Notch3 Null Mutation in Mice Causes Muscle Hyperplasia by Repetitive Muscle Regeneration

Repeated birth injuries lead to long-term pelvic floor muscle dysfunction in the preclinical rat model

BACKGROUND

Vaginal childbirth is a key risk factor for pelvic floor muscle injury and dysfunction, and subsequent pelvic floor disorders. Multiparity further exacerbates these risks. Using the rat model, validated for the studies of the human pelvic floor muscles, we have previously identified that a single simulated birth injury results in pelvic floor muscle atrophy and fibrosis.

OBJECTIVE

To test the hypothesis that multiple birth injuries would further overwhelm the muscle regenerative capacity, leading to functionally relevant pathological alterations long-term.

STUDY DESIGN

Sprague-Dawley rats underwent simulated birth injury and were allowed to recover for 8 weeks before undergoing additional birth injury. Animals were sacrificed at acute (3 and 7 days postinjury), subacute (21, 28, and 35 days postinjury), and long-term (8 and 12 weeks postinjury) time points post second injury (N=3-8/time point), and the pubocaudalis portion of the rat levator ani complex was harvested to assess the impact of repeated birth injuries on muscle mechanical and histomorphological properties. The accompanying transcriptional changes were assessed by a customized NanoString panel. Uninjured animals were used as controls. Data with a parametric distribution were analyzed by a 2-way analysis of variance followed by post hoc pairwise comparisons using Tukey's or Sidak's tests; nonparametrically distributed data were compared with Kruskal-Wallis test followed by pairwise comparisons with Dunn's test. Data, analyzed using GraphPad Prism v8.0, San Diego, CA, are presented as mean ± standard error of the mean or median (range).

RESULTS

Following the first simulated birth injury, active muscle force decreased acutely relative to uninjured controls (12.9±0.9 vs 25.98±2.1 g/mm2, P<.01). At 4 weeks, muscle active force production recovered to baseline and remained unchanged at 8 weeks after birth injury (P>.99). Similarly, precipitous decrease in active force was observed immediately after repeated birth injury (18.07±1.2 vs 25.98±2.1 g/mm2, P<.05). In contrast to the functional recovery after a single birth injury, a long-term decrease in muscle contractile function was observed up to 12 weeks after repeated birth injuries (18.3±1.6 vs 25.98±2.1 g/mm2, P<.05). Fiber size was smaller at the long-term time points after second injury compared to the uninjured group (12 weeks vs uninjured control: 1485 (60.7-5000) vs 1989 (65.6-4702) μm2, P<.0001). The proportion of fibers with centralized nuclei, indicating active myofiber regeneration, returned to baseline at 8 weeks post-first birth injury, (P=.95), but remained elevated as far as 12 weeks post-second injury (12 weeks vs uninjured control: 7.1±1.5 vs 0.84±0.13%, P<0.0001). In contrast to the plateauing intramuscular collagen content after 4 weeks post-first injury, fibrotic degeneration increased progressively over 12 weeks after repeated injury (12 weeks vs uninjured control: 6. 7±1.1 vs 2.03±0.2%, P<.001). Prolonged expression of proinflammatory genes accompanied by a greater immune infiltrate was observed after repeated compared to a single birth injury.

CONCLUSION

Overall, repeated birth injuries lead to a greater magnitude of pathological alterations compared to a single injury, resulting in more pronounced pelvic floor muscle degeneration and muscle dysfunction in the rat model. The above provides a putative mechanistic link between multiparity and the increased risk of pelvic floor dysfunction in women.

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.

The extracellular matrix niche of muscle stem cells

Preserving the potency of stem cells in adult tissues is very demanding and relies on the concerted action of various cellular and non-cellular elements in a precise stoichiometry. This balanced microenvironment is found in specific anatomical "pockets" within the tissue, known as the stem cell niche. In this review, we explore the interplay between stem cells and their niches, with a primary focus on skeletal muscle stem cells and the extracellular matrix (ECM). Quiescent muscle stem cells, known as satellite cells are active producers of a diverse array of ECM molecules, encompassing major constituents like collagens, laminins, and integrins, some of which are explored in this review. The conventional perception of ECM as merely a structural scaffold is evolving. Collagens can directly interact as ligands with receptors on satellite cells, while other ECM proteins have the capacity to sequester growth factors and regulate their release, especially relevant during satellite cell turnover in homeostasis or activation upon injury. Additionally, we explore an evolutionary perspective on the ECM across a range of multicellular organisms and discuss a model wherein satellite cells are self-sustained by generating their own niche. Considering the prevalence of ECM proteins in the connective tissue of various organs it is not surprising that mutations in ECM genes have pathological implications, including in muscle, where they can lead to myopathies. However, the particular role of certain disease-related ECM proteins in stem cell maintenance highlights the potential contribution of stem cell deregulation to the progression of these disorders.

Muscle stem cell dysfunction in rhabdomyosarcoma and muscular dystrophy

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

Insights into the Notch signaling pathway in degenerative musculoskeletal disorders: Mechanisms and perspectives

Degenerative musculoskeletal disorders are a group of age-related diseases of the locomotive system that severely affects the patient's ability to work and cause adverse sequalae such as fractures and even death. The incidence and prevalence of degenerative musculoskeletal disorders is rising owing to the aging of the world's population. The Notch signaling pathway, which is expressed in almost all organ systems, extensively regulates cell proliferation and differentiation as well as cellular fate. Notch signaling shows increased activity in degenerative musculoskeletal disorders and retards the progression of degeneration to some extent. The review focuses on four major degenerative musculoskeletal disorders (osteoarthritis, intervertebral disc degeneration, osteoporosis, and sarcopenia) and summarizes the pathophysiological functions of Notch signaling in these disorders, especially its role in stem/progenitor cells in each disorder. Finally, a conclusion will be presented to explore the research and application of the perspectives on Notch signaling in degenerative musculoskeletal disorders.

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.

Mechanical compression creates a quiescent muscle stem cell niche

Tissue stem cell niches are regulated by their mechanical environment, notably the extracellular matrix (ECM). Skeletal muscles consist of bundled myofibers for force transmission. Within this macroscopic architecture, quiescent Pax7-expressing (Pax7⁺) muscle stem cells (MuSCs) are compressed between ECM basally and myofiber apically. Muscle injury causes MuSCs to lose apical compression from the myofiber and re-enter the cell cycle for regeneration. While ECM elasticities have been shown to affect MuSC's renewal, the significance of apical compression remains unknown. To investigate the role of apical compression, we simulate the MuSCs' in vivo mechanical environment by applying physical compression to MuSCs' apical surface. We demonstrate that compression drives activated MuSCs back to a quiescent stem cell state, regardless of basal elasticities and chemistries. By mathematical modeling and cell tension manipulation, we conclude that low overall tension combined with high axial tension generated by compression leads to MuSCs' stemness and quiescence. Unexpectedly, we discovered that apical compression results in up-regulation of Notch downstream genes, accompanied by the increased levels of nuclear Notch1&3 in a Delta ligand (Dll) and ADAM10/17 independent manner. Our results fill a knowledge gap on the role of apical compression for MuSC fate and have implications to stem cells in other tissues.

