Phagocytosis of necrotic muscle in muscle isografts is influenced by the strain, age, and sex of host mice

Immunoengineering Biomaterials for Musculoskeletal Tissue Repair across Lifespan

Musculoskeletal diseases and injuries are among the leading causes of pain and morbidity worldwide. Broad efforts have focused on developing pro-regenerative biomaterials to treat musculoskeletal conditions; however, these approaches have yet to make a significant clinical impact. Recent studies have demonstrated that the immune system is central in orchestrating tissue repair and that targeting pro-regenerative immune responses can improve biomaterial therapeutic outcomes. However, aging is a critical factor negatively affecting musculoskeletal tissue repair and immune function. Hence, understanding how age affects the response to biomaterials is essential for improving musculoskeletal biomaterial therapies. This review focuses on the intersection of the immune system and aging in response to biomaterials for musculoskeletal tissue repair. The article introduces the general impacts of aging on tissue physiology, the immune system, and the response to biomaterials. Then, it explains how the adaptive immune system guides the response to injury and biomaterial implants in cartilage, muscle, and bone and discusses how aging impacts these processes in each tissue type. The review concludes by highlighting future directions for the development and translation of personalized immunomodulatory biomaterials for musculoskeletal tissue repair.

Resolution of Inflammation after Skeletal Muscle Ischemia-Reperfusion Injury: A Focus on the Lipid Mediators Lipoxins, Resolvins, Protectins and Maresins

Skeletal muscle ischemia reperfusion is very frequent in humans and results not only in muscle destruction but also in multi-organ failure and death via systemic effects related to inflammation and oxidative stress. In addition to overabundance of pro-inflammatory stimuli, excessive and uncontrolled inflammation can also result from defects in resolution signaling. Importantly, the resolution of inflammation is an active process also based on specific lipid mediators including lipoxins, resolvins and maresins that orchestrate the potential return to tissue homeostasis. Thus, lipid mediators have received growing attention since they dampen deleterious effects related to ischemia–reperfusion. For instance, the treatment of skeletal muscles with resolvins prior to ischemia decreases polymorphonuclear leukocyte (PMN) infiltration. Additionally, remote alterations in lungs or kidneys are reduced when enhancing lipid mediators’ functions. Accordingly, lipoxins prevented oxidative-stress-mediated tissue injuries, macrophage polarization was modified and in mice lacking DRV2 receptors, ischemia/reperfusion resulted in excessive leukocyte accumulation. In this review, we first aimed to describe the inflammatory response during ischemia and reperfusion in skeletal muscle and then discuss recent discoveries in resolution pathways. We focused on the role of specialized pro-resolving mediators (SPMs) derived from polyunsaturated fatty acids (PUFAs) and their potential therapeutic applications.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Disrupted macrophage metabolic reprogramming in aged soleus muscle during early recovery following disuse atrophy

Aged skeletal muscle is characterized by poor muscle recovery following disuse coinciding with an impaired muscle pro-inflammatory macrophage response. Macrophage inflammatory status is regulated by its metabolic state, but little is understood of macrophage metabolism and its relation to macrophage inflammation in the context of muscle recovery and aging. Therefore, the purpose of this study was to thoroughly characterize macrophage metabolism and inflammation in aged muscle during early recovery following disuse atrophy using single cell transcriptomics and functional assays. Young (4-5 months) and old (20-22 months) male C57BL/6 mice underwent 14 days of hindlimb unloading followed by 4 days of ambulatory recovery. CD45+ cells were isolated from solei muscles and analyzed using 10x Genomics single cell RNA sequencing. We found that aged pro-inflammatory macrophage clusters were characterized with an impaired inflammatory and glycolytic transcriptome, and this dysregulation was accompanied by a suppression of HIF-1α and its immediate downstream target, Glut1. As a follow-up, bone marrow-derived macrophages were isolated from a separate cohort of young and old mice at 4-d recovery and were polarized to a pro-inflammatory phenotype and used for glycolysis stress test, phagocytosis activity assay, and targeted GC-MS metabolomics. Aged bone marrow-derived pro-inflammatory macrophages were characterized with impaired glycolysis and phagocytosis function, decreased succinate and an accumulation of glycolytic metabolic intermediates overall supporting reduced glycolytic flux and macrophage function. Our results indicate that the metabolic reprograming and function of aged skeletal muscle pro-inflammatory macrophages are dysfunctional during early recovery from disuse atrophy possibly attributing to attenuated regrowth.

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

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

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.

Mechanisms Regulating Muscle Regeneration: Insights into the Interrelated and Time-Dependent Phases of Tissue Healing

Despite a massive body of knowledge which has been produced related to the mechanisms guiding muscle regeneration, great interest still moves the scientific community toward the study of different aspects of skeletal muscle homeostasis, plasticity, and regeneration. Indeed, the lack of effective therapies for several physiopathologic conditions suggests that a comprehensive knowledge of the different aspects of cellular behavior and molecular pathways, regulating each regenerative stage, has to be still devised. Hence, it is important to perform even more focused studies, taking the advantage of robust markers, reliable techniques, and reproducible protocols. Here, we provide an overview about the general aspects of muscle regeneration and discuss the different approaches to study the interrelated and time-dependent phases of muscle healing.

Fibrillar Collagen Organization Associated with Broiler Wooden Breast Fibrotic Myopathy

Wooden breast (WB) is a fibrotic myopathy affecting the pectoralis major (p. major) muscle in fast-growing commercial broiler lines. Birds with WB are phenotypically detected by the palpation of a hard p. major muscle. A primary feature of WB is the fibrosis of muscle with the replacement of muscle fibers with extracellular matrix proteins, such as collagen. The ability of a tissue to be pliable and stretch is associated with the organization of collagen fibrils in the connective tissue areas surrounding muscle fiber bundles (perimysium) and around individual muscle fibers (endomysium). The objective of this study was to compare the structure and organization of fibrillar collagen by using transmission electron microscopy in two fast-growing broiler lines (Lines A and B) with incidence of WB to a slower growing broiler Line C with no phenotypically detectable WB. In Line A, the collagen fibrils were tightly packed in a parallel organization, whereas in Line B, the collagen fibrils were randomly aligned. Tightly packed collagen fibrils arranged in parallel are associated with nonpliable collagen that is highly cross-linked. This will lead to a phenotypically hard p. major muscle. In Line C, the fibrillar collagen was sparse in its distribution. Furthermore, the average collagen fibril diameter and banding D-period length were altered in Line A p. major muscles affected with WB. Taken together, these data are suggestive of different fibrotic myopathies beyond just what is classified as WB in fast-growing broiler lines.

