Physiological role of tumor necrosis factor alpha in traumatic muscle injury

Effects of the immune system on muscle regeneration

Muscle regeneration is a complex process orchestrated by multiple steps. Recent findings indicate that inflammatory responses could play central roles in bridging initial muscle injury responses and timely muscle injury reparation. The various types of immune cells and cytokines have crucial roles in muscle regeneration process. In this review, we provide an overview of the functions of acute inflammation in muscle regeneration.

Regenerative inflammation: When immune cells help to re-build tissues

Inflammation is an essential immune response critical for responding to infection, injury and maintenance of tissue homeostasis. Upon injury, regenerative inflammation promotes tissue repair by a timed and coordinated infiltration of diverse cell types and the secretion of growth factors, cytokines and lipids mediators. Remarkably, throughout evolution as well as mammalian development, this type of physiological inflammation is highly associated with immunosuppression. For instance, regenerative inflammation is the consequence of an in situ macrophage polarization resulting in a transition from pro-inflammatory to anti-inflammatory/pro-regenerative response. Immune cells are the first responders upon injury, infiltrating the damaged tissue and initiating a pro-inflammatory response depleting cell debris and necrotic cells. After phagocytosis, macrophages undergo multiple coordinated metabolic and transcriptional changes allowing the transition and dictating the initiation of the regenerative phase. Differences between a highly efficient, complete ad integrum tissue repair, such as, acute skeletal muscle injury, and insufficient regenerative inflammation, as the one developing in Duchenne Muscular Dystrophy (DMD), highlight the importance of a coordinated response orchestrated by immune cells. During regenerative inflammation, these cells interact with others and alter the niche, affecting the character of inflammation itself and, therefore, the progression of tissue repair. Comparing acute muscle injury and chronic inflammation in DMD, we review how the same cells and molecules in different numbers, concentration and timing contribute to very different outcomes. Thus, it is important to understand and identify the distinct functions and secreted molecules of macrophages, and potentially other immune cells, during tissue repair, and the contributors to the macrophage switch leveraging this knowledge in treating diseases.

Effect of nonsteroidal anti-inflammatory drugs on pelvic floor muscle regeneration in a preclinical birth injury rat model

BACKGROUND

Pelvic floor muscle injury is a common consequence of vaginal childbirth. Nonsteroidal anti-inflammatory drugs are widely used postpartum analgesics. Multiple studies have reported negative effects of these drugs on limb muscle regeneration, but their impact on pelvic floor muscle recovery following birth injury has not been explored.

OBJECTIVE

Using a validated rat model, we assessed the effects of nonsteroidal anti-inflammatory drug on acute and longer-term pelvic floor muscle recovery following simulated birth injury.

STUDY DESIGN

Three-month old Sprague Dawley rats were randomly assigned to the following groups: (1) controls, (2) simulated birth injury, (3) simulated birth injury+nonsteroidal anti-inflammatory drug, or (4) nonsteroidal anti-inflammatory drug. Simulated birth injury was induced using a well-established vaginal balloon distension protocol. Ibuprofen was administered in drinking water (0.2 mg/mL), which was consumed by the animals ad libitum. Animals were euthanized at 1, 3, 5, 7, 10, and 28 days after birth injury/ibuprofen administration. The pubocaudalis portion of the rat levator ani, which, like the human pubococcygeus, undergoes greater parturition-associated strains, was harvested (N=3-9/time point/group). The cross-sectional areas of regenerating (embryonic myosin heavy chain+) and mature myofibers were assessed at the acute and 28-day time points, respectively. The intramuscular collagen content was assessed at the 28-day time point. Myogenesis was evaluated using anti-Pax7 and anti-myogenin antibodies to identify activated and differentiated muscle stem cells, respectively. The overall immune infiltrate was assessed using anti-CD45 antibody. Expression of genes coding for pro- and anti-inflammatory cytokines was assessed by quantitative reverse transcriptase polymerase chain reaction at 3, 5, and 10 days after injury.

RESULTS

The pubocaudalis fiber size was significantly smaller in the simulated birth injury+nonsteroidal anti-inflammatory drug compared with the simulated birth injury group at 28 days after injury (P<.0001). The median size of embryonic myosin heavy chain+ fibers was also significantly reduced, with the fiber area distribution enriched with smaller fibers in the simulated birth injury+nonsteroidal anti-inflammatory drug group relative to the simulated birth injury group at 3 days after injury (P<.0001), suggesting a delay in the onset of regeneration in the presence of nonsteroidal anti-inflammatory drugs. By 10 days after injury, the median embryonic myosin heavy chain+ fiber size in the simulated birth injury group decreased from 7 days after injury (P<.0001) with a tight cross-sectional area distribution, indicating nearing completion of this state of regeneration. However, in the simulated birth injury+nonsteroidal anti-inflammatory drug group, the size of embryonic myosin heavy chain+ fibers continued to increase (P<.0001) with expansion of the cross-sectional area distribution, signifying a delay in regeneration in these animals. Nonsteroidal anti-inflammatory drugs decreased the muscle stem cell pool at 7 days after injury (P<.0001) and delayed muscle stem cell differentiation, as indicated by persistently elevated number of myogenin+ cells 7 days after injury (P<.05). In contrast, a proportion of myogenin+ cells returned to baseline by 5 days after injury in the simulated birth injury group. The analysis of expression of genes coding for pro- and anti-inflammatory cytokines demonstrated only transient elevation of Tgfb1 in the simulated birth injury+nonsteroidal anti-inflammatory drug group at 5 but not at 10 days after injury. Consistently with previous studies, nonsteroidal anti-inflammatory drug administration following simulated birth injury resulted in increased deposition of intramuscular collagen relative to uninjured animals. There were no significant differences in any outcomes of interest between the nonsteroidal anti-inflammatory drug group and the unperturbed controls.

CONCLUSION

Nonsteroidal anti-inflammatory drugs negatively impacted pelvic floor muscle regeneration in a preclinical simulated birth injury model. This appears to be driven by the negative impact of these drugs on pelvic muscle stem cell function, resulting in delayed temporal progression of pelvic floor muscle regeneration following birth injury. These findings provide impetus to investigate the impact of postpartum nonsteroidal anti-inflammatory drug administration on muscle regeneration in women at high risk for pelvic floor muscle injury.

Agent-based model demonstrates the impact of nonlinear, complex interactions between cytokines on muscle regeneration

Muscle regeneration is a complex process due to dynamic and multiscale biochemical and cellular interactions, making it difficult to identify microenvironmental conditions that are beneficial to muscle recovery from injury using experimental approaches alone. To understand the degree to which individual cellular behaviors impact endogenous mechanisms of muscle recovery, we developed an agent-based model (ABM) using the Cellular Potts framework to simulate the dynamic microenvironment of a cross-section of murine skeletal muscle tissue. We referenced more than 100 published studies to define over 100 parameters and rules that dictate the behavior of muscle fibers, satellite stem cells (SSC), fibroblasts, neutrophils, macrophages, microvessels, and lymphatic vessels, as well as their interactions with each other and the microenvironment. We utilized parameter density estimation to calibrate the model to temporal biological datasets describing cross-sectional area (CSA) recovery, SSC, and fibroblast cell counts at multiple time points following injury. The calibrated model was validated by comparison of other model outputs (macrophage, neutrophil, and capillaries counts) to experimental observations. Predictions for eight model perturbations that varied cell or cytokine input conditions were compared to published experimental studies to validate model predictive capabilities. We used Latin hypercube sampling and partial rank correlation coefficient to identify in silico perturbations of cytokine diffusion coefficients and decay rates to enhance CSA recovery. This analysis suggests that combined alterations of specific cytokine decay and diffusion parameters result in greater fibroblast and SSC proliferation compared to individual perturbations with a 13% increase in CSA recovery compared to unaltered regeneration at 28 days. These results enable guided development of therapeutic strategies that similarly alter muscle physiology (i.e. converting ECM-bound cytokines into freely diffusible forms as studied in cancer therapeutics or delivery of exogenous cytokines) during regeneration to enhance muscle recovery after injury.

