Background: Mycolactone (ML) is a polyketide toxin secreted by the mycobacterium M.
ulcerans, responsible for the extensive hypoalgesic skin lesions characteristic of patients with
Buruli Ulcer. A recent pre-clinical study proposed that ML 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 ML 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 hours after plating, neurons were
incubated for 24 to 96 hours with synthetic ML A/B, followed by immunostaining with antibodies
to PGP9.5, Gap43,  tubulin, or Mitotracker dye staining. Acute functional effects were
examined by measuring capsaicin responses with calcium imaging in DRG neuronal cultures
treated with ML.
Results: Morphological effects: ML treated cultures showed dramatically reduced numbers of
surviving neurons and non-neuronal cells, reduced Gap43 and  tubulin expression, degenerating
neurites and reduced cell body diameter, compared with controls. Dose related reduction of
neurite length was observed in ML treated cultures. Mitochondria were distributed throughout the
length of neurites and soma of control neurons, but clustered in the neurites and soma of ML
treated neurons. Functional effects: ML treated human and rat DRG neurons showed dose
related inhibition of capsaicin responses, which were reversed by calcineurin inhibitor
cyclosporine and phosphodiesterase inhibitor IBMX, indicating involvement of cAMP/ATP
reduction. The morphological and functional effects of ML were not altered by Angiotensin II or
AT2R antagonist EMA401.
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Conclusion: ML 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