Autologous minced muscle grafts improve endogenous fracture healing and muscle strength after musculoskeletal trauma

Muscle injury in orthopaedic trauma

Muscle injury in the setting of orthopaedic trauma is common. Skeletal muscle injury can cause immune dysregulation and impair fracture healing both in patients and in various preclinical models. Muscle injury can also be associated with impaired vascularity and eliminate the muscle paracrine effect, both of which can impair fracture healing. Severe muscle injury can lead to volumetric muscle loss. While there are currently no effective treatments for volumetric muscle loss, minced muscle autograft has been shown to improve fracture healing, but not improve muscle function. Acute compartment syndrome can severely impact functional recovery and limit fracture healing. However, current animal models of compartment syndrome lack appropriate translation to the clinical injury.

Acrylated Hyaluronic-Acid Based Hydrogel for the Treatment of Craniofacial Volumetric Muscle Loss

Current treatment options for craniofacial volumetric muscle loss (VML) have disadvantages and cannot fully restore normal function. Bio-inspired semisynthetic acrylated hyaluronic acid (AcHyA) hydrogel, which fills irregularly shaped defects, resembles an extracellular matrix, and induces a minimal inflammatory response, has shown promise in experimental studies of extremity VML. We therefore sought to study AcHyA hydrogel in the treatment of craniofacial VML. For this, we used a novel model of masseter VML in the rat. Following the creation of a 5 mm × 5 mm injury to the superficial masseter and administration of AcHyA to the wound, masseters were explanted between 2 and 16 weeks postoperatively and were analyzed for evidence of muscle regeneration including fibrosis, defect size, and fiber cross-sectional area (FCSA). At 8 and 16 weeks, masseters treated with AcHyA showed significantly less fibrosis than nonrepaired controls and a smaller decrease in defect size. The mean FCSA among fibers near the defect was significantly greater among hydrogel-repaired than control masseters at 8 weeks, 12 weeks, and 16 weeks. These results show that the hydrogel mitigates the fibrotic healing response and wound contracture. Our findings also suggest that hydrogel-based treatments have potential use as a treatment for the regeneration of craniofacial VML and demonstrate a system for evaluating subsequent iterations of materials in VML injuries. Impact Statement Craniofacial volumetric muscle loss (VML) is a debilitating condition for which current treatment options are unable to restore normal appearance, or function. Tissue engineering approaches, such as hydrogel implants, may be an effective strategy to fill the volumetric defects and promote de novo muscle regeneration. In this study, we describe a novel rodent model for the study of craniofacial VML and a hyaluronic acid-based hydrogel that can be used as a treatment for the regeneration of craniofacial VML.

Age-associated functional healing of musculoskeletal trauma through regenerative engineering and rehabilitation

Traumatic musculoskeletal injuries that lead to volumetric muscle loss (VML) are challenged by irreparable soft tissue damage, impaired regenerative ability, and reduced muscle function. Regenerative rehabilitation strategies involving the pairing of engineered therapeutics with exercise have guided considerable advances in the functional repair of skeletal muscle following VML. However, few studies evaluate the efficacy of regenerative rehabilitation across the lifespan. In the current study, young and aged mice are treated with an engineered muscle, consisting of nanofibrillar-aligned collagen laden with myogenic cells, in combination with voluntary running activity following a VML injury. Overall, young mice perform at higher running volumes and intensities compared to aged mice but exhibit similar volumes relative to age-matched baselines. Additionally, young mice are highly responsive to the dual treatment showing enhanced force production (p < 0.001), muscle mass (p < 0.05), and vascular density (p < 0.01) compared to age-matched controls. Aged mice display upregulation of circulating inflammatory cytokines and show no significant regenerative response to treatment, suggesting a diminished efficacy of regenerative rehabilitation in aged populations. These findings highlight the restorative potential of regenerative engineering and rehabilitation for the treatment of traumatic musculoskeletal injuries in young populations and the complimentary need for age-specific interventions and studies to serve broader patient demographics.

Granular Hydrogels Improve Myogenic Invasion and Repair after Volumetric Muscle Loss

Skeletal muscle injuries including volumetric muscle loss (VML) lead to excessive tissue scarring and permanent functional disability. Despite its high prevalence, there is currently no effective treatment for VML. Bioengineering interventions such as biomaterials that fill the VML defect to support cell and tissue growth are a promising therapeutic strategy. However, traditional biomaterials developed for this purpose lack the pore features needed to support cell infiltration. The present study investigates for the first time, the impact of granular hydrogels on muscle repair - hypothesizing that their flowability will permit conformable filling of the defect site and their inherent porosity will support the invasion of native myogenic cells, leading to effective muscle repair. Small and large microparticle fragments are prepared from photocurable hyaluronic acid polymer via extrusion fragmentation and facile size sorting. In assembled granular hydrogels, particle size and degree of packing significantly influence pore features, rheological behavior, and injectability. Using a mouse model of VML, it is demonstrated that, in contrast to bulk hydrogels, granular hydrogels support early-stage (satellite cell invasion) and late-stage (myofiber regeneration) muscle repair processes. Together, these results highlight the promising potential of injectable and porous granular hydrogels in supporting endogenous repair after severe muscle injury.

Bioactive Glass and Silica Particles for Skeletal and Cardiac Muscle Tissue Regeneration

When skeletal and cardiac tissues are damaged, surgical approaches are not always successful and tissue regeneration approaches are investigated. Reports in the literature indicate that silica nanoparticles and bioactive glasses (BGs), including silicate bioactive glasses (e.g., 45S5 BG), phosphate glass fibers, boron-doped mesoporous BGs, borosilicate glasses, and aluminoborates, are promising for repairing skeletal muscle tissue. Silica nanoparticles and BGs have been combined with polymers to obtain aligned nanofibers and to maintain controlled delivery of nanoparticles for skeletal muscle repair. The literature indicates that cardiac muscle regeneration can be also triggered by the ionic products of BGs. This was observed to be due to the release of vascular endothelial growth factor and other growth factors from cardiomyocytes, which regulate endothelial cells to form capillary structures (angiogenesis). Specific studies, including both in vitro and in vivo approaches, are reviewed in this article. The analysis of the literature indicates that although the research field is still very limited, BGs are showing great promise for muscle tissue engineering and further research in the field should be carried out to expand our basic knowledge on the application of BGs in muscle (skeletal and cardiac) tissue regeneration. Impact statement This review highlights the potential of silica particles and bioactive glasses (BGs) for skeletal and cardiac tissue regeneration. These biomaterials create scaffolds triggering muscle cell differentiation. Ionic products from BGs stimulate growth factors, supporting angiogenesis in cardiac tissue repair. Further research is required to expand our know-how on silica particles and BGs in muscle tissue engineering.

