Satellite cells are essential for skeletal muscle regeneration: the cell on the edge returns centre stage

Mitochondrial stress response and myogenic differentiation

Regeneration and repair are prerequisites for maintaining effective function of skeletal muscle under high energy demands, and myogenic differentiation is one of the key steps in the regeneration and repair process. A striking feature of the process of myogenic differentiation is the alteration of mitochondria in number and function. Mitochondrial dysfunction can activate a number of transcriptional, translational and post-translational programmes and pathways to maintain cellular homeostasis under different types and degrees of stress, either through its own signaling or through constant signaling interactions with the nucleus and cytoplasm, a process known as the mitochondrial stress responses (MSRs). It is now believed that mitochondrial dysfunction is closely associated with a variety of muscle diseases caused by reduced levels of myogenic differentiation, suggesting the possibility that MSRs are involved in messaging during myogenic differentiation. Also, MSRs may be involved in myogenesis by promoting bioenergetic remodeling and assisting myoblast survival during myogenic differentiation. In this review, we will take MSRs as an entry point to explore its concrete regulatory mechanisms during myogenic differentiation, with a perspective to provide a theoretical basis for the treatment and repair of related muscle diseases.

MST1/2 regulates fibro/adipogenic progenitor fate decisions in skeletal muscle regeneration

Defective skeletal muscle regeneration is often accompanied by fibrosis. Fibroblast/adipose progenitors (FAPs) are important in these processes, however, the regulation of FAP fate decisions is unclear. Here, using inducible conditional knockout mice, we show that blocking mammalian Ste20-like kinases 1/2 (MST1/2) of FAPs prevented apoptosis and reduced interleukin-6 secretion in vivo and in vitro, which impaired myoblast proliferation and differentiation, as well as impaired muscle regeneration. Deletion of Mst1/2 increased co-localization of Yes-associated protein (YAP) with Smad2/3 in nuclei and promoted differentiation of FAPs toward myofibroblasts, resulting in excessive collagen deposition and skeletal muscle fibrosis. Meanwhile, inhibition of MST1/2 increased YAP/Transcriptional co-activator with PDZ-binding motif activation, which promoted activation of the WNT/β-catenin pathway and impaired the differentiation of FAPs toward adipocytes. These results reveal a new mechanism for MST1/2 action in disrupted skeletal muscle regeneration and fibrosis via regulation of FAP apoptosis and differentiation. MST1/2 is a potential therapeutic target for the treatment of some myopathies.

CREG1 deficiency impaired myoblast differentiation and skeletal muscle regeneration

BACKGROUND

CREG1 (cellular repressor of E1A-stimulated genes 1) is a protein involved in cellular differentiation and homeostasis regulation. However, its role in skeletal muscle satellite cells differentiation and muscle regeneration is poorly understood. This study aimed to investigate the role of CREG1 in myogenesis and muscle regeneration.

METHODS

RNA sequencing data (GSE8479) was analysed from the Gene Expression Omnibus database (GEO, https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi). We generated Creg1 knockdown and skeletal muscle satellite cells specific Creg1 overexpression mice mediated by adeno-associated virus serotype 9 (AAV9), skeletal muscle mature myofibre Creg1 knockout mice (myoblast/Creg1MKO), and control mice Creg1flox/flox (Creg1fl/fl) as in vivo models. The mice were injected into tibialis anterior (TA) muscle with 100 μL of 10 μM cardiotoxin to establish a muscle regeneration model. Creg1fl/fl and Creg1MKO mice were treated with AAV-sh-C-Cbl (2 × 1010 genomic copies/mouse) to silence C-Cbl in the TA muscle. 293T and C2C12 cells were transfected with plasmids using lipofectamine RNAi MAX in vitro. Mass spectrometry analyses and RNA sequencing transcriptomic assay were performed.

RESULTS

We analysed the transcriptional profiles of the skeletal muscle biopsies from healthy older (N = 25) and younger (N = 26) adult men and women in GSE8479 database, and the results showed that Creg1 was associated with human sarcopenia. We found that Creg1 knockdown mice regenerated less newly formed fibres in response to cardiotoxin injection (~30% reduction, P < 0.01); however, muscle satellite cells specific Creg1 overexpression mice regenerated more newly formed fibres (~20% increase, P < 0.05). AMPKa1 is known as a key mediator in the muscle regeneration process. Our results revealed that CREG1 deficiency inhibited AMPKa1 signalling through C-CBL E3-ubiquitin ligase-mediated AMPKa1 degradation (P < 0.01). C-CBL-mediated AMPKa1 ubiquitination was attributed to the K48-linked polyubiquitination of AMPKa1 at K396 and that the modification played an important role in the regulation of AMPKa1 protein stability. We also found that Creg1MKO mice regenerated less newly formed fibres compared with Creg1fl/fl mice (~30% reduction, P < 0.01). RNA-seq analysis showed that CREG1 deletion in impaired muscles led to the upregulation of inflammation and DKK3 expression. The TA muscles of Creg1MKO mice were injected with AAV-vector or AAV-shC-Cbl, silencing C-CBL (P < 0.01) in the skeletal muscles of Creg1MKO mice significantly improved muscle regeneration induced by CTX injury (P < 0.01).

CONCLUSIONS

Our findings suggest that CREG1 may be a potential therapeutic target for skeletal muscle regeneration.

Myogenic exosome miR-140-5p modulates skeletal muscle regeneration and injury repair by regulating muscle satellite cells

Muscle satellite cells (SCs) play a crucial role in the regeneration and repair of skeletal muscle injuries. Previous studies have shown that myogenic exosomes can enhance satellite cell proliferation, while the expression of miR-140-5p is significantly reduced during the repair process of mouse skeletal muscle injuries induced by BaCl2. This study aims to investigate the potential of myogenic exosomes carrying miR-140-5p inhibitors to activate SCs and influence the regeneration of injured muscles. Myogenic progenitor cell exosomes (MPC-Exo) and contained miR-140-5p mimics/inhibitors myogenic exosomes (MPC-Exo140+ and MPC-Exo140-) were employed to treat SCs and use the model. The results demonstrate that miR-140-5p regulates SC proliferation by targeting Pax7. Upon the addition of MPC-Exo and MPC-Exo140-, Pax7 expression in SCs significantly increased, leading to the transition of the cell cycle from G1 to S phase and an enhancement in cell proliferation. Furthermore, the therapeutic effect of MPC-Exo140- was validated in animal model, where the expression of muscle growth-related genes substantially increased in the gastrocnemius muscle. Our research demonstrates that MPC-Exo140- can effectively activate dormant muscle satellite cells, initiating their proliferation and differentiation processes, ultimately leading to the formation of new skeletal muscle cells and promoting skeletal muscle repair and remodeling.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

Muscle stem cells (MuSCs) are crucial to the repair and homeostasis of mature skeletal muscle. MuSC dysfunction and dysregulation of the myogenic program can contribute to the development of pathology ranging from cancers like rhabdomyosarcoma (RMS) or muscle degenerative diseases such as Duchenne muscular dystrophy (DMD). Both diseases exhibit dysregulation at nearly all steps of myogenesis. For instance, MuSC self-renewal processes are altered. In RMS, this leads to the creation of tumor propagating cells. In DMD, impaired asymmetric stem cell division creates a bias towards producing self-renewing stem cells instead of committing to differentiation. Hyperproliferation of these cells contribute to tumorigenesis in RMS and symmetric expansion of the self-renewing MuSC population in DMD. Both diseases also exhibit a repression of factors involved in terminal differentiation, halting RMS cells in the proliferative stage and thus driving tumor growth. Conversely, the MuSCs in DMD exhibit impaired differentiation and fuse prematurely, affecting myonuclei maturation and the integrity of the dystrophic muscle fiber. Finally, both disease states cause alterations to the MuSC niche. Various elements of the niche such as inflammatory and migratory signaling that impact MuSC behavior are dysregulated. Here we show how these seemingly distantly related diseases indeed have similarities in MuSC dysfunction, underlying the importance of considering MuSCs when studying the pathophysiology of muscle diseases.

Satellite cells in the growth and maintenance of muscle

Embryonic skeletal muscle growth is contingent upon a population of somite derived satellite cells, however, the contribution of these cells to early postnatal skeletal muscle growth remains relatively high. As prepubertal postnatal development proceeds, the activity and contribution of satellite cells to skeletal muscle growth diminishes. Eventually, at around puberty, a population of satellite cells escapes terminal commitment, continues to express the paired box transcription factor Pax7, and reside in a quiescent state orbiting the myofiber periphery adjacent to the basal lamina. After adolescence, some satellite cell contributions to muscle maintenance and adaptation occur, however, their necessity is reduced relative to embryonic, early postnatal, and prepubertal growth.

