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International audience; Mycobacterium ulcerans, the causative agent of Buruli ulcer disease, is unique among human pathogens in its capacity to produce mycolactone, a diffusible macrolide with immunosuppressive and cytotoxic properties. Recent studies have shown that mycolactone operates by inhibiting the host membrane translocation complex (Sec61), with an unprecedented potency compared to previously identified Sec61 blockers. Mycolactone binding to the pore-forming subunit of Sec61 inhibits its capacity to transport nascent secretory and membrane proteins into the endoplasmic reticulum, leading to their cytosolic degradation by the ubiquitin:proteasome system. In T lymphocytes, Sec61 blockade by mycolactone manifests as a sharp decrease in the cell's ability to express homing receptors and release cytokines following activation. Sustained exposure of human cells to mycolactone typically generates proteotoxic stress responses in their cytosol and endoplasmic reticulum (ER), ultimately inducing apoptosis. Here we describe cell-free systems for studying Sec61-mediated protein translocation that allow the impact of mycolactone on the biogenesis of secretory and membrane proteins to be probed. We also describe biological assays of mycolactone-driven inhibition of Sec61 providing rapid and sensitive means to quantitatively assess the presence of the toxin in biological samples.

International audience; 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.

International audience; Mycolactones are a family of polyketide synthase products made by the human pathogen Mycobacterium ulcerans that were recently identified as novel inhibitors of the host membrane translocation complex (Sec61). Here, we provide protocols for the purification of mycolactones from bacterial cultures, and for their quantitative assessment in biological samples.

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., Author summary Mycolactone is the major virulence factor of Mycobacterium ulcerans, the human pathogen causing Buruli ulcer (BU) disease. While it is now established that mycolactone is able to diffuse from infected tissues to exert immunomodulatory and analgesic effects at the systemic level, the in vivo spatiotemporal distribution of mycolactone remained largely unknown. Here, using the zebrafish larva, we describe the spatiotemporal distribution of a fluorescent derivative of mycolactone in vivo. We show that fluorescent mycolactone quickly diffuses from the blood circulation into various organs, and accumulates in muscles and brain. Mycolactone’s diffusion into the central nervous system was further confirmed in the mouse model of M. ulcerans infection. We reported previously that mycolactone suppresses the production of inflammatory mediators by primary microglia at nanomolar concentrations in vitro [1]. In the present study, we provide evidence suggesting that bacterially-produced mycolactone is able to reach the brain. While additional in vivo investigations will be required, we can speculate that the amount of mycolactone reaching the brain in the context of M. ulcerans infection is sufficient to modulate inflammation and pain transmission.

Mycobacterium ulcerans is a human pathogen that causes a necrotizing skin disease known as Buruli ulcer. Necrosis of infected skin is driven by bacterial production of mycolactone, a diffusible exotoxin targeting the host translocon (Sec61). By blocking Sec61, mycolactone prevents the transport of nascent secretory proteins into the endoplasmic reticulum of host cells. This triggers pro-apoptotic stress responses partially depending on activation of the ATF4 transcription factor. To gain further insight into the molecular pathways mediating the cytotoxic effects of mycolactone we conducted the first haploid genetic screen with the M. ulcerans toxin in KBM-7 cells. This approach allowed us to identify the histone methyltransferase SETD1B as a novel mediator of mycolactone-induced cell death. CRISPR/Cas9-based inactivation of SETD1B rendered cells resistant to lethal doses of the toxin, highlighting the critical importance of this gene’s expression. To understand how SETD1B contributes to mycolactone cytotoxicity, we compared the transcriptomes of wild-type (WT) and SETD1B knockout KBM-7 cells upon exposure to the toxin. While ATF4 effectors were upregulated by mycolactone in both WT and SETD1B knockout cells, mycolactone selectively induced the expression of pro-apoptotic genes in WT cells. Among those genes we identified CHAC1, which codes for a major glutathione (GSH)-degrading enzyme, and whose strong upregulation in mycolactone-treated WT cells correlated with a marked reduction in GSH protein level. Moreover, GSH supplementation conferred cells with substantial protection against the toxic effects of mycolactone. Our data thus identify SETD1B/CHAC1/GSH as a novel, epigenetic mechanism connecting Sec61 blockade with apoptotic cell death. They suggest that GSH-based treatments might have the capacity to limit skin necrosis in Buruli ulcer., Author summary The human pathogen Mycobacterium ulcerans causes a necrotizing skin disease known as Buruli ulcer. The major toxin of the mycobacteria, mycolactone, prevents the transport of secretory proteins into the endoplasmic reticulum, and thereby triggers a deadly stress response. We conducted the first haploid genetic screen to identify host factors with impact on mycolactone toxicity. This enabled us to identify the histone methyltransferase SETD1B as a novel mediator of mycolactone-induced cell death. RNA analyses of wild-type cells and resistant SETD1B knockout cells treated with mycolactone then showed a selective induction of genes implicated in programmed cell-death only in wild-type cells. This was accompanied by a marked reduction of the antioxidant glutathione, which might cause the mycolactone induced cell death.