Immunology Guides Skeletal Muscle Regeneration

Soft tissue trauma of skeletal muscle is one of the most common side effects in surgery. Muscle injuries are not only caused by accident-related injuries but can also be of an iatrogenic nature as they occur during surgical interventions when the anatomical region of interest is exposed. If the extent of trauma surpasses the intrinsic regenerative capacities, signs of fatty degeneration and formation of fibrotic scar tissue can occur, and, consequentially, muscle function deteriorates or is diminished. Despite research efforts to investigate the physiological healing cascade following trauma, our understanding of the early onset of healing and how it potentially determines success or failure is still only fragmentary. This review focuses on the initial physiological pathways following skeletal muscle trauma in comparison to bone and tendon trauma and what conclusions can be drawn from new scientific insights for the development of novel therapeutic strategies. Strategies to support regeneration of muscle tissue after injury are scarce, even though muscle trauma has a high incidence. Based on tissue specific differences, possible clinical treatment options such as local immune-modulatory and cell therapeutic approaches are suggested that aim to support the endogenous regenerative potential of injured muscle tissues.

"Known Unknowns": Current Questions in Muscle Satellite Cell Biology

Our understanding of satellite cells, now known to be the obligate stem cells of skeletal muscle, has increased dramatically in recent years due to the introduction of new molecular, genetic, and technical resources. In addition to their role in acute repair of damaged muscle, satellite cells are of interest in the fields of aging, exercise, neuromuscular disease, and stem cell therapy, and all of these applications have driven a dramatic increase in our understanding of the activity and potential of satellite cells. However, many fundamental questions of satellite cell biology remain to be answered, including their emergence as a specific lineage, the degree and significance of heterogeneity within the satellite cell population, the roles of their interactions with other resident and infiltrating cell types during homeostasis and regeneration, and the relative roles of intrinsic vs extrinsic factors that may contribute to satellite cell dysfunction in the context of aging or disease. This review will address the current state of these open questions in satellite cell biology.

Determination of cell fate in skeletal muscle following BMP gene transfer by in vivo electroporation

We previously developed a novel method for gene transfer, which combined a non-viral gene expression vector with transcutaneous in vivo electroporation. We applied this method to transfer the bone morphogenetic protein (BMP) gene and induce ectopic bone formation in rat skeletal muscles. At present, it remains unclear which types of cells can differentiate into osteogenic cells after BMP gene transfer by in vivo electroporation. Two types of stem cells in skeletal muscle can differentiate into osteogenic cells: muscle-derived stem cells, and bone marrow-derived stem cells in the blood. In the present study, we transferred the BMP gene into rat skeletal muscles. We then stained tissues for several muscle-derived stem cell markers (e.g., Pax7, M-cadherin), muscle regeneration-related markers (e.g., Myod1, myogenin), and an inflammatory cell marker (CD68) to follow cell differentiation over time. Our results indicate that, in the absence of BMP, the cell population undergoes muscle regeneration, whereas in its presence, it can differentiate into osteogenic cells. Commitment towards either muscle regeneration or induction of ectopic bone formation appears to occur five to seven days after BMP gene transfer.

Sex Differences in Muscle Wasting

With aging and other muscle wasting diseases, men and women undergo similar pathological changes in skeletal muscle: increased inflammation, enhanced oxidative stress, mitochondrial dysfunction, satellite cell senescence, elevated apoptosis and proteasome activity, and suppressed protein synthesis and myocyte regeneration. Decreased food intake and physical activity also indirectly contribute to muscle wasting. Sex hormones also play important roles in maintaining skeletal muscle homeostasis. Testosterone is a potent anabolic factor promoting muscle protein synthesis and muscular regeneration. Estrogens have a protective effect on skeletal muscle by attenuating inflammation; however, the mechanisms of estrogen action in skeletal muscle are less well characterized than those of testosterone. Age- and/or disease-induced alterations in sex hormones are major contributors to muscle wasting. Hence, men and women may respond differently to catabolic conditions because of their hormonal profiles. Here we review the similarities and differences between men and women with common wasting conditions including sarcopenia and cachexia due to cancer, end-stage renal disease/chronic kidney disease, liver disease, chronic heart failure, and chronic obstructive pulmonary disease based on the literature in clinical studies. In addition, the responses in men and women to the commonly used therapeutic agents and their efficacy to improve muscle mass and function are also reviewed.

Exercise Intensity and Recovery: Biomarkers of Injury, Inflammation, and Oxidative Stress

Biomarkers of inflammation, muscle damage, and oxidative stress after high-intensity exercise have been described previously; however, further understanding of their role in the postexercise recovery period is necessary. Because these markers have been implicated in cell signaling, they may be specifically related to the training adaptations induced by high-intensity exercise. Thus, a clear model showing their responses to exercise may be useful in characterizing the relative recovery status of an athlete. The purpose of this study was twofold: (a) to investigate the time course of markers of muscle damage and inflammation in the blood from 3 to 72 hours after combined training exercises and (b) to investigate indicators of oxidative stress and damage associated with increased reactive oxygen species production during high-intensity exercise in elite athletes. Nineteen male athletes performed a combination of high-intensity aerobic and anaerobic training exercises. Samples were acquired immediately before and at 3, 6, 12, 24, 48, and 72 hours after exercise. The appearance and clearance of creatine kinase and lactate dehydrogenase in the blood occurred faster than previous studies have reported. The neutrophil/lymphocyte ratio summarizes the mobilization of 2 leukocyte subpopulations in a single marker and may be used to predict the end of the postexercise recovery period. Further analysis of the immune response using serum cytokines indicated that high-intensity exercise performed by highly trained athletes only generated inflammation that was localized to the skeletal muscle. Biomarkers are not a replacement for performance tests, but when used in conjunction, they may offer a better indication of metabolic recovery status. Therefore, the use of biomarkers can improve a coach's ability to assess the recovery period after an exercise session and to establish the intensity of subsequent training sessions.

Comparative Study of Injury Models for Studying Muscle Regeneration in Mice

Background

A longstanding goal in regenerative medicine is to reconstitute functional tissus or organs after injury or disease. Attention has focused on the identification and relative contribution of tissue specific stem cells to the regeneration process. Relatively little is known about how the physiological process is regulated by other tissue constituents. Numerous injury models are used to investigate tissue regeneration, however, these models are often poorly understood. Specifically, for skeletal muscle regeneration several models are reported in the literature, yet the relative impact on muscle physiology and the distinct cells types have not been extensively characterised.

Methods

We have used transgenic Tg:Pax7nGFP and Flk1^GFP/+ mouse models to respectively count the number of muscle stem (satellite) cells (SC) and number/shape of vessels by confocal microscopy. We performed histological and immunostainings to assess the differences in the key regeneration steps. Infiltration of immune cells, chemokines and cytokines production was assessed in vivo by Luminex^®.

