EGFR-Aurka Signaling Rescues Polarity and Regeneration Defects in Dystrophin-Deficient Muscle Stem Cells by Increasing Asymmetric Divisions

The E3 ubiquitin ligase Nedd4L preserves skeletal muscle stem cell quiescence by inhibiting their activation

Adult stem cells play a critical role in tissue repair and maintenance. In tissues with slow turnover, including skeletal muscle, these cells are maintained in a mitotically quiescent state yet remain poised to re-enter the cell cycle to replenish themselves and regenerate the tissue. Using a panomics approach we show that the PAX7/NEDD4L axis acts against muscle stem cell activation in homeostatic skeletal muscle. Our findings suggest that PAX7 transcriptionally activates the E3 ubiquitin ligase Nedd4L and that the conditional genetic deletion of Nedd4L impairs muscle stem cell quiescence, with an upregulation of cell cycle and myogenic differentiation genes. Loss of Nedd4L in muscle stem cells results in the expression of doublecortin (DCX), which is exclusively expressed during their in vivo activation. Together, these data establish that the ubiquitin proteasome system, mediated by Nedd4L, is a key contributor to the muscle stem cell quiescent state in adult mice.

Mini-MEndR: a miniaturized 96-well predictive assay to evaluate muscle stem cell-mediated repair

Background

Functional evaluation of molecules that are predicted to promote stem cell mediated endogenous repair often requires in vivo transplant studies that are low throughput and hinder the rate of discovery. To offer greater throughput for functional validation studies, we miniaturized, simplified and expanded the functionality of a previously developed muscle endogenous repair (MEndR) in vitro assay that was shown to capture significant events of in vivo muscle endogenous repair.

Methods

The mini-MEndR assay consists of miniaturized cellulose scaffolds designed to fit in 96-well plates, the pores of which are infiltrated with human myoblasts encapsulated in a fibrin-based hydrogel to form engineered skeletal muscle tissues. Pre-adsorbing thrombin to the cellulose scaffolds facilitates in situ tissue polymerization, a critical modification that enables new users to rapidly acquire assay expertise. Following the generation of the 3D myotube template, muscle stem cells (MuSCs), enriched from digested mouse skeletal muscle tissue using an improved magnetic-activated cell sorting protocol, are engrafted within the engineered template. Murine MuSCs are fluorescently labeled, enabling co-evaluation of human and mouse Pax7+ cell responses to drug treatments. A regenerative milieu is introduced by injuring the muscle tissue with a myotoxin to initiate endogenous repair "in a dish". Phenotypic data is collected at endpoints with a high-content imaging system and is analyzed using ImageJ-based image analysis pipelines.

Results

The miniaturized format and modified manufacturing protocol cuts reagent costs in half and hands-on seeding time ~ threefold, while the image analysis pipelines save 40 h of labour. By evaluating multiple commercially available human primary myoblast lines in 2D and 3D culture, we establish quality assurance metrics for cell line selection that standardizes myotube template quality. In vivo outcomes (enhanced muscle production and Pax7+ cell expansion) to a known modulator of MuSC mediated repair (p38/β MAPK inhibition) are recapitulated in the miniaturized culture assay, but only in the presence of stem cells and the regenerative milieu.

Discussion

The miniaturized predictive assay offers a simple, scaled platform to co-investigate human and mouse skeletal muscle endogenous repair molecular modulators, and thus is a promising strategy to accelerate the muscle endogenous repair discovery pipeline.

Supplementary Information

The online version contains supplementary material available at 10.1186/s44330-024-00005-4.

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.

Interplay between Pitx2 and Pax7 temporally governs specification of extraocular muscle stem cells

Gene regulatory networks that act upstream of skeletal muscle fate determinants are distinct in different anatomical locations. Despite recent efforts, a clear understanding of the cascade of events underlying the emergence and maintenance of the stem cell pool in specific muscle groups remains unresolved and debated. Here, we invalidated Pitx2 with multiple Cre-driver mice prenatally, postnatally, and during lineage progression. We showed that this gene becomes progressively dispensable for specification and maintenance of the muscle stem (MuSC) cell pool in extraocular muscles (EOMs) despite being, together with Myf5, a major upstream regulator during early development. Moreover, constitutive inactivation of Pax7 postnatally led to a greater loss of MuSCs in the EOMs compared to the limb. Thus, we propose a relay between Pitx2, Myf5 and Pax7 for EOM stem cell maintenance. We demonstrate also that MuSCs in the EOMs adopt a quiescent state earlier that those in limb muscles and do not spontaneously proliferate in the adult, yet EOMs have a significantly higher content of Pax7+ MuSCs per area pre- and post-natally. Finally, while limb MuSCs proliferate in the mdx mouse model for Duchenne muscular dystrophy, significantly less MuSCs were present in the EOMs of the mdx mouse model compared to controls, and they were not proliferative. Overall, our study provides a comprehensive in vivo characterisation of MuSC heterogeneity along the body axis and brings further insights into the unusual sparing of EOMs during muscular dystrophy.

HB-EGF promotes progenitor cell proliferation and sensory neuron regeneration in the zebrafish olfactory epithelium

Maintenance and regeneration of the zebrafish olfactory epithelium (OE) are supported by two distinct progenitor cell populations that occupy spatially discrete stem cell niches and respond to different tissue conditions. Globose basal cells (GBCs) reside at the inner and peripheral margins of the sensory OE and are constitutively active to replace sporadically dying olfactory sensory neurons (OSNs). In contrast, horizontal basal cells (HBCs) are uniformly distributed across the sensory tissue and are selectively activated by acute injury conditions. Here we show that expression of the heparin-binding epidermal growth factor-like growth factor (HB-EGF) is strongly and transiently upregulated in response to OE injury and signals through the EGF receptor (EGFR), which is expressed by HBCs. Exogenous stimulation of the OE with recombinant HB-EGF promotes HBC expansion and OSN neurogenesis in a pattern that resembles the tissue response to injury. In contrast, pharmacological inhibition of HB-EGF membrane shedding, HB-EGF availability, and EGFR signaling strongly attenuate or delay injury-induced HBC activity and OSN restoration without affecting maintenance neurogenesis by GBCs. Thus, HB-EGF/EGFR signaling appears to be a critical component of the signaling network that controls HBC activity and, consequently, repair neurogenesis in the zebrafish OE.

Islr regulates satellite cells asymmetric division through the SPARC/p-ERK1/2 signaling pathway

Satellite cells (SCs) are adult muscle stem cells responsible for muscle regeneration after acute and chronic muscle injuries. The balance between stem cell self-renewal and differentiation determines the kinetics and efficiency of skeletal muscle regeneration. This study assessed the function of Islr in SC asymmetric division. The deletion of Islr reduced muscle regeneration in adult mice by decreasing the SC pool. Islr is pivotal for SC proliferation, and its deletion promoted the asymmetric division of SCs. A mechanistic search revealed that Islr bound to and degraded secreted protein acidic and rich in cysteine (SPARC), which activated p-ERK1/2 signaling required for asymmetric division. These findings demonstrate that Islr is a key regulator of SC division through the SPARC/p-ERK1/2 signaling pathway. These data provide a basis for treating myopathy.

