Skeletal muscle injury resulting in tissue loss poses unique challenges for surgical repair. Despite the regenerative potential of skeletal muscle, if a significant amount of tissue is lost, skeletal myofibers will not grow to fill the injured area completely. Prior work in our lab has shown the potential to fill the void with an extracellular matrix (ECM) scaffold, resulting in restoration of morphology, but not functional recovery. To improve the functional outcome of the injured muscle, a muscle-derived ECM was implanted into a 1 x 1 cm(2), full-thickness defect in the lateral gastrocnemius (LGAS) of Lewis rats. Seven days later, bone-marrow-derived mesenchymal stem cells (MSCs) were injected directly into the implanted ECM. Partial functional recovery occurred over the course of 42 days when the LGAS was repaired with an MSC-seeded ECM producing 85.4 +/- 3.6% of the contralateral LGAS. This was significantly higher than earlier recovery time points (p < 0.05). The specific tension returned to 94 +/- 9% of the contralateral limb. The implanted MSC-seeded ECM had more blood vessels and regenerating skeletal myofibers than the ECM without cells (p < 0.05). The data suggest that the repair of a skeletal muscle defect injury by the implantation of a muscle-derived ECM seeded with MSCs can improve functional recovery after 42 days.

The loss of a portion of skeletal muscle poses a unique challenge for the normal regeneration of muscle tissue. A transection injury with tissue loss will not heal due to the gap between muscle segments. A damage model was developed by removing a portion of the lateral gastrocnemius (GAS) of Sprague-Dawley rats. Maximal isometric, tetanic tension (P(o)) was measured after the removal of either a small defect (0.5 x 1.0 cm) or a large defect (1.0 x 1.0 cm) piece of the GAS. In situ P(o) immediately after creation of the defect was 88.3 +/- 2.0% of the nonoperated contralateral GAS force for small defect and 76.9 +/- 3.2% of control for large defect. No functional recovery occurred in either group over the course of 28 days. To enhance recovery, a homologous, decellularized, muscle extracellular matrix (ECM) was implanted into the 1 x 1 cm defect of the lateral GAS of Lewis rats. After 42 days, growth of blood vessels and myofibers into the ECM was apparent, but no restoration of P(o) occurred. These data demonstrate the ability of the ECM to support muscle and blood vessel regeneration, but full recovery of function does not occur after 42 days.

The regenerative response of skeletal muscle to mechanically induced damage is impaired with age. Previous work in our laboratory suggests this may result from higher proinflammatory signaling in aging muscle at rest and/or a greater inflammatory response to damage. We, therefore, assessed skeletal muscle proinflammatory signaling at rest and 24 h after unaccustomed, loaded knee extension contractions that induced modest muscle damage (72% increase in serum creatine kinase) in a cohort of 87 adults across three age groups (AGE40, AGE61, and AGE76). Vastus lateralis muscle gene expression and protein cell signaling of the IL-6 and TNF-α pathways were determined by quantitative PCR and immunoblot analysis. For in vitro studies, cell signaling and fusion capacities were compared among primary myoblasts from young (AGE28) and old (AGE64) donors treated with TNF-α. Muscle expression was higher (1.5- to 2.1-fold) in AGE76 and AGE61 relative to AGE40 for several genes involved in IL-6, TNF-α, and TNF-like weak inducer of apoptosis signaling. Indexes of activation for the proinflammatory transcription factors signal transducer and activator of transcription-3 and NF-κB were highest in AGE76. Resistance loading reduced gene expression of IL-6 receptor, muscle RING finger 1, and atrogin-1, and increased TNF-like weak inducer of apoptosis receptor expression. Donor myoblasts from AGE64 showed impaired differentiation and fusion in standard media and greater NF-κB activation in response to TNF-α treatment (compared with AGE28). We show for the first time that human aging is associated with muscle inflammation susceptibility (i.e., higher basal state of proinflammatory signaling) that is present in both tissue and isolated myogenic cells and likely contributes to the impaired regenerative capacity of skeletal muscle in the older population.

Age-related muscle loss (sarcopenia) is a major clinical problem affecting both men and women - accompanied by muscle weakness, dysfunction, disability, and impaired quality of life. Current definitions of sarcopenia do not fully encompass the age-related changes in skeletal muscle. We therefore examined the influence of aging and sex on elements of skeletal muscle health using a thorough histopathological analysis of myocellular aging and assessments of neuromuscular performance. Two-hundred and twenty-one untrained males and females were separated into four age cohorts [mean age 25 y (n = 47), 37 y (n = 79), 61 y (n = 51), and 72 y (n = 44)]. Total (-12%), leg (-17%), and arm (-21%) lean mass were lower in both 61 y and 72 y than in 25 y or 37 y (P < 0.05). Knee extensor strength (-34%) and power (-43%) were lower (P < 0.05) in the older two groups, and explosive sit-to-stand power was lower by 37 y (P < 0.05). At the histological/myocellular level, type IIx atrophy was noted by 37 y and type IIa atrophy by 61 y (P < 0.05). These effects were driven by females, noted by substantial and progressive type IIa and IIx atrophy across age. Aged female muscle displayed greater within-type myofiber size heterogeneity and marked type I myofiber grouping (~5-fold greater) compared to males. These findings suggest the predominant mechanisms leading to whole muscle atrophy differ between aging males and females: myofiber atrophy in females vs. myofiber loss in males. Future studies will be important to better understand the mechanisms underlying sex differences in myocellular aging and optimize exercise prescriptions and adjunctive treatments to mitigate or reverse age-related changes.