Results

We compared the 4 most commonly used injury models i.e. freeze injury (FI), barium chloride (BaCl₂), notexin (NTX) and cardiotoxin (CTX). The FI was the most damaging. In this model, up to 96% of the SCs are destroyed with their surrounding environment (basal lamina and vasculature) leaving a “dead zone” devoid of viable cells. The regeneration process itself is fulfilled in all 4 models with virtually no fibrosis 28 days post-injury, except in the FI model. Inflammatory cells return to basal levels in the CTX, BaCl₂ but still significantly high 1-month post-injury in the FI and NTX models. Interestingly the number of SC returned to normal only in the FI, 1-month post-injury, with SCs that are still cycling up to 3-months after the induction of the injury in the other models.

Conclusions

Our studies show that the nature of the injury model should be chosen carefully depending on the experimental design and desired outcome. Although in all models the muscle regenerates completely, the trajectories of the regenerative process vary considerably. Furthermore, we show that histological parameters are not wholly sufficient to declare that regeneration is complete as molecular alterations (e.g. cycling SCs, cytokines) could have a major persistent impact.

Hepatocyte Growth Factor and Satellite Cell Activation

Satellite cells are the "currency" for the muscle growth that is critical to meat production in many species, as well as to phenotypic distinctions in development at the level of species or taxa, and for human muscle growth, function and regeneration. Careful research on the activation and behaviour of satellite cells, the stem cells in skeletal muscle, including cross-species comparisons, has potential to reveal the mechanisms underlying pathological conditions in animals and humans, and to anticipate implications of development, evolution and environmental change on muscle function and animal performance.

Mitochondrial Regulation of the Muscle Microenvironment in Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe clinical presentation of peripheral arterial disease and manifests as chronic limb pain at rest and/or tissue necrosis. Current clinical interventions are largely ineffective and therapeutic angiogenesis based trials have shown little efficacy, highlighting the dire need for new ideas and novel therapeutic approaches. Despite a decade of research related to skeletal muscle as a determinant of morbidity and mortality outcomes in CLI, very little progress has been made toward an effective therapy aimed directly at the muscle myopathies of this disease. Within the muscle cell, mitochondria are well positioned to modulate the ischemic cellular response, as they are the principal sites of cellular energy production and the major regulators of cellular redox charge and cell death. In this mini review, we update the crucial importance of skeletal muscle to CLI pathology and examine the evolving influence of muscle and endothelial cell mitochondria in the complex ischemic microenvironment. Finally, we discuss the novelty of muscle mitochondria as a therapeutic target for ischemic pathology in the context of the complex co-morbidities often associated with CLI.

Effects of sex steroids on bones and muscles: Similarities, parallels, and putative interactions in health and disease

Estrogens and androgens influence the growth and maintenance of bones and muscles and are responsible for their sexual dimorphism. A decline in their circulating levels leads to loss of mass and functional integrity in both tissues. In the article, we highlight the similarities of the molecular and cellular mechanisms of action of sex steroids in the two tissues; the commonality of a critical role of mechanical forces on tissue mass and function; emerging evidence for an interplay between mechanical forces and hormonal and growth factor signals in both bones and muscles; as well as the current state of evidence for or against a cross-talk between muscles and bone. In addition, we review evidence for the parallels in the development of osteoporosis and sarcopenia with advancing age and the potential common mechanisms responsible for the age-dependent involution of these two tissues. Lastly, we discuss the striking difference in the availability of several drug therapies for the prevention and treatment of osteoporosis, as compared to none for sarcopenia. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Modulating exercise-induced hormesis: Does less equal more?

Hormesis encompasses the notion that low levels of stress stimulate or upregulate existing cellular and molecular pathways that improve the capacity of cells and organisms to withstand greater stress. This notion underlies much of what we know about how exercise conditions the body and induces long-term adaptations. During exercise, the body is exposed to various forms of stress, including thermal, metabolic, hypoxic, oxidative, and mechanical stress. These stressors activate biochemical messengers, which in turn activate various signaling pathways that regulate gene expression and adaptive responses. Historically, antioxidant supplements, nonsteroidal anti-inflammatory drugs, and cryotherapy have been favored to attenuate or counteract exercise-induced oxidative stress and inflammation. However, reactive oxygen species and inflammatory mediators are key signaling molecules in muscle, and such strategies may mitigate adaptations to exercise. Conversely, withholding dietary carbohydrate and restricting muscle blood flow during exercise may augment adaptations to exercise. In this review article, we combine, integrate, and apply knowledge about the fundamental mechanisms of exercise adaptation. We also critically evaluate the rationale for using interventions that target these mechanisms under the overarching concept of hormesis. There is currently insufficient evidence to establish whether these treatments exert dose-dependent effects on muscle adaptation. However, there appears to be some dissociation between the biochemical/molecular effects and functional/performance outcomes of some of these treatments. Although several of these treatments influence common kinases, transcription factors, and proteins, it remains to be determined if these interventions complement or negate each other, and whether such effects are strong enough to influence adaptations to exercise.

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.

The Basis of Muscle Regeneration

Muscle regeneration recapitulates many aspects of embryonic myogenesis and is an important homeostatic process of the adult skeletal muscle, which, after development, retains the capacity to regenerate in response to appropriate stimuli, activating the muscle compartment of stem cells, namely, satellite cells, as well as other precursor cells. Moreover, significant evidence suggests that while stem cells represent an important determinant for tissue regeneration, a “qualified” environment is necessary to guarantee and achieve functional results. It is therefore plausible that the loss of control over these cell fate decisions could lead to a pathological transdifferentiation, leading to pathologic defects in the regenerative process. This review provides an overview about the general aspects of muscle development and discusses the cellular and molecular aspects that characterize the five interrelated and time-dependent phases of muscle regeneration, namely, degeneration, inflammation, regeneration, remodeling, and maturation/functional repair.

Advancements in stem cells treatment of skeletal muscle wasting

Muscular dystrophies (MDs) are a heterogeneous group of inherited disorders, in which progressive muscle wasting and weakness is often associated with exhaustion of muscle regeneration potential. Although physiological properties of skeletal muscle tissue are now well known, no treatments are effective for these diseases. Muscle regeneration was attempted by means transplantation of myogenic cells (from myoblast to embryonic stem cells) and also by interfering with the malignant processes that originate in pathological tissues, such as uncontrolled fibrosis and inflammation. Taking into account the advances in the isolation of new subpopulation of stem cells and in the creation of artificial stem cell niches, we discuss how these emerging technologies offer great promises for therapeutic approaches to muscle diseases and muscle wasting associated with aging.

