Towards understanding skeletal muscle regeneration

Skeletal muscle regeneration with 3D bioprinted hyaluronate/gelatin hydrogels incorporating MXene nanoparticles

There has been significant progress in the field of three-dimensional (3D) bioprinting technology, leading to active research on creating bioinks capable of producing structurally and functionally tissue-mimetic constructs. Ti3C2Tx MXene nanoparticles (NPs), promising two-dimensional nanomaterials, are being investigated for their potential in muscle regeneration due to their unique physicochemical properties. In this study, we integrated MXene NPs into composite hydrogels made of gelatin methacryloyl (GelMA) and hyaluronic acid methacryloyl (HAMA) to develop bioinks (namely, GHM bioink) that promote myogenesis. The prepared GHM bioinks were found to offer excellent printability with structural integrity, cytocompatibility, and microporosity. Additionally, MXene NPs within the 3D bioprinted constructs encouraged the differentiation of C2C12 cells into skeletal muscle cells without additional support of myogenic agents. Genetic analysis indicated that representative myogenic markers both for early and late myogenesis were significantly up-regulated. Moreover, animal studies demonstrated that GHM bioinks contributed to enhanced regeneration of skeletal muscle while reducing immune responses in mice models with volumetric muscle loss (VML). Our results suggest that the GHM hydrogel can be exploited to craft a range of strategies for the development of a novel bioink to facilitate skeletal muscle regeneration because these MXene-incorporated composite materials have the potential to promote myogenesis.

Augmented mitochondrial apoptotic signaling impairs C2C12 myoblast differentiation following cellular aging through sequential passaging

Aging is associated with the steady decline of several cellular processes. The loss of skeletal muscle mass, termed sarcopenia, is one of the major hallmarks of aging. Aged skeletal muscle exhibits a robust reduction in its regenerative capacity due to dysfunction (i.e., senescence, lack of self-renewal, and impaired differentiation) of resident muscle stem cells, called satellite cells. To replicate aging in vitro, immortalized skeletal muscle cells (myoblasts) can be treated with various agents to mimic age-related dysfunction; however, these come with their own set of limitations. In the present study, we used sequential passaging of mouse myoblasts to mimic impaired differentiation that is observed in aged skeletal muscle. Further, we investigated mitochondrial apoptotic mechanisms to better understand the impaired differentiation in these "aged" cells. Our data shows that sequential passaging (>20 passages) of myoblasts is accompanied with significant reductions in differentiation and elevated cell death. Furthermore, high-passage (HP) myoblasts exhibit greater mitochondrial-mediated apoptotic signaling through mitochondrial BAX translocation, CYCS and AIFM1 release, and caspase-9 activation. Finally, we show that inhibition of mitochondrial outer membrane permeability partly recovered differentiation in HP myoblasts. Together, our findings suggests that mitochondrial apoptotic signaling is a contributing factor to the diminished differentiation that is observed in aged myoblasts.

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.

The cilium-centrosome axis in coupling cell cycle exit and cell fate

The centrosome is an evolutionarily conserved, ancient organelle whose role in cell division was first described over a century ago. The structure and function of the centrosome as a microtubule-organizing center, and of its extracellular extension - the primary cilium - as a sensory antenna, have since been extensively studied, but the role of the cilium-centrosome axis in cell fate is still emerging. In this Opinion piece, we view cellular quiescence and tissue homeostasis from the vantage point of the cilium-centrosome axis. We focus on a less explored role in the choice between distinct forms of mitotic arrest - reversible quiescence and terminal differentiation, which play distinct roles in tissue homeostasis. We outline evidence implicating the centrosome-basal body switch in stem cell function, including how the cilium-centrosome complex regulates reversible versus irreversible arrest in adult skeletal muscle progenitors. We then highlight exciting new findings in other quiescent cell types that suggest signal-dependent coupling of nuclear and cytoplasmic events to the centrosome-basal body switch. Finally, we propose a framework for involvement of this axis in mitotically inactive cells and identify future avenues for understanding how the cilium-centrosome axis impacts central decisions in tissue homeostasis.

The Role of Mitophagy in Skeletal Muscle Damage and Regeneration

Mitochondria are cellular organelles that play an essential role in generating the chemical energy needed for the biochemical reactions in cells. Mitochondrial biogenesis, i.e., de novo mitochondria formation, results in enhanced cellular respiration, metabolic processes, and ATP generation, while autophagic clearance of mitochondria (mitophagy) is required to remove damaged or useless mitochondria. The balance between the opposing processes of mitochondrial biogenesis and mitophagy is highly regulated and crucial for the maintenance of the number and function of mitochondria as well as for the cellular homeostasis and adaptations to metabolic demands and extracellular stimuli. In skeletal muscle, mitochondria are essential for maintaining energy homeostasis, and the mitochondrial network exhibits complex behaviors and undergoes dynamic remodeling in response to various conditions and pathologies characterized by changes in muscle cell structure and metabolism, such as exercise, muscle damage, and myopathies. In particular, the involvement of mitochondrial remodeling in mediating skeletal muscle regeneration following damage has received increased attention, as modifications in mitophagy-related signals arise from exercise, while variations in mitochondrial restructuring pathways can lead to partial regeneration and impaired muscle function. Muscle regeneration (through myogenesis) following exercise-induced damage is characterized by a highly regulated, rapid turnover of poor-functioning mitochondria, permitting the synthesis of better-functioning mitochondria to occur. Nevertheless, essential aspects of mitochondrial remodeling during muscle regeneration remain poorly understood and warrant further characterization. In this review, we focus on the critical role of mitophagy for proper muscle cell regeneration following damage, highlighting the molecular mechanisms of the mitophagy-associated mitochondrial dynamics and network reformation.

Genome-wide identification evolution and expression of vestigial-like gene family in chicken

Vestigial-like (Vgll) genes are widespread in vertebrates and play an important role in muscle development. In this study, we used bioinformatics methods to systematically identify the chicken VGLL family in the whole genome and investigated its evolutionary history and gene structure features. Tissue expression spectra combined with real-time PCR data were used to analyze the organizational expression pattern of the genes. Based on the maximum likelihood method, a phylogenetic tree of the VGLL family was constructed, and 94 VGLL genes were identified in 24 breeds, among which four VGLL family genes were identified in the chicken genome. Ten motifs were detected in the VGLL genes, and the analysis of introns combined with gene structure revealed that the family was conserved during evolution. Tissue expression analysis suggested that the expression profiles of the VGLL family genes in 16 tissues differed between LU Shi and AA broilers. In addition, a single gene (VGLL2) showed increased expression in chickens at embryonic days 10-16 and was involved in the growth and development of skeletal muscle in chickens in the embryonic stage. In summary, VGLL genes are involved in chicken muscle growth and development, which provides useful information for subsequent functional studies of VGLL genes.

