Fibrosis and adipogenesis originate from a common mesenchymal progenitor in skeletal muscle

Cell autonomous requirement of neurofibromin (Nf1) for postnatal muscle hypertrophic growth and metabolic homeostasis

BACKGROUND

Neurofibromatosis type 1 (NF1) is a multi-organ disease caused by mutations in neurofibromin 1 (NF1). Amongst other features, NF1 patients frequently show reduced muscle mass and strength, impairing patients' mobility and increasing the risk of fall. The role of Nf1 in muscle and the cause for the NF1-associated myopathy are mostly unknown.

METHODS

To dissect the function of Nf1 in muscle, we created muscle-specific knockout mouse models for NF1, inactivating Nf1 in the prenatal myogenic lineage either under the Lbx1 promoter or under the Myf5 promoter. Mice were analysed during prenatal and postnatal myogenesis and muscle growth.

RESULTS

Nf1^Lbx1 and Nf1^Myf5 animals showed only mild defects in prenatal myogenesis. Nf1^Lbx1 animals were perinatally lethal, while Nf1^Myf5 animals survived only up to approximately 25 weeks. A comprehensive phenotypic characterization of Nf1^Myf5 animals showed decreased postnatal growth, reduced muscle size, and fast fibre atrophy. Proteome and transcriptome analyses of muscle tissue indicated decreased protein synthesis and increased proteasomal degradation, and decreased glycolytic and increased oxidative activity in muscle tissue. High-resolution respirometry confirmed enhanced oxidative metabolism in Nf1^Myf5 muscles, which was concomitant to a fibre type shift from type 2B to type 2A and type 1. Moreover, Nf1^Myf5 muscles showed hallmarks of decreased activation of mTORC1 and increased expression of atrogenes. Remarkably, loss of Nf1 promoted a robust activation of AMPK with a gene expression profile indicative of increased fatty acid catabolism. Additionally, we observed a strong induction of genes encoding catabolic cytokines in muscle Nf1^Myf5 animals, in line with a drastic reduction of white, but not brown adipose tissue.

CONCLUSIONS

Our results demonstrate a cell autonomous role for Nf1 in myogenic cells during postnatal muscle growth required for metabolic and proteostatic homeostasis. Furthermore, Nf1 deficiency in muscle drives cross-tissue communication and mobilization of lipid reserves.

M-Ras is Muscle-Ras, Moderate-Ras, Mineral-Ras, Migration-Ras, and Many More-Ras

The Ras family of small GTPases comprises about 36 members in humans. M-Ras is related to classical Ras with regard to its regulators and effectors, but solely constitutes a subfamily among the Ras family members. Although classical Ras strongly binds Raf and highly activates the ERK pathway, M-Ras less strongly binds Raf and moderately but sustainedly activates the ERK pathway to induce neuronal differentiation. M-Ras also possesses specific effectors, including RapGEFs and the PP1 complex Shoc2-PP1c, which dephosphorylates Raf to activate the ERK pathway. M-Ras is highly expressed in the brain and plays essential roles in dendrite formation during neurogenesis, in contrast to the axon formation by R-Ras. M-Ras is also highly expressed in the bone and induces osteoblastic differentiation and transdifferentiation accompanied by calcification. Moreover, M-Ras elicits epithelial-mesenchymal transition-mediated collective and single cell migration through the PP1 complex-mediated ERK pathway activation. Activating missense mutations in the MRAS gene have been detected in Noonan syndrome, one of the RASopathies, and MRAS gene amplification occurs in several cancers. Furthermore, several SNPs in the MRAS gene are associated with coronary artery disease, obesity, and dyslipidemia. Therefore, M-Ras carries out a variety of cellular, physiological, and pathological functions. Further investigations may reveal more functions of M-Ras.

The role of neutrophil extracellular traps and TLR signaling in skeletal muscle ischemia reperfusion injury

Ischemia reperfusion (IR) injury results in devastating skeletal muscle fibrosis. Here, we recapitulate this injury with a mouse model of hindlimb IR injury which leads to skeletal muscle fibrosis. Injury resulted in extensive immune infiltration with robust neutrophil extracellular trap (NET) formation in the skeletal muscle, however, direct targeting of NETs via the peptidylarginine deiminase 4 (PAD4) mechanism was insufficient to reduce muscle fibrosis. Circulating levels of IL-10 and TNFα were significantly elevated post injury, indicating toll-like receptor (TLR) signaling may be involved in muscle injury. Administration of hydroxychloroquine (HCQ), a small molecule inhibitor of TLR7/8/9, following injury reduced NET formation, IL-10, and TNFα levels and ultimately mitigated muscle fibrosis and improved myofiber regeneration following IR injury. HCQ treatment decreased fibroadipogenic progenitor cell proliferation and partially inhibited ERK1/2 phosphorylation in the injured tissue, suggesting it may act through a combination of TLR7/8/9 and ERK signaling mechanisms. We demonstrate that treatment with FDA-approved HCQ leads to decreased muscle fibrosis and increased myofiber regeneration following IR injury, suggesting short-term HCQ treatment may be a viable treatment to prevent muscle fibrosis in ischemia reperfusion and traumatic extremity injury.

Bioengineered human skeletal muscle capable of functional regeneration

BACKGROUND

Skeletal muscle (SkM) regenerates following injury, replacing damaged tissue with high fidelity. However, in serious injuries, non-regenerative defects leave patients with loss of function, increased re-injury risk and often chronic pain. Progress in treating these non-regenerative defects has been slow, with advances only occurring where a comprehensive understanding of regeneration has been gained. Tissue engineering has allowed the development of bioengineered models of SkM which regenerate following injury to support research in regenerative physiology. To date, however, no studies have utilised human myogenic precursor cells (hMPCs) to closely mimic functional human regenerative physiology.

RESULTS

Here we address some of the difficulties associated with cell number and hMPC mitogenicity using magnetic association cell sorting (MACS), for the marker CD56, and media supplementation with fibroblast growth factor 2 (FGF-2) and B-27 supplement. Cell sorting allowed extended expansion of myogenic cells and supplementation was shown to improve myogenesis within engineered tissues and force generation at maturity. In addition, these engineered human SkM regenerated following barium chloride (BaCl2) injury. Following injury, reductions in function (87.5%) and myotube number (33.3%) were observed, followed by a proliferative phase with increased MyoD+ cells and a subsequent recovery of function and myotube number. An expansion of the Pax7+ cell population was observed across recovery suggesting an ability to generate Pax7+ cells within the tissue, similar to the self-renewal of satellite cells seen in vivo.

CONCLUSIONS

This work outlines an engineered human SkM capable of functional regeneration following injury, built upon an open source system adding to the pre-clinical testing toolbox to improve the understanding of basic regenerative physiology.

Cellular senescence-mediated exacerbation of Duchenne muscular dystrophy

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

Differentiation of skeletal muscle Mesenchymal progenitor cells to myofibroblasts is reversible

Accumulation of intramuscular adipose tissue (IMAT) and development of fibrous tissues due to accumulation of collagen both affect meat quality such as tenderness, texture, and flavor. Thus, it is important for the production of high‐quality meat to regulate the amount of adipose and fibrous tissues in skeletal muscle. IMAT is comprised of adipocytes, while collagens included in fibrous tissues are mainly produced by activated fibroblasts. Both adipocytes and fibroblasts are differentiated from their common ancestors, called mesenchymal progenitor cells (MPC). We previously established rat MPC clone, 2G11 cells. As several reports implicated the plasticity of fibroblast differentiation, in the present study, using 2G11 cells, we asked whether myofibroblasts differentiated from MPC are capable of re‐gaining adipogenic potential in vitro. By treating with bFGF, their αSMA expression was reduced and adipogenic potential was restored partially. Furthermore, by lowering cell density together with bFGF treatment, 2G11 cell‐derived myofibroblasts lost αSMA expression and showed the highest adipogenic potential, and this was along with their morphological change from flattened‐ to spindle‐like shape, which is typically observed with MPC. These results indicated that MPC‐derived myofibroblasts could re‐acquire adipogenic potential, possibly mediated through returning to an undifferentiated MPC‐like state.