Skeletal Muscle Stem Cells in Aging: Asymmetric/Symmetric Division Switching

In aged muscle, satellite cells’ symmetric and asymmetric divisions are impaired, and intrinsic and extrinsic complex mechanisms govern these processes. This review presents many updated aspects regarding muscle stem cells’ fate in normal and aging conditions. The balance between self-renewal and commitment divisions contributes to muscle regeneration, muscle homeostasis, aging, and disease. Stimulating muscle regeneration in aging could be a therapeutic target, but there is still a need to understand the many mechanisms that influence each other in satellite cells and their niche. We highlight here the general outlines regarding satellite cell divisions, the primary markers present in muscle stem cells, the aging aspects concerning signaling pathways involved in symmetric/asymmetric divisions, the regenerative capacity of satellite cells and their niche alteration in senescent muscle, genetics and epigenetics mechanisms implied in satellite cells aging and exercise effect on muscle regeneration in the elderly.

The Notch signaling pathway in skeletal muscle health and disease

The Notch signaling pathway is a key regulator of skeletal muscle development and regeneration. Over the past decade, the discoveries of three new muscle disease genes have added a new dimension to the relationship between the Notch signaling pathway and skeletal muscle: MEGF10, POGLUT1, and JAG2. We review the clinical syndromes associated with pathogenic variants in each of these genes, known molecular and cellular functions of their protein products with a particular focus on the Notch signaling pathway, and potential novel therapeutic targets that may emerge from further investigations of these diseases. The phenotypes associated with two of these genes, POGLUT1 and JAG2, clearly fall within the realm of muscular dystrophy, whereas the third, MEGF10, is associated with a congenital myopathy/muscular dystrophy overlap syndrome classically known as early-onset myopathy, areflexia, respiratory distress, and dysphagia. JAG2 is a canonical Notch ligand, POGLUT1 glycosylates the extracellular domain of Notch receptors, and MEGF10 interacts with the intracellular domain of NOTCH1. Additional genes and their encoded proteins relevant to muscle function and disease with links to the Notch signaling pathway include TRIM32, ATP2A1 (SERCA1), JAG1, PAX7, and NOTCH2NLC. There is enormous potential to identify convergent mechanisms of skeletal muscle disease and new therapeutic targets through further investigations of the Notch signaling pathway in the context of skeletal muscle development, maintenance, and disease.

Implications of notch signaling in duchenne muscular dystrophy

This review focuses upon the implications of the Notch signaling pathway in muscular dystrophies, particularly Duchenne muscular dystrophy (DMD): a pervasive and catastrophic condition concerned with skeletal muscle degeneration. Prior work has defined the pathogenesis of DMD, and several therapeutic approaches have been undertaken in order to regenerate skeletal muscle tissue and ameliorate the phenotype. There is presently no cure for DMD, but a promising avenue for novel therapies is inducing muscle regeneration via satellite cells (muscle stem cells). One specific target using this approach is the Notch signaling pathway. The canonical Notch signaling pathway has been well-characterized and it ultimately governs cell fate decision, cell proliferation, and induction of differentiation. Additionally, inhibition of the Notch signaling pathway has been directly implicated in the deficits seen with muscular dystrophies. Here, we explore the connection between the Notch signaling pathway and DMD, as well as how Notch signaling may be targeted to improve the muscle degeneration seen in muscular dystrophies.

Cultured Myoblasts Derived from Rat Soleus Muscle Show Altered Regulation of Proliferation and Myogenesis during the Course of Mechanical Unloading

The structure and function of soleus muscle fibers undergo substantial remodeling under real or simulated microgravity conditions. However, unloading-induced changes in the functional activity of skeletal muscle primary myoblasts remain poorly studied. The purpose of our study was to investigate how short-term and long-term mechanical unloading would affect cultured myoblasts derived from rat soleus muscle. Mechanical unloading was simulated by rat hindlimb suspension model (HS). Myoblasts were purified from rat soleus at basal conditions and after 1, 3, 7, and 14 days of HS. Myoblasts were expanded in vitro, and the myogenic nature was confirmed by their ability to differentiate as well as by immunostaining/mRNA expression of myogenic markers. The proliferation activity at different time points after HS was analyzed, and transcriptome analysis was performed. We have shown that soleus-derived myoblasts differently respond to an early and later stage of HS. At the early stage of HS, the proliferative activity of myoblasts was slightly decreased, and processes related to myogenesis activation were downregulated. At the later stage of HS, we observed a decrease in myoblast proliferative potential and spontaneous upregulation of the pro-myogenic program.

Functional Identification of Porcine DLK1 during Muscle Development

Simple Summary

Skeletal muscle is the largest tissue and serves as a protein reservoir and energy reservoir in the human and animal body. It also serves as the main metabolic activity site. The formation of skeletal muscle mainly depends on the differentiation and fusion of myocytes and other complex ordered processes; each step is regulated by various factors. In this study, we investigated the expression profiles, functional identification, and regulatory pathways of Delta-like 1 homolog (DLK1) in pigs and myocytes. We found that DLK1 was highly expressed in the muscle tissues of pigs. DLK1 promoted myocyte proliferation, migration, differentiation, fusion, and muscular hypertrophy, but suppressed muscle degradation. DLK1 also inhibited the Notch signaling pathway by regulating the expression of key factors in the pathway, thereby producing a phenotype in which DLK1 promotes muscle development. These findings provide valuable information to improve our understanding of the functional mechanisms of DLK1 that underly myogenesis to accelerate the process of animal genetic improvement.

Abstract

DLK1 is paternally expressed and is involved in metabolism switching, stem cell maintenance, cell proliferation, and differentiation. Porcine DLK1 was identified in our previous study as a candidate gene that regulates muscle development. In the present study, we characterized DLK1 expression in pigs, and the results showed that DLK1 was highly expressed in the muscles of pigs. In-vitro cellular tests showed that DLK1 promoted myoblast proliferation, migration, and muscular hypertrophy, and at the same time inhibited muscle degradation. The expression of myogenic and fusion markers and the formation of multinucleated myotubes were both upregulated in myoblasts with DLK1 overexpression. DLK1 levels in cultured myocytes were negatively correlated with the expression of key factors in the Notch pathway, suggesting that the suppression of Notch signaling pathways may mediate these processes. Collectively, our results suggest a biological function of DLK1 as an enhancer of muscle development by the inhibition of Notch pathways.

The Notch signaling network in muscle stem cells during development, homeostasis, and disease

Skeletal muscle stem cells have a central role in muscle growth and regeneration. They reside as quiescent cells in resting muscle and in response to damage they transiently amplify and fuse to produce new myofibers or self-renew to replenish the stem cell pool. A signaling pathway that is critical in the regulation of all these processes is Notch. Despite the major differences in the anatomical and cellular niches between the embryonic myotome, the adult sarcolemma/basement-membrane interphase, and the regenerating muscle, Notch signaling has evolved to support the context-specific requirements of the muscle cells. In this review, we discuss the diverse ways by which Notch signaling factors and other modifying partners are operating during the lifetime of muscle stem cells to establish an adaptive dynamic network.

