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, neglected and 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 Mycobaterium ulcerans (WT-MU) or the mycolactone-negative Mycobacterium ulcerans (M-neg-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 M-neg-MU induced no decrease at day 7 postinfection and a low but a significant 13% decrease in muscle force at day 42. A 13.2-fold and 4.3-fold increase in neutrophil and macrophage concentrations was observed on day 42 following the injection of WT-MU. However, the increases in neutrophil and macrophage concentrations were limited to 2.4-fold and 5.5-fold in M-neg-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 48h at a multiplicity of infection of 5:1. At the opposite, M-neg-MU had no significant effect. Interestingly, the addition of 1000 ng/ml of IGF-1 to the WT-MU/C2C12 coculture significantly improved all these biological parameters. The present investigation clearly established that muscle dysfunction and chronic inflammation in the presence of WT-MU is 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.