TGF-β-driven muscle degeneration and failed regeneration underlie disease onset in a DMD mouse model

Multiomic Analysis of Calf Muscle in Peripheral Artery Disease and Chronic Kidney Disease

BACKGROUND

Chronic kidney disease (CKD) has emerged as a significant risk factor that accelerates atherosclerosis, decreases muscle function, and increases the risk of amputation or death in patients with peripheral artery disease (PAD). However, the modulators underlying this exacerbated pathobiology are ill-defined. Recent work has demonstrated that uremic toxins are associated with limb amputation in PAD and have pathological effects in both the limb muscle and vasculature. Herein, we use multiomics to identify novel modulators of disease pathobiology in patients with PAD and CKD.

METHODS

A cross-sectional study enrolled 4 groups of participants: controls without PAD or CKD (n=28), patients with PAD only (n=46), patients with CKD only (n=31), and patients with both PAD and CKD (n=18). Both targeted (uremic toxins) and nontargeted metabolomics in plasma were performed using mass spectrometry. Calf muscle biopsies were used to measure histopathology, perform bulk and single-nucleus RNA sequencing, and assess mitochondrial function. Differential gene and metabolite analyses, as well as pathway and gene set enrichment analyses, were performed.

RESULTS

Patients with both PAD and CKD exhibited significantly lower calf muscle strength and smaller muscle fiber areas compared with controls and those with only PAD. Compared with controls, mitochondrial function was impaired in patients with CKD, with or without PAD, but not in PAD patients without CKD. Plasma metabolomics revealed substantial alterations in the metabolome of patients with CKD, with significant correlations observed between uremic toxins (eg, kynurenine and indoxyl sulfate) and both muscle strength and mitochondrial function. RNA sequencing analyses identified downregulation of mitochondrial genes and pathways associated with protein translation in patients with both PAD and CKD. Single-nucleus RNA sequencing further highlighted a mitochondrial deficiency in muscle fibers along with unique remodeling of fibro-adipogenic progenitor cells in patients with both PAD and CKD, with an increase in adipogenic cell populations.

CONCLUSIONS

CKD significantly exacerbates ischemic muscle pathology in PAD, as evidenced by diminished muscle strength, reduced mitochondrial function, and altered transcriptome profiles. The correlation between uremic toxins and muscle dysfunction suggests that targeting these metabolites may offer therapeutic potential for improving muscle health in PAD patients with CKD.

Integrating spatial transcriptomics and single-nucleus RNA-seq revealed the specific inhibitory effects of TGF-β on intramuscular fat deposition

Intramuscular fat (IMF) is a complex adipose tissue within skeletal muscle, appearing specially tissue heterogeneous, and the factors influencing its formation remain unclear. In conditions such as diabetes, aging, and muscle wasting, IMF was deposited in abnormal locations in skeletal muscle, damaged the normal physiological functions of skeletal muscle. Here, we used Longissimus dorsi muscles from pigs with different IMF contents as samples and adopted a method combining spatial transcriptome (ST) and single-nucleus RNA-seq to identify the spatial heterogeneity of IMF. ST revealed that genes involved in TGF-β signaling pathways were specifically highly enriched in IMF. In lean pigs, IMF autocrine produces more TGF-β2, while in obese pigs, IMF received more endothelial-derived TGF-β1. In vitro experiments have proven that porcine endothelial cells in a simulated high-fat environment released more TGF-β1 than TGF-β2. Moreover, under obesity mice, the addition of TGF-β after muscle injury abolished IMF production and slowed muscle repair, whereas TGF-β inhibition accelerated muscle repair. Our findings demonstrate that the TGF-β pathway specifically regulates these processes, suggesting it as a potential therapeutic target for managing muscle atrophy in obese patients and enhancing muscle repair while reducing IMF deposition.

Skeletal muscle stem cells modulate niche function in Duchenne muscular dystrophy mouse through YY1-CCL5 axis

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.

Failure to resolve inflammation contributes to juvenile onset cardiac damage in a mouse model of Duchenne Muscular Dystrophy

Absence of dystrophin protein causes cardiac dysfunction in patients with Duchenne muscular dystrophy (DMD). Unlike boys with DMD, the common mouse model of DMD (B10-mdx) does not manifest cardiac deficits until late adulthood. This has limited our understanding of the mechanism and therapeutic approaches to target the pediatric onset of cardiac pathology in DMD. Here we show that the mdx mouse model on the DBA/2J genetic background (D2-mdx) displays juvenile-onset cardiac degeneration. Molecular and histological analysis revealed that cardiac damage in this model is linked to increased leukocyte chemotactic signaling and an inability to resolve inflammation. These deficiencies result in chronic inflammation and fibrotic conversion of the extracellular matrix (ECM) in the juvenile D2-mdx heart. To address these pathologies, we tested the utility of pro-resolution therapy to clear chronic cardiac inflammation. Use of an N-formyl peptide receptor (FPR) agonist helped physiologically resolve inflammation and mitigate the downstream events that lead to fibrotic degeneration of cardiomyocytes, preventing juvenile onset cardiac muscle loss. These results establish the utility of D2-mdx model to study events associated with pediatric-onset cardiac damage and demonstrates pro-resolution therapy as an alternate to anti-inflammatory therapy for treating cardiac degenerative pathology responsible for cardiomyopathy in DMD patients.

Aptamer-conjugated gold nanoparticles enable oligonucleotide delivery into muscle stem cells to promote regeneration of dystrophic muscles

Inefficient targeting of muscle stem cells (MuSCs), also called satellite cells, represents a major bottleneck of current therapeutic strategies for muscular dystrophies, as it precludes the possibility of promoting compensatory regeneration. Here we describe a muscle-targeting delivery platform, based on gold nanoparticles, that enables the release of therapeutic oligonucleotides into MuSCs. We demonstrate that AuNPs conjugation to an aptamer against α7/β1 integrin dimers directs either local or systemic delivery of microRNA-206 to MuSCs, thereby promoting muscle regeneration and improving muscle functionality, in a mouse model of Duchenne Muscular Dystrophy. We show here that this platform is biocompatible, non-toxic, and non-immunogenic, and it can be easily adapted for the release of a wide range of therapeutic oligonucleotides into diseased muscles.

FNDC1 is a myokine that promotes myogenesis and muscle regeneration

Myogenesis is essential for skeletal muscle formation and regeneration after injury, yet its regulators are largely unknown. Here we identified fibronectin type III domain containing 1 (FNDC1) as a previously uncharacterized myokine. In vitro studies showed that knockdown of Fndc1 in myoblasts reduces myotube formation, while overexpression of Fndc1 promotes myogenic differentiation. We further generated recombinant truncated mouse FNDC1 (mFNDC1), which retains reliable activity in promoting myoblast differentiation in vitro. Gain- and loss-of-function studies collectively showed that FNDC1 promotes cardiotoxin (CTX)-induced muscle regeneration in adult mice. Furthermore, recombinant FNDC1 treatment ameliorated pathological muscle phenotypes in the mdx mouse model of Duchenne muscular dystrophy. Mechanistically, FNDC1 bound to the integrin α5β1 and activated the downstream FAK/PI3K/AKT/mTOR pathway to promote myogenic differentiation. Pharmacological inhibition of integrin α5β1 or of the downstream FAK/PI3K/AKT/mTOR pathway abolished the pro-myogenic effect of FNDC1. Collectively, these results suggested that myokine FNDC1 might be used as a therapeutic agent to regulate myogenic differentiation and muscle regeneration for the treatment of acute and chronic muscle disease.

