Oxidative pathophysiology following volumetric muscle loss injury in a porcine model

Neuromuscular Regeneration of Volumetric Muscle Loss Injury in Response to Agrin-Functionalized Tissue Engineered Muscle Grafts and Rehabilitative Exercise

Neuromuscular deficits compound the loss of contractile tissue in volumetric muscle loss (VML). Two avenues for promoting recovery are neuromuscular junction (NMJ)-promoting substrates (e.g., agrin) and endurance exercise. Although mechanical stimulation enhances agrin-induced NMJ formation, the two modalities have yet to be evaluated combinatorially. It is hypothesized that the implantation of human myogenic progenitor-seeded tissue-engineered muscle grafts (hTEMGs) in combination with agrin treatment and/or exercise will enhance neuromuscular recovery after VML. The hTEMGs alone transplant into VML defects promote significant regeneration with minimal scarring. A sex-appropriate, low-intensity continuous running exercise paradigm increases acetylcholine receptor (AChR) cluster density in male mice twofold relative to hTEMG alone after 7 weeks of treadmill training (p < 0.05). To further promote neuromuscular recovery, agrin is incorporated into the scaffolds via covalent tethering. In vitro, agrin increases the proliferation of hMPs, and trends toward greater myogenic maturity and AChR clustering. Upon transplantation, both hTEMGs + agrin and hTEMGs + exercise induce near 100% recovery of muscle mass and increase twitch and tetanic force output (p > 0.05). However, agrin treatment in combination with exercise produces no additional benefit. These data highlight the unprecedented regenerative potential of using hTEMGs together with either agrin or exercise supplementation to treat VML injuries.

Limitations in metabolic plasticity after traumatic injury are only moderately exacerbated by physical activity restriction

Following traumatic musculoskeletal injuries, prolonged bedrest and loss of physical activity may limit muscle plasticity and drive metabolic dysfunction. One specific injury, volumetric muscle loss (VML), results in frank loss of muscle and is characterized by whole-body and cellular metabolic dysfunction. However, how VML and restricted physical activity limit plasticity of the whole-body, cellular, and metabolomic environment of the remaining uninjured muscle remains unclear. Adult mice were randomized to posterior hindlimb compartment VML or were age-matched injury naïve controls, then randomized to standard or restricted activity cages for 8-wks. Activity restriction in naïve mice resulted in ~5% greater respiratory exchange ratio (RER); combined with VML, carbohydrate oxidation was ~23% greater than VML alone, but lipid oxidation was largely unchanged. Activity restriction combined with VML increased whole-body carbohydrate usage. Together there was a greater pACC:ACC ratio in the muscle remaining, which may contribute to decreased fatty acid synthesis. Further, β-HAD activity normalized to mitochondrial content was decreased following VML, suggesting a diminished capacity to oxidize fatty acids. The muscle metabolome was not altered by the restriction of physical activity. The combination of VML and activity restriction resulted in similar (~91%) up- and down-regulated metabolites and/or ratios, suggesting that VML injury alone is regulating changes in the metabolome. Data supports possible VML-induced alterations in fatty acid metabolism are exacerbated by activity restriction. Collectively, this work adds to the sequala of VML injury, exhausting the ability of the muscle remaining to oxidize fatty acids resulting in a possible accumulation of triglycerides.

Tibial bone strength is negatively affected by volumetric muscle loss injury to the adjacent muscle in male mice

This study's objective was to investigate how contractile strength loss associated with a volumetric muscle loss (VML) injury affects the adjacent tibial bone structural and functional properties in male C57BL/6J mice. Mice were randomized into one of two experimental groups: VML-injured mice that were injured at age 12 weeks and aged to 20 weeks (8 weeks postinjury, VML) and 20-week-old age-matched uninjured mice (Uninjured-20). Tibial bone strength, mid-diaphysis cortical geometry, intrinsic material properties, and metaphyseal trabecular bone structure were assessed by three-point bending and microcomputed tomography (µCT). The plantar flexor muscle group (gastrocnemius, soleus, plantaris) was analyzed for its functional capacities, that is, peak-isometric torque and peak-isokinetic power. VML-injured limbs had 25% less peak-isometric torque and 31% less peak-isokinetic power compared to those of Uninjured-20 mice (p < 0.001). Ultimate load, but not stiffness, was significantly less (10%) in tibias of VML-injured limbs compared to those from Uninjured-20 (p = 0.014). µCT analyses showed cortical bone thickness was 6% less in tibias of VML-injured limbs compared to Uninjured-20 (p = 0.001). Importantly, tibial bone cross-section moment of inertia, the primary determinant of bone ultimate load, was 16% smaller in bones of VML-injured limbs compared to bones from Uninjured-20 (p = 0.046). Metaphyseal trabecular bone structure was also altered up to 23% in tibias of VML-injured limbs (p < 0.010). These changes in tibial bone structure and function after a VML injury occur during a natural maturation phase between the age of 12 and 20 weeks, as evidenced by Uninjured-20 mice having greater tibial bone size and strength compared to uninjured-aged 12-week mice.