Current Thoughts of Notch's Role in Myoblast Regulation and Muscle-Associated Disease

The evolutionarily conserved signaling pathway Notch is unequivocally essential for embryogenesis. Notch’s contribution to the muscle repair process in adult tissue is complex and obscure but necessary. Notch integrates with other signals in a functional antagonist manner to direct myoblast activity and ultimately complete muscle repair. There is profound recent evidence describing plausible mechanisms of Notch in muscle repair. However, the story is not definitive as evidence is slowly emerging that negates Notch’s importance in myoblast proliferation. The purpose of this review article is to examine the prominent evidence and associated mechanisms of Notch’s contribution to the myogenic repair phases. In addition, we discuss the emerging roles of Notch in diseases associated with muscle atrophy. Understanding the mechanisms of Notch’s orchestration is useful for developing therapeutic targets for disease.

Scaffold-free tissue-engineered arterial grafts derived from human skeletal myoblasts

Tissue-engineered vascular grafts (TEVGs) are in urgent demand for both adult and pediatric patients. Although several approaches have utilized vascular smooth muscle cells (SMCs) and endothelial cells as cell sources for TEVGs, these cell sources have a limited proliferative capacity that results in an inability to reconstitute neotissues. Skeletal myoblasts are attractive cell sources as they possess high proliferative capacity, and they are already being tested in clinical trials for patients with ischemic cardiomyopathy. Our previous study demonstrated that periodic hydrostatic pressurization (PHP) promoted fibronectin fibrillogenesis in vascular SMCs, and that PHP-induced extracellular matrix (ECM) arrangements enabled the fabrication of implantable arterial grafts derived from SMCs without using a scaffold material. We assessed the molecular response of human skeletal myoblasts to PHP exposure, and aimed to fabricate arterial grafts from the myoblasts by exposure to PHP. To examine the PHP-response genes, human skeletal myoblasts were subjected to bulk RNA-sequencing after PHP exposure. Gene-set enrichment analysis revealed significant positive correlations between PHP exposure and vascular development-related genes. Real-time polymerase chain reaction (RT-PCR) demonstrated that PHP significantly upregulated collagen and elastic fiber formation-related gene expression, such as fibronectin, lysyl oxidase, collagen type I α1, collagen type IV α1, and tropoelastin. Based on these findings showing the potential role of PHP in vessel formation, we fabricated arterial grafts by repeated cell seeding and exposure to PHP every 24 hours. The resultant 15-layered myoblast grafts had high collagen content, which provided a tensile rupture strength of 899 ± 104 mm Hg. Human skeletal myoblast grafts were implanted as patch grafts in the aorta of immunosuppressed rats and found to be endothelialized and completely patent until the endpoint of 60 postoperative days. Implanted human myoblasts were gradually replaced by host-derived cells, which successfully formed vascular neotissues with layered elastic fibers. These findings suggest that human skeletal myoblasts have the potential to be a feasible cell source for scaffold-free implantable arterial grafts under PHP culture conditions.

FoxO maintains a genuine muscle stem-cell quiescent state until geriatric age

Tissue regeneration declines with ageing but little is known about whether this arises from changes in stem-cell heterogeneity. Here, in homeostatic skeletal muscle, we identify two quiescent stem-cell states distinguished by relative CD34 expression: CD34^High, with stemness properties (genuine state), and CD34^Low, committed to myogenic differentiation (primed state). The genuine-quiescent state is unexpectedly preserved into later life, succumbing only in extreme old age due to the acquisition of primed-state traits. Niche-derived IGF1-dependent Akt activation debilitates the genuine stem-cell state by imposing primed-state features via FoxO inhibition. Interventions to neutralize Akt and promote FoxO activity drive a primed-to-genuine state conversion, whereas FoxO inactivation deteriorates the genuine state at a young age, causing regenerative failure of muscle, as occurs in geriatric mice. These findings reveal transcriptional determinants of stem-cell heterogeneity that resist ageing more than previously anticipated and are only lost in extreme old age, with implications for the repair of geriatric muscle.

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

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

Notch3 in Development, Health and Disease

Notch3 is one of four mammalian Notch proteins, which act as signalling receptors to control cell fate in many developmental and adult tissue contexts. Notch signalling continues to be important in the adult organism for tissue maintenance and renewal and mis-regulation of Notch is involved in many diseases. Genetic studies have shown that Notch3 gene knockouts are viable and have limited developmental defects, focussed mostly on defects in the arterial smooth muscle cell lineage. Additional studies have revealed overlapping roles for Notch3 with other Notch proteins, which widen the range of developmental functions. In the adult, Notch3, in collaboration with other Notch proteins, is involved in stem cell regulation in different tissues in stem cell regulation in different tissues, and it also controls the plasticity of the vascular smooth muscle phenotype involved in arterial vessel remodelling. Overexpression, gene amplification and mis-activation of Notch3 are associated with different cancers, in particular triple negative breast cancer and ovarian cancer. Mutations of Notch3 are associated with a dominantly inherited disease CADASIL (cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy), and there is further evidence linking Notch3 misregulation to hypertensive disease. Here we discuss the distinctive roles of Notch3 in development, health and disease, different views as to the underlying mechanisms of its activation and misregulation in different contexts and potential for therapeutic intervention.

Stem Cell Aging in Skeletal Muscle Regeneration and Disease

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

Regulation of Cell Cycle Entry and Exit: A Single Cell Perspective

The transition between proliferating and quiescent states must be carefully regulated to ensure that cells divide to create the cells an organism needs only at the appropriate time and place. Cyclin-dependent kinases (CDKs) are critical for both transitioning cells from one cell cycle state to the next, and for regulating whether cells are proliferating or quiescent. CDKs are regulated by association with cognate cyclins, activating and inhibitory phosphorylation events, and proteins that bind to them and inhibit their activity. The substrates of these kinases, including the retinoblastoma protein, enforce the changes in cell cycle status. Single cell analysis has clarified that competition among factors that activate and inhibit CDK activity leads to the cell's decision to enter the cell cycle, a decision the cell makes before S phase. Signaling pathways that control the activity of CDKs regulate the transition between quiescence and proliferation in stem cells, including stem cells that generate muscle and neurons. © 2020 American Physiological Society. Compr Physiol 10:317-344, 2020.

A Three-Dimensional Culture Model of Reversibly Quiescent Myogenic Cells

Satellite cells (SC) are the stem cells of skeletal muscles. They are quiescent in adult animals but resume proliferation to allow muscle hypertrophy or regeneration after injury. The mechanisms balancing quiescence, self-renewal, and differentiation of SC are difficult to analyze in vivo owing to their complexity and in vitro because the staminal character of SC is lost when they are removed from the niche and is not adequately reproduced in the culture models currently available. To overcome these difficulties, we set up a culture model of the myogenic C2C12 cell line in suspension. When C2C12 cells are cultured in suspension, they enter a state of quiescence and form three-dimensional aggregates (myospheres) that produce the extracellular matrix and express markers of quiescent SC. In the initial phase of culture, a portion of the cells fuses in syncytia and abandons the myospheres. The remaining cells are mononucleated and quiescent but resume proliferation and differentiation when plated in a monolayer. The notch pathway controls the quiescent state of the cells as shown by the fact that its inhibition leads to the resumption of differentiation. Within this context, notch3 appears to play a central role in the activity of this pathway since the expression of notch1 declines soon after aggregation. In summary, the culture model of C2C12 in suspension may be used to study the cellular interactions of muscle stem cells and the pathways controlling SC quiescence entrance and maintenance.