Neurofibromin 1 controls metabolic balance and Notch-dependent quiescence of murine juvenile myogenic progenitors

Patients affected by neurofibromatosis type 1 (NF1) frequently show muscle weakness with unknown etiology. Here we show that, in mice, Neurofibromin 1 (Nf1) is not required in muscle fibers, but specifically in early postnatal myogenic progenitors (MPs), where Nf1 loss led to cell cycle exit and differentiation blockade, depleting the MP pool resulting in reduced myonuclear accretion as well as reduced muscle stem cell numbers. This was caused by precocious induction of stem cell quiescence coupled to metabolic reprogramming of MPs impinging on glycolytic shutdown, which was conserved in muscle fibers. We show that a Mek/Erk/NOS pathway hypersensitizes Nf1-deficient MPs to Notch signaling, consequently, early postnatal Notch pathway inhibition ameliorated premature quiescence, metabolic reprogramming and muscle growth. This reveals an unexpected role of Ras/Mek/Erk signaling supporting postnatal MP quiescence in concert with Notch signaling, which is controlled by Nf1 safeguarding coordinated muscle growth and muscle stem cell pool establishment. Furthermore, our data suggest transmission of metabolic reprogramming across cellular differentiation, affecting fiber metabolism and function in NF1.

Extraocular muscle stem cells exhibit distinct cellular properties associated with non-muscle molecular signatures

Skeletal muscle stem cells (MuSCs) are recognised as functionally heterogeneous. Cranial MuSCs are reported to have greater proliferative and regenerative capacity when compared with those in the limb. A comprehensive understanding of the mechanisms underlying this functional heterogeneity is lacking. Here, we have used clonal analysis, live imaging and single cell transcriptomic analysis to identify crucial features that distinguish extraocular muscle (EOM) from limb muscle stem cell populations. A MyogeninntdTom reporter showed that the increased proliferation capacity of EOM MuSCs correlates with deferred differentiation and lower expression of the myogenic commitment gene Myod. Unexpectedly, EOM MuSCs activated in vitro expressed a large array of extracellular matrix components typical of mesenchymal non-muscle cells. Computational analysis underscored a distinct co-regulatory module, which is absent in limb MuSCs, as driver of these features. The EOM transcription factor network, with Foxc1 as key player, appears to be hardwired to EOM identity as it persists during growth, disease and in vitro after several passages. Our findings shed light on how high-performing MuSCs regulate myogenic commitment by remodelling their local environment and adopting properties not generally associated with myogenic cells.

Molecular and Structural Alterations of Skeletal Muscle Tissue Nuclei during Aging

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. The mechanisms underlying this phenomenon are certainly multifactorial and still remain to be fully elucidated. Changes in the cell nucleus structure and function have been considered among the possible contributing causes. This review offers an overview of the current knowledge on skeletal muscle nuclei in aging, focusing on the impairment of nuclear pathways potentially involved in age-related muscle decline. In skeletal muscle two types of cells are present: fiber cells, constituting the contractile muscle mass and containing hundreds of myonuclei, and the satellite cells, i.e., the myogenic mononuclear stem cells occurring at the periphery of the fibers and responsible for muscle growth and repair. Research conducted on different experimental models and with different methodological approaches demonstrated that both the myonuclei and satellite cell nuclei of aged skeletal muscles undergo several structural and molecular alterations, affecting chromatin organization, gene expression, and transcriptional and post-transcriptional activities. These alterations play a key role in the impairment of muscle fiber homeostasis and regeneration, thus contributing to the age-related decrease in skeletal muscle mass and function.

Early mobilisation within 72 hours after admission of critically ill patients in the intensive care unit: A systematic review with network meta-analysis

INTRODUCTION

Early mobilisation within 72 hours of intensive care unit admission counteracts complications caused by critical illness. The effect of different interventions on intensive care unit length of stay and other outcomes is unclear. We aimed to investigate the effectiveness of various early mobilisation interventions within 72 hours of admission to the intensive care unit on length of stay and other outcomes.

METHODS

A systematic review and (network) meta-analysis examining the effect of early mobilisation on length of stay in the intensive care unit and other outcomes, conducting searches in four databases. Randomised controlled trials were included from inception to 10/08/2022. Early mobilisation was defined as interventions that initiates and/or supports passive/active range-of-motion exercises within 72 hours of admission. In multi-arm studies, interventions used in other studies were declared as early intervention and were included in subgroup meta-analysis. Risk-of-bias was assessed using RoB2.

RESULTS

Of 29,680 studies screened, 18 studies with 1923 patients (three high, eleven some, four low risk-of-bias) and seven discriminable interventions of early mobilisation met inclusion criteria. Early mobilisation alone (WMD 0.78 days, 95 %CI [-1.38;-0.18], 11 studies, n = 1124) and early mobilisation with early nutrition (WMD -1.19 days, 95 %CI [-2.34;-0.03], 1 study, n = 100) were able to significantly shorten length of stay. Early mobilisation alone could also substantially shorten hospital length of stay (WMD -1.05 days, 95 %CI [-1.74;-0.36], 8 studies, n = 977). This effect in hospital length of stay was furthermore seen in the early intervention group compared with standard care (WMD -1.71 days, 95 %CI [-2.99;-0.43], 14 studies, n = 1587). Also, functionality and quality of life could significantly be improved by an early start of mobilisation.

CONCLUSION

In the network meta-analysis, early mobilisation alone and early mobilisation with early nutrition demonstrated a significant effect on intensive care length of stay. Early mobilisation could also reduce hospital length of stay and positively influence functionality and quality of life.

IMPLICATION FOR CLINICAL PRACTICE

Early mobilisation and early mobilisation with early nutrition seemed to be beneficial compared to other interventions like cycling on intensive care length of stay.

Fibroblast growth factor 8b (FGF-8b) enhances myogenesis and inhibits adipogenesis in rotator cuff muscle cell populations in vitro

Fatty expansion is one of the features of muscle degeneration due to muscle injuries, and its presence interferes with muscle regeneration. Specifically, poor clinical outcomes have been linked to fatty expansion in rotator cuff tears and repairs. Our group recently found that fibroblast growth factor 8b (FGF-8b) inhibits adipogenic differentiation and promotes myofiber formation of mesenchymal stem cells in vitro. This led us to hypothesize that FGF-8b could similarly control the fate of muscle-specific cell populations derived from rotator cuff muscle involved in muscle repair following rotator cuff injury. In this study, we isolate fibro-adipogenic progenitor cells (FAPs) and satellite stem cells (SCs) from rat rotator cuff muscle tissue and analyzed the effects of FGF-8b supplementation. Utilizing a cell plating protocol, we successfully isolate FAPs-rich fibroblasts (FIBs) and SCs-rich muscle progenitor cells (MPCs). Subsequently, we demonstrate that FIB adipogenic differentiation can be inhibited by FGF-8b, while MPC myogenic differentiation can be enhanced by FGF-8b. We further demonstrate that phosphorylated ERK due to FGF-8b leads to the inhibition of adipogenesis in FIBs and SCs maintenance and myofiber formation in MPCs. Together, these findings demonstrate the powerful potential of FGF-8b for rotator cuff repair by altering the fate of muscle undergoing degeneration.

Isolation, identification, and induced differentiation of satellite cells from skeletal muscle of adult tree shrews

A method for the in vitro isolation, purification, identification, and induced differentiation of satellite cells from adult tree shrew skeletal muscle was established. The mixed enzyme digestion method and differential adhesion method were used to obtain skeletal muscle satellite cells, which were identified and induced to differentiate to verify their pluripotency. The use of a mixture of collagenase II, hyaluronidase IV, and DNase I is an efficient method for isolating adult tree shrew skeletal muscle satellite cells. The P3 generation of cells had good morphology, rapid proliferation, high viability, and an "S"-shaped growth curve. Reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence staining indicated that marker genes or proteins were expressed in skeletal muscle satellite cells. After myogenic differentiation was induced, multiple-nucleated myotubes were observed, and the MyHC protein was expressed. The expression of myogenic marker genes changed with the differentiation process. After the induction of adipogenic differentiation, orange-red lipid droplets were observed, and the expression of adipogenic marker genes increased gradually with the differentiation process. In summary, satellite cells from adult tree shrew skeletal muscle were successfully isolated using a mixed enzyme digestion method, and their potential for differentiation into myogenic and adipogenic cells was confirmed, laying a foundation for further in vitro study of tree shrew muscle damage.