International audience; In the past, it has proved challenging to generate antibodies against mycolactone, the primary lipidic toxin A of Mycobacterium ulcerans causing Buruli ulcer, due to its immunosuppressive properties. Here we show that in vitro display, comprising both phage and yeast display, can be used to select antibodies recognizing mycolactone from a large human naïve phage antibody library. Ten different antibodies were isolated, and hundreds more identified by next generation sequencing. These results indicate the value of in vitro display methods to generate antibodies against difficult antigenic targets such as toxins, which cannot be used for immunization unless inactivated by structural modification. The possibility to easily generate anti-mycolactone antibodies is an exciting prospect for the development of rapid and simple diagnostic/detection methods.

Funding Information: We thank the Arkansas Nano & Bio Materials Characterization Facility at the Institute for Nano Sciences & Engineering for our imaging studies, and Prof Yoshito Kishi (Harvard University) for the kind gift of synthetic mycolactone A/B used by S.H. and R.S. W.S. is supported by Grant No. R15GM116032 from the National Institute of General Medical Sciences of the National Institutes of Health (NIH) and startup funds from the University of Arkansas. This work was also supported in part by Grant No. P30 GM103450 from the National Institute of General Medical Sciences of the NIH and by seed money from the Arkansas Biosciences Institute (ABI). S.O’K. is the recipient of a Biotechnology and Biological Sciences Research Council (BBSRC) Doctoral Training Programme Award (BB/J014478/ 1), and S.H. holds a Welcome Trust Investigator Award in Science (204957/Z/16/Z). The alpha-1 antitrypsin work was supported by the Alpha-1 Foundation (J.I. and M.J.I.). J.I. and M.J.H. were supported by the intramural program of NCATS, National Institutes of Health, projects 1ZIATR000048-03 (J.I.) and ZIATR000063-04 (M.J.H.). R.S. holds a Welcome Trust Investigator Award in Science (202843/Z/16/Z). C.D. received funding from the Institut Pasteur, the Institut National de la Santé et de la Recherche Med́ icale, and the Fondation Raoul Follereau. N.B.’s synthesis and chemical biology studies of mycolactone were supported by CNRS, Université de Strasbourg, Fondations Potier et Follereau, and the Investisse-ment d’Avenir (Idex Unistra). V.O.P. is supported by the Academy of Finland (Grants 289737 and 314672) and the Sigrid Juselius Foundation. Funding Information: We thank the Arkansas Nano & Bio Materials Characterization Facility at the Institute for Nano Sciences & Engineering for our imaging studies, and Prof Yoshito Kishi (Harvard University) for the kind gift of synthetic mycolactone A/B used by S.H. and R.S. W.S. is supported by Grant No. R15GM116032 from the National Institute of General Medical Sciences of the National Institutes of Health (NIH) and startup funds from the University of Arkansas. This work was also supported in part by Grant No. P30 GM103450 from the National Institute of General Medical Sciences of the NIH and by seed money from the Arkansas Biosciences Institute (ABI). S.O'K. is the recipient of a Biotechnology and Biological Sciences Research Council (BBSRC) Doctoral Training Programme Award (BB/J014478/1), and S.H. holds a Welcome Trust Investigator Award in Science (204957/Z/16/Z). The alpha-1 antitrypsin work was supported by the Alpha-1 Foundation (J.I. and M.J.I.). J.I. and M.J.H. were supported by the intramural program of NCATS, National Institutes of Health, projects 1ZIATR000048-03 (J.I.) and ZIATR000063-04 (M.J.H.). R.S. holds a Welcome Trust Investigator Award in Science (202843/Z/16/Z). C.D. received funding from the Institut Pasteur, the Institut National de la Sante et de la Recherche Medicale, and the Fondation Raoul Follereau. N.B.'s synthesis and chemical biology studies of mycolactone were supported by CNRS, Universite de Strasbourg, Fondations Potier et Follereau and the Investissement d'Avenir (Idex Unistra). V.O.P. is supported by the Academy of Finland (Grants 289737 and 314672) and the Sigrid Juselius Foundation. Publisher Copyright: © 2019 American Chemical Society. Ipomoeassin F is a potent natural cytotoxin that inhibits growth of many tumor cell lines with single-digit nanomolar potency. However, its biological and pharmacological properties have remained largely unexplored. Building upon our earlier achievements in total synthesis and medicinal chemistry, we used chemical proteomics to identify Sec61 alpha (protein transport protein Sec61 subunit alpha isoform 1), the pore-forming subunit of the Sec61 protein translocon, as a direct binding partner of ipomoeassin F in living cells. The interaction is specific and strong enough to survive lysis conditions, enabling a biotin analogue of ipomoeassin F to pull down Sec61 alpha from live cells, yet it is also reversible, as judged by several experiments including fluorescent streptavidin staining, delayed competition in affinity pulldown, and inhibition of TNF biogenesis after washout. Sec61 alpha forms the central subunit of the ER protein translocation complex, and the binding of ipomoeassin F results in a substantial, yet selective, inhibition of protein translocation in vitro and a broad ranging inhibition of protein secretion in live cells. Lastly, the unique resistance profile demonstrated by specific amino acid single-point mutations in Sec61 alpha provides compelling evidence that Sec61 alpha is the primary molecular target of ipomoeassin F and strongly suggests that the binding of this natural product to Sec61 alpha is distinctive. Therefore, ipomoeassin F represents the first plant-derived, carbohydrate-based member of a novel structural class that offers new opportunities to explore Sec61 alpha function and to further investigate its potential as a therapeutic target for drug discovery.

