Exhaustion of Regenerative Potential Due to A Low Threshold for Exercise‐induced Myogenesis Might Explain Progressive Weakness in Dysferlin‐linked Muscle Diseases
Authors: Begam Morium , Galen Sujay S , Roche Joseph A 
Published: November 2023

BACKGROUNDThe absence of the protein dysferlin in skeletal muscle, leads to progressive muscle weakness that begins in the late‐teens. Anecdotal accounts from patients suggest that, the natural history of the disease involves athletic prowess prior to onset of symptoms, but poor tolerance to exercise once weakness has set in. Our earlier work shows a link between eccentric loading and severe muscle fiber damage in dysferlin‐deficient mouse muscle. We hypothesized that concentrically‐biased exercise is protective to dysferlin‐deficient muscle.METHODSWe studied the tibialis anterior (TA) muscle in dysferlin‐deficient A/J mice and dysferlin‐sufficient CRAJ mice (N = 4 per strain, males, 3–4 months at inclusion) for its response to 23 sessions of concentrically‐biased exercise over 12 weeks. Mice performed concentric exercise of the left TA (4 sets of 10 repetitions per set) under isoflurane anesthesia, on a custom‐built dynamometer for rodents. Each concentric contraction involved stimulating the TA tetanically (0.1 ms pulse duration, 150 Hz frequency, 450 ms pulse train) and simultaneously moving the foot into dorsiflexion (160° ‐ 90° at 600°/s) with a footplate connected to a stepper motor. The torque generated by the TA was monitored by a transducer placed in series between the footplate and the stepper motor.RESULTSAfter 12 weeks of exercise, in hematoxylin and eosin (H&E) stained cross sections of TA muscle (20X objective), centrally nucleated fibers (CNFs, a marker of myogenesis) were 7.6 ± 0.3 % in the unexercised right TA and 28.7 ± 3.0 % in the exercised left TA of dysferlin‐deficient mice (P < 0.05); whereas in dysferlin‐sufficient mice, CNFs were a low 1.2 ± 0.2 % and 3.3 ± 0.5 % in the unexercised and exercised TAs, respectively (P < 0.05). Through a bromodeoxyuridine (BrDU) nuclear‐incorporation assay, we confirmed that the increase in CNFs with concentric exercise was due to myogenesis and not migration of nuclei. In 20 visual fields (40X objective), of TA cross sections from 2 different dysferlin‐deficient mice, we found that the unexercised and exercised TAs had 1.5 ± 0.2 % and 7.8 ± 2 % BrDU+ nuclei (P < 0.05), respectively, after just 2 weeks of exercise. Despite the increase in CNFs, we found no significant muscle fiber damage after 12 weeks or a single session of exercise. Contractile torque also did not change significantly over 12 weeks.DISCUSSION AND CONCLUSIONOur data suggest that even with concentric exercise, that is not overtly damaging, there is recruitment of myogenesis in dysferlin‐deficient muscle. This could explain why patients are athletically gifted prior to onset of muscle weakness. Repeated triggering of myogenesis even for low exercise loads might therefore be exhausting the regenerative potential of involved muscles over time.Support or Funding InformationFunded by a grant to JAR from the Jain Foundation Inc. and a Faculty Startup Package to JAR from Wayne State University.