Hic1 Defines Quiescent Mesenchymal Progenitor Subpopulations with Distinct Functions and Fates in Skeletal Muscle Regeneration

Many adult tissues contain resident stem cells, such as the Pax7⁺ satellite cells within skeletal muscle, that regenerate parenchymal elements following damage. Tissue-resident mesenchymal progenitors (MPs) also participate in regeneration, although their function and fate in this process are unclear. Here, we identify Hypermethylated in cancer 1 (Hic1) as a marker of MPs in skeletal muscle and further show that Hic1 deletion leads to MP hyperplasia. Single-cell RNA-seq and ATAC-seq analysis of Hic1⁺ MPs in skeletal muscle shows multiple subpopulations, which we further show have distinct functions and lineage potential. Hic1⁺ MPs orchestrate multiple aspects of skeletal muscle regeneration by providing stage-specific immunomodulation and trophic and mechanical support. During muscle regeneration, Hic1⁺ derivatives directly contribute to several mesenchymal compartments including Col22a1-expressing cells within the myotendinous junction. Collectively, these findings demonstrate that HIC1 regulates MP quiescence and identifies MP subpopulations with transient and enduring roles in muscle regeneration.

Human Cell Modeling for Cardiovascular Diseases

The availability of appropriate and reliable in vitro cell models recapitulating human cardiovascular diseases has been the aim of numerous researchers, in order to retrace pathologic phenotypes, elucidate molecular mechanisms, and discover therapies using simple and reproducible techniques. In the past years, several human cell types have been utilized for these goals, including heterologous systems, cardiovascular and non-cardiovascular primary cells, and embryonic stem cells. The introduction of induced pluripotent stem cells and their differentiation potential brought new prospects for large-scale cardiovascular experiments, bypassing ethical concerns of embryonic stem cells and providing an advanced tool for disease modeling, diagnosis, and therapy. Each model has its advantages and disadvantages in terms of accessibility, maintenance, throughput, physiological relevance, recapitulation of the disease. A higher level of complexity in diseases modeling has been achieved with multicellular co-cultures. Furthermore, the important progresses reached by bioengineering during the last years, together with the opportunities given by pluripotent stem cells, have allowed the generation of increasingly advanced in vitro three-dimensional tissue-like constructs mimicking in vivo physiology. This review provides an overview of the main cell models used in cardiovascular research, highlighting the pros and cons of each, and describing examples of practical applications in disease modeling.

Human Rotator Cuff Tears Have an Endogenous, Inducible Stem Cell Source Capable of Improving Muscle Quality and Function After Rotator Cuff Repair

BACKGROUND

The muscle quality of the rotator cuff (RC), measured by atrophy and fatty infiltration (FI), is a key determinant of outcomes in RC injury and repair. The ability to regenerate muscle after repair has been shown to be limited.

PURPOSE

To determine if there is a source of resident endogenous stem cells, fibroadipogenic progenitor cells (FAPs), within RC injury patients, and if these cells are capable of adipogenic, fibrogenic, and pro-myogenic differentiation.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 20 patients between the ages of 40 and 75 years with partial- or full-thickness RC tears of the supraspinatus and evidence of atrophy and FI Goutallier grade 1, 2, or 3 were selected from 2 surgeons at an orthopaedic center. During the surgical repair procedure, supraspinatus muscle biopsy specimens were obtained for analysis as were deltoid muscle biopsy specimens to serve as the control. FAPs and satellite cells were quantified using fluorescence-activated cell sorting. Muscle FI and fibrosis was quantified using Oil Red O and Masson trichrome staining. FAP differentiation and gene expression profiles were compared across tear sizes after culture in adipogenic, fibrogenic, and beta-3 agonist (amibegron) conditions. Analysis of variance was used for statistical comparisons between groups, with P < .05 as statistically significant.

RESULTS

Histologic analysis confirmed the presence of fat in biopsy specimens from patients with full-thickness tears. There were more FAPs in the full-thickness tear group compared with the partial-thickness tear group (9.43% ± 4.25% vs 3.84% ± 2.54%; P < .01). Full-thickness tears were divided by tear size, with patients with larger tears having significantly more FAPs than those with smaller tears. FAPs from muscles with full-thickness tendon tears had more adipogenic and fibrogenic potential than those with partial tears. Induction of a beige adipose tissue (BAT) phenotype in FAPs was possible, as demonstrated by increased expression of BAT markers and pro-myogenic genes including insulin-like growth factor 1 and follistatin.

CONCLUSION

Endogenous FAPs are present within the RC and likely are the source of FI. These FAPs were increased in muscles with in larger tears but are capable of adopting a pro-myogenic BAT phenotype that could be utilized to improve muscle quality and patient function after RC repair.

Established and Emerging Mechanisms in the Pathogenesis of Arrhythmogenic Cardiomyopathy: A Multifaceted Disease

Arrhythmogenic cardiomyopathy (ACM) is a heritable myocardial disease that manifests with cardiac arrhythmias, syncope, sudden cardiac death, and heart failure in the advanced stages. The pathological hallmark of ACM is a gradual replacement of the myocardium by fibroadiposis, which typically starts from the epicardium. Molecular genetic studies have identified causal mutations predominantly in genes encoding for desmosomal proteins; however, non-desmosomal causal mutations have also been described, including genes coding for nuclear proteins, cytoskeleton componentsand proteins involved in excitation-contraction coupling. Despite the poor prognosis, currently available treatments can only partially control symptoms and to date there is no effective therapy for ACM. Inhibition of the canonical Wnt/β-catenin pathway and activation of the Hippo and the TGF-β pathways have been implicated in the pathogenesis of ACM. Yet, our understanding of the molecular mechanisms involved in the development of the disease and the cell source of fibroadiposis remains incomplete. Elucidation of the pathogenesis of the disease could facilitate targeted approaches for treatment. In this manuscript we will provide a comprehensive review of the proposed molecular and cellular mechanisms of the pathogenesis of ACM, including the emerging evidence on abnormal calcium homeostasis and inflammatory/autoimmune response. Moreover, we will propose novel hypothesis about the role of epicardial cells and paracrine factors in the development of the phenotype. Finally, we will discuss potential innovative therapeutic approaches based on the growing knowledge in the field.

Association of leptin mRNA expression with meat quality trait in Tianfu black rabbits

Leptin is a hormone synthesized and secreted primarily in adipocyte which can help to regulate energy balance. In this experiment, three tissue samples of Tianfu black rabbits at four growing periods were selected. The expression levels of leptin gene in different tissues were detected by real-time fluorescence quantitative PCR. The correlation analysis showed that the correlation coefficient between the expression levels of leptin gene in perirenal fat and intramuscular fat content in 84-day-old male rabbits was 0.73 (p < 0.05); the correlation coefficients between the expression levels of leptin gene in left biceps femoris and intramuscular fat and 24-hour pH in 84-day-old male rabbits were 0.95 (p < 0.01) and 0.85 (p < 0.05), respectively. Besides, the correlation coefficient between the expression levels of leptin gene in male left biceps femoris and cooked meat rate was 0.83 (p < 0.05). According to the analysis results, we inferred the expression levels of leptin gene in Tianfu black rabbits can influence meat quality and the meat quality of high expression levels of leptin gene in Tianfu black rabbits is better. These results revealed the leptin gene may be one of the important candidate genes for meat quality traits of molecular markers.