Aged skeletal muscle retains the ability to fully regenerate functional architecture

While the general understanding of muscle regenerative capacity is that it declines with increasing age due to impairments in the number of muscle progenitor cells and interaction with their niche, studies vary in their model of choice, indices of myogenic repair, muscle of interest and duration of studies. We focused on the net outcome of regeneration, functional architecture, compared across three models of acute muscle injury to test the hypothesis that satellite cells maintain their capacity for effective myogenic regeneration with age. Muscle regeneration in extensor digitorum longus muscle (EDL) of young (3 mo-old), old (22 mo-old) and senescent female mice (28 mo-old) was evaluated for architectural features, fiber number and central nucleation, weight, collagen and fat deposition. The 3 injury paradigms were: a myotoxin (notexin) which leaves the blood vessels and nerves intact, freezing (FI) that damages local muscle, nerve and blood vessels and denervation-devascularization (DD) which dissociates the nerves and blood vessels from the whole muscle. Histological analyses revealed successful architectural regeneration following notexin injury with negligible fibrosis and fully restored function, regardless of age. In comparison, the regenerative response to injuries that damaged the neurovascular supply (FI and DD) was less effective, but similar across the ages. The focus on net regenerative outcome demonstrated that old and senescent muscle has a robust capacity to regenerate functional architecture.

Satellite cells and the muscle stem cell niche

Adult skeletal muscle in mammals is a stable tissue under normal circumstances but has remarkable ability to repair after injury. Skeletal muscle regeneration is a highly orchestrated process involving the activation of various cellular and molecular responses. As skeletal muscle stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation of satellite cells not only maintains the stem cell population but also provides numerous myogenic cells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitution of a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscle regeneration is tightly regulated through the dynamic interplay between intrinsic factors within satellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. For the last half century, the advance of molecular biology, cell biology, and genetics has greatly improved our understanding of skeletal muscle biology. Here, we review some recent advances, with focuses on functions of satellite cells and their niche during the process of skeletal muscle regeneration.

Loss of STAT1 in bone marrow-derived cells accelerates skeletal muscle regeneration

Background

Skeletal muscle regeneration is a complex process which is not yet completely understood. Evidence suggested that the Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway may have a role in myogenesis. In this study, we aim to explore the possible role of STAT1 in muscle regeneration.

Methods

Wild-type and STAT1 knockout mice were used in this study. Tibialis anterior muscle injury was conducted by cardiotoxin (CTX) injection. Bone marrow transplantation and glucocorticoid treatment were performed to manipulate the immune system of the mice.

Results

Muscle regeneration was accelerated in STAT1−/− mice after CTX injury. Bone marrow transplantation experiments showed that the regeneration process relied on the type of donor mice rather than on recipient mice. Levels of pro-inflammatory cytokines, TNFα and IL-1β, were significantly higher in STAT1−/− mice at 1 day and/or 2 days post-injury, while levels of anti-inflammatory cytokine, IL-10, were lower in STAT1−/− mice at 2 days and 3 days post-injury. Levels of IGF-1 were significantly higher in the STAT1−/− mice at 1 day and 2 days post-injury. Furthermore, the muscle regeneration process was inhibited in glucocorticoid-treated mice.

Conclusions

Loss of STAT1 in bone marrow–derived cells accelerates skeletal muscle regeneration.

Skeletal muscle-specific genetic determinants contribute to the differential strain-dependent effects of hindlimb ischemia in mice

Genetics plays an important role in determining peripheral arterial disease (PAD) pathology, which causes a spectrum of clinical disorders that range from clinically silent reductions in blood flow to limb-threatening ischemia. The cell-type specificity of PAD pathology, however, has received little attention. To determine whether strain-dependent differences in skeletal muscle cells might account for the differential responses to ischemia observed in C57BL/6 and BALB/c mice, endothelial and skeletal muscle cells were subjected to hypoxia and nutrient deprivation (HND) in vitro, to mimic ischemia. Muscle cells were more susceptible to HND than were endothelial cells. In vivo, C57BL/6 and BALB/c mice displayed strain-specific differences in myofiber responses after hindlimb ischemia, with significantly greater myofiber atrophy, greater apoptosis, and attenuated myogenic regulatory gene expression and stress-responsive signaling in BALB/c mice. Strain-specific deficits were recapitulated in vitro in primary muscle cells from both strains after HND. Muscle cells from BALB/c mice congenic for the C57BL/6 Lsq-1 quantitative trait locus were protected from HND-induced atrophy, and gene expression of vascular growth factors and their receptors was significantly greater in C57BL/6 primary muscle cells. Our results indicate that the previously identified specific genetic locus regulating strain-dependent collateral vessel density has a nonvascular or muscle cell-autonomous role involving both the myogenic program and traditional vascular growth factor receptor expression.

Myoblast fusion: lessons from flies and mice

The fusion of myoblasts into multinucleate syncytia plays a fundamental role in muscle function, as it supports the formation of extended sarcomeric arrays, or myofibrils, within a large volume of cytoplasm. Principles learned from the study of myoblast fusion not only enhance our understanding of myogenesis, but also contribute to our perspectives on membrane fusion and cell-cell fusion in a wide array of model organisms and experimental systems. Recent studies have advanced our views of the cell biological processes and crucial proteins that drive myoblast fusion. Here, we provide an overview of myoblast fusion in three model systems that have contributed much to our understanding of these events: the Drosophila embryo; developing and regenerating mouse muscle; and cultured rodent muscle cells.

Impact of ageing on muscle cell regeneration

Skeletal muscle regeneration is a coordinate process in which several factors are sequentially activated to maintain and preserve muscle structure and function. The major role in the growth, remodeling and regeneration is played by satellite cells, a quiescent population of myogenic cells that reside between the basal lamina and plasmalemma and are rapidly activated in response to appropriate stimuli. However, in several muscle conditions, including aging, the capacity of skeletal muscle to sustain an efficient regenerative pathway is severely compromised. Nevertheless, if skeletal muscle possesses a stem cell compartment it is not clear why the muscle fails to regenerate under pathological conditions. Either the resident muscle stem cells are too rare or intrinsically incapable of repairing major damage, or perhaps the injured/pathological muscle is a prohibitive environment for stem cell activation and function. Although we lack definitive answers, recent experimental evidences suggest that the mere presence of endogenous stem cells may not be sufficient to guarantee muscle regeneration, and that the presence of appropriate stimuli and factors are necessary to provide a permissive environment that permits stem cell mediated muscle regeneration and repair. In this review we discuss the molecular basis of muscle regeneration and how aging impacts stem cell mediated muscle regeneration and repair.

Muscle resident macrophages control the immune cell reaction in a mouse model of notexin-induced myoinjury

OBJECTIVE

Skeletal muscle may be the site of a variety of poorly understood immune reactions, particularly after myofiber injury, which is typically observed in inflammatory myopathies. This study was undertaken to explore both the cell dynamics and functions of resident macrophages and dendritic cells (DCs) in damaged muscle, using a mouse model of notexin-induced myoinjury to study innate immune cell reactions.

