Arginine metabolism by macrophages promotes cardiac and muscle fibrosis in mdx muscular dystrophy

LED therapy modulates M1/M2 macrophage phenotypes and mitigates dystrophic features in treadmill-trained mdx mice

The mdx mouse phenotype, aggravated by chronic exercise on a treadmill, makes this murine model more reliable for the study of Duchenne muscular dystrophy (DMD) and allows the efficacy of therapeutic interventions to be evaluated. This study aims to investigate the effects of photobiomodulation by light-emitting diode (LED) therapy on functional, biochemical and morphological parameters in treadmill-trained adult mdx animals. Mdx mice were trained for 30 min of treadmill running at a speed of 12 m/min, twice a week for 4 weeks. The LED therapy (850 nm) was applied twice a week to the quadriceps muscle throughout the treadmill running period. LED therapy improved behavioral activity (open field) and muscle function (grip strength and four limb hanging test). Functional benefits correlated with reduced muscle damage; a decrease in the inflammatory process; modulation of the regenerative muscular process and calcium signalling pathways; and a decrease in oxidative stress markers. The striking finding of this work is that LED therapy leads to a shift from the M1 to M2 macrophage phenotype in the treadmill-trained mdx mice, enhancing tissue repair and mitigating the dystrophic features. Our data also imply that the beneficial effects of LED therapy in the dystrophic muscle correlate with the interplay between calcium, oxidative stress and inflammation signalling pathways. Together, these results suggest that photobiomodulation could be a potential adjuvant therapy for dystrophinopathies.

Integration of single-cell datasets depicts profiles of macrophages and fibro/adipogenic progenitors in dystrophic muscle

Single-cell technologies have recently expanded the possibilities for researchers to gain, at an unprecedented resolution level, knowledge about tissue composition, cell complexity, and heterogeneity. Moreover, the integration of data coming from different technologies and sources also offers, for the first time, the possibility to draw a holistic portrait of how cells behave to sustain tissue physiology during the human lifespan and disease. Here, we interrogated and integrated publicly available single-cell RNAseq data to advance the understanding of how macrophages, fibro/adipogenic progenitors, and other cell types establish gene regulatory networks and communicate with each other in the muscle tissue. We identified altered gene signatures and signaling pathways associated with the dystrophic condition, including an enhanced Spp1-Cd44 signaling in dystrophic macrophages. We shed light on the differences among dystrophic muscle aging, considering wild type, mdx, and more severe conditions as in the case of the mdx-2d model. Contextually, we provided details on existing communication relations between muscle niche cell populations, highlighting increased interactions and distinct signaling events that these cells stablish in the dystrophic microenvironment. We believe our findings can help scientists to formulate and test new hypotheses by moving towards a more complete understanding of muscle regeneration and immune system biology.

Loss of HNRNPU in Skeletal Muscle Increases Intramuscular Infiltration of Ly6C Positive Cells, leading to Muscle Atrophy through Activation of NF-κB Signaling

Heterogeneous nuclear ribonucleoprotein U (hnRNPU) is known to play multiple biological roles by regulating transcriptional expression, RNA splicing, RNA stability, and chromatin structure in a tissue-dependent manner. The role of hnRNPU in skeletal muscle development and maintenance has not been previously evaluated. In this study, skeletal muscle specific hnRNPU knock out mice is utilized and evaluated skeletal muscle mass and immune cell infiltration through development. By 4 weeks, muscle-specific hnRNPU knockout mice revealed Ly6C+ monocyte infiltration into skeletal muscle, which preceded muscle atrophy. Canonical NF-kB signaling is activated in a myofiber-autonomous manner with hnRNPU repression. Inducible hnRNPU skeletal muscle knockout mice further demonstrated that deletion of hnRNPU in adulthood is sufficient to cause muscle atrophy, suggesting that hnRNPU's role in muscle maintenance is not during development alone. Treatment with salirasib, to inhibit proliferation of immune cells, prevents muscle atrophy in muscle-specific hnRNPU knock out mice, indicating that immune cell infiltration plays causal role in muscle atrophy of hnRNPU knock out mice. Overall, the findings suggest that loss of hnRNPU triggers muscle inflammation and activates NF-κB signaling in a cell-autonomous manner, culminating in muscle atrophy.

Levels of asymmetric dimethylarginine in plasma and aqueous humor: a key risk factor for the severity of fibrovascular proliferation in proliferative diabetic retinopathy

Introduction

Proliferative diabetic retinopathy (PDR) is a common diabetes complication, significantly impacting vision and quality of life. Previous studies have suggested a potential link between arginine pathway metabolites and diabetic retinopathy (DR). Connective tissue growth factor (CTGF) plays a role in the occurrence and development of fibrovascular proliferation (FVP) in PDR patients. However, the relationship between arginine pathway metabolites and FVP in PDR remains undefined. This study aimed to explore the correlation between four arginine pathway metabolites (arginine, asymmetric dimethylarginine[ADMA], ornithine, and citrulline) and the severity of FVP in PDR patients.

Methods

In this study, plasma and aqueous humor samples were respectively collected from 30 patients with age-related cataracts without diabetes mellitus (DM) and from 85 PDR patients. The PDR patients were categorized as mild-to-moderate or severe based on the severity of fundal FVP. The study used Kruskal-Wallis test to compare arginine, ADMA, ornithine, and citrulline levels across three groups. Binary logistic regression identified risk factors for severe PDR. Spearman correlation analysis assessed associations between plasma and aqueous humor metabolite levels, and between ADMA and CTGF levels in aqueous humor among PDR patients.

Results

ADMA levels in the aqueous humor were significantly greater in patients with severe PDR than in those with mild-to-moderate PDR(P=0.0004). However, the plasma and aqueous humor levels of arginine, ornithine, and citrulline did not significantly differ between mild-to-moderate PDR patients and severe PDR patients (P>0.05). Binary logistic regression analysis indicated that the plasma (P=0.01) and aqueous humor (P=0.006) ADMA levels in PDR patients were risk factors for severe PDR. Furthermore, significant correlations were found between plasma and aqueous humor ADMA levels (r=0.263, P=0.015) and between aqueous humor ADMA and CTGF levels (r=0.837, P<0.001).

Conclusion

Elevated ADMA levels in plasma and aqueous humor positively correlate with the severity of FVP in PDR, indicating ADMA as a risk factor for severe PDR.

