An absolute requirement for Pax7-positive satellite cells in acute injury-induced skeletal muscle regeneration

Insights into human muscle biology from human primary skeletal muscle cell culture

This review arises from the symposium held in honour of Prof Jenny Morgan at UCL in 2024 and the authors would like to acknowledge the outstanding contribution that Prof Morgan has made to the field of translational muscle cell biology. Prof Morgan published a review article in 2010 entitled: Are human and mice satellite cells really the same? In which the authors highlighted differences between species which are still pertinent to skeletal muscle cell culture studies today. To our knowledge there are no comprehensive reviews which outline the considerable work that has been undertaken using human primary skeletal muscle origin cells as the main model system. This review highlights the multitude of muscle biology that has been investigated using human primary cells, as well as discussing the advantages and disadvantages over other cell models. We also discuss future directions for primary cell culture models utilising the latest technologies in cell type specificity and culture systems.

Mitochondrial fatty acid oxidation regulates adult muscle stem cell function through modulating metabolic flux and protein acetylation

During homeostasis and regeneration, satellite cells, the resident stem cells of skeletal muscle, have distinct metabolic requirements for fate transitions between quiescence, proliferation and differentiation. However, the contribution of distinct energy sources to satellite cell metabolism and function remains largely unexplored. Here, we uncover a role of mitochondrial fatty acid oxidation (FAO) in satellite cell integrity and function. Single-cell RNA sequencing revealed progressive enrichment of mitochondrial FAO and downstream pathways during activation, proliferation and myogenic commitment of satellite cells. Deletion of Carnitine palmitoyltransferase 2 (Cpt2), the rate-limiting enzyme in FAO, hampered muscle stem cell expansion and differentiation upon acute muscle injury, markedly delaying regeneration. Cpt2 deficiency reduces acetyl-CoA levels in satellite cells, impeding the metabolic flux and acetylation of selective proteins including Pax7, the central transcriptional regulator of satellite cells. Notably, acetate supplementation restored cellular metabolic flux and partially rescued the regenerative defects of Cpt2-null satellite cells. These findings highlight an essential role of fatty acid oxidation in controlling satellite cell function and suggest an integration of lipid metabolism and protein acetylation in adult stem cells.

Bioinstructive scaffolds enhance stem cell engraftment for functional tissue regeneration

Stem cell therapy is a promising approach for tissue regeneration after traumatic injury, yet current applications are limited by inadequate control over the fate of stem cells after transplantation. Here we introduce a bioconstruct engineered for the staged release of growth factors, tailored to direct different phases of muscle regeneration. The bioconstruct is composed of a decellularized extracellular matrix containing polymeric nanocapsules sequentially releasing basic fibroblast growth factor and insulin-like growth factor 1, which promote the proliferation and differentiation of muscle stem cells, respectively. When applied to a volumetric muscle loss defect in an animal model, the bioconstruct enhances myofibre formation, angiogenesis, innervation and functional restoration. Further, it promotes functional muscle formation with human or aged murine muscle stem cells, highlighting the translational potential of this bioconstruct. Overall, these results highlight the potential of bioconstructs with orchestrated growth factor release for stem cell therapies in traumatic injury.

Unveiling Key Genes and Crucial Pathways in Goose Muscle Satellite Cell Biology Through Integrated Transcriptomic and Metabolomic Analyses

Skeletal muscle satellite cells (SMSCs) are quiescent stem cells located in skeletal muscle tissue and function as the primary reservoir of myogenic progenitors for muscle growth and regeneration. However, the molecular and metabolic mechanisms governing their differentiation in geese remain largely unexplored. This study comprehensively examined the morphological, transcriptional, and metabolic dynamics of goose SMSCs across three critical differentiation stages: the quiescent stage (DD0), the differentiation stage (DD4), and the late differentiation stage (DD6). By integrating transcriptomic and metabolomic analyses, stage-specific molecular signatures and regulatory networks involved in SMSC differentiation were identified. Principal component analysis revealed distinct clustering patterns in gene expression and metabolite profiles across these stages, highlighting dynamic shifts in lipid metabolism and myogenesis. The PPAR signaling pathway emerged as a key regulator, with crucial genes such as PPARG, IGF1, ACSL5, FABP5, and PLIN1 exhibiting differentiation-dependent expression patterns. Notably, PPARG and IGF1 displayed negative correlations with adenosine and L-carnitine levels, suggesting their role in metabolic reprogramming during myotube formation. Additionally, MYOM2 and MYBPC1 exhibited stage-specific regulation and positively correlated with 2,3-dimethoxyphenylamine. This study provides a foundational framework for understanding muscle development and regeneration, offering valuable insights for both agricultural and biomedical research.

3D Mechanical Confinement Directs Muscle Stem Cell Fate and Function

Muscle stem cells (MuSCs) play a crucial role in skeletal muscle regeneration, residing in a niche that undergoes dimensional and mechanical changes throughout the regeneration process. This study investigates how 3D confinement and stiffness encountered by MuSCs during the later stages of regeneration regulate their function, including stemness, activation, proliferation, and differentiation. An asymmetric 3D hydrogel bilayer platform is engineered with tunable physical constraints to mimic the regenerating MuSC niche. These results demonstrate that increased 3D confinement maintains Pax7 expression, reduces MuSC activation and proliferation, inhibits differentiation, and is associated with smaller nuclear size and decreased H4K16ac levels, suggesting that mechanical confinement modulates both nuclear architecture and epigenetic regulation. MuSCs in unconfined 2D environments exhibit larger nuclei and higher H4K16ac expression compared to those in more confined 3D conditions, leading to progressive activation, expansion, and myogenic commitment. This study highlights the importance of 3D mechanical cues in MuSC fate regulation, with 3D confinement acting as a mechanical brake on myogenic commitment, offering novel insights into the mechano-epigenetic mechanisms that govern MuSC behavior during muscle regeneration.

Impact of Passaging Primary Skeletal Muscle Cell Isolates on the Engineering of Skeletal Muscle

Volumetric muscle loss (VML) is a clinical state that results in impaired skeletal muscle function. Engineered skeletal muscle can serve as a treatment for VML. Currently, large biopsies are required to achieve the cells necessary for the fabrication of engineered muscle, leading to donor-site morbidity. Amplification of cell numbers using cell passaging may increase the usefulness of a single muscle biopsy for engineering muscle tissue. In this study, we evaluated the impact of passaging cells obtained from donor muscle tissue by analyzing characteristics of in vitro cellular growth and tissue-engineered skeletal muscle unit (SMU) structure and function. Human skeletal muscle cell isolates from three separate donors (P0-Control) were compared with cells passaged once (P1), twice (P2), or three times (P3) by monitoring SMU force production and determining muscle content and structure using immunohistochemistry. Data indicated that passaging decreased the number of satellite cells and increased the population doubling time. P1 SMUs had slightly greater contractile force and P2 SMUs showed statistically significant greater force production compared with P0 SMUs with no change in SMU muscle content. In conclusion, human skeletal muscle cells can be passaged twice without negatively impacting SMU muscle content or contractile function, providing the opportunity to potentially create larger SMUs from smaller biopsies, thereby producing clinically relevant sized grafts to aid in VML repair.