International audience; From their observation of necrotic areas around bacterial foci in Buruli ulcers (BUs), Connor and co-workers were the first, back in 1965, to suggest that M. ulcerans may produce a diffusible cytotoxin [1]. This hypothesis was later confirmed by injecting mycobacterial culture filtrates into the skin of guinea pigs, showing that this causes focal necrosis resembling that of naturally occurring human infections [1–3]. In 1999, George et al. succeeded in isolating a cytotoxic factor from M. ulcerans lipid extracts, and deciphered its chemical nature [4]. The M. ulcerans toxin was named mycolactone, based on its mycobacterial origin and macrolactone structure: a 12-membered lactone ring, to which a C5-O-linked polyunsaturated acyl side chain and a C-linked upper side chain comprising C12–C20 are appended (Fig. 1). Follow-up studies showed that M. ulcerans-derived mycolactone was in reality a mixture of two stereoisomers called A and B [6, 7] (Fig. 1). Since the initial discovery of mycolactone A/B, eight additional mycolactone congeners have been identified that are either produced by M. ulcerans strains of different geographical origins or genetically related fish and frog pathogens, which were initially given different species designations such as M. pseudoshottsii and M. liflandii. Comparative genome analysis later revealed that all mycolactone-producing mycobacteria evolved from a common M. marinum-like progenitor, and are therefore ecovars of a single M. ulcerans species [8]. While the macrolide core structure and upper side chain are conserved, mycolactone populations from different M. ulcerans sub-lineages vary in the length, number and localization of hydroxyl groups and number of double bonds of the lower side chain. These modifications of the lower polyunsaturated acyl side chain cause pronounced changes in cytotoxicity [9–11]. The origin and chemistry of natural mycolactones, structure-activity relationships and the various approaches that were developed to generate synthetic mycolactones have been reviewed recently [12, 13]. These aspects will therefore not be further discussed here. Instead, this chapter provides an update on our understanding of mycolactone (A/B) biology, and discusses its proposed mechanisms of action in relation with BU pathogenesis.

