Temporal and spatial mRNA expression patterns of TGF-beta1, 2, 3 and TbetaRI, II, III in skeletal muscles of mdx mice

Expansion and pathogenic activation of skeletal muscle-resident macrophages in mdx5cv/Ccr2-/- mice

Infiltrating macrophages contribute to muscle dystrophic changes in Duchenne muscular dystrophy (DMD). In a DMD mouse model, mdx5cv mice, CC chemokine receptor type 2 (CCR2) deficiency diminishes Ly6Chi macrophage infiltration by blocking blood Ly6Chi inflammatory monocyte recruitment. This is accompanied by transient improvement of muscle damage, fibrosis, and regeneration. The benefit, however, is lost after the expansion of intramuscular Ly6Clo macrophages. To address the mechanisms underlying the Ly6Clo macrophage expansion, we compared mdx5cv/Nur77-/- and mdx5cv/Ccr2-/-/Nur7-/- mice with mdx5cv and mdx5cv/Ccr2-/- mice, respectively, and found no evidence to suggest Ly6Clo monocyte recruitment by dystrophic muscles. Single-cell RNA sequencing analysis and Flt3cre/Rosa26LSL-YFP-based lineage tracing of macrophage origins demonstrated the expansion and pathogenic activation of muscle resident macrophages in CCR2-deficient mdx5cv mice. The expansion was associated with increased cell proliferation, which appeared induced by colony-stimulating factor-1 (CSF-1) derived from fibro/adipogenic progenitors (FAPs). Our study establishes a pathogenic role for skeletal muscle resident macrophages and supports a regulatory role of FAPs in stimulating the expansion of resident macrophages in the DMD mouse model when the inflammatory macrophage infiltration is inhibited.

Macrophages in the Context of Muscle Regeneration and Duchenne Muscular Dystrophy

Macrophages are essential to muscle regeneration, as they regulate inflammation, carry out phagocytosis, and facilitate tissue repair. These cells exhibit phenotypic switching from pro-inflammatory (M1) to anti-inflammatory (M2) states during muscle repair, influencing myoblast proliferation, differentiation, and myofiber formation. In Duchenne Muscular Dystrophy (DMD), asynchronous muscle injuries disrupt the normal temporal stages of regeneration, leading to fibrosis and failed regeneration. Altered macrophage activity is associated with DMD progression and physiopathology. Gaining insight into the intricate relationship between macrophages and muscle cells is crucial for creating effective therapies aimed at treating this muscle disorder. This review explores the dynamic functions of macrophages in muscle regeneration and their implications in DMD.

Single cell RNA sequencing of human FAPs reveals different functional stages in Duchenne muscular dystrophy

Background: Duchenne muscular dystrophy is a genetic disease produced by mutations in the dystrophin gene characterized by early onset muscle weakness leading to severe and irreversible disability. Muscle degeneration involves a complex interplay between multiple cell lineages spatially located within areas of damage, termed the degenerative niche, including inflammatory cells, satellite cells (SCs) and fibro-adipogenic precursor cells (FAPs). FAPs are mesenchymal stem cell which have a pivotal role in muscle homeostasis as they can either promote muscle regeneration or contribute to muscle degeneration by expanding fibrotic and fatty tissue. Although it has been described that FAPs could have a different behavior in DMD patients than in healthy controls, the molecular pathways regulating their function as well as their gene expression profile are unknown. Methods: We used single-cell RNA sequencing (scRNAseq) with 10X Genomics and Illumina technology to elucidate the differences in the transcriptional profile of isolated FAPs from healthy and DMD patients. Results: Gene signatures in FAPs from both groups revealed transcriptional differences. Seurat analysis categorized cell clusters as proliferative FAPs, regulatory FAPs, inflammatory FAPs, and myofibroblasts. Differentially expressed genes (DEGs) between healthy and DMD FAPs included upregulated genes CHI3L1, EFEMP1, MFAP5, and TGFBR2 in DMD. Functional analysis highlighted distinctions in system development, wound healing, and cytoskeletal organization in control FAPs, while extracellular organization, degradation, and collagen degradation were upregulated in DMD FAPs. Validation of DEGs in additional samples (n = 9) using qPCR reinforced the specific impact of pathological settings on FAP heterogeneity, reflecting their distinct contribution to fibro or fatty degeneration in vivo. Conclusion: Using the single-cell RNA seq from human samples provide new opportunities to study cellular coordination to further understand the regulation of muscle homeostasis and degeneration that occurs in muscular dystrophies.

Association analysis of transcriptome and quasi-targeted metabolomics reveals the regulation mechanism underlying broiler muscle tissue development at different levels of dietary guanidinoacetic acid

The development and characteristics of muscle fibers in broilers are critical determinants that influence their growth performance, as well as serve as essential prerequisites for the production of high-quality chicken meat. Guanidinoacetic acid (GAA) is a crucial endogenous substance in animal creatine synthesis, and its utilization as a feed additive has been demonstrated the capabilities to enhance animal performance, optimize muscle yield, and augment carcass quality. The objective of this study was to investigate the regulation and molecular mechanism underlying muscle development in broilers at different levels of GAA via multiple omics analysis. The 90 Cobb broilers, aged 1 day, were randomly allocated into three treatments consisting of five replicates of six chickens each. The control group was provided with a basal diet, while the Normal GAA and High GAA groups received a basal diet supplemented with 1.2 g/kg and 3.6 g/kg of GAA, respectively. After a feeding period of 42 days, the pectoralis muscles were collected for histomorphological observation, transcriptome and metabolomic analysis. The results demonstrated that the addition of 1.2 g/kg GAA in the diet led to an augmentation in muscle fiber diameter and up-regulation of IGF1, IHH, ASB2, and ANKRD2 gene expression. However, a high dose of 3.6 g/kg GAA in the diet potentially reversed the beneficial effects on chicken breast development by excessively activating the TGF-β signaling pathway and reducing nucleotide metabolite content. These findings would provide a theoretical foundation for enhancing the performance and meat quality of broilers by incorporating GAA as a feed additive.

Interaction between Selected Adipokines and Musculoskeletal and Cardiovascular Systems: A Review of Current Knowledge

Adipokines are substances secreted by adipose tissue that are receiving increasing attention. The approach to adipose tissue has changed in recent years, and it is no longer looked at as just a storage organ but its secretion and how it influences systems in the human body are also looked at. The role of adipokine seems crucial in developing future therapies for pathologies of selected systems. In this study, we look at selected adipokines, leptin, adiponectin, chemerin, resistin, omentin-1, nesfatin, irisin-1, visfatin, apelin, vaspin, heparin-binding EGF-like growth factor (HB-EGF), and TGF-β2, and how they affect systems in the human body related to physical activity such as the musculoskeletal and cardiovascular systems.

The multifaceted role of macrophages in homeostatic and injured skeletal muscle

Skeletal muscle is essential for body physical activity, energy metabolism, and temperature maintenance. It has excellent capabilities to maintain homeostasis and to regenerate after injury, which indispensably relies on muscle stem cells, satellite cells (MuSCs). The quiescence, activation, and differentiation of MuSCs are tightly regulated in homeostatic and regenerating muscles. Among the important regulators are intramuscular macrophages, which are functionally heterogeneous with different subtypes present in a spatiotemporal manner to regulate the balance of different MuSC statuses. During chronic injury and aging, intramuscular macrophages often undergo aberrant activation, which in turn disrupts muscle homeostasis and regenerative repair. Growing evidence suggests that the aberrant activation is mainly triggered by altered muscle microenvironment. The trained immunity that affects myeloid progenitors during hematopoiesis may also contribute. Aged immune system may contribute, in part, to the aging-related sarcopenia and compromised skeletal muscle injury repair. As macrophages are actively involved in the progression of many muscle diseases, manipulating their functional activation has become a promising therapeutic approach, which requires comprehensive knowledge of the cellular and molecular mechanisms underlying the diverse activation. To this end, we discuss here the current knowledge of multifaceted role of macrophages in skeletal muscle homeostasis, injury, and repair.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

Duchenne muscular dystrophy (DMD) is a severe, progressive, and ultimately fatal disease of skeletal muscle wasting, respiratory insufficiency, and cardiomyopathy. The identification of the dystrophin gene as central to DMD pathogenesis has led to the understanding of the muscle membrane and the proteins involved in membrane stability as the focal point of the disease. The lessons learned from decades of research in human genetics, biochemistry, and physiology have culminated in establishing the myriad functionalities of dystrophin in striated muscle biology. Here, we review the pathophysiological basis of DMD and discuss recent progress toward the development of therapeutic strategies for DMD that are currently close to or are in human clinical trials. The first section of the review focuses on DMD and the mechanisms contributing to membrane instability, inflammation, and fibrosis. The second section discusses therapeutic strategies currently used to treat DMD. This includes a focus on outlining the strengths and limitations of approaches directed at correcting the genetic defect through dystrophin gene replacement, modification, repair, and/or a range of dystrophin-independent approaches. The final section highlights the different therapeutic strategies for DMD currently in clinical trials.

