Muscle injuries and strategies for improving their repair

P7C3 ameliorates barium chloride-induced skeletal muscle injury activating transcriptomic and epigenetic modulation of myogenic regulatory factors

Skeletal muscle injury affects the quality of life in many pathologies, including volumetric muscle loss, contusion injury, and aging. We hypothesized that the nicotinamide phosphoribosyltransferase (Nampt) activator P7C3 improves muscle repair following injury. In the present study, we tested the effect of P7C3 (1-anilino-3-(3,6-dibromocarbazol-9-yl) propan-2-ol) on chemically induced muscle injury. Muscle injury was induced by injecting 50 µL 1.2% barium chloride (BaCl2) into the tibialis anterior (TA) muscle in C57Bl/6J wild-type male mice. Mice were then treated with either 10 mg/kg body weight of P7C3 or Vehicle intraperitoneally for 7 days and assessed for histological, biochemical, and molecular changes. In the present study, we show that the acute BaCl2-induced TA muscle injury was robust and the P7C3-treated mice displayed a significant increase in the total number of myonuclei and blood vessels, and decreased serum CK activity compared with vehicle-treated mice. The specificity of P7C3 was evaluated using Nampt+/- mice, which did not display any significant difference in muscle repair capacity among treated groups. RNA-sequencing analysis of the injured TA muscles displayed 368 and 212 genes to be exclusively expressed in P7C3 and Veh-treated mice, respectively. There was an increase in the expression of genes involved in cellular processes, inflammatory response, angiogenesis, and muscle development in P7C3 versus Veh-treated mice. Conversely, there is a decrease in muscle structure and function, myeloid cell differentiation, glutathione, and oxidation-reduction, drug metabolism, and circadian rhythm signaling pathways. Chromatin immunoprecipitation-quantitative polymerase chain reaction (qPCR) and reverse transcription-qPCR analyses identified increased Pax7, Myf5, MyoD, and Myogenin expression in P7C3-treated mice. Increased histone lysine (H3K) methylation and acetylation were observed in P7C3-treated mice, with significant upregulation in inflammatory markers. Moreover, P7C3 treatment significantly increased the myotube fusion index in the BaCl2-injured human skeletal muscle in vitro. P7C3 also inhibited the lipopolysaccharide-induced inflammatory response and mitochondrial membrane potential of RAW 264.7 macrophage cells. Overall, we demonstrate that P7C3 activates muscle stem cells and enhances muscle injury repair with increased angiogenesis.

Novel therapeutic activities of dragon blood from palm tree Daemonorops draco for the treatment of chronic diabetic wounds

BACKGROUND

The clinical efficacy of Jinchuang Ointment, a traditional Chinese medicine (TCM), in treating chronic non-healing diabetic wounds has been demonstrated over the past decades. Both in vitro and in vivo angiogenic activities have been reported for its herbal ingredients, including dragon blood from the palm tree Daemonorops draco and catechu from Uncaria gambir Roxb. Additionally, crude extracts of dragon blood have exhibited hypoglycemic effects not only in animal studies but also in cell-based in vitro assays.

RESULTS

Our findings indicate that crude dragon blood extract promotes the differentiation of myoblasts into myotubes. Partially purified fractions of dragon blood crude extract significantly enhance the expression of muscle cell differentiation-related genes such as myoG, myoD, and myoHC. Our results also demonstrate that crude extracts of dragon blood can inhibit platelet-derived growth factor-induced PAI-1 expression in primary rat vascular smooth muscle cells, thereby favoring changes in hemostasis towards fibrinolysis. Consistent with previous reports, reduced expression of plasminogen activator inhibitor 1 (PAI-1) accelerates wound healing. However, further separation resulted in a significant loss of both activities, indicating the involvement of more than one compound in these processes. Stem cells play a crucial role in muscle injury repair. Neither dragon blood nor catechu alone stimulated the proliferation of human telomerase reverse transcriptase (hTERT)-immortalized and umbilical cord mesenchymal stem cells. Interestingly, the proliferation of both types of stem cells was observed when crude extracts of dragon blood and catechu were present together in the stem cell growth medium.

CONCLUSIONS

Dragon blood from D. draco offers multifaceted therapeutic benefits for treating chronic nonhealing diabetic wounds from various perspectives. Most drugs in Western medicine consist of small molecules with defined ingredients. However, this is not the case in TCM, as the activities of dragon blood reported in this study. Surprisingly, the activities documented here align with descriptions in ancient Chinese medical texts dating back to A.D. 1625.

Inflammatory pathway communication with skeletal muscle-Does aging play a role? A topical review of the current evidence

Skeletal muscle plays an integral role in locomotion, but also as part of the integrative physiological system. Recent progress has identified crosstalk between skeletal muscle and various physiological systems, including the immune system. Both the musculoskeletal and immune systems are impacted by aging. Increased age is associated with decreased muscle mass and function, while the immune system undergoes "inflammaging" and immunosenescence. Exercise is identified as a preventative medicine that can mitigate loss of function for both systems. This review summarizes: (1) the inflammatory pathways active in skeletal muscle; and (2) the inflammatory and skeletal muscle response to unaccustomed exercise in younger and older adults. Compared to younger adults, it appears older individuals have a muted pro-inflammatory response and elevated anti-inflammatory response to exercise. This important difference could contribute to decreased regeneration and recovery following unaccustomed exercise in older adults, as well as in chronic disease. The current research provides specific information on the role inflammation plays in altering skeletal muscle form and function, and adaptation to exercise; however, the pursuit of more knowledge in this area will delineate specific interventions that may enhance skeletal muscle recovery and promote resiliency in this tissue particularly with aging.

Platelet-Poor versus Platelet-Rich Plasma for the Treatment of Muscle Injuries

ABSTRACT

Muscle injury in sport results in significant lost time and potential for reinjury for athletes. Autologous blood product, namely, platelet-rich plasma (PRP), has been investigated for possible augmentation of the treatment timeline with prevention of reinjury; however, conflicting results have been identified. A growing body of basic science and clinical literature is forming that supports the use of platelet-poor plasma (PPP) for muscle injury. The purpose of this study was to provide a background of the basic science of PRP versus PPP for muscle injury and to identify and review the clinical evidence for both autologous blood products, including the author's clinical experience utilizing the blood products. At the tissue level, PRP causes myoblast proliferation while PPP has led to myoblast induction, potentially identifying improved native muscle healing. Conflicting studies have been identified for the use of PRP for muscle injury. A growing body of positive results for PPP was identified, but high-quality comparative studies are needed.

A Novel Minimally Invasive Surgically Induced Skeletal Muscle Injury Model in Sheep

Sports-related muscle injuries account for 10-55% of all injuries, which is a growing concern, especially given the aging world population. To evaluate the process of skeletal muscle injury and compare it with muscle lesions observed in humans, we developed a novel in vivo model in sheep. In this model, muscle injury was induced by an ultrasound-guided transverse biopsy at the myotendinous junction of the medial gastrocnemius muscle. Twelve male sheep were examined at 3, 7, 14, and 28 days post-injury. Histological, immunofluorescence, and MRI analyses indicate that our sheep model could resemble key human clinicopathological features. Statistically significant differences (p < 0.05) were observed in collagen I, dMHC, α-SMA, and CD68 immunohistochemical detection when comparing injured and healthy muscles. The injured gastrocnemius muscle exhibited elevated levels of type I collagen, infiltration of CD68(+) macrophages, angiogenesis, and the emergence of newly regenerated dMHC(+) myofibers, which persisted for up to 4 weeks post-injury. Similarly, the progression of muscle injury in the sheep model was assessed using advanced clinical 3 T MRI and compared with MRI scans from human patients. The data indicate that the sheep muscle injury model presents features similar to those observed in human skeletal muscle injuries. This makes it a valuable large animal model for studying muscle injuries and developing novel therapeutic strategies.

Cell culture on suspended fiber for tissue regeneration: A review

Cell culturing is a cornerstone of tissue engineering, playing a crucial role in tissue regeneration, drug screening, and the study of disease mechanisms. Among various culturing techniques, 3D culture systems, particularly those utilizing suspended fiber scaffolds, offer a more physiologically relevant environment than traditional 2D monolayer cultures. These 3D scaffolds enhance cell growth, differentiation, and proliferation by mimicking the in vivo cellular milieu. This review focuses on the critical role of suspended fiber scaffolds in tissue engineering. We compare the effectiveness of 3D suspended fiber scaffolds with 2D culture systems, discussing their respective benefits and limitations in the context of tissue regeneration. Furthermore, we explore the preparation methods of suspended fiber scaffolds and their potential applications. The review concludes by considering future research directions for optimizing suspended fiber scaffolds to address specific challenges in tissue regeneration, underscoring their significant promise in advancing tissue engineering and regenerative medicine.