Regulation of Satellite Cells Functions during Skeletal Muscle Regeneration: A Critical Step in Physiological and Pathological Conditions

Skeletal muscle regeneration is a complex process involving the generation of new myofibers after trauma, competitive physical activity, or disease. In this context, adult skeletal muscle stem cells, also known as satellite cells (SCs), play a crucial role in regulating muscle tissue homeostasis and activating regeneration. Alterations in their number or function have been associated with various pathological conditions. The main factors involved in the dysregulation of SCs' activity are inflammation, oxidative stress, and fibrosis. This review critically summarizes the current knowledge on the role of SCs in skeletal muscle regeneration. It examines the changes in the activity of SCs in three of the most common and severe muscle disorders: sarcopenia, muscular dystrophy, and cancer cachexia. Understanding the molecular mechanisms involved in their dysregulations is essential for improving current treatments, such as exercise, and developing personalized approaches to reactivate SCs.

Regeneration of Panniculus Carnosus Muscle in Fetal Mice Is Characterized by the Presence of Actin Cables

Mammalian skin, including human and mouse skin, does not regenerate completely after injury; it is repaired, leaving a scar. However, it is known that skin wounds up to a certain stage of embryonic development can regenerate. The mechanism behind the transition from regeneration to scar formation is not fully understood. Panniculus carnosus muscle (PCM) is present beneath the dermal fat layer and is a very important tissue for wound contraction. In rodents, PCM is present throughout the body. In humans, on the other hand, it disappears and becomes a shallow fascia on the trunk. Fetal cutaneous wounds, including PCM made until embryonic day 13 (E13), regenerate completely, but not beyond E14. We visualized the previously uncharacterized development of PCM in the fetus and investigated the temporal and spatial changes in PCM at different developmental stages, ranging from full regeneration to non-regeneration. Furthermore, we report that E13 epidermal closure occurs through actin cables, which are bundles of actomyosin formed at wound margins. The wound healing process of PCM suggests that actin cables may also be associated with PCM. Our findings reveal that PCM regenerates through a similar mechanism.

Pax7 reporter mouse models: a pocket guide for satellite cell research

Since their discovery, satellite cells have showcased their need as primary contributors to skeletal muscle maintenance and repair. Satellite cells lay dormant, but when needed, activate, differentiate, fuse to fibres and self-renew, that has bestowed satellite cells with the title of muscle stem cells. The satellite cell specific transcription factor Pax7 has enabled researchers to develop animal models against the Pax7 locus in order to isolate and characterise satellite cell-mediated events. This review focuses specifically on describing Pax7 reporter mouse models. Here we describe how each model was generated and the key findings obtained. The strengths and limitations of each model are also discussed. The aim is to provide new and current satellite cell enthusiasts with a basic understanding of the available Pax7 reporter mice and hopefully guide selection of the most appropriate Pax7 model to answer a specific research question.

Preventative and therapeutic potential of tocotrienols on musculoskeletal diseases in ageing

Musculoskeletal health is paramount in an ageing population susceptible to conditions such as osteoporosis, arthritis and fractures. Age-related changes in bone, muscle, and joint function result in declining musculoskeletal health, reduced mobility, increased risk of falls, and persistent discomfort. Preserving musculoskeletal wellbeing is essential for maintaining independence and enhancing the overall quality of life for the elderly. The global burden of musculoskeletal disorders is significant, impacting 1.71 billion individuals worldwide, with age-related muscle atrophy being a well-established phenomenon. Tocotrienols, a unique type of vitamin E found in various sources, demonstrate exceptional antioxidant capabilities compared to tocopherols. This characteristic positions them as promising candidates for addressing musculoskeletal challenges, particularly in mitigating inflammation and oxidative stress underlying musculoskeletal disorders. This review paper comprehensively examines existing research into the preventive and therapeutic potential of tocotrienols in addressing age-related musculoskeletal issues. It sheds light on the promising role of tocotrienols in enhancing musculoskeletal health and overall wellbeing, emphasizing their significance within the broader context of age-related health concerns.

Therapeutic Consequences of Targeting the IGF-1/PI3K/AKT/FOXO3 Axis in Sarcopenia: A Narrative Review

The high prevalence of sarcopenia in an aging population has an underestimated impact on quality of life by increasing the risk of falls and subsequent hospitalization. Unfortunately, the application of the major established key therapeutic-physical activity-is challenging in the immobile and injured sarcopenic patient. Consequently, novel therapeutic directions are needed. The transcription factor Forkhead-Box-Protein O3 (FOXO3) may be an option, as it and its targets have been observed to be more highly expressed in sarcopenic muscle. In such catabolic situations, Foxo3 induces the expression of two muscle specific ubiquitin ligases (Atrogin-1 and Murf-1) via the PI3K/AKT pathway. In this review, we particularly evaluate the potential of Foxo3-targeted gene therapy. Foxo3 knockdown has been shown to lead to increased muscle cross sectional area, through both the AKT-dependent and -independent pathways and the reduced impact on the two major downstream targets Atrogin-1 and Murf-1. Moreover, a Foxo3 reduction suppresses apoptosis, activates satellite cells, and initiates their differentiation into muscle cells. While this indicates a critical role in muscle regeneration, this mechanism might exhaust the stem cell pool, limiting its clinical applicability. As systemic Foxo3 knockdown has also been associated with risks of inflammation and cancer progression, a muscle-specific approach would be necessary. In this review, we summarize the current knowledge on Foxo3 and conceptualize a specific and targeted therapy that may circumvent the drawbacks of systemic Foxo3 knockdown. This approach presumably would limit the side effects and enable an activity-independent positive impact on skeletal muscle.

Collagen biomaterials promote the regenerative repair of abdominal wall defects in Bama miniature pigs

Due to adhesion and rejection of recent traditional materials, it is still challenging to promote the regenerative repair of abdominal wall defects caused by different hernias or severe trauma. However, biomaterials with a high biocompatibility and low immunogenicity have exhibited great potential in the regeneration of abdominal muscle tissue. Previously, we have designed a biological collagen scaffold material combined with growth factor, which enables a fusion protein-collagen binding domain (CBD)-basic fibroblast growth factor (bFGF) to bind and release specifically. Though experiments in rodent animals have indicated the regeneration function of CBD-bFGF modified biological collagen scaffolds, its translational properties in large animals or humans are still in need of solid evidence. In this study, the abdominal wall defect model of Bama miniature pigs was established by artificial operations, and the defective abdominal wall was sealed with or without a polypropylene patch, and unmodified and CBD-bFGF modified biological collagen scaffolds. Results showed that a recurrent abdominal hernia was observed in the defect control group (without the use of mesh). Although the polypropylene patch can repair the abdominal wall defect, it also induced serious adhesion and inflammation. Meanwhile, both kinds of collagen biomaterials exhibited positive effects in repairing abdominal wall defects and reducing regional adhesion and inflammation. However, CBD-bFGF-modified collagen biomaterials failed to induce the regenerative repair reported in rat experiments. In addition, unmodified collagen biomaterials induced abdominal wall muscle regeneration rather than fibrotic repair. These results indicated that the unmodified collagen biomaterials are a better option among translational patches for the treatment of abdominal wall defects.

Extracellular matrix: the critical contributor to skeletal muscle regeneration-a comprehensive review

The regenerative ability of skeletal muscle (SM) in response to damage, injury, or disease is a highly intricate process that involves the coordinated activities of multiple cell types and biomolecular factors. Of these, extracellular matrix (ECM) is considered a fundamental component of SM regenerative ability. This review briefly discusses SM myogenesis and regeneration, the roles played by muscle satellite cells (MSCs), other cells, and ECM components, and the effects of their dysregulations on these processes. In addition, we review the various types of ECM scaffolds and biomaterials used for SM regeneration, their applications, recent advances in ECM scaffold research, and their impacts on tissue engineering and SM regeneration, especially in the context of severe muscle injury, which frequently results in substantial muscle loss and impaired regenerative capacity. This review was undertaken to provide a comprehensive overview of SM myogenesis and regeneration, the stem cells used for muscle regeneration, the significance of ECM in SM regeneration, and to enhance understanding of the essential role of the ECM scaffold during SM regeneration.

CircLRRFIP1 promotes the proliferation and differentiation of chicken skeletal muscle satellite cells by sponging the miR-15 family via activating AKT3-mTOR/p70S6K signaling pathway

Skeletal muscle is important for animal meat production, regulating movements, and maintaining homeostasis. Circular RNAs (circRNAs) have been founded to play vital role in myogenesis. However, the effects of the numerous circRNAs on growth and development of the skeletal muscle are yet to be uncovered. Herein, we identified circLRRFIP1, which is a novel circular RNA that is preferentially expressed in the skeletal muscle. To study the role of circLRRFIP1 in the skeletal muscle, the skeletal muscle satellite cells (SMSCs) was used to silenced or overexpressed circLRRFIP1. The results obtained in this study showed that circLRRFIP1 play a positive role in the proliferation and differentiation of SMSCs. The SMSCs were generated with stable knockdown and overexpression of circLRRFIP1, and the results showed that circLRRFIP1 exerts a stimulatory effect on the proliferation and differentiation of SMSCs. We further generated SMSCs with stable knockdown and overexpression of circLRRFIP1, and the results revealed that circLRRFIP1 exerts a stimulatory effect on the proliferation and differentiation of SMSCs. Mechanistically, circLRRFIP1 targets the myogenic inhibitory factor-miR-15 family to release the suppression of the miR-15 family to AKT3. The knockdown of AKT inhibits SMSC differentiation through the mTOR/p70S6K pathway. Taken together, the results obtained in this present study revealed the important role and the regulatory mechanisms of circLRRFIP1 in the development of chicken skeletal muscle. Therefore, this study provides an attractive target for molecular breeding to enhance meat production in the chicken industry.