ZIP13 marks muscle satellite cells and contributes to their quiescent and active phase balance

Loss of ZIP13 causes Ehlers-Danlos syndrome spondylodysplastic type 3 involving connective tissue dysplasias associated with a reduction in muscular strength. However, ZIP13 role in skeletal muscle homeostasis, particularly for the regulation of muscle satellite cells (MuSCs), remains poorly understood. In this study, we investigated Zip13-knockout (KO) mice and found a reduction in MuSCs of Zip13-KO mice, in which the quiescent and activated phase balances were disrupted. To clarify the physiological role and dynamics of ZIP13 expression in MuSCs, we generated Zip13-GFP knock-in (KI) mice encoding GFP at the Zip13 locus, which showed that ZIP13 contributes to the phase balance regulation of quiescent and activated MuSCs and their functions. Indeed, Zip13-KO mice exhibited delayed recovery from skeletal muscle injury, indicating ZIP13 requirement for proper skeletal muscle regeneration. Moreover, GFP expression was reduced in the MuSCs of homozygous Zip13-GFP KI mice whose intact ZIP13 expression was perturbed, suggesting that positive feedback mechanisms exist to maintain ZIP13 expression. Altogether, our results illustrate that ZIP13 might be positively involved in skeletal muscle regeneration by controlling the quiescent/activated phase balance of MuSCs through autoregulatory ZIP13 expression, and that newly generated Zip13-GFP KI mice would be useful for investigating the roles and dynamics of ZIP13-expressing cells.

Biomaterial-Based Regenerative Strategies for Volumetric Muscle Loss: Challenges and Solutions

Significance: Volumetric muscle loss (VML) is caused by the loss of significant amounts of skeletal muscle tissue. VML cannot be repaired by intrinsic regenerative processes, resulting in permanent loss of muscle function and disability. Current rehabilitative-focused treatment strategies lack efficacy and do not restore muscle function, indicating the need for the development of effective regenerative strategies. Recent Advances: Recent developments implicate biomaterial-based approaches for promoting muscle repair and functional restoration post-VML. Specifically, bioscaffolds transplanted in the injury site have been utilized to mimic endogenous cues of the ablated tissue to promote myogenic pathways, increase neo-myofiber synthesis, and ultimately restore contractile function to the injured unit. Critical Issues: Despite the development and preclinical testing of various biomaterial-based regenerative strategies, effective therapies for patients are not available. The unique challenges posed for biomaterial-based treatments of VML injuries, including its scalability and clinical applicability beyond small-animal models, impede progress. Furthermore, production of tissue-engineered constructs is technically demanding, with reproducibility issues at scale and complexities in achieving vascularization and innervation of large constructs. Future Directions: Biomaterial-based regenerative strategies designed to comprehensively address the pathophysiology of VML are needed. Considerations for clinical translation, including scalability and regulatory compliance, should also be considered when developing such strategies. In addition, an integrated approach that combines regenerative and rehabilitative strategies is essential for ensuring functional improvement.

Increased muscle satellite cell content and preserved telomere length in response to combined exercise training in patients with FSHD

Facioscapulohumeral muscular dystrophy (FSHD) is an inherited muscle disease characterized by weakness and muscle wasting. In the absence of available treatments, exercise training has emerged as a potential strategy to attenuate muscle tissue deterioration. However, little is known about the impact of chronic exercise on degenerative events and regenerative capacity in FSHD muscle. Muscle biopsies were obtained from 16 FSHD patients before and after a 24 week training program combining aerobic-, strength- and high-intensity exercise (Control; n = 8, Training; n = 8). Histochemical and immunohistochemical approaches were applied to assess histopathological signs, markers of regeneration, inflammatory infiltrates and satellite cell content. Muscle telomere length was measured as an indicator of the remaining regenerative capacity. The proportion of muscle fibres expressing developmental myosins and centralized myonuclei was not exacerbated after the intervention. Similarly, no alterations were observed in the number of inflammatory infiltrates (CD68+ cells). Alongside muscle hypertrophy in slow (P = 0.022) and fast fibres (P = 0.022 and P = 0.008), satellite cell content increased specifically in fast fibres (+75 %, P = 0.015), indicating a functional satellite cell pool in FSHD muscle. Importantly, exercise training was not associated with a shortening of muscle telomere length, suggesting that muscle cell turnover was not accelerated despite an expansion of the satellite cell pool. Our findings suggest that combined exercise training elicits beneficial muscular adaptations without impairing important indicators of skeletal muscle regenerative capacity in patients with FSHD. KEY POINTS: A 24 week combined exercise training program is a safe and well-tolerated strategy to attenuate skeletal muscle deterioration in facioscapulohumeral muscular dystrophy (FSHD) patients. Markers of histopathology, muscle fibre regeneration and inflammatory infiltrates were not exacerbated following exercise training in FSHD muscle. Here, we show novel data that exercise training in FSHD patients induced muscle fibre hypertrophy and triggered an expansion of the satellite cell pool specifically in fast fibres. Exercise training in these patients is not associated with a shortening of muscle telomere length thereby indicating a preserved capacity for muscle regeneration.

Muscle fibroblasts and stem cells stimulate motor neurons in an age and exercise-dependent manner

Exercise preserves neuromuscular function in aging through unknown mechanisms. Skeletal muscle fibroblasts (FIB) and stem cells (MuSC) are abundant in skeletal muscle and reside close to neuromuscular junctions, but their relative roles in motor neuron maintenance remain undescribed. Using direct cocultures of embryonic rat motor neurons with either human MuSC or FIB, RNA sequencing revealed profound differential regulation of the motor neuron transcriptome, with FIB generally favoring neuron growth and cell migration and MuSC favoring production of ribosomes and translational machinery. Conditioned medium from FIB was superior to MuSC in preserving motor neurons and increasing their maturity. Lastly, we established the importance of donor age and exercise status and found an age-related distortion of motor neuron and muscle cell interaction that was fully mitigated by lifelong physical activity. In conclusion, we show that human muscle FIB and MuSC synergistically stimulate the growth and viability of motor neurons, which is further amplified by regular exercise.

Combined magnesium and silicon ions synergistically promote functional regeneration of skeletal muscle by regulating satellite cell fate

Muscle satellite cells (MuSCs) play a vital role in skeletal muscle regeneration. However, in intractable muscle diseases such as volumetric muscle loss (VML), the quantity and function of MuSCs are significantly reduced, severely limiting the body's inherent muscle regeneration capability. In this study, we propose a novel strategy to modulate the fate of MuSCs using a combination of bioactive magnesium (Mg) and silicon (Si) ions, sustainably delivered through magnesium silicate (MgSiO3, MS) bioceramic-based scaffolds. In vitro, Mg and Si ions synergistically promote the proliferation and differentiation of MuSCs. Similarly, Mg and Si ions derived from MS/poly(L-lactic acid) (MS/PLLA) composite scaffold also increase the proliferation and differentiation ability of MuSCs. Furthermore, MS/PLLA composite scaffolds facilitate the activation of MuSCs, regeneration of muscle fiber and neovascularization, while inhibiting fibrosis, thereby effectively restoring muscle function and promoting tibialis anterior muscle functional regeneration in a VML mouse model. Mechanistically, the combination of Mg and Si ions promotes the activation and proliferation of MuSCs by activating the Notch1-Hes1 pathway. Besides, the combination of Mg and Si ions also improves the differentiation of MuSCs by up-regulating Myod and Myog, and enhances fusion by up-regulating Mymk and Mymx expression. The outcomes of our research introduce a promising approach to the treatment of skeletal muscle injuries and related diseases.

