Mycolactone diffuses from Mycobacterium ulcerans-infected tissues and targets mononuclear cells in peripheral blood and lymphoid organs

Exploring Mycolactone-The Unique Causative Toxin of Buruli Ulcer: Biosynthetic, Synthetic Pathways, Biomarker for Diagnosis, and Therapeutic Potential

Mycolactone is a complex macrolide toxin produced by Mycobacterium ulcerans, the causative agent of Buruli ulcer. The aim of this paper is to review the chemistry, biosynthetic, and synthetic pathways of mycolactone A/B to help develop an understanding of the mode of action of these polyketides as well as their therapeutic potential. The synthetic work has largely been driven by the desire to afford researchers enough (≥100 mg) of the pure toxins for systematic biological studies toward understanding their very high biological activities. The review focuses on pioneering studies of Kishi which elaborate first-, second-, and third-generation approaches to the synthesis of mycolactones A/B. The three generations focused on the construction of the key intermediates required for the mycolactone synthesis. Synthesis of the first generation involves assignment of the relative and absolute stereochemistry of the mycolactones A and B. This was accomplished by employing a linear series of 17 chemical steps (1.3% overall yield) using the mycolactone core. The second generation significantly improved the first generation in three ways: (1) by optimizing the selection of protecting groups; (2) by removing needless protecting group adjustments; and (3) by enhancing the stereoselectivity and overall synthetic efficiency. Though the synthetic route to the mycolactone core was longer than the first generation, the overall yield was significantly higher (8.8%). The third-generation total synthesis was specifically aimed at an efficient, scalable, stereoselective, and shorter synthesis of mycolactone. The synthesis of the mycolactone core was achieved in 14 linear chemical steps with 19% overall yield. Furthermore, a modular synthetic approach where diverse analogues of mycolactone A/B were synthesized via a cascade of catalytic and/or asymmetric reactions as well as several Pd-catalyzed key steps coupled with hydroboration reactions were reviewed. In addition, the review discusses how mycolactone is employed in the diagnosis of Buruli ulcer with emphasis on detection methods of mass spectrometry, immunological assays, RNA aptamer techniques, and fluorescent-thin layer chromatography (f-TLC) methods as diagnostic tools. We examined studies of the structure-activity relationship (SAR) of various analogues of mycolactone. The paper highlights the multiple biological consequences associated with mycolactone such as skin ulceration, host immunomodulation, and analgesia. These effects are attributed to various proposed mechanisms of actions including Wiskott-Aldrich Syndrome protein (WASP)/neural Wiskott-Aldrich Syndrome protein (N-WASP) inhibition, Sec61 translocon inhibition, angiotensin II type 2 receptor (AT2R) inhibition, and inhibition of mTOR. The possible application of novel mycolactone analogues produced based on SAR investigations as therapeutic agents for the treatment of inflammatory disorders and inflammatory pain are discussed. Additionally, their therapeutic potential as anti-viral and anti-cancer agents have also been addressed.

Buruli ulcer in Australia: Evidence for a new endemic focus at Batemans Bay, New South Wales

We describe two locally acquired cases of Mycobacterium ulcerans infection (Buruli ulcer) in the town of Batemans Bay on the east coast of New South Wales (NSW), Australia, 150 km north of Eden, the only other place in NSW where Buruli ulcer has likely been locally acquired. Genomic analysis showed that the bacterial isolates from the cases were identical but belonged to a phylogenetically distinct M. ulcerans clade that was most closely related to the isolate from the earlier case in Eden to the south. It is proposed that Batemans Bay is a new endemic focus of human Buruli ulcer transmission.

Inflammasome-triggered IL-18 controls skin inflammation in the progression of Buruli ulcer

Buruli ulcer is an emerging chronic infectious skin disease caused by Mycobacterium ulcerans. Mycolactone, an exotoxin produced by the bacterium, is the only identified virulence factor so far, but the functions of this toxin and the mechanisms of disease progression remain unclear. By interfering Sec61 translocon, mycolactone inhibits the Sec61-dependent co-translational translocation of newly synthesized proteins, such as induced cytokines and immune cell receptors, into the endoplasmic reticulum. However, in regard to IL-1β, which is secreted by a Sec61-independent mechanism, mycolactone has been shown to induce IL-1β secretion via activation of inflammasomes. In this study, we clarified that cytokine induction, including that of IL-1β, in infected macrophages was suppressed by mycolactone produced by M. ulcerans subsp. shinshuense, despite the activation of caspase-1 through the inflammasome activation triggered in a manner independent of mycolactone. Intriguingly, mycolactone suppressed the expression of proIL-1β as well as TNF-α at the transcriptional level, suggesting that mycolactone of M. ulcerans subsp. shinshuense may exert additional inhibitory effect on proIL-1β expression. Remarkably, constitutively produced IL-18 was cleaved and mature IL-18 was actually released from macrophages infected with the causative mycobacterium. IL-18-deficient mice infected subcutaneously with M. ulcerans exhibited exacerbated skin inflammation during the course of disease progression. On the other hand, IL-1β controls bacterial multiplication in skin tissues. These results provide information regarding the mechanisms and functions of the induced cytokines in the pathology of Buruli ulcer.

Alternative boronic acids in the detection of Mycolactone A/B using the thin layer chromatography (f-TLC) method for diagnosis of Buruli ulcer

BACKGROUND

Mycobacterium ulcerans is the causative agent of Buruli ulcer. The pathology of M. ulcerans disease has been attributed to the secretion of a potent macrolide cytotoxin known as mycolactone which plays an important role in the virulence of the disease. Mycolactone is a biomarker for the diagnosis of BU that can be detected using the fluorescent-thin layer chromatography (f-TLC) technique. The technique relies on the chemical derivatization of mycolactone A/B with 2-naphthylboronic acid (BA) which acts as a fluorogenic chemosensor. However, background interferences due to co-extracted human tissue lipids, especially with clinical samples coupled with the subjectivity of the method call for an investigation to find an alternative to BA.

METHODS

Twenty-six commercially available arylboronic acids were initially screened as alternatives to BA using the f-TLC experiment. UV-vis measurements were also conducted to determine the absorption maximum spectra of mycolactone A/B and myco-boronic acid adducts followed by an investigation of the fluorescence-enhancing ability of the boronate ester formation between mycolactone A/B and our three most promising boronic acids (BA15, BA18, and BA21). LC-MS technique was employed to confirm the adduct formation between mycolactone and boronic acids. Furthermore, a comparative study was conducted between BA18 and BA using 6 Polymerase Chain Reaction (PCR) confirmed BU patient samples.

RESULTS

Three of the boronic acids (BA15, BA18, and BA21) produced fluorescent band intensities superior to BA. Complexation studies conducted on thin layer chromatography (TLC) using 0.1 M solution of the three boronic acids and various volumes of 10 ng/µL of synthetic mycolactone ranging from 1 µL - 9 µL corresponding to 10 ng - 90 ng gave similar results with myco-BA18 adduct emerging with the most visibly intense fluorescence bands. UV-vis absorption maxima (λ_max) for the free mycolactone A/B was observed at 362 nm, and the values for the adducts myco-BA15, myco-BA18, and myco-BA21 were at 272 nm, 270 nm, and 286 nm respectively. The comparable experimental λ_max of 362 nm for mycolactone A/B to the calculated Woodward-Fieser value of 367 nm for the fatty acid side chain of mycolactone A/B demonstrate that even though 2 cyclic boronates were formed, only the boronate of the southern side chain with the chromophore was excited by irradiation at 365 nm. Fluorescence experiments have demonstrated that coupling BA18 to mycolactone A/B along the 1,3-diols remarkably enhanced the fluorescence intensity at 537 nm. High-Resolution Mass Spectrometer (HR-MS) was used to confirm the formation of the myco-BA15 adduct. Finally, f-TLC analysis of patient samples with BA18 gave improved BA18-adduct intensities compared to the original BA-adduct.

CONCLUSION

Twenty-six commercially available boronic acids were investigated as alternatives to BA, used in the f-TLC analysis for the diagnosis of BU. Three (3) of them BA15, BA18, and BA21 gave superior fluorescence band intensity profiles. They gave profiles that were easier to interpret after the myco-boronic acid adduct formation and in experiments with clinical samples from patients with BA18 the best. BA18, therefore, has been identified as a potential alternative to BA and could provide a solution to the challenge of background interference of co-extracted human tissue lipids from clinical samples currently associated with the use of BA.