Diverse effector and regulatory functions of fibro/adipogenic progenitors during skeletal muscle fibrosis in muscular dystrophy

Fibrosis is a prominent pathological feature of skeletal muscle in Duchenne muscular dystrophy (DMD). The commonly used disease mouse model, mdx ^5cv , displays progressive fibrosis in the diaphragm but not limb muscles. We use single-cell RNA sequencing to determine the cellular expression of the genes involved in extracellular matrix (ECM) production and degradation in the mdx ^5cv diaphragm and quadriceps. We find that fibro/adipogenic progenitors (FAPs) are not only the primary source of ECM but also the predominant cells that express important ECM regulatory genes, including Ccn2, Ltbp4, Mmp2, Mmp14, Timp1, Timp2, and Loxs. The effector and regulatory functions are exerted by diverse FAP clusters which are different between diaphragm and quadriceps, indicating their activation by different tissue microenvironments. FAPs are more abundant in diaphragm than in quadriceps. Our findings suggest that the development of anti-fibrotic therapy for DMD should target not only the ECM production but also the pro-fibrogenic regulatory functions of FAPs.

The Many Roles of Macrophages in Skeletal Muscle Injury and Repair

Skeletal muscle is essential to physical activity and energy metabolism. Maintaining intact functions of skeletal muscle is crucial to health and wellbeing. Evolutionarily, skeletal muscle has developed a remarkable capacity to maintain homeostasis and to regenerate after injury, which indispensably relies on the resident muscle stem cells, satellite cells. Satellite cells are largely quiescent in the homeostatic steady state. They are activated in response to muscle injury. Activated satellite cells proliferate and differentiate into myoblasts. Myoblasts fuse to form myotubes which further grow and differentiate into mature myofibers. This process is tightly regulated by muscle microenvironment that consists of multiple cellular and molecular components, including macrophages. Present in both homeostatic and injured muscles, macrophages contain heterogeneous functional subtypes that play diverse roles in maintaining homeostasis and promoting injury repair. The spatial-temporal presence of different functional subtypes of macrophages and their interactions with myogenic cells are vital to the proper regeneration of skeletal muscle after injury. However, this well-coordinated process is often disrupted in a chronic muscle disease, such as muscular dystrophy, leading to asynchronous activation and differentiation of satellite cells and aberrant muscle regeneration. Understanding the precise cellular and molecular processes regulating interactions between macrophages and myogenic cells is critical to the development of therapeutic manipulation of macrophages to promote injury repair. Here, we review the current knowledge of the many roles played by macrophages in the regulation of myogenic cells in homeostatic, regenerating, and dystrophic skeletal muscles.

Selective ablation of Nfix in macrophages attenuates muscular dystrophy by inhibiting fibro-adipogenic progenitor-dependent fibrosis

Muscular dystrophies are genetic diseases characterized by chronic inflammation and fibrosis. Macrophages are immune cells that sustain muscle regeneration upon acute injury but seem deleterious in the context of chronic muscle injury such as in muscular dystrophies. Here, we observed that the number of macrophages expressing the transcription factor Nfix increases in two distinct mouse models of muscular dystrophies. We showed that the deletion of Nfix in macrophages in dystrophic mice delays the establishment of fibrosis and muscle wasting, and increases grasp force. Macrophages lacking Nfix expressed more TNFα and less TGFβ1, thus promoting apoptosis of fibro‐adipogenic progenitors. Moreover, pharmacological treatment of dystrophic mice with a ROCK inhibitor accelerated fibrosis through the increase of Nfix expression by macrophages. Thus, we have identified Nfix as a macrophage profibrotic factor in muscular dystrophies, whose inhibition could be a therapeutic route to reduce severity of the dystrophic disease. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Macrophages in Skeletal Muscle Dystrophies, An Entangled Partner

While skeletal muscle remodeling happens throughout life, diseases that result in its dysfunction are accountable for many deaths. Indeed, skeletal muscle is exceptionally capable to respond to stimuli modifying its homeostasis, such as in atrophy, hypertrophy, regeneration and repair. In particular conditions such as genetic diseases (muscular dystrophies), skeletal muscle’s capacity to remodel is strongly affected and undergoes continuous cycles of chronic damage. This induces scarring, fatty infiltration, as well as loss of contractibility and of the ability to generate force. In this context, inflammation, primarily mediated by macrophages, plays a central pathogenic role. Macrophages contribute as the primary regulators of inflammation during skeletal muscle regeneration, affecting tissue-resident cells such as myogenic cells and endothelial cells, but also fibro-adipogenic progenitors, which are the main source of the fibro fatty scar. During skeletal muscle regeneration their function is tightly orchestrated, while in dystrophies their fate is strongly disturbed, resulting in chronic inflammation. In this review, we will discuss the latest findings on the role of macrophages in skeletal muscle diseases, and how they are regulated.

Inflammation, fibrosis and skeletal muscle regeneration in LGMDR9 are orchestrated by macrophages

AIMS

Variable degrees of inflammation, necrosis, regeneration and fibrofatty replacement are part of the pathological spectrum of the dystrophic process in alpha dystroglycanopathy LGMDR9 (FKRP-related, OMIM #607155), one of the most prevailing types of LGMDs worldwide. Inflammatory processes and their complex interplay with vascular, myogenic and mesenchymal cells may have a major impact on disease development. The purpose of our study is to describe the specific immune morphological features in muscle tissue of patients with LGMDR9 to enable a better understanding of the phenotype of muscle damage leading to disease progression.

METHODS

We have analysed skeletal muscle biopsies of 17 patients genetically confirmed as having LGMDR9 by histopathological and molecular techniques.

RESULTS

We identified CD206⁺ MHC class II⁺ and STAT6⁺ immune-repressed macrophages dominating the endomysial infiltrate in areas of myofibre regeneration and fibrosis. Additionally, PDGFRβ⁺ pericytes were located around MHC class II⁺ activated capillaries residing in close proximity to areas of fibrosis and regenerating fibres. Expression of VEGF was found on many regenerating neonatal myosin⁺ fibres, myofibres and CD206⁺ macrophages also co-expressed VEGF.

CONCLUSION

Our results show characteristic immune inflammatory features in LGMDR9 and more specifically shed light on the predominant role of macrophages and their function in vascular organisation, fibrosis and myogenesis. Understanding disease-specific immune phenomena potentially inform about possibilities for anti-fibrotic and anti-inflammatory therapeutic strategies, which may complement Ribitol replacement and gene therapies for LGMDR9 that may be available in the future.

Rheumatoid cachexia: the underappreciated role of myoblast, macrophage and fibroblast interplay in the skeletal muscle niche

Although rheumatoid arthritis affects 1% of the global population, the role of rheumatoid cachexia, which occurs in up to a third of patients, is relatively neglected as research focus, despite its significant contribution to decreased quality of life in patients. A better understanding of the cellular and molecular processes involved in rheumatoid cachexia, as well as its potential treatment, is dependent on elucidation of the intricate interactions of the cells involved, such as myoblasts, fibroblasts and macrophages. Persistent RA-associated inflammation results in a relative depletion of the capacity for regeneration and repair in the satellite cell niche. The repair that does proceed is suboptimal due to dysregulated communication from the other cellular role players in this multi-cellular environment. This includes the incomplete switch in macrophage phenotype resulting in a lingering pro-inflammatory state within the tissues, as well as fibroblast-associated dysregulation of the dynamic control of the extracellular matrix. Additional to this endogenous dysregulation, some treatment strategies for RA may exacerbate muscle wasting and no multi-cell investigation has been done in this context. This review summarizes the most recent literature characterising clinical RA cachexia and links these features to the roles of and complex communication between multiple cellular contributors in the muscle niche, highlighting the importance of a targeted approach to therapeutic intervention.