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

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

HDAC inhibitors as pharmacological treatment for Duchenne muscular dystrophy: a discovery journey from bench to patients

Earlier evidence that targeting the balance between histone acetyltransferases (HATs) and deacetylases (HDACs), through exposure to HDAC inhibitors (HDACis), could enhance skeletal myogenesis, prompted interest in using HDACis to promote muscle regeneration. Further identification of constitutive HDAC activation in dystrophin-deficient muscles, caused by dysregulated nitric oxide (NO) signaling, provided the rationale for HDACi-based therapeutic interventions for Duchenne muscular dystrophy (DMD). In this review, we describe the molecular, preclinical, and clinical evidence supporting the efficacy of HDACis in countering disease progression by targeting pathogenic networks of gene expression in multiple muscle-resident cell types of patients with DMD. Given that givinostat is paving the way for HDACi-based interventions in DMD, next-generation HDACis with optimized therapeutic profiles and efficacy could be also explored for synergistic combinations with other therapeutic strategies.

The Acute, Short-, and Long-Term Effects of Endurance Exercise on Skeletal Muscle Transcriptome Profiles

A better understanding of the cellular and molecular mechanisms that are involved in skeletal muscle adaptation to exercise is fundamentally important to take full advantage of the enormous benefits that exercise training offers in disease prevention and therapy. The aim of this study was to elucidate the transcriptional signatures that distinguish the endurance-trained and untrained muscles in young adult males (24 ± 3.5 years). We characterized baseline differences as well as acute exercise-induced transcriptome responses in vastus lateralis biopsy specimens of endurance-trained athletes (ET; n = 8; VO2max, 67.2 ± 8.9 mL/min/kg) and sedentary healthy volunteers (SED; n = 8; VO2max, 40.3 ± 7.6 mL/min/kg) using microarray technology. A second cohort of SED volunteers (SED-T; n = 10) followed an 8-week endurance training program to assess expression changes of selected marker genes in the course of skeletal muscle adaptation. We deciphered differential baseline signatures that reflected major differences in the oxidative and metabolic capacity of the endurance-trained and untrained muscles. SED-T individuals in the training group displayed an up-regulation of nodal regulators of oxidative adaptation after 3 weeks of training and a significant shift toward the ET signature after 8 weeks. Transcriptome changes provoked by 1 h of intense cycling exercise only poorly overlapped with the genes that constituted the differential baseline signature of ETs and SEDs. Overall, acute exercise-induced transcriptional responses were connected to pathways of contractile, oxidative, and inflammatory stress and revealed a complex and highly regulated framework of interwoven signaling cascades to cope with exercise-provoked homeostatic challenges. While temporal transcriptional programs that were activated in SEDs and ETs were quite similar, the quantitative divergence in the acute response transcriptomes implicated divergent kinetics of gene induction and repression following an acute bout of exercise. Together, our results provide an extensive examination of the transcriptional framework that underlies skeletal muscle plasticity.

The satellite cell in skeletal muscle: A story of heterogeneity

Skeletal muscle is a highly represented tissue in mammals and is composed of fibers that are extremely adaptable and capable of regeneration. This characteristic of muscle fibers is made possible by a cell type called satellite cells. Adjacent to the fibers, satellite cells are found in a quiescent state and located between the muscle fibers membrane and the basal lamina. These cells are required for the growth and regeneration of skeletal muscle through myogenesis. This process is known to be tightly sequenced from the activation to the differentiation/fusion of myofibers. However, for the past fifteen years, researchers have been interested in examining satellite cell heterogeneity and have identified different subpopulations displaying distinct characteristics based on localization, quiescence state, stemness capacity, cell-cycle progression or gene expression. A small subset of satellite cells appears to represent multipotent long-term self-renewing muscle stem cells (MuSC). All these distinctions led us to the hypothesis that the characteristics of myogenesis might not be linear and therefore may be more permissive based on the evidence that satellite cells are a heterogeneous population. In this review, we discuss the different subpopulations that exist within the satellite cell pool to highlight the heterogeneity and to gain further understanding of the myogenesis progress. Finally, we discuss the long term self-renewing MuSC subpopulation that is capable of dividing asymmetrically and discuss the molecular mechanisms regulating MuSC polarization during health and disease.

MuSCs and IPCs: roles in skeletal muscle homeostasis, aging and injury

Skeletal muscle is a highly specialized tissue composed of myofibres that performs crucial functions in movement and metabolism. In response to external stimuli and injuries, a range of stem/progenitor cells, with muscle stem cells or satellite cells (MuSCs) being the predominant cell type, are rapidly activated to repair and regenerate skeletal muscle within weeks. Under normal conditions, MuSCs remain in a quiescent state, but become proliferative and differentiate into new myofibres in response to injury. In addition to MuSCs, some interstitial progenitor cells (IPCs) such as fibro-adipogenic progenitors (FAPs), pericytes, interstitial stem cells expressing PW1 and negative for Pax7 (PICs), muscle side population cells (SPCs), CD133-positive cells and Twist2-positive cells have been identified as playing direct or indirect roles in regenerating muscle tissue. Here, we highlight the heterogeneity, molecular markers, and functional properties of these interstitial progenitor cells, and explore the role of muscle stem/progenitor cells in skeletal muscle homeostasis, aging, and muscle-related diseases. This review provides critical insights for future stem cell therapies aimed at treating muscle-related diseases.

Wnt7a is Required for Regeneration of Dystrophic Skeletal Muscle

Intramuscular injection of Wnt7a has been shown to accelerate and augment skeletal muscle regeneration and to ameliorate dystrophic progression in mdx muscle, a model for Duchenne muscular dystrophy (DMD). However, loss-of-function studies to investigate the requirement for Wnt7a in muscle regeneration has not been evaluated. Here, we assessed muscle regeneration and function in wild type (WT) and mdx mice where Wnt7a was specifically deleted in muscle using a conditional Wnt7a floxed allele and a Myf5-Cre driver. We found that both WT and mdx mice with deletion of Wnt7a in muscle, exhibited marked deficiencies in muscle regeneration at 21 d following cardiotoxin (CTX) induced injury. Unlike WT, deletion of Wnt7a in mdx resulted in a marked decrease in specific force generation prior to CTX injury. However, both WT and mdx muscle lacking Wnt7a displayed decreased specific force generation following CTX injection. Notably the regeneration deficit observed in mdx mice lacking Wnt7a in muscle was rescued by a single tail vein injection of an extracellular vesicle preparation containing Wnt7a (Wnt7a-EVs). Therefore, we conclude that the regenerative capacity of muscle in mdx mice is due to the upregulation of endogenous Wnt7a following injury, and that systemic delivery of Wnt7a-EVs represents a therapeutic strategy for treating DMD.

Generation of highly pure pluripotent stem cell-derived myogenic progenitor cells and myotubes

Driving efficient and pure skeletal muscle cell differentiation from pluripotent stem cells (PSCs) has been challenging. Here, we report an optimized protocol that generates skeletal muscle progenitor cells with high efficiency and purity in a short period of time. Human induced PSCs (hiPSCs) and murine embryonic stem cells (mESCs) were specified into the mesodermal myogenic fate using distinct and species-specific protocols. We used a specific maturation medium to promote the terminal differentiation of both human and mouse myoblast populations, and generated myotubes associated with a large pool of cell-cycle arrested PAX7+ cells. We also show that myotube maturation is modulated by dish-coating properties, cell density, and percentage of myogenic progenitor cells. Given the high efficiency in the generation of myogenic progenitors and differentiated myofibers, this protocol provides an attractive strategy for tissue engineering, modeling of muscle dystrophies, and evaluation of new therapeutic approaches in vitro.