Genome-wide screening identified SEC61A1 as an essential factor for mycolactone-dependent apoptosis in human premonocytic THP-1 cells

Buruli ulcer is a chronic skin disease caused by a toxic lipid mycolactone produced by Mycobacterium ulcerans, which induces local skin tissue destruction and analgesia. However, the cytotoxicity pathway induced by mycolactone remains largely unknown. Here we investigated the mycolactone-induced cell death pathway by screening host factors using a genome-scale lenti-CRISPR mutagenesis assay in human premonocytic THP-1 cells. As a result, 884 genes were identified as candidates causing mycolactone-induced cell death, among which SEC61A1, the α-subunit of the Sec61 translocon complex, was the highest scoring. CRISPR/Cas9 genome editing of SEC61A1 in THP-1 cells suppressed mycolactone-induced endoplasmic reticulum stress, especially eIF2α phosphorylation, and caspase-dependent apoptosis. Although previous studies have reported that mycolactone targets SEC61A1 based on mutation screening and structural analysis in several cell lines, we have reconfirmed that SEC61A1 is a mycolactone target by genome-wide screening in THP-1 cells. These results shed light on the cytotoxicity of mycolactone and suggest that the inhibition of mycolactone activity or SEC61A1 downstream cascades will be a novel therapeutic modality to eliminate the harmful effects of mycolactone in addition to the 8-week antibiotic regimen of rifampicin and clarithromycin.

Buruli ulcer is a chronic skin disease caused by the bacterium Mycobacterium ulcerans. The disease mainly affects children in West Africa, and the skin ulcers are induced by mycolactone, a toxin produced by the bacteria. The mycolactone diffuses through the skin, killing cells, creating irreversible ulceration, and weakening host immune defenses. However, the cytotoxic pathway induced by mycolactone remains largely unknown. We evaluated the mycolactone-induced cell death pathway by screening host factors using a genome-scale knockout assay in human premonocytic THP-1 cells. We identified 884 genes that are potentially involved in mycolactone-induced cell death, of which SEC61A1, the α-subunit of the Sec61 translocon complex, was the highest ranking. Knockout of SEC61A1 in THP-1 cells resulted in suppression of endoplasmic reticulum stress and caspase-dependent apoptosis induced by mycolactone. These results suggest that SEC61A1 is an essential mediator of mycolactone-induced cell death.

Induced Synthesis of Mycolactone Restores the Pathogenesis of Mycobacterium ulcerans In Vitro and In Vivo

Mycobacterium ulcerans is the causative agent of Buruli ulcer (BU), the third most common mycobacterial infection. Virulent M. ulcerans secretes mycolactone, a polyketide toxin. Most observations of M. ulcerans infection are described as an extracellular milieu in the form of a necrotic ulcer. While some evidence exists of an intracellular life cycle for M. ulcerans during infection, the exact role that mycolactone plays in this process is poorly understood. Many previous studies have relied upon the addition of purified mycolactone to cell-culture systems to study its role in M. ulcerans pathogenesis and host-response modulation. However, this sterile system drastically simplifies the M. ulcerans infection model and assumes that mycolactone is the only relevant virulence factor expressed by M. ulcerans. Here we show that the addition of purified mycolactone to macrophages during M. ulcerans infection overcomes the bacterial activation of the mechanistic target of rapamycin (mTOR) signaling pathway that plays a substantial role in regulating different cellular processes, including autophagy and apoptosis. To further study the role of mycolactone during M. ulcerans infection, we have developed an inducible mycolactone expression system. Utilizing the mycolactone-deficient Mul::Tn118 strain that contains a transposon insertion in the putative beta-ketoacyl transferase (mup045), we have successfully restored mycolactone production by expressing mup045 in a tetracycline-inducible vector system, which overcomes in-vitro growth defects associated with constitutive complementation. The inducible mycolactone-expressing bacteria resulted in the establishment of infection in a murine footpad model of BU similar to that observed during the infection with wild-type M. ulcerans. This mycolactone inducible system will allow for further analysis of the roles and functions of mycolactone during M. ulcerans infection.

The Sec61 translocon is a therapeutic vulnerability in multiple myeloma

Multiple myeloma (MM) is an incurable malignancy characterized by the uncontrolled expansion of plasma cells in the bone marrow. While proteasome inhibitors like bortezomib efficiently halt MM progression, drug resistance inevitably develop, and novel therapeutic approaches are needed. Here, we used a recently discovered Sec61 inhibitor, mycolactone, to assess the interest of disrupting MM proteostasis via protein translocation blockade. In human MM cell lines, mycolactone caused rapid defects in secretion of immunoglobulins and expression of pro‐survival interleukin (IL)‐6 receptor and CD40, whose activation stimulates IL‐6 production. Mycolactone also triggered pro‐apoptotic endoplasmic reticulum stress responses synergizing with bortezomib for induction of MM cell death and overriding acquired resistance to the proteasome inhibitor. Notably, the mycolactone–bortezomib combination rapidly killed patient‐derived MM cells ex vivo, but not normal mononuclear cells. In immunodeficient mice engrafted with MM cells, it demonstrated superior therapeutic efficacy over single drug treatments, without inducing toxic side effects. Collectively, these findings establish Sec61 blockers as novel anti‐MM agents and reveal the interest of targeting both the translocon and the proteasome in proteostasis‐addicted tumors.

Conditions for Handling and Optimal Storage of Mycolactone

The successful isolation of mycolactone in a laboratory or from a clinical sample relies on proper handling and storage of the toxin. Mycolactone is a light-sensitive and an amphiphilic toxin produced by Mycobacterium ulcerans. The biochemistry of the toxin makes it unstable in aqueous matrices such as blood, which causes it to self-aggregate or present in complex with carrier molecules. This biochemistry also impacts the use of the toxin in vitro, in that it tends to aggregate and stick to substrates in an aqueous environment, which alters its physiological presentation and limits its availability in a sample. Glass materials (i.e., tubes, vials, syringes, plates) should be used when possible to avoid loss of mycolactone sticking to plastic surfaces. Dark containers such as amber vials or aluminum-foil wrapped tubes should be used to avoid photodegradation of the toxin upon exposure to light. Sample storage in organic solvents is ideal for mycolactone stability and recovery; however, this is not always amenable as multiple diagnostic assays might be performed on a single sample (such as PCR or ELISA). In these cases, samples can be stored in an aqueous solution containing a small amount of detergent to enhance recovery of the toxin, and in order to avoid aggregation. Therefore, the downstream manipulations should be carefully considered prior to sample collection and storage. Here we present considerations for the optimal handling and storage of mycolactone in order to obtain quality yield of the toxin for various research and diagnostic applications.

Molecular Mechanisms Underpinning the Circulation and Cellular Uptake of Mycobacterium ulcerans Toxin Mycolactone

Mycolactone is a diffusible lipid toxin produced by Mycobacterium ulcerans, the causative agent of Buruli ulcer disease. Altough bacterially derived mycolactone has been shown to traffic from cutaneous foci of infection to the bloodstream, the mechanisms underpinning its access to systemic circulation and import by host cells remain largely unknown. Using biophysical and cell-based approaches, we demonstrate that mycolactone specific association to serum albumin and lipoproteins is necessary for its solubilization and is a major mechanism to regulate its bioavailability. We also demonstrate that Scavenger Receptor (SR)-B1 contributes to the cellular uptake of mycolactone. Overall, we suggest a new mechanism of transport and cell entry, challenging the dogma that the toxin enters host cells via passive diffusion.

Host Immune-Metabolic Adaptations Upon Mycobacterial Infections and Associated Co-Morbidities

Mycobacterial diseases are a major public health challenge. Their causative agents include, in order of impact, members of the Mycobacterium tuberculosis complex (causing tuberculosis), Mycobacterium leprae (causing leprosy), and non-tuberculous mycobacterial pathogens including Mycobacterium ulcerans. Macrophages are mycobacterial targets and they play an essential role in the host immune response to mycobacteria. This review aims to provide a comprehensive understanding of the immune-metabolic adaptations of the macrophage to mycobacterial infections. This metabolic rewiring involves changes in glycolysis and oxidative metabolism, as well as in the use of fatty acids and that of metals such as iron, zinc and copper. The macrophage metabolic adaptations result in changes in intracellular metabolites, which can post-translationally modify proteins including histones, with potential for shaping the epigenetic landscape. This review will also cover how critical tuberculosis co-morbidities such as smoking, diabetes and HIV infection shape host metabolic responses and impact disease outcome. Finally, we will explore how the immune-metabolic knowledge gained in the last decades can be harnessed towards the design of novel diagnostic and therapeutic tools, as well as vaccines.

Genetics in the Host-Mycobacterium ulcerans interaction

Buruli ulcer is an emerging infectious disease associated with high morbidity and unpredictable outbreaks. It is caused by Mycobacterium ulcerans, a slow-growing pathogen evolutionarily shaped by the acquisition of a plasmid involved in the production of a potent macrolide-like cytotoxin and by genome rearrangements and downsizing. These events culminated in an uncommon infection pattern, whereby M. ulcerans is both able to induce the initiation of the inflammatory cascade and the cell death of its proponents, as well as to survive within the phagosome and in the extracellular milieu. In such extreme conditions, the host is sentenced to rely on a highly orchestrated genetic landscape to be able to control the infection. We here revisit the dynamics of M. ulcerans infection, drawing parallels from other mycobacterioses and integrating the most recent knowledge on its evolution and pathogenicity in its interaction with the host immune response.