Musculoskeletal crosstalk in chronic obstructive pulmonary disease and comorbidities: Emerging roles and therapeutic potentials

Chronic Obstructive Pulmonary Disease (COPD) is a multifaceted respiratory disorder characterized by progressive airflow limitation and systemic implications. It has become increasingly apparent that COPD exerts its influence far beyond the respiratory system, extending its impact to various organ systems. Among these, the musculoskeletal system emerges as a central player in both the pathogenesis and management of COPD and its associated comorbidities. Muscle dysfunction and osteoporosis are prevalent musculoskeletal disorders in COPD patients, leading to a substantial decline in exercise capacity and overall health. These manifestations are influenced by systemic inflammation, oxidative stress, and hormonal imbalances, all hallmarks of COPD. Recent research has uncovered an intricate interplay between COPD and musculoskeletal comorbidities, suggesting that muscle and bone tissues may cross-communicate through the release of signalling molecules, known as "myokines" and "osteokines". We explored this dynamic relationship, with a particular focus on the role of the immune system in mediating the cross-communication between muscle and bone in COPD. Moreover, we delved into existing and emerging therapeutic strategies for managing musculoskeletal disorders in COPD. It underscores the development of personalized treatment approaches that target both the respiratory and musculoskeletal aspects of COPD, offering the promise of improved well-being and quality of life for individuals grappling with this complex condition. This comprehensive review underscores the significance of recognizing the profound impact of COPD on the musculoskeletal system and its comorbidities. By unravelling the intricate connections between these systems and exploring innovative treatment avenues, we can aspire to enhance the overall care and outcomes for COPD patients, ultimately offering hope for improved health and well-being.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

Macrophages are essential for skeletal muscle homeostasis, but how their dysregulation contributes to the development of fibrosis in muscle disease remains unclear. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six clusters and unexpectedly found that none corresponded to traditional definitions of M1 or M2 macrophages. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 (gal-3) and osteopontin (Spp1). Spatial transcriptomics, computational inferences of intercellular communication, and in vitro assays indicated that macrophage-derived Spp1 regulates stromal progenitor differentiation. Gal-3+ macrophages were chronically activated in dystrophic muscle, and adoptive transfer assays showed that the gal-3+ phenotype was the dominant molecular program induced within the dystrophic milieu. Gal-3+ macrophages were also elevated in multiple human myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining their transcriptional programs and reveal Spp1 as a major regulator of macrophage and stromal progenitor interactions.

Inflammasome Activity in the Skeletal Muscle and Heart of Rodent Models for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is characterized by wasting of muscles that leads to difficulty moving and premature death, mainly from heart failure. Glucocorticoids are applied in the management of the disease, supporting the hypothesis that inflammation may be driver as well as target. However, the inflammatory mechanisms during progression of cardiac and skeletal muscle dysfunction are still not well characterized. Our objective was to characterize the inflammasomes in myocardial and skeletal muscle in rodent models of DMD. Gastrocnemius and heart samples were collected from mdx mice and DMDmdx rats (3 and 9-10 months). Inflammasome sensors and effectors were assessed by immunoblotting. Histology was used to assess leukocyte infiltration and fibrosis. In gastrocnemius, a tendency towards elevation of gasdermin D irrespective of the age of the animal was observed. The adaptor protein was elevated in the mdx mouse skeletal muscle and heart. Increased cleavage of the cytokines was observed in the skeletal muscle of the DMDmdx rats. Sensor or cytokine expression was not changed in the tissue samples of the mdx mice. In conclusion, inflammatory responses are distinct between the skeletal muscle and heart in relevant models of DMD. Inflammation tends to decrease over time, supporting the clinical observations that the efficacy of anti-inflammatory therapies might be more prominent in the early stage.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

The monocytic/macrophage system is essential for skeletal muscle homeostasis, but its dysregulation contributes to the pathogenesis of muscle degenerative disorders. Despite our increasing knowledge of the role of macrophages in degenerative disease, it still remains unclear how macrophages contribute to muscle fibrosis. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six novel clusters. Unexpectedly, none corresponded to traditional definitions of M1 or M2 macrophage activation. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 and spp1. Spatial transcriptomics and computational inferences of intercellular communication indicated that spp1 regulates stromal progenitor and macrophage interactions during muscular dystrophy. Galectin-3 + macrophages were chronically activated in dystrophic muscle and adoptive transfer assays showed that the galectin-3 + phenotype was the dominant molecular program induced within the dystrophic milieu. Histological examination of human muscle biopsies revealed that galectin-3 + macrophages were also elevated in multiple myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining the transcriptional programs induced in muscle macrophages, and reveal spp1 as a major regulator of macrophage and stromal progenitor interactions.

Vascular therapy for Duchenne muscular dystrophy (DMD)

Duchenne muscular dystrophy (DMD) is a progressive disease characterized by the wasting of the muscles that eventually lead to difficulty moving and, ultimately, premature death from heart and respiratory complications. DMD deficiency is caused by mutations in the gene encoding dystrophin, which prevents skeletal muscle, cardiac muscle, and other cells from producing the functional protein. Located on the cytoplasmic face of the plasma membrane of muscle fibers, dystrophin serves as a component of the dystrophin glycoprotein complex (DGC), mechanically reinforces the sarcolemma, and stabilizes the DGC, preventing it from contraction-mediated muscle degradation. In DMD muscle, dystrophin deficiency leads to progressive fibrosis, myofiber damage, chronic inflammation, and dysfunction of the mitochondria and muscle stem cells. Currently, DMD is incurable, and treatment involves the administration of glucocorticoids in order to delay disease progression. In the presence of developmental delay, proximal weakness, and elevated serum creatine kinase levels, a definitive diagnosis can usually be made after an extensive review of the patient's history and physical examination, as well as confirmation through muscle biopsy or genetic testing. Current standards of care include the use of corticosteroids to prolong ambulation and delay the onset of secondary complications, including respiratory muscle and cardiac functions. However, different studies have been carried out to show the relationship between vascular density and impaired angiogenesis in the pathogenesis of DMD. Several recent studies on DMD management are vascular targeted and focused on ischemia as a culprit for the pathogenesis of DMD. This review critically discusses approaches-such as modulation of nitric oxide (NO) or vascular endothelial growth factor (VEGF)-related pathways-to attenuate the dystrophic phenotype and enhance angiogenesis.

Re-examination of therapeutic management of muscular dystrophies using a vascular smooth muscle-centered approach

In contrast to the long-standing focus on the pathophysiology of skeletal muscles in the hunt for a cure for Duchenne muscular dystrophy (DMD), we opine that the malfunctioning of dystrophin produced by vascular smooth muscle is a major contributor to the pathology of the illness. We believe that a biological response modifier glucan (BRMG), which has been shown in clinical studies of DMD to boost the expression of vascular smooth muscle dystrophin and provide anti-fibrotic and anti-inflammatory effects, may play a key role in reducing the pathogenesis of DMD. According to the evaluation of biomarkers, this BRMG, which is safe and side-effect-free, reduces the pathogenesis of DMD. We describe the possible mechanisms of action by which this BRMG helps in alleviating the symptoms of DMD by targeting smooth muscle dystrophin, in addition to its advantages over other therapeutic modalities, as well as how it can serve as a valuable adjunct to existing therapies. We suggest that using BRMG adjuncts that target smooth muscle dystrophin would be a potential therapeutic approach that prolongs the lifespan and extends the duration of ambulation from the onset of DMD. Further studies are needed to validate this hypothesis.

Macrophages-microenvironment crosstalk in fibrostenotic inflammatory bowel disease: from basic mechanisms to clinical applications

INTRODUCTION

Intestinal fibrosis is a common complication of Inflammatory Bowel Disease (IBD) with no available drugs. The current therapeutic principle is surgical intervention as the core. Intestinal macrophages contribute to both the progression of inflammation and fibrosis. Understanding the role of macrophages in the intestinal microenvironment could bring new hope for fibrosis prevention or even reversal.