Exploring the therapeutic potential of fibroadipogenic progenitors in muscle disease

Skeletal muscle relies on its inherent self-repair ability to withstand continuous mechanical damage. Myofiber-intrinsic processes facilitate the repair of damage to sarcolemma and sarcomeres, but it is the coordinated interaction between muscle-resident satellite and stromal cells that are crucial in the regeneration of muscles to replace the lost muscle fibers. Fibroadipogenic progenitors (FAPs), are muscle-resident mesenchymal cells that are notable for their role in creating the dynamic stromal niche required to support long-term muscle homeostasis and regeneration. While FAP-mediated extracellular matrix formation and the establishment of a homeostatic muscle niche are essential for maintaining muscle health, excessive accumulation of FAPs and their aberrant differentiation leads to the fibrofatty degeneration that is a hallmark of myopathies and muscular dystrophies. Recent advancements, including single-cell RNA sequencing and in vivo analysis of FAPs, are providing deeper insights into the functions and specialization of FAPs, shedding light on their roles in both health and disease. This review will explore the above insights, discussing how FAP dysregulation contributes to muscle diseases. It will offer a concise overview of potential therapeutic interventions targeting FAPs to restore disrupted interactions among FAPs and muscle-resident cells, ultimately addressing degenerative muscle loss in neuromuscular diseases.

Mechanisms of muscle cells alterations and regeneration decline during aging

Skeletal muscles are essential for locomotion and body metabolism regulation. As muscles age, they lose strength, elasticity, and metabolic capability, leading to ineffective motion and metabolic derangement. Both cellular and extracellular alterations significantly influence muscle aging. Satellite cells (SCs), the primary muscle stem cells responsible for muscle regeneration, become exhausted, resulting in diminished population and functionality during aging. This decline in SC function impairs intercellular interactions as well as extracellular matrix production, further hindering muscle regeneration. Other muscle-resident cells, such as fibro-adipogenic progenitors (FAPs), pericytes, and immune cells, also deteriorate with age, reducing local growth factor activities and responsiveness to stress or injury. Systemic signaling, including hormonal changes, contributes to muscle cellular catabolism and disrupts muscle homeostasis. Collectively, these cellular and environmental components interact, disrupting muscle homeostasis and regeneration in advancing age. Understanding these complex interactions offers insights into potential regenerative strategies to mitigate age-related muscle degeneration.

Reduced myotube diameter induced by combined inhibition of transforming growth factor-β type I receptors Acvr1b and Tgfbr1 is associated with enhanced β1-syntrophin expression

Simultaneous inhibition of transforming growth factor-β (TGF-β) type I receptors Acvr1b and Tgfbr1 signalling has been associated with excessive skeletal muscle hypertrophy in vivo. However, it remains unclear whether the increased muscle mass in vivo is a direct result of inhibition of intracellular TGF-β signalling or whether this is an indirect effect of an altered extracellular anabolic environment. Here, we tested whether individual or simultaneous knockdown of TGF-β type I receptors in C2C12 myotubes was sufficient to induce muscle hypertrophy. The expression levels of TGF-β type I receptors Acvr1b and Tgfbr1 in myotubes were knocked down individually or in combination in the absence or presence of TGF-β1 and myostatin. Knocking down either Acvr1b or Tgfbr1 did not significantly change cell phenotype. Unexpectedly, simultaneous knockdown of both receptors reduced C2C12 myotube diameter, mRNA expression levels of Hgf, Ccn2 and Mymx with or without TGF-β1 and myostatin administration. In spite of decreased phosphorylation of Smad2/3, phosphorylation of P70S6K was reduced. In addition, the gene expression level of β1-syntrophin (Sntb1), which encodes a protein associated with the dystrophin-glycoprotein complex, was increased. Parallel experiments where Sntb1 gene expression was reduced showed an increase in myotube diameter and fusion of C2C12 myoblasts. Together, these results indicate that the knockdown of both TGF-β type I receptors reduced myotube diameter. This atrophic effect was attributed to reduced protein synthesis signalling and an increased expression of β1-syntrophin. These results have implications for our fundamental understanding of how TGF-β signalling regulates skeletal muscle size.

Spiny mice are primed but fail to regenerate volumetric skeletal muscle loss injuries

BACKGROUND

In recent years, the African spiny mouse Acomys cahirinus has been shown to regenerate a remarkable array of severe internal and external injuries in the absence of a fibrotic response, including the ability to regenerate full-thickness skin excisions, ear punches, severe kidney injuries, and complete transection of the spinal cord. While skeletal muscle is highly regenerative in adult mammals, Acomys displays superior muscle regeneration properties compared with standard laboratory mice following several injuries, including serial cardiotoxin injections of skeletal muscle and volumetric muscle loss (VML) of the panniculus carnosus muscle following full-thickness excision injuries. VML is an extreme muscle injury defined as the irrecoverable ablation of muscle mass, most commonly resulting from combat injuries or surgical debridement. Barriers to the treatment of VML injury include early and prolonged inflammatory responses that promote fibrotic repair and the loss of structural and mechanical cues that promote muscle regeneration. While the regeneration of the panniculus carnosus in Acomys is impressive, its direct relevance to the study of VML in patients is less clear as this muscle has largely been lost in humans, and, while striated, is not a true skeletal muscle. We therefore sought to test the ability of Acomys to regenerate a skeletal muscle more commonly used in VML injury models.

METHODS

We performed two different VML injuries of the Acomys tibialis anterior muscle and compared the regenerative response to a standard laboratory mouse strain, Mus C57BL6/J.

RESULTS

Neither Acomys nor Mus recovered lost muscle mass or myofiber number within three months following VML injury, and Acomys also failed to recover force production better than Mus. In contrast, Acomys continued to express eMHC within the injured area even three months following injury, whereas Mus ceased expressing eMHC less than one-month post-injury, suggesting that Acomys muscle was primed, but failed, to regenerate.

CONCLUSIONS

While the panniculus carnosus muscle in Acomys regenerates following VML injury in the context of full-thickness skin excision, this regenerative ability does not translate to regenerative repair of a skeletal muscle.

Distribution of MRI-derived T2 values as a biomarker for in vivo rapid screening of phenotype severity in mdx mice

BACKGROUND

The pathology in Duchenne muscular dystrophy (DMD) is characterized by degenerating muscle fibers, inflammation, fibro-fatty infiltrate, and edema, and these pathological processes replace normal healthy muscle tissue. The mdx mouse model is one of the most commonly used preclinical models to study DMD. Mounting evidence has emerged illustrating that muscle disease progression varies considerably in mdx mice, with inter-animal differences as well as intra-muscular differences in pathology in individual mdx mice. This variation is important to consider when conducting assessments of drug efficacy and in longitudinal studies. We developed a magnetic resonance imaging (MRI) segmentation and analysis pipeline to rapidly and non-invasively measure the severity of muscle disease in mdx mice.

METHODS

Wildtype and mdx mice were imaged with MRI and T2 maps were obtained axially across the hindlimbs. A neural network was trained to rapidly and semi-automatically segment the muscle tissue, and the distribution of resulting T2 values was analyzed. Interdecile range and Pearson Skew were identified as biomarkers to quickly and accurately estimate muscle disease severity in mice.

RESULTS

The semiautomated segmentation tool reduced image processing time approximately tenfold. Measures of Pearson skew and interdecile range based on that segmentation were repeatable and reflected muscle disease severity in healthy wildtype and diseased mdx mice based on both qualitative observation of images and correlation with Evans blue dye uptake.

CONCLUSION

Use of this rapid, non-invasive, semi-automated MR image segmentation and analysis pipeline has the potential to transform preclinical studies, allowing for pre-screening of dystrophic mice prior to study enrollment to ensure more uniform muscle disease pathology across treatment groups, improving study outcomes.

Diapocynin treatment induces functional and structural improvements in an advanced disease state in the mdx5Cv mice

Duchenne muscular dystrophy (DMD) is the most common muscular disorder affecting children. It affects nearly 1 male birth over 5000. Oxidative stress is a pervasive feature in the pathogenesis of DMD. Recent work shows that the main generators of ROS are NADPH oxidases (NOX), suggesting that they are an early and promising target in DMD. In addition, skeletal muscles of mdx mice, a murine model of DMD, overexpress NOXes. We investigated the impact of diapocynin, a dimer of the NOX inhibitor apocynin, on the chronic disease phase of mdx5Cv mice. Treatment of these mice with diapocynin from 7 to 10 months of age resulted in decreased hypertrophy of several muscles, prevented force loss induced by tetanic and eccentric contractions, improved muscle and respiratory functions, decreased fibrosis of the diaphragm and positively regulated the expression of disease modifiers. These encouraging results ensure the potential role of diapocynin in future treatment strategies.