Skeletal muscle differentiation induces wide-ranging nucleosome repositioning in muscle gene promoters

In a previous report, we demonstrated that Cbx1, PurB and Sp3 inhibited cardiac muscle differentiation by increasing nucleosome density around cardiac muscle gene promoters. Since cardiac and skeletal muscle express many of the same proteins, we asked if Cbx1, PurB and Sp3 similarly regulated skeletal muscle differentiation. In a C2C12 model of skeletal muscle differentiation, Cbx1 and PurB knockdown increased myotube formation. In contrast, Sp3 knockdown inhibited myotube formation, suggesting that Sp3 played opposing roles in cardiac muscle and skeletal muscle differentiation. Consistent with this finding, Sp3 knockdown also inhibited various muscle-specific genes. The Cbx1, PurB and Sp3 proteins are believed to influence gene-expression in part by altering nucleosome position. Importantly, we developed a statistical approach to determine if changes in nucleosome positioning were significant and applied it to understanding the architecture of muscle-specific genes. Through this novel statistical approach, we found that during myogenic differentiation, skeletal muscle-specific genes undergo a set of unique nucleosome changes which differ significantly from those shown in commonly expressed muscle genes. While Sp3 binding was associated with nucleosome loss, there appeared no correlation with the aforementioned nucleosome changes. In summary, we have identified a novel role for Sp3 in skeletal muscle differentiation and through the application of quantifiable MNase-seq have discovered unique fingerprints of nucleosome changes for various classes of muscle genes during myogenic differentiation.

Vitamin C regulates skeletal muscle post-injury regeneration by promoting myoblast proliferation through its direct interaction with the Pax7 protein

Previous studies have shown that vitamin C (VC), an essential vitamin for the human body, can promote the differentiation of muscle satellite cells (MuSCs) in vitro and play an important role in skeletal muscle post-injury regeneration. However, the molecular mechanism of VC regulating MuSC proliferation has not been elucidated. In this study, the role of VC in promoting MuSC proliferation and its molecular mechanism were explored using cell molecular biology and animal experiments. The results showed that VC accelerates the progress of skeletal muscle post-injury regeneration by promoting MuSC proliferation in vivo. VC can also promote skeletal muscle regeneration in the case of atrophy. Using the C2C12 myoblast murine cell line, we observed that VC also stimulated cell proliferation. In addition, after an in vitro study establishing the occurrence of a physical interaction between VC and Pax7, we observed that VC also upregulated the total and nuclear Pax7 protein levels. This mechanism increased the expression of Myf5 (Myogenic Factor 5), a Pax7 target gene. This study establishes a theoretical foundation for understanding the regulatory mechanisms underlying VC-mediated MuSC proliferation and skeletal muscle regeneration. Moreover, it develops the application of VC in animal muscle nutritional supplements and treatment of skeletal muscle-related diseases.

Novel Bioresorbable Drug-Eluting Mesh Scaffold for Therapy of Muscle Injury

A novel bioresorbable drug-eluting polycaprolactone (PCL) mesh scaffold was developed, utilizing a solvent-cast additive manufacturing technique, to promote therapy of muscle injury. The degradation rate and mechanical properties strength of the PCL mesh were characterized after immersion in a buffer solution for different times. The in vitro release characteristics of vancomycin, ceftazidime, and lidocaine from the prepared mesh were evaluated using a high-performance liquid chromatography (HPLC) assay. In addition, the in vivo efficacy of PCL meshes for the repair of muscle injury was investigated on a rat model with histological examinations. It was found that the additively manufactured PCL meshes degraded by 13% after submission in buffered solution for four months. All PCL meshes with different pore sizes exhibited greater strength than rat muscle and survived through 10,000 cyclic loadings. Furthermore, the meshes could offer a sustained release of antibiotics and analgesics for more than 3 days in vitro. The results of this study suggest that drug-loaded PCL mesh exhibits superior ability to pure PCL mesh in terms of effectively promoting muscle repair in rat models. The histological assay also showed adequate biocompatibility of the resorbable meshes. The additively manufactured biodegradable drug-eluting meshes may be adopted in the future in humans for the therapy of muscle injuries.

Semaphorins: Missing Signals in Age-dependent Alteration of Neuromuscular Junctions and Skeletal Muscle Regeneration

Skeletal muscle is characterized by a remarkable capacity to rearrange after physiological changes and efficiently regenerate. However, during aging, extensive injury, or pathological conditions, the complete regenerative program is severely affected, with a progressive loss of muscle mass and function, a condition known as sarcopenia. The compromised tissue repair program is attributable to the gradual depletion of stem cells and to altered regulatory signals. Defective muscle regeneration can severely affect re-innervation by motor axons, and neuromuscular junctions (NMJs) development, ultimately leading to skeletal muscle atrophy. Defects in NMJ formation and maintenance occur physiologically during aging and are responsible for the pathogenesis of several neuromuscular disorders. However, it is still largely unknown how neuromuscular connections are restored on regenerating fibers. It has been suggested that attractive and repelling signals used for axon guidance could be implicated in this process; in particular, guidance molecules called semaphorins play a key role. Semaphorins are a wide family of extracellular regulatory signals with a multifaceted role in cell-cell communication. Originally discovered as axon guidance factors, they have been implicated in cancer progression, embryonal organogenesis, skeletal muscle innervation, and other physiological and developmental functions in different tissues. In particular, in skeletal muscle, specific semaphorin molecules are involved in the restoration and remodeling of the nerve-muscle connections, thus emphasizing their plausible role to ensure the success of muscle regeneration. This review article aims to discuss the impact of aging on skeletal muscle regeneration and NMJs remodeling and will highlight the most recent insights about the role of semaphorins in this context.

Analysis of Infiltrating Immune Cells Following Intervertebral Disc Injury Reveals Recruitment of Gamma-Delta (γδ) T cells in Female Mice

Inadequate repair of injured intervertebral discs (IVD) leads to degeneration and contributes to low back pain. Infiltrating immune cells into damaged musculoskeletal tissues are critical mediators of repair, yet little is known about their identities, roles, and temporal regulation following IVD injury. By analyzing longitudinal changes in gene expression, tissue morphology, and the dynamics of infiltrating immune cells following injury, we characterize sex-specific differences in immune cell populations and identify the involvement of previously unreported immune cell types, γδ and NKT cells. Cd3+Cd4-Cd8- T cells are the largest infiltrating lymphocyte population with injury, and we identified the presence of γδ T cells in this population in female mice specifically, and NKT cells in males. Injury-mediated IVD degeneration was prevalent in both sexes, but more severe in males. Sex-specific degeneration may be associated with the differential immune response since γδ T cells have potent anti-inflammatory roles and may mediate IVD repair.

The Buried Guide Suture Method: A Novel Technique to Correct Traumatic Facial Dimples

SUMMARY

Facial dimpling, frequently occurring after blunt trauma, presents as soft-tissue depression, which is particularly apparent during facial expression. The displacement of subcutaneous tissue can be detected and measured by high-frequency ultrasound. Limited surgical methods have been applied in these closed-injury cases. Repositioning the subcutaneous tissue without incisions on unscarred skin is challenging. The authors propose a novel three-dimensional technique to suture and fix the subcutaneous tissue at a distance through a concealed incision. The buried guide suture method was used in the treatment of 22 patients with traumatic facial dimples on the cheek. All patients showed great improvement in their depressed deformity with minor complications. This technique provides an option to correct soft-tissue depression without leaving a visible scar, especially for mimetic rupture caused by blunt trauma.