Applications of bupivacaine in the non-surgical treatments of strabismus: a review

Bupivacaine (BUP) is an anesthetic from the family of aminoacyl anesthetics and has the highest myotoxicity among other groups of anesthetics. Intramuscular injection of BUP first causes acute libriform lysis and subsequently with the regeneration process, stronger myofibrils are formed within 3-4 weeks. Satellite cells, which are actually myogenic stem cells, are preserved in the early stage and during the destruction of muscle fibers. In fact, these cells are responsible for the subsequent regeneration of fibers. BUP is one of the few medicines that is able to increase muscle strength. In animal studies on rabbits, a decrease has been observed in the diameter of the global layer in the first week and an increase in type-I myosin occurs after 60 days, especially in the global muscle layer. There are numerous studies according to BUP injection for the non-surgical management of horizontal strabismus. To intensify the effects of the injection, botulinum toxin injection can also be used simultaneously in the antagonist muscles. In general, although the rate of improvement in strabismus varied among different studies, BUP injection alone corrects about 5-8 prism diopters. Together with botulinum toxin, BUP corrects about 15 prism diopters. The stability of this improvement is up to 10 years after injection. No significant difference has been observed in response rate between patients with esotropia and exotropia. Unlike the large molecule of botulinum toxin, which spreads slowly to its site of action, the BUP molecule is small and must be in direct contact with myofibrils before absorption into the bloodstream to exert its effect. Therefore, the injection volume should be about 3 cc with a concentration of 0.75 g per deciliter. Although BUP is promising non-surgical strabismus management, especially in small angle and residual horizontal strabismus, however, it has its own limitations. The need for direct infusion of a relatively large volume of BUP may be one of its major drawbacks that limits its usage in an office method.

Optimizing the Surface Structural and Morphological Properties of Silk Thin Films via Ultra-Short Laser Texturing for Creation of Muscle Cell Matrix Model

Temporary scaffolds that mimic the extracellular matrix’s structure and provide a stable substratum for the natural growth of cells are an innovative trend in the field of tissue engineering. The aim of this study is to obtain and design porous 2D fibroin-based cell matrices by femtosecond laser-induced microstructuring for future applications in muscle tissue engineering. Ultra-fast laser treatment is a non-contact method, which generates controlled porosity—the creation of micro/nanostructures on the surface of the biopolymer that can strongly affect cell behavior, while the control over its surface characteristics has the potential of directing the growth of future muscle tissue in the desired direction. The laser structured 2D thin film matrices from silk were characterized by means of SEM, EDX, AFM, FTIR, Micro-Raman, XRD, and 3D-roughness analyses. A WCA evaluation and initial experiments with murine C2C12 myoblasts cells were also performed. The results show that by varying the laser parameters, a different structuring degree can be achieved through the initial lifting and ejection of the material around the area of laser interaction to generate porous channels with varying widths and depths. The proper optimization of the applied laser parameters can significantly improve the bioactive properties of the investigated 2D model of a muscle cell matrix.

Efficacy of Serial Ultrasonographic Examinations in Predicting Return to Play in Agility Dogs with Shoulder Lameness

The aim of this study is to investigate the use of shoulder ultrasound as a method of predicting the likelihood of returning to competition in agility dogs with shoulder teno-muscular injuries after a standardised rehabilitation protocol. Thirty-two agility dogs with a clinical and ultrasonographic diagnosis of shoulder teno-muscular injury were included in a prospective study with physical and ultrasound examinations at the time of diagnosis (T0) and at two (T2), four (T4) and six (T6) months; during this period, the dogs received rehabilitation treatments. The endpoint of the study was to obtain information regarding participation in agility competitions 12 months after diagnosis, based on telephone interviews with the owners. The clinical lameness score (CLS) and the ultrasound lesion score (ULS) were used as outcome measurements. The CLS indicated partial recovery from a shoulder injury at T2 (78%), while the ULS indicated no satisfactory recovery at T2 in any patient. At 4 months, the CLS alone was not a valuable predictor of full recovery from a shoulder injury in agility dogs. Relative Risk indicated that, at T2, ultrasound was 23.8 times more valuable in identifying a shoulder lesion as compared to clinical lameness score (CLS), and it was 2.53 times more valuable at T4.

Mechanobiology in Tendon, Ligament, and Skeletal Muscle Tissue Engineering

Tendon, ligament, and skeletal muscle are highly organized tissues that largely rely on a hierarchical collagenous matrix to withstand high tensile loads experienced in activities of daily life. This critical biomechanical role predisposes these tissues to injury, and current treatments fail to recapitulate the biomechanical function of native tissue. This has prompted researchers to pursue engineering functional tissue replacements, or dysfunction/disease/development models, by emulating in vivo stimuli within in vitro tissue engineering platforms-specifically mechanical stimulation, as well as active contraction in skeletal muscle. Mechanical loading is critical for matrix production and organization in the development, maturation, and maintenance of native tendon, ligament, and skeletal muscle, as well as their interfaces. Tissue engineers seek to harness these mechanobiological benefits using bioreactors to apply both static and dynamic mechanical stimulation to tissue constructs, and induce active contraction in engineered skeletal muscle. The vast majority of engineering approaches in these tissues are scaffold-based, providing interim structure and support to engineered constructs, and sufficient integrity to withstand mechanical loading. Alternatively, some recent studies have employed developmentally inspired scaffold-free techniques, relying on cellular self-assembly and matrix production to form tissue constructs. Whether utilizing a scaffold or not, incorporation of mechanobiological stimuli has been shown to improve the composition, structure, and biomechanical function of engineered tendon, ligament, and skeletal muscle. Together, these findings highlight the importance of mechanobiology and suggest how it can be leveraged to engineer these tissues and their interfaces, and to create functional multitissue constructs.

Skeletal muscle regeneration in post-myofascial flap reconstruction specimens mimicking sarcoma: A potential diagnostic pitfall

Skeletal muscle regeneration (SMR) encompasses a sequence of events that unfolds after injury to muscle fibers. Nearby satellite cells become activated and function as precursor muscle cells by proliferating and differentiating into myoblasts, which eventually fuse to form myotubes and ultimately mature muscle fibers. Compared to other forms of mesenchymal repair, SMR has higher morphologic heterogeneity with the potential to show histopathologic similarities to sarcomas and other malignancies. It is important to recognize SMR and settings in which this can occur to avoid misdiagnosis. We report two cases where a diagnosis of SMR was made from tissue taken from locations previously treated with Mohs micrographic surgery followed by myofascial flap reconstruction. In both cases, histopathologic features identified with hematoxylin and eosin as well as immunostaining were used to support the diagnosis of SMR. These cases highlight the importance of recognizing this clinic entity to ensure accurate diagnosis.

