Regulation of muscle hypertrophy through granulin: Relayed communication among mesenchymal progenitors, macrophages, and satellite cells

Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.

Flow Cytometer Analyses, Isolation, and Staining of Murine Muscle Satellite Cells

Fluorescence-activated cell sorting (FACS) is a powerful and requisite tool for the analysis and purification of adult stem cells. However, it is difficult to separate adult stem cells from solid organs than from immune-related tissues/organs. This is because of the presence of large amounts of debris, which increases noise in the FACS profiles. In particular, it is extremely difficult for unfamiliar researchers to identify muscle stem cell (also known as muscle satellite cell: MuSC) fraction because all myofibers, which are mainly composed of skeletal muscle tissues, become debris during cell preparation. This chapter describes our FACS protocol, which we have used for more than a decade, to identify and purify MuSCs.

Differences in muscle satellite cell dynamics during muscle hypertrophy and regeneration

Skeletal muscle homeostasis and function are ensured by orchestrated cellular interactions among several types of cells. A noticeable aspect of skeletal muscle biology is the drastic cell–cell communication changes that occur in multiple scenarios. The process of recovering from an injury, which is known as regeneration, has been relatively well investigated. However, the cellular interplay that occurs in response to mechanical loading, such as during resistance training, is poorly understood compared to regeneration. During muscle regeneration, muscle satellite cells (MuSCs) rebuild multinuclear myofibers through a stepwise process of proliferation, differentiation, fusion, and maturation, whereas during mechanical loading-dependent muscle hypertrophy, MuSCs do not undergo such stepwise processes (except in rare injuries) because the nuclei of MuSCs become directly incorporated into the mature myonuclei. In this review, six specific examples of such differences in MuSC dynamics between regeneration and hypertrophy processes are discussed.

Optineurin promotes myogenesis during muscle regeneration in mice by autophagic degradation of GSK3β

Skeletal muscle regeneration is essential for maintaining muscle function in injury and muscular disease. Myogenesis plays key roles in forming new myofibers during the process. Here, through bioinformatic screen for the potential regulators of myogenesis from 5 independent microarray datasets, we identify an overlapping differentially expressed gene (DEG) optineurin (OPTN). Optn knockdown (KD) delays muscle regeneration in mice and impairs C2C12 myoblast differentiation without affecting their proliferation. Conversely, Optn overexpression (OE) promotes myoblast differentiation. Mechanistically, OPTN increases nuclear levels of β-catenin and enhances the T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription activity, suggesting activation of Wnt signaling pathway. The activation is accompanied by decreased protein levels of glycogen synthase kinase 3β (GSK3β), a negative regulator of the pathway. We further show that OPTN physically interacts with and targets GSK3β for autophagic degradation. Pharmacological inhibition of GSK3β rescues the impaired myogenesis induced by Optn KD during muscle regeneration and myoblast differentiation, corroborating that GSK3β is the downstream effector of OPTN-mediated myogenesis. Together, our study delineates the novel role of OPTN as a potential regulator of myogenesis and may open innovative therapeutic perspectives for muscle regeneration.

Uhrf1 governs the proliferation and differentiation of muscle satellite cells

DNA methylation is an essential form of epigenetic regulation responsible for cellular identity. In muscle stem cells, termed satellite cells, DNA methylation patterns are tightly regulated during differentiation. However, it is unclear how these DNA methylation patterns affect the function of satellite cells. We demonstrate that a key epigenetic regulator, ubiquitin like with PHD and RING finger domains 1 (Uhrf1), is activated in proliferating myogenic cells but not expressed in quiescent satellite cells or differentiated myogenic cells in mice. Ablation of Uhrf1 in mouse satellite cells impairs their proliferation and differentiation, leading to failed muscle regeneration. Uhrf1-deficient myogenic cells exhibited aberrant upregulation of transcripts, including Sox9, with the reduction of DNA methylation level of their promoter and enhancer region. These findings show that Uhrf1 is a critical epigenetic regulator of proliferation and differentiation in satellite cells, by controlling cell-type-specific gene expression via maintenance of DNA methylation.

