192 pathways, 14 of which were immune-related. Twelve DEGs were used to validate quantitative real-time PCR results with RNA-seq data. Time-course expression analysis of six genes demonstrated modulation of the snakehead immune response by A. schubertii. Furthermore, transcriptome analysis identified a substantial number of DEGs that were involved in the apoptosis signaling pathway. TUNEL analysis of infected spleens confirmed the presence of apoptotic cells. This study provided new information for a further understanding of the pathogenesis of A. schubertii in snakeheads, which can be used to prevent and possibly treat A. schubertii infections.

Histological and immunohistochemical features suggesting aetiological differences in lymph node and (muco)cutaneous feline tuberculosis lesions

OBJECTIVES

To identify and describe histological and immunohistochemical criteria that may differentiate between skin and lymph node lesions associated with Mycobacterium (M.) bovis and M. microti in a diagnostic pathology setting.

MATERIALS AND METHODS

Archived skin and lymph node biopsies of tuberculous lesions were stained with haematoxylin and eosin, Ziehl-Neelsen and Masson's Trichrome. Immunohistochemistry was performed to detect the expression of calprotectin, CD3 and Pax5. Samples were scored for histological parameters (i.e. granulomas with central necrosis versus small granulomas without central necrosis, percentage necrosis and/or multinucleated giant cells), number of acid-fast bacilli (bacterial index) and lesion percentage of fibrosis and positive immunohistochemical staining.

RESULTS

Twenty-two samples were examined (M. bovis n=11, M. microti n=11). When controlling for age, gender and tissue, feline M. bovis-associated lesions more often featured large multi-layered granulomas with central necrosis. Conversely, this presentation was infrequent in feline M. microti-associated lesions, where small granulomas without central necrosis predominated. The presence of an outer fibrous capsule was variable in both groups, as was the bacterial index. There were no differences in intralesional expression of immunohistochemical markers.

CLINICAL SIGNIFICANCE

Differences in the histological appearance of skin and lymph node lesions may help to infer feline infection with either M. bovis or M. microti at an earlier stage when investigating these cases, informing clinicians of the potential zoonotic risk. Importantly, cases of tuberculosis can present with numerous acid-fast bacilli. This implies that a high bacterial index does not infer infection with non-zoonotic non-tuberculous mycobacteria.

The pursuit of mechanism of action: uncovering drug complexity in TB drug discovery

Whole cell-based phenotypic screens have become the primary mode of hit generation in tuberculosis (TB) drug discovery during the last two decades. Different drug screening models have been developed to mirror the complexity of TB disease in the laboratory. As these culture conditions are becoming more and more sophisticated, unraveling the drug target and the identification of the mechanism of action (MOA) of compounds of interest have additionally become more challenging. A good understanding of MOA is essential for the successful delivery of drug candidates for TB treatment due to the high level of complexity in the interactions between Mycobacterium tuberculosis (Mtb) and the TB drug used to treat the disease. There is no single "standard" protocol to follow and no single approach that is sufficient to fully investigate how a drug restrains Mtb. However, with the recent advancements in -omics technologies, there are multiple strategies that have been developed generally in the field of drug discovery that have been adapted to comprehensively characterize the MOAs of TB drugs in the laboratory. These approaches have led to the successful development of preclinical TB drug candidates, and to a better understanding of the pathogenesis of Mtb infection. In this review, we describe a plethora of efforts based upon genetic, metabolomic, biochemical, and computational approaches to investigate TB drug MOAs. We assess these different platforms for their strengths and limitations in TB drug MOA elucidation in the context of Mtb pathogenesis. With an emphasis on the essentiality of MOA identification, we outline the unmet needs in delivering TB drug candidates and provide direction for further TB drug discovery.

Mycobacterial biofilms as players in human infections: a review

The role of biofilms in pathogenicity and treatment strategies is often neglected in mycobacterial infections. In recent years, the emergence of nontuberculous mycobacterial infections has necessitated the development of novel prophylactic strategies and elucidation of the mechanisms underlying the establishment of chronic infections. More importantly, the question arises whether members of the Mycobacterium tuberculosis complex can form biofilms and contribute to latent tuberculosis and drug resistance because of the long-lasting and recalcitrant nature of its infections. This review discusses some of the molecular mechanisms by which biofilms could play a role in infection or pathological events in humans.