Mitochondrial network remodeling: an important feature of myogenesis and skeletal muscle regeneration

The remodeling of the mitochondrial network is a critical process in maintaining cellular homeostasis and is intimately related to mitochondrial function. The interplay between the formation of new mitochondria (biogenesis) and the removal of damaged mitochondria (mitophagy) provide a means for the repopulation of the mitochondrial network. Additionally, mitochondrial fission and fusion serve as a bridge between biogenesis and mitophagy. In recent years, the importance of these processes has been characterised in multiple tissue- and cell-types, and under various conditions. In skeletal muscle, the robust remodeling of the mitochondrial network is observed, particularly after injury where large portions of the tissue/cell structures are damaged. The significance of mitochondrial remodeling in regulating skeletal muscle regeneration has been widely studied, with alterations in mitochondrial remodeling processes leading to incomplete regeneration and impaired skeletal muscle function. Needless to say, important questions related to mitochondrial remodeling and skeletal muscle regeneration still remain unanswered and require further investigation. Therefore, this review will discuss the known molecular mechanisms of mitochondrial network remodeling, as well as integrate these mechanisms and discuss their relevance in myogenesis and regenerating skeletal muscle.

Growth factors improve the proliferation of Jeju black pig muscle cells by regulating myogenic differentiation 1 and growth-related genes

OBJECTIVE

The growth rate of pigs is related to differentiation and proliferation of muscle cells, which are regulated by growth factors and expression of growth-related genes. Thus, the objective of this study was to establish optimal culture conditions for Jeju black pig (JBP) muscle cells and determine the relationship of various factors involved in muscle growth with the proliferation of JBP muscle cells.

METHODS

Muscles were taken from the femur skeletal muscle of JBP embryos. After isolation of the muscle cells, cells were cultured in a 6-well plate under four different culture conditions to optimize culture conditions for JBP muscle cells. To analyze proliferation rate of JBP muscle cells, these muscle cells were seeded into 6-well plates at a density of 1.5×105 cells per well and cultured for 3 days. Western blot and quantitative real-time polymerase chain reaction were applied to verify the myogenic differentiation 1 (MyoD) expression and growth-related gene expression in JBP muscle cells, respectively.

RESULTS

We established a muscle cell line from JBP embryos and optimized its culture conditions. These muscle cells were positive for MyoD, but not for paired box 7. The proliferation rate of these muscle cells was significantly higher in a culture medium containing bFGF and epidermal growth factor + basic fibroblast growth factor (EGF+bFGF) than that without a growth factor or containing EGF alone. Treatment with EGF and bFGF significantly induced the expression of MyoD protein, an important transcription factor in muscle cells. Moreover, we checked the changes of expression of growth-related genes in JBP muscle cells by presence or absence of growth factors. Expression level of collagen type XXI alpha 1 gene was changed only when EGF and bFGF were added together to culture media for JBP muscle cells.

CONCLUSION

Concurrent use of EGF and bFGF increased the expression of MyoD protein, thus regulating the proliferation of JBP muscle cells and the expression of growth-related genes.

Silkworm Silk Fiber Bundles as Improved In Vitro Scaffolds for Skeletal Muscle

To mimic skeletal muscle tissues in vitro, native and transgenic spider silk/silkworm silks were seeded with C2C12 myoblasts to observe if these three-dimensional substrates are preferable to a traditional two-dimensional polystyrene cell culture surface. Silks were wound around an acrylic chassis to produce a novel, three-dimensional cell culture device with suspended muscle fibers that genetically and morphologically resemble native skeletal muscle tissue. The transgenic spider silk/silkworm silk has never before been studied for this application. Genetic expression verified skeletal muscle lineage and differentiation, while fluorescent imaging verified contractile protein synthesis. Genetic analysis also revealed an increase in expression of the Myh2 contractile protein gene on silkworm silks, particularly on the transgenic silk. Mechanical properties and protein secondary structure content of the silks indicated correlation between substrate properties and Myh2 gene expression. This increase in contractile protein gene expression suggests that biologically derived silk substrates that are suspended may be a preferable substrate for in vitro muscle modeling because of the proteinaceous character and mechanical flexibility of the silk.

Combination of soluble factors and biomaterial scaffolds enhance human adipose-derived stem/stromal cell myogenesis

Volumetric muscle loss and muscle degeneration are conditions for which there are currently no effective treatment options. Human adipose stem cells (hASCs) offer promise in cell-based regenerative therapies to treat muscle damage due to their ability to self-renew and differentiate. However, in the absence of universal culture conditions that yield greater than 15% myogenic differentiation, the clinical potential of these cells is limited. Here we report on the evaluation of two different media recipes, three extracellular matrix (ECM) proteins, and a poly (ethylene glycol) (PEGDMA) hydrogel with a physiologically relevant elasticity to determine how the extracellular chemical and physical environment work together to enhance myogenic differentiation of hASCs. Our results identify a combination of unique biochemical and physical factors that promote myogenesis, laying the groundwork for creating a scaffold and culture medium that will effectively and efficiently direct myogenic differentiation of adult stem cells for clinical applications in the future.

Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss

Millions of Americans suffer from skeletal muscle injuries annually that can result in volumetric muscle loss (VML), where extensive musculoskeletal damage and tissue loss result in permanent functional deficits. In the case of small-scale injury skeletal muscle is capable of endogenous regeneration through activation of resident satellite cells (SCs). However, this is greatly reduced in VML injuries, which remove native biophysical and biochemical signaling cues and hinder the damaged tissue's ability to direct regeneration. The current clinical treatment for VML is autologous tissue transfer, but graft failure and scar tissue formation leave patients with limited functional recovery. Tissue engineering of instructive biomaterial scaffolds offers a promising approach for treating VML injuries. Herein, we review the strategic engineering of biophysical and biochemical cues in current scaffold designs that aid in restoring function to these preclinical VML injuries. We also discuss the successes and limitations of the three main biomaterial-based strategies to treat VML injuries: acellular scaffolds, cell-delivery scaffolds, and in vitro tissue engineered constructs. Finally, we examine several innovative approaches to enhancing the design of the next generation of engineered scaffolds to improve the functional regeneration of skeletal muscle following VML injuries.

Impaired ECM Remodeling and Macrophage Activity Define Necrosis and Regeneration Following Damage in Aged Skeletal Muscle

Regenerative capacity of skeletal muscle declines with age, the cause of which remains largely unknown. We investigated extracellular matrix (ECM) proteins and their regulators during early regeneration timepoints to define a link between aberrant ECM remodeling, and impaired aged muscle regeneration. The regeneration process was compared in young (three month old) and aged (18 month old) C56BL/6J mice at 3, 5, and 7 days following cardiotoxin-induced damage to the tibialis anterior muscle. Immunohistochemical analyses were performed to assess regenerative capacity, ECM remodeling, and the macrophage response in relation to plasminogen activator inhibitor-1 (PAI-1), matrix metalloproteinase-9 (MMP-9), and ECM protein expression. The regeneration process was impaired in aged muscle. Greater intracellular and extramyocellular PAI-1 expression was found in aged muscle. Collagen I was found to accumulate in necrotic regions, while macrophage infiltration was delayed in regenerating regions of aged muscle. Young muscle expressed higher levels of MMP-9 early in the regeneration process that primarily colocalized with macrophages, but this expression was reduced in aged muscle. Our results indicate that ECM remodeling is impaired at early time points following muscle damage, likely a result of elevated expression of the major inhibitor of ECM breakdown, PAI-1, and consequent suppression of the macrophage, MMP-9, and myogenic responses.