AREAS COVERED

This article reviewed the most relevant reports on macrophage in the field of intestinal fibrosis. The authors discussed current opinions about how intestinal macrophages function and interact with surrounding mediators during inflammation resolution and fibrostenotic IBD. Based on biological mechanisms findings, authors summarized related clinical trial outcomes.

EXPERT OPINION

The plasticity of intestinal macrophages allows them to undergo dramatic alterations in their phenotypes or functions when exposed to gastrointestinal environmental stimuli. They exhibit distinct metabolic characteristics, secrete various cytokines, express unique surface markers, and transmit different signals. Nevertheless, the specific mechanism through which the intestinal macrophages contribute to intestinal fibrosis remains unclear. It should further elucidate a novel therapeutic approach by targeting macrophages, especially distinct mechanisms in specific subgroups of macrophages involved in the progression of fibrogenesis in IBD.

Myeloid cell-specific mutation of Spi1 selectively reduces M2-biased macrophage numbers in skeletal muscle, reduces age-related muscle fibrosis and prevents sarcopenia

Intramuscular macrophages play key regulatory roles in determining the response of skeletal muscle to injury and disease. Recent investigations showed that the numbers and phenotype of intramuscular macrophages change during aging, suggesting that those changes could influence the aging process. We tested that hypothesis by generating a mouse model that harbors a myeloid cell-specific mutation of Spi1, which is a transcription factor that is essential for myeloid cell development. The mutation reduced the numbers of macrophages biased to the CD163+/CD206+ M2 phenotype in muscles of aging mice without affecting the numbers of CD68-expressing macrophages and reduced the expression of transcripts associated with the M2-biased phenotype. The mutation did not affect the colony-forming ability or the frequency of specific subpopulations of bone marrow hematopoietic cells and did not affect myeloid/lymphoid cell ratios in peripheral blood leukocyte populations. Cellularity of most myeloid lineage cells was not influenced by the mutation. The Spi1 mutation in bone marrow-derived macrophages in vitro also did not affect expression of transcripts that indicate the M2-biased phenotype. Thus, myeloid cell-targeted mutation of Spi1 influences macrophage phenotype in muscle but did not affect earlier stages of differentiation of cells in the macrophage lineage. The mutation reduced age-related muscle fibrosis, which is consistent with the reduction of M2-biased macrophages, and reduced expression of the pro-fibrotic enzyme arginase. Most importantly, the mutation prevented sarcopenia. Together, our observations indicate that intramuscular, M2-biased macrophages play significant roles in promoting detrimental, age-related changes in muscle.

Mitochondrial DNA on Tumor-Associated Macrophages Polarization and Immunity

As the richest immune cells in most tumor microenvironments (TMEs), tumor-associated macrophages (TAMs) play an important role in tumor development and treatment sensitivity. The phenotypes and functions of TAMs vary according to their sources and tumor progression. Different TAM phenotypes display distinct behaviors in terms of tumor immunity and are regulated by intracellular and exogenous molecules. Additionally, dysfunctional and oxidatively stressed mitochondrial-derived mitochondrial DNA (mtDNA) plays an important role in remodeling the phenotypes and functions of TAMs. This article reviews the interactions between mtDNA and TAMs in the TME and further discusses the influence of their performance on tumor genesis and development.

Myeloid cell-mediated targeting of LIF to dystrophic muscle causes transient increases in muscle fiber lesions by disrupting the recruitment and dispersion of macrophages in muscle

Leukemia inhibitory factor (LIF) can influence development by increasing cell proliferation and inhibiting differentiation. Because of its potency for expanding stem cell populations, delivery of exogenous LIF to diseased tissue could have therapeutic value. However, systemic elevations of LIF can have negative, off-target effects. We tested whether inflammatory cells expressing a LIF transgene under control of a leukocyte-specific, CD11b promoter provide a strategy to target LIF to sites of damage in the mdx mouse model of Duchenne muscular dystrophy, leading to increased numbers of muscle stem cells and improved muscle regeneration. However, transgene expression in inflammatory cells did not increase muscle growth or increase numbers of stem cells required for regeneration. Instead, transgene expression disrupted the normal dispersion of macrophages in dystrophic muscles, leading to transient increases in muscle damage in foci where macrophages were highly concentrated during early stages of pathology. The defect in inflammatory cell dispersion reflected impaired chemotaxis of macrophages to C-C motif chemokine ligand-2 and local increases of LIF production that produced large aggregations of cytolytic macrophages. Transgene expression also induced a shift in macrophage phenotype away from a CD206+, M2-biased phenotype that supports regeneration. However, at later stages of the disease when macrophage numbers declined, they dispersed in the muscle, leading to reductions in muscle fiber damage, compared to non-transgenic mdx mice. Together, the findings show that macrophage-mediated delivery of transgenic LIF exerts differential effects on macrophage dispersion and muscle damage depending on the stage of dystrophic pathology.

Beneficial effects of β-escin on muscle regeneration in rat model of skeletal muscle injury

BACKGROUND

Recent advancements in understanding β-escin action provide basis for new therapeutic claims for the drug. β-escin-evoked attenuation of NF-κB-dependent signaling, increase in MMP-14 and decrease in COUP-TFII content and a rise in cholesterol biosynthesis could be beneficial in alleviating muscle-damaging processes.

PURPOSE

The aim of this study was to investigate the effect of β-escin on skeletal muscle regeneration.

METHODS

Rat model of cardiotoxin-induced injury of fast-twich extensor digitorum longus (EDL) and slow-twich soleus (SOL) muscles and C2C12 myoblast cells were used in the study. We evaluated muscles obtained on day 3 and 14 post-injury by histological analyses of muscle fibers, connective tissue, and mononuclear infiltrate, by immunolocalization of macrophages and by qPCR to quantify the expression of muscle regeneration-related genes. Mechanism of drug action was investigated in vitro by assessing cell viability, NF-κB activation, MMP-2 and MMP-9 secretion, and ALDH activity.

RESULTS

In rat model, β-escin rescues regenerating muscles from atrophy. The drug reduces inflammatory infiltration, increases the number of muscle fibers and decreases fibrosis. β-escin reduces macrophage infiltration into injured muscles and promotes their M2 polarization. It also alters transcription of muscle regeneration-related genes: Myf5, Myh2, Myh3, Myh8, Myod1, Pax3 and Pax7, and Pcna. In C2C12 myoblasts in vitro, β-escin inhibits TNF-α-induced activation of NF-κB, reduces secretion of MMP-9 and increases ALDH activity.

CONCLUSIONS

The data reveal beneficial role of β-escin in muscle regeneration, particularly in poorly regenerating slow-twitch muscles. The findings provide rationale for further studies on β-escin repositioning into conditions associated with muscle damage such as strenuous exercise, drug-induced myotoxicity or age-related disuse atrophy.