Changing our view of the Schistosoma granuloma to an ecological standpoint

Schistosomiasis, a neglected parasitic tropical disease that has plagued humans for centuries, remains a major public health burden. A primary challenge to understanding schistosomiasis is deciphering the most remarkable pathological feature of this disease, the granuloma - a highly dynamic and self-organized structure formed by both host and parasite components. Granulomas are considered a remarkable example of how parasites evolved with their hosts to establish complex and intimate associations. However, much remains unclear regarding life within the granuloma, and strategies to restrain its development are still lacking. Here we explore current information on the hepatic Schistosoma mansoni granuloma in the light of Ecology and propose that this intricate structure acts as a real ecosystem. The schistosomal granuloma is formed by cells (biotic component), protein scaffolds, fibres, and chemical compounds (abiotic components) with inputs/outputs of energy and matter, as complex as in classical ecosystems. We review the distinct cell populations ('species') within the granuloma and examine how they integrate with each other and interact with their microenvironment to form a multifaceted cell community in different space-time frames. The colonization of the hepatic tissue to form granulomas is explained from the point of view of an ecological succession whereby a community is able to modify its physical environment, creating conditions and resources for ecosystem construction. Remarkably, the granuloma represents a dynamic evolutionary system that undergoes progressive changes in the 'species' that compose its community over time. In line with ecological concepts, we examine the granuloma not only as a place where a community of cells is settled (spatial niche or habitat) but also as a site in which the functional activities of these combined populations occur in an orchestrated way in response to microenvironmental gradients such as cytokines and egg antigens. Finally, we assert how the levels of organization of cellular components in a granuloma as conventionally defined by Cell Biology can fit perfectly into a hierarchical structure of biological systems as defined by Ecology. By rethinking the granuloma as an integrating and evolving ecosystem, we draw attention to the inner workings of this structure that are central to the understanding of schistosomiasis and could guide its future treatment.

Drug Susceptibility Screening Using In Vitro Models of Hypoxic Non-Replicating Persistent Mycobacteria

Non-replicating persistence (NRP) is a functional adaptation that mycobacteria undergo in response to the stresses of the granuloma, facilitating antibiotic tolerance and long-term infection. These stresses, or NRP-inducing factors, include hypoxia, nutrient deprivation, and nitric oxide assault, which mycobacteria are well evolved to tolerate through a series of metabolic and physiological adaptations producing the NRP state. Most attempts to replicate these conditions in vitro have focused on only one of these factors at a time for ease and simplicity, but as a result, do not necessarily produce physiologically relevant phenotypes. Here, we provide the methods for two different in vitro NRP strategies that are useful for drug susceptibility testing and high-throughput screening.

Analysis of the Efficiency of Different Antituberculous Drugs and Approaches to Treat BCG-Induced Granulomatosis in Mice and Abundance and Localization of Mycobacterium tuberculosis in the Liver

In 3 months after infection with Mycobacterium tuberculosis (MBT) from BCG vaccine, male BALB/с mice received intraperitoneal injections of isonicotinic acid hydrazide, dextrazide, or liposome-encapsulated dextrazide, or inhalation of liposome-encapsulated dextrazide 2 times a week for 6 months. In 6 months, no MBT were detected in macrophages outside granulomas in treated mice. Macrophages containing MBT can incorporate into granulomas and leave them after suppression of MBT persistence. Liposome-encapsulated dextrazide showed the maximum therapeutic efficiency: the total MBT level in granuloma macrophages and volume density of destruction foci in the liver parenchyma decreased by 5.1 and 5.3 times, respectively, in comparison with the corresponding parameters in mice treated with isonicotinic acid hydrazide. Inhalations of liposome-encapsulated dextrazide prevented the destructive processes in liver granulomas due to macrophage migration from granulomas, which reduced granuloma sizes and destructive potential of granuloma lysosomes and therefore improved their diffusion-dependent trophics.

Asap1 Affects the Susceptibility of Zebrafish to Mycobacterium by Regulating Macrophage Migration

The ADP ribosylation factor (ARF) GTPase activation protein ASAP1 possesses multiple biological functions, including regulation of cytoskeletal dynamics, small GTP-binding protein receptor recycling, and intracellular vesicle trafficking. Recently, ASAP1 polymorphisms have been reported to be associated with human susceptibility to tuberculosis (TB) according to a large-scale genome-wide association study (GWAS); ASAP1 expression affects dendritic cell migration, which may be involved in TB predisposition. However, it remains unclear whether ASAP1 affects TB in vivo. To address this issue, we used zebrafish as a model system to examine the effects of Asap1 against Mycobacterium marinum, an organism closely related to Mycobacterium tuberculosis. Two zebrafish asap1 homologs (asap1a and asap1b) were identified and characterized. By morpholino knockdown of asap1a and asap1b as a whole, we found that the asap1 morphants showed a higher mycobacterial load than the controls, which was almost rescued by injecting asap1 mRNA that confers resistance to mycobacterial infection. These Asap1-depleted zebrafish also exhibited decreased macrophage migration in response to tail injury or upon infection with M. marinum in the hindbrain ventricle, which was also proved in THP1-derived macrophages of knockdown ASAP1. Together, these findings represent a new perspective on the role of Asap1 in resistance to mycobacterial infection.