Engineering Aligned Skeletal Muscle Tissue Using Decellularized Plant-Derived Scaffolds

To achieve organization and function, engineered tissues require a scaffold that supports cell adhesion, alignment, growth, and differentiation. For skeletal muscle tissue engineering, decellularization has been an approach for fabricating 3D scaffolds that retain biological architecture. While many decellularization approaches are focused on utilizing animal muscle as the starting material, decellularized plants are a potential source of highly structured cellulose-rich scaffolds. Here, we assessed the potential for a variety of decellularized plant scaffolds to promote mouse and human muscle cell alignment and differentiation. After decellularizing a range of fruits and vegetables, we identified the green-onion scaffold to have appropriate surface topography for generating highly confluent and aligned C2C12 and human skeletal muscle cells (HSMCs). The topography of the green-onion cellulose scaffold contained a repeating pattern of grooves that are approximately 20 μm wide by 10 μm deep. The outer white section of the green onion had a microstructure that guided C2C12 cell differentiation into aligned myotubes. Quantitative analysis of C2C12 and HSMC alignment revealed an almost complete anisotropic organization compared to 2D isotropic controls. Our results demonstrate that the decellularized green onion cellulose scaffolds, particularly from the outer white bulb segment, provide a simple and low-cost substrate to engineer aligned human skeletal muscle.

Manganese influx and expression of ZIP8 is essential in primary myoblasts and contributes to activation of SOD2

Trace elements such as copper (Cu), zinc (Zn), iron (Fe), and manganese (Mn) function as enzyme cofactors and second messengers in cell signaling. Trace elements are emerging as key regulators of differentiation and development of mammalian tissues including blood, brain, and skeletal muscle. We previously reported an influx of Cu and dynamic expression of metal transporters during differentiation of skeletal muscle cells. Here, we demonstrate that during differentiation of skeletal myoblasts an increase of Mn, Fe and Zn also occurs. Interestingly the Mn increase is concomitant with increased Mn-dependent SOD2 levels. To better understand the Mn import pathway in skeletal muscle cells, we probed the functional relevance of the closely related proteins ZIP8 and ZIP14, which are implicated in Zn, Mn, and Fe transport. Partial depletion of ZIP8 severely impaired growth of myoblasts and led to cell death under differentiation conditions, indicating that ZIP8-mediated metal transport is essential in skeletal muscle cells. Moreover, knockdown of Zip8 impaired activity of the Mn-dependent SOD2. Growth defects were partially rescued only by Mn supplementation to the medium, suggesting additional functions for ZIP8 in the skeletal muscle lineage. Restoring wild type Zip8 into the knockdown cells rescued the proliferation and differentiation phenotypes. On the other hand, knockdown of Zip14, had only a mild effect on myotube size, consistent with a role for ZIP14 in muscle hypertrophy. Simultaneous knockdown of both Zip8 and Zip14 further impaired differentiation and led cell death. This is the first report on the functional relevance of two members of the ZIP family of metal transporters in the skeletal muscle lineage, and further supports the paradigm that trace metal transporters are important modulators of mammalian tissue development.

Loss of the adipokine lipocalin-2 impairs satellite cell activation and skeletal muscle regeneration

Lipocalin-2 (LCN2) is an adipokine previously described for its contribution to numerous processes, including innate immunity and energy metabolism. LCN2 has also been demonstrated to be an extracellular matrix (ECM) regulator through its association with the ECM protease matrix metalloproteinase-9 (MMP-9). With the global rise in obesity and the associated comorbidities related to increasing adiposity, it is imperative to gain an understanding of the cross talk between adipose tissue and other metabolic tissues, such as skeletal muscle. Given the function of LCN2 on the ECM in other tissues and the importance of matrix remodeling in skeletal muscle regeneration, we examined the localization and expression of LCN2 in uninjured and regenerating wild-type skeletal muscle and assessed the impact of LCN2 deletion (LCN2^-/-) on skeletal muscle repair following cardiotoxin injury. Though LCN2 was minimally present in uninjured skeletal muscle, its expression was increased significantly at 1 and 2 days postinjury, with expression present in Pax7-positive satellite cells. Although satellite cell content was unchanged, the ability of quiescent satellite cells to become activated was significantly impaired in LCN2^-/- skeletal muscles. Skeletal muscle regeneration was also significantly compromised as evidenced by decreased embryonic myosin heavy chain expression and smaller regenerating myofiber areas. Consistent with a role for LCN2 in MMP-9 regulation, regenerating muscle also displayed a significant increase in fibrosis and lower ( P = 0.07) MMP-9 activity in LCN2^-/- mice at 2 days postinjury. These data highlight a novel role for LCN2 in muscle regeneration and suggest that changes in adipokine expression can significantly impact skeletal muscle repair.

Why is Skeletal Muscle Regeneration Impaired after Myonecrosis Induced by Viperid Snake Venoms?

Skeletal muscle regeneration after myonecrosis involves the activation, proliferation and fusion of myogenic cells, and a coordinated inflammatory response encompassing phagocytosis of necrotic cell debris, and the concerted synthesis of cytokines and growth factors. Myonecrosis often occurs in snakebite envenomings. In the case of venoms that cause myotoxicity without affecting the vasculature, such as those of many elapid snakes, regeneration proceeds successfully. In contrast, in envenomings by most viperid snakes, which affect the vasculature and extracellular matrix in addition to muscle fibers, regeneration is largely impaired and, therefore, the muscle mass is reduced and replaced by fibro-adipose tissue. This review discusses possible causes for such poor regenerative outcome including: (a) damage to muscle microvasculature, which causes tissue hypoxia and affects the inflammatory response and the timely removal of necrotic tissue; (b) damage to intramuscular nerves, which results in atrophy of regenerating fibers; (c) degradation of muscle cell basement membrane, compromising the spatial niche for proliferating myoblasts; (d) widespread degradation of the extracellular matrix; and (e) persistence of venom components in the damaged tissue, which may affect myogenic cells at critical points in the regenerative process. Understanding the causes of poor muscle regeneration may pave the way for the development of novel therapeutic interventions aimed at fostering the regenerative process in envenomed patients.