Epigenetic integration of signaling from the regenerative environment

Skeletal muscle has an extraordinary capacity to regenerate itself after injury due to the presence of tissue-resident muscle stem cells. While these muscle stem cells are the primary contributor to the regenerated myofibers, the process occurs in a regenerative microenvironment where multiple different cell types act in a coordinated manner to clear the damaged myofibers and restore tissue homeostasis. In this regenerative environment, immune cells play a well-characterized role in initiating repair by establishing an inflammatory state that permits the removal of dead cells and necrotic muscle tissue at the injury site. More recently, it has come to be appreciated that the immune cells also play a crucial role in communicating with the stem cells within the regenerative environment to help coordinate the timing of repair events through the secretion of cytokines, chemokines, and growth factors. Evidence also suggests that stem cells can help modulate the extent of the inflammatory response by signaling to the immune cells, demonstrating a cross-talk between the different cells in the regenerative environment. Here, we review the current knowledge on the innate immune response to sterile muscle injury and provide insight into the epigenetic mechanisms used by the cells in the regenerative niche to integrate the cellular cross-talk required for efficient muscle repair.

A virtual simulation approach to assess the effect of trocar-site placement and scar characteristics on the abdominal wall biomechanics

Analyses of registries and medical imaging suggest that laparoscopic surgery may be penalized with a high incidence of trocar-site hernias (TSH). In addition to trocar diameter, the location of the surgical wound (SW) may affect TSH incidence. The intra-abdominal pressure (IAP) exerted on the abdominal wall (AW) might also influence the appearance of TSH. In the present study, we used finite element (FE) simulations to predict the influence of trocar location and SW characteristics (stiffness) on the mechanical behavior of the AW subject to an IAP. Two models of laparoscopy patterns on the AW, with trocars in the 5-12 mm range, were generated. FE simulations for IAP values within the 4 kPa-20 kPa range were carried out using the Code Aster open-source software. Different stiffness levels of the SW tissue were considered. We found that midline-located surgical wounds barely deformed, even though they moved outwards along with the regular LA tissue. Laterally located SWs hardly changed their location but they experienced significant variations in their volume and shape. The amount of deformation of lateral SWs was found to strongly depend on their stiffness. Trocar incisions placed in a LA with non-diastatic dimensions do not compromise its mechanical integrity. The more lateral the trocars are placed, the greater is their deformation, regardless of their size. Thus, to prevent TSH it might be advisable to close lateral trocars with a suture, or even use a prosthetic reinforcement depending on the patient's risk factors (e.g., obesity).

Targeting myostatin using quercetin as a media supplement to improve myogenesis for cultured meat production: An in silico and in vitro study

Cultured meat (CM) is an alternative protein food and is produced by cultivating muscle satellite (stem) cells (MSCs) derived from livestock animals (bovine, chickens, and porcine) through myogenesis leading to generate muscle mass. Myostatin (MSTN) is well well-known negative regulator of myogenesis, and in the present study, in silico screening of natural compounds was performed to identify MSTN inhibitors. Interestingly, quercetin was found to inhibit MSTN (binding energy -7.40 kcal/mol), and this was further validated by a 100 ns molecular dynamics simulation. Quercetin was added to culture media to boost myogenesis, and its potent antioxidant property helped maintain media pH. Furthermore, quercetin increased the myotube thickness and length, increased MSC differentiation, and upregulated the gene and protein expressions of myoblast determination protein 1 (MYOD), Myogenin (MYOG), and Myosin heavy chains (MYH) in vitro. In addition, quercetin inhibited the activities of MSTN, activin receptor type-2B (ACVR2B), and SMAD2 and 3, and thus significantly enhanced MSC differentiation and myotube formation. Overall, this study shows that quercetin might be useful for enhancing large-scale CM production. It is hoped that this study provides a starting point for research in the CM area aimed to enhancing product quality, nutritional values, and the efficacy of large-scale production.

2024 TSOC/TSPS Joint Consensus: Strategies for Advanced Vascular Wound Management in Arterial and Venous Diseases

The Taiwan Society of Cardiology (TSOC) and Taiwan Society of Plastic Surgery (TSPS) have collaborated to develop a joint consensus for the management of patients with advanced vascular wounds. The taskforce comprises experts including preventive cardiologists, interventionists, and cardiovascular and plastic surgeons. The consensus focuses on addressing the challenges in diagnosing, treating, and managing complex wounds; incorporates the perfusion evaluation and the advanced vascular wound care team; and highlights the importance of cross-disciplinary teamwork. The aim of this joint consensus is to manage patients with advanced vascular wounds and encourage the adoption of these guidelines by healthcare professionals to improve patient care and outcomes. The guidelines encompass a range of topics, including the definition of advanced vascular wounds, increased awareness, team structure, epidemiology, clinical presentation, medical treatment, endovascular intervention, vascular surgery, infection control, advanced wound management, and evaluation of treatment results. It also outlines a detailed protocol for assessing patients with lower leg wounds, provides guidance on consultation and referral processes, and offers recommendations for various wound care devices, dressings, and products. The 2024 TSOC/TSPS consensus for the management of patients with advanced vascular wounds serves as a catalyst for international collaboration, promoting knowledge exchange and facilitating advancements in the field of advanced vascular wound management. By providing a comprehensive and evidence-based approach, this consensus aims to contribute to improved patient care and outcomes globally.

Simulated altitude is medicine: intermittent exposure to hypobaric hypoxia and cold accelerates injured skeletal muscle recovery

Muscle injuries are the leading cause of sports casualties. Because of its high plasticity, skeletal muscle can respond to different stimuli to maintain and improve functionality. Intermittent hypobaric hypoxia (IHH) improves muscle oxygen delivery and utilization. Hypobaria coexists with cold in the biosphere, opening the possibility to consider the combined use of both environmental factors to achieve beneficial physiological adjustments. We studied the effects of IHH and cold exposure, separately and simultaneously, on muscle regeneration. Adult male rats were surgically injured in one gastrocnemius and randomly assigned to the following groups: (1) CTRL: passive recovery; (2) COLD: intermittently exposed to cold (4°C); (3) HYPO: submitted to IHH (4500 m); (4) COHY: exposed to intermittent simultaneous cold and hypoxia. Animals were subjected to these interventions for 4 h/day for 9 or 21 days. COLD and COHY rats showed faster muscle regeneration than CTRL, evidenced after 9 days at histological (dMHC-positive and centrally nucleated fibre reduction) and functional levels after 21 days. HYPO rats showed a full recovery from injury (at histological and functional levels) after 9 days, while COLD and COHY needed more time to induce a total functional recovery. IHH can be postulated as an anti-fibrotic treatment since it reduces collagen I deposition. The increase in the pSer473Akt/total Akt ratio observed after 9 days in COLD, HYPO and COHY, together with the increase in the pThr172AMPKα/total AMPKα ratio observed in the gastrocnemius of HYPO, provides clues to the molecular mechanisms involved in the improved muscle regeneration. KEY POINTS: Only intermittent hypobaric exposure accelerated muscle recovery as early as 9 days following injury at histological and functional levels. Injured muscles from animals treated with intermittent (4 h/day) cold, hypobaric hypoxia or a simultaneous combination of both stimuli regenerated histological structure and recovered muscle function 21 days after injury. The combination of cold and hypoxia showed a blunting effect as compared to hypoxia alone in the time course of the muscle recovery. The increased expression of the phosphorylated forms of Akt observed in all experimental groups could participate in the molecular cascade of events leading to a faster regeneration. The elevated levels of phosphorylated AMPKα in the HYPO group could play a key role in the modulation of the inflammatory response during the first steps of the muscle regeneration process.

Assessment of the effects of transforming growth factor beta1 (TGF-β1)-Smad2/3 on fibrosis in rat myofascial trigger points using point shear wave elastography

Background & Aims

Myofascial trigger points (MTrPs) are highly sensitive irritated points within a tense belt of skeletal muscle, and are the main cause of muscle pain and dysfunction. MTrPs can also cause paraesthesia and autonomic nervous dysfunction. Furthermore, long-term and chronic MTrPs can cause muscle atrophy and even disability, seriously affecting the quality of life and mental health of patients, and increasing the social and economic burden. However, to date, there have been few studies on fibrogenesis and changes in MTrPs. Therefore, this study investigated whether transforming growth factor beta1 (TGF-β1)-Smad2/3 participates in the formation of MTrPs and how it affects fibrosis using point shear wave elastography.

Methods

Forty Sprague‒Dawley rats were randomly divided into the MTrPs group and the control group. Blunt injury combined with eccentric exercise was used to establish an MTrPs model. Electromyography (EMG), haematoxylin and eosin (H&E) staining and transmission electron microscopy (TEM) were used to verify the model. The collagen volume fraction was measured by Masson staining, the protein expression of TGF-β1 and p-Smad2/3 was measured by Western blotting (WB) and immunohistochemistry (IHC), and the shear wave velocity (SWV) was measured by point shear wave elastography.