Electroacupuncture regulates inflammation, collagen deposition and macrophage function in skeletal muscle through the TGF-β1/Smad3/p38/ERK1/2 pathway

Skeletal muscle injury is one of the most common sports injury, which accounts for ~40% of all sports-related injuries among the elderly. In addition, cases of full recovery from treatment are rare. Although electroacupuncture (EA) is an integral aspect of traditional Chinese medicine, the effects of EA on skeletal muscle fibrosis and the possible underlying mechanism remain unclear. To investigate the effect and potential mechanism of EA on skeletal inflammation, collagen deposition and macrophage function, a skeletal muscle injury model was established by injecting 100 µl cardiotoxin into the anterior tibial muscle of Sprague Dawley rats. The animals were randomly divided into the following three groups: Control, model and EA. The expression of inflammation-related factors (IL-6, IL-4, IL-33, IL-10 and TNF-α) were measured using ELISA. H&E staining, Masson's staining and immunohistochemistry (collagen II, Axin2 and β-catenin) were performed to assess collagen deposition and fibrosis in the muscle tissues. Additionally, immunofluorescence was performed to measure the ratio of M₁ to M₂ macrophages. Western blotting was performed to examine the activity of the TGF-β1/Smad3/p38/ERK1/2 pathway. Compared with that in the control rats, the mental state, such as the degree of activity and excitement, of the model rats deteriorated, with clear activity limitations. Compared with those in the model rats, EA-treated rats exhibited improved mental status and activity, reduced levels of IL-6, IL-4 and TNF-α, reduced collagen deposition and fibrosis, in addition to increased expression of IL-33 and IL-10. This improvement became increasingly evident with prolonged intervention time. EA also promoted the transformation of macrophages from the M₁ into the M₂ sub-type, where the M₁/M₂ ratio on day 7 was lower compared with that on day 14. Western blotting results showed that compared with that in the model rats, the expression of TGF-β1, MMP-2, MMP-7 and the activation of Smad3 and p38 was decreased in EA-treated rats, whilst the activation of ERK1/2 was significantly elevated. In conclusion, EA can inhibit inflammation and collagen deposition whilst promoting the transformation of macrophages from the M₁ into the M₂ sub-type. The underlying mechanism was found to be associated with TGF-β1/Smad3/p38/ERK1/2 signaling.

Skeletal Muscle Regeneration by the Exosomes of Adipose Tissue-Derived Mesenchymal Stem Cells

Profound skeletal muscle loss can lead to severe disability and cosmetic deformities. Mesenchymal stem cell (MSC)-derived exosomes have shown potential as an effective therapeutic tool for tissue regeneration. This study aimed to determine the regenerative capacity of MSC-derived exosomes for skeletal muscle regeneration. Exosomes were isolated from human adipose tissue-derived MSCs (AD-MSCs). The effects of MSC-derived exosomes on satellite cells were investigated using cell viability, relevant genes, and protein analyses. Moreover, NOD-SCID mice were used and randomly assigned to the healthy control (n = 4), muscle defect (n = 6), and muscle defect + exosome (n = 6) groups. Muscle defects were created using a biopsy punch on the quadriceps of the hind limb. Four weeks after the surgery, the quadriceps muscles were harvested, weighed, and histologically analyzed. MSC-derived exosome treatment increased the proliferation and expression of myocyte-related genes, and immunofluorescence analysis for myogenin revealed a similar trend. Histologically, MSC-derived exosome-treated mice showed relatively preserved shapes and sizes of the muscle bundles. Immunohistochemical staining revealed greater expression of myogenin and myoblast determination protein 1 in the MSC-derived exosome-treated group. These results indicate that exosomes extracted from AD-MSCs have the therapeutic potential for skeletal muscle regeneration.

A review of applications of metabolomics in osteoarthritis

Osteoarthritis (OA) represents the most prevalent and disabling arthritis worldwide due to its heterogeneous and progressive articular degradation. However, effective and timely diagnosis and fundamental treatment for this disorder are lacking. Metabolomics, a growing field in life science research in recent years, has the potential to detect many metabolites and thus explains the underlying pathophysiological processes. Hence, new specific metabolic markers and related metabolic pathways can be identified for OA. In this review, we aimed to provide an overview of studies related to the metabolomics of OA in animal models and humans to describe the metabolic changes and related pathways for OA. The present metabolomics studies reveal that the pathogenesis of OA may be significantly related to perturbations of amino acid metabolism. These altered amino acids (e.g., branched-chain amino acids, arginine, and alanine), as well as phospholipids, were identified as potential biomarkers to distinguish patients with OA from healthy individuals.

The contemporary model of vertebral column joint dysfunction and impact of high-velocity, low-amplitude controlled vertebral thrusts on neuromuscular function

Purpose

There is growing evidence that vertebral column function and dysfunction play a vital role in neuromuscular control. This invited review summarises the evidence about how vertebral column dysfunction, known as a central segmental motor control (CSMC) problem, alters neuromuscular function and how spinal adjustments (high-velocity, low-amplitude or HVLA thrusts directed at a CSMC problem) and spinal manipulation (HVLA thrusts directed at segments of the vertebral column that may not have clinical indicators of a CSMC problem) alters neuromuscular function.

Methods

The current review elucidates the peripheral mechanisms by which CSMC problems, the spinal adjustment or spinal manipulation alter the afferent input from the paravertebral tissues. It summarises the contemporary model that provides a biologically plausible explanation for CSMC problems, the manipulable spinal lesion. This review also summarises the contemporary, biologically plausible understanding about how spinal adjustments enable more efficient production of muscular force. The evidence showing how spinal dysfunction, spinal manipulation and spinal adjustments alter central multimodal integration and motor control centres will be covered in a second invited review.

Results

Many studies have shown spinal adjustments increase voluntary force and prevent fatigue, which mainly occurs due to altered supraspinal excitability and multimodal integration. The literature suggests physical injury, pain, inflammation, and acute or chronic physiological or psychological stress can alter the vertebral column’s central neural motor control, leading to a CSMC problem. The many gaps in the literature have been identified, along with suggestions for future studies.

Conclusion

Spinal adjustments of CSMC problems impact motor control in a variety of ways. These include increasing muscle force and preventing fatigue. These changes in neuromuscular function most likely occur due to changes in supraspinal excitability. The current contemporary model of the CSMC problem, and our understanding of the mechanisms of spinal adjustments, provide a biologically plausible explanation for how the vertebral column’s central neural motor control can dysfunction, can lead to a self-perpetuating central segmental motor control problem, and how HVLA spinal adjustments can improve neuromuscular function.