Changes in Mass and Performance in Rabbit Muscles after Muscle Damage with or without Transplantation of Primary Satellite Cells

Changes in morphology, metabolism, myosin heavy chain gene expression, and functional performances in damaged rabbit muscles with or without transplantation of primary satellite cells were investigated. For this purpose, we damaged bilaterally the fast muscle tibialis anterior (TA) with either 1.5 or 2.6 ml cardiotoxin 10–5 M injections. Primary cultures of satellite cells were autotransplanted unilaterally 5 days after muscle degeneration. Two months postoperation, the masses of damaged TAs, with or without transplantation, were significantly larger than those of the controls. Furthermore, damaged transplanted muscles weighed significantly more than damaged muscles only. The increase in muscle mass was essentially due to increased fiber size. These results were independent of the quantity of cardiotoxin injected into the muscles. Maximal forces were similar in control and 2.6 ml damaged TAs with or without satellite cell transfer. In contrast, 1.5 ml damaged TAs showed a significant decrease in maximal forces that reached the level of controls after transplantation of satellite cells. Fatigue resistance was similar in control and 1.5 ml damaged TAs independently of satellite cell transfer. Fatigue index was significantly higher in 2.6 ml damaged muscles with or without cell transplantation. These changes could be explained in part by muscle metabolism, which shifted towards oxidative activities, and by gene expression of myosin heavy chain isoforms, which presented an increase in type IIa and a decrease in type I and IIb in all damaged muscles with or without cell transfer. Under our experimental conditions, these results show that muscle damage rather than satellite cell transplantation changes muscle metabolism, myosin heavy chain isoform gene expression, and, to a lesser extent, muscle contractile properties. In contrast, muscle weight and fiber size are increased both by muscle damage and by satellite cell transfer.

Development of Approaches to Improve the Healing following Muscle Contusion

Muscle injuries are a challenging problem in traumatology, and the most frequent occurrence in sports medicine. Muscle contusions are among the most common muscle injuries. Although this injury is capable of healing, an incomplete functional recovery often occurs, depending on the severity of the blunt trauma. We have developed an animal model of muscle contusion in mice (high energy blunt trauma) and characterized the muscle's ability to heal following this injury using histology and immunohistochemistry to determine the level of muscle regeneration and the development of scar tissue. We have observed a massive muscle regeneration occurring in the first 2 wk postinjury that is subsequently followed by the development of muscle fibrosis. Based on these observations, we propose that the enhancement of muscle growth and regeneration, as well as the prevention of fibrotic development, could be used as approach(es) to improve the healing of muscle injuries. In fact, we have identified three growth factors (bFGF, IGF-1, and NGF) capable of enhancing myoblast proliferation and differentiation in vitro and improving the healing of the injured muscle in vivo. Furthermore, the ability of adenovirus to mediate direct and ex vivo gene transfer of β-galactosidase into the injured site opens possibilities of delivering an efficient and persistent expression of these growth factors in the injured muscle. These studies should help in the development of strategies to promote efficient muscle healing with complete functional recovery following muscle contusion. © 1998 Elsevier Science Inc.

Transplantation of Adipose Tissue-Derived Stromal Cells Increases Mass and Functional Capacity of Damaged Skeletal Muscle

The regenerating skeletal muscle environment is capable of inducing uncommitted progenitors to terminally differentiate. The aim of this work was to determine whether adipose tissue-derived stromal cells were able to participate in muscle regeneration and to characterize the effect on muscle mass and functional capacities after transplantation of these cells. Adipose tissue stromal cells labeled with Adv cyto LacZ from 3-day-old primary cultures (SVF1) were autotransplanted into damaged tibialis anterior muscles. Fifteen days later, β-galactosidase staining of regenerated fibers was detected, showing participation of these cells in muscle regeneration. Two months after SVF1 cell transfer, muscles were heavier, showed a significantly larger fiber section area, and developed a significantly higher maximal force compared with damaged control muscles. These results are similar to those previously obtained after satellite cell transplantation. However, SVF1 transfer also generated a small amount of adipose tissue localized along the needle course. To minimize these adipose contaminants, we transferred cells from 7-day-old secondary cultures of the SVF1, containing only a small proportion of already engaged preadipocytes (SVF2). Under these conditions, no adipose tissue was observed in regenerated muscle but there was also no effect on muscle performances compared with damaged control muscles. This result provides further evidence for the existence of progenitor cells in the stromal fraction of freshly isolated adipose tissue cells, which, under our conditions, keep some of their pluripotent properties in primary cultures.

Muscle Injuries and Repair: What's New on the Horizon!

Although we recognize the many advantages of improved musculoskeletal health, we also note that our ability to sustain this health and to maintain quality of life in an aging population is currently deficient. However, global efforts have produced numerous advances in tissue engineering and regenerative medicine that will collectively serve to fill this deficiency in the near future. The purpose of this review is to highlight our current knowledge, to outline our recent advances, and to discuss the evolving paradigms in skeletal muscle injury and repair.

Myostatin in black Muscovy duck (Cairina moschata): full-length cDNA cloning and age-dependent mRNA expression compared with IGF-I

Insulin-like growth factor-I (IGF-I) and myostatin (MSTN) are a pair of critical positive and negative growth regulators. The aim of the current study was to examine the age-dependent and muscle-specific expression of IGF-I and MSTN mRNAs in black Muscovy ducks in order to understand their roles in regulating the postnatal muscle growth of domestic ducks. The full-length cDNA of the black Muscovy duck MSTN gene was cloned and the age-dependent mRNA expression profile was compared with that of the IGF-I mRNA in skeletal muscles. The cDNA sequence of the MSTN gene was 1128 bp in length and encodes 375 amino acids, with more than 94.9% homology with poultry MSTN genes, and 83.0-92.0% homology with that of human and mammals (accession: KR006339.1). The IGF-I and MSTN mRNA expression exhibited opposite trends in age-dependency and in different muscles: IGF-I mRNA level was high in the early postnatal stage and low in the late mature stage, corresponding positively to growth; while the MSTN mRNA was low in the early stage, increased gradually and reached the highest level in mature muscles, and was negatively related to muscle growth. In the breast muscles, IGF-I mRNA was much higher than in the leg muscles; the opposite effect was seen in MSTN mRNA. These data suggest that the relative expression levels of IGF-I and MSTN are essential determinants in the temporal and muscle-specific regulation of postnatal skeletal muscle growth in Muscovy duck and possibly in other poultry species as well.

Insulin-like Growth Factor-II Delays Early but Enhances Late Regeneration of Skeletal Muscle

This study tested whether administration of insulin-like growth factor-II (IGF-II) enhances muscle regeneration. Rat biceps femoris muscle was damaged with notexin and then IGF-II was administered for up to 7 days. Results show that the proportion of nuclei containing or surrounded by immunoreactivity to MyoD, myogenin, and developmental myosin heavy chain (dMHC) is less in the IGF-II treatment group relative to the control group on days 1 (p=0.057), 2 (p=0.034), and 3 (p=0.047), respectively. This indicates a delay in muscle precursor cell (MPC) proliferation and differentiation with IGF-II administration. This effect was not associated with decreased binding capacity of the type 1 IGF receptor, as determined by receptor autoradiography in day 1 muscle sections (NS), but was associated with inhibition of phagocytic processes. The cross-sectional area of regenerating muscle fibers was significantly greater in the IGF-II treatment group than in the control group by day 7 (p=0.0092). The enhancing effect of IGF-II on late muscle regeneration, when the main process taking place is fiber enlargement, coincides with the period in which IGF-II is normally expressed by regenerating muscle, indicating that greater endogenous production of IGF-II would be associated with improved regeneration.