Long-term longitudinal study on swine VML model

BACKGROUND

Volumetric Muscle Loss (VML), resulting from severe trauma or surgical ablation, is a pathological condition preventing myofibers regeneration, since skeletal muscle owns the remarkable ability to restore tissue damage, but only when limited in size. The current surgical therapies employed in the treatment of this pathology, which particularly affects military personnel, do not yet provide satisfactory results. For this reason, more innovative approaches must be sought, specifically skeletal muscle tissue engineering seems to highlight promising results obtained from preclinical studies in VML mouse model. Despite the great results obtained in rodents, translation into human needs a comparable animal model in terms of size, in order to validate the efficacy of the tissue engineering approach reconstructing larger muscle mass (human-like). In this work we aim to demonstrate the validity of a porcine model, that has underwent a surgical ablation of a large muscle area, as a VML damage model.

RESULTS

For this purpose, morphological, ultrasound, histological and fluorescence analyses were carried out on the scar tissue formed following the surgical ablation of the peroneus tertius muscle of Sus scrofa domesticus commonly called mini-pig. In particular, the replenishment of the damaged area, the macrophage infiltration and the vascularization at different time-points were evaluated up to the harvesting of the scar upon six months.

CONCLUSION

Here we demonstrated that following VML damage, there is an extremely poor regenerative process in the swine muscle tissue, while the formation of fibrotic, scar tissue occurs. The analyses performed up to 180 days after the injury revealed the development of a stable, structured and cellularized tissue, provided with vessels and extracellular matrix acquiring the status of granulation tissue like in human.

Response of terminal Schwann cells following volumetric muscle loss injury

An often-overlooked component of traumatic skeletal muscle injuries is the impact on the nervous system and resultant innervation of the affected muscles. Recent work in a rodent model of volumetric muscle loss (VML) injury demonstrated a progressive, secondary loss of neuromuscular junction (NMJ) innervation, supporting a role of NMJ dysregulation in chronic functional deficits. Terminal Schwann cells (tSCs) are known to be vital for the maintenance of NMJ structure and function, in addition to guiding repair and regeneration after injury. However, the tSC response to a traumatic muscle injury such as VML is not known. Thus, a study was conducted to investigate the effect of VML on tSC morphological characteristics and neurotrophic signaling proteins in adult male Lewis rats that underwent VML injury to the tibialis anterior muscle using a temporal design with outcome assessments at 3, 7, 14, 21, and 48 days post-injury. The following salient observations were made; first, although there is a loss of innervation over time, the number of tSCs per NMJ increases, significantly so at 48 days post-injury compared to control. The degree of NMJ fragmentation was positively correlated with tSC number after injury. Moreover, neurotrophic factors such as NRG1 and BDNF are elevated after injury through at least 48 days. These results were unanticipated and in contrast to neurodegenerative disease models, in which there is a reduction in tSC number that precedes denervation. However, we found that while there are more tSCs per NMJ after injury, they cover a significantly smaller percent of the post-synaptic endplate area compared to control. These findings support a sustained increase in neurotrophic activity and tSC number after VML, which is a maladaptive response occurring in parallel to other aspects of the VML injury, such as over-accumulation of collagen and aberrant inflammatory signaling.

The bioenergetic “CK Clamp” technique detects substrate-specific changes in mitochondrial respiration and membrane potential during early VML injury pathology