Neuromuscular electrical stimulation training induces myonuclear accretion and hypertrophy in mice without overt signs of muscle damage and regeneration

BACKGROUND

Skeletal muscle is a plastic tissue that adapts to increased mechanical loading/contractile activity through fusion of muscle stem cells (MuSCs) with myofibers, a physiological process referred to as myonuclear accretion. However, it is still unclear whether myonuclear accretion is driven by increased mechanical loading per se, or occurs, at least in part, in response to muscle injury/regeneration. Here, we developed a non-damaging protocol to evaluate contractile activity-induced myonuclear accretion/hypertrophy in physiological conditions.

METHODS

Contractile activity was generated by applying repeated electrical stimuli over the mouse plantar flexor muscles. This method is commonly referred to as NeuroMuscular Electrical Simulation (NMES) in Human. Each NMES training session consisted of 80 isometric contractions delivered at ∼15% of maximal tetanic force to avoid muscle damage. C57BL/6J male mice were submitted to either a short (i.e., 6 sessions) or long (i.e., 12 sessions) individualized NMES training program while unstimulated mice were used as controls. Histological investigations were performed to assess the impact of NMES on MuSC number and status, myonuclei content and muscle tissue integrity, typology and size.

RESULTS

NMES led to a robust proliferation of MuSCs and myonuclear accretion in the absence of overt signs of muscle damage/regeneration. NMES-induced myonuclear accretion was specific to type IIB myofibers and was an early event preceding muscle hypertrophy inasmuch as a mild increase in myofiber cross-sectional area was only observed in response to the long-term NMES training protocol.

CONCLUSION

We conclude that NMES-induced myonuclear accretion and muscle hypertrophy are driven by a mild increase in mechanical loading in the absence of overt signs of muscle injury.

C2C12 myoblasts differentiate into myofibroblasts via the TGF-β1 signaling pathway mediated by Fibulin2

Myoblasts play a critical role in the regeneration of skeletal muscle following injury. It has been reported that local elevation of transforming growth factor-β1 (TGF-β1) after skeletal muscle injury induces differentiation of myoblasts into myofibroblasts. However, the mechanisms underlying this differentiation process remain incompletely understood. In this study, we found that Fibulin2 expression significantly increases in myoblasts in response to TGF-β1 stimulation. Elevated Fibulin2 levels enhance the expression of fibrotic markers. Conversely, downregulation of Fibulin2 in myoblasts inhibits the upregulation of fibrotic markers induced by TGF-β1 stimulation. Extracellular secretion of Fibulin2 activates the TGF-β1-Smad2 pathway, thereby promoting the upregulation of fibrotic markers. Hence, Fibulin2 and TGF-β1 form a positive feedback loop that facilitates differentiation of myoblasts into myofibroblasts.

Molecular mechanisms and potential interventions during aging-associated sarcopenia

Sarcopenia, a common condition observed in the elderly, presenting a significant public health challenge due to its high prevalence, insidious onset and diverse systemic effects. Despite ongoing research, the precise etiology of sarcopenia remains elusive. Aging-related processes, which included inflammation, oxidative stress, compromised mitochondrial function and apoptosis, have been implicated in its development. Notably, effective pharmacological treatments for sarcopenia are currently lacking, highlighting the necessity for a deeper understanding of its pathogenesis and causative factors to enable proactive interventions. This article is aimed to provide an extensive overview of the pathogenesis of sarcopenia, along with a summary of current treatment and prevention strategies.

Ageing leads to selective type II myofibre deterioration and denervation independent of reinnervative capacity in human skeletal muscle

Age-related loss of muscle mass and function is underpinned by changes at the myocellular level. However, our understanding of the aged muscle phenotype might be confounded by factors secondary to ageing per se, such as inactivity and adiposity. Here, using healthy, lean, recreationally active, older men, we investigated the impact of ageing on myocellular properties in skeletal muscle. Muscle biopsies were obtained from young men (22 ± 3 years, n = 10) and older men (69 ± 3 years, n = 11) matched for health status, activity level and body mass index. Immunofluorescence was used to assess myofibre composition, morphology (size and shape), capillarization, the content of satellite cells and myonuclei, the spatial relationship between satellite cells and capillaries, denervation and myofibre grouping. Compared with young muscle, aged muscle contained 53% more type I myofibres, in addition to smaller (-32%) and misshapen (3%) type II myofibres (P < 0.05). Aged muscle manifested fewer capillaries (-29%) and satellite cells (-38%) surrounding type II myofibres (P < 0.05); however, the spatial relationship between these two remained intact. The proportion of denervated myofibres was ∼2.6-fold higher in old than young muscle (P < 0.05). Aged muscle had more grouped type I myofibres (∼18-fold), primarily driven by increased size of existing groups rather than increased group frequency (P < 0.05). Aged muscle displayed selective deterioration of type II myofibres alongside increased denervation and myofibre grouping. These data are key to understanding the cellular basis of age-related muscle decline and reveal a pressing need to fine-tune strategies to preserve type II myofibres and innervation status in ageing populations.

Adiponectin regulates proliferation and differentiation of chicken skeletal muscle satellite cells via ERK1/2 and p38 signaling pathways

Skeletal muscle satellite cells (SMSCs) are critical for postnatal skeletal muscle growth and regeneration. Adiponectin plays a pivotal role in regulating muscle glucose uptake and fatty acid metabolism. However, its function in the proliferation and differentiation of chicken SMSCs remains poorly understood. In this study, we investigated the effects of adiponectin on the proliferation and differentiation of in vitro cultured chicken SMSCs. Our results demonstrated that adiponectin promoted SMSCs proliferation while inhibiting myogenic differentiation and inducing adipogenic differentiation. RNA-seq analysis revealed enrichment of the MAPK signaling pathway, suggesting its potential involvement in the regulation of adiponectin on SMSCs activity. Western blot analysis revealed that adiponectin activated ERK1/2 phosphorylation and inhibited p38 phosphorylation during the process of the inhibition on myogenic differentiation in chicken SMSCs. Furthermore, suppression of ERK1/2 signaling with U0126 or activation of p38 signaling with SSK1 reversed the downregulated expression of myogenic differentiation marker MyHC, MyOD1, and MyOG induced by adiponectin. These findings validated that adiponectin impeded myogenic differentiation through activation of ERK1/2 and inhibition of p38 signaling pathways. Additionally, activation of p38 signaling pathway reduced the increased percentage of EdU-positive cells induced by adiponectin. Collectively, these findings demonstrated that adiponectin impedes myogenic differentiation of SMSCs through activating ERK1/2 and inhibiting p38 signaling pathways, while promoting proliferation by inhibiting p38 signaling pathway.

Multitasking muscle: engineering iPSC-derived myogenic progenitors to do more

The generation of myogenic progenitors from iPSCs (iMPs) with therapeutic potential for in vivo tissue regeneration has long been a goal in the skeletal muscle community. Today, protocols enable the production of potent, albeit immature, iMPs that resemble Pax7+ adult muscle stem cells. While muscular dystrophies are often the primary therapeutic target for these cells, an underexplored application is their use in treating traumatic muscle injuries. Notably absent from recent reviews on iMPs is the concept of engineering these cells to perform functions post-transplantation that non-transgenic cells cannot. Here, we highlight protocols to enhance the generation, purification, and maturation of iMPs, and introduce the idea of engineering these cells to perform functions beyond their normal capacities, envisioning novel therapeutic applications.

