Abstract Background Cancer cachexia (CC) is characterized by skeletal muscle atrophy and reduced strength, partly linked to dysfunction of muscle stem cells (MuSCs) and alterations in their niche. Although exercise may mitigate muscle loss, its effects in CC remain debated and its feasibility is often limited in advanced patients. Neuromuscular electrical stimulation (NMES) offers a promising alternative, by promoting MuSC proliferation and fusion, increasing muscle size and macrophage content in healthy muscle. This study investigated whether NMES, initiated at tumor onset, could improve MuSC regulation and its niche while limiting muscle atrophy and weakness in a tumor-bearing mouse model. Methods Ten-week-old male BALB/c mice were subcutaneously injected with C26 tumor cells or PBS. Tumor-bearing mice were divided into NMES-treated (C26 NMES) and non-stimulated controls (C26). NMES consisted of six sessions (two series of three consecutive daily sessions separated by one rest day), starting seven days post-inoculation when tumors became visible. Each session was delivered at a submaximal intensity corresponding to 15% of maximal strength. Muscle mass, myofiber size, strength and cellular composition were assessed. Results Muscle mass was decreased by 13% in C26 mice as compared to PBS controls, while C26 NMES mice showed a ∼7% improvement over C26 mice. Mean myofiber size decreased similarly in both tumor-bearing groups as compared to PBS controls (−12–14%). However, NMES reduced the proportion of small myofibers (400–600 µm²) as compared to C26 mice. Maximal torque loss was less severe in C26 NMES mice (−28%) than in C26 mice (−34%). As compared with PBS mice, C26 mice exhibited increased MuSC proliferation (+97%) but reduced differentiation (−61%), as indicated by fewer myogenin-positive cells. NMES normalized MuSC proliferation, restored myogenin-positive cell number, and enhanced MuSC fusion, reflected by an increased number of PCM1-positive myonuclei (+8-11%). NMES also modulated inflammation, reducing neutrophils (−42%) and increasing macrophages (+35%), through the proliferation of CD169-positive resident macrophages (+106%). In vitro , macrophages exposed to C26 muscle extracts showed elevated pro-inflammatory markers (COX2 and TNF-⍺; +21% and +16%) as compared to PBS controls. This effect was abolished with extracts from C26 NMES muscles. Additionally, C26 extracts reduced the expression of anti-inflammatory markers by macrophages (CD206 and IL-10; −23%), whereas NMES restored their levels to those of controls. Conclusion NMES-induced mild contractile activity is an effective stimulus for preserving muscle strength and mass, improving MuSC regulation, and modulating muscle inflammation in a mouse model of CC.

Muscle stem cells (MuSCs) fuse to form myofibers to repair skeletal muscle after injury. Within the regenerative MuSC niche, restorative macrophages stimulate MuSC fusion, although the molecular mechanisms involved are largely unknown. Here, we show that restorative macrophages secrete ribonuclease T2 (RNAseT2) to stimulate MuSC fusion. RNAseT2 enters MuSCs via the mannose receptor and induces the formation of actin bundles in MuSCs, enabling cell/cell fusion. Mechanistically, RNAseT2 binds to Ste20-like kinase (SLK), which itself triggers the phosphorylation-mediated activation of N-WASP, through Paxillin phosphorylation, allowing actin bundling necessary for MuSC fusion. In vivo, overexpressing RNAseT2 in regenerating muscle increases fusion in newly formed myofibers in mouse and zebrafish while macrophages deficient for RNAseT2 gene lead to fusion defect and smaller myofibers. This study reveals a new function for the highly conserved RNAseT2 and provides a new molecular mechanism by which restorative macrophages support MuSC fusion during muscle repair.