Results

EMG, H&E and TEM examination indicated that the modelling was successful. The collagen volume fraction and the protein expression of TGF-β1 and p-Smad2/3 were higher in the MTrPs group than in the control group. The SWV of the MTrPs group was also higher than that of the control group. These differences suggest that MTrPs may exhibit fibrosis. The correlations between the collagen volume fraction and SWV and between the collagen volume fraction and TGF-β1 were positive.

Conclusion

Fibrotic conditions may be involved in the formation of MTrPs. Ultrasound point shear wave elastography and assessment of TGF-β1 and p-Smad2/3 expression can reflect the degree of MTrPs fibrosis to some extent. Further exploration of the important role of TGF-β1 and Smad2/3 in the pathogenesis of MTrPs will be of great significance for clinical treatment.

Extracellular matrix: the critical contributor to skeletal muscle regeneration-a comprehensive review

The regenerative ability of skeletal muscle (SM) in response to damage, injury, or disease is a highly intricate process that involves the coordinated activities of multiple cell types and biomolecular factors. Of these, extracellular matrix (ECM) is considered a fundamental component of SM regenerative ability. This review briefly discusses SM myogenesis and regeneration, the roles played by muscle satellite cells (MSCs), other cells, and ECM components, and the effects of their dysregulations on these processes. In addition, we review the various types of ECM scaffolds and biomaterials used for SM regeneration, their applications, recent advances in ECM scaffold research, and their impacts on tissue engineering and SM regeneration, especially in the context of severe muscle injury, which frequently results in substantial muscle loss and impaired regenerative capacity. This review was undertaken to provide a comprehensive overview of SM myogenesis and regeneration, the stem cells used for muscle regeneration, the significance of ECM in SM regeneration, and to enhance understanding of the essential role of the ECM scaffold during SM regeneration.

[What if the origin of FAPs was contributing to their heterogeneity in muscle?]

Fibro-adipogenic progenitors (FAPs) are resident mesenchymal stromal cells (MSCs) of skeletal muscle. They play a crucial role in muscle homeostasis and regeneration through their paracrine activity. Recent technological advances in single-cell RNA sequencing have allowed the characterization of the heterogeneity within this cell population. In this article, we will present the different subpopulations of FAPs under basal, injury, or degenerative conditions, as well as their associated functions in mice and humans. We will then discuss the potential extramuscular origin of a post-injury FAP population. Indeed, our recent work demonstrates that MSCs from adipose tissue, infiltrating the muscle, could contribute to FAP heterogeneity.

BMP9 functions as a negative regulator in the myogenic differentiation of primary mouse myoblasts

BMP9, a member of the TGF-β superfamily, reveals the great translational promise for it has been shown to have the strong effect of osteogenic activity in vitro and in vivo. However, the implantation of certain BMPs (bone morphogenetic proteins) into muscular tissues induces ectopic bone formation. BMPs induce osteoblastic differentiation in skeletal muscle, suggesting that myogenic stem cells, such as myoblasts, are the potential progenitors of osteoblasts during heterotopic bone differentiation. Here, we investigate the role of BMP9 during primary mouse myoblasts differentiation. We found BMP9 enhanced cell proliferation and reduced myogenic differentiation of primary mouse myoblasts. In addition, adenovirus-mediated overexpression of BMP9 delayed muscle regeneration after BaCl2-induced injury. ALK1 knockdown reversed the inhibition of myoblast differentiation induced by BMP9. Our data indicate that BMP9 inhibits myogenic differentiation in primary mouse myoblasts and delays skeletal muscle regeneration after injury.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal muscle tissue engineering.

Traumatic Mid-substance Isolated Vastus Medialis Rupture in a Young Athlete: A Case Report

CASE

This is the first documented case of an isolated traumatic vastus medialis rupture that occurred in a 14-year-old athlete after direct impact during play. Imaging confirmed a rare mid-substance rupture. The patient underwent conservative management with physical therapy augmented by blood flow restriction therapy (BFRT) and platelet-poor plasma (PPP) injection to regain full function.

CONCLUSION

Isolated quadriceps mid-substance tears are rare. Determining an appropriate treatment technique for mid-substance tears is complex. This case documents the use of nonoperative management with BFRT and PPP to achieve full recovery and early return to play in a young athlete.

Vascular regeneration and skeletal muscle repair induced by long-term exposure to SDF-1α derived from engineered mesenchymal stem cells after hindlimb ischemia

Despite recent progress in medical and endovascular therapy, the prognosis for patients with critical limb ischemia (CLI) remains poor. In response, various stem cells and growth factors have been assessed for use in therapeutic neovascularization and limb salvage in CLI patients. However, the clinical outcomes of cell-based therapeutic angiogenesis have not provided the promised benefits, reinforcing the need for novel cell-based therapeutic angiogenic strategies to cure untreatable CLI. In the present study, we investigated genetically engineered mesenchymal stem cells (MSCs) derived from human bone marrow that continuously secrete stromal-derived factor-1α (SDF1α-eMSCs) and demonstrated that intramuscular injection of SDF1α-eMSCs can provide long-term paracrine effects in limb ischemia and effectively contribute to vascular regeneration as well as skeletal muscle repair through increased phosphorylation of ERK and Akt within the SDF1α/CXCR4 axis. These results provide compelling evidence that genetically engineered MSCs with SDF-1α can be an effective strategy for successful limb salvage in limb ischemia.

Regulatory Mechanisms That Guide the Fetal to Postnatal Transition of Cardiomyocytes

Heart disease remains a global leading cause of death and disability, necessitating a comprehensive understanding of the heart's development, repair, and dysfunction. This review surveys recent discoveries that explore the developmental transition of proliferative fetal cardiomyocytes into hypertrophic postnatal cardiomyocytes, a process yet to be well-defined. This transition is key to the heart's growth and has promising therapeutic potential, particularly for congenital or acquired heart damage, such as myocardial infarctions. Although significant progress has been made, much work is needed to unravel the complex interplay of signaling pathways that regulate cardiomyocyte proliferation and hypertrophy. This review provides a detailed perspective for future research directions aimed at the potential therapeutic harnessing of the perinatal heart transitions.

Complete Distal Rupture of the Rectus Femoris in an Elite Football Player: A Non-operative Treatment

Although muscle injuries represent the most frequent injury in professional football, isolated complete distal ruptures of the rectus femoris (RF) muscle are rare, and there is no consensus on their treatment and return to play (RTP). In this article, we report a clinical case of successful non-surgical management of an RF grade 4c muscle injury in a professional football player, in which the athlete was able to RTP 21 weeks after the injury, had no re-injury >1 year after RTP, and is playing at an elite level in the Portuguese Football First League.

Intramuscular Bleeding and Formation of Microthrombi during Skeletal Muscle Damage Caused by a Snake Venom Metalloprotease and a Cardiotoxin

The interactions between specific snake venom toxins and muscle constituents are the major cause of severe muscle damage that often result in amputations and subsequent socioeconomic ramifications for snakebite victims and/or their families. Therefore, improving our understanding of venom-induced muscle damage and determining the underlying mechanisms of muscle degeneration/regeneration following snakebites is critical to developing better strategies to tackle this issue. Here, we analysed intramuscular bleeding and thrombosis in muscle injuries induced by two different snake venom toxins (CAMP-Crotalus atrox metalloprotease (a PIII metalloprotease from the venom of this snake) and a three-finger toxin (CTX, a cardiotoxin from the venom of Naja pallida)). Classically, these toxins represent diverse scenarios characterised by persistent muscle damage (CAMP) and successful regeneration (CTX) following acute damage, as normally observed in envenomation by most vipers and some elapid snakes of Asian, Australasian, and African origin, respectively. Our immunohistochemical analysis confirmed that both CAMP and CTX induced extensive muscle destruction on day 5, although the effects of CTX were reversed over time. We identified the presence of fibrinogen and P-selectin exposure inside the damaged muscle sections, suggesting signs of bleeding and the formation of platelet aggregates/microthrombi in tissues, respectively. Intriguingly, CAMP causes integrin shedding but does not affect any blood clotting parameters, whereas CTX significantly extends the clotting time and has no impact on integrin shedding. The rates of fibrinogen clearance and reduction in microthrombi were greater in CTX-treated muscle compared to CAMP-treated muscle. Together, these findings reveal novel aspects of venom-induced muscle damage and highlight the relevance of haemostatic events such as bleeding and thrombosis for muscle regeneration and provide useful mechanistic insights for developing better therapeutic interventions.