Myofibers cultured in viscoelastic hydrogels reveal the effects of integrin-binding and mechanosensing on muscle satellite cells

Quiescent skeletal muscle satellite cells (SCs) located on myofibers activate in response to muscle injury to regenerate muscle; however, identifying the role of specific matrix signals on SC behavior in vivo is difficult. Therefore, we developed a viscoelastic hydrogel with tunable properties to encapsulate myofibers while maintaining stem cell niche polarity and SC-myofiber interactions to investigate how matrix signals, including viscoelasticity and the integrin-binding ligand arginyl-glycyl-aspartic acid (RGD), influence SC behavior during muscle regeneration. Viscoelastic hydrogels support myofiber culture while preserving SC stemness for up to 72 hours post-encapsulation, minimizing myofiber hypercontraction and SC hyperproliferation compared to Matrigel. Pax7 is continuously expressed in SCs on myofibers embedded in hydrogels with higher stress relaxation while SCs differentiate when embedded in elastic hydrogels. Increasing RGD concentrations activates SCs and translocates YAP/TAZ to the nucleus as revealed by photo-expansion microscopy. Deleting YAP/TAZ abrogates RGD-mediated activation of SCs, and thus, YAP/TAZ mediates RGD ligand-induced SC activation and subsequent proliferation. STATEMENT OF SIGNIFICANCE: Satellite cells (SCs) are responsible for muscle maintenance and regeneration, but how the extracellular matrix regulates SC function is less understood and would benefit from new biomaterial models that can recapitulate the complexity of SC niche in vitro. Upon isolation of myofibers, SCs exit quiescence, becoming activated. To circumvent this issue, we developed a viscoelastic hydrogel for encapsulating myofibers, which maintains SC quiescence and limits differentiation, allowing the study of RGD effects. We showed that increasing RGD concentration promotes activation and suppresses differentiation. Finally, to allow high resolution imaging for resolving the subcellular localization of YAP/TAZ transcriptional co-activators, we applied photo-expansion microscopy and gel-to-gel transfer techniques to quantify YAP/TAZ nuclear-cytoplasmic ratio, revealing that RGD-mediated activation relies on YAP/TAZ nuclear translocation.

Analyzing Muscle Stem Cell Function Ex Vivo

Muscle stem cells (MuSCs) lose a large proportion of their characteristics when removed from their niche, hampering the analysis of muscle stem cell functionality. However, the isolation and culture of single floating myofibers with their adjacent muscle stem cells allow the short-term culture and manipulation of muscle stem cells in conditions as close as possible to the endogenous niche. Here, the isolation, culture and transfection with siRNA of muscle stem cells on their adjacent myofibers from young as well as old mice are described.

Identification of a de-novo variant of the MEGF10 gene associated with EMARDD

Early-onset Myopathy, Areflexia, Respiratory Distress and Dysphagia (EMARDD) is a congenital neuromuscular disease with a progressive muscle weakness, respiratory failure, joint contractures, and scoliosis without any symptoms of functional brain anomalies caused by variants in the MEGF10 gene. Here, we report the clinical phenotype and genetic features of a Moroccan patient who carries a novel variant associated with EMARDD on the MEGF10 gene. The Whole Exome Sequencing analysis conducted on a 11 year old boy with respiratory and swallowing difficulties revealed the presence of the novel variant c.978T>A (p.Cys326Ter) on exon 9 of the MEGF10 gene; this variant is thought to be associated with EMARDD. Our study reports the first nonsense pathogenic de novo variant in MEGF10 associated with EMARDD worldwide, identified in a Moroccan patient.

Daily blood flow restriction does not affect muscle fiber capillarization and satellite cell content during 2 wk of bed rest in healthy young men

The present study assessed whether single-leg daily blood flow restriction (BFR) treatment attenuates the decline in muscle fiber size, capillarization, and satellite cell (SC) content during 2 wk of bed rest in healthy, young men. Twelve healthy, young men (age: 24 ± 3 yr; BMI: 23.7 ± 3.1 kg/m2) were subjected to 2 wk of bed rest, during which one leg was exposed to three times daily 5 min of BFR, whereas the contralateral leg received sham treatment [control (CON)]. Muscle biopsies were obtained from the m. vastus lateralis from both the BFR and CON legs before and immediately after 2 wk of bed rest. Types I and II muscle fiber size, myonuclear content, capillarization, and SC content were assessed by immunohistochemistry. No significant decline in either type I or type II muscle fiber size was observed following bed rest, with no differences between the CON and BFR legs (P > 0.05). Type I muscle fiber capillary density increased in response to bed rest in both legs (P < 0.05), whereas other muscle fiber capillarization measures remained unaltered. SC content decreased in both type I (from 7.4 ± 3.2 to 5.9 ± 2.7 per 100 fibers) and type II (from 7.2 ± 3.4 to 6.5 ± 3.2 per 100 fibers) muscle fibers (main effect of time P = 0.018), with no significant differences between the BFR and CON legs (P > 0.05). In conclusion, 2 wk of bed rest has no effect on muscle capillarization and decreases the SC content, and daily BFR treatment does not affect skeletal muscle fiber size and SC content in healthy, young men.NEW & NOTEWORTHY We recently reported that the application of daily blood flow restriction (BFR) treatment does not preserve muscle mass or strength and does not modulate daily muscle protein synthesis rates during 2 wk of bed rest. Here, we show that 2 wk of bed rest resulted in a decrease in satellite cell (SC) content. In addition, the BFR treatment did not affect muscle fiber size, capillarization, and SC content during 2 wk of bed rest.

Exploring the therapeutic potential of fibroadipogenic progenitors in muscle disease

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

Ilastik: a machine learning image analysis platform to interrogate stem cell fate decisions across multiple vertebrate species

Stem cells are the key cellular source for regenerating tissues and organs in vertebrate species. Historically, the investigation of stem cell fate decisions in vivo has been assessed in tissue sections using immunohistochemistry (IHC), where a trained user quantifies fluorescent signal in multiple randomly selected images using manual counting-which is prone to inaccuracies, bias, and is very labor intensive. Here, we highlight the performance of a recently developed machine-learning (ML)-based image analysis program called Ilastik using skeletal muscle as a model system. Interestingly, we demonstrate that Ilastik accurately quantifies Paired Box Protein 7 (PAX7)-positive muscle stem cells (MuSCs) before and during the regenerative process in whole muscle sections from mice, humans, axolotl salamanders, and short-lived African turquoise killifish, to a precision that exceeds human capabilities and in a fraction of the time. Overall, Ilastik is a free user-friendly ML-based program that will expedite the analysis of stained tissue sections in vertebrate animals.

The unique functions of Runx1 in skeletal muscle maintenance and regeneration are facilitated by an ETS interaction domain

The conserved Runt-related (RUNX) transcription factor family are master regulators of developmental and regenerative processes. Runx1 and Runx2 are expressed in satellite cells (SCs) and in skeletal myotubes. Here, we examined the role of Runx1 in mouse satellite cells to determine the role of Runx1 during muscle differentiation. Conditional deletion of Runx1 in adult SCs negatively impacted self-renewal and impaired skeletal muscle maintenance even though Runx2 expression persisted. Runx1 deletion in C2C12 cells (which retain Runx2 expression) identified unique molecular functions of Runx1 that could not be compensated for by Runx2. The reduced myoblast fusion in vitro caused by Runx1 loss was due in part to ectopic expression of Mef2c, a target repressed by Runx1. Structure-function analysis demonstrated that the ETS-interacting MID/EID region of Runx1, absent from Runx2, is essential for Runx1 myoblast function and for Etv4 binding. Analysis of ChIP-seq datasets from Runx1 (T cells, muscle)- versus Runx2 (preosteoblasts)-dependent tissues identified a composite ETS:RUNX motif enriched in Runx1-dependent tissues. The ETS:RUNX composite motif was enriched in peaks open exclusively in ATAC-seq datasets from wild-type cells compared to ATAC peaks unique to Runx1 knockout cells. Thus, engagement of a set of targets by the RUNX1/ETS complex define the non-redundant functions of Runx1 in mouse muscle precursor cells.