Delivering Microrobots in the Musculoskeletal System

Disorders of the musculoskeletal system are the major contributors to the global burden of disease and current treatments show limited efficacy. Patients often suffer chronic pain and might eventually have to undergo end-stage surgery. Therefore, future treatments should focus on early detection and intervention of regional lesions. Microrobots have been gradually used in organisms due to their advantages of intelligent, precise and minimally invasive targeted delivery. Through the combination of control and imaging systems, microrobots with good biosafety can be delivered to the desired area for treatment. In the musculoskeletal system, microrobots are mainly utilized to transport stem cells/drugs or to remove hazardous substances from the body. Compared to traditional biomaterial and tissue engineering strategies, active motion improves the efficiency and penetration of local targeting of cells/drugs. This review discusses the frontier applications of microrobotic systems in different tissues of the musculoskeletal system. We summarize the challenges and barriers that hinder clinical translation by evaluating the characteristics of different microrobots and finally point out the future direction of microrobots in the musculoskeletal system.

Multiprotein collagen/keratin hydrogel promoted myogenesis and angiogenesis of injured skeletal muscles in a mouse model

Volumetric loss is one of the challenging issues in muscle tissue structure that causes functio laesa. Tissue engineering of muscle tissue using suitable hydrogels is an alternative to restoring the physiological properties of the injured area. Here, myogenic properties of type I collagen (0.5%) and keratin (0.5%) were investigated in a mouse model of biceps femoris injury. Using FTIR, gelation time, and rheological analysis, the physicochemical properties of the collagen (Col)/Keratin scaffold were analyzed. Mouse C2C12 myoblast-laden Col/Keratin hydrogels were injected into the injury site and histological examination plus western blotting were performed to measure myogenic potential after 15 days. FTIR indicated an appropriate interaction between keratin and collagen. The blend of Col/Keratin delayed gelation time when compared to the collagen alone group. Rheological analysis revealed decreased stiffening in blended Col/Keratin hydrogel which is favorable for the extrudability of the hydrogel. Transplantation of C2C12 myoblast-laden Col/Keratin hydrogel to injured muscle tissues led to the formation of newly generated myofibers compared to cell-free hydrogel and collagen groups (p < 0.05). In the C2C12 myoblast-laden Col/Keratin group, a low number of CD31+ cells with minimum inflammatory cells was evident. Western blotting indicated the promotion of MyoD in mice that received cell-laden Col/Keratin hydrogel compared to the other groups (p < 0.05). Despite the increase of the myosin cell-laden Col/Keratin hydrogel group, no significant differences were obtained related to other groups (p > 0.05). The blend of Col/Keratin loaded with myoblasts provides a suitable myogenic platform for the alleviation of injured muscle tissue.

The local and systemic effects of immune function on fracture healing

The immune system plays an integral role in the regulation of cellular processes responsible for fracture healing. Local and systemic influences on fracture healing correlate in many ways with fracture-related outcomes, including soft tissue healing quality and fracture union rates. Impaired soft tissue healing, restricted perfusion of a fracture site, and infection also in turn affect the immune response to fracture injury. Modern techniques used to investigate the relationship between immune system function and fracture healing include precision medicine, using vast quantities of data to interpret broad patterns of inflammatory response. Early data from the PRECISE trial have demonstrated distinct patterns of inflammatory response in polytrauma patients, which thereby directly and indirectly regulate the fracture healing response. The clearly demonstrated linkage between immune function and fracture healing suggests that modulation of immune function has significant potential as a therapeutic target that can be used to enhance fracture healing.

The critical impact of traumatic muscle loss on fracture healing: Basic science and clinical aspects

Musculoskeletal trauma, specifically fractures, is a leading cause of patient morbidity and disability worldwide. In approximately 20% of cases with fracture and related traumatic muscle loss, bone healing is impaired leading to fracture nonunion. Over the past few years, several studies have demonstrated that bone and the surrounding muscle tissue interact not only anatomically and mechanically but also through biochemical pathways and mediators. Severe damage to the surrounding musculature at the fracture site causes an insufficiency in muscle-derived osteoprogenitor cells that are crucial for fracture healing. As an endocrine tissue, skeletal muscle produces many myokines that act on different bone cells, such as osteoblasts, osteoclasts, osteocytes, and mesenchymal stem cells. Investigating how muscle influences fracture healing at cellular, molecular, and hormonal levels provides translational therapeutic solutions to this clinical challenge. This review provides an overview about the contributions of surrounding muscle tissue in directing fracture healing. The focus of the review is on describing the interactions between bone and muscle in both healthy and fractured environments. We discuss current progress in identifying the bone-muscle molecular pathways and strategies to harness these pathways as cues for accelerating fracture healing. In addition, we review the existing challenges and research opportunities in the field.

Local IL-10 delivery modulates the immune response and enhances repair of volumetric muscle loss muscle injury

This study was designed to test the hypothesis that in addition to repairing the architectural and cellular cues via regenerative medicine, the delivery of immune cues (immunotherapy) may be needed to enhance regeneration following volumetric muscle loss (VML) injury. We identified IL-10 signaling as a promising immunotherapeutic target. To explore the impact of targeting IL-10 signaling, tibialis anterior (TA) VML injuries were created and then treated in rats using autologous minced muscle (MM). Animals received either recombinant rat IL-10 or phosphate buffered saline (PBS) controls injections at the site of VML repair beginning 7 days post injury (DPI) and continuing every other day (4 injections total) until 14 DPI. At 56 DPI (study endpoint), significant improvements to TA contractile torque (82% of uninjured values & 170% of PBS values), TA mass, and myofiber size in response to IL-10 treatment were detected. Whole transcriptome analysis at 14 DPI revealed activation of IL-10 signaling, muscle hypertrophy, and lymphocytes signaling pathways. Expression of ST2, a regulatory T (T_reg) cell receptor, was dramatically increased at the VML repair site in response to IL-10 treatment when compared to PBS controls. The findings suggest that the positive effect of delayed IL-10 delivery might be due to immuno-suppressive T_reg cell recruitment.