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Uhrf1 is activated in proliferating myogenic cells

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Uhrf1 in satellite cells is required for muscle regeneration

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Ablation of Uhrf1 in satellite cells impairs their proliferation and differentiation

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Uhrf1 controls cell-type-specific transcripts via maintenance of DNA methylation

Dlk1 regulates quiescence in calcitonin receptor-mutant muscle stem cells

Muscle stem cells, also called muscle satellite cells (MuSCs), are responsible for skeletal muscle regeneration and are sustained in an undifferentiated and quiescent state under steady conditions. The calcitonin receptor (CalcR)-protein kinase A (PKA)-Yes-associated protein 1 (Yap1) axis is one pathway that maintains quiescence in MuSCs. Although CalcR signaling in MuSCs has been identified, the critical CalcR signaling targets are incompletely understood. Here, we show the relevance between the ectopic expression of delta-like non-canonical Notch ligand 1 (Dlk1) and the impaired quiescent state in CalcR-conditional knockout (cKO) MuSCs. Dlk1 expression was rarely detected in both quiescent and proliferating MuSCs in control mice, whereas Dlk1 expression was remarkably increased in CalcR-cKO MuSCs at both the mRNA and protein levels. It is noteworthy that all Ki67+ non-quiescent CalcR-cKO MuSCs express Dlk1, and non-quiescent CalcR-cKO MuSCs are enriched in the Dlk1+ fraction by cell sorting. Using mutant mice, we demonstrated that PKA-activation or Yap1-depletion suppressed Dlk1 expression in CalcR-cKO MuSCs, which suggests that the CalcR-PKA-Yap1 axis inhibits the expression of Dlk1 in quiescent MuSCs. Moreover, the loss of Dlk1 rescued the quiescent state in CalcR-cKO MuSCs, which indicates that the ectopic expression of Dlk1 disturbs quiescence in CalcR-cKO. Collectively, our results suggest that ectopically expressed Dlk1 is responsible for the impaired quiescence in CalcR-cKO MuSCs.

Intravital imaging reveals cell cycle-dependent myogenic cell migration during muscle regeneration

During muscle regeneration, extracellular signal-regulated kinase (ERK) promotes both proliferation and migration. However, the relationship between proliferation and migration is poorly understood in this context. To elucidate this complex relationship on a physiological level, we established an intravital imaging system for measuring ERK activity, migration speed, and cell-cycle phases in mouse muscle satellite cell-derived myogenic cells. We found that in vivo, ERK is maximally activated in myogenic cells two days after injury, and this is then followed by increases in cell number and motility. With limited effects of ERK activity on migration on an acute timescale, we hypothesized that ERK increases migration speed in the later phase by promoting cell-cycle progression. Our cell-cycle analysis further revealed that in myogenic cells, ERK activity is critical for G1/S transition, and cells migrate more rapidly in S/G2 phase 3 days after injury. Finally, migration speed of myogenic cells was suppressed after CDK1/2-but not CDK1-inhibitor treatment, demonstrating a critical role of CDK2 in myogenic cell migration. Overall, our study demonstrates that in myogenic cells, the ERK-CDK2 axis promotes not only G1/S transition but also migration, thus providing a novel mechanism for efficient muscle regeneration.

Role of damage and management in muscle hypertrophy: Different behaviors of muscle stem cells in regeneration and hypertrophy

Skeletal muscle is a dynamic tissue with two unique abilities; one is its excellent regenerative ability, due to the activity of skeletal muscle-resident stem cells named muscle satellite cells (MuSCs); and the other is the adaptation of myofiber size in response to external stimulation, intrinsic factors, or physical activity, which is known as plasticity. Low physical activity and some disease conditions lead to the reduction of myofiber size, called atrophy, whereas hypertrophy refers to the increase in myofiber size induced by high physical activity or anabolic hormones/drugs. MuSCs are essential for generating new myofibers during regeneration and the increase in new myonuclei during hypertrophy; however, there has been little investigation of the molecular mechanisms underlying MuSC activation, proliferation, and differentiation during hypertrophy compared to those of regeneration. One reason is that 'degenerative damage' to myofibers during muscle injury or upon hypertrophy (especially overloaded muscle) is believed to trigger similar activation/proliferation of MuSCs. However, evidence suggests that degenerative damage of myofibers is not necessary for MuSC activation/proliferation during hypertrophy. When considering MuSC-based therapy for atrophy, including sarcopenia, it will be indispensable to elucidate MuSC behaviors in muscles that exhibit non-degenerative damage, because degenerated myofibers are not present in the atrophied muscles. In this review, we summarize recent findings concerning the relationship between MuSCs and hypertrophy, and discuss what remains to be discovered to inform the development and application of relevant treatments for muscle atrophy.