Targeting Hub Genes Involved in Muscle Injury Induced by Jumping Load Based on Transcriptomics

The purpose of this study was to find hub genes that may play key roles in skeletal muscle injury induced by jumping load. Twelve female Sprague Dawley rats were divided into the normal control (NC) group and the jumping-induced muscle injury (JI) group. After 6 weeks of jumping, transmission electron microscopy, hematoxylin-eosin staining, transcriptomics sequencing and genes analysis, interaction network prediction of multiple proteins, real-time PCR detection, and Western blotting were performed on gastrocnemius muscles from NC and JI groups. As compared with NC rats, excessive jumping can result in notable structural damage and inflammatory infiltration in JI rats. A total of 112 differentially expressed genes were confirmed in NC rats versus JI rats, with 59 genes upregulated and 53 genes downregulated. Using the online String database, four hub genes in the transcriptional regulatory network were targeted, including FOS, EGR1, ATF3, and NR4A3. All expression levels of FOS, EGR1, ATF3, and NR4A3 mRNAs were decreased in JI rats compared with NC rats (p < 0.05 or p < 0.01). All expression levels of c-Fos, EGR1, ATF3, and NOR1 proteins were upregulated in JI rats (p < 0.01, p < 0.05, p > 0.05, and p < 0.01, respectively). Collectively, these findings indicate that FOS, EGR1, ATF3, and NR4A3 genes may be functionally important in jumping-induced muscle injury.

Potential Local Mechanisms for Exercise-Induced Hypoalgesia in Response to Blood Flow Restriction Training

Overall, there is a great need within sports medicine to ensure that athletes can return from injury in an efficient, yet thorough manner. It is crucial to not avoid necessary difficulties in this process but also to ensure time-efficient rehabilitation. One of the more promising techniques to achieve timely recovery is blood flow restriction (BFR) training. BFR training is a growing and novel development that could be a vital tool to lighten the burden of recovery from injury in athletes. BFR utilizes a pneumatic tourniquet to limit blood flow in specific areas of the body. The use of BFR has been shown to potentially enhance the analgesic effects of exercise-induced hypoalgesia (EIH). By limiting pain, athletes will be less burdened by mobility and loading exercises required for them to effectively return to play. In a field where time away from sports can have massive implications, the need for tools to assist in the acceleration of the rehabilitation process is vital. Much of the work that has already been done in the field has been able to exploit the benefits of EIH and further enhance the body's capabilities through BFR. Studies have compared EIH at low- and high-intensity settings utilizing BFR with both resistance and aerobic exercise. The results of these studies show comparable beta-endorphin levels with high-intensity exercise without BFR and low-intensity exercise with BFR. Low-intensity training with BFR had greater local pain relief, perhaps indicating the promising effects of BFR in enhancing EIH. By reviewing the current literature on this topic, we hope that further progress can be made to better understand the mechanism behind BFR and its ability to enhance EIH. Currently, local metabolites are a major focus for the potential mechanism behind these effects. Mas-related G-protein-coupled receptors (Mrgprs) contribute to local pain pathways via mast cell degranulation. Similarly, chemokine receptor 2/chemokine ligand 2 (CCR2/CCL2) triggers mast cell degranulation and inflammation-induced pain. Finally, pain-reducing effects have been linked to anti-inflammatory IL-10 signaling and anaerobic metabolites via transient receptor potential vanilloid 1 (TRPV1). Through a better understanding of these metabolites and their mechanisms, it is possible to further exploit the use of BFR to not only serve athletes recovering from injury but also apply this information to better serve all patients.

Elevating athletic performance: Maximizing strength and power in long jumpers through combined low-intensity blood flow restriction and high-intensity resistance training

Purpose

This study aimed to evaluate the effects of low-intensity blood flow restriction (BFR) training and high-intensity resistance training (HI-RT) on the leaping performance of long-jumpers.

Materials and methods

Long jump players were divided into two groups; one group (group A) receiving HI-RT (n = 8) and the other group (group B) receiving combined low-intensity BFR training plus HI-RT (n = 8). Muscle power and knee muscle strength was assessed at baseline, 3 weeks and 6 weeks of intervention.

Results

1-RM was found to be significantly different between Group A and Group B at 3 and 6 weeks. Further, IKDQR, IKDHR and IKDQL was significantly improved in group B as compared to group A both at 3 and 6 weeks. There was significant time effect, group effect and time-group interaction in the strength of quadriceps and hamstring of both left and right leg measured through isokinetic device. Post-hoc analysis for 1-RM in group B showed a significant improvement at baseline and 6 weeks and the broad jump was significant at baseline and 3 weeks and at baseline and 6 weeks.

Conclusion

The combined effects of low-intensity BFR training and HI-RT is effective in improving the muscle strength and power of lower limbs in long jumpers.

Acellular collagen-glycosaminoglycan matrix promotes functional recovery in a rat model of volumetric muscle loss

Aim: Volumetric muscle loss (VML) is a composite loss of skeletal muscle, which heals with fibrosis, minimal muscle regeneration, and incomplete functional recovery. This study investigated whether collagen-glycosaminoglycan scaffolds (CGS) improve functional recovery following VML. Methods: 15 Sprague-Dawley rats underwent either sham injury or bilateral tibialis anterior (TA) VML injury, with or without CGS implantation. Results: In rats with VML injuries treated with CGS, the TA exhibited greater in vivo tetanic forces and in situ twitch and tetanic dorsiflexion forces compared with those in the non-CGS group at 4- and 6-weeks following injury, respectively. Histologically, the VML with CGS group demonstrated reduced fibrosis and increased muscle regeneration. Conclusion: Taken together, CGS implantation has potential augment muscle recovery following VML.

Microvascular Skeletal-Muscle Crosstalk in Health and Disease

As an organ system, skeletal muscle is essential for the generation of energy that underpins muscle contraction, plays a critical role in controlling energy balance and insulin-dependent glucose homeostasis, as well as vascular well-being, and regenerates following injury. To achieve homeostasis, there is requirement for "cross-talk" between the myogenic and vascular components and their regulatory factors that comprise skeletal muscle. Accordingly, this review will describe the following: [a] the embryonic cell-signaling events important in establishing vascular and myogenic cell-lineage, the cross-talk between endothelial cells (EC) and myogenic precursors underpinning the development of muscle, its vasculature and the satellite-stem-cell (SC) pool, and the EC-SC cross-talk that maintains SC quiescence and localizes ECs to SCs and angio-myogenesis postnatally; [b] the vascular-myocyte cross-talk and the actions of insulin on vasodilation and capillary surface area important for the uptake of glucose/insulin by myofibers and vascular homeostasis, the microvascular-myocyte dysfunction that characterizes the development of insulin resistance, diabetes and hypertension, and the actions of estrogen on muscle vasodilation and growth in adults; [c] the role of estrogen in utero on the development of fetal skeletal-muscle microvascularization and myofiber hypertrophy required for metabolic/vascular homeostasis after birth; [d] the EC-SC interactions that underpin myofiber vascular regeneration post-injury; and [e] the role of the skeletal-muscle vasculature in Duchenne muscular dystrophy.

Endothelial Progenitor Cells and Macrophage Subsets Recruitment in Postischemic Mouse Hind Limbs

INTRODUCTION

Peripheral arterial disease (PAD) occurs from atherosclerotic obstruction of arteries in the lower extremities. Restoration of perfusion requires angiogenesis and arteriogenesis through migration and differentiation of endothelial progenitor cells (EPCs) and macrophages at the site of injury. The time of recruitment has not been fully investigated. In this study, we investigated the infiltration of these cells in murine hind limb ischemia (HLI) model of PAD.

METHODS

EPCs and M1-like and M2-like macrophages from ischemic skeletal muscles were quantified by flow cytometry at day-0, 1, 3, 7, and 14 post-HLI.

RESULTS

The abundance of EPCs increased from day 1 and was highest on day 7 until day 14. M1-like population similarly increased and was highest on day 14 during the experiment. M2-like population was significantly greater than M1-like at baseline but surpassed the highest value of M1-like by day 7 during the experiment. Muscle regeneration and capillary density also increased and were highest at days 3 and 7, respectively, during the experiment. All mice achieved near full perfusion recovery by day 14.