Evaluation of pro-regenerative and anti-inflammatory effects of isolecanoric acid in the muscle: Potential treatment of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is a devastating degenerative disease of skeletal muscles caused by loss of dystrophin, a key protein that maintains muscle integrity, which leads to progressive muscle degeneration aggravated by chronic inflammation, muscle stem cells' (MuSCs) reduced regenerative capacity and replacement of muscle with fibroadipose tissue. Previous research has shown that pharmacological GSK-3β inhibition favors myogenic differentiation and plays an important role in modulating inflammatory processes. Isolecanoric acid (ILA) is a natural product isolated from a fungal culture displaying GSK-3β inhibitory properties. The present study aimed to investigate the proregenerative and anti-inflammatory properties of this natural compound in the DMD context. Our results showed that ILA markedly promotes myogenic differentiation of myoblasts by increasing β-Catenin signaling and boosting the myogenic potential of mouse and human stem cells. One important finding was that the GSK-3β/β-Catenin pathway is altered in dystrophic mice muscle and ILA enhances the myofiber formation of dystrophic MuSCs. Treatment with this natural compound improves muscle regeneration of dystrophic mice by, in turn, improving functional performance. Moreover, ILA ameliorates the inflammatory response in both muscle explants and the macrophages isolated from dystrophic mice to, thus, mitigate fibrosis after muscle damage. Overall, we show that ILA modulates both inflammation and muscle regeneration to, thus, contribute to improve the dystrophic phenotype.

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.

The muscle stem cell niche at a glance

Skeletal muscle stem cells (MuSCs, also called satellite cells) are the source of the robust regenerative capability of this tissue. The hallmark property of MuSCs at homeostasis is quiescence, a reversible state of cell cycle arrest required for long-term preservation of the stem cell population. MuSCs reside between an individual myofiber and an enwrapping basal lamina, defining the immediate MuSC niche. Additional cell types outside the basal lamina, in the interstitial space, also contribute to niche function. Quiescence is actively maintained by multiple niche-derived signals, including adhesion molecules presented from the myofiber surface and basal lamina, as well as soluble signaling factors produced by myofibers and interstitial cell types. In this Cell Science at a Glance article and accompanying poster, we present the most recent information on how niche signals promote MuSC quiescence and provide perspectives for further research.

Tenascin-C-EGFR activation induces functional human satellite cell proliferation and promotes wound-healing of skeletal muscles via oleanic acid

Human satellite cells (HuSCs) have been deemed to be the potential cure to treat muscular atrophy diseases such as Duchenne muscular dystrophy. However, the clinical trials of HuSCs were restricted to the inadequacy of donors because of that freshly isolated HuSCs quickly lost the Pax7 expression and myogenesis capacity in vivo after a few days of culture. Here we found that oleanic acid, a kind of triterpenoid endowed with diverse biological functions with treatment potential, could efficiently promote HuSCs proliferation. The HuSCs cultured in the medium supplement with oleanic acid could maintain a high expression level of Pax7 and retain the ability to differentiate into myotubes as well as facilitate muscle regeneration in injured muscles of recipient mice. We further revealed that Tenascin-C acts as the core mechanism to activate the EGFR signaling pathway followed by HuSCs proliferation. Taken together, our data provide an efficient method to expand functional HuSCs and a novel mechanism that controls HuSCs proliferation, which sheds light on the HuSCs-based therapy to treat muscle diseases.

Exploring traditional Chinese medicine as a potential treatment for sarcopenia: A network pharmacology and data mining analysis of drug selection and efficacy

Sarcopenia, as an increasingly pressing clinical issue, can be ameliorated through employment of traditional Chinese medicines. However, the current lack of specific pharmacological interventions for Sarcopenia necessitates further exploration of novel possibilities in traditional Chinese medicine for the treatment of this condition, utilizing advanced methodologies such as web pharmacology and data mining. Screening the essential targets of Sarcopenia, conducting matching between target and active molecules, as well as active molecules and herbs. Employing data mining techniques to analyze the screening outcomes, and molecular docking to compare the binding activities of active molecules with target proteins. The approach of using herbs for the treatment of Sarcopenia involves 13 targets, with 414 active compounds and 367 types of herbs. Data mining reveals that the herbs used in treating Sarcopenia are primarily characterized by their bitter taste, exerting their effects through dispelling dampness and promoting blood circulation. Moreover, 2 new formulas are postulated. Furthermore, molecular docking analysis indicates that the main active components of the herbs can be observed to tightly bind with the targets. Through network pharmacology and molecular docking, our findings reveal that herbs contain 15 key active components and 5 key targets, which correspond to 7 major herbs and 2 new formulas. Academically, these findings hold significant reference value for the development of novel drugs targeting Sarcopenia.

Reduced Expression of Septin7 Hinders Skeletal Muscle Regeneration

Septins are considered the fourth component of the cytoskeleton with the septin7 isoform playing a critical role in the formation of diffusion barriers in phospholipid bilayers and intra- and extracellular scaffolds. While its importance has already been confirmed in different intracellular processes, very little is known about its role in skeletal muscle. Muscle regeneration was studied in a Sept7 conditional knock-down mouse model to prove the possible role of septin7 in this process. Sterile inflammation in skeletal muscle was induced which was followed by regeneration resulting in the upregulation of septin7 expression. Partial knock-down of Sept7 resulted in an increased number of inflammatory cells and myofibers containing central nuclei. Taken together, our data suggest that partial knock-down of Sept7 hinders the kinetics of muscle regeneration, indicating its crucial role in skeletal muscle functions.

Transcriptional Profiling of Skeletal Muscle Stem Cells After In Vivo Engraftment into a Heterologous Niche Environment

Adult stem cells play a critical role in the maintenance and repair of the organs in which they reside. However, their function is highly dependent on the crosstalk with their niche environment that changes during development and in disease states. The niche provides signals to stem cells to activate, proliferate, self-renew, or remain in quiescence. In skeletal muscle, the niche is perturbed in disease contexts such as aging, muscular dystrophies, and cachexia. Therefore, it is important to develop methods that permit the decoupling of niche-mediated from cell-intrinsic changes that occur in muscle stem cells (MuSCs) in development and disease contexts. With the purpose of determining the effect of the niche environment on the MuSC transcriptome, function, or health, we have coupled an allogeneic stem cell transplantation system, meaning the transplantation of MuSCs from a donor mouse into a recipient host mouse, with Switching Mechanism at 5' End of RNA Template (SMART-Seq) to quantify the effects of the niche on the MuSC transcriptome in vivo. Briefly, MuSCs are isolated from a GFP reporter donor mouse (Pax7-nGFP) and transplanted into the irradiated muscles of immunocompromised allogeneic hosts. The MuSCs are re-isolated by fluorescence-activated cell sorting (FACS) after three weeks of inhabiting the heterologous niche, defined as a niche that is different from their originating niche, and sequencing-ready libraries are created. This method allows for the direct comparison of the transcriptome of stem cells before and after transplantation into a host of a different age, disease status, or genetic background. This method can be used to accurately quantify the direct effect of the niche environment on the stem cell gene expression profile and to decouple cell-intrinsic versus niche-mediated alterations in the stem cell transcriptome. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Allogeneic muscle stem cell transplantation.