MyD88 is required for satellite cell-mediated myofiber regeneration in dystrophin-deficient mdx mice

Duchenne muscular dystrophy (DMD), caused by mutations in the dystrophin gene, leads to severe muscle wasting and eventual death of the afflicted individuals, primarily due to respiratory failure. Deficit in myofiber regeneration, potentially due to an exhaustion of satellite cells, is one of the major pathological features of DMD. Myeloid differentiation primary response 88 (MyD88) is an adaptor protein that mediates activation of various inflammatory pathways in response to signaling from Toll-like receptors and interleukin-1 receptor. MyD88 also regulates cellular survival, proliferation and differentiation in a cell-autonomous manner. However, the role of MyD88 in satellite stem cell homeostasis and function in dystrophic muscle remains unknown. In this study, we demonstrate that tamoxifen-inducible deletion of MyD88 in satellite cells causes loss of skeletal muscle mass and strength in the mdx mouse model of DMD. Satellite cell-specific deletion of MyD88 inhibits myofiber regeneration and stimulates fibrogenesis in dystrophic muscle of mdx mice. Deletion of MyD88 also reduces the number of satellite cells and inhibits their fusion with injured myofibers in dystrophic muscle of mdx mice. Ablation of MyD88 in satellite cells increases the markers of M2 macrophages without having any significant effect on M1 macrophages and expression of inflammatory cytokines. Finally, we found that satellite cell-specific deletion of MyD88 leads to aberrant activation of Notch and Wnt signaling in skeletal muscle of mdx mice. Collectively, our results demonstrate that MyD88-mediated signaling in satellite cells is essential for the regeneration of injured myofibers in dystrophic muscle of mdx mice.

Skeletal muscle stem cells in comfort and stress

Investigations on developmental and regenerative myogenesis have led to major advances in decrypting stem cell properties and potential, as well as their interactions within the evolving niche. As a consequence, regenerative myogenesis has provided a forum to investigate intrinsic regulators of stem cell properties as well as extrinsic factors, including stromal cells, during normal growth and following injury and disease. Here we review some of the latest advances in the field that have exposed fundamental processes including regulation of stress following trauma and ageing, senescence, DNA damage control and modes of symmetric and asymmetric cell divisions. Recent studies have begun to explore the nature of the niche that is distinct in different muscle groups, and that is altered from prenatal to postnatal stages, and during ageing. We also discuss heterogeneities among muscle stem cells and how distinct properties within the quiescent and proliferating cell states might impact on homoeostasis and regeneration. Interestingly, cellular quiescence, which was thought to be a passive cell state, is regulated by multiple mechanisms, many of which are deregulated in various contexts including ageing. These and other factors including metabolic activity and genetic background can impact on the efficiency of muscle regeneration.

Mammary gland stem cells and their application in breast cancer

The mammary gland is an organ comprising two primary lineages, specifically the inner luminal and the outer myoepithelial cell layers. Mammary gland stem cells (MaSCs) are highly dynamic and self-renewing, and can give rise to these mammary gland lineages. The lineages are responsible for gland generation during puberty as well as expansion during pregnancy. In recent years, researchers have focused on understanding how MaSCs are regulated during mammary gland development and transformation of breast cancer. Here, we summarize the identification of MaSCs, and how they are regulated by the signaling transduction pathways, mammary gland microenvironment, and non-coding RNAs (ncRNAs). Moreover, we debate the evidence for their serving as the origin of breast cancer, and discuss the therapeutic perspectives of targeting breast cancer stem cells (BCSCs). In conclusion, a better understanding of the key regulators of MaSCs is crucial for the clinical treatment of breast cancer.

Delta-Like 4 Activates Notch 3 to Regulate Self-Renewal in Skeletal Muscle Stem Cells

Notch signaling is essential to maintain skeletal muscle stem cells in quiescence. However, the precise roles of different Notch receptors are incompletely defined. Here, we demonstrate a role for Notch3 (N3) in the self-renewal of muscle stem cells. We found that N3 is active in quiescent C2C12 reserve cells (RCs), and N3 over-expression and knockdown studies in C2C12 and primary satellite cells reveal a role in self-renewal. The Notch ligand Delta-like 4 (Dll4) is expressed by newly formed myotubes and interaction with this ligand is sufficient to maintain N3 activity in quiescent C2C12 RCs to prevent activation and progression into the cell cycle. Thus, our data suggest a model whereby during regeneration, expression of Dll4 by nascent muscle fibers triggers N3 signaling in associated muscle stem cells to recruit them to quiescence, thereby renewing the stem cell pool. Stem Cells 2018;36:458-466.

Pharmacological inhibition of REV-ERB stimulates differentiation, inhibits turnover and reduces fibrosis in dystrophic muscle

Duchenne muscular dystrophy (DMD) is a debilitating X-linked disorder that is fatal. DMD patients lack the expression of the structural protein dystrophin caused by mutations within the DMD gene. The absence of functional dystrophin protein results in excessive damage from normal muscle use due to the compromised structural integrity of the dystrophin associated glycoprotein complex. As a result, DMD patients exhibit ongoing cycles of muscle destruction and regeneration that promote inflammation, fibrosis, mitochondrial dysfunction, satellite cell (SC) exhaustion and loss of skeletal and cardiac muscle function. The nuclear receptor REV-ERB suppresses myoblast differentiation and recently we have demonstrated that the REV-ERB antagonist, SR8278, stimulates muscle regeneration after acute injury. Therefore, we decided to explore whether the REV-ERB antagonist SR8278 could slow the progression of muscular dystrophy. In mdx mice SR8278 increased lean mass and muscle function, and decreased muscle fibrosis and muscle protein degradation. Interestingly, we also found that SR8278 increased the SC pool through stimulation of Notch and Wnt signaling. These results suggest that REV-ERB is a potent target for the treatment of DMD.

