Regulation Of Muscle Regeneration Research Articles

Characterization of Age-Dependent Changes in Skeletal Muscle Repair and Regeneration Using a Mouse Model of Acute Muscle Injury.

Acute skeletal muscle injury initiates a process of necrosis, debris clearance, and ultimately tissue regeneration via myogenesis. While skeletal muscle stem cells (MuSCs) are responsible for populating the proliferative myogenic progenitor pool to fuel muscle repair, recruited and resident immune cells have a central role in the regulation of muscle regeneration via the execution of phagocytosis and release of soluble factors that act directly on MuSCs to regulate myogenic differentiation. Therefore, the timing of MuSC proliferation and differentiation is closely linked to the populations and behaviors of immune cells present within skeletal muscle. This has important implications for aging andmuscle repair, as systemic changes in immune system function contribute to a decline in muscle regenerative capacity. Here, we present adapted protocols for the isolation of mononuclear cells from skeletal muscles for the quantification of immune cell populations using flow cytometry. We also describe a cardiotoxin skeletal muscle injury protocol and detail the expected outcomes including immune cell infiltration to the injured sites and formation of new myocytes. As immune cell function is substantially influenced by aging, we extend these approaches and outcomes to aged mice.

Matrix mechanics regulates muscle regeneration by modulating kinesin-1 activity

Sarcopenia, a prevalent muscle disease characterized by muscle mass and strength reduction, is associated with impaired skeletal muscle regeneration. However, the influence of the biomechanical properties of sarcopenic skeletal muscle on the efficiency of the myogenic program remains unclear. Herein, we established a mouse model of sarcopenia and observed a reduction in stiffness within the sarcopenic skeletal muscle in vivo. To investigate whether the biomechanical properties of skeletal muscle directly impact the myogenic program, we established an in vitro system to explore the intrinsic mechanism involving matrix stiffness control of myogenic differentiation. Our findings identify the microtubule motor protein, kinesin-1, as a mechano-transduction hub that senses and responds to matrix stiffness, crucial for myogenic differentiation and muscle regeneration. Specifically, kinesin-1 activity is positively regulated by stiff matrices, facilitating its role in transporting mitochondria and enhancing translocation of the glucose transporter GLUT4 to the cell surface for glucose uptake. Conversely, the softer matrices significantly suppress kinesin-1 activity, leading to the accumulation of mitochondria around nuclei and hindering glucose uptake by inhibiting GLUT4 membrane translocation, consequently impairing myogenic differentiation. The insights gained from the in-vitro system highlight the mechano-transduction significance of kinesin-1 motor proteins in myogenic differentiation. Furthermore, our study confirms that enhancing kinesin-1 activity in the sarcopenic mouse model restores satellite cell expansion, myogenic differentiation, and muscle regeneration. Taken together, our findings provide a potential target for improving muscle regeneration in sarcopenia.

Skeletal muscle regeneration failure in ischemic-damaged limbs is associated with pro-inflammatory macrophages and premature differentiation of satellite cells

BackgroundChronic limb-threatening ischemia (CLTI), a severe manifestation of peripheral arterial disease (PAD), is associated with a 1-year limb amputation rate of approximately 15–20% and substantial mortality. A key feature of CLTI is the compromised regenerative ability of skeletal muscle; however, the mechanisms responsible for this impairment are not yet fully understood. In this study, we aim to delineate pathological changes at both the cellular and transcriptomic levels, as well as in cell–cell signaling pathways, associated with compromised muscle regeneration in limb ischemia in both human tissue samples and murine models of CLTI.MethodsWe performed single-cell transcriptome analysis of ischemic and non-ischemic muscle from the same CLTI patients and from a murine model of CLTI. In both datasets, we analyzed gene expression changes in macrophage and muscle satellite cell (MuSC) populations as well as differential cell–cell signaling interactions and differentiation trajectories.ResultsSingle-cell transcriptomic profiling and immunofluorescence analysis of CLTI patient skeletal muscle demonstrated that ischemic-damaged tissue displays a pro-inflammatory macrophage signature. Comparable results were observed in a murine CLTI model. Moreover, integrated analyses of both human and murine datasets revealed premature differentiation of MuSCs to be a key feature of failed muscle regeneration in the ischemic limb. Furthermore, in silico inferences of intercellular communication and in vitro assays highlight the importance of macrophage-MuSC signaling in ischemia induced muscle injuries.ConclusionsCollectively, our research provides the first single-cell transcriptome atlases of skeletal muscle from CLTI patients and a murine CLTI model, emphasizing the crucial role of macrophages and inflammation in regulating muscle regeneration in CLTI through interactions with MuSCs.