METHODS

The myeloid cell reaction to notexin-induced myoinjury was analyzed by microscopy and flow cytometry. Bone marrow (BM) transplantation studies were used to discriminate resident from exudate monocyte/macrophages. Functional tests included cytokine screening and an alloantigenic mixed leukocyte reaction to assess the antigen-presenting cell (APC) function. Selective resident macrophage depletion was obtained by injection of diphtheria toxin (DT) into CD11b-DT receptor-transgenic mice transplanted with DT-insensitive BM.

RESULTS

The connective tissue surrounding mouse muscle/fascicle tissue (the epimysium/perimysium) after deep muscle injury displayed a resident macrophage population of CD11b+F4/80+CD11c-Ly-6C-CX3CR1- cells, which concentrated first in the epimysium. These resident macrophages were being used by leukocytes as a centripetal migration pathway, and were found to selectively release 2 chemokines, cytokine-induced neutrophil chemoattractant and monocyte chemoattractant protein 1, and to crucially contribute to massive recruitment of neutrophils and monocytes from the blood. Early epimysial inflammation consisted of a predominance of Ly-6C(high)CX3CR1(low)CD11c- cells that were progressively substituted by Ly-6C(low)CX3CR1(high) cells displaying an intermediate, rather than high, level of CD11c expression. These CD11c(intermediate) cells were derived from circulating CCR2+ monocytes, functionally behaved as immature APCs in the absence of alloantigenic challenge, and migrated to draining lymph nodes while acquiring the phenotype of mature DCs (CD11c+Ia+CD80+ cells, corresponding to an inflammatory DC phenotype).

CONCLUSION

The results in this mouse model show that resident macrophages in the muscle epimysium/perimysium orchestrate the innate immune response to myoinjury, which is linked to adaptive immunity through the formation of inflammatory DCs.

Effect of nandrolone decanoate administration on recovery from bupivacaine-induced muscle injury

Although testosterone administration elicits well-documented anabolic effects on skeletal muscle mass, the enhancement of muscle regeneration after injury has not been widely examined. The purpose of this study was to determine whether anabolic steroid administration improves skeletal muscle regeneration from bupivacaine-induced injury. Male C57BL/6 mice were castrated 2 wk before muscle injury induced by an intramuscular bupivacaine injection into the tibialis anterior (TA) muscle. Control mice received an intramuscular PBS injection. Anabolic steroid [nandrolone decanoate (ND), 6 mg/kg] or sesame seed oil was administered at the time of initial injury and continued every 7 days for the study's duration. Mice were randomly assigned to one of four treatment groups for 5, 14, or 42 days of recovery, as follows: 1) control (uninjured); 2) ND only (uninjured + ND); 3) bupivacaine only (injured); or 4) bupivacaine + ND (injured + ND). TA morphology, protein, and gene expression were analyzed at 14 and 42 days after injury; protein expression was analyzed at 5 days after injury. After 14 days of recovery, the injury and injury + ND treatments induced small-diameter myofiber incidence and also decreased mean myofiber area. The increase in small-myofiber incidence was 65% greater in injury + ND muscle compared with injury alone. At 14 days, injury + ND induced a fivefold increase in muscle IGF-I mRNA expression, which was greater than injury alone. Muscle Akt activity and glycogen synthetase kinase-3beta activity were also induced by injury + ND at 14 days of recovery, but not by injury alone. ND had a main effect for increasing muscle MyoD and cyclin D1 mRNA expression at 14 days. After 42 days of recovery, injury + ND increased large-diameter myofiber incidence compared with injury only. Nandrolone decanoate (ND) administration can enhance castrated mouse muscle regeneration during the recovery from bupivacaine-induced injury.

Effects of phonophoresis with Arnica montana onto acute inflammatory process in rat skeletal muscles: an experimental study

This study aimed at verifying the effects of phonophoresis associated with Arnica montana on the acute phase of an inflammatory muscle lesion. Forty Wistar male rats (300+/-50 g), of which the Tibialis Anterior muscle was surgically lesioned, were divided into four groups (n=10 each): control group received no treatment; the ultrasound group (US) was treated in pulsed mode with 1-MHz frequency, 0.5 W/cm(2) intensity (spatial and temporal average - SATA), duty cycle of 1:2 (2 ms on, 4 ms off, 50%), time of application 3 min per session, one session per day, for 3 days; the phonophoresis or ultrasound plus arnica (US+A) group was treated with arnica with the same US parameters plus arnica gel; and the arnica group (A) was submitted to massage with arnica gel, also for 3 min, once a day, for 3 days. Treatment started 24h after the surgical lesion. On the 4th day after lesion creation, animals were sacrificed and sections of the lesioned, inflamed muscle were removed for quantitative (mononuclear and polymorphonuclear cell count) and qualitative histological analysis. Collected data from the 4 groups were statistically analyzed and the significance level set at p<0.05. Results show higher mononuclear cell density in all three treated groups with no significant difference between them, but values were significantly different (p<0.0001) when compared to control group's. As to polymorphonuclear cell density, significant differences were found between control group (p=0.0134) and US, US+A and A groups; the arnica group presented lesser density of polymorphonuclear cells when compared (p=0.0134) to the other groups. No significant difference was found between US and US+A groups. While the massage with arnica gel proved to be an effective anti-inflammatory on acute muscle lesion in topic use, these results point to ineffectiveness of Arnica montana phonophoresis, US having seemingly checked or minimized its anti-inflammatory effect.

Leukotriene B4 regulates proliferation and differentiation of cultured rat myoblasts via the BLT1 pathway

Skeletal muscle regeneration is a highly orchestrated process initiated by activation of adult muscle satellite cells. Upon muscle injury, the inflammatory process is always accompanied by muscle regeneration. Leukotriene B(4) is one of the essential inflammatory mediators. We isolated and cultured primary satellite cells. RT-PCR showed that myoblasts expressed mRNA for LTB(4) receptors BLT1 and BLT2, and LTB4 promoted myoblast proliferation and fusion. Quantitative real-time PCR and immunoblotting showed that LTB(4) treatment expedited the expression process of differentiation markers MyoD and M-cadherin. U-75302, a specific BLT1 inhibitor, but not LY2552833, a specific BLT2 inhibitor, blocked proliferation and differentiation of myoblasts induced by LTB(4), which implies the involvement of the BLT1 pathway. Overall, the data suggest that LTB(4) contributes to muscle regeneration by accelerating proliferation and differentiation of satellite cells.