Myogenesis modelled by human pluripotent stem cells: a multi-omic study of Duchenne myopathy early onset

BACKGROUND

Duchenne muscular dystrophy (DMD) causes severe disability of children and death of young men, with an incidence of approximately 1/5000 male births. Symptoms appear in early childhood, with a diagnosis made mostly around 4 years old, a time where the amount of muscle damage is already significant, preventing early therapeutic interventions that could be more efficient at halting disease progression. In the meantime, the precise moment at which disease phenotypes arise-even asymptomatically-is still unknown. Thus, there is a critical need to better define DMD onset as well as its first manifestations, which could help identify early disease biomarkers and novel therapeutic targets.

METHODS

We have used both human tissue-derived myoblasts and human induced pluripotent stem cells (hiPSCs) from DMD patients to model skeletal myogenesis and compared their differentiation dynamics with that of healthy control cells by a comprehensive multi-omic analysis at seven time points. Results were strengthened with the analysis of isogenic CRISPR-edited human embryonic stem cells and through comparisons against published transcriptomic and proteomic datasets from human DMD muscles. The study was completed with DMD knockdown/rescue experiments in hiPSC-derived skeletal muscle progenitor cells and adenosine triphosphate measurement in hiPSC-derived myotubes.

RESULTS

Transcriptome and miRnome comparisons combined with protein analyses demonstrated that hiPSC differentiation (i) leads to embryonic/foetal myotubes that mimic described DMD phenotypes at the differentiation endpoint and (ii) homogeneously and robustly recapitulates key developmental steps-mesoderm, somite, and skeletal muscle. Starting at the somite stage, DMD dysregulations concerned almost 10% of the transcriptome. These include mitochondrial genes whose dysregulations escalate during differentiation. We also describe fibrosis as an intrinsic feature of DMD skeletal muscle cells that begins early during myogenesis. All the omics data are available online for exploration through a graphical interface at https://muscle-dmd.omics.ovh/.

CONCLUSIONS

Our data argue for an early developmental manifestation of DMD whose onset is triggered before the entry into the skeletal muscle compartment, data leading to a necessary reconsideration of dystrophin roles during muscle development. This hiPSC model of skeletal muscle differentiation offers the possibility to explore these functions as well as find earlier DMD biomarkers and therapeutic targets.

Force dependent effects of chronic overuse on fibrosis-related genes and proteins in skeletal muscles

AIM

To examine the chronic effect of force on mRNA and protein expression levels of fibrosis-related genes in flexor digitorum muscles in a rat model of repetitive overuse injury that induces muscle fibrosis at high force levels.

MATERIALS AND METHODS

Two groups of rats were trained to perform a voluntary repetitive lever-pulling task at either a high (HFHR) or a low force (LFHR) for 18 weeks, while a control group (FRC) performed no task. RNA and protein were prepared from forelimb flexor digitorum muscles. Fibrosis-related gene RNA transcripts were evaluated using quantitative PCR (qPCR) and analyzed using the geometric mean of three housekeeping genes or the mean of each individually as reference. Protein levels were quantified using ELISA, western blot, or immunohistofluorescence.

RESULTS

Of eight fibrosis-related mRNAs examined, only FGF2 demonstrated a consistent significant increase in the HFHR group, compared to the FRC group. However, protein amounts of collagen type 1, collagen type 3, and TGFβ1 were significantly higher in the HFHR, compared to the FRC and LFHR groups, while CCN2 and FGF2 were higher in both HFHR and LFHR, compared to the FRC group.

CONCLUSIONS

Our results suggest that there is steady-state transcription of fibrogenic genes in muscles with established fibrosis, implying that post-transcriptional processes are responsible for the increased protein levels of fibrotic factors during muscle overuse conditions. We hypothesize that targeting such pathways represents a valid approach to treat overuse injury. Alternatively, FGF2 gene expression may represent a valid target for therapy.

Novel free-circulating and extracellular vesicle-derived miRNAs dysregulated in Duchenne muscular dystrophy

Aim: To perform cross-sectional and longitudinal miRNA profiling in plasma from Duchenne muscular dystrophy (DMD) subjects and find non-invasive biomarkers in DMD. Subjects/materials & methods: Plasma was collected from 14 age and sex matched controls and 46 DMD subjects. Free-circulating and extracellular vesicle (EV)-derived miRNA expression was measured by RT-qPCR. Results: Free-circulating and EVs derived miR-29c-3p and miR-133a-3p are dysregulated in DMD subjects. Free-circulating and EV-derived miR-29c-3p are reduced in DMD subjects undergoing daily corticosteroid treatment. Free-circulating miR-1-3p and miR-122-5p are longitudinally upregulated in ambulant DMD subjects. Conclusion: We detected novel free-circulating and EV-derived dysregulated miRNAs in plasma from DMD subjects and characterized the longitudinal profile of free-circulating miRNA on plasma from DMD subjects.

Cellular senescence-mediated exacerbation of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease characterised by chronic muscle degeneration and inflammation. Our previously established DMD model rats (DMD rats) have a more severe disease phenotype than the broadly used mouse model. We aimed to investigate the role of senescence in DMD using DMD rats and patients. Senescence was induced in satellite cells and mesenchymal progenitor cells, owing to the increased expression of CDKN2A, p16- and p19-encoding gene. Genetic ablation of p16 in DMD rats dramatically restored body weight and muscle strength. Histological analysis showed a reduction of fibrotic and adipose tissues invading skeletal muscle, with increased muscle regeneration. Senolytic drug ABT263 prevented loss of body weight and muscle strength, and increased muscle regeneration in rats even at 8 months—the late stage of DMD. Moreover, senescence markers were highly expressed in the skeletal muscle of DMD patients. In situ hybridization of CDKN2A confirmed the expression of it in satellite cells and mesenchymal progenitor cells in patients with DMD. Collectively, these data provide new insights into the integral role of senescence in DMD progression.

Extracellular matrix remodelling is associated with muscle force increase in overloaded mouse plantaris muscle

AIMS

Transforming growth factor-β (TGF-β) signalling is thought to contribute to the remodelling of extracellular matrix (ECM) of skeletal muscle and to functional decline in patients with muscular dystrophies. We wanted to determine the role of TGF-β-induced ECM remodelling in dystrophic muscle.

METHODS

We experimentally induced the pathological hallmarks of severe muscular dystrophy by mechanically overloading the plantaris muscle in mice. Furthermore, we determined the role of TGF-β signalling on dystrophic tissue modulation and on muscle function by (i) overloading myostatin knockout (Mstn^-/- ) mice and (ii) by additional pharmacological TGF-β inhibition via halofuginone.

RESULTS

Transcriptome analysis of overloaded muscles revealed upregulation predominantly of genes associated with ECM, inflammation and metalloproteinase activity. Histology revealed in wild-type mice signs of severe muscular dystrophy including myofibres with large variation in size and internalized myonuclei, as well as increased ECM deposition. At the same time, muscle weight had increased by 208% and muscle force by 234%. Myostatin deficiency blunted the effect of overload on muscle mass (59% increase) and force (76% increase), while having no effect on ECM deposition. Concomitant treatment with halofuginone blunted overload-induced muscle hypertrophy and muscle force increase, while reducing ECM deposition and increasing myofibre size.

CONCLUSIONS

ECM remodelling is associated with an increase in muscle mass and force in overload-modelled dystrophic muscle. Lack of myostatin is not advantageous and inhibition of ECM deposition by halofuginone is disadvantageous for muscle plasticity in response to stimuli that induce dystrophic muscle.