CONCLUSION

Thus, we observed a gradual increase in the percentage of EPC's and this was temporally paralleled with initial increase in M1-like followed by sustained increased in M2-like macrophages and perfusion recovered post-HLI.

Effects of energy drinks on myogenic differentiation of murine C2C12 myoblasts

Energy drinks, often advertised as dietary supplements that enhance physical and mental performance, have gained increasing popularity among adolescents and athletes. Several studies on individual ingredients such as caffeine or taurine have reported either adverse or favorable influences on myogenic differentiation, a key process in muscle regeneration to repair microtears after an intense workout session. However, the impact of different energy drinks with various formulas on muscle differentiation has never been reported. This study aims to examine the in vitro effects of various energy drink brands on myogenic differentiation. Murine C2C12 myoblast cells were induced to differentiate into myotubes in the presence of one of eight energy drinks at varying dilutions. A dose-dependent inhibition of myotube formation was observed for all energy drinks, supported by reduced percentage of MHC-positive nuclei and fusion index. Moreover, expression of myogenic regulatory factor MyoG and differentiation marker MCK were also decreased. Furthermore, given the variation in formulas of different energy drinks, there were remarkable differences in the differentiation and fusion of myotubes between energy drinks. This is the first study to investigate the impact of various energy drinks on myogenic differentiation and our results suggest an inhibitory effect of energy drinks in muscle regeneration.

FTY720 in immuno-regenerative and wound healing technologies for muscle, epithelial and bone regeneration

In 2010, the FDA approved the administration of FTY720, S1P lipid mediator, as a therapy to treat relapsing forms of multiple sclerosis. FTY720 was found to sequester pro-inflammatory lymphocytes within the lymph node, preventing them from causing injury to the central nervous system due to inflammation. Studies harnessing the anti-inflammatory properties of FTY720 as a pro-regenerative strategy in wound healing of muscle, bone and mucosal injuries are currently being performed. This in-depth review discusses the current regenerative impact of FTY720 due to its anti-inflammatory effect stratified into an assessment of wound regeneration in the muscular, skeletal, and epithelial systems. The regenerative effect of FTY720 in vivo was characterized in three animal models, with differing delivery mechanisms emerging in the last 20 years. In these studies, local delivery of FTY720 was found to increase pro-regenerative immune cell phenotypes (neutrophils, macrophages, monocytes), vascularization, cell proliferation and collagen deposition. Delivery of FTY720 to a localized wound environment demonstrated increased bone, muscle, and mucosal regeneration through changes in gene and cytokine production primarily by controlling the local immune cell phenotypes. These changes in gene and cytokine production reduced the inflammatory component of wound healing and increased the migration of pro-regenerative cells (neutrophils and macrophages) to the wound site. The application of FTY720 delivery using a biomaterial has demonstrated the ability of local delivery of FTY720 to promote local wound healing leveraging an immunomodulatory mechanism.

Delayed skeletal muscle repair following inflammatory damage in simulated agent-based models of muscle regeneration

Healthy skeletal muscle undergoes repair in response to mechanically localised strains during activities such as exercise. The ability of cells to transduce the external stimuli into a cascade of cell signalling responses is important to the process of muscle repair and regeneration. In chronic myopathies such as Duchenne muscular dystrophy and inflammatory myopathies, muscle is often subject to chronic necrosis and inflammation that perturbs tissue homeostasis and leads to non-localised, widespread damage across the tissue. Here we present an agent-based model that simulates muscle repair in response to both localised eccentric contractions similar to what would be experienced during exercise, and non-localised widespread inflammatory damage that is present in chronic disease. Computational modelling of muscle repair allows for in silico exploration of phenomena related to muscle disease. In our model, widespread inflammation led to delayed clearance of tissue damage, and delayed repair for recovery of initial fibril counts at all damage levels. Macrophage recruitment was delayed and significantly higher in widespread compared to localised damage. At higher damage percentages of 10%, widespread damage led to impaired muscle regeneration and changes in muscle geometry that represented alterations commonly observed in chronic myopathies, such as fibrosis. This computational work offers insight into the progression and aetiology of inflammatory muscle diseases, and suggests a focus on the muscle regeneration cascade in understanding the progression of muscle damage in inflammatory myopathies.

Altered T cell infiltration and enrichment of leukocyte regulating pathways within aged skeletal muscle are associated impaired muscle function following influenza infection

Older adults have diminished immune responses that increase susceptibility to infectious diseases, such as influenza (flu). In older adults, flu infection can lead to hospitalization, catastrophic disability, and mortality. We previously demonstrated severe and prolonged muscle degradation and atrophy in aged mice during flu infection. Here, we utilized an unbiased transcriptomic analysis to elucidate mechanisms of flu-induced muscular declines in a mouse model. Our results showed age-related gene expression differences including downregulation of genes associated with muscle regeneration and organization and upregulation of genes associated with pro-inflammatory cytokines and migratory immune pathways in aged mice when compared to young. Pathway analysis revealed significant enrichment of leukocyte migration and T cell activation pathways in the aged muscle during infection. Intramuscular CD4 T cells increased in both young and aged mice during infection, while intramuscular CD8 T cells increased exclusively in aged muscle. CD4 T cells in young muscle were regulatory T cells (Treg), while those in aged were T follicular helper (Tfh) and Th2 cells. Correspondingly, IL-33, an important cytokine for Treg accumulation within tissue, increased only in young flu-infected muscle. Conversely, CXCL10 (IP-10) increased only in aged muscle suggesting a continued recruitment of CD8 T cells into the aged muscle during flu infection. Overall, our findings elucidate a link between flu-induced disability and dysregulated intracellular T cell recruitment into flu-injured muscle with aging. Furthermore, we uncovered potential pathways involved that can be targeted to develop preventative and therapeutic interventions to avert disability and maintain independence following infection.

Development and Validation of a Multiparametric Semiquantitative Scoring System for the Histopathological Assessment of Ischaemia Severity in Skeletal Muscle

Skeletal muscle is one of the most abundant and dynamic tissues of the body, with a strong regenerative capacity. Muscle injuries can occur as a result of a variety of events, including tissue ischaemia. Lower limb ischaemia occurs when there is an insufficient nutrient and oxygen supply, often caused by stenosis of the arteries due to atherosclerosis. The aim of this study was to develop and validate a multiparametric scoring tool for assessing ischaemia severity in skeletal muscle in a commonly used preclinical animal model. Tissue ischaemia was surgically induced in mice by ligation and excision of the femoral artery. Calf muscles were carefully dissected, prepared for histological analysis, and scored for inflammation, fibrosis, necrosis, adipocyte infiltration, and muscle fibre degeneration/regeneration. Kendall’s coefficient of concordance (W) showed a very good agreement between the appraisers when scoring each individual histological feature: inflammation (W = 0.92, $p\le 0.001$ ), fibrosis (W = 0.94, $p\le 0.001$ ), necrosis (W = 0.77, $p\le 0.001$ ), adipocyte infiltration (W = 0.91, $p\le 0.001$ ), and fibre degeneration/regeneration (W = 0.86, $p\le 0.001$ ). Intrarater agreement was also excellent (W = 0.94 or more, $p\le 0.001$ ). There was a statistically significant negative association between the level of muscle ischaemia damage and the calf muscle weight and skeletal muscle fibre diameter. Here, we have developed and validated a new multiparametric, semiquantitative scoring system for assessing skeletal muscle damage due to ischaemia, with excellent inter- and intrarater reproducibility. This scoring system can be used for assessing treatment efficacy in preclinical models of hind limb ischaemia.

Functional Skeletal Muscle Regeneration Using Muscle Mimetic Tissue Fabricated by Microvalve-Assisted Coaxial 3D Bioprinting

3D-printed artificial skeletal muscle, which mimics the structural and functional characteristics of native skeletal muscle, is a promising treatment method for muscle reconstruction. Although various fabrication techniques for skeletal muscle using 3D bio-printers are studied, it is still challenging to build a functional muscle structure. A strategy using microvalve-assisted coaxial 3D bioprinting in consideration of functional skeletal muscle fabrication is reported. The unit (artificial muscle fascicle: AMF) of muscle mimetic tissue is composed of a core filled with medium-based C2C12 myoblast aggregates as a role of muscle fibers and a photo cross-linkable hydrogel-based shell as a role of connective tissue in muscles that enhances printability and cell adhesion and proliferation. Especially, a microvalve system is applied for the core part with even cell distribution and strong cell-cell interaction. This system enhances myotube formation and consequently shows spontaneous contraction. A multi-printed AMF (artificial muscle tissue: AMT) as a piece of muscle is implanted into the anterior tibia (TA) muscle defect site of immunocompromised rats. As a result, the TA-implanted AMT responds to electrical stimulation and represents histologically regenerated muscle tissue. This microvalve-assisted coaxial 3D bioprinting shows a significant step forward to mimicking native skeletal muscle tissue.