Potential therapeutic approaches for a sleeping pathogen: tuberculosis a case for bioinorganic chemistry

Mycobacterium tuberculosis (Mtb) has an old history as a human pathogen and still kills over one million people every year. One key feature of this bacterium is its dormancy: a phenomenon responsible for major changes in its metabolism and replication that have been associated with the need for a lengthy therapy for Mtb. This process is regulated by key heme-based sensors, particularly DosT and DevS (DosS), among other co-regulators, and also linked to nitrogen utilization (nitrate/nitrite) and stringent responses. In face of the current threat of tuberculosis, there is an urgent need to develop new therapeutic agents capable of targeting the dormant state, associated with the need for a lengthy therapy. Interestingly, many of those key proteins are indeed metallo-containing or metallo-dependent biomolecules, opening exciting bioinorganic opportunities. Here, we critically reviewed a series of small molecules targeting key proteins involved in these processes, including DosT/DevS/DevR, RegX3, MprA, MtrA, NarL, PknB, Rel, PPK, nitrate and nitrite reductases, GlnA1, aiming for new opportunities and alternative therapies. In the battle against Mycobacterium tuberculosis, new drug targets must be searched, in particular  those involved in dormancy. A series of exciting cases for drug development involving metallo-containing or metallo-dependent biomolecules are reviewed, opening great opportunities for the bioinorganic chemistry community.

A spectroelectrochemical investigation of the heme-based sensor DevS from Mycobacterium tuberculosis: a redox versus oxygen sensor

Tuberculosis is one of the oldest known infectious diseases, responsible for millions of deaths annually around the world. The ability of Mycobacterium tuberculosis (Mtb) to enter into a dormant state has been considered integral to the success of this bacterium as a human pathogen. One of the key systems involved in regulating the entrance into dormancy is the differentially expressed in virulent strain sensor protein (DevS) [(dormancy survival sensor protein (DosS)]. However, the physiological signal for DevS has remained unclear since it was first shown to be a heme-based sensor with conflicting reports on whether it is a redox or an oxygen sensor. To address this question and provide a better understanding of the electronic properties of this protein, we present here, for the first time, a series of spectroelectrochemistry measurements of the full-length holo DevS in anaerobic conditions as well as bound to CO, NO, imidazole (Imz), cyanide, and O₂ . An interesting feature of this protein is its ability to bind Imz even in the ferrous state, implying small-molecule analogues could be designed as potential regulators. Nonetheless, a midpoint potential (E_m ) value of +10 mV [vs normal hydrogen electrode (NHE)] for DevS as measured under anaerobic conditions is much higher than the expected cytosolic potential for Mtb or even within stimulated macrophages (~ -270 mV vs NHE), indicating this sensor works in a reduced ferrous state. These data, along with the high oxygen affinity and very slow auto-oxidation rate of DevS, provides evidence that it is not a redox sensor. Overall, this study validates the biological function of DevS as an oxygen sensor directly involved in the dormancy/latency of Mtb.

Comparing the Metabolic Capabilities of Bacteria in the Mycobacterium tuberculosis Complex

Pathogenic mycobacteria are known for their ability to maintain persistent infections in various mammals. The canonical pathogen in this genus is Mycobacterium tuberculosis and this bacterium is particularly successful at surviving and replicating within macrophages. Here, we will highlight the metabolic processes that M. tuberculosis employs during infection in macrophages and compare these findings with what is understood for other pathogens in the M. tuberculosis complex.

Tuberculosis and lung damage: from epidemiology to pathophysiology

A past history of pulmonary tuberculosis (TB) is a risk factor for long-term respiratory impairment. Post-TB lung dysfunction often goes unrecognised, despite its relatively high prevalence and its association with reduced quality of life. Importantly, specific host and pathogen factors causing lung impairment remain unclear. Host immune responses probably play a dominant role in lung damage, as excessive inflammation and elevated expression of lung matrix-degrading proteases are common during TB. Variability in host genes that modulate these immune responses may determine the severity of lung impairment, but this hypothesis remains largely untested. In this review, we provide an overview of the epidemiological literature on post-TB lung impairment and link it to data on the pathogenesis of lung injury from the perspective of dysregulated immune responses and immunogenetics.