Muscle Involvement in Amyotrophic Lateral Sclerosis: Understanding the Pathogenesis and Advancing Therapeutics

Amyotrophic lateral sclerosis (ALS) is a fatal condition characterized by the selective loss of motor neurons in the motor cortex, brainstem, and spinal cord. Muscle involvement, muscle atrophy, and subsequent paralysis are among the main features of this disease, which is defined as a neuromuscular disorder. ALS is a persistently progressive disease, and as motor neurons continue to degenerate, individuals with ALS experience a gradual decline in their ability to perform daily activities. Ultimately, muscle function loss may result in paralysis, presenting significant challenges in mobility, communication, and self-care. While the majority of ALS research has traditionally focused on pathogenic pathways in the central nervous system, there has been a great interest in muscle research. These studies were carried out on patients and animal models in order to better understand the molecular mechanisms involved and to develop therapies aimed at improving muscle function. This review summarizes the features of ALS and discusses the role of muscle, as well as examines recent studies in the development of treatments.

Pax7 haploinsufficiency impairs muscle stem cell function in Cre-recombinase mice and underscores the importance of appropriate controls

Ever since its introduction as a genetic tool, the Cre-lox system has been widely used for molecular genetic studies in vivo in the context of health and disease, as it allows time- and cell-specific gene modifications. However, insertion of the Cre-recombinase cassette in the gene of interest can alter transcription, protein expression, or function, either directly, by modifying the landscape of the locus, or indirectly, due to the lack of genetic compensation or by indirect impairment of the non-targeted allele. This is sometimes the case when Cre-lox is used for muscle stem cell studies. Muscle stem cells are required for skeletal muscle growth, regeneration and to delay muscle disease progression, hence providing an attractive model for stem cell research. Since the transcription factor Pax7 is specifically expressed in all muscle stem cells, tamoxifen-inducible Cre cassettes (CreERT2) have been inserted into this locus by different groups to allow targeted gene recombination. Here we compare the two Pax7-CreERT2 mouse lines that are mainly used to evaluate muscle regeneration and development of pathological features upon deletion of specific factors or pathways. We applied diverse commonly used tamoxifen schemes of CreERT2 activation, and we analyzed muscle repair after cardiotoxin-induced injury. We show that consistently the Pax7-CreERT2 allele targeted into the Pax7 coding sequence (knock-in/knock-out allele) produces an inherent defect in regeneration, manifested as delayed post-injury repair and reduction in muscle stem cell numbers. In genetic ablation studies lacking proper controls, this inherent defect could be misinterpreted as being provoked by the deletion of the factor of interest. Instead, using an alternative Pax7-CreERT2 allele that maintains bi-allelic Pax7 expression or including appropriate controls can prevent misinterpretation of experimental data. The findings presented here can guide researchers establish appropriate experimental design for muscle stem cell genetic studies.

Oral administration of a locally isolated Lactobacillus rhamnosus (NR_113332.1) improves regeneration of extensor digitorum longus muscle in mice

OBJECTIVE

The aim of this study was to examine the effects of probiotic supplementation on extensor digitorum longus (EDL) regeneration after grafting in mice.

METHODS

EDL muscles were ortho-transplanted in mice. The experimental group was given 1 ×  108 colony-forming units/g body weight of Lactobacillus rhamnosus daily after EDL muscle transplantation surgeries. EDL muscle transplants were recovered after 3, 5, 7, and 14 d post-transplantation from the control as well as the experimental animals and processed for histologic analysis.

RESULTS

At day 3 post-transplantation, the inflammatory cells had infiltrated into the grafted EDL muscles and the central section of the grafted tissue contained necrotic fibers. At day 5 post-transplantation, the concentration of inflammatory cells increased further and degenerative muscle fibers were being replaced with centrally nucleated muscle cells. The average cross-sectional area non-grafted EDL and grafted muscle in the probiotic supplemented mice at day 7 increased to 48% and 23% (P = 0.002), respectively, compared with the respective values in the control animals. Whereas in non-grafted and grafted EDL muscle it could approach 8% and 36% (P = 0.008), respectively at 14 d compared with the corresponding values of the control EDL muscle transplants. The number of muscle fibers in the non-grafted and grafted probiotic-supplemented groups increased to12% and 20% (P = 0.045) at day 7 compared with the control EDL muscle. In non-grafted and grafted EDL muscle, the number of regenerated muscle fibers increased to 73% and 64% (P = 0.110) at day 14 compared with control EDL grafted muscle.

CONCLUSION

Results of the present study regarding better regeneration of skeletal muscle fibers in the probiotic-supplemented mice than the control grafts warrant further molecular-level investigation to understand the underlying mechanism mediating the process of skeletal muscle fiber regeneration. Probiotics possibly modulate the process of muscle fiber regeneration by adjusting the composition of gut microbiota.

The emerging role of Piezo1 channels in skeletal muscle physiology

Piezo1 channels are mechanically activated (MA) cation channels that are involved in sensing of various mechanical perturbations, such as membrane stretch and shear stress, and play a crucial role in cell mechanotransduction. In response to mechanical stimuli, these channels open up and allow cations to travel into the cell and induce biochemical reactions that can change the cell's metabolism and function. Skeletal muscle cells/fibers inherently depend upon mechanical cues in the form of fluid shear stress and contractions (physical exercise). For example, an exposure of skeletal muscles to chronic mechanical loading leads to increased anabolism and fiber hypertrophy, while prolonged mechanical unloading results in muscle atrophy. MA Piezo1 channels have recently emerged as key mechanosensors that are capable of linking mechanical signals and intramuscular signaling in skeletal muscle cells/fibers. This review will summarize the emerging role of Piezo1 channels in the development and regeneration of skeletal muscle tissue as well as in the regulation of skeletal muscle atrophy. In addition, an overview of potential Piezo1-related signaling pathways underlying anabolic and catabolic processes will be provided. A better understanding of Piezo1's role in skeletal muscle mechanotransduction may represent an important basis for the development of therapeutic strategies for maintaining muscle functions under disuse conditions and in some disease states.

Injury-experienced satellite cells retain long-term enhanced regenerative capacity

BACKGROUND

Inflammatory memory or trained immunity is a recently described process in immune and non-immune tissue resident cells, whereby previous exposure to inflammation mediators leads to a faster and stronger responses upon secondary challenge. Whether previous muscle injury is associated with altered responses to subsequent injury by satellite cells (SCs), the muscle stem cells, is not known.

METHODS

We used a mouse model of repeated muscle injury, in which intramuscular cardiotoxin (CTX) injections were administered 50 days apart in order to allow for full recovery of the injured muscle before the second injury. The effect of prior injury on the phenotype, proliferation and regenerative potential of satellite cells following a second injury was examined in vitro and in vivo by immunohistochemistry, RT-qPCR and histological analysis.

RESULTS

We show that SCs isolated from muscle at 50 days post-injury (injury-experienced SCs (ieSCs)) enter the cell cycle faster and form bigger myotubes when cultured in vitro, compared to control SCs isolated from uninjured contralateral muscle. Injury-experienced SCs were characterized by the activation of the mTORC 1 signaling pathway, suggesting they are poised to activate sooner following a second injury. Consequently, upon second injury, SCs accumulate in greater numbers in muscle at 3 and 10 days after injury. These changes in SC phenotype and behavior were associated with accelerated muscle regeneration, as evidenced by an earlier appearance of bigger fibers and increased number of myonuclei per fiber at day 10 after the second injury.

CONCLUSIONS

Overall, we show that skeletal muscle injury has a lasting effect on SC function priming them to respond faster to a subsequent injury. The ieSCs have long-term enhanced regenerative properties that contribute to accelerated regeneration following a secondary challenge.