FK506 increases susceptibility to musculoskeletal infection in a rodent model

Background

Delayed fracture healing caused by soft tissue loss can be resolved by the administration of a Th1 immunosuppressant, such as FK506. Additionally, open fractures are at high risk for infection. We hypothesized that the inclusion of an immunosuppressant to a subject at risk for a musculoskeletal infection will increase the likelihood of infection.

Methods

A rat model of musculoskeletal infection was used. Sprague Dawley rats received a stabilized femur defect and were inoculated with 10⁴ CFU Staphylococcus aureus via a collagen matrix. Six hours after inoculation, the wounds were debrided of collagen and devitalized tissue and irrigated with sterile saline. The animals were randomized into two groups: carrier control and FK506, which were administered daily for 14 days and were euthanized and the tissues harvested to measure local bioburden.

Results

The dosing regimen of FK506 that restored bone healing increased the bioburden in the bone and on the fixation implant compared to the carrier control animals. As expected, the administration of FK506 decreased circulating white blood cells, lymphocytes, neutrophils, and monocytes. Additionally, the red blood cell count, hematocrit, and body weight were lower in those animals that received FK506 compared to carrier control.

Conclusions

FK506 administration decreased the systemic immune cell counts and increased the bacterial bioburden within a model of musculoskeletal infection. Collectively, these outcomes could be attributed to the overall T cell suppression by FK506 and the altered antimicrobial activity of innate cells, thereby allowing S. aureus to thrive and subsequently leading to infection of severe, musculoskeletal injuries. These observations reveal the crucial continued investigation for the clinical use of FK506, and other immunosuppressant compounds, in trauma patients who are at increased risk of developing infections.

Biomimetic sponges improve functional muscle recovery following composite trauma

There is a dearth of therapies that are safe and effective for the treatment of volumetric muscle loss (VML), defined as the surgical or traumatic loss of muscle tissue, resulting in functional impairment. To address this gap in orthopedic care, we developed a porous sponge-like scaffold composed of extracellular matrix (ECM) proteins (e.g., gelatin, collagen, and laminin-111) and an immunosuppressant drug, FK-506. While the majority of VML injuries occur in orthopedic trauma cases, preclinical models typically study muscle injuries in isolation without a concomitant bone fracture. The goal of this study was to investigate the extent to which FK506 loaded biomimetic sponges support functional muscle regeneration and fracture healing in a composite trauma model involving VML injury to the tibialis anterior muscle and osteotomy (OST) to the tibia. In this model, implantation of the FK-506 loaded biomimetic sponges limited the extent of inflammation while increasing the total number of myofibers, mean myofiber cross-sectional area, myosin-to-collagen ratio, and peak isometric torque compared to untreated VML+OST muscles on Day 28. Although all tibia fractures were bridged by Day 28 post-injury, fracture healing was impaired in response to an adjacent VML injury. Sponge treatment increased bone callus volume, yet the bridged mineralized bone volume was not significantly different. Taken together, these results suggest that biomimetic sponges primarily benefitted muscle repair and may provide a promising therapy for traumatized muscle.

Nanoengineered myogenic scaffolds for skeletal muscle tissue engineering

Extreme loss of skeletal muscle overwhelms the natural regenerative capability of the body, results in permanent disability and substantial economic burden. Current surgical techniques result in poor healing, secondary injury to the autograft donor site, and incomplete recuperation of muscle function. Most current tissue engineering and regenerative strategies fail to create an adequate mechanical and biological environment that enables cell infiltration, proliferation, and myogenic differentiation. In this study, we present a nanoengineered skeletal muscle scaffold based on functionalized gelatin methacrylate (GelMA) hydrogel, optimized for muscle progenitors' proliferation and differentiation. The scaffold was capable of controlling the release of insulin-like growth factor 1 (IGF-1), an important myogenic growth factor, by utilizing the electrostatic interactions with LAPONITE® nanoclays (NCs). Physiologically relevant levels of IGF-1 were maintained during a controlled release over two weeks. The NC was able to retain 50% of the released IGF-1 within the hydrogel niche, significantly improving cellular proliferation and differentiation compared to control hydrogels. IGF-1 supplemented medium controls required 44% more IGF-1 than the comparable NC hydrogel composites. The nanofunctionalized scaffold is a viable option for the treatment of extreme muscle injuries and offers scalable benefits for translational interventions and the growing field of clean meat production.

Laminin-111-Enriched Fibrin Hydrogels Enhance Functional Muscle Regeneration Following Trauma

Volumetric muscle loss (VML) is the surgical or traumatic loss of skeletal muscle, which can cause loss of limb function or permanent disability. VML injuries overwhelms the endogenous regenerative capacity of skeletal muscle and results in poor functional healing outcomes. Currently, there are no approved tissue engineering treatments for VML injuries. In this study, fibrin hydrogels enriched with laminin-111 (LM-111; 50-450 μg/mL) were used for the treatment of VML of the tibialis anterior in a rat model. Treatment with fibrin hydrogel containing 450 μg/mL of LM-111 (FBN450) improved muscle regeneration following VML injury. FBN450 hydrogel treatment increased the relative proportion of contractile to fibrotic tissue as indicated by the myosin: collagen ratio on day 28 post-VML injury. FBN450 hydrogels also enhanced myogenic protein expression and increased the quantity of small to medium size myofibers (500-2000 μm²) as well as innervated myofibers. Improved contractile tissue deposition due to FBN450 hydrogel treatment resulted in a significant improvement (∼60%) in torque production at day 28 postinjury. Taken together, these results suggest that the acellular FBN450 hydrogels provide a promising therapeutic strategy for VML that is worthy of further investigation.