Aging disrupts gene expression timing during muscle regeneration

Skeletal muscle function and regenerative capacity decline during aging, yet factors driving these changes are incompletely understood. Muscle regeneration requires temporally coordinated transcriptional programs to drive myogenic stem cells to activate, proliferate, fuse to form myofibers, and to mature as myonuclei, restoring muscle function after injury. We assessed global changes in myogenic transcription programs distinguishing muscle regeneration in aged mice from young mice by comparing pseudotime trajectories from single-nucleus RNA sequencing of myogenic nuclei. Aging-specific differences in coordinating myogenic transcription programs necessary for restoring muscle function occur following muscle injury, likely contributing to compromised regeneration in aged mice. Differences in pseudotime alignment of myogenic nuclei when comparing aged with young mice via dynamic time warping revealed pseudotemporal differences becoming progressively more severe as regeneration proceeds. Disruptions in timing of myogenic gene expression programs may contribute to incomplete skeletal muscle regeneration and declines in muscle function as organisms age.

Muscle stem cells contribute to long-term tissue repletion following surgical sepsis

BACKGROUND

Over the past decade, advances in sepsis identification and management have resulted in decreased sepsis mortality. This increase in survivorship has highlighted a new clinical obstacle: chronic critical illness (CCI), for which there are no effective treatment options. Up to half of sepsis survivors suffer from CCI, which can include multi-organ dysfunction, chronic inflammation, muscle wasting, physical and mental disabilities, and enhanced frailty. These symptoms prevent survivors from returning to regular day-to-day activities and are directly associated with poor quality of life.

METHODS

Mice were subjected to cecal ligation and puncture (CLP) with daily chronic stress (DCS) as an in vivo model to study sepsis late-effects/sequelae on skeletal muscle components. Longitudinal monitoring was performed via magnetic resonance imaging, skeletal muscle and/or muscle stem cell (MuSCs) assays (e.g., post-necropsy wet muscle weights, minimum Feret diameter measurements, in vitro MuSC proliferation and differentiation, number of regenerating myofibres and numbers of Pax7-positive nuclei per myofibre), post-sepsis whole muscle metabolomics and MuSC isolation and high-content transcriptional profiling.

RESULTS

We report several findings supporting the hypothesis that MuSCs/muscle regeneration are critically involved in post-sepsis muscle recovery. First, we show that genetic ablation of muscle stem cells (MuSCs) impairs post-sepsis muscle recovery (maintenance of 5-8% average lean mass loss compared with controls). Second, we observe impaired MuSCs expansion capacity and morphological defects at 26 days post-sepsis compared with control MuSCs (P < 0.001). Third, when subjected to an experimental muscle injury, sepsis-recovered mice exhibited evidence of impaired muscle regeneration compared with non-septic mice receiving the same muscle injury (CLP/DCS injured mean minimum Feret is 92.1% of control injured, P < 0.01). Fourth, we performed a longitudinal RNA sequencing study on MuSCs isolated from post-sepsis mice and found clear transcriptional differences in all post-sepsis samples compared with controls. At Day 28, CLP/DCS mice satellite cells have multiple altered metabolic pathways, such as oxidative phosphorylation, mitochondrial dysfunction, sirtuin signalling and oestrogen receptor signalling, compared with controls (P < 0.001).

CONCLUSIONS

Our data show that MuSCs and muscle regeneration are required for effective post-sepsis muscle recovery and that sepsis triggers morphological, functional, and transcriptional changes in MuSCs. Moving forward, we strive to leverage a more complete understanding of post-sepsis MuSC/regenerative defects to identify and test novel therapies that promote muscle recovery and improve quality of life in sepsis survivors.

A genome-wide association analysis of loss of ambulation in dystrophinopathy patients suggests multiple candidate modifiers of disease severity

The major determinant of disease severity in Duchenne muscular dystrophy (DMD) or milder Becker muscular dystrophy (BMD) is whether the dystrophin gene (DMD) mutation truncates the mRNA reading frame or allows expression of a partially functional protein. However, even in the complete absence of dystrophin, variability in disease severity is observed, and candidate gene studies have implicated several genes as modifiers. Here we present the largest genome-wide search to date for loci influencing severity in N = 419 DMD patients. Availability of subjects for such studies is quite limited, leading to modest sample sizes, which present a challenge for GWAS design. We have therefore taken special steps to minimize heterogeneity within our dataset at the DMD locus itself, taking a novel approach to mutation classification to effectively exclude the possibility of residual dystrophin expression, and utilized statistical methods that are well adapted to smaller sample sizes, including the use of a novel linear regression-like residual for time to ambulatory loss and the application of evidential statistics for the GWAS approach. Finally, we applied an unbiased in silico pipeline, utilizing functional genomic datasets to explore the potential impact of the best supported SNPs. In all, we obtained eight SNPs (out of 1,385,356 total) with posterior probability of trait-marker association (PPLD) ≥ 0.4, representing six distinct loci. Our analysis prioritized likely non-coding SNP regulatory effects on six genes (ETAA1, PARD6G, GALNTL6, MAN1A1, ADAMTS19, and NCALD), each with plausibility as a DMD modifier. These results support both recurrent and potentially new pathways for intervention in the dystrophinopathies.

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.

Inactivating IL34 promotes regenerating muscle stem cell expansion and attenuates Duchenne muscular dystrophy in mouse models

Background: The balance between the differentiation and self-renewal of satellite cells (SCs) is essential for skeletal muscle homeostasis and regeneration. Our knowledge of this regulatory process is incomplete. Methods: Using global and conditional knockout mice as in vivo models and isolated satellite cells as in vitro system, we investigated the regulatory mechanisms of IL34 in the process of skeletal muscle regeneration in vivo and in vitro. Results: Myocytes and regenerating fibers are major source of IL34. Deletion of interleukin 34 (IL34) sustains expansion by sacrificing the differentiation of SCs and leads to significant muscle regeneration defects. We further found that inactivating IL34 in SCs leads to hyperactivation of NFKB1 signaling; NFKB1 translocates to the nucleus and binds to the promoter region of Igfbp5 to synergistically disturb protein kinase B (Akt) activity. Notably, augmented Igfbp5 function in SCs led to deficient differentiation and Akt activity. Furthermore, disrupting Akt activity both in vivo and in vitro mimicked the phenotype of IL34 knockout. Finally, deleting IL34 or interfering Akt in mdx mice ameliorates dystrophic muscles. Conclusion: We comprehensively characterized regenerating myofibers-expressed IL34 plays a pivotal role in controlling myonuclear domain. The results also indicate that impairing IL34 function by promoting SC maintenance can lead to improved muscular performance in mdx mice in which the stem cell pool is compromised.