Immunity against Mycobacterium ulcerans: The subversive role of mycolactone

Mycobacterium ulcerans causes Buruli ulcer, a neglected tropical skin disease manifesting as chronic wounds that can leave victims with major, life-long deformity and disability. Differently from other mycobacterial pathogens, M ulcerans produces mycolactone, a diffusible lipid factor with unique cytotoxic and immunomodulatory properties. Both traits result from mycolactone targeting Sec61, the entry point of the secretory pathway in eukaryotic cells. By inhibiting Sec61, mycolactone prevents the host cell's production of secreted proteins, and most of its transmembrane proteins. This molecular blockade dramatically alters the functions of immune cells, thereby the generation of protective immunity. Moreover, sustained inhibition of Sec61 triggers proteotoxic stress responses leading to apoptotic cell death, which can stimulate vigorous immune responses. The dynamics of bacterial production of mycolactone and elimination by infected hosts thus critically determine the balance between its immunostimulatory and immunosuppressive effects. Following an introduction summarizing the essential information on Buruli ulcer disease, this review focuses on the current state of knowledge regarding mycolactone's regulation and biodistribution. We then detail the consequences of mycolactone-mediated Sec61 blockade on initiation and maintenance of innate and adaptive immune responses. Finally, we discuss the key questions to address in order to improve immunity to M ulcerans, and how increased knowledge of mycolactone biology may pave the way to innovative therapeutics.

Spatiotemporal analysis of mycolactone distribution in vivo reveals partial diffusion in the central nervous system

Mycobacterium ulcerans, the causative agent of Buruli ulcer (BU) disease, is unique amongst human pathogens in its capacity to produce a lipid toxin called mycolactone. While previous studies have demonstrated that bacterially-released mycolactone diffuses beyond infection foci, the spatiotemporal distribution of mycolactone remained largely unknown. Here, we used the zebrafish model to provide the first global kinetic analysis of mycolactone's diffusion in vivo, and multicellular co-culture systems to address the critical question of the toxin's access to the brain. Zebrafish larvae were injected with a fluorescent-derivative of mycolactone to visualize the in vivo diffusion of the toxin from the peripheral circulation. A rapid, body-wide distribution of mycolactone was observed, with selective accumulation in tissues near the injection site and brain, together with an important excretion through the gastro-intestinal tract. Our conclusion that mycolactone reached the central nervous system was reinforced by an in cellulo model of human blood brain barrier and a mouse model of M. ulcerans-infection. Here we show that mycolactone has a broad but heterogenous profile of distribution in vivo. Our investigations in vitro and in vivo support the view that a fraction of bacterially-produced mycolactone gains access to the central nervous system. The relative persistence of mycolactone in the bloodstream suggests that assays of circulating mycolactone are relevant for BU disease monitoring and treatment optimization.

The immunology of other mycobacteria: M. ulcerans, M. leprae

Mycobacterial pathogens can be categorized into three broad groups: Mycobacterium tuberculosis complex causing tuberculosis, M. leprae and M. lepromatosis causing leprosy, and atypical mycobacteria, or non-tuberculous mycobacteria (NTM), responsible for a wide range of diseases. Among the NTMs, M. ulcerans is responsible for the neglected tropical skin disease Buruli ulcer (BU). Most pathogenic mycobacteria, including M. leprae, evade effector mechanisms of the humoral immune system by hiding and replicating inside host cells and are furthermore excellent modulators of host immune responses. In contrast, M. ulcerans replicates predominantly extracellularly, sheltered from host immune responses through the cytotoxic and immunosuppressive effects of mycolactone, a macrolide produced by the bacteria. In the year 2018, 208,613 new cases of leprosy and 2713 new cases of BU were reported to WHO, figures which are notoriously skewed by vast underreporting of these diseases.

Understanding the Significance of Biochemistry in the Storage, Handling, Purification, and Sampling of Amphiphilic Mycolactone

Mycolactone, the amphiphilic macrolide toxin secreted by Mycobacterium ulcerans, plays a significant role in the pathology and manifestations of Buruli ulcer (BU). Consequently, it follows that the toxin is a suitable target for the development of diagnostics and therapeutics for this disease. Yet, several challenges have deterred such development. For one, the lipophilic nature of the toxin makes it difficult to handle and store and contributes to variability associated with laboratory experimentation and purification yields. In this manuscript, we have attempted to incorporate our understanding of the lipophilicity of mycolactone in order to define the optimal methods for the storage, handling, and purification of this toxin. We present a systematic correlation of variability associated with measurement techniques (thin-layer chromatography (TLC), mass spectrometry (MS), and UV-Vis spectrometry), storage conditions, choice of solvents, as well as the impact of each of these on toxin function as assessed by cellular cytotoxicity. We also compared natural mycolactone extracted from bacterial culture with synthesized toxins in laboratory (solvents, buffers) and physiologically relevant (serum) matrices. Our results point to the greater stability of mycolactone in organic, as well as detergent-containing, solvents, regardless of the container material (plastic, glass, or silanized tubes). They also highlight the presence of toxin in samples that may be undetectable by any one technique, suggesting that each detection approach captures different configurations of the molecule with varying specificity and sensitivity. Most importantly, our results demonstrate for the very first time that amphiphilic mycolactone associates with host lipoproteins in serum, and that this association will likely impact our ability to study, diagnose, and treat Buruli ulcers in patients.

Sec61 blockade by mycolactone: A central mechanism in Buruli ulcer disease

Infection with Mycobacterium ulcerans results in a necrotising skin disease known as a Buruli ulcer, the pathology of which is directly linked to the bacterial production of the toxin mycolactone. Recent studies have identified the protein translocation machinery of the endoplasmic reticulum (ER) membrane as the primary cellular target of mycolactone, and shown that the toxin binds to the core subunit of the Sec61 complex. Mycolactone binding strongly inhibits the capacity of the Sec61 translocon to transport newly synthesised membrane and secretory proteins into and across the ER membrane. Since the ER acts as the entry point for the mammalian secretory pathway, and hence regulates initial access to the entire endomembrane system, mycolactone-treated cells have a reduced ability to produce a range of proteins including secretory cytokines and plasma membrane receptors. The global effect of this molecular blockade of protein translocation at the ER is that the host is unable to mount an effective immune response to the underlying mycobacterial infection. Prolonged exposure to mycolactone is normally cytotoxic, since it triggers stress responses activating the transcription factor ATF4 and ultimately inducing apoptosis.

Mycobacterium ulcerans DNA in Bandicoot Excreta in Buruli Ulcer-Endemic Area, Northern Queensland, Australia

To identify potential reservoirs/vectors of Mycobacterium ulcerans in northern Queensland, Australia, we analyzed environmental samples collected from the Daintree River catchment area, to which Buruli ulcer is endemic, and adjacent coastal lowlands by species-specific PCR. We detected M. ulcerans DNA in soil, mosquitoes, and excreta of bandicoots, which are small terrestrial marsupials.

Buruli Ulcer: Review of a Neglected Skin Mycobacterial Disease

Buruli ulcer is caused by Mycobacterium ulcerans This neglected disease occurs in scattered foci around the world, with a higher concentration of cases in West Africa. The mycobacteria produce mycolactones that cause tissue necrosis. The disease presents as a painless skin nodule that ulcerates as necrosis expands. Finding acid-fast bacilli in smears or histopathology, culturing the mycobacteria, and performing M. ulcerans PCR in presumptive cases confirm the diagnosis. Medical treatment with oral rifampin and intramuscular streptomycin or oral treatment with rifampin plus clarithromycin for 8 weeks is supported by the World Health Organization. This review summarizes the epidemiology, pathogenesis, clinical presentation, diagnostic tests, and advances in treatment.

Membrane perturbing properties of toxin mycolactone from Mycobacterium ulcerans

Mycolactone is the exotoxin produced by Mycobacterium ulcerans and is the virulence factor behind the neglected tropical disease Buruli ulcer. The toxin has a broad spectrum of biological effects within the host organism, stemming from its interaction with at least two molecular targets and the inhibition of protein uptake into the endoplasmic reticulum. Although it has been shown that the toxin can passively permeate into host cells, it is clearly lipophilic. Association with lipid carriers would have substantial implications for the toxin’s distribution within a host organism, delivery to cellular targets, diagnostic susceptibility, and mechanisms of pathogenicity. Yet the toxin’s interactions with, and distribution in, lipids are unknown. Herein we have used coarse-grained molecular dynamics simulations, guided by all-atom simulations, to study the interaction of mycolactone with pure and mixed lipid membranes. Using established techniques, we calculated the toxin’s preferential localization, membrane translocation, and impact on membrane physical and dynamical properties. The computed water-octanol partition coefficient indicates that mycolactone prefers to be in an organic phase rather than in an aqueous environment. Our results show that in a solvated membrane environment the exotoxin mainly localizes in the water-membrane interface, with a preference for the glycerol moiety of lipids, consistent with the reported studies that found it in lipid extracts of the cell. The calculated association constant to the model membrane is similar to the reported association constant for Wiskott-Aldrich syndrome protein. Mycolactone is shown to modify the physical properties of membranes, lowering the transition temperature, compressibility modulus, and critical line tension at which pores can be stabilized. It also shows a tendency to behave as a linactant, a molecule that localizes at the boundary between different fluid lipid domains in membranes and promotes inter-mixing of domains. This property has implications for the toxin’s cellular access, T-cell immunosuppression, and therapeutic potential.