Skeletal muscle regeneration is driven by muscle stem cells (MuSCs), which proliferate, differentiate, and fuse to reform myofibers and restore muscle function. This myogenesis process is driven both by intrinsic MuSC properties and extrinsic cues. While coordinated inflammatory signals are necessary for healthy regeneration, chronic inflammation participates in various pathologies affecting the skeletal muscle. In the idiopathic inflammatory myopathy dermatomyositis (DM), MuSCs exhibit impaired myogenesis in vitro, indicating that they may have acquired intrinsic defects, contributing to the disease and providing a mechanism for sustained patient muscle weakness despite efficient anti-inflammatory treatments. Here, we investigated the transcriptomic regulation of DM-derived MuSCs, with a focus on the H3.3 histone variant that regulates myogenesis progression. DM-derived MuSCs were unable to effectively execute the myogenic transcriptional program during in vitro differentiation. They exhibited an activated canonical tumor necrosis factor (TNF)-⍺ signaling. They also showed reduced expression of H3.3 and its chaperone genes, coupled with a decrease in H3.3 deposition across the entire genome, and particularly at myogenic regulatory factor loci. The loss of H3.3 combined with elevated TNF-⍺ signaling was associated with a failure of DM-derived MuSCs to achieve myogenesis, suggesting a mechanistic link between epigenetic dysregulation and defective muscle regeneration in humans.

BackgroundDuchenne Muscular Dystrophy (DMD) is characterized by the formation of fibrosis and fat deposits that progressively replace muscle fibers, resulting in the loss of muscle function. Both fibrosis and adipogenesis are operated by fibroadipogenic precursors (FAPs), but the molecular regulation and interactions between the two processes are not fully understood.MethodsAdipogenesis was investigated in vivo in the D2-mdx mouse, and in vitro using FAPs isolated from WT (DBA/2) and D2-mdx muscles. Epithelial Growth Factor (EGF) was overexpressed in the D2-mdx muscle via electroporation of an expression plasmid.ResultsWe found that the D2-mdx gastrocnemius muscle showed fat deposition from 10 weeks of age and increased until 18 weeks of age, coinciding with fibrosis. Fat deposition was exclusively found within fibrotic areas. In vitro, D2-mdx FAPs proliferated more, and were more prone to adipogenesis than WT FAPs. Cells from both genotypes showed equal fibrogenesis. Analysis of normal muscle snRNAseq data showed that the Epithelial Growth Factor Receptor (EGFR) was primarily expressed by FAPs. Both EGFR expression and EGFR-phosphorylation were decreased in D2-mdx FAPs as compared with WT FAPs. Stimulating FAPs with EGF decreased adipogenesis, more efficiently in D2-mdx FAPs than in WT FAPs. However, EGF stimulation of EGFR had no effect on their fibrogenic differentiation. Finally, in vivo overexpression of EGF in D2-mdx gastrocnemius muscles reduced both adipogenesis and fibrosis, and was associated with an increased muscle force.ConclusionsIn a DMD context, FAPs are more likely to differentiate into adipocytes than in normal muscle, which is associated with decreased EGFR signaling. Stimulating EGFR signaling decreased adipogenesis in vitro and fat deposition in vivo. The impact of EGFR signaling on fibrogenesis is unclear, the reduced fibrosis observed in vivo may be due to indirect mechanisms. This study identifies EGFR signaling as a new molecular mechanism for controlling adipogenesis in skeletal muscle FAPs.

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disorder caused by gain-of-function mutations in ACVR1/ALK2, leading to progressive heterotopic ossification (HO) through endochondral bone formation. Inflammatory flare-ups often precede new ossification events, but the cellular and molecular mechanisms linking immune responses to progenitor cell fate remain incompletely understood. Here, using a tamoxifen-inducible Acvr1R206H mouse model of FOP and a reproducible muscle injury protocol, we combined single-cell RNA sequencing with in vitro assays to dissect early events during lesion formation. We identified an expansion of macrophages (MPs) and fibro-adipogenic progenitors (FAPs) in FOP mice, with both populations exhibiting inflammatory and osteochondrogenic transcriptional signatures. Cell–cell interaction analysis revealed a self-reinforcing network of cytokine signaling among MPs and a prominent MP–FAP communication axis centred on SPP1. Functional studies confirmed that SPP1 enhanced FAP osteogenic differentiation and that its inhibition partially reversed this phenotype in vitro and attenuated HO in vivo. Our findings highlight the critical role of inflammatory MPs in shaping the fate of resident stromal mesenchymal progenitors (e.g. FAP) and suggest that early immune–stromal interactions set the stage for HO. Targeting this immune–mesenchymal crosstalk may represent a potential complementary strategy for preventing or mitigating disease progression in FOP.