Molecular regulation of satellite cells via intercellular signaling

Somatic stem cells are tissue-specific reserve cells tasked to sustain tissue homeostasis in adulthood and/or effect tissue regeneration after traumatic injury. The stem cells of skeletal muscle tissue are the satellite cells, which were originally described and named after their localization beneath the muscle fiber lamina and attached to the multi-nucleated muscle fibers. During adult homeostasis, satellite cells are maintained in quiescence, a state of reversible cell cycle arrest. Yet, upon injury, satellite cells are rapidly activated, becoming highly mitotically active to generate large numbers of myoblasts that differentiate and fuse to regenerate the injured muscle fibers. A subset self-renews to replenish the pool of muscle stem cells.Complex intrinsic gene regulatory networks maintain the quiescent state of satellite cells, or upon injury, direct their activation, proliferation, differentiation and self-renewal. Molecular cues from the satellite cells' environment provide the essential information as to when and where satellite cells are to stay quiescent or break quiescence and effect regenerative myogenesis. Predominantly, these cues are secreted, diffusible or membrane-bound ligands that bind to and activate their specific cognate receptors on the satellite cell to activate downstream signaling cascades and elicit context-specific cell behavior. This review aims to offer a concise overview of major intercellular signaling pathways regulating satellite cells during quiescence and in injury-induced skeletal muscle regeneration.

Temporal static and dynamic imaging of skeletal muscle in vivo

A major challenge in the study of living systems is understanding how tissues and organs are established, maintained during homeostasis, reconstituted following injury or deteriorated during disease. Most of the studies that interrogate in vivo cell biological properties of cell populations within tissues are obtained through static imaging approaches. However, in vertebrates, little is known about which, when, and how extracellular and intracellular signals are dynamically integrated to regulate cell behaviour and fates, due largely to technical challenges. Intravital imaging of cellular dynamics in mammalian models has exposed surprising properties that have been missed by conventional static imaging approaches. Here we highlight some selected examples of intravital imaging in mouse intestinal stem cells, hematopoietic stem cells, hair follicle stem cells, and neural stem cells in the brain, each of which have distinct features from an anatomical and niche-architecture perspective. Intravital imaging of mouse skeletal muscles is comparatively less advanced due to several technical constraints that will be discussed, yet this approach holds great promise as a complementary investigative method to validate findings obtained by static imaging, as well as a method for discovery.

Three-dimensional chromatin re-organization during muscle stem cell aging

Age-related skeletal muscle atrophy or sarcopenia is a significant societal problem that is becoming amplified as the world's population continues to increase. The regeneration of damaged skeletal muscle is mediated by muscle stem cells, but in old age muscle stem cells become functionally attenuated. The molecular mechanisms that govern muscle stem cell aging encompass changes across multiple regulatory layers and are integrated by the three-dimensional organization of the genome. To quantitatively understand how hierarchical chromatin architecture changes during muscle stem cell aging, we generated 3D chromatin conformation maps (Hi-C) and integrated these datasets with multi-omic (chromatin accessibility and transcriptome) profiles from bulk populations and single cells. We observed that muscle stem cells display static behavior at global scales of chromatin organization during aging and extensive rewiring of local contacts at finer scales that were associated with variations in transcription factor binding and aberrant gene expression. These data provide insights into genome topology as a regulator of molecular function in stem cell aging.

The emergence of the stem cell niche

Stem cell niches are composed of dynamic microenvironments that support stem cells over a lifetime. The emerging niche is distinct from the adult because its main role is to support the progenitors that build organ systems in development. Emerging niches mature through distinct stages to form the adult niche and enable proper stem cell support. As a model of emerging niches, this review highlights how differences in the skeletal muscle microenvironment influence emerging versus satellite cell (SC) niche formation in skeletal muscle, which is among the most regenerative tissue systems. We contrast how stem cell niches regulate intrinsic properties between progenitor and stem cells throughout development to adulthood. We describe new applications for generating emerging niches from human pluripotent stem cells (hPSCs) using developmental principles and highlight potential applications for regeneration and therapeutics.

Stem Cell-Based Therapeutic Approaches in Genetic Diseases

Stem cells, which can self-renew and differentiate into different cell types, have become the keystone of regenerative medicine due to these properties. With the achievement of superior clinical results in the therapeutic approaches of different diseases, the applications of these cells in the treatment of genetic diseases have also come to the fore. Foremost, conventional approaches of stem cells to genetic diseases are the first approaches in this manner, and they have brought safety issues due to immune reactions caused by allogeneic transplantation. To eliminate these safety issues and phenotypic abnormalities caused by genetic defects, firstly, basic genetic engineering practices such as vectors or RNA modulators were combined with stem cell-based therapeutic approaches. However, due to challenges such as immune reactions and inability to target cells effectively in these applications, advanced molecular methods have been adopted in ZFN, TALEN, and CRISPR/Cas genome editing nucleases, which allow modular designs in stem cell-based genetic diseases' therapeutic approaches. Current studies in genetic diseases are in the direction of creating permanent treatment regimens by genomic manipulation of stem cells with differentiation potential through genome editing tools. In this chapter, the stem cell-based therapeutic approaches of various vital genetic diseases were addressed wide range from conventional applications to genome editing tools.

Quantification of Muscle Satellite Stem Cell Divisions by High-Content Analysis

High-content screening is commonly performed on 2D cultured cells, which is high throughput but has low biological relevance. In contrast, single myofiber culture assay preserves the satellite cell niche between the basal lamina and sarcolemma and consequently has high biological relevance but is low throughput. We describe here a high-content screening method that utilizes single myofiber culture that addresses the caveats of both techniques. Our method utilizes the transgenic reporter allele Myf5-Cre:R26R-eYFP to differentiate stem and committed cells within a dividing couplet that can be quantified by high-content throughput immunodetection and bioinformatic analysis.

Ageing and rejuvenation of tissue stem cells and their niches

Most adult organs contain regenerative stem cells, often organized in specific niches. Stem cell function is critical for tissue homeostasis and repair upon injury, and it is dependent on interactions with the niche. During ageing, stem cells decline in their regenerative potential and ability to give rise to differentiated cells in the tissue, which is associated with a deterioration of tissue integrity and health. Ageing-associated changes in regenerative tissue regions include defects in maintenance of stem cell quiescence, differentiation ability and bias, clonal expansion and infiltration of immune cells in the niche. In this Review, we discuss cellular and molecular mechanisms underlying ageing in the regenerative regions of different tissues as well as potential rejuvenation strategies. We focus primarily on brain, muscle and blood tissues, but also provide examples from other tissues, such as skin and intestine. We describe the complex interactions between different cell types, non-cell-autonomous mechanisms between ageing niches and stem cells, and the influence of systemic factors. We also compare different interventions for the rejuvenation of old regenerative regions. Future outlooks in the field of stem cell ageing are discussed, including strategies to counter ageing and age-dependent disease.

Genome-wide analysis of a cellular exercise model based on electrical pulse stimulation

Skeletal muscle communicates with other organs via myokines, which are secreted by muscle during exercise and exert various effects. Despite much investigation of the exercise, the underlying molecular mechanisms are still not fully understood. Here, we applied an in vitro exercise model in which cultured C2C12 myotubes were subjected to electrical pulse stimulation (EPS), which mimics contracting muscle. Based on the significantly up- and down-regulated genes in EPS, we constructed an in silico model to predict exercise responses at the transcriptional level. The in silico model revealed similarities in the transcriptomes of the EPS and exercised animals. Comparative analysis of the EPS data and exercised mouse muscle identified putative biomarkers in exercise signaling pathways and enabled to discover novel exercise-induced myokines. Biochemical analysis of selected exercise signature genes in muscle from exercised mice showed that EPS mimics in vivo exercise, at least in part, at the transcriptional level. Consequently, we provide a novel myokine, Amphiregulin (AREG), up-regulated both in vitro and in vivo, that would be a potential target for exercise mimetics.