HDAC inhibitors tune miRNAs in extracellular vesicles of dystrophic muscle-resident mesenchymal cells

We show that extracellular vesicles (EVs) released by mesenchymal cells (i.e., fibro-adipogenic progenitors-FAPs) mediate microRNA (miR) transfer to muscle stem cells (MuSCs) and that exposure of dystrophic FAPs to HDAC inhibitors (HDACis) increases the intra-EV levels of a subset of miRs, which cooperatively target biological processes of therapeutic interest, including regeneration, fibrosis, and inflammation. Increased levels of miR-206 in EVs released by FAPs of muscles from Duchenne muscular dystrophy (DMD) patients or mdx mice exposed to HDACi are associated with enhanced regeneration and decreased fibrosis. Consistently, EVs from HDACi-treated dystrophic FAPs can stimulate MuSC activation and expansion ex vivo, and promote regeneration, while inhibiting fibrosis and inflammation of dystrophic muscles, upon intramuscular transplantation in mdx mice, in vivo. AntagomiR-mediated blockade of individual miRs reveals a specific requirement of miR-206 for EV-induced expansion of MuSCs and regeneration of dystrophic muscles, and indicates that cooperative activity of HDACi-induced miRs accounts for the net biological effect of these EVs. These data point to pharmacological modulation of EV content as novel strategy for therapeutic interventions in muscular dystrophies.

Osteopontin: The Molecular Bridge between Fat and Cardiac-Renal Disorders

Osteopontin (OPN) is a multifaceted matricellular protein, with well-recognized roles in both the physiological and pathological processes in the body. OPN is expressed in the main organs and cell types, in which it induces different biological actions. During physiological conditioning, OPN acts as both an intracellular protein and soluble excreted cytokine, regulating tissue remodeling and immune-infiltrate in adipose tissue the heart and the kidney. In contrast, the increased expression of OPN has been correlated with the severity of the cardiovascular and renal outcomes associated with obesity. Indeed, OPN expression is at the “cross roads” of visceral fat extension, cardiovascular diseases (CVDs) and renal disorders, in which OPN orchestrates the molecular interactions, leading to chronic low-grade inflammation. The common factor associated with OPN overexpression in adipose, cardiac and renal tissues seems attributable to the concomitant increase in visceral fat size and the increase in infiltrated OPN⁺ macrophages. This review underlines the current knowledge on the molecular interactions between obesity and the cardiac–renal disorders ruled by OPN.

Murine Tissue-Resident PDGFRα+ Fibro-Adipogenic Progenitors Spontaneously Acquire Osteogenic Phenotype in an Altered Inflammatory Environment

Acquired heterotopic ossifications (HO) arising as a result of various traumas, including injury or surgical interventions, often result in pain and loss of motion. Though triggers for HO have been identified, the cellular source of these heterotopic lesions as well as the underlying mechanisms that drive the formation of acquired HO remain poorly understood, and treatment options, including preventative treatments, remain limited. Here, we explore the cellular source of HO and a possible underlying mechanism for their spontaneous osteogenic differentiation. We demonstrate that HO lesions arise from tissue-resident PDGFRα+ fibro/adipogenic progenitors (FAPs) in skeletal muscle and not from circulating bone marrow-derived progenitors. Further, we show that accumulation of these cells in the tissue after damage due to alterations in the inflammatory environment can result in activation of their inherent osteogenic potential. This work suggests a mechanism by which an altered inflammatory cell and FAP interactions can lead to the formation of HO after injury and presents potential targets for therapeutics in acquired HO. © 2020 American Society for Bone and Mineral Research.

Arrhythmogenic Cardiomyopathy and Skeletal Muscle Dystrophies: Shared Histopathological Features and Pathogenic Mechanisms

Arrhythmogenic cardiomyopathy (ACM) is a heritable cardiac disease characterized by fibrotic or fibrofatty myocardial replacement, associated with an increased risk of ventricular arrhythmias and sudden cardiac death. Originally described as a disease of the right ventricle, ACM is currently recognized as a biventricular entity, due to the increasing numbers of reports of predominant left ventricular or biventricular involvement. Research over the last 20 years has significantly advanced our knowledge of the etiology and pathogenesis of ACM. Several etiopathogenetic theories have been proposed; among them, the most attractive one is the dystrophic theory, based on the observation of similar histopathological features between ACM and skeletal muscle dystrophies (SMDs), such as progressive muscular degeneration, inflammation, and tissue replacement by fatty and fibrous tissue. This review will describe the pathophysiological and molecular similarities shared by ACM with SMDs.

Rotator cuff tear degeneration and the role of fibro-adipogenic progenitors

The high prevalence of rotator cuff tears poses challenges to individual patients and the healthcare system at large. This orthopedic injury is complicated further by high rates of retear after surgical repair. Outcomes following repair are highly dependent upon the quality of the injured rotator cuff muscles, and it is, therefore, crucial that the pathophysiology of rotator cuff degeneration continues to be explored. Fibro-adipogenic progenitors, a major population of resident muscle stem cells, have emerged as the main source of intramuscular fibrosis and fatty infiltration, both of which are key features of rotator cuff muscle degeneration. Improvements to rotator cuff repair outcomes will likely require addressing the muscle pathology produced by these cells. The aim of this review is to summarize the current rotator cuff degeneration assessment tools, the effects of poor muscle quality on patient outcomes, the role of fibro-adipogenic progenitors in mediating muscle pathology, and how these cells could be leveraged for potential therapeutics to augment current rotator cuff surgical and rehabilitative strategies.

mir-22-3p/KLF6/MMP14 axis in fibro-adipogenic progenitors regulates fatty infiltration in muscle degeneration

Fibro/adipogenic progenitors (FAPs) are the main cellular source of fatty degeneration in muscle injury; however, the underlying mechanism of FAP adipogenesis in muscle degeneration needs to be further examined. Matrix metalloproteinase 14 (MMP-14) has been reported to induce the adipogenesis of 3T3-L1 preadipocytes, but whether MMP-14 also regulates the differentiation of FAPs remains unclear. To investigate whether and how MMP-14 regulates FAP adipogenesis and fatty infiltration in muscle degeneration, we examined MMP-14 expression in degenerative muscles and tested the effect of MMP-14 on FAP adipogenesis in vitro and in vivo. As expected, MMP-14 enhanced FAP adipogenesis and fatty infiltration in degenerative muscles; moreover, blocking endogenous MMP-14 in injured muscles facilitated muscle repair. Further investigations revealed that Kruppel-like factor 6 (KLF6) was a transcription factor associated with MMP-14 and acted as an "on-off" switch in the differentiation of FAPs into adipocytes or myofibroblasts. Moreover, KLF6 was the target gene of miR-22-3p, which was downregulated during FAP adipogenesis both in vitro and in vivo, and overexpression of miR-22-3p markedly prevented FAP adipogenesis and attenuated fatty degeneration in muscles. Our study revealed that miR-22-3p/KLF6/MMP-14 is a novel pathway in FAP adipogenesis and that inhibiting KLF6 is a potential strategy for the treatment of muscular degenerative diseases.

High-Dimensional Single-Cell Quantitative Profiling of Skeletal Muscle Cell Population Dynamics during Regeneration

The interstitial space surrounding the skeletal muscle fibers is populated by a variety of mononuclear cell types. Upon acute or chronic insult, these cell populations become activated and initiate finely-orchestrated crosstalk that promotes myofiber repair and regeneration. Mass cytometry is a powerful and highly multiplexed technique for profiling single-cells. Herein, it was used to dissect the dynamics of cell populations in the skeletal muscle in physiological and pathological conditions. Here, we characterized an antibody panel that could be used to identify most of the cell populations in the muscle interstitial space. By exploiting the mass cytometry resolution, we provided a comprehensive picture of the dynamics of the major cell populations that sensed and responded to acute damage in wild type mice and in a mouse model of Duchenne muscular dystrophy. In addition, we revealed the intrinsic heterogeneity of many of these cell populations.