Soluble Factors Associated with Denervation-induced Skeletal Muscle Atrophy

Skeletal muscle tissue has the critical function of mechanical support protecting the body. In addition, its functions are strongly influenced by the balanced synthesis and degradation processes of structural and regulatory proteins. The inhibition of protein synthesis and/or the activation of catabolism generally determines a pathological state or condition called muscle atrophy, a reduction in muscle mass that results in partial or total loss of function. It has been established that many pathophysiological conditions can cause a decrease in muscle mass. Skeletal muscle innervation involves stable and functional neural interactions with muscles via neuromuscular junctions and is essential for maintaining normal muscle structure and function. Loss of motor innervation induces rapid skeletal muscle fiber degeneration with activation of atrophy-related signaling and subsequent disassembly of sarcomeres, altering normal muscle function. After denervation, an inflammation stage is characterized by the increased expression of pro-inflammatory cytokines that determine muscle atrophy. In this review, we highlighted the impact of some soluble factors on the development of muscle atrophy by denervation.

Severely Damaged Freeze-Injured Skeletal Muscle Reveals Functional Impairment, Inadequate Repair, and Opportunity for Human Stem Cell Application

BACKGROUND

The regeneration of severe traumatic muscle injuries is an unsolved medical need that is relevant for civilian and military medicine. In this work, we produced a critically sized nonhealing muscle defect in a mouse model to investigate muscle degeneration/healing phases.

MATERIALS AND METHODS

We caused a freeze injury (FI) in the biceps femoris of C57BL/6N mice. From day 1 to day 25 post-injury, we conducted histological/morphometric examinations, an analysis of the expression of genes involved in inflammation/regeneration, and an in vivo functional evaluation.

RESULTS

We found that FI activates cytosolic DNA sensing and inflammatory responses. Persistent macrophage infiltration, the prolonged expression of eMHC, the presence of centrally nucleated myofibers, and the presence of PAX7+ satellite cells at late time points and with chronic physical impairment indicated inadequate repair. By looking at stem-cell-based therapeutic protocols of muscle repair, we investigated the crosstalk between M1-biased macrophages and human amniotic mesenchymal stem cells (hAMSCs) in vitro. We demonstrated their reciprocal paracrine effects where hAMSCs induced a shift of M1 macrophages into an anti-inflammatory phenotype, and M1 macrophages promoted an increase in the expression of hAMSC immunomodulatory factors.

CONCLUSIONS

Our findings support the rationale for the future use of our injury model to exploit the full potential of in vivo hAMSC transplantation following severe traumatic injuries.

Stromal vascular fraction in the treatment of myositis

Muscle regeneration is a physiological process that converts satellite cells into mature myotubes under the influence of an inflammatory environment progressively replaced by an anti-inflammatory environment, with precise crosstalk between immune and muscular cells. If the succession of these phases is disturbed, the immune system can sometimes become auto-reactive, leading to chronic muscular inflammatory diseases, such as myositis. The triggers of these autoimmune myopathies remain mostly unknown, but the main mechanisms of pathogenesis are partially understood. They involve chronic inflammation, which could be associated with an auto-reactive immune response, and gradually with a decrease in the regenerative capacities of the muscle, leading to its degeneration, fibrosis and vascular architecture deterioration. Immunosuppressive treatments can block the first part of the process, but sometimes muscle remains weakened, or even still deteriorates, due to the exhaustion of its capacities. For patients refractory to immunosuppressive therapies, mesenchymal stem cells have shown interesting effects but their use is limited by their availability. Stromal vascular fraction, which can easily be extracted from adipose tissue, has shown good tolerance and possible therapeutic benefits in several degenerative and autoimmune diseases. However, despite the increasing use of stromal vascular fraction, the therapeutically active components within this heterogeneous cellular product are ill-defined and the mechanisms by which this therapy might be active remain insufficiently understood. We review herein the current knowledge on the mechanisms of action of stromal vascular fraction and hypothesise on how it could potentially respond to some of the unmet treatment needs of refractory myositis.

Global deletion of CCL2 has adverse impacts on recovery of skeletal muscle fiber size and function and is muscle specific

Timely and complete recovery of muscle mass and function following a bout of physical disuse are critical components of returning to normal activities of daily living and lifestyle. Proper cross talk between the muscle tissue and myeloid cells (e.g., macrophages) throughout the recovery period from disuse atrophy plays a significant role in the complete resolution of muscle size and function. Chemokine C-C motif ligand 2 (CCL2) has a critical function of recruiting macrophages during the early phase of muscle damage. However, the importance of CCL2 has not been defined in the context of disuse and recovery. Here, we utilized a mouse model of whole body CCL2 deletion (CCL2KO) and subjected them to a period of hindlimb unloading followed by reloading to investigate the importance of CCL2 on the regrowth of muscle following disuse atrophy using ex vivo muscle tests, immunohistochemistry, and fluorescence-activated cell sorting approaches. We show mice that lack CCL2 display an incomplete recovery of gastrocnemius muscle mass, myofiber cross-sectional area, and EDL muscle contractile characteristics during the recovery from disuse atrophy. The soleus and plantaris had limited impact as a result of CCL2 deficiency suggesting a muscle-specific effect. Mice that lack CCL2 have decreased skeletal muscle collagen turnover, which may be related to defects in muscle function and stiffness. In addition, we show that the recruitment of macrophages to gastrocnemius muscle was dramatically reduced in CCL2KO mice during the recovery from disuse atrophy, which likely precipitated poor recovery of muscle size and function and aberrant collagen remodeling.NEW & NOTEWORTHY We provide evidence that the whole body loss of CCL2 in mice has adverse impacts on whole body function and skeletal muscle-specific contractile characteristics and collagen content. These defects in muscle function worsened during the recovery from disuse atrophy and corresponded with decreased recovery of muscle mass. We conclude that the absence of CCL2 decreased recruitment of proinflammatory macrophages to the muscle during the regrowth phase following disuse atrophy resulting in impaired collagen remodeling events and full resolution of muscle morphology and function.

Single cell analysis reveals satellite cell heterogeneity for proinflammatory chemokine expression

Background: The expression of proinflammatory signals at the site of muscle injury are essential for efficient tissue repair and their dysregulation can lead to inflammatory myopathies. Macrophages, neutrophils, and fibroadipogenic progenitor cells residing in the muscle are significant sources of proinflammatory cytokines and chemokines. However, the inducibility of the myogenic satellite cell population and their contribution to proinflammatory signaling is less understood.

Methods: Mouse satellite cells were isolated and exposed to lipopolysaccharide (LPS) to mimic sterile skeletal muscle injury and changes in the expression of proinflammatory genes was examined by RT-qPCR and single cell RNA sequencing. Expression patterns were validated in skeletal muscle injured with cardiotoxin by RT-qPCR and immunofluorescence.

Results: Satellite cells in culture were able to express Tnfa, Ccl2, and Il6, within 2 h of treatment with LPS. Single cell RNA-Seq revealed seven cell clusters representing the continuum from activation to differentiation. LPS treatment led to a heterogeneous pattern of induction of C-C and C-X-C chemokines (e.g., Ccl2, Ccl5, and Cxcl0) and cytokines (e.g., Tgfb1, Bmp2, Il18, and Il33) associated with innate immune cell recruitment and satellite cell proliferation. One cell cluster was enriched for expression of the antiviral interferon pathway genes under control conditions and LPS treatment. Activation of this pathway in satellite cells was also detectable at the site of cardiotoxin induced muscle injury.

Conclusion: These data demonstrate that satellite cells respond to inflammatory signals and secrete chemokines and cytokines. Further, we identified a previously unrecognized subset of satellite cells that may act as sensors for muscle infection or injury using the antiviral interferon pathway.