An immunohistochemical evaluation of fast twitch muscle formation induced by repeated administration of flavan 3-ols in mice

Flavan-3-ols (FL) are poorly bioavailable astringent polyphenols that induce hyperactivation of the sympathetic nervous system. The aim of this study was to investigate the effects of repeated oral administration of FL on mice hindlimb skeletal muscle using immunohistochemical techniques. C57BL/6J male mice were orally administered 50 mg/kg of FL for a period of 2 weeks, and bromideoxyuridine (BrdU) was administered intraperitoneally 3 days prior to the dissection. The soleus and extensor digitorum longus (EDL) were excised and prepared for frozen sections. Myosin heavy chain (MHC) antibodies were used to classify muscle types, in addition, muscle cross-sectional areas (CSA) were measured. We observed a shift in the peak of CSA in the soleus muscle and to a larger extent in the EDL. In addition, a distinct shift toward fast muscle was detected, documented by a reduction in type I and an increase in type IIb in the soleus muscle, whereas in the EDL, we observed a decline in type IIa and an expansion in type IIb. Incorporation of BrdU into cells was significantly increased in all skeletal muscles, with a significant increase in cells co-expressing pair box 7 (Pax7), a marker of differentiation, as observed in the EDL. Given the evidence that β2-adrenergic receptors in skeletal muscles regulate differentiation and size, we measured plasma catecholamine (CA) concentrations following a single differentiation of FL. A single oral dose of FL was observed to significantly increase plasma CA. These findings indicate that catecholamines secreted into the bloodstream from the adrenal gland following oral administration of FL may influence skeletal muscle size and type via β2-receptors.

Mechanisms of muscle cells alterations and regeneration decline during aging

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

The L27 domain of MPP7 enhances TAZ-YY1 cooperation to renew muscle stem cells

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renew, to support a lifetime of regenerative capacity. Here we study the renewal of skeletal muscle stem cell (MuSC) during regeneration. The transcriptional co-factors TAZ/YAP (via the TEAD transcription factors) regulate cell cycle and growth while the transcription factor YY1 regulates metabolic programs for MuSC activation. We show that MPP7 and AMOT join TAZ and YY1 to regulate a selected number of common genes that harbor TEAD and YY1 binding sites. Among these common genes, Carm1 can direct MuSC renewal. We demonstrate that the L27 domain of MPP7 enhances the interaction as well as the transcriptional activity of TAZ and YY1, while AMOT acts as an intermediate to bridge them together. Furthermore, MPP7, TAZ and YY1 co-occupy the promoters of Carm1 and other common downstream genes. Our results define a renewal program comprised of two progenitor transcriptional programs, in which selected key genes are regulated by protein-protein interactions, dependent on promoter context.

The circular RNA circNFIX regulates MEF2C expression in muscle satellite cells in spastic cerebral palsy

Cerebral palsy (CP) is a pediatric onset disorder with poorly understood molecular causes and progression, making early diagnosis difficult. Circular RNAs are regulatory RNAs that show promise as biomarkers in various diseases but the role of circular RNAs in CP is beginning to be understood. This study identified the role of circNFIX in regulating the expression of myocyte-specific enhancer factor 2C (MEF2C), an important transcription factor for sarcomere development. We found that circNFIX is downregulated in the muscle cells of individuals with CP, and its localization shifts toward the nucleus as visualized using single-molecule resolution imaging. The decreased expression of circNFIX, MEF2C, and MEF2C targets persisted throughout myoblasts to myotubes differentiation, and in the skeletal muscle tissue. Bioinformatic and experimental validation confirmed that circNFIX acts as a sponge for miR373-3p, a microRNA that represses MEF2C translation. In normal muscle, circNFIX derepresses MEF2C translation by sponging miR373-3p, allowing for normal sarcomere generation. In CP, reduced circNFIX expression results in loss of miRNA sponging, leading to lower MEF2C expression and downregulation of sarcomere genes, potentially causing shortened and dysfunctional muscle fibers. Knockdown (KD) of circNFIX reduced myogenic capacity of myoblasts to fuse and form myotubes similar to CP cells evident from the lower fusion index in CP and KD as compared to control myotubes. This is the first study reporting reduction of MEF2C in CP and single-molecule resolution imaging of circNFIX's subcellular distribution and its role in CP, suggesting circNFIX as a potential therapeutic target and biomarker for early CP diagnosis.

Nicotinamide and pyridoxine stimulate muscle stem cell expansion and enhance regenerative capacity during aging

Skeletal muscle relies on resident muscle stem cells (MuSCs) for growth and repair. Aging and muscle diseases impair MuSC function, leading to stem cell exhaustion and regenerative decline that contribute to the progressive loss of skeletal muscle mass and strength. In the absence of clinically available nutritional solutions specifically targeting MuSCs, we used a human myogenic progenitor high-content imaging screen of natural molecules from food to identify nicotinamide (NAM) and pyridoxine (PN) as bioactive nutrients that stimulate MuSCs and have a history of safe human use. NAM and PN synergize via CK1-mediated cytoplasmic β-catenin activation and AKT signaling to promote amplification and differentiation of MuSCs. Oral treatment with a combination of NAM and PN accelerated muscle regeneration in vivo by stimulating MuSCs, increased muscle strength during recovery, and overcame MuSC dysfunction and regenerative failure during aging. Levels of NAM and bioactive PN spontaneously declined during aging in model organisms and interindependently associated with muscle mass and walking speed in a cohort of 186 aged people. Collectively, our results establish the NAM/PN combination as a nutritional intervention that stimulates MuSCs, enhances muscle regeneration, and alleviates age-related muscle decline with a direct opportunity for clinical translation.

GLUD1 determines murine muscle stem cell fate by controlling mitochondrial glutamate levels

Muscle stem cells (MuSCs) enable muscle growth and regeneration after exercise or injury, but how metabolism controls their regenerative potential is poorly understood. We describe that primary metabolic changes can determine murine MuSC fate decisions. We found that glutamine anaplerosis into the tricarboxylic acid (TCA) cycle decreases during MuSC differentiation and coincides with decreased expression of the mitochondrial glutamate deaminase GLUD1. Deletion of Glud1 in proliferating MuSCs resulted in precocious differentiation and fusion, combined with loss of self-renewal in vitro and in vivo. Mechanistically, deleting Glud1 caused mitochondrial glutamate accumulation and inhibited the malate-aspartate shuttle (MAS). The resulting defect in transporting NADH-reducing equivalents into the mitochondria induced compartment-specific NAD+/NADH ratio shifts. MAS activity restoration or directly altering NAD+/NADH ratios normalized myogenesis. In conclusion, GLUD1 prevents deleterious mitochondrial glutamate accumulation and inactivation of the MAS in proliferating MuSCs. It thereby acts as a compartment-specific metabolic brake on MuSC differentiation.