Progressive Degenerative Myopathy and Myosteatosis in ASNSD1-Deficient Mice

Mice with an inactivating mutation in the gene encoding asparagine synthetase domain containing 1 (ASNSD1) develop a progressive degenerative myopathy that results in severe sarcopenia and myosteatosis. ASNSD1 is conserved across many species, and whole body gene expression surveys show maximal expression levels of ASNSD1 in skeletal muscle. However, potential functions of this protein have not been previously reported. Asnsd1^-/- mice demonstrated severe muscle weakness, and their normalized body fat percentage on both normal chow and high fat diets was greater than 2 SD above the mean for 3651 chow-fed and 2463 high-fat-diet-fed knockout (KO) lines tested. Histologic lesions were essentially limited to the muscle and were characterized by a progressive degenerative myopathy with extensive transdifferentiation and replacement of muscle by well-differentiated adipose tissue. There was minimal inflammation, fibrosis, and muscle regeneration associated with this myopathy. In addition, the absence of any signs of lipotoxicity in Asnsd1^-/- mice despite their extremely elevated body fat percentage and low muscle mass suggests a role for metabolic dysfunctions in the development of this phenotype. Asnsd1^-/- mice provide the first insight into the function of this protein, and this mouse model could prove useful in elucidating fundamental metabolic interactions between skeletal muscle and adipose tissue.

Impaired ECM Remodeling and Macrophage Activity Define Necrosis and Regeneration Following Damage in Aged Skeletal Muscle

Regenerative capacity of skeletal muscle declines with age, the cause of which remains largely unknown. We investigated extracellular matrix (ECM) proteins and their regulators during early regeneration timepoints to define a link between aberrant ECM remodeling, and impaired aged muscle regeneration. The regeneration process was compared in young (three month old) and aged (18 month old) C56BL/6J mice at 3, 5, and 7 days following cardiotoxin-induced damage to the tibialis anterior muscle. Immunohistochemical analyses were performed to assess regenerative capacity, ECM remodeling, and the macrophage response in relation to plasminogen activator inhibitor-1 (PAI-1), matrix metalloproteinase-9 (MMP-9), and ECM protein expression. The regeneration process was impaired in aged muscle. Greater intracellular and extramyocellular PAI-1 expression was found in aged muscle. Collagen I was found to accumulate in necrotic regions, while macrophage infiltration was delayed in regenerating regions of aged muscle. Young muscle expressed higher levels of MMP-9 early in the regeneration process that primarily colocalized with macrophages, but this expression was reduced in aged muscle. Our results indicate that ECM remodeling is impaired at early time points following muscle damage, likely a result of elevated expression of the major inhibitor of ECM breakdown, PAI-1, and consequent suppression of the macrophage, MMP-9, and myogenic responses.

Molecular Phenotyping of White Striping and Wooden Breast Myopathies in Chicken

The White Striping (WS) and Wooden Breast (WB) defects are two myopathic syndromes whose occurrence has recently increased in modern fast-growing broilers. The impact of these defects on the quality of breast meat is very important, as they greatly affect its visual aspect, nutritional value, and processing yields. The research conducted to date has improved our knowledge of the biological processes involved in their occurrence, but no solution has been identified so far to significantly reduce their incidence without affecting growing performance of broilers. This study aims to follow the evolution of molecular phenotypes in relation to both fast-growing rate and the occurrence of defects in order to identify potential biomarkers for diagnostic purposes, but also to improve our understanding of physiological dysregulation involved in the occurrence of WS and WB. This has been achieved through enzymatic, histological, and transcriptional approaches by considering breast muscles from a slow- and a fast-growing line, affected or not by WS and WB. Fast-growing muscles produced more reactive oxygen species (ROS) than slow-growing ones, independently of WS and WB occurrence. Within fast-growing muscles, despite higher mitochondria density, muscles affected by WS or WB defects did not show higher cytochrome oxidase activity (COX) activity, suggesting altered mitochondrial function. Among the markers related to muscle remodeling and regeneration, immunohistochemical staining of FN1, NCAM, and MYH15 was higher in fast- compared to slow-growing muscles, and their amount also increased linearly with the presence and severity of WS and WB defects, making them potential biomarkers to assess accurately their presence and severity. Thanks to an innovative histological technique based on fluorescence intensity measurement, they can be rapidly quantified to estimate the injuries induced in case of WS and WB. The muscular expression of several other genes correlates also positively to the presence and severity of the defects like TGFB1 and CTGF, both involved in the development of connective tissue, or Twist1, known as an inhibitor of myogenesis. Finally, our results suggested that a balance between TGFB1 and PPARG would be essential for fibrosis or adiposis induction and therefore for determining WS and WB phenotypes.

Concurrent EPA and DHA Supplementation Impairs Brown Adipogenesis of C2C12 Cells

Maternal dietary supplementation of n−3 polyunsaturated fatty acids (n−3 PUFAs), especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), is considered to play positive roles in fetal neuro system development. However, maternal n−3 PUFAs may induce molecular reprogramming of uncommitted fetal myoblasts into adipocyte phenotype, in turn affecting lipid metabolism and energy expenditure of the offspring. The objective of this in vitro study was to investigate the combined effects of EPA and DHA on C2C12 cells undergoing brown adipogenic differentiation. C2C12 myoblasts were cultured to confluency and then treated with brown adipogenic differentiation medium with and without 50 μM EPA and 50 μM DHA. After differentiation, mRNA and protein samples were collected. Gene expression and protein levels were analyzed by real-time PCR and western blot. General Proteomics analysis was conducted using mass spectrometric evaluation. The effect of EPA and DHA on cellular oxygen consumption was measured using a Seahorse XFP Analyzer. Cells treated with n−3 PUFAs had significantly less (P < 0.05) expression of the brown adipocyte marker genes PGC1α, DIO2, and UCP3. Expression of mitochondrial biogenesis-related genes TFAM, PGC1α, and PGC1β were significantly downregulated (P < 0.05) by n−3 PUFAs treatment. Expression of mitochondrial electron transportation chain (ETC)-regulated genes were significantly inhibited (P < 0.05) by n−3 PUFAs, including ATP5J2, COX7a1, and COX8b. Mass spectrometric and western blot evaluation showed protein levels of enzymes which regulate the ETC and Krebs cycle, including ATP synthase α and β (F1F0 complex), citrate synthase, succinate CO-A ligase, succinate dehydrogenase (complex II), ubiquinol-cytochrome c reductase complex subunits (complex III), aconitate hydratase, cytochrome c, and pyruvate carboxylase were all decreased in the n−3 PUFAs group (P < 0.05). Genomic and proteomic changes were accompanied by mitochondrial dysfunction, represented by significantly reduced oxygen consumption rate, ATP production, and proton leak (P < 0.05). This study suggested that EPA and DHA may alter the BAT fate of myoblasts by inhibiting mitochondrial biogenesis and activity and induce white-like adipogenesis, shifting the metabolism from lipid oxidation to synthesis.

Cellular and molecular features of neurogenic skeletal muscle atrophy

Neurogenic atrophy refers to the loss of muscle mass and function that results directly from injury or disease of the peripheral nervous system. Individuals with neurogenic atrophy may experience reduced functional status and quality of life and, in some circumstances, reduced survival. Distinct pathological findings on muscle histology can aid in diagnosis of a neurogenic cause for muscle dysfunction, and provide indicators for the chronicity of denervation. Denervation induces pleiotypic responses in skeletal muscle, and the molecular mechanisms underlying neurogenic muscle atrophy appear to share common features with other causes of muscle atrophy, including activation of FOXO transcription factors and corresponding induction of ubiquitin-proteasomal and lysosomal degradation. In this review, we provide an overview of histologic features of neurogenic atrophy and a summary of current understanding of underlying mechanisms.

Perivascular Fibro-Adipogenic Progenitor Tracing during Post-Traumatic Osteoarthritis

Perivascular mural cells surround capillaries and microvessels and have diverse regenerative or fibrotic functions after tissue injury. Subsynovial fibrosis is a well-known pathologic feature of osteoarthritis, yet transgenic animals for use in visualizing perivascular cell contribution to fibrosis during arthritic changes have not been developed. Here, inducible Pdgfra-CreER^T2 reporter mice were subjected to joint-destabilization surgery to induce arthritic changes, and cell lineage was traced over an 8-week period with a focus on the joint-associated fat pad. Results showed that, at baseline, inducible Pdgfra reporter activity highlighted adventitial and, to a lesser extent, pericytic cells within the infrapatellar fat pad. Joint-destabilization surgery was associated with marked fibrosis of the infrapatellar fat pad, accompanied by an expansion of perivascular Pdgfra-expressing cellular descendants, many of which adopted α-smooth muscle actin expression. Gene expression analysis of microdissected infrapatellar fat pad confirmed enrichment in membrane-bound green fluorescent protein/Pdgfra-expressing cells, along with a gene signature that corresponded with injury-associated fibro-adipogenic progenitors. Our results highlight dynamic changes in joint-associated perivascular fibro-adipogenic progenitors during osteoarthritis.

