Kinetics of skeletal muscle regeneration after mild and severe muscle damage induced by electrically-evoked lengthening contractions

Ablation of satellite cell-specific clock gene, Bmal1, alters force production, muscle damage, and repair following contractile-induced injury

Following injury, skeletal muscle undergoes repair via satellite cell (SC)-mediated myogenic progression. In SCs, the circadian molecular clock gene, Bmal1, is necessary for appropriate myogenic progression and repair with evidence that muscle molecular clocks can also affect force production. Utilizing a mouse model allowing for inducible depletion of Bmal1 within SCs, we determined contractile function, SC myogenic progression and muscle damage and repair following eccentric contractile-induced injury. At baseline, SC-Bmal1iKO animals exhibited a ~20-25% reduction in normalized force production (ex vivo and in vivo) versus control SC-Bmal1Cntrl and SC-Bmal1iKO untreated littermates (p < .05). Following contractile injury, SC-Bmal1iKO animals displayed reduced muscle damage and subsequent repair post-injury (Dystrophinnegative fibers 24 h: SC-Bmal1Cntrl 199 ± 41; SC-Bmal1iKO 36 ± 13, p < .05) (eMHC+ fibers 7 day: SC-Bmal1Cntrl 217.8 ± 115.5; SC-Bmal1iKO 27.8 ± 17.3; Centralized nuclei 7 day: SC-Bmal1Cntrl 160.7 ± 70.5; SC-Bmal1iKO 46.2 ± 15.7). SC-Bmal1iKO animals also showed reduced neutrophil infiltration, consistent with less injury (Neutrophil content 24 h: SC-Bmal1Cntrl 2.4 ± 0.4; SC-Bmal1iKO 0.4 ± 0.2, % area fraction, p < .05). SC-Bmal1iKO animals had greater SC activation/proliferation at an earlier timepoint (p < .05) and an unexplained increase in activation 7 days post injury. Collectively, these data suggest SC-Bmal1 plays a regulatory role in force production, influencing the magnitude of muscle damage/repair, with an altered SC myogenic progression following contractile-induced muscle injury.

Impaired skeletal muscle regeneration in diabetes: From cellular and molecular mechanisms to novel treatments

Diabetes represents a major public health concern with a considerable impact on human life and healthcare expenditures. It is now well established that diabetes is characterized by a severe skeletal muscle pathology that limits functional capacity and quality of life. Increasing evidence indicates that diabetes is also one of the most prevalent disorders characterized by impaired skeletal muscle regeneration, yet underlying mechanisms and therapeutic treatments remain poorly established. In this review, we describe the cellular and molecular alterations currently known to occur during skeletal muscle regeneration in people with diabetes and animal models of diabetes, including its associated comorbidities, e.g., obesity, hyperinsulinemia, and insulin resistance. We describe the role of myogenic and non-myogenic cell types on muscle regeneration in conditions with or without diabetes. Therapies for skeletal muscle regeneration and gaps in our knowledge are also discussed, while proposing future directions for the field.

Traumatic muscle injury

Traumatic muscle injury represents a collection of skeletal muscle pathologies caused by trauma to the muscle tissue and is defined as damage to the muscle tissue that can result in a functional deficit. Traumatic muscle injury can affect people across the lifespan and can result from high stresses and strains to skeletal muscle tissue, often due to muscle activation while the muscle is lengthening, resulting in indirect and non-contact muscle injuries (strains or ruptures), or from external impact, resulting in direct muscle injuries (contusion or laceration). At a microscopic level, muscle fibres can repair focal damage but must be completely regenerated after full myofibre necrosis. The diagnosis of muscle injury is based on patient history and physical examination. Imaging may be indicated to eliminate differential diagnoses. The management of muscle injury has changed within the past 5 years from initial rest, immobilization and (over)protection to early activation and progressive loading using an active approach. One challenge of muscle injury management is that numerous medical treatment options, such as medications and injections, are often used or proposed to try to accelerate muscle recovery despite very limited efficacy evidence. Another challenge is the prevention of muscle injury owing to the multifactorial and complex nature of this injury.

Influence of sexual dimorphism on satellite cell regulation and inflammatory response during skeletal muscle regeneration

After injury, skeletal muscle regenerates thanks to the key role of satellite cells (SC). The regeneration process is supported and coordinated by other cell types among which immune cells. Among the mechanisms involved in skeletal muscle regeneration, a sexual dimorphism, involving sex hormones and more particularly estrogens, has been suggested. However, the role of sexual dimorphism on skeletal muscle regeneration is not fully understood, likely to the use of various experimental settings in both animals and human. This review aims at addressing how sex and estrogens regulate both the SC and the inflammatory response during skeletal muscle regeneration by considering the different experimental designs used in both animal models (i.e., ovarian hormone deficiency, estrogen replacement or supplementation, treatments with estrogen receptors agonists/antagonists and models knockout for estrogen receptors) and human (hormone therapy replacement, pre vs. postmenopausal, menstrual cycle variation…).