Chloroplast transformation for bioencapsulation and oral delivery using the immunoglobulin G fragment crystallizable (Fc) domain

Proinsulin Like Growth Factor I (prolGF-I) and myostatin (Mstn) regulate muscle regeneration and mass when intravenously delivered. We tested if chloroplast bioencapsulated forms of these proteins may serve as a non-invasive means of drug delivery through the digestive system. We created tobacco (Nicotiana tabacum) plants carrying GFP-Fc1, proIGF-I-Fc1, and Mstn-Fc1 fusion genes, in which fusion with the immunoglobulin G Fc domain improved both protein stability and absorption in the small intestine. No transplastomic plants were obtained with the Mstn-Fc1 gene, suggesting that the protein is toxic to plant cells. proIGF-I-Fc1 protein levels were too low to enable in vivo testing. However, GFP-Fc1 accumulated at a high level, enabling evaluation of chloroplast-made Fc fusion proteins for oral delivery. Tobacco leaves were lyophilized for testing in a mouse system. We report that the orally administered GFP-Fc1 fusion protein (5.45 µg/g GFP-Fc1) has been taken up by the intestinal epithelium cells, evidenced by confocal microscopy. GFP-Fc1 subsequently entered the circulation where it was detected by ELISA. Data reported here confirm that chloroplast expression and oral administration of lyophilized leaves is a potential delivery system of therapeutic proteins fused with Fc1, with the advantage that the proteins may be stored at room temperature.

Biomanufacturing Recombinantly Expressed Cripto-1 Protein in Anchorage-Dependent Mammalian Cells Growing in Suspension Bioreactors within a Three-Dimensional Hydrogel Microcarrier.

Biotherapeutic soluble proteins that are recombinantly expressed in mammalian cells can pose a challenge when biomanufacturing in three-dimensional (3D) suspension culture systems. Herein, we tested a 3D hydrogel microcarrier for a suspension culture of HEK293 cells overexpressing recombinant Cripto-1 protein. Cripto-1 is an extracellular protein that is involved in developmental processes and has recently been reported to have therapeutic effects in alleviating muscle injury and diseases by regulating muscle regeneration through satellite cell progression toward the myogenic lineage. Cripto-overexpressing HEK293 cell lines were cultured in microcarriers made from poly (ethylene glycol)-fibrinogen (PF) hydrogels, which provided the 3D substrate for cell growth and protein production in stirred bioreactors. The PF microcarriers were designed with sufficient strength to resist hydrodynamic deterioration and biodegradation associated with suspension culture in stirred bioreactors for up to 21 days. The yield of purified Cripto-1 obtained using the 3D PF microcarriers was significantly higher than that obtained with a two-dimensional (2D) culture system. The bioactivity of the 3D-produced Cripto-1 was equivalent to commercially available Cripto-1 in terms of an ELISA binding assay, a muscle cell proliferation assay, and a myogenic differentiation assay. Taken together, these data indicate that 3D microcarriers made from PF can be combined with mammalian cell expression systems to improve the biomanufacturing of protein-based therapeutics for muscle injuries.

Muscle Regeneration in Holothurians without the Upregulation of Muscle Genes.

The holothurian Eupentacta fraudatrix is capable of fully restoring its muscles after transverse dissection. Although the regeneration of these structures is well studied at the cellular level, the molecular basis of the process remains poorly understood. To identify genes that may be involved in the regulation of muscle regeneration, the transcriptome of the longitudinal muscle band of E. fraudatrix has been sequenced at different time periods post-injury. An analysis of the map of biological processes and pathways has shown that most genes associated with myogenesis decrease their expression during the regeneration. The only exception is the genes united by the GO term "heart valve development". This may indicate the antiquity of mechanisms of mesodermal structure transformation, which was co-opted into various morphogeneses in deuterostomes. Two groups of genes that play a key role in the regeneration have been analyzed: transcription factors and matrix metalloproteinases. A total of six transcription factor genes (Ef-HOX5, Ef-ZEB2, Ef-RARB, Ef-RUNX1, Ef-SOX17, and Ef-ZNF318) and seven matrix metalloproteinase genes (Ef-MMP11, Ef-MMP13, Ef-MMP13-1, Ef-MMP16-2, Ef-MMP16-3, Ef-MMP24, and Ef-MMP24-1) showing differential expression during myogenesis have been revealed. The identified genes are assumed to be involved in the muscle regeneration in holothurians.