Muscle regeneration in gilthead sea bream: Implications of endocrine and local regulatory factors and the crosstalk with bone

Fish muscle regeneration is still a poorly known process. In the present study, an injury was done into the left anterior epaxial skeletal muscle of seventy 15 g gilthead sea bream (Sparus aurata) juveniles to evaluate at days 0, 1, 2, 4, 8, 16 and 30 post-wound, the expression of several muscle genes. Moreover, transcripts' expression in the bone (uninjured tissue) was also analyzed. Histology of the muscle showed the presence of dead tissue the first day after injury and how the damaged fibers were removed and replaced by new muscle fibers by day 16 that kept growing up to day 30. Gene expression results showed in muscle an early upregulation of igf-2 and a downregulation of ghr-1 and igf-1. Proteolytic systems expression increased with capn2 and ctsl peaking at 1 and 2 days post-injury, respectively and mafbx at day 8. A pattern of expression that fitted well with active myogenesis progression 16 days after the injury was then observed, with the recovery of igf-1, pax7, cmet, and cav1 expression; and later on, that of cav3 as well. Furthermore, the first days post-injury, the cytokines il-6 and il-15 were also upregulated confirming the tissue inflammation, while tnfα was only upregulated at days 16 and 30 to induce satellite cells recruitment; overall suggesting a possible role for these molecules as myokines. The results of the bone transcripts showed an upregulation first, of bmp2 and ctsk at days 1 and 2, respectively; then, ogn1 and ocn peaked at day 4 in parallel to mstn2 downregulation, and runx2 and ogn2 increased after 8 days of muscle injury, suggesting a possible tissue crosstalk during the regenerative process. Overall, the present model allows studying the sequential involvement of different regulatory molecules during muscle regeneration, as well as the potential relationship between muscle and other tissues such as bone to control musculoskeletal development and growth, pointing out an interesting new line of research in this group of vertebrates.

Polyphenol Supplementation and Antioxidant Status in Athletes: A Narrative Review

Antioxidants in sports exercise training remain a debated research topic. Plant-derived polyphenol supplements are frequently used by athletes to reduce the negative effects of exercise-induced oxidative stress, accelerate the recovery of muscular function, and enhance performance. These processes can be efficiently modulated by antioxidant supplementation. The existing literature has failed to provide unequivocal evidence that dietary polyphenols should be promoted specifically among athletes. This narrative review summarizes the current knowledge regarding polyphenols' bioavailability, their role in exercise-induced oxidative stress, antioxidant status, and supplementation strategies in athletes. Overall, we draw attention to the paucity of available evidence suggesting that most antioxidant substances are beneficial to athletes. Additional research is necessary to reveal more fully their impact on exercise-induced oxidative stress and athletes' antioxidant status, as well as optimal dosing methods.

TNFα and IFNγ cooperate for efficient pro- to anti-inflammatory transition of macrophages during muscle regeneration

IFNγ is traditionally known as a proinflammatory cytokine with diverse roles in antimicrobial and antitumor immunity. Yet, findings regarding its sources and functions during the regeneration process following a sterile injury are conflicting. Here, we show that natural killer (NK) cells are the main source of IFNγ in regenerating muscle. Beyond this cell population, IFNγ production is limited to a small population of T cells. We further show that NK cells do not play a major role in muscle regeneration following an acute injury or in dystrophic mice. Surprisingly, the absence of IFNγ per se also has no effect on muscle regeneration following an acute injury. However, the role of IFNγ is partially unmasked when TNFα is also neutralized, suggesting a compensatory mechanism. Using transgenic mice, we showed that conditional inhibition of IFNGR1 signaling in muscle stem cells or fibro-adipogenic progenitors does not play a major role in muscle regeneration. In contrast to common belief, we found that IFNγ is not present in the early inflammatory phase of the regeneration process but rather peaks when macrophages are acquiring an anti-inflammatory phenotype. Further transcriptomic analysis suggests that IFNγ cooperates with TNFα to regulate the transition of macrophages from pro- to anti-inflammatory states. The absence of the cooperative effect of these cytokines on macrophages, however, does not result in significant regeneration impairment likely due to the presence of other compensatory mechanisms. Our findings support the arising view of IFNγ as a pleiotropic inflammatory regulator rather than an inducer of the inflammatory response.

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

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

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

Effect of Phytobiotic Composition on Production Parameters, Oxidative Stress Markers and Myokine Levels in Blood and Pectoral Muscle of Broiler Chickens

Simple Summary

Intensive rearing of broiler chickens is accompanied with pathological processes occurring in muscle tissue that decrease meat quality. The application of common spices as feed additives for chickens may improve the birds’ health and prevent the development of myopathies. Therefore, the aim of the study was to examine the effect of the dietary level of a phytobiotic composition on the production parameters, oxidative stress markers and myokine levels in the blood and pectoral muscle of broiler chickens. The composition consisted of red pepper fruit, white mustard seed, soapwort root, calamus rhizome, and thymol, and it was tested at two levels, i.e., 60 and 100 mg/kg diet. The results showed that dietary supplementation with phytobiotic composition at the level of 100 mg/kg diet improved feed efficiency in broiler chickens and might improve the quality and economy of broiler meat production. The plant constituents exerted their beneficial effects on meat via decreasing tumor necrosis factor-α concentration in pectoral muscle and increasing interleukin-6 content in the blood of chickens.

Abstract

The aim of this study was to evaluate the effect of dietary level of a phytobiotic composition (PBC) on production parameters, oxidative stress markers and cytokine levels in the blood and breast muscle of broiler chickens. The experiment was performed on 48 one-day-old female Ross 308 broiler chickens divided into three groups (n = 16) fed the control diet (without PBC), and a diet supplemented with 60 or 100 mg/kg of PBC. After 35 days of feeding, blood and breast muscle samples were collected for analyses. There was no effect on final body weight and feed intake but PBC addition (100 mg/kg) improved feed efficiency as compared to the control. Also, this dietary level of PBC contributed to an increase in interlukin-6 content in blood and a reduction in tumor necrosis factor-α concentrations in pectoral muscle in comparison with the control group. In conclusion, the addition of 100 mg/kg PBC improved the production parameters of broiler chickens and beneficially influenced the regeneration and protection of pectoral muscle against pathophysiological processes that may occur during intensive rearing.

Immunometabolism of macrophages regulates skeletal muscle regeneration

Sarcopenia is an age-related progressive loss of skeletal muscle mass, quality, and strength disease. In addition, sarcopenia is tightly correlated with age-associated pathologies, such as sarcopenic obesity and osteoporosis. Further understanding of disease mechanisms and the therapeutic strategies in muscle regeneration requires a deeper knowledge of the interaction of skeletal muscle and other cells in the muscle tissue. Skeletal muscle regeneration is a complex process that requires a series of highly coordinated events involving communication between muscle stem cells and niche cells, such as muscle fibro/adipogenic progenitors and macrophages. Macrophages play a critical role in tissue regeneration and the maintenance of muscle homeostasis by producing growth factors and cytokines that regulate muscle stem cells and myofibroblast activation. Furthermore, the aging-related immune dysregulation associated with the release of trophic factors and the polarization in macrophages transiently affect the inflammatory phase and impair muscle regeneration. In this review, we focus on the role and regulation of macrophages in skeletal muscle regeneration and homeostasis. The aim of this review is to highlight the important roles of macrophages as a therapeutic target in age-related sarcopenia and the increasing understanding of how macrophages are regulated will help to advance skeletal muscle regeneration.

Enhancing health span: muscle stem cells and hormesis

Sarcopenia is a significant public health and medical concern confronting the elderly. Considerable research is being directed to identify ways in which the onset and severity of sarcopenia may be delayed/minimized. This paper provides a detailed identification and assessment of hormetic dose responses in animal model muscle stem cells, with particular emphasis on cell proliferation, differentiation, and enhancing resilience to inflammatory stresses and how this information may be useful in preventing sarcopenia. Hormetic dose responses were observed following administration of a broad range of agents, including dietary supplements (e.g., resveratrol), pharmaceuticals (e.g., dexamethasone), endogenous ligands (e.g., tumor necrosis factor α), environmental contaminants (e.g., cadmium) and physical agents (e.g., low level laser). The paper assesses both putative mechanisms of hormetic responses in muscle stem cells, and potential therapeutic implications and application(s) of hormetic frameworks for slowing muscle loss and reduced functionality during the aging process.