Early Regeneration of Whole Skeletal Muscle Grafts Is Unaffected by Overexpression of IGF-1 in MLC/mIGF-1 Transgenic Mice

Early myogenic events in regenerating whole muscle grafts were compared between transgenic MLC/mIGF-1 mice with skeletal muscle-specific overexpression of the Exon-1 Ea isoform of insulin-like growth factor-1 (mIGF-1) and control FVB mice, from day 3 to day 21 after transplantation. Immunocytochemistry with antibodies against desmin showed that skeletal muscle-specific overexpression of IGF-1 did not affect the pattern of myoblast activation or proliferation or the onset and number of myotubes formed in regenerating whole muscle grafts. Hypertrophied myotubes were observed in MLC/mIGF grafts at day 7 after transplantation, although such hypertrophy was transient, and the transgenic and control grafts had a similar appearance at later time points (days 10, 14, and 21). Immunostaining with antibodies to platelet endothelial cell adhesion molecule-1, which identifies endothelial cells, demonstrated no difference in the formation of new vascular network in grafts of transgenic and control mice. Skeletal muscle-specific overexpression of mIGF-1 does not appear to stimulate the early events associated with myogenesis during regeneration of whole muscle grafts. (J Histochem Cytochem 52:873–883, 2004)

Cryotherapy Reduces Inflammatory Response Without Altering Muscle Regeneration Process and Extracellular Matrix Remodeling of Rat Muscle

The application of cryotherapy is widely used in sports medicine today. Cooling could minimize secondary hypoxic injury through the reduction of cellular metabolism and injury area. Conflicting results have also suggested cryotherapy could delay and impair the regeneration process. There are no definitive findings about the effects of cryotherapy on the process of muscle regeneration. The aim of the present study was to evaluate the effects of a clinical-like cryotherapy on inflammation, regeneration and extracellular matrix (ECM) remodeling on the Tibialis anterior (TA) muscle of rats 3, 7 and 14 days post-injury. It was observed that the intermittent application of cryotherapy (three 30-minute sessions, every 2 h) in the first 48 h post-injury decreased inflammatory processes (mRNA levels of TNF-α, NF-κB, TGF-β and MMP-9 and macrophage percentage). Cryotherapy did not alter regeneration markers such as injury area, desmin and Myod expression. Despite regulating Collagen I and III and their growth factors, cryotherapy did not alter collagen deposition. In summary, clinical-like cryotherapy reduces the inflammatory process through the decrease of macrophage infiltration and the accumulation of the inflammatory key markers without influencing muscle injury area and ECM remodeling.

Strategies for skeletal muscle tissue engineering: seed vs. soil

The most commonly used tissue engineering approach includes the ex vivo combination of site-appropriate cell(s) and scaffold material(s) to create three-dimensional constructs for tissue replacement or reconstruction. These three-dimensional combinations are typically subjected to a period of culture and conditioning (i.e., self-assembly and maturation) to promote the development of ex vivo constructs which closely mimic native target tissue. This cell-based approach is challenged by the host response to the engineered tissue construct following surgical implantation. As an alternative to the cell-based approach, acellular biologic scaffolds attract endogenous cells and remodel into partially functional mimics of native tissue upon implantation. The present review examines cell-types (i.e., seed), scaffold materials (i.e., soil), and challenges associated with functional tissue engineering. Skeletal muscle is used as the target tissue prototype but the discussed principles will largely apply to most body systems.

Bit-1 is an essential regulator of myogenic differentiation

Muscle differentiation requires a complex signaling cascade that leads to the production of multinucleated myofibers. Genes regulating the intrinsic mitochondrial apoptotic pathway also function in controlling cell differentiation. How such signaling pathways are regulated during differentiation is not fully understood. Bit-1 (also known as PTRH2) mutations in humans cause infantile-onset multisystem disease with muscle weakness. We demonstrate here that Bit-1 controls skeletal myogenesis through a caspase-mediated signaling pathway. Bit-1-null mice exhibit a myopathy with hypotrophic myofibers. Bit-1-null myoblasts prematurely express muscle-specific proteins. Similarly, knockdown of Bit-1 expression in C2C12 myoblasts promotes early differentiation, whereas overexpression delays differentiation. In wild-type mice, Bit-1 levels increase during differentiation. Bit-1-null myoblasts exhibited increased levels of caspase 9 and caspase 3 without increased apoptosis. Bit-1 re-expression partially rescued differentiation. In Bit-1-null muscle, Bcl-2 levels are reduced, suggesting that Bcl-2-mediated inhibition of caspase 9 and caspase 3 is decreased. Bcl-2 re-expression rescued Bit-1-mediated early differentiation in Bit-1-null myoblasts and C2C12 cells with knockdown of Bit-1 expression. These results support an unanticipated yet essential role for Bit-1 in controlling myogenesis through regulation of Bcl-2.

Muscle repair: platelet-rich plasma derivates as a bridge from spontaneity to intervention

Muscle injuries account for between 10% and 55% of all sporting injuries. Although the skeletal muscle is a plastic organ capable of responding efficiently to environmental changes, the appropriate treatment of muscle injuries remains a daunting clinical challenge in sports medicine. There is considerable evidence to indicate that growth factors, such as transforming growth factor-β (TGFβ), hepatocyte growth factor (HGF) or insulin-like growth factor (IGF), and fibrin matrix are key in cellular events required for muscle repair and regeneration, namely myogenesis, angiogenesis and fibrogenesis. An innovative biological approach to the treatment of muscle injuries is the application of Plasma Rich in Growth Factors (PRGF) in intramuscular infiltrations. PRGF delivers growth factors, cytokines and adhesive proteins present in platelets and plasma, as well as other biologically-active proteins conveyed by the plasma, such as fibrinogen, prothrombin and fibronectin. This autologous, mimetic biomaterial embedded with a pool of growth factors acts as a smart dynamic scaffold, and should be applied taking into account a biological approach. A clinical trial is required to assess the functional repair outcome of PRGF infiltrations in muscle injuries.