Interleukin-1beta (IL-1β)-induced Notch ligand Jagged1 suppresses mitogenic action of IL-1β on human dystrophic myogenic cells

Duchenne muscular dystrophy (DMD) is a severe X-linked recessive muscle disorder caused by mutations in the dystrophin gene. Nonetheless, secondary processes involving perturbation of muscle regeneration probably exacerbate disease progression, resulting in the fatal loss of muscle in DMD patients. A dysfunction of undifferentiated myogenic cells is the most likely cause for the reduction of regenerative capacity of muscle. To clarify molecular mechanisms in perturbation of the regenerative capacity of DMD muscle, we have established several NCAM (CD56)-positive immortalized human dystrophic and non-dystrophic myogenic cell lines from DMD and healthy muscles. A pro-inflammatory cytokine, IL-1β, promoted cell cycle progression of non-dystrophic myogenic cells but not DMD myogenic cells. In contrast, IL-1β upregulated the Notch ligand Jagged1 gene in DMD myogenic cells but not in non-dystrophic myogenic cells. Knockdown of Jagged1 in DMD myogenic cells restored the IL-1β-promoted cell cycle progression. Conversely, enforced expression of Jagged1-blocked IL-1β promoted proliferation of non-dystrophic myogenic cells. In addition, IL-1β prevented myogenic differentiation of DMD myogenic cells depending on Jagged1 but not of non-dystrophic myogenic cells. These results demonstrate that Jagged1 induced by IL-1β in DMD myogenic cells modified the action of IL-1β and reduced the ability to proliferate and differentiate. IL-1β induced Jagged1 gene expression may be a feedback response to excess stimulation with this cytokine because high IL-1β (200-1000 pg/ml) induced Jagged1 gene expression even in non-dystrophic myogenic cells. DMD myogenic cells are likely to acquire the susceptibility of the Jagged1 gene to IL-1β under the microcircumstances in DMD muscles. The present results suggest that Jagged1 induced by IL-1β plays a crucial role in the loss of muscle regeneration capacity of DMD muscles. The IL-1β/Jagged1 pathway may be a new therapeutic target to ameliorate exacerbation of muscular dystrophy in a dystrophin-independent manner.

Notch1 and Notch2 Coordinately Regulate Stem Cell Function in the Quiescent and Activated States of Muscle Satellite Cells

Satellite cells, the muscle tissue stem cells, express three Notch receptors (Notch1-3). The function of Notch1 and Notch2 in satellite cells has to date not been fully evaluated. We investigated the role of Notch1 and Notch2 in myogenic progression in adult skeletal muscle using tamoxifen-inducible satellite cell-specific conditional knockout mice for Notch1 (N1-scKO), Notch2 (N2-scKO), and Notch1/Notch2 (scDKO). In the quiescent state, the number of satellite cells was slightly reduced in N2-scKO, but not significantly in N1-scKO, and almost completely depleted in scDKO mice. N1-scKO and N2-scKO mice both exhibited a defect in muscle regeneration induced by cardiotoxin injection, while muscle regeneration was severely compromised with marked fibrosis in scDKO mice. In the activated state, ablation of either Notch1 or Notch2 alone in satellite cells prevented population expansion and self-renewal but induced premature myogenesis. Therefore, our results indicate that Notch1 and Notch2 coordinately maintain the stem-cell pool in the quiescent state by preventing activation and regulate stem-cell-fate decision in the activated state, governing adult muscle regeneration. Stem Cells 2018;36:278-285.

Area-Specific Regulation of Quiescent Neural Stem Cells by Notch3 in the Adult Mouse Subependymal Zone

In the adult mammalian brain, neural stem cells (NSCs) generate new neurons throughout the mammal's lifetime. The balance between quiescence and active cell division among NSCs is crucial in producing appropriate numbers of neurons while maintaining the stem cell pool for a long period. The Notch signaling pathway plays a central role in both maintaining quiescent NSCs (qNSCs) and promoting cell division of active NSCs (aNSCs), although no one knows how this pathway regulates these apparently opposite functions. Notch1 has been shown to promote proliferation of aNSCs without affecting qNSCs in the adult mouse subependymal zone (SEZ). In this study, we found that Notch3 is expressed to a higher extent in qNSCs than in aNSCs while Notch1 is preferentially expressed in aNSCs and transit-amplifying progenitors in the adult mouse SEZ. Furthermore, Notch3 is selectively expressed in the lateral and ventral walls of the SEZ. Knockdown of Notch3 in the lateral wall of the adult SEZ increased the division of NSCs. Moreover, deletion of the Notch3 gene resulted in significant reduction of qNSCs specifically in the lateral and ventral walls, compared with the medial and dorsal walls, of the lateral ventricles. Notch3 deletion also reduced the number of qNSCs activated after antimitotic cytosine β-D-arabinofuranoside (Ara-C) treatment. Importantly, Notch3 deletion preferentially reduced specific subtypes of newborn neurons in the olfactory bulb derived from the lateral walls of the SEZ. These results indicate that Notch isoforms differentially control the quiescent and proliferative steps of adult SEZ NSCs in a domain-specific manner.SIGNIFICANCE STATEMENT In the adult mammalian brain, the subependymal zone (SEZ) of the lateral ventricles is the largest neurogenic niche, where neural stem cells (NSCs) generate neurons. In this study, we found that Notch3 plays an important role in the maintenance of quiescent NSCs (qNSCs), while Notch1 has been reported to act as a regulator of actively cycling NSCs. Furthermore, we found that Notch3 is specifically expressed in qNSCs located in the lateral and ventral walls of the lateral ventricles and regulates neuronal production of NSCs in a region-specific manner. Our results indicate that Notch3, by maintaining the quiescence of a subpopulation of NSCs, confers a region-specific heterogeneity among NSCs in the adult SEZ.

Effects of omega-3 on matrix metalloproteinase-9, myoblast transplantation and satellite cell activation in dystrophin-deficient muscle fibers

In Duchenne muscular dystrophy (DMD), lack of dystrophin leads to progressive muscle degeneration, with DMD patients suffering from cardiorespiratory failure. Cell therapy is an alternative to life-long corticoid therapy. Satellite cells, the stem cells of skeletal muscles, do not completely compensate for the muscle damage in dystrophic muscles. Elevated levels of proinflammatory and profibrotic factors, such as metalloproteinase 9 (MMP-9), impair muscle regeneration, leading to extensive fibrosis and poor results with myoblast transplantation therapies. Omega-3 is an anti-inflammatory drug that protects against muscle degeneration in the mdx mouse model of DMD. In the present study, we test our hypothesis that omega-3 affects MMP-9 and thereby benefits muscle regeneration and myoblast transplantation in the mdx mouse. We observe that omega-3 reduces MMP-9 gene expression and improves myoblast engraftment, satellite cell activation, and muscle regeneration by mechanisms involving, at least in part, the regulation of macrophages, as shown here with the fluorescence-activated cell sorting technique. The present study demonstrates the benefits of omega-3 on satellite cell survival and muscle regeneration, further supporting its use in clinical trials and cell therapies in DMD.

AMPKα1-LDH pathway regulates muscle stem cell self-renewal by controlling metabolic homeostasis

Control of stem cell fate to either enter terminal differentiation versus returning to quiescence (self-renewal) is crucial for tissue repair. Here, we showed that AMP-activated protein kinase (AMPK), the master metabolic regulator of the cell, controls muscle stem cell (MuSC) self-renewal. AMPKα1^-/- MuSCs displayed a high self-renewal rate, which impairs muscle regeneration. AMPKα1^-/- MuSCs showed a Warburg-like switch of their metabolism to higher glycolysis. We identified lactate dehydrogenase (LDH) as a new functional target of AMPKα1. LDH, which is a non-limiting enzyme of glycolysis in differentiated cells, was tightly regulated in stem cells. In functional experiments, LDH overexpression phenocopied AMPKα1^-/- phenotype, that is shifted MuSC metabolism toward glycolysis triggering their return to quiescence, while inhibition of LDH activity rescued AMPKα1^-/- MuSC self-renewal. Finally, providing specific nutrients (galactose/glucose) to MuSCs directly controlled their fate through the AMPKα1/LDH pathway, emphasizing the importance of metabolism in stem cell fate.