Inflammatory myopathies and beyond: The dual role of neutrophils in muscle damage and regeneration

Skeletal muscle is one of the most abundant tissues of the human body and is responsible for the generation of movement. Muscle injuries can lead to severe disability. Skeletal muscle is characterized by an important regeneration capacity, which is possible due to the interaction between the myoblasts and immune cells. Neutrophils are fundamental as inducers of muscle damage and as promoters of the initial inflammatory response which eventually allows the muscle repair. The main functions of the neutrophils are phagocytosis, respiratory burst, degranulation, and the production of neutrophil extracellular traps (NETs). An overactivation of neutrophils after muscle injuries may lead to an expansion of the initial damage and can hamper the successful muscle repair. The importance of neutrophils as inducers of muscle damage extends beyond acute muscle injury and recently, neutrophils have become more relevant as part of the immunopathogenesis of chronic muscle diseases like idiopathic inflammatory myopathies (IIM). This heterogeneous group of systemic autoimmune diseases is characterized by the presence of muscle inflammation with a variable amount of extramuscular features. In IIM, neutrophils have been found to have a role as biomarkers of disease activity, and their expansion in peripheral blood is related to certain clinical features like interstitial lung disease (ILD) and cancer. On the other hand, low density granulocytes (LDG) are a distinctive subtype of neutrophils characterized by an enhanced production of NETs. These cells along with the NETs have also been related to disease activity and certain clinical features like ILD, vasculopathy, calcinosis, dermatosis, and cutaneous ulcers. The role of NETs in the immunopathogenesis of IIM is supported by an enhanced production and deficient degradation of NETs that have been observed in patients with dermatomyositis and anti-synthetase syndrome. Finally, new interest has arisen in the study of other phenotypes of LDG with a phenotype corresponding to myeloid-derived suppressor cells, which were also found to be expanded in patients with IIM and were related to disease activity. In this review, we discuss the role of neutrophils as both orchestrators of muscle repair and inducers of muscle damage, focusing on the immunopathogenesis of IIM.

Cathelicidin-related antimicrobial peptide mediates skeletal muscle degeneration caused by injury and Duchenne muscular dystrophy in mice

BACKGROUND

Cathelicidin, an antimicrobial peptide, plays a key role in regulating bacterial killing and innate immunity; however, its role in skeletal muscle function is unknown. We investigated the potential role of cathelicidin in skeletal muscle pathology resulting from acute injury and Duchenne muscular dystrophy (DMD) in mice.

METHODS

Expression changes and muscular localization of mouse cathelicidin-related antimicrobial peptide (Cramp) were examined in the skeletal muscle of normal mice treated with chemicals (cardiotoxin and BaCl₂ ) or in dystrophic muscle of DMD mouse models (mdx, mdx/Utrn^+/- and mdx/Utrn^-/- ). Cramp penetration into myofibres and effects on muscle damage were studied by treating synthetic peptides to mouse skeletal muscles or C2C12 myotubes. Cramp knockout (KO) mice and mdx/Utrn/Cramp KO lines were used to determine whether Cramp mediates muscle degeneration. Muscle pathophysiology was assessed by histological methods, serum analysis, grip strength and lifespan. Molecular factors targeted by Cramp were identified by the pull-down assay and proteomic analysis.

RESULTS

In response to acute muscle injury, Cramp was activated in muscle-infiltrating neutrophils and internalized into myofibres. Cramp treatments of mouse skeletal muscles or C2C12 myotubes resulted in muscle degeneration and myotube damage, respectively. Genetic ablation of Cramp reduced neutrophil infiltration and ameliorated muscle pathology, such as fibre size (P < 0.001; n = 6) and fibrofatty infiltration (P < 0.05). Genetic reduction of Cramp in mdx/Utrn^+/- mice not only attenuated muscle damage (35%, P < 0.05; n = 9-10), myonecrosis (53%, P < 0.05), inflammation (37-65%, P < 0.01) and fibrosis (14%, P < 0.05) but also restored muscle fibre size (14%, P < 0.05) and muscle force (18%, P < 0.05). Reducing Cramp levels led to a 63% (male, P < 0.05; n = 10-14) and a 124% (female, P < 0.001; n = 20) increase in the lifespan of mdx/Utrn^-/- mice. Proteomic and mechanistic studies revealed that Cramp cross-talks with Ca²⁺ signalling in skeletal muscle through sarcoplasmic/endoplasmic reticulum Ca²⁺ -ATPase1 (SERCA1). Cramp binds and inactivates SERCA1, leading to the activation of Ca²⁺ -dependent calpain proteases that exacerbate DMD progression.

CONCLUSIONS

These findings identify Cramp as an immune cell-derived regulator of skeletal muscle degeneration and provide a potential therapeutic target for DMD.

Myogenic commitment of human stem cells by myoblasts Co-culture: a static vs. a dynamic approach

An in-vitro model of human bone marrow mesenchymal stem cells (hBM-MSCs) myogenic commitment by synergic effect of a differentiation media coupled with human primary skeletal myoblasts (hSkMs) co-culture was developed adopting both conventional static co-seeding and perfused culture systems. Static co-seeding provided a notable outcome in terms of gene expression with a significant increase of Desmin (141-fold) and Myosin heavy chain II (MYH2, 32-fold) at day 21, clearly detected also by semi-quantitative immunofluorescence. Under perfusion conditions, myogenic induction ability of hSkMs on hBM-MSCs was exerted by paracrine effect with an excellent gene overexpression and immunofluorescence detection of MYH2 protein; furthermore, due to the dynamic cell culture in separate wells, western blot data were acquired confirming a successful cell commitment at day 14. A significant increase of anti-inflammatory cytokine gene expression, including IL-10 and IL-4 (15-fold and 11-fold, respectively) at day 14, with respect to the pro-inflammatory cytokines IL-12A (7-fold at day 21) and IL-1β (1.4-fold at day 7) was also detected during dynamic culture, confirming the immunomodulatory activity of hBM-MSCs along with commitment events. The present study opens interesting perspectives on the use of dynamic culture based on perfusion as a versatile tool to study myogenic events and paracrine cross-talk compared to the simple co-seeding static culture.

The Neuromusculoskeletal Epidemiological Outcome (NEO) matrix: An innovative injury classification based on anatomical location and primary tissue type

INTRODUCTION

The purpose of this paper is to propose the Neuromusculoskeletal Epidemiological Outcome (NEO) Matrix, an injury classification based on anatomical location and primary tissue type to classify NMSKI of the trunk and extremities in orthopaedic and sports medicine epidemiological research.

MATERIALS AND METHODS

In the proposed NEO matrix, NMSKI diagnosis (International Classification of Disease) codes are classified by the anatomical complex affected. These regional anatomical complexes are frequently employed in orthopaedic and sports medicine disciplines in biomechanical and clinical research. The complexes are arranged from distal to proximal in the lower extremity, inferior to superior in the trunk and head, and proximal to distal in the upper extremity. Injuries are characterized by the primary tissue type affected (Osseous; Articular; Musculotendinous; Neural; Non-specific) based on classified using International Classification of Disease codes.

DISCUSSION

The expected patterns of impairment and healing rates of bone, cartilage, ligaments, muscles, tendons, and neural structures vary considerably by the tissue type based on the differences in function, perfusion, and metabolic physiology. Using knowledge of the function of the tissues, expected minimum healing times, and expected sequelae of residual body system impairments, an estimation of morbidity and expected recovery times can be made. While heterogeneity is certainly expected due to varying pathomechanics involved and intrinsic and environmental factors, the NEO matrix classifies the ICD diagnosis codes in framework that qualifies the expected patterns of impairment based on the body system type.

CONCLUSION

The NEO matrix provides an alternative approach for characterizing NMSKI in epidemiological research.