In situ regeneration of skeletal muscle tissue through host cell recruitment

Standard reconstructive procedures for restoring normal function after skeletal muscle defects involve the use of existing host tissues such as muscular flaps. In many instances, this approach is not feasible and delays the rehabilitation process and restoration of tissue function. Currently, cell-based tissue engineering strategies have been used for reconstruction; however, donor tissue biopsy and ex vivo cell manipulation are required prior to implantation. The present study aimed to overcome these limitations by demonstrating mobilization of muscle cells into a target-specific site for in situ muscle regeneration. First, we investigated whether host muscle cells could be mobilized into an implanted scaffold. Poly(l-lactic acid) (PLLA) scaffolds were implanted in the tibialis anterior (TA) muscle of rats, and the retrieved scaffolds were characterized by examining host cell infiltration in the scaffolds. The host cell infiltrates, including Pax7+ cells, gradually increased with time. Second, we demonstrated that host muscle cells could be enriched by a myogenic factor released from the scaffolds. Gelatin-based scaffolds containing a myogenic factor were implanted in the TA muscle of rats, and the Pax7+ cell infiltration and newly formed muscle fibers were examined. By the second week after implantation, the Pax7+ cell infiltrates and muscle formation were significantly accelerated within the scaffolds containing insulin-like growth factor 1 (IGF-1). Our data suggest an ability of host stem cells to be recruited into the scaffolds with the capability of differentiating to muscle cells. In addition, the myogenic factor effectively promoted host cell recruitment, which resulted in accelerating muscle regeneration in situ.

MMP-10 is required for efficient muscle regeneration in mouse models of injury and muscular dystrophy

Matrix metalloproteinases (MMPs), a family of endopeptidases that are involved in the degradation of extracellular matrix components, have been implicated in skeletal muscle regeneration. Among the MMPs, MMP-2 and MMP-9 are upregulated in Duchenne muscular dystrophy (DMD), a fatal X-linked muscle disorder. However, inhibition or overexpression of specific MMPs in a mouse model of DMD (mdx) has yielded mixed results regarding disease progression, depending on the MMP studied. Here, we have examined the role of MMP-10 in muscle regeneration during injury and muscular dystrophy. We found that skeletal muscle increases MMP-10 protein expression in response to damage (notexin) or disease (mdx mice), suggesting its role in muscle regeneration. In addition, we found that MMP-10-deficient muscles displayed impaired recruitment of endothelial cells, reduced levels of extracellular matrix proteins, diminished collagen deposition, and decreased fiber size, which collectively contributed to delayed muscle regeneration after injury. Also, MMP-10 knockout in mdx mice led to a deteriorated dystrophic phenotype. Moreover, MMP-10 mRNA silencing in injured muscles (wild-type and mdx) reduced muscle regeneration, while addition of recombinant human MMP-10 accelerated muscle repair, suggesting that MMP-10 is required for efficient muscle regeneration. Furthermore, our data suggest that MMP-10-mediated muscle repair is associated with VEGF/Akt signaling. Thus, our findings indicate that MMP-10 is critical for skeletal muscle maintenance and regeneration during injury and disease.

MMP-14 is necessary but not sufficient for invasion of three-dimensional collagen by human muscle satellite cells

The twenty-five known matrix metalloproteases (MMPs) and their endogenous inhibitors, tissue inhibitors of metalloproteases (TIMPs), mediate cell invasion through the extracellular matrix (ECM). In a comparative three-dimensional assay, we analyzed human and mouse satellite cells' competence to invade an artificial ECM (collagen I). We identified a single MMP that 1) is expressed by human muscle satellite cells; 2) is induced at the mRNA/protein level by adhesion to collagen I; and 3) is necessary for invasion into a collagen I matrix. Interestingly, murine satellite cells neither express this MMP, nor invade the collagen matrix. However, exogenous human MMP-14 is not sufficient to induce invasion of a collagen matrix by murine cells, emphasizing species differences.

Dystropathology increases energy expenditure and protein turnover in the mdx mouse model of duchenne muscular dystrophy

The skeletal muscles in Duchenne muscular dystrophy and the mdx mouse model lack functional dystrophin and undergo repeated bouts of necrosis, regeneration, and growth. These processes have a high metabolic cost. However, the consequences for whole body energy and protein metabolism, and on the dietary requirements for these macronutrients at different stages of the disease, are not well-understood. This study used juvenile (4- to 5- wk-old) and adult (12- to 14-wk-old) male dystrophic C57BL/10ScSn-mdx/J and age-matched C57BL/10ScSn/J control male mice to measure total and resting energy expenditure, food intake, spontaneous activity, body composition, whole body protein turnover, and muscle protein synthesis rates. In juvenile mdx mice that have extensive muscle damage, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were higher than in age-matched controls. Adaptations in food intake and decreased activity were insufficient to meet the increased energy and protein needs of juvenile mdx mice and resulted in stunted growth. In (non-growing) adult mdx mice with less severe dystropathology, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were also higher than in age-matched controls. Food intake was sufficient to meet their protein and energy needs, but insufficient to result in fat deposition. These data show that dystropathology impacts the protein and energy needs of mdx mice and that tailored dietary interventions are necessary to redress this imbalance. If not met, the resultant imbalance blunts growth, and may limit the benefits of therapies designed to protect and repair dystrophic muscles.

Experimental model for the study of the effects of platelet-rich plasma on the early phases of muscle healing

BACKGROUND

There is abundant evidence suggesting that growth factors may play a key role in the healing process, especially in the early stages of inflammation. Despite the reported clinical successes with the use of growth factors there is still a lack of knowledge on the biological mechanism underlying the activity of platelet-rich plasma during the process of muscle healing. The aim of this study was to analyse the early effects of platelet- rich plasma in an easily reproducible animal model.

MATERIALS AND METHODS

Wistar male adult rats (n=102) were used in this study. The muscle lesion was created with a scalpel in the flexor sublimis muscles. Platelet-rich plasma was applied immediately after surgery. Treated, untreated and contralateral muscles were analysed by morphological evaluation and western blot assay.

RESULTS

Leucocyte infiltration was significantly greater in muscles treated with platelet-rich plasma than in both untreated and contralateral muscles. The latter showed greater leucocyte infiltration when compared to the untreated muscles. Platelet-rich plasma treatment also modified the cellular composition of the leucocyte infiltration leading to increased expression of CD3, CD8, CD19 and CD68 and to decreased CD4 antigen expression in both platelet-rich plasma treated and contralateral muscles. Blood vessel density and blood vessel diameters were not statistically significantly different between the three groups analysed.

DISCUSSION

The results of this study showed that treatment with platelet-rich plasma magnified the physiological early inflammatory response following a muscle injury, modifying the pattern of cellular recruitment. Local platelet-rich plasma treatment may exert a direct or, more plausibly, indirect systemic effect on healing processes, at least in the earliest inflammatory phase.

Adeno-associated virus serotype 8 (AAV8) delivery of recombinant A20 to skeletal muscle reduces pathological activation of nuclear factor (NF)-κB in muscle of mdx mice

Duchenne muscular dystrophy (DMD) is a genetic muscle disease caused by the absence of a functional dystrophin protein. Lack of dystrophin protein disrupts the dystrophin-glycoprotein complex causing muscle membrane instability and degeneration. One of the secondary manifestations resulting from lack of functional dystrophin in muscle tissue is an increased level of cytokines that recruit inflammatory cells, leading to chronic upregulation of the nuclear factor (NF)-κB. Negative regulators of the classical NF-κB pathway improve muscle health in the mdx mouse model for DMD. We have previously shown in vitro that a negative regulator of the NF-κB pathway, A20, plays a role in muscle regeneration. Here, we show that overexpression of A20 by using a muscle-specific promoter delivered with an adeno-associated virus serotype 8 (AAV8) vector to the mdx mouse decreases activation of the NF-κB pathway in skeletal muscle. Recombinant A20 expression resulted in a reduction in number of fibers with centrally placed nuclei and a reduction in the number of T cells infiltrating muscle transduced with the AAV8-A20 vector. Taken together, we conclude that overexpression of A20 in mdx skeletal muscle provides improved muscle health by reduction of chronic inflammation and muscle degeneration. These results suggest A20 is a potential therapeutic target to ameliorate symptoms of DMD.