Purpose The myriad consequences of age-related muscle atrophy include reduced muscular strength, power, and mobility; increased risk of falls, disability, and metabolic disease; and compromised immune function. At its root, aging muscle atrophy results from a loss of myofibers and atrophy of the remaining type II myofibers. The purpose of this trial (NCT02442479) was to titrate the dose of resistance training (RT) in older adults in an effort to maximize muscle regrowth and gains in muscle function. Methods A randomized, four-arm efficacy trial in which four, distinct exercise prescriptions varying in intensity, frequency, and contraction mode/rate were evaluated: (1) high-resistance concentric-eccentric training (H) 3 d/wk (HHH); (2) H training 2 d/wk (HH); (3) 3 d/wk mixed model consisting of H training 2 d/wk separated by 1 bout of low-resistance, high-velocity, concentric only (L) training (HLH); and (4) 2 d/wk mixed model consisting of H training 1 d/wk and L training 1 d/wk (HL). Sixty-four randomized subjects (65.5 ± 3.6y) completed the trial. All participants completed the same 4 wk of pre-training consisting of 3 d/wk followed by 30 wk of randomized RT. Results The HLH prescription maximized gains in thigh muscle mass (TMM, primary outcome) and total body lean mass. HLH also showed the greatest gains in knee extension maximum isometric strength, and reduced cardiorespiratory demand during steady-state walking. HHH was the only prescription that led to increased muscle expression of pro-inflammatory cytokine receptors and this was associated with a lesser gain in TMM and total body lean mass compared to HLH. The HL prescription induced minimal muscle regrowth and generally lesser gains in muscle performance vs. the other prescriptions. Major Conclusions The HLH prescription offers distinct advantages over the other doses, while the HL program is subpar. Although limited by a relatively small sample size, we conclude from this randomized dose-response trial that older adults benefit greatly from 2 d/wk high-intensity RT, and may further benefit from inserting an additional weekly bout of low-load, explosive RT. Trial Registration ClinicalTrials.gov NCT02442479

Objective Severe burn injuries lead to a prolonged hyper-catabolic state resulting in dramatic loss of skeletal muscle mass. Post-burn muscle loss is well documented, but the molecular signaling cascade preceding atrophy is not. Our purpose was to determine the response to burn injury of signaling pathways driving muscle inflammation and protein metabolism. Methods Muscle biopsies were collected in the early flow phase after burn injury from the vastus lateralis of a non-injured leg in patients with 20–60% total body surface area burns and compared to uninjured, matched controls. Circulating levels of pro-inflammatory cytokines were also compared. Immunoblotting was performed to determine protein levels of key signaling components for translation initiation, proteolysis, and TNF/NFκB and IL-6/STAT3 signaling. Results Burn subjects had significantly higher levels of circulating pro-inflammatory cytokines, but no difference in muscle STAT3 activity and lower NFκB activity. No differences were found in any translational signaling components. Regarding proteolytic signaling in burn, calpain-2 was 47% higher, calpastatin tended to be lower, and total ubiquitination was substantially higher. Conclusion Surprisingly, a systemic pro-inflammatory response 3–10 d post-burn did not lead to elevated muscle STAT3 or NFκB signaling. Signaling molecules governing translation initiation were unaffected while indices of calcium-mediated proteolysis and ubiquitin-proteasome activity were up-regulated. These novel findings are the first in humans to suggest the net catabolic effect of burn injury in skeletal muscle (i.e. atrophy) may be mediated, at least during the early flow phase, almost entirely by increased proteolytic activity in the absence of suppressed protein synthesis signaling.

Hammers DW, Merritt EK, Matheny W, Adamo ML, Walters TJ, Estep JS, Farrar RP. Functional deficits and insulin-like growth factor-I gene expression following tourniquet-induced injury of skeletal muscle in young and old rats.

The involvement of circulating insulin-like growth factor-I (IGF-I) in the skeletal muscle response to resistance exercise is currently unclear. To address this, we utilized the liver IGF-I-deficient (LID) mouse model, in which the igf1 gene has been disrupted in the hepatocytes, resulting in ~80% reduction in serum IGF-I. Twelve- to 13-month-old male LID and control (L/L) mice were subjected to 16 weeks of resistance training. Resistance exercise resulted in equal strength gains in both L/L and LID mice. Basal IGF-I mRNA levels were greater in LID muscles than in L/L, and exercise increased IGF-I mRNA in quadriceps, gastrocnemius, and plantaris muscles. LID mice had elevated tyrosine phosphorylation of IGF-IR and Stat5b, the latter possibly reflective of increased serum GH. Tyrosine phosphorylation of IGF-IR was increased, while phospho-Stat5b was reduced after resistance training of both wild-type and LID mice. These data suggest that: 1) performance and recovery in response to resistance training is normal even when there is severe deficiency of circulating IGF-I; and 2) upregulation of local IGF-I may be involved in the compensatory growth of muscle that occurs in response to resistance training. Decreased levels of p-Stat5b in exercised mice suggests that the upregulation of local IGF-I gene expression in response to exercise may be GH-independent.