Keep Your Friends Close: Cell-Cell Contact and Skeletal Myogenesis

Development of skeletal muscle is a multistage process that includes lineage commitment of multipotent progenitor cells, differentiation and fusion of myoblasts into multinucleated myofibers, and maturation of myofibers into distinct types. Lineage-specific transcriptional regulation lies at the core of this process, but myogenesis is also regulated by extracellular cues. Some of these cues are initiated by direct cell-cell contact between muscle precursor cells themselves or between muscle precursors and cells of other lineages. Examples of the latter include interaction of migrating neural crest cells with multipotent muscle progenitor cells, muscle interstitial cells with myoblasts, and neurons with myofibers. Among the signaling factors involved are Notch ligands and receptors, cadherins, Ig superfamily members, and Ephrins and Eph receptors. In this article we describe recent progress in this area and highlight open questions raised by the findings.

Canonical Wnt signalling regulates nuclear export of Setdb1 during skeletal muscle terminal differentiation

The histone 3 lysine 9 methyltransferase Setdb1 is essential for both stem cell pluripotency and terminal differentiation of different cell types. To shed light on the roles of Setdb1 in these mutually exclusive processes, we used mouse skeletal myoblasts as a model of terminal differentiation. Ex vivo studies on isolated single myofibres showed that Setdb1 is required for adult muscle stem cells expansion following activation. In vitro studies in skeletal myoblasts confirmed that Setdb1 suppresses terminal differentiation. Genomic binding analyses showed a release of Setdb1 from selected target genes upon myoblast terminal differentiation, concomitant to a nuclear export of Setdb1 to the cytoplasm. Both genomic release and cytoplasmic Setdb1 relocalisation during differentiation were dependent on canonical Wnt signalling. Transcriptomic assays in myoblasts unravelled a significant overlap between Setdb1 and Wnt3a regulated genetic programmes. Together, our findings revealed Wnt-dependent subcellular relocalisation of Setdb1 as a novel mechanism regulating Setdb1 functions and myogenesis.

Karyopherin Alpha 1 Regulates Satellite Cell Proliferation and Survival by Modulating Nuclear Import

Satellite cells are stem cells with an essential role in skeletal muscle repair. Precise regulation of gene expression is critical for proper satellite cell quiescence, proliferation, differentiation and self-renewal. Nuclear proteins required for gene expression are dependent on the nucleocytoplasmic transport machinery to access to nucleus, however little is known about regulation of nuclear transport in satellite cells. The best characterized nuclear import pathway is classical nuclear import which depends on a classical nuclear localization signal (cNLS) in a cargo protein and the heterodimeric import receptors, karyopherin alpha (KPNA) and beta (KPNB). Multiple KPNA1 paralogs exist and can differ in importing specific cNLS proteins required for cell differentiation and function. We show that transcripts for six Kpna paralogs underwent distinct changes in mouse satellite cells during muscle regeneration accompanied by changes in cNLS proteins in nuclei. Depletion of KPNA1, the most dramatically altered KPNA, caused satellite cells in uninjured muscle to prematurely activate, proliferate and undergo apoptosis leading to satellite cell exhaustion with age. Increased proliferation of satellite cells led to enhanced muscle regeneration at early stages of regeneration. In addition, we observed impaired nuclear localization of two key KPNA1 cargo proteins: p27, a cyclin-dependent kinase inhibitor associated with cell cycle control and lymphoid enhancer factor 1, a critical cotranscription factor for β-catenin. These results indicate that regulated nuclear import of proteins by KPNA1 is critical for satellite cell proliferation and survival and establish classical nuclear import as a novel regulatory mechanism for controlling satellite cell fate. Stem Cells 2016;34:2784-2797.

Notch Signaling Mediates Skeletal Muscle Atrophy in Cancer Cachexia Caused by Osteosarcoma

Skeletal muscle atrophy in cancer cachexia is mediated by the interaction between muscle stem cells and various tumor factors. Although Notch signaling has been known as a key regulator of both cancer development and muscle stem cell activity, the potential involvement of Notch signaling in cancer cachexia and concomitant muscle atrophy has yet to be elucidated. The murine K7M2 osteosarcoma cell line was used to generate an orthotopic model of sarcoma-associated cachexia, and the role of Notch signaling was evaluated. Skeletal muscle atrophy was observed in the sarcoma-bearing mice, and Notch signaling was highly active in both tumor tissues and the atrophic skeletal muscles. Systemic inhibition of Notch signaling reduced muscle atrophy. In vitro coculture of osteosarcoma cells with muscle-derived stem cells (MDSCs) isolated from normal mice resulted in decreased myogenic potential of MDSCs, while the application of Notch inhibitor was able to rescue this repressed myogenic potential. We further observed that Notch-activating factors reside in the exosomes of osteosarcoma cells, which activate Notch signaling in MDSCs and subsequently repress myogenesis. Our results revealed that signaling between tumor and muscle via the Notch pathway may play an important role in mediating the skeletal muscle atrophy seen in cancer cachexia.

KLF7 Regulates Satellite Cell Quiescence in Response to Extracellular Signaling

Retaining muscle stem satellite cell (SC) quiescence is important for the maintenance of stem cell population and tissue regeneration. Accumulating evidence supports the model where key extracellular signals play crucial roles in maintaining SC quiescence or activation, however, the intracellular mechanisms that mediate niche signals to control SC behavior are not fully understood. Here, we reported that KLF7 functioned as a key mediator involved in low-level TGF-β signaling and canonical Notch signaling-induced SC quiescence and myoblast arrest. The data obtained showed that KLF7 was upregulated in quiescent SCs and nonproliferating myoblasts. Silence of KLF7 promoted SCs activation and myoblasts proliferation, but overexpression of KLF7 induced myogenic cell arrest. Notably, the expression of KLF7 was regulated by TGF-β and Notch3 signaling. Knockdown of KLF7 diminished low-level TGF-β and canonical Notch signaling-induced SC quiescence. Investigation into the mechanism revealed that KLF7 regulation of SC function was dependent on p21 and acetylation of Lys227 and/or 231 in the DNA binding domain of KLF7. Our study provides new insights into the regulatory network of muscle stem cell quiescence. Stem Cells 2016;34:1310-1320.