Advances in electrospinning and 3D bioprinting strategies to enhance functional regeneration of skeletal muscle tissue

Skeletal muscles are essential for body movement, and the loss of motor function due to volumetric muscle loss (VML) limits the mobility of patients. Current therapeutic approaches are insufficient to offer complete functional recovery of muscle damages. Tissue engineering provides viable ways to fabricate scaffolds to regenerate damaged tissues. Hence, tissue engineering options are explored to address existing challenges in the treatment options for muscle regeneration. Electrospinning is a widely employed fabrication technique to make muscle mimetic nanofibrous scaffolds for tissue regeneration. 3D bioprinting has also been utilized to fabricate muscle-like tissues in recent times. This review discusses the anatomy of skeletal muscle, defects, the healing process, and various treatment options for VML. Further, the advanced strategies in electrospinning of natural and synthetic polymers are discussed, along with the recent developments in the fabrication of hybrid scaffolds. Current approaches in 3D bioprinting of skeletal muscle tissues are outlined with special emphasis on the combination of electrospinning and 3D bioprinting towards the development of fully functional muscle constructs. Finally, the current challenges and future perspectives of these convergence techniques are discussed.

Mechanisms of skeletal muscle repair and regeneration in health and disease

Skeletal muscle is a structurally and functionally remarkable tissue composed of multinucleated post-mitotic muscle fibres. These fibres are filled with an exquisite, near crystalline array of assembled contractile proteins, capable of coupling ATP utilization to mechanical muscle contraction. Fully differentiated muscle has an incredible ability to protect and repair itself from significant muscle injuries. In fact, through activation of a resident population of stem cells known as satellite cells, muscle fibres can be completely regenerated, and normal function can be restored in a matter of a few weeks after a major myocellular necrotic injury. The loss of key mechanisms to protect muscle from injuries or loss of the capacity to repair muscle after injury is thought to underlie several forms of muscular dystrophy and also the age-related decline of muscle function. In this Subject Collection, The FEBS Journal highlights articles that review or investigate key mechanisms of muscle repair and regeneration in response to injuries, and the contributions of these pathways to health and disease of skeletal muscle.

The regenerative potential of Pax3/Pax7 on skeletal muscle injury

Background

Skeletal muscle mishaps are the most well-known incidents in society, especially among athletes and the military population. From the various urgency, this accident needs to be cured more quickly. However, the current treatment still has some shortcomings and is less effective. In this case, Paired box 3 and Paired box 7 (Pax3/Pax7) proteins that induce stem cells could potentially be an alternative treatment for skeletal muscle injuries. This paper aimed to analyse the potential treatment of Pax3/Pax7 proteins inducing the stem cell for skeletal muscle injuries.

The main body of the abstract

We did a narrative review by gathering several scientific journals from several leading platforms like PubMed and Scopus. As common accidents, skeletal muscle disease could be due to workplace and non-workplace causes. The highest risk occurs in the athlete and military environment. The treatment of current skeletal muscle injuries is protection, rest, ice, compression, and elevation (PRICE), non-steroidal anti-inflammatory drugs (NSAIDs), and mechanical stimulation. However, it is considered less effective, especially in NSAIDs, inhibiting myogenic cell proliferation. The current finding indicates that the stem cells have markers known as Pax3/Pax7. The role of both markers in muscle injury, Pax3/Pax7, as transcription factors will induce cell division by H3K4 methylation mechanisms and chromatin modifications that stimulate gene activation.

Conclusion

Regulation by Pax3/Pax7 factors that affect stem cells and stem cell proliferation is one of the alternative treatments. This regulation can accelerate the healing of injury victims, especially injuries to the skeletal muscles. Finally, after being compared, Pax3/Pax7 induces stem cells to have the potential to be one of the skeletal muscle injury treatments.

Keywords

Pax3 and Pax7, Pax3/Pax7, Skeletal muscle, Athlete, Stem cells, Cell proliferation, Injuries.

Preclinical research studies for treating severe muscular injuries: focus on tissue-engineered strategies

Severe skeletal muscle injuries are a lifelong trauma with limited medical solutions. Significant progress has been made in developing in vitro surrogates for treating such trauma. However, more attention is needed when translating these approaches to the clinic. In this review, we survey the potential of tissue-engineered surrogates in promoting muscle healing, by critically analyzing data from recent preclinical models. The therapeutic advantages provided by a combination of different biomaterials, cell types, and biochemical mediators are discussed. Current therapies on muscle healing are also summarized, emphasizing their main advantages and drawbacks. We also discuss previous and ongoing clinical trials as well as highlighting future directions for the field.

A Metabolic Signature to Monitor Endothelial Cell Differentiation, Activation, and Vascular Organization

The formation of new blood vessels is an important step in the morphogenesis and organization of tissues and organs; hence, the success of regenerative medicine procedures is highly dependent on angiogenesis control. Despite the biotechnological advances, tissue engineering is still a challenge. Regarding vascular network formation, the regulators are well known, yet the identification of markers is pivotal in order to improve the monitoring of the differentiation and proliferation of endothelial cells, as well as the establishment of a vascular network supporting tissue viability for an efficacious implantation. The metabolic profile accompanies the physiological stages of cells involved in angiogenesis, being a fruitful hub of biomarkers, whose levels can be easily retrieved. Through NMR spectroscopy, we identified branched amino acids, acetate, and formate as central biomarkers of monocyte-to-endothelial-cell differentiation and endothelial cell proliferation. This study reinforces the successful differentiation process of monocytes into endothelial cells, allowing self-to-self transplantation of patient-derived vascular networks, which is an important step in tissue engineering, since monocytes are easily isolated and autologous transplantation reduces the immune rejection events.

A pilot study on nitration/dysfunction of NK1 segment of myogenic stem cell activator HGF

Protein tyrosine residue (Y) nitration, a post-translational chemical-modification mode, has been associated with changes in protein activity and function; hence the accumulation of specific nitrated proteins in tissues may be used to monitor the onset and progression of pathological disorders. To verify the possible impact of nitration on postnatal muscle growth and regeneration, a pilot study was designed to examine the nitration/dysfunction of hepatocyte growth factor (HGF), a key ligand that is released from the extracellular tethering and activates myogenic stem satellite cells to enter the cell cycle upon muscle stretch and injury. Exposure of recombinant HGF (a hetero-dimer of α- and β-chains) to peroxynitrite induces Y nitration in HGF α-chain under physiological conditions. Physiological significance of this finding was emphasized by Western blotting that showed the NK1 segment of HGF (including a K1 domain critical for signaling-receptor c-met binding) undergoes nitration with a primary target of Y198. Peroxynitrite treatment abolished HGF-agonistic activity of the NK1 segment, as revealed by in vitro c-met binding and bromodeoxyuridine-incorporation assays. Importantly, direct-immunofluorescence microscopy of rat lower hind-limb muscles from two aged-groups (2-month-old “young” and 12-month-old “retired/adult”) provided in vivo evidence for age-related nitration of extracellular HGF (Y198). Overall, findings provide the insight that HGF/NK1 nitration/dysfunction perturbs myogenic stem cell dynamics and homeostasis; hence NK1 nitration may stimulate progression of muscular disorders and diseases including sarcopenia.

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NK1 segment of hepatocyte growth factor (HGF) undergoes tyrosine (Y) nitration.

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Y198 was identified as a primary target for nitration of NK1.

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NK1 nitration may abolish HGF-agonistic activity that activates myogenic stem cells.

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Nitration of extracellular HGF-Y198 was detected in vivo at early aging-phase of rat.

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Findings may provide a possible strategy to combat progressive muscle-atrophy.

All for One and One for All: Regenerating Skeletal Muscle

Despite the evolutionary loss of tissue regenerative potential, robust skeletal muscle repair processes are largely retained even in higher vertebrates. In mammals, the skeletal muscle regeneration program is driven by resident stem cells termed satellite cells, guided by the coordinated activity of multiple intrinsic and extrinsic factors and other cell types. A thorough understanding of muscle repair mechanisms is crucial not only for combating skeletal myopathies, but for its prospective aid in devising therapeutic strategies to endow regenerative potential on otherwise regeneration-deficient organs. In this review, we discuss skeletal muscle regeneration from an evolutionary perspective, summarize the current knowledge of cellular and molecular mechanisms, and highlight novel paradigms of muscle repair revealed by explorations of the recent decade.