Attenuated muscle regeneration is a key factor in dysferlin-deficient muscular dystrophy

Skeletal muscle requires an efficient and active membrane repair system to overcome the rigours of frequent contraction. Dysferlin is a component of that system and absence of dysferlin causes muscular dystrophy (dysferlinopathy) characterized by adult onset muscle weakness, high serum creatine kinase levels and a prominent inflammatory infiltrate. We have observed that dysferlinopathy patient biopsies show an excess of immature fibres and therefore investigated the role of dysferlin in muscle regeneration. Using notexin-induced muscle damage, we have shown that regeneration is attenuated in a mouse model of dysferlinopathy, with delayed removal of necrotic fibres, an extended inflammatory phase and delayed functional recovery. Satellite cell activation and myoblast fusion appear normal, but there is a reduction in early neutrophil recruitment in regenerating and also needle wounded muscle in dysferlin-deficient mice. Primary mouse dysferlinopathy myoblast cultures show reduced cytokine release upon stimulation, indicating that the secretion of chemotactic molecules is impaired. We suggest an extension to the muscle membrane repair model, where in addition to fusing patch repair vesicles with the sarcolemma dysferlin is also involved in the release of chemotactic agents. Reduced neutrophil recruitment results in incomplete cycles of regeneration in dysferlinopathy which combines with the membrane repair deficit to ultimately trigger dystrophic pathology. This study reveals a novel pathomechanism affecting muscle regeneration and maintenance in dysferlinopathy and highlights enhancement of the neutrophil response as a potential therapeutic avenue in these disorders.

Context matters: in vivo and in vitro influences on muscle satellite cell activity

Skeletal muscle is formed during development by the progressive specification, proliferation, migration, and fusion of myoblasts to form terminally differentiated, contractile, highly patterned myofibers. Skeletal muscle is repaired or replaced postnatally by a similar process, involving a resident myogenic stem cell population referred to as satellite cells. In both cases, the activity of the myogenic precursor cells in question is regulated by local signals from the environment, frequently involving other, non-muscle cell types. However, while the majority of studies on muscle development were done in the context of the whole embryo, much of the current work on muscle satellite cells has been done in vitro, or on satellite cell-derived cell lines. While significant practical reasons for these approaches exist, it is almost certain that important influences from the context of the injured and regenerating muscle are lost, while potential tissue culture artifacts are introduced. This review will briefly address extracellular influences on satellite cells in vivo and in vitro that would be expected to impinge on their activity.

Análise qualitativa dos efeitos da sonoforese com Arnica montana sobre o processo inflamatório agudo do músculo esquelético de ratos

Este estudo visou verificar o efeito da sonoforese com Arnica montana sobre a fase inflamatória aguda de uma lesão muscular. Para isso, 40 ratos Wistar machos, lesados cirurgicamente, foram divididos em 4 grupos: controle (C), 10 ratos lesados e não tratados; grupo ultra-som (US), 10 lesados, tratados com US; grupo ultra-som com arnica (US+A), 10 ratos lesados, tratados com sonoforese de gel de arnica; grupo arnica (A), 10 ratos lesados, tratados com massagem de gel de arnica. O tratamento dos três grupos foi iniciado 24h após a lesão, sendo aplicado uma vez ao dia durante 3 minutos, por três dias. Quatro dias após a lesão, os animais foram sacrificados e o terço médio do músculo tibial anterior lesado foi removido e tratado histologicamente. Os resultados da análise qualitativa mostram que, no grupo C, formou-se um intenso infiltrado de células inflamatórias no espaço intersticial e um processo de regeneração apenas iniciado. Nos grupos US e US+A foi detectado um avançado processo inflamatório, com tecido conjuntivo mais organizado e consistente. No grupo A foi detectada diminuição no número de células inflamatórias e uma desorganização em sua disposição, o que poderia levar a um atraso no processo de regeneração. Conclui-se que os grupos que receberam a aplicação do ultra-som e ultra-som com arnica apresentaram semelhante aceleração do processo inflamatório agudo, sugerindo ineficácia da sonoforese quando comparada à aplicação de apenas ultra-som.

Age influences the early events of skeletal muscle regeneration: studies of whole muscle grafts transplanted between young (8 weeks) and old (13-21 months) mice

Injured skeletal muscle generally regenerates less efficiently with age, but little is understood about the effects of ageing on the very early inflammatory and neovascular events in the muscle repair process. This study used a total of 174 whole muscle grafts transplanted within and between young and old mice to analyse the effects of ageing on the early inflammatory response in two strains of mice (BALB/c and SJL/J). There was a very slight delay in the early inflammatory response, and in the appearance of myotubes at day 4 in BALB/c muscle grafted into an old host environment (implicating systemic events). In SJL/J mice, the initial speed of the inflammatory response was slightly delayed with old muscle grafts regardless of host age (implicating muscle-derived factors), while an old host environment transiently affected myogenesis (myotube formation). The slight delays in inflammatory and neovascular responses in old mice did not dramatically impact on the overall formation of new muscle. The neovascular response to injured young and old muscle tissue was further analysed using the corneal micropocket assay. This showed a very clear 1-2 day delay in angiogenesis induced by old versus young BALB/c muscle tissue implanted into the young rat cornea, indicating that new blood vessel formation is at least partly determined by muscle-derived factors. Taken together these results indicate that, while there are slight age-associated delays in inflammation and neovascularisation in response to injured muscle, there is no detrimental effect on myogenesis in the mouse model used in this study.

Inflammatory monocytes recruited after skeletal muscle injury switch into antiinflammatory macrophages to support myogenesis

Macrophages (MPs) are important for skeletal muscle regeneration in vivo and may exert beneficial effects on myogenic cell growth through mitogenic and antiapoptotic activities in vitro. However, MPs are highly versatile and may exert various, and even opposite, functions depending on their activation state. We studied monocyte (MO)/MP phenotypes and functions during skeletal muscle repair. Selective labeling of circulating MOs by latex beads in CX3CR1(GFP/+) mice showed that injured muscle recruited only CX3CR1(lo)/Ly-6C(+) MOs from blood that exhibited a nondividing, F4/80(lo), proinflammatory profile. Then, within muscle, these cells switched their phenotype to become proliferating antiinflammatory CX3CR1(hi)/Ly-6C(-) cells that further differentiated into F4/80(hi) MPs. In vitro, phagocytosis of muscle cell debris induced a switch of proinflammatory MPs toward an antiinflammatory phenotype releasing transforming growth factor beta1. In co-cultures, inflammatory MPs stimulated myogenic cell proliferation, whereas antiinflammatory MPs exhibited differentiating activity, assessed by both myogenin expression and fusion into myotubes. Finally, depletion of circulating MOs in CD11b-diphtheria toxin receptor mice at the time of injury totally prevented muscle regeneration, whereas depletion of intramuscular F4/80(hi) MPs at later stages reduced the diameter of regenerating fibers. In conclusion, injured skeletal muscle recruits MOs exhibiting inflammatory profiles that operate phagocytosis and rapidly convert to antiinflammatory MPs that stimulate myogenesis and fiber growth.