The aminopeptidase LAP3 suppression accelerates myogenic differentiation via the AKT-TFE3 pathway in C2C12 myoblasts

Skeletal muscle maintenance depends largely on muscle stem cells (satellite cells) that supply myoblasts required for muscle regeneration and growth. The ubiquitin-proteasome system is the major intracellular protein degradation pathway. We previously reported that proteasome dysfunction in skeletal muscle significantly impairs muscle growth and development. Furthermore, the inhibition of aminopeptidase, a proteolytic enzyme that removes amino acids from the termini of peptides derived from proteasomal proteolysis, impairs the proliferation and differentiation ability of C2C12 myoblasts. However, no evidence has been reported on the role of aminopeptidases with different substrate specificities on myogenesis. In this study, therefore, we investigated whether the knockdown of aminopeptidases in differentiating C2C12 myoblasts affects myogenesis. The knockdown of the X-prolyl aminopeptidase 1, aspartyl aminopeptidase, leucyl-cystinyl aminopeptidase, methionyl aminopeptidase 1, methionyl aminopeptidase 2, puromycine-sensitive aminopeptidase, and arginyl aminopeptidase like 1 gene in C2C12 myoblasts resulted in defective myogenic differentiation. Surprisingly, the knockdown of leucine aminopeptidase 3 (LAP3) in C2C12 myoblasts promoted myogenic differentiation. We also found that suppression of LAP3 expression in C2C12 myoblasts resulted in the inhibition of proteasomal proteolysis, decreased intracellular branched-chain amino acid levels, and enhanced mTORC2-mediated AKT phosphorylation (S473). Furthermore, phosphorylated AKT induced the translocation of TFE3 from the nucleus to the cytoplasm, promoting myogenic differentiation through increased expression of myogenin. Overall, our study highlights the association of aminopeptidases with myogenic differentiation.

Lineage tracing of newly accrued nuclei in skeletal myofibers uncovers distinct transcripts and interplay between nuclear populations

Multinucleated skeletal muscle cells have an obligatory need to acquire additional nuclei through fusion with activated skeletal muscle stem cells when responding to both developmental and adaptive growth stimuli. A fundamental question in skeletal muscle biology has been the reason underlying this need for new nuclei in syncytial cells that already harbor hundreds of nuclei. To begin to answer this long-standing question, we utilized nuclear RNA-sequencing approaches and developed a lineage tracing strategy capable of defining the transcriptional state of recently fused nuclei and distinguishing this state from that of pre-existing nuclei. Our findings reveal the presence of conserved markers of newly fused nuclei both during development and after a hypertrophic stimulus in the adult. However, newly fused nuclei also exhibit divergent gene expression that is determined by the myogenic environment to which they fuse. Moreover, accrual of new nuclei through fusion is required for nuclei already resident in adult myofibers to mount a normal transcriptional response to a load-inducing stimulus. We propose a model of mutual regulation in the control of skeletal muscle development and adaptations, where newly fused and pre-existing myonuclear populations influence each other to maintain optimal functional growth.

Identification of quiescent FOXC2+ spermatogonial stem cells in adult mammals

In adult mammals, spermatogenesis embodies the complex developmental process from spermatogonial stem cells (SSCs) to spermatozoa. At the top of this developmental hierarchy lie a series of SSC subpopulations. Their individual identities as well as the relationships with each other, however, remain largely elusive. Using single-cell analysis and lineage tracing, we discovered both in mice and humans the quiescent adult SSC subpopulation marked specifically by forkhead box protein C2 (FOXC2). All spermatogenic progenies can be derived from FOXC2+ SSCs and the ablation of FOXC2+ SSCs led to the depletion of the undifferentiated spermatogonia pool. During germline regeneration, FOXC2+ SSCs were activated and able to completely restore the process. Germ cell-specific Foxc2 knockout resulted in an accelerated exhaustion of SSCs and eventually led to male infertility. Furthermore, FOXC2 prompts the expressions of negative regulators of cell cycle thereby ensures the SSCs reside in quiescence. Thus, this work proposes that the quiescent FOXC2+ SSCs are essential for maintaining the homeostasis and regeneration of spermatogenesis in adult mammals.

Nanoenzyme-Reinforced Multifunctional Scaffold Based on Ti3C2Tx MXene Nanosheets for Promoting Structure-Functional Skeletal Muscle Regeneration via Electroactivity and Microenvironment Management

The completed volumetric muscle loss (VML) regeneration remains a challenge due to the limited myogenic differentiation as well as the oxidative, inflammatory, and hypoxic microenvironment. Herein, a 2D Ti3C2Tx MXene@MnO2 nanocomposite with conductivity and microenvironment remodeling was fabricated and applied in developing a multifunctional hydrogel (FME) scaffold to simultaneously conquer these hurdles. Among them, Ti3C2Tx MXene with electroconductive ability remarkably promotes myogenic differentiation via enhancing the myotube formation and upregulating the relative expression of the myosin heavy chain (MHC) protein and myogenic genes (MyoD and MyoG) in myogenesis. The MnO2 nanoenzyme-reinforced Ti3C2Tx MXene significantly reshapes the hostile microenvironment by eliminating reactive oxygen species (ROS), regulating macrophage polarization from M1 to M2 and continuously supplying O2. Together, the FME hydrogel as a bioactive multifunctional scaffold significantly accelerates structure-functional VML regeneration in vivo and represents a multipronged strategy for the VML regeneration via electroactivity and microenvironment management.

Unsaturated Long-Chain Fatty Acids Activate Resident Macrophages and Stem Cells in a Human Skeletal Muscle Tissue Model

Phenotypically heterogeneous populations of tissue-resident macrophages and stem cells play important roles in the regeneration of the skeletal muscle tissue. Previous studies using animal and cell culture models implied a beneficial effect of fatty acid (FA) species on tissue regeneration. Here, we applied a human experimental model using excised muscle tissues from reconstructive surgeries to study the effects of FAs on resident macrophages and stem cells in the natural environment of human skeletal muscle tissue. Muscle tissue samples from 20 donors were included in this study. The expression of 34 cytokines/chemokines was determined, using multiplex protein analysis. The phenotypes of macrophages and stem cells were determined immunohistochemically. The numbers of CD80+ macrophages correlated with the expression levels of IL-1α, IL-1RA, IL-8, IL-17A, and MCP-1, while the PAX7+ and MyoD+ stem cell counts were positively correlated with the expression level of CXCL12α, a recognized chemoattractant for muscle stem cells. Treatment of additional tissue sections with FAs revealed that CD80+ or MARCO+ macrophages- and PAX7+ or MyoD+ stem cells were simultaneously increased by unsaturated long-chain FAs. Taken together, this is the first experimental demonstration of a coordinated activation of macrophages and stem cells in human skeletal muscle tissue.

Targeting Hub Genes Involved in Muscle Injury Induced by Jumping Load Based on Transcriptomics

The purpose of this study was to find hub genes that may play key roles in skeletal muscle injury induced by jumping load. Twelve female Sprague Dawley rats were divided into the normal control (NC) group and the jumping-induced muscle injury (JI) group. After 6 weeks of jumping, transmission electron microscopy, hematoxylin-eosin staining, transcriptomics sequencing and genes analysis, interaction network prediction of multiple proteins, real-time PCR detection, and Western blotting were performed on gastrocnemius muscles from NC and JI groups. As compared with NC rats, excessive jumping can result in notable structural damage and inflammatory infiltration in JI rats. A total of 112 differentially expressed genes were confirmed in NC rats versus JI rats, with 59 genes upregulated and 53 genes downregulated. Using the online String database, four hub genes in the transcriptional regulatory network were targeted, including FOS, EGR1, ATF3, and NR4A3. All expression levels of FOS, EGR1, ATF3, and NR4A3 mRNAs were decreased in JI rats compared with NC rats (p < 0.05 or p < 0.01). All expression levels of c-Fos, EGR1, ATF3, and NOR1 proteins were upregulated in JI rats (p < 0.01, p < 0.05, p > 0.05, and p < 0.01, respectively). Collectively, these findings indicate that FOS, EGR1, ATF3, and NR4A3 genes may be functionally important in jumping-induced muscle injury.

Inflammatory, mitochondrial, and senescence-related markers: Underlying biological pathways of muscle aging and new therapeutic targets

The maintenance of functional health is pivotal for achieving independent life in older age. The aged muscle is characterized by ultrastructural changes, including loss of type I and type II myofibers and a greater proportion of cytochrome c oxidase deficient and succinate dehydrogenase positive fibers. Both intrinsic (e.g., altered proteostasis, DNA damage, and mitochondrial dysfunction) and extrinsic factors (e.g., denervation, altered metabolic regulation, declines in satellite cells, and inflammation) contribute to muscle aging. Being a hub for several cellular activities, mitochondria are key to myocyte viability and mitochondrial dysfunction has been implicated in age-associated physical decline. The maintenance of functional organelles via mitochondrial quality control (MQC) processes is, therefore, crucial to skeletal myofiber viability and organismal health. The autophagy-lysosome pathway has emerged as a critical step of MQC in muscle by disposing organelles and proteins via their tagging for autophagosome incorporation and delivery to the lysosome for clearance. This pathway was found to be altered in muscle of physically inactive older adults. A relationship between this pathway and muscle tissue composition of the lower extremities as well as physical performance was also identified. Therefore, integrating muscle structure and myocyte quality control measures in the evaluation of muscle health may be a promising strategy for devising interventions fostering muscle health.