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

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

The role of TGF-β signaling in muscle atrophy, sarcopenia and cancer cachexia

Skeletal muscle, comprising a significant proportion (40 to 50 percent) of total body weight in humans, plays a critical role in maintaining normal physiological conditions. Muscle atrophy occurs when the rate of protein degradation exceeds protein synthesis. Sarcopenia refers to age-related muscle atrophy, while cachexia represents a more complex form of muscle wasting associated with various diseases such as cancer, heart failure, and AIDS. Recent research has highlighted the involvement of signaling pathways, including IGF1-Akt-mTOR, MuRF1-MAFbx, and FOXO, in regulating the delicate balance between muscle protein synthesis and breakdown. Myostatin, a member of the TGF-β superfamily, negatively regulates muscle growth and promotes muscle atrophy by activating Smad2 and Smad3. It also interacts with other signaling pathways in cachexia and sarcopenia. Inhibition of myostatin has emerged as a promising therapeutic approach for sarcopenia and cachexia. Additionally, other TGF-β family members, such as TGF-β1, activin A, and GDF11, have been implicated in the regulation of skeletal muscle mass. Furthermore, myostatin cooperates with these family members to impair muscle differentiation and contribute to muscle loss. This review provides an overview of the significance of myostatin and other TGF-β signaling pathway members in muscular dystrophy, sarcopenia, and cachexia. It also discusses potential novel therapeutic strategies targeting myostatin and TGF-β signaling for the treatment of muscle atrophy.

Fibro-adipogenic progenitors in physiological adipogenesis and intermuscular adipose tissue remodeling

Excessive accumulation of intermuscular adipose tissue (IMAT) is a common pathological feature in various metabolic and health conditions and can cause muscle atrophy, reduced function, inflammation, insulin resistance, cardiovascular issues, and unhealthy aging. Although IMAT results from fat accumulation in muscle, the mechanisms underlying its onset, development, cellular components, and functions remain unclear. IMAT levels are influenced by several factors, such as changes in the tissue environment, muscle type and origin, extent and duration of trauma, and persistent activation of fibro-adipogenic progenitors (FAPs). FAPs are a diverse and transcriptionally heterogeneous population of stromal cells essential for tissue maintenance, neuromuscular stability, and tissue regeneration. However, in cases of chronic inflammation and pathological conditions, FAPs expand and differentiate into adipocytes, resulting in the development of abnormal and ectopic IMAT. This review discusses the role of FAPs in adipogenesis and how they remodel IMAT. It highlights evidence supporting FAPs and FAP-derived adipocytes as constituents of IMAT, emphasizing their significance in adipose tissue maintenance and development, as well as their involvement in metabolic disorders, chronic pathologies and diseases. We also investigated the intricate molecular pathways and cell interactions governing FAP behavior, adipogenesis, and IMAT accumulation in chronic diseases and muscle deconditioning. Finally, we hypothesize that impaired cellular metabolic flexibility in dysfunctional muscles impacts FAPs, leading to IMAT. A deeper understanding of the biology of IMAT accumulation and the mechanisms regulating FAP behavior and fate are essential for the development of new therapeutic strategies for several debilitating conditions.

The Role of MicroRNA in the Pathogenesis of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is an X-linked progressive disorder associated with muscle wasting and degeneration. The disease is caused by mutations in the gene that encodes dystrophin, a protein that links the cytoskeleton with cell membrane proteins. The current treatment methods aim to relieve the symptoms of the disease or partially rescue muscle functionality. However, they are insufficient to suppress disease progression. In recent years, studies have uncovered an important role for non-coding RNAs (ncRNAs) in regulating the progression of numerous diseases. ncRNAs, such as micro-RNAs (miRNAs), bind to their target messenger RNAs (mRNAs) to suppress translation. Understanding the mechanisms involving dysregulated miRNAs can improve diagnosis and suggest novel treatment methods for patients with DMD. This review presents the available evidence on the role of altered expression of miRNAs in the pathogenesis of DMD. We discuss the involvement of these molecules in the processes associated with muscle physiology and DMD-associated cardiomyopathy.

Studying intramuscular fat deposition and muscle regeneration: insights from a comparative analysis of mouse strains, injury models, and sex differences

Intramuscular fat (IMAT) infiltration, pathological adipose tissue that accumulates between muscle fibers, is a shared hallmark in a diverse set of diseases including muscular dystrophies and diabetes, spinal cord and rotator cuff injuries, as well as sarcopenia. While the mouse has been an invaluable preclinical model to study skeletal muscle diseases, they are also resistant to IMAT formation. To better understand this pathological feature, an adequate pre-clinical model that recapitulates human disease is necessary. To address this gap, we conducted a comprehensive in-depth comparison between three widely used mouse strains: C57BL/6J, 129S1/SvlmJ and CD1. We evaluated the impact of strain, sex and injury type on IMAT formation, myofiber regeneration and fibrosis. We confirm and extend previous findings that a Glycerol (GLY) injury causes significantly more IMAT and fibrosis compared to Cardiotoxin (CTX). Additionally, females form more IMAT than males after a GLY injury, independent of strain. Of all strains, C57BL/6J mice, both females and males, are the most resistant to IMAT formation. In regard to injury-induced fibrosis, we found that the 129S strain formed the least amount of scar tissue. Surprisingly, C57BL/6J of both sexes demonstrated complete myofiber regeneration, while both CD1 and 129S1/SvlmJ strains still displayed smaller myofibers 21 days post injury. In addition, our data indicate that myofiber regeneration is negatively correlated with IMAT and fibrosis. Combined, our results demonstrate that careful consideration and exploration are needed to determine which injury type, mouse model/strain and sex to utilize as preclinical model especially for modeling IMAT formation.

Identification of hub genes and therapeutic siRNAs to develop novel adjunctive therapy for Duchenne muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a devastating X-linked neuromuscular disorder caused by various defects in the dystrophin gene and still no universal therapy. This study aims to identify the hub genes unrelated to excessive immune response but responsible for DMD progression and explore therapeutic siRNAs, thereby providing a novel treatment.

METHODS

Top ten hub genes for DMD were identified from GSE38417 dataset by using GEO2R and PPI networks based on Cytoscape analysis. The hub genes unrelated to excessive immune response were identified by GeneCards, and their expression was further verified in mdx and C57 mice at 2 and 4 months (M) by (RT-q) PCR and western blotting. Therapeutic siRNAs were deemed as those that could normalize the expression of the validated hub genes in transfected C2C12 cells.

RESULTS

855 up-regulated and 324 down-regulated DEGs were screened from GSE38417 dataset. Five of the top 10 hub genes were considered as the candidate genes unrelated to excessive immune response, and three of these candidates were consistently and significantly up-regulated in mdx mice at 2 M and 4 M when compared with age-matched C57 mice, including Col1a2, Fbn1 and Fn1. Furthermore, the three validated up-regulated candidate genes can be significantly down-regulated by three rational designed siRNA (p < 0.0001), respectively.

CONCLUSION

COL1A2, FBN1 and FN1 may be novel biomarkers for DMD, and the siRNAs designed in our study were help to develop adjunctive therapy for Duchenne muscular dystrophy.

The extracellular matrix differentially directs myoblast motility and differentiation in distinct forms of muscular dystrophy: Dystrophic matrices alter myoblast motility

Extracellular matrix (ECM) pathologic remodeling underlies many disorders, including muscular dystrophy. Tissue decellularization removes cellular components while leaving behind ECM components. We generated "on-slide" decellularized tissue slices from genetically distinct dystrophic mouse models. The ECM of dystrophin- and sarcoglycan-deficient muscles had marked thrombospondin 4 deposition, while dysferlin-deficient muscle had excess decorin. Annexins A2 and A6 were present on all dystrophic decellularized ECMs, but annexin matrix deposition was excessive in dysferlin-deficient muscular dystrophy. Muscle-directed viral expression of annexin A6 resulted in annexin A6 in the ECM. C2C12 myoblasts seeded onto decellularized matrices displayed differential myoblast mobility and fusion. Dystrophin-deficient decellularized matrices inhibited myoblast mobility, while dysferlin-deficient decellularized matrices enhanced myoblast movement and differentiation. Myoblasts treated with recombinant annexin A6 increased mobility and fusion like that seen on dysferlin-deficient decellularized matrix and demonstrated upregulation of ECM and muscle cell differentiation genes. These findings demonstrate specific fibrotic signatures elicit effects on myoblast activity.