Mobilizing Endogenous Repair Through Understanding Immune Reaction With Biomaterials

With few exceptions, humans are incapable of fully recovering from severe physical trauma. Due to these limitations, the field of regenerative medicine seeks to find clinically viable ways to repair permanently damaged tissue. There are two main approaches to regenerative medicine: promoting endogenous repair of the wound, or transplanting a material to replace the injured tissue. In recent years, these two methods have fused with the development of biomaterials that act as a scaffold and mobilize the body's natural healing capabilities. This process involves not only promoting stem cell behavior, but by also inducing activity of the immune system. Through understanding the immune interactions with biomaterials, we can understand how the immune system participates in regeneration and wound healing. In this review, we will focus on biomaterials that promote endogenous tissue repair, with discussion on their interactions with the immune system.

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.

Administration of a selective retinoic acid receptor-γ agonist improves neuromuscular strength in a rodent model of volumetric muscle loss

PURPOSE

Volumetric muscle loss is a uniquely challenging pathology that results in irrecoverable functional deficits. Furthermore, a breakthrough drug or bioactive factor has yet to be established that adequately improves repair of these severe skeletal muscle injuries. This study sought to assess the ability of an orally administered selective retinoic acid receptor-γ agonist, palovarotene, to improve recovery of neuromuscular strength in a rat model of volumetric muscle loss.

METHODS

An irrecoverable, full thickness defect was created in the tibialis anterior muscle of Lewis rats and animals were survived for 4 weeks. Functional recovery of the tibialis anterior muscle was assessed in vivo via neural stimulation and determination of peak isometric torque. Histological staining was performed to qualitatively assess fibrous scarring of the defect site.

RESULTS

Treatment with the selective retinoic acid receptor-γ agonist, palovarotene, resulted in a 38% improvement of peak isometric torque in volumetric muscle loss affected limbs after 4 weeks of healing compared to untreated controls. Additionally, preliminary histological assessment suggests that oral administration of palovarotene reduced fibrous scarring at the defect site.

CONCLUSIONS

These results highlight the potential role of selective retinoic acid receptor-γ agonists in the design of regenerative medicine platforms to maximize skeletal muscle healing. Additional studies are needed to further elucidate cellular responses, optimize therapeutic delivery, and characterize synergistic potential with adjunct therapies.

Gait biomechanics: A clinically relevant outcome measure for preclinical research of musculoskeletal trauma

Traumatic injuries to the musculoskeletal system are the most prevalent of those suffered by United States Military Service members and accounts for two-thirds of initial hospital costs to the Department of Defense. These combat-related wounds often leave survivors with life-long disability and represent a significant impediment to the readiness of the fighting force. There are immense opportunities for the field of tissue engineering and regenerative medicine (TE/RM) to address these musculoskeletal injuries through regeneration of damaged tissues as a means to restore limb functionality and improve quality of life for affected individuals. Indeed, investigators have made promising advancements in the treatment for these injuries by utilizing small and large preclinical animal models to validate therapeutic efficacy of next-generation TE/RM-based technologies. Importantly, utilization of a comprehensive suite of functional outcome measures, particularly those designed to mimic data collected within the clinical setting, is critical for successful translation and implementation of these therapeutics. To that end, the objective of this review is to emphasize the clinical relevance and application of gait biomechanics as a functional outcome measure for preclinical research studies evaluating the efficacy of TE/RM therapies to treat traumatic musculoskeletal injuries. Specifically, common musculoskeletal injuries sustained by service members-including volumetric muscle loss, post-traumatic osteoarthritis, and composite tissue injuries-are examined as case examples to highlight the use of gait biomechanics as an outcome measure using small and large preclinical animal models.

Agent-based model provides insight into the mechanisms behind failed regeneration following volumetric muscle loss injury

Skeletal muscle possesses a remarkable capacity for repair and regeneration following a variety of injuries. When successful, this highly orchestrated regenerative process requires the contribution of several muscle resident cell populations including satellite stem cells (SSCs), fibroblasts, macrophages and vascular cells. However, volumetric muscle loss injuries (VML) involve simultaneous destruction of multiple tissue components (e.g., as a result of battlefield injuries or vehicular accidents) and are so extensive that they exceed the intrinsic capability for scarless wound healing and result in permanent cosmetic and functional deficits. In this scenario, the regenerative process fails and is dominated by an unproductive inflammatory response and accompanying fibrosis. The failure of current regenerative therapeutics to completely restore functional muscle tissue is not surprising considering the incomplete understanding of the cellular mechanisms that drive the regeneration response in the setting of VML injury. To begin to address this profound knowledge gap, we developed an agent-based model to predict the tissue remodeling response following surgical creation of a VML injury. Once the model was able to recapitulate key aspects of the tissue remodeling response in the absence of repair, we validated the model by simulating the tissue remodeling response to VML injury following implantation of either a decellularized extracellular matrix scaffold or a minced muscle graft. The model suggested that the SSC microenvironment and absence of pro-differentiation SSC signals were the most important aspects of failed muscle regeneration in VML injuries. The major implication of this work is that agent-based models may provide a much-needed predictive tool to optimize the design of new therapies, and thereby, accelerate the clinical translation of regenerative therapeutics for VML injuries.

The Role of Innate and Adaptive Immune Cells in Skeletal Muscle Regeneration

Skeletal muscle regeneration is highly dependent on the inflammatory response. A wide variety of innate and adaptive immune cells orchestrate the complex process of muscle repair. This review provides information about the various types of immune cells and biomolecules that have been shown to mediate muscle regeneration following injury and degenerative diseases. Recently developed cell and drug-based immunomodulatory strategies are highlighted. An improved understanding of the immune response to injured and diseased skeletal muscle will be essential for the development of therapeutic strategies.

Ionic Silicon Protects Oxidative Damage and Promotes Skeletal Muscle Cell Regeneration

Volumetric muscle loss injuries overwhelm the endogenous regenerative capacity of skeletal muscle, and the associated oxidative damage can delay regeneration and prolong recovery. This study aimed to investigate the effect of silicon-ions on C2C12 skeletal muscle cells under normal and excessive oxidative stress conditions to gain insights into its role on myogenesis during the early stages of muscle regeneration. In vitro studies indicated that 0.1 mM Si-ions into cell culture media significantly increased cell viability, proliferation, migration, and myotube formation compared to control. Additionally, MyoG, MyoD, Neurturin, and GABA expression were significantly increased with addition of 0.1, 0.5, and 1.0 mM of Si-ion for 1 and 5 days of C2C12 myoblast differentiation. Furthermore, 0.1-2.0 mM Si-ions attenuated the toxic effects of H2O2 within 24 h resulting in increased cell viability and differentiation. Addition of 1.0 mM of Si-ions significantly aid cell recovery and protected from the toxic effect of 0.4 mM H2O2 on cell migration. These results suggest that ionic silicon may have a potential effect in unfavorable situations where reactive oxygen species is predominant affecting cell viability, proliferation, migration, and differentiation. Furthermore, this study provides a guide for designing Si-containing biomaterials with desirable Si-ion release for skeletal muscle regeneration.

