The Macrolide Toxin Mycolactone Promotes Bim-Dependent Apoptosis in Buruli Ulcer through Inhibition of mTOR

Abstract

Mycolactone, the macrolide exotoxin produced by Mycobacterium ulcerans, is central to the pathogenesis of the chronic necrotizing skin disease Buruli ulcer (BU). Here we show that mycolactone acts as an inhibitor of the mechanistic Target of Rapamycin (mTOR) signaling pathway by interfering with the assembly of the two distinct mTOR protein complexes mTORC1 and mTORC2, which regulate different cellular processes. Inhibition of the assembly of the rictor containing mTORC2 complex by mycolactone prevents phosphorylation of the serine/threonine protein kinase Akt. The associated inactivation of Akt leads to the dephosphorylation and activation of the Akt-targeted transcription factor FoxO3. Subsequent up-regulation of the FoxO3 target gene BCL2L11 (Bim) increases expression of the pro-apoptotic regulator Bim, driving mycolactone treated mammalian cells into apoptosis. The central role of Bim-dependent apoptosis in BU pathogenesis deduced from our experiments with cultured mammalian cells was further verified in an experimental M. ulcerans infection model. As predicted by the model, M. ulcerans infected Bim knockout mice did not develop necrotic BU lesions with large clusters of extracellular bacteria, but were able to contain the mycobacterial multiplication. Our findings provide a new coherent and comprehensive concept of BU pathogenesis.

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

“…Short‐term (4–16 h) exposure to mycolactone induced rapid alterations in the actin cytoskeleton of HeLa cells, coinciding with a defective capacity of the cells to establish adhesive contacts and migrate directionally in wound‐healing assays in vitro [Guenin‐Mace et al., ]. In all skin cells studied, longer treatments (>48 h) induced cell retraction followed by detachment and apoptosis, albeit with slight differences in time‐to‐death across cell types [Bieri et al., ; Dangy et al., ; Gama et al., ; George et al., ; Guenin‐Mace et al., ; Ogbechi et al., ; Snyder and Small, ]. In human dermal microvascular endothelial cells, mycolactone treatment also resulted in the depletion of the blood coagulation regulator thrombomodulin from the cell surface [Ogbechi et al., ].…”

“…Importantly, both studies conclude that a sustained, mycolactone‐mediated, Sec61 blockade triggers cellular stress responses that eventually induce apoptosis via the ATF4/C/EBP homologous protein/Bim signalling pathway [Morel et al., ; Ogbechi et al., ]. The ability of mycolactone to induce apoptosis via Bim may be further enhanced as a consequence of its inhibition of the mammalian target of rapamycin signalling pathway [Bieri et al., ].…”

Challenges associated with the treatment of Buruli ulcer

“…It appears that adaptive immune responses associated with IFN‐γ secretion may be crucial. The cytopathic effect of the macrolide toxin causes apoptosis of mammalian cells and down‐regulates local and systemic immune responses by interfering with the activation of immune cells, which may account for poor inflammatory responses in BU lesions (Fig. ).…”

“…subsequently reported that BU patients with active ulcers showed distinctive profile of immune suppression marked by down modulation of selected cytokines and an impaired capacity to produce Th1, Th2, and Th17 cytokines when stimulated with mitogenic agents . Immunohistopathological studies of lesions from BU patients after SR regimen have demonstrated treatment‐associated initiation of vigorous immune responses and the development of ectopic lymphoid tissue in the BU lesions . Three different general types of infiltration, diffuse mixed infiltrates, granulomas, and dense lymphocyte aggregation, near vessels were observed.…”

Susceptibility to Mycobacterium ulcerans Disease (Buruli ulcer) Is Associated with IFNG and iNOS Gene Polymorphisms

“…Mycolactone increases expression of the pro-apoptotic regulator Bim in mammalian cells, driving them into apoptosis (Bieri et al, 2017). At low concentrations, mycolactone counteracts many functions of tissue-resident macrophages and monocytes by inhibiting the production of several cytokines and chemokines including TNF and IFNG (Simmonds et al, 2009; Torrado et al, 2010; Fraga et al, 2011).…”