Cardioprotective Effect of Whole Body Periodic Acceleration in Dystrophic Phenotype mdx Rodent

Duchenne muscular dystrophy (DMD) is characterized by progressive muscle wasting and the development of a dilated cardiomyopathy (DCM), which is the leading cause of death in DMD patients. Despite knowing the cause of DMD, there are currently no therapies which can prevent or reverse its inevitable progression. We have used whole body periodic acceleration (WBPA) as a novel tool to enhance intracellular constitutive nitric oxide (NO) production. WBPA adds small pulses to the circulation to increase pulsatile shear stress, thereby upregulating endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) and subsequently elevating the production of NO. Myocardial cells from dystrophin-deficient 15-month old mdx mice have contractile deficiency, which is associated with elevated concentrations of diastolic Ca²⁺ ([Ca²⁺]_d), Na⁺ ([Na⁺]_d), and reactive oxygen species (ROS), increased cell injury, and decreased cell viability. Treating 12-month old mdx mice with WBPA for 3 months reduced cardiomyocyte [Ca²⁺]_d and [Na⁺]_d overload, decreased ROS production, and upregulated expression of the protein utrophin resulting in increased cell viability, reduced cardiomyocyte damage, and improved contractile function compared to untreated mdx mice.

Periplocymarin protects against myocardial fibrosis induced by β-adrenergic activation in mice

Periplocymarin is an effective component of Periplocae Cortex, which was wildly used as an ingredient in Traditional Chinese Medicine. Our group previously reported that periplocymarin exerted cardiotonic role via promoting calcium influx. However, its exact role in the pathogenesis of myocardial fibrosis has not been elucidated yet. The present study was aimed at determining the potential effect and underlying mechanism of periplocymarin in isoproterenol (ISO)-induced myocardial fibrosis. C57BL/6 mice were subcutaneously injected with ISO (5 mg/kg/day) or saline for 1 week. The early-to-atrial wave ratio (E/A ratio) measured by echocardiography revealed that ISO-induced heart stiffness was remarkably reversed by administration of periplocymarin (5 mg/kg/day). Masson trichrome staining exhibited that treatment of periplocymarin reduced the excessive deposition of extracellular matrix (ECM). Further investigations employing real-time PCR and western blot demonstrated that periplocymarin suppressed the expression of fibrosis related genes (Col1a1, Col3a1, Acta2 and Tgfb1) and proteins (Collagen I, Collagen III, α-SMA and TGF-β1) induced by ISO. Metabolomics analysis demonstrated that periplocymarin ameliorated the disorders triggered by ISO and many of the differential metabolic substances were involved in amino acid, glucose and lipid metabolism. Further analysis using network pharmacology revealed that three key genes, namely NOS2, NOS3 and Ptgs2, may be the potential targets of periplocymarin and responsible for the disorders. Validation using heart tissues showed that the mRNA expression of NOS3 was decreased while Ptgs2 was increased upon ISO treatment, which were reversed by periplocymarin. Moreover, the expression of COX-2 (Ptgs2 encoded protein) was consistent with the aspect of Ptgs2 mRNA, while eNOS (NOS3 encoded protein) expression was unchanged. In vitro studies exhibited that periplocymarin exerts anti-fibrotic function via regulating at least eNOS and COX-2 in cardiomyocyte. Taken together, periplocymarin protects against myocardial fibrosis induced by β-adrenergic activation, the potential mechanism was that periplocymarin targeted on, at least eNOS and COX-2, to improve the metabolic processes of cardiomyocyte and thus attenuated the myocardial fibrosis. Our study highlighted that periplocymarin is a potential therapeutic agent for the prevention of myocardial fibrosis.

A stromal progenitor and ILC2 niche promotes muscle eosinophilia and fibrosis-associated gene expression

Despite the well-accepted view that chronic inflammation contributes to the pathogenesis of Duchenne muscular dystrophy (DMD), the function and regulation of eosinophils remain an unclear facet of type II innate immunity in dystrophic muscle. We report the observation that group 2 innate lymphoid cells (ILC2s) are present in skeletal muscle and are the principal regulators of muscle eosinophils during muscular dystrophy. Eosinophils were elevated in DMD patients and dystrophic mice along with interleukin (IL)-5, a major eosinophil survival factor that was predominantly expressed by muscle ILC2s. We also find that IL-33 was upregulated in dystrophic muscle and was predominantly produced by fibrogenic/adipogenic progenitors (FAPs). Exogenous IL-33 and IL-2 complex (IL-2c) expanded muscle ILC2s and eosinophils, decreased the cross-sectional area (CSA) of regenerating myofibers, and increased the expression of genes associated with muscle fibrosis. The deletion of ILC2s in dystrophic mice mitigated muscle eosinophilia and impaired the induction of IL-5 and fibrosis-associated genes. Our findings highlight a FAP/ILC2/eosinophil axis that promotes type II innate immunity, which influences the balance between regenerative and fibrotic responses during muscular dystrophy.

Immune cells that comprise type II innate immunity coalesce to regulate tissue repair and fibrosis. Kastenschmidt et al. report that ILC2s reside in skeletal muscle, are activated in muscular dystrophy, and promote muscle eosinophilia. Stromal progenitors expressed IL-33, which expanded ILC2s and promoted a transcriptional program associated with muscle fibrosis.

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

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

The linkage between inflammation and fibrosis in muscular dystrophies: The axis autotaxin-lysophosphatidic acid as a new therapeutic target?

Muscular dystrophies (MDs) are a diverse group of severe disorders characterized by increased skeletal muscle feebleness. In many cases, respiratory and cardiac muscles are also compromised. Skeletal muscle inflammation and fibrosis are hallmarks of several skeletal muscle diseases, including MDs. Until now, several keys signaling pathways and factors that regulate inflammation and fibrosis have been identified. However, no curative treatments are available. Therefore, it is necessary to find new therapeutic targets to fight these diseases and improve muscle performance. Lysophosphatidic acid (LPA) is an active glycerophospholipid mainly synthesized by the secreted enzyme autotaxin (ATX), which activates six different G protein-coupled receptors named LPA₁ to LPA₆ (LPARs). In conjunction, they are part of the ATX/LPA/LPARs axis, involved in the inflammatory and fibrotic response in several organs-tissues. This review recapitulates the most relevant aspects of inflammation and fibrosis in MDs. It analyzes experimental evidence of the effects of the ATX/LPA/LPARs axis on inflammatory and fibrotic responses. Finally, we speculate about its potential role as a new therapeutic pharmacological target to treat these diseases.

Role of the Renin-Angiotensin-Aldosterone System in Dystrophin-Deficient Cardiomyopathy

Dystrophin-deficient cardiomyopathy (DDC) is currently the leading cause of death in patients with dystrophinopathies. Targeting myocardial fibrosis (MF) has become a major therapeutic goal in order to prevent the occurrence of DDC. We aimed to review and summarize the current evidence about the role of the renin-angiotensin-aldosterone system (RAAS) in the development and perpetuation of MF in DCC. We conducted a comprehensive search of peer-reviewed English literature on PubMed about this subject. We found increasing preclinical evidence from studies in animal models during the last 20 years pointing out a central role of RAAS in the development of MF in DDC. Local tissue RAAS acts directly mainly through its main fibrotic component angiotensin II (ANG2) and its transducer receptor (AT1R) and downstream TGF-b pathway. Additionally, it modulates the actions of most of the remaining pro-fibrotic factors involved in DDC. Despite limited clinical evidence, RAAS blockade constitutes the most studied, available and promising therapeutic strategy against MF and DDC. Conclusion: Based on the evidence reviewed, it would be recommendable to start RAAS blockade therapy through angiotensin converter enzyme inhibitors (ACEI) or AT1R blockers (ARBs) alone or in combination with mineralocorticoid receptor antagonists (MRa) at the youngest age after the diagnosis of dystrophinopathies, in order to delay the occurrence or slow the progression of MF, even before the detection of any cardiovascular alteration.