Pleiotropic actions of Vitamin D in composite musculoskeletal trauma

Composite tissue injuries are the result of high energy impacts caused by motor vehicle accidents, gunshot wounds or blasts. These are highly traumatic injuries characterized by wide-spread, penetrating wounds affecting the entire musculoskeletal system, and are generally defined by frank volumetric muscle loss with concomitant segmental bone defects. At the tissue level, the breadth of damage to multiple tissue systems, and potential for infection from penetration, have been shown to lead to an exaggerated, often chronic inflammatory response with subsequent dysregulation of normal musculoskeletal healing mechanisms. Aside from the direct effects of inflammation on myogenesis and osteogenesis, frank muscle loss has been shown to directly impair fracture union and ultimately contribute to failed wound regeneration. Care for these injuries requires extensive surgical intervention and acute care strategies. However, often these interventions do not adequately mitigate inflammation or promote proper musculoskeletal injury repair and force amputation of the limb. Therefore, identification of factors that can promote tissue regeneration and mitigate inflammation could be key to restoring wound healing after composite tissue injury. One such factor that may directly affect both inflammation and tissue regeneration in response to these multi-tissue injuries may be Vitamin D. Beyond traditional roles, the pleiotropic and localized actions of Vitamin D are increasingly being recognized in most aspects of wound healing in complex tissue injuries - e.g., regulation of inflammation, myogenesis, fracture callus mineralization and remodeling. Conversely, pre-existing Vitamin D deficiency leads to musculoskeletal dysfunction, increased fracture risk or fracture non-unions, decreased strength/function and reduced capacity to heal wounds through increased inflammation. This Vitamin D deficient state requires acute supplementation in order to quickly restore circulating levels to an optimal level, thereby facilitating a robust wound healing response. Herein, the purpose of this review is to address the roles and critical functions of Vitamin D throughout the wound healing process. Findings from this review suggest that careful monitoring and/or supplementation of Vitamin D may be critical for wound regeneration in composite tissue injuries.

Transplantation of insulin-like growth factor-1 laden scaffolds combined with exercise promotes neuroregeneration and angiogenesis in a preclinical muscle injury model

The regeneration of skeletal muscle can be permanently impaired by traumatic injuries, despite the high regenerative capacity of native muscle. An attractive therapeutic approach for treating severe muscle inuries is the implantation of off-the-shelf engineered biomimetic scaffolds into the site of tissue damage to enhance muscle regeneration. Anisotropic nanofibrillar scaffolds provide spatial patterning cues to create organized myofibers, and growth factors such as insulin-like growth factor-1 (IGF-1) are potent inducers of both muscle regeneration as well as angiogenesis. The aim of this study was to test the therapeutic efficacy of anisotropic IGF-1-releasing collagen scaffolds combined with voluntary exercise for the treatment of acute volumetric muscle loss, with a focus on histomorphological effects. To enhance the angiogenic and regenerative potential of injured murine skeletal muscle, IGF-1-laden nanofibrillar scaffolds with aligned topography were fabricated using a shear-mediated extrusion approach, followed by growth factor adsorption. Individual scaffolds released a cumulative total of 1244 ng ± 153 ng of IGF-1 over the course of 21 days in vitro. To test the bioactivity of IGF-1-releasing scaffolds, the myotube formation capacity of murine myoblasts was quantified. On IGF-1-releasing scaffolds seeded with myoblasts, the resulting myotubes formed were 1.5-fold longer in length and contained 2-fold greater nuclei per myotube, when compared to scaffolds without IGF-1. When implanted into the ablated murine tibialis anterior muscle, the IGF-1-laden scaffolds, in conjunction with voluntary wheel running, significantly increased the density of perfused microvessels by greater than 3-fold, in comparison to treatment with scaffolds without IGF-1. Enhanced myogenesis was also observed in animals treated with the IGF-1-laden scaffolds combined with exercise, compared to control scaffolds transplanted into mice that did not receive exercise. Furthermore, the abundance of mature neuromuscular junctions was greater by approximately 2-fold in muscles treated with IGF-1-laden scaffolds, when paired with exercise, in comparison to the same treatment without exercise. These findings demonstrate that voluntary exercise improves the regenerative effect of growth factor-laden scaffolds by augmenting neurovascular regeneration, and have important translational implications in the design of off-the-shelf therapeutics for the treatment of traumatic muscle injury.

Time Course of Immune Response and Immunomodulation During Normal and Delayed Healing of Musculoskeletal Wounds

Single trauma injuries or isolated fractures are often manageable and generally heal without complications. In contrast, high-energy trauma results in multi/poly-trauma injury patterns presenting imbalanced pro- and anti- inflammatory responses often leading to immune dysfunction. These injuries often exhibit delayed healing, leading to fibrosis of injury sites and delayed healing of fractures depending on the intensity of the compounding traumas. Immune dysfunction is accompanied by a temporal shift in the innate and adaptive immune cells distribution, triggered by the overwhelming release of an arsenal of inflammatory mediators such as complements, cytokines and damage associated molecular patterns (DAMPs) from necrotic cells. Recent studies have implicated this dysregulated inflammation in the poor prognosis of polytraumatic injuries, however, interventions focusing on immunomodulating inflammatory cellular composition and activation, if administered incorrectly, can result in immune suppression and unintended outcomes. Immunomodulation therapy is promising but should be conducted with consideration for the spatial and temporal distribution of the immune cells during impaired healing. This review describes the current state of knowledge in the spatiotemporal distribution patterns of immune cells at various stages during musculoskeletal wound healing, with a focus on recent advances in the field of Osteoimmunology, a study of the interface between the immune and skeletal systems, in long bone fractures. The goals of this review are to (1) discuss wound and fracture healing processes of normal and delayed healing in skeletal muscles and long bones; (2) provide a balanced perspective on temporal distributions of immune cells and skeletal cells during healing; and (3) highlight recent therapeutic interventions used to improve fracture healing. This review is intended to promote an understanding of the importance of inflammation during normal and delayed wound and fracture healing. Knowledge gained will be instrumental in developing novel immunomodulatory approaches for impaired healing.