Sequential identification of a degradable phosphate glass scaffold for skeletal muscle regeneration

Tissue engineering has the potential to overcome limitations associated with current management of skeletal muscle defects. This study aimed to sequentially identify a degradable phosphate glass scaffold for the restoration of muscle defects. A series of glass compositions were investigated for the potential to promote bacterial growth. Thereafter, the response of human craniofacial muscle-derived cells was determined. Glass compositions containing Fe4- and 5 mol% did not promote greater Staphylococcus aureus and Staphylococcus epidermidis growth compared to the control (p > 0.05). Following confirmation of myogenicity, further studies assessed the biocompatibility of glasses containing Fe5 mol%. Cells seeded on collagen-coated disks demonstrated comparable cellular metabolic activity to control. Upregulation of genes encoding for myogenic regulatory factors (MRFs) confirmed myofibre formation and there was expression of developmental MYH genes. The use of 3-D aligned fibre scaffolds supported unidirectional cell alignment and upregulation of MRF and developmental MYH genes. Compared to the 2-D disks, there was also expression of MYH2 and MYH7 genes, indicating further myofibre maturation on the 3-D scaffolds and confirming the importance of key biophysical cues.

Alterations of biochemical marker levels and myonuclear numbers in rat skeletal muscle after ischemia-reperfusion

Prolonged ischemia-reperfusion results in various damages in skeletal muscle. Following reperfusion, although the damaged muscles undergo regeneration, the precise process and mechanism of regeneration have not yet been fully understood. Here, we show the altered levels of plasma biochemical markers of muscle damage, and the change in myonuclear numbers in adult rat skeletal muscle by ischemia-reperfusion. Male Wistar rats were subjected to unilateral hindlimb ischemia by clamping the anterior tibial artery for 2 h before reperfusion. Both plasma creatine kinase activity and C-reactive protein levels in plasma were increased significantly at 0.5 h of reperfusion and returned to the control level at 24 h. The transverse sectional area of muscle belly of the anterior tibial muscles in ischemic side was significantly decreased by 20 % compared with those in sham-ischemic (control) side at 2 days, and returned to the control level at 5 days of reperfusion. Moreover, the number of interstitial nuclei in the ischemic side were significantly increased at 5-14 days and returned to the control level at 21 days of reperfusion. Central nuclei that are specifically observed in regenerating muscle, appeared at 5 days, reached a peak at 14 days, and disappeared at 28 days of reperfusion. Furthermore, MyoD, a regulatory factor for myogenesis, showed a transient expression at 5 days of reperfusion. These results indicate that, although the size of muscle seems to be recovered by 5 days of reperfusion, the most active muscle regeneration occurs much later, as shown by the increase in central nuclei.

Biological approaches to improve skeletal muscle healing after injury and disease

Skeletal muscle injury and repair are complex processes, including well-coordinated steps of degeneration, inflammation, regeneration, and fibrosis. We have reviewed the recent literature including studies by our group that describe how to modulate the processes of skeletal muscle repair and regeneration. Antiinflammatory drugs that target cyclooxygenase-2 were found to hamper the skeletal muscle repair process. Muscle regeneration phase can be aided by growth factors, including insulin-like growth factor-1 and nerve growth factor, but these factors are typically short-lived, and thus more effective methods of delivery are needed. Skeletal muscle damage caused by traumatic injury or genetic diseases can benefit from cell therapy; however, the majority of transplanted muscle cells (myoblasts) are unable to survive the immune response and hypoxic conditions. Our group has isolated neonatal skeletal muscle derived stem cells (MDSCs) that appear to repair muscle tissue in a more effective manner than myoblasts, most likely due to their better resistance to oxidative stress. Enhancing antioxidant levels of MDSCs led to improved regenerative potential. It is becoming increasingly clear that stem cells tissue repair by direct differentiation and paracrine effects leading to neovascularization of injured site and chemoattraction of host cells. The factors invoked in paracrine action are still under investigation. Our group has found that angiotensin II receptor blocker (losartan) significantly reduces fibrotic tissue formation and improves repair of murine injured muscle. Based on these data, we have conducted a case study on two hamstring injury patients and found that losartan treatment was well tolerated and possibly improved recovery time. We believe this medication holds great promise to optimize muscle repair in humans.

Use of quantitative membrane proteomics identifies a novel role of mitochondria in healing injured muscles

Skeletal muscles are proficient at healing from a variety of injuries. Healing occurs in two phases, early and late phase. Early phase involves healing the injured sarcolemma and restricting the spread of damage to the injured myofiber. Late phase of healing occurs a few days postinjury and involves interaction of injured myofibers with regenerative and inflammatory cells. Of the two phases, cellular and molecular processes involved in the early phase of healing are poorly understood. We have implemented an improved sarcolemmal proteomics approach together with in vivo labeling of proteins with modified amino acids in mice to study acute changes in the sarcolemmal proteome in early phase of myofiber injury. We find that a notable early phase response to muscle injury is an increased association of mitochondria with the injured sarcolemma. Real-time imaging of live myofibers during injury demonstrated that the increased association of mitochondria with the injured sarcolemma involves translocation of mitochondria to the site of injury, a response that is lacking in cultured myoblasts. Inhibiting mitochondrial function at the time of injury inhibited healing of the injured myofibers. This identifies a novel role of mitochondria in the early phase of healing injured myofibers.

Experimental muscle injury: correlation between ultrasound and histological findings

INTRODUCTION

In this study we correlated ultrasound findings with histological changes taking place during experimentally induced degeneration-regeneration in rat skeletal muscle.

METHODS

Gastrocnemius muscles were injected with mepivacaine, and the progress of the muscle injury was monitored by ultrasound from day 1 to day 20. Muscles were extracted on the same days for histological examination.

RESULTS

The degenerative phase was characterized by increased echogenicity in the injured area; thereafter, echogenicity gradually diminished during the regenerative phase, attaining normal levels by 20 days postinjection. By this stage, histological examination revealed that regeneration was complete. The heteroechoic texture observed from day 4 to day 10 appeared to reflect the coexistence of degenerative and regenerative processes.

CONCLUSIONS

The results suggest that the degenerative and regenerative phases of muscle injury may be distinguished sonographically through differences in echogenicity and echotexture and, using Doppler ultrasound, differences in the degree of vascularization.