Photobiomodulation Therapy for Attenuating the Dystrophic Phenotype of Mdx Mice

This study analyzed photobiomodulation therapy (PBMT) effects on regenerative, antioxidative, anti-inflammatory and angiogenic markers in the dystrophic skeletal muscle of mdx mice, the experimental model of Duchenne muscular dystrophy (DMD), during the acute phase of dystrophy disease. The following groups were set up: Ctrl (control group of normal wild-type mice; C57BL/10); mdx (untreated mdx mice); mdxPred (mdx mice treated with prednisolone) and mdxLA (mdx mice treated with PBMT). The PBMT was carried out using an Aluminum Gallium Arsenide (AIGaAs; IBRAMED® laserpulse) diode, 830 nm wavelength, applied on the dystrophic quadriceps muscle. The mdxLA group showed a degenerative and regenerative area reduction simultaneously with a MyoD level increase, ROS production and inflammatory marker reduction and up-regulation in the VEGF factor. In addition, PBMT presented similar effects to prednisolone treatment in most of the parameters analyzed. In conclusion, our results indicate that PBMT in the parameters selected attenuated the dystrophic phenotype of mdx mice, improving skeletal muscle regeneration; reducing the oxidative stress and inflammatory process; and up-regulating the angiogenic marker.

Role of hypoxia in skeletal muscle fibrosis: Synergism between hypoxia and TGF-β signaling upregulates CCN2/CTGF expression specifically in muscle fibers

Several skeletal muscle diseases are characterized by fibrosis, the excessive accumulation of extracellular matrix. Transforming growth factor-β (TGF-β) and connective tissue growth factor (CCN2/CTGF) are two profibrotic factors augmented in fibrotic skeletal muscle, together with signs of reduced vasculature that implies a decrease in oxygen supply. We observed that fibrotic muscles are characterized by the presence of positive nuclei for hypoxia-inducible factor-1α (HIF-1α), a key mediator of the hypoxia response. However, it is not clear how a hypoxic environment could contribute to the fibrotic phenotype in skeletal muscle. We evaluated the role of hypoxia and TGF-β on CCN2 expression in vitro. Fibroblasts, myoblasts and differentiated myotubes were incubated with TGF-β1 under hypoxic conditions. Hypoxia and TGF-β1 induced CCN2 expression synergistically in myotubes but not in fibroblasts or undifferentiated muscle progenitors. This induction requires HIF-1α and the Smad-independent TGF-β signaling pathway. We performed in vivo experiments using pharmacological stabilization of HIF-1α or hypoxia-induced via hindlimb ischemia together with intramuscular injections of TGF-β1, and we found increased CCN2 expression. These observations suggest that hypoxic signaling together with TGF-β signaling, which are both characteristics of a fibrotic skeletal muscle environment, induce the expression of CCN2 in skeletal muscle fibers and myotubes.

Targeting a therapeutic LIF transgene to muscle via the immune system ameliorates muscular dystrophy

Many potentially therapeutic molecules have been identified for treating Duchenne muscular dystrophy. However, targeting those molecules only to sites of active pathology is an obstacle to their clinical use. Because dystrophic muscles become extensively inflamed, we tested whether expressing a therapeutic transgene in leukocyte progenitors that invade muscle would provide selective, timely delivery to diseased muscle. We designed a transgene in which leukemia inhibitory factor (LIF) is under control of a leukocyte-specific promoter and transplanted transgenic cells into dystrophic mice. Transplantation diminishes pathology, reduces Th2 cytokines in muscle and biases macrophages away from a CD163+/CD206+ phenotype that promotes fibrosis. Transgenic cells also abrogate TGFβ signaling, reduce fibro/adipogenic progenitor cells and reduce fibrogenesis of muscle cells. These findings indicate that leukocytes expressing a LIF transgene reduce fibrosis by suppressing type 2 immunity and highlight a novel application by which immune cells can be genetically modified as potential therapeutics to treat muscle disease.

Role of Transforming Growth Factor-β in Skeletal Muscle Fibrosis: A Review

Transforming growth factor-beta (TGF-β) isoforms are cytokines involved in a variety of cellular processes, including myofiber repair and regulation of connective tissue formation. Activation of the TGF-β pathway contributes to pathologic fibrosis in most organs. Here, we have focused on examining the evidence demonstrating the involvement of TGF-β in the fibrosis of skeletal muscle particularly. The TGF-β pathway plays a role in different skeletal muscle myopathies, and TGF-β signaling is highly induced in these diseases. In this review, we discuss different molecular mechanisms of TGF-β-mediated skeletal muscle fibrosis and highlight different TGF-β-targeted treatments that target these relevant pathways.

The Role of Metabolic Remodeling in Macrophage Polarization and Its Effect on Skeletal Muscle Regeneration

Significance: Macrophages are crucial for tissue homeostasis. Based on their activation, they might display classical/M1 or alternative/M2 phenotypes. M1 macrophages produce pro-inflammatory cytokines, reactive oxygen species (ROS), and nitric oxide (NO). M2 macrophages upregulate arginase-1 and reduce NO and ROS levels; they also release anti-inflammatory cytokines, growth factors, and polyamines, thus promoting angiogenesis and tissue healing. Moreover, M1 and M2 display key metabolic differences; M1 polarization is characterized by an enhancement in glycolysis and in the pentose phosphate pathway (PPP) along with a decreased oxidative phosphorylation (OxPhos), whereas M2 are characterized by an efficient OxPhos and reduced PPP. Recent Advances: The glutamine-related metabolism has been discovered as crucial for M2 polarization. Vice versa, flux discontinuities in the Krebs cycle are considered additional M1 features; they lead to increased levels of immunoresponsive gene 1 and itaconic acid, to isocitrate dehydrogenase 1-downregulation and to succinate, citrate, and isocitrate over-expression. Critical Issues: A macrophage classification problem, particularly in vivo, originating from a gap in the knowledge of the several intermediate polarization statuses between the M1 and M2 extremes, characterizes this field. Moreover, the detailed features of metabolic reprogramming crucial for macrophage polarization are largely unknown; in particular, the role of β-oxidation is highly controversial. Future Directions: Manipulating the metabolism to redirect macrophage polarization might be useful in various pathologies, including an efficient skeletal muscle regeneration. Unraveling the complexity pertaining to metabolic signatures that are specific for the different macrophage subsets is crucial for identifying new compounds that are able to trigger macrophage polarization and that might be used for therapeutical purposes.

Immunobiology of Inherited Muscular Dystrophies

The immune response to acute muscle damage is important for normal repair. However, in chronic diseases such as many muscular dystrophies, the immune response can amplify pathology and play a major role in determining disease severity. Muscular dystrophies are inheritable diseases that vary tremendously in severity, but share the progressive loss of muscle mass and function that can be debilitating and lethal. Mutations in diverse genes cause muscular dystrophy, including genes that encode proteins that maintain membrane strength, participate in membrane repair, or are components of the extracellular matrix or the nuclear envelope. In this article, we explore the hypothesis that an important feature of many muscular dystrophies is an immune response adapted to acute, infrequent muscle damage that is misapplied in the context of chronic injury. We discuss the involvement of the immune system in the most common muscular dystrophy, Duchenne muscular dystrophy, and show that the immune system influences muscle death and fibrosis as disease progresses. We then present information on immune cell function in other muscular dystrophies and show that for many muscular dystrophies, release of cytosolic proteins into the extracellular space may provide an initial signal, leading to an immune response that is typically dominated by macrophages, neutrophils, helper T-lymphocytes, and cytotoxic T-lymphocytes. Although those features are similar in many muscular dystrophies, each muscular dystrophy shows distinguishing features in the magnitude and type of inflammatory response. These differences indicate that there are disease-specific immunomodulatory molecules that determine response to muscle cell damage caused by diverse genetic mutations. © 2018 American Physiological Society. Compr Physiol 8:1313-1356, 2018.