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.

Recent advances toward understanding the role of transplanted stem cells in tissue-engineered regeneration of musculoskeletal tissues

Stem cell-based tissue engineering is poised to revolutionize the treatment of musculoskeletal injuries. However, in order to overcome scientific, practical, and regulatory obstacles and optimize therapeutic strategies, it is essential to better understand the mechanisms underlying the pro-regenerative effects of stem cells. There has been an attempted paradigm shift within the last decade to think of transplanted stem cells as "medicinal" therapies that orchestrate healing on the basis of their secretome and immunomodulatory profiles rather than acting as bona fide stem cells that proliferate, differentiate, and directly produce matrix to form de novo tissues. Yet the majority of current bone and skeletal muscle tissue engineering strategies are still premised on a direct contribution of stem cells as building blocks to tissue regeneration. Our review of the recent literature finds that researchers continue to focus on the quantification of de novo bone/skeletal muscle tissue following treatment and few studies aim to address this mechanistic conundrum directly. The dichotomy of thought is reflected in the diversity of new advances ranging from in situ three-dimensional bioprinting to a focus on exosomes and extracellular vesicles. However, recent findings elucidating the role of the immune system in tissue regeneration combined with novel imaging platform technologies will have a profound impact on our future understanding of how stem cells promote healing following biomaterial-mediated delivery to defect sites.

The role of PDIA3 in myogenesis during muscle regeneration

Beta 3 (β3) integrin plays an important role in the initiation of myogenesis in adult muscle. Protein disulfide isomerases (PDIs) can activate β3 integrin in various cells to promote cell migration, adhesion and fusion. However, the effect of PDIs on myogenesis during muscle regeneration has not been elucidated. Here, we report that PDIA3 expression is induced in regenerating myofibers. The inhibition of PDIA3 in muscle injuries in mice disrupts myoblast differentiation, impairs muscle regeneration, and ultimately aggravates muscle damage. Moreover, PDIA3 expression is upregulated and observed on the cell surfaces of myoblasts during differentiation and fusion. The inhibition of extracellular PDIA3 with an anti-PDIA3 monoclonal antibody attenuates β3 integrin/AKT/mTOR signal activity, inhibits myoblast differentiation, and blocks the fusion of myoblasts. Thus, PDIA3 may be a mediator of myoblast differentiation and fusion during muscle regeneration.

Myogenic differentiation of human amniotic mesenchymal cells and its tissue repair capacity on volumetric muscle loss

Stem cell-based tissue engineering therapy is the most promising method for treating volumetric muscle loss. Human amniotic mesenchymal cells possess characteristics similar to those of embryonic stem cells. In this study, we verified the stem cell characteristics of human amniotic mesenchymal cells by the flow cytometry analysis, and osteogenic and adipogenic differentiation. Through induction with the DNA demethylating agent 5-azacytidine, human amniotic mesenchymal cells can undergo myogenic differentiation and express skeletal muscle cell-specific markers such as desmin and MyoD. The Wnt/β-catenin signaling pathway also plays an important role. After 5-azacytidine-induced human amniotic mesenchymal cells were implanted into rat tibialis anterior muscle with volumetric muscle loss, we observed increased angiogenesis and improved local tissue repair. We believe that human amniotic mesenchymal cells can serve as a potential source of cells for skeletal muscle tissue engineering.

Cell therapy to improve regeneration of skeletal muscle injuries

Diseases that jeopardize the musculoskeletal system and cause chronic impairment are prevalent throughout the Western world. In Germany alone, ~1.8 million patients suffer from these diseases annually, and medical expenses have been reported to reach 34.2bn Euros. Although musculoskeletal disorders are seldom fatal, they compromise quality of life and diminish functional capacity. For example, musculoskeletal disorders incur an annual loss of over 0.8 million workforce years to the German economy. Among these diseases, traumatic skeletal muscle injuries are especially problematic because they can occur owing to a variety of causes and are very challenging to treat. In contrast to chronic muscle diseases such as dystrophy, sarcopenia, or cachexia, traumatic muscle injuries inflict damage to localized muscle groups. Although minor muscle trauma heals without severe consequences, no reliable clinical strategy exists to prevent excessive fibrosis or fatty degeneration, both of which occur after severe traumatic injury and contribute to muscle degeneration and dysfunction. Of the many proposed strategies, cell-based approaches have shown the most promising results in numerous pre-clinical studies and have demonstrated success in the handful of clinical trials performed so far. A number of myogenic and non-myogenic cell types benefit muscle healing, either by directly participating in new tissue formation or by stimulating the endogenous processes of muscle repair. These cell types operate via distinct modes of action, and they demonstrate varying levels of feasibility for muscle regeneration depending, to an extent, on the muscle injury model used. While in some models the injury naturally resolves over time, other models have been developed to recapitulate the peculiarities of real-life injuries and therefore mimic the structural and functional impairment observed in humans. Existing limitations of cell therapy approaches include issues related to autologous harvesting, expansion and sorting protocols, optimal dosage, and viability after transplantation. Several clinical trials have been performed to treat skeletal muscle injuries using myogenic progenitor cells or multipotent stromal cells, with promising outcomes. Recent improvements in our understanding of cell behaviour and the mechanistic basis for their modes of action have led to a new paradigm in cell therapies where physical, chemical, and signalling cues presented through biomaterials can instruct cells and enhance their regenerative capacity. Altogether, these studies and experiences provide a positive outlook on future opportunities towards innovative cell-based solutions for treating traumatic muscle injuries-a so far unmet clinical need.