Late-Stage Skeletal Muscle Transcriptome in Duchenne muscular dystrophy shows a BMP4-Induced Molecular Signature

Duchenne muscular dystrophy (DMD) is a fatal X-linked recessive disease due to loss-of-function mutations in the DYSTROPHIN gene. DMD-related skeletal muscle wasting is typified by an aberrant immune response involving upregulation of TGFβ family of cytokines. We previously demonstrated that bone morphogenetic protein 4 (BMP4) is increased in DMD and BMP4 stimulation induces a 20-fold upregulation of Smad8 transcription. However, the role of BMP4 in severely affected DMD skeletal muscle is unknown. We hypothesized that transcriptomic signatures in severely affected human DMD skeletal muscle are driven by BMP4 signaling. Transcriptomes from skeletal muscle biopsies of late-stage DMD vs. non-DMD controls and C2C12 muscle cells with or without BMP4 stimulation were generated by RNA-Seq and analyzed for single transcript differential expression as well as by Ingenuity Pathway Analysis and weighted gene co-expression network analyses. A total of 2,328 and 5,291 transcripts in the human muscle and C2C12 muscle cells, respectively, were differentially expressed. We identified an overlapping molecular signature of 1,027 genes dysregulated in DMD muscle that were induced in BMP4-stimulated C2C12 muscle cells. Highly upregulated DMD transcripts that overlapped with BMP4-stimulated C2C12 muscle cells included ADAMTS3, HCAR2, SERPING1, SMAD8 , and UNC13C. The DMD transcriptome was characterized by dysregulation of pathways involving immune function, extracellular matrix remodeling, and metabolic/mitochondrial function. In summary, we define a late-stage DMD skeletal muscle transcriptome that substantially overlaps with the BMP4-induced molecular signature in C2C12 muscle cells. This supports BMP4 as a disease-driving regulator of transcriptomic changes in late-stage DMD skeletal muscle and expands our understanding of the evolution of dystrophic signaling pathways and their associated gene networks that could be explored for therapeutic development.

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

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

The BALB/c.mdx62 mouse exhibits a dystrophic muscle pathology and is a model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a devastating monogenic skeletal muscle-wasting disorder. Although many pharmacological and genetic interventions have been reported in preclinical studies, few have progressed to clinical trials with meaningful benefit. Identifying therapeutic potential can be limited by availability of suitable preclinical mouse models. More rigorous testing across models with varied background strains and mutations can identify treatments for clinical success. Here, we report the generation of a DMD mouse model with a CRISPR-induced deletion within exon 62 of the dystrophin gene (Dmd) and the first generated in BALB/c mice. Analysis of mice at 3, 6 and 12 months of age confirmed loss of expression of the dystrophin protein isoform Dp427 and resultant dystrophic pathology in limb muscles and the diaphragm, with evidence of centrally nucleated fibers, increased inflammatory markers and fibrosis, progressive decline in muscle function, and compromised trabecular bone development. The BALB/c.mdx62 mouse is a novel model of DMD with associated variations in the immune response and muscle phenotype, compared with those of existing models. It represents an important addition to the preclinical model toolbox for developing therapeutic strategies.

Apelin stimulation of the vascular skeletal muscle stem cell niche enhances endogenous repair in dystrophic mice

Impaired skeletal muscle stem cell (MuSC) function has long been suspected to contribute to the pathogenesis of muscular dystrophy (MD). Here, we showed that defects in the endothelial cell (EC) compartment of the vascular stem cell niche in mouse models of Duchenne MD, laminin α2-related MD, and collagen VI-related myopathy were associated with inefficient mobilization of MuSCs after tissue damage. Using chemoinformatic analysis, we identified the 13-amino acid form of the peptide hormone apelin (AP-13) as a candidate for systemic stimulation of skeletal muscle ECs. Systemic administration of AP-13 using osmotic pumps generated a pro-proliferative EC-rich niche that supported MuSC function through angiocrine factors and markedly improved tissue regeneration and muscle strength in all three dystrophic mouse models. Moreover, EC-specific knockout of the apelin receptor led to regenerative defects that phenocopied key pathological features of MD, including vascular defects, fibrosis, muscle fiber necrosis, impaired MuSC function, and reduced force generation. Together, these studies provide in vivo proof of concept that enhancing endogenous skeletal muscle repair by targeting the vascular niche is a viable therapeutic avenue for MD and characterized AP-13 as a candidate for further study for the systemic treatment of MuSC dysfunction.

Skeletal muscle niche, at the crossroad of cell/cell communications

Skeletal muscle is composed of a variety of tissue and non-tissue resident cells that participate in homeostasis. In particular, the muscle stem cell niche is a dynamic system, requiring direct and indirect communications between cells, involving local and remote cues. Interactions within the niche must happen in a timely manner for the maintenance or recovery of the homeostatic niche. For instance, after an injury, pro-myogenic cues delivered too early will impact on muscle stem cell proliferation, delaying the repair process. Within the niche, myofibers, endothelial cells, perivascular cells (pericytes, smooth muscle cells), fibro-adipogenic progenitors, fibroblasts, and immune cells are in close proximity with each other. Each cell behavior, membrane profile, and secretome can interfere with muscle stem cell fate and skeletal muscle regeneration. On top of that, the muscle stem cell niche can also be modified by extra-muscle (remote) cues, as other tissues may act on muscle regeneration via the production of circulating factors or the delivery of cells. In this review, we highlight recent publications evidencing both local and remote effectors of the muscle stem cell niche.

A novel biomarker of fibrofatty replacement in dystrophinopathies identified by integrating transcriptome, magnetic resonance imaging, and pathology data

BACKGROUND

We aimed to analyse genome-wide transcriptome differences between Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) patients and identify biomarkers that correlate well with muscle magnetic resonance imaging (MRI) and histological fibrofatty replacement in both patients, which have not been reported.

METHODS

One hundred and one male patients with dystrophinopathies (55 DMD and 46 BMD) were enrolled. Muscle-derived genome-wide RNA-sequencing was performed in 31 DMD patients, 29 BMD patients, and 11 normal controls. Fibrofatty replacement was scored on muscle MRI and histological levels in all patients. A unique pipeline, single-sample gene set enrichment analysis combined with Spearman's rank correlations (ssGSEA-Cor) was developed to identify the most correlated gene signature for fibrofatty replacement. Quantitative real-time PCR (qRT-PCR) analysis, western blot analysis, and single-nucleus RNA-sequencing (snRNA-seq) were performed in the remaining patients to validate the most correlated gene signature.

RESULTS

Comparative transcriptomic analysis revealed that 31 DMD muscles were characterized by a significant increase of inflammation/immune response and extracellular matrix remodelling compared with 29 BMD muscles (P < 0.05). The ssGSEA-Cor pipeline revealed that the gene set of CDKN2A and CDKN2B was the most correlated gene signature for fibrofatty replacement (histological rs  = 0.744, P < 0.001; MRI rs  = 0.718, P < 0.001). Muscle qRT-PCR confirmed that CDKN2A mRNA expression in both 15 DMD (median = 25.007, P < 0.001) and 12 BMD (median = 5.654, P < 0.001) patients were significantly higher than that in controls (median = 1.101), while no significant difference in CDKN2B mRNA expression was found among DMD, BMD, and control groups. In the 27 patients, muscle CDKN2A mRNA expression respectively correlated with muscle MRI (rs  = 0.883, P < 0.001) and histological fibrofatty replacement (rs  = 0.804, P < 0.001) and disease duration (rs  = 0.645, P < 0.001) and North Star Ambulatory Assessment total scores (rs  = -0.698, P < 0.001). Muscle western blot analysis confirmed that both four DMD (median = 2.958, P < 0.05) and four BMD (median = 1.959, P < 0.01) patients had a significantly higher level of CDKN2A protein expression than controls (median = 1.068). The snRNA-seq analysis of two DMD muscles revealed that CDKN2A was mainly expressed in fibro-adipogenic progenitors, satellite cells, and myoblasts.

