Duchenne muscular dystrophy is the second most common fatal genetic disease in humans. In dystrophin-negative humans and rodents, regenerated skeletal muscle fibers show abnormal branching. The number of fibers with branches increases with each cycle of degeneration/regeneration.
Duchenne muscular dystrophy (DMD) is the second most common fatal genetic disease in humans and is characterized by the absence of a functional copy of the protein dystrophin from skeletal muscle. In dystrophin-negative humans and rodents, regenerated skeletal muscle fibers show abnormal branching. The number of fibers with branches and the complexity of branching increases with each cycle of degeneration/regeneration. Previously, using the mdx mouse model of DMD, we have proposed that once the number and complexity of branched fibers present in dystrophic fast-twitch EDL muscle surpasses a stable level, we term "tipping point" the branches, in and of themselves, mechanically weaken the muscle by rupturing when subjected to high forces during eccentric contractions. Here we use the slow-twitch soleus muscle from the dystrophic mdx mouse to study pre-diseased "peri-ambulatory" dystrophic at 2-3 weeks, the peak regenerative "adult" phase at 6-9 weeks and "old" at 58-112 weeks. Using isolated mdx soleus muscles we examined contractile function and response to eccentric contraction correlated with amount and complexity of regenerated branched fibers. The intact muscle was enzymatically dispersed into individual fibers in order to count fiber branching and some muscles were optically cleared to allow laser scanning confocal microscopy. We demonstrate throughout the lifespan of the mdx mouse dystrophic slow-twitch soleus muscle is no more susceptible to eccentric contraction induced injury than age matched littermate controls and that this is correlated with a reduction in the number and complexity of branched fibers compared to fast-twitch dystrophic EDL muscles.