Mitochondrial Dynamics Drive Muscle Stem Cell Progression from Quiescence to Myogenic Differentiation

From quiescence to activation and myogenic differentiation, muscle stem cells (MuSCs) experience drastic alterations in their signaling activity and metabolism. Through balanced cycles of fission and fusion, mitochondria alter their morphology and metabolism, allowing them to affect their decisive role in modulating MuSC activity and fate decisions. This tightly regulated process contributes to MuSC regulation by mediating changes in redox signaling pathways, cell cycle progression, and cell fate decisions. In this review, we discuss the role of mitochondrial dynamics as an integral modulator of MuSC activity, fate, and maintenance. Understanding the influence of mitochondrial dynamics in MuSCs in health and disease will further the development of therapeutics that support MuSC integrity and thus may aid in restoring the regenerative capacity of skeletal muscle.

Tuina Promotes Repair of Chronic Cervical Muscle Injury by Regulating Satellite Cell Proliferation and Differentiation and Inhibiting Myocyte Apoptosis

Purpose

Chronic cervical muscle injury is the first common cause of the development of cervical spondylosis, and Tuina can effectively promote the repair of chronic cervical muscle injury and alleviate neck pain, but the mechanism behind its efficacy is still unknown. The proliferation and differentiation of muscle satellite cells and the apoptosis of cervical myocytes play important roles in the repair of chronic cervical muscle injuries; therefore, this study aimed to explore the potential mechanisms of Tuina to promote the repair of cervical muscle injuries in terms of the proliferation and differentiation of satellite cells and the apoptosis of myocytes.

Patients and Methods

Twenty-eight Wistar rats were randomly divided into control group, model group, Tuina group, and meloxicam group, with 7 rats in each group. Except for the control group, each group were establish a chronic cervical muscle injury model (CCMI). Meloxicam (0.79 mg/kg) was administered by gavage, and in the Tuina group, pressure was applied to the Fengchi acupoint on the affected side once a day. Morphological changes of cervical muscle tissues were detected by ultrasonic diagnostic instrument and HE staining, electrophysiological recordings of electromyographic changes, apoptosis rate was detected by TUNEL staining, and positive expression of Bax, Bcl-2, IGF-1, MyoD, and Pax-7 were detected by Immunohistochemistry and Western blot.

Results

In CCMI model rats, we observed that the cervical muscle fibers were disorganized, with irregular morphology, and the amplitude of electromyography was significantly weakened, while Tuina could significantly improve these symptoms and effectively promote the repair of chronic cervical muscle injury. Meanwhile, compared with the model group, Tuina could significantly increase the expression levels of IGF-1 (P<0.01) and MyoD (P<0.05) and decrease the expression level of Pax7 (P<0.05). In addition, we found that the number of apoptotic cells in cervical myocytes was reduced after Tuina intervention (P<0.05), and Tuina inhibited the expression of pro-apoptotic factor Bax (P<0.01) and promoted the expression of anti-apoptotic factor Bcl-2 (P<0.05).

Conclusion

Tuina can promote the proliferation and differentiation of satellite cells to repair chronic cervical muscle injury by regulating the expression of Pax7, MyoD, and IGF-1, as well as inhibiting the expression of Bax and promoting the expression of Bcl-2 to ameliorate the apoptosis of cervical myocytes in CCMI model rats.

Preservation of masseter muscle until the end stage in the SOD1G93A mouse model for ALS

Amyotrophic lateral sclerosis (ALS) progressively impairs motor neurons, leading to muscle weakness and loss of voluntary muscle control. This study compared the effects of SOD1 mutation on masticatory and limb muscles from disease onset to death in ALS model mice. Notably, limb muscles begin to atrophy soon after ALS-like phenotype appear, whereas masticatory muscles maintain their volume and function in later stages. Our analysis showed that, unlike limb muscles, masticatory muscles retain their normal structure and cell makeup throughout most of the disease course. We found an increase in the number of muscle satellite cells (SCs), which are essential for muscle repair, in masticatory muscles. In addition, we observed no reduction in the number of muscle nuclei and no muscle fibre-type switching in masticatory muscles. This indicates that masticatory muscles have a higher resistance to ALS-related damage than limb muscles, likely because of differences in cell composition and repair mechanisms. Understanding why masticatory muscles are less affected by ALS could lead to the development of new treatments. This study highlights the importance of studying different muscle groups in ALS to clarify disease aetiology and mechanisms.

Overexpression of CPT1A disrupts the maintenance and regenerative function of muscle stem cells

The skeletal muscle satellite cells (SCs) mediate regeneration of myofibers upon injury. As they switch from maintenance (quiescence) to regeneration, their relative reliance on glucose and fatty acid metabolism alters. To explore the contribution of mitochondrial fatty acid oxidation (FAO) pathway to SCs and myogenesis, we examined the role of carnitine palmitoyltransferase 1A (CPT1A), the rate-limiting enzyme of FAO. CPT1A is highly expressed in quiescent SCs (QSCs) compared with activated and proliferating SCs, and its expression level decreases during myogenic differentiation. Myod1Cre-driven overexpression (OE) of Cpt1a in embryonic myoblasts (Cpt1aMTG) reduces muscle weight, grip strength, and contractile force without affecting treadmill endurance of adult mice. Adult Cpt1aMTG mice have reduced number of SC, impairing muscle regeneration and promoting lipid infiltration. Similarly, Pax7CreER-driven, tamoxifen-inducible Cpt1a-OE in QSCs of adult muscles (Cpt1aPTG) leads to depletion of SCs and compromises muscle regeneration. The reduced proliferation of Cpt1a-OE SCs is associated with elevated level of acyl-carnitine, and acyl-carnitine treatment impedes proliferation of wildtype SCs. These findings indicate that aberrant level of CPT1A elevates acyl-carnitine to impair the maintenance, proliferation and regenerative function of SCs.

Hotspots and trends in satellite cell research in muscle regeneration: A bibliometric visualization and analysis from 2010 to 2023

Background

The incidence of muscle atrophy or sports injuries is increasing with time and population aging, thereby attracting considerable attention to muscle generation research. Muscle satellite cells, which play an important role in this process, lack comprehensive literature regarding their use for muscle regeneration. Hence, this study aimed to analyze the hotspots and trends in satellite cell research from 2010 to 2023, providing a reference for muscle regeneration research.

Methods

Studies on satellite cells' role in muscle regeneration from 2010 to 2023 were retrieved from the Web of Science Core Collection. Using CiteSpace and VOSviewer, we analyzed annual publications, authors and co-citing authors, countries and institutions, journals and co-citing journals, co-citing references, and keywords.

Results

From 2010 to 2023, 1468 papers were retrieved, indicating an overall increasing trend in the number of annual publications related to satellite cells in muscle regeneration. The United States had the highest number of publications, while the Institut National de la Santé et de la Recherche Médicale was the institution with the most publications. Among journals, " PloS One" had the highest number of published papers, and "Cell" emerged as the most co-cited journal. A total of 7425 authors were involved, with Michael A. Rudnicki being the author with the highest number of publications and the most co-cited author. The most cited reference was "Satellite cells and the muscle stem cell niche." Among keywords, "satellite cells" was the most common, with "heterogeneity" having the highest centrality. Frontier themes included "Duchenne muscular dystrophy," "skeletal muscle," "in-vivo," "muscle regeneration," "mice," "muscle atrophy," "muscle fibers," "inflammation," " mesenchymal stem cells," and "satellite cell."

Conclusion

This study presents the current status and trends in satellite cell research on muscle regeneration from 2010 to 2023 using bibliometric analyses, providing valuable insights into numerous future research directions.