OR24-1 Paracrine Growth Factor Signaling Regulates Muscle Regeneration in Aged Mice via microRNA 501

Abstract Background Skeletal muscle is one of to the most dynamic and plastic tissues. The capacity to regenerate arises from adult muscle stem cells termed myogenic progenitors (MPs) that are activated in response to injury and differentiate to replenish damaged muscle fibers. The age-related loss of muscle mass is reflected in the reduced regenerative potential of MPs. The underlying mechanisms are still unclear and insights into alterations in adult muscle stem cells during ageing are urgently needed. We have previously identified a novel muscle-specific microRNA, miR-501-3p, that is enriched in activated MPs. Pharmacological inhibition of miR-501 during muscle regeneration promoted small-diameter neofibers. Here, we provide evidence for the regulation of miR-501 in skeletal muscle during aging and its effect on newly formed myofibers using genetic deletion in mice. Methods We analysed the regulation and upstream signalling pathways of miR-501 in primary myoblasts from mice and humans and in skeletal muscle from young and aged mice. We generated a novel Cre-loxP mouse model, which allowed for global and MP-specific deletion of miR-501. Skeletal muscle regeneration was induced using cardiotoxin (CTX) and assessed using immunofluorescence and transcriptomic analysis. Results miR-501 was upregulated in muscle cells by the paracrine growth factors platelet derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) through the Janus tyrosine kinase (JAK)-signal transducer and activator of transcription-3 (STAT3) signalling axis. Expression levels of miR-501 as well as its upstream regulators Pdgf and Vegf were significantly reduced in skeletal muscle from aged mice (22 months) compared to young mice (3 months) by 67.8% (p<0.001), 40.0% (p<0.01) and 36.3% (p<0.05), respectively. Genetic deletion of miR-501 in mice resulted in muscle fibers with decreased size in adult skeletal muscle and in newly formed myofibers during muscle regeneration. RNA-seq of regenerating muscle annotated a critical role for miR-501 in the organisation of muscle filaments. We identified a miR-501-dependent sarcomeric gene signature that involves genes such as myosin heavy chains, titin and tubulin polymerization-promoting protein. This was confirmed in vitro in myotubes from mice and humans in which miR-501 was deleted either genetically or pharmacologically using antagomirs. Altered sarcomeric gene expression correlated with an increased sarcomere length in myotubes lacking miR-501 (2.49 ± 0.08μm vs 3.22 ± 0.15μm, p<0.01). Conclusion Our data indicate that paracrine growth factor signalling contributes to muscle fiber formation via miR-501 in myogenic progenitor cells and its effect on the sarcomere. Restoring the Pdgf+Vegf/miR-501 axis might have the potential to improve muscle formation during aging. Presentation: Monday, June 13, 2022 11:00 a.m. - 11:15 a.m.

Role of irisin in physiology and pathology.

Irisin, out-membrane part of fibronectin type III domain–containing 5 protein (FNDC5), was activated by Peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) during physical exercise in skeletal muscle tissues. Most studies have reported that the concentration of irisin is highly associated with health status. For instance, the level of irisin is significantly lower in patients with obesity, osteoporosis/fractures, muscle atrophy, Alzheimer’s disease, and cardiovascular diseases (CVDs) but higher in patients with cancer. Irisin can bind to its receptor integrin αV/β5 to induce browning of white fat, maintain glucose stability, keep bone homeostasis, and alleviate cardiac injury. However, it is unclear whether it works by directly binding to its receptors to regulate muscle regeneration, promote neurogenesis, keep liver glucose homeostasis, and inhibit cancer development. Supplementation of recombinant irisin or exercise-activated irisin might be a successful strategy to fight obesity, osteoporosis, muscle atrophy, liver injury, and CVDs in one go. Here, we summarize the publications of FNDC5/irisin from PubMed/Medline, Scopus, and Web of Science until March 2022, and we review the role of FNDC5/irisin in physiology and pathology.