Regulation of muscle stem cell fate

Skeletal muscle plays a critical role in human health. Muscle stem cells (MuSCs) serve as the major cell type contributing to muscle regeneration by directly differentiating to mature muscle cells. MuSCs usually remain quiescent with occasionally self-renewal and are activated to enter cell cycle for proliferation followed by differentiation upon muscle injury or under pathological conditions. The quiescence maintenance, activation, proliferation, and differentiation of MuSCs are tightly regulated. The MuSC cell-intrinsic regulatory network and the microenvironments work coordinately to orchestrate the fate transition of MuSCs. The heterogeneity of MuSCs further complicates the regulation of MuSCs. This review briefly summarizes the current progress on the heterogeneity of MuSCs and the microenvironments, epigenetic, and transcription regulations of MuSCs.

Proteins implicated in muscular dystrophy and cancer are functional constituents of the centrosome

Aberrant expression of dystrophin, utrophin, dysferlin, or calpain-3 was originally identified in muscular dystrophies (MDs). Increasing evidence now indicates that these proteins might act as tumor suppressors in myogenic and non-myogenic cancers. As DNA damage and somatic aneuploidy, hallmarks of cancer, are early pathological signs in MDs, we hypothesized that a common pathway might involve the centrosome. Here, we show that dystrophin, utrophin, dysferlin, and calpain-3 are functional constituents of the centrosome. In myoblasts, lack of any of these proteins caused excess centrosomes, centrosome misorientation, nuclear abnormalities, and impaired microtubule nucleation. In dystrophin double-mutants, these defects were significantly aggravated. Moreover, we demonstrate that also in non-myogenic cells, all four MD-related proteins localize to the centrosome, including the muscle-specific full-length dystrophin isoform. Therefore, MD-related proteins might share a convergent function at the centrosome in addition to their diverse, well-established muscle-specific functions. Thus, our findings support the notion that cancer-like centrosome-related defects underlie MDs and establish a novel concept linking MDs to cancer.

Loss of full-length dystrophin expression results in major cell-autonomous abnormalities in proliferating myoblasts

Duchenne muscular dystrophy (DMD) affects myofibers and muscle stem cells, causing progressive muscle degeneration and repair defects. It was unknown whether dystrophic myoblasts—the effector cells of muscle growth and regeneration—are affected. Using transcriptomic, genome-scale metabolic modelling and functional analyses, we demonstrate, for the first time, convergent abnormalities in primary mouse and human dystrophic myoblasts. In Dmd^mdx myoblasts lacking full-length dystrophin, the expression of 170 genes was significantly altered. Myod1 and key genes controlled by MyoD (Myog, Mymk, Mymx, epigenetic regulators, ECM interactors, calcium signalling and fibrosis genes) were significantly downregulated. Gene ontology analysis indicated enrichment in genes involved in muscle development and function. Functionally, we found increased myoblast proliferation, reduced chemotaxis and accelerated differentiation, which are all essential for myoregeneration. The defects were caused by the loss of expression of full-length dystrophin, as similar and not exacerbated alterations were observed in dystrophin-null Dmd^mdx-βgeo myoblasts. Corresponding abnormalities were identified in human DMD primary myoblasts and a dystrophic mouse muscle cell line, confirming the cross-species and cell-autonomous nature of these defects. The genome-scale metabolic analysis in human DMD myoblasts showed alterations in the rate of glycolysis/gluconeogenesis, leukotriene metabolism, and mitochondrial beta-oxidation of various fatty acids. These results reveal the disease continuum: DMD defects in satellite cells, the myoblast dysfunction affecting muscle regeneration, which is insufficient to counteract muscle loss due to myofiber instability. Contrary to the established belief, our data demonstrate that DMD abnormalities occur in myoblasts, making these cells a novel therapeutic target for the treatment of this lethal disease.

Regulating the regulator: a survey of mechanisms from transcription to translation controlling expression of mammalian cell cycle kinase Aurora A

Aurora Kinase A (AURKA) is a positive regulator of mitosis with a strict cell cycle-dependent expression pattern. Recently, novel oncogenic roles of AURKA have been uncovered that are independent of the kinase activity and act within multiple signalling pathways, including cell proliferation, survival and cancer stem cell phenotypes. For this, cellular abundance of AURKA protein is per se crucial and must be tightly fine-tuned. Indeed, AURKA is found overexpressed in different cancers, typically as a result of gene amplification or enhanced transcription. It has however become clear that impaired processing, decay and translation of AURKA mRNA can also offer the basis for altered AURKA levels. Accordingly, the involvement of gene expression mechanisms controlling AURKA expression in human diseases is increasingly recognized and calls for much more research. Here, we explore and create an integrated view of the molecular processes regulating AURKA expression at the level of transcription, post-transcription and translation, intercalating discussion on how impaired regulation underlies disease. Given that targeting AURKA levels might affect more functions compared to inhibiting the kinase activity, deeper understanding of its gene expression may aid the design of alternative and therapeutically more successful ways of suppressing the AURKA oncogene.

Transplantation to study satellite cell heterogeneity in skeletal muscle

Skeletal muscle has a remarkable capacity to regenerate throughout life, which is mediated by its resident muscle stem cells, also called satellite cells. Satellite cells, located periphery to the muscle fibers and underneath the basal lamina, are an indispensable cellular source for muscle regeneration. Satellite cell transplantation into regenerating muscle contributes robustly to muscle repair, thereby indicating that satellite cells indeed function as adult muscle stem cells. Moreover, satellite cells are a heterogenous population in adult tissue, with subpopulations that can be distinguished based on gene expression, cell-cycle progression, ability to self-renew, and bi-potential ability. Transplantation assays provide a powerful tool to better understand satellite cell function in vivo enabling the separation of functionally distinct satellite cell subpopulations. In this review, we focus on transplantation strategies to explore satellite cells’ functional heterogeneity, approaches targeting the recipient tissue to improve transplantation efficiency, and common strategies to monitor the behaviour of the transplanted cells. Lastly, we discuss some recent approaches to overcome challenges to enhance the transplantation potential of muscle stem cells.

Satellite cell-specific deletion of Cipc alleviates myopathy in mdx mice

Skeletal muscle regeneration relies on satellite cells that can proliferate, differentiate, and form new myofibers upon injury. Emerging evidence suggests that misregulation of satellite cell fate and function influences the severity of Duchenne muscular dystrophy (DMD). The transcription factor Pax7 determines the myogenic identity and maintenance of the pool of satellite cells. The circadian clock regulates satellite cell proliferation and self-renewal. Here, we show that the CLOCK-interacting protein Circadian (CIPC) a negative-feedback regulator of the circadian clock, is up-regulated during myoblast differentiation. Specific deletion of Cipc in satellite cells alleviates myopathy, improves muscle function, and reduces fibrosis in mdx mice. Cipc deficiency leads to activation of the ERK1/2 and JNK1/2 signaling pathways, which activates the transcription factor SP1 to trigger the transcription of Pax7 and MyoD. Therefore, CIPC is a negative regulator of satellite cell function, and loss of Cipc in satellite cells promotes muscle regeneration.