Adult skeletal muscle stem cells (MuSCs) are indispensable for muscle regeneration and tightly regulated by macrophages (MPs) and fibro-adipogenic progenitors (FAPs) in their niche. Deregulated MuSC/MP/FAP interactions and the ensuing inflammation and fibrosis are hallmarks of dystrophic muscle. Here we demonstrate intrinsic deletion of transcription factor Yin Yang 1 (YY1) in MuSCs exacerbates dystrophic pathologies by altering composition and heterogeneity of MPs and FAPs. Further analysis reveals YY1 loss induces expression of immune genes in MuSCs, including C-C motif chemokine ligand 5 (Ccl5). Augmented CCL5 secretion promotes MP recruitment via CCL5/C-C chemokine receptor 5 (CCR5) crosstalk, which subsequently hinders FAP clearance through elevated Transforming growth factor-β1 (TGFβ1). Maraviroc-mediated pharmacological blockade of the CCL5/CCR5 axis effectively mitigates muscle dystrophy and improves muscle performance. Lastly, we demonstrate YY1 represses Ccl5 transcription by binding to its enhancer thus facilitating promoter-enhancer looping. Altogether, our study demonstrates the critical role of MuSCs in actively shaping their niche and provides novel insight into the therapeutic intervention of muscle dystrophy. Li et al. demonstrate the critical role of skeletal muscle stem cells (MuSCs) in actively shaping their niche in dystrophic muscle through YY1-CCL5-CCR5 axis mediated cellular interaction with macrophages.

Skeletal muscle is a plastic tissue that adapts to increased mechanical loading/contractile activity through fusion of muscle stem cells (MuSCs) with myofibers, a physiological process referred to as myonuclear accretion. However, it is still unclear whether myonuclear accretion is driven by increased mechanical loading per se, or occurs, at least in part, in response to muscle injury/regeneration. Here, we developed a non-damaging protocol to evaluate contractile activity-induced myonuclear accretion/hypertrophy in physiological conditions. Contractile activity was generated by applying repeated electrical stimuli over the mouse plantar flexor muscles. This method is commonly referred to as NeuroMuscular Electrical Simulation (NMES) in Human. Each NMES training session consisted of 80 isometric contractions delivered at ∼15 NMES led to a robust proliferation of MuSCs and myonuclear accretion in the absence of overt signs of muscle damage/regeneration. NMES-induced myonuclear accretion was specific to type IIB myofibers and was an early event preceding muscle hypertrophy inasmuch as a mild increase in myofiber cross-sectional area was only observed in response to the long-term NMES training protocol. We conclude that NMES-induced myonuclear accretion and muscle hypertrophy are driven by a mild increase in mechanical loading in the absence of overt signs of muscle injury.

Muscle regeneration is impaired with aging, due to both intrinsic defects of muscle stem cells (MuSCs) and alterations of their niche. Here, we monitor the cells constituting the MuSC niche over time in young and old regenerating mouse muscle. Aging alters the expansion of all niche cells, with prominent phenotypes in macrophages that show impaired resolution of inflammation. RNA sequencing of FACS-isolated mononucleated cells uncovers specific profiles and kinetics of genes and molecular pathways in old versus young muscle cells, indicating that each cell type responds to aging in a specific manner. Moreover, we show that macrophages have an altered expression of Selenoprotein P (Sepp1). Macrophage-specific deletion of Sepp1 is sufficient to impair the acquisition of their restorative profile and causes inefficient skeletal muscle regeneration. When transplanted in aged mice, bone marrow from young WT mice, but not Sepp1-KOs, restores muscle regeneration. This work provides a unique resource to study MuSC niche aging, reveals that niche cell aging is asynchronous and establishes the antioxidant Selenoprotein P as a driver of age-related decline of muscle regeneration.