Mycolactone is a macrolide exotoxin secreted by Mycobacterium ulcerans, which causes a skin disease called Buruli ulcer, a neglected emerging disease. It is the third most common mycobacterial disease after tuberculosis and leprosy. Studies have shown how mycolactone plays a pivotal role in Buruli ulcer pathogenesis, and identified it as an attractive therapeutic target. This multifunctional cytotoxin exerts multiple local and global responses, including ulcerative, analgesic, and anti-inflammatory effects. Prompted by its lipid-like structure, we used extensive multi-resolution simulations to probe mycolactone’s interactions with model membranes. Our results suggest that mycolactone is sequestered in membranes where it alters several dynamical, physical, and mechanical properties. It also behaves as a linactant, localizing at the interface between lipid domains and decreasing the inter-domain line tension. Our results shed light on how mycolactone permeates host cell membranes and is distributed between lipid and aqueous environments. These findings have significant implications for the toxin’s distribution in the host environment and mechanisms of pathogenicity. Understanding the toxin’s distribution and mechanism of trafficking will have ramifications for targeted diagnostics, therapeutic approaches, and our understanding of Buruli ulcer pathogenesis.

Buruli Ulcer, a Prototype for Ecosystem-Related Infection, Caused by Mycobacterium ulcerans

Buruli ulcer is a noncontagious disabling cutaneous and subcutaneous mycobacteriosis reported by 33 countries in Africa, Asia, Oceania, and South America. The causative agent, Mycobacterium ulcerans, derives from Mycobacterium marinum by genomic reduction and acquisition of a plasmid-borne, nonribosomal cytotoxin mycolactone, the major virulence factor. M. ulcerans-specific sequences have been readily detected in aquatic environments in food chains involving small mammals. Skin contamination combined with any type of puncture, including insect bites, is the most plausible route of transmission, and skin temperature of <30°C significantly correlates with the topography of lesions. After 30 years of emergence and increasing prevalence between 1970 and 2010, mainly in Africa, factors related to ongoing decreasing prevalence in the same countries remain unexplained. Rapid diagnosis, including laboratory confirmation at the point of care, is mandatory in order to reduce delays in effective treatment. Parenteral and potentially toxic streptomycin-rifampin is to be replaced by oral clarithromycin or fluoroquinolone combined with rifampin. In the absence of proven effective primary prevention, avoiding skin contamination by means of clothing can be implemented in areas of endemicity. Buruli ulcer is a prototype of ecosystem pathology, illustrating the impact of human activities on the environment as a source for emerging tropical infectious diseases.

Mycolactone subverts immunity by selectively blocking the Sec61 translocon

Mycolactone, an immunosuppressive macrolide released by the human pathogen Mycobacterium ulcerans, was previously shown to impair Sec61-dependent protein translocation, but the underlying molecular mechanism was not identified. In this study, we show that mycolactone directly targets the α subunit of the Sec61 translocon to block the production of secreted and integral membrane proteins with high potency. We identify a single-amino acid mutation conferring resistance to mycolactone, which localizes its interaction site near the lumenal plug of Sec61α. Quantitative proteomics reveals that during T cell activation, mycolactone-mediated Sec61 blockade affects a selective subset of secretory proteins including key signal-transmitting receptors and adhesion molecules. Expression of mutant Sec61α in mycolactone-treated T cells rescued their homing potential and effector functions. Furthermore, when expressed in macrophages, the mycolactone-resistant mutant restored IFN-γ receptor-mediated antimicrobial responses. Thus, our data provide definitive genetic evidence that Sec61 is the host receptor mediating the diverse immunomodulatory effects of mycolactone and identify Sec61 as a novel regulator of immune cell functions.

Recent advances: role of mycolactone in the pathogenesis and monitoring of Mycobacterium ulcerans infection/Buruli ulcer disease

Infection of subcutaneous tissue with Mycobacterium ulcerans can lead to chronic skin ulceration known as Buruli ulcer. The pathogenesis of this neglected tropical disease is dependent on a lipid‐like toxin, mycolactone, which diffuses through tissue away from the infecting organisms. Since its identification in 1999, this molecule has been intensely studied to elucidate its cytotoxic and immunosuppressive properties. Two recent major advances identifying the underlying molecular targets for mycolactone have been described. First, it can target scaffolding proteins (such as Wiskott Aldrich Syndrome Protein), which control actin dynamics in adherent cells and therefore lead to detachment and cell death by anoikis. Second, it prevents the co‐translational translocation (and therefore production) of many proteins that pass through the endoplasmic reticulum for secretion or placement in cell membranes. These pleiotropic effects underpin the range of cell‐specific functional defects in immune and other cells that contact mycolactone during infection. The dose and duration of mycolactone exposure for these different cells explains tissue necrosis and the paucity of immune cells in the ulcers. This review discusses recent advances in the field, revisits older findings in this context and highlights current developments in structure‐function studies as well as methodology that make mycolactone a promising diagnostic biomarker.

RNA Aptamer That Specifically Binds to Mycolactone and Serves as a Diagnostic Tool for Diagnosis of Buruli Ulcer

BACKGROUND

Buruli ulcer (BU) is a subcutaneous skin disease listed among the neglected tropical diseases by the World Health Organization (WHO). Early case detection and management is very important to reduce morbidity and the accompanied characteristic disfiguring nature of BU. Since diagnosis based on clinical evidence can lead to misdiagnosis, microbiological confirmation is essential to reduce abuse of drugs; since the anti-mycobacterial drugs are also used for TB treatment. The current WHO gold standard PCR method is expensive, requires infrastructure and expertise are usually not available at the peripheral centers where BU cases are managed. Thus one of the main research agendas is to develop methods that can be applied at the point of care. In this study we selected aptamers, which are emerging novel class of detection molecules, for detecting mycolactone, the first to be conducted in a BUD endemic country.

METHODS

Aptamers that bind to mycolactone were isolated by the SELEX process. To measure their affinity and specificity to mycolactone, the selected aptamers were screened by means of isothermal titration calorimetry (ITC) and an enzyme-linked oligonucleotide assay (ELONA). Selected aptamers were assessed by ELONA using swab samples from forty-one suspected BU patients with IS2404 PCR and culture as standard methods. ROC analysis was used to evaluate their accuracy and cutoff-points.

RESULTS

Five out of the nine selected aptamers bound significantly (p< 0.05) to mycolactone, of these, three were able to distinguish between mycolactone producing mycobacteria, M. marinum (CC240299, Israel) and other bacteria whilst two others also bounded significantly to Mycobacterium smegmatis. Their dissociation constants were in the micro-molar range. At 95% confidence interval, the ROC curve analysis among the aptamers at OD450 ranged from 0.5-0.7. Using this cut-off for the ELONA assay, the aptamers had 100% specificity and sensitivity between 0.0% and 50.0%. The most promising aptamer, Apt-3683 showed a discernible cleavage difference relative to the non-specific autocatalysis over a 3-minute time course.

CONCLUSION

This preliminary proof-of-concept indicates that diagnosis of BUD with RNA aptamers is feasible and can be used as point of care upon incorporation into a diagnostic platform.

Mycolactone-mediated neurite degeneration and functional effects in cultured human and rat DRG neurons: Mechanisms underlying hypoalgesia in Buruli ulcer

Background

Mycolactone is a polyketide toxin secreted by the mycobacterium Mycobacterium ulcerans, responsible for the extensive hypoalgesic skin lesions characteristic of patients with Buruli ulcer. A recent pre-clinical study proposed that mycolactone may produce analgesia via activation of the angiotensin II type 2 receptor (AT₂R). In contrast, AT₂R antagonist EMA401 has shown analgesic efficacy in animal models and clinical trials for neuropathic pain. We therefore investigated the morphological and functional effects of mycolactone in cultured human and rat dorsal root ganglia (DRG) neurons and the role of AT₂R using EMA401. Primary sensory neurons were prepared from avulsed cervical human DRG and rat DRG; 24 h after plating, neurons were incubated for 24 to 96 h with synthetic mycolactone A/B, followed by immunostaining with antibodies to PGP9.5, Gap43, β tubulin, or Mitotracker dye staining. Acute functional effects were examined by measuring capsaicin responses with calcium imaging in DRG neuronal cultures treated with mycolactone.