Sex-dependent role of Pannexin 1 in regulating skeletal muscle and satellite cell function.

The development and regeneration of skeletal muscle are mediated by satellite cells (SCs), which ensure the efficient formation of myofibers while repopulating the niche that allows muscle repair following injuries. Pannexin 1 (Panx1) channels are expressed in SCs and their levels increase during differentiation in vitro, as well as during skeletal muscle development and regeneration in vivo. Panx1 has recently been shown to regulate muscle regeneration by promoting bleb-based myoblast migration and fusion. While skeletal muscle is largely influenced in a sex-specific way, the sex-dependent roles of Panx1 in regulating skeletal muscle and SC function remain to be investigated. Here, using global Panx1 knockout (KO) mice, we demonstrate that Panx1 loss reduces muscle fiber size and strength, decreases SC number, and alters early SC differentiation and myoblast fusion in male, but not in female mice. Interestingly, while both male and female Panx1 KO mice display an increase in the number of regenerating fibers following acute injury, the newly formed fibers in male Panx1 KO mice are smaller. Overall, our results demonstrate that Panx1 plays a significant role in regulating muscle development, regeneration, and SC number and function in male mice and reveal distinct sex-dependent functions of Panx1 in skeletal muscle.

Sirt2 positively regulates muscle regeneration after Notexin-induced muscle injury

Sirt2 regulates various biological processes by deacetylating target genes. Despite roles in regulating proliferation, cell cycle, and glucose metabolism, which are closely associated with skeletal muscle physiology, Sirt2 functions in this tissue remain unclear. In this study, genetic deletion of Sirt2 delayed muscle regeneration after Notexin-induced muscle injury. Gene expressions of myogenic regulatory factors, including Myf5, MyoD, and Myogenin, and cell cycle regulators, such as cyclin D1 and CDK2, were repressed in Sirt2 knockout mice after injury. Also, Sirt2 knockout mice presented muscle atrophy after muscle injury which is associated with the down-regulation of anabolic signaling and the up-regulation of catabolic signaling, in particular, increased atrogin1 transcriptional expression. Thus, Sirt2 positively regulated skeletal muscle regeneration after muscle injury by regulating transcriptional expression involved in myogenesis, cell cycle, and anabolic and catabolic signaling. Based on the in vivo analyses, Sirt2 could function as an interventional therapeutic for chronic myopathy, which is characterized by impaired muscle regeneration and muscle atrophy.

Autophagy in muscle regeneration: potential therapies for myopathies.

Autophagy classically functions as a physiological process to degrade cytoplasmic components, protein aggregates, and/or organelles, as a mechanism for nutrient breakdown, and as a regulator of cellular architecture. Its biological functions include metabolic stress adaptation, stem cell differentiation, immunomodulation and diseases regulation, and so on. Current researches have proved that autophagy dysfunction may contribute to the pathogenesis of some myopathies through impairment of myofibres regeneration. Studies of autophagy inhibition also indicate the importance of autophagy in muscle regeneration, while activation of autophagy can restore muscle function in some myopathies. In this review, we aim to report the mechanisms of action of autophagy on muscle regeneration to provide relevant references for the treatment of regenerating defective myopathies by regulating autophagy. Results have shown that one key mechanism of autophagy regulating the muscle regeneration is to affect the differentiation fate of muscle stem cells (MuSCs), including quiescence maintenance, activation and differentiation. The roles of autophagy (organelle/protein degradation, energy facilitation, and/or other) vary at different myogenic stages of the repair process. When the muscle is in homeostasis, basal autophagy can maintain the quiescence state and stemness of MuSCs by renewing organelle and protein. After injury, the increased autophagy flux contributes to meet biological energy demand of MuSCs during activation and proliferation. By mitochondrial remodelling, autophagy during differentiation can promote the metabolic transformation and balance mitochondrial‐mediated apoptosis signals in myoblasts. Autophagy in mature myofibres is also essential for the degradation of necrotic myofibres, and may affect the dynamics of MuSCs by affecting the secretion spectrum of myofibres or the recruitment of supporting cells. Except for myogenic cells, autophagy also plays an important role in regulating the function of non‐myogenic cells in the muscle microenvironment, which is also essential for successful muscle recovery. Autophagy can regulate the immune microenvironment during muscle regeneration through the recruitment and polarization of macrophages, while autophagy in endothelial cells can regulate muscle regeneration in an angiogenic or angiogenesis‐independent manner. Drug or nutrition targeted autophagy has been preliminarily proved to restore muscle function in myopathies by promoting muscle regeneration, and further understanding the role and mechanism of autophagy in various cell types during muscle regeneration will enable more effective combinatorial therapeutic strategies.