Muscle miRNAome shows suppression of chronic inflammatory miRNAs with both prednisone and vamorolone

Corticosteroids are highly prescribed and effective anti-inflammatory drugs but the burden of side effects with chronic use significantly detracts from patient quality of life, particularly in children. Developing safer steroids amenable to long-term use is an important goal for treatment of chronic inflammatory diseases such as Duchenne muscular dystrophy (DMD). We have developed vamorolone (VBP15), a first-in-class dissociative glucocorticoid receptor (GR) ligand that shows the anti-inflammatory efficacy of corticosteroids without key steroid side effects in animal models. miRNAs are increasingly recognized as key regulators of inflammatory responses. To define effects of prednisolone and vamorolone on the muscle miRNAome, we performed a preclinical discovery study in the mdx mouse model of DMD. miRNAs associated with inflammation were highly elevated in mdx muscle. Both vamorolone and prednisolone returned these toward wild-type levels (miR-142-5p, miR-142-3p, miR-146a, miR-301a, miR-324-3p, miR-455-5p, miR-455-3p, miR-497, miR-652). Effects of vamorolone were largely limited to reduction of proinflammatory miRNAs. In contrast, prednisolone activated a separate group of miRNAs associated with steroid side effects and a noncoding RNA cluster homologous to human chromosome 14q32. Effects were validated for inflammatory miRNAs in a second, independent preclinical study. For the anti-inflammatory miRNA signature, bioinformatic analyses showed all of these miRNAs are directly regulated by, or in turn activate, the inflammatory transcription factor NF-κB. Moving forward miR-146a and miR-142 are of particular interest as biomarkers or novel drug targets. These data validate NF-κB signaling as a target of dissociative GR-ligand efficacy in vivo and provide new insight into miRNA signaling in chronic inflammation.

Downregulation of miRNA-29, -23 and -21 in urine of Duchenne muscular dystrophy patients

AIM

To study the signature of 87 urinary miRNAs in Duchenne muscular dystrophy (DMD) patients, select the most dysregulated and determine statistically significant differences in their expression between controls, ambulant (A) and nonambulant (NA) DMD patients, and patients on different corticosteroid regimens. Patients/materials & methods: Urine was collected from control (n = 20), A (n = 31) and NA (n = 23) DMD patients. miRNA expression was measured by reverse transcription-quantitative PCR.

RESULTS

miR-29c-3p was significantly downregulated in A DMD patients while miR-23b-3p and miR-21-5p were significantly downregulated in NA DMD patients compared with age-matched controls.

CONCLUSION

miR-29c-3p, miR-23b-3p and miR-21-5p are promising novel noninvasive biomarkers for DMD, and miR-29c-3p levels are differentially affected by different steroid regimens, supporting the antifibrotic effect of steroid therapy.

KLF10 Gene Expression Modulates Fibrosis in Dystrophic Skeletal Muscle

Dystrophic skeletal muscle is characterized by fibrotic accumulation of extracellular matrix components that compromise muscle structure, function, and capacity for regeneration. Tissue fibrosis is often initiated and sustained through transforming growth factor-β (TGF-β) signaling, and Krüppel-like factor 10 (KLF10) is an immediate early gene that is transcriptionally activated in response to TGF-β signaling. It encodes a transcriptional regulator that mediates the effects of TGF-β signaling in a variety of cell types. This report presents results of investigation of the effects of loss of KLF10 gene expression in wild-type and dystrophic (mdx) skeletal muscle. On the basis of RT-PCR, Western blot, and histological analyses of mouse tibialis anterior and diaphragm muscles, collagen type I (Col1a1) and fibronectin gene expression and protein deposition were increased in KLF10^-/- mice, contributing to increased fibrosis. KLF10^-/- mice displayed increased expression of genes encoding SMAD2, SMAD3, and SMAD7, particularly in diaphragm muscle. SMAD4 gene expression was unchanged. Expression of the extracellular matrix remodeling genes, MMP2 and TIMP1, was also increased in KLF10-deficient mouse muscle. Histological analyses and assays of hydroxyproline content indicated that the loss of KLF10 increased fibrosis. Dystrophic KLF10-null mice also had reduced grip strength. The effects of loss of KLF10 gene expression were most pronounced in dystrophic diaphragm muscle, suggesting that KLF10 moderates the fibrotic effects of TGF-β signaling in chronically damaged regenerating muscle.

α-smooth muscle actin is not a marker of fibrogenic cell activity in skeletal muscle fibrosis

α-Smooth muscle actin (α-SMA) is used as a marker for a subset of activated fibrogenic cells, myofibroblasts, which are regarded as important effector cells of tissue fibrogenesis. We address whether α-SMA-expressing myofibroblasts are detectable in fibrotic muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy (DMD), and whether the α-SMA expression correlates with the fibrogenic function of intramuscular fibrogenic cells. α-SMA immunostaining signal was not detected in collagen I (GFP)-expressing cells in fibrotic muscles of ColI-GFP/mdx^5cv mice, but it was readily detected in smooth muscle cells lining intramuscular blood vessel walls. α-SMA expression was detected by quantitative RT-PCR and Western blot in fibrogenic cells sorted from diaphragm and quadriceps muscles of the ColI-GFP/mdx^5cv mice. Consistent with the more severe fibrosis in the ColI-GFP/mdx^5cv diaphragm, the fibrogenic cells in the diaphragm exerted a stronger fibrogenic function than the fibrogenic cells in the quadriceps as gauged by their extracellular matrix gene expression. However, both gene and protein expression of α-SMA was lower in the diaphragm fibrogenic cells than in the quadriceps fibrogenic cells in the ColI-GFP/mdx^5cv mice. We conclude that myofibroblasts are present in fibrotic skeletal muscles, but their expression of α-SMA is not detectable by immunostaining. The level of α-SMA expression by intramuscular fibrogenic cells does not correlate positively with the level of collagen gene expression or the severity of skeletal muscle fibrosis in the mdx^5cv mice. α-SMA is not a functional marker of fibrogenic cells in skeletal muscle fibrosis associated with muscular dystrophy.

The role of TGF-β1 during skeletal muscle regeneration

The injury of adult skeletal muscle initiates series of well-coordinated events that lead to the efficient repair of the damaged tissue. Any disturbances during muscle myolysis or reconstruction may result in the unsuccessful regeneration, characterised by strong inflammatory response and formation of connective tissue, that is, fibrosis. The switch between proper regeneration of skeletal muscle and development of fibrosis is controlled by various factors. Amongst them are those belonging to the transforming growth factor β family. One of the TGF-β family members is TGF-β1, a multifunctional cytokine involved in the regulation of muscle repair via satellite cells activation, connective tissue formation, as well as regulation of the immune response intensity. Here, we present the role of TGF-β1 in myogenic differentiation and muscle repair. The understanding of the mechanisms controlling these processes can contribute to the better understanding of skeletal muscle atrophy and diseases which consequence is fibrosis disrupting muscle function.

PDGFRα signalling promotes fibrogenic responses in collagen-producing cells in Duchenne muscular dystrophy

Fibrosis is a characteristic of Duchenne muscular dystrophy (DMD), yet the cellular and molecular mechanisms responsible for DMD fibrosis are poorly understood. Utilizing the Collagen1a1-GFP transgene to identify cells producing Collagen-I matrix in wild-type mice exposed to toxic injury or those mutated at the dystrophin gene locus (mdx) as a model of DMD, we studied mechanisms of skeletal muscle injury/repair and fibrosis. PDGFRα is restricted to Sca1+, CD45- mesenchymal progenitors. Fate-mapping experiments using inducible CreER/LoxP somatic recombination indicate that these progenitors expand in injury or DMD to become PDGFRα+, Col1a1-GFP+ matrix-forming fibroblasts, whereas muscle fibres do not become fibroblasts but are an important source of the PDGFRα ligand, PDGF-AA. While in toxin injury/repair of muscle PDGFRα, signalling is transiently up-regulated during the regenerative phase in the DMD model and in human DMD it is chronically overactivated. Conditional expression of the constitutively active PDGFRα D842V mutation in Collagen-I+ fibroblasts, during injury/repair, hindered the repair phase and instead promoted fibrosis. In DMD, treatment of mdx mice with crenolanib, a highly selective PDGFRα/β tyrosine kinase inhibitor, reduced fibrosis, improved muscle strength, and was associated with decreased activity of Src, a downstream effector of PDGFRα signalling. These observations are consistent with a model in which PDGFRα activation of mesenchymal progenitors normally regulates repair of the injured muscle, but in DMD persistent and excessive activation of this pathway directly drives fibrosis and hinders repair. The PDGFRα pathway is a potential new target for treatment of progressive DMD. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Identification and Function of Fibrocytes in Skeletal Muscle Injury Repair and Muscular Dystrophy