Structural Changes in the Skeletal Muscle of Pigs after Long-Term Administration of Testosterone, Nandrolone and a Combination of the Two

Anabolic steroid hormones (AASs) are used in most countries of the world to accelerate the growth of animals, increase the volume of their muscles and thereby increase meat production. However, there is a strict ban on the use of AASs in the fattening of all animals in all countries of the European Union, and there must therefore be effective methods of detection and control of these substances. Methods based on chromatography and mass spectrometry may no longer be completely effective when faced with new synthetic steroids of unknown chemical structures and low concentrations. Therefore, there is an effort to develop new methods of AAS detection, based primarily on the monitoring of biological changes at the level of gene expression or changes in metabolism or structure at the cellular level. More detailed knowledge of the mechanisms of action of AASs on tissues is essential for these methods, and histological changes are one of them. In this study, we report histological changes in muscle structure after AAS application, specifically in the size of muscle fibers, the amount of endomysium and the number of nuclei and satellite cells in muscle fibers. A pig model was also intentionally used for the study, as no such study has been carried out on this species, and at the same time, pork is one of the most consumed meats across Europe. The results of histology and fluorescent antibody labeling showed that AASs increased the diameter and surface area of muscle fibers and also significantly increased the number of satellite cells on the fiber surface. The evident correlations between the number of satellite cells, all nuclei and the diameters of muscle fibers between some experimental groups provide evidence that the selected histological parameters could be additional detection mechanisms for screening a large number of samples and indicate the possibility of the presence of AASs in pork meat in the future.

Hedgehog signaling via its ligand DHH acts as cell fate determinant during skeletal muscle regeneration

Successful muscle regeneration relies on the interplay of multiple cell populations. However, the signals required for this coordinated intercellular crosstalk remain largely unknown. Here, we describe how the Hedgehog (Hh) signaling pathway controls the fate of fibro/adipogenic progenitors (FAPs), the cellular origin of intramuscular fat (IMAT) and fibrotic scar tissue. Using conditional mutagenesis and pharmacological Hh modulators in vivo and in vitro, we identify DHH as the key ligand that acts as a potent adipogenic brake by preventing the adipogenic differentiation of FAPs. Hh signaling also impacts muscle regeneration, albeit indirectly through induction of myogenic factors in FAPs. Our results also indicate that ectopic and sustained Hh activation forces FAPs to adopt a fibrogenic fate resulting in widespread fibrosis. In this work, we reveal crucial post-developmental functions of Hh signaling in balancing tissue regeneration and fatty fibrosis. Moreover, they provide the exciting possibility that mis-regulation of the Hh pathway with age and disease could be a major driver of pathological IMAT formation.

Scraping Therapy Improved Muscle Regeneration through Regulating GLUT4/Glycolytic and AMPK/mTOR/4EBP1 Pathways in Rats with Lumbar Multifidus Injury

Background

High morbidity of nonspecific low back pain (NLBP) and large consumption of medical resources caused by it have become a heavy social burden. There are many factors inducing NLBP, among which the damage and atrophy of multifidus (MF) are most closely related to NLBP. Scraping therapy can have significant treatment effects on NLBP with fewer adverse reactions and less medical fund input than other modalities or medications. However, the mechanism of scraping therapy treating NLBP remains unclarified. Here, we wanted to investigate the effects of scraping therapy on promoting MF regeneration and the underlying mechanisms.

Methods

A total of 54 male rats (SD, 6-7 weeks old) were randomly divided into nine groups, namely, K, M6h, M1d, M2d, M3d, G6h, G1d, G2d, and G3d, with six rats in each group. They were injected with bupivacaine (BPVC) to intentionally induce MF injury. We then performed scraping therapy on the rats that had been randomly chosen and compared treatment effects at different time points. In vitro data including skin temperature and tactile allodynia threshold were collected and histological sections were analyzed. mRNA sequencing was applied to distinguish the genes or signaling pathways that had been altered due to scraping therapy, and the results were further verified through reverse transcription polymerase chain reaction and Western blot analysis.

Results

Transitory petechiae and ecchymosis both on and beneath the rats' skin raised by scraping therapy gradually faded in about 3 d. Cross-sectional area (CSA) of MF was significantly smaller 30 h, 2 d, and 4 d after modeling (P=0.007, P=0.001, and P=0.015, respectively, vs. the blank group) and was significantly larger in the scraping group 1 d after treatment (P=0.002 vs. the model 1d group). Skin temperature significantly increased immediately after scraping (P < 0.001) and hindlimb pain threshold increased on the 2nd day after scraping (P=0.046 and P=0.028, respectively). 391 differentially expressed genes and 8 signaling pathways were characterized 6 h after scraping; only 3 differentially expressed genes and 3 signaling pathways were screened out 2 d after treatment. The amounts of mRNAs or proteins for GLUT4, HK2, PFKM, PKM, LDHA (which belong to the GLUT4/glycolytic pathway), p-mTOR, p-4EBP1 (which belong to the AMPK/mTOR/4EBP1 pathway), and BDH1 were enhanced, and p-AMPKα was decreased after scraping therapy.

Conclusions

Scraping therapy has therapeutic effects on rats with multifidus injury by promoting muscle regeneration via regulating GLUT4/glycolytic and AMPK/mTOR/4EBP1 signaling pathways.

Inhibition of type I PRMTs reforms muscle stem cell identity enhancing their therapeutic capacity

In skeletal muscle, muscle stem cells (MuSC) are the main cells responsible for regeneration upon injury. In diseased skeletal muscle, it would be therapeutically advantageous to replace defective MuSCs, or rejuvenate them with drugs to enhance their self-renewal and ensure long-term regenerative potential. One limitation of the replacement approach has been the inability to efficiently expand MuSCs ex vivo, while maintaining their stemness and engraftment abilities. Herein, we show that inhibition of type I protein arginine methyltransferases (PRMTs) with MS023 increases the proliferative capacity of ex vivo cultured MuSCs. Single cell RNA sequencing (scRNAseq) of ex vivo cultured MuSCs revealed the emergence of subpopulations in MS023-treated cells which are defined by elevated Pax7 expression and markers of MuSC quiescence, both features of enhanced self-renewal. Furthermore, the scRNAseq identified MS023-specific subpopulations to be metabolically altered with upregulated glycolysis and oxidative phosphorylation (OxPhos). Transplantation of MuSCs treated with MS023 had a better ability to repopulate the MuSC niche and contributed efficiently to muscle regeneration following injury. Interestingly, the preclinical mouse model of Duchenne muscular dystrophy had increased grip strength with MS023 treatment. Our findings show that inhibition of type I PRMTs increased the proliferation capabilities of MuSCs with altered cellular metabolism, while maintaining their stem-like properties such as self-renewal and engraftment potential.

Nanocarbon-Enforced Anisotropic MusCAMLR for Rapid Rescue of Mechanically Damaged Skeletal Muscles

Mechanical damages to skeletal muscles could be detrimental to the active work hours and lifestyle of athletes, mountaineers, and security personnel. In this regard, the slowness of conventional treatment strategies and drug-associated side effects greatly demand the design and development of novel biomaterials, which can rescue such mechanically damaged skeletal muscles. To accomplish this demand, we have developed a musculoresponsive polymer-carbon composite for assisting myotubular regeneration (MusCAMLR). The MusCAMLR is enforced to attain anisotropic muscle-like characteristics while incorporating a smartly passivated nanoscale carbon material in the PNIPAM gel under physiological conditions as a stimulus, which is not achieved by the pristine nanocarbon system. The MusCAMLR establishes a specific mechanical interaction with muscle cells, supports myotube regeneration, maintains excellent mechanical similarity with the myotube, and restores the structural integrity and biochemical parameters of mechanically damaged muscles in a delayed onset muscle soreness (DOMS) rat model within a short period of 72 h. Concisely, this study discloses the potential of smartly passivated nanocarbon in generating an advanced biomaterial system, MusCAMLR, from a regularly used polymeric hydrogel system. This engineered polymer-carbon composite reveals its possible potential to be used as a nondrug therapeutic alternative for rescuing mechanically damaged muscles and probably can be extended for therapy of various other diseases including muscular dystrophy.