TLR4 activation inhibits the proliferation and osteogenic differentiation of skeletal muscle stem cells by downregulating LGI1.

Skeletal muscle stem cells (SMSCs) are vital to the growth, maintenance, and repair of the muscles; emerging evidence has indicated that Toll-like receptor 4 (TLR4) can potentially regulate muscle regeneration. In present study, in vitro and in vivo experiments were performed to explore the correlation of TLR4 with leucine-rich glioma-inactivated 1 (LGI1) as well as their effects on the proliferation and osteogenesis potential of SMSCs. In order to examine the regulatory mechanisms of TLR4 and LGI1 in SMSCs, the obtained cells were treated with lipopolysaccharide (LPS, used as an activator of TLR4) of different concentration at different time points as well as the siRNA against LGI1. Subsequently, a series of detection was undertaken in order to measure the proliferation and differentiation potential of SMSCs, which involved detection of the related factors, cell activity, and the sphere-forming capability. Following LPS treatment, the increased TLR4 expression and reduced LGI1 expression were observed. Consequently, we also discovered that Erk signaling pathway was inactivated and cell proliferation and osteogenesis capabilities declined, presented by the downregulation of related factors such as cyclin B1 and runt-related transcription factor 2. Moreover, the cell activity and sphere-formation performance of SMSCs were also declined. These results were also validated in rats with cecal ligation and perforation-induced rat models with sepsis. In conclusion, the present study reveals a regulatory mechanism in SMSCs whereby LGI1 expression is reduced by TLR4, thus impeding cell proliferation and osteogenesis, highlighting TLR4 as a potential therapeutic target against many diseases related to SMSCs.

Female Outperformance in Voluntary Running Persists in Dystrophin-Null and Klotho-Overexpressing Mice.

Duchenne muscular dystrophy is a degenerative muscle disease that results from impairment of the dystrophin gene. The disease causes progressive loss in muscle mass and function. The anti-aging protein, α-klotho, has been implicated in the regulation of muscle regeneration. We previously discovered that mice harboring reduced α-klotho levels exhibited a decline in muscle strength and running endurance. To investigate the ability of α-klotho to improve overall endurance in a dystrophin null murine model, we examined the voluntary wheel running performance of dystrophin-null, mdx4cv mice overexpressing an α-klotho transgene. As expected, compared to wild type, both male and female dystrophic mice exhibited reduced running ability that was characterized by shorter running duration and longer periods of rest between cycles of activity. While our results did not detect an improvement in running performance with α-klotho overexpression, we identified distinct differences in the running patterns between females and males from all mouse strains analyzed (i.e., mdx4cv, mdx4cv overexpressing α-klotho, α-klotho overexpressing, α-klotho hypomorph, and wild type). For all strains, male mice displayed significantly reduced voluntary running ability compared to females. Further analysis of the mdx4cv strains demonstrated that male mice ran for shorter lengths of time and took longer breaks. However, we did not identify gender-associated differences in the actual speed at which mdx4cv mice ran. Our data suggest key differences in the running capabilities of female and male mice, which are of particular relevance to studies of dystrophin-null mice.

Evolutionary Characterization of the Short Protein SPAAR.

Microproteins (<100 amino acids) are receiving increasing recognition as important participants in numerous biological processes, but their evolutionary dynamics are poorly understood. SPAAR is a recently discovered microprotein that regulates muscle regeneration and angiogenesis through interactions with conserved signaling pathways. Interestingly, SPAAR does not belong to any known protein family and has known homologs exclusively among placental mammals. This lack of distant homology could be caused by challenges in homology detection of short sequences, or it could indicate a recent de novo emergence from a noncoding sequence. By integrating syntenic alignments and homology searches, we identify SPAAR orthologs in marsupials and monotremes, establishing that SPAAR has existed at least since the emergence of mammals. SPAAR shows substantial primary sequence divergence but retains a conserved protein structure. In primates, we infer two independent evolutionary events leading to the de novo origination of 5′ elongated isoforms of SPAAR from a noncoding sequence and find evidence of adaptive evolution in this extended region. Thus, SPAAR may be of ancient origin, but it appears to be experiencing continual evolutionary innovation in mammals.