Deciphering the cellular interplays underlying obesity-induced adipose tissue fibrosis

Obesity originates from an imbalance between caloric intake and energy expenditure that promotes adipose tissue expansion, which is necessary to buffer nutrient excess. Patients with higher visceral fat mass are at a higher risk of developing severe complications such as type 2 diabetes and cardiovascular and liver diseases. However, increased fat mass does not fully explain obesity's propensity to promote metabolic diseases. With chronic obesity, adipose tissue undergoes major remodeling, which can ultimately result in unresolved chronic inflammation leading to fibrosis accumulation. These features drive local tissue damage and initiate and/or maintain multiorgan dysfunction. Here, we review the current understanding of adipose tissue remodeling with a focus on obesity-induced adipose tissue fibrosis and its relevance to clinical manifestations.

Tissue cross talks governing limb muscle development and regeneration

For decades, limb development has been a paradigm of three-dimensional patterning. Moreover, as the limb muscles and the other tissues of the limb's musculoskeletal system arise from distinct developmental sources, it has been a prime example of integrative morphogenesis and cross-tissue communication. As the limbs grow, all components of the musculoskeletal system (muscles, tendons, connective tissue, nerves) coordinate their growth and differentiation, ultimately giving rise to a functional unit capable of executing elaborate movement. While the molecular mechanisms governing global three-dimensional patterning and formation of the skeletal structures of the limbs has been a matter of intense research, patterning of the soft tissues is less understood. Here, we review the development of limb muscles with an emphasis on their interaction with other tissue types and the instructive roles these tissues play. Furthermore, we discuss the role of adult correlates of these embryonic accessory tissues in muscle regeneration.

Intervertebral disc herniation effects on multifidus muscle composition and resident stem cell populations

BACKGROUND

Paraspinal muscles are crucial for vertebral stabilization and movement. These muscles are prone to develop fatty infiltration (FI), fibrosis, and atrophy in many spine conditions. Fibro-adipogenic progenitors (FAPs), a resident muscle stem cell population, are the main contributors of muscle fibrosis and FI. FAPs are involved in a complex interplay with satellite cells (SCs), the primary myogenic progenitor cells within muscle. Little is known about the stem cell composition of the multifidus. The aim of this study is to examine FAPs and SCs in the multifidus in disc herniation patients. Multifidus muscle samples were collected from 10 patients undergoing decompressive spine surgery for lumbar disc herniation. Hamstring muscle was collected from four patients undergoing hamstring autograft ACL reconstruction as an appendicular control. Multifidus tissue was analyzed for FI and fibrosis using Oil-Red-O and Masson's trichrome staining. FAPs and SCs were visualized using immunostaining and quantified with fluorescence-activated cell sorting (FACS) sorting. Gene expression of these cells from the multifidus were analyzed with reverse transcription-polymerase chain reaction and compared to those from hamstring muscle. FI and fibrosis accounted for 14.2%± 7.4% and 14.8%±4.2% of multifidus muscle, respectively. The multifidus contained more FAPs (11.7%±1.9% vs 1.4%±0.2%; P<.001) and more SCs (3.4%±1.6% vs 0.08%±0.02%; P=.002) than the hamstring. FAPs had greater α Smooth Muscle Actin (αSMA) and adipogenic gene expression than FAPs from the hamstring. SCs from the multifidus displayed upregulated expression of stem, proliferation, and differentiation genes.

CONCLUSION

The multifidus in patients with disc herniation contains large percentages of FAPs and SCs with different gene expression profiles compared to those in the hamstring. These results may help explain the tendency for the multifidus to atrophy and form FI and fibrosis as well as elucidate potential approaches for mitigating these degenerative changes by leveraging these muscle stem cell populations.

Intramuscular Brown Fat Activation Decreases Muscle Atrophy and Fatty Infiltration and Improves Gait After Delayed Rotator Cuff Repair in Mice

BACKGROUND

Successful repair of large and massive rotator cuff (RC) tears remains a challenge at least partially because of secondary muscle atrophy and fatty infiltration. β3 Adrenergic agonists are a group of drugs that promote fat resorption through "white fat browning" of intramuscular stem cells.

PURPOSE

To test the role of a β3 adrenergic receptor agonist, amibegron, in improving muscle quality and forelimb function in a delayed RC repair model via promoting brown/beige adipose tissue activation.

STUDY DESIGN

Controlled laboratory study.

METHODS

Three-month-old PDGFRα-GFP reporter mice, wild type C57BL/6J mice, and uncoupling protein 1 (UCP-1) knockout mice underwent unilateral supraspinatus tendon transection with a 6-week delayed tendon repair. Animals with sham surgery served as controls. Amibegron was given either immediately after tendon transection or after repair. Gait analysis was conducted to measure forelimb function at 6 weeks after tendon repair. Animals were sacrificed at 6 weeks after repair. Supraspinatus muscles were harvested and analyzed histologically. Reverse transcription polymerase chain reaction was performed to quantify gene expression related to atrophy, fibrosis, and fatty infiltration.

RESULTS

Histology of PDGFRα reporter mice showed significantly increased UCP-1 expression, suggesting white fat browning in muscle after RC repair. As administered either immediately after tendon transection or after tendon repair, amibegron significantly reduced muscle atrophy and fatty infiltration and resumed normal upper extremity gait in wild type mice. However, the effect of amibegron was not present in UCP-1 knockout mice, suggesting that the effect of amibegron in treating RC muscle atrophy and fatty infiltration is through a UCP 1-dependent mechanism.

CONCLUSION

Amibegron reduced muscle atrophy and fatty infiltration and improved forelimb function after delayed RC repair through a UCP 1-dependent mechanism. This may be an effective clinical treatment strategy for patients to improve muscle quality after RC repair.

CLINICAL RELEVANCE

β3 Adrenergic agonists may serve as a new pharmacologic modality to treat RC muscle atrophy and fatty infiltration to improve clinical outcome of RC repair.

Cardiac fibroblast diversity in health and disease

The cardiac stroma plays essential roles in health and following cardiac damage. The major player of the stroma with respect to extracellular matrix deposition, maintenance and remodeling is the poorly defined fibroblast. It has long been recognized that there is considerable variability to the fibroblast phenotype. With the advent of new, high throughput analytical methods our understanding and appreciation of this heterogeneity has grown dramatically. This review aims to explore the diversity of cardiac fibroblasts and highlights new insights into the diverse nature of these cells and their progenitors as revealed by single cell sequencing and fate mapping studies. We propose that at least in part the observed heterogeneity is related to the existence of a differentiation cascade within stromal cells. Beyond in-organ heterogeneity, we also discuss how the stromal response to damage differs between non-regenerating organs such as the heart and regenerating organs such as skeletal muscle. In exploring possible causes for these differences, we outline that although fibrogenic cells from different organs overlap in many properties, they still possess organ-specific transcriptional signatures and differentiation biases that make them functionally distinct.