Volumetric muscle loss (VML) injuries are characterized by non-recoverable loss of tissue resulting in contractile and metabolic dysfunction. The characterization of metabolic dysfunction in volumetric muscle loss-injured muscle has been interpreted from permeabilized myofiber respiration experiments involving saturating ADP levels and non-physiologic ATP:ADP concentration ratios. The extent to which this testing condition obscures the analysis of mitochondrial (dys) function after volumetric muscle loss injury is unclear. An alternative approach is described that leverages the enzymatic reaction of creatine kinase and phosphocreatine to assess mitochondrial respiration and membrane potential at clamped physiologic ATP:ADP ratios, “CK Clamp.” The objective of this study was to validate the CK Clamp in volumetric muscle loss-injured muscle and to detect differences that may exist between volumetric muscle loss-injured and uninjured muscles at 1, 3, 5, 7, 10, and 14 days post-injury. Volumetric muscle loss-injured muscle maintains bioenergetic features of the CK Clamp approach, i.e., mitochondrial respiration rate (JO2) titters down and mitochondrial membrane potential is more polarized with increasing ATP:ADP ratios. Pyruvate/malate/succinate-supported JO2 was significantly less in volumetric muscle loss-injured muscle at all timepoints compared to uninjured controls (−26% to −84%, p &lt; 0.001) and electron conductance was less at day 1 (−60%), 5 (−52%), 7 (−35%), 10 (−59%), and 14 (−41%) (p &lt; 0.001). Palmitoyl-carnitine/malate-supported JO2 and electron conductance were less affected following volumetric muscle loss injury. volumetric muscle loss-injury also corresponded with a more polarized mitochondrial membrane potential across the clamped ATP:ADP ratios at day 1 and 10 (pyruvate and palmitoyl-carnitine, respectively) (+5%, p &lt; 0.001). This study supports previous characterizations of metabolic dysfunction and validates the CK Clamp as a tool to investigate bioenergetics in traumatically-injured muscle.

Restricted physical activity after volumetric muscle loss alters whole-body and local muscle metabolism

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 β2 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 β2 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.

Early initiation of electrical stimulation paired with range of motion after a volumetric muscle loss injury does not benefit muscle function

NEW FINDINGS

What is the central question of this study? First, how does physical rehabilitation influence recovery from traumatic muscle injury? Second, how does physical activity impact the rehabilitation response for skeletal muscle function and whole-body metabolism? What is the main finding and its importance? The most salient findings were that rehabilitation impaired muscle function and range of motion, while restricting activity mitigated some negative effects but also impacted whole-body metabolism. These data suggest that first, work must continue to explore treatment parameters, including modality, time, type, duration and intensity, to find the best rehabilitation approaches for volumetric muscle loss injuries; and second, restricting activity acutely might enhance rehabilitation response, but whole-body co-morbidities should continue to be considered.

ABSTRACT

Volumetric muscle loss (VML) injury occurs when a substantial volume of muscle is lost by surgical removal or trauma, resulting in an irrecoverable deficit in muscle function. Recently, it was suggested that VML impacts whole-body and muscle-specific metabolism, which might contribute to the inability of the muscle to respond to treatments such as physical rehabilitation. The aim of this work was to understand the complex relationship between physical activity and the response to rehabilitation after VML in an animal model, evaluating the rehabilitation response by measurement of muscle function and whole-body metabolism. Adult male mice (n = 24) underwent a multi-muscle, full-thickness VML injury to the gastrocnemius, soleus and plantaris muscles and were randomized into one of three groups: (1) untreated; (2) rehabilitation (i.e., combined electrical stimulation and range of motion, twice per week, beginning 72 h post-injury, for ∼8 weeks); or (3) rehabilitation and restriction of physical activity. There was a lack of positive adaption associated with electrical stimulation and range of motion intervention alone; however, maximal isometric torque of the posterior muscle group was greater in mice receiving treatment with activity restriction (P = 0.008). Physical activity and whole-body metabolism were measured ∼6 weeks post-injury; metabolic rate decreased (P = 0.001) and respiratory exchange ratio increased (P = 0.022) with activity restriction. Therefore, restricting physical activity might enhance an intervention delivered to the injured muscle group but impair whole-body metabolism. It is possible that restricting activity is important initially post-injury to protect the muscle from excess demand. A gradual increase in activity throughout the course of treatment might optimize muscle function and whole-body metabolism.

Pharmaceutical Agents for Contractile-Metabolic Dysfunction After Volumetric Muscle Loss