The regenerating skeletal muscle niche drives satellite cell return to quiescence

Skeletal muscle stem cells, or satellite cells (SCs), are essential to regenerate and maintain muscle. Quiescent SCs reside in an asymmetric niche between the basal lamina and myofiber membrane. To repair muscle, SCs activate, proliferate, and differentiate, fusing to repair myofibers or reacquiring quiescence to replenish the SC niche. Little is known about when SCs reacquire quiescence during regeneration or the cellular processes that direct SC fate decisions. We find that most SCs reacquire quiescence 5–10 days after muscle injury, following differentiation and fusion of most cells to regenerate myofibers. Single-cell sequencing of myogenic cells in regenerating muscle identifies SCs reacquiring quiescence and reveals that noncell autonomous signaling networks influence SC fate decisions during regeneration. SC transplantation experiments confirm that the regenerating environment influences SC fate. We define a window for SC repopulation of the niche, emphasizing the temporal contribution of the regenerative muscle environment on SC fate.

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Satellite cells primarily produce myonuclei in the first 4 days following injury

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Most satellite cells reacquire quiescence 5–7 days following injury

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Asymmetric division pathways are elevated in satellite cells reacquiring quiescence

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The regenerating muscle environment directs satellite cell fate decisions

The Protective Effects of γ-Tocotrienol on Muscle Stem Cells Through Inhibiting Reactive Oxidative Stress Production

Pseudotrophic muscular dystrophy is a common clinical skeletal muscle necrotic disease, among which Duchenne muscular dystrophy (DMD) is the predominant. For such diseases, there is no clinically effective treatment, which is only symptomatic or palliative treatment. Oxidative stress and chronic inflammation are common pathological features of DMD. In recent years, it has been found that the pathophysiological changes of skeletal muscle in DMD mice are related to muscle stem cell failure. In the present study, we established a DMD mice model and provided tocotrienol (γ-tocotrienol, GT3), an antioxidant compound, to explore the relationship between the physiological state of muscle stem cells and oxidative stress. The results showed that the application of GT3 can reduce ROS production and cellular proliferation in the muscle stem cells of DMD mice, which is beneficial to promote the recovery of muscle stem cell function in DMD mice. GT3 treatment improved the differentiation ability of muscle stem cells in DMD mice with increasing numbers of MyoD⁺ cells. GT3 application significantly decreased percentages of CD45⁺ cells and PDGFRα⁺ fibro-adipogenic progenitors in the tibialis anterior of DMD mice, indicating that the increased inflammation and fibro-adipogenic progenitors were attenuated in GT3-treated DMD mice. These data suggest that increased ROS production causes dysfunctional muscle stem cell in DMD mice, which might provide a new avenue to treat DMD patients in the clinic.

The jam session between muscle stem cells and the extracellular matrix in the tissue microenvironment

Skeletal muscle requires a highly orchestrated coordination between multiple cell types and their microenvironment to exert its function and to maintain its homeostasis and regenerative capacity. Over the past decades, significant advances, including lineage tracing and single-cell RNA sequencing, have contributed to identifying multiple muscle resident cell populations participating in muscle maintenance and repair. Among these populations, muscle stem cells (MuSC), also known as satellite cells, in response to stress or injury, are able to proliferate, fuse, and form new myofibers to repair the damaged tissue. These cells reside adjacent to the myofiber and are surrounded by a specific and complex microenvironment, the stem cell niche. Major components of the niche are extracellular matrix (ECM) proteins, able to instruct MuSC behavior. However, during aging and muscle-associated diseases, muscle progressively loses its regenerative ability, in part due to a dysregulation of ECM components. This review provides an overview of the composition and importance of the MuSC microenvironment. We discuss relevant ECM proteins and how their mutations or dysregulation impact young and aged muscle tissue or contribute to diseases. Recent discoveries have improved our knowledge about the ECM composition of skeletal muscle, which has helped to mimic the architecture of the stem cell niche and improved the regenerative capacity of MuSC. Further understanding about extrinsic signals from the microenvironment controlling MuSC function and innovative technologies are still required to develop new therapies to improve muscle repair.

Muscle stem cell polarity requires QKI-mediated alternative splicing of Integrin Alpha-7 (Itga7)

The RNA-binding protein Quaking (QKI) is a post-transcriptional regulator of genes encoding polarity proteins in muscle stem cells. Loss of QKI in MuSCs results in reduced myogenic progenitors and a striking muscle regeneration defect.

Muscle stem cells (MuSCs) have the ability to carry out the specialized function of cell polarization, which is required for the production of one repopulating cell and one myogenic progenitor cell with muscle regeneration capabilities. The mechanisms which regulate proteins involved in establishing MuSC polarity such as Dmd and Itga7 are currently not well understood. Herein, we define the RNA-binding protein Quaking (QKI) as a major regulator alternative splicing of key MuSC polarity factors including Dmd, Itga7, Mark2, and Numb. We generate a conditional QKI knockout mouse, and for the first time it is shown in vivo that deficiency of QKI in MuSCs results in reduced asymmetric cell divisions, leading to a loss of the myogenic progenitor cell population and striking muscle regeneration defects. Transcriptomic analysis of QKI-deficient MuSCs identifies QKI as a regulator of the splicing events which give rise to the mutually exclusive Itga7-X1 and -X2 isoforms. We observe increased X1 expression in QKI-deficient MuSCs and recapitulate this splicing event using antisense oligonucleotide directed against a quaking binding site within the Itga7 mRNA. Interestingly, recreating this single splicing event is detrimental to the polarization of Itga7 and Dmd proteins, and leads to a drastic reduction of the myogenic progenitor population, highlighting the significance of QKI-mediated alternative splicing of Itga7 in maintaining MuSC polarity. Altogether, these findings define a novel role for QKI as a post-transcriptional regulator of MuSC polarity.

Muscle stem cell adaptations to cellular and environmental stress

Background

Lifelong regeneration of the skeletal muscle is dependent on a rare population of resident skeletal muscle stem cells, also named ‘satellite cells’ for their anatomical position on the outside of the myofibre and underneath the basal lamina. Muscle stem cells maintain prolonged quiescence, but activate the myogenic programme and the cell cycle in response to injury to expand a population of myogenic progenitors required to regenerate muscle. The skeletal muscle does not regenerate in the absence of muscle stem cells.

Main body

The notion that lifelong regeneration of the muscle is dependent on a rare, non-redundant population of stem cells seems contradictory to accumulating evidence that muscle stem cells have activated multiple stress response pathways. For example, muscle stem cell quiescence is mediated in part by the eIF2α arm of the integrated stress response and by negative regulators of mTORC1, two translational control pathways that downregulate protein synthesis in response to stress. Muscle stem cells also activate pathways to protect against DNA damage, heat shock, and environmental stress. Here, we review accumulating evidence that muscle stem cells encounter stress during their prolonged quiescence and their activation. While stress response pathways are classically described to be bimodal whereby a threshold dictates cell survival versus cell death responses to stress, we review evidence that muscle stem cells additionally respond to stress by spontaneous activation and fusion to myofibres.