CONCLUSIONS

We identify CDKN2A expression as a novel biomarker of fibrofatty replacement, which might be a new target for antifibrotic therapy in dystrophinopathies.

MEK-inhibitors decrease Nfix in muscular dystrophy but induce unexpected calcifications, partially rescued with Cyanidin diet

Muscular dystrophies (MDs) are incurable genetic myopathies characterized by progressive degeneration of skeletal muscles. Dystrophic mice lacking the transcription factor Nfix display morphological and functional improvements of the disease. Recently, we demonstrated that MAPK signaling pathway positively regulates Nfix in muscle development and that Cyanidin, a natural antioxidant molecule, strongly ameliorates the pathology. To explore a synergistic approach aimed at treating MDs, we administered Trametinib, a clinically approved MEK inhibitor, alone or combined with Cyanidin to adult Sgca null mice. We observed that chronic treatment with Trametinib and Cyanidin reduced Nfix in myogenic cells but, unexpectedly, caused ectopic calcifications exclusively in dystrophic muscles. The combined treatment with Cyanidin resulted in histological improvements by preventing Trametinib-induced calcifications in Diaphragm and Soleus. Collectively, this first pilot study revealed that Nfix is modulated by the MAPK pathway in MDs, and that Cyanidin partly rescued the unexpected ectopic calcifications caused by MEK inhibition.

The super-healing MRL strain promotes muscle growth in muscular dystrophy through a regenerative extracellular matrix

The Murphy Roths Large (MRL) mouse strain has "super-healing" properties that enhance recovery from injury. In mice, the DBA/2J strain intensifies many aspects of muscular dystrophy, so we evaluated the ability of the MRL strain to suppress muscular dystrophy in the Sgcg-null mouse model of limb girdle muscular dystrophy. A comparative analysis of Sgcg-null mice in the DBA/2J versus MRL strains showed greater myofiber regeneration, with reduced structural degradation of muscle in the MRL strain. Transcriptomic profiling of dystrophic muscle indicated strain-dependent expression of extracellular matrix (ECM) and TGF-β signaling genes. To investigate the MRL ECM, cellular components were removed from dystrophic muscle sections to generate decellularized myoscaffolds. Decellularized myoscaffolds from dystrophic mice in the protective MRL strain had significantly less deposition of collagen and matrix-bound TGF-β1 and TGF-β3 throughout the matrix. Dystrophic myoscaffolds from the MRL background, but not the DBA/2J background, were enriched in myokines like IGF-1 and IL-6. C2C12 myoblasts seeded onto decellularized matrices from Sgcg-/- MRL and Sgcg-/- DBA/2J muscles showed the MRL background induced greater myoblast differentiation compared with dystrophic DBA/2J myoscaffolds. Thus, the MRL background imparts its effect through a highly regenerative ECM, which is active even in muscular dystrophy.

A discrete 'early-responder' stromal-cell subtype orchestrates immunocyte recruitment to injured tissue

Following acute injury, stromal cells promote tissue regeneration by a diversity of mechanisms. Time-resolved single-cell RNA sequencing of muscle mesenchymal stromal cells (MmSCs) responding to acute injury identified an 'early-responder' subtype that spiked on day 1 and expressed a notable array of transcripts encoding immunomodulators. IL-1β, TNF-α and oncostatin M each strongly and rapidly induced MmSCs transcribing this immunomodulatory program. Macrophages amplified the program but were not strictly required for its induction. Transfer of the inflammatory MmSC subtype, tagged with a unique surface marker, into healthy hindlimb muscle induced inflammation primarily driven by neutrophils and macrophages. Among the abundant inflammatory transcripts produced by this subtype, Cxcl5 was stroma-specific and highly upregulated with injury. Depletion of this chemokine early after injury revealed a substantial impact on recruitment of neutrophils, a prolongation of inflammation to later times and an effect on tissue regeneration. Mesenchymal stromal cell subtypes expressing a comparable inflammatory program were found in a mouse model of muscular dystrophy and in several other tissues and pathologies in both mice and humans. These 'early-responder' mesenchymal stromal cells, already in place, permit rapid and coordinated mobilization and amplification of critical cell collaborators in response to injury.

Extracellular matrix contribution to disease progression and dysfunction in myopathy

Myopathic processes affect skeletal muscle and heart. In the muscular dystrophies, which are a subset of myopathies, muscle cells are gradually replaced by fibrosis and fat, impairing muscle function as well as regeneration and repair. In addition to skeletal muscle, these genetic disorders often also affect the heart, where fibrofatty infiltration progressively accumulates in the myocardium, impairing heart function. Although considerable effort has focused on gene-corrective and gene-replacement approaches to stabilize myofibers and cardiomyocytes, the continual and ongoing deposition of extracellular matrix itself contributes to tissue and organ dysfunction. Transcriptomic and proteomic profiling, along with high-resolution imaging and biophysical measurements, have been applied to define extracellular matrix components and their role in contributing to cardiac and skeletal muscle weakness. More recently, decellularization methods have been adapted to an on-slide format to preserve the spatial geography of the extracellular matrix, allowing new insight into matrix remodeling and its direct role in suppressing regeneration in muscle. This review highlights recent literature with focus on the extracellular matrix and molecular mechanisms that contribute to muscle and heart fibrotic disorders. We will also compare how the myopathic matrix differs from healthy matrix, emphasizing how the pathological matrix contributes to disease.

Cellular pathogenesis of Duchenne muscular dystrophy: progressive myofibre degeneration, chronic inflammation, reactive myofibrosis and satellite cell dysfunction

Duchenne muscular dystrophy is a highly progressive muscle wasting disease of early childhood and characterized by complex pathophysiological and histopathological changes in the voluntary contractile system, including myonecrosis, chronic inflammation, fat substitution and reactive myofibrosis. The continued loss of functional myofibres and replacement with non-contractile cells, as well as extensive tissue scarring and decline in tissue elasticity, leads to severe skeletal muscle weakness. In addition, dystrophic muscles exhibit a greatly diminished regenerative capacity to counteract the ongoing process of fibre degeneration. In normal muscle tissues, an abundant stem cell pool consisting of satellite cells that are localized between the sarcolemma and basal lamina, provides a rich source for the production of activated myogenic progenitor cells that are involved in efficient myofibre repair and tissue regeneration. Interestingly, the self-renewal of satellite cells for maintaining an essential pool of stem cells in matured skeletal muscles is increased in dystrophin-deficient fibres. However, satellite cell hyperplasia does not result in efficient recovery of dystrophic muscles due to impaired asymmetric cell divisions. The lack of expression of the full-length dystrophin isoform Dp427-M, which is due to primary defects in the DMD gene,  appears to affect key regulators of satellite cell polarity causing a reduced differentiation of myogenic progenitors, which are essential for myofibre regeneration. This review outlines the complexity of dystrophinopathy and describes the importance of the pathophysiological role of satellite cell dysfunction. A brief discussion of the bioanalytical usefulness of single cell proteomics for future studies of satellite cell biology is provided.

Thrown for a loop: fibro-adipogenic progenitors in skeletal muscle fibrosis

Fibro-adipogenic progenitors (FAPs) are key regulators of skeletal muscle regeneration and homeostasis. However, dysregulation of these cells leads to fibro-fatty infiltration across various muscle diseases. FAPs are the key source of extracellular matrix (ECM) deposition in muscle, and disruption to this process leads to a pathological accumulation of ECM, known as fibrosis. The replacement of contractile tissue with fibrotic ECM functionally impairs the muscle and increases muscle stiffness. FAPs and fibrotic muscle form a progressively degenerative feedback loop where, as a muscle becomes fibrotic, it induces a fibrotic FAP phenotype leading to further development of fibrosis. In this review, we summarize FAPs' role in fibrosis in terms of their activation, heterogeneity, contributions to fibrotic degeneration, and role across musculoskeletal diseases. We also discuss current research on potential therapeutic avenues to attenuate fibrosis by targeting FAPs.