Muscular dystrophies, such as Duchenne muscular dystrophy (DMD), are caused by permanent muscle injuries leading to chronic inflammation, with macrophages harboring an altered inflammatory profile contributing to fibrosis through the secretion of transforming growth factor β1 (TGF-β1). We previously showed that AMP-activated protein kinase (AMPK) activation reduces TGF-β1 secretion by macrophages and improves muscle homeostasis and muscle force in a DMD mouse model. However, direct AMPK activators like compound 991 show strong adverse effects in vivo. To overcome this toxicity, we encapsulated 991 into biodegradable polymeric poly(lactic-co-glycolic) acid (PLGA) nanoparticles for in vivo delivery. We show that 991-loaded PLGA nanoparticles retained drug activity on fibrotic macrophages in vitro and in vivo. In the D2-mdx DMD mouse model, intravenously injected PLGA nanoparticles reached macrophages in gastrocnemius and diaphragm muscles, two severely affected muscles in this model, but not in heart and quadriceps. Chronic intravenous injections of 991-loaded PLGA nanoparticles decreased inflammation in both gastrocnemius and diaphragm, which was associated with TGF-β1 level and fibrosis reduction and increase in myofiber size and muscle mass in the gastrocnemius, without toxicity. These results demonstrate that nanomedicine is an efficient strategy to deliver AMPK activators in vivo to target inflammation and improve the dystrophic muscle phenotype in the gastrocnemius.

Cardiotoxin injection is a commonly used method to induce muscle damage for studying skeletal muscle regeneration. Toxic injuries cause extensive myofiber damage and necrosis, leading to widespread muscle injury. This model results in an important macrophage infiltration, the release of chemical mediators and the activation of nociceptors, which may result in muscle pain. Managing pain in animal research is critical from both an ethical and scientific perspectives. The goal of this study was to evaluate whether two widely used analgesics, paracetamol and buprenorphine, affect mouse locomotor activity, mechanical sensitivity and muscle regeneration following cardiotoxin injection. Pain was assessed through the Von Frey test and voluntary wheel-running activity recordings. In parallel, we quantified histological and cellular markers of muscle regeneration and assessed the inflammatory response using flow cytometry. Although buprenorphine alleviated stimulus-evoked pain behaviors, none of the analgesics minimized cardiotoxin-induced reduction in wheel-running activity nor affected muscle regeneration and the inflammatory response following injury. Both paracetamol and buprenorphine may even negatively impact spontaneous activity. These findings demonstrate that cardiotoxin-induced muscle injury markedly impairs spontaneous locomotor activity, an effect that may be further amplified by the administration of analgesics. Our findings raise questions about the effectiveness of these pharmacological approaches in pain management after muscle injury, particularly when administrated within the first 24 h post-injury.

BACKGROUND AND AIMS:Up to 25 % of ischemic strokes are the consequence of a ruptured vulnerable carotid atherosclerotic plaque. Circulating pro-inflammatory classical and intermediate monocytes are known predictors of ischemic events and cardiovascular death. Interestingly, chronic physical activity can decrease inflammation, and is thereby associated with a reduction in pro-inflammatory monocytes. METHODS:In a randomized, controlled and monocentric trial, we recruited 56 patients (71 ± 8 years old) with asymptomatic carotid stenosis of ≥50 % who were ineligible for carotid endarterectomy. Blood analyses were completed to assess the phenotype of monocytes, as well as a multiplex assay and colorimetric assay for cytokines concentration and redox status, respectively. RESULTS:Classical monocyte count was increased in the control arm, while it remained stable in response to the physical activity intervention (-5215 ± 2307, [95 %CI -9878 to -551], p = 0.03). Moreover, the expression of most of pro-inflammatory cytokines was down-regulated in response to the physical activity intervention, while it was increased in the control arm. CONCLUSIONS:The stabilization of classical monocytes and down-regulation of the expression of pro-inflammatory cytokines in response to the physical activity intervention suggest that chronic physical activity alleviate the pro-inflammatory state, and might thereby reduce carotid plaque vulnerability and the subsequent risk of ischemic events. These results highlight that home-based physical activity intervention is an efficient clinical care that induce biological changes in carotid atherosclerotic patients.