Acid Sphingomyelinase Controls Early Phases of Skeletal Muscle Regeneration by Shaping the Macrophage Phenotype

Skeletal muscle regeneration is a complex process involving crosstalk between immune cells and myogenic precursor cells, i.e., satellite cells. In this scenario, macrophage recruitment in damaged muscles is a mandatory step for tissue repair since pro-inflammatory M1 macrophages promote the activation of satellite cells, stimulating their proliferation and then, after switching into anti-inflammatory M2 macrophages, they prompt satellite cells' differentiation into myotubes and resolve inflammation. Here, we show that acid sphingomyelinase (ASMase), a key enzyme in sphingolipid metabolism, is activated after skeletal muscle injury induced in vivo by the injection of cardiotoxin. ASMase ablation shortens the early phases of skeletal muscle regeneration without affecting satellite cell behavior. Of interest, ASMase regulates the balance between M1 and M2 macrophages in the injured muscles so that the absence of the enzyme reduces inflammation. The analysis of macrophage populations indicates that these events depend on the altered polarization of M1 macrophages towards an M2 phenotype. Our results unravel a novel role of ASMase in regulating immune response during muscle regeneration/repair and suggest ASMase as a supplemental therapeutic target in conditions of redundant inflammation that impairs muscle recovery.

Research progress in immune microenvironment regulation of muscle atrophy induced by peripheral nerve injury

Denervated skeletal muscular atrophy is primarily characterized by loss of muscle strength and mass and an unideal functional recovery of the muscle after extended denervation. This review emphasizes the interaction between the immune system and the denervated skeletal muscle. Immune cells such as neutrophils, macrophages and T-cells are activated and migrate to denervated muscle, where they release a high concentration of cytokines and chemokines. The migration of these immune cells, the transformation of different functional immune cell subtypes, and the cytokine network in the immune microenvironment may be involved in the regulatory process of muscle atrophy or repair. However, the exact mechanisms of the interaction between these immune cells and immune molecules in skeletal muscles are unclear. In this paper, the immune microenvironment regulation of muscle atrophy induced by peripheral nerve injury is reviewed.

Stem Cell and Macrophage Roles in Skeletal Muscle Regenerative Medicine

In severe muscle injury, skeletal muscle tissue structure and functionality can be repaired through the involvement of several cell types, such as muscle stem cells, and innate immune responses. However, the exact mechanisms behind muscle tissue regeneration, homeostasis, and plasticity are still under investigation, and the discovery of pathways and cell types involved in muscle repair can open the way for novel therapeutic approaches, such as cell-based therapies involving stem cells and peripheral blood mononucleate cells. Indeed, peripheral cell infusions are a new therapy for muscle healing, likely because autologous peripheral blood infusion at the site of injury might enhance innate immune responses, especially those driven by macrophages. In this review, we summarize current knowledge on functions of stem cells and macrophages in skeletal muscle repairs and their roles as components of a promising cell-based therapies for muscle repair and regeneration.

Met and Cxcr4 cooperate to protect skeletal muscle stem cells against inflammation-induced damage during regeneration

Acute skeletal muscle injury is followed by an inflammatory response, removal of damaged tissue, and the generation of new muscle fibers by resident muscle stem cells, a process well characterized in murine injury models. Inflammatory cells are needed to remove the debris at the site of injury and provide signals that are beneficial for repair. However, they also release chemokines, reactive oxygen species, as well as enzymes for clearance of damaged cells and fibers, which muscle stem cells have to withstand in order to regenerate the muscle. We show here that MET and CXCR4 cooperate to protect muscle stem cells against the adverse environment encountered during muscle repair. This powerful cyto-protective role was revealed by the genetic ablation of Met and Cxcr4 in muscle stem cells of mice, which resulted in severe apoptosis during early stages of regeneration. TNFα neutralizing antibodies rescued the apoptosis, indicating that TNFα provides crucial cell-death signals during muscle repair that are counteracted by MET and CXCR4. We conclude that muscle stem cells require MET and CXCR4 to protect them against the harsh inflammatory environment encountered in an acute muscle injury.

MiRNAs and Muscle Regeneration: Therapeutic Targets in Duchenne Muscular Dystrophy

microRNAs (miRNAs) are small non-coding RNAs required for the post-transcriptional control of gene expression. MicroRNAs play a critical role in modulating muscle regeneration and stem cell behavior. Muscle regeneration is affected in muscular dystrophies, and a critical point for the development of effective strategies for treating muscle disorders is optimizing approaches to target muscle stem cells in order to increase the ability to regenerate lost tissue. Within this framework, miRNAs are emerging as implicated in muscle stem cell response in neuromuscular disorders and new methodologies to regulate the expression of key microRNAs are coming up. In this review, we summarize recent advances highlighting the potential of miRNAs to be used in conjunction with gene replacement therapies, in order to improve muscle regeneration in the context of Duchenne Muscular Dystrophy (DMD).

Tailoring Materials for Modulation of Macrophage Fate

Human immune system acts as a pivotal role in the tissue homeostasis and disease progression. Immunomodulatory biomaterials that can manipulate innate immunity and adaptive immunity hold great promise for a broad range of prophylactic and therapeutic purposes. This review is focused on the design strategies and principles of immunomodulatory biomaterials from the standpoint of materials science to regulate macrophage fate, such as activation, polarization, adhesion, migration, proliferation, and secretion. It offers a comprehensive survey and discussion on the tunability of material designs regarding physical, chemical, biological, and dynamic cues for modulating macrophage immune response. The range of such tailorable cues encompasses surface properties, surface topography, materials mechanics, materials composition, and materials dynamics. The representative immunoengineering applications selected herein demonstrate how macrophage-immunomodulating biomaterials are being exploited for cancer immunotherapy, infection immunotherapy, tissue regeneration, inflammation resolution, and vaccination. A perspective on the future research directions of immunoregulatory biomaterials is also provided.

A Longitudinal Study of T2 Mapping Combined With Diffusion Tensor Imaging to Quantitatively Evaluate Tissue Repair of Rat Skeletal Muscle After Frostbite

Purpose: T2 mapping and diffusion tensor imaging (DTI) enable the detection of changes in the skeletal muscle microenvironment. We assessed T2 relaxation times, DTI metrics, performed histological characterization of frostbite-induced skeletal muscle injury and repair, and provided diagnostic imaging biomarkers.

Design and Methods: Thirty-six Sprague Dawley rats (200 ± 10 g) were obtained. Thirty rats were used for establishing a skeletal muscle frostbite model, and six were untreated controls. Functional MR sequences were performed on rats on days 0, 3, 5, 10, and 14 (n = 6 per time point). Rats were then sacrificed to obtain the quadriceps muscles. Tensor eigenvalues (λ1, λ2, and λ3), mean diffusivity (MD), fractional anisotropy (FA), and T2 values were compared between the frostbite model and control rats. ImageJ was used to measure the extracellular area fraction (EAF), muscle fiber cross-sectional area (fCSA), and skeletal muscle tumor necrosis factor α (TNF-α), and Myod1 expression. The correlation between the histological and imaging parameters of the frostbitten skeletal muscle was evaluated. Kolmogorov–Smirnoff test, Leven’s test, one-way ANOVA, and Spearman coefficient were used for analysis.

Results: T2 relaxation time of frostbitten skeletal muscle was higher at all time points (p < 0.01). T2 relaxation time correlated with EAF, and TNF-α and Myod1 expression (r = 0.42, p < 0.05; r = 0.86, p < 0.01; r = 0.84, p < 0.01). The average tensor metrics (MD, λ1, λ2, and λ3) of skeletal muscle at 3 and 5 days of frostbite increased (p < 0.05), and fCSA correlated with λ1, λ2, and λ3, and MD (r = 0.65, p < 0.01; r = 0.48, p < 0.01; r = 0.52, p < 0.01; r = 0.62, p < 0.01).

Conclusion: T2 mapping and DTI imaging detect frostbite-induced skeletal muscle injury early. This combined approach can quantitatively assess skeletal muscle repair and regeneration within 2 weeks of frostbite. Imaging biomarkers for the diagnosis of frostbite were suggested.

Computational Models Provide Insight into In Vivo Studies and Reveal the Complex Role of Fibrosis in mdx Muscle Regeneration

Duchenne muscular dystrophy is a pro-fibrotic, muscle wasting disease. Reducing fibrosis is a potential therapeutic target; however, its effect on muscle regeneration is not fully understood. This study (1) used an agent-based model to predict the effect of increased fibrosis in mdx muscle on regeneration from injury, and (2) experimentally tested the resulting model-derived hypothesis. The model predicted that increasing the area fraction of fibrosis decreased regeneration 28 days post injury due to limited growth factor diffusion and impaired cell migration. WT, mdx, and TGFβ-treated mdx mice were used to test this experimentally. TGFβ injections increased the extracellular matrix (ECM) area fraction; however, the passive stiffness of the treated muscle, which was assumed to correlate with ECM protein density, decreased following injections, suggesting that ECM protein density was lower. Further, there was no cross-sectional area (CSA) difference during recovery between the groups. Additional simulations revealed that decreasing the ECM protein density resulted in no difference in CSA, similar to the experiment. These results suggest that increases in ECM area fraction alone are not sufficient to reduce the regenerative capacity of mdx muscle, and that fibrosis is a complex pathological condition requiring further understanding.

