mycolactone and Fibrosis

Clinical observations from Buruli ulcer (BU) patients in West Africa suggest that severe Mycobacterium ulcerans infections can cause skeletal muscle contracture and atrophy leading to significant impairment in function. In the present study, male mice C57BL/6 were subcutaneously injected with M. ulcerans in proximity to the right biceps muscle, avoiding direct physical contact between the infectious agent and the skeletal muscle. The histological, morphological, and functional properties of the muscles were assessed at different times after the injection. On day 42 postinjection, the isometric tetanic force and the cross-sectional area of the myofibers were reduced by 31% and 29%, respectively, in the proximate-infected muscles relative to the control muscles. The necrotic areas of the proximate-infected muscles had spread to 7% of the total area by day 42 postinjection. However, the number of central nucleated fibers and myogenic regulatory factors (MyoD and myogenin) remained stable and low. Furthermore, Pax-7 expression did not increase significantly in mycolactone-injected muscles, indicating that the satellite cell proliferation is abrogated by the toxin. In addition, the fibrotic area increased progressively during the infection. Lastly, muscle-specific RING finger protein 1 (MuRF-1) and atrogin-1/muscle atrophy F-box protein (atrogin-1/MAFbx), two muscle-specific E3 ubiquitin ligases, were upregulated in the presence of M. ulcerans. These findings confirmed that skeletal muscle is affected in our model of subcutaneous infection with M. ulcerans and that a better understanding of muscle contractures and weakness is essential to develop a therapy to minimize loss of function and promote the autonomy of BU patients.

Topics: Animals; Bacterial Toxins; Buruli Ulcer; Cell Proliferation; Contracture; Disease Models, Animal; Fibrosis; Injections, Intramuscular; Isometric Contraction; Macrolides; Male; Mice; Mice, Inbred C57BL; Muscle Fatigue; Muscle Proteins; Muscle Strength; Muscle, Skeletal; Muscular Atrophy; Mycobacterium ulcerans; MyoD Protein; Necrosis; PAX7 Transcription Factor; Satellite Cells, Skeletal Muscle; SKP Cullin F-Box Protein Ligases; Time Factors; Tripartite Motif Proteins; Ubiquitin-Protein Ligases

Mycolactone produced by Mycobacterium ulcerans is the toxin responsible for most of the pathology in Buruli ulcer, the cutaneous signature of a complex disease. Although mycolactone cytopathicity is well described in various in vitro and in vivo models, the effect of this molecule on mammalian skeletal muscles has not been addressed. This is particularly surprising since muscle damage is characteristic of severe Buruli ulcer. We have thus investigated the impact of mycolactone on the mouse soleus muscle during degenerative and regenerative phases. Mice were intramuscularly injected with 300 microg of mycolactone and soleus muscles assessed histologically, biochemically and functionally at 7 and 42 days post-injection. Our results show that mycolactone induces local acute and chronic inflammatory responses which are respectively associated with a 65% and 68% decrease in maximal isometric force production (P(0)) relative to sham injections. In addition, muscle stiffness and total hydroxyproline content rose by 46% and 134% at day 42 relative to sham injections indicating an extensive fibrotic process in injured soleus muscles. Histological observations demonstrate significant muscle necrosis and atrophy with limited signs of regeneration. Together, our data indicate that mycolactone not only induces muscle damage but also prevents muscle regeneration to occur. These results may help to explain why patients with Buruli ulcer, experience muscle weakness and contracture.

Topics: Animals; Bacterial Toxins; Buruli Ulcer; Fibrosis; Humans; Inflammation; Macrolides; Male; Mice; Muscle Weakness; Muscle, Skeletal; Muscular Atrophy; Mycobacterium ulcerans; Necrosis