Correction of Glycogen Synthase Kinase 3β in Myotonic Dystrophy 1 Reduces the Mutant RNA and Improves Postnatal Survival of DMSXL Mice

Myotonic dystrophy type 1 (DM1) is a multisystem neuromuscular disease without cure. One of the possible therapeutic approaches for DM1 is correction of the RNA-binding proteins CUGBP1 and MBNL1, misregulated in DM1. CUGBP1 activity is controlled by glycogen synthase kinase 3β (GSK3β), which is elevated in skeletal muscle of patients with DM1, and inhibitors of GSK3 were suggested as therapeutic molecules to correct CUGBP1 activity in DM1. Here, we describe that correction of GSK3β with a small-molecule inhibitor of GSK3, tideglusib (TG), not only normalizes the GSK3β-CUGBP1 pathway but also reduces the mutant DMPK mRNA in myoblasts from patients with adult DM1 and congenital DM1 (CDM1). Correction of GSK3β in a mouse model of DM1 (HSA^LR mice) with TG also reduces the levels of CUG-containing RNA, normalizing a number of CUGBP1- and MBNL1-regulated mRNA targets. We also found that the GSK3β-CUGBP1 pathway is abnormal in skeletal muscle and brain of DMSXL mice, expressing more than 1,000 CUG repeats, and that the correction of this pathway with TG increases postnatal survival and improves growth and neuromotor activity of DMSXL mice. These findings show that the inhibitors of GSK3, such as TG, may correct pathology in DM1 and CDM1 via several pathways.

Orienting Muscle Stem Cells for Regeneration in Homeostasis, Aging, and Disease

Muscle stem cells, or satellite cells, are required for skeletal muscle maintenance, growth, and repair. Following satellite cell activation, several factors drive asymmetric cell division to generate a stem cell and a proliferative progenitor that forms new muscle. The balance between symmetric self-renewal and asymmetric division significantly impacts the efficiency of regeneration. In this Review, we discuss the relationship of satellite cell heterogeneity and the establishment of polarity to asymmetric division, as well as how these processes are impacted in homeostasis, aging, and disease. We also highlight therapeutic opportunities for targeting satellite cell polarity and self-renewal to stimulate muscle regeneration.

Sustained expression of HeyL is critical for the proliferation of muscle stem cells in overloaded muscle

In overloaded and regenerating muscle, the generation of new myonuclei depends on muscle satellite cells (MuSCs). Because MuSC behaviors in these two environments have not been considered separately, MuSC behaviors in overloaded muscle remain unexamined. Here, we show that most MuSCs in overloaded muscle, unlike MuSCs in regenerating muscle, proliferate in the absence of MyoD expression. Mechanistically, MuSCs in overloaded muscle sustain the expression of Heyl, a Notch effector gene, to suppress MyoD expression, which allows effective MuSC proliferation on myofibers and beneath the basal lamina. Although Heyl-knockout mice show no impairment in an injury model, in a hypertrophy model, their muscles harbor fewer new MuSC-derived myonuclei due to increased MyoD expression and diminished proliferation, which ultimately causes blunted hypertrophy. Our results show that sustained HeyL expression is critical for MuSC proliferation specifically in overloaded muscle, and thus indicate that the MuSC-proliferation mechanism differs in overloaded and regenerating muscle.

A novel long non-coding RNA Myolinc regulates myogenesis through TDP-43 and Filip1

Myogenesis is a complex process required for skeletal muscle formation during embryonic development and for regeneration and growth of myofibers in adults. Accumulating evidence suggests that long non-coding RNAs (lncRNAs) play key roles in regulating cell fate decision and function in various tissues. However, the role of lncRNAs in the regulation of myogenesis remains poorly understood. In this study, we identified a novel muscle-enriched lncRNA called 'Myolinc (AK142388)', which we functionally characterized in the C2C12 myoblast cell line. Myolinc is predominately localized in the nucleus, and its levels increase upon induction of the differentiation. Knockdown of Myolinc impairs the expression of myogenic regulatory factors and formation of multi-nucleated myotubes in cultured myoblasts. Myolinc also regulates the expression of Filip1 in a cis-manner. Similar to Myolinc, knockdown of Filip1 inhibits myogenic differentiation. Furthermore, Myolinc binds to TAR DNA-binding protein 43 (TDP-43), a DNA/RNA-binding protein that regulates the expression of muscle genes (e.g. Acta1 and MyoD). Knockdown of TDP-43 inhibits myogenic differentiation. We also show that Myolinc-TDP-43 interaction is essential for the binding of TDP-43 to the promoter regions of muscle marker genes. Finally, we show that silencing of Myolinc inhibits skeletal muscle regeneration in adult mice. Altogether, our study identifies a novel lncRNA that controls key regulatory networks of myogenesis.