Biomaterial and stem cell-based strategies for skeletal muscle regeneration

Adult skeletal muscle can regenerate effectively after mild physical or chemical insult. Muscle trauma or disease can overwhelm this innate capacity for regeneration and result in heightened inflammation and fibrotic tissue deposition resulting in loss of structure and function. Recent studies have focused on biomaterial and stem cell-based therapies to promote skeletal muscle regeneration following injury and disease. Many stem cell populations besides satellite cells are implicated in muscle regeneration. These stem cells include but are not limited to mesenchymal stem cells, adipose-derived stem cells, hematopoietic stem cells, pericytes, fibroadipogenic progenitors, side population cells, and CD133⁺ stem cells. However, several challenges associated with their isolation, availability, delivery, survival, engraftment, and differentiation have been reported in recent studies. While acellular scaffolds offer a relatively safe and potentially off-the-shelf solution to cell-based therapies, they are often unable to stimulate host cell migration and activity to a level that would result in clinically meaningful regeneration of traumatized muscle. Combining stem cells and biomaterials may offer a viable therapeutic strategy that may overcome the limitations associated with these therapies when they are used in isolation. In this article, we review the stem cell populations that can stimulate muscle regeneration in vitro and in vivo. We also discuss the regenerative potential of combination therapies that utilize both stem cell and biomaterials for the treatment of skeletal muscle injury and disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1246-1262, 2019.

High mobility group box 1 protein regulates osteoclastogenesis through direct actions on osteocytes and osteoclasts in vitro

Old age and Cx43 deletion in osteocytes are associated with increased osteocyte apoptosis and osteoclastogenesis. We previously demonstrated that apoptotic osteocytes release elevated concentrations of the proinflammatory cytokine, high mobility group box 1 protein (HMGB1) and apoptotic osteocyte conditioned media (CM) promotes osteoclast differentiation. Further, prevention of osteocyte apoptosis blocks osteoclast differentiation and attenuates the extracellular release of HMGB1 and RANKL. Moreover, sequestration of HMGB1, in turn, reduces RANKL production/release by MLO-Y4 osteocytic cells silenced for Cx43 (Cx43^def ), highlighting the possibility that HMGB1 promotes apoptotic osteocyte-induced osteoclastogenesis. However, the role of HMGB1 signaling in osteocytes has not been well studied. Further, the mechanisms underlying its release and the receptor(s) responsible for its actions is not clear. We now report that a neutralizing HMGB1 antibody reduces osteoclast formation in RANKL/M-CSF treated bone marrow cells. In bone marrow macrophages (BMMs), toll-like receptor 4 (TLR4) inhibition with LPS-RS, but not receptor for advanced glycation end products (RAGE) inhibition with Azeliragon attenuated osteoclast differentiation. Further, inhibition of RAGE but not of TLR4 in osteoclast precursors reduced osteoclast number, suggesting that HGMB1 produced by osteoclasts directly affects differentiation by activating TLR4 in BMMs and RAGE in preosteoclasts. Our findings also suggest that increased osteoclastogenesis induced by apoptotic osteocytes CM is not mediated through HMGB1/RAGE activation and that direct HMGB1 actions in osteocytes stimulate pro-osteoclastogenic signal release from Cx43^def osteocytes. Based on these findings, we propose that HMGB1 exerts dual effects on osteoclasts, directly by inducing differentiation through TLR4 and RAGE activation and indirectly by increasing pro-osteoclastogenic cytokine secretion from osteocytes.

RAGE Signaling in Skeletal Biology

PURPOSE OF REVIEW

The receptor for advanced glycation end products (RAGE) and several of its ligands have been implicated in the onset and progression of pathologies associated with aging, chronic inflammation, and cellular stress. In particular, the role of RAGE and its ligands in bone tissue during both physiological and pathological conditions has been investigated. However, the extent to which RAGE signaling regulates bone homeostasis and disease onset remains unclear. Further, RAGE effects in the different bone cells and whether these effects are cell-type specific is unknown. The objective of the current review is to describe the literature over RAGE signaling in skeletal biology as well as discuss the clinical potential of RAGE as a diagnostic and/or therapeutic target in bone disease.

RECENT FINDINGS

The role of RAGE and its ligands during skeletal homeostasis, tissue repair, and disease onset/progression is beginning to be uncovered. For example, detrimental effects of the RAGE ligands, advanced glycation end products (AGEs), have been identified for osteoblast viability/activity, while others have observed that low level AGE exposure stimulates osteoblast autophagy, which subsequently promotes viability and function. Similar findings have been reported with HMGB1, another RAGE ligand, in which high levels of the ligand are associated with osteoblast/osteocyte apoptosis, whereas low level/short-term administration stimulates osteoblast differentiation/bone formation and promotes fracture healing. Additionally, elevated levels of several RAGE ligands (AGEs, HMGB1, S100 proteins) induce osteoblast/osteocyte apoptosis and stimulate cytokine production, which is associated with increased osteoclast differentiation/activity. Conversely, direct RAGE-ligand exposure in osteoclasts may have inhibitory effects. These observations support a conclusion that elevated bone resorption observed in conditions of high circulating ligands and RAGE expression are due to actions on osteoblasts/osteocytes rather than direct actions on osteoclasts, although additional work is required to substantiate the observations. Recent studies have demonstrated that RAGE and its ligands play an important physiological role in the regulation of skeletal development, homeostasis, and repair/regeneration. Conversely, elevated levels of RAGE and its ligands are clearly related with various diseases associated with increased bone loss and fragility. However, despite the recent advancements in the field, many questions regarding RAGE and its ligands in skeletal biology remain unanswered.

Multiscale analysis of a regenerative therapy for treatment of volumetric muscle loss injury

Skeletal muscle possesses a remarkable capacity to regenerate when injured, but when confronted with major traumatic injury resulting in volumetric muscle loss (VML), the regenerative process consistently fails. The loss of muscle tissue and function from VML injury has prompted development of a suite of therapeutic approaches but these strategies have proceeded without a comprehensive understanding of the molecular landscape that drives the injury response. Herein, we administered a VML injury in an established rodent model and monitored the evolution of the healing phenomenology over multiple time points using muscle function testing, histology, and expression profiling by RNA sequencing. The injury response was then compared to a regenerative medicine treatment using orthotopic transplantation of autologous minced muscle grafts (~1 mm3 tissue fragments). A chronic inflammatory and fibrotic response was observed at all time points following VML. These results suggest that the pathological response to VML injury during the acute stage of the healing response overwhelms endogenous and therapeutic regenerative processes. Overall, the data presented delineate key molecular characteristics of the pathobiological response to VML injury that are critical effectors of effective regenerative treatment paradigms.