Volumetric muscle loss (VML) injuries represent a majority of military service member casualties and are common in civilian populations following blunt and/or penetrating traumas. Characterized as a skeletal muscle injury with permanent functional impairments, there is currently no standard for rehabilitation, leading to lifelong disability. Toward developing rehabilitative strategies, previous research demonstrates that the remaining muscle after a VML injury lacks similar levels of plasticity or adaptability as healthy, uninjured skeletal muscle. This may be due, in part, to impaired innervation and vascularization of the remaining muscle, as well as disrupted molecular signaling cascades commonly associated with muscle adaptation. The primary objective of this study was to assess the ability of four pharmacological agents with a strong record of modulating muscle contractile and metabolic function to improve functional deficits in a murine model of VML injury. Male C57BL/6 mice underwent a 15% multimuscle VML injury of the posterior hindlimb and were randomized into drug treatment groups (formoterol [FOR], 5-aminoimidazole-4-carboxamide riboside [AICAR], pioglitazone [PIO], or sildenafil [SIL]) or untreated VML group. At the end of 60 days, the injury model was first validated by comparison to age-matched injury-naive mice. Untreated VML mice had 22% less gastrocnemius muscle mass, 36% less peak-isometric torque, and 27% less maximal mitochondrial oxygen consumption rate compared to uninjured mice (p < 0.01). Experimental drug groups were, then, compared to VML untreated, and there was minimal evidence of efficacy for AICAR, PIO, or SIL in improving contractile and metabolic functional outcomes. However, FOR-treated VML mice had 18% greater peak isometric torque (p < 0.01) and permeabilized muscle fibers had 36% greater State III mitochondrial oxygen consumption rate (p < 0.01) compared to VML untreated mice, suggesting an overall improvement in muscle condition. There was minimal evidence that these benefits came from greater mitochondrial biogenesis and/or mitochondrial complex protein content, but could be due to greater enzyme activity levels for complex I and complex II. These findings suggest that FOR treatment is candidate to pair with a rehabilitative approach to maximize functional improvements in VML-injured muscle. Impact statement Volumetric muscle loss (VML) injuries result in deficiencies in strength and mobility, which have a severe impact on patient quality of life. Despite breakthroughs in tissue engineering, there are currently no treatments available that can restore function to the affected limb. Our data show that treatment of VML injuries with clinically available and FDA-approved formoterol (FOR), a beta-agonist, significantly improves strength and metabolism of VML-injured muscle. FOR is therefore a promising candidate for combined therapeutic approaches (i.e., regenerative rehabilitation) such as pairing FOR with structured rehabilitation or cell-seeded biomaterials as it may provide greater functional improvements than either strategy alone.

Multi-functional SiO32--releasing hydrogel with bioinspired mechanical properties and biodegradability for vascularized skeletal muscle regeneration

Vascularized skeletal muscle regeneration remains a great medical need but significant challenge. Biomaterial strategies that can facilitate the regeneration of muscle fibers and blood vessels are unavailable. Herein, we report...

Biomimetic sponges improve muscle structure and function following volumetric muscle loss

Skeletal muscle is inept in regenerating after traumatic injuries such as volumetric muscle loss (VML) due to significant loss of various cellular and acellular components. Currently, there are no approved therapies for the treatment of muscle tissue following trauma. In this study, biomimetic sponges composed of gelatin, collagen, laminin-111, and FK-506 were used for the treatment of VML in a rodent model. We observed that biomimetic sponge treatment improved muscle structure and function while modulating inflammation and limiting the extent of fibrotic tissue deposition. Specifically, sponge treatment increased the total number of myofibers, type 2B fiber cross-sectional area, myosin: collagen ratio, myofibers with central nuclei, and peak isometric torque compared to untreated VML injured muscles. As an acellular scaffold, biomimetic sponges may provide a promising clinical therapy for VML.

Independent of physical activity, volumetric muscle loss injury in a murine model impairs whole-body metabolism

Volumetric muscle loss (VML) injuries result in a non-recoverable loss of muscle tissue and function due to trauma or surgery. Reductions in physical activity increase the risk of metabolic comorbidities over time, and it is likely that VML may reduce whole-body activity. However, these aspects remain uncharacterized following injury. Our goal was to characterize the impact of VML on whole-body physical activity and metabolism, and to further investigate possible muscle-specific metabolic changes. Adult male C57Bl/6J (n = 28) mice underwent a standardized VML injury to the posterior compartment of the hind limb, or served as injury naïve controls. Mice underwent longitudinal evaluation of whole-body physical activity and metabolism in specialized cages up to three times over the course of 8 weeks. At terminal time points of 4- and 8-weeks post-VML in vivo muscle function of the posterior compartment was evaluated. Additionally, the gastrocnemius muscle was collected to understand histological and biochemical changes in the muscle remaining after VML. The VML injury did not alter the physical activity of mice. However, there was a noted reduction in whole-body metabolism and diurnal fluctuations between lipid and carbohydrate oxidation were also reduced, largely driven by lower carbohydrate utilization during active hours. Following VML, muscle-specific changes indicate a decreased proportion of fast (i.e., type IIb and IIx) and a greater proportion of slow (i.e., type I and IIa) fibers. However, there were minimal changes in the capillarity and metabolic biochemical activity properties of the gastrocnemius muscle, suggesting a miss-match in capacity to support the physiologic needs of the fibers. These novel findings indicate that following VML, independent of changes in physical activity, there is whole-body diurnal metabolic inflexibility. Supporting future investigations into the chronic and overlooked co-morbidities of VML injury.