Muscle functional recovery is driven by extracellular vesicles combined with muscle extracellular matrix in a volumetric muscle loss murine model

Biological scaffolds derived from decellularized tissues are being investigated as a promising approach to repair volumetric muscle losses (VML). Indeed, extracellular matrix (ECM) from decellularized tissues is highly biocompatible and mimics the original tissue. However, the development of fibrosis and the muscle stiffness still represents a major problem. Intercellular signals mediating tissue repair are conveyed via extracellular vesicles (EVs), biologically active nanoparticles secreted by the cells. This work aimed at using muscle ECM and human EVs derived from Wharton Jelly mesenchymal stromal cells (MSC EVs) to boost tissue regeneration in a VML murine model. Mice transplanted with muscle ECM and treated with PBS or MSC EVs were analyzed after 7 and 30 days. Flow cytometry, tissue analysis, qRT-PCR and physiology test were performed. We demonstrated that angiogenesis and myogenesis were enhanced while fibrosis was reduced after EV treatment. Moreover, the inflammation was directed toward tissue repair. M2-like, pro-regenerative macrophages were significantly increased in the MSC EVs treated group compared to control. Strikingly, the histological improvements were associated with enhanced functional recovery. These results suggest that human MSC EVs can be a naturally-derived boost able to ameliorate the efficacy of tissue-specific ECM in muscle regeneration up to the restored tissue function.

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20 years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Differential Effects of Myeloid Cell PPARδ and IL-10 in Regulating Macrophage Recruitment, Phenotype, and Regeneration following Acute Muscle Injury

Changes in macrophage phenotype in injured muscle profoundly influence regeneration. In particular, the shift of macrophages from a proinflammatory (M1 biased) phenotype to a proregenerative (M2 biased) phenotype characterized by expression of CD206 and CD163 is essential for normal repair. According to the current canonical mechanism regulating for M1/M2 phenotype transition, signaling through PPARδ is necessary for obtaining the M2-biased phenotype. Our findings confirm that the murine myeloid cell-targeted deletion of Ppard reduces expression in vitro of genes that are activated in M2-biased macrophages; however, the mutation in mice in vivo increased numbers of CD206+ M2-biased macrophages and did not reduce the expression of phenotypic markers of M2-biased macrophages in regenerating muscle. Nevertheless, the mutation impaired CCL2-mediated chemotaxis of macrophages and slowed revascularization of injured muscle. In contrast, null mutation of IL-10 diminished M2-biased macrophages but produced no defects in muscle revascularization. Our results provide two significant findings. First, they illustrate that mechanisms that regulate macrophage phenotype transitions in vitro are not always predictive of mechanisms that are most important in vivo. Second, they show that mechanisms that regulate macrophage phenotype transitions differ in different in vivo environments.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

Myogenic Cell Expression of Intercellular Adhesion Molecule-1 Contributes to Muscle Regeneration after Injury

This study investigated intercellular adhesion molecule-1 (ICAM-1), a membrane protein that mediates cell-to-cell adhesion and communication, as a mechanism through which the inflammatory response facilitates muscle regeneration after injury. Toxin-induced muscle injury to tibialis anterior muscles of wild-type mice caused ICAM-1 to be expressed by a population of satellite cells/myoblasts and myofibers. Myogenic cell expression of ICAM-1 contributed to the restoration of muscle structure after injury, as regenerating myofibers were more abundant and myofiber size was larger for wild-type compared with Icam1^-/- mice during 28 days of recovery. Contrastingly, restoration of muscle function after injury was similar between the genotypes. ICAM-1 facilitated the restoration of muscle structure after injury through mechanisms involving the regulation of myofiber branching, protein synthesis, and the organization of nuclei within myofibers after myogenic cell fusion. These findings provide support for a paradigm in which ICAM-1 expressed by myogenic cells after muscle injury augments their adhesive and fusogenic properties, which, in turn, facilitates regenerative and hypertrophic processes that restore structure to injured muscle.

Beneficial Effect of IL-4 and SDF-1 on Myogenic Potential of Mouse and Human Adipose Tissue-Derived Stromal Cells

Under physiological conditions skeletal muscle regeneration depends on the satellite cells. After injury these cells become activated, proliferate, and differentiate into myofibers reconstructing damaged tissue. Under pathological conditions satellite cells are not sufficient to support regeneration. For this reason, other cells are sought to be used in cell therapies, and different factors are tested as a tool to improve the regenerative potential of such cells. Many studies are conducted using animal cells, omitting the necessity to learn about human cells and compare them to animal ones. Here, we analyze and compare the impact of IL-4 and SDF-1, factors chosen by us on the basis of their ability to support myogenic differentiation and cell migration, at mouse and human adipose tissue-derived stromal cells (ADSCs). Importantly, we documented that mouse and human ADSCs differ in certain reactions to IL-4 and SDF-1. In general, the selected factors impacted transcriptome of ADSCs and improved migration and fusion ability of cells in vitro. In vivo, after transplantation into injured muscles, mouse ADSCs more eagerly participated in new myofiber formation than the human ones. However, regardless of the origin, ADSCs alleviated immune response and supported muscle reconstruction, and cytokine treatment enhanced these effects. Thus, we documented that the presence of ADSCs improves skeletal muscle regeneration and this influence could be increased by cell pretreatment with IL-4 and SDF-1.

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

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

Cobalt chloride, a chemical hypoxia-mimicking agent, suppresses myoblast differentiation by downregulating myogenin expression

Cobalt chloride can create hypoxia-like state in vitro (referred to as chemical hypoxia). Several studies have suggested that chemical hypoxia may cause deleterious effects on myogenesis. The intrinsic underlying mechanisms of myoblast differentiation, however, are not fully understood. Here, we show that cobalt chloride strongly suppresses myoblast differentiation in a dose-dependent manner. The impaired myoblast differentiation is accompanied by downregulation of myogenic regulatory factor myogenin. Under chemical hypoxia, myogenin stability is decreased at mRNA and protein levels. A muscle-specific E3 ubiquitin ligase MAFbx, which can target myogenin protein for proteasomal degradation, is upregulated along with changes in Akt/Foxo and AMPK/Foxo signaling pathways. A proteasome inhibitor completely prevents cobalt chloride-mediated decrease in myogenin protein. These results suggest that cobalt chloride might modulate myogenin expression at post-transcriptional and post-translational levels, resulting in the failure of the myoblasts to differentiate into myotubes.

Differential Macrophage Subsets in Muscle Damage Induced by a K49-PLA2 from Bothrops jararacussu Venom Modulate the Time Course of the Regeneration Process

Bothrops snakes cause around 80% of snakebites in Brazil, with muscle tissue damage as an important consequence, which may cause dysfunction on the affected limb. Bothropstoxin-I (BthTX-I) from Bothrops jararacussu is a K49-phospholipase A₂, involved in the injury and envenomation's inflammatory response. Immune system components act in the resolution of tissue damage and regeneration. Thus, macrophages exert a crucial role in the elimination of dead tissue and muscle repair. Here, we studied the cellular influx and presence of classical and alternative macrophages (M1 and M2) during muscle injury induced by BthTX-I and the regeneration process. BthTX-I elicited intense inflammatory response characterized by neutrophil migration, then increased influx of M1 macrophages followed by M2 population that declined, resulting in tissue regeneration. The high expressions of TNF-α and IL6 were changed by increased TGF-β expression after BthTX-I injection, coinciding with the iNOs and arginase expression and the peaks of M1 and M2 macrophages in muscle tissue. A coordinated sequence of PAX7, MyoD, and myogenin expression involved in muscle regenerative process appeared after BthTX-I injection. Together, these results demonstrate a direct correlation between the macrophage subsets, cytokine microenvironment, and the myogenesis process. This information may be useful for new envenomation and muscular dysfunction therapies.