Regulation of Notch-mediated transcription by a bovine herpesvirus 1 encoded protein (ORF2) that is expressed in latently infected sensory neurons

Bovine herpesvirus 1 (BoHV-1) is an Alphaherpesvirinae subfamily member that establishes life-long latency in sensory neurons. The latency-related RNA (LR-RNA) is abundantly expressed during latency. An LR mutant virus containing stop codons at the amino-terminus of open reading frame (ORF)2 does not reactivate from latency and replicates less efficiently in tonsils and trigeminal ganglia. ORF2 inhibits apoptosis, interacts with Notch family members, and interferes with Notch-dependent transcription suggesting ORF2 expression enhances survival of infected neurons. The Notch signaling pathway is crucial for neuronal differentiation and survival suggesting that interactions between ORF2 and Notch family members regulate certain aspects of latency. Consequently, for this study, we compared whether ORF2 interfered with the four mammalian Notch family members. ORF2 consistently interfered with Notch1-3-mediated transactivation of three cellular promoters. Conversely, Notch4-mediated transcription was not consistently inhibited by ORF2. Electrophoretic shift mobility assays using four copies of a consensus-DNA binding site for Notch/CSL (core binding factor (CBF)-1, Suppressor of Hairless, Lag-2) as a probe revealed ORF2 interfered with Notch1 and 3 interactions with a CSL family member bound to DNA. Additional studies demonstrated ORF2 enhances neurite sprouting in mouse neuroblastoma cells that express Notch1-3, but not Notch4. Collectively, these studies indicate that ORF2 inhibits Notch-mediated transcription and signaling by interfering with Notch interacting with CSL bound to DNA.

Comparative Study of Injury Models for Studying Muscle Regeneration in Mice

Background

A longstanding goal in regenerative medicine is to reconstitute functional tissus or organs after injury or disease. Attention has focused on the identification and relative contribution of tissue specific stem cells to the regeneration process. Relatively little is known about how the physiological process is regulated by other tissue constituents. Numerous injury models are used to investigate tissue regeneration, however, these models are often poorly understood. Specifically, for skeletal muscle regeneration several models are reported in the literature, yet the relative impact on muscle physiology and the distinct cells types have not been extensively characterised.

Methods

We have used transgenic Tg:Pax7nGFP and Flk1^GFP/+ mouse models to respectively count the number of muscle stem (satellite) cells (SC) and number/shape of vessels by confocal microscopy. We performed histological and immunostainings to assess the differences in the key regeneration steps. Infiltration of immune cells, chemokines and cytokines production was assessed in vivo by Luminex^®.

Results

We compared the 4 most commonly used injury models i.e. freeze injury (FI), barium chloride (BaCl₂), notexin (NTX) and cardiotoxin (CTX). The FI was the most damaging. In this model, up to 96% of the SCs are destroyed with their surrounding environment (basal lamina and vasculature) leaving a “dead zone” devoid of viable cells. The regeneration process itself is fulfilled in all 4 models with virtually no fibrosis 28 days post-injury, except in the FI model. Inflammatory cells return to basal levels in the CTX, BaCl₂ but still significantly high 1-month post-injury in the FI and NTX models. Interestingly the number of SC returned to normal only in the FI, 1-month post-injury, with SCs that are still cycling up to 3-months after the induction of the injury in the other models.

Conclusions

Our studies show that the nature of the injury model should be chosen carefully depending on the experimental design and desired outcome. Although in all models the muscle regenerates completely, the trajectories of the regenerative process vary considerably. Furthermore, we show that histological parameters are not wholly sufficient to declare that regeneration is complete as molecular alterations (e.g. cycling SCs, cytokines) could have a major persistent impact.

Notch signaling pathway activation in normal and hyperglycemic rats differs in the stem cells of visceral and subcutaneous adipose tissue

The precise mechanisms underlying the differential function and cardiometabolic risk of white adipose tissue (WAT) remain unclear. Visceral adipose tissue (VWAT) and subcutaneous adipose tissue (SCWAT) have different metabolic functions that seem to be ascribed to their different intrinsic expansion capacities. Here we have hypothesized that the WAT characteristics are determined by the resident adipose-derived stem cells (ASCs) found in the different WAT depots. Therefore, our objective has been to investigate adipogenesis in anatomically distinct fat depots. ASCs from five different WAT depots were characterized in both healthy lean and diabetic obese rats, showing significant differences in expression of some of genes governing the stemness and the earlier adipogenic differentiation steps. Notch-target genes [Hes (hairy and enhancer of split) and Hey (hairy/enhancer of split related with YRPW motif) families] were upregulated in ASCs derived from visceral depots. Upon adipogenic differentiation, adipocyte cell markers were downregulated in ASCs from VWAT in comparison to ASCs from SCWAT, revealing a lower adipogenic capacity in ASCs of visceral origin than in those of SCWAT in accordance with the differential activation of Notch signaling. Notch upregulation by its activator phenethyl isothiocyanate attenuated the adipogenic differentiation of ASCs from SCWAT whereas Notch inhibition by N-[N-(3,5-difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT) increased the adipogenic differentiation of ASCs from visceral origin. In conclusion, the differential activation of Notch in ASCs is the origin of the different intrinsic WAT expansion capacities that contribute to the regional variations in WAT homeostasis and to its associated cardiometabolic risk.

Changes in Communication between Muscle Stem Cells and their Environment with Aging

Aging is associated with both muscle weakness and a loss of muscle mass, contributing towards overall frailty in the elderly. Aging skeletal muscle is also characterised by a decreasing efficiency in repair and regeneration, together with a decline in the number of adult stem cells. Commensurate with this are general changes in whole body endocrine signalling, in local muscle secretory environment, as well as in intrinsic properties of the stem cells themselves. The present review discusses the various mechanisms that may be implicated in these age-associated changes, focusing on aspects of cell-cell communication and long-distance signalling factors, such as levels of circulating growth hormone, IL-6, IGF1, sex hormones, and inflammatory cytokines. Changes in the local environment are also discussed, implicating IL-6, IL-4, FGF-2, as well as other myokines, and processes that lead to thickening of the extra-cellular matrix. These factors, involved primarily in communication, can also modulate the intrinsic properties of muscle stem cells, including reduced DNA accessibility and repression of specific genes by methylation. Finally we discuss the decrease in the stem cell pool, particularly the failure of elderly myoblasts to re-quiesce after activation, and the consequences of all these changes on general muscle homeostasis.

Coaxing stem cells for skeletal muscle repair

Skeletal muscle has a tremendous ability to regenerate, attributed to a well-defined population of muscle stem cells called satellite cells. However, this ability to regenerate diminishes with age and can also be dramatically affected by multiple types of muscle diseases, or injury. Extrinsic and/or intrinsic defects in the regulation of satellite cells are considered to be major determinants for the diminished regenerative capacity. Maintenance and replenishment of the satellite cell pool is one focus for muscle regenerative medicine, which will be discussed. There are other sources of progenitor cells with myogenic capacity, which may also support skeletal muscle repair. However, all of these myogenic cell populations have inherent difficulties and challenges in maintaining or coaxing their derivation for therapeutic purpose. This review will highlight recent reported attributes of these cells and new bioengineering approaches to creating a supply of myogenic stem cells or implants applicable for acute and/or chronic muscle disorders.