Loss of compensation afforded by accessory muscles of breathing leads to respiratory system compromise in the mdx mouse model of Duchenne muscular dystrophy

Despite profound diaphragm weakness, peak inspiratory pressure-generating capacity is preserved in young mdx mice revealing adequate compensation by extra-diaphragmatic muscles of breathing in early dystrophic disease. We hypothesised that loss of compensation gives rise to respiratory system compromise in advanced dystrophic disease. Studies were performed in male wild-type (n = 196) and dystrophin-deficient mdx mice (n = 188) at 1, 4, 8, 12 and 16 months of age. In anaesthetised mice, inspiratory pressure and obligatory and accessory respiratory EMG activities were recorded during baseline and sustained tracheal occlusion for up to 30-40 s to evoke peak system activation to task failure. Obligatory inspiratory EMG activities were lower in mdx mice across the ventilatory range to peak activity, emerging in early dystrophic disease. Early compensation protecting peak inspiratory pressure-generating capacity in mdx mice, which appears to relate to transforming growth factor-β1-dependent fibrotic remodelling of the diaphragm and preserved accessory muscle function, was lost at 12 and 16 months of age. Denervation and surgical lesion of muscles of breathing in 4-month-old mice revealed a greater dependency on diaphragm for peak inspiratory performance in wild-type mice, whereas mdx mice were heavily dependent upon accessory muscles (including abdominal muscles) for peak performance. Accessory EMG activities were generally preserved or enhanced in young mdx mice, but peak EMG activities were lower than wild-type by 12 months of age. In general, ventilation was reasonably well protected in mdx mice until 16 months of age. Despite the early emergence of impairments in the principal obligatory muscles of breathing, peak inspiratory performance is compensated in early dystrophic disease due to diaphragm remodelling and facilitated contribution by accessory muscles of breathing. Loss of compensation afforded by accessory muscles underpins the emergence of respiratory system morbidity in advanced dystrophic disease. KEY POINTS: Despite diaphragm weakness, peak inspiratory performance is preserved in young dystrophin-deficient mdx mice revealing adequate compensation by extra-diaphragmatic muscles. Peak obligatory muscle (diaphragm, external intercostal, and parasternal intercostal) EMG activities are lower in mdx mice, emerging early in dystrophic disease, before the temporal decline in peak performance. Peak EMG activities of some accessory muscles are lower, whereas others are preserved. There is greater recruitment of the trapezius muscle in mdx mice during peak system activation. In phrenicotomised mice with confirmed diaphragm paralysis, there is a greater contribution made by extra-diaphragmatic muscles to peak inspiratory pressure in mdx compared with wild-type mice. Surgical lesion of accessory (including abdominal) muscles adversely affects peak pressure generation in mdx mice. Diaphragm remodelling leading to stiffening provides a mechanical advantage to peak pressure generation via the facilitated action of extra-diaphragmatic muscles in early dystrophic disease. Peak accessory EMG activities are lower in 12-month-old mdx compared to wild-type mice. Peak inspiratory pressure declines in mdx mice with advanced disease. We conclude that compensation afforded by accessory muscles of breathing declines in advanced dystrophic disease precipitating the emergence of respiratory system dysfunction.

Decoding the transcriptome of Duchenne muscular dystrophy to the single nuclei level reveals clinical-genetic correlations

Duchenne muscular dystrophy is a genetic disease produced by mutations in the dystrophin gene characterized by early onset muscle weakness leading to severe and irreversible disability. The cellular and molecular consequences of the lack of dystrophin in humans are only partially known, which is crucial for the development of new therapies aiming to slow or stop the progression of the disease. Here we have analyzed quadriceps muscle biopsies of seven DMD patients aged 2 to 4 years old and five age and gender matched controls using single nuclei RNA sequencing (snRNAseq) and correlated the results obtained with clinical data. SnRNAseq identified significant differences in the proportion of cell population present in the muscle samples, including an increase in the number of regenerative fibers, satellite cells, and fibro-adipogenic progenitor cells (FAPs) and a decrease in the number of slow fibers and smooth muscle cells. Muscle samples from the younger patients with stable mild weakness were characterized by an increase in regenerative fibers, while older patients with moderate and progressive weakness were characterized by loss of muscle fibers and an increase in FAPs. An analysis of the gene expression profile in muscle fibers identified a strong regenerative signature in DMD samples characterized by the upregulation of genes involved in myogenesis and muscle hypertrophy. In the case of FAPs, we observed upregulation of genes involved in the extracellular matrix regeneration but also several signaling pathways. Indeed, further analysis of the potential intercellular communication profile showed a dysregulation of the communication profile in DMD samples identifying FAPs as a key regulator of cell signaling in DMD muscle samples. In conclusion, our study has identified significant differences at the cellular and molecular levels in the different cell populations present in skeletal muscle samples of patients with DMD compared to controls.

Single-cell transcriptomic analysis of the identity and function of fibro/adipogenic progenitors in healthy and dystrophic muscle

Fibro/adipogenic progenitors (FAPs) are skeletal muscle stromal cells that support regeneration of injured myofibers and their maintenance in healthy muscles. FAPs are related to mesenchymal stem cells (MSCs/MeSCs) found in other adult tissues, but there is poor understanding of the extent of similarity between these cells. Using single-cell RNA sequencing (scRNA-seq) datasets from multiple mouse tissues, we have performed comparative transcriptomic analysis. This identified remarkable transcriptional similarity between FAPs and MeSCs, confirmed the suitability of PDGFRα as a reporter for FAPs, and identified extracellular proteolysis as a new FAP function. Using PDGFRα as a cell surface marker, we isolated FAPs from healthy and dysferlinopathic mouse muscles and performed scRNA-seq analysis. This revealed decreased FAP-mediated Wnt signaling as a potential driver of FAP dysfunction in dysferlinopathic muscles. Analysis of FAPs in dysferlin- and dystrophin-deficient muscles identified a relationship between the nature of muscle pathology and alteration in FAP gene expression.

Altered muscle niche contributes to myogenic deficit in the D2-mdx model of severe DMD

Lack of dystrophin expression is the underlying genetic basis for Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2-mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2-mdx muscles is associated with an enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports the excessive accumulation of fibroadipogenic progenitors (FAPs), leading to increased fibrosis. Unexpectedly, the extent of damage and degeneration in juvenile D2-mdx muscle is significantly reduced in adults, and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance regenerative myogenesis in the adult D2-mdx muscle, reaching levels comparable to the milder B10-mdx model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with juvenile D2-mdx FAPs reduces their fusion efficacy. Wild-type juvenile D2 mice also manifest regenerative myogenic deficit and glucocorticoid treatment improves their muscle regeneration. Our findings indicate that aberrant stromal cell responses contribute to poor regenerative myogenesis and greater muscle degeneration in juvenile D2-mdx muscles and reversal of this reduces pathology in adult D2-mdx muscle, identifying these responses as a potential therapeutic target for the treatment of DMD.

Myofiber directs macrophages IL-10-Vav1-Rac1 efferocytosis pathway in inflamed muscle following CTX myoinjury by activating the intrinsic TGF-β signaling

BACKGROUND

To explore the role of skeletal muscle specific TGF-β signaling on macrophages efferocytosis in inflamed muscle caused by Cardiotoxin (CTX) injection.

METHODS

CTX myoinjury was manipulated in TGF-βr2^flox/flox (control) mice or transgenic mice with TGF-β receptor 2 (TGF-βr2) being specifically deleted in skeletal muscle (SM TGF-βr2^-/-). Gene levels of TGF-β signal molecules, special inflammatory mediators in damaged muscle or in cultured and differentiated myogenic precursor cells (MPC-myotubes) were monitored by transcriptome microarray or qRT-PCR. TGF-β pathway molecules, myokines and embryonic myosin heavy chain in regenerating myofibers, the phenotype and efferocytosis of macrophages were evaluated by immunofluorescence, immunoblotting, Luminex, or FACS analysis. In vitro apoptotic cells were prepared by UV-irradiation.