Role of damage and management in muscle hypertrophy: Different behaviors of muscle stem cells in regeneration and hypertrophy

Skeletal muscle is a dynamic tissue with two unique abilities; one is its excellent regenerative ability, due to the activity of skeletal muscle-resident stem cells named muscle satellite cells (MuSCs); and the other is the adaptation of myofiber size in response to external stimulation, intrinsic factors, or physical activity, which is known as plasticity. Low physical activity and some disease conditions lead to the reduction of myofiber size, called atrophy, whereas hypertrophy refers to the increase in myofiber size induced by high physical activity or anabolic hormones/drugs. MuSCs are essential for generating new myofibers during regeneration and the increase in new myonuclei during hypertrophy; however, there has been little investigation of the molecular mechanisms underlying MuSC activation, proliferation, and differentiation during hypertrophy compared to those of regeneration. One reason is that 'degenerative damage' to myofibers during muscle injury or upon hypertrophy (especially overloaded muscle) is believed to trigger similar activation/proliferation of MuSCs. However, evidence suggests that degenerative damage of myofibers is not necessary for MuSC activation/proliferation during hypertrophy. When considering MuSC-based therapy for atrophy, including sarcopenia, it will be indispensable to elucidate MuSC behaviors in muscles that exhibit non-degenerative damage, because degenerated myofibers are not present in the atrophied muscles. In this review, we summarize recent findings concerning the relationship between MuSCs and hypertrophy, and discuss what remains to be discovered to inform the development and application of relevant treatments for muscle atrophy.

Adipogenesis of skeletal muscle fibro/adipogenic progenitors is affected by the WNT5a/GSK3/β-catenin axis

Fibro/Adipogenic Progenitors (FAPs) are muscle-interstitial progenitors mediating pro-myogenic signals that are critical for muscle homeostasis and regeneration. In myopathies, the autocrine/paracrine constraints controlling FAP adipogenesis are released causing fat infiltrates. Here, by combining pharmacological screening, high-dimensional mass cytometry and in silico network modeling with the integration of single-cell/bulk RNA sequencing data, we highlighted the canonical WNT/GSK/β-catenin signaling as a crucial pathway modulating FAP adipogenesis triggered by insulin signaling. Consistently, pharmacological blockade of GSK3, by the LY2090314 inhibitor, stabilizes β-catenin and represses PPARγ expression abrogating FAP adipogenesis ex vivo while limiting fatty degeneration in vivo. Furthermore, GSK3 inhibition improves the FAP pro-myogenic role by efficiently stimulating, via follistatin secretion, muscle satellite cell (MuSC) differentiation into mature myotubes. Combining, publicly available single-cell RNAseq datasets, we characterize FAPs as the main source of WNT ligands inferring their potential in mediating autocrine/paracrine responses in the muscle niche. Lastly, we identify WNT5a, whose expression is impaired in dystrophic FAPs, as a crucial WNT ligand able to restrain the detrimental adipogenic differentiation drift of these cells through the positive modulation of the β-catenin signaling.

Rotator Cuff Fibro-Adipogenic Progenitors Demonstrate Highest Concentration, Proliferative Capacity, and Adipogenic Potential Across Muscle Groups

Fatty infiltration (FI) of rotator cuff (RC) muscles is common in patients with RC tears. Studies have demonstrated that fibro-adipogenic progenitors (FAPs), a population of resident muscle stem cells, are the main contributors of FI, which adversely affects muscle quality and RC repair success. Although FI is common in RC injuries, it is not frequently reported after other musculotendinous injuries. Additionally, studies have shown the development of different pathology patterns across muscle groups suggestive of intrinsic differences in cellular composition and behavior. This study evaluates FAP distribution and differentiation properties across anatomic locations in mice. Muscles from seven different anatomic locations were harvested from PDGFRα-eGFP FAP reporter mice. FAPs were quantified using histology and FACS sorting with BD Aria II with CD31^- /CD45^- /Integrinα7^- /Sca-1⁺ and PDGFRα reporter signal (n = 3 per muscle). The cells were analyzed for adipogenesis using immunocytochemistry and for proliferation properties with Brdu-Ki67 staining. In a separate group of mice, RC and tibialis anterior muscles received glycerol injection and were harvested after 2 weeks for FI quantification (n = 4). One-way analysis of variance was used for statistical comparisons among groups, with significance at p < 0.05. FAPs from the RC, masseter, and paraspinal muscles were more numerous and demonstrated greater proliferative capacity and adipogenic potency than those from the tibialis anterior and gastrocnemius. The RC demonstrated significantly greater levels of FI than the tibialis anterior after glycerol-injection injury. Clinical Significance: This study suggests differences in FAP distribution and differentiation characteristics may account for the propensity to develop FI in RC tears as compared with other musculotendinous injuries. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1113-1121, 2020.

Beige FAPs Transplantation Improves Muscle Quality and Shoulder Function After Massive Rotator Cuff Tears

Rotator cuff (RC) tears are a common cause of upper extremity disability. Any tear size can result in subsequent muscle atrophy and fatty infiltration (FI). Preoperative muscle degeneration can predict repair and postoperative functional outcomes. Muscle residential fibro-adipogenic progenitors (FAPs) are found to be capable of differentiating into beige adipocytes that release factors to promote muscle growth. This study evaluated the regenerative potential of local cell transplantation of beige FAPs to mitigate muscle degeneration in a murine massive RC tear model. Beige FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺ /Sca1⁺ /PDGFR⁺ /CD31^- /CD45^- /integrin α7^- . C57/BL6J mice undergoing supraspinatus tendon tear with suprascapular nerve transection (TT + DN) received either no additional treatment, phosphate-buffered saline injection, or beige FAP injection 2 weeks after the initial injury. Forelimb gait analysis was used to assess shoulder function with DigiGait. Mice were sacrificed 6 weeks after cell transplantation. FI, fibrosis, fiber size, vascularity were analyzed and quantified via ImageJ. Our results showed that beige FAP transplantation significantly decreased fibrosis, FI, and atrophy, enhanced vascularization compared with saline injection and non-treatment groups. Beige FAP transplantation also significantly improved shoulder function as measured by gait analysis. This study suggests that beige-differentiated FAPs may serve as a treatment option for RC muscle atrophy and FI, thus improving shoulder function in patients with massive RC tendon tears. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1159-1166, 2020.

High-fat diet pre-conditioning improves microvascular remodelling during regeneration of ischaemic mouse skeletal muscle

AIM

Critical limb ischaemia (CLI) is characterized by inadequate angiogenesis, arteriolar remodelling and chronic myopathy, which are most severe in type 2 diabetic patients. Hypertriglyceridaemia, commonly observed in these patients, compromises macrovascular function. However, the effects of high-fat diet-induced increases in circulating lipids on microvascular remodelling are not established. Here, we investigated if high-fat diet would mimic the detrimental effect of type 2 diabetes on post-ischaemia vascular remodelling and muscle regeneration, using a mouse model of hindlimb ischaemia.

METHODS

Male C57Bl6/J mice were fed with normal or high-fat diets for 8 weeks prior to unilateral femoral artery ligation. Laser doppler imaging was used to assess limb perfusion recovery. Vascular recovery, inflammation, myofibre regeneration and fibrosis were assessed at 4 or 14 days post-ligation by histology and RNA analyses. Capillary-level haemodynamics were assessed by intravital microscopy of control and regenerating muscles 14 days post-ligation.

RESULTS

High-fat diet increased muscle succinate dehydrogenase activity and capillary-level oxygen supply. At 4 days post-ligation, no diet differences were detected in muscle damage, inflammatory infiltration or capillary activation. At 14 days post-ligation, high fat-fed mice displayed accelerated limb blood flow recovery, elevated capillary and arteriole densities as well as greater red blood cell supply rates and capillary-level oxygen supply. Regenerating muscles from high fat-fed mice displayed lower interstitial fat and collagen deposition.

CONCLUSION

The muscle-level adaptations to high-fat diet improved multiple aspects of muscle recovery in response to ischaemia and did not recapitulate the worse outcomes seen in diabetic CLI patients.

Adipose Stromal Cell Expansion and Exhaustion: Mechanisms and Consequences

Adipose tissue (AT) is comprised of a diverse number of cell types, including adipocytes, stromal cells, endothelial cells, and infiltrating leukocytes. Adipose stromal cells (ASCs) are a mixed population containing adipose progenitor cells (APCs) as well as fibro-inflammatory precursors and cells supporting the vasculature. There is growing evidence that the ability of ASCs to renew and undergo adipogenesis into new, healthy adipocytes is a hallmark of healthy fat, preventing disease-inducing adipocyte hypertrophy and the spillover of lipids into other organs, such as the liver and muscles. However, there is building evidence indicating that the ability for ASCs to self-renew is not infinite. With rates of ASC proliferation and adipogenesis tightly controlled by diet and the circadian clock, the capacity to maintain healthy AT via the generation of new, healthy adipocytes appears to be tightly regulated. Here, we review the contributions of ASCs to the maintenance of distinct adipocyte pools as well as pathogenic fibroblasts in cancer and fibrosis. We also discuss aging and diet-induced obesity as factors that might lead to ASC senescence, and the consequences for metabolic health.