Human macrophages rescue myoblasts and myotubes from apoptosis through a set of adhesion molecular systems

The mechanisms underlying stromal cell supportive functions are incompletely understood but probably implicate a mixture of cytokines, matrix components and cell adhesion molecules. Skeletal muscle uses recruited macrophages to support post-injury regeneration. We and others have previously shown that macrophages secrete mitogenic factors for myogenic cells. Here, we focused on macrophage-elicited survival signals. We demonstrated that: (1) macrophage influx is temporally correlated with the disappearance of TUNEL-positive apoptotic myogenic cells during post-injury muscle regeneration in mice; (2) direct cell-cell contacts between human macrophages and myogenic cells rescue myogenic cells from apoptosis, as assessed by decreased annexin V labelling and caspase-3 activity, and by increased DIOC-6 staining, Bcl-2 expression and phosphorylation of Akt and ERK1/2 survival pathways; (3) four pro-survival cell-cell adhesion molecular systems detected by DNA macroarray are expressed by macrophages and myogenic cells in vitro and in vivo - VCAM-1-VLA-4, ICAM-1-LFA-1, PECAM-1-PECAM-1 and CX3CL1-CX3CR1; (4) macrophages deliver anti-apoptotic signals through all four adhesion systems, as assessed by functional analyses with blocking antibodies; and (5) macrophages more strongly rescue differentiated myotubes, which must achieve adhesion-induced stabilisation of their structure to survive. Macrophages could secure these cells until they establish final association with the matrix.

Molecular forms of acetylcholinesterase in the rat extensor digitorum longus and soleus muscles regenerating from notexin-induced necrosis

The activity of acetylcholinesterase molecular forms were examined after separation on sucrose gradients during notexin-induced necrosis and the following regeneration in rat extensor digitorum longus (EDL) and soleus (SOL) muscles. All forms dropped rapidly in both muscles in the first few days after single notexin injection. After a delay small globular forms (G1+G2) started to regenerate from day 7 and larger forms (G4 and A12) from day 10 in EDL. The A8 form which cannot be detected in normal EDL was present between day 7 and day 28. In SOL the recovery of AChE forms begun already on day 3. The small globular forms displayed a more rapid increase between day 3 and day 7 then the other forms. In SOL we observed a temporary overshooting peak at day 7 in the activity of all molecular forms. Both muscles recovered their normal AChE pattern by that time when muscle fibres regained their normal diameter (day 28). Most of the events of regeneration of AChE forms resembled those of normal myogenesis.

Inflammatory processes in muscle injury and repair

Modified muscle use or injury can produce a stereotypic inflammatory response in which neutrophils rapidly invade, followed by macrophages. This inflammatory response coincides with muscle repair, regeneration, and growth, which involve activation and proliferation of satellite cells, followed by their terminal differentiation. Recent investigations have begun to explore the relationship between inflammatory cell functions and skeletal muscle injury and repair by using genetically modified animal models, antibody depletions of specific inflammatory cell populations, or expression profiling of inflamed muscle after injury. These studies have contributed to a complex picture in which inflammatory cells promote both injury and repair, through the combined actions of free radicals, growth factors, and chemokines. In this review, recent discoveries concerning the interactions between skeletal muscle and inflammatory cells are presented. New findings clearly show a role for neutrophils in promoting muscle damage soon after muscle injury or modified use. No direct evidence is yet available to show that neutrophils play a beneficial role in muscle repair or regeneration. Macrophages have also been shown capable of promoting muscle damage in vivo and in vitro through the release of free radicals, although other findings indicate that they may also play a role in muscle repair and regeneration through growth factors and cytokine-mediated signaling. However, this role for macrophages in muscle regeneration is still not definitive; other cells present in muscle can also produce the potentially regenerative factors, and it remains to be proven whether macrophage-derived factors are essential for muscle repair or regeneration in vivo. New evidence also shows that muscle cells can release positive and negative regulators of inflammatory cell invasion, and thereby play an active role in modulating the inflammatory process. In particular, muscle-derived nitric oxide can inhibit inflammatory cell invasion of healthy muscle and protect muscle from lysis by inflammatory cells in vivo and in vitro. On the other hand, muscle-derived cytokines can signal for inflammatory cell invasion, at least in vitro. The immediate challenge for advancing our current understanding of the relationships between muscle and inflammatory cells during muscle injury and repair is to place what has been learned in vitro into the complex and dynamic in vivo environment.

Cellular and molecular regulation of muscle regeneration

Under normal circumstances, mammalian adult skeletal muscle is a stable tissue with very little turnover of nuclei. However, upon injury, skeletal muscle has the remarkable ability to initiate a rapid and extensive repair process preventing the loss of muscle mass. Skeletal muscle repair is a highly synchronized process involving the activation of various cellular responses. The initial phase of muscle repair is characterized by necrosis of the damaged tissue and activation of an inflammatory response. This phase is rapidly followed by activation of myogenic cells to proliferate, differentiate, and fuse leading to new myofiber formation and reconstitution of a functional contractile apparatus. Activation of adult muscle satellite cells is a key element in this process. Muscle satellite cell activation resembles embryonic myogenesis in several ways including the de novo induction of the myogenic regulatory factors. Signaling factors released during the regenerating process have been identified, but their functions remain to be fully defined. In addition, recent evidence supports the possible contribution of adult stem cells in the muscle regeneration process. In particular, bone marrow-derived and muscle-derived stem cells contribute to new myofiber formation and to the satellite cell pool after injury.

Satellite cells attract monocytes and use macrophages as a support to escape apoptosis and enhance muscle growth

Once escaped from the quiescence niche, precursor cells interact with stromal components that support their survival, proliferation, and differentiation. We examined interplays between human myogenic precursor cells (mpc) and monocyte/macrophages (MP), the main stromal cell type observed at site of muscle regeneration. mpc selectively and specifically attracted monocytes in vitro after their release from quiescence, chemotaxis declining with differentiation. A DNA macroarray–based strategy identified five chemotactic factors accounting for 77% of chemotaxis: MP-derived chemokine, monocyte chemoattractant protein-1, fractalkine, VEGF, and the urokinase system. MP showed lower constitutive chemotactic activity than mpc, but attracted monocytes much strongly than mpc upon cross-stimulation, suggesting mpc-induced and predominantly MP-supported amplification of monocyte recruitment. Determination of [³H]thymidine incorporation, oligosomal DNA levels and annexin-V binding showed that MP stimulate mpc proliferation by soluble factors, and rescue mpc from apoptosis by direct contacts. We conclude that once activated, mpc, which are located close by capillaries, initiate monocyte recruitment and interplay with MP to amplify chemotaxis and enhance muscle growth.