RESULTS

In control mice, TGF-β-Smad2/3 signaling were significantly up-regulated in regenerating centronuclear myofibers after CTX-myoinjury. More severe muscle inflammation was caused by the deficiency of muscle TGF-β signaling, with the increased number of M1, but the decreased number of M2 macrophages. Notably, the deficiency of TGF-β signaling in myofibers dramatically affected on the ability of macrophages to conduct efferocytosis, marked by the decreased number of Annexin-V^-F4/80⁺Tunel⁺ macrophages in inflamed muscle, and the impaired uptake of macrophages to PKH67⁺ apoptotic cells transferred into damaged muscle. Further, our study suggested that, the intrinsic TGF-β signaling directed IL-10-Vav1-Rac1 efferocytosis signaling in muscle macrophages.

CONCLUSIONS

Our data demonstrate that muscle inflammation can be suppressed potentially by activating the intrinsic TGF-β signaling in myofibers to promote IL-10 dependent-macrophages efferocytosis. Video Abstract.

The super-healing MRL strain promotes muscle growth in muscular dystrophy through a regenerative extracellular matrix

Genetic background shifts the severity of muscular dystrophy. In mice, the DBA/2J strain confers a more severe muscular dystrophy phenotype, whereas the Murphy's Roth Large (MRL) strain has "super-healing" properties that reduce fibrosis. A comparative analysis of the Sgcg null model of Limb Girdle Muscular Dystrophy in the DBA/2J versus MRL strain showed the MRL background was associated with greater myofiber regeneration and reduced structural degradation of muscle. Transcriptomic profiling of dystrophic muscle in the DBA/2J and MRL strains indicated strain-dependent expression of the extracellular matrix (ECM) and TGF-β signaling genes. To investigate the MRL ECM, cellular components were removed from dystrophic muscle sections to generate decellularized "myoscaffolds". Decellularized myoscaffolds from dystrophic mice in the protective MRL strain had significantly less deposition of collagen and matrix-bound TGF-β1 and TGF-β3 throughout the matrix, and dystrophic myoscaffolds from the MRL background were enriched in myokines. C2C12 myoblasts were seeded onto decellularized matrices from Sgcg-/- MRL and Sgcg-/- DBA/2J matrices. Acellular myoscaffolds from the dystrophic MRL background induced myoblast differentiation and growth compared to dystrophic myoscaffolds from the DBA/2J matrices. These studies establish that the MRL background also generates its effect through a highly regenerative ECM, which is active even in muscular dystrophy.

Duchenne muscular dystrophy: disease mechanism and therapeutic strategies

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

New semi-automated tool for the quantitation of MR imaging to estimate in vivo muscle disease severity in mice

The pathology in Duchenne muscular dystrophy (DMD) is characterized by degenerating muscle fibers, inflammation, fibro-fatty infiltrate, and edema, and these pathological processes replace normal healthy muscle tissue. The mdx mouse model is one of the most commonly used preclinical models to study DMD. Mounting evidence has emerged illustrating that muscle disease progression varies considerably in mdx mice, with inter-animal differences as well as intra-muscular differences in pathology in individual mdx mice. This variation is important to consider when conducting assessments of drug efficacy and in longitudinal studies. Magnetic resonance imaging (MRI) is a non-invasive method that can be used qualitatively or quantitatively to measure muscle disease progression in the clinic and in preclinical models. Although MR imaging is highly sensitive, image acquisition and analysis can be time intensive. The purpose of this study was to develop a semi-automated muscle segmentation and quantitation pipeline that can quickly and accurately estimate muscle disease severity in mice. Herein, we show that the newly developed segmentation tool accurately divides muscle. We show that measures of skew and interdecile range based on segmentation sufficiently estimate muscle disease severity in healthy wildtype and diseased mdx mice. Moreover, the semi-automated pipeline reduced analysis time by nearly 10-fold. Use of this rapid, non-invasive, semi-automated MR imaging and analysis pipeline has the potential to transform preclinical studies, allowing for pre-screening of dystrophic mice prior to study enrollment to ensure more uniform muscle disease pathology across treatment groups, improving study outcomes.

Targeted regulation of TAK1 counteracts dystrophinopathy in a DMD mouse model

Muscular dystrophies make up a group of genetic neuromuscular disorders that involve severe muscle wasting. TGF-β-activated kinase 1 (TAK1) is an important signaling protein that regulates cell survival, growth, and inflammation. TAK1 has been recently found to promote myofiber growth in the skeletal muscle of adult mice. However, the role of TAK1 in muscle diseases remains poorly understood. In the present study, we have investigated how TAK1 affects the progression of dystrophic phenotype in the mdx mouse model of Duchenne muscular dystrophy (DMD). TAK1 is highly activated in the dystrophic muscle of mdx mice during the peak necrotic phase. While targeted inducible inactivation of TAK1 inhibits myofiber injury in young mdx mice, it results in reduced muscle mass and contractile function. TAK1 inactivation also causes loss of muscle mass in adult mdx mice. By contrast, forced activation of TAK1 through overexpression of TAK1 and TAB1 induces myofiber growth without having any deleterious effect on muscle histopathology. Collectively, our results suggest that TAK1 is a positive regulator of skeletal muscle mass and that targeted regulation of TAK1 can suppress myonecrosis and ameliorate disease progression in DMD.

Microdystrophin Gene Addition Significantly Improves Muscle Functionality and Diaphragm Muscle Histopathology in a Fibrotic Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a rare neuromuscular disease affecting 1:5000 newborn males. No cure is currently available, but gene addition therapy, based on the adeno-associated viral (AAV) vector-mediated delivery of microdystrophin transgenes, is currently being tested in clinical trials. The muscles of DMD boys present significant fibrotic and adipogenic tissue deposition at the time the treatment starts. The presence of fibrosis not only worsens the disease pathology, but also diminishes the efficacy of gene therapy treatments. To gain an understanding of the efficacy of AAV-based microdystrophin gene addition in a relevant, fibrotic animal model of DMD, we conducted a systemic study in juvenile D2.mdx mice using the single intravenous administration of an AAV8 system expressing a sequence-optimized murine microdystrophin, named MD1 (AAV8-MD1). We mainly focused our study on the diaphragm, a respiratory muscle that is crucial for DMD pathology and that has never been analyzed after treatment with AAV-microdystrophin in this mouse model. We provide strong evidence here that the delivery of AAV8-MD1 provides significant improvement in body-wide muscle function. This is associated with the protection of the hindlimb muscle from contraction-induced damage and the prevention of fibrosis deposition in the diaphragm muscle. Our work corroborates the observation that the administration of gene therapy in DMD is beneficial in preventing muscle fibrosis.

Altered muscle niche contributes to myogenic deficit in the D2- mdx model of severe DMD

Lack of dystrophin is the genetic basis for the Duchenne muscular dystrophy (DMD). However, disease severity varies between patients, based on specific genetic modifiers. D2- mdx is a model for severe DMD that exhibits exacerbated muscle degeneration and failure to regenerate even in the juvenile stage of the disease. We show that poor regeneration of juvenile D2- mdx muscles is associated with enhanced inflammatory response to muscle damage that fails to resolve efficiently and supports excessive accumulation of fibroadipogenic progenitors (FAPs). Unexpectedly, the extent of damage and degeneration of juvenile D2- mdx muscle is reduced in adults and is associated with the restoration of the inflammatory and FAP responses to muscle injury. These improvements enhance myogenesis in the adult D2- mdx muscle, reaching levels comparable to the milder (B10- mdx ) mouse model of DMD. Ex vivo co-culture of healthy satellite cells (SCs) with the juvenile D2- mdx FAPs reduced their fusion efficacy and in vivo glucocorticoid treatment of juvenile D2 mouse improved muscle regeneration. Our findings indicate that aberrant stromal cell response contributes to poor myogenesis and greater muscle degeneration in dystrophic juvenile D2- mdx muscles and reversal of this reduces pathology in adult D2- mdx mouse muscle, identifying these as therapeutic targets to treat dystrophic DMD muscles.