Muscle regeneration in adiponectin knockout mice showed early activation of anti-inflammatory response with perturbations in myogenesis

Activation, proliferation, and differentiation of satellite cells can be influenced by extracellular factors, such as adiponectin. This adipokine has been proposed as a regulator of in vitro myogenesis, but its action on in vivo regeneration is not still elucidated. We used C57BL/6 (wild-type [WT]) and adiponectin knockout (AdKO) mice injured with barium chloride at periods of 3, 7, and 14 days after injury. The AdKO presented a higher number of centralized nuclei after 7 days, and a reduction in myogenic genes was observed after 3 days. Moreover, these mice presented an increase in anti-inflammatory cytokines after 3 and 7 days, and an increase in the M2 gene marker and proinflammatory cytokines after 7 days. The WT demonstrated an increase in adiponectin messenger RNA after 7 days. These results demonstrate that adiponectin is important in tissue remodeling during regeneration and that its deficiency does not compromise the maturation of muscle fibers, due to an increase in anti-inflammatory response; however, there is a possible impairment in proinflammatory response and an increase in centralized myonuclei.

Comparisons of lung and gluteus transcriptome profiles between yaks at different ages

The yak, Bos grunniens, is the only large mammal in the Qinghai-Tibet Plateau and has been bred to provide meat, milk, and transportation. Previous studies indicate that the immune system contributes to the yak's adaptation to high-altitude environments. In order to further investigate changes in immune function during yak development, we compared the transcriptome profiles of gluteus and lung tissues among yaks at 6, 30, 60, and 90 months of age. Analyses of significantly differentially expressed genes (DEGs) in lung tissues revealed that immune function was more activated at 6-months and less activated at 90-months than in the 30 and 60-month-old animals. DEG exploration in gluteal tissues revealed that immune functions were more highly activated at both 6 and 90-months, compared with 30 and 60-months. Immune system activation in the muscle and lung tissues of 30-month-old yaks may increase their resistance to infections, while decreased may be due to aging. Furthermore, the higher immune activation status in the gluteal tissues in 90-month-old yaks could be due to muscle injury and subsequent regeneration, which is supported by the fact that 5 unigenes related with muscle injury and 3 related to muscle regeneration displayed greater expression levels at 90-months than at 30 and 60-months. Overall, the present study highlights the important role of the immune system in yak development, which will facilitate future investigations.

Eccentric exercise results in a prolonged increase in interleukin-6 and tumor necrosis factor-α levels in rat skeletal muscle

Interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) are well-known cytokines with pro-inflammatory capabilities, and have been shown to be involved in adaptation to exercise as multifaceted myokines. However, the precise role of IL-6 and TNF-α during exercise-induced skeletal muscle injury and subsequent repair processes is not fully understood. In this study, IL-6 and TNF-α were examined in soleus muscles at the gene and protein levels using in situ hybridization and immunohistochemical staining, respectively, and serum levels of IL-6 and TNF-α were determined before and after a 90-min downhill running session in rats. There were no changes in serum levels of IL-6 and TNF-α after exercise, but IL-6 and TNF-α mRNA increased and maintained high expression in muscles for 1-2 weeks after exercise. IL-6 and TNF-a mRNAs were identified in both the cytoplasm and the nuclei of myocytes, as well as in invading inflammatory cells. IL-6 and TNF-α protein mainly distributed in cytoplasm unevenly and had a prolonged expression until 2 weeks after eccentric exercise. Our results demonstrate that there is increased IL-6 and TNF-α expression in skeletal muscle that is induced by eccentric exercise and that the high expression of IL-6 and TNF-α in the long-term phase after eccentric exercise may be more involved in the subsequent recovery of damaged muscle.

Injectable and degradable methacrylic acid hydrogel alters macrophage response in skeletal muscle

After severe trauma, skeletal muscle cannot repair itself leading to scar tissue formation and functional impairment. A novel approach to overcome this issue is to alter the fibrotic response in muscle using regenerative biomaterials, such as those containing methacrylic acid (MAA). In the skin, MAA-based materials have been shown to promote wound healing and new vessel formation, through endogenous mechanisms, including macrophage polarization; however, MAA has yet to be studied outside the skin. To study the innate immune response to MAA in skeletal muscle, MAA-poly(ethylene glycol) (MAA-PEG) hydrogels were synthesized with degradation rates of either 2 (fast-degrading) or 7 days (slow-degrading). When injected into the tibialis anterior muscle of mice, both slow- and fast-degrading MAA hydrogels increased the expression of Il-10, Tnfα and M2 macrophage markers (Fizz1 and Arg for slow-and fast-degrading, respectively). Moreover, the slow degrading MAA hydrogel decreased the number of pro-inflammatory MHCII+ macrophages. An unbiased t-distributed stochastic neighbor embedding (tSNE) analysis suggested the involvement of other immune cells beyond just macrophages in the effect of MAA on skeletal muscle. Overall, this study shows that MAA hydrogels bias macrophages towards a pro-regenerative phenotype.

Macrophages in inflammation, repair and regeneration

Tissue injury triggers a complex series of cellular responses, starting from inflammation activated by tissue and cell damage and proceeding to healing. By clearing cell debris, activating and resolving inflammation and promoting fibrosis, macrophages play key roles in most, if not all, phases of the response to injury. Recent studies of the mechanisms underlying the initial inflammation and later tissue regeneration and repair revealed that macrophages bridge these processes in part by supporting and activating stem/progenitor cells, clearing damaged tissue, remodeling extracellular matrix to prepare scaffolding for regeneration and promoting angiogenesis. However, macrophages also have a central role in the development of pathology induced by failed resolution (e.g. chronic inflammation) and excessive scarring. In this review, we summarize the activities of macrophages in inflammation and healing in response to acute injury in tissues with differing regenerative capacities. While macrophages lead similar processes in response to tissue injury in these tissues, their priorities and the consequences of their activities differ among tissues. Moreover, the magnitude, nature and duration of injury also greatly affect cellular responses and healing processes. In particular, continuous injury and/or failed resolution of inflammation leads to chronic ailments in which macrophage activities may become detrimental.

KLF2 in Myeloid Lineage Cells Regulates the Innate Immune Response during Skeletal Muscle Injury and Regeneration

Skeletal muscle repair and regeneration after injury requires coordinated interactions between the innate immune system and the injured muscle. Myeloid cells predominate in these interactions. This study examined the role of KLF2, a zinc-finger transcription factor that regulates immune cell activation, in specifying myeloid cell functions during muscle regeneration. Loss of KLF2 in myeloid lineage cells (myeKlf2^-/- mice) dramatically enhanced the initial inflammatory response to acute muscle injury (cardiotoxin). Injured muscles showed dramatically elevated expression of inflammatory mediators and greater numbers of infiltrating, pro-inflammatory monocytes that matured earlier into activated macrophages. Notably, the inflammatory phase resolved earlier and regeneration progressed to myogenesis, marked by elevated expression of factors that promote the formation of new fibers from satellite cells. Regeneration was completed earlier, with phenotypically normal adult fibers integrated into the muscle syncytium. These findings identify myeloid KLF2 as a key regulator of myeloid cell functions in adult skeletal muscle regeneration.

Adipose Tissue-Derived Stromal Cells in Matrigel Impacts the Regeneration of Severely Damaged Skeletal Muscles

In case of large injuries of skeletal muscles the pool of endogenous stem cells, i.e., satellite cells, might be not sufficient to secure proper regeneration. Such failure in reconstruction is often associated with loss of muscle mass and excessive formation of connective tissue. Therapies aiming to improve skeletal muscle regeneration and prevent fibrosis may rely on the transplantation of different types of stem cell. Among such cells are adipose tissue-derived stromal cells (ADSCs) which are relatively easy to isolate, culture, and manipulate. Our study aimed to verify applicability of ADSCs in the therapies of severely injured skeletal muscles. We tested whether 3D structures obtained from Matrigel populated with ADSCs and transplanted to regenerating mouse gastrocnemius muscles could improve the regeneration. In addition, ADSCs used in this study were pretreated with myoblasts-conditioned medium or anti-TGFβ antibody, i.e., the factors modifying their ability to proliferate, migrate, or differentiate. Analyses performed one week after injury allowed us to show the impact of 3D cultured control and pretreated ADSCs at muscle mass and structure, as well as fibrosis development immune response of the injured muscle.