Conclusion

We propose a cellular stress test model whereby the prolonged state of quiescence and the microenvironment serve as selective pressures to maintain muscle stem cell fitness, to safeguard the lifelong regeneration of the muscle. Fit muscle stem cells that maintain robust stress responses are permitted to maintain the muscle stem cell pool. Unfit muscle stem cells are depleted from the pool first by spontaneous activation, or in the case of severe stress, by activating cell death or senescence pathways.

Key concepts in muscle regeneration: muscle "cellular ecology" integrates a gestalt of cellular cross-talk, motility, and activity to remodel structure and restore function

This review identifies some key concepts of muscle regeneration, viewed from perspectives of classical and modern research. Early insights noted the pattern and sequence of regeneration across species was similar, regardless of the type of injury, and differed from epimorphic limb regeneration. While potential benefits of exercise for tissue repair was debated, regeneration was not presumed to deliver functional restoration, especially after ischemia-reperfusion injury; muscle could develop fibrosis and ectopic bone and fat. Standard protocols and tools were identified as necessary for tracking injury and outcomes. Current concepts vastly extend early insights. Myogenic regeneration occurs within the environment of muscle tissue. Intercellular cross-talk generates an interactive system of cellular networks that with the extracellular matrix and local, regional, and systemic influences, forms the larger gestalt of the satellite cell niche. Regenerative potential and adaptive plasticity are overlain by epigenetically regionalized responsiveness and contributions by myogenic, endothelial, and fibroadipogenic progenitors and inflammatory and metabolic processes. Muscle architecture is a living portrait of functional regulatory hierarchies, while cellular dynamics, physical activity, and muscle-tendon-bone biomechanics arbitrate regeneration. The scope of ongoing research-from molecules and exosomes to morphology and physiology-reveals compelling new concepts in muscle regeneration that will guide future discoveries for use in application to fitness, rehabilitation, and disease prevention and treatment.

Silibinin Alleviates Muscle Atrophy Caused by Oxidative Stress Induced by Cisplatin through ERK/FoxO and JNK/FoxO Pathways

Cisplatin (DDP), a widely used chemotherapeutic drug in cancer treatment, causes oxidative stress, resulting in cancer cachexia and skeletal muscle atrophy. This study investigated the effects and activity of silibinin (SLI) in reducing DDP-induced oxidative stress and skeletal muscle atrophy in vivo and in vitro. SLI alleviated weight loss, food intake, muscle wasting, adipose tissue depletion, and organ weight reduction induced by DDP and improved the reduction of grip force caused by DDP. SLI can attenuated the increase in reactive oxygen species (ROS) levels, the decrease in Nrf2 expression, the decrease in the fiber cross-sectional area, and changes in fiber type induced by DDP. SLI regulated the ERK/FoxO and JNK/FoxO pathways by downregulating the abnormal increase in ROS and Nrf2 expression in DDP-treated skeletal muscle and C2C12 myotube cells. Further, SLI inhibited the upregulation of MAFbx and Mstn, the downregulation of MyHC and MyoG, the increase in protein degradation, and the decrease of protein synthesis. The protective effects of SLI were reversed by cotreatment with JNK agonists and ERK inhibitors. These results suggest that SLI can reduce DDP-induced skeletal muscle atrophy by reducing oxidative stress and regulating ERK/FoxO and JNK/FoxO pathways.

Transcriptome aberration associated with altered locomotor behavior of zebrafish (Danio rerio) caused by Waterborne Benzo[a]pyrene

Waterborne Benzo[a]pyrene (B[a]P) pollution is a global threat to aquatic organisms. The exposure to waterborne B[a]P can disrupt the normal locomotor behavior of zebrafish (Danio rerio), however, how it affect the locomotor behavior of adult zebrafish remains unclear. Herein, B[a]P at two concentrations (0.8 μg/L and 2.0 μg/L) were selected to investigate the molecular mechanisms of the affected locomotor behavior of zebrafish by B[a]P based on transcriptome profiling. Adverse effects of B[a]P exposure affecting locomotor behavior in zebrafish were studied by RNA sequencing, and the locomotion phenotype was acquired. The gene enrichment results showed that the differentially highly expressed genes (atp2a1, cdh2, aurka, fxyd1, clstn1, apoc1, mt-co1, tnnt3b, and fads2) of zebrafish are mainly enriched in adrenergic signaling in cardiomyocytes (dre04261) and locomotory behavior (GO:0007626). The movement trajectory plots showed an increase in the locomotor distance and velocity of zebrafish in the 0.8 μg/L group and the opposite in the 2.0 μg/L group. The results showed that B[a]P affects the variety of genes in zebrafish, including motor nerves, muscles, and energy supply, and ultimately leads to altered locomotor behavior.

Innovation in culture systems to study muscle complexity

Endogenous skeletal muscle development, regeneration, and pathology are extremely complex processes, influenced by local and systemic factors. Unpinning how these mechanisms function is crucial for fundamental biology and to develop therapeutic interventions for genetic disorders, but also conditions like sarcopenia and volumetric muscle loss. Ex vivo skeletal muscle models range from two- and three-dimensional primary cultures of satellite stem cell-derived myoblasts grown alone or in co-culture, to single muscle myofibers, myobundles, and whole tissues. Together, these systems provide the opportunity to gain mechanistic insights of stem cell behavior, cell-cell interactions, and mature muscle function in simplified systems, without confounding variables. Here, we highlight recent advances (published in the last 5 years) using in vitro primary cells and ex vivo skeletal muscle models, and summarize the new insights, tools, datasets, and screening methods they have provided. Finally, we highlight the opportunity for exponential advance of skeletal muscle knowledge, with spatiotemporal resolution, that is offered by guiding the study of muscle biology and physiology with in silico modelling and implementing high-content cell biology systems and ex vivo physiology platforms.

Neuromuscular Development and Disease: Learning From in vitro and in vivo Models

The neuromuscular junction (NMJ) is a specialized cholinergic synaptic interface between a motor neuron and a skeletal muscle fiber that translates presynaptic electrical impulses into motor function. NMJ formation and maintenance require tightly regulated signaling and cellular communication among motor neurons, myogenic cells, and Schwann cells. Neuromuscular diseases (NMDs) can result in loss of NMJ function and motor input leading to paralysis or even death. Although small animal models have been instrumental in advancing our understanding of the NMJ structure and function, the complexities of studying this multi-tissue system in vivo and poor clinical outcomes of candidate therapies developed in small animal models has driven the need for in vitro models of functional human NMJ to complement animal studies. In this review, we discuss prevailing models of NMDs and highlight the current progress and ongoing challenges in developing human iPSC-derived (hiPSC) 3D cell culture models of functional NMJs. We first review in vivo development of motor neurons, skeletal muscle, Schwann cells, and the NMJ alongside current methods for directing the differentiation of relevant cell types from hiPSCs. We further compare the efficacy of modeling NMDs in animals and human cell culture systems in the context of five NMDs: amyotrophic lateral sclerosis, myasthenia gravis, Duchenne muscular dystrophy, myotonic dystrophy, and Pompe disease. Finally, we discuss further work necessary for hiPSC-derived NMJ models to function as effective personalized NMD platforms.