Lifespan analysis of dystrophic mdx fast-twitch muscle morphology and its impact on contractile function

Abstract

Duchenne muscular dystrophy is caused by the absence of the protein dystrophin from skeletal muscle and is characterized by progressive cycles of necrosis/regeneration. Using the dystrophin deficient mdx mouse model we studied the morphological and contractile chronology of dystrophic skeletal muscle pathology in fast twitch EDL muscles from animals 4-22 months of age containing 100% regenerated muscle fibers. Catastrophically, the older age groups lost ~80% of their maximum force after one eccentric contraction of 20% strain, with the greatest loss ~93% recorded in senescent 22 month old mdx mice. In old age groups there was minimal force recovery ~24% after 120 minutes, correlated with a dramatic increase in the number and complexity of branched fibers. This data supports our two-stage model where a tipping point is reached when branched fibers rupture irrevocably on eccentric contraction. These findings have important implications for pre-clinical drug studies and genetic rescue strategies.

Six weeks of N-acetylcysteine antioxidant in drinking water decreases pathological fiber branching in MDX mouse dystrophic fast-twitch skeletal muscle

“…Figure 3A shows that there was a graded reduction of force with each EC for both NAC treated and untreated muscles. As has been previously reported mdx muscle were significantly more susceptible to EC induced force loss when Frontiers in Physiology frontiersin.org compared to age matched LC (Kiriaev et al, 2018;Kiriaev et al, 2021a) and this was not altered by treatment with NAC. Figure 3B shows the rate of recovery of force over 60 min, mdx muscles recovered ~20% and this recovery was not different in EDL muscles from NAC treated mdx.…”

“…The fast twitch EDL muscles of mdx mice have long been known to be more susceptible to damage from EC compared to age match WT controls, as measured by a reduction in Po post EC (Head et al, 1992) and this was confirmed in later studies which utilized LCs for the mdx mice (Kiriaev et al, 2018). In the present study we use a strong eccentric contraction protocol that produced a ~90% Po force loss in mdx which only recovered to ~20% of Po after 60 min demonstrating that it was likely the EC had caused significant membrane damage (Olthoff et al, 2018).…”

“…Previous studies from our laboratory and others have demonstrated a characteristic feature of the dystrophic pathology in mdx mice and DMD boys are the presence of abnormally regenerated branched limb skeletal muscle fibers which increase in number and complexity with age (Bell and Conen, 1968;Schmalbruch, 1976;Head et al, 1992;Chan and Head, 2011;Massopust et al, 2020;Kiriaev et al, 2021b). We and others have demonstrated the increase of these branched fibers mechanically weakens the muscle in the later stages of the dystrophic disease (Head et al, 1992;Chan and Head, 2011;Head, 2012;Pichavant et al, 2016;Kiriaev et al, 2018).…”

“…Since the first reports of the mdx mouse it has been noted that the fast-twitch muscles are more susceptible to eccentric (lengthening) contraction (EC) damage (Head et al, 1992;Moens et al, 1993). This susceptibility to EC damage increases as the dystrophic animal ages (Chan et al, 2007;Chan and Head, 2011;Head, 2012;Kiriaev et al, 2018;Kiriaev et al, 2021a). The reason for this increased susceptibility to EC damage is not clear and there have been several hypotheses put forward to explain this effect (Allen et al, 2016); 1) The absence of dystrophin mechanically weakens the sarcolemma making it more susceptible to rupture during EC, 2) Absence of dystrophin is responsible for cycles of necrosis/regeneration resulting in the increase of abnormally regenerated branched fibers structurally compromising the dystrophic membranes ability to resist EC damage 3) The absence of dystrophin results in an increased susceptibility to oxidative damage triggered by EC or 4) The absence of dystrophin sensitizes the ionic mechanisms of the electrical pathways that trigger muscle contraction to damage from EC.…”

Loss of α-actinin-3 confers protection from eccentric contraction damage in fast-twitch EDL muscles from aged mdx dystrophic mice by reducing pathological fibre branching

“…(2017) in a new cohort of mice aged to 12 months. The 12 month age group was selected because we have previously demonstrated that by this age mdx fast-twitch EDL muscles are irreversibly damaged by a single eccentric contraction, and this irreversible eccentric force deficit is correlated with the number and complexity of branched dystrophic fibres present (Kiriaev et al ., 2018; Kiriaev et al ., 2021b). We found convincing correlative evidence that the age related protective effect conferred by the absence of α-actinin-3 is due to the reduction in the number and complexity of pathologically branched dystrophic fibres.…”