The FRiND Model: A Mathematical Model for Representing Macrophage Plasticity in Muscular Dystrophy Pathogenesis

Muscular dystrophy describes generalized progressive muscular weakness due to the wasting of muscle fibers. The progression of the disease is affected by known immunological and mechanical factors, and possibly other unknown mechanisms. This article introduces a new mathematical model, the FRiND model, to further elucidate these known immunological actions. We will perform stability and sensitivity analyses on this model. The models time course results will be verified by biological studies in the literature. This model could be the foundation for further understanding of immunological muscle repair.

Expression and Functional Analyses of Dlk1 in Muscle Stem Cells and Mesenchymal Progenitors during Muscle Regeneration

Delta like non-canonical Notch ligand 1 (Dlk1) is a paternally expressed gene which is also known as preadipocyte factor 1 (Pref-1). The accumulation of adipocytes and expression of Dlk1 in regenerating muscle suggests a correlation between fat accumulation and Dlk1 expression in the muscle. Additionally, mice overexpressing Dlk1 show increased muscle weight, while Dlk1-null mice exhibit decreased body weight and muscle mass, indicating that Dlk1 is a critical factor in regulating skeletal muscle mass during development. The muscle regeneration process shares some features with muscle development. However, the role of Dlk1 in regeneration processes remains controversial. Here, we show that mesenchymal progenitors also known as adipocyte progenitors exclusively express Dlk1 during muscle regeneration. Eliminating developmental effects, we used conditional depletion models to examine the specific roles of Dlk1 in muscle stem cells or mesenchymal progenitors. Unexpectedly, deletion of Dlk1 in neither the muscle stem cells nor the mesenchymal progenitors affected the regenerative ability of skeletal muscle. In addition, fat accumulation was not increased by the loss of Dlk1. Collectively, Dlk1 plays essential roles in muscle development, but does not greatly impact regeneration processes and adipogenic differentiation in adult skeletal muscle regeneration.

Implication of basal lamina dependency in survival of Nrf2-null muscle stem cells via an antioxidative-independent mechanism

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a master regulator for the induction of antioxidative genes and plays roles in diverse cellular functions. The roles of Nrf2 in muscle regeneration have been investigated, and both important and unimportant roles of Nrf2 for muscle regeneration have been reported. Here, using aged Nrf2-null and Nrf2-dystrophic double-null mice, we showed nonsignificant phenotypes in the muscle regeneration ability of Nrf2-null mice. In contrast with these results, strikingly, almost all Nrf2-null muscle stem cells (MuSCs) isolated by fluorescence-activated cell sorting died in vitro of apoptosis and were not rescued by antioxidative reagents. Although their proliferation was still impaired, the Nrf2-null MuSCs attached to myofibers activated and divided normally, at least in the first round. To elucidate these discrepancies of MuSCs behaviors, we focused on the basal lamina, because both in vivo and single myofiber culture allow MuSCs within the basal lamina to become activated. In a basal lamina-disrupted model, Nrf2-null mice exhibited remarkable regeneration defects without increased levels of reactive oxidative species in MuSCs, suggesting that the existence of the basal lamina affects the survival of Nrf2-null MuSCs. Taken together, these results suggest that the basal lamina compensates for the loss of Nrf2, independent of the antioxidative roles of Nrf2. In addition, experimental conditions might explain the discrepant results of Nrf2-null regenerative ability.