Beige fibro-adipogenic progenitor transplantation reduces muscle degeneration and improves function in a mouse model of delayed repair of rotator cuff tears

BACKGROUND

Muscle atrophy and fatty infiltration (FI) are common occurrences following rotator cuff (RC) tears. Tears of all sizes are subject to muscle degeneration. The degree of muscle degeneration following RC tears is highly correlated with repair success and functional outcomes. We have recently discovered that muscle fibro-adipogenic progenitors (FAPs) can differentiate into uncoupling protein 1 (UCP-1)-expressing beige adipocytes and induce muscle regeneration. This study evaluated the potential of local cell transplantation of beige adipose FAPs (BAT-FAPs) to treat RC muscle degeneration in a murine model of RC repair.

METHODS

BAT-FAPs were isolated from muscle in UCP-1 reporter mice by flow cytometry as UCP-1⁺/Sca1⁺/PDGFR⁺/CD31^-/CD45^-/integrin α7^-. C57/BL6J mice underwent supraspinatus tendon tear with suprascapular nerve transection followed by repair 2 or 6 weeks after the initial injury. At the time of repair, mice received either no additional treatment, phosphate-buffered saline injection, or BAT-FAP injection. Functional outcomes were assessed by gait analysis. Mice were humanely killed at 6 weeks after cell transplantation. Supraspinatus muscle FI, fibrosis, muscle fiber size, and vascularity were analyzed and quantified via ImageJ. Analysis of variance with post hoc Tukey test and P <.05 was used to determine statistical significance.

RESULTS

Cell transplantation diminished fibrosis, FI, and atrophy and enhanced vascularization in both delayed repair models. Cell transplantation resulted in improved shoulder function as assessed with gait analysis in both the delayed repair models.

CONCLUSIONS

BAT-FAPs significantly reduced muscle degeneration and improved shoulder function after RC repair. BAT-FAPs hold significant promise as a therapeutic adjunct to repair for patients with advanced RC pathology.

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

Duchenne muscular dystrophy (DMD) is a chronic muscle disease characterized by poor myogenesis and replacement of muscle by extracellular matrix. Despite the shared genetic basis, severity of these deficits varies among patients. One source of these variations is the genetic modifier that leads to increased TGF-β activity. While anti-TGF-β therapies are being developed to target muscle fibrosis, their effect on the myogenic deficit is underexplored. Our analysis of in vivo myogenesis in mild (C57BL/10ScSn-mdx/J and C57BL/6J-mdxΔ52) and severe DBA/2J-mdx (D2-mdx) dystrophic models reveals no defects in developmental myogenesis in these mice. However, muscle damage at the onset of disease pathology, or by experimental injury, drives up TGF-β activity in the severe, but not in the mild, dystrophic models. Increased TGF-β activity is accompanied by increased accumulation of fibroadipogenic progenitors (FAPs) leading to fibro-calcification of muscle, together with failure of regenerative myogenesis. Inhibition of TGF-β signaling reduces muscle degeneration by blocking FAP accumulation without rescuing regenerative myogenesis. These findings provide in vivo evidence of early-stage deficit in regenerative myogenesis in D2-mdx mice and implicates TGF-β as a major component of a pathogenic positive feedback loop in this model, identifying this feedback loop as a therapeutic target.

Janus effect of glucocorticoids on differentiation of muscle fibro/adipogenic progenitors

Muscle resident fibro-adipogenic progenitors (FAPs), support muscle regeneration by releasing cytokines that stimulate the differentiation of myogenic stem cells. However, in non-physiological contexts (myopathies, atrophy, aging) FAPs cause fibrotic and fat infiltrations that impair muscle function. We set out to perform a fluorescence microscopy-based screening to identify compounds that perturb the differentiation trajectories of these multipotent stem cells. From a primary screen of 1,120 FDA/EMA approved drugs, we identified 34 compounds as potential inhibitors of adipogenic differentiation of FAPs isolated from the murine model (mdx) of Duchenne muscular dystrophy (DMD). The hit list from this screen was surprisingly enriched with compounds from the glucocorticoid (GCs) chemical class, drugs that are known to promote adipogenesis in vitro and in vivo. To shed light on these data, three GCs identified in our screening efforts were characterized by different approaches. We found that like dexamethasone, budesonide inhibits adipogenesis induced by insulin in sub-confluent FAPs. However, both drugs have a pro-adipogenic impact when the adipogenic mix contains factors that increase the concentration of cAMP. Gene expression analysis demonstrated that treatment with glucocorticoids induces the transcription of Gilz/Tsc22d3, an inhibitor of the adipogenic master regulator PPARγ, only in anti-adipogenic conditions. Additionally, alongside their anti-adipogenic effect, GCs are shown to promote terminal differentiation of satellite cells. Both the anti-adipogenic and pro-myogenic effects are mediated by the glucocorticoid receptor and are not observed in the presence of receptor inhibitors. Steroid administration currently represents the standard treatment for DMD patients, the rationale being based on their anti-inflammatory effects. The findings presented here offer new insights on additional glucocorticoid effects on muscle stem cells that may affect muscle homeostasis and physiology.

Stem Cell Aging in Skeletal Muscle Regeneration and Disease

Skeletal muscle comprises 30-40% of the weight of a healthy human body and is required for voluntary movements in humans. Mature skeletal muscle is formed by multinuclear cells, which are called myofibers. Formation of myofibers depends on the proliferation, differentiation, and fusion of muscle progenitor cells during development and after injury. Muscle progenitor cells are derived from muscle satellite (stem) cells (MuSCs), which reside on the surface of the myofiber but beneath the basement membrane. MuSCs play a central role in postnatal maintenance, growth, repair, and regeneration of skeletal muscle. In sedentary adult muscle, MuSCs are mitotically quiescent, but are promptly activated in response to muscle injury. Physiological and chronological aging induces MuSC aging, leading to an impaired regenerative capability. Importantly, in pathological situations, repetitive muscle injury induces early impairment of MuSCs due to stem cell aging and leads to early impairment of regeneration ability. In this review, we discuss (1) the role of MuSCs in muscle regeneration, (2) stem cell aging under physiological and pathological conditions, and (3) prospects related to clinical applications of controlling MuSCs.

Elucidating the Regulatory Role of Melatonin in Brown, White, and Beige Adipocytes

The high prevalence of obesity and its associated metabolic diseases has heightened the importance of understanding control of adipose tissue development and energy metabolism. In mammals, 3 types of adipocytes with different characteristics and origins have been identified: white, brown, and beige. Beige and brown adipocytes contain numerous mitochondria and have the capability to burn energy and counteract obesity, while white adipocytes store energy and are closely associated with metabolic disorders and obesity. Thus, regulation of the development and function of different adipocytes is important for controlling energy balance and combating obesity and related metabolic disorders. Melatonin is a neurohormone, which plays multiple roles in regulating inflammation, blood pressure, insulin actions, and energy metabolism. This article summarizes and discusses the role of melatonin in white, beige, and brown adipocytes, especially in affecting adipogenesis, inducing beige formation or white adipose tissue browning, enhancing brown adipose tissue mass and activities, improving anti-inflammatory and antioxidative effects, regulating adipokine secretion, and preventing body weight gain. Based on the current findings, melatonin is a potential therapeutic agent to control energy metabolism, adipogenesis, fat deposition, adiposity, and related metabolic diseases.