Metabolic Signature of Articular Cartilage Following Mechanical Injury: An Integrated Transcriptomics and Metabolomics Analysis

Mechanical injury to the articular cartilage is a key risk factor in joint damage and predisposition to osteoarthritis. Integrative multi-omics approaches provide a valuable tool to understand tissue behavior in response to mechanical injury insult and help to identify key pathways linking injury to tissue damage. Global or untargeted metabolomics provides a comprehensive characterization of the metabolite content of biological samples. In this study, we aimed to identify the metabolic signature of cartilage tissue post injury. We employed an integrative analysis of transcriptomics and global metabolomics of murine epiphyseal hip cartilage before and after injury. Transcriptomics analysis showed a significant enrichment of gene sets involved in regulation of metabolic processes including carbon metabolism, biosynthesis of amino acids, and steroid biosynthesis. Integrative analysis of enriched genes with putatively identified metabolite features post injury showed a significant enrichment for carbohydrate metabolism (glycolysis, galactose, and glycosylate metabolism and pentose phosphate pathway) and amino acid metabolism (arginine biosynthesis and tyrosine, glycine, serine, threonine, and arginine and proline metabolism). We then performed a cross analysis of global metabolomics profiles of murine and porcine ex vivo cartilage injury models. The top commonly modulated metabolic pathways post injury included arginine and proline metabolism, arginine biosynthesis, glycolysis/gluconeogenesis, and vitamin B6 metabolic pathways. These results highlight the significant modulation of metabolic responses following mechanical injury to articular cartilage. Further investigation of these pathways would provide new insights into the role of the early metabolic state of articular cartilage post injury in promoting tissue damage and its link to disease progression of osteoarthritis.

Duchenne muscular dystrophy (DMD) cardiomyocyte-secreted exosomes promote the pathogenesis of DMD-associated cardiomyopathy

Cardiomyopathy is a leading cause of early mortality in Duchenne muscular dystrophy (DMD). There is a need to gain a better understanding of the molecular pathogenesis for the development effective therapies. Exosomes (exo) are secreted vesicles and exert effects via their RNA, lipid and protein cargo. The role of exosomes in disease pathology is unknown. Exosomes derived from stem cells have demonstrated cardioprotection in the murine DMD heart. However, it is unknown how the disease status of the donor cell type influences exosome function. Here, we sought to determine the phenotypic responses of DMD cardiomyocytes (DMD-iCMs) after long-term exposure to DMD cardiac exosomes (DMD-exo). DMD-iCMs were vulnerable to stress, evidenced by production of reactive oxygen species, the mitochondrial membrane potential and cell death levels. Long-term exposure to non-affected exosomes (N-exo) was protective. By contrast, long-term exposure to DMD-exo was not protective, and the response to stress improved with inhibition of DMD-exo secretion in vitro and in vivo The microRNA (miR) cargo, but not exosome surface peptides, was implicated in the pathological effects of DMD-exo. Exosomal surface profiling revealed N-exo peptides associated with PI3K-Akt signaling. Transcriptomic profiling identified unique changes with exposure to either N- or DMD-exo. Furthermore, DMD-exo miR cargo regulated injurious pathways, including p53 and TGF-beta. The findings reveal changes in exosomal cargo between healthy and diseased states, resulting in adverse outcomes. Here, DMD-exo contained miR changes, which promoted the vulnerability of DMD-iCMs to stress. Identification of these molecular changes in exosome cargo and effectual phenotypes might shed new light on processes underlying DMD cardiomyopathy.This article has an associated First Person interview with the first author of the paper.

Macrophages potentiate STAT3 signaling in skeletal muscles and regulate pancreatic cancer cachexia

Incidence of cachexia is highly prevalent in pancreatic ductal adenocarcinoma (PDAC); advanced disease stage directly correlates with decreased muscle and fat mass in PDAC patients. The pancreatic tumor microenvironment is central to the release of systemic factors that govern lipolysis, proteolysis, and muscle and fat degeneration leading to the cachectic phenotype in cancer patients. The current study explores the role of macrophages, a key immunosuppressive player in the pancreatic tumor microenvironment, in regulating cancer cachexia. We observed a negative correlation between CD163-positive macrophage infiltration and muscle-fiber cross sectional area in human PDAC patients. To investigate the role of macrophages in myodegeneration, we utilized conditioned media transplant assays and orthotopic models of PDAC-induced cachexia in immune-competent mice with and without macrophage depletion. We observed that macrophage-derived conditioned medium, in combination with tumor cell-conditioned medium, promoted muscle atrophy through STAT3 signaling. Furthermore, macrophage depletion attenuated systemic inflammation and muscle wasting in pancreatic tumor-bearing mice. Targeting macrophage-mediated STAT3 activation or macrophage-derived interleukin-1 alpha or interleukin-6 diminished myofiber atrophy. Taken together, the current study identified the critical association between macrophages and cachexia phenotype in pancreatic cancer.

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.

Macrophages Are Key Regulators of Stem Cells during Skeletal Muscle Regeneration and Diseases

Muscle regeneration is a closely regulated process that involves a variety of cell types such as satellite cells, myofibers, fibroadipogenic progenitors, endothelial cells, and inflammatory cells. Among these different cell types, macrophages emerged as a central actor coordinating the different cellular interactions and biological processes. Particularly, the transition of macrophages from their proinflammatory to their anti-inflammatory phenotype was shown to regulate inflammation, myogenesis, fibrosis, vascularization, and return to homeostasis. On the other hand, deregulation of macrophage accumulation or polarization in chronic degenerative muscle disorders was shown to impair muscle regeneration. Considering the key roles of macrophages in skeletal muscle, they represent an attractive target for new therapeutic approaches aiming at mitigating various muscle disorders. This review aims at summarizing the novel insights into macrophage heterogeneity, plasticity, and functions in skeletal muscle homeostasis, regeneration, and disease.

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.

Targeting angiogenesis in Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) represents one of the most devastating types of muscular dystrophies which affect boys already at early childhood. Despite the fact that the primary cause of the disease, namely the lack of functional dystrophin is known already for more than 30 years, DMD still remains an incurable disease. Thus, an enormous effort has been made during recent years to reveal novel mechanisms that could provide therapeutic targets for DMD, especially because glucocorticoids treatment acts mostly symptomatic and exerts many side effects, whereas the effectiveness of genetic approaches aiming at the restoration of functional dystrophin is under the constant debate. Taking into account that dystrophin expression is not restricted to muscle cells, but is present also in, e.g., endothelial cells, alterations in angiogenesis process have been proposed to have a significant impact on DMD progression. Indeed, already before the discovery of dystrophin, several abnormalities in blood vessels structure and function have been revealed, suggesting that targeting angiogenesis could be beneficial in DMD. In this review, we will summarize current knowledge about the angiogenesis status both in animal models of DMD as well as in DMD patients, focusing on different organs as well as age- and sex-dependent effects. Moreover, we will critically discuss some approaches such as modulation of vascular endothelial growth factor or nitric oxide related pathways, to enhance angiogenesis and attenuate the dystrophic phenotype. Additionally, we will suggest the potential role of other mediators, such as heme oxygenase-1 or statins in those processes.