The roles of muscle stem cells in muscle injury, atrophy and hypertrophy

Skeletal muscle is composed of multinuclear cells called myofibers. Muscular dystrophy (a genetic muscle disorder) induces instability in the cell membrane of myofibers and eventually causes myofibre damage. Non-genetic muscle disorders, including sarcopenia, diabetes, bedridden immobility and cancer cachexia, lead to atrophy of myofibres. In contrast, resistance training induces myofibre hypertrophy. Thus, myofibres exhibit a plasticity that is strongly affected by both intrinsic and extrinsic factors. There is no doubt that muscle stem cells (MuSCs, also known as muscle satellite cells) are indispensable for muscle repair/regeneration, but their contributions to atrophy and hypertrophy are still controversial. The present review focuses on the relevance of MuSCs to (i) muscle diseases and (ii) hypertrophy. Further, this review addresses fundamental questions about MuSCs to clarify the onset or progression of these diseases and which might lead to development of a MuSC-based therapy.

Gm7325 is MyoD-dependently expressed in activated muscle satellite cells

The Gm7325 gene, bioinformatically identified in the mouse genome, encodes a small protein but has not been characterized until recently. Our gene expression analysis revealed that Gm7325 transcription is remarkably upregulated in injured skeletal muscle tissues. Activated satellite cells and immature myotubes were densely decorated with positive signals for Gm7325 mRNA in in situ hybridization analysis, while no obvious signals were observed in quiescent satellite cells and mature myofibers. In the 5'-flanking regions of mouse Gm7325 and its human homologue, conserved E-box motifs for helix-loop-helix transcription factors are repeatedly arranged around the putative promoter regions. Reporter gene assays suggested that MyoD, a master transcription factor for myogenesis, binds to the conserved E-box motifs to activate Gm7325 expression. Therefore, Gm7325, as a novel MyoD-target gene, is specifically induced in activated satellite cells, and may have an important role in skeletal myogenesis.

Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is a progressive neuromuscular disease caused by expanded CUG repeats, which misregulate RNA metabolism through several RNA-binding proteins, including CUG-binding protein/CUGBP1 elav-like factor 1 (CUGBP1/CELF1) and muscleblind 1 protein. Mutant CUG repeats elevate CUGBP1 and alter CUGBP1 activity via a glycogen synthase kinase 3β (GSK3β)-cyclin D3-cyclin D-dependent kinase 4 (CDK4) signaling pathway. Inhibition of GSK3β corrects abnormal activity of CUGBP1 in DM1 mice [human skeletal actin mRNA, containing long repeats ( HSA^LR) model]. Here, we show that the inhibition of GSK3β in young HSA^LR mice prevents development of DM1 muscle pathology. Skeletal muscle in 1-yr-old HSA^LR mice, treated at 1.5 mo for 6 wk with the inhibitors of GSK3, exhibits high fiber density, corrected atrophy, normal fiber size, with reduced central nuclei and normalized grip strength. Because CUG-GSK3β-cyclin D3-CDK4 converts the active form of CUGBP1 into a form of translational repressor, we examined the contribution of CUGBP1 in myogenesis using Celf1 knockout mice. We found that a loss of CUGBP1 disrupts myogenesis, affecting genes that regulate differentiation and the extracellular matrix. Proteins of those pathways are also misregulated in young HSA^LR mice and in muscle biopsies of patients with congenital DM1. These findings suggest that the correction of GSK3β-CUGBP1 pathway in young HSA^LR mice might have a positive effect on the myogenesis over time.-Wei, C., Stock, L., Valanejad, L., Zalewski, Z. A., Karns, R., Puymirat, J., Nelson, D., Witte, D., Woodgett, J., Timchenko, N. A., Timchenko, L. Correction of GSK3β at young age prevents muscle pathology in mice with myotonic dystrophy type 1.

interleukin-11 induces and maintains progenitors of different cell lineages during Xenopus tadpole tail regeneration

Unlike mammals, Xenopus laevis tadpoles possess high ability to regenerate their lost organs. In amphibians, the main source of regenerated tissues is lineage-restricted tissue stem cells, but the mechanisms underlying induction, maintenance and differentiation of these stem/progenitor cells in the regenerating organs are poorly understood. We previously reported that interleukin-11 (il-11) is highly expressed in the proliferating cells of regenerating Xenopus tadpole tails. Here, we show that il-11 knockdown (KD) shortens the regenerated tail length, and the phenotype is rescued by forced-il-11-expression in the KD tadpoles. Moreover, marker genes for undifferentiated notochord, muscle, and sensory neurons are downregulated in the KD tadpoles, and the forced-il-11-expression in intact tadpole tails induces expression of these marker genes. Our findings demonstrate that il-11 is necessary for organ regeneration, and suggest that IL-11 plays a key role in the induction and maintenance of undifferentiated progenitors across cell lineages during Xenopus tail regeneration. Xenopus laevis tadpoles have maintained their ability to regenerate various organs. Here, the authors show that interleukin-11 is necessary for organ regeneration, by inducing and maintaining undifferentiated progenitors across cell lineages during Xenopus tail regeneration.