Hic1 deletion unleashes quiescent connective tissue stem cells and impairs skeletal muscle regeneration

Skeletal muscle fibro-adipogenic progenitors (FAPs) are tissue-resident connective tissue cells and the main cellular source of pathological fibro-fatty scar associated with muscle disorders. Although our knowledge about skeletal muscle mesenchymal progenitor cells has exploded in the past decade, we still lack information about their origin, fate, gene regulation, function, and stemness. A recent study by Underhill and colleagues, published in Cell Stem Cell, described the last census of Hic1 mesenchymal progenitor/stem cells in skeletal muscle regeneration, providing valuable results and data to the ever-expanding community of scientists interested in tissue regeneration and fibrosis. This commentary contextualizes and summarizes these exciting new findings.

Model systems for regeneration: the spiny mouse, Acomys cahirinus

The spiny mouse, Acomys spp., is a recently described model organism for regeneration studies. For a mammal, it displays surprising powers of regeneration because it does not fibrose (i.e. scar) in response to tissue injury as most other mammals, including humans, do. In this Primer article, we review these regenerative abilities, highlighting the phylogenetic position of the spiny mouse relative to other rodents. We also briefly describe the Acomys tissues that have been used for regeneration studies and the common features of their regeneration compared with the typical mammalian response. Finally, we discuss the contribution that Acomys has made in understanding the general principles of regeneration and elaborate hypotheses as to why this mammal is successful at regenerating.

Exercise enhances skeletal muscle regeneration by promoting senescence in fibro-adipogenic progenitors

Idiopathic inflammatory myopathies cause progressive muscle weakness and degeneration. Since high-dose glucocorticoids might not lead to full recovery of muscle function, physical exercise is also an important intervention, but some exercises exacerbate chronic inflammation and muscle fibrosis. It is unknown how physical exercise can have both beneficial and detrimental effects in chronic myopathy. Here we show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise-induced muscle damage is needed to establish a state of regenerative inflammation that induces muscle regeneration. In chronic inflammatory myopathy model mice, exercise does not promote FAP senescence or resistance against tumor necrosis factor–mediated apoptosis. Pro-senescent intervention combining exercise and pharmacological AMPK activation reverses FAP apoptosis resistance and improves muscle function and regeneration. Our results demonstrate that the absence of FAP senescence after exercise leads to muscle degeneration with FAP accumulation. FAP-targeted pro-senescent interventions with exercise and pharmacological AMPK activation may constitute a therapeutic strategy for chronic inflammatory myopathy.

Some exercises exacerbate chronic inflammation and muscle fibrosis in chronic myopathy. Here, the authors show that senescence of fibro-adipogenic progenitors (FAPs) in response to exercise induces muscle regeneration, and impaired FAP senescence worsens inflammation and fibrosis in chronic myopathy in mice.

Skeletal muscle in health and disease

Skeletal muscle fibres are multinucleated cells that contain postmitotic nuclei (i.e. they are no longer able to divide) and perform muscle contraction. They are formed by fusion of muscle precursor cells, and grow into elongating myofibres by the addition of further precursor cells, called satellite cells, which are also responsible for regeneration following injury. Skeletal muscle regeneration occurs in most muscular dystrophies in response to necrosis of muscle fibres. However, the complex environment within dystrophic skeletal muscle, which includes inflammatory cells, fibroblasts and fibro-adipogenic cells, together with the genetic background of the in vivo model and the muscle being studied, complicates the interpretation of laboratory studies on muscular dystrophies. Many genes are expressed in satellite cells and in other tissues, which makes it difficult to determine the molecular cause of various types of muscular dystrophies. Here, and in the accompanying poster, we discuss our current knowledge of the cellular mechanisms that govern the growth and regeneration of skeletal muscle, and highlight the defects in satellite cell function that give rise to muscular dystrophies.

Summary: The mechanisms of skeletal muscle development, growth and regeneration are described. We discuss whether these processes are dysregulated in inherited muscle diseases and identify pathways that may represent therapeutic targets.

Single-cell revolution unveils the mysteries of the regenerative mammalian digit tip

The digit tip is an exciting model for studying regeneration in mammals, but the precise mechanisms and the populations of cells involved in the formation and remodeling of the blastema remain unknown. In an exciting new work, Storer et al. take advantage of single-cell RNAseq combined with Pdgfra+ ​lineage-tracing to open the way into the enigmatic world of mammalian tissue regeneration.

Metabolic reprogramming of fibro/adipogenic progenitors facilitates muscle regeneration

High-fat diet ameliorates muscle dystrophic phenotype by promoting the FAP-dependent myogenesis of satellite cells.

In Duchenne muscular dystrophy (DMD), the absence of the dystrophin protein causes a variety of poorly understood secondary effects. Notably, muscle fibers of dystrophic individuals are characterized by mitochondrial dysfunctions, as revealed by a reduced ATP production rate and by defective oxidative phosphorylation. Here, we show that in a mouse model of DMD (mdx), fibro/adipogenic progenitors (FAPs) are characterized by a dysfunctional mitochondrial metabolism which correlates with increased adipogenic potential. Using high-sensitivity mass spectrometry–based proteomics, we report that a short-term high-fat diet (HFD) reprograms dystrophic FAP metabolism in vivo. By combining our proteomic dataset with a literature-derived signaling network, we revealed that HFD modulates the β-catenin–follistatin axis. These changes are accompanied by significant amelioration of the histological phenotype in dystrophic mice. Transplantation of purified FAPs from HFD-fed mice into the muscles of dystrophic recipients demonstrates that modulation of FAP metabolism can be functional to ameliorate the dystrophic phenotype. Our study supports metabolic reprogramming of muscle interstitial progenitor cells as a novel approach to alleviate some of the adverse outcomes of DMD.

Role of Metabolic Stress and Exercise in Regulating Fibro/Adipogenic Progenitors

Obesity is a major public health concern and is associated with decreased muscle quality (i.e., strength, metabolism). Muscle from obese adults is characterized by increases in fatty, fibrotic tissue that decreases the force producing capacity of muscle and impairs glucose disposal. Fibro/adipogenic progenitors (FAPs) are muscle resident, multipotent stromal cells that are responsible for muscle fibro/fatty tissue accumulation. Additionally, they are indirectly involved in muscle adaptation through their promotion of myogenic (muscle-forming) satellite cell proliferation and differentiation. In conditions similar to obesity that are characterized by chronic muscle degeneration, FAP dysfunction has been shown to be responsible for increased fibro/fatty tissue accumulation in skeletal muscle, and impaired satellite cell function. The role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle is just beginning to be unraveled. Thus, the present review aims to summarize the recent literature on the role of metabolic stress in regulating FAP differentiation and paracrine function in skeletal muscle, and the mechanisms responsible for these effects. Furthermore, we will review the role of physical activity in reversing or ameliorating the detrimental effects of obesity on FAP function.

Role of noncanonical Wnt ligands and Ror-family receptor tyrosine kinases in the development, regeneration, and diseases of the musculoskeletal system

The Ror-family receptor tyrosine kinases (RTKs), consisting of Ror1 and Ror2, play crucial roles in morphogenesis and formation of various tissues/organs, including the bones and skeletal muscles, the so-called musculoskeletal system, during embryonic development, by acting as receptors or coreceptors for a noncanonical Wnt protein Wnt5a. Furthermore, several lines of evidence have indicated that Ror1 and/or Ror2 play critical roles in the regeneration and maintenance of the musculoskeletal system in adults. Considering the anatomical and functional relationship between the skeleton and skeletal muscles, their structural and functional association might be tightly regulated during their embryonic development, development after birth, and their regeneration after injury in adults. Importantly, in addition to their congenital anomalies, much attention has been paid onto the age-related disorders of the musculoskeletal system, including osteopenia and sarcopenia, which affect severely the quality of life. In this article, we overview recent advances in our understanding of the roles of Ror1- and/or Ror2-mediated signaling in the embryonic development, regeneration in adults, and congenital and age-related disorders of the musculoskeletal system and discuss possible therapeutic approaches to locomotive syndromes by modulating Ror1- and/or Ror2-mediated signaling.