Cardiac and skeletal muscle effects in the randomized HOPE-Duchenne trial

OBJECTIVE

To assess the feasibility, safety, and efficacy of intracoronary allogeneic cardiosphere-derived cells (CAP-1002) in patients with Duchenne muscular dystrophy (DMD).

METHODS

The Halt Cardiomyopathy Progression (HOPE)-Duchenne trial is a phase I/II, randomized, controlled, open-label trial (NCT02485938). Patients with DMD >12 years old, with substantial myocardial fibrosis, were randomized (1:1) to usual care (control) or global intracoronary infusion of CAP-1002 (75 million cells). Participants were enrolled at 3 US medical centers between January and August 2016 and followed for 12 months. An independent Data and Safety Monitoring Board provided safety oversight. Cardiac function and structure were assessed by MRI, and analyzed by a blinded core laboratory. Skeletal muscle function was assessed by performance of the upper limb (PUL).

RESULTS

Twenty-five eligible patients (mean age 17.8 years; 68% wheelchair-dependent) were randomized to CAP-1002 (n = 13) or control (n = 12). Incidence of treatment-emergent adverse events was similar between groups. Compared to baseline, MRI at 12 months revealed significant scar size reduction and improvement in inferior wall systolic thickening in CAP-1002 but not control patients. Mid-distal PUL improved at 12 months in 8 of 9 lower functioning CAP-1002 patients, and no controls (p = 0.007).

CONCLUSIONS

Intracoronary CAP-1002 in DMD appears safe and demonstrates signals of efficacy on both cardiac and upper limb function for up to 12 months. Thus, future clinical research on CAP-1002 treatment of DMD cardiac and skeletal myopathies is warranted.

CLASSIFICATION OF EVIDENCE

This phase I/II study provides Class II evidence that for patients with DMD, intracoronary CAP-1002 is feasible and appears safe and potentially effective.

Determination of a Critical Size Threshold for Volumetric Muscle Loss in the Mouse Quadriceps

IMPACT STATEMENT

The goal of this study was to determine the threshold for a critically sized, nonhealing muscle defect by characterizing key components in the balance between fibrosis and regeneration as a function of injury size in the mouse quadriceps. There is currently limited understanding of what leads to a critically sized muscle defect and which muscle regenerative components are functionally impaired. With the substantial increase in preclinical VML models as testbeds for tissue engineering therapeutics, defining the critical threshold for VML injuries will be instrumental in characterizing therapeutic efficacy and potential for subsequent translation.

Macrophage and Fibroblast Interactions in Biomaterial-Mediated Fibrosis

Biomaterial-mediated inflammation and fibrosis remain a prominent challenge in designing materials to support tissue repair and regeneration. Despite the many biomaterial technologies that have been designed to evade or suppress inflammation (i.e., delivery of anti-inflammatory drugs, hydrophobic coatings, etc.), many materials are still subject to a foreign body response, resulting in encapsulation of dense, scar-like extracellular matrix. The primary cells involved in biomaterial-mediated fibrosis are macrophages, which modulate inflammation, and fibroblasts, which primarily lay down new extracellular matrix. While macrophages and fibroblasts are implicated in driving biomaterial-mediated fibrosis, the signaling pathways and spatiotemporal crosstalk between these cell types remain loosely defined. In this review, the role of M1 and M2 macrophages (and soluble cues) involved in the fibrous encapsulation of biomaterials in vivo is investigated, with additional focus on fibroblast and macrophage crosstalk in vitro along with in vitro models to study the foreign body response. Lastly, several strategies that have been used to specifically modulate macrophage and fibroblast behavior in vitro and in vivo to control biomaterial-mediated fibrosis are highlighted.

The Metabolome and Osteoarthritis: Possible Contributions to Symptoms and Pathology

Osteoarthritis (OA) is a progressive, deteriorative disease of articular joints. Although traditionally viewed as a local pathology, biomarker exploration has shown that systemic changes can be observed. These include changes to cytokines, microRNAs, and more recently, metabolites. The metabolome is the set of metabolites within a biological sample and includes circulating amino acids, lipids, and sugar moieties. Recent studies suggest that metabolites in the synovial fluid and blood could be used as biomarkers for OA incidence, prognosis, and response to therapy. However, based on clinical, demographic, and anthropometric factors, the local synovial joint and circulating metabolomes may be patient specific, with select subsets of metabolites contributing to OA disease. This review explores the contribution of the local and systemic metabolite changes to OA, and their potential impact on OA symptoms and disease pathogenesis.

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.

Aging of the immune system causes reductions in muscle stem cell populations, promotes their shift to a fibrogenic phenotype, and modulates sarcopenia

Aging is associated with diminished muscle mass, reductions in muscle stem cell functions, and increased muscle fibrosis. The immune system, especially macrophages, can have important roles in modulating muscle growth and regeneration, suggesting that the immune system may also have significant influences on muscle aging. Moreover, the immune system experiences changes in function during senescence, suggesting that regulatory interaction between muscle cells and the immune system may also change during aging. In this study, we performed bone marrow transplantations between age-mismatched donor and recipient mice to test the influence of the age of the immune system on muscle aging. Transplantation of young bone marrow cells into old recipients prevented sarcopenia and prevented age-related change in muscle fiber phenotype. Transplantation of old bone marrow cells into young animals reduced satellite cell numbers and promoted satellite cells to switch toward a fibrogenic phenotype. We also demonstrated that conditioned media from young, but not old, bone marrow cells promoted myoblast proliferation in vitro, and we found that factors released by young bone marrow cells were more supportive of myotube differentiation in vitro. Together, our results demonstrate that aging of bone marrow cells promotes the age-related reduction of satellite cell number and function and contributes to sarcopenia.-Wang, Y., Wehling-Henricks, M., Welc, S. S., Fisher, A. L., Zuo, Q., Tidball, J. G. Aging of the immune system causes reductions in muscle stem cell populations, promotes their shift to a fibrogenic phenotype, and modulates sarcopenia.

Macrophage polarization contributes to local inflammation and structural change in the multifidus muscle after intervertebral disc injury

PURPOSE

Intervertebral disk (IVD) lesion and its subsequent degeneration have a profound effect on the multifidus muscle. The subacute/early chronic phase of multifidus remodeling after IVD lesion has been proposed to be regulated by inflammatory processes. The balance between pro-inflammatory (M1) and anti-inflammatory (M2) macrophages plays an important role in maintaining tissue integrity after injury. The localization, polarization of macrophage subtypes and their mediation of the pro-inflammatory cytokine tumor necrosis factor (TNF) are unknown in paraspinal muscles during IVD degeneration. A sheep model of IVD degeneration was used to investigate the role of macrophages and TNF in the structural alterations that occur within the multifidus muscle.