Mechanosensing of matrix by stem cells: From matrix heterogeneity, contractility, and the nucleus in pore-migration to cardiogenesis and muscle stem cells in vivo

Stem cells are particularly 'plastic' cell types that are induced by various cues to become specialized, tissue-functional lineages by switching on the expression of specific gene programs. Matrix stiffness is among the cues that multiple stem cell types can sense and respond to. This seminar-style review focuses on mechanosensing of matrix elasticity in the differentiation or early maturation of a few illustrative stem cell types, with an intended audience of biologists and physical scientists. Contractile forces applied by a cell's acto-myosin cytoskeleton are often resisted by the extracellular matrix and transduced through adhesions and the cytoskeleton ultimately into the nucleus to modulate gene expression. Complexity is added by matrix heterogeneity, and careful scrutiny of the evident stiffness heterogeneity in some model systems resolves some controversies concerning matrix mechanosensing. Importantly, local stiffness tends to dominate, and 'durotaxis' of stem cells toward stiff matrix reveals a dependence of persistent migration on myosin-II force generation and also rigid microtubules that confer directionality. Stem and progenitor cell migration in 3D can be further affected by matrix porosity as well as stiffness, with nuclear size and rigidity influencing niche retention and fate choices. Cell squeezing through rigid pores can even cause DNA damage and genomic changes that contribute to de-differentiation toward stem cell-like states. Contraction of acto-myosin is the essential function of striated muscle, which also exhibit mechanosensitive differentiation and maturation as illustrated in vivo by beating heart cells and by the regenerative mobilization of skeletal muscle stem cells.

Dnmt3a Regulates Proliferation of Muscle Satellite Cells via p57Kip2

Cell differentiation status is defined by the gene expression profile, which is coordinately controlled by epigenetic mechanisms. Cell type-specific DNA methylation patterns are established by chromatin modifiers including de novo DNA methyltransferases, such as Dnmt3a and Dnmt3b. Since the discovery of the myogenic master gene MyoD, myogenic differentiation has been utilized as a model system to study tissue differentiation. Although knowledge about myogenic gene networks is accumulating, there is only a limited understanding of how DNA methylation controls the myogenic gene program. With an aim to elucidate the role of DNA methylation in muscle development and regeneration, we investigate the consequences of mutating Dnmt3a in muscle precursor cells in mice. Pax3 promoter-driven Dnmt3a-conditional knockout (cKO) mice exhibit decreased organ mass in the skeletal muscles, and attenuated regeneration after cardiotoxin-induced muscle injury. In addition, Dnmt3a-null satellite cells (SCs) exhibit a striking loss of proliferation in culture. Transcriptome analysis reveals dysregulated expression of p57Kip2, a member of the Cip/Kip family of cyclin-dependent kinase inhibitors (CDKIs), in the Dnmt3a-KO SCs. Moreover, RNAi-mediated depletion of p57Kip2 replenishes the proliferation activity of the SCs, thus establishing a role for the Dnmt3a-p57Kip2 axis in the regulation of SC proliferation. Consistent with these findings, Dnmt3a-cKO muscles exhibit fewer Pax7⁺ SCs, which show increased expression of p57Kip2 protein. Thus, Dnmt3a is found to maintain muscle homeostasis by epigenetically regulating the proliferation of SCs through p57Kip2.

How muscle homeostasis is maintained is not completely elucidated yet. Epigenetic disorders such as Beckwith-Wiedemann syndrome, which causes hypergrowth of skeletal muscles and rhabdomyosarcoma, indicate that epigenetic regulations such as DNA methylation, contribute to this homeostasis control. DNA methylation is mediated by DNA methyltransferases, such as Dnmt3a and Dnmt3b, which are de novo DNA methyltransferases. The role of DNA methylation in somatic stem cells is not completely understood, although it has been shown to be indispensable in differentiation of primordial germ cells and embryonic stem cells. In this report, we investigated the role of Dnmt3a in muscle satellite cells by analyzing Dnmt3a-conditional knockout (cKO) mice in which Dnmt3a loci are deleted utilizing Cre-recombinase driven by Pax7 or Pax3 promoters that are specifically activated in the muscle precursor lineage. The loss of Dnmt3a in cKO mice causes decreased muscle mass and significantly impaired muscle regeneration. Moreover, Dnmt3a loss also results in a striking loss of proliferation of SCs, which is caused by mis-expression of a cyclin-dependent kinase inhibitor, p57Kip2. Therefore, our findings suggest that DNA methylation plays an essential role in muscle homeostasis.