On the use of D2.B10-Dmdmdx/J (D2.mdx) Versus C57BL/10ScSn-Dmdmdx/J (mdx) Mouse Models for Preclinical Studies on Duchenne Muscular Dystrophy: A Cautionary Note from Members of the TREAT-NMD Advisory Committee on Therapeutics

The C57BL/10ScSn-Dmdmdx/J (mdx) mouse model has been used by researchers for decades as a model to study pathology of and develop therapies for Duchenne muscular dystrophy. However, the model is relatively mildly affected compared to the human situation. Recently, the D2.B10-Dmdmdx/J (D2.mdx) mouse model was suggested as a more severely affected and therefore better alternative. While the pathology of this model is indeed more pronounced early in life, it is not progressive, and increasing evidence suggest that it actually partially resolves with age. As such, caution is needed when using this model. However, as preclinical experts of the TREAT-NMD advisory committee for therapeutics (TACT), we frequently encounter study designs that underestimate this caveat. We here provide context for how to best use the two models for preclinical studies at the current stage of knowledge.

Humanization of the mdx Mouse Phenotype for Duchenne Muscular Dystrophy Modeling: A Metabolic Perspective

Duchenne muscular dystrophy (DMD) is a severe form of muscular dystrophy (MD) that is characterized by early muscle wasting and lethal cardiorespiratory failure. While the mdx mouse is the most common model of DMD, it fails to replicate the severe loss of muscle mass and other complications observed in patients, in part due to the multiple rescue pathways found in mice. This led to several attempts at improving DMD animal models by interfering with these rescue pathways through double transgenic approaches, resulting in more severe phenotypes with mixed relevance to the human pathology. As a growing body of literature depicts DMD as a multi-system metabolic disease, improvements in mdx-based modeling of DMD may be achieved by modulating whole-body metabolism instead of muscle homeostasis. This review provides an overview of the established dual-transgenic approaches that exacerbate the mild mdx phenotype by primarily interfering with muscle homeostasis and highlights how advances in DMD modeling coincide with inducing whole-body metabolic changes. We focus on the DBA2/J strain-based D2.mdx mouse with heightened transforming growth factor (TGF)-β signaling and the dyslipidemic mdx/apolipoprotein E (mdx/ApoE) knock-out (KO) mouse, and summarize how these novel models emulate the metabolic changes observed in DMD.

Histological Methods to Assess Skeletal Muscle Degeneration and Regeneration in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a progressive disease caused by the loss of function of the protein dystrophin. This protein contributes to the stabilisation of striated cells during contraction, as it anchors the cytoskeleton with components of the extracellular matrix through the dystrophin-associated protein complex (DAPC). Moreover, absence of the functional protein affects the expression and function of proteins within the DAPC, leading to molecular events responsible for myofibre damage, muscle weakening, disability and, eventually, premature death. Presently, there is no cure for DMD, but different treatments help manage some of the symptoms. Advances in genetic and exon-skipping therapies are the most promising intervention, the safety and efficiency of which are tested in animal models. In addition to in vivo functional tests, ex vivo molecular evaluation aids assess to what extent the therapy has contributed to the regenerative process. In this regard, the later advances in microscopy and image acquisition systems and the current expansion of antibodies for immunohistological evaluation together with the development of different spectrum fluorescent dyes have made histology a crucial tool. Nevertheless, the complexity of the molecular events that take place in dystrophic muscles, together with the rise of a multitude of markers for each of the phases of the process, makes the histological assessment a challenging task. Therefore, here, we summarise and explain the rationale behind different histological techniques used in the literature to assess degeneration and regeneration in the field of dystrophinopathies, focusing especially on those related to DMD.

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

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

Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

The absence of dystrophin in Duchenne muscular dystrophy disrupts the dystrophin-associated glycoprotein complex resulting in skeletal muscle fiber fragility and atrophy, associated with fibrosis as well as microtubule and neuromuscular junction disorganization. The specific, non-conventional cytoplasmic histone deacetylase 6 (HDAC6) was recently shown to regulate acetylcholine receptor distribution and muscle atrophy. Here, we report that administration of the HDAC6 selective inhibitor tubastatin A to the Duchenne muscular dystrophy, mdx mouse model increases muscle strength, improves microtubule, neuromuscular junction, and dystrophin-associated glycoprotein complex organization, and reduces muscle atrophy and fibrosis. Interestingly, we found that the beneficial effects of HDAC6 inhibition involve the downregulation of transforming growth factor beta signaling. By increasing Smad3 acetylation in the cytoplasm, HDAC6 inhibition reduces Smad2/3 phosphorylation, nuclear translocation, and transcriptional activity. These findings provide in vivo evidence that Smad3 is a new target of HDAC6 and implicate HDAC6 as a potential therapeutic target in Duchenne muscular dystrophy.

Single cell sequencing maps skeletal muscle cellular diversity as disease severity increases in dystrophic mouse models

Duchenne muscular dystrophy (DMD) is caused by out-of-frame mutations in the DMD gene resulting in the absence of a functional dystrophin protein, leading to a devastating and progressive lethal muscle-wasting disease. Little is known about cellular heterogeneity as disease severity increases. Advances in single-cell RNA sequencing (scRNA-seq) enabled us to explore skeletal muscle-resident cell populations in healthy, dystrophic, and severely dystrophic mouse models. We found increased frequencies of activated fibroblasts, fibro-adipogenic progenitor cells, and pro-inflammatory macrophages in dystrophic gastrocnemius muscles and an upregulation of extracellular matrix genes on endothelial cells in dystrophic and severely dystrophic muscles. We observed a pronounced risk of clotting, especially in the severely dystrophic mice with increased expression of plasminogen activator inhibitor-1 in endothelial cells, indicating endothelial cell impairment as disease severity increases. This work extends our understanding of the severe nature of DMD which should be considered when developing single or combinatorial approaches for DMD.

Dystrophin Restoration after Adeno-Associated Virus U7-Mediated Dmd Exon Skipping Is Modulated by Muscular Exercise in the Severe D2-Mdx Duchenne Muscular Dystrophy Murine Model

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by Dmd mutations, resulting in the absence of dystrophin in skeletal muscle, and a greater susceptibility to damage during contraction (exercise). The current study evaluated whether voluntary exercise impacts a Dmd exon skipping and muscle physiology in a severe DMD murine model. D2-mdx mice were intramuscularly injected with an adeno-associated virus (AAV) U7 snRNA to correct Dmd reading frame, and allowed to voluntary run on a wheel for 1 month. Voluntary running did not induce muscle fiber regeneration, as indicated by the percentage of centronucleated fibers, Myh3 and Myh4 expression, and maximal force production, and thus possibly did not compromise the gene therapy approach. Voluntary running did not impact the number of viral genomes and the expression of U7 and Dmd 1 month after injection of AAV-U7 injected just before exercise initiation, but reduced the amount of dystrophin in dystrophin-expressing fibers from 80% to 65% of the muscle cross-sectional area. In conclusion, voluntary running did not induce muscle damage and had no drastic detrimental effect on the AAV gene therapy exon skipping approach in a severe murine DMD model. Moreover, these results suggest considering exercise as an additional element in the design and conception of future therapeutic approaches for DMD.

NOX4 inhibition promotes the remodeling of dystrophic muscle

The muscular dystrophies (MDs) are genetic muscle diseases that result in progressive muscle degeneration followed by the fibrotic replacement of affected muscles as regenerative processes fail. Therapeutics that specifically address the fibrosis and failed regeneration associated with MDs represent a major unmet clinical need for MD patients, particularly those with advanced-stage disease progression. The current study investigated targeting NAD(P)H oxidase 4 (NOX4) as a potential strategy to reduce fibrosis and promote regeneration in disease-burdened muscle that models Duchenne muscular dystrophy (DMD). NOX4 was elevated in the muscles of dystrophic mice and DMD patients, localizing primarily to interstitial cells located between muscle fibers. Genetic and pharmacological targeting of NOX4 significantly reduced fibrosis in dystrophic respiratory and limb muscles. Mechanistically, NOX4 targeting decreased the number of fibrosis-depositing cells (myofibroblasts) and restored the number of muscle-specific stem cells (satellite cells) localized to their physiological niche, thereby rejuvenating muscle regeneration. Furthermore, acute inhibition of NOX4 was sufficient to induce apoptotic clearing of myofibroblasts within dystrophic muscle. These data indicate that targeting NOX4 is an effective strategy to promote the beneficial remodeling of disease-burdened muscle representative of DMD and, potentially, other MDs and muscle pathologies.