Results

Morphological effects: Mycolactone-treated cultures showed dramatically reduced numbers of surviving neurons and non-neuronal cells, reduced Gap43 and β tubulin expression, degenerating neurites and reduced cell body diameter, compared with controls. Dose-related reduction of neurite length was observed in mycolactone-treated cultures. Mitochondria were distributed throughout the length of neurites and soma of control neurons, but clustered in the neurites and soma of mycolactone-treated neurons. Functional effects: Mycolactone-treated human and rat DRG neurons showed dose-related inhibition of capsaicin responses, which were reversed by calcineurin inhibitor cyclosporine and phosphodiesterase inhibitor 3-isobutyl-1-Methylxanthine, indicating involvement of cAMP/ATP reduction. The morphological and functional effects of mycolactone were not altered by Angiotensin II or AT₂R antagonist EMA401.

Conclusion

Mycolactone induces toxic effects in DRG neurons, leading to impaired nociceptor function, neurite degeneration, and cell death, resembling the cutaneous hypoalgesia and nerve damage in individuals with M. Ulcerans infection.

Shaping mycolactone for therapeutic use against inflammatory disorders

Inflammation adversely affects the health of millions of people worldwide, and there is an unmet medical need for better anti-inflammatory drugs. We evaluated the therapeutic interest of mycolactone, a polyketide-derived macrolide produced by Mycobacterium ulcerans. Bacterial production of mycolactone in human skin causes a combination of ulcerative, analgesic, and anti-inflammatory effects. Whereas ulcer formation is mediated by the proapoptotic activity of mycolactone on skin cells via hyperactivation of Wiskott-Aldrich syndrome proteins, analgesia results from neuronal hyperpolarization via signaling through angiotensin II type 2 receptors. Mycolactone also blunts the capacity of immune cells to produce inflammatory mediators by an independent mechanism of protein synthesis blockade. In an attempt to isolate the structural determinants of mycolactone's immunosuppressive activity, we screened a library of synthetic subunits of mycolactone for inhibition of cytokine production by activated T cells. The minimal structure retaining immunosuppressive activity was a truncated version of mycolactone, missing one of the two core-branched polyketide chains. This compound inhibited the inflammatory cytokine responses of human primary cells at noncytotoxic doses and bound to angiotensin II type 2 receptors comparably to mycolactone in vitro. Notably, it was considerably less toxic than mycolactone in human primary dermal fibroblasts modeling ulcerative activity. In mouse models of human diseases, it conferred systemic protection against chronic skin inflammation and inflammatory pain, with no apparent side effects. In addition to establishing the anti-inflammatory potency of mycolactone in vivo, our study therefore highlights the translational potential of mycolactone core-derived structures as prospective immunosuppressants.

Bacterial Toxins as Pathogen Weapons Against Phagocytes

Bacterial toxins are virulence factors that manipulate host cell functions and take over the control of vital processes of living organisms to favor microbial infection. Some toxins directly target innate immune cells, thereby annihilating a major branch of the host immune response. In this review we will focus on bacterial toxins that act from the extracellular milieu and hinder the function of macrophages and neutrophils. In particular, we will concentrate on toxins from Gram-positive and Gram-negative bacteria that manipulate cell signaling or induce cell death by either imposing direct damage to the host cells cytoplasmic membrane or enzymatically modifying key eukaryotic targets. Outcomes regarding pathogen dissemination, host damage and disease progression will be discussed.

A Sero-epidemiological Approach to Explore Transmission of Mycobacterium ulcerans

The debilitating skin disease Buruli ulcer (BU) is caused by infection with Mycobacterium ulcerans. While various hypotheses on potential reservoirs and vectors of M. ulcerans exist, the mode of transmission has remained unclear. Epidemiological studies have indicated that children below the age of four are less exposed to the pathogen and at lower risk of developing BU than older children. In the present study we compared the age at which children begin to develop antibody responses against M. ulcerans with the age pattern of responses to other pathogens transmitted by various mechanisms. A total of 1,352 sera from individuals living in the BU endemic Offin river valley of Ghana were included in the study. While first serological responses to the mosquito transmitted malaria parasite Plasmodium falciparum and to soil transmitted Strongyloides helminths emerged around the age of one and two years, sero-conversion for M. ulcerans and for the water transmitted trematode Schistosoma mansoni occurred at around four and five years, respectively. Our data suggest that exposure to M. ulcerans intensifies strongly at the age when children start to have more intense contact with the environment, outside the small movement range of young children. Further results from our serological investigations in the Offin river valley also indicate ongoing transmission of Treponema pallidum, the causative agent of yaws.

Metabolomic profiles delineate mycolactone signature in Buruli ulcer disease

Infection of human skin with Mycobacterium ulcerans, the causative agent of Buruli ulcer, is associated with the systemic diffusion of a bacterial macrolide named mycolactone. Patients with progressive disease show alterations in their serum proteome, likely reflecting the inhibition of secreted protein production by mycolactone at the cellular level. Here, we used semi-quantitative metabolomics to characterize metabolic perturbations in serum samples of infected individuals, and human cells exposed to mycolactone. Among the 430 metabolites profiled across 20 patients and 20 healthy endemic controls, there were significant differences in the serum levels of hexoses, steroid hormones, acylcarnitines, purine, heme, bile acids, riboflavin and lysolipids. In parallel, analysis of 292 metabolites in human T cells treated or not with mycolactone showed alterations in hexoses, lysolipids and purine catabolites. Together, these data demonstrate that M. ulcerans infection causes systemic perturbations in the serum metabolome that can be ascribed to mycolactone. Of particular importance to Buruli ulcer pathogenesis is that changes in blood sugar homeostasis in infected patients are mirrored by alterations in hexose metabolism in mycolactone-exposed cells.

Pleiotropic molecular effects of the Mycobacterium ulcerans virulence factor mycolactone underlying the cell death and immunosuppression seen in Buruli ulcer

Mycolactone is a polyketide macrolide lipid-like secondary metabolite synthesized by Mycobacterium ulcerans, the causative agent of BU (Buruli ulcer), and is the only virulence factor for this pathogen identified to date. Prolonged exposure to high concentrations of mycolactone is cytotoxic to diverse mammalian cells (albeit with varying efficiency), whereas at lower doses it has a spectrum of immunosuppressive activities. Combined, these pleiotropic properties have a powerful influence on local and systemic cellular function that should explain the pathophysiology of BU disease. The last decade has seen significant advances in our understanding of the molecular mechanisms underlying these effects in a range of different cell types. The present review focuses on the current state of our knowledge of mycolactone function, and its molecular and cellular targets, and seeks to identify commonalities between the different functional and cellular systems. Since mycolactone influences fundamental cellular processes (cell division, cell death and inflammation), getting to the root of how mycolactone achieves this could have a profound impact on our understanding of eukaryotic cell biology.

Synthetic variants of mycolactone bind and activate Wiskott-Aldrich syndrome proteins

Mycolactone is a complex macrolide toxin produced by Mycobacterium ulcerans, the causative agent of skin lesions called Buruli ulcers. Mycolactone-mediated activation of neural (N) Wiskott-Aldrich syndrome proteins (WASP) induces defects in cell adhesion underpinning cytotoxicity and disease pathogenesis. We describe the chemical synthesis of 23 novel mycolactone analogues that differ in structure and modular assembly of the lactone core with its northern and southern polyketide side chains. The lactone core linked to southern chain was the minimal structure binding N-WASP and hematopoietic homolog WASP, where the number and configuration of hydroxyl groups on the acyl side chain impacted the degree of binding. A fluorescent derivative of this compound showed time-dependent accumulation in target cells. Furthermore, a simplified version of mycolactone mimicked the natural toxin for activation of WASP in vitro and induced comparable alterations of epithelial cell adhesion. Therefore, it constitutes a structural and functional surrogate of mycolactone for WASP/N-WASP-dependent effects.

Combined inflammatory and metabolic defects reflected by reduced serum protein levels in patients with Buruli ulcer disease

Buruli ulcer is a skin disease caused by Mycobacterium ulcerans that is spreading in tropical countries, with major public health and economic implications in West Africa. Multi-analyte profiling of serum proteins in patients and endemic controls revealed that Buruli ulcer disease down-regulates the circulating levels of a large array of inflammatory mediators, without impacting on the leukocyte composition of peripheral blood. Notably, several proteins contributing to acute phase reaction, lipid metabolism, coagulation and tissue remodelling were also impacted. Their down-regulation was selective and persisted after the elimination of bacteria with antibiotic therapy. It involved proteins with various functions and origins, suggesting that M. ulcerans infection causes global and chronic defects in the host's protein metabolism. Accordingly, patients had reduced levels of total serum proteins and blood urea, in the absence of signs of malnutrition, or functional failure of liver or kidney. Interestingly, slow healers had deeper metabolic and coagulation defects at the start of antibiotic therapy. In addition to providing novel insight into Buruli ulcer pathogenesis, our study therefore identifies a unique proteomic signature for this disease.