BMSC-Derived Exosomes Inhibit Dexamethasone-Induced Muscle Atrophy via the miR-486-5p/FoxO1 Axis.

Sarcopenia, characterized by reduced muscle function as well as muscle mass, has been a public health problem with increasing prevalence. It might result from aging, injury, hormone imbalance and other catabolic conditions. Recently, exosomes were considered to regulate muscle regeneration and protein synthesis. In order to confirm the effect of BMSC-derived exosomes (BMSC-Exos) on muscle, dexamethasone-induced muscle atrophy was built both in vitro and in vivo. In the present research, BMSC-Exos attenuated the decrease of myotube diameter induced by dexamethasone, indicating that BMSC-Exos played a protective role in skeletal muscle atrophy. Further mechanism analysis exhibited that the content of miR-486-5p in C2C12 myotubes was up-regulated after treated with BMSC-Exos. Meanwhile, BMSC-Exos markedly downregulated the nuclear translocation of FoxO1, which plays an important role in muscle differentiation and atrophy. Importantly, the miR-486-5p inhibitor reversed the decreased expression of FoxO1 induced by BMSC-Exos. In animal experiments, BMSC-Exos inhibited dexamethasone-induced muscle atrophy, and miR-486-5p inhibitor reversed the protective effect of BMSC-Exos. These results indicating that BMSC-derived exosomes inhibit dexamethasone-induced muscle atrophy via miR486-5p/Foxo1 Axis.

Delineating the relationship between immune system aging and myogenesis in muscle repair.

Recruited immune cells play a critical role in muscle repair, in part by interacting with local stem cell populations to regulate muscle regeneration. How aging affects their communication during myogenesis is unclear. Here, we investigate how aging impacts the cellular function of these two cell types after muscle injury during normal aging or after immune rejuvenation using a young to old (Y‐O) or old to old (O‐O) bone marrow (BM) transplant model. We found that skeletal muscle from old mice (20 months) exhibited elevated basal inflammation and possessed fewer satellite cells compared with young mice (3 months). After cardiotoxin muscle injury (CTX), old mice exhibited a blunted inflammatory response compared with young mice and enhanced M2 macrophage recruitment and IL‐10 expression. Temporal immune and cytokine responses of old mice were partially restored to a young phenotype following reconstitution with young cells (Y‐O chimeras). Improved immune responses in Y‐O chimeras were associated with greater satellite cell proliferation compared with O‐O chimeras. To identify how immune cell aging affects myoblast function, conditioned media (CM) from activated young or old macrophages was applied to cultured C2C12 myoblasts. CM from young macrophages inhibited myogenesis while CM from old macrophages reduced proliferation. These functional differences coincided with age‐related differences in macrophage cytokine expression. Together, this study examines the infiltration and proliferation of immune cells and satellite cells after injury in the context of aging and, using BM chimeras, demonstrates that young immune cells retain cell autonomy in an old host to increase satellite cell proliferation.

Nogo-A regulates myogenesis via interacting with Filamin-C

Among the three isoforms encoded by Rtn4, Nogo-A has been intensely investigated as a central nervous system inhibitor. Although Nogo-A expression is increased in muscles of patients with amyotrophic lateral sclerosis, its role in muscle homeostasis and regeneration is not well elucidated. In this study, we discovered a significant increase in Nogo-A expression in various muscle-related pathological conditions. Nogo−/− mice displayed dystrophic muscle structure, dysregulated muscle regeneration following injury, and altered gene expression involving lipid storage and muscle cell differentiation. We hypothesized that increased Nogo-A levels might regulate muscle regeneration. Differentiating myoblasts exhibited Nogo-A upregulation and silencing Nogo-A abrogated myoblast differentiation. Nogo-A interacted with filamin-C, suggesting a role for Nogo-A in cytoskeletal arrangement during myogenesis. In conclusion, Nogo-A maintains muscle homeostasis and integrity, and pathologically altered Nogo-A expression mediates muscle regeneration, suggesting Nogo-A as a novel target for the treatment of myopathies in clinical settings.