Mycolactone is a diffusible lipid produced by the human pathogen Mycobacteriumulcerans, the causative agent of a tropical skin disease called Buruli ulcer. Bacterial production of mycolactone in infected skin causes local tissue necrosis, while inducing immunosuppressive defects at the systemic level. When I started my PhD, the molecular mechanism(s) underpinning these effects were unknown. Over the course of my thesis, I contributed to demonstrate that mycolactoneis a novel inhibitor of the Sec61 translocon, a channel regulating the biogenesis of most secretedand membrane proteins in eukaryotic cells. Indeed, a single point mutation in the alpha subunit ofSec61 protected cells from the cytotoxic and immunosuppressive effects of mycolactone. I showed that mycolactone-mediated blockade of the Sec61 translocon efficiently prevents the synthesis ofkey immune receptors and signaling molecules, impeding the communication between immunecells that is required for the development of anti-mycobacterial immunity. Through a series of larges caleproteomic studies, I demonstrated that mycolactone is a broad-acting inhibitor of Sec61 and identified the Sec61 clients that are primarily down regulated by mycolactone in physiologicallyrelevant cell types. These analyses also allowed me to describe a unique stress response,encompassing elements of the unfolded protein response and integrated stress response, that isinduced upon protein translocation blockade and ultimately causes cell apoptosis. The Sec61 translocon has been proposed to play a role in other cell functions that require the retrograde transport of proteins across membranes, namely Endoplasmic Reticulum-Associated Degradation(ERAD), an essential process in protein quality control, and antigen export to the cytosol during cross-presentation, a pathway essential to the activation of adaptive immunity to intracellular pathogens and cancer. Using mycolactone, I showed that Sec61 blockade does not affect protein export to the cytosol in either of these pathways, arguing against Sec61 operating as are trotranslocon. Altogether, my work provided a molecular mechanism for the diverse effects of mycolactone in Buruli Ulcer patients, and thus for M. ulcerans virulence. Mycolactone representing the most potent Sec61 blocker identified to date, my studies also revealed the key importance of Sec61-mediated protein translocation in the regulation of immune responses and protein homeostasis.; La mycolactone est un lipide diffusible produit par Mycobacterium ulcerans, la bactérie responsable d'une maladie tropicale de la peau appelée ulcère de Buruli. La production de mycolactone dans la peau infectée par M. ulcerans provoque une nécrose tissulaire locale, tout en induisant des anomalies immunosuppressives au niveau systémique. Lorsque j'ai commencé mon doctorat, les mécanismes moléculaires à l'origine de ces effets étaient inconnus. Au cours de ma thèse, j'ai contribué à démontrer que la mycolactone est un nouvel inhibiteur du translocon Sec61,le canal régulant la biogenèse de la plupart des protéines sécrétées et membranaires dans les cellules eucaryotes. J’ai démontré qu’une mutation ponctuelle dans la sous-unité alpha de Sec61 protège les cellules des effets cytotoxiques et immunosuppresseurs de la mycolactone. J'ai montré que le blocage du translocon Sec61 par la mycolactone empêche efficacement la synthèse des principaux récepteurs immunitaires et des molécules de signalisation du système immunitaire,blockant la communication entre les cellules immunitaires et inhibant l'immunité antimycobactérienne. Par une série d'études protéomiques à grande échelle, j'ai démontré que la mycolactone est un inhibiteur à large action de Sec61 et j’ai identifié les substrats de Sec61 les plus impactés dans différents types cellulaires. Ces analyses m'ont également permis de décrire la réponse au stress induite par le blocage de la translocation des protéines, qui inclut des éléments de la réponse au stress protéostatique (UPR) et de la réponse intégrée au stress (ISR), provoquant finalement l'apoptose cellulaire. Plusieurs études ont impliqué le translocon Sec61 dans des processus qui requièrent le transport rétrograde de protéines à travers les membranes : la Dégradation Associée au Réticulum Endoplasmique (ERAD), processus essentiel du contrôle de la qualité des protéines et la cross-présentation, une voie essentielle à l'activation de l'immunité adaptative aux pathogènes intracellulaires et au cancer. J'ai montré que le blocage de Sec61 par la mycolactone n'affecte pas l'export de protéines vers le cytosol dans ces deux voies, suggérant que Sec61 ne peut pas fonctionner comme un rétrotranslocon. Mes travaux ont permis d’élucider le moléculaire responsable des divers effets de la mycolactone observés chez les patients atteints d’ulcère de Buruli et de révéler l’importance majeure de la translocation des protéines dans la régulation des réponses immunitaires et de l’homéostasie des protéines.

