RESUMO
Objective: Mitigation of local pathologic fibrotic tissue deposition is a target area of interest for volumetric muscle loss (VML); nintedanib has shown promise for reduction of fibrosis following VML. Herein studies investigate how in sequence anti-fibrotic treatment administered immediately following VML and delayed rehabilitation could improve functional recovery after VML. Approach: Adult male C57BL/6 mice (n=36) were VML injured or Naïve, and randomly assigned to nintedanib (6 mg/kg/day) for 2 weeks or were left untreated; additionally, mice were given access to a running wheel beginning at 2 weeks until 8 weeks. Terminally, mice underwent maximal in vivo functional testing in addition to quantification of muscle collagen content and fibrotic and myogenic markers. Results: Daily running distances (p=0.17) were similar across groups, but weekly averages were greatest in the VML anti-fibrotic group (p<0.01). As expected, 2 weeks post-VML, all VML-injured mice had lower maximal torque normalized to body and muscle mass than Naïve. By 8 weeks, running alone after VML did not recover function, but mice that received the anti-fibrotic treatment prior to running, had greater torque than those untreated (p<0.01), with functional measurements similar to naïve muscle thar ran, indicating improved functional recovery. Innovation: The ability to translate current FDA-approved pharmaceuticals, in a repurposing approach, is critical to mitigate the pathophysiologic consequences of VML in support of functional recovery. However, foundational and translational studies are still needed to understand feasibility and efficacy. Conclusions: Early prevention of fibrotic tissue deposition supports improvements in muscle quality and force chronically after VML injury.
RESUMO
Volumetric muscle loss (VML) is the traumatic loss of skeletal muscle, resulting in chronic functional deficits and pathological comorbidities, including altered whole-body metabolic rate and respiratory exchange ratio (RER), despite no change in physical activity in animal models. In other injury models, treatment with ß2 receptor agonists (e.g. formoterol) improves metabolic and skeletal muscle function. We aimed first to examine if restricting physical activity following injury affects metabolic and skeletal muscle function, and second, to enhance the metabolic and contractile function of the muscle remaining following VML injury through treatment with formoterol. Adult male C57Bl/6J mice (n = 32) underwent VML injury to the posterior hindlimb compartment and were randomly assigned to unrestricted or restricted activity and formoterol treatment or no treatment; age-matched injury naïve mice (n = 4) were controls for biochemical analyses. Longitudinal 24 h evaluations of physical activity and whole-body metabolism were conducted following VML. In vivo muscle function was assessed terminally, and muscles were biochemically evaluated for protein expression, mitochondrial enzyme activity and untargeted metabolomics. Restricting activity chronically after VML had the greatest effect on physical activity and RER, reflected in reduced lipid oxidation, although changes were attenuated by formoterol treatment. Formoterol enhanced injured muscle mass, while mitigating functional deficits. These novel findings indicate physical activity restriction may recapitulate following VML clinically, and adjunctive oxidative treatment may create a metabolically beneficial intramuscular environment while enhancing the injured muscle's mass and force-producing capacity. Further investigation is needed to evaluate adjunctive oxidative treatment with rehabilitation, which may augment the muscle's regenerative and functional capacity following VML. KEY POINTS: The natural ability of skeletal muscle to regenerate and recover function is lost following complex traumatic musculoskeletal injury, such as volumetric muscle loss (VML), and physical inactivity following VML may incur additional deleterious consequences for muscle and metabolic health. Modelling VML injury-induced physical activity restriction altered whole-body metabolism, primarily by decreasing lipid oxidation, while preserving local skeletal muscle metabolic activity. The ß2 adrenergic receptor agonist formoterol has shown promise in other severe injury models to improve regeneration, recover function and enhance metabolism. Treatment with formoterol enhanced mass of the injured muscle and whole-body metabolism while mitigating functional deficits resulting from injury. Understanding of chronic effects of the clinically available and FDA-approved pharmaceutical formoterol could be a translational option to support muscle function after VML injury.