The role of Notch signaling in muscle progenitor cell depletion and the rapid onset of histopathology in muscular dystrophy

Although it has been speculated that stem cell depletion plays a role in the rapid progression of the muscle histopathology associated with Duchenne Muscular Dystrophy (DMD), the molecular and cellular mechanisms responsible for stem cell depletion remain poorly understood. The rapid depletion of muscle stem cells has not been observed in the dystrophin-deficient model of DMD (mdx mouse), which may explain the relatively mild dystrophic phenotype observed in this animal model. In contrast, we have observed a rapid occurrence of stem cell depletion in the dystrophin/utrophin double knockout (dKO) mouse model, which exhibits histopathological features that more closely recapitulate the phenotype observed in DMD patients compared with the mdx mouse. Notch signaling has been found to be a key regulator of stem cell self-renewal and myogenesis in normal skeletal muscle; however, little is known about the role that Notch plays in the development of the dystrophic histopathology associated with DMD. Our results revealed an over-activation of Notch in the skeletal muscles of dKO mice, which correlated with sustained inflammation, impaired muscle regeneration and the rapid depletion and senescence of the muscle progenitor cells (MPCs, i.e. Pax7+ cells). Consequently, the repression of Notch in the skeletal muscle of dKO mice delayed/reduced the depletion and senescence of MPCs, and restored the myogenesis capacity while reducing inflammation and fibrosis. We suggest that the down-regulation of Notch could represent a viable approach to reduce the dystrophic histopathologies associated with DMD.

Barx2 and Pax7 have antagonistic functions in regulation of wnt signaling and satellite cell differentiation

The canonical Wnt signaling pathway is critical for myogenesis and can induce muscle progenitors to switch from proliferation to differentiation; how Wnt signals integrate with muscle-specific regulatory factors in this process is poorly understood. We previously demonstrated that the Barx2 homeobox protein promotes differentiation in cooperation with the muscle regulatory factor (MRF) MyoD. Pax7, another important muscle homeobox factor, represses differentiation. We now identify Barx2, MyoD, and Pax7 as novel components of the Wnt effector complex, providing a new molecular pathway for regulation of muscle progenitor differentiation. Canonical Wnt signaling induces Barx2 expression in muscle progenitors and perturbation of Barx2 leads to misregulation of Wnt target genes. Barx2 activates two endogenous Wnt target promoters as well as the Wnt reporter gene TOPflash, the latter synergistically with MyoD. Moreover, Barx2 interacts with the core Wnt effectors β-catenin and T cell-factor 4 (TCF4), is recruited to TCF/lymphoid enhancer factor sites, and promotes recruitment of β-catenin. In contrast, Pax7 represses the Wnt reporter gene and antagonizes the activating effect of Barx2. Pax7 also binds β-catenin suggesting that Barx2 and Pax7 may compete for interaction with the core Wnt effector complex. Overall, the data show for the first time that Barx2, Pax7, and MRFs can act as direct transcriptional effectors of Wnt signals in myoblasts and that Barx2 and Wnt signaling participate in a regulatory loop. We propose that antagonism between Barx2 and Pax7 in regulation of Wnt signaling may help mediate the switch from myoblast proliferation to differentiation.

Adult skeletal muscle stem cells

Skeletal muscles in vertebrates have a phenomenal regenerative capacity. A muscle that has been crushed can regenerate fully both structurally and functionally within a month. Remarkably, efficient regeneration continues to occur following repeated injuries. Thousands of muscle precursor cells are needed to accomplish regeneration following acute injury. The differentiated muscle cells, the multinucleated contractile myofibers, are terminally withdrawn from mitosis. The source of the regenerative precursors is the skeletal muscle stem cells-the mononucleated cells closely associated with myofibers, which are known as satellite cells. Satellite cells are mitotically quiescent or slow-cycling, committed to myogenesis, but undifferentiated. Disruption of the niche after muscle damage results in their exit from quiescence and progression towards commitment. They eventually arrest proliferation, differentiate, and fuse to damaged myofibers or make de novo myofibers. Satellite cells are one of the well-studied adult tissue-specific stem cells and have served as an excellent model for investigating adult stem cells. They have also emerged as an important standard in the field of ageing and stem cells. Several recent reviews have highlighted the importance of these cells as a model to understand stem cell biology. This chapter begins with the discovery of satellite cells as skeletal muscle stem cells and their developmental origin. We discuss transcription factors and signalling cues governing stem cell function of satellite cells and heterogeneity in the satellite cell pool. Apart from satellite cells, a number of other stem cells have been shown to make muscle and are being considered as candidate stem cells for amelioration of muscle degenerative diseases. We discuss these "offbeat" muscle stem cells and their status as adult skeletal muscle stem cells vis-a-vis satellite cells. The ageing context is highlighted in the concluding section.

Regulation of a Notch3-Hes1 pathway and protective effect by a tocopherol-omega alkanol chain derivative in muscle atrophy

Muscular atrophy, a physiopathologic process associated with severe human diseases such as amyotrophic lateral sclerosis (ALS) or cancer, has been linked to reactive oxygen species (ROS) production. The Notch pathway plays a role in muscle development and in muscle regeneration upon physical injury. In this study, we explored the possibility that the Notch pathway participates in the ROS-related muscular atrophy occurring in cancer-associated cachexia and ALS. We also tested whether hybrid compounds of tocopherol, harboring antioxidant activity, and the omega-alkanol chain, presenting cytoprotective activity, might reduce muscle atrophy and impact the Notch pathway. We identified one tocopherol-omega alkanol chain derivative, AGT251, protecting myoblastic cells against known cytotoxic agents. We showed that this compound presenting antioxidant activity counteracts the induction of the Notch pathway by cytotoxic stress, leading to a decrease of Notch1 and Notch3 expression. At the functional level, these regulations correlated with a repression of the Notch target gene Hes1 and the atrophy/remodeling gene MuRF1. Importantly, we also observed an induction of Notch3 and Hes1 expression in two murine models of muscle atrophy: a doxorubicin-induced cachexia model and an ALS murine model expressing mutated superoxide dismutase 1. In both models, the induction of Notch3 and Hes1 were partially opposed by AGT251, which correlated with ameliorations in body and muscle weight, reduction of muscular atrophy markers, and improved survival. Altogether, we identified a compound of the tocopherol family that protects against muscle atrophy in various models, possibly through the regulation of the Notch pathway.

Antagonistic regulation of p57kip2 by Hes/Hey downstream of Notch signaling and muscle regulatory factors regulates skeletal muscle growth arrest

A central question in development is to define how the equilibrium between cell proliferation and differentiation is temporally and spatially regulated during tissue formation. Here, we address how interactions between cyclin-dependent kinase inhibitors essential for myogenic growth arrest (p21(cip1) and p57(kip2)), the Notch pathway and myogenic regulatory factors (MRFs) orchestrate the proliferation, specification and differentiation of muscle progenitor cells. We first show that cell cycle exit and myogenic differentiation can be uncoupled. In addition, we establish that skeletal muscle progenitor cells require Notch signaling to maintain their cycling status. Using several mouse models combined with ex vivo studies, we demonstrate that Notch signaling is required to repress p21(cip1) and p57(kip2) expression in muscle progenitor cells. Finally, we identify a muscle-specific regulatory element of p57(kip2) directly activated by MRFs in myoblasts but repressed by the Notch targets Hes1/Hey1 in progenitor cells. We propose a molecular mechanism whereby information provided by Hes/Hey downstream of Notch as well as MRF activities are integrated at the level of the p57(kip2) enhancer to regulate the decision between progenitor cell maintenance and muscle differentiation.