Mycolactone is a diffusible lipid produced by the human pathogen Mycobacteriumulcerans, the causative agent of a tropical skin disease called Buruli ulcer. Bacterial production of mycolactone in infected skin causes local tissue necrosis, while inducing immunosuppressive defects at the systemic level. When I started my PhD, the molecular mechanism(s) underpinning these effects were unknown. Over the course of my thesis, I contributed to demonstrate that mycolactoneis a novel inhibitor of the Sec61 translocon, a channel regulating the biogenesis of most secretedand membrane proteins in eukaryotic cells. Indeed, a single point mutation in the alpha subunit ofSec61 protected cells from the cytotoxic and immunosuppressive effects of mycolactone. I showed that mycolactone-mediated blockade of the Sec61 translocon efficiently prevents the synthesis ofkey immune receptors and signaling molecules, impeding the communication between immunecells that is required for the development of anti-mycobacterial immunity. Through a series of larges caleproteomic studies, I demonstrated that mycolactone is a broad-acting inhibitor of Sec61 and identified the Sec61 clients that are primarily down regulated by mycolactone in physiologicallyrelevant cell types. These analyses also allowed me to describe a unique stress response,encompassing elements of the unfolded protein response and integrated stress response, that isinduced upon protein translocation blockade and ultimately causes cell apoptosis. The Sec61 translocon has been proposed to play a role in other cell functions that require the retrograde transport of proteins across membranes, namely Endoplasmic Reticulum-Associated Degradation(ERAD), an essential process in protein quality control, and antigen export to the cytosol during cross-presentation, a pathway essential to the activation of adaptive immunity to intracellular pathogens and cancer. Using mycolactone, I showed that Sec61 blockade does not affect protein export to the cytosol in either of these pathways, arguing against Sec61 operating as are trotranslocon. Altogether, my work provided a molecular mechanism for the diverse effects of mycolactone in Buruli Ulcer patients, and thus for M. ulcerans virulence. Mycolactone representing the most potent Sec61 blocker identified to date, my studies also revealed the key importance of Sec61-mediated protein translocation in the regulation of immune responses and protein homeostasis., La mycolactone est un lipide diffusible produit par Mycobacterium ulcerans, la bactérie responsable d'une maladie tropicale de la peau appelée ulcère de Buruli. La production de mycolactone dans la peau infectée par M. ulcerans provoque une nécrose tissulaire locale, tout en induisant des anomalies immunosuppressives au niveau systémique. Lorsque j'ai commencé mon doctorat, les mécanismes moléculaires à l'origine de ces effets étaient inconnus. Au cours de ma thèse, j'ai contribué à démontrer que la mycolactone est un nouvel inhibiteur du translocon Sec61,le canal régulant la biogenèse de la plupart des protéines sécrétées et membranaires dans les cellules eucaryotes. J’ai démontré qu’une mutation ponctuelle dans la sous-unité alpha de Sec61 protège les cellules des effets cytotoxiques et immunosuppresseurs de la mycolactone. J'ai montré que le blocage du translocon Sec61 par la mycolactone empêche efficacement la synthèse des principaux récepteurs immunitaires et des molécules de signalisation du système immunitaire,blockant la communication entre les cellules immunitaires et inhibant l'immunité antimycobactérienne. Par une série d'études protéomiques à grande échelle, j'ai démontré que la mycolactone est un inhibiteur à large action de Sec61 et j’ai identifié les substrats de Sec61 les plus impactés dans différents types cellulaires. Ces analyses m'ont également permis de décrire la réponse au stress induite par le blocage de la translocation des protéines, qui inclut des éléments de la réponse au stress protéostatique (UPR) et de la réponse intégrée au stress (ISR), provoquant finalement l'apoptose cellulaire. Plusieurs études ont impliqué le translocon Sec61 dans des processus qui requièrent le transport rétrograde de protéines à travers les membranes : la Dégradation Associée au Réticulum Endoplasmique (ERAD), processus essentiel du contrôle de la qualité des protéines et la cross-présentation, une voie essentielle à l'activation de l'immunité adaptative aux pathogènes intracellulaires et au cancer. J'ai montré que le blocage de Sec61 par la mycolactone n'affecte pas l'export de protéines vers le cytosol dans ces deux voies, suggérant que Sec61 ne peut pas fonctionner comme un rétrotranslocon. Mes travaux ont permis d’élucider le moléculaire responsable des divers effets de la mycolactone observés chez les patients atteints d’ulcère de Buruli et de révéler l’importance majeure de la translocation des protéines dans la régulation des réponses immunitaires et de l’homéostasie des protéines.