We identified and characterized the function of CD45⁺/collagen I⁺ fibrocytes in acutely injured skeletal muscle of wild-type (WT) and Ccr2^-/- mice, and in quadriceps and diaphragm muscles of mdx^5cv mice, a mouse model for Duchenne muscular dystrophy. Fibrocytes were not detected in peripheral blood of WT mice after acute muscle injury or mdx^5cv mice. Fibrocytes were detected in acutely injured muscles and in mdx^5cv quadriceps and diaphragm muscles. These cells expressed F4/80 and CCR2, and they were mostly Ly6C^lo They expressed a low level of collagens but a high level of profibrotic growth factors as compared with i.m. fibroblasts. Fibrocyte expression of collagens and profibrotic growth factors was not increased in Ccr2^-/- mice as compared with WT controls. Fibrocyte expression of both proinflammatory and profibrotic cytokines was significantly higher in mdx^5cv diaphragm than in mdx^5cv quadriceps. In cocultures, fibrocytes from the mdx^5cv diaphragm stimulated a higher level of fibroblast expression of extracellular matrix genes than did those from the mdx^5cv quadriceps. Our findings suggest that i.m. fibrocytes most likely originate from infiltrating monocytes/macrophages and differentiate within injured muscles. They likely contribute to the normal muscle injury repair by producing growth factors. They do not appear to contribute to the persistent muscle fibrosis associated with poor injury repair in Ccr2^-/- mice. However, they likely contribute to the persistent inflammation and progressive fibrosis in the mdx^5cv diaphragm.

Pharmacological therapeutics targeting the secondary defects and downstream pathology of Duchenne muscular dystrophy

INTRODUCTION

Since the identification of the dystrophin gene in 1986, a cure for Duchenne muscular dystrophy (DMD) has yet to be discovered. Presently, there are a number of genetic-based therapies in development aimed at restoration and/or repair of the primary defect. However, growing understanding of the pathophysiological consequences of dystrophin absence has revealed several promising downstream targets for the development of therapeutics.

AREAS COVERED

In this review, we discuss various strategies for DMD therapy targeting downstream consequences of dystrophin absence including loss of muscle mass, inflammation, fibrosis, calcium overload, oxidative stress, and ischemia. The rationale of each approach and the efficacy of drugs in preclinical and clinical studies are discussed.

EXPERT OPINION

For the last 30 years, effective DMD drug therapy has been limited to corticosteroids, which are associated with a number of negative side effects. Our knowledge of the consequences of dystrophin absence that contribute to DMD pathology has revealed several potential therapeutic targets. Some of these approaches may have potential to improve or slow disease progression independently or in combination with genetic-based approaches. The applicability of these pharmacological therapies to DMD patients irrespective of their genetic mutation, as well as the potential benefits even for advanced stage patients warrants their continued investigation.

MicroRNA deep sequencing in two adult stem cell populations identifies miR-501 as a novel regulator of myosin heavy chain during muscle regeneration

MicroRNAs (miRNAs) are important regulators of skeletal muscle regeneration, but the underlying mechanisms are still incompletely understood. Here, comparative miRNA sequencing analysis of myogenic progenitor cells (MPs) and non-myogenic fibroblast-adipocyte progenitors (FAPs) during cardiotoxin (CTX)-induced muscle injury uncovered miR-501 as a novel muscle-specific miRNA. miR-501 is an intronic miRNA and its expression levels in MPs correlated with its host gene, chloride channel, voltage-sensitive 5 (Clcn5). Pharmacological inhibition of miR-501 dramatically blunted the induction of embryonic myosin heavy chain (MYH3) and, to a lesser extent, adult myosin isoforms during muscle regeneration, and promoted small-diameter neofibers. An unbiased target identification approach in primary myoblasts validated gigaxonin as a target of miR-501 that mimicked the effect of miR-501 inhibition on MYH3 expression. In the mdx mouse model, which models a pathological disease state, not only was miR-501 induced in regenerating skeletal muscle, but also its serum levels were increased, which correlated with the disease state of the animals. Our results suggest that miR-501 plays a key role in adult muscle regeneration and might serve as a novel serum biomarker for the activation of adult muscle stem cells.

Summary: MicroRNA 501 is a novel muscle-specific microRNA that is induced during muscle regeneration and regulates the transition of myosin heavy chains during early myogenesis.

CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx5cv mice

Genetic ablation or pharmacologic inhibition of CC chemokine receptor type 2 (CCR2) reduced macrophage (MP) infiltration and improved muscle pathology and function in mdx diaphragm muscle at early stages. We addressed whether CCR2 deficiency resulted in sustained improvement of mdx^5cv-Ccr2^-/- diaphragm. Compared to mdx^5cv controls, CCR2 deficiency in mdx^5cv-Ccr2^-/- mice markedly reduced intramuscular Ly6C^hi MPs at all stages, but it reduced Ly6C^low MPs only at early stages (4 and 9 wk). CCR2 deficiency reduced quadriceps and diaphragm muscle damage and fibrosis at 14 wk but not at 6 mo, and it improved diaphragm muscle regeneration and respiratory function at 14 wk but not at 6 mo. Intramuscular MPs in mdx^5cv-Ccr2^-/- diaphragm expressed a low level of IL-1β, IL-6, and IFN-γ genes, a similar level of TNF-α, TGF-β1, and platelet-derived growth factor α genes, and a high level of IGF-1 and osteopontin genes compared to mdx^5cv controls. Diaphragm fibroblasts at 14 wk showed a similar cell number with a similar level of collagen and profibrogenic growth factor gene expression in mdx^5cv-Ccr2^-/- and mdx^5cv mice. Diaphragm MPs from both mdx^5cv-Ccr2^-/- and mdx^5cv mice stimulated collagen gene expression by cocultured fibroblasts. The findings suggest that CCR2 deficiency does not provide a sustained benefit and that Ly6C^low MPs may contribute to the progressive fibrosis and dysfunction of mdx^5cv diaphragm.-Zhao, W., Wang, X., Ransohoff, R. M., Zhou, L. CCR2 deficiency does not provide sustained improvement of muscular dystrophy in mdx^5cv mice.

Tenotomy immobilization as a model to investigate skeletal muscle fibrosis (with emphasis on Secreted frizzled-related protein 2)

The pathological endpoint of congenital and senile myopathies is chronic muscle degeneration characterized by the atrophy of contractile elements, accompanied by fibrosis and fatty infiltration of the interstitium. Tenotomy is the release of preload that causes abrupt shortening of the muscle and models atrophy and fibrosis without prominent inflammatory response. Fibrosis in the skeletal muscle is known to be triggered by transforming growth factor (TGF)-β, which is activated by inflammatory events. As these were lacking, tenotomy provided an opportunity to investigate transcriptional events on a background without inflammation. An unbiased look at the transcriptome of tenotomy-immobilized soleus muscle revealed that the majority of the transcriptional changes took place in the first 4 wk. Regarding atrophy, proteasomal and lysosomal pathways were actively involved in accompanying cathepsins and calpains in the breakdown of the macromolecular contractile machinery. The transcriptome provided clear-cut evidence for the upregulation of collagens and several extracellular matrix components that define fibrotic remodeling of the skeletal muscle architecture as well as activation of the fibro-adipogenic precursors. Concomitantly, Sfrp2, a Wnt antagonist as well as a procollagen processor, accompanied fibrosis in skeletal muscle with an expression that was stringently confined to the slow-twitch fibers. An interpreted mechanistic scenario construed the kinetic events initiated through the abnormal shortening of the muscle fibers as enough to trigger the resident latent TGF-β in the extracellular matrix, leading to the activation of fibroadipogenic precursors. As in the heart, Sfrp2 shows itself to be a therapeutic target for the prevention of irreversible fibrosis in degenerative skeletal muscle conditions.

A Wnt-TGFβ2 axis induces a fibrogenic program in muscle stem cells from dystrophic mice

We have previously observed that Wnt signaling activates a fibrogenic program in adult muscle stem cells, called satellite cells, during aging. We genetically labeled satellite cells in a mouse model of Duchenne muscular dystrophy to follow their fate during the progression of the disease. We observed that a fraction of satellite cells had a reduced myogenic potential and showed enhanced expression of profibrotic genes compared to age-matched controls. By combining in vitro and in vivo results, we found that expression of transforming growth factor-β2 (TGFβ2) was induced in response to elevated canonical Wnt signaling in dystrophic muscles and that the resulting increase in TGFβ activity affected the behavior of satellite cells in an autocrine or paracrine fashion. Indeed, pharmacological inhibition of the TGFβ pathway in vivo reduced the fibrogenic characteristics of satellite cells. These studies shed new light on the cellular and molecular mechanisms responsible for stem cell dysfunction in dystrophic muscle and may contribute to the development of more effective and specific therapeutic approaches for the prevention of muscle fibrosis.