The pathogenic mechanism of the Mycobacterium ulcerans virulence factor, mycolactone, depends on blockade of protein translocation into the ER

Infection with Mycobacterium ulcerans is characterised by tissue necrosis and immunosuppression due to mycolactone, the necessary and sufficient virulence factor for Buruli ulcer disease pathology. Many of its effects are known to involve down-regulation of specific proteins implicated in important cellular processes, such as immune responses and cell adhesion. We have previously shown mycolactone completely blocks the production of LPS-dependent proinflammatory mediators post-transcriptionally. Using polysome profiling we now demonstrate conclusively that mycolactone does not prevent translation of TNF, IL-6 and Cox-2 mRNAs in macrophages. Instead, it inhibits the production of these, along with nearly all other (induced and constitutive) proteins that transit through the ER. This is due to a blockade of protein translocation and subsequent degradation of aberrantly located protein. Several lines of evidence support this transformative explanation of mycolactone function. First, cellular TNF and Cox-2 can be once more detected if the action of the 26S proteasome is inhibited concurrently. Second, restored protein is found in the cytosol, indicating an inability to translocate. Third, in vitro translation assays show mycolactone prevents the translocation of TNF and other proteins into the ER. This is specific as the insertion of tail-anchored proteins into the ER is unaffected showing that the ER remains structurally intact. Fourth, metabolic labelling reveals a near-complete loss of glycosylated and secreted proteins from treated cells, whereas cytosolic proteins are unaffected. Notably, the profound lack of glycosylated and secreted protein production is apparent in a range of different disease-relevant cell types. These studies provide a new mechanism underlying mycolactone's observed pathological activities both in vitro and in vivo. Mycolactone-dependent inhibition of protein translocation into the ER not only explains the deficit of innate cytokines, but also the loss of membrane receptors, adhesion molecules and T-cell cytokines that drive the aetiology of Buruli ulcer.

Buruli ulcer is a progressive necrotic skin lesion caused by infection with the human pathogen Mycobacterium ulcerans. Mycolactone, a small compound produced by the mycobacterium, is the root cause of the disease pathology, but until now there has been no unifying mechanism explaining why. We have been using a model system to investigate the reason for the selective loss of protein that is a common feature of mycolactone exposure. Specifically, this involves identifying the point at which it stops immune cells making inflammatory mediators. In this work, we demonstrate that mycolactone inhibits production of such proteins by blocking the first step of protein export: translocation into a cellular compartment called the endoplasmic reticulum (ER). Proteins due for export are instead made in the cell cytosol where they are recognised as being in the wrong place and are rapidly degraded, causing a general cessation of the production of proteins that have to travel through the ER, including almost all secreted and surface proteins. This has a profound effect on basic cell functions such as growth, adhesion and survival. Therefore, we have identified the molecular basis underlying the key features of Buruli ulcer, and this will transform our understanding of disease progression.

History, biology and chemistry of Mycobacterium ulcerans infections (Buruli ulcer disease)

Mycobacterium ulcerans infections (Buruli ulcer disease) have a long history that can be traced back 150 years. The successive discoveries of the mycobacteria in 1948 and of mycolactone A/B in 1999, the toxin responsible for this dramatic necrotic skin disease, resulted in a paradigm shift concerning the disease itself and in a broader sense, delineated an entirely new role for bioactive polyketides as virulence factors. The fascinating history, biology and chemistry of M. ulcerans infections are discussed in this review.

Analysis of the vaccine potential of plasmid DNA encoding nine mycolactone polyketide synthase domains in Mycobacterium ulcerans infected mice

There is no effective vaccine against Buruli ulcer. In experimental footpad infection of C57BL/6 mice with M. ulcerans, a prime-boost vaccination protocol using plasmid DNA encoding mycolyltransferase Ag85A of M. ulcerans and a homologous protein boost has shown significant, albeit transient protection, comparable to the one induced by M. bovis BCG. The mycolactone toxin is an obvious candidate for a vaccine, but by virtue of its chemical structure, this toxin is not immunogenic in itself. However, antibodies against some of the polyketide synthase domains involved in mycolactone synthesis, were found in Buruli ulcer patients and healthy controls from the same endemic region, suggesting that these domains are indeed immunogenic. Here we have analyzed the vaccine potential of nine polyketide synthase domains using a DNA prime/protein boost strategy. C57BL/6 mice were vaccinated against the following domains: acyl carrier protein 1, 2, and 3, acyltransferase (acetate) 1 and 2, acyltransferase (propionate), enoylreductase, ketoreductase A, and ketosynthase load module. As positive controls, mice were vaccinated with DNA encoding Ag85A or with M. bovis BCG. Strongest antigen specific antibodies could be detected in response to acyltransferase (propionate) and enoylreductase. Antigen-specific Th1 type cytokine responses (IL-2 or IFN-γ) were induced by vaccination against all antigens, and were strongest against acyltransferase (propionate). Finally, vaccination against acyltransferase (propionate) and enoylreductase conferred some protection against challenge with virulent M. ulcerans 1615. However, protection was weaker than the one conferred by vaccination with Ag85A or M. bovis BCG. Combinations of these polyketide synthase domains with the vaccine targeting Ag85A, of which the latter is involved in the integrity of the cell wall of the pathogen, and/or with live attenuated M. bovis BCG or mycolactone negative M. ulcerans may eventually lead to the development of an efficacious BU vaccine.

The cell wall-associated mycolactone polyketide synthases are necessary but not sufficient for mycolactone biosynthesis

Mycolactones are polyketide-derived lipid virulence factors made by the slow-growing human pathogen, Mycobacterium ulcerans. Three unusually large and homologous plasmid-borne genes (mlsA1: 51 kb, mlsB: 42 kb and mlsA2: 7 kb) encode the mycolactone type I polyketide synthases (PKS). The extreme size and low sequence diversity of these genes has posed significant barriers for exploration of the genetic and biochemical basis of mycolactone synthesis. Here, we have developed a truncated, more tractable 3-module version of the 18-module mycolactone PKS and we show that this engineered PKS functions as expected in the natural host M. ulcerans to produce an additional polyketide; a triketide lactone (TKL). Cell fractionation experiments indicated that this 3-module PKS and the putative accessory enzymes encoded by mup045 and mup038 associated with the mycobacterial cell wall, a finding supported by confocal microscopy. We then assessed the capacity of the faster growing, Mycobacterium marinum to harbor and express the 3-module Mls PKS and accessory enzymes encoded by mup045 and mup038. RT-PCR, immunoblotting, and cell fractionation experiments confirmed that the truncated Mls PKS multienzymes were expressed and also partitioned with the cell wall material in M. marinum. However, this heterologous host failed to produce TKL. The systematic deconstruction of the mycolactone PKS presented here suggests that the Mls multienzymes are necessary but not sufficient for mycolactone synthesis and that synthesis is likely to occur (at least in part) within the mycobacterial cell wall. This research is also the first proof-of-principle demonstration of the potential of this enzyme complex to produce tailored small molecules through genetically engineered rearrangements of the Mls modules.

Investigation of wild-type and mycolactone-negative mutant Mycobacterium ulcerans on skeletal muscle: IGF-1 protects against mycolactone-induced muscle catabolism

Buruli ulcer (BU), which is caused by Mycobacterium ulcerans (MU), is an endemic and neglected tropical disease that affects mostly subcutaneous tissues. Skeletal muscle under infected skin is also subject to serious dysfunctions and contractures. The goal of this study was to investigate the effects of an infection with the wild-type M. ulcerans (WT-MU) or the mycolactone-negative Mycobacterium ulcerans (Mneg-MU) mutant strains on myotubes or fully differentiated skeletal muscles. WT-MU infection decreased by 22% and 29% the maximal muscle force at days 7 and 42 postinfection, respectively, while Mneg-MU induced no decrease at day 7 postinfection and a small but significant 13% decrease in muscle force at day 42. A 13.2-fold and 4.3-fold increase in neutrophil and macrophage concentrations, respectively, was observed on day 42 following the injection of WT-MU. However, the increases in neutrophil and macrophage concentrations were 2.4-fold and 5.5-fold in Mneg-MU. Myoblast proliferation decreased by 20%, myotube diameter by 45%, MyHC levels by 32%, while MuRF-1 levels increased by 22.8% when C2C12 cells and WT-MU were cocultured for 48 h at a multiplicity of infection of 5:1. In contrast, Mneg-MU had no significant effect. Interestingly, the addition of 1,000 ng/ml of IGF-1 to the WT-MU/C2C12 coculture significantly improved all of these biological parameters. The present investigation clearly established that muscle dysfunction and chronic inflammation in the presence of WT-MU are largely caused by the release of mycolactone, and the addition of recombinant IGF-1 was sufficient to alleviate some of the antiproliferative and atrophic effects of mycolactone.