Satellite cell contribution to disease pathology in Duchenne muscular dystrophy

Progressive muscle weakness and degeneration characterize Duchenne muscular dystrophy (DMD), a lethal, x-linked neuromuscular disorder that affects 1 in 5,000 boys. Loss of dystrophin protein leads to recurrent muscle degeneration, progressive fibrosis, chronic inflammation, and dysfunction of skeletal muscle resident stem cells, called satellite cells. Unfortunately, there is currently no cure for DMD. In this mini review, we discuss how satellite cells in dystrophic muscle are functionally impaired, and how this contributes to the DMD pathology, and the tremendous potential of restoring endogenous satellite cell function as a viable treatment strategy to treat this debilitating and fatal disease.

Gut-muscle axis and sepsis-induced myopathy: The potential role of gut microbiota

Sepsis is described as an immune response disorder of the host to infection in which microorganisms play a non-negligible role. Most survivors of sepsis experience ICU-acquired weakness, also known as septic myopathy, characterized by skeletal muscle atrophy, weakness, and irreparable damage/regenerated or dysfunctional. The mechanism of sepsis-induced myopathy is currently unclear. It has been believed that this state is triggered by circulating pathogens and their related harmful factors, leading to impaired muscle metabolism. Sepsis and its resulting alterations in the intestinal microbiota are associated with sepsis-related organ dysfunction, including skeletal muscle wasting. There are also some studies on interventions targeting the flora, including fecal microbiota transplants, the addition of dietary fiber and probiotics in enteral feeding products, etc., aiming to improve sepsis-related myopathy. In this review, we critically assess the potential mechanisms and therapeutic prospects of intestinal flora in the development of septic myopathy.

Diffusion-tensor magnetic resonance imaging captures increased skeletal muscle fibre diameters in Becker muscular dystrophy

BACKGROUND

Becker muscular dystrophy (BMD) is an X-linked disorder characterized by slow, progressive muscle damage and muscle weakness. Hallmarks include fibre-size variation and replacement of skeletal muscle with fibrous and adipose tissues, after repeated cycles of regeneration. Muscle histology can detect these features, but the required biopsies are invasive, are difficult to repeat and capture only small muscle volumes. Diffusion-tensor magnetic resonance imaging (DT-MRI) is a potential non-invasive alternative that can calculate muscle fibre diameters when applied with the novel random permeable barrier model (RPBM). In this study, we assessed muscle fibre diameters using DT-MRI in BMD patients and healthy controls and compared these with histology.

METHODS

We included 13 BMD patients and 9 age-matched controls, who underwent water-fat MRI and DT-MRI at multiple diffusion times, allowing RPBM parameter estimation in the lower leg muscles. Tibialis anterior muscle biopsies were taken from the contralateral leg in 6 BMD patients who underwent DT-MRI and from an additional 32 BMD patients and 15 healthy controls. Laminin and Sirius-red stainings were performed to evaluate muscle fibre morphology and fibrosis. Twelve ambulant patients from the MRI cohort underwent the North Star ambulatory assessment, and 6-min walk, rise-from-floor and 10-m run/walk functional tests.

RESULTS

RPBM fibre diameter was significantly larger in BMD patients (P = 0.015): mean (SD) = 68.0 (25.3) μm versus 59.4 (19.2) μm in controls. Inter-muscle differences were also observed (P ≤ 0.002). Both inter- and intra-individual RPBM fibre diameter variability were similar between groups. Laminin staining agreed with the RPBM, showing larger median fibre diameters in patients than in controls: 72.5 (7.9) versus 63.2 (6.9) μm, P = 0.006. However, despite showing similar inter-individual variation, patients showed more intra-individual fibre diameter variability than controls-mean variance (SD) = 34.2 (7.9) versus 21.4 (6.9) μm, P < 0.001-and larger fibrosis areas: median (interquartile range) = 21.7 (5.6)% versus 14.9 (3.4)%, P < 0.001. Despite good overall agreement of RPBM and laminin fibre diameters, they were not associated in patients who underwent DT-MRI and muscle biopsy, perhaps due to lack of colocalization of DT-MRI with biopsy samples.

CONCLUSIONS

DT-MRI RPBM metrics agree with histology and can quantify changes in muscle fibre size that are associated with regeneration without the need for biopsies. They therefore show promise as imaging biomarkers for muscular dystrophies.

Macrophage Involvement in Aging-Associated Skeletal Muscle Regeneration

The skeletal muscle is a dynamic organ composed of contractile muscle fibers, connective tissues, blood vessels and nerve endings. Its main function is to provide motility to the body, but it is also deeply involved in systemic metabolism and thermoregulation. The skeletal muscle frequently encounters microinjury or trauma, which is primarily repaired by the coordinated actions of muscle stem cells (satellite cells, SCs), fibro-adipogenic progenitors (FAPs), and multiple immune cells, particularly macrophages. During aging, however, the capacity of skeletal muscle to repair and regenerate declines, likely contributing to sarcopenia, an age-related condition defined as loss of muscle mass and function. Recent studies have shown that resident macrophages in skeletal muscle are highly heterogeneous, and their phenotypes shift during aging, which may exacerbate skeletal muscle deterioration and inefficient regeneration. In this review, we highlight recent insight into the heterogeneity and functional roles of macrophages in skeletal muscle regeneration, particularly as it declines with aging.

3D human induced pluripotent stem cell-derived bioengineered skeletal muscles for tissue, disease and therapy modeling

Skeletal muscle is a complex tissue composed of multinucleated myofibers responsible for force generation that are supported by multiple cell types. Many severe and lethal disorders affect skeletal muscle; therefore, engineering models to reproduce such cellular complexity and function are instrumental for investigating muscle pathophysiology and developing therapies. Here, we detail the modular 3D bioengineering of multilineage skeletal muscles from human induced pluripotent stem cells, which are first differentiated into myogenic, neural and vascular progenitor cells and then combined within 3D hydrogels under tension to generate an aligned myofiber scaffold containing vascular networks and motor neurons. 3D bioengineered muscles recapitulate morphological and functional features of human skeletal muscle, including establishment of a pool of cells expressing muscle stem cell markers. Importantly, bioengineered muscles provide a high-fidelity platform to study muscle pathology, such as emergence of dysmorphic nuclei in muscular dystrophies caused by mutant lamins. The protocol is easy to follow for operators with cell culture experience and takes between 9 and 30 d, depending on the number of cell lineages in the construct. We also provide examples of applications of this advanced platform for testing gene and cell therapies in vitro, as well as for in vivo studies, providing proof of principle of its potential as a tool to develop next-generation neuromuscular or musculoskeletal therapies.

The Role of the Piezo1 Mechanosensitive Channel in the Musculoskeletal System

Since the recent discovery of the mechanosensitive Piezo1 channels, many studies have addressed the role of the channel in various physiological or even pathological processes of different organs. Although the number of studies on their effects on the musculoskeletal system is constantly increasing, we are still far from a precise understanding. In this review, the knowledge available so far regarding the musculoskeletal system is summarized, reviewing the results achieved in the field of skeletal muscles, bones, joints and cartilage, tendons and ligaments, as well as intervertebral discs.

Stem cell therapy combined with controlled release of growth factors for the treatment of sphincter dysfunction

Sphincter dysfunction often occurs at the end of tubule organs such as the urethra, anus, or gastroesophageal sphincters. It is the primary consequence of neuromuscular impairment caused by trauma, inflammation, and aging. Despite intensive efforts to recover sphincter function, pharmacological treatments have not achieved significant improvement. Cell- or growth factor-based therapy is a promising approach for neuromuscular regeneration and the recovery of sphincter function. However, a decrease in cell retention and viability, or the short half-life and rapid degradation of growth factors after implantation, remain obstacles to the translation of these therapies to the clinic. Natural biomaterials provide unique tools for controlled growth factor delivery, which leads to better outcomes for sphincter function recovery in vivo when stem cells and growth factors are co-administrated, in comparison to the delivery of single therapies. In this review, we discuss the role of stem cells combined with the controlled release of growth factors, the methods used for delivery, their potential therapeutic role in neuromuscular repair, and the outcomes of preclinical studies using combination therapy, with the hope of providing new therapeutic strategies to treat incontinence or sphincter dysfunction of the urethra, anus, or gastroesophageal tissues, respectively.

Effect of heme oxygenase-1 on the differentiation of human myoblasts and the regeneration of murine skeletal muscles after acute and chronic injury

BACKGROUND

Impaired muscle regeneration is a hallmark of Duchenne muscular dystrophy (DMD), a neuromuscular disorder caused by mutations in the DMD gene encoding dystrophin. The lack of heme oxygenase-1 (HO-1, Hmox1), a known anti-inflammatory and cytoprotective enzyme, was shown to aggravate DMD pathology.