Changes in calsequestrin, TNF-α, TGF-β and MyoD levels during the progression of skeletal muscle dystrophy in mdx mice: a comparative analysis of the quadriceps, diaphragm and intrinsic laryngeal muscles

In Duchenne muscular dystrophy (DMD), the search for new biomarkers to follow the evolution of the disease is of fundamental importance in the light of the evolving gene and pharmacological therapies. In addition to the lack of dystrophin, secondary events including changes in calcium levels, inflammation and fibrosis greatly contribute to DMD progression and the molecules involved in these events may represent potential biomarkers. In this study, we performed a comparative evaluation of the progression of dystrophy within muscles that are differently affected by dystrophy (diaphragm; DIA and quadriceps; QDR) or spared (intrinsic laryngeal muscles) using the mdx mice model of DMD. We assessed muscle levels of calsequestrin (calcium-related protein), tumour necrosis factor (TNF-α; pro-inflammatory cytokine), tumour growth factor (TGF-β; pro-fibrotic factor) and MyoD (muscle proliferation) vs. histopathology at early (1 and 4 months of age) and late (9 months of age) stages of dystrophy. Fibrosis was the primary feature in the DIA of mdx mice (9 months: 32% fibrosis), which was greater than in the QDR (9 months: 0.6% fibrosis). Muscle regeneration was the primary feature in the QDR (9 months: 90% of centrally nucleated fibres areas vs. 33% in the DIA). The QDR expressed higher levels of calsequestrin than the DIA. Laryngeal muscles showed normal levels of TNF-α, TGF-β and MyoD. A positive correlation between histopathology and cytokine levels was observed only in the diaphragm, suggesting that TNF-α and TGF-β serve as markers of dystrophy primarily for the diaphragm.

Sulforaphane mitigates muscle fibrosis in mdx mice via Nrf2-mediated inhibition of TGF-β/Smad signaling

Sulforaphane (SFN), an activator of NF-E2-related factor 2 (Nrf2), has been found to have an antifibrotic effect on liver and lung. However, its effects on dystrophic muscle fibrosis remain unknown. This work was undertaken to evaluate the effects of SFN-mediated activation of Nrf2 on dystrophic muscle fibrosis. Male mdx mice (age 3 mo) were treated with SFN by gavage (2 mg/kg body wt per day) for 3 mo. Experimental results demonstrated that SFN remarkably attenuated skeletal and cardiac muscle fibrosis as indicated by reduced Sirius Red staining and immunostaining of the extracellular matrix. Moreover, SFN significantly inhibited the transforming growth factor-β (TGF-β)/Smad signaling pathway and suppressed profibrogenic gene and protein expressions such as those of α-smooth muscle actin (α-SMA), fibronectin, collagen I, plasminogen activator inhibitor-1 (PAI-1), and tissue inhibitor metalloproteinase-1 (TIMP-1) in an Nrf2-dependent manner. Furthermore, SFN significantly decreased the expression of inflammatory cytokines CD45, TNF-α, and IL-6 in mdx mice. In conclusion, these results show that SFN can attenuate dystrophic muscle fibrosis by Nrf2-mediated inhibition of the TGF-β/Smad signaling pathway, which indicates that Nrf2 may represent a new target for dystrophic muscle fibrosis.

Genome-wide Mechanosensitive MicroRNA (MechanomiR) Screen Uncovers Dysregulation of Their Regulatory Networks in the mdm Mouse Model of Muscular Dystrophy

Muscular dystrophies (MDs) are a heterogeneous group of genetic and neuromuscular disorders, which result in severe loss of motor ability and skeletal muscle mass and function. Aberrant mechanotransduction and dysregulated-microRNA pathways are often associated with the progression of MD. Here, we hypothesized that dysregulation of mechanosensitive microRNAs (mechanomiRs) in dystrophic skeletal muscle plays a major role in the progression of MD. To test our hypothesis, we performed a genome-wide expression profile of anisotropically regulated mechanomiRs and bioinformatically analyzed their target gene networks. We assessed their functional roles in the advancement of MD using diaphragm muscles from mdm (MD with myositis) mice, an animal model of human tibial MD (titinopathy), and their wild-type littermates. We were able to show that ex vivo anisotropic mechanical stretch significantly alters the miRNA expression profile in diaphragm muscles from WT and mdm mice; as a result, some of the genes associated with MDs are dysregulated in mdm mice due to differential regulation of a distinct set of mechanomiRs. Interestingly, we found a contrasting expression pattern of the highly expressed let-7 family mechanomiRs, let-7e-5p and miR-98-5p, and their target genes associated with the extracellular matrix and TGF-β pathways, respectively, between WT and mdm mice. Gain- and loss-of-function analysis of let-7e-5p in myocytes isolated from the diaphragms of WT and mdm mice confirmed Col1a1, Col1a2, Col3a1, Col24a1, Col27a1, Itga1, Itga4, Scd1, and Thbs1 as target genes of let-7e-5p. Furthermore, we found that miR-98 negatively regulates myoblast differentiation. Our study therefore introduces additional biological players in the regulation of skeletal muscle structure and myogenesis that may contribute to unexplained disorders of MD.

Effects of low-level laser therapy on skeletal muscle repair: a systematic review

A review of the literature was performed to demonstrate the most current applicability of low-level laser therapy (LLLT) for the treatment of skeletal muscle injuries, addressing different lasers, irradiation parameters, and treatment results in animal models. Searches were performed in the PubMed/MEDLINE, SCOPUS, and SPIE Digital Library databases for studies published from January 2006 to August 2013 on the use of LLLT for the repair of skeletal muscle in any animal model. All selected articles were critically appraised by two independent raters. Seventeen of the 36 original articles on LLLT and muscle injuries met the inclusion criteria and were critically evaluated. The main effects of LLLT were a reduction in the inflammatory process, the modulation of growth factors and myogenic regulatory factors, and increased angiogenesis. The studies analyzed demonstrate the positive effects of LLLT on the muscle repair process, which are dependent on irradiation and treatment parameters. The findings suggest that LLLT is an excellent therapeutic resource for the treatment of skeletal muscle injuries in the short-term.

Transforming growth factor type-β inhibits Mas receptor expression in fibroblasts but not in myoblasts or differentiated myotubes; Relevance to fibrosis associated to muscular dystrophies

Duchenne muscular dystrophy is a genetic disorder characterized by myofiber degeneration, muscle weakness, and increased fibrosis. Transforming growth factor type-β (TGF-β), a central mediator of fibrosis, is upregulated in fibrotic diseases. Angiotensin-(1-7) [Ang-(1-7)] is a peptide with actions that oppose those of angiotensin-II (Ang II). Ang-(1-7) effects are mediated by the Mas receptor. Treatment with Ang-(1-7) produce positive effects in the mdx mouse, normalizing skeletal muscle architecture, decreasing local fibrosis, and fibroblasts, and improving muscle function. Mdx mice deficient for the Mas receptor showed the opposite effects. To identify the cell type(s) responsible for Mas receptor expression, and to characterize whether profibrotic effectors had any effect on its expression, we determined the effect of profibrotic agents on Mas expression. TGF-β, but not connective tissue growth factor or Ang-II, reduced the expression of Mas receptor in fibroblasts isolated from skeletal muscle cells and fibroblasts from two established cell lines. In contrast, no effects were observed in myoblasts and differentiated myotubes. This inhibition was mediated by the Smad-dependent (canonical) and the PI3K and MEK1/2 (noncanonical) TGF-β signaling pathways. When both canonical and noncanonical inhibitors of the TGF-β-dependent pathways were added together, the inhibitory effect of TGF-β on Mas expression was lost. The decrease in Mas receptor induced by TGF-β in fibroblasts reduced the Ang-(1-7) mediated stimulation of phosphorylation of AKT pathway proteins. These results suggest that reduction of Mas receptor in fibroblasts, by TGF-β, could increase the fibrotic phenotype observed in dystrophic skeletal muscle decreasing the beneficial effect of Ang-(1-7).