Aging of the immune system and impaired muscle regeneration: A failure of immunomodulation of adult myogenesis.

Skeletal muscle regeneration that follows acute injury is strongly influenced by interactions with immune cells that invade and proliferate in the damaged tissue. Discoveries over the past 20years have identified many of the key mechanisms through which myeloid cells, especially macrophages, regulate muscle regeneration. In addition, lymphoid cells that include CD8+ T-cells and regulatory T-cells also significantly affect the course of muscle regeneration. During aging, the regenerative capacity of skeletal muscle declines, which can contribute to progressive loss of muscle mass and function. Those age-related reductions in muscle regeneration are accompanied by systemic, age-related changes in the immune system, that affect many of the myeloid and lymphoid cell populations that can influence muscle regeneration. In this review, we present recent discoveries that indicate that aging of the immune system contributes to the diminished regenerative capacity of aging muscle. Intrinsic, age-related changes in immune cells modify their expression of factors that affect the function of a population of muscle stem cells, called satellite cells, that are necessary for normal muscle regeneration. For example, age-related reductions in the expression of growth differentiation factor-3 (GDF3) or CXCL10 by macrophages negatively affect adult myogenesis, by disrupting regulatory interactions between macrophages and satellite cells. Those changes contribute to a reduction in the numbers and myogenic capacity of satellite cells in old muscle, which reduces their ability to restore damaged muscle. In addition, aging produces changes in the expression of molecules that regulate the inflammatory response to injured muscle, which also contributes to age-related defects in muscle regeneration. For example, age-related increases in the production of osteopontin by macrophages disrupts the normal inflammatory response to muscle injury, resulting in regenerative defects. These nascent findings represent the beginning of a newly-developing field of investigation into mechanisms through which aging of the immune system affects muscle regeneration.

Basement membrane recovery process in rat soleus muscle after exercise-induced muscle injury

ABSTRACT Purpose: Collagen IV is a component of the basement membrane (BM) that provides mechanical support for muscle fibers. In addition, transcription factor 4 (TCF4) is highly expressed in muscle connective tissue fibroblasts and regulates muscle regeneration. However, the expression of collagen IV and TCF4 (+) cells in response to exercise-induced muscle injury is not well known. Here, we investigated the expression and localization of collagen IV and TCF4 (+) cells during the recovery process after muscle injury induced by different exercise loads. Materials and Methods: Muscle injury was observed in the soleus muscle of young Wistar rats after 12 or 18 sets—downhill running (DR) on a treadmill. After running, the rats were permitted to recover for a period of 0.5 days, 2 days, or 7 days. Results: Ectopic localization of collagen IV in injured muscle fibers was observed after DR, and the number increased at 0.5 days after 18 sets DR and at 2 days after 12 or 18 sets DR as compared to the number observed at baseline. BM disruption was observed after DR. TCF4 (+) cells appeared in the inside and around injured muscle fibers at 0.5 day of recovery. After 18 sets DR, TCF4 (+) cells were more abundant for a longer period than that observed after 12 sets DR. Conclusions: DR induces BM disruption accompanied by muscle fiber damage. It is possible that BM destruction may be accompanied by muscle damage and that TCF4 (+) cells contribute to muscle fiber and BM recovery.

Mechanisms Regulating Muscle Regeneration: Insights into the Interrelated and Time-Dependent Phases of Tissue Healing.

Despite a massive body of knowledge which has been produced related to the mechanisms guiding muscle regeneration, great interest still moves the scientific community toward the study of different aspects of skeletal muscle homeostasis, plasticity, and regeneration. Indeed, the lack of effective therapies for several physiopathologic conditions suggests that a comprehensive knowledge of the different aspects of cellular behavior and molecular pathways, regulating each regenerative stage, has to be still devised. Hence, it is important to perform even more focused studies, taking the advantage of robust markers, reliable techniques, and reproducible protocols. Here, we provide an overview about the general aspects of muscle regeneration and discuss the different approaches to study the interrelated and time-dependent phases of muscle healing.