Targeting intramuscular adipose tissue expansion to preserve contractile function in volumetric muscle loss: A potentially novel therapy?

In volumetric muscle loss (VML), the severity of trauma exceeds a muscle's regenerative capacity. VML causes permanent functional impairments for which there are no rehabilitative, pharmacological, or regenerative medicine interventions. Driving failed regeneration in VML is a hostile microenvironment characterized by heightened inflammation, fibrosis, and denervation, which may reduce the remaining muscle tissue's quality, and stimulate intramuscular adipose tissue (IMAT) expansion. IMAT is increased in various muscle disease states, and has known lipotoxic effects on regeneration and contractile function. The contribution of ectopic fat deposition to the hostile VML microenvironment at the injury site and in the remaining tissue warrants further investigation. Targeting IMAT may lead to novel therapeutic strategies for improving functional outcomes in VML.

Secondary denervation is a chronic pathophysiologic sequela of volumetric muscle loss

Volumetric muscle loss (VML) is the traumatic loss of muscle tissue that results in long-term functional impairments. Despite the loss of myofibers, there remains an unexplained significant decline in muscle function. VML injury likely extends beyond the defect area, causing negative secondary outcomes to the neuromuscular system, including the neuromuscular junctions (NMJs), yet the extent to which VML induces denervation is unclear. This study systematically examined NMJs surrounding the VML injury, hypothesizing that the sequela of VML includes denervation. The VML injury removed ∼20% of the tibialis anterior (TA) muscle in adult male inbred Lewis rats (n = 43), the noninjured leg served as an intra-animal control. Muscles were harvested up to 48 days post-VML. Synaptic terminals were identified immunohistochemically, and quantitative confocal microscopy evaluated 2,613 individual NMJ. Significant denervation was apparent by 21 and 48 days post-VML. Initially, denervation increased ∼10% within 3 days of injury; with time, denervation further increased to ∼22% and 32% by 21 and 48 days post-VML, respectively, suggesting significant secondary denervation. The appearance of terminal axon sprouting and polyinnervation were observed as early as 7 days post-VML, increasing in number and complexity throughout 48 days. There was no evidence of VML-induced NMJ size alteration, which may be beneficial for interventions aimed at restoring muscle function. This work recognizes VML-induced secondary denervation and poor remodeling of the NMJ as part of the sequela of VML injury; moreover, secondary denervation is a possible contributing factor to the chronic functional impairments and potentially an overlooked treatment target.NEW & NOTEWORTHY This work advances our understanding of the pathophysiologic complexity of volumetric muscle loss injury. Specifically, we identified secondary denervation in the muscle remaining after volumetric muscle loss injuries as a novel aspect of the injury sequela. Denervation increased chronically, in parallel with the appearance of irregular morphological characteristics and destabilization of the neuromuscular junction, which is expected to further confound chronic functional impairments.

Advances in biomaterials for skeletal muscle engineering and obstacles still to overcome

Repair of injured skeletal muscle is a sophisticated process that uses immune, muscle, perivascular, and neural cells. In acute injury, the robust endogenous repair process can facilitate complete regeneration with little to no functional deficit. However, in severe injury, the damage is beyond the capacity for self-repair, often resulting in structural and functional deficits. Aside from the insufficiencies in muscle function, the aesthetic deficits can impact quality of life. Current clinical treatments are significantly limited in their capacity to structurally and functionally repair the damaged skeletal muscle. Therefore, alternative approaches are needed. Biomaterial therapies for skeletal muscle engineering have leveraged natural materials with sophisticated scaffold fabrication techniques to guide cell infiltration, alignment, and differentiation. Advances in biomaterials paired with a standardized and rigorous assessment of resulting tissue formation have greatly advanced the field of skeletal muscle engineering in the last several years. Herein, we discuss the current trends in biomaterials-based therapies for skeletal muscle regeneration and present the obstacles still to be overcome before clinical translation is possible. With millions of people affected by muscle trauma each year, the development of a therapy that can repair the structural and functional deficits after severe muscle injury is pivotal.