International audience; 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.

A modular total synthesis of mycolactone A/B, the exotoxin produced by Mycobacterium ulcerans, has been achieved through the orchestration of several Pd-catalyzed key steps. While this route leads to a mixture of the natural product and its C12 epimer (4 : 1 ratio), this was inconsequential from the biological activity standpoint. Compared to the previously reported routes, this synthetic blueprint allows the late-stage modification of the toxin, as exemplified by the preparation of [22,22,22-2H3]-mycolactone A/B.

Background Mycolactone is a macrolide produced by the skin pathogen Mycobacterium ulcerans, with cytotoxic, analgesic and immunomodulatory properties. The latter were recently shown to result from mycolactone blocking the Sec61-dependent production of pro-inflammatory mediators by immune cells. Here we investigated whether mycolactone similarly affects the inflammatory responses of the nervous cell subsets involved in pain perception, transmission and maintenance. We also investigated the effects of mycolactone on the neuroinflammation that is associated with chronic pain in vivo. Methodology/ Principle findings Sensory neurons, Schwann cells and microglia were isolated from mice for ex vivo assessment of mycolactone cytotoxicity and immunomodulatory activity by measuring the production of proalgesic cytokines and chemokines. In all cell types studied, prolonged (>48h) exposure to mycolactone induced significant cell death at concentrations >10 ng/ml. Within the first 24h treatment, nanomolar concentrations of mycolactone efficiently suppressed the cell production of pro-inflammatory mediators, without affecting their viability. Notably, mycolactone also prevented the pro-inflammatory polarization of cortical microglia. Since these cells critically contribute to neuroinflammation, we next tested if mycolactone impacts this pathogenic process in vivo. We used a rat model of neuropathic pain induced by chronic constriction of the sciatic nerve. Here, mycolactone was injected daily for 3 days in the spinal canal, to ensure its proper delivery to spinal cord. While this treatment failed to prevent injury-induced neuroinflammation, it decreased significantly the local production of inflammatory cytokines without inducing detectable cytotoxicity. Conclusion/ Significance The present study provides in vitro and in vivo evidence that mycolactone suppresses the inflammatory responses of sensory neurons, Schwann cells and microglia, without affecting the cell viability. Together with previous studies using peripheral blood leukocytes, our work implies that mycolactone-mediated analgesia may, at least partially, be explained by its anti-inflammatory properties., Author summary Mycolactone is a complex macrolidic polyketide produced by the skin pathogen Mycobacterium ulcerans, with cytotoxic, analgesic and anti-inflammatory properties. Peripheral nerve destruction and activation of type 2 angiotensin II receptors on sensory neurons have been proposed to mediate bacteria-induced hypoesthesia in infected skin. In addition, mycolactone was recently shown to block the co-translational translocation of secretory proteins into the endoplasmic reticulum in host cells, leading to defective inflammatory responses. Here we examined if this last mechanism may also contribute to inhibit neuro-inflammation and particularly in the context of neuropathic pain. Using a representative panel of primary cells from the central and peripheral nervous systems, we found that indeed, mycolactone potently inhibits the production of pro-inflammatory mediators at non-cytotoxic concentrations. Notably, mycolactone prevented the polarization and pro-inflammatory functions of cortical microglia, which are critical inducers of neuroinflammation. Consistent with our in vitro findings, mycolactone had potent anti-inflammatory effects on the spinal cord of rats injected in the spinal canal, with no apparent side effects. Our data show that mycolactone suppresses inflammatory responses in the nervous system similarly as in the immune system, suggesting that mycolactone-mediated analgesia may, at least partially, be explained by its anti-inflammatory properties.

Baron et al. show that mycolactone, an immunosuppressive macrolide produced by the pathogen Mycobacterium ulcerans, operates by targeting the Sec61 translocon. This identifies the most potent Sec61 inhibitor reported to date and the potential of inhibiting Sec61 for immune modulation., 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.

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.