METHODS

We evaluated the role of HO-1 overexpression in human induced pluripotent stem cell (hiPSC)-derived skeletal muscle cells (hiPSC-SkM) in vitro and in the regeneration process in vivo in wild-type mice. Furthermore, the effect of cobalt protoporphyrin IX (CoPP), a pharmacological inducer of HO-1 expression, on regeneration markers during myogenic hiPSC differentiation and progression of the dystrophic phenotype was analysed in the mdx mouse DMD model.

RESULTS

HO-1 has an impact on hiPSC-SkM generation by decreasing cell fusion capacity and the expression of myogenic regulatory factors and muscle-specific microRNAs (myomiRs). Also, strong induction of HO-1 by CoPP totally abolished hiPSC-SkM differentiation. Injection of HO-1-overexpressing hiPSC-SkM into the cardiotoxin (CTX)-injured muscle of immunodeficient wild-type mice was associated with decreased expression of miR-206 and Myh3 and lower number of regenerating fibers, suggesting some advanced regeneration. However, the very potent induction of HO-1 by CoPP did not exert any protective effect on necrosis, leukocyte infiltration, fibrosis, myofiber regeneration biomarkers, and exercise capacity of mdx mice.

CONCLUSIONS

In summary, HO-1 inhibits the expression of differentiation markers in human iPSC-derived myoblasts. Although moderate overexpression of HO-1 in the injected myoblast was associated with partially advanced muscle regeneration, the high systemic induction of HO-1 did not improve muscle regeneration. The appropriate threshold of HO-1 expression must be established for the therapeutic effect of HO-1 on muscle regeneration.

Cas9-induced single cut enables highly efficient and template-free repair of a muscular dystrophy causing founder mutation

With thousands of patients worldwide, CAPN3 c.550delA is the most frequent mutation causing severe, progressive, and untreatable limb girdle muscular dystrophy. We aimed to genetically correct this founder mutation in primary human muscle stem cells. We designed editing strategies providing CRISPR-Cas9 as plasmid and mRNA first in patient-derived induced pluripotent stem cells and applied this strategy then in primary human muscle stem cells from patients. Mutation-specific targeting yielded highly efficient and precise correction of CAPN3 c.550delA to wild type for both cell types. Most likely a single cut generated by SpCas9 resulted in a 5' staggered overhang of one base pair, which triggered an overhang-dependent base replication of an A:T at the mutation site. This recovered the open reading frame and the CAPN3 DNA sequence was repaired template-free to wild type, which led to CAPN3 mRNA and protein expression. Off-target analysis using amplicon sequencing of 43 in silico predicted sites demonstrates the safety of this approach. Our study extends previous usage of single cut DNA modification since our gene product has been repaired into the wild-type CAPN3 sequence with the perspective of a real cure.

P2Y1R and P2Y2R: potential molecular triggers in muscle regeneration

Muscle regeneration is indispensable for skeletal muscle health and daily life when injury, muscular disease, and aging occur. Among the muscle regeneration, muscle stem cells' (MuSCs) activation, proliferation, and differentiation play a key role in muscle regeneration. Purines bind to its specific receptors during muscle development, which transmit environmental stimuli and play a crucial role of modulator of muscle regeneration. Evidences proved P2R expression during development and regeneration of skeletal muscle, both in human and mouse. In contrast to P2XR, which have been extensively investigated in skeletal muscles, the knowledge of P2YR in this tissue is less comprehensive. This review summarized muscle regeneration via P2Y1R and P2Y2R and speculated that P2Y1R and P2Y2R might be potential molecular triggers for MuSCs' activation and proliferation via the p-ERK1/2 and PLC pathways, explored their cascade effects on skeletal muscle, and proposed P2Y1/2 receptors as potential pharmacological targets in muscle regeneration, to advance the purinergic signaling within muscle and provide promising strategies for alleviating muscular disease.

CPNE1 regulates myogenesis through the PERK-eIF2α pathway mediated by endoplasmic reticulum stress

Sarcopenia is characterized by a progressive reduction in muscle mass or muscle physiological function associated with aging, but the relevant molecular mechanisms are not clear. Here, we identify the role of the myogenesis modifier CPNE1 in sarcopenia. CPNE1 is upregulated in aged skeletal muscles and young skeletal muscle satellite cells with palmitate-induced atrophy. The overexpression of CPNE1 hinders proliferation and differentiation and increases muscle atrophy characteristics in young skeletal muscle-derived satellite cells. In addition, CPNE1 overexpression disrupts the balance of mitochondrial fusion and division and causes endoplasmic reticulum stress. We found that the effects of CPNE1 on mitochondrial function are dependent on the PERK/eIF2α/ATF4 pathway. The overexpression of CPNE1 in young muscles alters membrane lipid composition, reduces skeletal muscle fibrosis regeneration, and exercise capacity in mice. These effects were reversed by PERK inhibitor GSK2606414. Moreover, immunoprecipitation indicates that CPNE1 overexpression greatly increased the acetylation of PERK. Therefore, CPNE1 is an important modifier that drives mitochondrial homeostasis to regulate myogenic cell proliferation and differentiation via the PERK-eIF2α pathway, which could be a valuable target for age-related sarcopenia.

The Role of Mitophagy in Skeletal Muscle Damage and Regeneration

Mitochondria are cellular organelles that play an essential role in generating the chemical energy needed for the biochemical reactions in cells. Mitochondrial biogenesis, i.e., de novo mitochondria formation, results in enhanced cellular respiration, metabolic processes, and ATP generation, while autophagic clearance of mitochondria (mitophagy) is required to remove damaged or useless mitochondria. The balance between the opposing processes of mitochondrial biogenesis and mitophagy is highly regulated and crucial for the maintenance of the number and function of mitochondria as well as for the cellular homeostasis and adaptations to metabolic demands and extracellular stimuli. In skeletal muscle, mitochondria are essential for maintaining energy homeostasis, and the mitochondrial network exhibits complex behaviors and undergoes dynamic remodeling in response to various conditions and pathologies characterized by changes in muscle cell structure and metabolism, such as exercise, muscle damage, and myopathies. In particular, the involvement of mitochondrial remodeling in mediating skeletal muscle regeneration following damage has received increased attention, as modifications in mitophagy-related signals arise from exercise, while variations in mitochondrial restructuring pathways can lead to partial regeneration and impaired muscle function. Muscle regeneration (through myogenesis) following exercise-induced damage is characterized by a highly regulated, rapid turnover of poor-functioning mitochondria, permitting the synthesis of better-functioning mitochondria to occur. Nevertheless, essential aspects of mitochondrial remodeling during muscle regeneration remain poorly understood and warrant further characterization. In this review, we focus on the critical role of mitophagy for proper muscle cell regeneration following damage, highlighting the molecular mechanisms of the mitophagy-associated mitochondrial dynamics and network reformation.

Cellular interplay in skeletal muscle regeneration and wasting: insights from animal models

Skeletal muscle wasting, whether related to physiological ageing, muscle disuse or to an underlying chronic disease, is a key determinant to quality of life and mortality. However, cellular basis responsible for increased catabolism in myocytes often remains unclear. Although myocytes represent the vast majority of skeletal muscle cellular population, they are surrounded by numerous cells with various functions. Animal models, mostly rodents, can help to decipher the mechanisms behind this highly dynamic process, by allowing access to every muscle as well as time-course studies. Satellite cells (SCs) play a crucial role in muscle regeneration, within a niche also composed of fibroblasts and vascular and immune cells. Their proliferation and differentiation is altered in several models of muscle wasting such as cancer, chronic kidney disease or chronic obstructive pulmonary disease (COPD). Fibro-adipogenic progenitor cells are also responsible for functional muscle growth and repair and are associated in disease to muscle fibrosis such as in chronic kidney disease. Other cells have recently proven to have direct myogenic potential, such as pericytes. Outside their role in angiogenesis, endothelial cells and pericytes also participate to healthy muscle homoeostasis by promoting SC pool maintenance (so-called myogenesis-angiogenesis coupling). Their role in chronic diseases muscle wasting has been less studied. Immune cells are pivotal for muscle repair after injury: Macrophages undergo a transition from the M1 to the M2 state along with the transition between the inflammatory and resolutive phase of muscle repair. T regulatory lymphocytes promote and regulate this transition and are also able to activate SC proliferation and differentiation. Neural cells such as terminal Schwann cells, motor neurons and kranocytes are notably implicated in age-related sarcopenia. Last, newly identified cells in skeletal muscle, such as telocytes or interstitial tenocytes could play a role in tissular homoeostasis. We also put a special focus on cellular alterations occurring in COPD, a chronic and highly prevalent respiratory disease mainly linked to tobacco smoke exposure, where muscle wasting is strongly associated with increased mortality, and discuss the pros and cons of animal models versus human studies in this context. Finally, we discuss resident cells metabolism and present future promising leads for research, including the use of muscle organoids.