Introduction Les myopathies inflammatoires idiopathiques sont un groupe de maladies musculaires rares dont le diagnostic repose sur l’évaluation clinique et les biomarqueurs sérologiques et histologiques. Alors que l’immunomarquage HLA-I (MHC-I) est utilisé en routine comme marqueur diagnostic de myosite, le rôle de l’immunomarquage HLA-DR (MHC-II) reste à préciser. Résultats Dans cette étude, nous avons examiné les profils d’expression d’HLA-DR par les myofibres et les capillaires, sur les biopsies musculaires de 103 patients, appartenant à cinq sous-groupes de myopathies inflammatoires idiopathiques : dermatomyosite (DM, n=31), myosite à inclusions (IBM, n=24), syndrome des anti-synthétase (ASyS, n=10), myopathie nécrosante auto-immune (MNAI, n=18) et myosite de chevauchement (OM, n=20). Une expression anormale de l’HLA-DR par les myofibres a été observée chez 63 des 103 patients (61 %), avec des profils distincts dans les différents sous-groupes : diffuse dans les IBM (96 %), négative dans les MNAI (83 %), périfasciculaire dans les ASyS (70 %), à la fois négative (61 %) et périfasciculaire (32 %) dans les DM, et groupée (40 %), périfasciculaire (30 %) ou hétérogène diffuse (15 %) dans les OM. En outre, le marquage capillaire HLA-DR a révélé des anomalies capillaires quantitatives, comme une perte en capillaires (47 sur 88, 53 %), et des anomalies qualitatives comme les capillaires dilatés et les marquages capillaires « baveux » (79 sur 98, 81 %). Conclusion Ces résultats indiquent que le marquage HLA-DR représente un outil complémentaire intéressant pour le diagnostic des sous-groupes de myopathies inflammatoires idiopathiques. Nos résultats suggèrent que l’ajout de l’immunomarquage HLA-DR dans les évaluations histologiques de routine pourrait faciliter le diagnostic entre les différents sous-types de myopathies inflammatoires idiopathiques.

Muscle regeneration is impaired in the aged organism, due to both intrinsic defects of muscle stem cells (MuSCs) and alterations of their environmental niche. However, the latter has still been poorly explored. Here, we compared and analyzed the time course of the various cell types constituting the MuSC niche during muscle generation in young and old mice. Aging altered the amplification of all niche cells with particularly prominent phenotypes in macrophages that impaired the resolution of inflammation in the old regenerating muscle. RNAsequencing of FACs-isolated MuSCs and non-myogenic niche cells during regeneration uncovered specific profiles and kinetics of genes and molecular pathways differentially regulated in old versus young regenerating muscle, indicating that each cell type responded to aging in a specific manner. Through this, we discovered that macrophages have a strong signature of aging with altered the activation of Selenoprotein P (Sepp1) expression in macrophages during the resolution of inflammation in regenerating muscle. Macrophage-specific deletion of Sepp1 gene was sufficient to impair the acquisition of the repair inflammatory profile, perturbed the support of macrophages to MuSCs in vitro and in vivo, and to cause inefficient skeletal muscle regeneration. When transplanted in aged mice, bone marrow from young WT mice, but not Sepp1 KOs, restored muscle regeneration to youthful levels. Altogether this work provides a unique resource to study the aging of the MuSC niche, reveals that aging of niche cells is asynchronous and establishes impaired macrophage dynamics/polarization and the anti-oxidant Selenoprotein P expression as drivers of age-related decline of muscle regeneration.