METHODS

Anterolateral lesions were induced at L3-4 IVD in sheep. Multifidus muscle tissue at L4 was harvested 3 and 6 months after lesion and used for immunofluorescence assays to examine total macrophage number, macrophage polarization between M1 and M2, and to assess the localization of TNF expression in muscle, adipose and connective tissues from injured and naïve control animals.

RESULTS

A greater proportion of M1 macrophages is present in muscle at both 3 and 6 months after IVD lesion, and adipose tissue at 6 months. Total number of macrophages is unchanged. At 6 months, expression of TNF is increased in adipose and connective tissue and the proportion of TNF expressed by M1 macrophages is increased.

CONCLUSIONS

These data support the proposal that macrophages and TNF (pro-inflammatory cytokine) play an active role in the subacute/early chronic phase of remodeling in muscle, adipose and connective tissues of the multifidus during IVD degeneration. This presents a novel target for treatment. These slides can be retrieved under Electronic Supplementary Material.

Skeletal muscle regeneration is modulated by inflammation

Skeletal 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 briefly summarise the functions of acute inflammation in muscle regeneration.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

Immune system is closely relevant to the muscle regeneration. Understanding the mechanisms of inflammation in muscle regeneration is therefore critical for the development of effective regenerative, and therapeutic strategies in muscular disorders. This review provides information for muscle regeneration research regarding the effects of inflammation on muscle regeneration.

Exosome-Mediated Benefits of Cell Therapy in Mouse and Human Models of Duchenne Muscular Dystrophy

Genetic deficiency of dystrophin leads to disability and premature death in Duchenne muscular dystrophy (DMD), affecting the heart as well as skeletal muscle. Here, we report that clinical-stage cardiac progenitor cells, known as cardiosphere-derived cells (CDCs), improve cardiac and skeletal myopathy in the mdx mouse model of DMD. Injection of CDCs into the hearts of mdx mice augments cardiac function, ambulatory capacity, and survival. Exosomes secreted by human CDCs reproduce the benefits of CDCs in mdx mice and in human induced pluripotent stem cell-derived Duchenne cardiomyocytes. Surprisingly, CDCs and their exosomes also transiently restored partial expression of full-length dystrophin in mdx mice. The findings further motivate the testing of CDCs in Duchenne patients, while identifying exosomes as next-generation therapeutic candidates.

Macrophages escape Klotho gene silencing in the mdx mouse model of Duchenne muscular dystrophy and promote muscle growth and increase satellite cell numbers through a Klotho-mediated pathway

Duchenne muscular dystrophy (DMD) is a muscle wasting disease in which inflammation influences the severity of pathology. We found that the onset of muscle inflammation in the mdx mouse model of DMD coincides with large increases in expression of pro-inflammatory cytokines [tumor necrosis factor-α (TNFα); interferon gamma (IFNγ)] and dramatic reductions of the pro-myogenic protein Klotho in muscle cells and large increases of Klotho in pro-regenerative, CD206+ macrophages. Furthermore, TNFα and IFNγ treatments reduced Klotho in muscle cells and increased Klotho in macrophages. Because CD206+/Klotho+ macrophages were concentrated at sites of muscle regeneration, we tested whether macrophage-derived Klotho promotes myogenesis. Klotho transgenic macrophages had a pro-proliferative influence on muscle cells that was ablated by neutralizing antibodies to Klotho and conditioned media from Klotho mutant macrophages did not increase muscle cell proliferation in vitro. In addition, transplantation of bone marrow cells from Klotho transgenic mice into mdx recipients increased numbers of myogenic cells and increased the size of muscle fibers. Klotho also acted directly on macrophages, stimulating their secretion of TNFα. Because TNFα is a muscle mitogen, we tested whether the pro-proliferative effects of Klotho on muscle cells were mediated by TNFα and found that increased proliferation caused by Klotho was reduced by anti-TNFα. Collectively, these data show that pro-inflammatory cytokines contribute to silencing of Klotho in dystrophic muscle, but increase Klotho expression by macrophages. Our findings also show that macrophage-derived Klotho can promote muscle regeneration by expanding populations of muscle stem cells and increasing muscle fiber growth in dystrophic muscle.

Treatment with the anti-IL-6 receptor antibody attenuates muscular dystrophy via promoting skeletal muscle regeneration in dystrophin-/utrophin-deficient mice

Background

Chronic increases in the levels of the inflammatory cytokine interleukin-6 (IL-6) in serum and skeletal muscle are thought to contribute to the progression of muscular dystrophy. Dystrophin/utrophin double-knockout (dKO) mice develop a more severe and progressive muscular dystrophy than the mdx mice, the most common murine model of Duchenne muscular dystrophy (DMD). In particular, dKO mice have smaller body sizes and muscle diameters, and develop progressive kyphosis and fibrosis in skeletal and cardiac muscles. As mdx mice and DMD patients, we found that IL-6 levels in the skeletal muscle were significantly increased in dKO mice. Thus, in this study, we aimed to analyze the effects of IL-6 receptor (IL-6R) blockade on the muscle pathology of dKO mice.

Methods

Male dKO mice were administered an initial injection (200 mg/kg intraperitoneally (i.p.)) of either the anti-IL-6R antibody MR16-1 or an isotype-matched control rat IgG at the age of 14 days, and were then given weekly injections (25 mg/kg i.p.) until 90 days of age.

Results

Treatment of dKO mice with the MR16-1 antibody successfully inhibited the IL-6 pathway in the skeletal muscle and resulted in a significant reduction in the expression levels of phosphorylated signal transducer and activator of transcription 3 in the skeletal muscle. Pathologically, a significant increase in the area of embryonic myosin heavy chain-positive myofibers and muscle diameter, and reduced fibrosis in the quadriceps muscle were observed. These results demonstrated the therapeutic effects of IL-6R blockade on promoting muscle regeneration. Consistently, serum creatine kinase levels were decreased. Despite these improvements observed in the limb muscles, degeneration of the diaphragm and cardiac muscles was not ameliorated by the treatment of mice with the MR16-1 antibody.

Conclusion

As no adverse effects of treatment with the MR16-1 antibody were observed, our results indicate that the anti-IL-6R antibody is a potential therapy for muscular dystrophy particularly for promoting skeletal muscle regeneration.

Electronic supplementary material

The online version of this article (10.1186/s13395-017-0140-z) contains supplementary material, which is available to authorized users.

Regulation of muscle growth and regeneration by the immune system

Diseases of muscle that are caused by pathological interactions between muscle and the immune system are devastating, but rare. However, muscle injuries that involve trauma and regeneration are fairly common, and inflammation is a clear feature of the regenerative process. Investigations of the inflammatory response to muscle injury have now revealed that the apparently nonspecific inflammatory response to trauma is actually a complex and coordinated interaction between muscle and the immune system that determines the success or failure of tissue regeneration.