Use of nonsteroidal anti-inflammatory drugs following exercise-induced muscle injury

The objective of this article is to examine the use of NSAIDs for attenuating exercise-induced muscle injuries (EIMI), with an emphasis on their safety and usefulness for improving muscle function and reducing soreness. NSAIDs are some of the most widely consumed medications in the world, and NSAID use as therapy for EIMI has increased dramatically over the last 20 years. However, there is a lack of agreement concerning NSAID effectiveness for this purpose. The lack of consensus about the efficacy of NSAID use in relation to EIMI has spawned a recent interest in sports medicine research regarding NSAIDs. The application of a variety of methods used to induce, assess and quantify muscle injury has contributed to the inconsistency among the findings regarding the efficacy of NSAIDs for EIMI. Therefore, future studies should focus on the evaluation of muscle injury and function, with the use of better functional measurement tools and more uniformity in the assessment tools used. However, from review of the current literature, it is concluded that NSAID use for brief periods of time is beneficial for short-term recovery of muscle function and is an important laboratory tool for the study of EIMI.

Dysferlin expression after normal myoblast transplantation in SCID and in SJL mice

Limb girdle muscular dystrophy type 2B form and Miyoshi myopathy are both caused by mutations in the recently cloned gene dysferlin. In the present study, we have investigated whether cell transplantation could permit dysferlin expression in vivo. Two transplantation models were used: SCID mice transplanted with normal human myoblasts, and SJL mice, the mouse model for limb girdle muscular dystrophy type 2B and Miyoshi myopathy, transplanted with allogeneic primary mouse muscle cell cultures expressing the beta-galactosidase gene under control of a muscle promoter of Troponin I. FK506 immunosuppression was used in the non-compatible allogeneic model. One month after transplantation, human and mouse dysferlin proteins were detected in all transplanted SCID and SJL muscles, respectively. Co-localization of dysferlin and human dystrophin or beta-galactosidase-positive fibers was observed following the transplantation of myoblasts. Dysferlin proteins were monitored by immunocytochemistry and Western blot. The number of dysferlin-positive fibers was 40-50% and 20-30% in SCID and SJL muscle sections, respectively. Detection of dysferlin in both SCID mice and dysferlin-deficient SJL mouse shows that myoblast transplantation permits the expression of the donor dysferlin protein.

Adaptation to lengthening contractions is independent of voluntary muscle recruitment but relies on inflammation

Lengthening contractions trigger an adaptive response decreasing the susceptibility to exercise-induced muscle damage (EIMD). We hypothesized that 1) this adaptation can be observed when voluntary muscle recruitment is bypassed and 2) inflammation repression lessens the adaptive response. Rat ankle dorsiflexors were submitted to two bouts of elicited lengthening contractions 14 days apart; in vitro force production and macrophage concentrations were obtained before and 2 days after each bout in rats treated or not for 2 or 7 days with diclofenac. The first bout caused a 45% force deficit in the placebo group vs. 25% in the diclofenac group, whereas the ED1+ macrophage concentration increased by 10- and 5-fold, respectively. After the second bout, only diclofenac-treated rats (2 or 7 days) presented significant force deficits and increases in ED1+ and ED2+ macrophage concentrations, but this was more pronounced in the 7-day group. We conclude that adaptation to lengthening contractions does not depend on neural components but is likely mediated by strengthening of muscle structural/cellular elements and that inflammation is important for this process.

Skeletal muscle regeneration in very old rats

This study was undertaken to assess the regenerative capacity of skeletal muscle in rats near the end of their normal life span. Two experiments were performed. In the first, extensor digitorum longus (EDL) muscles were cross-age transplanted from 32-month-old male inbred Wistar (WI/HicksCar) rats in place of an EDL muscle in 4-month-old hosts. The other EDL muscle in the hosts was autotransplanted. After 60 days, the old-into-young muscle transplants regenerated as well as the young-into-young autotransplants. In the second experiment, EDL muscles in young adult (4 months) and old rats (32 and 34 months) of WI/HicksCar and Brown Norway (BN) were injected with a local anesthetic, bupivacaine, and allowed to regenerate for 41 days. In all cases, the masses and absolute maximum tetanic force of the regenerates equaled or exceeded those of untouched contralateral control muscles. These experiments showed that under appropriate conditions, very old muscles can regenerate to equal or exceed the contralateral control values, which in old rats are much less than those in muscles of young rats.

Myostatin regulation during skeletal muscle regeneration

Myostatin, a member of the TGF-beta superfamily, is a key negative regulator of skeletal muscle growth. The role of myostatin during skeletal muscle regeneration has not previously been reported. In the present studies, normal Sprague-Dawley and growth hormone (GH)-deficient (dw/dw) rats were administered the myotoxin, notexin, in the right M. biceps femoris on day 0. The dw/dw rats then received either saline or human-N-methionyl GH (200microg/100g body weight/day) during the ensuing regeneration. Normal and dw/dw M. biceps femoris were dissected on days 1, 2, 3, 5, 9 and 13, formalin-fixed, then immunostained for myostatin protein. Immunostaining for myostatin revealed high levels of protein within necrotic fibres and connective tissue of normal and dw/dw damaged muscles. Regenerating myotubes contained no myostatin at the time of fusion (peak fusion on day 5), and only low levels of myostatin were observed during subsequent myotube enlargement. Fibres which survived assault by notexin (survivor fibres) contained moderate to high myostatin immunostaining initially. The levels in both normal and dw/dw rat survivor fibres decreased on days 2-3, then increased on days 9-13. In dw/dw rats, there was no observed effect of GH administration on the levels of myostatin protein in damaged muscle. The low level of myostatin observed in regenerating myotubes in these studies suggests a negative regulatory role for myostatin in muscle regeneration.

Skeletal muscle regeneration is not impaired in Fgf6 -/- mutant mice

FGF6 is a member of the fibroblast growth factor family. The Fgf6 gene is almost exclusively expressed in adult and developing skeletal muscle. We have obtained mice deficient in FGF6 by targeting the Fgf6 gene by homologous recombination. We studied regeneration of adult skeletal muscle in Fgf6 -/- mice derived on a standard inbred background. Muscle degeneration was induced by notexin drug or crush injury. The defect in FGF6 did not modify the kinetics of muscle regeneration. We bred Fgf6 -/- mice with mdx dystrophin deficient mice; Fgf6 -/-:mdx and mdx muscles were similar. Our study suggests that FGF6 does not play a role in muscle regeneration, i.e., in satellite cell proliferation and fusion, or that this role is strictly compensated by other factors, possibly other FGFs.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.