CXCL10 increases in human skeletal muscle following damage but is not necessary for muscle regeneration

CXCL10 is a chemokine for activated and memory T cells with many important immunological functions. We recently found that CXCL10 is upregulated in human muscle following contraction‐induced damage. No information is available on the role of CXCL10 in the context of muscle damage or repair. In this study, we confirm that CXCL10 is elevated in human muscle at 2 and 3 days following damage and perform cell culture and animal studies to examine the role of CXCL10 in muscle repair. CXCL10 did not impact proliferation rates of human primary myoblasts but it did promote myogenic differentiation in vitro, suggesting a possible direct impact on muscle regeneration. To test if CXCL10 was dispensable for effective muscle regeneration in vivo, we measured functional and histological markers of muscle repair out to 14 days postmuscle injury caused by a myotoxin in wild‐type (WT) mice and CXCL10 knockout (KO) mice. Between genotypes, no significant differences were found in loss or restoration of in situ muscle force, cross‐sectional area of newly formed myofibers, or the number of embryonic myosin heavy chain‐positive myofibers. In addition, KO animals were not deficient in T‐cell accumulation in the damaged muscle following injury. Gene expression of the other two ligands (CXCL9 and 11) that bind to the same receptor as CXCL10 were also elevated in the damaged muscle of KO mice. Thus, other ligands may have compensated for the lack of CXCL10 in the KO mice. We conclude that CXCL10 is not necessary for effective muscle regeneration.

Dilated cardiomyopathy-mediated heart failure induces a unique skeletal muscle myopathy with inflammation

Background

Skeletal muscle myopathy and exercise intolerance are diagnostic hallmarks of heart failure (HF). However, the molecular adaptations of skeletal muscles during dilated cardiomyopathy (DCM)-mediated HF are not completely understood.

Methods

Skeletal muscle structure and function were compared in wild-type (WT) and cardiac myosin binding protein-C null mice (t/t), which develop DCM-induced HF. Cardiac function was examined by echocardiography. Exercise tolerance was measured using a graded maximum treadmill running test. Hindlimb muscle function was assessed in vivo from measurements of plantar flexor strength. Inflammatory status was evaluated from the expression of inflammatory markers and the presence of specific immune cell types in gastrocnemius muscles. Muscle regenerative capacityat days 3, 7, and 14 after eccentric contraction-induced injury was determined from the number of phenotypically new and adult fibers in the gastrocnemius, and functional recovery of plantar flexion torque.

Results

t/t mice developed DCM-induced HF in association with profound exercise intolerance, consistent with previous reports. Compared to WT, t/t mouse hearts show significant hypertrophy of the atria and ventricles and reduced fractional shortening, both systolic and diastolic. In parallel, the skeletal muscles of t/t mice exhibit weakness and myopathy. Compared to WT, plantar flexor muscles of t/t null mice produce less peak isometric plantar torque (Po), develop torque more slowly (+ dF/dt), and relax more slowly (− dF/dt, longer half-relaxation times,1/2RT). Gastrocnemius muscles of t/t mice have a greater number of fibers with smaller diameters and central nuclei. Oxidative fibers, both type I and type IIa, show significantly smaller cross-sectional areas and more central nuclei. These fiber phenotypes suggest ongoing repair and regeneration under homeostatic conditions. In addition, the ability of muscles to recover and regenerate after acute injury is impaired in t/t mice.

Conclusions

Our studies concluded that DCM-induced HF induces a unique skeletal myopathy characterized by decreased muscle strength, atrophy of oxidative fiber types, ongoing inflammation and damage under homeostasis, and impaired regeneration after acute muscle injury. Furthermore, this unique myopathy in DCM-induced HF likely contributes to and exacerbates exercise intolerance. Therefore, efforts to develop therapeutic interventions to treat skeletal myopathy during DCM-induced HF should be considered.

Electronic supplementary material

The online version of this article (10.1186/s13395-019-0189-y) contains supplementary material, which is available to authorized users.

Effects of Dandelion Extract on the Proliferation of Rat Skeletal Muscle Cells and the Inhibition of a Lipopolysaccharide-Induced Inflammatory Reaction

Background

Dandelion is commonly used in traditional Chinese medicine with several active compounds found in extracts. It has a variety of pharmacological effects, such as a reduction in swelling and inflammation, and detoxification. The mechanism by which dandelion extract inhibits the inflammatory response in skeletal muscle cells remains unknown; therefore, the aim of this study was to investigate the effects of dandelion extract root on the proliferation of skeletal muscle cells and the alleviation of lipopolysaccharide (LPS)-induced inflammatory response in vitro.

Methods

Rat skeletal muscle cells were isolated from Sprague-Dawley rat and cultured in vitro which were cultured in basal medium, or medium containing LPS or dandelion extract. Cell counting kit-8 (CCK-8) was employed to measure cell proliferation; meanwhile, the optimal concentration of dandelion extract and treatment time were selected. Crystal violet staining was used to detect the proliferation of muscle cells. Western blotting analysis was used to detect the levels of inflammatory factors, myogenic factor, and p-AKT protein expression.

Results

The optimal concentration and treatment time of dandelion extract for the following study were 5 mg/ml and 4 days, respectively. Dandelion extract was found to increase proliferation of rat skeletal muscle cells (t = 3.145, P < 0.05), with the highest effect observed at 5 mg/ml. LPS was found to decrease proliferation of skeletal muscle cells (t = -131.959, P < 0.001), and dandelion extract could against this affection (t = 19.466, P < 0.01). LPS could induce expression of inflammatory factors, including interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α (IL-1β: t = 9.118, P < 0.01; IL-6: t = 4.346, P < 0.05; TNF-α: t = 15.806, P < 0.05), and dandelion extract was shown to reduce LPS-induced expression of IL-1β, IL-6 and TNF-α (IL-1β: t = -2.823, P < 0.05; IL-6: t = -3.348, P < 0.01; and TNF-α: t = -3.710, P < 0.01). Furthermore, LPS was also shown to decrease expression of myogenic factor, including myod1 and myogenin (MyoD1: t = 4.039, P < 0.05 and myogenin: t = 3.300, P < 0.01), but dandelion extract was shown to against this effect of LPS (MyoD1: t = -3.160, P < 0.05 and myogenin: t = -3.207, P < 0.01). And then, LPS was found to increase expression of p-AKT protein (p-AKT/AKT: t = 4.432, P < 0.05). Moreover, expression of p-AKT protein was found to decrease, with 5 mg/ml of dandelion extract (p-AKT/AKT: t = -3.618, P < 0.05).

Conclusions

The findings indicate that dandelion extract plays an important role in skeletal muscle cells viability regulation, promote cells proliferation by increasing level of p-AKT protein expression, and reduce LPS-induced expression of inflammatory factors, inhibiting the inflammatory response of rat skeletal muscle cells.

A moderate oestradiol level enhances neutrophil number and activity in muscle after traumatic injury but strength recovery is accelerated

KEY POINTS

The female hormone oestrogen may protect muscle from injury by reducing inflammation but this is debatable. In this study, the inflammatory response of injured muscle from oestrogen-replete mice was comprehensively compared to that from oestrogen-deficient mice. We show that oestrogen markedly promotes movement of neutrophils, an inflammatory white blood cell type, into muscle over the first few days after injury but has only a minor effect on the movement of macrophages, another inflammatory cell type. Despite the enhancement of inflammation by oestrogen in injured muscle, we found strength in oestrogen-replete mice to recover faster and to a greater extent than it does in oestrogen-deficient mice. Our study and others indicate that lower doses of oestrogen, such as that used in our study, may affect muscle inflammation and injury differently from higher doses.