Macrophages in the Context of Muscle Regeneration and Duchenne Muscular Dystrophy

Macrophages are essential to muscle regeneration, as they regulate inflammation, carry out phagocytosis, and facilitate tissue repair. These cells exhibit phenotypic switching from pro-inflammatory (M1) to anti-inflammatory (M2) states during muscle repair, influencing myoblast proliferation, differentiation, and myofiber formation. In Duchenne Muscular Dystrophy (DMD), asynchronous muscle injuries disrupt the normal temporal stages of regeneration, leading to fibrosis and failed regeneration. Altered macrophage activity is associated with DMD progression and physiopathology. Gaining insight into the intricate relationship between macrophages and muscle cells is crucial for creating effective therapies aimed at treating this muscle disorder. This review explores the dynamic functions of macrophages in muscle regeneration and their implications in DMD.

Organoid culture promotes dedifferentiation of mouse myoblasts into stem cells capable of complete muscle regeneration

Experimental cell therapies for skeletal muscle conditions have shown little success, primarily because they use committed myogenic progenitors rather than true muscle stem cells, known as satellite cells. Here we present a method to generate in vitro-derived satellite cells (idSCs) from skeletal muscle tissue. When transplanted in small numbers into mouse muscle, mouse idSCs fuse into myofibers, repopulate the satellite cell niche, self-renew, support multiple rounds of muscle regeneration and improve force production on par with freshly isolated satellite cells in damaged skeletal muscle. We compared the epigenomic and transcriptional signatures between idSCs, myoblasts and satellite cells and used these signatures to identify core signaling pathways and genes that confer idSC functionality. Finally, from human muscle biopsies, we successfully generated satellite cell-like cells in vitro. After further development, idSCs may provide a scalable source of cells for the treatment of genetic muscle disorders, trauma-induced muscle damage and age-related muscle weakness.

Effect of Chicken Age on Proliferation and Differentiation Abilities of Muscle Stem Cells and Nutritional Characteristics of Cultured Meat Tissue

This study aimed to investigate effects of chicken age on proliferation and differentiation capacity of muscle satellite cells (MSCs) and to determine total amino acid contents of cultured meat (CM) produced. Chicken MSCs (cMSCs) were isolated from hindlimb muscles of broiler chickens at 5-week-old (5W) and 19-embryonic-day (19ED), respectively. Proliferation abilities (population doubling time and cell counting kit 8) of cMSCs from 19ED were significantly higher than those from 5W (p<0.05). Likewise, both myotube formation area and expression of myosin heavy chain heavy of cMSCs from 19ED were significantly higher than those from 5W (p<0.05). After cMSCs were serially subcultured for long-term cultivation in 2D flasks to produce cultured meat tissue (CMT), total amino acid contents of CMT showed no significant difference between 5W and 19ED chickens (p>0.05). This finding suggests that cMSCs from chicken embryos are more suitable for improving the production efficiency of CM than those derived from young chickens.

Satellite cell-derived TRIM28 is pivotal for mechanical load- and injury-induced myogenesis

Satellite cells are skeletal muscle stem cells that contribute to postnatal muscle growth, and they endow skeletal muscle with the ability to regenerate after a severe injury. Here we discover that this myogenic potential of satellite cells requires a protein called tripartite motif-containing 28 (TRIM28). Interestingly, different from the role reported in a previous study based on C2C12 myoblasts, multiple lines of both in vitro and in vivo evidence reveal that the myogenic function of TRIM28 is not dependent on changes in the phosphorylation of its serine 473 residue. Moreover, the functions of TRIM28 are not mediated through the regulation of satellite cell proliferation or differentiation. Instead, our findings indicate that TRIM28 regulates the ability of satellite cells to progress through the process of fusion. Specifically, we discover that TRIM28 controls the expression of a fusogenic protein called myomixer and concomitant fusion pore formation. Collectively, the outcomes of this study expose the framework of a novel regulatory pathway that is essential for myogenesis.

Sexual dimorphisms in skeletal muscle: current concepts and research horizons

The complex compositional and functional nature of skeletal muscle makes this organ an essential topic of study for biomedical researchers and clinicians. An additional layer of complexity is added with the consideration of sex as a biological variable. Recent research advances have revealed sexual dimorphisms in developmental biology, muscle homeostasis, adaptive responses, and disorders relating to skeletal muscle. Many of the observed sex differences have hormonal and molecular mechanistic underpinnings, whereas others have yet to be elucidated. Future research is needed to investigate the mechanisms dictating sex-based differences in the various aspects of skeletal muscle. As such, it is necessary that skeletal muscle biologists ensure that both female and male subjects are represented in biomedical and clinical studies to facilitate the successful testing and development of therapeutics for all patients.

We need to talk-how muscle stem cells communicate

Skeletal muscle is one of the tissues with the highest ability to regenerate, a finely controlled process which is critically depending on muscle stem cells. Muscle stem cell functionality depends on intrinsic signaling pathways and interaction with their immediate niche. Upon injury quiescent muscle stem cells get activated, proliferate and fuse to form new myofibers, a process involving the interaction of multiple cell types in regenerating skeletal muscle. Receptors in muscle stem cells receive the respective signals through direct cell-cell interaction, signaling via secreted factors or cell-matrix interactions thereby regulating responses of muscle stem cells to external stimuli. Here, we discuss how muscle stem cells interact with their immediate niche focusing on how this controls their quiescence, activation and self-renewal and how these processes are altered in age and disease.

A dual-color PAX7 and MYF5 in vivo reporter to investigate muscle stem cell heterogeneity in regeneration and aging

Increasing evidence suggests that the muscle stem cell (MuSC) pool is heterogeneous. In particular, a rare subset of PAX7-positive MuSCs that has never expressed the myogenic regulatory factor MYF5 displays unique self-renewal and engraftment characteristics. However, the scarcity and limited availability of protein markers make the characterization of these cells challenging. Here, we describe the generation of StemRep reporter mice enabling the monitoring of PAX7 and MYF5 proteins based on equimolar levels of dual nuclear fluorescence. High levels of PAX7 protein and low levels of MYF5 delineate a deeply quiescent MuSC subpopulation with an increased capacity for asymmetric division and distinct dynamics of activation, proliferation, and commitment. Aging primarily reduces the MYF5Low MuSCs and skews the stem cell pool toward MYF5High cells with lower quiescence and self-renewal potential. Altogether, we establish the StemRep model as a versatile tool to study MuSC heterogeneity and broaden our understanding of mechanisms regulating MuSC quiescence and self-renewal in homeostatic, regenerating, and aged muscles.

Resveratrol and Vitamin D: Eclectic Molecules Promoting Mitochondrial Health in Sarcopenia

Sarcopenia refers to the progressive loss and atrophy of skeletal muscle function, often associated with aging or secondary to conditions involving systemic inflammation, oxidative stress, and mitochondrial dysfunction. Recent evidence indicates that skeletal muscle function is not only influenced by physical, environmental, and genetic factors but is also significantly impacted by nutritional deficiencies. Natural compounds with antioxidant properties, such as resveratrol and vitamin D, have shown promise in preventing mitochondrial dysfunction in skeletal muscle cells. These antioxidants can slow down muscle atrophy by regulating mitochondrial functions and neuromuscular junctions. This review provides an overview of the molecular mechanisms leading to skeletal muscle atrophy and summarizes recent advances in using resveratrol and vitamin D supplementation for its prevention and treatment. Understanding these molecular mechanisms and implementing combined interventions can optimize treatment outcomes, ensure muscle function recovery, and improve the quality of life for patients.