Visualizing the Functional Heterogeneity of Muscle Stem Cells

Skeletal muscle stem cells are satellite cells that play crucial roles in tissue repair and regeneration after muscle injury. Accumulating evidence indicates that satellite cells are genetically and functionally heterogeneous, even within the same muscle. A small population of satellite cells possesses "stemness" and exhibits the remarkable ability to regenerate through robust self-renewal when transplanted into a regenerating muscle niche. In contrast, not all satellite cells self-renew. For example, some cells are committed myogenic progenitors that immediately undergo myogenic differentiation with minimal cell division after activation. Recent studies illuminate the cellular and molecular characteristics of the functional heterogeneity among satellite cells. To evaluate heterogeneity and stem cell dynamics, here we describe methods to conduct a clonal analysis of satellite cells and to visualize a slowly dividing cell population.

Intrinsic and extrinsic mechanisms regulating satellite cell function

Muscle stem cells, termed satellite cells, are crucial for skeletal muscle growth and regeneration. In healthy adult muscle, satellite cells are quiescent but poised for activation. During muscle regeneration, activated satellite cells transiently re-enter the cell cycle to proliferate and subsequently exit the cell cycle to differentiate or self-renew. Recent studies have demonstrated that satellite cells are heterogeneous and that subpopulations of satellite stem cells are able to perform asymmetric divisions to generate myogenic progenitors or symmetric divisions to expand the satellite cell pool. Thus, a complex balance between extrinsic cues and intrinsic regulatory mechanisms is needed to tightly control satellite cell cycle progression and cell fate determination. Defects in satellite cell regulation or in their niche, as observed in degenerative conditions such as aging, can impair muscle regeneration. Here, we review recent discoveries of the intrinsic and extrinsic factors that regulate satellite cell behaviour in regenerating and degenerating muscles.

Summary: This Review discusses how satellite stem cell behaviour is regulated during regeneration and degeneration by a complex balance between extrinsic cues and intrinsic regulatory mechanisms.

Pro-Insulin-Like Growth Factor-II Ameliorates Age-Related Inefficient Regenerative Response by Orchestrating Self-Reinforcement Mechanism of Muscle Regeneration

Sarcopenia, age-related muscle weakness, increases the frequency of falls and fractures in elderly people, which can trigger severe muscle injury. Rapid and successful recovery from muscle injury is essential not to cause further frailty and loss of independence. In fact, we showed insufficient muscle regeneration in aged mice. Although the number of satellite cells, muscle stem cells, decreases with age, the remaining satellite cells maintain the myogenic capacity equivalent to young mice. Transplantation of young green fluorescent protein (GFP)-Tg mice-derived satellite cells into young and aged mice revealed that age-related deterioration of the muscle environment contributes to the decline in regenerative capacity of satellite cells. Thus, extrinsic changes rather than intrinsic changes in satellite cells appear to be a major determinant of inefficient muscle regeneration with age. Comprehensive protein expression analysis identified a decrease in insulin-like growth factor-II (IGF-II) level in regenerating muscle of aged mice. We found that pro- and big-IGF-II but not mature IGF-II specifically express during muscle regeneration and the expressions are not only delayed but also decreased in absolute quantity with age. Supplementation of pro-IGF-II in aged mice ameliorated the inefficient regenerative response by promoting proliferation of satellite cells, angiogenesis, and suppressing adipogenic differentiation of platelet derived growth factor receptor (PDGFR)α(+) mesenchymal progenitors. We further revealed that pro-IGF-II but not mature IGF-II specifically inhibits the pathological adipogenesis of PDGFRα(+) cells. Together, these results uncovered a distinctive pro-IGF-II-mediated self-reinforcement mechanism of muscle regeneration and suggest that supplementation of pro-IGF-II could be one of the most effective therapeutic approaches for muscle injury in elderly people.