Co-delivery of a laminin-111 supplemented hyaluronic acid based hydrogel with minced muscle graft in the treatment of volumetric muscle loss injury

Minced muscle autografting mediates de novo myofiber regeneration and promotes partial recovery of neuromuscular strength after volumetric muscle loss injury (VML). A major limitation of this approach is the availability of sufficient donor tissue for the treatment of relatively large VMLs without inducing donor site morbidity. This study evaluated a laminin-111 supplemented hyaluronic acid based hydrogel (HA+LMN) as a putative myoconductive scaffolding to be co-delivered with minced muscle grafts. In a rat tibialis anterior muscle VML model, delivery of a reduced dose of minced muscle graft (50% of VML defect) within HA+LMN resulted in a 42% improvement of peak tetanic torque production over unrepaired VML affected limbs. However, the improvement in strength was not improved compared to a 50% minced graft-only control group. Moreover, histological analysis revealed that the improvement in in vivo functional capacity mediated by minced grafts in HA+LMN was not accompanied by a particularly robust graft mediated regenerative response as determined through donor cell tracking of the GFP+ grafting material. Characterization of the spatial distribution and density of macrophage and satellite cell populations indicated that the combination therapy damps the heightened macrophage response while re-establishing satellite content 14 days after VML to a level consistent with an endogenously healing ischemia-reperfusion induced muscle injury. Moreover, regional analysis revealed that the combination therapy increased satellite cell density mostly in the remaining musculature, as opposed to the defect area. Based on the results, the following salient conclusions were drawn: 1) functional recovery mediated by the combination therapy is likely due to a superposition of de novo muscle fiber regeneration and augmented repair of muscle fibers within the remaining musculature, and 2) The capacity for VML therapies to augment regeneration and repair within the remaining musculature may have significant clinical impact and warrants further exploration.

Inflammatory and Physiological Consequences of Debridement of Fibrous Tissue after Volumetric Muscle Loss Injury

Volumetric muscle loss (VML) injuries present chronic loss of muscle fibers followed by expansive fibrotic tissue deposition. Regenerative medicine therapies are under development to promote regeneration. However, mitigation of the expansive fibrous tissue is required for integration with the remaining muscle. Using a porcine VML model, delayed debridement of injury fibrosis was performed 3 months post‐VML and observed for an additional 4 weeks. A second group underwent the initial VML and was observed for 4 weeks, allowing comparison of initial fibrosis formation and debrided groups. The following salient observations were made: (i) debridement neither exacerbated nor ameliorated strength deficits; (ii) debridement results in recurrent fibrotic tissue deposition of a similar magnitude and composition as acute VML injury; and (iii) similarly upregulated transcriptional fibrotic and transcriptional pathways persist 4 weeks after initial VML or delayed debridement. This highlights the need for future studies to investigate adjunctive antifibrotic treatments for the fibrosed musculature.

Tacrolimus as an adjunct to autologous minced muscle grafts for the repair of a volumetric muscle loss injury

BACKGROUND

Volumetric muscle loss (VML) following extremity orthopaedic trauma or surgery results in chronic functional deficits and disability. A current translational approach to address the devastating functional limitations due to VML injury is the use of an autologous minced muscle graft (~1 mm3 pieces of muscle tissue) replacement into the injured defect area, although limitations related to donor site morbidity are still unaddressed. This study was designed to explore adjunct pharmacological immunomodulation to enhance graft efficacy and promote muscle function following VML injury, and thereby reduce the amount of donor tissue required.

FINDINGS

Using a validated VML porcine injury model in which 20% of the muscle volume was surgically removed, this study examined muscle function over 3 months post-VML injury. In vivo isometric torque of the peroneus teritus (PT) muscle was not different before surgery among sham, non-repaired, non-repaired with tacrolimus, graft-repaired, and graft-repaired with tacrolimus VML groups. Bi-weekly torque analysis of the VML injured musculature presented a significant strength deficit of ~26% compared to pre-injury in the non-repaired, non-repaired with tacrolimus, and graft-repaired groups. Comparatively, the strength deficit in the graft-repair with systemic tacrolimus was marginally improved (~19%; p = 0.056). Both of the minced graft repaired groups presented a greater proportion of muscle tissue in full-thickness histology specimen.

CONCLUSIONS

We demonstrate that adjunctive use of tacrolimus with an ~50% minced muscle graft replacement resulted in modest improvements in muscle function 3 months after injury and repair, but the magnitude of improvement is not expected to elicit clinically meaningful functional improvements.

Autologous minced muscle grafts improve endogenous fracture healing and muscle strength after musculoskeletal trauma

The deleterious impact of concomitant muscle injury on fracture healing and limb function is commonly considered part of the natural sequela of orthopedic trauma. Recent reports suggest that heightened inflammation in the surrounding traumatized musculature is a primary determinant of fracture healing. Relatedly, there are emerging potential therapeutic approaches for severe muscle trauma (e.g., volumetric muscle loss [VML] injury), such as autologous minced muscle grafts (1 mm3 pieces of muscle; GRAFT), that can partially prevent chronic functional deficits and appear to have an immunomodulatory effect within VML injured muscle. The primary goal of this study was to determine if repair of VML injury with GRAFT rescues impaired fracture healing and improves the strength of the traumatized muscle in a male Lewis rat model of tibia open fracture. The most salient findings of the study were: (1) tibialis anterior (TA) muscle repair with GRAFT improved endogenous healing of fractured tibia and improved the functional outcome of muscle regeneration; (2) GRAFT repair attenuated the monocyte/macrophage (CD45+CDllb+) and T lymphocyte (CD3+) response to VML injury; (3) TA muscle protein concentrations of MCP1, IL-10, and IGF-1 were augmented in a proregenerative manner by GRAFT repair; (4) VML injury concomitant with osteotomy induced a heightened systemic presence of alarmins (e.g., soluble RAGE) and leukocytes (e.g., monocytes), and depressed IGF-1 concentration, which GRAFT repair ameliorated. Collectively, these data indicate that repair of VML injury with a regenerative therapy can modulate the inflammatory and regenerative phenotype of the treated muscle and in association improve musculoskeletal healing.