Structure-activity relationship studies on the macrolide exotoxin mycolactone of Mycobacterium ulcerans

Background

Mycolactones are a family of polyketide-derived macrolide exotoxins produced by Mycobacterium ulcerans, the causative agent of the chronic necrotizing skin disease Buruli ulcer. The toxin is synthesized by polyketide synthases encoded by the virulence plasmid pMUM. The apoptotic, necrotic and immunosuppressive properties of mycolactones play a central role in the pathogenesis of M. ulcerans.

Methodology/Principal Findings

We have synthesized and tested a series of mycolactone derivatives to conduct structure-activity relationship studies. Flow cytometry, fluorescence microscopy and Alamar Blue-based metabolic assays were used to assess activities of mycolactones on the murine L929 fibroblast cell line. Modifications of the C-linked upper side chain (comprising C12–C20) caused less pronounced changes in cytotoxicity than modifications in the lower C5-O-linked polyunsaturated acyl side chain. A derivative with a truncated lower side chain was unique in having strong inhibitory effects on fibroblast metabolism and cell proliferation at non-cytotoxic concentrations. We also tested whether mycolactones have antimicrobial activity and found no activity against representatives of Gram-positive (Streptococcus pneumoniae) or Gram-negative bacteria (Neisseria meningitis and Escherichia coli), the fungus Saccharomyces cerevisae or the amoeba Dictyostelium discoideum.

Conclusion

Highly defined synthetic compounds allowed to unambiguously compare biological activities of mycolactones expressed by different M. ulcerans lineages and may help identifying target structures and triggering pathways.

Buruli ulcer is a chronic necrotizing skin disease caused by Mycobacterium ulcerans. The characteristic histopathological features of Buruli ulcer, severe destruction of subcutaneous tissue with minimal inflammation in the core of the lesion, are primarily attributed to the cytotoxic activity of mycolactone, the macrolide exotoxin of M. ulcerans. Different geographical lineages of M. ulcerans produce different structural variants of mycolactone. By using highly defined synthetic mycolactones, including both naturally occurring molecular species and additional non-natural variants, we have assessed the influence of the structure of the C-linked upper side chain and the lower C5-O-linked polyunsaturated acyl side chain on biological activity. Changes in the lower side chain affected the cytotoxic activity against mammalian cells more profoundly than changes in the upper side chain. Mycolactone A/B had no antimicrobial activity against Gram-positive and Gram-negative bacteria and was also inactive against Saccharomyces and Dictyostelium.

Mycolactone activation of Wiskott-Aldrich syndrome proteins underpins Buruli ulcer formation

Mycolactone is a diffusible lipid secreted by the human pathogen Mycobacterium ulcerans, which induces the formation of open skin lesions referred to as Buruli ulcers. Here, we show that mycolactone operates by hijacking the Wiskott-Aldrich syndrome protein (WASP) family of actin-nucleating factors. By disrupting WASP autoinhibition, mycolactone leads to uncontrolled activation of ARP2/3-mediated assembly of actin in the cytoplasm. In epithelial cells, mycolactone-induced stimulation of ARP2/3 concentrated in the perinuclear region, resulting in defective cell adhesion and directional migration. In vivo injection of mycolactone into mouse ears consistently altered the junctional organization and stratification of keratinocytes, leading to epidermal thinning, followed by rupture. This degradation process was efficiently suppressed by coadministration of the N-WASP inhibitor wiskostatin. These results elucidate the molecular basis of mycolactone activity and provide a mechanism for Buruli ulcer pathogenesis. Our findings should allow for the rationale design of competitive inhibitors of mycolactone binding to N-WASP, with anti-Buruli ulcer therapeutic potential.

Microbiological, histological, immunological, and toxin response to antibiotic treatment in the mouse model of Mycobacterium ulcerans disease

Mycobacterium ulcerans infection causes a neglected tropical disease known as Buruli ulcer that is now found in poor rural areas of West Africa in numbers that sometimes exceed those reported for another significant mycobacterial disease, leprosy, caused by M. leprae. Unique among mycobacterial diseases, M. ulcerans produces a plasmid-encoded toxin called mycolactone (ML), which is the principal virulence factor and destroys fat cells in subcutaneous tissue. Disease is typically first manifested by the appearance of a nodule that eventually ulcerates and the lesions may continue to spread over limbs or occasionally the trunk. The current standard treatment is 8 weeks of daily rifampin and injections of streptomycin (RS). The treatment kills bacilli and wounds gradually heal. Whether RS treatment actually stops mycolactone production before killing bacilli has been suggested by histopathological analyses of patient lesions. Using a mouse footpad model of M. ulcerans infection where the time of infection and development of lesions can be followed in a controlled manner before and after antibiotic treatment, we have evaluated the progress of infection by assessing bacterial numbers, mycolactone production, the immune response, and lesion histopathology at regular intervals after infection and after antibiotic therapy. We found that RS treatment rapidly reduced gross lesions, bacterial numbers, and ML production as assessed by cytotoxicity assays and mass spectrometric analysis. Histopathological analysis revealed that RS treatment maintained the association of the bacilli with (or within) host cells where they were destroyed whereas lack of treatment resulted in extracellular infection, destruction of host cells, and ultimately lesion ulceration. We propose that RS treatment promotes healing in the host by blocking mycolactone production, which favors the survival of host cells, and by killing M. ulcerans bacilli.

Mycobacterium ulcerans infection causes Buruli ulcer (BU), a disfiguring skin disease now found principally in poor rural areas of West Africa. M. ulcerans produces a toxin called mycolactone (ML), which destroys fat cells in skin tissue. BU typically first shows as a nodule that eventually ulcerates. The lesions may continue to spread over limbs or occasionally the trunk. The current standard treatment is 8 weeks of daily rifampin and injections of streptomycin (RS). The treatment kills the bacilli and wounds gradually heal. We tried to determine if RS treatment actually stops mycolactone production before killing bacilli. Using a mouse footpad model of M. ulcerans infection where the time of infection and lesion development can be followed in a controlled manner before and after antibiotic treatment, we found that RS treatment rapidly reduced footpad swelling, M. ulcerans numbers, and ML production. Microscopic analysis of footpads revealed that RS treatment resulted in bacilli being destroyed by host cells whereas lack of treatment resulted in extracellular infection, destruction of host cells, and lesion ulceration. We propose that RS treatment promotes healing in the host by blocking mycolactone production, which favors the survival of host cells, and by killing M. ulcerans.

Development of a temperature-switch PCR-based SNP typing method for Mycobacterium ulcerans

Mycobacterium ulcerans (M. ulcerans), the causative agent of the devastating skin disease Buruli ulcer (BU), is characterized by an extremely low level of genetic diversity. Recently, we have reported the first discrimination of closely related M. ulcerans variants in the BU endemic Densu River Valley of Ghana. In the study real-time PCR-based single nucleotide polymorphism (SNP) typing at 89 predefined loci revealed the presence of ten M. ulcerans haplotypes circulating in the BU endemic region. Here we describe the development of temperature-switch PCR (TSP) assays that allow distinguishing these haplotypes by conventional agarose gel-based analysis of the PCR products. After validation of the accuracy of typing results, the TSP assays were successfully established in a reference laboratory in Ghana. Development of the cost-effective and rapid TSP-based genetic fingerprinting method will thus allow investigating the spread of M. ulcerans clones by regular genetic monitoring in BU endemic countries.

On the origin of Mycobacterium ulcerans, the causative agent of Buruli ulcer

Background

Mycobacterium ulcerans is an unusual bacterial pathogen with elusive origins. While closely related to the aquatic dwelling M. marinum, M. ulcerans has evolved the ability to produce the immunosuppressive polyketide toxin mycolactone and cause the neglected tropical disease Buruli ulcer. Other mycolactone-producing mycobacteria (MPM) have been identified in fish and frogs and given distinct species designations (M. pseudoshottsii, M. shinshuense, M. liflandii and M. marinum), however the evolution of M. ulcerans and its relationship to other MPM has not been defined. Here we report the comparative analysis of whole genome sequences from 30 MPM and five M. marinum.

Results

A high-resolution phylogeny based on genome-wide single nucleotide polymorphisms (SNPs) showed that M. ulcerans and all other MPM represent a single clonal group that evolved from a common M. marinum progenitor. The emergence of the MPM was driven by the acquisition of the pMUM plasmid encoding genes for the biosynthesis of mycolactones. This change was accompanied by the loss of at least 185 genes, with a significant overrepresentation of genes associated with cell wall functions. Cell wall associated genes also showed evidence of substantial adaptive selection, suggesting cell wall remodeling has been critical for the survival of MPM. Fine-grain analysis of the MPM complex revealed at least three distinct lineages, one of which comprised a highly clonal group, responsible for Buruli ulcer in Africa and Australia. This indicates relatively recent transfer of M. ulcerans between these continents, which represent the vast majority of the global Buruli ulcer burden. Our data provide SNPs and gene sequences that can differentiate M. ulcerans lineages, suitable for use in the diagnosis and surveillance of Buruli ulcer.