Host factors driving lung injury in TB likely contribute to variable patterns of pulmonary impairment after TBhttp://ow.ly/a3of30hBsxB

c-Abl-TWIST1 Epigenetically Dysregulate Inflammatory Responses during Mycobacterial Infection by Co-Regulating Bone Morphogenesis Protein and miR27a

Mycobacteria propelled modulation of host responses is of considerable interest in the face of emerging drug resistance. Although it is known that Abl tyrosine kinases affect entry and persistence of mycobacteria, mechanisms that couple c-Abl to proximal signaling pathways during immunity are poorly understood. Loss-of-function of c-Abl through Imatinib, in a mouse model of tuberculosis or RNA interference, identified bone morphogenesis protein (BMP) signaling as its cellular target. We demonstrate that c-Abl promotes mycobacterial survival through epigenetic modification brought about by KAT5-TWIST1 at Bmp loci. c-Abl-BMP signaling deregulated iNOS, aggravating the inflammatory balance. Interestingly, BMP signaling was observed to have far-reaching effects on host immunity, as it attenuated TLR3 pathway by engaging miR27a. Significantly, these events were largely mediated via WhiB3 and DosR/S/T but not SecA signaling pathway of mycobacteria. Our findings suggest molecular mechanisms of host pathways hijacked by mycobacteria and expand our understanding of c-Abl inhibitors in potentiating innate immune responses.

Pathology of the Emerging Mycobacterium tuberculosis Complex Pathogen, Mycobacterium mungi, in the Banded Mongoose ( Mungos mungo)

Wild banded mongooses ( Mungos mungo) in northeastern Botswana and northwest Zimbabwe are infected with a novel Mycobacterium tuberculosis complex (MTC) pathogen, Mycobacterium mungi. We evaluated gross and histologic lesions in 62 infected mongooses (1999-2017). Many tissues contained multifocal irregular, lymphohistiocytic to granulomatous infiltrates and/or multifocal or coalescing noncaseating to caseating granulomas with variable numbers of intralesional acid-fast bacilli. Over one-third of nasal turbinates examined had submucosal lymphohistiocytic to granulomatous infiltrates, erosion and ulceration of the nasal mucosa, bony remodeling, and nasal distortion. Similar inflammatory cell infiltrates expanded the dermis of the nasal planum with frequent ulceration. However, even in cases with intact epidermis, acid-fast bacilli were present in variable numbers among dermal infiltrates and on the epidermal surface among desquamated cells and debris, most commonly in small crevices or folds. In general, tissue involvement varied among cases but was highest in lymph nodes (50/54, 93%), liver (39/53, 74%), spleen (37/51, 73%), and anal glands/sacs (6/8, 75%). Pulmonary lesions were present in 67% of sampled mongooses (35/52) but only in advanced disseminated disease. The pathological presentation of M. mungi in the banded mongoose is consistent with pathogen shedding occurring through scent-marking behaviors (urine and anal gland secretions) with new infections arising from contact with these contaminated olfactory secretions and percutaneous movement of the pathogen through breaks in the skin, nasal planum, and/or skin of the snout. Given the character and distribution of lesions and the presence of intracellular acid-fast bacilli, we hypothesize that pathogen spread occurs within the body through a hematogenous and/or lymphatic route. Features of prototypical granulomas such as multinucleated giant cells and peripheral fibrosis were rarely present in affected mongooses. Acid-fast bacilli were consistently found intracellularly, even in regions of necrosis. The mongoose genome has a unique deletion (RD1^mon) that includes part of the encoding region for PPE68 (Rv3873), a gene co-operonic with PE35. These proteins can influence the host's cellular immune response to mycobacterial infections, and it remains uncertain how this deletion might contribute to observed patterns of pathology. M. mungi infection in banded mongooses is characterized by both a unique transmission and exposure route, as well as accompanying pathological features, providing an opportunity to increase our understanding of MTC pathogenesis across host-pathogen systems.

Target DNA stabilizes Mycobacterium tuberculosis DevR/DosR phosphorylation by the full-length oxygen sensors DevS/DosS and DosT