Understanding the process of fibrosis in Duchenne muscular dystrophy

Fibrosis is the aberrant deposition of extracellular matrix (ECM) components during tissue healing leading to loss of its architecture and function. Fibrotic diseases are often associated with chronic pathologies and occur in a large variety of vital organs and tissues, including skeletal muscle. In human muscle, fibrosis is most readily associated with the severe muscle wasting disorder Duchenne muscular dystrophy (DMD), caused by loss of dystrophin gene function. In DMD, skeletal muscle degenerates and is infiltrated by inflammatory cells and the functions of the muscle stem cells (satellite cells) become impeded and fibrogenic cells hyperproliferate and are overactivated, leading to the substitution of skeletal muscle with nonfunctional fibrotic tissue. Here, we review new developments in our understanding of the mechanisms leading to fibrosis in DMD and several recent advances towards reverting it, as potential treatments to attenuate disease progression.

Fibrosis and inflammation are greater in muscles of beta-sarcoglycan-null mouse than mdx mouse

The Sgcb-null mouse, with knocked-down β-sarcoglycan, develops severe muscular dystrophy as in type 2E human limb girdle muscular dystrophy. The mdx mouse, lacking dystrophin, is the most used model for Duchenne muscular dystrophy (DMD). Unlike DMD, the mdx mouse has mild clinical features and shows little fibrosis in limb muscles. To characterize ECM protein deposition and the progression of muscle fibrosis, we evaluated protein and transcript levels of collagens I, III and VI, decorin, and TGF-β1, in quadriceps and diaphragm, at 2, 4, 8, 12, 26, and 52 weeks in Sgcb-null mice, and protein levels at 12, 26, and 52 weeks in mdx mice. In Sgcb-null mice, severe morphological disruption was present from 4 weeks in both quadriceps and diaphragm, and included conspicuous deposition of extracellular matrix components. Histopathological features of Sgcb-null mouse muscles were similar to those of age-matched mdx muscles at all ages examined, but, in the Sgcb-null mouse, the extent of connective tissue deposition was generally greater than mdx. Furthermore, in the Sgcb-null mouse, the amount of all three collagen isoforms increased steadily, while, in the mdx, they remained stable. We also found that, at 12 weeks, macrophages were significantly more numerous in mildly inflamed areas of Sgcb-null quadriceps compared to mdx quadriceps (but not in highly inflamed regions), while, in the diaphragm, macrophages did not differ significantly between the two models, in either region. Osteopontin mRNA was also significantly greater at 12 weeks in laser-dissected highly inflamed areas of the Sgcb-null quadriceps compared to the mdx quadriceps. TGF-β1 was present in areas of degeneration-regeneration, but levels were highly variable and in general did not differ significantly between the two models and controls. The roles of the various subtypes of macrophages in muscle repair and fibrosis in the two models require further study. The Sgcb-null mouse, which develops early fibrosis in limb muscles, appears more promising than the mdx mouse for probing pathogenetic mechanisms of muscle fibrosis and for developing anti-fibrotic treatments. Highlights • The Sgcb-null mouse develops severe muscular dystrophy, the mdx mouse does not. • Fibrosis developed earlier in Sgcb-null quadriceps and diaphragm than mdx. • Macrophages were commoner in mildly inflamed parts of Sgcb-null quadriceps than mdx. • The Sgcb-null model appears more useful than mdx for studying fibrotic mechanisms. • The Sgcb-null model also appears more useful for developing anti-fibrotic treatments.

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.

Coronary adventitial cells are linked to perivascular cardiac fibrosis via TGFβ1 signaling in the mdx mouse model of Duchenne muscular dystrophy

In Duchenne muscular dystrophy (DMD), progressive accumulation of cardiac fibrosis promotes heart failure. While the cellular origins of fibrosis in DMD hearts remain enigmatic, fibrotic tissue conspicuously forms near the coronary adventitia. Therefore, we sought to characterize the role of coronary adventitial cells in the formation of perivascular fibrosis. Utilizing the mdx model of DMD, we have identified a population of Sca1+, PDGFRα+, CD31-, and CD45- coronary adventitial cells responsible for perivascular fibrosis. Histopathology of dystrophic hearts revealed that Sca1+ cells extend from the adventitia and occupy regions of perivascular fibrosis. The number of Sca1+ adventitial cells increased two-fold in fibrotic mdx hearts vs. age matched wild-type hearts. Moreover, relative to Sca1-, PDGFRα+, CD31-, and CD45- cells and endothelial cells, Sca1+ adventitial cells FACS-sorted from mdx hearts expressed the highest level of Collagen1α1 and 3α1, Connective tissue growth factor, and Tgfβr1 transcripts. Surprisingly, mdx endothelial cells expressed the greatest level of the Tgfβ1 ligand. Utilizing Collagen1α1-GFP reporter mice, we confirmed that the majority of Sca1+ adventitial cells expressed type I collagen, an abundant component of cardiac fibrosis, in both wt (71%±4.1) and mdx (77%±3.5) hearts. In contrast, GFP+ interstitial fibroblasts were PDGFRα+ but negative for Sca1. Treatment of cultured Collagen1α1-GFP+ adventitial cells with TGFβ1 resulted in increased collagen synthesis, whereas pharmacological inhibition of TGFβR1 signaling reduced the fibrotic response. Therefore, perivascular cardiac fibrosis by coronary adventitial cells may be mediated by TGFβ1 signaling. Our results implicate coronary endothelial cells in mediating cardiac fibrosis via transmural TGFβ signaling, and suggest that the coronary adventitia is a promising target for developing novel anti-fibrotic therapies.

Wnt signaling in skeletal muscle dynamics: myogenesis, neuromuscular synapse and fibrosis

The signaling pathways activated by Wnt ligands are related to a wide range of critical cell functions, such as cell division, migration, and synaptogenesis. Here, we summarize compelling evidence on the role of Wnt signaling on several features of skeletal muscle physiology. We briefly review the role of Wnt pathways on the formation of muscle fibers during prenatal and postnatal myogenesis, highlighting its role on the activation of stem cells of the adult muscles. We also discuss how Wnt signaling regulates the precise formation of neuromuscular synapses, by modulating the differentiation of presynaptic and postsynaptic components, particularly regarding the clustering of acetylcholine receptors on the muscle membrane. In addition, based on previous evidence showing that Wnt pathways are linked to several diseases, such as Alzheimer's and cancer, we address recent studies indicating that Wnt signaling plays a key role in skeletal muscle fibrosis, a disease characterized by an increase in the extracellular matrix components leading to failure in muscle regeneration, tissue disorganization and loss of muscle activity. In this context, we also discuss the possible cross-talk between the Wnt/β-catenin pathway with two other critical profibrotic pathways, transforming growth factor β and connective tissue growth factor, which are potent stimulators of the accumulation of connective tissue, an effect characteristic of the fibrotic condition. As it has emerged in other pathological conditions, we suggests that muscle fibrosis may be a consequence of alterations of Wnt signaling activity.

Loss of SPARC in mouse skeletal muscle causes myofiber atrophy

INTRODUCTION

The expression of secreted protein acidic and rich in cysteine (SPARC) in skeletal muscle decreases with age. Here, we examined the role of SPARC in skeletal muscle by reducing its expression.

METHODS

SPARC expression was suppressed by introducing short interfering RNA (siRNA) into mouse tibialis anterior muscle. Myofiber diameter, atrogin1, and muscle RING-finger protein 1 (MuRF1) expression, and tumor necrosis factor-α (TNFα) and transforming growth factor-β (TGFβ) signaling were then analyzed.

RESULTS

Reduced SPARC expression caused decreases in the diameter of myofibers, especially fast-type ones, accompanied by upregulation of atrogin1, but not MuRF1, at 10 days after siRNA transfection. The expression of TNFα and TGFβ and the phosphorylation status of p38 were not affected by SPARC knockdown, whereas Smad3 phosphorylation was increased at 2 days after siRNA transfection.

CONCLUSIONS

The loss of SPARC not only upregulates atrogin1 expression but also enhances TGFβ signaling, which may in turn cause muscle atrophy.