Conclusions

M. ulcerans and all mycolactone-producing mycobacteria are specialized variants of a common Mycobacterium marinum progenitor that have adapted to live in restricted environments. Examination of genes lost or retained and now under selective pressure suggests these environments might be aerobic, and extracellular, where slow growth, production of an immune suppressor, cell wall remodeling, loss or modification of cell wall antigens, and biofilm-forming ability provide a survival advantage. These insights will guide our efforts to find the elusive reservoir(s) of M. ulcerans and to understand transmission of Buruli ulcer.

Sero-epidemiology as a tool to screen populations for exposure to Mycobacterium ulcerans

Background

Previous analyses of sera from a limited number of Ghanaian Buruli ulcer (BU) patients, their household contacts, individuals living in BU non-endemic regions as well as European controls have indicated that antibody responses to the M. ulcerans 18 kDa small heat shock protein (shsp) reflect exposure to this pathogen. Here, we have investigated to what extent inhabitants of regions in Ghana regarded as non-endemic for BU develop anti-18 kDa shsp antibody titers.

Methodology/Principal Findings

For this purpose we determined anti-18 kDa shsp IgG titers in sera collected from healthy inhabitants of the BU endemic Densu River Valley and the Volta Region, which was so far regarded as BU non-endemic. Significantly more sera from the Densu River Valley contained anti-18 kDa shsp IgG (32% versus 12%, respectively). However, some sera from the Volta Region also showed high titers. When interviewing these sero-responders, it was revealed that the person with the highest titer had a chronic wound, which was clinically diagnosed and laboratory reconfirmed as active BU. After identification of this BU index case, further BU cases were clinically diagnosed by the Volta Region local health authorities and laboratory reconfirmed. Interestingly, there was neither a difference in sero-prevalence nor in IS2404 PCR positivity of environmental samples between BU endemic and non-endemic communities located in the Densu River Valley.

Conclusions

These data indicate that the intensity of exposure to M. ulcerans in endemic and non-endemic communities along the Densu River is comparable and that currently unknown host and/or pathogen factors may determine how frequently exposure is leading to clinical disease. While even high serum titers of anti-18 kDa shsp IgG do not indicate active disease, sero-epidemiological studies can be used to identify new BU endemic areas.

Sero-epidemiological analyses revealed that a higher proportion of sera from individuals living in the Buruli ulcer (BU) endemic Densu River Valley of Ghana contain Mycobacterium ulcerans 18 kDa small heat shock protein (shsp)-specific IgG than sera from inhabitants of the Volta Region, which was regarded so far as BU non-endemic. However, follow-up studies in the Volta Region showed that the individual with the highest anti-18 kDa shsp-specific serum IgG titer of all participants from the Volta Region had a BU lesion. Identification of more BU patients in the Volta Region by subsequent active case search demonstrated that sero-epidemiology can help identify low endemicity areas. Endemic and non-endemic communities along the Densu River Valley differed neither in sero-prevalence nor in positivity of environmental samples in PCR targeting M. ulcerans genomic and plasmid DNA sequences. A lower risk of developing M. ulcerans disease in the non-endemic communities may either be related to host factors or a lower virulence of local M. ulcerans strains.

Mycolactone diffuses into the peripheral blood of Buruli ulcer patients--implications for diagnosis and disease monitoring

BACKGROUND

Mycobacterium ulcerans, the causative agent of Buruli ulcer (BU), is unique among human pathogens in its capacity to produce a polyketide-derived macrolide called mycolactone, making this molecule an attractive candidate target for diagnosis and disease monitoring. Whether mycolactone diffuses from ulcerated lesions in clinically accessible samples and is modulated by antibiotic therapy remained to be established.

METHODOLOGY/PRINCIPAL FINDING

Peripheral blood and ulcer exudates were sampled from patients at various stages of antibiotic therapy in Ghana and Ivory Coast. Total lipids were extracted from serum, white cell pellets and ulcer exudates with organic solvents. The presence of mycolactone in these extracts was then analyzed by a recently published, field-friendly method using thin layer chromatography and fluorescence detection. This approach did not allow us to detect mycolactone accurately, because of a high background due to co-extracted human lipids. We thus used a previously established approach based on high performance liquid chromatography coupled to mass spectrometry. By this means, we could identify structurally intact mycolactone in ulcer exudates and serum of patients, and evaluate the impact of antibiotic treatment on the concentration of mycolactone.

CONCLUSIONS/SIGNIFICANCE

Our study provides the proof of concept that assays based on mycolactone detection in serum and ulcer exudates can form the basis of BU diagnostic tests. However, the identification of mycolactone required a technology that is not compatible with field conditions and point-of-care assays for mycolactone detection remain to be worked out. Notably, we found mycolactone in ulcer exudates harvested at the end of antibiotic therapy, suggesting that the toxin is eliminated by BU patients at a slow rate. Our results also indicated that mycolactone titres in the serum may reflect a positive response to antibiotics, a possibility that it will be interesting to examine further through longitudinal studies.

Mycolactone impairs T cell homing by suppressing microRNA control of L-selectin expression

Mycolactone is a macrolide produced by Mycobacterium ulcerans with immunomodulatory properties. Here, we describe that in mouse, mycolactone injection led to a massive T-cell depletion in peripheral lymph nodes (PLNs) that was associated with defective expression of L-selectin (CD62-L). Importantly, preexposure to mycolactone impaired the capacity of T cells to reach PLNs after adoptive transfer, respond to chemotactic signals, and expand upon antigenic stimulation in vivo. We found that mycolactone-induced suppression of CD62-L expression by human primary T cells was induced rapidly at both the mRNA and protein levels and correlated with the reduced expression of one miRNA: let-7b. Notably, silencing of let-7b was sufficient to inhibit CD62-L gene expression. Conversely, its overexpression tended to up-regulate CD62-L and counteract the effects of mycolactone. Our results identify T-cell homing as a biological process targeted by mycolactone. Moreover, they reveal a mechanism of control of CD62-L expression involving the miRNA let-7b.

Buruli ulcer: Advances in understanding Mycobacterium ulcerans infection

Buruli ulcer (BU), caused by the environmental organism Mycobacterium ulcerans and characterized by necrotizing skin and bone lesions, poses important public health issues as the third most common mycobacterial infection in humans. Pathogenesis of M ulcerans is mediated by mycolactone, a necrotizing immunosuppressive toxin. First-line therapy for BU is rifampin plus streptomycin, sometimes with surgery. New insights into the pathogenesis of BU should improve control strategies.

Chemistry of mycolactones, the causative toxins of Buruli ulcer

Buruli ulcer is a severe and devastating skin disease caused by Mycobacterium ulcerans infection, yet it is one of the most neglected diseases. The causative toxin, referred to as mycolactone A/B, was isolated and characterized as a polyketide-derived macrolide in 1999. The current status of the mycolactone chemistry is described, highlighting the stereochemistry assignment of mycolactone A/B; total synthesis; the structure determination of mycolactone congeners from the human pathogen M. ulcerans, the frog pathogen Mycobacterium liflandii, and the fish pathogen Mycobacterium marinum; the structural diversity in the mycolactone class of natural products; the highly sensitive detection/structure-analysis of mycolactones; and some biological activity.

Mycobacterium ulcerans infections cause progressive muscle atrophy and dysfunction, and mycolactone impairs satellite cell proliferation

Clinical observations from Buruli ulcer (BU) patients in West Africa suggest that severe Mycobacterium ulcerans infections can cause skeletal muscle contracture and atrophy leading to significant impairment in function. In the present study, male mice C57BL/6 were subcutaneously injected with M. ulcerans in proximity to the right biceps muscle, avoiding direct physical contact between the infectious agent and the skeletal muscle. The histological, morphological, and functional properties of the muscles were assessed at different times after the injection. On day 42 postinjection, the isometric tetanic force and the cross-sectional area of the myofibers were reduced by 31% and 29%, respectively, in the proximate-infected muscles relative to the control muscles. The necrotic areas of the proximate-infected muscles had spread to 7% of the total area by day 42 postinjection. However, the number of central nucleated fibers and myogenic regulatory factors (MyoD and myogenin) remained stable and low. Furthermore, Pax-7 expression did not increase significantly in mycolactone-injected muscles, indicating that the satellite cell proliferation is abrogated by the toxin. In addition, the fibrotic area increased progressively during the infection. Lastly, muscle-specific RING finger protein 1 (MuRF-1) and atrogin-1/muscle atrophy F-box protein (atrogin-1/MAFbx), two muscle-specific E3 ubiquitin ligases, were upregulated in the presence of M. ulcerans. These findings confirmed that skeletal muscle is affected in our model of subcutaneous infection with M. ulcerans and that a better understanding of muscle contractures and weakness is essential to develop a therapy to minimize loss of function and promote the autonomy of BU patients.