Could Mycolactone Inspire New Potent Analgesics? Perspectives and Pitfalls

Abstract

Pain currently represents the most common symptom for which medical attention is sought by patients. The available treatments have limited effectiveness and significant side-effects. In addition, most often, the duration of analgesia is short. Today, the handling of pain remains a major challenge. One promising alternative for the discovery of novel potent analgesics is to take inspiration from Mother Nature; in this context, the detailed investigation of the intriguing analgesia implemented in Buruli ulcer, an infectious disease caused by the bacterium Mycobacterium ulcerans and characterized by painless ulcerative lesions, seems particularly promising. More precisely, in this disease, the painless skin ulcers are caused by mycolactone, a polyketide lactone exotoxin. In fact, mycolactone exerts a wide range of effects on the host, besides being responsible for analgesia, as it has been shown notably to modulate the immune response or to provoke apoptosis. Several cellular mechanisms and different targets have been proposed to account for the analgesic effect of the toxin, such as nerve degeneration, the inhibition of inflammatory mediators and the activation of angiotensin II receptor 2. In this review, we discuss the current knowledge in the field, highlighting possible controversies. We first discuss the different pain-mimicking experimental models that were used to study the effect of mycolactone. We then detail the different variants of mycolactone that were used in such models. Overall, based on the results and the discussions, we conclude that the development of mycolactone-derived molecules can represent very promising perspectives for new analgesic drugs, which could be effective for specific pain indications.

Keywords: AT2R; analgesia; drug development; mycolactone; neurons.

Figure 1

Mechanisms of pain perception upon a skin injury and reported effects of mycolactone on different cell types. Pain perception is mainly mediated by the signal transduction of nociceptors with cell bodies located in the dorsal root ganglion and axons of the Aδ- or C-fiber type extending to the skin. Inflammatory cells (neutrophils, mast cells, T cells, macrophages) release mediators (chemokines, cytokines, lipids, interleukins (IL)) that are detected by nociceptor terminals to modulate neuronal excitation and the transduction of pain signals. Immune mediators can also impact ion channel trafficking to the membrane or ion channel transcriptional expression. The overall result of these immune-mediated pathways in nociceptors is the lowering of the threshold for responses to an external stimulus, leading to increased pain sensitivity. Nociceptors release substance P and CGRP (calcitonin-gene related peptide), which act on inflammatory cells leading to the release of inflammatory mediators. Mycolactone causes neuronal hyperpolarization by inducing angiotensin II receptor 2 (AT2R) signaling. Mycolactone also induces a Sec61 translocon blockade and inhibits protein translocation into the endoplasmic reticulum (ER). The inhibition of Sec61 thus prevents the production of key mediators of innate and adaptive immune responses. AT2R and Sec61 were both shown to mediate the mycolactone-induced inhibition of pain perception. K+: potassium, GPCR: G-protein coupled receptor, TRP: transient receptor potential ion channel, R: receptors, K2P: two-pore domain potassium channel.

Figure 2

Mycolactone bio-extraction versus synthesis: pros and cons.