ABSTRACT

Oestrogen has been shown to protect against skeletal muscle injury and a reduced inflammatory response has been suggested as a possible protective mechanism. There are, however, dissenting reports. Our objective was to conduct an unbiased, comprehensive study of the effect of oestradiol on the inflammatory response following muscle injury. Female C57BL6/J mice were ovariectomized and supplemented with and without oestradiol. Tibialis anterior muscles were freeze injured and studied primarily at 1-4 days post-injury. Oestradiol supplementation increased injured muscle gene expression of neutrophil chemoattractants (Cxcl1 and Cxcl5) and to a lesser extent that of monocyte/macrophage chemoattractants (Ccl2 and Spp1). Oestradiol markedly increased gene expression of the neutrophil cell surface marker (Ly6g) but had less consistent effects on the monocyte/macrophage cell surface markers (Cd68, Cd163 and Cd206). These results were confirmed at the protein level by immunoblot with oestradiol increasing LY6G/C content and having no significant effect on CD163 content. These findings were confirmed with fluorescence-activated cell sorting counts of neutrophils and macrophages in injured muscles; oestradiol increased the proportion of CD45+ cells that were neutrophils (LY6G+ ) but not the proportion that were macrophages (CD68+ or CD206+ ). Physiological impact of the oestradiol-enhanced neutrophil response was assessed by strength measurements. There was no significant difference in strength between oestradiol-supplemented and -unsupplemented mice until 2 weeks post-injury; strength was 13-24% greater in supplemented mice at 2-6 weeks post-injury. In conclusion, a moderate level of oestradiol supplementation enhances neutrophil infiltration in injured muscle and this is associated with a beneficial effect on strength recovery.

Agent-based model illustrates the role of the microenvironment in regeneration in healthy and mdx skeletal muscle

Duchenne muscular dystrophy (DMD) is a progressive muscle-wasting disease with no effective treatment. Multiple mechanisms are thought to contribute to muscle wasting, including increased susceptibility to contraction-induced damage, chronic inflammation, fibrosis, altered satellite stem cell (SSC) dynamics, and impaired regenerative capacity. The goals of this project were to 1) develop an agent-based model of skeletal muscle that predicts the dynamic regenerative response of muscle cells, fibroblasts, SSCs, and inflammatory cells as a result of contraction-induced injury, 2) calibrate and validate the model parameters based on comparisons with published experimental measurements, and 3) use the model to investigate how changing isolated and combined factors known to be associated with DMD (e.g., altered fibroblast or SSC behaviors) influence muscle regeneration. Our predictions revealed that the percent of injured muscle that recovered 28 days after injury was dependent on the peak SSC counts following injury. In simulations with near-full cross-sectional area recovery (healthy, 4-wk mdx, 3-mo mdx), the SSC counts correlated with the extent of initial injury; however, in simulations with impaired regeneration (9-mo mdx), the peak SSC counts were suppressed relative to initial injury. The differences in SSC counts between these groups were emergent predictions dependent on altered microenvironment factors known to be associated with DMD. Multiple cell types influenced the peak number of SSCs, but no individual parameter predicted the differences in SSC counts. This finding suggests that interventions to target the microenvironment rather than SSCs directly could be an effective method for improving regeneration in impaired muscle. NEW & NOTEWORTHY A computational model predicted that satellite stem cell (SSC) counts are correlated with muscle cross-sectional area (CSA) recovery following injury. In simulations with impaired CSA recovery, SSC counts are suppressed relative to healthy muscle. The suppressed SSC counts were an emergent model prediction, because all simulations had equal initial SSC counts. Fibroblast and anti-inflammatory macrophage counts influenced SSC counts, but no single factor was able to predict the pathological differences in SSC counts that lead to impaired regeneration.

Caveolae-mediated effects of TNF-α on human skeletal muscle cells

Chronic diseases are characterized by the production of pro-inflammatory cytokines such than TNF-α and are frequently correlated with muscle wasting conditions. Among the pleiotropic effects of TNF-α within the cell, its binding to TNFR1 receptor has been shown to activate sphingomyelinases leading to the production of ceramides. Sphingomyelinases and TNF receptor have been localized within caveolae which are specialized RAFT enriched in cholesterol and sphingolipids. Because of their inverted omega shape, maintained by the oligomerization of specialized proteins, caveolins and cavins, caveolae serve as membrane reservoir therefore providing mechanical protection to plasma membranes. Although sphingolipids metabolites, caveolins and TNF-α/TNFR1 have been shown to independently interfere with muscle physiology, no data have clearly demonstrated their concerted action on muscle cell regeneration. In this context, our study aimed at studying the molecular mechanisms induced by TNF-α at the level of caveolae in LHCN-M2 human muscle satellite cells. Here we showed that TNF-α-induced production of ROS and nSMase activation requires caveolin. More strikingly, we have demonstrated that TNF-α induces the formation of additional caveolae at the plasma membrane of myoblasts. Furthermore, TNF-α prevents myoblast fusion suggesting that inflammation could modulate caveolae organization/function and satellite cell function.

Muscle Fn14 gene expression is associated with fat-free mass retention during energy deficit at high altitude

Intramuscular factors that modulate fat-free mass (FFM) loss in lowlanders exposed to energy deficit during high-altitude (HA) sojourns remain unclear. Muscle inflammation may contribute to FFM loss at HA by inducing atrophy and inhibiting myogenesis via the tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) and its receptor, fibroblast growth factor-inducible protein 14 (Fn14). To explore whether muscle inflammation modulates FFM loss reportedly developing during HA sojourns, muscle inflammation, myogenesis, and proteolysis were assessed in 16 men at sea level (SL) and following 21 days of energy deficit (-1862 ± 525 kcal/days) at high altitude (HA, 4300 m). Total body mass (TBM), FFM, and fat mass (FM) were assessed using DEXA. Gene expression and proteolytic enzymatic activities were assessed in muscle samples collected at rest at SL and HA. Participants lost 7.2 ± 1.8 kg TBM (P < 0.05); 43 ± 30% and 57 ± 30% of the TBM lost was FFM and FM, respectively. Fn14, TWEAK, TNF alpha-receptor (TNFα-R), TNFα, MYOGENIN, and paired box protein-7 (PAX7) were upregulated (P < 0.05) at HA compared to SL. Stepwise linear regression identified that Fn14 explained the highest percentage of variance in FFM loss (r2  = 0.511, P < 0.05). Dichotomization of volunteers into HIGH and LOW Fn14 gene expression indicated HIGH lost less FFM and more FM (28 ± 28% and 72 ± 28%, respectively) as a proportion of TBM loss than LOW (58 ± 26% and 42 ± 26%; P < 0.05) at HA. MYOGENIN gene expression was also greater for HIGH versus LOW (P < 0.05). These data suggest that heightened Fn14 gene expression is not catabolic and may protect FFM during HA sojourns.

Minimal Evidence for a Secondary Loss of Strength After an Acute Muscle Injury: A Systematic Review and Meta-Analysis

BACKGROUND

An immediate loss of strength follows virtually all types of muscle injury but there is debate whether the initial strength loss is maximal or if a secondary loss of strength occurs during the first 3 days post-injury.

OBJECTIVE

The objective of this analysis was to conduct a systematic review and meta-analysis of the research literature to determine if a secondary loss of strength occurs after an injurious initiating event.

METHODS

Literature searches were performed using eight electronic databases (e.g., PubMed, Cochrane Library). Search terms included skeletal muscle AND (injur* OR damage*) AND (strength OR force OR torque). The extracted strength data were converted to a standard format by calculating the standardized mean difference, which is reported as the effect size (ES) along with its 95 % confidence interval (CI). The calculation of ES was designed so that a negative ES that was statistically less than zero would be interpreted as indicating a secondary loss of strength.

RESULTS

A total of 223 studies with over 4000 human and animal subjects yielded data on 262 independent groups and a total of 936 separate ESs. Our overall meta-analysis yielded a small-to-medium, positive overall ES that was statistically greater than zero (overall ES = +0.34, 95 % CI 0.27-0.40; P < 0.00000001). Considerable variation in ES was observed among studies (I ² = 86 %), which could be partially explained by the research group conducting the study, sex of the subject, day of post-injury strength assessment, whether fatigue was present immediately post-injury, and the muscle group injured. From the subgroup meta-analyses probing these variables, 36 subgroup ESs were calculated and none were statistically less than zero.

CONCLUSION

Overall, our findings do not support the presence of a secondary loss of strength following an acute muscle injury, and strongly suggest that strength, on average, recovers steadily over the first 3 days post-injury.