Three-dimensional imaging studies in mice identify cellular dynamics of skeletal muscle regeneration

The function of many organs, including skeletal muscle, depends on their three-dimensional structure. Muscle regeneration therefore requires not only reestablishment of myofibers but also restoration of tissue architecture. Resident muscle stem cells (SCs) are essential for regeneration, but how SCs regenerate muscle architecture is largely unknown. We address this problem using genetic labeling of mouse SCs and whole-mount imaging to reconstruct, in three dimensions, muscle regeneration. Unexpectedly, we found that myofibers form via two distinct phases of fusion and the residual basement membrane of necrotic myofibers is critical for promoting fusion and orienting regenerated myofibers. Furthermore, the centralized myonuclei characteristic of regenerated myofibers are associated with myofibrillogenesis and endure months post injury. Finally, we elucidate two cellular mechanisms for the formation of branched myofibers, a pathology characteristic of diseased muscle. We provide a synthesis of the cellular events of regeneration and show that these differ from those used during development.

Cell-specific functions of androgen receptor in skeletal muscles

Androgens play a vital role not only in promoting the development of male sexual characteristics but also in exerting diverse physiological effects, including the regulation of skeletal muscle growth and function. Given that the effects of androgens are mediated through androgen receptor (AR) binding, an understanding of AR functionality is crucial for comprehending the mechanisms of androgen action on skeletal muscles. Drawing from insights gained using conditional knockout mouse models facilitated by Cre/loxP technology, we review the cell-specific functions of AR in skeletal muscles. We focus on three specific cell populations expressing AR within skeletal muscles: skeletal muscle cells, responsible for muscle contraction; satellite cells, which are essential stem cells contributing to the growth and regeneration of skeletal muscles; and mesenchymal progenitors, situated in interstitial areas and playing a crucial role in muscle homeostasis. Furthermore, the indirect effects of androgens on skeletal muscle through extra-muscle tissue are essential, especially for the regulation of skeletal muscle mass. The regulation of genes by AR varies across different cell types and contexts, including homeostasis, regeneration and hypertrophy of skeletal muscles. The varied mechanisms orchestrated by AR collectively influence the physiology of skeletal muscles.

Deletion of TECRL promotes skeletal muscle repair by up-regulating EGR2

Myogenic regeneration relies on the proliferation and differentiation of satellite cells. TECRL (trans-2,3-enoyl-CoA reductase like) is an endoplasmic reticulum protein only expressed in cardiac and skeletal muscle. However, its role in myogenesis remains unknown. We show that TECRL expression is increased in response to injury. Satellite cell-specific deletion of TECRL enhances muscle repair by increasing the expression of EGR2 through the activation of the ERK1/2 signaling pathway, which in turn promotes the expression of PAX7. We further show that TECRL deletion led to the upregulation of the histone acetyltransferase general control nonderepressible 5, which enhances the transcription of EGR2 through acetylation. Importantly, we showed that AAV9-mediated TECRL silencing improved muscle repair in mice. These findings shed light on myogenic regeneration and muscle repair.

Lineage tracing reveals a novel PDGFRβ+ satellite cell subset that contributes to myo-regeneration of chronically injured rotator cuff muscle

Massive rotator cuff (RC) tendon tears are associated with progressive fibro-adipogenesis and muscle atrophy that altogether cause shoulder muscle wasting. Platelet derived growth factor β (PDGFRβ) lineage cells, that co-express PDGFRα have previously been shown to directly contribute to scar formation and fat accumulation in a mouse model of irreversible tendon and nerve transection (TTDN). Conversely, PDGFRβ+ lineage cells have also been  shown to be myogenic in cultures and in other models of skeletal muscle injury. We therefore hypothesized that PDGFRβ demarcates two distinct RC residing subpopulations, fibro-adipogenic and myogenic, and aimed to elucidate the identity of the PDGFRβ myogenic precursors and evaluate their contribution, if any, to RC myo-regeneration. Lineage tracing revealed increasing contribution of PDGFRβ+ myo-progenitors to the formation of GFP+ myofibers, which were the most abundant myofiber type in regenerated muscle at 2 weeks post-TTDN. Muscle regeneration preceded muscle atrophy and both advanced from the lateral site of tendon transection to the farthest medial region. GFP+/PDGFRβ+Sca-1-lin-CXCR4+Integrin-β1+ marked a novel subset of satellite cells with confirmed myogenic properties. Further studies are warranted to identify the existence of PDGFRβ+ satellite cells in human and other mouse muscles and to define their myo-regenerative potential following acute and chronic muscle injury.

Inhibition of p53-MDM2 binding reduces senescent cell abundance and improves the adaptive responses of skeletal muscle from aged mice

Skeletal muscle adaptation to external stimuli, such as regeneration following injury and hypertrophy in response to resistance exercise, are blunted with advanced age. The accumulation of senescent cells, along with defects in myogenic progenitor cell (MPC) proliferation, have been strongly linked as contributing factors to age-associated impairment in muscle adaptation. p53 plays an integral role in all these processes, as upregulation of p53 causes apoptosis in senescent cells and prevents mitotic catastrophe in MPCs from old mice. The goal of this study was to determine if a novel pharmaceutical agent (BI01), which functions by upregulating p53 through inhibition of binding to MDM2, the primary p53 regulatory protein, improves muscle regeneration and hypertrophy in old mice. BI01 effectively reduced the number of senescent cells in vitro but had no effect on MPC survival or proliferation at a comparable dose. Following repeated oral gavage with 2 mg/kg of BI01 (OS) or vehicle (OV), old mice (24 months) underwent unilateral BaCl2 injury in the tibialis anterior (TA) muscle, with PBS injections serving as controls. After 7 days, satellite cell number was higher in the TA of OS compared to OV mice, as was the expression of genes involved in ATP production. By 35 days, old mice treated with BI01 displayed reduced senescent cell burden, enhanced regeneration (higher muscle mass and fiber cross-sectional area) and restoration of muscle function relative to OV mice. To examine the impact of 2 mg/kg BI01 on muscle hypertrophy, the plantaris muscle was subjected to 28 days of mechanical overload (MOV) in OS and OV mice. In response to MOV, OS mice had larger plantaris muscles and muscle fibers than OV mice, particularly type 2b + x fibers, associated with reduced senescent cells. Together our data show that BI01 is an effective senolytic agent that may also augment muscle metabolism to enhance muscle regeneration and hypertrophy in old mice.

Cadherin-dependent adhesion is required for muscle stem cell niche anchorage and maintenance

Adhesion between stem cells and their niche provides stable anchorage and signaling cues to sustain properties such as quiescence. Skeletal muscle stem cells (MuSCs) adhere to an adjacent myofiber via cadherin-catenin complexes. Previous studies on N- and M-cadherin in MuSCs revealed that although N-cadherin is required for quiescence, they are collectively dispensable for MuSC niche localization and regenerative activity. Although additional cadherins are expressed at low levels, these findings raise the possibility that cadherins are unnecessary for MuSC anchorage to the niche. To address this question, we conditionally removed from MuSCs β- and γ-catenin, and, separately, αE- and αT-catenin, factors that are essential for cadherin-dependent adhesion. Catenin-deficient MuSCs break quiescence similarly to N-/M-cadherin-deficient MuSCs, but exit the niche and are depleted. Combined in vivo, ex vivo and single cell RNA-sequencing approaches reveal that MuSC attrition occurs via precocious differentiation, re-entry to the niche and fusion to myofibers. These findings indicate that cadherin-catenin-dependent adhesion is required for anchorage of MuSCs to their niche and for preservation of the stem cell compartment. Furthermore, separable cadherin-regulated functions govern niche localization, quiescence and MuSC maintenance.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.