Mycobacterium tuberculosis strongly relies on a latency, or nonreplicating persistence, to escape a human host's immune system. The DevR (DosR), DevS (DosS), and DosT proteins are key components of this process. Like the rhizobial FixL oxygen sensor, DevS and DosT are histidine protein kinases with a heme-binding domain. Like the FixJ partner and substrate of FixL, DevR is a classical response regulator of the two-component class. When activated by DevS or DosT during hypoxia in vivo, DevR induces a dormancy regulon of more than 40 genes. To investigate the contributions of DevS, DosT, and target DNA to the phosphorylation of DevR, we developed an in vitro assay in which the full-length, sensing, DevS and DosT proteins were used to phosphorylate DevR with ATP, in the presence of target DNAs that were introduced as oligonucleotides linked to magnetic nanoparticles. We found that the DevR phosphorylations proceeded only for the deoxy states of the sensors. The reaction was strongly inhibited by O₂ , but not CO or NO. The production of phospho-DevR was enhanced sixfold by target consensus DNA or acr-DNA. The phospho-DevR bound tightly to that DNA (K_d ~ 0.8 nm toward acr-DNA), and it was only slightly displaced by a 200-fold excess of unphosphorylated DevR or of a truncated DevR with only a DNA-binding domain. To our knowledge, this represents the first in vitro study of the ligand regulation of DevR phosphorylation by full-length DevS and DosT, and demonstration of a positive effect of DNA on this reaction.

Phenotypic Heterogeneity in Mycobacterium tuberculosis

The interaction between the host and the pathogen is extremely complex and is affected by anatomical, physiological, and immunological diversity in the microenvironments, leading to phenotypic diversity of the pathogen. Phenotypic heterogeneity, defined as nongenetic variation observed in individual members of a clonal population, can have beneficial consequences especially in fluctuating stressful environmental conditions. This is all the more relevant in infections caused by Mycobacterium tuberculosis wherein the pathogen is able to survive and often establish a lifelong persistent infection in the host. Recent studies in tuberculosis patients and in animal models have documented the heterogeneous and diverging trajectories of individual lesions within a single host. Since the fate of the individual lesions appears to be determined by the local tissue environment rather than systemic response of the host, studying this heterogeneity is very relevant to ensure better control and complete eradication of the pathogen from individual lesions. The heterogeneous microenvironments greatly enhance M. tuberculosis heterogeneity influencing the growth rates, metabolic potential, stress responses, drug susceptibility, and eventual lesion resolution. Single-cell approaches such as time-lapse microscopy using microfluidic devices allow us to address cell-to-cell variations that are often lost in population-average measurements. In this review, we focus on some of the factors that could be considered as drivers of phenotypic heterogeneity in M. tuberculosis as well as highlight some of the techniques that are useful in addressing this issue.

Host-directed therapies offer novel opportunities for the fight against tuberculosis

Tuberculosis (TB) remains a leading global health problem that is exacerbated by the emergence of multidrug and extensively drug-resistant Mycobacterium tuberculosis strains. Control of the disease requires novel therapeutic strategies. Modulating host homeostasis appears to be a promising approach, and recent studies have identified novel potential host targets and compounds that could be investigated for host-directed therapies (HDTs). Moreover, the recent development of intracellular high-throughput phenotypic assays makes it possible to screen large libraries of compounds to identify more rapidly new effectors for mycobacterial elimination. Technological advances combined with the novel HDT concept opens an interesting and promising research area that could ultimately deliver personalized TB treatment.

Evolutionary Aspects of Macrophages Polarization

Macrophages constitute a heterogeneous population of myeloid cells that are essential for maintaining homeostasis and as a first line of innate responders controlling and organizing host defenses against pathogens. Monocyte-macrophage lineage cells are among the most functionally diverse and plastic cells of the immune system. They undergo specific activation into functionally distinct phenotypes in response to immune signals and microbial products. In mammals, macrophage functional heterogeneity is defined by two activation states, M1 and M2, which represent two polar ends of a continuum exhibiting pro-inflammatory and tissue repair activities, respectively. While the ancient evolutionary origin of macrophages as phagocytic defenders is well established, the evolutionary roots of the specialized division of macrophages into subsets with polarized activation phenotypes is less well defined. Accordingly, this chapter focuses on recent advances in the understanding of the evolution of macrophage polarization and functional heterogeneity with a focus on ectothermic vertebrates.

Toll-like receptor 2 in host defense against Mycobacterium tuberculosis: to be or not to be-that is the question

Toll-like receptor (TLR) 2 is expressed on immune cells and respiratory epithelial cells lining the lung. TLR2 is not critical for protection during acute Mycobacterium tuberculosis (Mtb) infection but it has a significant multi-faceted role in containing chronic infection. This review highlights the contribution of TLR2 to host protection, immune evasion by Mtb and immune regulation during chronic Mtb infection. The TLR2-triggered pro-inflammatory cytokines initiate protective mechanisms and limit Mtb replication while the immune evasion pathways counterattack anti-bacterial effector mechanisms. The immune regulation pathways that are activated dampen TLR2 signaling. The combinatorial effect of these functional responses is persistence of Mtb with minimal immunopathology.